Tysonite-type solid state electrolyte for fluoride ion batteries: Highly dense thin film by PVD processing

Abstract

Dense thin films with tysonite-type structure La1−xBaxF3−x (LBF, x = 0.08–0.09) and a thickness of ≈ 1.5 µm were deposited through physical vapor deposition (PVD) technique. For a better understanding of the film/substrate interface, layers with e ≈ 100 nm have also been studied. Grazing incidence X-ray diffraction analysis of these very thin films revealed the presence of BaxLa1−xO1−uF1−x+2u (0 ≤ u ≤ x) oxyfluoride phase formed at the film/substrate interface. The thickness of the resistive oxyfluoride layer decreases with increasing of the substrate temperature Ts. The effect of Ts on the compactness, morphology and conductivity of the fabricated PVD films was established. The ionic conductivity was evaluated by electrochemical impedance spectroscopy in both configurations: in- and cross-plane. An intrinsic conductivity of 9.10−5 S cm−1 at 40 °C with an activation energy of ≈ 0.45 eV related to the fluoride ion mobility was estimated from in-plane measurements for a thin film elaborated at Ts = 450 °C. This anionic conductivity value is close to that reported previously on a single crystal (2.10−4 S cm–1 at 40 °C for x = 0.08). The improvement of the conductivity of LBF films with the growing substrate temperature is directly related to the increase in compactness which reaches 99% for Ts = 450 °C.