Synergistic Charge Compensation Process in P2‐Type Layered Oxides Enables High‐Reversibility Sodium‐Ion Batteries

Abstract P2‐type layered oxides have emerged as an attractive cathode candidate for sodium‐ion batteries (SIBs), demonstrating notable specific capacity and superior working voltages. However, the inherent Jahn–Teller effect associated with Mn³⁺ coordination at reduced potentials, coupled with irreversible oxygen evolution and transition metal (TM) cation displacement under high‐voltage operation, critically undermines the electrochemical cyclability of these systems. This study reveals that Cu/Ti co‐substitution in P2‐type cathodes establishes synergistic charge compensation mechanisms, enhancing structural reversibility across full voltage ranges. Under high‐voltage operation, Ti─O covalent bonding stabilizes oxygen redox through suppressed oxygen evolution. Cu^2+/3+ redox elevates operational voltage while alleviating Ni valence fluctuation. Furthermore, Cu²⁺ doping increases the average oxidation state of manganese based on the electroneutrality principle, which simultaneously suppresses the Jahn–Teller distortion under low‐voltage operation and consequently proves structural stability. This multi‐ion cooperation achieves comprehensive optimization of layered oxide cathodes for sodium‐ion batteries. The optimized Na_0.7Ni_0.2Mn_0.6Cu_0.15Ti_0.05O₂ (TC‐NNMO) cathode delivers a discharge capacity of 151.88 mAh g⁻¹ at 0.5 C (1.5–4.2 V) with remarkable cyclability, retaining 80.29% capacity after 200 cycles alongside enhanced voltage plateau maintenance. This work demonstrates how dual cationic substitution regulates charge compensation synergistically. The findings establish a paradigm for multi‐ion cooperative design in high‐performance P2‐type cathodes.