A test method for calendar life

Abstract

The invention provides a calendar life testing method, which comprises the following steps: a control cell testing step: performing an actual calendar life test on the control cell, and recording a cycle life T1 with a capacity retention rate of C%; a test cell testing step: performing a charge-discharge cycle treatment on the cell to be tested until the capacity retention rate of the cell is C%-1% to C%+1%; performing an actual calendar life test on the cell to be tested after the charge-discharge cycle treatment, and obtaining a cycle life T2; and a calculation step: adding T1 and T2 to obtain the calendar life of the cell to be tested, which significantly shortens the test period, quickly identifies the calendar life of the cell, and has a small difference between the test result and that of a traditional method, which is within an acceptable range, thereby ensuring the accuracy of the test result.

Description

Technical Field

[0001] The present invention relates to the field of batteries, and in particular to a method for testing calendar life. Background Art

[0002] Like other types of batteries (or cells), lithium-ion batteries face lifespan issues. This refers to the gradual degradation of the battery's internal structure during use, which affects its discharge capacity. This includes deterioration of the positive and negative electrode materials and loss of the electrolyte solvent. The end of a battery's lifespan results in the loss of functionality across the entire system, potentially leading to serious consequences. Therefore, predicting the lifespan of batteries (or cells) in advance will improve battery quality and significantly contribute to battery development.

[0003] Battery aging mainly includes cycle aging and calendar aging. Correspondingly, battery life includes calendar life and cycle life. Calendar life is the most important, which refers to the period from the date of production to the end of the battery life, measured in years. This period includes different links such as storage, aging, high and low temperature, circulation, and working condition simulation. The traditional calendar life test method is to adopt storage-periodic testing-storage-periodic testing until the capacity retention rate drops to the cutoff condition (for example, 70-80%). However, this test method has a very long cycle, and the test cannot be completed in 1 year. It is impossible to quickly observe the failure of the battery cell, and thus it is impossible to quickly identify the calendar life of the battery cell. Summary of the Invention

[0004] Therefore, the technical problem to be solved by the present invention is to overcome the defects of the prior art in that the test cycle is long and the calendar life of the battery cell cannot be quickly identified, thereby providing a calendar life testing method.

[0005] To this end, the present invention provides a calendar life testing method, which comprises the following steps:

[0006] Test steps for control cells: subject the control cells to actual calendar life test and record the cycle life T1 at which the capacity retention rate is C%;

[0007] The test steps of the battery cell to be tested are as follows: subjecting the battery cell to be tested to a charge-discharge cycle treatment until the capacity retention rate of the battery cell is C%-1% to C%+1%; performing an actual calendar life test on the battery cell to be tested after the charge-discharge cycle treatment to obtain a cycle life T2;

[0008] Calculation steps: Add T1 and T2 to get the calendar life of the battery cell to be tested.

[0009] Furthermore, the charge-discharge cycle process includes the following steps:

[0010] (1) a first resting step, (2) a discharging step, (3) a second resting step, (4) a charging step, and steps (1) to (4) are repeated.

[0011] Furthermore, the charge-discharge cycle treatment further satisfies at least one of the following 1)-6):

[0012] 1) In step (1) and / or (3), the standing time is 30-60 min and the temperature is 25-60° C., preferably, the temperature is 25-30° C.;

[0013] 2) In step (2), the discharge mode is constant current discharge, the current is 0.33C-2C, and the discharge is performed to the lower cut-off voltage; preferably, the current is 0.8-1.2C;

[0014] 3) In step (4), the charging mode is constant current to constant voltage charging, constant current charging to the upper limit cutoff voltage at a charging current of 0.33C-2C, and then constant voltage charging to 0.01C-0.05C; preferably, the charging current is 0.8-1.2C;

[0015] 4) In step (2), discharging is performed at a temperature of 25-60° C., preferably, the temperature is 25-30° C.;

[0016] 5) In step (4), charging is performed at a temperature of 25-60° C., preferably, at a temperature of 25-30° C.;

[0017] 6) Repeat 100-300 times.

[0018] Furthermore, C%≤101%; preferably, C% is 99%-100%.

[0019] Furthermore, the control battery cell and the battery cell to be tested are battery cells of the same model.

[0020] Furthermore, the actual calendar life test of the test step of the control cell and / or the test step of the cell to be tested includes the following steps:

[0021] A. Charging step, B. Storage step, C. Testing step, repeat steps A to C.

