Reassessing anionic redox in conventional layered oxide cathodes for Li-ion batteries: ionic and covalent mechanisms

Efforts to improve the specific capacity and energy density of lithium nickel-cobalt-manganese oxide (NCM) cathodes focus on operating at high voltages or increasing nickel content. However, both approaches necessitate a thorough understanding of the charge compensation mechanism. Traditional ionic-bonding models which separate transition metal (TM) and oxygen redox processes prove inadequate as anionic redox becomes significant, ignoring crucial metal-oxygen interactions. In this study, we systematically investigate the charge compensation process in low-nickel and high-nickel NCMs under high-voltage conditions. Here, the involvement of oxygen is critical in redox, as it shares electrons with TM to form a strong TM-O covalent bond. Compared to low-Ni NCMs, high-Ni NCMs exhibit an oxygen dimerization stage with trapped O₂, which leads to the aggregation of vacancies in the transition metal layer, thereby accelerating structural destabilization. This variation in oxygen dimerization behavior among NCMs is closely correlated with differences in elemental composition, spin states, and stacking faults. Our findings comprehensively reveal the redox behaviors of transition metals and oxygen, particularly highlighting oxygen behavior at each delithiation state, helping to optimize the utilization of oxygen redox reactions in commercial NCM compounds for high-capacity and high-energy-density lithium-ion batteries.