Role of site-disorder in energy materials: case of Li$_x$Nb$_2$O$_5$ pseudocapacitor and $\beta$-Li$_3$PS$_4$ solid electrolyte*

In this study, we will present computational studies to elucidate the importance of site-disorder in energy materials. We will specifically focus on two recently discovered materials: a Li-ion intercalation pseudocapacitor Li$_x$Nb$_2$O$_5$ (Nature Materials, {\bf 12} 518 (2013)) and a Li-ion solid-electrolyte.( JACS, {\bf 135} 975 (2013)). A combination of theoretical methods, such as density functional theory (DFT) based cluster-expansion, basin hopping, {\em ab initio} molecular dynamics, and nudged-elastic-bands calculations were employed to understand the origin of intercalation pseudocapacitance in the niobate-system.( J. Materials Chem. {\bf 1}14951 (2013)). It was found that having multiple sites with similar energies for ion-adsorption, lead to a site-occupancy disorder that eventually lead to a capacitative slope in the voltage profile over the entire range of ion intercalation, as seen in experiments. A similar site-occupancy induced sublattice melting in the $\beta$-Li$_3$PS$_4$ solid-electrolyte, which when ``frozen'' to RT, lead to high Li-ion conductivity.(G.K.P.Dathar et al, submitted (2014)). Further, we will elucidate how to take advantage of this control over site-disorder to better engineer improved energy materials for batteries and fuel-cells.