Separator, and secondary battery and electrical device therefor

Abstract

A separator comprising a porous substrate and a coating arranged on at least one surface of the porous substrate, wherein the coating comprises a three-dimensional framework structure and a filler, wherein at least a portion of the filler is filled into the three-dimensional framework structure; and wherein, in a thickness direction of the separator, a mean pore area of ​​the coating is designated as S1 and a mean pore area of ​​the porous substrate is designated as S2, wherein 0 < S1 / S2 < 1.

Description

TECHNICAL FIELDThis application relates to the field of battery technologies and specifically relates to a separator, and a secondary battery and associated electrical devices.GENERAL STATE OF THE ARTIn recent years, secondary batteries have been widely used in energy storage systems such as hydroelectric, thermal, wind and solar power plants and many other fields including electric tools, electric bicycles, electric bicycles, electric vehicles, military equipment and aerospace. With the application and popularity of secondary batteries, safety issues, particularly thermal safety issues, of secondary batteries have gained increasing attention. However, at present, methods for improving the thermal safety performance of secondary batteries are generally disadvantageous for compensating energy density and durability of secondary batteries. Therefore, how secondary batteries can achieve a balance between high energy density, high thermal safety performance, as well as good cycle performance and kinetic performance is a central challenge in the design of secondary batteries.SUMMARYThis application is intended to provide a separator, a manufacturing method thereof, and a secondary battery and an electric device therefor, for allowing the secondary battery to balance between high energy density, high thermal safety performance, and good cycle performance and kinetic performance.A first aspect of this application provides a separator including a porous substrate and a coating disposed on at least one surface of the porous substrate, the coating including a three-dimensional framework structure and a filler, at least a part of the filler being filled in the three-dimensional framework structure; and in a thickness direction of the separator, a center pore area of the coating is denoted by S 1 and a center pore area of the porous substrate is denoted by S 2 where 0<S 1 / S 2< 1.The inventors of this application have surprisingly discovered, in their research, that when the coating includes the three-dimensional framework structure and the filler, at least a part of the filler is filled in the three-dimensional framework structure, and when the average pore area S 1 of the coating and the average pore area S 2 of the porous substrate are set to 0<S 1 / S 2< 1 the separator can achieve a balance between high heat resistance and high ionic conductivity, and a secondary battery can also achieve a balance between high energy density, high thermal safety performance, and good cycle performance and kinetic performance.In each embodiment of this application, 0.06≤S 1 / S 2< 1 and optionally 0.30≤S 1 / S 2≤0.97.In each embodiment of this application, 0.0002 μm 2 ≤ S 1 ≤ 0.0080 μm 2 and optionally 0.0004 μm 2 ≤ S 1 ≤ 0.0050 μm 2. The average pore area S 1 of the coating falling within the above range can increase the heat resistance of the separator without hindering the active ion transport, whereby the secondary battery can achieve a better balance between high energy density, high thermal safety performance, and good cycle performance and kinetic performance.In each embodiment of this application, 0.0005 μm 2 ≤ S 2 ≤ 0.0100 μm 2 and optionally 0.0008 μm 2 ≤ S 2 ≤ 0.0080 μm 2. The average pore area S 2 of the porous substrate falling within the above range is promoted to increase the ionic conductivity of the separator and the extractable capacity of the secondary battery.In each embodiment of this application, an average pore diameter of the separator is referred to as d 1 and an average pore diameter of the porous substrate is referred to as d 2 where d 1 / d 2< 1 and optionally 0.3≤d 12≤0.8. By the ratio d 1 / d 2 of the average pore diameter d 1 of the separator to the average pore diameter d 2 of the porous substrate falling within the above range, it can be ensured that the separator has high ionic conductivity under the condition of high heat resistance.In each embodiment of this application, 15 nm ≤ d 1 ≤ 50 nm and optionally 20 nm ≤ d 2 ≤ 40 nm. The average pore diameter d 1 of the separator falling within the above range can increase the heat resistance of the separator without hindering the active ion transport, whereby the secondary battery can achieve a better balance between high energy density, high thermal safety performance, and good cycle performance and kinetic performance.In each embodiment of this application, 25 nm ≤ d 2 ≤ 60 nm and optionally 30 nm ≤ d 2 ≤ 50 nm. The average pore diameter d 2 of the porous substrate falling within the above range is useful for increasing the ionic conductivity of the separator and the extractable capacity of the secondary battery.In each embodiment of this application, the surface density of the coating is denoted as ρ 1 and the surface density of the porous substrate is denoted as ρ 2 where 0.15≤p 1 / ρ 2≤0.80, and optionally 0.20 ρ 1 / ρ 2≤0.50. ρ 1 / ρ 2, which falls within the above range, is conducive to high heat resistance of the coating and also helps the secondary battery achieve a balance between high energy density and high safety thermal performance.In each embodiment of this application, optionally 0.50 g / m 2 ≤ ρ 1 ≤ 1.50 g / m 2, and more optionally 0.75 g / m 2 ≤ ρ 1 ≤ 1.40 g / m 2. The surface density ρ 1 of the coating falling within the above range helps the secondary battery achieve a balance between high energy density and high safety thermal performance, allows the coating to have a corresponding average pore area, and can further increase the heat resistance and ion conductivity of the separator.In each embodiment of this application, optionally 1.50 g / m 2 ≤ ρ 2 ≤ 4.50 g / m 2 and more optionally 2.00 g / m 2 ≤ ρ 2 ≤ 4.00 g / m 2. The surface density ρ 2 of the porous substrate falling within the above range allows the porous substrate to have a corresponding average pore area, and is also conducive to increase the ion conductivity of the separator and the extractable capacity of the secondary battery.In each embodiment of this application, porosity of the separator is referred to as P 1 and porosity of the porous substrate is referred to as P 2 where 0.4≤P 2 / P 1< 1 and optionally, 0.55≤P 2 / P 1≤0.85, P 2 / P 1, which fall within the above range, may help the secondary battery achieve a balance between high thermal safety performance as well as good cycle performance and kinetic performance.In each embodiment of this application, 20% ≤ P 1 ≤ 60%, and optionally 25% ≤ P 1 ≤ 45%. The porosity P 1, of the separator falling within the above range may help the secondary battery achieve a balance between high safety thermal performance as well as good cycle performance and kinetic performance.In each embodiment of this application, 15% ≤ P 2 ≤ 45% and optionally 20% ≤ P 2 ≤ 40%. The porosity P 2 of the porous substrate falling within the above range is conducive to increase the ionic conductivity of the separator and the extractable capacity of the secondary battery.In each embodiment of this application, the filler includes a first filler, wherein an average particle size of the first filler is less than or equal to 150 nm, optionally 15 nm to 120 nm. The first filler having advantages of a small average particle size, a large specific surface area, and a good affinity to the three-dimensional framework structure can be better interlocked with the three-dimensional framework structure, whereby the coating can have a more stable spatial network structure. As a result, not only the ionic conductivity of the separator is increased, but also the heat resistance and electrolyte infiltration as well as the retention of the separator. Moreover, the average particle size of the first filler falling within the above range also enables a corresponding average pore area of the coating and further enhances the heat resistance and ion conductivity of the separator.In each embodiment of this application, the first filler includes at least one of primary and secondary particles.In each embodiment of this application, an average particle size of the first filler of primary particle morphology is 15 nm to 80 nm, optionally 30 nm to 75 nm.In each embodiment of this application, an average particle size of the first filler of secondary particle morphology is 50 nm to 150 nm, optionally 55 nm to 120 nm.In each embodiment of this application, the first filler includes at least one of an inorganic particle and an organic particle. Optionally, the inorganic particle includes at least one of boehmite, alumina, barium sulfate, magnesia, magnesium hydroxide, silicon oxygen compound, tin dioxide, titanium oxide, calcium oxide, zinc oxide, zirconium oxide, yttrium oxide, nickel oxide, hafnium dioxide, cerium oxide, zirconium titanate, barium titanate, and magnesium fluoride, and even more optionally, the inorganic particle includes at least one of boehmite, alumina, barium sulfate, magnesium oxide, silicon oxygen compound, titanium oxide, zinc oxide, cerium oxide, and barium titanate; and optionally, the organic particle includes at least one of polystyrene particles, polypropylene wax particles, melamine formaldehyde resin particles, phenol resin particles, polyester particles, polyimide particles, polyamideimide particles, polyaramid particles, polyphenylene sulfide particles, polysulfone particles, polyethersulfone particles, polyetheretherketone particles, and polyaryletherketone particles.In each embodiment of this application, the first filler includes inorganic particles, wherein a crystalline form of the inorganic particles includes at least one of a θ-crystalline form, γ-crystalline form, and η-crystalline form, and optionally wherein the crystalline form of the inorganic particles includes at least one of the θ-crystalline form and the γ-crystalline form.In each embodiment of this application, based on a total weight of the inorganic particles in the first filler, a proportion of the inorganic particles in a θ-crystalline form is greater than or equal to 50% by weight, optionally 60% by weight to 82% by weight.In each embodiment of this application, based on the total weight of the inorganic particles in the first filler, a proportion of the inorganic particles in a γ-crystalline form is greater than or equal to 10 wt %, optionally 17 wt % to 38 wt %.In each embodiment of this application, based on a total weight of the inorganic particles in the first filler, a proportion of the inorganic particles in an η-crystalline form is less than or equal to 5 wt %, optionally less than or equal to 1.5 wt %.The selection of the first filler in various crystalline forms contributes to the increase in the heat resistance, ionic conductivity and / or electrolyte infiltration and retention of the separator.In each embodiment of this application, the BET specific surface area of the first filler is greater than or equal to 15 m 2 / g, optionally 18 m 2 / g to 65 m 2 / g. The BET specific surface area of the first filler falling within the above range allows a corresponding average pore area of the coating and further increases the heat resistance and ionic conductivity of the separator; and further increases the affinity between the first filler and the three-dimensional framework structure, thereby allowing a more stable spatial network structure of the coating and, in turn, better heat resistance and higher ionic conductivity of the separator.In each embodiment of this application, based on a total weight of the coating, a proportion of the first filler is greater than or equal to 55 wt %, optionally 60 wt % to 90 wt %. The proportion of the first filler falling within the above range enables a corresponding average pore area of the coating and further enhances the heat resistance and ion conductivity of the separator; and additionally enables a more stable spatial network structure of the coating, thereby further enhancing the heat resistance and ion conductivity of the separator.In each embodiment of this application, the filler further includes a second filler, wherein an average particle size of the second filler is greater than the average particle size of the first filler. The second filler has a larger particle size and can play its auxiliary role in the coating more, reduce shrinkage of the first filler, and reduce the amount of a binder, thereby improving the heat resistance of the separator. The second filler has a larger particle size, thereby allowing more pore structures and a lower water content of the coating when a small amount of the second filler is used, thereby further improving the ionic conductivity and electrolyte infiltration and retention of the separator.In each embodiment of this application, the average particle size of the second filler is less than or equal to 400 nm, optionally 100 nm to 300 nm.In each embodiment of this application, the second filler has a primary particle morphology.In each embodiment of this application, the BET specific surface area of the second filler is greater than or equal to 15 m 2 / g, optionally 7 m 2 / g to 12 m 2 / g. The BET specific surface area of the second filler falling within the above range allows an appropriate average pore area of the coating and further increases the heat resistance and ionic conductivity of the separator; and allows the second filler to play its supporting role better, reduce the water content in the coating, and maintain the coating a stable pore structure during a long-term charging and discharging process, thereby facilitating ion transport and simultaneously increasing the heat resistance of the separator.In each embodiment of this application, based on a total weight of the coating, a proportion of the second filler is less than or equal to 20 wt %, optionally 2 wt % to 15 wt %. The proportion of the second filler falling within the above range allows an appropriate average pore area of the coating and further increases the heat resistance and ionic conductivity of the separator; and allows the second filler to play its supporting role better, reduce the water content in the coating, and maintain the coating a stable pore structure during a long-term charging and discharging process, thereby facilitating ion transport and simultaneously increasing the heat resistance of the separator.In each embodiment of this application, the second filler includes at least one of an inorganic particle and an organic particle.In each embodiment of this application, the second filler includes inorganic particles having primary particle morphology, and a crystalline form of the inorganic particles having primary particle morphology includes at least one of an α-crystalline form and a γ-crystalline form, and optionally includes an α-crystalline form. The second filler in α-crystalline form has advantages of high hardness, good heat resistance, low dielectric constant, high safety and high clean density, thereby further enhancing the heat resistance of the coating.In each embodiment of this application, the second filler includes inorganic particles having primary particle morphology, and a crystalline form of the inorganic particles having primary particle morphology includes an α-crystalline form, wherein, based on a total weight of the inorganic particles having primary particle morphology in the second filler, a proportion of the inorganic particles in α-crystalline form is greater than or equal to 80 wt %, optionally 90 wt % to 100 wt %.In each embodiment of this application, based on a total weight of the coating, a proportion of the three-dimensional framework structure is less than or equal to 40 wt %, optionally 5 wt % to 25 wt %. The proportion of the three-dimensional framework structure falling within the above range enables a corresponding average pore area of the coating and further enhances the heat resistance and ion conductivity of the separator.In each embodiment of this application, the three-dimensional framework structure is formed by a fibrous substance, wherein the morphology of the fibrous substance optionally includes at least one of a rod shape, a tube shape, a rod shape, and a fibrous shape.In each embodiment of this application, an average diameter of a material constituting the three-dimensional framework structure is less than or equal to 40 nm, optionally 10 nm to 35 nm. The average diameter of the material constituting the three-dimensional framework structure falling within the above range helps the material constituting the three-dimensional framework structure to interlock with the filler as a whole.In each embodiment of this application, an average length of the material forming the three-dimensional framework structure is 100 nm to 800 nm, optionally 200 nm to 600 nm. The average length of the material constituting the three-dimensional framework structure falling within the corresponding range can further increase the heat resistance and the ion conductivity of the separator.In each embodiment of this application, a length-diameter ratio of the material constituting the three-dimensional framework structure is 5 to 60, optionally 10 to 30, and the average length of the material constituting the three-dimensional framework structure falling within the corresponding range can further increase the heat resistance and the ion conductivity of the separator.In each embodiment of this application, a material constituting the three-dimensional framework structure includes an organic material and / or an inorganic material. Optionally, the organic material includes nano-cellulose, polytetrafluoroethylene nano-fiber, and / or polyamide nano-fiber, and optionally, the nano-cellulose includes at least one of cellulose nano-fibrils, cellulose nano-crystals, and / or bacterial nano-cellulose. Optionally, the inorganic material includes at least one of halogenite nanotubes, nano-rod alumina, nano-rod boehmite, nano-rod silica, and glass fiber.In each embodiment of this application, a material forming the three-dimensional framework structure includes nanocellulose.In each embodiment of this application, the nanocellulose includes a hydroxyl group and an anionically modified group.In each embodiment of this application, the anionically modified group includes at least one of an amino group, a carboxyl group, a sulfonic acid group, a boric acid group and a phosphoric acid group, and optionally at least one of a sulfonic acid group, a boric acid group and a phosphoric acid group. The nanocellulose having the above-mentioned anionic modified group can effectively increase the heat resistance of the separator and improve the thermal safety performance of the secondary battery; and can also improve the adhesion strength between the coating and the porous substrate. The nanocellulose having the above-mentioned anionic modified group may also help the nanocellulose interact with the filler as a whole. This enables a more stable spatial network structure of the coating, thereby improving electrolyte infiltration and posture of the separator and enhancing ionic conductivity and withstand voltage of the separator. This also facilitates the adaptation to high-voltage positive electrode active materials, whereby the energy density of the secondary battery is further increased. Moreover, by the presence of the anionic modified group, the hydroxyl group content can also be reduced, which can ensure a corresponding viscosity of the coating slurry for facilitating coating, as well as improve the production efficiency of the separator and the uniformity of coating.In each embodiment of this application, a molar ratio of the anionic modified group to the hydroxyl group is 1:4 to 4:1, optionally 2:3 to 7:3.In each embodiment of this application, the coating further includes a non-granular binder. Optionally, the non-granular binder includes an aqueous solution type binder.In each embodiment of this application, based on a total weight of the coating, a proportion of the non-granular binder in the coating is less than or equal to 2% by weight. The three-dimensional framework structure, the filler, and the like in the coating can form a stable spatial network structure, whereby the separator can maintain high adhesion provided that the binder amount is reduced.In each embodiment of this application, the thickness of the porous substrate is less than or equal to 8 μm, optionally 3 μm to 6 μm. The coating in this application can significantly increase the heat resistance of the separator, thereby enabling selection of thinner porous substrates, which contributes to increase the energy density of the secondary battery.In each embodiment of this application, the thickness of the coating is less than or equal to 2 μm, optionally 0.5 μm to 1.3 μm. The coating in this application has high heat resistance, thereby enabling a smaller thickness of the coating and further increasing the energy density of the secondary battery. In this application, the thickness of the coating refers to the thickness of the coating on one side of the porous substrate.In each embodiment of this application, the separator further includes an adhesive layer, the adhesive layer is disposed on at least a part of a surface of the coating, and the adhesive layer includes a granular binder. The adhesive layer can not only prevent falling of the coating and improve the safety performance of the secondary battery, but also improve an interface between the separator and an electrode and improve the cycle performance of the secondary battery.In each embodiment of this application, the granular binder includes at least one of acrylate monomer homo- or copolymer, acrylic monomer homo- or copolymer, and fluorine-containing olefin monomer homo- or copolymer.In each embodiment of this application, a machine direction thermal shrinkage rate of the separator at 150° C. for 1 h is less than or equal to 6%, optionally 0.5% to 4%.In each embodiment of this application, a transverse thermal shrinkage rate of the separator at 150° C. for 1 h is less than or equal to 6%, optionally 0.5% to 4%.The separator in this application has a low thermal shrinkage rate at a high temperature of 150° C. in both the cross and machine directions, thereby improving the safety performance of the secondary battery.In each embodiment of this application, the ionic conductivity of the separator is greater than or equal to 0.6 ms / cm 2, optionally greater than or equal to 0.9 ms / cm 2. The separator in this application has high ionic conductivity, thereby improving cycle performance and / or kinetic performance of the secondary battery.In each embodiment of this application, a resistance value of the separator is less than or equal to 1.3 Ω, optionally less than or equal to 1.0 Ω. The separator in this application has a low resistance value, thereby improving cycle performance and / or kinetic performance of the secondary battery.In each embodiment of this application, the machine direction tensile strength of the separator is greater than or equal to 2000 kg / cm 2, optionally 2500 kg / cm 2 to 4500 kg / cm 2.In each embodiment of this application, the transverse tensile strength of the separator is greater than or equal to 2000 kg / cm 2, optionally 2500 kg / cm 2 to 4500 kg / cm 2.The separator in this application has high tensile strength in both the transverse direction and the machine direction, thereby providing a low possibility of damage to the separator upon swelling of the secondary battery, thereby improving safety performance of the secondary battery.In each embodiment of this application, the infiltration length of the separator is greater than or equal to 30 mm, optionally 30 mm to 80 mm.In each embodiment of this application, the infiltration rate of the separator is greater than or equal to 3 mm / s, optionally 3 mm / s to 10 mm / s.The separator in this application has good electrolyte infiltration and retention, thereby improving the ionic conductivity of the separator and the extractable capacity of the secondary battery.In each embodiment of this application, the air permeability of the separator is less than or equal to 300 s / 100 ml, optionally 100 s / 100 ml to 230 s / 100 ml. The separator in this application has good air permeability, thereby improving the ion conductivity and extractable capacity of the secondary battery.A second aspect of this application provides a manufacturing method of the separator according to the first aspect of this application. The method includes the steps of: providing a porous substrate; mixing a material forming the three-dimensional framework structure and a filler in a solvent at a predetermined ratio, and then stirring for uniformity at a certain shear rate to produce a coating slurry; and applying the coating slurry to at least one surface of the porous substrate, followed by drying to obtain the separator; wherein the separator includes the porous substrate and a coating disposed on at least one surface of the porous substrate; the coating includes the three-dimensional framework structure and the filler; at least a part of the filler is filled in the three-dimensional framework structure; and in a thickness direction of the separator, a mean pore area of the coating is denoted by S 1 and a mean pore area of the porous substrate is denoted by S 2 where 0<S 1 / S 2< 1.In each embodiment of this application, the shear rate is less than or equal to 30 m / s, optionally 15 m / s to 30 m / s. The shear rate falling within the above range allows a corresponding average pore area of the dried coating and further increases the heat resistance and ionic conductivity of the separator.In each embodiment of this application, a solid content of the coating slurry is 8% to 30%, optionally 10% to 20%.In each embodiment of this application, the coating surface density of the coating slurry on one side is 0.50 g / m 2 to 1.50 g / m 2, optionally 0.75 g / m 2 to 1.40 g / m 2. The coating surface density of the coating slurry falling within the above range enables a corresponding average pore area of the dried coating and further enhances the heat resistance and ion conductivity of the separator.In each embodiment of this application, the coating ceiling of the coating slurry on one side is less than or equal to 2 μm, optionally 0.5 μm to 1.3 μm. The coating thickness of the coating slurry falling within the above range enables a corresponding average pore area of the dried coating and further enhances the heat resistance and ionic conductivity of the separator.In one embodiment of this application, the method further includes the step of: applying a slurry containing a granular binder to at least a portion of a surface of the coating, followed by drying to form an adhesive layer.A third aspect of this application provides a secondary battery including the separator according to the first aspect of this application or a separator manufactured using the method according to the second aspect of this application.A fourth aspect of this application provides an electric device including the secondary battery according to the third aspect of this application.The separator in this application allows the secondary battery to achieve a balance between high energy density, high thermal safety performance, and good cycle performance and kinetic performance. The electric device in this application includes the secondary battery provided in this application, and thus has at least the same advantages as the secondary battery.BRIEF DESCRIPTION OF THE DRAWINGSIn order to describe the technical solutions of embodiments of this application in more detail, the accompanying drawings for describing the embodiments of this application will be briefly described below. Obviously, the accompanying drawings in the following descriptions show only some embodiments of this application, and those skilled in the art can derive still other drawings from these accompanying drawings without any creative efforts. FIG. 1 is a schematic diagram of an embodiment of a secondary battery in this application. FIG. 2 is an exploded schematic view of the embodiment of the secondary battery in FIG. 1. FIG. 3 is a schematic illustration of an embodiment of a battery module in this application. FIG. 4 is a schematic illustration of an embodiment of a battery pack in this application. FIG. 5 is an exploded schematic view of the embodiment of the battery pack shown in FIG. 4. FIG. 6 is a schematic diagram of an embodiment of an electric device having a secondary battery as a power source in this application.In the accompanying drawings, the figures are not necessarily drawn to scale. Reference numerals are: 1. battery pack, 2. upper case body, 3. lower case body, 4. battery module, 5. secondary battery, 51th case, 52th electrode assembly, 53rd cover plate.DESCRIPTION OF EMBODIMENTSHereinafter, particularly, embodiments of the separator, its manufacturing method, and the secondary battery and the related electric device of this application will be disclosed in detail with reference to the accompanying drawings, respectively. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known facts and repeated descriptions of actually identical structures have been omitted. In order that the following description be not unnecessarily implied by a person skilled in the art for the sake of better understanding, it is provided that the accompanying drawings and the following descriptions serve to fully understand this application by a person skilled in the art and are not intended to limit the subject matter recited in the claims.The "ranges" disclosed in this application are defined in terms of lower and upper limits. A predetermined range is defined by a lower limit and an upper limit selected, the selected lower and upper limits defining limits of that particular range. Ranges defined in this method may or may not include final values, and any combinations may be used, i.e., any lower limit may be combined with any upper limit to form an area. For example, if ranges of 60-120 and 80-110 are provided for a particular parameter, it will be appreciated that ranges of 60-110 and 80-120 may also be provided. Moreover, when minimum values of a range are indicated as 1 and 2 and maximum values of the range are indicated as 3, 4 and 5, all of the following ranges may be provided: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise stated, a range of values "a-b" is a short representation of any combination of real numbers between a and b, where both a and b are real numbers. A range of values of "0-5" means, for example, that all real numbers in the range of "0-5" are listed here, and "0-5" is only a brief representation of a combination of these values. Further, a parameter expressed as an integer greater than or equal to 2 is equivalent to the disclosure that the parameter is, for example, an integer between 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and so on.Unless otherwise indicated, all embodiments and optional embodiments of this application may be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of this application.Unless otherwise indicated, all technical features and optional technical features of this application may be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of this application.Unless otherwise indicated, all steps in this application may be performed in the order described or in random order, preferably in the order described. For example, a method including steps (a) and (b) indicates that the method includes steps (a) and (b) in order or includes steps (b) and (a) in order. For example, the above method may further include step (c) indicating that step (c) may be added to the method at any ordinal position, for example, the method may include steps (a), (b), and (c), steps (a), (c), and (b), steps (c), (a), and (b), or the like.Unless otherwise stated, the terms "include" and "contain" mentioned in this application are inclusive or may be exclusive. For example, the terms "include" and "include" may mean that other unlisted components may also be included or included, or that only listed components are included or included.Unless otherwise indicated, the term "or" in this application is inclusive. For example, the phrase "A or B" means "A, B, or both A and B.". More specifically, each of the following conditions satisfies the condition "A or B": A is true (or present) and B is false (or absent); A is false (or absent) and B is true (or present); or both A and B are true (or present).Unless otherwise indicated, the terms "first", "second", and the like in this application are intended to distinguish between different objects and not to indicate a particular order or relative meaning.Unless otherwise indicated, the terms used in this application have known meanings as generally understood by those skilled in the art.Unless otherwise indicated, numerical values of parameters mentioned in this application can be measured using various measurement methods customary in the art, for example using the methods provided in the embodiments of this application.Typically, a secondary battery includes an electrode assembly and an electrolyte. The electrode assembly includes a positive electrode plate, a negative electrode plate, and a separator. The separator is disposed between the positive electrode plate and the negative electrode plate mainly to prevent a short circuit between the positive electrode and the negative electrode and to let active ions freely pass to form a closed loop.With the application and popularity of secondary batteries, increasingly high demands are being placed on the energy density and kinetic performance of secondary batteries. Thinning separators is an effective measure for increasing the energy density of secondary batteries. A separator used in a commercially available secondary battery is typically a polyolefin porous film such as a polyethylene porous film, a polypropylene porous film, or a polypropylene / polyethylene / polypropylene three-layered composite film having a melting point of 130° C. to 160° C. Therefore, the heat resistance of the separator has deteriorated after its thickness is reduced, and it will undergo a substantial thermal shrinkage upon heating, which causes an internal short circuit by direct contact between a positive electrode and a negative electrode inside the battery, thereby increasing the safety risks of the secondary battery.In order to solve the above problem, at present, a heat-resistant inorganic ceramic layer is mainly applied to the porous polyolefin sheet, which can increase the mechanical strength of the separator, reduce the shrinkage degree of the separator upon heating, and reduce the risk of a short circuit between the positive and negative electrodes in the battery. However, the inventors have found in further research that the improvement effect of the inorganic ceramic layer is limited to the heat resistance of the separator. In order to ensure high heat resistance of the separator, the layer thickness of the inorganic ceramic layer needs to be increased, but this leads to an extended active ion transport distance, affects cycle performance and kinetic performance of the secondary battery, and also cannot achieve the desired energy density of the secondary battery.Therefore, in the related art for separators, it is usually difficult to make secondary batteries achieve a balance among high energy density, high safety thermal performance, and good cycle performance and kinetic performance.The inventors of this application have surprisingly found in research that, by configuring a coating including a three-dimensional skeletal structure and a filler on a surface of a porous substrate of a separator and reducing an average pore area of the coating from an average pore area of the porous substrate, the separator can achieve a balance between high heat resistance and high ionic conductivity, and a secondary battery can also achieve a