Lithium ion battery self-discharge consistency screening method

Abstract

The invention relates to a lithium ion battery self-discharge consistency screening method. The method comprises the following steps: S1, fully charging n formed battery cells, carrying out constant-current discharge to 2.5 V after shelving, obtaining a discharge curve, and recording inflection point voltage U1 on the curve; s2, recording a corresponding voltage value on the curve, which is less than or equal to U1 and greater than 3% SOC, as U2; charging the battery cell to U2; s3, placing all the battery cells T1, placing the battery cells T2 at a high temperature, continuously placing the battery cells T3 at a normal temperature, and testing an open-circuit voltage OCV2; placing the battery cell at normal temperature for T4, and testing open-circuit voltage OCV3; s4, calculating a K value of each battery cell; s5, calculating an average value alpha and a standard deviation sigma of the K values of all the battery cells; and S6, comparing the K value of each battery cell with the value of alpha + x sigma, and screening out the battery cells of which Ki is greater than alpha + x sigma in the group. According to the method, the test time is shortened, the influence of temperature and polarization on the test is reduced, and the screening result is more accurate.

Description

Technical Field

[0001] The invention relates to the technical field of lithium ion batteries, and in particular to a method for screening self-discharge consistency of lithium ion batteries. Background Art

[0002] As society's demand for clean and sustainable energy continues to increase, the electric vehicle and energy storage industries are booming. In the actual use of electric vehicles and energy storage base stations, single cells are grouped in series or parallel and used as power modules or energy storage media. In a battery pack, the electrochemical performance of the battery pack is similar to the "barrel" principle. Generally, the lowest-performing cell determines the performance upper limit of the entire battery pack. Therefore, each battery must ensure good consistency, otherwise there will be phenomena such as overcharge and discharge of single cells, different battery attenuation rates, and temperature differences between batteries, resulting in reduced battery pack performance, shortened life, and increased safety hazards. The sorting indicators for judging the consistency of battery packs include capacity, internal resistance, self-discharge rate, and voltage difference of batteries within the group. Among them, self-discharge refers to the phenomenon that the battery energy is consumed by itself when the battery does not do work to the outside. It is characterized by the self-reduction of the charge of the battery during storage in a non-use state, which is manifested as a decrease in the capacity and voltage of the battery. The self-discharge process occurs inside the battery and is related to battery materials and processes. In addition, the test of the self-discharge rate is affected by battery polarization, SOC status, aging temperature and time, foreign matter short circuit, chemical reaction, etc., and may result in missed judgment or misjudgment. Therefore, accurate and rapid measurement of the self-discharge rate is crucial for the screening of battery consistency. In addition, severe self-discharge may lead to thermal runaway of the battery, thus causing safety problems. The self-discharge rate is also an important reference indicator in the battery safety management process.

[0003] Among the existing self-discharge screening methods, the commonly used methods are the open circuit voltage decay test method and the tolerance method. Generally, after the battery cell is charged / discharged to the preset SOC through a certain current, the battery is placed at high temperature or room temperature for a period of time to test the voltage drop of a single voltage, or placed at high temperature or room temperature for a period of time to test the capacity loss of a single battery. The voltage drop or capacity loss of a single battery cell is compared with the standard voltage drop value or standard capacity loss rate to determine the size of the battery self-discharge. This method not only takes a long time to test, but the test accuracy may be affected by temperature, battery polarization or battery production batch, resulting in misjudgment or missed judgment.

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

[0005] The purpose of the present invention is to provide a lithium ion battery self-discharge consistency screening method, which reduces the test time, reduces the influence of temperature and polarization on the test, and makes the screening result more accurate.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

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

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

[0009] S2: The voltage value corresponding to the inflection point voltage U1 and greater than 3% SOC on the capacity-voltage curve is recorded as U2; all the cells that have been discharged according to step S1 are charged to 90-95% U2 with a current of 0.5-1C, left for 5-10 minutes, and then charged to U2 with a current of 0.05-0.1C;

[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 tested and determined as OCV2; after the cells are placed at room temperature for T4, the open circuit voltage is tested and determined as OCV3;

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

[0012] S5: Calculate the average value α and standard deviation σ of each battery cell K value in the group formed by n formed batteries;

[0013] S6: Compare the K value of each battery cell with the value of α+xσ within the group to screen out the K value within the group. i The battery cells with >α+xσ are the unqualified battery cells screened out.

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

[0015] Furthermore, the full charge in step S1 refers to charging to 3.65V at 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, all the cells that have completed step S2 are placed at room temperature for T1 and then tested for a voltage of OCV1.

[0019] Furthermore, the normal temperature is 22-28°C; the high temperature is 40-45°C.

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

[0021]

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

[0023]

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

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

[0026] 1. The present invention first obtains the discharge curve of the lithium iron phosphate lithium-ion battery by discharging the battery cell to 2.5V at room temperature, and records the voltage value corresponding to the inflection point U1 and greater than 3% SOC as U2 through the capacity-voltage curve. Then, the battery cell is charged to U2, and the battery cell with large self-discharge can be quickly screened out.

