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Investigation of Polymers Used in Lithium
Oxygen Batteries as Electrolyte and
Cathode Materials
A thesis presented for the degree of Master by Research
By
Jinqiang Zhang, B. Sc.
University of Technology, Sydney
2013
i
Certificate of original authorship
I certify that the work in this thesis has not previously been submitted for a degree nor has it been submitted as part of requirements for a degree except as fully acknowledged within the text.
I also certify that the thesis has been written by me. Any help that I have received in my research work and the preparation of the thesis itself has been acknowledged. In addition, I certify that all information sources and literature used are indicated in the thesis.
Jinqiang Zhang
May 2013
ii
Acknowledgements
Although it’s only been one and half year since I started my Master research, I have
received many help from many people who supported me finishing this Master degree
project. First of all, I would like to express my sincere gratitude to my supervisor, Prof
Guoxiu Wang, for the guidance, support, and encouragement. I cannot thank you
enough for all the advice that leads to the improvement of me, all the patience when I
made a mistake, and all the concern about my research and life. I’m really grateful to
have you as my teacher and supervisor.
I wish to thank all my colleagues in the research team of Centre for Clean Energy
Technology, Dr Hao Liu, Dr Xiaodan Huang, Dr Bei Wang, Dr Bing Sun, Dr Ying
Wang, Mr Dawei Su, Mr Kefei Li, Mr Anjon Kumar Mondal, Mr Shuangqiang Chen,
Mr Yiying Wei, and Mr Xiuqiang Xie for the help both in my research and life during
my Master period. Special thanks would address to Dr Yueping (Jane) Yao, for the
administrative assistance and lab management as well as the great support for our life. It
is a great pleasure to work with all of you and I wish you all the best luck.
I wish to thank Rochelle Seneviratne for the assistance and patience during my study. I
am also grateful for the training and support in Faculty of Science from Dr Ronald
Shimmon, Dr Linda Xiao, and all the MAU staff. The help from all the teachers and
professors from school of chemistry and forensic science are much appreciated.
Financial support provided by the Australian Research Council (ARC) through the ARC
Linkage project (LP0989134), and ARC Discovery Project (DP1093855) is gratefully
acknowledged.
iii
Finally I would like to thank my family, my parents and my brother, for your help and
support for me. You always have faith in me even when I was confused about my life
and future in the toughest time. It is your loves that make me the person I am and allow
me to chase my dreams. Thank you and I love you all.
iv
Table of Contents
Certificate of original authorship ....................................................................................... i
Acknowledgements ........................................................................................................... ii
Table of Contents ............................................................................................................. iv
List of Figures ................................................................................................................ viii
List of Tables.................................................................................................................. xiii
Abstract .......................................................................................................................... xiv
Introduction ....................................................................................................................... 1
Chapter 1 Literature Review ............................................................................................. 4
1.1 Li-O2 batteries ......................................................................................................... 4
1.1.1 Anode ................................................................................................................ 7
1.1.2 Electrolyte ......................................................................................................... 8
1.1.3 Cathode ........................................................................................................... 16
1.1.4 Catalyst ........................................................................................................... 20
1.2 Polymer electrolyte ................................................................................................ 25
1.2.1 Solid polymer electrolyte ................................................................................ 26
1.2.2 Gel polymer electrolyte .................................................................................. 29
1.3 Conducting polymer .............................................................................................. 33
1.3.1 Synthesis method ............................................................................................ 36
1.3.2 Application...................................................................................................... 40
1.4 Summary ............................................................................................................... 46
v
Chapter 2 Experimental Methods.................................................................................... 47
2.1 Overview ............................................................................................................... 47
2.2 Materials and chemicals ........................................................................................ 48
2.3 Material preparation .............................................................................................. 49
2.3.1 In situ oxidation .............................................................................................. 50
2.3.2 Solution casting method.................................................................................. 50
2.4 Material characterization ....................................................................................... 51
2.4.1 X-ray Diffraction (XRD) ................................................................................ 51
2.4.2 Scanning electron microscope (SEM) ............................................................ 53
2.4.3 Fourier transform infrared spectroscopy (FT-IR) ........................................... 53
2.4.4 Thermogravimetric Analysis (TGA) .............................................................. 54
2.5 Electrode preparation and cell assembly ............................................................... 55
2.5.1 Electrode preparation ...................................................................................... 55
2.5.2 Cell assembly .................................................................................................. 55
2.6 Electrochemical characterization........................................................................... 56
2.6.1 Cyclic Voltammetry (CV) .............................................................................. 56
2.6.2 Electrochemical Impedance Spectroscopy (EIS) ............................................ 57
2.6.3 Linear Sweep Voltammetry (LSV) ................................................................. 58
2.6.4 Galvanostatic Charge and Discharge .............................................................. 59
Chapter 3 Low Molecular Weight Polyethylene Glycol Based Gel Polymer Electrolyte
Used in Li-O2 Batteries ................................................................................................... 61
3.1 Introduction ........................................................................................................... 61
vi
3.2 Experiment ............................................................................................................ 62
3.2.1 Preparation of PEG based GPEs ..................................................................... 62
3.2.2 Material characterization ................................................................................ 63
3.2.3 Electrochemical testing ................................................................................... 63
3.3 Results and discussion ........................................................................................... 64
3.4 Summary ............................................................................................................... 78
Chapter 4 Investigation of PVDF-HFP Based Gel Polymer Electrolyte Used in Li-O2
Batteries........................................................................................................................... 80
4.1 Introduction ........................................................................................................... 80
4.2 Experiment ............................................................................................................ 81
4.2.1 Preparation of PVDF-HFP based GPEs.......................................................... 81
4.2.2 Material characterization ................................................................................ 82
4.2.3 Electrochemical testing ................................................................................... 82
4.3 Results and discussion ........................................................................................... 83
4.4 Summary ............................................................................................................... 93
Chapter 5 Conducting Polymer-Doped Polypyrrole as An Effective Cathode Catalyst
for Li-O2 Batteries ........................................................................................................... 94
5.1 Introduction ........................................................................................................... 94
5.2 Experiment ............................................................................................................ 95
5.2.1 Synthesis of materials ..................................................................................... 95
5.2.2 Characterization of samples ............................................................................ 96
5.2.3 Electrochemical measurements....................................................................... 96
vii
5.3 Results and discussion ........................................................................................... 97
5.4 Summary ............................................................................................................. 106
Chapter 6 Conducting Polymer Coated CNT Used in Li-O2 Batteries with Enhanced
Electrochemical Performance ....................................................................................... 107
6.1 Introduction ......................................................................................................... 107
6.2 Experiment .......................................................................................................... 109
6.2.1 Synthesis of materials ................................................................................... 109
6.2.2 Characterization of samples .......................................................................... 110
6.2.3 Electrochemical measurements..................................................................... 110
6.3 Results and discussion ......................................................................................... 111
6.4 Summary ............................................................................................................. 116
Chapter 7 Conclusions .................................................................................................. 118
7.1 General conclusion .............................................................................................. 118
7.2 Outlook and future work ..................................................................................... 120
References ..................................................................................................................... 122
viii
List of Figures
Figure 1- 1 The gravimetric energy density of commonly used rechargeable batteries. .. 5
Figure 1- 2 Schematic mechanism of Li-O2 batteries during discharge and charge
process. .............................................................................................................................. 6
Figure 1- 3 Different types of Li-O2 batteries based on different architectures................ 9
Figure 1- 4 Two models of reaction mechanisms of Li-O2 batteries, (A) aqueous system
and (B) non-aqueous system ........................................................................................... 10
Figure 1- 5 Three different types of electrolyte filling on cathodes, (A) flooding, (B) dry
and (C) wetting ................................................................................................................ 11
Figure 1- 6 Schematic mechanism of decomposition of PC electrolyte in Li-O2 batteries
......................................................................................................................................... 12
Figure 1- 7 Cycle performance of Li-O2 batteries with TEGDME as electrolytes ......... 13
Figure 1- 8 Schematic mechanism of discharge process on porous carbon cathodes ..... 17
Figure 1- 9 The morphology study, discharge performance and discharge mechanism of
a hierarchical graphene ................................................................................................... 18
Figure 1- 10 Schematic mechanism of (a) side reactions of carbon cathode and
discharge products and (b) side reactions between electrolyte and carbon cathode ....... 19
Figure 1- 11 Discharge/charge profiles (left) and cycle performance (right) of nano gold
cathode in DMSO based electrolyte ................................................................................ 20
Figure 1- 12 Discharge/charge profile (left) and cycle performance (right) of graphene
cathode and carbon black cathode................................................................................... 21
Figure 1- 13 First galvanostatic charge of Li2O2 oxidation for various Li–O2 cells ....... 23
Figure 1- 14 Schematic mechanism of Li2O2 and Li2O forming on MnO2 catalyst ....... 24
Figure 1- 15 Schematic mechanism of Li+ movement through PEO based polymer
electrolyte ........................................................................................................................ 27
ix
Figure 1- 16 Schematic mechanism of the addition of ceramic fillers and the effect of
different particle sizes, (a) macro-size and (b) nano-size ............................................... 29
Figure 1- 17 Schematic presentation for functional role of PDMITFSI ionic liquid on
lithium deposition, (a) without and (b) with ionic liquid ................................................ 32
Figure 1- 18 The structures of the most commonly used conducting polymers ............. 33
Figure 1- 19 Conjugated orbitals formed in polyacetylene ............................................. 34
Figure 1- 20 Schematic illustration of synthesis mechanism of PPy .............................. 37
Figure 1- 21 Schematic illustration of synthesis mechanism of (A) PPy nanotube and
(B) PANI nanowire ......................................................................................................... 40
Figure 1- 22 Cycling performance of PPy/FC at (a) constant current density of 50 mAg-1
and (b) different current densities ................................................................................... 42
Figure 1- 23 Discharge/charge profiles (left) and resistance (right) of the LiFePO4
cathode (a) coated with PEDOT, (b) coated with PPy, (c) coated with C, and (d) pristine
particles ........................................................................................................................... 43
Figure 1- 24 a) Electron-transfer pathway for LiFePO4 particles partially coated with
carbon. b) Designed ideal structure for LiFePO4 particles with typical nano-size and a
complete carbon coating. c) Preparation process for the LiFePO4/carbon composite
including an in situ polymerization reaction and two typical restriction processes ........ 44
Figure 1- 25 (A) Morphology and cycle performance of PPy cathode [70], (B) nitrogen-
doped graphene derived from PANI and (C) Performance of PEDOT catalyst ............. 45
Figure 2- 1 Schematic illustration of the whole experiment process .............................. 47
Figure 2- 2 The preparation process of PPy when (NH4)2S2O8 was used as oxidant ..... 50
Figure 2- 3 Schematic drawing of Bragg’s law .............................................................. 52
Figure 2- 4 An example TGA result of polypyrrole coated silicon ................................ 54
Figure 2- 5 The structure of a Li-O2 battery ................................................................... 56
x
Figure 2- 6 A typical ESI Nyquist curve of a battery system ......................................... 58
Figure 2- 7 A typical result of LSV measurement .......................................................... 59
Figure 2- 8 An example charge and discharge curve of a Li-O2 battery ......................... 60
Figure 3- 1 The typical molecular structure of PEG or PEO .......................................... 61
Figure 3- 2 Cyclic voltammetry results of Li/GPE/Li type cells with (a) PEG and (b)
PEG with SiO2 addition .................................................................................................. 65
Figure 3- 3 Linear sweep voltammetry results of (a) Li/GPE/SS and (b) Li/GPE/CB-air
type cells.......................................................................................................................... 67
Figure 3- 4 The impedance spectra of PEG at different temperatures. (b) The calculated
ionic conductivity of PEG at different temperatures....................................................... 69
Figure 3- 5 (a) First discharge and charge profiles of Li-O2 batteries with PEG, PEG-
SiO2, TEGDME as electrolytes. (b) Partial enlarged view of first discharge and charge
profiles from 0-1500 mAhg-1 .......................................................................................... 71
Figure 3- 6 Discharge and charge profiles of Li-O2 batteries with (a) PEG, (b) PEG-
SiO2, and (c) TEGDME as electrolytes at fixed capacity to 500 mAhg-1 ....................... 72
Figure 3- 7 Cycle profiles of Li-O2 batteries with PEG, PEG-SiO2, and TEGDME as
electrolytes ...................................................................................................................... 74
Figure 3- 8 Structures of (a) PEG-based electrolyte and (b) PEG-SiO2-based electrolyte
......................................................................................................................................... 76
Figure 3- 9 XRD pattern of PEG before and after made into polymer electrolyte ......... 77
Figure 3- 10 XRD pattern of cathode after discharge in PEG polymer electrolyte ........ 78
Figure 4- 1 The typical structure of PVDF-HFP ............................................................. 80
Figure 4- 2 The cyclic voltammetry curve of Li/GPE/Li typed cell with TEGDME based
GPE as electrolyte ........................................................................................................... 84
xi
Figure 4- 3 Linear sweep voltammetry results of (a) Li/GPE/SS and (b) Li/GPE/CB-air
type cells.......................................................................................................................... 85
Figure 4- 4 The calculated ionic conductivity of PEG at different temperatures ........... 87
Figure 4- 5 The discharge and charge profiles in the first cycle of Li-O2 batteries using
different electrolyte ......................................................................................................... 88
Figure 4- 6 Discharge and charge profiles of Li-O2 batteries with TEGDME based GPE
as electrolytes at fixed capacity to 500 mAhg-1 .............................................................. 89
Figure 4- 7 Cycle profiles of Li-O2 batteries with PVDF-HFP based GPE and TEGDME
as electrolytes .................................................................................................................. 90
Figure 4- 8 Proposed structure of PVDF-HFP based GPE ............................................. 91
Figure 4- 9 The discharge and charge profiles in the first cycle of PC (a) and DMSO (c)
based GPEs and the cycling performance of PC (b) and DMSO (d) based GPEs .......... 92
Figure 5- 1 The typical structure of PPy ......................................................................... 94
Figure 5- 2 SEM images of the as-prepared (a) PPy-Cl and (b) PPy-ClO4, and (c) FT-IR
spectra of both PPy polymers .......................................................................................... 97
Figure 5- 3 The discharge-charge profiles and (b) cycling performance of PPy-Cl, PPy-
ClO4 and carbon black electrodes ................................................................................... 99
Figure 5- 4 The mechanism of (a) oxygen activation of PPy and (b) doping-undoping
process of PPy-Cl and PPy-ClO4 .................................................................................. 101
Figure 5- 5 FT-IR spectra of (a) PPy-Cl, (b) PPy-ClO4, and (c) carbon black electrodes
before discharge, after discharge and after charge process ........................................... 101
Figure 5- 6 (a) The charge-discharge profiles and (b) the cycling performance of carbon
black electrodes with and without LiCl additive........................................................... 104
Figure 5- 7 Schematic mechanism of discharge process on cathode with LiCl addition
....................................................................................................................................... 105
xii
Figure 6- 1 The typical structure of PEDOT ................................................................. 107
Figure 6- 2 The SEM images of (a) the bare CNT, the as-prepared (b) PPy/CNT1:2, (c)
PPy/CNT1:1, (d) PPy/CNT2:1, (e) PEDOT/CNT1:1, and (f) FT-IR spectra .................... 112
Figure 6- 3 The TGA spectra of as-prepared (a) PPy/CNT1:2, (b)PPy/CNT1:1, (c)
PPy/CNT2:1, and (d) PEDOT/CNT1:1 ............................................................................ 113
Figure 6- 4 (a) The discharge and charge profiles of as-prepared PPy/CNT1:2,
PPy/CNT1:1, PPy/CNT2:1, PEDOT/CNT1:1, and CNT electrodes. (b) Partially enlarged
profiles of as-prepared electrodes with capacity of 500 mAh g-1 ................................. 114
Figure 6- 5 The cycling performance of as-prepared prepared PPy/CNT1:2, PPy/CNT1:1,
PPy/CNT2:1, PEDOT/CNT1:1, and CNT electrodes ...................................................... 115
Figure 6- 6 6 The schematic mechanism of (a) PPy/CNT during cycling, and (b) the
block of O2 from PEDOT structure ............................................................................... 116
xiii
List of Tables
Table 2- 1 Materials and chemicals used in the research project .................................... 49
Table 5- 1 EDS results of PPy-Cl and PPy-ClO4 electrodes after cycling.................... 103
xiv
Abstract
It has been well established that the electrolytes and cathodes have a significant effect
on the electrochemical performance of lithium oxygen batteries. In this Master project,
polymers were employed as electrolyte and cathode materials due to their unique
superior properties. Using different methods, we synthesized suitable gel polymer
electrolytes and conducting polymer catalysts for lithium oxygen batteries. Techniques
such as field emission gun scanning electron microscopy, X-ray diffraction, and Fourier
transform infrared spectroscopy were used to characterize the physical properties.
Electrochemical analyses including the galvanostatic discharge and charge method, the
cyclic voltammetry, the linear sweep voltammetry and the impedance spectra were
conducted to determine the electrochemical performance for the as-prepared materials.
Gel polymer electrolytes based on low molecular weight polyethylene glycol were
prepared and used as electrolyte in lithium oxygen batteries. The as-prepared polymer
electrolytes showed improved stability compared with liquid electrolytes and exhibited
good performance in lithium oxygen batteries. Additionally, the addition of ceramic
filler SiO2 was found to reduce the stability of polymer electrolyte towards oxygen
reduction reaction although higher ionic conductivity was obtained. Polyethylene glycol
based gel polymer electrolyte without SiO2 addition exhibited excellent cycling
performance and it could be used for achieving long-life lithium oxygen batteries.
Poly(vinylidene fluoride-co-hexafluoropropylene) based gel polymer electrolytes were
prepared by solvent casting and employed as electrolytes in lithium oxygen batteries.
The stability of the gelled electrolyte with tetraethylene glycol dimethyl ether has been
greatly increased than the liquid one. The as-prepared polymer electrolyte was
demonstrated excellent cycling performances. This thesis also investigated the effect of
xv
different plasticizers on the performance of lithium oxygen batteries. The reason could
lie on the interactions among the components when the gelled structure was set. The
tetraethylene glycol dimethyl ether based gel polymer electrolyte showed the best
electrochemical performance and can be used for long-life lithium oxygen batteries.
Polypyrrole conducting polymers with different dopants have been synthesized and
applied as the cathode catalysts in lithium oxygen batteries. Polypyrrole polymers
exhibited an effective catalytic activity for oxygen reduction in lithium oxygen batteries.
It was discovered that dopant significantly influenced the electrochemical performance
of polypyrrole. The polypyrrole doped with Cl- demonstrated higher capactity and more
stable cyclability than that doped with ClO4-. Polypyrrole conducting polymers also
exhibited higher capacity and better cycling performance than that of carbon catalyst.
Conducting polymer coated carbon nanotubes were synthesized and used as catalysts in
lithium oxygen batteries. It was found that both polypyrrole and poly(3,4-
ethylenedioxythiophene) coated carbon nanotubes could provide high cycling
performance while polypyrrole based one exhibited higher capacities. The ratio of
conducting polymer coating also affected the electrochemical performance of lithium
oxygen batteries. The conducting polymer coated carbon nanotubes also showed better
performance than the bare carbon nanotubes.
1
Introduction
Due to the wide use of fossil fuels as a source of the energy supply in the last few
centuries, pollution, the limiting sources of fossil fuels and global warming are the main
issues for today’s society. Green energy devices such as batteries are in great demand to
suppress CO2 emission and air pollution, and electric vehicles are designed to replace
internal combustion engine cars. Finding alternative energy storages has never been so
important ever.
Lithium-ion (Li-ion) batteries, are one form of green energy storage devices, and have
been demonstrated as the most advanced alternative electrochemical power sources in
the last two decades. However, conventional Li-ion batteries cannot meet all the
requirements for usage in large-scaled applications such as the electric vehicles due to a
limitation on its theoretical energy density. Lithium-oxygen (Li-O2) batteries have
attracted intensive research recently as an alternative choice. The use of oxygen directly
drawn from air as the cathode reactant makes their theoretical energy densities almost
equal to gasoline. However, the commercialization of Li-O2 batteries has not yet been
realized. The development of effective electrolytes and air cathodes are the critical
challenges to achieve high performance Li-O2 batteries. Many approaches have been
made and polymers have been researched as the main components for Li-O2 batteries
due to their superior properties such as stability and unique electrochemical properties.
The main purpose of this thesis is to find effective ways to improve the performance of
Li-O2 batteries by employing polymer electrolytes and conducting polymers in this
battery system. The materials were successfully prepared and characterized as
electrolyte and cathode materials. The outline of each chapter is listed below,
2
Chapter 1 contains an introduction to Li-O2 batteries, polymer electrolytes, and
conducting polymers. Recent progresses in anode, electrolyte, cathode and
catalyst materials used in Li-O2 batteries are reviewed. Research in polymer
electrolytes, both solid and gel ones, used in lithium batteries are also
introduced. This chapter includes recent studies on conducting polymers such as
polypyrrole, polyamine, and poly(3,4-ethylenedioxythiophene) and the
application in the lithium batteries at the same time.
Chapter 2 provides the methodology of research applied in this thesis. The
material preparation methods, physical characterization methods, the electrode
preparation methods and electrochemical testing methods are illustrated in this
chapter. The preparation methods were solution casting for polymer electrolytes
and in situ chemical synthesis method for conducting polymers. Field emission
gun scanning electron microscopy was used for investigating morphology and
X-ray diffraction was employed to determine the crystal structures of as-
prepared polymer electrolytes, conducting polymers and discharge products.
