Method for short-circuit inspection of secondary battery cell

Abstract

Disclosed herein is a method for short circuit inspection of a secondary battery cell, the method including preparing a fully encapsulated secondary battery cell, micro-charging the secondary battery cell to a state of charge (SOC) in a range of 0.03% to 0.05%, measuring an open circuit voltage drop caused by self-discharge of the micro-charged secondary battery cell, and determining a short circuit of the secondary battery cell. And determining the secondary battery cell as a defective product if the measured rate of change per hour of the open circuit voltage drop exceeds a predetermined reference value.

Description

Technical Field

[0001] This disclosure relates to an inspection method that can detect with high precision the presence of short circuits in the positive and / or negative electrodes caused by defects in the separator at an early stage before the activation process of a fully encapsulated secondary battery cell.

[0002] This application claims the benefit of priority to Korean Patent Application No. 10-2023-0146407, filed on October 30, 2023, the contents of which are incorporated herein by reference. Background Technology

[0003] Unlike primary batteries, secondary batteries are rechargeable and have been extensively researched and developed in recent years due to their potential for miniaturization and high capacity. Furthermore, with the technological advancements and increasing demands of mobile devices, the need for secondary batteries as an energy source is rapidly growing in response to current environmental protection requirements, particularly in electric vehicles and energy storage systems.

[0004] Secondary batteries are classified based on the shape of their casing into button cells, cylindrical cells, prismatic cells, and pouch cells. In a secondary battery, the electrode assembly installed inside the casing is a power-generating element capable of charging and discharging, consisting of a stacked structure of electrodes and separators.

[0005] Electrode assemblies can be broadly classified as: roll-up type, in which a separator is inserted and wound between sheet-like positive and negative electrodes coated with active material; stacked type, in which multiple positive and negative electrodes are stacked sequentially with separators inserted; and stacked and folded type, in which the stacked unit cells are wound with long-length separation membranes.

[0006] After the electrode assembly is inserted into the battery casing and the electrolyte is injected, the secondary battery is completely sealed. The assembled secondary battery undergoes an activation process involving repeated charging and discharging for a certain period of time, and after various inspections during or after the activation process, including charging and discharging performance, electrolyte leakage, and appearance defects, it is shipped as a product.

[0007] The activation process involves repeatedly charging and discharging the secondary battery over a long period of several days to bring its functionality to a factory-ready level. However, if defects such as tearing or folding of the separator occur in the secondary battery cells during various manufacturing processes, short circuits may occur in the positive and / or negative electrodes, and secondary batteries with low-voltage defects due to such separator problems cannot be shipped as good products. Therefore, if low-voltage defects are only detected after packaging and activation, there is a significant loss in time and cost in secondary battery manufacturing, so it is necessary to filter out low-voltage defective cells as early as possible, especially before the time-consuming activation process. Summary of the Invention

[0008] Technical issues

[0009] The purpose of this disclosure is to provide a method for high-precision detection of low-voltage defects caused by separator problems in an early stage immediately following encapsulation, prior to insertion into a time-consuming activation process.

[0010] However, the technical problems to be solved by this disclosure are not limited to those described above, and those skilled in the art will clearly understand from the description of this disclosure below that there are other problems not mentioned.

[0011] Technical solution

[0012] This disclosure relates to a method for short-circuit inspection of a secondary battery cell. In one example, the method includes: preparing a fully encapsulated secondary battery cell; micro-charging the secondary battery cell to a state of charge (SOC) in the range of 0.03% to 0.05%; measuring the open-circuit voltage drop caused by self-discharge of the micro-charged secondary battery cell; and identifying the secondary battery cell as a defective product if the hourly rate of change of the measured open-circuit voltage drop exceeds a predetermined reference value.

[0013] In one embodiment of this disclosure, a secondary battery cell can be microcharged for 2 to 5 minutes at a current of 1 / 200 C to 1 / 50 C (where 1C is the current that fully charges the secondary battery cell in 1 hour).

