Preparation And Electrochemical Studies Of Li₃V₂（PO₄）₃Cathode Material For Lithium-ion Batteries

Lithium-ion battery has become the most favorite power supply for its highenergy density, long life span and no memory effect. Here the most importantrequirement of the cathode material for a power battery is good safety performance.Compared to lithium metal oxides, the polyanion compounds offer excellent thermalstability. Among the polyanion compounds, the monocline Li₃V₂（PO₄）₃cathodematerial has attracted significant interest because of its large theoretical capacity （197mAh/g）, high operate voltage （3.62,3.68,4.08and4.55V）, good ion mobility andexcellent thermal stability. However, the monocline Li₃V₂（PO₄）₃suffers the intrinsicdisadvantages of poor electronic conductivity （2.3×108S/cm）, which make difficultthe large-scale application of pure Li₃V₂（PO₄）₃. An effective way to improve theconductivity and electrochemical performance of Li₃V₂（PO₄）₃is by introduction ofconductive carbon. In this work, the carbon-coated Li₃V₂（PO₄）₃monoclinic cathodematerials were synthesized by a solid-state reaction process using different carbonsources. The physical properties and electrochemical performances of as-preparedLi₃V₂（PO₄）₃/C composites are investigated. The main contents are as follows:1. The pristine Li₃V₂（PO₄）₃sample was synthesized using N2（95%）+H2（5%）gas as reducing and protective atmosphere. According to the electrochemical tests, therate performance of the pristine Li₃V₂（PO₄）₃is good, but the long-term cyclability isvery poor, the higher the rate is, the larger the capacity decay. There is a rapidcapacity fade on cycling due to vanadium dissolution into electrolyte, and the poorelectronic conductivity of the pristine Li₃V₂（PO₄）₃could be influenced theelectrochemical performances, too.2. The monoclinic Li₃V₂（PO₄）₃/C composite material was synthesized by asimple solid-state reaction route, and PEG-10000was used as a carbon source. Thecomposite with the optimum electrochemical performance is obtained by850°Ctreatment for8h. The Raman spectra show that the coating carbon has a goodstructure with a low ID/IGand sp3/sp2ratios. The images of SEM and TEM show thatthe carbon is dispersed between the Li₃V₂（PO₄）₃particles, which improves theelectrical contact between the corresponding particles. The electronic conductivity ofthe Li₃V₂（PO₄）₃/C composite is7.0×10-1S/cm. A thin carbon layer on the surface of the particles could significantly prevent the vanadium dissolution from Li₃V₂（PO₄）₃.The Li₃V₂（PO₄）₃/C material exhibits an excellent rate capability with high dischargecapacities of131.2mAh/g （0.28C）,115.2mAh/g （1C） and106.4mAh/g （5C）,furthermore, the long-term cyclability is excellent.3. The Li₃V₂（PO₄）₃/C composite cathode material is synthesized via a simplecarbothermal reduction reaction route using polyvinyl alcohol （PVA） as bothreduction agent and carbon source. Polyvinyl alcohol （PVA） is ultimately convertedinto substituted aromatics and substituted olefins after heat-treatment. Thehigh-quality carbon can not only enhance the electronic conductivity of theLi₃V₂（PO₄）₃/C composite but also prevent the growth of the particle size. TheLi₃V₂（PO₄）₃/C material exhibits an very well rate capability with high dischargecapacities of125.3mAh/g （0.3C）,115mAh/g （1C）,108mAh/g （3C） and104mAh/g （5C）. The electrochemical performance, which is especially notable for itshigh-rate performance, is excellent. It delivers an initial discharge capacity of105.3mAh/g at5C, which is retained as high as90%after2000cycles. No capacity losscan be observed up to300cycles under20C rate condition.4. The Li₃V₂（PO₄）₃/C composite cathode material is synthesized via a simplecarbothermal reduction reaction route using ascorbic acid as both reduction agent andcarbon source. The composite with the optimum electrochemical performance isobtained by10%ascorbic acid adding. The morphology of the Li₃V₂（PO₄）₃/Ccomposite and the structure of the carbon are very good. The rate performance of theLi₃V₂（PO₄）₃/C composite is excellent, however, the long-term cyclability is poor.5. The Li₃V₂（PO₄）₃/C composite cathode material is synthesized via a simplecarbothermal reduction reaction route using both polyvinyl alcohol （PVA） andascorbic acid as carbon source, to improve the long-term cyclability of the abovesample. According to the electrochemical tests, the long-term cyclability ofLi₃V₂（PO₄）₃/C composite is improved. The conductive carbon network could explainthe electrical continuity between Li₃V₂（PO₄）₃particles as well as improves theelectronic conductivity of the material, thus leading to the enhancement ofelectrochemical performance. The completely coated carbon also acts as a stabilizingbuffer layer that prevent further unwanted reactions on higher cycling by reducing thecontact areas between electrode and electrolyte.6. The Li₃V₂（PO₄）₃/C composite cathode material is synthesized via a simplecarbothermal reduction reaction route using methyl orange as carbon source. Thecomposite with the optimum electrochemical performance is obtained by8%methyl orange adding. The Li3V2（PO4）3/C composite with the particle size in the range30-50nm. The Raman spectra show that the coating carbon has a good structure with a lowID/IGand sp3/sp2ratios. The electrochemical performance, which is especially notablefor its high-rate performance, is excellent. It delivers an initial discharge capacity of97.9mAh/g at5C, which is retained as high as96%after1159cycles.