Method for evaluating long cycle lithium battery capacity retention rate

Abstract

This invention provides a method for evaluating the capacity retention of a long-cycle lithium battery, comprising the following steps: performing a preset number of cycle charge-discharge tests on the battery to be evaluated, recording the number of cycles, cycle charge capacity, and cycle discharge capacity; calculating the constant current charge ratio based on the cycle charge capacity; calculating the capacity retention of the battery to be evaluated within the preset number of cycles based on the cycle discharge capacity; fitting the relationship between the number of cycles and the constant current charge ratio for a preset number of weeks within the preset number of cycles to obtain a first linear regression equation; fitting the relationship between the constant current charge ratio and the capacity retention rate for a preset number of weeks within the preset number of cycles to obtain a second linear regression equation; and calculating the capacity retention of the battery to be evaluated outside the preset number of cycles based on the first linear regression equation and the second linear regression equation. The evaluation method provided by this invention significantly reduces cycle testing time and improves the economic efficiency of lithium batteries.

Description

[Technical Field]

[0001] This invention relates to the field of battery technology, and in particular to a method for evaluating the capacity retention rate of long-cycle lithium batteries. [Background Technology]

[0002] Lithium-ion batteries offer advantages such as long cycle life, energy efficiency, environmental friendliness, low maintenance costs, complete charge / discharge cycles, and light weight. Market demands for cycle life are increasingly stringent, ranging from 2000 to 3000 cycles. During lithium-ion battery production, cycle performance testing is necessary. Current testing methods involve placing the batteries in a testing cabinet for repeated charge / discharge cycles, which is time-consuming and results in a long development-to-trial-production-to-mass-production cycle, impacting the economic viability of lithium-ion batteries.

[0003] Therefore, it is necessary to provide a novel method for evaluating the capacity retention of long-cycle lithium batteries to overcome the aforementioned shortcomings. [Summary of the Invention]

[0004] The purpose of this invention is to provide a method for evaluating the capacity retention of long-cycle lithium batteries, which reduces the number of cycles required for testing, greatly reduces the cycle testing time, and improves the economic efficiency of lithium batteries.

[0005] To achieve the above objectives, the present invention provides a method for evaluating the capacity retention rate of a long-cycle lithium battery, comprising the following steps: performing a preset cycle charge-discharge test on the battery to be evaluated, recording the number of cycles, cycle charge capacity, and cycle discharge capacity; calculating the constant current charge ratio based on the cycle charge capacity; calculating the capacity retention rate of the battery to be evaluated within the preset cycle based on the cycle discharge capacity; fitting the relationship between the number of cycles and the constant current charge ratio for a preset number of weeks within the preset cycle to obtain a first linear regression equation; fitting the relationship between the constant current charge ratio and the capacity retention rate for a preset number of weeks within the preset cycle to obtain a second linear regression equation; and calculating the capacity retention rate of the battery to be evaluated outside the preset cycle based on the first linear regression equation and the second linear regression equation.

[0006] In a preferred embodiment, the step of performing a preset cycle charge-discharge test on the battery to be evaluated and recording the number of cycles, cycle charge capacity, and cycle discharge capacity includes the following steps: first, constant current 1C charging to 3.65V, then constant voltage charging to 0.02C cutoff; then constant current discharge at 1C with a cutoff voltage of 2.5V.

[0007] In a preferred embodiment, the preset cycle is 800 weeks.

[0008] In a preferred embodiment, in the step of calculating the constant current charge ratio based on the cycle charging capacity, the constant current charge ratio = current cycle constant current charging capacity / (current cycle constant current charging capacity + current cycle constant voltage charging capacity).

[0009] In a preferred embodiment, in the step of calculating the capacity retention rate of the battery to be evaluated within the preset period based on the cycle discharge capacity, the capacity retention rate is defined as the current cycle discharge capacity divided by the initial cycle discharge capacity.

[0010] In a preferred embodiment, the preset number of weeks is 450-800 weeks.

