A method and system for testing the performance of lithium batteries used in lithium battery production.

Abstract

This invention relates to the field of lithium battery performance testing, and more particularly to a method and system for testing lithium battery performance in lithium battery production. The method includes: acquiring the surface temperature and electrochemical impedance spectrum of a qualified lithium battery at each acquisition moment during a single charge-discharge cycle; obtaining a first comprehensive temperature sequence and a first complex impedance sequence at each frequency corresponding to the single charge-discharge cycle after preprocessing; similarly obtaining a second comprehensive temperature sequence and a second complex impedance sequence at each frequency for the lithium battery under test; calculating the performance score of the lithium battery under test; if the performance score is less than a preset qualified threshold, stopping the charge-discharge cycle; and determining whether the total number of charge-discharge cycles is greater than a preset number; if it is greater than the preset number, the lithium battery under test is deemed to be qualified. This invention can consider the mid-to-high frequency irregularities and low-frequency irregularities in the distribution of complex impedance values ​​during the charge-discharge cycle of the lithium battery under test, thereby improving the confidence level of lithium battery performance evaluation.

Description

Technical Field

[0001] This invention relates to the field of lithium battery performance testing. In particular, it relates to a method and system for testing the performance of lithium batteries used in lithium battery production. Background Technology

[0002] Lithium-ion batteries are a high-performance rechargeable battery technology widely used in consumer electronics, electric vehicles, energy storage systems, and other fields. During the production of lithium-ion batteries, their performance needs to be tested to determine their qualification.

[0003] Chinese patent application CN112327186A discloses a method for detecting the electrochemical polarization impedance of a lithium battery, comprising the following steps: before performing electrochemical impedance detection on the lithium battery, the lithium battery is placed in a ventilated and dry environment to ensure the electrolyte is balanced, avoiding deviations in the battery's electrochemical polarization impedance caused by electrolyte imbalance; the current flowing through the lithium battery also flows through a resistor Rr, with the potential at point "L" maintained at 0V, and an IV converter amplifier used to balance the current on Rr with the current in the lithium battery; the detection operation is performed using an LCR meter and an impedance analyzer, measuring the high-side voltage and the voltage on Rr, thereby calculating the impedance value of the lithium battery.

[0004] Because the electrochemical reaction rates differ at different locations within a lithium-ion battery cell, temperature variations occur at different points in the cell. The impedance values ​​of the cell at different frequencies are affected by temperature, leading to measurement biases. The aforementioned technical solution neglects the impact of temperature differences on impedance values ​​during lithium-ion battery performance testing, resulting in significant deviations in performance evaluation and affecting the accuracy of lithium-ion battery performance testing. Summary of the Invention

[0005] To address the technical problem of significant deviations in the performance evaluation of lithium batteries by the aforementioned testing methods, this invention provides solutions in the following aspects.

[0006] In the first aspect, a method for testing the performance of lithium batteries used in lithium battery production includes:

[0007] The surface temperature and electrochemical impedance spectrum of a qualified lithium battery at each acquisition moment during a single charge and discharge process are obtained. After preprocessing, the first comprehensive temperature sequence and the first complex impedance sequence at each frequency corresponding to the single charge and discharge process are obtained. Similarly, the second comprehensive temperature sequence and the second complex impedance sequence at each frequency of the lithium battery under test are obtained.

[0008] Calculate the performance score of the lithium battery under test. If the performance score is greater than or equal to the preset qualified threshold, proceed to the next charge-discharge cycle. Otherwise, stop the charge-discharge cycle and determine whether the total number of charge-discharge cycles is greater than the preset number. If it is greater than the preset number, the lithium battery under test is deemed to be qualified.

[0009] The calculation method for the performance score of the lithium battery under test includes:

[0010] The second complex impedance sequence is corrected based on the first comprehensive temperature sequence and the first complex impedance sequence to obtain the corrected complex impedance sequence for each frequency. The mid-to-high frequency impedance spectrum at each acquisition time is fitted using the corrected complex impedance sequence in the mid-to-high frequency region to obtain the fitting center and fitting radius of each mid-to-high frequency impedance spectrum. The mid-to-high frequency irregularity at each time is calculated using the corrected complex impedance sequence in the mid-to-high frequency region and the fitting center. The low-frequency irregularity at each acquisition time is calculated using the corrected complex impedance sequence in the low-frequency region. The performance score of the lithium battery under test is calculated based on the average of the fitting radius, mid-to-high frequency irregularity, and low-frequency irregularity at all acquisition times during a single charge-discharge process.

[0011] Preferably, before correcting the second complex impedance sequence, the method further includes: performing a first-order difference processing on the first comprehensive temperature sequence to obtain a first comprehensive temperature difference sequence, and performing a first-order difference processing on the first complex impedance sequence to obtain a first complex impedance difference sequence; during the electrochemical impedance spectroscopy test, the frequency region is divided into a low-frequency region, a mid-frequency region, and a high-frequency region. For a single frequency region, a first real part temperature coefficient and a first imaginary part temperature coefficient are calculated based on the first complex impedance sequence and the first comprehensive temperature sequence for each frequency in that frequency region during each charge and discharge process, and a second real part temperature coefficient and a second imaginary part temperature coefficient are calculated based on the first complex impedance difference sequence and the first comprehensive temperature difference sequence.

