Battery formation method and secondary battery

Abstract

The application provides a battery formation method and a secondary battery, and the method comprises the following steps: an injection step of injecting an electrolyte into the battery; a clamping step of clamping the side walls of the shell on both sides by a clamp to press the large surfaces on both sides of the bare battery cell in the thickness direction of the shell; a formation step of charging the battery to a pre-formation voltage under a first air pressure, and charging the battery to each formation voltage in turn under a second air pressure; wherein the electrode assembly comprises a positive pole piece, and the positive pole piece comprises a positive pole material and at least one lithium supplementing agent; the first air pressure and the second air pressure are negative pressures, and the second air pressure is greater than the first air pressure. According to the formation method, the large surfaces of the bare battery cell are pressed by the clamp to accelerate the formation gas discharge without wrinkling the pole piece, and the double-negative-pressure breathing formation process is used to complete the extraction of the formation gas from the battery and reduce the electrolyte loss caused by the exhaust, so that the formation gas is completely discharged and the problem of lithium precipitation at the interface of the pole piece is eliminated.

Description

Technical Field

[0001] This invention relates to the field of battery technology, specifically to a battery formation method and a secondary battery. Background Technology

[0002] During the first week of charging of a lithium-ion battery, a solid electrolyte interface (SEI) forms on the surface of the negative electrode. This consumes active lithium in the positive electrode, leading to irreversible capacity loss. The most widely used graphite negative electrode can experience irreversible capacity loss of up to 10%, while for silicon-based and tin-based negative electrodes with high specific capacity, this irreversible capacity loss can even exceed 30%. This means that Li₂ extracted from the positive electrode material... + Some lithium is irreversibly consumed, and the loss of lithium leads to a decrease in battery capacity, a decrease in coulombic efficiency, and a deterioration in cycle performance.

[0003] Currently, lithium replenishers are typically added to electrode assemblies to replenish the active lithium consumed during electrolyte formation, thereby increasing the energy density of the bare cells and extending battery life. However, lithium replenishers release a large amount of gas during charging formation. The industry-standard negative pressure formation process not only removes a significant amount of electrolyte from the battery, leading to insufficient electrolyte, but also leaves behind incompletely expelled air bubbles. Both of these situations can cause lithium deposition at the electrode interfaces of the bare cells, resulting in the loss of active lithium and even posing safety hazards. Although the active lithium lost in the battery can be replenished through secondary electrolyte injection, the air bubbles between the electrodes will remain in the bare cells. During use, this will increase polarization and further cause lithium deposition at the cell interfaces, thus affecting the cell's cycle life and even causing safety risks such as short circuits.

[0004] Therefore, it is necessary to design a battery formation method and a secondary battery to solve the above problems. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the present invention provides a battery formation method and a secondary battery, which can completely remove the formation gas during the electrolyte formation process, thereby solving the technical problem that the gas generated by the lithium replenishment reaction in the existing formation process cannot be completely removed, and the gas bubbles remain in the interface of the bare cell electrode, resulting in lithium deposition at the cell interface.

[0006] To achieve the above and other related objectives, the present invention provides a battery formation method. Before the formation process, the battery includes a casing and a bare cell located in the casing. The bare cell includes an electrode assembly and tabs electrically connected to the electrode assembly.

[0007] The battery formation method includes the following steps:

[0008] Electrolyte injection step: Inject electrolyte into the battery;

[0009] Clamping step: clamping the side walls of the shell on both sides by a clamp to press the large surfaces of the bare battery cell on both sides in the thickness direction of the shell;

[0010] Formation step: charging the battery to a pre-formation voltage under a first air pressure; and charging the battery to each formation voltage in turn under a second air pressure;

[0011] The electrode assembly comprises a positive electrode sheet, the positive electrode sheet comprises a positive electrode material and at least one lithium supplementing agent; the first air pressure and the second air pressure are negative pressures, and the second air pressure is greater than the first air pressure.

[0012] In an example of the present application, the opposite ends of the clamp are provided with bosses, the clamp clamps the side walls of the shell on both sides through the bosses, and the clamping surfaces of the bosses are flat.

[0013] In an example of the present application, the length and width of the boss are greater than or equal to the length and width of the bare battery cell respectively, and the length and width of the boss are less than the length and width of the shell respectively.

[0014] In an example of the present application, the clamping force of the clamp on the shell is 0.15 MP to 0.5 MPa.

[0015] In an example of the present application, the first air pressure is -70 kPa to -85 kPa, and the second air pressure is -30 kPa to -60 kPa.

[0016] In an example of the present application, the positive electrode material comprises lithium iron phosphate, and / or the lithium supplementing agent comprises lithium ferrite.

