Rapid and secondary evaluation method for battery performance

Abstract

The invention belongs to the field of battery testing, and particularly relates to a rapid and secondary evaluation method for battery performance. The rapid evaluation method comprises the following steps: carrying out initial capacity testing on a to-be-tested battery and a reference battery; performing an overcharge test on the to-be-tested battery and the reference battery to obtain the overcharged capacities of the to-be-tested battery and the reference battery; and evaluating the battery performance through the capacity retention ratio of the to-be-tested battery and the reference battery. According to the method, firstly, the capacity test is performed on the fresh battery to obtain the initial capacity of the battery, secondly, the overcharge test is performed on the battery, the capacity test of the same system is performed again after the overcharge is finished, the capacity retention rate of the battery at the moment is calculated, and the difference of the battery performance can be preliminarily determined by comparing the difference of the capacity retention rates. When the difference of the battery capacity retention ratio is within 1%, secondary evaluation is continued, and the battery performance is accurately evaluated through the Fe content and / or the L content in the battery negative plate.

Description

Technical Field

[0001] The present invention belongs to the field of battery testing, and particularly relates to a method for quickly and secondarily evaluating the performance of a battery. Background Art

[0002] As a low-cost cathode material, lithium iron phosphate uses sustainable and abundant iron and is a more attractive battery material compared with ternary materials. It is statistically predicted that the global battery demand will increase from 0.7 TWh in 2022 to 2.6 - 6.0 TWh in 2030. Although the increasingly high production growth rate enables the battery industry to transform the global energy infrastructure into renewable energy, it is necessary to prioritize whether the battery performance can meet human needs.

[0003] In the actual testing process, the performance testing of lithium iron phosphate batteries often takes a long time. For the 1000 - 3000 cycle tests required for lithium-ion batteries in fields such as power, the time required is about 100 - 300 days, and for the batteries required for energy storage power stations to reach 3000 - 5000 cycles, the cycle time is even longer, which requires a large amount of testing resources and cannot meet the product development needs. Therefore, the research on the rapid testing of the performance of lithium iron phosphate batteries has become very important and urgent. The performance of lithium iron phosphate batteries can be quickly evaluated by means of accelerated aging and establishing a mathematical model. The significance of rapid performance evaluation can save battery resources on the one hand and shorten the development cycle of researchers on the other hand, promoting the rapid development of the battery industry.

[0004] Currently, in the methods for rapidly evaluating the performance of lithium iron phosphate batteries, the test is mainly accelerated by changing the current and temperature. For example, the publication number CN108051753A discloses an accelerated test method for a long-life fast-charging lithium iron phosphate battery. This method involves leaving the battery at different temperatures and charging or discharging at a constant power, then cycling multiple times and calculating the charge energy retention rate and discharge energy retention rate, and finally obtaining the charge energy retention rate and discharge energy retention rate of the battery. However, the test steps shown in this patent are complex and still require a long test time. The publication number CN111190114B discloses an accelerated test method for a long-cycle lithium iron phosphate battery for energy storage. This method involves selecting the parameters of charge and discharge energy, selecting constant power charge and discharge, and this invention utilizes the battery polarization phenomenon to judge the difference before and after cycling by calculating the change rate of the simulated internal resistance, and combines the capacity loss and energy loss parameters to use any two as the basis for judging the end of the cycle. However, this invention requires 100 cycles as the judgment basis, and the time used is only shorter than that of the conventional test method, and still requires a lot of manpower and material resources for testing.

