Polypropylene porous membrane and preparation method thereof, battery diaphragm and secondary battery

Abstract

The invention relates to the technical field of battery diaphragms, in particular to a polypropylene porous membrane and a preparation method thereof, a battery diaphragm and a secondary battery, the polypropylene porous membrane comprises a middle layer and surface layers arranged on the two surfaces of the middle layer, the surface layers are made of polypropylene A, the middle layer is made of polypropylene B, the melt index of the polypropylene A is smaller than that of the polypropylene B. The melt index of the raw material for preparing the surface layer is smaller than that of the raw material of the middle layer, so that the molecular fluidity of the surface layer in contact with the pole piece is lower, and the crystallization of the surface layer is insufficient during annealing at the same temperature, so that the crystallinity is lower, and the cohesiveness of the surface layer can be improved; therefore, the surface bonding capacity of the polypropylene porous membrane is improved, the polypropylene porous membrane has a good bonding effect with a pole piece, the problem of short circuit caused by poor bonding effect of a diaphragm and the pole piece of a battery can be solved, and the safety performance of the battery in use can be improved.

Description

Technical Field

[0001] The present application relates to the technical field of battery separators, and more specifically to a polypropylene porous membrane and a preparation method thereof, a battery separator, and a secondary battery. Background Art

[0002] As one of the important components of lithium-ion batteries, the diaphragm should not only have good chemical stability and safety, but also good mechanical strength. The performance of the diaphragm has an important impact on the cycle life of lithium-ion batteries. There are two main production processes for lithium-ion battery diaphragms: wet process and dry process. Among them, the dry uniaxial stretching technology is to melt and extrude the polymer melt under certain conditions to form a highly oriented structure in the extrusion direction, and then form a porous diaphragm through annealing and stretching. The dry uniaxial stretching film usually uses PP raw material, has good heat shrinkage and temperature resistance, and can be used directly without adding ceramic coating. It has certain performance and cost advantages in the application of lithium-ion batteries.

[0003] The polypropylene porous membrane itself has low adhesion to the positive and negative electrodes, which can easily cause battery short circuit problems. In order to improve the bonding effect between the dry-process diaphragm and the electrode, a polymer bonding layer, such as PVDF, PMMA and other materials, is usually coated on the surface of the diaphragm to improve the bonding effect. The method of using an adhesive coating has a certain effect on improving the interface, but coating the diaphragm is to add a coating process and coating materials to the original base film preparation process, and the corresponding cost is also increased.

[0004] Therefore, how to improve the adhesion between the separator and the positive and negative electrodes is a technical problem that needs to be solved urgently in this field. Summary of the invention

[0005] The present application provides a polypropylene porous membrane and a preparation method thereof, a battery separator, and a secondary battery, which can improve the adhesion between the separator and the positive and negative electrode sheets, thereby reducing the short circuit problem.

[0006] The present application provides a polypropylene porous membrane, comprising an intermediate layer and surface layers arranged on two surfaces of the intermediate layer, the raw material of the surface layer comprises polypropylene A, the raw material of the intermediate layer comprises polypropylene B, and the melting index of the polypropylene A is smaller than the melting index of the polypropylene B.

[0007] In an optional embodiment, the melt index of the polypropylene A is in the range of 0.3 g / 10 min-2 g / 10 min, and the weight average molecular weight is 500,000-600,000.

[0008] In an optional embodiment, the melt index of the polypropylene B satisfies a value >2g / 10min, and the weight average molecular weight is 300,000-500,000.

[0009] In an optional embodiment, the polypropylene porous membrane satisfies at least one of the conditions (1)-(7):

[0010] (1) In the polypropylene porous membrane, the mass ratio of the polypropylene A to the polypropylene B is 1:1-1:4;

[0011] (2) In the polypropylene porous membrane, the thickness of the surface layer accounts for 20% to 50% of the total thickness;

[0012] (3) The porosity of the polypropylene porous membrane is 35%-60%;

[0013] (4) The average pore size of the polypropylene porous membrane is 20nm-40nm;

[0014] (5) The air permeability of the polypropylene porous membrane is 50s / 100mL-300s / 100mL;

[0015] (6) The thickness of the polypropylene porous membrane is 10 μm-25 μm;

[0016] (7) The puncture strength of the polypropylene porous membrane is ≥250g.

