MURDOCH RESEARCH REPOSITORY

This is the author’s final version of the work, as accepted for publication following peer review but without the publisher’s layout or pagination.

The definitive version is available at http://dx.doi.org/10.1016/j.electacta.2011.11.028

Kandhasamy, S., Pandey, A. and Minakshi, M. (2012)

Polyvinylpyrrolidone assisted sol–gel route LiCo1/3Mn1/3Ni1/3PO4 composite cathode for aqueous

rechargeable battery. Electrochimica Acta, 60 (15 Jan). pp. 170-176.

http://researchrepository.murdoch.edu.au/6542/

Copyright: © 2011 Elsevier Ltd. It is posted here for your personal use. No further distribution is permitted.

Accepted Manuscript

Title: Polyvinylpyrrolidone assisted sol-gel routeLiCo1/3Mn1/3Ni1/3PO4 composite cathode for aqueousrechargeable battery

Authors: Kandhasamy Sathiyaraj, Akanksha Pandey,Manickam Minakshi

PII: S0013-4686(11)01699-9DOI: doi:10.1016/j.electacta.2011.11.028Reference: EA 17875

To appear in: Electrochimica Acta

Received date: 12-8-2011Revised date: 5-11-2011Accepted date: 7-11-2011

Please cite this article as: K. Sathiyaraj, A. Pandey, M. Minakshi, Polyvinylpyrrolidoneassisted sol-gel route LiCo1/3Mn1/3Ni1/3PO4 composite cathode for aqueousrechargeable battery, Electrochimica Acta (2010), doi:10.1016/j.electacta.2011.11.028

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Polyvinylpyrrolidone assisted sol-gel route LiCo1/3Mn1/3Ni1/3PO4 composite

cathode for aqueous rechargeable battery

Kandhasamy Sathiyaraj1, Akanksha Pandey

2 and Manickam Minakshi

1*

1School of Chemical and Mathematical Sciences, Murdoch University, Murdoch, WA 6150, Australia.

2Department of Chemistry, Indian Institute of Technology, Powai, Mumbai 400076, India

Abstract

Olivine-phosphate LiCo1/3Mn1/3Ni1/3PO4, synthesised via conventional solid state (SS) and

polymer assisted sol-gel (SG) methods, is reported as a cathode material for aqueous

rechargeable batteries. X-ray diffraction (XRD) analysis confirms phase pure compounds in an

orthorhombic structure for both these methods. However, microstructural images show

polyvinylpyrrolidone (PVP) assisted SG method aids colloidal growth giving evenly distributed

microparticles while SS method tend to an agglomeration during high-temperature processing.

The electrochemical properties for SG are seen to be dramatically superior to those of SS. The

galvanostatic analysis of the SG shows improved specific capacity over the initial cycles (45

mAh/g and 60 mAh/g for the 1st and 20

th cycles) and stabilized upon cycling. This unique

capability is due to the trapped organic products from the thermal decomposition of PVP that

facilitates Li+ transfer. The trapped organic coating on the cathode surface without affecting the

bulk olivine is evidenced by XRD, FTIR and XPS spectroscopic techniques.

Keywords: Olivine; PVP; sol-gel; electrolyte; aqueous battery.

* Corresponding author

Fax: +61-8-9360 6452. E-mail: minakshi@murdoch.edu.au

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1. Introduction

Energy source for a variety of portable and modern electronic devices is provided through a

battery. The ever growing demand for batteries with improved specific capacity and high

energy density has promoted strong interest on developing other potential electrode materials

for battery applications. In recent years, ordered olivine structure LiMPO4 (M = Fe, Mn, Co

or Ni) are considered as next generation cathodes due to its high specific capacity and

improved coulombic efficiency. The olivine intercalants perform superior due to their strong

covalent bonding between phosphorous and oxygen atoms in phosphate polyanions [1, 2].

However, the available organic solvents (restricted the charge regime to ~4.8 V) limit the

utilization of olivines [3].

