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Solid State Ionics 179 (2

Preparation and characterization of LiV3O8 cathode material for lithiumsecondary batteries through an EDTA-sol-gel method

Yuan Zhou a,⁎, Hong-Fei Yue a,b, Xin-Yue Zhang a,b, Xiao-Yu Deng a,b

a Institute of Salt Lakes, Chinese Academy of Sciences (ISLCAS), Xi'ning 810008, Qinghai, People's Republic of Chinab Graduate University of Chinese Academy of Sciences (GUCAS), Beijing 100049, People's Republic of China

Received 13 July 2007; received in revised form 6 March 2008; accepted 13 March 2008

Abstract

LiV3O8 is a very promising cathode material for lithium secondary batteries. An EDTA-sol-gel method was proposed for the first time to obtainLiV3O8 at low temperature. LiAc·2H2O, NH4VO3 and EDTA were used as raw materials to prepare the precursor, and then the precursor wassintered to obtain the layered structure of LiV3O8. The effect of sintering temperature and duration on the quality of LiV3O8 was investigated. Wealso tested the electrochemical performances of LiV3O8 obtained through the method. The sample heated at 550 °C for 15 h has a single-phaselayered structure and it has a first cycle discharge capacity of 251.7 mA h g−1 at the range of 1.8–3.6 V at 0.1 mA and an average capacity decayrate of 0.43% per cycle after 30 cycles.© 2008 Elsevier B.V. All rights reserved.

Keywords: Lithium ion battery; LiV3O8; Sol–gel; EDTA

1. Introduction

LiV3O8 with layered structure was thought to be one of themost promising cathode materials for lithium ion batteries dueto its low cost, large specific capacity, high discharge rate andlong cycle life [1]. Preparation process and post-treatment con-dition have a significant influence on the electrochemical per-formances of LiV3O8 cathode material [2]. There are mainlytwo routes for the preparation of LiV3O8: solid-state reactionsand sol–gel synthesis [3]. Sol–gel synthesis has been named forits low temperature for reaction, short reacting time, easiness toobtain uniform products etc [4]. Using ascorbic acid as chela-ting agent, Deptula et al. [5] synthesized LiV3O8 through a sol-gel process. The material obtained through this method has alarge discharging capacity and amazing cycling performance.Keli Zhang et al. [6] obtained LiV3O8 through a sol-gel methodwith citric acid as chelating agent, which has a first dischargespecific capacity of 350 mA h g−1.

Carboxylic ions can reduce V(V) to VO2+. EDTA has fourcarboxylic ions in one molecular, and has a stability constant [7]

⁎ Corresponding author.E-mail address: zhouy@isl.ac.cn (Y. Zhou).

0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.ssi.2008.03.025

of 18.8 while chelating with VO2+ and much lower solubility[8] in water compared with ascorbic acid and citric acid. Wehave expected it would have a good performance in producingLiV3O8 with layered structure by using EDTA as chelatingagent in the sol-gel process, because when it is added into thesystem it would first reduce VO3

− into VO2+ and then chelate theVO2+, which plus its lower solubility would make it equivalentto adding EDTA to the system slowly and homogeneously. Thiswould result in a uniform precursor and finally uniform product.In this study it was used as chelating agent to prepare LiV3O8.X-ray diffraction (XRD) patterns and scanning electron micro-scope (SEM) results indicate that single-phase LiV3O8 withlayered structure has been obtained. Constant current charge/discharge experiments show that when the sample heated at550 °C for 15 h it has the best electrochemical performanceamong all other materials we prepared.

2. Experimental

2.1. Synthesis of LIV3O8

With a mole ratio of Li:V:EDTA of 1:3:3, certain amount ofLiAc·2H2O, NH4VO3 and EDTA were mixed uniformly with

1764 Y. Zhou et al. / Solid State Ionics 179 (2008) 1763–1767

appropriate amount of deionized water, then the temperature ofthe mixture was controlled to be at 80 °C for 12 h. The mixturehas been stirred. The solution was then dried at 80 °C to obtainblue sol. Precursors were attained by drying the blue sol in avacuum desiccator at 120 °C, about 24 kPa of vacuum for 4 h.Precursors were then grinded in corundum crucible before theywere sintered in a tubular furnace in air for 20 h at 500 °C,550 °C and 600 °C. Another precursor was sintered at 550 °Cfor 15 h. The product obtained under 500 °C, 550 °C, 600 °C for20 h to be marked as a, b and c, and the product obtainedat 550 °C for 15 h to be marked as d. Then the obtained LiV3O8

was grinded 400 eyes to sift in corundum crucible again.

