Li ion transport and interface percolation in nano- and microcrystalline composites

Abstract

We use model calculations to study the ionic conductivity in micro- and nanocrystalline composites of the type (1 − x)Li₂O∶xB₂O₃. Experimentally, such composites show a significant grain size effect. Microcrystalline samples (grain diameters in the range of some μm) show a strong monotonic decrease of the dc conductivity with increasing insulator content x, while nanocrystalline composites (grain sizes in the range of several nanometers) display a pronounced maximum in the conductivity at x ≈ 0.6. Above x = 0.9 the conductivity of the nanocrystalline materials drops sharply below the detection limit. We assume that neighbouring grains of conducting Li₂O and insulating B₂O₃ are separated by a highly conducting interface with a constant thickness of about 1 nm, irrespective of the grain size. By using Monte Carlo simulations and percolation theory we show that the overall features of the ionic conductivity in both nano- and microcrystalline composites can be well described by a brick-layer type model that explicitly takes into account the different cross sectional areas for ionic transport between neighbouring Li₂O grains, without additional free parameters involved.