Synergistic Bulk and Interface Modulation of LiMn 0.6 Fe 0.4 PO ₄ Cathodes through Mg ²⁺ Gradient Insertion for Enhanced Li ⁺ Diffusion

Olivine-type LiMnxFe1–xPO4 (LMFP) is a promising high-voltage cathode for next-generation lithium-ion batteries; however, its commercialization is hindered by structural instability from Mn3+ Jahn–Teller distortion and sluggish ionic kinetics. Herein, we develop a novel gradient-insertion strategy to address these limitations simultaneously. Through a facile solid-state process, Mg2+ is incorporated into LMFP microspheres, creating a concentration profile. Structural characterization confirms the successful incorporation of Mg2+, predominantly occupying the transition-metal sites, without altering the primary structure or forming a coating layer. This targeted modification significantly improves electrochemical performance: the Mg-insertion cathode (LMFP-Mg) delivers a high capacity of 149.78 mAh g–1 at 0.1 C, with the Mn redox contribution increasing significantly from 40.3% to 53.2%. After 300 cycles at 1 C, LMFP-Mg retains 91.2% of its capacity, significantly outperforming pristine LMFP (80.5%). At a high rate of 5 C, LMFP-Mg demonstrates a 26.4% improvement in discharge capacity compared to LMFP. Mechanistic studies reveal that gradient Mg2+ insertion cooperatively stabilizes the bulk lattice, suppresses Mn3+ Jahn–Teller distortion, and enhances Li+ diffusion coefficient, while the Mg-rich surface mitigates transition-metal dissolution and interfacial side reactions. This work demonstrates that spatially controlled insertion is a promising strategy for designing high-energy-density, long-life olivine cathodes.