[0017] In an optional embodiment, the surface layer includes a first surface layer and a second surface layer, the first surface layer and the second surface layer are respectively arranged on two surfaces of the intermediate layer, the first surface layer includes polypropylene A1, the second surface layer includes polypropylene A2, and the melting indexes of the polypropylene A1 and the polypropylene A2 are different.

[0018] In an optional embodiment, the melt index of the polypropylene A1 is in the range of 0.3 g / 10 min to 0.8 g / 10 min, and the melt index of the polypropylene A2 is in the range of 0.8 g / 10 min to 1.5 g / 10 min.

[0019] The present application provides a method for preparing a polypropylene porous membrane, which comprises the following steps:

[0020] The polypropylene A for the surface layer and the polypropylene B for the middle layer are mixed according to a ratio, and then co-extruded and cooled to form a three-layer cast film in which two surface layers are respectively attached to two surfaces of the middle layer;

[0021] annealing the cast film;

[0022] The cast film after annealing is sequentially stretched and heat-set to obtain a polypropylene porous film.

[0023] In an optional embodiment, the method for preparing the polypropylene porous membrane satisfies at least one of the conditions (1) to (4):

[0024] (1) During the cooling of the co-extrusion process, the extrusion temperature is 180°C-250°C, and the cooling temperature is 60°C-100°C;

[0025] (2) During annealing, the annealing temperature is 125°C-155°C and the annealing time is 6h-12h;

[0026] (3) The stretching includes cold drawing and hot drawing, the cold drawing temperature is 80°C-100°C, and the cold drawing ratio is 1.1-1.5; the hot drawing temperature is 135°C-153°C, and the hot drawing ratio is 1.6-2.4;

[0027] (4) The temperature of the heat setting is 150°C-165°C.

[0028] The present application provides a battery separator, comprising the polypropylene porous membrane as described above, or a polypropylene porous membrane prepared by the method for preparing the polypropylene porous membrane as described above.

[0029] The present application provides a secondary battery, comprising the polypropylene porous membrane as described above, or a polypropylene porous membrane prepared by the method for preparing a polypropylene porous membrane as described above, or a battery separator as described above.

[0030] According to the polypropylene porous membrane in the above embodiment, it includes an intermediate layer and surface layers arranged on the two surfaces of the intermediate layer. Since the melting index of the raw material for preparing the surface layer is lower than the melting index of the raw material for the intermediate layer, the molecular fluidity of the surface layer in contact with the electrode is lower. When annealed at the same temperature, the crystallization of the surface layer is insufficient, so the crystallinity is low, which can improve the adhesion of the surface layer, thereby improving the bonding ability of the surface of the polypropylene porous membrane, so that it has a better bonding effect with the electrode, and can solve the short circuit problem of the battery caused by poor bonding between the diaphragm and the electrode, thereby improving the safety performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Schematic diagram of the structure of a polypropylene porous membrane in an embodiment.

[0032] Among them: 100, surface layer; 110, first surface layer; 120, second surface layer; 200, middle layer. DETAILED DESCRIPTION

[0033] The present application is further described in detail below by specific embodiments in conjunction with the accompanying drawings. Wherein similar elements in different embodiments adopt associated similar element numbers. In the following embodiments, many detailed descriptions are intended to enable the present application to be better understood. However, those skilled in the art can easily recognize that some of the features can be omitted in different situations, or can be replaced by other elements, materials, and methods. In some cases, some operations related to the present application are not shown or described in the specification, in order to avoid the core part of the present application being overwhelmed by too much description, and for those skilled in the art, it is not necessary to describe these related operations in detail, and they can fully understand the related operations based on the description in the specification and the general technical knowledge in the art.

[0034] In addition, the features, operations or characteristics described in the specification can be combined in any appropriate manner to form various implementations, and the operation steps involved in each embodiment can also be replaced or adjusted in a sequence in a manner that is obvious to those skilled in the art. Therefore, the specification and drawings are only for the purpose of clearly describing a certain embodiment and do not mean that the composition and / or sequence are necessary.

[0035] The serial numbers of the components in this document, such as "first", "second", etc., are only used to distinguish the objects described and do not have any order or technical meaning. The "connection" and "coupling" mentioned in this application, unless otherwise specified, include direct and indirect connections (couplings).