An alternative approach is aqueous solutions with LiOH electrolyte [3]. A pioneering

work on intercalation materials such as LiCoO2 and LiMn2O4 in aqueous solutions is also

well reported [4-7]. The ionic conductivity of aqueous electrolytes is two orders of

magnitude greater than that of some organic electrolytes, allowing higher discharge rates and

lower voltage drops due to electrolyte impedance. The main disadvantage of using an

aqueous electrolyte is that the battery voltage is limited to ~2 V. To offset the effects of this

voltage limit, the storage capacity and the charge-discharge characteristics of the aqueous

electrolyte system must be significantly superior to those of the organic (non-aqueous)

system. The suitability of olivines as cathodes for aqueous rechargeable batteries has been

well studied and reported by us [3, 8-11]. These were focused on the electrochemical

performance of lithium insertion/extraction into an olivine cathode in aqueous LiOH

electrolyte. One of the main drawbacks with using these olivines is their poor electronic

conductivity, [2, 12] and this limitation had to be overcome through materials processing

[12]. These include i.e. reducing particle size, laying conductive coating with carbon, and

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mixed metal atom dopants in the olivine structure [12]. The olivines have been generally

synthesized by conventional solid state (SS) reaction involving prolonged heat treatment at

high temperature to attain phase pure compound. This solid state processes lead to

agglomeration of particles, unevenly distributed particle size [13] and final products

containing contaminations from the crucibles like silica or alumina particles.

Compared to the solid-state method, sol-gel (SG) (wet chemistry) is a promising

technique to synthesize materials at lower temperature to obtain controlled particles size, that

is evenly distributed and in-situ conductive coatings by addition of suitable precursors [13].

Owing to the mechanism of molecular level tuning of dispersed metal ions in network

formation (gelation), even small fractional amount of metal atom substitution at preferred

positions is obtained. Most commonly, organic solvents (gels) like citric acid, ascorbic acid

and ethylene glycol are used as chelating agents [14] in the sol-gel process for synthesizing

in-situ carbon coated cathode. Degradation of these gels leaves uneven carbon coating on the

active material that does not show substantial improvement in electrochemical performance

while compared to uncoated cathode [15]. This drawback is due to a lack of network

between precursor and the cathode particles [15].

Recently, sol-gel derived organic – inorganic hybrid materials using polymer assisted

transition metal composites have attracted great interest as a potential electrode material for

battery systems [16, 17]. Here, we propose to overcome the drawbacks seen in the above

said chelating agents by choosing a novel homopolymer polyvinylpyrrolidone (PVP)

chelating agent. When PVP is added to the metal colloidal dispersion, its non-ionic

surfactant property makes PVP to readily dissolve in water and the imide group (N and O

atoms) establishes strong affinity to a single unit of metal colloid. This imide and metal

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bonding stabilize the particle growth and increase the probability of nucleus formation [18-

19]. Further, sintering of this complex melts PVP and decomposes. The decomposed product

is expected to be a new organic molecule instead of simple carbon residue that is trapped on

the cathode surface. This trapped organic interface between the solid electrode particles and

electrolyte increases the electrolyte penetration (lithium ion mobility) and gives improved

conductivity [15, 20].

In this paper, we report PVP assisted sol-gel method to synthesise

LiCo1/3Mn1/3Ni1/3PO4 cathode that will be an organic – inorganic hybrid composite and for

comparison the identical olivine compound was also synthesized by conventional solid-state

(SS) method. Preferred olivine phase LiCo1/3Mn1/3Ni1/3PO4 was obtained for both SS and SG

routes, but the SG derivation showed small primary particle size with a well distribution of

carbon coating in the composites. Improvement in physical and electrochemical properties of

the synthesized hybrid composite cathode is demonstrated with Zn anode in LiOH

electrolyte in an aqueous rechargeable battery.

2. Experimental

2.1. Materials

Solid state (SS) synthesis of LiCo1/3Mn1/3Ni1/3PO4 was performed while mixing a

stoichiometric amount of precursors containing lithium carbonate, cobalt oxalate, manganese

oxide, nickel oxide and ammonium dihydrogen phosphate. The stoichiometric reactants were

mixed with ethanol in a mortar and pestle. After mixing the ingredients for 30 min, the

mixture was dried at 80°C for 1 h in a hot air oven. The dried powder was collected in a

crucible and given a two-step heat treatment in a muffle furnace in air atmosphere. In first

step the powder was heated to 375°C for 12 h, and then it was crushed and ground. Then the

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calcined powder was sintered at 800°C for 24 h in air and cooled to room temperature. The

olivine phase LiCo1/3Mn1/3Ni1/3PO4 powder thus obtained was ground.

2.2. Synthesis

Sol-gel (SG) synthesis of LiCo1/3Mn1/3Ni1/3PO4 was performed while mixing a

stoichiometric amount of precursors containing lithium acetate, cobalt acetate, manganese

acetate, nickel acetate, ammonium dihydrogen phosphate and polyvinylpyrrolidone (PVP) as

chelating agent. Primarily, the metal and phosphate reactants were dissolved in water at 80°C

with an effective stirring to obtain a homogeneous solution. Then the PVP was added in 1:1

weight ratio to the metal ions, pH of the solution was adjusted to 3.5 by adding nitric acid.

