Boosting the Energy Density of Aqueous Batteries via Facile Grotthuss Proton Transport

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 alpha-MnO2 which features wide [2x2] tunnels. Exemplified by the substitution doping of Ni (approximate to 5 at.%) in alpha-MnO2 that increases the energy density of the electrode by approximate to 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 alpha-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.