Method for determining optimal liquid injection quantity of battery cell

Abstract

The invention belongs to the technical field of secondary batteries, and particularly relates to a method for determining the optimal liquid injection amount of a battery cell. From the perspective of electrolyte infiltration and electrolyte consumption of SEI film interface film formation, the electrolyte loss amount, the core package net liquid amount and the free liquid amount are determined through capacity grading full-electricity interface disassembly, and the optimal core package net liquid amount and the optimal loss amount are determined according to a broken line graph of cell free liquid amount changes of different liquid injection amounts. And the optimal free liquid amount is determined by evaluating the capacity retention ratio and the full-charge interface of the accelerated circulation, and finally the optimal liquid injection amount is obtained. Compared with the prior art, the determination method provided by the invention considers the performance of battery cell aging, conventional circulation and even accelerated circulation and electrolyte consumption, accurately evaluates the optimal free liquid amount and the optimal liquid injection amount of the battery cell, reduces the risk of circulation diving caused by insufficient electrolyte in the circulation process, and improves the reliability of the battery cell. And meanwhile, raw materials and time cost are saved.

Description

Technical Field

[0001] The present application belongs to the technical field of secondary batteries, and specifically relates to a method for determining an optimal liquid injection amount for a battery cell. Background Art

[0002] Lithium-ion batteries generally include positive electrodes, negative electrodes, separators, and electrolytes, among which the electrolyte plays the role of conducting ions between the positive and negative electrodes, and is an important guarantee for lithium-ion batteries to obtain excellent electrical properties such as high voltage and high specific energy. If there is too little electrolyte in the battery cell, the transmission channels will be reduced, which can easily cause the battery performance to deteriorate. However, if there is too much electrolyte, the amount of electrolyte free between the electrodes will increase, which will increase the probability of electrolyte leakage and reaction with active metals, leading to an increase in battery side reactions, increased gas production, and poor cycle performance. In addition, too much free electrolyte will also participate in violent reactions such as combustion and explosion, resulting in reduced safety performance. Therefore, the amount of electrolyte injection in secondary batteries is a very important design parameter in the battery.

[0003] At present, in the design and development process of secondary batteries, the optimal free liquid amount of both soft-pack and square-shell lithium-ion batteries is generally verified by performing different gradient injection amounts on the battery cells during the trial production of the battery cells, and then disassembling the battery cells in the fully charged state at different capacities to determine the optimal free liquid amount of the battery cells. Then, the electrolyte loss during the battery life cycle is obtained through conventional cycles, and then the electrolyte injection amount is obtained.

[0004] However, this evaluation method not only consumes a large amount of battery cell raw materials and time costs, but also cannot accurately evaluate the optimal filling amount of the battery cell, because it does not take into account the entire life cycle of the battery cell, such as aging, and the electrolyte loss caused by the battery cell itself, and does not consider the performance of high-temperature accelerated cycling and electrolyte consumption. Summary of the invention

[0005] Therefore, the technical problem to be solved by the present application is to overcome the defects in the prior art such as the inability to accurately evaluate the optimal liquid filling amount of the battery cell or the long evaluation cycle, thereby providing a method for determining the optimal liquid filling amount of the battery cell.

[0006] To this end, this application provides the following technical solutions:

[0007] According to one aspect of the present application, a method for determining an optimal liquid injection amount of a battery cell is provided, comprising the following steps:

[0008] S1, take several groups of unfilled battery cells, record the mass m0 of the unfilled battery cells, and inject electrolyte into each group of unfilled battery cells with different injection amounts;

[0009] S2, after liquid injection, carry out infiltration, formation, aging, and exhaust to remove excess electrolyte to obtain a liquid-injected battery cell, record the mass m5 of the liquid-injected battery cell, and obtain the core package liquid volume m1, m1=m5-m0;

[0010] S3, the battery cell obtained in S2 is divided into different capacities, fully charged after the divided capacity is removed from the production line, disassembled, the electrolyte is poured out to obtain the free liquid volume, the electrolyte adhering to the battery cell shell is scraped off to obtain the loss volume m2, the electrolyte m4 between the pole pieces and between the pole pieces and the diaphragm is squeezed out to obtain the net liquid volume m3 of the core package, m3=m1-m2-m4;

[0011] S4, establishing a line graph of the free liquid amount change of the battery cells in different injection amount groups, determining the group with a significant increase in the free liquid amount, and obtaining the optimal core package net liquid amount and the optimal loss amount;

[0012] S5, subjecting the group determined in S4 and the corresponding group of liquid-filled cells before and after it to an accelerated cycle test for more than 700 cycles, disassembling the cells after full charge, pouring out the free electrolyte, and determining the group with the highest capacity retention rate and intact interface wetting after full charge disassembly. The mass of the free electrolyte poured out corresponding to the group is the optimal free liquid amount;

[0013] S6, calculating the optimal liquid injection amount of the battery cell according to the optimal net liquid amount and optimal loss amount of the core package obtained in S4 and the optimal free liquid amount obtained in S5, wherein the optimal liquid injection amount of the battery cell = optimal net liquid amount of the core package + optimal loss amount + optimal free liquid amount.

[0014] In some optional embodiments, in S5, the accelerated cycle test includes: charging to an upper limit voltage at a constant current and voltage of 1.5-2.5C at a temperature of 42-48°C, with a cutoff current of 0.05C; discharging to a lower limit voltage at 1.5-2.5C, and cycling for more than 700 cycles.

[0015] In some optional embodiments, the infiltration step includes: standing at 42-48°C for 22-26 hours, and standing at 32-38°C for 10-14 hours.

[0016] In some optional embodiments, the formation step includes: 0.02-0.05C constant current charging to 3.5-3.6V; leaving for 2-10min, 0.05-0.1C constant current charging to 3.5-3.6V; leaving for 2-10min, 0.2-0.33C constant current charging to 3.7-3.8V, leaving for 2-5min.

[0017] In some optional embodiments, the aging step comprises: standing at 43-47° C. for 22-24 hours.

[0018] In some optional embodiments, the capacity division step includes: 1) standing at 22-28°C for 2-10 min, charging at 0.2-0.5C constant current and constant voltage to 4.19-4.2V, with a cut-off current of 0.02-0.05C; 2) standing for 2-10 min, discharging at 0.33-1.0C to 2.7V; 3) standing for 2-10 min, charging at 0.2-0.5C constant current and constant voltage to 3.65-3.67V, with a cut-off current of 0.02-0.05C; 4) standing for 2-10 min.

