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. 2020 Sep 29;5(40):26287–26294. doi: 10.1021/acsomega.0c04307

Synthesis of Sulfide Solid Electrolytes through the Liquid Phase: Optimization of the Preparation Conditions

Kentaro Yamamoto †,*, Masakuni Takahashi †,‡, Koji Ohara ‡, Nguyen Huu Huy Phuc §, Seunghoon Yang †, Toshiki Watanabe †, Tomoki Uchiyama †, Atsushi Sakuda ∥, Akitoshi Hayashi ∥, Masahiro Tatsumisago ∥, Hiroyuki Muto §, Atsunori Matsuda §, Yoshiharu Uchimoto †
PMCID: PMC7557990  PMID: 33073156

Abstract

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All-solid-state lithium batteries using inorganic sulfide solid electrolytes have good safety properties and high rate capabilities as expected for a next-generation battery. Presently, conventional preparation methods such as mechanical milling and/or solid-phase synthesis need a long time to provide a small amount of the product, and they have difficult in supplying a sufficient amount to meet the demand. Hence, liquid-phase synthesis methods have been developed for large-scale synthesis. However, the ionic conductivity of sulfide solid electrolytes prepared via liquid-phase synthesis is typically lower than that prepared via solid-phase synthesis. In this study, we have controlled three factors: (1) shaking time, (2) annealing temperature, and (3) annealing time. The factors influencing lithium ionic conductivity of Li3PS4 prepared via liquid-phase synthesis were quantitatively evaluated using high-energy X-ray diffraction (XRD) measurement coupled with pair distribution function (PDF) analysis. It was revealed from PDF analysis that the amount of Li2S that cannot be detected by Raman spectroscopy or XRD decreased the ionic conductivity. Furthermore, it was revealed that the ionic conductivity of Li3PS4 is dominated by other parameters, such as remaining solvent in the sample and high crystallinity of the sample.

Introduction

Lithium-ion rechargeable batteries (LIBs) have been expected for application to electric vehicles and to play the role of peak shaving and load shifting in a grid.1−5 These power source and power storage demand to improve energy density, rate characteristics, safety, and lifetime.3−5 However, current LIBs using organic liquid electrolytes have a safety issue, and the operating temperature is limited because of combustion hazards.6−11 During past decades, all-solid-state batteries using inorganic solid electrolytes have been developed.12−21 The inorganic solid electrolytes are nonleaking and nonvolatile12,13 and have a wide potential window14−18 and operating temperature,16,19 ensuring safety and durability. Moreover, the lithium-ion transfer number of inorganic solid electrolytes is 1, leading to high-power batteries.20,21

Solid electrolytes are of several types such as oxide and sulfide solid electrolytes; especially, sulfide solid electrolytes show high ionic conductivities.16,22−25 Furthermore, the sulfide solid electrolytes can easily contact the active material via mechanical pressing because of their lower elastic moduli26,27 compared to oxide solid electrolytes.28,29 Therefore, the commercial viability of the all-solid-state battery using sulfide solid electrolytes is expected as the first popularization stage.26,27

Sulfide solid electrolytes are commonly synthesized by solid-phase methods, classified into the mechanical milling method and the method synthesizing via heating the evacuated quartz ampoule. The mechanical milling method is widely used in the synthesis of Li2S–P2S5 sulfide glasses and glass ceramics22,30 and lithium argyrodite Li6PS5X (X = Cl, Br, or I).25,31 Lithium superionic conductor-like crystalline compounds including Li10GeP2S12 have been prepared via heating of the evacuated quartz ampoule.16,32,33 These methods have disadvantages from the aspect of economical demand for large-scale synthesis and low energy because they cannot provide a large amount of the product and take a long time.16,22,25,30,31 Therefore, developing a novel manufacturing method for large-scale synthesis is desired.

