CN105576318A - Multi-parameter comprehensive determination method for determining consistency of electric automobile retired lithium batteries - Google Patents

Abstract

The invention relates to a multi-parameter comprehensive judgment method for determining the consistency of the decommissioned lithium battery of an electric vehicle. Taking the decommissioned lithium battery of an electric vehicle as the research object, the performance of the decommissioned lithium battery is tested through appearance inspection, capacity measurement, pulse characteristic curve and electrochemical impedance spectrum test. Indicators are used to characterize, and the decommissioned power batteries are gradually screened and graded to evaluate the consistency of the performance of each decommissioned battery, laying the foundation for the sequential utilization of more and more decommissioned power batteries.

Description

A multi-parameter comprehensive judgment method for determining the consistency of retired lithium batteries for electric vehicles

technical field

The invention relates to a battery detection method, in particular to a multi-parameter comprehensive judgment method for determining the consistency of a decommissioned lithium battery of an electric vehicle.

Background technique

With the gradual industrialization of electric vehicles, the output of electric vehicles in my country has grown rapidly, and the stock of electric vehicle power batteries will also increase sharply. In 2015, domestic sales of new energy vehicles reached more than 330,000, a four-fold increase from the previous year. Normally, in order to ensure the safety and performance of the vehicle, electric vehicle manufacturers require that the power battery be replaced when the capacity of the power battery has decayed to 70-80%. However, decommissioned power batteries still have a certain remaining capacity and service life, and can still be further used in other fields to tap their residual value, such as power supplies for electric bicycles, tour buses, etc., general life lighting power supplies, or power storage Energy, including renewable energy output power smoothing, distributed power supply in remote areas, charging and swapping station energy storage, power quality regulation and other fields. Due to the complex environment in which the power battery is in service, the performance degradation of the lithium battery is different, which increases the inconsistency between the batteries. Therefore, if decommissioned power batteries are to be reused, it is necessary to conduct research on the performance of decommissioned power batteries, evaluate the consistency between batteries, and perform screening and grouping in order to maximize the use of decommissioned batteries under the premise of safety. The remaining capacity.

At present, the research of many scholars at home and abroad on decommissioned power batteries mainly focuses on the analysis of battery attenuation mechanism, battery performance testing, and battery cascade utilization technology and economic model. Taking decommissioned power batteries and scrapped power batteries as research objects, Wang Chaofeng and Tan Li found that the performance of power lithium-ion batteries was significantly reduced due to the dissolution and shedding of electrode active materials and conductive agents, the change of electrode material grains, and the repeated regeneration of the SEI film on the surface of the negative electrode. attenuation. MatthieuDubarry et al. used non-destructive analysis methods such as dynamic response test, quantified capacity decay and capacity decay under peak power to conclude that the main reason for the capacity decay of decommissioned power lithium batteries is the loss of lithium in the electrodes. Xu Jing et al. compared the advantages, disadvantages and correlations of the hybrid pulse power characteristic (HPPC) test method, electrochemical impedance test method and current conversion method to measure the internal resistance characteristics of decommissioned power batteries. Jeremy Neubauer and others analyzed the necessity of secondary utilization of power batteries from the high initial cost of power batteries, and believed that this would prolong the life cycle of power batteries, reduce battery costs, and benefit the market promotion of environmentally friendly electric vehicles.

The operating conditions of electric vehicle power batteries are quite different, and it is difficult to correctly judge the internal consistency of batteries from the evaluation of internal resistance and electrical performance alone. How to gradually screen and grade retired power batteries and evaluate the consistency of performance of each retired battery There is no good solution, but it is the key to reuse the remaining energy of decommissioned power batteries.

Contents of the invention

The present invention aims at the problem of reuse of decommissioned lithium batteries of electric vehicles, and proposes a multi-parameter comprehensive judgment method for determining the consistency of decommissioned lithium batteries of electric vehicles. The pulse characteristic curve and electrochemical impedance spectroscopy test and other performance indicators are used to characterize, and the decommissioned power batteries are gradually screened and graded to evaluate the consistency of the performance of each decommissioned battery.