[0019] In some optional embodiments, in S1, the method for determining different injection amounts includes: determining the injection amount range according to the capacity of the battery cell, the pore conditions of the positive electrode sheet, the negative electrode sheet and the diaphragm, and dividing the range into several injection amount groups;

[0020] And / or, the number of groups of the non-liquid-injected battery cells is 6-8 groups.

[0021] In some optional embodiments, in S1, the battery cell is a soft-pack battery cell, a square-shell battery cell, a cylindrical battery cell or a special-shaped battery cell;

[0022] When the battery cell is a square shell battery cell, S2 includes: after liquid injection, infiltration, formation, secondary liquid injection, aging, degassing, removing excess electrolyte, recording the mass m5 of the battery cell, and obtaining the core package liquid volume m1, m1=m5-m0.

[0023] In some optional implementations, in S3, the full charging step includes: 0.33-0.5C constant current charging.

[0024] In some optional implementations, in S5, the full charging step includes: 0.33-0.5C constant current charging; and the accelerated cycle test for 700-800 cycles.

[0025] The technical solution of this application has the following advantages:

[0026] The method for determining the optimal liquid injection amount of the battery cell provided in the present application starts from the perspective of electrolyte infiltration and electrolyte consumption in the film formation of the SEI membrane interface, and determines the electrolyte loss, the net liquid amount of the core package and the free liquid amount by disassembling the full-charge interface by volume division, and determines the optimal net liquid amount of the core package and the optimal loss amount according to the line graph of the change in the free liquid amount, and determines the optimal free liquid amount by evaluating the capacity retention rate and the full-charge interface of the accelerated cycle, and finally obtains the optimal liquid injection amount. Compared with the prior art, the determination method provided in the present application takes into account the performance and electrolyte consumption of the battery cell aging, conventional cycles, and even accelerated cycles, accurately evaluates the optimal free liquid amount and the optimal liquid injection amount of the battery cell, and reduces the risk of cycle jumps due to insufficient electrolyte during the cycle process; at the same time, the number of battery cells required for the liquid retention verification battery cell is reduced from the liquid injection to the volume division offline process, and the time cost is saved and the evaluation cycle is shortened by the accelerated cycle test; at the same time, the optimal free liquid amount of the battery cell is accurately determined. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the specific implementation methods of the present application or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0028] Figure 1 It is a line graph showing the change in free liquid amount of cells in different groups in Example 1 of the present application;

[0029] Figure 2 It is a line graph showing the change in free liquid amount of cells in different groups in Example 2 of the present application;

[0030] Figure 3 It is a line graph showing the change in free liquid amount of cells in different groups in Example 3 of the present application;

[0031] Figure 4 It is a line graph showing the change in free liquid amount of cells in different groups in Example 4 of the present application;

[0032] Figure 5 It is a line graph showing the change in free liquid amount in different groups of battery cells in Example 5 of the present application. DETAILED DESCRIPTION

[0033] The following examples are provided for a better understanding of the present application, but are not limited to the best implementation mode described, and do not limit the content and protection scope of the present application. Any product identical or similar to the present application obtained by anyone under the inspiration of the present application or by combining the features of the present application with other prior arts shall fall within the protection scope of the present application.

[0034] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in the field can be used. If no manufacturer is specified for the reagents or instruments used, they are all conventional reagent products that can be obtained commercially.

[0035] As described in the background technology, in order to overcome the defects of the prior art such as the inability to accurately evaluate the optimal liquid injection amount of the battery cell or the long evaluation cycle, the following technical solutions are provided:

[0036] According to one aspect of the present application, a method for determining an optimal liquid injection amount of a battery cell is provided, comprising the following steps:

[0037] S1, take several groups of unfilled battery cells, record the mass m0 of the unfilled battery cells, and inject electrolyte into each group of unfilled battery cells with different injection amounts;

[0038] S2, after liquid injection, carry out infiltration, formation, aging, and exhaust to remove excess electrolyte to obtain a liquid-injected battery cell, record the mass m5 of the liquid-injected battery cell, and obtain the core package liquid volume m1, m1=m5-m0;

[0039] S3, the battery cell obtained in S2 is divided into different capacities, fully charged after the divided capacity is removed from the production line, disassembled, the electrolyte is poured out to obtain the free liquid volume, the electrolyte adhering to the battery cell shell is scraped off to obtain the loss volume m2, the electrolyte m4 between the pole pieces and between the pole pieces and the diaphragm is squeezed out to obtain the net liquid volume m3 of the core package, m3=m1-m2-m4;

[0040] S4, establishing a line graph of the free liquid amount change of the battery cells in different injection amount groups, determining the group with a significant increase in the free liquid amount, and obtaining the optimal core package net liquid amount and the optimal loss amount;

[0041] S5, subjecting the group determined in S4 and a group of corresponding liquid-filled cells before and after it to an accelerated cycle test for more than 700 cycles, disassembling the cells after full charge, pouring out the free electrolyte, and determining the group with the highest capacity retention rate and intact interface wetting after full charge disassembly, and the amount of free electrolyte poured out corresponding to the group is the optimal free liquid amount;

[0042] S6, calculating the optimal liquid injection amount of the battery cell according to the optimal net liquid amount and optimal loss amount of the core package obtained in S4 and the optimal free liquid amount obtained in S5, wherein the optimal liquid injection amount of the battery cell = optimal net liquid amount of the core package + optimal loss amount + optimal free liquid amount.

[0043] In the present application, a significant increase in the amount of free liquid indicates that the electrolyte that the positive and negative electrodes and the diaphragm included in the electrode core should absorb has been saturated, and even if more electrolyte is injected, it cannot be absorbed. Before this group, the amount of free liquid increased slowly or did not increase, and starting from this group, the amount of free liquid increased steadily.

[0044] The method for determining the optimal liquid injection amount of the battery cell provided in the present application starts from the perspective of electrolyte infiltration and electrolyte consumption in the film formation of the SEI membrane interface, and determines the electrolyte loss, the net liquid amount of the core package and the free liquid amount by disassembling the full-charge interface by volume division, and determines the optimal net liquid amount of the core package and the optimal loss amount according to the line graph of the change in the free liquid amount, and determines the optimal free liquid amount by evaluating the capacity retention rate and the full-charge interface of the accelerated cycle, and finally obtains the optimal liquid injection amount. Compared with the prior art, the determination method provided in the present application takes into account the performance and electrolyte consumption of the battery cell aging, conventional cycles, and even accelerated cycles, accurately evaluates the optimal free liquid amount and the optimal liquid injection amount of the battery cell, and reduces the risk of cycle jumps due to insufficient electrolyte during the cycle process; at the same time, the number of battery cells required for the liquid retention verification battery cell is reduced from the liquid injection to the volume division offline process, and the time cost is saved and the evaluation cycle is shortened by the accelerated cycle test; at the same time, the optimal free liquid amount of the battery cell is accurately determined.

