Secondary battery, battery module, battery pack, and electric device

Abstract

The application provides a secondary battery, a battery module, a battery pack and a power utilization device. The secondary battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; the positive electrode sheet comprises a positive electrode active material; the negative electrode sheet comprises a negative electrode active material; the separator is arranged between the positive electrode sheet and the negative electrode sheet; and the electrolyte comprises a first organic solvent and a film-forming additive configured to form an interface film on the surface of the positive electrode active material and / or the negative electrode active material, wherein the porosity of the separator is denoted as ε%; the mass percentage content of the film-forming additive relative to the total mass of the electrolyte is denoted as b%; the viscosity value of the electrolyte at 25 DEG C is denoted as c (mPa s), and the secondary battery satisfies: 4≤(b*ε) / c≤240; and the application can improve the rapid charge-discharge performance and cycle life of the secondary battery.

Description

Technical Field

[0001] This application relates to the field of batteries, specifically to a secondary battery, a battery module, a battery pack, and an electrical device. Background Technology

[0002] Secondary batteries have the characteristics of high capacity and long life, so they are widely used in electronic devices such as mobile phones, laptops, electric vehicles, electric cars, electric airplanes, electric ships, electric toy cars, electric toy ships, electric toy airplanes, and power tools.

[0003] As batteries are used more and more widely, the requirements for the performance of secondary batteries are becoming increasingly stringent. For example, they are required to have good fast charge and discharge performance and cycle life. Therefore, improving the fast charge and discharge performance and cycle life of secondary batteries is an urgent problem to be solved. Summary of the Invention

[0004] This application is made in view of the above-mentioned issues, and its purpose is to provide a secondary battery, a battery module, a battery pack, and an electrical device.

[0005] The first aspect of this application provides a secondary battery, the secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte; the positive electrode comprises a positive active material; the negative electrode comprises a negative active material; the separator is disposed between the positive electrode and the negative electrode; and the electrolyte comprises a first organic solvent and a film-forming additive, the film-forming additive being configured to form an interfacial film on the surface of the positive active material and / or the negative active material, wherein the porosity of the separator is denoted as ε%; the mass percentage of the film-forming additive relative to the total mass of the electrolyte is denoted as b%; the viscosity of the electrolyte at 25°C is denoted as c (mPa·s), and the secondary battery satisfies: 4≤(b*ε) / c≤240; optionally, 4≤(b*ε) / c≤180.

[0006] Therefore, this application controls the porosity of the separator, the viscosity of the electrolyte, and the film-forming additives to meet the aforementioned ranges, comprehensively regulating the migration rate of lithium ions in both solid-phase and liquid-phase mass transfer, thereby improving the average migration rate of lithium ions and thus enhancing the fast-charging capability of the secondary battery. Furthermore, the film-forming additives not only regulate the viscosity of the electrolyte to further adjust the migration rate of lithium ions in the liquid phase, but also form a protective layer on the surface of the active material to passivate the surface of the active material, reducing the risk of side reactions between the active material surface and the electrolyte. This improves the structural stability of the active material and increases the cycle life of the secondary battery. In particular, lithium salt additives form a solid electrolyte interface film on the surface of the negative electrode active material with lower impedance, which is more conducive to the insertion and extraction of lithium ions.

[0007] In any embodiment, the secondary battery also satisfies at least one of conditions (1) to (3): (1) 25 ≤ ε ≤ 55; optionally, 30 ≤ ε ≤ 50; (2) 0.1 ≤ b ≤ 8; optionally, 0.1 ≤ b ≤ 6; (3) 1 ≤ c ≤ 6; optionally, 2 ≤ c ≤ 5.

[0008] Therefore, when the porosity of the separator in this application is within the above-mentioned range, the porosity of the separator will not be too small, its liquid permeability is good, and its lithium-ion penetration ability is strong, which is conducive to the migration of lithium ions through the separator to the adjacent active material, and can further improve the fast charging capability of the secondary battery. When the mass percentage content of the film-forming additive is within the above-mentioned range, the film-forming additive can react with the active material to form a dense and uniform film layer, thereby providing good protection for the active material. At the same time, the film-forming additive, such as lithium salt additive, can form an SEI film with low film-forming resistance, balancing lifespan and power. When the viscosity of the electrolyte is within the above-mentioned range, the viscosity of the electrolyte will not be too high, and its ionic conductivity is relatively high, which is conducive to improving the ion transport rate, thereby improving the fast charging capability of the secondary battery. In addition, when the viscosity of the electrolyte is within the above-mentioned range, the compatibility of the electrolyte with the positive and negative electrode sheets is relatively good, and it is not easy for it to undergo side reactions with the active materials in the positive and negative electrode sheets, thereby improving the cycle stability of the secondary battery.

