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Synthesis and Characterizations of Carbon Coated Li3V2（PO4）3Cathode Materials for Lithium Ion Batteries

Author: ChenYongSheng
Tutor: WangChunZhong
School: Jilin University
Course: Materials Physics and Chemistry
Keywords: Li-ion batteries Li3V2（PO4）3 carbon coating SEI film electrolyte
CLC: TM912.9
Type: Master's thesis
Year: 2012
Downloads: 65
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Abstract


Great concerns have been aroused on lithium ion batteries since their firstintroduction into the market in1991. Especially in recent years the lithium iontechnology is experienced increased demands for a large market share in high powertools, electric vehicles and renewable energy storage. In this context, cost and safetycome as two major parameters apart from satisfactory electrochemical performance.The early time of lithium ion technology was solely devoted to oxide cathodematerials, such as LiCoO2, LiMn2O4and LiNiO2. Since the original work ofGoodenough et al. the olivine type phosphate materials LiMPO4（M=Fe,Co,Ni） haveattracted much intention as low cost and high safety cathode materials. Besides theLiMPO4olivines, monoclinic Li3V2（PO4）3is also a promising phosphate cathodematerial. Except the numerous attempts in preparing high performance Li3V2（PO4）3,there are limited attention focusing on the electrode/electrolyte interfacial propertiesof the material. Not only the performance of the cathode material itself，but also thematching between electrode and electrolyte have important impact on theperformance of the lithium ion battery. Therefore, the main work of this thesis is tostudy the electrode/electrolyte interface of carbon coated Li3V2（PO4）3cathodematerial in LiPF6based electrolyte.Firstly, we successfully prepared carbon coated Li3V2（PO4）3usingLi2CO3,NH4VO3and NH4H2PO4as raw materials and sucrose as the carbon source atthe temperature of750oC,800oC and850oC, respectively. XRD analysis shows thatthe Li3PO4impurities appears in the Li3V2（PO4）3at750oC However, no impuritieshave been found in the other two samples. After camparing the electrochemicalperformance, we choose800oC as the best senstering temperature of carbon coatedLi3V2（PO4）3cathode material.Then, for the Li3V2（PO4）3/C using the temperature of800oC. Raman scatteringshowed the amorphous nature of the residual carbon. SEM illustrates that the particlesdispersed well except for small quantity agglomerated. The average particle size ofthe powders is about1μm. HRTEM and XPS showed that a small amount of carbonpenetrated into the Li3V2（PO4）3crystallite to form a Li3V2（PO4）3-C interface film withthickness about10nm. The material showed a reversible discharge capacity of120mAh/g（C/4rate）,115mAh/g （1C rate） and110mAh/g （2C rate） in the voltage window of3.0-4.3V. The material suffered from capacity loss in the initial tencharge-discharge cycles. The irreversible capacity loss was mainly related to theprogressive formation of a solid electrolyte interface （SEI） film which was clearlyobserved by HRTEM. FTIR analysis showed that the chemical species of SEI mainlycontained some organic compounds such as ROCO2Li, RCO2Li, and aliphatic, as wellas some inorganic compounds such as Li2CO3, LixPFyor LixPOyFz. EIS shows the SEIfilm tended to be stabilized in the initial several cycles. Contraction and/or crack ofsome specific areas which can be seen from the HRTEM image. This leaves openchannels on the electrode interface to soak more electrolytes, which facilitates Li+intercalation/deintercalation as reflected by the decreasing charge transfer resistanceafter five cycles.Finally, we study the influence of two differents electrolyte, EC/DMC/EMC（1/1/1） and EC/DMC/EMC（1/1/8）, on the Li3V2（PO4）3/C electrochemical properties.CV and EIS test shows that when using EC/DMC/EMC（1/1/1） electrolyte, theelectrode polarization and charge-transfer resistance are much lower and lithium iondiffusion coefficient is much higer than using EC/DMC/EMC（1/1/8） electrolyte.Therefore, EC/DMC/EMC（1/1/1） is much suitable electrolyte system forLi3V2（PO4）3/C material.All in all, through this research we have a deep understanding of theelectrode/electrolyte interface of carbon coated Li3V2（PO4）3cathode material in LiPF6based electrolyte. This work showed that an appropriate amount of SEI interface ishelpful for the Li3V2（PO4）3cathode to maintain a long cycle life for hundreds ofcycles.

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