Thermogravimetric analysis was used to determine the content of each
component in composite materials. Galvanostatic discharge and charge and a
series of electrochemical characterizations such as cyclic voltammetry, linear
sweep voltammetry, and impedance spectra were performed to evaluate the
properties of as-prepared materials.
Chapters 3 and 4 report the use of polyethylene glycol and poly(vinylidene
fluoride-co-hexafluoropropylene) based gel polymer electrolytes used in Li-O2
batteries, respectively. In Chapter 3, the polyethylene glycol based polymer
electrolyte was prepared and investigated. The effect of nano-sized silica
addition was also investigated. In Chapter 4, the electrochemical performances
3
of poly(vinylidene fluoride-co-hexafluoropropylene) based polymer electrolytes
with different plasticizer such as propylene carbonate, dimethyl sulfoxide, and
tetraethylene glycol dimethyl ether were investigated.
Chapters 5 and 6 describe the experiments on the use of conducting polymers in
Li-O2 batteries. The synthesis and comparison of Polypyrrole with different
dopants were reported in Chapter 5. The catalytic mechanism was also
discussed. In Chapter 6, polypyrrole coated carbon nanotubes with different
polymer ratio were prepared and tested for Li-O2 batteries. Poly(3,4-
ethylenedioxythiophene) coated carbon nanotubes were also synthesized and
compared with polypyrrole based ones.
Chapter 7 concludes this thesis, and provide a brief summary of the previous
work on the gel polymer electrolytes and conducting polymers and an outlook
for future research of polymers used in Li-O2 batteries.
4
Chapter 1 Literature Review
1.1 Li-O2 batteries
Due to the fast growth of economy and technology, finding large-scale energy storage is
much more important today than ever. Coal-based and oil-based sources have been used
for centuries because they can provide large quantity of energy to meet energy demands.
However, due to the limited sources of coal and oil and pollution by CO2 and other
waste gas emissions, there is the great urgency to find more sustainable and
environmentally friendly energy. Therefore, electrochemical devices for energy storage
and conversion become more and more important. Devices including super capacitors,
fuel cells and batteries are used to solve the problems raised by using coal- and oil-
based energy sources. Rechargeable batteries are considered as good candidates for next
generation energy storage devices. Figure 1-1 shows the energy densities of commonly
used rechargeable batteries. Among all energy storage technologies, lithium batteries
have drawn great interest because of their high energy density and light weight [1, 2].
Li-ion batteries have been already used in all kinds of portable electronic devices such
as cell phones and laptops [3]. However, the theoretical energy density of Li-ion
batteries is not high enough for electrical vehicles, which has limited their application.
In order to meet the demands of society, new generations of batteries are urgently
needed.
5
Figure 1- 1 The gravimetric energy density of commonly used rechargeable batteries.
Figure 1-1 The gravimetric energy density of commonly used rechargeable batteries
[4].
Li-O2 batteries, also known as Li-air batteries, have shown potentials for large scale
applications such as electric vehicles because of their extremely high theoretic energy
density (11140 kW kg-1), which is comparable to gasoline [4-8]. Another advantage of
Li-O2 batteries is that they do not need to storage cathode reactants inside the batteries.
The fact that the reactant is oxygen and oxygen can be replenished at any time from air
provides the possibility to reduce the weight of batteries and make them suitable for
portable devices. A Li-O2 battery is the combination of a lithium battery and a fuel cell,
which has a lithium ion system and an oxygen system at the same time. A typical Li-O2
battery consists of a lithium foil used as the anode, electrolyte which provides lithium
6
Figure 1- 2 Schematic mechanism of Li-O2 batteries during discharge and charge process.
ion pathways between the anode and the cathode, and an air electrode to allow the flow
of air into the system. During the discharge process, oxygen is consumed on the cathode
and forms discharge products while Li+ ions in electrolyte can be replenished by lithium
anode. The reaction can be reversed during the charging process. The whole process is
exhibited in Figure 1-2.
Figure 1-2 Schematic mechanism of Li-O2 batteries during the discharge and charge
process [4].
Despite all the superiorities they can provide, current Li-O2 batteries are far from
satisfactory [4-8]. The main issues of Li-O2 batteries are listed below,
Low practical capacity due to insufficient porosity in the cathode electrodes for
accommodating the discharge products
Poor cycleablility due to the decomposition of organic electrolytes
High reactivity of lithium metal in aqueous electrolyte
Large discharge-charge over-potentials
Side-reactions due to contaminates such as CO2 and H2O
7
In order to solve all the above issues, a proper knowledge Li-O2 battery is necessary. In
the next section, each component of Li-O2 batteries will be reviewed.
1.1.1 Anode
The anode in a Li-O2 battery provides Li+ for electrochemical reactions. Lithium metal
is usually used directly as anode. The anode reaction is shown below,
(1-1)
During the discharge process, lithium metal is oxidized and releases Li+ into the
electrolyte. The reaction is reversed during the charge process.
Although metallic lithium is light-weight and has a very high energy density, the direct
use of lithium metal as the anode material in Li-O2 batteries still causes problems
including dendrite growth during cycling and side reactions towards electrolyte and O2
crossover from the porous cathodes [2, 8]. This may have a detrimental influence on the
electrochemical performance of Li-O2 batteries. Some research groups tried to replace
lithium metal anode with lithium alloying compounds such as LixSi [9] or partially
charged LiFeO4 [10] which are proven to have good cycling performance. Anther
strategy is to process the lithium metal before use. Bruce et al. used 0.1 M LiClO4-
propylene carbonate (PC) electrolyte to process lithium foil before it was used as the
anode in a Dimethyl sulfoxide (DMSO) electrolyte [11]. It is believed that this process
is effective to stabilize the lithium metal in DMSO electrolyte and to ensure good
cycling performance at the same time.
Besides processing the anode materials, a lithium protection layer can also be used,
especially in aqueous electrolyte system, because lithium metal can aggressively react
with water electrolyte and cause severe safety issues. Many Li+ conducting but
8
electronically insulating membranes, such as LiSICON glass ceramics [8, 12, 13], are
employed in aqueous Li-O2 batteries. Polymer electrolyte is also a good candidature and
will be introduced in the following section.
1.1.2 Electrolyte
Electrolytes serve as a Li+ pathway as well as a separator between anodes and cathodes.
Therefore it is believed that ideal electrolytes for Li-O2 batteries should have superior
properties including high conductivity, wide electrochemical stability, acceptable
lithium transference number, compatibility with both electrodes, stability towards
battery reactions, high oxygen solubility, and low volatility as well [4, 5, 7].
There are four types of Li-O2 batteries, depending on the architecture differences as
shown in Figure 1-3. They are aprotic, aprotic, aqueous, solid, and mixed aqueous-
aprotic systems [2, 4, 8]. Although in all cases the discharge process involves the
reactions with oxygen, the mechanism of each system is different from others
depending on the electrolyte used. For simplicity, all four systems are divided into two
types, aqueous and non-aqueous systems. In the non-aqueous systems, the oxygen
reduction reaction products are insoluble in the electrolytes, which mean the products
will deposited in the porous structures of cathodes and may hinder the oxygen
transference and cause termination of battery reactions. Therefore, the actual capacity of
non-aqueous Li-O2 batteries mainly depends on the porosity of cathodes. In aqueous
systems, the discharge products are soluble in the water systems. Figure 1-4 shows two
models of reaction mechanisms of Li-O2 batteries, which illustrates the difference
clearly. The reactions in aqueous systems are often described as three-phase reactions,
consisting of solid phase (cathodes and catalysts), liquid phase (electrolyte), and gas
phase (O2), while the ones in non-aqueous systems are called two-phase reactions
9
Figure 1- 3 Different types of Li-O2 batteries based on different architectures.
without the gas phase which means only the oxygen dissolved in the liquid electrolyte
may be involved in the oxygen reduction reactions. The difference here between two
systems is the solubility of discharge products which has been mentioned above. The
battery reactions are listed below [14-16],
(Aqueous acidic media) (1-2)
(Aqueous alkaline media) (1-3)
(Non-aqueous media) (1-4)
(Non-aqueous media) (1-5)
Figure 1-3 Different types of Li-O2 batteries based on different architectures [4].
10
Figure 1- 4 Two models of reaction mechanisms of Li-O2 batteries, (A) aqueous system and (B) non-aqueous system
Figure 1-4 Two models of reaction mechanisms of Li-O2 batteries, (A) aqueous system
and (B) non-aqueous system [7].
Despite the difference of reactions of both non-aqueous and aqueous media, the
performance of Li-O2 batteries with both electrolytes are affected by the amount of
electrolytes filling of the cathodes. Generally there are three kinds of filling, known as
flooding, dry, and wetting. Figure 1-5 shows the three fillings of electrolytes. It is
believed that both oxygen dissolved in electrolytes and in gas phase can participate in
the battery reactions. However, it is obvious that oxygen in solution is less mobile than
one in the gas phase. Therefore, when the electrolyte floods the cathode, the kinetic is
very low because of the slow dissolving process of oxygen. The oxygen reduction
reaction more likely happens on the air side of the cathode. On the other hand when the
amount of electrolyte is not sufficient, or dry electrolyte, the gas oxygen is very easy to
penetrate into cathode. However, the Li+ cannot reach the interface of electrolyte and
cathode, which also leads to the low capacity of Li-O2 batteries. Therefore, the ideal
amount of electrolyte is just wetting the cathode to provide sufficient Li+ for oxygen
reduction reaction and allow oxygen transferring into cathode at the same time [17].
Zhang et al. has provided evidence about the effects of electrolyte filling towards
performance of Li-O2 batteries [18].
11
Figure 1- 5 Three different types of electrolyte filling on cathodes, (A) flooding, (B) dry and (C) wetting
Figure 1-5 Three different types of electrolyte filling on cathodes, (A) flooding, (B) dry
and (C) wetting [7].
Since the first report on non-aqueous electrolytes used in Li-O2 batteries by Abraham
and Jiang [19], this field has been a heated topic and attracted much attention. Generally
speaking, there are three different types of non-aqueous electrolytes, such as organic
liquid solvent electrolyte, hydrophobic ionic liquids and polymer electrolyte. All three
types share the same mechanism displayed in equation (4) and (5).
Electrolytes based on alkyl carbonate solvents including propylene carbonate (PC),
dimethyl carbonate (DMC), ethylene carbonate (EC) and so on, are initially used in Li-
O2 battery owing to their low volatility, good Li compatibility, high ionic conductivity,
oxygen solubility, and large operating window [5]. These carbonate-based electrolytes
have been widely used in the conventional Li-ion batteries However, recent studies
proved that organic carbonates are not suitable for long-life Li-O2 batteries because the
discharge by-products other than Li2O2 are formed in these systems [20-22]. They
believe superoxide radicals are formed during discharge process and these species
attack the carbonates through nucleophilic reactions. It results in the formation of H2O,
CO2, Li2CO3 and lithium alky-carbonates including HCO2Li and CH3CO2Li. A possible
mechanism of decomposition of PC is exhibited in Figure 1-6. Although these products
12
Figure 1- 6 Schematic mechanism of decomposition of PC electrolyte in Li-O2 batteries
can be oxidized in the charge process with evolution of H2O and CO2, the whole battery
suffers from low cycleability and fading of capacity because of the starvation of
electrolyte and accumulation of products on the cathode surface.
Figure 1-6 Schematic mechanism of decomposition of PC electrolyte in Li-O2 batteries
[22].
Due to the side reactions exhibited above, electrolyte based on carbonate solvents are no
longer favourable in Li-O2 batteries despite its superiorities. In order to achieve long life
Li-O2 batteries, stability of electrolyte should be sufficient. The use of additives or co-
solvents is a way to improve the stability and performance of carbonate-based
electrolytes. For instance, trifluoroethlphosphates and phosphite have been added into
the Li-triflate-PC/DME or Li-triflate-PC system and is found to have superior
performance during discharge and charge process [23, 24]. However, addition of such
additives in electrolyte causes increase of the viscosity which finally affects the ionic
conductivity. Therefore finding a substitute for carbonate-based electrolyte is needed.
Ether-based electrolytes, known for their better stability than carbonate-based ones, are
tested in Li-O2 batteries and they have been proven that Li2O2 are formed in first few
cycles [25, 26]. For instance, Read employed 1,3-dioxolane (DOL) and 1,2-
13
Figure 1- 7 Cycle performance of Li-O2 batteries with TEGDME as electrolytes
dimethoxyethane (DME) in Li-O2 batteries in 2006 [27]. Xu et al. also used several
ether-based solvents such as DME, diethylene glycol dimethyl ether (DG), diethylene
glycol diethyl ether (EDG), diethylene glycol dibutyl ether (BDG), 1,2-diethoxyethane
(DEE), and 1-tert-butoxy-2-ethoxyethane (BEE) and compared the performance of each
candidate. In 2012, tetra(ethylene) glycol dimethyl ether (TEGDME) was found to be
stable towards oxygen reduction reaction [28] and Jun et al. could even achieve at least
30 cycles at 500 mAg-1 [29]. The result is exhibited in Figure 1-7. However, ether-based
electrolytes are found to have some negative properties limiting their applications in Li-
O2 batteries, such as high viscosity for TEGDME, and high volatility for DME.
Moreover, recent researches indicate that ether-based electrolytes are still not stable
enough towards superoxide radicals. They claimed that even though Li2O2 was detected
in first discharge, the amount of Li2O2 reduces significantly and the discharge products
were then dominated by electrolyte composition products [30]. Bryantsev et al. used
theoretical calculation to predict the autoxidation stability of ether- and amide-based
electrolyte solvents and found that every electrolyte, more or less, suffers from
decomposition in Li-O2 batteries [31]. In this way, ether-based electrolytes may not be
the best choices for Li-O2 batteries.
Figure 1-7 Cycle performance of Li-O2 batteries with TEGDME as electrolytes [29].
Current density is 500mA g-1.
14
Dimethyl sulfoxide (DMSO) based electrolyte was used by Xu et al. and showed the
possibility of long-term use in Li-O2 batteries [32]. Thotiyl et al. also demonstrated
although accompanied by small decomposition during cycling, DMSO is much more
stable towards oxygen reduction reaction than ether-based ones [33]. Xu et al. also used
tetramethylene sulfone (TMS) based electrolytes in Li-O2 batteries and showed the
stability towards cycles [34].
Another approach is the use of silanes as electrolytes. It is believed that silane-based
electrolytes can provide higher stability in Li-O2 batteries due to the presence of Si-O
groups [35, 36]. Hydrophobic ionic liquids (ILs) are also considered to be used in Li-O2
batteries. ILs are known for their lithium compatibility, low volatility, and also stability
and showed superior performance when employed in Li-O2 batteries [37-39]. Recently,
ILs have also been used in polymer electrolytes which will be mentioned later in
polymer electrolyte section.
Although stability of electrolytes is an important issue for practical usage, it is not the
only factor that can affect the performance of Li-O2 batteries. It is believed that oxygen
solubility, oxygen diffusion rate [40], water contents [41], lithium salts [42-44], and
even binders [45] all have great influence on the performance. Therefore, choosing
suitable electrolytes for non-aqueous Li-O2 batteries is complicated and still needs great
efforts.
Unlike non-aqueous electrolytes with so many choices, the choice of aqueous
electrolyte is limited only to acidic or basic solutions, either weak or strong. The most
attractive properties of Li-O2 batteries with aqueous electrolytes are the stability of
electrolyte solution and the solubility of discharge products which differs from products
in non-aqueous ones. However, due to the highly reactive property of lithium metal
15
towards water solution, it is not surprised that most research focus on the protection of
the lithium anode. Typically, Li+-conducting but electronically insulating membranes
like LISICON or NASICON are used between anodes and electrolytes [46, 47]. Without
these protecting layers, the aqueous Li-O2 batteries cannot function.
Another difference between aqueous electrolytes and non-aqueous electrolytes is the
participation of electrolyte solvents in the battery reactions. As discharge process
proceeds, the solvents are continuously consumed and the amount of discharge product
LiOH gradually increases. However, although LiOH can dissolve into aqueous
electrolyte, saturation can be finally reached as the battery reaction proceeds. As a
result, the product LiOH precipitates on the surface of cathodes and protective ceramic
membranes which eventually leads to the clogging of porous structure and termination
of battery reaction. This often causes low capacity and efficiency. In order to improve
the performance of aqueous Li-O2 batteries, several solutions have been come up with
[48]. A flow cell construction has been used to replenish the electrolyte with fresh
solution continuously and LiOH is drawn from electrolyte to make lithium metal to
replenish the lithium anode [12, 49]. In this way the battery capacity is highly improved
and it can also achieve good efficiency. Besides mechanical method to refresh
electrolytes, blocking membranes can be employed between cathodes and electrolytes.
These blocking membranes are also known as anion-exchange membranes which allow
OH- formed in the cathode to be transport out and keep blocking Li+ from entering the
cathode at the same time. Many have also been demonstrated to have the ability to block
any carbonates which can form Li2CO3 in the electrolyte [8]. Another approach is to add
a third electrode into the system for oxygen evolution on recharging. It is found to be
much more efficient oxidizing LiOH than allowing LiOH precipitate onto the cathodes
[15]. Using an acidic media as electrolyte is also a way to prevent LiOH from blocking
16
the cathode porosity corresponding to equation (2). However, when the battery reaction
proceeds, the pH of electrolyte gradually increases and finally leads to the precipitate of
LiOH in the electrolytes. This means a great achievement for producing long-life and
effective aqueous Li-O2 batteries is still needed.
Among all designs of Li-O2 batteries, the system with non-aqueous media is more
preferred because of the stability, rechargeability and also safety.
1.1.3 Cathode
It is believed that cathodes, also known as air electrodes, play important roles in Li-O2
batteries because the battery reactions, both oxygen reduction and oxygen evolution
reactions, happen in the cathodes. The most commonly used cathode matrix is carbon
due to its intrinsic conductivity and light weight. According to the mechanisms of Li-O2
batteries shown in Figure 1-8, the characteristics such as porosity, surface area and
morphology of carbon can affect the performance greatly. Since the discharge products
of non-aqueous Li-O2 batteries cannot dissolve into electrolyte and precipitate in the
pores of cathodes which usually hinder the diffusion of oxygen, electrolyte and even
electrons, the capacity of Li-O2 batteries in practical use is far less than theoretical
calculation. Therefore the porosity and pore volume of electrodes can be very important
for maintain low over-potentials and high capacity [50, 51]. However, the relationship
between surface area and discharge capacity is still not clear [52, 53].
17
Figure 1- 8 Schematic mechanism of discharge process on porous carbon cathodes
Figure 1-8 Schematic mechanism of discharge process on porous carbon cathodes [4].
However, it is believed that the effective use of pores, not the size or total volume of
pores determines the highest performance of Li-O2 batteries. It is believed that ideal
pore structures of cathodes should be able to accommodate a large amount of insoluble
discharge products without hindering the transportation of oxygen, Li+ and even
electrons. Among all porous structures, mesoporous structures seem to have the most
suitable properties for Li-O2 batteries. For instance, mesoporous carbon template [54,
55], mesocellular carbon foam [52], and highly mesoporous nitrogen-doped carbon [56,
57] are used in Li-O2 batteries and proven to have enhanced performance because of the
mesoporous structures. Carbon nantubes [58-62], and carbon nanofibers [59, 63] are
also used in Li-O2 batteries. It is believed that 1-Dimensional structures like nanofibers
or nanotubes can also help maintain structures with sufficient porosity for oxygen
diffusion and accommodation of discharge products.
Graphene nanosheets (GNSs), as one type of carbon materials, have attracted great
attention for Li-O2 battery application because of their unique morphology and
18
Figure 1- 9 The morphology study, discharge performance and discharge mechanism of a hierarchical graphene
structure. Besides the microporous channels for oxygen transportation, GNSs also have
plenty of defects which help facilitate the formation of discharge products which can
provide very high capacity and good cycle performance at the same time [64-67]. Xiao
et al. used a hierarchical graphene structure in Li-O2 battery and achieved extremely
high capacity [64]. The result is shown in Figure 1-9.
Figure 1-9 The morphology study, discharge performance and discharge mechanism of
a hierarchical graphene [64]. Current density is 0.1 mA cm-2.
However, there are also some issues for using carbon matrices in Li-O2 batteries. The
intrinsic reactive properties of carbon make it possible to react with electrolytes and
discharge products. The side reactions affect the performance of Li-O2 batteries greatly.
McCloskey et al. have demonstrated that during discharge carbon matrix can react with
Li2O2 to form Li2CO3 and this leads to higher charge over-potential and low cycleability
[68]. This idea was further confirmed by Bruce’s group [33]. They believed that the
stability of carbon also depended on the hydrophobicity/hydrophilicity of the carbon
surface. Both of these results are shown in Figure 1-10.
19
Figure 1- 10 Schematic mechanism of (A) side reactions of carbon cathode and discharge products and (B) side reactions between electrolyte and carbon cathode
Figure 1-10 Schematic mechanism of (A) side reactions of carbon cathode and
discharge products [68] and (B) side reactions between electrolyte and carbon cathode
[33].
In order to overcome this problem, carbon-free electrodes are used to replace carbon as
cathodes in Li-O2 batteries. Cui et al. successfully prepared a free-standing Co3O4
electrode and used it in Li-O2 [69]. The capacity and cycle performance are acceptable.
