Rapid Synthesis of Li₁₀GeP₂S₁₂-type Li-Si-P-S-Cl Solid Electrolytes via a Solution Method

Abstract

Owing to its high ionic conductivity, Li₁₀GeP₂S₁₂ (LGPS)-type Li-Si-P-S-Cl (LSiPSCl) solid electrolytes are promising candidates for all-solid-state batteries. This study introduces an LGPS-type LSiPSCl solid electrolyte synthesized rapidly via a solution method using excess sulfur and a solvent mixture of acetonitrile, tetrahydrofuran, and ethanol to enable large-scale production. X-ray diffraction patterns reveal an LGPS-type structure as the primary phase, while FE-SEM analysis confirms the presence of few large particles exceeding 5 µm. The LSiPSCl solid electrolyte synthesized via the solution method exhibits an ionic conductivity of 2.7 mS cm⁻¹, which is comparable to that of the sample synthesized using the mechanical milling method (3.1 mS cm⁻¹). In addition, the all-solid-state battery incorporating LSiPSCl synthesized using the solution method exhibits a slightly higher discharge capacity and similar cycle stability compared with the battery containing LSiPSCl synthesized using the mechanical milling method. These results confirm that the solution method successfully produces an LSiPSCl solid electrolyte. Raman and X-ray photoelectron spectroscopy analyses reveal a carbon surface layer on the particles originating from the solvent. This surface layer is identified as a key factor contributing to the higher discharge capacity of the all-solid-state battery containing the LSiPSCl solid electrolyte synthesized using the solution method. These findings suggest that the surface layer on the particles and/or particle characteristics are critical advantages of solution synthesis for improving battery performance.

1. Introduction

All-solid-state batteries have gained considerable interest owing to their numerous advantages, including higher safety, greater energy density, superior rate performance, and the ability to operate over a wider temperature range than conventional lithium-ion batteries.¹ These batteries include inorganic solid electrolytes instead of organic liquid electrolytes, making the selection and optimization of the solid electrolyte crucial for overall battery performance. Notably, among the various options, sulfide-based solid electrolytes exhibit high ionic conductivities ranging from 10⁻⁴ to 10⁻² S cm⁻¹ at room temperature.

Li₁₀GeP₂S₁₂ (LGPS) is a well-known sulfide-based electrolyte with a high ionic conductivity, 12 mS cm⁻¹ at 300 K, which is similar to that of organic liquid electrolytes.² This high ionic conductivity can be attributed to the LGPS-type crystal structure, which consists of one-dimensional chains comprising LiS₆ octahedra and [Ge/P]S₄ tetrahedra sharing a common edge and interconnected by PS₄ tetrahedra at a common corner. These polyhedral structural units form a framework in which Li⁺ ions are extensively distributed along the c-axis. As a second-generation electrolyte, Li_9.54Si_1.74P_1.44S_11.7Cl_0.3 (LiSiPSCl) with an LGPS-type structure, in which Si and Cl substitute parts of Ge and S, exhibits an ionic conductivity of 25 mS cm⁻¹ at 298 K.¹ Furthermore, a third-generation electrolyte, Li_9.54[Si_0.6Ge_0.4]_1.74P_1.44S_11.1Br_0.3O_0.6, has also been reported to show an ionic conductivity of 32 mS cm⁻¹ at 300 K.³

