Li-based antiperovskites (LiAP, Li_3-xOH_xX, X = Cl, Br) are an emergent class of Li-ion conductors that are potential candidates for electrolytes in all-solid-state batteries. As a material class, pLiAP shows vast compositional design freedom; however, the resulting properties are susceptible to synthesis and processing methodologies. For example, proton incorporation and halide mixing stabilize the perovskite cubic phase near room temperature, and halides mixtures near the eutectic points drive the solid-state reaction temperature down, allowing for faster synthesis and processing conditions (< 1 h). The mixed halogen compositions, such as Li₂OHCl_0.37Br_0.63, also show a 30-fold improvement in room temperature ionic conductivity of a single halide structure, 1.5 x 10^-6 vs. 4.9 x 10^-8 S cm^-1 (Li₂OHCl). Despite the growing interest in these materials, important questions remain about LiAPs on the structure-property correlation upon halide substitution and the correlations between the OH/halide dynamics and the Li-ion transport. We thus attempted to deconvolute how proton dynamics and halide substitution enhance or impede ionic conduction in pLiAP at compositions near the halide salts' eutectic points.

We combined infrared spectroscopy and nuclear magnetic resonance (NMR) with first-principles density functional theory (DFT) calculations to deconvolute halide mixing effects from local proton dynamics on Li-ion transport. The NMR results and ab initio molecular dynamics suggest that Li⁺ transport is more strongly correlated with halide dynamics. While the hydroxide does stabilize the highly conductive cubic structure, it limits correlative ionic transport and thus lowers Li⁺ conductivity.

Experiment design, data analysis, and manuscript preparation (RLS) were supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering. Synthesis (THB and JN) were supported by Asst. Secretary, Energy Efficiency and Renewable Energy (EERE), Vehicle Technologies Office (VTO) through the Advanced Battery Materials Research (BMR) Program. P. J. acknowledges partial support by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-FG02-96ER45579. H. F. was supported from U.S. Department of Energy (Award No. DE-EE0008865). This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The NMR characterization part of the work is supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, and Basic Energy Sciences. The NMR work was performed at the W. R. Wiley Environmental Molecular Sciences Laboratory, a DOE User Facility sponsored by the Office of Biological and Environmental Research, located at Pacific Northwest National Laboratory.