Investigation on crystallographic orientation dependent ion transportation in Li3YBr6 superionic conductor for lithium ion battery

Abstract

Enhancing ionic conductivity in solid electrolytes poses a significant hurdle in advancing all-solid-state batteries. Researchers have explored a variety of materials and techniques to elevate ionic conductivity. Lithium halide-based materials are emerging as a promising option for solid electrolytes due to their capacity for rapid ion conduction and robust electrochemical stability. Various methods, including substitution strategies and defect chemistry, have been deployed to enhance ionic conductivity within lithium halide solid electrolytes. Ionic conductivity is intricately linked to the substructure of migrating ions; therefore, ionic conductivity can be significantly enhanced by the alteration of the substructure. This work focuses on the impact of crystallographic orientation on ionic conductivity. Halide superionic conductor of structure Li₃YX₆ (X = halogen) has been used for the study. First, the ion transport mechanism in Li₃YBr₆ (in the C2/c phase) crystal has been examined and sequentially the impact of the crystallographic orientation has been investigated by creating two surfaces of different crystal orientations of pristine Li₃YBr₆. Findings of the present work show that among two different crystal-oriented surfaces (010) and (100) the (010) crystal-oriented structure Li₃YBr₆ has remarkable room temperature ionic conductivity (11.8 mS/cm) which is nearly fourteen times higher than that of pristine Li₃YBr₆ crystal while (100) oriented structure shows a little increment in room temperature ionic conductivity. To examine the mechanism behind this exceptional ionic conductivity in different structures energy barriers along different migration pathways have been evaluated. For the determination of the energy barrier nudged elastic band simulation was carried out. The results demonstrate that the (010) crystal-oriented surface structure possesses the lowest energy barrier, at 0.16 eV, which is 0.10 eV lower than the (100) oriented structure and 0.19 eV less than pristine Li₃YBr₆ (C2/c). This reduced energy barrier in the (010) crystal-oriented structure can be attributed to the prevalence of the tetrahedron-to-tetrahedron migration pathway.