Analysis of Diffusion Behavior in Lithium-Air Batteries Via Simulated and Experimental Impedance Spectroscopy and Equivalent Circuit Modeling

Abstract

Porous electrodes offer the advantage of high surface area in a compact structure and play an important role in the diffusion process in batteries. Particular to lithium-oxygen (Li-O2) batteries, diffusion of oxygen from an external source complicates transport properties due to the consideration of multiple active species at multiple phases and oxygen diffusion complexity. General diffusion from transport processes that normally result in relaxation time constants then may not apply. Instead a complex frequency response may occur, which may be explained by blocking electrode behavior. In Li-O2 cells featuring binder-free electrodes with metal oxide catalyst material (1), rough surfaces and possible non-uniform current distribution may generate interesting impedance spectra from stochastic errors and depressed time constants. Previous studies by de Levie (2) and Bisquert (3-5) generally investigate via transmission line circuit models the impedance exhibited by porous electrodes. This work aims to apply the basis of a transmission line model like Figure 1 with the experimental impedance spectra of a Li-O2 cell with a lithium metal electrode and a carbon cloth coated cobalt, manganese composite oxide discharged and charged more than ten times (6). The circuit model estimates the thickness of the oxide, or length of the model pore, in terms of the number of resistor-constant phase elements (R-CPE), and the impedances along the line. The observed diffusion impedance and its frequency dispersion estimated by repeated R-CPE elements includes a terminating impedance at the end of the line, indicating what the impedance at the boundary where diffusion of the species to the electrode surface should occur. The impedance spectra of the experimental cell has high negative reactance values at the lower frequencies as shown in Figure 2. This capacitive nature is suggestive of a blocking behavior exhibited by electrodes where little faradaic current or charge transfer occurs, such as in the case of electrodes completely coated by a nonconductive oxide. Acknowledgements This work was supported by the ERC program of National Science Foundation under award number EEC-08212121.