Battery diaphragm, preparation method thereof and secondary battery

Abstract

The invention discloses a battery diaphragm, a preparation method thereof and a secondary battery, and relates to the field of battery diaphragms. The battery diaphragm comprises a diaphragm base material, a polymer coating A is arranged on the surface A, close to a positive electrode, of the diaphragm base material, a polymer coating B is arranged on the surface B, close to a negative electrode, of the diaphragm base material, the area ratio of the polymer coating A to the surface A is 60%-85%, the average pore diameter is 1-1.6 microns, the area ratio of the polymer coating B to the surface B is 70%-90%, and the average pore diameter is 0.4-1.3 microns. The polymer coatings are intermittently coated on the two sides of the diaphragm, the grooves are formed by the adjacent coating belts to increase the electrolyte storage space, and different average pore sizes and coating areas of the polymer coatings on the two sides are designed, so that the requirements of the diaphragm on different adhesive force and liquid retention capacity on the surfaces of the two sides of the positive electrode and the negative electrode are met at the same time; and the liquid retention capacity of the battery, the cell expansion and the capacity retention rate of the fast charge cycle are improved.

Description

Technical Field

[0001] The present application relates to the field of battery separators, and in particular to a battery separator, a preparation method thereof, and a secondary battery. Background Art

[0002] In recent years, the rapid growth of new energy vehicles and portable electronic devices has driven increasingly stringent demands for fast-charging and fast-discharging battery performance. The ion conduction efficiency of the separator directly impacts this performance. Currently, commonly used lithium-ion battery separators are typically ceramic separators coated with an adhesive polymer, such as PVDF or PMMA. This creates an adhesive layer, enhancing the bond between the separator and the positive and negative electrodes, thereby improving battery cell performance. Conventional oil-based PVDF coatings have an average pore size of less than 0.5 μm, and the pore size of the PVDF coating on both sides of the substrate is designed to be identical, resulting in a low porosity. During fast charging, lithium-ion batteries using these separators can experience excessive electrolyte consumption and insufficient electrolyte retention, resulting in a shortened cycle life. This can also lead to the formation of metal dendrites on the negative electrode, compromising cycle life and safety. Sharp dendrites, when grown to a certain size, can pierce the separator, compromising safety. In lithium-ion batteries, the formation of metal dendrites is known as lithium plating.

[0003] A key reason for a short battery cycle life, or the rapid decline in capacity retention, is the rapid consumption of electrolyte during fast charging, which simultaneously causes the electrodes to expand and the gap between the electrodes to decrease. This leads to insufficient electrolyte retention in the middle and late stages of the cycle, causing the cycle to drop. Furthermore, the areas of cell expansion stress that are greatest are the corners, where electrolyte consumption is most pronounced and the risk of dendrite formation is particularly high. Therefore, developing separators with high electrolyte retention and improved dendrite formation is crucial. Summary of the Invention

[0004] The present application provides a battery separator, a preparation method thereof, and a secondary battery, by intermittently coating a polymer coating on both sides of the separator substrate and designing different pore size ranges for the polymer coating on both sides to simultaneously meet the different adhesion and liquid retention capacity requirements of the separator on both sides of the positive and negative electrodes, thereby improving the liquid retention capacity and ion conduction efficiency of the battery cell, taking into account the improvement of the battery liquid retention, battery cell expansion, improvement of dendrite precipitation, and capacity retention rate of fast charging cycles.

[0005] In order to solve the above technical problems, one of the purposes of the present application is to provide a battery separator, including a separator substrate, wherein the separator substrate includes a surface A close to the positive electrode and a surface B close to the negative electrode, the surface A is provided with a polymer coating A, and the surface B is provided with a polymer coating B;

[0006] The polymer coating A comprises at least two spaced apart coating bands A, and the ratio S of the area of ​​the polymer coating A to the area of ​​the surface A isA 60%-85%;

[0007] The polymer coating B comprises at least two coating bands B spaced apart, and the ratio S of the area of ​​the polymer coating B to the area of ​​the surface B is B 70%-90%;

[0008] The angles formed by the coating strips A and B with the length direction of the diaphragm substrate are independent of each other, and one of the angles is an acute angle;

[0009] The polymer coating A and the polymer coating B both contain micropores, and the average pore size P of the micropores contained in the polymer coating A is A is 1-1.6 μm, and the pore size is (P A -0.1)~(P A +0.1)μm pore size concentration distribution rate D A ≥80%; the average pore size P of the micropores contained in the polymer coating B B is 0.4-1.3 μm, and the pore size is (P B -0.1)~(P B +0.1)μm pore size concentration distribution rate D B ≥80%;

[0010] The S A 、S B 、P A 、P B 、D A 、D B The following relationship is satisfied:

[0011] M A =S A ×P A ×D A ;M B =S B ×P B ×D B ; and 0.5≤M A / M B ≤3.5.

