Battery cell system self-discharge evaluation method, system, equipment and medium

Abstract

The invention discloses a self-discharge evaluation method, system and equipment for a battery cell system and a medium, relates to the technical field of lithium ion batteries, and aims to quickly and accurately screen out self-discharge of a battery cell. A positive electrode, a negative electrode and a diaphragm of a battery cell system are assembled into a small soft package laminated battery cell, and liquid is injected for capacity grading; carrying out a low-current charging and discharging test under a high-temperature condition, obtaining an SOC-OCV curve, and screening an SOC range; adjusting the battery cell to a target SOC, taking out the positive pole piece and the negative pole piece, packaging the positive pole piece and the negative pole piece, tightening the packaged positive pole piece and negative pole piece to remove gas, placing the packaged positive pole piece and negative pole piece at different temperatures, recording volume change values, and drawing a temperature-volume change value curve; testing voltage values of the battery cell before and after the battery cell is placed at different temperatures under the target SOC, and drawing a temperature-voltage change value curve; and calculating a correlation coefficient of the slopes of the two curves, and carrying out self-discharge screening on the battery cells. According to the method, effective self-discharge screening is carried out in the early stage, rework and the rejection rate in subsequent production can be reduced, and therefore time and cost are saved.

Description

Technical Field

[0001] The present invention relates to the field of lithium-ion batteries, and in particular to a method, system, equipment and medium for evaluating self-discharge of a battery cell system. Background Art

[0002] With the widespread application of lithium-ion batteries in power and energy storage, the consistency of battery cells plays a decisive role in the performance and life of the entire battery pack. After the battery cells are later assembled into modules, if the self-discharge performance of the battery cells is inconsistent, it will lead to a decline in the overall performance of the module, shorten the life, and even cause safety hazards. Therefore, self-discharge screening is usually carried out in the early stages of battery cell production.

[0003] Self-discharge refers to the gradual loss of stored energy in a lithium-ion battery when it is not connected to an external circuit and no current is flowing. This phenomenon not only affects the battery's effective energy storage capacity but also significantly impacts its overall performance and life expectancy. Therefore, a deep understanding of the mechanisms of self-discharge is crucial for improving battery performance, extending its lifespan, and ensuring safe use.

[0004] The main mechanisms of self-discharge usually include the following aspects:

[0005] 1. Formation and decomposition of the solid electrolyte interface (SEI) layer: During the charge and discharge cycle of lithium-ion batteries, a protective thin film, the SEI layer, forms on the surface of the negative electrode. This film effectively prevents direct contact between the electrode material and the electrolyte, reducing unnecessary side reactions, thereby protecting the battery and extending its life. However, when the battery is in an unused static state, the SEI layer may undergo spontaneous chemical decomposition, releasing lithium ions and resulting in energy loss, which is a source of self-discharge.

[0006] 2. Local short circuits in positive and negative electrode materials: Under normal circumstances, the positive and negative electrodes of lithium-ion batteries are kept at a certain distance by a separator to prevent direct contact and avoid short circuits. However, in certain circumstances, such as battery manufacturing defects or damage caused by prolonged use, a channel formed by tiny metal particles or other conductive materials can form between the positive and negative electrodes, causing a local short circuit. This local short circuit causes lithium ions inside the battery to continuously migrate between the positive and negative electrodes, leading to self-discharge.

[0007] 3. Impurity reaction: During the battery production and assembly process, some impurities, such as moisture and oxygen, may inevitably be introduced. These impurities can react with the active materials in the battery to generate insoluble or electrochemically inert substances, consuming some lithium ions, which is also a cause of self-discharge.

[0008] 4. Electrolyte Redox Reactions: The electrolyte in lithium-ion batteries contains organic solvent molecules that can undergo redox reactions under certain conditions. These chemical reactions can cause the electrolyte to decompose, forming gases and other byproducts, which consume energy within the battery and affect its overall performance and stability, becoming another significant factor in self-discharge.

