Isolating membrane, preparation method thereof, and related secondary battery and electric device

Abstract

The present application provides an isolation membrane, a preparation method thereof, and a related secondary battery and electrical device, the isolation membrane comprises a porous substrate and a coating disposed on at least one surface of the porous substrate, the coating comprises a fibrous material, a first filler and a second filler, and the average particle size of the second filler is greater than that of the first filler. The secondary battery has the advantages of high energy density, high thermal safety performance, long cycle life and good dynamic performance.

Description

[0001] This application is a divisional application of an invention titled "Separator, Preparation Method Thereof, and Related Secondary Battery and Electrical Device", with the application number 202280018076.X, the filing date of December 05, 2022, and the applicant Contemporary Amperex Technology Co., Limited. Technical Field

[0002] This application belongs to the technical field of batteries, and particularly relates to a separator, a preparation method thereof, and related secondary batteries and electrical devices. Background Art

[0003] In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power stations, as well as in multiple fields such as power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, and aerospace. With the application and popularization of secondary batteries, their safety issues, especially thermal safety issues, have received increasing attention. However, the current methods for improving the thermal safety performance of secondary batteries often do not facilitate the balance of the energy density and service life of secondary batteries. Therefore, how to enable secondary batteries to achieve high energy density, high thermal safety performance, long cycle life, and good kinetic performance is the key challenge in the design of secondary batteries. Summary of the Invention

[0004] The purpose of this application is to provide a separator, a preparation method thereof, and related secondary batteries and electrical devices, which can enable secondary batteries to achieve high energy density, high thermal safety performance, long cycle life, and good kinetic performance.

[0005] In the first aspect of this application, a separator is provided, which includes a porous substrate and a coating disposed on at least one surface of the porous substrate. The coating includes fibrous materials, a first filler, and a second filler, and the average particle size of the second filler is larger than that of the first filler.

[0006] Materials with suitable shapes are conducive to forming a more stable spatial network structure with the first filler for the three-dimensional framework structure, thereby further improving the heat resistance, ion conductivity, and wetting and retention characteristics of the separator for the electrolyte. The average particle size of the second filler is larger, so that its supporting role in the coating can be better exerted, the shrinkage of the first filler can be reduced, and the amount of binder can be decreased, thereby improving the heat resistance of the separator; the larger particle size of the second filler also helps to make the coating have more pore structures and less water content with a smaller amount, which can further improve the ion conductivity, and the wetting and retention characteristics of the separator for the electrolyte, and at the same time can also improve the cycle performance and / or kinetic performance of the secondary battery. Among them, the three-dimensional framework structure is formed by fibrous materials.

[0007] In any embodiment of the present application, the first filler includes at least one of primary particles and secondary particles.

[0008] In any embodiment of the present application, the second filler has a primary particle morphology.

[0009] In any embodiment of the present application, the average particle size of the first filler with a primary particle morphology is 15 nm to 80 nm, and optionally 30 nm to 65 nm.

[0010] In any embodiment of the present application, the average particle size of the first filler with a secondary particle morphology is 50 nm to 200 nm, and optionally 55 nm to 150 nm.

[0011] In any embodiment of the present application, the first filler includes at least one of primary particles and secondary particles, and the average particle size of the first filler with a primary particle morphology is denoted as d 11 , and the average particle size of the first filler with a secondary particle morphology is denoted as d 12 , and the average particle size of the second filler is denoted as d2, 3.0 ≤ d2 / d 11 ≤ 10.0, optionally, 3.5 ≤ d2 / d 11 ≤ 8.0; and / or, 1.2 ≤ d2 / d 12 ≤ 6.0, optionally, 2.0 ≤ d2 / d 12 ≤ 5.5.

[0012] Through the combined action of the first filler and the second filler, it helps to reduce the moisture content of the coating, keep the pore structure of the coating stable during long-term charge and discharge processes, and at the same time improve the heat resistance of the separator, thereby enabling the secondary battery to better balance high energy density, high thermal safety performance, long cycle life and good kinetic performance.

[0013] In any embodiment of the present application, the average particle size of the first filler is less than or equal to 200 nm, optionally 15 nm to 180 nm, and more optionally 30 nm to 150 nm. When the average particle size of the first filler is within the above range, the first filler can have a relatively high specific surface area, making the particle size of the first filler better match the three-dimensional skeleton structure. Thus, the first filler can better overlap with the three-dimensional skeleton structure to form an integrated effect, increasing the affinity between the first filler and the three-dimensional skeleton structure, increasing the heat resistance and ion conductivity of the separator, and at the same time increasing the wetting and retention characteristics of the separator for the electrolyte.

[0014] In any embodiment of the present application, the average particle size of the second filler is 120 nm to 350 nm, and may be optionally 150 nm to 300 nm. Thereby, the supporting effect of the second filler can be better exerted, the moisture content of the coating can be reduced, the coating can maintain a stable pore structure during long-term charge and discharge, and at the same time, the heat resistance of the separator can be improved.

[0015] In any embodiment of the present application, the first filler includes a combination of primary particles and secondary particles.

[0016] In any embodiment of the present application, based on the total weight of the first filler, the content of the first filler with the morphology of primary particles is less than the content of the first filler with the morphology of secondary particles.

[0017] In any embodiment of the present application, based on the total weight of the first filler, the content of the first filler with the morphology of primary particles is less than or equal to 30 wt%, and may be optionally 8 wt% to 30 wt%.

[0018] In any embodiment of the present application, the BET specific surface area of the first filler is ≥25 m 2 / g, and may be optionally 30 m 2 / g to 65 m 2 / g. When the specific surface area of the first filler is within the above range, its affinity with the three-dimensional framework structure is better, which can increase the heat resistance and ion conductivity of the separator, and at the same time, can also increase the wetting and retention properties of the separator for the electrolyte.

[0019] In any embodiment of the present application, the first filler includes at least one of inorganic particles and organic particles. Optionally, the inorganic particles include at least one of boehmite, alumina, barium sulfate, magnesia, magnesium hydroxide, silicon oxide, stannic oxide, titanium oxide, calcium oxide, zinc oxide, zirconium oxide, yttrium oxide, nickel oxide, hafnium dioxide, cerium oxide, zirconium titanate, barium titanate, and magnesium fluoride. More optionally, the inorganic particles include at least one of boehmite, alumina, barium sulfate, magnesia, silicon oxide, titanium oxide, zinc oxide, cerium oxide, and barium titanate. Optionally, the organic particles include at least one of polystyrene particles, polyacrylic acid wax particles, melamine formaldehyde resin particles, phenolic resin particles, polyester particles, polyimide particles, polyamideimide particles, polyaramide particles, polyphenylene sulfide particles, polysulfone particles, polyethersulfone particles, polyetheretherketone particles, and polyaryletherketone particles.

[0020] In any embodiment of the present application, the first filler includes inorganic particles, and the crystal form of the inorganic particles includes at least one of θ crystal form, γ crystal form, and η crystal form; optionally, the crystal form of the inorganic particles includes at least one of θ crystal form and γ crystal form.

[0021] In any embodiment of the present application, optionally, the content of the inorganic particles in the θ crystal form is ≥50 wt%, more optionally 55 wt% to 84 wt%, based on the total weight of the inorganic particles in the first filler.

[0022] In any embodiment of the present application, optionally, the content of the inorganic particles in the γ crystal form is ≥10 wt%, more optionally 15 wt% to 44 wt%, based on the total weight of the inorganic particles in the first filler.

[0023] In any embodiment of the present application, optionally, the content of the inorganic particles in the η crystal form is ≤5 wt%, more optionally ≤2.5 wt%, based on the total weight of the inorganic particles in the first filler.

[0024] Selecting the first filler with different crystal forms helps to improve at least one of the heat resistance, ion conductivity, adhesion strength, and electrolyte wetting and retention characteristics of the separator membrane.

[0025] In any embodiment of the present application, the BET specific surface area of the second filler is ≤20 m 2 / g, optionally 6 m 2 / g to 15 m 2 / g. Thereby, the supporting effect of the second filler can be better exerted, the moisture content of the coating can be reduced, the coating can maintain a stable pore structure during long-term charge and discharge processes, and at the same time, the heat resistance of the separator membrane can be improved.

[0026] In any embodiment of the present application, the second filler includes at least one of inorganic particles and organic particles.

[0027] In any embodiment of the present application, the second filler includes inorganic particles in the form of primary particles, and the crystal forms of the inorganic particles in the form of primary particles include at least one of the α crystal form and the γ crystal form, optionally including the α crystal form. The second filler in the α crystal form has the advantages of high hardness, good heat resistance, low dielectric constant, high safety, and large true density, and thereby can further improve the heat resistance of the coating.

[0028] In any embodiment of the present application, the second filler includes inorganic particles in the form of primary particles, and the crystal forms of the inorganic particles in the form of primary particles include the α crystal form, and the content of the α crystal form is ≥70 wt%, optionally 85 wt% to 100 wt%, based on the total weight of the inorganic particles in the form of primary particles in the second filler.

[0029] In any embodiment of the present application, the morphology of the fibrous material includes at least one of rod-shaped, tubular, rod-shaped, and fibrous.

[0030] In any embodiment of the present application, the average diameter of the fibrous material is ≤40 nm, and may be optionally 10 nm to 35 nm. Thereby, the ion conductivity and voltage breakdown resistance characteristics of the separator can be further improved, and at the same time, it is also helpful to form an integrated effect by overlapping with the first filler, and thereby the heat resistance of the separator can be further improved.

[0031] In any embodiment of the present application, the average length of the fibrous material is 100 nm to 600 nm, and may be optionally 200 nm to 500 nm. Thereby, the heat resistance and ion conductivity of the separator can be further improved.

[0032] In any embodiment of the present application, the aspect ratio of the fibrous material is 5 to 60, and may be optionally 10 to 30. Thereby, the ion conductivity of the separator and the wetting and retention characteristics of the electrolyte can be further improved.

[0033] In any embodiment of the present application, the fibrous material includes at least one of an organic material and an inorganic material. Optionally, the organic material includes at least one of nanocellulose, polytetrafluoroethylene nanofibers, and polyamide nanofibers. Optionally, the nanocellulose includes at least one of cellulose nanofibers, cellulose nanocrystals, and bacterial nanocellulose. Optionally, the inorganic material includes at least one of halloysite nanotubes, nanorod-shaped alumina, nanorod-shaped boehmite, nanorod-shaped silica, and glass fibers.

[0034] In any embodiment of the present application, the fibrous material includes nanocellulose, and the nanocellulose includes at least one of unmodified nanocellulose and modified nanocellulose.

[0035] In any embodiment of the present application, optionally, the modified nanocellulose includes a modifying group, and the modifying group includes at least one of an amino group, a carboxyl group, an aldehyde group, a sulfonic acid group, a boric acid group, and a phosphoric acid group, and more optionally includes at least one of a sulfonic acid group, a boric acid group, and a phosphoric acid group.

[0036] When the nanocellulose has the above specific modifying groups, on the one hand, it can effectively improve the heat resistance of the separator and the thermal safety performance of the secondary battery; on the other hand, it can also improve the bonding strength between the coating and the porous substrate. When the nanocellulose has the above specific modifying groups, it is also beneficial to form an integrated effect by overlapping the nanocellulose with the first filler, thereby enabling the coating to have a more stable spatial network structure, so that the wetting and retention characteristics of the separator for the electrolyte can be improved, and the ion conductivity and voltage breakdown resistance characteristics of the separator can be improved. In addition, the presence of the modifying group can also reduce the proportion of hydroxyl groups, thereby ensuring that the coating slurry has a suitable viscosity, which is more conducive to coating, and thus can also improve the production efficiency of the separator and the uniformity of the coating.

[0037] In any embodiment of the present application, optionally, the modified nanocellulose includes hydroxyl groups and modified groups, and the molar ratio of the modified groups to the hydroxyl groups is from 1:4 to 4:1, more preferably from 2:3 to 7:3. When the molar ratio of the modified groups to the hydroxyl groups is within the above range, the heat resistance, ion conductivity, and electrolyte wetting and retention properties of the separator membrane can be further improved.

[0038] In any embodiment of the present application, the fibrous material includes sulfonic acid groups, and the sulfur element content in the fibrous material is ≥ 0.1 wt%, preferably 0.2 wt% to 0.5 wt%, based on the total weight of the fibrous material.

[0039] In any embodiment of the present application, the content of the first filler is ≥ 50 wt%, preferably 60 wt% to 85 wt%, based on the total weight of the coating. When the content of the first filler is within the above range, it can ensure that the coating slurry has an appropriate viscosity, which is more conducive to coating; in addition, it is also conducive to forming an integrated effect with the three-dimensional skeleton structure, thereby enabling the coating to have a more stable spatial network structure, and further improving the heat resistance and ion conductivity of the separator membrane.

[0040] In any embodiment of the present application, the content of the second filler is ≤ 30 wt%, preferably 5 wt% to 25 wt%, based on the total weight of the coating. When the content of the second filler is within the above range, the supporting role of the second filler can be better exerted, the moisture content of the coating can be reduced, and the coating can maintain a stable pore structure during long-term charge and discharge processes.