[0012] Preferably, the method for calculating the first real part temperature coefficient within a single frequency region includes: dividing the first complex impedance sequence of each frequency within the frequency region into a first real part sequence and a first imaginary part sequence; for each charge-discharge process, calculating the Pearson correlation coefficient value between the first real part sequence and the first comprehensive temperature sequence of each frequency within the frequency region; calculating the sum of the Pearson correlation coefficient values ​​obtained in all charge-discharge processes of the qualified lithium battery; calculating the product of the number of charge-discharge cycles of the qualified lithium battery and the number of frequencies contained in the frequency region; and multiplying the sum of the obtained Pearson correlation coefficient values ​​by the reciprocal of the obtained product to obtain the first real part temperature coefficient within the frequency region.

[0013] Preferably, the method for correcting a second complex impedance sequence at a single frequency includes: obtaining a first real temperature coefficient, a first imaginary temperature coefficient, a second real temperature coefficient, and a second imaginary temperature coefficient for the frequency region to which the frequency belongs; each complex impedance value in the second complex impedance sequence includes a real part and an imaginary part; the real part is corrected based on the first real temperature coefficient, the second real temperature coefficient, and the combined temperature value; the imaginary part is corrected based on the first imaginary temperature coefficient and the second imaginary temperature coefficient, to obtain a corrected complex impedance value for each complex impedance value.

[0014] Preferably, the second comprehensive temperature sequence contains multiple comprehensive temperature values. The second comprehensive temperature sequence is subjected to first-order difference processing to obtain a second comprehensive temperature difference sequence formed by comprehensive temperature difference values. The correction method for the real part includes: for the acquisition time corresponding to the complex impedance value, calculating the product of the first real part temperature coefficient and the comprehensive temperature value corresponding to the acquisition time, calculating the product of the second real part temperature coefficient and the comprehensive temperature difference value corresponding to the acquisition time, calculating the sum of the two products, and adding the real part of the complex impedance value to the obtained sum to complete the correction of the real part.

[0015] Preferably, the method for calculating the mid-to-high frequency irregularity at a single acquisition time includes: calculating the Euclidean distance between the corrected complex impedance value of each frequency in the mid-to-high frequency region at the acquisition time and the center of the fitting circle of the mid-to-high frequency impedance spectrum at the acquisition time, and calculating the standard deviation of all Euclidean distances; calculating a first ratio of the distance between a single frequency and the center of the fitting circle of the left nearest neighbor frequency, calculating a second ratio of the distance between a single frequency and the center of the fitting circle of the right nearest neighbor frequency, calculating the absolute value of the difference between the first ratio and the second ratio, summing the absolute values ​​of all frequencies in the mid-to-high frequency region, and multiplying the sum by the standard deviation of the Euclidean distance to obtain the mid-to-high frequency region irregularity at the acquisition time.

[0016] Preferably, the method for calculating the low-frequency irregularity at a single acquisition moment includes: calculating a third ratio of the difference between the imaginary and real parts of the modified complex impedance values ​​of a single frequency and its left nearest neighbor frequency at that acquisition moment; calculating a fourth ratio of the difference between the imaginary and real parts of the modified complex impedance values ​​of a single frequency and its right nearest neighbor frequency at that acquisition moment; multiplying the absolute value of the difference between the third ratio and 1 and the absolute value of the difference between the fourth ratio and 1 to obtain the product corresponding to the single frequency; and summing the products corresponding to all frequencies in the low-frequency region at that acquisition moment to obtain the low-frequency irregularity at that acquisition moment.

[0017] Preferably, the method for calculating the performance score of the lithium battery under test includes: for a single charge-discharge process, calculating the average value of the fitting radius of the mid-to-high frequency impedance spectrum, the average value of the mid-to-high frequency irregularity, and the average value of the low-frequency irregularity at all acquisition times; calculating the product of the average fitting radius and the average value of the mid-to-high frequency irregularity, and performing negative correlation normalization on the product to obtain a first normalized value; performing negative correlation normalization on the average value of the low-frequency irregularity to obtain a second normalized value; and calculating the sum of the first normalized value and the second normalized value to obtain the performance score of the lithium battery under test.

[0018] Preferably, the method for obtaining the first comprehensive temperature sequence includes: using the DBSCAN algorithm to cluster all surface temperature values ​​collected at a single acquisition time to form multiple clusters; averaging the surface temperature values ​​within each cluster, calculating the ratio of the number of surface temperature values ​​contained in a single cluster to the total number of surface temperature values, multiplying this ratio by the average surface temperature values ​​of the corresponding cluster to obtain a single temperature value for the single cluster, summing the single temperature values ​​of all clusters corresponding to a single acquisition time to obtain the comprehensive temperature value corresponding to the single acquisition time; and sorting all comprehensive temperature values ​​according to the order of acquisition time to obtain the first comprehensive temperature sequence of a qualified lithium battery in a single charge-discharge cycle.

[0019] Secondly, a lithium battery performance testing system for lithium battery production includes: a processor and a memory, wherein the memory stores computer program instructions, and when the computer program instructions are executed by the processor, the above-mentioned lithium battery performance testing method for lithium battery production is implemented.

