Hysteresis abated P2-type NaCoO2 cathode reveals highly reversible multiple phase transitions for high-rate sodium-ion batteries

Abstract

Despite the multiple phase transitions that occur during Na⁺ ion intercalation and deintercalation, an enhanced charge–discharge rate and a long cycle life are achieved with a hexagonal shaped P2-type NaCoO₂ cathode for sodium-ion batteries (SIBs). The hysteresis abated crystalline phase was obtained by tuning the calcination temperature/time factors for the citric acid assisted sol–gel technique precursor. Powder X-ray diffraction data confirmed that the studied material belongs to a hexagonal crystal system and that, among the materials, the material synthesized at 750 °C/28 h in an air atmosphere in particular showed pure phase formation with an ordered structure. Furthermore, the local atomic arrangement of the synthesized NaCoO₂ cathodes was monitored using a Raman spectroscopy technique, revealing five active vibration modes of E_1g(O), E_2g(O), 2 E_2g (Na), and A_1g(O), in NaCoO₂ to confirm the existence of the hexagonal crystal structure. The surface morphology of the designed materials exhibited a hexagonal shape with a size of 2–5 μm. By tuning the calcination temperature/time factors, the optimized critical parameters of the P2-type NaCoO₂ cathode were 750 °C and 28 h, resulting in a well-ordered structure, which enhances Na⁺ ion storage capacity at a high-rate. Thus, a hysteresis abated P2-type NaCoO₂ cathode demonstrated high-rate, stable charge–discharge cycles with 79 mA h g⁻¹ capacity at a rate of 1C with a retention of 99% coulombic efficiency for the 100th cycle.