Electrochemical and Surface Analysis of Li10GeP2S12 Solid Electrolytes Synthesized Using a Solution-Based Method
Authors: Hikima Kazuhiro , Kishi Ryota , Matsuda Atsunori 
Published: December 2024

All-solid-state batteries using flame-retardant inorganic solid electrolytes are expected to be the next-generation batteries with high safety and high output characteristics. For these, solid sulfide electrolytes with high ionic conductivity are required. Li10GeP2S12 represents a promising solid-electrolyte material for all-solid-state batteries owing to its high Li ionic conductivity (12 mS cm-1) at 27 °C [1]. To achieve practical application, Li10GeP2S12 should be synthesized using the liquid-phase method, which facilitates mass production. Our group has succeeded in a rapid synthesis of Li10GeP2S12 using excess sulfur as a solubilizer and a mixture of acetonitrile (ACN), tetrahydrofuran (THF), and a trace of ethanol (EtOH) as solvents [2]. However, Li10GeP2S12 synthesized using this approach exhibits lower ionic conductivity than that achieved using mechanical milling. In this study, Li10GeP2S12 were synthesized through the solution synthesis as the improved version. In addition, the particle characteristics of the sample synthesized by the solution synthesis were analyzed by transmission electron microscopy, particle size analysis, impedance measurements at low temperatures at 190 K, and X-ray photoelectron spectroscopy.
Starting materials Li2S (Mitsuwa, 99.9 %), GeS2 (Wako, 99.9 %), P2S5 (Aldrich, 99%), and S (Aldrich, 99.8%) were mixed at a molar ratio of 5:1:1:10. The mixed powder was then added to an ACN-THF-EtOH solvent. After stirring and dissolution for 30 min, the obtained solution was vacuum-dried at 130 °C for 1 h. The precursor powder was then pelletized via uniaxial pressing. The pellet was then placed in a SiO2 tube using boats of different materials, such as quartz (SiO2), alumina (Al2O3), and titanium (Ti). Subsequently, the precursor pellet was subjected to heat treatment in a tube furnace at different temperatures (550, 650, and 750 °C) for 8 h to obtain the Li10GeP2S12 solid electrolyte powder. For comparison, this sulfite was also synthesized by mechanical milling. The crystal structure was evaluated by X-ray diffraction (XRD) measurements, the surface state was determined by X-ray photoelectron spectroscopy (XPS), and the grain size and morphology were determined by field-emission scanning electron microscopy. The ionic conductivity was also evaluated using alternating-current impedance measurements, and particle size distribution measurements and transmission electron microscopy observations were performed.
The solution synthesis sample with the Ti boat exhibited an ionic conductivity of 5.5 mS cm-1, which is the highest among the extant liquid phase synthesis studies. According to the XRD patterns, impurity peaks (e.g., at SiS2) were hardly observed in the sample heat-treated in the Ti boat, and a decrease in the full-width-at-half-maximum metric was observed (Fig. 1). This result indicates that the use of a Ti boat can improve crystallinity and suppress side reactions. In comparison, the Li10GeP2S12 samples synthesized by mechanical milling exhibited ionic conductivities of 7.9 mS cm-1, higher than those achieved using solution synthesis, although the solution-synthesized sample exhibited a smaller particle size. This smaller particle size correlates with a higher grain boundary resistance, which leads to a lower total ionic conductivity. Moreover, a surface layer from the solvent was detected on the solution-synthesized particle surfaces according to XPS measurements. This surface layer contributes to a more stable interface of Li-In/Li10GeP2S12. We expect that these findings will enable the effective harnessing of particle states to develop sulfide solid electrolytes with high ionic conductivity and fine particles.

Acknowledgments: This work was supported by JSPS KAKENHI (Grant Number JP 21K14716 and JP 22H04614), and the SOLiD-EV project (JPNP18003), SOLiD-NEXT project (JPNP23005) of the New Energy and Industrial Technology Development Organization (NEDO), and the GteX Program (JPMJGX23S5) of the Japan Science and Technology Agency (JST), Japan. The XPS experiments were conducted at the BL7U beamline in the Aichi Synchrotron Radiation Center, Aichi Science & Technology Foundation, Aichi, Japan (Proposal Nos. 202204118 and 202302109).

References:

[1] N. Kamaya, K. Homma, Y. Yamakawa, M. Hirayama, R. Kanno, M. Yonemura, T. Kamiyama, Y. Kato, S. Hama, K. Kawamoto and A. Mitsui, Nat. Mater.,2011, 10, 682.
[2] K. Hikima, K. Ogawa, H. Gamo and A. Matsuda, Chem. Commun.,2023, 59, 6564–6567.

Figure 1