LGPS-type solid electrolytes are typically synthesized using the mechanical milling method, which is effective for material screening. However, this method consumes a significant amount of energy and is difficult to scale up in controlled atmospheres. One solution to these challenges is the development of a liquid-phase synthesis method. In a previous study, LiSiPSCl solid electrolytes were synthesized using acetonitrile (ACN) in two steps over 120 h; however, a shorter synthesis method is still needed.⁴ Our group has previously attempted to rapidly synthesize LGPS solid electrolytes using a solution method with excess sulfur and a solvent mixture of ACN, tetrahydrofuran (THF), and ethanol (EtOH).^5,6 The LGPS solid electrolyte was prepared in 15 h, which is the fastest synthesis time achieved to date. This solution synthesis method was employed in the current study to rapidly synthesize LiSiPSCl solid electrolytes as second-generation LGPS-type solid electrolytes. The particle characteristics of LiSiPSCl solid electrolytes synthesized using the solution and mechanical milling methods were analyzed and compared through particle size analysis, impedance measurements at low temperatures (200 K), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Furthermore, the mechanical property was assessed using indentation tests. Finally, the all-solid-state battery performances incorporating the LiSiPSCl solid electrolytes were evaluated.

2. Experimental

2.1 Solid electrolyte synthesis

In the solution method, raw materials, including Li₂S (Mitsuwa Chemicals, 99.9 % purity), P₂S₅ (Merck, 99 % purity), SiS₂ (Mitsuwa Chemicals, 99 % purity), LiCl (Sigma-Aldrich, 99.98 % purity), and elemental sulfur (Sigma-Aldrich, 99.98 % purity), were mixed in a molar ratio of 4.86 : 0.78 : 1.47 : 0.3 : x (where x = 5–15) to achieve the chemical composition of Li_10.02Si_1.47P_1.56S_11.7Cl_0.3, as reported in a previous study.⁴ The mixed powder was added to a solvent consisting of ACN (Fujifilm Wako Pure Chemical, 99.8 % purity), THF (Sigma-Aldrich, 99.9 % purity), and EtOH (Konishi Chemical, 99.5 % purity) in a volume ratio of 1 : 1 : 0.005.⁶ Molecular sieves were used to remove water from the THF and EtOH solvents before use. After stirring and dissolving for 30 min, the solution was vacuum-dried at 130 °C for 2 h, with optimization performed using various temperatures and drying times. The precursor powder was pelletized via uniaxial pressing at 25 °C. The pellets were then placed in a SiO₂ tube using an Al₂O₃ boat. For comparison, heat treatment was conducted on the powder instead of the pellets. Argon gas was supplied to the tube through the inlet and allowed to pass through the outlet. The precursor pellet underwent heat treatment in a tube furnace at different temperatures (475 °C, 550 °C, 600 °C, and 650 °C for 12 h) to synthesize the LSiPSCl solid-electrolyte powder. Noted that excess sulfur was removed by heat treatment. In the mechanical milling method, raw materials were mixed in a molar ratio of 4.86 : 0.78 : 1.47 : 0.3. The powder was mixed using a planetary ball mill (Pulverisette 7, Fritsch) with a zirconia pot with zirconia balls (φ = 10 mm × 15 mm) at a rotation speed of 370 rpm for 40 h to obtain the precursor. The precursor powder was then pelletized via uniaxial pressing, and the precursor pellet was heat-treated in a tube furnace at 550 °C for 12 h.¹

2.2 Characterization

The crystal structures were evaluated using X-ray diffraction (XRD, Smartlab SE, Rigaku) with the samples sealed in holders (Rigaku) inside an Ar-filled glove box. Particle size and morphology were examined using scanning electron microscopy (SEM S-4800, Hitachi High-Tech). The particle size distributions of the LSiPSCl solid electrolytes dispersed by a particle disperser tool (HORIBA, XD-100) were determined using an SEM (JEOL, JSM-7900F) and a particle analysis application (Oxford Instruments, AZtecLive). In addition, XPS measurements were conducted to analyze the surface states of the LSiPSCl solid electrolytes. Specifically, XPS measurements of the S 2p and C 1s orbital profiles were performed at the BL7U beamline of the Aichi Synchrotron Radiation Center, Japan. An incident X-ray energy of 260 eV was used to evaluate the S 2p spectra, whereas 650 eV X-rays were used for the C 1s spectral analyses. The kinetic energy was converted to binding energy prior to analysis, and the data were analyzed using Casa XPS software. The obtained spectra were calibrated using the C 1s peak of hydrocarbon species at 285 eV.⁷ According to a previous study,⁸ the probing depth is approximately 2 nm. The local structure and carbon content were investigated using Raman spectroscopy (NRS-4500, Jasco) with a 532 nm laser, with the samples sealed within an Ar-filled glove box.

