Invited: Investigation of the Solid Electrolyte Interface (SEI) on Silicon Nano-Particle Anodes

The surface reactions of electrolytes with a silicon anode in lithium ion cells have been investigated. The investigation utilizes two novel techniques that are enabled by the use of binder-free silicon (BF-Si) nanoparticle anodes. The first method, transmission electron microscopy with energyndispersive X-ray spectroscopy, allows straightforward analysis of the BF-Si solid electrolyte interphase (SEI). The second method utilizes multi-nuclear magnetic resonance spectroscopy of D2O extracts from the cycled anodes. The TEM and NMR data are complemented by XPS and FTIR data, which are routinely used for SEI studies. Coin cells (BF-Si/Li) were cycled in electrolytes containing LiPF6 salt and ethylene carbonate or fluoroethylene carbonate solvent. Capacity retention was significantly better for cells cycled with LiPF6/FEC electrolyte than for cells cycled with LiPF6/EC electrolyte. Our unique combination of techniques establishes that for LiPF6/EC electrolyte the BF-Si SEI continuously grows during the first 20 cycles and the SEI becomes integrated with the BF-Si nanoparticles. The SEI predominantly contains lithium ethylene dicarbonate, LiF, and LixSiOy. BF-Si electrodes cycled with LiPF6/FEC electrolyte have a different behavior; the BF-Si nanoparticles remain relatively distinct from the SEI. The SEI predominantly contains LiF, LixSiOy, and an insoluble polymeric species.  The depth dependence of structure of the SEI was further investigated by Hard X-ray photoelectron spectroscopy (HAXPES).  Finally, the effect of different binders including PVDF, CMC, and PAA on the surface of silicon nanoparticles and on the structure of the SEI will be discussed.

We gratefully acknowledge funding from Department of Energy office of Basic Energy Sciences EPSCoR Implementation award (DE-SC0007074).