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 gamma-MnO2. Ab initio computational methods based on density functional theory (DFT) were applied to study the mechanism of hydrogen ion insertion into the pyrolusite (beta), ramsdellite (R), and nsutite (gamma) MnO2 polymorphs. Itwas found that hydrogen ion insertion induced significant distortion in the crystal structures of gamma-MnO2 polymorphs. Calculations demonstrated that the hydrogen ions inserted into gamma-MnO2 initially occupied the larger 2x1 ramsdellite tunnels. The protonated form of gamma-MnO2 was found to be stable over the discharge range during which up to two hydrogen ions were inserted into each 2x1 tunnel. At the same time, the study showed that the insertion of hydrogen ions into the 1x1 pyrolusite tunnels of gamma-MnO2 created instability leading to the structural breakdown of gamma-MnO2. The results of this study explain the presence of groutite (alpha-MnOOH) and the absence of manganite (gamma-MnOOH) among the reaction products of partially reduced gamma-MnO2. (c) The Author(s) 2018. Published by ECS.