Phase stability and charge compensation in disordered rock salt compounds based on nickel and titanium

Abstract

Disordered rock salt compounds (DRX) are increasingly investigated as an alternative to established layered oxides, due to the significant compositional flexibility allowing the use of cheap elements, as well as the demonstrated high specific discharge capacities. In this work we investigate DRXs based on Ni as the main redox active element and Ti as a charge compensator and structure stabilizer, according to composition Li_2yTi_yNi_2−3yO₂ (0.50 ≤ y ≤ 0.67). We prepare such materials by solid state synthesis and carefully investigate their stability as single phases by combining theoretical calculations based on density functional theory and in situ X-ray diffraction, elucidating the role of entropy in the synthesis process. By a combination of SEM, ICP-AES and X-ray and neutron diffraction we reveal that the above-mentioned formula indicating a solid solution is only appropriate at high temperatures. When increasing the Li content and lowering the temperature, a secondary Li₂TiO₃-like phase emerges. The electrochemical performances showed a low capacity in the first cycle, which appears to be kinetically limited as it strongly increases at 50 °C. A strong voltage hysteresis is nonetheless present regardless of the temperature and is the main bottleneck limiting energy efficiently in these compounds. By combined operando X-ray diffraction and X-ray absorption near edge structure (XANES) spectroscopy, we demonstrate such hysteresis can be observed both structurally in the unit cell parameters as well as electronically at the Ni K-edge, hence in the Ni oxidation state. Our study confirms the peculiar, coupled behaviour of Ni and O redox in this class of materials, which needs to be overcome if new improved compositions are to be designed. We therefore provide a clear structural analysis of the synthesis and electrochemical behaviour of the Li_2yTi_yNi_2−3yO₂ family of compounds, identifying the main bottlenecks towards their improvement.