The mechanical properties of the LSiPSCl solid electrolytes were evaluated by an indentation method, and indentation tests were performed using a Tensilon universal material-testing instrument (RTF-1250, A&D), on samples immersed in silicon oil.⁹ The pellets (φ = 10 mm) of the LSiPSCl solid electrolyte powders were prepared via uniaxial pressing under 561 MPa. The indentation load was measured with a load cell (RTF series, A&D), and the penetration depth was measured as the relative displacement between the plate and test specimen using a non-contact capacitance displacement meter (VT-5210, Ono Sokki Technology). Further tests at the same position with a smaller spherical indenter (φ = 1.2 mm) were conducted after the cycle indentation test with a larger spherical indenter (φ = 12 mm).¹⁰

2.3 Electrochemical properties

The temperature dependence of the ionic conductivities of the solid electrolytes was examined using electrochemical impedance spectroscopy (EIS) (Hz-Pro; Hokuto Denko) in the frequency range of 1 MHz to 10 Hz. The samples for the EIS measurement were prepared by uniaxially pressing 80 mg of the LSiPSCl solid electrolyte powders into pellets at a pressure of 254 MPa. The electronic conductivities of the solid electrolytes were measured using the direct current (DC) polarization technique (Hz-Pro; Hokuto Denko). Impedance measurements at low temperatures (200 K) were conducted by applying an amplitude of 50 mV in the frequency range of 100 MHz to 20 Hz using a Keysight E4990A frequency-response analyzer.

2.4 Battery performance

An all-solid-state battery was assembled to evaluate the battery performance of the solid electrolyte synthesized by the mechanical milling and solution method. A cathode composite comprising LiNbO₃-coated LiCoO₂ and LSiPSCl solid electrolyte synthesized by the solution method (LSiPSCl-15S heat treated at 550 °C for 12 h) and mechanical milling (70 : 30 wt%) was prepared using an agate mortar. The Li₁₀GeP₂S₁₂ solid electrolyte synthesized by mechanical milling,⁶ was used as the separator, with a Li–In alloy (φ = 8 mm In foil and φ = 5 mm Li foil) serving as the anode. Bilayer pellets (φ = 10 mm), comprising a cathode composite (5 mg) and separator layer (80 mg), were prepared via uniaxial pressing at 254 MPa, after which the Li–In alloy was mounted onto the pellets. Two stainless steel rods were used as current collectors. The constant current charge–discharge measurements were conducted using a charge–discharge device (BST-2004H, Nagano) in an Ar atmosphere at 0.05 C. The cutoff voltages were 2.0–3.6 V. All cells were placed in an insulation box maintained at 30 ± 2 °C to ensure a consistent temperature.

3. Results and Discussion

3.1 XRD and ionic conductivity analysis

Figure S1a shows the UV-Vis spectra of the LSiPSCl-10S precursor solutions synthesized using the solution method. The spectra exhibit features corresponding to chemical species such as S₄²⁻, S₆²⁻, and S₃^·−, consistent with previous reports on Li₇P₃S₁₁,¹¹ Li₆PS₅X (X = Cl, Br, I),¹² and LGPS.⁵ The high polarity of EtOH stabilizes polysulfides, particularly the S₃^·− radical anions, which enhances the reaction kinetics and solubility in the LSiPSCl precursor solution. In addition, our previous study demonstrated that halogen elements can accelerate the dissociation of P₂S₅. Consequently, the chemical reactions involved in LSiPSCl synthesis with excess sulfur proceeded via nucleophilic attack of halide ions and S₃^·− radical anions,¹² resulting LSiPSCl precursors were obtained rapidly in the solution method.

