Battery diaphragm, preparation method thereof and secondary battery

Abstract

The invention relates to the technical field of battery materials, in particular to a battery diaphragm, a preparation method thereof and a secondary battery. The battery diaphragm comprises a base membrane, and a first coating and a second coating arranged on the surface of the base membrane, the first coating comprises a first granular binding material and an inorganic heat-resistant material; the second coating comprises a second granular binding material; the softening points T1 and T2 of the first and second particulate binder materials satisfy 25 DEG C < = T1 < = 45 DEG C, 45 DEG C < = T2 < = 65 DEG C, and the D50 particle sizes D1 and D2 of the first and second particulate binder materials satisfy 2.5 [mu] m < = D1 < = 6.5 [mu] m, 0.1 [mu] m < = D2 < = 0.5 [mu] m; and 0.15 < = (T2-T1) * D2 / D1 < = 8. The battery diaphragm disclosed by the invention has good low-temperature bonding performance, and can meet the bonding requirements of the diaphragm on a negative electrode and a positive electrode in a thermal composite lamination process at the temperature of 65 DEG C or below, so that the energy consumption is reduced; the process window can be widened, and the product stability is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of battery materials, and in particular to a battery separator and its preparation method, and a secondary battery. Background Art

[0002] The lithium battery separator is one of the four main materials of lithium ion batteries, which plays a role in isolating the positive and negative electrodes to prevent short circuit caused by the contact of the positive and negative electrodes. Currently, polyethylene (PE) and polypropylene (PP) are mainly used as the base films for large-scale commercial separator materials. To improve the high-temperature resistance performance, ceramic or high-temperature resistant organic materials are usually coated on the surface of the base film; to increase the hardness of the battery core and improve the interfacial performance between the separator and the electrode sheet, a bonding coating is added on the surface of the base film, and the commonly used coating material is mainly polyvinylidene fluoride (PVDF).

[0003] The assembly process of lithium batteries mainly includes two methods: winding and stacking. These two methods usually assemble the coated separator with the positive and negative electrodes of the battery into a battery core, and then fix it through hot pressing assembly. To improve production efficiency, a thermal composite stacking process has been developed based on the traditional stacking process. In this process, the separator and the negative electrode are bonded by roller pressing with a hot roller, and the separator and the positive electrode are bonded by flat hot pressing. No matter which process is adopted, a certain temperature is required to achieve the pressing process. Since the battery core has a certain thickness, it takes a long time to achieve sufficient heat conduction to complete the hot pressing process, resulting in the hot press needing to maintain a high temperature state for a long time, thus causing huge energy consumption and high production costs, which is not conducive to energy conservation and cost reduction. Therefore, developing a separator that can achieve effective bonding at a lower temperature and meet the requirements of positive and negative electrode fitting is an urgent problem to be solved currently.

[0004] In view of this, the present invention is specifically proposed. Summary of the Invention

[0005] An object of the present invention is to provide a battery separator with good low-temperature bonding performance, which can achieve the composite of the electrode sheet and the separator and the pressing of the entire electrode group at a lower temperature, while reducing energy consumption while ensuring or even improving the product stability.

[0006] Another object of the present invention is to provide a preparation method of the battery separator, which is simple to operate and has mild conditions.

[0007] Another object of the present invention is to provide a secondary battery including the above battery separator, which has good cycle performance.

[0008] To achieve the above objects of the present invention, in the first aspect, the present invention provides a battery separator, including a base film, a first coating is provided on one side surface of the base film, and a second coating is provided on the other side surface of the base film;

[0009] The first coating includes a first particulate binder material and an inorganic heat-resistant material; the second coating includes a second particulate binder material;

[0010] The softening points of the first particulate binder material and the second particulate binder material are T 1 and T 2 , respectively, and satisfy: 25°C ≤ T 1 ≤ 45°C, 45°C ≤ T 2 ≤ 65°C;

[0011] The D 50 particle sizes of the first particulate binder material and the second particulate binder material are D 1 and D 2 , respectively, and satisfy: 2.5 μm ≤ D 1 ≤ 6.5 μm, 0.1 μm ≤ D 2 ≤ 0.5 μm;

[0012] The first particulate binder material and the second particulate binder material satisfy the following relationship:

[0013] 0.15 ≤ (T 2 - T 1 ) × D 2 / D 1 ≤ 8.

[0014] In a specific embodiment of the present invention, the softening points of the first particulate binder material and the second particulate binder material satisfy: T 2 - T 1 ≥ 10°C.

[0015] In a specific embodiment of the present invention, in the first coating, the inorganic heat-resistant material is continuously distributed, and the first particulate binder material is distributed in an island form in the inorganic heat-resistant material and protrudes from the surface formed by the inorganic heat-resistant material. Further, in the first coating, the thickness H of the inorganic heat-resistant material satisfies: 1.5 μm ≤ H ≤ 3 μm.