[0012] The present application provides a battery separator with high liquid retention and high ion conductivity. Polymer coatings A and B are intermittently provided on the surface A and surface B of the battery separator, respectively. The relationship between the coating area ratio of the separator substrate surface and the formed pore size is controlled to meet the adhesion requirements of the polymer coating and ensure the high porosity and high ionic conductivity of the battery separator. At the same time, some uncoated areas are reserved on the surface of the separator substrate to form intermittent grooves, which can increase the storage space of the electrolyte, improve the liquid retention capacity of the battery, reduce the consumption of circulating electrolyte in the corner area of ​​the battery, and alleviate the precipitation dendrite phenomenon in the negative electrode material area.

[0013] Among them, since polymer coating A faces the positive electrode of the battery and polymer coating B faces the negative electrode of the battery, the expansion of the positive electrode is smaller than that of the negative electrode. Therefore, in order to suppress the expansion of the battery cell, the adhesion of polymer coating B needs to be higher than that of polymer coating A. In order to ensure a greater adhesion, usually more polymer coating is required, which reduces the porosity and liquid retention capacity of the micropores. Therefore, by simultaneously improving the porosity or pore size of polymer coating A, it is possible to suppress the expansion of the battery cell while obtaining a high liquid retention capacity. When the average pore size of the polymer coating A is controlled to be above 1 μm and the average pore size of the polymer coating B is controlled to be above 0.4 μm, it helps to further improve the battery's liquid retention capacity, reduce the precipitation of dendrites in the negative electrode material area, and effectively improve the battery cell cycle capacity retention rate; however, when the average pore size of the polymer coating is too large, the adhesion of the polymer coating will decrease. Therefore, when the average pore size of the polymer coating A is controlled to be below 1.6 μm and the average pore size of the polymer coating B is controlled to be below 1.3 μm, the adhesion of the coating can be effectively guaranteed, the ion conduction efficiency of the diaphragm can be improved, and the excessive expansion of the battery cell during the cycle can be avoided. At the same time, the pore size is (P A -0.1)~(P A +0.1)μm and (P B -0.1)~(P B A pore size concentration distribution rate of ≥80% of +0.1) μm can ensure uniform pore size of the polymer coating, which is beneficial to the stability and consistency of the coating adhesion and liquid retention capacity in various areas of the diaphragm, thereby ensuring stable performance of the battery cell.

[0014] In addition, when the coating area of ​​the polymer coating of the battery separator of the present application is less than 60%, the adhesion of the coating decreases, the expansion rate of the battery cell increases, and the liquid retention capacity of the battery cell is also weakened. When the coating area of ​​the polymer coating is greater than 90%, the uncoated area of ​​the separator substrate surface is small, and the intermittent grooves formed cannot effectively store electrolyte, resulting in poor liquid retention of the separator, dendrite precipitation of the negative electrode, and low cycle capacity retention of the battery cell. Therefore, controlling the coating area of ​​the polymer coating within the range of 60%-90% can take into account the improvement of the battery's liquid retention, battery cell expansion and capacity retention, and extend the battery's fast charge cycle life.

[0015] This application controls S A 、S B 、P A 、P B 、D A 、D B The relationship between 0.5≤M A / M B ≤3.5, it can achieve the best performance balance between cell expansion suppression and cycle liquid retention. A / MB The ratio is less than 0.5, then the adhesion of polymer coating B is too poor, resulting in excessive expansion of the battery cell; if M A / M B The ratio is greater than 3.5. Although the battery cell expands less, the risk of lithium plating is greater.

[0016] As a preferred embodiment, the average pore size P of the polymer coating A is A The range value is any one or any two of 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, and 1.6 μm.

[0017] As a preferred embodiment, the average pore size P of the polymer coating B is B The range value is any one or any two of 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, and 1.3 μm.

[0018] As a preferred embodiment, the average pore size P of the polymer coating A is A The average pore size P of the polymer coating B is 1.2-1.4 μm. B 0.6-0.8μm.

[0019] As a preferred solution, the ratio S of the area of ​​the polymer coating A to the area of ​​the surface A is A The range value is any one of 60%, 65%, 70%, 75%, 80%, 85%, or any two of them.

[0020] As a preferred solution, the ratio S of the area of ​​the polymer coating B to the area of ​​the surface B is B The range value is any one of 70%, 75%, 80%, 85%, 90%, or any two of them.

[0021] As a preferred solution, the ratio S of the area of ​​the polymer coating A to the area of ​​the surface A is A The ratio S of the area of ​​the polymer coating B to the area of ​​the surface B is 65%-75%. B It is 75%-85%.

[0022] When the present application controls the average pore size and coating area of ​​the polymer coating A and the polymer coating B on the diaphragm within the above range, it can further take into account the improvement of the battery's liquid retention, battery cell expansion and capacity retention rate. The larger average pore size of the double-sided polymer coating combined with the adapted coating area can effectively improve the battery cell's liquid retention capacity, improve the battery cell's liquid retention capacity in the corner area, and reduce the precipitation of dendrites. At the same time, the average pore size and coating area of ​​the polymer coating within the above range can meet the adhesion requirements of different surfaces of the diaphragm to further reduce the expansion of the battery cell, and the battery can maintain a high capacity retention rate during the fast charging cycle, thereby improving the fast charging performance.

