Kinetic Control of Polymorphism in Li10.5Si1.5P1.5S12–Based Solid Electrolytes

Li10GeP2S12 (LGPS)-type solid electrolytes (SEs) are among the most promising materials for next-generation solid-state batteries (SSBs) due to their high lithium-ion conductivity. However, challenges related to phase stability and polymorphism, especially in Ge-free, Si-based systems, limit their practical implementation. Here, the crystallization and phase transition behavior of materials with a nominal composition of Li10.5–xSi1.5P1.5S12–xIx (x = 0, 0.2, 0.4) are systematically investigated. Operando high-temperature synchrotron X-ray diffraction, total scattering with pair distribution function analysis, and differential scanning calorimetry measurements reveal the formation of a metastable orthorhombic (o) phase, followed by transformation into the thermodynamically stable tetragonal (t) phase at elevated temperatures. Our findings indicate that lithium iodide (LiI) as an additive regulates the crystallization of the o-phase and thereby governs the transition kinetics. This enables the o-to-t phase transition to occur at much reduced temperatures and faster rates. Neutron powder diffraction further uncovers minor modifications to the Si/P ratio and lithium sublattice, as well as the fact that iodine is not incorporated into the t-phase; instead, a secondary Li4PS4I-type phase forms. Collectively, this work highlights the critical role of microstructural engineering on crystallization and polymorphism in LGPS-type SEs and also demonstrates the applicability of these dual-phase materials by enabling high-performance lithium–sulfur SSBs.