Sodium Polysulfides during Charge/Discharge of the Room-Temperature Na/S Battery Using TEGDME Electrolyte

The charge-discharge process of the room-temperature Na/S battery is studied using the Na/S cell with tetraethylene glycol dimethyl ether (TEGDME) and the solid electrolyte. The solid electrolyte is a barrier for chemical mass transport between the anode and the cathode. The sodium polysulfides as reaction products in TEGDME could therefore be investigated without interference from the anode side. During discharge, the color of the TEGDME electrolyte changes from transparent to brown, yellowish-green, and then back to transparent, and in the reverse order during charge. The first three colors are well-matched with those of prepared TEGDME solutions of Na2Sn (6 ≤ n ≤ 8), Na2S4 and Na2S, respectively. The solubility of sodium polysulfides (Na2Sn, 1 ≤ n ≤ 8) in TEGDME are measured systematically. The dissolution of sodium polysulfides is found to depend on the molar concentration of sulfur. The optimized Na/S cell using activated carbon delivers a high capacity of 1070 mAh g−1 at the first discharge, and remains at 782 mAh g−1 after 37 cycles. It also shows good rate capability with a discharge capacity of 569 mAh g−1 at 1/2 C. The coulombic efficiency is nearly 100% after the 5th cycle.