The technical solution of the present invention is: a multi-parameter comprehensive judgment method for determining the consistency of the retired lithium battery of an electric vehicle. The consistency of the battery is determined as follows:

1) Appearance inspection: batteries with deformation or swelling, batteries with damage or leakage, batteries with serious corrosion or pole damage are rejected and cannot be reused;

2) Capacity test: The power battery selected by visual inspection is tested for capacity. The capacity test method is to first charge with 1xI ₃ constant current to 3.65V and then perform constant voltage charging. When the current decreases to 0.1xI ₃ , the battery stops charging , then stand still for 1 hour, and finally discharge with 1xI ₃ until the end-of-discharge voltage reaches 2.70V, calculate the battery capacity according to the current value of 1xI ₃ and the discharge time data, the capacity is in Ah, where I ₃ is the 1/3C rate current ;

Calibration of I ₃ : According to three calibration conditions assuming that the battery capacity is not attenuated, attenuated to 80%, and attenuated to 67%, use the above capacity test method to test the capacity of the battery, and substitute the tested capacity into the following formula to calculate the average relative error of each battery capacity rate δ,

$\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&Delta;</span>}}{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; x</span>}}}<span\; class="notranslate">\; \&times;</span>\mathrm{<span\; class="notranslate">\; 100</span>}\mathrm{<span\; class="notranslate">\; \%</span>}$

is the average value of the capacity under the three calibration conditions, Δ is the absolute value of the difference between the average value and each group value, and the average relative error rate δ of each battery capacity is obtained, and the calibration condition with the smallest average relative error rate range of the measured capacity is taken The battery capacity below is the calibrated capacity, and the I ₃ current value is calibrated with this capacity;

3) Pulse characteristic curve analysis: the battery is charged and discharged according to different rate pulses, and the charge and discharge voltage curves of each battery are compared. The charge and discharge process is: the lithium battery is charged to 3.65V at a constant current of 1/3C, and then charged at a constant voltage to 0.1xI ₃ , stand still for 2 hours; 1C discharge for 10s, stand still for 40s; 3C discharge for 10s, stand for 40s; 5C discharge for 10s, stand for 40s; Afterwards, carry out constant voltage charging to 0.1xI ₃ , and the pulse charge and discharge ends; obtain the pulse charge and discharge voltage curve, and when the discharge rate is 5C, the voltage is greater than 2.7V, between 2.7-2.5V, and less than 2.5V to judge the consistency of the battery. Batteries with a voltage greater than 2.7V are grouped into one group, batteries with a voltage between 2.7-2.5V are classified into another group, and batteries with a voltage lower than 2.5V are excluded. 2.7V is the end-of-discharge voltage of the battery test, and 2.5V is set by the battery manufacturer. The lowest safe discharge voltage;

4) Electrochemical impedance spectroscopy: Use the Swiss AutolabPGSTAT302 electrochemical workstation to perform electrochemical impedance tests on decommissioned batteries. The electrochemical impedance test frequency is between 0.01Hz and 100kHz. Use ZSimpWin software to perform equivalent circuits on the electrochemical impedance test data. Fitting, analyze the internal electrochemical impedance characteristics of the battery by fitting parameters, use the ohmic internal resistance R _s , charge transfer resistance R _ct and lithium ion diffusion coefficient D _Li+ to group the batteries, where the lithium ion diffusion coefficient D _Li+ reflects the concentration difference The smaller the D _Li+ value is, the larger the concentration polarization will be. The formula for calculating the lithium ion diffusion coefficient D _Li+ is:

${\mathrm{<span\; class="notranslate">\; D.</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; +</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; A</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}^{\mathrm{<span\; class="notranslate">\; 4</span>}}{\mathrm{<span\; class="notranslate">\; f</span>}}^{\mathrm{<span\; class="notranslate">\; 4</span>}}{\mathrm{<span\; class="notranslate">\; C</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; ,</span>$

