Activating Reversible V4+/V5+ Redox Couple in NASICON‐Type Phosphate Cathodes by High Entropy Substitution for Sodium‐Ion Batteries

Abstract Vanadium‐based Na superionic conductor (NASICON) type materials (Na x VM(PO 4 ) 3 , M = transition metals) have attracted extensive attention when used as sodium‐ion batteries (SIBs) cathodes due to their stable structures and large Na + diffusion channels. However, the materials have poor electrical conductivity and mediocre energy density, which hinder their practical applications. Activating the V 4+ /V 5+ redox couple (V 4+ /V 5+ ≈4.1 V vs Na + /Na) is an effective way to elevate the energy density of SIBs, whereas the irreversible phase transition of V 4+ /V 5+ and severe structural distortion will inevitably result in fast capacity fading and unsatisfactory rate capability. Herein, a high entropy regulation strategy is proposed to optimize the detailed crystal structure and improve the reversibility of crystalline phase transformation and electrical conductivity of the material. With the activated reversible V 4+ /V 5+ redox couple, stable structure, and fast electrochemical kinetics, the high entropy material Na 3.2 V 1.5 Fe 0.1 Al 0.1 Cr 0.1 Mn 0.1 Cu 0.1 (PO 4 ) 3 (NVMP‐HE) exhibits an outstanding electrochemical performance with highly reversible specific capacity of 120.1 mAh g −1 at 0.1 C and excellent cycling stability (92.4% retention after 1000 cycles at 20 C). Besides, the in situ X‐ray diffraction (XRD) measurement reveals that a smooth three‐phase transition reaction is involved in this high‐entropy cathode and the existence of mesophase facilitates a fast phase transition.