[0041] In any embodiment of the present application, the content of the fibrous material is 5 wt% to 40 wt%, preferably 8 wt% to 25 wt%, based on the total weight of the coating. This can ensure that the coating slurry has an appropriate viscosity, which is more conducive to coating; in addition, it is also conducive to the three-dimensional skeleton structure and the first filler forming an integrated effect, thereby enabling the coating to have a more stable spatial network structure, and further improving the heat resistance, ion conductivity, electrolyte wetting and retention properties, and voltage breakdown resistance of the separator membrane.

[0042] In any embodiment of the present application, the coating further includes a non-granular binder. Optionally, the non-granular binder includes an aqueous solution binder.

[0043] In any embodiment of the present application, the content of the non-granular binder in the coating is ≤ 2 wt%, based on the total weight of the coating. The three-dimensional skeleton structure and the first filler in the coating of the present application can form a stable spatial network structure, thereby enabling the separator membrane to maintain high adhesiveness while reducing the amount of binder used.

[0044] In any embodiment of the present application, the thickness of the porous substrate is ≤6 μm, and may be optionally 3 μm to 5 μm. The coating of the present application can significantly improve the heat resistance of the separator, so that a thinner porous substrate can be selected, which helps to improve the energy density of the secondary battery.

[0045] In any embodiment of the present application, the thickness of the coating is ≤2 μm, and may be optionally 0.5 μm to 1.5 μm. This helps to improve the energy density of the secondary battery.

[0046] In any embodiment of the present application, the separator further includes an adhesive layer, the adhesive layer is disposed on at least a part of the surface of the coating, and the adhesive layer includes particulate binders. The adhesive layer can not only prevent the coating from falling off, improve the safety performance of the secondary battery, but also improve the interface between the separator and the electrode, and improve the cycle performance of the secondary battery.

[0047] In any embodiment of the present application, the particulate binders include at least one of homopolymers or copolymers of acrylate monomers, homopolymers or copolymers of acrylic monomers, and homopolymers or copolymers of fluoroolefin monomers.

[0048] In any embodiment of the present application, the longitudinal thermal shrinkage rate of the separator at 150 °C for 1 h is ≤6%, and may be optionally 0.5% to 4%.

[0049] In any embodiment of the present application, the transverse thermal shrinkage rate of the separator at 150 °C for 1 h is ≤6%, and may be optionally 0.5% to 4%.

[0050] The separator of the present application has a low thermal shrinkage rate in both the transverse and longitudinal directions at a high temperature of 150 °C, thereby improving the safety performance of the secondary battery.

[0051] In any embodiment of the present application, the longitudinal tensile strength of the separator is ≥2000 kg / cm 2 , and may be optionally 2500 kg / cm 2 to 4500 kg / cm 2 .

[0052] In any embodiment of the present application, the transverse tensile strength of the separator is ≥2000 kg / cm 2 , and may be optionally 2500 kg / cm 2 to 4500 kg / cm 2 .

[0053] The separator of the present application has high tensile strength in both the transverse and longitudinal directions. Therefore, when the secondary battery expands, the probability of the separator being damaged is small, thereby improving the safety performance of the secondary battery.

[0054] In any embodiment of the present application, the wetting length of the separator is ≥30 mm, and can be optionally 30 mm to 80 mm.

[0055] In any embodiment of the present application, the wetting speed of the separator is ≥3 mm / s, and can be optionally 3 mm / s to 10 mm / s.

[0056] The separator of the present application has good infiltration and retention characteristics for the electrolyte, thereby improving the ion conductivity of the separator and the capacity performance of the secondary battery.

[0057] In any embodiment of the present application, the air permeability of the separator is ≤300 s / 100 mL, and can be optionally 100 s / 100 mL to 230 s / 100 mL. The separator of the present application has good air permeability, thereby improving the ion conductivity and the capacity performance of the secondary battery.

[0058] In any embodiment of the present application, the voltage breakdown strength of the separator is ≥1 kV. The separator of the present application has a relatively high voltage breakdown strength, thereby improving the safety performance of the secondary battery.

[0059] The second aspect of the present application provides a method for preparing the separator of the first aspect of the present application, including the following steps: providing a porous substrate; mixing fibrous materials, a first filler, and a second filler in a solvent according to a predetermined ratio to prepare a coating slurry; coating the coating slurry on at least one surface of the porous substrate, and drying to obtain a separator, wherein the separator includes a porous substrate and a coating provided on at least one surface of the porous substrate, the coating includes fibrous materials, a first filler, and a second filler, and the average particle size of the second filler is greater than that of the first filler.

[0060] The third aspect of the present application provides a secondary battery, including the separator of the first aspect of the present application or the separator prepared by the method of the second aspect of the present application.

[0061] The fourth aspect of the present application provides an electrical device, including the secondary battery of the third aspect of the present application.

[0062] The separator of the present application enables the secondary battery to have both high energy density, high thermal safety performance, and good cycle performance and kinetic performance. The electrical device of the present application includes the secondary battery provided by the present application, and thus has at least the same advantages as the secondary battery. Description of the Drawings

[0063] To more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings required for the embodiments of the present application. Obviously, the following described drawings are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on the drawings.

[0064] Figure 1 It is a schematic diagram of an embodiment of the secondary battery of the present application.

[0065] Figure 2 is Figure 1 exploded view of the embodiment of the secondary battery of

[0066] Figure 3 It is a schematic diagram of an embodiment of the battery module of the present application.

[0067] Figure 4 It is a schematic diagram of an embodiment of the battery pack of the present application.

[0068] Figure 5 is Figure 4 exploded view of the embodiment of the battery pack shown in

[0069] Figure 6 It is a schematic diagram of an embodiment of an electrical device using the secondary battery of the present application as a power source.

[0070] In the drawings, the drawings are not necessarily drawn to actual scale. The reference numerals are explained as follows: 1 battery pack, 2 upper box body, 3 lower box body, 4 battery module, 5 secondary battery, 51 housing, 52 electrode assembly, 53 cover plate. Specific Embodiments

[0071] Hereinafter, embodiments of the separator of the present application, its preparation method, and related secondary batteries and electrical devices will be specifically disclosed with appropriate reference to the drawings. However, there may be cases where unnecessary details are omitted. For example, there are cases where the detailed description of well-known matters and the repeated description of actually identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0072] The "ranges" disclosed in this application are defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of a particular range. The ranges defined in this way can include the end values or not include the end values, and can be combined arbitrarily, that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60 - 120 and 80 - 110 are listed for a specific parameter, ranges of 60 - 110 and 80 - 120 are also contemplated. In addition, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, then the following ranges are all contemplated: 1 - 3, 1 - 4, 1 - 5, 2 - 3, 2 - 4, and 2 - 5. In this application, unless otherwise specified, the numerical range "a - b" represents an abbreviated representation of any real number combination between a and b, where both a and b are real numbers. For example, the numerical range "0 - 5" means that all real numbers between "0 - 5" have been fully listed herein, and "0 - 5" is only an abbreviated representation of these numerical combinations. Additionally, when stating that a certain parameter is an integer ≥2, it is equivalent to disclosing that the parameter is, for example, the integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0073] If there is no special instruction, all embodiments and optional embodiments of this application can 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.

[0074] If there is no special instruction, all technical features and optional technical features of this application can 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.

[0075] If there is no special instruction, all steps of this application can be carried out in sequence or randomly, preferably in sequence. For example, the method includes steps (a) and (b), which means that the method can include steps (a) and (b) carried out in sequence, or can also include steps (b) and (a) carried out in sequence. For example, it is mentioned that the method may further include step (c), which means that step (c) can be added to the method in any order. For example, the method can include steps (a), (b), and (c), or can also include steps (a), (c), and (b), or can also include steps (c), (a), and (b), etc.

[0076] If there is no special instruction, the "including" and "comprising" mentioned in this application mean open-ended, and can also be closed-ended. For example, the "including" and "comprising" can mean that other components not listed can also be included or comprised, or can also only include or comprise the listed components.

[0077] Unless otherwise specified, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any 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) while B is true (or present); or both A and B are true (or present).

[0078] Unless otherwise specified, in this application, terms such as "first", "second", etc. are used to distinguish different objects, rather than to describe a specific order or primary-secondary relationship.

[0079] Unless otherwise indicated, the terms used in this application have the well-known meanings commonly understood by those skilled in the art.

[0080] Unless otherwise specified, the numerical values of the various parameters mentioned in this application can be determined by various commonly used testing methods in the art. For example, they can be determined according to the testing methods given in the embodiments of this application.

[0081] Generally, 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 serving to prevent short circuit between the positive electrode and the negative electrode, and at the same time allowing active ions to freely pass through to form a circuit.

[0082] With the application and popularization of secondary batteries, people's requirements for the energy density, service life, and kinetic performance of secondary batteries are getting higher and higher. Thinning the separator is an effective measure to improve the energy density of secondary batteries. Currently, the separators used in commercial secondary batteries are usually polyolefin porous membranes, such as polyethylene porous membranes, polypropylene porous membranes, or polypropylene / polyethylene / polypropylene three-layer composite membranes, whose melting points are between 130°C and 160°C. Therefore, when their thickness is thinned, the heat resistance of the separator becomes poor, and obvious thermal shrinkage effects will occur when heated, resulting in direct contact between the positive electrode and the negative electrode inside the battery, leading to internal short circuit, and further increasing the safety risk of secondary batteries.

[0083] To solve the above problems, the current measures mainly involve coating a heat-resistant inorganic ceramic layer on the polyolefin porous membrane, which can increase the mechanical strength of the separator, reduce the shrinkage of the separator when heated, and lower the risk of short circuit between the positive and negative electrodes inside the battery. However, the commercially available inorganic ceramic particles have a relatively large particle size, which will increase the overall thickness of the separator, resulting in an imbalance in the energy density of the secondary battery. Especially in the field of power batteries, it is not conducive to improving the cruising range. In addition, the improvement effect of commercially available inorganic ceramic particles on the heat resistance of the separator is also limited. Nanonization of inorganic ceramic particles can reduce the coating thickness and alleviate the adverse impact on the energy density of the secondary battery. However, the nanonized inorganic ceramic particles are prone to clogging the polyolefin porous membrane, leading to poor capacity performance and kinetic performance of the secondary battery. At the same time, due to the relatively high specific surface area of the nanonized inorganic ceramic particles and the point contact form between the particles, a large amount of binder needs to be used to ensure the adhesion between the particles. However, when the amount of binder is large, pore clogging problems are likely to occur, which is adverse to the kinetic performance of the secondary battery.

[0084] Therefore, it is often difficult for the separators in the prior art to balance the high energy density, high thermal safety performance, long cycle life, and good kinetic performance of secondary batteries.

[0085] Surprisingly, the inventors of the present application found during the research process that by providing a coating including a three-dimensional skeleton structure and a first filler with an average particle size less than or equal to 200 nm on the surface of the porous substrate of the separator, the separator can achieve low weight, high heat resistance, and high ion conductivity, and further enable the secondary battery to achieve high energy density, high thermal safety performance, long cycle life, and good kinetic performance.

[0086] Separator

[0087] Specifically, in the first aspect of the embodiment of the present application, a separator is provided, including a porous substrate and a coating provided on at least one surface of the porous substrate, wherein the coating includes a three-dimensional skeleton structure and a first filler, at least a part of the first filler is filled in the three-dimensional skeleton structure, and the average particle size of the first filler is less than or equal to 200 nm. In the present application, the "three-dimensional skeleton structure" refers to a structure with a three-dimensional spatial shape and certain pores, which can be formed by the materials constituting the three-dimensional skeleton structure overlapping each other.

[0088] The average particle size of the first filler is less than or equal to 200 nm, which has the advantages of a large specific surface area and good affinity with the three-dimensional skeleton structure, and is conducive to forming a stable spatial network structure with the three-dimensional skeleton structure, thereby increasing the ion conductivity of the separator and improving the heat resistance of the separator.

[0089] At least a part of the first filler is filled in the three-dimensional framework structure, which helps the first filler and the three-dimensional framework structure to form a nesting effect. As a result, it can not only increase the heat resistance of the separator, reduce the shrinkage degree of the separator when heated, reduce the risk of short circuit between the positive electrode and the negative electrode, and endow the secondary battery with high thermal safety performance, but also maintain a high bonding strength between the coating and the porous substrate, preventing the first filler from falling off during the long-term charge and discharge process of the secondary battery. At the same time, at least a part of the first filler is filled in the three-dimensional framework structure, so there are also many contact sites between the first filler and the three-dimensional framework structure, which can reduce the amount of binder used in the coating, effectively reduce the risk of binder plugging the pores, and further improve the cycle performance and kinetic performance of the secondary battery.

[0090] The coating of the present application has high heat resistance, which can reduce the thickness of the coating (for example, the thickness of the coating can be less than or equal to 2 μm), shorten the active ion transport distance, and thus the secondary battery can also achieve high energy density as well as good cycle performance and kinetic performance; in addition, the coating of the present application has high heat resistance, so a thinner porous substrate can be selected, thereby further improving the energy density of the secondary battery.

