The battery is the power source of electric vehicles and one of the core technologies of electric vehicles. Using modern testing methods to study the performance of lithium-ion batteries is an important way to reduce battery costs and increase cruising range.

　　The battery is the power source of electric vehicles and one of the core technologies of electric vehicles. Using modern testing methods to study the performance of lithium-ion batteries is an important way to reduce battery costs and increase cruising range.

　　Electrochemical impedance spectroscopy is widely used in the analysis of positive and negative electrode materials of lithium-ion batteries, research on lithium-ion deintercalation kinetic parameters, solid electrolytes, interface reactions, and SOC prediction. It is a powerful tool for analyzing the performance of lithium-ion batteries. This article synthesizes the results of electrochemical impedance spectroscopy in studying the performance of lithium-ion batteries, and looks forward to the application progress and development direction of electrochemical impedance spectroscopy.

　　1 Introduction to electrochemical impedance spectroscopy

　　Electrochemical impedance spectroscopy (EIS) is a non-destructive method for parameter determination and effective battery kinetic behavior determination. A sine wave voltage signal with a small amplitude of frequency w1 is applied to the battery system, and the system generates a sine wave current response with frequency w2. The change in the ratio of the excitation voltage and the response current is the impedance spectrum of the electrochemical system.

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　　EIS has high practicability. This testing method can scan from very low frequency (several μHz) to very high frequency (several MHz) to realize electrochemical interface reaction research in a wide frequency range. At present, most domestic research is still in the initial exploration stage, and most of them focus on EIS curve analysis and related electrochemical interpretation. Foreign research has made breakthroughs in the establishment of EIS mathematical models and practical applications of EIS (such as battery temperature prediction based on EIS).

　　The first part is the ultra-high frequency part, the part where the impedance curve intersects with the horizontal axis: ohmic impedance Rb;

　　The second part is the high-frequency part, semicircle: lithium ions pass through the solid electrolyte impedance Rsei;

　　The third part is the intermediate frequency part, semicircle: charge transfer impedance, also called electrode polarization impedance Rct;

　　The fourth part is the low-frequency part, 45° straight line: lithium ion diffusion impedance, also called concentration polarization impedance W.

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　　2 Introduction to Equivalent Circuit Model

　　A lithium-ion battery can be understood as a circuit system including resistance, inductance and capacitance. The establishment of an equivalent model is to simplify the battery into a circuit system to simulate the change process in the electrochemical system.

　　Corresponding to the impedance components of each frequency in the impedance spectrum, Rb represents the ohmic resistance; Rsei and Csei represent the resistance and capacitance of the SEI film, corresponding to the semicircle in the high-frequency part; Rct and Cdl represent the charge transfer resistance and electric double-layer capacitance respectively, and The intermediate frequency part corresponds to the semicircle; W is the Warburg impedance, that is, the diffusion impedance of lithium ions in the electrode material, which is represented by a straight line at 45° to the real axis on the complex plane.

　　3 Current research status at home and abroad

　　At present, research on electrochemical impedance spectroscopy mainly focuses on the prediction of SOC, analysis of electrode materials, lithium ion deintercalation process and research on solid electrolyte membranes. A large amount of research has been devoted to exploring the relationship between ohmic impedance, charge transfer impedance, diffusion impedance and SOC, SOH, temperature, charge and discharge rate, and providing relevant electrochemical explanations.

　　The establishment of the equivalent circuit model relies on the curve form of the electrochemical impedance spectrum. Some scholars have proposed pure mathematical models to replace the equivalent circuit model, providing multiple solutions for data fitting. Comprehensive related research shows that ohmic resistance is slightly affected by SOC, temperature, rate and other factors, while charge transfer resistance and lithium ion diffusion resistance are significantly affected by these factors.

　　4Research progress

　　4.1The impact of SOC

　　SOC is the battery state of charge and also reflects the battery power usage status. The impedance curve fitted by EIS can be used to determine the changes in impedance inside the battery. At the same time, EIS can also provide a basis for selecting the optimal SOC range for the battery.

　　Xi Jingjing et al. studied the changes in impedance of lithium iron phosphate batteries with SOC, focusing on the mid-frequency impedance. She found that the ohmic impedance remained unchanged at different SOCs, and the charge transfer impedance and diffusion impedance were significantly affected by SOC. It also verified the feasibility of using series capacitance, electric double layer capacitance and charge transfer impedance to predict battery SOC.

　　Zhang Wenhua et al. used a C/LiFePO4 battery with a capacity of 60Ah as the research object, and conducted full charge and full discharge experiments on four groups of batteries with different cycle times at a charge and discharge rate of 1.0C. The research results were similar to Xi Jinping's research. They believe that the ohmic impedance is basically unchanged under different SOC states. The charge transfer resistance and diffusion resistance showed a trend of first decreasing, then stabilizing and then increasing. They were significantly larger in the SOC range of 0-25% and 75%-100%, and tended to be flat in the middle range. They believe that this is caused by the weak electrode response between low SOC and high SOC.

