Synthesis And Characterizations Of Carbon Coated Li₃V₂（PO₄）₃Cathode Materials For Lithium Ion Batteries

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, Li_xPF_yor Li_xPO_yF_z. 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.