Separator and preparation method therefor, battery, and electrical device

Abstract

A separator (31) and a preparation method therefor, a battery, and an electrical device. The separator (31) comprises a base film (311), a first coating (312), and a second coating (313), the first coating (312) and the second coating (313) being respectively provided on two sides of the base film (311). The pore size distribution η(2) of the second coating (313) and the pore size distribution η(1) of the first coating (312) satisfy: η(2) / η(1)>1.0. By using the separator (31), the cycling performance of the battery can be improved, and the cycle life of the battery can be extended.

Description

Diaphragm and preparation method thereof, battery and electrical equipment

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to the Chinese patent application filed with the State Intellectual Property Office of China on July 31, 2023, with application number 202310953462.4 and application name “Diaphragm and its preparation method, battery and electrical equipment”, all contents of which are incorporated by reference into this application. Technical Field

[0003] The present application relates to the field of batteries, and in particular to a diaphragm and a preparation method thereof, as well as batteries and electrical equipment. Background Art

[0004] Rechargeable batteries, such as lithium-ion batteries, are widely used in consumer devices such as mobile phones, laptops, wearables, and power banks. They also have broad application prospects in electric vehicles and energy storage. As one of the four main components of a battery, the separator not only serves as a safety medium but also as a transport channel for the electrolyte, acting as a bridge between the positive and negative electrodes. Lithium ions in the electrolyte shuttle back and forth between the positive and negative electrodes through the separator's porous structure. The more efficient this shuttle, the longer the battery cell's cycle life. Therefore, both sides of the separator must possess excellent lyophilic properties. However, during cycling, the negative electrode material repeatedly forms a SEI film on its surface, continuously consuming electrolyte. Consequently, the separator closer to the negative electrode is more susceptible to electrolyte depletion, which in turn causes a broken electrolyte bridge between the negative electrode and the separator, resulting in a reduction in the battery's cycle life.

[0005] Summary of the Invention

[0006] The present application provides a diaphragm and a preparation method thereof, as well as a battery and an electrical device to improve the cycle performance of the battery and extend the cycle life of the battery.

[0007] In a first aspect, the present application provides a diaphragm comprising a base film, a first coating layer and a second coating layer, wherein the first coating layer and the second coating layer are respectively provided on both sides of the base film. (2) The pore size distribution η of the first coating (1) Satisfy: η (2) / η (1) >1.0, where η (1) =[D90 (1) -D25 (1) ] / D50 (1) , η (2) =[D90 (2) -D25 (2) ] / D50 (2) ;

[0008] D25 (1) Indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the first coating reaches 25%;

[0009] D50 (1) Indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the first coating reaches 50%;

[0010] D90 (1) Indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the first coating reaches 90%;

[0011] D25 (2) Indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the second coating reaches 25%;

[0012] D50 (2) Indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the second coating reaches 50%;

[0013] D90 (2) It indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the second coating reaches 90%.

[0014] In the diaphragm of the present application, when there is a difference in the pore size distribution between the first coating and the second coating and the pore size distribution of the second coating is larger than that of the first coating, the electrolyte is more likely to accumulate on the side of the second coating, so that the second coating stores more electrolyte relative to the first coating. Uniform pores are formed on the first coating side, which has good lyophilicity and heat resistance. Pores of varying sizes and a wider distribution are formed on the second coating side. The small pores are used to transmit electrolyte, and the large pores are used to store electrolyte. When assembling the battery, the second coating is arranged toward the negative electrode plate. During the charge and discharge cycle of the battery, both the first coating and the second coating have high lyophilicity, and the second coating can enrich more electrolyte to effectively solve the problem of liquid depletion caused by repeated formation of SEI film on the negative electrode plate side. Since the second coating can always maintain a high lyophilicity, the problem of electrolyte bridging in the second coating can be avoided in the later stages of the battery cycle, so that the diaphragm always serves as an electrolyte transmission channel, improving the battery's cycle performance and extending the battery's cycle life.

[0015] In an optional implementation, the pore size D90 of the second coating is (2) Larger than the pore size D90 of the first coating (1) When the pore size of the second coating is larger than that of the first coating, the electrolyte is more likely to be enriched on the second coating side during the battery manufacturing process. In the later stage of the battery cycle, the electrolyte enriched on the second coating side can continuously replenish the consumed electrolyte, thereby improving the cycle performance of the battery.

[0016] In an optional embodiment, the pore size D90 of the first coating layer is (1) The pore size D90 of the second coating is less than 0.8 μm. (2) Greater than or equal to 0.5μm. When the pore size D90 in the first coating (1) Less than 0.8μm, the pore size D90 of the second coating (2) When the thickness is greater than or equal to 0.5 μm, both the first coating and the second coating can obtain higher liquid retention performance, so that the diaphragm is always infiltrated with electrolyte during the battery charge and discharge cycle, ensuring that the active ions of the battery can shuttle in the diaphragm.

[0017] In an optional embodiment, the pore size D90 of the first coating layer is (1) The pore size D90 of the second coating is 0.1-0.5 μm. (2) When the pore sizes of the first coating layer and the second coating layer are limited to the above sizes, the first coating layer and the second coating layer can each have a high electrolyte retention capacity, while at the same time, the first coating layer and the second coating layer themselves will not form a broken bridge due to the pore size being too large, thereby affecting the transmission of lithium ions.

[0018] In an optional implementation, the n (2) / η (1) The value of is 1.2 to 3. The greater the difference in pore size distribution between the first and second coating layers, the more electrolyte the second coating layer can consistently store, and the lower the likelihood of subsequent electrolyte depletion on the second coating layer, thus preventing electrolyte depletion on the negative electrode side of the battery from affecting the battery's cycle life. At the same time, the pore size distribution of the first and second coating layers should not be too large, otherwise, overly large pores will easily appear in the second coating layer, affecting the contact between the second coating layer and the electrolyte.

[0019] In one optional implementation, both the first and second coating layers comprise an organic resin; the organic resin content of the first coating layer is less than 50% by weight, while the organic resin content of the second coating layer is greater than that of the first coating layer. A higher organic resin content in the second coating layer facilitates the formation of a suitable pore size distribution in the second coating layer. A lower organic resin content in the first coating layer facilitates the formation of small, uniformly sized pores in the first coating layer.

