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Rechargeable aqueous Zn/manganese dioxide (Zn/MnO 2 ) batteries are attractive energy storage technology owing to their merits of low cost, high safety, and environmental friendliness. However, the b-MnO 2 cathode is still plagued by the sluggish ion insertion kinetics due to the relatively narrow tunneled pathway. Furthermore, the energy storage mechanism is under debate as well. Here, b-MnO 2 cathode with enhanced ion insertion kinetics is introduced by the efficient oxygen defect engineering strategy. Density functional theory computations show that the b-MnO 2 host structure is more likely for H + insertion rather than Zn 2+ , and the introduction of oxygen defects will facilitate the insertion of H + into b-MnO 2 . This theoretical conjecture is confirmed by the capacity of 302 mA h g À1 and capacity retention of 94% after 300 cycles in the assembled aqueous Zn/ b-MnO 2 cell. These results highlight the potentials of defect engineering as a strategy of improving the electrochemical performance of b-MnO 2 in aqueous rechargeable batteries.

Despite the substantial progress in cathode materials in the past few years, rechargeable zinc batteries (RZBs) are plagued by rapid performance degradation due to dendrite formation and notorious side reactions at the Zn anode side. Here, an optimized hydrated eutectic electrolyte (HEE) system containing methylsulfonylmethane, zinc perchlorate, and water, in which an organic ligand coordinated the solvation shell of Zn ions with water molecules constituting the eutectic network, is proposed. Compared to common aqueous solutions, this HEE system is proven effective in promoting the smooth Zn deposition and plating/stripping reversibility as well as suppressing side reactions. The vanadium-based zinc batteries based on this new HEE exhibit exceptionally high-capacity retention (≈100% retention even after 1600 cycles at a relatively small current density of 1000 mA g −1 ). This study offers a new type of electrolyte for RZBs and a deep understanding of the effect of Zn 2+ solvent sheath structure on the cycle reversibility.