Fluoroethylene carbonate (FEC) is known to be an effective electrolyte additive in silicon-based anodes for lithium ion batteries, but reported to be consumed, eventually resulting in a sudden capacity fade. We quantified the amount of remaining FEC in separators harvested from the cycled cells using infrared (IR) spectroscopy. FEC is linearly depleted over cycles until complete consumption. When the amount of FEC additive is nearly zero, capacity of the cell suddenly drops. It is first shown that IR spectroscopy is a facile method for quantitative analysis of electrolyte additives. (C) The Author(s) 2018. Published by ECS.