Monoclinic VO2(D) hollow nanospheres with super-long cycle life for aqueous zinc ion batteries.

Vanadium dioxide (VO2) is a very promising cathode material for aqueous zinc ion batteries (AZIBs) because of its high reversible specific capacity, excellent rate performance and fast diffusion kinetics. However, its long-term cycle stability and compatibility with electrolytes have not met expectations. In this study, another metastable phase of vanadium dioxide-monoclinic VO2(D)-is demonstrated to be a better choice as a cathode for AZIBs. Electrochemical results revealed that the as-prepared VO2(D) hollow nanospheres delivered high reversible discharge capacity (up to 408 mA h g-1 at 0.1 A g-1), exceptional rate performance (200 mA h g-1 at 20 A g-1), and long cyclic endurance stability (cycling for 30 000 cycles with a low capacity fading rate of 0.0023% per cycle) in inexpensive 3 M ZnSO4 electrolyte. Furthermore, the electrochemical reaction mechanism was corroborated using ex situ XRD, HRTEM and XPS, showing that an interesting electrochemically induced phase transition from VO2(D) to V2O5·xH2O occured with the insertion/extraction of zinc ions. Finally, the prototype batteries assembled with our as-prepared VO2(D) hollow nanospheres and the impressive performance of this electrode under high active material mass loading further reveal its high potential in practical applications.