Research On Performance Of Lithium Ion Batteries By Using Silicon/carbon Composite As Anode

The development of lithium ion batteries is vital to the promotion of electronic vehicle.In order to keep pace with the development of electronic vehicle,electrode material with high performance and good stability should be researched.The traditional material used in anode of lithium ion batteries is graphite,which has a low energy density and cannot meet the demand of electronic vehicle.Silicon-based material is much promising compared to graphite because its high theoretical capacity.However,there are several problems that inhibits the commercialization of silicon-based material.The first one is that the formation of lithium-silicon alloy accompanying with the huge expansion,which will easily destroy the silicon material.Another problem is that the poor conductivity of silicon material.This will inhibit the transfer and diffusion of lithium ions.The last problem is that the solid electrolyte interface formed on the surface of silicon material is very important to maintain the stability of electrode.This solid electrolyte interface will enable the lithium ion transfer and prevent the electrolyte from reacting with the silicon material.The solid electrolyte interface can also reduce the side reaction between electrolyte and silicon.However,the huge expansion during charging and discharging will destroy the solid electrolyte interface.In order to solve these problems,a new approach is proposed to synthesize a kind of silicon/carbon composite.The principle we use in this thesis is to drive asphaltene onto the surface of silicon by changing the property of the solution.The performance effected by different parameters in the synthesis procedure were investigated.It has been proved that the structure and mechanical property of the coating-layer could be modified by adjusting the property of the solvent while the cycling performance of silicon/carbon anode is dependent on the structure and mechanical property of the coating layer.In another word,the cycling performance of silicon/carbon could be optimized by changing the solvent property.Furthermore,the mechanical property of coating layer on nano-silison would be adjusted by changing the temperature of heat treatment.Plenty of batteries were assembled to test the electrochemistry performance of synthesized material.High-performance batteries were obtained by using this approach.One of our best batteries can maintain 92.9% reversible capacity after cycling for 400 cycles in 0.2 C current density,the capacity los was 0.02% per cycle.The carbon in the silicon/carbon composite is different from other carbon materials reported before.The elastic modulus was different from area to area.In a micro horizon,some area can be defined as hard area while others are defined as soft area.This coating layer can prevent the cracking of inner material and enhance the stability of batteries.This kind of coating layer can be used not only in lithium ion batteries,but can also be applied to other kinds material by compositing together,which could effectively buffer the expansion.