Effect of Surface Oxide Layer to Si-based Materials for Li-ion Batteries

Abstract

The global energy demand is continually increasing, and it has been accelerated recently by the depletion of fossil fuels and by climate change caused by human activities. Hence, energy devices such as solar cells, fuel cells, rechargeable batteries, and supercapacitors have become increasingly important for energy harvesting, conversion and storage. Among them, Li-ion batteries have been the leading rechargeable battery and with the rapid development of mobile devices and EVs, their market has continued to expand. Currently, graphite (LiC6: 372 mAh g-1) is used as an anode material in rechargeable Li-ion batteries. To meet the requirements such as high capacity, high power, and stable cycle performance, many researchers have focused their attention on Si-based materials (Si,SiO and SiO2). However, the main problem associated with the use of Li-alloys in rechargeable batteries is the large volume change during alloying-dealloying with Li, resulting in cracking and fracturing of active materials, causing deterioration of the anodes. Also another important factor is silicon oxide layer on surface of Si-based materials because a thick oxide layer hindered the Li–ion diffusion, reacting with active material. In this work, we investigated the oxide layer effect to Si-based materials (Si and SiO) for Li-ion batteries. The oxide layer of Si and SiO material was confirmed using HRTEM and XPS, respectively. And the oxide layer was etched by NaOH solution and etched samples were tested as an anode. As the oxide layer of Si and SiO reduced, the electrochemical performance was also significantly enhanced. Also, the self-limiting reaction behavior of nano-Si with oxide layer and a new mechanism of SiO with Li during first cycle were proposed. Finally, a new concept for the preparation of porous SiOx was suggested adopting Si as a pore generating agent and Si oxides as template using NaOH solution. The porous SiOx was tested as an anode for Li-ion batteries, and it showed excellent electrochemical performance without any carbon coating. These studies of oxide layer could provide basic information when the Si-based materials were used as an anode for Li-ion batteries.