Since Liu et al. first reported liquid-phase synthesis of sulfide solid electrolytes,17 many researchers have attempted to synthesize them via liquid-phase synthesis to solve these current issues.17,34−42 Several studies reported that the Li2S–P2S5 solid electrolyte was synthesized via stirring Li2S and P2S5 in polar organic solvents.17,34,38−41 In all cases, the ionic conductivity of Li3PS4 prepared via liquid-phase synthesis was lower than that via solid-phase synthesis.17,34,38−41 The relatively low ionic conductivity of Li3PS4 in the liquid phase is hypothesized to be due to two factors: the residual unreacted Li2S and/or its relatively high crystallinity.39,43,44 Shin et al. reported that the reason for the low ionic conductivity was the unreacted starting material, Li2S.39 However, the unreacted Li2S was not confirmed using X-ray diffraction (XRD) and/or Raman spectroscopy in other studies.17,34−38,40−42 Crystallinity is another factor for influencing the lithium ionic conductivity. Other researchers reported that the ionic conductivity decreased with increasing crystallinity.43,44 In the case of the liquid-phase synthesis, Li3PS4 is highly crystallized during the annealing process for removing the solvent, resulting in the decrease of the ionic conductivity.17,34,38−41

These factors are influenced by synthesis and annealing conditions, which depend on the solvent properties such as solubility and boiling point. However, their contributions to the ionic conductivity have not been totally clarified because quantitative evaluation of the amount of the raw materials such as Li2S and the degree of crystallinity is difficult by using conventional analytical methods such as Raman spectroscopy and XRD measurements. Pair distribution function (PDF) analysis is a method of structural analysis using total scattering data including not only Bragg but also diffuse scattering which can obtain both periodic and nonperiodic structural information. In addition, PDF analysis can quantitatively evaluate the crystallinity of the materials.43,44 Therefore, PDF analysis is expected to be a powerful technique to clarify the reason of the relatively low ionic conductivity of Li3PS4 prepared by liquid-phase synthesis.

In this study, we prepared Li3PS4 electrolytes with various mixing times and annealing conditions via liquid-phase synthesis. High-energy XRD measurement coupled to PDF analysis is performed for the Li3PS4 electrolytes to evaluate quantitatively the amount of the raw material and crystallinity. This study is useful to optimize the liquid-phase synthesis process for sulfide solid electrolytes.

Experimental Section

Synthesis Methods of Li3PS4

Liquid-phase synthesis was used for the preparation of Li3PS4, which was reported by Matsuda et al.37 Li2S (Mitsuwa Chemical Co., Ltd.) and P2S5 (Merck & Co., Inc.), at a molar ratio of 3:1, were shaken with a zirconia ball for up to 360 min at 1500 rpm in ethyl propionate (Sigma-Aldrich Co., LLC). The residual solvent was removed via centrifugation and reduced pressure drying at room temperature. Li3PS4 was synthesized by annealing of the precursor at 80, 100, 120, and 170 °C. In this study, the shaking time varied from 5, 10, 30, 60, 120, 240, to 360 min. In addition, another Li3PS4 was prepared via the mechanical milling method for a reference material. Li2S (Mitsuwa chemical Co., Ltd.), P2S5 (Merck & Co., Inc.), and zirconia balls were added to a zirconia pot and mixed via a planetary ball mill apparatus at 370 rpm for 20 h.

Raman Spectroscopy, Thermogravimetry, XRD Measurements, and Scanning Electron Microscopy Observation

Raman spectroscopy was conducted using a MultiRAM (Bruker Optics Co., Ltd.) equipped with a Nd-YAG laser (1064 nm) at room temperature. Raman spectra of the precursor were recorded between 3600 and 100 cm–1. All samples were sealed in glass vessels in an argon-filled glovebox and measured without air exposure. Thermogravimetry (TG) measurement was performed for the precursor with a shaking time of 360 min. The precursor was loaded on an alumina pan in a glovebox. The alumina pan was transferred into the chamber of the TG instrument, and the chamber was purged with argon gas. The weight loss of the precursor was measured between 25 and 350 °C. XRD measurement was conducted using an X-ray diffractometer (RINIT-Ultima III, Rigaku) with Cu Kα radiation. Samples were sealed in a holder in a glovebox to prevent air exposure. Scanning electron microscopy (SEM) observation was performed using a field emission scanning electron microscope (SU8220, Hitachi High-Technologies) to observe the sample morphology.

