Preparation And Electrochemical Properties Of Modified Silicon Based Anode Composites

Developing high energy density lithium ion batteries(LIBs)plays an important role in the development of electric vehicles and portable electronic equipment.The limited capacity of cathode and anode materials is the major obstacle for the high energy density LIBs.Silicon-based materials are regarded as one of the most promising next-generation anode materials due to their high capacity(?4200 mA h g^-1,Li_4.4Si).Compared with the traditional graphite anode(?372 mA h g^-1),silicon-based materials exhibit high capacity,low lithium intercalation potential,high abundance,and environmental friendliness.However,the application of silicon-based materials in practice still needs to face several challenges,including the obvious volume expansion during charging and discharging,increasing the number of unstable solid electrolyte interface(SEI)film on the surface of silicon and theirs poor conductivity.In order to solve the problems,the silicon-based materials were compounded with other materials by solution coating method,electrospinning technology and sol-gel method,multiple Si/C composites were obtained by high temperature heat treatment.Structure characterization and electrochemical properties of the products were investigated.(1)Si@void@NC composites were successfully prepared by using polydopamine(PDA)and metal organic framework(ZIF-8)as carbon sources via a solution coating and high-temperature pyrolysis method.The composites show better electrochemical performance compared with that of Si/NC.At a current density of 0.2A g^-1,the discharge capacity remains at 697.7 mA h g^-1 after 100 cycles.The improvement of electrochemical performance can be attributed to the strong interface interaction between ZIF-8 and PDA,which can inhibit the shrinkage of ZIF-8 during the heat treatment process and provide space for the volume expansion of composites during the charging and discharging process.Meanwhile,the N-doped carbon layer derived from PDA and ZIF-8 after high-temperature carbonization,which not only further buffers the volume expansion of silicon anode,but also helps to improve the conductivity of the materials.(2)The Si@C/CNFs materials were obtained by electrospinning technology and high-temperature pyrolysis,polyacrylonitrile(PAN)and ZIF-8 were used as carbon sources.The electrochemical performance of Si@C/CNFs were compared with that of Si/C materials without electrospinning process.At the current density of 0.2 A g^-1,the discharge capacity of Si@C/CNFs remains at 945.5 mA h g^-1 after 150 cycles,and the materials capacity retention is 64%from the 2nd to the 150th cycles.The better electrochemical performances of the materials are due to the fact that the pumpkin-like structure alleviates the volume expansion of silicon nanoparticles during charge/discharge process.The N-doped carbon layer not only improves the conductivity of the material,but also forms a stable SEI film.In addition,the self-supporting electrode can reduce the resistance of electron and Li⁺ion.(3)Si@C/Ti O₂ materials were prepared by sol-gel method and heat treatment using phenolic resin(RF)and butyl titanate(TBT)as raw materials,which were applied to the anode material of LIBs and compared with the pure silicon materials.The results exhibit that the electrochemical properties of the composites are improved significantly compared with that of pure silicon.The discharge capacity of Si@C/Ti O₂is 1597.6 mA h g^-1 for the first cycle at the current density of 0.5 A g^-1,and remains at 723.2 mA h g^-1 after 200 cycles,and its initial coulombic efficiency is74%.The performance improvement is due to the elastic of titanium dioxide,which makes the SEI grow stably on the outer surface of silicon.Meanwhile,the carbon layer on the surface of the silicon particles not only greatly improves the overall conductivity of the materials as a conductive layer,but also gives the silicon particles volume expansion space,which enhances cycle stability greatly.