Method for measuring short circuit boundary parameter in lithium ion battery, safety evaluation method

Abstract

The application discloses a kind of lithium ion battery internal short circuit boundary parameter determination method, safety evaluation method, in the state of the test battery occurs internal short circuit, test battery is charged to cut-off voltage by constant current, then constant voltage charging is cut-off current;During constant voltage charging stage, when current is stable at a certain current value and continues for a set time, switch to constant current discharge and discharge to cut-off voltage;Repeat the cycle operation of constant current charging, constant voltage charging and constant current discharge of test battery until triggering battery thermal runaway, the current value corresponding to the current value that current is stable at a certain current value and continues for a set time in constant voltage charging stage before triggering thermal runaway is regarded as boundary internal short circuit current.The application adopts the way of implanting conductive metal in battery, so that battery can gradually approach thermal runaway boundary during cyclic charging and discharging, so that the internal short circuit boundary parameter of battery under different conditions can be accurately determined, and the accurate evaluation of battery safety state is realized.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery safety testing technology, specifically relating to a method for determining internal short-circuit boundary parameters and a safety assessment method for lithium-ion batteries. Background Technology

[0002] Lithium-ion batteries are widely used in mobile communications, laptops, and even electric and hybrid vehicles due to their advantages such as high energy density, long cycle life, no memory effect, and low self-discharge rate. During use, however, the cells may experience thermal runaway due to the unavoidable introduction of metallic impurities from external sources or during manufacturing. To minimize the negative effects of internal short circuits and reduce their hazards, it is necessary to conduct controlled internal short circuit simulation tests on lithium-ion pouch batteries using experimental methods. This allows for the assessment of the severity of internal short circuits and the optimization of relevant design parameters and manufacturing processes.

[0003] Currently, the main method for simulating short-term triggered thermal runaway is needle acupuncture, that is, using... A 5mm-8mm high-temperature resistant steel needle is used to puncture the battery at a certain speed. The conductive needle connects the positive and negative electrodes internally, creating an internal short circuit and triggering thermal runaway. However, this method has the following drawbacks: 1. The internal short circuit is caused internally, but the needle puncture is from the outside, making this method deviate from the actual internal short-induced thermal runaway scenario; 2. In the needle puncture test, the temperature consistency during the puncture process is related to various factors such as the needle surface condition and the shape of the puncture point. For example, different needle surface conditions may have different thermal conductivity, resulting in different detected maximum puncture temperatures; 3. The needle puncture test can only be performed at different locations on the battery surface, which is equivalent to only short-circuiting gradually from the surface inwards, and cannot simulate the thermal runaway caused by a short circuit source inside the cell; 4. After the steel needle is inserted, the battery will simultaneously experience multiple different types of internal short circuits, resulting in multiple different types of internal short thermal runaway processes. This makes the internal short types in the needle-triggered thermal runaway process complex and inseparable, making it impossible to simulate single / different types of internal short-circuit triggering methods.

[0004] Patent document CN108390112A discloses a method and apparatus for simulating an internal short circuit in a battery by incorporating metal particles that are not part of the battery components, thereby further triggering thermal runaway. However, during the battery manufacturing process, larger foreign particles can be detected, making this simulation method less applicable and exhibiting low controllability and repeatability. Patent document CN101276943A discloses a method for inducing an internal short circuit in a battery by mixing in a specific conductive foreign object (in contact with the positive electrode but not the negative electrode). However, this method requires the conductive foreign object to be specifically implanted on the positive electrode side, and the implantation must be performed after disassembling the battery or during the manufacturing process, which places high demands on the implantation and can have uncertain effects on the battery.

[0005] In the above test methods, the internal short circuit is triggered instantaneously, which can only measure the temperature after the battery thermal runaway. It is difficult to measure the relevant boundary parameters of the battery thermal runaway caused by the internal short circuit, such as the internal short circuit current, internal resistance, heat and so on when the battery thermal runaway is triggered. Summary of the Invention

[0006] The purpose of this invention is to provide a method for determining the boundary parameters of internal short circuits in lithium-ion batteries and a method for safety assessment, so as to solve the problems existing in the determination of the relevant boundary parameters of thermal runaway caused by internal short circuits in batteries.

[0007] This invention is achieved through the following technical solution: A method for determining the internal short-circuit boundary parameters of a lithium-ion battery includes the following steps: The foil tab of the test battery is short-circuited with the positive tab, and the test battery is charged and discharged at a constant current. When the battery voltage reaches a specific potential, the conductive metal foil implanted in the test battery dissolves and deposits, triggering a short circuit inside the test battery (different conductive metal materials have different deposition potentials). The test battery is a battery cell with a conductive metal foil covered with a separator film implanted on its surface. The foil tab is connected to the end of the conductive metal foil that extends out of the battery. When the test battery is in the state of internal short circuit, the test battery is charged with constant current to the cutoff voltage, and then charged with constant voltage to the cutoff current. During the constant voltage charging stage, when the current stabilizes at a certain current value and continues for a set time, it is switched to constant current discharge and discharged to the cutoff voltage. The current value when the current stabilizes at a certain current value is taken as the internal short circuit current corresponding to this charge and discharge cycle. Repeat the constant current charging, constant voltage charging and constant current discharging cycle of the test battery, and make the cut-off current of the constant voltage charging stage of the later charge and discharge cycle greater than the internal short circuit current corresponding to the previous charge and discharge cycle, until the battery thermal runaway is triggered. The current value corresponding to the current stabilizing at a certain current value and lasting for a set time during the constant voltage charging stage before the thermal runaway is triggered is taken as the boundary internal short circuit current.