[0017] In an example of the present application, the formation step further comprises, after the battery is charged to any of the formation voltages, placing the battery under the second air pressure for standing and exhausting.

[0018] In an example of the present application, the formation step further comprises, after the battery is charged to any of the formation voltages, placing the battery under a third air pressure for standing and exhausting; wherein the third air pressure is greater than the second air pressure.

[0019] In an example of the present application, the third air pressure is -10 kPa to 10 kPa.

[0020] In an example of the present application, in the formation step, the battery is charged to the pre-formation voltage at a first charging current, and the battery is charged to each of the formation voltages at a second charging current, the first charging current is greater than the second charging current, and the charging rate of the first charging current and the second charging current is less than or equal to 0.3C.

[0021] The application also provides a secondary battery prepared by the preparation method comprising the battery formation method of any one of the examples.

[0022] The battery formation method of the application, in the formation process, clamps the two side walls of the battery shell by the clamping surface on the clamp to uniformly press the two side surfaces of the bare battery cell in the thickness direction of the shell, thereby accelerating the escape of the formation gas bubbles between the electrode plates and improving the stress on the interface of the bare battery cell, and relieving the wrinkling phenomenon of the electrode plate in the charging and discharging process. At the same time, the double negative pressure breathing formation process is adopted, high vacuum negative pressure exhaust is used before the voltage interval of the lithium supplement reaction, and low vacuum air pressure exhaust is used in the voltage interval of the lithium supplement reaction, so as to reduce the loss of electrolyte and completely extract the gas generated in the formation process, thereby eliminating the lithium precipitation problem of the interface of the bare battery cell in the formation process, and making the battery cell have a good full charge interface after formation. BRIEF DESCRIPTION OF DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description only constitute some embodiments of the application, and for those skilled in the art, other embodiments can also be obtained from these drawings without creative labor.

[0024] Figure 1 It is a flowchart of the battery formation method in an embodiment of the application;

[0025] Figure 2 It is a flowchart of the formation step in an embodiment of the application;

[0026] Figure 3 It is a flowchart of the formation step in another embodiment of the application;

[0027] Figure 4 It is a flowchart of the formation step in another embodiment of the application;

[0028] Figures 5a to 5f It is an interface photo of the bare battery cell after formation in embodiment 1 of the application;

[0029] Figures 6a to 6f It is an interface photo of the bare battery cell after formation in comparative example 1 of the application;

[0030] Figures 7a to 7f It is an interface photo of the bare battery cell after formation in comparative example 2 of the application. DETAILED DESCRIPTION

[0031] The present application is described in greater detail by the following specific examples, and other advantages and permutations of the present application will become apparent to those skilled in the art upon reading the following description with reference to the drawings and claims. The present application may be carried out in other specific ways than those herein set forth without departing from the essential spirit and fundamental principles of the application. Embodiments of the present application illustratively described herein are by way of example only and various modifications and changes can be made without departing from the spirit and scope of the application. It should be noted that the following examples and features thereof can be combined with each other, where appropriate, without conflict. It is also to be understood that the terminology used herein is for the purpose of describing the particular embodiments only and is not intended to limit the scope of the present application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The materials, methods, and examples provided herein are illustrative only and are not intended to be limiting on the scope of the application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Any methods, devices and materials similar or equivalent to those described herein can be used in the practice of the present application.

[0032] When a numerical range is given in the embodiments, it should be understood that, unless otherwise specified by the present application, each numerical range has two endpoints and any number between the two endpoints can be selected. Unless otherwise defined, all technical and scientific terms used in the present application have the same meaning as the present technical field of the skilled in the art and the description of the present application, and any method, equipment and material of the prior art similar or equivalent to the method, equipment and material described in the embodiments of the present application can be used to realize the present application.

[0033] The inventor found that, in the prior art, directly pressing the interface of the bare cell pole piece during formation can accelerate the formation reaction of the lithium supplement, thereby reducing the side reaction of the electrolyte with the lithium supplement at high voltage and improving the cycle life of the lithium ion battery. However, the existing pressing method cannot make the multiple layers of electrode pole pieces in the bare cell tightly adhere to each other, resulting in a gap between the electrode pole pieces that can accumulate gas. This makes the reaction gas of the lithium supplement, which is difficult to completely discharge, remain between the interfaces of the bare cell pole pieces, thereby deteriorating the cycle life of the battery and causing lithium precipitation short circuit at the interface of the cell. In particular, for the square cell with a large electrode interface of the bare cell, the existing formation process is more likely to cause lithium precipitation at the electrode interface of the battery, thereby causing safety risks during use of the battery.

