Synergistic Mg/Cu co-doping toward optimizing phase transitions across the full voltage window in P2-type sodium-ion cathodes

P2-type layered transition metal oxides (TMOs) are among the most promising cathode materials for sodium-ion batteries due to their high capacity and energy density. However, their practical application is hindered by irreversible phase transitions and lattice oxygen loss at high voltages, while Mn 3+ -induced Jahn–Teller distortion and P2–P′2 phase transitions at low voltages accelerate structural degradation and capacity fading. Here, we present a Mg/Cu co-doping strategy to systematically regulate structural evolution and redox chemistry, addressing these key degradation mechanisms. Mg 2+ , with strong electrostatic interactions and a small ionic radius, acts as an “anchoring” agent during deep sodiation (low potential), stabilizing the transition-metal layers, suppressing Mn disproportionation and Jahn–Teller distortion, and mitigating the P2–P′2 phase transition. Cu 2+ , in contrast, functions during deep desodiation (high potential) by enhancing Ni O bond ionicity and lowering the charge-transfer barrier between O 2p and transition-metal 3d orbitals, thereby reducing lattice oxygen participation, transition-metal dissolution, and interlayer gliding, and delaying the P2–O2 phase transition. Benefiting from this synergistic effect, Na 0.67 Ni 0.2 Mn 0.65 Mg 0.1 Cu 0.05 O 2 exhibits excellent electrochemical performance, retaining 80% of its capacity after 300 cycles at 1C in the 2.0–4.5 V window and maintaining a high reversible capacity of 78.5 mAh g −1 at 20C. This work not only provides a solution to the dual high-/low-voltage degradation challenge in P2-type TMOs but also elucidates the underlying mechanism of multi-element co-doping, offering a strategy for designing high-energy-density, long-life sodium-ion battery cathodes.