[0020] In an optional implementation, the mass content of the organic resin in the first coating layer is 10-30%, and the mass content of the organic resin in the second coating layer is 20-80%. When the organic resin content in the first coating layer is 10-30%, in addition to helping the first coating layer form small and uniform pores, it can also ensure that the first coating layer has high stability, making it difficult for the first coating layer to fall off from the base film, and at the same time, it can also make the first coating layer have suitable heat resistance. When the organic resin content in the second coating layer is 20-80%, the second coating layer can be controlled through process control to form pores with large pores and a wide pore size distribution. At the same time, it can also make the second coating layer have high adhesion stability and thermal stability to the base film.

[0021] In an optional implementation, the organic resin content in the second coating layer is 1.2 to 5 times the organic resin content in the first coating layer. When the organic resin content in the first coating layer and the second coating layer satisfies the above relationship, the diaphragm can have both a suitable pore size distribution and high heat resistance, and an asymmetric distribution of pore sizes on both sides of the diaphragm can be achieved. Preferably, the organic resin content in the second coating layer is 1.5 to 4 times the organic resin content in the first coating layer. By optimizing the content of the organic resin in the first coating layer and the second coating layer, the pore size distribution and heat resistance of the diaphragm can be further optimized.

[0022] In an optional implementation, both the first coating and the second coating include inorganic particles; the particle size D50 of the inorganic particles in the first coating is 0.05 to 0.6 μm, preferably 0.1 to 0.4 μm. When the particle size of the inorganic particles in the first coating is within the above numerical range, combined with a specific content of organic resin, a uniform and dense ceramic coating can be formed. At the same time, the organic resin is distributed in a network-like manner between the particles of the inorganic particles, thereby ensuring that the pores formed in the first coating are relatively uniform, giving the first coating good thermal shrinkage properties and a high film rupture temperature.

[0023] In addition, the particle size D50 of the inorganic particles in the second coating layer is 0.2 to 2 μm, preferably 0.3 to 1.3 μm. The inorganic particles of the above particle size in the second coating layer, combined with the specific content of organic resin in the second coating layer, are more likely to form a suitable pore size distribution, with both abundant small pores for electrolyte transmission and suitable large pores for electrolyte storage, thereby ensuring that the separator is always in contact with the electrolyte during the charge and discharge cycle of the battery.

[0024] In an optional implementation, the particle size D50 of the inorganic particles in the second coating layer is 1.5 to 5 relative to the particle size D50 of the inorganic particles in the first coating layer. When the particle size ratio of the inorganic particles in the first coating layer to the second coating layer is 1.5 to 5, the separator can achieve both heat resistance and a suitable pore size distribution.

[0025] In a second aspect, the present application provides a method for preparing a diaphragm, the method comprising the following steps:

[0026] Applying a first coating slurry on one side of the base film; applying a second coating slurry on the other side of the base film; the first coating slurry and the second coating slurry are different slurries;

[0027] The base film coated with the first coating slurry and the second coating slurry is subjected to multi-stage extraction, and then dried to obtain the separator;

[0028] In the obtained diaphragm, the pore size distribution η of the second coating is (2) The pore size distribution η of the first coating (1) Satisfy: η (2) / η (1) >1.0, where

[0029] η (1) =[D90 (1) -D25 (1) ] / D50 (1) , η (2) =[D90 (2) -D25 (2) ] / D50 (2) ;

[0030] D25 (1) Indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the first coating reaches 25%;

[0031] D50 (1) Indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the first coating reaches 50%;

[0032] D90 (1) Indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the first coating reaches 90%;

[0033] D25 (2) Indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the second coating reaches 25%;

[0034] D50 (2) Indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the second coating reaches 50%;

[0035] D90 (2) It indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the second coating reaches 90%.

[0036] The technical effects that can be achieved in the second aspect mentioned above can be described with reference to the corresponding effects in the first aspect mentioned above, and will not be repeated here.

[0037] In an optional implementation, the first coating slurry and the second coating slurry may include organic resins of different mass proportions, or inorganic particles of different mass proportions, or both organic resins of different mass proportions and inorganic particles of different mass proportions. As long as the separator finally obtained satisfies η (2) / η (1) >1.0 is sufficient.

[0038] In an optional implementation, the multi-stage extraction includes three-stage extraction, the mass proportion of the good solvent in the first-stage extraction is 35-60%, the mass proportion of the good solvent in the second-stage extraction tank is 0-35%, and the mass proportion of the good solvent in the third-stage extraction tank is 0-2.5%. In the first-stage extraction, the coating slurry is not completely solidified. By controlling the mass concentration of the good solvent in the first-stage extraction within the range of 35-60%, it can help to form a pore structure with uniform pore size in the thickness direction of the coating. In the second-stage extraction, the coating slurry is completely solidified. By controlling the mass concentration of the good solvent in the second-stage extraction within the range of 20-35%, it can be beneficial to the formation of pores and avoid the formation of a dense coating. By controlling the mass concentration of the good solvent in the third-stage extraction to below 2.5%, it can be used to remove the residual extraction stage on the coated diaphragm.

[0039] In an optional implementation, during the first stage extraction, the concentration of the good solvent gradually decreases along the direction of movement of the base membrane in the first extraction tank, and the difference between the highest concentration of the good solvent and the lowest concentration of the good solvent in the first extraction tank is 5-15%. Multi-stage extraction can be performed in a multi-stage extraction tank. The multi-stage extraction tank includes at least a first extraction tank, a second extraction tank, and a third extraction tank, which are arranged in sequence. During the multi-stage extraction, the base membrane coated with the coating slurry is extracted in the first extraction tank, the second extraction tank, and the third extraction tank in sequence. The first extraction tank is also called a coagulation bath, and the coating slurry coagulates in the first extraction tank. Along the direction of movement of the base membrane in the first extraction tank, the concentration of the good solvent in the first extraction tank gradually decreases, and the difference between the highest concentration value and the lowest concentration value of the good solvent in the first extraction tank is 5-15wt%, preferably 6-13wt%, and more preferably 7-12wt%. By controlling the above parameters and adopting the formulations of the first and second coatings, an asymmetric pore structure of the first and second coatings can be obtained.

[0040] In an optional implementation, the time for the first stage extraction and the second stage extraction is 5 to 15 seconds respectively.

[0041] Among them, the data in the above-mentioned possible implementation methods of the present application, such as the pore size of the first coating, the pore size of the second coating, the content of organic resin, the particle size of inorganic particles, etc., when measured, the values ​​within the scope of engineering measurement error should be understood to be within the range specified in the present application.

