Method for determining primary liquid injection condition of battery and method for injecting liquid into battery

Abstract

The invention provides a method for determining a primary liquid injection condition of a battery. The method comprises the following steps: injecting liquid; performing formation; determining the liquid loss amount; and one-time liquid injection quantity is determined. By adopting the technical scheme, on one hand, the determined one-time liquid injection amount of the battery can still ensure that the battery is infiltrated by sufficient electrolyte, and on the other hand, when the determined one-time liquid injection amount is adopted for liquid injection of the battery, due to the fact that the one-time liquid injection amount is reduced, the electrolyte pumped out during formation is also reduced, and the volatilization amount of the electrolyte is also reduced; and the liquid loss amount is reduced. Therefore, the proportion unbalance of the first-injection electrolyte and the second-injection electrolyte actually injected into the battery can be effectively avoided, the content change of the additives in the first-injection electrolyte and the second-injection electrolyte actually injected into the battery is avoided, and the cycle life of the battery can be further prolonged. The invention also provides a method for injecting liquid into the battery. And the battery after liquid injection has relatively long cycle life.

Description

Technical Field

[0001] The invention belongs to the technical field of batteries, and in particular relates to a method for determining a battery primary liquid injection condition and a method for injecting liquid into a battery. Background Art

[0002] With the continuous development of batteries, users have put forward higher requirements for the performance of batteries, such as battery capacity and cycle life. The injection volume is closely related to the above performance. In order to improve the above performance, it is often necessary to inject the battery twice. The main purpose of the first injection (also known as the first injection, referred to as "one injection") is to form a stable SEI film (Solid Electrolyte Interphase, i.e. solid electrolyte interface film). In order to ensure the film formation effect, some low-temperature film-forming additives (such as methylformamide sulfonyl imide MMDS) and additives to improve the film formation effect will be added during the first injection. Since there is always a local rupture of SEI during the use of the battery, some additives with good repair effects, such as vinylene carbonate (VC), will be added during the second injection (also known as the second injection, referred to as "two injections") to repair the damaged SEI film. Through the synergistic effect of additives in the first injection and the second injection, the SEI film can be optimized, thereby improving the cycle life of the battery.

[0003] Based on the different purposes of the primary injection and the secondary injection, the types and contents of additives in the electrolyte used for the two injections will also be different. On this basis, in order to ensure that the electrolyte can fully infiltrate the electrode materials of the battery, the amount of the primary injection is usually significantly higher than the amount of the secondary injection. For example, the ratio of the primary injection amount to the secondary injection amount is roughly between 84:16 and 90:10. Generally speaking, after the primary injection, after the electrolyte fully infiltrates the battery, the battery needs to be formed to form a stable SEI. During the formation process, the electrolyte may decompose on the electrode surface and produce gas. In order to facilitate gas discharge, the formation is usually carried out under negative pressure conditions. However, this will also cause the free electrolyte in the battery (such as the electrolyte not absorbed by the pole piece and the liquid film on the pole piece) to be extracted. The extracted electrolyte is easy to volatilize, which causes electrolyte loss. Among them, the more electrolyte is injected during the primary injection process, the more electrolyte is extracted during the formation process, the more electrolyte is volatilized, and the more electrolyte is lost.

[0004] When the loss is large, in order to ensure that the battery has sufficient electrolyte to maintain long-term operation. The subsequent secondary injection needs to adopt a variable injection method to increase the secondary injection volume, so as to replenish the electrolyte lost in the battery after the primary injection during the secondary injection stage. However, if the secondary injection volume increases too much, it will affect the cycle performance of the battery, that is, reduce the cycle life of the battery, and even cause a diving problem (after a certain number of charge and discharge cycles, the capacity or performance of the battery suddenly drops sharply without obvious warning). In addition, the battery obtained by the above variable injection method also has a poor SEI film formation effect.

[0005] When the loss amount is large, if the secondary injection adopts quantitative injection, that is, the electrolyte lost in the battery after the first injection is no longer replenished in the second injection stage, and the battery is directly injected and injected once and twice according to the preset ratio of one injection and two injections, then this will also cause the ratio of the actual amount of the first injection of electrolyte and the amount of the second injection of electrolyte injected into the battery to seriously deviate from the preset ratio, thereby causing an imbalance in the ratio. At the same time, it cannot ensure that the battery has sufficient electrolyte to maintain long-term operation, which in turn affects the battery's cycle performance. Summary of the invention

[0006] The present invention provides the following technical solutions to solve the above technical problems.

[0007] The present invention provides a method for determining a primary liquid injection condition of a battery, the method comprising:

[0008] Injection: injecting a preset amount of electrolyte into the sample battery so that the sample battery is completely soaked by the electrolyte, wherein the preset amount is a preset amount of electrolyte injected into the sample battery once;

[0009] Formation: Formation of fully soaked sample batteries;

[0010] Determine the amount of liquid loss: determine the amount of liquid loss of the sample battery injected with electrolyte according to the mass of the sample battery before formation and the mass of the sample battery after formation;

[0011] Determine the amount of liquid injected at one time: determine the amount of liquid injection reduction at one time based on the amount of liquid loss. The amount of liquid injection reduction at one time is within the allowable error range of the liquid loss amount. Determine the amount of liquid injected at one time based on the difference between the preset amount and the reduced amount.

[0012] By adopting the above technical solution, on the one hand, the determined amount of liquid injected into the battery once can still ensure that the battery is fully infiltrated with electrolyte. On the other hand, when the battery is injected with the determined amount of liquid injected once, the amount of electrolyte drawn out during formation is reduced, and the amount of electrolyte volatilization is also reduced, that is, the amount of liquid loss is reduced. This can effectively avoid the imbalance of the ratio of the first injection of electrolyte and the second injection of electrolyte actually injected into the battery, avoid the change of the content of additives in the first injection and the second injection of electrolyte actually injected into the battery, and thus improve the cycle life of the battery.

[0013] Optionally, the liquid injection step further comprises:

[0014] Determine the battery infiltration time: select multiple sample batteries, inject preset amounts of electrolyte into the multiple sample batteries respectively, and infiltrate the multiple sample batteries injected with preset amounts of electrolyte at different test times respectively, with the test time increasing from low to high, determine the sample batteries that are completely infiltrated with the electrolyte and the corresponding test times, select the lowest time among the test times corresponding to all the completely infiltrated sample batteries as the battery infiltration time, and in the injection step, after injecting the electrolyte into the sample battery, let it stand for the infiltration time so that the sample battery is completely infiltrated with the electrolyte.

