In situTEM observation of lithiation of two smallc-Si nanoparticles without fracture.^[17](a) Two pristinec-Si nanoparticles, marked by “A” and “B” with diameters of 80 and 40 nm, respectively. There is a twin boundary (TB) in the center of particle A. (b) A partially lithiated state with a core–shell, two-phase microstructure in the two nanoparticles. (c) Neither cracking nor fracture occur in the two Si nanoparticles after full lithiation, and but both nanoparticles have ∼ 280% volume expansion.