In 2012, the same group introduced a tubular polypyrrole based cathode for Li-O2 and
exhibited excellent performance [70]. Peng et al. used nano gold particles instead of
carbon as the matrix of cathode and achieved 100 cycles with only 5% drop of capacity
which is shown in Figure 1-11. Using surface coating of carbon is also a good idea for
preventing side reactions [71].
(A)
(B)
20
Figure 1- 11 Discharge/charge profiles (left) and cycle performance (right) of nano gold cathode in DMSO based electrolyte
Figure 1-11 Discharge/charge profiles (left) and cycle performance (right) of nano gold
cathode in DMSO based electrolyte [11]. Current density is 500 mAg-1.
Besides the decomposition of carbon matrix during discharge and charge, other factors
also influence the performance of Li-O2 batteries greatly. Discharge products deposited
on the surface of cathodes are usually insulators. As a result, the resistance to electron
transport of cathode continuously increases as the battery reaction proceeds. This leads
to the increase of electrode polarization and over-potential [59, 72]. Other factors such
as decomposition of binders also influence the performance greatly [73, 74].
1.1.4 Catalyst
Li-O2 batteries nowadays still suffer from many drawbacks, limiting their applications,
such as low cycleability, large over-potential, low discharge-charge efficiency, and low
practical capacity. In order to achieve good performance, catalysts are employed in Li-
O2 batteries. It is known that electrocatalysts can sufficiently reduce the over-potential
of charge and discharge reactions, thus increasing round trip efficiency and improving
cycling performance [2, 75]. The most commonly used catalysts in Li-O2 batteries are
carbon catalysts, noble metals, transition metals and transition metal oxides.
Some carbon materials have the ability to catalyse the battery reactions. Graphene (GE),
known for its unique structure and property, has been widely studied in Li-O2 batteries
[65, 66]. The edge effect and defects of GE is considered to have promoting effect for
21
Figure 1- 12 Discharge/charge profile (left) and cycle performance (right) of graphene cathode and carbon black cathode
oxygen reduction reaction and formation of discharge products which is beneficent for
the performance of Li-O2 batteries [64]. For instance, Sun et al. reported the GE catalyst
used in PC-based electrolyte and achieved good performance for at least 5 cycles with
lower over-potential in both discharge and charge process [67] as shown in Figure 1-12.
GE composites such as GE/CNT [76] and MnO2/GE [77], have also been used and
demonstrated to have superior properties.
Figure 1-12 Discharge/charge profile (left) and cycle performance (right) of graphene
cathode and carbon black cathode. Current density is 50 mAg-1 [67].
Noble metals such as Pt and Au are employed as catalysts because of their intrinsic
conductivity, stability and also excellent catalytic properties in fuel cells [78]. The
ability of noble metals to reduce over-potential in Li-O2 batteries is very obvious. Lu et
al. compared the trend in the catalytic activity of noble metals for the oxygen reduction
reactions in the Li-O2 systems [79]. Same research was carried out by Gopi et al. and
achieved similar results [80]. To further increase the performance of Li-O2 batteries,
noble metals composites are also used. Metal oxide composites [81, 82] showed
excellent catalytic properties due to the bi-catalyst structures while carbon composites
[83-85] due to the increase of conductivity. However, noble metals are often very
expensive and heavy. The synthesis methods of nanostructured noble metals are usually
22
very complex. All the drawbacks limit the application of noble metals in Li-O2 batteries.
Therefore, finding cheap and effective substitutes are still a challenge for Li-O2
batteries.
Transition metal oxides are demonstrated to have good catalytic properties towards
oxygen reduction reaction in fuel cells [78]. They are famous for their activity,
availability, low cost, thermodynamic stability, low electrical resistance and
environmental friendliness. Due to these superiorities, transition metal oxides are
widely employed in Li-O2 batteries. For instance, Debart et al. investigated the catalytic
activity of Fe2O3, Fe3O4, CuO, CoFe2O4, and Co3O4 in O2 cathodes and found Co3O4
gave the best compromise between capacity and capacity retention [86]. Similar results
about the cobalt oxides are represented by other groups [87, 88]. Sun et al. also used
composites consisting of CoO and CMK-3 and achieved excellent cycling performance
for at least 15 cycles [54]. V2O5 is also a good candidate for Li-O2 batteries and is
reported several time used as oxygen reduction reaction catalyst [89, 90]. Besides
employing single metal oxides, multi-metal oxide composites are also favourable for Li-
O2 batteries. Zhao et al. reported hierarchical mesoporous perovskite La0.5Sr0.5CoO2.91
nanowires for using as catalysts in Li-O2 batteries and achieved ultrahigh capacity up to
more than 11000 mAh g-1 [91]. Similar compound was shown by other groups [92].
Various catalysts have been come up with and provided more opportunities to find
suitable catalysts for Li-O2 batteries. Giordani et al. developed a method using the H2O2
decomposition reaction as a tool to find better catalysts for oxygen reduction reaction
[93]. They have investigated most of transition metal oxides using this method. The
results are shown in Figure 1-13. Due to the similar structure of H2O2 and Li2O2, this
tool may be a reliable, useful, and fast screening tool for materials that promote the
23
Figure 1- 13 First galvanostatic charge of Li2O2 oxidation for various Li–O2 cells
charging process of the Li-O2 batteries and may ultimately give insight into the charging
mechanisms.
Figure 1-13 First galvanostatic charge of Li2O2 oxidation for various Li–O2 cells [93].
Among all transition metals, manganese oxides are considered the best choice for
employing as catalysts due to their easy preparation, excellent catalytic properties and
variety. Numerous researches focusing on manganese oxides carried on these days [94].
Debart et al. examined different phases of MnO2 used as catalysts and found α-MnO2
had the best catalytic property to catalyse the oxygen reduction reaction and support
higher capacity retention than MnO2 in other forms [95]. This indicates the crystal
structure of MnO2 plays very important role in affecting the catalytic performance. They
also demonstrated nanostructured MnO2 such as nanowires showed even better ability
to form and decompose Li2O2 and to support higher capacity at the same time than the
amorphous ones. The properties of promoting of Li2O2 formation were confirmed by
Trahey et al. and they even believed the employment of MnO2 in carbonate electrolytes
24
Figure 1- 14 Schematic mechanism of Li2O2 and Li2O forming on MnO2 catalyst
could help suppress the decomposition of electrolyte [96]. Result is shown in Figure 1-
14. Due to superior properties of MnO2, all kinds of morphologies were made such as
nanorods [97], hollow clews [98], and unique card-house-like structures [99]. Besides
morphologies, composites such as GE/MnO2 [77], Au-Pd/MnO2 [81], and MnO2/C
[100], were also researched. Doping methods are also good ways to improve the
performance of MnO2. Benbow et al. found the addition of Ni2+ could enhance the
catalytic properties of MnO2 [101]. Lee et al. showed the similar result with the doping
of Na+ [102].
Figure 1-14 Schematic mechanism of Li2O2 and Li2O forming on MnO2 catalyst [96].
Both metals and metal oxides have excellent catalytic properties towards oxygen
reduction reaction. Cheng et al. compared the performance of Li-O2 batteries with
metals and metal oxides as catalysts [103]. They found catalysts in their metal forms
were more catalytic in first few cycles with high capacity and low discharge over-
potential. However, after few cycles, the catalysts in oxide forms showed better
performance than the metal ones. This is because the stability of oxides is superior to
that of metals. It makes metal oxides more favourable in Li-O2 batteries. As it is so hard
to choose between metals and oxides, Thapa et al. mixed Pd with oxides together and
25
the mixture showed significant lower charge over-potential and good cycling
performance [82, 104-106].
Although using catalysts in Li-O2 batteries can support high capacity and low over-
potential, the mechanisms of catalytic reaction are still not fully understood. Some
researchers even believed catalysts may be unnecessary or detrimental in non-aqueous
electrolytes. The addition of catalysts in Li-O2 batteries may lead to side reactions such
as binder decomposition [73]. Moreover, some believed the beneficial effects of catalyst
in charge process are not attributed to the catalysts but to the electrolyte decomposition
[107]. These issues will certainly be paid great attention in future’s research.
In summary, Li-O2 batteries have been studied over the last few years. Numerous
progresses have been made to improve the performance of Li-O2 batteries. However, the
performance of current Li-O2 batteries is still far from satisfactory and they are not
ready for industrial application. Therefore a great breakthrough is still urgently needed.
1.2 Polymer electrolyte
Considering the typical properties, electrolytes used in Li-O2 batteries must be stable
enough against oxygen reduction reactions and able to provide sufficient Li+ for ionic
conductivity as well as battery reactions. Polymer electrolytes, known for their stability,
have great potential application for Li-O2 batteries.
Polymer electrolytes are considered to be divided into two classes [108]. Those based
on polymers which serve as both solvents to dissolve salts and mechanical support are
known as solid polymer electrolytes [109, 110]. Those based on gel polymer gels in
which polymer matrix are encaged in liquid electrolyte solutions are called gel polymer
electrolytes [111, 112]. Polymer electrolytes act as separators preventing cathodes and
26
anodes from contact with each other and also media to transport ions involved in
discharging and charging process. Compared with liquid electrolytes, polymer
electrolytes exhibit the superior mechanical, thermal, and electrochemical stability. Due
to these superiorities, polymer electrolytes have been widely used in lithium batteries.
1.2.1 Solid polymer electrolyte
Since the first discovery that ether-based polymer polyethylene oxide was able to
dissolve inorganic salts and exhibit certain ionic conductivity was published in 1973,
polymer electrolytes have been widely applied in batteries [113]. When compared with
liquid ones, polymer electrolytes can offer excellent processability and flexibility that
can help adjust various geometric shapes of batteries. Moreover, the stability of polymer
electrolytes can not only ensure the safety of batteries when cycling, but also help
eliminate the use of separator. The lack of organic liquid in solid polymer electrolytes
provides the possibility to prevent the growth of lithium dendrites [108]. Due to all
these advantages, solid polymer electrolytes show the possibility to be employed as
electrolytes in all kinds of batteries. Since lithium batteries are widely researched during
recent years, employing polymer electrolytes in lithium batteries become more and
more heated.
Ionic conductivity of solid polymer electrolytes is considered critical when applied to
practical use. The ionic conductivity of solid polymer electrolytes can be roughly
determined by the effective number of mobile ions, the elementary electric charge, and
the ion mobility [114]. The mobile ions are usually known as free ions that are
responsible for ionic conductivity. Therefore, a high degree of dissociation of the
lithium salts in polymer matrix is needed in order to obtain high conductivity. Besides
the dissociation ratio, the Li+ transference number is also critical since large amount of
27
Figure 1- 15 Schematic mechanism of Li+ movement through PEO based polymer electrolyte
Li+ are required in battery reactions [110, 115, 116]. The ionic motion of Li+ in polymer
matrix is closely associated with local segmental motion of polymer chains. It is
believed the interaction between Li+ and atoms such as oxygen and fluorine is the
driving force. For instance, molecular dynamics simulations suggest that in
poly(ethylene oxide) (PEO) polymer electrolytes, the best ratio between Li+ and ether
oxygen of PEO chain are approximately 1 to 5. It is believed Li+ moves from PEO chain
to chain through complexation between each other [116]. The schematic mechanism is
shown in Figure 1-15.
Figure 1-15 Schematic mechanism of Li+ movement through PEO based polymer
electrolyte [114].
The segmental motion of polymer chain is often characterized by the glass transition
temperature (Tg) of polymer matrix, which is also responsible for the mechanical
properties. Various polymers with low Tg have been investigated. For instance,
polypropylene oxide hosts are known for their amorphousness even at room temperature
[108]. Polymers such as PEO, PPO with low Tg have the conductivities that are
comparable with some of the liquid solutions [117-119]. However, those polymer
electrolytes have some drawbacks such as complex chemistry and lack of mechanical
stability. In order to solve this problem, comb-type polyethers with ether linkages
attached as side-chains to the stiff backbone are employed to make polymer electrolytes
28
[120, 121]. This design makes it possible for polymer electrolytes to have more flexible
segmental motion.
There are many polymer matrices that have been employed to make solid polymer
electrolytes. PEO-based polymer electrolytes are first discovered in 1973 [113].
However the ionic conductivity was far from satisfactory. The poly(acrylonitrile) (PAN)
based electrolytes were investigated because of the acceptable ionic conductivity and
wide electrochemical stability window of PAN. However, PAN based electrolytes are
found not suitable for directly using in lithium batteries due to the severe passivation
upon contact with lithium metal anodes [122, 123]. Other matrices such as poly(methyl
methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF) are demonstrated as
potential hosts for lithium batteries.
Although so many possible polymer matrices can be used to improve the properties of
polymer electrolytes, the conductivity of prepared polymer electrolytes is still big issue
for practical use. Operating at high temperature is believed to be a solution, though it is
not practical when employed in lithium batteries. The addition of ceramic fillers is also
considered as an efficient solution. It has been demonstrated that the ceramic fillers can
significantly improve the electrochemical properties including conductivity [124-126].
The most commonly used ceramic fillers are Al2O3, SiO2, MgO, LiAl2O3 and
TiO2.They believed the reason of such improvement is the acid-base type interactions
involving oxygen atoms or fluorine atoms, filler acid or base centres and alkali metal
cations [127]. Another hypothesis is the addition of ceramic fillers can significantly
reduce the possible side reactions between lithium metals and polymer chains as shown
in Figure 1-16 [128]. The particle size of fillers also plays an important role.
29
Figure 1- 16 Schematic mechanism of the addition of ceramic fillers and the effect of different particle sizes, (a) macro-size and (b) nano-size
Figure 1-16 Schematic mechanism of the addition of ceramic fillers and the effect of
different particle sizes, (a) macro-size and (b) nano-size [128].
Although solid polymer electrolytes have so many superiorities, Anderman questioned
the application of solid polymer electrolytes in a review article published in 1994 [129].
He believed that lithium batteries with solid polymer electrolytes didn’t share such
practical advantages considering the flexibility, thickness, manufacture and mechanical
strength. Therefore, the application of solid polymer electrolytes in lithium batteries is
still limited.
1.2.2 Gel polymer electrolyte
Since the ionic conductivity of solid polymer electrolytes is the main issue which limits
their application, liquid solutions are added as plasticizer in the system to form gel-like
30
structures which is known as gel polymer electrolytes. Compared with solid polymer
electrolytes, gel polymer electrolytes have been commercialized in many lithium battery
industries. Gel polymer electrolytes have been widely studied due to their superior
properties including high ionic conductivity, electrochemical stability, safety and
tolerance against mechanical and electric abuse [108]. It is believed that the use of gel
polymer electrolytes in lithium batteries can effectively suppress the growth of lithium
dendrite which is the main issue in lithium batteries since it can lower the cycling
efficiency and cause internal short-circuiting. Polymer electrolytes can also have
excellent ability to endure the volume change of electrodes during cycling which will
further improve the flexibility of designed cells. Another advantage of gel polymer
electrolytes used in lithium batteries is their ability to reduce the reactivity of liquid
electrolytes towards battery reactions and lithium anodes. This makes gel polymer
electrolytes more suitable than liquid electrolytes due to the safety issues. Besides, the
manufacturing integrity is also an advantage.
Similar to solid polymer electrolytes, PEO can also be used in gel polymer electrolytes.
The addition of liquid plasticizer into the systems results in reducing of the crystalline
content of PEO, and the increasing of polymer segmental mobility which lead to the
increase of ionic conductivity. Generally speaking, polyethers with low molecular
weight and polar organic solvents are usually used as plasticizers [130-132]. Since the
percentage of polymer is low in the whole electrolyte system, the function of polymer
matrix is no longer a solvent for lithium salts. The polymer matrices swelling by the
liquid solvents only act to provide dimensional stability [111]. This means the
interactions between lithium ions and oxygen or fluorine atoms are no longer necessities
to form electrolytes and the choices of polymer matrix become various. PAN based
electrolytes have been used in the lithium batteries due to their low crystalline content.
31
Abraham and Jiang also used a PAN based electrolyte in rechargeable Li-O2 battery and
last for 3 cycles which is also known as the first publication of using organic electrolyte
in Li-O2 batteries [19]. The use of PMMA in the polymer electrolyte was first published
in 1985 [133]. The conductivity of PMMA based electrolyte reached 10-3 S cm-1 at
room temperature. Since then, PMMA based electrolytes have been widely studied
[134-138]. However, the poor mechanical strength of PMMA after swelling by liquid
plasticizers limits its applications. Additives, such as poly(vinyl chloride) (PVC), have
been used to improve the mechanical properties of PMMA [139-141]. Other polymer
matrices such as PVDF, PVC are also used in polymer electrolytes [142-145].
Furthermore, poly(vinylidene fluoride-hex fluoropropylene) (PVDF-HFP) draws great
attention because the amorphous phase of HFP can help encage large amount of liquid
electrolytes and the crystalline phase of PVDF can provide mechanical support as
polymer matrix. The research of PVDF-HFP based electrolytes has also been widely
carried on in Li-ion and Li-O2 batteries [146-149].
However, most of the polymer hosts lose their mechanical properties after swelling by
liquid plasticizers. Moreover the gain of conductivity is often accompanied by the loss
of mechanical strength, the decrease of compatibility with lithium metals and the
reducing of safety. In order to solve these problems, various ceramic fillers such as
zeolites, ionites, and some neutral fillers are added into the systems [150]. They are
known as composite polymer electrolytes [128]. It is believed that the addition of
ceramic fillers can significantly improve the conductivity of polymer hosts and also
their interfacial properties in contact with lithium metals by suppressing the degree of
crystallinity of polymer matrices.
32
Figure 1- 17 Schematic presentation for functional role of PDMITFSI ionic liquid on lithium deposition, (a) without and (b) with ionic liquid
Ionic liquids (ILs) have also been used as plasticizers for gel polymer electrolytes. ILs
are liquids comprised entirely of ions at room temperature. Their unique properties such
as low vapour pressure, high ionic conductivity, and good thermal and electrochemical
properties make them potential candidates as plasticizers in polymer electrolytes used in
Li-ion batteries [151, 152]. The function of ILs towards lithium deposition has also been
discussed which is shown in Figure 1-17 [152]. Moreover, polymer electrolytes based
on ILs have also been used in Li-O2 batteries [153, 154].
Although polymer electrolytes exhibit so many superiorities, the use of polymer
electrolytes in Li-O2 batteries is still far from satisfactory. Most batteries with polymer
electrolytes can only last for 3 cycles or less [153, 154]. In order to obtain effective Li-
O2 batteries with extraordinary performance, great efforts are still needed in the future.
Figure 1-17 Schematic presentation for functional role of PDMITFSI ionic liquid on
lithium deposition, (a) without and (b) with ionic liquid [152].
33
Figure 1- 18 The structures of the most commonly used conducting polymers
1.3 Conducting polymer
Since the use of carbon may cause problems during cycling in Li-O2 batteries, it is eager
to find substitutes for carbon cathodes. Many solutions have been brought up, one of
which is use conducting polymers to replace carbon. Conducting polymers have been
widely used in all kinds of areas and it is possible to be used in Li-O2 batteries.
Conducting polymers, or known as intrinsically conducting polymers such as
polypyrrole (PPy), polythiophene (PT), polyaniline (PANI), and poly(3,4-
ethylenedioxythiophene) (PEDOT) have drawn great attention since the Noble Prize
Award in 2000. The structures of most commonly used conducting polymers are shown
in Figure 1-18. The conducting polymers show superior properties and applications than
other organic polymers because of the processability and conductivity. It is known that
conducting polymers can possess very high electrical conductivity in their doped states.
Figure 1-18 The structures of the most commonly used conducting polymers.
34
Figure 1- 19 Conjugated orbitals formed in polyacetylene
Most of the conducting polymers share the similar unique structures which differ from
other organic compounds. The carbon atoms and sometimes other heteroatoms such as
sulphur and nitrogen in the backbones are usually sp2 hybridized. This makes all the
atoms in the backbones on the same plane. All the atoms have pz orbitals which are
orthogonal to the σ-bonds formed by sp2 hybridised orbitals. The conjugated structure
can be formed by all the pz orbitals when they are standing side by side obtaining a
delocalized π-bond [155]. For instance, the polyacetylene possesses a long chain-like
conjugated structure with all the pz orbitals standing side by side as shown in Figure 1-
19. Technically, the delocalized π-bonds provide the pathways for electrons to move
from side to side. However, it is proved that the undoped conducting polymers are
usually insulators or semiconductors, which is far from expectation. This is because the
energy gaps of undoped conducting polymers are too high to let pz electrons move
freely. Due to this reason, doped conducting polymers obtained either by oxidized or
reduced from undoped ones are synthesized. By introducing electrons into (reducing) or
removing electrons from (oxidizing) the conjugated structures of conducting polymer
backbones, the energy gaps are greatly reduced and the delocalized electrons can be
very easy to move along the chains form side to side. In this way, the polymers obtain
high conductivity.
Figure 1-19 Conjugated orbitals formed in polyacetylene.
35
There are two types of doping, p-doping and n-doping, referred to as oxidizing and
reducing states, respectively [156]. P-doping means the polymers are the electron
donors and electrons are taken away from the structure backbones. In order to keep
electroneutrality, same amount of anions have to be introduced as counterions.
