Highly efficient atomic-scale design of CaF2 for ultrafast fluoride-ion conduction†

Fluorite-type CaF₂, as one of the most promising solid-state electrolytes, has attracted great research attention due to its electrically insulated nature and low cost for fluoride-ion batteries. However, it suffers from poor fluoride-ion conductivity at room temperature (300 K) and grain boundary effects on its fluoride-ion transport are not fully understood at the atomic scale. Therefore, we performed first-principles calculations and large-scale molecular dynamics simulations to reveal the atomic-scale impact of the dopant K on the physicochemical properties of CaF₂, such as its structural stability, electronic structure, and fluoride-ion conductivity. These were aimed at determining the optimal structure of K-doped CaF₂ with considerable fluoride-ion conductivity at 300 K. Moreover, we further clarified and optimized the relationship between its grain boundaries (GBs) and fluoride-ion conductivity. Our results predict that crystalline Ca_0.90625K_0.09375F_1.90625 has the highest fluoride-ion conductivity of 1.13 × 10⁻² S cm⁻¹ at 300 K. It also clearly exhibits excellent insulation and a wide electrochemical window. Moreover, through optimization of its grain boundaries, it is revealed that the preferred Σ3(111) grain boundary (GB) orientation can promote the fluoride-ion conductivity of polycrystalline Ca_0.90625K_0.09375F_1.90625 to a great extent. The order of magnitude of the conductivity can exceed 10⁻³ S cm⁻¹ as the working temperature is decreased to 15 °C, thereby overcoming the limitation of high working temperature (150 °C) in frequently used solid-state electrolytes for fluoride-ion batteries. Additionally, our results pave the way for quick and accurate design of polycrystalline CaF₂-based solid-state electrolytes. They can also afford valuable methods for the optimization and screening of other solid electrolytes for fluoride-ion batteries.