[0027] 2. The present invention discharges the battery cell at 0.33-1C and then discharges it again to 2.5V with a small current of 0.05-0.1C, so that the capacity in the battery cell can be released as completely as possible, avoiding the influence of the discharge temperature causing incomplete discharge of the battery cell and large voltage error after subsequent charging.

[0028] 3. The present invention charges the fully discharged battery cell twice and uses a smaller current after 95% U2, thereby eliminating the polarization phenomenon of the battery cell in the high current stage and reducing the error caused by voltage polarization during K value screening.

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

[0030] 5. After calculating the K value of the battery, the present invention compares the K value of each battery cell with the average value α and standard deviation σ of the K value in the group, and K i ≤α+xσ means the K value is qualified, which improves the battery matching consistency.

[0031] 6. The present invention improves the consistency of batteries in a group by comparing and screening the K value of a single battery cell and the standard deviation within the group, thereby improving the consistency of battery cell matching in electric vehicles or energy storage base station applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

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

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

[0035] Figure 3 This is a graph of the screening results of α+2σ provided in Example 1 of the present invention;

[0036] Figure 4 This is a graph of the screening results of α+2σ provided in Comparative Example 1 of the present invention;

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

[0038] Figure 6 This is a graph of the screening results of α+2σ provided for the battery cell tested in comparative example 2 of the present invention after the process screening in example 1. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme in the embodiment of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present invention. The process parameters of the following embodiments that do not specify specific conditions are usually based on conventional conditions.

[0040] The endpoints and any values ​​of the ranges disclosed in the present 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 each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in the present invention.

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

[0042] S1: fully charge n formed cells, place them at room temperature for 25-30 minutes, then discharge them to 2.5V at a current of 0.33-1C (e.g. 0.33C, 0.5C, 0.6C, 0.7C, 0.8C, 0.9C), place them at room temperature for 5-10 minutes (e.g. 5min, 6min, 8min, 10min), and then discharge them to 2.5V at 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 greatly at a lower SOC);

[0043] S2: The voltage value corresponding to the inflection point voltage U1 and greater than 3% SOC on the capacity-voltage curve is recorded as U2; all the cells that have been discharged according to step S1 are charged to 90-95% U2 with a current of 0.5-1C (for example, 0.6C, 0.7C, 0.8C, 0.9C), left at room temperature for 5-10 minutes, and then charged to U2 with a current of 0.05-0.1C (for example, 0.06C, 0.07C, 0.08C, 0.09C);

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

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

[0046] S5: Calculate the average value α and standard deviation σ of each battery cell K value in the group formed by n formed batteries;

[0047] S6: By comparing the K value of each battery cell with the value of α+xσ within the group, the battery cells with Ki>α+xσ within the group are screened out, which are the unqualified battery cells. The unqualified battery cells are the battery cells with larger K value deviation.

[0048] The present invention firstly discharges a group of cells at a constant current to 2.5V to obtain a discharge curve of the cells, and finds the inflection point voltage U1 (see Figure 2 ), and then record the voltage corresponding to the SOC that is equal to or lower than U1 and greater than 3% as U2, and then charge all the cells to U2, which can ensure that the screening voltage of the cell is at a position with a larger slope in the discharge curve, and can quickly screen out the cells with large self-discharge. Before charging or discharging to the end voltage, charging and discharging with a small current can reduce the effects of temperature and polarization on the cell and improve the accuracy of K value screening. The OCV2 test went through two stages before high temperature and room temperature. High temperature can accelerate chemical side reactions and shorten the time for K value screening, while room temperature can reduce the effect of temperature on the OCV2 test, making the OCV2 test value more accurate, so that the calculated K value is closer to the true value. After calculating the battery K value, by comparing the K value of each cell with the average value α and standard deviation σ of the K value in the group, that is, K i ≤α+xσ means that the K value is qualified, which improves the battery matching consistency.

[0049] In addition, in S2, the 5-10 min standby period is to reduce the polarization phenomenon caused by the large current, so that the subsequently measured OCV is more accurate.

[0050] As an optional implementation manner of the present invention, in S1, n is an integer ≥100.

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

[0052] As an optional implementation manner of the present invention, the full charging in step S1 refers to charging to 3.65V at 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 the screening purpose.

[0054] And / or, the normal temperature is 22-28°C (such as 23°C, 24°C, 25°C, 26°C, 27°C); the high temperature is 40-45°C (such as 41°C, 42°C, 43°C, 44°C).

[0055] As an optional implementation manner of the present invention, in 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] As an optional implementation manner of the present invention, in S6, the x is any constant from 1 to 3.

