Detection method of foil fracture at welding point of lithium battery cell

Abstract

The present invention relates to a method for detecting a broken foil at a welding position of a lithium battery cell. The method comprises establishing a calibration voltage range, preforming a cell to be tested and collecting voltage data after preforming and comparing the voltage data with the calibration voltage range. If the difference between the value of the voltage data and the upper limit value of the calibration voltage range is not less than a judgment value, it is judged that the foil at the welding position of the cell to be tested is broken. The method for detecting a broken foil at a welding position of a lithium battery cell provided by the present invention does not require additional steps, is simple to operate, can effectively detect cells with broken foil at welding positions, and prevent abnormal cells from flowing out.

Description

Technical Field

[0001] The invention relates to the field of lithium ion batteries, and in particular to a method for detecting the breakage of foil at a welding position of a lithium battery cell. Background Art

[0002] Welding is one of the key processes in the preparation of lithium-ion batteries. A common problem in the welding process is that the foil at the welding joint breaks due to sudden fluctuations in welding parameters or defects in individual incoming foils.

[0003] At present, the more common way to detect foil breakage at welding spots is to test the internal resistance of lithium-ion cells. However, the internal resistance of lithium-ion cells is at the milliohm level, and experiments have shown that when laminated batteries have the undesirable phenomenon of foil breakage at welding spots, the internal resistance of the cells only fluctuates slightly. That is, the method of measuring the internal resistance of cells has low sensitivity and accuracy, and it is difficult to completely pick out lithium-ion cells with broken foil at welding spots, which poses a risk of misjudgment and missed judgment.

[0004] Once lithium-ion cells with broken foil at the welding spot are shipped out of the factory, customers may experience poor voltage or electrochemical performance when using the cells to assemble battery modules. More seriously, if abnormal batteries are assembled in new energy vehicles, there will be safety hazards during the use of the vehicles. Therefore, the industry is in urgent need of finding a method to effectively pick out cells with broken foil at the welding spot without adding additional testing steps. Summary of the invention

[0005] In view of the technical problems existing in the prior art, the present invention provides a method for detecting the breakage of foil at the welding position of a lithium battery cell. The detection method pre-forms the cell to be tested and collects voltage data after the pre-formation, and judges whether the foil at the welding position of the cell is broken according to the voltage data. The detection method does not require additional steps and is simple to operate. It can effectively detect cells with broken foil at the welding position to prevent abnormal cells from flowing out.

[0006] A method for detecting the breakage of foil at the welding position of a lithium battery cell comprises establishing a calibrated voltage range, pre-forming a cell to be tested and collecting voltage data after the pre-formation for comparison with the calibrated voltage range; if the difference between the value of the voltage data and the upper limit value of the calibrated voltage range is not less than a judgment value, it is judged that the foil at the welding position of the cell to be tested is broken.

[0007] Furthermore, establishing the calibrated voltage range includes: selecting a batch of calibrated cells, performing pre-forming treatment on the calibrated cells, collecting the voltage of the calibrated cells after pre-forming, obtaining a voltage distribution diagram according to the number of the calibrated cells and the voltage, and obtaining the calibrated voltage range according to the voltage distribution diagram.

[0008] Furthermore, a rectangular coordinate system is established with the number of the calibrated cells as the horizontal coordinate and the voltage as the vertical coordinate, and scattered points consisting of the number of the calibrated cells and the voltage are plotted in the rectangular coordinate system to obtain the voltage distribution diagram.

[0009] Further, in the voltage distribution diagram, the value of the ordinate at the lowest position scatter point is the lower limit of the calibrated voltage range, and the value of the ordinate of the lowest position scatter point plus 32-38 is the upper limit of the calibrated voltage range.

[0010] Furthermore, the number of the calibrated cells is not less than 500.

[0011] Furthermore, the determination value is 5.

[0012] Furthermore, the battery cell to be tested with intact foil at the welding location is a normal battery cell, and the normal battery cell satisfies: the voltage value of the normal battery cell is within the calibrated voltage range or the voltage value of the normal battery cell exceeds the upper limit of the calibrated voltage range but the difference between the two is less than the judgment value.

[0013] Furthermore, the detection method comprises the following steps:

[0014] S1: Select a batch of calibration cells and perform pre-forming treatment on the calibration cells;

[0015] S2: Collecting the voltage of the calibration cell after pre-formation and drawing a voltage distribution diagram;

[0016] S3: establishing the calibrated voltage range according to the voltage distribution diagram;

[0017] S4: performing the pre-forming treatment on the battery cell to be tested to obtain corresponding voltage data, and comparing the voltage data with the calibrated voltage range to determine the fracture condition of the foil at the welding position of the battery cell to be tested;

[0018] The calibration cell has the same specification as the cell to be tested.

