Method for inspecting short circuit in secondary battery cell

Abstract

This disclosed method for inspecting a short circuit in a secondary battery cell comprises the steps of: preparing a secondary battery cell which is completely packaged; micro-charging the secondary battery cell in a state-of-charge (SOC) range of 0.03-0.05%; measuring an open-circuit voltage drop due to self-discharge of the micro-charged secondary battery cell; and determining the secondary battery cell to be a defective product when the hourly rate of change of the measured open-circuit voltage drop exceeds a predetermined reference value.

Description

Short circuit inspection method for secondary battery cells

[0001] The present invention relates to an inspection method capable of detecting with high accuracy whether a positive electrode and / or negative electrode is short-circuited due to a defective separator before performing an activation process on a secondary battery cell whose packaging has been completed.

[0002] This application claims the benefit of priority from Republic of Korea Patent Application No. 10-2023-0146407, dated October 30, 2023, the entire contents of which are incorporated herein by reference.

[0003] Unlike primary batteries, secondary batteries are rechargeable and, due to their potential for miniaturization and large capacity, have been the subject of extensive research and development in recent years. With increasing technological development and demand for mobile devices, and the emergence of electric vehicles and energy storage systems in response to the era's growing environmental concerns, demand for secondary batteries as an energy source is rapidly increasing.

[0004] Secondary batteries are classified into coin-shaped, cylindrical, square, and pouch-shaped batteries, depending on the shape of their battery cases. The electrode assembly mounted inside the battery case of a secondary battery is a rechargeable power generating element comprised of a laminated structure of electrodes and a separator.

[0005] Electrode assemblies can be roughly classified into a jellyroll type in which a separator is interposed between sheet-shaped positive and negative electrodes coated with active materials, a stack type in which a plurality of positive and negative electrodes are sequentially stacked with a separator interposed between them, and a stack & folding type in which stack-type unit cells are wound with a long separator film.

[0006] Secondary batteries are manufactured by placing the electrode assembly in a battery case, adding electrolyte, and then completely sealing it. The assembled secondary battery goes through an activation process of repeating charging and discharging for a certain period of time. During or after the activation process, the secondary battery undergoes various tests, such as for charge and discharge performance, electrolyte leakage, and appearance defects, before being shipped as a product.

[0007] The activation process involves repeated charging and discharging over a long period of several days to bring the secondary battery's functionality up to a shippable level. However, if a defect, such as a tear or fold in the separator, occurs in the secondary battery cell during the numerous processes involved in manufacturing the secondary battery, this can cause a short circuit between the positive and / or negative electrodes. Consequently, secondary batteries with low-voltage defects due to these separator problems cannot be shipped as good products. Thus, if low-voltage defects are detected only after packaging and the activation process, secondary battery manufacturing incurs significant losses in time and cost. Therefore, it is crucial to filter out low-voltage defective cells as early as possible, especially before the lengthy activation process.

[0008] The purpose of the present invention is to provide a method capable of detecting low voltage failure due to a separator problem with high discriminative power early on, immediately after packaging and before the activation process that takes a long time is initiated.

[0009] However, the technical problems to be solved by the present invention are not limited to the problems described above, and other problems not mentioned can be clearly understood by those skilled in the art from the description of the invention described below.

[0010] The present invention relates to a method for inspecting a short circuit of a secondary battery cell, and in one example, the method comprises the steps of: preparing a secondary battery cell whose packaging has been completed; micro-charging the secondary battery cell to a SOC (State Of Charge) range of 0.03 to 0.05%; measuring an open circuit voltage drop due to self-discharge of the micro-charged secondary battery cell; and determining the cell as a defective product if the hourly rate of change of the measured open circuit voltage drop exceeds a preset reference value.

[0011] In one embodiment of the present invention, the secondary battery cell can be micro-charged for 2 to 5 minutes at a current of 1 / 200 C to 1 / 50 C (1 C is the current that fully charges the secondary battery cell in 1 hour).

