Similarity between the redox potentials of 3d transition-metal ions in polyanionic insertion materials and aqueous solutions

Abstract

The redox potentials of the transition-metal ions in polyanionic materials with three distinct structures (olivine, NASICON, and MOXO4-type structures) were very similar to those in aqueous solution (aqua-complexes of [M(H2O)6]n+).

Quantum Chemical Analysis of the Correlation between Electrode Potential and Redox Center of Li‐Insertion Materials: Olivine, Layered and Spinel Structures, and Aqua‐Complexes

“…As regards the electrode potential in solid‐state redox reactions, our laboratory has discovered two unexpected phenomena related to the crystal structure of Li‐insertion materials and their electrode potential. The first phenomenon is that the electrode potentials of polyanion materials (e. g. olivine, NASICON, and MOXO 4 ) are highly similar to the standard electrode potentials of their transition‐metal ions (i. e. transition‐metal aqua‐complexes in aqueous solutions) [7] . This finding means that the thermodynamic properties of the corresponding redox reactions are similar despite differences in the local environments of transition‐metal ions in solids and aqueous solutions.…”

“…The first phenomenon is that the electrode potentials of polyanion materials (e. g. olivine, NASICON, and MOXO 4 ) are highly similar to the standard electrode potentials of their transition-metal ions (i. e. transition-metal aqua-complexes in aqueous solutions). [7] This finding means that the thermodynamic properties of the corresponding redox reactions are similar despite differences in the local environments of transition-metal ions in solids and aqueous solutions. The second phenomenon is that the electrode potential of olivine materials is higher than that of oxide-based materials, although the M 2 + /3 + solid-state redox reaction occurs in olivine materials [8][9][10] in contrast to M 3 + /4 + for layered and spinel materials.…”