Among them, the ideal gas constant R=8.314J/(mol K), the absolute temperature T=298.15K, A is the cross-sectional area of the electrode, n is the electron transfer number, F is the Faraday constant F=96487C/mol, C is the electrode The concentration of lithium ions, σ is the Warburg Weber factor, and the relationship between σ and the real part Z _re of the impedance spectrum is as follows:

Z _re =R _s +R _ct +σω ^−1/2 , ω is the angular frequency during the electrochemical impedance test, ω=2πf, f is the frequency during the EIS test.

The beneficial effect of the present invention is that: the present invention determines the multi-parameter comprehensive judgment method of the consistency of the decommissioned lithium battery of electric vehicles, provides a suitable screening method for the reuse of decommissioned power batteries, and provides more and more decommissioned power batteries for the post-sequence Lay the foundation for step-by-step utilization.

Description of drawings

Fig. 1 is the actual capacity diagram of 20 decommissioned batteries with qualified appearance of the present invention;

Fig. 2 is the average actual capacity distribution figure of the decommissioned lithium batteries of 20 qualified outward appearances of the present invention;

Fig. 3 is the pulse charging and discharging voltage curve diagram of part of the decommissioned battery of the present invention;

Fig. 4 is the electrochemical impedance spectrogram of the representative decommissioned lithium battery of the present invention;

Fig. 5 is the electrochemical impedance spectroscopy equivalent circuit model diagram of decommissioned lithium battery of the present invention;

Fig. 6 is a distribution diagram of the decommissioned battery capacity, ohmic internal resistance R _s and charge transfer resistance R _ct of the present invention;

Fig. 7 is a graph showing the relationship between the diffusion coefficient D _Li+ value and the capacity of the present invention.

detailed description

Take the 60 lithium iron phosphate power batteries eliminated from an electric vehicle as an example, and their nominal capacity is 15Ah.

The multi-parameter comprehensive judgment method for determining the consistency of retired lithium batteries for electric vehicles includes four parts: appearance inspection, capacity test, pulse characteristic curve analysis, and electrochemical impedance spectroscopy test. The details are as follows:

1. After the visual inspection, some unusable retired lithium power batteries are removed. The following three types of retired batteries cannot be reused in stages, but can only be disassembled and recycled: (1) Batteries with deformation or swelling; (2) Damaged or leaking batteries; (3) batteries with severe corrosion or pole damage.

The inconsistency of lithium power batteries after decommissioning is prominent. Changes in physical and chemical properties of batteries may be manifested through appearance characteristics, such as deformation, bulging, damage, leakage, rust, pole damage, etc. Decommissioned lithium batteries with these characteristics cannot be reused. From the 60 retired batteries, 20 batteries with better appearance were selected and marked as 1-20 respectively.

2. Capacity test:

Use the American Bitrode MCV2-200-5 single battery test system to test the capacity of the power battery screened out through appearance inspection and analysis. First charge it to 3.65V with a constant current of 1xI ₃ (I ₃ is a 1/3C rate current) to 3.65V. Voltage charging, when the current decreases to 0.1xI ₃ , the battery stops charging, then stands still for 1 hour, and finally discharges with 1xI ₃ until the end-of-discharge voltage reaches 2.70V, according to the current value and discharge time of 1xI ₃ (A) The data calculates the battery capacity (in Ah).

The actual capacity of the decommissioned power battery must have a certain degree of attenuation, so its capacity needs to be recalibrated. However, due to the uncertainty of the degree to which the capacity of the decommissioned battery has decayed, the charge and discharge current 1xI ₃ set during capacity measurement cannot be set. Referring to the test requirements of "Technical Specifications for Performance Testing of Energy Storage Batteries for Smart Grid", we assume that the battery capacity has not decayed (15Ah), decayed to 80% (12Ah), and decayed to 67% (10Ah) to calibrate the battery The actual capacity, that is, set _I3 to 5A, 4A, 3.3A at this time to conduct a capacity test, to see the difference in the actual capacity of the same battery in the above three cases. The actual capacity of the 20 decommissioned batteries with qualified appearance is shown in Figure 1.

