Investigating the ion conductivity and synthesis conditions of calcium monocarborane solid-state electrolytes†

Multivalent-ion and all-solid-state batteries have emerged as potential solutions to address resource concerns and safety issues. Calcium is a promising element for multivalent-ion batteries owing to its abundance in the Earth's crust and low reduction potential. In addition, complex hydrides exhibit both high ion conductivity and reduction stability, making them suitable materials for solid-state ion conductors. In this study, we investigated the thermal stability and optimised the synthesis conditions of calcium monocarborane, namely, Ca(CB₁₁H₁₂)₂, which is a closo-type calcium complex hydride. In addition, we conducted electrochemical analysis to assess its performance as a solid-state divalent-ion conductor. The results indicate that a heat-treatment temperature of 433 K provides Ca(CB₁₁H₁₂)₂ with higher ion conductivity (σ = 1.42 × 10⁻⁴ S cm⁻¹) than the other heating temperatures. Thus, 433 K is considered optimal because [CB₁₁H₁₂]⁻ anions decompose when heat-treated at and above 453 K. Furthermore, the insertion and deinsertion of Ca²⁺ ions are stable and reversible in symmetric cells employing Ca–Sn alloy electrodes, representing the first time this has been observed for an inorganic solid-state calcium-ion conductor. Such insertion and deinsertion highlight the potential of Ca(CB₁₁H₁₂)₂ as a solid-state electrolyte for battery applications.