As cathode materials of sodium-ion batteries (SIBs), O3-type NaTMO 2 (TM = transition metal) have attracted wide interest. However, the irreversible structural change during the cycling and the unsatisfactory rate performance hinder their practical application. In this work, we designed sodium-deficient O3 phase-Na 0.75 Fe 0.5- x Cu x Mn 0.5 O 2 as the cathode materials of SIBs by simple sol-gel method to improve the electrochemical performance. The suitable copper ion doping can stabilize crystal structure, deliver higher discharge capacity, and restrain the nonreversible phase transition from O3 to P3. The Na 0.75 Fe 0.25 Cu 0.25 Mn 0.5 O 2 can provide a reversible capacity of 100 mAh g −1 with a capacity retention of 91% between 2.5 and 4.1 V at 0.1 C. The sodium storage mechanism and phase transition process of Na 0.75 Fe 0.25 Cu 0.25 Mn 0.5 O 2 were revealed by the ex-situ XRD and XPS. When charging to 3.2V, the phase change from single-phase O3 to biphase P3/O3 is found. After a complete cycle, the biphase P3/O3 regains O3 phase, demonstrating the phase transition of Na 0.75 Fe 0.25 Cu 0.25 Mn 0.5 O 2 is completely reversible, which is mainly based on the solid-solution reaction during the whole electrochemical reaction process. Meanwhile, the Mössbauer spectroscopy also displays the iron atoms in two different coordination environments when charged to 4.1 V, indicating a coexistence of P3 and O3 phases. This sodium deficient cathode material possesses advantages of high capacity from the O3 phase and the excellent stability from the P2 phase simultaneously. This work provides a new idea for the design of cathode materials for SIBs. • Sodium-deficient O3 phase-Na 0. 7 5 Fe 0.5- x Cu x Mn 0.5 O 2 are constructed for the first time. • The Na storage mechanism of Na 0. 7 5 Fe 0.5- x Cu x Mn 0.5 O 2 is revealed by ex-situ XRD. • Na 0. 7 5 Fe 0.25 Cu 0.25 Mn 0.5 O 2 with excellent cycling stability and low cost shows a potential application prospect.

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