a conductive binder for high-performance sn electrodes in ......pulverization of electrode and loss...
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Article
A Conductive Binder for High-PerformanceSn Electrodes in Lithium-ion Batteries
Yan Zhao, Luyi Yang, Dong Liu, Jiangtao Hu, Lei Han, Zijian Wang, and Feng PanACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13692 • Publication Date (Web): 21 Dec 2017
Downloaded from http://pubs.acs.org on December 24, 2017
Just Accepted
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A Conductive Binder for High-Performance Sn
Electrodes in Lithium-ion Batteries
Yan Zhaoa, Luyi Yang
a*, Dong Liu
b, Jiangtao Hu
a, Lei Han
a, Zijian Wang
a and Feng Pan
a*
a School of Advanced Materials, Peking University, Shenzhen Graduate School, Shenzhen
518055, People’s Republic of China
b BUCT-CWRU International Joint Laboratory, College of Energy, Beijing University of
Chemical Technology, Beijing 100029, People’s Republic of China
KEYWORDS
Tin anode, Li-ion batteries, conductive binder, solid electrolyte interface, tin pulverization.
ABSTRACT
Tin (Sn) has been widely studied as a promising anode material for high energy and power
density Li-ion batteries owing to its high specific capacity. In this work, a water-soluble
conductive polymer is studied as a binder for nano-sized Sn anodes. Unlike conventional
binders, this conductive polymer formed a conductive network, which maintained the
mechanical integrity during the repeated charge and discharge processes despite the inevitable Sn
particle pulverization. The resultant Sn anode without conductive additives showed a specific
capacity of 593 mA h g-1
after 600 cycles at the current density of 500 mA g-1
, exhibiting better
cycling stability as well as rate performance compared to the Sn anodes with conventional
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binders. Furthermore, it was also found that the conductive binder enhanced the formation of
stable solid electrolyte interphase (SEI) layers.
1. INTRODUCTION
Improving the energy density of lithium ion batteries (LIBs) is crucial to the development of
electronic vehicles and consumer electronics.1–4
The application of commercially available
graphite anode is limited by its low theoretical capacity (372 mA h g-1
) and poor rate
performance.5 These drawbacks have limited the application of LIBs in electric vehicles.
Therefore, alternative anode materials with higher theoretical capacities are highly desired. As a
non-toxic and abundant element, tin anode has attracted much attention because of its appealing
theoretical capacity. In theory, one tin atom can store 4.4 lithium atoms to form Li22Sn5, resulting
in a capacity of 992 mA h g-1
.6 However, similar to silicon anode, tin anode suffers from massive
volume change due to the large amount of lithium insertion and extraction, which leads to
pulverization of electrode and loss of active material.7,8
In order to improve the cycling stability of tin anode, some strategies have been implemented:
(1) reduce the size of Sn particles to nanoscale to endure the high degree of volume change,6,9–11
(2) introduce Sn into a conductive matrix (e.g. carbon) to cope with volume change and maintain
the integrity of the electrode.12–17
However, these mentioned methods emphasis on the design of
Sn or Sn composite materials, the volume expansion is still inevitable. By using conductive
binder instead of conventional binders such as polyacrylate acid (PAA),18
poly(vinyldene
difluoride) (PVDF)19
and caboxymethyl cellulose (CMC)20
, good electrical contact can be
maintained despite the volume expansion.21
It has been previously reported that the conductive
binder significantly improved the electrochemical performance of Sn anodes.22,23
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Sodium poly(9,9-bis(3-propanoate)fluorine) (PF-COONa) has been successfully applied in Si
anodes as a conductive binder.24
In the previous work, PF-COONa exhibited good mechanical
adhesion, high electrolyte uptake and certain electric conductivity, and at the same time the polar
groups of the binder could form strong chemical bonds with the hydroxyls on nanosized silicon
particles. By employing it in Si anode, superior electrochemical performance was obtained,
suggesting that PF-COONa is a promising binder for various anode materials that could be
affected by the volume expansion.