[0045] In some optional embodiments, in S5, the accelerated cycle test includes: charging to an upper limit voltage at a constant current and voltage of 1.5-2.5C at a temperature of 42-48°C, with a cutoff current of 0.05C; discharging to a lower limit voltage at 1.5-2.5C, and cycling for more than 700 cycles.

[0046] As an example, the temperature of the accelerated cycle test can be 42°C, 44°C, 45°C, 46°C, 48°C, or within the range of any of the above values; the constant current constant voltage charging rate can be 1.5C, 1.8C, 2.0C, 2.2C, 2.5C, or within the range of any of the above values; the discharge rate can be 1.5C, 1.8C, 2.0C, 2.2C, 2.5C, or within the range of any of the above values.

[0047] In some optional embodiments, the infiltration step includes: standing at 42-48°C for 22-26 hours, or standing at room temperature 35±3°C for 10-14 hours.

[0048] In some optional embodiments, the formation step includes: 0.02-0.05C constant current charging to 3.5-3.6V; leaving for 2-10min, 0.05-0.1C constant current charging to 3.5-3.6V; leaving for 2-10min, 0.2-0.33C constant current charging to 3.7-3.8V, leaving for 2-5min.

[0049] In some optional embodiments, the formation step further includes standing for 2-10 minutes before constant current charging. The purpose of adding the standing step is mainly to collect the voltage change of the cell after immersion aging and to reject the short-circuited cell.

[0050] In some optional embodiments, the aging step comprises: standing at 43-47° C. for 22-24 hours.

[0051] In some optional embodiments, the step of dividing the capacity includes: 1) standing at 22-28°C for 2-10 minutes, charging at 0.2-0.5C constant current and constant voltage to 4.19-4.2V, with a cut-off current of 0.02-0.05C; 2) standing for 2-10 minutes, discharging at 0.33-1.0C to 2.7V; 3) standing for 2-10 minutes, charging at 0.2-0.5C constant current and constant voltage to 3.65-3.67V, with a cut-off current of 0.02-0.05C; 4) standing for 2-10 minutes;

[0052] Further, the step of capacity division includes: 1) standing at 22-28°C for 2-10min, 0.2-0.5C constant current and constant voltage charging to 4.19-4.2V, cut-off current 0.02-0.05C-time cut-off 200-240min; 2) standing at 2-10min, 0.33-1.0C discharge to 2.7V-time cut-off 300-330min; 3) standing at 2-10min, 0.2-0.5C constant current and constant voltage charging to 3.65-3.67V, cut-off current 0.02-0.05C-time cut-off 120-150min; 4) standing at 2-10min. In this application, the time cut-off setting is to add a safety protection when setting the process on the charging and discharging equipment. If the cut-off voltage and current protection of the equipment fail, there is also cut-off time protection to reduce safety hazards.

[0053] In some optional embodiments, in S1, the method for determining different injection amounts includes: empirically determining the injection amount range according to the capacity of the battery cell, the pore conditions of the positive electrode sheet, the negative electrode sheet and the diaphragm, and dividing the range into several injection amount groups;

[0054] And / or, the number of groups of the unfilled battery cells is 6 to 8. As an example, the number of groups of the unfilled battery cells may be 6, 7, or 8, each group including at least two battery cells, one of which is used for determining the optimal core package net liquid volume and the optimal loss volume in steps S2 and S3, and the other is used for subsequent accelerated cycle testing in S5.

[0055] In some optional embodiments, in S1, the battery cell is a soft-pack battery cell, a square-shell battery cell, a cylindrical battery cell, or a special-shaped battery cell;

[0056] When the battery cell is a square shell battery cell, S2 includes: after liquid injection, infiltration, formation, secondary liquid injection, aging, degassing, removing excess electrolyte, recording the mass m5 of the battery cell, and obtaining the core package liquid volume m1, m1=m5-m0.

[0057] Those skilled in the art can understand that the main purpose of the secondary injection is to ensure that when the battery cell is a large-capacity battery cell or a relatively large-sized battery cell, the chemical substances in the electrolyte can be fully absorbed by the positive and negative electrode materials. The injection amount of the secondary injection can be adjusted by those skilled in the art according to actual conditions. The secondary injection amount is the amount of electrolyte that satisfies the battery cell to be fully charged, plus the electrolyte required for full capacity charging and cycling to end of life (EOL).

[0058] In some optional implementations, in said S3, the step of fully charging comprises: 0.33-0.5C constant current charging;

[0059] And / or, in S5, the step of fully charging includes: 0.33-0.5C constant current charging; the accelerated cycle test is 700-800 cycles. As an example, the number of cycles of the accelerated cycle can be 700 cycles, 720 cycles, 750 cycles, 760 cycles, 780 cycles, 800 cycles, or any range of the above values.

[0060] The present application is further described in detail below in conjunction with specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in the present application.

[0061] Example 1

[0062] This embodiment provides a method for determining the optimal free liquid amount of a battery cell, and the specific steps and operating parameters are as follows:

[0063] (1) A soft-pack 15Ah battery cell (designed capacity median 15.8Ah) is used, the positive electrode active material is NCM88 / 9 / 3, and the positive electrode compaction is 3.3g / cm 3 The negative electrode active material is a mixture of artificial graphite and silicon, the mass ratio of the two is 93:7, and the negative electrode compaction is 1.55g / cm 3 , the thickness of the diaphragm is 12μm PE base film + 4μm ceramic layer, the number of laminated layers is 30 layers of positive electrode sheets / 31 layers of negative electrode sheets, and the mass m0 of the dry battery cell without liquid injection is recorded. According to the electrolyte demand for filling the full pores of the positive and negative electrode sheets and the pores of the diaphragm, 8 groups (two batteries in each group, the same below) with different gradient injection amounts are designed, which are 39.6g, 40.5g, 41.4g, 42.2g, 43.1g, 44g, 44.8g, and 45.7g respectively, for injection; wherein, the composition of the electrolyte is solvent (the mass ratio of ethylene carbonate EC and diethyl carbonate DEC is 1:1), 13.75mol / L lithium salt (LiPF6), and additives (0.3% of vinylene carbonate VC, 0.6% of 1,3-propane sultone PS);

[0064] (2) soaking the cell after injection, standing at 45°C for 24 hours, and standing at 35°C for 12 hours. After soaking, forming is performed to activate the cell for the first time to form a SEI film, consuming a certain amount of free electrolyte; the forming steps include: standing for 10 minutes, charging at 0.05C constant current to 3.6V; standing for 10 minutes, charging at 0.1C constant current to 3.6V; standing for 10 minutes, charging at 0.33C constant current to 3.8V; standing for 5 minutes;

