Dispersed Solid Conductors: Fast Interfacial Li-Ion Dynamics in Nanostructured LiF and LiF:γ-Al2O3 Composites

Lithium fluoride serves as a model substance to study Li and F hopping processes in a material solely composed of mobile ions with an opposite charge. In its microcrystalline form, it is known to be a very poor ionic conductor. Here, we tried to boost ion dynamics in LiF by taking advantage of size effects and the introduction of structural disorder. Compared to micro-LiF, we observed an increase of the ion conductivity by 2 orders of magnitude for nanocrystalline LiF prepared by high-energy ball milling. A further boost might be achieved in nanocrystalline two-phase systems consisting of LiF and an insulator, such as amphoteric gamma-Al2O3. In such dispersed ionic conductors, percolating conductor/insulator pathways are anticipated enabling the ions to move quickly over long distances. Indeed, for nano-LiF:Al2O3, another drastic increase of ionic conductivity by 3 orders of magnitude (393 K) is achieved by interface engineering. The activation energy characterizing long-range ion transport is reduced from 0.98 eV (nanocrystalline LiF) to 0.79 eV for (LiF)(0.86)(Al2O3)(0.14). Li-7 nuclear magnetic resonance (NMR) measurements showed that Li+ is mainly responsible for this increase seen for nano-LiF:Al2O3. Al-27 magic angle spinning NMR revealed that pentacoordinated Al species act as anchor sites for F- anions (and Li+). This mechanism is assumed to lead to a 3D network of fast Li+ diffusion pathways along the conductor/insulator interfaces.