Figure 1a shows the XRD patterns of the LGPS-type LSiPSCl-xS precursors with x = 5–20. As the sulfur content increased, the intensity of the Li₂S-derived peak decreased, indicating that the amount of the soluble polysulfide Li₂S_x increased. Notably, the XRD patterns of the LSiPSCl-10S and LSiPSCl-15S precursors show the lowest peak intensities for Li₂S. Figure S1b shows the TG-DTA curve of the LSiPSCl-5S precursor powder. Significant weight loss occurred at approximately 250 °C, indicating that elemental sulfur evaporated at approximately 300 °C, which is consistent with the results of a previous report.^11,12 Figure 1b shows the XRD patterns after heat treatment of the LGPS-type LSiPSCl-xS solid electrolytes (x = 5–20) synthesized using the solution method with heat-treated at 550 °C for 12 h and the corresponding one using the mechanical milling method. All the samples exhibit XRD patterns consistent with the LGPS-type phase, along with several impurities, such as Li₆PS₅Cl,¹² Li₃PO₄,¹² and β-Li₃PS₄.⁴ Li₃PO₄ formation was derived from oxygen elements of organic solvents such as THF and EtOH, similar to Li₆PS₅X (X = Cl, Br, I) cases.¹² Notably, the XRD pattern of the LSiPSCl-20S solid electrolyte shows a lower peak intensity.

Figure 1.

(a) XRD patterns of LGPS-type LSiPSCl-xS precursors (x = 5–20) synthesized via the solution method. (b) XRD patterns of LGPS-type LSiPSCl-xS solid electrolytes (x = 5–20) synthesized using the solution method and LSiPSCl solid electrolytes synthesized via mechanical milling with heat treatment at 550 °C for 12 h.

The ionic conductivities at 25 °C for the LSiPSCl-5S, LSiPSCl-10S, LSiPSCl-15S, and LSiPSCl-20S samples were 0.93, 1.1, 1.6, and 0.96 mS cm⁻¹, respectively. Thus, the LSiPSCl-15S solid electrolyte exhibits the highest ionic conductivity among the samples synthesized using the solution method. This can be attributed to the efficient reaction process, which results in minimal amounts of Li₂S and S residues in the precursor solution of LSiPSCl-15S. The appropriate amount of the excess sulfur does not depend on the heat treatment condition because the excess sulfur helps to form the soluble Li₂S_x in the precursor solution. In addition, the effects of heat-treatment temperature on the LSiPSCl-15S synthesized using the solution method were investigated. Figure S2 shows the XRD patterns of LSiPSCl-15S heat-treated at 475 °C, 550 °C, 600 °C, and 650 °C for 12 h. The sample heat-treated at 475 °C for 12 h exhibits lower crystallinity, while heat treatment above 600 °C decreases the purpose crystal phase. The LSiPSCl-15S samples heat-treated at 475, 550, 600 °C, and 650 °C for 12 h exhibited ionic conductivities of 0.67, 1.4, 0.23, and 0.23 mS cm⁻¹, respectively, indicating that 550 °C for 12 h is the optimal heat-treatment condition for the LSiPSCl solid electrolyte synthesized via the solution method. An additional experiment was conducted to compare the effects of heat treatment on the material in different forms (powder and pellet). Figure S3 shows the XRD patterns of the LSiPSCl solid electrolyte synthesized using heat treatment of the powder-form precursor. This shows that the amount of powder-form precursors does not affect the crystallinity in the heat-treatment process. The ionic conductivities at room temperature were 2.4, 1.7, and 1.6 mS cm⁻¹ for 450, 600, and 800 mg of precursor powder used during heat treatment, respectively, showing similar trends. These results indicate that neither the pellet/powder form nor the amount of precursor powder in the case of the powder form during heat treatment significantly affects the ionic conductivities.

