Research On Synthesis And Electrochemical Characteristics Of Lithium Vanadium Phosphate As Cathode Materials For Lithium Ion Batteries

Li₃V₂（PO₄）₃ with monoclinic structure is thought to be a promising cathode material because of its high operating voltage,large theoretical capacity and high reversible capacity.However,the poor electrical conductivity and low lithium ion diffusivity affect its electrochemical performance and limit its large-scale application for its intrinsic crystal structure.Therefore,it is necessary to carry out a series of studies on Li₃V₂（PO₄）₃ to improve its electrochemical performance.In this paper,the precursor was prepared by ball milling and the Li₃V₂（PO₄）₃/C was synthesized by carbon-thermal reduction method.The micro structure and morphology of Li₃V₂（PO₄）₃/C was investigated by XRD and SEM and the electrochemical performances were characterized by charge/discharge test and electrochemical workstation.The effects of synthesis conditions,the doping content of titanium dioxide and the addition of stearic acid on the structure and performance of materials were studied in detail.The main contents are summarized as follows:Firstly,the effect of synthesis technics on electrochemical properties of Li₃V₂（PO₄）₃/C were investigated.Li₃V₂（PO₄）₃/C precursor was synthesized using LiH₂PO₄,V₂O₅ and stearic acid as the raw materials by carbon-thermal reduction method.The results showed that the sample which was preparated at 700℃for 10h in nitrogen ambience had a well-developed crystallite structure,uniform particle sizes and excellent electrochemical performance.In the potential range of 3.0–4.3V,the Li₃V₂（PO₄）₃/C composite showed the highest discharge capacity of 119.1mAh/g at 0.1C.The discharge capacity of the sample was stable at115.2mAh/g even after the 20th cycles.Secondly,the influence of the addition of stearic acid on the electrochemical performance of Li₃V₂（PO₄）₃/C composite were investigated.The amount of stearic acid was7.9wt.%,10wt.%,12.3wt.%and 13.8wt.%.Li₃V₂（PO₄）₃/C was synthesized by carbon thermal reduction method.The research results revealed that when the amount of stearic acid was12.3%showed the best electrochemical performance.The third discharge capacity of the sample was 119.4mAh/g at 0.1C,after 50 cycles the capacity retention was 98.5%.At the rates of 0.5C、1C、2C、5C、0.5C in proper order,the sample discharge capacity only showed slight decay,appearing favourable cycle performance.Finally,the effect of Ti-doping on the electrochemical properties of Li₃V₂（PO₄）₃/C was investigated on the basis of the carbon coating.Li₃V_2-xTi_x（PO₄）₃/C（x=0,0.02,0.06,0.08）composites were synthesized by balling mill and carbon-thermal reduction method.Among all the samples,the Li₃V_1.94Ti_0.06（PO₄）₃/C composite delivers the highest presented comprehensive electrochemical performance in the voltage rang of 3.0V⁴.3V.The initial discharge capacities were 121.9mAh/g and 123.5mAh/g at 0.2C and 0.5C,and the capacity retention ratios were 101.3%and 98.38%after15 and 50 cycles respectively.