balance between high energy density, high thermal safety performance, and good cycle performance and kinetic performance.SeparatorSpecifically, a first aspect of the embodiments of this application provides a separator.The separator includes a porous substrate and a coating disposed on at least one surface of the porous substrate, the coating including a three-dimensional framework structure and a filler, at least a part of the filler being filled in the three-dimensional framework structure; and in a thickness direction of the separator, a mean pore area of the coating is referred to as S 1 and a mean pore area of the porous substrate is referred to as S 2 where 0<S 1 / S 2< 1. In this application, the "three-dimensional framework structure" refers to a structure having a three-dimensional spatial shape and a certain porosity, and may be formed by intermeshing the materials constituting the three-dimensional framework structure.In investigating the heat resistance of separators, the inventors have found that a coating (e.g., an inorganic ceramic layer) on a surface of a porous substrate has poor heat resistance, and generally, the thickness of the coating needs to be increased to ensure high heat resistance of the coating. However, this results in a loss of energy density, cycle power, and kinetic performance of secondary batteries.The inventors have found in further research that when the configuration in preparing the coating includes the three-dimensional framework structure and the filler, at least a part of the filler is filled in the three-dimensional framework structure, and when the average pore area S 1 of the coating and the average pore area S 2 of the porous substrate satisfy 0<S 1 / S 2< 1 the separator can achieve a balance between high heat resistance and high ion conductivity, and a secondary battery can also achieve a balance between high energy density, high thermal safety performance, and good cycle performance and kinetic performance.The coating includes the three-dimensional framework structure and the filler, at least a portion of the filler being filled in the three-dimensional framework structure, thereby providing an interlocking effect between the filler and the three-dimensional framework structure. This not only increases the heat resistance of the separator, reduces the degree of shrinkage of the separator upon heating, reduces the risk of a short circuit between a positive electrode and a negative electrode, and enables the secondary battery to have high thermal safety performance, but also enables high adhesive strength maintained between the coating and the porous substrate, and prevents the filler from dropping in a long-term charging and discharging operation of the secondary battery. In addition, when at least a part of the filler is filled in the three-dimensional framework structure, there are more contact points between the filler and the three-dimensional framework structure that can reduce the amount of a binder used in the coating, thereby effectively reducing the risk that pores are blocked by the binder, and further improving cycle performance and kinetic performance of the secondary battery. In some embodiments, at least a portion of the filler is filled in the three-dimensional framework structure; other portions of the filler may be located on a surface of the three-dimensional framework structure and / or an interface between the three-dimensional framework structure and the porous substrate; and at an interface position between the three-dimensional framework structure and the porous substrate, a small amount of the filler may also be embedded in the porous substrate, for example, in a winding process of an electrode assembly, wherein a small amount of the filler is embedded at an interface position in the matrix and / or pores of the porous substrate due to the action of external pressure.The average pore area S 1 of the coating is smaller than the average pore area S 2 of the porous substrate, enabling high heat resistance of the coating. The coating in this application has high heat resistance, so that the thickness of the coating can be reduced (for example, the thickness of the coating can be less than or equal to 2 μm) to shorten the active ion transport distance, and the secondary battery can also achieve a balance between high energy density as well as good cycle performance and kinetic performance. In addition, the coating in this application has high heat resistance, so that a thinner porous substrate can also be selected, thereby further increasing the energy density of the secondary battery.The inventors have also found in further research that the ratio of the average pore area S 1 of the coating to the average pore area S 2 of the porous substrate, S 1 / S 2, should not be too small. If S 1 / S 2 is too small, the average pore area of the coating tends to be small, which may impede the transport of active ions and impair cycle performance and / or kinetic performance of the secondary battery. In some embodiments, 0.06≤S 1 / S 2< 1 and optionally 0.10≤S 1 / S 2≤0.99, 0.20≤S 1 / S 2≤0.98, 0.30≤S 1 / S 2≤0.97, 0.30≤S 1 / S 2≤0.80, 0.30≤S1 / S2≤0.70, 0.35≤S1 / S2≤0.94, 0.35 ≤ S 1 / S 2 ≤ 0.85, 0.40 ≤ S 1 / S 2 ≤ 0.92, 0.40 ≤ S 1 / S 2 ≤ 0.80, 0.40 ≤ S 1 / S 2 ≤ 0.70 or 0.45 ≤ S 1 / S 2 ≤ 0.90.In some embodiments, the pore area S 1 of the coating satisfies 0.0002 μm 2 ≤ S 1 ≤ 0.0080 μm 2 and optionally 0.0004 μm 2 ≤ S 1 ≤ 0.0050 μm 2 or 0.0008 μm 2 ≤ S 1 ≤ 0.0048 μm 2. The average pore area S 1 of the coating falling within the above range can increase the heat resistance of the separator without hindering the active ion transport, whereby the secondary battery can achieve a better balance between high energy density, high thermal safety performance, and good cycle performance and kinetic performance.In some embodiments, the average pore area S 2 of the porous substrate satisfies 0.0005 μm 2 ≤ S 2 ≤ 0.0100 μm 2 and optionally 0.0008 μm 2 ≤ S 2 ≤ 0.0080 μm 2. The average pore area S 2 of the porous substrate falling within the above range is promoted to increase the ionic conductivity of the separator and the extractable capacity of the secondary battery.In this application, the average pore area S 1 of the coating of the separator and the average pore area S 2 of the porous substrate of the separator may be obtained by examining a cross-sectional image of the separator.The average pore area S 1 of the coating of the separator refers to a ratio of a total pore area of the coating in the cross-sectional image of the separator to the number of pores in the coating.The average pore area S 2 of the porous substrate of the separator refers to a ratio of a total pore area of the porous substrate in the cross-sectional image of the separator to the number of pores in the porous substrate.In this application, the cross-sectional image of the separator refers to an image in the thickness direction of the separator. From a region selected randomly at the separator, a separator sample to be tested is cut to a predetermined size (e.g., 15 mm×15 mm), and a cross section of the separator is obtained with an ion beam polisher (e.g., Hitachi Arblast 5000) by cutting under a frozen state, e.g., -80° C. Referring to JY / T010-1996, an SEM image (magnification may be 1000X to 30000X) of the cross section of the separator is obtained using a scanning electron microscope (e.g., a Sigma 300 model scanning electron microscope from ZEISS in Germany). The average pore area S 1 of the coating of the separator and the average pore area S 2 of the porous substrate of the separator are obtained by an image processing recognition system (e.g., Fully Separator Detection System 2022-0408) using a multi-segment binarization method.When the ion beam polisher is used for cutting, the sample may be wound with copper foil or aluminum foil before cutting. In the case of acquiring the SEM image of the cross section of the separator, the sample may be subjected to metal spray treatment.The pore area data of the coating and porous substrate of the separator may be separately obtained with the image processing acquisition system, and then the pore area distribution plots and average pore areas of the coating and porous substrate of the separator may be separately obtained with the mintab software. The ratio of the total pore area of the coating of the separator to the number of pores in the coating is the average pore area S 1 of the coating of the separator, and the ratio of the total pore area of the porous substrate of the separator to the number of pores in the porous substrate is the average pore area S 2 of the porous substrate of the separator.In some embodiments, a mean pore diameter of the separator is referred to as d 1 and a mean pore diameter of the porous substrate is referred to as d 2 where d 1 / d 2< 1 and optionally 0.2≤d 1 / d 2≤0.9, 0.3≤d 1 / d 2≤0.8, or 0.4≤d 1 / d 2≤0.7, Falling within the above range, it can be ensured that the separator has high ionic conductivity under the condition of high heat resistance.The average pore diameter d 1 of the separator satisfies 15 nm≤d 1≤50 nm and optionally 20 nm≤d 2≤40 nm in some embodiments. The average pore diameter d 1 of the separator falling within the above range can increase the heat resistance of the separator without hindering the active ion transport, whereby the secondary battery can achieve a better balance between high energy density, high thermal safety performance, and good cycle performance and kinetic performance.In some embodiments, the average pore diameter d 2 of the porous substrate satisfies 25 nm≤d 2≤60 nm, and optionally 30 nm≤d 2≤50 nm. The average pore diameter d 2 of the porous substrate falling within the above range is useful for increasing the ionic conductivity of the separator and the extractable capacity of the secondary battery.The average pore diameter d 1 of the separator and the average pore diameter d 2 of the porous substrate can be obtained by testing with a capillary porosity tester (bubble point method). An example of a test method is as follows: taking a circular sample having a diameter of 25 mm, adding 3-5 drops of an infiltrating liquid onto the sample, and placing the sample in a mold after the infiltrating liquid completely infiltrates the sample; then extruding the infiltrating liquid in the pores of the sample to be tested with inert gas, wherein the extrusion pressure and the flow rate are inversely proportional to the pore diameter; and obtaining the average pore diameter of the sample to be tested by software sampling and pressure-to-pore diameter conversion analysis. The test instrument may be a CFP 1500 model pore diameter analyzer from PMI, where the test pressure may be 100 psi to 350 psi.In some embodiments, the surface density of the coating is denoted as ρ 1 and the surface density of the porous substrate is denoted as ρ 2 where 0.15≤p 1 / ρ 2≤0.80, and optionally, 0.20≤p 1 / ρ 2≤0.50. ρ 1 / ρ 2, which falls within the above range, is conducive to high heat resistance of the coating and also helps the secondary battery achieve a balance between high energy density and high safety thermal performance.In some embodiments, the surface density ρ 1 of the coating satisfies 0.50 g / m 2 ≤ ρ 1 ≤ 1.50 g / m 2 and optionally 0.60 g / m 2 ≤ ρ 1 ≤ 1.40 g / m 2, 0,75 g / m 2 ≤ ρ 1 ≤ 1.40 g / m 2 or 0.75 g / m2≤ ρ1≤ 1.30 g / m2. The surface density ρ 1 of the coating falling within the above range contributes to the secondary battery attaining a balance between high energy density and high safety thermal performance. The surface density ρ 1 of the coating falling within the above range also enables an appropriate average pore area of the coating and further improves the heat resistance and ion conductivity of the separator. In this application, the surface density of the coating refers to the surface density of the coating on one side of the porous substrate.In some embodiments, the surface density ρ 2 of the porous substrate satisfies 1.50 g / m 2 ≤ ρ 2 ≤ 4.50 g / m 2 and optionally 2.00 g / m 2 ≤ ρ 2 ≤ 4.00 g / m 2. The surface density ρ 2 of the porous substrate falling within the above range allows the porous substrate to have a corresponding average pore area, and is also conducive to increase the ion conductivity of the separator and the extractable capacity of the secondary battery.In some embodiments, the porosity of the separator is referred to as P 1 and the porosity of the porous substrate is referred to as P 2 where 0.4≤P 2 / P 1< 1 and optionally 0.55≤P 2 / P 1≤0.85. P 2 / P 1, which fall within the above range, can help the secondary battery achieve a balance between high safety thermal performance as well as good cycle performance and kinetic performance.In some embodiments, the porosity P 1 of the separator satisfies 20%≤P 1≤60%, and optionally, 25%≤P 1≤45%. The porosity P 1, of the separator falling within the above range may help the secondary battery achieve a balance between high safety thermal performance as well as good cycle performance and kinetic performance.In some embodiments, the porosity P 2 of the porous substrate satisfies 15%≤P 2≤45%, and optionally 20%≤P 2≤40%. The porosity P 2 of the porous substrate falling within the above range is conducive to increase the ionic conductivity of the separator and the extractable capacity of the secondary battery.The porosity P 1 of the separator and the porosity P 2 of the porous substrate may be obtained by examination with reference to GB / T 24586-2009. A test method is as follows: punching a separator or porous substrate into small round samples having a diameter of 14 mm, measurement thickness, and calculating an apparent volume V 1 of the separator or porous substrate according to the cylindrical volume calculation formula; and with reference to GB / T 24586-2009, measuring a real volume V 2 of the separator or porous substrate by a clean density measuring apparatus using the gas exchange method in which an inert gas such as helium or nitrogen is used as the medium. Porosity of separator or porous substrate = (V 1- V 2) / V 1 × 100 %. The test device can be an AccuPyc II 1340 automatic clean density measuring device from Micromeritics of the USA.In some embodiments, the thickness of the porous substrate is less than or equal to 8 μm, optionally 3 μm to 6 μm. The coating in this application can significantly increase the heat resistance of the separator, thereby enabling selection of thinner porous substrates, which contributes to increase the energy density of the secondary battery.In some embodiments, the thickness of the coating is less than or equal to 2 μm, optionally 0.5 μm to 1.3 μm. The coating in this application has high heat resistance, thereby enabling a smaller thickness of the coating and further increasing the energy density of the secondary battery. In this application, the thickness of the coating refers to the thickness of the coating on one side of the porous substrate.The porous substrate in this application is not limited to a specific material, and may be any known substrate having good chemical and mechanical stability. For example, the porous substrate may include a polyolefin-based porous resin film (e.g., polyethylene, polypropylene, and / or polyvinylidene fluoride), porous glass fibers, and / or porous nonwoven fabric. The porous substrate may be a monolayer film or a multilayer composite film. When the porous substrate is a laminated multilayer film, each layer may be made of the same material or different materials.In each embodiment of this application, the filler includes a first filler, wherein an average particle size of the first filler is less than or equal to 150 nm, optionally 15 nm to 150, or 15 nm to 120 nm.The first filler having advantages of a small average particle size, a large specific surface area, and a good affinity to the three-dimensional framework structure can be better interlocked with the three-dimensional framework structure, whereby the coating can have a more stable spatial network structure. As a result, not only the ionic conductivity of the separator is increased, but also the heat resistance and electrolyte infiltration as well as the retention of the separator. In this way, the secondary battery with the separator of this application can achieve a balance among high thermal safety performance, long life, and good cycle performance and kinetic performance. Moreover, the average particle size of the first filler falling within the above range also enables a corresponding average pore area of the coating and further enhances the heat resistance and ion conductivity of the separator.In some embodiments, the first filler includes at least one of a primary particle and a secondary particle.In some embodiments, an average particle size of the first filler of primary particle morphology is 15 nm to 80 nm, optionally 20 nm to 80 nm or 30 nm to 75 nm.In some embodiments, an average particle size of the first filler having secondary particle morphology is 50 nm to 150 nm, optionally 50 nm to 135 nm, 50 nm to 120 nm, 55 nm to 150 nm, 55 nm to 135 nm, or 55 nm to 120 nm.In some embodiments, the BET specific surface area of the first filler is greater than or equal to 15 m 2 / g, optionally 18 m 2 / g to 65 m 2 / g. The BET specific surface area of the first filler falling within the above range allows a corresponding average pore area of the coating and further increases the heat resistance and ionic conductivity of the separator; and further increases the affinity between the first filler and the three-dimensional framework structure, thereby allowing a more stable spatial network structure of the coating and in turn, better heat resistance and higher ionic conductivity of the separator.In some embodiments, the first filler includes at least one of an inorganic particle and an organic particle, and optionally, the inorganic particle includes or a combination of an inorganic particle and an organic particle. The inorganic particle has properties of high hardness, high thermal stability and less tendency to decompose. Moreover, the inorganic particle typically has a hydroxyl group on the surface, and thus easily establishes a