Medium Entropy Tuning Improved Multiple Electron Redox in Polyanion Cathode for High-Rate Sodium-Ion Battery
Sodium vanadium phosphate (Na3V2(PO4)3, NVP) with NASICON structure is a promising cathode for sodium-ion batteries but suffers from low electronic conductivity and a high energy barrier for the V4+/V5+ redox couple, limiting practical energy density. A medium-entropy tuning strategy yields the multi-element substituted Na3.2V1.5Cr0.1Fe0.1Mn0.1Ni0.1Ti0.1(PO4)3 (ME-NVP). Entropy modulation tailors the microscopic electronic structure, enabling reversible V4+/V5+ redox at 4.0 V. Analyses reveal a synergistic diffusion mechanism that accelerates Na+ transport and enhances multiple-electron redox kinetics. Ex-situ X-ray diffraction confirms highly reversible structural evolution during cycling. The ME-NVP cathode delivers 116.8 mAh g-1 at 0.1C and retains 83.9% of initial capacity after 1000 cycles at 20C, with excellent performance from -12 to 50 °C. This work demonstrates that configurational entropy regulation unlocks high-energy polyanion cathodes for advanced sodium-ion batteries.