Method for evaluating quality of internal moisture control of lithium ion battery

Abstract

The invention relates to the technical field of lithium ion batteries, and particularly discloses a lithium ion battery internal moisture control quality evaluation method, which comprises the following steps: respectively adjusting to-be-evaluated lithium ion batteries to different charge states, and enabling the to-be-evaluated lithium ion batteries to be in a shelving state; after a preset time, measuring the expansion rate of the battery in the shelving state under each charge state; when the expansion rate of the low-state-of-charge battery is higher than or equal to the expansion rate of the high-state-of-charge battery, outputting that water control in the battery has defects; and when the expansion rate of the low-state-of-charge battery is lower than that of the high-state-of-charge battery, the moisture in the output battery is well controlled. By comparing the expansion rates of the batteries in different states of charge after being placed, the control of the moisture in the batteries can be quickly and effectively evaluated.

Description

Technical Field

[0001] The invention relates to the technical field of lithium ion batteries, and in particular to a method for evaluating the quality of internal moisture control of lithium ion batteries. Background Art

[0002] In the manufacturing process of lithium-ion batteries, dust, metal particles and moisture are crucial quality control points. Improper control of dust and metal particles can easily cause internal short circuits in the battery, and even lead to serious safety accidents such as fires; while poor moisture control can cause battery swelling, increased internal resistance and shortened cycle life. On the contrary, reasonable moisture control has a positive effect on improving battery performance and safety.

[0003] In the lithium battery manufacturing process, moisture comes from a wide range of sources, mainly including moisture carried by raw materials such as positive and negative electrode materials, electrolytes, and moisture in the workshop environment. Currently, the moisture content of raw materials is usually determined offline by Karl Fischer titration, and the moisture content of the workshop environment is achieved by controlling the dew point of the environment. However, this method cannot accurately grasp the actual residual moisture content inside the battery. Currently, it can only be comprehensively evaluated based on the performance of the battery after it is offline. There is a lack of a direct and effective method to evaluate the pros and cons of moisture control inside the battery. Summary of the invention

[0004] The purpose of the present invention is to provide a method for evaluating the quality of internal moisture control of a lithium-ion battery, so as to solve the problem that it is difficult to accurately evaluate the internal moisture control of the battery in the prior art.

[0005] The purpose of the present invention can be achieved through the following technical solutions: A method for evaluating the quality of internal moisture control of a lithium-ion battery comprises the following steps: The lithium-ion batteries to be evaluated are adjusted to different states of charge respectively and placed in a standby state; After a preset time, the expansion rate of the battery in each state of charge is measured when it is stored; When the expansion rate of the low state-of-charge battery is higher than or equal to the expansion rate of the high state-of-charge battery, the internal moisture control of the output battery is defective; When the expansion rate of a low state-of-charge battery is lower than that of a high state-of-charge battery, the internal moisture of the output battery is well controlled.

[0006] As a further solution of the present invention: the specific method of selecting batteries with different charge states is as follows: The value range of low state of charge is set to 0%-25% SOC; The high state of charge ranges from 75% to 100% SOC.

[0007] As a further solution of the present invention: when the type of the lithium-ion battery is a soft-pack battery or a square aluminum shell battery, the expansion rate is calculated according to the following formula: expansion rate = (battery thickness after storage - battery thickness before storage) ÷ battery thickness before storage × 100%; When measuring battery thickness, a measuring tool with an accuracy of no less than 0.001 mm must be used, and measurements must be made at multiple evenly distributed locations on the battery, and the average value must be taken as the final thickness measurement result.

[0008] As a further solution of the present invention: when the type of lithium-ion battery is a cylindrical battery, the battery expansion rate is equivalently obtained by testing the gas production of the battery; Methods for testing battery gas production include gas chromatography and pressure volume temperature method. Under the same storage conditions, the more gas the battery produces, the greater the battery expansion rate.

[0009] As a further solution of the present invention: the expansion rate can also be monitored by a pressure sensor. The pressure sensor is installed on the surface of the battery. The pressure sensor records in real time the pressure changes of the battery during the storage process. Through the pre-established correspondence between the pressure change and the expansion rate, the pressure change data is converted to obtain the expansion rate of the battery.

