Method for evaluating internal resistance performance of lithium ion battery

Abstract

The application provides a method for evaluating internal resistance performance of a lithium ion battery. The method comprises the following steps: S1: performing a charge-discharge cycle test on the lithium ion battery to be tested, the charging process is constant-current constant-voltage charging, and the discharging process is constant-current discharging; and S2: evaluating the internal resistance performance of the lithium ion battery to be tested by taking the growth rate of the constant-voltage charging capacity ratio in the xth cycle as B x (B x =(A x -A1) / A1*100%, A x =C 2x / C 1x *100%). The application takes the change rate of the constant-voltage charging capacity ratio in the charging process of the lithium ion battery as the internal resistance growth rate of the material, can continuously monitor the internal resistance change in the cycle process of the battery, and can accurately judge the internal resistance growth rate of different materials. The method is simple and convenient. That is, the method provided by the application has the advantages of short period, low cost, simplicity and convenience, and is suitable for evaluating and screening multiple materials in the material research and development stage.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology and relates to a method for evaluating the internal resistance performance of lithium-ion batteries. Background Technology

[0002] Lithium-ion batteries possess advantages such as small size, high energy density, long cycle life, low self-discharge rate, and no memory effect, making them widely used in portable electronic devices, electric vehicles, and energy storage systems. With the increasing demands for driving range in new energy vehicles and the phasing out of government subsidies for these vehicles, improving the energy density of power batteries while reducing costs has become an inevitable trend. As the most crucial component of lithium-ion batteries, the cathode material largely determines the battery's energy density and cost; therefore, cathode materials are developing towards lower cobalt content and higher voltage.

[0003] Internal resistance is a key indicator for evaluating lithium-ion batteries, directly affecting their energy density, power performance, cycle performance, and safety characteristics. Especially for low-cobalt materials, the cobalt content affects the conductivity of the material. When the cobalt content decreases, the internal resistance of the material increases, and the internal resistance increases relatively quickly during cycling, which seriously affects battery performance. Therefore, characterizing the internal resistance and the rate of increase of internal resistance during cycling is crucial in the development of low-cobalt materials.

[0004] At present, the general test method for the internal resistance growth rate during the cycle is as follows: (1) Stop the test on the battery in the cycle and remove the battery. The online measurement of internal resistance usually has a large error and is greatly affected by the battery temperature, the duration of the measurement pulse current, and the size of the pulse current. Often, the online estimation conditions cannot be the same as the offline test method for the internal resistance of a new battery, so the online estimated internal resistance is not comparable to the internal resistance value of a new battery and cannot accurately measure the growth rate of internal resistance; (2) Use specific devices and conditions to perform DCR test on the battery; (3) Continue to perform charge and discharge cycle test on the battery, and repeat the cycle to obtain the growth rate of the battery's internal resistance. The method is complicated.

[0005] CN103018566A discloses a method for testing the DC internal resistance of a lithium-ion battery, comprising the following steps: Step 1: placing the lithium-ion battery at room temperature; Step 2: continuously discharging the battery with a first preset discharge current I1 within a first preset discharge time interval, and measuring the first discharge cutoff voltage U1 of the battery in real time after the discharge ends; Step 3: continuing to discharge the battery with a second preset discharge current I2 within a second preset discharge time interval, and measuring the second discharge cutoff voltage U2 of the battery in real time after the discharge ends; Step 4: calculating the DC internal resistance of the battery in real time according to a preset DC internal resistance calculation formula. However, directly measuring the internal resistance online results in a large error, making it difficult to accurately assess the rate of increase of internal resistance.

