| Summary: | High-energy cathode materials that are Li- and Mn-rich lithiated oxides—for instance, 0.35Li<sub>2</sub>MnO<sub>3</sub><b><sup>.</sup></b>0.65LiNi<sub>0.35</sub>Mn<sub>0.45</sub>Co<sub>0.20</sub>O<sub>2</sub> (HE-NCM)—are promising for advanced lithium-ion batteries. However, HE-NCM cathodes suffer from severe degradation during cycling, causing gradual capacity loss, voltage fading, and low-rate capability performance. In this work, we applied an effective approach to creating a nano-sized surface layer of Li<sub>2</sub>SO<sub>4</sub> on the above material, providing mitigation of the interfacial side reactions while retaining the structural integrity of the cathodes upon extended cycling. The Li<sub>2</sub>SO<sub>4</sub> coating was formed on the surface of the material by mixing it with nanocrystalline Li<sub>2</sub>SO<sub>4</sub> and annealing at 600 °C. We established enhanced electrochemical behavior with ~20% higher discharge capacity, improved charge-transfer kinetics, and higher rate capability of HE-NCM cathodes due to the presence of the Li<sub>2</sub>SO<sub>4</sub> coating. Online electrochemical mass spectrometry studies revealed lower CO<sub>2</sub> and H<sub>2</sub> evolution in the treated samples, implying that the Li<sub>2</sub>SO<sub>4</sub> layer partially suppresses the electrolyte degradation during the initial cycle. In addition, a ~28% improvement in the thermal stability of the Li<sub>2</sub>SO<sub>4</sub>-treated samples in reactions with battery solution was also shown by DSC studies. The post-cycling analysis allowed us to conclude that the Li<sub>2</sub>SO<sub>4</sub> phase remained on the surface and retained its structure after 100 cycles.
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