A two-stage rapid screening method for K-values ​​in lithium-ion batteries

Abstract

This invention provides a two-stage rapid screening method for K-values ​​of lithium-ion batteries, belonging to the field of battery testing technology. The method involves: performing capacity-controlled charge-discharge processing on the battery cell to obtain its charge-discharge curve, determining the inflection point voltage, and finally charging the cell to above the inflection point voltage; allowing the cell to cool and settle; performing a voltage test on the cell to obtain a first test voltage and recording the test time; placing the cell in a high-temperature constant-temperature chamber for settling, and then performing a voltage test to obtain a second test voltage and recording the test time; calculating the high-temperature K-value using the difference between the first and second test voltages and the test time difference, and discarding cells with high-temperature K-values ​​greater than the high-temperature K-value threshold; placing the qualified cells in a room-temperature settling area, and then performing a voltage test to obtain a third test voltage and recording the test time; calculating the room-temperature K-value using the difference between the second and third test voltages and the test time difference, and discarding cells with room-temperature K-values ​​greater than the room-temperature K-value threshold, with the remaining cells being deemed qualified.

Description

Technical Field

[0001] This invention belongs to the field of battery testing technology. Specifically, this invention relates to a two-stage rapid screening method for K-values ​​of lithium-ion batteries. Background Technology

[0002] The self-discharge performance of lithium-ion batteries is one of the core indicators for evaluating battery quality and battery pack lifespan. The industry typically uses the K-value (the rate of decrease of open-circuit voltage per unit time, unit: mV / h) to quantify the degree of battery self-discharge. A higher K-value indicates higher self-discharge, potentially indicating internal issues such as micro-short circuits, separator defects, and metallic foreign objects. Long-term use will lead to a significant decline in battery consistency, electrical performance, and safety. Therefore, K-value screening is a crucial step in quality control after battery formation and capacity testing.

[0003] Chinese Patent 118080396A discloses a battery self-discharge screening method, apparatus, and storage medium. The battery self-discharge screening method includes: determining a set of batteries to be screened; first, according to production time sequence, extracting multiple batteries from the set to be screened to form a parent group, and determining an initial range of K-value specifications based on the K-value distribution of the batteries within the parent group; then, sequentially extracting individual batteries as the current screening batteries and performing individual battery screening operations. This embodiment of the invention extracts the current screening batteries one by one from the set of batteries to be screened according to production time sequence, and continuously updates the composition of the parent groups and updates the K-value specifications based on the new parent groups.

[0004] The existing technology uses a long-term, room-temperature settling method, which involves placing the battery in an environment of 25±2℃ for 7-14 days and calculating the K-value by measuring the change in open-circuit voltage before and after the settling period. While this method can accurately eliminate abnormal cells, the testing cycle is too long. Currently, there is also a high-temperature accelerated settling method, which uses a high-temperature environment of 45-60℃ to shorten the settling time. However, a single high-temperature test has the problems of damaging battery performance and insufficient screening accuracy. Summary of the Invention

[0005] The present invention aims to provide a two-stage rapid screening method for K-values ​​of lithium-ion batteries, so as to improve the detection efficiency and accuracy of the battery cells.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] This invention provides a two-stage rapid screening method for K-values ​​in lithium-ion batteries, comprising the following steps:

[0008] Step 1: Perform capacity grading and charging / discharging on the battery cells to obtain the battery charge / discharge curve and determine the inflection point voltage (V0). Charge the battery to above the inflection point voltage during the last charge-discharge process.

[0009] Step 2: Allow the battery cells to cool to room temperature to obtain the test group of battery cells;

[0010] Step 3: Perform the first open-circuit voltage test on the test group cells to obtain the first test voltage (V1), and record the first voltage test time (T1).

[0011] Step 4: Place the battery cell in a high-temperature constant temperature chamber and let it stand in the high-temperature environment for a preset time. After the standing time is completed, obtain the second test voltage (V2) through the second open circuit voltage test and record the second voltage test time (T2).

[0012] Step 5: Calculate the high-temperature K value by the ratio of the difference between the first test voltage and the second test voltage to the time difference between the two tests. Compare the high-temperature K value with the high-temperature K value threshold and remove cells with a high-temperature K value greater than the high-temperature K value threshold.

