Method for evaluating cycle performance of battery cell

Abstract

The invention provides a method for evaluating the cycle performance of a battery cell. Specifically, the evaluation method comprises the following steps: based on a preset cyclic charging and discharging method, carrying out cyclic charging and discharging on a battery cell, and testing the discharge capacities of a first cycle and at least one target cycle; according to the discharge capacity of the first cycle and the discharge capacity of any target cycle, determining a cell capacity retention rate corresponding to the target cycle; wherein the cyclic charging and discharging method comprises an acceleration method, and the acceleration method comprises the following steps: continuously discharging for a preset duration under a preset voltage; wherein the preset voltage is smaller than the minimum working voltage of the battery cell. By controlling the continuous discharge preset duration of the battery cell under the preset voltage, the capacity loss is accelerated, and the capacity distinction degree in unit time is improved, so that the cycle capacity retention ratio difference of different design battery cells is identified in a shorter time, and the iteration period is shortened.

Description

Technical Field

[0001] The present disclosure relates to the field of battery technology, and in particular to a method for evaluating the cycle performance of a battery cell. Background Art

[0002] With the expansion of the energy storage market and the continuous increase in demand, the market's requirements for energy storage cells are also constantly increasing. Energy storage cells generally use lithium iron phosphate batteries, which have extremely high requirements for cycle performance, generally requiring a service life of 20 to 30 years, and the number of cycles in the life cycle may reach tens of thousands of times. In the battery development process, the industry often measures the cycle life of the battery by testing the cycle capacity retention rate of the battery cell. However, since energy storage cells have extremely high requirements for cycle life, the evaluation of cycle performance often requires a long test cycle, and the test time can be as long as several years. In this process, by comparing the capacity recovery rate of cells of different designs in the same number of cycles, the advantages and disadvantages of different designs can be evaluated, thereby optimizing the battery performance. Summary of the invention

[0003] In view of this, the purpose of the present disclosure is to provide an efficient method for evaluating the cycle performance of a battery cell.

[0004] Based on the above objectives, the present disclosure provides a method for evaluating the cycle performance of a battery cell, comprising:

[0005] Based on a preset cyclic charge and discharge method, the battery cell is cyclically charged and discharged and the discharge capacity of the first cycle and at least one target cycle is tested;

[0006] According to the discharge capacity of the first cycle and the discharge capacity of any target cycle, the cell capacity retention rate corresponding to the target cycle is determined; wherein,

[0007] The cyclic charge and discharge method includes an acceleration method, and the acceleration method includes: continuously discharging for a preset time at a preset voltage; wherein the preset voltage is less than the minimum operating voltage of the battery cell.

[0008] In some embodiments, the acceleration method further includes: discharging the battery cell from a fully charged state to a preset voltage in a constant current discharge manner.

[0009] In some embodiments, the difference between the minimum operating voltage and the preset voltage is ≥0.3V.

[0010] In some embodiments, the acceleration method is applied to at least one cycle and the cycle executes the acceleration method at least once.

[0011] In some embodiments, the acceleration method is executed no more than 5 times in any loop.

[0012] In some embodiments, the preset duration is 5 to 30 minutes.

[0013] In some embodiments, the positive electrode material of the battery cell is lithium iron phosphate material; and the preset voltage is 1.5V to 2.2V.

[0014] In some embodiments, the cyclic charge and discharge method further comprises a cyclic method;

[0015] The cycle method comprises: discharging the battery cell from a fully charged state to a minimum operating voltage in a constant current discharge manner.

[0016] In some embodiments, the testing of the discharge capacity of the first cycle and at least one target cycle is performed after the battery cell is discharged from a fully charged state to a minimum operating voltage.

[0017] In some embodiments, the cycling method and the acceleration method further include a charging process; the charging process includes constant current charging and constant voltage charging.

