Enhancing Structure Stability by Mg/Cr Co-Doped for High-Voltage Sodium-Ion Batteries.

P2-NaNiMnO cathode materials have garnered significant attention due to their high cationic and anionic redox capacity under high voltage. However, the challenge of structural instability caused by lattice oxygen evolution and P2-O2 phase transition during deep charging persists. A breakthrough is achieved through a simple one-step synthesis of Cr, Mg co-doped P2-NaNMCM, resulting in a bi-functional improvement effect. P2-NaNMCM-0.01 exhibits an impressive capacity retention rate of 82% after 100 cycles at 1 C. In situ X-ray diffraction analysis shows that the "pillar effect" of Mg mitigates the weakening of the electrostatic shielding and effectively suppresses the phase transition of P2-O2 during the charging and discharging process. This successfully averts serious volume expansion linked to the phase transition, as well as enhances the Na migration. Simultaneously, in situ Raman spectroscopy and ex situ X-ray photoelectron spectroscopy tests demonstrate that the strong oxygen affinity of Cr forms a robust TM─O bond, effectively restraining lattice oxygen evolution during deep charging. This study pioneers a novel approach to designing and optimizing layered oxide cathode materials for sodium-ion batteries, promising high operating voltage and energy density.