Development of Equivalent Circuits for Litihum-Air Battery Impedance Using Mixed and Isolated Diffusion Models
A graphical analysis of the complex impedance add additional support for a proposed physical model and its circuit parameters. With this method, the imaginary part of the impedance versus frequency, on the logarithmic scale, are plotted to determine the frequency ranges where the impedance is linear. The linear portions of these plots indicate separate electrochemical processes and distributed time constants, or constant phase element (CPE) behavior, with the slope of the line being the value of the CPE coefficient, α. This is helpful in determining the most accurate fit model with reasonable results. Since the focus is on the placement of the diffusion component in the circuit model, more emphasis will be on the low frequency range, where the diffusion impedance dominates.
In this study, the impedance of a lithium-air battery is measured after various cycles, and Nyquist plot and the impedance versus frequency plot are presented. Fitted equivalent circuit models of mixed diffusion and isolated diffusion are fitted to the Nyquist plots and are compared to the results of the graphical analysis technique.
This work was supported by the FREEDM ERC program of the National Science Foundation under award number EEC-08212121.