Ab Initio Studies of Hydrogen Ion Insertion into β-, R-, and γ-MnO2 Polymorphs and the Implications for Shallow-Cycled Rechargeable Zn/MnO2 Batteries.

At a low depth of discharge, the performance of rechargeable alkaline Zn/MnO₂ batteries is determined 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 (DFT) were applied to study the mechanism of hydrogen ion insertion into the pyrolusite (β), ramsdellite (R), and nsutite (γ)MnO₂ polymorphs. Itwas found that hydrogen ion insertion induced significant distortion in the crystal structures ofMnO₂ polymorphs. Calculations demonstrated that the hydrogen ions inserted into γ-MnO₂ initially occupied the larger 2×1 ramsdellite tunnels. The protonated form of γ-MnO₂ 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-MnO₂ created instability leading to the structural breakdown of γ-MnO₂. 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-MnO₂. [ABSTRACT FROM AUTHOR]