Nanoconfined vanadium nitride in 3D porous reduced graphene oxide microspheres as high-capacity cathode for aqueous zinc-ion batteries

• VN-rGO microspheres are synthesized through a facile spray pyrolysis process. • Electrochemical reaction of VN-rGO microspheres with zinc ions is studied. • VN-rGO shows peculiar behavior as a cathode for Zn-ion batteries. Aqueous zinc-ion batteries (ZIBs) are receiving considerable research highlights owing to their high safety and environment-friendliness. To implement this promising technology for grid-scale energy storage, effective cathode materials with high capacity, cycle stability, and electrochemical kinetics should be developed. Herein, the synthesis of uniquely structured porous VN-reduced graphene oxide composite (VN-rGO) microspheres through a facile spray pyrolysis process and their application as cathodes for ZIBs are introduced. The electrochemical reaction mechanism of VN-rGO microspheres with zinc ions is investigated through various in situ and ex situ analyses. During the initial charge process, VN phase transforms into the Zn 3 (OH) 2 V 2 O 7 ·2H 2 O (ZVOH) phase. From the second cycle and on, the ZVOH phase undergoes zinc-ion ingress and egress processes. VN-rGO microspheres exhibit an unprecedented high capacity (809 mA h g −1 at 0.1 A g −1 ), high energy density (613 W h kg −1 ), and good rate capability (467 mA h g −1 at 2.0 A g −1 ). The cathode delivers a reversible capacity of 445 mA h g −1 after 400 cycles at 1.0 A g −1 , which ascertains the robustness of the structure. The 3D porous rGO matrix to which VN nanocrystals are homogenously anchored accelerates the zinc-ion storage kinetics and endows the cathode with structural robustness.