[0042] In a third aspect, the present application provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet, and a separator disposed between the positive electrode sheet and the negative electrode sheet, wherein the separator may be the separator of the first aspect of the present application.

[0043] The secondary battery provided by the present application includes the diaphragm of the first aspect of the present application. When the diaphragm of the present application has high heat resistance, the secondary battery of the present application can obtain better safety. At the same time, since the diaphragm of the present application has good air permeability, the secondary battery of the present application can also obtain higher fast charging performance.

[0044] In a fourth aspect, the present application provides an electrical device, which includes the secondary battery according to the third aspect of the present application.

[0045] Among them, electrical equipment includes but is not limited to electronic devices, electric vehicles, power storage systems, etc. On the basis that the secondary batteries of each possible embodiment of this application have good safety and fast charging performance, electrical equipment using the secondary batteries of this application as a driving power source can also achieve the same effect. The technical effects that can be achieved in the third and fourth aspects above can be described with reference to the corresponding effects in the first aspect above, and will not be repeated here. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] FIG1 is a schematic structural diagram of a battery provided in an embodiment of the present application;

[0047] FIG2 is a schematic structural diagram of a diaphragm provided in an embodiment of the present application;

[0048] FIG3 is a schematic diagram showing the relative relationship between the pore size distribution diagram of the first coating layer and the pore size distribution diagram of the second coating layer according to an embodiment of the present application.

[0049] Reference numerals: 11 - positive electrode sheet; 21 - negative electrode sheet; 31 - separator; 311 - base film; 312 - first coating layer; 313 - second coating layer; 314 - inorganic particles. DETAILED DESCRIPTION

[0050] In order to make the purpose, technical solutions and advantages of this application clearer, this application will be further described in detail below with reference to the accompanying drawings.

[0051] The terms used in the following embodiments are only for the purpose of describing specific embodiments and are not intended to limit the present application. As used in the specification and appended claims of this application, the singular expressions "a", "an", "above", "the", and "this" are intended to also include expressions such as "one or more", unless the context clearly indicates otherwise.

[0052] References to "one embodiment" or "some embodiments" in this specification mean that a particular feature, structure, or characteristic described in conjunction with that embodiment is included in one or more embodiments of the present application. Thus, phrases such as "in one embodiment," "in some embodiments," "in other embodiments," and "in yet other embodiments" appearing in various places in this specification do not necessarily refer to the same embodiment, but rather mean "one or more but not all embodiments," unless otherwise specifically emphasized. The terms "including," "comprising," "having," and variations thereof mean "including but not limited to," unless otherwise specifically emphasized.

[0053] Figure 1 is a schematic diagram of a battery structure. As shown in Figure 1, the battery includes a positive electrode sheet 11, a negative electrode sheet 21, a separator 31, and an electrolyte. Separator 31 is located between positive and negative electrode sheets 11 and 21. The electrolyte permeates separator 31, positive electrode sheet 11, and negative electrode sheet 21. During the battery's charge and discharge cycles, separator 31 serves as a transport channel for active ions and blocks the conduction of electrons.

[0054] Typically, a SEI film is repeatedly formed on the surface of the negative electrode plate 21 of the battery during the charge and discharge cycle, thereby continuously consuming the electrolyte. Therefore, in order to avoid the problem of liquid depletion on the surface of the diaphragm 31 close to the negative electrode plate 21, which causes the electrolyte bridge to break between the negative electrode plate 21 and the diaphragm 31, an embodiment of the present application provides a diaphragm 31. The diaphragm 31 of the embodiment of the present application is a diaphragm 31 with a double-sided asymmetric pore size distribution. The battery assembled using the diaphragm 31 can have better liquid retention performance on the side close to the negative electrode plate 21, which can alleviate the problem of battery cycle performance degradation due to liquid depletion on the side of the diaphragm 31 facing the negative electrode plate 21.

[0055] Figure 2 is a schematic diagram of the structure of a diaphragm. As shown in Figure 2, the diaphragm 31 includes a base film 311, one side of which is provided with a first coating 312, and the other side of which is provided with a second coating 313. The base film 311 is a porous film, and its material can be a polyolefin substrate such as polyethylene or polypropylene. The structure and material selection of the base film 311 are not specifically limited in this application, and can be a conventional base film 311 in the art. The structures of the first coating 312 and the second coating 313 are described in detail below.

[0056] First coating

[0057] The first coating layer 312 may include an organic resin. The mass content of the organic resin in the first coating layer 312 may be less than 50%. The mass content of the organic resin in the first coating layer 312 is the ratio of the mass of the organic resin to the total mass of the first coating layer 312. In a preferred embodiment, the mass content of the organic resin in the first coating layer 312 may be, for example, 10% to 30%. This content of organic resin may help form uniform and fine pores in the first coating layer 312. For example, the mass content of the organic resin in the first coating layer 312 may be, for example, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, or 30%, or any value in between.

[0058] The organic resin can be a heat-resistant polymer resin with a melting point or decomposition temperature greater than 200°C to improve the heat resistance of the diaphragm 31, such as the membrane rupture temperature. Organic resins include, but are not limited to, aromatic polyamides, polyimides, polyamide-imides, polysulfones, polyketones, polyetherketones, and polytetrafluoroethylene. The organic resin can also be selected from polymers having adhesive properties, such as polyvinylidene fluoride and its modifications, polyacrylonitrile, polyacrylates, and styrene-butadiene copolymers.

[0059] In addition to the organic resin, the first coating layer 312 may also include inorganic particles 314. The mass content of the inorganic particles 314 in the first coating layer 312 may be greater than 50%. The inorganic particles 314 include, but are not limited to, at least one of SrTiO3, SnO2, Mg(OH)2, MgO, Al(OH)3, Al2O3, SiO2, BaSO4, or TiO2, boehmite, and oxide solid electrolytes (including but not limited to lithium aluminum titanium phosphate, lithium lanthanum tantalate, lithium lanthanum zirconium oxide, etc.). The addition of the inorganic particles 314 can improve the heat resistance of the separator 31, such as improving its thermal shrinkage. Furthermore, the addition of the inorganic particles 314 can also facilitate forming the first coating layer 312 with a desired pore size structure.

