Method for diagnosing physical micro short circuit of battery cell

Abstract

The invention relates to the technical field of lithium ion batteries, and discloses a method for diagnosing physical micro short circuit of a battery cell. The method comprises the following steps: applying a predetermined direct current detection voltage to two electrodes of a to-be-detected cell in a cryogenic state; monitoring whether a breakdown hole is formed in the to-be-detected battery cell or not; if the to-be-detected battery cell has the breakdown hole, determining that a physical micro short circuit point exists in the to-be-detected battery cell, and disassembling the to-be-detected battery cell; observing and recording the morphology and the position of each breakdown hole, detecting the element composition of each breakdown hole through an energy disperse spectroscopy, and constructing destructive physical analysis data; and according to the destructive physical analysis data, positioning the process defects of the battery cell to be detected in the manufacturing process. According to the method, the physical micro short circuit and the chemical micro short circuit can be accurately distinguished, the specific position of the physical micro short circuit point is positioned, and the root of the physical micro short circuit is diagnosed, so that a direct and accurate basis is provided for improvement of a manufacturing process, and a reference is provided for improvement of the problem of poor low voltage / K value in the manufacturing process.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a diagnostic method for physical micro-short circuits in battery cells. Summary of the Invention

[0002] The purpose of this invention is to provide a diagnostic method for physical micro-short circuits in battery cells, so as to solve or at least partially solve the technical problems existing in the prior art.

[0003] To achieve this objective, the present invention adopts the following technical solution: This invention provides a method for diagnosing physical micro-short circuits in battery cells, comprising: Cells with low voltage or poor K-value that occurred during the manufacturing process were selected as cells to be tested; The battery cell under test is immersed in a liquid nitrogen cryogenic device; A predetermined DC detection voltage is applied to the two poles of the battery cell under test, which is in a cryogenic state. Monitor whether there are breakdown holes on the battery cell under test; If a breakdown hole is found on the battery cell under test, it is determined that there is a physical micro short circuit point inside the battery cell under test, and the battery cell under test is disassembled. During the disassembly of the battery cell under test, the morphology and location of each breakdown hole were observed and recorded, and the elemental composition of each breakdown hole was detected by an energy dispersive spectrometer to construct destructive physical analysis data. Based on the destructive physical analysis data, the manufacturing defects of the battery cell under test are located.

[0004] Optionally, the battery cell under test is a wound battery cell; The breakdown hole in the arc region of the battery cell under test is defined as the first breakdown hole, and the breakdown hole in the main body of the battery cell under test is defined as the second breakdown hole. The step of locating the manufacturing defects of the battery cell under test based on the destructive physical analysis data specifically includes: When the first breakdown hole appears on the cell under test, and its morphology is irregularly radial, if the aluminum content in the detected elemental composition is greater than or equal to a predetermined first content threshold, then the root cause of the physical micro short circuit is determined to be aluminum burrs generated during the positive electrode slitting process that pierce the separator; if the copper content in the detected elemental composition is greater than or equal to a predetermined first content threshold, then the root cause of the physical micro short circuit is determined to be copper burrs generated during the negative electrode slitting process that pierce the separator. When a second breakdown hole appears on the battery cell under test, and its shape is circular or elliptical, if the iron / chromium content in the elemental composition is greater than or equal to the predetermined second content threshold, then the root cause of the physical micro short circuit is determined to be laser cleaning sputtering contamination or failure of environmental cleanliness control.

[0005] Optionally, locating process defects in the battery cell under test based on the destructive physical analysis data further includes: When the first breakdown hole appears on the cell under test, if the detected elemental composition does not contain aluminum and copper, but contains a third-party metal element, then the root cause of the physical micro short circuit is determined to be the diaphragm puncture caused by the third-party metal contaminant being pressed into the edge of the positive electrode during the slitting process. If the elemental composition does not contain aluminum and copper, and no third-party metal element is found, then the root cause of the physical micro-short circuit is determined to be the conductive path formed by the incorporation of non-metallic conductive foreign matter. The third-party metal element is a metal element that should not be present in the raw materials used to make the battery cell, including metal elements contained in stainless steel particles from laser cleaning sputtering and hard alloy particles from mold wear; the non-metallic conductive foreign matter includes graphite dust agglomerates or carbon fibers doped in recycled materials.