[0016] In a specific embodiment of the present invention, in the first coating, the surface density M 1 of the first particulate binder material satisfies: 0.14 g / m 2 ≤ M 1 ≤ 0.56 g / m 2 .

[0017] In a specific embodiment of the present invention, in the second coating, the surface density M 2 of the second particulate binder material satisfies: 0.2 g / m 2 ≤ M 2 ≤ 0.6 g / m2 。

[0018] In a specific embodiment of the present invention, the battery separator satisfies: 0.39 ≤ (D 1 / H) × (M 1 / M 2 ) ≤ 12.

[0019] In a specific embodiment of the present invention, both the first particulate binder material and the second particulate binder material are particulate polyacrylate binder materials.

[0020] In a specific embodiment of the present invention, the inorganic heat-resistant material includes at least one of boehmite, alumina, magnesium hydroxide, and barium titanate.

[0021] In a specific embodiment of the present invention, the first coating further includes an aqueous binder and an optional wetting agent; the second coating further includes an aqueous binder and an optional wetting agent.

[0022] In the first coating of the present invention, the mass ratio of the first particulate binder material, the inorganic heat-resistant material, the aqueous binder, and the wetting agent is (0.5 - 2.0) : 10 : (0.3 - 0.8) : (0.05 - 0.1).

[0023] In the second coating of the present invention, the mass ratio of the second particulate binder material, the aqueous binder, and the wetting agent is 10 : (0.5 - 1.0) : (0.05 - 0.1).

[0024] In a specific embodiment of the present invention, the base film is a polyolefin base film. Further, the base film includes at least one of polyethylene and polypropylene.

[0025] In a specific embodiment of the present invention, the thickness of the base film is 3 - 16 μm.

[0026] The second aspect of the present invention provides a method for preparing the battery separator described in the first aspect of the present invention, including the following steps:

[0027] (a) Mix the first particulate binder material, the inorganic heat-resistant material, the aqueous binder, and the optional wetting agent with a solvent to form a first coating slurry, coat it on one side surface of the base film, and perform a drying treatment;

[0028] (b) Mix the second particulate binder material, the aqueous binder, and the optional wetting agent with a solvent to form a second coating slurry, coat it on the other side surface of the base film, and perform a drying treatment to obtain the battery separator.

[0029] In a specific embodiment of the present invention, the solid content of the first coating slurry is 30% - 40%, and the solid content of the second coating slurry is 4% - 10%.

[0030] The third aspect of the present invention provides a secondary battery, including the battery separator described in the first aspect of the present invention.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0032] (1) The battery separator of the present invention has good low-temperature adhesion performance, and can meet the adhesion requirements of the separator to the negative electrode and the positive electrode in the thermal composite lamination process at a temperature below 65°C, greatly reducing the energy consumption in production and manufacturing; at the same time, the reduction of the minimum temperature requirement in the process helps to broaden the process window and improve product stability;

[0033] (2) Based on the battery separator of the present invention, the secondary battery prepared by the thermal composite lamination process has a high adhesion strength and a uniform interface between the separator and the electrode sheet, and can improve the battery cycle performance. Specific Embodiments

[0034] The technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. However, those skilled in the art will understand that the following described embodiments are some embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention. Those not specified in the embodiments are carried out under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments not specified by the manufacturer are all conventional products that can be obtained through commercial purchase.

[0035] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as limiting the present invention. In addition, the terms "first", "second", "third" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance.

[0036] In the current thermal composite laminate process, the separator is bonded to the negative electrode by rolling with a hot roller, and the separator is bonded to the positive electrode by flat hot pressing. To meet the bonding requirements of the separator for the positive and negative electrodes, the currently commonly used rolling and hot pressing temperatures are 80-120°C, and the process window is narrow. As the thickness of the battery cell increases, the time-consuming of the hot pressing process increases, further increasing the energy consumption. Based on this, the present invention provides a battery separator with good low-temperature bonding performance, which can meet the bonding requirements of the separator for the negative and positive electrodes in the thermal composite laminate process at a temperature below 65°C, greatly reducing the energy consumption in production and manufacturing; at the same time, the reduction of the minimum temperature requirement in the manufacturing process broadens the process window and is beneficial to improving the product stability.

