High‐rate sodium‐ion storage of vanadium nitride via surface‐redox pseudocapacitance

Abstract

Vanadium nitride (VN) electrode displays high‐rate, pseudocapacitive responses in aqueous electrolytes, however, it remains largely unclear in nonaqueous, Na + ‐based electrolytes. The traditional view supposes a conversion‐type mechanism for Na + storage in VN anodes but does not explain the phenomena of their size‐dependent specific capacities and underlying causes of pseudocapacitive charge storage behaviors. Herein, we insightfully reveal the VN anode exhibits a surface‐redox pseudocapacitive mechanism in nonaqueous, Na + ‐based electrolytes, as demonstrated by kinetics analysis, experimental observations, and first‐principles calculations. Through ex situ X‐ray photoelectron spectroscopy and semiquantitative analyses, the Na + storage is characterized by redox reactions occurring with the V 5+ /V 4+ to V 3+ at the surface of VN particles, which is different from the well‐known conversion reaction mechanism. The pseudocapacitive performance is enhanced through nanoarchitecture design via oxidized vanadium states at the surface. The optimized VN‐10 nm anode delivers a sodium‐ion storage capability of 106 mAh g −1 at the high specific current of 20 A g −1 , and excellent cycling performance of 5000 cycles with negligible capacity losses. This work demonstrates the emerging opportunities of utilizing pseudocapacitive charge storage for realizing high‐rate sodium‐ion storage applications.