Separator and its preparation method, battery cell and power device

Abstract

This application relates to a separator and its preparation method, a battery cell, and an electrical device. The separator includes a substrate layer and a functional layer disposed on at least one surface of the substrate layer along its thickness direction. The functional layer includes polymethyl methacrylate and a stabilizer. The volume swelling rate of the stabilizer after immersion in an electrolyte at 45°C for 12 hours is less than or equal to 8%. The electrolyte includes LiPF6 and a solvent, wherein the concentration of LiPF6 in the electrolyte is 1 mol / L, and the solvent is a mixture of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1. The polymethyl methacrylate comprises primary particles, and the stabilizer comprises secondary particles. The average particle size of the primary particles of the polymethyl methacrylate is smaller than the average particle size of the secondary particles of the stabilizer. Thus, through the synergistic effect of polymethyl methacrylate and the stabilizer, the adhesion between the separator and the electrode can be ensured, the electrolyte storage capacity of the separator can be improved, and the problem of battery cell bulging can be solved.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a separator and its preparation method, a battery cell, and an electrical device. Background Technology

[0002] Lithium-ion batteries possess advantages such as high energy density, long cycle life, low self-discharge rate, high operating voltage, fast charge / discharge capability, no memory effect, and environmental friendliness, and are widely used in consumer electronics, electric vehicles, and energy storage. A lithium-ion battery typically consists of a positive electrode, a negative electrode, a separator, and an electrolyte. The separator, positioned between the positive and negative electrodes, facilitates active ion conduction and electronic isolation, preventing short circuits between the electrodes. Therefore, the performance of the separator directly affects the battery's electrochemical performance.

[0003] Currently, the commonly used materials for separators are mainly polyolefin materials such as polyethylene (PE) and polypropylene (PP). Polyolefin separators have hydrophobic properties, resulting in poor wettability and electrolyte retention. Furthermore, polyolefin separators have poor adhesion to the electrodes, which can easily lead to problems such as wrinkling and lithium plating on the electrodes, thereby affecting the cycle performance and safety performance of the battery. Summary of the Invention

[0004] In view of the above, this application provides a separator, a method for preparing the same, a battery cell, and an electrical device to solve at least one problem existing in the prior art.

[0005] In a first aspect, embodiments of this application provide a diaphragm, including a substrate layer and a functional layer disposed on at least one surface of the substrate layer along the thickness direction; The functional layer includes polymethyl methacrylate and a stabilizer. The volume swelling rate of the stabilizer after soaking in an electrolyte at 45°C for 12 hours is less than or equal to 8%. The electrolyte includes LiPF6 and a solvent. The concentration of LiPF6 in the electrolyte is 1 mol / L. The solvent is a mixture of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1. The polymethacrylate comprises primary particles, and the stabilizer comprises secondary particles, wherein the average particle size of the primary particles of the polymethacrylate is smaller than the average particle size of the secondary particles of the stabilizer.

[0006] In conjunction with the first aspect of this application, in an alternative embodiment, the polymethacrylate comprises polymethyl methacrylate; and / or, The stabilizer includes at least one of polyacrylic acid, polyacrylamide, and poly(p-phenylene terephthalamide).

[0007] In conjunction with the first aspect of this application, in an alternative embodiment, the stabilizer comprises polyacrylamide.

[0008] In conjunction with the first aspect of this application, in an optional embodiment, the mass ratio of the polymethacrylate to the stabilizer is (3:7) to (7:3).

[0009] In conjunction with the first aspect of this application, in an optional embodiment, the polymethacrylate is primary particles, and the stabilizer is secondary particles; and / or, The ratio of the average particle size of the secondary particles of the stabilizer to the average particle size of the primary particles of the polymethyl methacrylate is 2.5 to 7.5; and / or, The average particle size of the primary particles of the polymethacrylate is 600 nm to 1000 nm; and / or, The average particle size of the secondary particles of the stabilizer is 2μm to 6μm.

[0010] In conjunction with the first aspect of this application, in an optional embodiment, the average thickness of the functional layer located on one side of the substrate layer is 1.5 μm to 4.0 μm; and / or, The areal density of the functional layer located on one side of the substrate layer is 0.13 g / m². 2 ~0.39g / m 2 .

[0011] In conjunction with the first aspect of this application, in an alternative embodiment, the substrate layer comprises a polyethylene film and / or a polypropylene film.

[0012] Secondly, embodiments of this application provide a method for preparing a diaphragm as described in any of the first aspects, the method comprising the following steps: S1: Acrylic acid monomer and initiator are added to deionized water, ammonia gas is introduced, and polymerization reaction is carried out under heating and pressure. After filtration, washing and drying, primary polyacrylamide particles are obtained. The primary polyacrylamide particles and wetting agent are added to deionized water, and addition reaction is carried out under heating and stirring to obtain secondary polyacrylamide particles. The secondary polyacrylamide particles serve as the stabilizer. S2: Add the polymethyl methacrylate and the polyacrylamide secondary particles to deionized water, mix them evenly, and obtain the functional layer slurry; S3: The functional layer slurry is coated on at least one surface of the substrate layer along the thickness direction, and after drying, the functional layer is formed to obtain the diaphragm.

[0013] In conjunction with the second aspect of this application, in an alternative embodiment, the method satisfies at least one of the following features: (1) The initiator includes at least one of azobisisobutyronitrile, cumene hydrogen peroxide, and methyl ethyl ketone peroxide; (2) The initiator accounts for 0.3% to 2.0% of the mass of the acrylic acid monomer; (3) The polymerization reaction is carried out at a temperature of 80℃~90℃ and a pressure of 3MPa~4MPa; (4) The average particle size of the primary polyacrylamide particles is 600 nm to 1000 nm; (5) The wetting agent accounts for 2% to 6% of the mass of the primary polyacrylamide particles; (6) The wetting agent includes at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, thiols, and polyoxyethylene alkyl ethers; (7) The addition reaction is carried out at a temperature of 70℃~80℃ and a stirring speed of 200r / min~300r / min.

[0014] Thirdly, embodiments of this application provide a battery cell including the separator described in any one of the first aspects or a separator prepared by a method including the separator described in the second aspect.

[0015] In conjunction with a third aspect of this application, in an optional embodiment, the battery cell further includes a positive electrode and a negative electrode, the separator is located between the positive electrode and the negative electrode, and the functional layer is at least disposed on the side of the substrate layer facing the negative electrode.

[0016] Fourthly, embodiments of this application provide an electrical device including a battery cell as described in any of the third aspects.

[0017] Compared with the prior art, the embodiments of this application have the following beneficial effects: The separator, its preparation method, battery cell, and power device provided in this application embodiment are as follows: The separator includes a substrate layer and a functional layer disposed on at least one surface of the substrate layer along its thickness direction. The functional layer includes polymethyl methacrylate and a stabilizer. Polymethyl methacrylate has strong adhesion, which can improve the adhesion between the separator and the electrode, and increase the hardness of the electrode assembly. This can effectively improve problems such as wrinkling and lithium plating on the electrode. The stabilizer has a volume swelling rate of less than or equal to 8% after soaking in electrolyte at 45°C for 12 hours, and has good swelling stability. The average particle size of the primary particles of polymethyl methacrylate is smaller than the average particle size of the secondary particles of the stabilizer. Thus, the secondary particles of the stabilizer can form gaps between the separator and the electrode, providing a buffer space for the continuous swelling of polymethyl methacrylate. This improves the problem of bulging of the battery cell caused by excessive swelling of the separator functional layer during long-cycle operation. At the same time, it can also improve the wettability and liquid retention of the separator to the electrolyte, promote the transport of active ions, and thus improve the cycle performance of the battery cell. In other words, the embodiments of this application, through the synergistic effect of polymethyl methacrylate and stabilizer in the separator functional layer, can ensure the adhesion between the separator and the electrode, improve the liquid storage capacity of the separator, and solve the problem of battery cell bulging caused by the continuous swelling of polymethyl methacrylate, thereby improving the cycle performance and safety performance of the battery cell.

[0018] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0019] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings: Figure 1 A schematic flowchart illustrating a method for preparing a diaphragm according to an embodiment of this application; Figure 2 This is a scanning electron microscope image of the secondary polyacrylamide particles prepared in Example 1 after drying. Figure 3 for Figure 2 Scanning electron microscope image of polyacrylamide secondary particles after soaking in electrolyte at 45℃ for 12 hours; Figure 4 This is a scanning electron microscope image of the surface of the diaphragm prepared in Example 1; Figure 5 This is a scanning electron microscope image of the surface of the diaphragm prepared in Example 1 after it has been immersed in electrolyte. Detailed Implementation

[0020] To make the technical solution and beneficial effects of the present invention more apparent and understandable, a detailed description is provided below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following embodiments, unless otherwise specified, are generally performed under conventional experimental conditions. Unless otherwise specified, all reagents and raw materials used in this invention are commercially available.