[0005] In addition, in the existing technology, the test method for iron dissolution in lithium iron phosphate batteries is mainly the chemical method. Researchers soak lithium iron phosphate powder in water for 1 to 2 days, or soak lithium iron phosphate in the electrolyte for a long time, and judge the content of iron ion dissolution in lithium iron phosphate by measuring the content of iron ions in the filtrate. However, such test methods have problems of long test cycle and low test efficiency. Other researchers configured an acidic solution with a hydrogen ion concentration range of 10 -2 to 10 -6 mol / L by simulating the solution environment when iron elements are dissolved in the electrolyte under normal use conditions of lithium iron phosphate, weighed lithium iron phosphate powder and mixed it with the acidic solution and reacted under a certain temperature condition, filtered to obtain a filtrate containing iron ions, and measured the content of iron ions in the filtrate containing iron ions. This method has improved the test efficiency to a certain extent, but it is impossible to verify whether the content of iron ions in the sample dissolved by acid includes the iron ions in the lithium iron phosphate body, there are large human errors in the test results, and the simulated acidic solution environment cannot guarantee a high degree of similarity with the real environment of the battery. Summary of the Invention

[0006] The purpose of the present invention is to overcome the shortcomings in the existing technology and provide a method for quickly and secondarily evaluating battery performance.

[0007] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0008] A method for quickly evaluating battery performance includes the following steps: Step 1: Perform an initial capacity test on the battery under test and the reference battery; Step 2: Perform an overcharge test on the battery under test and the reference battery to obtain the capacities of the battery under test and the reference battery after overcharge; Step 3: Evaluate the battery performance through the capacity retention rates of the battery under test and the reference battery.

[0009] The specific steps of the first step are as follows: With a current I 1 , perform charge and discharge tests of the same format on the battery under test and the reference battery in the normal voltage range, and record the initial discharge capacity X 1 of the battery under test and the initial discharge capacity Y 1 of the reference battery; Preferably, 0.1C ≤ I 1 ≤ 1C.

[0010] The specific steps of the second step are as follows:

[0011] 2.1) Charge the battery under test and the reference battery at a constant current I 2 to 4.9 - 5.3V, keep the voltage constant for 0.5 - 1h, and fully charge the battery to transfer all lithium ions and iron ions to the negative electrode; Preferably, 0.5C ≤ I 2 ≤ 1C;

[0012] 2.2) Then at a current I3 Perform constant current discharge, and the cut-off voltage is the same as that in the first step; preferably, 0.5C ≤ I 3 ≤ 1C;

[0013] 2.3) Perform charge and discharge tests on the battery under test and the reference battery in the normal voltage range with current I 1 and record the discharge capacities X 2 and Y 2 of the battery under test and the reference battery after overcharge respectively.

[0014] The specific steps of the third step are as follows: If the capacity retention rate of the battery under test is higher than that of the reference battery, it indicates that the performance of the battery under test is better than that of the reference battery; if the capacity retention rate of the battery under test is lower than that of the reference battery, it indicates that the performance of the battery under test is inferior to that of the reference battery; the battery capacity retention rate = the discharge capacity after overcharge / the initial discharge capacity.

[0015] The battery mentioned above is a lithium iron phosphate battery. Preferably, the normal voltage range of the lithium iron phosphate battery is 2.5 - 3.65V.

[0016] The present invention also includes a secondary evaluation method for battery performance, including the rapid evaluation method for the performance of the lithium iron phosphate battery mentioned above; when the difference between the capacity retention rate of the battery under test and the capacity retention rate of the reference battery in the third step is within 1%, continue with the secondary evaluation.

[0017] The secondary evaluation method for battery performance includes a fourth step of testing the Fe content and / or Li content in the negative electrode sheets of the battery under test and the reference battery, and performing a secondary evaluation of the performance of the lithium iron phosphate battery based on the Fe content and / or Li content in the negative electrode sheets of the battery under test and the reference battery.

[0018] The specific steps of the fourth step are as follows:

[0019] 4.1) After discharging the battery under test and the reference battery in the same mode, dissect them to obtain the negative electrode sheets at the same position of the batteries;

[0020] 4.2) Put the negative electrode sheets into DMC solution for cleaning and then drying, scrape an appropriate amount of sample from the negative electrode sheets, place it in a clean beaker and weigh it to record the mass;

[0021] 4.3) After fully digesting the sample with acid, filter and make up the volume, and use ICP to obtain the Fe content and Li content in the negative electrode sheets of the battery under test and the reference battery; preferably, the acid mentioned above is hydrochloric acid or nitric acid or their mixed acid.