[0036] The "diaphragm" referred to in this application is an important component of the secondary battery cell. The diaphragm is a thin film used to separate the positive and negative electrodes during the electrolysis reaction to prevent the loss of energy in the direct reaction. The performance of the diaphragm determines the interface structure and internal resistance of the battery, and directly affects the battery's capacity, cycle and safety performance. A diaphragm with excellent performance plays an important role in improving the overall performance of the battery.

[0037] Secondary batteries refer to batteries that can be used again by recharging the active materials after the battery is discharged. This type of battery usually uses the reversibility of chemical reactions, that is, after a chemical reaction is converted into electrical energy, the chemical system can be repaired with electrical energy, and then converted into electrical energy by chemical reaction again. Among them, the more common secondary batteries include but are not limited to nickel-metal hydride batteries, nickel-cadmium batteries, lead-acid (or lead storage) batteries, lithium-ion batteries, polymer lithium-ion batteries, sodium-ion batteries, etc.

[0038] See also Figure 1The present application discloses a polypropylene porous membrane, including an intermediate layer 200 and a surface layer 100 arranged on two surfaces of the intermediate layer 200, the raw material of the surface layer 100 includes polypropylene A, the raw material of the intermediate layer 200 includes polypropylene B, and the melting index of polypropylene A is less than the melting index of polypropylene B.

[0039] Since the melting index of the raw material for preparing the surface layer 100 is lower than the melting index of the raw material for the middle layer 200, the molecular fluidity of the surface layer 100 in contact with the electrode is lower. When annealed at the same temperature, the crystallization of the surface layer 100 is insufficient, and thus the degree of crystallinity is low. This can improve the adhesion of the surface layer 100, thereby improving the bonding ability of the surface of the polypropylene porous membrane, so that it has a better bonding effect with the electrode, and can solve the short circuit problem of the battery caused by poor bonding between the diaphragm and the electrode.

[0040] In some embodiments, the melt index of polypropylene A ranges from 0.3 g / 10 min to 2 g / 10 min. For example, the melt index of polypropylene A can range from 0.3 g / 10 min, 0.5 g / 10 min, 0.8 g / 10 min, 1.0 g / 10 min, 1.2 g / 10 min, 0.3 g / 10 min, 1.5 g / 10 min, 1.8 g / 10 min or 2 g / 10 min. Optionally, the melt index of polypropylene A can range from 0.3 g / 10 min to 1.5 g / 10 min.

[0041] In some embodiments, the weight average molecular weight of polypropylene A is 500,000-600,000. For example, the weight average molecular weight of polypropylene A can be 500,000, 520,000, 550,000, 580,000 or 600,000. When the molecular weight of polypropylene A is within this range, the adhesion of the surface layer 100 can be improved by properly controlling the annealing process.

[0042] In some embodiments, the melt index of polypropylene B satisfies a value > 2g / 10min, for example, the melt index of polypropylene B may be 2.1g / 10min, 2.5g / 10min, 2.8g / 10min, 3g / 10min, 3.5g / 10min, 4g / 10min, 5g / 10min or 6g / 10min. Optionally, the melt index of polypropylene B may range from 2.5g / 10min to 4g / 10min. Polypropylene B has a larger melt index, better melt fluidity, and better processing and molding properties.

[0043] In some embodiments, the weight average molecular weight of polypropylene B is 300,000-500,000. For example, the weight average molecular weight of polypropylene B can be 300,000, 320,000, 350,000, 400,000, 450,000 or 500,000.

[0044] In some embodiments, in the surface layer 100, the mass percentage of polypropylene A is ≥95wt%, that is, the raw material of the surface layer 100 is mainly polypropylene A, for example, the mass percentage of polypropylene A can be 95wt%, 96wt%, 97wt% or 98wt%. Similarly, in the middle layer 200, the raw material of the middle layer 200 is mainly polypropylene B, and the mass percentage of polypropylene B is ≥95wt%, for example, the mass percentage of polypropylene B can be 95wt%, 96wt%, 97wt% or 98wt%. It is understood that the surface layer 100 and the middle layer 200 may also include functional additives or polypropylenes with different melt indexes as needed, and their mass percentages should be ≤5wt%.

[0045] In some embodiments, in the polypropylene porous membrane, the mass ratio of polypropylene A to polypropylene B is 1:1-1:4. For example, the mass ratio of polypropylene A to polypropylene B can be 1:1, 1:2, 1:3 or 1:4. By reasonably controlling the mass ratio of polypropylene A to polypropylene B, the raw materials of the intermediate layer 200 and the surface layer 100 have better processing properties, and the performance of the formed polypropylene porous membrane can be improved.