[0009] 5. Temperature Effects: Changes in ambient temperature significantly impact the self-discharge of lithium-ion batteries. Rising temperatures promote chemical reactions within the battery, accelerating self-discharge. Self-discharge is particularly severe in high-temperature environments. Therefore, when designing battery operating and storage conditions, the importance of temperature control must be considered to slow self-discharge and extend battery life.

[0010] In summary, lithium-ion battery self-discharge is the result of the interaction of multiple internal mechanisms, involving complex physical and chemical reactions. Chemical self-discharge is crucial for the cell's subsequent performance. Currently, existing self-discharge screening methods use voltage as a single criterion. However, evaluating solely based on voltage changes can miss some important self-discharge characteristics, leading to inaccurate screening results. Therefore, an effective and convenient testing method is needed to quickly and accurately screen for cell self-discharge. Summary of the Invention

[0011] The technical solution of the present invention is used to solve the problem of needing to quickly and accurately screen the self-discharge of battery cells at the electrode stage before mass production of battery cells.

[0012] The present invention solves the above technical problems through the following technical means: a method for evaluating the self-discharge of a battery cell system, the method comprising:

[0013] S1. Assemble the positive electrode, negative electrode, and separator of the battery system into a small soft-pack laminated battery cell, and perform liquid injection, formation, and capacity division. After capacity division, perform a low-current charge and discharge test on the battery cell under high temperature conditions to obtain the SOC-OCV curve under different temperature conditions, and use the curve to screen the SOC range;

[0014] S2. Adjust the battery cell to the target SOC within the SOC range, remove the positive and negative electrode sheets of the battery cell in this state, respectively, encapsulate the positive and negative electrode sheets in sealed packaging bags, tighten them to remove air, place them at different temperatures for a certain period of time, record the volume change of the packaging bag, and draw a temperature-volume change curve;

[0015] S3. Test the voltage change of the small soft-pack laminated battery cell with the target SOC before and after storage at different temperatures, and draw a temperature-voltage change curve;

[0016] S4. Calculate the correlation coefficient of the slopes of the temperature-volume change curve and the temperature-voltage change curve, and perform cell self-discharge screening based on the correlation coefficient.

[0017] By performing low-current charge-discharge tests at different temperatures and obtaining SOC-OCV curves, this method can accurately screen the SOC range suitable for self-discharge testing, providing a scientific basis for subsequent testing. The volume change value directly reflects the self-discharge of the battery cell at different temperatures, providing an intuitive quantitative indicator for self-discharge evaluation. By recording the voltage change value before and after storage, the degree of self-discharge of the battery cell at different temperatures can be quantified, providing a key parameter for self-discharge evaluation. By calculating the correlation coefficient between the volume change and voltage change curve slopes, the self-discharge characteristics of the battery cell can be comprehensively evaluated, avoiding misjudgments caused by a single indicator.

[0018] Furthermore, the capacity of the small soft-pack laminated battery cell in S1 is 2 to 10 Ah, and excess electrolyte and air are extracted and removed after the battery cell is filled and divided, and the electrolyte retention coefficient is 3 to 4 mAh / g.

[0019] Furthermore, the battery cells after capacity division are subjected to charge and discharge tests at a temperature of 25°C to 55°C and a current of 0.05C to 0.2C, with an SOC adjustment interval of 5% to 10%, and at each SOC value, the battery cells are left standing for 1h to 3h.

[0020] Furthermore, the SOC range in S1 is an SOC segment in which the overlap rate of the SOC-OCV curves at different temperatures is less than 98%.

[0021] Furthermore, S2 includes: after adjusting the battery cell to the target SOC, placing the battery cell under conditions of a temperature of 25°C to 55°C, setting the shelf time to 24 to 72 hours, taking out the positive and negative electrodes of the battery cell and packaging them with aluminum film, sealing the air tube on the side, using a vacuum exhaust device to extract the air in the package, recording the volume change value in the packaging bag, the sum of the volume changes of the positive and negative electrodes is recorded as △v, drawing a temperature-volume change value curve, and fitting the slope K1 of the curve.

