Medium‐Entropy Engineering Enhances Na⁺/Electron Transport in Na ₃ Fe _0.1 Mn _0.2 Co _0.2 Ni _0.3 V _1.2 (PO ₄ ) ₂ F ₃ @CNTs Cathode for Sodium‐Ion Batteries

Abstract Sodium vanadium fluorophosphate (Na ₃ V ₂ (PO ₄ ) ₂ F ₃ , NVPF), a promising cathode material for sodium‐ion batteries, exhibits high energy density and a stable voltage plateau, yet its practical application is hindered by intrinsic low electronic conductivity. Here, a medium‐entropy engineering strategy is introduced to address this limitation by developing a novel Na ₃ Fe _0.1 Mn _0.2 Co _0.2 Ni _0.3 V _1.2 (PO ₄ ) ₂ F ₃ @CNTs (ME‐NV _1.2 PF@CNTs) composite. The medium‐entropy design synergistically optimizes structural stability and charge transport kinetics, while carbon nanotubes (CNTs) coating enhances surface conductivity. Systematic investigation reveals a parabolic relationship between electrochemical performance and entropy, with the optimal entropy value (1.2 R ) delivering a discharge capacity of 120 mAh g ⁻¹ at 0.1 C and remarkable cycling stability (60% capacity retention after 3000 cycles at 5 C ). Structural characterization demonstrates a uniform granular morphology with surface nanopores, facilitating rapid Na ⁺ diffusion. Kinetic analysis and theoretical calculations confirm that entropy‐induced lattice distortion and CNT networks synergistically accelerate Na⁺/electron transport (diffusion coefficient: ≈10 ⁻¹⁰ cm ² s ⁻¹ ) and ensure highly reversible redox reactions. In situ XRD further elucidates a dual‐phase reaction mechanism involving both single‐phase and two‐phase transitions during (de)sodiation. This work provides fundamental insights into entropy‐performance correlations and demonstrates the superiority of medium‐entropy materials in overcoming intrinsic limitations of polyanionic cathodes.