Preparation Of Silicon-Carbon Composite Anode Materials For Lithium-Ion Batteries By Surface Aggregation And Their Electrochemical Properties

Silicon-based materials are regarded as one of the most promising anode materials for the next generation of lithium-ion batteries due to their ultra-high theoretical specific capacity(4200mAh g-1),suitable voltage platform and easy access to materials.However,in the process of charging and discharging silicon-based materials,the volume becomes larger,the SEI film is damaged,the material powder,the active substance falls off and the electrolyte is exhausted,which leads to the battery failure.Since silicon carbon composites can effectively solve the above problems,because carbon materials have good conductivity,ductility and can buffer the volume expansion of silicon,we often use different methods to prepare silicon carbon composites,and make different silicon carbon composites is the focus of current research.In order to obtain silicon carbon anode materials with a high specific capacity,good cycle stability,and good rate performance,a series of sulfur grafted carbon based materials were prepared by introducing S atoms with electrochemical activity and covalently combining with carbon based materials,and their properties were studied.The main research contents and results are as follows:Using polyacrylonitrile as a precursor,sulfur-doped carbon derived from sulfuration of polyacrylonitrile was used as the carbon-based material to encapsulate nano-silicon particles through a one-step ball milling method.It was found that sulfur doping can effectively improve the cycling stability of silicon-based composite materials.The carbon-coated nanosilicon material with sulfur doping can provide space for the volume expansion of silicon during charging and discharging.The prepared silicon/sulfur-doped carbon composite material with moderate silicon content(～16.7 wt%)had an initial discharge capacity of 1735 mA h g-1,which remained at 699 mA h g-1 after 200 cycles.Even at a current density of 1 A g-1,the 0.5gSi@SPANdC composite material exhibited good cyclic stability and relatively high reversible capacity of 546.1mAh g-1 after 400 cycles.Building on the previous chapter,silicon-carbon composite negative electrode materials were synthesized using a wet method by adding sulfur-doped copolymerized graphite as a conductive medium.The doped sulfur can change the internal structure of the material,increase its defect level,and accelerate the electrochemical reaction rate.Additionally,adding graphite to the silicon-carbon negative electrode material can improve its conductivity,mechanical stability,capacity retention rate,and cycle life because graphite has good conductivity and high chemical stability and can reduce the volume expansion and contraction of silicon-carbon materials during charging and discharging,thereby improving their cyclic stability.However,it may have some impact on the phonological performance of the material.The prepared 3gSi@Gr@SPANdC material showed a reversible specific capacity of 564.6mAh g-1 after 200 cycles at a current density of 0.2A g-1,with a first Coulombic efficiency of 70%.