Diaphragm and secondary battery

Abstract

The invention discloses a diaphragm and a secondary battery, and belongs to the field of batteries. According to the diaphragm, a functional coating containing polymer particles with two sizes and glass transition temperatures as key components is arranged on an organic substrate, and meanwhile, the size gradient of the polymer particles and the porosity of the diaphragm are correspondingly adjusted; when the electrolyte is applied to a winding type secondary battery, ideal electrolyte absorption and retention effects can be realized, and meanwhile, the lithium precipitation condition at the corner bending part of the secondary battery can be effectively inhibited.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a diaphragm and a secondary battery. Background Art

[0002] The force strength of the pole piece and separator of the wound secondary battery at the corner bend is higher than that of the flat area, and the lithium ion transmission path in this area is more complicated, which is very easy to cause local lithium precipitation during the secondary battery cycle, and even internal short circuit in severe cases. For this reason, people try to set a separator with a specific coating or a specific composition in the wound secondary battery to replace the stress at the corner bend of the device, while improving the lithium ion transmission efficiency in this area, thereby inhibiting the lithium precipitation phenomenon. However, this type of separator may have the defects of low electrolyte impregnation rate and low liquid retention coefficient during injection after setting, and the cycle performance of the prepared secondary battery cannot be guaranteed. Summary of the invention

[0003] The purpose of the present application is to overcome the shortcomings of the prior art and provide a diaphragm, wherein a functional coating containing polymer particles of two sizes and glass transition temperatures as key components is arranged on an organic substrate, and the size gradient of the polymer particles and the porosity of the diaphragm are adjusted accordingly. When applied to a wound secondary battery, it can not only achieve ideal electrolyte absorption and retention effects, but also effectively inhibit lithium deposition at the corners of the secondary battery.

[0004] To achieve the above-mentioned object, in a first aspect of the present application, the present application provides a membrane, comprising an organic substrate and a coating;

[0005] The coating comprises polymer particles 1 and polymer particles 2;

[0006] The average particle size a of the polymer particles 1 is ≥4 μm, and the average particle size b of the polymer particles 2 is ≤3 μm;

[0007] The test spectrum obtained by performing DSC analysis test on the polymer particles 1 and 2 after mixing includes a first exothermic peak and a second exothermic peak, the peak position of the first exothermic peak is ≤85°C, and the peak position of the second exothermic peak is ≥150°C;

[0008] The diaphragm satisfies D / H=0.03-0.12, wherein D=a / b, and H% is the porosity of the diaphragm.

[0009] As an embodiment of the present application, a=4-6 μm.

[0010] As an embodiment of the present application, b=1-3 μm.

[0011] As an implementation scheme of the present application, D=1-4.5.

[0012] As an embodiment of the present application, H=35 to 50%.

[0013] As an embodiment of the present application, the surface density of the coating is 0.1 to 0.5 g / m 2 .

[0014] As an embodiment of the present application, the thickness of the coating layer is 4 to 6.5 μm.

[0015] As an embodiment of the present application, the thickness of the separator is 8 to 11.5 μm.

[0016] As an embodiment of the present application, the polymer particles 1 include at least one of polymethyl methacrylate particles and polyethylene particles.

[0017] As an embodiment of the present application, the polymer particles 2 include at least one of polyetheretherketone particles, polytetrafluoroethylene particles, and polystyrene particles.

[0018] In a second aspect of the present application, the present application provides a secondary battery, comprising the separator described in the present application.

[0019] In a third aspect of the present application, the present application provides an electrical device, comprising the secondary battery described in the present application.

[0020] The beneficial effects of this application are:

[0021] The present application provides a diaphragm, wherein a functional coating containing polymer particles of two sizes and glass transition temperatures as key components is arranged on an organic substrate, and the size gradient of the polymer particles and the porosity of the diaphragm are adjusted accordingly. When applied to a wound secondary battery, it can not only achieve ideal electrolyte absorption and retention effects, but also effectively inhibit lithium deposition at the corners and bends of the secondary battery. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application are described clearly and completely below. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present application.

[0023] In the present application, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions containing the listed features.