[0018] From the above, it can be seen that the present disclosure provides a method for evaluating the cycle performance of a battery cell, which controls the battery cell to discharge continuously for a preset time at a preset voltage; wherein the preset voltage is less than the minimum operating voltage of the battery cell, thereby accelerating capacity loss and improving capacity differentiation per unit time, thereby accurately identifying the difference in cycle capacity retention rates of battery cells of different designs in a shorter time and shortening the iteration cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the present disclosure or related technologies, the drawings required for use in the embodiments or related technical descriptions are briefly introduced below. Obviously, the drawings described below are only embodiments of the present disclosure. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0020] Figure 1 A flow chart showing a method for evaluating the cycle performance of a battery cell provided in an embodiment of the present disclosure is shown. DETAILED DESCRIPTION

[0021] In order to make the objectives, technical solutions and advantages of the present disclosure more clearly understood, the present disclosure is further described in detail below in combination with specific embodiments and with reference to the accompanying drawings.

[0022] It should be noted that, unless otherwise defined, the technical terms or scientific terms used in the embodiments of the present disclosure should be understood by people with ordinary skills in the field to which the present disclosure belongs. The "first", "second" and similar words used in the embodiments of the present disclosure do not indicate any order, quantity or importance, but are only used to distinguish different components. "Including" or "comprising" and similar words mean that the elements or objects appearing before the word cover the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0023] In order to facilitate understanding of the technical solutions of the present disclosure, some technical terms involved in the present disclosure are introduced below.

[0024] Cycle life refers to the number of cycles that a battery undergoes when it undergoes charge and discharge cycles until its capacity reaches 80% of its initial capacity.

[0025] Capacity calibration refers to determining the actual capacity of a battery under specific conditions through charge and discharge tests.

[0026] "C number" refers to the discharge rate of the battery, which indicates the ratio of the current intensity that the battery can release in a specific time to the battery capacity. Specifically, 1C means the current intensity when the battery is fully discharged in one hour, 2C means the current intensity when it is fully discharged in half an hour, and 0.5C means the current intensity when it is fully discharged in two hours. For example, for a battery with a capacity of 1200mAh, the current is 1200mA when discharged at 1C, and the current is 2400mA when discharged at 2C.

[0027] Constant current (CC) charging means that the current value is constant and the battery voltage rises.

[0028] Constant current discharge means that the current value is constant and the voltage decreases.

[0029] Constant current and constant voltage (CV) process: It includes two stages: constant current and constant voltage. First, the current is charged (discharged) at a constant level until the voltage reaches a certain cut-off voltage, and then the voltage is charged (discharged) at a constant level to a certain cut-off current.

[0030] The cell voltage range refers to the operating voltage range of a battery or battery pack, specifically the interval between the maximum voltage (corresponding to the maximum operating voltage) and the minimum voltage (corresponding to the minimum operating voltage) at which the cell can work normally. Outside the voltage range of the cell, the cell may not work properly, or even pose a safety hazard. Exemplarily, the cell voltage range of a lithium-ion battery can be 2.5V to 4.2V. When the cell voltage is lower than 2.5V, the battery enters a protection state to prevent the cell from over-discharging. When the cell voltage is higher than 4.2V, in order to protect the safety and life of the cell, a charging management system is generally used for charging control.

[0031] In the early stage of battery cell cycle testing, the capacity retention rates of different designs may not differ much. If the capacity retention rates of two solutions differ by 2% after 10,000 cycles, the difference may be less than 0.2% when converted to 1,000 cycles according to the linear trend, so it is basically impossible to distinguish the advantages and disadvantages of the two solutions based on short-term data. Therefore, it often takes a long time to test the cycle performance to distinguish the differences between different solutions, which greatly prolongs the product iteration cycle, occupies a lot of resources and leads to a long development cycle.

[0032] In order to speed up product iteration and shorten the development cycle, it is urgent to develop more efficient evaluation methods for battery cell cycle performance.

