New Insights into the Passivation Behavior of Silicon Electrodes in Silicon-Air Batteries with Alkaline Electrolytes

Abstract

The chemical etching of silicon has been investigated thoroughly during the past 40 years to achieve a fast and controlled process for texturing surfaces. Nonetheless, still little is known about the electrochemical reaction of silicon with KOH for the use in batteries. The high theoretical capacity density of 3817 mAh cm-2 and potential of 2.09 V make the silicon-air system a promising candidate for energy storage. However, a crucial constraint to the use of an aqueous alkaline electrolyte, such as KOH, is related to the fact that the system discharges only once. Overpotentials reduce the practical potential to 1.4 V, and the competing chemical corrosion of the silicon leads to a strong self-discharge, resulting in a low conversion efficiency. The discharge ends with a sudden potential drop, due to the passivation of the silicon surface before the complete consumption of the electrode. We believe that understanding the mechanisms behind that potential drop and the processes that lead to it are essential to the further improvement of this silicon-air battery. Therefore, we studied cells with different KOH concentrations and with flat phosphor-doped silicon (100) wafers as the Si-electrodes. To investigate the potential drop and increase the conversion efficiency of silicon electrodes, we performed electrochemical measurements in a full cell setup. Galvanostatic discharge tests revealed that the ratio of electrochemical to chemical reaction is influenced by the KOH concentration when the battery is discharged till the passivation. While the highest etching rate for silicon in KOH solution reaches its maximum around 6 mol L-1 KOH, the electrochemical reaction seems to have a peak at 2 mol L-1 KOH in our experimental conditions. We compared the performance of cells with various concentrations of pre-dissolved silicon in KOH and we found a correlation between the silicon content in the electrolyte and the time before the potential drop occurred. The conductivity of the solution decreases with increasing silicon content, probably as a result of the presence of a network of silicate species, which might hinder the diffusion of the Si(OH)4 species. To prove this hypothesis, we tested different volumes of the electrolyte with the same cell parameters. The use of an excess of electrolyte solution enabled the complete consumption of the 500 μm Si-electrode in any KOH concentration used. During these experiments, the silicate species do not reach the critical concentration, and the reaction products can dissolve into the bulk electrolyte, not leading to the formation of a passivation layer. Figure 1