Heterogeneous F anion transport, local dynamics and electrochemical stability of nanocrystalline La_1−xBa_xF_3−x

Solid state ion conductors exhibiting fast ion dynamics are of utmost importance for the development of modern energy storage systems or sensors. For the realization of fluoride ion batteries (FIBs), that is, batteries relying on F⁻ ions as charge carriers, materials with fast fluorine anion dynamics are needed. Here we used high-energy ball milling to incorporate BaF₂ into the tysonite structure of LaF₃ and studied ion dynamics by both broadband impedance and ¹⁹F nuclear magnetic resonance (NMR) spectroscopy. The nanocrystalline solid solution formed revealed increased ion conductivity compared to pure LaF₃. Apart from the formation of F vacancies through aliovalent substitution, the increase in conductivity is also stimulated by the introduction of lattice strain and the high degree of local disorder produced. While long-range ion dynamics has been studied recently, less information is known about local ion hopping processes to which ¹⁹F NMR spin-lattice relaxation is sensitive. In this work, we tried to correlate short-range ion diffusion with long-range transport properties measured by conductivity spectroscopy. Additionally, we used electrochemical polarization and cyclic voltammetry to judge whether the ternary fluoride shows a sufficiently high electrochemical stability to work in FIBs.