Understanding the Role of Fe/Mn Ratio on the Structural and Electrochemical Performance of LiMn _x Fe

The recent commercialization of next‐generation blade batteries, as exemplified by BYD’s second‐generation systems, has reaffirmed the relevance of LiFePO ₄ ‐based cathodes for high‐safety lithium‐ion batteries, yet their limited energy density remains a key bottleneck. Partial substitution of Fe with Mn to form LiMn _x Fe _{1− x} PO ₄ is considered an effective strategy to enhance energy density, but is hindered by Jahn–Teller distortion of Mn (III). Although Fe/Mn solid‐solution engineering has been widely explored, considerable inconsistency persists regarding the optimal Fe/Mn ratio, with conflicting reports of 4:6, 5:5, and 6:4. Here, nanocrystalline LiMn _x Fe _{1– x} PO ₄ solid solutions are synthesized by a microwave‐assisted hydrothermal route, and their composition‐dependent electrochemical behavior and phase transition mechanisms are elucidated. A non‐monotonic dependence on Fe/Mn ratio is identified, with the equimolar composition (Fe/Mn = 1:1) delivering the optimal balance of energy density, polarization, and cycling stability. A transition from biphasic to solid‐solution‐like behavior is revealed by in situ X‐ray diffraction at this composition, enabling improved structural stability and reaction kinetics. These results provide a systematic composition‐dependent understanding of how Fe/Mn stoichiometry regulates reaction kinetics and apparent phase‐transition behavior in nanocrystalline LiMn _x Fe _{1− x} PO ₄ /C, offering guidance for the rational design of high‐energy‐density olivine cathodes.