[0065] (3) After the battery cell is formed and off the line, it is aged at 45°C for 24 hours to allow the battery cell to produce gas completely. Then, it is evacuated to remove the gas produced by the formation and the excess electrolyte. The mass m5 of the battery cell is recorded, and the core package liquid volume m1 of the battery cell with different gradient injection amounts is obtained, where m1 = m5-m0. The specific results are shown in the table below:

[0066] Table 1

[0067] Group 1 2 3 4 5 6 7 8 m0,g 220 220 220 219 219.5 220 220 219.5 m5,g 259.4 260.3 260.9 261.1 262.2 263.4 264.2 264.9 m1, g 39.4 40.3 40.9 42.1 42.7 43.4 44.2 45.4

[0068] (4) Capacity division of the battery cell, i.e., secondary activation; the capacity division steps include: 1) placing at 25°C for 10 minutes, charging at 0.5C constant current and constant voltage to 4.2V, with a cut-off current of 0.05C for 200 minutes; 2) placing at 10 minutes, discharging at 1.0C to 2.7V for 300 minutes; 3) placing at 10 minutes, charging at 0.5C constant current and constant voltage to 3.67V, with a cut-off current of 0.05C for 300 minutes. 120min; 4) set aside for 10min; after the cell capacity is separated and offline, 0.33C is fully charged to disassemble the interface, and the electrolyte is poured out to obtain the free liquid volume of each of the 8 groups of cells; then, the cell shell is removed, and the electrolyte adhering to the cell shell is scraped off to obtain the loss volume m2, and the electrolyte m4 between the cell pole pieces and between the pole pieces and the diaphragm is squeezed out to obtain the net liquid volume m3 of the core package of the 8 groups of cells, m3=m1-m2-m4; the specific results are shown in the table below:

[0069] Table 2

[0070]

[0071] (5) Establish a line graph of the changes in free fluid volume in different groups (e.g. Figure 1 As shown), the net liquid volume and loss amount of the core package corresponding to the group with significantly increased free liquid volume are determined as the optimal net liquid volume and optimal loss amount of the core package; according to Figure 1 It can be seen that the group with a significant increase in free liquid volume is Group 6. The significant increase in free liquid volume indicates that the electrolyte that the positive and negative electrodes and the diaphragm in the electrode core should absorb is saturated, and even if more electrolyte is injected, it cannot be absorbed;

[0072] (6) The group determined in step (5) and the corresponding group of liquid-filled cells before and after it (i.e., another cell in the same group) are subjected to accelerated cycle test. The remaining cell groups are discarded and no further verification is required. The steps of accelerated cycle are: charging at 2.5C constant current and constant voltage at 45°C to an upper voltage limit of 4.2V, with a cut-off current of 0.05C; discharging at 2.5C to a lower voltage limit of 2.7V, and cycling for 700 cycles (700 cycles take 42 days, plus 144 hours to determine the optimal net liquid volume and the optimal loss volume, a total of 48 days), disassembling after full charge, pouring out the free electrolyte, and recording the mass, paying attention to the capacity retention rate of the accelerated cycle and whether the interface of the fully charged cell is well wetted. The group with the highest cycle capacity retention rate and good wetness corresponds to the mass of the free electrolyte poured out in the above disassembly step as the optimal free liquid volume; the specific results are shown in the following table:

[0073] Table 3

[0074]

[0075]

[0076] (7) According to the formula: optimal liquid filling amount of the battery cell = optimal net liquid amount of the core package + optimal loss amount + optimal free liquid amount, the optimal liquid filling amount of the battery cell is calculated. In this embodiment, the optimal liquid filling amount of the battery cell = 42 + 0.5 + 5.2 = 47.7 g.

[0077] Example 2

[0078] This embodiment provides a method for determining the optimal free liquid amount of a battery cell, and the specific steps and operating parameters are as follows:

[0079] (1) Using soft-pack 15Ah battery cells (designed capacity median 15.8Ah), the positive electrode active material is NCM88 / 9 / 3, and the positive electrode compaction is 3.4g / cm 3 The negative electrode active material is a mixture of artificial graphite and silicon, the mass ratio of the two is 93:7, and the negative electrode compaction is 1.6g / cm 3 , the thickness of the diaphragm is 12μm PE base film + 4μm ceramic layer, the number of laminated layers is 30 layers of positive electrode sheets / 31 layers of negative electrode sheets, the mass m0 of the dry battery cell without liquid injection is recorded, and according to the electrolyte demand for filling the full pores of the positive and negative electrode sheets and the pores of the diaphragm, 8 groups of different gradient injection amounts are designed, which are 31.7g, 32.6g, 33.5g, 34.3g, 35.2g, 36.1g, 36.9g, and 37.8g respectively, and the injection is performed; wherein the composition of the electrolyte is the same as that of Example 1;

[0080] (2) soaking the cell after injection, standing at 45°C for 24 hours, and standing at 35°C for 12 hours. After soaking, forming is performed to activate the cell for the first time to form a SEI film, consuming a certain amount of free electrolyte; the forming steps include: standing for 10 minutes, charging at 0.05C constant current to 3.6V; standing for 10 minutes, charging at 0.1C constant current to 3.6V; standing for 10 minutes, charging at 0.33C constant current to 3.8V; standing for 5 minutes;

[0081] (3) After the battery cell is formed and off the line, it is aged at 45°C for 24 hours to allow the battery cell to produce gas completely. Then, it is evacuated to remove the gas produced by the formation and the excess electrolyte. The mass m5 of the battery cell is recorded, and the core package liquid volume m1 of the battery cell with different gradient injection amounts is obtained, where m1 = m5-m0. The specific results are shown in the table below:

[0082] Table 4

[0083] Group 1 2 3 4 5 6 7 8 m0,g 220 219.5 220 220 219.5 220 220 219.5 m5,g 251.3 251.5 252.5 253.5 253.5 254.5 255.1 255.6 m1, g 31.3 32 32.5 33.5 34.0 34.5 35.1 36.1

[0084] (4) Capacity division of the battery cell, i.e., secondary activation; the capacity division steps include: 1) placing at 25°C for 10 minutes, charging at 0.5C constant current and constant voltage to 4.2V, with a cut-off current of 0.05C for 200 minutes; 2) placing at 10 minutes, discharging at 1.0C to 2.7V for 300 minutes; 3) placing at 10 minutes, charging at 0.5C constant current and constant voltage to 3.67V, with a cut-off current of 0.05C for 300 minutes. 120min; 4) set aside for 10min; after the cell capacity is separated and offline, 0.33C is fully charged to disassemble the interface, and the electrolyte is poured out to obtain the free liquid volume of each of the 8 groups of cells; then, the cell shell is removed, and the electrolyte adhering to the cell shell is scraped off to obtain the loss volume m2, and the electrolyte m4 between the cell pole pieces and between the pole pieces and the diaphragm is squeezed out to obtain the net liquid volume m3 of the core package of the 8 groups of cells, m3=m1-m2-m4; the specific results are shown in the table below:

