Synergistic V−Mg Co-Doping for a High-Manganese LiMn0.9Fe0.1PO4 Cathode with Superior Cycling Stability and Rate Capability

High-manganese lithium manganese iron phosphate (LiMn0.9Fe0.1PO4, 9Mn-LMFP) offers a high-voltage platform for high-energy-density Li-ion batteries but suffers from poor electronic conductivity, Mn dissolution, and structural degradation. Herein, we report a V-Mg co-doping strategy via one-step ball milling combined with solid-state sintering to address these challenges. V3+ incorporation narrows the band gap from 3.39 to 1.34 eV and promotes a uniform and dense carbon coating on the particle surface. Mg2+ stabilizes the crystal framework, as evidenced by shortened Mn-O bonds and reduced Mn dissolution (from 6.128 to 3.139 ppm). Furthermore, V-Mg co-doping expands the solid-solution region (from 44.1 to 48.1% state of charge) while shrinking the two-phase region, indicating a lowered phase-transformation barrier. Consequently, the 9Mn-LMFP-V-Mg∥Li half-cells exhibit optimal electrochemical performance with a discharge capacity of 157.0 mAh g-1 at 0.1C, 123.7 mAh g-1 at 10C, and 89.7% capacity retention after 400 cycles at 1C, significantly outperforming the undoped counterpart (with a capacity retention of only 58.2%). When paired with the Li4Ti5O12 anode, 9Mn-LMFP-V-Mg delivers 118.6 mAh g-1 at 2C with 82.5% retention after 100 cycles. Kinetic analyses reveal reduced charge transfer resistance, enhanced Li+ diffusivity (approximately one order of magnitude higher), and increased pseudocapacitive contribution, leading to minimized polarization and efficient capacity release under high rates. This work provides a viable strategy for developing high-energy-density, long-life olivine cathodes.