Enhanced Conductivity at the Interface ofLi2O∶B2O3Nanocomposites: Atomistic Models

Abstract

A theoretical investigation at density-functional level of Li ion conduction at the interfaces in Li 2 O:B 2 O 3 nanocomposites is presented. The structural disorder at the Li 2 O111:B 2 O 3 001 interface leads to reduced defect formation energies for Li vacancies and Frenkel defects compared to Li 2 O surfaces. The average activation energy for Li diffusion in the interface region is in the range of the values for Li 2 O. It is therefore concluded that the enhanced Li conductivity of Li 2 O:B 2 O 3 nanocomposites is mainly due to the increased defect concentration. DOI: 10.1103/PhysRevLett.99.145502 PACS numbers: 61.72.ÿy, 61.82.Rx, 66.30.Pa, 71.15.Mb In the last decade, ion diffusion in nanocrystalline ceramics has received considerable interest. Conductivity in ceramic oxides has been observed in single-phase systems as well as in composites of different components [1][2][3][4][5][6][7]. Nanocomposite materials often show enhanced conductivity compared to the single-phase ceramic oxides which is attractive with respect to possible applications in battery systems, fuel cells or sensors. In recent experiments, it has been observed that the conductivity in Several classical models have been employed to describe the enhanced ionic conductivity in composite materials. The continuum percolation model was used to describe the dependence of the dc conductivity of Li 2 O:B 2 O 3 nanocrystalline composites on the insulator concentration [1]. A brick-layer type percolation model treating both the micro-and the nanocrystalline composites on the same footing [4] is also able to reproduce the experimental results for the conductivity as a function of composition. In a recent investigation [5], a more sophisticated Voronoi approach was used. All these stochastic models do not explicitly take into account the structure of the nanoparticles and of their surfaces. The aim of the present study was to investigate the effect of the atomic structure in the interface region on the ion mobility. We have developed atomistic models of the Li 2 O:B 2 O 3 nanocomposite based on periodic slabs. Investigations were performed to clarify whether the observed enhancement of Li conductivity in the Li 2 O:B 2 O 3 interface is due to a higher defect concentration (thermodynamically controlled) or to smaller activation barriers for local hopping processes (kinetically controlled). Models as proposed in the present study allow a direct simulation of the defect formation and mobility at atomic scale without any experimental input. They can give insight into the local bonding situation at the interface which is difficult to obtain from experiments.We have modeled Thus with the present combination of surface supercells, the lattice mismatch is only 1.3%. It is assumed that the nanoparticles can adjust their structure to minimize surface stress. Therefore the interlayer distance Z and the common surface lattice parameters a b were optimized [16]. Our model is infinite in two dimensions but nonperiodic in the direction of the surface normal. The a...

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