The average relative error rate δ of each battery capacity calibrated according to the three capacities of 10Ah, 12Ah and 15Ah is calculated by formula (1).

$\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&Delta;</span>}}{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; x</span>}}}<span\; class="notranslate">\; \&times;</span>\mathrm{<span\; class="notranslate">\; 100</span>}\mathrm{<span\; class="notranslate">\; \%</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

is the average value of capacity under three calibration conditions, and Δ is the absolute value of the difference between the average value and the values of each group. After calculation, the average relative error rate δ of each battery capacity calibrated according to the three capacities of 10Ah, 12Ah, and 15Ah is between -6.93% to 3.61%, -1.30% to 4.85%, and -3.02% to 3.55, respectively. It can be seen that under the condition of 12Ah The measured capacity error range is small. Figure 2 is a distribution map of the average actual capacity of 20 retired lithium batteries with acceptable appearance. Divide these batteries into 3 groups according to the capacity level, 12-14Ah group has 1, 2, 3, 4, 8, 9, 10, 12, 15, 16, 17, 19, 20 batteries, a total of 13; 10 -The 12Ah group has 5, 7, and 14 batteries, a total of 3; the 8-10Ah group has 6, 11, 13, 18, a total of 4, in order to facilitate subsequent performance evaluation and classification. At the same time, taking these 20 retired power lithium batteries as a whole, the distribution characteristics of their capacity were studied. Through the calculation of the "Statistical Product and Service Solutions" software (Statistical Product and Service Solutions, SPSS), we get: the mean value of the battery capacity is 12.06Ah, and the standard deviation (arithmetic square root of the variance) is 1.71. A non-parametric test is performed on 20 sample size data, assuming that the sample size obeys a normal distribution. When the sig (significance coefficient) is greater than 0.05, it means that the data obeys the specified distribution, that is, the normal distribution. Since the sample size N<2000 this time, the SW test was used to conduct a non-parametric test on the data of the 20 sample volumes, and the result was: sig=1.151>0.05, indicating that the distribution of the 20 sample volumes conformed to the normal distribution. The normal distribution means that the battery capacity has a bell-shaped distribution. When the number of samples increases, the average value of the samples tends to a fixed value, which is the expected value of the distribution. This is the law of large numbers or the law of central limit; In the end, it will be found in the sample that the frequency of samples appearing near the expected value is higher, and the farther away from the expected value, the smaller the frequency of occurrence.

3. Pulse characteristic curve analysis:

It is an intuitive and accurate method to evaluate the consistency of batteries by studying the characteristic curves of high-current pulse charge and discharge of retired lithium power batteries. Using the feedback of the battery itself to the high-rate current, the decommissioned power battery is subjected to a series of charge-discharge and static steps, and the measured charge-discharge curve can truly reflect the voltage change of the battery during actual operation. The test steps are as follows: choose 2 lithium batteries to be charged to 3.65V according to 1/3C constant current, then charge to 0.1xI ₃ at constant voltage, and stand for 2 hours; discharge at 1C for 10s, and stand for 40s; ;Discharge at 5C for 10s, then rest for 40s; charge at 1C for 10s, and rest for 40s; finally, charge to 3.65V with a constant current of 1/3C, then charge at a constant voltage to 0.1xI ₃ , and the pulse charge and discharge ends. At this time, 1/3C is charged according to 4A, and then it is tested according to the newly calibrated capacity.

