The irreversible mechanisms of lithium insertion in amorphous tin composite oxides SnB0.6P0.4O2.9 have been studied by X-ray diffraction (XRD) and 119 Sn Mössbauer spectroscopy. The determination of the Lamb–Mössbauer factor has allowed us to evaluate the relative numbers of different tin atoms (SnII, Sn0). We show that insertion of lithium reduces the SnII into Sn0 atoms, which form nanoparticles of active species. The lithium ions act as glass modifiers, breaking the bonds within MOM′ (M, M′=B, P, Sn) bridges and forming non-bridging MO δ− bonds.