Elucidating the Surface Reactions of an Amorphous Si Thin Film as a Model Electrode for Li-Ion Batteries

We investigated during the first lithiation/delithiation process the electrochemical reaction mechanisms at the surface of 30 nm n-doped amorphous silicon (a-Si) thin film used as a negative model electrode for Li-ion batteries. Usage of thin film allowed us to accurately discern the different reaction mechanisms occurring :at the Surface by avoiding interference from carbon and binder components. The potential dependency of the evolution of the solid electrolyte interphase (SEI) and the reactions on the a-Si and on the copper current collector were elucidated by coupling galvanostatic cycling with postmortem X-ray photoemission spectroscopy and scanning electron microscopy analyses. Our approach revealed the clear reversibility of lithiation/delithiation in the a-Si-and native SiO2 layers; such a reaction for SiO2 has not been previously detected and was considered to be an irreversible process. Quantitative and qualitative analyses of the potential-dependent surface evolution revealed the decomposition products of both the salt (LiPF6) and solvent (dimethyl carbonate/ethylene carbonate)) :giving insight into the complex SEI formation mechanism on the a-Si film but also underlining the strong influence of inert materials such as the role of the current collector in the irreversible charge loss. A model mechanism describing the evolutionary complexity of the a-Si surface during the first galvanostatic cycle is proposed and discussed.