stable spatial network structure with a material (e.g., nanocellulose) constituting the three-dimensional framework structure. The organic particle has properties of good thermal stability and is less susceptible to decomposition. In addition, when the internal temperature of the secondary battery reaches a melting point of the organic particle by overcharge, thermal abuse, and the like, the organic particle can melt and be drawn into micropores of the porous substrate under the action of capillarity, thereby effectively blocking pores and breaking continuity, thereby contributing to improvement in safety performance of the secondary battery.Optionally, the inorganic particle includes at least one of boehmite (γ-AlOOH), alumina (Al 2 O 3), barium sulfate (BaSO 4), magnesium oxide (MgO), magnesium hydroxide (Mg(OH) 2), silicon oxygen SiO x(0 < × ≤ 2), tin dioxide (SnO 2), titanium oxide (TiO 2), calcium oxide (CaO), zinc oxide (ZnO), zirconium oxide (ZrO 2), yttrium oxide (Y 2 O 3), nickel oxide (NiO), Hafnium dioxide (HfO 2), cerium oxide (CeO 2), zirconium titanate (ZrTiO 3), barium titanate (BaTiO 3) and magnesium fluoride (MgF 2). Still optionally, the inorganic particle includes at least one of boehmite (γ-AlOOH), alumina (Al 2 O 3), barium sulfate (BaSO 4), magnesium oxide (MgO), silicon oxygen compound SiO x(0 < × ≤ 2), titanium oxide (TiO 2), zinc oxide (ZnO), cerium oxide (CeO 2) and barium titanate (BaTiO 3).Optionally, the organic particle includes at least one of polystyrene particles, polypropylene wax particles, melamine formaldehyde resin particles, phenol resin particles, polyester particles, polyimide particles, polyamideimide particles, polyaramide particles, polyphenylene sulfide particles, polysulfone particles, polyethersulfone particles, polyether ether ketone particles, and polyaryl ether ketone particles.In some embodiments, the first filler includes inorganic particles, wherein a crystalline form of the inorganic particles includes at least one of the θ-crystalline form, the γ-crystalline form, and the η-crystalline form. Optionally, the crystalline form of the inorganic particles includes at least one of the θ-crystalline form and the γ-crystalline form.The inorganic particles in θ crystalline form show diffraction peaks at 2θ of 36.68°±0.2° and 31.21°±0.2° in an X-ray diffraction pattern determined by an X-ray diffractometer. In some embodiments of this application, based on a total weight of the inorganic particles in the first filler, a proportion of the inorganic particles in a θ-crystalline form is greater than or equal to 50% by weight, optionally 60% by weight to 82% by weight.The inorganic particles in γ-crystalline form show diffraction peaks at 2θ of 66.95°±0.2° and 45.91°±0.2° in an X-ray diffraction pattern determined with an X-ray diffractometer. In some embodiments, the proportion of the inorganic particles in γ-crystalline form in the first filler, based on the total weight of the inorganic particles, is greater than or equal to 10 wt %, optionally 17 wt % to 38 wt %.The inorganic particles in η-crystalline form show diffraction peaks at 2θ of 31.89°±0.2° and 19.37°±0.2° in an X-ray diffraction pattern determined by an X-ray diffractometer. In some embodiments, based on the total weight of the inorganic particles in the first filler, a proportion of the inorganic particles in η-crystalline form in the first filler is less than or equal to 5 wt %, optionally less than or equal to 1.5 wt %, and even more optionally less than or equal to 1 wt %.The inorganic particles in θ crystalline form have moderate specific surface area and hardness, and therefore can improve both the heat resistance and the ion conductivity of the separator more. The inorganic particles in γ-crystalline form and in η-crystalline form have the advantage of a large specific surface area.The selection of the first filler in various crystalline forms contributes to the increase in the heat resistance, ionic conductivity and / or electrolyte infiltration and retention of the separator.In some embodiments, the first filler may include inorganic particles, wherein a crystalline form of the inorganic particles includes θ-crystalline form, γ-crystalline form, and η-crystalline form; and in the first filler, based on a total weight of the inorganic particles in the first filler, a proportion of the inorganic particles in the θ-crystalline form may be 60 wt % to 82 wt %, a proportion of the inorganic particles in the γ-crystalline form may be 17 wt % to 38 wt %, and a proportion of the inorganic particles in the η-crystalline form may be less than or equal to 1.5 wt %.An X-ray diffraction pattern of the inorganic particles can be obtained by the following method: After drying the inorganic particles, they are crushed in a mortar (e.g., an agate mortar) for 30 minutes, and then tested with an X-ray diffractometer (e.g., Miniflex600-C) to obtain an X-ray diffraction pattern. In the test, a Cu target material and Ni filter having a tube pressure of 40 KV, a tube flow of 15 mA and a continuous scan range of 5-80° may be used.In some embodiments, the first filler may include inorganic particles, and the inorganic particles may be produced by the following method: performing an oxidation reaction on a precursor solution of inorganic particles by high pressure sputtering, then heating to 600° C. to 900° C. (for example, for 1 hour to 3 hours) to generate inorganic particles of the primary particle morphology, and then performing drying and shaping at 150° C. to 250° C. (for example, for 30 minutes to 60 minutes) to obtain inorganic particles of the secondary particle morphology formed by self-assembly of primary particles.In some embodiments, based on a total weight of the coating, a proportion of the first filler is greater than or equal to 55 wt %, optionally 60 wt % to 90 wt %. The proportion of the first filler falling within the above range enables a corresponding average pore area of the coating and further enhances the heat resistance and ion conductivity of the separator; and additionally enables a more stable spatial network structure of the coating, thereby further enhancing the heat resistance and ion conductivity of the separator.In some embodiments, the filler includes the first filler and a second filler, wherein the average particle size of the second filler is greater than the average particle size of the first filler.The second filler has a larger particle size and can play its auxiliary role in the coating more, reduce shrinkage of the first filler, and reduce the amount of a binder, thereby improving the heat resistance of the separator. The second filler has a larger particle size, thereby allowing more pore structures and a lower water content of the coating when a small amount of the second filler is used, thereby further improving the ionic conductivity and electrolyte infiltration and retention of the separator.In some embodiments, the average particle size of the second filler is less than or equal to 400 nm, optionally 100 nm to 300 nm. The average particle size of the second filler falling within the above range allows an appropriate average pore area of the coating and further increases the heat resistance and ionic conductivity of the separator; and allows the second filler to play its supporting role better, reduce the water content in the coating, and maintain the coating a stable pore structure during a long-term charging and discharging process, thereby facilitating ion transport and simultaneously increasing the heat resistance of the separator.In some embodiments, the BET specific surface area of the second filler is greater than or equal to 15 m 2 / g, optionally 7 m 2 / g to 12 m 2 / g. The BET specific surface area of the second filler falling within the above range allows an appropriate average pore area of the coating and further increases the thermal resistance and ionic conductivity of the separator; and allows the second filler to play its supporting role better, reduce the water content in the coating, and maintain the coating a stable pore structure during a long-term charging and discharging process, thereby facilitating ion transport and simultaneously increasing the thermal resistance of the separator.In some embodiments, the second filler includes at least one of an inorganic particle and an organic particle.In some embodiments, the inorganic particle may include at least one of an inorganic particle having a dielectric constant above 5, an inorganic particle having ionic conductivity but not storing ions, and an inorganic particle capable of electrochemical reactions.Optionally, the inorganic particle having a dielectric constant above 5 includes boehmite, alumina, zinc oxide, silicon oxide, titanium oxide, zirconium oxide, barium oxide, calcium oxide, magnesium oxide, nickel oxide, tin oxide, cerium oxide, yttrium oxide, hafnium oxide, aluminum hydroxide, magnesium hydroxide, silicon carbide, boron carbide, aluminum nitride, silicon nitride, boron nitride, magnesium fluoride, calcium fluoride, barium fluoride, barium sulfate, magnesium aluminum silicate, lithium magnesium silicate, sodium magnesium silicate, bentonite, hectorite, zirconium titanate, barium titanate, Pb(Zr,Ti)O 3( PZT for short), Pb 1-m La m Zr 1-n Ti n O 3( PLZT for short, where 0<m<1, and 0<n<1), Pb(Mg 3 Nb 2 / 3) O 3- PbTiO 3( PMN-PT for short) Optionally, a modification method for the inorganic particles may be a chemical modification and / or physical modification. The chemical modification method includes a coupling agent modification (for example, using a silane coupling agent, a titanate coupling agent, and the like), a surfactant modification, a polymer graft modification, and the like. The physical modification method may include mechanical force dispersion, ultrasonic dispersion, high energy treatment, and the like. By the modification treatment, agglomeration of the inorganic particles can be reduced, so that the coating obtains a more stable and uniform spatial network structure. Moreover, the choice of a coupling agent, surfactant or polymer having a particular functional group to modify the inorganic particles also helps to improve electrolyte infiltration and retention of the coating and to improve adhesion of the coating to the porous substrate.Optionally, the inorganic particle having ionic conductivity but without ion storage includes at least one of Li 3 PO 4, lithium titanium phosphate Li x1 Ti y1( PO 4)3, lithium aluminum titanium phosphate Li x2 Al y2 Ti z1( PO 4)3, ( LiAlTiP) x3 Oy3vom type glass, lithium lanthanum titanate Lix4Lay4TiO3, Lithium germanium thiophosphate Li x5 Ge y5 P z2 S w, lithium nitride Li x6 N y6, SiS 2 of the type glass Li x7 Si y7 S z3 and P2S5vom type glass Lix8Py8Sz4, where 0<x1<2.0<y1<3.0<x2<2.0<y2<1, 0<z 1<3.0<x 3<4.0<y 3<13.0<x 4<2, 0<y 4<3, 0<x 5<4, 0<y 5<1, 0<z 2<1, 0<w<5, 0<x 6<4, 0<y 6<2, 0<x 7<3, 0<y7<2, 0<z3<4, 0<x8<3, 0<y8<3, and 0<z4<7.Optionally, the inorganic particle capable of electrochemical reactions includes at least one of lithium-containing transition metal oxide, lithium-containing phosphate, a carbon-based material, a silicon-based material, a tin-based material, and a lithium titanium compound.In some embodiments, the organic particle includes at least one of polyethylene particles, polypropylene particles, cellulose, cellulose modifiers (such as carboxymethyl cellulose), melamine resin particles, phenol resin particles, polyester particles (such as polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate), organosilicon resin particles, polyimide particles, polyamideimide particles, polyaramide particles, polyphenylene sulfide particles, polysulfone particles, polyethersulfone particles, polyether ether ketone particles, polyaryl ether ketone particles, and butyl acrylate ethyl methacrylate copolymer (such as crosslinking copolymer of butyl acrylate and ethyl methacrylate), but is not limited thereto.In some embodiments, the second filler has a primary particle morphology.In some embodiments, the second filler includes inorganic particles having primary particle morphology, and a crystalline form of the inorganic particles having primary particle morphology includes at least one of an α-crystalline form and a γ-crystalline form, and optionally includes an α-crystalline form. The second filler in α-crystalline form has advantages of high hardness, good heat resistance, low dielectric constant, high safety and high clean density, thereby further enhancing the heat resistance of the coating.In some embodiments, the second filler includes inorganic particles having primary particle morphology, and a crystalline form of the inorganic particles having primary particle morphology includes an α-crystalline form, wherein, based on a total weight of the inorganic particles having primary particle morphology in the second filler, a proportion of the inorganic particles in α-crystalline form is greater than or equal to 80 wt %, optionally 85 wt % to 100 wt %, 90 wt % to 100 wt %, or 95 wt % to 100 wt %.The inorganic particles in α-crystalline form show diffraction peaks at 2θ of 57.48°±0.2° and 43.34°±0.2° in an X-ray diffraction pattern determined with an X-ray diffractometer.In some embodiments, the proportion of the second filler, based on the total weight of the coating, is less than or equal to 20 wt %, optionally 2 wt % to 15 wt %. The proportion of the second filler falling within the above range allows an appropriate average pore area of the coating and further increases the heat resistance and ionic conductivity of the separator; and allows the second filler to play its supporting role better, reduce the water content in the coating, and maintain the coating a stable pore structure during a long-term charging and discharging process, thereby facilitating ion transport and simultaneously increasing the heat resistance of the separator.In some embodiments, the three-dimensional framework structure may be formed by a fibrous substance, wherein the morphology of the fibrous substance optionally includes at least one of a rod shape, a tubular shape (for example, a hollow tubular shape), a rod shape, and a fibrous shape. The material having a corresponding shape helps the three-dimensional framework structure to form a stable spatial network structure with the filler, thereby further enhancing the heat resistance and ion conductivity of the separator. In this application, the "pulp" refers to a material having a length-to-diameter ratio of over 5.In some embodiments, the average diameter of a material forming the three-dimensional framework structure is less than or equal to 40 nm, optionally 10 nm to 35 nm. The average diameter of the material constituting the three-dimensional framework structure falling within the above range helps the material constituting the three-dimensional framework structure to interlock with the filler as a whole. Moreover, the following situation can be effectively avoided: too large an average diameter of the material constituting the three-dimensional framework structure results in an insufficient entanglement effect of the formed three-dimensional framework structure, which may result in that the heat resistance, the breakdown voltage resistance, and the like of the separator are not excellent enough.In some embodiments, the average length of the material forming the three-dimensional framework structure is 100 nm to 800 nm, optionally 200 nm to 600 nm. The average length of the material constituting the three-dimensional framework structure falling within the corresponding range can further increase the heat resistance and the ion conductivity of the separator. Moreover, the following situations can be effectively avoided: When the average length of the material constituting the three-dimensional framework structure is too short, the meshing effect with the filler is poor, the heat resistance of the coating deteriorates, and during the drying process of the coating, the three-dimensional framework structure tends to collapse due to the lack of support by the filler, which easily leads to blocking of the pores and hinders the ion transport and the moisture discharge, which in turn may impair the thermal safety performance, the cycle performance, and the kinetic performance of the secondary battery. If the average length of the material forming the three-dimensional framework structure is too long, the coating slurry has a high viscosity and a poor flowability, which can influence the application of the coating slurry and thus, for example, the quality of the coating and influence the heat resistance and ionic conductivity of the separator.In some embodiments, a length-diameter ratio of the material forming the three-dimensional framework structure is 5 to 60, optionally 10 to 30. The average length of the material forming the three-dimensional framework structure falling within the corresponding range may further increase the heat resistance and the ion conductivity of the separator. When the length-diameter ratio of the material constituting the three-dimensional framework structure is too small, the meshing effect with the filler is poor, the heat resistance of the coating deteriorates, and during the drying process of the coating, the three-dimensional framework structure tends to collapse due to the lack of support by the filler, which easily leads to blocking of the pores and hinders the ion transport and the moisture discharge, which in turn may impair the safety thermal performance, the cycle performance, and the kinetic performance of the secondary battery. When the length-diameter ratio of the material constituting the three-dimensional framework structure is too large, the ionic conductivity of the separator may be lowered, resulting in deterioration of the cycle performance and / or the kinetic performance of the secondary battery.The average length and the average diameter of the material forming the three-dimensional framework structure can be determined using the following method: a sample having a size of 3.6 mm×3.6 mm is cut out from a randomly selected region on the separator, the micromorphology and structure of the coating in the sample is measured