[0006] CN111525202A discloses a method, system, device, and medium for monitoring the discharge coefficient (DCR) of a lithium-ion battery during cycling. The monitoring method includes acquiring data during the charge-discharge cycles of the lithium-ion battery, including time and the voltage, temperature, and charge / discharge current values ​​of the lithium-ion battery at corresponding times during the charge / discharge process. It then extracts test data from several charge-discharge cycles, including voltage and current values ​​for a period at the start of charging and for a period before the start of discharging. The method calculates the discharge DCR and charge DCR using the DCR calculation formula. This patent application provides a better understanding of the changing trends of charge and discharge DCR during lithium-ion battery cycling, enabling early identification of cell performance abnormalities and saving time and testing costs. Research shows that battery internal resistance is affected by temperature and state of charge (SOC). Therefore, internal resistance testing should be conducted under the same SOC and temperature conditions. However, current DCR testing methods do not consider the influence of temperature on DCR measurement, leading to inaccurate DCR calculation results.

[0007] Therefore, there is an urgent need to develop a testing and evaluation method for the internal resistance growth rate during the development of lithium-ion battery materials, so as to accelerate the development progress and reduce the development cost. Summary of the Invention

[0008] The purpose of this invention is to provide a method for evaluating the internal resistance performance of lithium-ion batteries. This invention characterizes the internal resistance growth rate of a material by measuring the rate of change in the percentage of capacity charged at constant voltage during the charging process of a coin-type lithium-ion battery. It allows for continuous monitoring of internal resistance changes during battery cycling, and the evaluation results can accurately determine the internal resistance growth rate of different materials. Furthermore, the method is simple and convenient. In short, the method provided by this invention has a short cycle time, low cost, and is simple and convenient, making it suitable for evaluating and screening multiple materials during the material development stage.

[0009] To achieve this objective, the present invention adopts the following technical solution:

[0010] In a first aspect, the present invention provides a method for evaluating the internal resistance performance of a lithium-ion battery, the method comprising the following steps:

[0011] S1: Perform charge-discharge cycle tests on the lithium-ion battery under test. The charging process is constant current and constant voltage charging, and the discharging process is constant current discharging. The total capacity charged during the x-th charge-discharge cycle is taken as C. 1x The capacity during the xth charge-discharge cycle under constant voltage charging is C. 2x The number of cycles is at least 30 times, for example, 30 times, 33 times, 35 times, 38 times, 40 times, 43 times, 45 times, 48 ​​times, 50 times, 60 times, 70 times, 80 times, 90 times, or 100 times, etc.

[0012] S2: Let A be the percentage of capacity achieved during constant-voltage charging in the x-th charge-discharge cycle. x A x =C 2x / C 1x *100%;

[0013] S3: Taking the first charge-discharge process as the first cycle, and A1 as the comparison benchmark, the growth rate of the constant-voltage charging capacity percentage in the x-th cycle is B. x B x =(A x -A1) / A1*100%, the final B x Used to evaluate the internal resistance performance of coin-type lithium-ion batteries under test.

[0014] The constant current and constant voltage charging process refers to the following: In the first stage of charging, a constant current is used to charge the battery. As the charging process progresses, the voltage increases. Once the voltage reaches a set value, the process switches to the second stage of constant voltage charging. During constant voltage charging, the current gradually decreases until it drops to a set cutoff current, at which point charging ends and the battery is fully charged. When the battery is charged at a constant current until the voltage reaches the set value, this voltage is not the battery's true voltage. It is a falsely high voltage due to battery polarization. Therefore, constant voltage charging is used to reduce the charging current and improve the charging efficiency. Thus, the constant voltage charging process can also be understood as a process of removing battery polarization. The capacity percentage during constant voltage charging can characterize the magnitude of polarization, which is equivalent to internal resistance. As the cycling process progresses, the internal resistance of the material continuously increases, leading to a continuous increase in the capacity percentage charged during constant voltage charging. Therefore, the growth rate of the capacity percentage charged during cycling is calculated, and this growth rate is the internal resistance growth rate during cycling, which can be used to characterize the material's internal resistance performance.

