Origin of Optimal Composition and Density for Li‐Ion Diffusion in Amorphous Li–P–S Electrolytes

Amorphous solid electrolytes (SEs) are promising candidates for next‐generation solid‐state batteries owing to their mechanical softness and isotropic ion transport. However, the absence of long‐range order has hindered a mechanistic understanding of their ionic conductivity. Here, we employ AI‐based simulations to investigate Li 2 S–P 2 S 5 glasses across a broad composition–density space. We show that conventional descriptors—density, lithium content, and polyanion distribution—explain only part of the diffusivity trends. By decomposing transport into short‐ and long‐range components, we find that conduction is dominated by short‐range dynamics. Further analysis identifies two complementary structural determinants: Li–S 4 coordination environments, which stabilize hopping events, and pore evolution, which distinguishes accessible diffusion channels from inactive voids. This dual framework explains the emergence of optimal conductivity near Li 3 PS 4 stoichiometry and clarifies why excessive free volume can suppress transport. Overall, our results provide a mechanistic basis for ion conduction in amorphous solids and guiding principles for the design of high‐performance glassy electrolytes.