[0037] In a first aspect of the present invention, there is provided a battery separator, comprising a base film, a first coating is provided on one side surface of the base film, and a second coating is provided on the other side surface of the base film;

[0038] The first coating includes a first particulate bonding material and an inorganic heat-resistant material; the second coating includes a second particulate bonding material;

[0039] The softening points of the first particulate bonding material and the second particulate bonding material are T 1 and T 2 , respectively, and satisfy: 25°C ≤ T 1 ≤ 45°C, 45°C ≤ T 2 ≤ 65°C;

[0040] The D 50 particle sizes of the first particulate bonding material and the second particulate bonding material are D 1 and D 2 , respectively, and satisfy: 2.5 μm ≤ D 1 ≤ 6.5 μm, 0.1 μm ≤ D 2 ≤ 0.5 μm;

[0041] The first particulate bonding material and the second particulate bonding material satisfy the following relationship:

[0042] 0.15 ≤ (T 2 - T 1 ) × D 2 / D 1 ≤ 8.

[0043] In the battery separator of the present invention, a first coating and a second coating are respectively provided on the two side surfaces of the base film, and particulate bonding materials with certain softening points and particle sizes are respectively used in the first coating and the second coating, taking into account ensuring the lamination of the positive electrode sheet and the negative electrode sheet under low-temperature conditions and ensuring the bonding strength and bonding stability.

[0044] For example, in different embodiments, the softening point T 1It can be 25°C, 28°C, 30°C, 32°C, 35°C, 38°C, 40°C, 42°C, 45°C or a range composed of any two of them; the softening point T of the second particulate binder 2 It can be 45°C, 48°C, 50°C, 52°C, 55°C, 58°C, 60°C, 62°C, 65°C or a range composed of any two of them. The softening points of the materials in the present invention are all determined by the DSC method.

[0045] For example, in different embodiments, the D 50 particle size (i.e., median particle size) D 1 of the first particulate binder can be 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm or a range composed of any two of them; the D 50 particle size D 2 of the second particulate binder can be 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm or a range composed of any two of them.

[0046] The relationship formula (T 50 -T 2 ) × D 1 / D 2 / D 1 of the softening points and D 50 particle sizes of the first particulate binder and the second particulate binder can be 0.15, 0.5, 1, 2, 3, 4, 5, 6, 7, 8 or a range composed of any two of them. Through research, it is found that when the softening points and D 1 particle sizes of the first particulate binder and the second particulate binder satisfy the above relationship formula, the lamination of the positive electrode sheet and the negative electrode sheet under low temperature conditions can be ensured, and the bonding strength and bonding stability can be ensured. It should be noted that when calculating the result corresponding to the above relationship formula, the units represented by T 2 , T 1 , D 2 and D 1 are not considered, and only the values corresponding to the case where the units of T 2 and T 1 are °C and the units of D 2 are μm are used to calculate the result corresponding to the corresponding relationship formula.

[0047] In a specific embodiment of the present invention, the softening points of the first particulate binder and the second particulate binder satisfy: T 2 -T 1 ≥10°C, for example, T 2 -T 1It can be 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, 40°C or a range composed of any two of them, which is more conducive to taking into account the low-temperature bonding effects on both the negative electrode sheet and the positive electrode sheet.

[0048] In a specific embodiment of the present invention, in the first coating, the inorganic heat-resistant material is continuously distributed, and the first particulate binder material is distributed in the inorganic heat-resistant material in an island form and protrudes from the surface formed by the inorganic heat-resistant material. Further, in the first coating, the thickness H of the inorganic heat-resistant material satisfies: 1.5 μm ≤ H ≤ 3 μm. For example, H can be 1.5 μm, 1.8 μm, 2 μm, 2.2 μm, 2.5 μm, 2.8 μm, 3 μm or a range composed of any two of them. By controlling H within the above range, the thermal stability of the battery separator, the low-temperature bonding effect of the first coating on the electrode sheet, and the bonding stability are taken into account. In the first coating of the present invention, the inorganic heat-resistant material is continuously and substantially uniformly distributed on the surface of the base film, and the thickness H of the inorganic heat-resistant material refers to the distance between the surface formed by the inorganic heat-resistant material and the surface of the base film.

[0049] In a specific embodiment of the present invention, in the first coating, the areal density M of the first particulate binder material 1 satisfies: 0.14 g / m 2 ≤ M 1 ≤ 0.56 g / m 2 For example, M 1 can be 0.14 g / m 2 、0.2 g / m 2 、0.25 g / m 2 、0.3 g / m 2 、0.35 g / m 2 、0.4 g / m 2 、0.45 g / m 2 、0.5 g / m 2 、0.56 g / m 2 or a range composed of any two of them; the areal density M 1 here refers to the mass of the first particulate binder material contained in the first coating per unit area.

