Synergistic Mn-Cr Co-doping in Na3V2(PO4)2O2F Promising Cathode: Unlocking Superior Performance for Next-Generation Sodium-Ion Batteries

Sodium vanadium fluorophosphate (Na₃V₂O₂(PO₄)₂F, NVOPF), with a NASICON framework, is a promising cathode material due to its robust 3D structure, high operating potential (∼3.8 V), and theoretical energy density (≈494 Wh kg⁻¹). However, its commercial viability is limited by low electronic conductivity and a reduced practical energy density. To address these limitations, vanadium in NVOPF is partially substituted with cost-effective Mn and Cr via a one-pot solvothermal method. This co-doping induces lattice distortion, enhances Na⁺ diffusion kinetics, and improves ionic/electronic conductivity, as confirmed by DFT calculations. The optimized Na₃V_1.9Mn_0.095Cr_0.005O₂(PO₄)₂F (NVMC-95) cathode delivers an initial discharge capacity of 120 mAh g⁻¹ at 0.1 C and 87 mAh g⁻¹ at 20 C, with 94% capacity retention after 500 cycles at 1 C and 76% after 2000 cycles at 5 C. The co-doped NVOPF exhibits excellent thermal stability at 40 °C, retaining 91% of its capacity over 400 cycles at 2 C. In a full-cell configuration (NVMC-95//hard carbon), the system delivers 110 mAh g⁻¹ at 3.75 V, retaining 97% capacity over 100 cycles at 0.2 C. Mn/Cr co-doping synergy in Na₃V₂O₂(PO₄)₂F enhances Na⁺ transport, reduces impedance, accelerates diffusion kinetics, and stabilizes cycling, enabling durable NASICON-type cathodes with extended cycle life for practical applications.