Lithium loss resulting from positive electrode deterioration and lithium accumulation within the solid electrolyte interphase (SEI) at the negative electrode leads to charge capacity loss in lithium-ion batteries (LIBs). We present a novel method to quantify origins of capacity loss resulting from positive electrode degradation and lithium immobilization within the SEI by comparative lithium depth profiles using Lithium Nuclear Reaction Analysis (Li-NRA). Li(Ni0.4Mn0.4Co0.2)O2/artificial graphite (NMC442/AG) pouch cells were cycled with electrolytes containing 1M LiPF6 in either 1:1 ethylene carbonate (EC)/dimethyl carbonate (DMC) or DMC with 3% vinylene carbonate (VC) SEI former. Lithium accumulated rapidly within the SEI in cells containing EC as compared to cells containing VC. Lithium loss from the positive electrode occurred at a linear rate, after formation, which was independent of electrolyte composition tested. The technique and methods provide quantitative insight into the impact of materials on capacity loss.