Method for testing diving point of lithium battery

Abstract

The embodiment of the invention discloses a lithium battery diving point test method, and the method comprises the steps: carrying out the cycle test of different cycle times on to-be-tested cells of M groups of lithium batteries, and M is greater than or equal to 5; obtaining the mass of FEC in the residual electrolyte of each group of to-be-tested cells after the cycle test; generating a linear relation curve between the quality of the residual FEC and the cycle number according to the cycle number of each group of to-be-detected battery cells and the quality of the FEC in the corresponding residual electrolyte; and based on the linear relation curve of the residual FEC quality and the cycle number, obtaining the cycle number when the FEC is completely consumed so as to obtain the lithium battery diving point. According to the method for testing the diving point of the lithium battery provided by the invention, the consumption speed of the electrolyte additive FEC which must be used in the lithium battery cell is cyclically tested, so that the cycle number of the lithium battery cell required by complete consumption of the FEC can be predicted, and the capacity diving point of the lithium battery cell can be accurately predicted.

Description

Technical Field

[0001] The present application relates to the field of lithium battery technology, and in particular to a method for testing a lithium battery diving point. Background Art

[0002] Lithium-ion batteries have the advantages of high energy density, long life, and environmental friendliness. In recent years, they have been widely used in electric vehicles, portable electronic devices, energy storage systems, and other fields. The positive and negative electrode materials and electrolyte are important factors affecting the performance of lithium-ion batteries.

[0003] The electrolyte, a crucial conductive medium within lithium-ion batteries, directly impacts their service life. During use, lithium-ion batteries can experience a sudden capacity drop due to electrolyte consumption. This phenomenon occurs when a lithium-ion battery's performance rapidly deteriorates and its capacity decays after a certain point in time. This capacity drop significantly reduces the battery's performance, making it susceptible to failure and even thermal runaway. This poses a significant safety risk to lithium-ion batteries and poses a challenge to the safety and reliability of lithium-ion battery systems.

[0004] Therefore, there is an urgent need for a method to predict the capacity drop point of lithium battery cells. Summary of the Invention

[0005] This application aims to address at least one of the technical problems existing in the prior art. To this end, this application proposes a method for testing the capacity drop point of a lithium battery to address the current difficulty in predicting the capacity drop point of a lithium battery cell, which results in poor safety and reliability of lithium battery system operation.

[0006] Specifically, the present application provides a method for testing the diving point of a lithium battery, the testing method comprising:

[0007] Perform cycle tests on M groups of lithium battery cells under test for different cycles, M ≥ 5;

[0008] Obtaining the mass of FEC in the residual electrolyte after the cycle test of each group of battery cells to be tested;

[0009] Generating a linear relationship curve between the residual FEC mass and the cycle number according to the cycle number of each group of the battery cells to be tested and the mass of the FEC in the corresponding residual electrolyte;

[0010] The number of cycles when the FEC is completely consumed is obtained based on the linear relationship curve between the residual FEC mass and the cycle number, so as to obtain the lithium battery diving point.

[0011] The test method for the lithium battery diving point provided in this application can predict the number of cycles of the lithium battery cell required for the complete consumption of FEC by cyclically testing the consumption rate of the electrolyte additive FEC that must be used in the lithium battery cell, and thus accurately predict the capacity diving point of the lithium battery cell.

[0012] In one embodiment of the above-mentioned method for testing the water drop point of a lithium battery, generating a linear relationship curve between the residual FEC mass and the cycle number according to the cycle number of each group of the battery cells to be tested and the corresponding mass of FEC in the residual electrolyte includes:

[0013] Generate a relationship curve between the residual FEC mass and the cycle number according to the different cycle numbers of each group of the battery cells to be tested and the corresponding FEC mass in the residual electrolyte;

[0014] A linear segment curve in which the residual FEC quality and the cycle number are in a linear relationship is obtained from the residual FEC quality and the cycle number relationship curve, and a linear relationship curve between the residual FEC quality and the cycle number is generated.

[0015] A relationship curve between the residual FEC mass and the number of cycles is generated based on the number of cycles and the corresponding FEC mass in the residual electrolyte. A linear segment curve in which the residual FEC mass and the number of cycles are linearly related is then selected. This can accurately reflect the linear relationship between the residual FEC mass and the number of cycles of the battery cell to be tested after cycling. Therefore, the number of cycles corresponding to the cell diving point when the residual FEC mass is 0 can be predicted based on the linear relationship between the residual FEC mass and the number of cycles.

