Figure 6

Bulk diffusion rate of Na _x CoO₂ as a function of sodium concentration, x. The measurements performed at ambient temperature using the Potentiostatic Intermittent Titration Technique (PITT) are shown as green circles. The diffusion rate determined at room temperature in our QENS study in the striped phase (Fig. 2(b)) and composition in the vicinity of x ~ 0.8 is shown as a red dashed line, and this value is comparable to the optimum rate for x > 0.5 (green dashed line). Sharp drops are observed at simple fractional values of x that correspond to Na-ion superstructures, and these structures are shown for x = 1/2, 5/9 and 5/7²⁴. PITT data reproduced from Fig. 4(c) in ref.²¹.