Engineering local chemistry via Cu/Al Co-doped Na0.67Ni0.33Mn0.67O2 cathode for suppressed Na-vacancy ordering and P2-O2 transition with enhanced sodium-ion storage stability

P2-type cathode materials are promising for sodium-ion batteries (SIBs) due to their high theoretical capacity. However, sluggish Na + kinetics and irreversible phase transitions hinder their practicality. Herein, we reported Cu/Al co-doping strategy to modify the local chemical environment and optimize Na + ​diffusion of Na 0.67 Ni 0.33 Mn 0.67 O 2 cathode. The resulting Na 0.76 Ni 0.33- x Cu x Mn 0.57 Al l0 O 2 ( x ​= ​0.10, 0.15) cathodes exhibited superior cycling performance, with 93 ​% capacity retention after 50 cycles at 0.1 ​C and 88 ​% after 150 cycles at 1 ​C. DFT analysis indicated that the formation of Cu-O and Al-O bonds significantly enhanced oxygen binding strength in TM, expanding the NaO 2 layer and reducing energy barriers for Na + ​migration, which is further supported by GITT measurements. The improved stability suggests that the substitution of Al 3+ for Mn 3+ effectively mitigates the Jahn-Teller distortions linked to Mn 3+ , thereby strengthening the layered structure during the cycling by suppressing the irreversible P2-O2 phase transition, as confirmed by in-situ XRD. The NaMAC-10//HC and NaMAC-15//HC full cell demonstrates a high energy density of (399.0 and 386.51 ​Wh kg −1 ) with output voltages of (3.56 and 3.6 ​V), with excellent long-term cycling, reflecting the efficacy of chemical environment engineering. Cu and Al co-doping in Na 0.67 Ni 0.33 Mn 0.67 O 2 significantly expands the NaO 2 interlayer spacing, improving Na + diffusion kinetics and suppressing Na-vacancy ordering and irreversible phase transitions. This chemical environment engineering achieves superior stability and enhanced cycling performance, as verified by experimental and DFT results.