Anionic
redox chemistry offers a transformative approach for significantly
increasing specific energy capacities of cathodes for rechargeable
Li-ion batteries. This study employs operando electron paramagnetic
resonance (EPR) to simultaneously monitor the evolution of both transition
metal and oxygen redox reactions, as well as their intertwined couplings
in Li<sub>2</sub>MnO<sub>3</sub>, Li<sub>1.2</sub>Ni<sub>0.2</sub>Mn<sub>0.6</sub>O<sub>2</sub>, and Li<sub>1.2</sub>Ni<sub>0.13</sub>Mn<sub>0.54</sub>Co<sub>0.13</sub>O<sub>2</sub> cathodes. Reversible
O<sup>2–</sup>/O<sub>2</sub><sup><i>n</i>–</sup> redox takes place above 3.0 V, which is clearly distinguished from
transition metal redox in the operando EPR on Li<sub>2</sub>MnO<sub>3</sub> cathodes. O<sup>2–</sup>/O<sub>2</sub><sup><i>n</i>–</sup> redox is also observed in Li<sub>1.2</sub>Ni<sub>0.2</sub>Mn<sub>0.6</sub>O<sub>2</sub>, and Li<sub>1.2</sub>Ni<sub>0.13</sub>Mn<sub>0.54</sub>Co<sub>0.13</sub>O<sub>2</sub> cathodes,
albeit its overlapping potential ranges with Ni redox. This study
further reveals the stabilization of the reversible O redox by Mn
and e<sup>–</sup> hole delocalization within the Mn–O
complex. The interactions within the cation–anion pairs are
essential for preventing O<sub>2</sub><sup><i>n</i>–</sup> from recombination into gaseous O<sub>2</sub> and prove to activate
Mn for its increasing participation in redox reactions. Operando EPR
helps to establish a fundamental understanding of reversible anionic
redox chemistry. The gained insights will support the search for structural
factors that promote desirable O redox reactions