[0024] In this application, when it comes to numerical ranges, unless otherwise specified, the above numerical ranges are deemed to be continuous and include the minimum and maximum values ​​of the range, as well as each value between such minimum and maximum values. Further, when a range refers to an integer, each integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe features or characteristics, the ranges can be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges included therein.

[0025] The present application is further described below with specific embodiments:

[0026] A separator includes an organic substrate and a coating;

[0027] The coating comprises polymer particles 1 and polymer particles 2;

[0028] The average particle size a of the polymer particles 1 is ≥4 μm, and the average particle size b of the polymer particles 2 is ≤3 μm;

[0029] The test spectrum obtained by performing DSC analysis test on the polymer particles 1 and 2 after mixing includes a first exothermic peak and a second exothermic peak, the peak position of the first exothermic peak is ≤85°C, and the peak position of the second exothermic peak is ≥150°C;

[0030] The diaphragm satisfies D / H=0.03-0.12, wherein D=a / b, and H% is the porosity of the diaphragm.

[0031] In order to overcome the problem of lithium plating at the corners of the existing wound secondary batteries, the technical solution of the present application has developed a diaphragm with a special coating structure design, in which two polymer particles of different sizes are arranged in the diaphragm coating, and the coating has two exothermic peaks at specific sites during DSC analysis, and the exothermic peak is mainly the glass transition characteristic peak of the polymer particles, that is, in the coating, the polymer particles have gradients in both particle size and glass transition temperature. When the diaphragm is set in the wound secondary battery, the polymer particles with low glass transition temperature in the diaphragm coating can soften in the formation stage and play a bonding and buffering role, thereby suppressing the stress of the device at the corners of the wound secondary battery in the formation stage, while the polymer particles with high glass transition temperature serve as a supporting skeleton to ensure the dimensional stability and pore structure integrity of the diaphragm coating, thereby avoiding obvious deformation of the diaphragm or even clogging of the pores.

[0032] In addition, polymer particles of different size gradients will affect the porosity of the diaphragm, and the porosity of the diaphragm will directly affect the electrolyte retention rate of the diaphragm and the ion / electron transmission efficiency of the diaphragm. Even if the stress problem of the secondary battery at the corner bend is solved, if the porosity of the diaphragm is too small and the electrolyte retention rate is too low, which leads to the ion / electron transmission efficiency of the diaphragm is too low, lithium deposition will still occur due to the electrolyte bridge breaking effect at the corner bend or the slow retention of lithium ions. Therefore, it is necessary to ensure that the diaphragm has sufficient porosity, but if the porosity is too small, the electrolyte retention rate will be too low, which will lead to the ion / electron transmission efficiency of the diaphragm being too low. If the rate is too high, the elastic strength of the diaphragm will become low, and the stress cannot be effectively relieved. Therefore, in the present application, the size ratio of the two polymer particles and the porosity of the diaphragm are synchronously regulated. When the ratio of the size ratio D to the porosity H is between 0.03 and 0.12, the diaphragm can take into account both the ideal stress improvement performance and the structural stability performance when applied to a wound secondary battery. The dimensional change after formation is small, and at the same time, it has a high liquid retention coefficient. The secondary battery can maintain a high level of cycle performance under the condition of low corner lithium precipitation probability.

[0033] In some embodiments, a=4-6 μm.

[0034] More preferably, a is in the range of one or any two of 4 μm, 4.2 μm, 4.5 μm, 4.8 μm, 5 μm, 5.2 μm, 5.5 μm, 5.8 μm, and 6 μm.

[0035] In some embodiments, b=1-3 μm.

[0036] More preferably, b is in the range of one or any two of 1 μm, 1.2 μm, 1.5 μm, 1.8 μm, 2 μm, 2.2 μm, 2.5 μm, 2.8 μm, and 3 μm.

[0037] It should be noted that the test method for a and b described in this application is: the coating on the surface of the diaphragm is scraped off with a scraper, and the resulting powder is then immersed in N-methylpyrrolidone and stirred for 30 minutes, solid-liquid separation is performed, and then washed 3 times with deionized water and dried. The resulting powder is treated with gold spraying and a FEI TF20 transmission electron microscope energy spectrometer is used to identify polymer particles at a magnification of 3000, and 5 samples with a particle size ≥4μm and a sample with a particle size ≤3μm are randomly selected from the upper, lower, left, right and middle areas of the image, 25 in each case, and then magnified to 10,000 times and measured at this magnification using mapping software The particle size of the samples with a carbon particle size ≥4μm and the sample with a particle size ≤3μm, and the average value is calculated, which is a and b.