[0050] In a specific embodiment of the present invention, in the second coating, the areal density M of the second particulate binder material 2 satisfies: 0.2 g / m 2 ≤ M 2 ≤ 0.6 g / m 2 For example, M 2 can be 0.2 g / m 2 、0.25 g / m 2 、0.3 g / m 2 、0.35 g / m 2, 0.4 g / m 2 , 0.45 g / m 2 , 0.5 g / m 2 , 0.55 g / m 2 , 0.6 g / m 2 or the range composed of any two of them. On the one hand, it ensures the low-temperature bonding effect of the second coating on the electrode sheet, and on the other hand, it avoids problems such as material dropping during the composite process caused by too high content; the areal density M 2 here refers to the mass of the second particulate binder material contained in the second coating per unit area.

[0051] In a specific embodiment of the present invention, the battery separator satisfies: 0.39 ≤ (D 1 / H) × (M 1 / M 2 ), and the corresponding relational expression (D 1 / H) × (M 1 / M 2 ) can be 0.39, 1, 3, 4, 5, 6, 8, 10, 12 or the range composed of any two of them. Through further research, it is found that by coordinately regulating the D 50 particle size of the first particulate binder material, the thickness H of the inorganic heat-resistant material in the first coating, the areal density of the first particulate binder material in the first coating, and the areal density of the second particulate binder material in the second coating, the bonding property between the first particulate binder material in the first coating and the inorganic heat-resistant material layer is ensured, and problems such as poor bonding effect caused by the shedding of the first particulate binder material are avoided.

[0052] In a specific embodiment of the present invention, both the first particulate binder material and the second particulate binder material are particulate polyacrylate binder materials. Using particulate polyacrylate binder materials, the cost is relatively low, and it helps to achieve composite and hot pressing at low temperature.

[0053] In a specific embodiment of the present invention, the inorganic heat-resistant material includes at least one of boehmite, alumina, magnesium hydroxide, and barium titanate. Further, the average particle size of the inorganic heat-resistant material is 0.2 - 1.0 μm, such as 0.2 μm, 0.5 μm, 0.8 μm, 1 μm, etc.

[0054] In a specific embodiment of the present invention, the first coating further includes an aqueous binder and an optional wetting agent; the second coating further includes an aqueous binder and an optional wetting agent.

[0055] In a specific embodiment of the present invention, the aqueous binder includes at least one of ethyl polyacrylate and ethyl polymethacrylate.

[0056] In a specific embodiment of the present invention, the wetting agent includes, but is not limited to, fatty alcohol lubricants, for example, it can be at least one of polyvinyl alcohol, glycerol, and propylene glycol.

[0057] In a specific embodiment of the present invention, in the first coating, the mass ratio of the first particulate binder material, the inorganic heat-resistant material, the aqueous binder, and the wetting agent is (0.5 - 2.0) : 10 : (0.3 - 0.8) : (0.05 - 0.1). For example, the mass ratio of the first particulate binder material to the inorganic heat-resistant material can be 0.5 : 10, 1 : 10, 1.5 : 10, 2 : 10, or the range composed of any two of them; the mass ratio of the inorganic heat-resistant material to the aqueous binder can be 10 : 0.3, 10 : 0.5, 10 : 0.6, 10 : 0.8, or the range composed of any two of them; the mass ratio of the inorganic heat-resistant material to the wetting agent can be 10 : 0.05, 10 : 0.06, 10 : 0.08, 10 : 0.1, or the range composed of any two of them.

[0058] In a specific embodiment of the present invention, in the second coating, the mass ratio of the second particulate binder material, the aqueous binder, and the wetting agent is 10 : (0.5 - 1.0) : (0.05 - 0.1). For example, the mass ratio of the second particulate binder material to the aqueous binder can be 10 : 0.5, 10 : 0.6, 10 : 0.8, 10 : 1, or the range composed of any two of them; the mass ratio of the second particulate binder material to the wetting agent can be 10 : 0.05, 10 : 0.06, 10 : 0.08, 10 : 0.1, or the range composed of any two of them.

[0059] In a specific embodiment of the present invention, the base film is a polyolefin base film. Further, the base film includes at least one of polyethylene and polypropylene.

[0060] In a specific embodiment of the present invention, the thickness of the base film is 3 - 16 μm. For example, it can be 3 μm, 5 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, or the range composed of any two of them.

[0061] The second aspect of the present invention provides a method for preparing the battery separator of the first aspect of the present invention, including the following steps:

[0062] (a) Mix the first particulate binder material, the inorganic heat-resistant material, the aqueous binder, and an optional wetting agent with a solvent to form a first coating slurry, coat it on one side surface of the base film, and perform a drying treatment;

[0063] (b) Mix the second particulate binder material, the aqueous binder, and an optional wetting agent with a solvent to form a second coating slurry, coat it on the other side surface of the base film, and perform a drying treatment to obtain the battery separator.

