Medium-Entropy Engineering and 3D Carbon Network Synergy in Na ₃ V _1.4 Fe _0.1 Mn _0.2 Cr _0.2 Zr _0.1 (PO ₄ ) ₂ O ₂ F@C for High-Performance Sodium-Ion Batteries

This study introduces ME-Na 3 V 1.4 Fe 0.1 Mn 0.2 Cr 0.2 Zr 0.1 (PO 4 ) 2 O 2 F@C (ME-NVOPF 1.01@C), a medium-entropy engineered sodium-ion battery (SIB) cathode material, addressing low conductivity of NASICON-type Na 3 V 2 (PO 4 ) 2 O 2 F. Medium-entropy doping regulates the energy band structure and vanadium coordination, enhancing electronic conductivity and enabling a nonphase-change reaction with minimal volume change during Na + insertion/extraction. Density functional theory calculations demonstrate that Na + migration barriers and a narrowed bandgap were reduced, improving redox reversibility. The 3D carbon network further boosts the conductivity and sodium storage kinetics, achieving a Na + diffusion coefficient of ∼10 –11 cm 2 ·s –1 . ME-NVOPF 1.01@C delivers 97.2 mAh·g –1 at 0.1 C, with 72% capacity retention after 4500 cycles at 5 C, confirming exceptional cycling stability. This work elucidates the entropy-performance correlation and highlights the superiority of medium-entropy materials in overcoming the inherent limitations of polyanion cathodes, providing a strategic pathway for designing higher electrochemical performance sodium-ion battery materials.