[0022] Furthermore, the actual calendar life test also satisfies at least one of the following (1) to (3):

[0023] (1) The charging mode is constant current to constant voltage charging, charging at a constant current of 0.33C-2C to the upper cut-off voltage, and then charging at a constant voltage of 0.01C-0.05C;

[0024] (2) Storage temperature is 25-60°C for 7-30 days;

[0025] (3) The test steps include 1) a first resting step, 2) a discharging step, 3) a second resting step, and 4) a charging step.

[0026] Furthermore, the test steps in the actual calendar life test meet at least one of the following AEs:

[0027] A. In step 1) and / or step 3), the standing time is 30-60 min and the temperature is 25-60°C;

[0028] B. In step 2), the discharge mode is constant current discharge, the current is 0.33C-2C, and the discharge is performed to the lower cut-off voltage;

[0029] C. In step 4), the charging mode is constant current to constant voltage charging, constant current charging or upper limit cutoff voltage at a charging current of 0.33C-2C, and then constant voltage charging to 0.01-0.05C;

[0030] D. In step 2), discharging is performed at a temperature of 25-60° C.;

[0031] E. In step 4), charging is performed at a temperature of 25-60°C.

[0032] Furthermore, the test step of the control cell also includes performing a polynomial fitting on the correlation between the capacity retention rate of the control cell and the total storage time of the actual calendar life test process to obtain the model shown in formula (I), Y1=a1X1 2 +b1X1+c1(I);

[0033] The test step of the battery cell to be tested also includes fitting a polynomial of the correlation between the capacity retention rate and the charge-discharge cycle treatment time during the charge-discharge cycle treatment of the battery cell to be tested, and obtaining the model shown in formula (II): Y2=a2X2 2 +b2X2+c2(II);

[0034] Where Y1 and Y2 are capacity retention rates, X1 is the charge and discharge cycle processing time, X2 is the total storage time of the actual calendar life test, and a1, b1, c1, a2, b2, and c2 are fitting parameters.

[0035] Preferably, the charge-discharge cycle treatment time includes the total time of repeating the following steps: a first resting step, a discharging step, a second resting step, and a charging step.

[0036] Preferably, the total storage time is the sum of the time taken to perform the storage steps multiple times.

[0037] Further, the ratio of 100 times a1 to a2 is 1:9-9:1; and / or the ratio of 10 times b1 to b2 is 1:9-9:1.

[0038] Furthermore, the testing steps of the battery cell to be tested also include performing polynomial fitting on the correlation between the capacity retention rate of the battery cell to be tested during the actual calendar life test and the total storage time in the actual calendar life test when the storage time is at least 120 days (for example, 120-210 days), to obtain the model shown in formula (III), Y3=a3X3+b3X3+c3(III); wherein Y3 is the capacity retention rate, X3 is the actual calendar life test processing time, and a3, b3, and c3 are fitting parameters; according to the model calculation, the test time corresponding to the capacity retention rate reaching 70-80% is the cycle life T2.

[0039] Furthermore, a nominal capacity test step is included.

[0040] In the nominal capacity determination step, the test temperature is controlled at 15-45°C, the cycle rate is 0.05-1.0C, the number of charge cycles is 1-5 times, and the number of discharge cycles is 1-5 times.

[0041] This patent applies to lithium-ion batteries, including but not limited to soft-pack, square and L-shaped batteries; the thickness of the battery cell is 7-80mm; the width of the battery cell is 90-300mm; and the height of the battery cell is 90-300mm.

[0042] In certain preferred embodiments, the positive electrode of the battery cell to be tested includes a current collector and a positive electrode active material bonded to the current collector, and the positive electrode active material is selected from at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium nickel manganese oxide material, lithium nickel oxide material, lithium cobalt oxide material, and lithium nickel manganese cobalt oxide material. The bonding process can adopt the existing coating and cold pressing process. Specifically, the positive electrode active material, the conductive agent, and the binder are mixed uniformly in a conventional proportion and added to a solvent to form a positive electrode slurry; the positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, dried and cold pressed, and then die-cut and stripped to form a positive electrode sheet. The solid content of the positive electrode slurry can be 70-75%, the conductive agent can be a conventional conductive agent, such as acetylene black, the binder can be a conventional binder, such as styrene-butadiene rubber or vinylidene fluoride PVDF, and the solvent can be a conventional organic solvent, such as N-methylpyrrolidone NMP.

