A Non-Gassing Online Electrochemical Mass Spectrometry Strategy for Isolating Electrode-Derived Gas Evolution in Sodium-Ion Batteries

Understanding how nanoscale structure governs interfacial reactivity remains a central challenge in alkali-metal battery materials. Accurate attribution of gas evolution processes is particularly difficult in conventional online electrochemical mass spectrometry (OEMS), where electrolyte decomposition and ionization-induced fragmentation often obscure structure-dependent contributions from electrode materials and typically necessitate isotopically labeled electrolytes. Here, we introduce a strategy for non-gassing OEMS that enables structure-resolved analysis of electrode-derived gas evolution at electrode-electrolyte interfaces. By employing a gas-inert, partially desodiated Na₃V₂(PO₄)₃ counter electrode in combination with an oxidation-stable sulfolane-based electrolyte, electrolyte-derived gassing is suppressed, allowing unambiguous isolation of gas formation originating from the working electrode without isotopic labeling. The capability of this platform is demonstrated using commercially relevant conductive carbon materials as model nanostructured interfaces. Under anodic polarization, CO₂ is identified as the dominant gaseous decomposition product for all investigated carbons. The total amount of evolved CO₂ exhibits a clear qualitative correlation with the specific surface area of the carbon materials, indicating that gas formation is primarily governed by available interfacial surface. Beyond surface area effects, systematic trends in CO₂ evolution are observed with carbon bonding configuration and degree of graphitic order. These results reveal that both nanoscale surface architecture and local bonding jointly influence the oxidative stability of carbon interfaces in alkali-metal battery environments. Overall, this work establishes a robust and broadly applicable non-gassing OEMS platform for structure-resolved investigation of interfacial gas evolution and highlights the non-negligible role of carbon additives to the total gas-evolution.