A method for quickly evaluating the cycle performance of lithium-ion power batteries

Abstract

The present invention discloses a method for quickly evaluating the cycle performance of lithium-ion power batteries, belonging to the technical field of lithium batteries. The battery to be tested is first activated by charging and discharging at a small current of 0.1-0.2C for 2-3 weeks at room temperature, and then subjected to cycle aging. The cycle conditions are: the temperature is 30-45°C, 0-100% SOC, constant current charge and discharge, the charge and discharge current is 0.8C-2.0C, and the charge and discharge interval time is 15-30 minutes, so that the battery reaches an electrochemical stable state. During the above cycle, a symmetric pulse charge and discharge is applied synchronously to measure the pseudo-cycle life of the lithium-ion battery. The present invention quickly measures the "pseudo-cycle life" of the lithium-ion battery by using symmetric pulse charge and discharge, and uses conventional charge and discharge equipment to quickly and quantitatively evaluate the cycle performance of the battery in about 10 days. The operation cost is low and no additional equipment is required, providing strong technical support for accelerating product development.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and specifically to a method for rapidly evaluating the cycle performance of lithium-ion power batteries. Background Art

[0002] Rapidly evaluating the cycle performance of lithium-ion power batteries is a technical challenge. In particular, the trade-off between long-term cycle performance and high energy density remains one of the technical difficulties of current new lithium-ion battery products. Therefore, it is very necessary to develop an effective test method to evaluate the cycle life, especially the starting point at which the performance of lithium-ion batteries significantly decays. Currently, the methods mainly used include long-term cycle testing, thermal analysis, or differential capacitance method, etc. These methods have a long evaluation period or require special equipment, which prolongs the product development cycle and has high operating costs, and is not conducive to the rapid development of new products. For example, the patent with the application number CN201710495131.5 proposes a method for rapidly predicting the cycle life of lithium-ion batteries. The empty-state batteries after different cycle numbers are disassembled, the electrode powder on the negative electrode sheet is scraped off, and the graphitization degree of the graphite negative electrode material is tested by the XRD internal standard method; fitting calculations are performed based on the three test data of the cycle number, capacity retention rate, and graphitization degree, so as to predict the cycle life of the battery.

[0003] The cycle life is crucial for the performance evaluation of lithium-ion batteries. The existing technical solutions have a long evaluation period, cumbersome steps, expensive detection equipment required, and high operating costs, which are not suitable for the rapid development of new lithium-ion battery products and have certain technical limitations. Summary of the Invention

[0004] Aiming at the defects of the existing technical solutions, the present invention proposes a method for rapidly evaluating the cycle performance of lithium-ion power batteries. By using symmetric pulse charge and discharge, the "pseudo-cycle life" of lithium-ion batteries is rapidly measured. With conventional charge and discharge equipment, the rapid and quantitative evaluation of the battery cycle performance can be achieved in about 10 days, with low operating costs and no need to use additional equipment, providing strong technical support for accelerating product development.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A method for rapidly evaluating the cycle performance of lithium-ion power batteries. The battery to be tested is first activated at room temperature by charging and discharging with a small current of 0.1 - 0.2C for 2 - 3 weeks, and then cycle aging is carried out. The cycle conditions are: the temperature is 30 - 45°C, 0 - 100% SOC, constant current charge and discharge, the charge and discharge current is 0.8C - 2.0C, and the charge and discharge interval time is 15 - 30 minutes, so that the battery reaches an electrochemical stable state. During the above cycle, symmetric pulse charge and discharge is synchronously applied to measure the pseudo-cycle life of the lithium-ion battery.

[0007] As a further technical solution of the present invention, the charging process is pulse charging for 4 - 8 s at different SOCs, and the pulse current is 3 - 5C.

[0008] As a further technical solution of the present invention, the discharging process is pulse discharging for 4 - 10 s at different SOCs, and the pulse current is 3 - 5C.

[0009] As a further technical solution of the present invention, at the same SOC during the charge - discharge process, the pulse time and pulse current need to be kept consistent respectively to ensure that the charged and discharged capacities are the same, that is, the net pulse capacity is 0 Ah.

[0010] As a further technical solution of the present invention, to avoid safety accidents caused by lithium plating during high - current charging, the impedance of the lithium - ion battery to be measured is monitored by the alternating current impedance method (EIS) during the cycling process. The frequency range is 0.1 HZ - 1000 HZ, and the applied voltage amplitude is � mV.

