Silicon is a promising anode material for Li-ion batteries¹. Silicon’s ability to alloy with lithium results in a theoretical capacity ten times higher than that for graphite. However, since this high capacity involves several lithium ions per silicon atom, silicon will expand during lithiation and contract during delithiation. The volume changes causes the solid-electrolyte interphase (SEI) to crack, leading to continuous SEI formation. This eventually results in electronic insulation of the Si particles and large amounts of consumed lithium². Hence, there is a need for an electrolyte composition that forms an SEI with increased ability to withstand the volume changes.

By changing the electrolyte salt, the reactions at the interphase between electrode and electrolyte also changes. Inspired by the work of Philippe et al.³, we have evaluated lithium bis(fluorosulfonyl)imide (LiFSI) as an alternative electrolyte salt to the commercial lithium hexafluorophosphate (LiPF₆). Promising results were achieved with LiFSI as electrolyte salt in half-cells with micron-sized silicon as active material against circular Li foil as counter electrode.

The silicon based anodes used are made of 60 wt% Si (Silgrain®, e-Si 400, a commercially available battery grade silicon from Elkem), with an average particle size of 3 µm, 10 wt% graphite (Timcal, KS6L), 15 wt% carbon black (Timcal, C-Nergy C65, CB) and 15 wt% Na-CMC binder (Sigma Aldrich Mw ~90000). Slurries were cast onto dendritic copper foil and the electrodes were cycled in 2016 coin cells. The reference electrolyte composition is 1M LiFSI in EC:PC:DMC (1:1:3) + 5 wt% FEC and 1 wt% VC.

In this work, the performance of the silicon anodes are evaluated in full-cells with NMC as cathode. Performance in the reference electrolyte is compared to when LiPF₆ is the electrolyte salt. In addition, the effect of increasing the LiFSI concentration is evaluated. The results include electrochemical performance and post mortem characterization of the silicon electrodes. The post mortem characterization includes XPS and cross sectional analysis to investigate how the electrolyte compositions affects the SEI. Cross sectional analysis is performed using focused ion beam in combination with scanning electron microscopy.

[1] J. Ling et al. Journal of the Electrochemical Society, 154 (3), A156-A161, 2007.

[2] H. Wu et al. Nano Today, 7, (5), 414-429, 2012.

[3] B. Philippe et al. Journal of the American Chemical Society, 135, 9829-9842, 2013.