[0033] The inventors of the present disclosure have analyzed the relevant technologies and noticed that for lithium iron phosphate batteries, both the positive and negative electrode materials are very stable, so during long-term cycling, the capacity decay of the battery mainly comes from the loss of active lithium. Active lithium refers to the reversible lithium inventory in the battery. When the battery is charged, due to the extremely low potential of the negative electrode, Li + +xSol+e - →xLiSol reaction leads to the growth of Solid Electrolyte Interface (SEI) (whose main component is LiSol). It can be seen that this reaction will increase the loss of active lithium in the battery, thereby causing capacity decay. The reaction rate depends on the integrity of SEI. If the SEI stability of the battery is reduced, the loss of active lithium can be aggravated, thereby accelerating the loss of capacity, which is conducive to finding differences in battery cell cycle performance in fewer cycles.

[0034] Based on the above analysis, the inventors of the present invention propose a method for evaluating the cycle performance of a battery cell, by controlling the battery cell to discharge continuously for a preset time at a preset voltage during the cycle process; wherein the preset voltage is lower than the minimum operating voltage of the battery cell, thereby promoting partial decomposition of SEI at a high potential at the negative electrode, reducing SEI stability, and accelerating side reactions between the negative electrode and the electrolyte, thereby accelerating capacity loss and improving capacity differentiation per unit time, thereby accurately identifying differences in cycle capacity retention rates of battery cells of different designs in a shorter time and shortening the iteration cycle.

[0035] Figure 1 A schematic diagram of a flow chart of a method for evaluating the cycle performance of a battery cell provided by an embodiment of the present disclosure is shown. Figure 1 As shown, the evaluation method includes:

[0036] S101: Based on a preset cyclic charge and discharge method, the battery cell is cyclically charged and discharged and the discharge capacity of a first cycle and at least one target cycle is tested.

[0037] In some embodiments, the cyclic charge and discharge method includes a cyclic method and an accelerated method. Further, the cyclic method includes a charging process and a first discharging process.

[0038] Optionally, the charging process may include constant current charging and constant voltage charging performed sequentially.

[0039] As for the cycle method, the present disclosure embodiment takes a lithium iron phosphate battery as an example for illustrative description. Here, the standard voltage of the lithium iron phosphate battery can be 3.26V, the minimum discharge voltage is 2.5V, and the maximum charge voltage is 3.65V.

[0040] Exemplarily, the charging process may be: charging the battery cell at 25° C. and leaving it to stand for a certain period of time (e.g., 10 minutes). Optionally, the charging condition may be first charging at 1C constant current to 3.65V, and then charging at 3.65V constant voltage to 0.05C.

[0041] Exemplarily, the first discharge process may be: performing constant current discharge on the battery cell to a minimum operating voltage, for example, the discharge condition may be 1C constant current discharge to 2.5V.

[0042] It should be noted that after the cycle discharge reaches the minimum working voltage, the discharge capacity of the battery cell can be detected and recorded as C n Here, n is the number of cycles, which can be any natural number > 0, such as 1, 2, 3, 4, etc. For example, C1 represents the discharge capacity after 1 cycle; C5 represents the discharge capacity after 5 cycles.

[0043] It should be noted that those skilled in the art may adjust the above-mentioned charging and discharging conditions according to different types of battery cells, and the present disclosure does not limit this.

[0044] In some embodiments, the acceleration method includes a charging process and a second discharging process, wherein the charging process is similar to the charging process of the cycle method and is not described in detail.

[0045] Exemplarily, the second discharging process includes:

[0046] Discharging the battery cell from a fully charged state to a preset voltage in a constant current discharge manner;

[0047] The battery is continuously discharged for a preset time at a preset voltage, wherein the preset voltage is less than the minimum operating voltage of the battery cell.

[0048] Optionally, the difference between the minimum operating voltage and the preset voltage is ≥ 0.3 V. Such a voltage difference can ensure the rate of partial decomposition of SEI and reduce SEI stability, thereby accelerating capacity loss and improving capacity differentiation per unit time.

[0049] Taking lithium iron phosphate battery as an example, the minimum operating voltage is 2.5V, so the preset voltage can be 2.2V, 2.15V, 2.0V, 1.8V, 1.5V, etc.