[0019] Further, in step S1, the operation of the pre-forming treatment includes: charging the calibrated battery cell at a constant current of 0.01-0.05C to a state of charge of 30%.

[0020] Furthermore, the battery cell to be tested is a laminated lithium battery cell.

[0021] The internal structure of a laminated lithium battery cell is composed of alternating positive and negative electrode plates. The stacking of the plates can be regarded as multiple small batteries in parallel. When a fixed charging capacity value is set to pre-charge the cell, if the welding joints of individual layers in the cell are broken, the charging capacity of other layers will inevitably increase. In this case, the voltage of the abnormal cell will exceed the upper limit of the pre-forming process parameters before the set charge state is reached. The voltage value of the cell after pre-forming can be compared with the calibrated voltage range to determine the breakage of the foil at the welding joint of the cell to be tested.

[0022] Compared with the prior art, the technical solution of the present invention has at least the following beneficial effects: the detection method detects the fracture of the foil at the welding position during the pre-forming process of the lithium battery cell, and the operation is simple, and no additional detection equipment or detection process is required; the detection method establishes a calibration voltage range by pre-forming the calibration cell, and only needs to collect the voltage data of the cell to be tested after pre-forming, and compare the voltage data with the calibration voltage range to determine whether the foil at the welding position of the cell is broken, which can effectively detect the cell with broken foil at the welding position and avoid the outflow of abnormal cells. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The accompanying drawings further illustrate the present invention, but the embodiments in the accompanying drawings do not constitute any limitation to the present invention.

[0024] Figure 1 A flow chart of a method for detecting a break in foil at a welding point of a lithium battery cell provided by the present invention;

[0025] Figure 2 A voltage distribution diagram provided in an embodiment. DETAILED DESCRIPTION

[0026] It should be understood that the specific embodiments described herein are only used to explain the present invention, and are not used to limit the present invention.

[0027] In order to effectively detect the battery cells with broken foil at the welding place and avoid the outflow of abnormal battery cells, the present invention provides a detection method for the broken foil at the welding place of lithium battery cells, which includes establishing a calibration voltage range, pre-forming the battery cells to be tested and collecting the voltage data after pre-forming and comparing it with the calibration voltage range. If the difference between the value of the voltage data and the upper limit value of the calibration voltage range is not less than the judgment value, it is judged that the foil at the welding place of the battery cells to be tested is broken. Figure 1 As shown, the specific steps of the detection method are:

[0028] S101: Select a batch of calibration cells and perform pre-forming treatment on the calibration cells;

[0029] The calibration cells in the present invention are all qualified products, that is, the foil materials at their welding places are all intact. No less than 500 laminated lithium battery cells of the same specification and model are selected as calibration cells, and the calibration cells are uniformly pre-formed. Cell pre-formation is an important process in the production process of lithium batteries. The purpose of pre-formation is to activate the lithium battery and activate the positive and negative electrode materials inside it. After the cell is pre-formed, a layer of solid electrolyte interface film (SEI film) will be formed on the surface of the negative electrode. The quality of the SEI film directly affects the electrochemical properties of the lithium battery, such as the cycle life, stability, and self-discharge.

[0030] In the industry, the pre-forming treatment of the battery cell is generally to use a small current to charge the battery cell to a 30% state of charge at a constant current. The small current generally refers to 0.01-0.05C. The purpose of the present invention is to solve the problem of how to effectively detect the breakage of the foil at the welding point of the battery cell in the current production and manufacturing. In order to make the battery cell closer to the actual production and application state, the pre-forming operation of the battery cell is carried out according to the common operation in the industry. Preferably, the calibration battery cell is charged with a constant current of 0.05C to a 30% state of charge.

[0031] S102: collecting the voltage of the calibrated battery cell after pre-formation and drawing a voltage distribution diagram;

[0032] After the calibration cell is pre-formed, it has a certain voltage. The voltage data is collected, and a rectangular coordinate system is established with the number of calibration cells as the horizontal coordinate and the voltage as the vertical coordinate. The voltage distribution diagram can be obtained by plotting the scattered points composed of the number of calibration cells and the voltage in the rectangular coordinate system. Converting the collected voltage data into a voltage distribution diagram can intuitively reflect the distribution of the voltage data, which is conducive to quickly finding the extreme value of the voltage data in order to determine the range of the calibration voltage.

[0033] S103: Establishing a calibration voltage range according to the voltage distribution diagram;

[0034] Due to the differences in the battery cells themselves and the error range in the operating process, there is a reasonable fluctuation range in the voltage of the battery cells after pre-formation. The inventors have determined through a large number of experiments that after the calibration battery cells are pre-formed, the difference between the maximum and minimum values ​​of the corresponding voltage is 32-38, that is, in the voltage distribution diagram, the value of the ordinate of the scatter point at the lowest position is the lower limit of the calibration voltage range, and the value of the ordinate of the scatter point at the lowest position plus 32-38 is the upper limit of the calibration voltage range.