[0043] In certain preferred embodiments, the negative electrode of the battery cell to be tested includes a current collector and a negative electrode active material bonded to the current collector, and the negative electrode active material is selected from at least one of graphite, hard carbon, soft carbon, and mesophase carbon microbeads. The bonding process can adopt the existing coating and cold pressing process. Specifically, the negative electrode active material, conductive agent, thickener, and binder are mixed in conventional proportions, added to solvent water, mixed evenly, and made into a negative electrode slurry; the negative electrode slurry is evenly coated on the negative electrode current collector copper foil, dried, and cold pressed to form a negative electrode sheet. The solid content of the negative electrode slurry can be 50-55%, the conductive agent can be a conventional conductive agent, such as acetylene black, the binder can be a conventional binder, such as styrene-butadiene rubber or vinylidene fluoride PVDF, and the thickener can be a conventional thickener, such as sodium hydroxymethyl cellulose.

[0044] The electrode liquid of the present invention can adopt conventional commercially available lithium ion electrolyte, or can be homemade using existing conventional materials. For example, an electrolyte comprising a solvent, a lithium salt and an additive can be adopted, wherein the solvent is selected from at least one of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate. The lithium salt is selected from lithium hexafluorophosphate and / or lithium tetrafluoroborate; the additive is selected from at least one of vinylene carbonate, propylene carbonate, vinyl sulfate and lithium difluorophosphate. The molar concentration of the lithium salt is 0.8-1.2 mol / L, and a mixed solution of ethylene carbonate (EC), dimethyl carbonate (DEC) and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1-1:2:2 can be used as the solvent. The volume percentage of the additive can be 0.5-5%. The present invention can adopt existing traditional diaphragms, such as PE diaphragms, PP diaphragms, PP / PE composite films, or other commercially available diaphragms.

[0045] The technical solution of the present invention has the following advantages:

[0046] 1. The calendar life testing method provided by the present invention includes the following steps: a control cell testing step: performing an actual calendar life test on the control cell, and recording a cycle life T1 with a capacity retention rate of C%; a test cell testing step: performing a charge-discharge cycle treatment on the test cell until the capacity retention rate of the cell is C%-1% to C%+1%; performing an actual calendar life test on the test cell after the charge-discharge cycle treatment to obtain a cycle life T2; and a calculation step: adding T1 and T2 to obtain the calendar life of the test cell. Because the control cell testing step and the test cell testing step do not affect each other, they can be tested simultaneously, which significantly shortens the test cycle and quickly identifies the calendar life of the cell. In addition, the difference between the test result and the traditional method is small and within an acceptable range, thereby ensuring the accuracy of the test result.

[0047] 2. The test method for calendar life provided by the present invention can further shorten the difference value from the traditional method by controlling 85% < C% ≤ 101%; in particular, C% is 99% - 100%; and / or, by controlling the reference cell and the cell to be tested to be of the same model, and the accuracy of the test result is higher.

[0048] 3. The test method for calendar life provided by the present invention, by adopting the charge-discharge cycle process including the following steps: (1) the first static step, (2) the discharge step, (3) the second static step, (4) the charge step, repeating steps (1) to (4), adopting this method, and by controlling conditions such as the static time, charge-discharge current, and charge-discharge temperature, etc., can further shorten the difference value from the traditional method and improve the accuracy of the test result. In particular, during the charge-discharge cycle process of the cell to be tested, by controlling the current in the discharge step to be 0.8 - 1.2C, or controlling the charge current in the charge step to be 0.8 - 1.2C, or controlling the temperature in the first static step, discharge step or charge step to be 25 - 30°C, the accuracy of the test result is higher.

[0049] 4. The test method for calendar life provided by the present invention, in step S1, it also includes performing polynomial fitting on the correlation between the capacity retention rate of the reference cell and the total storage time of the actual calendar life test process; in step S2, it also includes performing polynomial fitting on the correlation between the capacity retention rate and the charge-discharge cycle process time during the charge-discharge cycle process of the cell to be tested. It is found that when the fitting parameters, the ratio of 100 times a1 to a2 is 1:9 - 9:1; and / or, the ratio of 10 times b1 to b2 is 1:9 - 9:1, the accuracy of the test result is higher.

