Insulator:conductor interfacial regions — Li ion dynamics in the nanocrystalline dispersed ionic conductor LiF:TiO₂

Lithium fluoride is known to be a very poor ionic conductor. Here, we used it as a model substance to investigate the influence of the insulator TiO₂ on ion dynamics in nanocrystalline composites of LiF and TiO₂. In such two-phase systems a percolation network of fast Li⁺ pathways at the conductor:insulator interfaces may lead to enhanced overall, through-going ionic (and electronic) transport. Indeed, we observed such an effect and characterized it by conductivity spectroscopy as well as by variable-temperature ⁷Li and ¹⁹F Nuclear Magnetic Resonance (NMR) line shape measurements and diffusion-induced NMR relaxometry. Compared to nanocrystalline LiF, a remarkable increase in total conductivity of almost four orders of magnitude, that is, from 6×10⁻¹¹ S cm⁻¹ up to 2×10⁻⁷ S cm⁻¹ (100 °C) was observed for LiF:TiO₂ containing 40 vol.-% of TiO₂. Direct current polarization measurements revealed that principally ionic charge carriers are responsible for this enhancement. As both ions in LiF, Li⁺ and F⁻, might be mobile, NMR helped revealing that Li⁺ is the charge carrier being mainly responsible for the increase in ionic conductivity. Compared to SiO₂ and Al₂O₃ [see, S. Breuer et al. J. Phys. Chem. C, 2019, 123, 5222], with TiO₂ the largest increase in conductivity was achieved. Hence the introduction of heterogeneous conductor:insulator contacts turned out to be a highly suitable tool to effectively engineer the interfacial regions in such two-phase systems. In general, such a composite effect is important for ion transport in components for energy storage devices and in the solid electrolyte interphase region that generally passivates the anode material in lithium-ion batteries.