Surface Film Characterization of a Li-Air Cell Using Electrochemical Impedance Spectroscopy

The development of Li-air batteries has presented many problematic shortcomings that effect the cyclability and efficiency of the cell. More specifically, it is known that as the cell cycles the discharge capacity of Li-air gets much smaller than its theoretical capacity. This can come from both the irreversible and reversible products that build up on the cathode surface, inhibiting the flow of reactants to the active surface. [1] This paper provides preliminary results on a technique used to characterize the surface film build-up on the cathode of a Li-air cell.  The surface film capacitance and resistance are extracted from high frequency impedance measurements of the cell. 

Experimental Method

The Gamry Reference 3000 was used to perform electrochemical impedance spectroscopy (EIS) measurements on a Li-air cell employing a porous carbon cathode with a electroless deposited  nickel and molybdenum (NiMo) composite that was also electrolytically oxidized.  The Nyquist plot of the pre-discharge data is presented in Figure 1.  The inset in Figure 1 shows a small semi-circle in the high frequency region which is indicative of a film on the surface of the cathode. In this region an associated interfacial capacitance (C_int) and interfacial resistance (R_int) was investigated.

Results

The effective capacitance (C_eff) of the film on the cathode was calculated from the high frequency reactance data in Figure 1 based upon equation (1) and is plotted in Figure 2 and 3  [2]. Z_j is the imaginary impedance, and ω is the angular frequency.  C_int can be approximated in Figure 2 where C_eff is seen to be frequency independent. Equation (2) was then used to determine R_int with respect to C_int and C_eff. From the calculated C_int and R_int values, the characteristic frequency (f_RC) and relaxation characteristic frequency (f_C ) were determined using equations (3) and (4) where R_e is the solution resistance. These two frequencies are identified in Figure 3 versus C_eff and can be used to help verify the correctness of C_int and R_int.  For future work, this technique will be applied to EIS data obtained both after discharge and charge to extract the full set of cathode interfacial parameters versus cycling.

Acknowledgement

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

References

1. Girishkumar, G, B. McCloskey, A. C. Luntz, S. Swanson, and W. Wilcke. "Lithium-Air Battery: Promise and Challenges." Physical Chemistry (2010): 2193-2203. Web.
2. Orazem, Mark E., and Bernard Tribollet. "Methods for representing Impedance."Electrochemical Impedance Spectroscopy (2008)