Contrarily, n-doping means the polymers are the electron accepters with same amount
of cations as counterions. The type of doping usually depends on the gaps, or known as
the position of the HOMO and LUMO levels (HOMO corresponds to the upper edge of
the valence band and LUMO to the lower edge of the conduction band) [157]. Some
polymers such as polyacetylene can be n-doped and p-doped. However, most
conducting polymers such as PPy have very high position of their HOMO and LUMO
levels, which means it would be much easier to obtain p-doped ones and n-doping
would be quite difficult. Moreover, the oxidized conducting polymers possess higher
stability than the reduced ones [158]. Besides, the chemical synthesis methods of most
conducting polymers involve oxidation of polymer monomers. Therefore, it is easier to
achieve the oxidized p-doped forms than the reduced n-doped forms of conducting
polymers.
The doped conducting polymers have various properties such as stable, environmentally
friendly and conductive. The most unique property of conducting polymers that
distinguishing from other materials is the doping-dedoping redox performance. In
certain environment, the doped conducting polymers can be reduced into their undoped
form which can be oxidized quickly at the same time [159, 160]. The redox reaction is
shown below where CP stands for conducting polymers and A for anions,
(1-6)
36
Due to these superiorities of conducting polymers, they have been considered to have
great potential applications in various areas such as solar cells [161], fuel cells [162],
batteries [163], corrosion protection coatings [164] and supercapacitors [165].
Among all the conducting polymers that have been investigated, PPy, PANI, PEDOT
are most commonly used in most areas.
1.3.1 Synthesis method
Conducting polymers are usually synthesized from monomers such as pyrrole (Py),
aniline (ANI), and 3, 4-ethylenedioxythiophene (EDOT). The synthesis processes can
be through either electrochemical or chemical methods. However, the synthesis
mechanism is still not fully understood. Lots of mechanisms have been proposed to
explain the process [166-169]. The most acceptable mechanism is that the polymer
monomer is activated by removing an electron from the molecule and forms a radical
monomer structure [166]. When two radical monomers encounter with each other, they
quickly combine with each other and form a conjugated long chain. Then the chain can
be easily oxidized to its doped phase with anions balancing the charge. In this way, the
doped conducting polymer is obtained. The synthesis mechanism of PPy as an example
is shown in Figure 1-20. It is known that electrochemical synthesis produces thin films
on an electrode surface while chemical oxidation provides grained materials.
37
Figure 1- 20 Schematic illustration of synthesis mechanism of PPy
Figure 1-20 Schematic illustration of synthesis mechanism of PPy [166].
Electrochemical Synthesis methods are considered quick and easy to control. Proposed
conducting polymer films are achieved through electropolymerization, also known as
electrodeposition of monomers on suitable substrates and working electrodes. The
electropolymerization methods don’t provide undoped polymers but oxidized
conducting forms. The final polymer chain carries a positive charge every several
monomer units and is counter balanced by same amount of anion. In order to achieve
ideal polymer films, various electrochemical techniques are employed in
electropolymerization. The most commonly used methods are cyclic voltammetry,
potentiodynamic, galvanostatic, potentiostatic, and reversal potential pulsing technique.
For instance, Wang et al. obtained highly flexible and bendable free-standing PF6 doped
PPy films by electrochemical polymerization method [170]. They also found out the
morphology of PPy changed as the deposition time increased while the conductivity
decreased with the increase of thickness of polymer film. Another example is the work
Sultana et al. carried out [171]. They obtained similar soft, light-weighted, mechanically
38
robust and highly conductive free-standing para(toluene sulfonic acid) (pTSA) doped
PPy film by electropolymerization methods. They discovered the porosity of PPy films
increases while prolonging deposition time. Dubal et al. employed potentiodynamic
methods to synthesize nanostructured PPy, such as nanobelts, nanobricks and
nanosheets on stainless steel substrates by simply changing the scanning rate during
deposition [165]. PEDOT with different doping anions have also been synthesized by
Spanninga et al. through electrochemical deposition [172-174]. They further discussed
what ions are more favourable for counterions when the deposition was carried in
solutions with different ions.
However, electrochemical synthesis methods usually need equipment to gain
conducting polymers, which greatly depends on many parameters such as deposition
time, current density and scanning rates. This leads to the complex synthesis processes.
Compared with electrochemical synthesis methods, chemical synthesis methods are
often known as simpler ways to obtain proposed polymers. In a typical chemical
synthesis process, conducting polymers are obtained through chemical oxidation by
oxidants, such as FeCl3, (NH4)2S2O8, H2O2, KIO3, and K2GrO7 [175-177]. Usually the
polymerization is going in aqueous solutions with acidic additions. The anions from
acids can be employed as counterions for oxidized polymers. It is believed that the
participation of acids in the process of polymerization can significantly improve the
conductivity and other properties of conducting polymers [178, 179]. The choice of acid
can be various, such as HCl, H2SO4 and pTSA. All of them provide the counterions as
well as pH the polymerizations need.
However, we can only obtain amorphous conducting polymers by using in situ chemical
methods. The amorphous conducting polymers are usually not conductive enough. Thus
39
the application of conducting polymers may be limited. In order to achieve conducting
polymers with higher conductivity and electrochemical properties, nanostructured
materials are made for use. It is well known that the nanostructured materials have
superior properties than bulk and amorphous ones due to the unique structures. It is
believed the defects and edge effects of these structures can provide possibility for
better performance in various applications. One-dimensional conducting polymers show
unusual physical and chemical behaviour due to the nanosize effects [180, 181]. There
are various ways to synthesize nanostructured conducting polymers, such as
electrospining [182-185], hard template synthesis [186-190], soft template synthesis
[191-195] and a variety of lithography techniques [196-198]. With rational synthesis
design, nanostructured conducting polymers with different diameters, thickness, and
length can be obtained under control. Jang et al. used reverse microemulsion
polymerization by adding a surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT),
into a apolar solution hexane and prepared very regular PPy with nanotube morphology
[199]. The whole process is shown in Figure 1-21. PEDOT nanotube has also been
synthesized using the same soft template method. Zhang et al. employed this method
with diluted concentration of AOT and EDOT monomer to synthesize PEDOT
nanotubes [200]. Huang et al. presented a mechanistic study about the formation of
PANI nanofiber [175]. They believed the aniline had intrinsic potential to form
nanofiber- or nanowire-like structures. In order to obtain regular structure and prevent
secondary growth of PANI, they used fast-mixing method to achieve PANI nanofiber
which is also shown in Figure 1-21. Interfacial method was also employed to synthesis
PANI nanofiber by the same group [201]. Qi et al. also exhibited a freezing interfacial
polymerization method to prepare highly conductive free standing PPy films [202].
40
Figure 1- 21 Schematic illustration of synthesis mechanism of (A) PPy nanotube and (B) PANI nanowire
However, most of the methods need strict conditions and they are difficult to control the
parameters of conducting polymers. Therefore, a more efficient, simple and controllable
synthesis method is still urgently needed.
Figure 1-21 Schematic illustration of synthesis mechanism of (a) PPy nanotube [199]
and (b) PANI nanowire [175].
1.3.2 Application
Due to all the superior properties mentioned above, conducting polymers have been
widely used in most areas such as solar cells, fuel cells, supercapacitors and
rechargeable batteries. Since rechargeable lithium batteries draw great attentions these
days, here we would like to focus mainly on the rechargeable lithium batteries.
(a)
(b)
41
Rechargeable lithium batteries are extensively studied due to their extremely high
theoretical capacity, light weight, low toxicity and relatively high safety [181]. Typical
lithium batteries consist of anodes, cathodes, electrolytes and separators. During
discharge process, the lithium ions are released from anodes and inserted into or
consumed on the cathodes and it reverses in the charge process. Usually, the anode and
cathode materials are made of carbon or metal oxides which can perform the lithium
insertion and extraction.
Conducting polymers, known for their unique π-conjugated structures and
electrochemical properties, can also be used for rechargeable lithium batteries. There
are many examples of using conducting polymers directly as cathode materials in
lithium batteries because of their intrinsic nature of doping-dedoping redox reactions.
For instance, Osaka et al. have proposed a Li-PPy battery with PEO based polymer
electrolyte and achieved at least 1500 cycles [203]. Zhou et al used Fe(CN)64- doped
PPy as cathode material in Li-ion battery and achieved greatly enhanced capacity [204].
The result is shown in Figure 1-22. Other groups also obtained similar results when
employed PPy as cathode materials in lithium batteries [170, 171].
42
Figure 1- 22 Cycling performance of PPy/FC at (a) constant current density of 50 mAg-1 and (b) different current densities
Figure 1-22 Cycling performance of PPy/FC at (a) constant current density of 50 mAg-1
and (b) at different current densities [204].
Many research groups also used conducting polymers as coating materials on anodes
and cathodes due to the high conductivity and chemical stability [205, 206]. It is
obvious the resistance of batteries has been significantly reduced after the introducing of
conducting polymers into the whole systems. For instance, LiFePO4 has been widely
studied in Li-ion batteries. However, the semi conductivity of LiFePO4 has limited its
performance. In order to improve the conductivity of LiFePO4 electrodes, PEDOT was
used and coated on the surface of LiFePO4 and the charge transference resistances have
been significantly reduced [207]. The performance of these batteries was considered
very good as shown in Figure 1-23. Another strategy to use conducting polymers in
43
Figure 1- 23 Discharge/charge profiles (left) and resistance (right) of the LiFePO4 cathode (a) coated with PEDOT, (b) coated with PPy, (c) coated with C, and (d) pristine particles
lithium batteries is through preparing composites. Conducting polymers play as
conductive media in the composites for electron movement. Composites such as
LiFePO4/PPy [208], LiN1/3Co1/3Mn1/3O2/PPy [209], PPy/Fe/O [210], Sn/PPy [211],
PEDOT/V2O5 [212, 213], PEDOT/PDBM [214] and PPy/graphene [215] have been
prepared and showed improved performance. Conducting polymers can also be used as
media to obtain carbon coating or carbon composites. Wang et al. proposed a method to
synthesize carbon composite using PANI as media [216]. The mechanism has been
discussed and shown in Figure 1-24.
Figure 1-23 Discharge/charge profiles (left) and resistance (right) of the LiFePO4
cathode (a) coated with PEDOT, (b) coated with PPy, (c) coated with carbon, and (d)
pristine particles [207].
44
Figure 1- 24 a) Electron-transfer pathway for LiFePO4 particles partially coated with carbon. b) Designed ideal structure for LiFePO4 particles with typical nano-size and a complete carbon coating. c) Preparation process for the LiFePO4/carbon composite including an in situ polymerization reaction and two typical restriction processes
Figure 1-24 a) Electron-transfer pathway for LiFePO4 particles partially coated with
carbon. b) Designed ideal structure for LiFePO4 particles with typical nano-size and a
complete carbon coating. c) Preparation process for the LiFePO4/carbon composite
including an in situ polymerization reaction and two typical restriction processes [216].
Conducting polymers can also be used in Li-S batteries as coating materials on the
surface of sulphur to reduce impedance of whole electrodes. Liang et al. showed the
improvement of the cycling stability by introducing nanostructured ordered PPy/S
45
Figure 1- 25 (A) Morphology and cycle performance of PPy cathode [70], (B) nitrogen-doped graphene derived from PANI and (C) Performance of PEDOT catalyst
cathode in Li-S battery [217]. The same group then further investigated PPy/CNT/S in
Li-S battery and achieved even better performance [218].
There have also been some reports about conducting polymers used in Li-O2 batteries.
Cui et al. employed a tubular PPy baed air electrode in Li-O2 battery as the substitute of
carbon based electrodes, and showed better cycleability and higher capacity due to the
tubular structure and hydrophilic properties of PPy [219]. PANI has also been employed
as waterproof barrier in Li-O2 batteries [220] as well as media to make nitrogen-doped
graphene [76]. Nasybulin et al. investigated PEDOT as catalyst in Li-O2 batteries and
found the charge over-potential was greatly reduced [221]. These results are shown in
Figure 1-25.
Figure 1-25 (a) Morphology and cycle performance of PPy cathode [70], (b) nitrogen-
doped graphene derived from PANI [76] and (c) Performance of PEDOT catalyst [221].
(a)
(b)
(c)
46
However, despite the few reports listed above about the use of conducting polymers in
Li-O2 batteries, there have been seldom reports on this area. Due to the very unique
properties of conducting polymers, they should have the potential applications in Li-O2
batteries. Great efforts should be made to obtain highly efficient Li-O2 batteries with
conducting polymers in future research.
1.4 Summary
Great efforts have been made in order to obtain Li-O2 batteries with high specific
capacity and reasonable cycle performance. According to the literature above, the
electrolytes and cathode materials are the most critical parts in Li-O2 batteries because
the decomposition of electrolytes and low efficiency of catalysts greatly limit the
performance. The stability of polymer electrolytes and unique redox properties of
conducting polymers provides the possibility to be applied in high-performance Li-O2
batteries. Polyethylene glycol (PEG) and PVDF-HFP were chosen as the matrices of
polymer electrolytes and had been investigated in this work. Gel polymer electrolytes
based on these two matrices with different additions were studied in Chapters 3 and 4.
Furthermore, PPy was also chosen as the catalyst material in Li-O2 batteries. Batteries
based on PPy and PPy/CNT were investigated to have high catalytic performance which
is shown in Chapters 5 and 6, respectively.
47
Figure 2- 1 Schematic illustration of the whole experiment process
Chapter 2 Experimental Methods
2.1 Overview
In order to obtain the usable results of proposed hypothesis, experiments should be
conducted in the laboratory. The main process of each experiment can be divided into
four parts as listed below (Figure 2-1),
The preparation of proposed materials
The characterization of as-prepared materials
The electrochemical analysis of as-prepared materials
The characterization of products and materials
During most of the procedures, electric equipment such as SEM, XRD, FT-IR and CHI
will be used for characterizing and analysing which will be introduced later in the
following parts of this chapter.
Figure 2-1 Schematic illustration of the whole experiment process.
Preparation
Characterization
meexxxppp
Performance atic illustratio of the whole i f h h l
Characterization
Fi 2 1
enent procemmmmmmment procemme ssssseesssssss
g
In situ reaction
Solution casting FT-IR
XRD
SEM
CV
Discharge/charge
EIS
48
2.2 Materials and chemicals
A list of materials and chemicals which were used in the research project are shown
below in Table 2-1 along with their formula, purity and supplier.
Table 2-1 Materials and chemicals used in the research project
Materials and chemicals Formula Purity Supplier 3,4-Ethylenedioxythiophene
(EDOT) C6H6O2S 97% Aldrich
Ammonium persulphate (NH4)2S2O8 98% Aldrich
Acetone CH3COCH3 99.9% Sigma-
Aldrich
Bis(trifluoromethane)sulfonimide
lithium salt (LiTFSI) CF3SO2NLiSO2CF3 99.95%
Sigma-
Aldrich
Carbon black (Super-P) C 99% Lexel
Dimethyl carbonate (CH3O)2CO 99.0% Sigma-
Aldrich
Dimethyl sulfoxide (CH3)2SO 99.5% Sigma-
Aldrich
Ethanol C2H5OH 95% Sigma-
Aldrich
Hydrochloric acid HCl 32% Sigma-
Aldrich
Iron chloride FeCl3 97% Sigma-
Aldrich
Lithium perchlorate LiClO4 99.99% Sigma-
Aldrich
Lithium chloride LiCl 99.0% Sigma-
Aldrich
49
Lithium foil Li 99.999%
Hohsen
Corporation
Japan
Pyrrole C4H5N 98% Aldrich
Poly(tetrafluoroethylene)
(CF2CF2)n 60%
Guangzhou
Chunting
Industrial
Co., Ltd
Perchloric acid HClO4 70%
AJAX
Chemicals
PTY
Limited
Propylene carbonate C4H6O3 99.7% Sigma-
Aldrich
Polyethylene glycol (CH2CH2O)n -
Sigma-
Aldrich
Fluka
Poly(vinylidene fluoride-co-
hexafluoropropylene)
(CH2CF2)x(CH2CF(CF3))y - Sigma-
Aldrich
Silicon dioxide nanoparticle SiO2 99.5% Sigma-
Aldrich
Tetraethylene glycol dimethyl
ether CH3O(CH2CH2O)4CH3 99%
Sigma-
Aldrich Table 2- 1 Materials and chemicals used in the research project
2.3 Material preparation
The experiments in this thesis are mainly focus on the conducting polymers and
polymer electrolytes used in Li-O2 batteries. The preparation methods of these two
materials are in situ oxidation and solution casting method, respectively.
50
Figure 2- 2 The preparation process of PPy when (NH4)2S2O8 was used as oxidant
2.3.1 In situ oxidation
In situ oxidation simply represents mixture oxidation reaction when applied in chemical
reactions. In situ oxidation is carried on in a container such as a flask with mixed
reactants. In this thesis, PPy was synthesized by in situ oxidation. First, certain amount
of pyrrole monomers was added into aqueous solution consisting of dopand acids and
the solution was kept for stirring for 30 min. At same time, a solution consisting of
certain amount of oxidant and dopand acids was prepared and it was added into the
previous solution drop by drop. The mixture was kept stirring at room temperature for
certain time. The black precipitation was then filtered and washed with distilled water
and ethanol for several times and dried under vacuum at a certain temperature for
certain time. In this process, the amounts of reactants, temperature and time vary from
different materials. The dopands can be HCl and HClO4 while the oxidants can be FeCl3
and (NH4)2S2O8 depending on the experiments. The whole process is illustrated in
Figure 2-2 when using (NH4)2S2O8 as oxidant.
Figure 2-2 The preparation process of PPy when (NH4)2S2O8 was used as oxidant.
2.3.2 Solution casting method
Polymer electrolytes in this thesis were prepared by solution casting methods. Solution
casting method simply represents casting a solution mixed with polymer matrices and
solvents or other components onto a flat glass surface. However, the process differs
according to the polymer matrix used. For polymers with low melting points such as
51
PEO and PEG, the solution casting method involves no solvents. Polymer matrix was
first heated to a certain temperature to obtain a liquid phase. Then, a pre-prepared liquid
electrolyte with certain concentration of lithium salt was added into the melting polymer
system. The mixture was kept stirring until a homogeneous mixture was obtained. The
mixture was then casted onto a flat glass surface and allowed to solidify into a gel-like
membrane. This membrane was the as-prepared polymer electrolyte. On the other hand,
for polymers with high melting points such as PVDF-HFP, solvents to dissolve the
polymer matrix are necessary to obtain electrolyte membrane. The whole preparation
process was similar to the one described above except for the addition of solvents and
elimination of heating process. The solvent can be acetone when PVDF-HFP was used
as polymer matrix.
2.4 Material characterization
The following techniques were employed to characterize the status, phase, morphology
and structure of the as-prepared materials. In this project, XRD, FT-IR and SEM were
mainly used.
2.4.1 X-ray Diffraction (XRD)
X-ray diffraction (XRD) is a non-destructive analytical characterization method that
reveals detailed information about the chemical composition and the crystal phase and
structure of a wide range of materials. An X-ray diffractometer generates an X-ray beam
hitting a sample as a function of incident and scattered angle, polarization, and
wavelength or energy. A certain sample has a particular atom arrangement within the
unit cell and this will lead to particular relative intensities of the recorded diffraction
peaks upon X-ray hitting. Therefore, the unit cell size and geometry may be resolved
from the angular positions of the X-ray diffraction results. The resultant diffraction lines
52
Figure 2- 3 Schematic drawing of Bragg’s law
with obvious peaks together are called an XRD pattern which can provide information
on crystal stricture, chemical composition, and physical properties of materials and thin
films. Each crystal has a unique XRD pattern according to Bragg’s law,
(2-1)
Where n stands for integer, λ is the wavelength of the incident X-ray beam, d is the
distance between atomic layers in a crystal and θ stands for the incident angle. The
theory of Bragg’s law is shown in Figure 2-3.
Figure 2-3 Schematic drawing of Bragg’s law [222].
The basic use of XRD in this research project was to determine the composition and
phase of products by comparing the obtained XRD patterns to known standard
diffraction lines. The XRD instrument used in this research project was Siemens D5000
with a monochromatized Cu Kα radiation (λ=0.15406 nm) at a scan rate of 1º min-1 and
step size of 0.02º.
53
2.4.2 Scanning electron microscope (SEM)
The scanning electron microscope (SEM) is a characterization technique that can image
a sample by scanning it in a raster scan pattern with the high-energy electron beam.
After each scan, the sample producing signals that contain information about the
sample’s surface topography, composition, and other properties such as electrical
conductivity were made up by electrons interacting with the sample atoms. Generally,
SEM are used for preliminary analysis.
The basic use of SEM in this research project was observing the morphology of as-
prepared materials and surface of electrodes during cycling. The SEM instrument used
in this project was Zeiss Supra 55VP field emission SEM (FE-SEM) with an
accelerating voltage of 5-20 kV and 10-30 mm aperture. The images were taken by an
in-lens secondary detector. A thin layer of carbon was used to deposite on the surface of
the materials if the conductivity was too low.
2.4.3 Fourier transform infrared spectroscopy (FT-IR)
Fourier transform infrared spectroscopy (FT-IR) is used for obtaining infrared spectra of
absorption, emission, photoconductivity or Raman scattering of a solid, liquid or gas.
An FTIR spectrometer simultaneously collects spectral data in a wide spectral range.
This confers a significant advantage over a dispersive spectrometer which measures
intensity over a narrow range of wavelengths at a time.
The basic use of FT-IR in this research project was to analyse the structure of as-
prepared materials and products that were produced during cycling. The FT-IR
instrument used in this project was Nicolet Magna 6700FT-IR spectrometer using 4 cm-
1 resolution and 64 scans at room temperature.