In contrast, the mechanically milled sample exhibited XRD patterns consistent with the LGPS-type phase, with no discernible peaks corresponding to impurities such as Li₃PO₄¹² and β-Li₃PS₄.⁴ Thus, the XRD patterns of the solution-synthesized sample are similar to those of the mechanically milled sample, which exhibits slightly higher purity. The maximum ionic conductivities at room temperature for the LSiPSCl solid electrolytes synthesized via the solution method and mechanical milling were 2.7 and 3.1 mS cm⁻¹, respectively. In contrast, the electronic conductivities at room temperature were 3.3 × 10⁻⁸ and 2.2 × 10⁻⁹ S cm⁻¹ for the solution-synthesized and mechanically milled samples, respectively. Figure S4 shows the temperature dependence of the ionic conductivity for the LSiPSCl solid electrolytes synthesized using the solution method and mechanical milling. The samples showed similar activation energies of 21 and 18 kJ mol⁻¹, and the ionic conductivity values of 2.7 mS cm⁻¹ at 27 °C and 2.4 mS cm⁻¹ at 30 °C. The solution synthesis method resulted in a comparable ionic conductivity and an electronic conductivity one order of magnitude higher than the mechanical milling method. The LGPS-type LSiPSCl solid electrolytes were successfully synthesized using the solution method with excess sulfur and a solvent mixture of ACN, THF, and EtOH in approximately 15 h, which, to the best of our knowledge, represents the fastest synthesis time to date.

3.2 Particle size analysis

Figure 2 shows the particle size distributions of the LSiPSCl solid electrolytes, as analyzed using SEM (Fig. S5). The LSiPSCl solid electrolyte synthesized via the solution method exhibits a size distribution with an average particle size (d₅₀) of 2.3 µm, which is significantly smaller than that of the mechanically milled sample (4.9 µm). These results align with the SEM images (Fig. S6), which show fewer large particles (over 5 µm) in the solution-synthesized LSiPSCl solid electrolyte. In contrast, mechanical milling resulted in larger particles. Previous research on LGPS-type solid electrolytes indicates that excess sulfur coats the precursor particles, preventing grain growth during heat treatment and promoting the formation of smaller particles, which is a key advantage of solution synthesis.

Figure 2.

Particle size distribution of LGPS-type LSiPSCl solid electrolytes synthesized via the solution method and mechanical milling.

3.3 Electrochemical analysis

Figure 3 shows the AC impedance measurements at 200 K, which were used to distinguish between bulk and grain boundary resistances. The inset shows an enlarged graph of the high-frequency region along with the equivalent circuit.⁴ A reasonable fit was achieved using this equivalent circuit. The values of both types of resistance in the best matching cases were calculated by fitting the Nyquist plot using the equivalent circuit (Table 1). The bulk resistance of the solution-synthesized sample was approximately 1.03 times that of the mechanically milled sample. However, the grain boundary resistance of the solution-synthesized sample was approximately 1.35 times higher than that of the mechanically milled sample. The fitting results with the same grain boundary resistance value of the solution-synthesized and mechanically milled samples did not match the raw data. These results indicate that grain boundary resistance plays a more significant role in the total resistance change. Because the number of grain boundaries is correlated with particle size, this result aligns with the particle size analysis. The previous study also revealed that grain boundary resistance changes when the average particle sizes of the sulfide electrolyte decrease to several micrometers.¹³ Compared with previous reports on LGPS-type solid electrolytes,⁶ a smaller difference was observed between the solution-synthesized and mechanically milled samples, reflecting a smaller particle size difference. In addition, the smaller particle size, which means a higher grain boundary resistance, causes a decrease in ionic conductivity for the solution-synthesized sample compared with the mechanically milled sample.

Figure 3.

Nyquist plots of LGPS-type LSiPSCl solid electrolytes synthesized via mechanical milling and the solution method at 200 K. The insets show enlarged views of the high-frequency region and the equivalent circuit.

Table 1. Calculated bulk (R_b) and grain boundary (R_gb) resistances of LGPS-type LSiPSCl solid electrolytes at 200 K.

