Multiscale characterizations boosting understanding of gassing behavior in rechargeable batteries: Progress and outlook

Gas evolution caused by parasitic reactions, together with the subsequent migration, consumption, and accumulation of gaseous products, severely affects the performance, lifetime, and safety of rechargeable lithium/sodium (Li/Na) batteries. Advanced characterization techniques allow for probing the mechanisms and impacts of these gas-related processes, thereby providing a substantive understanding of gassing behavior. As these techniques continue to develop, new insights will undoubtedly emerge. Herein, from the perspective of advanced characterization techniques, including both online gas analysis and gas visualization, gassing behavior in rechargeable Li/Na batteries is comprehensively reviewed. Gas analysis is presented according to the dominant driving force of gas evolution. For voltage-driven gas evolution, battery-level online gas analysis techniques are discussed, with emphasis on the mechanistic insights obtained from isotope labeling and the decoupling of gas evolution reactions at individual electrodes. For thermal-driven gas evolution, gas release and crosstalk mechanisms during thermal runaway are elucidated based on material-level gas analysis. In parallel, the principles of gas visualization at both the interface and battery scales are summarized, together with insights into the dynamic behavior of gaseous products. Finally, remaining challenges and future directions in gas-related characterizations are outlined. This review provides a comprehensive understanding of the overall gassing behavior in rechargeable Li/Na batteries, thereby advancing their safe and stable operation.