Regulating Phase Transition and Oxygen Redox to Achieve Stable High‐Voltage O3‐Type Cathode Materials for Sodium‐Ion Batteries

O3‐type layered oxides are promising cathode materials for sodium‐ion batteries. However, unsatisfying cyclic stability hinders its practical application, mainly resulting from harmful phase transition and irreversible oxygen redox, especially in high‐voltage regions. Herein, a co‐doped strategy by incorporating Li + , Mg 2+ , Ca 2+ , and Sb 5+ into the O3‐Na 0.8 Ni 0.4 Fe 0.2 Mn 0.4 O 2 cathode materials is proposed. Both suppressing the undesired phase transition over 4.1 V (vs Na/Na + ) and reducing the anisotropic strain are achieved with the novel Na 0.8 Ni 0.3 Fe 0.2 Mn 0.3 Li 0.1 Mg 0.02 Ca 0.05 Sb 0.03 O 2 (LMCS NFM). Moreover, restricted but highly reversible oxygen redox is observed due to strong attraction from Sb 5+ and special “Li–O‐vacancy” and “Mg–O‐vacancy” configurations. The strategy brings about excellent high‐voltage cyclic stability with a reversible capacity of 130 mAh g −1 and a capacity retention of 85% after 250 cycles at 4.2 V, and less thermal runaway risk and moisture sensitivity, increasing the probability of O3‐type oxide cathode practical applications.