Method for judging whether cycle failure modes of power battery are consistent or not

Abstract

The invention belongs to the field of batteries, and particularly relates to a method for judging whether cycle failure modes of power batteries are consistent or not, which comprises the following steps of: S1) performing cycle test on a plurality of batteries at different temperatures in the same sequential mode, and stopping when the capacity retention rate reaches any consistent SOH; s2) discharging the plurality of batteries to an empty electricity state in a consistent discharging mode, and disassembling the batteries; s3) testing the lithium element content and the iron element content of the negative electrode of the disassembled battery to obtain the lithium element / iron element molar ratio of the negative electrode; and S4) drawing by taking the temperature as an abscissa and the lithium element / iron element molar ratio of the negative electrode as an ordinate, and analyzing whether the cycle failure modes are consistent or not. According to the method, whether the cycle failure modes of the power battery are consistent or not is rapidly and accurately compared, and good assistance is provided for early judgment, cycle intervention and evaluation of different power lithium ion battery systems of the battery.

Description

Technical Field

[0001] The present invention belongs to the field of batteries, and particularly relates to a method for judging the cycle failure mode of a power battery and whether they are consistent. Background Art

[0002] Lithium-ion batteries have gradually become the main power sources for digital products and new energy vehicles due to their advantages such as high energy storage density, high specific power, and long service life. In high-power systems such as electric vehicles and power tools, multiple single cells with small capacity and low voltage need to be combined into a high-capacity and high-voltage battery pack through parallel / series connection. To better utilize the role of single lithium-ion batteries and maintain and use them better, it is particularly important to evaluate the consistency of the cycle failure mode of single lithium-ion batteries. Summary of the Invention

[0003] The purpose of the present invention is to overcome the shortcomings in the prior art and provide a method for judging the cycle failure mode of a power battery and whether they are consistent.

[0004] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0005] A method for judging whether the cycle failure modes of power batteries are consistent includes the following steps: S1) Perform cycle tests on multiple batteries at different temperatures with the same cycle pattern until stopped at any SOH when the capacity retention rate reaches consistency; S2) Discharge multiple batteries to the empty state with a consistent discharge pattern and disassemble the batteries; S3) Test the contents of lithium and iron elements in the negative electrode of the disassembled batteries to obtain the molar ratio of lithium / iron elements in the negative electrode; S4) Plot the temperature as the abscissa and the molar ratio of lithium / iron elements in the negative electrode as the ordinate to analyze whether the cycle failure modes are consistent.

[0006] In step S3), the test method for the contents of lithium and iron elements is to cut the pole pieces at the same position for each battery, soak and clean the cut pole pieces with DMC, dry them, then scrape the powder, and then seal the samples for elemental composition testing.

[0007] The elemental composition testing is ICP testing.

[0008] The judgment criterion for consistency in step S4) is judged by the growth rate of the molar ratio of lithium / iron elements at different temperatures; if the growth rate is within ±20%, it is judged that the cycle failure modes of the batteries are consistent; if the growth rate exceeds this range, it is judged that the cycle failure modes of the batteries are inconsistent.

[0009] The temperature in step S1) is 25 - 65°C, and the interval range of different temperatures tested is 5 - 10°C.

[0010] In step S1), SOH ≥ 90%; preferably 90%.

[0011] The present invention also includes a method for judging the cycle failure mode of a power battery, comprising the following steps: 1) Using the method for judging whether the cycle failure modes of the power battery are consistent for testing; 2) After the test, when the cycle failure modes are consistent, using the relationship between the different battery capacity retention rates and the molar ratio of lithium element / iron element at any temperature for different batteries to test so as to judge the cycle failure mode of the power battery; 3) After the test, when the cycle failure modes are inconsistent, using any two temperatures under different failure modes to test the relationship between the different battery capacity retention rates and the molar ratio of lithium element / iron element for different batteries so as to judge the battery cycle failure mode.

