Electrochemical properties of P2-phase Na0.74CoO2 compounds as cathode material for rechargeable sodium-ion batteries

P2-phase Na0.74CoO2 cathode material prepared by a solid-state method exhibits the specific discharge capacity of 107 mAh g−1 at 0.1 C with good cycling performance for rechargeable sodium ion batteries. The voltage polarization between charging and discharging at 0.1 C rate is about 150–250 mV and the coulombic efficiency in each cycle is about 89%. The expansion and compression in c-axis of the NaxCoO2 unit cell during the Na intercalation/deintercalation is revealed by ex situ XRD. XPS and in situ XAS data directly confirm that deintercalation/intercalation of Na ions from/into the layered structure proceeds with the Co3+/Co4+ redox reaction.

► In this paper, we synthesized Na0.74CoO2 as cathode for rechargeable sodium ion batteries. ► We examined its electrochemical performance. ► A reversible capacity of 107 mAh/g with 0.1% capacity fading for the first 40 cycles was obtained. ► The deintercalation/intercalation reactions of Na ions from/into Na0.74CoO2 proceeded while Co3+/Co4+ redox.