In situ ambient pressure x-ray photoelectron spectroscopy study on O2/H2O-assisted Na–CO2 batteries

Abstract

Na–CO2 battery is considered a high-energy-density storage system with the ability to utilize CO2. However, unlike its Li counterpart, it suffers from an unclear reaction mechanism and poor battery performance. CO2 is an inert species and requires high activation energy. Moreover, the understanding of how additives such as O2 and H2O aid the cathode reaction remains lacking. Herein, the mechanism of the CO2 reduction reaction is unraveled through in situ ambient pressure X-ray photoelectron spectroscopy (APXPS). The oxidation states of the discharged products can be well revealed to understand the reactions. Unlike previous studies, a pure CO2 environment was found to have poor electrochemical activity. When additives, such as O2 and H2O, were introduced to the system, some Csp2 was formed. We propose that Csp2 should be attributed to the alkene formation as the decomposition product of ionic liquid, which serves as the electrolyte. The formation of elemental carbon as the discharge product is unlikely, which is in stark contrast to previous studies. As a result, the poor electrochemical activity of the pure CO2 system leads to the formation of CO, which escapes from the electrode surface and results in poor reversibility. Additives such as O2 and H2O are electrochemically active themselves, while CO2 participated in the reaction chemically, reducing the chemical reversibility. Therefore, a system without CO2 is more beneficial to the Na–air batteries.