[0010] As a further solution of the present invention: during the storage process of the battery, the storage conditions are set as follows: The temperature range is set at 20℃-85℃ and the shelf time is not less than 12h.

[0011] As a further solution of the present invention: before comparing the expansion rates, the battery needs to be calibrated in capacity. The capacity calibration process is carried out at room temperature. The battery is charged to an upper limit voltage using a constant current-constant voltage charging method. The cut-off current is set to 0.05-0.15 times the rated current. After charging is completed, the battery is left for 15-35 minutes to stabilize the internal state of the battery. The battery is then discharged to a lower limit voltage with a constant current. This is repeated for 3-6 cycles, and the stable discharge capacity is used as the calibration capacity.

[0012] Beneficial effects of the invention: The method for evaluating the quality of internal moisture control of lithium-ion batteries provided by the invention has the advantages of simple operation and intuitive results. By comparing the expansion rates of batteries with different charge states after being shelved, the quality of internal moisture control of the battery can be quickly and effectively evaluated, providing a new evaluation method for the production, quality inspection and research and development of lithium-ion batteries, which is helpful to improve the quality and performance of lithium-ion batteries and promote the development of the lithium-ion battery industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present invention will be further described below in conjunction with the accompanying drawings.

[0014] Figure 1 It is a schematic flow chart of a method for evaluating the quality of internal moisture control of a lithium-ion battery according to the present invention; Figure 2 The voltage change of the 21Ah lithium iron phosphate battery in the embodiment of the present invention after 30 days under different storage temperatures and SOC conditions; Figure 3 It is the expansion characteristic of the 21Ah battery of the present invention after being shelved through thickness monitoring; Figure 4 It is the expansion characteristic of the 21Ah battery of the present invention after being placed on hold by monitoring the expansion force; Figure 5 The expansion rate and voltage change of the 314Ah lithium iron phosphate battery under different SOC conditions of the present invention after being placed at 65°C for 10 days. DETAILED DESCRIPTION

[0015] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0016] In the manufacturing process of lithium-ion batteries, dust, metal particles and moisture are three very critical quality control points that must be strictly controlled. Failure to control dust and metal particles will directly lead to safety accidents such as internal short circuits and fires in the battery, while poor moisture control will lead to battery swelling, increased internal resistance and shortened cycle life. Proper moisture control will help improve battery performance and safety.

[0017] In the manufacturing process of lithium batteries, the sources of moisture are mainly the moisture brought in by raw materials such as positive and negative electrode materials and electrolytes, as well as moisture in the workshop environment. Therefore, in the manufacturing process of lithium batteries, moisture control in various links such as environmental humidity, storage and exposure time of pole pieces, baking and dehydration process of pole pieces and diaphragms, shelf life of electrolytes, and water content testing are essential to ensure that the battery moisture is always in a controlled and qualified state. Generally speaking, the moisture content of raw materials is obtained by offline determination by Karl Fischer titration, while workshop environmental moisture control is achieved by controlling the environmental dew point. As a result, it is difficult to accurately grasp the actual residual moisture content inside the battery, and it can only be comprehensively evaluated by the performance of the battery after it is offline.

[0018] See also Figure 1As shown, the present invention provides a method for evaluating the quality of internal moisture control of lithium-ion batteries, that is, comparing the expansion rates of batteries with different states of charge (SOC) after being placed on hold. If the expansion rate of a low SOC battery is higher than that of a high SOC battery, it indicates that the internal moisture control of the battery is not good. Otherwise, it indicates that the internal moisture control quality of the battery is good.

[0019] The expansion rate can be calculated by testing the volume change of soft-pack batteries or square aluminum shell batteries, where the volume change is easy to observe. The expansion rate = (thickness after storage - thickness before storage) / thickness before storage × 100%; the expansion rate can also be obtained by monitoring the pressure change of the battery through a pressure sensor. The volume change of cylindrical batteries is not easy to monitor, but the battery expansion rate can be calculated by testing the gas production of the battery. That is, under the same conditions, the more gas production of the battery, the greater the battery expansion rate.

[0020] Generally speaking, the gas production of batteries at high SOC is higher than that of batteries at low SOC, and high temperatures above 40°C will accelerate the gas production of batteries. However, the presence of excessive moisture will cause the gas production of low SOC batteries to be higher than that of high SOC batteries, which is more obvious at high temperatures.

