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(10-7S cm-1at 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 10-5S cm-1; the(Li5.7Al0.1)PS5I had the highest ionic conductivity in the studied system, at 2.6×10-5S 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 betterperforming SEs in the argyrodite system.