The battery is charged and discharged according to different rate pulses, and the charge and discharge voltage curves of each battery are compared. Figure 3 is the pulse charge and discharge voltage curve of some retired batteries. It can be seen from Figure 3 that although the five batteries (1, 3, 4, 9, 12) are grouped together by capacity, the capacity is relatively close, and the voltage is basically the same when charging and discharging at low rates, but at 3C When discharged at a high rate such as 5C, the voltage of No. 1 battery and the other 4 batteries showed obvious inconsistency, and the greater the discharge rate, the more obvious the inconsistency.

As the discharge current increases, the degree of polarization (or polarization internal resistance) is different due to the different degrees of deterioration of retired batteries, and the inconsistency of the batteries is also highlighted. In Figure 3, the maximum voltage difference between No. 1 battery and the other 4 batteries is 0.013V at 1C rate discharge; the maximum voltage difference between No. 1 battery and the other 4 batteries is 0.101V and 0.23V at 3C and 5C rate discharge respectively. V; when charging at 1C rate, the maximum voltage difference between No. 1 battery and the other 4 batteries is reduced to 0.02V again. This shows that when the battery is actually working, even if the ohmic internal resistance is consistent, the difference in polarization internal resistance caused by the complex physical and chemical changes inside the battery will significantly affect the consistency of the battery's operating voltage. Table 1 shows the voltage of decommissioned batteries under different pulse charge and discharge rates, the unit is V,

Table 1

Table 1 shows the voltage of the decommissioned lithium battery under different pulse charge and discharge rates (take the voltage value of the pulse 5s). It can be seen from Table 1 that the maximum voltage difference of these 20 batteries is 0.263V at 1C rate, 0.41V at 3C rate, and 0.505V at 5C rate. The maximum voltage difference at 1C rate is 0.081V. As the current increases, the voltage difference between the batteries increases, and when the current decreases, the voltage difference decreases. The battery consistency is judged based on the voltage greater than 2.7V, between 2.7-2.5V, and less than 2.5V when discharged at a rate of 5C, and is divided into 3 groups. As can be seen from Table 1, batteries with a voltage greater than 2.7V are 1, 2, 3, 5, 7, 8, 9, 11, 12, 13, 14, 16, 18, and 19, and the voltage is between 2.7-2.5V There are 4, 6, 10, 15, and 17 batteries among them, and 20 batteries with a voltage less than 2.5V. 2.7V is the end-of-discharge voltage of our battery test, and 2.5V is the lowest safe discharge voltage set by the battery manufacturer.

4. Electrochemical impedance spectroscopy:

The electrochemical impedance test was carried out on the decommissioned batteries by using the Swiss Autolab PGSTAT302 electrochemical workstation. The electrochemical impedance test frequency is between 0.01Hz and 100kHz. Use ZSimpWin software to perform equivalent circuit fitting on the test data of electrochemical impedance, and study the internal electrochemical impedance characteristics of the battery through fitting parameters.

In order to further explore the consistency of decommissioned batteries, electrochemical impedance spectroscopy was carried out on these 20 batteries. Figure 4 is the electrochemical impedance spectroscopy of a representative decommissioned lithium battery. In Figure 4, the straight line part of the fourth quadrant is the hysteresis current in the battery system caused by inductance, which is the embodiment of inductive reactance. Therefore, it can be inferred that there should be an inductance in the electrochemical impedance equivalent circuit of the decommissioned lithium battery under test. The resistance element L exists; in the high frequency band, the Z _re value corresponds to the ohmic internal resistance R _s related to mass transfer when Z _im is 0, Z _im is the imaginary part in the electrochemical impedance spectrum, that is, the vertical axis, and Z _re is The real part in the electrochemical impedance spectrum, that is, the horizontal axis, R _s is the ohmic internal resistance; the low frequency band reflects the solid diffusion process of lithium ions inside the active material particles, which is characterized by a slanted line. ) Impedance Z _W component said. Because the slope of the slope line in the low-frequency region of the battery measured is not 45°, which is not a standard Weber impedance, therefore, Z _W in the equivalent circuit is replaced by a general constant phase angle element Q _Zw . There is no clear junction point at the junction of the mid-frequency band and the low-frequency band, because in this area the battery has concentration polarization and electrochemical polarization at the same time, and the high-frequency capacitive reactance arc corresponds to the charge transfer resistance R _ct and the electrode double layer capacitance Q _Cdl . Therefore, the equivalent circuit model of the electrochemical impedance spectroscopy of the decommissioned lithium battery is shown in Figure 5. The constant phase angle element Q _Zw and the charge transfer resistance R _ct are connected in series, then connected in parallel with the electrode electric double layer capacitance Q _Cdl , and then connected in series within the ohm resistance R _s and inductance L.