[0021] In the following description, numerous specific details are set forth in order to provide a more thorough understanding of this application. However, it will be apparent to those skilled in the art that this application can be practiced without one or more of these details. In other instances, to avoid confusion with this application, some technical features well-known in the art have not been described; that is, not all features of actual embodiments are described herein, nor are well-known functions and steps described in detail.

[0022] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of this application. When used herein, the singular forms “a,” “an,” and “the” are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the terms “comprising” and / or “including,” when used in this specification, identify the presence of the stated features, integers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups. When used herein, the term “and / or” includes any and all combinations of the associated listed items.

[0023] To fully understand this application, detailed steps and structures will be presented in the following description to illustrate the technical solution of this application. Preferred embodiments of this application are described in detail below; however, in addition to these detailed descriptions, this application may have other implementation methods.

[0024] Unless otherwise defined, the technical and scientific terms used in this application have the same meanings as those in the technical and scientific field to which this application pertains.

[0025] Unless otherwise specified, the techniques or conditions described in the following embodiments are generally performed in accordance with conventional techniques or conditions described in the literature in this field, or in accordance with the product manual and the manufacturer's recommendations. All numerical ranges in the following embodiments include endpoint values.

[0026] In this application, the electrical device can be a vehicle, mobile phone, portable device, laptop computer, ship, spacecraft, electric toy, power tool, energy storage device, amusement equipment, elevator and lifting equipment, etc. The electrical device includes a battery and is capable of providing electrical energy. The battery can be composed of one or more battery cells. The battery cell in this application can include an electrode assembly, electrolyte, casing, and top cover. The casing has a receiving cavity and an opening. The electrode assembly and electrolyte are placed in the receiving cavity of the casing, and the opening of the casing is sealed by the top cover. The electrode assembly consists of a positive electrode, a negative electrode, and a separator. The battery cell mainly relies on active ions (such as lithium ions, sodium ions, magnesium ions, etc.) to intercalate and deintercalate between the positive and negative electrodes. The separator is disposed between the positive and negative electrodes, which can prevent short circuits between the positive and negative electrodes while allowing active ions to pass through.

[0027] The positive electrode sheet may include a positive electrode current collector and a positive electrode active material layer, the positive electrode active material layer being coated on one or both surfaces of the positive electrode current collector along its thickness direction. The positive electrode current collector may be a metal foil (such as aluminum foil) or a composite current collector (such as a copper layer composited on a polymer base film). Taking lithium-ion batteries as an example, the positive electrode active material may be lithium iron phosphate, ternary materials (LiNi... a Co b Mn c O2, wherein a+b+c=1, a>0, b>0, c>0), lithium cobalt oxide, and lithium manganese oxide are at least one of them.

[0028] The negative electrode sheet may include a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer being coated on one or both surfaces of the negative electrode current collector along its thickness direction. The negative electrode current collector may be a metal foil (such as copper foil) or a composite current collector (such as a copper layer composited on a polymer base film surface). The negative electrode active material may be at least one of graphite, hard carbon, soft carbon, and silicon-based materials.

[0029] The diaphragm can be a single-layer polyethylene or polypropylene membrane, or it can be a composite diaphragm, such as coating a functional layer onto a polyethylene or polypropylene membrane to improve the overall performance of the diaphragm. The functional layer can be, for example, a ceramic coating or a polymer coating.

[0030] Taking lithium-ion batteries as an example, the electrolyte may include lithium salt, solvent, and optional additives; wherein, the lithium salt may be selected from at least one of LiPF6, LiFSI, LiTFSI, LiBOB, and LiDFOB, the solvent may be selected from at least two of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl acetate (MA), and ethyl acetate (EA), and the additives may be selected from at least one of vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propane sulpholactone (PS), vinyl sulfate (DTD), tris(trimethylsilane) phosphite (TMSP), tris(trimethylsilane) phosphate (TTSP), and lithium difluorophosphate (LiPO2F2).

[0031] If the adhesion between the positive and negative electrodes and the separator is insufficient, the electrode assembly (also known as the cell in the industry) is prone to softening during hot pressing in the battery manufacturing process, leading to wrinkling of the positive and negative electrodes. Furthermore, wrinkling of the positive and negative electrodes is also likely to occur during the initial charge and discharge process in the capacity stage, causing problems such as lithium plating. In related technologies, polymethyl methacrylate (PMA) is coated on the surface of the separator base film as a functional layer. On the one hand, the adhesion between PMA and the positive and negative electrodes is much higher than that between the commonly used binder polyvinylidene fluoride (PVDF) and the positive and negative electrodes. On the other hand, PMA has good wettability with the electrolyte, which can improve the wettability of the separator and the positive and negative electrodes to the electrolyte. However, PMA is an amorphous polymer, and it will continue to swell until it dissolves when immersed in the electrolyte. After hot pressing and electrolyte injection, the functional layer between the separator and the positive and negative electrodes does not have sufficient effective space to buffer the swelling of PMA, thus causing bulging of the battery cells, affecting the cycle performance and safety performance of the battery cells.

[0032] Based on this, embodiments of this application provide a diaphragm, which includes a substrate layer and a functional layer disposed on at least one surface of the substrate layer along the thickness direction; the functional layer includes polymethyl methacrylate and a stabilizer, wherein the volume swelling rate of the stabilizer after soaking in an electrolyte at 45°C for 12 hours is less than or equal to 8%, the electrolyte includes LiPF6 and a solvent, wherein the concentration of LiPF6 in the electrolyte is 1 mol / L, and the solvent is a mixture of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1; the polymethyl methacrylate includes primary particles, the stabilizer includes secondary particles, and the average particle size of the primary particles of the polymethyl methacrylate is smaller than the average particle size of the secondary particles of the stabilizer.

[0033] In this embodiment, polymethyl methacrylate (PMMA) has strong adhesive properties, which can improve the adhesion between the separator and the electrode, and increase the hardness of the electrode assembly. This can effectively improve problems such as wrinkling and lithium plating on the electrode. The volume swelling rate of the stabilizer after soaking in electrolyte at 45°C for 12 hours is less than or equal to 8%, which shows good swelling stability. The average particle size of the primary particles of PMMA is smaller than the average particle size of the secondary particles of the stabilizer. Thus, the secondary particles of the stabilizer can form gaps between the separator and the electrode, providing a buffer space for the continuous swelling of PMMA. This can improve the problem of bulging of the battery cell caused by excessive swelling of the separator functional layer during long-cycle operation. At the same time, it can also improve the wettability and liquid retention of the separator in the electrolyte, promote the transport of active ions, and thus improve the cycle performance of the battery cell. In other words, the embodiments of this application, through the synergistic effect of polymethyl methacrylate and stabilizer in the separator functional layer, can ensure the adhesion between the separator and the electrode, improve the liquid storage capacity of the separator, and solve the problem of battery cell bulging caused by the continuous swelling of polymethyl methacrylate, thereby improving the cycle performance and safety performance of the battery cell.

[0034] It should be noted that in the diaphragm of this application, the functional layer can be disposed on one surface of the substrate layer along the thickness direction, or it can be disposed on two surfaces of the substrate layer along the thickness direction.

[0035] When the functional layer is disposed on two surfaces of the substrate layer along the thickness direction, the adhesion between the separator and the positive and negative electrodes can be improved simultaneously, especially the adhesion in the electrolyte, as well as the interfacial contact between the positive and negative electrodes and the separator. This significantly increases the electrolyte storage capacity of the separator and provides a certain buffer space for the expansion of the positive and negative electrodes, thereby better improving the electrochemical performance of the battery cell.

[0036] To save costs, the functional layer can be disposed on one surface of the substrate layer along the thickness direction. In this case, in the above-mentioned battery cell, the functional layer can be disposed on either the side of the substrate layer facing the negative electrode or the side of the substrate layer facing the positive electrode.

