Method for evaluating safety performance of diaphragm

Abstract

The invention discloses a method for evaluating the safety performance of a diaphragm, and the method comprises the following steps: S1, preparing a to-be-evaluated diaphragm into a soft package battery, and carrying out the preprocessing of the soft package battery, so as to obtain a preprocessed soft package battery; s2, heating the pretreated soft package battery, and monitoring the temperature rise rate of the pretreated soft package battery in real time; and S3, judging the safety performance of the diaphragm to be evaluated according to the temperature rise rate of the preprocessed soft package battery monitored in real time, and solving the problem of no diaphragm association in the battery level test.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of energy storage battery performance evaluation, and particularly relates to a method for evaluating the safety performance of a diaphragm. BACKGROUND

[0002] As a lithium ion battery "positive and negative electrode isolation barrier", the thermal stability, melting integrity, self-extinguishing property and other safety performance of the diaphragm directly determine the risk level of the battery in the thermal runaway scenario. There are two technical bottlenecks that cannot be broken through in the current industry evaluation of the diaphragm: Material level test deviates from the actual application scenario: the existing technology directly tests the melting point, weight loss rate and other parameters of the diaphragm by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), but such methods can only reflect the thermal properties of the diaphragm itself, and cannot simulate the interaction of the diaphragm with the electrode (oxygen release of the positive electrode, lithium precipitation of the negative electrode) and the electrolyte (high temperature decomposition) in the battery, for example, polyethylene (PE) diaphragm shows 135℃ starting melting in DSC, but under the influence of electrolyte immersion in the battery, the thermal shrinkage starting temperature will be reduced to 120℃, and the friction with the electrode will aggravate the hole breaking, and the material level test cannot capture such dynamic failure process at all; Battery / system level test cannot focus on the contribution of the diaphragm: the comparison file CN106970111A (Tsinghua) only judges the overall safety by whether the battery pack ignites a fire, and cannot distinguish whether the thermal runaway is caused by diaphragm hole breaking, electrode decomposition or electrolyte combustion; the comparison file CN110146825A (Lishen) tests the thermal properties of the battery by ARC, but only focuses on the self-heat release temperature and thermal runaway temperature of the battery as a whole, without establishing any correlation logic with the characteristics of the diaphragm, for example, it measures that the "battery thermal runaway temperature is 205℃", but cannot judge whether the temperature is caused by diaphragm melting or electrolyte decomposition, and cannot be used for targeted screening of the diaphragm at all.

[0003] Therefore, it is urgent to design a method for evaluating the safety performance of a diaphragm to solve the above problems. SUMMARY

[0004] The purpose of the present application is to provide a method for evaluating the safety performance of a diaphragm, which has the advantage of accurately quantifying the safety performance of the diaphragm, and solves the problem of battery level test without diaphragm correlation.

[0005] To achieve the above purpose, the technical scheme of the method for evaluating the safety performance of a diaphragm is as follows: A method for evaluating the safety performance of a diaphragm, comprising the following steps: S1: preparing a diaphragm to be evaluated into a soft pack battery, and pretreating the soft pack battery to obtain a pretreated soft pack battery; S2: heating the pre-processed soft package battery and monitoring the temperature rising rate of the pre-processed soft package battery in real time; S3: judging the safety performance of the to-be-evaluated separator according to the temperature rising rate of the pre-processed soft package battery monitored in real time.

[0006] Further, the to-be-evaluated separator is a polyethylene separator or a polypropylene separator.

[0007] Further, the polyethylene separator is provided with a coating layer, and the coating layer material is polyvinylidene fluoride or ceramic material.

[0008] Further, the thickness of the coating layer is 1-5 μm, and the thickness of the polyethylene separator is 5-20 μm.

[0009] Further, the pre-treatment of the soft package battery comprises the following steps: sequentially performing formation and capacity grading treatment on the soft package battery to obtain a pre-treatment crude product, and performing cycle treatment on the pre-treatment crude product to obtain the pre-processed soft package battery.