In Fig. 4, batteries No. 7, No. 12 and No. 14 are grouped into one group in the pulse discharge test, which is considered to have better consistency. However, it can be seen from the electrochemical impedance spectrum that there are still relatively large differences between the No. 7 battery and the No. 12 and No. 14 batteries, especially in the low-frequency diffusion part of the impedance spectrum. Through the equivalent circuit in Figure 5, the parameters of the equivalent circuit components of these 20 decommissioned lithium batteries were analyzed to obtain the ohmic internal resistance R _s value and the charge transfer resistance R _ct value, and the capacity, R _s value and The R _ct values are plotted in Fig. 6. We use the ohmic internal resistance R _s and the charge transfer resistance R _ct to be less than 7mΩ, between 8-10mΩ, and greater than 10mΩ as the basis for judging the battery consistency. From Figure 6, it can be seen that the batteries with Rs and _Rct values less than _7mΩ are composed of 1, 2, 4, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 19, 20 No. 3, 6, and 18 for batteries between 8-10mΩ, and No. 5 for batteries greater than 10mΩ.

In addition to the ohmic internal resistance R _s and the charge transfer resistance R _ct , the concentration polarization impedance is a more important factor affecting the capacity fading of the battery, and the lithium ion diffusion coefficient D _Li+ can reflect the concentration polarization impedance, D _Li+ The smaller the value, the greater the concentration polarization. The calculation formula of lithium ion diffusion coefficient D _Li+ is:

${\mathrm{<span\; class="notranslate">\; D.</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; +</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; A</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}^{\mathrm{<span\; class="notranslate">\; 4</span>}}{\mathrm{<span\; class="notranslate">\; f</span>}}^{\mathrm{<span\; class="notranslate">\; 4</span>}}{\mathrm{<span\; class="notranslate">\; C</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

Among them: ideal gas constant R=8.314J/(mol K), absolute temperature T=298.15K, cross-sectional area of electrode A=0.01m ³ , electron transfer number n=1, Faraday constant F=96487C/mol, C is the concentration of lithium ions in the electrode (the concentration of lithium iron phosphate is 7.69×10 ³ mol/m ³ ), and σ is the Warburg Weber factor. And σ has the following relationship with Z _re :

Z _re =R _s +R _ct +σω ^-1/2 (3)

Among them: ω is the angular frequency during the electrochemical impedance test, ω=2πf, f is the frequency in the EIS test, Z _re is the real part of the impedance spectrum corresponding to ω, R _s is the ohmic internal resistance, and R _ct is charge transfer resistance. According to formula (3), with ω ^-1/2 as the abscissa and Z _re as the ordinate, the slope of the obtained line is the Warburg factor σ, and then σ is substituted into formula (2) to calculate the lithium ion diffusion coefficient . Table 2 shows the σ value and diffusion coefficient D _Li+ value of lithium ions in 20 retired lithium batteries. The relationship between the diffusion coefficient D _Li+ value and capacity is shown in Figure 7. It can be seen from Figure 7 that the battery capacity is positively correlated with the lithium ion diffusion coefficient. The consistency of the battery is judged based on the diffusion coefficient D _Li+ value greater than 6×10 ^-14 cm ² /s. Based on this, it can be seen from Table 2 that the consistency of batteries 1, 2, 3, 4, 5, 7, 8, 9, 10, 11, 12, 14, 15, 16, 17, 19, and 20 Better, and the D _Li+ values of batteries No. 6, No. 13 and No. 18 are all less than 4×10 ^-14 cm ² /s.

