Exploring the Synthesis of Alkali Metal Anti-perovskites

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, Delta G(r)(T), for synthesis reactions involving 36 alkali metal-based APs, X(3)AZ (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 Delta G(r)(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.

Automated summary

The development of solid-state batteries has been hindered by limited understanding of ion mobility control in solid electrolytes (SEs). However, the present study combines theory and experiments to explore the synthesizability of marginally stable anti-perovskite (AP) SEs, which are predicted to exhibit high ionic mobility for Li+, Na+, and K+. The study finds a linear correlation between lattice distortion and stabilization temperature, indicating that APs with the highest ionic mobility require the highest synthesis temperature. The experimental results align well with the computational predictions, suggesting that a compound's zero K decomposition energy is an efficient descriptor for synthesizing new SEs.