Insights into Li-S Battery Cathode Capacity Fading Mechanisms: Irreversible Oxidation of Active Mass during Cycling

Abstract

The solid components deposited in sulfur cathode during cycling for Li-S battery is studied in this work. ROLi, HCO₂Li, Li_xSO_y and Li₂S (or Li₂S₂) are proved to be the main components by the methods of Fourier transform infrared (FTIR), Raman spectra and X-ray photoelectron spectroscopy (XPS). ROLi and HCO₂Li are solvent degradation products existed in electrolyte. The reversibility of Li₂S and Li₂S₂ are not serious as in previous reports. ROLi, HCO₂Li and Li_xSO_y co-deposited with Li₂S or Li₂S₂ in discharge process lead to the cathodes performance deterioration. Lithium salts such as LiNO₃ and LiTFSI can oxidize sulfur compounds to higher oxidation states, and Li_xSO_y species increased with cycling indicates the active mass irreversible oxidation that may be another important reason for the capacity fading of Li-S battery.