Enhancement of Electrochemical Performance of Lithium and Manganese-Rich Cathode Materials via Thermal Treatment with SO2

In this study, we present a novel surface modification approach via SO2 gas treatment at 200 °C–400 °C to enhance the electrochemical performance of Li and Mn-rich cathode materials 0.35Li2MnO3·0.65LiNi0.35Mn0.45Co0.20O2 (HE-NCM) for advanced lithium-ion batteries. It was established by X-ray photoelectron spectroscopy that the SO2 treatment leads to the formation of surface sulfates and sulfites on the material, while the bulk remains unaffected, as confirmed by X-ray and electron diffraction studies. Based on the results obtained, we proposed possible mechanisms of the SO2 thermal treatment that include partial reduction of manganese (however, we could not find any substantial evidence for it in the XPS data) and oxidation of sulfur. The electrochemical performance was evaluated by testing the materials as cathodes in coin-type half-cells with metallic lithium anodes at 25 °C and 30 °C. The main findings are as follows: the SO2-treated materials demonstrate ∼10% higher capacity at all C-rates and lower the voltage hysteresis during prolonged cycling compared to the untreated samples. The proposed approach to modify the surface of HE-NCM materials by SO2 treatment is demonstrated to be a promising method to enhance the electrochemical performance of these cathodes.