[0034] To solve the above problems, the present application provides a battery formation method, which uses a clamp tool to clamp the two side walls of the shell on the outside during the formation process to uniformly press the two sides of the bare cell along the thickness direction of the shell, so that the multiple layers of electrode pole pieces in the bare cell are uniformly stressed and tightly adhere to each other below the clamping line. This effectively alleviates the wrinkling phenomenon of the pole piece interface during the formation process, and makes the reaction gas of the lithium supplement be squeezed out of the bare cell by the tightly adhered pole pieces, and then the reaction gas of the lithium supplement in the battery is discharged under negative pressure, thereby avoiding the accumulation of bubbles between the electrode pole pieces and eliminating the problem of lithium precipitation at the interface of the bare cell during the formation process.

[0035] In the present application, before the formation process, the battery comprises a shell and a bare cell assembled into the shell. The shell has a receiving cavity for accommodating the bare cell, electrolyte and other components. The shell can be cylindrical, prismatic or square in shape. In the embodiment of the present application, the shell is square, one end of the shell has an opening, and the bare cell is installed in the shell, the positive and negative tabs on the bare cell extending out of the opening.

[0036] The bare cell comprises an electrode assembly and tabs electrically connected to the electrode assembly. The electrode assembly is mainly formed by winding or stacking a positive electrode tab and a negative electrode tab, and a separator is usually arranged between the positive electrode tab and the negative electrode tab. The positive electrode tab comprises a positive current collector and a positive active material layer coated on the surface of the positive current collector; the positive current collector comprises a positive coating area coated with the positive active material layer and a positive tab not coated with the positive active material layer. The negative electrode tab comprises a negative current collector and a negative active material layer coated on the surface of the negative current collector; the negative current collector comprises a negative coating area coated with the negative active material layer and a negative tab not coated with the negative active material layer. Taking a lithium ion battery as an example, the material of the positive current collector can be aluminum, and the positive active material layer comprises a positive material, a lithium supplement agent, a conductive agent and a binder. The positive material can be lithium cobaltate, lithium iron phosphate, ternary lithium or lithium manganate, etc. The lithium supplement agent comprises lithium ironate, lithium nickelate and lithium cobaltate, etc. For example, in some embodiments, the positive material comprises lithium iron phosphate (LiFePO4), and the lithium supplement agent comprises lithium ironate (Li5FeO4). The material of the negative current collector can be copper, and the negative active material layer comprises a negative material, a conductive agent, a thickening agent and a binder. The negative material can be carbon or silicon, etc. The material of the separator can be PP (polypropylene), PE (polyethylene) or the like. In order to protect and insulate the bare cell, an insulating film can be coated on the outside of the bare cell, which can be synthesized by PP, PE, PET, PVC or other high polymer materials.

[0037] As shown in FIG. 1, the formation method of the above battery comprises the following steps: Figure 1

[0038] S1, a liquid injection step: injecting electrolyte into the battery;

[0039] S2, a clamping step: clamping the side walls of the shell on both sides by a clamp to uniformly press the two large surfaces of the bare cell in the thickness direction of the shell;

[0040] S3, a formation step: charging the battery to a pre-formation voltage at a first air pressure; and charging the battery to each formation voltage in sequence at a second air pressure. ​

[0041] The positive electrode tab in the bare battery cell has a positive electrode material and at least one lithium supplementing agent. In order to exert the supplementing function of the lithium supplementing agent on the active lithium in the battery, each formation voltage of the first charging of the battery is greater than or equal to the formation reaction voltage of the lithium supplementing agent and the electrolyte, so that the at least one lithium supplementing agent in the bare battery cell can react with the electrolyte to be delithiated at the corresponding formation voltage.

[0042] In addition, in the formation step, the first gas pressure and the second gas pressure are both negative pressures. In the formation reaction voltage interval of the delithiation reaction of the lithium supplementing agent and the electrolyte, the second gas pressure applied to the battery is greater than the first gas pressure, so as to slow down the discharge speed of the reaction gas of the lithium supplementing agent in the battery, and avoid the excessive discharge of the electrolyte in the battery due to the violent discharge of the lithium supplementing agent reaction under the clamping of the tab interface, thereby effectively reducing the loss of the electrolyte in the formation process.

[0043] S1, the liquid injection step specifically includes placing the battery in a formation cabinet, vacuumizing the formation cabinet, driving a liquid injector into the shell of the battery in the formation cabinet, and positively driving the liquid injector to inject electrolyte into the battery. After injection, the battery is left in the vacuum formation cabinet for a period of time to ensure that the injected electrolyte in the battery can fully soak the interfaces between the various electrode tabs in the bare battery cell. The soaking time of the battery in the vacuum formation cabinet is 5-30 minutes, for example, it can be 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes or 30 minutes.

