||In general, hard carbons are composed of two characteristic domains: stacked hexagonally bounded carbon sheets and nano-sized pores, i.e. interstitial spaces of the stacked carbons. Because of this structure, capacity can be more delivered from hard carbon than graphite as an anode for sodium ion battery. However, since it exhibits limited lifetime characteristics, there is a limitation in the commercialization part. The reason of this problem is due to the unstable SEI (solid electrolyte interphase) layer formed on the hard carbon surface. Therefore, to compensate the SEI layer, additives such as a fluoroethylene carbonate and a succinic acid were added to the electrolyte to observe the change of the SEI layer structure and investigate its effect on the electrochemical performance. As a result, it was confirmed that when the additives were added, the film resistance increased but the lifetime of it decreased.