Operando Muon Spin Rotation and Relaxation Measurement on LiCoO₂ Half-Cell

The self-diffusion coefficient of Li+ ions (DLiJ) in LixCoO2 has been determined during charge and discharge reactions of a LiCoO2 half-cell using a positive muon spin rotation and relaxation (μ+SR) technique through the observation of the fluctuating nuclear magnetic field at room temperature: specifically, with an operando μ+SR technique in the half-cell potential range between 3.0 and 4.7 V. The estimated DLiJ was found to range between 10–12 and 10–11 cm2/s and increase as Li content (x) decreased. Although the slope (dDLiJ/dx) was relatively steep below the vicinity of x = 1, DLiJ slowly increases with decreasing x at x < 0.8 with two small minima at x ∼ 2/3 and x ∼ 1/2. The obtained result in the initial charging process agreed with that obtained in the second charging process. Furthermore, it was discovered that the change in DLiJ with x is similar to that of the reported self-diffusion coefficient estimated using an electrochemical technique for a film sample. By combining with the electrochemical measurement, the x dependence of the reactive surface area of the LixCoO2 electrode was first determined, while it was assumed to be independent of x.