[0038] In some embodiments, the test spectrum obtained when the polymer particles 1 and polymer particles 2 are subjected to DSC analysis after mixing includes a first exothermic peak and a second exothermic peak, the first exothermic peak has a peak position of 45 to 85°C, and the second exothermic peak has a peak position of 150 to 350°C. It should be noted that the DSC analysis test of the polymer particles 1 and polymer particles 2 described in the present application is carried out in the following manner: the coating on the surface of the diaphragm is scraped off with a scraper, and then the obtained powder is placed in N-methylpyrrolidone and immersed and stirred for 30 minutes, solid-liquid separation, and then washed with deionized water 3 times, dried, and the obtained powder is placed in a DSC analyzer, and a nitrogen atmosphere is used to test at a heating rate of 10°C / min.

[0039] In some embodiments, the mass ratio of the polymer particles 1 to the polymer particles 2 is (1:9) to (9:1).

[0040] In some embodiments, the coating further comprises a coating matrix, the coating matrix comprises at least one of aramid, polyvinylidene fluoride, polyimide, polyetherimide, and polyacrylate polymers, and the mass ratio of the coating matrix to the polymer particles is (1:9) to (9:1).

[0041] In the coating described in the present application, the coating matrix acts as a binder for the polymer particles. Different matrix ratios can be selected based on different preparation processes and are not subject to specific limitations.

[0042] In some embodiments, D=1-4.5.

[0043] Further preferably, D is a range value of one or any two of 1, 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, 4.2, 4.5.

[0044] In some embodiments, D=2-3.5;

[0045] The size gradient setting of the polymer particles affects the buffering effect of the coating itself on the stress at the corner bend, the porosity and dimensional stability of the coating. When the size setting ratio of the two polymer particles is further preferably within the above range, the comprehensive performance of the coating is better.

[0046] In some embodiments, H=35-50%.

[0047] More preferably, H is in the range of one or any two of 35%, 38%, 40%, 42%, 45%, 48%, and 50%.

[0048] It should be noted that the H is obtained by testing in the following manner: the porosity of the diaphragm is directly tested by a PMI water pressure tester.

[0049] In some embodiments, the D / H=0.04-0.085.

[0050] The size gradient setting of the polymer particles will directly affect the porosity of the diaphragm, and the porosity of the diaphragm is also related to the surface density of the coating itself, the coating coating method and even the thickness of the coating. By synchronously regulating the size gradient setting of the polymer particles and maintaining the porosity of the overall diaphragm within the above-mentioned preferred range, its application in wound secondary batteries can achieve better liquid retention and lithium precipitation inhibition effects, and better cycle stability.

[0051] In some embodiments, the surface density of the coating is 0.1 to 0.5 g / m 2 .

[0052] In some embodiments, the coating has a thickness of 4 to 6.5 μm.

[0053] In some embodiments, the thickness of the separator is 8 to 11.5 μm.

[0054] In the present application, when testing the thickness of the diaphragm, the diaphragm is cut into a size of 50*100mm and stacked in ten layers. The total thickness H1 is tested and recorded, and the diaphragm thickness = H1 / 10. The testing equipment is a Marr thickness gauge. When testing the thickness and surface density of the coating, conventional confirmation can be adopted by conventional methods commonly used in the industry. For example, the coating is positioned and the thickness is measured on the diaphragm cross section through an optical microscope and surveying and mapping software (the points are the left end point, the middle end point and the right end point of the coating in the cross section, and the average of the measured values ​​is the thickness of the coating on the single diaphragm. The thickness of the coating on ten diaphragms is tested in parallel and recorded. The data is averaged, that is, the coating thickness of the diaphragm is recorded). At the same time, the weight loss rate of the base polyolefin layer in the coating is confirmed by thermogravimetric-DSC combined analysis, so that the mass of the coating is confirmed by calculation, and finally the surface density of the coating in the diaphragm is calculated.

[0055] In some embodiments, the polymer particles 1 include at least one of polymethyl methacrylate particles and polyethylene particles.

