Exploring Mg^2+ and Ca^2+ 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^2+- and Ca^2+-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 Li_4MgTeO_6, Na_2Mg_2TeO_6, and Na_4MgTeO_6 with MgCl_2/Mg(NO_3)_2 or Ca(NO_3)_2 at temperatures not exceeding 500^∘C to produce Mg_3TeO_6 polymorphs, ilmenite-type CaMg_2TeO_6 (Mg_2CaTeO_6), and double perovskite Ca_2MgTeO_6. Thus, we demonstrate that these materials, conventionally requiring gigascale pressures and high temperatures (> 1000 ^∘C) 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 Mg_3TeO_6 and double perovskite Ca_2M TeO_6 (M = Mg,Ca,Zn) materials exhibit remarkable bulk ionic conductivity at room temperature, marking them as promising compositional spaces for exploring novel Mg^2+ and Ca^2+ conductors. Furthermore, this study extends the applicability of metathetical reactions to attain Mg- or Ca-based bismuthates, antimonates, ruthenates, tungstates, titanates, phosphates, and silicates, thus opening avenues to novel multifunctional nanomaterial platforms with utility in energy storage and beyond.