Li Ion Self-Diffusion in Metastable Li15Si4 prepared from Monocrystalline Si — an Ex Situ 7Li NMR Relaxometry Study

New energy systems became one of the basic needs in modern society. The explosive increase of portable electronic devices, such as cell phones, laptop computers etc., and the storage of electricity from renewable resources calls for reliable batteries with extraordinary capacities. For this purpose, tremendous research effort has been devoted to novel high-capacity electrode materials.

Silicon containing electrodes are one of the most promising anode materials, not only due to their extremely large theoretical charge capacity but also owing to the availability of state-of-the-art fabrication methods. To overcome, however, the hurdles of Si-based batteries an in-depth understanding of the fundamental ion transport processes in such systems is expected to be highly useful. In the past, mainly macroscopic electrochemical methods have been used to estimate Li ion diffusivities in amorphous Si and metastable Li₁₅Si₄. By comparing the studies published so far, in some cases the resulting diffusivities differ from each other by many orders of magnitude. In the present contribution, we used an atomic-scale technique to probe Li self-diffusion parameters viz time-domain nuclear magnetic resonance (NMR) relaxometry.

Here, temperature-variable ⁷Li NMR relaxometry is known to be a powerful tool for the investigation of Li ion dynamics in crystalline as well as amorphous solids and enables, in the ideal case, direct access to both Li⁺ jump rates and activation energies [1-3]. For that purpose, half-cells of boron doped, monocrystalline Si (100) on Cu were cycled at 20 mV vs. Li/Li⁺ (2.5 cycles) with charge cut-off and current limitation. X-ray powder diffraction confirmed the formation of phase pure crystalline, metastable Li₁₅Si₄. The samples prepared are also expected to contain amorphous Si. ⁷Li NMR line shape measurements point to heterogeneous dynamics with relatively fast Li ions at temperatures well below 20 °C. A comprehensive NMR spin-lattice relaxation (SLR) study including both measurements in the laboratory (R₁, 116 MHz) and the rotating frame of reference (R_1ρ, 20 kHz) was used to deduce microscopic Li ion diffusion parameters. Fortunately, the temperature variable R_1ρ rates recorded at locking frequencies in the kHz range reveal a diffusion induced rate peak when the rates are plotted versus inverse temperature 1/T in an Arrhenius diagram. From the low-temperature flank of this peak an activation energy of 0.55(5) eV can be estimated. Moreover, the position of the maximum on the 1/T-axis leads to a Li jump rate of approximately 2.5 × 10⁵ 1/s at –30 °C. According to the Einstein-Smoluchowski equation this corresponds to a Li ion self-diffusion coefficient in the order of 10^–11 cm²/s which points to relatively fast Li diffusivity in the sample studied, see also ref. [4]. For comparison, ongoing NMR studies also include the investigation of a completely amorphous Si sample prepared by electrochemical reaction with Li as described above.

Acknowledgement  Financial support by the Federal Ministry of Economy, Family and Youth 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, S. Narayanan, L. Spencer, G. Goward, V. Thangadurai, M. Wilkening, Phys. Rev. B. 83 (2011) 094302.
[3]   V. Epp, Ö. Gün, H.-J. Deiseroth, M. Wilkening, J. Phys. Chem. Lett. 4 (2013) 2118.
[4]   A. Kuhn, P. Sreeraj, R. Pöttgen, H.-D. Wiemhöfer, M. Wilkening, P. Heitjans, J. Am. Chem. Soc. 113 (2011) 11018.