The spinel Li4Ti5O12, a stable phase of the Li2O–TiO2 system, allows to insert three Li atoms per formula unit at a potential of 1.5V on the basis of a spinel ↔ NaCl phase transition. This mechanism leads to a reduction of three Ti(IV) atoms out of five, corresponding to a theoretical capacity of 175mAh/g. The influence of structural defaults on the spinel → NaCl phase transition and its reversibility during charge/discharge cycles have been studied. Solid solutions formed from chemical insertion of lithium or substitutions Ti/V, Ti/Mn, Ti/Fe modify the cation distribution on the crystallographic sites (tetrahedral 8a, octahedral 16d, space group Fd3m) and influence the electrochemical performances. A structural analysis by X-ray and neutron diffraction, X-ray absorption, 57 Fe Mössbauer spectroscopy and first principle calculations have allowed to establish a relationship between the structure and the electrochemical properties.