Method for determining pre-tightening force of battery cell

Abstract

The invention provides a method for determining the pre-tightening force of a battery cell, and the method comprises the following steps: providing a plurality of initial-state battery cells which have the same chemical system as a target battery cell; and circulating the plurality of initial-state battery cells for a preset number of times at a preset charge-discharge rate to obtain a plurality of test battery cells. Different pre-tightening forces are applied to the multiple test battery cells, battery cell impedance data of the multiple test battery cells under the condition that the different pre-tightening forces are applied are obtained, and the pre-tightening forces are applied in the stacking direction of the pole pieces in the test battery cells. And determining the pre-tightening force of the target battery cell based on the battery cell impedance data. According to the method, a battery cell in an initial state is preprocessed to obtain a test battery cell, different pre-tightening forces are applied to the test battery cell, the impedance of the battery cell is obtained, and then an appropriate pre-tightening force range is obtained. The method avoids obtaining proper pre-tightening force by depending on long-term circulation data, remarkably shortens the verification period, reduces the test cost, and is flexible, convenient and reliable.

Description

Technical Field

[0001] The present application belongs to the field of battery technology and relates to a method for determining the preload force of a battery cell. Background Art

[0002] During the development of lithium-ion batteries (cells), optimizing cell design parameters (such as electrode compaction density, assembly gap, and preload force) is crucial for improving cycle life and preventing capacity drop. Applying an appropriate preload force is one of the key factors in improving cell cycle life, preventing capacity drop, and maintaining good electrochemical performance.

[0003] However, there are obvious limitations in the selection or design of the preload force of battery cells in the relevant technologies. On the one hand, there is a strong reliance on experience and the complexity of variables leads to deviations. The current setting of preload force is often based on experience or limited experimental data. However, since the battery cell design and assembly process involves many interrelated variables (such as electrode materials, compaction density, electrolyte volume, diaphragm characteristics, shell / fixture rigidity, etc.), it is difficult for empirical methods to accurately grasp the complex coupling relationship between the variables, which can easily lead to a large deviation between the set preload force and the actual demand, and it is impossible to achieve the ideal preload state. On the other hand, the evaluation of whether the preload force is appropriate and whether it can effectively cope with the maximum expansion force in the later stage of the cycle is highly dependent on actual measurement verification. To obtain the key data for verifying the effect of the preload force (especially the maximum expansion force), it is usually necessary to cycle the battery cell until the capacity dive stage occurs before it can be extracted, resulting in an extremely long test cycle, which seriously restricts the development and iteration speed of battery cells and battery modules.

[0004] Therefore, how to accurately and quickly obtain the preload range of the battery cell has become an important technical problem that needs to be solved urgently by those skilled in the art.

[0005] It should be noted that the above introduction to the technical background is merely intended to provide a clear and complete description of the technical solutions of this application and facilitate understanding by those skilled in the art. Simply because these solutions are described in the background technology section of this application, it should not be assumed that the above technical solutions are well known to those skilled in the art. Summary of the Invention

[0006] In view of the shortcomings of the related technologies mentioned above, the purpose of the present application is to provide a method for determining the pre-tightening force of a battery cell, so as to improve the problem that the pre-tightening force design of the battery cell needs to be improved.

[0007] To achieve the above-mentioned and other related purposes, the present application provides a method for determining the preload force of a battery cell, comprising the following steps:

[0008] Providing a plurality of initial state battery cells, wherein the initial state battery cells have the same chemical system as the target battery cells;

[0009] Cycling the plurality of initial state cells at a preset charge and discharge rate for a preset number of times to obtain a plurality of test cells;

[0010] Applying different preload forces to the plurality of test cells, and obtaining cell impedance data of the plurality of test cells under the different preload forces, wherein the preload forces are applied along a stacking direction of the electrode pieces in the test cells;

[0011] Based on the battery cell impedance data, a preload force of the target battery cell is determined.

[0012] In an optional embodiment, the preset number of times includes 50 to 200 times; and / or,

[0013] The chemical system is a lithium iron phosphate system, and the preset charge and discharge rates are: the charge rate range is 0.5C to 3C, and the discharge rate range is 0.5C to 3C; or,

[0014] The chemical system is a nickel-cobalt-manganese ternary system, and the preset charge and discharge rates are: the range of the charge rate is 0.5C to 5C, and the range of the discharge rate is 0.5C to 5C.

[0015] In an optional embodiment, the method includes the following steps:

[0016] Dispersively selecting a plurality of first preload test values ​​within the first preload range, and applying preload to the plurality of first test cells according to the first preload test values ​​based on the first preload application rule;

[0017] Obtaining, based on the first test method, a first impedance test value of each first test cell under the action of a first preload test value, and obtaining a corresponding relationship between the first preload test value and the first impedance test value;

[0018] Based on the corresponding relationship between each first preload test value and the first impedance test value, a first cell impedance-preload fitting curve is obtained by fitting, where the first cell impedance-preload fitting curve represents the relationship between the preload of the target cell and the cell impedance of the target cell;

[0019] According to the first battery cell impedance-preload force fitting curve, a preload force that enables the battery cell impedance of the target battery cell to meet the design requirements is determined as the preload force of the target battery cell.

[0020] In an optional embodiment, determining the pre-tightening force that makes the cell impedance of the target cell meet the design requirements as the pre-tightening force of the target cell further includes the following steps:

[0021] determining, according to the first cell impedance-preload force fitting curve, a preload force range that makes the cell impedance of the target cell lower than a predetermined threshold as a second preload force range;

[0022] Dispersively selecting a plurality of second preload test values ​​within the second preload range, and applying preload to the plurality of second test cells according to the second preload test values ​​based on the second preload application rule;

[0023] Obtaining a second impedance test value of each second test cell under the action of a second preload test value based on a second test method, and obtaining a corresponding relationship between the second preload test value and the second impedance test value;

[0024] A second cell impedance-preload fitting curve is obtained based on the corresponding relationship between each second preload test value and the second impedance test value, where the second cell impedance-preload fitting curve represents the relationship between the preload of the target cell and the cell impedance of the target cell;

[0025] According to the second battery cell impedance-preload force fitting curve, a preload force that enables the battery cell impedance of the target battery cell to meet the design requirements is determined as the preload force of the target battery cell.

[0026] In an optional embodiment, based on the preload force application rule, preload forces are applied to the plurality of test cells respectively according to respective preload force test values, including:

[0027] Apply preload force to several test cells according to each preload force test value;

[0028] The corresponding relationship between the preload test value and the impedance test value is determined as follows:

[0029] The average value of the impedance test values ​​of multiple test cells subjected to the same preload test value is used as the impedance test value corresponding to the preload test value.

