(592h) Investigations on the Effect of Current Density on the Performance of Si@SiOx Anode

Silicon with a surficial oxide layer is one of the most promising anode materials with high capacity and excellent durability. A thick surficial oxide layer could greatly improve the cycle life, but also significantly reduce the capacity of Si anodes. In this study, silicon nanoparticles with a thick surficial oxide layer of ~18 nm (Si@SiO_x) were fabricated through the thermal oxidation and characterized as anodes of lithium-ion batteries. The results from the electrochemical characterization revealed that the current density applied in the initial plays a crucial role in the behavior of Si@SiO_x anodes and, there exists a threshold value for the current density, below which the Si@SiO_x anode could deliver an unprecedently high capacity (~1832 mAh/g) in the initial discharge process. In further, the low current density applied in the initial cycle also has an “activating” effect, which enables the utilization of Si phase wrapped in the oxide layer and cell with high capacity of >700 mAh/g during the following cycles, even at higher current densities. The results from the electrochemical impedance spectroscopy (EIS) showed that the charge-transfer resistance of the cell applied with a low current density, was much lower than that with high current density in the initial cycle, indicating higher degree of the lithiation of the surficial oxide layer in the presence of low current density. The analysis on EIS also revealed that the cell applied with low current density possessed almost identical and low resistance at the end of discharge and charge processes in the initial cycle. This implies that the lithiation of surficial oxide layer is an irreversible process. Thus, the existence of threshold value and “activating” effect of the current density applied in the initial cycle are associated with the formation of lithium silicate with high conductivity.