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

High Resolution Image Download MS PowerPoint Slide In this study, the self-diffusion coefficient of Na + ( D Na J ) in P2-Na x CoO 2 was measured during charge and discharge reactions between P2-Na x CoO 2 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. D Na J was found to range between 7 × 10 –12 and 2 × 10 –11 cm 2 /s, which roughly decreased with decreasing Na content x . However, the apparent self-diffusion coefficient determined with an electrochemical technique ( D Na J,app ) was approximately 1 order of magnitude greater than D Na J 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 D Na J ( 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-Na 0.75 CoO 2 powder sample indicated a strong temperature dependence of the Na dynamics, particularly near room temperature.