[0015] This invention characterizes the internal resistance growth rate of a material by measuring the rate of change in the percentage of capacity charged at constant voltage during lithium-ion battery charging. It allows for continuous monitoring of internal resistance changes during battery cycling, and the evaluation results accurately determine the internal resistance growth rate of different materials. Furthermore, the method is simple and convenient. In short, the method provided by this invention is time-efficient, low-cost, and easy to use, making it suitable for evaluating and screening multiple materials during the material development stage.

[0016] In this invention, the final B x The results can characterize the internal resistance performance of materials in a battery. By evaluating batteries made of different materials, we can assess and select materials with superior performance, which is of great significance in the battery material research and development stage.

[0017] In this invention, too few cycles are not conducive to determining the growth law of internal resistance.

[0018] Preferably, the temperature in the charge-discharge cycle test is -20 to 60°C, such as -20°C, -15°C, -10°C, -5°C, 0°C, 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, or 60°C.

[0019] The evaluation method provided by this invention is applicable to batteries in different temperature environments, from low temperature to high temperature, and can achieve accurate testing and evaluation of internal resistance growth rate.

[0020] Preferably, the constant current and constant voltage charging process includes:

[0021] First, perform constant current charging to the cutoff voltage, and then perform constant voltage charging at the cutoff voltage.

[0022] Preferably, the cutoff voltage during the constant current charging process is 4.3 to 4.6V, such as 4.3V, 4.35V, 4.4V, 4.45V, 4.5V, 4.55V, or 4.6V.

[0023] Preferably, the charging current during the constant current charging process is 0.1 to 2C, such as 0.1C, 0.3C, 0.5C, 0.8C, 1C, 1.3C, 1.5C, 1.8C, or 2C.

[0024] Preferably, the cutoff current during the constant voltage charging process is 0.005 to 0.1C, such as 0.005C, 0.008C, 0.01C, 0.03C, 0.05C, 0.08C, or 0.1C.

[0025] Preferably, the discharge current during the constant current discharge process is 0.1 to 2C, such as 0.1C, 0.3C, 0.5C, 0.8C, 1C, 1.3C, 1.5C, 1.8C, or 2C.

[0026] Preferably, the discharge cutoff voltage during the constant current discharge process is 2.5 to 3V, such as 2.5V, 2.6V, 2.7V, 2.8V, 2.9V, or 3V.

[0027] Preferably, the device is left to stand after each constant current and constant voltage charging.

[0028] Preferably, the settling time is ≥60s, such as 60s, 70s, 80s, 90s, 100s, 110s, 120s, 130s, 140s or 150s.

[0029] Preferably, the system is left to stand after each constant current discharge.

[0030] Preferably, the settling time is ≥60s, such as 60s, 70s, 80s, 90s, 100s, 110s, 120s, 130s, 140s or 150s.

[0031] In this invention, the battery is left to stand after both the charging and discharging processes are completed. The purpose of this standing period is to allow the electrochemical reaction process inside the battery to reach a stable state. At the same time, when using high-rate charging and discharging, standing can also help dissipate heat. However, if the standing time is too short, it will not be conducive to the stability of the battery's internal state.

[0032] Preferably, the lithium-ion battery is a lithium-ion half-cell.

[0033] In existing technologies, full cells are generally used for internal resistance testing. However, full cell fabrication is time-consuming, requires a large amount of raw materials, and is costly, making it unsuitable for the evaluation and screening of cathode materials during the material development process. In contrast, half-cell fabrication is simpler, has a shorter cycle time, and is less expensive.

[0034] As a preferred technical solution, the evaluation method includes the following steps:

[0035] S1: The lithium-ion half-cell under test is subjected to charge-discharge cycle test at -20 to 60℃. The charging process is constant current and constant voltage charging, and the discharging process is constant current discharging. First, constant current charging is performed at a charging current of 0.1 to 2C until the cutoff voltage is 4.3 to 4.6V. Then, constant voltage charging is performed at the cutoff voltage until the cutoff current is 0.005 to 0.1C. After resting for ≥60s, constant current discharging is performed at a discharging current of 0.1 to 2C until the cutoff voltage is 2.5 to 3V. After resting for ≥60s, the next charge-discharge cycle process is carried out.