[0020] The present invention has the following effects:

[0021] 1. This invention analyzes the influence of temperature on the real and imaginary parts of the complex impedance value in different frequency regions of a lithium battery, and obtains the temperature coefficient to perform frequency adaptive compensation for the real and imaginary parts of the complex impedance value. This method can more accurately reflect the true performance of the battery under different temperature conditions, reduce measurement errors caused by temperature changes, and improve the accuracy of lithium battery performance evaluation.

[0022] 2. This invention not only considers the single fitting radius of the complex impedance value, but also analyzes the irregularity of the distribution of complex impedance values ​​of the lithium battery under test at different frequencies. It takes into account the difference and smoothing phenomenon of the fitting radius due to the local distribution irregularity. This comprehensive analysis method can more accurately evaluate the performance of the lithium battery under test and improve the confidence of the performance evaluation of the lithium battery under test. Attached Figure Description

[0023] Figure 1This is a flowchart of steps S1-S6 in a lithium battery performance testing method for lithium battery production according to an embodiment of the present invention. Detailed Implementation

[0024] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0025] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0026] Reference Figure 1 A method for testing the performance of lithium batteries used in lithium battery production includes steps S1-S6, as detailed below:

[0027] S1: Obtain the surface temperature value and electrochemical impedance spectrum of a qualified lithium battery at each acquisition moment during a single charge and discharge process. After preprocessing, obtain the first comprehensive temperature sequence and the first complex impedance sequence at each frequency corresponding to the single charge and discharge process. Similarly, obtain the second comprehensive temperature sequence and the second complex impedance sequence at each frequency of the lithium battery under test.

[0028] The process of obtaining the electrochemical impedance spectroscopy (EIS) of a qualified lithium battery includes: connecting an electrochemical workstation to the quadrupole of the qualified lithium battery cell; for each charge-discharge cycle, applying an AC signal to the qualified lithium battery at preset intervals within a set frequency range (e.g., 0.01 Hz to 100 kHz), and measuring the impedance response of the qualified lithium battery at each frequency point, thereby obtaining the EIS from low frequency to high frequency at each acquisition moment (i.e., the moment when the AC signal is applied). The low-frequency region (<1 Hz) mainly reflects the diffusion process of the battery, such as the diffusion of lithium ions in the electrolyte and the insertion / extraction process in the electrode material; the mid-frequency region (1 Hz to 10 kHz) mainly reflects the charge transfer process, such as the charge transfer at the electrode-electrolyte interface; and the high-frequency region (>10 kHz) mainly reflects the ohmic resistance of the battery, such as the resistance of the electrolyte and the resistance of the electrode material.

[0029] During a single charge-discharge cycle, the complex impedance values ​​in the electrochemical impedance spectrum at each acquisition time for a single frequency are obtained. These complex impedance values ​​are then sorted according to the acquisition time to obtain the first complex impedance sequence for each single frequency, thus yielding the first complex impedance sequence for each frequency. A first-order difference processing is performed on the first complex impedance sequence to obtain the first complex impedance difference sequence.

[0030] Similarly, the second complex impedance sequence and the second complex impedance difference sequence corresponding to each frequency during a single charge and discharge cycle of the lithium battery under test are obtained.

[0031] The process of obtaining the surface temperature of a qualified lithium battery includes: suspending an infrared thermal imager directly above the qualified lithium battery, collecting the temperature at different locations on the surface of the qualified lithium battery at each acquisition moment, thereby obtaining multiple surface temperature values ​​at each acquisition moment.

[0032] The DBSCAN algorithm (Density-Based Spatial Clustering of Applications with Noise) is used to cluster all surface temperature values ​​collected at a single acquisition time, forming multiple clusters. The average surface temperature values ​​within each cluster are calculated, and the ratio of the number of surface temperature values ​​in a single cluster to the total number of collected surface temperature values ​​is calculated. This ratio is multiplied by the average surface temperature values ​​of the corresponding cluster to obtain the individual temperature value of the single cluster. The individual temperature values ​​of all clusters corresponding to a single acquisition time are summed to obtain the comprehensive temperature value corresponding to that single acquisition time. All comprehensive temperature values ​​are sorted according to the chronological order of acquisition time to obtain the first comprehensive temperature sequence of a qualified lithium battery in a single charge-discharge cycle. A first-order difference processing is performed on the first comprehensive temperature sequence to obtain the first comprehensive temperature difference sequence.

[0033] Similarly, the second comprehensive temperature sequence and the second comprehensive temperature difference sequence during a single charge and discharge process of the lithium battery under test are obtained.

[0034] Thus, the first comprehensive temperature sequence, the first comprehensive temperature difference sequence, and the first complex impedance sequence and the first complex impedance difference sequence for each frequency of a qualified lithium battery are obtained. These are used for subsequent temperature coefficient calculations, so as to compensate and correct the complex impedance values ​​in the second complex impedance sequence of the lithium battery under test by combining the second comprehensive temperature sequence and the second comprehensive temperature difference sequence.

[0035] S2: Based on the first comprehensive temperature sequence and the first complex impedance sequence, the second complex impedance sequence is corrected to obtain the corrected complex impedance sequence for each frequency.

[0036] During the charge-discharge cycle of lithium battery performance testing, the charge-discharge cycle causes certain temperature fluctuations on the surface of the cell. These temperature fluctuations result in different changes in the real and imaginary parts of the complex impedance value at different locations within the cell. Furthermore, this effect varies under voltage signals of different frequencies. Therefore, analyzing the temperature-dependent effects on the real and imaginary parts of the complex impedance value in different frequency regions of the lithium battery and obtaining the corresponding temperature coefficients is crucial for improving the accuracy of lithium battery performance evaluation.

