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

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 V5+/V4+ to V3+ 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-10nm anode delivers a sodium-ion storage capability of 106mAhg(-1) at the high specific current of 20Ag(-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.