[0050] 5. The test method for calendar life provided by the present invention, in step S2, it also includes performing polynomial fitting on the correlation between the capacity retention rate of the cell to be tested during the actual calendar life test and the total storage time in the actual calendar life test when the storage time is at least 120 days (for example, 120 - 210 days), to obtain the model shown in formula (III), Y3 = a3X3 2 + b3X3 + c3 (III); where Y3 is the capacity retention rate, X3 is the actual calendar life test process time, and a3, b3, c3 are fitting parameters; according to the model, the test time corresponding to the capacity retention rate reaching 70 - 85% is the cycle life T2. By the above steps, the test cycle can be further shortened. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0052] Figure 1 is a relationship curve between the capacity retention rate during the charge-discharge cycle treatment of the battery cell to be tested and the total charge-discharge cycle treatment time (days) in Example 1 of the present invention;

[0053] Figure 2 : is a curve showing the relationship between the capacity retention rate and the total storage time (days) in the control cell test experiment in Example 1 of the present invention;

[0054] Figure 3 1 is a curve showing the relationship between the capacity retention rate and the total storage time (months) in the actual calendar life test of the battery cell to be tested in Example 1 of the present invention. DETAILED DESCRIPTION

[0055] The following examples are provided for a better understanding of the present invention and are not intended to limit the best mode of implementation. They do not limit the content and scope of protection of the present invention. Any product identical or similar to the present invention obtained by anyone under the guidance of the present invention or by combining the features of the present invention with other prior arts shall fall within the scope of protection of the present invention.

[0056] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in this field can be used. If the manufacturer of the reagents or instruments is not specified, they are all commercially available conventional reagents.

[0057] The test cells and control cells in the examples and comparative examples were prepared by the following method: The preparation of the test cells includes: (1) Preparation of the positive electrode sheet: taking the positive electrode active material (nickel cobalt manganese material NCM613), the conductive agent acetylene black, and the binder polyvinylidene fluoride PVDF according to a mass ratio of 96:2:2 and mixing them evenly to obtain a mixture, adding the mixture to the solvent N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode slurry (solid content of 70%), and the positive electrode slurry was heated at 19 mg / cm 2 The surface density is evenly coated on the positive electrode current collector aluminum foil with a thickness of 12μm. After drying at 100℃, it is cold pressed and then die-cut and slit to make the positive electrode sheet of lithium-ion battery.

[0058] (2) Preparation of negative electrode sheet: Take the negative electrode active material graphite, the conductive agent acetylene black, the thickener sodium carboxymethyl cellulose (CMC), and the binder styrene butadiene rubber (SBR) in a mass ratio of 95:1.5:1.5:2 to obtain a mixture, add the mixture into the solvent water and mix well to prepare the negative electrode slurry (solid content of 50%); the negative electrode slurry is heated to 11 mg / cm 2 The surface density is evenly coated on the negative electrode current collector copper foil, the thickness of the copper foil is 6 μm, and it is dried at 90°C and then cold pressed to make the negative electrode sheet of the lithium ion battery to be manufactured.

[0059] (3) Preparation of electrolyte: Lithium hexafluorophosphate was dissolved in a mixed solvent of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate in a volume ratio of 5:3:2 to obtain a lithium hexafluorophosphate solution with a concentration of 1.15 mol / L. 1% by volume of vinylene carbonate, 0.5% by volume of lithium difluorophosphate, and 0.5% by volume of vinyl sulfate DTD were added to obtain a lithium ion battery electrode solution.

[0060] (5) The positive electrode sheet, PE separator (purchased from Enjie Company, model: SV12), and negative electrode sheet were assembled in a stacked manner to obtain a battery electrode group, dried in a vacuum drying oven, injected with electrolyte, and sealed to obtain a battery cell (model 33220102, thickness 33, width 220, height 102, nominal voltage 3.72V).

[0061] The preparation method of the control cell is the same as that of the cell to be tested.

[0062] Test steps for the control cell of Example 1:

[0063] The test steps for the control battery cell are as follows: (1) Take the control battery cell for nominal capacity test, the test temperature is 25 ° C, the cycle rate is 1C, and the charge and discharge cycle (first charge to 4.3V, then discharge to 2.8V, cycle 3 times), and measure the nominal capacity of the control battery cell to be 115AH.

[0064] (2) The control cell was charged to 4.3 V (upper cutoff voltage) at 1 C current, and then charged to 0.01 C at a constant voltage, with a state of charge of 100%.

