Decoding the Role of Oxygen Partial Pressure in Steering Anionic Redox Reactivities of Single‐Crystal Li‐Rich Mn‐Based Cathodes

Li‐rich Mn‐based oxides (LRMOs) are highly attractive cathodes for next‐generation lithium‐ion batteries due to their substantial capacity enabled by anionic redox reactions (ARR). However, balancing ARR activity with structural stability remains a major bottleneck. Here, we identify oxygen partial pressure during synthesis as a decisive factor governing this balance. Using single‐crystal Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 , we systematically regulate the calcination atmosphere—argon (LRMO‐0), air (LRMO‐20), and oxygen (LRMO‐100)—to tune oxygen‐vacancy levels, transition‐metal valence states, and cation disorder. Low oxygen partial pressure results in excessive oxygen vacancies and suppressed reversible ARR, leading to poor capacity and rate performance. Conversely, high oxygen partial pressure over‐activates ARR, triggering irreversible oxygen release and structural degradation. Notably, LRMO‐20 synthesized in air achieves the optimal compromise, delivering a 259 mAh g −1 initial discharge capacity, 90.1% retention after 500 cycles, and markedly reduced phase transformation. This work clarifies how atmospheric control modulates ARR and structural evolution, offering an effective strategy for developing high‐performance Li‐rich cathodes.