[0008] In some embodiments, the corresponding intra-boundary short-circuit resistance and intra-boundary short-circuit Joule heat are obtained based on the intra-boundary short-circuit current.

[0009] In some embodiments, the test battery is obtained by embedding conductive metal foil into a fresh battery, a finished battery, or an aged battery.

[0010] In some embodiments, the fresh battery is formed by stacking or winding a positive electrode sheet, a separator, and a negative electrode sheet arranged in sequence, and the conductive metal foil is implanted into the battery cell during the stacking or winding process.

[0011] In some embodiments, before triggering an internal short circuit, the prepared fresh battery is charged and discharged with a small current, which is 1 / 25C to 1 / 10C, ​​where nC represents the time required for the battery to be charged from an empty state to a fully charged state in 1 / n hours.

[0012] In some embodiments, the number of implanted conductive metal foils is one or more.

[0013] In some embodiments, the conductive metal foil is embedded on the positive or negative side of the battery cell.

[0014] In some embodiments, the conductive metal foil is made of Cu, Zn, Fe, or Ni, etc.

[0015] In some embodiments, the area of ​​the conductive metal foil is 10%-50% of the area of ​​the battery cell electrode.

[0016] On the other hand, the present invention also provides a lithium-ion battery safety assessment method, which uses the lithium-ion battery internal short-circuit boundary parameter determination method to obtain the battery's internal short-circuit boundary parameters and assess the battery's safety status.

[0017] Compared with the prior art, the present invention has the following advantages and beneficial effects: This invention employs a method of implanting conductive metal within the battery. By controlling the potential between the implanted conductive metal and the positive electrode of the battery, the metal is dissolved and deposited inside the battery, effectively controlling the internal short-circuit process. Furthermore, by cyclic charging and discharging and increasing the number of internal short-circuit contact points, the degree of internal short-circuit in the battery can be gradually aggravated, allowing the battery to gradually approach the thermal runaway boundary during cyclic charging and discharging. This enables accurate measurement of the internal short-circuit boundary parameters of the battery under different states, achieving an accurate assessment of the battery's safety status. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the battery cell structure in an embodiment of the present invention.

[0020] Figure 2 This is a schematic diagram of the conductive metal foil implantation structure in an embodiment of the present invention.

[0021] Figure 3 This is a schematic diagram of the structure for testing the battery's test state in an embodiment of the present invention.

[0022] Figure 4 This is a schematic diagram illustrating the internal short circuit state of the test battery in an embodiment of the present invention when lithium plating exists on the simulated battery surface.

[0023] in: 1. Negative current collector, 2. Positive electrode sheet, 3. Positive active material, 4. Negative active material, 5. Positive current collector, 6. Negative electrode sheet, 7. Separator, 8. Conductive metal foil, 9. Separating membrane, 10. Foil tab, 11. Positive electrode tab, 12. Negative electrode tab, 13. Wire. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this application clearer, specific embodiments of this application will be described in further detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are merely for explaining this application and not for limiting it. It should also be noted that, for ease of description, only the parts relevant to this application are shown in the drawings, not all of them. Before discussing exemplary embodiments in more detail, it should be mentioned that some exemplary embodiments are described as processes or methods depicted as flowcharts. Although the flowcharts describe operations (or steps) as sequential processes, many of these operations can be performed in parallel, concurrently, or simultaneously. Furthermore, the order of the operations can be rearranged. The process can be terminated when its operation is completed, but may also have additional steps not included in the drawings. The process can correspond to a method, function, procedure, subroutine, subprogram, etc.

[0025] When a battery experiences an internal short circuit, the electrochemical substances inside are damaged, leading to abnormal self-discharge and a corresponding internal short-circuit current. Therefore, when charging a battery with an internal short circuit using constant current and constant voltage, if the charging current in the constant current phase is less than the internal short-circuit current, the battery will not charge normally, and the voltage will remain constant or decrease. If the charging current in the constant current phase equals the internal short-circuit current, the battery will exhibit abnormal charging, with the voltage fluctuating around a certain value without increasing or decreasing. If the charging current in the constant current phase is greater than the internal short-circuit current, the battery will charge normally, with the voltage increasing normally. After reaching the cutoff voltage, it will switch to constant voltage charging. During constant voltage charging, the charging current gradually decreases. When the charging current matches the internal short-circuit current, the current will not decrease normally but will fluctuate continuously around that current value.

[0026] Based on the above characteristics, it is possible to measure the internal short-circuit current generated when an internal short circuit occurs in the battery, and further obtain relevant parameters of the internal short-circuit resistance and the heat generated by the internal short circuit. This allows for the determination of the relevant boundary parameters corresponding to thermal runaway caused by the internal short circuit. However, the following conditions must be met during the internal short circuit triggering process: 1) The internal short circuit process should continue to occur during the charging process, and with the aging of the battery, the internal short circuit will gradually intensify, eventually leading to thermal runaway; 2) The intensification process of the internal short circuit should be slow, gradual, and controllable, so as to gradually approach the boundary of thermal runaway; 3) The internal short circuit triggering scheme needs to have good repeatability and accuracy.