[0050] Optionally, the preset voltage is 1.5 V to 2.2 V. If it is lower than 1.5 V, the stability of the battery cell electrode will be affected; if it is higher than 2.2 V, the SEI decomposition degree will be insufficient and the acceleration efficiency will be poor.

[0051] In some embodiments, the preset time is 5 to 30 minutes, such as 5 minutes, 10 minutes, 15 minutes, 20 minutes, 30 minutes, etc. If the preset time is less than 5 minutes, the preset voltage does not act on the battery cell for enough time, and SEI is difficult to decompose; if the preset time is greater than 30 minutes, SEI decomposition has reached a balanced state, and increasing the time is unlikely to produce better results. Optionally, the preset time is 5 to 10 minutes.

[0052] It should be understood that the above acceleration method is applied to at least one cycle and the cycle executes the acceleration method at least once. In other words, in multiple cycles, the acceleration method can be executed in each cycle, or in selected cycles. When executing the acceleration method, the acceleration method can be executed once in one cycle, or the charging process and the second discharging process of the acceleration method can be executed cyclically (i.e., the acceleration method is executed multiple times).

[0053] Optionally, the acceleration method is executed no more than 5 times in any cycle, such as 2 times, 3 times, etc. Here, if the execution times are within 5 times, SEI has a better degradation effect, and there is no need to continue to increase the execution times.

[0054] Taking lithium iron phosphate batteries as an example, the acceleration method can be to charge the battery cell at 25°C and let it stand for a certain period of time (e.g. 10 minutes). Optionally, the charging condition can be to first charge at 1C constant current to 3.65V, and then charge at 3.65V constant voltage to 0.05C. After standing still, discharge the battery cell and let it stand for a certain period of time (e.g. 10 minutes). Optionally, the discharge condition can be to discharge at 1C constant current to 2.0V, and then maintain 2.0V constant voltage discharge for 10 minutes.

[0055] It should be noted that within a cycle, the cycling method may be performed first and then the acceleration method, or the acceleration method may be performed first and then the cycling method, and the present disclosure does not limit this. It should be understood that regardless of the order in which the cycling method is performed, the discharge capacity of the detection cycle is performed after the cycling method is performed to discharge to the minimum working voltage.

[0056] It should be noted that those skilled in the art may choose to measure the discharge capacity of each cycle, or may take part of the cycles as target cycles and only detect the discharge capacity of the cycles, such as 100 cycles, 500 cycles, etc., which is not limited in the present disclosure.

[0057] It should be noted that, during the evaluation process, each charging (discharging) process may be left to stand for a certain period of time, such as 10 minutes.

[0058] S103: Determine a cell capacity retention rate corresponding to a target cycle according to the discharge capacity of the first cycle and the discharge capacity of any target cycle.

[0059] For example, the cell capacity retention rate C r =C n / C1×100%.

[0060] Example

[0061] Hereinafter, embodiments of the present disclosure are described. The embodiments described below are exemplary and are only used to explain the present disclosure, and should not be construed as limiting the present disclosure. If specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in the art or the product instructions are used. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be obtained commercially.

[0062] Evaluation object

[0063] Cell A: LiFePO4 is used as the positive electrode material, artificial graphite is used as the negative electrode material, the electrolyte composition is 1M LiPF6 dissolved in 1M LiPF6 dissolved in a solvent of EC:EMC=3:7 (mass ratio), and VC accounting for 2% of the total mass ratio of the electrolyte is added as an additive. Among them, EC is ethylene carbonate, EMC is ethyl methyl carbonate, and VC is vinylene carbonate.

[0064] Cell B: The difference from cell A is that the amount of VC added is 3%.

[0065] It should be noted that the above-mentioned evaluation objects are only exemplary, and the evaluation method provided in the embodiments of the present disclosure is not limited to identifying the performance differences caused by differences in electrolytes, but can also identify the performance differences caused by differences in negative electrode material design.

[0066] Example 1

[0067] (1) Cyclic charging and discharging of the battery cell and testing the discharge capacity of at least two cycles Before charging and discharging, place the battery cell in a constant temperature box, adjust the temperature of the constant temperature box to 25℃, and let it stand for 30 minutes.