[0016] In one embodiment of the above-mentioned method for testing the water drop point of a lithium battery, generating a curve of the relationship between the residual FEC mass and the number of cycles according to the different cycle numbers of each group of the battery cells to be tested and the corresponding FEC mass in the residual electrolyte includes:

[0017] A relationship curve between the residual FEC mass and the cycle number is generated with the increasing cycle number of the battery cell to be tested as the horizontal axis coordinate and the mass of FEC in the residual electrolyte corresponding to the cycle number of the battery cell to be tested as the vertical axis coordinate.

[0018] A relationship curve between the residual FEC mass and the cycle number is generated with the increasing cycle number of the battery cell to be tested as the horizontal axis coordinate and the mass of FEC in the residual electrolyte corresponding to the cycle number of the battery cell to be tested as the vertical axis coordinate. This can intuitively show the changing trend of the residual FEC mass of the battery cell to be tested with increasing cycle number after cycling.

[0019] In one embodiment of the above-mentioned method for testing the water-diving point of a lithium battery, the number of cycles of the battery cells to be tested in different groups during the cycle test is an arithmetic progression;

[0020] The obtaining of a linear segment curve showing a linear relationship between the residual FEC quality and the cycle number in the curve of the relationship between the residual FEC quality and the cycle number, and generating a linear curve of the relationship between the residual FEC quality and the cycle number comprises:

[0021] Calculating the difference in residual FEC quality corresponding to each two adjacent horizontal axis coordinate points in the linear relationship curve between the residual FEC quality and the number of cycles;

[0022] Determining whether a difference in residual FEC qualities corresponding to the two adjacent horizontal axis coordinate points is within a preset range;

[0023] If so, a line segment starting from the previous horizontal axis coordinate point in the linear relationship curve between the residual FEC quality and the cycle number is obtained as the linear relationship curve between the residual FEC quality and the cycle number.

[0024] The FEC quality test is performed at the same cycle intervals. When the error of the FEC amount consumed at the same cycle intervals is within a preset range, it is judged that the relationship between the residual FEC quality and the cycle number is linear. Based on the linear relationship between the residual FEC quality and the cycle number, the changing trend of the residual FEC quality of the battery cell to be tested as the cycle number increases can be accurately predicted.

[0025] In one embodiment of the above-mentioned method for testing the water-diving point of a lithium battery, obtaining the number of cycles when the FEC is completely consumed based on the linear relationship curve between the residual FEC mass and the number of cycles includes:

[0026] Linearly fitting the linear relationship curve between the residual FEC mass and the cycle number to obtain a target slope;

[0027] The number of cycles corresponding to when the residual FEC mass is 0 is calculated according to the target slope, and the number of cycles when the FEC is completely consumed is obtained.

[0028] The linear fitting shows the residual FEC mass and cycle number data after linearity is obtained to obtain the slope of the linear relationship between the residual FEC mass and the cycle number. The relationship between the FEC consumption rate and the cycle number is calculated based on the obtained slope, so as to obtain the cycle number at which the FEC is completely consumed, that is, the cell capacity drops, and accurately evaluate the cycle life of the lithium-ion battery cell.

[0029] In one embodiment of the above-mentioned method for testing the water drop point of a lithium battery, the method for obtaining the mass of FEC in the residual electrolyte of each group of cells to be tested after the cycle test includes:

[0030] Before performing a cycle test on the M groups of cells to be tested, obtain the initial total mass of each cell to be tested;

[0031] After the cycle test is completed, the battery cells to be tested are disassembled and dried to obtain the residual mass of the battery cells;

[0032] Calculating the difference between the initial total mass of the battery cell to be tested and the residual mass of the battery cell to obtain the residual electrolyte mass;

[0033] Detecting the mass proportion of FEC in the residual electrolyte in the disassembled battery cell based on GC-MS;

[0034] The mass of FEC in the residual electrolyte is obtained based on the ratio of the mass of the residual electrolyte to the mass of FEC in the residual electrolyte.

[0035] In one embodiment of the above-mentioned method for testing the water drop point of a lithium battery, the disassembling and drying the battery cell to be tested to obtain the residual mass of the battery cell includes:

[0036] Disassembling the battery cell to be tested in a dry environment and collecting the free residual electrolyte in the battery cell to be tested;

[0037] Separate and immerse the positive electrode sheet, negative electrode sheet and separator of the battery cell to be tested in a solvent for a preset time;

[0038] Cleaning the positive electrode sheet, the negative electrode sheet and the separator of the battery cell to be tested to remove residual lithium salt and solvent in the positive electrode sheet, the negative electrode sheet and the separator of the battery cell to be tested;

[0039] Baking the positive electrode sheet, the negative electrode sheet and the separator of the battery cell to be tested to remove the positive electrode sheet, the negative electrode sheet and the separator of the battery cell to be tested until the mass remains unchanged;

[0040] Weigh all disassembled components of the battery cell to be tested to obtain the residual mass of the battery cell.