The process of stirring and heating was continued till getting a thick transparent gel and the

gel was dried at 110°C in hot air oven for 12 h. Furnace heating was carried out at 300oC for

8 h and at 600oC for 6 h in air with intermittent grinding. The olivine phase

LiCo1/3Mn1/3Ni1/3PO4 powder obtained was ground for further analysis.

LiCo1/3Mn1/3Ni1/3PO4 obtained by both the methods was subjected to systematic physical and

electrochemical studies.

2.3. Instrumental Methods

Thermo gravimetric analysis (TGA) for the precursor phase of SS and the transparent

gel phase in SG was conducted using a TA instruments (SDT 2960). The structural

determination of the synthesized LiCo1/3Mn1/3Ni1/3PO4 was done by a Siemens D500 X-ray

diffractometer 5635 using Cu-Kα radiation. The voltage and current were 30 kV and 28 mA

respectively with scan rate of 1 degree per minute. The transmittance FTIR spectrum was

recorded by using a Nicolet Magna-IR 850 spectrometer. LiCo1/3Mn1/3Ni1/3PO4 (5 mg)

powder was mixed with KBr (95 mg) powder for this FTIR transmission studies. The surface

analysis of the materials was conducted using a scanning electron microscope (Philips

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Analytical XL series 20). X-ray photoelectron spectroscope (Kratos Ultra Axis

Spectrometer) was used to analyze the chemical binding energy of the samples using

monochromatic AlKα (1486.6 eV) radiation. The X-ray photoelectron spectroscopy (XPS)

analysis was started when the pressure in the analysis chamber fell below 1 x 10-9

hPa.

2.4. Electrochemical measurements

An EG&G Princeton Applied Research Versa Stat III model was used to scan the potential

at 25 µV/s in all cyclic voltammetric (CV) experiments. Hg/HgO purchased from Koslow

Scientific served as the standard reference electrode. The cell design and its experimental

procedures for the galvanostatic and slow scan cyclic voltammetric studies were made in a

similar way to that described in our earlier report [21]. An electrochemical cell was

constructed with a disk-like pellet of cathode (pellet was prepared by mixing 75 wt.%

synthesized material, 15 wt.% acetylene black (A-99, Asbury, USA) and 10 wt.% poly

(vinylidene difluoride) (Sigma Aldrich) binder), metallic zinc foil as anode and Whatman

filter paper as separator. The electrolyte was a saturated solution of lithium hydroxide

(LiOH) containing 1 mol. L-1

zinc sulphate (ZnSO4) with a pH equal to 10.5. For

galvanostatic experiments, the cell was discharged/ charged galvanostatically using an 8

channel battery analyzer from MTI Corp., USA, operated by a battery testing system (BTS).

3. Results and Discussion

Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC) curves of

the precursor phase and transparent gel phase to olivine LiCo1/3Mn1/3Ni1/3PO4 for SS and SG

routes are shown in Figs. 1a and b respectively. The TG curves for solid state (SS) in Fig. 1a

show two main steps of mass loss, whereas for sol-gel (SG) in Fig. 1b, four short steps of

mass loss are observed. The mass loss in between 100°C and 220°C in both SS and SG are

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attributed to the release of ammonia and physically adsorbed water from the precursor mix.

The DSC curves indicate that these processes are endothermic and the corresponding peaks

are found at 218°C and 299°C for SS, 130°C and 225°C for SG [22]. Finally, TG curve

shows steep weight loss in between 310°C and 650°C for SS, 320°C and 450°C for SG. The

corresponding DSC curve indicates that these processes are highly exothermic around 350°C

in both the samples. This is due to the thermal decomposition of the ingredients to form

LiCo1/3Mn1/3Ni1/3PO4 olivine phase. In the case of SG, weight loss in this region is high

(22%), due to combustion of nitrate ions present in PVP [23]. The second DSC curve

indicating the exothermic reaction in SG at 439°C is due to degradation of pyrrolidone ring

in PVP [20, 24]. The TG curves clearly show that in SG only a trace amount of weight loss

(due to slow decomposition of carbon and nitrogen) was found beyond 450°C, however, in

SS weight loss was extended till 650°C. In SS, the broad exothermic peak found at 801°C

has no significant weight loss evidenced in the curve TG, which is just ascribed to

reorganization of the crystal lattice [12]. Although TG-DSC analysis showed 650°C (SS) and

450°C (SG) are suitable enough for the formation of the solid olivine phase

LiCo1/3Mn1/3Ni1/3PO4, we have extended until 800°C (SS) and 600°C (SG) in order to

achieve better crystallization. The SG sample synthesised at 600°C, contains a trace amount

of carbon and nitrogen.