[0006] Optionally, when a second breakdown hole appears on the battery cell under test, if the elemental composition contains zinc, then the root cause of the physical micro short circuit is determined to be the volatilization of the zinc plating layer on the battery cell shell during the laser welding process, or the wear of the zinc plating fixture during use. If the elemental composition contains a third-party non-metallic element, then the root cause of the physical micro-short circuit is determined to be environmental dust from the workshop, biological contamination from workers, and / or degradation and shedding of cleanroom HEPA filter materials. If the elemental composition contains nickel, then the root cause of the physical micro-short circuit is determined to be abnormal wear of the stainless steel material. If the elemental composition contains uncommon metallic elements, the root cause of the physical micro-short circuit is determined to be solder residue or abnormal wear of a specific fixture. The third-party non-metallic elements refer to non-metallic elements that should not be present in the raw materials used to make battery cells, including silicon, calcium, and sulfur; the uncommon metallic elements include tin and silver.

[0007] Optionally, applying a predetermined DC detection voltage to the two poles of the battery cell under test, which is in a cryogenic state, specifically includes: While maintaining the battery cell under test in a cryogenic state using a liquid nitrogen cryogenic device, a predetermined DC detection voltage is applied to the positive and negative tabs of the battery cell under test through a DC power supply; wherein, the DC detection voltage must be able to excite a physical micro-short circuit in the battery cell under test, and strictly avoid the risk of causing the membrane body to break down. The setting of the DC detection voltage is based on experimental determination, and the specific method is as follows: A normal battery cell with the same energy density as the battery cell under test was selected as the experimental battery cell. The intrinsic breakdown voltage of the separator of the experimental battery cell was measured at cryogenic temperatures. Set the DC detection voltage between 60% and 80% of the intrinsic breakdown voltage.

[0008] Optionally, a current sensor is also provided in the detection circuit where the DC power supply is located; The monitoring of whether a breakdown hole appears on the battery cell under test also includes: The system monitors whether a discharge arc occurs at the connection between the positive and negative tabs of the battery cell under test, and captures and records the current when the battery cell under test is short-circuited using a current sensor.

[0009] Optionally, after locating the manufacturing defects of the battery cell under test based on the destructive physical analysis data, the method further includes: Improve the battery manufacturing process accordingly based on the identified process defects.

[0010] Optionally, for LCO system cells with an energy density of 800Wh / L or higher, the DC detection voltage is designed to be 25V; for ternary system cells with an energy density of 700Wh / L or higher, the DC detection voltage is designed to be 30V.

[0011] Optionally, the step of improving the battery manufacturing process according to the identified process defects includes: Based on the data of the breakdown holes of all the cells under test, the occurrence ratio of the first breakdown hole and the second breakdown hole were calculated respectively. Based on the proportion of breakdown holes, key areas for improvement in battery manufacturing processes can be determined. The method of determining key areas for improvement in battery manufacturing processes based on the proportion of breakdown holes includes: If the proportion of first-stage perforation holes exceeds a predetermined threshold, then the focus will be on optimizing the winding process parameters. If the proportion of second-stage perforations exceeds the predetermined threshold, then the focus will be on strengthening cleanliness control measures.

[0012] Optionally, immersing the battery cell under test in a liquid nitrogen cryogenic device specifically includes: First, the battery cell under test is completely discharged. Subsequently, the battery cell under test after discharge is immersed in a liquid nitrogen cryogenic device for more than 10 minutes to reduce the overall temperature of the battery cell under test to a predetermined temperature threshold, thereby putting the battery cell under test into a cryogenic state. Among them, the electrolyte of the battery cell under test, which is in a cryogenic state, is basically solidified, and the ionic conductivity drops to a negligible level.

[0013] Compared with the prior art, the present invention has the following beneficial effects: This application provides a diagnostic method for physical micro-short circuits in battery cells, which can accurately distinguish between physical micro-short circuits and chemical micro-short circuits, locate the specific position of the physical micro-short circuit point, and diagnose the root cause of the physical micro-short circuit. This provides a direct and accurate basis for improving the manufacturing process and a reference for improving the problem of low voltage / K value defects that occur during the manufacturing process.

[0014] The present invention has other features and advantages, which will be apparent from or will be set forth in detail in the accompanying drawings and the following detailed description, which together serve to explain the particular principles of the invention. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This is a flowchart of a method for diagnosing physical micro-short circuits in battery cells provided by an embodiment of the present invention.

