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_xVM(PO₄)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 V⁴⁺/V⁵⁺ redox couple (V⁴⁺/V⁵⁺≈4.1 V vs Na⁺/Na) is an effective way to elevate the energy density of SIBs, whereas the irreversible phase transition of V⁴⁺/V⁵⁺ 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 V⁴⁺/V⁵⁺ redox couple, stable structure, and fast electrochemical kinetics, the high entropy material Na_3.2V_1.5Fe_0.1Al_0.1Cr_0.1Mn_0.1Cu_0.1(PO₄)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.