[0085] Table 5

[0086]

[0087] (5) Establish a line graph of the changes in free fluid volume in different groups (e.g. Figure 2 As shown in the figure, the point where the free liquid volume increases significantly is determined as the optimal net liquid volume and the optimal loss volume; according to Figure 2 It can be seen that the group with a significant increase in free fluid volume was Group 6;

[0088] (6) The group determined in step (5) and the corresponding group of liquid-filled cells before and after it (i.e., another cell in the same group) were subjected to accelerated cycle test. The remaining cell experimental groups were discarded and no further verification was required. The steps of accelerated cycle were: charging at 2.5C constant current and constant voltage at 45°C to an upper voltage limit of 4.2V, with a cut-off current of 0.05C; discharging at 2.5C to a lower voltage limit of 2.7V, and cycling for 700 cycles (700 cycles took 42 days, plus 144 hours to determine the optimal net liquid volume and the optimal loss volume, a total of 48 days), disassembling after full charge, pouring out the free electrolyte, and recording the mass, paying attention to the capacity retention rate of the accelerated cycle and whether the interface infiltration was intact. The group with the highest cycle capacity retention rate and intact infiltration corresponded to the mass of the free electrolyte poured out in the above disassembly step as the optimal free liquid volume; the specific results are shown in the following table:

[0089] Table 6

[0090]

[0091] (7) According to the formula: optimal liquid filling amount of the battery cell = optimal net liquid amount of the core package + optimal loss amount + optimal free liquid amount, the optimal liquid filling amount of the battery cell is calculated. In this embodiment, the optimal liquid filling amount of the battery cell = 33.4 + 0.4 + 5.0 = 38.8 g.

[0092] Example 3

[0093] This embodiment provides a method for determining the optimal free liquid amount of a battery cell, and the specific steps and operating parameters are as follows:

[0094] (1) A soft-pack 15Ah battery cell (designed capacity median 15.8Ah) is used, the positive electrode active material is NCM88 / 9 / 3, and the positive electrode compaction is 3.3g / cm 3 The negative electrode active material is a mixture of artificial graphite and silicon, the mass ratio of the two is 93:7, and the negative electrode compaction is 1.55g / cm 3 , the thickness of the diaphragm is 9μm PE base film + 3μm ceramic layer, the number of laminated layers is 30 layers of positive electrode sheets / 31 layers of negative electrode sheets, the mass m0 of the dry battery cell without liquid injection is recorded, and according to the electrolyte demand for filling the full pores of the positive and negative electrode sheets and the pores of the diaphragm, 8 groups of different gradient injection amounts are designed, which are 29.7g, 30.6g, 31.4g, 32.3g, 33.1g, 34g, 34.9g, and 35.7g respectively, and the injection is performed; wherein the composition of the electrolyte is the same as that of Example 1;

[0095] (2) soaking the cell after injection, standing at 45°C for 24 hours, and standing at 35°C for 12 hours. After soaking, forming is performed to activate the cell for the first time to form a SEI film, consuming a certain amount of free electrolyte; the forming steps include: standing for 10 minutes, charging at 0.05C constant current to 3.6V; standing for 10 minutes, charging at 0.1C constant current to 3.6V; standing for 10 minutes, charging at 0.33C constant current to 3.8V; standing for 5 minutes;

[0096] (3) After the battery cell is formed and off the line, it is aged at 45°C for 24 hours to allow the battery cell to produce gas completely. Then, it is evacuated to remove the gas produced by the formation and the excess electrolyte. The mass m5 of the battery cell is recorded, and the core package liquid volume m1 of the battery cell with different gradient injection amounts is obtained, where m1 = m5-m0. The specific results are shown in the table below:

[0097] Table 7

[0098] Group 1 2 3 4 5 6 7 8 m0,g 219.5 220 220 219.3 219.5 220.2 220 219.3 m5,g 248.8 250 250.5 250.7 251.3 252.5 252.9 253.1 m1, g 29.3 30.0 30.5 31.4 31.8 32.3 32.9 33.8

[0099] (4) Capacity division of the battery cell, i.e., secondary activation; the capacity division steps include: 1) placing at 25°C for 10 minutes, charging at 0.5C constant current and constant voltage to 4.2V, with a cut-off current of 0.05C for 200 minutes; 2) placing at 10 minutes, discharging at 1.0C to 2.7V for 300 minutes; 3) placing at 10 minutes, charging at 0.5C constant current and constant voltage to 3.67V, with a cut-off current of 0.05C for 300 minutes. 120min; 4) set aside for 10min; after the cell capacity is separated and offline, 0.33C is fully charged to disassemble the interface, and the electrolyte is poured out to obtain the free liquid volume of each of the 8 groups of cells; then, the cell shell is removed, and the electrolyte adhering to the cell shell is scraped off to obtain the loss volume m2, and the electrolyte m4 between the cell pole pieces and between the pole pieces and the diaphragm is squeezed out to obtain the net liquid volume m3 of the core package of the 8 groups of cells, m3=m1-m2-m4; the specific results are shown in the table below:

[0100] Table 8

[0101]

[0102] (5) Establish a line graph of the changes in free fluid volume in different groups (e.g. Figure 3 As shown in the figure, the point where the free liquid volume increases significantly is determined as the optimal net liquid volume and the optimal loss volume; according to Figure 3 It can be seen that the group with a significant increase in free fluid volume was Group 6;

[0103] (6) The group determined in step (5) and the corresponding group of liquid-filled cells before and after it (i.e., another cell in the same group) were subjected to accelerated cycle test. The remaining cell experimental groups were discarded and no further verification was required. The steps of accelerated cycle were: charging at 2.5C constant current and constant voltage at 45°C to an upper voltage limit of 4.2V, with a cut-off current of 0.05C; discharging at 2.5C to a lower voltage limit of 2.7V, and cycling for 700 cycles (700 cycles took 42 days, plus 144 hours to determine the optimal net liquid volume and the optimal loss volume, a total of 48 days), disassembling after full charge, pouring out the free electrolyte, and recording the mass, paying attention to the capacity retention rate of the accelerated cycle and whether the interface infiltration was intact. The group with the highest cycle capacity retention rate and intact infiltration corresponded to the mass of the free electrolyte poured out in the above disassembly step as the optimal free liquid volume; the specific results are shown in the following table:

[0104] Table 9

[0105]

[0106] (7) According to the formula: optimal liquid filling amount of the battery cell = optimal net liquid amount of the core package + optimal loss amount + optimal free liquid amount, the optimal liquid filling amount of the battery cell is calculated. In this embodiment, the optimal liquid filling amount of the battery cell = 31.3 + 0.3 + 4.8 = 36.4 g.