[0065] (3) Store the battery cells in a 45°C incubator for 30 days.

[0066] (4) The battery cell was subjected to a charge and discharge cycle, including a first resting step, a discharge step, a second resting step and a charging step, and the capacity retention rate was measured, wherein the resting time of the first resting step and the second resting step was 30 minutes, the temperature was 25°C, the discharge step was constant current discharge, and constant current discharge was performed at a temperature of 25°C and a current of 1C to 2.8V (lower cutoff voltage). The charging method was constant current to constant voltage charging, and constant current charging was performed at a charging current of 1C to 4.3V (upper cutoff voltage), and then constant voltage charging was performed to 0.01C.

[0067] (5) Repeat steps (2)-(4) until the capacity retention rate is measured to be ≤100% for the first time (in this embodiment, the capacity retention rate is 99.4% after storage for 330 days through charge and discharge cycles). Record the time and record it as the cycle life T1. The charge and discharge cycle time is short and can be ignored. Only the storage time is recorded. In this comparative example, T1 is 330 days.

[0068] (6) The correlation between the capacity retention rate of the control cell and the total storage time was fitted with a polynomial to obtain the model shown in formula (I), Y1 = a1X1 2 +b1X1+c1(I), see Figure 1 As shown, Y = -6 × 10 -07 X 2 +0.0002X+1.0019, R 2 =0.9206, Y is the capacity retention rate, and X is the total storage time (days).

[0069] Example 2 Testing steps for the battery cell to be tested

[0070] The test steps of the battery cell to be tested in this embodiment are as follows:

[0071] (1) Charge and discharge cycle treatment

[0072] Take the battery cell to be tested for nominal capacity test, the test temperature is 25℃, the cycle rate is 1C, and the charge and discharge cycle is (first charge to 4.3V, then discharge to 2.8V, cycle 3 times). The nominal capacity of the battery cell to be tested is measured to be 116AH.

[0073] The battery cell to be tested is subjected to a charge-discharge cycle, including a first resting step, a discharge step, a second resting step, and a charge step, and the above four steps are repeated until the capacity retention rate is ≤100%. In this embodiment, 160 cycles are performed, the charge-discharge cycle treatment time is 28.3 days, and the capacity retention rate is 99.9%. Among them, the resting time of the first and second resting steps is 30 minutes, the temperature is 25°C, the discharge step is constant current discharge, constant current charging is performed at a temperature of 25°C and a current of 1C to 2.8V (lower cut-off voltage), and the charging mode is constant current to constant voltage charging, constant current charging is performed at a charging current of 1C to 4.3V (upper cut-off voltage), and then constant voltage charging is performed to 0.01C.

[0074] The correlation between the capacity retention rate and the charge-discharge cycle time during the charge-discharge cycle of the battery cell to be tested is fitted with a polynomial to obtain the model shown in formula (II): Y2=a2X2 2 +b2X2+c2(II), see Figure 2 As shown, Y = -4 × 10 - 05 X 2 +0.0009X+1.0023, R 2 =0.9223, Y is the capacity retention rate, and X is the charge-discharge cycle treatment time (days).

[0075] (2) Actual calendar life test

[0076] 1) Charge the cell to 4.3V (upper cutoff voltage) at 1C, then charge at a constant voltage of 0.01C to a state of charge of 100%.

[0077] 2) Store the battery cells in a 45°C incubator for 30 days.

[0078] 3) The battery cell was subjected to a charge and discharge cycle, including a first resting step, a discharge step, a second resting step and a charging step, and the capacity retention rate was measured, wherein the resting time for the first resting step and the second resting step was 30 min, the temperature was 25°C, the discharge step was constant current discharge, and the constant current was charged to 2.8 V (lower cutoff voltage) at a temperature of 25°C and a current of 1C. The charging mode was constant current to constant voltage charging, and the constant current was charged to 4.3 V (upper cutoff voltage) at a charging current of 1C, and then constant voltage was charged to 0.01C.

[0079] 4) Repeat steps 1)-3) until the total storage time reaches 7 months. Perform a polynomial fit on the correlation between the capacity retention rate of the battery cell under test during the actual calendar life test and the total storage time of multiple times to obtain the model shown in formula (III): Y3=a3X3 2 +b3X3+c3(III), see Figure 3 As shown, Y3 = -0.0001X 2 -0.0038X+1.0028, R 2 =0.9951, Y is the capacity retention rate, and X is the total storage time (months). The charge-discharge cycle time is short and can be ignored. According to the model, the total storage time corresponding to the capacity retention rate reaching 83% is calculated and recorded as the cycle life T2. In this embodiment, T2 is 27 months.

