The Preparation And Improved Characteristics Of Silicon Carbon Composite Materials For Lithium Ion Battery

With the decreasing of petrol resources and more and more intense demand for new energy, lithium-ion secondary battery as the representative has been widely used. However, the traditional graphite cathode materials cannot have satisfied people's demand for high capacity battery; people have turned to developing new high capacity battery materials. Silicon has theoretical capacity of 4200 mAh·g^-1, so it has become the hotspot for research. Silicon as lithium ion battery cathode materials will produce serious inflation in the intercalated-li process, causing pulverization after repeated charging and discharging, which has seriously restricted its commercial application.This paper focuses on the modification of silicon/graphite/carbon composites by high temperature pyrolysis method, and improvement of cyclic stability of silicon cathode materials in the process of making silicon/carbon composites by sol-gel method. Silicon/graphite/carbon composites synthesized by high temperature pyrolysis method can improve cyclic stability by changing the drying temperature, pH value, anointing conductive agent and second coating. Research shows that the cycle performance of silicon/graphite/carbon composite was greatly improved by reducing the drying temperature. When drying at 80℃, the composites have a delithiation capacity of 1000 mAh·g^-1, initial efficiency of 87.9%, capacity retention of 54% after 100 times cycling. When using carbon nanotube(CNT) as the conductive agent, the composites have a delithiation capacity of 1148 mAh·g^-1, initial efficiency of 87.5%, capacity retention of 67.3% after 100 times cycling. These modifications all can effectively increase cyclic stability of the material.Doping nickel for silicon/graphite/carbon composite material can improve its electrical conductivity. Mechanically ball milling silicon and nickel will produce some NixSi alloy, and milled silicon/nickel mixture replaces pure silicon to produce silicon/nickel/graphite/carbon composites. After doping, the composites have a delithiation capacity of 500 mAh·g^-1, initial efficiency of 70%, capacity retention of 92% after 100 times cycling. It shows that doping nickel with good conductivity, can effectively improve cyclic stability of silicon/graphite/carbon composites.Carbon aero gels and silicon/carbon composite materials can be produced by the sol-gel method. The carbon aero gels have first discharge capacity of 162 mAh·g^-1, remaining 127 mAh·g^-1 after 50 times cycling, but first efficiency just has 37%. The silicon/carbon composites have first reversible capacity of 188 mAh·g^-1, rising up to 228 mAh·g^-1 after 100 times cycling, but first efficiency just has 48%. The silicon/graphite/carbon composites have first reversible capacity of 650 mAh·g^-1, remaining 412 mAh·g^-1 after 100 times cycling, but first efficiency had risen to 62%.