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

The transition-metal ions in a solid matrix are oxidised and reduced via a solid-state redox reaction during the charge/discharge process of lithium insertion materials, which are commonly used as positive and negative electrodes in lithium-ion batteries. Therefore, the electrode potentials of lithium insertion materials should be different from the redox potentials of transition-metal ions in aqueous solution (i.e., the standard electrode potential). In this study, the solid-state redox potentials of the transition-metal ions in polyanionic materials with three distinct structures (i.e., olivine, NASICON-type, and MOXO4-type structures, where M = 3d transition-metal ion, and X = P or S) were surveyed to understand the electrode potentials of lithium insertion materials. The redox potentials of the transition-metal ions in polyanionic materials were very similar to those in aqueous solution despite the differences between the environments of these ions in the MO6 octahedron in polyanionic materials and the aqua complexes of [M(H2O)6]n+ in aqueous solutions. The high coefficient of determination (R2 ≈ 0.990) of these two potentials indicated that the solid-state redox potential for the lithium insertion reaction in polyanionic materials can be estimated using the standard electrode potential of the corresponding transition-metal ion in aqueous solution. Finally, the similarity between the redox potentials of the transition-metal ions in polyanionic materials and those in aqua complexes is discussed from the thermodynamic perspective. The present findings on the similarity of the redox potentials of transition-metal ions in different media could provide useful insights into the design of novel insertion materials for rechargeable batteries based on lithium, sodium, potassium, and magnesium, among other metals.