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

Argyrodites, Li 6 PS 5 X ( 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 Li 6 PS 5 Cl (651Cl) and Li 6 PS 5 Br (651Br), Li 6 PS 5 I (651I) shows poor ionic conductivity (10 −7 S cm −1 at 298 K). Herein, we present Al-doped 651I with I − /S 2− site disordering to lower activation energy ( E a ) and improve ionic conductivity. They formed argyrodite-type solid solutions with a composition of (Li 6−3 x Al x )PS 5 I in 0⩽ x ⩽0.10, and structural analysis revealed that Al 3+ is located at Li sites. Also, the Al-doped samples contained anion I − /S 2− 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, E a , and increased the ionic conductivity to 10 −5 S cm −1 ; the (Li 5.7 Al 0.1 )PS 5 I had the highest ionic conductivity in the studied system, at 2.6×10 −5 S cm −1 . In a lab-scale ASSB, with (Li 5.7 Al 0.1 )PS 5 I 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 − /S 2− 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.