[0012] Here, microcharging of the secondary battery cell can be performed below the reaction potential that activates the secondary battery cell.

[0013] In addition, the open circuit voltage drop due to self-discharge can be measured while the secondary battery cell is heated to 50 to 70°C.

[0014] 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 to 70°C and pressurized.

[0015] For example, the pouch cell can be pressurized at a pressure of 5 to 14 kgf / ㎠.

[0016] Alternatively, the pouch cell may be pressurized at a pressure range of 14 kgf / cm2 or more and less than the damage pressure of the pouch cell.

[0017] And, pressure can be applied across the entire surface of the pouch cell.

[0018] In one embodiment of the present invention, the secondary battery cell is a pouch cell, and the pouch cell is heated to 50 to 60°C at a state of SOC (State Of Charge) of 0.05%, a current amount of 1 / 50C (1C is the current amount for fully charging the secondary battery cell in 1 hour), and a pressure of 14 kgf / cm2, and the entire surface of the pouch cell is pressed, and an open circuit voltage drop due to self-discharge can be measured for 5 minutes.

[0019] And, the preset reference value for the hourly rate of change of the open circuit voltage drop can be set to a 4 sigma level for the hourly rate of change of the open circuit voltage drop of a good secondary battery cell.

[0020] According to the short-circuit inspection method of the secondary battery cell of the present invention, which performs a series of steps as described above, by measuring the open circuit voltage drop due to self-discharge for a short period of time after micro-charging the secondary battery cell before performing the activation process, it is possible to reliably detect low-voltage defects of positive / negative short-circuit caused by tearing or folding of the separator.

[0021] Accordingly, defective products can be effectively sorted out before the long-term activation process, thereby preventing time and cost losses in secondary battery manufacturing.

[0022] However, the technical effects that can be obtained through the present invention are not limited to the above-described effects, and other effects that are not mentioned can be clearly understood by those skilled in the art from the description of the invention described below.

[0023] The following drawings attached to this specification illustrate preferred embodiments of the present invention, and together with the detailed description of the invention described below, serve to further understand the technical idea of ​​the present invention, and therefore, the present invention should not be interpreted as being limited to matters described in such drawings.

[0024] FIG. 1 is a schematic drawing of a device for performing a short circuit inspection method of a secondary battery cell according to the present invention.

[0025] Figure 2 is a flowchart of a short circuit inspection method for a secondary battery cell of the present invention.

[0026] Figure 3 is a graph showing the hourly drop rate of open circuit voltage versus the charge amount of a secondary battery cell.

[0027] Figure 4 is a graph showing the open circuit voltage drop according to the quality of a secondary battery cell.

[0028] Figure 5 is a graph showing the gap difference between a good product and a defective product according to changes in SOC and charging current.

[0029] Figure 6 is a graph showing the gap difference between high and low quality products as pressure increases.

[0030] Figure 7 is a graph showing changes in short circuit detection capability according to pressure increase.

[0031] The present invention can be modified in various ways and has many embodiments, and specific embodiments will be described in detail below.

[0032] However, this is not intended to limit the present invention to a specific embodiment, but should be understood to include all modifications, equivalents, or substitutes included in the spirit and technical scope of the present invention.

[0033] In the present invention, it should be understood that terms such as “include” or “have” are intended to specify the presence of a feature, number, step, operation, component, part or combination thereof described in the specification, but do not exclude in advance the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts or combinations thereof.

[0034] Additionally, in the present invention, when a part such as a layer, film, region, or plate is described as being "on" another part, this includes not only cases where it is "directly above" the other part, but also cases where there is another part in between. Conversely, when a part such as a layer, film, region, or plate is described as being "under" another part, this includes not only cases where it is "directly below" the other part, but also cases where there is another part in between. Furthermore, in the present application, "being placed on" may include cases where it is placed below as well as above.