[0041] The total mass of the battery cell - the mass of the battery cell after disassembly, soaking in solvent, cleaning and drying = the mass of the residual electrolyte. The mass proportion of FEC in the residual electrolyte of the battery cell is measured by gas chromatography-mass spectrometry, and then the mass of FEC in the residual electrolyte is calculated, so that the accurate mass of FEC in the residual electrolyte can be obtained.

[0042] In one embodiment of the above-mentioned method for testing the water drop point of a lithium battery, the method for obtaining the mass of FEC in the residual electrolyte of each group of the cells to be tested after the cycle test further includes:

[0043] After the cycle test of the battery cell to be tested, disassembling the battery cell to be tested to obtain residual electrolyte;

[0044] Adding an internal standard of known mass to the residual electrolyte and soaking the residual electrolyte for a preset time to obtain a soaking solution;

[0045] The internal standard and FEC peak areas and relative response factors are obtained by GC-MS testing the soaking solution;

[0046] The mass of FEC in the residual electrolyte was obtained based on the known mass of the internal standard, the peak areas of the internal standard and FEC, and the relative response factor.

[0047] An internal standard is added to the residual electrolyte and soaked, and the soaked liquid is taken for gas chromatography-mass spectrometry testing. When the mass of the added internal standard is known, the peak area of the internal standard and FEC and the relative response factor obtained by gas chromatography-mass spectrometry testing can be used to calculate the accurate FEC mass in the residual electrolyte in the battery cell.

[0048] In one embodiment of the above-mentioned method for testing the water drop point of a lithium battery, each group of the M groups of lithium battery cells to be tested has at least two cells to be tested, and obtaining the mass of FEC in the residual electrolyte of each group of cells to be tested after the cycle test includes:

[0049] Obtain the mean mass of FEC in the residual electrolyte after the cycle test of each group of battery cells to be tested.

[0050] Each group has at least two battery cells to be tested, and the average mass value of the FEC in the residual electrolyte after the testing of at least two battery cells to be tested is used as the mass value of the FEC in the residual electrolyte after the cycle test of the group. This can avoid the error in the result of a single battery cell to be tested causing an excessively large mass error in the FEC in the residual electrolyte after the cycle test of the group, thereby affecting the accuracy of the generated linear relationship curve between the residual FEC mass and the number of cycles.

[0051] In one embodiment of the above-mentioned method for testing the water drop point of a lithium battery, the method further comprises:

[0052] Before performing cycle tests of different cycles on the M groups of lithium battery cells to be tested, the M groups of cells to be tested having performance index values within a preset range are screened out based on preset performance indexes.

[0053] By pre-screening M groups of test cells with performance index values within a preset range, M groups of test cells with similar performance are screened out for cycle testing to ensure that the linear curve drawn based on the result values of the cycle test of the M groups of test cells is accurate.

[0054] Additional aspects and advantages of the present application will be given in part in the description below, and in part will become apparent from the description below, or will be learned through practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] The disclosure of this application will be more easily understood with reference to the accompanying drawings. Those skilled in the art will readily appreciate that these drawings are for illustrative purposes only and are not intended to limit the scope of protection of this application. Furthermore, similar numbers in the figures represent similar components, where:

[0056] Figure 1 This is a flow chart of a method for testing a lithium battery diving point provided by one embodiment of the present application;

[0057] Figure 2 1 is a diagram showing the relationship between the residual FEC quality and the number of cycles generated in Example 1 of the present application;

[0058] Figure 3 Schematic diagram of the linear relationship curve between the residual FEC quality and the number of cycles generated in Example 1 of the present application;

[0059] Figure 4 1 is a graph showing the relationship between the residual FEC quality and the number of cycles in the cycle test of the verification group H in Example 1 of the present application;

[0060] Figure 5 Schematic diagram of the relationship between the residual FEC quality and the number of cycles generated in Example 2 of the present application;

[0061] Figure 6 Schematic diagram of the linear relationship curve between the residual FEC quality and the number of cycles generated in Example 2 of the present application;

[0062] Figure 7 This is a diagram showing the relationship between the residual FEC quality and the number of cycles in the cycle test of the verification group F in Example 2 of the present application. DETAILED DESCRIPTION

[0063] As described in the background section, the electrolyte, as a crucial conductive medium within lithium-ion batteries, directly impacts their service life. As lithium-ion batteries are recycled, the electrolyte within the cells depletes. When the electrolyte depletion reaches a threshold, the battery capacity plummets, rapidly deteriorating performance and becoming susceptible to failure or even thermal runaway, posing a significant safety risk to lithium-ion batteries.