[0056] In some embodiments, the polymer particles 2 include at least one of polyetheretherketone particles, polytetrafluoroethylene particles, and polystyrene particles.

[0057] It should be noted that there is no special setting for the source of the polymer particles 1 and 2 described in the present invention. Homemade products can be selected, or specific polymer particles with limited particle size and glass transition temperature can be purchased. They can be compounded according to actual conditions for preparing the coating. In the technical scheme of the present application, there are no special restrictions on other components of the coating and the preparation process. In the technical scheme of the present application, based on the selection of specific polymer particles 1 and 2 in the coating, after appropriate regulation to make the porosity in a suitable range, the resulting diaphragm can achieve ideal dimensional stability and high liquid retention coefficient when applied to secondary batteries. The secondary battery can achieve low corner lithium precipitation and excellent cycle performance. According to general knowledge, those skilled in the art can select conventional polymer system coatings for the present application scheme. In addition to the optional types mentioned above, other types are not limited if they can also achieve the same technical effects, and the coating can be prepared by conventional methods. According to common knowledge, those skilled in the art can control various conventional parameters including but not limited to material stirring rate, solid content of prepared slurry, coating rate, etc. during the preparation process to achieve coating construction and diaphragm porosity regulation, and the regulation does not belong to unclear or unconventional technical operations.

[0058] In some embodiments, the organic coating includes an organic polymer, and the organic polymer includes at least one of polyvinylidene fluoride, polyvinylidene fluoride-based copolymers, polyamide, polyimide, polyacrylonitrile, polyethylene oxide, polyurethane, polyphenylene ether, acrylate copolymers, and polymethyl methacrylate.

[0059] In some embodiments, the coating layer is disposed on the organic substrate layer by coating.

[0060] Specifically, the coating includes at least one of gravure coating, extrusion coating, immersion coating, spray coating, spot coating, and wire rod coating.

[0061] The coating in the technical solution of the present application can be constructed on the organic base layer by coating methods including but not limited to the above-mentioned types, and is not limited as long as it does not affect the limitations of the membrane on inorganic particles and thickness.

[0062] In a second aspect of the present application, the present application provides a secondary battery, comprising the separator described in the present application.

[0063] In some specific embodiments, the secondary battery further comprises a positive electrode sheet, the positive electrode sheet comprises a current collector and a positive electrode material layer, the positive electrode material layer comprises a positive electrode active material, the positive electrode active material includes but is not limited to LiCoO2, LiNiO2, LiVO2, LiCrO2, LiMn2O4, LiCoMnO4, Li2NiMn3O8, LiNi 0.5 Mn 1.5At least one of O4, LiCoPO4, LiMnPO4, LiFePO4, LiNiPO4, LiCoFSO4 and lithium nickel cobalt manganese oxide, and the mass content of the positive electrode material in the positive electrode material layer is 90-99%.

[0064] In some embodiments, the positive electrode material layer further includes at least one of a conductive agent and a binder.

[0065] Furthermore, the binder includes at least one of polyvinylidene fluoride, polyvinyl butyral, polytetrafluoroethylene, ethylene-vinyl acetate copolymer, and polyvinyl alcohol.

[0066] Furthermore, the conductive agent includes at least one of graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0067] In some embodiments, the compaction density of the positive electrode sheet is 2.5 to 3.5 g / cm 3 .

[0068] In some embodiments, the thickness of the positive electrode material layer is 70 to 150 μm.

[0069] In some embodiments, the secondary battery includes a negative electrode sheet.

[0070] In some embodiments, the compaction density of the negative electrode plate is 1.5 to 1.7 g / cm 3 .

[0071] In some embodiments, the negative electrode plate includes a current collector, a negative electrode material layer disposed on the current collector, the negative electrode material layer includes a negative electrode material, and the mass content of the negative electrode material in the negative electrode material layer is 94-98%.

[0072] In some embodiments, the negative electrode material includes at least one of artificial graphite and silicon-doped graphite.

[0073] In some embodiments, the negative electrode material layer further includes at least one of a conductive agent, a binder, and a thickener.

[0074] Further preferably, the binder and / or thickener includes at least one of styrene-butadiene rubber, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyvinyl butyral, polytetrafluoroethylene, ethylene-vinyl acetate copolymer, and polyvinyl alcohol.

