Study Of Silicon-based Film As Anode Material For Lithium-ion Batteries

Silicon is a promising candidate for lithium ion battery anode because of its high energy/power density,good compatibility during the fabrication process and easy integration with electronic devices.Si has a high specific capacity of 4200 mAh g^-1,however,the volume of Si expands and shrinks severely upon insertion and extraction of lithium.This phenomenon will damage the structure of electrode,leading to capacity fading and even battery failure.In order to prolong the lifespan of Silicon-based lithium-ion battery,the electrochemical properties of common materials（silicon,silicon nitride and silicon carbide）in the semiconductor industry are studied and a freestanding silicon film structure manufacturing method is proposed.The electrochermical and physiochemical properties of SiC film prepared by ICP-CVD process as anode materials for lithium-ion batteries are studied.Due to the low reaction temperature,the SiC film is formed into an amorphous state,which is conductive to relieve the stress generation in the electrochemical reation.An irreversible reaction of SiC with Li⁺occurs with a formation of lithium silicon carbide（Li_xSi_yC）and elemental Si,followed by a reversible alloying/dealloying reaction of the elemental Si with Li⁺.The low coulombic efficiency of the first cycle is due to the irreversible reaction.The better capacity retention of the 500 nm SiC anode is mainly because that residual SiC exists in the film due to its incomplete reaction caused by its lower reaction kinetics,which has high hardness and can act as a buffer matrix to alleviate the anode volume change,thus improving the mechanical stability and capacity retention of the SiC anode.The effects of thermal annealing on the performance of hydrogenated silicon nitride anode prepared by PECVD are investigated by varying annealing temperatures.With increasing the annealing temperature to 450 ^oC,the thermal treatment can break the hydrogen-containing bonds（induce dehydrogenation）and activate the anodes.therefore,the anodes with relatively high-temperature annealing（?450 ^oC）exhibit a high capacity even with no electrochemical activation processes needed.The mechanical properties of the anodes also depend on the thermal treatment and the anode elastic modulus and hardness increase with increasing the annealing temperature,which results in a poor mechanical stability and thus poor cycling performance of the anode with high temperature annealing（⁶00 ^oC）.Compared with the anode with lower or higher temperature annealing,the anode with an appropriate annealing（⁴50 ^oC）shows higher and more stable reversible capacity and better rate performance.In addition,the freestanding silicon film are prepared by using sodium chloride crystal as sacrificial layer.The silicon generated by sputtering is amorphous,and the amorphous silicon can form an infiltration channel,which can promote the diffusion of lithium ions.The capacity of freestanding silicon film has a negligible loss after 200 cycles while the capacity of silicon film on elastic substrate and fixed substrate both decreased rapidly and the cycling performance of silicon film on fixed substrate is worse than the elastic one.Therefore,the freestanding silicon film structure can significantly improve the electrochemical performance of silicon-based lithium-ion batteries.