[0037] In a preferred embodiment, the functional layer is disposed on the side of the substrate layer facing the negative electrode. Because the negative electrode provides storage space for active ions (such as lithium ions, sodium ions, etc.), the rate of active ion extraction and insertion in the negative electrode directly determines the kinetic performance of the battery cell. By placing the functional layer on the side of the substrate layer facing the negative electrode, and utilizing the functional layer's superior liquid absorption and retention properties, the electrolyte can be better applied to the negative electrode, accelerating the transport of active ions on the negative electrode side. This improves the kinetic performance of the negative electrode, suppresses lithium plating, and effectively increases the charge / discharge rate of the battery cell. Furthermore, the negative electrode expands more than the positive electrode during charge / discharge. The opposing arrangement of the negative electrode and the functional layer also helps to restrain each other's expansion through mutual restraint, further improving the battery's cycle performance.

[0038] In electrode assemblies, the separator typically completely covers the edges of both the positive and negative electrode active material layers and extends outwards to ensure effective insulation between the positive and negative electrodes and prevent short circuits. In the separator of this application, the functional layer can completely cover the surface of the substrate layer, or it can cover only a portion of the substrate layer's surface. Optionally, the functional layer covers only a portion of the substrate layer's surface, and the projections of the positive and negative electrode active material layers along the thickness direction of the electrode assembly fall within the functional layer. This allows for both cost control and improved electrochemical performance of the electrode assembly.

[0039] For example, polymethacrylate may include polymethyl methacrylate (PMMA). In a specific example, the polymethacrylate is PMMA.

[0040] For example, the stabilizer may include at least one of polyacrylic acid (PAA), polyacrylamide (PAM), and poly(p-phenylene terephthalamide) (PPTA). In a specific example, the stabilizer is at least one of PAA, PAM, and PPTA.

[0041] As a preferred embodiment, the stabilizer may include polyacrylamide. Further alternatively, the stabilizer may be polyacrylamide.

[0042] Polyacrylamide has an R-NH2 functional group, which can capture H+ in the electrolyte through a coupling reaction. + This reduces the content of highly corrosive substances such as HF in the electrolyte, thereby reducing the damage of these substances to the active material layers of the positive and negative electrodes inside the battery. At the same time, the R-NH2 functional group is a highly polar functional group with a high ion migration rate, which can compensate for the low ion migration rate of polymethyl methacrylate in the functional layer, thus helping to improve the kinetic performance of the battery.

[0043] It is understandable that when the mass ratio of polymethyl methacrylate (PMMA) to stabilizer is too low, the stabilizer accounts for a large proportion of the functional layer, reducing the adhesion between the functional layer and the electrode, leading to a decrease in the hardness of the electrode assembly. This may cause electrode wrinkling during the hot-pressing and initial charge-discharge processes in battery fabrication. Conversely, when the mass ratio of PMMA to stabilizer is too high, the PMMA accounts for a large proportion of the functional layer. While this may improve the hardness of the electrode assembly, the reduced stabilizer proportion in the functional layer decreases the space for large-particle stabilizer to form between the separator and the electrode, reducing the buffer space for continuous swelling of PMMA. Simultaneously, the separator's ability to store electrolyte also decreases, which is detrimental to the long-term electrochemical performance of the battery cell. Therefore, in an optional embodiment, the mass ratio of PMMA to stabilizer can be (3:7) to (7:3), for example, 3:7, 4:7, 5:7, 6:7, 1:1, 4:3, 5:3, 1:2, 7:3, or any value within any two of the above ranges. This approach effectively improves the problem of electrode wrinkling while also taking into account the cycle performance of individual battery cells.

[0044] In an optional embodiment, the polymethacrylate is a primary particle and the stabilizer is a secondary particle.

[0045] Here, primary particles and secondary particles have the same meanings known in the art. Primary particles refer to non-agglomerated particles, while secondary particles refer to agglomerated particles formed by the aggregation of two or more primary particles.

[0046] Polymethyl methacrylate (PMMA) exhibits a high swelling rate in electrolytes. Therefore, using primary particles can reduce the volume expansion of the functional layers. However, if secondary particle agglomerates of PMMA are used, their average particle size is larger, and combined with their high swelling rate, battery cells are more prone to bulging. In contrast, stabilizers have a low swelling rate in electrolytes, allowing for the use of larger-diameter secondary particles. This effectively creates a gap between the separator and the electrode, providing a buffer for the continuous swelling of PMMA and increasing the separator's electrolyte storage capacity.

[0047] In the functional layer of the membrane, the ratio of the average particle size of the secondary particles of the stabilizer to the average particle size of the primary particles of polymethyl methacrylate can be 2.5 to 7.5, for example, it can be 2.5, 3.5, 4.5, 5.5, 6.5, 7.5 or any value between any two of the above ranges.

[0048] The average particle size here refers to the average particle size of the secondary particles of the stabilizer and the primary particles of polymethyl methacrylate in the prepared diaphragm, that is, the average particle size of the secondary particles of the stabilizer and the primary particles of polymethyl methacrylate before the diaphragm is immersed in the electrolyte.

[0049] Polymethyl methacrylate (PMMA) continuously swells in the electrolyte. The stabilizer initially expands slightly but then stabilizes. Therefore, in the initial stage, the large-particle-size stabilizer provides a framework, offering sufficient swelling space for the small-particle-size PMMA. Over time, the large-particle-size stabilizer stabilizes and stops swelling. When the average particle size ratio of the secondary stabilizer particles to the primary PMMA particles is too large, the secondary stabilizer particles create excessive space between the electrode and separator, resulting in wasted space. Simultaneously, the excessive increase in the gap between the positive and negative electrodes enlarges the lithium-ion transport path, affecting the kinetic performance of the battery cell and reducing the charging speed. Conversely, when the average particle size ratio of the secondary stabilizer particles to the primary PMMA particles is too small, the space created by the secondary stabilizer particles between the electrode and separator cannot adequately accommodate the high swelling volume of PMMA, potentially leading to bulging of the battery cell during long-term use. Therefore, by controlling the ratio of the average particle size of the secondary particles of the stabilizer to the average particle size of the primary particles of polymethyl methacrylate within the aforementioned range, and considering the maximum volume swelling rate of the primary particles of polymethyl methacrylate (130%~150%), even after long-term immersion in the electrolyte, the average particle size of the secondary particles of the stabilizer can still be larger than that of the primary particles of polymethyl methacrylate, thus still providing space for the swelling of polymethyl methacrylate. Moreover, although the small-diameter polymethyl methacrylate particles continue to swell in the later stages, the change in swelling volume is relatively small, and their particle size remains smaller than that of the stabilizer. The stabilizer continuously provides a skeletal function for swelling, thereby effectively alleviating the problem of bulging of battery cells during long-term use, while also taking into account the dynamic performance of the battery cells.

[0050] The maximum volume swelling rate of the polymethyl methacrylate primary particles mentioned above refers to the maximum volume swelling rate of the polymethyl methacrylate primary particles after being soaked in an electrolyte at 45°C for 12 hours. The electrolyte includes LiPF6 and a solvent, wherein the concentration of LiPF6 in the electrolyte is 1 mol / L, and the solvent is a mixture of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1.

[0051] When the average particle size of the primary particles of polymethyl methacrylate (PMMA) is too large, the average particle size ratio of the stabilizer to PMMA becomes smaller. The larger volume swelling of PMMA can easily cause the battery cells to bulge. Conversely, when the average particle size of the primary PMMA particles is too small, on the one hand, the adhesion between the small-sized PMMA particles and the separator is poor, and PMMA powdering is very likely to occur on the surface of the substrate layer during actual separator preparation. On the other hand, the small-sized PMMA particles cannot provide sufficient adhesion between the separator and the electrode, which may cause wrinkling of the positive and negative electrodes. Therefore, in a specific embodiment, the average particle size of the primary PMMA particles can be 600 nm to 1000 nm, for example, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm, or any value between any two of the above ranges. This makes it easy to control the average particle size ratio of stabilizer to polymethyl methacrylate within a suitable range, while avoiding excessive volume expansion of polymethyl methacrylate and ensuring its bonding performance, thereby effectively suppressing electrode wrinkling and battery cell bulging.