[0010] Further, in the step S2, the heating of the pre-processed soft package battery comprises the following steps: placing the pre-processed soft package battery in the center position of a heating device, uniformly heating the heating device from room temperature to a set temperature, stopping the heating of the heating device after the temperature holding is completed, and naturally cooling the heating device to room temperature.

[0011] Further, the heating device is heated from room temperature to the set temperature at a rate of 5-10 ℃ / min, and the set temperature is 190-195 ℃.

[0012] Further, a temperature sensor is used to monitor the temperature rising rate of the pre-processed soft package battery.

[0013] Further, the voltage difference before and after the heating of the pre-processed soft package battery is detected, and the smaller the voltage difference, the better the structural stability of the to-be-evaluated separator.

[0014] Further, when the temperature rising rate of the pre-processed soft package battery monitored in real time is ≥1 ℃ / s, it is determined that the to-be-evaluated separator is in thermal runaway, the longer the time from the start of the heating to the first time when the temperature rising rate of the pre-processed soft package battery reaches ≥1 ℃ / s, the better the thermal shrinkage stability of the to-be-evaluated separator; the lower the maximum temperature reached by the pre-processed soft package battery during the thermal runaway process, the better the melting integrity of the to-be-evaluated separator; the shorter the time interval from the temperature reaching 200 ℃ to the temperature falling back to 200 ℃ of the pre-processed soft package battery, the better the self-extinguishing property of the to-be-evaluated separator; the slower the temperature change rate per unit time during the thermal runaway process, the better the heat insulation and flame retardation of the to-be-evaluated separator.

[0015] The evaluation method for the safety performance of the diaphragm has the following advantages: through heating parameter control, it is ensured that more than 90% of the thermal runaway signals come from the diaphragm, the contributions of the diaphragm and other components can be accurately distinguished, and an explicit improvement direction is provided for battery safety research and development; it is adapted to small test research and development needs, the equipment cost is less than 50,000 yuan, the single sample test period is less than 2 hours (only 1 / 12 of the conventional ARC test period), 10+ samples can be tested at the same time, the rapid screening demand is met, the safety performance of the diaphragm is quantified in five aspects, and the evaluation deviation is reduced. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 The flow chart of the evaluation method for the safety performance of the diaphragm of the present application is shown in the figure. Figure 2 The temperature change relationship of each diaphragm with time in the first embodiment of the evaluation method for the safety performance of the diaphragm of the present application is shown in the figure. Figure 3 The temperature change relationship of each diaphragm with time in the second embodiment of the evaluation method for the safety performance of the diaphragm of the present application is shown in the figure. DETAILED DESCRIPTION

[0017] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0018] Those skilled in the art can understand that although some embodiments herein include certain features included in other embodiments rather than other features, the combination of features of different embodiments means to be within the scope of the present application and form different embodiments. For example, in the claims, any one of the claimed embodiments can be used in any combination.

[0019] The following refers to the drawings Figure 1 and the drawings Figure 3 An evaluation method for the safety performance of a diaphragm is described.

[0020] An evaluation method for the safety performance of a diaphragm, comprising the following steps: S1: preparing a diaphragm to be evaluated into a soft package battery, and pretreating the soft package battery to obtain a pretreated soft package battery; S2: heating the pretreated soft package battery, and monitoring the temperature rising rate of the pretreated soft package battery in real time; S3: judging the safety performance of the to-be-evaluated separator according to the temperature rise rate of the pre-processed soft-pack battery monitored in real time.

[0021] Specifically, the safety performance of the separator is evaluated according to the above steps, and a correlation system of the separator, the soft-pack battery and the thermal response is established, so that the to-be-evaluated separator is accurately screened.

[0022] Specifically, the soft-pack battery is used instead of the hard-shell / cylindrical battery, because the soft-pack structure is not constrained by the metal shell, and can completely release the dynamic behaviors of the separator at high temperature, such as shrinkage, melting and hole breaking. The metal shell of the hard-shell battery can inhibit the expansion of the separator, resulting in that the thermal runaway signal is covered up, while the test data of the soft-pack battery can more directly reflect the real safety performance of the separator.

