High Entropy Enabling the Reversible Redox Reaction of V⁴⁺/V⁵⁺ Couple in NASICON-Type Sodium Ion Cathode

NASICON-type Na₃VM(PO₄)₃ (M: transition metals) cathodes usually suffer from poor cycling stability in the voltage region of above 4 V versus Na⁺/Na owing to irreversible phase transition and severe structural distortion. Herein, the high entropy concept is extended to NASICONs and Na₃VAl_0.2Cr_0.2Fe_0.2In_0.2Ga_0.2(PO₄)₃ (NVMP) with high purity is obtained. The NVMP achieves a highly reversible specific capacity of 102 mAh g⁻¹ (2.5–4.4 V vs Na⁺/Na) via the successive redox reaction of V³⁺/V⁴⁺/V⁵⁺ together with a long-term lifespan of 5000 cycles at 20 C (a capacity retention of 86.8%). Even at an extreme temperature of −20 °C, the NVMP cathode can still provide excellent cycling performance (a capacity retention of 94.2% at 5 C after 1000 cycles). Moreover, the increased configurational entropy in the electrode renders a quite small cell volume change of 1.1%. The sodium ion storage mechanism containing solid solution-type in the voltage range of 2.5–3.8 V and bi-phasic in 3.8–4.4 V is revealed by ex situ XRD analysis. Pairing with a hard carbon anode, NVMP//HC cell offers a specific capacity of 81 mAh g⁻¹ at 0.2 C based on the cathode mass. This high-entropy engineering is expected to be widely applicable for the development of polyanionic electrode materials.