[0052] When the average particle size of the secondary particles of the stabilizer is too large, the gap between the separator and the electrode becomes too large, which may weaken the adhesion between the separator and the electrode. During cycle testing, this can lead to electrode wrinkling and lithium plating, resulting in a decrease in battery cycle capacity retention. Conversely, when the average particle size of the secondary particles of the stabilizer is too small, the large particle size cannot create a sufficient buffer gap between the separator and the electrode. When the polymethyl methacrylate, after being soaked in the electrolyte, continues to swell, it will compress the space between the separator and the electrode in the lateral direction (the thickness direction of the electrode assembly), causing battery bulging and affecting the battery cycle capacity retention. Therefore, in a specific embodiment, the average particle size of the secondary particles of the stabilizer can be 2μm to 6μm, for example, 2μm, 3μm, 4μm, 5μm, 6μm, or any value between any two of the above ranges. The stabilizer particles do not exhibit continuous swelling when immersed in the electrolyte, demonstrating good stability. The stabilizer particles, with an average particle size in the micrometer range, can create a certain gap between the separator and the electrode. This gap is sufficient to provide a buffer space for the continuous swelling of polymethyl methacrylate, preventing the battery cells from bulging. At the same time, it can provide space for the electrolyte.

[0053] When the functional layer is too thin, the adhesion between the separator and the electrode may be affected, leading to insufficient hardness of the electrode assembly. This can cause problems such as electrode wrinkling and lithium plating during cycling, affecting the performance of the battery cell. When the functional layer is too thick, it will affect the permeability of the separator, affecting the migration of active ions and thus the kinetic performance of the battery cell. It will also increase the gap between the separator and the electrode, lengthening the lithium ion migration path and increasing the internal resistance of the battery cell, which is detrimental to the electrochemical performance of the battery cell. Therefore, in a specific embodiment, the average thickness of the functional layer on one side of the substrate layer can be 1.5 μm to 4.0 μm, for example, 1.5 μm, 2.0 μm, 2.5 μm, 3.0 μm, 3.5 μm, 4.0 μm, or any value between any two of the above ranges. When the functional layer is disposed on both surfaces of the substrate layer along the thickness direction, the average thickness of the functional layer on both sides of the substrate layer is 1.5 μm to 4.0 μm. This is beneficial for improving problems such as electrode wrinkling and lithium plating while better ensuring the electrochemical performance of the battery cell.

[0054] It is understandable that, since the functional layer includes primary polymethacrylate particles and secondary stabilizer particles, the surface of the functional layer away from the substrate layer is typically uneven. The average thickness of the functional layer was measured as follows: a 10cm x 10cm membrane was randomly selected, and 20 test points were randomly chosen on its surface. The thickness of the functional layer at each test point was measured, and the values ​​of the 20 test points were recorded. The arithmetic mean of these measurements was then calculated as the average thickness of the functional layer. Furthermore, the average thickness of the functional layer mentioned above refers to the average thickness of the functional layer before the membrane is immersed in the electrolyte.

[0055] When the areal density of the functional layer is too low, the adhesion between the separator and the electrode may be affected, leading to insufficient hardness of the electrode assembly. This can result in problems such as electrode wrinkling and lithium plating during cycling, affecting the performance of the battery cell. Conversely, when the areal density of the functional layer is too high, the compaction density of the functional layer will be greater to maintain the same thickness. This, in turn, will affect the permeability of the separator, hindering the migration of active ions and consequently impacting the kinetic performance of the battery cell. Furthermore, it will make it difficult for active ions to pass through the separator, increasing the internal resistance of the battery cell and negatively impacting its electrochemical performance. Therefore, in one specific embodiment, the areal density of the functional layer located on one side of the substrate layer can be 0.13 g / m³. 2 ~0.39g / m 2 For example, it can be 0.13g / m 2 0.26g / m 2 0.39g / m 2Or any value between any two of the above ranges. This is beneficial for improving electrode wrinkling and lithium plating issues while better ensuring the electrochemical performance of the battery cells.

[0056] The areal density of the aforementioned functional layer refers to the areal density of the functional layer before the diaphragm is immersed in the electrolyte. When the functional layer is disposed on two surfaces of the substrate layer along the thickness direction, the areal density of the functional layer on both sides of the substrate layer is 0.13 g / m². 2 ~0.39g / m 2 .

[0057] The substrate layer of the diaphragm in this application may include a polyethylene film and / or a polypropylene film. In a specific example, the substrate layer may be a polyethylene film or a polypropylene film.

[0058] Using polyethylene film and / or polypropylene film for the substrate layer can not only ensure the insulation between the positive and negative electrode sheets, but also help ensure the compatibility between the substrate layer and the functional layer.

[0059] This application also provides a method for preparing the diaphragm described in any of the foregoing embodiments. Please refer to [link / reference]. Figure 1 The method includes the following steps: S1: Acrylic acid monomer and initiator are added to deionized water, ammonia gas is introduced, and polymerization reaction is carried out under heating and pressure. After filtration, washing and drying, primary polyacrylamide particles are obtained. Primary polyacrylamide particles and wetting agent are added to deionized water, and addition reaction is carried out under heating and stirring to obtain secondary polyacrylamide particles. Secondary polyacrylamide particles are used as stabilizers. S2: Add polymethyl methacrylate and polyacrylamide secondary particles to deionized water, mix evenly, and obtain functional layer slurry; S3: The functional layer slurry is coated on at least one surface of the substrate layer along the thickness direction, and after drying, a functional layer is formed to obtain a diaphragm.

[0060] In step S1, deionized water is first added to the reactor, followed by acrylic monomer and initiator. Ammonia gas is then introduced to purge oxygen. Once the oxygen purge is complete, the temperature is increased to 80°C to 90°C (e.g., 80°C, 85°C, 90°C, or any two of these ranges) at a rate of 2°C / min to 5°C / min, while simultaneously pressurizing to 3MPa to 4MPa (e.g., 3MPa, 3.5MPa, 4MPa, or any two of these ranges) to initiate the polymerization reaction. After the reaction, the polymerization suspension is allowed to settle and precipitate, yielding large molecular aggregates. These aggregates are then filtered and washed with deionized water, and finally separated using a centrifuge to obtain polyacrylamide powder (primary polyacrylamide particles). The average particle size of the obtained primary polyacrylamide particles can be 600nm to 1000nm, for example, 600nm, 700nm, 800nm, 900nm, 1000nm, or any two of these ranges.

[0061] In the above steps, the reaction to generate primary polyacrylamide particles through polymerization is divided into two stages: The first stage reaction is: CH2=CHCOOH+NH3→CH2=CHCONH2+H2O; The second-stage reaction is: nCH2=CH-CONH2→[-CH2-CH(CONH2)] n ; The reaction formula for generating secondary polyacrylamide particles through an addition reaction is: [-CH2-CH(CONH2)] n -+[-CH2-CH(CONH2)] n → [-CH2-CH(CONH2)] 2n -

[0062] The initiator may include at least one of azobisisobutyronitrile, cumene hydroperoxide, and methyl ethyl ketone peroxide. For example, the initiator may be at least one of azobisisobutyronitrile, cumene hydroperoxide, and methyl ethyl ketone peroxide.

[0063] When the initiator dosage is too high, on the one hand, as the initiator concentration increases, the number of reactive free radicals increases, raising the probability of chain termination. This leads to a decrease in the average molecular weight and particle size of the macromolecular polymer. In other words, while excessive initiator can accelerate the initial reaction rate, it negatively impacts product performance, process safety, and system stability. On the other hand, when the initiator dosage is too low, the fewer free radicals result in a relatively prolonged chain growth process, leading to an increase in the average molecular weight and particle size of the macromolecular polymer. Simultaneously, the polymerization reaction may be incomplete, leaving a large amount of unreacted monomers, which affects product purity and performance. Therefore, the initiator mass can be 0.3% to 2.0% of the acrylic acid monomer mass, for example, 0.3%, 0.5%, 0.7%, 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, or any value within any two of these ranges. This balances the polymerization rate, the average molecular weight of the macromolecular polymer, and the properties of the product (polyacrylamide primary particles).

[0064] Next, the obtained primary polyacrylamide particles can be added to a reaction vessel along with a wetting agent. The temperature is raised to 70°C to 80°C (for example, 70°C, 75°C, 80°C, or any value between any two of the above ranges), and an addition reaction is carried out at a stirring speed of 200 r / min to 300 r / min (for example, 200 r / min, 250 r / min, 300 r / min, or any value between any two of the above ranges) to obtain secondary polyacrylamide particles, which can be used as the stabilizer in the aforementioned examples.

