AC impedance is a commonly used lithium ion battery detection method. The basic principle is to use different impedance types inside the lithium ion battery to respond to different time. By applying different frequency sine wave signals, according to the feedback (current signal or Voltage signal) Differentiating between different impedance types, for example, the high-frequency phase is mainly the electronic contact impedance in the lithium-ion battery and the diffusion resistance of Li+ in the electrolyte. The intermediate-frequency phase reaction is mainly the electrode/electrolyte interface charge. Exchange impedance, the main reaction in the low frequency phase is the diffusion resistance of Li+ in the active material and SEI film.
Lithium-ion battery self-discharge screening is a very important task for lithium-ion batteries. It is directly related to the reliability of battery packs. In general, battery manufacturers will store lithium-ion batteries at room temperature or high temperature for 7-28 days. Screening of batteries with different self-discharge rates by detecting voltage and capacity decay has made self-discharge a bottleneck in the production of lithium-ion batteries. Pierrot S. Attidekou, University of Newcastle, UK (first author, newsletter) Authors) Through the application of AC impedance method, the self-discharge screening time of lithium-ion batteries has been shortened from several weeks to 10 minutes. By continuing optimization, it is expected to shorten the screening time to 1 min.
Pierrot S.Attidekou used two 40Ah cylindrical batteries from the well-known military and space lithium-ion battery manufacturer SAFT (the battery information is shown in the table below), one of which is a normal battery (self-discharge rate of 2.108) mV / day, battery 2), another self-discharge is large (self-discharge rate 3.940mV / day, battery 1), they were tested in 0% SoC state (C/10 discharge to 3.2V), different temperatures ( AC impedance spectrum at 15, 20, 25, 30 ° C).
The EIS spectrum of the two batteries at different temperatures is shown in the figure below. It can be seen from the figure that the EIS spectrum of the two batteries is mainly composed of two arcs, the first one is composed of a small arc of the middle frequency band, and the second one. For the large arc of the low frequency band, the radius of the arc decreases as the temperature increases, and the entire curve moves to the left (the impedance is smaller), indicating that not only the charge exchange inside the battery increases with the increase of temperature. The impedance is significantly reduced, and the diffusion resistance of Li+ in the electrolyte also shows a significant downward trend.
According to the characteristics of the above EIS map, PierrotS. Attidekou designed the following equivalent circuit, where L1 is the inductance, R1 ohmic impedance, and the latter two parallel resistances represent the two semicircles in the figure, where CPE is the constant phase angle element, mainly Some capacitance characteristics of the reaction electrode interface, Rp, a and Rp, c are the negative and positive charge exchange impedance, and Wa and Wc are the solid-phase diffusion impedance of Li+ at the negative electrode and the positive electrode.
The EIS spectra of the two batteries are fitted by the equivalent circuit shown in the above figure. The results are shown in the following table. All the fitting errors are between 0.6 and 2.4%, and the following table a is a self-discharged battery. Table b below shows the battery with slower self-discharge. In the table, R1 represents the ohmic impedance inside the lithium ion battery, such as the contact impedance between the electrolyte, the current collector, the diaphragm and the active material particles. The increase in temperature, R1, shows a downward trend, mainly because the diffusion resistance of Li+ in the electrolyte decreases as the temperature rises. The graph below shows the relationship between R1 and temperature T, as can be seen from the figure. A normal self-discharged battery has a linear relationship between log (1/R1) and 1000/T, while a self-discharged battery exhibits a non-linear characteristic, indicating that it is in a self-discharged battery. There are some defects.
The principle of EIS work is to use different impedances with different time constants (as shown in the following equation). The following figure shows the time constants of the positive (triangle) and negative (square) of two batteries as a function of temperature. Only the battery shows a tendency that the positive time constant is significantly larger than that of the negative electrode, but as the battery temperature increases, the time constant of the positive and negative electrodes decreases. For the self-discharged battery 1, when the battery temperature reaches 25 ° C In the future, the time constant of the positive electrode is smaller than that of the negative electrode. For the battery 2 with slow self-discharge, the time constant of the positive electrode is only smaller than the time constant of the negative electrode when the temperature reaches 30 ° C. From this point, self-discharge can also be seen. There are some problems with the faster battery 1.
The graph below shows the relationship between the logarithm of the charge exchange resistance Rp of the positive and negative electrodes of the two batteries and the battery temperature. Pierrot S. Attidekou believes that the impedance of the first semicircle in the EIS diagram is mainly due to the SEI membrane impedance of the negative electrode and the negative electrode. The charge exchange impedance is composed, and the second semicircle in the EIS diagram is mainly composed of the charge exchange impedance of the positive electrode. The author makes the logarithm of the charge exchange impedance and the temperature curve (as shown in the figure below). It can be seen that at lower temperatures, the impedance of the negative electrode is significantly higher than that of the positive electrode, but this phenomenon reverses with increasing temperature. For battery 1 with faster self-discharge, at 25 ° C The impedance of the rear negative electrode is lower than the positive electrode impedance, and the negative electrode impedance of the battery 2 with slow self-discharge is lower than the positive electrode impedance at 30 ° C, which can also be used as a basis for distinguishing the self-discharge of the lithium ion battery.
The following figure shows the data of Li+ diffusion coefficient obtained by Pierrot S. Attidekou based on AC impedance data. It can be seen that the Li+ diffusion coefficients of both batteries increase with increasing temperature, but it is still possible to see that two batteries are obvious. Gap, this can also be used as a basis for judging different self-discharge rate batteries.
AC impedance is a powerful tool for studying the internal reaction and chemical changes of lithium-ion batteries. The work of Pierrot S. Attidekou shows that the self-discharge of different lithium-ion batteries has obvious ohmic impedance, charge exchange impedance and interface capacitance with temperature. The difference can be used to screen lithium ion batteries with different self-discharge rates, thereby accelerating the self-discharge screening of lithium-ion batteries and improving production efficiency.