In this paper, the impact of the conductive binder on the electrochemical performance of Sn
electrode is investigated. PF-COONa can firmly adhere to both Sn particles and Cu current
collector to form an integrated structure. Above all, owing to the good conductivity of the binder,
a carbon-free conductive network was formed. As a result, the electrode could still maintain
good electric contact despite the significant volume changes after repeated cycling. The
conductive binder can accommodate the huge volume change of tin particles as well as
contribute to the formation of stable SEI films, hence greatly improved the electrochemical
properties of Sn electrode. Using this conductive binder, the Sn electrode exhibited excellent
long-term cycling capacity, stability as well as rate capability, which outperformed other
conventional binders for pure Sn anode (see Table S1), indicating its great potential for high-
capacity anode material with large volume changes.
2. RESULTS AND DISSCUSSION
Sn nanoparticles are used as-purchased without further treatments. Figure S1 displays the X-ray
diffraction pattern of Sn nanoparticle with average size of 100 nm, where the major peaks can be
well indexed to crystalline tin. The weak peaks of tetragonal SnO indicate the presence of a very
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small amount of oxidized impurities. PF-COONa is prepared using the same method as described
in the previous work. The infrared spectrum of Sn/PF-COONa in Figure S2 shows that different
from the Si/PF-COONa electrodes, where the binder chemically bonds with Si particles, there is
no chemical bond formed between Sn and PF-COONa. Therefore, the Sn particles are physically
cohered by PF-COONa.The Sn/PF-COONa electrode consist of 80 wt% of Sn particles and 20
wt% of PF-COONa. PF-COONa electrodes with different average Sn loading were prepared for
different testing purposes.
0 400 800 1200 16000.0
0.5
1.0
1.5
2.0
2.5
100 mA g-1
1st
2nd
100th
200th
E/ V vs Li/Li+
Specific capacity/ mA h g-1 0 50 100 150 200
0
300
600
900
1200
1500
Specific capacity/ mA h g
-1
Cycle Number
100 mA g-1
0 200 400 6000
300
600
900
1200
1500
Specific capacity/ mA h g
-1
Cycle Number
500 mA g-1
(a) (b)
(d)
0 400 800 1200 16000.0
0.5
1.0
1.5
2.0
2.5
1st
100th
200th
600th
E/ V vs Li/Li+
Specific capacity/ mA h g-1
500 mA g-1(c)
Figure 1. The 1st, 2nd, 100th and 200th cycle voltage profiles of the cells at the current densities
of (a) 100 mA g-1
and (c) 500 mA g-1
; cycling capacities (red-discharge, black-charge) of Sn/PF-
COONa electrode at current densities of (b) 100 mA g-1
and (d) 500 mA g-1
. The loading of
active material is approximately 0.6 mg cm-2
.
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Figure 1 shows The cycling performances of Sn/PFCOO-Na electrode (with Sn areal loading of
approximately 0.6 mg cm-2
) at different cycling current densities. The voltage profiles of Sn/PF-
COONa are shown in Figure 1a and Figure 1c, it can be seen that as the current density
increased, very little overpotential due to DC polarization can be observed, which is an
indication of good electronic contact in the electrode. For both cells, an irreversible discharge
capacity of approximately 1500 mA h g-1
was obtained at the first cycle, this phenomenon will be
discussed in the following content. Figure 1b and Figure 1d demonstrate the cycling capacities
of Sn/PF-COONa electrodes at current densities of 100 mA g-1
and 500 mA g-1
, respectively.
After long cycling, stable capacities of 762 mA h g-1
(200 cycles, 100 mA g-1
) and 593 mA h g-1
(600 cycles, 500 mA g-1
) have been delivered. The stable capacities showed that the integrity of
the electrodes was well maintained after long cycling, suggesting good adhesive property of PF-
COONa for Sn nanoparticles. In addition, it can be observed that both cells exhibit excellent
coulombic efficiency during long cycling, indicating good electrochemical stability of the binder.