[0017] Figure 2 This is a schematic diagram of the structure of the first perforation hole provided in an embodiment of the present invention.

[0018] Figure 3 This is a schematic diagram of the structure of the second perforation hole provided in an embodiment of the present invention.

[0019] Figure 4 This is an energy dispersive spectral analysis diagram of the elemental composition of a perforated hole provided in an embodiment of the present invention. Detailed Implementation

[0020] To illustrate the possible application scenarios, technical principles, implementable specific solutions, and achievable objectives and effects of this application in detail, the following description, in conjunction with the listed specific embodiments and accompanying drawings, provides a detailed explanation. The embodiments described herein are merely illustrative of the technical solutions of this application and are therefore intended to limit the scope of protection of this application.

[0021] In this document, the term "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The term "embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment, nor does it specifically limit its independence or connection with other embodiments. In principle, in this application, as long as there are no technical contradictions or conflicts, the technical features mentioned in each embodiment can be combined in any way to form corresponding implementable technical solutions.

[0022] Unless otherwise defined, the technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the use of related terms herein is merely for the purpose of describing particular embodiments and is not intended to limit this application.

[0023] In the description of this application, the term "and / or" is used to describe the logical relationship between objects, indicating that three relationships can exist. For example, A and / or B means: A exists, B exists, and A and B exist simultaneously. Additionally, the character " / " in this document generally indicates that the preceding and following objects have an "or" logical relationship.

[0024] In this application, terms such as “first” and “second” are used only to distinguish one entity or operation from another, and do not necessarily require or imply any actual quantity, hierarchy or order relationship between these entities or operations.

[0025] Unless otherwise specified, the use of terms such as “comprising,” “including,” “having,” or other similar expressions in this application is intended to cover non-exclusive inclusion, which does not exclude the presence of additional elements in a process, method, or product that includes the stated elements, such that a process, method, or product that includes a list of elements may include not only those defined elements but also other elements not expressly listed, or elements inherent to such a process, method, or product.

[0026] As understood in the Examination Guidelines, in this application, expressions such as "greater than," "less than," and "exceeding" are understood to exclude the stated number; expressions such as "above," "below," and "within" are understood to include the stated number. Furthermore, in the description of the embodiments in this application, "multiple" means two or more (including two), and similar expressions related to "multiple" are also understood in this way, such as "multiple groups" and "multiple times," unless otherwise explicitly specified.

[0027] In the description of the embodiments of this application, the space-related expressions used, such as "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "vertical," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential," indicate the orientation or positional relationship based on the orientation or positional relationship shown in the specific embodiments or drawings. They are only for the purpose of describing the specific embodiments of this application or for the reader's understanding, and do not indicate or imply that the device or component referred to must have a specific position, a specific orientation, or be constructed or operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0028] Unless otherwise expressly specified or limited, the terms "installation," "connection," "linking," "fixing," and "setting," as used in the description of the embodiments of this application, should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral setting; it can be a mechanical connection, an electrical connection, or a communication connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be the internal connection of two components or the interaction between two components. For those skilled in the art to which this application pertains, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0029] Example 1: Please see Figure 1 , Figure 1 This is a flowchart of a diagnostic method for physical micro-short circuits in battery cells provided by an embodiment of the present invention. This diagnostic method achieves accurate diagnosis of physical micro-short circuits by combining liquid nitrogen treatment with electrical detection, specifically including: Step 100: Select cells with low voltage or poor K-value that occurred during the manufacturing process as cells to be tested.

[0030] This step is a pre-set step, which means that before diagnosis, low-voltage or K-value defective cells that occurred during the production process must be selected as the analysis objects. For ease of explanation, the selected analysis objects will be referred to as the cells to be tested.

[0031] It should be noted that the cells to be tested in this step are randomly sampled from cells with low voltage or poor K-value. The number of samples needs to reach the predetermined analysis base (e.g., 10% of the total) to ensure the accuracy of the diagnostic results.

[0032] Step 110: Immerse the battery cell to be tested in a liquid nitrogen cryogenic device.