54
Figure 2- 4 An example TGA result of polypyrrole coated silicon
2.4.4 Thermogravimetric Analysis (TGA)
Thermogravimetric analysis (TGA), also known as thermal gravimetric analysis (TGA),
is a type of measurement usually used to determine the weight changes of a certain
sample during the changing of temperature in a controlled atmosphere. Such analysis
can detect the weight of a sample as temperature elevates accurately. The results are
often concluded in a figure with a continuous line to identify weight loss processes
which are related to the chemical reactions occurring. Figure 2-4 shows an example
result of a TGA measurement. The TGA measurement can be conducted in atmosphere
of air or noble gases for different applications.
Figure 2-4 An example TGA result of polypyrrole coated silicon [223].
In this project, the TGA was used to determine the weight ratio of each component in
as-prepared materials. The TGA instrument used in this project was Simultaneous TG-
DTA (SDT 2960) with a platinum plate as the sample holder. The temperature was set
to increase to 1000 ºC in air atmosphere with a speed of 5-10 ºC/min.
55
2.5 Electrode preparation and cell assembly
2.5.1 Electrode preparation
The cathode used in Li-O2 batteries was prepared as follows: the catalyst mash was
prepared by mixing the as-prepared materials, conducting matrices and binders together
in certain ratio in isopropanol with continuous stirring. The mixture was then pressed
onto the stainless steel mesh or a nickel mesh to form the air cathode. The cathode film
was punched into discs with a diameter of 14 mm and dried at 80ºC in a vacuum oven
for 12 h and then kept in the glove box. The typical loading of the air electrode is about
2 mg cm-2.
2.5.2 Cell assembly
A Swagelok type cell with an air hole (0.785 cm2) on the cathode side was used to
investigate the electrochemical performance. The cell was assembled in Ar filled glove
box (Mbraun) with water and oxygen level less than 0.1 ppm. The as-prepared air
cathode was used as the working electrode and a lithium foil was used as the counter
and reference electrode. The electrodes were separated by a glass microfiber filter
(Watman). The electrolytes depended on the experiments operated. The cell was gas-
tight except for the air side window that exposed the porous cathode film to the oxygen
atmosphere. The brief diagram of the structure of the Li-O2 battery is shown in Figure
2-5. All experiments were tested in 1 atm dry oxygen atmosphere to avoid any negative
effects of humidity and CO2.
56
Figure 2- 5 The structure of a Li-O2 battery
Figure 2-5 The structure of a Li-O2 battery.
2.6 Electrochemical characterization
The electrochemical properties of as-prepared materials are obtained by performance
characterization techniques on the assembled batteries. These techniques usually include
cyclic voltammetry, electrochemical impedance spectroscopy, linear sweep
voltammetry, and galvanostatic charge-discharge testing cyclic voltammetry. The
characterization of as-prepared materials can be further evaluated from these results.
2.6.1 Cyclic Voltammetry (CV)
Cyclic voltammetry or known as CV is a type of potentiodynamic electrochemical
measurement which has been widely used in electrochemical characterization of
guguuuuuuuuuuuuurrreee 22-- 55 TTThee struucttuuree ooff aa LLii--OOOO22 bbababababababababbabb t
Current collector Cathode Separator Lithium foil
57
materials in Li-ion and Li-O2 batteries. It records the relationship of current and voltage
when the potential of the working electrode is ramped linearly versus time. This
ramping is known as scan rate (V/s). CV can be conducted in two electrode and three
electrode systems. In a typical three electrode system, the potential is applied between a
reference electrode and a working electrode and the current is measured between a
working electrode and a counter electrode while in a typical two electrode system, the
potential and current are both measured between a working electrode and counter
electrode. In this project, the CV measurements were mainly performed by two
electrode systems, where a lithium anode was used as both reference and counter
electrode. During the scanning, an analyte can be reduced or oxidized and re-oxidized
or re-reduced on the return scan, which is known as a sign of highly reversible redox
couples. The peaks on the CV results indicate the oxidation and reduction potentials.
The CV measurements in this project were mainly conducted via a CHI 660C or CHI
660D electrochemical workstation (CH Instrument, Cordova, TN).
2.6.2 Electrochemical Impedance Spectroscopy (EIS)
Electrochemical impedance spectroscopy (EIS) has also been known as Alternating
current (AC) Impedance. EIS is often used to characterize the dynamics of an
electrochemical process in terms of an electrochemical cell’s response to an applied
potential at different frequencies. By observing the current response, the resistance
within different frequencies can be examined. The resistance can be read from a typical
Nyquist curve. Figure 2-6 shows the typical impedance Nyquist curve of a battery
system consisting of a compressed semicircle in a medium frequency region which
represents the charge-transfer resistance (Rct) and an inclined line in the low frequency
range which represents the Warburg impedance attributed to a diffusion-controlled
58
Figure 2- 6 A typical ESI Nyquist curve of a battery system
process. The high frequency intercept at the real axis corresponds to the electrolyte bulk
resistance and electronic resistance of the current collector [55, 59, 224].
Figure 2-6 A typical ESI Nyquist curve of a battery system.
ESI data were mainly conducted form a CHI 660C or CHI660D electrochemical
workstation (CH Instrument, Cordova, TN) in this project. The frequency range was set
between 100 kHz and 0.01 Hz at controlled temperature when the amplitude of the AC
signal applied was set to 5 mV.
2.6.3 Linear Sweep Voltammetry (LSV)
Linear sweep voltammetry (LSV) is also a commonly used potentiodynamic
electrochemical measurement where the current at a working electrode is measured
while the potential between the working electrode and a reference electrode is swept
linearly in time. The whole scanning process is carried on through constant scanning
rate (V/s). During the scanning, materials on the working electrode will undertake redox
reactions. Oxidation or reduction of species is registered as a peak or trough in the
current signal at the potential at which the species begins to be oxidized or reduced. The
59
Figure 2- 7 A typical result of LSV measurement
scanning process can be operated only once or repeatedly. The basic mechanism is
similar to CV measurement. Figure 2-7 shows the typical result of a LSV measurement.
Figure 2-7 A typical result of LSV measurement.
The LSV measurements in this project were mainly conducted via a CHI 660 C or CHI
660 D electrochemical workstation (CH Instrument, Cordova, TN).
2.6.4 Galvanostatic Charge and Discharge
Generally, Galvanostatic charge and discharge is used as electrochemical tests in which
a constant current density is used to determine the electrochemical performances such as
capacity. The capacities during charge or discharge can be calculated through the
following equation,
(2-1)
I represents for current density, t is the charge or discharge time and Q stands for the
capacity during charge or discharge process. In a three electrode system, the
galvanostatic charge and discharge performance is tested through a chronoamperometry
technique on an electrochemistry workstation with an aqueous electrolyte in open
60
Figure 2- 8 An example charge and discharge curve of a Li-O2 battery
circumstances. For a two electrode system, a sealed or open battery cell is used for the
testing. Usually the galvanostatic charge and discharge tests can exhibit electrochemical
information such as capacities, charge/discharge profiles, Columbic efficiency, and
cycle properties. Figure 2-8 shows an example discharge and charge curve obtained
through galvanostatic charge and discharge tests.
Figure 2-8 An example charge and discharge curve of a Li-O2 battery [225].
In this project, the galvanostatic charge and discharge measurements were conducted on
a computer-controlled Neware battery testing system or a Land Battery testing system.
61
Figure 3- 1 The typical molecular structure of PEG or PEO
Chapter 3 Low Molecular Weight Polyethylene Glycol Based
Gel Polymer Electrolyte Used in Li-O2 Batteries
3.1 Introduction
Polyethylene glycol (PEG) is a polyether compound with many applications from
industry manufacture to medicine. It is also known as polyethylene oxide (PEO),
depending on the molecular weight. Usually PEO refers to polymer with a molecular
weight higher than 20,000 g/mol while PEG prefers oligomers and polymers with a
molecular weight lower than 20,000 g/mol. PEG are liquids or solids with low melting
point, also depending on the molecular weight. The typical molecular structure of PEG
or PEO is shown in Figure 3-1.
Figure 3-1 The typical molecular structure of PEG or PEO.
PEO has been widely used in polymer electrolytes, both solid states and gel states due
to its semicrystalline properties with low glass transition temperature and low melting
point [226]. In the solid polymer electrolyte systems, several PEOn-LiX systems have
been studied and the ion conduction mechanism is closely related to the oxygen-assisted
hopping process with the long range segmental motion [227]. However, due to the low
conductivity of solid polymer electrolytes, gel polymer electrolytes (GPEs) are more
Fiiiiiguguguguguuguguguguguuguguguguggggug rrrrrerrr 33333333333333- 111111111111111111 ThTThThTTTThhThhhhheeeeee eeeeeeeeeeeee tytytytytytytytytytytttytypical moleeeeeeecucucucucucucucucucucucucucucuculallllalalalalalalalalalar structtttttttttururururururuururuurururrururuuuureeeeeeeeeeee of PEG GGGGGG or PEO
62
acceptable for lithium batteries. PEO has also been widely used as polymer matrix in
GPEs [228-231]. Appetecchi et al. investigated the PEO based GPEs with Polyethylene
glycol dimethyl ether (PEGDME) as plasticizer with different ratio [132]. However,
most of these reports use PEO with high molecular weight as the polymer matrix. Few
focused on the low molecular weight PEG based ones and even less was employed in
Li-O2 batteries. Moreover, the ones that were used in Li-O2 batteries cannot provide
sufficient capacity and cycling performance.
In this chapter, GPEs based on low molecular weight PEG were prepared and used in
Li-O2 batteries. Tetraethylene glycol dimethyl ether (TEGDME) was used as plasticizer
and solvent for lithium salts. Silica nanoparticle (SiO2) was used as addictive to increase
the ionic conductivity. The electrochemical performances of all GPEs have been tested
and liquid TEGDME electrolyte was used for comparison.
3.2 Experiment
3.2.1 Preparation of PEG based GPEs
PEG-based gel polymer electrolyte (GPE) was prepared by hot solution casting method.
The liquid TEGDME electrolyte was prepared by dissolving
Bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) into tetraethylene glycol
dimethyl ether (TEGDME). The concentration was kept at 1 M. The solid PEG was
heating up to 70 oC and melting completely. After mixing the melting PEG, liquid
TEGDME electrolyte and certain amount of ceramic fillers, SiO2 nanoparticle, together,
the whole solution was mechanically stirred at 70 oC until a homogenous solution was
obtained. The weight ratio of PEG, liquid solution and SiO2 was kept at 20: 70: 5. The
homogenous solution was then cast into a glass microfiber filter as the mechanical
support at room temperature and allowed to solidify. This process resulted in a white gel
63
like membrane and it was ready to characterize. The same process was used to make
SiO2-free GPE only without the addition of ceramic filler. All the chemicals were dried
before use and all the process was operated in a glovebox with water and oxygen
content lower than 0.1 ppm.
3.2.2 Material characterization
X-ray diffraction (XRD) was conducted on a Siemens D5000 X-ray diffractionmeter.
During the XRD analysis process, all materials and cathodes were protected without
exposure to the ambient atmosphere.
3.2.3 Electrochemical testing
All the electrochemical characterization of as-prepared GPEs was carried on
electrochemical workstation. The lithium ion plating/stripping was characterised by
cyclic voltammetry measurements using Li/GPE/Li cell over a wide voltage range (-4.5
V to 4.5 V) at medium scanning rate (10 mV s-1). Linear sweep voltammetry
measurements were used to determine the stability of as-prepared GPEs with
Li/GPE/stainless steel (SS) sealed cell and Li/GPE/carbon black electrode (CB) cell
exposed to ambient oxygen over a wide scanning range (open circuit potential to 6.5 V)
at slow scanning rate (1.0 mV s-1). The preparation of carbon black electrode will be
mentioned later. Liquid TEGDME electrolyte was characterised in the same way for
comparison.
The carbon electrode slurries were prepared by mixing carbon black (90 wt%) and
poly(tetrafluoroethylene) (PTFE) (10 wt%) together in propanol. The mixture was then
coated on a nickel mesh substrate. The cathode film was then obtained by punched into
disc and dried under vacuum at 80 oC for 12 h. A Swagelok cell with an air hole (0.785
cm2) on the cathode side was used to investigate the discharge and charge performance.
64
The Li-air cells were assembled in a glovebox with water and oxygen level less than 0.1
ppm. The as-prepared GPEs were sandwiched between lithium foils and carbon black
electrodes and a glass microfiber filter was used for keeping the structure from
collapsing, as mentioned before. For comparison, a cell with a glass microfiber filter
soaked in liquid TEGDME electrolyte was also made. All the cells were gas-tight
except for the cathode side window. All the measurements were conducted in 1 atm in
dry oxygen atmosphere to avoid any negative effects of humidity and CO2.
3.3 Results and discussion
The most important factors that affect the performance of electrolytes used in Li-O2
batteries are Li+ transportation, the stability, and the conductivity. When GPEs are used
in lithium batteries, they have to have sufficient properties to support the acceptable
performance. It is very essential for GPEs used in Li-O2 batteries to have a very good
reversibility for Li+ plating and stripping. Good reversibility can ensure a smoothly
running of battery reactions. Figure 3-2 shows the typical cyclic voltammetry curves
obtained in a Li/GPE/Li typed cell for both GPEs. There was only single peak found
during the cathodic and anodic reaction process, respectively, which was related to the
Li+ plating and stripping and the curves were symmetrical. The anodic and cathodic
peaks were almost identical both for current and potential. All those results indicated
the good reversibility of both GPEs and demonstrated the usability of them in the Li-O2
batteries. From the comparison of Figure 3-2 (a) and (b), it was very clear that the
current was higher when GPE with SiO2 addition was used. This indicated that the
addition of SiO2 did help improve the conductivity as many references claimed [108,
111, 128]. However, after 20 cycles the current was greatly reduced when GPE with
SiO2 addition was used while the one was almost the same when no addition was added.
65
Figure 3- 2 Cyclic voltammetry results of Li/GPE/Li type cells with (a) PEG and (b) PEG with SiO2 addition
This is probably due to the addition of SiO2 into the system could reduce the stability of
the GPEs, according to some references [117, 128].
Figure 3-2 Cyclic voltammetry results of Li/GPE/Li type cells with (a) PEG and (b)
PEG with SiO2 addition. The scanning rate is 10 mV s-1.
It is also very critical for electrolyte used in lithium rechargeable batteries to have a
wide operating potential window to ensure minimal side reactions. During the charge
process in Li-O2 batteries, the large over-potentials cause many side reactions such as
the decomposition of electrolytes. This makes the batteries very difficult to have
reasonable cycling performance. In order to investigate the stability of both as-prepared
PEG based GPEs, linear sweep voltammetry (LSV) method was used with
-4 -2 0 2 4
-0.004
-0.002
0.000
0.002
0.004
Cycle 20
Cur
rent
(A)
Voltage (V)
Li/PEG/Li
Cycle 1
(a)
-4 -2 0 2 4-0.02
-0.01
0.00
0.01
0.02
Cycle 1
Cur
rent
(A)
Voltage (V)
Li/PEG-SiO2/Li
Cycle 20
(b)
66
Li/GPE/cathode cells and the results are shown in Figure 3-3. Liquid TEGDME
electrolyte was also measured through the same method for comparison. In Figure 3-3
(a), the measurement was carried out in a sealed cell with a stainless steel as cathode.
Both the two GPEs exhibited high electrochemical stability up to 5.5 V vs. Li+/Li while
the liquid TEGDME electrolyte showed increasing current since 5 V. This indicated
electrolytes gelled with PEG polymer exhibited a significant increase of stability in a
sealed system and normally a stability window up to 4 V is sufficient for usage in Li-ion
batteries. However, because the cathodes and electrolytes are exposed to the ambient
oxygen in the Li-O2 batteries, the stability results of GPEs in sealed cells shown in
Figure 3-3 (a) are not convincing enough. In Figure 3-3 (b), the measurement was
carried in a cell with an open hole at the cathode side allowing the oxygen flowing
inside of cell and a porous carbon black electrode was used as cathode. Both the GPEs
were stable up to 4.6 V vs. Li+/Li while the liquid TEGDME electrolyte suffered from
decomposition at lower voltage. It was very obvious that exposure to oxygen
atmosphere caused the instability of electrolyte systems. This indicates that choosing
electrolytes for Li-O2 batteries is much more difficult than the regular Li-ion batteries
and both GPEs showed the potential qualification to be used in Li-O2 batteries. It was
also clearly seen from Figure 3-3 that the addition of Nano-scaled SiO2 could reduce the
stability of the whole electrolyte systems. But the stability of GPEs with the addition of
SiO2 was still sufficient enough for usage as electrolyte in Li-O2 batteries.
67
Figure 3- 3 Linear sweep voltammetry results of (a) Li/GPE/SS and (b) Li/GPE/CB-air type cells
Figure 3-3 Linear sweep voltammetry results of (a) Li/GPE/SS and (b) Li/GPE/CB-air
type cells. The scanning rate is 1 mV s-1.
The ionic conductivity was also considered as an important concern. The conductivities
were calculated from the membrane resistances obtained from impedance spectra. The
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
0.0000
-0.0005
-0.0010
-0.0015
-0.0020
Cur
rent
(A)
Potential (V)
Li-PEG-SS Li-PEG-SiO2-SS Li-TEGDME-SS
(a)
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
0.0000
-0.0005
-0.0010
-0.0015
-0.0020
-0.0025
-0.0030
Cur
rent
(A)
Potential (V)
Li-PEG-air Li-PEG-SiO2-air Li-TEGDME-air
(b)
68
typical impedance spectra obtained at different temperature are shown in Figure 3-4 (a).
It is clearly seen that the spectra have the shape of slanted straight lines and intercept the
real axis on the high-frequency side. According to the literatures, this represents an
equivalent circuit in which a resistor is in series with the electrode capacitance and the
intercept on the real axis gives the resistance from which the conductivity of the
electrolyte is calculated [149]. The ionic conductivity is calculated through Equation 3-
1,
(3-1)
where σ stands for ionic conductivity, Rb represents the bulk resistance, d is the
thickness of the gelled polymer electrolyte, and S is the area of the electrodes. The
calculated result is shown in Figure 3-4 (b). It was seen that the ionic conductivity
slightly increased with the temperature. However there was a dramatic increase 55 to 75
ºC. This is probably due to the melting of PEG polymer matrix. The melting point of
PEG was estimated at 60 to 70 ºC and the gelling process lowered the temperature
furthermore. After the melting of PEG polymer matrix, the ionic conductivity was
greatly increased because the gelled structure was destroyed and the number of free ions
would greatly increase. After then the conductivity stayed still even the temperature was
kept increasing. It also fits the theory that operating at high temperature is a good way
to increase conductivity for polymer electrolytes. The conductivity of GPE shown in
Figure 3-4 was demonstrated to be enough for Li-O2 batteries at room temperature.
69
Figure 3-4 (a) The impedance spectra of PEG at different temperatures. (b) The
calculated ionic conductivity of PEG at different temperatures.
Figure 3- 4 The impedance spectra of PEG at different temperatures. (b) The calculated ionic conductivity of PEG at different temperatures
0 1,000 2,000 3,0000
1,000
2,000
3,000
Z" (
)
Z' ( )
105oC 95oC 85oC 75oC 65oC 55oC 45oC 35oC 25oC
(a)
20 40 60 80 1000.000
0.001
0.002
0.003
0.004
0.005
Con
duct
ivity
(cm
-1)
Temperature (oC)
PEG(b)
70
The electrochemical performance of as-prepared GPEs was investigated through Li-O2
batteries. Figure 3-5 (a) shows the discharge and charge profiles of Li-O2 batteries using
the as-prepared GPEs. The specific discharge capacity of battery with PEG based GPE
was 3,667 mAh g-1. The battery with SiO2 addition showed much higher capacity of
6,477 mAh g-1 while the one with liquid TEGDME electrolyte showed even higher
capacity of 7,921 mAh g-1. It was clearly seen that liquid electrolytes had better ability
to support higher capacity than the gelled ones and the addition of SiO2 could be a good
choice to increase the specific capacity of polymer electrolytes. Figure 3-5 (b) displays
the partial enlarged view of the discharge and charge profiles in Figure 3-5 (a) from 0 to
1,500 mAh g-1 in the first cycle. The discharge plateau of battery with liquid TEGDME
electrolyte was the highest while the one of battery with PEG based GPE without any
addition was the lowest. Similar order was shown during the charge process. It was very
obvious that although the GPEs could support acceptable electrochemical performance,
the addition of SiO2 into the polymer system could help further reduce the over-
potential and increase specific capacity during the discharge and charge process.
The possible reason should be related to their different intrinsic properties such as
conductivity and phase structure. The TEGDME electrolyte is in its liquid phase which
means the ions that dissolved in the solution can move freely through the whole
electrolyte system and this provides relatively high ionic conductivity. It is believed the
higher ionic conductivity can help support higher capacity and lower over-potential due
to the increase of the oxygen reduction reaction kinetics. For both GPEs obtained in this
experiment, the gelled structures would hinder the transfer of ions through the systems
and this might lead to a lower conductivity. Furthermore, gel structures are in their solid
phase which means they don’t have the same penetrability as the liquid electrolytes. The
contact surface between electrolytes and cathodes will be reduced and this may result in
71
the decrease of the active sites which can also cause the reduction of specific capacity.
This could explain the differences of behaviour between GPEs and liquid TEGDME
electrolyte. For PEG based GPEs, the addition of SiO2 nanoparticle can help provide
higher ionic conductivity to the GPE system which has been demonstrated [117, 119].
