Investigation of the Electron Transfer at Si Electrodes: Impact and Removal of the Native SiO2 Layer

Silicon is considered as one of the promising alternatives to graphite as negative electrode material in lithium-ion batteries. The electron transfer at uncharged microstructured and planar Si was characterized using the feedback mode of scanning electrochemical microscopy (SECM) and 2,5-di-tert-butyl-1,4-dimethoxybenzene as redox mediator. Approach curves and images demonstrate that the electron transfer rate constants at pristine Si are relatively small due to the native SiO2 surface layer. In addition, the electron transfer rate constants show local variations because of the heterogeneous coverage of SiO2. The SiO2 layer is at least partially removed by mechanical contact and abrasion with the microelectrode probe. After SiO2 removal by the microelectrode or by a hydrofluoric acid dip, the electron transfer rate constants increase strongly and remain heterogeneous. Moreover, the surface of the Si electrodes is at least stable over hours after SiO2 removal. The consequences for investigating the formation of the solid electrolyte interphase (SEI) on Si are discussed.