0 100 200 300 400 500
0
300
600
900
1200
1500
Sn/AB/PVDF
Sn/PF-COONa
Specific capacity/ mA h g
-1
Cycle Number
200 mA g-1
Sn/AB/CMC-Na
0 5 10 15 20 250
300
600
900
1200
1500
Sn/AB/PVDF
100 mA g-11000 mA g
-1500 mA g-1200 mA g-1
Specific capacity/ mA h g
-1
Cycle Number
100 mA g-1
Sn/PF-COONa
Sn/AB/CMC-Na
40 μm
(c)
(a) (b)
(d)
40 μm
(e)
20 μm
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Figure 2. Cycling performances (a) and rate performances (b) of Sn/PF-COONa, Sn/AB/CMC-
Na and Sn/AB/PVDF electrode. SEM images of Sn/PF-COONa electrode (c), Sn/AB/CMC-Na
electrode (d) and Sn/AB/PVDF electrode (e) after cycling. The loading of active material is
approximately 1 mg cm-2
.
In order to prove the advantage of using this conductive binder over traditional binder,
Sn/acetylene black (AB)/CMC-Na and Sn/AB/PVDF (8:1:1 in weight) electrodes were prepared
and compared with Sn/PF-COONa electrode. In this case, all electrodes were prepared with
higher loading of active material (approximately 1 mg cm-1
). From the long cycling
performances shown in Figure 2a, it can be seen that the first cycle coulombic efficiency of
Sn/PF-COONa is lower than that of Sn/AB/CMC-Na and Sn/AB/PVDF, which could be
attributed to several factors: 1. the n-type doping of PF-COONa; 2. the formation of more stable
SEI layer. After 50 cycles, the capacity of Sn/PF-COONa trended to become stable and a
capacity of 518 mA h g-1
was achieved after 500 cycles; in contrast, the capacity of
Sn/AB/CMC-Na and Sn/AB/PVDF showed rapid declining trends after initial cycles, especially
for PVDF, the capacity dropped to less than 100 mA h g-1
after 10 cycles, which can be
attributed to the swelling property of PVDF18,26
. This wide difference indicates that PF-COONa
has superior cycling stability over the traditional combination of AB/CMC-Na and AB/PVDF.
Figure 2b compares the rate performances of Sn/PF-COONa, Sn/AB/CMC-Na and
Sn/AB/PVDF electrodes. It can be seen that at 100 mA g-1
, 200 mA g-1
, 500 mA g-1
and 1000
mA g-1
, similar capacity variation trends were exhibited by Sn/PF-COONa (1087, 1036, 883 and
767 mA h g-1
) and Sn/AB/CMC-Na (892, 793, 679 and 592 mA h g-1
), indicating similar rate
capabilities of two electrodes. Similar to the results from Figure 2a, Sn/AB/PVDF anode resulted
in the poorest capacities at all currents. This result shows that without conductive additives, PF-
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COONa not only acts as binder, but also exhibits excellent conductivity that could accommodate
high-rate tests. Furthermore, the post-mortem SEM images (Figure 2c and Figure 2d) showed
that after cycling the Sn/PF-COONa electrode demonstrated a smooth surface while a much
more uneven surface is resulted from the Sn/AB/CMC-Na and Sn/AB/PVDF electrodes. For the
first time, the conductive polymer has been found to promote the formation of stable SEI layer.
This difference in surface topography could be attributed to that the electronic conductivity of
PF-COONa facilitated the formation of a homogeneous and stable SEI layer on the Sn electrode.