[0060] The particle size D50 of the inorganic particles 314 in the first coating layer 312 (1)It can be 0.05 to 0.6 μm, preferably 0.1 to 0.4 μm. For example, the particle size of the inorganic particles 314 in the first coating 312 can be 0.05 μm, 0.1 μm, 0.15 μm, 0.2 μm, 0.25 μm, 0.3 μm, 0.35 μm, 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm or 0.6 μm or a value between any two of the above values. When the particle size of the inorganic particles 314 in the first coating 312 is within the above numerical range, a uniform and dense ceramic coating can be formed by combining with a specific content of organic resin. At the same time, the organic resin is distributed in a network-like manner between the particles of the inorganic particles 314, thereby ensuring that the pores formed in the first coating 312 are relatively uniform, so that the first coating 312 has good thermal shrinkage properties and a higher film rupture temperature.

[0061] The pore size distribution η of the first coating 312 is (1) =[D90 (1) -D25 (1) ] / D50 (1) , among which D25 (1) D50 represents the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the first coating 312 reaches 25%; (1) D90 represents the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the first coating 312 reaches 50%; (1) It represents the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the first coating layer 312 reaches 90%.

[0062] The pore size D90 of the first coating layer 312 (1) As a preferred embodiment, the pore size D90 of the first coating 312 may be less than 0.8 μm. (1) The value range of D90 can be 0.1 to 0.5 μm. For example, the D90 of the first coating 312 is (1) The value of can be, for example, 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, or 0.7 μm or any value between any two of the above values. (1) When the pore size D90 of the first coating layer 312 is less than 0.8 μm, especially 0.1 to 0.5 μm, the first coating layer 312 can have a higher lyophilic property, which is conducive to the electrolyte infiltration of the first coating layer 312. (1) It cannot be too small, otherwise the air permeability will be poor and the ability to transmit lithium ions will be poor. On the other hand, the pore size D90 of the first coating 312 (1) It should not be too large. If it is too large, the formed coating will be loose and not dense, and the heat resistance will be poor. At the same time, the liquid retention capacity of the first coating 312 and the second coating 313 will be dedifferentiated, which will worsen the poor liquid short board effect of the second coating 313.

[0063] The first coating 312 with the above structure can obtain good lyophilicity, electrolyte transport performance and heat resistance. During the battery charge and discharge cycle, the diaphragm 31 is always infiltrated by the electrolyte, ensuring that the active ions of the battery can shuttle in the diaphragm 31.

[0064] Second coating

[0065] The second coating layer 313 may include an organic resin. The mass content of the organic resin in the second coating layer 313 is greater than the mass content of the organic resin in the first coating layer 312. The mass content of the organic resin in the second coating layer 313 is the ratio of the mass of the organic resin to the total mass of the second coating layer 313. The mass content of the organic resin in the second coating layer 313 may be greater than or equal to 20%. In a preferred embodiment, the mass content of the organic resin in the second coating layer 313 may be, for example, 20% to 80%. This mass content of organic resin may help form pores with a wide range of pore size distribution in the second coating layer 313. For example, the mass content of the organic resin in the second coating layer 313 may be 20%, 30%, 40%, 50%, 55%, 58%, 60%, 63%, 65%, 68%, 70%, 72%, 75%, 77%, or 80%, or any value therebetween.

[0066] The organic resin in the second coating 313 can be a heat-resistant polymer resin with a melting point or decomposition temperature greater than 200°C to improve the heat resistance of the diaphragm 31, such as the membrane rupture temperature. The organic resin includes, but is not limited to, at least one of aromatic polyamide, polyimide, polyamide-imide, polysulfone, polyketone, polyetherketone, or polytetrafluoroethylene.

[0067] The organic resin content of the second coating layer 313 can be 1.2 to 5 times, preferably 1.5 to 4 times, the organic resin content of the first coating layer 312. When the organic resin content of the first and second coating layers 312 and 313 satisfies the aforementioned relationship, the separator 31 can achieve both a suitable pore size distribution and high heat resistance. By optimizing the organic resin content of the first and second coating layers 312 and 313, the asymmetric pore size distribution on both sides of the separator 31 can be further optimized, which can also improve the separator 31's heat resistance.

[0068] In addition to the organic resin, the second coating layer 313 may also include inorganic particles 314. The mass content of the inorganic particles 314 in the second coating layer 313 may be less than 80%. The inorganic particles 314 include, but are not limited to, at least one of SrTiO3, SnO2, Mg(OH)2, MgO, Al(OH)3, Al2O3, SiO2, BaSO4, or TiO2, boehmite, and oxide solid electrolytes (including but not limited to lithium aluminum titanium phosphate, lithium lanthanum tantalate, lithium lanthanum zirconium oxide, etc.). The addition of the inorganic particles 314 can improve the heat resistance of the separator 31, such as improving its thermal shrinkage. Furthermore, the addition of the inorganic particles 314 can also facilitate the formation of the desired pore structure in the second coating layer 313.

[0069] The particle size D50 of the inorganic particles 314 in the second coating layer 313 (2) The particle size of the inorganic particles 314 in the second coating layer 313 can be 0.2 to 2 μm, preferably 0.3 to 1.3 μm. For example, the particle size of the inorganic particles 314 in the second coating layer 313 can be 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, or 2.0 μm, or any value between any two of the above values. When the particle size of the inorganic particles 314 in the second coating layer 313 is within the above numerical range, combined with a specific content of the organic resin, pores with a wide pore size distribution can be formed.

[0070] The pore size distribution η of the second coating 313 is (2) =[D90 (2) -D25 (2) ] / D50 (2) , among which D25 (2) D50 represents the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the second coating 313 reaches 25%; (2) D90 represents the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the second coating 313 reaches 50%; (2) It represents the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the second coating layer 313 reaches 90%.

[0071] The pore size D90 of the second coating layer 313 (2) Larger than the pore size D90 of the first coating 312 (1) , so as to generate a driving force between the first coating 312 and the second coating 313, so as to drive the electrolyte to be more easily enriched on the side of the second coating 313 during the preparation of the battery cell. Among them, the pore size D90 of the second coating 313 (2) As a preferred embodiment, the pore size D90 of the second coating 313 may be greater than or equal to 0.5 μm. (2)The value range of D90 can be 0.6 to 2 μm. For example, the D90 of the second coating 313 is (2) The value of can be, for example, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm or 2.0 μm or a value between any two of the above values. (2) When the thickness is greater than or equal to 0.5 μm, especially when it is 0.6 to 2 μm, the second coating layer 313 can have a higher lyophilic property, which can facilitate the electrolyte to infiltrate the second coating layer 313 .