[0068] 1.1 Charging and Discharging

[0069] Charge to 3.65V at 1C constant current, charge to current 0.05C at constant voltage 3.65V, and let stand for 10 minutes; then, discharge (DC) to 2.5V at 1C constant current, and the discharge capacitance at this time is detected and recorded as C1.

[0070] After the test is completed, let it stand for 10 minutes and perform the following steps:

[0071] 1.2 Acceleration

[0072] Charge to 3.65V at 1C constant current, charge to current 0.05C at constant voltage 3.65V, and let stand for 10 minutes; then, discharge (DC) to 2.0V at 1C constant current, discharge at constant voltage 2.0V for 10 minutes, and let stand for 10 minutes.

[0073] 1.1 and 1.2 are executed 2000 times in total, and C is detected at the 100th, 500th and 2000th cycles respectively. 100 , C 500 and C 2000 Here, the 100th, 500th and 2000th laps are the target number of cycles.

[0074] (2) Determine the cell capacity retention rate

[0075] Based on C1, C 100 , C 500 and C 2000 The capacity retention rate corresponding to each cycle number was determined, and the results are shown in Table 1.

[0076] Example 2

[0077] The difference between this embodiment and embodiment 1 is that in step 1.2, the battery is discharged at a constant current of 1C to 1.5V, and then discharged at a constant voltage of 1.5V for 10 minutes.

[0078] Example 3

[0079] The difference between this embodiment and embodiment 1 is that in step 1.2, the battery is discharged at a constant current of 1C to 2.2V, and then discharged at a constant voltage of 2.2V for 10 minutes.

[0080] Example 4

[0081] The difference between this embodiment and embodiment 1 is that in step 1.2, the battery is discharged at a constant current of 1C to 2.2V, and then discharged at a constant voltage of 2.2V for 5 minutes.

[0082] Comparative Example 1

[0083] The difference between Comparative Example 1 and Example 1 is that in step 1.2, 1C constant current discharge is sufficient until 2.5V is sufficient.

[0084] Comparative Example 2

[0085] The difference between this comparative example and Example 1 is that in step 1.2, the battery is discharged at a constant current of 1C to 2.5V, and then discharged at a constant voltage of 2.5V for 10min.

[0086] Comparative Example 3

[0087] The difference between this comparative example and Example 1 is that in step 1.2, the battery is discharged at a constant current of 1C to 2.0V, and then discharged at a constant voltage of 2.0V for 3min.

[0088] Table 1

[0089]

[0090] It can be seen from Table 1 that, using the evaluation method of Comparative Example 1, the difference between battery cell A and battery cell B only showed 0.3% after 2000 cycles, while using the accelerated evaluation method of Example 1, the difference between battery cell A and battery cell B showed 0.5% after 500 cycles, indicating that the evaluation method provided in the embodiments of the present disclosure can accelerate the evaluation of the cyclability of battery cells.

[0091] By comparing Example 1, Example 2, Example 3 and Comparative Example 2, it can be seen that the higher the discharge cut-off voltage, the smaller the capacity difference between battery cell A and battery cell B under the same number of cycles, indicating that the higher the discharge cut-off voltage, the milder the SEI decomposition, the less lithium loss, and the more difficult it is to show the capacity loss difference, and the capacity difference between battery cell A and battery cell B is more significant when the discharge cut-off voltage is in the range of 1.5 to 2.2V.

[0092] By comparing Example 1 and Comparative Example 3, and Example 3 and Example 4, it can be seen that the shorter the constant voltage discharge time is, the smaller the capacity difference between battery cell A and battery cell B is under the same number of cycles, which means that the shorter the time is, the milder the SEI decomposition is, the less lithium loss is, and the more difficult it is to show the capacity loss difference.

[0093] In addition, by comparing the cycle capacity change trends of battery cells A and B in the embodiments of the present application with those in the comparative examples, it can be seen that the embodiments of the present disclosure, by limiting the discharge cut-off voltage and the constant-voltage discharge time within a reasonable range, can accelerate the test while reducing the impact of changes in the test process on the observed parameter change trends, thereby obtaining more accurate test results.