[0012] The method for testing the relationship between the different battery capacity retention rates and the molar ratio of lithium element / iron element in steps 2) and 3) is as follows:

[0013] a. Conduct cycle tests on multiple batteries with the same cycling regime and stop at different remaining capacity retention rates SOH;

[0014] b. Discharge multiple batteries to the empty state with a consistent discharging regime and disassemble the batteries;

[0015] c. Test the lithium element and iron element contents of the negative electrode of the disassembled batteries to obtain the molar ratio of lithium element / iron element of the negative electrode;

[0016] d. Plot a graph with the different battery capacity retention rates as the abscissa and the molar ratio of lithium element / iron element of the negative electrode as the ordinate to obtain the relationship between the different battery capacity retention rates and the molar ratio of lithium element / iron element of the negative electrode.

[0017] The method for judging the cycle failure mode of the battery in 2) and 3) is as follows: Perform cycling on the battery to be tested with the same cycling regime as in step a), discharge it to the empty state with the same discharging regime as in step b) and disassemble the battery, and test the lithium element and iron element contents of the negative electrode of the disassembled battery using the same method as in step c) to obtain the molar ratio of lithium element / iron element of the negative electrode; Substitute the molar ratio of lithium element / iron element of the negative electrode obtained in step e) into the graph obtained in step d) to judge the failure mode.

[0018] When the molar ratio of lithium element / iron element of the negative electrode is in the upper right of the graph obtained in step d), it indicates that the failure mode is mainly lithium deposition on the negative electrode; when the molar ratio of lithium element / iron element of the negative electrode is in the lower left of the graph obtained in step d), it indicates that the failure mode is mainly positive electrode metal dissolution.

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] The present invention provides a test method to determine whether the cycle failure modes of power batteries are consistent. Taking the batteries at the initial stage or during the cycle as a reference, an X-Y diagram of temperature and the molar ratio of lithium element to iron element at the negative electrode is drawn, so as to conveniently see the differences among different series. This method involves the reverse analysis of batteries and the rapid analysis at the initial stage of the cycle. By combining the cycle data and the reverse analysis data, it can quickly and accurately compare whether the cycle failure modes of power batteries are consistent, which provides good assistance for the early judgment of batteries, cycle intervention, and the evaluation of different power lithium-ion battery systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of the test data of the molar ratio of Li / Fe at the negative electrode corresponding to different temperatures in Embodiment 1 of the present invention;

[0022] Figure 2 Comparison diagram of the cycle data of batteries at different temperatures in Embodiment 1 of the present invention;

[0023] Figure 3 Diagram of the situation of the negative electrode sheets of the batteries at different temperatures disassembled in Embodiment 1 of the present invention.

[0024] Figure 4 Schematic diagram of the battery capacity retention rate and the molar ratio of Li / Fe at the negative electrode of the batteries at different temperatures in Embodiment 1 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] In order to enable those skilled in the art of the present technology to better understand the technical solutions of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and the best embodiments.

[0026] Embodiment 1

[0027] A method for determining whether the cycle failure modes of power batteries are consistent, and the specific steps are as follows:

[0028] S1) Select a lithium iron phosphate power battery of about 200 Ah and perform cycle tests according to the same cycle pattern at 25 °C, 35 °C, 50 °C, and 55 °C respectively, and stop the cycle when the capacity retention rate reaches 90% of consistency.

[0029] S2) Discharge the battery to the empty state with a consistent discharge pattern and temperature, and after standing for the same time, disassemble the battery. The battery cannot introduce metal debris and other pollutants, and disassemble it in a drying room;

[0030] S3) After disassembly, the same positions are selected on each battery for pole piece cutting. The cut pole pieces are soaked and cleaned in DMC for the same period of time, and then dried at the same temperature. After that, powder scraping is performed with a non-metallic blade. The areas and sizes of powder scraping should be the same, and scraping is carried out from the surface layer to the inner layer of the battery without scraping the copper foil. Then, the samples are sealed and subjected to elemental composition testing. It is recommended but not limited to using ICP for testing to determine the compositions of element lithium and element iron in the negative pole piece, and calculate the molar ratio of lithium element / iron element in the negative pole. As shown in Table 1 below;

[0031] Table 1

[0032]

[0033] S4) Using temperature as the abscissa and the molar ratio of lithium element / iron element in the negative pole as the ordinate, a scatter plot is drawn. As Figure 1 shown, the differences between each series can be visually observed. When comparing, taking the molar ratio data of lithium element / iron element at one temperature as the reference, calculate the growth rate of the molar ratio of lithium element / iron element at other temperatures. If the growth rate is within ±20%, it is determined that the battery cycle failure modes are the same; if the growth rate exceeds this range, it is determined that the battery cycle failure modes are different. As Figure 1 shown, in this embodiment, the data point at 25°C is significantly more than 20% different from that of other data batteries, while other data points are within 20%.