[0013] Step 6: Place the qualified battery cells from Step 5 into the room temperature resting area and let them rest for a preset time. Then, perform the third open circuit voltage test to obtain the third test voltage (V3) and record the third voltage test time (T3).

[0014] Step 7: Calculate the room temperature K value by the ratio of the difference between the second and third test voltages to the time difference between the two tests. Compare the room temperature K value with the room temperature K value threshold, and remove cells whose room temperature K value is greater than the room temperature K value threshold. The remaining cells are judged to be qualified.

[0015] In step one, the method for determining the inflection point voltage is as follows: the inflection point is the point on the battery charge-discharge curve where the voltage-capacity relationship changes from the plateau region to the slope region.

[0016] In step four, the temperature of the high-temperature constant temperature chamber is set to 45℃, and the preset settling time is set to 48 hours.

[0017] In step five, the formula for calculating the high-temperature K value is:

[0018] ;

[0019] Where V1 is the first test voltage, V2 is the second test voltage, T1 is the first voltage test time, and T2 is the second voltage test time.

[0020] In step five, the high-temperature K value threshold is set to 0.15mV / h.

[0021] In step seven, the formula for calculating the K value at room temperature is:

[0022] ;

[0023] Where V2 is the second test voltage, V3 is the third test voltage, T3 is the test time for the third voltage, and T2 is the test time for the second voltage.

[0024] In step seven, the threshold value for K at room temperature is set to 0.015 mV / h.

[0025] The open-circuit voltage test in steps three, four, and six uses a four-wire measurement method.

[0026] In step six, the temperature of the room temperature settling area is 25°C, and the preset settling time is 72 hours.

[0027] The technical effects of this invention are as follows:

[0028] (1) This invention effectively reduces the safety risks of long-term use of battery cells. The qualified battery cells selected by this method have high-temperature K values ​​and room-temperature K values ​​that are both below the threshold, indicating that there are no obvious micro-short circuits, separator defects or metal foreign objects inside the battery cells. After these battery cells are assembled, the voltage consistency decay is slower during long-term cycling, and the risk of thermal runaway is significantly reduced, thereby improving the life and safety reliability of the battery pack.

[0029] (2) This invention shortens the testing cycle and improves production line efficiency. By adopting a two-stage strategy of high-temperature screening and room-temperature screening, this invention reduces the total testing time to about five days, which is highly compatible with the high-efficiency operation requirements of soft-pack lithium iron phosphate battery production lines, significantly reducing waiting time and capital occupation, and improving production efficiency.

[0030] (3) The present invention has high screening accuracy and low misjudgment rate. The present invention effectively eliminates the interference of side reactions on self-discharge assessment under high temperature environment and avoids misjudgment caused by the difference between high temperature and room temperature self-discharge mechanisms. Compared with single room temperature long-term static placement, the present invention shortens the test cycle while ensuring higher accuracy.

[0031] (4) This invention introduces the concept of inflection point voltage, charging the battery after capacity grading to above the inflection point voltage, placing it in a low SOC state. In this state, the voltage-capacity curve is in the slope region, voltage changes are sensitive to capacity loss, and the self-discharge current is mainly micro-short-circuit leakage current. This enables the high-temperature initial screening stage to more sensitively identify cells with micro-short-circuit hazards, improving the defect detection probability.

[0032] (5) The present invention sets a high temperature K value threshold and a normal temperature K value threshold respectively. The former is used to quickly eliminate cells with high temperature self-discharge risk, and the latter is used to finely screen cells with excessive self-discharge at normal temperature, thus avoiding the problem of missed detection caused by screening with a single threshold. Attached Figure Description

[0033] This manual includes the following figures, which illustrate the following:

[0034] Figure 1 This is a flowchart of a two-stage rapid K-value screening method for lithium-ion batteries according to the present invention;

[0035] Figure 2 This invention provides a method for rapid screening of lithium-ion battery values ​​using a two-stage K-value method, showing the voltage-capacity curves of a soft-pack lithium iron phosphate battery in different SOC ranges.

[0036] Figure 3 This is a scatter plot of high-temperature K-values ​​for a two-stage rapid K-value screening method for lithium-ion batteries according to the present invention.

[0037] Figure 4 This is a scatter plot of K-values ​​at room temperature for a two-stage rapid K-value screening method for lithium-ion batteries according to the present invention.

