Towards Solid-State Batteries Using a Calcium Hydridoborate Electrolyte

Solid-state batteries created from abundant elements, such as calcium, may pave the way for cheaper and safer electrical energy storage. Here we report a new type of solid calcium hydridoborate electrolyte, Ca(BH₄)₂ ⋅ 2NH₂CH₃, with a high ionic conductivity of σ(Ca²⁺) ~10^-5 S cm^-1 at T=70 °C, which is assigned to a relatively open and flexible structure with apolar moieties and weak dihydrogen bonds that facilitate migration of Ca²⁺ ions in the solid state. The compound display a low electronic conductivity, providing an ionic transport number close to unity (t_ion=0.9916). Calcium plating is observed for a Ca|Ca(BH₄)₂ ⋅ 2NH₂CH₃|Pt electrochemical cell and the electrodes are investigated using scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS) that reveal a rugged Ca anode surface owing to the stripping process and the presence of Ca-containing domains on the Pt working electrode from the plating process. Improved electrochemical reversiblity was achieved in a three-electrode cell configuration using a Ca_xSn counter and reference electrode and a Sn working electrode, Ca_xSn | Ca(BH₄)₂ ⋅ 2NH₂CH₃ | Sn, providing reversible Ca plating and stripping.