A Li-Li₂S₄ battery with improved discharge capacity and cycle life at low electrolyte/sulfur ratios

Lithium sulfur (Li-S) batteries are promising candidates to replace lithium ion batteries as the next-generation energy storage devices. However, the operation of conventional Li-S batteries at low electrolyte/sulfur ratios is rather challenging with many obstacles to be addressed. In this work we use solid-state Li₂S₄ as the cathode active material to construct a new type of Li-Li₂S₄ battery. With the aid of electrochemical impedance spectrum, we find that the use of Li₂S_4(S) in the cathode alleviates the issue of surface coverage encountered under the lean electrolyte condition. We also find that the discharge capacity is not determined by the solubility of Li₂S₄ on the lower plateau of discharge. Consequently, the Li-Li₂S₄ cell with a high sulfur content of 84% and a low E/S ratio of 4.4 ml g⁻¹ delivered a much higher discharge capacity compared to a conventional Li-S cell. In addition, improved cycle retention was achieved when controlling the Li-Li₂S₄ cell cycling between Li₂S₄ and Li₂S. We believe that the reported results open a new direction of cathode development for high energy density Li-S batteries.