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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.