  Sodium-ion batteries (SIBs) have received significant attention as promising
alternative for energy storage applications owing to the large availability and
low cost of sodium. In this paper we study the electrochemical behavior of
Na$_{0.74}$Co$_{1-x}$Nb$_x$O$_2$ ($x=$ 0 and 0.05 samples), synthesized via
solid-state reaction. The Rietveld refinement of x-ray diffraction patterns
reveals the hexagonal crystal symmetry with P63/mmc space group. The
Na$_{0.74}$Co$_{0.95}$Nb$_{0.05}$O$_2$ cathode exhibits a specific capacity of
about 91 mAhg$^{-1}$ at a current density of 6 mAg$^{-1}$, whereas
Na$_{0.74}$CoO$_2$ exhibits comparatively low specific capacity (70 mAhg$^{-1}$
at a current density of 6 mAg$^{-1}$). The cyclic voltammetry (CV) and electron
impedance spectroscopy (EIS) were performed to determine the diffusion
coefficient of Na, which found to be in the range of 10$^{-10}$ cm$^2$s$^{-1}$.
