Discharge Mechanism of the γ-MnO₂ Electrode in Shallow-Cycled Zn/MnO₂ Batteries: An Ab Initio Study

Alkaline Zn/MnO₂ batteries hold great promise for electrical energy storage due to their high energy density, non-toxicity, and low cost. At a low depth of discharge, the reduction reaction in the Zn/MnO₂ battery cathode is governed by hydrogen trapping in the solid phase of γ-MnO₂. We applied ab initio computational methods based on density functional theory to study the mechanism of hydrogen insertion into the pyrolusite and ramsdellite tunnels of γ-MnO₂. Our calculations were carried out using the Quantum ESPRESSO electronic structure code combined with Vanderbilt ultrasoft pseudopotentials. We found that the trapped hydrogen initially occupied the 2x1 ramsdellite tunnels of γ-MnO₂. Our study showed that the insertion of hydrogen into the 1x1 pyrolusite tunnels induced significant structural distortions leading to the breakdown of the crystal structure of γ-MnO₂. These results could explain the presence of groutite and the absence of manganite among the reaction products of partially reduced γ-MnO₂.