[0033] Specifically, the battery cell under test (fully discharged, 30% SOC, 100% SOC voltage, etc. are all acceptable; this embodiment uses a fully discharged battery cell as an example) is first immersed in a liquid nitrogen cryogenic device for more than 10 minutes, causing a significant drop in the overall temperature of the battery cell (the corresponding temperature threshold can be set according to the battery cell type). Under these cryogenic conditions, the electrolyte system is basically solidified, and the ionic conductivity drops to a negligible level, thereby effectively suppressing all ion migration-dependent chemical micro-short-circuit processes and related electrochemical side reactions.

[0034] Step 120: Apply a predetermined DC detection voltage to the two poles of the cell under test, which is in a cryogenic state.

[0035] While maintaining the battery cell under test in a cryogenic state using a liquid nitrogen cryogenic device, a predetermined DC detection voltage is applied to the positive and negative tabs of the battery cell under test through a DC power supply. The DC detection voltage must be able to excite a physical micro-short circuit in the battery cell under test, and strictly avoid the risk of causing the diaphragm body to break down.

[0036] The setting of the DC detection voltage is based on experimental determination, and the specific method is as follows: A normal battery cell with the same energy density as the battery cell under test was selected as the experimental battery cell; the intrinsic breakdown voltage of the separator of the experimental battery cell was measured at cryogenic temperature; the DC detection voltage was set between 60% and 80% of the intrinsic breakdown voltage.

[0037] Therefore, the DC detection voltage in this embodiment is specially designed for cell systems with different energy densities. This voltage design ensures that physical micro-short circuits can be effectively triggered, while strictly avoiding the risk of diaphragm breakdown.

[0038] For example, let's take two common battery cell systems used in actual production as examples: For LCO system cells with an energy density of 800Wh / L or higher, the DC detection voltage obtained based on the above experiments is 25V; for ternary system cells with an energy density of 700Wh / L or higher, the DC detection voltage obtained based on the above experiments is 30V. However, it should be noted that different base films, coatings, and thicknesses will affect this voltage value, and it can be adjusted as needed in actual operation.

[0039] Step 130: Monitor whether there are breakdown holes on the cell under test; if yes, proceed to step 131; if no, proceed to step 132.

[0040] After applying a predetermined DC detection voltage to the two poles of the battery cell under test in a cryogenic state, if there is a physical micro-short circuit point inside the battery cell, the unique low-impedance discharge path formed at the short circuit point (the electrolyte is solidified at low temperature and cannot conduct electricity) will generate a transient maximum current concentrated at the short circuit point under the applied DC detection voltage. This current is concentrated at the short circuit point, and according to the Joule effect, it instantly generates extremely high local heat, causing the diaphragm, active material, and even metal foreign objects at that point to be vaporized or melted, forming an irreversible breakdown hole; at the same time, a significant discharge arc appears at the connection between the positive and negative tabs of the battery cell under test.

[0041] Therefore, as an optional implementation, step 130 further includes: while monitoring whether a breakdown hole appears on the battery cell under test, monitoring whether a discharge arc appears at the connection between the positive and negative tabs of the battery cell under test.

[0042] Furthermore, a current sensor is also installed in the detection circuit where the DC power supply is located; Step 130 also includes: capturing and recording the current when a short circuit occurs in the cell under test using a current sensor.

[0043] The breakdown event is captured and recorded in real time by the current sensor, serving as direct electrical evidence of the existence of a physical micro-short circuit, and is corroborated by obvious discharge arc phenomena.

[0044] Step 131: Determine if there is a physical micro-short circuit inside the cell under test, and disassemble the cell under test.

[0045] After completing the electrical tests and confirming the presence of a physical micro-short circuit, a destructive physical analysis was performed on the battery cell under test.

[0046] During disassembly, characteristic breakdown holes formed by the concentrated release of a large instantaneous current during previous testing can be observed in specific locations such as the arc area (corner area) and the main body (surface area) of the battery cell under test. For example... Figure 2 and Figure 3 As shown, the breakdown hole typically exhibits a micromorphology of edge melting and carbonization, and its location highly coincides with the location of the initial defect that caused the micro-short circuit.

[0047] Step 132: Determine whether the monitoring duration has reached the preset duration threshold.

[0048] The duration threshold can be set according to specific needs. Because if there is a micro short circuit inside the battery cell under test, an arc or breakdown can occur quickly once power is applied. Therefore, the duration threshold should not be set too long. For example, 10 seconds, 30 seconds, or 1 minute are sufficient.