[0046] In some embodiments, in the polypropylene porous membrane, the thickness of the surface layer 100 accounts for 20%-50% of the total thickness. For example, the thickness of the surface layer 100 accounts for 20%, 25%, 30%, 35%, 40%, 45% or 50% of the total thickness. The intermediate layer 200 made of polypropylene B is the main structure in the polypropylene porous membrane, and the surface layer 100 made of polypropylene A with a relatively small melt index is arranged on the surface thereof, which can improve the adhesion of the surface layer.

[0047] In some embodiments, the porosity of the polypropylene porous membrane is 35%-60%. For example, the porosity of the polypropylene porous membrane is 35%, 40%, 45%, 50%, 55% or 60%. Optionally, the porosity of the polypropylene porous membrane is 40%-55%. The transmission of lithium ions can be effectively guaranteed to ensure the normal operation of the separator.

[0048] In some embodiments, the average pore size of the polypropylene porous membrane is 20nm-40nm. For example, the average pore size of the polypropylene porous membrane is 20nm, 25nm, 30nm, 35nm or 40nm. When the pore size is too large, the diaphragm is easily punctured and short-circuited. When the pore size is too small, the liquid absorption and retention performance of the diaphragm is poor, which affects the transmission effect of lithium ions. The average pore size of the polypropylene porous membrane is controlled within the above range. While ensuring the transmission of lithium ions, the diaphragm prepared therefrom has good mechanical strength, which can reduce the short-circuit problem in the battery.

[0049] It should be noted that, since the pores of the polypropylene porous membrane are formed by stretching, the pores may be slit-shaped (such as elliptical) or cylindrical, and the size of each pore diameter may vary.

[0050] In some embodiments, the air permeability of the polypropylene porous membrane is 50s / 100mL-300s / 100mL. For example, the air permeability of the polypropylene porous membrane is 50s / 100mL, 60s / 100mL, 70s / 100mL, 80s / 100mL, 90s / 100mL, 100s / 100mL, 200s / 100mL or 300s / 100mL. Optionally, the air permeability of the polypropylene porous membrane is 150s / 100mL-250s / 100mL. When the air permeability is controlled within the above range, the polypropylene porous membrane has both good lithium ion transmission capacity and good mechanical strength, and has better overall performance.

[0051] In some embodiments, the thickness of the polypropylene porous membrane is 10 μm-25 μm. For example, the thickness of the polypropylene porous membrane is 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, 22 μm or 25 μm. The smaller the thickness, the smaller the mechanical strength of the membrane may be, and the membrane is prone to rupture. Controlling the thickness of the polypropylene porous membrane within the above range can effectively ensure the mechanical strength of the prepared membrane.

[0052] In some embodiments, the puncture strength of the polypropylene porous membrane is ≥ 250 g. For example, the puncture strength of the polypropylene porous membrane can be 250 g, 260 g, 300 g, 400 g or 500 g. Effective control of the puncture strength can avoid the problem of short circuit caused by puncture of the diaphragm.

[0053] In some embodiments, the surface layer 100 includes a first surface layer 110 and a second surface layer 120 , which are respectively disposed on two surfaces of the intermediate layer 200 , and the first surface layer 110 and the second surface layer 120 may be mainly made of the same polypropylene A.

[0054] In other embodiments, since the two surfaces of the diaphragm made of polypropylene porous film are respectively bonded to the positive and negative electrodes of the battery, and the positive and negative electrodes require different bonding forces due to their different materials, in order to effectively ensure that the bonding forces between the positive and negative electrodes and the diaphragm are consistent and avoid short circuit problems caused by poor bonding effects, the first surface layer 110 and the second surface layer 120 can be mainly made of different polypropylenes A. For example, the first surface layer 110 includes polypropylene A1, and the second surface layer 120 includes polypropylene A2. The melting indexes of polypropylene A1 and polypropylene A2 are different. The bonding force with the electrode sheet can be improved by adjusting the melting index, so that the two sides of the polypropylene porous film have similar bonding forces to the positive and negative electrode sheets, respectively, thereby improving the stability within the battery.

