Published online by Cambridge University Press: 20 March 2015
Na2Ti3O7, a potential negative electrode for Na batteries, is investigated by combining experiments and first-principles calculations at the Density Functional Theory (DFT) level. A structural model is proposed for the reduced phases (A2+xTi3O7), with all alkali ions in octahedral coordination, leading to a distorted rocksalt type structure. The calculated elastic constants support the mechanical stability of the proposed Na4Ti3O7 structure. Calculated average intercalation potentials are 0.37 V for Na insertion in Na2Ti3O7 and 1.46 V for Li insertion in Li2Ti3O7, being in very good agreement with the values observed experimentally (0.3 V and 1.6 V respectively). The higher polarizing character of Li ions vs Na ions acts as a key-factor to bring the Li intercalation voltage 0.7 V above that of Na intercalation in layered-A2Ti3O7 materials.