The X-ray diffraction (XRD) patterns of the synthesised olivine

LiCo1/3Mn1/3Ni1/3PO4 compounds by both SS and SG methods are shown in Fig. 2. All of the

observed diffraction peaks are assigned to the olivine structure indexed by orthorhombic

Pmna (JCPDS: 01-085-0002). As each of our mixed dopants has a comparable ionic radius it

is preferred to crystallize in an olivine phase. It could be also confirmed that although the

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thermal decomposition was observed so high with a corresponding weight loss (22%) for SG

sample (in Fig 1b) but the XRD patterns (in Fig. 2b) are well crystallized and phase pure

without any detectable impurities. However, a small peak split was observed (Fig. 2b) in all

the peaks of LiCo1/3Mn1/3Ni1/3PO4 synthesized by SG. This can be explained by formation of

LiCoPO4, LiMnPO4, and LiNiPO4 multiple olivine phases. The polymer metal ion formation

depends on the oxidation state and the ionic radii of the metal ions. In our synthesized

LiCo1/3Mn1/3Ni1/3PO4 compound, the metal cations (Co, Mn and Ni) are all in the 2+

oxidation state, therefore, the complex formation depends on the ionic radii. The smaller

ionic radius interacts faster with PVP, Ni the earliest and Mn the last. This mechanism

resulted in the formation of discrete olivine phases. The existence of multiple olivine phase

compounds with different lattice parameter leads to the peak splitting [25-26]. It is also

observed that XRD pattern of SG (Fig. 2b) has broader and higher intensity peaks as

compared to those of SS (Fig. 2a) showing the ability of PVP assisted sol-gel resulted in high

crystalline finer particles even at the low temperature synthesis route. The FT-IR spectra

(Fig. 3) recorded in between 400 and 1400 cm-1

for the SS and SG synthesized samples show

all the four PO43-

fundamental modes of vibration. The 1200-475 cm-1

region of the IR

spectrum is essentially related to internal (stretching and bending for both symmetric and

antisymmetric) modes of the PO4 tetrahedron whereas most of the lower frequency bands are

assigned to external modes. The peak values observed for υ1, υ2, υ3 and υ4 are in good

agreement with the PO43-

vibrations modes for LiMPO4 (M = Mn, Co, Ni) [27-28]. The

absence of bands at 1664 cm-1

[20, 24] (in Fig. 3b) related to C=O group (due to PVP)

confirmed that the tailoring of mixed dopants and organic PVP assisted SG synthesised

composite cathode does not affect the olivine structure [11, 29].

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Microstructural images of synthesized olivine LiCo1/3Mn1/3Ni1/3PO4 cathode

observed by SEM analysis are shown in Figure 3. It shows typical powder morphologies for

SS sample (Fig. 4a), showing irregular particle size distribution with agglomerations

compared to SG samples (Fig. 4b). The SG composites have much smaller primary

microparticles size that is evenly distributed. The smaller particle size could reduce the

diffusion length within the composite cathode, in turn improving the ionic diffusivity. The

difference in particle size and its distribution observed within SS and SG were mainly due to

the reaction temperature and the nature of precursors [13] that is shown schematically in Fig.

5 [15, 30]. In SG, the precursors are homogeneously dispersed by vigorous stirring. The

addition of PVP chelating agent bonds the discrete active material dispersion onto the PVP

backbone and stabilizes particle growth by surrounding the particles with its long polymeric

chain. Moreover, the strong hydrogen bond between PVP amide group and metal ion

hydroxyl group hinders the condensation reaction and promotes the stress relaxation in the

heating-up processes. Hence, during sintering process, PVP is trapped as an organic layer

over the active material that extends the homogeneity between the particles [31].

We also examined the presence of elements and the type of carbon present in the

composite cathode. X-ray photoelectron spectrometry (XPS), a powerful tool for surface

studies is employed for this study. A wide scan XPS spectrum of the surface (0-1000 eV) of

the LiCo1/3Mn1/3Ni1/3PO4 cathode synthesised by SS and SG are shown in Fig. 6. The

important feature to notice in Fig. 6 is dominant O (1s) peak, while the signals of other

elements corresponding to mixed dopants [32-34] i.e. Co, Mn and Ni and P, N are also

present. To confirm the presence of carbon trapped by PVP, narrow scan for the region

corresponding to the element C was recorded and curve fitted by applying multiple

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Gaussians. Fig. 7a-b show the XPS spectra of C (1s) for the LiCo1/3Mn1/3Ni1/3PO4 cathode

synthesised by SS and SG methods. Two types of carbon peaks were observed for SG (Fig.