[0023] As a preferred embodiment, the polymer coating A is formed by coating a polymer coating slurry A and then drying it, wherein the polymer coating slurry A comprises a solvent A1 and a solvent A2 in a mass percentage of (84-92): (8-16);

[0024] The polymer coating B is formed by coating a polymer coating slurry B and then drying it, wherein the polymer coating slurry B comprises a solvent B1 and a solvent B2 in a mass percentage of (88-95): (5-12);

[0025] The solvents A1 and B1 are each independently at least one of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and acetone, and the solvents A2 and B2 are each independently at least one of methanol, ethanol, isopropanol, n-butanol, and water.

[0026] As a preferred solution, the polymer coating slurry A and the polymer coating solvent B are independent of each other and further include a polymer with a mass fraction of 8 wt% to 15 wt%.

[0027] As a preferred embodiment, the polymer coating slurry A and the polymer coating solvent B are independent of each other and also include a polymer with a mass fraction of any one or any two of 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, and 15wt%.

[0028] As a preferred embodiment, the polymers of the polymer coating A and the polymer coating B are each independent and include at least one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinylidene fluoride-trifluoroethylene copolymer, polyvinylidene fluoride-chlorotrifluoroethylene copolymer, polyvinylidene fluoride-tetrafluoroethylene copolymer, polyvinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene copolymer, polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene copolymer, polyetherimide, polyimide, polymethyl methacrylate, para-aramid, meta-aramid, and acrylate copolymer.

[0029] As a preferred solution, the thickness of the polymer coating A and the polymer coating B are independent of each other and are 0.5-3 μm.

[0030] As a preferred embodiment, at least two coating bands A contained in the polymer coating A are distributed in parallel, and the distance between two adjacent coating bands A is 0.2-3 mm. One of the angles formed between the coating band A and the diaphragm substrate along the length direction is 30-60 degrees.

[0031] As a preferred embodiment, at least two coating bands B contained in the polymer coating B are distributed in parallel, and the distance between two adjacent coating bands B is 0.2-3 mm. One of the angles formed between the coating bands B and the diaphragm substrate along the length direction is 30-60 degrees.

[0032] The polymer coating of the battery separator of the present application is distributed in strip-shaped coating bands, which can form effective gaps between adjacent coating bands to store electrolyte, and the polymer coating coating area on both sides of the substrate accounts for different proportions. Combined with the different average pore size designs of polymer coatings A and B, the liquid retention capacity of the battery cell is synergistically improved, the cycle capacity retention rate of the battery cell is improved, and lithium plating and battery cell expansion are simultaneously improved.

[0033] As a preferred embodiment, the diaphragm substrate is at least one of polyethylene, polypropylene, polyethylene terephthalate, polyimide, aramid, and polybutylene terephthalate.

[0034] As a more preferred embodiment, surface A and / or surface B of the diaphragm substrate is coated with at least one of aluminum oxide, boehmite, magnesium oxide, magnesium hydroxide, barium titanate, aluminum hydroxide, silicon dioxide, aluminum nitride, lithium aluminum titanium phosphate, lithium lanthanum zirconium oxide solid electrolyte, melamine cyanurate, melamine polyphosphate, ammonium polyphosphate, triphenyl phosphate, decabromodiphenyl ethane, tetrabromobisphenol A, brominated polystyrene, cellulose, and polyimide.

[0035] As a preferred embodiment, the surface A and / or the surface B of the separator substrate is a polyethylene wet-process base film coated with boehmite with a thickness of 0.3-3 μm.

[0036] In order to solve the above technical problems, the second object of this application is to provide a method for preparing a battery separator, comprising the following steps:

[0037] The polymer coating slurry A is applied to the surface A of the diaphragm substrate, which is then immersed in a coagulation bath and dried to form a polymer coating A. The polymer coating slurry B is applied to the surface B of the diaphragm substrate, which is then immersed in a coagulation bath and dried to form a polymer coating B to obtain a battery diaphragm.

[0038] This application adopts non-solvent induced phase separation pore-forming technology, which mainly adjusts the ratio and type of solvent A1 or B1 and solvent A2 or B2, and also assists in adjusting the component ratio of the coagulation bath, so as to achieve the purpose of regulating the pore size and distribution of the coating.

[0039] As a preferred embodiment, the coagulation bath includes at least one of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, acetone, ethanol, isopropanol, n-butanol, and water.

[0040] As a preferred solution, the immersion time in the coagulation bath is 1-20 minutes.

[0041] As a preferred solution, the drying temperature is 50-90° C. and the drying time is 1-30 minutes.

[0042] In order to solve the above technical problems, the third object of the present application is to provide a secondary lithium-ion battery, including a positive electrode sheet, a negative electrode sheet, an electrolyte and the battery separator.

[0043] As a preferred embodiment, the positive electrode sheet includes aluminum foil and a positive electrode material coated on both sides of the aluminum foil, wherein the positive electrode material includes a positive electrode active material, a conductive agent and a binder A in a mass ratio of (94-98): (1-2): (1-2).

