Electrochemical Analysis and Novel Characterization of Porous Electrodes By Using Transmission Line Model for Lithium-Ion Batteries

 

The excellent performance battery with high power and high energy density is required for automobile application. To realize such batteries, it is necessary to comprehend the internal resistance of battery. In this study the novel electrochemical analysis method are applied to porous electrode of Lithium-ion battery by using transmission line model.To study on the internal resistances of porous electrodes of Lithium-ion batteries in more detail, we examined how to separate constituent of resistance in porous electrode, such as electric resistance (R_e), ionic resistance inside pore (R_ion), and charge-transfer resistance (R_ct). The real electrode of Lithium-ion batteries is composed of insertion material (improved NCA based cathode material), conductive carbon, binder and pore filled with the electrolyte^{1, 2}. We investigated electrochemical impedance spectroscopy using symmetric cell and treated the electrolyte/porous electrode interface by using transmission line model with or without charge transfer resistance³. From transmission line model⁴, the overall impedance (Z) is derived as Eq.1 without charge transfer reaction.                  

(L: pore radius, X: pore length, C_dL:double-layer capacitance)

The overall impedance is derived as Eq.2 with charge transfer reaction.

Fig.1 shows the experimental impedance spectra and the spectra calculated by inputting each parameter into Eq.1 and Eq.2. The experimental results of the impedance spectra agree well with that of the theoretical calculations. This result shows that the resistance constituents inside electrode are separated well by this method.

To investigate the factors that affect R_ion and R_ct inside electrode, the analysis methods as mentioned above were applied to various kinds of electrode of Lithium-ion battery. Fig.2 shows the relation between the resistance constitution (R_ion and R_ct) and thickness of electrode with different composition ratio of the insertion material, conductive carbon and binder.  (The area of the electrodes in symmetric cell is constant.) The value of R_ion increases with increasing in thickness of electrode, and the R_ion is directly proportional to thickness. The value of R_ct decreases with the increase in thickness, and the value of R_ct are inversely proportional to thickness. Similar results are derived from Eq.1 and Eq.2. These results mean that the rate of R_ion becomes larger among total resistance in the thick electrodes. Correlation between the R_ion and the thickness vary depending on composition of the electrodes, whereas the R_ct does not vary so much. The results show that the ionic resistance was dependents on the structure of electrode, and the charge transfer resistance dependents on state of the insertion material. We conclude that these results are very important. These results indicate that the decrease of the R_ion is key point in the high energy density battery with thick electrode for excellent performance battery with high power and high energy density. The R_ion represents the ease of movement of the lithium-ion inside pore filled with the electrolyte, and the pore size and distribution inside electrode are closely related.

In the following, the values of each resistance were measured in a wide temperature range and the values of activation energies were calculated. Especially the value of R_ct increased as the low temperature and the activation energy of R_ct is greater than that of R_ion. The results show that the decrease of R_ct was important under low temperature. In summary, the R_ion is more important on thick electrode in normal temperature. It is necessary to improve electrode structure for excellent performance battery in this case. The R_ct is more important on thin electrode under low temperature. It is necessary to improve insertion material for excellent performance battery in that case.

These analytical tools as mentioned above can provide very useful information in electrode design for excellent performance battery. More quantitative analyses and applications will be discussed in the presentation.

Reference

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