[0027] This invention provides a controllable method for simulating an internal short circuit in a battery. It utilizes the property that different metals dissolve and deposit on the negative electrode under the influence of the positive electrode potential, thereby triggering an internal short circuit between the positive and negative electrodes. This internal short circuit triggering method also ensures that the internal short circuit process gradually intensifies during battery cycling.

[0028] By controlling the battery voltage, when the battery voltage reaches a specific potential, the metal implanted inside the battery begins to dissolve and deposit, triggering an internal short circuit (different conductive metal materials have different deposition potentials); when the battery voltage falls below a specific potential, the internal short circuit process automatically stops, thus achieving controllability of the internal short circuit process.

[0029] This internal short-circuit triggering method based on a single short-circuit point can achieve controllability of the internal short circuit, and the internal short circuit process can be further aggravated by adding more short-circuit points. Therefore, this triggering method can meet the testing requirements of relevant boundary parameters when internal short circuits induce thermal runaway, and can well ensure the accuracy of the measurement results.

[0030] In some embodiments of the present invention, the method for determining the short-circuit boundary parameters of a lithium-ion battery includes the following steps: 1. Prepare test batteries; The steps for preparing and testing batteries using batteries from the manufacturing process are as follows: S011. A positive electrode active material 3 slurry is coated onto the positive electrode current collector 5 to prepare a positive electrode sheet 2; a negative electrode active material 4 slurry is coated onto the negative electrode current collector 1 to prepare a negative electrode sheet 6; a separator 7 is provided between the positive electrode sheet and the negative electrode sheet; as shown. Figure 1 As shown.

[0031] S012. Select conductive metal foils 8 with a thickness of 5~100μm, such as Cu (dissolution potential > 3.6V), Zn (dissolution potential > 3.0V), Fe (dissolution potential > 3.8V), and Ni (dissolution potential > 4.3V). Polish the surface of the conductive metal foils 8 with sandpaper to remove the oxide layer. Cover one end of the conductive metal foils 8 with an isolation film 9 to prevent the conductive metal foils from directly contacting the positive / negative electrode plates. The area of ​​the conductive metal foil is approximately 10-50% of the electrode area, and the foil tab 10 is welded to the end that is not covered with the separator.

[0032] S013. During the battery stacking or winding process, conductive metal foil with a surface-coated separator is implanted into the battery, such as... Figure 2 As shown; Then, the cells are packaged. For soft-pack cells, the tabs of the conductive metal foil extend from the edge of the cell before packaging. For hard-pack cells, the tabs of the conductive metal foil extend from the explosion-proof valve. The number of conductive metal foils implanted inside the battery can be one or more.

[0033] The steps for preparing test batteries using batteries that have already completed the manufacturing process (including batteries that have undergone long-term aging tests): If a hard-shell battery is used, open the battery's explosion-proof valve, insert one end of the conductive metal foil covered with the separator into the battery cell, either into the positive or negative side of the cell. After insertion, use aluminum foil tape to pre-seal the explosion-proof valve, and then use epoxy resin for sealing. Similarly, the number of conductive metal foils implanted inside the battery can be one or more.

[0034] If a pouch battery is used, open the aluminum-plastic film on the edge of the pouch battery, and insert the end of the conductive metal foil covered with the separator into the battery cell. It can be inserted into the positive or negative side of the battery cell. After insertion, reseal the aluminum-plastic film on the edge of the battery. Similarly, the number of conductive metal foils implanted inside the battery can be one or more.

[0035] After the test battery was prepared, it was charged and discharged with a small current to stabilize the battery structure and ensure that there were no abnormalities. The value of the small current here is 1 / 25C to 1 / 10C, ​​and nC means that it takes 1 / n hours for the battery to be charged from an empty state to a fully charged state.

[0036] 2. Perform cyclic charge and discharge tests on the test battery and determine the internal short-circuit boundary parameters. The steps for charging and discharging the battery are as follows: At 25°C, charge the battery with a constant current at a rate of 0.5~10C until it reaches 3.6~4.5V, then charge it with a constant voltage until the cutoff current is 0.05C; After charging, discharge the battery with a constant current at a rate of 0.5~2C until the discharge cutoff voltage is 2.0~3V; and perform multiple charge and discharge cycles on the test battery.

[0037] Different upper and lower voltage limits can be used for charging and discharging batteries of different systems.

[0038] This invention utilizes the property that different metals can dissolve under the influence of potential difference, which enables the slow and controllable triggering of internal short circuits in lithium-ion batteries, and the degree of internal short circuit will gradually increase as the cycle test proceeds.

[0039] During the charging process of the test battery, when the potential difference between the positive and negative electrodes reaches the metal dissolution potential, an internal short circuit is automatically triggered. During the discharging process, when the potential difference between the positive and negative electrodes falls below the metal dissolution potential, the triggering of the internal short circuit automatically stops until the next cycle. When the potential difference between the positive and negative electrodes reaches the metal dissolution potential again, a new short circuit point is triggered, further triggering the internal short circuit, thus gradually increasing the degree of internal short circuit. After multiple cycles, this eventually leads to thermal runaway.

[0040] When an internal short circuit occurs within a battery, if the generated internal short-circuit current matches the charging current, it will cause abnormal charging, especially when the battery is in the constant-voltage charging stage. In the constant-voltage charging stage, the battery's charging current gradually decreases, but when the charging current and the internal short-circuit current reach equilibrium, the current will not decrease normally and will fluctuate continuously around that value, allowing for the determination of the current internal short-circuit current of the battery under test.

