Solid Electrolyte Interphase Layer Formation during Lithiation of Single-Crystal Silicon Electrodes with a Protective Aluminum Oxide Coating.

The lithiation of crystalline silicon was studied over several cycles using operando neutron reflectometry over six cycles. A thin layer of aluminum oxide was employed as an artificial coating on the silicon to suppress the solid electrolyte interphase (SEI) layer-related aging effects. Initially, the artificial SEI prevented side effects but led to increased lithium trapping. This layer degraded after two cycles, followed by side reactions, which decrease the coulombic efficiency. No hint for electrode fracturization was found even though the lithiation depth exceeded 1 μm. Two distinct zones with high and low lithium concentrations were found, initially separated by a sharp interface, which broadens with cycling. The correlation of the reflectometry results with the electrochemical current showed the lithium fraction that is lithiated in the silicon and the lithium consumed in side reactions. Also, neutron reflectometry was used to quantify the amount of lithium that remained inside of the silicon. Additional electrochemical impedance spectroscopy was used to gain insights into the electrical properties of the sample via fitting to an equivalent circuit.