Zn-H 2 O fuel cells, [8] etc.), rechargeable aqueous Zn-ion batteries (ZIBs) with a mild electrolyte are particularly attractive as zinc is more compatible with water than alkaline metals, Zn-ions are divalent, and the production and recycling of these batteries is relatively simple. [9][10][11][12] A variety of manganese dioxide (MnO 2 ) polymorphs (α-, β-, γ-, δ-, and amorphous) have been investigated as cathodes materials for ZIBs. [10,11,[13][14][15][16][17][18][19] Several studies have demonstrated that the storage capacity of MnO 2 in a neutral or mildly acidic aqueous electrolyte is partly induced by a reversible Zn 2+ insertion/extraction reaction and partly by the reversible H + insertion/extraction. [10,11,19] This process is governed by the reversible phase transition of the MnO 2 polymorphs from a tunneled to layered structure, driven by the electrochemical reaction. During this phase transition, even though the crystalline structure is maintained, Mn 3+ in the tunnel chains are reduced to soluble Mn 2+ leading to a capacity fading, a short cycle life and low Coulombic efficiency of aqueous Zn/MnO 2 batteries. [17,20] To prevent this dissolution of Mn ions, alternative electrolytes were identified, for example, Zhang et al., found that by using a Zn(CF 3 SO 3 ) 2 aqueous electrolyte, Mn-ion dissolution was suppressed and a high capacity retention with a ZnMn 2 O 4 cathode was achieved. [21] Alternatively by using a MnSO 4 additive to a mild ZnSO 4 aqueous electrolyte, Pan et al. reported that the Mn 2+ dissolution in the aqueous Zn/MnO 2 electrolyte was inhibited. [11] Though research into MnO 2 cathodes for aqueous ZIBs has gained momentum, it is of great importance to develop alternative high capacity cathode materials for ZIBs that are stable in an aqueous electrolyte.Very recently, vanadium oxide and its related compounds have been reported as cathodes for ZIBs, showing higher energy densities and better capacity retention compared to MnO 2 cathodes. Senguttuvan et al. reported the reversible Zn 2+ insertion/extraction process in a V 2 O 5 cathode for a nonaqueous ZIBs. [22] Kundu et al. developed a highly stable vanadium bronze (Zn 0.25 V 2 O 5 ·nH 2 O) cathode for aqueous ZIBs, which displayed a high energy density and capacity retention through a reversible Zn 2+ (de)intercalation process. [12] In addition, LiV 3 O 8 , [23] H 2 V 3 O 8 , [24] and V 2 S [25] cathodes exhibit good capacity reversibility and power density for aqueous ZIBs. Although Oberholzer et al. [26] observed a proton intercalation Rechargeable aqueous zinc-ion batteries (ZIBs) are promising for cheap stationary energy storage. Challenges for Zn-ion insertion hosts are the large structural changes of the host structure upon Zn-ion insertion and the divalent Zn-ion transport, challenging cycle life and power density respectively. Here a new mechanism is demonstrated for the VO 2 cathode toward proton insertion accompanied by Zn-ion storage through the reversible deposition of Zn 4 (OH) 6 SO 4 ·5H 2 O on the cathode surface, sup...
