Decoupling the roles of Ni and Co in anionic redox activity of Li-rich NMC cathodes

Li[Li_xNi_yMn_zCo_1-x-y-z]O₂ (lithium-rich NMCs) are benchmark cathode materials receiving considerable attention due to the abnormally high capacities resulting from their anionic redox chemistry. Although their anionic redox mechanisms have been much investigated, the roles of cationic redox processes remain underexplored, hindering further performance improvement. Here we decoupled the effects of nickel and cobalt in lithium-rich NMCs via a comprehensive study of two typical compounds, Li_1.2Ni_0.2Mn_0.6O₂ and Li_1.2Co_0.4Mn_0.4O₂. We discovered that both Ni^3+/4+ and Co⁴⁺, generated during cationic redox processes, are actually intermediate species for triggering oxygen redox through a ligand-to-metal charge-transfer process. However, cobalt is better than nickel in mediating the kinetics of ligand-to-metal charge transfer by favouring more transition metal migration, leading to less cationic redox but more oxygen redox, more O₂ release, poorer cycling performance and more severe voltage decay. Our work highlights a compositional optimization pathway for lithium-rich NMCs by deviating from using cobalt to using nickel, providing valuable guidelines for future high-capacity cathode design.