[0034] It is concluded that the failure mode of the cycle of this type of battery at 25°C is significantly different from that at other temperatures, while the failure modes of the cycles at other higher temperatures are similar. In the Figure 3 battery cycle data graph, and the Figure 4 graph of the disassembled negative pole piece situation, corresponding verification can be obtained.

[0035] Example 2

[0036] A method for judging the cycle failure mode of a power battery includes the following steps: 1) Using the method described above for testing whether the cycle failure modes of power batteries are the same; 2) When the cycle failure modes are the same after the test, using the relationship test between the different battery capacity retention rates and the molar ratio of lithium element / iron element at any temperature to judge the cycle failure mode of the power battery; 3) When the cycle failure modes are different after the test, using the relationship test between the different battery capacity retention rates and the molar ratio of lithium element / iron element at any two temperatures under different failure modes to judge the cycle failure mode of the battery.

[0037] The method for the relationship test between the different battery capacity retention rates and the molar ratio of lithium element / iron element in steps 2) and 3) is as follows:

[0038] a. Perform cyclic tests on multiple batteries using the same cycling regime and stop when the remaining capacity retention rate (SOH) is different;

[0039] b. Discharge multiple batteries to the empty state using a consistent discharging regime and then disassemble the batteries;

[0040] c. Test the contents of lithium and iron elements in the negative electrode of the disassembled batteries to obtain the lithium / iron element molar ratio of the negative electrode;

[0041] d. Plot a graph with different battery capacity retention rates as the abscissa and the lithium / iron element molar ratio of the negative electrode as the ordinate to obtain the relationship between different battery capacity retention rates and the lithium / iron element molar ratio of the negative electrode. To verify the results, fit the data relationship graphs of the battery capacity retention rate and the lithium / iron element molar ratio of the negative electrode at 25°C, 35°C, 45°C, and 60°C respectively.

[0042] Table 2 shows the results of the Li / Fe molar ratio of the negative electrode at a 95% capacity retention rate at different temperatures. Fit these results with the results of the Li / Fe molar ratio of the negative electrode at a 90% capacity retention rate in Table 1, and the results are as Figure 4 shown.

[0043] Table 2

[0044]

[0045]

[0046] 2). The method for judging the battery cycling failure mode in 3) is as follows: Discharge the battery to be tested to the empty state and disassemble the battery. Test the contents of lithium and iron elements in the negative electrode of the disassembled battery using the same test method as in step c) to obtain the lithium / iron element molar ratio of the negative electrode; Substitute the lithium / iron element molar ratio of the negative electrode obtained in step e) into the graph obtained in step d) to judge the failure mode.

[0047] When the lithium / iron element molar ratio of the negative electrode is in the upper right of the graph obtained in step d), it indicates that the failure mode is mainly lithium deposition on the negative electrode. When the lithium / iron element molar ratio of the negative electrode is in the lower left of the graph obtained in step d), it indicates that the failure mode is mainly metal dissolution from the positive electrode. Above the trend line is mainly the attenuation mode of lithium deposition on the negative electrode; As the temperature increases, the lithium deposition area on the negative electrode decreases, and the metal dissolution area increases. The results can be correspondingly verified in the Figure 3 battery cycling data graph, and Figure 4 the graph of the disassembled negative electrode sheet situation.

[0048] In summary, the present invention provides a test method to determine whether the cycle failure modes of power batteries are consistent. Taking the batteries at the initial stage or during the cycle as a reference, an X-Y graph of temperature and the molar ratio of lithium element to iron element at the negative electrode is plotted, so that the differences between different series can be easily seen. This method involves the reverse analysis of batteries and the rapid analysis at the initial stage of the cycle. By combining the cycle data and the reverse analysis data, it can quickly and accurately compare whether the cycle failure modes of power batteries are consistent, and provides good assistance for the early judgment of batteries, cycle intervention, and the evaluation of different power lithium-ion battery systems.