[0064] In actual operation, the above solvent is water. Moreover, during the preparation of the first coating slurry and the second coating slurry, operations such as conventional stirring and dispersion can be carried out to ensure the uniformity of the material mixture, which will not be elaborated here.

[0065] In a specific embodiment of the present invention, the solid content of the first coating slurry is 30% - 40%, for example, it can be 30%, 32%, 35%, 38%, 40% or the range composed of any two of them; the solid content of the second coating slurry is 4% - 10%, for example, it can be 4%, 5%, 6%, 8%, 10% or the range composed of any two of them; the specific solid content can be adjusted conventionally according to the actual coating situation to balance ensuring coating uniformity and subsequent drying efficiency, etc.

[0066] In a specific embodiment of the present invention, the temperature of the drying treatment in step (a) and step (b) is independently selected from 60 - 80°C, for example, it can be 60°C, 65°C, 70°C, 75°C, 80°C or the range composed of any two of them.

[0067] The third aspect of the present invention provides a secondary battery, including the battery separator of the first aspect of the present invention.

[0068] In a specific embodiment of the present invention, the preparation of the secondary battery includes thermal composite lamination; the thermal composite lamination includes: (a) respectively disposing paired battery separators on both sides of the negative electrode sheet, making the first coating of the battery separator face the negative electrode sheet, and roll-pressing and laminating to form an intermediate; (b) alternately laminating the intermediate and the positive electrode sheet, and hot-pressing to form an electrode group.

[0069] In a specific embodiment of the present invention, in the roll-pressing and laminating, the lamination temperature is 25 - 45°C, and the lamination pressure is 0.2 - 2 MPa. For example, in different embodiments, the lamination temperature can be 25°C, 30°C, 35°C, 40°C, 45°C or the range composed of any two of them, and the lamination pressure can be 0.2 MPa, 0.6 MPa, 1 MPa, 1.5 MPa, 2 MPa or the range composed of any two of them.

[0070] In a specific embodiment of the present invention, in the hot-pressing, the pressing temperature is 25 - 65°C, the pressing pressure is 1 - 5 MPa, and the pressing time is 25 - 300 s. For example, in different embodiments, the pressing temperature can be 25°C, 30°C, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C or the range composed of any two of them, the pressing pressure can be 1 MPa, 2 MPa, 3 MPa, 4 MPa, 5 MPa or the range composed of any two of them, and the pressing time can be 25 s, 50 s, 100 s, 150 s, 200 s, 250 s, 300 s or the range composed of any two of them.

[0071] Example 1

[0072] This embodiment provides a battery separator, which includes a base film. A first coating is provided on one side surface of the base film, and a second coating is provided on the other side surface of the base film; the first coating includes a first granular polyacrylate binder material and an inorganic heat-resistant material; the second coating includes a second granular polyacrylate binder material. The softening point of the first granular polyacrylate binder material is T 1 , D 50 and the particle size is D 1 ; the softening point of the second granular polyacrylate binder material is T 2 , D 50 and the particle size is D 2 ; wherein, T 1 = 25 °C, T 2 = 45 °C, D 1 = 2.5 μm, D 2 = 0.25 μm. In the first coating, the thickness H of the inorganic heat-resistant material is 1.5 μm, and the areal density M 1 of the first granular polyacrylate binder material is 0.14 g / m 2 ; in the second coating, the areal density M 2 of the second granular polyacrylate binder material is 0.2 g / m 2 .

[0073] The base film of this embodiment is a wet-process separator, with a thickness of 7 μm and a porosity of 36%. The inorganic heat-resistant material is boehmite with an average particle size of 0.5 μm.

[0074] The preparation method of the battery separator of this embodiment includes the following steps:

[0075] (1) Weigh the materials according to the mass ratio of the first granular polyacrylate binder material, inorganic heat-resistant material, water-based binder (ethyl polyacrylate), and wetting agent (propylene glycol) of 0.5:10:0.6:0.08, and then mix them with deionized water and stir and disperse to make the materials evenly mixed, obtaining a first coating slurry with a solid content of 35%; weigh the materials according to the mass ratio of the second granular polyacrylate binder material, water-based binder (ethyl polyacrylate), and wetting agent (propylene glycol) of 10:0.8:0.1, and then mix them with deionized water and stir and disperse to make the materials evenly mixed, obtaining a second coating slurry with a solid content of 7%.

[0076] (2) Coating the first coating slurry on one side of the base film by gravure roll coating, drying at 70 °C for 5 min; then coating the second coating slurry on the other side of the above base film by gravure roll coating, drying at 70 °C for 5 min to obtain the battery separator.