[0075] Further preferably, the conductive agent includes at least one of graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0076] In some embodiments, the thickness of the negative electrode material layer is 50 to 150 μm.

[0077] In some embodiments, the secondary battery also includes an electrolyte, the electrolyte includes a lithium salt and a solvent, the lithium salt includes at least one of LiPF6, LiBOB, LiBF4, LiPF6, LiBF4, LiPF6, LiTFSI, LiClO4, LiAsF6, LiCF3SO3, and LiN(CF3SO2)2, and the solvent includes at least one of EC, PC, EMC, DMC, DEC, THF, 2-MeTHF, TMS, EA, and MP.

[0078] The present invention is further described below with specific examples, which are not to be construed as limiting the scope of the present invention:

[0079] Example 1

[0080] A diaphragm and a secondary battery, wherein the preparation method comprises the following steps:

[0081] Preparation of diaphragm: VALIANT@TPI-VAT002G / P model polyetherimide is dissolved in N-methylpyrrolidone as a coating matrix and stirred evenly at a speed of 500 r / min. Then, polymer particles 1 and polymer particles 2 with a specific average particle size after screening are added to the obtained mixed solution, and stirred at a speed of 1000 r / min for 30 minutes. The obtained coating slurry is gravure-coated on one side of a polyethylene base film with a thickness of 5 μm using a single-sided anilox roller at a specific speed, dried, and cut to obtain the diaphragm, wherein the mass ratio of the coating matrix to the polymer particles is 10:4.

[0082] The parameters for preparing each membrane are shown in Table 1, and the porosity of the membrane is adjusted accordingly.

[0083] Preparation of positive electrode:

[0084] Lithium cobalt oxide was used as the positive electrode active material. The material was then mixed with the conductive agent acetylene black and the binder polyvinylidene fluoride in a mass ratio of 95:3:2 in NMP solvent to prepare a slurry, which was then coated on the current collector aluminum foil, dried, cold pressed, and slit to obtain a positive electrode sheet. The thickness of the positive electrode material layer on the sheet was 100 μm and the compaction density was 3 g / cm 3 ;

[0085] Preparation of negative electrode sheet:

[0086] Artificial graphite, conductive agent acetylene black, binder styrene butadiene rubber and thickener sodium carboxymethyl cellulose were mixed in water at a mass ratio of 96:1:1.5:1.5 to prepare a slurry, which was coated on the current collector copper foil, dried, cold pressed and stripped to obtain a negative electrode sheet. The thickness of the negative electrode material layer on the sheet was 100 μm and the compaction density was 1.6 g / cm 3 .

[0087] Preparation of electrolyte:

[0088] The lithium salt LiPF6 and the non-aqueous organic solvent (the mass ratio of ethylene carbonate: diethyl carbonate: propylene carbonate: propyl propionate: vinylene carbonate = 20:30:20:28:2) are prepared in a mass ratio of 8:92.

[0089] Preparation of secondary batteries:

[0090] The positive electrode sheet, the separator (the coating side faces the positive electrode sheet), and the negative electrode sheet are stacked in order, and then wound. The obtained electrode assembly is placed in a packaging shell and injected with electrolyte, and then packaged to obtain the secondary battery.

[0091] Embodiments 2 to 11

[0092] A diaphragm and a secondary battery, which are different from those in Example 1 only in that the preparation parameters of the diaphragm are different, which leads to different porosity of the diaphragm, as shown in Table 1.

[0093] Embodiments 12 to 15

[0094] A diaphragm and a secondary battery, which differ from embodiments 1 to 11 in that the preparation method of the diaphragm is as follows: 0.5wt% sodium polyacrylate is dispersed in water as a dispersant, and stirred at a rate of 1000rpm for 20min, then 3wt% sodium carboxymethyl cellulose is added and mixed and continued to stir for 30min, polymer particles 1 and 2 are added and stirred at 1000rpm for 60min, and finally a coating matrix polyacrylate is added and stirred at a rate of 500rpm for 20min, 0.5wt% polyether silicone is added as a wetting agent and mixed, the obtained coating slurry is gravure-coated on a polyethylene base film with a thickness of 5μm by an anilox roller at a specific vehicle speed, dried, and cut to obtain the diaphragm, wherein the mass ratio of the coating matrix to the polymer particles is 1:2.