[0009] In any embodiment, the mass percentage of the first organic solvent relative to the total mass of the electrolyte is denoted as a%; the secondary battery also satisfies: 2≤c+2*a%≤8.

[0010] Therefore, by controlling the mass percentage of the first organic solvent, this application can adjust the viscosity of the electrolyte within a suitable range, and can form a stable interfacial film during the film formation process, which can completely cover the surface of the active material.

[0011] In any implementation, 60 ≤ a ≤ 90; alternatively, 65 ≤ a ≤ 85.

[0012] In any embodiment, the first organic solvent includes one or more of linear carbonate solvents, carboxylic acid ester solvents, and nitrile solvents; optionally, the linear carbonate solvent includes one or more of ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and methyl propyl carbonate (MPC); optionally, the carboxylic acid ester solvent includes one or more of ethyl acetate (EA), methyl acetate (MA), ethyl propionate (EP), methyl formate (MF), ethyl butyrate (EB), butyl acetate (BA), methyl propionate (MP), methyl butyrate (MB), propyl butyrate (PB), and butyl butyrate (BB); optionally, the nitrile solvent includes one or more of acetonitrile (AN), glutaronitrile (GLN), and adiponitrile (ADN).

[0013] Therefore, the solvents described in this application have low viscosity, which can control the overall viscosity range of the electrolyte within a suitable range; and when the first organic solvent includes two or more, the viscosity and ionic conductivity of the electrolyte can be synergistically controlled, and multiple solvents can participate in the film-forming reaction so that the film structure contains multiple components, thereby improving the stability of the interfacial film.

[0014] In any embodiment, the film-forming additive includes: a negative electrode film-forming additive configured to form an interfacial film on the surface of the negative electrode active material, wherein the mass percentage of the negative electrode film-forming additive relative to the total mass of the electrolyte is denoted as b1%; and a positive electrode film-forming additive configured to form an interfacial film on the surface of the positive electrode active material, wherein the mass percentage of the positive electrode film-forming additive relative to the total mass of the electrolyte is denoted as b2%, wherein the secondary battery satisfies: 1≤b2 / b1≤60; optionally, 1≤b2 / b1≤40.

[0015] Therefore, when the content of the negative electrode film-forming additive and the positive electrode film-forming additive in this application is adjusted to meet the above formula, the electrolyte can form an SEI film on the surface of the negative electrode active material and a CEI film on the surface of the positive electrode active material. The interfacial impedance of the two film layers is relatively low, the kinetic activity is good, the charge transfer impedance in the secondary battery is small, which is conducive to the rapid migration of lithium ions, thereby improving the fast charging performance of the secondary battery.

[0016] In any embodiment, 0.01 ≤ b1 ≤ 1.5; and / or 0.1 ≤ b2 ≤ 7. When the film-forming additive is within the above range, it can form a dense and stable interfacial film on the surface of the active material, thereby providing sufficient protection for the active material, improving the structural stability of the active material, and thus ensuring the cycle stability of the secondary battery.

[0017] In any embodiment, the negative electrode film-forming additive includes one or more of boron-containing lithium salts, phosphorus-containing lithium salts, and sulfur-containing lithium salts; optionally, the boron-containing lithium salt includes one or more of lithium tetrafluoroborate LiBF4, lithium bis(oxalate-borate) borate LiBOB, and lithium bis(oxalate-borate) borate LiDFOB; optionally, the phosphorus-containing lithium salt includes one or more of lithium difluorophosphate LiPO2F2, lithium fluorophosphate Li2PO3F, and lithium phosphate Li3PO4; optionally, the sulfur-containing lithium salt includes one or more of lithium fluorosulfonate LiFSO3, lithium sulfate Li2SO4, and lithium aminosulfonate LiSO3NH2.

[0018] Therefore, the boron-containing lithium salt of this application can form a structurally stable SEI film with relatively low impedance on the surface of the negative electrode active material, thereby improving the high and low temperature performance of the secondary battery. The phosphorus-containing lithium salt can form a Li-rich film on the surface of the negative electrode active material. x PO y F zA LiF-based SEI film exhibits low interfacial impedance, significantly improving the cycle performance of secondary batteries. Sulfur-containing lithium salts can form a dense and stable SEI film on the surface of the negative electrode active material, further enhancing its protective properties.