Table 2

After analyzing the electrochemical impedance spectroscopy data of these 20 retired lithium batteries, non-parametric tests were performed on R _s , R _ct and D _Li+ respectively, and the correlation between R _s , R _ct and D _Li+ and battery capacity was analyzed. Due to the small number of samples of decommissioned lithium batteries, the values under the precise test conditions are taken, among which Sig of R _s = 0.862>0.05, Sig of R _ct = 0.186>0.05, and Sig of D _Li+ = 0.834>0.05. It can be considered that R _s , The R _ct and D _Li+ values both conform to the normal distribution, and the above analysis shows that the capacity of the decommissioned battery also conforms to the normal distribution, and both meet the application conditions of the Pearson correlation coefficient. The Pearson correlation coefficient is used to measure R _s , R _ct and D The degree of correlation between _Li+ and battery capacity. The positive or negative of the Pearson correlation coefficient of the variable indicates the positive or negative of the correlation, but the correlation is not established when the significance Sig value is greater than 0.05, and the correlation is established when it is less than 0.05. The calculated Pearson correlation coefficient between capacity and R _s is 0.364, but Sig=0.114>0.05, no significant relationship; the Peason correlation coefficient between capacity and R _ct value is -0.538, Sig=0.014<0.05, Therefore, there is a significant negative correlation between capacity and R _ct , indicating that the larger the R _ct value, the smaller the battery capacity; the Peason correlation coefficient between capacity and D _Li+ is 0.729, Sig=0.00<0.05, so the capacity and D _Li + There is a strong positive correlation between them, indicating that the larger the D _Li+ value, the larger the battery capacity. Therefore, the factors that affect the capacity fading of retired lithium batteries, from the perspective of impedance analysis, it can be seen that the concentration polarization impedance is the first, followed by the charge transfer resistance, while the ohmic internal resistance has less influence.

Based on the consistency analysis of battery capacity, pulse discharge voltage, resistance, and diffusion coefficient, the 20 retired lithium batteries can be divided into 3 groups, with a capacity between 12-14Ah, pulse discharge voltage greater than 2.7V, ohmic internal resistance and charge transfer resistance. All of which are less than 7mΩ and the lithium ion diffusion coefficient is greater than 6×10 ^-14 cm ² s ^-1 , there are 1, 2, 8, 9, 12, 14, 16, and 19 batteries, which are the first group; the capacity is between 10-14Ah, The pulse discharge voltage is greater than 2.5V, the ohmic internal resistance and charge transfer resistance are both less than 10mΩ, and the lithium ion diffusion coefficient is greater than 6×10 ^-14 cm ² /s. There are 3, 4, 7, 10, 15, and 17 batteries, which are the second group; the remaining No. 5, 6, 11, 13, 18, and 20 batteries are the third group, with low battery capacity, low pulse discharge voltage, large charge transfer resistance, or small lithium ion diffusion coefficient Case. For the first or second set of batteries, from the perspective of safety, the charge and discharge system after the batteries are grouped can be designed according to the battery with the lowest capacity in the set of batteries. For the third group of batteries, due to the large drop in battery performance and poor consistency, it is not recommended to reuse them in groups.

