Unraveling Phase Transition Dynamics in Carbon-Modulated Synthesis of LMFP Cathodes from Hydrated Phosphate Precursors

Conventional LiFePO4 cathodes face limitations due to their low operating voltage (3.4 V) and discharge capacity. Partial Mn substitution in LiMn0.6Fe0.4PO4 (LMFP) elevates the discharge potential to 4.1 V, yet challenges such as Mn dissolution and structural degradation persist during charge/discharge cycling. Traditional solid-state approaches often produce inhomogeneous phases and impurities, while hydrated phosphate precursors synthesized via coprecipitation offer a promising alternative─though their thermal evolution remains poorly characterized. This study deciphers the phase transformation mechanisms during LMFP synthesis from Mn0.6Fe0.4PO4·0.25H2O precursors, employing in situ heating X-ray diffraction (XRD) to map structural evolution under thermal treatment. Carbon integration promotes structural decoupling, enabling efficient Li+ insertion into the olivine lattice. Contrasting with the conventional single-step process, the gradient-sintered (multistage heating) based on the phase transition mechanism enhances phase purity, suppresses impurities, and improves crystallinity by optimizing intermediate phase transitions. Electrochemical evaluations confirm that gradient-sintered LMFP delivers a higher specific capacity than the non-gradient counterparts. These findings establish a paradigm for controlled reaction pathways in LMFP synthesis, emphasizing the critical role of staged thermal treatment and carbon mediation in minimizing parasitic reactions. This work advances scalable strategies for high-performance manganese-stable lithium-ion battery cathodes.