Unexpected Anion Segregation Enabling High Conductivity in Argyrodite Li6–xPS5–xClBrx Solid Electrolytes

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 on the Li5.75PS4.75ClBr0.25 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 Li5.75PS4.75ClBr0.25, 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 Li5.75PS4.75ClBr0.25. 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 Li6–xPS5–xClBrx and offers a new perspective for understanding superionic conductivity in Li6PS5Cl-based argyrodite solid electrolytes.