Solid-state electrolytes are anticipated to enable the construction of batteries with increased volumetric energy densities. However, depleting reserves of sources of lithium has motivated the exploration of next-generation alkali-metal-conducting solid-state electrolytes such as solid-state sodium-ion conductors. We recently explored a water-mediated synthesis of Na₄P₂S₆, which could allow for the scalable fabrication of this Na ion conductor.¹ Na₄P₂S₆ has been shown to crystalize² in the based-centered monoclinic structure C2/m (#12) and its room temperature ionic conductivity has been measured¹ to be 3x10^-6S/cm. Previous simulations³ suggested that the monoclinic structure is meta-stable relative to structures found for the similar material Li₄P₂S₆, but further work reported here shows that vibrational free energy contributes to the stabilization of the C2/m structure of Na₄P₂S₆ at room temperature. Our analysis also investigates the mixed alkali electrolyte Li₂Na₂P₂S₆ which may have increased Na ion conductivity compared with that of Na₄P₂S₆.

1. Z. D. Hood et al., to be published.
2. A. Kuhn et al., Z. Anorg. Allg. Chem., 640, 689–692 (2014)
3. E. Rush Jr. et al., Solid State Ionics, 286, 45-50 (2016)

Acknowledgements: YL and NAWH were supported by NSF grant DMR-1507942. A portion of the work by ZDH was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.