Lithium-site substituted argyrodite-type Li6PS5I solid electrolytes with enhanced ionic conduction for all-solid-state batteries

Argyrodites, Li6PS5X (X=Cl, Br, I), have piqued the interest of researchers by offering promising lithium ionic conductivity for their application in all-solid-state batteries (ASSBs). However, other than Li6PS5Cl (651Cl) and Li6PS5Br (651Br), Li6PS5I (651I) shows poor ionic conductivity (10−7 S cm−1 at 298 K). Herein, we present Al-doped 651I with I−/S2− site disordering to lower activation energy (Ea) and improve ionic conductivity. They formed argyrodite-type solid solutions with a composition of (Li6−3xAlx)PS5I in 0⩽x⩽0.10, and structural analysis revealed that Al3+ is located at Li sites. Also, the Al-doped samples contained anion I−/S2− site disorders in the crystal structures and smaller lattice parameters than the non-doped samples. Impedance spectroscopy measurements indicated that Al-doping reduced the ionic diffusion barrier, Ea, and increased the ionic conductivity to 10−5 S cm−1; the (Li5.7Al0.1)PS5I had the highest ionic conductivity in the studied system, at 2.6×10−5 S cm−1. In a lab-scale ASSB, with (Li5.7Al0.1)PS5I functioned as a solid electrolyte, demonstrating the characteristics of a pure ionic conductor with negligible electronic conductivity. The evaluated ionic conduction is due to decreased Li+ content and I−/S2− disorder formation. Li-site cation doping enables an in-depth understanding of the structure and provides an additional approach to designing better-performing SEs in the argyrodite system.