Sodium ion diffusion in layered NaxMnO2 (0.49 ≤ x ≤ 0.75): Comparison with NaxCoO2

The temperature (T) dependence of the sodium ion diffusion coefficient D of a NaxMnO2 thin film is extensively investigated versus x and T using electrochemical impedance spectroscopy. D is found to be 5.0 x 10(-11) cm(2)/s at x = 0.54 at 301 K, which is much larger than the value of 0.7 x 10(-11) cm(2)/s at x = 0.54 at 303 K of a NaxCoO2 thin film. In addition, the activation energy E-a(D) of D (= 0.26-0.32 eV) is smaller than that of NaxCoO2 (= 0.31-0.44 eV). The improved Na+ diffusion properties of NaxMnO2 are interpreted in terms of the larger interlayer distance (d = 5.54 angstrom) of NaxMnO2. (C) 2014 The Japan Society of Applied Physics