Chemical Origin of in Situ Carbon Dioxide Outgassing from a Cation-Disordered Rock Salt Cathode

In situ carbon dioxide (CO₂) outgassing is a common phenomenon in lithium-ion batteries (LiBs), primarily due to parasitic side reactions at the cathode-electrolyte interface. However, little is known about the chemical origins of the in situ CO₂ released from emerging Li-excess cation-disordered rock salt (DRX) cathodes. In this study, we selectively labeled various carbon sources with ¹³C in cathodes containing a representative DRX material, Li_1.2Mn_0.4Ti_0.4O₂ (LMTO), and performed differential electrochemical mass spectrometry (DEMS) during galvanostatic cycling in a carbonate-based electrolyte. When charging LMTO cathodes, electrolyte solvent (EC) decomposition is the dominant source of the CO₂ outgassing. The amount of EC-originated CO₂ is strongly correlated with the total surface area of carbon black in the electrode, revealing the critical role of electron-conducting carbon additives in the electrolyte degradation mechanisms. In addition, unusual bimodal CO₂ evolution during the first cycle is found to originate from carbon black oxidation. Overall, the underlying chemical origin of in situ CO₂ release during battery cycling is highly voltage- and cycle-dependent. This work further provides insights into improving the stability of DRX cathodes in LiBs and is envisioned to help guide future relevant material design to mitigate parasitic reactions in DRX-based batteries.