[0091] In some embodiments, at least a part of the first filler is filled in the three-dimensional framework structure, and the other part of the first filler can be located on the surface of the three-dimensional framework structure and / or at the interface between the three-dimensional framework structure and the porous substrate. And at the interface position between the three-dimensional framework structure and the porous substrate, a small part of the first filler may be embedded in the porous substrate. For example, during the winding process of the electrode assembly, due to the action of external pressure, a small part of the first filler at the interface position is embedded in the matrix and / or pores of the porous substrate.

[0092] [Three-dimensional framework structure]

[0093] In some embodiments, the three-dimensional framework structure can be formed by fibrous materials, and the morphology of the fibrous materials can optionally include at least one of rod-shaped, tubular (such as hollow tubular), rod-shaped and fibrous. Materials with suitable shapes are beneficial to the three-dimensional framework structure and the first filler to form a more stable spatial network structure, which can further improve the heat resistance, ion conductivity, and electrolyte infiltration and retention characteristics of the separator. In the present application, "fibrous material" refers to a material with an aspect ratio of 5 or more.

[0094] In some embodiments, the material constituting the three-dimensional framework structure includes at least one of organic materials and inorganic materials.

[0095] Optionally, the organic material includes at least one of nanocellulose, polytetrafluoroethylene nanofibers, and polyamide nanofibers. Optionally, the inorganic material includes at least one of halloysite nanotubes, nanorod-shaped alumina, nanorod-shaped boehmite, nanorod-shaped silica, and glass fibers.

[0096] In some embodiments, the material constituting the three-dimensional framework structure may include nanocellulose. Optionally, the nanocellulose includes at least one of cellulose nanofibrils (CNF, also known as nanofibrillated cellulose or microfibrillated cellulose), cellulose nanocrystals (CNC, also known as cellulose nanocrystals or nanocrystalline cellulose), and bacterial nanocellulose (BNC, also known as bacterial cellulose or microbial cellulose).

[0097] Nanocellulose refers to the general term for cellulose with any one-dimensional dimension in the nanoscale (e.g., within 100 nm). It has both the properties of cellulose and the properties of nanoparticles. Nanocellulose can be a polymer nanomaterial extracted from natural materials such as wood and cotton through one or more means of chemistry, physics, biology, etc. It has the advantages of wide sources, low cost, biodegradability, high modulus, and high specific surface area. Therefore, it is an excellent substitute for traditional petrochemical resources, which can effectively alleviate problems such as environmental pollution and petrochemical resource shortages. Nanocellulose also has good high-temperature resistance characteristics and small volume changes after heating, thereby improving the heat resistance of the separator membrane. At the same time, compared with traditional inorganic ceramic particles, nanocellulose has a smaller density, which can also reduce the weight of the secondary battery and improve the weight energy density of the secondary battery. In addition, the three-dimensional framework structure formed by nanocellulose can also have tiny nanopores to prevent current leakage, thereby enabling the separator membrane to have good infiltration and retention characteristics for the electrolyte and good voltage breakdown resistance characteristics.

[0098] In some embodiments, the nanocellulose may include at least one of unmodified nanocellulose (also known as hydroxy nanocellulose) and modified nanocellulose, optionally modified nanocellulose.

[0099] Modified nanocellulose refers to nanocellulose that includes both hydroxyl groups and modified groups. In some embodiments, the modified nanocellulose includes modified groups, and the modified groups include at least one of amino groups, carboxyl groups, aldehyde groups, sulfonic acid groups, boric acid groups, and phosphoric acid groups, optionally including at least one of sulfonic acid groups, boric acid groups, and phosphoric acid groups.

[0100] In further research, the inventors found that when the nanocellulose has the above-mentioned specific modification groups, on the one hand, it can effectively improve the heat resistance of the separator and enhance the thermal safety performance of the secondary battery; on the other hand, it can also improve the bonding strength between the coating and the porous substrate. When the nanocellulose has the above-mentioned specific modification groups, it is also beneficial for the nanocellulose to overlap with the first filler to form an integrated effect, thereby enabling the coating to have a more stable spatial network structure, and thus improving the wetting and retention characteristics of the separator for the electrolyte, and enhancing the ion conductivity and voltage breakdown resistance characteristics of the separator. In addition, the presence of the modification groups can also reduce the proportion of hydroxyl groups, thereby ensuring that the coating slurry has an appropriate viscosity, which is more conducive to coating, and thus can also improve the production efficiency of the separator and the uniformity of the coating.

[0101] In some embodiments, the molar ratio of the modification group to the hydroxyl group can be from 1:4 to 4:1, and can be optionally from 2:3 to 7:3. When the molar ratio of the modification group to the hydroxyl group is within the above range, the heat resistance, ion conductivity, and wetting and retention characteristics of the separator for the electrolyte can be further improved. And it can effectively avoid the following situations: when the molar ratio of the modification group to the hydroxyl group is too small, the further improvement effect of the modification group on the heat resistance and ion conductivity of the separator may not be obvious; when the molar ratio of the modification group to the hydroxyl group is too large, the wetting and retention characteristics of the separator for the electrolyte may deteriorate, which may in turn affect the cycle performance and safety performance of the secondary battery, and may also cause a decrease in the heat resistance of the separator, and may further affect the improvement effect on the thermal safety performance of the secondary battery.

[0102] The type of the modification group in the nanocellulose can be determined by infrared spectroscopy. For example, the infrared spectrum of the material can be tested to determine the characteristic peaks it contains, so as to determine the type of the modification group. Specifically, the material can be analyzed by infrared spectroscopy using the instruments and methods well-known in the art. For example, an infrared spectrometer (such as the IS10 type Fourier transform infrared spectrometer of Thermo Fisher Scientific) can be used to perform the test according to the General Rules for Infrared Spectral Analysis GB / T 6040-2019.

[0103] In some embodiments, the material constituting the three-dimensional framework structure includes a sulfonic group, and the sulfur element content in the material constituting the three-dimensional framework structure is ≥0.1 wt%, and can be optionally 0.2 wt% to 0.5 wt%, based on the total weight of the material constituting the three-dimensional framework structure. Optionally, the material constituting the three-dimensional framework structure includes nanocellulose.

[0104] The content of sulfur element in the material constituting the three-dimensional skeleton structure can be obtained by testing according to the following method: After drying the material constituting the three-dimensional skeleton structure, grind it in a mortar (such as an agate mortar) for 30 minutes, and then use an X-ray diffractometer (such as Miniflex600-C) for testing to obtain the content of sulfur element. When testing, a Cu target and a Ni filter can be used, with a tube voltage of 40 KV, a tube current of 15 mA, and a continuous scanning range of 5°-80°.

[0105] In some embodiments, the average diameter of the material constituting the three-dimensional skeleton structure can be ≤ 40 nm, and can be selected from 10 nm to 35 nm. When the average diameter of the material constituting the three-dimensional skeleton structure is within the above range, the ion conductivity and voltage breakdown resistance of the separator can be further improved. At the same time, it is also helpful to form an integrated effect with the first filler, and thus the heat resistance of the separator can be further improved. And it can effectively avoid the following situation: When the average diameter of the material constituting the three-dimensional skeleton structure is too large, the mutual entanglement effect of the formed three-dimensional skeleton structure is insufficient and the pores are larger, which may lead to insufficient heat resistance and voltage breakdown resistance of the separator. At the same time, it is not conducive to forming an integrated effect with the first filler, and during the drying process of the coating, the three-dimensional skeleton structure is prone to collapse due to the lack of the supporting effect of the first filler, and then it is easy to directly contact the porous substrate to cause pore blocking problems, which may affect the ion conductivity of the separator.

[0106] In some embodiments, the average length of the material constituting the three-dimensional skeleton structure can be from 100 nm to 600 nm, and can be selected from 200 nm to 500 nm. When the average length of the material constituting the three-dimensional skeleton structure is within the above range, the heat resistance and ion conductivity of the separator can be further improved. And it can effectively avoid the following situation: When the average length of the material constituting the three-dimensional skeleton structure is too short, the overlapping effect with the first filler is poor, the heat resistance of the coating becomes worse, and during the drying process of the coating, the three-dimensional skeleton structure is prone to collapse due to the lack of the supporting effect of the first filler, and then it is easy to cause pore blocking problems, hindering ion transport and water discharge, which may affect the thermal safety performance, cycle performance and kinetic performance of the secondary battery; When the average length of the material constituting the three-dimensional skeleton structure is too long, the viscosity of the coating slurry is large and the fluidity is poor, which may affect the coating of the coating slurry and thus affect the quality of the coating, such as may affect the heat resistance and ion conductivity of the separator.

[0107] In some embodiments, the aspect ratio of the material constituting the three-dimensional skeleton structure may be 5 to 60, preferably 10 to 30. When the aspect ratio of the material constituting the three-dimensional skeleton structure is within the above range, the ion conductivity of the separator and the wetting and retention characteristics of the electrolyte can be further improved. And the following situations can be effectively avoided: when the aspect ratio of the material constituting the three-dimensional skeleton structure is too small, the overlapping effect with the first filler is poor, the heat resistance of the coating is deteriorated, and during the drying process of the coating, the three-dimensional skeleton structure is prone to collapse due to the lack of the supporting effect of the first filler, and thus the problem of pore blockage is likely to occur, hindering ion transport and water discharge, which may affect the thermal safety performance, cycle performance and kinetic performance of the secondary battery; when the aspect ratio of the material constituting the three-dimensional skeleton structure is too large, the pores of the formed three-dimensional skeleton structure are small, and thus the ion conductivity of the separator may be reduced.

[0108] The average length and average diameter of the material constituting the three-dimensional skeleton structure can be measured by the following method: cut out a sample of 3.6 mm × 3.6 mm from an arbitrary area of the separator, and use a scanning electron microscope (such as ZEISS Sigma300) to map the microscopic morphological structure of the coating in the sample. Select the high vacuum mode, the working voltage is 3 kV, and the magnification is 30,000 times to obtain an SEM image; according to the obtained SEM image, select multiple (for example, more than 5) test areas for length statistics, and the size of each test area is 0.5 μm × 0.5 μm, and then take the average value of the lengths obtained from each test area as the average length of the material constituting the three-dimensional skeleton structure; according to the obtained SEM image, use Nano Measurer particle size distribution statistics software, select multiple (for example, more than 5) test areas for diameter statistics, and the size of each test area is 0.5 μm × 0.5 μm, and then take the average value of the diameters obtained from each test area as the average diameter of the material constituting the three-dimensional skeleton structure.

[0109] In some embodiments, the content of the three-dimensional skeleton structure may be 5 wt% to 40 wt%, optionally 8 wt% to 25 wt%, 10 wt% to 25 wt%, based on the total weight of the coating. The material forming the three-dimensional skeleton structure has a relatively large specific surface area. Therefore, under the same mass, the coating formed has a large specific surface area and many pores, resulting in poor heat resistance of the separator membrane. At the same time, the hydrogen bonding of the material (such as nanocellulose) forming the three-dimensional skeleton structure is extremely strong. When the content is relatively high, it will lead to a relatively high viscosity of the coating slurry, which is not conducive to achieving thin coating and is also not conducive to commercial production. When the content of the three-dimensional skeleton structure is within the above range, it can ensure that the coating slurry has an appropriate viscosity, which is more conducive to coating. In addition, it is also conducive to the three-dimensional skeleton structure and the first filler to overlap and form an integrated effect, thereby enabling the coating to have a more stable spatial network structure, and further improving the heat resistance, ion conductivity, electrolyte infiltration and retention characteristics, and voltage breakdown resistance of the separator membrane.

[0110] [First filler]

[0111] In some embodiments, the average particle size of the first filler is 15 nm to 180 nm, optionally 20 nm to 170 nm, 25 nm to 160 nm, 30 nm to 150 nm, 40 nm to 140 nm, 50 nm to 135 nm. When the average particle size of the first filler is within the above range, the first filler can have a relatively high specific surface area, and the particle size of the first filler can be better matched with the three-dimensional skeleton structure. Thus, the first filler and the three-dimensional skeleton structure can better overlap and form an integrated effect, increasing the affinity between the first filler and the three-dimensional skeleton structure, increasing the heat resistance and ion conductivity of the separator membrane, and at the same time, also increasing the electrolyte infiltration and retention characteristics of the separator membrane.

[0112] In some embodiments, the first filler includes at least one of primary particles and secondary particles. Optionally, the first filler includes a combination of primary particles and secondary particles. The first filler with a primary particle morphology is beneficial to reducing the moisture content of the coating and improving the ion conductivity of the coating, thereby better improving the cycling performance of the secondary battery. The first filler with a secondary particle morphology can better overlap with the three-dimensional skeleton structure to form an integrated effect, thereby enabling the coating to have a more stable spatial network structure, and further improving the heat resistance of the separator membrane.

[0113] In some embodiments, the first filler includes a combination of primary particles and secondary particles, and based on the total weight of the first filler, the content of the first filler with a primary particle morphology is less than the content of the first filler with a secondary particle morphology.