Samples	Rb (kΩ)	Rgb (kΩ)
Mechanical milling	7.0	1.7 × 101
Solution method	7.2	2.3 × 101

Figure 4 shows the charge–discharge curves of all-solid-state batteries incorporating LGPS-type LSiPSCl solid electrolytes synthesized via (a) the solution method and (b) mechanical milling. The all-solid-state batteries exhibited reversible capacities of approximately 100 mAh g⁻¹, with no significant degradation over 20 cycles. Notably, the all-solid-state batteries with solution-synthesized LSiPSCl solid electrolytes showed a slightly higher capacity of 110 mAh g⁻¹. These findings highlight the suitability of LSiPSCl solid electrolytes synthesized via the solution method for all-solid-state batteries. These battery performance differences are later discussed in a separate section.

Figure 4.

Charge–discharge curves of all-solid-state batteries incorporating LGPS-type LSiPSCl solid electrolytes synthesized via (a) the solution method and (b) mechanical milling.

3.4 Particle surface analysis

Figure 5 shows the Raman spectra of the LGPS-type LSiPSCl solid electrolytes synthesized using the solution method and mechanical milling. The Raman spectra exhibit peaks corresponding to the Si–S bonds of the SiS₄ unit at 190 cm⁻¹,⁴ P–S bonds of the PS₄ unit at 280, 440, and 570 cm⁻¹,¹⁴ Si–S and Li–S bonds of the SiS₄ and LiS₆ units at 390 cm⁻¹,^15,16 and S–S bonds at 500 cm⁻¹.¹⁷ The peak intensities of the LSiPSCl solid electrolytes synthesized using the solution method were lower than those of the mechanically milled sample. In contrast, the peak intensities of the solution-synthesized samples at approximately 1440 and 1530 cm⁻¹, corresponding to the D- and G-bands of carbon-related species,¹⁸ were higher than those of the mechanically milled samples. These results suggest the formation of a solvent-induced carbon surface layer on the particles synthesized via the solution method, confirming the carbonization of the organic solvent.¹⁹ In addition, the peak positions differed between the samples, indicating that carbon-related species were formed from the solvent used in the solution method.

Figure 5.

XPS profiles of (a) C 1s and (b) S 2p for LGPS-type LSiPSCl solid electrolytes synthesized using the solution method and mechanical milling.

Figure 6 shows the C 1s and S 2p XPS profiles of the LGPS-type LSiPSCl solid electrolytes. The C 1s spectra of the solution-synthesized and mechanically milled samples exhibit peaks at approximately 285 eV, corresponding to C–C and C–H bonds. The S 2p peaks at ∼162 eV were observed in both samples, which correspond to the Li₆PS₅Cl impurity phase.²⁰ No other peaks were observed, indicating that no excess sulfur residue existed after heat treatment. The relative integrated C 1s peak intensity at 285 eV to the integrated S 2p peak intensity at 162 eV for the solution-synthesized sample (I_C1s/I_S2p = 1.2) is higher than the one obtained with the mechanically milled sample (I_C1s/I_S2p = 0.89), indicating that the particle surface of the LSiPSCl solid electrolytes synthesized via the solution method could contain more carbon elements. This XPS result also supports the hypothesis that LSiPSCl solid electrolytes have more surface elements, such as carbon, at the particle surface, consistent with the Raman spectrum. These carbon-related species could enhance the electronic conductivity in the solution-synthesized LSiPSCl solid electrolyte. Notably, these changes in the XPS spectrum were minor, similar to previously reported results for LGPS,⁶ and differed from the liquid-phase synthesis of Li₇P₂S₈I,²¹ which was heat-treated at a lower temperature of 110 °C. This lower temperature, compared with the 550 °C heat treatment of LSiPSCl, increased the amount of organic solvent residue in the Li₇P₂S₈I case.

Figure 6.

Raman spectra of LGPS-type LSiPSCl solid electrolytes synthesized via the solution method and mechanical milling.