7b), one at 282 eV could be assigned to hydrocarbons from XPS instrument, and the other at

286 eV could be associated to carbon from PVP. Whereas for SS sample (Fig. 7a), only one

carbon peak at 282 eV peak due to hydrocarbon is observed. The presence of N(1s) peak at

409.62 eV in the wide scan spectrum (Fig. 6b) is quite well observed for SG and no

significant peak was observed (Fig. 6a) for SS. The peak N(1s) is due to nitrogen atoms from

PVP, this is consistent with the proposed synthesis mechanism [35-36]. The presence of PVP

component in XPS spectra clearly shows that the role of PVP is unique and different from

the other chelating agents like citric acid and ascorbic acid etc. For an example, in citric acid

assisted sol-gel, thermal decomposition of citric acid leads to residual carbon as a coating

over the cathode material. In the case of PVP, the polymer gradually melts and then

decomposed into a new organic molecule (combination of nitrogen and carbon) which gets

coated uniformly over active material thereby having a colloidal formation (as seen in

schematic diagram Fig. 5) [15, 19]. This affinity between the particles enhances the

intercalation rate in the cathode.

To examine the electrochemical activity of these synthesized cathodes, cyclic

voltammetry (Fig. 8) was performed in the voltage range from 0.2 to -0.3 V vs Hg/HgO (as a

standard electrode) with a slow scan rate of 25 µV/s. The scan was performed initially in the

anodic direction and then reversed it back to the cathodic direction. For SS, an anodic peak (-

100 mV) corresponding to extraction of lithium from LiCo1/3Mn1/3Ni1/3PO4 working

electrode was observed. However, during the cathodic sweep, the absence of any reduction

peak indicates that the process is not reversible. Whereas in the case of SG, well-defined

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oxidation/reduction peaks were observed. The redox mechanism was identified as lithium

intercalation/de-intercalation from/into the olivine cathode. Two anodic peaks (A1 = -95mV

and A2 = -22mV) and two cathodic peaks (C1 = -282mV and C2 = -217mV) were observed.

The redox potentials observed for this SG composite is consistent with the values reported

earlier [11, 37]. Controlled particle growth and particle to particle utilization uniformity

arising from the presence of PVP as chelating agent resulted in the improved electrochemical

performance.

The charge-discharge capacities for olivine LiCo1/3Mn1/3Ni1/3PO4 synthesised by

SS and SG processes and its efficiency observed in continuous cycling are shown in Figs. 9-

10. A charge reaction is conducted before the first discharge reaction. The unique behavior

for SG (Fig. 9b & 10b) shown here is the specific capacity of the composite cathode started

with an initial discharge capacity of 45 mAh.g-1

then increases to 60 mAh.g-1

over the initial

25 cycles and gets stabilized there on. The observed capacity for the aqueous electrolyte

within the 1.0 V cut-off region is 60 mAh/g which is 40 % lower than the theoretical

capacity. This enhanced behavior is not widely reported, however, similar trend was

observed with PPy/LiFePO4 composite [38] and the authors suggested it could be due to the

change in volume of coated polymer layer for an initial oxidation and then the structure is

stabilized [38]. This organic interface between electrode and electrolyte, leads to the high

rate of lithium ion diffusivity with better capacity retention. In contrast to the reversible

capacity of the SG sample, the SS samples (Fig. 9a & 10a) yields declining capacities of 30

and 20 mAh/g for the 10th

and 20th

cycles. The improved electrochemical performance for

SG sample proves that the affinity between the particles with the aid of PVP enhances the

intercalation rate in the cathode [39].

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Conventional SS derived olivine sample causes particle agglomeration (Fig. 4a)

which slows Li+ diffusion into the cathode. Furthermore, a poor surface conductivity

hampers the reversibility upon multiple cycling that resulted in quick capacity fading (in Fig.

10a). While SG derived olivine sample is homogenous (Fig. 4b) and ionically conductive

that facilitates Li+ transfer from the LiOH electrolyte to the olivine cathode particles leading

to an improved capacity as seen in Fig. 10b.