[0015] Optionally, the method for determining that the sample battery is completely wetted by the electrolyte includes one or more of the following (1) to (3):

[0016] (1) Detect the change of voltage of each sample battery over time during the infiltration process. If the voltage of the sample battery tends to be stable over time, the sample battery is completely infiltrated by the electrolyte;

[0017] (2) After the infiltration is completed, each sample battery is disassembled and the free electrolyte is collected, and the amount of free electrolyte collected from each sample battery at different infiltration times is compared. If the amount of free electrolyte collected from the sample battery tends to be stable as the infiltration time increases, the sample battery is completely infiltrated with the electrolyte;

[0018] (3) After the immersion is completed, each sample battery is disassembled and the amount of liquid absorbed by the electrode in each sample battery is determined. The amount of liquid absorbed by the electrode in each sample battery at different immersion times is compared. If the amount of liquid absorbed by the electrode tends to be stable as the immersion time changes, the sample battery is completely infiltrated with the electrolyte.

[0019] Optionally, the methods for determining a sample battery that is completely soaked in electrolyte include (1) to (3), and a sample battery that is completely soaked in electrolyte determined by all three methods (1) to (3) is selected, and then the minimum test time is determined from the test times corresponding to the selected sample batteries that are completely soaked in electrolyte as the soaking time of the battery.

[0020] Optionally, in determining the soaking time of the battery, the difference between adjacent test times is controlled within 12-18 hours.

[0021] Optionally, the formation step includes: charging the fully soaked sample battery for 60 to 120 minutes at a rate of 0.15C to 0.35C under a negative pressure of -85 to -75KPa.

[0022] Optionally, the injection step includes:

[0023] Vacuuming: Place the sample battery into the vacuum system and evacuate to a vacuum degree of -90 to -70 KPa, then maintain for 2 to 3 minutes;

[0024] Positive pressure: Inert gas is filled into the vacuum system where the sample battery is located and pressurized until the positive pressure reaches 400Kpa, and then maintained for 4 to 6 minutes.

[0025] Optionally, the allowable error range of the liquid loss amount is: ±2% to ±6% of the preset amount.

[0026] Optionally, the type of the sample battery is a square case battery.

[0027] The present invention also provides a method for injecting liquid into a battery, wherein the method adopts the method for determining the primary injection conditions of the battery in any of the above embodiments to determine the primary injection amount, determines the secondary injection amount according to the total injection amount of the battery, and uses the determined primary injection amount and secondary injection amount to perform primary injection, formation and secondary injection on the battery to complete the injection of the battery.

[0028] By adopting the above technical solution, the battery after liquid injection has a longer cycle life. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The open circuit voltage of the sample battery during the soaking period in one embodiment of the present invention is shown as a function of time. Figure 1 ;

[0030] Figure 2 The open circuit voltage of the sample battery during the soaking period varies with time in one embodiment of the present invention. Figure 2 ;

[0031] Figure 3 The content of free electrolyte in Group 1 to Group 6 in one embodiment of the present invention is shown;

[0032] Figure 4 The figure shows the liquid absorption amount of the electrode pieces of group 1 to group 6 in one embodiment of the present invention;

[0033] Figure 5 A flow chart showing a method for determining a battery primary injection condition in one embodiment of the present invention is shown;

[0034] Figure 6 A cross-sectional view showing a device for accelerating infiltration by a vacuum cart at equal pressure in one embodiment of the present invention;

[0035] Figure 7 A three-dimensional diagram showing a device for accelerating infiltration by a vacuum cart at equal pressure in one embodiment of the present invention;

[0036] Figure 8 A schematic diagram showing the wetting interface of the electrode after the wetting of Group 1 to Group 6 is completed in one embodiment of the present invention;

[0037] Fig. 9 A diagram showing the gas generation rate and charging SOC during formation charging in one embodiment of the present invention is shown;

[0038] Fig.10 A schematic diagram showing the amount of electrolyte in the collection cup during negative pressure formation charging in one embodiment of the present invention;

[0039] Fig.11 A process flow chart showing the formation process;

[0040] Fig.12 A schematic diagram showing the interface state of a battery after the battery is injected with liquid using a certain primary injection condition in one embodiment of the present invention;

[0041] Fig.13 A schematic diagram showing the interface state of the battery pole piece when the battery is filled with liquid in a one-injection to two-injection ratio of 80:20 is used;

[0042] Fig.14 The cycle life curve of the battery is shown when the battery is filled with liquid at different one-injection to two-injection ratios;

[0043] Fig.15 A schematic diagram showing the wetting interface of the electrode after wetting of Group 1 is completed. DETAILED DESCRIPTION

[0044] The following is an explanation of the embodiments of the present invention by specific specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. Although the description of the present invention will be introduced in conjunction with the preferred embodiment, this does not mean that the features of this invention are limited to this embodiment. On the contrary, the purpose of introducing the invention in conjunction with the embodiment is to cover other options or modifications that may be extended based on the claims of the present invention. In order to provide a deep understanding of the present invention, the following description will include many specific details. The present invention can also be implemented without using these details. In addition, in order to avoid confusion or blurring the focus of the present invention, some specific details will be omitted in the description. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0045] The terms “first”, “second”, etc. are only used for distinguishing descriptions and should not be understood as indicating or implying relative importance.

[0046] It should be noted that in this specification, similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, it does not need to be further defined and explained in the subsequent drawings.

[0047] In order to make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0048] When the injection stage is divided into primary injection and secondary injection, there are two methods for secondary injection, namely quantitative injection and variable injection. Quantitative injection means that the primary injection and the secondary injection are injected according to the established design ratio, wherein the calculation formula of the secondary injection amount is "secondary injection amount = total injection amount - primary injection amount"; variable injection means that the electrolyte lost by formation will be used to make up for it in the second injection, and the calculation formula of the secondary injection amount is "secondary injection amount = total injection amount - primary injection amount + formation loss amount". When the loss of electrolyte in one injection is large, whether the second injection adopts quantitative injection or variable injection, it is impossible to avoid the problem of imbalance in the ratio of electrolyte in one injection and electrolyte in two injections. Specifically, when the loss of electrolyte in one injection is large, if the secondary injection needs to adopt variable injection to increase the secondary injection amount, the electrolyte lost in the battery after the primary injection will be replenished in the secondary injection stage. This will cause the secondary injection volume to increase too much, resulting in the ratio of the amount of electrolyte actually injected into the battery to the amount of electrolyte injected into the second injection deviating from the preset ratio, resulting in an imbalance in the ratio, which in turn affects the battery's cycle performance. For example, assuming that the total injection of the battery is 1000g, the preset ratio of the primary injection volume to the secondary injection volume is 88:12. If the amount of liquid loss in one injection is 40g, then the secondary injection volume needs to be increased by 40g, which will cause the ratio of the amount of electrolyte actually injected into the battery to the amount of electrolyte injected into the second injection to be 84:16. This ratio deviates from the preset ratio of 88:12. This imbalance will cause the content of additives in the first and second injections of electrolyte actually injected into the battery to change (deviate from the preset content), which will affect the battery's charge and discharge performance and cycle life.