[0038] Figure 5 This is a high-temperature voltage variation curve of a two-stage rapid K-value screening method for lithium-ion batteries according to the present invention. Detailed Implementation

[0039] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings, in order to help those skilled in the art to have a more complete, accurate and in-depth understanding of the inventive concept and technical solution of the present invention, and to facilitate its implementation.

[0040] This invention provides a two-stage rapid screening method for K-values ​​in lithium-ion batteries, comprising the following steps:

[0041] Step 1: Perform capacity grading and charging / discharging on the battery cells to obtain the battery charge / discharge curve and determine the inflection point voltage (V0). Charge the battery to above the inflection point voltage during the last charge-discharge process.

[0042] Step 2: Allow the battery cells to cool to room temperature to obtain the test group of battery cells;

[0043] Step 3: Perform the first open-circuit voltage test on the test group cells to obtain the first test voltage (V1), and record the first voltage test time (T1).

[0044] Step 4: Place the battery cell in a high-temperature constant temperature chamber and let it stand in the high-temperature environment for a preset time. After the standing time is completed, obtain the second test voltage (V2) through the second open circuit voltage test and record the second voltage test time (T2).

[0045] Step 5: Calculate the high-temperature K value by the ratio of the difference between the first test voltage and the second test voltage to the time difference between the two tests. Compare the high-temperature K value with the high-temperature K value threshold and remove cells with a high-temperature K value greater than the high-temperature K value threshold.

[0046] Step 6: Place the qualified battery cells from Step 5 into the room temperature resting area and let them rest for a preset time. Then, perform the third open circuit voltage test to obtain the third test voltage (V3) and record the third voltage test time (T3).

[0047] Step 7: Calculate the room temperature K value by the ratio of the difference between the second and third test voltages to the time difference between the two tests. Compare the room temperature K value with the room temperature K value threshold, and remove cells whose room temperature K value is greater than the room temperature K value threshold. The remaining cells are judged to be qualified.

[0048] In step one, the method for determining the inflection point voltage is as follows: the inflection point is the point on the battery charge-discharge curve where the voltage-capacity relationship changes from the plateau region to the slope region.

[0049] In step four, the temperature of the high-temperature constant temperature chamber is set to 45℃, and the preset settling time is set to 48 hours.

[0050] In step five, the formula for calculating the high-temperature K value is:

[0051] ;

[0052] Where V1 is the first test voltage, V2 is the second test voltage, T1 is the first voltage test time, and T2 is the second voltage test time.

[0053] In step five, the high-temperature K value threshold is set to 0.15mV / h.

[0054] In step seven, the formula for calculating the K value at room temperature is:

[0055] ;

[0056] Where V2 is the second test voltage, V3 is the third test voltage, T3 is the test time for the third voltage, and T2 is the test time for the second voltage.

[0057] In step seven, the threshold value for K at room temperature is set to 0.015 mV / h.

[0058] The open-circuit voltage test in steps three, four, and six uses a four-wire measurement method.

[0059] In step six, the temperature of the room temperature settling area is 25°C, and the preset settling time is 72 hours.

[0060] The present invention will now be described in detail with reference to the embodiments.

[0061] The battery cells undergo a capacity grading and charging / discharging process to obtain the battery charge / discharge curve and determine the inflection point voltage (V0). The selection of the inflection point voltage V0 is based on the following criteria: Soft-pack lithium iron phosphate batteries exhibit a clear voltage-capacity correlation within the 2.5V-V0 range, facilitating accurate reflection of capacity loss through voltage changes; simultaneously, at low SOC, the self-discharge current is primarily leakage current, resulting in higher sensitivity for detecting micro-short circuit defects. The cells are then charged to above the inflection point voltage during the final capacity grading process.

[0062] In this embodiment, 200 50Ah soft-pack lithium iron phosphate A-grade batteries were selected and subjected to charge-discharge capacity testing in a capacity testing cabinet according to standard procedures to obtain charge-discharge curves, such as... Figure 2 As shown, the last charge to 5% SOC corresponds to an open-circuit voltage of 3.25V.

[0063] The cells were allowed to cool to room temperature (25±2℃) to obtain the test group cells; the cooling time was 3 hours to ensure uniform internal temperature of the cells.

