Advancing Solid‐State Calcium Batteries: Achieving Fast Ionic Conductivity at Near Ambient Conditions in Calcium Hydridoborates

Abstract All‐solid‐state batteries based on abundant elements, such as calcium, offer a promising route to safer, cheaper, and more sustainable energy storage. Here, we report a series of fast Ca 2+ ‐conducting compounds of methylamine calcium tetrahydridoborates, Ca(BH 4 ) 2 · x CH 3 NH 2 (0 < x < 4.9) and related nanocomposites stabilized by inert MgO nanoparticles. Three new crystal structures are identified: a three‐dimensional network of octahedrally coordinated Ca 2+ complexes for x = 1, a molecular structure of neutral complexes for x = 4, and a structure of cationic complexes for x = 6. The thermal stability generally decreases with increasing CH 3 NH 2 content, and samples with x > 2 slowly release CH 3 NH 2 in “open” atmosphere at room temperature, but are stabilized in “closed” environments, e.g. capillaries. The ionic conductivity increases with CH 3 NH 2 content and correlates with increased void space and structural flexibility, reaching σ (Ca 2+ ) = 5.0·10 −5 S cm −1 at 60 °C for x = 4. Moreover, the effect of nanocomposite formation provides mechanical stability and a doubling of the ionic conductivity for Ca(BH 4 ) 2 ·4CH 3 NH 2 − MgO (50 wt%), reaching σ (Ca 2+ ) = 1.3·10 −4 S cm −1 at 60 °C. These findings demonstrate how local structure and nanoscale interfacial effects govern calcium transport, offering new design principles for functional calcium solid electrolytes.