[0019] In any embodiment, the positive electrode film-forming additive includes carbonate additives and / or sulfate additives; optionally, the carbonate additive includes one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), vinyl ethylene carbonate (VEC), and dioctyl carbonate (CC); optionally, the sulfate additive includes cyclic sulfonate additives and / or sulfated hydrocarbon ester additives; further optionally, the cyclic sulfonate additive includes one or more of 1,3-propanesulfonate lactone (PS), propenesulfonate lactone (PES), and 3-fluoro-1,3-propanesulfonate lactone (FPS); the sulfated hydrocarbon ester additive includes one or more of vinyl sulfate (DTD), diethyl sulfate (DES), and dimethyl sulfate (DMS).

[0020] Therefore, the carbonate additives of this application possess a strong dielectric constant, enabling them to dissolve lithium salts to a greater extent, thus facilitating the dissociation of lithium salts into lithium ions and improving the conductivity of the electrolyte. Their interaction with the first organic solvent allows for better control of the dielectric constant and viscosity of the electrolyte, thereby improving the ionic conductivity of the secondary battery and further enhancing the electrochemical window of the secondary battery. Sulfate additives can form a dense and stable film layer, thus providing excellent protection for the active materials.

[0021] In any embodiment, the electrolyte further includes a lithium salt, comprising one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Optionally, the mass percentage of the lithium salt relative to the total mass of the electrolyte is denoted as d%, with a mass percentage of 5% ≤ d ≤ 25%; more preferably, 10% ≤ d ≤ 20%. The lithium salt can be considered as a complex of lithium ions and anionic groups, which plays a significant role in the electrolyte composition. The aforementioned lithium salts exhibit thermal stability and high electrical conductivity.

[0022] In any embodiment, the separator includes a substrate material layer and a coating disposed on the surface of the substrate material layer. The substrate material of the substrate material layer includes one or more of polyethylene, polypropylene, poly(p-phenylene terephthalamide), polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, and polyamide. The coating includes a ceramic coating and / or a polymer coating. Optionally, the ceramic particles in the ceramic coating include one or more of SiO2, Al2O3, AlOOH, CaO, TiO2, MgO, ZnO, ZrO2, Mg(OH)2, and BaSO4. The polymer material of the polymer coating includes one or more of polyethylene PE, polypropylene PP, poly(p-phenylene terephthalamide) PPTA, polyethylene terephthalate PET, polytetrafluoroethylene PTFE, polyacrylonitrile PAN, polyimide PI, and polyamide PA.

[0023] Therefore, the substrate material layer of this application has good lithium ion permeability, which is beneficial to lithium ion migration; the surface of the substrate material layer is provided with a coating, which can further improve the mechanical properties of the separator.

[0024] A second aspect of this application also provides a battery module including a secondary battery as described in any embodiment of the first aspect of this application.

[0025] A third aspect of this application also provides a battery pack including a battery module as described in the second aspect of this application.

[0026] The fourth aspect of this application also provides an electrical device, including a secondary battery as described in any embodiment of the first aspect of this application, a battery module as described in the second aspect of this application, or a battery pack as described in the third aspect of this application. Attached Figure Description

[0027] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments of this application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0028] Figure 1 This is a schematic diagram of one embodiment of the secondary battery of this application.

[0029] Figure 2 yes Figure 1 An exploded view of the implementation method of the secondary battery.

[0030] Figure 3 This is a schematic diagram of one embodiment of the battery module of this application.

[0031] Figure 4This is a schematic diagram of one embodiment of the battery pack of this application.

[0032] Figure 5 yes Figure 4 An exploded view of an embodiment of the battery pack shown.

[0033] Figure 6 This is a schematic diagram of one embodiment of an electrical device that uses a secondary battery as a power source, as described in this application.

[0034] The accompanying drawings may not be drawn to scale.

[0035] The annotations in the attached figures are explained as follows:

[0036] 1. Battery pack; 2. Upper casing; 3. Lower casing; 4. Battery module;

[0037] 5. Secondary battery; 51. Housing; 52. Electrode assembly;

[0038] 53. Cover plate;

[0039] 6. Electrical appliances. Detailed Implementation

[0040] The following detailed description discloses embodiments of the secondary battery, battery module, battery pack, and power-consuming device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of actually identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0041] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; 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, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0042] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions. Unless otherwise specified, all technical features and optional technical features of this application can be combined to form new technical solutions.

