Ionic Conductivity and Crystal Structure of Li₅GaS₄ Solid Electrolytes Prepared via Mechanochemical Process

High-performance batteries attract attention as the power sources for electric cars with cruising long distance and Smart Grid to balance supply with demand in electric power. All-solid-state batteries using solid electrolytes without leakage are attracting with high safety and reliability. For realization of all-solid-state batteries, the solid electrolytes are needed to have a high ionic conductivity, good ductility, and chemical stability.

Various sulfide solid electrolytes were reported as Li₂S-P₂S₅ glass-based electrolytes, thio-LISICON-type crystals, and the class of argyrodites. We reported Li₂S-P₂S₅ glass and glass-ceramic electrolytes have high ionic conductivities, wide potential windows, and good ductility [1]. In other systems, the electrolytes with thio-LISICON-type and Li₁₀GeP₂S₁₂-type crystal structure were reported to have high conductivity [2-4].

The solid electrolytes of Li₄GeS₄-Li₃PS₄ and Li₄GeS₄-Li₅GaS₄ with thio-LISICON-type crystal structure have the conductivity of 2.2×10⁻³ and 6.5×10⁻⁵ S cm⁻¹ at 25°C, respectively [2,3]. The crystal structures and conductivities at room temperature of Li₄GeS₄ and Li₃PS₄, which are the terminal composition in the binary systems, were reported [3,5]. Another terminal composition Li₅GaS₄ had the low conductivity of 5.1×10⁻⁸ S cm⁻¹ at 100°C, but has not been reported its crystal structure [3].

In this study, we prepared Li₅GaS₄ electrolytes with low crystallinity by mechanochemical process in order to increase their conductivity.

The Li₅GaS₄ solid electrolytes were prepared by mechanochemical process and subsequent heat treatment at 600°C. In the XRD pattern of the sample after mechanochemical process, weak unidentified peaks were observed and different from the peaks of the starting material. This indicated that the sample with low crystallinity including amorphous matrix was obtained. On the other hand, the diffraction pattern of the heated sample was not reported and different from that after mechanochemical process. The peaks in the pattern were able to be indexed in monoclinic system. The sample of Li₅GaS₄ after mechanochemical treatment showed the highest ionic conductivity of 2.2×10⁻⁵ S cm⁻¹ at 25°C. The ionic conductivity of the sample decreased with increasing the crystallinity by heat treatment, and the sample heated at 600°C showed a low value of 2.1×10⁻⁸ S cm⁻¹ at 25°C. This suggests that the structure of Li₅GaS₄ crystal is not suitable for ionic conduction. On the other hand, the ionic conductivity of the Li₅GaS₄ sample increased due to the amorphization by mechanochemical process, and increasing of the free volume in the sample is effective in increasing the conductivity.

References:

[1] M. Tatsumisago and A. Hayashi, Int. J. Appl. Glass Sci., 5, 226 (2014).

[2] R. Kanno and M. Murayama, J. Electrochem. Soc., 148, A742 (2001).

[3] R. Kanno et al., Solid State Ionics, 130, 97 (2000).

[4] N. Kayama et al., Nat. Mater., 10, 682 (2011).

[5] M. Tachez et al., Solid State Ionics, 14, 181 (1984).