[0041] As the cycling process continues, the degree of internal short circuit intensifies, and the internal short circuit current gradually increases, eventually triggering thermal runaway. The final value that balances the constant-voltage charging current with the current internal short circuit current before thermal runaway is the boundary internal short circuit current value that triggers thermal runaway. Based on Ohm's law and Joule's law, the corresponding boundary internal short circuit resistance and the heat generated by the boundary internal short circuit can be obtained.

[0042] The specific steps for determining the internal short-circuit boundary parameters are as follows: S021, such as Figure 3 Connect the foil tab 10 of the conductive metal foil to the positive electrode tab 11 through the wire 13 so that the potential of the conductive metal foil is the same as the potential of the positive electrode. Let the battery stand for 30~120 minutes until the battery is stable. The test battery was charged with constant current at a rate of 0.5~10C until it reached 3.6~4.5V; then charged with constant voltage until the cutoff current was 0.05C, so that the battery was kept at 100% SOC. After charging is completed, a constant current discharge of 0.5~10C is used, and the discharge cutoff voltage is 2.0~3V. Due to the influence of the potential between the positive and negative electrodes, the conductive metal foil will undergo dissolution and deposition, thus forming an internal short circuit.

[0043] As the test progresses, the number of short-circuit contact points inside the battery gradually increases, leading to a decrease in internal short-circuit resistance and an increase in internal short-circuit current, ultimately triggering thermal runaway.

[0044] S022. When the battery is in the state of internal short circuit, recharge the test battery at a rate of 0.5~10C to 3.6~4.5V; then charge it at a constant voltage until the cutoff current is 0.05~5 mA. Due to an internal short circuit in the battery, during the constant voltage charging phase, when the current decreases to a certain value, it cannot continue to decrease to the cutoff current and gradually stabilizes. At this time, if the current value remains stable within 30~90 minutes, this current value is the current internal short circuit current I1.

[0045] During the constant voltage charging stage, after the current value remains stable at I1 for 30~90 minutes, switch to constant current discharge, with a discharge cutoff voltage of 2.0~3V.

[0046] According to Ohm's law, the current internal short-circuit resistance R1 can be calculated as R1 = U / I1; and the Joule heat generated by the current short circuit can also be calculated. for Where t1 is the start time of the current stage, t2 is the end time of the current stage, I1 is the internal short-circuit current, and R1 is the internal short-circuit resistance.

[0047] S023. Perform constant current charging and constant voltage charging on the test battery again, and increase the cutoff current of the constant voltage charging section to make it greater than the internal short circuit current I1. When the current value that reappears in the constant voltage charging stage can no longer decrease and remains stable within 30 to 90 minutes, it indicates that the internal short circuit within the battery has been aggravated, and the current current value is the current internal short circuit current I2.

[0048] After the current value remains stable at I2 for 30~90 minutes, switch to constant current discharge with a discharge cutoff voltage of 2.0~3V.

[0049] Similarly, we can obtain the current internal short-circuit resistance R2 as R2 = U / I2; and the Joule heat generated by the current short circuit. for Where t1 is the start time of the current stage, t2 is the end time of the current stage, I2 is the internal short-circuit current, and R2 is the internal short-circuit resistance.

[0050] S024. Increase the cutoff current of the constant voltage charging section again to make it greater than the internal short-circuit current I2, and perform charge and discharge cycle tests in sequence until the test battery experiences thermal runaway.

[0051] The current value of the constant-voltage charging phase before thermal runaway is the internal short-circuit current boundary value I for internal short-circuit-triggered thermal runaway. i Accordingly, based on Ohm's law, the short-circuit resistance R within the boundary can be calculated. i =U / I i Joule heating generated under the current short-circuit triggered thermal runaway boundary conditions for Where t1 is the start time of the current stage, t2 is the end time of the current stage, and I i For the short-circuit current within the boundary, R i This is the short-circuit resistance within the boundary.

[0052] Because the battery itself dissipates heat, the heat dissipated by the battery itself needs to be subtracted when calculating the heat required for thermal runaway. Different battery specifications have different heat dissipation effects; thus, the actual heat required for thermal runaway, Q, can be obtained as Q = Q0. J -Q 自身散热 .

[0053] The test battery of this invention can also be used to simulate an internal short circuit when lithium plating is present on the battery surface. After lithium plating, the battery's stability deteriorates, and an internal short circuit makes it more prone to triggering thermal runaway. If both lithium plating and an internal short circuit are required, during battery cycling, the foil tab 10 and the battery negative electrode tab 12 are connected via a wire 13, such as... Figure 4 As shown, the battery is continuously charged and discharged in cycles, causing lithium to deposit on the surface of the foil to simulate lithium plating.

[0054] The following describes the process of measuring internal short-circuit boundary parameters using the lithium-ion battery internal short-circuit boundary parameter measurement method of the present invention, with reference to specific embodiments.

