Sodium storage property and mechanism of NaCr1/4Fe1/4Ni1/4Ti1/4O2 cathode at various cut-off voltages

Room-temperature sodium-ion batteries (SIBs) have attracted extensive interest in large-scale energy storage applications for renewable energy and smart grid, owing to abundant sodium resources and low cost. O3-layered sodium transition metal oxides (i.e., NaMO2, M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, etc.) are considered as a promising class of cathode materials for SIBs due to their high capacity and ease of synthesis. In this work, a quaternary layered material O3-NaCr1/4Fe1/4Ni1/4Ti1/4O2 (O3-NCFNT) is successfully synthesized and investigated as a new cathode material for SIBs. Within the voltage range of 1.5-4.1 V, O3-NCFNT delivers an ultrahigh charge capacity of 213 mA h g(-1) but a limited discharge capacity of 107 mA h g(-1) in the initial cycle. Ex situ X-ray diffraction and X-ray absorption spectroscopy results reveal that an irreversible phase transformation as well as the irreversible redox of Cr3+/Cr6+ within the voltage range of 1.5-4.1 V should be responsible for the capacity decay in the initial cycle. While, O3-NCFNT exhibits the initial charge/discharge capacities of 135.4 and 129.2 mA h g(-1) with a high coulombic efficiency of 95.4% as well as good cyclic performance within the voltage range of 1.5-3.4 V at current rate of 0.1C. Especially, O3-NCFNT shows a capacity retention of 77.1% after 300 cycles at a high rate of 1C, indicating that the structural origins of capacity decay caused by excessive sodium extraction are confirmed by XRD and XAS measurements to unlock the potential of this material for sodium-ion battery application.