Synergistic H+/Zn2+ dual ion insertion mechanism in high-capacity and ultra-stable hydrated VO2 cathode for aqueous Zn-ion batteries

Rechargeable aqueous zinc ion batteries (ZIB) with near-neutral electrolytes are a promising candidate for stationary energy storage owing to their high-energy-density, high-safety, low-cost and environmental-friendliness. However, the development of ZIBs is currently hindered by the lack of high-performance cathode materials and a good understanding of the true ionic storage mechanism in cathodes. Herein, using a promising ZIB cathode, hydrated VO2 (denoted as H-VO2), as a model material, we carried out a systematic experimental and theoretical work to elucidate the ionic storage mechanisms. We show strong evidence that H+ and Zn2+ are synergistically involved in the ionic storage in H-VO2. The H+-insertion/extraction, which leads to a pH swing of the electrolyte, can be viewed as an indirect Zn2+-storage through a reversible precipitation/dissolution of Zn(OH)2 on the surface of H-VO2 cathode. The first-principles DFT calculations further reveal that H+ and Zn2+ have their own favorable insertion sites and migration pathways, but H+-insertion predominates in the initial discharge stage whereas Zn2+-insertion controls in the late discharge stage. Because of the synergetic H+/Zn2+ co-insertion, H-VO2-based ZIB exhibits a high capacity and stability at both low and high rates, e.g. 410 and 200 ​mAh g-1, 88% and 70% retention rate for 200 (~1500 ​h) and 3000 cycles (~215 ​h) at 0.1 and 5.0 ​A ​g-1, respectively. The new fundamental insights gained from this study deepen the understanding of aqueous Zn-ion battery chemistry for future development of advanced ZIB cathodes.