The room temperature Na/S battery with tetraethylene glycol dimethyl ether (TEGDME) electrolyte (hereafter referred to as the Na/S battery) can overcome these drawbacks. Furthermore, assuming Na2S as the final discharge product, sulfur cathode has a high theoretical specific capacity of 1672 mAh g-1 and the Na/S battery has a high theoretical energy density of 1230 Wh kg-1.
Sulfur in the Na/S cell has been reported to involve complicated transition processes during charge/discharge. It evolves from S8 to Na2S through various sodium polysulfides such as Na2S6, Na2S5, Na2S4, Na2S3 and Na2S2 during the discharge and does reversely during charge. Especially, the high-order polysulfides (Na2Sn, 4 ≤ n ≤ 8) are generally believed to dissolve into the TEGDME electrolyte, bringing a number of drawbacks such as low discharge capacity, fast capacity decay during cycling and deterioration of the sodium anode due to the shuttle effect. Thus, some strategies such as a solid electrolyte separator (β"-Al2O3), ion selective polymer membranes, and interlayers have been applied to suppress the shuttle effect of the high-order polysulfides (Na2Sn, 4 ≤ n ≤ 8). Inversely, there have also been several studies such as catholyte system to use their soluble properties. The low-order ones (Na2Sn, 1 ≤ n ≤ 3) are considered to generally be in the solid state and contribute to the low discharge capacity and high polarization due to the low electronic and ionic conductivities. These mean that understanding the evolution of sulfur in the cathode to sodium polysulfides (Na2Sn, 1 ≤ n ≤ 8), based on solubility, is critical for understanding and developing the Na/S battery. However, previous studies have focused only on identifying the chemical species of sodium polysulfides (Na2Sn, 1 ≤ n ≤ 8), which were even partially identified, and there has been no study on the Na/S battery in viewpoint of the solubilities of sodium polysulfides (Na2Sn, 1 ≤ n ≤ 8).
In the present study, we aim to investigate the formation and variation of sodium polysulfides during charge/discharge. Sulfur cathode and sodium anode are examined at different points of the charge/discharge by separating both electrodes with the beta alumina solid electrolyte separator (β"-Al2O3). The solubilities of sodium polysulfides of Na2Sn (1 ≤ n ≤ 8) are measured and compared to the sulfur cathode visually.