Ionic Conductivity and Crystal Structure of Li5GaS4 Solid Electrolytes Prepared via Mechanochemical Process
Authors: Kimura Takuya , Hotehama Chie , Sakuda Atsushi , Tatsumisago Masahiro , Hayashi Akitoshi 
Published: December 2020

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.

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