Exploring Magnesium- and Calcium-Ion Conductors Via Solid-State Metathesis Reactions

Magnesium and calcium batteries offer promising energy storage solutions characterised by cost-effectiveness, safety, and high energy density. However, the scarcity of viable electrode and electrolyte materials vastly hinders their advancement. This study utilises solid-state metathetical reactions involving predominantly chalcogen- and pnictogen-based honeycomb layered oxides with alkaline-earth halides/nitrates to synthesise Mg- and Ca-based materials previously achievable only under high-temperature/high-pressure conditions, as well as new metastable materials with unique crystal versatility. Particularly, we employ metathetical reactions involving Li4MgTeO6, Na2Mg2TeO6, and Na4MgTeO6 with MgCl/Mg(NO3)2 or Ca(NO3)2 at temperatures not exceeding 773 K to produce Mg3TeO6 polymorphs, ilmenite-type CaMg2TeO6 (Mg2CaTeO6), and double perovskite Ca2MgTeO6. Thus, we demonstrate that these materials, conventionally requiring gigascale pressures and high temperatures (>1273 K) for their proper synthesis, are now readily accessible at ambient pressure and considerably lower temperatures. Meanwhile, despite sub-optimal pellet densities, the synthesised ilmenite-type Mg3TeO6 and double perovskite Ca2MTeO6 (M=Mg,Ca,Zn) materials exhibit remarkable bulk ionic conductivity at room temperature, marking them as promising compositional spaces for exploring novel Mg2+ and Ca2+ conductors. Furthermore, this study extends the applicability of metathetical reactions to attain Mg- or Ca-based antimonates, bismuthates, ruthenates, tungstates, titanates, phosphates, and silicates, thus opening avenues to novel multifunctional nanomaterial platforms with utility in energy storage and beyond.