[0072] The pore size D90 of the second coating 313 is (1) It cannot be too small, otherwise it will be detrimental to the enrichment of the electrolyte in the second coating 313. On the other hand, the pore size D90 of the second coating 313 (2) It should not be too large. If it is too large, it will be difficult to form a liquid retention space for the electrolyte, and it will be easy to form a broken bridge between the second coating 313 and the electrolyte. In the later stage of the cycle, it will be unfavorable for the electrolyte to adhere to the surface of the second coating 313, thereby affecting the cycle performance of the battery.

[0073] Furthermore, the second coating layer 313 has a wide pore size distribution, indicating that the second coating layer 313 contains both small and large pores. The small pores in the second coating layer 313 can form lithium ion transport channels for lithium ion transmission. The large pores in the second coating layer 313 can be used to absorb and store electrolyte. Later in the battery's cycle life, the electrolyte in the large pores can replenish the electrolyte lost from the small pores, allowing the separator 31 to maintain its function as a lithium ion transport channel, thereby extending the battery's service life.

[0074] The second coating 313 with the above structure can achieve higher liquid retention performance, so that the diaphragm 31 is always infiltrated with electrolyte during the battery charge and discharge cycle, ensuring that the active ions of the battery can shuttle in the diaphragm 31.

[0075] FIG3 is a schematic diagram showing the relative relationship between the pore size distribution of the first coating layer and the pore size distribution of the second coating layer in one embodiment. As shown in FIG2 and FIG3, there is a certain difference in the pore size distribution of the first coating layer 312 and the second coating layer 313. The pore size distribution of the second coating layer 313 is wider than that of the first coating layer 312, and the corresponding pore size distribution difference is greater. The pore size distribution η of the second coating layer 313 of the diaphragm 31 of the embodiment of the present application is (2) The pore size distribution η of the first coating 312 (1) Ratio: η (2) / η (1) >1.0, preferably, η (2) / η (1)>1.1, further preferably, η (2) / η (1) The value of is 1.2 to 3. The greater the difference in pore size distribution between the first coating layer 312 and the second coating layer 313, the greater the difference in electrolyte retention between the first coating layer 312 and the second coating layer 313. The second coating layer 313 can always store more electrolyte, preventing electrolyte depletion on the negative electrode 21 side of the battery and affecting the battery's cycle life. At the same time, the pore size distribution of the first coating layer 312 and the second coating layer 313 cannot be too large. Otherwise, overly large pores will easily appear in the second coating layer 313, thereby affecting the contact between the second coating layer 313 and the electrolyte.

[0076] The structure and composition of the diaphragm are explained above, and the preparation method of the diaphragm is explained below.

[0077] The preparation method of the diaphragm of the embodiment of the present application may include the following steps: coating a first coating slurry on the surface of one side of the base film; coating a second coating slurry on the surface of the other side of the base film; the first coating slurry and the second coating slurry are different slurries; the base film coated with the first coating slurry and the second coating slurry is subjected to multi-stage extraction, and then the diaphragm is obtained after drying.

[0078] In the obtained diaphragm, the pore size distribution η of the second coating is (2) The pore size distribution η of the first coating (1) Satisfy: η (2) / η (1) >1.0, where

[0079] η (1) =[D90 (1) -D25 (1) ] / D50 (1) , η (2) =[D90 (2) -D25 (2) ] / D50 (2) ;

[0080] D25 (1) Indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the first coating reaches 25%;

[0081] D50 (1) Indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the first coating reaches 50%;

[0082] D90 (1) Indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the first coating reaches 90%;

[0083] D25 (2)Indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the second coating reaches 25%;

[0084] D50 (2) Indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the second coating reaches 50%;

[0085] D90 (2) It indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the second coating reaches 90%.

[0086] The coating slurry may include a first coating slurry and a second coating slurry. The first coating slurry is used to form a first coating layer. The second coating slurry is used to form a second coating layer. The organic resins in both the first coating slurry and the second coating slurry may be aramid resins. The aramid resin may be at least one of meta-, para-, or other heterocyclic, or modified aramid resins. When preparing the first coating slurry, the organic resin may be first dissolved in one or more good solvents, and then the inorganic particles may be dispersed therein to obtain a first coating slurry containing the organic resin and the inorganic particles. The second coating slurry may be prepared according to the method for preparing the first coating slurry. The good solvent includes, but is not limited to, one or a combination of at least two of N-methylpyrrolidone (NMP), dimethylacetamide (DMAC), or N,N-dimethylformamide (DMF).

[0087] In addition to the organic resin and inorganic particles, the coating slurry may also contain a cosolvent / pore-forming agent, and a poor solvent for the organic resin. Cosolvents / pore-forming agents include, but are not limited to, one or a combination of at least two of lithium chloride, magnesium chloride, calcium chloride, calcium carbonate, and calcium chloride. Poor solvents include, but are not limited to, one or a combination of at least two of water, methanol, ethanol, propanol, acetone, ethyl acetate, dichloromethane, and petroleum ether.

[0088] In the coating slurry, the weight proportion of organic resin is 2-20%, the weight proportion of inorganic particles can be 0-30%, the weight proportion of co-solvent can be 0-8%, the weight proportion of poor solvent can be 0-5%, and the weight proportion of good solvent can be 60-85%. Among them, the amount of organic resin and inorganic particles added in the first coating slurry and the second coating slurry is different. The specific amount can be formulated with reference to the weight content of organic resin and inorganic particles in the composition structure of the diaphragm, and will not be explained one by one here. The first coating slurry and the second coating slurry are respectively applied to both sides of the base film, and then subjected to multi-stage extraction. Among them, the coating method of the coating slurry can be gravure roller, wire rod, extrusion or dip coating, etc.

[0089] In one embodiment, the multi-stage extraction can be performed in multiple extraction tanks. The extraction tanks contain a mixed solution of a good solvent and a poor solvent for the organic resin. The concentration of the good solvent in different extraction tanks during the multi-stage extraction can vary. The poor solvent in the extraction tanks can be water.

[0090] The multi-stage extraction tank is at least a three-stage extraction tank, i.e., it has three extraction sections. The good solvent content in the first-stage extraction tank is 35-60% by weight, the good solvent content in the second-stage extraction tank is 0-35% by weight, and the good solvent content in the third-stage extraction tank is 0-2.5% by weight. The extraction times for the first and second stages are 5-15 seconds, respectively.

[0091] The good solvent concentration distribution within the first-stage extraction tank is uneven. The good solvent concentration gradually decreases as the base film moves within the first-stage extraction tank, and the difference between the highest and lowest good solvent concentrations within the first-stage extraction tank is 5-15%. Controlling the uneven good solvent concentration within the first-stage extraction tank can be achieved in a variety of ways, such as by placing baffles in the direction of the base film's movement to slow the diffusion of the good solvent within the extraction tank, thereby varying the good solvent concentration across the extraction tank.