[0022] Furthermore, S3 includes: placing the battery cell at a temperature of 25°C to 55°C, recording the voltage values ​​OCV1 and OCV2 of the battery cell of the target SOC before and after the volume change test at different temperatures, K=OCV2-OCV1, drawing a temperature-voltage change value curve, and fitting the slope K2 of the curve.

[0023] Furthermore, the S4 includes: combining the volume change values ​​and voltage change values ​​of the battery cell at different temperatures to determine the consistency correlation coefficient S, S=K1×K2, and performing system screening based on the coefficient.

[0024] The present invention also provides a self-discharge evaluation system for a battery cell system, the system comprising:

[0025] Cell assembly and testing module: used to assemble the positive electrode, negative electrode and separator of the cell system into small soft-pack laminated cells, and perform liquid injection, formation and capacity separation. The cells after capacity separation are subjected to low-current charge and discharge tests under high temperature conditions to obtain SOC-OCV curves under different temperature conditions, and the SOC range is screened based on the curves.

[0026] Electrode packaging and volume monitoring module: used to adjust the battery cell to the target SOC within the SOC range, remove the positive and negative electrode plates of the battery cell in this state, respectively, package the positive and negative plates in sealed packaging bags, tighten them to remove gas, place them at different temperatures for a certain period of time, record the volume change of the packaging bag, and draw a temperature-volume change value curve;

[0027] Voltage change detection module: used to test the voltage change of small soft-pack laminated cells of the target SOC before and after being stored at different temperatures, and draw a temperature-voltage change curve;

[0028] Data analysis and screening module: used to calculate the correlation coefficient of the slope of the temperature-volume change value curve and the temperature-voltage change value curve, and perform battery cell self-discharge screening based on the correlation coefficient.

[0029] The present invention also provides a processing device, comprising at least one processor and at least one memory communicatively connected to the processor, wherein: the memory stores program instructions that can be executed by the processor, and the processor can execute the above method steps by calling the program instructions.

[0030] The present invention also provides a computer-readable storage medium, wherein the computer-readable storage medium stores computer instructions, and the computer instructions enable the computer to execute the above method steps.

[0031] The advantages of the present invention are: the present invention determines the target SOC value of the battery cell through a self-discharge test, defines the relationship between the battery cell voltage, volume change value and temperature under the SOC, and uses the correlation coefficient S=K1×K2 of the slope of the comprehensive degree-volume change value curve and the temperature-voltage change value curve as the final screening standard. It can accurately evaluate the self-discharge characteristics of the battery cell under different temperature conditions, and the screening standard can be given at the electrode stage before mass production of the battery cell, saving costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a flow chart of a method for evaluating battery cell self-discharge according to embodiment 1 of the present invention;

[0033] Figure 2 The SOC-OCV curves at different temperatures of Example 1 of the present invention are shown;

[0034] Figure 3 This is the temperature-volume change value curve of Embodiment 1 of the present invention;

[0035] Figure 4 This is the temperature-voltage change value curve of Embodiment 1 of the present invention. Specific Embodiments

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are some, rather than all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0037] Embodiment 1

[0038] As Figure 1 shown is a flowchart of a method for evaluating the self-discharge of an electric cell, and the method includes:

[0039] S1. Assemble the positive electrode, negative electrode, and separator of the electric cell system into a small soft-pack laminated electric cell, and perform liquid injection, formation, and grading; after grading, the electric cell is subjected to a small-current charge-discharge test under high-temperature conditions to obtain the SOC-OCV curve under different temperature conditions, and the SOC range is screened out through this curve;