This leads to the reduction of over-potential and the improvement of discharge specific
capacity.
Figure 3-5 (a) First discharge and charge profiles of Li-O2 batteries with PEG, PEG-
SiO2, TEGDME as electrolytes. (b) Partial enlarged view of first discharge and charge
profiles from 0-1,500 mAh g-1. The current density is 50 mA g-1.
Figure 3- 5 (a) First discharge and charge profiles of Li-O2 batteries with PEG, PEG-SiO2, TEGDME as electrolytes. (b) Partial enlarged view of first discharge and charge profiles from 0-1500 mAhg-1
0 2,000 4,000 6,000 8,000
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Vol
tage
(V)
Specific capacity (mAh g-1)
PEG PEG-SiO2
TEGDME
(a)
0 500 1000 15002.5
3.0
3.5
4.0
4.5
Vol
tage
(V)
Specific capacity (mAh g-1)
PEG PEG-SiO2
TEGDME
(b)
72
In order to determine whether the reversibility of batteries with different electrolytes is
suitable for Li-O2 batteries, the batteries were cycled at a fixed capacity of 500 mAhg-1
for 25 cycles. Figure 3-6 shows the cycling performances of the batteries with different
electrolytes. It is clearly seen that the battery with PEG based GPE without addition
could survive at least 25 cycles with no obvious fading. This indicates the sufficient
stability of PEG based GPE to support acceptable cycling performance for Li-O2
batteries. However, the one with SiO2 addition could only last for 21 cycles and the
capacity faded quickly since then. This was probably due to the reduction of stability of
electrolyte system when the SiO2 nanoparticle was added. Therefore, when considering
the cycling performance, SiO2 addition is not suitable for Li-O2 batteries. For
comparison, the battery with liquid TEGDME was also cycled and found to be able to
proceed at least 25 cycles.
Figure 3-6 Discharge and charge profiles of Li-O2 batteries with (a) PEG, (b) PEG-
SiO2, and (c) TEGDME as electrolytes at fixed capacity to 500 mAhg-1. Current density
is 50 mAg-1.
Figure 3- 6 Discharge and charge profiles of Li-O2 batteries with (a) PEG, (b) PEG-SiO2, and (c) TEGDME as electrolytes at fixed capacity to 500 mAhg-1
0 100 200 300 400 500
2.5
3.0
3.5
4.0
4.5
5.0
Vol
tage
(V)
Specicif capacity (mAh g-1)
1st 5th 10th 15th 20th 25th
(a)
73
Figure 3-6 Discharge and charge profiles of Li-O2 batteries with (a) PEG, (b) PEG-
SiO2, and (c) TEGDME as electrolytes at fixed capacity to 500 mAh g-1. Current
density is 50 mA g-1.
0 100 200 300 400 500
2.5
3.0
3.5
4.0
4.5
5.0V
olta
ge (V
)
Specific capacity (mAh g-1)
1st 5th 10th 15th 20th 21st
(b)
0 100 200 300 400 500
2.5
3.0
3.5
4.0
4.5
5.0
Vol
tage
(V)
Specific capacity (mAh g-1)
1st 5th 10th 15th 20th 25th
(c)
74
To further investigate the stability of PEG based GPEs, cycles at fully discharge and
charge were performed. The results are displayed in Figure 3-7. The battery with PEG
based GPE without any addition as electrolyte could last at least 5 cycles with 86%
retention of discharge capacity, while the capacity of battery with liquid TEGDME
electrolyte dropped quickly after the first cycle with only 38% retention even it
possessed extremely high capacity at the first discharge process. This provides the
evidence that the gelled polymer electrolyte has much higher stability than the
plasticizer before the addition of polymer matrix. The stability leads to the stable
cycling performance in Li-O2 batteries. Figure 3-7 also shows the fast fading of battery
with addition of SiO2, which also demonstrated that the addition of SiO2 could reduce
the stability of the polymer electrolyte system.
Figure 3-7 Cycle profiles of Li-O2 batteries with PEG, PEG-SiO2, and TEGDME as
electrolytes. Current density is 50 mA g-1.
The reason of such different performance of GPEs comparing to liquid electrolytes is
due to their unique gelled structures. It is believed that the interaction between polymer
Figure 3- 7 Cycle profiles of Li-O2 batteries with PEG, PEG-SiO2, and TEGDME as electrolytes
1 2 3 4 5
2,000
3,000
4,000
5,000
6,000
7,000
8,000
Spe
cific
cap
acity
(mA
h g-1
)
Cycle numbers
PEG PEG-SiO2
TEGDME
75
matrix and plasticizer through Li+ can help maintain a stable gel structure [149]. In this
report, TEGDME was used for plasticizer as well as solvent for lithium salt. The
possible structure is displayed in Fighure 3-8 (a). Li+ acted as a cross-link between
polymer matrix PEG and plasticizer TEGDME. The whole structure was bonded by
Lewis acid-base force which indicated that the Li+ interacted with oxygen atoms at the
polyether structures from polymer matrix and plasticizer. The gelled structure was held
up in this way. Moreover, the stability of structures like this could also be improved
greatly towards reactants, such as superoxide radicals. The interactions of oxygen atoms
in the electrolyte systems with Li+ might cause electrons donated from oxygen atoms to
Li+. In this way, the stability of α-hydrogen atoms adjacent to oxygen atoms is greatly
improved. According to the previous reports, the oxidation of polyether preferred to
happen at the α-carbon to oxidize the α-hydrogen atoms [31, 232]. The resistance of the
interacted structures would have higher capability to suffer from superoxide radicals.
Bryantsev et al. proved that electrolyte with a similar structure consisting of 1,2-
Dimethoxyethane (DME) and Li+ was much more stable than the DME itself [31]. After
the structure was set up, the excess Li+ could move along the electrolyte system and
provide ionic conductivity and ions for battery reactions. When SiO2 nanoparticle was
added into the electrolyte systems, partial Li+ used in the structures would be replaced
and the amount of excess Li+ would be increased as SiO2 is a known Lewis acid which
is shown in Figure 3-8 (b). In this way, the ionic conductivity can be improved.
However, the stability of SiO2-added GPE system towards superoxide radicals reduced
greatly. The strong Lewis acid property made it possible for radicals to attack the
electrolyte systems instead of free Li+ in the electrolytes. This might lead to the collapse
of the interacted structures in GPE systems, which explains the instability of GPE with
the addition of SiO2 nanoparticle.
76
Figure 3-8 Structures of (a) PEG-based electrolyte and (b) PEG-SiO2-based electrolyte.
Figure 3-9 shows the XRD patterns of PEG and GPEs, respectively. The peak related to
polymer matrix PEG broadened and the intensity decreased. It indicated that the
crystallinity of PEG after made into GPEs was greatly reduced by introducing Li+ and
plasticizer in the structures. This provided the evidence that there might be interactions
among these three components which is also consistent with the previous theory.
Another possible explanation of such improved cycleability of battery with GPE as
electrolyte than the one with liquid TEGDME electrolyte is the protection of lithium
anode. As we know, the anodes in Li-O2 batteries use lithium metals directly and
lithium metals are considered very reactive towards oxygen. Ether based electrolytes are
believed to have great ability to dissolve and transport oxygen, which may lead to the
corrosion of lithium anode. This would cause the fast fading of capacity. The gelled
Figure 3- 8 Structures of (a) PEG-based electrolyte and (b) PEG-SiO2-based electrolyte
(a) (b)
77
structure of GPE could help lower the kinetics of transporting oxygen through the
battery system. This could help maintain a longer cycle life for Li-O2 batteries.
Figure 3-9 XRD pattern of PEG before and after made into polymer electrolyte.
As showed above, the liquid TEGDME could help deliver higher initial specific
capacity while PEG based GPE could provide better cycling performance. Since the
main issue for applying rechargeable Li-O2 batteries in practical application is their
cycle life and stability [103]. It is believed that the cycleability of Li-O2 batteries at a
certain current is more important than the initial discharge capacity. Therefore, the use
of PEG based GPE instead of liquid TEGDME is much more favourable for Li-O2
batteries. Due to the same reason, Nano SiO2 addition is also considered unfavourable
for Li-O2 batteries.
To further investigate the usability of PEG based GPE in Li-O2 batteries, XRD
measurement was conducted to detect the discharge products after first discharge
process. Figure 3-10 shows the XRD patterns of cathodes after first discharge in
Figure 3- 9 XRD pattern of PEG before and after made into polymer electrolyte
20 40 60 80
Inte
nsity
(a.u
)
2
before after
78
batteries with PEG based GPE as electrolyte compared with pure Li2O2 and PEG. It is
clearly seen the discharge products were dominated by Li2O2. This demonstrated the
high stability of PEG based GPE towards oxygen reduction reaction and confirmed the
usability in Li-O2 batteries. Therefore, PEG based GPE is suitable for Li-O2 batteries.
Further investigation of increasing the capacity of such batteries is still going.
Figure 3-10 XRD pattern of cathode after discharge in PEG polymer electrolyte.
3.4 Summary
Gel polymer electrolytes with low molecular weight polyethylene glycol were
successfully prepared by hot solution casting method. The Li+ plating/stripping
reversibility, stability and conductivity were considered acceptable for usage in Li-O2
batteries. The cycling performance of batteries using PEG based GPE as electrolyte was
better than the direct use of liquid tetraethylene glycol dimethyl ether plasticizer. The
addition of SiO2 nanoparticle into the GPE system has also been demonstrated not
Figure 3- 10 XRD pattern of cathode after discharge in PEG polymer electrolyte
20 30 40 50 60 70
***
**
Inte
nsity
(a.u
.)
2
PEG electrode Li2O2
PEG*
79
favourable for Li-O2 batteries because of the decrease of stability. Therefore, we expect
that PEG based GPE could be used as promising electrolyte in long-life Li-O2 batteries.
80
Chapter 4 Investigation of PVDF-HFP Based Gel Polymer
Electrolyte Used in Li-O2 Batteries
4.1 Introduction
Poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP) is known as a copolymer
of vinylidene fluoride with hexafluoropropene. The copolymer process provides low
crystallinity and glass transition temperature and helps improve the flexibility.
However, the content of HFP in the copolymer systems is very critical, which is
basically kept 8% to 25%. PVDF-HFP has been largely employed in many industry
applications including battery industry. Figure 4-1 shows the typical structure of PVDF-
HFP.
Figure 4-1 The typical structure of PVDF-HFP.
Poly(vinylidene fluoride) (PVDF) has been widely applied as polymer host in gel
polymer electrolytes because of the high stability due to the strong electro-withdrawing
function group (-C-F), and high dielectric constant which can help provide a high
concentration of charge carrier [111, 233-235]. However, the mechanical properties of
PVDF based GPE are often very weak and need cross-linking treatment. PVDF-HFP is
Figure 4- 1 The typical structure of PVDF-HFP Figure 4- 1 The tyyyyyyypical strurrrrruuuctuuureeeeereeeerrreeeereererr ooooooooooooooooooooooooooooof ffffffffffffffffffffffff ffffffff ff PVPVVPVPVPVPVVVPVVPVPVVVVVPVVPVPPVPPPVPVPVPVPVVVPVPVPVPPPVPPPPPPPPPP DDFDDDDDDDDDFDFFFDDDDFFDFDFDFFFFDDDDDFDFD --------HFHHFHHHHHFHFHFHFHFHFHFHHFHHHFHFHHFHHHHHFHHFHFHHFFFFFFHHHHHHH P
81
considered to have very good capability of trapping large amount of liquid electrolytes
and maintaining sufficient mechanical integrity at the same time without the need for
cross-linking process. This makes PVDF-HFP more favourable as polymer matrix for
GPEs. It is known that the PVDF-HFP GPE system can be described as a
heterogeneous, phase-separated, plasticized polymer electrolyte and there are four
phases in the system. They are semi-crystalline polymer with low degree of
crystallinity, amorphous part plasticized with the electrolyte solution, large volume of
nanopores, and interfacial regions of the nanoparticle filler filled/coated by the liquid
solution of electrolyte [111]. This system has already been employed in the battery
industry [236].
In this chapter, GPE based on PVDF-HFP with Tetratheylene glycol dimethyl ether
(TEGDME) as plasticizer and solvents for lithium salts was prepared and used in Li-O2
batteries. The liquid TEGDME electrolyte was used for comparison. Other plasticizer
such as propylene carbonate (PC) and dimethyl sulfoxide (DMSO) were also used since
they were most commonly used liquid electrolytes in Li-O2 batteries.
4.2 Experiment
4.2.1 Preparation of PVDF-HFP based GPEs
PVDF-HFP based polymer electrolyte (GPE) was prepared by solvent casting method.
The liquid TEGDME electrolyte was prepared by dissolving
Bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) into tetraethylene glycol
dimethyl ether (TEGDME). The concentration was kept at 1 M. The solid PVDF-HFP
was dissolved in certain amount of acetone and kept stirring for 2 h. After all PVDF-
HFP was dissolved, liquid TEGDME electrolyte was added into the solution and the
solution was kept mechanically stirring for 2 h until a homogenous solution was
82
obtained. The weight ratio of PVDF-HFP and liquid solution was kept at 2: 7. The
homogenous solution was then cast onto a flat glass at room temperature and allowed to
solidify. This process resulted in a transparent gel like membrane and it was ready to
characterize. The same process was used to make GPEs containing propylene carbonate
(PC) and dimethyl sulfoxide (DMSO) as plasticizers. All the chemicals were dried
before use and all the process was operated in a glovebox with water and oxygen
content lower than 0.1 ppm.
4.2.2 Material characterization
X-ray diffraction (XRD) was conducted on a Siemens D5000 X-ray diffractionmeter.
During the XRD analysis process, all materials and cathodes were protected without
exposure to the ambient atmosphere.
4.2.3 Electrochemical testing
All the electrochemical characterization of as-prepared GPE was carried on
electrochemical workstation. The lithium ion plating/stripping was characterised by
cyclic voltammetry measurements using Li/GPE/Li cell over a wide voltage range (-4.5
V to 4.5 V) at medium scanning rate (10 mV s-1). Linear sweep voltammetry
measurements were used to determine the stability of as-prepared GPEs with
Li/GPE/stainless steel (SS) sealed cell and Li/GPE/carbon black electrode (CB) cell
exposed to ambient oxygen over a wide scanning range (open circuit potential to 6.5 V)
at slow scanning rate (1.0 mV s-1). The preparation of carbon black electrode will be
mentioned later. Liquid TEGDME electrolyte was characterise the same way for
comparison. The as-prepared GPE was sandwiched between two symmetrical stainless
steel electrodes and sealed in a Swagelok cell. The impedance was conducted by
electrochemical workstation CH Instrument 660D in the frequency range 0.1- 100 kHz.
83
The carbon electrode slurries were prepared by mixing carbon black (90 wt%) and
poly(tetrafluoroethylene) (PTFE) (10 wt%) together in propanol. The mixture was then
coated on a nickel mesh substrate. The cathode film was then obtained by punched into
disc and dried under vacuum at 80 oC for 12 h. A Swagelok cell with an air hole (0.785
cm2) on the cathode side was used to investigate the discharge and charge performance.
The Li-air cells were assembled in a glovebox with water and oxygen level less than 0.1
ppm. The as-prepared GPEs were sandwiched between lithium foils and carbon black
electrodes and a glass microfiber filter was used for keeping the structure from
collapsing. For comparison, a cell with a glass microfiber filter soaked in liquid
TEGDME electrolyte was also made. Same measurements were also performed with
GPEs with PC and DMSO as plasticizers. All the cells were gas-tight except for the
cathode side window. All the measurements were conducted in 1 atm in dry oxygen
atmosphere to avoid any negative effects of humidity and CO2.
4.3 Results and discussion
The most three important factors that determine whether one electrolyte can be used in
lithium battery or not are the reversibility of Li+ insertion and extraction reaction,
electrochemical stability, and ionic conductivity. Figure 4-2 displays the cyclic
voltammetry curve using TEGDME based GPE sandwiched between two lithium
electrodes. The curve shows only one anodic peak and one cathodic peak and the values
of potential and current density were almost the same, which indicated the reversibility
of Li+ insertion and extraction. The sufficient reversibility of Li+ insertion and
extraction reaction can ensure the smoothly running of battery reaction during cycling.
Therefore, the as-prepared TEGDME based GPE could be applied in Li-O2 batteries.
84
Figure 4-2 The cyclic voltammetry curve of Li/GPE/Li typed cell with TEGDME based
GPE as electrolyte
The second concern is the electrochemical stability of an electrolyte when using in
lithium batteries. The demand of wide operating window can eliminate unwanted side
reactions and ensure an acceptable electrochemical performance. Figure 4-3 (a) shows
the result of linear sweep voltammetry measurement using sealed battery with
TEGDME based GPE sandwiched between a lithium electrode and a stainless steel
electrode. For comparison, the one with liquid TEGDME electrolyte was also measured
using the same method. TEGDME based GPE could stay stable up to 5.5 V while the
liquid electrolyte suffered decomposition at the voltage of 4.5 V. This indicates the
gelled polymer electrolyte had higher stability than the liquid one. Normally, a lithium-
ion battery requires a stability window up to 4 V and these results were sufficient
enough. However, as Li-O2 batteries must be operated in an open atmosphere because
oxygen is the reactant for battery reactions, the stability data in the sealed batteries is
Figure 4- 2 The cyclic voltammetry curve of Li/GPE/Li typed cell with TEGDME based GPE as electrolyte
-6 -4 -2 0 2 4 6-0.03
-0.02
-0.01
0.00
0.01
0.02
0.03
Cur
rent
(A)
Potential (V)
Li/PVDF-HFP/Li
85
not persuadable enough. In order to investigate the stability, linear sweep voltammetry
measurement was conducted with an open battery using TEGDME based GPE
sandwiched between a lithium electrode and a porous carbon black electrode with a hole
at the carbon black side as the gas channel. The liquid TEGDME electrolyte was also
measured for comparison. The results are shown in Figure 4-3 (b). The current density
of battery with TEGDME based GPE didn’t increase until 4.5 V while the one of
battery with liquid electrolyte increased from 4 V. The results indicate the exposure to
oxygen atmosphere can decrease the stability of electrolyte systems greatly and
choosing a suitable electrolyte for Li-O2 battery is much difficult than Li-ion battery.
The stability of TEGDME based GPE showed in Figure 4-3 ensured the usability in Li-
O2 batteries.
Figure 4- 3 Linear sweep voltammetry results of (a) Li/GPE/SS and (b) Li/GPE/CB-air type cells
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
0.0000
-0.0005
-0.0010
-0.0015
-0.0020
Cur
rent
(A)
Potential (V)
Li-PVDF-HFP-SS Li-TEGDME-SS
(a)
86
Figure 4-3 Linear sweep voltammetry results of (a) Li/GPE/SS and (b) Li/GPE/CB-air
type cells.
The ionic conductivity is another concern for the usage of GPE in Li-O2 systems.
Sufficient ionic conductivity can ensure a smooth running of battery reactions. The
ionic conductivity was measured through A.C Impedance measurement and the values
were calculated from the membrane resistances obtained from the impedance spectra.
The results are displayed in Figure 4-4. It was seen that the ionic conductivity increase
with the increase of temperature. This is probably due to the reason that the capability of
ions to move freely in the electrolyte system was greatly improved when the
temperature was increased. The ionic conductivity at the room temperature (25 ºC) was
roughly 10-3 Ωcm-1 and it was considered sufficient for Li-O2 batteries.
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
0.000
-0.001
-0.002
-0.003
Cur
rent
(A)
Potential (V)
Li-PVDF-HFP-air Li-TEGDME-air
(b)
87
Figure 4-4 The calculated ionic conductivity of PEG at different temperatures.
All the measurement confirmed the properties of TEGDME based GPE could be used in
Li-O2 batteries. The electrochemical performance of as-prepared GPE was investigated
in a battery with TEGDME based GPE sandwiched between a lithium anode and a
porous carbon black electrode. Same battery construction was used for liquid TEGDME
electrolyte. The results of discharge and charge profiles in first cycle are shown in
Figure 4-5. Battery with TEGDME based GPE as electrolyte could deliver specific
capacity of 2,988 mAh g-1 and this result was acceptable according to the previous
reports of other groups. However, the battery with liquid TEGDME as electrolyte
showed even higher capacity of 7,921 mAh g-1 and lower over-potential. The possible
reason for such differences of electrochemical performance in the first cycle for both
electrolytes is that the performance was greatly influenced by their intrinsic properties
such as ionic conductivity and phase structure. The liquid TEGDME was in its liquid
phase which means the ions in the solution could move freely and provide relatively
high ionic conductivity. On the other hand, the GPE was in its solid phase which
Figure 4- 4 The calculated ionic conductivity of PEG at different temperatures
20 40 60 80 100
0.001
0.002
0.003
0.004
0.005
Con
duct
ivity
( c
m-1)
Temperature (oC)
PVDF-HFP
88
indicated the movement of the ions in the electrolyte system could be restricted.
Although the impedance measurement confirmed the ionic conductivity of GPE was
sufficient for Li-O2 batteries, it was still lower than the plasticizer before it was made
into GPE. This could explain the lower capacity and higher over-potential of battery
with TEGDME based GPE. Another reaction is the solid state GPE did not have the
same penetrability as the liquid state TEGDME. This meant the contact area of
electrolyte and cathode was smaller for GPE. The active site on the cathode for oxygen
reduction reaction was reduced and this led to the reduction of specific capacity. Despite
these drawbacks, the first cycle performance of battery with TEGDME based GPE was
acceptable for Li-O2 batteries.
