Sodium-ion diffusion and ordering in single-crystal P2-NaxCoO2

Sodium-ion self-diffusion in P2(gamma)-Na(x)CoO(2) has been studied using a single crystal as an electrode. Mass transport and thermodynamic properties of Na ions were measured by the potentiostatic intermittent titration technique on a single-crystal electrode with greater accuracy under well-defined diffusion geometry. Excluding the significantly slower diffusion behavior near particular Na ordered phases, average diffusion coefficients D(Na) are extracted to be 1.2 +/- 0.5 X 10(-7) cm(2)/s for x>0.5 and 4.1 +/- 0.5 X 10(-8) cm(2)/s for x<0.5, respectively. The existence of stable phases with special Na ordering of x similar to 0.25, 0.33, 0.43, 0.5, 0.55, and 0.71 are verified by either the maximum or diverging signature of the x-dependent self-diffusion coefficients.