[0011] In a preferred embodiment, the first linear regression equation is: constant current charge ratio = 99.32 - 0.000965 * number of cycles.

[0012] In a preferred embodiment, the second linear regression equation is: Capacity retention rate = -468.3 + 5.698 * constant current charge ratio.

[0013] Compared to existing technologies, the method for evaluating the capacity retention of long-cycle lithium batteries provided by this invention first conducts a preset number of charge-discharge tests on the battery to be evaluated, recording the number of cycles, the charge capacity, and the discharge capacity. Then, it calculates the constant current charge ratio based on the charge capacity and the capacity retention rate of the battery within the preset number of cycles based on the discharge capacity. Next, by fitting the relationship between the number of cycles and the constant current charge ratio for the preset number of cycles, and the relationship between the constant current charge ratio and the capacity retention rate for the preset number of cycles, a first linear regression equation and a second linear regression equation are derived, respectively. Finally, the capacity retention rate of the battery outside the preset number of cycles is calculated based on the first and second linear regression equations. In other words, by conducting cycle tests on the battery to be evaluated within a preset number of cycles, and using linear fitting of the number of cycles and the constant current charge ratio, as well as linear fitting of the constant current charge ratio and the capacity retention rate, regression equations are established to ultimately simulate the capacity retention rate corresponding to a larger number of cycles in a long-cycle lithium battery. This reduces the number of cycles tested, significantly reduces the cycle testing time, and improves the economic efficiency of lithium batteries.

[0014] To make the above-mentioned objects, features and advantages of the invention more apparent and understandable, preferred embodiments of the invention are described below in detail with reference to the accompanying drawings. [Attached Image Description]

[0015] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 A flowchart of the method for evaluating the capacity retention of long-cycle lithium batteries provided by the present invention;

[0017] Figure 2 A graph showing the relationship between the number of cycles and the constant current charge ratio in the 0-800 cycle charge-discharge test of the battery to be evaluated.

[0018] Figure 3 A graph showing the relationship between the constant current charge ratio and capacity retention rate during 0-800 cycle charge-discharge tests of the battery to be evaluated.

[0019] Figure 4 A graph showing the relationship between the number of cycles and the constant current charge ratio in the 450-800 cycle charge-discharge test of the battery to be evaluated;

[0020] Figure 5 A graph showing the relationship between the constant current charge ratio and capacity retention rate for 450-800 cycles of charge-discharge testing of the battery to be evaluated.

[0021] Figure 6 A graph showing the relationship between the number of cycles and the constant current charge ratio in a 2000-cycle charge-discharge test of the battery to be evaluated;

[0022] Figure 7 A graph showing the relationship between the constant current charge ratio and capacity retention rate during a 2000-cycle charge-discharge test of the battery to be evaluated.

Detailed Implementation Methods

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0024] Please see Figure 1This invention provides a method for evaluating the capacity retention of a long-cycle lithium battery, used to assess the capacity retention of a long-cycle lithium battery. Specifically, the method for evaluating the capacity retention of a long-cycle lithium battery includes the following steps:

[0025] Step S10: Perform a preset number of charge-discharge cycles on the battery to be evaluated, and record the number of cycles, the charge capacity, and the discharge capacity. Specifically, take several batteries to be evaluated and perform a preset number of charge-discharge cycles on them. During the cycle test, record the number of cycles, the charge capacity for each cycle, and the discharge capacity for each cycle. Specifically, during charging, record the constant current charging capacity and constant voltage charging capacity for that cycle; during discharging, record the discharge capacity for that cycle and the initial discharge capacity.

[0026] Step S20: Calculate the constant current charge ratio based on the cycle charging capacity. Specifically, the constant current charge ratio can be calculated based on the cycle charging capacity recorded in step S10. It can be understood that the constant current charge ratio is the ratio of the current cycle constant current charging capacity to the sum of the current cycle constant current charging capacity and the current cycle constant voltage charging capacity.

