Boron Nitride-Dispersed Calcium Borohydride Tetrahydrofuran Composite Electrolytes for Solid-State Calcium Batteries

Abstract

All-solid-state calcium-metal batteries have gained significant interest due to their advantages of low reduction potential, high energy capacity, and promising resource availability by using the Ca-metal anode. To date, hydrides have attracted attention as solid electrolytes due to their high Ca-ion conductivity, and research is being conducted to further improve their conductivity. However, most of them show low thermal stability because of complexing with volatile ligands such as NH3 and THF (tetrahydrofuran), resulting in a limited operating temperature range. Here, we aim to achieve high ionic conductivity and excellent thermal stability of Ca(BH4)2·2THF by compositing it with hexagonal boron nitride (h-BN) because h-BN is an insulating material that can form dispersed ionic conductors, and the interactions between this layered compound and the volatile ligands may functionalize the h-BN interface with fast ionic conductivity. Remarkably, the Ca(BH4)2·2THF−h-BN composites retain their pellet shape even after heating to 200 °C and exhibit fast conductivity on the order of 10−3 S cm−1 at 160 °C, whereas the pristine Ca(BH4)2·2THF material and Ca(BH4)2·2THF−mesoporous Al2O3 composites collapse after heating. Structural and nuclear magnetic resonance analyses reveal that the pelletized form suppresses the complete desorption of the THF ligands. Compositing Ca(BH4)2·2THF with h-BN is an effective strategy for simultaneously improving the ionic conductivity and high-temperature pellet durability, enabling the creation of a Ca-ion-conductive solid electrolyte.