Comparing the Gassing of Hard Carbon During Lithiation and Sodiation via a Novel Two-Compartment Online Electrochemical Mass Spectrometry Cell for Sodium-Ion Batteries

Interfacial degradation processes in lithium-ion and sodium-ion batteries frequently result in gas formation, making online electrochemical mass spectrometry (OEMS) a powerful tool to gain mechanistic insights. This work introduces a novel two-compartment OEMS cell for Na-ion systems, using a Na-β′′-alumina solid electrolyte (BASE) separator, whose resistance is sufficiently low to enable measurements at battery-relevant current densities. This enables the measurement of the gas evolution from an individual electrode by preventing the exchange of gases between the anode and cathode compartments, which is validated in the first part of this study. Subsequently, the gassing behavior of hard carbon (HC) anodes during sodiation (with 1.5 M NaPF₆ in EC) is compared to that during lithiation (with 1.5 M LiPF₆ in EC; using the established two-compartment OEMS cell for Li-ion systems). The HC anodes in both the Li-ion and Na-ion cells show comparable overall gas evolution profiles, with the characteristic evolution of ethylene, H₂, and CO; a striking difference is the ∼300 mV lower onset potential for gas formation in Na-ion cells (vs Na⁺/Na) vs Li-ion cells (vs Li⁺/Li), which will be discussed. Overall, this novel two-compartment OEMS cell for Na-ion systems offers a potent platform for mechanistic studies on sodium-ion batteries.