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

Abstract

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.

9 + 5 pages (5 + 8 figures). Published version. Open access version available from the Chemical Science website

References in corpus (2)

• First-Principles Studies of the Atomic, Electronic, and Magnetic Structure of a-MnO2 (Cryptomelane)
• Charge Localization and Ordering in A${}_2$Mn${}_8$O${}_{16}$ Hollandite Group Oxides: Impact of Density Functional Theory Approaches

Cited by in corpus (1)

• Proton-transfer dynamics in ionized water chains using real-time Time Dependent Density Functional Theory