In the present study, we explore the use of lithium oxalate as a "sacrificial salt" in combination with lithium nickel manganese spinel (LNMO) cathodes. By online electrochemical mass spectrometry (OEMS), we demonstrate that the oxidation of lithium oxalate to CO2 (corresponding to 525 mAh/g) occurs quantitatively around 4.7 V vs. Li/Li+. LNMO/graphite cells containing 2.5 or 5 wt% lithium oxalate show an up to similar to 11% higher initial discharge capacity and less capacity fade over 300 cycles (12% and 8% vs. 19%) compared to cells without lithium oxalate. In LNMO/SiG full-cells with an FEC-containing electrolyte solution, lithium oxalate leads to a better capacity retention (45% vs 20% after 250 cycles) and a higher coulombic efficiency throughout cycling (similar to 1%) compared to cells without lithium oxalate. When CO2 from lithium oxalate oxidation is removed after formation, a similar capacity fading as in LNMO/SiG cells without lithium oxalate is observed. Hence, we attribute the improved cycling performance to the presence of CO2 in the cells. Further analysis (e.g., FEC consumption by F-19-NMR) indicate that CO2 is an effective SEI-forming additive for SiG anodes, and that a combination of FEC and CO2 has a synergistic effect on the lifetime of full-cells with SiG anodes. (C) The Author(s) 2018. Published by ECS.