The electrolyte degradation process on nano Si-based negative electrodes prepared with a carboxymethylcellulose (CMC) binder is studied by comparing the irreversible loss to ex-situ measurements of the weight and the thickness of the electrode along cycling. The electrode thickness and mass increase in close relationship to the irreversible loss increase, due to the continuous accumulation of insoluble electrolyte degradation products in the Si electrode. The use of a pH 3 buffer solution during the slurry electrode preparation, and the presence of fluoroethylene carbonate (FEC) + vinylidene carbonate (VC) results in much less electrolyte decomposition. The double role of the CMC binder with respect to the mechanical and chemical stability of the composite electrode is highlighted.