Hydrothermal Preparation And Modification Of The Li₂FeSiO₄ Cathode For Lithium-ion Batteries

Lithium-ion batteries are widely used in electronic mobile devices such as mobile phones and laptops because of their high energy density,long life,and low pollution.At present,the limited specific capacity of commercialized cathode materials,is the bottleneck of development of high energy density lithium-ion batteries,Compared with the traditional layered oxide and spinel structure lithium intercalation compound cathode materials,the silicate-based polyanionic cathode material Li2FeSiO4 has the advantages of much higher theoretical specific capacity,and better environmental friendliness.It is considered to be one of the most promising cathode materials for lithium-ion batteries.However,due to its own low electronic conductivity,low lithium-ion diffusion rate,impurities are easily generated during the preparation process,which reduce the utilization rate of its high theoretical specific capacity.In this thesis,the conditions for preparing Li2FeSiO4 cathode material by hydrothermal method were studied,and based on this,the study of carbon coating and active agent compound phase change process to control the avtive lattice plane were carried out.The key results are listed as below:1.Li2FeSiO4 is prepared by hydrothermal method.The effects of different reaction temperatures and time on product purity during hydrothermal process were investigated.The results show that when the hydrothermal reaction condition is 180℃ 24h,Pmn21-type Li2FeSiO4 with higher purity can be prepared,and the specific discharge capacity can be reached 132.9 mAh·g-1 under 1C(1C=166 mA·g-1)at 55℃.2.Carbon-coated Li2FeSiO4 prepared by hydrothermal method at 180℃ 24h,the purity of the product and the degree of graphitization of the carbon layer after carbon coating with different carbon sources were studied.The results show that high-purity P21/n-type Li2FeSiO4/C composite materials can be prepared with citric acid as the carbon source,and the surface carbon layer has the highest degree of graphitization.and the specific discharge capacity can be reached 161 mAh·g-1 under 1C at 55℃.3.By interfering the growth of polymorphic transition(Pmn21→P21/n)intermediate with surfactant,a nanoplate like particles of Li2FeSiO4 with Li+active facets large exposed has been successfully synthesized,which has shown greatly improved electrochemical activity.And a large specific discharge capacity of 211.3 mAh·g-1 is achieved under 1C at 55℃.