[0055] A battery cell for simulating an internal short circuit is fabricated. This cell consists of a cell impregnated with electrolyte and an aluminum-plastic film or rigid shell surrounding the cell for encapsulation. Figure 1 The battery cell consists of a positive electrode 2, a negative electrode 6, and a separator 7 located between the positive and negative electrodes. The positive electrode 2 consists of a positive current collector 5 and a positive active material 3 coated on the surface of the positive current collector. The negative electrode 6 consists of a negative current collector 1 and a negative active material 4 coated on the surface of the negative current collector. The separator 7 is an organic polymer film with a microporous structure, allowing lithium ions to pass through but preventing electrons from passing through.

[0056] Example 1 Fresh lithium iron phosphate (LFP) hard-shell batteries with dimensions of 70×26×165mm were prepared. The battery cells were assembled using a winding method. The positive electrode material was LFP, the negative electrode material was Gr, and the implanted conductive metal foil was Cu foil. The portion of the conductive metal foil wrapped with a separator was implanted into the battery cell before encapsulation, with the tabs of the conductive metal foil protruding from the explosion-proof valve.

[0057] After the battery was manufactured, it was charged and discharged at a low current at 25°C. It was charged at a constant current rate of 1C until it reached 3.75V, and then charged at a constant voltage until the cutoff current was 0.05C. After charging, it was discharged at a constant current of 1C with a discharge cutoff voltage of 2.5V. This cycle was repeated 20 times to ensure that the battery was in normal condition.

[0058] Short-circuit the positive electrode of the battery and the tab of the conductive metal foil, and let it stand for 60 minutes until the battery stabilizes. Under the potential difference between the positive and negative electrodes, copper is deposited from the surface of the positive electrode to the negative electrode, forming an internal short circuit in the battery. Then perform charge and discharge cycles. The cycle steps are 1C constant current charging to 3.65V, constant voltage charging to 0.05mA, 1C constant current discharging to 2.5V, and repeat the cycle.

[0059] During the constant-voltage charging phase, the current drops to 80mA and cannot decrease further, remaining stable for 60 minutes. The current internal short-circuit current is I1 = 80mA. Based on Ohm's law, the current internal short-circuit resistance is calculated to be R1 = 45.6Ω; the generated Joule heat Q... J1 =I1 2 R1Δt (Δt is taken as 60min=3600s), Q is calculated. J1 =0.08 2 45.6×3600≈1036.8J.

[0060] During the next cycle, the cutoff current of the constant voltage charging phase is increased to 150mA, and the charge-discharge cycle continues. After 25 cycles, the current in the constant voltage charging phase stabilizes at 150mA for 60 minutes, resulting in a current of I2 = 150mA; the current internal short-circuit resistance is calculated as R2 = 3.65V / 0.15A = 24.3Ω; the generated Joule heat Q... J2 =0.15 2 ×24.3×3600≈1971J.

[0061] Continue increasing the cutoff current of the constant voltage charging section, successively increasing it to 500mA, 1000mA..., until the 86th cycle, when the cutoff current of the constant voltage charging section increases to 20000mA, which is the current internal short-circuit circuit I8. Calculate the current internal short-circuit resistance R8 = 3.65V / 20A = 0.183Ω, and the current generated Joule heat Q. J8 =20 2 ×0.183×3600≈262800J; By continuously increasing the cutoff current of the constant voltage charging stage, thermal runaway occurs in the 87th cycle, thus obtaining the boundary short-circuit current I before the thermal runaway is triggered. i =20000mA, short-circuit resistance within the boundary is R i =0.183Ω, boundary Joule heat is Q i =262800J (After deducting the heat dissipation of the battery itself, the actual heat that triggers thermal runaway is about 235700J).

[0062] Example 2 An aged ternary hard-shell battery with dimensions of 220×42×101mm was used. CT results showed that lithium plating had already occurred inside the battery. The battery cell was assembled by winding. The positive electrode material was NCM, the negative electrode material was Gr, and the implanted conductive metal foil was Cu foil.

[0063] Open the battery's explosion-proof valve, insert the conductive metal foil with the insulating film wrapped around its surface into the battery cell, pre-seal it with aluminum foil tape, and then further seal it with epoxy resin glue.

[0064] After the battery was manufactured, it was charged and discharged at a low current at 25°C. It was charged at a constant current rate of 1C until it reached 4.3V, and then charged at a constant voltage until the cutoff current was 0.05C. After charging, it was discharged at a constant current of 1C until the discharge cutoff voltage was 2.8V. This cycle was repeated 20 times to ensure that the battery was in normal condition.

[0065] Charge the battery to full capacity. While the battery is resting, short-circuit the positive terminal of the battery to the tab of the conductive metal foil. Let it rest for 60 minutes until the battery stabilizes. Under the potential difference between the positive and negative terminals, copper is deposited from the surface of the positive terminal to the negative terminal, forming an internal short circuit in the battery. Then perform charge and discharge cycles. The cycle steps are 1C constant current charging to 4.3V, constant voltage charging to 0.05mA, and 1C constant current discharging to 2.8V, and repeat the cycle.

[0066] During the constant-voltage charging phase, the current drops to 60mA and cannot decrease further, remaining stable for 60 minutes. The current internal short-circuit current is I1 = 60mA. Based on Ohm's law, the current internal short-circuit resistance R1 = 71.7Ω; the generated Joule heat Q... J1 =I1 2 R1Δt (Δt is taken as 60min=3600s), Q is calculated. J1 =0.06 2 ×71.7×3600≈928.8J.