[0044] As a preferred embodiment, the positive electrode active material is at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel manganese oxide, lithium iron phosphate, and lithium manganese iron phosphate.

[0045] As a preferred embodiment, the conductive agent is at least one of conductive carbon black, conductive graphite, carbon nanotubes, acetylene black, and graphene.

[0046] As a preferred embodiment, the binder A is at least one of polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, ethylene-tetrafluoroethylene copolymer, and ethylene chlorotrifluoroethylene.

[0047] As a preferred embodiment, the negative electrode sheet includes copper foil and a negative electrode material coated on both sides of the copper foil, wherein the negative electrode material includes a negative electrode active material, a binder B and sodium carboxymethyl cellulose in a mass ratio of (96-99): (0.5-2): (0.5-2).

[0048] As a preferred embodiment, the negative electrode active material is at least one of a carbon-based compound, a silicon-based compound, a titanium-based compound, a tin-based alloy, and a transition metal nitride.

[0049] As a preferred solution, the negative electrode active material is graphite.

[0050] As a preferred solution, the binder B is at least one of polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, ethylene-tetrafluoroethylene copolymer, and ethylene chlorotrifluoroethylene.

[0051] As a preferred solution, the electrolyte includes a lithium salt with a concentration of 0.5-3 mol / L and the balance an organic solvent.

[0052] As a preferred embodiment, the organic solvent is at least one of ethylene carbonate, propylene carbonate and dimethyl carbonate.

[0053] As a preferred embodiment, the organic solvent includes ethylene carbonate, propylene carbonate and dimethyl carbonate in a mass ratio of (2-4):(1-3):(3-5).

[0054] As a preferred embodiment, the lithium salt is lithium hexafluorophosphate.

[0055] As a preferred embodiment, the method for preparing the secondary lithium-ion battery comprises the following steps:

[0056] (1) The positive electrode active material, the conductive agent, and the binder are mixed, and the solvent C is added and stirred evenly to obtain a positive electrode slurry with a solid content of 60 wt% to 80 wt%. The positive electrode slurry is coated on both sides of an aluminum foil and dried to obtain a positive electrode sheet;

[0057] (2) mixing the negative electrode active material, the binder, and sodium carboxymethyl cellulose, adding water and stirring evenly to obtain a negative electrode slurry with a solid content of 50 wt% to 70 wt%, coating the negative electrode slurry on both sides of a copper foil, and drying to obtain a negative electrode sheet;

[0058] (3) The positive electrode sheet, battery separator, and negative electrode sheet are assembled in order, wound and placed in an aluminum-plastic film packaging bag, and then injected with electrolyte after drying. After vacuum packaging, standing, forming, degassing, and trimming, a secondary lithium-ion battery is obtained.

[0059] As a preferred embodiment, the solvent C is at least one of N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide.

[0060] As a preferred solution, in steps (1) and (2), the drying temperature is 100-130° C. and the drying time is 1-3 hours.

[0061] As a preferred embodiment, the electrolyte is prepared by mixing organic solvents in an argon atmosphere glove box with a water content of <10 ppm, and then adding lithium salt to the organic solvent to obtain the electrolyte.

[0062] In order to solve the above technical problems, the fourth object of the present application is to provide an electrical device, which includes the above-mentioned secondary lithium-ion battery.

[0063] The secondary lithium-ion battery of the present invention can be applied to electrical devices that use batteries as power sources or various energy storage systems that use batteries as energy storage elements. The electrical devices include but are not limited to mobile phones, tablet computers, laptops, electric toys, electric tools, battery vehicles, electric cars, ships, spacecraft, etc.

[0064] Compared with the existing technology, this application has the following beneficial effects:

[0065] The present application controls the intermittent coating of 60%-85% of the area of ​​polymer coating A near the positive electrode side of the battery separator, and intermittent coating of 70%-90% of the area of ​​polymer coating B near the negative electrode side, and reserves some uncoated areas to form intermittent grooves, which can increase the storage space of the electrolyte and improve the battery's liquid retention capacity. At the same time, combined with the different average pore sizes and pore size distribution ratios of polymer coatings A and B, it can take into account both the adhesion and liquid retention of the coating, avoid the average pore size being too large to affect the adhesion or too small to affect the battery's liquid retention capacity, synergistically reduce the consumption of circulating electrolyte in the corner area of ​​the battery, alleviate the precipitation dendrite phenomenon in the negative electrode material area, and improve the problem of long-cycle capacity retention rate of the battery cell, ensure the high ion conduction efficiency of the battery separator, effectively suppress the expansion rate of the battery cell, and extend the fast charge cycle life. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] Figure 1 : is a schematic cross-sectional view of a battery separator according to an embodiment of the present application;

[0067] Figure 2 : This is a schematic top view of a polymer coating in a battery separator in an embodiment of the present application. DETAILED DESCRIPTION

[0068] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0069] It should be understood that the terms described in this application are only for describing particular embodiments and are not intended to limit this application. In addition, for numerical ranges in this application, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. The intermediate value within any stated value or stated range, and each smaller range between any other stated value or intermediate value within the stated range are also included in this application. The upper and lower limits of these smaller ranges may be independently included or excluded in the range.