Figure 4-5 The discharge and charge profiles in the first cycle of Li-O2 batteries using
different electrolyte. The current density was 50 mAh g-1.
The cycling performance of the battery with TEGDME based GPE was also
investigated with a fixed capacity of 500 mAh g-1. The result is shown in Figure 4-6. It
Figure 4- 5 The discharge and charge profiles in the first cycle of Li-O2 batteries using different electrolyte
0 2,000 4,000 6,000 8,000
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Vol
tage
(V)
Specific capacity (mAh g-1)
PVDF-HFP TEGDME
89
is clearly seen that the battery with TEGDME based GPE could last 25 cycles with
nearly no differences in discharge and charge potentials. The reversibility was great
according to this result and this demonstrated the usability of TEGDME based GPE in
Li-O2 batteries.
Figure 4-6 Discharge and charge profiles of Li-O2 batteries with TEGDME based GPE
as electrolytes at fixed capacity to 500 mAh g-1. Current density is 50 mA g-1.
In order to further demonstrate the sufficient stability of GPE in Li-O2 batteries, cycles
with fully discharge and charge were operated. For comparison, the battery with liquid
electrolyte was also been cycled under the same condition. The results are displayed in
Figure 4-7. The battery with TEGDME based GPE could last for at least 5 cycles with
only a slight drop of capacity. The capacity of battery with liquid TEGDME electrolyte
faded quickly from the second cycle although it could provide extremely high initial
discharge capacity. This provided the evidence that the stability of the plasticizer was
Figure 4- 6 Discharge and charge profiles of Li-O2 batteries with TEGDME based GPE as electrolytes at fixed capacity to 500 mAhg-1
0 100 200 300 400 5002.5
3.0
3.5
4.0
4.5
5.0
Vol
tage
(V)
Specifi capacity (mAh g-1)
1st cycle 5th cycle 10th cycle 15th cycle 20th cycle 25th cycle
90
greatly improved after made in to GPE. The stability of TEGDME based GPE system
made it possible to be used in Li-O2 batteries.
Figure 4-7 Cycling profiles of Li-O2 batteries with PVDF-HFP based GPE and
TEGDME as electrolytes. Current density is 50 mA g-1.
The superior cycling performance of TEGDME based GPE is probably due to the gelled
structure of the polymer electrolyte system which is proposed to be set up by the
interaction between polymer matrix PVDF-HFP and plasticizer TEGDME through Li+.
A brief schematic mechanism is shown in Figure 4-8. According to the reference, the
Li+ serves as a cross-link between polyether and PVDF or HFP chains with the Li+
bonding the former via oxygen atoms and the latter via fluorine atoms [149]. As Li+ was
used as cross-link in the structure, the amount of free Li+ in the polymer electrolyte
system was greatly reduced, which explained the decrease of ionic conductivity. At the
same time, the interactions could also help increase the stability of the α-hydrogen and
Figure 4- 7 Cycle profiles of Li-O2 batteries with PVDF-HFP based GPE and TEGDME as electrolytes
1 2 3 4 50
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
Spe
cific
Cap
acity
(mA
h g-1
)
Cycle numbers
PVDF-HFP TEGDME
91
α-carbon atoms. This resulted in the improving stability of the plasticizer TEGDME.
The mechanism was detailedly discussed in Chapter 3. In this way the stability of the
whole electrolyte system was significantly improved.
Figure 4-8 Proposed structure of PVDF-HFP based GPE
In order to find out more about the interactions, DMSO and PC were used to replace
TEGDME to break the interactions. The batteries with DMSO based and PC based
GPEs were discharged and charged under the same condition. The results are shown in
Figure 4-9. It was clearly seen that the specific capacities of both batteries increased but
the cycleability was greatly reduced. This confirmed the existence of unique interactions
between TEGDME and polymer matrix PVDF-HFP. The results listed in Figure 4-9
also indicate the plasticizer could affect the electrochemical performance. The battery
with DMSO based GPE as electrolyte could deliver higher specific capacity than the
Figure 4- 8 Proposed structure of PVDF-HFP based GPE
92
one with PC based GPE. The differences should lie on the different properties of DMSO
and PC plasticizers.
Figure 4-9 The discharge and charge profiles in the first cycle of PC (a) and DMSO (c)
based GPEs and the cycling performance of PC (b) and DMSO (d) based GPEs. The
current density was 50 mAh g-1.
It is very difficult to choose between initial capacity and cycleability because they both
characterize the performance of Li-O2 batteries. According to the reference, the cycling
performance is the main issue that limits the practical application of Li-O2 batteries
[103]. In that case, TEGDME based GPE showed better cycling performance and was
demonstrated suitable for Li-O2 batteries.
Figure 4- 9 The discharge and charge profiles in the first cycle of PC (a) and DMSO (c) based GPEs and the cycling performance of PC (b) and DMSO (d) based GPEs
0 1,000 2,000 3,000 4,0002.0
2.4
2.8
3.2
3.6
4.0
4.4
4.8
Vol
tage
(V)
Specific capacity (mAh g-1)
PC(a)
1 2 3 4 50
1,000
2,000
3,000
4,000
5,000
Spe
cific
cap
acity
(mA
h g-1
)
Cycle number
PC(b)
0 2,500 5,000 7,500 10,000 12,500 15,0002.0
2.4
2.8
3.2
3.6
4.0
4.4
4.8
Vol
tage
(V)
Specific capacity (mAh g-1)
DMSO (c)
1 2 3 4 50
2,000
4,000
6,000
8,000
10,000
12,000
14,000S
peci
fic c
apac
ity (m
Ahg
-1)
Cycle number
DMSO(d)
93
4.4 Summary
Gel polymer electrolytes with PVDF-HFP as polymer matrix have been successfully
prepared and applied in Li-O2 batteries. PVDF-HFP based GPE showed sufficient
reversibility of Li+ insertion and extraction, good stability during discharge and charge
process, and high ionic conductivity. The battery with PVDF-HFP based GPE exhibited
high cycling performance which was much better than the liquid TEGDME electrolyte.
It is believed the interactions between the components in GPE helped stabilize the
electrolyte system. The electrochemical performance of batteries with GPEs based on
different plasticizer was also investigated. It is found that the performances of GPEs
were significantly affected by the use of plasticizer and the best choice is TEGDME as
plasticizer when compared with PC and DMSO. Therefore, we expect that PVDF-HFP-
TEGDME based GPE could be used as promising electrolyte in long-life Li-O2
batteries.
94
Chapter 5 Conducting Polymer-Doped Polypyrrole as An
Effective Cathode Catalyst for Li-O2 Batteries
5.1 Introduction
The Nobel Prize in Chemistry was awarded in 2000 for work on conductive polymers,
including polypyrrole (PPy) [237, 238]. PPy is a conjugated compound formed from a
number of connected pyrrole ring structures. The pure PPy is considered as insulator.
However, the oxidized PPy is very good electrical conductor. The conductivity is
mainly based on the conditions and reagents used when the polymerization process is
carried on. The conductivity can be ranged from 1 to 100 Scm-1. The typical structure of
PPy is displayed in Figure 5-1.
Figure 5-1 The typical structure of PPy.
PPy has attracted extensive investigations recent years due to the high electrical
conductivity, high mechanical strength, and high chemical stability. It has been applied
into several areas such as electric devices and chemical sensors. Li-ion batteries are
considered heated research areas recent days and PPy has been widely used in this areas
[239-246]. It is believed that the PPy can be used as the coating materials or directly
used as cathode materials. Owing to the intrinsic reversible redox properties, doped PPy
Figure 5- 1 The typical structure of PPy
95
could be potentially used as catalysts for lithium batteries. The electrode reaction is
considered as below,
(5-1)
where A- stands for the dopant ions [155, 160, 247].
PPy is also considered to have great catalytic properties for oxygen reduction reactions
[248-250]. Cui et al. reported replacement of carbon with tubular PPy as catalyst for Li-
O2 batteries and achieved a very good cycling performance [70]. However, the influence
of different dopants on the electrochemistry performance of PPy as catalyst in Li-O2
batteries has not yet been reported. Despite all the superiorities PPy shared, there have
been seldom reports in Li-O2 batteries.
In this chapter, we report the use of polypyrrole with different dopants as catalyst in
non-aqueous Li-O2 batteries for the first time. Polypyrrole doped with Cl- and ClO4-
were synthesized and applied as the cathode catalyst in Li-O2 batteries.
5.2 Experiment
5.2.1 Synthesis of materials
Polypyrrole with different dopants was synthesized by an in situ chemical
polymerization method. In a typical synthesis process, 10 mmol pyrrole monomers were
added into 40 mL aqueous solution consisting of 1 M dopant acid under stirring. The
dopant acids are HCl and HClO4, respectively. After stirring for 30 min, 20 mL aqueous
solution consisting of 5 mmol (NH4)2S2O8 and 1 M dopant acid was added into the
previous solution. The mixture was kept stirring at room temperature for 6 h. The black
precipitate was filtered and washed with distilled water and ethanol for several times
and then dried in a vacuum oven at 60 oC for 12 h.
96
5.2.2 Characterization of samples
Field emission scanning electron microscope (FESEM, Zeiss Supra 55 VP) was used to
investigate the morphology of the as-prepared PPy polymers. Infrared spectra were
measured using a Nicolet Magna 6700 FT-IR spectrometer. All spectra were obtained
using 4 cm-1 resolution and 64 scans at room temperature.
5.2.3 Electrochemical measurements
Cathode slurry was prepared by mixing the as-prepared PPy, poly(tetrafluoroethylene)
(PTFE) with Super-P carbon black together in isopropanol with the weight ratio of
60:10:30. The mixture was coated on a stainless steel mesh substrate and then cut into
discs with a diameter of 14 mm. The electrodes were dried at 80 oC under vacuum for
12 h. The loading of the cathode materials is about 2 mg cm-2. For comparison, pure
carbon black electrodes were fabricated by mixing PTFE and Super-P carbon black in
isopropanol with the weight ratio of 10:90. Some carbon black electrodes were chosen
to soak into 0.5 M LiCl ethanol solution under vacuum for 3 hours and dried under
vacuum at 150 oC for 6 h. A Swagelok type cell with an air hole (0.785 cm2) on the
cathode side was used to test the electrochemical performance. The cell was assembled
in an argon filled glove box (Mbrau) with water and oxygen level less than 0.1 ppm. A
lithium foil was used as the anode and was separated from cathode by a glass microfiber
filter (Whatman) soaked in electrolyte (1 M LiClO4 in propylene carbonate). The cell
was gas tight except for the cathode side window that exposed the cathode film to the
oxygen atmosphere. All measurements were conducted in 1 atm in dry oxygen
atmosphere to avoid any negative effects of humidity and CO2.
97
5.3 Results and discussion
Figure 5-2 (a) and (b) show the SEM images of the as-prepared PPy-Cl and PPy-ClO4
polymers. Both of them showed similar morphology, which consists of nanoparticles
with a diameter about 200 400 nm. The FT-IR spectra of the as-prepared PPy are
shown in Figure 5-2 (c). The pyrrole ring vibrations can be observed at 1,544 and 1,456
cm-1 and =C-H vibrations appeared at 1,298 and 1,042 cm-1 in the PPy spectra. The
vibration at 1175 cm-1 can be assigned to the stretching of C-N group.
Figure 5- 2 SEM images of the as-prepared (a) PPy-Cl and (b) PPy-ClO4, and (c) FT-IR spectra of both PPy polymers
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98
Figure 5-2 SEM images of the as-prepared (a) PPy-Cl and (b) PPy-ClO4, and (c) FT-IR
spectra of both PPy polymers.
The electrocatalytic activity of the as-prepared PPy was examined in Li-O2 battery and
compared with carbon black at room temperature. The charge-discharge voltage range
was set between 2.0 and 4.5 V for all the measurements. The voltage profiles of the
electrodes in the first cycle are shown in Figure 5-3 (a). The discharge capacities of
PPy-Cl and PPy-ClO4 electrodes are 6,208 mAh g-1 and 5,164 mAh g-1, respectively.
The carbon black electrode delivered a lower capacity of 1,365 mAh g-1. The discharge
plateaus of both conducting polymer electrodes were higher than that of carbon black
electrode indicating PPy had better catalytic activity towards ORR than carbon black.
The charge potential plateaus of conducting polymer electrodes were also lower
comparing to carbon black electrode indicating PPy had better catalytic activity towards
oxygen evolution reaction (OER) than carbon black. The cycling performances of both
PPy and carbon black electrodes are shown in Figure 5-3 (b). PPy-Cl and PPy-ClO4
electrodes exhibited much better cycling stability than carbon black electrodes in the
1800 1600 1400 1200 1000 800 600
Tran
smiti
on (a
. u.)
Wavenumbers (cm-1)
PPy-Cl PPy-ClO
4
(C)=C-H
-C-NFudamental vibrationsof pyrrole ring
99
first five cycles. Furthermore, PPy doped with Cl- showed better cycling stability than
the PPy-ClO4 electrode.
Figure 5-3 (a) The discharge-charge profiles and (b) cycling performance of PPy-Cl,
PPy-ClO4 and carbon black electrodes. The discharge-charge current density is 100 mA
g-1 in 1 atm O2 at room temperature.
Figure 5- 3 The discharge-charge profiles and (b) cycling performance of PPy-Cl, PPy-ClO4 and carbon black electrodes 0 2,000 4,000 6,000
2.0
2.5
3.0
3.5
4.0
4.5
PPy-Cl PPy-ClO4
Carbon black
Volta
ge (V
)
Specific capacity (mA h g-1carbon)
(a)
1 2 3 4 5
0
2,000
4,000
6,000
8,000
carbon black PPy-Cl PPy-ClO4
Spec
ific
capa
city
(mAh
g-1
carb
on)
Cycle number
(b)
100
The ORR reaction with the presence of PPy on cathode in Li-O2 batteries could involve
two steps:
(5-2)
(5-3)
As shown in Figure 5-4 (a), the π-bond orbitals of the oxygen molecule have the
tendency to accept one electron by overlapping with pyrrole rings, which consist of
delocalized π-bonds formed by carbon atoms in the state of sp2 hybridization,
corresponding to the equation (3). Khomenko et al. demonstrated that the carbon atoms
in 3 and 4 positions of pyrrole rings can combine with oxygen atoms. The polypyrrole is
the electron density donor and the oxygen is electron density acceptor [155], which
makes the ORR occurring on PPy catalysts. In the whole discharge process on the PPy-
ClO4 cathode, PPy-ClO4 is firstly reduced into its undoped state and releases ClO4- ions
into the electrolyte and then the undoped PPy is oxidized to PPy-ClO4 by O2 with ClO4-
ions from electrolyte. After the reduction of PPy-Cl, Cl- will go into electrolyte and
LiCl will precipitate on the surface of PPy. When the reduced PPy reacts with O2, it will
react with ClO4- from the electrolyte to form PPy-ClO4 instead of PPy-Cl (Figure 5-4
(b)). To further investigate the ORR on the cathodes, the FTIR spectra of all electrodes
after discharge and charge are shown in Figure 5-5. The discharge products were
dominated by Li2CO3 and after charge, the peaks corresponding to Li2CO3 disappeared.
Recently, many investigations demonstrated that ORR in alkyl carbonate electrolytes is
more complicated than simply forming Li2O2 as the discharge product [21, 26]. The
discharge reaction also involves the decomposition of the electrolyte and the oxidation
of carbon materials to form other products, such as Li2CO3, CH3COOLi, HCOOLi,
C3H6(OCOOLi)2, CO2 and H2O. Although these products can be decomposed in the
101
charge process, the loss of electrolyte and the accumulation of discharge products will
cause capacity loss during cycling. Although the PPy-ClO4 electrode showed better
capacity retention than that of carbon black electrode, it still suffered from severe
capacity fading during cycling. On the other hand, the PPy-Cl electrode exhibited a
better cycling stability than the PPy-ClO4 electrode, which is probably due to the
formation of LiCl on the surface of the electrode materials.
Figure 5-4 The mechanism of (a) oxygen activation of PPy and (b) doping-undoping
process of PPy-Cl and PPy-ClO4.
Figure 5- 5 FT-IR spectra of (a) PPy-Cl, (b) PPy-ClO4, and (c) carbon black electrodes before discharge, after discharge and after charge process
Figure 5- 4 The mechanism of (a) oxygen activation of PPy and (b) doping-undoping process of PPy-Cl and PPy-ClO4
Figuguguuuuuuguuuuuuuuguguuuuuguuuuuuuuuuguguuuguguuuguuuuguuuguguuguguuuugguuugguure 5- 4 The mechanism of (a) oxygen actiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiivavavvvvvvvvvvvvvvvvvvvvvvvvvvvvavvvvvvvvvavvvvvatititititionononoonnnnnnonoonnnnoonnnnnoonnnooononnnnnnnnnnnonnnnonnnnnnonn of PPy aaand (b) doping-undoping process oooooooooooooof fff fffffffffffffffffffffffffff ffffffffffffff PPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPP y-Cl and PPy-ClO4
1,800 1,600 1,400 1,200 1,000 800 600
*
*****
Tran
smiti
on (a
.u.)
Wavenumber (cm-1)
Pristine electrode Discharged electrode Charged electrode
Li2CO3
(a)
102
Figure 5-5 FT-IR spectra of (a) PPy-Cl, (b) PPy-ClO4, and (c) carbon black electrodes
before discharge, after discharge and after charge process.
1,800 1,600 1,400 1,200 1,000 800 600
*
***
**
Tran
smiti
on (a
.u.)
Wavenumber (cm-1)
Pristine electrode Discharged electrode Charged electrode
Li2CO3
(b)
1,800 1,600 1,400 1,200 1,000 800 600
*
* ***
pristine electrode discharged electrode charged electrode
Li2CO3
Tran
smiti
on (a
.u.)
wavenumber (cm-1)
*
(c)
103
As the amount of LiCl formation on the surface of PPy-Cl cathode is very small, it is
very difficult to detect LiCl by X-ray diffraction analysis. In order to confirm the
formation of LiCl on the PPy-Cl cathode after discharge and charge cycling, energy
dispersive X-ray spectroscopy (EDS) was performed on the cycled electrodes. The EDS
results of chloride element are shown in Table 5-1. We found that the content of Cl in
PPy-Cl electrodes is about 4-5 times higher than that in PPy-ClO4 electrodes. Therefore,
a LiCl precipitation layer formed on the surface of PPy-Cl electrode during cycling
process.
Table 5-1 EDS results of PPy-Cl and PPy-ClO4 electrodes after cycling
Cl %
PPy-Cl
electrode
Atomic
percentage
3.36
Weight
percentage
8.06
PPy-ClO4
electrode
Atomic
percentage
0.77
Weight
percentage
1.93
Table 5- 1 EDS results of PPy-Cl and PPy-ClO4 electrodes after cycling
In order to explain the differences of the electrochemical performances between PPy-Cl
and PPy-ClO4, a carbon black electrode with the addition of LiCl was tested and
compared with pure carbon black electrode. The charge-discharge voltage profiles and
cycling performances are shown in Figure 5-6. After the addition of LiCl, the carbon
104
black electrode exhibited reduced charge-discharge over-potential and better cycling
stability than the bare carbon black electrode. The improved performance could be
attributed to the formation of LiCl layer on the surface of the electrode. It is believed the
LiCl layer can help protect the electrode from reacting with the electrolyte to form by-
products [68].
Figure 5-6 (a) The charge-discharge profiles and (b) the cycling performance of carbon
black electrodes with and without LiCl additive. The current density is 100 mA g-1 in 1
atm O2 at room temperature.
Figure 5- 6 (a) The charge-discharge profiles and (b) the cycling performance of carbon black electrodes with and without LiCl additive
0 500 1,000 1,500 2,000
2.0
2.5
3.0
3.5
4.0
4.5
5.0
V
olta
ge (V
)
Specific capacity (mA h-1carbon)
Carbon black + LiCl Carbon black
(a)
1 2 3 4 50
500
1,000
1,500
2,000
2,500
3,000
Spe
cific
cap
acity
(mA
h g-1
carb
on)
Cycle number
carbon black + LiCl carbon black
(b)
105
The formation of LiCl layers on the cathode surface not only can protect carbon from
reacting with electrolyte to form discharge byproducts, but also can participate in the
discharge and charge reactions. It is well known that LiCl is a commonly used Lewis
acid catalyst in many organic reactions. The O2- radical formed on the cathode is a
strong Lewis base. Therefore, LiCl may interact with O2- and stabilize the superoxide
radical. This process may lower the barrier of the reaction and provide a reduced over-
potential. The schematic mechanism is shown in Figure 5-7. Therefore, the presence of
LiCl layer on the cathode could lead to better cycling performance and lower over-
potential.
Figure 5-7 Schematic mechanism of discharge process on cathode with LiCl addition.
Figure 5- 7 Schematic mechanism of discharge process on cathode with LiCl addition
106
5.4 Summary
Polypyrrole with different dopants have been synthesized and applied as cathode
catalysts in Li-O2 batteries in alkyl carbonated electrolyte. The PPy electrodes showed
higher discharge capacities and lower over-potential than that of the carbon black
electrode. The mechanism of this phenomenon suggested that the PPy has an excellent
redox property and a capability to activate oxygen reduction reaction. Furthermore, PPy
doped with Cl- exhibited better cycling stability than that of PPy doped with ClO4-. We
expect that PPy-Cl polymer could be a promising cathode catalyst for Li-O2 batteries.
