Sulfide electrolytes are key materials for all-solid-state batteries. However, the synthesis of sulfide electrolytes needs care to avoid moisture and to prevent the loss of sulfur species. In this minireview, we introduce the chronological history, property, and synthesis of sulfide solid electrolytes, especially that using organic solvents. We show our recent achievements in the synthesis mechanism of β-Li₃PS₄ via solution process by analyzing intermediates.

A mixed cation phosphate, KCo_1−xH_2x(PO₃)₃ • yH₂O, with a tunnel structure was synthesized by a coprecipitation method and investigated as a proton conductor. KCo_1−xH_2x(PO₃)₃ • yH₂O formed a solid solution in the range of x = 0–0.18, and the enhancement of the absorption peaks corresponding to the vibration and stretching modes of OH and POH were observed for increasing x. The sample with x = 0.18 exhibited a high proton conductivity in the temperature range of 25–250°C, and the value of 2.6 x 10⁻³ Scm⁻¹ was obtained at 250°C. This sample maintained a high proton conductivity of over 10⁻³ Scm⁻¹ at 150°C for 9 h, and no electron conductivity was confirmed in a hydrogen or oxygen gas atmosphere.

We investigated the electrochemically activity of formed LiCoO₂ (LCO) on oxide-based solid electrolytes; Li_0.29La_0.57TiO₃ (LLTO) sintered body by heating the molten salts as starting materials. The effect of surface area and microstructure of LLTO on the electrochemical performance of an all-solid-state Li secondary battery was examined. The surface structure of LLTO was machined mechanically by using a pico-second laser and obtained in various forms of LLTO. Then, LCO on the LLTO was prepared by molten salts. From the microstructural observation, it was confirmed that LCO was formed on the surface of the machined LLTO. A discharge capacity was enlarged as an amount of loading LCO increased. These results suggested that the LCO layer formed on LLTO by molten salts was active electrochemically regardless of the various shape of microstructure of solid electrolyte.

A machine learning method was developed, which predicts ionic conductivity based on chemical composition alone, aiming to develop an efficient method to search for lithium conductive oxides. Under the obtained guideline, the material search was focused on the Li₂O-SiO₂-MoO₃ pseudo-ternary phase diagram, which is predicted to have high ionic conductivity (>10⁻⁴ S • cm⁻¹). We investigated the formation range, ionic conductivity, and crystal structure of the lithium superionic conductor (LISICON) solid solution on the Li₄SiO₄-Li₂MoO₄ tie line. The ionic conductivity of the LISICON phases is about 10⁻⁷ S • cm⁻¹, which is higher than that of the end members; however, two orders of magnitude lower than that of the analogous LISICON materials. In addition, the experimental values were two or three orders of magnitude lower than the predicted conductivity values by machine learning. The crystal structure analysis revealed that the distance between the lithium sites and the occupancy of each lithium site in the crystal structure contributed to the decrease in ionic conductivity. This strong correlation between crystal structure and ionic conductivity was one of the reasons for the discrepancy between the predicted ionic conductivity based on chemical composition alone and the experimental value.

A crystalline solid electrolyte, Li₁₀GeP₂S₁₂ (LGPS), was synthesized by a two-step wet process. In the first step, solvated Li₄GeS₄ was prepared by mixing Li₂S and GeS₂ powders in EtOH solvent for 3 days at 60°C. The solvated Li₄GeS₄ was dried in vacuum at room temperature. In the second step, the obtained Li₄GeS₄ was reacted with Li₂S and P₂S₅ in THF solvent for 3 days at 60°C. After a vacuum drying of the precursor, the crystalline phase of Li₁₀GeP₂S₁₂ was obtained by a heat treatment at 550°C for 1hr in vacuum. The Li₁₀GeP₂S₁₂ sample showed high ion conductivity of 1.0 x 10^-2 Scm^-1 at room temperature. The wet synthesis of the solid electrolyte has advantages for mass-production of all-solid-state batteries.