High-Energy XRD Measurement Coupled to PDF Analysis

High-energy XRD measurements of all samples were carried out at the SPring-8 (Hyogo, Japan) beamline BL08W. The X-ray energy was 115 keV (0.108 Å), and the scattering X-ray was detected using a flat panel detector. The samples were sealed in a 2ϕ quartz capillary to prevent air exposure. Scattering data of each sample were corrected for background, absorption, multiple scattering, and inelastic scattering. The structure factor S(Q) was obtained from the standardization of the corrected scattering data by the number of atoms and the scattering intensity from one atom. Moreover, the reduced PDF, G(r), was calculated by the Fourier transform of S(Q). G(r) is defined by the equation G(r) = 4πrρ0{ρ(r)/ρ0 – 1}, where r, ρ0, and ρ(r) are the real space distances, average atomic number density, and local atomic number density, respectively. Therefore, the information regarding the existing probability to the real distance is obtained using high-energy XRD measurement coupled to PDF analysis.

Ionic Conductivity

The ionic conductivity of the solid electrolytes was obtained by alternating current impedance measurements. The measurements were conducted at 25 °C within a frequency range from 1 MHz to 1 Hz with an amplitude of 100 mV using an impedance analyzer (ModuLab XM ECS, Solartron Analytical). Pellets of samples, approximately 0.7 mm in thickness and 10.0 mm in diameter, were prepared via uniaxial pressing of a powdery sample at a pressure of 360 MPa in a cell. The pellets were held by two stainless-steel rods. These operations were conducted in a dry argon gas atmosphere.

Results and Discussion

Raman spectra for the precursors with different shaking times are shown in Figure S1. Li3PS4 of solid-phase synthesis and Li2S as the raw material are listed for comparison. Peaks at 372, 422, and 407 cm–1 were observed in the sample after shaking for 5 min. These peaks were attributed to the vibration of the sublattice of Li2S, the PS43– anion, and the P2S74– anion, respectively.45 The peak intensities of Li2S and the P2S74– anion decreased with the increase of the shaking time and completely disappeared after 30 min. In contrast, the peak intensity of the PS43– anion increased with the increase of the shaking time.

Figure S2a shows the TG curve for the precursor prepared after a shaking time of 360 min. The TG curve showed a dramatic weight loss from 50 to 130 °C. The dramatic weight loss in our synthesis may be related to ethyl propionate volatilization. Liang et al. reported a similar result using the tetrahydrofuran solvent during liquid-phase synthesis and mentioned that the weight loss was derived from solvent volatilization.17 According to the TG data, we fixed the annealing temperature at 170 °C for 2 h.

The XRD pattern of the precursor is shown in Figure S2b. The peaks from raw materials were not observed, and the observed Bragg peaks could not be assigned to any known structure. Similar results have been reported.17,36,37

Figure S3 shows the XRD patterns of the precursor with different shaking times after annealing at 170 °C for 2 h. Strong diffraction peaks of Li2S were observed at 27.0 and 31.3°, and weak diffraction peaks of β-Li3PS4 were also observed at 27.4, 29.1, and 29.8° for a sample prepared by shaking for 5 min. The peak intensity of Li2S decreased with the increase of shaking time and disappeared after 60 min, while the peak intensity for β-Li3PS4 increased with the increase of shaking time. These results indicate β-Li3PS4 formation, which is consistent with the result of Raman spectroscopy. XRD and Raman spectroscopy measurements showed the formation of β-Li3PS4; however, Li2S can be detected within 30 min via Raman measurement and within 60 min via XRD measurement. In the previous report, Li3PS4 formed from the surface of Li2S via the conversion reaction of Li2S and dissolved species.41 Raman spectroscopy is sensitive to the information of the surface structure, whereas XRD measurement can obtain information of the bulk structure. Therefore, Raman measurement may not detect the peak derived from Li2S after Li3PS4 formation at the surface, which caused the different disappearance times between the XRD and Raman measurements.

