Facilitating the oxygen redox chemistry in O3-type layered oxide cathode material for sodium-ion batteries by Fe substitution

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Facilitating anion redox chemistry is an effective strategy to increase the capacity of layered oxides for sodium-ion batteries. Nevertheless, there remains a paucity of literature pertaining to the oxygen redox chemistry of O3-type layered oxide cathode materials. This work systematically investigates the effect of Fe doping on the anionic oxygen redox chemistry and electrochemical reactions in O3-NaNi0.4Cu0.1Mn0.4Ti0.1O2. The results of the density functional theory (DFT) calculations indicate that the electrons of the O 2p occupy a higher energy level. In the ex-situ X-ray photoelectron spectrometer (XPS) of O 1s, the addition of Fe facilitates the lattice oxygen (On−) to exhibit enhanced activity at 4.45 V. The in-situ X-ray diffraction (XRD) demonstrates that the doping of Fe effectively suppresses the Y phase transition at high voltages. Furthermore, the Galvanostatic Intermittent Titration Technique (GITT) data indicate that Fe doping significantly increases the Na+ migration rate at high voltages. Consequently, the substitution of Fe can elevate the cut-off voltage to 4.45 V, thereby facilitating electron migration from O2−. The redox of O2−/On− (n < 2) contributes to the overall capacity. O3-Na(Ni0.4Cu0.1Mn0.4Ti0.1)0.92Fe0.08O2 provides an initial discharge specific capacity of 180.55 mA h g−1 and 71.6% capacity retention at 0.5 C (1 C = 240 mA g−1). This work not only demonstrates the beneficial impact of Fe substitution for promoting the redox activity and reversibility of O2− in O3-type layered oxides, but also guarantees the structural integrity of the cathode materials at high voltages (>4.2 V). It offers a novel avenue for investigating the anionic redox reaction in O3-type layered oxides to design advanced cathode materials.