[0058] The present invention will be further described in detail below 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 at a constant current and constant voltage of 0.5C, leave them at room temperature for 30 minutes, discharge them to 2.5V at 0.33C, leave them at room temperature for 10 minutes, and then discharge them to 2.5V at a current of 0.1C. The discharge curve of lithium iron phosphate is obtained ( Figure 2 ), here we can select a discharge curve of a battery cell to determine U1 and U2, because the discharge curves of batteries in the same batch are not much different; the inflection point voltage corresponding to the relatively low SOC on the record curve is U1;

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

[0062] S3: After all the cells are placed at room temperature for 12 hours, the OCV1 voltage is tested. After that, they are placed at a high temperature of 40°C for 48 hours. After that, they are placed at room temperature for 24 hours, and the voltage OCV2 is tested. After that, they are placed at room temperature for 72 hours, and the voltage OCV3 is tested.

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

[0064] S5: Calculate the average value 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 of each battery cell with the α+2σ value within the group, the battery cells with K values ​​greater than α+2σ within the group are screened out, that is, the battery cells with larger offset values.

[0067] For the specific K value and α+2σ screening results of each battery cell, see Figure 3 .

[0068] from Figure 3 It can be seen that the K values ​​of three battery cells are greater than α+2σ, that is, the K values ​​of these three battery cells are unqualified.

[0069] Comparative Example 1

[0070] 100 lithium iron phosphate batteries from the same batch as Example 1 were screened according to the method of Example 1. Except that U2 in this comparative example 1 was changed to above the inflection point U1, such as 3.30V, the other conditions were the same as those in Example 1. The K value of the battery was calculated and compared with α+2σ. Figure 4 ,from Figure 4 It can be seen that there are no unqualified cells, and obviously its accuracy is not as high as that of Example 1.

[0071] Comparative Example 2

[0072] 100 lithium iron phosphate batteries from the same batch as Example 1 were screened according to the method of Example 1, except that U2 in this comparative example 2 was changed to 2.56V corresponding to the voltage at 0.2% SOC state of charge, and the other conditions were the same as those in Example 1. Due to the low charge of the battery in this comparative example, in the subsequent screening process, the battery only measured a small K value before the power was exhausted, and the battery self-discharge screening could not be continued, that is, the effectiveness of the 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 at a constant current and constant voltage of 0.5C, leave them 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: According to the discharge curve of the battery cell, the voltage U2 is 3.10V, which is less than the inflection point voltage U1. Charge the battery cell to 3.10V at 1C;

[0076] S3: Place all cells at room temperature for 12 hours and then test OCV1. Place them at room temperature for another 144 hours and then test the voltage OCV2.

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

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

[0079] S6: Comparison of Ki and α+2σ values ​​within the group Figure 5 ;

[0080] from Figure 5 It can be seen that there are no unqualified batteries.

[0081] For a clearer comparison, this batch of cells was tested for OCV according to the screening process of Example 1, and the K value, the average value α and the standard deviation σ of the K value were calculated again. The K value was obtained by comparing the Ki and α+2σ values ​​within the group. Figure 6 .from Figure 6 It can be seen that there are 2 unqualified cells among 100 cells.

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

[0083] Because small current charge and discharge treatment is not performed to eliminate polarization before K value screening, and high-temperature storage is not performed to accelerate chemical side reactions during the storage stage, battery cells with large K values ​​cannot 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 aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned 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: The following steps are involved: S1: fully charge n formed cells, place them at room temperature for 25-30 minutes, then discharge them to 2.5V at a current of 0.33-1C, place them for 5-10 minutes, and then discharge them to 2.5V at a current of 0.05-0.1C to obtain a capacity-voltage curve, and record the inflection point voltage corresponding to the relatively low SOC on the curve as U1; S2: The voltage value corresponding to the inflection point voltage U1 and greater than 3% SOC on the capacity-voltage curve is recorded as U2; all the cells that have been discharged according to step S1 are charged to 90-95% U2 with a current of 0.5-1C, left for 5-10 minutes, and then charged to U2 with a current of 0.05-0.1C; 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 tested and determined as OCV2; after the cells are placed at room temperature for T4, the open circuit voltage is tested and determined as OCV3; S4: Calculate the K value of each battery cell: K i =(OCV2-OCV3) / T4, i=any integer from 1 to n; S5: Calculate the average value α and standard deviation σ of each battery cell K value in the group formed by n formed batteries; S6: Compare the K value of each battery cell with the value of α+xσ within the group to screen out the K value within the group. i The battery cells with >α+xσ are the unqualified battery cells screened out.

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

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

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

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

6. The method for screening the self-discharge consistency of lithium-ion batteries according to claim 1, characterized in that: All cells that have completed step S2 are placed at room temperature for T1 and then tested at a voltage of OCV1.

7. The method for screening the self-discharge consistency of lithium-ion batteries according to claim 1, characterized in that: The normal temperature is 22-28°C; the high temperature is 40-45°C.

8. The method for screening the self-discharge consistency of lithium-ion batteries according to claim 1, characterized in that: In S5, the average value α is calculated according to the following formula:

9. The method for screening the self-discharge consistency of lithium-ion batteries according to claim 1, characterized in that: In S5, the standard deviation σ is calculated according to the following formula:

10. The method for screening the self-discharge consistency of lithium-ion batteries according to claim 1, characterized in that: In S6, the x is any constant from 1 to 3.

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