Synthesis and detailed electrochemical analysis of sulfide-based solid electrolyte with practical level ionic conductivity via liquid phase method

<Overview>

　The research team, including master’s student Ryota Kishi, Assistant Professor Kazuhiro Hikima, and Professor Atsunori Matsuda from the Department of Electrical and Electronic Information Engineering, and Professor Hiroyuki Muto from the Institute of Liberal Arts and Sciences at Toyohashi University of Technology; along with Specially Appointed Associate Professor (at the time) Hirofumi Tsukasaki and Professor Shigeo Mori from the Department of Materials Science at Osaka Metropolitan University, successfully synthesized the sulfide-based solid electrolyte Li₁₀GeP₂S₁₂ with practical room-temperature ionic conductivity by optimizing the heat treatment process in the solution method. The Li₁₀GeP₂S₁₂ synthesized solution method exhibits unique electrochemical properties compared to ball-mill-synthesized samples, such as a (1) small particle size with high grain boundary resistance and (2) surface layer from the organic solvent that is highly stable with respect to Li-In anodes. The results of this research were published online in ACS Applied Energy Materials on September 25, 2024.

<Details>

　All-solid-state batteries using flame-retardant inorganic solid electrolytes are expected to be the next-generation batteries for electric vehicles (EVs) owing to their high safety and output characteristics. Much research and development is being conducted on sulfide-based solid electrolytes that exhibit excellent ionic conductivity and plasticity to apply these batteries to EVs. However, sulfide-based solid electrolytes are unstable in the atmosphere; hence, they must be synthesized without exposure to air, which necessitates the development of a low-cost liquid-phase synthesis method suitable for mass production.

 Our group has been actively conducting research on liquid-phase synthesis processes. A previous study in May 2023 indicated that adding an excess amount of sulfur along with Li₂S, P₂S₅, and GeS₂, which are the starting materials for the Li₁₀GeP₂S₁₂ solid electrolyte, to a mixed solvent of acetonitrile, tetrahydrofuran, and a trace amount of ethanol reduced the total synthesis time from approximately three days, to just 7.5 h. Li₁₀GeP₂S₁₂ solid electrolyte synthesized by this method showed a relatively high ionic conductivity (room-temperature conductivity 1.6 mS/cm) (https://doi.org/10.1039/D3CC01018J). However, the ionic conductivity of Li₁₀GeP₂S₁₂ obtained by solution synthesis was lower than that of ball-mill-synthesized samples.

　First, the synthesis conditions were improved to obtain higher ionic conductivity in solution-synthesized samples. The conventional heat treatment process uses a quartz boat (SiO₂). However, the SiO₂ boat resulted in the formation of impurities, such as SiS₂. Therefore, the various boat materials were examined. A Ti boat resulted in a room-temperature conductivity of 5.5 mS/cm. However, the ball-mill-synthesized sample under the same conditions showed an ionic conductivity of 7.9 mS/cm at room temperature; whereas the solution-synthesized sample had a lower ionic conductivity, the reasons were unclear. Therefore, the electrochemical properties (1: ionic conductivity, 2: Li-In anode stability) of the ball-mill-synthesized and solution-synthesized Li₁₀GeP₂S₁₂ were compared to analyze the characteristics and factors of the electrochemical properties of the solution-synthesized sample.

1. Ionic conductivity: The particle size of the solution-synthesized sample was smaller than that of the ball-mill-synthesized sample. In addition, the AC impedance measurements obtained at a low temperature of 190 K were aimed at distinguishing between the bulk resistance and grain boundary resistance. This result showed that the grain boundary resistance of the solution-synthesized sample was significantly higher than that of the ball-mill-synthesized sample. Based on the results, it was concluded that this is the reason why the ionic conductivity calculated from the total resistance became lower.
2. Li-In anode stability: The stability of the Li-In/Li₁₀GeP₂S₁₂ interface was investigated by evaluating the voltage change of a Li-In/Li₁₀GeP₂S₁₂ symmetric cell. Despite its lower ionic conductivity, the solution-synthesized sample showed a lower overvoltage, and the Li₁₀GeP₂S₁₂ interface had a higher stability. X-ray photoelectron spectroscopy also suggested the presence of an organic solvent-derived particle surface layer, which contributes to the interface stability between Li-In and Li₁₀GeP₂S₁₂.

<Future perspective>

　This study presented a liquid-phase synthesis method for sulfide-based solid electrolytes that demonstrates practical room-temperature ionic conductivity while also revealing the unique electrochemical properties and mechanisms associated with the liquid-phase approach. The results highlight the significance of the particle surface state, which has not been considered in previous studies. This study focused on Li₁₀GeP₂S₁₂ as a sulfide-based solid electrolyte with high ionic conductivity. A future goal is to extend this approach to synthesize sulfide-based solid electrolytes beyond Li₁₀GeP₂S₁₂, aiming for higher ionic conductivity and improved anode stability and to analyze the particle surface state, which could significantly enhance performance.

<Paper information>

Kazuhiro Hikima, Ryota Kishi, Hirofumi Tsukasaki, Shigeo Mori, Hiroyuki Muto, and Atsunori Matsuda, Electrochemical properties of Li₁₀GeP₂S₁₂ solid electrolytes synthesized using a solution-based method, ACS Applied Energy Materials, 7, 19, 8788–8796 (2024).

This study was supported by JSPS KAKENHI (JP 21K14716, JP 22H04614), the SOLiD-EV (JPNP 18003) and SOLID-NEXT (JPNP23005) of the New Energy and Industrial Technology Development Organization (NEDO), and the GteX program (JPMJGX23S5) of the Japan Science and Technology Agency (JST).