[0055] In some embodiments, the melt index of polypropylene A1 ranges from 0.3 g / 10 min to 0.8 g / 10 min. For example, the melt index of polypropylene A1 may be 0.3 g / 10 min, 0.4 g / 10 min, 0.5 g / 10 min, 0.6 g / 10 min, 0.7 g / 10 min, or 0.8 g / 10 min.

[0056] In some embodiments, the melt index of polypropylene A2 ranges from 0.8 g / 10 min to 1.5 g / 10 min. For example, the melt index of polypropylene A2 can be 0.8 g / 10 min, 0.9 g / 10 min, 1.0 g / 10 min, 1.1 g / 10 min, 1.2 g / 10 min, 1.3 g / 10 min, 1.4 g / 10 min, or 1.5 g / 10 min.

[0057] The present application also provides a method for preparing a polypropylene porous membrane, which mainly prepares a three-layer polypropylene porous membrane through the steps of co-extrusion cooling, annealing heat treatment, stretching and heat setting, and the polypropylene porous membrane can be used as a battery separator. Of course, the above method can also be used to prepare five-layer, seven-layer and other structures according to needs.

[0058] The co-extrusion cooling process specifically involves mixing the polypropylene A for preparing the surface layer 100 and the polypropylene B for preparing the middle layer 200 according to a ratio, and then co-extruding and cooling the mixture to form a three-layer structure cast film in which the two surface layers 100 are respectively attached to the two surfaces of the middle layer 200. During the co-extrusion cooling process, the polypropylene A and the polypropylene B are melted and then extruded in three cavities, the polypropylene B is arranged in the middle cavity, and the polypropylene A is arranged in the cavities on both sides of the middle cavity, and then cooled by a casting cooling roller to obtain the cast film.

[0059] Specifically, during extrusion, the extrusion temperature is 180°C-250°C, for example, the extrusion temperature may be 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C or 250°C. Since different layers of materials are placed in different cavities during extrusion, the extrusion temperature of each layer may be different depending on the characteristics of the material. The cooling temperature during cooling at the casting cooling roller is 60°C-100°C. For example, the cooling temperature may be 60°C, 70°C, 80°C, 90°C or 100°C.

[0060] When the cast film is annealed, the annealing temperature is 125°C-155°C, for example, the annealing temperature may be 125°C, 130°C, 135°C, 140°C, 145°C, 150°C or 155°C. Preferably, the annealing temperature is 130°C-140°C. The annealing time is 6h-12h, for example, the annealing time may be 6h, 6.5h, 7h, 8h, 9h, 10h, 11h or 12h. Controlling the annealing temperature to 130°C-140°C and the annealing time to 6h-12h can further reduce the crystallinity of the surface layer 100, thereby improving the adhesion between the polypropylene porous film and the pole piece.

[0061] When stretching and heat setting, stretching includes cold drawing and hot drawing.

[0062] The cold drawing temperature is 80-100° C., for example, the cold drawing temperature may be 80, 85, 90, 95 or 80° C. The cold drawing ratio is 1.1-1.5, for example, the cold drawing ratio may be 1.1, 1.2, 1.3, 1.4 or 1.5.

[0063] The temperature of hot drawing is 135°C-153°C, for example, the temperature of hot drawing can be 135°C, 138°C, 140°C, 145°C, 150°C or 153°C, and the hot drawing ratio is 1.6-2.4, for example, the cold drawing ratio can be 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3 or 2.4.

[0064] The temperature of heat setting is 150°C-165°C, for example, the temperature of heat drawing may be 150°C, 155°C, 157°C, 160°C, 162°C or 165°C.

[0065] The present application provides a battery separator, which can be used in a battery (such as a lithium-ion battery), arranged between the positive electrode and the negative electrode thereof, used to block the positive and negative electrodes, and capable of transmitting ions (such as lithium ions) between the positive and negative electrodes, including the above-mentioned polypropylene porous membrane, or a polypropylene porous membrane prepared by the method for preparing a polypropylene porous membrane. The battery separator has a good bonding effect with the positive and negative electrodes, avoiding the problem of internal short circuit of the battery caused by weak bonding.

[0066] The present application provides a secondary battery, comprising the above-mentioned polypropylene porous membrane, or the polypropylene porous membrane prepared by the above-mentioned preparation method, or the above-mentioned battery separator. Since the polypropylene porous membrane has a good bonding effect with the positive and negative electrodes, the problem of internal short circuit of the battery caused by weak bonding is avoided. Specifically, the secondary battery includes a lithium ion battery, a sodium ion battery, etc.