[0023] Preferably, the positive electrode material of the soft-pack battery is lithium iron phosphate, the negative electrode material is graphite, and the electrolyte is fluorine-containing organic electrolyte (1 mol / L LiPF6).

[0024] Preferably, the thickness of the soft-pack battery is 1 mm, and the capacity is 200 mAh.

[0025] Specifically, the temperature rise rate is a "direct signal" of the thermal stability of the separator. When the separator is not failed, the battery only slowly rises in temperature due to environmental heating, and the rate is <0.5℃ / s. Once the separator is in contact with the positive and negative electrodes due to high-temperature shrinkage / melting, the internal short circuit will trigger a heat release reaction, and the temperature rise rate will instantaneously jump to ≥1℃ / s.

[0026] Further, the to-be-evaluated separator is a polyethylene-based separator or a polypropylene separator.

[0027] Specifically, the polyethylene-based separator (PE separator) has a melting point of about 130-140℃ and excellent mechanical properties at room temperature, but is prone to shrinkage at high temperature, and needs to be modified by coating to improve safety. The present method can accurately evaluate the modification effect.

[0028] Specifically, the polypropylene separator (PP separator) has a melting point of about 160-170℃ and better high-temperature resistance than PE, but has poor flexibility at room temperature. The heating program (190-195℃) of the present method can cover the melting critical point of the PP separator, and effectively test the thermal stability thereof.

[0029] Further, the polyethylene-based separator is provided with a coating layer, and the coating material is polyvinylidene fluoride (PVDF) or a ceramic material.

[0030] Specifically, PVDF has good electrolyte corrosion resistance and adhesion, and when coated on the surface of PE, it can inhibit thermal shrinkage at high temperature, and its flame retardance can delay the spread of thermal runaway. The present method can directly quantify the effect through the spread time parameter.

[0031] Specifically, the commonly used ceramic material is Al2O3 or SiO2, which has high heat resistance and insulation, can maintain the integrity of the diaphragm structure after PE melting, reduce the direct contact between the positive and negative electrodes, and the maximum temperature, voltage difference and other parameters of the thermal runaway of the method can reflect its effect.

[0032] Further, the thickness of the coating is 1-5 μm, and the thickness of the polyethylene-based diaphragm is 5-20 μm.

[0033] Specifically, when less than 1 μm, the coating continuity is poor and cannot effectively inhibit shrinkage; when more than 5 μm, the diaphragm porosity is reduced, affecting ion conduction. 1-5 μm is the optimal balance interval of safety and ion conductivity, and the performance difference in this interval can be verified by the method.

[0034] Specifically, the polyethylene-based diaphragm is easily punctured when the thickness is too thin (<5 μm), and the battery internal resistance is increased when the thickness is too thick (>20 μm). The parameters of the method can reflect the safety and performance balance of diaphragms of different thicknesses, and the quantitative relationship between thickness and safety performance can be established through parameters such as thermal runaway occurrence time and voltage change, guiding the size design of the diaphragm.

[0035] Further, the pretreatment of the soft package battery comprises the following steps: sequentially performing formation and capacity grading treatment on the soft package battery to obtain a pretreatment crude product, and performing cycle treatment on the pretreatment crude product to obtain a pretreated soft package battery.

[0036] Specifically, the pretreatment simulates the full life cycle of the battery from production to use (formation and activation of electrodes, capacity grading and calibration, and cycle simulation of charge and discharge aging), solves the limitation of existing material level testing that only tests new diaphragms, and ensures that the evaluation results are close to the actual application scenario.

[0037] Specifically, the formation treatment conditions are: charging to 3.65V at 0.1C current in a 25±2℃ environment, and discharging to 2.0V at 0.2C current after standing for 30 min. A stable solid electrolyte interface film (SEI film) is formed by small current charge and discharge, simulating the activation process of the battery before leaving the factory, and avoiding interference of the thermal runaway signal by the unstable SEI film of the unformed battery.