[0049] When the loss of one injection of electrolyte is large, if the second injection adopts quantitative injection, the battery is directly injected once and twice according to the preset ratio of one injection and two injections, which will cause the ratio of the amount of electrolyte actually injected into the battery and the amount of electrolyte in the second injection to seriously deviate from the preset ratio, resulting in an imbalance in the ratio. At the same time, it is impossible to ensure that the battery has sufficient electrolyte to maintain long-term operation, thereby affecting the battery's cycle performance. For example, assuming that the total injection of the battery is 1000g, the preset ratio of the amount of the first injection and the amount of the second injection is 88:12. If the loss of the first injection is 40g, the second injection adopts quantitative injection, which will make the ratio of the amount of electrolyte actually injected into the battery and the amount of electrolyte in the second injection 84:12. This ratio deviates from the preset ratio of 88:12. This imbalance in the ratio will also cause the content of the additives in the first and second injections of the electrolyte actually injected into the battery to change (deviate from the preset content), and at the same time, it is impossible to ensure that the battery has sufficient electrolyte to maintain long-term operation, thereby affecting the battery's charge and discharge performance and cycle life.

[0050] One injection needs to ensure that the electrolyte fully infiltrates (completely infiltrates) the electrode materials of the battery, so as to prevent the appearance of black spots in the battery (usually a local phenomenon caused by the electrode materials not being completely infiltrated by the electrolyte during charging) and avoid poor battery performance. At present, the setting of the one-injection ratio can already ensure complete infiltration. However, in order to ensure complete infiltration, the injection amount of one injection is required to be excessive, that is, after one injection, in addition to fully penetrating the pores of the electrode materials in the battery, there will also be a certain amount of free electrolyte and electrolyte (liquid film) attached to the electrode plates. During formation, the free electrolyte and part of the liquid film will be drawn away due to the negative pressure state. Although the drawn electrolyte will be re-injected (refluxed) into the battery, the process of drawing out and re-injecting will cause the volatilization of one injection of electrolyte, resulting in the loss of one injection of electrolyte, and the more electrolyte drawn out during formation, the greater the loss. Therefore, it is necessary to ensure that the amount of liquid injected is completely soaked, and to reduce the amount of liquid injected as much as possible, thereby reducing the loss of electrolyte. This requires setting the excess amount of liquid injected within an appropriate range to take into account the above two requirements. In the case of an excess amount of liquid injected in the existing practice, how much should be reduced and how to determine this reduction amount have become difficulties in the current method for determining the amount of liquid injected. Based on this, the present invention provides the following technical solutions to solve the above technical problems.

[0051] The present invention provides a method for determining a battery primary injection condition, wherein the method is applicable to a battery that needs to undergo a primary injection, a formation, and a secondary injection process, and the method comprises:

[0052] Injection: Inject a preset amount of electrolyte into the sample battery so that the sample battery is completely soaked in the electrolyte, wherein the preset amount is the preset amount of the sample battery for one injection. The preset amount generally refers to the usual one-time injection standard, and the standard of the one-time electrolyte injection meets the requirement of complete soaking. Usually, the one-time injection volume is 84% ​​to 90% of the total injection volume, and the corresponding two-time injection volume is 10% to 16% of the total injection volume. For example, if the total injection volume of the battery is 1000g, the ratio of the one-time injection volume to the two-time injection volume is 88:12, then the preset amount of one-time injection is 880g.

[0053] Formation: The fully soaked sample battery is formed. Specifically, the sample battery is formed under negative pressure conditions, wherein the negative pressure process returns the extracted electrolyte to the sample battery.

[0054] Determine the amount of electrolyte loss: Since the electrolyte extracted during the formation process will volatilize, even if it flows back into the sample battery, some electrolyte will still be lost. That is, compared with the sample battery before formation, the mass of the sample battery after formation will be reduced. The reduced mass is the mass of the volatilized electrolyte. Based on this, the amount of electrolyte loss injected into the sample battery can be determined based on the mass of the sample battery before formation and the mass of the sample battery after formation.

[0055] Determine the amount of liquid injected at one time: determine the amount of liquid injection reduction at one time based on the amount of liquid loss. The amount of liquid injection reduction at one time is within the allowable error range of the liquid loss amount. Determine the amount of liquid injected at one time based on the difference between the preset amount and the reduced amount.

[0056] The present invention uses the amount of liquid loss as a reference, determines the amount of liquid reduction in one injection within its allowable error range, and then determines the amount of liquid injection in one injection of the battery according to the difference between the preset amount and the amount of reduction. On the one hand, the determined amount of liquid injection in one injection of the battery can still ensure that the battery is sufficiently infiltrated with electrolyte. On the other hand, when the battery is injected with the determined amount of liquid injection, the electrolyte drawn out during formation is also reduced due to the reduction in the amount of liquid injection, and the amount of electrolyte volatilization is also reduced, that is, the amount of liquid loss is reduced. This can effectively avoid an imbalance in the ratio of the one injection of electrolyte and the two injections of electrolyte actually injected into the battery, avoid changes in the content of additives in the one injection and the two injections of electrolyte actually injected into the battery, and thus improve the cycle life of the battery.

[0057] The method provided by the present invention can be applied to batteries that need to undergo one shot, formation and two shot processes. The method of the present invention adjusts the amount of one shot liquid on the basis of ensuring the excess amount of one shot liquid for complete infiltration, which will not cause excessive adjustment so that one shot cannot be completely infiltrated, and can minimize the degree of excess of one shot liquid, minimize the loss of one shot liquid, avoid imbalance in the ratio of one shot and two shots, and the method is simple and widely applicable.

