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The recent developments in rechargeable aqueous batteries have witnessed a burgeoning interest in the mechanism of proton transport in the cathode materials. Herein, for the first time, we report the Grotthuss proton transport mechanism in α‐MnO2 which features wide [2×2] tunnels. Exemplified by the substitution doping of Ni (≈5 at.%) in α‐MnO2 that increases the energy density of the electrode by ≈25 %, we reveal a close link between the tetragonal‐orthorhombic (TO) distortion of the lattice and the diffusion kinetics of protons in the tunnels. Experimental and theoretical results verify that Ni dopants can exacerbate the TO distortion during discharge, thereby facilitating the hydrogen bond formation in bulk α‐MnO2. The isolated direct hopping mode of proton transport is switched to a facile concerted mode, which involves the formation and concomitant cleavage of O−H bonds in a proton array, namely via Grotthuss proton transport mechanism. Our study provides important insight towards the understanding of proton transport in MnO2 and can serve as a model for the compositional design of cathode materials for rechargeable aqueous batteries.

the past decades, some issues of MnO 2based cathodes still remain due to the low electronic conductivity, [19-21] low utilization of reversible discharge depth, [22,23] sluggish diffusion kinetics, [24-26] and poor structural stability upon cycling, [27-29] which restricts their practical application in the commercial secondary batteries. Taking the Zn-ion batteries as example, the MnO 2 cathode seriously suffer from the above issues, especially the sluggish Zn 2+ diffusion, [30] and structural collapse issue during H + /Zn 2+ intercalation/ extraction cycles. [31-33] Regarding these bottlenecks, researchers have strived to develop strategies that can realize optimizations in capacity, rate, and cycling properties of MnO 2 cathodes, such as surface coating, [34] metal-doping, [35] preintercalation, [36] etc. Among all the strategies, preintercalation strategy provides a basic and effective method for optimizing the structure and electrochemical performance of MnO 2-based cathodes. In recent years, the preintercalation strategy has attracted much attention as an effective approach to enhance the electrochemical performance of cathode materials, including vanadate, [37] manganese oxides, [23] layered LiCoO 2 , [38] etc. Several reviews and prospects have been conducted for MnO 2 materials. Some reviews have mentioned the electrochemical properties and correlated reaction mechanism of MnO 2 materials in aqueous Zn batteries, [29,39,40] and a review by Mai's group offers insights into the rational design of preintercalation electrodes in next-generation rechargeable batteries. [36] However, a review or prospect on the application and mechanism of the preintercalation strategy in MnO 2 materials for nextgeneration batteries is lacking. For MnO 2 electrode materials, many reports on improving the electrochemical properties of materials by applying preintercalation strategy have been emerged in the last 5 years (Table S1 in the Supporting Information). The main feature of the preintercalated MnO 2 materials is that some ions/molecules are preintercalated into the tunnel or interlayer hosts of MnO 2 materials prior to the battery cycling (or during synthesis process). These intercalated guest species, including ions, inorganic/organic molecules, as well as polymers, present electrostatic and physical interactions with the host framework and the inserted carrier ions via chemical bonding or coordination, presenting significant benefiting effect on the inherent structure of hosts and the transport kinetics of carrier ions. Generally, there are several Manganese oxides (MnO 2) are promising cathode materials for various kinds of battery applications, including Li-ion, Na-ion, Mg-ion, and Zn-ion batteries, etc., due to their low-cost and high-capacity. However, the practical application of MnO 2 cathodes has been restricted by some critical issues including low electronic conductivity, low utilization of discharge depth, sluggish diffusion kinetics, and structural instability upon cycling. Preintercalation of ions/molecules ...

The recent developments in rechargeable aqueous batteries have witnessed a burgeoning interest in the mechanism of proton transport in the cathode materials. Herein, for the first time, we report the Grotthuss proton transport mechanism in α‐MnO2 which features wide [2×2] tunnels. Exemplified by the substitution doping of Ni (≈5 at.%) in α‐MnO2 that increases the energy density of the electrode by ≈25 %, we reveal a close link between the tetragonal‐orthorhombic (TO) distortion of the lattice and the diffusion kinetics of protons in the tunnels. Experimental and theoretical results verify that Ni dopants can exacerbate the TO distortion during discharge, thereby facilitating the hydrogen bond formation in bulk α‐MnO2. The isolated direct hopping mode of proton transport is switched to a facile concerted mode, which involves the formation and concomitant cleavage of O−H bonds in a proton array, namely via Grotthuss proton transport mechanism. Our study provides important insight towards the understanding of proton transport in MnO2 and can serve as a model for the compositional design of cathode materials for rechargeable aqueous batteries.