[0064] The first open-circuit voltage test was performed on the battery cells in the test group to obtain the first test voltage (V1), and the test time (T1) was recorded. The open-circuit voltage test adopted a four-wire measurement method with a test accuracy of ±0.1mV. Before the test, the battery cells were kept at a constant temperature in the test environment for more than 30 minutes to ensure voltage stability. In the embodiment, the average value of the first test voltage was 3.251V, with a standard deviation of 0.8mV.

[0065] The battery cell is placed in a high-temperature constant temperature chamber and left to stand in the high-temperature environment for a preset time. After the standing time is completed, a second test voltage (V2) is obtained through a second open-circuit voltage test, and the test time (T2) of the second voltage is recorded. In this embodiment, the temperature of the high-temperature constant temperature chamber is set to 45°C, the humidity is 50%RH, and the preset standing time is 48 hours.

[0066] The high-temperature K value is calculated by the ratio of the difference between the first and second test voltages to the time difference between the two tests. The formula for calculating the high-temperature K value is as follows:

[0067] ;

[0068] Where V1 is the first test voltage, V2 is the second test voltage, T1 is the first voltage test time, and T2 is the second voltage test time.

[0069] The high-temperature K-value is compared with a high-temperature K-value threshold. Cells with a high-temperature K-value greater than the high-temperature K-value threshold are identified as cells with high side reaction rates and are discarded. In this embodiment, the high-temperature K-value threshold is set to 0.15mV / h.

[0070] The battery cells that passed the high-temperature test were placed in a room-temperature resting area (temperature 25±2℃, humidity 60%RH) and left for a preset time. After that time, a third open-circuit voltage test was performed to obtain the third test voltage (V3), and the test time (T3) was recorded. In this embodiment, the preset resting time was 72 hours.

[0071] The K value at room temperature is calculated by the ratio of the difference between the second and third test voltages to the time difference between the two tests. The formula for calculating the K value at room temperature is:

[0072] ;

[0073] Where V2 is the second test voltage, V3 is the third test voltage, T3 is the test time for the third voltage, and T2 is the test time for the second voltage.

[0074] The room temperature K value is compared with a room temperature K value threshold. Cells with room temperature K values ​​greater than the room temperature K value threshold are discarded, and the remaining cells are determined to be cells that pass the self-discharge test. In this example, the room temperature K value is 0.015mV / h.

[0075] The present invention also includes two comparative examples.

[0076] Comparative Example 1: 100 50Ah soft-pack lithium iron phosphate batteries from the same batch were selected, and K-value tests were conducted by standing at 45℃ for 120 hours and at room temperature for 48 hours. The specific implementation method is based on the example above.

[0077] Comparative Example 2: 100 50Ah soft-pack lithium iron phosphate batteries from the same batch were selected, and K-value tests were conducted by simply leaving them at room temperature for 240 hours. Only the K-value at room temperature was calculated.

[0078] The following table compares the false positive rates of cell qualification screening with those of the present invention.

[0079] Example 1 120h <0.2% Comparative Example 1 168h <15% Comparative Example 2 240h <1%

[0080] Table 1

[0081] The beneficial effects of the present invention are described in detail below.

[0082] This invention effectively reduces the safety risks of long-term battery cell use. Qualified cells selected using this method have high-temperature K-values ​​and room-temperature K-values ​​both below the threshold, indicating the absence of significant micro-short circuits, separator defects, or metallic foreign objects within the cell. When these cells are assembled into a battery pack, the voltage consistency decays more slowly during long-term cycling, significantly reducing the risk of thermal runaway, thereby improving the battery pack's lifespan and reliability.

[0083] This invention shortens the testing cycle and improves production line efficiency. Through a two-stage strategy of high-temperature screening and room-temperature screening, the total testing time is reduced to approximately five days, which highly matches the high-efficiency operation requirements of pouch lithium iron phosphate battery production lines, significantly reducing waiting time and capital tied up, and improving production efficiency.

[0084] This invention offers high screening accuracy and a low false positive rate. It effectively eliminates the interference of side reactions on self-discharge assessment under high-temperature conditions, avoiding misjudgments caused by differences in self-discharge mechanisms between high and room temperature. Compared to prolonged static placement at room temperature, this invention shortens the testing cycle while maintaining higher accuracy.