[0021] In a specific embodiment of the present invention, the battery expansion rate is tested by a shelf aging method. Before the shelf aging test, the battery capacity is calibrated at room temperature (25±2° C.) using a battery tester (BTS 4008-5V12A).

[0022] First, a commercial 21Ah lithium iron phosphate graphite square aluminum shell battery was selected as the test object (the moisture content of the positive and negative electrodes and the diaphragm of the battery before injection was about 300ppm, and the dew point of the injection workshop was about -20℃). The battery was charged to 3.65V by constant current-constant voltage (CC-CV) using a current of 1.05A, and the cut-off current was 0.11A. Subsequently, after standing for 10 minutes, it was discharged to 2.5V through CC using a current of 1.05A. After 3 cycles, the stable discharge capacity was used as the calibration capacity. Before the SOC adjustment process, the battery was charged to 3.65V by CC-CV using a current of 1.05A. When the cut-off current reached 0.11A again, the SOC was adjusted by controlling the discharge time. For example, if the rated capacity is 21Ah, a battery with a 90% SOC is obtained by discharging with a current of 1.05A for 2 hours; a battery with a 70% SOC is obtained by discharging with a current of 1.05A for 6 hours, and so on. The characteristics of five SOC (10%, 30%, 50%, 70% and 90%) batteries stored at 25°C, 45°C and 65°C for 30 days were studied by orthogonal experiments. During the 30-day shelf period, the battery parameters such as internal resistance, open circuit voltage and thickness were initially tested, and then tested every 10 days. The batteries subjected to different temperature and SOC tests were then labeled as (temperature / SOC / storage time). For example, a battery with 10% SOC stored at 25°C for 10 days is labeled as 25°C / 10%SOC / 10d.

[0023] Figure 2 The voltage change of 21Ah lithium iron phosphate battery after 30 days under different storage temperature and SOC conditions is shown, indicating that the effect of temperature on battery storage performance is greater than that of battery SOC. At the same time, the battery open circuit voltage (OCV) decreases with the increase of storage temperature, except for 10% SOC and 50% SOC (such as Figure 2 As shown in the figure, especially at 65°C, the self-discharge rate of 30% SOC and 70% SOC batteries is higher than that of 10% SOC and 50% SOC batteries, but the voltage drop of 90% SOC batteries is not particularly obvious. Therefore, it is difficult to characterize the difference in the storage state of batteries at different SOCs by testing the voltage drop method.

[0024] According to the battery thickness data recorded every 10 days during the shelf period, Figure 3 The expansion rate of the battery after being stored at different temperatures for 30 days is shown. Figure 3 In the figure, the end points of each broken line are, from top to bottom, 65℃ / 30d, 65℃ / 20d, 65℃ / 10d, 45℃ / 30d, 45℃ / 20d, 45℃ / 10d, 25℃ / 30d, 25℃ / 20d, and 25℃ / 10d.

[0025] The expansion rate increases with the increase of storage temperature and decreases with the increase of SOC. The battery stored at 25℃ for 30 days has almost no expansion, but as the storage temperature increases, the battery expansion rate gradually increases. But unexpectedly, the expansion rate of the low SOC battery is much higher than that of the high SOC battery. The expansion rate of the 10% SOC battery after being stored at 65℃ / 30 days is as high as 11%, which is almost twice that of the 90% SOC battery. In addition, the expansion rate of the 10% SOC battery after being stored at 65℃ for 10 days is almost the same as the expansion rate after being stored for 30 days, while the expansion rate of the 90% SOC battery increases significantly with the increase of storage time (see Figure 3 ).

[0026] In order to further confirm the effect of SOC on the high temperature expansion rate of the battery during storage, a pressure sensor device was used to monitor the expansion force of the 65℃ / 10%SOC / 30d and 65℃ / 90%SOC / 30d batteries in real time. Figure 4 As shown, it can also be observed that the expansion force of the 10% SOC battery rises rapidly from 310N (preload force applied to the battery) to 415N in the first ten days, and then stabilizes at about 395N. At the same time, the expansion force of the 90% SOC battery gradually increases from 310N to about 380N during the 30-day shelf period. The in-situ pressure monitoring results are very consistent with the intermittent expansion rate calculation results. Therefore, the difference in battery expansion rate can be seen by shelving at 65°C for 3-10 days. The expansion rate of the low SOC battery after high temperature shelving is significantly higher than that of the high SOC battery.