[0070] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art described in this application. Although this application describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of this application. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In the event of any conflict with any incorporated document, the content of this specification shall prevail.

[0071] As used herein:

[0072] "Prepared from" is synonymous with "comprising." As used herein, the terms "comprising," "including," "having," "containing," or any other variations thereof, are intended to cover a non-exclusive inclusion. For example, a composition, process, method, article, or apparatus that comprises the listed elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition, process, method, article, or apparatus.

[0073] The conjunction "consisting of" excludes any unspecified element, step, or component. If used in a claim, this phrase renders the claim closed, excluding materials other than those described, except for conventional impurities associated therewith. When the phrase "consisting of" appears in a clause of the body of a claim, rather than immediately following the subject matter, it limits only the elements described in that clause; other elements are not excluded from the claim as a whole.

[0074] In the description of this application, it should be understood that the terms "up", "down", "left", "right", "top", "bottom", etc., which indicate directions or positional relationships, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction. Therefore, they should not be understood as limitations on this application.

[0075] To further illustrate the present application, the present application is described in detail below in conjunction with the examples, but they should not be construed as limiting the scope of protection of the present application. Unless otherwise specified, the raw materials used in the following examples and comparative examples of the present application are all commercially available, and the same raw materials are used in parallel experiments.

[0076] Examples 1-10 and Comparative Examples 1-9

[0077] A battery separator includes a separator substrate, the separator substrate including a surface A close to a positive electrode and a surface B close to a negative electrode, the surface A is coated with a polymer coating A, and the surface B is coated with a polymer coating B;

[0078] like Figure 1-2 As shown, the polymer coating A comprises at least two coating bands A applied at intervals, and the ratio of the area of ​​the polymer coating A to the area of ​​the surface A is S A , as shown in Table 1; the polymer coating B includes at least two spaced coating strips B, and the ratio of the area of ​​the polymer coating B to the area of ​​the surface B is S B , as shown in Table 1;

[0079] Both polymer coating A and polymer coating B contain micropores, and the average pore size of the micropores contained in polymer coating A is P A , and the aperture is (P A -0.1)~(P A +0.1)μm pore size concentration distribution rate D A ≥90%; the average pore size of the micropores contained in the polymer coating B is P B , and the aperture is (P B -0.1)~(P B +0.1)μm pore size concentration distribution rate D B ≥90%, as shown in Table 1; the above S A 、S B 、P A 、P B 、D A 、D B The following relationship is satisfied:

[0080] M A =S A ×P A ×D A ;M B =S B ×P B ×D B ; Examples 1-10 satisfy 0.5≤M A / M B ≤3.5;

[0081] The thickness of polymer coating A and polymer coating B are both 1 μm, the spacing between adjacent coating strips A and adjacent coating strips B is 1 mm, and one of the angles formed by coating strips A, coating strips B and the diaphragm substrate along the length direction is 45 degrees.

[0082] The above-mentioned method for preparing a battery separator comprises the following steps:

[0083] (1) Adding polyvinylidene fluoride-hexafluoropropylene copolymer (Arkema, France, brand LBG) to N,N-dimethylacetamide (DMAC), stirring and dissolving at 60°C, adding ethanol, wherein the mass percentages of DMAC and ethanol are as shown in Table 1, and stirring until completely dissolved, to obtain a polymer coating slurry A with a polyvinylidene fluoride-hexafluoropropylene copolymer mass concentration of 10 wt%; adding polyvinylidene fluoride-hexafluoropropylene copolymer to DMAC, stirring and dissolving at 60°C, adding ethanol, wherein the mass percentages of DMAC and ethanol are as shown in Table 1, and stirring until completely dissolved, to obtain a polymer coating slurry B with a polyvinylidene fluoride-hexafluoropropylene copolymer mass concentration of 10 wt%;

[0084] (2) The polymer coating slurry A was uniformly coated on the surface A of the diaphragm substrate by intermittent coating, and then immersed in deionized water at 25°C for 3 minutes, and then dried at 60°C for 20 minutes to form a polymer coating A. The average pore size of the polymer coating A is shown in Table 1 below;

[0085] The diaphragm substrate is a polyethylene wet-process base film that has been pre-coated with 1 μm thick boehmite on both sides.

[0086] (3) The polymer coating slurry B was uniformly coated on the surface B of the diaphragm substrate by intermittent coating, and then immersed in deionized water at 25°C for 3 minutes, and then dried at 60°C for 20 minutes to form a polymer coating B to prepare a battery diaphragm. The average pore size of the polymer coating B is shown in Table 1 below.

[0087] Comparative Example 10

[0088] A battery separator, wherein the reagents, equipment, and process parameters used in each step of the preparation method are the same as those in Example 1, except that, in step (2), the surface A of the separator substrate is not coated with the polymer coating slurry A.