[0067] The technical solution of the present application is further described below through specific embodiments.

[0068] Embodiment 1

[0069] A three-layer polypropylene porous membrane was prepared, wherein the raw material of the surface layer 100 was polypropylene A with a melt index of 0.8 g / 10 min and a weight average molecular weight of 550,000; the raw material of the middle layer 200 was polypropylene B with a melt index of 2.5 g / 10 min and a weight average molecular weight of 400,000; the mass ratio of polypropylene A to polypropylene B was 1:1.

[0070] The preparation method of the polypropylene porous membrane comprises the following steps:

[0071] Step 1: Put polypropylene A, polypropylene B, and polypropylene A into a co-extrusion device in a mass ratio of 1:2:1, and form an A / B / A structure by co-extrusion, wherein the A layer is extruded at 230°C and the B layer is extruded at 220°C; then cool through an 80°C casting roller to form a cast film, which is rolled up for use.

[0072] Step 2: Anneal the cast film at 135°C for 12 hours, and then stretch it in the MD (machine direction, parallel to the direction of film travel, also called longitudinal) direction, wherein the cold drawing temperature is 85°C, the stretching ratio is 1.2, the hot drawing temperature is 145°C, the stretching ratio is 2.5, and after stretching, heat set at 155°C to finally obtain a polypropylene porous film with a thickness of 14 μm and a porosity of 43%.

[0073] The differences between Embodiments 2 to 7 and Embodiment 1 are shown in Table 1.

[0074] Comparative Example 1

[0075] The difference between Comparative Example 1 and Example 1 is that the raw materials of the surface layer 100 and the middle layer 200 are both polypropylene B, with a melt index of 2.5 g / 10 min and a weight average molecular weight of 400,000.

[0076] Comparative Example 2

[0077] The difference between Comparative Example 2 and Example 1 is that the raw material of the surface layer 100 is polypropylene B, and the raw material of the middle layer 200 is polypropylene A.

[0078] Comparative Example 3

[0079] The difference between comparative example 3 and embodiment 3 is that the raw materials of the intermediate layer 200 and the raw materials of the surface layer 100 are interchanged, that is, the surface layer 100 of comparative example 3 is prepared with the intermediate layer 200 of embodiment 3, and the intermediate layer 200 of comparative example 3 is prepared with the surface layer 100 of embodiment 3.

[0080] Comparative Example 4

[0081] The difference between Comparative Example 4 and Example 1 is that the surface layer 100 and the middle layer 200 are both prepared from a blend of polypropylene A and polypropylene B.

[0082] The method for preparing the three-layer polypropylene porous membrane in this comparative example comprises the following steps:

[0083] Step 1: Mix polypropylene A and polypropylene B in a mass ratio of 5:1 to prepare a mixture, put the mixture into a co-extrusion device, form a three-layer structure through co-extrusion, cool the cast film with a cast cooling roller, and roll it up for use.

[0084] Step 2: The cast film is annealed at 135°C for 12 hours, and then stretched in the MD direction, wherein the cold drawing temperature is 85°C, the stretching ratio is 1.2, the hot drawing temperature is 145°C, and the stretching ratio is 2.5. After the stretching is completed, it is heat-set at 155°C to obtain a polypropylene porous membrane.

[0085] Comparative Example 5

[0086] The difference between Comparative Example 5 and Comparative Example 4 is that the mass ratio of polypropylene A and polypropylene B in preparing the blend is different. In this comparative example, the mass ratio of polypropylene A and polypropylene B is 0.5:1.

[0087] The compositions and preparation processes of the polypropylene porous membranes of Examples 1 to 7 and Comparative Examples 1 to 5 are shown in Table 1 below.

[0088] Table 1

[0089]

[0090]

[0091] The polypropylene porous membranes of the above embodiments and comparative examples were tested. The test method is as follows:

[0092] Thickness test: Use a Mahr thickness gauge for measurement, refer to GB / T 18287-2013 "Lithium-ion Battery", the specific test method is: use the thickness gauge to place its probe on the sample surface and record the measurement results, repeat the above steps, measure the thickness of the polypropylene porous membrane at multiple positions, such as 3 or 5, etc., take the average of multiple data as the thickness of the sample to obtain more accurate data.

