Investigation of Electrode Contributions to the Impedance of a Li-Air Cell

Electrochemical impedance spectroscopy (EIS) has the great advantage of non-invasive measurement of any electrochemical system’s impedance.  The source of impedance behavior over the course of one or many discharges may give insight to changes in trends between one spectrum and others.  Beyond that, given the knowledge of the electrochemical system, spectra analysis and/or circuit models may give some detail as to the inner workings of the system.

This study aims to present a unique study between a two and three electrode impedance measurement of a lithium-air (Li-O₂) cell with a lithium metal anode and a porous, binderless air cathode.  Most EIS studies of a Li-O₂ cell are a two electrode measurement, which provides impedance of the full cell.  Although the impedance contribution in a cell is primarily from the air cathode, there are some contributions from the lithium anode that change over the course of single and multiple cycles. A third electrode, a lithium metal reference, helps to isolate the activity of one electrode, the air cathode specifically, for in situ impedance measurements. Indirectly, this provides insight into the activity of the anode’s contribution to full cell impedance by comparing the two and three electrode Nyquist and Bode plots. Analytically, the measured impedance can then be assessed via circuit models, graphical methods, or mathematically for understanding of dispersive capacitive, charge transfer, interfacial, and ohmic characteristics.

Cells with mirrored, identical cathodes and anodes, respectively, in addition to a full Li-O₂ cell were constructed in an ECC-Air electrochemical test structure by EL-CELL.  Preliminary EIS measurements were performed using the Gamry Reference 3000 between a frequency range of 1 MHz to 0.1 Hz at a 10 mV amplitude perturbation signal.  The initial Nyquist plot of the full Li-O₂ cell in Figure 1 is typical of a binder-free Li-O₂ cell [1]. The initial Nyquist plot of an identical Li/Li cell presented in Figure 2 shows the interfacial impedance contribution associated with the lithium-electrolyte interface [2] [3].  The initial Nyquist plot of an identical cathode/cathode cell presented in Figure 3 is indicative of the capacitive impedance that is dominant throughout most of the frequency sweep of a two electrode impedance measurement [1].  Future impedance measurements with a three electrode configuration of the Li-O₂ cell should demonstrate the observed two electrode impedance behavior in some capacity.

 Acknowledgement

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

References

[1] R. Nelson, M. H. Weatherspoon, J. Gomez, E. E. Kalu, J. P. Zheng, Electrochem. Comm., 34 (2013) 77.

[2] M. Mirzaeian, P. J. Hall, J. Power Sources, 195 (2010) 6817.

[3] J.Y. Song, H.H Lee, Y.Y Wang, C.C Wan, J. Power Sources, 111 (2002) 255.

Figure1: Impedance of full Li-O₂ cell with lithium anode and cathode with magnified inset of high to mid-frequency

Figure 2: Impedance of Li/Li cell with magnified inset

Figure 3: Impedance of cathode/cathode cell with magnified inset