Fluoride-Excess Synthesis of CaF₂ Nano-powder: Tuning Conductivity for Fluoride Ion Batteries (FIB)

Calcium fluoride (CaF₂) nano-powders were prepared by a polyol-mediated synthesis under fluoride-excess conditions. By varying the initial fluorine precursor concentration, we induced fluoride anti-Frenkel defects (fluoride-ion interstitials with corresponding vacancies) and increased the concentration of fluoride-diethylene glycol (DEG) complexes at the nanoparticle surface. Precursors were mixed at Ca:F ratios of 0.50, 0.40, and 0.35 (denoted samples DE8, DE9, and DE10, respectively) to study the effect of stoichiometry on the final CaF₂ nano-powders. The structure, morphology, and ionic conductivity of these materials were examined. X-ray diffraction (XRD) confirmed that the CaF₂ nano-powders crystallize in the fluorite-type face-centered cubic (FCC) structure (space group Fm-3m) with an average core crystallite size of ~8 nm, and scanning electron microscopy (SEM) showed particle sizes around 14-18 nm. Nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) analyses revealed the presence of fluorideexcess defect structures in the material. The best-performing composition (Ca:F = 0.35) exhibited an improved ionic conductivity of approximately 2×10 −5 S•cm −1 at 40 °C. This solid electrolyte was successfully implemented in a fluoride ion battery (FIB) test cell using a magnesium anode and a BiF₃ cathode, demonstrating room-temperature battery operation.