[0049] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for judging whether the cycle failure modes of power batteries are consistent, characterized in that, It includes the following steps: S1) Perform cycle tests on multiple batteries at different temperatures in the same sequential pattern and stop when the capacity retention rate reaches any SOH with consistent results; S2) Discharge multiple batteries to the empty state in a consistent discharge pattern and disassemble the batteries; S3) Test the lithium and iron element contents in the negative electrode of the disassembled batteries to obtain the lithium / iron element molar ratio of the negative electrode; S4) Plot the temperature on the abscissa and the lithium / iron element molar ratio of the negative electrode on the ordinate to analyze whether the cycle failure modes are consistent.

2. The method for judging whether the cycle failure modes of power batteries are consistent according to claim 1, characterized in that In step S3), the test method for the lithium and iron element contents is to cut the electrode sheets at the same position for each battery. The cut electrode sheets are soaked and cleaned with DMC, dried, then powdered, and sealed for elemental composition testing.

3. The method for judging whether the cycle failure modes of power batteries are consistent according to claim 2, characterized in that The said elemental composition testing is ICP testing.

4. The method for determining whether the cycle failure modes of power batteries are consistent according to claim 1, characterized in that In step S4), the judgment criterion for consistency is determined by the growth rate of the lithium / iron element molar ratio at different temperatures; if the growth rate is within ±20%, it is judged that the battery cycle failure modes are consistent; if the growth rate exceeds this range, it is judged that the battery cycle failure modes are inconsistent.

5. The method for judging whether the cycle failure modes of power batteries are consistent according to claim 1, characterized in that, In step S1), the temperature is 25 - 65 °C, and the interval range of the different temperatures tested is 5 - 15 °C.

6. The method for judging whether the cycle failure modes of power batteries are consistent according to claim 1, characterized in that In step S1), SOH ≥ 90%; preferably 90%.

7. A method for judging the cycle failure mode of a power battery, characterized in that, It includes the following steps: 1) Test using the method for judging whether the cycle failure modes of power batteries are consistent described in any one of claims 1 - 6; 2) When the cycle failure modes are consistent after the test, test the relationship between the different battery capacity retention rates and the lithium / iron element molar ratio of the negative electrode at any temperature to judge the cycle failure modes of the power batteries; 3) When the cycle failure modes are inconsistent after the test, test the relationship between the different battery capacity retention rates and the lithium / iron element molar ratio of the negative electrode at any two temperatures under different failure modes to judge the battery cycle failure modes.

8. The method for determining the cycle failure mode of a power battery according to claim 7, wherein In steps 2) and 3), the test method for the relationship between the different battery capacity retention rates and the lithium / iron element molar ratio of the negative electrode is as follows: a. Perform cycle tests on multiple batteries in the same sequential pattern and stop at different remaining capacity retention rates SOH; b. Discharge multiple batteries to the empty state in a consistent discharge pattern and disassemble the batteries; c. Test the lithium and iron element contents in the negative electrode of the disassembled batteries to obtain the lithium / iron element molar ratio of the negative electrode; d. Plot the different battery capacity retention rates on the abscissa and the lithium / iron element molar ratio of the negative electrode on the ordinate to obtain the relationship between the different battery capacity retention rates and the lithium / iron element molar ratio of the negative electrode.

9. The method for judging the cycle failure mode of a power battery according to claim 8, characterized in that, In 2) and 3), the judgment method for the battery cycle failure modes is as follows: Perform cycling on the battery to be tested in the same cycle pattern as in step a), discharge it to the empty state in the same discharge pattern as in step b) and disassemble the battery, test the lithium and iron element contents in the negative electrode of the disassembled battery using the same test method as in step c) to obtain the lithium / iron element molar ratio of the negative electrode; Substitute the lithium / iron element molar ratio of the negative electrode obtained in step e) into the graph obtained in step d) to judge the failure mode.

10. The method for judging the cycle failure mode of a power battery according to claim 9, characterized in that, When the lithium / iron element molar ratio of the negative electrode is in the upper right of the graph obtained in step d), it indicates that the failure mode is mainly lithium deposition on the negative electrode. When the lithium / iron element molar ratio of the negative electrode is in the lower left of the graph obtained in step d), it indicates that the failure mode is mainly metal dissolution of the positive electrode.

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