0 500 1000 1500 20000
500
1000
1500
2000
1st cycle
10th cycles
20th cycles
initial
-Z'' (ohm)
Z' (ohm)
Sn/AB/CMC-Na
0 500 1000 1500 2000 25000
500
1000
1500
2000
2500
1st cycle
10th cycle
20th cycle
initial
-Z'' (ohm)
Z' (ohm)
Sn/CNT/PF-COONa
0 500 1000 1500 2000 2500
500
1000
1500
2000
2500
1st cycle
10th cycle
20th cycle
initial
-Z'' (ohm)
Z' (ohm)
Sn/CNT/PVDF
(a)(a)(a)(a) (b)(b)(b)(b)
(c)(c)(c)(c) (d)(d)(d)(d)
0 50 100 1500
400
800
1200
Sn/CNT/PVDF
Sn/CNT/CMC-Na
Specific capacity/ mA h g
-1
Cycle Number
200 mA h g-1
Sn/CNT/PF-COONa
Figure 3. Cycling performances of cells using different binders (a). Impedance spectra of cells
using PF-COONa (b), CMC-Na (c) and PVDF (d) at different cycle numbers. The loading of
active material is approximately 2.5 mg cm-2
.
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Furthermore, the cycling performance of Sn/carbon nanotube (CNT)/PF-COONa,
Sn/CNT/CMC-Na and Sn/CNT/PVDF electrodes with higher Sn loading (2.5 mg cm-2
) was also
tested. As shown in Figure 3, at the current density of 200 mA g-1
, the capacity of Sn/CNT/PF-
COONa was retained at around 555 mA h g-1
after 150 cycles. In contrast, capacities of 254 mA
h g-1
and 87 mA h g-1
were obtained from Sn/CNT/CMC-Na and Sn/CNT/PVDF respectively.
Therefore, the electrochemical performance of Sn electrode can be further improved by adding
conductive CNT. In addition, alternative current (AC) impedance spectroscopy was used to
measure the impedance of the cells at different cycle number. As shown in Figure 3, three cells
showed similar charge transfer impedance initially. However, as cycle number increased, the cell
using PF-COONa exhibited much lower impedance compared to the cells using CMC-Na and
PVDF. This result not only proved the formation of stable SEI layer, but also indicated that PF-
COONa served as a conductive network in the electrode.
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(a) (b)
400 nm
(c)
200 nm 200 nm
0.0 0.5 1.0 1.5 2.0 2.5-30
-20
-10
0
10
20
I/ µµ µµA
E/ V vs Li/Li+
Cycle 1
Cycle 2
Cycle 3
Cycle 4
(d)
Figure 4. (a) Cyclic voltammogram of single-dispersed Sn/PF-COONa electrode at the scanning
rate of 0.1 mV s-1
; SEM images of single-dispersed Sn/PF-COONa electrode (b) before and (c)
after cycling; SEM image of highly pulverized Sn nano particles after cycling (d).
To observe the morphology, change of Sn nanoparticles after cycling, single-dispersed electrodes
were prepared where the Sn weight content is 12.5 %. Figure 4a shows the cyclic
voltammogram (CV) of the single-dispersed electrode at the scan rate of 0.1 mV s-1
. During the
first cathodic sweep, two broad peaks at 1.5 V and 0.75 V vs Li/Li+ can be attributed to the n-
type doping of the polyfluorene structure, which enhances the conductivity of PF-COONa;24
while the cathodic peak and the at 0.3 V vs Li/Li+ can be attributed to both Sn reduction and the
formation of the solid-electrolyte interface film. This result corresponds to the irreversible
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discharge capacity during the first galvanostatic cycling. From the second cycle, three
reproducible cathodic peaks can be found at 0.66 V, 0.62 V and 0.41 V vs Li/Li+, which can be
explained by the formation of LixSn alloy.8 As for the anodic sweep, four oxidation peaks can be
found at 0.45 V, 0.60 V, 0.72 V and 0.78 V vs Li/Li+, which can be assigned to the dealloying of
LixSn.8 As a result, Sn particles underwent a full lithiation/delithiation process in this system.
Owing to the low thickness of the electrode, the morphology of Sn particles can be observed
using SEM. By comparing the SEM images of Sn/PF-COONa electrodes before and after cycling
in Figure 4b and Figure 4c, it can be seen that after cycling, the surface of Sn nanoparticles has
become much rougher, suggesting the start of pulverization. It is also observed for some particles,
smaller subgrains are formed on the surface, indicating a higher degree of pulverization (see
Figure 4d). Therefore, it can be inferred that after long-term galvanostatic cycling, highly
pulverized nanoparticles will be inevitably resulted and the use of conductive binder is a
practical solution.