[0014] Here, the micro-charging of the secondary battery cell can be performed at a reaction potential lower than that of the activated secondary battery cell.

[0015] In addition, the open-circuit voltage drop caused by self-discharge can be measured while the secondary battery cell is heated to 50°C to 70°C.

[0016] In one embodiment, the secondary battery cell is a pouch cell, and the open-circuit voltage drop due to self-discharge can be measured while the pouch cell is heated to 50°C to 70°C and pressure is applied.

[0017] For example, bag monomers can be pressurized to 5 kgf / cm². 2 Up to 14 kgf / cm 2 The pressure.

[0018] Alternatively, the bag unit can be 14 kgf / cm². 2 It may be pressurized within a pressure range that is greater than or less than the damage pressure of the bag unit.

[0019] In addition, pressure can be applied across the entire surface of the bag monomer.

[0020] In one embodiment of this disclosure, the secondary battery cell is a pouch cell, and can be charged by subjecting the pouch cell to 0.05% state of charge (SOC), a current of 1 / 50 C (where 1C is the current required to fully charge the secondary battery cell in 1 hour), heating to 50°C to 60°C, and at 14 kgf / cm². 2 Pressure was applied to the entire surface of the bag unit for 5 minutes to measure the open-circuit voltage drop caused by self-discharge.

[0021] Furthermore, a predetermined reference value for the hourly rate of change of open-circuit voltage drop can be set to the 4 sigma level of the hourly rate of change of open-circuit voltage drop of a good quality secondary battery cell.

[0022] Beneficial effects

[0023] According to the short-circuit inspection method for secondary battery cells disclosed herein, which performs a series of steps as described above, by micro-charging the secondary battery cell before performing the activation process and measuring the open-circuit voltage drop due to self-discharge within a short period of time, low voltage defects caused by short circuits between the positive and negative electrodes due to tearing or folding of the separator can be detected with high reliability.

[0024] Therefore, defective products can be effectively screened out before the time-consuming activation process, thus preventing time and cost losses in the manufacturing of secondary batteries.

[0025] However, the technical effects that can be obtained through this disclosure are not limited to those described above, and other effects not mentioned will be clearly understood by those skilled in the art from the description of this disclosure below. Attached Figure Description

[0026] Because the accompanying drawings illustrate exemplary embodiments of the present disclosure and are intended to enhance understanding of the technical concept of the present disclosure together with the detailed description of the present disclosure described below, the present disclosure should not be interpreted restrictively based on the drawings.

[0027] Figure 1 This is a schematic diagram of an apparatus for performing a short-circuit test method for a secondary battery cell according to the present disclosure.

[0028] Figure 2 This is a flowchart of a short-circuit testing method for a secondary battery cell according to this disclosure.

[0029] Figure 3 This is a graph showing the rate of decrease of open-circuit voltage per hour compared to the charge level of a single secondary battery cell.

[0030] Figure 4This is a graph showing the open-circuit voltage drop of a high-quality secondary battery cell and a defective secondary battery cell.

[0031] Figure 5 This is a graph illustrating the difference between a product of good quality and one with variations in SOC and charging current.

[0032] Figure 6 It is a graph illustrating the difference between defective products and good quality products based on increased pressure.

[0033] Figure 7 This is a graph illustrating how short-circuit detection capability changes with increasing pressure. Detailed Implementation

[0034] This disclosure can have various modifications and various embodiments, and therefore specific embodiments thereof will be described in detail below.

[0035] However, it should be understood that this disclosure is not limited to the specific embodiments, but includes all modifications, equivalents or substitutions within the spirit and technical scope of this disclosure.

[0036] The terms “comprising,” “including,” and “having” as used herein refer to the presence of features, numbers, steps, actions, parts, or components or combinations thereof described in the specification, and it should be understood that the possibility of the presence or addition of one or more other features, numbers, steps, actions, parts, components or combinations thereof is not excluded in advance.