4. Conclusions

Olivine-type LiCo1/3Mn1/3Ni1/3PO4 was synthesised through polyvinylpyrrolidone (PVP)

assisted sol-gel (SG) and compared their performance with conventional solid state (SS)

method. The electrochemical behaviour of synthesized cathodes was studied with Zn anode

in LiOH electrolyte. The homogeneous metal atom mixing in SG showed well defined redox

peaks that correspond to substituted transition metal atoms while SS showed poorly defined

electrochemical performance due to uneven distribution and particle agglomeration. The

decomposed PVP coated as a fragment/layer interface for lithium ion diffusion leading to an

increase in specific capacity over initial cycles (from 45 to 60 mAh.g-1

) and then stabilizes at

60 mAhg-1

for multiple cycles. This unique behaviour is due to the coating of the trapped

polymer layer allowing higher Li+ transfer into the cathode. Thus, LiCo1/3Mn1/3Ni1/3PO4

composite hybrid material was successfully synthesized and its improved electrochemical

performance was demonstrated by fabricating an aqueous battery.

Acknowledgements

The author (M. M) wishes to acknowledge the Australian Research Council (ARC). This

research was supported under Australian Research Council (ARC) Discovery Project

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funding scheme (DP1092543) and Centre for Research into Energy for Sustainable Transport

(CREST) (Center of Excellence, Project 1.1.5). The views expressed herein are those of the

author (M. M) and are not necessarily those of the ARC and CREST.

References

[1] A. Yamada, M. Hosoya, S.C. Chung, Y. Kudo, K. Hinokuma, K.Y. Liu and Y. Nishi, J.

Power Sources 119-121 (2003) 232.

[2] A. K. Padhi, K. S. Nanjundaswamy and J. B. Goodenough, J. Electrochem. Soc. 144 (1997)

1188.

[3] M. Minakshi, P. Singh, N. Sharma, M. Blackford and M. Ionescu, Ind. Eng. Chem. Res. 50

(2011) 1899.

[4] W. Tang, S. Tian, L.L. Liu, L. Li, H.P. Zhang, Y.B. Yue, Y. Bai, Y.P. Wu and K. Zhu,

Electrochem. Commun. 13 (2011) 205.

[5] G.-J. Wang, L. Fu, N. Zhao, L. Yang, Y. Wu and H. Wu, Angew. Chem. Int. Ed. 46 (2007)

295.

[6] G.-J. Wang, N. H. Zhao, L.C. Yang, Y.P. Wu, H.Q. Wu and R. Holze, Electrochim. Acta 52

(2007) 4911.

[7] Q. Qu, L. Fu, X. Zhan, D. Samuelis, J. Maier, L. Li, S. Tian, Z. Li and Y. Wu, Energy

Environ. Sci. 4 (2011) 3985.

[8] M. Minakshi, P. Singh, D. Appadoo and D. E. Martin, Electrochim. Acta 56 (2011) 4356.

[9] M. Minakshi, P. Singh, S. Thurgate and K. Prince, Electrochem. Solid State Lett. 9 (2006)

A471.

[10] M. Minakshi, P. Singh, S. Thurgate and K. Prince, J. Power Sources 158 (2006) 646.

Page 14 of 25

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ted

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uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

14

[11] M. Minakshi, N. Sharma, D. Ralph, D. Appadoo and K. Nallathamby, Electrochem. Solid

State Lett. 14 (2011) A86.

[12] A.F. Liu, Z.H. Hu, Z.B. Wen, L. Lei and J. An, Ionics 16 (2010) 311.

[13] X.J. Zhu, H.H. Chen, and H. Zhan, H.X. Liu, D.L. Yang and Y.H. Zhou, Chinese J. Chem.

23 (2005) 491.

[14] H. Liu, Y. P. Wu, E. Rahm, R. Holze and H. Q. Wu, J. Solid State Electrochem. 8 (2004)

450.

[15] S.H. Lim and J. Cho, Electrochem.Commun. 10 (2008) 1478.

[16] S. Tsukuda, S. Seki, M. Sugimoto, A. Idesaki and S.I. Tanaka, Surf. Coat. Technol. 206

(2011) 825.

[17] Pierre Audebert, Alain Walcarius, in Functional Hybrid Materials, ed. P. Gómez-Romero

and C. Sanchez, wiley – VCH, 6 (2003) 172.

[18] J.M. Lee, Y.D. Jun, D.W. Kim, Y.H. Lee and S.G. Oh, Mat. Chem. Phy. 114 (2009) 549.

[19] F. Bonet, V. Delmas, S. Grugeon, R.H. Urbina, P.Y. Silvert and K. T. Elhsissen,

Nanostruct. Mater. 11 (1999) 1277.

[20] Y. K. Du, P. Yang, Z. G. Mou, N. P. Hua and L. Jiang, J. App. Polym. Sci. 99 (2006) 23.

[21] M. Minakshi, Electrochim. Acta, 55 (2010) 9174.

[22] V. Koleva, E. Zhecheva and R. Stoyanova, Eur. J. Inorg. Chem. (2010) 4091.