2.2. Characterization of the product

Powder X-ray diffraction (XRD, X'Pert Pro/PANalytical)(using Cu Kα radiation, 40 kV, 30 mA, at 4° min−1 with 2θ atthe range of 5°–80°) was used to identify the crystalline phase andcrystal lattice parameters of the synthesized LiV3O8 powders. Theparticle morphology, particle size and particle size distribution ofLiV3O8 powders and precursors were examined using a scanningelectron microscopy (SEM, JSM-5600L/JEOL).

2.3. Preparation of cell

Test cathode electrodes consisted of 80wt.% LiV3O8, 10 wt.%acetylene black, and 10 wt.% PVDF. Celguard 2400microporouspolypropylene membrane was used as the separator. The elec-trolyte was 1M LiPF6 EC+DEC (1:1 by volume). The cells wereassembled in a Unilab glove box (Braun Ltd. Germany) filledwith argon. Electrochemical tests were carried out on a LANDCT2001A electrochemical station (WuhanKINGNUOElectronicCo., Ltd.).

Fig. 1. XRD patterns of samples sintered at different temperature for differenttimes. Li0.3V2O5(●), LiV2.5O6.75− x(★).

3. Results and discussion

3.1. XRD analysis of synthesized LiV3O8

The XRD patterns of LiV3O8 powders sintered under differenttemperature for different durations are shown in Fig. 1. It can beobserved that the principal crystalline phases of the samplessintered at different temperature are LiV3O8, which is characteristicfor the 2θ of 14.0°, 28.4°, 30.7°, 40.9° and 42.5°. The XRD patternof the sample sintered under 500 °C shows a small amount ofLi0.3V2O5 and LiV2O5, which are impurities. It can be concludedthat the reaction for the generation of LiV3O8 has been undertakenin such a case but has not completed. The XRD pattern of thesample sintered under 550 °C shows a single-phase layeredstructure of LiV3O8. The major phase of sample sintered under600 °C is also LiV3O8, but a minute quality of LiV2.5O6.75−x couldalso be observed from XRD pattern, which may be due to thedecomposition of LiV3O8 under such a temperature but this needsfurther research. The patterns of the LiV3O8 synthesized at differenttemperatures are very similar. This may be attributed to the small,isotropic crystallites and low crystallinity of low temperatureLiV3O8 [11]. We may come to the conclusion that single-phaseLiV3O8 with layered structure can be synthesized through themethod described above. It was also found that the relative intensityof diffraction peak (100) enhanced with increasing temperatures,degree of crystallinity increased with temperature and d100decreased with temperature. Decrease of d100 narrows the diffusionpassage and lengthens the evolving path of Li+ in LiV3O8, whichmakes it difficult for Li+ to deintercalate [2].

In order to investigate the influence of sintering time on thequality of LiV3O8, we sintered samples at 550 °C for 15 h and 20 hrespectively. Their XRD patterns are also shown in Fig. 1. Thecharacteristic peaks of Li0.3V2O5 can be observed from the XRDpattern of sample sintered for 15 h, but the amount is minus. Weconcluded that the reaction for the production of LiV3O8 has notfinished yet under this condition. When we prolong the sinteringtime to 20 h, single-phase LiV3O8 with layered structure can beobtained. Fig. 1 shows that main peaks of the two samples matcheach other almost exactly, which imply that the crystal structure ofthe samples doesn't change with sintering time. But the value ofd100 changes distinctly. It is 6.405 Å when it was sintered for 15 h,the value ofd100 reduces to 6.370Åwhen it was sintered for 5morehours. Samples sintered at 550 °C for 15 h demonstrated the largestinterlayer spacing. The increase of interlayer spacing could improvethe mobility and distribution of Li+ between layers [10], andfollows the improvement of electrochemical performance oflithium ion batteries such as specific capacity, reversibility andcharge/discharge rate.

3.2. Morphology characteristic of synthesized LiV3O8

The upper part of Fig. 2 shows the SEM image of theprecursor of LiV3O8 through EDTA-sol-gel method. It alsoillustrated that the grains of the precursor are small and uniform.Their diameter varies between 1 and 5 μm and most of them arearound 3 μm. The particle surface of the precursor is clear andsmooth and all the particles are in accumulation state.

Fig. 2. SEM photographs of the precursor and samples synthesized under different conditions; a: 500 °C 20 h; b: 550 °C 20 h; c: 600 °C 20 h; d: 550 °C 15 h.