[0043] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0044] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0045] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0046] In this application, the terms "multiple" or "various" refer to two or more kinds.

[0047] In this application, the secondary battery may include lithium-ion batteries, lithium-sulfur batteries, sodium-lithium-ion batteries, sodium-ion batteries, or magnesium-ion batteries, etc., and the embodiments of this application are not limited to this.

[0048] A secondary battery consists of an electrode assembly and an electrolyte. The electrode assembly comprises a positive electrode, a negative electrode, and a separator. The secondary battery primarily functions by the movement of metal ions between the positive and negative electrodes. The positive electrode includes a positive active material, and the negative electrode includes a negative active material. In this paper, the metal ions can be lithium ions, sodium ions, etc.; the charging process will be explained using lithium ions as an example.

[0049] During the charging process of a secondary battery, the electrode kinetics process usually includes the following steps: (1) Lithium ion extraction step: Lithium ions are extracted from the positive electrode active material and migrate into the electrolyte phase; (2) Liquid phase mass transfer step in the electrolyte phase: Solvated lithium ions in the electrolyte diffuse and transfer to the surface of the negative electrode active material; (3) Surface conversion step: During the first charge, solvated lithium ions are adsorbed on the surface of the negative electrode active material and react to form a solid electrolyte interphase (SEI) film. During subsequent charging, solvated lithium ions are adsorbed on the surface of the SEI film. After the desolvation process, lithium ions reach the surface of the negative electrode active material; (4) Charge exchange step: Lithium ions gain electrons from the surface of the negative electrode active material and form lithium intercalation products; (5) Solid phase mass transfer step of lithium intercalation products: Lithium intercalation products diffuse from the surface of the negative electrode active material to the interior in a solid phase, completing the charging process.

[0050] As the charging rate increases, during charging and discharging, lithium ions are required to rapidly extract from the positive electrode active material and embed into the negative electrode active material, and vice versa. In this process, both solid-phase and liquid-phase mass transfer significantly influence the lithium ion migration rate, substantially impacting the fast-charging performance of the rechargeable battery. Furthermore, side reactions may occur between the active material surface and the electrolyte during charging and discharging, damaging the active material's structure and severely shortening its lifespan, potentially leading to a reduction in the cycle life of the rechargeable battery.

[0051] In view of this, the inventors have improved the secondary battery, proposing a secondary battery that improves the fast-charging performance of the secondary battery by improving the migration rate of lithium ions in the solid-phase mass transfer and liquid-phase mass transfer; and by protecting the surface of the active material, the structural stability of the active material is improved, thereby increasing the cycle stability of the secondary battery and thus improving its cycle life. The technical solution of this application will now be described in detail.

[0052] Secondary batteries

[0053] Firstly, this application proposes a secondary battery. A secondary battery, also known as a rechargeable battery or storage battery, refers to a battery that can be recharged after discharge to activate the active materials and continue to be used.

[0054] The secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte; the positive electrode includes a positive active material; the negative electrode includes a negative active material; the separator is disposed between the positive and negative electrodes; the electrolyte includes a first organic solvent and a film-forming additive, the film-forming additive being configured to form an interfacial film on the surface of the positive and / or negative active materials; the porosity of the separator is denoted as ε%; the mass percentage of the film-forming additive relative to the total mass of the electrolyte is denoted as b%; the viscosity of the electrolyte at 25°C is denoted as c (mPa·s), and the secondary battery satisfies 4 ≤ (b*ε) / c ≤ 240.

[0055] Although the mechanism is not entirely clear, this application is able to improve both the fast charging performance and cycle life of secondary batteries. The inventors speculate the reasons are as follows:

[0056] During the migration of lithium ions from the positive electrode to the negative electrode, they first need to migrate from the positive electrode to the separator, and then from the separator to the negative electrode. Therefore, the porosity of the separator has a certain influence on the migration rate of lithium ions. Thus, this application uses the porosity of the separator as one of the variables controlled in this application.

[0057] In the liquid-phase mass transfer process, lithium ions migrate in the electrolyte. The viscosity of the electrolyte has a certain influence on the migration rate of lithium ions; the lower the viscosity, the more favorable the migration of lithium ions. Therefore, the viscosity of the electrolyte has a certain impact on the migration rate of lithium ions, and it is used as another variable controlled in this application. The first organic solvent, as the main component of the electrolyte, has a significant impact on the viscosity of the electrolyte. Therefore, the viscosity of the electrolyte can be controlled by selecting the first organic solvent. Of course, film-forming additives also have a certain influence on the viscosity of the electrolyte. Therefore, the final viscosity of the electrolyte can be determined by controlling the mass percentage of film-forming additives based on the selection of the first organic solvent. When the viscosity of the electrolyte is within a suitable range, it can better wet the separator membrane. During the charging and discharging process of the secondary battery, it is beneficial for the rapid reflux of the electrolyte and facilitates the rapid transport of lithium ions.