[0030] In an optional embodiment, based on the preload force application rule, applying preload forces to the plurality of test cells respectively according to respective preload force test values ​​includes:

[0031] Applying multiple different preload forces to each test cell in ascending order according to multiple preload force test values, and obtaining an impedance test value of the test cell after applying each preload force;

[0032] The corresponding relationship between the preload test value and the impedance test value is determined as follows:

[0033] The average value of the impedance test values ​​of different test cells under the same preload test value is taken as the impedance test value corresponding to the corresponding preload test value.

[0034] In an optional embodiment, the second pre-tightening force applying rule is the same as the first pre-tightening force applying rule, or the second pre-tightening force applying rule is different from the first pre-tightening force applying rule; and / or,

[0035] The second test method is the same as the first test method, or the second test method is different from the first test method.

[0036] In an optional embodiment, any one of the first test method and the second test method is a DC resistance method, and obtaining the impedance test value of each test cell under the preload test value based on the impedance test method includes the following steps:

[0037] After applying a preload test value to the test cell, setting the test cell to a preset capacity state;

[0038] Applying a pulse current to the test cell, and obtaining an initial voltage of the test cell and a cut-off voltage of the test cell;

[0039] An impedance test value of the test cell is calculated based on the pulse current, the initial voltage, and the cut-off voltage.

[0040] In an optional embodiment, any one of the first test method and the second test method is an electrochemical impedance method, and obtaining the impedance test value of each test cell under the preload test value based on the impedance test method includes the following steps:

[0041] After applying the preload test value to the test cell, charging the test cell;

[0042] Applying an alternating current signal disturbance to the test cell to obtain an impedance spectrum of the test cell;

[0043] Fitting the impedance spectrum based on a second-order equivalent circuit model to obtain a preset impedance of the test cell, wherein the preset impedance includes at least one of ohmic impedance, membrane impedance, electrochemical reaction impedance, and diffusion impedance;

[0044] The cell impedance of the test cell is calculated based on the preset impedance.

[0045] In an optional embodiment, during the charging process of the test cell, an AC signal disturbance is synchronously applied to the test cell, and the impedance spectrum of the test cell under different capacity states is obtained. The cell impedance of the test cell is the average value of the cell impedance of the test cell under different capacity states.

[0046] As described above, the method of obtaining a comfortable preload force of a battery cell of the present application is to pre-treat the battery cell in the initial state (cycle a preset number of times at a preset charge and discharge rate) to obtain a test battery cell, and generate a certain amount of side reaction products inside the test battery cell and fill the pores of the electrode material, so that the impedance difference of the test battery cell under different preload forces is obvious, and the influence of the preload force on the internal state of the battery cell can be reflected by the impedance data, so as to obtain a suitable preload force range based on the impedance data. More importantly, this method can obtain a suitable preload force range in a relatively short time, avoid relying on long-term cycle data to obtain the preload force range, significantly shorten the verification cycle and reduce the testing cost. In addition, the testing method is flexible and convenient, and its reliability and accuracy are guaranteed, which is suitable for practical application. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 A flowchart of the steps of the method for determining the preload force of a battery cell provided in an embodiment of the present application;

[0048] Figure 2 A schematic diagram comparing the EIS curves of the initial state battery cell and the test battery cell under different preload forces provided in an embodiment of the present application;

[0049] Figure 3 A schematic diagram of the structure of the expansion force tooling provided in an embodiment of the present application;

[0050] Figure 4 The cell impedance-preload fitting curve obtained based on the DC resistance method provided in the embodiment of the present application;

[0051] Figure 5 The cell impedance-preload fitting curve obtained based on the electrochemical impedance spectroscopy provided in the embodiment of the present application;

[0052] Figure 6 A schematic diagram comparing the cycle curves of the test cells under different preload conditions provided in the embodiment of the present application.

[0053] Description of reference numerals:

[0054] 10. Test cell; 20. Expansion force tooling; 21. Tooling bottom plate; 22. Buffer pad; 23. Guide rail column; 24. Rotating shaft; 25. Pressure sensor; 26. First fixing plate; 27. Second fixing plate; 28. Tooling top plate. DETAILED DESCRIPTION

[0055] The following describes the embodiments of the present application through specific examples. Those skilled in the art will readily understand the other advantages and benefits of the present application from the disclosure herein. The present application may also be implemented or applied through various other specific embodiments, and the details in this specification may be modified or altered based on different viewpoints and applications without departing from the spirit of the present application.

[0056] See also Figures 1 to 6 It should be noted that the diagrams provided in this embodiment are only for schematically illustrating the basic concept of the present application. Therefore, the diagrams only show components related to the present application and are not drawn according to the number, shape and size of components in actual implementation. In actual implementation, the type, quantity and proportion of each component may be changed at will, and the component layout may also be more complex.

[0057] The mechanism analysis of the capacity drop caused by the maximum expansion force mentioned in the background art is as follows: the battery cell will produce volume changes (expansion, contraction) during charging and discharging, and the volume change includes a reversible part and an irreversible part. Among them, the reversible part is the expansion or contraction of the crystal structure of the positive and negative active materials during the charging and discharging process due to the deintercalation of lithium in the material. This part can be restored when the battery cell returns to its initial state (i.e., the reversible part); the irreversible part is mainly the oxidation reaction of the positive electrode and the reduction reaction of the negative electrode during the charging and discharging process, generating by-products, and the generated by-products are continuously deposited on the surface of the active material or in the gaps between the positive and negative electrodes, causing the battery cell to gradually thicken. After the by-products fill the surface pores, they will hinder the transmission of most lithium ions, so that as the number of by-products increases, the impedance of the battery cell continues to increase, which in turn causes the capacity of the battery cell to drop significantly. The battery cell after cycling is disassembled, and the surface electron microscope image of the negative electrode shows layered by-products. The amount of by-products generated by the Coulomb efficiency calculation and the volume calculated by the porosity all verify the capacity drop mechanism analyzed above. In addition to by-products, the expansion force will also affect the porosity between the active materials of the battery cell. The gradual increase in the expansion force will further compress the porosity and aggravate the capacity drop process.

[0058] Based on the above analysis, this application provides a method for determining the preload force of a battery cell. By applying an appropriate preload force to the battery cell, the electrodes in the battery cell are tightly fitted, keeping the impedance in the battery cell at a low level. This prolongs the time it takes for the battery cell to reach its maximum expansion force, slows the process of capacity drop, and improves the cycle life of the battery cell. This method can also determine the preload force range of the battery cell in a relatively short time, effectively shortening the battery cell verification and development cycle and reducing costs. The specific technical solution is described below.