[0093] Porosity test: Porosity is defined as the ratio of the volume of micropores in the polypropylene porous membrane to the total volume of the polypropylene porous membrane. It is calculated using the mass method. The calculation formula is as follows:

[0094] P = (1-M / ρV) × 100%, P is the porosity, M is the sample mass, V is the sample volume, and ρ is the sample density (raw material density).

[0095] Air permeability test: The air permeability is tested by Gurley air permeability instrument, and the reference standards are ISO5636.5, ASTMD726, TAPPI T460, GB / T 36363-2018. The specific test method is: fix the sample, apply air pressure on one side of the sample, measure the air pressure drop and the time used, and compare the time taken for the pressure to drop from the initial pressure to the final pressure to know the air permeability.

[0096] Puncture strength test: refer to GB / T 21302-2007 General Rules for Composite Films and Bags for Packaging. Take 5 circular samples with a diameter greater than 40 mm along the TD direction of the film, clamp them in the puncture fixture, and move the flat-head needle downward at a speed of 100 mm / min. The maximum force of puncturing the sample is the puncture strength. The average value of the 5 points is the puncture strength.

[0097] Positive electrode bonding strength test: the sample is cut into 20mm×200mm strips, and the positive electrode sheet is cut into 20mm×80mm as the base plate. After hot pressing for 1min under a pressure of 3MPa, the test sample is made. This example tests the bonding strength between the sample and the lithium iron phosphate positive electrode sheet at a hot pressing temperature of 100℃. Then the 180° direction peel strength test method is used. The test is carried out on an electronic tensile machine at a speed of 100mm / min to separate the strip and the positive electrode sheet. After the experiment, the software automatically generates peel strength data, that is, the electrode sheet bonding strength data. In this example, the average value of 5 parallel test samples is taken as the electrode sheet bonding strength of the sample.

[0098] Negative electrode bonding strength test: the sample is cut into 20mm×200mm strips, and the negative electrode sheet is cut into 20mm×80mm as the base plate. After hot pressing for 1min at a pressure of 3MPa, the test sample is made. This example tests the bonding strength between the sample and the artificial graphite negative electrode sheet at a hot pressing temperature of 105℃. Then the 180° direction peel strength test method is used. The test is carried out on an electronic tensile machine at a speed of 100mm / min to separate the strip and the negative electrode sheet. After the experiment, the software automatically generates peel strength data, that is, the electrode sheet bonding strength data. In this example, the average value of 5 parallel test samples is taken as the electrode sheet bonding strength of the sample.

[0099] The test results are shown in Table 2 below.

[0100] Table 2

[0101]

[0102]

[0103] As can be seen from the data in Table 2, the melting index of the raw material of the surface layer 100 is less than the raw material of the intermediate layer 200, which can improve the bonding strength of the polypropylene porous film to the positive and negative electrodes, such as Example 1 to Example 7, Comparative Example 1 to Comparative Example 3, Comparative Example 1 is made of the same raw materials into a three-layer structure, the melting index of the surface layer 100 in Comparative Example 2 and Comparative Example 3 is greater than the melting index of the intermediate layer 200, and its positive electrode bonding strength and negative electrode bonding strength are both less than the bonding strength values ​​of Example 1 to Example 7, such as Comparative Example 4 and Comparative Example 5 are to blend polypropylene A and polypropylene B into a three-layer structure, and its positive and negative electrode bonding strengths are much smaller than Example 1 to Example 7, although it has a certain adhesive force, it is difficult to meet the bonding requirements in the battery. With the increase of the melting index of the intermediate layer 200, the air permeability of the polypropylene porous film can be increased, so that it has a good ion passing effect, for example, Example 1 is compared with Example 3. Reasonable control of the mass ratio of the surface layer 100 and the intermediate layer 200 can regulate the polypropylene porous film to have good bonding properties. For example, in the comparison of Example 1, Example 4 and Example 5, the mass of the intermediate layer 200 gradually increases in Example 1, Example 4 and Example 5, but the bonding strength of the positive and negative electrodes is slightly deteriorated, and the mechanical strength is relatively deteriorated. With the increase of the melt index of the surface layer 100, its fluidity increases, and the mechanical strength is relatively deteriorated, such as the comparison of Example 1 and Example 2. The raw materials of the two surface layers 100 can be different, and they can be mainly made of polypropylene A with different melt indexes, for example, in Example 7, two surface layers 100 are made of polypropylene A1 and polypropylene A2, respectively, and the bonding force of the positive and negative electrodes can be regulated to be consistent. Comparing with Example 1 and Example 6, with the extension of annealing time and the increase of annealing temperature, the crystallinity of the polypropylene porous film increases, so that its bonding properties deteriorate.