[0092] The diaphragm of the present application will be further described in detail with reference to specific embodiments.

[0093] Example 1

[0094] The preparation method of the diaphragm of this embodiment includes the following steps:

[0095] S11. Preparation of first coating slurry

[0096] Add 0.5 kg of calcium chloride, a dissolution aid and pore-forming agent, to 8.7 kg of DMAC and stir thoroughly to dissolve. Then, add 0.8 kg of meta-aramid powder and stir in a 40°C water bath for 1 hour until the aramid is completely dissolved. Add 2.4 kg of 0.2 μm alumina to 7.6 kg of DMAC and grind thoroughly to obtain an alumina dispersion. The aramid solution and alumina dispersion are mixed to form the first coating slurry; the mass ratio of aramid to alumina is 1:3.

[0097] S12. Preparation of second coating slurry

[0098] Add 0.9 kg of calcium chloride, a dissolution aid and pore-forming agent, to 15 kg of DMAC and stir thoroughly to dissolve. Then, add 1.5 kg of meta-aramid powder and stir in a 40°C water bath for 1 hour until the aramid is completely dissolved. Add 1.0 kg of 0.5 μm alumina to 3 kg of DMAC and grind thoroughly to obtain an alumina dispersion. The aramid solution and alumina dispersion are mixed to form a second coating slurry; the mass ratio of aramid to alumina is 1.5:1.

[0099] S13, diaphragm preparation

[0100] Using a gravure roller, the first and second coating slurries were applied to both sides of a 7μm PE base film, with the upper roller applying the first coating slurry and the lower roller applying the second coating slurry at a coating speed of 30 m / min. After coating, the film was cured in a mixture of DMAC and water. The DMAC content in the first extraction tank was 40-50%, in the second extraction tank was 30%, and in the third extraction tank was <2.5%. The membrane passed through each extraction tank for 8 seconds. After washing, it was dried in a three-stage oven at 50°C, 55°C, and 60°C. After drying, a 10μm aramid coating film was obtained. The first coating layer of this film contained 25% aramid by weight and 75% alumina by weight. The second coating layer contained 60% aramid by weight and 40% alumina by weight.

[0101] Example 2

[0102] This embodiment is a diaphragm, and the specific preparation method is the same as that of Example 1, except that in the second coating layer, the mass ratio of aramid to alumina is 7:3, and the alumina particle size D50 is 0.6 μm.

[0103] Example 3

[0104] This embodiment is a diaphragm, and the specific preparation method is the same as that of Example 1, except that in the second coating, the mass ratio of aramid to alumina is 5.5:4.5, and the alumina particle size D50 is 0.4 μm.

[0105] Example 4

[0106] This embodiment is a diaphragm, and the specific preparation method is the same as that of Example 1, except that in the first coating layer, the mass ratio of aramid to aluminum oxide is 3.5:6.5.

[0107] Example 5

[0108] This embodiment is a diaphragm, and the specific preparation method is the same as that of Example 1, except that in the first coating layer, the particle size of aluminum oxide is 0.4 μm.

[0109] Example 6

[0110] This embodiment is a diaphragm, and the specific preparation method is the same as that of Example 1, except that in the first coating layer, the inorganic particles are barium sulfate.

[0111] Example 7

[0112] This embodiment is a diaphragm, and the specific preparation method is the same as that of Example 1, except that in the second coating layer, the mass ratio of aramid to aluminum oxide is 9:1.

[0113] Example 8

[0114] This embodiment is a diaphragm, and the specific preparation method is the same as that of Example 1, except that in the second coating layer, the aluminum oxide particle size D50 is 1.4 μm.

[0115] Comparative Example 1

[0116] The preparation method of the diaphragm of this comparative example comprises the following steps:

[0117] S11. Preparation of coating slurry

[0118] Add 0.5 kg of calcium chloride, a dissolution aid and pore-forming agent, to 8.7 kg of DMAC and stir thoroughly to dissolve. Then, add 0.8 kg of meta-aramid powder and stir in a 40°C water bath for 1 hour until the aramid is completely dissolved. Add 2.4 kg of 0.2 μm alumina to 7.6 kg of DMAC and grind thoroughly to obtain an alumina dispersion. The aramid solution and alumina dispersion are mixed to form a coating slurry.

[0119] S12, diaphragm preparation

[0120] A gravure roller was used to coat both sides of a 7μm PE-based film with an aramid slurry. Both the upper and lower rollers were coated with the same coating slurry at a coating speed of 30 m / min. After coating, the membrane was placed in a mixture of DMAC and water for curing. The DMAC content in the first extraction tank was 40-50%, the second extraction tank had a DMAC content of 30%, and the third extraction tank had a DMAC content of <2.5%. The membrane passed through each extraction tank for 8 seconds. After washing, it was dried in a three-stage oven at 50°C, 55°C, and 60°C, respectively. After drying, a 10μm aramid coating membrane was obtained, with both sides showing a symmetrical coating with the same composition and pore size distribution.

[0121] Comparative Example 2

[0122] This comparative example is a diaphragm, and the specific preparation method is the same as that of comparative example 1, except that in the coating, the mass ratio of aramid to alumina is 7:3, and the particle size of alumina is 0.5 μm.

[0123] Comparative Example 3

[0124] This comparative example is a diaphragm prepared in the same manner as Example 1, except that in the first coating layer, the organic resin is PVDF, the mass ratio of the organic resin to alumina is 4:6, and the particle size of the inorganic ion alumina is 0.6 μm. In the second coating layer, the particle size of the inorganic ion alumina is 0.4 μm.

[0125] Comparative Example 4

[0126] This comparative example is a diaphragm prepared in the same manner as in Comparative Example 1, except that in the first coating layer, the mass ratio of aramid to alumina is 3.5:6.5, and the alumina particle size is 0.2 μm. In the second coating layer, the mass ratio of organic resin to alumina is 5.5:4.5, and the inorganic ion alumina particle size is 0.4 μm.

[0127] Among them, during the preparation process of the diaphragms of each embodiment and comparative example, the dissolution mixing time, the addition amount of good solvent and poor solvent, the extraction process parameters, and the drying process parameters can be adjusted accordingly according to the specific material composition and ingredients to match the relevant process parameters with the raw material composition.

