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

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 Ca²⁺-conducting compounds of methylamine calcium tetrahydridoborates, Ca(BH₄)₂·xCH₃NH₂ (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 Ca²⁺ 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 CH₃NH₂ content, and samples with x > 2 slowly release CH₃NH₂ in "open" atmosphere at room temperature, but are stabilized in "closed" environments, e.g. capillaries. The ionic conductivity increases with CH₃NH₂ content and correlates with increased void space and structural flexibility, reaching σ(Ca²⁺) = 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(BH₄)₂·4CH₃NH₂ - MgO (50 wt%), reaching σ(Ca²⁺) = 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.