[0022] Evaluate based on the Fe content in the negative electrode sheet and analyze the reasons for battery capacity decay: If the Fe content in the negative electrode sheet of the battery under test is higher than that in the negative electrode sheet of the reference battery, it indicates that the dissolution degree of the positive electrode of the battery under test is relatively high, and the performance of the battery under test is inferior to that of the reference battery. Conversely, the performance of the battery under test is superior to that of the reference battery.

[0023] Evaluate based on the Li content in the negative electrode sheet and analyze the reasons for battery capacity decay: If the Li content in the negative electrode sheet of the battery under test is higher than that in the negative electrode sheet of the reference battery, it indicates that the side reaction degree on the negative electrode side of the battery under test is relatively high, and the consumption of active lithium is relatively high. The performance of the battery under test is inferior to that of the reference battery. Conversely, the performance of the battery under test is superior to that of the reference battery.

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] In this application, in-situ XRD tests were conducted on lithium iron phosphate batteries with different capacity retention rates. It was found that even when the battery capacity decayed and the battery was at 0% SOC, the characteristic peaks of the positive electrode material had already changed from 100% lithium iron phosphate (corresponding to a fresh battery) to a mixture of lithium iron phosphate and iron phosphate. This indicates that as the cycle progresses, the crystal structure of lithium iron phosphate has undergone an irreversible phase change due to slow over-discharging (micro overcharge), resulting in the decay of battery performance.

[0026] Based on this, this application first conducts a capacity test on a fresh battery to obtain the initial capacity of the battery. Secondly, an overcharge test is conducted on the battery. After the overcharge ends, a capacity test of the same format is conducted again, and the capacity retention rate of the battery at this time is calculated. By comparing the differences in the capacity retention rates, the differences in battery performance can be preliminarily determined.

[0027] Furthermore, during the overcharge process, the iron ions dissolved from the positive electrode migrate to the negative electrode through the electrolyte. The battery is disassembled, and the Fe content in the negative electrode sheet is tested using ICP, which can more accurately evaluate the battery performance. At the same time, the degree of side reactions triggered by the battery can be evaluated through the Li content in the negative electrode sheet.

[0028] To sum up, (1) the present invention's solution first proposes to use an overcharge test to evaluate the battery performance. The test method is simple, and the time required for battery performance evaluation is shortened by more than 10 times;

[0029] (2) The present invention's solution first proposes to use an electrochemical method to test the iron dissolution content in lithium iron phosphate materials. During overcharge, the dissolved Fe is transferred to the negative electrode, and the iron dissolution content in lithium iron phosphate materials is determined by testing the Fe content in the negative electrode sheet of the battery. The test results are true and reliable;

[0030] (3) The solution of the present invention is simple and reliable. By combining the test results of the macroscopic charge-discharge capacity and the microscopic element content, the performance evaluation is completed, shortening the product verification cycle, and the evaluation results are proven to be reliable.

[0031] (4) The solution of the present invention is based on the normal failure of lithium iron phosphate batteries, and can provide a verification method for the design and optimal combination of the battery's positive electrode material, negative electrode material, electrolyte, and battery N / P ratio. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a comparison diagram of the cycle performance of the battery to be tested and the reference battery in Example 1; DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] In order to enable those skilled in the art of the present technology to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the drawings and the best embodiments.

[0034] Example 1

[0035] In this embodiment, the active main materials of the positive electrode sheets of the battery to be tested and the reference battery are both lithium iron phosphate, and the active main materials of the negative electrode sheets are both artificial graphite. Now it is necessary to evaluate the performance of batteries using different types of artificial graphite.

