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.