We investigate chemical and structural modifications occurring in homogeneous crystalline Si nanoparticles (NPs) used as anode material in Li cells. We exploit state‐of‐the‐art high‐resolution scanning transmission electron microscopy coupled with electron energy loss spectroscopy resolved at the nanoscale. We directly highlight by electron spectro‐microscopy that, above 0.1 V vs Li, the electrochemical activity of Si electrodes involves a complex interplay between Li incorporation, electrolyte degradation and Si reduction/oxidation. These redox processes occur upon cycling through partially reversible reactions mediated by the solid electrolyte interphase. Overall, a SiO2 amorphous layer forms in the oxidized electrodes at the Si NPs interface with the electrolyte: this oxide shell partially dissolves upon reduction to give Li2CO3 and amorphous Si. Si NPs cores are therefore eroded upon cycling as their outer …