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

Abstract In this work, almost pure Na x CoO 2 ( 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) A, b =4.89983(4) A, and c =10.92334(14) A. 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 Na x CoO 2 . 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.