Understanding the Origin of the Redox Potential Shift of Transition Metal in LiFexMn1–xPO4 Cathodes by the Molecular Orbital Theory

Abstract

Olivine-structured LiFe_xMn_1–xPO₄ cathodes exhibiting higher redox potentials than their layer oxide counterparts have been utilized in commercial lithium-ion batteries, but the origin of the systematical shifts of the redox potential of transition metal couples with the variation of the Fe–Mn molar ratio is not clear, at least on the electronic scale. In the current work, we carried out experiments and theoretical calculations to study the molecular orbital characteristics of metal–ligand and determined the origin of transition metal redox potential shifts in LiFe_1–xMn_xPO₄ cathodes on the electronic scale. The systematic shifts of redox potential of Fe³⁺/Fe²⁺ and Mn³⁺/Mn²⁺ couples in LiFe_1–xMn_xPO₄ cathodes are not only because of the decreased energies of e_g* antibonding orbitals with regard to the enlarged metal–ligand atomic distances but also due to almost the same slopes of the e_g* antibonding orbital energies as a function of atomic distance. This chemistry picture of the metal–ligand atomic distance-dependent e_g bonding/e_g* antibonding splitting provides a new perspective to understand the redox potential variations of the electrode upon element substitution.