3.5 Mechanical property analysis

The density of the LSiPSCl solid electrolyte pellets was 1.55 and 1.61 g cm⁻³ for the solution-synthesized and mechanically milled samples, respectively, with relative densities of 80.2 % and 83.2 %. No significant differences were observed between the synthesis methods, making both pellets suitable for indentation measurements. Figure 7 shows P-h curves of the LGPS-type LSiPSCl solid electrolytes synthesized via the solution method and mechanical milling, before and after the cyclic indentation test. For the solution-synthesized sample, the penetration depth before the cyclic indentation test was 40 µm, which is larger than the value obtained after the test for 25 µm. In contrast, P-h curves for the mechanically milled sample before and after the indentation test overlap, and the penetration depth had a value of 25 µm (Fig. 7b). A significant difference was observed between the solution-synthesized and mechanically milled samples before the cyclic indentation test, although the same LSiPSCl solid electrolytes were evaluated. These results indicate that LSiPSCl solid electrolytes synthesized via the solution method were deformed more plastically than mechanically milled samples before the cyclic indentation test. The plastic deformation involved in powder deformation under compressive stress consists of (i) shear deformation due to the motion of dislocations within crystalline particles and (ii) densification resulting from the movement of particles and the reduction of voids.¹⁰ Therefore, the P-h curve for the solution-synthesized sample before the cyclic indentation test includes a significant contribution from the movement of particles and the reduction of voids, which means that the powder of the solution-synthesized sample can more easily move than the mechanically milled sample when pressure is applied. These different powder characteristics may affect the microstructure changes of cathode composites.

Figure 7.

P-h curves of LGPS-type LSiPSCl solid electrolytes synthesized using (a) the solution method and (b) mechanical milling, before and after cyclic indentation tests.

3.6 Comparison of synthesis methods

Based on these results, differences in battery performance are discussed in this section. First, the carbon particle surface layer was a critical factor contributing to the electronic conductivity of the LSiPSCl solid electrolytes synthesized via the solution method, consistent with previous studies.^4,5,19 Thus, the carbon particle surface layer is one of the factors for a higher discharge capacity of the all-solid-state battery with the solution-synthesized LSiPSCl solid electrolyte. Second, the particle characteristic of the solution-synthesized LSiPSCl solid electrolytes, resulting from smaller particle sizes and/or changes in the particle surface state, differed from that of the mechanically milled samples. The previous study suggests that the size distribution of Li₁₀GeP₂S₁₂ particles also affected the charge/discharge properties of the composite cathodes, and smaller particles improved the battery performance.¹³ In this study, the microstructure changes derived from the particle characteristics of LSiPSCl solid electrolytes, which were observed at the indentation test, may also affect battery performance. Therefore, these two factors influence the battery performance of LSiPSCl solid electrolytes synthesized via the solution method.

4. Conclusions

LGPS-type LSiPSCl solid electrolytes were synthesized rapidly using a solution method with excess sulfur and a solvent mixture of ACN, THF, and EtOH. The XRD patterns reveal an LGPS-type structure as the primary phase, and FE-SEM analysis confirmed the presence of a small amount of large particles (over 5 µm). LSiPSCl solid electrolytes synthesized using the solution method exhibited an ionic conductivity of 2.7 mS cm⁻¹, which is similar to that of the sample synthesized via mechanical milling (3.1 mS cm⁻¹). The all-solid-state battery with solution-synthesized LSiPSCl exhibited a slightly higher discharge capacity and similar cycle stability compared with the battery containing mechanically milled LSiPSCl. XPS and Raman spectroscopy revealed the presence of a carbon layer on the particle surface derived from the solvent. Indentation test results indicate the powder of the solution-synthesized sample was deformed more plastically, which means it more easily moved than the mechanically milled sample when pressure was applied. The surface carbon layer and/or particle characteristics could contribute to the higher discharge capacity of the all-solid-state battery containing the solution-synthesized LSiPSCl solid electrolyte.