[0036] Let C be the total capacity charged during the xth charge-discharge cycle. 1x The capacity during the xth charge-discharge cycle under constant voltage charging is C. 2x The number of cycles is ≥30.

[0037] S2: Let A be the percentage of capacity achieved during constant-voltage charging in the x-th charge-discharge cycle. x A x =C 2x / C 1x *100%;

[0038] S3: Taking the first charge-discharge process as the first cycle, and A1 as the comparison benchmark, the growth rate of the constant-voltage charging capacity percentage in the x-th cycle is B. x B x =(A x -A1) / A1*100%, the final B x Used to evaluate the internal resistance performance of coin-type lithium-ion batteries under test.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] This invention characterizes the internal resistance growth rate of a material by measuring the rate of change in the percentage of capacity charged at constant voltage during lithium-ion battery charging. This allows for continuous monitoring of internal resistance changes during battery cycling, and the evaluation results accurately determine the internal resistance growth rate of different materials. This enables the accurate identification and screening of battery materials with superior performance, and the method is simple and convenient. Furthermore, using half-cells for evaluation further shortens the preparation cycle. In short, the method provided by this invention is short-cycle, low-cost, simple, and convenient, making it suitable for evaluating and screening multiple materials during the material development stage. Attached Figure Description

[0041] Figure 1 This is a line graph showing the growth rate of internal resistance of button cell 1 (material M1) and button cell 2 (material M2) in Example 1.

[0042] Figure 2 This is a dotted line graph showing the growth rate of internal resistance of button cell 1 (material M1) and button cell 2 (material M2) in Comparative Example 1. Detailed Implementation

[0043] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0044] In one specific embodiment, the present invention provides a method for evaluating the internal resistance performance of a lithium-ion battery, the testing method comprising:

[0045] S1: The lithium-ion half-cell under test is subjected to charge-discharge cycle test at -20 to 60℃. The charging process is constant current and constant voltage charging, and the discharging process is constant current discharging. First, constant current charging is performed at a charging current of 0.1 to 2C until the cutoff voltage is 4.3 to 4.6V. Then, constant voltage charging is performed at the cutoff voltage until the cutoff current is 0.005 to 0.1C. After resting for ≥60s, constant current discharging is performed at a discharging current of 0.1 to 2C until the cutoff voltage is 2.5 to 3V. After resting for ≥60s, the next charge-discharge cycle process is carried out.

[0046] Let C be the total capacity charged during the xth charge-discharge cycle. 1x The capacity during the xth charge-discharge cycle under constant voltage charging is C. 2x The number of cycles is ≥30.

[0047] S2: Let A be the percentage of capacity achieved during constant-voltage charging in the x-th charge-discharge cycle. x A x =C 2x / C1x *100%;

[0048] S3: Taking the first charge-discharge process as the first cycle, and A1 as the comparison benchmark, the growth rate of the constant-voltage charging capacity percentage in the x-th cycle is B. x B x =(A x -A1) / A1*100%, the final B x Used to evaluate the internal resistance performance of coin-type lithium-ion batteries under test.

[0049] Example 1

[0050] This embodiment evaluates the internal resistance performance of coin cell 1 (positive electrode material is M1) and coin cell 2 (positive electrode material is M2), based on the evaluation method provided in the specific implementation method (both are half-cells):

[0051] S1: Perform charge-discharge cycle tests on button cell 1 and button cell 2 respectively. The test process is as follows:

[0052] The test temperature was 45℃. The device was charged to 4.5V using a 1C constant current, and then charged to 4.5V using a constant voltage. The cutoff current for constant voltage charging was 0.05C. After resting for 120s, the device was discharged using a 1C constant current. The cutoff voltage for discharge was 3.0V. After resting for 120s, the total capacity during charging and the capacity during constant voltage charging were recorded in each cycle of the test. The test was repeated 50 times.

