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arXiv · 2007.11223

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

Shaobo Cheng·Vidushi Sharma·Altug S. Poyraz·Lijun Wu·Xing Li·Amy C. Marschilok·Esther S. Takeuchi·Kenneth J. Takeuchi·Marivi Fernández-Serra·Yimei Zhu
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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.

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Shaobo Cheng, Vidushi Sharma, Altug S. Poyraz, Lijun Wu, Xing Li, Amy C. Marschilok, Esther S. Takeuchi, Kenneth J. Takeuchi, Marivi Fernández-Serra, Yimei Zhu. 2020-07-22. Water-induced formation of an alkali-ion dimer in cryptomelane nanorods. https://doi.org/10.1039/d0sc01517b

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