[0095] In addition, the preparation parameters of the diaphragm are different, which leads to different porosities of the diaphragm, as shown in Table 1.

[0096] Comparative Examples 1 to 7

[0097] A diaphragm and a secondary battery, which are different from Examples 1 to 11 only in that the preparation parameters of the diaphragm are different, which leads to different porosity of the diaphragm, as shown in Table 1.

[0098] Comparative Examples 8 to 9

[0099] A diaphragm and a secondary battery, which are different from Examples 12 to 15 only in that the preparation parameters of the diaphragm are different, which leads to different porosity of the diaphragm, as shown in Table 1.

[0100] In Table 1, the PMMA1 particles are FS400 produced by Sichuan Yindile Technology Co., Ltd., PMMA2 is FS405 produced by Sichuan Yindile Technology Co., Ltd., PE1 particles are PEW-0261 produced by Nanjing Tianshi New Material Technology Co., Ltd., PE2 particles are PEW-0278X produced by Nanjing Tianshi New Material Technology Co., Ltd., PEEK particles are PEEK30 produced by Shanghai Yumingshen Plastic Co., Ltd., and PTFE particles are PTFE0108 produced by Nanjing Tianshi New Material Technology Co., Ltd. All of them were ball milled and screened when preparing the slurry.

[0101] Table 1

[0102]

[0103]

[0104]

[0105]

[0106] Effect Example 1

[0107] In order to verify the performance effect of the secondary battery obtained by the technical solution of the present application, the secondary batteries prepared in each embodiment and comparative example were subjected to the following performance tests:

[0108] (1) Test of the change of the thickness of the diaphragm before and after formation: The diaphragm before assembling the secondary battery is measured by a Mahr thickness gauge, and then the secondary battery is disassembled after formation, and the part of the winding head that does not contact the active material is taken for secondary testing, wherein the formation steps of the secondary battery are: at 80°C and 1749kgf pressure, the secondary battery is pre-charged to 4.5V at a constant current rate of 0.2C in a charge and discharge device at a constant current rate of 1C to 4.5V, and finally charged to 4.5V at a constant current rate of 1.2C, and the formation is completed;

[0109] (2) Room temperature cycle performance test: In an environment of 25°C, the first charge and discharge are carried out, and constant current and constant voltage charging is carried out at a charging current of 0.1C (i.e., the current value at which the theoretical capacity is completely discharged within 10 hours) until the upper limit voltage is 4.3V, and then constant current discharge is carried out at a discharge current of 1C until the final voltage is 3V, and the discharge capacity of the first cycle is recorded; then 100 charge and discharge cycles are carried out, and the discharge capacity of the 100th cycle is recorded.

[0110] Cycle capacity retention rate = (discharge capacity of the 100th cycle / discharge capacity of the first cycle) × 100%;

[0111] The test samples were set as 5 groups of parallel samples, and the final test results were calculated as the average value of the 5 groups of parallel samples.

[0112] (3) Liquid retention coefficient test: When preparing each product, weigh the mass of the secondary battery cell before injection M1, weigh the mass after injection M2, and drain the excess electrolyte after 24 hours to weigh the mass of the battery cell M3. Injection volume = M2-M1, liquid loss volume = M2-M3, liquid retention coefficient = liquid retention volume / capacity, where liquid retention volume = injection volume-liquid loss volume.

[0113] (4) Corner lithium deposition level test: Take parallel samples of the secondary battery cells obtained in each embodiment and comparative example, and perform step (1) for a cycle, then disassemble them, remove the packaging film from the secondary battery cells, unfold them along the winding direction, separate the separator and the electrode, and observe the interface of the negative electrode. If there is no lithium deposition at the corner, it is marked as level 0; if there is point-like lithium deposition at the corner, it is marked as level 1; if there is linear lithium deposition at the corner, but it is not continuous, it is marked as level 2; if there is linear lithium deposition at the corner and it is continuous, it is marked as level 3; each group of parallel samples has 5 samples, and the highest level among the samples is counted.

[0114] The test results are shown in Table 2.