[0035]

[0036] The present invention relates to a method for inspecting a short circuit of a secondary battery cell, and in one example, the method comprises the steps of: preparing a secondary battery cell whose packaging has been completed; micro-charging the secondary battery cell to a SOC (State Of Charge) range of 0.03 to 0.05%; measuring an open circuit voltage drop due to self-discharge of the micro-charged secondary battery cell; and determining the cell as a defective product if the hourly rate of change of the measured open circuit voltage drop exceeds a preset reference value.

[0037] According to the short-circuit inspection method of the secondary battery cell of the present invention, which performs a series of steps as described above, by measuring the open circuit voltage drop due to self-discharge for a short period of time after micro-charging the secondary battery cell before performing the activation process, it is possible to reliably detect low-voltage defects of positive / negative short-circuit caused by tearing or folding of the separator.

[0038] Accordingly, defective products can be effectively sorted out before the long-term activation process, thereby preventing time and cost losses in secondary battery manufacturing.

[0039] Hereinafter, with reference to the attached drawings, a specific embodiment of a short-circuit inspection method for a secondary battery cell according to the present invention (hereinafter, simply referred to as the "short-circuit inspection method") will be described in detail. For reference, if there are instructions regarding directions such as front, back, up, down, left, and right that designate relative positions in the following description, this is to help understanding of the invention, and unless otherwise defined, the directions are based on the directions depicted in the drawings.

[0040]

[0041] [First Embodiment]

[0042] FIG. 1 schematically illustrates an apparatus for performing a short circuit inspection method of a secondary battery cell according to the present invention. The secondary battery cell illustrated in FIG. 1 is a pouch cell (10), which houses an electrode assembly and an electrolyte therein, and electrode leads of a positive electrode (12) and a negative electrode (14) extend outward from the pouch, which is a packaging material. Charging and discharging are performed through the electrode leads of the positive electrode (12) and the negative electrode (14) exposed to the outside. For sealing, the edges of the pouch are joined by heat fusion. For example, the lower part of the pouch cell (10) is a folding area where the pouch is folded in half, and no heat fusion edge is formed, while the other three edges can form heat fusion edges. Of these three heat fusion edges, the edge from which the electrode leads (12, 14) extend is called a terrace portion (16), and the remaining heat fusion edges form a gas pocket portion (18). After the activation process, the gas generated inside the pouch is discharged through a hole in the gas pocket (18).

[0043] And, Fig. 1 illustrates an MVT (Micro-charge Voltage Tracking) device (100) for performing a short circuit inspection method. The MVT device (100) performs micro-charging on a secondary battery cell (10) and also measures the open circuit voltage drop due to self-discharge of the secondary battery cell (10) after charging. During micro-charging, the MVT device (100) can perform constant current charging by limiting the maximum charging current and charging at a constant current according to the load characteristics of the secondary battery cell (10).

[0044] In addition, the device of FIG. 1 includes a pressurizing device (200) that applies pressure to the secondary battery cell (10). The pressurizing device (200) may be optionally used when performing the short circuit inspection method of the present invention, but it is preferable to use the pressurizing device (200) in order to improve the accuracy of detecting a defective cell. In particular, since the pressurizing device (200) applies pressure to the surface of the secondary battery cell (10) to pressurize the electrode assembly inside, it may be preferable to necessarily use the pressurizing device (200) when targeting a pouch cell (10) to which a flexible packaging material is applied.

[0045] When applying a pressurizing device (200) to a pouch cell (10), it may be desirable for the pressurizing device (200) to apply pressure across the entire surface of the pouch cell (10). By applying pressure to the entire surface of the pouch cell (10) rather than just the edges, it becomes possible to detect not only separator folding occurring at the edges of the electrode assembly but also separator tearing occurring at the edges and center of the electrode assembly. In addition, by pressurizing the pouch cell (10), it becomes possible to detect small-sized separator defects that were not detected when no pressure was applied or when only the edges were pressurized.

