Sulfide solid electrolytes represent a critical class of materials for next-generation solid-state batteries due to their high ionic conductivity and favorable mechanical properties. Their performance is intrinsically linked to their crystal structures, which dictate ion transport pathways, phase stability, and interfacial compatibility. This article examines the atomic arrangements, lattice parameters, and structure-property relationships of key sulfide electrolyte families, including argyrodite, LGPS, and thio-LISICON, while highlighting recent advances in structural engineering.

The argyrodite family, with the general formula Li6PS5X (X = Cl, Br, I), exhibits a face-centered cubic structure (space group F-43m) characterized by a highly disordered lithium sublattice. The structure consists of PS4 tetrahedra and X anions forming a framework, with lithium ions distributed across partially occupied sites. The lattice parameter typically ranges from 9.8 to 10.3 Å, depending on the halide substitution. The ionic conductivity in argyrodites is strongly influenced by the degree of lithium disorder and halide positioning, with Li6PS5Cl demonstrating conductivities approaching 10-3 S/cm at room temperature. The halogen site plays a crucial role in stabilizing the cubic phase, with larger anions like iodine promoting higher disorder but potentially compromising electrochemical stability.

LGPS-type materials (Li10GeP2S12) crystallize in a tetragonal structure (space group P42/nmc) with lattice parameters a ≈ 8.7 Å and c ≈ 12.6 Å. The structure features a three-dimensional framework of (Ge/P)S4 tetrahedra interconnected by LiS6 octahedra, creating continuous lithium migration channels along the c-axis. This unique arrangement enables exceptionally high lithium-ion conductivity exceeding 10-2 S/cm. The Ge/P ratio influences the lattice dimensions, with higher germanium content expanding the framework and enhancing ionic transport. However, the presence of germanium also increases material costs and reduces oxidative stability, prompting research into Sn and Si substitutions.

Thio-LISICON electrolytes adopt structures derived from the γ-Li3PO4 type, with compositions such as Li4-xGe1-xPxS4. These materials exhibit orthorhombic or monoclinic symmetry, with lattice parameters varying significantly based on composition. The structure comprises PS4 and GeS4 tetrahedra forming a rigid framework, with lithium ions occupying interstitial sites. Ionic conductivity in thio-LISICONs generally ranges from 10-4 to 10-3 S/cm, with optimal performance achieved at specific Ge/P ratios that balance lattice distortion and lithium site connectivity. The mechanical properties of thio-LISICONs tend to be superior to LGPS, making them more resistant to dendrite penetration.

Structural variations profoundly impact ionic transport mechanisms. In argyrodites, lithium migration occurs through a concerted hopping between tetrahedral and octahedral sites, with the activation energy strongly correlated to the halide identity and local coordination environment. LGPS materials exhibit a unique paddle-wheel mechanism where the rotation of (Ge/P)S4 tetrahedra facilitates lithium motion along one-dimensional channels. Thio-LISICONs rely on a more conventional vacancy-assisted diffusion process, with the activation energy heavily dependent on the concentration of lithium vacancies.

Electrochemical stability is closely tied to structural features. The highest occupied molecular orbital (HOMO) levels in these materials are primarily determined by the sulfur 3p states, but structural factors such as bond lengths and angles modify the electronic structure. Argyrodites with chloride substitution typically demonstrate better oxidative stability than bromide or iodide variants due to stronger P-Cl bonding. LGPS shows limited stability against high-voltage cathodes due to the easily oxidizable germanium centers. Thio-LISICONs exhibit intermediate stability, with compositions rich in phosphorus showing better resistance to oxidation.

Mechanical properties vary significantly across structural families. Argyrodites possess relatively low shear moduli (15-25 GPa), making them more susceptible to dendrite penetration but easier to process into dense pellets. LGPS has anisotropic mechanical properties due to its layered-like structure, with higher stiffness along the c-axis. Thio-LISICONs generally display higher hardness and fracture toughness, attributed to their more three-dimensionally connected framework.

Recent advances in structural engineering have focused on optimizing these trade-offs. Strain engineering through epitaxial growth or lattice mismatch has been employed to modify lithium transport pathways in thin-film electrolytes. Interface engineering at the atomic scale has demonstrated success in stabilizing metastable high-conductivity phases, such as the cubic modification of Li7P3S11. Computational studies have identified new structural motifs, including lithium-rich anti-perovskite sulfides, that combine high conductivity with improved stability.

Doping strategies have been developed to tailor structural parameters. In argyrodites, partial substitution of phosphorus with silicon or aluminum modifies the lattice parameter and enhances phase stability. LGPS analogs with mixed Ge/Sn compositions show improved mechanical properties while maintaining high conductivity. Thio-LISICON derivatives incorporating oxygen in the sulfur sublattice demonstrate enhanced electrochemical stability without significant conductivity loss.

The understanding of structure-property relationships has advanced through sophisticated characterization techniques. Neutron diffraction with isotopic substitution has precisely located lithium positions in these materials. Pair distribution function analysis has revealed local structural distortions that influence ion transport. In situ X-ray diffraction during electrochemical cycling has provided insights into phase transitions and degradation mechanisms.

Future development of sulfide electrolytes will require deeper understanding of structure dynamics under operating conditions. The correlation between lattice vibrations and ionic transport needs further exploration. The role of grain boundaries and defects in polycrystalline materials remains an active area of investigation. Advances in atomic-resolution microscopy promise to reveal previously inaccessible structural details at interfaces and defects.

The crystal structure serves as the foundation for all functional properties of sulfide solid electrolytes. Continued progress in structural design and control will be essential for realizing their full potential in solid-state batteries. By systematically engineering atomic arrangements and lattice parameters, researchers can develop materials that simultaneously achieve high ionic conductivity, wide electrochemical stability, and robust mechanical properties.