Lithium Diffusion in Ion-Beam Sputter-Deposited Lithium–Silicon Layers

Abstract

Lithium−silicon compounds are used as active material in the negative electrodes of Li-ion batteries. The knowledge of Li diffusion in these materials is of importance for the optimization of charging/discharging rates and achievable maximum specific capacity as well as for an understanding of the basic lithiation mechanism. We carried out Li tracer self-diffusion experiments on ion-beam sputterdeposited Li x Si(O) thin films for x ≈ 0.25 and 4.5 using Li x Si/ 6 Li x Si heterostructures in combination with secondary-ion mass spectrometry in the line-scan-like mode. Measurements with elastic recoil detection analysis revealed the presence of a considerable amount of oxygen in the films. The diffusivities follow the Arrhenius law in the temperature range of 300−500 °C with an activation energy of 0.8−0.9 eV. The film containing a higher amount of Li shows faster diffusion by 1 order of magnitude. The Li diffusivities in the investigated Li-rich materials are several orders of magnitude higher than in Li-poor Li x Si films (x = 0.02−0.06) as given in the literature because of a lower activation energy. This indicates the presence of a direct interstitial-like mechanism. The oxygen present in samples with the same Li concentration of x = 0.06 also enhances diffusion but does not lead to a reduction in the activation energy.

Li Diffusion in Thin-Film LiySi Electrodes: Galvanostatic Intermittent Titration Technique and Tracer Diffusion Experiments

“…The Li tracer diffusivities as derived from GITT experiments of this work (first pulse) are 2 orders of magnitude lower than the chemical diffusivities of Li and show values of ∼1 × 10 −18 m 2 /s (Li 0.02 Si) and ∼1 × 10 −19 m 2 /s (Li 0.25 Si). Strauß and coworkers recently determined in our laboratory Li tracer selfdiffusivities on ion-beam sputter-deposited Li y Si thin films for y ≈ 0.02 19 and 0.25 20 using Li y Si/ 6 Li y Si heterostructures in combination with SIMS at temperatures above 100 °C. The samples were produced in the same way as those of this work.…”

“…Note that the Li tracer diffusivity of 10 −18 m 2 /s from the GITT data for Li 0.02 Si is more similar to the extrapolated Li tracer diffusivity from SIMS data of the composition of Li 0.25 Si than to that of Li 0.02 Si. Further information on Li diffusion in Li y Si compounds with different compositions of y can be found in ref 20.…”

“…In Figure , GITT measurements done on Li y Si samples with compositions of y ≈ 0.02 and 0.25 recorded during lithiation are shown. For a comparison to the Li tracer diffusion experiments carried out by our group on the same types of samples, , especially the results at the beginning of the GITT measurements are important, where low amounts of δ x are introduced into the sample and the initial relative Li concentration is modified not much. For the Li 0.02 Si sample, two types of measurements were done with pulse times of t p = 15 min (long pulse) and t p = 3 min (short pulse) (Figure a,b).…”

“…The general technique has been outlined in detail in a previous publication of our group . For the present study, Li y Si thin films with a thickness of 100 nm with compositions of y ∼ 0.02 and 0.25 were produced and characterized as described in refs , . After deposition, the samples were introduced into an electrochemical cell without coming in contact with air.…”

“…Consequently, a solid experimental determination of Li diffusivities in silicon-based electrode materials is necessary. Recently, we investigated Li tracer self-diffusion in amorphous Li y Si films containing low (y = 0.02 and 0.06) 19 and high (y = 0.25 and ≈4.5) 20 relative Li concentrations at temperatures between 150 and 500 °C using secondary ion mass spectrometry (SIMS). We found diffusivities considerably lower by orders of magnitude for the first group of materials.…”