[0089] Comparative Example 11

[0090] A battery separator, wherein the reagents, equipment, and process parameters used in each step of the preparation method are the same as those in Example 1, except that, in step (3), the surface B of the separator substrate is not coated with the polymer coating slurry B.

[0091] Comparative Example 12

[0092] A battery separator, wherein the reagents, equipment, and process parameters used in each step of the preparation method are the same as those in Example 1, except that, in steps (2) and (3), coating strips A and B are each arranged parallel to the separator substrate along the length direction.

[0093] Comparative Example 13

[0094] A battery separator, wherein the reagents, equipment, and process parameters used in each step of the preparation method are the same as those in Example 1, except that, in steps (2) and (3), coating strips A and B are each arranged perpendicular to the separator substrate along the length direction.

[0095] Table 1 - Polymer coating materials and coating area ratios of battery separators in Examples and Comparative Examples

[0096]

[0097]

[0098] Application Examples 1-10 and Comparative Application Examples 1-13

[0099] A secondary battery comprising a positive electrode sheet, a negative electrode sheet, a battery separator and an electrolyte, wherein the battery separator is the battery separator prepared in Examples 1-10 or Comparative Examples 1-13;

[0100] The positive electrode sheet includes aluminum foil and positive electrode materials coated on both sides of the aluminum foil. The thickness of the aluminum foil is 8μm, and the positive electrode specifications are 65mm×860mm. The positive electrode materials include lithium cobalt oxide, carbon black and polyvinylidene fluoride in a mass ratio of 97:1.5:1.5.

[0101] The negative electrode sheet includes a copper foil and a negative electrode material coated on both sides of the copper foil. The thickness of the copper foil is 6μm, the negative electrode specification is 69mm×868mm, and the negative electrode material includes graphite, styrene-butadiene rubber and sodium carboxymethyl cellulose in a mass ratio of 98:1:1.

[0102] The electrolyte includes 1 mol / L lithium hexafluorophosphate and the balance organic solvent, and the organic solvent includes ethylene carbonate (EC), propylene carbonate (PC) and dimethyl carbonate (DMC) in a mass ratio of 3:2:4.

[0103] The above-mentioned method for preparing a secondary battery comprises the following steps:

[0104] (1) Lithium cobalt oxide, carbon black, and polyvinylidene fluoride were mixed in a mass ratio of 97:1.5:1.5, N-methylpyrrolidone was added, and the mixture was stirred evenly in a vacuum mixer to obtain a positive electrode slurry with a solid content of 75 wt%. The positive electrode slurry was evenly coated on both sides of an aluminum foil, dried at 125° C. for 2 h, and then cold pressed, cut, and slit to obtain a positive electrode sheet;

[0105] (2) Graphite, styrene-butadiene rubber, and sodium carboxymethyl cellulose were mixed in a mass ratio of 98:1:1, deionized water was added, and the mixture was stirred uniformly in a vacuum mixer to obtain a negative electrode slurry with a solid content of 60 wt%. The negative electrode slurry was evenly coated on both sides of a copper foil, dried at 120° C. for 2 h, and cold pressed, cut, and slit to obtain a negative electrode sheet.

[0106] (3) In an argon atmosphere glove box with a water content of <10 ppm, EC, PC, and DMC were mixed in a mass ratio of 3:2:4 to obtain an organic solvent, and then lithium hexafluorophosphate was added to the organic solvent to obtain an electrolyte;

[0107] (4) Assemble the positive electrode sheet, battery separator, and negative electrode sheet in order, place the battery separator between the positive electrode sheet and the negative electrode sheet to play an isolating role, and wind to obtain an electrode assembly. Place the electrode assembly in an aluminum-plastic film packaging bag, inject electrolyte after drying, and obtain a secondary battery after vacuum packaging, standing, formation, degassing, and trimming.

[0108] Performance testing

[0109] 1. Example and Comparative Example Pore size test method for polymer coating of battery separator: A one-meter range was selected on the surface of the battery separator, and samples were randomly selected at three positions within the range. The surface morphology of the samples was observed by scanning electron microscopy and photographed. The pore size of the coating in the photograph was analyzed using scanning electron microscope analysis software, the pore size of all micropores was measured, and the average pore size and pore size distribution were calculated. The test results are shown in Table 1.

[0110] 2. Battery charge and discharge cycle test: The lithium-ion batteries prepared in the application examples and comparative application examples were placed in a 25°C environment. After standing for 30 minutes, they were charged to 4.50V at a constant current of 4C, then charged to 0.05V at a constant voltage of 4.50V, stood for 5 minutes, and discharged to 3.0V at a constant current of 0.7C. The charge and discharge cycle was repeated 800 times. The capacity retention rate and cell thickness expansion rate of the battery after 800 cycles at room temperature and 4C were tested (PPG test, full charge vs. full charge). The lithium plating in the negative electrode material area was observed. The lithium plating level determination method is shown in Table 2, and the test results are shown in Table 3.

