High Entropy Enabling the Reversible Redox Reaction of V<sup>4+</sup>/V<sup>5+</sup> Couple in NASICON‐Type Sodium Ion Cathode
Meng Li, Chen Sun, Qing Ni, Zheng Sun, Yang Liu, Yang Li, Lei Li, Haibo Jin, Yongjie Zhao
Abstract
Abstract NASICON‐type Na 3 VM(PO 4 ) 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 Na 3 VAl 0.2 Cr 0.2 Fe 0.2 In 0.2 Ga 0.2 (PO 4 ) 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 V 3+ /V 4+ /V 5+ 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.