[0058] Further, the allowable error may be ±2% to ±6% of the preset amount. In particular, the allowable error may be ±5% of the preset amount. For example, if the preset amount is in the order of 1000g, the allowable error is ±50g. Assuming that the amount of liquid loss is 100g, the amount of liquid reduction per injection may be selected within the range of 50g to 150g. If the preset amount is in the order of 100g, the allowable error is ±5g. Assuming that the amount of liquid loss is 10g, the amount of liquid reduction per injection may be selected within the range of 5g to 15g. If the preset amount is in the order of 10g, the allowable error is ±0.5g. Assuming that the amount of liquid loss is 1g, the amount of liquid reduction per injection may be selected within the range of 0.5g to 1.5g. The allowable error is set to 5% of the preset amount, thereby determining the reduction amount of one injection, and then determining the one-time injection amount of the battery based on the difference between the preset amount and the reduction amount. This will neither cause excessive callback that results in one injection not being fully soaked, nor minimize the degree of one injection excess, and minimize the loss of one injection as much as possible, thus avoiding an imbalance in the ratio of one injection and two injections.

[0059] Furthermore, in the above embodiment, the liquid injection step further includes:

[0060] Determine the battery infiltration time: select multiple sample batteries, inject preset amounts of electrolyte into the multiple sample batteries respectively, and infiltrate the multiple sample batteries injected with preset amounts of electrolyte at different test times respectively, with the test time increasing from low to high, determine the sample batteries that are completely infiltrated with the electrolyte and the corresponding test times, select the lowest time among the test times corresponding to all the completely infiltrated sample batteries as the battery infiltration time, and in the injection step, after injecting the electrolyte into the sample battery, let it stand for the infiltration time so that the sample battery is completely infiltrated with the electrolyte.

[0061] For example, select 6 sample batteries and label them as: Group 1-Group 6 (where the number of cells in each group of sample batteries is 10 pcs). Inject a preset amount of 880g of electrolyte into each of the 6 sample batteries. Set the temperature to 45°C, and infiltrate the sample batteries of Group 1-Group 6 according to the infiltration conditions shown in Table 1 below. Then determine the sample batteries that are completely infiltrated by the electrolyte and the corresponding test time. Assuming that after the infiltration is completed, it is detected that the sample batteries in Group 2-Group 6 are completely infiltrated, then the minimum time of 48h in the test time corresponding to Group 2-Group 6 is used as the battery infiltration time (that is, the infiltration time corresponding to Group 2). In the above injection step, after injecting the electrolyte into the sample battery, let it stand for 48h of infiltration time, so that the sample battery can be completely infiltrated by the electrolyte, and then perform the above steps of formation, determination of liquid loss, and determination of the injection amount at one time.

[0062] By adopting the above technical scheme, while determining the injection volume at one time, the optimal infiltration time of the battery can also be determined. The optimal infiltration time can not only ensure that the battery is completely infiltrated with the electrolyte, but also avoid the decomposition of the electrolyte. Specifically, the infiltration of the battery mainly relies on the high porosity of the diaphragm and the glue layer or ceramic layer on the surface of the diaphragm, which has extremely high wettability to the electrolyte, so that the electrolyte gradually infiltrates from the edges around the stacked core to the internal area through capillary force. Under a high temperature of 45°, the viscosity of the cyclic ester solvent in the electrolyte will decrease, which is more conducive to the diffusion of the electrolyte. Since low-temperature additives such as MMDS (methylene disulfonate) and DTD (vinyl sulfate) may exist in the electrolyte, free acid is easily generated when the infiltration time is long at high temperature, so the high-temperature infiltration time needs to be controlled. Therefore, it is necessary to take the minimum time among the test times corresponding to all the fully infiltrated sample batteries as the infiltration time of the battery.

[0063] Table 1 Infiltration conditions of different groups

[0064]

[0065] / represents not adopting the infiltration condition, and √ represents adopting the infiltration condition.

[0066] Furthermore, in the above embodiment, the method for determining that the sample battery is completely wetted by the electrolyte includes one or more of the following (1) to (3):

[0067] (1) Detect the change of voltage of each sample battery over time during the infiltration process. If the voltage of the sample battery tends to be stable over time, the sample battery is completely infiltrated by the electrolyte.

[0068] (2) After the infiltration is completed, each sample battery is disassembled and the free electrolyte is collected. The amount of free electrolyte collected from each sample battery at different infiltration times is compared. If the amount of free electrolyte collected from the sample battery tends to be stable as the infiltration time increases, the sample battery is completely infiltrated with the electrolyte.

[0069] (3) After the immersion is completed, each sample battery is disassembled and the amount of liquid absorbed by the electrode in each sample battery is determined. The amount of liquid absorbed by the electrode in each sample battery at different immersion times is compared. If the amount of liquid absorbed by the electrode tends to be stable as the immersion time changes, the sample battery is completely infiltrated with the electrolyte.

[0070] For method (1), for example, the voltage variation of sample batteries of groups 1 to 6 over time during the immersion process can be detected, wherein the time interval between each measurement is 2 to 4 hours. In some other possible embodiments, it can be 1 to 2 hours. The voltage detection result of group 6 is as follows: Figure 1 and Figure 2 As shown ( Figure 1 The box plot can be used to evaluate and compare the voltage variation and distribution. Figure 2 The curve in is the dV / dT change of a battery in group 6, that is, the derivative of voltage (V) with respect to time (T), that is, the rate of change of voltage over time. Specifically, it describes the rate of voltage change at a specific point in time). Figure 1 and Figure 2 It can be seen that after immersion at high temperature of 45°C for 48 hours, the voltage of the sample battery tends to be stable over time, for example, dv / dt tends to 0. Therefore, it can be determined that the optimal immersion time of the battery is 48 hours, and it is concluded that the sample batteries of Group 2 to Group 6 are completely infiltrated by the electrolyte.

[0071] Regarding method (2), for example, after the immersion is completed, the sample batteries of Group 1 to Group 6 can be disassembled and the free electrolyte can be collected. The free electrolyte refers to the electrolyte that can flow freely in the battery without being affected by external forces. In the experiment, the aluminum shell can be disassembled and the core stack can be taken out without pressurizing the core stack, and then the free electrolyte can be collected. The amount of free electrolyte collected from the sample batteries of Group 1 to Group 6 at different immersion times is then compared. The results of the collected free electrolyte are as follows: Figure 3 As shown (where Figure 3 The values ​​in the left column represent the amount of free electrolyte in g, and the percentage values ​​in the right column are the "fixed base ratio". Fixed base ratio: the group with the smallest free electrolyte is selected as the fixed group, and the free electrolyte level of each group is compared with the fixed group. The calculation formula is: fixed base ratio = (free amount of the measured experimental group - free amount of the fixed group) / free amount of the fixed group * 100, for example, the fixed base ratio of group 1 is 19.8% = (56.2g-46.9g) / 46.9g * 100%). Figure 3 The change of the free amount is intuitively fed back. Among them, the amount of free electrolyte collected from the sample batteries in groups 2 to 6 tends to be stable as the immersion time increases (reference base ratio ≤ 10%), which can determine that the sample batteries in groups 2 to 6 are completely infiltrated by the electrolyte. After that, it can be determined that the optimal immersion time of the battery is 48h.