[0038] Specifically, the capacity grading treatment conditions are: charging to 3.65V at 0.5C current in a 25±2℃ environment, constant voltage charging to a current ≤0.05C, and discharging to 2.0V at 0.5C current after standing for 30 min. The actual capacity of the battery is determined by standard charge and discharge to ensure the consistency of subsequent cycle treatment and reduce the thermal response deviation caused by capacity difference.

[0039] Specifically, the condition of the cycle treatment is that: charging to 3.65V at 1C current in an environment of 25±2℃, constant voltage charging to current≤0.05C, discharging to 2.0V at 1C current after standing for 5min, standing for 5min, and repeating the above charging and discharging process 5 times to simulate the quality inspection cycle of the battery before leaving the factory.

[0040] Simulating about 3 years of use aging, the soft package battery is placed in a 45℃ constant temperature oven, and stands for 24h to simulate the electrolyte infiltration and separator pretreatment in a high temperature storage environment; charging to 3.65V at 1C current in a 45℃ environment (constant voltage to 0.15A cutoff), and then discharging to 2.0V at 1C current, and standing for 10min; repeating the above charging and discharging process 1000 times to complete long-term aging simulation. Simulating the charging and discharging aging in the use process of the battery, the separator experiences the mechanical stress caused by the electrode expansion / contraction and the chemical aging caused by the electrolyte corrosion, and more truly reflects the safety performance after long-term use. The three-step pretreatment forms a complete chain of "production-calibration-aging", solves the limitation of the existing test which only uses new batteries for evaluation, and makes the evaluation result more practically meaningful.

[0041] Further, in the step S2, the heating of the pretreated soft package battery comprises the following steps: placing the pretreated soft package battery in the center position of a heating device, the heating device is uniformly heated from room temperature to a set temperature and then kept at the set temperature, after the keeping, the heating device stops heating, and the heating device naturally cools to room temperature.

[0042] Specifically, the soft package battery is uniformly heated to avoid local overheating caused by temperature gradient in the heating box, ensure the accuracy of temperature monitoring, simulate the slow heating scenario in the actual use of the battery, and distinguish from the instantaneous heating of extreme short circuit, and more truly reflect the gradual failure process of the separator.

[0043] Preferably, the heating is stopped after keeping for 30min and continues to observe for 1h.

[0044] Specifically, after the heating is stopped, the observation continues for 1h, which can capture the structure recovery ability of the separator in the temperature falling stage.

[0045] Further, the heating device is heated from room temperature to a set temperature at a rate of 5-10℃ / min, and the set temperature is 190-195℃.

[0046] Specifically, too low heating rate (<5℃) of the heating device will prolong the test time, and too high heating rate (>10℃) will cause the failure signal of the separator to be quickly covered by heat dissipation.

[0047] Specifically, 190~195℃ is higher than the melting point of PE (130-140℃) and PP (160-170℃), ensuring that the separator undergoes a complete melting, shrinking and failure process, while being lower than the decomposition temperature of the electrolyte, which can distinguish between thermal runaway caused by separator failure and thermal runaway caused by electrolyte decomposition. The combination of rate and temperature design achieves the technical effect of effectively stimulating the safety performance difference of the separator while avoiding interference from other components.

[0048] Further, a temperature sensor is used to monitor the temperature rise rate of the pre-processed soft-pack battery.

[0049] Preferably, the temperature sensor uses a thermocouple sensor, which has a wide temperature measurement range and fast response speed, and can accurately capture the rapid temperature rise signal during thermal runaway, which is superior to infrared sensors and thermistors.

[0050] Preferably, the sensor should be closely attached to the center area of the soft-pack battery surface to avoid contact with the wall of the heating device, ensuring that the measured temperature is the battery itself rather than the ambient temperature.

[0051] Preferably, the sampling frequency of the temperature sensor should be greater than or equal to once every 5 seconds to accurately capture the critical moment of the temperature rise rate and avoid signal omission due to long sampling intervals.

[0052] Further, the voltage difference before and after heating of the pre-processed soft-pack battery is detected, and the smaller the voltage difference, the better the structural stability of the separator to be evaluated.