[0059] Example 1

[0060] This embodiment provides a method for determining the preload force of a battery cell. Figure 1 , Figure 1 A flow chart of the steps of the method is shown, and the method comprises the following steps:

[0061] S1: Provide multiple initial state cells, the chemical system of the initial state cells is the same as the target cell;

[0062] S2: Cycling the plurality of initial state batteries at a preset charge and discharge rate for a preset number of times to obtain a plurality of test batteries;

[0063] S3: applying different preload forces to the plurality of test cells, and obtaining cell impedance data of the plurality of test cells under the different preload forces, wherein the preload forces are applied along a stacking direction of the electrodes in the test cells;

[0064] S4: Determine the preload force of the target battery cell based on the battery cell impedance data.

[0065] First, execute step S1 to provide a plurality of initial state battery cells, and the chemical system of the initial state battery cells is the same as that of the target battery cells. That is, a plurality of initial state battery cells are provided according to the chemical system of the target battery cells in the development stage, and a preload range is further obtained based on the initial state battery cells, and the preload range is used as a reference range for applying the preload of the target battery cells. Among them, the initial state battery cells refer to "fresh" battery cells that have not been circulated after assembly, injection, packaging and formation (that is, the initial state battery cells are battery cells that have not undergone circulation after production). The "chemical system" includes at least one of the positive electrode material system, negative electrode material system and electrolyte system of the battery.

[0066] In some embodiments, the structural parameters of the initial state battery cell and the target battery cell are the same, and the structural parameters include at least one of the specifications of the positive electrode sheet, the specifications of the negative electrode sheet, the specifications of the diaphragm, the specifications of the electrolyte, the electrode composition molding process (for example, lamination, winding), and the packaging form (for example, soft pack, square shell). Furthermore, the initial state battery cell is manufactured based on the design parameters of the target battery cell. For example, the initial state battery cell is a soft pack battery cell and the target battery cell is a square shell battery cell, but the chemical systems of the two are the same. Therefore, the preload range obtained based on the soft pack battery cell can be used as the preload range of the target battery cell.

[0067] Furthermore, without considering the assembly or sale of the target battery cells in actual applications, the initial state battery cells and the target battery cells can belong to the same production batch, so that the initial state battery cells are closest to the target battery cells. In actual production, since the target battery cells obtained in mass production may be stored for a short time after production and then enter the packing process, if a portion of the target battery cells from the same production batch are selected as the initial state battery cells for preload range testing, it may be difficult to obtain the preload range in time. Therefore, a small batch of initial state battery cells can be produced based on the design parameters of the target battery cells before mass production, so as to obtain a more reliable preload range.

[0068] Then, step S2 is performed to cycle the plurality of initial state battery cells at a preset charge and discharge rate for a preset number of times to obtain a plurality of test battery cells.

[0069] In the embodiment of the present application, since there are very few side reaction products inside the initial state battery cell, after applying different pressures to the initial state battery cell, the compression state of the battery cell is relatively close, and the effect on the impedance is also relatively close, and the accuracy of the obtained preload range is poor. Before obtaining the preload range with an initial state battery cell with the same chemical system as the target battery cell, the initial state battery cell is first subjected to a preset number of charge and discharge cycles to produce a certain amount of by-products inside the battery cell during the cycle, and the by-products are filled in the pores of the electrode (its modulus is much smaller than the size of the negative electrode material), which can amplify the effect of the subsequent preload applied to the test battery cell on the impedance of the battery cell. Compared with directly obtaining the preload range based on the initial state battery cell, the obtained preload range is more accurate. The charge and discharge rates and number of cycles of multiple initial state batteries are kept consistent to avoid differences in the number of products due to different cycle parameters, thereby affecting the test accuracy.

[0070] See also Figure 2 , Figure 2 The figure shows the EIS impedance comparison curve of the initial state battery cell and the test battery cell. Figure 2 The preload force applied by the initial state cell 1 and the initial state cell 2 is the same, and the preload force applied by the test cell 1 and the test cell 2 is the same. Figure 2It can be seen that although different preload forces are applied, the EIS impedance curves of the initial state cell 1 and the initial state cell 2 are close (basically overlap). In contrast, the EIS impedance curves of the test cell 1 and the test cell 2 are significantly different. This shows that the internal compression state (or the degree of fit of the electrode, porosity, etc.) of the initial state cell is relatively close under different preload forces. After the initial state cell is cycled a preset number of times, a certain amount of organic byproducts are generated inside the test cell and distributed around the electrode material. The modulus is much smaller than the size of the negative electrode material and will fill the pores of the negative electrode material. At this time, the difference between the cell impedances under different preload forces will increase, that is, the effect of the preload force on the cell impedance can be amplified by a preset number of cycles.

[0071] In some embodiments, the chemical system is a lithium iron phosphate system, and the preset charge and discharge rates are: the charge rate range is 0.5C to 3C, and the discharge rate range is 0.5C to 3C. When the chemical system of the initial state battery cell (and the target battery cell) is a lithium iron phosphate system, when the charge and discharge rate cycle of the initial state battery cell is performed, the charge rate can be 0.5C, 1C, 1.5C, 2C or 3C, and the discharge rate can be 0.5C, 1C, 1.5C, 2C or 3C.

[0072] In other embodiments, the chemical system is a nickel-cobalt-manganese ternary system, and the preset charge and discharge rates are: the charge rate range is 0.5C to 5C, and the discharge rate range is 0.5C to 5C. When the chemical system of the initial state battery cell (and the target battery cell) is a nickel-cobalt-manganese ternary system, in the charge and discharge rate cycle of the initial state battery cell, the charge rate can be 0.5C, 1C, 1.5C, 2C, 3C, 4C or 5C, and the discharge rate can be 0.5C, 1C, 1.5C, 2C, 3C, 4C or 5C.

[0073] Specifically, since the negative electrode material system and the electrolyte system in the chemical system of the battery cell usually adopt the mainstream system (for example, the negative electrode material system is mainly a graphite system), and the influence of the two on the expansion behavior of the battery cell and the number and generation rate of side reaction products produced during the cycle may not be as significant as the influence of the positive electrode material system, therefore, in the embodiments of the present application, targeted experimental verification of the preset charge and discharge rates was carried out for two types of the mainstream positive electrode material systems, and the charge rates and discharge rates corresponding to the above two different chemical systems were summarized.