[0072] Regarding method (3), for example, after the infiltration is completed, the batteries of the sample batteries of Group 1 to Group 6 can be disassembled to obtain the electrodes and determine the amount of liquid absorbed by the electrodes in the sample batteries of Group 1 to Group 6. Specifically, after disassembly, first measure the thickness of the positive electrode sheet, the thickness of the negative electrode sheet, the weight of the positive electrode sheet, and the weight of the negative electrode sheet. Then, based on the measured thickness of the positive and negative electrode sheets and the material ratio in the battery design table, calculate the porosity of the positive and negative electrode sheets, and then the theoretical liquid absorption of the positive and negative electrode sheets can be calculated. Finally, the initial weight of the single positive and negative electrode sheets after die cutting is calculated based on the designed surface density and electrode sheet size. The actual weight of the electrolyte absorbed by the positive and negative electrodes is determined based on the difference between the weight of the positive and negative electrode sheets after disassembly and the initial weight of the positive and negative electrode sheets. The test results are as follows. Figure 4 As shown ( Figure 4 The data in the left column represent the actual liquid absorption of the electrode, in g, and the data in the right column are the liquid absorption ratios, where liquid absorption ratio = actual liquid absorption / theoretical liquid absorption * 100%). The maximum value (maximum liquid absorption ratio) is selected from the liquid absorption ratio data of the 6 groups of positive electrode plates, and the difference between the liquid absorption ratio and the maximum liquid absorption ratio of each group is calculated. The smaller the difference, the smaller the difference in the wetting effect between the groups at different wetting times. In particular, for the positive electrode plate, when the difference is ≤2%; for the negative electrode plate, when the difference is ≤5%, the wetting effects between these groups are close to the same, which can determine that the electrolyte is completely infiltrated. Therefore, according to Figure 4 Combined with Figure 8 ( Figure 8 In each group, the left side is the diaphragm and the positive electrode sheet, and the right side is the negative electrode sheet, wherein the diaphragm is attached to the positive electrode sheet) It can be seen that there are slight dot bubbles on the surface of each positive electrode sheet and the left side of the diaphragm in group 1 (reference Fig.15 ), while the wetness of the diaphragms of Groups 2 to 6 is basically the same visually, so in comparison, the wetting effect of Group 1 is worse than that of other groups. It should be noted that the electrolyte on the surface of the negative electrode plate evaporates quickly, and the electrolyte evaporates at the moment the plate is torn, which cannot be effectively compared. Therefore, the side where the positive electrode and the diaphragm are in contact is selected as the reference surface to analyze the electrolyte infiltration. It can be determined that the sample batteries of Groups 2-6 are completely infiltrated by the electrolyte. After that, it can be determined that the effective infiltration time of the battery is 48h.

[0073] In particular, in order to reduce measurement errors, when disassembling the battery, the number of positive and negative electrodes collected is no less than 5, and then the thickness of the 5 positive and 5 negative electrodes is measured and the average value is taken as the thickness of the electrode. Similarly, when measuring the weight of the positive and negative electrodes, the weight of each electrode should be measured, and the total weight should be accumulated, and then the average value is taken as the weight of each electrode. Finally, the liquid absorption of the positive and negative electrodes is determined based on the difference between the actual weight of a single electrode and the theoretical weight of a single electrode (theoretical weight of the electrode = (double-sided surface density of the material + foil surface density) * height of the electrode * width of the electrode).

[0074] Furthermore, in the above-mentioned embodiment, the methods for determining a sample battery that is completely soaked in electrolyte include (1) to (3), and a sample battery that is completely soaked in electrolyte determined by all three methods (1) to (3) is selected, and then the minimum test time is determined from the test times corresponding to the selected sample batteries that are completely soaked in electrolyte as the soaking time of the battery.

[0075] By adopting the above technical solution, through cross-validation of multiple methods, it is possible to more accurately determine whether the battery is completely infiltrated by the electrolyte, reducing the error that may be caused by a single method, thereby improving the accuracy of the detection.

[0076] For example, in the above embodiment, when selecting sample batteries that are completely soaked in electrolyte as determined by the three methods (1) to (3) at the same time, sample batteries of group 2 to group 6 can be determined. Then, it can be determined that the optimal effective soaking time of the battery is 48 hours.

[0077] Furthermore, in the above embodiment, in determining the soaking time of the battery, the difference between adjacent test times is controlled within 12-18 hours. If the time difference is too large, the optimal soaking time may be missed. If the difference is too small, unnecessary testing burden will be added. By controlling the difference between adjacent test times within 12-18 hours, the soaking time can be determined more quickly without excessively extending the detection time, thereby improving efficiency.

[0078] Furthermore, in the above embodiment, the formation step includes: charging the fully infiltrated sample battery for 60 to 120 minutes at a rate of 0.15C to 0.35C under a negative pressure of -85 to -75Kpa for formation. Under this condition, the SEI film-forming effect can be improved during the formation process, and the maximum amount of electrolyte can be ensured to be extracted. In particular, the charging rate is controlled at 0.15C to 0.35C, so that the gas production rate reaches the maximum, and the liquid loss amount obtained on this basis is also the maximum value. When the reduction amount of one injection is determined based on the liquid loss amount, the reduction amount of one injection is also the maximum value. Finally, after determining the reduction amount of one injection based on the liquid loss amount, when the liquid is injected with the determined one-time injection amount, the liquid loss amount is significantly reduced, thereby avoiding the problem of reduced cycle life caused by the imbalance of the ratio of one injection and two injections.

[0079] Further, in the above embodiment, referring to Fig. 9 and Fig.11 The charging process of negative pressure formation is divided into two steps. The first step is to charge the battery at a charging rate of 0.02C until the SOC (State of Charge) is 2%. The second step is to charge the battery at a charging rate of 0.15C until the SOC is 30%, that is, the total charging SOC in the formation step is 32%. A 10-minute normal pressure standby is set between the two charging steps. Fig. 9 Combined with Fig.10 When the battery 3 is charged in the second step, the gas generation rate of the process is the largest when the charging SOC is between 2% and 8%. The bubbles generated between the electrodes will drive more electrolyte to flow toward the outside of the battery. At this time, the volume of electrolyte in the collection cup 7 (connected to the negative pressure pipe 8) is the largest. For example, the volume of the collection cup 7 is 80 mL (refer to Fig.10 ).