[0011] As a further technical solution of the present invention, if within the first 10 weeks of cycling, the battery impedance suddenly drops by more than 15% and rebounds during the subsequent charge - discharge interval time, then both the cycling charge - discharge current and the pulse current are adjusted downwards until the above - mentioned sudden impedance drop no longer occurs in the battery.

[0012] As a further technical solution of the present invention, the charge - discharge pulse conditions are the same at the same SOC, and the net charged capacity of the battery is 0 Ah. If no side reaction occurs inside the battery at this time, the error between the discharge capacity in the nth week and the discharge capacity of the control group without pulse charge - discharge is within ±0.3%. If a side reaction occurs, the charged capacity part will be consumed in the side reaction, resulting in a decrease in the discharge capacity, and the decrease percentage > 0.3%. Then the measured pseudo - cycle life of the lithium - ion battery is confirmed as n weeks. At this time, the side reaction inside the battery causes the cycling performance to start to decrease significantly, which is considered the starting point of lithium - ion battery aging.

[0013] Compared with the prior art, the beneficial effects of the present invention are:

[0014] The present invention uses symmetric pulse charge - discharge to quickly measure the "pseudo - cycle life" of lithium - ion batteries. Using conventional charge - discharge equipment, it can quickly and quantitatively evaluate the cycling performance of batteries in about 10 days. The operation cost is low, and no additional equipment is required, providing strong technical support for accelerating product development. Description of the Drawings

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the accompanying drawings required for the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on these drawings.

[0016] Figure 1 It is a relationship diagram of the "false cycle life" and the actual cycle life of a lithium-ion battery.

[0017] Figure 2 It is a comparison data diagram of pulse current and pulse time under different SOCs during the charge and discharge process.

[0018] Figure 3 It is a comparison data diagram of the cycle life test of a lithium-ion battery. Specific Embodiments

[0019] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0020] Embodiment 1: Please refer to Figures 1-3 As shown, a method for quickly evaluating the cycle performance of a lithium-ion power battery. The lithium-ion battery used is a square metal case, cylindrical or square soft pack, and uses a stacked or wound structure. The positive electrode active material is one or several of high-nickel layered transition metal oxides NCM523, NCM622 or NCM811, and the negative electrode is one or several of silicon-carbon, graphite, soft carbon, hard carbon or silicon oxide. The lithium-ion battery used is in a fresh state without any aging, and the time from the end of self-capacitance to the test and evaluation is no more than 60 days.

[0021] The test method is as follows: All batteries to be tested are first activated by charging and discharging at a small current of 0.1-0.2C for 2-3 weeks at room temperature, and then cycle aging is carried out. The cycle conditions are: the temperature is 30°C, 0-100% SOC, constant current charge and discharge, the charge and discharge current is 0.8C, and the charge and discharge interval time is 15 minutes, so that the battery reaches an electrochemically stable state. During the above cycle, symmetric pulse charge and discharge are applied synchronously to measure the "false cycle life" of the lithium-ion battery.

[0022] The charging process is pulsed charging for 4 - 8 s at different SOCs with a pulsed current of 3 - 5C; the subsequent discharging process is pulsed discharging for 4 - 10 s at different SOCs with a pulsed current of 3 - 5C. In the highest and lowest SOC ranges, both the pulsed current and the pulse time are relatively small to avoid additional electrochemical polarization of the lithium-ion battery in this region and reduce the accuracy of the measurement results. At the same SOC during the charge and discharge processes, the pulse time and the pulsed current need to be kept consistent respectively to ensure that the charged and discharged capacities are the same, that is, the net pulse capacity is 0 Ah. The specific operation is as Figure 2 shown.

[0023] Example 2, a method for rapidly evaluating the cycle performance of lithium-ion power batteries, the test method is as follows: All the batteries to be tested are first activated by charging and discharging at a small current of 0.2C for 2 - 3 weeks at room temperature, and then cycle aging is carried out. The cycle conditions are: the temperature is 45°C, 0 - 100% SOC, constant current charge and discharge, the charge and discharge current is 0.8C - 2.0C, and the charge and discharge interval time is 15 - 30 minutes to make the battery reach the electrochemical stable state. Symmetric pulsed charge and discharge are applied synchronously during the above cycle to measure the "pseudo cycle life" of the lithium-ion battery.