[0036] A rapid evaluation method for battery performance includes the following steps:

[0037] The first step: Conduct an initial capacity test on the battery to be tested and the reference battery

[0038] With I 1 = 0.5C, perform 3 cycles of charge-discharge tests with the same system on the battery to be tested and the reference battery in the voltage range of 2.5 - 3.65V, and record the initial discharge capacity X 1 = 1.16Ah of the battery to be tested and the initial discharge capacity Y 1 = 1.15Ah of the reference battery.

[0039] The second step: Conduct an overcharge test on the battery to be tested and the reference battery

[0040] Step 2.1) With a current of I 2 = 0.5C, overcharge the battery to be tested and the reference battery from the open-circuit voltage to 5V respectively, then perform constant-voltage charging at 5V for 30 minutes and let it stand for 30 minutes; fully charge the battery to transfer all lithium ions and iron ions to the negative electrode;

[0041] Step 2.2) With a current of I 3 = 0.5C, discharge the battery, and the discharge cut-off voltage is 2.5V;

[0042] Step 3) Perform 3 - cycle charge - discharge tests on the battery under test and the reference battery with the same regime as in the first step at 0.5C, with the voltage range being 2.5 - 3.65V, and record the discharge capacities X 2 = 1.03Ah and Y 2 = 0.96Ah after over - charging the battery under test and the reference battery respectively, and calculate the capacity retention rate R 待测 = 1.03 / 1.16 = 88.79%, R 参比 = 0.96 / 1.15 = 83.48%.

[0043] Step 3: Battery performance evaluation

[0044] Taking the capacity retention rates of the battery under test and the reference battery after over - charging as the evaluation criteria, since the capacity retention rate of the battery under test (88.79%) is higher than that of the reference battery (83.48%), it indicates that the performance of the battery under test is better than that of the reference battery;

[0045] Take the battery under test and the reference battery made in the same batch for cyclic testing, and verify the above - mentioned evaluation results with the cyclic curve. From the cyclic test data for two months in the normal voltage range, it is found that the performance of the battery under test is better than that of the reference battery.

[0046] Figure 1 It is a comparison chart of the cyclic performance of the battery under test and the reference battery, and the results are consistent with the test method of this application.

[0047] Example 2

[0048] In this example, the active main material of the positive electrode plates of the battery under test and the reference battery is lithium iron phosphate, and the active main material of the negative electrode plates is artificial graphite. Now it is necessary to evaluate the battery performance of the positive electrode materials using different types of precursors.

[0049] The secondary evaluation method for battery performance includes the following steps:

[0050] Step 1: Perform initial capacity tests on the battery under test and the reference battery

[0051] At I 1 = 0.5C, perform 3 - cycle charge - discharge tests on the battery under test and the reference battery in the voltage range of 2.5 - 3.65V with the same regime, and record the initial discharge capacity X 1 = 1.80Ah of the battery under test and the initial discharge capacity Y 1 = 1.82Ah of the reference battery.

[0052] Step 2: Perform over - charge tests on the battery under test and the reference battery

[0053] Step 2.1) At I 2With a current of 0.5C, after overcharging the battery under test and the reference battery from the open-circuit voltage to 5V respectively, perform constant-voltage charging at 5V for 30 minutes and then let it stand for 30 minutes; fully charge the battery to transfer all lithium ions and iron ions to the negative electrode;

[0054] Step 2.2) With I 3 = 0.5C current, discharge the battery, and the discharge cut-off voltage is 2.5V;

[0055] Step 3) Perform 3 cycles of charge-discharge tests with the same regime as in the first step on the battery under test and the reference battery at 0.5C, the voltage range is 2.5 - 3.65V, and record the discharge capacities X 2 = 1.62Ah, Y 2 = 1.63Ah of the battery under test and the reference battery after overcharging respectively, and calculate the capacity retention rate R 待测 = 1.62 / 1.80 = 90%, R 参比 = 1.63 / 1.82 = 89.56%.