[0053] S2: A under different number of iterations x x =C x / C1*100% is shown in Table 1;

[0054] S3: B under different number of iterations x x =(A x -A1) / A1*100% is shown in Table 2, through B x The data results are used to evaluate the internal resistance performance of materials M1 and M2.

[0055] Table 1

[0056]

[0057] Table 2

[0058]

[0059] Combining the data results in Tables 1 and 2, it can be seen that the internal resistance growth rate of coin cell 1 during the cycle is significantly lower than that of coin cell 2, which indicates that the internal resistance performance of cathode material M1 is superior to that of cathode material M2.

[0060] at the same time, Figure 1 The diagram shows a dotted line graph illustrating the rate of increase in internal resistance for coin cell 1 (material M1) and coin cell 2 (material M2) in Example 1, in conjunction with... Figure 1 This indicates that the internal resistance growth rate in the coin cell 1 is not only lower, but its change process is also more gradual, which also indicates that the structure of the cathode material M1 is more stable.

[0061] Comparative Example 1

[0062] Comparative Example 1 uses the conventional DCR growth rate test method to evaluate the internal resistance performance of cathode materials M1 and M2, and the evaluation is conducted using a full cell.

[0063] S1: The initial DCR of full cells made of the two materials was tested using the Blue Electric Test System;

[0064] S2: Perform routine cycle performance tests on full batteries made of the two materials. The test method is as follows:

[0065] The test temperature was 45℃. The device was charged to 4.5V using a constant current of 1C, and then charged to 4.5V using a constant voltage. The cutoff current for constant voltage charging was 0.05C. After resting for 120s, the device was discharged using a constant current of 1C. The cutoff voltage for discharge was 3.0V. After resting for 120s, the device was cycled 200 times.

[0066] S3: Every 20 cycles, the battery is removed, and the DCR is tested using the Blue Charge testing system. The initial DCR is used as a baseline to obtain the DCR growth rate during the cycle. The data is as follows:

[0067] Table 3

[0068]

[0069]

[0070] According to Table 3 and Figure 2 The results show that the internal resistance growth rate of cathode material M1 is significantly lower than that of material M2, indicating that the internal resistance performance of cathode material M1 is superior to that of cathode material M2.

[0071] Figure 2 The diagram shows a dotted line graph illustrating the rate of increase in internal resistance for full cell 1 (material M1) and full cell 2 (material M2) in Comparative Example 1, combined with... Figure 1 This indicates that the internal resistance growth rate in the coin cell 1 is not only lower, but its change process is also more gradual, which also indicates that the structure of the cathode material M1 is more stable.

[0072] And from Table 2, Table 3, Figure 1 and Figure 2As can be seen from the results, the conclusions drawn by the conventional DCR test method are consistent with the evaluation method provided by this invention, indicating that the method of this invention can accurately determine the internal resistance growth rate during material cycling, thereby screening out battery materials with superior performance.

[0073] In summary, this invention characterizes the internal resistance growth rate of a material by measuring the rate of change in the percentage of capacity charged at constant voltage during lithium-ion battery charging. This allows for continuous monitoring of internal resistance changes during battery cycling, and the evaluation results accurately determine the internal resistance growth rate of different materials. This enables the accurate identification and screening of battery materials with superior performance, and the method is simple and convenient. Furthermore, using half-cells for evaluation further shortens the preparation cycle. In short, the method provided by this invention is short-cycle, low-cost, simple, and convenient, making it suitable for evaluating and screening multiple materials during the material development stage.

