The influence of the Si particle size (85 nm versus 230 nm) on the mechanical stability of composite Si/C/carboxymethyl cellulose (CMC) electrodes is evaluated from in-operando dilatometry and acoustic emission measurements. A lower, more progressive and more reversible expansion/contraction of the electrode is observed with the Si 230 nm powder, with a maximum expansion of ∼140% and a residual irreversible expansion of ∼25% measured during the first cycle compared to ∼350% and ∼90% for the Si 85 nm based electrode. Moreover, during the 2nd cycle, an abrupt and very large expansion/contraction (up to ∼400%) is observed for the Si 85 nm based electrode, which results in the irreversible cracking and exfoliation of the electrode as confirmed by post-mortem scanning electron microscopy observations. This is also in accordance with the more intensive acoustic activity measured during the Si 85 nm electrode cycling. The lower mechanical strength of the Si 85 nm electrode is interpreted as the consequence of an insufficient amount of CMC binder relative to the larger specific surface area of the Si 85 nm powder. This tends to be confirmed by the significant improvement of its electrochemical cycling performance as its CMC content is increased.