Study On Dry Preparation And Modification Of Silicon Carbon-Graphite Anode Materials

Graphite is the most commonly used negative electrode material for commercial lithium-ion batteries.However,the lower theoretical specific capacity of the graphite limits the further improvement of battery energy density.With many merits such as abundant reserves,suitable voltage platform and higher theoretical specific capacity,silicon material has become a promising new anode material for lithium-ion batteries.Nevertheless,silicon will produce a huge volume effect during the charging and discharging process,resulting the failure of silicon particle pulverization and the continuous formation of solid electrolyte film on the particle surface,which will lead to a rapid decrease in electrode capacity.Aiming at the technical problems of silicon-based materials,this paper improves the lithium storage performance of silicon by preparing silicon/graphite/carbon composite materials,and mixes composite materials and artificial graphite into silicon-carbon-graphite products to meet the technical standards for lithium ion performance of commercial battery anode materials.The research content of the paper is as follows:（1）The molded silicon/graphite/carbon composite materials and the unmolded silicon/graphite/carbon composite materials are mixed with artificial graphite to form the silicon-carbon-graphite products.It is found that the silicon-carbon-graphite products prepared by the molding method have the best electrochemical performance.The first charge specific capacity of unmolded silicon carbon-graphite is 426.5m Ah/g the capacity retention rate after 100^thcycles is 86.5%in the button half-cell test,while it is 74.36%after 300^thcycles in the soft-pack full battery test.The silicon-carbon-graphite composites prepared under molded exhibites a first charge specific capacity of 429.4 m Ah/g the capacity retention rate after 100^thcycles is90.6%in the button half-cell test,while it is 76.7%after 300^thcycles in the soft-pack full battery test.Furthermore,the analysis shows that the molding process is helpful to the particle forming of the composite materials,which effectively increases the vibrational density and reduces the specific surface area of the materials.After molding,the silicon/graphite/carbon composite materials has a graphite core and a pitch pyrolytic carbon shell structure,which effectively alleviates the volume effect of nano-silicon and improves the cycle stability of the material.（2）The silicon/graphite/carbon composites were prepared by molding,high-temperature pyrolysis and jet pulverization,and then the composites with artificial graphite were mixed into silicon-carbon-graphite products.Also,the effects of different pyrolysis temperatures and particle sizes on the properties of silicon-graphite products were studied.It indicates that the electrochemical performance of silicon-carbon-graphite composites is the best when the pyrolysis temperature is 1000℃and the average particle size of silicon/graphite/carbon composites is 8μm.The silicon-carbon-graphite composites prepared under optimized conditions exhibites a first charge specific capacity of 429.4 m Ah/g and a first coulombic efficiency of 94.5%.In addition,the capacity retention rate after 100^thcycles is 90.6%in the button half-cell test,while it is 76.7%after 300^thcycles in the soft-pack full battery test.（3）In the first,taking coal tar pitch as the carbon source,the prepared silicon/graphite/carbon composite materials are modified by secondary carbon coating.Afterwards,the composite materials after the secondary carbon coating with artificial graphite are mixed into silicon carbon-graphite products in order to study the influence of different amount of secondary carbon coating on the properties of silicon carbon-graphite composites.It turns out that the silicon carbon-graphite composites with 5 wt%secondary carbon coating have the best electrochemical performance.The obtained charge capacities of the products is 425.2 m Ah/g,while the first coulombic efficiency is 94.8%.The capacity retention rate is 92.9%after 100^thcycles in the button half-cell test and 82.3%after 300^thcycles in the soft-pack full battery test,respectively.