[0058] This application regulates the porosity of the separator, the viscosity of the electrolyte, and the film-forming additives to meet the aforementioned ranges, comprehensively controlling the migration rate of lithium ions in both solid-phase and liquid-phase mass transfer. This improves the average migration rate of lithium ions, thereby enhancing the fast-charging capability of the secondary battery. Furthermore, the film-forming additives not only regulate the viscosity of the electrolyte to further adjust the migration rate of lithium ions in the liquid phase, but also form a protective layer on the surface of the active material to passivate the surface, reducing the risk of side reactions between the active material surface and the electrolyte. This improves the structural stability of the active material and extends the cycle life of the secondary battery. The film-forming additives, especially lithium salt additives, form a solid electrolyte interface (SEI) film on the surface of the negative electrode active material with low impedance, facilitating lithium ion insertion and extraction. Of course, other types of additives, such as ester additives, can also be used.

[0059] Optionally, 4 ≤ (b*ε) / c ≤ 180; for example, (b*ε) / c can be 4, 5, 8, 10, 12, 15, 18, 20, 25, 30, 40, 50, 60, 70, 80, 90, 100, 120, 150, 160, 170, 180, 190, 200, 210, 220 or 240; or a range consisting of any two of the above values.

[0060] In some implementations, 25 ≤ ε ≤ 55.

[0061] When the porosity of the separator is within the above-mentioned range, the porosity is not too small, its liquid permeability is good, and its lithium-ion penetration ability is strong, which is conducive to the migration of lithium ions through the separator to adjacent active materials, and can further improve the fast charging capability of the secondary battery. The porosity of the separator is not too large, reducing the risk of short circuits caused by direct contact between the positive and negative electrodes; and the separator has good mechanical properties, so even if dendritic crystals form inside the secondary battery, the separator is not easily punctured, thus ensuring the safety performance of the secondary battery. Optionally, 30 ≤ ε ≤ 50; for example, the porosity ε% of the separator can be 25%, 28%, 30%, 32%, 35%, 38%, 40%, 42%, 45%, 46%, 48%, 50%, 52%, 53%, 54%, or 55%; or any range of any two of the above values.

[0062] In some implementations, 0.1 ≤ b ≤ 8.

[0063] When the mass percentage of the film-forming additive is within the above range, the film-forming additive can react with the active material to form a dense and uniform film layer, thereby providing good protection for the active material. Simultaneously, the film-forming additive, such as lithium salt additives, can form an SEI film with low film-forming resistance, balancing lifetime and power. Optionally, 0.1 ≤ b ≤ 6; exemplaryly, the mass percentage b% of the film-forming additive can be 0.1%, 0.5%, 0.8%, 1%, 1.5%, 1.8%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, or 8%; or a range consisting of any two of the above values.

[0064] In some implementations, 1 ≤ c ≤ 6.

[0065] When the viscosity of the electrolyte is within the above-mentioned range, the viscosity is not excessive, and its ionic conductivity is relatively high, which is beneficial to improving the ion transport rate and thus enhancing the fast-charging capability of the secondary battery. Furthermore, when the viscosity of the electrolyte is within the above-mentioned range, the compatibility between the electrolyte and the positive and negative electrode plates is relatively good, and it is less likely to undergo side reactions with the active materials in the positive and negative electrode plates, thereby improving the cycle stability of the secondary battery. Optionally, 2 ≤ c ≤ 5; for example, the viscosity c (mPa·s) of the electrolyte can be 1 mPa·s, 1.5 mPa·s, 2 mPa·s, 2.5 mPa·s, 3 mPa·s, 3.5 mPa·s, 4 mPa·s, 4.5 mPa·s, 5 mPa·s, 5.5 mPa·s, or 6 mPa·s; or a range consisting of any two of the above values.

[0066] In some embodiments, the mass percentage of the first organic solvent relative to the total mass of the electrolyte is denoted as a; the secondary battery also satisfies: 2≤c+2*a%≤8.

[0067] By adjusting the mass percentage of the first organic solvent, the viscosity of the electrolyte can be adjusted to a suitable range, enabling the formation of a stable interfacial film during film formation that completely covers the surface of the active material. For example, c+2*a% can be 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, or 8; or within any range of two of the above values.

