Preparation And Performance Study Of Fe-Si@C Composite Anode Materials For Lithium-ion Batteries

In recent years,new energy vehicles have developed rapidly.The demand for lithium-ion battery capacity in energy density,rate,and cycle performance is increasing.Silicon is considered to be one of the most promising anode materials because of it's high theoretical capacity,but it's disadvantage of large volume expansion makes it difficult to use directly as a negative electrode material.The ferrosilicon(Fe-Si)has FeSi_x phase,which does not chemically react with lithium ions,and buffer volume changes in the material.However,the coulombic efficiency of first cycle,cycle stability and rate performance of Fe-Si need to be further improved.In this thesis,Fe-Si@C composite anode material was prepared by mechanical ball milling and high temperature pyrolysis.The influences of organic carbon source types,pyrolysis temperature and other factors on its properties were studied.In addition,conductive agents and other anode materials were added to improve its electrochemical properties.The main contents are as follows:(1)Different carbon sources are combined with Fe-Si,then Fe-Si@C composite anode material were prepared after high temperature pyrolysis.The results show that asphalt can form more amorphous carbon after complete pyrolysis,which effectively reduces the formation of SEI film and limits the volume increase.In addition,when the pyrolysis temperature of Fe-Si@C is 850°C,there is a reasonable alloy phase distribution inside Fe-Si,showing better electrochemical performance.It's first discharge specific capacity reached 1376.3 mAh/g at 100 mA/g,the first coulombic efficiency was 86.4%,and after 76 cycles,76.3%capacity was maintained compared to the second cycle,and the rate performance is well.(2)Modification of Fe-Si@C by using graphene and carbon nanotubes(CNTs)as conductive agents.The results show that graphene can enhance the conductive contact of the particles and has better structural support for the material due to it's large specific surface area and high strength.The first discharge specific capacity of Fe-Si@C/graphene is 916.3 mAh/g,the first coulombic efficiency is 82.4%,and the discharge capacity is maintained at 76.0%after 100 cycles.(3)The Fe-Si/SiO_x@C composite anode material was prepared by combining SiO_x with Fe-Si.The results show that Fe-Si/SiO_x@C exhibits good electrochemical performance due to its triple buffer structure.The first discharge specific capacity of Fe-Si/SiO_x@C was 1164.9 mAh/g.After 100 cycles,the discharge capacity was kept at66.5%.When Fe-Si@C was combined with graphite,it was found that graphite effectively improved the cycle stability of Fe-Si@C,and Fe-Si@C effectively improved the stable capacity of graphite,which fully combined the advantages of both.