Li2GeS3: New Structural Type of Lithium Solid Electrolyte for All-Solid-State Batteries

Abstract

Sulfide inorganic materials exhibit favorable qualities such as high ionic conductivity and mechanical softness, making them promising candidates for solid electrolytes in Li-ion all-solid-state batteries. Sulfide solid electrolytes exist in various structural types, including β-Li3PS4, Li10GeP2S12, argyrodite, Li4PS4I, Li7P3S11, and Li1.82SiP0.036S3. While these materials exhibit high ionic conductivities of up to ~10− 2 S cm− 1 at room temperature and possess mechanical softness, they also have limitations, such as a narrow electrochemical stability window and air instability, which can hinder their scalability. The electrochemical and air stabilities of a solid electrolyte (Li–M–S, where M = non-Li metal ions) are largely governed by its stability of crystal structure consisting of MSn polyhedra and Li ions. A material with a different crystal structure would show its distinct properties. However, despite extensive research, no single structure known to date has proven superior in all aspects of performance. Given the importance of crystal structure in developing high-performance solid electrolytes, exploring and identifying novel ion-conducting materials with unique crystal structures is imperative. Additionally, once a new structural type of material is discovered, its performance can be adjusted, enhanced, or optimized through various techniques, such as chemical substitution, morphology and particle size control, and surface coating. In this work, we discovered a new crystal structural type of Li-ion conductor Li2GeS3. It was first known in 2000 by Kanno et al., with only orthorhombic unit cell dimension reported. However, our study using ab initio structure determination techniques from powder X-ray diffraction data, similar to our previous works, unveiled its structure possessing a hexagonal P61 symmetry with lattice parameters of a = 6.80098(4) Å and c = 17.92827(11) Å, revealing a new type of crystal structure. Its structure is comprised of a distorted hexagonal close-packed arrangement of sulfur anions with three asymmetric metal atoms: Ge and Li2 are in tetrahedral cavities surrounded by sulfur atoms; Li1 is in the trigonal bipyramidal cavity. The material displayed an ionic conductivity of 1.63 × 10− 8 S cm− 1 at 303 K and 2.45 × 10− 7 S cm− 1 at 383 K. Bond valence energy landscape calculations revealed three-dimensional lithium diffusion pathways within the structure. These results suggest that Li2GeS3 is expected to be a host structure from which improved ionic conductors can be derived through appropriate chemical substitution.