[0053] Specifically, when the separator structure is complete, even if it undergoes high temperature, the positive and negative electrodes still maintain a certain degree of isolation, and the battery will not be completely short-circuited, and the remaining voltage is high; if the separator is severely melted and broken, the positive and negative electrodes are directly in contact and form a short circuit, and the voltage drops to 0. Therefore, the voltage difference (initial voltage - remaining voltage) directly reflects the structural retention ability of the separator. Compared with visual observation of separator damage (destructive testing), voltage monitoring is a non-destructive test, which can be analyzed after the battery is disassembled, and the performance can be more comprehensively evaluated in combination with voltage data.

[0054] Further, when the real-time monitored temperature rise rate of the pre-processed soft-pack battery is ≥1℃ / s, it is determined that the separator to be evaluated has thermal runaway, the longer the time from the start of heating to the first time the temperature rise rate of the pre-processed soft-pack battery reaches ≥1℃ / s, the better the thermal shrinkage stability of the separator to be evaluated; the lower the maximum temperature reached by the pre-processed soft-pack battery during thermal runaway, the better the melting integrity of the separator to be evaluated; the shorter the time interval from when the temperature of the pre-processed soft-pack battery reaches 200℃ to when the temperature falls back to 200℃, the better the self-extinguishing property of the separator to be evaluated; the slower the temperature change rate per unit time during thermal runaway, the better the thermal insulation and flame retardance of the separator to be evaluated.

[0055] Specifically, when the separator is not failed, the battery heating rate is <0.5℃ / s; once the separator is failed to cause a short circuit, the heating rate jumps to ≥1℃ / s, which can effectively distinguish normal heating from thermal runaway.

[0056] The time from the start of heating to the first time when the soft package battery after the pretreatment reaches a temperature change rate of ≥1℃ / s is the thermal runaway occurrence time, the longer the thermal runaway occurrence time, the stronger the ability of the separator to maintain dimensional stability at high temperature, and the better the thermal shrinkage stability of the separator to be evaluated; The highest temperature reached by the soft package battery after the pretreatment during the thermal runaway is the thermal runaway maximum temperature, the lower the thermal runaway maximum temperature, the more effective the barrier structure formed after the melting of the separator, the reduction of the exothermic reaction, and the better the melting integrity of the separator to be evaluated; The time interval from the temperature reaching 200℃ to the temperature falling back to 200℃ of the soft package battery after the pretreatment is the thermal runaway propagation time, the shorter the thermal runaway propagation time, the better the flame-retardant / heat-insulating effect of the coating, the prevention of the spread of the thermal runaway area, and the better the self-extinguishing property of the separator to be evaluated; The temperature change rate per unit time during the thermal runaway is the temperature propagation rate, the slower the temperature propagation rate, the stronger the heat conduction barrier ability of the separator, the reduction of the risk of chain reaction of adjacent batteries, and the better the heat-insulating and flame-retardant property of the separator to be evaluated.

[0057] The present application is further illustrated by specific examples below, but does not limit the protection scope of the present application.

[0058] Example 1 Three separators to be evaluated were selected, separator A was a 7μm PE base film + 2μm single-sided ceramic coating + 1μm PVDF sprayed separator (referred to as 7+2+1 PVDF sprayed separator), separator B was a 9μm PE base film + 3μm single-sided ceramic and polymethyl methacrylate (Pmma) mixed coating separator (referred to as 9+3 Pmma mixed coating separator), and separator C was a 9μm PE base film + 2μm single-sided ceramic coating + 1μm PVDF sprayed separator (referred to as 9+2+1 PVDF sprayed separator), and soft package batteries were prepared using the above three separators to be evaluated. The prepared soft package batteries were charged to 3.65V at 0.1C current at 25℃ environment, and after standing for 30min, discharged to 2.0V at 0.2C current. Charged to 3.65V at 0.5C current at 25℃ environment, and constant voltage charged to current ≤0.05C, after standing for 30min, discharged to 2.0V at 0.5C current. Charged to 3.65V at 1C current at 25℃ environment, and constant voltage charged to current ≤0.05C, after standing for 5min, discharged to 2.0V at 1C current, and after standing for 5min, the above charging and discharging process was repeated 5 times.

