Concentration Effect of Fluoroethylene Carbonate on the Formation of Solid Electrolyte Interphase Layer in Sodium-Ion Batteries.

Fluoroethylene carbonate (FEC) is an effective additive to improve the performance of Na-ion batteries (NIB). A recent experimental study has shown that a small amount of FEC enhances the NIB performance, whereas increasing the FEC amount deteriorates the performance. Toward understanding the microscopic mechanism of this observation, the dependency of the solid electrolyte interphase (SEI) film formation on the FEC concentration has been investigated in a propylene carbonate (PC)-based electrolyte solution by using the Red Moon method. This method was able to reproduce successfully the experimental observations where a small amount of FEC makes SEI film stable. Further, the increase in FEC amounts decreased the stability of the SEI film and should lead to the decrease in the NIB lifetime during charge-discharge cycles. It was revealed that this is because of the insufficient organic dimer formation between the monomer products at the higher FEC concentration. Finally, it was reconfirmed theoretically that the appropriate adjustment of FEC additive amount is essential to develop the high-performance of NIB.