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 Na3VAl0.2Cr0.2Fe0.2In0.2Ga0.2(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 degrees 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.

Automated summary

By introducing the concept of high entropy, researchers have obtained high-purity high-entropy Na3VAl0.2Cr0.2Fe0.2In0.2Ga0.2(PO4)3 (NVMP) cathodes, which exhibit excellent cycling stability and capacity retention above 4V voltage range, achieving a specific capacity of 102 mAh g(-1) in the voltage range of 2.5-4.4V. This study is of great significance for the development of polyanionic electrode materials.