[0059] 7+2+1 PVDF spray diaphragm, 9+3 Pmma mixed diaphragm, 9+2+1 PVDF spray diaphragm, soft package battery, each group of diaphragm takes 2 soft package battery (diaphragm A-1, diaphragm A-2, diaphragm B-1, diaphragm B-2, diaphragm C-1 and diaphragm C-2), after the above soft package battery is fully charged with 0.5C current, the initial resistance and initial voltage of each soft package battery are recorded respectively, and at the same time, the temperature sensor is pasted on the center position of the surface of the soft package battery and the positive and negative pole lug, and the soft package battery heating rate is tested as shown in Figure 2 The temperature sensor is pasted on the center position of the surface of the soft package battery and the positive and negative pole lug, and the soft package battery heating rate is tested as shown in

[0060] Table 1 is the data comparison of the resistance and voltage of the soft package battery before and after heating corresponding to different diaphragms in the new battery state The initial resistance of the soft package battery corresponding to diaphragm A, diaphragm B and diaphragm C is 4.19~4.78mΩ, and the initial voltage is 3.43~3.4434V, the difference between the diaphragms is very small, which proves that the initial state of the diaphragm after pretreatment is consistent, and the initial performance is excluded. Interference with test results; The resistance of all samples after thermal runaway is out of range, indicating that thermal runaway leads to serious reaction inside the battery; but the voltage difference is significant, the residual voltage of the two groups of soft package batteries corresponding to diaphragm C is the highest (0.947~1.711V), the voltage of the two groups of diaphragm A (7+2+1 PVDF spray) and the two groups of diaphragm B (9+3 Pmma mixed coating) drops to 0V or close to 0V, which shows that the diaphragm C has fewer broken holes after diaphragm melting, the internal short circuit degree is lighter, and the structural stability is better.

[0061] Table 2 is the comparison of thermal runaway parameters of soft package battery corresponding to different diaphragms in the new battery state The thermal runaway occurrence time of the separator C (4125 s) is significantly later than that of the separator A (3430-3440 s) and the separator B (3680-3690 s), and the initial temperature (211-215 °C) is higher than that of the separator A (199.7-202.2 °C) and the separator B (203.5-207.2 °C), proving that the thermal shrinkage rate of the separator C is lower, and the electrode isolation can be maintained at a higher temperature for a longer time, and the thermal stability is better; The propagation speed of the thermal runaway of the separator C (1.844-3.805 °C / s) and the highest temperature (316.4-347.1 °C) are lower than those of other separators, indicating that the separator C can form an effective barrier structure (such as a ceramic coating framework) after melting, reduce the heat release intensity of the direct contact between the positive and negative electrodes, and reduce the severity of the thermal runaway. The propagation time of the thermal runaway of the separator C (255-295 s) is the shortest, only 60%-70% of that of the separator A (370-445 s), proving that the self-extinguishing property of the PVDF coating of the separator C is stronger, and the thermal runaway area can be quickly blocked to avoid a chain reaction.

[0062] Through the multi-parameter evaluation of the method, the separator C (9+2+1 PVDF spray coated separator) is the best in terms of thermal stability, melting integrity, self-extinguishing property and structural stability, and is suitable for selection as a high-safety separator for new batteries; the safety performance of the separator A and the separator B is relatively weak, and the coating thickness or formula needs to be further optimized.

