Interfacial Impedance Characterization and Equivalent Circuit Modeling Analysis of a Li-O2 Battery
Authors: Nelson Ruben , Weatherspoon Mark H 
Published: February 2020

Lithium-oxygen (Li-O2) battery electrodes are known to form a film of alkyl carbonates and lithium products over cycling between the electrode/electrolyte interface, known as the solid electrolyte interface (SEI). That dielectric mostly comprise of products such as HCO2Li, CH3CO2Li, or Li2CO3 [1-3] if there are carbonate solvents, LiO2 from O2
- superoxide formation [2], stable Li2O, or unstable Li2O2 [1,2,4]. SEI film is a significant influence to cell performance, cycle life, self-discharge, safety, faradaic efficiency and physical electrode quality [5,6]. That build-up of lithium product on the cathode attributes to impedance growth with both reversible and irreversible product formation on the electrode surface. Furthermore, charge migration and dipole relaxation processes that dominate at high frequencies are greatly influenced by the type and severity of product deposition [7]. Past studies involving EIS measurements of Li-O2 batteries are accompanied by an equivalent circuit model (ECM), similar to the simplified model in figure (a). Furthermore, ECMs of surface films may utilize constant phase elements (CPE) when impedance measurements exhibit non-deal, CPE-like behavior. This may be physically due to surface heterogeneities, porosity, non-uniform coating, or slow adsorption. In any or all cases, this CPE behavior reflects a resistive and capacitive dispersion that results in a distribution of time constants, τ. Hence, the ECM that is proposed to describe the electrolyte-electrode surface film interface comprise of a solution resistance, Rs
, in series with serial Voigt type parallel RC elements, as shown in figure (b), distributed normal to the bare electrode surface. Each Voigt element is a portion of the layer, each with its resistance, capacitance, and a corresponding τ for the process that produces that portion of the microfilm, such as from oxygen reduction, aging, or other side reactions from electrolyte, adding additional product. However, there may be some ambiguity since fitted models are determined by mathematical optimization and not electrochemical phenomena [8].
This work introduces a method for extracting resistive and capacitive values directly from high frequency EIS measurements after both charge and discharge. The results are compared to the parameters of an ECM interpretation of the same EIS data. The objective of this work is to take a non-ECM approach to high frequency impedance characterization of a binder-free Li-O2 battery, which would then be compared to the more common ECM approach to modeling interfacial impedance. Furthermore, a statistical analysis of the impedance distribution throughout the

RC network will be discussed, where impedance from dispersive soruces may indicate distributed impedance like that of a normal or chi-squared distribution, rather than uniform.

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Acknowledgement:

This work was supported by the ERC program of the National Science Foundation under award number EEC-08212121.

Figure 1