Electrode performance of hard-carbon electrodes in non-aqueous Na cells is examined with or without fluoroethylene carbonate (FEC) as an electrolyte additive. It is found that electrode performance is highly improved by using FEC as additives. To study the mechanisms for the improvement of electrode performance, solid electrolyte interphase (SEI) of hard-carbon electrodes is investigated by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS). Surface analysis reveals that fluorine-containing SEI is formed at the hard carbon surface, which suppresses the decomposition of electrolyte solution in Na cells. From these results, the role and importance of SEI for the development of Na-ion batteries are also discussed.