[0074] In some embodiments, the preset number of times includes 50 to 200 times, for example, the preset number of times can be 50 times, 100 times, 150 times, or 200 times. Typically, the cycle life of the initial state battery cell (or target battery cell) is much higher than 200 times (for example, greater than 1000 times). Although cycling the initial state battery cell a few times (or a small number of times) will extend the acquisition time of the preload range to a certain extent, compared to the related art that requires cycling the battery cell until the capacity dive occurs before data can be extracted, it still significantly shortens the verification time and battery cell development cycle, making it suitable for practical applications.

[0075] Then, step S3 is executed to apply different preload forces to the multiple test cells and obtain cell impedance data of the multiple test cells under different preload forces. The preload forces are applied along the stacking direction of the electrode sheets in the test cells. It is easy to understand that the stacking direction of the electrode sheets is the direction in which the positive electrode sheet, the separator, and the negative electrode sheet are stacked in sequence in the battery cell.

[0076] In some embodiments, step S3 includes the following steps: selecting a plurality of first preload test values ​​within a first preload range; applying preload to each of the plurality of first test cells according to the first preload test values ​​based on a first preload application rule; obtaining a first impedance test value for each first test cell under the first preload test value based on a first testing method; and obtaining a correspondence between the first preload test value and the first impedance test value (i.e., cell impedance data for the first test cell).

[0077] Furthermore, the first preload range is set based on the target battery cell's operating pressure range. Since the actual operating pressure of the battery cell has a pressure threshold (for example, the safety threshold of the extrusion pressure perpendicular to the electrode is 5 MPa), the battery cell needs to be used within this pressure threshold, otherwise the structural integrity and safety of the battery will be damaged. Therefore, the upper limit of the first preload range needs to be less than this pressure threshold to obtain the preload range without damaging the battery's performance.

[0078] In the embodiment of the present invention, based on the preload force application rule, applying preload forces to the plurality of test cells according to the respective preload force test values ​​includes the following two methods:

[0079] Method 1: Applying preload to multiple test cells according to each preload test value. Correspondingly, determining the correspondence between the preload test value and the impedance test value includes taking the average of the impedance test values ​​of multiple test cells subjected to the same preload test value as the impedance test value corresponding to the preload test value.

[0080] In some embodiments, multiple first test cell groups are obtained based on the division of multiple test cells, each first test cell group includes at least one first test cell, and the first preload force application rule includes: applying different preload forces to the first test cells in different first test cell groups with a first preload force gradient, wherein the preload forces applied to the first test cells in the same first test cell group are the same, and the preload forces applied to the first test cells in different first test cell groups are different. The average value of the cell impedances of the first test cells in the same first test cell group is used as the cell impedance under the corresponding preload force.

[0081] In a specific example, the first preload range is 0N to 7000N, and the first preload gradient is 1000N, then multiple test cells need to be divided into eight first test cell groups, each test cell group includes three first test cells, and each first test cell in the eight first test cell groups is respectively applied with a preload of 0N, 1000N, ..., 7000N, and the cell impedance of each first test cell is obtained, and the average of the cell impedances of the three first test cells in the same first test cell group is taken as the cell impedance value of the preload corresponding to the first test cell group. At this time, the first cell impedance data consists of eight different preload values ​​and eight corresponding cell impedance values.

[0082] Method 2: Apply multiple different preload forces to each test cell in ascending order according to multiple preload force test values, and obtain the impedance test value of the test cell after each preload force application. Accordingly, determining the correspondence between the preload force test value and the impedance test value includes taking the average of the impedance test values ​​of different test cells at the same preload force test value as the impedance test value corresponding to the corresponding preload force test value.

[0083] In some embodiments, at least one first test cell group is obtained based on the division of multiple test cells, each first test cell group includes at least one first test cell, and a first preload force application rule includes: applying a preload force to each first test cell in the same first test cell group at least twice from low to high; and obtaining the cell impedance of the first test cell after each application of the preload force. The average cell impedance of different first test cells under the same preload force is used as the cell impedance under the corresponding preload force.

[0084] In some embodiments, a first test cell group is obtained based on multiple test cells, and a preload force is applied m times to each first test cell in the first test cell group, where m is: the ratio of the difference between the upper limit of the first preload force range and the lower limit of the first preload force range to the numerical value corresponding to the first preload force gradient.

[0085] In a specific example, the first pre-load force range is 0N to 7000N, and the first pre-load force gradient is 1000N. The number of first test cells in the first test cell group is eight. These eight first test cells are applied with pre-load forces of 0N, 1000N, ..., 7000N from low to high, and the cell impedance of the first test cell under the corresponding pre-load force state is obtained after each application of the pre-load force, and the average of the cell impedances of the eight first test cells under each pre-load force state is taken as the cell impedance of the pre-load force.

[0086] In other embodiments, multiple first test cell groups are obtained based on the plurality of test cells. Each first test cell in each first test cell group is subjected to n preload forces, where n < m. Among the n preload forces applied to the first test cells in different first test cell groups, at least two of the preload forces have different values.

[0087] In a specific example, the first preload range is 0N to 7000N, and the first preload gradient is 1000N. Four first test cell groups are obtained based on multiple test cells, and each first test cell group has three first test cells. 0N is first applied to each first test cell in the first first test cell group to obtain the cell impedance under the 0N preload state, and then 4000N is applied to each first test cell to obtain the cell impedance under the 4000N preload state. 1000N / 5000N, 2000N / 6000N, and 3000N / 7000N are applied to the first test cells in the next three first test cell groups, respectively, to obtain the cell impedance value of the first test cell under each preload within the first preload range, and then obtain the first cell impedance data.

[0088] In another specific example, the first preload range is 0N to 7000N, and the first preload gradient is 1000N. Four first test cell groups are obtained based on multiple test cells. There are three first test cells in each first test cell group. 0N / 2000N / 4000N, 1000N / 3000N / 5000N, 2000N / 4000N / 6000N and 3000N / 5000N / 7000N are applied to the first test cells in the four first test cell groups respectively, so as to obtain the cell impedance value of the first test cell under each preload within the first preload range, and then obtain the first cell impedance data.

[0089] It should be noted that in the above two specific examples, different preload forces are uniformly applied to each first test cell for illustration (i.e., according to the first preload gradient). In actual application, on the premise of obtaining the cell impedance value corresponding to each gradient preload force within the first preload range, the number and size of preload forces applied to each test cell can be selected based on actual needs (i.e., the preload force can be uniformly distributed or non-uniformly distributed). Similarly, the number of first test cells in different first test cell groups can also be the same or different. In addition, the first preload range, the first preload gradient, the number of first test cell groups, and the number of first test cells in each first test cell group in the above specific examples are all used as examples for illustration. In actual application, they can be selected and adjusted according to actual needs. For example, in order to maximize the accuracy and reliability of the preload range, multiple groups of first test cell groups can be set and the number of first test cells in each first test cell group can be set to multiple, the first preload gradient can be as small as possible, and so on.