with a scanning electron microscope (e.g., ZEISS Sigma 300) to acquire an SEM image, wherein the high vacuum mode is selected, the working voltage is 3 kV, and the magnification is 30000X; Based on the obtained SEM image, a plurality (e.g., more than 5) of test regions for length statistics are selected, the size of each test region is 0.5 μm×0.5 μm, and then the average value of the lengths obtained from the test regions is taken as the average length of the material constituting the three-dimensional framework structure; and based on the obtained SEM image, a plurality (e.g., more than 5) of test regions for diameter statistics are selected by the nano Mea particle size statistical software, the size of each test region is 0.5 μm×0.5 μm, and then the average value of the diameters obtained from the test regions is taken as the average diameter of the material constituting the three-dimensional framework structure.In some embodiments, a material forming the three-dimensional framework structure includes at least one of an organic material and an inorganic material. The material constituting the three-dimensional framework structure only needs to have electrical insulation, electrochemical stability, and stability to electrolyte, is not particularly limited to any material, and may be both an organic material and an inorganic material.In some embodiments, the organic material includes at least one of nanocellulose, polytetrafluoroethylene nano-fiber, and / or polyamide nano-fiber. In some embodiments, the inorganic material includes at least one of halloysite nanotubes, nano-rod-shaped alumina, nano-rod-shaped boehmite, nano-rod-shaped silica, and glass fibers.In some embodiments, the material forming the three-dimensional framework structure includes nanocellulose. Optionally, the nanocellulose includes at least one of cellulose nanofibrils (cellulose nanofibrils, CNF, also known as nanofibrillated cellulose or microfibrillated cellulose), cellulose nanocrystals (cellulose nanocrystals, CNC, also known as cellulose nanocrystals or nanocrystalline cellulose) and bacterial nanocellulose (bacterial nanocellulose, BNC, also known as bacterial cellulose or microbial cellulose). Optionally, the nanocellulose includes cellulose nanocrystals having the advantage of high crystallinity, thereby better enhancing the heat resistance of the separator.Nanocellulose is a general term for cellulose with a nanoscale size (e.g. within 100 nm) in each dimension that has both the characteristics of cellulose and the characteristics of nanoparticles. Nanocellulose may be a high molecular nanomaterial obtained from natural sources such as wood and cotton by one or more chemical, physical or biological methods, has advantages such as wide availability, low cost, biodegradability, high modulus, and high specific surface area, and is therefore an excellent substitute for conventional petrochemical resources, thereby effectively alleviating problems such as environmental pollution and shortage of petrochemical resources. Nanocellulose also has good high temperature resistance and small volume change upon heating, which increases the heat resistance of the separator. Moreover, nanocellulose has a lower density compared to conventional inorganic ceramic particles, thereby reducing the weight of the secondary battery and increasing the gravimetric energy density of the secondary battery. Moreover, the three-dimensional framework structure formed by nanocellulose can also have minute nanopores, thereby preventing current leaks, whereby the separator can achieve a balance between good electrolyte infiltration and retention and good withstand voltage.In some embodiments, the nanocellulose includes a hydroxyl group and an anionically modified group.In some embodiments, the anionically modified group optionally includes at least one of an amino group, a carboxyl group, a sulfonic acid group, a boric acid group, and a phosphoric acid group, and even more optionally at least one of a sulfonic acid group, a boric acid group, and a phosphoric acid group.The inventors have found in further research that the nanocellulose having the above-mentioned anionic modified group can effectively increase the heat resistance of the separator and improve the thermal safety performance of the secondary battery; and it can also improve the adhesion strength between the coating and the porous substrate. The nanocellulose having the above-mentioned anionic modified group may also help the nanocellulose interact with the filler as a whole. This enables a more stable spatial network structure of the coating, thereby improving electrolyte infiltration and posture of the separator and enhancing ionic conductivity and withstand voltage of the separator. This also facilitates the adaptation to high-voltage positive electrode active materials, whereby the energy density of the secondary battery is further increased. Moreover, by the presence of the anionic modified group, the hydroxyl group content can also be reduced, which can ensure a corresponding viscosity of the coating slurry for facilitating coating, as well as improve the production efficiency of the separator and the uniformity of coating.In some embodiments, a molar ratio of the anionic modified group to the hydroxyl group is 1:4 to 4:1, optionally 2:3 to 7:3. Moreover, the following situations can be effectively avoided: If the molar ratio of the anionically modified group to the hydroxyl group is too small, further improvement of the effect of the anionically modified group on the heat resistance and ionic conductivity of the separator may not be significant. If the molar ratio of the anionically modified group to the hydroxyl group is too large, electrolyte infiltration and the retention degree of the separator may deteriorate, which in turn may impair cycle performance and / or kinetic performance of the secondary battery; and heat resistance of the separator may also be lowered, which in turn may further impair the improvement effect on the thermal safety performance of the secondary battery.The type of anionically modified group in nanocellulose can be determined by infrared spectroscopy. For example, the infrared spectrum of the material can be examined for characteristic peaks containing the material to determine the type of the anionic modified group. More specifically, the infrared spectroscopy procedure can be performed on the material using instruments and methods known in the art, the test is performed using an infrared spectrometer (e.g., an IS10 Fourier Transform infrared spectrometer from Nicote of the United States) according to the general rules of the GB / T 6040-2019 infrared spectroscopy procedure.In some embodiments, the material constituting the three-dimensional framework structure includes a sulfonic acid group, and based on a total weight of the material constituting the three-dimensional framework structure, a content of element sulfur in the material constituting the three-dimensional framework structure is greater than or equal to 0.1% by weight, optionally 0.2% by weight to 0.5% by weight.The percentage of sulfur element in the material forming the three-dimensional skeletal structure can be determined by the following method: After drying the material forming the three-dimensional skeletal structure, it is ground in a mortar (e.g., an agate mortar) for 30 minutes, and then an X-ray diffractometer (e.g., Miniflex600-C) test is performed to determine the percentage of sulfur element. In the test, a Cu target material and Ni filter having a tube pressure of 40 KV, a tube flow of 15 mA and a continuous scanning range of 5-80° may be used.In some embodiments, based on a total weight of the coating, a proportion of the three-dimensional framework structure is less than or equal to 40 wt %, optionally 5 wt % to 25 wt %. The proportion of the three-dimensional framework structure falling within the above range enables a corresponding average pore area of the coating and further enhances the heat resistance and ion conductivity of the separator.In some embodiments, the coating further includes a non-granular binder. The non-granular binder in this application is not limited to a specific type and may be any known material having good adhesion. Optionally, the non-granular binder includes an aqueous solution type binder which has the advantages of good thermodynamic stability and environmental friendliness, thereby facilitating the preparation and application of the coating slurry. For example, the aqueous solution type binder may include at least one of aqueous soluble acrylic resin (e.g., acrylic acid, methacrylic acid, sodium acrylate monomer homo- or copolymer with another comonomer), polyvinyl alcohol (PVA), isobutylene-maleic anhydride copolymer, and polyacrylamide.In some embodiments, based on a total weight of the coating, a proportion of the non-granular binder in the coating is less than or equal to 2% by weight. The three-dimensional framework structure, the filler, and the like in the coating can form a stable spatial network structure, whereby the separator can maintain high adhesion provided that the binder amount is reduced.In some embodiments, the separator further includes an adhesive layer, wherein the adhesive layer is disposed on at least a portion of a surface of the coating, and the adhesive layer includes a granular binder. The adhesive layer can not only prevent falling of the coating and improve the safety performance of the secondary battery, but also improve an interface between the separator and an electrode and improve the cycle performance of the secondary battery.Optionally, the granular binder includes at least one of acrylate monomer homopolymer or copolymer, acrylic monomer homopolymer or copolymer, and fluorine-containing olefin monomer homopolymer or copolymer. The comonomer includes, but is not limited to, at least one of the following: acrylate monomer, acrylic monomer, olefin monomer, halogen-containing olefin monomer, and fluoroether monomer.Optionally, the granular binder includes a vinylidene fluoride-based polymer, for example, a homopolymer of vinylidene fluoride (VDF) monomer and / or a copolymer of vinylidene fluoride monomer with a comonomer. The comonomer may be an olefin monomer, a fluorine-containing olefin monomer, a chlorine-containing olefin monomer, an acrylate monomer, an acrylic monomer or a fluoroether monomer. Optionally, the comonomer may include at least one of trifluoroethylene (VF3), chlorotrifluoroethylene (CTFE), 1,2-difluoroethylene, tetrafluoroethylene (TFE), hexafluoropropylene (HFP), perfluoro(alkyl vinyl ethers) (e.g., perfluoro(methyl vinyl ether) PMVE, perfluoro(ethyl vinyl ether) PEVE, and perfluoro(propyl vinyl ether) PPVE), perfluoro(1,3-dioxole), and perfluoro(2,2-dimethyl-1,3-dioxole) (PDD).In some embodiments, a machine direction thermal shrinkage rate of the separator at 150° C. for 1 h is less than or equal to 6%, optionally 0.5% to 4%.In some embodiments, a transverse thermal shrinkage rate of the separator at 150° C. for 1 h is less than or equal to 6%, optionally 0.5% to 4%.The separator in this application has a low thermal shrinkage rate at a high temperature of 150° C. in both the cross and machine directions, thereby improving the safety performance of the secondary battery.In some embodiments, the ion conductivity of the separator is greater than or equal to 0.6 ms / cm 2, optionally greater than or equal to 0.9 ms / cm 2.The separator in this application has high ionic conductivity, thereby improving cycle performance and / or kinetic performance of the secondary battery.In some embodiments, a resistance value of the separator is less than or equal to 1.3 Ω, optionally less than or equal to 1.0 Ω.The separator in this application has a low resistance value, thereby improving cycle performance and / or kinetic performance of the secondary battery.In some embodiments, the machine direction tensile strength of the separator is greater than or equal to 2000 kg / cm 2, optionally 2500 kg / cm 2 to 4500 kg / cm 2.In some embodiments, the transverse tensile strength of the separator is greater than or equal to 2000 kg / cm 2, optionally 2500 kg / cm 2 to 4500 kg / cm 2.The separator in this application has high tensile strength in both the transverse direction and the machine direction, thereby providing a low possibility of damage to the separator upon swelling of the secondary battery, thereby improving safety performance of the secondary battery.In some embodiments, the infiltration length of the separator is greater than or equal to 30 mm, optionally 30 mm to 80 mm.In some embodiments, the infiltration rate of the separator is greater than or equal to 3 mm / s, optionally 3 mm / s to 10 mm / s.The separator in this application has good electrolyte infiltration and retention, thereby improving the ionic conductivity of the separator and the extractable capacity of the secondary battery.In some embodiments, the air permeability of the separator is less than or equal to 300 s / 100 mL, optionally 100 s / 100 mL to 230 s / 100 mL.The separator in this application has good air permeability, thereby improving the ion conductivity and extractable capacity of the secondary battery.The average particle size of a material (e.g., the first filler or the second filler) is as defined in the art in this application and can be tested with an instrument and method known in the art. For example, the material or separator may be measured with a scanning electron microscope, a transmission electron microscope, or a particle size distribution instrument to obtain an image, a plurality (e.g., more than 10) of test particles (such as those containing the first filler or the second filler) may be randomly selected on the image, and the average value of the shortest diagonal lengths of the particles is taken as the average particle size.In this application, the specific surface area of a material (e.g., the first filler or the second filler) has a meaning well known in the art and can be tested with an instrument and a method known in the art. For example, the specific surface area can be measured according to GB / T 19587-2017 using the nitrogen adsorption specific surface analysis test method and calculated using the BET (BrunaauerEm Teller) method. Optionally, the nitrogen adsorption specific surface analysis test can be performed using the Tri-Star 3020 specific surface and pore diameter analyzer from Micromeritics in the USA.In this application, the thermal shrinkage rate, tensile strength and air permeability of the separator have the meanings generally known in the art and can be tested by known methods in the art, for example, the testing can be carried out according to GB / T 36363-2018.In this application, the infiltration length and infiltration rate of the separator have the meanings known in the art and can be tested by known methods of the art. An example of a test method is: cutting the separator into samples having a width of 5 mm and a length of 100 mm, fixing two ends of the sample, and horizontally placing the sample; dropping 0.5 mg of an electrolyte into the center of the sample; and after a certain time (1 min in this application), a photograph is taken and the length of electrolyte diffusion is measured to obtain the infiltration length and infiltration speed of the separator. To ensure accuracy of the test results, multiple (e.g., 5-10) samples may be tested and the test results obtained by calculating the mean. The electrolyte may be prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) at a mass ratio of 30:50:20 to obtain an organic solvent, and dissolving through-dried LiPF 6 in the organic solvent to prepare an electrolyte having a concentration of 1 mol / L.The ion conductivity and resistance value of the separator may be obtained by an AC impedance spectroscopy test. Specifically, the separator is cut into a disk having a certain area, dried, and placed between two stainless steel electrodes; and after containing a sufficient amount of electrolyte, the separator is sealed to form a button cell for the AC impedance spectroscopy test.Note that parameters (such as surface density and thickness) of the above-mentioned coating of the separator are all for the coating on one side of the porous substrate. When the coating is disposed on both sides of the porous substrate, the coating is considered to fall within the scope of this application as long as parameters of the coating on each side of this application satisfy.Production MethodA second aspect of the embodiments of this application provides a manufacturing method for the separator according to the first aspect of the embodiments of this application. The method includes the steps of: providing a porous substrate; mixing a material forming the three-dimensional framework structure and a filler in a solvent at a predetermined ratio, and then stirring for uniformity at a certain shear rate to produce a coating slurry; and applying the coating slurry to at least one surface of the porous substrate, followed by drying to obtain the separator; wherein the separator includes the porous substrate and a coating disposed on at least one surface of the porous substrate; the coating includes the three-dimensional framework structure and the filler; at least a part of the filler is filled in the three-dimensional framework structure; and in a thickness direction of the separator, a mean pore area of the coating is denoted by S 1 and a mean pore area of the porous substrate is denoted by S 2 where 0<S 1 / S 2< 1.As solvent for the preparation of the coating sludge, water, for example deionized water, can be used in some embodiments.In some embodiments, the coating slurry may include still other components, for example, additionally a dispersant, a wetting agent, a binder, and the like.In some embodiments, the shear rate is less than or equal to 30 m / s, optionally 15 m / s to 30 m / s. The shear rate falling within the above range allows a corresponding average pore area of the dried coating and further increases the heat resistance and ionic conductivity of the separator.In some embodiments, a solids content of the