[0064] To solve the above problems, the present application creatively proposes a method for testing the diving point of a lithium battery. By cyclically testing the consumption rate of FEC, an electrolyte additive that must be used in lithium battery cells, it is possible to predict the number of cycles of the lithium battery cells required for complete consumption of FEC, and then accurately predict the capacity diving point of the lithium battery cells, thereby facilitating timely replacement of the lithium battery when the capacity diving phenomenon is about to occur, thereby avoiding safety accidents such as lithium battery failure or even thermal runaway due to capacity diving.

[0065] Some embodiments of the present application are described below with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are only used to explain the technical principles of the present application and are not intended to limit the scope of protection of the present application.

[0066] The present application will be described in detail below through specific embodiments.

[0067] Specifically, the present invention provides a method for testing the water drop point of a lithium battery. Figure 1 As shown, the test methods include:

[0068] S110. Perform cycle tests of different numbers of cycles on the M groups of lithium battery cells to be tested, where M is ≥ 5.

[0069] S120: Obtain the mass of FEC in the residual electrolyte after the cycle test of each group of battery cells to be tested.

[0070] S130 , generating a linear relationship curve between the residual FEC mass and the cycle number according to the cycle number of each group of cells to be tested and the mass of FEC in the corresponding residual electrolyte.

[0071] S140. Obtain the cycle number when the FEC is completely consumed based on the linear relationship curve between the residual FEC mass and the cycle number, so as to obtain the lithium battery diving point.

[0072] The present application provides a method for testing the capacity drop point of a lithium battery. By cyclically testing the consumption rate of FEC, an electrolyte additive that must be used in lithium battery cells, the method can predict the number of cycles of the lithium battery cells required for complete consumption of FEC, and thus accurately predict the capacity drop point of the lithium battery cells. The lower limit of the number of groups of cells to be tested is set to 5 groups; otherwise, a complete and effective relationship curve cannot be formed. For example, if there are only one or two groups of data, an effective linear relationship curve cannot be formed.

[0073] Among them, FEC (Fluoroethylene carbonate) is an electrolyte additive that must be used in the silicon-based negative electrode of lithium battery cells. When FEC is completely consumed, the lithium battery cell will immediately experience a diving phenomenon.

[0074] The M groups of lithium-ion batteries have identical or similar performance. Similar performance of the tested cells means that the numerical error of the performance indicators of the tested cells is within a preset range, for example, the numerical error of the performance indicators of the tested cells is within ±5%. The performance indicator of the tested cells refers to at least one of the capacity, energy density, charge / discharge rate, and voltage of the lithium-ion battery cells.

[0075] In some embodiments, step S130 includes:

[0076] Generate a relationship curve between residual FEC mass and cycle number based on the different cycle numbers of each group of cells to be tested and the corresponding FEC mass in the residual electrolyte;

[0077] A linear segment curve showing a linear relationship between the residual FEC quality and the cycle number is obtained from the curve showing a relationship between the residual FEC quality and the cycle number, and a linear relationship curve between the residual FEC quality and the cycle number is generated.

[0078] The relationship curve between the residual FEC mass and the number of cycles includes a nonlinear segment curve and a linear segment curve. When the lithium battery cell undergoes the initial cycle cycle, the FEC mass in the residual electrolyte decreases with the increase of the cycle cycle, but the FEC mass in the residual electrolyte does not show a linear relationship with the cycle cycle. After the cycle cycle reaches a certain amount, the FEC mass in the residual electrolyte shows a linear correlation with the cycle cycle.

[0079] A relationship curve between the residual FEC mass and the number of cycles is generated based on the number of cycles and the corresponding FEC mass in the residual electrolyte. A linear segment curve in which the residual FEC mass and the number of cycles are linearly related is then selected. This can accurately reflect the linear relationship between the residual FEC mass and the number of cycles of the battery cell to be tested after cycling. Therefore, the number of cycles corresponding to the cell diving point when the residual FEC mass is 0 can be predicted based on the linear relationship between the residual FEC mass and the number of cycles.

[0080] In some embodiments, generating a curve of the relationship between the residual FEC mass and the number of cycles according to the different number of cycles of each group of cells to be tested and the corresponding mass of FEC in the residual electrolyte includes:

[0081] A relationship curve between the residual FEC mass and the cycle number is generated with the increasing cycle number of the battery cell to be tested as the horizontal axis coordinate and the mass of FEC in the residual electrolyte corresponding to the cycle number of the battery cell to be tested as the vertical axis coordinate.

[0082] A relationship curve between the residual FEC mass and the cycle number is generated with the increasing cycle number of the battery cell to be tested as the horizontal axis coordinate and the mass of FEC in the residual electrolyte corresponding to the cycle number of the battery cell to be tested as the vertical axis coordinate. This can intuitively show the changing trend of the residual FEC mass of the battery cell to be tested with increasing cycle number after cycling.

