To improve the poor solid electrolyte interface (SEI) formed with propylene carbonate (PC)-based electrolyte on a graphite electrode, 2(5H)-furanone was assessed as a novel electrolyte additive together with fluoroethylene carbonate (FEC). The mass change during the formation of the SEI was monitored with different 2(5H)-furanone to FEC ratio using an electrochemical quartz crystal microbalance. The single addition of FEC was insufficient to prevent the decomposition of the electrolyte and damage of graphite electrode, due to FEC's more negative reduction potential compared to PC, leading to serious irreversible charge loss during the 1st charge-discharge process. On the other hand, 2(5H)-Furanone suppressed the graphite damage effectively, which resulted in relatively higher discharge capacity. However, for the electrolyte containing only 2(5H)-furanone, minor charge loss due to electrolyte reduction continued even after the SEI formation. This led to SEI with excessive amount and resistance, which resulted in a severe capacity drop at rapid discharge rates. The mixed additive of 2(5H)-furanone and FEC suppressed both the damage of the graphite electrode and residual reduction currents. The LIBs with this co-additive system exhibited much lower film resistance associated with the SEI and showed superior electrochemical performances.