[0037] Furthermore, in this disclosure, when a portion of a layer, film, region, plate, etc., is disposed "on" another portion, this includes not only the case where one portion is disposed "directly" on another portion, but also the case where the other portion is inserted therein. Conversely, when a portion of a layer, film, region, plate, etc., is disposed "below" another portion, this includes not only the case where one portion is disposed "directly" below another portion, but also the case where the other portion is inserted therein. Moreover, in this application, "on" can include not only the case where it is disposed on the upper part, but also the case where it is disposed on the lower part.

[0038] This disclosure relates to a method for short-circuit testing of a secondary battery cell. In one example, the method includes: preparing a fully packaged secondary battery cell; micro-charging the secondary battery cell to a state of charge (SOC) in the range of 0.03% to 0.05%; measuring the open-circuit voltage drop caused by self-discharge of the micro-charged secondary battery cell; and identifying the secondary battery cell as a defective product if the hourly rate of change of the measured open-circuit voltage drop exceeds a predetermined reference value.

[0039] According to the short-circuit inspection method for secondary battery cells disclosed herein, which performs a series of steps as described above, by micro-charging the secondary battery cell before performing the activation process and measuring the open-circuit voltage drop due to self-discharge within a short period of time, low voltage defects caused by short circuits between the positive and negative electrodes due to tearing or folding of the separator can be detected with high reliability.

[0040] Therefore, defective products can be effectively screened out before the time-consuming activation process, thus preventing time and cost losses in the manufacturing of secondary batteries.

[0041] Detailed description

[0042] In the following, specific embodiments of the short-circuit testing method for a secondary battery cell according to the present disclosure (hereinafter, it may be simply referred to as the "short-circuit testing method") will be described in detail with reference to the accompanying drawings. For reference, unless otherwise specified, the directions used in the following description to refer to relative positions—front, back, up, down, left, and right—are for the purposes of understanding the present disclosure and refer to the directions shown in the accompanying drawings.

[0043] First Embodiment

[0044] Figure 1 This is a schematic diagram of an apparatus for performing a short-circuit test method for a secondary battery cell according to the present disclosure. Figure 1 The secondary battery cell illustrated in the diagram is a pouch cell 10, which internally houses the electrode assembly and electrolyte, and has electrode leads 12 and 14 extending to the outside of the pouch containing the encapsulating material. Charging and discharging are performed through the exposed electrode leads 12 and 14. For sealing, the pouch boundaries are joined by thermal fusion. For example, the bottom of the pouch cell 10 is a folded area where the pouch is folded in half and no thermally bonded boundary is formed, while three other boundaries can form thermally bonded boundaries. Of these three thermally bonded boundaries, the boundary from which the electrode leads 12 and 14 are extracted is called the plateau portion 16, and the remaining thermally bonded boundaries form a cavitation portion 18. After the activation process, gas generated inside the pouch is discharged through perforations in the cavitation portion 18.

[0045] Then, in Figure 1 The image illustrates a micro-charge voltage tracking (MVT) device 100 for performing a short-circuit check method. The MVT device 100 performs a micro-charge on a secondary battery cell 10 and measures the open-circuit voltage drop caused by the self-discharge of the secondary battery cell 10 after charging. During the micro-charge, the MVT device can perform constant-current charging by limiting the maximum charging current, thereby charging at a constant current based on the load characteristics of the secondary battery cell.

[0046] also, Figure 1The apparatus includes a pressurizing device 200 that applies pressure to the secondary battery cell 10. While the pressurizing device 200 may be selectively used when performing the short-circuit testing method of this disclosure, it is preferably used to improve the accuracy of detecting defective cells. In particular, since the pressurizing device 200 is directed to apply pressure to the surface of the secondary battery cell 10 to pressurize the internal electrode assembly, it may be preferable to use the pressurizing device 200 when the target cell 10 is covered with a flexible encapsulation material.

