Boosting Ionic Conductivity and Air Stability in Bromide‐Rich Thioarsenate Argyrodite Solid Electrolytes

Abstract Lithium argyrodite thiophosphate superionic conductors are being explored as promising solid electrolytes for all‐solid‐state batteries, primarily due to their high ionic conductivity and ease of processing. Yet, these electrolytes present challenges such as chemical instability in humid conditions and incompatibility with cathode materials. Although some lithium argyrodites show improved air stability, their ionic conductivity deteriorates below the practically required value. Herein, based on hard soft acid base theory, a new family of lithium argyrodite, as solid solution Li_6−xAsS_5−xBr_1+x (for 0.0 ≤ x ≤ 0.6), has been proposed to address these issues. Through a combination of neutron diffraction, NMR spectroscopy, and electrochemical impedance spectroscopy, it has been determined that the partial substitution of S²⁻ by Br⁻ weakens interactions within the Li⁺ “cage”, facilitating long lithium‐ion movement throughout the structure. An additional T4 Li⁺ site is identified, offering a lower energy barrier for inter‐cage jumps. Consequently, the Li_5.5AsS_4.5Br_1.5 member of the composition series exhibits a higher Li‐ion diffusivity resulting in a remarkable ionic conductivity of 15.4 mS cm⁻¹. Compared with lithium thiophosphates, the Li_5.5AsS_4.5Br_1.5 also shows excellent air stability. This research opens a new avenue for developing air‐stable sulfide solid electrolytes with high ionic conductivity necessitated for practical application in solid‐state batteries.