恒相位元件特征分析与应用

Analysis of the impedance at Li/separator/Li symmetric cell using constantphase elements:Multi-compressed semicircles

恒相位元件（CPE）是处理电化学阻抗数据时经常使用的电化学元器件，用于处理和分析Nyquist图上出现的不规则半圆型阻抗。很少有文献详细报道过恒相位元件的特点，关于阻抗图谱上存在多个不规则半圆时恒相位元件的应用规则则研究的更少，此时如何判定拟合的结果与图谱之间的一一对应关系对于正确分析阻抗图谱十分重要。本文从恒相位元件的基本公式出发，利用软件编程分析恒相位元件的基本特征，得到复平面上不规则半圆顶点所对应的特征频率与电荷转移电阻、以及拟合参数之间的定量关系。如果测量的阻抗图谱在复平面上包含多个不规则压扁的半圆，那么这种定量关系可以用于判定拟合参数、不规则半圆和电化学过程之间的一一对应关系。以Li／隔膜／Li对称电池作为例子，将上述分析应用于实际电池的阻抗图谱分析中，分析了每一个压扁半圆所代表的电化学过程，表明金属锂／隔膜界面的阻抗响应是由两个不规则半圆组成，分别代表固体电解质界面中锂离子的传输和界面上的电荷转移过程。隔膜改性后与电荷传递过程对应的界面电阻从4．32×10^5降到2．11×10^5欧姆，相应的界面电容没有明显变化：与锂离子在固体电解质界面中电迁移过程对应的界面电阻从2．24×10^4降到1．19×10^4欧姆。

The constant-phase element （CPE） is extensively used to analyze the impedance data, when the compressed semicircles are obtained in the Nyquist plots. The analysis of the impedance data will become more complex and difficult if there exist two or more oblate semicircles in the Nyquist plots. Until now there are few of literatures reporting its characteristics. The characteristics of constant-phase elements have been analyzed in details using computer software in this article. The quantitative relationship between the character frequency of the vertex point of the compressed semicircle in complex-plane plot, the parallel resistance and the simulated parameters has been obtained. This relationship can be used to determine the one-to-one correspondence between the simulated parameters, the measured chords and the electrochemical reaction steps when there exist muiti-compressed semicircles. In addition, the constant-phase elements are used to analyze the impedance spectroscopy that is obtained at the symmetric Li/separator/Li cells. There exist two oblate semicircles in the high-medium frequency and one horizontal straight line in the low-frequency. The electrochemical process that is represented by every oblate semicircle has been proposed： one semicircle corresponds to the migration process of lithium ion through the solid electrolyte interface and another represents the charge ~.nsfer process. For the modified PP separator, the interface resistance decreased from 4.32x 105 to 2.1 lx 10Sohms while the migration resistance of the lithium ion through the solid electrolyte interface decreased from 2.24x104 to 1.19x104 ohms. The interface capacitance that corresponds to the charge transfer process does not have a significant variation.