[0037] Before correcting the second complex impedance sequence, the process includes: performing a first-order difference processing on the first comprehensive temperature sequence to obtain a first comprehensive temperature difference sequence, and performing a first-order difference processing on the first complex impedance sequence to obtain a first complex impedance difference sequence; during the electrochemical impedance spectroscopy test, the frequency region is divided into a low-frequency region, a mid-frequency region, and a high-frequency region. For a single frequency region, the first real part temperature coefficient and the first imaginary part temperature coefficient are calculated based on the first complex impedance sequence and the first comprehensive temperature sequence for each frequency in that frequency region during each charge and discharge process, and the second real part temperature coefficient and the second imaginary part temperature coefficient are calculated based on the first complex impedance difference sequence and the first comprehensive temperature difference sequence.

[0038] The method for calculating the first real part temperature coefficient within a single frequency region includes: dividing the first complex impedance sequence at each frequency within the frequency region into a first real part sequence and a first imaginary part sequence; for each charge-discharge process, calculating the Pearson correlation coefficient between the first real part sequence and the first comprehensive temperature sequence at each frequency within the frequency region; summing the Pearson correlation coefficient values ​​obtained during all charge-discharge processes of a qualified lithium battery; calculating the product of the number of charge-discharge cycles of the qualified lithium battery and the number of frequencies contained in the frequency region; and multiplying the sum by the reciprocal of the product to obtain the first real part temperature coefficient within the frequency region. The specific calculation formula is as follows:

[0039]

[0040] In the formula, Indicates the first The temperature coefficient of the first real part in each frequency region; This indicates the charge / discharge cycle coefficient of a qualified lithium battery; Indicates the first The number of frequencies contained within a frequency region; The first grade of qualified lithium battery The first charging and discharging process The Pearson correlation coefficient between the first real part of the first complex impedance sequence at each frequency and the first comprehensive temperature sequence.

[0041] Within the same frequency range, the impedance is less affected by temperature. Therefore, all Pearson correlation coefficient values ​​within the same frequency range can be combined to obtain a more comprehensive temperature coefficient. To further improve the accuracy of the temperature coefficient, multiple charge-discharge cycles need to be considered to reduce random errors and the abnormal effects of individual cycles.

[0042] The calculation method for the first imaginary part temperature coefficient within a single frequency region is the same as that for the first real part temperature coefficient. The Pearson correlation coefficient between the first imaginary part sequence and the first comprehensive temperature sequence is used in the calculation.

[0043] The method for calculating the second real part temperature coefficient within a single frequency region includes: dividing the first complex impedance difference sequence at each frequency into a first real part difference sequence and a first imaginary part difference sequence; for each charge-discharge process, calculating the Pearson correlation coefficient value between the first real part difference sequence and the first comprehensive temperature difference sequence at each frequency within the frequency region; calculating the sum of the Pearson correlation coefficient values ​​obtained in all charge-discharge processes of a qualified lithium battery; calculating the product of the number of charge-discharge cycles of a qualified lithium battery and the number of frequencies contained in the frequency region; and multiplying the sum by the reciprocal of the product to obtain the second real part temperature coefficient within the frequency region.

[0044] The calculation method for the second imaginary part temperature coefficient within a single frequency region is the same as that for the second real part temperature coefficient. The Pearson correlation coefficient between the first imaginary part difference sequence and the first comprehensive temperature difference sequence is used in the calculation.

[0045] Since temperature directly affects the electrochemical processes in lithium battery cells, such as electrolyte conductivity, charge transfer rate, and ion diffusion, a single static temperature value (comprehensive temperature value) cannot analyze the effects of thermal hysteresis and unsteady thermal stress. Furthermore, temperature differences at different locations exhibit time-delayed thermal diffusion. Therefore, by using both static temperature values ​​and dynamic temperature changes (comprehensive temperature difference value) for two-dimensional compensation, the influence of temperature on complex impedance values ​​can be comprehensively compensated, reducing errors in complex impedance values ​​and improving the accuracy of lithium battery performance evaluation.

[0046] The method for correcting a second complex impedance sequence at a single frequency includes: obtaining a first real temperature coefficient, a first imaginary temperature coefficient, a second real temperature coefficient, and a second imaginary temperature coefficient for the frequency region to which the frequency belongs; each complex impedance value in the second complex impedance sequence includes a real part and an imaginary part; the real part is corrected based on the first real temperature coefficient, the second real temperature coefficient, and the combined temperature value; the imaginary part is corrected based on the first imaginary temperature coefficient and the second imaginary temperature coefficient, to obtain the corrected complex impedance value for each complex impedance value.

[0047] The method for correcting the real part includes: for the acquisition time corresponding to the complex impedance value, calculating the product of the first real part temperature coefficient and the comprehensive temperature value corresponding to that acquisition time; calculating the product of the second real part temperature coefficient and the comprehensive temperature difference value corresponding to that acquisition time; calculating the sum of the two products; and adding the real part of the complex impedance value to the sum to complete the correction of the real part. The specific formula is as follows:

[0048]

[0049] In the formula, Indicates a single frequency at the time of acquisition. The correction value for the real part of the complex impedance value; Indicates a single frequency at the time of acquisition. The measured value of the real part of the complex impedance; Indicates a single frequency at the time of acquisition. The overall temperature value; Indicates the frequency range in which the frequency is located. The temperature coefficient of the first real part within the range; Indicates a single frequency at the time of acquisition. The overall temperature difference score; Indicates the frequency range in which the frequency is located. The second real part temperature coefficient within.