[0115] Table 2

[0116]

[0117]

[0118] It can be seen from Table 2 that the separator in the secondary battery described in the present application is based on the two specific gradient polymer particles set in the coating, which can not only effectively suppress the stress effect at the corner bending of the secondary battery, but also play an obvious buffering role during the battery cycle. At the same time, based on the supporting effect of the particles with high glass transition temperature, the size change amplitude of the separator is always maintained in the range of 1 to 2 μm, and the separator can achieve good electrolyte absorption effect, and the liquid retention coefficient can reach above 1.440. During the cycle, based on the control setting of the gradient of polymer particles in the separator coating and the porosity of the separator, the problem of lithium deposition at the corner is effectively solved, and the lithium deposition level is 0. Moreover, the capacity retention rate of the secondary battery can reach more than 93% after 100 cycles, and the comprehensive performance is excellent. In contrast, due to improper regulation and setting of the polymer particle gradient and the membrane porosity in the diaphragm coating, the products of Comparative Examples 1 to 3 and Comparative Examples 8 to 9 not only have a large size change range, but also are difficult to balance the electrolyte absorption and retention rate, and are also difficult to solve the problem of lithium deposition at the corners during battery cycling, resulting in poor cycle performance; in the products of Comparative Examples 4 to 5, the polymer particle gradient is improperly set, and the particle size setting of the two particles does not meet the requirements, and the prepared products are also unable to optimize the lithium deposition problem at the corners of the battery; when setting the products of Comparative Examples 6 to 7, since there are no particles with two different glass transition temperature gradients, although the particle size gradient and the membrane porosity are similar to the products of the embodiments of the present application, they cannot balance the effective buffering effect and the supporting effect of the diaphragm coating, and the lithium deposition problem of the secondary battery is still serious. Although the product of Comparative Example 10 is set with polymer particles with obvious glass transition temperature gradients, it does not meet the gradient limitation of the present application, and it is obvious that this product cannot balance the guarantee of electrochemical cycle performance and the control of lithium deposition of the electrode.

[0119] It can be seen from Examples 1 to 15 that the size gradient setting of the polymer particles directly affects the porosity of the diaphragm, and the porosity of the diaphragm is also related to the surface density of the coating itself, the coating coating method and even the thickness of the coating. When the gradient setting D is preferably D=2-3.5, especially by synchronously regulating the size gradient setting of the polymer particles and maintaining the porosity of the overall diaphragm within D / H=0.04-0.085, applying it to wound secondary batteries can achieve better inhibition of lithium precipitation and better cycle stability.

[0120] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of this article rather than to limit the scope of protection of this article. Although the present application is described in detail with reference to the preferred embodiments, ordinary technicians in this field should understand that the technical solution of this article can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of this article.

Claims

1. A diaphragm, characterized in that: including organic substrates and coatings; The coating comprises polymer particles 1 and polymer particles 2; The average particle size a of the polymer particles 1 is ≥4 μm, and the average particle size b of the polymer particles 2 is ≤3 μm; The test spectrum obtained by performing DSC analysis test on the polymer particles 1 and 2 after mixing includes a first exothermic peak and a second exothermic peak, the peak position of the first exothermic peak is ≤85°C, and the peak position of the second exothermic peak is ≥150°C; The diaphragm satisfies D / H=0.03-0.12, wherein D=a / b, and H% is the porosity of the diaphragm.

2. The diaphragm according to claim 1, characterized in that: The a=4-6 μm.

3. The diaphragm according to claim 1, characterized in that: The b=1-3 μm.

4. The diaphragm according to claim 1, characterized in that: The D=1-4.

5.

5. The diaphragm according to claim 1, characterized in that: The H=35-50%.

6. The diaphragm according to claim 1, characterized in that: The surface density of the coating is 0.1 to 0.5 g / m 2 , and / or, the coating has a thickness of 4 to 6.5 μm.

7. The diaphragm according to claim 1, characterized in that: The thickness of the separator is 8 to 11.5 μm.

8. The diaphragm according to claim 1, characterized in that: The polymer particles 1 include at least one of polymethyl methacrylate particles and polyethylene particles, and / or the polymer particles 2 include at least one of polyetheretherketone particles, polytetrafluoroethylene particles, and polystyrene particles.

9. A secondary battery, characterized in that: The invention comprises the diaphragm according to any one of claims 1 to 8.

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

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