[0094] In summary, the embodiments of the present disclosure limit the discharge cut-off voltage and the constant voltage discharge time within a reasonable range, thereby being able to obtain accurate test results while accelerating the test, which is beneficial to quickly optimize product performance and shorten the product development cycle.

[0095] Those skilled in the art should understand that the discussion of any of the above embodiments is merely illustrative and is not intended to imply that the scope of the present disclosure (including the claims) is limited to these examples. Based on the concept of the present disclosure, the technical features in the above embodiments or different embodiments may be combined, the steps may be implemented in any order, and there are many other variations of the different aspects of the embodiments of the present disclosure as described above, which are not provided in detail for the sake of simplicity.

[0096] In addition, to simplify the description and discussion, and in order not to make the embodiments of the present disclosure difficult to understand, the known power / ground connections to the integrated circuit (IC) chips and other components may or may not be shown in the provided figures. In addition, the device can be shown in the form of a block diagram to avoid making the embodiments of the present disclosure difficult to understand, and this also takes into account the fact that the details of the implementation of these block diagram devices are highly dependent on the platform on which the embodiments of the present disclosure will be implemented (that is, these details should be fully within the scope of understanding of those skilled in the art). Where specific details (e.g., circuits) are set forth to describe exemplary embodiments of the present disclosure, it is apparent to those skilled in the art that the embodiments of the present disclosure can be implemented without these specific details or with changes in these specific details. Therefore, these descriptions should be considered illustrative rather than restrictive.

[0097] Although the present disclosure has been described in conjunction with specific embodiments of the present disclosure, many replacements, modifications and variations of these embodiments will be apparent to those skilled in the art from the foregoing description. For example, other memory architectures (e.g., dynamic RAM (DRAM)) may use the embodiments discussed.

[0098] The embodiments of the present disclosure are intended to cover all such substitutions, modifications and variations that fall within the broad scope of the appended claims. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the embodiments of the present disclosure should be included in the scope of protection of the present disclosure.

Claims

1. A method for evaluating the cycle performance of a battery cell, characterized in that: include: Based on a preset cyclic charge and discharge method, the battery cell is cyclically charged and discharged and the discharge capacity of the first cycle and at least one target cycle is tested; According to the discharge capacity of the first cycle and the discharge capacity of any target cycle, the cell capacity retention rate corresponding to the target cycle is determined; wherein, The cyclic charge and discharge method includes an acceleration method, and the acceleration method includes: continuously discharging for a preset time at a preset voltage; wherein the preset voltage is less than the minimum operating voltage of the battery cell.

2. The evaluation method according to claim 1, characterized in that: The acceleration method further includes: discharging the battery cell from a fully charged state to a preset voltage in a constant current discharge manner.

3. The evaluation method according to claim 1, characterized in that: The difference between the minimum operating voltage and the preset voltage is ≥0.3V.

4. The evaluation method according to claim 1, characterized in that: The acceleration method is applied to at least one cycle and the cycle executes the acceleration method at least once.

5. The evaluation method according to claim 4, characterized in that: The acceleration method is executed no more than 5 times in any loop.

6. The evaluation method according to claim 1, characterized in that: The preset duration is 5 to 30 minutes.

7. The evaluation method according to claim 1, characterized in that: The positive electrode material of the battery cell is lithium iron phosphate material; the preset voltage is 1.5V to 2.2V.

8. The evaluation method according to claim 1, characterized in that: The cyclic charge and discharge method also includes a cyclic method; The cycle method comprises: discharging the battery cell from a fully charged state to a minimum operating voltage in a constant current discharge manner.

9. The evaluation method according to claim 8, characterized in that: The testing of the discharge capacity of the first cycle and at least one target cycle is performed after the battery cell is discharged from a fully charged state to a minimum operating voltage.

10. The evaluation method according to claim 8, characterized in that: The cycle method and the acceleration method also include a charging process; the charging process includes constant current charging and constant voltage charging.

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