Electrochemical properties of NaxCoO2 (x~0.71) cathode for rechargeable sodium-ion batteries

In this work, almost pure NaxCoO2 (x=0.71) sample has been prepared via a solid-state route. The stoichiometric composition has been estimated by ICP-AES analysis and the Rietveld refinement performed on the respective XRD pattern led to an orthorhombic structure in the Cmcm space group with a=2.83218(3) Å, b=4.89983(4) Å, and c=10.92334(14) Å. The insertion/deinsertion of sodium into the well-crystallized particles leads to capacities as high as 70.4 mAhg−1 with an initial Coulombic efficiency of 90%, and excellent capacity retention of almost ~100% after the 90th cycle between a 2.0 and 3.5 V voltage window for a 0.08 C–rate. The insertion ratio (x) varied between 0.592 and 0.673 during the deinsertion/insertion process within NaxCoO2. The voltage profile shows numerous voltage steps being mainly associated to the slight ordering of the Na ions within the layers. By coupling the electrochemical process with an in-situ X-ray diffraction experiment, the c-axis and a-axis peak positions such as (002), (004) and (100) are shifted with the accompanying deintercalation/intercalation of sodium, suggesting that the extracted sodium has been reversibly occupied with a small variation in sodium composition ranges due to peculiar Na+/vacancy ordering.