[0080] Furthermore, in the above embodiment, the injection can be performed by isobaric injection or differential pressure injection. In particular, the injection is performed by isobaric injection. The isobaric injection steps include:

[0081] Vacuuming: Place the sample battery into the vacuum system and evacuate to a vacuum degree of -90 to -70 KPa, then maintain for 2 to 3 minutes;

[0082] Positive pressure: Inert gas is filled into the vacuum system where the sample battery is located and pressurized until the positive pressure reaches 400Kpa, and then maintained for 4 to 6 minutes.

[0083] The above technical solution can ensure that the electrolyte fully infiltrates the inside of the battery and improve the infiltration effect. Therefore, a more accurate and appropriate amount of liquid loss can be obtained under the premise of ensuring the infiltration effect, and then the amount of liquid injection can be determined based on the amount of liquid loss. When the battery is subsequently injected with the determined amount of liquid injection, the amount of electrolyte in the first injection can be increased as much as possible while ensuring a good infiltration effect, thereby improving the efficiency of the subsequent second injection.

[0084] Specifically, before the injection, the injection cup must be matched with the battery cell injection hole and the air tightness of the cup and the battery cell must be tested by the equipment. After the air tightness is qualified, a shot of electrolyte is injected into the cup, and then the sample battery is transferred to the vacuum system (such as a cylinder-type static station) and evacuated to a vacuum degree of -90 to -70Kpa, and then maintained for 2 to 3 minutes. Next, the cylinder-type static station can be filled with inert gas to add positive pressure in steps. The positive pressure changes to 200Kpa, 300Kpa and 400Kpa. The positive pressure is maintained for 4 to 6 minutes at each step. After the positive pressure reaches the set value, it jumps to the next positive pressure value until the maximum positive pressure setting value is reached.

[0085] Furthermore, in the above embodiment, the vacuuming step and the positive pressure step are cycled sequentially after the positive pressure step, and the number of cycles is 4 to 6. Multiple cycles help the electrolyte to penetrate deeper into the micropores of the electrode material and improve the wetting efficiency.

[0086] Furthermore, in the above-mentioned embodiments, the type of the sample battery is a square shell battery. Lithium-ion batteries have the advantages of high energy density, long cycle life, low self-discharge, and no memory effect. Since commercialization, they have received widespread attention and application. Currently, lithium-ion batteries can be mainly divided into square shell batteries, soft-pack batteries, and cylindrical batteries by packaging methods. Among them, square shell batteries have become the mainstream product of power batteries due to their good safety performance, high energy density, and good cycle performance. The technical solutions in the above-mentioned embodiments provided by the present invention are particularly suitable for square shell batteries, that is, the amount of liquid loss can be reduced while ensuring the infiltration effect, avoiding the imbalance of the ratio of one injection and two injections.

[0087] Furthermore, the present invention also provides a method for injecting liquid into a battery, which method adopts the method for determining the conditions for the first injection of a battery in any of the above embodiments to determine the first injection amount, determines the second injection amount according to the total injection amount of the battery, and uses the determined first injection amount and second injection amount to inject liquid into the battery once, form and inject liquid twice, thereby completing the injection of the battery. That is, the battery is injected once using the determined first injection amount; formed; the battery is injected twice using the second injection amount, thereby completing the injection of the battery. Specifically, after determining the first injection amount, the battery can be injected with the determined first injection amount, and the determined first injection amount can not only ensure the battery wetting effect, but also reduce the amount of electrolyte lost in the formation stage, thereby avoiding the problem of low cycle life caused by the above-mentioned imbalance in proportions. On this basis, since the amount of liquid loss is small when the determined primary injection volume is used for injection, the subsequent secondary injection can be carried out directly using a quantitative injection method or a variable injection method. Both of these injection methods will not cause an imbalance in the ratio of the first injection of electrolyte and the second injection of electrolyte.

[0088] Taking quantitative injection as an example, if the primary injection volume of the battery is determined to be 820g, then the secondary injection volume can be determined to be 180g based on the total injection volume of 1000g. When the battery is injected later, it is only necessary to inject the battery once according to the determined primary injection volume of 820g, and then inject the battery a second time according to the determined secondary injection volume of 180g.

[0089] Taking variable injection as an example, if the primary injection volume of the battery is determined to be 820g, the total injection volume is 1000g, and the electrolyte loss of the battery cell after formation is 3g, according to the calculation logic of variable injection, the secondary injection is performed at 180g+3g=183g. By comparing the above two secondary injection methods, it can be seen that after the primary injection volume is determined by the method of the present invention, the amount of liquid loss during the formation process is small and even negligible, so that the second injection can directly use quantitative injection or variable injection, thereby avoiding the problem of imbalance in the ratio of the first injection and the second injection of electrolyte.

[0090] The present invention is further described in detail below through specific embodiments in conjunction with the accompanying drawings.

[0091] Example 1

[0092] Six square-shell lithium iron phosphate batteries were selected as sample batteries (the ratio of one injection to two injections of this battery is usually 88:12 on the market), with a design capacity of 330Ah and a total injection volume of 1000g. According to industry standards, the injection volume of one injection is tentatively set at 880g. The selected square-shell lithium iron phosphate sample batteries were processed as follows (refer to Figure 5 ):

[0093] Baking: Select square-shell lithium-ion batteries produced in the same batch and measure the moisture content after baking to ensure that it is qualified.

[0094] Liquid injection: isobaric liquid injection is adopted. The steps of isobaric liquid injection include: installing the liquid injection sleeve cup on the top of the battery cell and then conducting an airtightness test. After the airtightness is qualified, a shot of electrolyte is injected into the sleeve cup, and then the sample battery is transferred to the cylinder static station, and the pressure is maintained for 3 minutes under the vacuum condition of -85kPa, and then the vacuum is broken and the normal pressure is restored and maintained for 1 minute; then the pressure is maintained for 5 minutes under each condition of the stepped positive pressure of 200kPa / 300kPa / 400kPa (the positive pressure jumps to the next positive pressure setting value after reaching the set value), and then the positive pressure is broken and the normal pressure is placed for 1 minute. The number of liquid injection cycles under this condition is 2, and then it is placed at normal pressure to avoid electrolyte reflux;

