Estimation of Diffusion Coefficient of Lithium in Carbon Using AC Impedance Technique

The validity of estimating the solid phase diffusion coefficient, Ds, of a lithium intercalation electrode from impedance measurement by a modified electrochemical impedance spectroscopy (EIS) method is studied. A macroscopic porous electrode model and concentrated electrolyte theory are used to simulate the synthetic impedance data. The modified EIS method is applied for estimating Ds. The influence of parameters such as the exchange current density, radius of active material particle, solid phase conductive, porosity, volume fraction of inert material, and thickness of the porous carbon intercalation electrode, the solution phase diffusion coefficient, and transference number, on the validity of Ds estimation, is evaluated. A simple dimensionless group is developed to correlate all the results. It shows that the accurate estimation of Ds requires large particle size, small electrode thickness, large solution diffusion coefficient, and low active material loading. Finally, a "full model" method is developed for the cases where the modified EIS method does not work well.