In-Situ Electrochemical Formation of Protective SEI on Sulfur-Carbon Cathode to Improve the Stability of a RT-Sodium Sulfur Battery
Authors: Lee HwonKi , Chang UiJin , Eom KwangSup 
Published: February 2020

Presently, lithium-ion batteries (LiBs) are commercially used in electronic vehicles as well as various electronic devices. Rechargeable lithium-sulfur battery (Li-S) is one of the promising LiBs for the next generation energy storage system because of high theoretical specific capacity (1675 mAh g-1). However, because of the significantly increased demand for lithium resources in energy storage system, a use of sodium is being considered as an alternative of lithium. Specifically, a room temperature sodium sulfur battery (RT-NaS) is receiving great attention for the large-scale energy storage system such as electrical grids because the electrode materials (sodium and sulfur) are resource-abundant and cost-effective in comparison with Li-S battery. Moreover, RT-Nas has the theoretical capacity of 3467 mAh cm-3 and 1675 mAh g-1 This energy storage system, however, has the critical challenge that the RT-NaS battery showed a low initial reversible capacity and a fast capacity decay. It is due to production of the intermediate products, long-chain sodium polysulfides formed during the electrochemical reaction which is easily dissolved into the electrolyte. During the cycling, the long-chain polysulfides diffuse between the anode and the cathode through a separator, and electrochemically deposited on the surface of the electrode. This phenomenon which called Shuttle Effect causes a significant decrease in coulomb efficiency. Therefore, it is necessary to restrain a dissolution of intermediate products, in order to enhance the performance of RT-NaS.

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.

Figure 1