[0074] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for evaluating the internal resistance performance of a lithium-ion battery, characterized in that, The assessment method includes the following steps: S1: Perform charge-discharge cycle tests on the lithium-ion battery under test. The charging process is constant current and constant voltage charging, and the discharging process is constant current discharging. The total capacity charged during the x-th charge-discharge cycle is taken as C. 1x The capacity during the xth charge-discharge cycle under constant voltage charging is C. 2x The number of cycles is at least 30. S2: Let A be the percentage of capacity achieved during constant-voltage charging in the x-th charge-discharge cycle. x A x =C 2x / C 1x ×100%; S3: Taking the first charge-discharge process as the first cycle, and A1 as the comparison benchmark, the growth rate of the constant-voltage charging capacity percentage in the x-th cycle is B. x B x = (A x -A1) / A1×100%, the final B x Used to evaluate the internal resistance performance of coin-type lithium-ion batteries under test.

2. The method for evaluating the internal resistance performance of a lithium-ion battery according to claim 1, characterized in that, The temperature during the charge-discharge cycle test is -20~60℃.

3. The method for evaluating the internal resistance performance of a lithium-ion battery according to claim 1 or 2, characterized in that, The constant current and constant voltage charging process includes: First, perform constant current charging to the cutoff voltage, and then perform constant voltage charging at the cutoff voltage.

4. The method for evaluating the internal resistance performance of a lithium-ion battery according to claim 3, characterized in that, The cutoff voltage during the constant current charging process is 4.3~4.6V.

5. The method for evaluating the internal resistance performance of a lithium-ion battery according to claim 3, characterized in that, The charging current during the constant current charging process is 0.1~2C.

6. The method for evaluating the internal resistance performance of a lithium-ion battery according to claim 1, characterized in that, The cutoff current during the constant voltage charging process is 0.005~0.1C.

7. The method for evaluating the internal resistance performance of a lithium-ion battery according to claim 1, characterized in that, The discharge current during the constant current discharge process is 0.1~2C.

8. The method for evaluating the internal resistance performance of a lithium-ion battery according to claim 1, characterized in that, The discharge cutoff voltage during the constant current discharge process is 2.5~3V.

9. The method for evaluating the internal resistance performance of a lithium-ion battery according to claim 1, characterized in that, After each constant current and constant voltage charge, allow the device to stand still.

10. The method for evaluating the internal resistance performance of a lithium-ion battery according to claim 9, characterized in that, The settling time is ≥60s.

11. The method for evaluating the internal resistance performance of a lithium-ion battery according to claim 1, characterized in that, After each constant current discharge, allow the mixture to stand still.

12. The method for evaluating the internal resistance performance of a lithium-ion battery according to claim 11, characterized in that, The settling time is ≥60s.

13. The method for evaluating the internal resistance performance of a lithium-ion battery according to claim 1, characterized in that, The lithium-ion battery is a lithium-ion half-cell.

14. The method for evaluating the internal resistance performance of a lithium-ion battery according to claim 1, characterized in that, The assessment method includes the following steps: S1: The lithium-ion half-cell under test is subjected to charge-discharge cycle test at -20~60℃. The charging process is constant current and constant voltage charging, and the discharging process is constant current discharging. First, constant current charging is performed with a charging current of 0.1~2C until the cutoff voltage is 4.3~4.6V. Then, constant voltage charging is performed with the cutoff voltage until the cutoff current is 0.005~0.1C. After resting for ≥60s, constant current discharging is performed with a discharging current of 0.1~2C until the cutoff voltage is 2.5~3V. After resting for ≥60s, the next charge-discharge cycle process is carried out. Let C be the total capacity charged during the xth charge-discharge cycle. 1x The capacity during the xth charge-discharge cycle under constant voltage charging is C. 2x The number of cycles is ≥30. S2: Let A be the percentage of capacity achieved during constant-voltage charging in the x-th charge-discharge cycle. x A x =C 2x / C 1x ×100%; S3: Taking the first charge-discharge process as the first cycle, and A1 as the comparison benchmark, the growth rate of the constant-voltage charging capacity percentage in the x-th cycle is B. x B x = (A x -A1) / A1×100%, the final B x Used to evaluate the internal resistance performance of coin-type lithium-ion batteries under test.

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