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.

arXiv comment9 + 5 pages (5 + 8 figures). Published version. Open access version available from the Chemical Science website

References in corpus (3)

• An exchange functional that tests the robustness of the plasmon description of the van der Waals density functional
• 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