[0107] Example 4

[0108] This embodiment provides a method for determining the optimal free liquid amount of a battery cell, and the specific steps and operating parameters are as follows:

[0109] (1) Using soft-pack 103Ah battery cells (designed capacity median 105.6Ah), the positive electrode active material is NCM88 / 9 / 3, and the positive electrode compaction is 3.4g / cm 3 The negative electrode active material is a mixture of artificial graphite and silicon, the mass ratio of the two is 93:7, and the negative electrode compaction is 1.6g / cm 3 , the thickness of the diaphragm is 12μm PE base film + 4μm ceramic layer, the material is PE, the number of layers of a single battery cell is 66 positive electrode sheets / 67 negative electrode sheets, and the mass m0 of the dry battery cell without liquid injection is recorded. According to the electrolyte demand for filling the full pores of the positive and negative electrode sheets and the pores of the diaphragm, 8 groups of different gradients of primary injection amount are designed, which are 192g, 198g, 203g, 209g, 215g, 221g, 226g, and 232g respectively, and the injection is performed; wherein, the composition of the electrolyte is the same as that of Example 1;

[0110] (2) soaking the cell after injection, standing at 45°C for 24 hours, and standing at 35°C for 12 hours. After soaking, forming is performed to activate the cell for the first time to form a SEI film, consuming a certain amount of free electrolyte; the forming steps include: standing for 10 minutes, charging at 0.05C constant current to 3.6V; standing for 10 minutes, charging at 0.1C constant current to 3.6V; standing for 10 minutes, charging at 0.33C constant current to 3.8V; standing for 5 minutes;

[0111] (3) After the cell is formed and off the line, a secondary injection is performed with a secondary injection volume of 20 g. The cell is then aged at 45°C for 23 h to allow the cell to completely produce gas. The cell is then returned to helium for nailing to remove the gas produced and excess electrolyte. The mass m5 of the cell is recorded to obtain the core package liquid volume m1 of the cell with different gradient injection volumes, where m1 = m5 - m0. The specific results are shown in the table below:

[0112] Table 10

[0113] Group 1 2 3 4 5 6 7 8 m0,g 1718 1718 1719 1718 1720 1718.5 1719 1718 m5,g 1928.8 1933.7 1938.3 1943.4 1949 1951 1955.8 1961.1 m1, g 210.8 215.7 219.3 225.4 229 232.5 236.8 243.1

[0114] (4) Capacity division of the battery cell, i.e., secondary activation; the capacity division steps include: 1) placing at 25°C for 10 minutes, charging at 0.5C constant current and constant voltage to 4.2V, with a cut-off current of 0.05C for 200 minutes; 2) placing at 10 minutes, discharging at 1.0C to 2.7V for 300 minutes; 3) placing at 10 minutes, charging at 0.5C constant current and constant voltage to 3.67V, with a cut-off current of 0.05C for 300 minutes. 120min; 4) set aside for 10min; after the cell capacity is separated and offline, 0.33C is fully charged to disassemble the interface, and the electrolyte is poured out to obtain the free liquid volume of each of the 8 groups of cells; then, the cell shell is removed, and the electrolyte adhering to the cell shell is scraped off to obtain the loss volume m2, and the electrolyte m4 between the cell pole pieces and between the pole pieces and the diaphragm is squeezed out to obtain the net liquid volume m3 of the core package of the 8 groups of cells, m3=m1-m2-m4; the specific results are shown in the table below:

[0115] Table 11

[0116]

[0117]

[0118] (5) Establish a line graph of the changes in free fluid volume in different groups (e.g. Figure 4 As shown in the figure, the point where the free liquid volume increases significantly is determined as the optimal net liquid volume and the optimal loss volume; Figure 4 It can be seen that the group with a significant increase in free fluid volume was Group 6;

[0119] (6) The group determined in step (5) and the corresponding group of liquid-filled cells before and after it (i.e., another cell in the same group) were subjected to accelerated cycle test. The remaining cell experimental groups were discarded and no further verification was required. The steps of accelerated cycle were: charging at 2.5C constant current and constant voltage at 45°C to an upper voltage limit of 4.2V, with a cut-off current of 0.05C; discharging at 2.5C to a lower voltage limit of 2.7V, and cycling for 700 cycles (700 cycles took 42 days, plus 144 hours to determine the optimal net liquid volume and the optimal loss volume, a total of 48 days), disassembling after full charge, pouring out the free electrolyte, and recording the mass, paying attention to the capacity retention rate of the accelerated cycle and whether the interface infiltration was intact. The group with the highest cycle capacity retention rate and intact infiltration corresponded to the mass of the free electrolyte poured out in the above disassembly step as the optimal free liquid volume; the specific results are shown in the following table:

[0120] Table 12

[0121]

[0122] (7) According to the formula: optimal liquid filling amount of the battery cell = optimal net liquid amount of the core package + optimal loss amount + optimal free liquid amount, the optimal liquid filling amount of the battery cell is calculated. In this embodiment, the optimal liquid filling amount of the battery cell = 225.1 + 2.5 + 10 = 237.6 g.

[0123] Example 5

[0124] This embodiment provides a method for determining the optimal free liquid amount of a battery cell, and the specific steps and operating parameters are as follows:

[0125] (1) Using soft-pack 103Ah battery cells (designed capacity median 105.6Ah), the positive electrode active material is NCM88 / 9 / 3, and the positive electrode compaction is 3.4g / cm 3 The negative electrode active material is a mixture of artificial graphite and silicon, the mass ratio of the two is 93:7, and the negative electrode compaction is 1.6g / cm 3 , the thickness of the diaphragm is 9μm thick PE base film + 2μm thick ceramic layer + 1μm glue layer, the number of layers of a single battery cell is 66 positive electrode sheets / 67 negative electrode sheets, and the mass m0 of the dry battery cell without liquid injection is recorded. According to the electrolyte demand for filling the full pores of the positive and negative electrode sheets and the pores of the diaphragm, 8 groups of injection amounts with different gradients are designed, which are 178g, 184g, 190g, 195g, 201g, 207g, 213g, and 218g respectively, for injection; wherein the composition of the electrolyte is the same as that of Example 1;

