Enhancing electrochemical performance of ultra-nickel-rich layered oxides cathodes via tailored Co/Mn-rich shell structures

Driven by the growing demand for energy density of lithium-ion batteries, ultra-nickel- rich layered oxides LiNi x Co y Mn z O 2 (x > 0.9) (ultra-Ni-rich NCM) cathodes attract considerable attention. However, these cathode materials suffer from severe structural degradation during cycling, primarily due to phase transitions, which significantly hinders their long-term application. In this work, ultra-Ni-rich NCM cathode materials with Co- and Mn-rich surface architectures are synthesized using Co/Mn-rich shell precursors and are systematically investigated to evaluate the suitability of different surface chemistries under various electrochemical conditions. The results reveal that the Co-rich surface significantly enhances the electrochemical kinetics by mitigating the Li + concentration gradient between the surface and bulk, thereby effectively suppressing structural degradation. In contrast, the Mn-rich surface, due to the presence of electrochemically inert Mn 4+ , leads to sluggish kinetics and incomplete phase transitions within the voltage window of 2.8–4.3 V. This results in a lower capacity but notably improved structural integrity. Upon increasing the upper cutoff voltage to 4.4V, the Mn-rich shell cathode undergoes more complete phase transitions, leading to a substantial improvement in capacity while maintaining superior structural stability.