Preparation And Electrochemical Studies On Carbonizing Polyacrylonitrile Coated Silicon Composites

With the progress of human civilization and the development of society,the development and application of clean energy is an inevitable trend in the future.Silicon has the highest theoretical specific capacity(4200 mAh/g)and moderate operating potential(<0.4V vs Li/Li~+),making it an ideal anode material for next-generation lithium-ion batteries.However,silicon has a huge volume expansion in the process of lithium ion intercalation,the electrode is powdered,the active material loses electrical contact with the current collector,and cannot be re-intercalated with lithium ions,and the capacity is abruptly attenuated,which is the biggest obstacle to the commercial application of silicon anode materials.For the problem of silicon negative electrode,three schemes were adopted in this article,aiming at alleviating the volume expansion of silicon and improving the cycle stability of silicon anode.The experimental results obtained nano silicon(Nano Si),polyacrylonitrile coated silicon-carbon composite(CGS),polyaniline modified polyacrylonitrile coated silicon-carbon composite(PCGS).The following are the results of this article:(1)In this article,micron silicon was changed into nano silicon by ball milling.Nanocrystallization effectively alleviated the volume effect of silicon and promoted the cycle stability of silicon negative electrodes.The experimental results showed that the particle size of silicon was about 200 nm,which had good electrochemical performance:At a test current density of 0.2 A/g,the first discharge specific capacity of the silicon negative electrode was3217.3 mAh/g,the first charge specific capacity was 2264.2 mAh/g,the first coulombic efficiency was 70.38%,after 100 cycles,it still had a reversible capacity of 900 mAh/g or more.(2)The volume expansion of pure silicon materials was still very large,and further solutions were needed to improve stability.In this article,nano-silicon was first combined with graphite,and then coated with polyacrylonitrile.The composite material had good performance.We studied the effects of carbonization temperature,polyacrylonitrile coating ratio,and ratio of silicon to graphite on the electrochemical properties of composites,and concluded the following:The preferred carbonization temperature was 1000?,the preferred polyacrylonitrile coating ratio was 40%of the solid material,and the preferred ratio of silicon to graphite was 5:95.The CGS composite prepared had goog performance.At the test current density of 189.24 mA/g,the initial discharge specific capacity was 557.6 mAh/g,the first charge specific capacity was 513.9 mAh/g,and the first coulombic efficiency was 92.16%.After 100 cycles,the reversible capacity was 435.38 mAh/g,and the capacity retention rate was 84.72%.(3)In order to further improve the stability of the silicon anode material,we modified the 0.4C-85G-15Si composite with conductive polyaniline.The highly elastic conductive polyaniline was porous network structure on the surface of 0.4C-85G-15Si material,which not only improved the electrical conductivity of the material,but also further stabilized the structure of the 0.4C-85G-15Si material.The experimental results showed that when the mass ratio of aniline to 0.4C-85G-15Si was 1.4:6,the performance of PCGS composites was the best:At the current density of 189.24 mA/g,the first discharge specific capacity was 854.8 mAh/g,the first charge specific capacity was739.8 mAh/g,and the first efficiency was 86.55%.After 100 cycles,the maintained capacity was 586.96 mAh/g,and the capacity retention rate was 79.34%.The reversible capacity of 431.3 mAh/g was maintained even at the current density of 946.2 mA/g.