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 Batteri

Sodium vanadium fluorophosphate (Na₃V₂(PO₄)₂F₃, 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 Na₃Fe_0.1Mn_0.2Co_0.2Ni_0.3V_1.2(PO₄)₂F₃@CNTs (ME-NV_1.2PF@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 (1.2R) 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 cm² 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.