CO2 has been shown to be an effective additive to improve the cycling characteristics of silicon negative electrodes for Li-ion batteries. However, a quantitative technique for measuring the CO2 content in electrolyte is not readily available. Here, FTIR was used to accurately determine the CO2 content of various carbonate-based Li-ion battery electrolytes. The accuracy of this method was validated with the weight variation method. Using this method, it was found that in EC/DEC electrolytes with 1M LiPF6 and LiTFSI salts, CO2 was found to have a maximum solubility of 0.37 wt% when the LiPF6:LiTFSI molar ratio is 3:1. In 1M LiPF6 solutions, CO2 solubility is in the order of PC approximate to EMC > DEC for the pure solvents. Mixed non-polar solvents are also favorable for CO2 dissolution. When used in silicon alloy cells, electrolytes saturated with CO2 ( 0.33% CO2 in EC/ DEC) resulted in the best cycling stability. (C) The Author(s) 2019. Published by ECS.