Understanding the Origin of the Redox Potential Shift of Transition Metal in LiFe_<i>x</i>Mn_1–<i>x</i>PO₄ Cathodes by the Molecular Orbital Theory

Olivine-structured LiFexMn1–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 LiFe1–xMnxPO4 cathodes on the electronic scale. The systematic shifts of redox potential of Fe3+/Fe2+ and Mn3+/Mn2+ couples in LiFe1–xMnxPO4 cathodes are not only because of the decreased energies of eg* antibonding orbitals with regard to the enlarged metal–ligand atomic distances but also due to almost the same slopes of the eg* antibonding orbital energies as a function of atomic distance. This chemistry picture of the metal–ligand atomic distance-dependent eg bonding/eg* antibonding splitting provides a new perspective to understand the redox potential variations of the electrode upon element substitution.