The addition of methylamine to calcium tetrahydridoborate enhances calcium ion conductivity significantly by increasing structural flexibility and void space as a function of increasing methylamine content. However, this results in decreasing mechanical stability. This drawback is overcome by forming nanocomposites through the addition of inert MgO nanoparticles, enabling both high ionic conductivity and enhanced mechanical stability.

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 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.