[0047] When the pressurizing device 200 is applied to the bag monomer 10, it is preferable that the pressurizing device 200 applies pressure across the entire surface of the bag monomer 10. By applying pressure to the entire surface of the bag monomer 10, rather than just to the boundaries, it becomes possible to detect septum folding occurring at the edges of the electrode assembly and septum tearing occurring at the edges and center of the electrode assembly. Furthermore, by pressurizing the bag monomer 10, it becomes possible to detect small-sized septum defects that would not be detected when no pressure is applied or when only the boundaries are pressurized.

[0048] Figure 2 This is a flowchart of the short-circuit checking method disclosed herein. (Reference) Figure 2 The short-circuit inspection method disclosed herein includes the following steps: preparing a fully encapsulated secondary battery cell, micro-charging the secondary battery cell, measuring the open-circuit voltage drop of the secondary battery cell, and determining good quality secondary battery cells and defective secondary battery cells based on the hourly rate of change of the measured open-circuit voltage drop.

[0049] A fully encapsulated secondary battery cell refers to a cell in which electrode components and electrolyte are contained within an encapsulation material—such as… Figure 1 As shown—and sealed so that the positive and negative electrodes (or electrode leads) are exposed to the outside of the secondary battery cell. In other words, the short-circuit check method begins by preparing the externally completed secondary battery cell immediately before insertion into the activation process.

[0050] The fully encapsulated secondary battery cells are micro-charged using an MVT device. If the level of micro-charge is quantitatively defined, it corresponds to the state of charge (SOC) of the secondary battery cell in the range of 0.03% to 0.05%. Here, the micro-charging of the secondary battery cell is performed using a constant current charging method.

[0051] When microcharging is complete, the open-circuit voltage drop caused by the self-discharge of the secondary battery cells is measured using an MVT device. Figure 3 This is a graph showing the rate of decrease in open-circuit voltage per hour compared to the charge level of a single secondary battery cell. (Reference) Figure 3The rate of decrease of open-circuit voltage drop per hour shows an irregular pattern depending on the charge level of the secondary battery cell, especially in the region of very low SOC, for example, in the region of 0.03% to 0.05%—which is the micro-charge range of this disclosure—where the rate of decrease of open-circuit voltage drop per hour shows a steep slope. In other words, in the SOC segment of 0.03% to 0.05%, the open-circuit voltage drops rapidly over a short period of time, and therefore measuring the rate of decrease of open-circuit voltage per hour in this segment can sensitively determine the good quality or defects of the secondary battery cell.

[0052] Based on this fact, when the hourly rate of change of the measured open-circuit voltage drop exceeds a predetermined reference value, it is identified as a defective product. Figure 4 This graph compares the open-circuit voltage drop of a good-quality secondary battery cell and a defective secondary battery cell. Defective products are those with man-made separator defects. The left side shows the voltage under micro-charging and self-discharging conditions, and the right side shows the rate of decrease of the open-circuit voltage per hour under self-discharging conditions. As shown in the right-hand graph, a good-quality secondary battery cell has a good voltage drop rate below the passing specification (red line), but a defective secondary battery cell has a high voltage drop rate exceeding the passing specification. Therefore, a secondary battery cell can be identified as a defective product when the measured rate of change of the open-circuit voltage drop per hour exceeds a predetermined reference value.

[0053] (Experimental Example 1) Based on the difference between the charging amount and charging current of the micro-charger and that of a good quality product.

[0054] Table 1 below summarizes six experimental conditions for three levels of SOC (%) and two levels of charging current. Under these six experimental conditions, the pressure applied to the bag cell was 5 kgf / cm². 2 same.

[0055] [Table 1]

[0056]

[0057] Experiments were conducted using 67 defective pouch cells, which had folded or torn septa artificially created at the edges or center of the electrode assembly. As shown in Table 1, the basic unit of charging current, "1C," refers to the current required to fully charge a secondary battery cell in one hour, and is measured by maintaining 5 kgf / cm². 2 The experiment was conducted under constant pressure conditions while varying two variables: SOC (%) and charging current (C). Additionally, the open-circuit voltage drop due to self-discharge was measured while heating the individual secondary battery cells to an appropriate temperature (e.g., 55°C) within the range of 50°C to 70°C.