[0027] Step S30: Calculate the capacity retention rate of the battery under evaluation within the preset period based on the cycle discharge capacity. Specifically, the capacity retention rate of the battery under evaluation within the preset period can be calculated based on the cycle discharge capacity recorded in step S10. It can be understood that the capacity retention rate is the ratio of the current cycle discharge capacity to the initial cycle discharge capacity.

[0028] Step S40: Fit the relationship between the number of cycles and the constant current charge ratio within the preset period (preset number of weeks) to obtain a first linear regression equation. Understandably, in the early stages of cycling, battery performance is unstable, aging time is short, electrolyte is not fully wetted, and the SEI film is unstable; therefore, the cycling data will fluctuate significantly. Thus, data from later preset weeks within the preset period are used for fitting to make the fitting relationship more accurate and the obtained first linear regression equation more precise. In the first linear regression equation, the number of cycles is the independent variable, and the constant current charge ratio is the dependent variable. That is, given the number of cycles, substituting the number of cycles into the first linear regression equation yields the constant current charge ratio.

[0029] Step S50: Fit the relationship between the constant current charge ratio and the capacity retention rate for a preset number of weeks within the preset cycle to obtain a second linear regression equation. It is also understandable that in the early stages of cycling, battery performance is unstable, aging time is short, electrolyte is not fully wetted, and the SEI film is unstable, resulting in significant fluctuations in cycle data. Therefore, data from later preset weeks within the preset cycle are used for fitting to make the fitting relationship more accurate and the obtained second linear regression equation more precise. In the second linear regression equation, the constant current charge ratio is used as the independent variable, and the capacity retention rate is the dependent variable. That is, given the constant current charge ratio, substituting the constant current charge ratio into the second linear regression equation yields the capacity retention rate.

[0030] Step S60: Calculate the capacity retention rate of the battery under evaluation outside the preset cycle based on the first linear regression equation and the second linear regression equation. Understandably, "outside the preset cycle" refers to a number of cycles far exceeding the preset cycle, such as 1500, 2000, or 3000 cycles for long-cycle batteries. Specifically, after deriving the first and second linear regression equations, for long-cycle lithium batteries, it is only necessary to first substitute the number of cycles into the first linear regression equation to calculate the constant current charge ratio, and then substitute the calculated constant current charge ratio into the second linear regression equation to calculate the capacity retention rate of the battery under evaluation outside the preset cycle. This reduces the number of cycles required for testing and significantly reduces the cycle testing time.

[0031] Therefore, the method for evaluating the capacity retention of long-cycle lithium batteries provided by this invention first performs a preset period of cyclic charge-discharge testing on the battery to be evaluated, recording the number of cycles, cyclic charging capacity, and cyclic discharging capacity. Then, it calculates the constant current charge ratio based on the cyclic charging capacity and the capacity retention rate of the battery to be evaluated within the preset period based on the cyclic discharging capacity. Next, by fitting the relationship between the number of cycles and the constant current charge ratio for the preset number of weeks, and the relationship between the constant current charge ratio and the capacity retention rate for the preset number of weeks, a first linear regression equation and a second linear regression equation are obtained, respectively. Finally, the capacity retention rate of the battery to be evaluated outside the preset period is calculated based on the first and second linear regression equations. In other words, by performing cyclic testing on the battery to be evaluated within a preset period, and using linear fitting of the number of cycles and the constant current charge ratio, and linear fitting of the constant current charge ratio and the capacity retention rate, regression equations are established to ultimately simulate the capacity retention rate corresponding to a larger number of cycles in a long-cycle lithium battery. This reduces the number of cycles required for testing, significantly reduces the cyclic testing time, and improves the economic efficiency of lithium batteries.

[0032] Further, in step S10, the cyclic charge-discharge test includes:

[0033] First, charge at a constant current of 1C to 3.65V, then switch to constant voltage charging at 0.02C to cut off; finally, discharge at a constant current of 1C with a cutoff voltage of 2.5V. Understandably, one charging and one discharging cycle together constitute one cycle. Using the above-mentioned constant current followed by constant voltage charging, and constant current discharging, can extend the lifespan of the lithium battery through its electrochemical performance.