[0083] In some embodiments, the number of cycles of different groups of cells to be tested during the cycle test is an arithmetic progression;

[0084] Obtaining a linear segment curve showing a linear relationship between the residual FEC quality and the cycle number in the curve of the relationship between the residual FEC quality and the cycle number, and generating a linear relationship curve between the residual FEC quality and the cycle number includes:

[0085] Calculate the difference in residual FEC quality corresponding to each two adjacent horizontal axis coordinate points in the linear relationship curve between residual FEC quality and cycle number;

[0086] Determine whether the difference between the residual FEC qualities corresponding to two adjacent horizontal axis coordinate points is within a preset range;

[0087] If so, a line segment starting from the previous horizontal axis coordinate point in the linear relationship curve between the residual FEC quality and the cycle number is obtained as the linear relationship curve between the residual FEC quality and the cycle number.

[0088] The FEC quality test is performed at the same cycle intervals. When the error of the FEC amount consumed at the same cycle intervals is within a preset range, it is judged that the relationship between the residual FEC quality and the cycle number is linear. Based on the linear relationship between the residual FEC quality and the cycle number, the changing trend of the residual FEC quality of the battery cell to be tested as the cycle number increases can be accurately predicted.

[0089] For example only, when the error of the amount of FEC consumed at the same interval of cycles is within a range of ±5%, it is determined that the relationship between the residual FEC quality and the number of cycles is linear.

[0090] In some embodiments, obtaining the number of cycles when the FEC is completely consumed based on a linear relationship curve between the residual FEC quality and the number of cycles includes:

[0091] The target slope is obtained by linear fitting the linear relationship curve between residual FEC mass and cycle number;

[0092] The number of cycles corresponding to when the residual FEC mass is 0 is calculated according to the target slope, and the number of cycles when the FEC is completely consumed is obtained.

[0093] The linear fitting shows the residual FEC mass and cycle number data after linearity is obtained to obtain the slope of the linear relationship between the residual FEC mass and the cycle number. The relationship between the FEC consumption rate and the cycle number is calculated based on the obtained slope, so as to obtain the cycle number at which the FEC is completely consumed, that is, the cell capacity drops, and accurately evaluate the cycle life of the lithium-ion battery cell.

[0094] In some embodiments, each group of M groups of lithium battery cells to be tested has at least two cells to be tested, and step S120 includes:

[0095] Obtain the average mass of FEC in the residual electrolyte after the cycle test of each group of tested cells.

[0096] Each group has at least two battery cells to be tested, and the average mass value of the FEC in the residual electrolyte after the testing of at least two battery cells to be tested is used as the mass value of the FEC in the residual electrolyte after the cycle test of the group. This can avoid the error in the result of a single battery cell to be tested causing an excessively large mass error in the FEC in the residual electrolyte after the cycle test of the group, thereby affecting the accuracy of the generated linear relationship curve between the residual FEC mass and the number of cycles.

[0097] In some embodiments, the mass of FEC in the residual electrolyte after the cycle test of each group of cells to be tested is obtained by differential weight method, and the specific steps include:

[0098] Before performing a cycle test on the M groups of cells to be tested, obtaining the initial total mass of each cell to be tested;

[0099] After the cycle test is completed, the battery cells to be tested are disassembled and dried to obtain the residual mass of the battery cells;

[0100] Calculate the difference between the initial total mass of the battery cell to be tested and the residual mass of the battery cell to obtain the residual electrolyte mass;

[0101] The mass ratio of FEC in the residual electrolyte in the disassembled battery cell to be tested is detected based on GC-MS;

[0102] The mass of FEC in the residual electrolyte was obtained based on the mass ratio of the residual electrolyte to the mass of FEC in the residual electrolyte.

[0103] Among them, before implementing the cycle test on the M groups of battery cells to be tested, obtaining the initial total mass of each battery cell to be tested refers to the mass of the battery cell obtained by weighing the battery cell to be tested after directly discharging the battery cell to be tested to the lower limit voltage.

[0104] The term "lower voltage" refers to the point during discharge when the battery voltage drops to approximately 3.0 to 3.2 volts. At this point, the battery is nearly depleted and needs to be recharged.

[0105] Preferably, disassembling and drying the battery cell to be tested to obtain the residual mass of the battery cell includes:

[0106] Disassemble the battery cell to be tested in a dry environment and collect the residual electrolyte in the battery cell to be tested;

[0107] Separate and immerse the positive electrode sheet, negative electrode sheet and separator of the battery cell to be tested in a solvent for a preset time;

[0108] Clean the positive electrode sheet, negative electrode sheet and diaphragm of the battery cell to be tested to remove residual lithium salt and solvent in the positive electrode sheet, negative electrode sheet and diaphragm of the battery cell to be tested;

[0109] Bake the positive electrode sheet, negative electrode sheet and separator of the battery cell to be tested to remove the positive electrode sheet, negative electrode sheet and separator of the battery cell to be tested until the mass remains unchanged;

[0110] Weigh all disassembled components of the battery cell to be tested to obtain the residual mass of the battery cell.

