Unraveling the Role of Fluorinated Alkyl Carbonate Additives in Improving Cathode Performance in Sodium-Ion Batteries

A key issue in the development of sustainable Na-ion batteries (NIBs) is the stability of the electrolyte solution and its ability to form effective passivation layers on both cathode and anode. In this regard, the use of fluorine-based additives is considered a promising direction for improving electrode performance. Fluoroethylene carbonate (FEC) and trans-difluoroethylene carbonate (DFEC) were demonstrated as additives or cosolvents that form effective passivating surface films in Li-ion batteries. Their effect is evaluated for the first time with cathodes in NIBs. By application of systematic electrochemical and postmortem investigations, the role of fluorinated additives in the good performance of Na0.44MnO2 (NMO) cathodes was deciphered. Despite the significant improvement in the performance of Li-ion cells enabled by the use of FEC and FEC + DFEC, the highest stability for NIBs was observed when only FEC was used as an additive. Mechanistic insights and analytical characterizations were carried out to shed light on the inferior effect of FEC + DFEC in NIBs, in contrast to its positive effect on the stability of Li-ion batteries.