[0065] In this embodiment, a hydrothermal method is used to synthesize large-particle-size polyacrylamide secondary particles. To address the issues of large particle agglomeration and sedimentation that easily occur during the subsequent pulping process, a wetting agent is added during the preparation of the (large-particle-size polymer) polyacrylamide secondary particles. This allows for in-situ coating of the large-particle-size polymer with the wetting agent, reducing its surface energy and facilitating faster wetting of these large-particle-size polymers by deionized water. Consequently, during the subsequent preparation of the membrane functional layer slurry, the large-particle-size polyacrylamide secondary particles can be more uniformly dispersed in deionized water, forming a stable suspension. This promotes the formation of a high-quality coating film from the functional layer slurry. Because the obtained polyacrylamide secondary particles are in-situ coated with a wetting agent, it is not necessary to add a separate wetting agent during the subsequent preparation of the membrane functional layer slurry, effectively saving preparation time. The in-situ coating of the wetting agent also better reduces the risk of agglomeration during the pulping process of the large-particle-size polymer.

[0066] The wetting agent may include at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, thiols, and polyethylene oxide alkyl ethers. For example, the wetting agent may be at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, thiols, and polyethylene oxide alkyl ethers.

[0067] In a preferred embodiment, the wetting agent may be polyethylene oxide alkyl ether (R-C6H4-O-(CH2CH2O)). n H (where R represents alkyl), specifically, a polyoxyethylene alkyl ether can be, for example, CH3-C6H4-O-(CH2CH2O). n H、CH3CH2-C6H4-O-(CH2CH2O) n H.

[0068] Polyethylene oxide alkyl ethers have a chain-like structure, which can be oriented to the surface of large-diameter polyacrylamide secondary particles (large-diameter agglomerates), reducing surface energy and allowing deionized water to better wet the large-diameter polyacrylamide secondary particles. Consequently, in the subsequent preparation of the membrane functional layer slurry, the large-diameter polyacrylamide secondary particles can be more uniformly dispersed in deionized water to form a stable suspension, which is beneficial for the formation of a high-quality coating film. In addition, polyethylene oxide alkyl ethers can also act as chelating agents to promote the addition reaction of polyacrylamide primary particles, that is, to promote the formation of polyacrylamide secondary particles.

[0069] When the amount of wetting agent added is too small, it is insufficient to completely cover the secondary polyacrylamide particles, leading to poor wettability of the large-particle polymer surface. This can cause agglomeration during the preparation of the diaphragm functional layer slurry, hindering the formation of a stable suspension and affecting subsequent coating results. Conversely, when the amount of wetting agent added is too large, excess residue on the surface of the secondary polyacrylamide particles can cause excessive air bubbles in the diaphragm functional layer slurry, increasing the difficulty of coating. Therefore, the mass of the wetting agent can be 2% to 6% of the mass of the primary polyacrylamide particles, for example, 2%, 3%, 4%, 5%, 6%, or any value within any two of these ranges. This promotes the formation of a stable suspension, ensuring a better coating effect for the functional layer slurry while reducing coating difficulty.

[0070] In step S2, polymethyl methacrylate and polyacrylamide secondary particles are added to deionized water and mixed evenly to obtain functional layer slurry.

[0071] In actual preparation processes, polymethyl methacrylate and polyacrylamide secondary particle powders can be directly added to deionized water. Alternatively, the two slurries, each containing polymethyl methacrylate and polyacrylamide secondary particles respectively, can be mixed to improve the uniformity of the mixture.

[0072] Specifically, the slurry after the addition reaction in step S1 can be added to a mixing tank and mixed with polymethyl methacrylate slurry (polymethyl methacrylate is dissolved in deionized water). The mixing tank is evacuated to -80MPa~90MPa, the temperature is controlled at 20℃~40℃, the stirring speed is controlled at 25r / min~35r / min, and the stirring is continued for 40min~60min to obtain the functional layer slurry.

[0073] In step S3, the functional layer slurry is coated on at least one surface of the substrate layer along the thickness direction, and after drying, a functional layer is formed to obtain a diaphragm.

[0074] In actual manufacturing processes, functional layer slurry can be coated onto the surface of the substrate layer along its thickness direction using methods such as spin coating, thereby obtaining a diaphragm containing a functional layer. The coating amount can be adjusted to achieve an average thickness of 1.5 μm to 4.0 μm and an areal density of 0.13 g / m² on one side of the substrate layer. 2 ~0.39g / m 2 .

[0075] This application also provides a battery cell, which includes the separator described in any of the foregoing embodiments or a separator prepared by a method including the separator described in any of the foregoing embodiments.

[0076] It is understood that the beneficial effects of the separator described in any of the foregoing embodiments or the separator prepared by the method described in any of the foregoing embodiments are applicable to the battery cells in the embodiments of this application.

[0077] In some embodiments, the method for preparing a battery cell may include: first, stacking a positive electrode, a separator, and a negative electrode, wherein the separator is located between the positive and negative electrode, and then preparing an electrode assembly by winding or stacking; then, placing the electrode assembly and electrolyte in a housing, sealing it, and then sequentially performing production processes such as standing at high temperature (e.g., 45°C) for 12h~24h, formation process, and capacity process to obtain the final finished battery cell.

[0078] In some embodiments, the separator is located between the positive electrode and the negative electrode, and the functional layer is at least disposed on the side of the substrate layer facing the negative electrode.

[0079] This application also provides an electrical device, which includes the battery cell described in any of the foregoing embodiments.

[0080] The technical solution of this application will be further described below with reference to several embodiments and comparative examples.

[0081] Example 1 The steps for preparing the battery in this embodiment include: Step S101: Preparation of stabilizer: First, add 1500 parts of deionized water to the reactor, then add 500 parts of acrylic acid monomer and 5 parts of initiator azobisisobutyronitrile; after adding the above reactants, repeatedly purge oxygen into the reactor with ammonia gas. After the oxygen is properly purged, raise the temperature to 85℃ at 3.5℃ / min and simultaneously pressurize to 3.5MPa to carry out the polymerization reaction; after the reaction, let the polymerization suspension stand for 12 hours to obtain macromolecular aggregates. Wash and filter the macromolecular aggregates with deionized water, and then separate them using a centrifuge to obtain macromolecular polyacrylamide powder (polyacrylamide primary particles) with an average particle size of 800nm; add the above polyacrylamide primary particles to the reactor, and simultaneously add 4 parts of wetting agent polyethylene oxide alkyl ether (CH3-C6H4-O-(CH2CH2O)). n H, n is 210~230), heated to 75℃, and stirred at 250r / min for addition reaction. The temperature was maintained at 75℃ during the reaction, and the reaction time was 116min. After the reaction, a PAM emulsion with a polymer (polyacrylamide secondary particles) solid content of 30% and an average particle size of 4μm was obtained. Polyoxyethylene alkyl ether was purchased from Guangzhou Yike Juneng New Materials Co., Ltd. Step S102, Preparation of the diaphragm: In a 100L stirred tank, add 84 parts deionized water, 30 parts PAM emulsion, and 22.5 parts PMMA emulsion with a polymer solid content of 40% sequentially. The stirred tank is evacuated to -85MPa, the temperature is controlled at 30℃, and the stirring speed is controlled at 25r / min. Stirring is continued for 50min to obtain the functional layer slurry. The functional layer slurry is then sprayed onto both sides of a 14μm thick polypropylene membrane (substrate layer) using a rotary spraying device. The spraying flow rate is 0.53L / min, and the rotation speed is 150r / min, resulting in a total areal density of 0.52g / m² for the functional layers on both sides of the polypropylene membrane. 2 Simultaneously, the average thickness of the functional layer reaches 6.0 μm (the average thickness of the functional layer on both sides is 3.0 μm), thus obtaining a separator. In this embodiment, the mass ratio of PAM to PMMA is 5:5, and the average particle size ratio of PAM to PMMA is 5. The PMMA was purchased from Shenzhen Haodian Technology Co., Ltd.

[0082] Step S103, Battery Assembly: The negative electrode active material (graphite), conductive carbon (carbon black Super), binder (styrene-butadiene rubber), and thickener (sodium carboxymethyl cellulose) are added to deionized water in a mass ratio of 96:1:1.5:1.5 and homogenized to obtain a negative electrode slurry. The negative electrode slurry is coated onto copper foil and then dried and rolled to obtain a negative electrode sheet for later use. The positive electrode active material (lithium iron phosphate), conductive agent (carbon black Super), and binder (polyvinyl alcohol) are added to N-methylpyrrolidone (NMP) in a mass ratio of 97:2:1 and stirred evenly to obtain a positive electrode slurry. The positive electrode slurry is coated onto aluminum foil and then rolled and separated. Cut the positive electrode sheet to obtain a ready-to-use sheet; place the prepared separator between the negative and positive electrode sheets, and use a cell winding mechanism to form a bare cell (electrode assembly). Then, use hot pressing at a temperature of 90℃, a surface pressure of 3.2 MPa, and a hot pressing time of 120 s for shaping. Place the bare cell into an aluminum shell for encapsulation, and place it in a vacuum baking oven at a temperature of 100℃ and a vacuum degree of -80 MPa for 18 hours. After baking, inject 18g of electrolyte (1.0M LiPF6 solution, with a volume ratio of 1:1 ethylene carbonate and dimethyl carbonate as solvents). After high-temperature standing at 45℃ for 18 hours, formation process, high-temperature aging for 12 hours, capacity process, and other production processes, the battery is manufactured.