[0080] Example 3 Testing steps for the battery cell to be tested

[0081] (1) Charge and discharge cycle treatment

[0082] The battery cell to be tested was taken for nominal capacity test. The test temperature was 25°C, the cycle rate was 0.05C, and the charge and discharge cycle was performed (first charge to 4.3V, then discharge to 2.8V, and cycled 3 times). The nominal capacity of the battery cell to be tested was measured to be 117AH.

[0083] The cell under test was subjected to a charge-discharge cycle, including a first resting step, a discharge step, a second resting step, and a charge step. These four steps were repeated until the capacity retention rate was ≤100%. After 161 cycles, the capacity retention rate was 99.8%. The resting time for the first and second resting steps was 30 minutes at a temperature of 25°C. The discharge step was a constant current discharge, with a constant current charge at a temperature of 25°C and a current of 1C to 2.8V (lower cut-off voltage). The charging mode was a constant current to constant voltage charge, with a constant current charge at a charging current of 1C to 4.3V (upper cut-off voltage), followed by a constant voltage charge to 0.05C.

[0084] (2) Actual calendar life test

[0085] 1) Charge the cell to 4.3V (upper cutoff voltage) at 1C, then charge at a constant voltage of 0.01C to a state of charge of 100%.

[0086] 2) Store the battery cells in a 45°C incubator for 30 days.

[0087] 3) The battery cell was subjected to a charge and discharge cycle, including a first resting step, a discharge step, a second resting step and a charging step, and the capacity retention rate was measured, wherein the resting time for the first resting step and the second resting step was 30 min, the temperature was 45°C, the discharge step was constant current discharge, and the constant current was charged to 2.8 V (lower cutoff voltage) at a temperature of 45°C and a current of 1C. The charging mode was constant current to constant voltage charging, and the constant current was charged to 4.3 V (upper cutoff voltage) at a charging current of 1C, and then constant voltage charged to 0.01C.

[0088] 4) Repeat steps 1)-3) until the total storage time reaches 5 months. Perform a polynomial fit on the correlation between the capacity retention rate and the total storage time during the actual calendar life test of the battery cell to be tested, and obtain the model shown in formula (III): Y3=a3X3 2 +b3X3+c3(III), Y3 is the capacity retention rate, X3 is the total storage time (months), a3, b3, and c3 are fitting parameters; according to the model calculation, the corresponding test time when the capacity retention rate reaches 80% is the cycle life T2.

[0089] Example 4 Testing steps for the battery cell to be tested

[0090] It is basically the same as Example 2, except that the resting time of the first resting step and the second resting step in the charge-discharge cycle treatment in step (1) is different. In this embodiment, the resting time of both steps is increased from 30 min to 60 min.

[0091] Example 5 Testing steps for the battery cell to be tested

[0092] It is basically the same as Example 2, except that the temperature of the first resting step, the discharging step, the second resting step and the charging step in step (1) of the charge-discharge cycle treatment is different. The temperature in this embodiment is 35°C.

[0093] Example 6 Testing steps for the battery cell to be tested

[0094] It is basically the same as Example 2, except that the temperature of the first resting step, the discharging step, the second resting step and the charging step in step (1) of the charge-discharge cycle treatment is different. The temperature in this embodiment is 20°C.

[0095] Example 7 Testing steps for the battery cell to be tested

[0096] The method is basically the same as Example 2, except that the current of the discharge step in the charge-discharge cycle treatment in step (1) is 0.33C, and the charging current of the constant current charging step is 0.5C.

[0097] Example 8 Testing steps for the battery cell to be tested

[0098] The method is basically the same as Example 2, except that the current of the discharge step in the charge-discharge cycle treatment in step (1) is 1.5C, and the charging current of the constant current charging step is 1.5C.

[0099] Example 9 Testing steps for the battery cell to be tested

[0100] The method is basically the same as Example 2, except that the capacity retention rate is 100.4% after 23.0 days of charge-discharge cycle treatment in step (1).

