Unexpected anion segregation enabling high conductivity in argyrodite Li6−xPS5−xClBrx solid electrolytes†

Abstract

Site disorder between S and Cl anions at Wyckoff 4a and 4d sites is a key structural feature of the argyrodite Li₆PS₅Cl solid electrolyte in all-solid-state batteries. This first-principles study of the Li_5.75PS_4.75ClBr_0.25 system is the first to report that S/Cl/Br anion disorder leads to an unexpected segregation of two anion sublattice domains, one composed of S anions and the other composed of Cl/Br anions. Increasing the degree of anion disorder enhances the stability of Li_5.75PS_4.75ClBr_0.25, resulting in the formation of the most stable structure with anion segregation when the highest degree of anion disorder is reached. The anion segregation causes a greater distribution of Li ions in the S domain than in the Cl/Br domain, which greatly reduces the repulsion between S²⁻ ions compared to the repulsion between Cl⁻/Br⁻ ions and effectively stabilizes Li_5.75PS_4.75ClBr_0.25. Li ions in the S domain move with difficulty due to the strong Li–S bonds, whereas Li ions in the Cl/Br domain can move relatively freely due to the weak Li–Cl (Li–Br) bonds, resulting in a high conductivity of 10.1 mS cm⁻¹. This study suggests that domain-dependent Li migration due to anion segregation is the fundamental ion transport mechanism in Li_6−xPS_5−xClBr_x and offers a new perspective for understanding superionic conductivity in Li₆PS₅Cl-based argyrodite solid electrolytes.