[0128] The composition parameters of the membranes of Examples 1-8 and Comparative Examples 1-4 are listed in Table 1.

[0129] Lithium-ion batteries were assembled using the separators of Examples 1-8 and Comparative Examples 1-4. The specific assembly process included the following steps:

[0130] S21. Production of positive electrode

[0131] The positive electrode active material, lithium iron phosphate, the conductive agent, carbon black, and the binder, polyvinylidene difluoride (PVDF), were mixed with NMP solvent at a mass ratio of 96.5:1.0:2.5 and stirred evenly to form a positive electrode slurry. The slurry was evenly coated on both sides of aluminum foil using a coating device and oven-dried to remove the NMP solvent. The coated electrode was then cold-pressed, slit, and tab-welded to form the positive electrode sheet.

[0132] S22, negative electrode sheet production

[0133] The negative electrode active material artificial graphite, conductive agent SP, thickener carboxymethyl cellulose (CMC), and adhesive polymerized styrene butadiene rubber (SBR) were stirred and evenly mixed in deionized water at a weight ratio of 96:1.0:1.5:1.5 to form a negative electrode slurry. The negative electrode slurry was evenly coated on both sides of the copper foil using a coating device. The electrode sheet after oven drying was cold pressed, slit, and tab welding processes to produce a negative electrode sheet.

[0134] S23. Winding the positive electrode sheet, separator, and negative electrode sheet together to form a bare cell, wherein the first coating of the separator is disposed toward the positive electrode sheet and the second coating is disposed toward the negative electrode sheet. The bare cell is then packaged, baked, injected, allowed to stand, formed, vented, and measured for capacitance to form a lithium-ion battery.

[0135] The pore size distribution of each embodiment and comparative example separator and the thermal and cycling performance of the corresponding lithium-ion battery were tested. The test results are listed in Table 1. The specific testing procedures for the pore size distribution test, battery hot box test, and cycling performance test are as follows.

[0136] 1. Pore size distribution test

[0137] 1) Divide a roll of separator into three equal parts in the TD and MD directions to form a nine-square grid. Randomly select and cut a 1 cm*1 cm sample from each grid, for a total of 9 samples;

[0138] 2) Use SEM to take pictures of the cross section of each sample and select the pictures at a magnification of 20K;

[0139] 3) For each sample, a 5.0 μm*1.5 μm rectangular area was selected for cross-section and surface electron microscopy. The entire area was divided into 500*150 pixels, each pixel was 10 nm long and 100 nm in area. 2 The color level of each pixel is identified and summarized. A is the maximum color level of the area, and B is the minimum color level of the area. The color level value of any pixel in the area is an:

[0140] C=(an-B) / (AB)×100%,

[0141] Wherein, C is the discrimination value of the pixel point. If C is less than 40%, the pixel point is identified as a hole structure; otherwise, it is identified as a boundary structure.

[0142] The above judgment is performed on all pixels in the entire area. All pore structures and boundary structure distributions are distinguished. For a closed area surrounded by a boundary structure, it is judged as a pore. The area of ​​the pore containing the pixels is the pore area S. The pore diameter d is calculated as:

[0143] d=2*(S / π)^0.5

[0144] 4) The cross-sectional pore diameters of the nine samples were accumulated to produce a cross-sectional pore diameter distribution diagram. D50 is the pore size when the cumulative volume of all pore diameters of the nine samples accounts for 50%.

[0145] 2. Battery hot box test:

[0146] After fully charging the battery using the standard charging mode, perform the test within 12-24 hours. Heat the battery using convection or a circulating hot air oven, starting at 25±3°C at a rate of 5±2°C / min, to 180±2°C. Hold the temperature for 30 minutes before the test is complete. The battery passes the test if it does not catch fire or explode.

[0147] 3. Cycle performance test:

[0148] After the battery cell is tested for core capacity at 0.2C at 25°C, it is discharged to 2.5V at a constant current and left to stand for 30 minutes;

[0149] The battery cell was placed in a 55°C environment for 4 hours. The battery was cycled according to the following steps, and the capacity retention rate after 1000 cycles was calculated: 0.5C constant current and constant voltage charging to 3.65V / 0.05C cut-off, standing for 30 minutes, and 0.5C constant current discharge to 2.5V and standing for 30 minutes.

[0150] Table 1

[0151] Table 1 (continued)

[0152] As can be seen from the data in Table 1, both sides of Comparative Examples 1 and 2 have coatings with the same composition and structure. In the lithium-ion batteries corresponding to the separators of Comparative Examples 1 and 2, new SEI films are repeatedly formed on the surface of the negative electrode material during cycling, continuously consuming the electrolyte. Therefore, the side of the separator close to the negative electrode plate is more susceptible to electrolyte depletion, which in turn forms an electrolyte bridge between the negative electrode plate and the separator, resulting in a reduction in the cycle life of the battery cell and a decrease in the capacity retention rate of the lithium-ion battery after 100 cycles.

[0153] In Comparative Example 2, both sides of the separator coating used the same coating formula. Inorganic particles with a large particle size (0.5 μm) and a low content (30%) were used on both sides. This resulted in a loose coating, which resulted in a larger thermal shrinkage of the separator, adversely affecting the safety performance of the battery cell. Furthermore, the pore structure and pore distribution of the coatings on both sides of the separator were identical, and the negative electrode still experienced liquid starvation in the later stages of the cycle, resulting in poor cycling performance.

[0154] The capacity retention rates of the lithium-ion batteries corresponding to Examples 1-8 of the present application after 100 cycles are all higher than those of Comparative Examples 1-2, indicating that when the coatings on both sides of the separator meet the requirements defined in the present application, its cycle life can be significantly improved.

[0155] The test data of Examples 1-8 show that the capacity retention rate of Example 2 is better than that of Examples 1, 4, and 6, and even better than that of Examples 3, 5, and 8. The data of Example 7 is relatively low. This indicates that the cycle life of lithium-ion batteries can be improved by optimizing the pore size distribution ratio of the first coating layer and the second coating layer.

[0156] In addition, from the comparative data of Examples 1-8 and Example 9, it can be seen that in Example 9, although the pore size distribution ratio of the second coating layer and the first coating layer of the diaphragm satisfies η (2) / η (1) >1.0, however, due to the pore size D90 of the second coating of the diaphragm (2) Smaller than the pore size D90 of the first coating (1) , and the effects of Examples 1-8 of the present application cannot be achieved. The main reason may be that when the pore size D90 of the second coating is (2) Smaller than the pore size D90 of the first coating (1) When the electrolyte is retained by the second coating, the electrolyte retention capacity of the second coating is insufficient. In the later stage of the battery cycle, the electrolyte on the side of the diaphragm facing the negative electrode cannot be effectively replenished, resulting in a decrease in the capacity retention rate of the lithium-ion battery.