[0114] In some embodiments, the first filler comprises a combination of primary particles and secondary particles, and based on the total weight of the first filler, the content of the first filler with the primary particle morphology is less than or equal to 30 wt%, optionally 8 wt% to 30 wt%, 8 wt% to 28 wt%, 10 wt% to 30 wt%, 10 wt% to 28 wt%, 12 wt% to 30 wt%, 12 wt% to 28 wt%, 15 wt% to 30 wt%, 15 wt% to 28 wt%, 17.5 wt% to 30 wt%, 17.5 wt% to 28 wt%.

[0115] In some embodiments, the average particle size of the first filler with the primary particle morphology is 15 nm to 95 nm, optionally 15 nm to 80 nm, 20 nm to 80 nm, 30 nm to 75 nm, 35 nm to 75 nm, 35 nm to 70 nm, 30 nm to 70 nm, 30 nm to 65 nm.

[0116] In some embodiments, the average particle size of the first filler with the secondary particle morphology is 50 nm to 200 nm, optionally 50 nm to 180 nm, 50 nm to 150 nm, 50 nm to 135 nm, 50 nm to 120 nm, 55 nm to 180 nm, 55 nm to 150 nm, 55 nm to 135 nm, 55 nm to 120 nm, 65 nm to 180 nm, 65 nm to 150 nm, 65 nm to 135 nm, 65 nm to 120 nm.

[0117] In some embodiments, the BET specific surface area of the first filler is ≥ 25 m 2 / g, optionally 30 m 2 / g to 80 m 2 / g, 30 m 2 / g to 65 m 2 / g. When the specific surface area of the first filler is within the above range, its affinity with the three-dimensional skeleton structure is better, and it can form an integrated effect with the three-dimensional skeleton structure, thereby increasing the heat resistance and ion conductivity of the separator membrane, and at the same time increasing the wetting and retention characteristics of the separator membrane for the electrolyte.

[0118] In some embodiments, the first filler includes at least one of inorganic particles and organic particles, optionally including inorganic particles, or a combination of inorganic particles and organic particles. The inorganic particles have the characteristics of high hardness, high thermal stability and being not easily decomposed, and their surfaces usually have hydroxyl groups, thus it is easy to form a stable spatial network structure with the materials (such as nanocellulose, etc.) constituting the three-dimensional framework structure. The organic particles have good thermal stability and are not easily decomposed. At the same time, when the internal temperature of the secondary battery reaches the melting point of the organic particles due to overcharge abuse, thermal abuse, etc., the organic particles can also melt and be sucked into the micropores of the porous substrate by capillary action to play a role in closing pores and opening circuits, thereby being beneficial to improving the safety performance of the secondary battery.

[0119] Optionally, the inorganic particles include at least one of boehmite (γ - AlOOH), alumina (Al2O3), barium sulfate (BaSO4), magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), silicon oxide SiO x (0 < x ≤ 2), tin dioxide (SnO2), titanium dioxide (TiO2), calcium oxide (CaO), zinc oxide (ZnO), zirconium oxide (ZrO2), yttrium oxide (Y2O3), nickel oxide (NiO), hafnium dioxide (HfO2), cerium dioxide (CeO2), zirconium titanate (ZrTiO3), barium titanate (BaTiO3), and magnesium fluoride (MgF2). More optionally, the inorganic particles include at least one of boehmite (γ - AlOOH), alumina (Al2O3), barium sulfate (BaSO4), magnesium oxide (MgO), silicon oxide SiO x (0 < x ≤ 2), titanium dioxide (TiO2), zinc oxide (ZnO), cerium dioxide (CeO2), and barium titanate (BaTiO3).

[0120] Optionally, the organic particles include at least one of polystyrene particles, polyacrylic acid wax particles, melamine formaldehyde resin particles, phenolic resin particles, polyester particles, polyimide particles, polyamideimide particles, polyaramide particles, polyphenylene sulfide particles, polysulfone particles, polyethersulfone particles, polyetheretherketone particles, and polyaryletherketone particles.

[0121] In some embodiments, the first filler includes inorganic particles, and the crystal form of the inorganic particles includes at least one of θ crystal form, γ crystal form, and η crystal form. Optionally, the crystal form of the inorganic particles includes at least one of θ crystal form and γ crystal form.

[0122] The inorganic particles of the θ crystal form have diffraction peaks at 2θ of 36.68° ± 0.2° and 31.21° ± 0.2° in the X-ray diffraction pattern measured by using an X-ray diffractometer. In some embodiments, the content of the inorganic particles of the θ crystal form in the first filler may be ≥ 50 wt%, and may be optionally 55 wt% to 84 wt% based on the total weight of the inorganic particles in the first filler.

[0123] The inorganic particles of the γ crystal form have diffraction peaks at 2θ of 66.95° ± 0.2° and 45.91° ± 0.2° in the X-ray diffraction pattern measured by using an X-ray diffractometer. In some embodiments, the content of the inorganic particles of the γ crystal form in the first filler may be ≥ 10 wt%, and may be optionally 15 wt% to 44 wt% based on the total weight of the inorganic particles in the first filler.

[0124] The inorganic particles of the η crystal form have diffraction peaks at 2θ of 31.89° ± 0.2° and 19.37° ± 0.2° in the X-ray diffraction pattern measured by using an X-ray diffractometer. In some embodiments, the content of the inorganic particles of the η crystal form in the first filler may be ≤ 5 wt%, may be optionally ≤ 2.5 wt%, and more optionally ≤ 1.5 wt% based on the total weight of the inorganic particles in the first filler.

[0125] The inorganic particles of the θ crystal form have a moderate specific surface area and hardness, and thus can better improve the heat resistance and ion conductivity of the separator membrane simultaneously; the inorganic particles of the γ crystal form and the η crystal form have the advantage of a large specific surface area.

[0126] Selecting the first filler of different crystal forms helps to improve at least one of the heat resistance, ion conductivity, bonding strength, and electrolyte wetting and retention characteristics of the separator membrane.

[0127] In some embodiments, the first filler may include inorganic particles, and the crystal forms of the inorganic particles include the θ crystal form, the γ crystal form, and the η crystal form, and the content of the inorganic particles of the θ crystal form in the first filler may be 55 wt% to 84 wt%, the content of the inorganic particles of the γ crystal form may be 15 wt% to 44 wt%, and the content of the inorganic particles of the η crystal form may be ≤ 2.5 wt%, all based on the total weight of the inorganic particles in the first filler.

[0128] The X-ray diffraction pattern of the inorganic particles can be obtained by testing according to the following method: After drying the inorganic particles, grind them in a mortar (such as an agate mortar) for 30 min, and then use an X-ray diffractometer (such as Miniflex600-C) for testing to obtain the X-ray diffraction pattern. When testing, a Cu target and a Ni filter can be used, the tube voltage is 40 KV, the tube current is 15 mA, and the continuous scanning range is 5° - 80°.

[0129] In some embodiments, the first filler may include inorganic particles, and the inorganic particles can be prepared by the following method: subjecting a precursor solution of the inorganic particles to an oxidation reaction by means of high-pressure sputtering, and then heating at 600 °C to 900 °C (for example, for 1 hour to 3 hours) to form inorganic particles with a primary particle morphology, and then further drying and shaping at 150 °C to 250 °C (for example, for 30 minutes to 60 minutes) to obtain inorganic particles with a secondary particle morphology (obtained by assembling primary particles).

[0130] In some embodiments, the content of the first filler is ≥50 wt%, and may be optionally 50 wt% to 90 wt%, 55 wt% to 90 wt%, 60 wt% to 90 wt%, 50 wt% to 85 wt%, 55 wt% to 85 wt%, 60 wt% to 85 wt%, 50 wt% to 82.5 wt%, 55 wt% to 82.5 wt%, 60 wt% to 82.5 wt%, based on the total weight of the coating. When the content of the first filler is within the above range, it can ensure that the coating slurry has an appropriate viscosity, which is more conducive to coating; in addition, it is also conducive to overlapping with the three-dimensional skeleton structure to form an integrated effect, thereby enabling the coating to have a more stable spatial network structure, and further improving the heat resistance and ion conductivity of the separator membrane.

[0131] [Second filler]

[0132] In some embodiments, the coating further includes a second filler, and at least a part of the second filler is embedded in the coating. In addition, there may also be a part of the second filler protruding from the surface of the coating.

[0133] In some embodiments, the coating includes a first filler and a second filler. The average particle size of the first filler is denoted as d1, and the average particle size of the second filler is denoted as d2, then d2 / d1 > 1. The second filler has a relatively large average particle size, so that it can better play its supporting role in the coating, reduce the shrinkage of the first filler, reduce the amount of binder used, and thus improve the heat resistance of the separator membrane; the relatively large particle size of the second filler also helps to make the coating have more pore structures and less water content when the amount used is small, and further improve the ion conductivity of the separator membrane and the wetting and retention characteristics of the electrolyte, while also improving the cycle performance and / or kinetic performance of the secondary battery.

[0134] The first filler includes at least one of primary particles and secondary particles, and the average particle size of the first filler with a primary particle morphology is denoted as d 11 , and the average particle size of the first filler with a secondary particle morphology is denoted as d 12 .

[0135] In some embodiments, 3.0 ≤ d2 / d 11 ≤ 10.0. Optionally, 3.5 ≤ d2 / d 11 ≤ 8.0, 3.5 ≤ d2 / d 11 ≤ 6.0. Through the combined action of the first filler and the second filler, it helps to reduce the moisture content of the coating, keep the coating in a stable pore structure during long-term charge and discharge processes, and at the same time improve the heat resistance of the separator, thereby enabling the secondary battery to better balance high energy density, high thermal safety performance, long cycle life, and good kinetic performance.

[0136] In some embodiments, 1.2 ≤ d2 / d 12 ≤ 6.0. Optionally, 2.0 ≤ d2 / d 12 ≤ 5.5, 2.0 ≤ d2 / d 12 ≤ 5.0, 2.0 ≤ d2 / d 12 ≤ 4.5. Through the combined action of the first filler and the second filler, it helps to reduce the moisture content of the coating, keep the coating in a stable pore structure during long-term charge and discharge processes, and at the same time improve the heat resistance of the separator, thereby enabling the secondary battery to better balance high energy density, high thermal safety performance, long cycle life, and good kinetic performance.

[0137] In some embodiments, the average particle size d2 of the second filler is 120 nm to 350 nm, and optionally 150 nm to 300 nm. Thereby, the supporting effect of the second filler can be better exerted, the moisture content of the coating can be reduced, the coating can maintain a stable pore structure during long-term charge and discharge processes, and at the same time the heat resistance of the separator can be improved.

[0138] In some embodiments, the BET specific surface area of the second filler is ≤ 20 m 2 / g, and optionally 6 m 2 / g to 15 m 2 / g. Thereby, the supporting effect of the second filler can be better exerted, the moisture content of the coating can be reduced, the coating can maintain a stable pore structure during long-term charge and discharge processes, and at the same time the heat resistance of the separator can be improved.

[0139] In some embodiments, the second filler includes at least one of inorganic particles and organic particles.

[0140] In some embodiments, the inorganic particles may include at least one of inorganic particles having a dielectric constant of more than 5, inorganic particles having ionic conductivity but not storing ions, and inorganic particles capable of undergoing an electrochemical reaction.

[0141] Optionally, the inorganic particles having a dielectric constant of more than 5 include 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)O3 (abbreviated as PZT), Pb 1-m La m Zr 1-n Ti n O3 (abbreviated as PLZT, 0 < m < 1, 0 < n < 1), Pb(Mg3Nb 2 / 3 )O3-PbTiO3 (abbreviated as PMN-PT), and at least one of their respective modified inorganic particles. Optionally, the modification method of each inorganic particle can be chemical modification and / or physical modification. The chemical modification methods include coupling agent modification (such as using silane coupling agents, titanate coupling agents, etc.), surfactant modification, polymer grafting modification, etc. The physical modification methods can be mechanical force dispersion, ultrasonic dispersion, high-energy treatment, etc. Through the modification treatment, the agglomeration of inorganic particles can be reduced, thereby enabling the coating to have a more stable and uniform spatial network structure; in addition, by selecting a coupling agent, surfactant or polymer with specific functional groups to modify the inorganic particles, it also helps to improve the infiltration and retention characteristics of the coating for the electrolyte and the adhesion of the coating to the porous substrate.

[0142] Optionally, the inorganic particles having ion conductivity but not storing ions include Li3PO4, lithium titanium phosphate Li x1 Ti y1 (PO4)3, lithium aluminum titanium phosphate Li x2 Al y2 Ti z1 (PO4)3, (LiAlTiP) x3 O y3 -type glass, lithium lanthanum titanate Li x4 La y4 TiO3, lithium germanium thiophosphate Li x5 Ge y5 P z2 S w , lithium nitride Li x6 N y6 , SiS2-type glass Li x7 Si y7 S z3 and P2S5-type glass Li x8 P y8 S z4at least one of them, 0 < x1 < 2, 0 < y1 < 3, 0 < x2 < 2, 0 < y2 < 1, 0 < z1 < 3, 0 < x3 < 4, 0 < y3 < 13, 0 < x4 < 2, 0 < y4 < 3, 0 < x5 < 4, 0 < y5 < 1, 0 < z2 < 1, 0 < w < 5, 0 < x6 < 4, 0 < y6 < 2, 0 < x7 < 3, 0 < y7 < 2, 0 < z3 < 4, 0 < x8 < 3, 0 < y8 < 3, 0 < z4 < 7. Thereby, the ion conductivity of the separator can be further improved.

