Exploring the Synthesis of Alkali Metal Anti-perovskites

Abstract

The development of solid-state batteries has been slowed by limited understanding of the features that control ion mobility in solid electrolytes (SEs). In the case of anti-perovskite (AP) SE, lattice distortions have been proposed as one such controlling factor: APs that exhibit distortions of the octahedral building blocks are predicted to exhibit enhanced ionic mobility. Nevertheless, large distortions come at the cost of stability, implying a tradeoff between stability and ionic mobility. The present study combines theory and experiments to explore the synthesizability of several marginally stable APs predicted to exhibit high mobility for Li+, Na+, and K+. Density functional theory calculations, in combination with the quasi-harmonic approximation, were used to predict the free energy change, ΔGr(T), for synthesis reactions involving 36 alkali metal-based APs, X3AZ (X = Li, Na, or K; A = O, S, or Se; and Z = F, Cl, Br, or I). A linear correlation is observed between the degree of lattice distortion and the stabilization temperature, at which ΔGr(T) = 0. Hence, APs with the highest ionic mobility generally require the highest synthesis temperature. These data were used to guide experimental synthesis efforts of APs by estimating the temperatures above which a given AP is expected to be thermodynamically stable. Attempts were made to synthesize several AP compositions; overall, good agreement is obtained between experiments and computation. These data suggest that a compound’s zero K decomposition energy is an efficient descriptor for predicting the ease and likelihood of synthesizing new SEs.