[0040] Specifically, the capacity of the small soft-pack laminated electric cell is 2-10 Ah. After liquid injection and grading of the electric cell, excess electrolyte and air are extracted and removed. The liquid retention coefficient of the electrolyte is 3-4 mAh / g. The graded electric cell is subjected to a charge-discharge test at a temperature of 25°C to 55°C and a current of 0.05C to 0.2C. The interval of SOC adjustment is 5% to 10%. At each SOC value, the standing time of the electric cell is 1h to 3h. The SOC range is the SOC segment where the overlap rate of the SOC-OCV curves at different temperatures is less than 98%. As Figure 2 shown are the SOC-OCV curves at different temperatures; the calculation method of the overlap rate: set temperatures T1 and T2, where T1 < T2. Under the same SOC condition, the open-circuit voltage V1 of the electric cell at temperature T1 and the open-circuit voltage V2 of the electric cell at temperature T2 are respectively measured. The calculation formula of the overlap rate is: overlap rate = V2 / V1.

[0041] The temperature of the electric cell is 25°C to 55°C. The temperature setting mainly considers the self-discharge of the electric cell. Currently, for the ternary or lithium iron phosphate system of lithium-ion batteries, it is necessary to consider within this temperature range. Too high or too low temperature will affect the production and performance of the electric cell. The current is 0.05C to 0.2C because small-current charging reduces the polarization of the electric cell.

[0042] S2. Adjust the battery cell to the target SOC within the SOC range, remove the positive and negative electrode sheets of the battery cell in this state, respectively, encapsulate the positive and negative electrode sheets in sealed packaging bags, tighten them to remove air, place them at different temperatures for a certain period of time, record the volume change of the packaging bag, and draw a temperature-volume change curve;

[0043] Specifically, after adjusting the battery cell to the target SOC, the battery cell is placed in a temperature condition of 25°C to 55°C, and the shelf time is set to 24 to 72 hours. The positive and negative electrodes of the battery cell are taken out and packaged with aluminum film, the air tube is sealed on the side, and the air in the package is extracted using a vacuum exhaust device. The volume change value in the packaging bag is recorded, and the sum of the volume changes of the positive and negative electrodes is recorded as △v. A temperature-volume change value curve is drawn, and the slope K1 of the curve is fitted.

[0044] The shelf time is set to 24 to 72 hours. This time range mainly considers the battery cell design and production process. If the time is too short, the performance will be affected and the battery cell consistency will be poor. If the time is too long, the battery cell production and design research time will be longer, and the cost will increase.

[0045] S3 tests the voltage change of a small soft-pack laminated battery cell with target SOC before and after storage at different temperatures, and draws a temperature-voltage change curve;

[0046] Specifically, the battery cell is placed at a temperature of 25°C to 55°C, and the voltage values ​​OCV1 and OCV2 of the battery cell with the target SOC before and after the volume change test at different temperatures are recorded, K = OCV2-OCV1, and a temperature-voltage change curve is drawn, and the slope K2 of the curve is fitted.

[0047] S4. Calculate the correlation coefficient of the slopes of the temperature-volume change curve and the temperature-voltage change curve, and perform self-discharge screening of the battery cells based on the correlation coefficient. Combine the volume change and voltage change values ​​of the battery cells at different temperatures to determine the consistency correlation coefficient S, S = K1 × K2, and perform system screening.

[0048] For example: obtain the positive electrode, negative electrode sheets and electrolyte of the battery cell system to be evaluated, the positive electrode is lithium iron phosphate, the negative electrode is a graphite system, the battery cells are assembled into soft-pack stacked cells, the battery cell capacity is 10Ah, and injection, formation and capacity separation are performed; after injection and capacity separation of the battery cell, excess electrolyte and air are extracted and removed, the electrolyte retention coefficient is 3.5mAh / g, and the retention volume is 35mAh / g.

[0049] The cells were then charged and discharged at 25°C, 35°C, and 45°C using a 0.1C current. Three cycles of charge and discharge were performed at 0.1C with a voltage range of 2.5-3.65V. The cell capacity was determined and the SOC was adjusted based on the capacity of the third cycle, which was recorded as C3.

