Water-induced formation of an alkali-ion dimer in cryptomelane nanorods.

Tunneled metal oxides such as α-MnO (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 HO 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 α-MnO (cryptomelane). The combined experimental and theoretical analyses show that for high enough concentrations of water and tunnel-ions, HO displaces K ions from their natural binding sites. This displacement becomes energetically favorable due to the formation of K 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.