Mechanistic Insights into O₂/H₂O-Assisted Na–CO₂ Batteries via In Situ Ambient Pressure X-Ray Photoelectron Spectroscopy

Sodium–carbon dioxide (Na–CO₂) batteries are a promising high-energy-density energy storage system with significant potential for CO₂ utilization. To address the safety concerns associated with liquid electrolytes, the development of solid-state metal–air batteries has also become an important goal. However, the reaction mechanisms of Na–CO₂ batteries remain unclear, and their overall performance is relatively poor. Furthermore, the role of additives such as O₂ and H₂O in facilitating cathodic reactions is not well understood. In this study, the reduction mechanisms under a pure CO₂ environment and with additives such as O₂ and H₂O were investigated using in-situ ambient-pressure X-ray photoelectron spectroscopy (APXPS). The oxidation states of the discharge products were identified, providing deeper insights into the reaction processes. When O₂ and H₂O additives were introduced, the system showed the formation of Csp² species. This study suggests that the formation of Csp² is attributed to the decomposition of the ionic liquid electrolyte, specifically through the generation of olefins. The formation of elemental carbon as a discharge product is considered unlikely. In a pure CO₂ system, poor electrochemical activity was observed, resulting in the generation of CO, which escaped from the electrode surface and led to poor reversibility. Additives such as O₂ and H₂O exhibited inherent electrochemical activity, whereas CO₂ contributed to the reaction through chemical processes. thereby reducing chemical reversibility. These findings provide a detailed reaction pathway that may guide the future development of solid-state Na–CO₂ batteries.