Amongst known families of solid electrolytes of potential interest for solid-state batteries, the anti-perovskites (A₃BX) are of interest for the high ionic conductivities observed in certain compositions (see Figure), and, like perovskites, the compositional flexibility provided by the possibility for ion substitution onto multiple lattice sites. For example, variations in the relative size of ions A, B and X result in changes in the Goldschmidt tolerance factor, or extent of disorder, that are accompanied by changes in the distribution of lithium migration energies. [1]

In surveying transport in this family of ion conductors, we observed a trend wherein numerous compositions show upwards curvature on an Arrhenius plot (see Figure), indicating a non-constant activation energy. This is unusual as most solids exhibiting non-Arrhenius behavior exhibit a negative deviation. We have synthesized a wide range of Li and Na anti-perovskites, and have confirmed non-Arrhenius conductivity with a positive deviation in compositions such as Na₃OBr_0.6I_0.4. This talk will discuss compositional trends related to this behavior, and our attempts to understand the structural origin(s) of this behavior through temperature-dependent X-ray and neutron scattering, calorimetry, and other methods. Implications for the design of solid electrolytes for solid state batteries will be discussed.

This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.

[1] Kwangnam Kim and Donald J. Siegel. "Correlating lattice distortions, ion migration barriers, and stability in solid electrolytes." Journal of Materials Chemistry A 7, no. 7 (2019): 3216-3227.