Direct Determination of Solid-Electrolyte Interphase Thickness and Composition as a Function of State of Charge on a Silicon Anode

With the use of neutron reflectometry, we have determined the thickness and chemistry of the solid-electrolyte interphase (SEI) layer grown on a silicon anode as a function of state of charge and during cycling. We show the chemistry of this SEI layer becomes more LIP like with increasing lithiation and more Li-C-O-F like with delithiation. More importantly, the SET layer thickness appears to increase (about 250 angstrom) as the electrode becomes less lithiated and thins to 180 angstrom with increasing Li content (Li3.7Si). We attribute this breathing to the continual consumption of electrolyte with cycling.