[0104] The above specific examples are used to illustrate the present application, which is only used to help understand the present application and is not intended to limit the present application. For technicians in the technical field to which the present application belongs, they can also make some simple deductions, deformations or substitutions based on the ideas of the present application.

Claims

1. A polypropylene porous membrane, characterized in that: It comprises an intermediate layer and surface layers arranged on two surfaces of the intermediate layer. The raw material of the surface layer comprises polypropylene A, the raw material of the intermediate layer comprises polypropylene B, and the melting index of the polypropylene A is smaller than the melting index of the polypropylene B.

2. The polypropylene porous membrane according to claim 1, characterized in that The melt index of the polypropylene A ranges from 0.3 g / 10 min to 2 g / 10 min, and the weight average molecular weight ranges from 500,000 to 600,000.

3. The polypropylene porous membrane according to claim 1, characterized in that The melt index of the polypropylene B satisfies a value >2g / 10min, and the weight average molecular weight is 300,000-500,000.

4. The polypropylene porous membrane according to claim 1, characterized in that The polypropylene porous membrane satisfies at least one of the conditions (1) to (7): (1) In the polypropylene porous membrane, the mass ratio of the polypropylene A to the polypropylene B is 1:1-1:4; (2) In the polypropylene porous membrane, the thickness of the surface layer accounts for 20% to 50% of the total thickness; (3) The porosity of the polypropylene porous membrane is 35%-60%; (4) The average pore size of the polypropylene porous membrane is 20nm-40nm; (5) The air permeability of the polypropylene porous membrane is 50s / 100mL-300s / 100mL; (6) The thickness of the polypropylene porous membrane is 10 μm-25 μm; (7) The puncture strength of the polypropylene porous membrane is ≥250g.

5. The polypropylene porous membrane according to any one of claims 1 to 4, characterized in that: The surface layer includes a first surface layer and a second surface layer, the first surface layer and the second surface layer are respectively arranged on two surfaces of the middle layer, the first surface layer includes polypropylene A1, the second surface layer includes polypropylene A2, and the polypropylene A1 and the polypropylene A2 have different melting indexes.

6. The polypropylene porous membrane according to claim 5, characterized in that The melting index of the polypropylene A1 ranges from 0.3 g / 10 min to 0.8 g / 10 min, and the melting index of the polypropylene A2 ranges from 0.8 g / 10 min to 1.5 g / 10 min.

7. The method for preparing a polypropylene porous membrane according to any one of claims 1 to 6, characterized in that: The steps include: The polypropylene A for the surface layer and the polypropylene B for the middle layer are mixed according to a ratio, and then co-extruded and cooled to form a three-layer cast film in which two surface layers are respectively attached to two surfaces of the middle layer; annealing the cast film; The cast film after annealing is sequentially stretched and heat-set to obtain a polypropylene porous film.

8. The method for preparing a polypropylene porous membrane according to claim 7, characterized in that: The method for preparing the polypropylene porous membrane satisfies at least one of the conditions (1) to (4): (1) During the cooling of the co-extrusion process, the extrusion temperature is 180°C-250°C, and the cooling temperature is 60°C-100°C; (2) During annealing, the annealing temperature is 125°C-155°C and the annealing time is 6h-12h; (3) The stretching includes cold drawing and hot drawing, the cold drawing temperature is 80°C-100°C, and the cold drawing ratio is 1.1-1.5; the hot drawing temperature is 135°C-153°C, and the hot drawing ratio is 1.6-2.4; (4) The temperature of the heat setting is 150°C-165°C.

9. A battery separator, characterized in that: The invention comprises the polypropylene porous membrane according to any one of claims 1 to 6, or a polypropylene porous membrane prepared by the method for preparing a polypropylene porous membrane according to claim 7 or 8.

10. A secondary battery, characterized in that: It comprises the polypropylene porous membrane according to any one of claims 1 to 6, or a polypropylene porous membrane prepared by the method for preparing a polypropylene porous membrane according to claim 7 or 8, or a battery separator according to claim 9.

Images