[0049] Step 140: During the disassembly of the battery cell under test, observe and record the morphology and location of each breakdown hole, and use an energy dispersive spectrometer to detect the elemental composition of each breakdown hole to construct destructive physical analysis data.

[0050] The breakdown holes observed in the arc region typically exhibit an irregular radial pattern, generally located near the edge of the positive electrode sheet, with a diameter usually ranging from 1 to 5 millimeters. SEM-EDS analysis (a material analysis method combining scanning electron microscopy and energy dispersive spectroscopy) of the breakdown holes at the positive electrode sheet reveals significant aluminum element signals at the hole edges. These characteristics directly point to membrane perforation caused by slitting burrs. For example, the results of SEM-EDS analysis are as follows... Figure 4 As shown.

[0051] The perforations observed in the main body are distributed in the flat areas of the electrode sheet, far from the edge of the electrode sheet. They are usually circular or elliptical with relatively neat edges, and traces of molten metal foreign matter can be seen around them. Energy dispersive spectroscopy analysis often shows abnormal content of foreign metal elements such as iron and chromium. These compositional characteristics clearly point to process problems such as laser cleaning sputtering contamination or failure of environmental cleanliness control (dust contamination).

[0052] Step 150: Based on the destructive physical analysis data, locate the process defects in the battery cell under test during the manufacturing process.

[0053] For ease of description, the breakdown hole in the arc region of the battery cell under test is defined as the first breakdown hole, and the breakdown hole in the main body of the battery cell under test is defined as the second breakdown hole. Step 150 specifically includes: When the first breakdown hole appears on the cell under test, and its morphology is irregularly radial, if the aluminum content in the detected elemental composition is greater than or equal to a predetermined first content threshold (e.g., 70%), then the root cause of the physical micro short circuit is determined to be aluminum burrs generated during the positive electrode slitting process that pierce the separator; if the copper content in the detected elemental composition is greater than or equal to a predetermined first content threshold, then the root cause of the physical micro short circuit is determined to be copper burrs generated during the negative electrode slitting process that pierce the separator.

[0054] When the first breakdown hole appears on the cell under test, if the detected elemental composition does not contain aluminum and copper (content less than 1%), but contains a third-party metal element (content greater than 1%), then the root cause of the physical micro short circuit is determined to be the diaphragm puncture caused by the third-party metal contaminant being pressed into the edge of the positive electrode during the slitting process. If the elemental composition does not contain aluminum and copper, and no third-party metal element is found, then the root cause of the physical micro-short circuit is determined to be the conductive path formed by the incorporation of non-metallic conductive foreign matter. Among them, third-party metal elements are metal elements that should not be present in the raw materials used to make battery cells, including metal elements contained in stainless steel particles from laser cleaning sputtering and hard alloy particles from mold wear; non-metallic conductive foreign matter includes graphite dust agglomerates or carbon fibers doped in recycled materials.

[0055] When a second breakdown hole appears on the battery cell under test, and its shape is circular or elliptical, if the iron / chromium content in the elemental composition is greater than or equal to the predetermined second content threshold, then the root cause of the physical micro short circuit is determined to be laser cleaning sputtering contamination or failure of environmental cleanliness control.

[0056] When a second breakdown hole appears on the battery cell under test, if the elemental composition contains zinc, then the root cause of the physical micro short circuit is determined to be the volatilization of the zinc plating layer on the battery cell shell during the laser welding process, or the wear of the zinc plating fixture during use. If the elemental composition contains a third-party non-metallic element, then the root cause of the physical micro-short circuit is determined to be environmental dust from the workshop, biological contamination from workers, and / or degradation and shedding of cleanroom HEPA filter materials. If the elemental composition contains nickel, then the root cause of the physical micro-short circuit is determined to be abnormal wear of the stainless steel material. If the elemental composition contains uncommon metallic elements, the root cause of the physical micro-short circuit is determined to be solder residue or abnormal wear of a specific fixture. Among them, third-party non-metallic elements refer to non-metallic elements that should not be present in the raw materials used to make battery cells, including silicon, calcium and sulfur; uncommon metallic elements include tin and silver.