[0034] Furthermore, in step S10, the preset cycle is 800 cycles. That is, the battery to be evaluated is subjected to an 800-cycle charge-discharge test, and the parameters of the 800-cycle cycle are recorded, specifically including the number of cycles, the cycle charge capacity, and the cycle discharge capacity.

[0035] Further, in step S20, the constant current charging ratio = constant current charging capacity of the current cycle / (constant current charging capacity of the current cycle + constant voltage charging capacity of the current cycle), that is, the constant current charging ratio corresponding to 800 cycles can be calculated based on the 800-cycle charging capacity obtained in step S10.

[0036] Further, in step S30, the capacity retention rate = current cycle discharge capacity / initial cycle discharge capacity, that is, the capacity retention rate corresponding to 800 cycles can be calculated based on the 800-cycle discharge capacity obtained in step S10.

[0037] Furthermore, in steps S40 and S50, the preset number of cycles is 450-800 cycles. That is, when fitting the relationship between the number of cycles and the constant current charge ratio calculated in step S20, and when fitting the relationship between the constant current charge ratio calculated in step S20 and the capacity retention rate calculated in step S30, the data from 0-450 cycles is discarded, and the data from 450-800 cycles is used for fitting. This is because in the early stage of cycling, the battery performance is unstable, the aging time is short, the electrolyte is not fully wetted, and the SEI film is unstable, so the cycle data will fluctuate greatly. Therefore, the data from the later part of 800 cycles, 450-800 cycles, is used for fitting, making the fitting relationship more accurate and the obtained linear regression equation more precise.

[0038] Please see Figure 2 and Figure 3 To verify the relationship between cycle number and constant current charge ratio, and between constant current charge ratio and capacity retention, an 800-cycle charge-discharge test was conducted on the battery to be evaluated. Relevant graphs (e.g., 0-800 cycle number and corresponding constant current charge ratio (%) data were fitted and plotted) were generated. Figure 2 ), and plot the relevant graphs (e.g., constant current charge ratio (%) and corresponding capacity retention rate (%) data. Figure 3It can be determined that there is a clear linear relationship between the number of cycles and the corresponding constant current charge ratio, but there are many outliers. The linear relationship between the constant current charge ratio and the corresponding capacity retention rate is not obvious, and there are many outliers.

[0039] However, the constant current charge ratio reaches its maximum at cycle 450, which also marks the inflection point of the linear relationship. Therefore, we selected cycle numbers from 450 to 800 and the corresponding constant current charge ratio (%) data to plot a relevant graph (e.g., Figure 4 ), and plot the relevant graphs (e.g., constant current charge ratio (%) and corresponding capacity retention rate (%) data. Figure 5 It can be determined that there is a clear linear relationship between the number of cycles and the corresponding constant current charge ratio, with few outliers. The constant current charge ratio and the corresponding capacity retention rate also show a clear linear relationship with few outliers.

[0040] based on Figure 4 and Figure 5 The first and second linear regression equations can be derived. In this embodiment, the first linear regression equation is: constant current charge ratio = 99.32 - 0.000965 * number of cycles; the second linear regression equation is: capacity retention rate = -468.3 + 5.698 * constant current charge ratio. Therefore, for the evaluation of the capacity retention rate of long-cycle lithium batteries, the constant current charge ratio is calculated by substituting the number of cycles into the first linear regression equation, and then the capacity retention rate is calculated by substituting the calculated constant current charge ratio into the second linear regression equation. That is, the capacity retention rate can be calculated using the first and second linear regression equations, without the need for testing with a large number of cycles, which greatly reduces the cycle testing time and improves the economic efficiency of lithium batteries.

[0041] To verify the above evaluation method, the battery to be evaluated was subjected to a 2000-cycle charge-discharge test. Specifically, the charge-discharge test process was as follows: first, constant current 1C charging was performed to 3.65V, then constant voltage charging was performed to 0.02C cutoff; then constant current discharging was performed with a current of 1C and a cutoff voltage of 2.5V. The test data such as the number of cycles, cycle charging capacity, and cycle discharging capacity were recorded.