Figure 1 shows the ionic conductivities for the sample prepared with different shaking times after annealing at 170 °C for 2 h. The lithium ionic conductivity logarithmically increased with the increase of the shaking time. It was observed that the particle size of the sample was a few micrometers from 5 to 60 min with shaking time and decreased to sub-micrometer from 60 to 360 min with shaking time by SEM (Figure S4). This particle size tendency cannot explain the tendency of the ionic conductivity change with shaking time. Although the XRD result shown in Figure S3 suggests that Li2S disappeared after 60 min shaking time, the lithium ionic conductivity increased above 60 min shaking time. The reason for the low ionic conductivity at the short shaking time is the remaining Li2S; our XRD results did not seem to agree well with the ionic conductivity result. In addition, the ionic conductivity of the liquid-phase synthesis was lower than that of the mechanical milling preparation. To clarify the reason, XRD measurement is not a sufficiently powerful tool because of the trace amount of the raw material and the low crystallinity of products. We attempted to confirm the reason for the low ionic conductivity using PDF analysis.

Figure 1.

Figure 1

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Correlation between the shaking time and the sample ionic conductivity after annealing at 170 °C for 2 h at 25 °C.

Figure S5 shows the results of the PDF analysis for Li3PS4 prepared via liquid-phase synthesis with various shaking times and starting materials. After shaking for 360 min, the sample did not include starting materials and was nearly all Li3PS4. Li3PS4 had peaks at approximately 2.0, 3.3, and 4.1 Å and a broad peak around 7.0 Å corresponding to the P–S bond in the PS43– tetrahedral anion, S–S correlation in the PS43– tetrahedral anion, S–S correlation between the PS43– tetrahedral anions, and P–P correlation between the PS43– tetrahedral anions, respectively. Li2S had characteristic peaks at approximately 2.5, 4.0, and 7.0 Å, as shown by the brown line in Figure S5. The peaks of Li2S were observed in the samples prepared by shaking from 5 to 10 min. Then, the peak intensity of Li2S decreased with the increase of the shaking time, whereas the peak intensity of Li3PS4 increased. It was reported in a previous study that differential PDF analysis can be used as a method for evaluating the ratio of glassy and crystalline phases.43 We applied this method for estimating the ratio of the unreacted substance Li2S and the product substance Li3PS4. We describe the details of the analytical method in the Supporting Information. The ratio of Li2S to Li3PS4 estimated using the method is shown in Figure 2a. The ratio of Li2S decreased, while the ratio of Li3PS4 increased with the increase of the shaking time. Intriguingly, it was shown using this analysis method that Li2S, which was not detected in the Raman spectroscopy and XRD measurements, remained at several percentage points in the samples prepared by shaking up to 120 min. The sample prepared by shaking for 360 min did not have the unreacted material Li2S. The correlation between the volume ratio of Li2S and the ionic conductivity is shown in Figure 2b. The ionic conductivity increased with a decreasing amount of Li2S with the increase of shaking time. However, Li3PS4 prepared via liquid-phase synthesis showed a lower ionic conductivity than that of the mechanical milling method despite not including Li2S. These results clearly indicate that high-energy XRD coupled to PDF analysis is a powerful tool to detect trace amounts of compounds which cannot be detected by XRD.

Figure 2.

Figure 2

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(a) Correlation of the ratio of Li3PS4 to Li2S obtained from G(r) in samples after annealing at 170 °C for 2 h to the shaking time from 5 to 360 min. (b) Correlation between volume ratio of Li2S and ionic conductivity at 25 °C of Li3PS4 prepared using annealing precursors with different shaking times at 170 °C for 2 h.

