Understanding the\nOrigin of the Redox Potential Shift\nof Transition Metal in LiFe<sub><i>x</i></sub>Mn<sub>1–<i>x</i></sub>PO<sub>4</sub> Cathodes by the Molecular Orbital\nTheory

Olivine-structured LiFe<sub><i>x</i></sub>Mn<sub>1–<i>x</i></sub>PO<sub>4</sub> cathodes exhibiting\nhigher redox potentials\nthan their layer oxide counterparts have been utilized in commercial\nlithium-ion batteries, but the origin of the systematical shifts of\nthe redox potential of transition metal couples with the variation\nof the Fe–Mn molar ratio is not clear, at least on the electronic\nscale. In the current work, we carried out experiments and theoretical\ncalculations to study the molecular orbital characteristics of metal–ligand\nand determined the origin of transition metal redox potential shifts\nin LiFe<sub>1–<i>x</i></sub>Mn<sub><i>x</i></sub>PO<sub>4</sub> cathodes on the electronic scale. The systematic\nshifts of redox potential of Fe<sup>3+</sup>/Fe<sup>2+</sup> and Mn<sup>3+</sup>/Mn<sup>2+</sup> couples in LiFe<sub>1–<i>x</i></sub>Mn<sub><i>x</i></sub>PO<sub>4</sub> cathodes are\nnot only because of the decreased energies of e<sub>g</sub>* antibonding\norbitals with regard to the enlarged metal–ligand atomic distances\nbut also due to almost the same slopes of the e<sub>g</sub>* antibonding\norbital energies as a function of atomic distance. This chemistry\npicture of the metal–ligand atomic distance-dependent e<sub>g</sub> bonding/e<sub>g</sub>* antibonding splitting provides a new\nperspective to understand the redox potential variations of the electrode\nupon element substitution.