[0111] Table 2 - Determination method of lithium plating level in negative electrode material area

[0112]

[0113] Table 3 - Battery charge and discharge cycle test results of application examples and comparative application examples

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[0115]

[0116] As shown in Table 3, the average pore size of the polymer coating A of the diaphragms in Examples 1-3 and 8-9 is 1-1.6 μm, and that of the polymer coating B is 0.7-1.3 μm, and the pore size satisfies 0.5≤M A / M B ≤3.5, the capacity retention rate of the battery is above 82%, the cell expansion rate is low, and there is no lithium deposition or only a trace amount of lithium deposition in the negative electrode material area. This shows that when the pore diameter of both sides of the diaphragm is within the above range, it is possible to achieve a balance between improving the battery capacity retention rate, cell expansion and lithium deposition. The average pore diameter of the polymer coating A of the separators of Comparative Examples 1 and 3-4 is relatively low, both at 0.5 μm. Since the diaphragm A faces the positive electrode sheet of the battery and the diaphragm B faces the negative electrode sheet of the battery, the A side needs to meet a higher liquid retention capacity than the B side, and the B side needs to meet a higher adhesion force than the A side. The smaller the pore diameter, the higher the adhesion force and the lower the liquid retention capacity. However, when the average pore diameter of the A side is less than 1 μm or the average pore diameter of the B side is less than 0.4 μm, the liquid retention capacity is too poor. Although the expansion rate of the cell is improved, the negative electrode material area is prone to lithium deposition, and the fast charging process easily causes the battery cycle capacity retention rate to be low, and the battery performance cannot be effectively balanced.

[0117] In addition, the average pore size of the polymer coating B of the separators of Comparative Examples 2 and 6 is relatively low at 0.3 μm, and the average pore size of the polymer coating of the separators of Comparative Examples 4-5 is relatively low at 0.3 μm. A / M B The ratio is less than 0.5, and the polymer coating of the separator of Comparative Example 7 has M A / M B The ratio is greater than 3.5. If M A / M B The ratio is less than 0.5, then the adhesion of polymer coating B is too poor, resulting in excessive expansion of the battery cell; if M A / M B The ratio is greater than 3.5. Although the battery cell expansion is small, the risk of lithium plating is greater. Therefore, it is also impossible to effectively balance the battery's lithium plating and battery cell expansion performance.

[0118] The average pore size of the polymer coating A in the separator of Example 2 is relatively large, at 1.6 μm, and the average pore size of the polymer coating B in the separator of Example 3 is relatively large, at 1 μm. The larger particle size of the polymer coatings in Examples 2-3 results in the coating's adhesion failing to meet the requirements of the A or B sides, which in turn causes the cell to expand more during the charge and discharge cycle than in Example 1. Therefore, the average pore size of the separator polymer coating A is preferably 1.3 μm, and the average pore size of the polymer coating B is preferably 0.7 μm, so that the battery can further improve the capacity retention rate, cell expansion rate, and negative electrode lithium plating.

[0119] As shown in Table 3, compared with Example 1, the coating area of ​​the polymer coating of the diaphragm of Example 6 is reduced, and the coating area of ​​the polymer coating of the diaphragm of Example 7 is increased. It can be seen that due to the reduction in the coating area of ​​Example 6, the adhesion of the coating is reduced, and the battery cell is prone to slight expansion during the charge and discharge cycle. The reduction in the coating area leads to excessive spacing between the strip polymer coatings, which reduces the liquid retention capacity of the diaphragm and causes slight salt precipitation in the negative electrode material area. In Example 7, due to the increase in the coating area, the spacing between the strip polymer coatings gradually decreases, resulting in a reduction in the interval liquid storage capacity, and the liquid retention capacity of the battery cell is reduced, making it easy for slight lithium precipitation to occur in the negative electrode material area, and the fast charging process may cause the battery cycle capacity retention rate to decrease. Therefore, the coating area of ​​polymer coatings A and B is preferably 70% and 80%. The polymer coating of the diaphragm of Comparative Example 9 is fully coated, and there is no intermittent space on the surface of the diaphragm to store electrolyte. The liquid storage capacity of the coating pore size is limited, resulting in insufficient liquid retention capacity of the diaphragm and moderate lithium precipitation during the cycle. The separator of Comparative Example 10-11 is coated with a polymer coating on only one side, which cannot meet the battery's requirements for the separator's liquid retention capacity, nor can it effectively inhibit the expansion of the battery cell, resulting in severe lithium deposition during the battery cycle and a high battery cell expansion rate.

[0120] As shown in Table 3, compared with Example 1, the coating belt in the diaphragm of Comparative Example 12 is parallel to the length direction of the diaphragm. At this time, the coating belt will cause low injection efficiency of the battery cell in the injection link, and poor capillary action, and lithium is easily deposited at the interface, resulting in poor performance; the coating belt in the diaphragm of Comparative Example 13 is perpendicular to the length direction of the diaphragm, and the coating belt is likely to cause lithium deposition in the corner area of ​​the battery cell, resulting in poor performance.

