Ab Initio Studies of the Electrochemical Properties of β, R, and 𝛾-MnO₂ Polymorphs

At a low depth of discharge, the performance of rechargeable alkaline Zn/MnO₂ batteries is governed by the concomitant processes of hydrogen ion insertion and electro-reduction in the solid phase of 𝛾-MnO₂. Ab initio computational methods based on density functional theory were applied to study the mechanism of hydrogen ion insertion into the pyrolusite (𝛽), ramsdellite (R), and nsutite (𝛾) MnO₂ polymorphs. It was found that the hydrogen ions inserted into 𝛾-MnO₂ initially occupied the 2x1 ramsdellite tunnels. The study showed that the insertion of hydrogen ions into the 1x1 pyrolusite tunnels of 𝛾-MnO₂ created instability leading to the breakdown of the crystal structure of 𝛾-MnO₂.