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 cm 2/s (0:54 ≤ x ≤ 0:85) at 303 K, which is comparable to that of LiCoO2. The activation energy (ED a ) of D gradually increases from 0.32 at x = 0:54 to 0.51 eV at x = 0:85. The activation energy (ER a ) of charge-transfer resistance (R ct) 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).
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