Effect of Discharge Cutoff Voltage on Reversibility of Lithium/Sulfur Batteries with LiNO3-Contained Electrolyte

LiNO3 is unique in suppressing the redox shuttle of lithium polysulfides in Li/S battery. In this paper we evaluate the effect of discharge cutoff voltage on the reversibility of Li/S battery with a LiNO3-contained electrolyte. Based on a study of the liquid Li/S cells with a 0.05 m Li2S9 solution as the catholyte, we find that the highly reversible discharge and charge cycling of Li/S cells occurs at higher than 1.8 V vs. Li/Li+, which corresponds to a multistage redox process of the sulfur cathode between elemental sulfur and insoluble Li2S2. At lower potentials, not only the Li2S2 is reduced into poorly reversible Li2S, but also the LiNO3 is reduced irreversibly. Moreover, the irreversible reduction of LiNO3 further reduces the reversibility of the poorly reversible Li2S, resulting in a significant slowdown in the electrode reaction kinetics and a permanent loss in the reversibility of the Li/S cell. This work suggests that deep discharge must be avoided for a long cycle life Li/S battery when LiNO3 is used as an additive or a co-salt of the electrolyte.