Intercalating layered materials for energy storage

Layered materials are widely used as energy-storage media in applications such as hydrogen storage and batteries. Computational approaches can provide valuable insights into the underlying storage mechanisms and shed light on strategies to improve materials performance. We have employed advanced hybrid functional calculations to study two types of intercalated layered materials: (1) hydrogen-intercalated MoS$_2$ and (2) sodium-intercalated MnO$_2$. Our goal is to elucidate intrinsic materials properties that affect energy storage. \\ \\ We have studied the interactions of hydrogen with MoS$_2$ by exploring the equilibrium geometry, formation energy, and electronic behavior of interstitial H and H$_2$ molecules inside layered MoS$_2$ structures [1]. Interstitial H is identified to be a deep donor while H$_2$ molecules are electrically inactive and energetically more stable in MoS$_2$. To further shed light on the hydrogen-storage capacity of MoS$_2$, we have also explored the insertion energies of H$_2$ molecules as a function of hydrogen concentration and found that up to 13 H$_2$ molecules can be accommodated within the same interlayer spacing of an areal $3\times3$ supercell. \\ \\ In the second part of the talk, I will discuss electronic and ionic conductions in layered NaMnO$_2$, a cathode material for sodium ion batteries. Free carriers are trapped to form small hole or electron polarons; hence, electronic conduction is through polaron hopping. Ionic conduction is in the form of sodium vacancy migration. Both electronic and ionic conduction can be significantly affected by the presence of point defects. We will discuss strategies, such as optimizing synthesis conditions and impurity doping, to improve electrical conduction and storage performance of NaMnO$_2$. \newline\newline The work was performed in collaboration with H. Peelaers and C. G. Van de Walle, and supported by DOE. \newline\newline [1] Z. Zhu, H. Peelaers, and C. G. Van de Walle, Phys. Rev. B 94, 085426 (2016).