[0143] Optionally, the inorganic particles capable of undergoing an electrochemical reaction include at least one of lithium-containing transition metal oxides, lithium-containing phosphates, carbon-based materials, silicon-based materials, tin-based materials, and lithium titanium compounds.

[0144] In some embodiments, the organic particles include, but are not limited to, polyethylene particles, polypropylene particles, cellulose, cellulose modifiers (such as carboxymethyl cellulose), melamine resin particles, phenolic resin particles, polyester particles (such as polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate), silicone resin particles, polyimide particles, polyamideimide particles, polyaramide particles, polyphenylene sulfide particles, polysulfone particles, polyethersulfone particles, polyetheretherketone particles, polyaryletherketone particles, copolymers of butyl acrylate and ethyl methacrylate (such as crosslinked polymers of butyl acrylate and ethyl methacrylate), etc.

[0145] In some embodiments, the second filler has a primary particle morphology.

[0146] In some embodiments, the second filler includes inorganic particles with a primary particle morphology, and the crystal form of the inorganic particles with a primary particle morphology includes at least one of α-crystal form and γ-crystal form, and optionally includes α-crystal form. The second filler with α-crystal form has the advantages of high hardness, good heat resistance, low dielectric constant, high safety, and large true density. Thereby, the heat resistance of the coating can be further improved.

[0147] In some embodiments, the second filler includes inorganic particles with a primary particle morphology, and the crystal form of the inorganic particles with a primary particle morphology includes α-crystal form, and the content of α-crystal form is ≥ 70 wt%, optionally 75 wt% to 100 wt%, 85 wt% to 100 wt%, 95 wt% to 100 wt%, based on the total weight of the inorganic particles with a primary particle morphology in the second filler.

[0148] The inorganic particles with α-crystal form have diffraction peaks at 2θ of 57.48° ± 0.2° and 43.34° ± 0.2° in the X-ray diffraction pattern measured by an X-ray diffractometer.

[0149] In some embodiments, the content of the second filler is ≤30 wt%, optionally 5 wt% to 25 wt%, 6 wt% to 22 wt%, 6 wt% to 20 wt%, 8 wt% to 18 wt%, based on the total weight of the coating. When the content of the second filler is within the above range, the supporting effect of the second filler can be better exerted, the moisture content of the coating can be reduced, and the coating can maintain a stable pore structure during long-term charge and discharge processes.

[0150] In some embodiments, the coating may further include a non-granular binder. The present application does not particularly limit the type of the non-granular binder, and any well-known material with good adhesiveness can be selected. Optionally, the non-granular binder includes an aqueous solution type binder, which has the advantages of good thermodynamic stability and environmental friendliness, thus being beneficial to the preparation and coating of the coating slurry. As an example, the aqueous solution type binder may include at least one of an aqueous solution type acrylic resin (for example, a homopolymer of acrylic acid, methacrylic acid, sodium acrylate monomers or a copolymer with other comonomers), polyvinyl alcohol (PVA), isobutene-maleic anhydride copolymer, and polyacrylamide.

[0151] Optionally, the content of the non-granular binder in the coating is ≤2 wt%, based on the total weight of the coating. The three-dimensional skeleton structure in the coating of the present application and the first filler, etc. can form a stable spatial network structure, thereby enabling the separator to still maintain high adhesiveness while reducing the amount of the binder.

[0152] In some embodiments, the thickness of the coating may be ≤2 μm, optionally 0.5 μm to 1.5 μm. This helps to improve the energy density of the secondary battery. In the present application, the thickness of the coating refers to the thickness of the coating on one side of the porous substrate.

[0153] In some embodiments, the thickness of the porous substrate may be ≤6 μm, optionally 3 μm to 5 μm. The coating of the present application can significantly improve the heat resistance of the separator, so that a thinner porous substrate can be selected, thereby helping to improve the energy density of the secondary battery.

[0154] The present application does not particularly limit the material of the porous substrate, and any well-known substrate with good chemical stability and mechanical stability can be selected. For example, the porous substrate may include at least one of a porous polyolefin-based resin film (for example, at least one of polyethylene, polypropylene, and polyvinylidene fluoride), porous glass fiber, and porous non-woven fabric. The porous substrate may be a single-layer thin film or a multi-layer composite thin film. When the porous substrate is a multi-layer composite thin film, the materials of each layer may be the same or different.

[0155] In some embodiments, the separator membrane may further include an adhesive layer disposed on at least a portion of the surface of the coating layer, and the adhesive layer includes particulate binders. The adhesive layer can not only prevent the coating layer from peeling off and improve the safety performance of the secondary battery, but also improve the interface between the separator membrane and the electrode and enhance the cycling performance of the secondary battery.

[0156] Optionally, the particulate binder includes at least one of homopolymers or copolymers of acrylate monomers, homopolymers or copolymers of acrylic monomers, and homopolymers or copolymers of fluoroolefin monomers. The comonomers include, but are not limited to, at least one of the following: acrylate monomers, acrylic monomers, olefin monomers, halogen-containing olefin monomers, fluoroether monomers, etc.

[0157] Optionally, the particulate binder includes a vinylidene fluoride-based polymer, such as a homopolymer of vinylidene fluoride monomer (VDF) and / or a copolymer of vinylidene fluoride monomer and a comonomer. The comonomer can be at least one of an olefin monomer, a fluoroolefin monomer, a chloroolefin monomer, an acrylate monomer, an acrylic monomer, and a fluoroether monomer. Optionally, the comonomer may include at least one of the following: trifluoroethylene (VF3), chlorotrifluoroethylene (CTFE), 1,2-difluoroethylene, tetrafluoroethylene (TFE), hexafluoropropylene (HFP), perfluoro(alkyl vinyl ether) (such as perfluoro(methyl vinyl ether) PMVE, perfluoro(ethyl vinyl ether) PEVE, perfluoro(propyl vinyl ether) PPVE), perfluoro(1,3-dioxolene), and perfluoro(2,2-dimethyl-1,3-dioxolene) (PDD).

[0158] In some embodiments, the longitudinal thermal shrinkage rate of the separator membrane at 150 °C for 1 h is ≤6%, and may be optionally 0.5% to 4%.

[0159] In some embodiments, the transverse thermal shrinkage rate of the separator membrane at 150 °C for 1 h is ≤6%, and may be optionally 0.5% to 4%.

[0160] The separator membrane of the present application has a low thermal shrinkage rate in both the transverse and longitudinal directions at a high temperature of 150 °C, thereby improving the safety performance of the secondary battery.

[0161] In some embodiments, the longitudinal tensile strength of the separator membrane is ≥2000 kg / cm 2 , and may be optionally 2500 kg / cm 2 to 4500 kg / cm 2 .

[0162] In some embodiments, the transverse tensile strength of the separator membrane is ≥2000 kg / cm 2 , and may be optionally 2500 kg / cm2 to 4500 kg / cm 2 。

[0163] The separator of the present application has high tensile strength in both the transverse and longitudinal directions. Therefore, when the secondary battery expands, the probability of the separator being damaged is small, thereby improving the safety performance of the secondary battery.

[0164] In some embodiments, the wetting length of the separator is ≥ 30 mm, and may be optionally 30 mm to 80 mm.

[0165] In some embodiments, the wetting speed of the separator is ≥ 3 mm / s, and may be optionally 3 mm / s to 10 mm / s.

[0166] The separator of the present application has good infiltration and retention characteristics for the electrolyte, thereby improving the ion conductivity and the secondary battery capacity performance of the separator.

[0167] In some embodiments, the air permeability of the separator is ≤ 300 s / 100 mL, and may be optionally 100 s / 100 mL to 230 s / 100 mL. The separator of the present application has good air permeability, thereby improving the ion conductivity and the secondary battery capacity performance.

[0168] In some embodiments, the breakdown voltage resistance of the separator is ≥ 1 KV. The separator of the present application has a relatively high breakdown voltage resistance, thereby improving the safety performance of the secondary battery.

[0169] In the present application, the average particle size of the material has the meaning well-known in the art and can be measured by instruments and methods known in the art. For example, pictures can be obtained by measuring the material or the separator through a scanning electron microscope, a transmission electron microscope, or a particle size distribution instrument. From the pictures, multiple (e.g., more than 10) test particles (e.g., having a first filler, a second filler, etc.) are randomly selected, and the average value of the shortest diagonal lengths of the particles is statistically used as the average particle size.

[0170] In the present application, the specific surface area of the material has the meaning well-known in the art and can be measured by instruments and methods known in the art. For example, it can be measured by referring to GB / T 19587-2017 using the nitrogen adsorption specific surface area analysis test method and calculated by the BET (Brunauer Emmett Teller) method. Optionally, the nitrogen adsorption specific surface area analysis test can be performed by a Tri-Star 3020 type specific surface area and pore size analyzer from Micromeritics, USA.

[0171] In this application, the thermal shrinkage rate, tensile strength, and air permeability of the separator membrane all have the meanings well-known in the art and can be measured by methods known in the art. For example, they can all be tested with reference to Standard GB / T 36363-2018.

[0172] In this application, the wetting length and wetting speed of the separator membrane both have the meanings well-known in the art and can be measured by methods known in the art. The exemplary test method is as follows: Cut the separator membrane into samples with a width of 5 mm and a length of 100 mm, fix both ends of the sample and place it horizontally; Drop 0.5 mg of electrolyte in the center of the sample. After reaching the specified time (1 minute in this application), take a photo and measure the length of the electrolyte diffusion, thereby obtaining the wetting length and wetting speed of the separator membrane. To ensure the accuracy of the test results, multiple (for example, 5 to 10) samples can be taken for testing, and the test results are obtained by calculating the average value. The electrolyte can be prepared as follows: Mix ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a mass ratio of 30:50:20 to obtain an organic solvent, and dissolve the fully dried LiPF6 in the above organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0173] In this application, the breakdown voltage resistance strength of the separator membrane has the meaning well-known in the art and can be measured by methods known in the art. For example, it can be measured with reference to GB / T 13542.2-2009 and GB / T 1408-2006 using a breakdown voltage tester. The exemplary test method is as follows: Cut the separator membrane into a rectangular specimen of 450 mm × 650 mm and measure it using a breakdown voltage tester. The test instrument can use a CS2671AX type breakdown voltage tester.

[0174] It should be noted that the coating parameters (such as thickness, etc.) of the above-mentioned separator membrane are the coating parameters on one side of the porous substrate. When the coating is provided on both sides of the porous substrate, as long as the coating parameters on any one side meet the requirements of this application, it is considered to fall within the protection scope of this application.

[0175] Preparation method

[0176] The second aspect of the embodiments of the present application provides a method for preparing the separator membrane of the first aspect of the embodiments of the present application, including the following steps: providing a porous substrate; mixing the materials for forming a three-dimensional skeleton structure and a first filler in a solvent in a predetermined ratio to prepare a coating slurry; coating the coating slurry on at least one surface of the porous substrate, and drying to obtain a separator membrane, wherein the separator membrane includes a porous substrate and a coating provided on at least one surface of the porous substrate, the coating includes a three-dimensional skeleton structure and a first filler, at least a part of the first filler is filled in the three-dimensional skeleton structure, and the average particle size of the first filler is less than or equal to 200 nm.

[0177] In some embodiments, the coating slurry further includes a second filler. If the average particle size of the first filler is denoted as d1 and the average particle size of the second filler is denoted as d2, then d2 / d1 > 1.

[0178] In some embodiments, the solvent used for preparing the coating slurry can be water, such as deionized water.

[0179] In some embodiments, the coating slurry may further include other components. For example, it may further include a dispersant, a wetting agent, a binder, etc.

[0180] In some embodiments, the materials for forming the three-dimensional skeleton structure include at least one of an organic material and an inorganic material. Optionally, the organic material includes at least one of nanocellulose, polytetrafluoroethylene nanofibers, and polyamide nanofibers. Optionally, the inorganic material includes at least one of halloysite nanotubes, nanorod-shaped alumina, nanorod-shaped boehmite, nanorod-shaped silica, and glass fibers.

[0181] In some embodiments, the material forming the three-dimensional skeleton structure includes nanocellulose.

[0182] In some embodiments, the nanocellulose can be obtained by the following method: providing cellulose powder with a whiteness ≥ 80%; mixing the obtained cellulose powder with a modification solution and reacting, then washing to remove impurities, then adjusting the pH to neutral, and subjecting it to grinding and cutting to obtain nanocellulose.

[0183] Optionally, the cellulose powder with a whiteness of ≥80% can be obtained commercially, or by chemical methods (such as acid hydrolysis, alkali treatment, Tempo-catalyzed oxidation), biological methods (such as enzyme treatment), mechanical methods (such as ultrafine grinding, ultrasonic fragmentation, high-pressure homogenization), etc. The fiber raw materials used to prepare the cellulose powder with a whiteness of ≥80% can include plant fibers, such as cotton fibers (such as cotton fiber, kapok fiber), hemp fibers (such as sisal fiber, ramie fiber, jute fiber, flax fiber, hemp fiber, abaca fiber, etc.), palm fibers, wood fibers, bamboo fibers, grass fibers, etc., at least one of which.

