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

Abstract

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 Li2S4(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.