Lithium Ion Dynamics in Amorphous Li-Si Electrochemically Prepared from Semiconductor Grade, Monocrystalline Silicon - an NMR Study about Local Structures and Li+ Self-Diffusivity

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. Noteworthy, a second sample, which is a mixture of amorphous Li-Si and metastable, crystalline Li₁₅Si₄ that forms at lower discharge potentials, points to slightly enhanced Li diffusivity (0.51 eV). Interestingly, first high-resolution ⁶Li MAS NMR experiments on X-ray amorphous Li-Si, which have been carried out using isotope enriched samples, indicate the presence of structural motifs that resemble those of Li₁₂Si₇. The data obtained might help optimizing Li-based silicon batteries whose performance critically depend on fast Li-ion transport.

 

Acknowledgement  Financial support by the Federal Ministry of Science, Research and
Economy and the National Foundation for Research, Technology and Development is gratefully acknowledged.

 

References

[1]   V. Epp, M. Wilkening, Phys. Rev. B 82 (2010) 020301.

[2]   A. Kuhn, P. Sreeraj, R. Pöttgen, H.-D. Wiemhöfer, M. Wilkening, P. Heitjans, J. Am. Chem. Soc. 113 (2011) 11018.