coating slurry is 8% to 30%, optionally 10% to 20%.In some embodiments, the coating surface density of the coating slurry on one side is 0.50 g / m 2 to 1.50 g / m 2, optionally 0.75 g / m 2 to 1.40 g / m 2. The coating surface density of the coating slurry falling within the above range enables a corresponding average pore area of the dried coating and further enhances the heat resistance and ion conductivity of the separator.In some embodiments, the coating thickness of the coating slurry on one side is less than or equal to 2 μm, optionally 0.5 μm to 1.3 μm. The coating thickness of the coating slurry falling within the above range enables a corresponding average pore area of the dried coating and further enhances the heat resistance and ionic conductivity of the separator.In some embodiments, the material forming the three-dimensional framework structure may include at least one of an organic material and an inorganic material. Optionally, the organic material includes nano cellulose, polytetrafluoroethylene nano fiber, and / or polyamide nano fiber. Optionally, the inorganic material includes at least one of halogenite nanotubes, nano-rod alumina, nano-rod boehmite, nano-rod silica, and glass fiber. In some embodiments, a material forming the three-dimensional framework structure may include nano-cellulose, optionally cellulose nano-crystals (CNC, also known as cellulose nano-crystals or microfibrillated cellulose).In some embodiments, the nanocellulose may be obtained by the following method: providing cellulose powder having a whiteness greater than or equal to 80%; mixing the obtained cellulose powder with a modifying solution for reaction, followed by washing to remove impurities to obtain cellulose nanocrystals; and adjusting the pH of the obtained cellulose nanocrystals to neutral, followed by grinding and cutting to obtain nanocellulose.Optionally, the cellulose powder having a whiteness greater than or equal to 80% is commercially available or can be obtained by a chemical method (such as acidic hydrolysis, alkali treatment or tempo catalytic oxidation), a biological method (such as enzyme treatment), a mechanical method (such as pulverization, ultrasonic pulverization or high pressure homogenization), or the like. A fibrous raw material for producing the cellulose powder having a whiteness greater than or equal to 80% may include at least one of plant fibers such as cotton fiber (such as cotton linter and kapok fiber), fibrils (such as sisal fiber, ramie fiber, jute fiber, flax fiber, hemp fiber and abaca fiber), palm fiber, wood fiber, bamboo fiber and grass fiber.In some embodiments, the cellulose powder having a whiteness greater than or equal to 80% may also be prepared by the following process: after loosening the fibrous raw material and removing impurities, pulping with an alkali solution (e.g., NaOH aqueous solution, having a concentration of 4 wt % to 20 wt %, optionally 5 wt % to 15 wt %), followed by the following processes in sequence: water washing to remove impurities (e.g., 3-6 times water washing), bleaching (e.g., sodium hypochlorite and / or hydrogen peroxide may be used), acid washing to remove impurities, water washing to remove impurities, dehydration, and air drying to obtain cellulose powder.In some embodiments, the modifying solution may be an acidic solution (e.g., aqueous sulfuric acid solution, aqueous boric acid solution, aqueous phosphoric acid solution, or aqueous acetic acid solution) or an alkali solution (e.g., urea-organic solvent solution). Optionally, the modifying solution is an acidic solution.Optionally, a concentration of the acidic solution may be 5 wt % to 80 wt %. When the modifying solution is an aqueous sulfuric acid solution, a concentration of the acid solution may be 40 wt % to 80 wt %, thereby obtaining cellulose powder having sulfonic acid group. When the modifying solution is an aqueous boric acid solution, the concentration of the acid solution may be 5 to 10 wt %, thereby obtaining cellulose powder having boric acid group. When the modifying solution is an aqueous phosphoric acid solution, the concentration of the acid solution may be 45 wt % to 75 wt %, thereby obtaining cellulose powder having a phosphoric acid group. When the modifying solution is an aqueous acetic acid solution, the concentration of the acid solution may be 40 wt % to 80 wt %, thereby obtaining cellulose powder having a carboxylic acid group.Optionally, the urea-organic solvent solution is a urea-xylene solution, thereby obtaining cellulose powder having an amino group.In some embodiments, a mass ratio of the cellulose powder to the modifying solution may be 1:2.5 to 1:50, optionally 1:5 to 1:30.When the modifying solution is an aqueous sulfuric acid solution, the mass ratio between the cellulose powder and the acid solution may be 1:5 to 1:30. When the modifying solution is an aqueous boric acid solution, the mass ratio between the cellulose powder and the acid solution may be 1:20 to 1:50. When the modifying solution is an aqueous phosphoric acid solution, the mass ratio between the cellulose powder and the acid solution may be 1:5 to 1:30. When the modifying solution is an aqueous acetic acid solution, the mass ratio between the cellulose powder and the acid solution may be 1:5 to 1:30. When the modifying solution is an organic urea solution, the mass ratio between the cellulose powder and the organic urea solution may be 1:4 to 1:40.In some embodiments, when the modifying solution is an acidic solution, the reaction may be carried out at a temperature of not higher than 80° C., optionally at 30° C. to 60° C., and the reaction time of the cellulose powder with the modifying solution may be 0.5 h to 4 h, optionally 1 h to 3 h.In some embodiments, when the modifying solution is an alkali solution, the reaction may be performed at a temperature of 100° C. to 145° C., and the reaction time of the cellulose powder with the modifying solution may be 1 h to 5 h.In some embodiments, milling may be performed with a grinder and cutting may be performed with a high pressure homogenizer. The mill parameters (such as the number of mills and the milling time) and the high pressure homogenizer cutting parameters can be adjusted to obtain nanocellulose with different average diameters and / or different average lengths.In some embodiments, a coater may be used to apply the coating slurry. In this application, a model of the coater is not particularly limited, for example, a commercially available coater may be used. The coater includes an gravure roll and the gravure roll is used to transfer the slurry to the porous substrate.In some embodiments, the coating slurry may be applied in a process such as transfer coating, roto-spraying, or dip coating.In some embodiments, the method further includes the step of: applying a slurry containing a granular binder to at least a portion of a surface of the coating followed by drying to form an adhesive layer.According to the manner of manufacturing the separator in this application, the coating is obtained by coating once, thereby greatly simplifying the manufacturing process of the separator.For some raw materials and their parameters, such as a proportion used in the manufacturing method of the separator in this application, reference may be made to the separator according to the first aspect of the embodiments of this application. Details are not described again here.Unless otherwise stated, all raw materials used in the production of the separator in this application are commercially available.Secondary BatteryA third aspect of the embodiments of this application provides a secondary battery.Secondary batteries, also called rechargeable batteries or rechargeable batteries, are batteries whose active material can be activated by charging after the batteries have been discharged for continuous use. Typically, a secondary battery includes an electrode assembly and an electrolyte. The electrode assembly includes a positive electrode plate, a negative electrode plate, and a separator. The separator is disposed between the positive electrode plate and the negative electrode plate mainly to prevent a short circuit between the positive electrode and the negative electrode and to let active ions freely pass.The secondary battery in this application is not limited to a specific type. For example, the secondary battery may be a lithium ion battery, a sodium ion battery, or the like, and in particular, the secondary battery may be a lithium ion secondary battery.The secondary battery according to the third aspect of the embodiments of this application includes the separator according to the first aspect of the embodiments of this application or a separator manufactured using the method according to the second aspect of the embodiments of this application, wherein the separator is sandwiched between the positive electrode plate and the negative electrode plate. Optionally, at least one side of the separator that is closer to the negative electrode plate is provided with the coating of this application. The separator in this application allows the secondary battery to achieve a balance between high energy density, high thermal safety performance, and good cycle performance and kinetic performance.[Positive electrode plate]In some embodiments, the positive electrode plate includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector and including a positive electrode active material. For example, the positive electrode current collector has two opposing faces in its thickness direction, and the positive electrode film layer is disposed on one of the two or both opposing faces of the positive electrode current collector.When the secondary battery in this application is a lithium ion battery, the positive electrode active material may include, but is not limited to, lithium-containing transition metal oxide, lithium-containing phosphate, and / or modified compounds thereof. Examples of lithium-containing transition metal oxides may include, but are not limited to, at least one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium nickel manganese oxide, Lithiumnickelkobaltmanganoxid lithium nickel cobalt aluminum oxide, and their respective modified compounds. Examples of lithium-containing phosphates include, but are not limited to, at least one of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite material of lithium manganese iron phosphate and carbon, and their respective modified compounds.In some embodiments, to further increase the energy density of the secondary battery, the positive electrode active material for lithium ion batteries may include at least one of lithium transition metal oxides having the general formula Li a Ni b Co c M d O e A f and modified compounds thereof, wherein 0.8≤a≤1.2, 0.5≤b<1, 0<c<1, 0<d<1, 1≤e≤2, 0≤f≤1, M is at least one selected from Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, and B, and A is at least one selected from N, F, s and Cl.For example, for lithium ion batteries, the positive electrode active material may include at least one of LiCoO 2, LiNiO 2, LiMnO 2, LiMn 2 O 4, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2( NCM333), LiNi 0,5 Co_NERmn_Mn_NERmn_O2(NCM523), LiNi 0,6 Co 0,2 Mn 0.2 O 2( NCM622), LiNi 0,8 Co 0,1 Mn 0,1 O 2( NCM811), LiNi 0,85 Co 0,15 Al0,05O2, LiFePO4and LiMnPO4boneWhen the secondary battery in this application is a sodium ion battery, the positive electrode active material may include, but is not limited to, at least one of a sodium-containing transition metal oxide, a polyanionic material (such as phosphate, fluorophosphate, pyrophosphate, and sulfate), and a Prussian blue material.For example, the positive electrode active material for sodium ions may include at least one of NaFeO 2, NaCoO 2, NaCrO 2, NaMnO 2, NaNiO 2, NaNi 1 / 2 Ti 1 / 2 O 2, NaNi 1 / 2 Mn 1 / 2 O_NERmn_, Na_NERmn_Fe_NERmn_Mn2 / 3O_NERmn_, NaNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2, NaFePO 4, NaMnPO 4, NaCoPO 4, a prussian blue material and a material having the general formula X p M' q( PO 4)r O_NERmn_Y3-x. In the general formula, X p M' q( PO 4)r O x Y 3-x, 0 < p ≤ 4, 0<q ≤ 2, 1≤r ≤ 3, 0≤x ≤ 2, X is at least one selected from H +, Li +, Na +, K +, and NH 4+, M' is a transition metal cation, optionally at least one of V, Ti, Mn, Fe, Co, Ni, Cu and Zn, and Y is a halogen anion, optionally at least one of F, Cl and Br.In this application, the modified compounds of the above positive electrode active materials may be modified by doping and / or surface coating on the positive electrode active material.In some embodiments, the positive electrode film layer further optionally includes a positive electrode conductive agent. The positive electrode conductive means in this application is not limited to a specific type. For example, the positive electrode conductor includes at least one of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nano-fiber. In some embodiments, a mass proportion of the positive electrode conductor agent is less than or equal to 5 wt % based on the total mass of the positive electrode film layer.In some embodiments, the positive electrode film layer further optionally includes a positive electrode binder. The positive electrode binder is not limited to a specific type in this application. For example, the positive electrode binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and / or fluorine-containing acrylic resin. In some embodiments, a mass proportion of the positive electrode binder is less than or equal to 5 wt % based on the total mass of the positive electrode film layer.In some embodiments, the positive electrode current collector may be a metal foil current collector or a composite current collector. An aluminum foil, for example, can be used as the metal foil. The composite current collector may include a polymeric material matrix and a metal layer formed on at least one surface of the polymeric material matrix. For example, the metal material may include aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and / or silver alloy. For example, the polymeric material matrix may include at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and / or polyethylene (PE).The positive electrode film layer is typically formed by applying a positive electrode slurry to the positive electrode current collector and then drying and cold pressing. The positive electrode slurry is typically formed by dispersing and stirring the positive electrode active material, optional conducting agent, optional binder, and all other components in a solvent until the mixture is homogeneous. The solvent may be, but is not limited to, N-methylpyrrolidone (NMP).[Negative Electrode Plate]In some embodiments, the negative electrode plate includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector and including a negative electrode active material. For example, the negative electrode current collector has two opposing faces in its thickness direction, and the negative electrode film layer is disposed on one of the two or both opposing faces of the negative electrode current collector.The negative electrode active material may be a negative electrode active material for secondary batteries well known in the art. For example, the negative electrode active material may include, but is not limited to, natural graphite, artificial graphite, soft carbon, hard carbon, a silicon-based material, a tin-based material, and lithium titanate. The silicon-based material may include at least one of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and / or a silicon alloy material. The tin-based material may include at least one of elemental tin, tin oxide, and / or a tin alloy material.In some embodiments, the negative electrode film layer further optionally includes a negative electrode conductive agent. The negative electrode conductive means in this application is not limited to a specific type. For example, the negative electrode conductor may include at least one of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nano-fiber. In some embodiments, a mass proportion of the negative electrode conductive agent is less than or equal to 5 wt % based on the total mass of the negative electrode film layer.In some embodiments, the negative electrode film layer further optionally includes a negative electrode binder. The negative electrode binder is not limited to a specific type in this application. For example, the negative electrode binder may include at least one of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, water-borne acrylic resin (e.g., polyacrylic acid PAA, polymethylacrylic acid PMAA, polyacrylic acid sodium PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethylchitosan (CMCS). In some embodiments, a mass proportion of the negative electrode binder is less than or equal to 5 wt % based on the total mass of the negative electrode film layer.In some embodiments, the negative electrode film layer further optionally includes another additive. For example, the other additive may include a thickener, for example sodium carboxymethylcellulose (CMC) or PTC thermistor material. In some embodiments, a mass proportion of the other additive is less than or equal to 2% by weight based on the total mass of the negative electrode film layer.In some embodiments, the negative electrode current collector may be a metal foil current collector or a composite current collector. As the metal foil, for example, a copper foil may be used. The composite current collector may include a polymeric material matrix and a metal layer formed on at least one surface of the polymeric material matrix. For example, the metal material may include at least one of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and / or silver alloy. For example, the polymeric material matrix may include at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and / or polyethylene (PE).The negative electrode film layer is typically formed by applying a negative electrode slurry to the negative electrode current collector and then drying and cold pressing. The negative electrode slurry is typically formed by dispersing and uniformly stirring the negative electrode active material, the optionally conductive agent, the optionally present binder, and optionally the further additive in a solvent. The solvent may be, but is not limited to, N-methylpyrrolidone (NMP) or deionized water.The negative electrode plate includes still further functional layers in addition to the negative electrode layer. In some embodiments, the negative electrode plate described in this application further includes a conductive undercoat layer (e.g., consisting of a conductive agent and a binder) disposed on the surface of the negative electrode current collector and sandwiched between the negative electrode current collector and the negative electrode film layer. In some other embodiments, the negative electrode plate of this application further includes a protective layer covering the surface of the negative electrode film layer.