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Scheme 1. Schematic illustration of the Sn anodes in the processes of lithiation/delithiation using
(upper) AB and non-conductive polymer and (lower) conductive binder.
As shown in Scheme 1, when using the conventional binder/conductive material combination,
the pulverized particles may will lose electronic contact with the conducting network, leading to
capacity fading. By contrast, the conductive material will provide a conducting network, which
facilitates electron transfer pathways for pulverized Sn particles, hence greatly reducing the
capacity loss.
3. CONCLUSION
In summary, a water-soluble conductive polymer (PF-COONa) is investigated as a promising
binder for nano-sized Sn anode. The Sn/PF-COONa electrodes demonstrated excellent reversible
capacities of 762 mA g-1
and 593 mA g-1
at the current density of 100 mA h g-1
and 500 mA h g-
1after long cycling, respectively. By comparing the electrochemical performances of Sn electrode
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using PF-COONa as binder and those using conventional binder, PF-COONa delivered
significantly better cycling capacities and very similar rate capability. After examining the
cycled electrodes, it can be seen that pulverization of Sn particles generally exists during the
cycling. It is believed that the superior electrochemical performance is due to the conductive
binder provides a conductive network, which will keep the pulverized Sn particles from losing
electronic contact. Furthermore, the use of PF-COONa promotes the formation of stable SEI
layer, which can be also attributed to its excellent performance. In this case, it is reasonable to
speculate that this type of binder can generally boost the performance of anode materials with
large volume changes. Therefore, this work not only demonstrates a polymeric binder for Sn
anode with great commercialization potential, but also provide guidance on designing Sn-based
electrodes.
4. EXPERIMETAL PROCEDURES
Preparation and characterization of materials:
Preparation of 2,7-Dibromo-9,9-bis(3-tert-butyl propanoate)fluorine (M1): Firstly, 5 g
(76.1mmol) of 2,7-dibromofluorene and 300mg (0.94 mmol) of tetrabutylammonium bromide
(TBAB) were mixed in 35 mL toluene solution. Then 8 mL of 50 wt% NaOH aqueous solution
was injected dropwise into the above solution under N2 atmosphere. After half an hour, 8 g (62.5
mmol) of tert-butyl acrylate was slowly added. The solution was vigorously stirred at RT for 12h.
After the reaction completed, the products were extracted by dichloromethane and washed with
water three times. The organic solution was dried over aqueous Na2SO4 and further concentrated
under reduced pressure. The crude product was purified by column chromatography using ethyl
acetate: petroleum ether (1:50) in order to obtain a white solid (M1) with a 60% yield. 1H NMR
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(400 M Hz, CDCl3), δ(ppm): 7.68-7.50 (m, 6H); 2.30 (t, 4H); 1.46 (t, 4H); 1.33 (s, 18H). 13
C
NMR (400 MHz, CDCl3), δ(ppm): 172.2, 150.1, 139.2, 131.2, 126.2, 122.2, 121.6, 80.5, 54.2,
34.5, 29.9, 28.
Preparation of 2,7-bis(4,4,5,5-tetrameth-yl-1,3,2-dioxaborolan-2-yl)-9,9-bis (3-tert- butyl
propanoate)fluorine (M2): 6.912 g (12 mmol) of M1, 6.0 g (60 mmol) of anhydrous KOAc and
6.4 g (25.2 mmol) of bis(pinacolato)diboron were added into 80 mL anhydrous DMF,
subsequently 300 mg of Pd(dppf)2Cl2 was added quickly under a nitrogen atmosphere. The
reaction was conducted in the dark condition at 90 °C for 10 h. The completed mixture was
poured into deionized water and extracted with dichloromethane. The obtained organic solution
was washed with deionized water seven times and then dried with aqueous MgSO4. After
concentration under reduced pressure, the product was purified via column chromatography
(ethyl acetate: hexane = 1: 20) to obtain a white product (M2) with a 75% yield. 1H NMR (400
M Hz, CDCl3), δ(ppm): 7.84-7.72 (m, 6H); 2.39 (t, 4H); 1.44-1.39 (m, 28H); 1.31 (s, 18H). 13
C
NMR (400 MHz, CDCl3), δ(ppm): 172.8, 147.8, 143.8, 134.3, 129.0, 119.6, 83.8, 76.68, 53.5,
34.4, 29.92, 28.0, 25.0.
