Structural modulation by Zn dopants towards air-stable layered oxide cathodes for sodium-ion batteries

P2-type layered oxides have higher specific capacities among cathodes for sodium-ion batteries. However, these materials gradually degrade in air due to the surface reaction. In this work, we modulate the P2-type Na_0.75(Li_0.25Mn_0.75)O₂ (NLMO) into a honeycomb superstructure by Zn dopants (NLMZO), enabling more sodium storage. Despite the rapid capacity decay in initial cycles, NLMZO still delivers a stable capacity higher than NLMO. The reaction of NLMZO with air occurs mainly at metastable storage sites without destroying the structure and passivates the degradation on the stable storage sites. In contrast, the degradation of NLMO in air leads to the formation of NaOH and Na₂CO₃ on the surface and thus the loss of stable capacity. The octahedral distortions make the superstructure less stable, but enhance the air stability. Density functional theory calculation suggests the lower surface energy of (100) plane in NLMZO than that in NLMO. The work paves an innovative way to stabilize the layered oxides in air by modulating the superstructure.