Carbon Coating Modification And Electrochemical Properties Of Si/MG Composite Anode Materials

The theoretical capacity of Si is up to 4200 mA g^-1.However,it suffers from volume expansion rate?300%?in the charge and discharge cycle and a low intrinsic conductivity.These defects limit the industrial application of the silicon negative electrode.The theoretical capacity of the graphite anode material is only 372 mAh g^-1,but it has very excellent cycle performance.The surface roughness of the microcrystalline graphite?MG?can provide a site for the adhesion of silicon while facilitating carbon coating.In order to comprehensively utilize the advantages of both silicon and microcrystalline graphite,the Si/MG@C composite anode materials were prepared by liquid phase blending and hydrothermal methods respectively.The effect of different reaction conditions on the carbon coating and electrochemical performance of the products under were investigated.The content and main results of research are as follows:?1?Si/MG@C composites were prepared by liquid phase blending method,which using commercial nano silicon powder and spherical microcrystalline graphite as raw materials.The results show that the carbon coating formed by sucrose is continuous and smooth,while the surface of the carbon coating formed by citric acid?CA?and polyvinylpyrrolidone?PVP?is rough and a large number of fine particles are attached.At a current density of 50 mA g^-1,the capacity retention of citric acid,PVP and sucrose electrodes were 53,46 and 88.6%,respectively,after 50 cycles.The study also found that the carbon coating content was higher and the reversible capacity was higher too,but the capacity decayed very quickly.With the increase of carbonization temperature,the prepared composite electrode has a reversible specific capacity at a small current density of 50 mA g^-1,and the sample with a carbonization temperature of 600? exhibits the best electrochemical performance.The reversible capacity is up to 782 mAh g^-1,and it remained at 676 mAh g^-1 after 50 cycles.?2?Si/MG@C composites were prepared by using hydrothermal method,which use glucose as carbon source and commercial nano-silica powder and spherical microcrystalline graphite as raw materials.The shape of the hydrothermal product is spherical-like particles,and the particle size is uniformly dispersed.The silicon particles are evenly distributed on the surface of the graphite particles by means of the amorphous carbon layer,and the surfaces of the silicon and graphite particles are covered with the amorphous carbon layer.By optimizing experiment condition,it was determined the mass ratio of silicon/graphite to 2:15,the hydrothermal time was 6h,the hydrothermal temperature was 160?,and the carbonization temperature was 800?.The composite exhibited the best electrochemical performance with a reversible specific capacity of 556 mAh g^-1 at a current density of 200 mA g^-1 and a first coulombic efficiency of 79.9%.