[0067] During the next cycle, the cutoff current of the constant voltage charging phase is increased to 120mA, and the charge-discharge cycle continues. After 18 cycles, the current in the constant voltage charging phase stabilizes at 120mA for 60 minutes, resulting in a current of I2 = 120mA; the current internal short-circuit resistance is calculated to be R2 = 35.8Ω; and the generated Joule heat Q... J2 =0.12 2 ×35.8×3600≈1857.6J.

[0068] Continue increasing the cutoff current of the constant voltage charging section, successively increasing it to 300mA, 1000mA..., until the 45th cycle, when the cutoff current of the constant voltage charging section increases to 15000mA, which is the current internal short-circuit circuit I8. At this point, according to Ohm's law, the current internal short-circuit resistance R8 = 4.3V / 15A = 0.287Ω, and the current generated Joule heat Q... J8 =15 2 ×0.287×3600≈232470J.

[0069] By continuously increasing the cutoff current of the constant voltage charging stage, thermal runaway occurred in the battery during the 46th cycle, yielding the boundary short-circuit current I before the thermal runaway was triggered. i =15000mA, short-circuit resistance R within the boundary i =0.287Ω, boundary Joule heat Q i =232470J (After deducting the heat dissipation of the battery itself, the actual heat that triggers thermal runaway is about 207538J).

[0070] Example 3 It uses a fresh ternary hard-shell battery with dimensions of 220×42×101mm. The battery cells are assembled by winding. The positive electrode material is NCM, the negative electrode is Gr, and the implanted conductive metal foil is Cu foil.

[0071] Open the battery's explosion-proof valve, insert the conductive metal foil with the insulating film wrapped around its surface into the battery cell, pre-seal it with aluminum foil tape, and then further seal it with epoxy resin glue.

[0072] After the battery was manufactured, it was charged and discharged at a low current at 25°C. It was charged at a constant current rate of 1C until it reached 4.3V, and then charged at a constant voltage until the cutoff current was 0.05C. After charging, it was discharged at a constant current of 1C until the discharge cutoff voltage was 2.8V. This cycle was repeated 20 times to ensure that the battery was in normal condition.

[0073] Charge the battery to full capacity. While the battery is resting, short-circuit the positive terminal of the battery to the tab of the conductive metal foil. Let it rest for 60 minutes until the battery stabilizes. Under the potential difference between the positive and negative terminals, copper is deposited from the surface of the positive terminal to the negative terminal, forming an internal short circuit in the battery. Then perform charge and discharge cycles. The cycle steps are 1C constant current charging to 4.3V, constant voltage charging to 0.05mA, and 1C constant current discharging to 2.8V, and repeat the cycle.

[0074] During the constant-voltage charging phase, the current drops to 77mA and cannot decrease further, remaining stable for 60 minutes. The current internal short-circuit current is I1 = 77mA. Based on Ohm's law, the current internal short-circuit resistance R1 = 55.84Ω; the generated Joule heat Q... J1 =I1 2 R1Δt (Δt is taken as 60min=3600s), Q is calculated. J1 =0.077 2 ×55.84×3600≈1191.96J.

[0075] During the next cycle, the cutoff current of the constant voltage charging phase is increased to 200mA, and the charge-discharge cycle continues. After 28 cycles, the current in the constant voltage charging phase stabilizes at 200mA for 60 minutes, resulting in a current of I2 = 200mA; the current internal short-circuit resistance is calculated to be R2 = 21.5Ω; and the generated Joule heat Q... J2 =0.2 2 ×21.5×3600≈3096J.

[0076] Continue increasing the cutoff current of the constant voltage charging section, successively increasing it to 300mA, 1000mA... By the 53rd cycle, the cutoff current of the constant voltage charging section has increased to 17800mA, which is the current internal short-circuit circuit I.10 At this point, the current internal short-circuit resistance R is calculated according to Ohm's law. 10 =4.3V / 17.8A=0.242Ω, the Q generated under this short-circuit current J10 =17.8 2 ×0.242×3600≈275544J.

[0077] By continuously increasing the cutoff current of the constant voltage charging stage, thermal runaway occurred in the battery during the 54th cycle, yielding the boundary short-circuit current I before the thermal runaway was triggered. i =17800mA, short-circuit resistance R within the boundary i =0.242Ω, boundary Joule heat Q i =275544J (After deducting the heat dissipation of the battery itself, the actual heat that triggers thermal runaway is about 237369J).

[0078] Compared to the lithium-plated aged battery used in Example 2, the fresh battery has better stability and requires more heat to trigger thermal runaway.

[0079] Example 4 Example 4 uses a fresh ternary hard-shell battery of the same specifications as Example 3, the difference being that in this example, an internal short circuit is triggered by implanting Fe foil.

[0080] It uses a fresh ternary hard-shell battery with dimensions of 220×42×101mm. The battery cells are assembled by winding. The positive electrode material is NCM, the negative electrode is Gr, and the implanted conductive metal foil is Fe foil.

[0081] Open the battery's explosion-proof valve, insert the conductive metal foil with the insulating film wrapped around its surface into the battery cell, pre-seal it with aluminum foil tape, and then further seal it with epoxy resin glue.

[0082] After the battery was manufactured, it was charged and discharged at a low current at 25°C. It was charged at a constant current rate of 1C until it reached 4.3V, and then charged at a constant voltage until the cutoff current was 0.05C. After charging, it was discharged at a constant current of 1C until the discharge cutoff voltage was 2.8V. This cycle was repeated 20 times to ensure that the battery was in normal condition.

