Fig.1

(a) XRD patterns of Na3PS4 powders and the powders sintered respectively at 270 °C and 420 °C. The filled circles and rhombuses represent cubic and tetragonal phases, respectively[34]; (b) XRD patterns of Na3PS4 prepared using Na2S with different purities[47]; (c) Ionic conductivity of Na3PS4 prepared using Na2S with different purities[47]; (d) Molecular dynamics simulation of Na-distribution for Na3PS4 with 0% and 2% Na vacancies[48]"

Fig.2

(a) Crystal structure of Na3PSe4. The sodium ions located at the diffusion channels formed by PSe4 tetrahedral[40]; (b) Fourier map of the difference pattern on the xy (z=0) plane[40]; (c) Room temperature ionic conductivity and lattice parameters with various S contents in Na3PSe4-xSx[40]; (d) Schematic diagram if Na+ diffusion channels of different electrolytes[53]; (e) Arrhenius plots of Na3PSxSe4-x (x=0,1,2,3,4)[55]; (f) X-ray diffraction pattern of Na3PSxSe4-x (x=0,1,2,3,4)[55]"

Fig.3

(a) Room temperature ion conductivity of (100-x)Na3PS4?xNa4SiS4 glass-ceramic[58]; (b) Crystal structure of cubic Na3PS4 (left panel) and the interstitial sodium ions (right panel)[59]; (c) Room temperature ion conductivity of Na3P0.62As0.38S4 (i) and Arrhenius plots and the corresponding impedance plots of Na3P0.62As0.38S4 (ii)[36]; (d) Room temperature ion conductivity and activation energy of cubic Na3-2xCaxPS4 as a function of x[61]"

Table 1

Summary of sodium sulfide solid electrolytes"

Fig.4

(a) Crystal structure of Na3SbS4[41]; (b) SEM image of Na3SbS4 coated NaCrO2[63]; (c) Lattice parameters of Na3SbSe4-xSx as function of x[65]; (d) Crystal structure of Na4-xSn1-xSbxS4[67]; (e) Arrhenius diagrams of various sodium-ion conductors[69]"

Fig.5

Crystal structure of Na11Sn2PS12[76]. (a) The Na1、Na2 sites are fractional occupancy while the Na3、Na4、Na5 ion sites are almost full occupied sites; (b) The small tetragonal cell (a×a×c) equivalent to Li10GeP2S12 is related to the actual tetragonal cell (a′×a′×c′) of Na11Sn2PS12 as follows: a=2a′，c=2c′; (c)、(d) View of the ab planes consisting of interconnected NaS6 octahedron; (e) view of the sodium atoms chains that run along c"

Fig.6

(a) Schematic diagram of positive electrode assembling and the corresponding battery performance[44]; (b) Schematic diagram of the uniform coating of Na3SbS4[91]; (c) CV curves and cycling performance of the cells with and without coating layer[91]"

Fig.7

(a) Schematic diagram of NaBr coating on sodium disk[98]; (b) Schematic diagram of ion and electron transport before and after the passivation layer formed at the interface between Na3SbS4 electrolyte and Na electrode (top panel). The galvanostatic cycling of the symmetric cells with and without surface hydration[99]; (c) Schematic diagram of electron transport with and without CPEO at the interface between Na3SbS4 electrolyte and Na electrode[100]"