Silicon is a promising anode candidate in Li-ion batteries with a theoretical capacity more than ten times that of the commercial graphite anodes (372 mAh/g). However, Si electrodes suffer from rapid capacity fade during electrochemical cycling. One of the more successful strategies for dealing with the capacity loss and continuous solid electrolyte interphase (SEI) formation on Si electrodes has been the use of battery electrolyte additives such as fluoroethylene carbonate (FEC) [1-4]. In this work we used a combination of Annular Dark Field - Scanning Transmission Electron Microscope (ADF-STEM) imaging and Electron Energy Loss Spectroscopy (EELS) techniques in order to understand the effect of FEC on the chemical and morphology evolution of Silicon and SEI layer during cycling. We observed that upon lithiation crystalline Si converted into the amorphous Li_xSi alloy and two layers formed on the outermost surface of the electrode including: 1- SEI and 2- Li_xSiO_y. It was found that the presence of FEC suppresses the formation of Li_xSiO_y layer and leads to the formation of uniform SEI layer around the Li_xSi particles with high LiF content. Such STEM/EELS study correlates the morphology and chemistry of the Silicon and SEI layer for the first time and provides valuable insights into the electrochemicalperformance of Si electrodes.

Acknowledgements

This research is funded by the Office of Vehicle Technologies, U.S. Department of Energy under the Advanced Battery materials Research (BMR) Program.

References

[1] Philippe B., G. G. (2013), Chemistry of Materials, 25: 394-404.

[2] Xu C., F., et. al. (2015),  Chemistry of Materials, 27:2591-2599..

[3McDowell M. T., et al. (2012), Advanced Materials 24: 6034-6041.

[4] Sina M., et al. (2015), Journal of Phys. Chem. C, 119: 9762–9775.