[0058] The reason for limiting the state of charge (SOC) of the secondary battery cells to a range of 0.03% to 0.05% is because, as referenced... Figure 3 This corresponds to the micro-charging range where the rate of decrease in open-circuit voltage per hour shows the steepest slope. Additionally, corresponding to the SOC range of 0.03% to 0.05%, the charging current is limited to 1 / 50 C or less because micro-charging of the secondary battery cell should occur below the activation potential of the secondary battery. For example, if the activation potential (reaction potential) of the secondary battery cell is approximately 2V or greater, the charging potential needs to be maintained at approximately 1.5V during micro-charging.

[0059] Figure 5 This graph illustrates the difference between defective and good quality products based on variations in SOC and charging current. The more pronounced the open-circuit voltage drop of a defective product compared to a good product—in other words, the greater the difference in open-circuit voltage drop between a defective and a good product—the higher the ability to filter out defective products. From this perspective, when observing... Figure 5 As the charging current of the secondary battery cells increases, the difference in open-circuit voltage drop between defective products and good-quality products appears to widen. However, while the difference increases as the SOC increases from 0.033% to 0.042%, it tends to return to the 0.033% level when further increased to 0.050%. Based on these results, and assuming that the process occurs below the activation potential of the secondary battery, setting the charging current as high as possible to micro-charge the secondary battery cells appears to be beneficial for short-circuit checks.

[0060] (Experimental Example 2) Based on the difference between the pressure of micro-inflation and that of a good quality product

[0061] Table 2 below summarizes the experimental conditions under which the pressure applied across the entire surface of the transbag monomer was used as the primary factor.

[0062] [Table 2]

[0063]

[0064] Figure 6 This is a graph illustrating the difference between defective and high-quality products under increased pressure. (Example) Figure 6 As shown, with Figure 5 In comparison, the effect of increasing the gap as pressure increases is the most pronounced.

[0065] Figure 7 This is a graph showing how short-circuit detection capability changes with increasing pressure. The pressure is limited to 14 kgf / cm². 2The upper limit of the horizontal dimension is the maximum value that the current device can apply. Then, the defective dimensions of the separator are fabricated to have a diameter of 2.0 mm at the center of the electrode assembly. 2 The dimensions are 2.0 / 4.0 mm, and the electrode assembly has a 2.0 / 4.0 mm diameter at the edge. 2 Two sizes are available.

[0066] like Figure 7 As shown, as expected, only a lower-than-standard open-circuit voltage drop is observed in high-quality products, and the width of the open-circuit voltage drop increases with increasing pressure in both the central region and the edges of the electrode assembly. Separator defects that were previously undetectable by increasing pressure can be detected, and this detection capability is found to increase almost proportionally with increasing pressure. Therefore, if a pressurization device is added, it is expected that the bag cells can be pressurized to 14 kgf / cm² within a pressure range below the damage pressure of the bag cell. 2 Or even larger, which can further improve the ability to detect defects in the partition.

[0067] Furthermore, based on the experimental results, it was found that increasing the charging current and pressure has a positive impact on short-circuit detection, while the state of charge (SOC) has no significant effect on the detection results when it is in the range of 0.03% to 0.05%—where the rate of decrease in open-circuit voltage is greatest per hour. In this regard, it may be desirable to set the charging current and state of charge so that microcharging can be completed within a short time period—such as 2 to 5 minutes.

[0068] Second Embodiment

[0069] In the first embodiment, three factors are established as the main factors for detecting separator defects: charge amount, charging current and pressure. These are the preferred process conditions for the short-circuit inspection method of the bag unit, i.e., conditions that can be applied to the current production facility without major problems. These conditions are as follows.

[0070] The secondary battery cell involves a pouch cell, in which externally applied pressure acts directly on the internal electrode assembly, and process conditions can be established to heat the pouch cell to 50°C to 60°C at 0.05% state of charge (SOC), using a current of 1 / 50 C (where 1 C is the current required to fully charge the secondary battery cell in one hour), and at 14 kgf / cm². 2 While applying pressure to the entire surface of the bag unit, the open-circuit voltage drop caused by self-discharge for up to 5 minutes was measured.