[0083] The inventors tested the particle size of PAM in the PAM emulsion obtained in step S101, and the test method is as follows: First, a portion of the PAM emulsion obtained in step S101 was taken and coated onto a polypropylene membrane. After drying at 85°C for 12 hours, microscopic images of the PAM particles were taken using a scanning electron microscope (SEM). Figure 2 As shown); based on Figure 2 The particle size of all 22 visible particles in the center of the particle in the figure was measured and the average particle size was calculated to obtain the average particle size of the PAM particles before swelling, which is denoted as diameter D1, D1=3.92μm; Then, the polypropylene membrane coated with PAM particles was immersed in an electrolyte at 45°C for 12 hours. The electrolyte consisted of LiPF6 and a solvent, with the concentration of LiPF6 in the electrolyte being 1 mol / L and the solvent being a mixture of ethylene carbonate and dimethyl carbonate in a 1:1 volume ratio. After immersion, the polypropylene membrane coated with PAM particles was removed and allowed to stand for 10 minutes (to allow the solvent in the electrolyte to largely evaporate). Microscopic images of the PAM particles were then taken using a scanning electron microscope (SEM) (e.g., ...). Figure 3 As shown); based on Figure 3 The particle size of all 16 visible particles in the center of the particle in the figure was measured and the average particle size was calculated to obtain the average particle size of the PAM particles after swelling, which is denoted as diameter D2, D2=4.01μm; Volume swelling ratio of PAM particles = .

[0084] Example 2 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: The step of preparing the stabilizer was omitted. Accordingly, in step S102, the PAM emulsion was replaced with a PAA emulsion with equal solids content. The polyacrylic acid was purchased from Shenzhen Haodian Technology Co., Ltd., with an average particle size of 4 μm.

[0085] In this embodiment, the same method as in Example 1 was used to measure the volume swelling rate of PAA particles in the PAA emulsion as 7.81%.

[0086] Example 3 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: The addition reaction time in step S101 was increased to 205 min, which increased the average particle size of the secondary polyacrylamide particles in the obtained PAM emulsion to 7 μm and adjusted the average thickness of the functional layer to 8.0 μm (the average thickness of the functional layer on both the front and back sides is 4.0 μm).

[0087] In this embodiment, the same method as in Example 1 was used, and the volume swelling rate of the PAM particles was measured to be 6.75%.

[0088] Example 4 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: The addition reaction time in step S101 was reduced to 28 min, which reduced the average particle size of the secondary polyacrylamide particles in the obtained PAM emulsion to 1 μm, and the average thickness of the functional layer was adjusted to 3.0 μm (the average thickness of the functional layer on both the front and back sides is 1.5 μm).

[0089] In this embodiment, the same method as in Example 1 was used, and the volume swelling rate of the PAM particles was measured to be 7.52%.

[0090] Example 5 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: The addition reaction time in step S101 was increased to 183 min, which increased the average particle size of the polyacrylamide secondary particles in the obtained PAM emulsion to 6 μm and adjusted the average thickness of the functional layer to 7.5 μm (the average thickness of the functional layer on both the front and back sides is 3.75 μm).

[0091] In this embodiment, the same method as in Example 1 was used, and the volume swelling rate of the PAM particles was measured to be 6.88%.

[0092] Example 6 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: The addition reaction time in step S101 was reduced to 67 min, which reduced the average particle size of the secondary polyacrylamide particles in the obtained PAM emulsion to 2 μm, and the average thickness of the functional layer was adjusted to 3.3 μm (the average thickness of the functional layer on both the front and back sides is 1.65 μm).

[0093] In this embodiment, the same method as in Example 1 was used, and the volume swelling rate of the PAM particles was measured to be 7.22%.

[0094] Example 7 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: In step S102, the mass ratio of PAM to PMMA is adjusted to 3:7, and the average thickness of the functional layer is adjusted to 7.3 μm (the average thickness of the functional layer on both the front and back sides is 3.65 μm).

[0095] The preparation method of PAM emulsion in this embodiment is the same as that in Example 1, that is, the volume swelling rate of PAM particles is 7.05%.

[0096] Example 8 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: In step S102, the mass ratio of PAM to PMMA is adjusted to 7:3, and the average thickness of the functional layer is adjusted to 4.7 μm (the average thickness of the functional layer on both the front and back sides is 2.35 μm).

[0097] The preparation method of PAM emulsion in this embodiment is the same as that in Example 1, that is, the volume swelling rate of PAM particles is 7.05%.

[0098] Example 9 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: In step S102, the mass ratio of PAM to PMMA is adjusted to 1:9, and the average thickness of the functional layer is adjusted to 7.6 μm (the average thickness of the functional layer on both the front and back sides is 3.8 μm).

[0099] The preparation method of PAM emulsion in this embodiment is the same as that in Example 1, that is, the volume swelling rate of PAM particles is 7.05%.

[0100] Example 10 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: In step S102, the mass ratio of PAM to PMMA is adjusted to 9:1, and the average thickness of the functional layer is adjusted to 3.3 μm (the average thickness of the functional layer on both the front and back sides is 1.65 μm).

[0101] The preparation method of PAM emulsion in this embodiment is the same as that in Example 1, that is, the volume swelling rate of PAM particles is 7.05%.

[0102] Example 11 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: In step S102, the spray flow rate in the rotary spraying process is adjusted to 0.76 L / min, the rotation speed is adjusted to 300 r / min, and the total areal density of the functional layers obtained on both sides of the polypropylene film is adjusted to 0.78 g / m³. 2 The average thickness of the functional layer was adjusted to 9.0 μm (the average thickness of the functional layers on both sides is 4.5 μm).

[0103] The preparation method of PAM emulsion in this embodiment is the same as that in Example 1, that is, the volume swelling rate of PAM particles is 7.05%.

[0104] Example 12 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: In step S102, the spray flow rate in the rotary spraying process is adjusted to 0.24 L / min, the rotation speed is adjusted to 100 r / min, and the total areal density of the functional layers obtained on both sides of the polypropylene film is adjusted to 0.26 g / m³. 2 The average thickness of the functional layer was adjusted to 2.5 μm (the average thickness of the functional layer on both sides is 1.25 μm).

[0105] The preparation method of PAM emulsion in this embodiment is the same as that in Example 1, that is, the volume swelling rate of PAM particles is 7.05%.

[0106] Example 13 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: Replace the wetting agent in step S101 with sodium dodecyl sulfate.

[0107] The preparation method of PAM emulsion in this embodiment is the same as that in Example 1, that is, the volume swelling rate of PAM particles is 7.05%.

[0108] Example 14 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: In step S102, the functional layer slurry is adjusted to be sprayed only on a single surface of the polypropylene film, with a single-sided coating density of 0.26 g / m². 2 The average thickness of the functional layer was adjusted to 3 μm; In step S103, the side of the diaphragm with the functional layer is positioned facing the negative electrode.

[0109] The preparation method of PAM emulsion in this embodiment is the same as that in Example 1, that is, the volume swelling rate of PAM particles is 7.05%.

[0110] Example 15 The battery preparation steps in this embodiment are basically the same as those in Example 10, with the main difference being: In step S103, the side of the diaphragm with the functional layer is adjusted to face the positive electrode.

[0111] The preparation method of PAM emulsion in this embodiment is the same as that in Example 1, that is, the volume swelling rate of PAM particles is 7.05%.

[0112] Comparative Example 1 The steps for preparing the battery in this comparative example are basically the same as step S103 in Example 1, except that: The diaphragm is replaced with a polypropylene membrane, meaning that no functional layer is set on the polypropylene membrane.

[0113] Comparative Example 2 The steps for preparing the battery in this comparative example are basically the same as those in Example 1, with the following differences: Step S101 was omitted. In step S102, no PAM emulsion was added when preparing the functional layer slurry, meaning that the formed functional layer only contains PMMA and not PAM.

