Synthesis And Electrochemical Performance Of Li₂FeSiO₄Cathode Materials For Lithium Ion Battery

Li2FeSi04, as a cathode material of lithium ion batteries, is one of the most promising candidates for LiFePO4because of its high specific capacity, excellent chemical stability, inexpensive and abundant materials and environmental benignancy. Orthosilicate Li2MSi04(M=Fe, Mn) could store and release two lithium ion along with transfer reaction of M2+/M4+redox couples, suggesting that Li2FeSiO4possess a high theoretical capacity of166mAh·g-1for one lithium ion exchange reversibly,332mAh·g-1for two lithium ion exchange. However, as a member of polyanion materials, Li2FeSiO4has some drawbacks just as same as LiFePO4:lower electronic and ionic conductivity. Usually, three main means have been used to overcome those shortcomings:1) nanocrystallization, reducing size of particle avail shorten the distance for lithium diffusion;2) doping with supervalent or isovalent cation in crystal, which could increase the conductivity of bulk material, directly;3) coating with carbon or conductive polymers to improve the conductivity of composites. What’s more, various syntheses of Li2FeSiO4had been exploited and exhibit different electrochemical performances. In this paper, we main used hydrothermal synthesis to prepare Li2FeSiO4with a special morphology. And their electrochemical performances have been investigated, too. The major research works are as follows:1) Nano-particle Li2FeSi04was prepared by hydrothermal synthesis. Then, nano-particle Li2FeSiO4/C was obtained with dopamine as carbon source coated. The particle size of obtained Li2FeSiO4/C is about50-200nm that characterized by SEM and TEM. The carbon layers on particle surface were further explored by HRTEM. What’s more, their corresponding electrochemical performance also had been investigated. As a result, the sample LFSC, which was obtained under a condition of a mass ratio of Li2FeSiO4to DPA is3:1and the carbon layer thickness is5nm, showed the best electrochemical performance. At a rate of0.1C, its initial specific capacity is148mAh·g-1at room temperature. In addition, at a high temperature, its specific capacity could reach240mAh·g-1at current density of0.2C.2) Contrasting to solid materials, hollow structure shows some advantages of good mono-disperse, stable structure and large specific surface. The major advantage is a shorter ion diffusion path which means a better rate performance. In this study, hollow sphere Li2FeSiO4/C was harvested by one-step hydrothermal synthesize Li2FeSi04and DPA as a carbon source to do further coating. The sample we obtained was characterized by XRD, SEM and TEM to prove the formation of hollow sphere structural Li2FeSi04/C. what’s more, the formation process for Li2FeSi04hollow sphere were investigated in detail. It is found that the electrochemical performance was affect by particle size and thickness of hollow sphere. As a result, Li2FeSi04hollow sphere with hydrothermal for3days has a best electrochemical performance with a specific capacity of152mAh·g-1at a current density of0.05C.