Effect of transition metal cations on the local structure and lithium transport in disordered rock-salt oxides

Abstract

Lithium-excess oxides Li_1.2Ti_0.4Mn_0.4O₂ and Li_1.3Nb_0.3Mn_0.4O₂ with a disordered rock-salt structure and Mn³⁺/Mn⁴⁺ as a redox couple were compared to analyze the effect of different d⁰ metal ions on the local structure and Li⁺ ion migration. These cathode materials were obtained by mechanochemically assisted solid-state synthesis. Using XRD, ⁷Li NMR and EPR spectroscopy and transmission electron microscopy it was shown that the Mn ions are prone to form clusters, while d⁰ metal ions are evenly distributed in the crystal lattice. The presence of Nb⁵⁺ ions contributes to the formation of noticeably larger Mn clusters and larger gaps in the Li⁺ migration maps as compared to Ti⁴⁺. These results were confirmed by the geometrical-topological method, BVSE simulation and DFT calculations, and are in good agreement with the Li diffusion coefficient determined by GITT, which is 1.5 orders of magnitude higher in Li_1.2Ti_0.4Mn_0.4O₂ than that in Li_1.3Nb_0.3Mn_0.4O₂.