Structure and interface properties of the electrolyte material Li$_4$P$_2$S$_6$*

Li$_4$P$_2$S$_6$ has been identified in several high temperature preparations of lithium thiophosphate electrolytes as a synthesis or decomposition product. Its characteristic P$-$P bond may be partly responsible for its relative stability. Early structural analysis\footnote{R. Mercier {\em{et. al.}}, {\em{J. Solid State Chem.}} {\bf{43}} 151 (1982)} found the P sites to be disordered. Our previous simulations,\footnote{N. A. W. Holzwarth, {\em{J. Power Sources}} {\bf{196}} 6970 (2011) } found a related low energy structure with ordered P sites. We report here a re-examination of the simulation results and new $X$-ray measurements which indicate that the lowest energy structure of Li$_4$P$_2$S$_6$ is different from that determined in previous analysis. Ionic conductivity and thermal stability are also reported. In addition to examining the bulk electrolyte, we have simulated idealized interfaces of Li$_4$P$_2$S$_6$ and lithium metal representing an electrolyte/anode system.