Ab Initio Studies of Hydrogen Ion Insertion into β-, R-, and γ-MnO₂Polymorphs and the Implications for Shallow-Cycled Rechargeable Zn/MnO₂Batteries

At a low depth of discharge, the performance of rechargeable alkaline Zn/MnO 2 batteries is determined by the concomitant processes of hydrogen ion insertion and electro-reduction in the solid phase of γ-MnO 2 . Ab initio computational methods based on density functional theory (DFT) were applied to study the mechanism of hydrogen ion insertion into the pyrolusite (β), ramsdellite (R), and nsutite (γ) MnO 2 polymorphs. It was found that hydrogen ion insertion induced significant distortion in the crystal structures of MnO 2 polymorphs. Calculations demonstrated that the hydrogen ions inserted into γ-MnO 2 initially occupied the larger 2 × 1 ramsdellite tunnels. The protonated form of γ-MnO 2 was found to be stable over the discharge range during which up to two hydrogen ions were inserted into each 2 × 1 tunnel. At the same time, the study showed that the insertion of hydrogen ions into the 1 × 1 pyrolusite tunnels of γ-MnO 2 created instability leading to the structural breakdown of γ-MnO 2 . The results of this study explain the presence of groutite (α-MnOOH) and the absence of manganite (γ-MnOOH) among the reaction products of partially reduced γ-MnO 2 .