[0114] Comparative Example 3 The steps for preparing the battery in this comparative example are basically the same as those in Example 1, with the following differences: In step S102, no PMMA emulsion was added when preparing the functional layer slurry, meaning that the formed functional layer contains only PAM and not PMMA.

[0115] The preparation method of the PAM emulsion in this comparative example is the same as that in Example 1, that is, the volume swelling rate of the PAM particles is 7.05%.

[0116] Comparative Example 4 The steps for preparing the battery in this comparative example are basically the same as those in Example 1, with the following differences:

[0117] In step S101, only polymerization reaction is carried out, without addition reaction. By shortening the polymerization reaction time, the average particle size of the primary polyacrylamide particles is adjusted to 200 nm, that is, the polyacrylamide in the obtained PAM emulsion is primary particle with an average particle size of 200 nm.

[0118] In this comparative example, the same method as in Example 1 was used, and the volume swelling rate of the primary PAM particles was measured to be 7.61%.

[0119] Comparative Example 5 The steps for preparing the battery in this comparative example are basically the same as those in Example 1, with the following differences: Step S101 is omitted. In step S102, the PAM emulsion is replaced with a polyacrylonitrile emulsion with equal solid content. The polyacrylonitrile in the polyacrylonitrile emulsion is purchased from Shenzhen Haodian Technology Co., Ltd., and the average particle size is 4μm.

[0120] In this comparative example, the same method as in Example 1 was used, and the volume swelling rate of the polyacrylonitrile particles was measured to be 80.22%.

[0121] It should be noted that the average particle size of the particles involved in the above embodiments and comparative examples was measured using a particle size analyzer. The method for testing the average thickness of the functional layer is as follows: randomly select a diaphragm with a length and width of 10cm*10cm after the functional layer has been coated, randomly measure the thickness of the functional layer at 20 points using a height gauge, and then take the arithmetic mean of the thicknesses at the 20 points as the average thickness of the functional layer.

[0122] The performance of the separators and batteries prepared in the above embodiments and comparative examples was tested, and the specific tests are as follows: (1) Diaphragm storage performance test: Take a diaphragm with a length and width of 20cm*20cm, weigh the diaphragm and record the mass as m1. Soak the diaphragm in lithium salt electrolyte (1.0M LiPF6 solution, solvent is ethylene carbonate and dimethyl carbonate with a volume ratio of 1:1) at 45℃ for 12h, weigh the diaphragm after soaking and record the mass as m2. Diaphragm storage rate = (m2 / m1)*100%; (2) Adhesion test between diaphragm and negative electrode: The diaphragm and negative electrode were hot-pressed under the conditions of hot-pressing temperature of 90℃, hot-pressing pressure of 3.2Mpa and hot-pressing time of 120s. The adhesion between the two after hot pressing was measured using a universal testing machine. (3) Battery thickness expansion coefficient: The thickness of the battery is measured and recorded as d1. The battery is stored at a high temperature of 60℃ for 30 days, and the thickness d2 of the battery after storage is measured. The battery thickness expansion coefficient = ((d2-d1) / d1)*100%; (4) Initial state battery DC internal resistance: The initial state battery was adjusted to 50% SOC (State of Charge) under 25℃ conditions, and discharged for 10s using a 3C (16.5A) discharge current. The initial state battery DC internal resistance was then tested. (5) The battery was charged and discharged at 25°C within the range of 2V~3.65V, and after 500 cycles at a charge-discharge rate of 1C (1C=5.5Ah), the initial discharge capacity C1 and the discharge capacity C2 after 500 cycles were obtained. The capacity retention rate after 500 cycles = (C2 / C1)*100%; (6) Disassemble the battery after 500 cycles in step (5) and observe the wrinkling and lithium plating of the negative electrode; among which, The criteria for judging the degree of wrinkling of the negative electrode sheet are as follows: If the number of wrinkles n≤3 in a single battery, it is judged as slight wrinkling; if the number of wrinkles 3<n≤10, it is judged as moderate wrinkling; if the number of wrinkles d>10, it is judged as severe wrinkling. Criteria for judging the degree of lithium plating on the negative electrode: In a single cell, the area of ​​lithium plating on the negative electrode is 100 mm². 2 ≤M≤400mm 2 If the lithium plating area on the negative electrode is 400 mm², it is considered a slight lithium plating. 2 <M≤800mm 2 If the lithium plating area on the negative electrode is greater than 800 mm², it is considered moderate lithium plating. 2 If so, it is judged as severe lithium plating.

[0123] (7) 45℃, 100% SOC storage capacity retention test for 180 days: Step 1: Charge the battery at 0.33C (1C=5.5Ah) to 3.65V at 25℃, then charge at a constant voltage until 0.05C is cut off, and discharge at 0.33C to 2V to obtain the battery discharge capacity C3; Step 2: Charge the battery to 3.65V at 0.33C under 25℃ conditions, and then charge it to 0.05C at constant voltage until it reaches 100% SOC. Then, place the battery with 100% SOC in a 45℃ high-temperature furnace and let it stand for 180 days. Step 3: After the battery has been left to stand in a 45℃ high-temperature furnace for 180 days, place it at 25℃, discharge it to 2V at 0.33C, charge it to 3.65V at 0.33C, charge it at a constant voltage until 0.05C is cut off, and then discharge it to 2V at 0.33C to obtain the battery discharge capacity C4. Capacity retention rate of 45℃, 100% SOC storage for 180 days = (C4 / C3) * 100%.

[0124] The test results are shown in Tables 1 and 2.

[0125] Table 1

[0126] Table 2

[0127] Figure 4 This is a scanning electron microscope image of the surface of the diaphragm prepared in Example 1. Figure 4 The large-diameter particles in the membrane are PAM in the functional layer, and the small-diameter particles are PMMA, both of which are relatively evenly distributed on the surface of the substrate layer. Typically, the large-diameter PAM particles have poor adhesion to the membrane. Figure 4 It can be seen that when PMMA and PAM are coated on the surface of the substrate layer, the small-particle-size and strongly adhesive PMMA can bind the large-particle-size PAM, preventing the problem of powder shedding.

[0128] Figure 5 This is a scanning electron microscope (SEM) image of the surface of the membrane prepared in Example 1 after it has been immersed in electrolyte. Figure 4 and Figure 5 The comparison shows that after the separator is immersed in electrolyte, the particle size difference between PAM and PMMA in the functional layer is significantly reduced. This indicates that the volume swelling rate of primary PMMA particles is significantly greater than that of secondary PAM particles. Therefore, PAM, with its advantages of low volume swelling rate and large particle size, can pre-construct sufficient space between the separator and the electrode to accommodate the volume expansion of small-particle-size PMMA with a high volume swelling rate. This effectively prevents the battery from bulging due to the large volume swelling rate of PMMA.

[0129] As can be seen from the data in Table 1, the electrolyte storage capacity of the separators in Examples 1 to 15 is significantly improved compared to Comparative Example 1, while the battery thickness expansion coefficient is significantly lower. This indicates that the large-particle-size secondary stabilizer particles in the functional layer are beneficial to improving the separator's electrolyte storage capacity. Furthermore, the volume swelling rate of the stabilizer after soaking in electrolyte at 45°C for 12 hours is less than or equal to 8%, allowing the secondary stabilizer particles to form gaps between the separator and the electrode, providing a buffer space for the continuous swelling of polymethyl methacrylate (PMMA). This effectively improves the problem of battery cell bulging caused by excessive swelling of the separator functional layer during long-cycle operation. The PMMA in the functional layer has strong adhesive properties, which can improve the adhesion between the separator and the electrode. In other words, through the synergistic effect of PMMA and the stabilizer in the separator functional layer, the adhesion between the separator and the electrode can be guaranteed, improving the separator's electrolyte storage capacity and solving the battery cell bulging problem caused by the continuous swelling of PMMA. As can be seen from the data in Table 2, the batteries of Examples 1 to 15 have low internal resistance, no serious lithium plating on the negative electrode, and good cycle performance and storage stability.

