Role of Structural Disorder on Phonon Transport and Thermal Conductivity in Li 6 PS 5 Br

Sulfidic solid electrolytes pose a promising material class for enabling solid‐state batteries. While their defect chemistry and ion transport have been studied extensively, thermal transport and its interplay with ion dynamics remain poorly understood, despite the exceptionally low thermal conductivities of these materials. Here, we investigate the relationship between structural disorder, ion dynamics, and thermal properties in the argyrodite Li 6 PS 5 Br using neural network potential molecular dynamics, mesoscale modeling, and experimental techniques. By systematically varying Br − /S 2− site inversion, we demonstrate its strong influence on lattice dynamics and Li + diffusion. Increasing anion disorder broadens the vibrational density of states and promotes liquid‐like Li + dynamics resulting in significantly enhanced ionic transport. In contrast, thermal transport remains largely unaffected by the degree of site inversion. Both anion‐ordered and anion‐disordered Li 6 PS 5 Br exhibit similarly low bulk thermal conductivities of approximately 0.4 W∙m −1 ∙K −1 with only weak temperature dependence between 100 and 900 K. Experimental and computational analyses of the transport mechanisms reveal that thermal transport in this temperature range is primarily governed by diffuson‐like modes. Excellent agreement between experimental and computational results is achieved by bridging molecular dynamics simulations with mesoscale modeling to incorporate porosity effects.