[0121] The specific embodiments described above further illustrate the objectives, technical solutions, and beneficial effects of this application. It should be understood that the above descriptions are merely specific embodiments of this application and are not intended to limit the scope of protection of this application. In particular, it should be noted that any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of this application by those skilled in the art should be included within the scope of protection of this application.

Claims

1. A battery separator, characterized in that: The diaphragm substrate includes a surface A close to the positive electrode and a surface B close to the negative electrode, the surface A is provided with a polymer coating A, and the surface B is provided with a polymer coating B; The polymer coating A comprises at least two spaced apart coating bands A, and the ratio S of the area of ​​the polymer coating A to the area of ​​the surface A is A 60%-85%; The polymer coating B comprises at least two coating bands B spaced apart, and the ratio S of the area of ​​the polymer coating B to the area of ​​the surface B is B 70%-90%; The angles formed by the coating strips A and B with the length direction of the diaphragm substrate are independent of each other, and one of the angles is an acute angle; The polymer coating A and the polymer coating B both contain micropores, and the average pore size P of the micropores contained in the polymer coating A is A is 1-1.6 μm, and the pore size is (P A -0.1)~(P A +0.1)μm pore size concentration distribution rate D A ≥80%; the average pore size P of the micropores contained in the polymer coating B B is 0.4-1.3 μm, and the pore size is (P B -0.1)~(P B +0.1)μm pore size concentration distribution rate D B ≥80%; The S A 、S B 、P A 、P B 、D A 、D B The following relationship is satisfied: M A =S A ×P A ×D A ;M B =S B ×P B ×D B ; and 0.5≤M A / M B ≤3.

5.

2. The battery separator according to claim 1, wherein The polymer coating A is formed by coating a polymer coating slurry A and then drying it, wherein the polymer coating slurry A comprises a solvent A1 and a solvent A2 in a mass percentage of (84-92): (8-16); The polymer coating B is formed by coating a polymer coating slurry B and then drying it, wherein the polymer coating slurry B comprises a solvent B1 and a solvent B2 in a mass percentage of (88-95): (5-12); The solvents A1 and B1 are each independently at least one of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and acetone, and the solvents A2 and B2 are each independently at least one of methanol, ethanol, isopropanol, n-butanol, and water.

3. A battery separator according to claim 1, characterized in that: The polymer coating slurry A and the polymer coating solvent B are independent of each other and further include a polymer with a mass fraction of 8 wt% to 15 wt%.

4. The battery separator according to claim 3, wherein The polymers in the polymer coating A and the polymer coating B are each independent and include at least one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinylidene fluoride-trifluoroethylene copolymer, polyvinylidene fluoride-chlorotrifluoroethylene copolymer, polyvinylidene fluoride-tetrafluoroethylene copolymer, polyvinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene copolymer, polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene copolymer, polyetherimide, polymethyl methacrylate, para-aramid, meta-aramid, and acrylate copolymer.

5. The battery separator according to claim 1, wherein The thickness of the polymer coating A and the polymer coating B are each independent and range from 0.5 to 3 μm.

6. The battery separator according to claim 1, wherein The polymer coating A contains at least two coating bands A distributed in parallel, and the distance between two adjacent coating bands A is 0.2-3 mm, and one of the angles formed between the coating bands A and the diaphragm substrate along the length direction is 30-60 degrees; And / or, at least two coating bands B contained in the polymer coating B are distributed in parallel, and the distance between two adjacent coating bands B is 0.2-3 mm, and one of the angles formed between the coating bands B and the diaphragm substrate along the length direction is 30-60 degrees.

7. The battery separator according to claim 1, wherein The diaphragm substrate is at least one of polyethylene, polypropylene, polyethylene terephthalate, polyimide, aramid, and polybutylene terephthalate; And / or, surface A and / or surface B of the diaphragm substrate is coated with at least one of aluminum oxide, boehmite, magnesium oxide, magnesium hydroxide, barium titanate, aluminum hydroxide, silicon dioxide, aluminum nitride, lithium aluminum titanium phosphate, lithium lanthanum zirconium oxide solid electrolyte, melamine cyanurate, melamine polyphosphate, ammonium polyphosphate, triphenyl phosphate, decabromodiphenyl ethane, tetrabromobisphenol A, brominated polystyrene, cellulose, and polyimide; And / or, surface A and / or surface B of the separator substrate is a polyethylene wet-process base film coated with boehmite with a thickness of 0.3-3 μm.

8. A method for preparing a battery separator according to any one of claims 1 to 7, characterized in that: The following steps are involved: The polymer coating slurry A is applied to the surface A of the diaphragm substrate, which is then immersed in a coagulation bath and dried to form a polymer coating A. The polymer coating slurry B is applied to the surface B of the diaphragm substrate, which is then immersed in a coagulation bath and dried to form a polymer coating B to obtain a battery diaphragm.

9. A secondary battery, characterized in that: The battery comprises a positive electrode sheet, a negative electrode sheet, an electrolyte and a battery separator as claimed in any one of claims 1 to 7.

10. An electrical device, characterized in that: Comprising the secondary battery as claimed in claim 9.

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