[0024] The charging process is pulsed charging for 4 - 8 s at different SOCs with a pulsed current of 3 - 5C; the subsequent discharging process is pulsed discharging for 4 - 10 s at different SOCs with a pulsed current of 3 - 5C. In the highest and lowest SOC ranges, both the pulsed current and the pulse time are relatively small to avoid additional electrochemical polarization of the lithium-ion battery in this region and reduce the accuracy of the measurement results. At the same SOC during the charge and discharge processes, the pulse time and the pulsed current need to be kept consistent respectively to ensure that the charged and discharged capacities are the same, that is, the net pulse capacity is 0 Ah. The specific operation is as Figure 2 shown.

[0025] Example 3, a method for rapidly evaluating the cycle performance of lithium-ion power batteries, the test method is as follows: All the batteries to be tested are first activated by charging and discharging at a small current of 0.1C for 2 - 3 weeks at room temperature, and then cycle aging is carried out. The cycle conditions are: the temperature is 40°C, 0 - 100% SOC, constant current charge and discharge, the charge and discharge current is 1C, and the charge and discharge interval time is 25 minutes to make the battery reach the electrochemical stable state. Symmetric pulsed charge and discharge are applied synchronously during the above cycle to measure the "pseudo cycle life" of the lithium-ion battery.

[0026] The charging process is pulsed charging for 4 - 8 s at different SOCs with a pulsed current of 3 - 5C; the subsequent discharging process is pulsed discharging for 4 - 10 s at different SOCs with a pulsed current of 3 - 5C. In the highest and lowest SOC ranges, both the pulsed current and the pulse time are relatively small to avoid additional electrochemical polarization of the lithium-ion battery in this area and reduce the accuracy of the measurement results. At the same SOC during the charge and discharge processes, the pulse time and the pulsed current need to be kept consistent respectively to ensure that the charged and discharged capacities are the same, that is, the net pulse capacity is 0 Ah. The specific operation is as Figure 2 shown.

[0027] Example 4, a method for quickly evaluating the cycle performance of lithium-ion power batteries, the test method is as follows: all batteries to be tested are first activated by charging and discharging at a small current of 0.15C for 2 - 3 weeks at room temperature, and then cycle aging is carried out. The cycle conditions are: the temperature is 35°C, 0 - 100% SOC, constant current charge and discharge, the charge and discharge current is 1.5C, and the charge and discharge interval time is 22 minutes to make the battery reach the electrochemical stable state. During the above cycle, symmetric pulsed charge and discharge are applied synchronously to measure the "false cycle life" of the lithium-ion battery.

[0028] The charging process is pulsed charging for 4 - 8 s at different SOCs with a pulsed current of 3 - 5C; the subsequent discharging process is pulsed discharging for 4 - 10 s at different SOCs with a pulsed current of 3 - 5C. In the highest and lowest SOC ranges, both the pulsed current and the pulse time are relatively small to avoid additional electrochemical polarization of the lithium-ion battery in this area and reduce the accuracy of the measurement results. At the same SOC during the charge and discharge processes, the pulse time and the pulsed current need to be kept consistent respectively to ensure that the charged and discharged capacities are the same, that is, the net pulse capacity is 0 Ah. The specific operation is as Figure 2 shown.

[0029] To avoid safety accidents caused by lithium plating during high-current charging, the impedance of the lithium-ion battery to be tested is monitored by the electrochemical impedance spectroscopy (EIS) method during the cycle, the frequency range is 0.1 HZ - 1000 HZ, and the applied voltage amplitude is 5 mV. If within the first 10 cycles, the battery impedance suddenly drops by more than 15% and rebounds during the subsequent charge and discharge intervals, then both the cycle charge and discharge current and the pulsed current are adjusted downward until the above sudden impedance drop no longer occurs in the battery.