[0042] Figure 6 A graph showing the relationship between the number of charge-discharge cycles and the constant current charge ratio during a 2000-cycle charge-discharge test of the battery to be evaluated. Figure 7 A graph showing the relationship between the constant current charge ratio and capacity retention rate during 2000-cycle charge-discharge testing of the battery under evaluation. Figure 6 and Figure 7 It can be seen that the error between the capacity retention rate calculated using the evaluation method of the present invention and the actual capacity retention rate of 2000 cycles obtained by testing is within 1%, which confirms the accuracy of the evaluation method provided by the present invention.

[0043] In summary, the method for evaluating the capacity retention of long-cycle lithium batteries provided by this invention first performs a preset number of charge-discharge tests on the battery to be evaluated, recording the number of cycles, the charge capacity, and the discharge capacity. Then, it calculates the constant current charge ratio based on the charge capacity and the capacity retention of the battery within the preset number of cycles based on the discharge capacity. Next, by fitting the relationship between the number of cycles and the constant current charge ratio for the preset number of cycles, and the relationship between the constant current charge ratio and the capacity retention for the preset number of cycles, a first linear regression equation and a second linear regression equation are derived, respectively. Finally, the capacity retention of the battery outside the preset number of cycles is calculated based on the first and second linear regression equations. In other words, by performing cycle tests on the battery to be evaluated within a preset number of cycles, and using linear fitting of the number of cycles and the constant current charge ratio, and linear fitting of the constant current charge ratio and the capacity retention, regression equations are established to ultimately simulate the capacity retention corresponding to a larger number of cycles in a long-cycle lithium battery. This reduces the number of cycles required for testing, significantly reduces the cycle testing time, and improves the economic efficiency of lithium batteries.

[0044] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for evaluating the capacity retention rate of a long-cycle lithium battery, characterized in that, Includes the following steps: Perform a preset number of charge-discharge cycles on the battery to be evaluated, and record the number of cycles, cycle charge capacity, and cycle discharge capacity. Calculate the constant current charging ratio based on the cycle charging capacity; Calculate the capacity retention rate of the battery to be evaluated within the preset cycle based on the cycle discharge capacity; Fit the relationship between the number of cycles and the constant current charging ratio within the preset period and the preset number of weeks to obtain the first linear regression equation; the first linear regression equation is: constant current charging ratio = 99.32 - 0.000965 * number of cycles; Fit the relationship between the constant current charge ratio and the capacity retention rate for a preset number of weeks within the preset period to obtain a second linear regression equation; the second linear regression equation is: capacity retention rate = -468.3 + 5.698 * constant current charge ratio; The preset number of weeks is 450-800 weeks; The capacity retention rate of the battery to be evaluated outside the preset period is calculated based on the first linear regression equation and the second linear regression equation.

2. The method for evaluating the capacity retention rate of a long-cycle lithium battery as described in claim 1, characterized in that, In the step of performing a preset number of charge-discharge cycles on the battery to be evaluated and recording the number of cycles, the cycle charge capacity, and the cycle discharge capacity, the charge-discharge cycle test includes: First, charge at a constant current of 1C to 3.65V, then switch to constant voltage charging at 0.02C to cut off; then discharge at a constant current of 1C with a cutoff voltage of 2.5V.

3. The method for evaluating the capacity retention rate of a long-cycle lithium battery as described in claim 2, characterized in that, In the step of calculating the constant current charging ratio based on the cycle charging capacity, the constant current charging ratio = current cycle constant current charging capacity / (current cycle constant current charging capacity + current cycle constant voltage charging capacity).

4. The method for evaluating the capacity retention rate of a long-cycle lithium battery as described in claim 2, characterized in that, In the step of calculating the capacity retention rate of the battery to be evaluated within the preset period based on the cycle discharge capacity, the capacity retention rate = current cycle discharge capacity / initial cycle discharge capacity.

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