[0090] In the embodiments of the present application, the two aforementioned methods can be used to simply and accurately obtain the cell impedance values ​​under different preload conditions within the first preload range. Specifically, when testing using method 2, differences between parallel samples (i.e., slight differences between different test cells) can be eliminated, further reducing test errors. Furthermore, while achieving multiple measurements at the same preload to reduce test errors, the number and cost of test cells can also be effectively reduced, making this method suitable for efficient and low-cost testing.

[0091] In some embodiments, applying the preload force to the test cell is achieved based on an expansion force fixture. Figure 3 , Figure 3 A structural schematic diagram of an expansion force tooling is shown, and the expansion force tooling 20 includes a tooling base plate 21, a buffer pad 22 (for example, a foam pad), a guide rail assembly, a rotating shaft 24, a pressure sensor 25, a fixed assembly and a tooling top plate 28. Among them, the guide rail assembly is arranged on the tooling base plate 21, and the guide rail assembly includes a plurality of discrete guide rail columns 23. The fixed assembly is movably connected to the guide rail columns 23. The fixed assembly includes a first fixed plate 26 and a second fixed plate 27 spaced apart, and the first fixed plate 26 is located between the tooling base plate 21 and the second fixed plate 27, and there is a certain space between the first fixed plate 26 and the second fixed plate 27. The pressure sensor 25 is arranged between the first fixed plate 26 and the second fixed plate 27. The tooling top plate 28 is arranged on the side of the second fixed plate 27 away from the first fixed plate 26. The rotating shaft 24 is telescopically and rotatably arranged between the tooling top plate 28 and the second fixed plate 27.

[0092] Furthermore, applying a pre-tightening force to the test cell 10 includes the following steps: arranging a buffer pad 22 between the test cell 10 and the tooling base plate 21, and between the test cell 10 and the first fixed plate 26, so as to provide a buffering protection for the test cell 10. Rotating the rotating shaft 24 to drive the fixing assembly to move along the guide rail assembly toward the tooling base plate 21, and generating a force (i.e., a pre-tightening force) on the test cell 10. During the rotation of the rotating shaft 24, the value of the pre-tightening force applied to the test cell 10 based on the rotation of the rotating shaft is determined by the pressure sensor 25. When the pre-tightening force applied to the test cell 10 reaches a preset value, the cell impedance of the test cell 10 is subsequently tested.

[0093] It should be noted that the embodiments of this application are only based on Figure 3 The expansion force fixture shown in the figure is used to illustrate the process of applying the preload force. In actual application, it is not limited to Figure 3 The expansion force shown applies pre-tightening force to the test cell. On the premise of achieving accurate and stable pre-tightening force application to the test cell, other forms or structures of expansion force work or mechanical structures can also be used to perform the above steps, which are not illustrated one by one here.

[0094] In some embodiments, the impedance test method in the first test method (and / or the second test method described below) is a DC resistance method or an electrochemical impedance method, wherein the DC resistance method is relatively fast and simple, but mainly reflects the ohmic impedance inside the battery cell and is suitable for quickly and roughly determining the range of the preload force; the electrochemical impedance method can more comprehensively reflect the impedance state of different characteristics inside the battery cell, and is more accurate in evaluating the impact of the preload force on the inside of the battery cell, but the test process is more complicated and takes a long time. Next, the implementation methods of the DC resistance method and the electrochemical impedance method are illustrated.

[0095] In some embodiments, the impedance test method in the first test method is a DC resistance method, and obtaining the impedance test value of each test cell under the preload test value based on the impedance test method includes the following steps: after applying the preload test value to the test cell, setting the test cell to a preset capacity state. Apply a pulse current to the test cell, and obtain the initial voltage of the test cell and the cut-off voltage of the test cell. Based on the pulse current, the initial voltage and the cut-off voltage, calculate the impedance test value of the test cell, and the calculation formula is: (V1-V0) / I, wherein V0 is the initial voltage (i.e., the voltage at the moment the pulse current is applied), V1 is the cut-off voltage (i.e., the voltage at the moment the pulse current ends), and I is the applied pulse current value.

[0096] Furthermore, the range of the preset capacity state is 70% SOC to 90% SOC (State of Charge, abbreviated as SOC), and SOC refers to the ratio of the current capacity of the first test battery cell to the rated capacity, that is, when the test battery cell is charged to 70% to 90% of the rated capacity, it is the preset capacity state. For example, the preset capacity state is 70% SOC, 80% SOC or 90% SOC.

[0097] In the embodiment of the present application, the reason for setting the above-mentioned range of the preset capacity state is that: within the above-mentioned range, the chemical reaction inside the battery cell is in a relatively balanced state, and the battery cell impedance obtained based on this can accurately reflect the actual state inside the battery cell (including the degree of fit and porosity of the electrode), and after applying the pulse current, the voltage response is timely and stable, and the test accuracy and reliability are high. When the first test cell is in a low-capacity state (for example, less than 70% SOC) or a high-capacity state (for example, >90% SOC), the obtained battery cell impedance may be too high, which is not conducive to obtaining a suitable preload range. For example, when the first test cell is in a low-capacity state, after applying a pulse current to the test cell, the voltage response is delayed, thereby affecting the accuracy of the battery cell impedance calculated based on the voltage and pulse current.

[0098] Furthermore, the pulse current range is 0.2C to 1C. For example, the pulse current can be 0.2C, 0.5C, or 1C. It should be noted that the application time of the pulse current needs to be strictly controlled to avoid the voltage not being able to feedback in a timely and effective manner, thereby affecting the test accuracy. For example, the application time of the pulse current is 5s to 30s.

[0099] In some embodiments, the impedance test method in the first test method is an electrochemical impedance method, and obtaining the impedance test value of each test cell under the preload test value based on the impedance test method includes the following steps: after applying the preload test value to the test cell, charging the test cell. Applying an AC signal disturbance to the test cell to obtain the impedance spectrum of the test cell. Fitting the impedance spectrum based on the second-order equivalent circuit model to obtain a preset impedance of the test cell, the preset impedance includes at least one of ohmic impedance, membrane impedance, electrochemical reaction impedance and diffusion impedance. The cell impedance of the test cell is calculated based on the preset impedance. Wherein, before charging the test cell, the step of leaving the test cell to stand for a preset time (for example, the preset time is 1 hour) is also included to allow the internal state of the test cell to reach equilibrium, so as to avoid interference with the cell impedance caused by the unstable internal state of the test cell due to transportation or movement.

