Permeation, Solubility, Diffusion and Segregation of Lithium in Amorphous Silicon Layers

The permeation, solubility, diffusion and segregation of lithium in amorphous silicon layers (∼100 nm) were experimentally determined at temperatures up to 500 °C (773 K). For the experiments, silicon was embedded between 6Li and 7Li enriched solid state Li reservoir layers and the exchange of the isotopes was monitored by secondary ion mass spectrometry. The silicon layers effectively block Li permeation for more than four years at room temperature. At higher temperatures, a fast and a slow Li permeation process can be discriminated. The activation enthalpy for the two processes amounts to (0.9 ± 0.2) and (1.9 ± 0.2) eV, respectively. The first process is based on fast interstitial diffusion leading to Li segregation at the surface and at the substrate interface. The second slower process is due to trap-limited Li diffusion in silicon. An activation enthalpy of Li solubility of (0.5 ± 0.1) eV is found to be identical to that in crystalline silicon, even if absolute values are higher. The activation enthalpy of trap-limited diffusion is determined to be (1.5 ± 0.2) eV. The results are discussed in framework of literature with impact on the lithiation mechanism of silicon electrodes in lithium-ion batteries.