A method for screening the self-discharge consistency of lithium-ion batteries

Abstract

This invention relates to a method for screening the self-discharge consistency of lithium-ion batteries, comprising: S1: fully charging n formed cells, letting them rest, and then discharging them at a constant current to 2.5V to obtain a discharge curve, and recording the inflection point voltage U1 on the curve; S2: recording the voltage value corresponding to ≤U1 and greater than 3% SOC on the curve as U2; charging the cells to U2; S3: letting all cells rest for T1, then letting them rest at high temperature for T2, and then letting them rest at room temperature for T3, and then testing the open circuit voltage OCV2; then letting them rest at room temperature for T4, and then testing the open circuit voltage OCV3; S4: calculating the K value of each cell; S5: calculating the average value α and standard deviation σ of all cell K values; S6: comparing the K value of each cell with the value of α+xσ to screen out the K values ​​within the group. i Cells with a polarization greater than α+xσ. This method reduces testing time, minimizes the impact of temperature and polarization on the test, and yields more accurate screening results.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a method for screening the self-discharge consistency of lithium-ion batteries. Background Technology

[0002] With the increasing demand for clean and sustainable energy, the electric vehicle and energy storage industries are booming. In the actual use of electric vehicles and energy storage base stations, individual batteries are connected in series and parallel to form groups, serving as power modules or energy storage media. Within a battery pack, the electrochemical performance of the pack is similar to the "barrel principle," where the lowest-performing cell generally determines the upper limit of the entire pack's performance. Therefore, each battery must maintain good consistency; otherwise, phenomena such as overcharging and discharging of individual cells, different battery degradation rates, and temperature differences between batteries will occur, leading to decreased battery pack performance, shortened lifespan, and increased safety hazards. Sorting indicators for judging battery pack consistency include capacity, internal resistance, self-discharge rate, and voltage difference between cells within the pack. Self-discharge refers to the phenomenon of a battery consuming energy on its own without performing external work. It is characterized by a decrease in battery charge during non-use storage, manifested as a reduction in battery capacity and voltage. Self-discharge occurs inside the battery and is related to battery materials and manufacturing processes. Furthermore, the self-discharge rate is affected by factors such as battery polarization, state of charge (SOC), aging temperature and time, foreign object short circuits, and chemical reactions, which can lead to missed or false positives. Therefore, accurate and rapid measurement of the self-discharge rate is crucial for screening battery consistency. In addition, severe self-discharge can cause thermal runaway, leading to safety issues; thus, the self-discharge rate is also an important reference indicator in battery safety management.

[0003] Among existing self-discharge screening methods, the commonly used methods include the open-circuit voltage decay test and the tolerance method. Generally, after charging / discharging the battery cell to a preset SOC with a certain current, the battery is placed at high temperature or room temperature for a period of time to test the voltage drop of individual cells, or placed at high temperature or room temperature for a period of time to test the capacity loss of individual cells. The self-discharge level of the battery is determined by comparing the voltage drop or capacity loss of a single cell with a standard voltage drop value or standard capacity loss rate. This method is not only time-consuming, but its accuracy may also be affected by temperature, battery polarization, or battery manufacturing batch, leading to misjudgments or omissions.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a method for screening the self-discharge consistency of lithium-ion batteries, which reduces testing time, reduces the impact of temperature and polarization on testing, and makes the screening results more accurate.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] This invention provides a method for screening the self-discharge consistency of lithium-ion batteries, comprising the following steps:

[0008] S1: Fully charge n formed cells, let them rest at room temperature for 25-30 minutes, then discharge them to 2.5V with a current of 0.33-1C, let them rest for 5-10 minutes, and then discharge them to 2.5V with a current of 0.05-0.1C. Obtain the capacity-voltage curve and record the inflection point voltage corresponding to the relatively low SOC on the curve as U1.

[0009] S2: Record the voltage value on the capacity-voltage curve that is less than or equal to the inflection point voltage U1 and greater than 3% SOC as U2; charge all the cells that have been discharged according to step S1 with a current of 0.5-1C to 90-95% U2, let them rest for 5-10 minutes, and then charge them with a current of 0.05-0.1C to U2.

