Clarifying the microscopic origin of Mn³⁺ ion instability in cathode oxides

The pronounced mobility of Mn³⁺ ions in oxygen sublattice critically limits the utilization of Mn³⁺/Mn⁴⁺ redox couples for cathode materials. While Mn³⁺ instability has historically long been associated with their inherent Jahn-Teller (JT) effect, the microstructural features and electronic-state evolutions underlying the Mn migration remain insufficiently understood, hindering the development of effective Mn stabilization strategies. Here, we demonstrate that the Mn³⁺ site instability is not an inherent property of the JT effect but closely depends on their local coordination environment. Using spinel LiMn₂O₄ as a research model, we experimentally demonstrate that Mn migration induced by coordination instability preferentially occurs within the 0–50% SOC, where both Li vacancies and a high concentration of Mn³⁺ ions coexist. Under these structural conditions, weakly hybridized oxygen orbitals aligned with elongated Mn³⁺–O bonds act as electronic donors, stabilizing the linear Mn–O–Mn configuration in the degraded state. This electronic stabilization reduces the energetic penalty for Mn migration, thereby uncovering the microscopic origin of site instability that drives Mn³⁺ migration.