Redox Behavior in Li Insertion Materials Based on a Hypothetical Reaction Model Using the DV-Xα Method

The redox behavior of the lithium insertion materials used as electrodes in lithium-ion batteries (LIBs) is complicated and poorly understood because both transition metal and oxide ions can act as redox-active species. Therefore, this study used discrete variational (DV)-Xα calculations to systematically investigate the electronic structure and redox behavior of lithium insertion materials with olivine, layered, and spinel structures. The analysis used a hypothetical reaction pathway that assumed no structural changes occurred during Li insertion/extraction, which allowed diverse materials to be compared consistently. Mulliken population and bond overlap analyses revealed that redox reactions mainly occur at the transition metal ions in olivine materials, whereas oxide ions serve as the main redox centers in layered and spinel materials. Furthermore, the electrode potential of the lithium insertion materials correlated with changes in the electronic states, particularly changes in the net charges of the transition metal and oxide ions. The results highlight the importance of both transition metals and oxide ions in determining the redox potential. Moreover, they provide insight into the origin of the electrode potential of solid-state redox reactions in lithium insertion materials. This study offers a unified framework for predicting redox-active species and understanding potential trends based on electronic structures.