Unlocking a new storage mechanism of high performance VN@N/S–C cathode: Role of electrochemical reduction

Abstract

Vanadium-based compounds have earned incremental attention as cathode for aqueous zinc ion batteries (AZIBs) due to their ideal electrical conductivity, rich valence states and flexible crystal structures. Unfortunately, their storage mechanisms are confusing and not well-established, especially for nitride. Herein, we have constructed a cubic vanadium nitride and nitrogen/sulfur doped carbon composite (VN@N/S–C) as a cathode for AZIBs. An in-depth examination of ex-situ XRD and XPS reveal that the electrochemical reduction during the first discharge process promotes a structural transformation of cubic VN to layered Zn3+x(OH)2V2O7·2H2O, thus permitting subsequent insertion of Zn2+ in such layered structure. Ex-situ SEM studies of electrodes further identify a morphology change of nanoparticles to nanosheets, corresponding to VN and Zn3+x(OH)2V2O7·2H2O, respectively. New storage mechanism-the electrochemical reduction combined with the contributions from Zn2+ (de)intercalation, furnishes the VN@N/S–C cathode a high discharge capacity of 232 mA h g−1 after 400 cycles at 4 A g−1 and good rate performance (252 mA h g−1 at 6 A g−1). Even at a low temperature of 0 °C, the cathode still delivers an excellent capacity of 361 mA h g−1 after 300 cycles at 1 A g−1. This study offers a hint to improve the understanding on cubic structure for AZIBs.