All-solid-state batteries are one of the most promising candidates for next generation batteries because of their safety property and high power density¹. The sulfur solid electrolyte is prepared conventionally by hand-mixing in the all-solid-state batteries. The ball-milling method is not suitable for large-scale preparation. Therefore it is important to develop the large-scale synthesis method of solid electrolyte. Recently, some research groups have reported on the synthesis of Li₃PS₄ electrolyte via liquid phase by dissolving the starting sulfide materials such as Li₂S and P₂S₅ in organic solvents². However, the formation mechanism has not been clearly understood. In this study, we evaluate a local structure change of Li₃PS₄ (LPS) in the liquid phase synthesis process by total X-ray scattering measurement coupled with pair distribution function (PDF) analysis to clarify the formation mechanism.

Experimental

Li₃PS₄ was prepared by liquid-phase shaking method as previously reported². Li₂S (Sigma-Aldrich), P₂S₅(Sigma-Aldrich), dehydrated ethyl propionate (Sigma-Aldrich) and zirconia balls were added in centrifugation tube and shaken under Ar atmosphere from 0 min to 360 min at 1500 rpm. The tube was centrifuged at 10000 rpm for 5 min. The supernatant was decanted and the residual solvent was evacuated at room temperature. Then the precursor was annealed at 170 ºC in low pressure for 2 h.

The high-energy X-ray diffraction experiments were carried out at the beamline BL08W using a two-dimensional detector in SPring-8, Japan. The incident X-ray energy was 115 keV. The diffraction patterns of the samples and an empty tube were measured in the transmission geometry. The obtained data of each sample was corrected for background, absorption correction, multiple scattering and inelastic scattering.

Results and discussion

Figure 1 shows that reduced pair distribution functions, G(r), obtained from a Fourier transformation of total structure factors for each liquid phase sample. In the initial stage of the synthesis process, peaks were attributed to the starting materials of P₂S₅ and Li₂S. In the middle stage of the synthesis process, although the intensity of the peak from Li₂S decrease, intensity of the first peak from P-S bond of PS₄ tetrahedral anion increased. In the end stage of the synthesis process, two peaks, which were observed in Li₃PS₄ obtained by subsequent annealing treatment, appeared around 3.3 Å, 4.0 Å. These results indicate that the locally ordered structure of Li₂S is broken while the locally ordered structure at short range is grown in the middle stage of the synthesis process. In the end stage of the synthesis process, locally ordered structure at middle range of Li₃PS₄ forms with keeping locally ordered structure at the short range.

Acknowledgement

This research was financially supported by the Japan Science and Technology Agency (JST), Advanced Low Carbon Technology Research and Development Program (ALCA), Specially Promoted Research for Innovative Next Generation Batteries (SPRING) Project.