[0095] Determine the battery soaking time: Divide the sample batteries after injection into 6 groups, each group of sample batteries has 10 cells, recorded as Group 1, Group 2, Group 3, Group 4, Group 5, and Group 6. Inject a preset amount of 880g of electrolyte into each of the 6 sample batteries, and soak the 6 sample batteries injected with the preset amount of electrolyte at different test times. The test time increases from low to high. For specific test time, refer to Table 1 above. It should be noted that, refer to Figure 6-Figure 7 After 12 hours of high-temperature soaking at 45°C for group 5, the battery 3 is taken out and placed under the sleeve cup 2 to ensure that the injection nozzle of the sleeve cup 2 fits tightly with the injection hole of the battery 3. Then, the tooling 1 with the battery is transferred to the vacuum trolley 6, the door 5 is closed, and negative pressure treatment is started. Specifically, negative pressure is pumped into the vacuum trolley 6 through the vacuum pipe 4 to effectively discharge the bubbles inside the battery pole piece and improve the soaking efficiency. For specific operations, please refer to the first step, negative pressure extraction: follow the following gradient negative pressure: -0.02Mpa→-0.04Mpa→-0.06Mpa→-0.08Mpa (maintain negative pressure for 5 minutes under each negative pressure gradient, and then switch to the next negative pressure gradient until the maximum negative pressure state is reached), the second step, normal pressure: normal pressure is maintained for 5 minutes, during which the first step of negative pressure extraction and the second step of normal pressure process need to be repeated twice. After that, the tooling 1 is taken out, and after removing the sleeve cup 2, the battery 3 is transferred back to the high-temperature environment for continued soaking for 48 hours. After the infiltration is completed, the tooling 1 is taken out from the vacuum trolley 6 and the cup 2 is removed.

[0096] Determine the sample battery that is completely soaked in electrolyte: During the soaking process, the voltage of the sample battery of test group 6 changes with time. The time interval between each measurement is 2h. The measurement results are as follows: Figure 1 and Figure 2 After the immersion, the sample batteries of Group 1 to Group 6 were disassembled and the free electrolyte was collected. The results of the collected free electrolyte are shown in Figure 3As shown; after the immersion, the batteries of the sample batteries of Group 1 to Group 6 were disassembled to obtain the electrodes and determine the liquid absorption of the electrodes in the sample batteries of Group 1 to Group 6. (After calculation, the porosity of the positive electrode and the negative electrode were 28.59% and 25.8% respectively, the theoretical liquid absorption of the positive electrode and the negative electrode were 2.48g and 1.66g respectively, and the initial weight of the single positive electrode and the negative electrode were 18.672g and 10.011g respectively) The test results are shown in Figure 4 As shown. Figure 1-Figure 4 Combined with Figure 8 It can be determined that the optimal immersion time for the battery is 48h.

[0097] Formation: Transfer the soaked batteries of groups 1 to 5 to the formation cabinet for negative pressure charging. The number of working batteries in each group is 4 to 8 pcs. Before starting the formation charging, conduct a leak detection negative pressure test and set the negative pressure value to -60kPa. Under the negative pressure condition of -60kPa, maintain the pressure for 60 seconds. During the pressure maintenance period, monitor the leakage rate of the batteries of groups 1 to 5 to ensure that the leakage rate is less than 0.5%. Among them, the specific parameters of the formation charging process are as follows: Fig.11 The gas generation rate during the formation charging process is shown as Fig. 9 The electrolyte extracted during the formation process is shown in Fig.10 As shown (the volume of electrolyte collected by the collection cup 7 is 80 mL).

[0098] Determine the amount of liquid loss: The mass of the sample battery before formation is 5402g, and the mass of the sample battery after formation is 5382.2g, from which the amount of liquid loss can be determined to be 19.8g.

[0099] Determine the amount of liquid injected at one time: the allowable error of the liquid loss is ±2% of the preset amount, so the amount of liquid injected at one time of the battery is 860g.

[0100] Application Example 1:

[0101] Liquid injection: a square-shell lithium iron phosphate battery (total liquid injection amount is 1000g) is injected once with a liquid injection amount of 860g determined in Example 1, and the injection condition is isobaric injection, which is the same as the injection condition in Example 1;

[0102] Formation: Same as Example 1 (refer to Fig.11 )

[0103] Secondary injection: inject the battery with a secondary injection volume of 140g.

[0104] Comparative application example 1:

[0105] The steps are the same as those in Application Example 1, except that the first injection volume is 880 g, the battery liquid loss after formation is 25.6 g, the second injection is variable injection, and the second injection volume is determined to be 120 g + 25.6 g = 145.6 g (the amount of electrolyte lost in one injection is compensated by the second injection of electrolyte).

[0106] Comparative application example 2:

[0107] The steps are the same as those in Application Example 1, except that the first injection volume is 800 g, the battery liquid loss after formation is 3.1 g, the second injection is variable injection, and the second injection volume is 200 g + 3.1 g = 203.1 g (the amount of electrolyte lost in the first injection is compensated by the second injection of electrolyte).

[0108] The batteries after secondary liquid injection in Application Example 1 and Comparative Application Examples 1-2 were packaged, and then a cycle performance test was performed. The test conditions were room temperature 25°C, and the charge / discharge current was 0.33C / 0.5C, respectively. The test results are shown in Table 2 below (wherein the number of cycles at which cycle attenuation starts refers to: the number of cycles corresponding to when the capacity starts to show a decay trend. For example, 670 cycles in Comparative Application Example 1 means that the battery cycled 670 times when the battery capacity began to decay. When the battery cycled 670 times in Application Example 1, the capacity did not decay, so it was marked as "-"; the number of cycles of the battery when the retention rate was 93% indicates the number of charge and discharge cycles that the battery can complete when the battery capacity retention rate is 93%). The interface of Comparative Application Example 2 was then observed, and the results are as follows: Fig.12 shown.

[0109] Table 2

[0110]

[0111] According to Table 2, Application Example 1 uses the primary injection amount determined by the present invention to inject the battery once, so that the liquid loss is small and can be ignored. After that, the secondary injection can be directly quantitatively injected without the problem of imbalance in proportion, and finally the battery interface is good (such as Fig.12 As shown, Fig.12 Each battery cell in the embodiment is injected once and injected twice in the manner of Application Example 1. The battery has no capacity decay after 670 cycles. The battery has been cycled for 788 cycles when the retention rate is 93%. That is, the cycle performance of Application Example 1 is better than that of Application Examples 1-2.

[0112] In comparison with Application Example 1, the amount of liquid injected in the first injection is too much, resulting in excessive liquid loss in the first injection. When the second injection adopts variable injection, the ratio of the amount of electrolyte injected into the battery in the first injection to the amount of electrolyte injected in the second injection deviates from the preset ratio of 88:12. This imbalance causes the content of additives in the first and second injections of electrolyte actually injected into the battery to change (deviate from the preset content), which in turn affects the charge and discharge performance and cycle life of the battery. The final manifestation is that when the battery capacity of Comparative Application Example 1 begins to decay, the battery cycles 670 times, and when the retention rate is 93%, the battery cycles 733 times, and the cycle performance is worse than that of Application Example 1.