[0111] The total mass of the battery cell - the mass of the battery cell after disassembly, soaking in solvent, cleaning and drying = the mass of the residual electrolyte. The mass proportion of FEC in the residual electrolyte of the battery cell is measured by gas chromatography-mass spectrometry, and then the mass of FEC in the residual electrolyte is calculated, so that the accurate mass of FEC in the residual electrolyte can be obtained.

[0112] In some embodiments, the mass of FEC in the residual electrolyte after the cycle test of each group of cells to be tested is obtained by an internal standard method, and the specific steps include:

[0113] After the cycle test of the battery cell to be tested, the battery cell to be tested is disassembled to obtain the residual electrolyte;

[0114] Add an internal standard of known mass to the residual electrolyte and soak for a preset time to obtain a soaking solution;

[0115] The internal standard and FEC peak areas and relative response factors were obtained based on GC-MS testing of the soaking solution;

[0116] The mass of FEC in the residual electrolyte was obtained based on the known mass of the internal standard, the peak areas of the internal standard and FEC, and the relative response factor.

[0117] An internal standard is added to the residual electrolyte and soaked, and the soaked liquid is taken for gas chromatography-mass spectrometry testing. When the mass of the added internal standard is known, the peak area of the internal standard and FEC and the relative response factor obtained by gas chromatography-mass spectrometry testing can be used to calculate the accurate FEC mass in the residual electrolyte in the battery cell.

[0118] GC-MS (Gas Chromatography-Mass Spectrometry).

[0119] In some embodiments, the M groups of lithium battery cells to be tested with the same or similar performance can be obtained by screening out through preliminary performance testing, and the M groups of cells to be tested with performance index values within a preset range are screened based on preset performance indicators.

[0120] By pre-screening M groups of test cells with performance index values within a preset range, M groups of test cells with similar performance are screened out for cycle testing to ensure that the linear curve drawn based on the result values of the cycle test of the M groups of test cells is accurate.

[0121] The present application will be further described in detail below with reference to the examples. It should be noted that the following examples are intended only to illustrate the present application and are not intended to limit the scope of protection claimed in the present application and should not be construed as limiting the scope of protection of the present application. Any non-essential improvements and adjustments made by those skilled in the art based on the contents of the present application above still fall within the scope of protection of the present application.

[0122] In the following examples, the reagents, materials and instruments used are all commercially available products and can be purchased unless otherwise specified.

[0123] Example 1: 1. Prepare a 1.7Ah small soft-pack battery cell, wherein the positive electrode active material is NCM622 ternary material, the negative electrode active material is graphite / silicon composite material, and the battery cell injection amount is 12g.

[0124] 2. Through performance testing, select 16 cells whose initial performance is within the specifications as the cells to be tested. These cells to be tested are divided into 8 groups (A / B / C / D / E / F / G / H), with 2 cells in each group. Take the cells in group A and perform the following operations:

[0125] 1) Discharge to the lower limit voltage with 0.1C, weigh the battery cell, and record the battery cell mass A0;

[0126] 2) Disassemble the battery cell in a dry environment, collect the free residual electrolyte in the battery cell during the disassembly process, and perform GC-MS analysis. The FEC mass fraction obtained by the test is recorded as a0;

[0127] 3) Keep all the disassembled battery cell components, separate the positive electrode sheet, negative electrode sheet and separator, and soak them in DMC (dimethyl carbonate) solvent at room temperature for 12 hours to clean the residual lithium salt and solvent in the positive electrode sheet, negative electrode sheet and separator;

[0128] 4) Take out the soaked positive electrode sheet, negative electrode sheet and separator and bake them at a baking temperature of 45°C until the mass of the positive electrode sheet, negative electrode sheet and separator remains unchanged. Weigh all components retained during the disassembly process and record the mass A1.

[0129] A0-A1 is the total mass of the residual electrolyte inside the battery cell, (A0-A1)*a0 is the remaining FEC mass inside the battery cell, and the data values are shown in Table 1:

[0130] Table 1

[0131]

[0132]

[0133] 3. Cycle the above 7 groups of cells B / C / D / E / F / G / H. The cycle mode is 0.5C constant current charge to 4.25V, constant voltage to 0.05C, and 1C constant current discharge to 2.8V.

