Lithium-ion concentration polarization and slow ionic transport in the electrolyte phase hinder fast charging of electric vehicle batteries and can cause cell degradation. Although pseudo-2D (P2D) models capture concentration polarization phenomena during fast charging, experimental measurements of Li-ion concentration in the electrolyte phase have been limited. In this research, Fourier transform infrared spectroscopy (FTIR) and attenuated total reflection (ATR) were used to collect operando optical measurements of Li-ion concentration in a high-loading graphite/NMC811 full cell with 1.2M LiPF6 in 3/7 (wt./wt.) ethylene carbonate (EC)/ethyl methyl carbonate (EMC). For this research, an optically accessible operando battery was developed that enables real-time measurements of Li-ion concentration at the back of the graphite anode near the current collector. Significant Li-ion concentration changes up to 0.5M were observed during charging at 1C, 2C, and 3C, as well as discharging at C/2, and compared with a P2D model. Both the model and the experiments showed Li-ion depletion and sinuous fluctuations at the graphite/current collector interface that indicate non-ideal behavior. These gradients likely develop due to insufficient electrolyte transport properties. This research enables novel Li-ion concentration measurements with FTIR. This research also validates P2D model physics for measuring and analyzing Li-ion concentration polarization at fast charging rates.