[0100] Furthermore, the AC signal includes AC voltage or AC current, and the frequency range of the AC signal is 100 kHz to 50 MHz. The wider frequency range can more comprehensively obtain impedance information corresponding to the main physical and chemical processes inside the battery cell.

[0101] In some embodiments, during the charging process of the test cell, an AC signal disturbance is synchronously applied to the test cell (i.e., an in-situ testing method). On this basis, the impedance of the test cell under different capacity states is also obtained. The cell impedance of the test cell is the average value of the cell impedance of the first test cell under different capacity states. The in-situ testing method can realize the charging of the cell and the measurement of the cell impedance without disassembling the cell, avoid the test deviation caused by the change of the position of the test clamp clamped on the test cell, and help to ensure the reliability of the test.

[0102] Furthermore, obtaining the impedance spectrum of the test cell under different capacity states includes: during the charging process of the test cell, collecting the cell impedance of the test cell under the corresponding preload force every time the capacity of the test cell increases by a certain value (for example, 10% SOC), and then taking the average of the impedances obtained under different capacity states under multiple same preload forces.

[0103] Next, step S4 is executed to determine the preload force of the target battery cell based on the battery cell impedance data.

[0104] In some embodiments, step S4 includes the following steps: based on the correspondence between each first preload test value and the first impedance test value, fitting a first cell impedance-preload fitting curve, the first cell impedance-preload fitting curve representing the relationship between the preload of the target cell and the cell impedance of the target cell. Based on the first cell impedance-preload fitting curve, determining the preload that makes the cell impedance of the target cell meet the design requirements as the preload of the target cell.

[0105] Furthermore, based on the first cell impedance-preload fitting curve, determining the preload that makes the cell impedance of the target cell meet the design requirements as the preload of the target cell, including: obtaining the cell impedance value x corresponding to the lowest point in the first cell impedance-preload fitting curve, determining the cell impedance threshold y (y>x) based on the cell impedance value x, selecting the preload corresponding to the cell impedance in the first cell impedance-preload fitting curve being less than the cell impedance threshold as the initial preload range of the target cell, and the preload of the target cell is selected within the initial preload range. Furthermore, due to the selectivity of the first test method (either the DC resistance method or the electrochemical impedance method can be selected) and the large range of resistance values ​​obtained by different methods, the cell impedance threshold y also needs to be set specifically. After experimental verification, when the first test method is the DC resistance method, y = (x + 1) mΩ. When the first test method is the electrochemical impedance method, y = (x + 0.03) Ω.

[0106] In some embodiments, determining the pre-tightening force that makes the cell impedance of the target cell meet the design requirements as the pre-tightening force of the target cell further includes the following steps:

[0107] According to the first cell impedance-preload fitting curve, the preload range (i.e., the initial preload range mentioned above) that makes the cell impedance of the target cell lower than a predetermined threshold is determined as the second preload range. Multiple second preload test values ​​are selected in a dispersed manner in the second preload range, and based on the second preload application rule, preload is applied to the multiple second test cells according to each second preload test value. Based on the second test method, the second impedance test value of each second test cell under the action of the second preload test value is obtained, and the correspondence between the second preload test value and the second impedance test value is obtained. Based on the correspondence between each second preload test value and the second impedance test value, a second cell impedance-preload fitting curve is obtained by fitting, and the second cell impedance-preload fitting curve represents the relationship between the preload of the target cell and the cell impedance of the target cell. According to the second cell impedance-preload fitting curve, the preload that makes the cell impedance of the target cell meet the design requirements is determined as the preload of the target cell. In other words, the above steps are used to optimize the initial preload range obtained based on the first cell impedance-preload fitting curve, so that the final preload range is more suitable for the actual application of the target cell. Furthermore, the value density of the second preload test value is higher than that of the first preload test value, or the gradient of the second preload test value is smaller than the interval of the first preload test value.

[0108] In some embodiments, the method further includes the following steps: further dividing the plurality of test cells to obtain at least one second test cell group, each second test cell group including at least one second test cell. Based on a second preload application rule, within a second preload range, a preload is applied to each second test cell with a second preload gradient, and the second preload gradient is less than the first preload gradient (for example, the second preload gradient is half of the first preload gradient).

[0109] In some embodiments, the second preload force application rule is the same as the first preload force application rule. Alternatively, the second preload force application rule is different from the first preload force application rule. In a specific example, the first preload force application rule selects method one, and the second preload force application rule selects method two.

[0110] In some embodiments, the second test method is the same as the first test method, for example, both the first test method and the second test method use a DC resistance method or an electrochemical impedance spectroscopy method. Alternatively, the second test method is different from the first test method, for example, the first test method uses a DC resistance method and the second test method uses an electrochemical impedance spectroscopy method. Of course, the first test method may also use an electrochemical impedance spectroscopy method and the second test method may use a DC resistance method.

[0111] In the embodiment of the present application, after taking the aforementioned steps to obtain a relatively suitable initial preload range, the initial preload range is further optimized to obtain a target preload range, which can improve acquisition efficiency and reduce the time and cost of testing battery cells. Of course, the aforementioned steps can also be designed more carefully, so that the initial preload range is the target preload range, for example, reducing the first preload gradient to the second preload gradient, or directly performing a more intensive first preload test value setting. However, compared with the further optimized solution, the number of tested battery cells, test time, and test cost are all increased.

[0112] It should be noted that, in order to facilitate a detailed introduction to the method provided in the embodiments of the present application, multiple test cells are divided into first test cells and second test cells, wherein a first cell impedance-preload fitting curve is obtained based on at least a portion of the multiple test cells (multiple first test cells), and an initial preload range is obtained. When the application scenario of the target cell has relatively loose requirements for the preload applied to the target cell, the preload applied to the target cell can be directly selected from the initial preload range, which can basically achieve an effect of improving the cycle life of the target cell (at this time, all of the multiple test cells can be used as first test cells). In the case where the application scenario of the target battery cell has more stringent performance requirements (for example, cycle life and reliability), a part of the multiple test cells can be used as the first test cells, and another part of the multiple test cells can be used as the second test cells. The initial preload force range is first obtained based on the multiple first test cells, and then the initial preload force range is further optimized and limited based on the multiple second test cells to obtain a better preload force range (target preload force range), and then the preload force applied to the target battery cell is selected from the preload force range.

