Method for predicting cycle life of lithium iron phosphate battery

Abstract

The invention discloses a method for predicting the cycle life of a lithium iron phosphate battery. The method comprises the following steps: (1) testing the cycle performance of the lithium iron phosphate battery, establishing a relation curve of the cycle number and the capacity retention ratio, and solving a relational expression of the cycle number and the capacity retention ratio; (2) respectively carrying out cycle performance test on the lithium iron phosphate battery according to the set cycle number, carrying out empty electricity and full electricity treatment after the test is finished, and disassembling a positive pole piece and a negative pole piece; measuring the expansion rate of the negative pole piece, and establishing a relational expression between the capacity retention rate and the expansion rate of the negative pole piece; and 3) carrying out cycle performance test on the lithium iron phosphate battery to be predicted according to a set cycle number, then disassembling the lithium iron phosphate battery to be predicted, calculating the expansion rate of the negative pole piece, substituting the expansion rate into the relational expression in the step 2) for calculation to obtain a capacity retention ratio, and then substituting the capacity retention ratio into the relational expression in the step 1) for calculation to obtain the cycle life of the lithium iron phosphate battery to be predicted. The method can achieve the precise prediction of the service life of the lithium iron phosphate battery.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion battery cycle life testing and relates to a method for predicting the cycle life of a lithium iron phosphate battery. Background Art

[0002] Lithium batteries have the advantages of low cost, high energy density, and long cycle life, and are widely used in fixed, portable, and transportation fields. In recent years, with the widespread application of lithium-ion batteries in various fields, how to quickly evaluate the life of the battery, so as to judge the reliability of the battery and effectively control the use of the battery, has become a hot topic of research. Lithium iron phosphate batteries are not only becoming more and more popular in the automotive field, but also play a very important role in the field of energy storage, but a series of problems caused by their life are beginning to emerge. Life prediction technology plays an important role in accelerating the research and development of new materials, new structures, and new management systems for lithium-ion batteries, as well as the safe operation, predictive maintenance, and secondary use of lithium-ion batteries in practical applications. The significance of lithium-ion battery life prediction is that it can shorten the verification cycle of developers, save battery testing resources, and obtain a more reliable full life cycle life model.

[0003] Among the existing disclosed technical solutions, the methods for predicting the life of iron phosphate batteries are mostly based on the Kalman filter lithium-ion battery life prediction of data or the prediction model combining particle filtering with a mechanism model. In view of the shortcomings of the existing technical solutions such as cumbersome modeling process and complex models, how to objectively predict the cycle life of lithium-ion batteries remains a technical problem that needs to be solved urgently in this field. Summary of the invention

[0004] The purpose of the present invention is to provide a method for predicting the cycle life of a lithium iron phosphate battery. The present invention can solve the problem that the existing method for predicting the life of a lithium ion battery is complicated, time-consuming, and affected by many factors, resulting in the inability to objectively predict the cycle life of a lithium ion battery.

[0005] The present invention provides a method for predicting the cycle life of a lithium iron phosphate battery, comprising the following steps: 1) Select multiple lithium iron phosphate batteries with consistency, perform cycle performance tests, record the capacity retention rate of each lithium iron phosphate battery, establish a relationship curve between the number of cycles and the capacity retention rate, and find a relationship between the number of cycles and the capacity retention rate; 2) Selecting a number of consistent lithium iron phosphate batteries from the same batch, increasing the number of cycles of each lithium iron phosphate battery at a fixed gradient, performing cycle performance tests according to the set number of cycles, and recording the capacity retention rate of each lithium iron phosphate battery, performing empty and full-charge treatments on the tested lithium iron phosphate batteries, and disassembling the positive and negative electrode sheets; measuring the expansion rate of the negative electrode sheet, and establishing a relationship between the capacity retention rate and the expansion rate of the negative electrode sheet; 3) After the lithium iron phosphate battery to be predicted is subjected to a cycle performance test according to a set number of cycles, it is disassembled, and the expansion rate of the negative electrode plate is calculated. The capacity retention rate is substituted into the relationship between the capacity retention rate and the negative electrode plate expansion rate in step 2) to obtain the capacity retention rate, which is then substituted into the relationship between the number of cycles and the capacity retention rate in step 1) to obtain the cycle life of the lithium iron phosphate battery to be predicted.

[0006] In the method of the present invention, the change in the expansion rate of the negative electrode plate during the cycle of the lithium iron phosphate battery is used to predict the battery life through fitting calculation of the measured data after short-term cycling, which has better consistency with the actual test results, can achieve accurate prediction of the life of the lithium iron phosphate battery, and improve the reliability, safety and durability of the lithium iron phosphate battery.