[23] S. Soltanmohammad and S. Asgari, J. Nanomater. 104012 (2010) 4012.

[24] V. M. Bogatyrev, N. V. Borisenko and V. A. Pokrovskii, Russ. J. App. Chem.74 (2001)

839.

[25] R.S. Okojie, T. Holzheu, X.R. Huang and M. Dudley, Appl. Phys. Lett. 83 (2003) 1971.

Page 15 of 25

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

15

[26] T. Nagasaki, K. Kok, A.H. Yahaya, N. Igawa, K. Noda and H. Ohno, Solid State Ionics 96

(1997) 61.

[27] K.P. Korona, J. Papierska, M. Kamińska, A. Witowski, M. Michalska and L. Lipińska,

Mat. Chem. Phy. 127 (2011) 391.

[28] D. Shanmukaraj and R. Murugan, Ionics 10 (2004) 88.

[29] N. Jayaprakash, N. Kalaiselvi and P. Periasamy, Int. J. Electrochem. Sci. 3 (2008) 476.

[30] J.J. Zhu, C.X. Kan, J.G. Wan, M. Han and G.-H. Wang, J. Nanomater.

doi:10.1155/2011/982547.

[31] M.P. Zheng, M.Y. Gu, Y.P. Jin, H.H. Wang, P.F. Zu, P.Tao and J.B. He, Mat. Sci.Eng.B

87 (2001) 197.

[32] J.W. Lee, M.S. Park, B. Anass, J.H. Park, M.S. Paika and S.G. Doo, Electrochimica. Acta

55 (2010) 4162.

[33] K. Sathiyaraj, Gangulibabu and D. Bhuvaneswari, N. Kalaiselvi, Ionics 17 (2011) 49.

[34] Perkin Elmer Corporation, Handbook of X-ray Photoelectron Spectrscopy, Physical

Electronics Division, Perkin Elmer Corporation, USA, October 1992.

[35] P. Jiang, S.Y. Li, S.S. Xie, Y. Gao and L. Song, Chem. Eur. J. 10 (2004) 4817.

[36] T. Du, H. Song and O.J. Ilegbusi, Mat. Sci. Eng. C 27 (2007) 414.

[37] M. Minakshi, A. Pandey, M. Blackford and M. Ionescu, Energy Fuels, 24 (2010) 6193.

[38] K.S. Park, S. B. Schougaard and J.B. Goodenough, Adv. Mater. 19 (2007) 848.

[39] H. Sasaki, K. Takada, T. Inada, A. Kajiyama, S. Kondo and M. Watanabe, J. Power

Sources 119-121 (2003) 774.

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16

Figure Captions

Fig. 1 TG-DSC curves of (a) oxide precursor phase and (b) transparent gel phase to olivine

LiCo1/3Mn1/3Ni1/3PO4.

Fig. 2 X-ray diffraction patterns of olivine LiCo1/3Mn1/3Ni1/3PO4 synthesised by (a) solid

state and (b) sol-gel methods.

Fig. 3 FT IR spectra of LiCo1/3Mn1/3Ni1/3PO4 synthesized by (a) solid state and (b) sol-gel

method.

Fig. 4 SEM images of LiCo1/3Mn1/3Ni1/3PO4 synthesized by (a) solid state and (b) sol-gel

method.

Fig. 5 Schematic representation of the synthesis mechanism of LiCo1/3Mn1/3Ni1/3PO4 by

solid state and sol-gel method.

Fig. 6 XPS spectrum (wide scan) of the olivine LiCo1/3Mn1/3Ni1/3PO4 synthesized by (a)

solid state and (b) sol-gel method.

Fig. 7 XPS spectra of C (1s) of the olivine LiCo1/3Mn1/3Ni1/3PO4 synthesized by (a) solid

state and (b) sol-gel method.

Fig. 8 Cyclic voltammogram of LiCo0.33Mn0.33Ni0.33PO4 synthesized by (a) solid state and

(b) sol-gel method in aqueous LiOH electrolyte.

Fig. 9 Charge-discharge behavior of LiCo0.33Mn0.33Ni0.33PO4 synthesized by (a) solid state and

(b) sol-gel method in aqueous LiOH electrolyte at a constant current of 0.2 mA.

Fig. 10 Cycling behavior of LiCo0.33Mn0.33Ni0.33PO4 synthesized by (a) solid state and (b) sol-

gel method in aqueous LiOH electrolyte at a constant current of 0.2 mA.

