Preparation And Modification Of Silicon-Carbon Composite Anode Materials

Graphite-based materials are the anode materials for lithium-ion batteries that have been commercially used today,but the theoretical specific capacity of graphite is only 372 m Ah/g,and the low theoretical specific capacity will greatly limit its further development in the field of anode materials for lithium-ion batteries.The theoretical specific capacity of silicon is as high as 4200 m Ah/g,which is a promising anode material.However,silicon material has a huge volume effect during charging and discharging,and has serious instability.In view of this shortcoming,compounding silicon with other materials to form a composite material can significantly remedy this issue.In this paper,silicon-carbon composites were prepared by mixing nano-silicon powder,artificial graphite and pitch,and on this basis,the effects of molding preparation technology,addition of different conductive materials and different graphite carriers on the properties of silicon-carbon composites were studied.The specific research contents are as follows:(1)Two types of silicon-carbon composite materials were prepared by molding process and without molding process,and then mixed with artificial graphite to obtain 420 products(design capacity: 420 m Ah/g),they were tested respectively and it was found that the silicon-carbon composite precursor prepared by the molding process had a higher charge specific capacity in the first cycle,reaching 2223.5 m Ah/g.The charge specific capacity of its 420 product in the first cycle is also higher than that of the 420 product without molding process.After 20 cycles,the charge specific capacity of the molded 420 product is 416.0 m Ah/g,however,the charge specific capacity of the unmolded 420 product after 20 cycles is only 387.4 m Ah/g.At the same time,comparing the cycle data of the soft pack battery,when the capacity retention rate reaches 80%,the cycle number of the 420 product in the molding process is 372 cycles.The number of cycles of the unmolded420 product is only 207.It shows that the improvement of the performance of silicon carbon composites by the molding process is very obvious.(2)Under the condition of molding process,conductive materials(CNTs and SP)with mass fractions of 0.3%,0.5% and 0.7% were added during the preparation of silicon-carbon composite precursors,and prepare 420 products separately.The test results show that the addition of 0.3%CNTs has the most obvious improvement in the performance of the composites.After adding 0.3%CNTs to the precursor,the first cycle charge specific capacity of the precursor is 2092.7 m Ah/g,after 20 cycles,the charge specific capacity is still 1021.3 m Ah/g.Its 420 product soft pack battery has a capacity retention rate of 83.39% when it is cycled for 405 cycles.Compared with the silicon-carbon composite material without CNTs(373 cycles of capacity retention rate of 80%),there is a significant improvement.(3)In view of the problem that more fine powder in the process affects the cycle performance,two kinds of graphite carriers with different particle sizes were used to prepare the composite materials under the molding process conditions.Compared with the 420 product,it is found that the fine powder phenomenon in the precursor prepared by using the small particle size graphite carrier is improved,and its cycle performance is also improved.S420(the small particle size graphite carrier420 product)has a specific charge capacity of 402.7 m Ah/g after 20 cycles,and a capacity retention rate of 95.59%.L420(the large particle size graphite carrier 420product)has a specific charge capacity of only 387.4 m Ah/g after 20 cycles,and the capacity retention rate is 90.49%.The data of the soft pack battery shows that the capacity retention rate of the S420 is 80% when it is cycled for 405 cycles,and the data is better than the 373 cycles of the L420.It shows that the use of small particle size graphite carrier has a positive effect on the performance improvement of silicon carbon composites.