[0126] (2) soaking the cell after injection, standing at 45°C for 24 hours, and standing at 35°C for 12 hours. After soaking, forming is performed to activate the cell for the first time to form a SEI film, consuming a certain amount of free electrolyte; the forming steps include: standing for 10 minutes, charging at 0.05C constant current to 3.6V; standing for 10 minutes, charging at 0.1C constant current to 3.6V; standing for 10 minutes, charging at 0.33C constant current to 3.8V; standing for 5 minutes;

[0127] (3) After the cell is formed and off the line, a secondary injection is performed with a secondary injection volume of 20 g. The cell is then aged at 45°C for 23 h to allow the cell to completely produce gas. The cell is then returned to helium for nailing to remove the gas produced and excess electrolyte. The mass m5 of the cell is recorded to obtain the core package liquid volume m1 of the cell with different gradient injection volumes, where m1 = m5 - m0. The specific results are shown in the table below:

[0128] Table 13

[0129] Group 1 2 3 4 5 6 7 8 m0,g 1718 1719 1719.5 1718 1720 1718 1718 1718 m5,g 1914.8 1920.4 1924.2 1928.4 1933.7 1935 1939 1944.9 m1, g 196.8 201.4 204.7 210.4 213.7 217 221 226.9

[0130] (4) Capacity division of the battery cell, i.e., secondary activation; the capacity division steps include: 1) placing at 25±3°C for 10 minutes, charging at 0.5C constant current and constant voltage to 4.2V, with a cut-off current of 0.05C for 200 minutes; 2) placing at 10 minutes, discharging at 1.0C to 2.7V for 300 minutes; 3) placing at 10 minutes, charging at 0.5C constant current and constant voltage to 3.67V, with a cut-off current of 0.05C for 300 minutes. 120 minutes were cut off; 4) it was set aside for 10 minutes; after the cell was offline, the interface was fully charged at 0.33C and the electrolyte was poured out to obtain the free liquid volume of each of the 8 groups of cells; then, the cell shell was removed, the electrolyte adhering to the cell shell was scraped off, and the loss volume m2 was obtained. The electrolyte m4 between the cell pole pieces and between the pole pieces and the diaphragm was squeezed out to obtain the net liquid volume m3 of the core package of the 8 groups of cells, m3 = m1-m2-m4; the specific results are shown in the table below:

[0131] Table 14

[0132]

[0133] (5) Establish a line graph of the changes in free fluid volume in different groups (e.g. Figure 5 As shown in the figure, the point where the free liquid volume increases significantly is determined as the optimal net liquid volume and the optimal loss volume; according to Figure 5 It can be seen that the group with a significant increase in free fluid volume was Group 6;

[0134] (6) The group determined in step (5) and the corresponding group of liquid-filled cells before and after it (i.e., another cell in the same group) were subjected to accelerated cycle test. The remaining cell experimental groups were discarded and no further verification was required. The steps of accelerated cycle were: charging at 2.5C constant current and constant voltage at 45°C to an upper voltage limit of 4.2V, with a cut-off current of 0.05C; discharging at 2.5C to a lower voltage limit of 2.7V, and cycling for 700 cycles (700 cycles took 42 days, plus 144 hours to determine the optimal net liquid volume and the optimal loss volume, a total of 48 days), disassembling after full charge, pouring out the free electrolyte, and recording the mass, paying attention to the capacity retention rate of the accelerated cycle and whether the interface infiltration was intact. The group with the highest cycle capacity retention rate and intact infiltration corresponded to the mass of the free electrolyte poured out in the above disassembly step as the optimal free liquid volume; the specific results are shown in the following table:

[0135] Table 15

[0136]

[0137] (7) According to the formula: optimal liquid filling amount of the battery cell = optimal net liquid amount of the core package + optimal loss amount + optimal free liquid amount, the optimal liquid filling amount of the battery cell is calculated. In this embodiment, the optimal liquid filling amount of the battery cell = 210.1 + 2.3 + 9.8 = 222.2 g.

[0138] Comparative Example 1

[0139] This comparative example provides a method for determining the optimal free liquid amount of a battery cell. Compared with Example 1, no attention is paid to the loss amount on the battery cell shell, and no accelerated cycle is performed. The specific steps and operating parameters are as follows:

[0140] Steps (1) to (3) are the same as steps (1) to (3) of Example 1;

[0141] (4) performing capacity division on the battery cells, i.e., secondary activation; the capacity division steps include: 1) placing at 25±3°C for 10 minutes, charging at 0.5C constant current and constant voltage to 4.2V, with a current cutoff of 0.05C and a time cutoff of 200 minutes; 2) placing for 10 minutes, discharging at 1.0C to 2.7V and a time cutoff of 300 minutes; 3) placing for 10 minutes, charging at 0.5C constant current and constant voltage to 3.67V, with a current cutoff of 0.05C and a time cutoff of 120 minutes; 4) placing for 10 minutes; after the battery cells are separated and offline, fully charge at 0.33C to disassemble the interface, pour out the electrolyte, and obtain the free liquid volume of each of the 8 groups of battery cells;

[0142] (5) After the cell is offline, the 0.33C fully charged disassembly interface is used, and the liquid retention amount (39.4, 40.3, 40.9, 42.1, 42.7, 43.4, 44.2, 45.4) and free liquid amount (3.1, 3.0, 3.0, 3.3, 3.3, 4.0, 4.5, 5.4) of the cells with different gradient injection amounts are determined. The optimal free liquid amount of 4.0 g is finally determined based on the group with a significant increase in the free liquid amount. The optimal injection amount = optimal core package liquid retention amount + free liquid amount + liquid loss amount = 43.4 g + 4 g + 2 g (empirical value) = 49.4 g.

[0143] Comparative Example 2

[0144] This comparative example provides a method for determining the optimal free liquid amount of a battery cell. Compared with Example 4, no attention is paid to the loss amount on the battery cell shell, and no accelerated cycle is performed. The specific steps and operating parameters are as follows:

[0145] Steps (1) to (3) are the same as steps (1) to (3) of Example 4;

[0146] (4) performing capacity division on the battery cells, i.e., secondary activation; the capacity division steps include: 1) placing at 25±3°C for 10 minutes, charging at 0.5C constant current and constant voltage to 4.2V, with a current cutoff of 0.05C and a time cutoff of 200 minutes; 2) placing for 10 minutes, discharging at 1.0C to 2.7V and a time cutoff of 300 minutes; 3) placing for 10 minutes, charging at 0.5C constant current and constant voltage to 3.67V, with a current cutoff of 0.05C and a time cutoff of 120 minutes; 4) placing for 10 minutes; after the battery cells are separated and offline, fully charge at 0.33C to disassemble the interface, pour out the electrolyte, and obtain the free liquid volume of each of the 8 groups of battery cells;

[0147] (5) After the cell is offline, the interface is fully charged at 0.33C and the liquid retention amount (210.8, 215.7, 219.3, 225.4, 229, 232.5, 236.8, 243.1) and free liquid amount (3.7, 3.4, 3.6, 4.9, 5.6, 8.8, 11.5, 15.3) of the cells with different gradient injection amounts are determined. The optimal free liquid amount of 8.8 is finally determined according to the group with a significant increase in the free liquid amount. The optimal injection amount = liquid retention amount + free liquid amount + liquid loss amount = 232.5 + 8.8 + 5 (empirical value) = 246.3 g.

