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

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 Zn 3+ x (OH) 2 V 2 O 7 ·2H 2 O, thus permitting subsequent insertion of Zn 2+ in such layered structure. Ex-situ SEM studies of electrodes further identify a morphology change of nanoparticles to nanosheets , corresponding to VN and Zn 3+ x (OH) 2 V 2 O 7 ·2H 2 O, respectively. New storage mechanism-the electrochemical reduction combined with the contributions from Zn 2+ (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.