Modeling, simulation, and experimental verification of impedance spectra in Li-air batteries

There has been a growing interest in electrochemical storage devices such as batteries, fuel cells and supercapacitors in recent years. This interest is due to our increasing dependence on portable electronic devices and on the high demand for energy storage from the electric transport vehicles and electrical power grid industries. As we transition towards cleaner renewable fuel sources such as solar, wind, tidal, etc. our dependence on energy storage devices will continue to grow. Li- air offers much higher energy density than all other batteries based on electrochemical storage. However, these batteries currently suffer from a number of issues such as a low cyclability and a reduced practical energy density compared to the theoretical energy density. The deposition of lithium peroxide on the surface of the cathode is one of the main causes for the low practical specific capacity of lithium-air batteries with organic electrolyte. Electrochemical impedance spectroscopy (EIS) has been used in the past to extract physical parameters such as chemical diffusion coefficient, effective diffusion coefficient, Faradaic reaction rate, degradation and stability of an electrochemical device. In this dissertation, a physics based analytical model is developed to study the EIS of Li-air batteries, in which the mass transport inside the cathode is limited by oxygen diffusion, during charge and discharge. The model takes into consideration the effects of double layer, Faradaic processes, and oxygen diffusion in the cathode, but neglects the effects of anode, separator, conductivity of the deposit layer, and Li-ion transport. The analytical model predicts that the effects of Faradaic impedance can be hidden by the double layer capacitance. Therefore, the dissertation focuses separately on two cases: 1) then case when the Faradaic process and the double layer capacitance are separate and can be observed as two different semicircles on the Nyquist plot and 2) the case when the Faradaic process is shadowed by the double layer capacitance and shows up as only one large semicircle on the Nyquist plot. A simple expression is developed to extract physical parameters such as the values of the diffusion coefficient of oxygen and Faradaic reaction rate from experimental impedance spectrum for each of the two cases. The diffusion coefficient can be determined by using the resistances (real impedance intercept on the Nyquist plot) of both the semicircles for the first case and by using the combined resistance for the second case. Once, the effective oxygen diffusion coefficient is estimated, it can be used to estimate the value of the reaction constant. This method of extracting the values of the diffusion coefficient and reaction constant can serve as a tool in identifying an effective electrolyte or cathode material. It can also serve as a noninvasive technique to identify and also quantify the use of the catalyst to improve the reaction kinetics in an electrochemical system. Finally, finite element simulations are used to validate the analytical models and to study the effects of discharge products on the impedance spectra of Li-air batteries with organic electrolyte. The finite element simulations are based on the theory of concentrated solutions and the complex impedance spectra are computed by linearizing the partial differential equations that describe the mass and charge transport in Li-air batteries. These equations include the oxygen diffusion equation, the Li drift-diffusion equation, and the electron conduction equation. The reaction at the anode and cathode are described by Butler-Volmer kinetics. The total impedance of a Li-air battery increases by more than 200% when the response is measured near the end of the discharge cycle as compared to on a fresh battery. The resistivity of the deposition layer significantly affects the deposition profile and the total impedance. Using electrolytes with high oxygen solubility and concentrated O2 gas at high pressures will reduce the total impedance of Li-air batteries.