[0050] The method for correcting the imaginary part includes: for the acquisition time corresponding to the complex impedance value, calculating the product of the first imaginary part temperature coefficient and the comprehensive temperature value corresponding to the acquisition time, calculating the product of the second imaginary part temperature coefficient and the comprehensive temperature difference value corresponding to the acquisition time, calculating the sum of the two products, adding the imaginary part of the complex impedance value to the obtained sum, and completing the correction of the imaginary part.

[0051] Since the first element in each second complex impedance sequence is the complex impedance value corresponding to the first acquisition time, it is only affected by the static first real part temperature coefficient and the first imaginary part temperature coefficient, and no compensation is performed for the second real part temperature coefficient and the second imaginary part temperature coefficient.

[0052] A complex impedance value is a complex number consisting of a real part and an imaginary part, with the imaginary part having an imaginary unit. After correcting the real and imaginary parts of the complex impedance value, the corrected imaginary part is multiplied by the original imaginary unit, and the product is added to the corrected real part to obtain the corrected complex impedance value.

[0053] Compared with existing technologies that directly test performance based on measured complex impedance values, this invention considers the degree to which different parts (real and imaginary parts) of the complex impedance value are affected by temperature in different frequency regions. By analyzing the influence of static and dynamic temperature changes, a two-dimensional temperature comprehensive compensation is performed, which reduces the deviation of the corrected complex impedance value and improves the accuracy of lithium battery performance evaluation.

[0054] S3: Fit the mid-to-high frequency impedance spectrum at each acquisition time using the corrected complex impedance sequence of the frequency in the mid-to-high frequency region, and obtain the fitting center and fitting radius of each mid-to-high frequency impedance spectrum. Calculate the mid-to-high frequency irregularity at each acquisition time using the corrected complex impedance sequence of the frequency in the mid-to-high frequency region and the fitting center.

[0055] In the electrochemical impedance spectroscopy of lithium batteries, due to the rapid electrochemical processes inside the battery, the complex impedance values ​​in the mid-to-high frequency region exhibit a semi-circular distribution in the Nyquist plot. When the battery is damaged or undergoes multiple charge-discharge cycles, this distribution becomes irregular. Existing technologies typically evaluate performance based on the fitted radius, neglecting the irregularity of the complex impedance value distribution. Therefore, analyzing the irregularity of the complex impedance value distribution in the mid-to-high frequency region of lithium batteries is beneficial for accurately evaluating lithium battery performance.

[0056] In step S2, the corrected complex impedance sequence of the lithium battery under test at each frequency during this charge and discharge process is obtained. The corrected complex impedance sequence of all frequencies in the mid-frequency region and high-frequency region (hereinafter referred to as mid-high frequency region) is obtained. The corrected complex impedance value of all frequencies in the mid-high frequency region at the same acquisition time is fitted by the equivalent circuit to obtain the mid-high frequency impedance spectrum at that acquisition time, thereby obtaining the fitting center of the mid-high frequency impedance spectrum at each acquisition time.

[0057] The method for calculating the mid-to-high frequency irregularity at a single acquisition moment includes: calculating the Euclidean distance between the corrected complex impedance value of each frequency in the mid-to-high frequency region at that acquisition moment and the center of the fitted circle of the mid-to-high frequency impedance spectrum at that acquisition moment, and calculating the standard deviation of all Euclidean distances; calculating the first ratio of the distance between a single frequency and its left nearest neighbor frequency to the center of the fitted circle, calculating the second ratio of the distance between a single frequency and its right nearest neighbor frequency to the center of the fitted circle, calculating the absolute value of the difference between the first ratio and the second ratio, summing the absolute values ​​of all frequencies in the mid-to-high frequency region, and multiplying the sum by the standard deviation of the Euclidean distances to obtain the mid-to-high frequency region irregularity at that acquisition moment. The specific formula is as follows:

[0058]

[0059] In the formula, This indicates the time of data collection during the charge and discharge process of the lithium battery under test. Mid-to-high frequency irregularity; This indicates that all frequencies in the mid-to-high frequency region are at the time of acquisition. The standard deviation of the Euclidean distance between the corrected complex impedance value and the fitting center of the mid-to-high frequency impedance spectrum; Indicates the time of data collection The number of frequencies included in the mid-to-high frequency region; Indicates the first Each frequency at the time of acquisition The first ratio of the Euclidean distance of the corrected complex impedance value from the center of the fitted circle to the Euclidean distance of the corrected complex impedance value of its left nearest neighbor frequency from the center of the fitted circle. Indicates the first Each frequency at the time of acquisition The ratio of the Euclidean distance of the corrected complex impedance value from the center of the fitted circle to the second ratio of the Euclidean distance of the corrected complex impedance value of the right nearest neighbor frequency from the center of the fitted circle. If the first frequency in the mid-to-high frequency region at a single acquisition time has no left nearest neighbor frequency, then its first ratio is 0; if the last frequency has no right nearest neighbor frequency, then its second ratio is 0.

