Computer modeling of crystalline electrolytes -- lithium thiophosphates and phosphates*

During the last 5 years, lithium thiophosphate solid electrolyte materials have been developed\footnote{H. Yamane, M. Shibata, Y. Shimane, et al., {\em{Solid State Ionics}} {\bf{178}}, 1162-1167 (2007).} for use in all-solid-state rechargeable batteries. In particular, crystalline Li$_7$P$_3$S$_{11}$ has been characterized as a superionic conducting material having room temperature conductivities as high as 10$^{-3}$ S/cm, which is 1000 times greater than that of the commercial solid electrolyte material LiPON. Building on our previous work,\footnote{N. A. W. Holzwarth, N. D. Lepley, Y. A. Du, {\em{J. Power Sources}} (2010) [in press: doi:10.1016/j.jpowsour.2010.08.042]} we report computer modeling studies of this material as well as those of related phosphates and phosphonitrides. We present results on meta-stable crystal structures, formation energies, and mechanisms of Li ion migration. The calculational methods are based on density functional theory. The calculations were carried out using the Quantum Espresso (PWSCF) package.\footnote{P. Giannozzi, S. Baroni, et al., {\em{J. Phys.: Condens. Matter.}} {\bf{21}}, 394402 (2009); available from the website: http://www.pwscf.org/.}