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

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.