[0010] S3: After all the cells that have completed step S2 are placed at room temperature for T1, the cells are placed at high temperature for T2, and then placed at room temperature for T3. The open circuit voltage is then tested and set as OCV2. After the cells are placed at room temperature for T4, the open circuit voltage is then tested and set as OCV3.

[0011] S4: Calculate the K value for each cell: K i = (OCV2-OCV3) / T4, where i = any integer from 1 to n;

[0012] S5: Calculate the average value α and standard deviation σ for each cell in the group formed by n formed cells;

[0013] S6: By comparing the K value of each cell with the value of α+xσ within the group, select the K values ​​within the group. i Cells with a value greater than α+xσ are considered unqualified cells and are selected for screening.

[0014] Furthermore, in S1, n is an integer ≥100.

[0015] Furthermore, the full charge in step S1 refers to charging to 3.65V with a constant current and constant voltage of 0.5C.

[0016] Furthermore, in step S1, the battery cell is a lithium iron phosphate battery cell.

[0017] Further, in step S3, T1 is 12-24h, T2 is 46-50h, T3 is 12-24h, and T4 is 70-74h;

[0018] Furthermore, the test voltage of all cells that have completed step S2 after being placed at room temperature for T1 is set as OCV1.

[0019] Furthermore, the ambient temperature is 22-28℃; the high temperature is 40-45℃.

[0020] Furthermore, in step S5, the average value α is calculated according to the following formula:

[0021]

[0022] Furthermore, in step S5, the standard deviation σ is calculated according to the following formula:

[0023]

[0024] Furthermore, in S6, x is any constant from 1 to 3.

[0025] The present invention provides a method for screening the self-discharge consistency of lithium-ion batteries, which has at least one of the following beneficial effects:

[0026] 1. This invention first obtains the discharge curve of a lithium iron phosphate battery by discharging the battery cell to 2.5V at room temperature. The voltage value corresponding to the inflection point U1 and greater than 3% SOC is then denoted as U2 based on the capacity-voltage curve. The battery cell is then charged to U2, allowing for rapid screening of cells with high self-discharge.

[0027] 2. In this invention, after discharging the battery cell at 0.33-1C, it is then discharged again with a small current of 0.05-0.1C to 2.5V. This allows the capacity in the battery cell to be discharged as completely as possible, avoiding the influence of discharge temperature that could lead to incomplete discharge of the battery cell and large voltage errors after subsequent charging.

[0028] 3. This invention involves charging the fully discharged battery cell twice, using a smaller current after 95% U2, thus eliminating the polarization phenomenon of the battery cell during the high current stage and reducing the error caused by voltage polarization during K-value screening.

[0029] 4. The OCV2 test of this invention went through two stages: high temperature and room temperature. High temperature can accelerate the chemical side reaction and shorten the K value screening time, while room temperature can reduce the influence of temperature on OCV2 test, making the OCV2 test value more accurate, and thus the calculated K value is closer to the true value.

[0030] 5. After calculating the battery K-value, this invention compares the K-value of each cell with the average K-value α and standard deviation σ of the group's K-values. i A value ≤α+xσ indicates that the K value is acceptable, which improves the consistency of battery packing.

[0031] 6. This invention improves the consistency of batteries within a group by comparing the K value of an individual cell with the standard deviation within the group, thereby improving the consistency of cell grouping in electric vehicles or energy storage base station applications. Attached Figure Description

[0032] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0033] Figure 1 The process roadmap for screening the self-discharge consistency of lithium-ion batteries provided by this invention;

[0034] Figure 2 The discharge curve of the lithium iron phosphate battery cell provided in Embodiment 1 of the present invention;

[0035] Figure 3 This is a screening result diagram of α+2σ provided in Embodiment 1 of the present invention;

[0036] Figure 4 The image shows the screening results of α+2σ provided in Comparative Example 1 of this invention;

[0037] Figure 5 The screening results diagram of α+2σ provided by the screening process of Comparative Example 2 of the present invention;

[0038] Figure 6 This is a graph showing the screening results of α+2σ for the battery cell tested in Comparative Example 2 of the present invention after screening using the process in Example 1. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Those skilled in the art should understand that the embodiments described are merely illustrative of the invention and should not be considered as specific limitations thereof. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. Process parameters not specifically specified in the following embodiments are generally performed under conventional conditions.

