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 LiF like with increasing lithiation and more Li-C-O-F like with delithiation. More importantly, the SEI layer thickness appears to increase (about 250 Å) as the electrode becomes less lithiated and thins to 180 Å with increasing Li content (Li_3.7Si). We attribute this "breathing" to the continual consumption of electrolyte with cycling.