AbstractOxygen-anion redox in lithium-rich layered oxides can boost the capacity of lithium-ion battery cathodes. However, the over-oxidation of oxygen at highly charged states aggravates irreversible structure changes and deteriorates cycle performance. Here, we investigate the mechanism of surface degradation caused by oxygen oxidation and the kinetics of surface reconstruction. Considering Li₂MnO₃, we show through density functional theory calculations that a high energy orbital (lO_2p’) at under-coordinated surface oxygen prefers over-oxidation over bulk oxygen, and that surface oxygen release is then kinetically favored during charging. We use a simple strategy of turning under-coordinated surface oxygen into polyanionic (SO₄)²⁻, and show that these groups stabilize the surface of Li₂MnO₃ by depressing gas release and side reactions with the electrolyte. Experimental validation on Li_1.2Ni_0.2Mn_0.6O₂ shows that sulfur deposition enhances stability of the cathode with 99.0% capacity remaining (194 mA h g⁻¹) after 100 cycles at 1 C. Our work reveals a promising surface treatment to address the instability of highly charged layered cathode materials.