[0044] S2, the clamping step specifically includes clamping the two side walls of the shell on the outside in the formation cabinet by the clamp, and the clamp uniformly applies pressure to the two sides of the bare battery cell through the two side walls. The pressure drives the various layers of electrode tabs in the bare battery cell to be uniformly stressed and closely attached, and forces the gas generated between the tab interfaces of the bare battery cell to be extruded and accelerated.

[0045] The inventor found that the wound or stacked electrode tabs in the bare battery cell have a certain elasticity. The existing binding and pressing method of the bare battery cell is limited by the thickness direction group margin of the bare battery cell, and cannot drive the multiple layers of electrode tabs in the bare battery cell to completely adhere. Moreover, the binding and pressing method is also limited by the tab interface size of the bare battery cell, which is easy to cause the local pressure of the tab interface of the bare battery cell to be too large when the pressure is applied, resulting in irregular rebound of the tab interface in the bare battery cell due to uneven stress, and further causing the electrode tabs in the bare battery cell to wrinkle. The above two situations will make the electrode tabs in the bare battery cell unable to completely adhere, and the multiple layers of electrode tabs will still have gaps between the relative interfaces, which will make it difficult for the battery to completely extrude the reaction gas of the lithium supplementing agent under negative pressure, and the bubbles of the electrode tabs will continue to remain in the battery cell, increasing the polarization and affecting the cycle life of the battery.

[0046] In the clamping step of the present application, the clamps are opposite to the side walls of the clamping shell, the clamping surfaces of the clamps clamping the side walls are matched with the large surfaces of the bare battery cell, and the clamps apply pressure to the large surfaces of the bare battery cell in the shell along the thickness direction by means of the side walls of the clamping shell, so that the multiple electrode plates in the bare battery cell are tightly attached. This clamping method is different from the existing binding and pressing method, and is not limited by the thickness direction tolerance of the bare battery cell. Instead, the clamping force is uniformly applied to the opposite interfaces of the electrode plates along the thickness direction of the shell, avoiding irregular rebound of the electrode plates in the bare battery cell due to uneven stress on the interface, effectively suppressing the wrinkling of the electrode plates in the bare battery cell due to clamping, and realizing the tight attachment of the multiple electrode plates in the bare battery cell on the opposite interfaces.

[0047] In addition, during the formation and full charging process, due to the tight attachment of the electrode plates in the bare battery cell on the opposite interfaces, the reaction gas of the lithium supplement and the electrolyte in the bare battery cell is accelerated to escape between the electrode plates under the extrusion of the plate attachment, and is quickly pumped out of the bare battery cell by the negative pressure air pumping in the large corner gap area of the bare battery cell, so that there is no gas bubble left between the electrode plates in the bare battery cell, ensuring that the bare battery cell has a good formed and fully charged electrode plate interface (without lithium precipitation and purple stain, without wrinkling and other abnormal phenomena), thereby improving the cycle performance and life of the battery.

[0048] In some embodiments, the clamps used in the clamping step are provided with bosses opposite to each other, the clamping surfaces of the opposite bosses are flat, and the length and width of the clamping surfaces should be greater than or equal to the length and width of the bare battery cell respectively, and less than the length and width of the shell respectively, so as to ensure that the clamps can apply clamping force to the large surfaces of the bare battery cell on the side walls of the shell rather than the shell frame.

[0049] It should be noted that the clamping force of the clamps clamping the tabs is any value in the range of 0.15-0.5 MPa, for example, it can be 0.15 MPa, 0.2 MPa, 0.25 MPa, 0.3 MPa, 0.35 MPa, 0.4 MPa, 0.45 MPa or 0.5 MPa; wherein the specific value of the clamping force can be determined according to the battery model and the chemical system.

[0050] S3, the formation step, is to activate the positive and negative materials in the battery by charging the battery for the first time in a negative pressure vacuum formation cabinet. In this process, the electrolyte reacts on the surface of the negative material to form an SEI film, and at the same time, the lithium supplement reacts with the electrolyte on the positive plate to supplement the active lithium consumed by the formation of the SEI film. In addition, during the formation process, the lithium supplement reacts with the electrolyte on the positive plate interface of the bare battery cell and produces gas, and the reaction gas of the lithium supplement is accelerated to escape between the electrode plates under the extrusion of the plate attachment, and is pumped out of the bare battery cell by the negative pressure air pumping of the formation cabinet, so as to avoid the residual gas bubbles in the bare battery cell causing lithium precipitation in the bare battery cell.

[0051] The formation step in the present application adopts a double negative pressure breathing formation process. High vacuum negative pressure air extraction is used in the pre-formation stage of the battery to promote the SEI film formation of the electrolyte in the bare battery and discharge the generated gas. Low vacuum negative pressure air extraction is used in the formation stage of the battery to slow down the air extraction speed under the premise of ensuring the effective discharge of the reaction gas generated by the lithium supplement agent, thereby reducing the loss of electrolyte caused by the violent gas discharge in the formation process.