[0060] By calculating the standard deviation of the Euclidean distance, the overall irregularity of the distribution of complex impedance values ​​at all frequency points in the mid-to-high frequency region can be assessed. A larger standard deviation indicates a greater difference in the distance between the complex impedance values ​​at different frequencies and the center of the fitted circle, and a more irregular overall distribution. By calculating the difference between the ratio of the Euclidean distance of each frequency point to its nearest neighbor frequency point, the irregularity of the distribution at local frequency points can be assessed. A larger difference indicates a greater difference in the distance between the complex impedance values ​​at local frequency points and the center of the fitted circle, and a more irregular local distribution.

[0061] Compared with existing technologies that use the fitting radius as the evaluation standard for lithium battery performance, this invention considers the radius difference of different frequencies in the mid-to-high frequency region, which can identify the phenomenon of irregular semi-circular distribution and avoid the phenomenon of large performance evaluation error caused by low performance or smoothed capacitance of the lithium battery under test, resulting in a small fitting radius.

[0062] S4: Calculate the low-frequency irregularity at each acquisition moment using the corrected complex impedance sequence of frequencies in the low-frequency region.

[0063] In the electrochemical impedance spectroscopy (EIS) of lithium-ion batteries, particularly in the low-frequency region, the slow lithium-ion diffusion process, controlled by concentration polarization, restricts ion migration, causing the imaginary part of the complex impedance value to increase linearly with decreasing frequency, forming a slope of approximately 45°. When the diffusion process is hindered, such as by membrane blockage, changes in electrode material structure, or electrolyte depletion, the angle of the slope will either decrease or increase. Specifically, membrane blockage obstructs ion migration paths, resulting in a smaller slope angle; while electrolyte depletion increases ion migration resistance, resulting in a larger slope angle. This irregular change becomes increasingly apparent with increasing charge-discharge cycles and is closely related to the degree of battery aging. Therefore, by analyzing the low-frequency irregularities of the complex impedance value of lithium-ion batteries, the effective lifespan and performance of lithium-ion batteries can be accurately assessed, providing an important basis for diagnosing battery health.

[0064] The method for calculating the low-frequency irregularity at a single acquisition moment includes: calculating the third ratio of the difference between the imaginary and real parts of the modified complex impedance values ​​of a single frequency and its left nearest neighbor frequency at that acquisition moment; calculating the fourth ratio of the difference between the imaginary and real parts of the modified complex impedance values ​​of a single frequency and its right nearest neighbor frequency at that acquisition moment; multiplying the absolute value of the difference between the third ratio and 1 and the absolute value of the difference between the fourth ratio and 1 to obtain the product corresponding to the single frequency; and summing the products corresponding to all frequencies in the low-frequency region at that acquisition moment to obtain the low-frequency irregularity at that acquisition moment. The specific formula is as follows:

[0065]

[0066] In the formula, Indicates the time of data collection for the lithium battery under test. Low-frequency irregularity; Indicates the time of data collection The number of frequencies contained in the low-frequency region; Indicates the first The frequency and its left nearest neighbor frequency at the acquisition time The third ratio of the difference between the imaginary and real parts of the corrected complex impedance value; No. The frequency and its right nearest neighbor frequency at the acquisition time The fourth ratio of the difference between the imaginary and real parts of the corrected complex impedance value.

[0067] Since the starting point of the low-frequency sloping line is not the origin of the Nyquist plot coordinate axis, the ratio of the difference between the imaginary part and the real part of the difference with the adjacent frequency is used to determine whether it fits 45°. The closer the ratio is to 1, the closer it is to 45°. The further the ratio deviates from 1, the more irregular the sloping line is, which may be related to the obstruction of the diffusion process inside the battery, such as membrane blockage, changes in electrode material structure, or electrolyte depletion.

[0068] Compared with existing technologies that use the fitting radius as the evaluation standard for lithium battery performance, this invention analyzes the performance differences of the lithium battery under test at low frequencies, identifies diffusion anomalies caused by multiple charge-discharge cycles, and accurately evaluates the performance of lithium batteries.

[0069] S5: Calculate the performance score of the lithium battery under test based on the fitting radius, mid-to-high frequency irregularity, and low-frequency irregularity averaged at all acquisition times during a single charge-discharge process.

[0070] In the Nyquist plot of a lithium battery, the semicircular diameter of the mid-to-high frequency impedance spectrum is close to the charge transfer resistance. Its size indicates the charge transfer rate. The smaller the value, the faster the transfer and the longer the cycle life. Therefore, the smaller the average fitting radius, the better the performance. The more regular the distribution of its complex impedance values, the larger the performance score and the better the performance.

[0071] Therefore, the calculation method for the performance score of the lithium battery under test includes: for a single charge-discharge process, calculating the average value of the fitting radius of the mid-to-high frequency impedance spectrum, the average value of the mid-to-high frequency irregularity, and the average value of the low-frequency irregularity at all sampling times; calculating the product of the average fitting radius and the average value of the mid-to-high frequency irregularity, and performing negative correlation normalization on the resulting product to obtain the first normalized value; performing negative correlation normalization on the average value of the low-frequency irregularity to obtain the second normalized value; and summing the first and second normalized values ​​to obtain the performance score of the lithium battery under test. The specific formula is as follows:

[0072]

[0073] In the formula, This indicates the performance score of the lithium battery under test during this charge and discharge process; This represents the average value of the fitted radius of the mid-to-high frequency impedance spectrum at all acquisition times; This represents the average value of the mid-to-high frequency irregularity across all data acquisition times. This represents the average low-frequency irregularity across all data acquisition times. Represented by natural numbers An exponential function with base 1. The smaller the average value of the fitted radius, the average value of the mid-to-high frequency irregularity, and the average value of the low-frequency irregularity, the higher the performance score of the lithium battery under test.

