Rationalization of Intercalation Potential and Redox Mechanism for A2Ti3O7 (A = Li, Na)

Na2Ti3O7 was recently reported to be highly promising as a negative electrode material for Na-ion batteries, thanks to its very low intercalation voltage (0.3 V vs Na+/Na-0) and high capacity (200 mAh/g). The investigation of the redox mechanism in A(2)Ti(3)O(7) (A = Li, Na) upon additional alkali ion intercalation concomitant to reduction of titanium is reported in this paper. Even if the low stability of the reduced phases (A(2+x),Ti3O7) precluded a direct study, density functional theory (DFT) calculations allowed us to propose structural models for the reduced phases (A(2+x)Ti(3)O(7)), which were further successfully confronted to the available experimental data. The alkali atoms are octahedrally coordinated in the reduced A(2+x)Ti(3)O(7) phase, so that the whole resulting structure can be considered rocksalt type. The very different potentials at which ion insertion is observed for A = Li or A = Na clearly prove that analogies between lithium and sodium systems cannot be taken for granted. In this case, such findings can be fully rationalized through DFT and arise from the differences in polarizing character between lithium and sodium ions that cause, respectively, contraction and expansion of the cell volume and the destabilization of the inserted materials derived from the larger size of sodium ions.