As cathode materials of sodium-ion batteries (SIBs), O3-type NaTMO₂ (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_0.75Fe_0.5-xCu_xMn_0.5O₂ 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_0.75Fe_0.25Cu_0.25Mn_0.5O₂ 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_0.75Fe_0.25Cu_0.25Mn_0.5O₂ 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_0.75Fe_0.25Cu_0.25Mn_0.5O₂ 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.

作为钠离子电池（SIBs）的正极材料，O3型NaTMO ₂（TM=过渡金属）引起了广泛关注。然而，循环过程中不可逆的结构变化和不理想的倍率性能阻碍了它们的实际应用。在这项工作中，我们通过简单的溶胶-凝胶法设计了缺钠O3相-Na _0.75 Fe _{0.5- x} Cu _x Mn _0.5 O ₂作为SIBs的正极材料，以提高电化学性能。合适的铜离子掺杂可以稳定晶体结构，提供更高的放电容量，并抑制从O3到P3的不可逆相变。Na _0.75 Fe _0.25 Cu_0.25 Mn _0.5 O ₂可提供100 mAh g ^-1的可逆容量，在0.1 C、2.5和4.1 V之间的容量保持率为91%。 Na _0.75 Fe _0.25 Cu _0.25 Mn _0.5 O的储钠机理和相变过程₂由非原位 XRD 和 XPS 揭示。充电至3.2V时，发现从单相O3到双相P3/O3的相变。经过一个完整的循环后，双相 P3/O3 重新回到 O3 相，表明 Na _0.75 Fe _0.25 Cu _0.25 Mn _0.5 O _2的相变是完全可逆的，主要是基于整个电化学反应过程中的固溶反应。同时，穆斯堡尔光谱还显示当充电至 4.1 V 时铁原子在两种不同的配位环境中，表明 P3 和 O3 相共存。这种缺钠正极材料同时具有O3相的高容量和P2相的优异稳定性的优点。该工作为SIBs正极材料的设计提供了新思路。