Antiperovskite Superionic Conductors: A Critical Review.

Antiperovskites of composition M 3 AB (M = Li, Na, K; A = O; B = Cl, Br, I, NO 2 , etc.) have recently been investigated as solid-state electrolytes for all-solid-state batteries. Inspired by the impressive ionic conductivities of Li 3 OCl 0.5 Br 0.5 and Na 3 OBH 4 as high as 10 -3 S/cm at room temperature, many variants of antiperovskite-based Li-ion and Na-ion conductors have been reported, and K-ion antiperovskites are emerging. These materials exhibit low melting points and thus have the advantages of easy processing into films and intimate contacts with electrodes. However, there are also issues in interpreting the stellar materials and reproducing their high ionic conductivities. Therefore, we think a critical review can be useful for summarizing the current results, pointing out the potential issues, and discussing best practices for future research. In this critical review, we first overview the reported compositions, structural stabilities, and ionic conductivities of antiperovskites. We then discuss the different conduction mechanisms that have been proposed, including the partial melting of cations and the paddlewheel mechanism for cluster anions. We close by reviewing the use of antiperovskites in batteries and suggest some practices for the community to consider.