[0085] This invention introduces the concept of inflection point voltage, charging the battery after capacity grading to above the inflection point voltage, placing it in a low state of charge (SOC) state. In this state, the voltage-capacity curve is in a sloping region, voltage changes are sensitive to capacity loss, and the self-discharge current is mainly composed of micro-short-circuit leakage current. This allows for more sensitive identification of cells with potential micro-short-circuit hazards during the high-temperature initial screening stage, improving the defect detection probability.

[0086] This invention sets a high-temperature K-value threshold and a room-temperature K-value threshold, respectively. The former is used to quickly eliminate battery cells with high-temperature self-discharge risk, while the latter is used to finely screen battery cells with excessive self-discharge at room temperature, thus avoiding the problem of missed detection caused by screening with a single threshold.

[0087] The present invention has been described above by way of example with reference to the accompanying drawings. Obviously, the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvements made using the inventive concept and technical solution; or the direct application of the inventive concept and technical solution to other situations without modification, are all within the protection scope of the present invention.

Claims

1. A two-stage rapid screening method for K-values ​​in lithium-ion batteries, characterized in that, Includes the following steps: Step 1: Perform capacity grading and charging / discharging on the battery cells to obtain the battery charge / discharge curve and determine the inflection point voltage (V0). Charge the battery to above the inflection point voltage during the last charge-discharge process. Step 2: Allow the battery cells to cool to room temperature to obtain the test group of battery cells; Step 3: Perform the first open-circuit voltage test on the test group cells to obtain the first test voltage (V1), and record the first voltage test time (T1). Step 4: Place the battery cell in a high-temperature constant temperature chamber and let it stand in the high-temperature environment for a preset time. After the standing time is completed, obtain the second test voltage (V2) through the second open circuit voltage test and record the second voltage test time (T2). Step 5: Calculate the high-temperature K value by the ratio of the difference between the first test voltage and the second test voltage to the time difference between the two tests. Compare the high-temperature K value with the high-temperature K value threshold and remove cells with a high-temperature K value greater than the high-temperature K value threshold. Step 6: Place the qualified battery cells from Step 5 into the room temperature resting area and let them rest for a preset time. Then, perform the third open circuit voltage test to obtain the third test voltage (V3) and record the third voltage test time (T3). Step 7: Calculate the room temperature K value by the ratio of the difference between the second and third test voltages to the time difference between the two tests. Compare the room temperature K value with the room temperature K value threshold, and remove cells whose room temperature K value is greater than the room temperature K value threshold. The remaining cells are judged to be qualified.

2. The two-stage rapid K-value screening method for lithium-ion batteries as described in claim 1, characterized in that: In step one, the method for determining the inflection point voltage is as follows: the inflection point is the point on the battery charge-discharge curve where the voltage-capacity relationship changes from the plateau region to the slope region.

3. The two-stage rapid K-value screening method for lithium-ion batteries as described in claim 1, characterized in that: In step four, the temperature of the high-temperature constant temperature chamber is set to 45℃, and the preset settling time is set to 48 hours.

4. The two-stage rapid K-value screening method for lithium-ion batteries as described in claim 1, characterized in that: In step five, the formula for calculating the high-temperature K value is: ； Where V1 is the first test voltage, V2 is the second test voltage, T1 is the first voltage test time, and T2 is the second voltage test time.

5. The two-stage rapid K-value screening method for lithium-ion batteries as described in claim 1, characterized in that: In step five, the high-temperature K value threshold is set to 0.15mV / h.

6. The two-stage rapid screening method for K-values ​​of lithium-ion batteries as described in claim 1, characterized in that: In step seven, the formula for calculating the K value at room temperature is: ； Where V2 is the second test voltage, V3 is the third test voltage, T3 is the test time for the third voltage, and T2 is the test time for the second voltage.

7. The two-stage rapid K-value screening method for lithium-ion batteries as described in claim 1, characterized in that: In step seven, the threshold value for K at room temperature is set to 0.015 mV / h.

8. The two-stage rapid screening method for K-values ​​of lithium-ion batteries as described in claim 1, characterized in that: The open-circuit voltage test in steps three, four, and six uses a four-wire measurement method.

9. The two-stage rapid screening method for K-values ​​of lithium-ion batteries as described in claim 1, characterized in that: In step six, the temperature of the room temperature settling area is 25°C, and the preset settling time is 72 hours.

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