Understanding Na plus ion diffusion in 1 T-MO2 (M = Mn, Fe, and Ni) via potential energy surface calculation

The cooperative Jahn-Teller effect (CJTE) plays a very important role in the study of sodium ion batteries with layered transition metal oxides 1 T-MO2 (M = Mn, Fe, and Ni) as electrodes. We have used the potential energy surface (PES) approach to describe the diffusion process of Na+ ion in these electrode materials. The successive distorting-and-hopping process was identified. For pure electrodes, the diffusion barrier of Na+ ion in FeO2 is smaller than MnO2 and NiO2. We have also investigated the diffusion mechanism of Na+ in the substitutional doped system. In MnO2, the Jahn-Teller (JT) effect of Mn3+ is modulated by the substitution of Ni and Co, and the diffusion barrier increases. In FeO2, the JT effect of Fe4+ is suppressed by the substitution of Co, Mn and Ni, and the diffusion barriers are reduced. We find that the substitutional doping of transition metal ions is an effective way to modulate the cooperative Jahn-Teller effect which helps to improve the batteries' cycling sta-bility or rate performance.