[0050] The cell was fully charged at a 0.1C rate, and the SOC was adjusted at intervals of C3*5% based on the C3 capacity. The cell was then discharged at a 0.1C rate, with the discharge capacity controlled at C3*5% each time. The rest time for each discharge was set to 1 hour, and this process was repeated 20 times until the cell was discharged to the lower voltage limit. The voltage after each SOC adjustment and rest for 1 hour was recorded as the final voltage to obtain the SOC-OCV curve for the cell under this system. The curve was used to calculate the points of the self-discharge screening SOC of the cell. The SOC segments with an overlap rate of less than 98% of the SOC-OCV curves at different temperatures were selected for testing. Analysis of the data slope in the figure shows that the self-discharge screening SOC range for the lithium iron phosphate-graphite system cell is 5% to 20%, and 20% SOC was selected for the self-discharge test.

[0051] After the battery cell is adjusted to 20% SOC through a 0.5C rate, it is left to stand for 24 hours until the battery cell stabilizes and then the volume change test during the self-discharge process is performed. The volume change test method is to take out 8 battery cells of this model, take out the positive and negative plates of the battery cell and encapsulate them separately with aluminum-plastic film, encapsulate the positive and negative plates of each battery cell separately, seal the exhaust pipe on the side of the aluminum-plastic film bag, use a vacuum exhaust device to tighten the air in the package, put the sample under certain temperature conditions, set the temperature to 25℃, 35℃, 45℃, 55℃, set the shelf time to 48h, place 2 parallel samples at each temperature, take them out after the shelf is completed, wait for the temperature to drop to about 25℃, use a vacuum exhaust device to tighten the battery cell again, collect the gas, and record the volume change value in the package bag at the same time. The sum of the positive and negative electrodes is recorded as △v, draw a temperature-volume change value curve, fit the curve slope K1, the volume value and the curve reference Figure 3 , Figure 3 is the temperature-volume change curve; the linear curve is fitted according to the measured temperature-volume change value, and tools such as EXCEL or Origin can be used.

[0052] Prepare 8 finished 10Ah cells, adjust them to 20% SOC, record the voltage OCV1 after 24 hours, place the cells at 25℃, 35℃, 45℃, and 55℃, and place 2 cells at each temperature for 48 hours. After completion, take them out and place them for 12 hours. Test OCV2 and calculate the voltage change value K=OCV1-OCV2. Draw the temperature-voltage change value curve and calculate the slope K2 of the curve. For specific values ​​and curve references Figure 4 , Figure 4 This is the temperature-voltage change curve.

[0053] Finally, the correlation coefficient S=K1×K2 of the two curves is calculated, and the chemical self-discharge of the system is judged by the correlation coefficient.

[0054] Verification system design

[0055] In the cell system design verification, two systems were designed:

[0056] System 1: The positive electrode is made of lithium iron phosphate material, the negative electrode uses artificial graphite secondary particles, the electrolyte is ethylene carbonate, diethyl carbonate, dimethyl carbonate, lithium hexafluorophosphate, and film-forming additive VC, and the diaphragm uses 9um PE base film plus 3um ceramic coating film

[0057] System 2: The positive electrode is made of lithium iron phosphate material, the negative electrode uses artificial graphite secondary particles, the electrolyte is ethylene carbonate, diethyl carbonate, dimethyl carbonate, lithium hexafluorophosphate, and the film-forming additive VC, and the diaphragm uses 9um PE base film plus 3um ceramic coating film plus 1un PVDF film

[0058] The electrode surface density and compaction design of the two systems are consistent. Except for the different separators, the battery cell production process remains the same. The main purpose is to verify the impact of the separator on the self-discharge of the battery cell. In actual operation, different design verifications can be carried out according to needs.

[0059] The test results are shown in Table 1:

[0060] system K1 (temperature-volume change slope) K2 (temperature-voltage change slope) Correlation coefficient (S) System 1 0.0594 0.641 0.038 System 2 0.0231 0.384 0.009

[0061] By comparing the correlation coefficients, we can find that S1>S2. Therefore, the correlation design of system 2 is better.

