Elucidation of Gas Evolution in Model Sodium Battery Cells By Online Electrochemical Mass Spectrometry

Abstract

Na-ion batteries are regaining attention as the geopolitically independent option, especially for large-scale stationary energy storage. Numerous Na-based materials have been discovered, synthesized, and verified in the past decade as promising active material candidates. However, long-term cyclability of Na-based cells is still generally inferior to the Li-based ones, hindering their wide-spread commercialization. Moreover, cycling performance obtained in half-cells is not sufficiently accurate, which is largely attributed to the much higher reactivity of Na-metal, as compared to Li-metal, and the unstable solid electrolyte interface (SEI), inevitably leading to rapid performance degradation. The parasitic side-reactions between electrodes and electrolyte are most often associated with the release of gaseous products, such as H2, CO2, C2H4, O2, and so on. Their evolution rates, onsets, and amounts disclose vital information concerning the formation of SEI and stability of electrode materials. However, there are only limited studies focusing on gassing behaviors of individual Na-based systems.In this study, we embarked on a systematic investigation on the gassing of model sodium battery cells by means of online electrochemical mass spectrometry (OEMS). Gaseous species generated within the cell were quantitatively analyzed in real-time. We compared oxidative and reductive stability of common alky-carbonate-based solvents using Na3V2(PO4)2F3, Na3V2(PO4)3, and hard carbon as model electrodes. Results indicate that linear carbonates cannot form a stable SEI layer, as supported by continuous electrolyte reduction and formation of alkane and H2. On the other hand, cyclic carbonates form a more stable SEI layer with characteristic alkene formation, similarly to Li-ion case. In this presentation, we will discuss the SEI stability of sodium cells and the origin of gas formed upon cycling. In addition, we will compare gassing from polyanionic-type cathodes with layered oxides. The impact of electrolyte additives on SEI stability and related gas evolution will also be addressed. Presenting author present address: Department of Chemistry, Ångström Laboratory, Uppsala University, Box 538, SE-751 21, Uppsala, Sweden. Figure 1