[0007] In the above method for predicting the cycle life of a lithium iron phosphate battery, the number of the lithium iron phosphate batteries selected in step 1) can be 3 to 15, specifically 10.

[0008] In the above method for predicting the cycle life of a lithium iron phosphate battery, in step 2), the total cycle number of the lithium iron phosphate battery is 300 to 800 cycles, and the cycle number is 50 to 150 cycles as a gradient (specifically, 50, 100, 150 cycles). In the above method for predicting the cycle life of lithium iron phosphate batteries, the number of the lithium iron phosphate batteries selected in step 2) is 40 to 120.

[0009] In the above-mentioned method for predicting the cycle life of a lithium iron phosphate battery, the negative electrode sheet expansion rate measured in step 2) is obtained by measuring the thickness change of the negative electrode sheet of the lithium iron phosphate during the charging process or the discharging process.

[0010] In the present invention, the cycle number of the lithium iron phosphate battery to be predicted in step 3) is generally set to a multiple of 50.

[0011] In the above-mentioned method for predicting the cycle life of a lithium iron phosphate battery, the negative electrode plate of the lithium iron phosphate battery is selected from at least one of a graphite plate, a SiC plate, a SiO plate, and a Si and graphite composite plate.

[0012] The present invention has the following beneficial effects: The method of the present invention is simple and reliable, and can complete the life prediction work by measuring the number of cycles, the capacity retention rate of the lithium iron phosphate battery and the negative electrode plate expansion rate data fitting model, which greatly shortens the product verification cycle, and the results are accurate, reliable and stable. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention.

[0014] Figure 1 This is a flow chart of a method for predicting the life of a lithium iron phosphate battery according to an embodiment of the present invention; Figure 2 is a fitting relationship curve diagram between the number of cycles and the capacity retention rate in the present invention; Figure 3 is a fitting relationship curve diagram between the expansion rate of the negative electrode plate and the capacity retention rate in the present invention; Figure 4 It is a curve diagram of the actual test relationship between the capacity retention rate and the number of cycles of batteries in the same batch as the battery to be predicted in the present invention. DETAILED DESCRIPTION

[0015] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0016] Unless otherwise specified, the materials and reagents used in the following examples can be obtained from commercial sources.

[0017] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0018] The cycle life of a lithium iron phosphate battery refers to the number of times the battery can be charged and discharged before its capacity drops to a certain level. Generally speaking, if the battery capacity is only 80% of the rated capacity, it means that the cycle life of the lithium iron phosphate battery has ended, but the actual life depends on the charging / discharging conditions.

[0019] Figure 1 A method for predicting the cycle life of a lithium iron phosphate battery according to an embodiment of the present invention is shown, comprising the following steps: The first step is to select multiple lithium iron phosphate batteries with good consistency, conduct cycle performance tests on lithium-ion batteries, record the capacity retention rate of each battery, establish a relationship curve between the number of cycles and the capacity retention rate, and find the relationship between the number of cycles and the capacity retention rate; The second step is to select several lithium iron phosphate batteries with good consistency from the same batch, increase the number of cycles of each lithium battery at a fixed gradient, conduct cycle performance tests on the lithium-ion batteries according to the set number of cycles, and record the capacity retention rate of each battery. The tested lithium iron phosphate batteries are empty and fully charged, and the positive and negative pole pieces are disassembled; the expansion rate of the negative pole piece is measured, and a relationship between the capacity retention rate and the expansion rate of the negative pole piece is established; The third step is to disassemble the lithium iron phosphate battery to be predicted after a certain number of cycles, calculate the expansion rate of the negative electrode plate, substitute it into the relationship between the capacity retention rate and the negative electrode plate expansion rate, calculate the capacity retention rate and then substitute it into the relationship between the number of cycles and the capacity retention rate, and the cycle life of the lithium-ion battery can be calculated.

[0020] Example 1 The selected lithium-ion battery has a charge and discharge mode of 1P constant power charge and discharge, and a charge voltage cutoff voltage of 2.5-3.65V.

[0021] As an embodiment of the present invention, a method for predicting the cycle life of a lithium iron phosphate battery based on the expansion rate of a negative electrode plate, the specific steps of completing the life prediction are as follows: a. Select 100 lithium-ion batteries with good consistency, increase the number of cycles by 50 cycles, and test the cycle performance of the lithium-ion batteries according to the set number of cycles. Record the capacity retention rate of each lithium-ion battery, establish the relationship curve between the number of cycles and the capacity retention rate, and find the relationship between the capacity retention rate and the number of cycles: Y1=-0.000028X+0.9755054, such as Figure 2 Shown b. After the test, the lithium iron phosphate battery with the same number of cycles is discharged and fully charged, and the electrode pieces are disassembled. The thickness is measured with a laser thickness gauge, and the expansion rate is calculated as α = L2 (full-charge negative electrode thickness) / L1 (empty negative electrode thickness).