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The lower part of Fig. 2 shows the morphology of sampleLiV3O8 obtained under different synthesis condition using thesame precursor. It is important to control the morphology of thematerial because the energy density of a material will beinfluenced by its morphology and particle size. We can learnfrom the image that samples sintered under different tempera-tures have diverse grain shapes. Particles of sample sinteredunder 500 °C for 20 h are small and uniform, within a narrow

diameter from 2–5 μm. With the increasing of sintering tem-perature, degree of crystallinity is enlarged, the grain size isincreasing and particles are tend to become rod-like granular.The particle size of samples sintered at 550 °C for 20 h rangesfrom 4 to 8 μm, but the particle size of samples sintered at600 °C for 20 h are about 10 μm and are not in accumulationstate. No significant difference is seen between the samplessintered at 550 °C for 15 h and for 20 h (Fig. 2b and d).

Fig. 4. Variation of discharge capacity with cycle number of samples.

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Samples sintered for 15 h have a smaller particle size, which isbetween 3 and 6 μm. With small particle size, materials canhave higher activated specific surface area, which wouldshorten the diffusion path of lithium ion and increase the num-ber of diffusion path, and enlarge discharge specific capacityconsequently.

3.3. Test of electrochemical performance

Fig. 3 shows the first batch of discharge curve of the LiV3O8

obtained by heating at different temperatures. As shown inFig. 3, the sample heated at 500 °C has a first cycle dischargecapacity of 247.1 mA h g−1. However, the sample heated at550 °C has a first cycle discharge capacity of 236.0 mA h g−1

and the sample heated at 600 °C has only 214.7 mA h g−1. Thismight be explained by the degree of difficulty or easiness forlithium ion to diffuse. The sample sintered at 500 °C has a lowerdegree of crystallinity, larger activated specific surface area andfurther layered distance, which favors for the diffusion oflithium ion. On the contrary, samples heated at higher tem-perature have a larger particle size, smaller activated specificsurface area and what's more, part of the samples may under-take decomposition. Furthermore, we can see from Fig. 3 thatthe sample heated at 550 °C has the clearest plateau near 2.5 V,which seems to have no clear relation with sintering temperatureas reported in literature [9]. This requires further research.

Fig. 3 also shows the first discharge curve of the LiV3O8

obtained by heating at the same temperature for different hours.The sample heated at 550 °C for 15 h has a first cycle dischargecapacity of 251.7 mA h g−1, which is the highest. All the threedischarge plateaus of the sample heated for 15 h are higher thanthose heated for 20 h. This may be also on account of the degreeof difficulty or easiness for lithium ion to diffuse. We get fromthe result of XRD patterns that the d100 of sample heated at550 °C for 15 h is larger than the sample heated for 20 h. Theimage of SEM shows that the sample heated at 550 °C for 15 hhas a lower degree of crystallinity and smaller grain size when

Fig. 3. First discharge curves of samples synthesized at different temperature fordifferent time.

compared with the sample heated for 20 h. All of which arefavored for the diffusion of lithium ion.

Fig. 4 shows the cycle performance of the LiV3O8 obtainedby heating at different temperatures. The sample sintered at550 °C and 600 °C has a better cycle performance when com-pared with the sample sintered at 500 °C. The average decayrate of cycle performance of the sample sintered at 550 °C and600 °C is 0.32% and 0.38% per cycle and the total decay ofcycle performance is 9.7% and 11.3% after 30 cycles, but thesample sintered at 500 °C has a decay rate of cycle performanceof 0.52% per cycle. The reasons for the poorer cycle per-formance of the sample sintered at 500 °C may attribute to itsimpurities of Li0.3V2O5 and LiV2O5, and its lower degree ofcrystallinity.

Fig. 4 also shows the cycle performance of the LiV3O8

obtained by heating at 550 °C for different hours. The specificcapacity of the sample sintered for 15 h decayed 12.8% after30 cycles and there is no great difference when comparedwith the sample sintered for 20 h which decayed 11.3% after30 cycles. On the other hand, the sample sintered for 15 h has ahigher specific capacity than that sintered for 20 h. When takingdischarge specific capacity and cycle performance intoconsideration, the sample heated at 550 °C for 15 h is the bestamong the four samples mentioned above.

4. Conclusions

An EDTA-sol-gel method was used for the first time toobtain LiV3O8 with layered structure at low temperature. Thesample heated at 550 °C for 15 h has a first cycle dischargecapacity of 251.7 mA h g−1 and an average capacity decay rateof 0.43 per cycle after 30 cycles. We conclude that the LiV3O8

prepared through this method has a fine crystallinity, small rod-like grain size, a good distribution in size and fine electro-chemical performances. Sintering temperature and durationhave a complicated influence on the electrochemical perfor-mances of LiV3O8 prepared through this method.

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