High-energy XRD coupled to PDF analysis showed that the starting materials were completely removed in the sample obtained with a 360 min shaking time. Therefore, we compared the structure of Li3PS4 prepared by the mechanical milling method and liquid-phase synthesis after annealing. The XRD patterns of Li3PS4 prepared by annealing the precursor synthesized via liquid-phase synthesis and the mechanical milling method are shown in Figure S8. The XRD pattern of Li3PS4 prepared by liquid-phase synthesis was assigned to β-Li3PS4 (Figure S8a). In contrast, the XRD pattern of Li3PS4 prepared by mechanical milling showed broad peaks that were difficult to assign (Figure S8b). Therefore, samples prepared via liquid-phase and solid-phase synthesis were compared using high-energy XRD measurement coupled to PDF analysis, which can analyze both crystalline and amorphous structures to examine the local structure. The results of PDF analysis using the data are shown in Figure 3. Both the samples prepared via mechanical milling and liquid-phase synthesis after annealing showed the characteristic peaks of Li3PS4 at approximately 2, 3.3, and 4.1 Å and a broad peak around 7.0 Å, which were related to the P–S correlation, S–S correlation in the PS43– anions, S–S correlation between the PS43– anions, and P–P correlation between the PS43– anions, respectively. The previous study reported that increasing the peak intensity of the S–S correlation and P–P correlation between the PS43– anions was in accordance with increasing crystallinity.43 These peaks of the sample prepared via liquid-phase synthesis were higher than those via solid-phase synthesis. This result implies that the annealing sample prepared via liquid-phase synthesis had higher crystallinity than the sample prepared by the mechanical milling method. This is the reason for the lower ionic conductivity of the solid electrolyte prepared via liquid-phase synthesis than that of the solid electrolyte prepared by solid-phase synthesis because the ionic conductivity of Li3PS4 decreased with increasing crystallinity.

Figure 3.

Figure 3

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Reduced PDFs of Li3PS4 prepared via (a) liquid-phase synthesis after annealing at 170 °C for 2 h (black line) or (b) solid-phase synthesis (red line).

Therefore, we annealed the precursor not including raw materials, which was confirmed by PDF analysis, at low temperature for the suppression of crystallization and optimized the annealing time. Figure 4 shows the lithium ionic conductivity of samples annealed at 80, 100, and 120 °C for different times under an Ar atmosphere. In the case of the sample annealed at 80 °C, the lithium ionic conductivity increased with the increase of the annealing time and reached a maximum value (0.109 mS cm–1) at 12 h. Subsequently, it decreased with the increase of the annealing time. In the case of the sample annealed at 100 °C, the lithium ionic conductivity showed the same tendency as the samples annealed at 80 °C. In addition, it showed a higher ionic conductivity (0.167 mS cm–1) than that of 80 °C. The ionic conductivity of the samples annealed at 120 °C decreased with the increase of the annealing time, the tendency of which was different from the others. The lithium ionic conductivity of all samples prepared via liquid-phase synthesis was lower than the ionic conductivity of the mechanical milling method. We focused on the samples annealed at 100 °C and examined the relationship between the structure and the ionic conductivity. The XRD patterns of the samples annealed at 100 °C in each annealing time are shown in Figure S9. The Bragg peaks at 27.4, 29.1, and 29.8° attributed to Li3PS4 (space group: Pnma) were observed from all samples. The Bragg peaks were obtained from all annealing samples, and the peaks increased with annealing time and temperature. It is reported that Li3PS4 with an amorphous structure shows higher lithium ionic conductivity than Li3PS4 with a crystal structure.43,44 This result can explain the reason why the lithium ionic conductivity was decreased by annealing over 12 h but cannot explain the reason why the lithium ionic conductivity increased with annealing time and reached a maximum value at 12 h.

Figure 4.

Figure 4

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Lithium ionic conductivity of samples prepared by liquid-phase synthesis after annealing at 80 °C (black plots), 100 °C (red plots), and 120 °C (green plots) for each time.