[0057] By systematically recording and analyzing the location distribution, macroscopic morphology, and microscopic composition characteristics of these breakdown holes, combined with the obvious discharge arc phenomenon mentioned above and the changes in the system's current sensors, a complete chain of evidence for process defects is established.

[0058] Based on the above diagnostic results, the established correlation between breakdown holes and process defects in the cell manufacturing process is shown in Table 1 below: Table 1 Specifically, after step 150, the following steps are also included: Step 160: Improve the battery manufacturing process accordingly based on the identified process defects.

[0059] Furthermore, in this embodiment, step 160 specifically includes: Step 161: Based on the data of the breakdown holes of all the cells under test, calculate the occurrence ratio of the first breakdown hole and the second breakdown hole respectively. Step 162: Based on the proportion of breakdown holes, determine the key areas for improvement in the battery manufacturing process.

[0060] Step 162 specifically includes: If the proportion of first-stage perforation holes is greater than the predetermined proportion threshold, it indicates that the focus is on optimizing the winding process parameters (because there are many burrs during winding and slitting). If the proportion of second-stage perforations exceeds the predetermined threshold, it indicates that cleanliness control measures need to be strengthened (due to the large number of foreign metal objects).

[0061] In this embodiment, the predetermined ratio threshold is 60%.

[0062] This system analysis method based on the characteristics of breakdown holes establishes a reliable correlation between macroscopic electrical performance failures and microscopic manufacturing process defects, providing accurate technical basis for the assessment and improvement of process capabilities. The manufacturing department can quickly locate process bottlenecks and take targeted improvement measures to achieve continuous improvement in product quality.

[0063] The above-described embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for diagnosing physical micro-short circuits in battery cells, characterized in that, include: Cells with low voltage or poor K-value that occurred during the manufacturing process were selected as cells to be tested; The battery cell under test is immersed in a liquid nitrogen cryogenic device; A predetermined DC detection voltage is applied to the two poles of the battery cell under test, which is in a cryogenic state. Monitor whether there are breakdown holes on the battery cell under test; If a breakdown hole is found on the battery cell under test, it is determined that there is a physical micro short circuit point inside the battery cell under test, and the battery cell under test is disassembled. During the disassembly of the battery cell under test, the morphology and location of each breakdown hole were observed and recorded, and the elemental composition of each breakdown hole was detected by an energy dispersive spectrometer to construct destructive physical analysis data. Based on the destructive physical analysis data, the manufacturing defects of the battery cell under test are located.

2. The diagnostic method for a physical micro-short circuit in a battery cell according to claim 1, characterized in that, The battery cell under test is a wound battery cell; The breakdown hole in the arc region of the battery cell under test is defined as the first breakdown hole, and the breakdown hole in the main body of the battery cell under test is defined as the second breakdown hole. The step of locating the manufacturing defects of the battery cell under test based on the destructive physical analysis data specifically includes: When the first breakdown hole appears on the cell under test, and its morphology is irregularly radial, if the aluminum content in the detected elemental composition is greater than or equal to a predetermined first content threshold, then the root cause of the physical micro short circuit is determined to be aluminum burrs generated during the positive electrode slitting process that pierce the separator; if the copper content in the detected elemental composition is greater than or equal to a predetermined first content threshold, then the root cause of the physical micro short circuit is determined to be copper burrs generated during the negative electrode slitting process that pierce the separator. When a second breakdown hole appears on the battery cell under test, and its shape is circular or elliptical, if the iron / chromium content in the elemental composition is greater than or equal to the predetermined second content threshold, then the root cause of the physical micro short circuit is determined to be laser cleaning sputtering contamination or failure of environmental cleanliness control.

3. The method for diagnosing a physical micro-short circuit in a battery cell according to claim 2, characterized in that, The step of locating process defects in the battery cell under test during manufacturing based on the destructive physical analysis data further includes: When the first breakdown hole appears on the cell under test, if the detected elemental composition does not contain aluminum and copper, but contains a third-party metal element, then the root cause of the physical micro short circuit is determined to be the diaphragm puncture caused by the third-party metal contaminant being pressed into the edge of the positive electrode during the slitting process. If the elemental composition does not contain aluminum and copper, and no third-party metal element is found, then the root cause of the physical micro-short circuit is determined to be the conductive path formed by the incorporation of non-metallic conductive foreign matter. The third-party metal element is a metal element that should not be present in the raw materials used to make the battery cell, including metal elements contained in stainless steel particles from laser cleaning sputtering and hard alloy particles from mold wear; the non-metallic conductive foreign matter includes graphite dust agglomerates or carbon fibers doped in recycled materials.