[0113] Comparative Application Example 2 Although the amount of liquid loss in one shot is small, the amount of liquid injected in one shot is too small, resulting in incomplete battery infiltration and black spots on the interface (such as Fig.13 As shown, Fig.13 Each battery cell in the battery is injected once and twice in the same manner as in Comparative Application Example 2, which also causes the ratio of the amount of electrolyte injected into the battery and the amount of electrolyte injected twice to deviate seriously from the preset ratio, thereby causing the content of additives in the electrolyte injected into the battery to change (deviate from the preset content), thereby affecting the battery's charge and discharge performance and cycle life. The battery cycled 313 times when the battery capacity began to decay, and the battery cycled 644 times when the retention rate was 93%, and the cycle performance was worse than that of Application Example 1.

[0114] In summary, when performing one injection on the battery, if the method for determining the one-time injection amount provided by the present invention is used to determine the one-time injection amount and perform one-time injection, the amount of liquid loss is significantly reduced, and the ratio of the amount of one injection of electrolyte actually injected into the battery to the amount of the two injections of electrolyte will not be unbalanced, and the content of additives in the one injection and the two injections of electrolyte actually injected into the battery will not change. In this way, the synergistic effect of the additives in the one injection of electrolyte and the additives in the two injections of electrolyte can be fully utilized. In other words, after determining the one-time injection amount according to the method for determining the one-time injection amount provided by the present invention, it is only necessary to determine the two injection amounts based on the total injection amount, and then adjust the additive content in the one injection and the two injections of electrolyte accordingly. Subsequently, the battery only needs to be injected according to the ratio of the one injection amount to the two injection amounts determined above to fully utilize the synergistic effect of the additives and improve the cycle performance of the battery.

[0115] Although the present invention has been illustrated and described with reference to certain preferred embodiments of the present invention, it should be understood by those skilled in the art that the above is a further detailed description of the present invention in conjunction with specific embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. Those skilled in the art may make various changes in form and details, including making several simple deductions or substitutions, without departing from the spirit and scope of the present invention.

Claims

1. A method for determining the primary liquid injection conditions of a battery, characterized in that: include: Injection: injecting a preset amount of electrolyte into the sample battery so that the sample battery is completely soaked by the electrolyte, wherein the preset amount is a preset amount of electrolyte for one injection of the sample battery; Formation: forming the completely infiltrated sample battery; Determine the amount of liquid loss: determine the amount of liquid loss of the sample battery injected with electrolyte according to the mass of the sample battery before formation and the mass of the sample battery after formation; Determine the amount of liquid injected once: determine the amount of liquid injected reduced according to the amount of liquid loss, wherein the amount of liquid injected reduced is within an allowable error range of the amount of liquid loss, and determine the amount of liquid injected once according to the difference between the preset amount and the reduced amount.

2. The method for determining the primary liquid injection condition of a battery according to claim 1, characterized in that: The liquid injection step also includes: Determine the battery infiltration time: select multiple sample batteries, inject the preset amount of electrolyte into the multiple sample batteries respectively, and infiltrate the multiple sample batteries injected with the preset amount of electrolyte at different test times, wherein the test time increases from low to high, determine the sample batteries that are completely infiltrated with the electrolyte and the corresponding test time, and select the lowest time among the test times corresponding to all the completely infiltrated sample batteries as the battery infiltration time. In the injection step, after the electrolyte is injected into the sample battery, let it stand for the infiltration time so that the sample battery is completely infiltrated with the electrolyte.

3. The method for determining the primary liquid injection condition of a battery according to claim 2, characterized in that: Methods for determining whether a sample battery is completely wetted by the electrolyte include one or more of the following (1) to (3): (1) Detect the change of voltage of each sample battery over time during the infiltration process. If the voltage of the sample battery tends to be stable over time, the sample battery is completely infiltrated by the electrolyte; (2) After the infiltration is completed, each sample battery is disassembled and the free electrolyte is collected, and the amount of free electrolyte collected from each sample battery at different infiltration times is compared. If the amount of free electrolyte collected from the sample battery tends to be stable as the infiltration time increases, the sample battery is completely infiltrated with the electrolyte; (3) After the immersion is completed, each sample battery is disassembled and the amount of liquid absorbed by the electrode in each sample battery is determined. The amount of liquid absorbed by the electrode in each sample battery at different immersion times is compared. If the amount of liquid absorbed by the electrode tends to be stable as the immersion time changes, the sample battery is completely infiltrated with the electrolyte.

4. The method for determining the primary liquid injection condition of a battery according to claim 3, characterized in that: The methods for determining a sample battery that is completely soaked in electrolyte include (1) to (3). A sample battery that is completely soaked in electrolyte and that satisfies all three methods (1) to (3) is selected, and then the minimum test time is determined from the test times corresponding to the selected sample batteries that are completely soaked in electrolyte as the soaking time of the battery.

5. The method for determining the primary liquid injection condition of a battery according to claim 2, characterized in that: In determining the battery soaking time, the difference between adjacent test times is controlled within 12-18 hours.

6. The method for determining the primary liquid injection condition of a battery according to claim 1, characterized in that: The formation step comprises: charging the completely soaked sample battery for 60 to 120 minutes at a rate of 0.15C to 0.35C under a negative pressure of -85 to -75Kpa for formation.

7. The method for determining the primary liquid injection condition of a battery according to claim 1, characterized in that: The liquid injection step comprises: Vacuuming: Place the sample battery into the vacuum system and evacuate to a vacuum degree of -90 to -70 KPa, then maintain for 2 to 3 minutes; Positive pressure: Inert gas is filled into the vacuum system where the sample battery is located and positive pressure is applied until the positive pressure reaches 400 KPa, and then maintained for 4 to 6 minutes.

8. The method for determining the primary liquid injection condition of a battery according to claim 1, characterized in that: The allowable error range of the liquid loss amount is: ±2% to ±6% of the preset amount.

9. The method for determining the primary liquid injection condition of a battery according to any one of claims 1 to 8, characterized in that: The type of the sample battery is a square shell battery.

10. A method for injecting liquid into a battery, characterized in that: The method for determining the primary injection conditions of a battery according to any one of claims 1 to 8 is used to determine the primary injection amount, the secondary injection amount is determined according to the total injection amount of the battery, and the battery is injected once, formed and injected twice using the determined primary injection amount and secondary injection amount to complete the injection of the battery.

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