Real-time gas evolution analysis of SEI formation in sodium-ion batteries using chip based electrochemistry mass...

The stability of the solid electrolyte interphase (SEI) in sodium-ion batteries remains a critical challenge for achieving long cycle lifetimes. In hard-carbon anodes, the interplay between microstructure, solvation chemistry, and electrolyte decomposition dictates SEI evolution, yet the mechanistic link between electrolyte decomposition, gas evolution, and interphase formation remains poorly resolved. Here, we apply chip-based electrochemistry-mass spectrometry (EC-MS) with picomole-per-second sensitivity to monitor real-time gas evolution during SEI formation on hard carbon in different solvent systems, representing one of the first applications of EC-MS to sodium-ion cells. Carbonate-based (EC:DMC) electrolytes exhibit pronounced gas evolution, dominated by C₂H₄ during the first discharge, consistent with reductive decomposition of EC and the formation of sodium ethylene dicarbonate and Na₂CO₃ rich interphase species. In contrast, ether-based electrolytes (diglyme) show strongly suppressed gas evolution, with only trace C₂H₄ detected, alongside improved rate capability and enhanced cycling stability over 100 cycles at 0.5 C. These differences are attributed to solvent-dependent solvation and interfacial kinetics, arising from the lower effective desolvation barrier and higher reductive stability of diglyme relative to carbonate solvents, leading to the formation of a thinner and more ion-permeable SEI. Collectively, these findings establish a direct connection between solvent chemistry, gas evolution, and SEI formation in sodium-ion systems and demonstrate the utility of operando chip-based EC-MS for resolving electrolyte degradation pathways.