Cycling-Driven Electrochemical Activation of Li-Rich NMC Positive Electrodes for Li-Ion Batteries

For over a decade, Li-rich layered metal oxides have been intensively investigated as promising positive electrode materials for Li-ion batteries. Despite substantial progress in understanding of their electrochemical properties and (de)intercalation mechanisms, certain aspects of their chemical and structural transformations still remain unclear. In this work, we investigated the so-called cycling-driven electrochemical activation, which manifests itself as a gradual increase of reversible capacity upon cycling when the Li-to-transition metal atomic ratio exceeds 1.5 in the Li(LixMn1–x–y–zNiyCoz)O2 formula. We found that initially, transition metals in this material are in high oxidation states and cannot be further oxidized during charge, which explains low initial capacity. The cycling-driven activation process proceeds through partial O2–/n– oxidation on charge, followed by reduction of oxygen and transition metals on discharge. The activated area gradually advances from the surface to the center of secondary Li-rich NMC particles through evolution of a core–shell structure. In this model, the slow anionic redox is a rate-limiting step, which also explains substantial dependence of the cycling-driven electrochemical activation on cycling rate.