In this study, we will introduce a new method to suppress the dissolution of the intermediate products into the electrolyte, which is artificially forming a protective solid electrolyte interphase (SEI) on the surface of carbon-sulfur (C/S) composite cathode by an in-situ electrochemical treatment (in-situ ET) with a carbonate additive (FEC). To assess the electrochemical properties of RT-NaS battery, the coin-cells were prepared: i) precycled RT-NaS (Not-treated NaS), ii) in-situ ET RT-NaS (EC-treated NaS) iii) precycled RT-NaS with FEC (Not-treated NaS with FEC) and iv) in-situ ET RT-NaS with FEC (EC-treated NaS with FEC). The schematic of the in-situ electrochemical treatment is displayed in Fig. 1A. An SEI is formed during the in-situ ET discharge by the electrochemical reduction of the carbonate based electrolyte especially when decreasing the potential down to 0.05 V vs Na/Na+. In particular, this protective SEI layer is formed on the surface of C/S composite particles when the sulfur has a maximum volume. Therefore, it is believed that the layer is well maintained. As shown in Fig. 1B, it is confirmed that the layer is successfully formed on the surface of cathode with a thickness of 45 to 60 nm. Fig 2. exhibits the cyclic performances of various RT-NaSs (i-iv) at 0.4 A g-1 in the voltage range from 0.6 to 2.6 VNa/Na+. Among them, EC-treated NaS with FEC (iv) shows the best cyclic performance. Regardless of EC treatment, the NaS without FEC presents the low discharge capacity of about 208 ~270 mAh g-1 after 50th cycle. The NaS with FEC shows the higher initial capacity, but during the sequent cycling it shows a rapid capacity decay. In contrast, the EC-treated NaS with FEC maintains the highest discharge capacity, and has the capacity of 590 mAh g-1 even after the 50th cycle.