Ion Dynamics in P2-Na_xCoO₂ Detected with Operando Muon Spin Rotation and Relaxation

In this study, the self-diffusion coefficient of Na+ (DNaJ) in P2-NaxCoO2 was measured during charge and discharge reactions between P2-NaxCoO2 and Na metal electrodes in the electrochemical half-cell using an operando muon spin rotation and relaxation (μ+SR) technique through the observation of a fluctuating nuclear magnetic field. The measurements were performed at room temperature [297(1) K] in the half-cell potential range of 2.25–4.0 V. DNaJ was found to range between 7 × 10–12 and 2 × 10–11 cm2/s, which roughly decreased with decreasing Na content x. However, the apparent self-diffusion coefficient determined with an electrochemical technique (DNaJ,app) was approximately 1 order of magnitude greater than DNaJ in the whole range of x measured, besides three sharp minima at x = 1/2, 2/3, and approx. 0.72. The absence of the sharp minima in the μ+SR-determined DNaJ(x) curve is attributed to the fact that μ+SR provides local diffusive information, while electrochemical techniques are based on macroscopic Na+ flow. The μ+SR measurements on the P2-Na0.75CoO2 powder sample indicated a strong temperature dependence of the Na dynamics, particularly near room temperature.