[0056] Third step: Battery performance evaluation

[0057] Taking the capacity retention rates of the battery under test and the reference battery after overcharging as the evaluation criteria, the capacity retention rate of the battery under test (90%) is slightly higher than that of the reference battery (89.56%), and the difference is within 1%. It is impossible to accurately determine whether the performance of the battery under test is superior to that of the reference battery;

[0058] Fourth step: Test the Fe content and Li content in the negative electrode sheets of the battery under test and the reference battery

[0059] Step 4.1) Perform constant-current discharge on the battery under test and the reference battery at 0.1C, the discharge cut-off voltage is 2.5V. After the discharge is completed, dissect the battery in a drying room to obtain the negative electrode sheets at the same position of the battery;

[0060] Step 4.2) Put the negative electrode sheet into DMC solution for cleaning and then drying. Scrap an appropriate amount of sample from the negative electrode sheet, place it in a clean beaker and weigh it to record the mass;

[0061] Step 4.3) Use hydrochloric acid or nitric acid or any other mixed acid, etc. to fully digest the sample, then filter and make up the volume, and use ICP to obtain the Fe content and Li content in the negative electrode sheets of the battery under test and the reference battery;

[0062] Step 4.4) Further, perform a secondary evaluation of the battery performance and analyze the reasons for the battery performance attenuation: Evaluating with the Fe content in the negative electrode sheet, the Fe content (142 ppm) in the negative electrode sheet of the battery under test is lower than the Fe content (191 ppm) in the negative electrode sheet of the reference battery, which indicates that the dissolution degree of the positive electrode of the battery under test is lower, and the performance of the battery under test is superior to that of the reference battery;

[0063] Step 5) Optionally, evaluate based on the Li content in the negative electrode sheet and analyze the reasons for the battery performance degradation: If the Li content (6502 ppm) in the negative electrode sheet of the battery under test is lower than the Li content (9409 ppm) in the negative electrode sheet of the reference battery, it indicates that the side reaction degree on the negative electrode side of the battery under test is lower, the consumption of active lithium is less, and the performance of the battery under test is better than that of the reference battery.

[0064] The results are shown in Table 1.

[0065] Table 1

[0066] Sample mass / g Fe content / ppm Li content / ppm Reference battery 0.401 191 9409 Battery to be measured 0.404 142 6502

[0067] In summary, (1) The present invention's solution first proposes to evaluate the battery performance using overcharge testing. The testing method is simple, and the time required for battery performance evaluation is shortened by more than 10 times.

[0068] (2) The present invention's solution first proposes to test the iron dissolution content in lithium iron phosphate materials using an electrochemical method. During overcharge, the dissolved Fe is transferred to the negative electrode, and the iron dissolution content in the lithium iron phosphate material is determined by testing the Fe content in the negative electrode sheet of the battery. The test results are true and reliable.

[0069] (3) The present invention's solution is simple and reliable. By combining the macroscopic charge-discharge capacity and the test results of microscopic element content, performance evaluation is completed, the product verification cycle is shortened, and the evaluation results are proven to be reliable.

[0070] (4) The present invention's solution is based on the normal failure of lithium iron phosphate batteries and can provide a verification method for the design and optimal combination of battery positive electrode materials, negative electrode materials, electrolytes, and battery N / P ratios.

[0071] The above is only the preferred embodiment 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 rapid evaluation of battery performance, characterized in that: The steps include: Step 1: Perform initial capacity test on the battery to be tested and the reference battery; Step 2: Perform an overcharge test on the battery to be tested and the reference battery to obtain the overcharged capacity of the battery to be tested and the reference battery; Step 3: Evaluate battery performance by measuring the capacity retention rate of the test battery and the reference battery.