[0035] S104: The battery cell to be tested is pre-formed to obtain corresponding voltage data, and the voltage data is compared with a calibrated voltage range to determine the fracture condition of the foil at the welding position of the battery cell to be tested.

[0036] The battery cell to be tested is subjected to the same pre-forming treatment as the calibrated battery cell to obtain corresponding voltage data. The voltage data is compared with the calibrated voltage range obtained in step S103 to quickly determine the fracture condition of the foil at the welding part of the battery cell to be tested. The inventor has found through a large number of experiments that the judgment value is 5. When the difference between the voltage value corresponding to the battery cell to be tested after pre-forming and the upper limit value of the calibrated voltage range is greater than or equal to 5, the battery cell to be tested is an abnormal battery cell, and it can be determined that the foil at the welding part of the battery cell is fractured; when the voltage value corresponding to the battery cell to be tested after pre-forming is within the calibrated voltage range or the voltage value exceeds the upper limit value of the calibrated range but the difference between the two is less than 5, the battery cell to be tested is a normal battery cell, that is, the foil at the welding part of the battery cell is intact and there is no fracture.

[0037] Example 1

[0038] Establish a calibration voltage range: select 500 1000EA laminated lithium battery cells with a designed capacity of 50Ah and 50 positive electrode layers as calibration cells, charge the calibration batteries to a 30% state of charge at a constant current of 0.05C, that is, charge the calibration cells to a capacity of 15Ah at a current of 2.5A, collect voltage data of the calibration cells, and establish a rectangular coordinate system with the number of calibration cells as the horizontal coordinate and the collected voltage as the vertical coordinate, draw a voltage distribution diagram in the established rectangular coordinate system, and determine the range of the calibration voltage according to the voltage distribution diagram, wherein the vertical coordinate value of the scattered point at the lowest position is the lower limit of the calibration voltage range, and the difference between the vertical coordinate value of the scattered point at the highest position and the scattered point at the lowest position is in the range of 32-38, then the vertical coordinate value of the scattered point is the upper limit of the calibration voltage range, otherwise, select the second highest point to continue verification to determine the upper limit of the calibration voltage range.

[0039] Test the battery cell to be tested: select the battery cell 1 to be tested of the same specification and model, charge it to 30% state of charge at a constant current of 0.05C and collect the corresponding voltage data, compare the voltage data with the upper limit of the calibrated voltage range, if the value is within the calibrated voltage range or the voltage value exceeds the upper limit of the calibrated range but the difference between the two is less than 5, then the battery cell to be tested is a normal battery cell, and there is no break in the foil at the welding place; if the value is greater than the upper limit of the calibrated voltage range and the difference is greater than or equal to 5, then the battery cell is an abnormal battery cell, and the foil at the welding place is broken.

[0040] Example 2

[0041] This embodiment only performs pre-forming treatment on the battery cell 2 to be tested of the same specification and model as the calibrated battery cell and collects the corresponding voltage data, and compares the voltage data with the upper limit value of the calibrated voltage range obtained in Example 1. The specific operation steps and parameters are the same as those in Example 1.

[0042] Example 3

[0043] This embodiment only performs pre-forming treatment on the battery cell 3 to be tested of the same specification and model as the calibrated battery cell and collects the corresponding voltage data, and compares the voltage data with the upper limit value of the calibrated voltage range obtained in Example 1. The specific operation steps and parameters are the same as those in Example 1.

[0044] Example 4

[0045] This embodiment only performs pre-forming treatment on the battery cell 4 to be tested of the same specification and model as the calibrated battery cell and collects the corresponding voltage data, and compares the voltage data with the upper limit value of the calibrated voltage range obtained in Example 1. The specific operation steps and parameters are the same as those in Example 1.

[0046] Example 5

[0047] This embodiment only performs pre-forming treatment on the battery cell 5 to be tested of the same specification and model as the calibrated battery cell and collects the corresponding voltage data, and compares the voltage data with the upper limit value of the calibrated voltage range obtained in Example 1. The specific operation steps and parameters are the same as those in Example 1.

[0048] The voltage data of the 500 calibration cells in the above embodiment 1 after pre-forming are shown in Table 1. The voltage data is plotted into a voltage distribution diagram, as shown in Table 1. Figure 2 As shown, the horizontal axis is the number of calibrated battery cells, and the vertical axis is the voltage value. In the voltage distribution diagram, the vertical axis value of the scattered point at the lowest position is 3607.422, and the vertical axis value of the scattered point at the highest position is 3640.277. The difference between the two is 32.835. If the difference between the vertical axis value of the scattered point at the highest position and the scattered point at the lowest position is within the range of 32-38, then the calibrated voltage range is 3607.422-3640.277.

