Abstract
The temperature dependence of sodium ion diffusion coefficient (D) of NaxCoO2 is extensively investigated in the temperature range of 303≤T≤349 K using electrochemical impedance spectroscopy. The magnitude of D is found to be (0.7–1.9)×10-11 cm2/s (0.54≤x≤0.85) at 303 K, which is comparable to that of LiCoO2. The activation energy (EaD) of D gradually increases from 0.32 at x=0.54 to 0.51 eV at x=0.85. The activation energy (EaR) of charge-transfer resistance (Rct) also increases from 0.34 at x=0.54 to 0.59 eV at x=0.85. These behaviors are ascribed to an increase in potential barrier due to the decrease in interlayer distance (d).