Boosting the Structural Reversibility of Layered Oxide Cathode for Realizing Long‐Term Cycle Life Through Electronic Structure Regulation

O3‐type layered oxide cathode exhibits great application potential for practical sodium‐ion batteries, due to its cost‐effectiveness, abundant sodium and manganese resources, and high theoretical capacity. However, the irreversible phase transition, coupled with rapid capacity decay, which is primarily attributed to the Jahn‐Teller effect of Mn3+, remains a significant bottleneck for commercial application. Additionally, the sluggish kinetics during the (de)sodiation process require urgent improvement. Herein, an electronic structure regulation strategy is proposed by low‐valence Li/Cu co‐substitution to address these issues. The roles of Li/Cu on the electronic structure, structural evolution, and electrochemical properties in the Na0.96Ni0.22Fe0.2Mn0.5Li0.04Cu0.04O2 (NFMLC) cathode are comprehensively explored through systematic in situ/ex situ characterization techniques and theoretical calculations. The results reveal that this strategy effectively activates more Ni2+/3+ and Fe3+/4+ redox reactions above 2.5 V, while suppressing Mn3+/4+ redox activity below 2.5 V, thereby achieving highly structural reversibility. Therefore, the NFMLC electrode displays excellent long‐term cycling stability (81.5% capacity retention after 2000 cycles at 5 C), and significantly enhanced rate performance (from 45.5% to 80.4% under a ratio of 5 C to 0.5 C). This work provides a valuable perspective on the design of low‐cost, long‐life, and high‐performance layered oxide cathodes for practical sodium‐ion batteries. By optimizing the electronic structure, Li/Cu co‐substitution effectively activates more Ni2+ and Fe3+ ions to participate in charge compensation above 2.5 V, while suppressing the redox activity of Mn3+ below 2.5 V. This strategy effectively mitigates the Jahn‐Teller distortion of Mn3+ and inhibits the irreversible O3–P3 phase transition, realizing long‐term cycle life for the O3‐type layered oxide cathode.