Li Ion-Sulfur Batteries with Glyme-Lithium Salt Solvate Ionic Liquid Electrolytes

We have reported that equimolar mixtures of certain Li salts and glymes (triglyme (G3) and tetraglyme (G4)) form molten complexes at ambient temperature, and behave like typical ionic liquids (ILs).^1,2 In the mixtures, ligand glyme molecules strongly coordinate (or solvate) to Li⁺ ions to form complex cations, hence, this family is classified as “solvate” ILs. Because complexation of appropriate Li salts with appropriate glymes allows realizing ILs involving Li⁺ ions as single cationic species, lithium solvate ILs possess high Li⁺ ion conductivity and transference number. In addition, the dissolution of lithium polysulfides (Li₂S_m), the discharge products of elemental sulfur, was suppressed in certain solvate ILs due to their extremely low coordinating nature. Owing to these remarkable properties, they are fascinating electrolytes for Li-sulfur batteries.^3,4

  Recently, we found that solvate IL electrolyte allows successful charge-discharge of graphite electrode. These results inspire us to develop low-cost and safe batteries without Li metal anode. In this study, the battery performance of the graphite-sulfur full cells with the solvate IL electrolytes was evaluated employing a lithiated graphite (LiC₆) anode or a lithiated sulfur (lithium sulfide, Li₂S) cathode as a lithium source. A LiC₆ anode and a Li₂S cathode were prepared by chemical and electrochemical methods. Using these electrodes, two kinds of full cells in both charge and discharge states were fabricated.

  Reversible charge-discharge of both full cells can be performed at 60 ºC, however, the capacities decrease as repeating cycles and the coulombic efficiency for these cells is substantially low compared to our previous results. This is because relatively high operation temperature facilitate dissolution of elemental sulfur and/or Li₂S_m, leading to loss of active materials and undesired side-reactions. To overcome these problems, solvate IL electrolytes were diluted by an appropriate solvent with low donor ability. As a result, a stable cycle operation with a discharge capacity of 700–800 mA h g⁻¹ and a coulombic efficiency of 90–95 % at 30 ºC was achieved over 100 cycles (Figure 1), irrespective of starting charge-discharge states.