[0052] As shown in Figure 2 , S3, the formation step specifically includes the following steps:

[0053] S31, charge the battery to a pre-formation voltage under a first air pressure of high vacuum negative pressure; wherein the pre-formation voltage is less than the formation reaction voltage of the lithium supplement agent and the electrolyte. In the charging process of step S31, the lithium supplement agent does not react with the electrolyte to generate gas, and the high vacuum negative pressure mainly promotes the discharge of the gas generated by the film formation reaction of the electrolyte on the surface of the negative electrode material;

[0054] S32, sequentially charge the battery to each preset formation voltage under a second air pressure of low vacuum negative pressure until the battery is charged to a formation cutoff voltage, which is the highest voltage of the full charge of the battery in this embodiment and is determined according to the type of lithium supplement agent added in the bare battery; wherein each formation voltage is greater than or equal to the formation reaction voltage of the corresponding lithium supplement agent and the electrolyte. Step S32 adopts a staged charging to ensure the full reaction of each type of lithium supplement agent and to avoid the occurrence of side reactions between the lithium supplement agent and the electrolyte under high pressure, while also ensuring that the reaction gas generated by each type of lithium supplement agent can be fully discharged.

[0055] It should be noted that when the battery charging voltage is higher than the pre-formation voltage, the lithium supplement agent in the bare battery will react with the electrolyte to rapidly produce a large amount of gas. Using high vacuum negative pressure formation will quickly extract a large amount of gas from the battery, thereby causing a large amount of electrolyte to be taken away from the battery, and further causing the bare battery to be short of electrolyte. In this state, full charging can easily cause lithium precipitation at the interface of the bare battery, therefore, in step S32, using low vacuum negative pressure air extraction can slow down the discharge speed of the reaction gas generated by the lithium supplement agent, effectively reducing the loss of electrolyte in the battery.

[0056] As shown in Figure 2 , in some embodiments, step S3 further includes S33, a negative pressure standing step. Step S33 includes, after the battery is charged to any formation voltage, allowing the battery to stand and discharge gas under the negative pressure environment of the second air pressure at the formation voltage, to ensure the full reaction of the lithium supplement agent and the full discharge of the reaction gas. Wherein the standing time of the battery under the second air pressure is 5-10 min, for example, it can be 5 min, 6 min, 7 min, 8 min, 9 min or 10 min.

[0057] In some embodiments, in the formation cabinet, the first air pressure of the high vacuum negative pressure is -70 to -85 kPa, for example, the first air pressure can be -70 kPa, -75 kPa, -80 kPa or -85 kPa; the second air pressure of the low vacuum negative pressure is -30 to -60 kPa, for example, the second air pressure can be -30 kPa, -35 kPa, -40 kPa, -45 kPa, -50 kPa, -55 kPa or -60 kPa.

[0058] In some embodiments, in step S3, the battery is charged to the pre-formation voltage at a constant first charging current, and then the battery is sequentially charged to each preset formation voltage from the pre-formation voltage at a constant second charging current, until the battery is charged to the formation cut-off voltage. Wherein, the first charging current is greater than the second charging current, and the second charging current can be set differently according to the charging speed in each formation voltage charging stage of step S32, and does not have to be set as the same current. In an example, the charging rate of the first charging current and the second charging current is less than or equal to 0.3C, for example, the charging rate of the first charging current is 0.3C, and the charging rate of the second charging current is 0.1C.

[0059] In addition, the inventors have found that although low vacuum negative pressure gas extraction can remove most of the reaction gas of the lithium supplementing agent, due to the surface tension of the bubbles generated between the electrode sheets by the lithium supplementing agent, part of the bubbles between the electrode sheets are difficult to be squeezed out due to the elastic tension, and cannot be extracted by negative pressure. In some embodiments, step S3 further comprises S34, a positive pressure standing step, wherein step S34 is performed after the low vacuum negative pressure gas extraction of the battery in the formation cabinet, and a third air pressure close to positive pressure is applied to the battery, so that the residual bubbles in the bare cell are subjected to the squeezing action of the clamp along the thickness direction of the bare cell and the air pressure along the height direction of the bare cell, thereby promoting the flow of bubbles between the electrode sheet interfaces and constantly breaking the surface tension of the bubbles from the inside and outside directions, and then breaking the bubbles between the electrode sheet interfaces to completely remove the residual bubbles from the bare cell.

[0060] It should be noted that, as shown in Figure 3 and Figure 4 Step S34 can be performed after the battery is charged to any formation voltage in step S32, or can be performed after the battery is standing under the second air pressure ring in step S33.