[0083] Charge the battery to full capacity. While the battery is resting, short-circuit the positive terminal of the battery to the tab of the conductive metal foil. Let it rest for 60 minutes until the battery stabilizes. Under the potential difference between the positive and negative terminals, Fe is deposited from the surface of the positive terminal to the negative terminal, forming an internal short circuit in the battery. Then perform charge and discharge cycles. The cycle steps are 1C constant current charging to 4.3V, constant voltage charging to 0.05mA, and 1C constant current discharging to 2.8V, and repeat the cycle.

[0084] During the constant-voltage charging phase, the current drops to 65mA and cannot decrease further, remaining stable for 60 minutes. The current internal short-circuit current is I1 = 65mA. Based on Ohm's law, the current internal short-circuit resistance R1 = 66.15Ω; the generated Joule heat Q... J1 =I1 2 R1Δt (Δt is taken as 60min=3600s), Q is calculated. J1 =0.065 2 ×66.15×3600≈1006.14J.

[0085] During the next cycle, the cutoff current of the constant voltage charging phase is increased to 150mA, and the charge-discharge cycle continues. After 28 cycles, the current in the constant voltage charging phase stabilizes at 150mA for 60 minutes, resulting in a current I2 = 150mA; the current internal short-circuit resistance R2 is calculated to be 28.67Ω; and the generated Joule heat Q... J2 =0.15 2 ×28.67×3600≈2322J.

[0086] Continue increasing the cutoff current of the constant voltage charging section, successively increasing it to 300mA, 1000mA... By the 78th cycle, the cutoff current of the constant voltage charging section has increased to 18400mA, which is the current internal short-circuit circuit I. 10 At this point, the current internal short-circuit resistance R is calculated according to Ohm's law. 10 =4.3V / 18.4A=0.234Ω, the Q generated under this short-circuit current J10 =18.4 2 ×0.234×3600≈284832J.

[0087] By continuously increasing the cutoff current of the constant voltage charging stage, thermal runaway occurred in the battery during the 79th cycle, yielding the boundary short-circuit current I before the thermal runaway was triggered. i =18400mA, short-circuit resistance R within the boundary i =0.234Ω, boundary Joule heat Q i =284382 (After deducting the heat dissipation of the battery itself, the actual heat that triggers thermal runaway is about 246207J).

[0088] Comparative Example 1 Comparative Example 1 uses a fresh ternary hard-shell battery of the same specifications as Example 3, the difference being that the internal short circuit is triggered by a needle penetration method in this comparative example.

[0089] At 25℃, the battery was charged and discharged using a constant current charging rate of 1C until it reached 4.3V. Then, it was charged at a constant voltage until the cutoff current was 0.05C. After charging, it was discharged using a constant current of 1C until the discharge cutoff voltage was 2.8V. This process was repeated 20 times to ensure the battery was in normal condition.

[0090] A short circuit within the battery was simulated using a needle penetration test. The test conditions for the needle penetration were as follows: A stainless steel needle of grade 1 is inserted at a 30° angle at a needle penetration speed of 0.1 mm / s. During the needle penetration process, the battery is simultaneously charged and discharged. The cycle steps are: 1C constant current charging to 4.3V, constant voltage charging to 0.05mA, and 1C constant current discharging to 2.8V, and the cycle is repeated to test the short-circuit boundary parameters of the battery.

[0091] During the constant-voltage charging phase of the first charging cycle, the current dropped to 90mA and could not decrease further, remaining stable for 60 minutes. The current internal short-circuit current I1 = 90mA. Based on Ohm's law, the current internal short-circuit resistance R1 = 47.78Ω; the generated Joule heat Q... J1 =I1 2 R1Δt (Δt is taken as 60min=3600s), Q is calculated. J1 =0.09 2 ×47.78×3600≈1393.26J.

[0092] During the next cycle, the cutoff current of the constant voltage charging phase is increased to 180mA, and the charge-discharge cycle continues. After 5 cycles, the current in the constant voltage charging phase stabilizes at 180mA for 60 minutes, resulting in a current I2 = 180mA; the current internal short-circuit resistance R2 is calculated to be 23.89Ω; and the generated Joule heat Q... J2 =0.18 2 ×23.89×3600≈2786.53J. In subsequent tests, the cutoff current of the constant voltage charging section was continuously increased, successively to 400mA, 1000mA… On the 27th cycle, the cutoff current of the constant voltage charging section increased to I7=12000mA, which is the current internal short-circuit circuit I7. At this point, according to Ohm's law, the internal short-circuit resistance R7=4.3V / 12A=0.358Ω. The Q generated under this short-circuit current… J7 =12 2 ×0.358×3600≈185760J.

[0093] By continuously increasing the cutoff current of the constant voltage charging stage, thermal runaway occurred in the battery during the 28th cycle, yielding the boundary short-circuit current I before the thermal runaway was triggered. i =12000mA, which is the short-circuit resistance R within the boundary. i=0.358Ω, boundary Joule heat Q i =185760J (After deducting the heat dissipation of the battery itself, the actual heat that triggers thermal runaway is about 167172.8J).

[0094] Compared with the results in Example 3, the accuracy of the internal short-circuit boundary parameters obtained by the needle punching method is not high.

[0095] In the above embodiments, relevant parameters such as battery voltage, center temperature, and tab temperature can be monitored in real time during the test.