[0027] Commercial 314Ah lithium iron phosphate graphite square aluminum shell batteries were selected as comparison test objects again (the moisture content of the positive and negative electrodes and the diaphragm of the battery was about 180ppm before injection, and the dew point of the injection workshop was about -30°C). After adjusting the SOC, the battery was placed in a 65°C oven for 10 days. Figure 5 The battery thickness expansion rate and voltage data after storage are given, and it can be seen that the battery exhibits the characteristics of high SOC and high expansion rate after storage at 65°C.

[0028] contrast Figure 2-Figure 3 and Figure 5 It can be seen from the battery expansion rate data that the presence of high moisture content in the battery will cause the battery to show a higher expansion rate after high-temperature storage than the low-SOC battery. Preferably, the expansion rates of 10% SOC and 90% SOC batteries can be used as comparison to judge the quality of the internal moisture control level of the battery.

[0029] The above is a detailed description of an embodiment of the present invention, but the content is only a preferred embodiment of the present invention and cannot be considered to limit the scope of implementation of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the patent coverage of the present invention.

Claims

1. A method for evaluating the quality of internal moisture control of a lithium-ion battery, characterized in that: The following steps are involved: The lithium-ion batteries to be evaluated are adjusted to different states of charge respectively and placed in a standby state; After a preset time, the expansion rate of the battery in each state of charge is measured when it is stored; When the expansion rate of the low state-of-charge battery is higher than or equal to the expansion rate of the high state-of-charge battery, the internal moisture control of the output battery is defective; When the expansion rate of a low state-of-charge battery is lower than that of a high state-of-charge battery, the internal moisture of the output battery is well controlled.

2. The method for evaluating the quality of internal moisture control of a lithium-ion battery according to claim 1, characterized in that: The specific method for selecting batteries with different states of charge is as follows: The value range of low state of charge is set to 0%-25% SOC; The high state of charge ranges from 75% to 100% SOC.

3. The method for evaluating the quality of internal moisture control of a lithium-ion battery according to claim 1, characterized in that: When the type of lithium-ion battery is a soft-pack battery or a square aluminum shell battery, the expansion rate is calculated according to the following formula: Expansion rate = (battery thickness after storage - battery thickness before storage) ÷ battery thickness before storage × 100%; When measuring battery thickness, a measuring tool with an accuracy of no less than 0.001 mm must be used, and measurements must be made at multiple evenly distributed locations on the battery, and the average value must be taken as the final thickness measurement result.

4. The method for evaluating the quality of internal moisture control of a lithium-ion battery according to claim 1, characterized in that: When the lithium-ion battery is a cylindrical battery, the battery expansion rate is equivalently obtained by testing the battery gas production; Methods for testing battery gas production include gas chromatography and pressure volume temperature method. Under the same storage conditions, the more gas the battery produces, the greater the battery expansion rate.

5. The method for evaluating the quality of internal moisture control of a lithium-ion battery according to claim 1, characterized in that: The expansion rate can also be monitored by a pressure sensor. The pressure sensor is installed on the surface of the battery. The pressure sensor records the pressure changes of the battery during storage in real time. The pressure change data is converted into the expansion rate of the battery through the pre-established correspondence between the pressure change and the expansion rate.

6. The method for evaluating the quality of internal moisture control of a lithium-ion battery according to claim 1, characterized in that: During the storage process, the storage conditions are set as follows: The temperature range is set at 20℃-85℃ and the shelf time is not less than 12h.

7. The method for evaluating the quality of internal moisture control of a lithium-ion battery according to claim 1, characterized in that: Before comparing the expansion rates, the battery needs to be calibrated in capacity. The capacity calibration process is carried out at room temperature. The battery is charged to the upper limit voltage using a constant current-constant voltage charging method. The cut-off current is set to 0.05-0.15 times the rated current. After charging is completed, it is left for 15-35 minutes to stabilize the internal state of the battery. The battery is then discharged to the lower limit voltage at a constant current. This is repeated for 3-6 cycles, and the stable discharge capacity is used as the calibration capacity.

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