A new insight on capacity fading of lithium-sulfur batteries: The effect of Li2S phase structure

In this work, a significant difference in Li2S phase structure, depending on the discharge rate, is reported for lithium sulfur (Li-S) batteries, and their performances are interpreted in terms of the phase structures. The production of crystalline Li2S is preferred at a low discharge rate, while the formation of amorphous Li2S is favored at a high discharge rate. The differences in the morphologies of the charged electrodes and the charging overvoltages demonstrate that the amorphous Li2S is more easily oxidized to soluble PS during charging compared to the crystalline Li2S. Impedance spectroscopic analysis reveals a faster Li ion diffusion through the amorphous Li2S discharge product is responsible for the smaller overvoltage and higher reversibility at the higher discharge rate. The slower capacity fade rate at a higher discharge rate indicates the importance of the Li2S phase structure on Li-S battery performances. The formation of amorphous Li2S is more favored with cycling, which provides a feasible explanation for widely observed capacity saturation behavior.