[0134] The cells in groups B / C / D / E / F / G were cycled 100, 200, 300, 400, 500, and 600 times, respectively. The cells were then tested and calculated according to 1)-4) in step 2, yielding the data shown in Table 2. The cells in group H were cycled continuously until the capacity retention rate accelerated.

[0135] Table 2

[0136]

[0137] 4. Plot the data in Table 2, with the cycle number of the battery cells in each group as the horizontal axis and the mean mass of FEC in the residual electrolyte as the vertical axis, and obtain the following: Figure 2 The results show that after 400 cycles, the consumption rate of FEC with the charge and discharge cycle is basically linear; the data of 400-600 cycles are linearly fitted to obtain Figure 3 The results shown are: the slope is -0.205; therefore, the FEC consumption rate is 0.205 mg / cycle; from this, it can be inferred that after 1052 cycles, the FEC is completely consumed and the capacity decay accelerates.

[0138] 5.H group cycle data such as Figure 4 As shown, the accelerated inflection point of its capacity decay rate is consistent with the above-mentioned prediction result, which proves that the prediction method of the lithium battery diving point provided in this application can accurately predict the capacity diving point of the lithium battery silicon-based negative electrode cell.

[0139] Example 2: 1. Prepare a 1.5Ah small soft-pack battery cell, wherein the positive electrode active material is NCM622 ternary material, the negative electrode active material is graphite / silicon composite material, and the battery cell injection amount is 10g.

[0140] 2. Select 10 cells whose initial performance is within the specifications as the cells to be tested;

[0141] Divide the above 10 cells into 6 groups (A / B / C / D / E / F), with 2 cells in each group. Take the cells in group A and perform the following operations:

[0142] 1) Discharge to the lower limit voltage with 0.1C, weigh the battery cell, and record the battery cell mass A0;

[0143] 2) Disassemble the battery cell in a dry environment, separate the positive electrode sheet, negative electrode sheet and separator, and soak the positive and negative electrode sheets, separator and other components with electrolyte in DMC solvent, add a known mass of internal standard, and soak at room temperature for 12 hours;

[0144] 3) Take the soaking solution for GC-MS testing. The mass of the added internal standard is known. The FEC residual mass can be calculated based on the internal standard and FEC peak areas and relative response factors obtained by GC-MS testing.

[0145] 3. The four groups of cells (B / C / D / E) were cycled using a 0.5C constant current charge to 4.25V, constant voltage to 0.05C, and 1C constant current discharge to 2.8V. Cells in groups B / C / D / E were cycled 200, 400, 500, and 600 times, respectively. The test calculations were performed according to 1)-3) in step 2, yielding the data shown in Table 3 below. Cells in group F were cycled continuously until the capacity retention rate accelerated.

[0146] Table 3

[0147]

[0148] The data in Table 3 are plotted with the cycle number of each group of cells as the horizontal axis and the mean mass of FEC in the residual electrolyte as the vertical axis. Figure 5 The results show that after 400 cycles, the consumption rate of FEC with the charge and discharge cycle is basically linear; the data of 400-600 cycles are linearly fitted to obtain Figure 6 The results show that the slope is -0.191; therefore, the FEC consumption rate is 0.191 mg / cycle; from this, it can be inferred that after 935 cycles, FEC is completely consumed and the capacity decay is accelerated. Figure 7 As shown, the accelerated inflection point of its capacity decay rate is consistent with the above-mentioned prediction result, which proves that the prediction method of the lithium battery diving point provided in this application can accurately predict the capacity diving point of the lithium battery silicon-based negative electrode cell.

[0149] Throughout this specification, reference to terms such as "one embodiment," "some embodiments," "examples," "specific examples," or "some examples" means that a specific feature, structure, material, or characteristic described in conjunction with that embodiment or example is included in at least one embodiment or example of the present application. In this specification, schematic representations of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0150] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of such features. Throughout the description of this application, "plurality" means at least two, for example, two, three, etc., unless otherwise specifically defined.

[0151] Although the embodiments of the present application have been shown and described above, it can be understood that the above embodiments are exemplary and cannot be understood as limitations on the present application. Ordinary technicians in this field can change, modify, replace and modify the above embodiments within the scope of the present application.

Claims

1. A method for testing the diving point of a lithium battery, characterized in that: The test method includes: Perform cycle tests of different cycles on M groups of lithium battery cells to be tested, where the number of groups of cells to be tested, M, is ≥ 5; Obtaining the mass of FEC in the residual electrolyte after the cycle test of each group of battery cells to be tested; Generating a linear relationship curve between the residual FEC mass and the cycle number according to the cycle number of each group of the battery cells to be tested and the mass of the FEC in the corresponding residual electrolyte; The number of cycles when the FEC is completely consumed is obtained based on the linear relationship curve between the residual FEC mass and the cycle number, so as to obtain the lithium battery diving point.

