(Invited) Analysis of Li-Ion Battery By EIS Response By Equivalent Circuit and Verification of Estimated Parameters

Abstract

Analyzing technique of Li-ion batteries (LIB) is strongly demanded both in the field of developments of LIB with improved properties such as safety, long life, and high capacity with high power, and in the field of checking the condition of the installed LIB in the power unit. Electrochemical impedance spectroscopy (EIS) is one of the most effective methods of investigation of LIB without disassembling of the cell. We have proposed an equivalent circuit for analysis of electrochemical reactions, diffusion parameters, and resistivity of solid electrolyte interphase (SEI), and demonstrated analysis of impedance spectra with using the proposed circuit.[1-3] Since the impedance responses of the LIB are the results of impedance of each anode and cathode connected in series, it is hard to distinguish them into elemental steps for each electrode. In this paper, our strategy for the analysis of LIB impedance with resolving the response classified by time constant into elemental steps of each electrode is demonstrated. The impedance of whole LIB cell was analyzed based on the equivalent circuit with the values of time constant of impedance response for elemental steps of each electrode. The impedance response of elemental step for each electrode was confirmed with two symmetric cells which have a set of two LIB anodes or two LIB cathodes. In order to confirm the correspondence between the each elemental step and the estimated parameters, impedance analyses of symmetric cells to obtain each value of time constant for response of each elemental step. Furthermore, parameter analysis with shifting profile of time constant of impedance responses by sequence change in temperature of the LIB also helps to verify the reliability of the estimated values. Figure 1 shows the estimated values of each resistance in the equivalent circuit with the analysis of ac response of LIB under various temperatures. The values are aligned straightly against the LIB temperature, while the values obtained at 20 degree C locates out of the lines. These values at 20 degree C could be considered to be difficult to estimate adequately since the time constant of the response were over-lapping. At the presentation, detailed explanation of these analyses will be introduced and discussed their potential for reliable analysis of impedance spectra. This work was partly supported by “Research & Development Initiative for Scientific Innovation of New Generation Batteries” from NEDO, Japan. [1] T. Osaka, et al., J. Power Sources, 119 (2003) 929-933. [2] T. Osaka, et al., J. Power Sources, 205 (2012) 483-486. [3] D. Mukoyama, et al., Chem. Lett., 41 (2012) 444-446. [4] T. Momma, et al., J. Power Sources, 216 (2012) 304-307. Figure 1