Analysis of Impedance Spectrum by Electrochemical Approach

Electrochemical impedance spectroscopy (EIS) and electrochemical approach are two major tools in analyzing the performance of a battery, where the former one is commonly used to measure frequency response of a battery. However, the impedance spectrum does not clearly reveal the information about the change in material parameters of electrochemical models like electrolyte conductivity, solid-phase diffusivity, and double-layer capacity. To gain more information about impedance spectrum, the link between the two methods should be established.

By constructing an isothermal one-dimensional electrochemical model of lithium iron phosphate (LFP) cell, we have simulated the discharge curve of a single 26650 LFP cell, which agrees well with the experimental data [1], as shown in the inset of Fig. 1. Figure 1 also shows that the simulation physical model can be further used to calculate the impedance spectrum. Although there exists slight difference between the experiment and the simulation, which could be caused by the use of distinct electrode materials, using the physical model to explore battery impedance still provides a new way to have a deeper understanding of discharge behavior.

It is known that double layer on the surface of solid electrodes significantly contributes to impedance spectrum, whereas it exerts little influence on the discharge curve whether it exists or not. Base on the electrochemical model, we find in Fig. 2 that there are few variations between the case which has double layer in cathode only and the other case which has double layer in both electrodes. This result shows an agreement with an experiment, which indicates that cathode contributes to impedance spectrum magnitude [2].

In spectrum analysis the irreversible resistance, coming from electrolyte conductivity and resistance between the porous electrode surface and the current collector [3], is frequently treated as a constant value. Figure 3 shows that decreasing electrolyte conductivity in the electrochemical model drives the impedance curve to the right. This proves that the difference in electrolyte conductivity will lead to a change in the real-part resistance. More detailed discussion on the electrochemical parameters in the impedance spectrum will be presented in this work. 

REFERENCES

[1] M. Safari and C. Delacourt, J. Electrochem. Soc.,158, A562 (2011).

[2] G. Nagasubramanian, J. Power Sources, 87, 226 (2000)

[3] D. D. Macdonald, Electrochemica Acta, 51, 1376, (2006)