Direct Detection and Visualization of the H⁺ Reaction Process in a VO₂ Cathode for Aqueous Zinc-Ion Batteries

Because they are safer and less costly than state-of-the-art Li-ion batteries, aqueous zinc-ion batteries (AZIBs) have been attracting more attention in stationary energy storage and industrial energy storage. However, the electrochemical reaction of H⁺ in all of the cathode materials of AZIBs has been puzzling until now. Herein, highly oriented VO₂ monocrystals grown on a Ti current collector (VO₂-Ti) were rationally designed as the research model, and such a well-aligned VO₂ cathode also displayed excellent zinc-ion storage capability (e.g., a reversible capacity of 148.4 mAh/g at a current density of 2 A/g). To visualize the H⁺ reaction process, we used time-of-flight secondary-ion mass spectrometry. With the benefit of such a binder-free and conductor-free electrode design, a clear and intuitive reaction of H⁺ in a VO₂ cathode is obtained, which is quite significant for unraveling the accurate reaction mechanism of VO₂ in AZIBs.