[0077] This embodiment also provides a method for preparing a lithium battery, comprising the following steps:

[0078] (1) Preparation of positive electrode sheets: lithium iron phosphate active material, binder PVDF, and conductive agent carbon black are mixed in a mass ratio of 96.5:2:1.5 and added to solvent NMP. The mixture is stirred thoroughly to obtain positive electrode slurry. The positive electrode slurry is then coated on both sides of carbon-coated aluminum foil, dried, compacted, and cut into sheets to obtain positive electrode sheets.

[0079] (2) Preparation of negative electrode sheets: artificial graphite, binder SBR, and conductive agent carbon black are mixed in a mass ratio of 96:2.5:1.5 and added to solvent water. The mixture is stirred thoroughly to obtain negative electrode slurry. The negative electrode slurry is then coated on both sides of the copper foil, dried, compacted, and cut into sheets to obtain negative electrode sheets.

[0080] (3) Preparation of electrode group: The battery separator and the negative electrode plate are bonded by hot roller hot pressing using a hot composite lamination process, wherein the first coating is applied to the negative electrode plate, the hot roller lamination temperature is 25-45°C, and the lamination pressure is 0.2-2 MPa; the unit formed after lamination is cut into single pieces by a hot cutter to form a sandwich structure of separator-negative electrode plate-separator, the sandwich structure and the positive electrode plate are stacked into a electrode group in the form of a laminate, and the electrode group is completed by hot pressing using a flat hot press, wherein the pressing temperature during hot pressing is 25-65°C, the pressing pressure is 1-5 MPa, and the pressing time is 25-300 s.

[0081] (4) Encapsulating the prepared electrode group, and then injecting electrolyte; precharging and forming the battery cell to obtain a lithium battery. The electrolyte is a 1 mol / L lithium hexafluorophosphate solution, and the solvent is ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate, and the volume ratio of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate is 3:4:3.

[0082] Embodiments 2 to 8

[0083] Examples 2 to 8 refer to the battery separator, lithium battery and preparation method thereof of Example 1, except that: the softening point T of the first granular polyacrylate bonding material is 1 , D 50 Particle size D 1 , the surface density M of the first particulate polyacrylate bonding material in the first coating 1 , the thickness H of the inorganic heat-resistant material, the softening point T of the second granular polyacrylate bonding material 2 , D 50 Particle size D 2 , the surface density M of the second particulate polyacrylate bonding material in the second coating 2 Any one or more of the above are different. The specific information is shown in Table 1.

[0084] In the preparation method of the battery separator, the amounts of the aqueous binder and the wetting agent in the first coating slurry can be kept unchanged, and the solid content of the first coating slurry can be kept unchanged. By adjusting the ratio of the first granular polyacrylate binder material to the inorganic heat-resistant material, the surface density M of the first granular polyacrylate binder material in the first coating slurry 1 , and the thickness H of the inorganic heat-resistant material are corresponding values (the same applies to the subsequent comparative examples).

[0085] Table 1 Information of Different Examples

[0086]

[0087] Comparative Example 1

[0088] Comparative Example 1 provides a commercially available coated separator product, and a lithium battery is assembled with this coated separator product. The assembly method refers to Example 1.

[0089] The specifications of the coated separator product in Comparative Example 1 are 7 + 2 + 1 + 1. 7 represents that the base film is a wet-process separator with a thickness of 7 μm and a porosity of 36%; 2 represents that a pure boehmite material with a thickness of 2 μm is coated on one side; 1 + 1 represents that a symmetrically structured PVDF coating product is coated on the outermost two sides of the separator coated with the pure boehmite material on one side, and the coating thickness of PVDF on both sides is 1 μm.

[0090] Comparative Example 2

[0091] Comparative Example 2 refers to the battery separator, lithium battery and their preparation methods in Example 1, the differences are: the softening point and D 50 particle size of the first granular polyacrylate binder material in the first coating are different, the thickness of the inorganic heat-resistant material is different, and the surface density of the first granular polyacrylate binder material in the first coating is different; the second coating includes a PVDF material, and the second coating is formed by coating a commercially available PVDF powder for separators (Sinochem Lantian PVDF). The surface density of PVDF in the second coating is 0.5 g / m 2 , and the thickness of the second coating is 1 μm.

[0092] In Comparative Example 2, the softening point T 1 of the first granular polyacrylate binder material = 65 °C, D 50 particle size D 1 = 4.5 μm, the thickness H of the inorganic heat-resistant material = 2 μm, and the surface density M 1 of the first granular polyacrylate binder material in the first coating is 0.28 g / m 2 .

[0093] Comparative Examples 3 - 7

[0094] Comparative Examples 3 to 7 refer to the battery separator, lithium battery and its preparation method of Example 1, the difference being that: the softening point T of the first granular polyacrylate binder 1 , D 50 , the particle size D 1 , the surface density M of the first granular polyacrylate binder in the first coating 1 , the thickness H of the inorganic heat-resistant material, the softening point T of the second granular polyacrylate binder 2 , D 50 , the particle size D 2 , the surface density M of the second granular polyacrylate binder in the second coating 2 is different in any one or more of them. The specific information is shown in Table 2.

