Lithium Storage Electrochemical Properties Of Si-C Composites Prepared By Plasma Assisted Ball-milling

Si-based composites have been widely studied as promising anode materials for Li-ionbatteries due to their high specific capacity, moderate operation potential and low cost.However, their problems of low electric conductivity and large volume expansion duringlithium insertion still remain unsolved. To improve the performances of Si-based anodes,dielectric barrier discharge plasma （DBDP） assisted high energy ball milling has been usedfor the first time to prepare Si-C composites in this dissertation. The influence of millingprocess, compoent and milling time on the microstructure and electrochemical performance ofthe composite materials were investigated.With respect to the milling process, a stable structure in which nano-sized Si particleshomogenously dispersed in micro-sized carbon matrix was achieved by the two-step p-millingincluding the pre-milling of Si and followed combine milling of Si-C. It is due to that thebuffering effect of C can be fully utilized in that structure, the as-prepared Si20C80compositeshows slightly capacity fading after20cycles under the current density of200μA/cm²,exhibits good cycle stability.It has been found that the increasing of Si content in the composite from20%to70%brings obvious enhancement on the initial coulombic efficiency while serious decline on thecycle stability. This is because the Si with high hardness leads to severe damage of the carbonmatrix during milling. Among them, the Si50C50composite exhibited higher performance,based on which we compared the electrochemical properties under different milling time. Asa result, when the time extends from5h to10h, the discharge capacity at the current density of400μA/cm²can increase as high as274.2mAh/g due to the further refining and dispersingeffect on Si particles; However, when the milling time extends to15h, Si agglomeration andiron contamination appeared in the composites, indicating that there’s critical value of millingtime. Compared to the adding Si content, extending milling time in a fixed scope can betterimprove the performance of Si-C anode composite.The Li diffusion kinetics of the Si50C50composite prepared by the two-step p-millingfor10h has been evaluated. The diffusion coefficient of Li-ion was calculated to be1.25×10^-13cm²/s, which is nine orders higher than that of the pure Si anode. Thus it is evidentthat the combination of Si with C can enormously enhance the kinetics of Li intercalation andde-intercalation of Si-based anode materials. Furthermore, a Si50C50-nano composite wasprepared by the utilization of nano-sized spherical Si particles. It has relative low ion diffusion impedance, and maintains a high reversible capacity of1000mAh/g after100cyclesat the current rate of1C, indicating that the uniform fine morphology of Si is benefit todeliver higher reactivity and make the Li-Si reaction more sufficient.Finally, In an attempt to reduce the initial irreversible capacity loss and enhance thestructural integrity between the Si and the carbon matrix, Cu has been combined to the Si-Ccomposite prepared by DBDP assist milling for the first time. It has been found that the Cushows little modification to pure Si, while however, can react with Si in the Si-C compositeunder high energy milling and form a inactive Cu₉Si phase. The presence of Cu₉Si phasegains a10percent increase in cycle stability and nearly doubles the rate of Li-ion diffusion byenhancing the structural stability of the Si-C-Cu composite.