[0046] Figure 2 is a flowchart of a short circuit inspection method of the present invention. Referring to Figure 2, the short circuit inspection method of the present invention includes a step of preparing a secondary battery cell whose packaging has been completed, a step of micro-charging the secondary battery cell, a step of measuring the open circuit voltage drop of the secondary battery cell, and a step of judging the quality of the secondary battery cell based on the hourly rate of change of the measured open circuit voltage drop.

[0047] A packaged secondary battery cell, as illustrated in Fig. 1, refers to a secondary battery cell that is sealed so that the electrode assembly and electrolyte are housed within the packaging material and the positive and negative electrodes (or electrode leads) are exposed to the outside. In other words, the short circuit inspection method begins by preparing a secondary battery cell that is externally completed just before being put into the activation process.

[0048] After packaging, secondary battery cells are microcharged using an MVT device. Quantitatively, the level of microcharge corresponds to a secondary battery cell's SOC (State of Charge) range of 0.03–0.05%. Microcharge of secondary battery cells is performed using a constant current charging method.

[0049] When the micro-charging is completed, the open-circuit voltage drop due to self-discharge of the secondary battery cell is measured through the MVT device. Fig. 3 is a graph showing the hourly drop rate of the open-circuit voltage versus the state of charge (SOC) of the secondary battery cell. Referring to Fig. 3, the hourly drop rate of the open-circuit voltage shows an irregular pattern depending on the state of charge of the secondary battery cell. In particular, in a region where the SOC is very low, for example, in the region of 0.03 to 0.05%, which is the micro-charging range of the present invention, the hourly drop rate of the open-circuit voltage shows a steep slope. In other words, since the open-circuit voltage drops rapidly in a short period of time in the SOC range of 0.03 to 0.05%, measuring the hourly drop rate of the open-circuit voltage in this region can sensitively determine the quality of the secondary battery cell.

[0050] Based on these facts, if the hourly rate of change of the measured open circuit voltage drop exceeds a preset reference value, it is determined as a defective product. Fig. 4 is a graph showing the open circuit voltage drop according to the quality of a secondary battery cell. A defective product is a high-quality secondary battery cell in which a separator defect is artificially created. The left side is a graph showing the voltage according to micro-charge and self-discharge, and the right side is a graph showing the hourly rate of drop of the open circuit voltage during self-discharge. As shown in the graph on the right, a good secondary battery cell shows a good voltage drop rate that is below the pass specification (red line), but a defective secondary battery cell shows a high voltage drop rate that exceeds the pass specification. Therefore, when the hourly rate of change of the measured open circuit voltage drop exceeds a preset reference value, the secondary battery cell can be determined as a defective product.

[0051]

[0052] (Experimental Example 1) Gap difference between good and bad products according to the charge amount and charge current of micro-charge

[0053] [Table 1] below is a diagram summarizing six experimental conditions according to SOC(%) 3 levels and charging current 2 levels. In the six experimental conditions, the pressure applied to the pouch cell was the same at 5 kgf / ㎠.

[0054] Experimental number SOC (%) Charging current (C) Pressure (kgf / ㎠) Micro-charging time (min) 10.0331 / 2005420.0331 / 505130.0421 / 2005540.0421 / 505250.0501 / 2005660.0501 / 5052

[0055] To conduct the above experiment, 67 high-quality pouch cells were prepared in which the separator was artificially folded or torn at the edge or center of the electrode assembly, respectively. In addition, in the above [Table 1], the basic unit of charging current, "1C," refers to the current that fully charges the secondary battery cell in 1 hour, and the experiment was conducted under conditions in which two variables, SOC (%) and charging current (C), were varied while the pressure condition was fixed at 5 kgf / ㎠. In addition, the open-circuit voltage drop due to self-discharge was measured while the secondary battery cell was heated to an appropriate temperature in the range of 50 to 70℃, for example, 55℃.