[0157] In Comparative Example 4, the ratio of the pore size distribution of the second coating layer to the first coating layer does not satisfy η (2) / η (1) >1.0. The corresponding lithium-ion battery capacity retention rate is also low. This indicates that when the pore size distribution ratio of the first coating layer to the second coating layer does not meet the range value specified in this application, the second coating layer is insufficiently wetted by the electrolyte, thereby causing a decrease in the cycle performance of the lithium-ion battery.

[0158] In summary, in the diaphragm of the present application, by optimizing the content of organic resin in the first coating and the second coating and the particle size of inorganic particles, the second coating has a higher organic resin content relative to the first coating, and the inorganic particles with larger particle size can help the second coating obtain a wider pore size distribution relative to the first coating, thereby achieving an asymmetric distribution of pore size and an asymmetric distribution of electrolyte, and the electrolyte is more enriched in the second coating. The small-sized inorganic particles in the first coating can give the diaphragm a higher heat resistance. Therefore, the diaphragm of the embodiment of the present application can obtain a longer cycle life on the basis of having higher heat resistance.

[0159] The above are only specific embodiments of the present application, but the scope of protection of this application is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in this application should be included in the scope of protection of this application. Therefore, the scope of protection of this application should be based on the scope of protection of the claims.

Claims

1. A diaphragm, characterized in that: It includes a base film, a first coating layer and a second coating layer, wherein the first coating layer and the second coating layer are respectively arranged on both sides of the base film; The pore size distribution η of the second coating (2) The pore size distribution η of the first coating (1) Satisfy: η (2) / η (1) >1.0, where or (1) =[D90 (1) -D25 (1) ] / D50 (1) ,or (2) =[D90 (2) -D25 (2) ] / D50 (2) ; D25 (1) It indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the first coating reaches 25%; D50 (1) It indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the first coating reaches 50%; D90 (1) It indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the first coating reaches 90%; D25 (2) It indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the second coating reaches 25%; D50 (2) It indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the second coating reaches 50%; D90 (2) It indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the second coating reaches 90%.

2. The diaphragm according to claim 1, characterized in that The pore size D90 of the second coating (2) Larger than the pore size D90 of the first coating (1) .

3. The diaphragm according to claim 2, characterized in that The pore size D90 of the first coating (1) The pore size D90 of the second coating is less than 0.8 μm. (2) Greater than or equal to 0.5μm.

4. The diaphragm according to claim 3, characterized in that The pore size D90 of the first coating (1) The pore size D90 of the second coating is 0.1-0.5 μm. (2) 0.6~2μm.

5. The diaphragm according to any one of claims 1 to 4, characterized in that: The η (2) / η (1) The value ranges from 1.2 to 3.

6. The diaphragm according to any one of claims 1 to 5, characterized in that: The first coating and the second coating both include organic resin; wherein the mass content of the organic resin in the first coating accounts for less than 50% of the total mass of the first coating, and the mass content of the organic resin in the second coating accounts for a greater proportion of the total mass of the second coating than the mass content of the organic resin in the first coating.

7. The diaphragm according to claim 6, characterized in that The mass content of the organic resin in the first coating layer is 10-30%, and the mass content of the organic resin in the second coating layer is 20-80%.

8. The diaphragm according to claim 6 or 7, characterized in that: The content of the organic resin in the second coating layer is 1.2 to 5 times the content of the organic resin in the first coating layer.

9. The diaphragm according to any one of claims 6 to 8, characterized in that: The first coating layer and the second coating layer both include inorganic particles; the particle size D50 of the inorganic particles in the first coating layer is 0.05 to 0.6 μm, and the particle size D50 of the inorganic particles in the second coating layer is 0.2 to 2 μm.

10. The diaphragm according to claim 9, characterized in that The particle size D50 of the inorganic particles in the second coating layer and the particle size D50 of the inorganic particles in the first coating layer are 1.5 to 5.

11. A method for preparing a diaphragm, characterized in that: include: Applying a first coating slurry on one side of the base film; Applying a second coating slurry on the other side surface of the base film; The first coating slurry and the second coating slurry are different slurries; The base film coated with the first coating slurry and the second coating slurry is subjected to multi-stage extraction, and then dried to obtain the separator; In the obtained diaphragm, the pore size distribution η of the second coating is (2) The pore size distribution η of the first coating (1) Satisfy: η (2) / η (1) >1.0, where or (1) =[D90 (1) -D25 (1) ] / D50 (1) ,or (2) =[D90 (2) -D25 (2) ] / D50 (2) ; D25 (1) It indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the first coating reaches 25%; D50 (1) It indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the first coating reaches 50%; D90 (1) It indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the first coating reaches 90%; D25 (2) It indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the second coating reaches 25%; D50 (2) It indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the second coating reaches 50%; D90 (2) It indicates the pore size corresponding to when the volume percentage of the cumulative pore size distribution in the second coating reaches 90%.

12. The preparation method according to claim 11, characterized in that: The multi-stage extraction includes three-stage extraction, the mass proportion of good solvent in the first-stage extraction is 35-60%, the mass proportion of good solvent in the second-stage extraction tank is 0-35%, and the mass proportion of good solvent in the third-stage extraction tank is 0-2.5%.

13. The preparation method according to claim 12, characterized in that: In the first stage extraction, the concentration of the good solvent gradually decreases in the moving direction of the base film in the first extraction tank, and the difference between the highest concentration of the good solvent and the lowest concentration of the good solvent in the first extraction tank is 0.1-15%.

14. The preparation method according to claim 12 or 13, characterized in that: The time of the first stage extraction and the second stage extraction are respectively 5 to 15 seconds.

15. A battery, characterized in that: It includes a positive electrode sheet, a negative electrode sheet, an electrolyte and a diaphragm as described in any one of claims 1 to 10, wherein the diaphragm is arranged between the positive electrode sheet and the negative electrode sheet, and the first coating of the diaphragm is arranged toward the positive electrode sheet, the second coating of the diaphragm is arranged toward the negative electrode sheet, and the electrolyte impregnates the diaphragm.

16. An electrical equipment, characterized in that: Comprising the battery of claim 15.

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