Preparation of silicon/graphite/carbon composites with fiber carbon and their application in lithium-ion batteries

Lithium-ion batteries have been widely used in various industries because of their high energy density, long life cycle, and green ring. In recent years, with the rapid development of consumer electronics, mobile wearable devices, and especially electric vehicles, the energy density requirements of the lithium-ion battery have progressively increased, promoting the development of lithium-ion batteries of higher specific capacity and longer life cycle. The commonly used graphite negative electrodes have a low theoretical capacity of 372 mA·h·g^-1, which does not meet the current requirements. Silicon is a very promising lithium-ion battery anode material because of its high theoretical specific capacity of 4200 mA·h·g^-1, low price, and eco-friendliness. However, silicon experiences high volume expansion (~300%) during charging and discharging, leading to severe loss of electrical contact with conductive agents and current collectors along with capacity degradation. Thus, using pitch as a soft carbon raw material and nano-Si and commercial graphite as active materials, a silicon/graphite/carbon composite was successfully synthesized using the high-temperature pyrolysis method, and micron-scale carbon fiber was formed in situ. The silicon/graphite/carbon composite material has many advantages: the void between the graphite sheet provides an effective space for the volume expansion of silicon, the coating of the asphalt pyrolysis carbon material can inhibit the volume effect in the nano-Si and increase its electronic conductivity to a certain extent, and the micro-sized carbon fiber enhances the long-range conductivity and structural stability of the material, thus greatly improving the cycle performance of the negative electrode material. The electrochemical test show that the silicon/graphite/carbon composite anode material delivers a reversible capacity of 650 mA·h·g^-1 at 200 mA·g^-1 and a capacity retention rate of 92.8% after 500 cycles at a current density of 500 mA·g^-1. The capacity decay rate per cycle was only 0.014%, indicating excellent cyclic performance.