[Electrolyte]In a charging and discharging operation of the secondary battery, active ions intercalate and deintercalate between the positive electrode plate and the negative electrode plate. The electrolyte is disposed between the positive electrode plate and the negative electrode plate to mainly conduct active ions. The electrolyte in this application is not specifically limited to a specific type and may be selected according to actual needs.The electrolyte includes an electrolyte salt and a solvent. Electrolyte salt and solvent are not specifically limited to any types and may be selected according to actual needs.For example, in the case where the secondary battery in this application is a lithium ion battery, the electrolysis salt may be at least one of lithium hexafluorophosphate (LiPF 6), lithium tetrafluoroborate (LiF 4), lithium perchlorate (LiClO 4), lithium hexafluoroarsenate (LiAsF 6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bistrifluoromethanesulfonimide (LiTFSI), lithium ethanesulfonate (LiTFS), lithium difluoro(oxalato)borate (LiDFOB), lithium dioxalate borate (LiBO), Lithium difluorophosphate (LiPO 2 F 2), lithium difluoro(dioxalato)phosphate (LiDFOP), and Lithiumtetrafluor(dioxalato)phosphat (LiTFOP), include, but are not limited to, lithium difluorophosphate.For example, in the case where the secondary battery in this application is a sodium ion battery, the electrolysis salt may be at least one of sodium hexafluorophosphate (NaPF 6), sodium tetrafluoroborate (NaB 4), sodium perchlorate (NaClO 4), sodium hexafluoroarsenate (NaAsF 6), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bistrifluoromethanesulfonimide (NaTFSI), sodium trifluoromethanesulfonate (NaTFS), sodium difluoro(oxalato)borate (NaDFOB), sodium dioxalate borate (NaBO), Sodium difluorophosphate (NaPO 2 F 2) sodium difluoro(dioxalato)phosphate (NaDFOP), and sodium tetrafluorooxalatophosphate (NaTFOP), may include, but are not limited to, sodium difluorophosphate (NaTFOP).For example, the solvent may include at least one of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-gamma-butyrolactone (GBL), sulfolane (SF), Methylsulfonylmethane (MSM), ethylmethylsulfone (EMS), and diethylsulfone (ESE) include, but are not limited to, methylsulfonylmethane (MSM).In some embodiments, the electrolyte further optionally includes an additive. For example, the additive may include a negative electrode film-forming additive, or may include a positive electrode film-forming additive, or may include an additive capable of improving a certain performance of the battery, for example, an additive for improving overcharge performance of the battery, an additive for improving high temperature performance of the battery, or an additive for improving low temperature performance of the battery.In some embodiments, the positive electrode plate, the separator, and the negative electrode plate may be made into an electrode assembly by winding and / or lamination.In some embodiments, the secondary battery may include an outer package. The outer package may be used to package the electrode assembly and the electrolyte.In some embodiments, the outer packaging of the secondary battery may be a hard shell, for example a hard plastic shell, an aluminum shell or a steel shell. The outer packaging of the secondary battery can alternatively also be a soft packaging, for example a soft pouch. A material of the soft pack can be plastic, for example polypropylene (PP), polybutylene terephthalate (PBT) and / or polybutylene succinate (PBS).This application does not particularly limit the shape of the secondary battery, and the secondary battery may be formed cylindrically, rectangularly, or in another shape. FIG. 1 exemplarily shows a secondary battery 5 having a rectangular configuration.In some embodiments, as shown in FIG. 2, the outer package may include a housing 51 and a cover plate 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and the side plates enclosing a receiving cavity. The housing 51 has an opening communicating with the accommodating cavity, and the cover plate 53 is configured to cover the opening for closing the accommodating cavity. The positive electrode plate, the negative electrode plate, and the separator may be joined together into an electrode assembly 52 by winding and / or lamination. The electrode assembly 52 is packaged in the accommodation space. The electrolyte infiltrates the electrode assembly 52. the secondary battery 5 may include one or more electrode assemblies 52, and the amount may be adjusted as needed.The manufacturing method of the secondary battery is well known in this application. In some embodiments, a positive electrode plate, a separator, a negative electrode plate, and an electrolyte may be assembled into a secondary battery. For example, the positive electrode plate, the separator, and the negative electrode plate may be made into an electrode assembly by winding and / or lamination; and the electrode assembly is placed in an outer package which is filled with electrolyte after drying, followed by processes such as vacuum packaging, standing, forming, and shaping to obtain a secondary battery.In some embodiments of this application, such secondary batteries of this application may be assembled into a battery module. The battery module may include a plurality of secondary batteries, and a certain amount may be adjusted based on application and capacity of the battery module.FIG. 3 is a schematic diagram of an exemplary battery module 4. Of course, the batteries may be arranged in any other manner. Further, the plurality of secondary batteries 5 may be fixed by fastening members.Optionally, the battery module 4 may further include a housing having a storage space, and the plurality of secondary batteries 5 are stored in the storage space.In some embodiments, the battery modules may be further assembled into a battery pack, and a quantity of battery modules included in the battery pack may be adjusted based on application and capacity of the battery pack.FIGS. 4 and 5 are schematic diagrams of an exemplary battery pack 1. The battery box includes an upper box body 2 and a lower box body 3. the upper box body 2 is configured to be engaged with the lower box body 3 to form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 may be disposed in the battery box in any manner.Electrical DeviceA fourth aspect of the embodiments of this application provides an electrical device. The electric device includes at least one of the secondary battery, the battery module, or the battery pack according to this application. The secondary battery, the battery module, or the battery pack may be used as a power source for the electric device or an energy storage of the electric device. The electrical device may include, but is not limited to, a mobile device (e.g., a mobile phone or notebook computer), an electric vehicle (e.g., a rechargeable electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf vehicle, or an electric truck), an electric train, a ship, a satellite system, or an energy storage system.The secondary battery, the battery module, or the battery pack may be selected for the electric device based on requirements for use of the electric device.FIG. 6 is a schematic illustration of an example electrical device. This electric device is a battery electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, or the like. In order to meet the requirements of the electric device for high power and high energy density, a battery pack or a battery module may be used.In another example, the electrical device may be a mobile phone, a tablet computer, a notebook computer, or the like. Such an electric device is usually required to be light and thin, and may use a secondary battery as a power source.ExamplesThe content disclosed in this application is described in more detail in the examples which follow. These examples are illustrative only, as various modifications and changes will be apparent to those skilled in the art without departing from the scope of the contents disclosed in this application. Unless otherwise indicated, all parts, proportions and proportions indicated in the examples below are by mass, all reagents used in the examples are commercially available or synthesized in a manner known per se and can be used directly without further treatment, and all instruments used in the examples are commercially available.Preparation of nanocellulose C1Production of cellulose pulpThe cotton feed was decontaminated with a cotton cutter and then disintegrated with a 5 wt% NaOH aqueous solution at 150°C for 2 hours, followed by the following processes in sequence: water washing for removing impurities (washing with water 3 times), bleaching with sodium hypochlorite, acid washing with dilute hydrochloric acid for removing impurities, water washing for removing impurities (washing with water once), dehydration and air drying to obtain cotton cellulose powder having a whiteness of at least 85%.Esterification of cellulose1 kg of the obtained cotton cellulose powder was mixed with 30 kg of a 60 wt % aqueous sulfuric acid solution and reacted at 55° C. to 60° C. for 1.5 hours, and after the reaction, the resultant product sequentially undergone the following processes: water washing to remove impurities (washing with water 3 times), filtering, and washing with acid to remove impurities to obtain cellulose nanocrystals having sulfonic acid group.Neutralization of celluloseThe cellulose nanocrystals with sulfonic acid group were adjusted to a neutral pH with 10% by weight aqueous NaOH solution, then ground with a grinder and subjected to nano-scale cutting with a high-pressure homogenizer to form nanocellulose with sulfonic acid-modified group having an average length of 500 nm and an average diameter of 22 nm, where a molar ratio of the sulfonic acid group to the hydroxyl group was 5:3.The molar ratio of the anionic modified group to the hydroxyl group in nanocellulose could be measured by the following method: The hydroxyl values (the number of milligrams of potassium hydroxide equivalent to the content of the hydroxyl group per gram of sample) of the raw materials cellulose and nanocellulose were tested by the phthalic anhydride method according to GB / T 12008.3-2009, the obtained values were measured in mg KOH / g, and then reacted in mmol / g as the content of the hydroxyl group. The content of the anionic modified group (i.e., the content of the modified hydroxyl group) could be obtained by subtracting the content of the hydroxyl group in nanocellulose from the content of the hydroxyl group in the raw material cellulose, and then the molar ratio of the anionic modified group to the hydroxyl group was calculated.Preparation of nanocellulose C2Unmodified nanocellulose having an average length of 500 nm and an average diameter of 22 nm was used. The product model was CNWS-50, and was obtained from Zhongkegering (Beijing) Technology Co. Ltd. The nanocellulose could be further treated with a grinder and / or high pressure homogenizer to obtain nanocellulose with different mean diameters and / or different mean lengths.Example 1Production of SeparatorA porous PE substrate having a thickness of 4.8 μm and a porosity of 32% was provided.Preparation of Coating Slurry: The above-prepared nanocellulose C1, a first filler alumina (having secondary particle morphology and an average particle size of 75 nm), and a binder-aqueous solution-like polyacrylic acid were uniformly mixed in an appropriate amount of demineralized water in a mass ratio of 16:83:1, and then stirred at a shear rate of 23 m / s to obtain a coating slurry having a solid content of 15% by weight. Percentages of particles in α-crystalline form, θ-crystalline form, γ-crystalline form and η-crystalline form in the first filler were 1.1 wt %, 68.7 wt %, 29.6 wt % and 0.6 wt %, respectively, based on the total weight of the first filler.Coating: The prepared coating slurry was applied to both surfaces of the PE porous substrate with a coater, followed by drying and slitting processes to obtain a separator. The thickness of the coating on one side of the PE porous substrate was 0.8 μm, and a surface density was 1.25 g / m 2.Production of positive electrode plateA positive electrode active material LiNi 0,8 Co 0,1 Mn 0,1 O 2( NCM811), a conductive carbon black (Super P), and a binder polyvinylidene fluoride (PVDF) were uniformly mixed in a mass ratio of 96.2:2.7:1.1 in a solvent N-methylpyrrolidone (NMP) to obtain a positive electrode slurry. The positive electrode slurry was applied to a positive electrode current collector aluminum foil, followed by processes such as drying, cold pressing, slitting, and cutting to obtain a positive electrode plate.Production of Negative Electrode PlateA negative electrode active material, artificial graphite, a conductive carbon (Super P), a binder, styrene butadiene rubber (SBR), and sodium carboxymethylcellulose (CMC) were uniformly mixed in a corresponding amount of deionized water in a mass ratio of 96,4:0,7:1,8:1,1 in a solvent to obtain a negative electrode slurry. The negative electrode slurry was applied to a negative electrode current collector copper foil, followed by processes of drying, cold pressing, slitting, and cutting to obtain a negative electrode plate.Electrolyte PreparationEthylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a mass ratio of 30:70 to obtain an organic solvent. Subsequently, completely dried LiPF 6 was dissolved in the organic solvent to prepare an electrolyte having a concentration of 1 mol / L.Production of Secondary BatteryThe positive electrode plate, the separator, and the negative electrode plate were sequentially stacked and wound to obtain an electrode assembly. The electrode assembly was placed in an outer package and dried. Subsequently, the electrolyte was injected, and processes such as vacuum packaging, standing, molding, and molding were performed to obtain a secondary battery.Examples 2 to 16Secondary batteries were produced by a similar method as in Example 1, the differences being shown in Table 1. In Examples 12 to 15, the second filler had a primary particle morphology, and the crystalline form of the second filler was mainly the α-crystalline form with a weight percent of over 99% based on the total weight of the second filler.Comparative Example 1The secondary batteries were manufactured by a method similar to that of Example 1, with differences in the manufacturing method of the separator.A porous PE substrate having a thickness of 4.8 μm and a porosity of 32% was provided.Preparation of the coating slurry: alumina (having an average particle size of 600 nm, primary particle morphology, and a proportion of particles in α-crystalline form over 99%) and a binder were mixed in a mass ratio of 94:6, dissolved in deionized water, and then stirred at a shear rate of 23 m / s to obtain a coating slurry having a solid content of 38% by weight.Coating: The prepared coating slurry was applied to both surfaces of the PE porous substrate with a coater, followed by drying and slitting processes to obtain the separator. The thickness of the coating on one side of the PE porous substrate was 1.7 μm, and a surface density was 1.80 g / m 2.Tests Test(1) Test for Ionic Conductivity of the SeparatorThe ionic conductivity of the separator was obtained by an AC impedance spectroscopy test. Specifically, the separator was cut into a small round sheet having a certain area, dried, and put between two stainless steel electrodes. After the separator had taken up a sufficient amount of electrolyte, it was sealed to form a coin cell and the alternating current impedance spectroscopy test was performed with an electrochemical work station to obtain the ionic conductivity of the separator. The electrochemical workstation could be a CHI 660C electrochemical workstation from Shanghai CH Instruments, with an AC signal frequency range of 0.01 Hz to 1 MHz and a sine wave potential amplitude of 5 mV. For accuracy, the mean of 5 parallel samples was taken as the test result.The electrolyte used was prepared as follows: ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a mass ratio of 30:50:20 to obtain an organic solvent. Subsequently, completely dried LiPF 6 was dissolved in the organic solvent to prepare an electrolyte having a concentration of 1 mol / L.(2) Test on Average Pore Area of Coating and Porous SubstrateA separator sample to be tested having a certain size (e.g., 15 mm×15 mm) was cut out from a randomly selected region of the separator, the separator sample was wound with copper foil, and a cross section of the separator sample was obtained with an ion beam polisher (e.g., Hitachi ArLade 5000) by cutting under frozen conditions, for example, -80° C. The separator sample was subjected to metal spray treatment. Referring to JY / T010-1996, an SEM image (magnification may be 1000X to 30000X) of the cross section of the separator was obtained using a scanning electron microscope (e.g., a Sigma 300 model scanning electron microscope from ZEISS in Germany). The average pore area S 1 of the coating of the separator and the average pore area S 2 of the porous substrate of the separator were obtained by an image processing recognition system (e.g., Fully Separator Detection System 2022-0408) using a multi-segment binarization method. The ratio of the total pore area of the coating of the separator to the number of pores in the coating was the average pore area S 1 of the coating of the separator, and the ratio of the total pore area of the porous substrate of the separator to the number of pores in the porous substrate was the average pore area S 2 of the porous substrate of the separator. For accuracy, the mean of 5 parallel samples was taken as the test result.