Relationship between Crystal Structure and Ionic Conductivity in Structural Polymorphs of Li2MS3 (M = Si, Ge) Solid Electrolytes
Authors: Sasaki Yuki , Hori Satoshi , Suzuki Kota , Hirayama Masaaki , Jang Yongjun , Kanno Ryoji 
Published: December 2024

Abstract

All-solid-state Li-ion secondary batteries have been under development as a candidate for next-generation batteries. Among solid electrolytes, sulfide-based one, such as Li7P3S11,1 Li10GeP2S12
2 and Li7-xPS6-xClx
3 are promising because they have high ionic conductivities equivalent to those of liquid electrolytes. These sulfides possess framework structure consisting of PS4 tetrahedra. Similarly, Li-Si-S and Li-Ge-S systems have SiS4 or GeS4 tetrahedra, as compounds with the compositions of Li4
MS4 (M = Si, Ge)4,5 and Li2
MS3
4-6 have been reported. However, their ionic conductivities were low, which was below 2 × 10-6 S cm-1. In 2022, a tetragonal polymorph of Li2SiS3, which shows the high ionic conductivity of 2.4 mS cm-1 at room temperature, was discovered at the P substituted composition of Li1.82PS0.036S3.7 In this study, we aimed to discover solid electrolytes with higher ionic conductivity than that of Li1.82PS0.036S3, via material search in the Li2
MS3 composition region including Li2GeS3 composition containing Ge4+, which has a larger ionic radius than Si4+.

The tetragonal and monoclinic forms were synthesized at Li2SiS3. They showed ionic conductivities of 3.0 × 10-4 S cm-1 and 7.0 × 10-5 S cm-1 at room temperature, respectively. For Li2GeS3, hexagonal and orthorhombic phases were synthesized. Their ionic conductivities were 8.6 × 10-9 S cm-1 and 1.5 × 10-7 S cm-1, respectively. These values for the Li2GeS3 polymorphs are more than two orders of magnitude lower than that of Li2SiS3.

In order to investigate this difference in ionic conductivity, synchrotron radiation XRD measurements and crystal structure analyses were performed. The results revealed that Li2SiS3 has isolated Si2S6 polyhedra with edged-shared SiS4 tetrahedra, while Li2GeS3 has a one-dimensional chain with corner-shared GeS4 tetrahedra. The possible connection styles of these MS4 tetrahedra was explained from the electrostatic structural stability of the coordination polyhedra, by considering the ionic radius of the cation at the center of the MS4 tetrahedra.

In this presentation, we will clarify the relationship between the connection styles of MS4 tetrahedra in Li2
MS3 composition and ionic conductivity, and discuss guidelines for achieving high ionic conductivity in Li2S-SiS2-GeS2 pseudo-ternary crystalline solid electrolytes.

Acknowledgment

Synchrotron XRD measurements were performed at the BL02B2 beamline of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (proposal no. 2023A1647). R.K. acknowledges support from the Open Innovation Platform with Enterprises, Research Institute, and Academia (OPERA) Program (JPMJOP1862) of the Japan Science and Technology Agency (JST). M.H. acknowledges support from a Grant-in-Aid for Scientific Research on Innovative Areas (19H05785).

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