While many organic liquid and electrolyte salt formulations have been explored to improve the stability of lithium-ion batteries, less research has been done to determine the role of evolved gasses in the performance of commercial Li-ion battery chemistries, let alone novel combinations of gas additives that may affect the solid-electrolyte interphase (SEI) formation and evolution.[1,2] Many gases are generated during Li-ion battery cycling, any of which could have possible beneficial or harmful effects on performance. Currently, only the effect of carbon dioxide (CO₂) gas addition to Li-ion batteries with Si anodes has been studied.[3-4] In addition to CO₂, ethylene gas (C₂H₄) is a promising additive for Si anode systems because it is a precursor in polyethylene polymerization reactions, nCH₂=CH₂ --> [-CH₂-CH₂-]_n. In addition, polyethylene oxide (PEO) has been observed in the Si-anode SEI and contributes to its passivation while maintaining flexibility.[5] Therefore, creating polyethylene and increasing PEO concentration in situ via ethylene gas doping may yield an improved SEI and more-stable battery. This work investigates the potential of gas-phase additives, including CO₂ and C₂H₄, to improve the performance of Si anodes for Li-ion batteries. The pressure decay of the gasses at different starting pressures dissolving in GenF3 electrolyte (1.2 M LiPF₆ in 3:7 wt:wt ethylene carbonate to ethyl methyl carbonate + 3 wt% fluoroethylene carbonate) was monitored to determine the saturation concentration of dissolved gas. The experimental pressure decay curve was fit to a model and extrapolated to predict the final pressure at equilibrium.[6] The relationship between partial pressure and concentration of dissolved gas in GenF3 at equilibrium was plotted and a curve was drawn to determine the Henry’s law constant. Varying volumes of gas were injected into battery pouch cells containing Si nanoparticle anodes and LiFePO₄ (LFP) cathodes to pressurize them to different pressures. Electrochemical cycling and subsequent multi-phase analyses, including vibrational spectroscopy and X-ray characterization of the SEI surface layer and GC-MS/FID were conducted to determine the impact of gas doping on the capacity, SEI, and gas phase reaction products. References: [1] G. G. Eshetu and E. Figgemeier, ChemSusChem, 12 (12), 2515-2539 (2019). [2] L. Bläubaum, P. Röse, L. Schmidt, and U. Krewer, ChemSusChem, 14 (14), 2943-2951 (2021). [3] L. J. Krause, V. L. Chevrier, L. D. Jensen, and T. Brandt, J. Electrochem. Soc., 164 (12), A2527-A2533 (2017). [4] E. J. Hopkins, S. Frisco, R. T. Pekarek, C. Stetson, Z. Huey, S. Harvey, X. Li, B. Key, C. Fang, G. Liu, G. Yang, G. Teeter, N. R. Neale, and G. M. Veith, J. Electrochem. Soc., 168 (3), 030534 (2021). [5] M. C. Schulze, G. M. Carroll, T. R. Martin, K. Sanchez-Rivera, F. Urias, and N. R. Neale, ACS Applied Energy Materials, 4 (2), 1628-1636 (2021). [6] E. Behzadfar and S. G. Hatzikiriakos, Energy & Fuels, 28 (2), 1304-1311 (2014).