(3) Test for Thermal Shrinkage Rate of the SeparatorSample Preparation: The above-prepared separator was punched into samples having a width of 50 mm and a length of 100 mm with a punching machine, 5 parallel samples were put on an A4 paper and fixed, and then the A4 paper with the samples was put on corrugated paper having a thickness of 1 mm to 5 mm.Test of Sample: The A4 paper placed on the corrugated paper was placed in a blast furnace, with the temperature of the blast furnace set at 150°C. After reaching and maintaining the predetermined temperature for 30 minutes, the time measurement was started. After the predetermined time (1 hour for this application) had been reached, the length and width of the separator were measured and the values were designated a and b, respectively.Calculation of thermal shrinkage rate: machine direction heat shrinkage rate (MD) = [(100 - a) / 100] × 100%, transverse direction heat shrinkage rate (TD) = [(50 - b) / 50] × 100%, and the average value of 5 parallel samples was taken as a test result.(4) Hot Box Test for Secondary BatteryAt 25° C., the secondary battery was charged to 4.2 V at a constant current of 1 C., charged to a current of less than or equal to 0.05 C. at a constant voltage, and allowed to stand for 5 min. Subsequently, each secondary battery was tested in a DHG-9070A DHG series high temperature furnace with a clamp, and the temperature was raised from room temperature to 80° C.±2° C. at a rate of 5° C. / min. This temperature was maintained for 30 minutes. Then, the temperature was raised at a rate of 5°C / min, with the temperature being maintained for 30 min for each rise of 5°C. During the temperature increase process, the surface temperature change of the secondary battery was monitored. With the temperature greatly increasing, the corresponding furnace temperature was the hot box failure temperature of the secondary battery. A higher hot box failure temperature of the secondary battery indicates a better safety thermal performance of the secondary battery. For accuracy, the mean of 5 parallel samples was taken as the test result.(5) Cycle Performance Test for Secondary BatteryAt 25° C., the secondary battery was charged to 4.2 V at a constant current of 1 C. and charged to a current of less than or equal to 0.05 C. at a constant voltage. At this time, the secondary battery was fully charged, and a charge capacity at this time was recorded as a charge capacity of the first cycle. The secondary battery was allowed to stand for 5 minutes and discharged to 2.8 V at a constant current of 1C. This was a charge and discharge cycle, and a discharge capacity at this time was recorded as a discharge capacity of the first cycle. The secondary battery was subjected to the charge and discharge cycle test according to the above method, and a discharge capacity of each cycle was recorded. Storage capacity (%) of secondary battery after 1000. Cycle at 25° C.= Entlade capacity of 1000. Cycle / Discharge Capacity of First Cycle × 100 %. For accuracy, the mean of 5 parallel samples was taken as the test result.As is apparent from Table 2 in Examples 1 to 16, the separator having the nanocellulose (constituting the three-dimensional framework structure) disposed on both surfaces of the porous substrate of the separator and filler and the average pore area S 1 of the coating and the average pore area S 2 of the porous substrate, which are reasonably set to 0<S 1 / S 2< 1 can achieve a balance between low thermal shrinkage rate and high ionic conductivity, and the secondary battery can achieve a balance between high thermal safety performance and good cycle performance.It should be noted that this application is not limited to the above embodiments. The above embodiments are merely examples, and embodiments having substantially the same constructions and the same effects as the technical idea within the technical solutions of this application are all included in the technical scope of this application. Moreover, without departing from the scope of this application, various modifications of the embodiments conceivable by those skilled in the art as well as other methods constructed by combining some of the constituent elements in the embodiments fall within the scope of this application. Table 1 Table 1Example 1C116Alumina7583 / / / 12315 %1,250,8Example 2C116Alumina7583 / / / 12310 %1,000,6Example 3C116Alumina7583 / / / 12510 %1,300,6Example 4C116Alumina7583 / / / 12515 %1,400,9Example 5C116Alumina7583 / / / 12515 %1,501,1Example 6C116Alumina7583 / / / 12815 %1,501,2Example 7C116Alumina7583 / / / 11810 %0,750,4Example 8C15Alumina7594 / / / 12315 %1,250,8Example 9C110Alumina7589 / / / 12315 %1,250,8Example 10C125Alumina7574 / / / 12315 %1,250,8Example 11C140Alumina7559 / / / 12315%1,250,8Example 12C116Alumina7580Aluminum oxide250312315%1,250,8Example 13C116Alumina7572Aluminum oxide2501112315%1,251,0Example 14C116Alumina7564Aluminum oxide2501912315%1,251,3Example 15C116Alumina7552Aluminum oxide2503112315%1,251,6Example 16C216Alumina7583 / / / 12315%1,251,5Comparative Example 1 / / / / / Aluminum oxide6009462338%1,801,7Table 2Table 2Example 10,00190,00290,660,9561,50 %1,80 %15187,0 %Example 20,00250,00290,860,9583,50 %3,80 %14788,0 %Example 30,00180,00290,62 %0,9251,50 %1,80 %15085,0 %Example 40,00150,00290,520,9201,50 %1,70 %15084,0 %Example 50,00120,00290,410,9181,50 %1,60 %15182,0 %Example 60,00090,00290,310,8891,40 %1,50 %15180,0 %Example 70,00280,00290,970,9625,70 %6,00 %14589,0 %Example 80,00120,00290,410,9054,40 %4,70 %14680,0 %Example 90,00150,00290,520,9422,80 %3,00 %14885,0 %Example 100,00250,00290,860,9402.50 %2,80 %14984,8 %Example 110,00280,00290,970,9322,90 %3,20 %14882,0 %Example 120,00250,00290,860,9621,50 %1,70 %15188,0 %Example 130,00280,00290,970,9582,10 %2,30 %14987,0 %Example 140,00220,00290,760,9263,00 %3,20 %14881,0%Example 150,00120,00290,410,9154,00 %4,20 %14780,0 %Example 160,00260,00290,900,9234,50 %4,80 %14681,0%Comparative Example 10,04090,002914,100,92913,00 %15,00 %14278,0 %References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Cited Non-Patent LiteratureJY / T010-1996

[0104]

Claims

A separator comprising a porous substrate and a coating disposed on at least one surface of the porous substrate, wherein the coating comprises a three-dimensional framework structure and a filler, wherein at least a part of the filler is filled in the three-dimensional framework structure; and wherein, in a thickness direction of the separator, a mean pore area of the coating is referred to as S 1 and a mean pore area of the porous substrate is referred to as S 2 wherein 0<S 1 / S 2< 1.The separator of claim 1, wherein 0.06 ≤ S 1 / S 2< 1 and optionally 0.30 ≤ S 1 / S 2 ≤ 0.97.The separator according to claim 1 or 2, wherein 0.0002 μm 2 ≤ S 1 ≤ 0.0080 μm 2 and optionally 0.0004 μm 2 ≤ S 1 ≤ 0.0050 μm 2; and / or 0.0005 μm 2 ≤ S 2 ≤ 0.0100 μm 2 and optionally 0.0008 μm 2 ≤ S2 ≤ 0.0080 μm2.The separator of any one of claims 1 to 3, wherein an average pore diameter of the separator is designated as d 1 and an average pore diameter of the porous substrate is designated as d 2 wherein d 1 / d 2< 1; optionally 0.3 ≤ d 1 / d 2 ≤ 0.8; optionally 15 nm ≤ d 1 ≤ 50 nm, and more optionally 20 nm ≤ d 1 ≤ 40 nm; and optionally 25 nm ≤ d 2 ≤ 60 nm, and more optionally 30 nm ≤ d 2 ≤ 50 nm.The separator according to any one of claims 1 to 4, wherein the surface density of the coating is designated as ρ 1 and the surface density of the porous substrate is designated as ρ 2 wherein 0.15 ≤ ρ 1 / ρ 2 ≤ 0.80; optionally 0.20 ≤ ρ 1 / ρ 2 ≤ 0.50; optionally 0.50 g / m 2 ≤ ρ 1 ≤ 1.50 g / m 2 and even optionally 0.75 g / m 2 ≤ ρ1 ≤ 1.40 g / m2; and optionally 1.50 g / m 2 ≤ ρ 2 ≤ 4.50 g / m 2 and even more optionally 2.00 g / m 2 ≤ ρ 2 ≤ 4.00 g / m 2.The separator according to any one of claims 1 to 5, wherein the porosity of the separator is designated as P 1 and the porosity of the porous substrate is designated as P 2 wherein 0.4 ≤ P 2 / P 1< 1; optionally 0.55 ≤ P 2 / P 1 ≤ 0.85; optionally 20% ≤ P 1 ≤ 60% and even more optionally 25% ≤ P 1 ≤ 45%; and optionally 15% ≤ P 2 ≤ 45% and even more optionally 20% ≤ P 2 ≤ 40%.The separator of any one of claims 1 to 6, wherein the filler comprises a first filler and an average particle size of the first filler is less than or equal to 150 nm, optionally 15 nm to 120 nm.The separator of claim 7, wherein the first filler comprises at least one of a primary particle and a secondary particle; optionally wherein an average particle size of the first filler of primary particle morphology is 15 nm to 80 nm, optionally 30 nm to 75 nm; and optionally wherein an average particle size of the first filler of secondary particle morphology is 50 nm to 150 nm, optionally 55 nm to 120 nm.The separator according to claim 7 or 8, wherein the first filler comprises at least one of an inorganic particle and an organic particle; optionally wherein the inorganic particle comprises at least one of boehmite, alumina, barium sulfate, magnesia, magnesium hydroxide, silicon oxygen compound, tin dioxide, titanium oxide, calcium oxide, zinc oxide, zirconium oxide, yttrium oxide, nickel oxide, hafnium dioxide, cerium oxide, zirconium titanate, barium titanate, and magnesium fluoride, and still optionally the inorganic particle comprises at least one of boehmite, alumina, barium sulfate, magnesium oxide, silicon oxygen compound, titanium oxide, zinc oxide, cerium oxide, and barium titanate; and optionally, the organic particle comprises at least one of a polystyrene particle, polypropylene wax particle, melamine formaldehyde resin particle, phenol resin particle, polyester particle, polyimide particle, polyamideimide particle, polyaramid particle, polyphenylene sulfide particle, polysulfone particle, polyethersulfone particle, polyether ether ketone particle, and polyaryl ether ketone particle.The separator of any one of claims 7 to 9, wherein the first filler material comprises inorganic particles, and a crystalline form of the inorganic particles comprises at least one of θ-crystalline form, γ-crystalline form, and η-crystalline form; optionally wherein the crystalline form of the inorganic particles comprises at least one of θ-crystalline form and γ-crystalline form; optionally, based on the total weight of the inorganic particles in the first filler, a proportion of the inorganic particles in θ-crystalline form is greater than or equal to 50% by weight, more optionally, 60% by weight to 82% by weight; optionally, based on the total weight of the inorganic particles in the first filler, a proportion of the inorganic particles in γ-crystalline form is greater than or equal to 10% by weight, more optionally, 17% by weight to 38% by weight; and optionally, based on the total weight of the inorganic particles in the first filler, a proportion of the inorganic particles in η-crystalline form is less than or equal to 5 wt %, more optionally less than or equal to 1.5 wt %.The separator according to any one of claims 7 to 10, wherein the BET specific surface area of the first filler is greater than or equal to 15 m 2 / g, optionally 18 m 2 / g to 65 m 2 / g; and / or based on the total weight of the coating, a proportion of the first filler is greater than or equal to 55 wt%, optionally 60 wt% to 90 wt%.The separator of any one of claims 7 to 11, wherein the filler further comprises a second filler, and an average particle size of the second filler is greater than the average particle size of the first filler; and optionally, the average particle size of the second filler is less than or equal to 400 nm, optionally 100 nm to 300 nm.The separator of claim 12, wherein the second filler satisfies at least one of the following conditions (1) to (6): (1) the second filler has a primary particle morphology; (2) the BET specific surface area of the second filler is less than or equal to 15 m 2 / g, optionally 7 m 2 / g to 12 m 2 / g; (3) based on the total weight of the coating, the proportion of the second filler is less than or equal to 20 wt%, optionally 2 wt% to 15 wt%; (4) the second filler comprises at least one of an inorganic and an organic particle; (5) the second filler includes inorganic particles having primary particle morphology, and a crystalline form of the inorganic particles having primary particle morphology includes at least one of α-crystalline form and γ-crystalline form, and optionally includes α-crystalline form; and (6) the second filler includes inorganic particles having primary particle morphology, and a crystalline form of the inorganic particles having primary particle morphology includes α-crystalline form, wherein, based on the total weight of the inorganic particles having primary particle morphology in the second filler, the percentage of the inorganic particles in α-crystalline form is greater than or equal to 80% by weight, optionally 90% by weight to 100% by weight.The separator according to any one of claims 1 to 13, wherein, based on the total weight of the coating, the percentage of the three-dimensional framework structure is less than or equal to 40 wt%, optionally 5 wt% to 25 wt%.The separator according to any one of claims 1 to 14, wherein the three-dimensional framework structure is formed by a fibrous substance, and the morphology of the fibrous substance optionally comprises at least one of a rod shape, a tube shape, a rod shape, and a fiber shape.The separator according to any one of claims 1 to 15, wherein the average diameter of a material constituting the three-dimensional framework structure is less than or equal to 40 nm, optionally 10 nm to 35 nm; the average length of the material constituting the three-dimensional framework structure is 100 nm to 800 nm, optionally 200 nm to 600 nm; and the length-to-diameter ratio of the material constituting the three-dimensional framework structure is 5 to 60, optionally 10 to 30.The separator of any one of claims 1 to 16, wherein the material forming the three-dimensional framework structure comprises at least one of an organic material and an inorganic material; optionally wherein the organic material comprises at least one of nano-cellulose, polytetrafluoroethylene nano-fiber, and polyamide nano-fiber, and optionally the nano-cellulose comprises at least one of cellulose nano-fibrils, cellulose nano-crystals, and bacterial nano-cellulose; and optionally the inorganic material comprises at least one of halloysite nano-tubes, nano-rod shaped alumina, nano-rod shaped boehmite, nano-rod shaped silica, and glass fiber.The separator according to any one of claims 1 to 17, wherein a material forming the three-dimensional framework structure comprises nanocellulose; wherein the nanocellulose optionally comprises a hydroxyl group and an anionically modified group; the anionically modified group optionally comprises at least one of an amino group, a carboxyl group, a sulfonic acid group, a boric acid group and a phosphoric acid group, and even more optionally comprises at least one of a sulfonic acid group, a boric acid group and a phosphoric acid group; and optionally the molar ratio of the anionically modified group to the hydroxyl group is 1:4 to 4:1, optionally 2:3 to 7:3.The separator of any one of claims 1 to 18, wherein the coating further comprises a non-granular binder; optionally wherein the non-granular binder comprises an aqueous solution type binder; and optionally, based on the total weight of the coating, the proportion of the non-granular binder in the coating is less than or equal to 2% by weight.The separator according to any one of claims 1 to 19, wherein the thickness of the porous substrate is less than or equal to 8 μm, optionally 3 μm to 6 μm; and / or the thickness of the coating is less than or equal to 2 μm, optionally 0.5 μm to 1.3 μm.The separator of any one of claims 1 to 20, wherein the separator further comprises an adhesive layer, the adhesive layer is disposed on at least a portion of a surface of the coating, and the adhesive layer comprises a granular binder; optionally wherein the granular binder comprises at least one of acrylate monomer homopolymer or copolymer, acrylic monomer homopolymer or copolymer, and fluorine-containing olefin monomer homopolymer or copolymer.The separator according to any one of claims 1 to 21, wherein the separator satisfies at least one of the following conditions (1) to (9): (1) a thermal shrinkage rate of the separator in the machine direction at 150°C for 1 hour is less than or equal to 6%, optionally 0.5% to 4%; (2) a thermal shrinkage rate of the separator in the transverse direction at 150°C for 1 hour is less than or equal to 6%, optionally 0.5% to 4%; (3) the ionic conductivity of the separator is greater than or equal to 0.6 ms / cm 2, optionally greater than or equal to 0.9 ms / cm 2; (4) a resistance value of the separator is less than or equal to 1.3 Ω, optionally less than or equal to 1.0 Ω; (5) The machine direction tensile strength of the separator is greater than or equal to 2000 kg / cm 2, optionally 2500 kg / cm 2 to 4500 kg / cm 2; (6) the cross direction tensile strength of the separator is greater than or equal to 2000 kg / cm 2, optionally 2500 kg / cm 2 to 4500 kg / cm 2; (7) the infiltration length of the separator is greater than or equal to 30 mm, optionally 30 mm to 80 mm; (8) the infiltration rate of the separator is greater than or equal to 3 mm / s, optionally 3 mm / s to 10 mm / s; and (9) the air permeability of the separator is less than or equal to 300 s / 100 ml, optionally 100 s / 100 ml to 230 s / 100 ml.A secondary battery comprising the separator according to any one of claims 1 to 22.An electrical device comprising the secondary battery according to claim 23.

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