Impact of thermal gas treatment on the surface modification of Li-rich Mn-based cathode materials for Li-ion batteries

High energy density Li-rich 0.33Li₂MnO₃·0.67LiNi_0.4Co_0.2Mn_0.4O₂ (HE-NCM) layered structure cathodes for Li-ion batteries provide higher capacity gain via incorporation of an excess of lithium into the host. As a serious drawback, these cathodes suffer from continuous voltage fade upon cycling. Recently, high capacity retention, rate capability and low voltage hysteresis were achieved for HE-NCM by new thermal double gases SO₂ and NH₃ treatment. However, so far a fundamental understanding of the mechanisms responsible for this improved stability is missing. Herein, a comprehensive study of the chemical composition and electronic structure modifications of a series of HE-NCM (untreated, treated, carbon- and binder- free) is performed using advanced electron spectroscopy techniques supported by theoretical calculations. We demonstrate that the double gases treatment process leads to a partial reduction of Co³⁺ and Mn⁴⁺. The suggested chemical reactions include electron transfer from SO₂, which behaves as a Lewis acid, to the transition metal sites accompanied by decomposition of SO₂ and a characteristic surface modification which acts as protective layer for the HE-NCM.