107
Chapter 6 Conducting Polymer Coated CNT Used in Li-O2
Batteries with Enhanced Electrochemical Performance
6.1 Introduction
Polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) are considered two
most commonly used conducting polymers. PPy is formed by a number of connected
pyrrole rings. PEDOT is a derivative of polythiophen (PT) based on the connected
structures of 3,4-ethylenedioxythiophene (EDOT) monomers. Figure 6-1 shows the
typical structure of PEDOT. Both conducting polymers have great chemical, mechanical
and electrochemical properties. Due to these superior properties, PPy and PEDOT have
been widely applied in many areas such as electric devices, anticorrosion coating, and
chemical sensors.
Figure 6-1 The typical structure of PEDOT.
Figure 6- 1 The typical structure of PEDOT
108
The conducting polymers used as catalysts in electric devices are believed to be
involved in a unique redox reaction, which is also known as doping-dedoping process.
The doped conducting polymers can be reduced into their undoped forms and this
process can be reversed. The process is show in the equation below,
(6-1)
where CP stands for conducting polymer, and A stands for anion.
In Chapter 5, PPy was used as catalyst in Li-O2 batteries and showed high catalytic
activity towards battery reactions. However, from Chapter 5, it is easy to find out that
the capacity of batteries with PPy as catalyst still faded after few cycles. One reason was
the instability of propylene carbonate electrolyte towards oxygen reduction reaction.
The other was the partial reversibility of the redox reaction of PPy. Because only
oxidized PPy or known as doped PPy was conductive and it was very difficult to
oxidize the PPy once it was reduced because the undoped one was an insulator. Due to
this reason, finding a way to maintain the conductivity of PPy while cycling is very
important. In order to overcome this drawback, conducting polymer coated carbon
materials are prepared. Carbon materials can be used as conductive matrix for electron
transportation and conducting polymers can be used as catalyst for battery reactions.
In recent years, multiwall carbon nanotubes (CNT) have been studied intensively
because of their high conductivity along with their mechanical strength. Combining
with conducting polymers coated on the surface, the composite shows improved
electrochemical properties. There have been many researches carried on in this area
[251-259]. Some of these researchers believed that CNT can be used as dopant for
conducting polymers. This provides the possibility that composite materials consisting
of CNT and conducting polymers can find specific applications in the field of energy
109
storage, optical limiting and electron field emission devices, transistors, conducting
textiles, sensors, biomedical and food domains [260].
In this chapter, PPy and PEDOT were used to coat on the surface of CNT. The
composites were employed as catalysts in the Li-O2 batteries for the first time. The
effects of different ratio and different conducting polymers were also investigated.
6.2 Experiment
6.2.1 Synthesis of materials
Polypyrrole coated carbon nanotubes (PPy/CNTs) with different ratio were prepared by
an in situ chemical polymerization method. CNT was pre-treated by HNO3 to add
function groups such as –COOH to the surface of CNT. Acid-treated CNT (30 mg) was
dispersed into 20 mL aqueous solution consisting of 1 M HCl by the supersonic method.
Pyrrole monomers (0.5 mmol) were added into this suspension. After mechanically
stirring for 30 min, 20 mL aqueous solution consisting of 2 mmol FeCl3 (4 times of
pyrrole) and 1 M HCl was added into the previous suspension. The mixture was kept
stirring at room temperature for 6 h. The black precipitate was filtered and washed with
distilled water and ethanol for several times and then dried in a vacuum oven at 60 ºC
for 12 h. The same process was used only with different amount of pyrrole and FeCl3.
The amount of pyrrole was kept at 0.25 mmol and 1mmol. The as-prepared PPy/CNTs
were weighed after drying process and the results were roughly 60, 45 and 90 mg,
respectively. Since the pristine CNT was 30 mg, the as-prepared PPy/CNTs were named
PPy/CNT1:1, PPy/CNT1:2, and PPy/CNT2:1, respectively. The same synthesis process
was used to prepare poly(3,4-ethylenedioxythiophene) coated carbon nanotube
(PEDOT/CNT) with the ratio of 1:1. The as-prepared materials were ready to
characterize.
110
6.2.2 Characterization of samples
Field emission scanning electron microscope (FESEM, Zeiss Supra 55 VP) was used to
investigate the morphology of as-prepared materials. Infrared spectroscopy was
conducted on a Nicolet Magna 6700 FT-IR spectrometer. All spectra were obtained
using 4 cm-1 resolution and 64 scans at room temperature. The conducting polymers and
CNT content in the composite materials was determined by TGA on a Mettler Toledo
TGA/DSC instrument in air at 10 ºC min-1 at temperature range of 30-1000 ºC.
6.2.3 Electrochemical measurements
Cathode slurry was prepared by mixing the as-prepared materials,
poly(tetrafluoroethylene) (PTFE) with Super-P carbon black together in isopropanol
with the weight ratio of 60:10:20. The mixture was coated on a stainless steel mesh
substrate and the cut into discs with a diameter of 14 mm. The electrodes were dried at
80 ºC under vacuum for 12 h. The loading of the cathode materials is about 1 mg cm-2.
For comparison, CNT cathode was also prepared with the same process.
A Swagelok type cell with an air hole (0.785 cm2) on the cathode side was used to test
the electrochemical performance. The cell was assembled in an argon filled glove box
(Mbrau) with water and oxygen level less than 0.1 ppm. A lithium foil was used as the
anode and was separated from cathode by a glass microfiber filter (Whatman) soaked in
electrolyte (1 M LiTFSI in Diethylene glycol dimethyl ether). The cell was gas tight
except for the cathode side window that exposed the cathode film to the oxygen
atmosphere. All measurements were conducted in 1 atm in dry oxygen atmosphere to
avoid any negative effects of humidity and CO2.
111
6.3 Results and discussion
Figure 6-2 (a)-(e) show the SEM images of the as-prepared PPy/CNTs and
PEDOT/CNT. All of them showed the similar tubular morphology only with different
diameters. The pristine CNT had the diameter of 50 nm and the PPy/CNT1:2 shared the
similar diameter. The diameters of PPy/CNT1:1 and PPy/CNT2:1 were larger than the
previous ones, which are 100 and 200 nm, respectively. This indicated that PPy was
successfully coated onto these CNTs and higher content of PPy resulted in larger
diameter. The morphology of as-prepared PEDOT/CNT1:1 was similar to PPy/CNT1:1
with the same diameter. The FT-IR specta of the as-prepared conducting polymer
coated CNTs are shown in Figure 6-2 (f). The pyrrole ring vibrations can be observed at
1,544 and 1,456 cm-1 and the thiophen ring vibrations can be seen at 1,365 cm-1. =C-H
vibrations appeared at 1,298 and 1,042 cm-1 in the spectra. The vibration at 1,175 cm-1
can be assigned to the stretching of C-N group and the vibration at 1,100 cm-1 can be
assigned to the stretching of C-O-C in the ethylenedioxy group. The results clearly
indicate the successful synthesis of the conducting polymer coated CNTs.
Although the weight ratios were roughly determined by the weight comparison before
and after synthesis, TGA method was still employed to further confirm the exact results.
The TGA spectra are shown in Figure 6-3. Due to the similar decomposition
temperatures of PPy and CNT, it is quite difficult to tell the weight ratio only referring
to the weight ratio curves which is shown in Figure 6-3 (a)-(c). However, the results can
be still found in the temperature difference curves. It was found that there were two
peaks in each curve and the position indicated the weight content of CNTs in the
composites were 34%, 48%, and 65% for PPy/CNT1:2, PPy/CNT1:1, PPy/CNT2:1,
respectively, which are consistent with the previous results. For PEDOT/CNT1:1, it is
112
easy to tell the weight ratio because of the large difference decomposition temperatures
of PEDOT and CNT shown in Figure 6-3 (d). The ratio was roughly 47%, which was
consistent of previous result.
Figure 6-2 The SEM images of (a) the bare CNT, the as-prepared (b) PPy/CNT1:2, (c)
PPy/CNT1:1, (d) PPy/CNT2:1, (e) PEDOT/CNT1:1, and (f) FT-IR spectra.
Figure 6- 2 The SEM images of (a) the bare CNT, the as-prepared (b) PPy/CNT1:2, (c) PPy/CNT1:1, (d) PPy/CNT2:1, (e) PEDOT/CNT1:1, and (f) FT-IR spectra
1800 1600 1400 1200 1000 800 600
Tran
smiti
on (a
.u.)
Wavenumber (cm-1)
CNT PPy/CNT1:2
PPy/CNT1:1
PPy/CNT2:1
PEDOT/CNT1:1
(f)
113
Figure 6-3 The TGA spectra of as-prepared (a) PPy/CNT1:2, (b)PPy/CNT1:1, (c)
PPy/CNT2:1, and (d) PEDOT/CNT1:1.
The electrochemical performances were tested with conducting polymer coated CNTs
through Li-O2 batteries at room temperature. The discharge and charge profiles of the
first cycle are shown in Figure 6-4 (a). The discharge capacities of PPy/CNTs with
different weight ratio were 3,793, 4,346, and 3,531 mAh g-1, respectively. The
PEDOT/CNT1:1 electrode provided a capacity of 2,646 mAh g-1 while the CNT
electrode delivered a lower capacity of 2,082 mAh g-1. These results indicated that all
conducting polymer coated CNT electrodes could deliver higher capacities than the
pristine CNT electrode, and PPy coated CNT could provide higher capacity than the
PEDOT coated one. Figure 6-4 (b) showed the partially enlarged profiles of discharge
and charge profiles with capacity to 500 mAh g-1. It was seen that the discharge plateaus
were very close to each other. However, the charge plateaus were very different. The
Figure 6- 3 The TGA spectra of as-prepared (a) PPy/CNT1:2, (b)PPy/CNT1:1, (c) PPy/CNT2:1, and (d) PEDOT/CNT1:1
0 200 400 600 800 1000
0
20
40
60
80
100
Wei
ght r
atio
(%)
Temperature (oC)
PPy/CNT1:2
-8
0
8
16
Tem
pera
ture
diff
eren
ce (
V)
(a)
0 200 400 600 800 1000-20
0
20
40
60
80
100
Wei
ght r
atio
(%)
Temperature (oC)
PPy/CNT1:1
-8
-4
0
4
Tem
pera
ture
diff
eren
ce (
V)
(b)
0 200 400 600 800 1000
0
20
40
60
80
100
Wei
ght r
atio
(%)
Temperature (oC)
PPy/CNT2:1
-12
-8
-4
0
4
8
Tem
pera
ture
diff
eren
ce (
V)
(c)
0 200 400 600 800 1000
0
20
40
60
80
100
Wei
ght r
atio
(%)
Temperature (oC)
PEDOT/CNT
-10
0
10
20
30
40
Tem
pera
ture
diff
eren
ce (μ
V)
(d)
114
PPy/CNT1:1 electrode had the lowest charge voltage while CNT electrode showed the
highest. The second was PEDOT/CNT1:1 electrode and PPy/CNT2:1 and PPy/CNT1:2
followed behind. These results provided the evidence that all conducting polymer
coated CNTs showed higher catalytic activity towards battery reactions than CNT.
The cycling performances are displayed in Figure 6-5. It was seen that PPy/CNT1:1
showed the best cycling performance and PEDOT/CNT1:1 followed behind. Although
the other two PPy/CNT electrodes had high capacities at the first cycle, the cycling
performances in the following cycles were far from satisfactory. As the comparison,
CNT electrode also exhibited poor cycling performance. It was observed that most
electrodes suffered the fast drop of capacities after first cycle. During the first cycle, the
electrolyte was kept evaporating. When in the second cycle, although the evaporation of
electrolyte was almost stoped because of the saturation of electrolyte vapour, some of
the discharge products could not reach the electrolyte and the capacity would certainly
decrease. The experiments with longer resting time are still proceeding.
Figure 6-4 (a) The discharge and charge profiles of as-prepared PPy/CNT1:2,
PPy/CNT1:1, PPy/CNT2:1, PEDOT/CNT1:1, and CNT electrodes. (b) Partially enlarged
profiles of as-prepared electrodes with capacity of 500 mAh g-1. The current density is
200mA g-1 in 1 atm O2 at room temperature.
Figure 6- 4 (a) The discharge and charge profiles of as-prepared PPy/CNT1:2, PPy/CNT1:1, PPy/CNT2:1, PEDOT/CNT1:1, and CNT electrodes. (b) Partially enlarged profiles of as-prepared electrodes with capacity of 500 mAh g-1
0 1,000 2,000 3,000 4,0002
3
4
5
Vol
tage
(V)
Specific capacity (mAh g-1)
PPy/CNT1:2
PPy/CNT1:1
PPy/CNT2:1
PEDOT/CNT1:1
CNT
(a)
0 100 200 300 400 5002
3
4
5
Vol
tage
(V)
Specific capacity (mAh g-1)
PPy/CNT1:2
PPy/CNT1:1
PPy/CNT2:1
PEDOT/CNT1:1
CNT
(b)
115
Figure 6-5 The cycling performance of as-prepared prepared PPy/CNT1:2, PPy/CNT1:1,
PPy/CNT2:1, PEDOT/CNT1:1, and CNT electrodes. The current density is 200mA g-1 in
1 atm O2 at room temperature.
The conducting polymer coated CNTs showed higher catalytic activity towards battery
reactions in Li-O2 batteries than bare CNT. The reason should be the unique redox
reaction which was discussed in the introduction part. During discharge and charge
process, CNT acted as the conductive matrix for electrons while conducting polymers
on the surface could perform as catalysts. The mechanism was shown in Figure 6-6 (a)
using PPy/CNT as an example. However, the content of coating also affected the
performances. With low content of PPy, the improvement of electrochemical
performance was not obvious while with high content of PPy, the performance was
mainly dominated by the conductivity, which explained the poor electrochemical
performances of PPy/CNT1:2 and PPy/CNT2:1. Therefore, PPy/CNT1:1 showed the
Figure 6- 5 The cycling performance of as-prepared prepared PPy/CNT1:2, PPy/CNT1:1, PPy/CNT2:1, PEDOT/CNT1:1, and CNT electrodes
1 2 3 4 50
1,000
2,000
3,000
4,000
5,000S
peci
fic c
apac
ity (m
Ahg
-1)
Cycle number
PPy/CNT1:2
PPy/CNT1:1
PPy/CNT2:1
PEDOT/CNT1:1
CNT
116
highest activity due to the appropriate combination of the conductivity and catalytic
properties. On the other hand, the use of different conducting polymer could also affect
the performances. The difference between PPy/CNT1:1 and PEDOT/CNT1:1 was mainly
due to the good ability of PPy to absorb oxygen from ambient air. The ethylenedioxy
group on the 3 and 4 position of thiophen ring blocked the combination of oxygen with
PEDOT. The mechanism is shown in Figure 6-6 (b). This explained the lower capacity
of PEDOT/CNT1:1. In all, the PPy/CNT1:1 was the best choice for Li-O2 batteries among
all the as-prepared conducting polymer coated CNTs.
Figure 6-6 The schematic mechanism of (a) PPy/CNT during cycling, and (b) the block
of O2 from PEDOT structure.
6.4 Summary
Conducting polymer coated carbon nanotubes have been prepared by in situ
polymerization and applied in Li-O2 batteries in ether based electrolyte. The
PPy/CNT1:1 showed the highest discharge capacity and lowest over-potential due to the
Figure 6- 6 6 The schematic mechanism of (a) PPy/CNT during cycling, and (b) the block of O2 from PEDOT structure
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117
best combination of conductivity and catalytic activity. The PEDOT/CNT1:1 also
showed good cycling performance but low capacity mainly because of the lower ability
of PEDOT to absorb oxygen than PPy. The outstanding properties of conducting
polymer coated CNTs may lead to the application in high performance Li-O2 batteries.
118
Chapter 7 Conclusions
7.1 General conclusion
Organic polymers have unique properties that are quite different from inorganic
compounds. Recent research on lithium oxygen batteries mainly focus on the inorganic
areas. In this Master project, polymers were investigated as the electrolyte and cathode
materials in lithium oxygen batteries. Using different synthesis methods, electrolyte
membranes based on semi-crystal polymers were successfully prepared and exhibited
increased stability along with great cycling performance, which have been illustrated in
Chapter 3 and Chapter 4. Conducting polymers were also prepared and employed as
catalysts which provided high capacities and good cycling performance. The detailed
illustrations were shown from Chapter 5 to Chapter 6.
Low molecular weight polyethylene glycol based gel polymer electrolyte was prepared
through hot solution casting. This gel polymer electrolyte showed sufficient properties
such as Li+ plating/stripping reversibility, stability, and ionic conductivity for usage in
lithium oxygen batteries. The battery with this electrolyte could deliver an initial
capacity of 3,667 mAh g-1 and could last for 5 cycles with 86% retention of discharge
capacity, which was much higher than 38% retention of liquid tetraethylene glycol
dimethyl ether electrolyte. It is believed that interactions between polymer matrix and
plasticizer through Li+ contributed mainly to the high stability and low conductivity.
The addition of silica nanoparticles into the electrolyte system was also investigated and
the stability of the electrolyte was greatly reduced which indicated silica was not
favourable for long-life lithium oxygen batteries.
119
A Poly(vinylidene fluoride-co-hexafluoropropene) based free-standing gel polymer
electrolyte membrane was prepared by solvent casting method. This membrane gelled
with tetraethylene glycol dimethyl ether and exhibited high Li+ insertion/extraction
reversibility, high stability both in oxygen atmosphere up to 4.5 V, and high ionic
conductivity which was acceptable for lithium oxygen batteries. The battery with this
membrane as electrolyte showed initial capacity of 2,988 mAh g-1 and the capacity
dropped slightly after 5 cycles. The effects of using different plasticizers were also
investigated. Except tetraethylene glycol, gel polymer electrolytes with other solvents
displayed high initial discharge capacity but poor cycling performance. This was mainly
because only tetraethylene glycol dimethyl ether could interact with polymer matrix due
to the relatively large molecular structure.
Polypyrrole with different dopants were prepared through in situ polymerization and
employed in lithium oxygen batteries. Batteries of polypyrrole with Cl- as dopant
exhibited extremely high initial capacity of 6,208 mAh g-1, while one with ClO4-
showed 5,164 mAh g-1. The over-potentials were also significantly reduced. The redox
doping-undoping process and strong capability to absorb oxygen of polypyrrole are the
main reason for such high performance. However, the polypyrrole with Cl- as dopant
showed even higher performance because of LiCl precipitation formed on the surface of
cathode during cycling. The Lewis acid LiCl could act as a catalyst to improve the
performance of lithium oxygen batteries.
Different conducting polymer coated carbon nanotubes were also prepared by in situ
polymerization. The effect of conducting polymer content was investigated through
polypyrrole coated carbon nanotubes. It is believed 50% loading of polypyrrole is the
best choice due to the appropriate combination of conductivity and catalytic activity.
120
The initial capacity of 50% loading material was 4,346 mAh g-1 and the cycling
performance was the best of all as-prepared conducting polymer coated carbon
nanotubes even after 5 cycles. Poly(3,4-ethylenedioxythiophene) coated carbon
nanotube with the same ratio was also prepared and showed 2,646 mAh g-1 in the first
cycle. It is believed that the carbon nanotube could work as conductive matrix while
conducting polymers act as catalysts. The differences in the performance of different
conducting polymer were contributed by the different structure of both conducting
polymers on the 3 and 4 positions of conjugated rings.
7.2 Outlook and future work
This Master project mainly focused on the use of polymers in lithium oxygen batteries
as electrolyte and cathode materials instead of inorganic materials. The synthesis
methods were solution casting and in situ polymerization. The electrochemical
performances were measured in this Master work.
Recently, the research on lithium oxygen batteries indicates the stability of electrolytes
is the main issue for capacity fading. The stability of gel polymer electrolytes based on
polyethylene glycol and poly(vinylidene fluoride-co-hexafluoropropene) studied in this
project was demonstrated to be high enough for long-term lithium oxygen batteries.
Especially poly(vinylidene fluoride-co-hexafluoropropene) based one was found to have
both high free-standing property and good electrochemical performance at the same
time. This helps build efficient and stable lithium oxygen batteries for commercial use.
However, ionic conductivity was the biggest issue for gel polymer electrolyte
application. Further, the synthesis methods used were too complicated for large-scaled
industrial production. Therefore, finding a gel polymer electrolyte with high ionic
conductivity and discovering easy ways to manufacture are more and more urgent.
121
Recently, a class of porous gel polymer electrolytes has been studied. It was found to
have high ionic conductivity which almost equals to liquid ones and the synthesis
methods are simple to achieve. Therefore, porous gel polymer electrolytes are
considered promising.
For conducting polymers used in this Master work, the catalytic activity towards oxygen
reduction reaction and oxygen evolution reaction was investigated and demonstrated to
be better than commonly used carbon catalysts. These materials are considered good
candidates for high performance lithium oxygen batteries. However, the loss of
conductivity of conducting polymers when used as cathode materials is still an issue. A
promising approach to solving this issue is to use a carbon matrix as conductive matrix
for the conducting polymers. For future work, different conducting polymer coated on
different conducting polymer coated on different carbon substrates by variety improved
methods will be prepared, to find more appropriate combination between conductivity
and catalytic activity.
In summary, the materials used in this Master project were demonstrated to be suitable
for lithium oxygen batteries and might be applied in other batteries. However, there are
still many issues to be solved. Future work should focus on improving the stability of
polymer electrolytes and conductivity of conducting polymers.
122
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