The high temperature Na/S battery working above 300°C has been highlighted as a promising candidate for large scale energy storage systems. It carries many advantages such as low-cost raw materials, high energy density (760 Wh kg^-1), high coulombic efficiency, and a long cycle life. However, it suffers from critical problems of the safety issue, etc. For example, sulfur and sodium polysulfides are in the liquid state and react with sodium vigorously. In the case of a failure of solid electrolyte separator (β"-Al₂O₃), a fire or an explosion can take place.

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 (Na₂S_n, 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 (Na₂S_n, 4 ≤ n ≤ 8). Inversely, there have also been several studies such as catholyte system to use their soluble properties. The low-order ones (Na₂S_n, 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 (Na₂S_n, 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 (Na₂S_n, 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 (Na₂S_n, 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 Na₂S_n (1 ≤ n ≤ 8) are measured and compared to the sulfur cathode visually.