[0130] In Comparative Examples 2 and 3, the separator functional layer only included PMMA or only PAM, lacking the synergistic effect of both. This resulted in severe lithium plating on the negative electrode, leading to poor cycle performance and storage stability. In Comparative Example 4, although a combination of PMMA and PAM was used in the functional layer, the average particle size of PAM particles was smaller than that of PMMA particles. This prevented the formation of a gap between the separator and the electrode to provide buffer space for the continuous swelling of polymethyl methacrylate. Consequently, the battery thickness expansion coefficient was large, resulting in severe lithium plating on the negative electrode and poor cycle performance and storage stability. In Comparative Example 5, polyacrylonitrile polymer was used instead of PAM. Because polyacrylonitrile has a large volume swelling rate in the electrolyte (approximately 80%) and continues to swell, it compresses both the positive and negative electrodes. This easily leads to bulging of the battery, reducing its kinetic performance and causing severe lithium plating on the negative electrode, resulting in poor cycle performance and storage stability.

[0131] Comparing the data from Examples 1 and 3 through 6 in Tables 1 and 2, it can be seen that in Example 3, the particle size of the secondary PAM particles is relatively large, and the ratio of the average particle size of the secondary PAM particles to the average particle size of PMMA is relatively large (greater than 7.5). In Example 4, the particle size of the secondary PAM particles is relatively small, and the ratio of the average particle size of the secondary PAM particles to the average particle size of PMMA is relatively small (less than 2.5). The thickness expansion coefficients of the batteries in Examples 3 and 4 are relatively large, and the negative electrode sheets of the batteries show slight wrinkling and slight lithium plating. The capacity retention rate after 500 cycles at 25°C and the reversible capacity retention rate after 180 days of storage at 45°C and 100% SOC are relatively low. This indicates that a ratio of the average particle size of the secondary stabilizer particles to the average particle size of the primary polymethyl methacrylate particles of 2.5 to 7.5 allows the stabilizer and polymethyl methacrylate to exert a better synergistic effect.

[0132] Comparing the data from Examples 1 and 7 to 10 in Tables 1 and 2, it can be seen that in Example 9, the mass ratio of PMMA to PAM is relatively large (greater than 7:3), while in Example 10, the mass ratio of PMMA to PAM is relatively small (less than 3:7). The battery thickness expansion coefficients of Examples 9 and 10 are relatively large, and the negative electrode sheet exhibits moderate wrinkling and moderate lithium plating. The capacity retention rate after 500 cycles at 25°C and the reversible capacity retention rate after 180 days of storage at 45°C and 100% SOC are both relatively low. This indicates that a mass ratio of polymethyl methacrylate to stabilizer of (3:7) to (7:3) allows the stabilizer and polymethyl methacrylate to exert a better synergistic effect.

[0133] A comparison of the data from Examples 1, 11, and 12 in Tables 1 and 2 shows that in Example 11, the functional layer is too thick, which affects lithium-ion transport; while in Example 12, the functional layer is too thin, which may affect the adhesion between the separator and the electrode, resulting in insufficient hardness of the electrode assembly and affecting the electrolyte storage capacity of the separator. The negative electrode sheets of the batteries in Examples 11 and 12 exhibit some wrinkling and lithium plating, leading to a decrease in the electrochemical performance of the batteries. This indicates that controlling the average thickness of the functional layer on one side of the substrate layer to 1.5 μm to 4.0 μm (the total average thickness of the functional layers on both sides of the substrate layer to 3.0 μm to 8.0 μm) is a superior technical solution.

[0134] A comparison of the data from Examples 1 and 13 shows that the battery exhibits better electrochemical performance when polyoxyethylene alkyl ether is used as the wetting agent, indicating that using polyoxyethylene alkyl ether as the wetting agent is a superior technical solution.

[0135] A comparison of the data from Examples 1, 14, and 15 shows that the electrochemical performance of the battery is optimal when functional layers are provided on both surfaces of the substrate layer along the thickness direction. When functional layers are provided on only one surface of the substrate layer along the thickness direction, the better technical solution is to position the functional layers in the separator towards the negative electrode.

[0136] It should be noted that the diaphragm embodiments, diaphragm preparation method embodiments, battery cell embodiments, and power device embodiments provided in this application belong to the same concept; the technical features in the technical solutions described in each embodiment can be arbitrarily combined without conflict.

[0137] It should be understood that the above embodiments are exemplary and not intended to encompass all possible implementations. Various modifications and changes can be made to the above embodiments without departing from the scope of this disclosure. Similarly, the various technical features of the above embodiments can be arbitrarily combined to form other embodiments of the present invention that may not be explicitly described. Therefore, the above embodiments only illustrate several implementations of the present invention and do not limit the scope of protection of this patent.

Claims

1. A diaphragm, characterized in that, It includes a substrate layer and a functional layer disposed on at least one surface of the substrate layer along the thickness direction; The functional layer includes polymethyl methacrylate and a stabilizer. The volume swelling rate of the stabilizer after soaking in an electrolyte at 45°C for 12 hours is less than or equal to 8%. The electrolyte includes LiPF6 and a solvent. The concentration of LiPF6 in the electrolyte is 1 mol / L. The solvent is a mixture of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:

1. The polymethacrylate comprises primary particles, and the stabilizer comprises secondary particles, wherein the average particle size of the primary particles of the polymethacrylate is smaller than the average particle size of the secondary particles of the stabilizer.

2. The diaphragm according to claim 1, characterized in that, The polymethacrylate includes polymethyl methacrylate; and / or, The stabilizer includes at least one of polyacrylic acid, polyacrylamide, and poly(p-phenylene terephthalamide).

3. The diaphragm according to claim 2, characterized in that, The stabilizer includes polyacrylamide.

4. The diaphragm according to claim 1, characterized in that, The mass ratio of the polymethacrylate to the stabilizer is (3:7) to (7:3).

5. The diaphragm according to any one of claims 1 to 4, characterized in that, The polymethacrylate is in the form of primary particles, and the stabilizer is in the form of secondary particles; and / or, The ratio of the average particle size of the secondary particles of the stabilizer to the average particle size of the primary particles of the polymethyl methacrylate is 2.5 to 7.5; and / or, The average particle size of the primary particles of the polymethacrylate is 600 nm to 1000 nm; and / or, The average particle size of the secondary particles of the stabilizer is 2μm to 6μm.

6. The diaphragm according to any one of claims 1 to 4, characterized in that, The average thickness of the functional layer located on one side of the substrate layer is 1.5 μm to 4.0 μm; and / or, The areal density of the functional layer located on one side of the substrate layer is 0.13 g / m². 2 ~0.39g / m 2 .

7. The diaphragm according to claim 1, characterized in that, The substrate layer includes a polyethylene film and / or a polypropylene film.

8. A method for preparing a diaphragm as described in any one of claims 1 to 7, characterized in that, The method includes the following steps: S1: Acrylic acid monomer and initiator are added to deionized water, ammonia gas is introduced, and polymerization reaction is carried out under heating and pressure. After filtration, washing and drying, primary polyacrylamide particles are obtained. The primary polyacrylamide particles and wetting agent are added to deionized water, and addition reaction is carried out under heating and stirring to obtain secondary polyacrylamide particles. The secondary polyacrylamide particles serve as the stabilizer. S2: Add the polymethyl methacrylate and the polyacrylamide secondary particles to deionized water, mix them evenly, and obtain the functional layer slurry; S3: The functional layer slurry is coated on at least one surface of the substrate layer along the thickness direction, and after drying, the functional layer is formed to obtain the diaphragm.

9. The method for preparing the diaphragm according to claim 8, characterized in that, The method satisfies at least one of the following characteristics: (1) The initiator includes at least one of azobisisobutyronitrile, cumene hydrogen peroxide, and methyl ethyl ketone peroxide; (2) The initiator accounts for 0.3% to 2.0% of the mass of the acrylic acid monomer; (3) The polymerization reaction is carried out at a temperature of 80℃~90℃ and a pressure of 3MPa~4MPa; (4) The average particle size of the primary polyacrylamide particles is 600 nm to 1000 nm; (5) The wetting agent accounts for 2% to 6% of the mass of the primary polyacrylamide particles; (6) The wetting agent includes at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, thiols, and polyoxyethylene alkyl ethers; (7) The addition reaction is carried out at a temperature of 70℃~80℃ and a stirring speed of 200r / min~300r / min.

10. A single battery cell, characterized in that, The membrane includes the membrane according to any one of claims 1 to 7, or the membrane prepared by the method of the membrane according to claim 8 or 9.

11. The battery cell according to claim 10, characterized in that, The battery cell further includes a positive electrode and a negative electrode, the separator is located between the positive electrode and the negative electrode, and the functional layer is at least disposed on the side of the substrate layer facing the negative electrode.

12. An electrical appliance, characterized in that, Includes the battery cell described in claim 10 or 11.

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