Surface Structure and Stability in Li$_3$PS$_4$ and Li$_3$PO$_4$ Electrolytes from First Principles*

Crystalline solid electrolyte materials continue to show considerable promise for lithium ion battery applications. Recent experiments on these materials\footnote{Chengdu Liang, ORNL, (private communication).} suggest that in some cases surface effects may play a significant role with regard to both stability and ionic conductivity. In this study, we extend our previous modeling work\footnote{N. A. W. Holzwarth, N. D. Lepley, Y. A. Du, {\em{J. Power Sources}} {\bf{196}}, 6870 (2011).} to an examination of idealized surfaces of several phases of Li$_3$PX$_4$ (X=O,S). Our preliminary results suggest that energy contributions from the surface affect the relative phase stability in Li$_3$PS$_4$, although this is not observed in the phosphate analogue. Our presentation will focus on surface energies and structures, as well as examining the calculated stability of the interface between the electrolyte and lithium metal.