First-Principles Characterization of the Unknown Crystal Structure and Ionic Conductivity of Li7P2S8I as a Solid Electrolyte for High-Voltage Li Ion Batteries

Using first-principles density functional theory calculations and ab initio molecular dynamics (AIMD) simulations, we demonstrate the crystal structure of the Li7P2S8I (LPSI) and Li ionic conductivity at room temperature with its atomic-level mechanism. By successively applying three rigorous conceptual approaches, we identify that the LPSI has a similar symmetry class as Li10GeP2S12 (LGPS) material and estimate the Li ionic conductivity to be 0.3 mS cm–1 with an activation energy of 0.20 eV, similar to the experimental value of 0.63 mS cm–1. Iodine ions provide an additional path for Li ion diffusion, but a strong Li–I attractive interaction degrades the Li ionic transport. Calculated density of states (DOS) for LPSI indicate that electrochemical instability can be substantially improved by incorporating iodine at the Li metallic anode via forming a LiI compound. Our methods propose the computational design concept for a sulfide-based solid electrolyte with heteroatom doping for high-voltage Li ion batteries.