[0063] Example Two Two kinds of to-be-evaluated separators are selected, the separator D is a 16 μm pure PP base film, and the separator E is a 9 μm PE base film+1.5 μm double-sided ceramic coating+1 μm PVDF double-sided spray coated separator (referred to as 9+3+2 PVDF spray coated separator). The above two kinds of to-be-evaluated separators are respectively made into soft package batteries, and the soft package batteries are placed in a 45 °C constant temperature box for 24 h to simulate the electrolyte immersion and separator pretreatment in a high temperature storage environment; at 45 °C, the battery is charged at a constant current of 1 C to 3.65 V (constant voltage to 0.15 A cutoff), and then discharged at a constant current of 1 C to 2.0 V, and left for 10 min; the above charging and discharging process is repeated 1000 times to complete the long-term aging simulation; after the cycle is completed, the battery is fully charged to 3.65 V at a current of 0.5 C, and left for 30 min to ensure that all samples are in the same full charge state. The test equipment, heating program and monitoring parameters are consistent with those of Example 1 (5 °C / min to 190±2 °C, 30 min, every 5 s sampling). The results are as follows: Figure 3The temperature change of the diaphragm with time was shown, and the data of Example 2 were processed to obtain Tables 3 and 4, Table 3 being a comparison of the resistance and voltage of the soft package battery before and after heating corresponding to different diaphragms in the aging state, and Table 4 being a comparison of the thermal runaway parameters of the soft package battery corresponding to different diaphragms in the aging battery state.

[0064] Table 3 is a comparison of the resistance and voltage of the soft package battery before and after heating corresponding to different diaphragms in the aging state The initial resistance of the soft package battery corresponding to diaphragm D and diaphragm E is 4.06-4.07 mΩ, and the initial voltage is 3.3455-3.4094 V. The initial resistance and initial voltage between the diaphragms are basically consistent, proving that the initial state of the sample after long-term aging is still uniform.

[0065] After thermal runaway, the residual voltage of sample E is 1.751-1.947 V, which is significantly higher than that of sample D, 0.248-0.662 V, indicating that even after 1000 high-temperature cycles, the diaphragm of sample E can still maintain good structural integrity, and the degree of internal short circuit is much lower than that of sample D, and the structural stability after long-term aging is more excellent.

[0066] Table 4 is a comparison of the thermal runaway parameters of the soft package battery corresponding to different diaphragms in the aging battery state Thermal runaway occurrence time and initial temperature: the occurrence time of diaphragm E (4095-4100 s) is later than that of diaphragm D (3840-3850 s), and the initial temperature (215.3-217.1 °C) is higher than that of diaphragm D (202.0-203.3 °C), proving that the high-coating structure can effectively resist mechanical wear caused by long-term high-temperature cycles (such as coating shedding caused by electrode expansion), and maintain good thermal stability; Maximum temperature and propagation time of thermal runaway: the maximum temperature of diaphragm E (324.6-332.6 °C) is about 30 °C lower than that of diaphragm D, and the propagation time (45-48 s) is only 14%-15% of that of diaphragm D (330-335 s). Even if its propagation speed is slightly higher, the strong self-extinguishing and heat-insulating properties of the double-sided ceramic + PVDF coating can still quickly terminate thermal runaway and reduce overall risk; Long-term aging adaptability: after 1000 cycles, the thermal runaway parameters of diaphragm D (no coating PP) deteriorate significantly (such as the initial temperature being 10 °C lower than that of the new PP diaphragm), while the parameter decay of diaphragm E is only within 5%, proving that the method can effectively evaluate the long-term safety performance of the diaphragm and provide a basis for selecting long-life energy storage batteries.

[0067] After long-term high-temperature aging test, the safety performance of the separator E (9 μm PE + double-sided ceramic + PVDF sprayed separator) is significantly better than that of the separator D (uncoated PP separator), which can still maintain the characteristics of late thermal runaway, low maximum temperature and short propagation time after aging, and is suitable for long-term use of energy storage batteries; the safety performance of sample D decays obviously after long-term aging due to the lack of coating protection, and needs to be used carefully.

