Sodium-deficient O3-Na0.75Fe0.5-xCuxMn0.5O2 as high-performance cathode materials of sodium-ion batteries

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 per-formance hinder their practical application. In this work, we designed sodium-deficient O3 phase-Na0.75Fe0.5-xCuxMn0.5O2 as the cathode materials of SIBs by simple sol-gel method to improve the electrochemical perfor-mance. The suitable copper ion doping can stabilize crystal structure, deliver higher discharge capacity, and restrain the nonreversible phase transition from O3 to P3. The Na0.75Fe0.25Cu0.25Mn0.5O2 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 Na0.75Fe0.25Cu0.25Mn0.5O2 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 Na0.75Fe0.25Cu0.25Mn0.5O2 is completely reversible, which is mainly based on the solid-solution reaction during the whole electrochemical reaction process. Meanwhile, the Modssbauer spectroscopy also displays the iron atoms in two different coordi-nation 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.