A Computational Chemical Insight into Microscopic Additive Effect on Solid Electrolyte Interphase Film Formation in Sodium-Ion Batteries: Suppression of Unstable Film Growth by Intact Fluoroethylene Carbonate

Sodium (Na)-ion batteries (NIB) have recently attracted special attention as a substitute for Li-ion batteries due to the increase in the cost of Li. The performance of NIB with liquid electrolytes, for example, propylene carbonate (PC), is strongly dependent on a stable solid electrolyte interphase (SEI) film on the anode surface. According to a recent experiment, fluoroethylene carbonate (FEC) can be an efficient electrolyte additive to improve the SEI film formation in NIB. However, the molecular mechanism of such additive effect remains unknown. To investigate this mechanism, atomistic reaction simulations in PC-based electrolyte with and without FEC additives were performed using the hybrid Monte Carlo/molecular dynamics reaction method and successfully reproduced experimental observations such as the smaller irreversible capacity and the smoother SEI film in FEC-added electrolyte. Further, this study showed for the first time that intact FEC molecules can improve SEI film formation so as to enhance the network formation of organic species owing to the large electronegativity of their fluorine atoms. This new microscopic insight may provide an important guiding principle for use in designing the most suitable electrolytes for developing high-performance NIB.