[0061] Specifically, the step S34 comprises, after the battery is charged to any formation voltage each time, allowing the battery to stand at the formation voltage in a negative pressure environment of a third air pressure, the third air pressure being greater than the second air pressure, the third air pressure being set near zero air pressure, the third air pressure being -10-10 kPa, for example, -10 kPa, -7 kPa, -5 kPa, -3 kPa, 0 kPa, 3 kPa, 5 kPa, 7 kPa or 10 kPa. Wherein, the standing time of the battery at the third air pressure is 5-10 min, for example, 5 min, 6 min, 7 min, 8 min, 9 min or 10 min.

[0062] The application further provides a secondary battery prepared by the preparation method, which comprises the steps of shell assembly, liquid injection formation, sealing and aging, wherein the liquid injection formation is the battery formation method of any one of the embodiments of the application. And since the preparation method of the secondary battery comprises the above battery formation method, it has the corresponding beneficial effects, which are not described here again.

[0063] The technical solutions of the application will be described in detail below through the following examples. Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available or can be prepared by conventional methods in the art, and the instruments used in the examples are commercially available.

[0064] Example 1

[0065] The square case battery is taken as the formation research object, which comprises a shell and a bare cell assembled into the shell, the shell is a 52148 square aluminum shell, and the bare cell is formed by winding process.

[0066] In the bare cell, the preparation of the positive electrode sheet is that the slurry mixed by the positive electrode material lithium iron phosphate (LiFePO4), lithium iron oxide (Li5FeO4) as a lithium supplement, conductive agent carbon black (Super P) and binder polyvinylidene fluoride (PVDF) is coated on the surface of the positive electrode current collector aluminum foil, and then the positive electrode sheet is made through processes such as drying, cold pressing and slitting;

[0067] The preparation of the negative electrode sheet is that the slurry mixed by the negative electrode material artificial graphite, conductive agent carbon black (Super P), thickening agent sodium carboxymethyl cellulose (CMC) and binder styrene butadiene rubber (SBR) is coated on the surface of the negative electrode current collector copper foil, and then the negative electrode sheet is made through processes such as drying, cold pressing and slitting;

[0068] The separator is selected as a 9 μm polyethylene separator coated with a 2 μm ceramic coating on both sides;

[0069] The bare battery cell is prepared by sequentially stacking the positive electrode sheet, the separator and the negative electrode sheet, so that the separator is between the positive electrode sheet and the negative electrode sheet to play a role of isolation, then the aluminum plastic film is externally packaged, and the bare battery cell in a roll shape is prepared after rolling and hot pressing.

[0070] The formation method of the battery includes the following steps:

[0071] S1, liquid injection step: placing the prepared battery in a vacuum formation cabinet, injecting electrolyte into the battery through a liquid injector, and standing for 5 minutes; the electrolyte is lithium salt LiPF6 dissolved in an organic solvent to prepare an electrolyte with a concentration of 1 mol / L, and the organic solvent is a mixed organic solvent of ethylene carbonate (EC), methyl ethyl carbonate (EMC) and diethyl carbonate (DEC) in a volume ratio of 1:1:1.

[0072] S2, end face clamping step: in the formation cabinet, clamping the two side walls of the shell with a clamp to press the two large faces of the bare battery cell in the thickness direction of the shell. The clamping force of the clamp clamping the tab is 0.2 MPa.

[0073] S3, formation step:

[0074] (1) the battery is charged to a pre-formation voltage of 3.65V at a constant rate of 0.3C, and the first gas pressure in the formation cabinet is continuously high vacuum of-80kPa during the process;

[0075] (2) the battery is charged to a formation voltage of 3.75V at a constant rate of 0.1C, and the second gas pressure in the formation cabinet is continuously low vacuum of-45kPa during the process;

[0076] (3) the battery is placed at 3.75V in the formation cabinet for 5 minutes, and the second gas pressure in the formation cabinet is continuously low vacuum of-45kPa during the process;

[0077] (4) the battery is placed at 3.75V in the formation cabinet for 5 minutes, and the third gas pressure in the formation cabinet is continuously near 5kPa during the process;

[0078] (5) the battery is charged to a formation voltage of 3.85V at a constant rate of 0.1C, and the second gas pressure in the formation cabinet is continuously low vacuum of-45kPa during the process;

[0079] (6) the battery is placed at 3.85V in the formation cabinet for 5 minutes, and the second gas pressure in the formation cabinet is continuously low vacuum of-45kPa during the process;

[0080] (7) the battery is placed at 3.85V in the formation cabinet for 5 minutes, and the third gas pressure in the formation cabinet is continuously near 5kPa during the process;

[0081] (8) the battery is charged at a second charging current of constant rate 0.1C to a formation voltage 3.95V, during which the second gas pressure of low vacuum is continuously kept at -45 kPa in the formation cabinet;