[0040] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0041] A specific embodiment of the present invention provides a method for screening the self-discharge consistency of lithium-ion batteries, comprising the following steps:

[0042] S1: Fully charge n formed cells, let them rest at room temperature for 25-30 minutes, then discharge them to 2.5V with a current of 0.33-1C (e.g., 0.33C, 0.5C, 0.6C, 0.7C, 0.8C, 0.9C), let them rest at room temperature for 5-10 minutes (e.g., 5min, 6min, 8min, 10min), and then discharge them to 2.5V with a current of 0.05-0.1C (e.g., 0.06C, 0.07C, 0.08C, 0.09C). Obtain the capacity-voltage curve, and record the inflection point voltage corresponding to the relatively low SOC on the curve as U1 (i.e., the range where the cell voltage drop changes significantly at a lower SOC).

[0043] S2: Record the voltage value on the capacity-voltage curve that is less than or equal to the inflection point voltage U1 and greater than 3% SOC as U2; charge all the cells that have been discharged according to step S1 with a current of 0.5-1C (e.g., 0.6C, 0.7C, 0.8C, 0.9C) to 90-95% U2, let them rest at room temperature for 5-10 minutes, and then charge them with a current of 0.05-0.1C (e.g., 0.06C, 0.07C, 0.08C, 0.09C) to U2;

[0044] S3: After all the cells that have completed step S2 are placed at room temperature for T1, the test voltage is set as OCV1. Then, the cells are placed at high temperature for T2, and then at room temperature for T3. The open circuit voltage is set as OCV2. Finally, the cells are placed at room temperature for T4. The open circuit voltage is set as OCV3.

[0045] S4: Calculate the K value for each cell: K i = (OCV2-OCV3) / T4, where i = any integer from 1 to n;

[0046] S5: Calculate the average value α and standard deviation σ for each cell in the group formed by n formed cells;

[0047] S6: By comparing the K value of each cell with the value of α+xσ within the group, cells with Ki > α+xσ within the group are selected as unqualified cells. Unqualified cells are those with a large K value deviation.

[0048] This invention first obtains the discharge curves of a group of battery cells by constant current discharge to 2.5V, and then finds the inflection point voltage U1 from the discharge curve of a randomly selected battery cell (see [link to invention]). Figure 2 Then, the voltage corresponding to a self-discharge rate equal to or lower than U1 and greater than 3% SOC is recorded as U2. All cells are then charged to U2. This ensures that the cell screening voltage is located at a position with a steep slope in the discharge curve, allowing for rapid screening of cells with high self-discharge. Before charging or discharging to the final voltage, a small current is used for charging and discharging to reduce the influence of temperature and polarization on the cells, improving the accuracy of K-value screening. The OCV2 test undergoes two stages: high temperature and room temperature. High temperature accelerates chemical side reactions, shortening the K-value screening time, while room temperature reduces the influence of temperature on the OCV2 test, making the OCV2 test value more accurate, thus the calculated K-value is closer to the true value. After calculating the battery K-value, the K-value of each cell is compared with the average K-value α and standard deviation σ of the group's K-values, i.e., K... i ≤α+xσ indicates that the K value is qualified, which improves the consistency of battery packing.

[0049] In addition, in S2, the resting time of 5-10 minutes is to reduce the polarization caused by the large current, so that the OCV measured later is more accurate.

[0050] In an optional embodiment of the present invention, in S1, n is an integer ≥100.

[0051] In a preferred embodiment of the present invention, in step S1, the battery cell is a lithium iron phosphate battery cell.

[0052] As an optional embodiment of the present invention, the full charge in step S1 refers to charging to 3.65V with a constant current and constant voltage of 0.5C.

[0053] As an optional embodiment of the present invention, in S3, T1 is 12-24h (e.g., 14h, 16h, 18h, 20h, 22h), T2 is 46-50h (e.g., 47h, 48h, 49h), T3 is 12-24h (e.g., 14h, 16h, 18h, 20h, 22h, 24h), and T4 is 70-74h (e.g., 70h, 72h, 74h). The time settings of T1, T2, T3, and T4 are based on achieving the filtering purpose.