[0062] Example 2

[0063] The present invention also provides a battery cell self-discharge evaluation system, the system comprising:

[0064] The cell assembly and testing module is used to assemble the positive electrode, negative electrode, and separator of the cell system into small soft-pack laminated cells, perform electrolyte injection, formation, and capacity separation. The separated cells are then subjected to low-current charge and discharge testing under high-temperature conditions to obtain SOC-OCV curves at different temperature conditions, which are used to filter the SOC range. The capacity of the small soft-pack laminated cells ranges from 2-10Ah. Excess electrolyte and air are extracted and removed after both electrolyte injection and capacity separation, and the electrolyte retention coefficient is 3-4mAh / g. The separated cells are then charged and discharged at a current of 0.05C to 0.2C at temperatures between 25°C and 55°C. The SOC adjustment interval is 5% to 10%, and the cells are allowed to rest for 1 to 3 hours at each SOC value. The SOC range in the cell assembly and testing module is the SOC range where the SOC-OCV curves at different temperatures overlap by less than 98%.

[0065] Pole piece packaging and volume monitoring module: used to adjust the battery cell to the target SOC within the SOC range, take out the positive and negative pole pieces of the battery cell in this state respectively, package the positive and negative pole pieces in sealed packaging bags respectively, and tighten them to remove gas, place them at different temperatures for a certain period of time, record the volume change value of the packaging bag, and draw a temperature-volume change value curve; after the battery cell is adjusted to the target SOC in the pole piece packaging and volume monitoring module, the battery cell is placed at a temperature of 25°C to 55°C, and the shelf time is set to 24 to 72 hours. The positive and negative pole pieces of the battery cell are taken out and packaged with aluminum film, the air tube is sealed on the side, and the air in the package is extracted using a vacuum exhaust device, the volume change value in the packaging bag is recorded, and the sum of the volume changes of the positive and negative electrodes is recorded as △v. The temperature-volume change value curve is drawn, and the slope K1 of the curve is fitted.

[0066] Voltage change detection module: Test the voltage change value of the small soft-pack laminated battery cell of the target SOC before and after being placed at different temperatures, and draw the temperature-voltage change value curve; place the battery cell at a temperature of 25℃~55℃, record the voltage values ​​OCV1 and OCV2 of the battery cell of the target SOC before and after the volume change test at different temperatures, K=OCV2-OCV1, draw the temperature-voltage change value curve, and fit the slope K2 of the curve.

[0067] Data Analysis and Screening Module: Calculates the correlation coefficients of the temperature-volume change curve and the temperature-voltage change curve slopes, and uses these correlation coefficients to screen for cell self-discharge. Combined with the cell volume change and voltage change values ​​at different temperatures, the consistency correlation coefficient S = K1 * K2 is determined for system screening.

[0068] Example 3

[0069] Based on Example 1, Example 3 of the present invention also provides a processing device, including at least one processor and at least one memory communicatively connected to the processor, wherein: the memory stores program instructions that can be executed by the processor, and the processor calls the program instructions to execute the method steps described in Example 1.

[0070] Example 4

[0071] Based on Example 1, Example 4 of the present invention further provides a computer-readable storage medium, wherein the computer-readable storage medium stores computer instructions, and the computer instructions enable the computer to execute the method steps described in Example 1.

[0072] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. 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 make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for evaluating the self-discharge of a battery cell system, characterized in that: The method comprises: S1. Assemble the positive electrode, negative electrode, and separator of the battery system into a small soft-pack laminated battery cell, and perform liquid injection, formation, and capacity division. After capacity division, perform a low-current charge and discharge test on the battery cell under high temperature conditions to obtain the SOC-OCV curve under different temperature conditions, and use the curve to screen the SOC range; S2. Adjust the battery cell to the target SOC within the SOC range, remove the positive and negative electrode sheets of the battery cell in this state, respectively, encapsulate the positive and negative electrode sheets in sealed packaging bags, tighten them to remove air, place them at different temperatures for a certain period of time, record the volume change of the packaging bag, and draw a temperature-volume change curve; S3. Test the voltage change of the small soft-pack laminated battery cell with the target SOC before and after storage at different temperatures, and draw a temperature-voltage change curve; S4. Calculate the correlation coefficient of the slopes of the temperature-volume change curve and the temperature-voltage change curve, and perform cell self-discharge screening based on the correlation coefficient.