2. The method for testing the water-diving point of a lithium battery according to claim 1, wherein: Generating a linear relationship curve between the residual FEC mass and the cycle number according to the cycle number of each group of the battery cells to be tested and the corresponding FEC mass in the residual electrolyte includes: Generate a relationship curve between the residual FEC mass and the cycle number according to the different cycle numbers of each group of the battery cells to be tested and the corresponding FEC mass in the residual electrolyte; A linear segment curve in which the residual FEC quality and the cycle number are in a linear relationship is obtained from the residual FEC quality and the cycle number relationship curve, and a linear relationship curve between the residual FEC quality and the cycle number is generated.

3. The method for testing the water-diving point of a lithium battery according to claim 2, wherein: Generating a curve of the relationship between the residual FEC mass and the cycle number according to the different cycle numbers of each group of the battery cells to be tested and the corresponding FEC mass in the residual electrolyte includes: A relationship curve between the residual FEC mass and the cycle number is generated with the increasing cycle number of the battery cell to be tested as the horizontal axis coordinate and the mass of FEC in the residual electrolyte corresponding to the cycle number of the battery cell to be tested as the vertical axis coordinate.

4. The method for testing the water-diving point of a lithium battery according to claim 2, wherein: The number of cycles of the battery cells to be tested in different groups during the cycle test is an arithmetic progression.

5. The method for testing the water-diving point of a lithium battery according to claim 2, wherein: The obtaining of a linear segment curve showing a linear relationship between the residual FEC quality and the cycle number in the curve of the relationship between the residual FEC quality and the cycle number, and generating a linear curve of the relationship between the residual FEC quality and the cycle number comprises: Calculating the difference in residual FEC quality corresponding to each two adjacent horizontal axis coordinate points in the linear relationship curve between the residual FEC quality and the number of cycles; Determining whether a difference in residual FEC qualities corresponding to the two adjacent horizontal axis coordinate points is within a preset range; If so, a line segment starting from the previous horizontal axis coordinate point in the linear relationship curve between the residual FEC quality and the cycle number is obtained as the linear relationship curve between the residual FEC quality and the cycle number.

6. The method for testing the water drop point of a lithium battery according to any one of claims 1 to 5, characterized in that: The obtaining of the number of cycles when the FEC is completely consumed based on the linear relationship curve between the residual FEC mass and the number of cycles includes: Linearly fitting the linear relationship curve between the residual FEC mass and the cycle number to obtain a target slope; The number of cycles corresponding to when the residual FEC mass is 0 is calculated according to the target slope, and the number of cycles when the FEC is completely consumed is obtained.

7. The method for testing the water-diving point of a lithium battery according to claim 1, wherein: Obtaining the mass of FEC in the residual electrolyte after the cycle test of each group of cells to be tested includes: Before performing a cycle test on the M groups of cells to be tested, obtain the initial total mass of each cell to be tested; After the cycle test is completed, the battery cells to be tested are disassembled and dried to obtain the residual mass of the battery cells; Calculating the difference between the initial total mass of the battery cell to be tested and the residual mass of the battery cell to obtain the residual electrolyte mass; Detecting the mass proportion of FEC in the residual electrolyte in the disassembled battery cell based on GC-MS; The mass of FEC in the residual electrolyte is obtained based on the ratio of the mass of the residual electrolyte to the mass of FEC in the residual electrolyte.

8. The method for testing the water-diving point of a lithium battery according to claim 1, wherein: The obtaining of the mass of FEC in the residual electrolyte after the cycle test of each group of cells to be tested further comprises: After the cycle test of the battery cell to be tested, disassembling the battery cell to be tested to obtain residual electrolyte; Adding an internal standard of known mass to the residual electrolyte and soaking the residual electrolyte for a preset time to obtain a soaking solution; The internal standard and FEC peak areas and relative response factors are obtained by GC-MS testing the soaking solution; The mass of FEC in the residual electrolyte was obtained based on the known mass of the internal standard, the peak areas of the internal standard and FEC, and the relative response factor.

9. The method for testing the water-diving point of a lithium battery according to claim 1, wherein: Each of the M groups of lithium battery cells to be tested has at least two cells to be tested, and obtaining the mass of FEC in the residual electrolyte after the cycle test of each group of cells to be tested includes: Obtain the mean mass of FEC in the residual electrolyte after the cycle test of each group of battery cells to be tested.

10. The method for testing the water-diving point of a lithium battery according to claim 1, wherein: The method further comprises: Before performing cycle tests of different cycles on the M groups of lithium battery cells to be tested, the M groups of cells to be tested having performance index values within a preset range are screened out based on preset performance indexes.

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