　　Jiang Jiuchun and others tested the impedance spectrum of lithium iron phosphate batteries under different SOC. Compared with the research by Zhang Wenhua et al., the impedance spectrum curve obtained by Jiang Jiuchun et al. can distinguish charge transfer impedance and diffusion impedance with high precision, which well confirms the electrode polarization and electrochemical characteristics caused by lithium ion concentration and electrochemical properties of electrode materials. Significant changes in concentration polarization. Analysis of charge transfer impedance characteristics at 10%, 50% and 90% SOC based on the Arrhenius equation provides a theoretical basis and estimation method for the selection of SOC usage intervals for battery energy management strategies.

　　Yuan Xiang et al. conducted experimental studies on the impedance characteristics of power lithium-ion batteries under charging and discharging conditions. Coinciding with the research of Zhang Wenhua et al., the ohmic impedance they measured almost did not change with SOC, but the charge transfer impedance was very different during charging and discharging. As the charging depth increases, the charge transfer impedance decreases, and the change is most obvious in the 0.1-0.2 SOC range, as shown in Figure 3. During discharge, the consumption of electrolyte active material causes the charge transfer resistance to increase, which rises sharply at low SOC. For the diffusion impedance, the change pattern of the charging process and the discharging process is that the values in the high SOC interval and the low SOC interval are small, and the value in the middle SOC interval is relatively large. However, when the discharge reaches below 10% SOC, the diffusion coefficient decreases rapidly, as shown in Figure 4. The diffusion resistance is inversely proportional to the diffusion coefficient, so the diffusion resistance increases significantly.

　　4.2 Effect of temperature

　　In lithium-ion batteries, almost all diffusion processes are affected by temperature. The self-heating during the battery charging and discharging process and the ambient temperature all affect the transfer of charge inside the battery and the deintercalation of lithium ions in the electrode active material.

　　Xie Yuanyuan et al. conducted an experimental study on the thermal impact of battery impedance spectrum and tested the battery impedance spectrum under different cycle times. In the first cycle, temperature has little impact on mid- and high-frequency impedance, but has a large impact on low-frequency impedance. Under high temperature conditions, the low-frequency impedance changes very little, and the mid-frequency impedance changes greatly. This is because the SEI is destroyed and reacts with the electrolyte to generate new SEI, causing oscillations in the impedance spectrum.

　　Jiang Jiuchun et al. studied the impedance spectrum under temperature conditions of 263~318K. Research shows that as the temperature increases, the electrochemical polarization resistance decreases, and the curve is almost oblique at 318K, making it difficult to distinguish the various impedance components, as shown in Figure 5.

　　Considering the effects of SOC and temperature comprehensively, it can be concluded that under low temperature conditions, the activity of the electrolyte inside the battery is low and the polarization is serious. At high temperatures, the high activity of the reactants reduces the interface resistance and charge transfer resistance, and is accompanied by battery side reactions—interface degradation. Their research can be used for battery management systems to select a reasonable temperature range (for example, 5 to 45°C). The impedance at other temperatures can be estimated based on impedance data at a certain temperature, and a reasonable temperature range control strategy can also be formed.

　　If the internal impedance of the battery is too large, it may cause abnormal temperature rise of the battery during high current discharge, causing the battery to become thermally out of control. In order to ensure the thermal safety of the battery, battery temperature prediction and estimation is particularly important. J.G. Zhu et al. used electrochemical impedance spectroscopy to predict battery internal temperature. Impedance spectroscopy is used to explore the excitation frequency range that can be used for battery internal temperature estimation. Since SOC is difficult to estimate, it was found that SOC characteristics such as low frequency and high frequency are not friendly. However, the impedance spectrum with only temperature changes allowed them to find the optimal excitation frequency range and establish a temperature prediction mathematical model related to the excitation frequency. They concluded that low frequency is better than high frequency and phase shift is better than impedance spectrum amplitude in temperature estimation.

　　H.P.G.J.Beelen et al. proposed a measurement system for estimating battery temperature based on the set temperature, using calculation formulas containing parameters such as excitation frequency f, impedance amplitude, etc. to estimate battery temperature. The temperature estimation method of impedance spectrum is divided into two steps. One is to determine the excitation frequency that acts on the experimental set value, and the other is to use the impedance amplitude to estimate the battery temperature. The combination of experimental design and parameter estimation allowed the researchers to obtain the most accurate temperature values.

　　Regarding the accuracy of temperature estimation, the researchers used the Monte-Carlo method to study the accuracy of battery temperature estimation and found that it had an absolute deviation of 0.4°C and a standard deviation of 0.7°C, so the accuracy was good. The researchers' research has a good reference value for temperature control of battery thermal management systems.