Therefore, high-energy XRD measurement coupled to PDF analysis was applied to compare the local structure of each sample for revealing the reason. The result of PDF analysis of samples annealed at 100 °C is shown in Figure S10. The samples prepared via liquid-phase synthesis and the mechanical milling method showed peaks of Li3PS4 around 2.0, 3.3, and 4.1 Å and a broad peak around 7.0 Å. In addition, the peak corresponding to S–S correlation between the PS43– tetrahedral anions increased with the increase of annealing time. It is reported that the peak intensity of S–S correlation between PS43– units increased with the increase of crystallinity and the crystallinity can be quantitatively evaluated by reproducing the experimental PDF with amorphous Li3PS4 and crystalline Li3PS4 by the following equation.43 We quantitatively evaluated the crystallinity of the samples by using same technique.

graphic file with name ao0c04307_m001.jpg

where G(r)experimental data, G(r)amorphous, G(r)crystal, and x are the reduced PDF of the sample prepared by liquid-phase synthesis after annealing at 100 °C, reduced PDF of Li3PS4 prepared by mechanical milling, reduced PDF of Li3PS4 prepared by mechanical milling after annealing at 270 °C, and crystallinity, respectively. Figure 5a shows the annealing time dependence of the crystallinity. The crystallinity of the samples annealed at 100 °C increased with the increase of the annealing time. These results indicated that the crystallinity of Li3PS4 prepared via liquid-phase synthesis increased as annealing time increased. The correlation between the crystallinity and the ionic conductivity of the samples annealed at 100 °C is shown in Figure 5b. The lithium ionic conductivity decreased in the annealing time over 12 h because the crystallinity of Li3PS4 increased. This result was consistent with the results of XRD measurement. Although this result can explain the reason why the lithium ionic conductivity was decreased by annealing over 12 h, it cannot explain why the lithium ionic conductivity increased by the annealing time of 12 h.

Figure 5.

Figure 5

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(a) Correlation between the annealing time and the crystallinity of the samples annealed at 100 °C. (b) Correlation between the crystallinity and the lithium ionic conductivity of the samples annealed at 100 °C.

We compared the reduced PDF of samples prepared via liquid-phase synthesis and the mechanical milling method for revealing the reason of the increase of the ionic conductivity by the annealing time of 12 h. The results of PDF analysis of the sample annealed at 100 °C for 6 and 12 h and Li3PS4 prepared by the mechanical milling method are shown in Figure 6a. We focused on the shoulder peak observed at around 4.22 Å in the case of the sample annealed at 100 °C. Figure 6b shows an enlarged view around 4.0 Å. The sample prepared by liquid-phase synthesis showed the peak of S–S correlation between PS43– units at 3.97 Å and a shoulder peak at around 4.22 Å. The shoulder peak was not obtained from the PDF of Li3PS4 prepared by the mechanical milling method. As shown in Figure S10, the shoulder peak decreased with the increase of the annealing time. In the case of liquid-phase synthesis, Li3PS4 was formed by removing the solvent from the complex of Li3PS4 during the annealing process. In the result of TG–differential thermal analysis (DTA) measurement as shown in Figure S11a, the TG curve showed a significant weight loss between 80 and 120 °C and the DTA curve showed the two endothermic peaks at 85 and 121 °C. This result indicates that some intermediate states existed during the annealing process. The Raman spectra of the precursor and samples after annealing at 100 °C are shown in Figure S11b. The Raman spectrum of the precursor was recorded, and the peak around 2900 cm–1 was attributed to ethyl propionate.37 In addition, Raman spectra of the samples after annealing at 100 °C were recorded; the peak was attributed to ethyl propionate, and the peak decreased with the increase of annealing time. This result indicates that the intermediate remained in the sample after annealing at 100 °C and the amount of the intermediate decreased with the increase of annealing time. Considering these results, we considered that the shoulder peak, as seen in Figure 6b, was attributed to the intermediate states formed during the annealing process and that the intermediate remained in the samples in the short annealing time and decreased as the annealing time increased. Therefore, the lithium ionic conductivity was increased by decreasing intermediate with annealing time between 6 and 12 h. These results showed that the decrease of the intermediate and the increase of the crystallinity occurred simultaneously as annealing time increased, indicating that ionic conductivity shows a volcano behavior.