[0184] In some embodiments, the cellulose powder with a whiteness of ≥80% can also be prepared as follows: After the fiber raw material is opened and deflated, it is cooked with an alkali solution (such as an aqueous NaOH solution, the concentration of which can be 4wt% to 20wt%, and can be optionally 5wt% to 15wt%), and then sequentially undergoes washing and impurity removal (such as the number of washing times is 3 to 6 times), bleaching (such as using sodium hypochlorite and / or hydrogen peroxide), acid washing and impurity removal, washing and impurity removal, water expulsion, and air flow drying to obtain cellulose powder.

[0185] In some embodiments, the modification solution can be an acid solution (such as sulfuric acid aqueous solution, boric acid aqueous solution, phosphoric acid aqueous solution, acetic acid aqueous solution) or an alkali solution (such as urea organic solvent solution). Optionally, the modification solution is an acid solution.

[0186] Optionally, the concentration of the acid solution can be 5wt% to 80wt%. When the modification solution is a sulfuric acid aqueous solution, the concentration of the acid solution can be 40wt% to 80wt%, whereby cellulose powder with sulfonic acid groups can be obtained. When the modification solution is a boric acid aqueous solution, the concentration of the acid solution can be 5wt% to 10wt%, whereby cellulose powder with boric acid groups can be obtained. When the modification solution is a phosphoric acid aqueous solution, the concentration of the acid solution can be 45wt% to 75wt%, whereby cellulose powder with phosphoric acid groups can be obtained. When the modification solution is an acetic acid aqueous solution, the concentration of the acid solution can be 40wt% to 80wt%, whereby cellulose powder with carboxylic acid groups can be obtained.

[0187] Optionally, the urea organic solvent solution is a urea xylene solution, whereby cellulose powder with amino groups can be obtained.

[0188] In some embodiments, optionally, the mass ratio of the cellulose powder to the modification solution can be 1:2.5 to 1:50, and can be optionally 1:5 to 1:30.

[0189] When the modifying solution is an aqueous sulfuric acid solution, the mass ratio of the cellulose powder to the acid solution can be 1:5 to 1:30. When the modifying solution is an aqueous boric acid solution, the mass ratio of the cellulose powder to the acid solution can be 1:20 to 1:50. When the modifying solution is an aqueous phosphoric acid solution, the mass ratio of the cellulose powder to the acid solution can be 1:5 to 1:30. When the modifying solution is an aqueous acetic acid solution, the mass ratio of the cellulose powder to the acid solution can be 1:5 to 1:30. When the modifying solution is a urea organic solvent solution, the mass ratio of the cellulose powder to the urea organic solvent solution can be 1:4 to 1:40.

[0190] In some embodiments, when the modifying solution is an acid solution, the reaction can be carried out under the condition of not higher than 80°C, and can be optionally carried out under the condition of 30°C to 60°C. The reaction time of the cellulose powder and the modifying solution can be 0.5 h to 4 h, and can be optionally 1 h to 3 h.

[0191] In some embodiments, when the modifying solution is an alkaline solution, the reaction can be carried out under the condition of 100°C to 145°C, and the reaction time of the cellulose powder and the modifying solution can be 1 h to 5 h.

[0192] In some embodiments, grinding can be carried out using a grinder, and cutting can be carried out using a high-pressure homogenizer. By adjusting the grinding parameters (such as the number of grinding times, grinding time, etc.) of the grinder and the cutting parameters of the high-pressure homogenizer, nanocellulose with different average diameters and / or different average lengths can be obtained.

[0193] In some embodiments, a coater can be used when coating the coating slurry. There is no special limitation on the model of the coater in this application. For example, a commercially available coater can be used. The coater includes an intaglio roll; the intaglio roll is used to transfer the slurry onto a porous substrate.

[0194] In some embodiments, the coating method of the coating slurry can adopt transfer coating, rotary spraying, dip coating, etc.

[0195] In some embodiments, the method further includes the following steps: coating a slurry containing particulate binder on at least a part of the surface of the coating, and forming an adhesive layer after drying.

[0196] The preparation method of the separator membrane of this application obtains a coating by one-time coating, which greatly simplifies the production process flow of the separator membrane.

[0197] Some raw materials used in the preparation method of the separator membrane of this application and parameters such as their contents can refer to the separator membrane in the first aspect of the embodiment of this application, which will not be elaborated here.

[0198] Unless otherwise specified, all raw materials used in the method for preparing the separator of the present application can be obtained commercially.

[0199] Secondary battery

[0200] The third aspect of the embodiments of the present application provides a secondary battery.

[0201] A secondary battery, also known as a rechargeable battery or a storage battery, refers to a battery that can be recharged to activate the active materials for continued use after discharging. Generally, 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 serving to prevent short circuit between the positive electrode and the negative electrode, and allowing active ions to pass through simultaneously.

[0202] The present application places no particular limitation on the type of the secondary battery. For example, the secondary battery may be a lithium-ion battery, a sodium-ion battery, etc. In particular, the secondary battery may be a lithium-ion secondary battery.

[0203] The secondary battery according to the third aspect of the embodiments of the present application includes the separator according to the first aspect of the embodiments of the present application or the separator prepared by the method according to the second aspect of the embodiments of the present application. The separator is disposed between the positive electrode plate and the negative electrode plate. Optionally, at least one side of the separator close to the negative electrode plate has the coating of the present application. Thus, the secondary battery of the present application can achieve a balance among high energy density, high thermal safety performance, long cycle life, and good kinetic performance.

[0204] [Positive electrode plate]

[0205] 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 surfaces opposite to each other in its own thickness direction, and the positive electrode film layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector.

[0206] When the secondary battery of the present application is a lithium-ion battery, the positive electrode active material may include, but is not limited to, at least one of lithium-containing transition metal oxides, lithium-containing phosphates, and their respective modified compounds. Examples of the lithium 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 manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and their respective modified compounds. Examples of the lithium-containing phosphates may 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.

[0207] In some embodiments, in order to further improve the energy density of the secondary battery, the positive electrode active material for the lithium-ion battery 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 their modified compounds. 0.8 ≤ a ≤ 1.2, 0.5 ≤ b < 1, 0 < c < 1, 0 < d < 1, 1 ≤ e ≤ 2, 0 ≤ f ≤ 1, M is selected from at least one of Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, and B, and A is selected from at least one of N, F, S, and Cl.

[0208] As an example, the positive electrode active material for the lithium-ion battery may include at least one of LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2 (NCM523), LiNi 0.6 Co 0.2 Mn 0.2 O2 (NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811), LiNi 0.85 Co 0.15 Al 0.05 O2, LiFePO4, LiMnPO4.

[0209] When the secondary battery of the present application is a sodium-ion battery, the positive electrode active material may include, but is not limited to, at least one of sodium-containing transition metal oxides, polyanion materials (such as phosphates, fluorophosphates, pyrophosphates, sulfates, etc.), and Prussian blue-based materials.

[0210] As an example, the positive electrode active material for a sodium-ion battery may include NaFeO2, NaCoO2, NaCrO2, NaMnO2, NaNiO2, NaNi 1 / 2 Ti 1 / 2 O2, NaNi 1 / 2 Mn 1 / 2 O2, Na 2 / 3 Fe 1 / 3 Mn 2 / 3 O2, NaNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, NaFePO4, NaMnPO4, NaCoPO4, Prussian blue-based materials, materials with the general formula X p M’ q (PO4) r O x Y 3-x At least one of. In the general formula X p M’ q (PO4) r O x Y 3-x , 0 < p ≤ 4, 0 < q ≤ 2, 1 ≤ r ≤ 3, 0 ≤ x ≤ 2, X is selected from at least one of H + , Li + , Na + , K + and NH4 + , 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.

[0211] In the present application, the modified compounds of the above positive electrode active materials may be doping modification and / or surface coating modification of the positive electrode active material.

[0212] In some embodiments, the positive electrode film layer may further optionally include a positive electrode conductive agent. The present application does not particularly limit the type of the positive electrode conductive agent. As an example, the positive electrode conductive agent includes at least one of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, based on the total weight of the positive electrode film layer, the mass percentage content of the positive electrode conductive agent is ≤ 5 wt%.

[0213] In some embodiments, the positive electrode film layer may further optionally include a positive electrode binder. There is no particular limitation on the type of the positive electrode binder in this application. As an 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 fluorinated acrylate resin. In some embodiments, based on the total weight of the positive electrode film layer, the mass percentage content of the positive electrode binder is ≤5 wt%.

[0214] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. As an example of the metal foil, aluminum foil may be used. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material may include at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. As an example, the polymer material base layer may include at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0215] The positive electrode film layer is generally formed by coating a positive electrode slurry on a positive electrode current collector and then drying and cold pressing. The positive electrode slurry is generally formed by dispersing a positive electrode active material, an optional conductive agent, an optional binder, and any other components in a solvent and stirring evenly. The solvent may be N-methylpyrrolidone (NMP), but is not limited thereto.

[0216] [Negative electrode tab]

[0217] In some embodiments, the negative electrode tab includes a negative electrode current collector and a negative electrode film layer provided 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 surfaces opposite to each other in its own thickness direction, and the negative electrode film layer is provided on any one or both of the two opposite surfaces of the negative electrode current collector.

[0218] The negative electrode active material may be a negative electrode active material known in the art for secondary batteries. As an example, the negative electrode active material may include, but is not limited to, at least one of natural graphite, artificial graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based materials may include at least one of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy materials. The tin-based materials may include at least one of elemental tin, tin oxide, and tin alloy materials.

[0219] In some embodiments, the negative electrode film layer may further optionally include a negative electrode conductive agent. There is no particular limitation on the type of the negative electrode conductive agent in the present application. As an example, the negative electrode conductive agent may include at least one of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, based on the total weight of the negative electrode film layer, the mass percentage content of the negative electrode conductive agent is ≤5 wt%.

[0220] In some embodiments, the negative electrode film layer may further optionally include a negative electrode binder. There is no particular limitation on the type of the negative electrode binder in the present application. As an example, the negative electrode binder may include at least one of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, aqueous acrylic resin (for example, polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS). In some embodiments, based on the total weight of the negative electrode film layer, the mass percentage content of the negative electrode binder is ≤5 wt%.

[0221] In some embodiments, the negative electrode film layer may further optionally include other additives. As an example, the other additives may include thickeners, such as sodium carboxymethyl cellulose (CMC), PTC thermistor materials, etc. In some embodiments, based on the total weight of the negative electrode film layer, the mass percentage content of the other additives is ≤2 wt%.

[0222] In some embodiments, the negative electrode current collector may be made of a metal foil or a composite current collector. As an example of the metal foil, copper foil may be used. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material may include at least one of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. As an example, the polymer material base layer may include at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0223] The negative electrode film layer is generally formed by coating a negative electrode paste on a negative electrode current collector and then drying and cold pressing. The negative electrode paste is generally formed by dispersing a negative electrode active material, an optional conductive agent, an optional binder, and other optional additives in a solvent and stirring evenly. The solvent may be N-methylpyrrolidone (NMP) or deionized water, but is not limited thereto.

[0224] The negative electrode sheet does not exclude other additional functional layers in addition to the negative electrode film layer. For example, in some embodiments, the negative electrode sheet described in the present application further includes a conductive bottom coating (e.g., composed 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 sheet described in the present application further includes a protective layer covering the surface of the negative electrode film layer.

[0225] [Electrolyte solution]

[0226] During the charge and discharge process of the secondary battery, active ions are embedded and extracted back and forth between the positive electrode sheet and the negative electrode sheet, and the electrolyte solution plays a role in conducting active ions between the positive electrode sheet and the negative electrode sheet. The present application does not particularly limit the type of the electrolyte solution, and it can be selected according to actual needs.

[0227] The electrolyte solution includes an electrolyte salt and a solvent. The types of the electrolyte salt and the solvent are not specifically limited and can be selected according to actual needs.

[0228] When the secondary battery of the present application is a lithium-ion battery, by way of example, the electrolyte salt may include but is not limited to at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium bis(oxalato)borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP).

[0229] When the secondary battery of the present application is a sodium-ion battery, by way of example, the electrolyte salt may include but is not limited to at least one of sodium hexafluorophosphate (NaPF6), sodium tetrafluoroborate (NaBF4), sodium perchlorate (NaClO4), sodium hexafluoroarsenate (NaAsF6), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium trifluoromethanesulfonate (NaTFS), sodium difluorooxalate borate (NaDFOB), sodium bis(oxalato)borate (NaBOB), sodium difluorophosphate (NaPO2F2), sodium difluorodioxalate phosphate (NaDFOP), and sodium tetrafluorooxalate phosphate (NaTFOP).

[0230] As an example, the solvent may include, but is not limited to, 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 - butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS), and diethyl sulfone (ESE).