[0096] The method of this invention features controllable short-circuit triggering and high repeatability. It is applicable to the testing of batteries of different forms (such as pouch batteries, hard-shell batteries, or cylindrical batteries) and different systems (such as lithium iron phosphate system, lithium nickel cobalt manganese oxide system, lithium manganese iron phosphate system, etc.). It can be used for batteries in the manufacturing process, as well as finished batteries and aged batteries. Furthermore, there are no restrictions on the implantation position of the conductive metal foil, and the implantation operation is convenient.

[0097] The method of this invention triggers the internal short circuit at a point inside the battery, which can better reflect the true state of the internal short circuit and make the measured internal short circuit boundary parameters more accurate.

[0098] By utilizing the current characteristics of the battery during the constant voltage charging phase, the internal short-circuit current when the internal short circuit occurs can be determined. Furthermore, the resulting internal short circuit is a single-layer / multi-layer local internal short circuit, which is more consistent with the actual situation of battery internal short circuits.

[0099] This invention triggers internal short circuits based on the different dissolution and deposition potentials of different metals. The degree of internal short circuit can be aggravated by increasing the number of cycles or gradually increasing the contact points of the internal short circuit, ultimately triggering thermal runaway. This allows for accurate measurement of the internal short circuit boundary parameters of the battery under different conditions (such as cycle aging, before and after lithium plating), enabling accurate assessment of the battery's safety status.

[0100] On the other hand, some embodiments of the present invention also provide a lithium-ion battery safety assessment method, which uses the lithium-ion battery internal short-circuit boundary parameter measurement method in the above embodiments to obtain the battery's internal short-circuit boundary parameters, including the internal short-circuit current, internal short-circuit resistance, and internal short-circuit Joule heat, and assesses the battery's safety status based on the internal short-circuit current, internal short-circuit resistance, and internal short-circuit Joule heat.

[0101] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications or equivalent changes made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for determining the internal short-circuit boundary parameters of a lithium-ion battery, characterized in that, Includes the following steps: The foil tab of the test battery is short-circuited with the positive tab, and the test battery is charged and discharged at a constant current, causing the conductive metal foil implanted in the test battery to dissolve and deposit, triggering a short circuit in the test battery; the test battery is a battery cell with a conductive metal foil covered with a separator film implanted on its surface, and the foil tab is connected to the end of the conductive metal foil that extends out of the battery. When the test battery is in the state of internal short circuit, the test battery is charged with constant current to the cutoff voltage, and then charged with constant voltage to the cutoff current. During the constant voltage charging stage, when the current stabilizes at a certain current value and continues for a set time, it switches to constant current discharge and discharges to the cutoff voltage. The current value when the current stabilizes at a certain current value is taken as the internal short-circuit current corresponding to this charge and discharge cycle. Repeat the constant current charging, constant voltage charging and constant current discharging cycle of the test battery, and make the cut-off current of the constant voltage charging stage of the later charge and discharge cycle greater than the internal short circuit current corresponding to the previous charge and discharge cycle, until the battery thermal runaway is triggered. The current value corresponding to the current stabilizing at a certain current value and lasting for a set time during the constant voltage charging stage before the thermal runaway is triggered is taken as the boundary internal short circuit current.

2. The method for determining the internal short-circuit boundary parameters of a lithium-ion battery according to claim 1, characterized in that, The corresponding short-circuit resistance and short-circuit Joule heat within the boundary are obtained based on the short-circuit current within the boundary.

3. The method for determining the internal short-circuit boundary parameters of a lithium-ion battery according to claim 1, characterized in that, The test battery was obtained by embedding conductive metal foil into a fresh battery, a finished battery, or an aged battery.

4. The method for determining the internal short-circuit boundary parameters of a lithium-ion battery according to claim 3, characterized in that, The fresh battery is formed by stacking or winding a positive electrode sheet, a separator, and a negative electrode sheet arranged in sequence, and the conductive metal foil is implanted into the battery cell during the stacking or winding process.

5. The method for determining the internal short-circuit boundary parameters of a lithium-ion battery according to claim 1 or 3, characterized in that, Before triggering an internal short circuit, the freshly prepared battery is charged and discharged with a small current of 1 / 25C to 1 / 10C, ​​where nC represents the time required for the battery to be charged from an empty state to a fully charged state in 1 / n hours.

6. The method for determining the internal short-circuit boundary parameters of a lithium-ion battery according to claim 1, characterized in that, The number of implanted conductive metal foils is one or more.

7. The method for determining the internal short-circuit boundary parameters of a lithium-ion battery according to claim 1, characterized in that, The conductive metal foil is implanted on the positive or negative side of the battery cell.

8. The method for determining the internal short-circuit boundary parameters of a lithium-ion battery according to claim 1, characterized in that, The conductive metal foil is made of Cu, Zn, Fe, or Ni, etc.

9. The method for determining the internal short-circuit boundary parameters of a lithium-ion battery according to claim 1, characterized in that, The area of ​​the conductive metal foil is 10%-50% of the area of ​​the battery cell electrode.

10. A method for safety assessment of lithium-ion batteries, characterized in that, The internal short-circuit boundary parameters of a lithium-ion battery are obtained using the method for determining the internal short-circuit boundary parameters of a lithium-ion battery as described in any one of claims 1-9, and the battery safety status is evaluated.

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