Acknowledgment

XPS experiments were conducted at the BL7U beamline of the Aichi Synchrotron Radiation Center, Aichi Science & Technology Foundation, Aichi, Japan (Proposal Nos. 202204118, 202302109, 202306131, and 202402093). We also thank Dr. Satoshi Obokata for assistance with the indentation experiments.

The authors were waived from the APC with the support of The Committee of Battery Technology, ECSJ.

Data Availability Statement

The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.28622132.

• 1) Owing to its high ionic conductivity, Li₁₀GeP₂S₁₂ (LGPS)-type Li-Si-P-S-Cl (LSiPSCl) solid electrolytes are promising candidates for all-solid-state batteries. This study introduces an LGPS-type LSiPSCl solid electrolyte synthesized rapidly via a solution method using excess sulfur and a solvent mixture of acetonitrile, tetrahydrofuran, and ethanol to enable large-scale production. X-ray diffraction patterns reveal an LGPS-type structure as the primary phase, while FE-SEM analysis confirms the presence of few large particles exceeding 5 µm. The LSiPSCl solid electrolyte synthesized via the solution method exhibits an ionic conductivity of 2.7 mS cm⁻¹, which is comparable to that of the sample synthesized using the mechanical milling method (3.1 mS cm⁻¹). In addition, the all-solid-state battery incorporating LSiPSCl synthesized using the solution method exhibits a slightly higher discharge capacity and similar cycle stability compared with the battery containing LSiPSCl synthesized using the mechanical milling method. These results confirm that the solution method successfully produces an LSiPSCl solid electrolyte. Raman and X-ray photoelectron spectroscopy analyses reveal a carbon surface layer on the particles originating from the solvent. This surface layer is identified as a key factor contributing to the higher discharge capacity of the all-solid-state battery containing the LSiPSCl solid electrolyte synthesized using the solution method. These findings suggest that the surface layer on the particles and/or particle characteristics are critical advantages of solution synthesis for improving battery performance.

CRediT Authorship Contribution Statement

Kazuhiro Hikima: Conceptualization (Lead), Data curation (Equal), Formal analysis (Equal), Funding acquisition (Equal), Investigation (Equal), Methodology (Equal), Project administration (Equal), Supervision (Supporting), Visualization (Equal), Writing – original draft (Lead), Writing – review & editing (Lead)

Ikuyo Kusaba: Data curation (Equal), Formal analysis (Lead), Methodology (Equal), Visualization (Supporting), Writing – review & editing (Equal)

Masaki Shimada: Data curation (Equal), Formal analysis (Equal), Methodology (Equal), Software (Equal), Visualization (Equal), Writing – review & editing (Equal)

Yuhei Horisawa: Data curation (Equal), Formal analysis (Equal), Methodology (Equal), Writing – review & editing (Equal)

Shunsuke Kawaguchi: Formal analysis (Equal), Methodology (Equal), Writing – review & editing (Equal)

Minoru Kuzuhara: Funding acquisition (Equal), Project administration (Equal), Resources (Equal), Supervision (Equal), Writing – review & editing (Equal)

Hiroyuki Muto: Funding acquisition (Supporting), Project administration (Supporting), Supervision (Supporting), Writing – review & editing (Supporting)

Atsunori Matsuda: Funding acquisition (Lead), Project administration (Equal), Resources (Equal), Supervision (Equal), Writing – review & editing (Equal)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Funding

New Energy and Industrial Technology Development Organization: SOLiD-EV (JPNP18003)

New Energy and Industrial Technology Development Organization: SOLiD-NEXT (JPNP23005)

Footnotes

A part of this paper has been presented in the 65th Battery Symposium in Japan in 2024 (Presentation #3F03).

K. Hikima, M. Shimada, Y. Horisawa, S. Kawaguchi, M. Kuzuhara, and A. Matsuda: ECSJ Active Members

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