Medium‐Entropy Engineering Enhances Na⁺/Electron Transport in Na₃Fe0.1Mn0.2Co0.2Ni0.3V1.2(PO₄)₂F₃@CNTs Cathode for Sodium‐Ion Batteries

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 Na3Fe0.1Mn0.2Co0.2Ni0.3V1.2(PO4)2F3@CNTs (ME-NV1.2PF@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.