Synthesis of sodium Poly[9,9-bis(3-propanoate)fluorine)] (PF-COONa): A mixture containing
1.741 g (3mmol) of M1 and 2.023 g (3 mmol) of M2, 35 mg Pd(PPh3)4, and several drops of
Aliquat 336 was added to a flask. Then 12 mL 2 M Na2CO3 solution and 36 mL THF was added
and the flask was degassed by three freeze–pump–thaw cycles. The mixture was heated to 85 oC
for 72 h under Ar and after cooling down to RT, the crude product was precipitated from
methanol and dried under vacuum. Then the obtained material was dissolved in 200 mL
dichloromethane containing 15% trifluoroacetic acid. The mixture was stirred at room
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temperature for 24h. And after the solvent removed under reduced pressure, 200 mL 0.5 M
aqueous Na2CO3 solution was added, stirred for 6h and dialyzed against water for several times.
The product was obtained via freeze-dry with a 75% yield. 1H NMR (400 M Hz, CDCl3) δ (ppm):
7.92-7.76 (br, 6H); 2.45 (br, 4H); 1.45 (br, 4H).
Fourier transform infrared (FTIR) spectroscopy was performed on a Nicolet Avatar 360
spectrophotometer. The surface morphology of the electrodes was characterized by a scanning
electron microscope (SEM, ZEISS Supra 55). The crystal structure of Sn nanoparticles was
analyzed by XRD using a Bruker D8-Advantage powder diffractometer (Cu-Kradiation) with 2θ
from 20° to 90° at 0.2 s per step.
Electrochemical measurements:
All electrodes in this work were prepared using casting method. Firstly, Sn nanoparticles
(Aladdin, 99.99% metals basis ≤100nm) was dispersed in the 2% PF-COONa solution and
stirred vigorously for 24 hours. Then slurry is casted onto the Cu foil current collector and dried
naturally at room temperature. After cut into pieces, the electrodes are dried at 110 °C under
vacuum for removing remained H2O content. The electrodes with PVDF, CMC-Na binders and
CNT additives were prepared by using same procedures.
All of coin cells were fabricated in an Ar-filled dry-box. Coin cells (2032) were used as to
assemble half cells. Li foil (99.9%) was used as the negative electrode. 1.2 M LiPF6 in ethylene
carbonate (EC): diethylene carbonate (DEC) (1:1 w/w) with additive of 10 wt% fluoroethylene
carbonate (FEC) was used as electrolyte. Galvanostatic cycling was conducted in the voltage
range between 0.01 V and 1 V at room temperature using a battery test system (Newell, China).
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The CV measurements were performed at the scan rate of 0.1 mV s-1
from 0.01 V to 2 V on an
electrochemical workstation (CHI 604E, CH Instruments).
ASSOCIATED CONTENT
Supporting Information
Performance comparison of PF-COONa with other reported binders; XRD pattern of Sn; infrared
spectra of Sn, Sn/PF-COONa and PF-COONa; cycling capacities of Sn/SWCNT/PF-COONa
electrode.
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]
*E-mail: [email protected]
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
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The research was financially supported by National Materials Genome Project
(2016YFB0700600), Guangdong Innovation Team Project (No. 2013N080) and Shenzhen
Science and Technology Research Grant (peacock plan KYPT20141016105435850, No.
JCYJ20151015162256516, JCYJ20150729111733470).
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Table of Contents:
0 100 200 300 400 5000
300
600
900
Specific capacity/ mA h g
-1
Cycle Number
Conventional binder
AB
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