[0074] S6: Compare the obtained performance score of the lithium battery under test with the preset qualified threshold. If the performance score is greater than or equal to the preset qualified threshold, proceed to the next charge-discharge cycle. Otherwise, stop the charge-discharge cycle and determine whether the total number of charge-discharge cycles is greater than the preset number. If yes, the lithium battery under test is deemed to be qualified; otherwise, it is deemed unqualified.

[0075] After obtaining the performance score of the lithium battery under test during this charge-discharge process, the performance score is normalized. In this embodiment, the preset pass threshold is 0.9. If the normalized performance score is greater than or equal to 0.9, the next charge-discharge cycle continues. If the normalized performance score is less than 0.9, it is determined whether the total number of charge-discharge cycles up to the current moment is greater than the preset number. If the total number of charge-discharge cycles is greater than the preset number, the lithium battery under test is deemed to be qualified; otherwise, it is deemed unqualified. The preset pass threshold and the preset number of cycles can be set based on previous experimental experience.

[0076] Compared with existing technologies that use the fitting radius as the evaluation standard for lithium battery performance, this invention can take into account the mid-to-high frequency irregularities and low-frequency irregularities of the complex impedance distribution in time sequence during each charge-discharge cycle of the lithium battery, thereby improving the confidence of lithium battery performance evaluation.

[0077] The lithium battery performance testing system for lithium battery production of the present invention includes a processor and a memory. The memory stores computer program instructions, and when the computer program instructions are executed by the processor, the lithium battery performance testing method for lithium battery production according to the above embodiments of the present invention is implemented.

[0078] The system also includes other components well known to those skilled in the art, such as communication buses and communication interfaces, the settings and functions of which are known in the art and will not be described in detail here.

[0079] It should be noted that those skilled in the art can make various modifications and improvements without departing from the inventive concept, and these all fall within the scope of protection of this invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for detecting performance of a lithium battery for production of the lithium battery, characterized by, The method comprises the following steps: Obtain the surface temperature and electrochemical impedance spectrum of the qualified lithium battery at each collection time in a single charging and discharging process, and obtain the first comprehensive temperature sequence corresponding to the single charging and discharging process and the first complex impedance sequence of each frequency after preprocessing, and obtain the second comprehensive temperature sequence and the second complex impedance sequence of each frequency of the lithium battery to be tested in the same way; Calculate the performance score of the lithium battery to be tested, if the performance score is greater than or equal to the preset qualified threshold, the next charging and discharging cycle is carried out, otherwise the charging and discharging cycle is stopped, and whether the total number of charging and discharging cycles is greater than the preset number is judged, and if the total number of charging and discharging cycles is greater than the preset number, it is determined that the performance of the lithium battery to be tested is qualified; The calculation method of the performance score of the lithium battery to be tested comprises: According to the first comprehensive temperature sequence and the first complex impedance sequence, the second complex impedance sequence is corrected to obtain the corrected complex impedance sequence of each frequency; the corrected complex impedance sequence of the frequency in the middle and high frequency region is used to fit the middle and high frequency impedance spectrum at each collection time to obtain the fitting center and fitting radius of each middle and high frequency impedance spectrum, and the corrected complex impedance sequence of the frequency in the middle and high frequency region and the fitting center are used to calculate the middle and high frequency irregularity at each time; the corrected complex impedance sequence of the frequency in the low frequency region is used to calculate the low frequency irregularity at each collection time; and the average values of the fitting radius, the middle and high frequency irregularity and the low frequency irregularity at all collection times in the single charging and discharging process are used to calculate the performance score of the lithium battery to be tested.

2. The method for detecting performance of a lithium battery for production of a lithium battery according to claim 1, characterized by, Before correcting the second complex impedance sequence, the first comprehensive temperature sequence is subjected to first-order differential processing to obtain the first comprehensive temperature differential sequence, and the first complex impedance sequence is subjected to first-order differential processing to obtain the first complex impedance differential sequence; during the electrochemical impedance spectrum test, the frequency region is divided into a low frequency region, a middle frequency region and a high frequency region, for a single frequency region, the first real part temperature coefficient and the first imaginary part temperature coefficient are calculated based on the first complex impedance sequence and the first comprehensive temperature sequence of each frequency in the frequency region during each charging and discharging process, and the second real part temperature coefficient and the second imaginary part temperature coefficient are calculated based on the first complex impedance differential sequence and the first comprehensive temperature differential sequence. 3.The lithium battery performance detection method for lithium battery production of claim 2, wherein, The calculation method of the first real part temperature coefficient in the single frequency region comprises: dividing the first complex impedance sequence of each frequency in the frequency region into a first real part sequence and a first imaginary part sequence; for each charging and discharging process, the Pearson correlation coefficient value of the first real part sequence and the first comprehensive temperature sequence of each frequency in the frequency region is calculated, the sum of the Pearson correlation coefficient values obtained in all charging and discharging processes of the qualified lithium battery is calculated, the product of the number of charging and discharging cycles of the qualified lithium battery and the number of frequencies contained in the frequency region is calculated, the sum of the Pearson correlation coefficient values is multiplied by the reciprocal of the obtained product to obtain the first real part temperature coefficient in the frequency region.

