The changes in net charge during the redox reaction indicated that the transition metal serves as the redox center in olivine materials and aqua-complexes, whereas the O atom serves as the redox center in oxide-based layered and spinel materials. A correlation exists between the redox center and electrode potential of these materials.

Abstract

Li-insertion materials employed as electrode materials in Li-ion batteries undergo solid-state redox reactions wherein ions within a solid matrix are oxidized and reduced, in contrast to the conventional redox reactions of ions in solution. However, owing to the lack of a comprehensive theory for solid-state redox reactions, the electrode potential of Li-insertion materials remains unexplained from a theoretical standpoint. This limitation impedes the rational design of positive and negative electrodes with higher and lower potentials, respectively. This study employs the DV-Xα method to calculate the electronic structures of various Li-insertion materials and transition-metal aqua-complexes associated with the redox reaction to shed light on the corresponding solid-state redox potentials. Notably, the transition-metal ion is identified as the redox center in olivine materials and aqua-complexes, which exhibit similar electrode potentials, whereas the oxide ion is identified as the redox center in layered and spinel oxide materials, which show significant differences in electrode potential compared with olivine materials. These findings imply a correlation between the electrode potential and redox center in Li-insertion materials. The results of this study reveal that the electrode potential of Li-insertion materials is determined by their redox center rather than their constituent elements.