Claims (1)

1. determine the conforming multi-parameter comprehensive decision method of the retired lithium battery of electric automobile for one kind, it is characterized in that, test the consistency of four retired lithium batteries of part synthetic determination electric automobile from visual examination, volume test, pulse characteristic tracing analysis, electrochemical impedance spectroscopy, concrete judge as follows:

1) visual examination: the battery of there is the battery of distortion or bulge situation, there is the battery of breakage or leakage, exist serious corrosion or pole damage is disallowable, not recycling;

2) volume test: carry out capacity check through external morphology Analysis and Screening electrokinetic cell out, capacity test method is for first to use 1xI ₃constant voltage charge is carried out, when electric current reduces to be reduced to 0.1xI after constant current charge to 3.65V ₃time battery stop charging, then leave standstill 1 hour, finally use 1xI ₃discharge, until final discharging voltage reaches 2.70V, according to 1xI ₃current value and discharge time data calculate battery capacity, capacity in Ah, wherein I ₃for 1/3C multiplying power electric current;

I ₃demarcation: by hypothesis battery capacity do not decay, decay to 80%, decay to 67% 3 kind of demarcation condition, carry out the capacity of test battery with above-mentioned capacity test method, the capacity of test substitutes into each battery capacity average relative error of formulae discovery below rate δ,

$\mathrm{\&delta;}=\frac{\mathrm{\&Delta;}}{\stackrel{\&OverBar;}{X}}\&times;100\%$

be the capacity mean value under three kinds of demarcation conditions, Δ is the absolute value of difference between mean value and each class value, obtain each battery capacity average relative error rate δ, the battery capacity of getting under the minimum demarcation condition of the capacity average relative error rate scope that records is calibrated capacity, demarcates I with this capacity ₃current value;

3) pulse characteristic tracing analysis: battery is by different multiplying pulse discharge and recharge, and compare the charging/discharging voltage curve of each battery, charge and discharge process is: lithium battery is according to carrying out constant voltage charge after 1/3C constant current charge to 3.65V to 0.1xI ₃, leave standstill 2 hours; 1C discharges 10s, leaves standstill 40s; 3C discharges 10s, leaves standstill 40s; 5C discharges 10s, leaves standstill 40s; 1C charges 10s, leaves standstill 40s; Finally, according to carrying out constant voltage charge after 1/3C constant current charge to 3.65V to 0.1xI ₃, pulse discharge and recharge terminates; Obtain pulse charging/discharging voltage curve, be greater than 2.7V with voltage during 5C multiplying power discharging, between 2.7-2.5V, be less than 2.5V to judge battery consistency, the battery that voltage is greater than 2.7V is classified as one group, voltage is classified as another group between the battery of 2.7-2.5V, the battery that voltage is less than 2.5V is rejected, 2.7V is battery testing final discharging voltage, and 2.5V is the minimum electric discharge safe voltage of battery factory settings;

4) electrochemical impedance spectroscopy: utilize Switzerland AutolabPGSTAT302 type electrochemical workstation to carry out electrochemical impedance test to retired battery, electrochemical impedance test frequency is between 0.01Hz ~ 100kHz, ZSimpWin software is utilized to carry out Equivalent Circuit Fitting to the test data of electrochemical impedance, analyze inside battery electrochemical impedance characteristic by fitting parameter, use ohmic internal resistance R _s, charge transfer resistance R _ctwith lithium ion diffusion coefficient D _li+come to battery group, wherein lithium ion diffusion coefficient D _li+reflection concentration polarization impedance size, D _li+be worth less, concentration polarization is larger, lithium ion diffusion coefficient D _li+computing formula be:

$D_{Li+}=\frac{R^2{T}^2}{2A^2{n}^4{F}^4{C}^2{\mathrm{\&sigma;}}^2},$

Wherein ideal gas constant R=8.314J/ (molK), absolute temperature T=298.15K, A are the cross-sectional area of electrode, n is electron transfer number, the concentration of F to be Faraday constant F=96487C/mol, C be lithium ion in electrode, σ is the real part Z of the Warburg weber factor, σ and impedance spectrum _rerelation as follows:

Z _re=R _s+ R _ct+ σ ω ^-1/2, ω carries out the angular frequency in electrochemical impedance test process, and ω=2 π f, f are the frequencies in EIS test.