[0022] c. Establish a relationship curve between the calculated negative electrode expansion rate and the capacity retention rate of the lithium-ion battery, and find the relationship between the negative electrode expansion rate and the capacity retention rate: Y2 = -0.744158α + 1.718993. Figure 3 shown.

[0023] d. After the lithium-ion battery to be predicted has been cycled for 6112 cycles, the expansion rate of the negative electrode sheet is set to 123.51% according to step b. The capacity retention rate is substituted into the relationship between the capacity retention rate and the negative electrode sheet expansion rate Y2, and the capacity retention rate is calculated to be 79.9883%. The calculated capacity retention rate is substituted into the relationship between the number of cycles and the capacity retention rate Y1, and the cycle life of the lithium iron phosphate battery is calculated to be 6098 weeks. Figure 4The figure shows the fitting curve of the actual test cycle life and capacity retention rate of the same batch of lithium-ion batteries to be predicted. The capacity retention rate is cut off at 80%, and the actual test cycle life is 6112 weeks, which is only 14 weeks different from the calculated cycle life, with an error of 0.23%. It can be seen that the cycle life predicted by the present invention is very close to the actual test result, and has obvious beneficial effects.

[0024] As described above, on the one hand, the present invention can predict the battery life by fitting the measured data after short-term cycling, which has better consistency with the actual test results, and realizes accurate prediction of the life of lithium iron phosphate batteries. On the other hand, the present invention is simple and easy to implement, greatly shortens the product verification cycle, and the results are proven to be reliable and the model is robust. Although the present invention has been disclosed as above with a preferred embodiment, it is not used to limit the present invention. Any technician familiar with this profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical content disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of the technical solution of the present invention.

Claims

1. A method for predicting the cycle life of a lithium iron phosphate battery, characterized in that: The steps include: 1) Select multiple lithium iron phosphate batteries with consistency, perform cycle performance tests, record the capacity retention rate of each lithium iron phosphate battery, establish a relationship curve between the number of cycles and the capacity retention rate, and find a relationship between the number of cycles and the capacity retention rate; 2) Selecting a number of consistent lithium iron phosphate batteries from the same batch, increasing the number of cycles of each lithium iron phosphate battery at a fixed gradient, performing cycle performance tests according to the set number of cycles, and recording the capacity retention rate of each lithium iron phosphate battery, performing empty and full-charge treatments on the tested lithium iron phosphate batteries, and disassembling the positive and negative electrode sheets; measuring the expansion rate of the negative electrode sheet, and establishing a relationship between the capacity retention rate and the expansion rate of the negative electrode sheet; 3) After the lithium iron phosphate battery to be predicted is subjected to a cycle performance test according to a set number of cycles, it is disassembled, and the expansion rate of the negative electrode plate is calculated. The capacity retention rate is substituted into the relationship between the capacity retention rate and the negative electrode plate expansion rate in step 2) to obtain the capacity retention rate, which is then substituted into the relationship between the number of cycles and the capacity retention rate in step 1) to obtain the cycle life of the lithium iron phosphate battery to be predicted.

2. The method for predicting the cycle life of a lithium iron phosphate battery according to claim 1, characterized in that: In step 1), the number of the lithium iron phosphate batteries selected is 3 to 15.

3. The method for predicting the cycle life of a lithium iron phosphate battery according to claim 1, characterized in that: In step 2), the total cycle number of the lithium iron phosphate battery is 300 to 800 cycles, with a gradient of 50 to 150 cycles.

4. The method for predicting the cycle life of a lithium iron phosphate battery according to claim 1 or 3, characterized in that: The number of the lithium iron phosphate batteries selected in step 2) is 40 to 120.

5. The method for predicting the cycle life of a lithium iron phosphate battery according to claim 1 or 3, characterized in that: The negative electrode sheet expansion rate measured in step 2) is obtained by measuring the thickness change of the negative electrode sheet expansion rate of the lithium iron phosphate sheet during the charging process or the discharging process.

6. The method for predicting the cycle life of a lithium iron phosphate battery according to claim 1, characterized in that: The negative electrode plate of the lithium iron phosphate battery is selected from at least one of a graphite plate, a SiC plate, a SiO plate, and a Si and graphite composite plate.

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