[0056] Here, the reason why the state of charge (SOC) of the secondary battery cell is limited to the range of 0.03 to 0.05% is because, as explained with reference to Fig. 3, this corresponds to the micro-charge range in which the hourly drop rate of the open circuit voltage shows the steepest slope. Furthermore, the reason why the charging current is limited to 1 / 50C or less in response to the SOC range of 0.03 to 0.05% is because the micro-charge for the secondary battery cell must be performed below the reaction potential that activates the secondary battery. For example, if the activation potential (reaction potential) of the secondary battery cell is approximately 2 V or higher, the charging potential during micro-charge needs to be maintained at approximately 1.5 V.

[0057] Figure 5 is a graph showing the gap difference with respect to the SOC (%) and charging current of a good product. The open-circuit voltage drop of a high-quality product is clearly visible compared to a good product, which means that the larger the gap in the open-circuit voltage drop of a high-quality product compared to a good product, the higher the discrimination ability to filter out defective products. From this perspective, when looking at Figure 5, the gap in the open-circuit voltage drop of a high-quality product compared to a good product was found to be larger as the charging current of the secondary battery cell increased. However, when the SOC increased from 0.033% to 0.042%, the gap increased, but when it was further increased to 0.050%, the gap tended to return to the 0.033% level. Based on these results, under the premise that micro-charging of a secondary battery cell is performed below the reaction potential that activates the secondary battery, it appears that setting the charging current as high as possible is advantageous for short-circuit inspection of the secondary battery cell.

[0058]

[0059] (Experimental Example 2) Gap difference between good and bad products according to micro-filling pressure

[0060] [Table 2] below is a diagram summarizing the experimental conditions conducted with the pressure applied across the entire surface of the pouch cell as the main factor.

[0061] Experimental number SOC (%) Charging current (C) Pressure (kgf / ㎠) Micro-charging time (min) 70.0421 / 2008580.0421 / 508290.0501 / 5082100.0501 / 50112110.0501 / 50142

[0062] Figure 6 is a graph depicting the gap difference between high-quality and good-quality products as pressure increases. Compared to Figure 5, Figure 6 demonstrates the greatest increase in gap difference as pressure increases.

[0063] Figure 7 is a graph illustrating changes in short-circuit detection capability as pressure increases. The upper limit of the pressure applied by the current equipment is 14 kgf / cm2. Furthermore, the defect size of the separator was set at the center of the electrode assembly, with a 2.0 (㎟) size, and at the edges, two sizes of 2.0 / 4.0 (㎟) were provided.

[0064] As shown in Fig. 7, only an open-circuit voltage drop below the standard is observed in the good product, and the open-circuit voltage drop increases with increasing pressure in both the central region and the edge of the electrode assembly. The pressure increase enables detection of membrane defects that were previously undetectable, and this detection ability appears to increase almost proportionally with the pressure increase. Therefore, if the pressurization equipment is supplemented, it is expected that the ability to detect membrane defects can be further improved by pressurizing the pouch cell to 14 kgf / cm2 or more in a pressure range below the damage pressure of the pouch cell.

[0065] Furthermore, a comprehensive review of the experimental results suggests that increased charging current and pressure have a positive effect on short-circuit detection. Conversely, the state of charge (SOC) appears to have little impact on detection results within the 0.03–0.05% range, where the open-circuit voltage decline rate per hour is at its maximum. Therefore, it may be desirable to set the charging current and charge amount so that micro-charging can be completed within a short period of time, typically 2–5 minutes.

[0066]

[0067] [Second Embodiment]

[0068] In the first embodiment, the three factors of charge amount, charge current, and pressure, which are considered as major factors in detecting whether or not a separator is defective, are established as desirable process conditions for a short-circuit inspection method for pouch cells, i.e., conditions that do not pose any problems in applying them to current production facilities, as follows.

