Differentiating grain and grain boundary ionic conductivities of Li-ion antiperovskite electrolytes

Recently, antiperovskites such as Li2OHX (X ​= ​Cl, Br) have gained attention as possible solid-state electrolytes for use in all-solid-state batteries. Their low melting point allows for scalable manufacturing, making them more attractive than other inorganic solid-state electrolytes, and their high ion selectivity and good mechanical rigidity are superior to those of polymer electrolytes. However, there is significant variation of up to three orders of magnitude between reported ionic conductivities in different studies. One of the likely reasons is that studies have not separated Li-ion conduction contributions from the grain and the grain boundaries. Therefore, in this study, we present an electrochemical impedance analysis of Li-ion antiperovskites (Li2OHBr and Li2OHCl) prepared by different methods: cold press, hot press, and melt casting. We thereby separate the contributions from grain ionic conduction and grain boundary ionic conduction. While each method gives the same grain conductivity, the grain boundary conductivity depends on the preparation method. The largest improvement in grain boundary conductivity was found using the melt-casting method. These results provide an explanation for the reported variations in ionic conductivity.