[0095] Table 2 Information of Different Comparative Examples

[0096]

[0097] Experimental Example

[0098] The state of the electrode groups prepared by the thermal composite process (the specific composite process is shown in Table 3) for the battery separators of different Examples and Comparative Examples was observed, and the adhesion between the separator and the negative electrode sheet in the single-piece structure of separator-negative electrode sheet-separator after the first step of thermally pressing the battery separator and the negative electrode sheet together through a hot roller during the thermal composite process was tested. The test results are shown in Table 3.

[0099] Table 3 Different Thermal Composite Processes and Test Results

[0100]

[0101]

[0102] Note: Among them, "<1 / 3 length" means that the length of the cracking area is less than 1 / 3 of the total length of the electrode group. When the length of the cracking area is less than 1 / 3 of the total length of the electrode group, the requirements for subsequent processes such as the transfer and shelling during the cell assembly process can be met.

[0103] According to the test results of the thermal composite process, the lithium batteries corresponding to the battery separators that meet the requirements of the thermal composite process were subjected to a 25°C cycle test of the battery at a cycle test rate of 1C. The test results are shown in Table 4. Among them, for Example 1, the electrode group prepared with a composite pressure of 0.2 MPa was selected; for Example 2, the electrode group prepared with a pressing pressure of 1 MPa was selected.

[0104] Table 4 Different Battery Cycle Test Results

[0105]

[0106]

[0107] As can be seen from Table 3, when the battery separator of the present invention is applied to the thermal composite lamination process, thermal composite can be achieved under the conditions of a composite temperature of 25 to 45 °C and a composite pressure of 0.2 to 2 MPa. Moreover, in the composite single sheet, the adhesion between the separator and the negative electrode sheet is between 0.33 and 4.2 N / m, the adhesion between the separator and the electrode sheet is uniform, and no material drops from the negative electrode. After composite, thermal press forming can be achieved under the conditions of a press temperature of 25 to 65 °C, a press pressure of 1 to 5 MPa, and a press time of 25 to 300 s. The pressed electrode group is not soft, without cracking or slightly cracked (<1 / 3 length), and can meet the assembly transfer and shell insertion of the battery cell. For the time and pressure during the press process, they can be adjusted according to the size of the battery cell. As the size of the battery cell increases and the thickness becomes thicker, the corresponding press time and pressure can be appropriately increased. And the assembled battery has good cycle performance. When the fully charged battery cell of the present invention is disassembled, the negative electrode interface is uniform and there is no problem of material dropping.

[0108] According to Example 1 and Example 4, it can be seen that by increasing the composite pressure and using a D 50 particle size of the first granular polyacrylate binder material increased within the specified range, the adhesion between the separator and the negative electrode sheet increases; combined with Example 5, it can be seen that within the D 50 particle size and thickness ranges specified in the present invention, the composite effect can be satisfied.

[0109] According to Example 1, Example 2 and Example 3, it can be seen that by increasing the press pressure and increasing the surface density of the second granular polyacrylate binder material in the second coating, the hardness of the electrode group can be improved; and within the press pressure and surface density ranges specified in the present invention, the press effect of the electrode group can be satisfied.

[0110] According to Example 6 to Example 8, when the softening points of the first granular polyacrylate binder material and the second granular polyacrylate binder material meet certain requirements, the composite effect and the press effect can be satisfied, and problems affecting production such as the sticking of the rubber particles to the roller during the composite process will not occur.

[0111] According to Comparative Example 3, when the two granular polyacrylate binder materials in the first coating and the second coating do not satisfy (T 2 -T 1 )×D 2 / D 1 is 0.15 to 8, during the composite process, since the softening points of the granular polyacrylate binder materials in the two coatings are too close, problems affecting production such as the sticking and dropping of the second granular polyacrylate binder material to the hot roller are likely to occur during the contact with the hot roller.

[0112] According to Comparative Example 4, when the particle size of the first particulate polyacrylate binder material in the first coating is too small and does not satisfy (T 2 -T 1 )×D 2 / D 1 is between 0.15 and 8, the binding effect of the first particulate polyacrylate binder material in the first coating is significantly reduced, and the composite requirements cannot be met.

[0113] According to Comparative Example 5, when the particle size of the first particulate polyacrylate binder material in the first coating is too small, the thickness of the inorganic heat-resistant material in the first coating is too large; at the same time, the surface density of the second particulate polyacrylate binder material in the second coating is relatively high, such that (D 1 / H)×(M 1 / M 2 ) is not within the range of 0.39 to 12. Not only can the composite requirements not be met, but problems such as the second particulate polyacrylate binder material sticking to the roller and falling off will occur.

