Outgassing of active materials in Li-ion batteries provides a route to quantitatively study degradation processes that occur during cycling. In particular, we are primarily interested in quantifying the individual and coupled decomposition/transformations of the cathode – a lithiated transition metal oxide (TMO) – and the electrolyte – most commonly carbonate blends (ethylene carbonate, diethyl carbonate, etc.) with lithium hexafluorophosphate (LiPF₆) as the salt. Previous observations of high-voltage instabilities include TMO surface reconstruction, transition metal dissolution, electrolyte decomposition, and formation of surface species. However, this picture is still incomplete, with the dependence on electrolyte and TMO composition not yet fully understood. We will present results in which isotopic labeling of ¹⁸O in Ni-rich and Li/Mn-rich NMCs is combined with quantitative gas evolution analysis to show that residual solid lithium carbonate (Li₂CO₃) on the surface of TMOs has a direct impact on electrolyte and electrode degradation. In particular, oxygen release from the TMO lattice is related to the amount of Li₂CO₃ present in the cathodes. Our results suggest that the role of impurities on interfacial reactivity in batteries is critically important and should be a key parameter considered in similar future studies.