[0231] In some embodiments, the electrolyte may optionally further include additives. For example, the additives may include negative electrode film - forming additives, or may include positive electrode film - forming additives, or may also include additives that can improve certain battery performance, such as additives for improving the overcharge performance of the battery, additives for improving the high - temperature performance of the battery, additives for improving the low - temperature power performance of the battery, etc.

[0232] In some embodiments, the positive electrode sheet, the separator, and the negative electrode sheet may be made into an electrode assembly by a winding process and / or a stacking process.

[0233] In some embodiments, the secondary battery may include an outer package. The outer package can be used to encapsulate the above - mentioned electrode assembly and electrolyte.

[0234] In some embodiments, the outer package of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery may also be a soft package, such as a pouch - type soft package. The material of the soft package may be plastic, such as at least one of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0235] This application has no particular limitation on the shape of the secondary battery, and it can be cylindrical, square, or any other arbitrary shape. As Figure 1 is a secondary battery 5 with a square structure as an example.

[0236] In some embodiments, such as Figure 2As shown, the outer package may include a housing 51 and a cover plate 53. The housing 51 may include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates enclose to form a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 is used to cover the opening to close the receiving cavity. The positive electrode plate, the negative electrode plate and the separator may be formed into an electrode assembly 52 through a winding process and / or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. The number of electrode assemblies 52 included in the secondary battery 5 may be one or more, which can be adjusted according to requirements.

[0237] The preparation method of the secondary battery of the present application is well-known. In some embodiments, a positive electrode plate, a separator, a negative electrode plate and an electrolyte may be assembled to form a secondary battery. As an example, a positive electrode plate, a separator and a negative electrode plate may be formed into an electrode assembly through a winding process and / or a stacking process, the electrode assembly is placed in an outer package, dried and then injected with an electrolyte, and after processes such as vacuum packaging, standing, forming and shaping, a secondary battery is obtained.

[0238] In some embodiments of the present application, the secondary battery according to the present application may be assembled into a battery module. The number of secondary batteries included in the battery module may be multiple, and the specific number may be adjusted according to the application and capacity of the battery module.

[0239] Figure 3 is a schematic diagram of a battery module 4 as an example. As Figure 3 shown, in the battery module 4, a plurality of secondary batteries 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other way. Further, the plurality of secondary batteries 5 may be fixed by fasteners.

[0240] Optionally, the battery module 4 may further include a housing having a receiving space, and a plurality of secondary batteries 5 are received in the receiving space.

[0241] In some embodiments, the above battery module may also be assembled into a battery pack. The number of battery modules included in the battery pack may be adjusted according to the application and capacity of the battery pack.

[0242] Figure 4 and Figure 5 is a schematic diagram of a battery pack 1 as an example. As Figure 4 and Figure 5 shown, the battery pack 1 may include a battery box and a plurality of battery modules 4 arranged in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 is used to cover the lower box body 3 and form a closed space for receiving the battery modules 4. The plurality of battery modules 4 may be arranged in the battery box in any way.

[0243] Electric device

[0244] A fourth aspect of the embodiments of the present application provides an electrical device, which includes at least one of the secondary battery, battery module or battery pack of the present application. The secondary battery, battery module or battery pack can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device can be, but is not limited to, a mobile device (such as a mobile phone, laptop computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship, a satellite, an energy storage system, etc.

[0245] The electrical device can select a secondary battery, battery module or battery pack according to its usage requirements.

[0246] Figure 6 It is a schematic diagram of an electrical device as an example. The electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the requirements of the electrical device for high power and high energy density, a battery pack or battery module can be used.

[0247] An electrical device as another example can be a mobile phone, a tablet computer, a laptop computer, etc. This electrical device usually requires thin and light, and a secondary battery can be used as the power source.

[0248] Example

[0249] The following embodiments more specifically describe the content disclosed in the present application. These embodiments are only for illustrative purposes, because various modifications and changes within the scope of the content disclosed in the present application are obvious to those skilled in the art. Unless otherwise stated, all parts, percentages and ratios reported in the following embodiments are based on mass, and all reagents used in the embodiments are commercially available or synthesized according to conventional methods and can be used directly without further treatment, and the instruments used in the embodiments are all commercially available.

[0250] Preparation of nanocellulose C1

[0251] After the cotton linter is loosened and deslagged by a cotton opener, it is cooked with a 5wt% NaOH aqueous solution at 150°C for 2h, and then sequentially washed to remove impurities (the number of washing times is 3 times), bleached with sodium hypochlorite, washed with dilute hydrochloric acid to remove impurities, washed to remove impurities (the number of washing times is 1 time), dewatered, and dried by air flow to obtain cotton cellulose powder with a whiteness of ≥85%.

[0252] Mix 1 kg of the obtained cotton cellulose powder with 30 kg of 60 wt% sulfuric acid aqueous solution, react at 55 °C to 60 °C for 1.5 h. After the reaction, carry out impurity removal by water washing (the number of water washing times is 3 times), filtration, acid removal and impurity removal.

[0253] Then adjust the pH to neutral with 10 wt% NaOH aqueous solution, then grind with a grinder, and then use a high-pressure homogenizer equipment for nanoscale cutting to obtain nanocellulose C1 with sulfonic acid group modification groups, with an average length of 350 nm and an average diameter of 18 nm, and the molar ratio of sulfonic acid group to hydroxyl group is 5:3.

[0254] Preparation of nanocellulose C2 to C4

[0255] Nanocellulose C2 to C4 are prepared in a similar method to nanocellulose C1, and the differences are shown in Table 1 in detail. During the preparation process, nanocellulose with different average diameters and / or different average lengths can be obtained by adjusting the parameters of the grinder treatment and the cutting parameters of the high-pressure homogenizer equipment.

[0256] Preparation of nanocellulose C5

[0257] After loosening and removing slag from cotton linter by a cotton opener, cook it with 5 wt% NaOH aqueous solution at 150 °C for 2 h, and then sequentially carry out impurity removal by water washing (the number of water washing times is 3 times), sodium hypochlorite bleaching, dilute hydrochloric acid washing for impurity removal, water washing for impurity removal (the number of water washing times is 1 time), water driving, and air flow drying to obtain cotton cellulose powder with a whiteness of ≥85%. At 10 °C, mix the obtained cotton cellulose powder with 20 wt% NaOH aqueous solution, stir for 2 h, filter, and wash with water 2 times to obtain cellulose powder.

[0258] Put 50 g of the obtained cellulose powder and 200 g of urea into a three-necked reactor with an oil-water separator. After the urea is dissolved, add 5 g of xylene, heat up to 137 °C under stirring, terminate the reaction after 4 h, and then carry out water washing (the number of water washing times is 3 times), filtration, and drying to obtain cellulose carbamate.

[0259] Dissolve the obtained cellulose carbamate in 5 wt% NaOH aqueous solution to obtain a uniform cellulose carbamate solution, then grind with a grinder, and then use a high-pressure homogenizer equipment for nanoscale cutting to obtain nanocellulose C5 with amine group modification groups, with an average length of 350 nm and an average diameter of 18 nm, and the molar ratio of amine group to hydroxyl group is 4:3.

[0260] Preparation of nanocellulose C6

[0261] Unmodified nanocellulose with an average length of 350 nm and an average diameter of 18 nm, product model CNWS-50, is purchased from Zhongke Leiming (Beijing) Technology Co., Ltd. It can be further processed using a grinder and / or a high-pressure homogenizer to obtain nanocellulose with different average diameters and / or different average lengths.

[0262] The molar ratio of the modifying group to the hydroxyl group in nanocelluloses C1 to C5 can be measured by the following method: According to the phthalic anhydride method in GB / T 12008.3-2009, the hydroxyl values (the milligrams of potassium hydroxide equivalent to the hydroxyl content per gram of the sample) of the raw cellulose and nanocelluloses C1 to C5 are measured respectively. The obtained numerical unit is mg KOH / g, which is converted to mmol / g as the hydroxyl content. Subtracting the hydroxyl content of nanocelluloses C1 to C5 from the hydroxyl content of the raw cellulose gives the content of the modifying group (i.e., the content of the modified hydroxyl group), and from this, the molar ratio of the modifying group to the hydroxyl group is calculated.

[0263] Table 1

[0264]

[0265] Example 1

[0266] Preparation of separator

[0267] Provide a PE porous substrate: the thickness is 5.2 μm.

[0268] Prepare a coating slurry: Mix the above-prepared nanocellulose C1, the first filler, the second filler, and the binder aqueous solution type polyacrylic acid in a mass ratio of 16.0:62.5:20.0:1.5 in an appropriate amount of deionized water solvent to obtain a coating slurry.

[0269] The first filler is a mixture of alumina primary particles (average particle size of 50 nm, content of 12.5 wt%, based on the total weight of the coating) and alumina secondary particles (average particle size of 100 nm, content of 50 wt%, based on the total weight of the coating). And the contents of α-crystalline form, θ-crystalline form, γ-crystalline form, and η-crystalline form in the first filler are 1.5 wt%, 70.7 wt%, 27.3 wt%, and 0.5 wt% respectively, based on the total weight of the first filler. The second filler is alumina primary particles (average particle size of 240 nm), and the crystalline form of the second filler is mainly α-crystalline form, with a mass ratio of more than 99.5%, based on the total weight of the second filler.

[0270] Coating: Coat the prepared coating slurry on both surfaces of the PE porous substrate with a coater, and through the drying and slitting processes, an isolation film is obtained. The coating thickness on one side of the PE porous substrate is 1.0 μm.

[0271] Preparation of positive electrode sheet

[0272] The positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811), the conductive agent carbon black (Super P), and the binder polyvinylidene fluoride (PVDF) are mixed uniformly in an appropriate amount of solvent N-methylpyrrolidone (NMP) according to a mass ratio of 96.2:2.7:1.1 to obtain a positive electrode paste; the positive electrode paste is coated on an aluminum foil positive electrode current collector, and through processes such as drying, cold pressing, slitting, and cutting, a positive electrode plate is obtained.

[0273] Preparation of negative electrode sheet

[0274] The negative electrode active material artificial graphite, the conductive agent carbon black (Super P), the binder styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC) are mixed uniformly in an appropriate amount of solvent deionized water according to a mass ratio of 96.4:0.7:1.8:1.1 to obtain a negative electrode paste; the negative electrode paste is coated on a copper foil negative electrode current collector, and through processes such as drying, cold pressing, slitting, and cutting, a negative electrode plate is obtained.

[0275] Preparation of electrolyte

[0276] Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed according to a mass ratio of 30:70 to obtain an organic solvent, and fully dried LiPF6 is dissolved in the above organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0277] Preparation of secondary battery

[0278] The positive electrode plate, the separator, and the negative electrode plate are stacked and wound in sequence to obtain an electrode assembly; the electrode assembly is placed in an outer package, dried, and then the electrolyte is injected, and through processes such as vacuum packaging, standing, forming, and shaping, a secondary battery is obtained.

[0279] Examples 2 - 4

[0280] The secondary battery is prepared by a method similar to that of Example 1, except that the particle size of the first filler in the preparation of the separator is different, and the specific parameters are shown in Table 2.

[0281] Examples 5 - 14

[0282] The secondary battery is prepared by a method similar to that of Example 1, except that the types and / or addition amounts of nanocellulose and the first filler in the preparation of the separator are different, and the specific parameters are shown in Table 2.

[0283] Example 15

[0284] The secondary battery was prepared by a method similar to that of Example 1, except that in the preparation of the separator, the first filler was alumina secondary particles with an average particle size of 100 nm, and the contents of α-crystalline form, θ-crystalline form, γ-crystalline form and η-crystalline form in the first filler were 1.5 wt%, 70.7 wt%, 27.3 wt% and 0.5 wt% respectively, based on the total weight of the first filler.

[0285] Example 16

[0286] The secondary battery was prepared by a method similar to that of Example 1, except that in the preparation of the separator, the first filler was alumina primary particles. The average particle size of the alumina primary particles was 50 nm, and the contents of α-crystalline form, θ-crystalline form, γ-crystalline form and η-crystalline form were 1.5 wt%, 70.7 wt%, 27.3 wt% and 0.5 wt% respectively, based on the total weight of the alumina primary particles.

[0287] Comparative Example 1

[0288] The secondary battery was prepared by a method similar to that of Example 1, except for the preparation process of the separator.

[0289] A PE porous substrate was provided: the thickness was 5.2 μm.

[0290] A coating slurry was prepared: alumina primary particles (average particle size of 700 nm, with the mass ratio of α-crystalline form more than 99.5%) and a binder were mixed in a mass ratio of 94:6 and dissolved in deionized water to obtain the coating slurry.

[0291] Coating: The prepared coating slurry was coated on both surfaces of the PE porous substrate by a coater, and through the processes of drying and slitting, a separator was obtained. The coating thickness on one side of the PE porous substrate was 1.8 μm.

[0292] Test part

[0293] (1) Test of the thermal shrinkage rate of the separator

[0294] Sample preparation: The separator prepared above was punched into samples with a width of 50 mm and a length of 100 mm by a punching machine. Five parallel samples were taken and placed on an A4 paper and fixed, and then the A4 paper with the samples was placed on a corrugated paper with a thickness of 1 mm to 5 mm.