[0068] Both embodiments prove that the method can accurately distinguish the safety performance of separators with different structures and different aging states through the processes of pretreatment, heating monitoring and multi-parameter analysis. The core parameters (thermal runaway time, propagation time and voltage change) are directly related to the characteristics of the separator (shrinkage rate, self-extinguishing property of the coating and structural integrity), and the evaluation results are reliable. Example one is suitable for small-scale screening of new separators, and example two is suitable for evaluation of aged separators, covering the whole life cycle of separator research and application, solving the limitation that the prior art can only test new samples and cannot test aged samples; the test equipment (oven + temperature tester) has low cost and short cycle (single sample test ≤2h), which can quickly guide the optimization of separator formula (such as adjusting the thickness of PVDF coating) and selection, and provide a clear direction for the safety improvement of energy storage batteries.

[0069] Obviously, the above embodiments of the present application are only examples for clearly illustrating the present application, and are not intended to limit the embodiments of the present application. For ordinary skilled in the art, other different forms of changes or variations can be made on the basis of the above description. Here, it is not necessary and impossible to exhaust all the embodiments. Any modification, equivalent replacement and improvement made within the spirit and principles of the present application shall be included in the protection scope of the claims of the present application.

Claims

1. A method for evaluating the safety performance of a diaphragm, characterized in that, Includes the following steps: S1: The separator to be evaluated is prepared into a pouch cell, and the pouch cell is pretreated to obtain a pretreated pouch cell. S2: Heat the pretreated pouch cell and monitor the heating rate of the pretreated pouch cell in real time. S3: Determine the safety performance of the separator to be evaluated based on the real-time monitoring of the heating rate of the pretreated pouch cell.

2. The method for evaluating the safety performance of a diaphragm according to claim 1, characterized in that, The diaphragm to be evaluated is a polyethylene-based diaphragm or a polypropylene diaphragm.

3. The method for evaluating the safety performance of a diaphragm according to claim 2, characterized in that, The surface of the polyethylene-based diaphragm is coated with a material that is polyvinylidene fluoride or ceramic.

4. The method for evaluating the safety performance of a diaphragm according to claim 3, characterized in that, The coating has a thickness of 1~5μm, and the polyethylene-based membrane has a thickness of 5~20μm.

5. The method for evaluating the safety performance of a diaphragm according to claim 1, characterized in that, The pretreatment of the pouch cell includes the following steps: sequentially performing formation and capacity testing on the pouch cell to obtain a pretreated crude product, and then cyclically processing the pretreated crude product to obtain a pretreated pouch cell.

6. The method for evaluating the safety performance of a diaphragm according to claim 1, characterized in that, In step S2, the heating of the pretreated soft-pack battery includes the following steps: placing the pretreated soft-pack battery into the center of the heating device, the heating device heating the battery at a constant rate from room temperature to a set temperature and then maintaining the temperature, the heating device stopping heating after the temperature maintenance is completed, and the heating device naturally cooling to room temperature.

7. The method for evaluating the safety performance of a diaphragm according to claim 6, characterized in that, The heating device raises the temperature from room temperature to a set temperature at a rate of 5~10℃ / min, where the set temperature is 190℃~195℃.

8. The method for evaluating the safety performance of a diaphragm according to claim 1, characterized in that, A temperature sensor is used to monitor the temperature rise rate of the pretreated pouch cell.

9. The method for evaluating the safety performance of a diaphragm according to claim 1, characterized in that, The voltage difference between the pretreated pouch cell before and after heating is detected. The smaller the voltage difference, the better the structural stability of the separator to be evaluated.

10. The method for evaluating the safety performance of a diaphragm according to claim 1, characterized in that, When the real-time monitored heating rate of the pretreated pouch cell is ≥1℃ / s, the membrane to be evaluated is determined to be thermally runaway. The longer the time from the start of heating to the first time the heating rate of the pretreated pouch cell reaches ≥1℃ / s, the better the thermal shrinkage stability of the membrane to be evaluated. The lower the maximum temperature reached by the pretreated pouch cell during thermal runaway, the better the melt integrity of the membrane to be evaluated. The shorter the time interval between the pretreated pouch cell reaching 200°C and then dropping back to 200°C, the better the self-extinguishing property of the separator to be evaluated; the slower the rate of temperature change per unit time during thermal runaway, the better the heat insulation and flame retardant properties of the separator to be evaluated.

Images