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 LiMn_0.6Fe_0.4PO₄ (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 Mn_0.6Fe_0.4PO₄·0.25H₂O 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.