Preparation And Properties Of Si/C Composite As Anode Material For Lithium-ion Batteries

Silicon which is abundant,low-costand environmental protection materials is the most potential candidate of anode material for lithium ion batteries because of its high theoretical capacity of(4200 mAh g-1)and moderate discharge potential(~0.5 V vs Li+/Li).However,the huge volume expansion(up to 300%)of silicon during Li+ intercalation and deintercalation process and the low intrinsic electronic conductivity restrict the development of silicon anode materials.In this paper,the silica anode materials were synthesized by ball-milling to alleviate the volume effect and improve electrochemical performance of silicon-based anode materials for lithiumion batteries.The silicon/polyaniline(Si/PANI)composites were successfully synthesized by ball-milling to disperse the nano-silicon particles in polyaniline conductive matrix.Variations of conductivities of the conductive polyaniline in lithium ion electrolyte were investigated.Meanwhile,the impact of the doping PANI content and milling time on the physical morphology and electrochemical performance of Si/PANI composites were also investigated.The results show that PANI in lithium ion electrolyte still keeps certain electrical conductivity and ball milling can make long chain PANI molecules and silica on the surface of silicon particles interactions to form reticular structure through hydrogen bonding,so as to accommodate the volume effect of silicon materials and improve cycling stability of the Si/PANI composites.The Si/PANI composite with the Si/PANI mass ratio of 3:1,the milling time for 4 h,exhibits the optimum electrochemical performance.The electrode still maintains reversible specific capacity of 849.5 mAh g-1 after 50 cycles at current density of 400 mAg-1.Silicon/pyrolysis polyaniline(Si/p-PANI)composites were prepared by high temperature pyrolysis of Si/PANI composites synthesized by ball milling.The effects of pyrolysis temperature on the structure and electrochemical properties of Si/p-PANI composites were investigated.The results show that the p-PANI coating on the surface of nano-silicon can suppress agglomeration of the nano-silicon particles,form good conductive network and effectively accommodatethe volume change and improve the conductivity of silicon electrodes.The Si/p-PANI composite pyrolysed at 700oC has the reversible capacity of 1051.1 mAh g-1 at density of 400 mAg-1 over 50 cycles.The activated grapheme nanoplatelets(a-GNs)were prepared by the facile hydrothermal method with zinc chloride as activator.In the synthesis of the material as the foundation,The Si/p-PANI/a-GNs composite was synthesized by ball-milling and high temperature pyrolysis,using Si/PANI composite as raw material and a-GNs as flexible matrix.The results show that as an anode material for lithium-ion batteries,the Si/p-PANI/a-GNs composite delivers a reversible capacity of 1664.5 mAh g-1 at 400 mA g-1 after 50 cycles with initial coulombic efficiency of 87.5%.The enhanced performance of Si/p-PANI/a-GNs could be attributed to this unique structure,which is beneficial for providing the dual protection from pyrolysis polyaniline layer and activated graphene overcoat to buffer the volume variationand maintain the structural and electrical integrity of the electrode,accelerating electron transferand stabilizing the SEI films.