  Tunneled metal oxides such as $\alpha-$Mn$_8$O$_{16}$ (hollandite) have
proven to be compelling candidates for charge-storage materials in high-density
batteries. In particular, the tunnels can support one-dimensional chains of
K$^+$ ions (which act as structure-stabilizing dopants) and H$_2$O molecules,
as these chains are favored by strong H-bonds and electrostatic interactions.
In this work, we examine the role of water molecules in enhancing the stability
of K$^+$-doped $\alpha-$Mn$_8$O$_{16}$ (cryptomelane). The combined
experimental and theoretical analyses show that for high enough concentrations
of water and tunnel-ions, H$_2$O displaces K$^+$ ions from their natural
binding sites. This displacement becomes energetically favorable due to the
formation of K$_2^+$ dimers, thereby modifying the stoichiometric charge of the
system. These findings have potentially significant technological implications
for the consideration of cryptomelane as a Li$^+$/Na$^+$ battery electrode. Our
work establishes the functional role of water in altering the energetics and
structural properties of cryptomelane, an observation that has frequently been
overlooked in previous studies.