[0049] The voltage data of the cells to be tested in Examples 1-5 are shown in Table 2. Among them, the voltage value of Example 2 is within the calibrated voltage range, so the battery cell 2 to be tested in Example 2 is a normal battery cell, and the foil at its welding place is not broken; the voltage value of Example 1 is greater than the upper limit value of the calibrated voltage range, but the difference between the two is 2.493, that is, the difference between the two is less than the judgment value 5, so the battery cell 1 to be tested in Example 1 is also a normal battery cell, and the foil at its welding place is not broken; the voltage values ​​of Examples 3-5 are all greater than the upper limit value of the calibrated voltage range, and the difference between the two is 7.723, 14.723, and 18.723, respectively, and the difference is greater than the judgment value 5, then the battery cell 3 to be tested, the battery cell 4 to be tested, and the battery cell 5 to be tested are all abnormal batteries, and the foil at their welding place is broken. In addition, in the abnormal battery cell, the greater the difference between the voltage and the upper limit value of the calibrated voltage range, the more layers of broken foil at the welding place in the battery cell, and in Examples 3-5, the number of broken foil layers at the welding place in the battery cell 5 to be tested is the largest.

[0050] Table 2 Voltage data of the cells to be tested in Examples 1-4

[0051] Serial number Voltage mV Example 1 3642.77 Example 2 3636.673 Example 3 3648 Example 4 3655 Example 5 3659

[0052] The method for detecting the breakage of foil at the welding position of a lithium battery cell provided by the present invention has the following advantages:

[0053] 1. This detection method detects the fracture of the foil at the welding point during the pre-forming process of the lithium battery cell. The operation is simple and does not require additional detection equipment or additional detection processes.

[0054] 2. Collect the voltage of the calibration cell after pre-formation and convert the voltage data into a voltage distribution diagram. Through a large number of tests, determine the reasonable fluctuation range of the voltage, that is, confirm the calibration voltage range. The data is true and reliable, which improves the accuracy of the detection method;

[0055] 3. Through a large number of tests, the standard for judging whether the foil at the welding point of the tested battery cell is broken was determined and verified, which effectively avoided misjudgment and missed judgment and improved the effectiveness of the detection method;

[0056] 4. This detection method establishes a calibration voltage range by pre-forming the calibration battery cell. It only needs to collect the voltage data of the battery cell to be tested after pre-forming, and compare the voltage data with the calibration voltage range to determine whether the foil at the welding point of the battery cell is broken. It can effectively detect the battery cell with broken foil at the welding point and avoid the outflow of abnormal batteries.

[0057] Table 1 Voltage data of the calibration cell after pre-formation

[0058]

[0059]

[0060]

[0061] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0062] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for detecting the breakage of foil at the welding position of a lithium battery cell, characterized in that: The detection method comprises the following steps: S1: Select a batch of calibration cells and perform pre-forming treatment on the calibration cells, the number of the calibration cells being not less than 500; S2: collecting the voltage of the calibration cell after pre-formation and drawing a voltage distribution diagram, establishing a rectangular coordinate system with the number of the calibration cells as the horizontal coordinate and the voltage as the vertical coordinate, and drawing the scattered points composed of the number of the calibration cells and the voltage in the rectangular coordinate system to obtain the voltage distribution diagram; S3: establishing a calibration voltage range according to the voltage distribution diagram, wherein the value of the ordinate of the lowest position scatter point in the voltage distribution diagram is the lower limit of the calibration voltage range, and the value of the ordinate of the lowest position scatter point plus 32-38 is the upper limit of the calibration voltage range; S4: The cell to be tested is subjected to the pre-forming treatment to obtain corresponding voltage data, and the voltage data is compared with the calibrated voltage range to determine the fracture of the foil at the welding position of the cell to be tested. The cell to be tested with intact foil at the welding position is a normal cell, and the normal cell satisfies: the voltage value of the normal cell is within the calibrated voltage range or the voltage value of the normal cell exceeds the upper limit of the calibrated voltage range but the difference between the two is less than the judgment value, and the judgment value is 5; The calibration cell has the same specification and model as the cell to be tested; In step S1, the operation of the pre-forming treatment includes: charging the calibration cell at a constant current of 0.01-0.05C to a state of charge of 30%.

2. The method for detecting the breakage of foil at the welding position of a lithium battery cell according to claim 1, characterized in that: The battery cell to be tested is a laminated lithium battery cell.

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