Unified Picture of (Non)Hysteretic Oxygen Capacity in O3‐Type Sodium 3d Layered Oxides

Abstract A unified picture of the generalized mechanism that elucidates the (non)hysteretic oxygen capacities of O3‐type Na_1−x[Li_2/6Mn_3/6M_1/6]O₂ (M = Mn⁴⁺, Ni⁴⁺, and Ti⁴⁺) layered oxides is suggested to provide a critical factor in inducing ideal reversibility for an oxygen redox (OR) reaction using the new concept, the “potential‐pillar” effect. Considering that there is no formation of interlayer oxygen–oxygen (OO) dimers at x = 1.0 in Na_1−x[Li_2/6Mn_3/6Ti_1/6]O₂, the phase stabilities reveal that the biphasic reaction occurs in Na_1−x[Li_2/6Mn_3/6Ni_1/6]O₂ (0.5 ≤ x ≤ 1.0), and the monophasic reaction takes place in Na_1−x[Li_2/6Mn_3/6Ti_1/6]O₂ during desodiation. The electronic structures of cations and anions unambiguously show OR reactions over the full vacancy range, and the oxygen ions comprising TiO₆ are relatively deactivated compared with those of NiO₆ upon electrochemical OR reaction. This is deemed as an intriguing “potential‐pillar” effect, in which the chemically stiff Ti⁴⁺(3d)O(2p) bond locally retains a strong electrostatic repulsion between the mixed layers. This unified concept based on an in‐depth understanding of the three cathode models not only accounts for the origin of the (non)hysteretic oxygen capacities, but also provides an exciting local structure viewpoint for harnessing the full potential of OR reactions for high energy‐density sodium‐ion batteries.