Synergistically Engineering Low-Coordination Mn Sites via Cu/Sb Co-Doping toward Low-Strain O3-Type Sodium Oxide Cathodes

O3-type layered oxides are prone to severe structural distortion and irreversible phase transitions during deep Na + deintercalation/intercalation, leading to rapid capacity decay and thus limiting their practical application. Herein, a lattice-doping strategy is employed to synthesize an O3-type cathode, Na 0.93 Ni 0.37 Mn 0.49 Cu 0.13 Sb 0.01 O 2, with pronounced low-strain behavior. At 1C, the material exhibits excellent cycling stability, retaining 92.6% of its capacity after 200 cycles. In addition, it delivers a discharge capacity of 96.2 mAh g –1 at 5C, corresponding to 84.9% of its 1C capacity, and maintains 91.0% capacity retention after 200 cycles at this rate. In situ XRD combined with Rietveld refinement reveals that the unit-cell volume variation of NNMCS during cycling is only 0.34%, indicating highly reversible phase evolution with minimal lattice strain. The superior electrochemical performance is attributed to local structural/electronic reconstruction within the transition-metal layer induced by Cu/Sb codoping. XPS and XAFS analyses demonstrate that Cu/Sb codoping increases the average Mn oxidation state and induces local coordination rearrangement around Mn, leading to the formation of a low-coordination configuration. This structural feature effectively mitigates lattice distortion, suppresses irreversible structural evolution during cycling, and promotes the synergistic optimization of Na + transport kinetics and structural stability. This work provides useful insight into the design of high-performance and long-lifetime layered cathodes for sodium-ion batteries.