[0113] Example 2

[0114] This embodiment also provides a specific example of the method for determining the preload force of the battery cell described in Implementation 1, and the specific steps are as follows:

[0115] Multiple initial state battery cells are provided. The initial state battery cells are in the form of small soft packs. The positive electrode material is a lithium iron phosphate system, and the negative electrode material is a graphite system. The length of the initial state battery cell is 96 mm and the width is 51 mm.

[0116] A plurality of initial state battery cells were cycled with a voltage of 2.6V-3.65V and a capacity of 3.8Ah at 25°C with a 1C charge rate and a 1C discharge rate, and the cycle was terminated after 100 cycles to obtain a plurality of test battery cells. Compared with the initial state battery cells, a certain amount of side reaction products are generated inside the test battery cells, and the side reaction products are filled in the pores of the active material, which can achieve the impedance amplification effect of the battery cells under different preload forces. It should be noted that since the initial preload force range obtained was not further optimized in this specific example, the above-mentioned multiple test battery cells were divided into five test battery cell groups, each test battery cell group included five test battery cells, and the test battery cells were in an empty state after the cycle was completed.

[0117] According to the actual application range of the battery cell, the preload range is set (1000N~7000N), and the preload forces applied to different test battery cell groups are set to 0N, 1000N, 3000N, 5000N and 7000N respectively (where the preload force of 0N is used as a comparison reference, and the preload force gradient is 2000N). Figure 3The expansion force fixture shown applies the corresponding preload force to the test cell.

[0118] Since the porosity in the test cell changes under different preloads, the impedance changes corresponding to different preloads are shown. The steps for obtaining cell impedance data using the DC resistance method are as follows: when each test cell is charged to 90% SOC, let it stand for 1 hour, record the initial voltage V0, and apply a pulse current to the test cell (0.5C charging for 30s), and record the cut-off voltage V1 when the pulse current is withdrawn, and calculate the cell impedance based on Ohm's law: (V1-V0) / (0.5×Q), where 0.5×Q=I, Q is the cell capacity), to obtain Table 1, and further fit the data based on Table 1 to obtain Figure 4 The cell impedance-preload fitting curve is shown. It should be noted that in Table 1 (and Table 2 below), the preload is converted to preload pressure based on the pressure formula. In actual application, the target preload range can be converted to preload pressure for application to avoid errors caused by different preload-bearing areas of the battery cells.

[0119] Table 1. Cell impedance data obtained by DC resistance method

[0120] Preload pressure / Mpa 0 0.204 0.613 1.021 1.43 Cell internal resistance / mΩ 29.344 28.073 27.073 28.800 30.028

[0121] Alternatively, the electrochemical impedance method can be used to obtain resistance data. The steps are as follows: let the test cell stand for 1 hour to allow the test cell to reach equilibrium, charge each test cell, and apply 300mAh AC current interference with a frequency range of 100kHz to 50MHz to the test cell during the charging process, and test the electrochemical impedance of the test cell at every 10% SOC interval. Then, a second-order equivalent circuit model is used to fit the ohmic impedance, membrane impedance, electrochemical impedance, and diffusion impedance respectively to calculate the total impedance at each capacity state, and then the average of the total impedance of multiple capacity states is taken as the cell impedance, and Table 2 is obtained, and further fitting is performed based on the data in Table 2 to obtain Figure 5 The cell impedance-preload fitting curve is shown.

[0122] Table 2. Cell impedance data obtained by electrochemical impedance spectroscopy

[0123] Preload pressure / Mpa 0 0.204 0.613 1.02 1.43 Cell internal resistance / Ω 0.111417987 0.064198407 0.03019077 0.113692372 0.12099687

[0124] After obtaining the preload range (i.e., target preload) using the above steps, the test cells under different preload conditions were cycled for a long period (1C / 1C rate cycling), and the capacity retention rate of each test cell was compared with the number of cycles (e.g., Figure 6As shown), to verify whether the cycle life of the test cell within the target preload range is improved, thereby verifying the reliability of the method of the embodiment of the present application.

[0125] See also Figure 4 and Figure 5 ,in, Figure 4 The figure shows the cell impedance-preload fitting curve obtained by the DC resistance method. The preload range obtained by the DC resistance method is a to b. Figure 5 The figure shows the cell impedance-preload fitting curve obtained by electrochemical impedance spectroscopy. The preload range obtained by electrochemical impedance spectroscopy is c to d. Figure 4 and Figure 5 It can be seen that as the preload applied to the test cell increases, the cell impedance shows a trend of first increasing and then decreasing. The reason for this trend is that when the preload is small, the electrodes inside the cell are not tightly fitted, the ionic conductivity and electronic conductivity are low, and the overall cell impedance is high. When the preload gradually increases, the overall porosity of the cell decreases (for example, the pores between the electrode materials, the gaps between the electrodes, etc.), the ion (for example, lithium ion) conductivity decreases, and the electronic conductivity increases, so that the cell will have a maximum conductivity during the gradual increase of the preload, and the cell impedance is the smallest at the maximum conductivity. Therefore, according to the size of the cell impedance, the preload corresponding to the smaller cell impedance is selected to form the preload range, which can effectively extend the time it takes for the cell to reach the maximum expansion force, thereby extending the cycle life of the cell. Comparison Figure 4 and Figure 5 It can be seen that a and c (b and d) are relatively close, that is, different first test methods can be used to obtain similar initial preload ranges. Figure 4 and Figure 5 It can be seen that the preload force obtained in this embodiment ranges from 0.2 MPa to 0.8 MPa.

[0126] Please refer to Figures 4 to 6 ,in, Figure 6 Shows a schematic diagram of the cycle curve of the test cell, Figure 6 The number of cycles in the test cell does not refer to the number of cycles during the conversion from the initial state cell to the test cell. Figure 6 It can be seen that for the test cell without preload (preload is 0 MPa), the capacity retention rate drops to below 90% after about 1,000 cycles, while the test cell with appropriate preload (appropriate preload refers to the one selected from the initial preload range) still has a capacity retention rate above 90% after 1,100 cycles. That is, when the preload is gradually increased to the appropriate range, the capacity attenuation of the test cell is improved. After exceeding the preload range, the capacity of the test cell decays rapidly with the increase of the preload, and the capacity retention rate drops to below 80% after less than 600 cycles. Therefore, Figure 6 It can be proved that the preload force range obtained by the method according to the embodiment of the present application can significantly improve the cycle life of the battery cell.

