Anharmonic Lattice Dynamics in Sodium Ion Conductors

Abstract

We employ THz-range temperature-dependent Raman spectroscopy and first-principles latticedynamical calculations to show that the undoped sodium ion conductors Na3PS4 and isostructural Na3PSe4 both exhibit anharmonic lattice dynamics. The anharmonic effects in the compounds involve coupled host lattice -Na + ion dynamics that drive the tetragonal-to-cubic phase transition in both cases, but with a qualitative difference in the anharmonic character of the transition. Na3PSe4 shows almost purely displacive character with the soft modes disappearing in the cubic phase as the change of symmetry shifts these modes to the Raman-inactive Brillouin zone boundary. Na3PS4 instead shows order-disorder character in the cubic phase, with the soft modes persisting through the phase transition and remaining active in Raman in the cubic phase, violating Raman selection rules for that phase. Our findings highlight the important role of coupled host lattice -mobile ion dynamics in vibrational instabilities that are coincident with the exceptional conductivity in these Na + ion conductors.

Na metal anodes for liquid and solid-state Na batteries

“…The development of high-performance SSNBs is contingent on the feasibility of the SSEs, which should possess Fig. 3 Representative crystal structures of the most common inorganic solid electrolytes: (a) oxide-based, 63,64 (b) sulfide-based, 65,66 (c) hydroboratebased, 67 and (d) halide-based electrolytes. 68,69 high ionic conductivity, wide electrochemical windows, excellent ductility, and air stability.…”

“…3c. 65,66 More recently, halide-based SSEs have been revived with much improved ionic conductivity, air stability and wide electrochemical windows. Then, the new Na halide SSE Na 2 ZrCl 6 was developed.…”