4. The diagnostic method for a physical micro-short circuit in a battery cell according to claim 2, characterized in that, When a second breakdown hole appears on the battery cell under test, if the elemental composition contains zinc, then the root cause of the physical micro short circuit is determined to be the volatilization of the zinc plating layer on the battery cell shell during the laser welding process, or the wear of the zinc plating fixture during use. If the elemental composition contains a third-party non-metallic element, then the root cause of the physical micro-short circuit is determined to be environmental dust from the workshop, biological contamination from workers, and / or degradation and shedding of cleanroom HEPA filter materials. If the elemental composition contains nickel, then the root cause of the physical micro-short circuit is determined to be abnormal wear of the stainless steel material. If the elemental composition contains uncommon metallic elements, the root cause of the physical micro-short circuit is determined to be solder residue or abnormal wear of a specific fixture. The third-party non-metallic elements refer to non-metallic elements that should not be present in the raw materials used to make battery cells, including silicon, calcium, and sulfur; the uncommon metallic elements include tin and silver.

5. The diagnostic method for a physical micro-short circuit in a battery cell according to claim 2, characterized in that, Applying a predetermined DC detection voltage to the two poles of the battery cell under test, which is in a cryogenic state, specifically includes: While maintaining the battery cell under test in a cryogenic state using a liquid nitrogen cryogenic device, a predetermined DC detection voltage is applied to the positive and negative tabs of the battery cell under test through a DC power supply; wherein, the DC detection voltage must be able to excite a physical micro-short circuit in the battery cell under test, and strictly avoid the risk of causing the membrane body to break down. The setting of the DC detection voltage is based on experimental determination, and the specific method is as follows: A normal battery cell with the same energy density as the battery cell under test was selected as the experimental battery cell. The intrinsic breakdown voltage of the separator of the experimental battery cell was measured at cryogenic temperatures. Set the DC detection voltage between 60% and 80% of the intrinsic breakdown voltage.

6. The diagnostic method for a physical micro-short circuit in a battery cell according to claim 5, characterized in that, A current sensor is also provided in the detection circuit where the DC power supply is located; The monitoring of whether a breakdown hole appears on the battery cell under test also includes: The system monitors whether a discharge arc occurs at the connection between the positive and negative tabs of the battery cell under test, and captures and records the current when the battery cell under test is short-circuited using a current sensor.

7. The method for diagnosing a physical micro-short circuit in a battery cell according to claim 6, characterized in that, After locating the manufacturing defects of the battery cell under test based on the destructive physical analysis data, the process further includes: Improve the battery manufacturing process accordingly based on the identified process defects.

8. The method for diagnosing a physical micro-short circuit in a battery cell according to claim 5, characterized in that, For LCO system cells with an energy density of 800Wh / L and above, the DC detection voltage is designed to be 25V; for ternary system cells with an energy density of 700Wh / L and above, the DC detection voltage is designed to be 30V.

9. The method for diagnosing a physical micro-short circuit in a battery cell according to claim 7, characterized in that, The improvement of the battery manufacturing process based on the identified process defects includes: Based on the data of the breakdown holes of all the cells under test, the occurrence ratio of the first breakdown hole and the second breakdown hole were calculated respectively. Based on the proportion of breakdown holes, key areas for improvement in battery manufacturing processes can be determined. The method of determining key areas for improvement in battery manufacturing processes based on the proportion of breakdown holes includes: If the proportion of first-stage perforation holes exceeds a predetermined threshold, then the focus will be on optimizing the winding process parameters. If the proportion of second-stage perforations exceeds the predetermined threshold, then the focus will be on strengthening cleanliness control measures.

10. The method for diagnosing a physical micro-short circuit in a battery cell according to claim 1, characterized in that, The step of immersing the battery cell under test in a liquid nitrogen cryogenic device specifically includes: First, the battery cell under test is completely discharged. Subsequently, the battery cell under test after discharge is immersed in a liquid nitrogen cryogenic device for more than 10 minutes to reduce the overall temperature of the battery cell under test to a predetermined temperature threshold, thereby putting the battery cell under test into a cryogenic state.

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