Understanding abnormal potential behaviors at the 1st charge in Li2S cathode material for rechargeable Li–S batteries

Abstract

In this study, electrochemical behaviors of Li₂S such as a large potential barrier at the beginning of the 1st charging process and a continuous increase in potential to ∼4 V during the rest of this process were understood through X-ray photoelectron spectroscopy measurements and electrochemical evaluations for a full utilization of Li₂S. The large potential barrier to the 1st charge in Li₂S can be caused by the presence of insulating oxidized products (Li₂SO₃ or Li₂SO₄-like structures) on the surface; simple surface etching can remove them and thereby reduce the potential barrier. Even though the potential barrier was substantially reduced, the electrochemical activity of Li₂S might not be improved due to the continuous increase in potential. This increase in potential was related to the polarization caused by the Li₂S-conversion reaction; the polarization can affect the utilization of Li₂S in subsequent cycles. We speculate that the increase in potential is related to the decomposition of oxidized products such as Li₂CO₃-like or Li₂O-like structures on the surface of the Li₂S particles. These findings indicate that the full utilization of Li₂S can be achieved by controlling their surface characteristics, especially the surface oxidation products.