Synthesis And Electrochemical Performances Of Li₃V₂（PO₄）₃and Lithium Sulfur Battery For Lithium-ion Battery

Since the commercialization of lithium-ion battery, lithium has become one ofthe mainpower supplies. Monoclinic Li₃V₂（PO₄）₃, as a lithium battery cathodematerial with high stability, high capacity and high-potential characteristics, has beenextensively studied recently. Meanwhile, the sulfur cathode material has beenconcerned due to its high energy density, excellent performance and low pricecharacteristics. In this paper, Li₃V₂（PO₄）₃was synthetized by sol-gel method, and themicrostructure and performance of the lithium storage was analysed. The porouscarbon with an ultra-high specific surface area was prepared by alkality method. Themicrostructure, electrochemical capacitance performance, and the application incarbon/sulfur composite cathode material were also studied.In chapter3, Li₃V_2-x(M_0.5Ti_0.5)_x（PO₄）₃was prepared by sol-gel method withLiOH·H₂O，NH₄VO₃，NH₄H₂PO₄as raw materials. Citric acid was selected as boththe carbon source and the complexing agent. The preparation process and the impactof metal doping to the sample performance were investigated by scanning electronmicroscopy （SEM）, X-ray diffraction （XRD）, constant current charge/discharge testmethods. Compared with pure-phase material with capacity of150mA/g at0.1C,Li₃V_2-x(M_0.5Ti_0.5)_x（PO₄）₃doped with Co and Ti showed an improved electrochemicalperformance with capacity of172mA/g. When the doping content about Co and Tiwas0.15, the sample’s capacity reached161mA/g at0.2C. It was confirmed thatappropriate doped metal cations would not change the crystal structure, and cansignificantly change the conductive properties, increase the discharge capacity, reducethe impedance of the electrode charge transfer.In chapter4, the carbon/sulfur composite cathode material based on the porouscarbon was synthesized. The morphology, electrochemical performance and the effectof sulfur loading on the electrochemical properties of carbon/sulfur composite cathodematerial were investigated by XRD, BET, constant current charge-discharge test and AC impedance test. It was found that the initial discharge capacity of carbon/sulfurcathode materials was1400mA/g at the density of10mA/g; The sample with37.5%sulfur loading showed the best electrochemical performance. The discharge capacitymaintained389mA/g after50cycles. The results showed that the sulfur can beintroduced into the pore structures of activated carbon by heat treatment.The excess ofsulfur loaded on the carbon substrate surface would affect the discharge capacity ofthe sample. The insoluble polysulfides generated during the reaction were believed tobe the main reason for capacity fading.In chapter5, the porous carbon materials were prepared with phenolic resin,asphalt, and natural plant pollen as carbon sources, KOH and ZnCl2as activators. Theprepared carbon materials were characterized by XRD, SEM, etc. Theelectrochemical properties of carbon-based supercapacitors were tested by cyclicvoltammetry and galvanostatic charge/discharge test. The porous carbon samplesynthesized with the carbon/alkali ratio of1:5and phenolic resin as carbon sourceshowd the best property. The carbon resulted from phenolic resin, lotus pollen,camellia pollen and asphalt showed the initial discharge capacity of272.6,164,207and151F/g, respectivlely. Cyclic voltammetry showed a good stability and ACimpedance showed smaller impedance. The experimental results revealed that theporous structure could improve the electrochemical performance. The stability of thecarbon mirostructure was favorable for the cycling performance of the capacitor.