　　There are many factors that affect impedance. If multiple factors are considered comprehensively, the impact of different factors on impedance can be explored. Alexander Farmann et al. studied the kinetic parameters of new and old batteries at different SOC and different temperatures. They believe that the sensitivity of the electrolyte to temperature results in high impedance at low temperatures and low impedance at high temperatures. During the service life of the battery, the shape of the curves of ohmic impedance and charge transfer impedance as they change with SOC and temperature remains basically unchanged, and the effect of temperature on impedance is greater than the effect of SOC on impedance. They also fit the overall impedance versus SOC and temperature curves, which can be used for battery voltage prediction in electric vehicles.

　　Similarly, DAndre et al. used electrochemical impedance spectroscopy to explore the effects of temperature and SOC on the performance of high-power lithium-ion batteries, and considered the low-temperature starting conditions of electric vehicles, clarifying that the battery design needs to meet certain low-temperature conditions. Similar to Alexander Farmann's research, it was also concluded that the internal impedance of the battery is mainly affected by temperature and is less affected by SOC.

　　4.3 Influence of charge and discharge rate

　　Lithium-ion power batteries often encounter working conditions with different power requirements, and the required charge and discharge currents vary greatly, which also affects the charge transfer process and electrochemical reaction process inside the battery.

　　In order to explore the battery impedance under different charge and discharge rates, Xie Yuanyuan et al. used lithium-ion batteries as the research object and tested the impedance spectra under 0.1C, 0.2C and 0.5C charge and discharge rates. Researchers believe that when charging and discharging with small current, the battery impedance does not change much under a certain number of cycles, and the small current has the effect of reducing the low-frequency impedance of the battery. When charging and discharging with large current, the semicircle of the intermediate frequency part increases, and the charge transfer impedance increases. It was also found that although low charge and discharge rates can greatly reduce the impact of cycling on battery impedance in the mid-to-high frequency range, its impact on the low-frequency component of the impedance spectrum is still significant.

　　Electrochemical impedance spectroscopy is one of the powerful tools for studying electrochemical reactions at the electrode/electrolyte interface. It is widely used in research on the impedance of positive and negative electrode materials and the insertion and extraction of lithium ions in positive and negative electrode materials. Masayuki Itagaki et al. focused on the charge transfer resistance and ohmic impedance of battery positive and negative electrode materials at 0.5C, 1.0C and 1.5C charge and discharge rates. Research shows that at 1.5C rate, the change in charge transfer impedance of the positive and negative electrodes shows a certain hysteresis phenomenon, and the influencing factor is the direction of the current. Regarding ohmic impedance, whether it is a positive electrode material or a negative electrode material, the effect of magnification rate on its size and change trend is not obvious. It can be considered that in the electrodes of lithium-ion batteries, the charge transfer impedance during the delithiation process is greater than the charge transfer impedance during the lithium insertion process.

　　4.4Influence of SOH

　　SOH is a reflection of the health status of the battery and an indicator of the aging status of the battery. After a battery has undergone a certain number of charge and discharge cycles, the decline of the battery will obviously intensify, mainly manifested in the reduction of discharge voltage and discharge capacity, which will challenge the performance of the battery.

　　Zhang Wenhua et al. explored the relationship between the aging state of lithium iron phosphate batteries and battery impedance, and analyzed in detail the changes in each impedance component with the number of cycles. It was found that more than 800 cycles have a great impact on the charge transfer impedance and have a minimal impact on the ohmic impedance and diffusion impedance. They believe that SOH is between 95% and 100%, ohmic impedance, charge transfer impedance and diffusion impedance remain basically stable, and the battery is in a stable state of charge and discharge. When SOH is reduced below 90%, the charge transfer resistance and diffusion resistance increase significantly, the interface structure between the electrolyte and the electrode is gradually destroyed, and a new arc appears in the low-frequency region of the impedance spectrum. The reason may be that the battery negative electrode material is damaged Destroyed, the lithium insertion reaction slows down. Their research shows the correlation between AC impedance and battery degradation, which can be used to screen out aging batteries, which is beneficial to the sequential utilization of lithium-ion batteries.

　　Based on electrochemical impedance spectroscopy, Zhang Caiping and others analyzed the aging characteristics of the battery and proposed a way to use lithium-ion batteries in stages to extend their life. Comparing the impedance spectrum curves of new and old batteries, it was found that the degradation of battery performance after use was mainly caused by the increase in electrochemical polarization resistance and concentration polarization resistance. A method was proposed to control the degree of polarization by controlling the charge and discharge rate. The research by Zhang Caiping and others considers the recycling issue of lithium-ion batteries, which is of great significance to reducing the full life cycle cost of batteries and promoting the healthy and green development of the battery industry.

　　In terms of battery aging life research, Xu Xinmin and others conducted aging experiments and electrochemical impedance spectroscopy tests on lithium iron phosphate battery samples using cyclic charge and discharge methods. They proposed a SOH calculation formula based on AC impedance and verified the feasibility of testing battery AC impedance with current perturbation excitation. Based on the obtained impedance data, it was found that the low-frequency impedance and SOH showed a monotonically increasing pattern. Finally, a linear fitting method was used to obtain the battery aging curve, which provided algorithm support and theoretical basis for using impedance data to calculate SOH and predict battery service life.

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