2. A method for evaluating self-discharge of a battery cell system according to claim 1, characterized in that: The capacity of the small soft-pack laminated battery cell in S1 is 2-10Ah. Excess electrolyte and air are extracted and removed after the battery cell is filled and divided. The electrolyte retention coefficient is 3-4mAh / g.

3. The method for evaluating self-discharge of a battery cell system according to claim 1, wherein: The battery cells after capacity division are subjected to charge and discharge tests at a temperature of 25°C to 55°C and a current of 0.05C to 0.2C. The SOC adjustment interval is 5% to 10%, and at each SOC value, the battery cells are left standing for 1h to 3h.

4. A method for evaluating self-discharge of a battery cell system according to claim 1, characterized in that: The SOC range in S1 is an SOC segment in which the overlap rate of the SOC-OCV curves at different temperatures is less than 98%.

5. The method for evaluating self-discharge of a battery cell system according to claim 1, wherein: The S2 includes: after adjusting the battery cell to the target SOC, placing the battery cell under conditions of a temperature of 25°C to 55°C, setting the shelf time to 24 to 72 hours, taking out the positive and negative electrode sheets of the battery cell and encapsulating them with aluminum film, sealing the air tube on the side, using a vacuum exhaust device to extract the air in the package, recording the volume change value in the packaging bag, recording the total volume change of the positive and negative electrodes as △v, drawing a temperature-volume change value curve, and fitting the slope K1 of the curve.

6. A method for evaluating self-discharge of a battery cell system according to claim 5, characterized in that: The S3 includes: placing the battery cell at a temperature of 25°C to 55°C, recording the voltage values ​​OCV1 and OCV2 of the battery cell of the target SOC before and after the volume change test at different temperatures, the voltage change value K=OCV2-OCV1, drawing a temperature-voltage change value curve, and fitting the slope K2 of the curve.

7. A method for evaluating self-discharge of a battery cell system according to claim 6, characterized in that: The S4 includes: combining the volume change value and voltage change value of the battery cell at different temperatures to determine the consistency correlation coefficient S, S=K1×K2, and performing system screening based on the coefficient.

8. A self-discharge evaluation system for a battery cell system, characterized in that: The system comprises: Cell assembly and testing module: used to assemble the positive electrode, negative electrode and separator of the cell system into small soft-pack laminated cells, and perform liquid injection, formation and capacity separation. The cells after capacity separation are subjected to low-current charge and discharge tests under high temperature conditions to obtain SOC-OCV curves under different temperature conditions, and the SOC range is screened based on the curves. Electrode packaging and volume monitoring module: used to adjust the battery cell to the target SOC within the SOC range, remove the positive and negative electrode plates of the battery cell in this state, respectively, package the positive and negative plates in sealed packaging bags, tighten them to remove gas, place them at different temperatures for a certain period of time, record the volume change of the packaging bag, and draw a temperature-volume change value curve; Voltage change detection module: used to test the voltage change of small soft-pack laminated cells of the target SOC before and after being stored at different temperatures, and draw a temperature-voltage change curve; Data analysis and screening module: used to calculate the correlation coefficient of the slope of the temperature-volume change value curve and the temperature-voltage change value curve, and perform battery cell self-discharge screening based on the correlation coefficient.

9. A processing device, characterized in that The method comprises at least one processor and at least one memory in communication with the processor, wherein the memory stores program instructions executable by the processor, and the processor calls the program instructions to execute the method according to any one of claims 1 to 7.

10. A computer-readable storage medium, characterized in that The computer-readable storage medium stores computer instructions, and the computer instructions enable the computer to execute the method according to any one of claims 1 to 7.

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