[0082] (9) the battery is left at 3.95V voltage in the formation cabinet for 5 min, during which the second gas pressure of low vacuum is continuously kept at -45 kPa in the formation cabinet;

[0083] (10) the battery is left at 3.95V voltage in the formation cabinet for 5 min, during which the third gas pressure is continuously kept at 5 kPa around positive pressure in the formation cabinet;

[0084] (11) the battery is charged at a second charging current of constant rate 0.1C to a formation voltage 4.1V, during which the second gas pressure of low vacuum is continuously kept at -45 kPa in the formation cabinet;

[0085] (12) the battery is left at 4.1V voltage in the formation cabinet for 5 min, during which the second gas pressure of low vacuum is continuously kept at -45 kPa in the formation cabinet;

[0086] (13) the battery is left at 4.1V voltage in the formation cabinet for 5 min, during which the third gas pressure is continuously kept at 5 kPa around positive pressure in the formation cabinet;

[0087] (14) the battery is charged at a second charging current of constant rate 0.1C to a formation voltage 4.3V, during which the second gas pressure of low vacuum is continuously kept at -45 kPa in the formation cabinet;

[0088] (15) the battery is left at 4.3V voltage in the formation cabinet for 5 min, during which the second gas pressure of low vacuum is continuously kept at -45 kPa in the formation cabinet;

[0089] (16) the battery is left at 4.3V voltage in the formation cabinet for 5 min, during which the third gas pressure is continuously kept at 5 kPa around positive pressure in the formation cabinet.

[0090] Comparative Example 1

[0091] Comparative Example 1 is the same as Example 1 in the formation process, the difference between Comparative Example 1 and Example 1 is that S3, the formation step uses high vacuum negative pressure formation process.

[0092] Specifically, S3, the formation step includes:

[0093] (1) the battery is charged at a first charging current of constant rate 0.3C to a pre-formation voltage 3.65V, during which the first gas pressure of high vacuum is continuously kept at -80 kPa in the formation cabinet;

[0094] (2) the battery is charged to a formation voltage of 3.75 V at a second charging current of a constant ratio of 0.1 C, and in the process, the first gas pressure of a high vacuum in the formation cabinet is continuously -80 kPa;

[0095] (3) the battery is left to stand at a voltage of 3.75 V in the formation cabinet for 10 min, and in the process, the first gas pressure of a high vacuum in the formation cabinet is continuously -80 kPa;

[0096] (4) the battery is charged to a formation voltage of 3.85 V at a second charging current of a constant ratio of 0.1 C, and in the process, the first gas pressure of a high vacuum in the formation cabinet is continuously -80 kPa;

[0097] (5) the battery is left to stand at a voltage of 3.85 V in the formation cabinet for 10 min, and in the process, the first gas pressure of a high vacuum in the formation cabinet is continuously -80 kPa;

[0098] (6) the battery is charged to a formation voltage of 3.95 V at a second charging current of a constant ratio of 0.1 C, and in the process, the first gas pressure of a high vacuum in the formation cabinet is continuously -80 kPa;

[0099] (7) the battery is left to stand at a voltage of 3.95 V in the formation cabinet for 10 min, and in the process, the first gas pressure of a high vacuum in the formation cabinet is continuously -80 kPa;

[0100] (8) the battery is charged to a formation voltage of 4.1 V at a second charging current of a constant ratio of 0.1 C, and in the process, the first gas pressure of a high vacuum in the formation cabinet is continuously -80 kPa;

[0101] (9) the battery is left to stand at a voltage of 4.1 V in the formation cabinet for 10 min, and in the process, the first gas pressure of a high vacuum in the formation cabinet is continuously -80 kPa;

[0102] (10) the battery is charged to a formation voltage of 4.3 V at a second charging current of a constant ratio of 0.1 C, and in the process, the first gas pressure of a high vacuum in the formation cabinet is continuously -80 kPa;

[0103] (11) the battery is left to stand at a voltage of 4.3 V in the formation cabinet for 10 min, and in the process, the first gas pressure of a high vacuum in the formation cabinet is continuously -80 kPa.

[0104] Comparative Example 2

[0105] Comparative Example 2 carries out the formation process on the same battery as in Example 1, and the difference between Comparative Example 2 and Example 1 is that the tab of the bare battery is not clamped by the clamp, but is directly placed on the formation tray for formation, and the remaining steps are the same as those in Example 1.

[0106] The batteries in Example 1 and Comparative Examples 1 and 2 were weighed before and after the formation step to test the electrolyte loss during the formation process in Example 1 and Comparative Examples 1 and 2. Experimental data showed that the electrolyte loss after the formation process in Example 1 was approximately 4g, while the electrolyte losses after the formation processes in Comparative Examples 1 and 2 were approximately 20g and 11g, respectively. This indicates that the dual negative pressure breathing formation process in Example 1 can effectively reduce electrolyte loss during the formation process.