Figure 6.

Figure 6

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(a) Reduced PDF G(r) of samples annealed at 100 °C for 6 h (red line) and 12 h (blue line) and Li3PS4 prepared by the mechanical milling method (black line). (b) Enlarged view of the reduced PDF from 3.0 to 5.0 Å.

Conclusions

In this study, the factors determining lithium ionic conductivity in Li3PS4 prepared via liquid-phase synthesis were studied. High-energy XRD coupled to PDF analysis can detect trace amounts of the Li2S starting material, which were difficult to detect using Raman spectroscopy and XRD measurements (Figure 7a). It takes a longer sample shaking time than 360 min to completely remove the starting material. In addition, it was shown that the Li3PS4 prepared via liquid-phase synthesis after a long shaking time and annealed at 170 °C had higher crystallinity than that prepared via solid-phase synthesis.

Figure 7.

Figure 7

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(a) Scheme of detection limits of Li2S in the case of Raman and XRD measurements and high-energy XRD coupled to PDF analysis. (b) Scheme of the effect of annealing conditions on ionic conductivity of Li3PS4 prepared by the liquid-phase synthesis.

Therefore, we optimized the annealing process in the liquid-phase synthesis of Li3PS4 for the suppression of crystallization. The sample annealed at 100 °C showed the highest ionic conductivity (0.167 mS cm–1) at 12 h for the annealing time, whose ionic conductivity was higher than the ionic conductivity of the sample annealed at 170 °C. However, the sample prepared by annealing at 100 °C shows lower ionic conductivity than that prepared by the mechanical milling method. The high-energy XRD measurement coupled to PDF analysis indicates that the residual amount of the intermediate in the sample decreased with the increase of the annealing time, while the crystallinity of Li3PS4 increased. This trade-off relationship between the crystallinity and the residual amount of the solvent in the sample determines the lithium ionic conductivity as shown in Figure 7b.

Acknowledgments

This work was partially supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan. The synchrotron radiation experiments were performed at BL04B2 and BL08W of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposals 2017A1023, 2018A1023, 2018B1027, 2018B1030, and 2019A1017).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c04307.

  • Raman spectra of raw materials and precursors at different shaking times; TG curves of Li3PS4 and the precursor prepared by shaking for 360 min; XRD patterns of samples prepared at different shaking times after annealing at 170 °C for 2 h; SEM images of Li3PS4 prepared by shaking for each time; reduced PDF, G(r), of Li3PS4 prepared by heating precursors with different shaking times at 170 °C for 2 h; details of the differential PDF analysis method; XRD patterns of Li3PS4 prepared via liquid-phase synthesis and solid-phase synthesis; XRD patterns of samples annealed at 100 °C; reduced PDF of Li3PS4 prepared by the mechanical milling method and the precursor and the samples annealed at 100 °C for each time; and TG–DTA curve of the precursor and Raman spectra of the samples annealed at 100 °C with different annealing times (PDF)

Author Contributions

K.Y., A.H., M.T., H.M., A.M., and Y.U. conceived and directed the project. K.Y., M.T., N.H.H.P., and A.S. synthesized the sulfide solid electrolyte and measured the electrochemical properties. M.T., K.O., T.W., and T.U. performed high-energy XRD. M.T. and K.O. analyzed the local structure of the solid electrolyte by PDF analysis.

The authors declare no competing financial interest.

Notes

⊥ K.Y. and M.T. co-first author.

Supplementary Material

ao0c04307_si_001.pdf (518.4KB, pdf)

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