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17

50 250 450 650 85050

60

70

80

90

100

Temperature / οοοο

C

Weig

ht

(%)

(a)

75

50

25

0

-25

Hea

t F

low

(m

W)

Endo

100 200 300 400 500 600 7000

20

40

60

80

100

25

0

-25

-50

-75

Temperature / οοοο

C

Wei

gh

t (%

)

Heat

Flo

w (m

W)

(b)

Endo

Fig. 1 TG-DSC curves of (a) oxide precursor phase and (b) transparent gel phase to olivine

LiCo1/3Mn1/3Ni1/3PO4.

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18

15 20 25 30 35 40 45 50 55 60

0

300

600

900

10000

1000

2000

3000

4000

6013

31

610

412

402

222

13

12

30

420212

32

1112

401

22

11

02002121

311

301

02

0

11

101

121

0

101

200

Inte

nsi

ty /

cp

s

2 theta / degree

(a)

(b)

Fig. 2 X-ray diffraction patterns of olivine LiCo1/3Mn1/3Ni1/3PO4 synthesised by (a) solid

state and (b) sol-gel methods.

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19

1800 1600 1400 1200 1000 800 600 400

474

.5

Wavenumber / cm-1

551.9

581

.6

64

7.0

993.4

10

70.0

1103

.4

1146

.9

(a)

(b)

νννν2222

νννν4444

νννν1111

476.6

Tra

nsm

itta

nce

/ a

.u.

554.3

57

8.9

645.79

98.4

10

58.5

1101.4

1145

.6

νννν3333

Fig. 3 FT IR spectra of LiCo1/3Mn1/3Ni1/3PO4 synthesized by (a) solid state and (b) sol-gel

method.

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20

Fig. 4 SEM images of LiCo1/3Mn1/3Ni1/3PO4 synthesized by (a) solid state and (b) sol-gel

method.

Fig. 5 schematic representation of the synthesis mechanism of LiCo1/3Mn1/3Ni1/3PO4 by

solid state and sol-gel method.

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21

1000 800 600 400 200 00

10000

20000

30000

40000

50000

60000

10000

20000

30000

40000

50000

60000

Zn

(3

p1/2)

P (

2p

3/2)

(b)

P (

2s)

C1s

O1s

Binding Energy / eV

Mn

2p

3/2

Mn

2p

1/2

Mn

(3p

)

Li

(1s)

Ni2

p3

/2

N1s

Ni2

p1

/2

C1s

Inte

nsit

y / a

.u.

Co2p

3/2

Co

2p

1/2

(a)

Fig. 6 XPS spectrum (wide scan) of the olivine LiCo1/3Mn1/3Ni1/3PO4 synthesized by (a)

solid state and (b) sol-gel method.

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22

290 288 286 284 282 280 278

Binding Energy / eV

284.65eV

C1s

282.61eV(a)

290 288 286 284 282 280 278

282.82eV

286.23eV

C1s

(b)

Binding Energy / eV

Fig. 7 XPS spectra of C (1s) of the olivine LiCo1/3Mn1/3Ni1/3PO4 synthesized by (a) solid

state and (b) sol-gel method.

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23

-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2

-1.0m

-500.0µ

0.0

500.0µ

1.0m

1.5m

(a)

(- 217 mV)

(- 22 mV)

(- 95 mV)

C1

C2

A2

Cu

rren

t /

A

Potential vs. Hg/HgO / V

A1

(- 282 mV)

(b)

Fig. 8 Cyclic voltammogram of LiCo0.33Mn0.33Ni0.33PO4 synthesized by (a) solid state and

(b) sol-gel method in aqueous LiOH electrolyte.

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24

0 10 20 30 40 50 60 700.0

0.5

1.0

1.5

2.0

2.5

Discharge

201

20 10 1

Ce

ll P

ote

nti

al / V

Cell Capacity / mAh.g-1

(a)

Charge

0 15 30 45 60 750.0

0.5

1.0

1.5

2.0

7

Charge

4020105

Ce

ll P

ote

nti

al / V

Cell Capacity / mAh.g-1

1

Cycle numbers

Discharge

(b)

Fig. 9 Charge-discharge behavior of LiCo0.33Mn0.33Ni0.33PO4 synthesized by (a) solid state and (b)

sol-gel method in aqueous LiOH electrolyte at a constant current of 0.2 mA.

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0 10 20 30 40

20

30

40

50

60

Declined

Dis

ch

arg

e C

ap

ac

ity

/ m

Ah

.g-1

Cycle number

(a)

(b)

Improved

Fig. 10 Cycling behavior of LiCo0.33Mn0.33Ni0.33PO4 synthesized by (a) solid state and (b) sol-gel

method in aqueous LiOH electrolyte at a constant current of 0.2 mA.