4. The method for detecting performance of a lithium battery for lithium battery production according to claim 3, characterized in that, The method for correcting the second complex impedance sequence of a single frequency includes: obtaining a first real part temperature coefficient, a first imaginary part temperature coefficient, a second real part temperature coefficient and a second imaginary part temperature coefficient of a frequency region to which the frequency belongs; each complex impedance value in the second complex impedance sequence includes a real part and an imaginary part, the real part is corrected based on the first real part temperature coefficient, the second real part temperature coefficient and a comprehensive temperature value, and the imaginary part is corrected based on the first imaginary part temperature coefficient and the second imaginary part temperature coefficient, to obtain a corrected complex impedance value of each complex impedance value.

5. The method for detecting performance of a lithium battery for production of a lithium battery according to claim 4, characterized by, The second comprehensive temperature sequence includes a plurality of comprehensive temperature values, a first-order difference processing is performed on the second comprehensive temperature sequence to obtain a second comprehensive temperature difference sequence formed by comprehensive temperature difference values, and the correction method of the real part includes: for a collection time corresponding to the complex impedance value, calculating a product of the first real part temperature coefficient and a comprehensive temperature value corresponding to the collection time, calculating a product of the second real part temperature coefficient and a comprehensive temperature difference value corresponding to the collection time, calculating a sum value of the two products, and adding the real part of the complex impedance value to the obtained sum value to complete the correction of the real part.

6. The method for detecting performance of a lithium battery for lithium battery production according to claim 1, characterized in that, The calculation method of the middle-high frequency irregularity of a single collection time includes: calculating the Euclidean distance between the corrected complex impedance value of each frequency in the middle-high frequency region at the collection time and the fitting circle center of the middle-high frequency impedance spectrum at the collection time, and calculating the standard deviation of all Euclidean distances; calculating a first ratio of the Euclidean distance between the single frequency and the fitting circle center of the distance between the left nearest neighbor frequency and the right nearest neighbor frequency, calculating a second ratio of the Euclidean distance between the single frequency and the fitting circle center of the distance between the left nearest neighbor frequency and the right nearest neighbor frequency, calculating the absolute value of the difference between the first ratio and the second ratio, adding all the absolute values of all frequencies in the middle-high frequency region, and multiplying the obtained sum value by the standard deviation of the Euclidean distance to obtain the middle-high frequency region irregularity at the collection time.

7. The method for detecting performance of lithium battery for lithium battery production according to claim 1, characterized in that, The calculation method of the low frequency irregularity of a single collection time includes: calculating a third ratio of the difference between the imaginary part and the real part of the corrected complex impedance value of the single frequency and the left nearest neighbor frequency at the collection time; calculating a fourth ratio of the difference between the imaginary part and the real part of the corrected complex impedance value of the single frequency and the right nearest neighbor frequency at the collection time; multiplying the absolute value of the difference between the third ratio and 1 by the absolute value of the difference between the fourth ratio and 1 to obtain a product corresponding to the single frequency; adding the products corresponding to all frequencies in the low frequency region at the collection time to obtain the low frequency irregularity at the collection time. 8.The method for detecting performance of lithium battery for lithium battery production according to claim 1, characterized in that, The calculation method of the performance score of the to-be-tested lithium battery includes: for a single charge-discharge process, calculating the average value of the fitting radius of the middle-high frequency impedance spectrum, the average value of the middle-high frequency irregularity and the average value of the low frequency irregularity at all collection times; calculating the product of the average value of the fitting radius and the average value of the middle-high frequency irregularity, and performing negative correlation normalization processing on the obtained product to obtain a first normalized value, and performing negative correlation normalization processing on the average value of the low frequency irregularity to obtain a second normalized value; and calculating the sum of the first normalized value and the second normalized value to obtain the performance score of the to-be-tested lithium battery. 9.The lithium battery performance detection method for lithium battery production of claim 1, wherein, The method for obtaining the first comprehensive temperature sequence comprises: adopting a DBSCAN algorithm to cluster all surface temperature values collected at a single collection time to form a plurality of clustering clusters; calculating the ratio of the number of surface temperature values contained in each clustering cluster to the number of all surface temperature values, multiplying the ratio by the average value of the surface temperature values of the corresponding clustering cluster to obtain a single temperature value of the single clustering cluster, summing up the single temperature values of all clustering clusters corresponding to the single collection time to obtain a comprehensive temperature value corresponding to the single collection time; and sorting all comprehensive temperature values according to the chronological order of collection time to obtain the first comprehensive temperature sequence of the qualified lithium battery in a single charge-discharge cycle.

10. A lithium battery performance detection system for lithium battery production, characterized in that, Comprise: A processor and a memory, the memory stores computer program instructions, when the computer program instructions are executed by the processor, the method for detecting the performance of the lithium battery for the production of the lithium battery according to any one of claims 1-9 is realized.

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