Abstract
Electrochemical impedance spectroscopy (EIS) is widely used to obtain the impedance parameters inside Lithium-ion batteries (LIBs) without its destruction. The evaluation of the impedance of the LIBs during charge and discharge is useful to estimate the degradation state of the LIBs. Wavelet Transformation (WT) is one of the frequency analysis methods. Our group1-3) applied to degradation monitoring of LIBs. The WT can determine the impedance from time domain data. The wavelet coefficient X(a,b) of a signal x(t) is calculated with the convolution integral of x(t) and the function Ψ(t) called “mother wavelet”. The a is the scale parameter and the b [s] is the time parameter. In this calculation, the impedance of the LIBs during the voltage relaxation process was determined by the WT without using an external input signal. In addition, this impedance determination method was applied to the diagnostics technique of LIBs degradation.The three-electrode cell was used for the measurement. A positive electrode was LiCoO2 and a negative electrode was spherical graphite. The electrolyte solution was a mixture of ethylene carbonate (EC) and ethylmethylcarbonate (EMC) (3:7 by volume) containing 1 M LiPF6. A reference electrode was a lithium ring. The charge and discharge were performed under a constant current-constant voltage (CC-CV) charge and constant current (CC) discharge. We measured 0.5 C-rate charge-discharge test at 1st cycle and 1 C-rate charge-discharge tests from 2nd to 6th cycles. We determined the impedance spectra of the cell during the voltage relaxation process after the charging at 3rd, 4th, 5th and 6th cycles by the WT. The frequency range for the determination of electrochemical impedance by the WT was from 0.0631 to 19.9 Hz.The impedance spectrum determined by the WT had a capacitive semicircle in the high frequency range and a straight line in the low frequency range. The capacitive semicircle appearing in the high frequency range is considered to be related to the time constant of charge transfer resistance and electric double-layer capacitance, while the straight line is attributable to the diffusion of Li+2). In addition, the charge transfer resistance of the LIBs increased with degradation.The result indicates that the impedance of the LIBs during the voltage relaxation process of the charging curve can be determined by the WT without using an external input signal. Furthermore, it is possible to detect degraded LIBs with the convergence value of the capacitive semicircle at the low frequency side of the determined impedance spectrum by the WT.
Published Version
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