[0101] Example 10 Testing steps for the battery cell to be tested

[0102] The method is basically the same as Example 2, except that the capacity retention rate is measured to be 97.8% after 44.3 days of charge and discharge cycle treatment in step (1).

[0103] Comparative Example 1 Traditional method

[0104] This comparative example provides a calendar life testing method, comprising the following steps:

[0105] (1) The nominal capacity test of the battery cell A to be tested, which is the same system and model as the battery cell in Example 1, is carried out at a test temperature of 25°C, a cycle rate of 1C, and a charge-discharge cycle (first charging to 4.3V, then discharging to 2.8V, and cycling for 3 times). The nominal capacity of the battery cell to be tested is measured to be 115Ah.

[0106] (2) Charge the cell to be tested to 4.3V (upper cut-off voltage) at 1C current, then charge to 0.01C at constant voltage, with the state of charge being 100% SOC.

[0107] (3) Store the battery cells in a 45°C incubator for 30 days.

[0108] (4) The battery cell was subjected to a charge and discharge cycle, including a first resting step, a discharge step, a second resting step and a charging step, and the capacity retention rate was measured, wherein the resting time of the first resting step and the second resting step was 30 min, the temperature was 25°C, the discharge step was a constant current discharge, and the constant current was charged to 2.8 V (lower cut-off voltage) at a temperature of 25°C and a current of 1C. The charging mode was a constant current to constant voltage charging, and the constant current was charged to 4.3 V (upper cut-off voltage) at a charging current of 1C, and then the constant voltage was charged to 0.01C.

[0109] (5) Repeat steps (2) to (4) until the capacity retention rate reaches 83%. Record the storage time and record it as the calendar life T.

[0110] Experimental Example 1

[0111] The calendar lives measured for the above-mentioned embodiments and comparative examples are shown in the table below. The calendar lives of Examples 2-10 and their deviations from Comparative Example 1 are calculated according to the following formula: Calendar life = T1 of Example 1 + T2 of each embodiment; Deviation = (calendar life of each embodiment - calendar life of Comparative Example 1) / calendar life of Comparative Example 1 × 100%.

[0112] Table 1 Test results of various embodiments and comparative examples

[0113]

[0114]

[0115] It can be seen from the above table that compared with the conventional test method of comparative example 1, the absolute value of the deviation of the calendar life of embodiments 1-11 of the present patent is <10%, indicating that the test results are accurate and reliable.

[0116] Experimental Example 2

[0117] In Examples 2-10, the tests of the control cell and the cell to be tested can be carried out simultaneously without affecting each other. The charge-discharge cycle treatment time of the cell to be tested and the test time of the actual calendar life are recorded, and the sum of the two is the total test time.

[0118] Table 2 Test time

[0119]

[0120] It can be seen from the above table that, compared with the traditional calendar life test method, the method of the present invention can significantly shorten the detection time.

[0121] Experimental Example 3

[0122] According to the method of Examples 2-10, 5 batches of tests were carried out respectively, and the calendar life of each test was recorded. The standard deviation of the deviation between the 5 batches of experiments and Comparative Example 1 and the RSD value of the calendar life of the 5 batches of experiments were calculated, as shown in the following table.

[0123] Table 3 Test results of 5 batches of experiments

[0124]

[0125] As can be seen from the above table, the RSD values ​​of the 5 calendar life tests of Examples 2-10 of this patent and the standard deviation of the deviation compared with Comparative Example 1 are all <2.5%, indicating that the test results are stable, reliable and highly reproducible.

[0126] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will readily appreciate that other variations or modifications based on the above descriptions are possible. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A calendar life testing method, characterized in that: The testing method comprises the following steps: Test steps for control cells: Perform actual calendar life test on the control cells and record the cycle life T1 at which the capacity retention rate is C%; Testing steps for the battery cell to be tested: subject the battery cell to charge and discharge cycles until the capacity retention rate of the battery cell is C%-1%~C%+1%; perform an actual calendar life test on the battery cell after the charge and discharge cycles to obtain the cycle life T2; Calculation steps: Add T1 and T2 to get the calendar life of the battery cell to be tested; Among them, 85% <C%≤101%。 2. The calendar life testing method according to claim 1, characterized in that: C% is 99%-100%.

3. The calendar life testing method according to claim 1, characterized in that: The charge-discharge cycle process comprises the following steps: (1) First resting step, (2) discharging step, (3) second resting step, (4) charging step, and repeating steps (1) to (4).