[0069] The secondary battery cell is a pouch cell in which pressure is applied to the internal electrode assembly by external pressure application. The process conditions can be established by measuring the open circuit voltage drop due to self-discharge for 5 minutes while the pouch cell is heated to 50-60℃, with an SOC of 0.05%, a current of 1 / 50C (1C is the current that fully charges the secondary battery cell in 1 hour), and a pressure of 14 kgf / ㎠ while the entire surface of the pouch cell is pressed.

[0070] In addition, the preset reference value for the hourly rate of change of the open circuit voltage drop can be set at the 4-sigma level for the hourly rate of change of the open circuit voltage drop of a good secondary battery cell. Since the manufacturer's risk of defects in the secondary battery cell is expected if it is managed at the 3-sigma level, the hourly rate of change of the open circuit voltage drop can preferably be applied at the 4-sigma level. The voltage drop rate at the 4-sigma level is indicated by a dashed line in Fig. 7.

[0071]

[0072] The present invention has been described in more detail through drawings and examples. However, the configurations described in the drawings or examples described in this specification are merely embodiments of the present invention and do not represent all of the technical ideas of the present invention. Therefore, it should be understood that various equivalents and modified examples may exist as of the time of this application.

[0073]

[0074] [Explanation of symbols]

[0075] 10: Secondary battery cell (pouch cell)

[0076] 12: Bipolar

[0077] 14: Cathode

[0078] 16: Terrace

[0079] 18: Gas pocket section

[0080] 100: MVT device

[0081] 200: Pressurization device

Claims

1. Step of preparing a secondary battery cell after packaging is completed; A step of micro-charging the secondary battery cell to a SOC (State Of Charge) range of 0.03 to 0.05%; A step of measuring the open circuit voltage drop due to self-discharge of a micro-charged secondary battery cell; and A step of determining a product as defective if the hourly rate of change of the measured open circuit voltage drop exceeds a preset reference value; A method for inspecting a short circuit in a secondary battery cell, including:

2. In paragraph 1, The above secondary battery cell, A method for short-circuit inspection of secondary battery cells by micro-charging them for 2 to 5 minutes at a current of 1 / 200C to 1 / 50C (1C is the current that fully charges a secondary battery cell in 1 hour).

3. In paragraph 2, Micro-charging for the above secondary battery cell is: A method for inspecting a secondary battery cell for short circuit, the method being performed below a reaction potential that activates the secondary battery cell.

4. In paragraph 2, A method for inspecting a secondary battery cell for short circuit, which measures the open circuit voltage drop due to self-discharge while heating the secondary battery cell to 50 to 70°C.

5. In paragraph 4, The above secondary battery cell is a pouch cell, A method for inspecting a secondary battery cell for short circuit, wherein the pouch cell is heated to 50 to 70°C and pressurized, and the open circuit voltage drop due to self-discharge is measured.

6. In paragraph 5, A method for inspecting a short circuit in a secondary battery cell, wherein the above pouch cell is pressurized at a pressure of 5 to 14 kgf / ㎠.

7. In paragraph 5, A method for inspecting a short circuit of a secondary battery cell, wherein the pouch cell is pressurized at a pressure range of 14 kgf / ㎠ or more and less than the damage pressure of the pouch cell.

8. In any one of paragraphs 5 to 7, A method for inspecting a short circuit in a secondary battery cell, wherein pressure is applied across the entire surface of the pouch cell.

9. In paragraph 1, The above secondary battery cell is a pouch cell, A method for inspecting a secondary battery cell for short circuit, wherein the pouch cell is heated to 50 to 60°C at a state of SOC (State Of Charge) of 0.05%, a current of 1 / 50C (1C is the current that fully charges a secondary battery cell in 1 hour), and a pressure of 14 kgf / cm2 while the entire surface of the pouch cell is pressed, and the open circuit voltage drop due to self-discharge is measured for 5 minutes.

10. In paragraph 1, The preset reference value for the hourly rate of change of the open circuit voltage drop is: A method for inspecting short circuits in secondary battery cells, wherein the hourly rate of change in open circuit voltage drop of a good secondary battery cell is set at a 4 sigma level.

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