[0030] As the aging process intensifies, the performance and microstructure of lithium-ion batteries gradually deteriorate. Under the same conditions, the likelihood of lithium plating and side reactions increases. In this method, the charge-discharge pulse conditions are the same at the same state of charge (SOC), and the net charge capacity of the battery is 0 Ah. If no side reactions occur inside the battery at this time, the discharge capacity in the nth week should be similar to that of the control group without pulsed charge-discharge (within ±0.3%). If side reactions occur, the charged capacity will be consumed in the side reactions, resulting in a decrease in the discharge capacity (the percentage decrease > 0.3%). Then, the "pseudo cycle life" of the measured lithium-ion battery is confirmed as n weeks. At this time, the side reactions inside the battery cause the cycle performance to start decreasing significantly, which is also considered the starting point of lithium-ion battery aging. Considering the accuracy of the charge-discharge equipment and the consistency of the lithium-ion batteries to be tested, with ±0.3% as the control standard, the test results are as Figure 3 shown. The "pseudo cycle life" of the 5 experimental batteries was confirmed within 10 days, significantly shortening the evaluation period. Moreover, there is an approximately linear relationship between the "pseudo cycle life" and the actual cycle life. The longer the "pseudo cycle life", the better the corresponding actual cycle performance, as Figure 1 shown. This result indicates that the method proposed in the present invention can confirm the "pseudo cycle life" of lithium-ion batteries and the starting point of lithium-ion battery cycle aging in a short time, realizing the quantitative evaluation of the cycle performance of lithium-ion batteries, which is beneficial to timely adjusting the development plan, saving costs, accelerating the development of new products, and improving the economic benefits of the company.

[0031] The present invention uses symmetric pulse charge-discharge to quickly measure the "pseudo cycle life" of lithium-ion batteries. Using conventional charge-discharge equipment, it can quickly and quantitatively evaluate the cycle performance of the battery in about 10 days, with low operating costs and no need to use additional equipment, providing strong technical support for accelerating product development.

[0032] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be included in the present invention. Any reference signs in the claims should not be regarded as limiting the claimed rights.

[0033] In addition, it should be understood that although this specification is described in terms of embodiments, not every embodiment contains only one independent technical solution. This narrative way of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for quickly evaluating the cycle performance of lithium-ion power batteries, characterized in that, The battery under test is first activated by charging and discharging at a small current of 0.1-0.2C for 2-3 weeks at room temperature, and then cycle-aged. The cycling conditions are: temperature is 30-45°C, 0-100% SOC, constant current charge and discharge, charge and discharge current is 0.8C-2.0C, charge and discharge interval time is 15-30 minutes, so that the battery reaches an electrochemically stable state. During the above cycling, symmetric pulse charge and discharge are applied synchronously to measure the pseudo-cycle life of the lithium-ion battery; During the charge and discharge process at the same SOC, the pulse time and pulse current need to be kept consistent respectively to ensure that the charged and discharged capacities are the same, that is, the net pulse capacity is 0 Ah.

2. The method for rapidly evaluating the cycle performance of a lithium-ion power battery according to claim 1, characterized in that, The charging process is pulse charging for 4-8 s at different SOCs, and the pulse current is 3-5C.

3. The method for rapidly evaluating the cycle performance of a lithium-ion power battery according to claim 2, characterized in that, The discharging process is pulse discharging for 4-10 s at different SOCs, and the pulse current is 3-5C.

4. A method for quickly evaluating the cycle performance of a lithium-ion power battery according to claim 1, characterized in that, To avoid safety accidents caused by lithium plating during large current charging, the impedance of the lithium-ion battery under test is monitored by the AC impedance method EIS during the cycling process. The frequency range is 0.1 HZ-1000 HZ, and the applied voltage amplitude is 5 mV.

5. A method for quickly evaluating the cycle performance of a lithium-ion power battery according to claim 4, characterized in that, If within the first 10 weeks of cycling, the battery impedance suddenly drops by more than 15% and rebounds during the subsequent charge and discharge intervals, then both the cycling charge and discharge current and the pulse current are lowered until the above sudden impedance drop no longer occurs in the battery.

6. The method for rapidly evaluating the cycle performance of a lithium-ion power battery according to claim 4, characterized in that, Under the same SOC, the charge and discharge pulse conditions are the same, and the net charged capacity of the battery is 0 Ah. If no side reactions occur inside the battery at this time, the error between the discharge capacity in the nth week and the discharge capacity of the control group without pulse charge and discharge is within ±0.3%. If side reactions occur, the charged capacity part will be consumed in the side reactions, resulting in a decrease in the discharge capacity, and the decrease percentage > 0.3%. Then the pseudo-cycle life of the measured lithium-ion battery is confirmed as n weeks. At this time, the side reactions inside the battery cause the cycling performance to start to decrease significantly, which is considered the starting point of lithium-ion battery aging.

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