4. The calendar life testing method according to claim 3, characterized in that: The charge-discharge cycle treatment further satisfies at least one of the following 1) to 6): 1) In step (1) and / or step (3), the standing time is 30-60 min and the temperature is 25-60°C; 2) In step (2), the discharge mode is constant current discharge, the current is 0.33C-2C, and the discharge is performed to the lower cut-off voltage; 3) In step (4), the charging mode is constant current to constant voltage charging, constant current charging to the upper cut-off voltage at a charging current of 0.33C-2C, and then constant voltage charging to 0.01-0.05C; 4) In step (2), discharging is performed at a temperature of 25-60°C; 5) In step (4), charging is performed at a temperature of 25-60°C; 6) Repeat 100-300 times.

5. The calendar life testing method according to claim 4, characterized in that: In step (1) and / or step (3), the standing temperature is 25-30°C.

6. The calendar life testing method according to claim 4, characterized in that: In step (2), the discharge current is 0.8-1.2C.

7. The calendar life testing method according to claim 4, characterized in that: In step (4), the charging current is 0.8-1.2C.

8. The calendar life testing method according to claim 4, characterized in that: In step (2), the discharge is performed at a temperature of 25-30°C.

9. The calendar life testing method according to claim 4, characterized in that: In step (4), charging is performed at a temperature of 25-30°C.

10. The calendar life testing method according to any one of claims 1 to 9, characterized in that: The actual calendar life test of the control cell test steps and / or the test steps of the test cell includes the following steps: A. Charging step, B. Storage step, C. Testing step, repeat steps A to C.

11. The calendar life testing method according to claim 10, characterized in that: The actual calendar life test also satisfies at least one of the following (1)-(3): (1) The charging mode is constant current to constant voltage charging. The charging current is 0.33C-2C and the constant current is charged to the upper cut-off voltage, and then the constant voltage is charged to 0.01C-0.05C. (2) Storage temperature is 25-60℃ and storage time is 7-30 days; (3) The test steps include 1) the first rest step, 2) the discharge step, 3) the second rest step, and 4) the charge step.

12. The calendar life testing method according to claim 11, characterized in that: The test steps in the actual calendar life test meet at least one of the following AEs: A. In step 1) and / or step 3), the standing time is 30-60 minutes and the temperature is 25-60°C; B. In step 2), the discharge mode is constant current discharge with a current of 0.33C-2C, and the discharge is performed to the lower cut-off voltage; C. In step 4), the charging mode is constant current to constant voltage charging, constant current charging to the upper cut-off voltage at a charging current of 0.33C-2C, and then constant voltage charging to 0.01-0.05C; D. In step 2), discharging is performed at a temperature of 25-60°C; E. In step 4), charging is performed at a temperature of 25-60°C.

13. The calendar life testing method according to claim 10, characterized in that: The test steps for the control cell also include polynomial fitting of the correlation between the capacity retention rate of the control cell and the total storage time of the actual calendar life test treatment, and obtaining the model shown in formula (I), Y1=a1X1 2 +b1X1+c1(I); The test steps of the battery cell to be tested also include polynomial fitting of the correlation between the capacity retention rate and the charge and discharge cycle treatment time during the charge and discharge cycle treatment of the battery cell to be tested, and obtaining the model shown in formula (II), Y2=a2X2 2 +b2X2+c2(II); Where Y1 and Y2 are capacity retention rates, X1 is the total storage time of the actual calendar life test, X2 is the charge and discharge cycle processing time, and a1, b1, c1, a2, b2, and c2 are fitting parameters.

14. The calendar life testing method according to claim 13, characterized in that: The ratio of 100 times a1 to a2 is 1:9-9:1; and / or, the ratio of 10 times b1 to b2 is 1:9-9:

1.

15. The calendar life testing method according to any one of claims 1 to 9, characterized in that: The test steps of the battery cell to be tested also include performing a polynomial fit on the correlation between the capacity retention rate of the battery cell to be tested during the actual calendar life test and the total storage time in the actual calendar life test when the storage time is at least 120 days, to obtain the model shown in formula (III), Y3=a3X3 2 +b3X3+c3 (III); where Y3 is the capacity retention rate, X3 is the actual total storage time, and a3, b3, and c3 are fitting parameters; according to the model shown in formula (III), the test time corresponding to the capacity retention rate reaching 70-85% is calculated as the cycle life T2.

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