[0295] Sample test: The A4 paper placed on the corrugated paper was put into a forced-air oven, and the temperature of the forced-air oven was set at 150 °C. After the temperature reached the set temperature and was stable for 30 minutes, timing started. After reaching the set time (1 hour in this application), the length and width of the separator were measured, and the values were marked as a and b respectively.

[0296] Calculation of thermal shrinkage rate: Longitudinal (MD) thermal shrinkage rate = [(100 - a) / 100] × 100%, Transverse (TD) thermal shrinkage rate = [(50 - b) / 50] × 100%. The average value of 5 parallel samples is taken as the test result.

[0297] (2) Ion conductivity test of the separator

[0298] The ion conductivity of the separator is obtained through an alternating current impedance spectroscopy experiment. Specifically, the separator is cut into circular wafers of a certain area, dried, and placed between two stainless steel electrodes. After absorbing a sufficient amount of electrolyte, it is sealed to form a button cell, and an alternating current impedance spectroscopy experiment is carried out using an electrochemical workstation to obtain the ion conductivity of the separator. The electrochemical workstation can use the Shanghai Chenhua CHI 660C electrochemical workstation, the alternating current signal frequency range is from 0.01 Hz to 1 MHz, and the sine wave potential amplitude is 5 mV. For accuracy, the average value of 5 parallel samples is taken as the test result.

[0299] The electrolyte used is prepared as follows: Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a mass ratio of 30:50:20 to obtain an organic solvent, and fully dried LiPF6 is dissolved in the above organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0300] (3) Thermal box test of the secondary battery

[0301] At 25°C, the secondary battery is charged at a constant current of 1C to 4.2V, and then charged at a constant voltage until the current is ≤ 0.05C, and left standing for 5 min; then each secondary battery is tested with a fixture in a DHG - 9070A DHG series high - temperature oven, heated from room temperature to 80°C ± 2°C at a rate of 5°C / min, and maintained for 30 min; then it is heated again at a heating rate of 5°C / min, and for every 5°C increase in temperature, it is maintained for 30 min. Monitor the surface temperature change of the secondary battery during the heating process, and the oven temperature corresponding to when the temperature starts to rise sharply is the thermal box failure temperature of the secondary battery. The higher the thermal box failure temperature of the secondary battery, the better the thermal safety performance of the secondary battery. For accuracy, the average value of 5 parallel samples is taken as the test result.

[0302] (4) Cycle performance test of the secondary battery

[0303] At 25°C, charge the secondary battery to 4.2V at 1C constant current, and continue to charge at constant voltage until the current is ≤0.05C. At this time, the secondary battery is fully charged. Record the charging capacity at this time, which is the charging capacity of the first cycle; after the secondary battery is left to stand for 5 minutes, discharge it to 2.8V at 1C constant current. This is a cyclic charge and discharge process. Record the discharge capacity at this time, which is the discharge capacity of the first cycle. Perform a cyclic charge and discharge test on the secondary battery according to the above method, and record the discharge capacity after each cycle. The capacity retention rate (%) of the secondary battery after 1000 cycles at 25°C = discharge capacity after 1000 cycles / discharge capacity of the first cycle × 100%. For accuracy, take the average value of 5 parallel samples as the test result.

[0304] As can be seen from Table 2, by arranging a coating comprising nanocellulose (constituting a three-dimensional skeleton structure) and a first filler having an average particle size of less than or equal to 200 nm on both surfaces of the porous substrate of the isolation membrane, the isolation membrane can have both low thermal shrinkage and high ion conductivity, and the secondary battery can have both high thermal safety performance and good cycle performance.

[0305] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and the same effect as the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

[0306]

Claims

1. An isolation film, comprising a porous substrate and a coating disposed on at least one surface of the porous substrate, wherein, The coating includes fibrous materials, a first filler, and a second filler, and the average particle size of the second filler is greater than that of the first filler.

2. The separator film according to claim 1, wherein the first filler includes at least one of primary particles and secondary particles; and / or the second filler has a primary particle morphology.

3. The separator film according to claim 2, wherein the average particle size of the first filler with a primary particle morphology is 15 nm to 80 nm, optionally 30 nm to 65 nm; and / or the average particle size of the first filler with a secondary particle morphology is 50 nm to 200 nm, optionally 55 nm to 150 nm.

4. The separator film according to any one of claims 1-3, wherein The first filler includes at least one of primary particles and secondary particles, and the average particle size of the first filler with the morphology of primary particles is denoted as d 11 , and the average particle size of the first filler with the morphology of secondary particles is denoted as d 12 , and the average particle size of the second filler is denoted as d2 3.0 ≤ d2 / d 11 ≤ 10.0, optionally, 3.5 ≤ d2 / d 11 ≤ 8.0; and / or, 1.2 ≤ d2 / d 12 ≤ 6.

0. Optionally, 2.0 ≤ d2 / d 12 ≤ 5.

5.

5. The separator film according to any one of claims 1-4, wherein the average particle size of the first filler is less than or equal to 200 nm, optionally 15 nm to 180 nm, more optionally 30 nm to 150 nm; and / or the average particle size of the second filler is 120 nm to 350 nm, optionally 150 nm to 300 nm.

6. The separator film according to any one of claims 1-5, wherein, The first filler includes a combination of primary particles and secondary particles; Optionally, based on the total weight of the first filler, the content of the first filler with a primary particle morphology is less than that of the first filler with a secondary particle morphology; Optionally, based on the total weight of the first filler, the content of the first filler with a primary particle morphology is less than or equal to 30 wt%, optionally 8 wt% to 30 wt%.

7. The separator according to any one of claims 1-6, wherein, The BET specific surface area of the first filler is ≥ 25 m 2 / g, and can be optionally 30 m 2 / g to 65 m 2 / g.

8. The separator film according to any one of claims 1-7, wherein, The first filler includes at least one of inorganic particles and organic particles; Optionally, the inorganic particles include at least one of boehmite, alumina, barium sulfate, magnesium oxide, magnesium hydroxide, silicon oxide, 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. More optionally, the inorganic particles include at least one of boehmite, alumina, barium sulfate, magnesium oxide, silicon oxide, titanium oxide, zinc oxide, cerium oxide, and barium titanate; Optionally, the organic particles include at least one of polystyrene particles, polyacrylic acid wax particles, melamine formaldehyde resin particles, phenolic resin particles, polyester particles, polyimide particles, polyamideimide particles, polyaramide particles, polyphenylene sulfide particles, polysulfone particles, polyethersulfone particles, polyetheretherketone particles, and polyaryletherketone particles.

9. The separator according to any one of claims 1-8, wherein, The first filler includes inorganic particles, and the crystal form of the inorganic particles includes at least one of θ crystal form, γ crystal form, and η crystal form; Optionally, the crystal form of the inorganic particles includes at least one of θ crystal form and γ crystal form; Optionally, the content of the inorganic particles with θ crystal form is ≥50 wt%, more optionally 55 wt% to 84 wt%, based on the total weight of the inorganic particles in the first filler; Optionally, the content of the inorganic particles with γ crystal form is ≥10 wt%, more optionally 15 wt% to 44 wt%, based on the total weight of the inorganic particles in the first filler; Optionally, the content of the inorganic particles in the η crystal form is ≤5 wt%, more optionally ≤2.5 wt%, based on the total weight of the inorganic particles in the first filler.

10. The separator according to any one of claims 1-9, wherein, The second filler satisfies at least one of the following conditions (1) to (4): (1) The BET specific surface area of the second filler is ≤ 20 m 2 / g, and can be optionally 6 m 2 / g to 15 m 2 / g; (2) The second filler includes at least one of inorganic particles and organic particles; (3) The second filler includes inorganic particles in the morphology of primary particles, and the crystal form of the inorganic particles in the morphology of primary particles includes at least one of α crystal form and γ crystal form, optionally including α crystal form; (4) The second filler includes inorganic particles in the morphology of primary particles, and the crystal form of the inorganic particles in the morphology of primary particles includes α crystal form, and the content of the α crystal form is ≥70 wt%, optionally 85 wt% to 100 wt%, based on the total weight of the inorganic particles in the morphology of primary particles in the second filler.

11. The separator membrane according to any one of claims 1-10, wherein the morphology of the fibrous material includes at least one of rod-like, tubular, rod-shaped and fibrous; and / or, the average diameter of the fibrous material is ≤40 nm, optionally 10 nm to 35 nm; and / or, the average length of the fibrous material is 100 nm to 600 nm, optionally 200 nm to 500 nm; and / or, the aspect ratio of the fibrous material is 5 to 60, optionally 10 to 30.

12. The separator according to any one of claims 1-11, wherein, The fibrous material includes at least one of an organic material and an inorganic material; Optionally, the organic material includes at least one of nanocellulose, polytetrafluoroethylene nanofibers and polyamide nanofibers. Optionally, the nanocellulose includes at least one of cellulose nanofibers, cellulose nanocrystals and bacterial nanocellulose; Optionally, the inorganic material includes at least one of halloysite nanotubes, nanorod-shaped alumina, nanorod-shaped boehmite, nanorod-shaped silica and glass fibers.

13. The separator film according to any one of claims 1-12, wherein The fibrous material includes nanocellulose, and the nanocellulose includes at least one of unmodified nanocellulose and modified nanocellulose; Optionally, the modified nanocellulose includes a modifying group, and the modifying group includes at least one of an amino group, a carboxyl group, an aldehyde group, a sulfonic acid group, a boric acid group and a phosphoric acid group, more optionally at least one of a sulfonic acid group, a boric acid group and a phosphoric acid group; Optionally, the modified nanocellulose includes a hydroxyl group and a modifying group, and the molar ratio of the modifying group to the hydroxyl group is 1:4 to 4:1, more optionally 2:3 to 7:

3.

14. The separator according to any one of claims 1 to 13, wherein, The fibrous material includes a sulfonic acid group, and the content of sulfur element in the fibrous material is ≥0.1 wt%, optionally 0.2 wt% to 0.5 wt%, based on the total weight of the fibrous material.

15. The separator membrane according to any one of claims 1-14, wherein the content of the first filler is ≥50 wt%, optionally 60 wt% to 85 wt%, based on the total weight of the coating; and / or, the content of the second filler is ≤30 wt%, optionally 5 wt% to 25 wt%, based on the total weight of the coating; and / or, The content of the fibrous material is 5 wt% to 40 wt%, optionally 8 wt% to 25 wt%, based on the total weight of the coating.

16. The separator according to any one of claims 1-15, wherein, The coating further includes a non-granular binder; Optionally, the non-granular binder includes an aqueous solution binder; Optionally, the content of the non-granular binder in the coating is ≤ 2 wt%, based on the total weight of the coating.

17. The separator according to any one of claims 1 - 16, wherein, The thickness of the porous substrate is ≤ 6 μm, optionally 3 μm to 5 μm; and / or, The thickness of the coating is ≤ 2 μm, optionally 0.5 μm to 1.5 μm.

18. The separator film according to any one of claims 1-17, wherein, The separator further includes an adhesive layer disposed on at least a part of the surface of the coating, and the adhesive layer includes a granular binder; Optionally, the granular binder includes at least one of a homopolymer or copolymer of an acrylate monomer, a homopolymer or copolymer of an acrylic monomer, and a homopolymer or copolymer of a fluorinated olefin monomer.

19. The separator according to any one of claims 1-18, wherein, The separator satisfies at least one of the following conditions (1) to (8): (1) The longitudinal thermal shrinkage rate of the separator at 150 °C for 1 h is ≤ 6%, optionally 0.5% to 4%; (2) The transverse thermal shrinkage rate of the separator at 150 °C for 1 h is ≤ 6%, optionally 0.5% to 4%; (3) The longitudinal tensile strength of the separator membrane is ≥ 2000 kg / cm 2 , and can be optionally 2500 kg / cm 2 to 4500 kg / cm 2 ; (4) The transverse tensile strength of the separator membrane is ≥ 2000 kg / cm 2 , optionally 2500 kg / cm 2 to 4500 kg / cm 2 ; (5) The wetting length of the separator is ≥ 30 mm, optionally 30 mm to 80 mm; (6) The wetting speed of the separator is ≥ 3 mm / s, optionally 3 mm / s to 10 mm / s; (7) The air permeability of the separator is ≤ 300 s / 100 mL, optionally 100 s / 100 mL to 230 s / 100 mL; (8) The breakdown voltage strength of the separator is ≥ 1 KV.

20. A method for preparing the separator membrane according to any one of claims 1-19, comprising the following steps: Providing a porous substrate; mixing a fibrous material, a first filler, and a second filler in a solvent in a predetermined ratio to prepare a coating slurry; coating the coating slurry on at least one surface of the porous substrate, and obtaining a separator after drying, wherein the separator includes a porous substrate and a coating disposed on at least one surface of the porous substrate, the coating includes a fibrous material, a first filler, and a second filler, and the average particle size of the second filler is greater than that of the first filler.

21. A secondary battery, which includes the separator according to any one of claims 1 - 19 or the separator prepared by the method according to claim 20.

22. An electrical device, which includes the secondary battery according to claim 21.

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