Composition regulation of ternary rare-earth halide solid-state electrolytes and its influence on their ionic conducting and electrochemical properties

Abstract

Ternary rare-earth halide solid-state electrolytes (SSEs) have attracted widespread attention owing their good ionic conductivity, wide electrochemical stability windows, and excellent mechanical properties. However, the relationship of how the composition of ternary rare-earth halide SSEs influences their ionic conducting and electrochemical properties has not been fully investigated. In this study, three groups of ternary rare-earth halide SSEs were synthesized and compared to investigate this relationship. The first group of halides was Li₃YX₆ (X = Cl, Br or I) with the same rare-earth element of yttrium but different types of halogens. The second group of halides was Li₃MCl₆ (M = Dy, Ho, Er, Tm, Yb, or Lu) with the same halogen element of Cl but different rare-earth elements. The third group of halides was Li_2.8MCl_5.8 (M = Dy, Ho, Er, Tm, Yb, or Lu), which contains the same rare-earth and halogen elements as the second group but with a slight difference in stoichiometry. From the first group, it was found that the halogen element had a crucial influence on the structure, ionic conductivity, and electrochemical stability windows of the SSEs, and the mixed halogen species were more conductive than the single halogen analogues. Two mixed halide SSEs, i.e., Li₃YCl_1.2Br_4.8 and Li₃YBr_4.8I_1.2, were found to have high ionic conductivities of up to 2.75 × 10⁻³ S cm⁻¹ and 3.28 × 10⁻³ S cm⁻¹ at 25 °C, respectively. The second group revealed that altering the rare-earth elements had a very weak influence on the ionic conductivity or electrochemical stability windows of the SSEs. In contrast, a slight lithium deficiency in stoichiometry led to a significant boost in the ionic conductivity, as revealed by the third group. Based on the above results, several guidelines for the future design of rare-earth halide SSEs were proposed.