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

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] 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.