Preparation Of Si/C Anode Materials And Their Electrochemical Properties Based On Rectorite

Compared with graphite anode materials widely used at present,silicon anode materials have become one of the most promising new anode materials due to their high theoretical capacity(about 4200mA/g).However,due to the poor intrinsic conductivity of silicon anode materials and the large volume expansion during charging and discharging,the silicon anode materials have such disadvantages as low coulomb efficiency and poor cycling stability,which seriously restrict the application of the materials.How to improve the electrical conductivity of the material as well as the inhibition of silicon materials caused by the volume expansion pulverization is the key to improve the electrochemical performance of silicon anode materials.In order to solve these problems,this paper synthesized silicon-carbon composites by magnesium thermal reduction and carbon cladding using natural clay mineral rectorite as raw material.Firstly,the effect of magnesium powder dosage on the properties of nano-silicon was investigated.Secondly,the effects of different carbon coating processes on the properties of silicon-carbon composites were investigated.Finally,the effects of purified modified rectorite and graphite addition on the properties of the composites were investigated.The research results are as follows:(1)magnesium thermal reduction process: the reversible capacity of the first cycle of silicon carbon anode material prepared with magnesium powder dosage of 20%wt,30%wt,40%wt and 50%wt respectively is 676mAh/g,842mAh/g,1110mAh/g and 1045mAh/g.The silicon carbon composite material prepared with magnesium powder dosage of 40% has the best cycling stability.The material can still maintain the reversible specific capacity of 390mAh/g after 200 charging and discharging cycles,and the capacity retention rate is as high as 90%.(The capacity retention rate of material cycle to 200 cycles is calculated by taking the third cycle capacity as the stable specific capacity,the same as below.)Silicon-carbon composites prepared with different amounts of magnesium powder showed good doubling property under the current density of 0.5-1-2A /g,which was attributed to the special alternating layer structure of rectorite,and the nano-silicon particles prepared by magnesium thermal reduction had good morphology and pore structures.(2)When sucrose was used as carbon source and mass ratio of carbon source to silicon particles was 1:2 and the carbon coating process was once carried out,the electrochemical performance of the synthesized silicon-carbon composites was higher than that of the material from the carbon coating process secondary carried out.The specific capacity of the first lap is up to 1226mAh/g,and the specific capacity of the second lap is 595mAh/g,and the stable specific capacity after the third lap is 512mAh/g,and the specific capacity of 481mAh/g can be maintained after 200 cycles,and the specific capacity retention rate is up to 94%.The amorphous coated carbon not only improves the electrical conductivity of the prepared materials but also alleviates the expansion effect of the silicon particles.(3)the silicon content in the purified rectorite-based composite material is 15.9%wt,and it is increased by 4.54% compared with before purification;and the initial specific capacity of the purified rectorite-based silicon-carbon composite material is 1510 mAh/g,and it is increased by 23% compared with before purification;but the increase of silicon content in the composite materials caused the increase of the expansion effect,which resulted in the obvious decrease of the cycle stability of the purified rectorite-based silicon-carbon composite material.A good "sandwich" structure formed by doping graphite during the coating carbon process could well alleviate the expansion effect of the silicon particles.The silicon/carbon/graphite materials not only has the initial specific capacity 1585 mAh/g,but also can still maintain the specific capacity of 640 mAh/g after 200 cycles.It has good cycle stability while maintaining high specific capacity.