[0107] In addition to weighing, the fully charged interfaces of the bare cells after formation in Example 1, Comparative Example 1, and Comparative Example 2 were disassembled. The electrode interfaces after disassembly in Example 1, Comparative Example 1, and Comparative Example 2 are shown below. Figures 5a to 5f As shown in the above images, it is clear that in Example 1, the fully charged electrode interface produced by the double negative pressure breathing formation process using a fixture is free from lithium plating, purple spots, and wrinkles; Figures 6a to 6f As shown, compared to Comparative Example 1, which used high-vacuum negative pressure formation instead of dual negative pressure breathing formation in Example 1, severe U-shaped lithium plating occurred at the fully charged electrode interface of the battery. Gas path residues were clearly observed below the large surface of the electrode tab and at the corners, and the lithium plating on the electrode interface was also formed primarily along these gas paths. The comparison of these test results demonstrates that dual negative pressure breathing formation can promote the complete extraction of air bubbles from the bare cell, avoiding lithium plating at the electrode interface caused by residual air bubbles. Furthermore, as... Figures 7a to 7f As shown, compared to Example 1 and Comparative Example 2 (which does not use a clamp for pressurization), lithium plating occurs on the large surfaces and corners of the fully charged electrode interface (more large-area lithium plating compared to Comparative Example 1, and more large-area and corner lithium plating compared to Example 1), and the inner ring where the electrode tab is located is severely wrinkled. The above test results show that clamp pressurization can accelerate the release of bubbles between the electrodes to prevent large-area lithium plating, and can also alleviate electrode wrinkling.

[0108] Therefore, the combination of clamping the casing sidewall pressurization and the dual negative pressure breathing formation process can completely remove the formation gas generated in the bare cell during the formation process and eliminate the problem of interface lithium plating, ensuring that the battery has a good formation full charge interface, thereby ensuring that the battery has good capacity performance and high safety.

[0109] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A battery formation method, the battery comprising a case and a bare cell located in the case, the bare cell comprising an electrode assembly and a tab electrically connecting the electrode assembly, characterized by, The electrode assembly comprises a positive electrode tab comprising a positive electrode material and at least one lithium supplementing agent, and the battery formation method comprises: a liquid injection step of injecting electrolyte into the battery; a clamping step of clamping the side walls of the shell on both sides by a clamp to press the large surfaces of the bare battery cell on both sides in the thickness direction of the shell; a formation step of charging the battery to a pre-formation voltage under a first air pressure, the pre-formation voltage being less than the formation reaction voltage of the lithium supplementing agent and the electrolyte; and charging the battery to each formation voltage in turn under a second air pressure until the battery is charged to a formation cut-off voltage, each formation voltage being greater than or equal to the formation reaction voltage of the corresponding lithium supplementing agent and electrolyte; wherein the first air pressure and the second air pressure are negative pressures, the second air pressure is greater than the first air pressure, the first air pressure is -70 kPa to -85 kPa, and the second air pressure is -30 kPa to -60 kPa.

2. The battery formation method of claim 1, wherein, The clamp is provided with a boss in opposition, the clamp clamps the side walls of the shell on both sides through the boss, and the clamping surface of the boss is a plane; wherein the length and width of the boss are greater than or equal to the length and width of the bare battery cell, respectively, and the length and width of the boss are less than the length and width of the shell, respectively.

3. The battery formation method of claim 1, wherein, The clamping force of the clamp clamping the shell is 0.15 MPa to 0.5 MPa.

4. The battery formation method of claim 1, wherein, The positive electrode material comprises lithium iron phosphate, and / or the lithium supplementing agent comprises lithium ferrite.

5. The battery formation method of claim 1, wherein, The formation step further comprises, after the battery is charged to any of the formation voltages, placing the battery under the second air pressure for standing and exhausting.

6. The battery formation method of claim 1 or 5, wherein, The formation step further comprises, after the battery is charged to any of the formation voltages, placing the battery under a third air pressure for standing and exhausting; wherein the third air pressure is greater than the second air pressure.

7. The battery formation method of claim 6, wherein, The third air pressure is -10 kPa to 10 kPa.

8. The battery formation method of claim 1, wherein, In the formation step, the battery is charged to the pre-formation voltage at a first charging current, and the battery is charged to each of the formation voltages at a second charging current, the first charging current being greater than the second charging current, and the charging rate of the first charging current and the second charging current being less than or equal to 0.3C.

9. A secondary battery characterized by comprising: The secondary battery is prepared by a preparation method comprising the battery formation method of any one of claims 1 to 8.

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