[0071] Furthermore, a predetermined reference value for the hourly rate of change of open-circuit voltage drop can be set to a 4-sigma level of the hourly rate of change of open-circuit voltage drop for a good quality secondary battery cell. Since managing at a 3-sigma level would present a producer risk of secondary battery cell defects, it is preferable that the hourly rate of change of open-circuit voltage drop be applied at a 4-sigma level. The voltage drop rate at the 4-sigma level is determined by… Figure 7 The dashed line indicates this.

[0072] As described above, this disclosure has been described in more detail with reference to the accompanying drawings, embodiments, etc. However, since the configurations described in the accompanying drawings or embodiments are merely one embodiment of this disclosure and do not represent the overall technical spirit of this disclosure, it should be understood that this disclosure covers various equivalents, modifications, and substitutions at the time of filing this application.

[0073] [Explanation of reference numerals in the attached figures]

[0074] 10: Secondary battery cells (bag cells)

[0075] 12: Positive electrode

[0076] 14: Negative electrode

[0077] 16: Platform Section

[0078] 18: Cavitation section

[0079] 100: MVT equipment

[0080] 200: Pressurization equipment

Claims

1. A method for short-circuit testing of a single secondary battery cell, comprising: Prepare fully encapsulated secondary battery cells; The secondary battery cell is micro-charged to a state of charge (SOC) in the range of 0.03% to 0.05%. Measure the open-circuit voltage drop caused by the self-discharge of a micro-charged secondary battery cell; as well as If the hourly rate of change of the measured open-circuit voltage drop exceeds a predetermined reference value, the secondary battery cell is identified as a defective product.

2. The method for short-circuit testing of a secondary battery cell according to claim 1, wherein, The secondary battery cell is micro-charged for 2 to 5 minutes at a current of 1 / 200 C to 1 / 50 C, where 1C is the current that fully charges the secondary battery cell in 1 hour.

3. The method for short-circuit testing of a secondary battery cell according to claim 2, wherein, The micro-charging of the secondary battery cell is performed at a reaction potential lower than that used to activate the secondary battery cell.

4. The method for short-circuit testing of a secondary battery cell according to claim 2, wherein, The open-circuit voltage drop caused by self-discharge was measured while the secondary battery cell was heated to 50°C to 70°C.

5. The method for short-circuit testing of a secondary battery cell according to claim 4, wherein, The secondary battery cell is a pouch cell, and The open-circuit voltage drop due to self-discharge was measured while the bag monomer was heated to 50°C to 70°C and pressure was applied.

6. The method for short-circuit testing of a secondary battery cell according to claim 5, wherein, The bag unit was pressurized to 5 kgf / cm². 2 Up to 14 kgf / cm 2 The pressure.

7. The method for short-circuit testing of a secondary battery cell according to claim 5, wherein, The bag unit has a strength of 14 kgf / cm³. 2 It may be pressurized within a pressure range that is greater than or less than the damage pressure of the bag unit.

8. The method for short-circuit testing of a secondary battery cell according to any one of claims 5 to 7, wherein, The pressure is applied across the entire surface of the bag monomer.

9. The method for short-circuit testing of a secondary battery cell according to claim 1, wherein, The secondary battery cell is a pouch cell, and By subjecting the bag cells to a state of charge (SOC) of 0.05%, a current of 1 / 50 C, heating to 50°C to 60°C, and applying a current of 14 kgf / cm², 2 Pressure is applied to the entire surface of the cell for 5 minutes to measure the open-circuit voltage drop due to self-discharge, where 1C is the amount of current required to fully charge the secondary battery cell in 1 hour.

10. The method for short-circuit testing of a secondary battery cell according to claim 1, wherein, The predetermined reference value for the hourly rate of change of the open-circuit voltage drop is set to the 4-sigma level of the hourly rate of change of the open-circuit voltage drop of a good-quality secondary battery cell.

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