Institute of Solid State Physics
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Lithium ion dynamics in amorphous Li-Si electrochemically prepared from semiconductor grade, monocrystalline silicon — An NMR Study Silicon is one of the most promising anode materials for lithium-based rechargeable batteries. Provided the volume changes during Li uptake can be brought under control, Li ion diffusivity is expected to crucially determine the performance of such energy storage systems. Therefore, studying diffusion properties in amorphous Li-Si underpins applied research that is being directed towards the development of powerful storage devices. So far, only little information is available on Li self-diffusion in amorphous Si. Here, we used 7Li NMR spectroscopy [1] to precisely quantify microscopic activation energies and Li jump rates in amorphous Li-Si which is primarily formed if monocrystalline Si is lithiated electrochemically. Our results reveal relatively fast Li diffusivity with an average activation energy for long-range ion transport as high as ca. 0.65 eV; jump rates turn out to be in the order of 2.5 105 1/s at 246 K, see also ref. [2]. Comparisons with data from laboratory frame NMR relaxometry, which is sensitive to more localized ion hopping, points to complex dynamics that is most likely governed by non-exponential motional correlation functions originating from a large distribution of activation energies. Preliminary 6Li MAS NMR measurements helped characterize local structures in the amorphous sample. We found indications that at temperatures below 373 K, Li12Si7 is formed during our MAS NMR experiments performed at a spinning speed of 30 kHz. The formation of structural motifs of the binary Zintl phase was verified by ex situ X-ray powder diffraction. Noteworthy, a second sample, which is a mixture of amorphous Li-Si and metastable, crystalline Li15Si4 that forms at lower discharge potentials, points to Li diffusivity (0.51 eV) being comparable with that of the amorphous sample. The data obtained might help optimizing Li-based silicon batteries whose performance critically depend on fast Li-ion transport. |
