Online Mass Spectrometry Investigation of SEI Formation on Carbon Electrode Surfaces in Sodium-Ion Batteries: Oxygen and Additive Effects

Lithium-ion batteries (LIBs) are widely used due to their high cyclability and stability. However, the high cost of raw materials has spurred interest in sodium-ion batteries (SIBs) as a cost-effective alternative. Despite this, SIBs suffer from lower energy density and poorer cyclability, leaving them in a developmental stage. The formation of the solid electrolyte interphase (SEI) is critical for battery performance as it stabilizes the electrode and prevents further solvent degradation during cycling. This study investigated the SEI formation mechanism in sodium-based EC/DEC electrolyte by monitoring the evolution of volatile products and comparing them with those in lithium-based electrolytes under Ar and O2 atmospheres using online electrochemical mass spectrometry and vibrational spectroscopy. It was demonstrated that the SEI forms via similar reaction pathways in both sodium- and lithium-based electrolytes. However, Na+ lead to more significant ethylene (C2H4) evolution during this process. Upon introducing O2 into the operating environment, carbon dioxide (CO2) and water were produced during the initial cycle. At the same time, the amount of C2H4 evolved was significantly lower than under Ar. The results indicate that in the presence of O2, the superoxide (O2–) generated during the oxygen reduction reaction (ORR) alters the decomposition of EC molecules into two pathways. Because O2– is more stable in sodium-based electrolytes, a larger fraction of O2– participates in the decomposition of EC. This alters the electrochemical reduction reaction (ERR) pathway of C2H4 generated from EC decomposition to an ORR pathway that yields CO2. We also found that specific additives, such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC), can improve SEI stability in SIBs by inhibiting solvent decomposition and promoting the formation of a denser, stronger SEI. These findings provide valuable insights into the development of stable SEI, particularly for advancing the performance of SIBs.