2. The rapid evaluation method for battery performance according to claim 1, characterized in that: The specific steps of the first step are: The battery under test and the reference battery are subjected to the same charging and discharging test in the normal voltage range with the current I1, and the initial discharge capacity X1 of the battery under test and the initial discharge capacity Y1 of the reference battery are recorded; preferably, 0.1C≤I1≤1C.

3. The rapid evaluation method for battery performance according to claim 1, characterized in that: The specific steps of the second step are: 2.1) The test battery and the reference battery are charged to 4.9-5.3V with a constant current of I2 and a constant voltage of 0.5-1h, and the battery is fully charged so that all lithium ions and iron ions are transferred to the negative electrode; preferably, 0.5C≤I2≤1C; 2.2) Then, a constant current discharge is performed with a current I3, and the cut-off voltage is consistent with the cut-off voltage in the first step; 0.5C≤I3≤1C; 2.3) Perform charge and discharge tests on the battery under test and the reference battery in the normal voltage range with current I1, and record the discharge capacities X2 and Y2 of the battery under test and the reference battery after overcharging respectively.

4. The method for rapid evaluation of battery performance according to claim 1, characterized in that: The specific steps of the third step are: if the capacity retention rate of the battery under test is higher than that of the reference battery, it means that the performance of the battery under test is better than that of the reference battery; if the capacity retention rate of the battery under test is lower than that of the reference battery, it means that the performance of the battery under test is worse than that of the reference battery; Battery capacity retention rate = discharge capacity after overcharge / initial discharge capacity.

5. The method for rapid evaluation of battery performance according to claim 1, characterized in that: The battery is a lithium iron phosphate battery. Preferably, the normal voltage range of the lithium iron phosphate battery is 2.5-3.65V.

6. A secondary evaluation method for battery performance, characterized in that: A rapid evaluation method for lithium iron phosphate battery performance comprising the method described in any one of claims 1 to 5; when the capacity retention rate of the battery to be tested and the capacity retention rate of the reference battery in the third step are within 1%, a secondary evaluation is continued.

7. The secondary evaluation method of battery performance according to claim 6, characterized in that: The method comprises the fourth step of testing the Fe content and / or Li content in the negative electrode sheets of the test battery and the reference battery, and conducting a secondary evaluation on the performance of the lithium iron phosphate battery through the Fe content and / or Li content in the negative electrode sheets of the test battery and the reference battery.

8. The secondary evaluation method of battery performance according to claim 6, characterized in that: The specific steps of the fourth step are: 4.1) Perform the same discharge dissection on the test battery and the reference battery to obtain the negative electrode sheets at the same position of the battery; 4.2) Place the negative electrode sheet in the DMC solution for cleaning and drying, scrape an appropriate amount of sample from the negative electrode sheet, place it in a clean beaker and weigh it, and record the mass; 4.3) After fully digesting the sample with acid, filtering and fixing the volume, the Fe content and Li content in the negative electrode sheets of the test battery and the reference battery are obtained by ICP; preferably, the acid is hydrochloric acid or nitric acid or a mixed acid thereof.

9. The secondary evaluation method of battery performance according to claim 6, characterized in that: The evaluation is based on the Fe content in the negative electrode sheet. If the Fe content in the negative electrode sheet of the battery to be tested is higher than that in the negative electrode sheet of the reference battery, it means that the degree of dissolution of the positive electrode of the battery to be tested is higher and the performance of the battery to be tested is inferior to that of the reference battery. Otherwise, the performance of the battery to be tested is better than that of the reference battery.

10. The secondary evaluation method of battery performance according to claim 6, characterized in that: The Li content in the negative electrode sheet is evaluated, and the reasons for battery capacity attenuation are analyzed: if the Li content in the negative electrode sheet of the battery to be tested is higher than that in the negative electrode sheet of the reference battery, it means that the degree of side reactions on the negative electrode side of the battery to be tested is high, the consumption of active lithium is high, and the performance of the battery to be tested is inferior to that of the reference battery. Otherwise, the performance of the battery to be tested is better than that of the reference battery.

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