[0114] According to Comparative Example 6, when the particle size of the first particulate polyacrylate binder material in the first coating is too large, (D 1 / H)×(M 1 / M 2 ) is not within the range of 0.39 to 12, and the height of the corresponding first particulate polyacrylate binder material protruding from the inorganic heat-resistant material is too large, and it cannot be firmly fixed in the first coating, is prone to falling off, affects the bonding effect of the separator, and cannot be used.

[0115] According to Comparative Example 7, when the thickness H of the inorganic heat-resistant material in the first coating is relatively high, (D 1 / H)×(M 1 / M 2 ) is not within the range of 0.39 to 12, and the bonding effect of the first particulate polyacrylate binder material in the corresponding first coating becomes poor, affecting its use.

[0116] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A battery separator, characterized in that: The invention comprises a base film, wherein a first coating is disposed on one surface of the base film, and a second coating is disposed on the other surface of the base film; The first coating layer comprises a first granular bonding material and an inorganic heat-resistant material; the second coating layer comprises a second granular bonding material; The softening points of the first granular bonding material and the second granular bonding material are T1 and T2 respectively, and satisfy the following conditions: 25°C≤T1≤45°C, 45°C≤T2≤65°C; The D of the first granular bonding material and the second granular bonding material 50 The particle sizes are D1 and D2 respectively, and satisfy the following conditions: 2.5μm≤D1≤6.5μm, 0.1μm≤D2≤0.5μm; The first granular bonding material and the second granular bonding material satisfy the following relationship: 0.15≤(T2-T1)×D2 / D1≤8.

2. The battery separator according to claim 1, characterized in that: The softening points of the first granular bonding material and the second granular bonding material satisfy: T2-T1≥10°C.

3. The battery separator according to claim 1, characterized in that: In the first coating layer, the inorganic heat-resistant material is continuously distributed, and the first granular bonding material is distributed in the inorganic heat-resistant material in the form of islands and protrudes from the surface formed by the inorganic heat-resistant material.

4. The battery separator according to claim 1, characterized in that: Having at least one of the following characteristics: (1) In the first coating layer, the thickness H of the inorganic heat-resistant material satisfies: 1.5 μm ≤ H ≤ 3 μm; (2) In the first coating layer, the surface density M1 of the first granular bonding material satisfies: 0.14 g / m 2 ≤M1≤0.56g / m 2 ; (3) In the second coating layer, the surface density M2 of the second granular bonding material satisfies: 0.2 g / m 2 ≤M2≤0.6g / m 2 .

5. The battery separator according to claim 4, characterized in that: The battery separator satisfies: 0.39≤(D1 / H)×(M1 / M2)≤12.

6. The battery separator according to claim 1, characterized in that: Having at least one of the following characteristics: (1) The first granular bonding material and the second granular bonding material are both granular polyacrylate bonding materials; (2) The inorganic heat-resistant material includes at least one of boehmite, alumina, magnesium hydroxide and barium titanate; (3) The first coating further comprises an aqueous binder and an optional wetting agent; the second coating further comprises an aqueous binder and an optional wetting agent; (4) In the first coating, the mass ratio of the first granular bonding material, the inorganic heat-resistant material, the aqueous bonding agent and the wetting agent is (0.5-2.0) : 10 : (0.3-0.8) : (0.05-0.1); (5) In the second coating layer, the mass ratio of the second granular binder, the aqueous binder and the wetting agent is 10:(0.5-1.0):(0.05-0.1).

7. The battery separator according to claim 1, characterized in that: The base film is a polyolefin base film; the base film satisfies at least one of the following characteristics: (1) The base film comprises at least one of polyethylene and polypropylene; (2) The thickness of the base film is 3 to 16 μm.

8. The method for preparing a battery separator according to any one of claims 1 to 7, characterized in that: The steps include: (a) mixing a first granular bonding material, an inorganic heat-resistant material, an aqueous bonding agent and an optional wetting agent with a solvent to prepare a first coating slurry, coating the first coating slurry on one side of the base film, and drying the first coating slurry; (b) mixing a second granular adhesive material, an aqueous adhesive and an optional wetting agent with a solvent to prepare a second coating slurry, coating the second coating slurry on the other side of the base film, and drying the slurry to obtain the battery separator.

9. The preparation method according to claim 8, characterized in that: The solid content of the first coating slurry is 30% to 40%, and the solid content of the second coating slurry is 4% to 10%.

10. A secondary battery, characterized in that: A battery separator comprising the battery separator according to any one of claims 1 to 7.

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