[0127] In summary, the method for determining the preload force of the battery cell of the present application is to obtain a test battery cell by pre-treating the battery cell in the initial state (cycling a preset number of times at a preset charge and discharge rate), and generating a certain amount of side reaction products inside the test battery cell and filling the pores of the electrode material, so that the impedance difference of the test battery cell under different preload forces is obvious, and the influence of the preload force on the internal state of the battery cell can be reflected by the impedance data, so as to obtain a suitable preload force range based on the impedance data. More importantly, this method can obtain a suitable preload force range in a relatively short period of time, avoid relying on long-term cycle data to obtain the preload force range, significantly shorten the verification cycle and reduce the testing cost. In addition, the test method is flexible (not limited to a single type of battery cell test, nor limited to a single method step), convenient, and its reliability and accuracy are guaranteed, making it suitable for practical application.

[0128] The above embodiments are merely illustrative of the principles and effects of this application and are not intended to limit this application. Anyone skilled in the art may modify or alter the above embodiments without departing from the spirit and scope of this application. Therefore, all equivalent modifications or alterations made by one of ordinary skill in the art without departing from the spirit and technical concepts disclosed in this application shall be covered by the claims of this application.

Claims

1. A method for determining the preload force of a battery cell, characterized in that: The following steps are involved: Providing a plurality of initial state battery cells, wherein the initial state battery cells have the same chemical system as the target battery cells; Cycling the plurality of initial state battery cells at a preset charge and discharge rate for a preset number of times to obtain a plurality of test battery cells; Applying different preload forces to the plurality of test cells, and obtaining cell impedance data of the plurality of test cells under the different preload forces, wherein the preload forces are applied along a stacking direction of the electrode pieces in the test cells; Based on the battery cell impedance data, a preload force of the target battery cell is determined.

2. The method for determining the preload force of a battery cell according to claim 1, wherein: The preset number of times includes 50 to 200 times; and / or, The chemical system is a lithium iron phosphate system, and the preset charge and discharge rates are: the charge rate range is 0.5C to 3C, and the discharge rate range is 0.5C to 3C; or, The chemical system is a nickel-cobalt-manganese ternary system, and the preset charge and discharge rates are: the charge rate range is 0.5C to 5C, and the discharge rate range is 0.5C to 5C.

3. The method for determining the preload force of a battery cell according to claim 1, wherein: The method comprises the following steps: Dispersively selecting a plurality of first preload test values ​​within the first preload range, and applying preload to the plurality of first test cells according to the first preload test values ​​based on the first preload application rule; Obtaining, based on the first test method, a first impedance test value of each first test cell under the action of a first preload test value, and obtaining a corresponding relationship between the first preload test value and the first impedance test value; Based on the corresponding relationship between each first preload test value and the first impedance test value, a first cell impedance-preload fitting curve is obtained by fitting, where the first cell impedance-preload fitting curve represents the relationship between the preload of the target cell and the cell impedance of the target cell; According to the first battery cell impedance-preload force fitting curve, a preload force that enables the battery cell impedance of the target battery cell to meet the design requirements is determined as the preload force of the target battery cell.

4. The method for determining the preload force of a battery cell according to claim 3, wherein: Determining the pre-tightening force that makes the cell impedance of the target cell meet the design requirements as the pre-tightening force of the target cell further includes the following steps: determining, according to the first cell impedance-preload force fitting curve, a preload force range that makes the cell impedance of the target cell lower than a predetermined threshold as a second preload force range; Dispersively selecting a plurality of second preload test values ​​within the second preload range, and applying preload to the plurality of second test cells according to the second preload test values ​​based on the second preload application rule; Obtaining a second impedance test value of each second test cell under the action of a second preload test value based on a second test method, and obtaining a corresponding relationship between the second preload test value and the second impedance test value; A second cell impedance-preload fitting curve is obtained based on the corresponding relationship between each second preload test value and the second impedance test value, where the second cell impedance-preload fitting curve represents the relationship between the preload of the target cell and the cell impedance of the target cell; According to the second battery cell impedance-preload force fitting curve, a preload force that enables the battery cell impedance of the target battery cell to meet the design requirements is determined as the preload force of the target battery cell.

5. The method for determining the preload force of a battery cell according to claim 3 or 4, characterized in that: Based on the preload force application rules, preload forces are applied to multiple test cells according to various preload force test values, including: Apply preload force to several test cells according to each preload force test value; The corresponding relationship between the preload test value and the impedance test value is determined as follows: The average value of the impedance test values ​​of multiple test cells subjected to the same preload test value is used as the impedance test value corresponding to the preload test value.

6. The method for determining the preload force of a battery cell according to claim 3 or 4, characterized in that: Based on the preload force application rules, each preload force test value applies preload force to multiple test cells respectively, including: Applying multiple different preload forces to each test cell in ascending order according to multiple preload force test values, and obtaining an impedance test value of the test cell after applying each preload force; The corresponding relationship between the preload test value and the impedance test value is determined as follows: The average value of the impedance test values ​​of different test cells under the same preload test value is taken as the impedance test value corresponding to the corresponding preload test value.

7. The method for determining the preload force of a battery cell according to claim 4, wherein: The second preload force applying rule is the same as the first preload force applying rule, or the second preload force applying rule is different from the first preload force applying rule; and / or, The second test method is the same as the first test method, or the second test method is different from the first test method.

8. The method for determining the preload force of a battery cell according to claim 3 or 4, characterized in that: If any impedance test method in the first test method or the second test method is a DC resistance method, obtaining an impedance test value of each test cell under a preload test value based on the impedance test method includes the following steps: After applying a preload test value to the test cell, setting the test cell to a preset capacity state; Applying a pulse current to the test cell, and obtaining an initial voltage of the test cell and a cut-off voltage of the test cell; An impedance test value of the test cell is calculated based on the pulse current, the initial voltage, and the cut-off voltage.

9. The method for determining the preload force of a battery cell according to claim 3 or 4, characterized in that: Any impedance test method in the first test method and the second test method is an electrochemical impedance method, and obtaining an impedance test value of each test cell under a preload test value based on the impedance test method includes the following steps: After applying the preload test value to the test cell, charging the test cell; Applying an alternating current signal disturbance to the test cell to obtain an impedance spectrum of the test cell; Fitting the impedance spectrum based on a second-order equivalent circuit model to obtain a preset impedance of the test cell, wherein the preset impedance includes at least one of ohmic impedance, membrane impedance, electrochemical reaction impedance, and diffusion impedance; The cell impedance of the test cell is calculated based on the preset impedance.

10. The method for determining the preload force of a battery cell according to claim 9, characterized in that: During the charging process of the test cell, an AC signal disturbance is synchronously applied to the test cell, and the impedance spectrum of the test cell under different capacity states is obtained. The cell impedance of the test cell is the average value of the cell impedance of the test cell under different capacity states.

Images