[0148] Test Case

[0149] The battery cells were injected with the optimal injection amounts determined according to Examples 1 to 5 and Comparative Examples 1 to 2, respectively, and 700 cycles of high-temperature accelerated cycling were performed with reference to the high-temperature accelerated cycle parameters of Example 1. The data results of the capacity retention rate are shown in the table below.

[0150] Table 16

[0151]

[0152] It can be seen from the data in the above table that after the injection cycle is performed according to the optimal injection amount determined by the method of the embodiment of the present application, the capacity retention rate is higher. The method for determining the optimal free liquid amount of the battery cell provided by Examples 1-5, firstly, saves the number of cells required for verifying the liquid retention amount of the battery cell; secondly, shortens the evaluation cycle through accelerated cycle testing; thirdly, accurately evaluates the optimal free liquid amount of the battery cell, takes into account the aging of the battery cell and the performance of the accelerated cycle and the consumption of the electrolyte, and reduces the risk of cycle jump caused by insufficient electrolyte during the cycle process.

[0153] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from these are still within the scope of protection of the invention.

Claims

1. A method for determining the optimal liquid injection amount of a battery cell, characterized in that: The steps include: S1, take several groups of unfilled battery cells, record the mass m0 of the unfilled battery cells, and inject electrolyte into each group of unfilled battery cells with different injection amounts; S2, after liquid injection, carry out wetting, formation, aging, and exhaust to remove excess electrolyte to obtain a liquid-injected battery cell, record the mass m5 of the liquid-injected battery cell, and obtain the core package liquid volume m1, m1=m5-m0; S3, the battery cell obtained in S2 is divided into different capacities, fully charged after the divided capacity is removed from the production line, disassembled, the electrolyte is poured out to obtain the free liquid volume, the electrolyte adhering to the battery cell shell is scraped off to obtain the loss volume m2, the electrolyte m4 between the pole pieces and between the pole pieces and the diaphragm is squeezed out to obtain the net liquid volume m3 of the core package, m3=m1-m2-m4; S4, establishing a line graph of the free liquid amount change of the battery cells in different injection amount groups, determining the group with a significant increase in the free liquid amount, and obtaining the optimal core package net liquid amount and the optimal loss amount; S5, subjecting the group determined in S4 and the corresponding group of liquid-filled cells before and after it to an accelerated cycle test for more than 700 cycles, disassembling the cells after full charge, pouring out the free electrolyte, and determining the group with the highest capacity retention rate and intact interface wetting after full charge disassembly. The mass of the free electrolyte poured out corresponding to the group is the optimal free liquid amount; S6, calculate the optimal liquid injection amount of the battery cell according to the optimal net liquid amount and optimal loss amount of the core package obtained in S4 and the optimal free liquid amount obtained in S5, and the optimal liquid injection amount of the battery cell = optimal net liquid amount of the core package + optimal loss amount + optimal free liquid amount.

2. The method for determining the optimal liquid injection amount of a battery cell according to claim 1, characterized in that: In S5, the accelerated cycle test includes: charging to an upper limit voltage at a constant current and voltage of 1.5-2.5C at a temperature of 42-48°C, with a cut-off current of 0.05C; discharging to a lower limit voltage at 1.5-2.5C, and cycling for more than 700 cycles.

3. The method for determining the optimal liquid injection amount of a battery cell according to claim 1, characterized in that: The infiltration step includes: standing at 42-48° C. for 22-26 hours, and standing at 32-38° C. for 10-14 hours.

4. The method for determining the optimal liquid injection amount of a battery cell according to claim 1, characterized in that: The formation steps include: 0.02-0.05C constant current charging to 3.5-3.6V; leaving for 2-10min, 0.05-0.1C constant current charging to 3.5-3.6V; leaving for 2-10min, 0.2-0.33C constant current charging to 3.7-3.8V, leaving for 2-5min.

5. The method for determining the optimal liquid injection amount of a battery cell according to claim 1, characterized in that: The aging step includes: standing at 43-47° C. for 22-24 hours.

6. The method for determining the optimal liquid injection amount of a battery cell according to claim 1, characterized in that: The steps of capacity division include: 1) standing at 22-28°C for 2-10 minutes, charging at 0.2-0.5C constant current and constant voltage to 4.19-4.2V, with a cut-off current of 0.02-0.05C; 2) standing for 2-10 minutes, discharging at 0.33-1.0C to 2.7V; 3) standing for 2-10 minutes, charging at 0.2-0.5C constant current and constant voltage to 3.65-3.67V, with a cut-off current of 0.02-0.05C; 4) standing for 2-10 minutes.

7. The method for determining the optimal liquid injection amount of a battery cell according to any one of claims 1 to 6, characterized in that: In S1, the method for determining different injection amounts includes: determining the injection amount range according to the capacity of the battery cell, the pore conditions of the positive electrode sheet, the negative electrode sheet and the diaphragm, and dividing the range into several injection amount groups; And / or, the number of groups of the non-liquid-injected battery cells is 6-8 groups.

8. The method for determining the optimal liquid injection amount of a battery cell according to claim 1, characterized in that: In S1, the battery cell is a soft-pack battery cell, a square-shell battery cell, a cylindrical battery cell or a special-shaped battery cell; When the battery cell is a square shell battery cell, S2 includes: after liquid injection, infiltration, formation, secondary liquid injection, aging, degassing, removing excess electrolyte, recording the mass m5 of the battery cell, and obtaining the core package liquid volume m1, m1=m5-m0.

9. The method for determining the optimal liquid injection amount of a battery cell according to claim 8, characterized in that: In S3, the step of fully charging includes: 0.33-0.5C constant current charging.

10. The method for determining the optimal liquid injection amount of a battery cell according to any one of claims 1 to 6 or claims 8 to 9, characterized in that: In the S5, the step of fully charging includes: 0.33-0.5C constant current charging; and the accelerated cycle test for 700-800 cycles.

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