A sodium-ion sulfide solid electrolyte with unprecedented conductivity at room temperature

Abstract

Solid electrolytes are key materials to enable solid-state rechargeable batteries, a promising technology that could address the safety and energy density issues. Here, we report a sulfide sodium-ion conductor, Na_2.88Sb_0.88W_0.12S₄, with conductivity superior to that of the benchmark electrolyte, Li₁₀GeP₂S₁₂. Partial substitution of antimony in Na₃SbS₄ with tungsten introduces sodium vacancies and tetragonal to cubic phase transition, giving rise to the highest room-temperature conductivity of 32 mS cm^-1 for a sintered body, Na_2.88Sb_0.88W_0.12S₄. Moreover, this sulfide possesses additional advantages including stability against humid atmosphere and densification at much lower sintering temperatures than those (>1000 °C) of typical oxide sodium-ion conductors. The discovery of the fast sodium-ion conductors boosts the ongoing research for solid-state rechargeable battery technology with high safety, cost-effectiveness, large energy and power densities.

Fig. 1

Structural data for Na_3−xSb_1−xW_xS₄ (x = 0 and 0.12) electrolytes prepared by heat treatment (H.T.) at 275 °C for 1.5–12 h. a X-ray diffraction patterns, b Raman spectra, and c cross-sectional SEM images (scale bar: 1 μm) of the prepared Na_3−xSb_1−xW_xS₄

Fig. 2

Crystal structure of Na_2.88Sb_0.88W_0.12S₄ heated at 275 °C for 12 h. a Rietveld refinement profiles of X-ray powder diffraction data for Na_2.88Sb_0.88W_0.12S₄. Red dots and black lines denote the observed and calculated XRD patterns, respectively. The green sticks mark the position of the reflections for Na_2.88Sb_0.88W_0.12S₄. The difference between the observed and calculated patterns is indicated by the blue line. b Crystal structure of cubic Na_2.88Sb_0.88W_0.12S₄ with the unit cell outlined. The Na, Sb, W, and S sites are represented by blue, orange, gray, and yellow balls, respectively. Na is linearly arranged, and distortion of the Sb₄/WS₄ tetrahedra is very small compared to the tetragonal structure of Na₃SbS₄ (Supplementary Fig. 3)

Fig. 3

Conductive behavior of Na_2.88Sb_0.88W_0.12S₄ electrolyte. a Conductivity at 25 °C (σ₂₅) and activation energy E_a for the conduction of the pelletized Na_2.88Sb_0.88W_0.12S₄ electrolytes as a function of heat treatment (H.T.) time at 275 °C. The pellets were prepared by cold-pressing the electrolyte powder at 1080 MPa, followed by heat treatment at 275 °C. b The Arrhenius plot of the Na ion conductivity of the Na_2.88Sb_0.88W_0.12S₄ electrolyte developed in this study and representative Na ion conductors reported so far

Fig. 4

Monitoring H₂S gas generated from Na_2.88Sb_0.88W_0.12S₄ as a function of exposure time to humid air (relative humidity (R.H.) = 70%). The H₂S content is normalized by the weight of sulfur in the samples. For comparison, the same measurement was carried out for Na₃PS₄ under a less-humid condition of R.H. = 50%