[0054] And / or, the ambient temperature is 22-28℃ (e.g., 23℃, 24℃, 25℃, 26℃, 27℃); the high temperature is 40-45℃ (e.g., 41℃, 42℃, 43℃, 44℃).

[0055] As an optional embodiment of the present invention, in step S5, the average value α is calculated according to the following formula:

[0056] And / or, the standard deviation σ is calculated according to the following formula:

[0057] In an optional embodiment of the present invention, in step S6, x is any constant from 1 to 3.

[0058] The present invention will now be described in further detail with reference to specific embodiments and comparative examples.

[0059] Example 1

[0060] S1: Take 100 lithium iron phosphate cells from the same batch, charge them to 3.65V with a constant current and constant voltage of 0.5C, let them rest at room temperature for 30 minutes, discharge them to 2.5V at 0.33C, let them rest at room temperature for 10 minutes, and then discharge them to 2.5V with a current of 0.1C. This yields the discharge curve of the lithium iron phosphate cells. Figure 2 Here, we can select the discharge curve of a battery cell to determine U1 and U2, because the discharge curves of batteries in the same batch are not significantly different; record the inflection point voltage corresponding to the relatively low SOC on the curve as U1;

[0061] S2: Determine the value of voltage U2 to be 3.05V based on the value of voltage U1 at the inflection point of the cell's discharge curve. Charge the cell to 95% U2 using 0.5C, let it rest at room temperature for 10 minutes, and then charge it to 3.05V using 0.1C.

[0062] S3: After all the cells are left at room temperature for 12 hours, test OCV1, then leave them at a high temperature of 40℃ for 48 hours, then leave them at room temperature for 24 hours, and test the voltage OCV2; then leave them at room temperature for 72 hours and test the voltage OCV3.

[0063] S4: Calculate the K value for each cell based on the test results of OCV2 and OCV3, where the formula for calculating the K value is K. i =(OCV2-OCV3) / 72h;

[0064] S5: Calculate the average α and standard deviation σ of the K values ​​of the 100 cells measured in step S4, where...

[0065]

[0066] S6: By comparing the K value and α+2σ value of each cell within the group, cells with a K value greater than α+2σ are selected, which are cells with larger offset values.

[0067] For specific K-values ​​and α+2σ screening results for each cell, see [link to details]. Figure 3 .

[0068] from Figure 3 It can be seen that the K value of 3 cells is greater than α+2σ, meaning that the K value of these 3 cells is unqualified.

[0069] Comparative Example 1

[0070] One hundred lithium iron phosphate cells from the same batch as in Example 1 were screened according to the method in Example 1, except that U2 in Comparative Example 1 was changed to above the inflection point U1, such as 3.30V. All other conditions were the same as in Example 1. The K value of the cells was calculated and compared with α+2σ. (See...) Figure 4 ,from Figure 4 It can be seen that there are no substandard battery cells, and its accuracy is obviously not as high as that of Example 1.

[0071] Comparative Example 2

[0072] One hundred lithium iron phosphate cells from the same batch as in Example 1 were screened using the method described in Example 1. Except for changing U2 in Comparative Example 2 to 2.56V, which corresponds to a 0.2% SOC state of charge, all other conditions were the same as in Example 1. Due to the low charge capacity of the batteries in this comparative example, the batteries depleted their charge after only a few K values ​​were measured during the subsequent screening process, making it impossible to continue screening for battery self-discharge. In other words, the effectiveness of battery self-discharge screening could not be ensured.

[0073] Comparative Example 3

[0074] S1: Take 100 lithium iron phosphate cells from the same batch, charge them to 3.65V with a constant current and constant voltage of 0.5C, let them rest at room temperature for 30 minutes, and then discharge them to 2.5V at 0.33C to obtain the discharge curve of lithium iron phosphate and record the inflection point voltage U1 of the curve.

[0075] S2: Based on the cell's discharge curve, select a voltage U2 that is less than the inflection point voltage U1, which is 3.10V. Charge the cell to 3.10V using 1C.

[0076] S3: After leaving all the cells at room temperature for 12 hours, test OCV1, and then leave them at room temperature for 144 hours to test the voltage OCV2.

[0077] S4: Calculate the K value for each cell based on the test results of OCV1 and OCV2, where the formula for calculating the K value is K. i = (OCV1-OCV2) / 144h;

[0078] S5: Calculate the mean α and standard deviation σ of the K value;

[0079] S6: Compare Ki with the α+2σ value within each group (see below) Figure 5 ;

[0080] from Figure 5 Therefore, there are no substandard battery cells.

[0081] For a clearer comparison, this batch of cells was subjected to OCV testing again following the screening steps in Example 1, and the K value, the average value α, and the standard deviation σ of the K value were calculated again. The results were obtained by comparing Ki with the α+2σ value within the same group. Figure 6 .from Figure 6 It can be seen that there are 2 substandard cells out of 100 cells.

[0082] Therefore, it can be seen that the screening method of the present invention is more scientific and reasonable, and has higher accuracy.

[0083] Because the low-current charge-discharge treatment was not performed to eliminate polarization before K-value screening, and high-temperature storage was not carried out during the storage stage to accelerate chemical side reactions, cells with large K-values ​​could not be accurately screened in a short period of time, resulting in misjudgment in the actual production process.

[0084] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for screening the self-discharge consistency of lithium-ion batteries, characterized in that, Includes the following steps: S1: Fully charge n formed cells, let them rest at room temperature for 25-30 minutes, then discharge them to 2.5V with a current of 0.33-1C, let them rest for 5-10 minutes, and then discharge them to 2.5V with a current of 0.05-0.1C. Obtain the capacity-voltage curve and record the inflection point voltage corresponding to the relatively low SOC on the curve as U1. S2: Record the voltage value on the capacity-voltage curve that is less than or equal to the inflection point voltage U1 and greater than 3% SOC as U2; charge all the cells that have been discharged according to step S1 with a current of 0.5-1C to 90-95% U2, let them rest for 5-10 minutes, and then charge them with a current of 0.05-0.1C to U2. S3: After all the cells that have completed step S2 are placed at room temperature for T1, the cells are placed at high temperature for T2, and then placed at room temperature for T3. The open circuit voltage is then tested and set as OCV2. After the cells are placed at room temperature for T4, the open circuit voltage is then tested and set as OCV3. S4: Calculate the K value for each cell: K i =(OCV2-OCV3) / T4, where i = any integer from 1 to n; S5: Calculate the average value α and standard deviation σ for each cell in the group formed by n formed cells; S6: By comparing the K value of each cell with the value of α+xσ within the group, select the K values ​​within the group. i Cells with a value greater than α+xσ are the unqualified cells that have been screened out. In S6, x is any constant from 1 to 3.

2. The lithium-ion battery self-discharge consistency screening method according to claim 1, characterized in that, In S1, n is an integer ≥ 100.

3. The lithium-ion battery self-discharge consistency screening method according to claim 1, characterized in that, The full charge mentioned in step S1 refers to charging to 3.65V with a constant current and constant voltage of 0.5C.

4. The lithium-ion battery self-discharge consistency screening method according to claim 1, characterized in that, In step S1, the battery cell is a lithium iron phosphate battery cell.

5. The lithium-ion battery self-discharge consistency screening method according to claim 1, characterized in that, In step S3, T1 is 12-24h, T2 is 46-50h, T3 is 12-24h, and T4 is 70-74h.

6. The lithium-ion battery self-discharge consistency screening method according to claim 1, characterized in that, After all the cells that have completed step S2 are placed at room temperature for T1, the test voltage is set as OCV1.

7. The lithium-ion battery self-discharge consistency screening method according to claim 1, characterized in that, The ambient temperature is 22-28℃; the high temperature is 40-45℃.

8. The lithium-ion battery self-discharge consistency screening method according to claim 1, characterized in that, In step S5, the average value α is calculated according to the following formula: 。 9. The lithium-ion battery self-discharge consistency screening method according to claim 1, characterized in that, In S5, the standard deviation σ is calculated according to the following formula: 。

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