[0068] In some embodiments, 60 ≤ a ≤ 90; alternatively, 65 ≤ a ≤ 85. Exemplarily, the mass percentage a% of the first organic solvent relative to the total mass of the electrolyte can be 60%, 65%, 70%, 75%, 80%, 85%, or 90%; or a range of any two of the above values.

[0069] In some embodiments, the first organic solvent includes one or more of linear carbonate solvents, carboxylic acid ester solvents, and nitrile solvents. These solvents have low viscosity, allowing the overall viscosity range of the electrolyte to be controlled within a suitable range. Furthermore, when the first organic solvent includes two or more types, the viscosity and ionic conductivity of the electrolyte can be synergistically controlled. Moreover, multiple solvents can participate in the film-forming reaction, resulting in a film structure containing multiple components, which improves the stability of the interfacial film.

[0070] As examples of linear carbonate solvents, these linear carbonate solvents include one or more of ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and methyl propyl carbonate (MPC). These linear carbonate solvents have relatively low viscosity, which is beneficial for ionic conductivity; and they also have high electrochemical stability and a wide electrochemical window.

[0071] Examples of carboxylic acid ester solvents include one or more of ethyl acetate (EA), methyl acetate (MA), ethyl propionate (EP), methyl formate (MF), ethyl butyrate (EB), butyl acetate (BA), methyl propionate (MP), methyl butyrate (MB), propyl butyrate (PB), and butyl butyrate (BB). Carboxylic acid ester solvents have relatively lower viscosity, and when multiple carboxylic acid ester solvents are used in combination, the electrolyte can have a lower surface tension.

[0072] As an example of a nitrile solvent, the nitrile solvent includes one or more of acetonitrile (AN), glutaronitrile (GLN), and adiponitrile (ADN). These nitrile solvents have low viscosity and relatively high dielectric constant, which is beneficial for improving the migration rate of lithium ions; moreover, these nitrile solvents have high stability and are less likely to undergo side reactions with the surface of the active material, thus ensuring the structural stability of the active material and consequently guaranteeing the cycle stability of the secondary battery.

[0073] In some embodiments, the electrolyte may further include a second organic solvent, which may include ether solvents, etc. Ether solvents also have relatively low viscosity, which is beneficial for further improving the overall viscosity of the electrolyte; and ether solvents can enable lithium in the secondary battery to maintain a good morphological structure during charge-discharge cycles, which is beneficial for improving the cycle stability of the secondary battery, thereby increasing the cycle life of the secondary battery.

[0074] As an example of an ether solvent, the ether solvent may include diethyl ether, etc.

[0075] In some embodiments, the film-forming additives include a negative electrode film-forming additive and a positive electrode film-forming additive; the negative electrode film-forming additive is configured to form an interface film on the surface of the negative electrode active material, and the mass percentage of the negative electrode film-forming additive relative to the total mass of the electrolyte is denoted as b1%; the positive electrode film-forming additive is configured to form an interface film on the surface of the positive electrode active material, and the mass percentage of the positive electrode film-forming additive relative to the total mass of the electrolyte is denoted as b2%, and the secondary battery satisfies: 1≤b2 / b1≤60.

[0076] Anode film-forming additives can undergo film-forming reactions on the surface of anode active materials, thereby forming an SEI film on the surface of the anode active materials. The SEI film in situ coats the surface of the anode active materials, which can stabilize the structure of the anode active materials and reduce the risk of continuous side reactions between the anode active materials and the electrolyte, thus ensuring the electrochemical performance of the anode active materials. At the same time, anode film-forming additives can include lithium salt additives, and the SEI film formed by them on the anode has low impedance, which is more conducive to the insertion and extraction of lithium ions.

[0077] The cathode film-forming additive can undergo a film-forming reaction on the surface of the cathode active material, thereby forming a cathode electrolyte interphase (CEI) film on the surface of the cathode active material. The CEI film coats the surface of the cathode active material in situ, which can stabilize the structure of the cathode active material and reduce the risk of electrolyte corrosion of the cathode active material due to electrolyte decomposition, which may cause the transition metal in the cathode active material to dissolve. This further protects the cathode active material and ensures its electrochemical performance.

[0078] In this application, CEI membrane and SEI membrane are collectively referred to as interface membrane.

[0079] When the content of the negative electrode film-forming additive and the positive electrode film-forming additive in this application is adjusted to satisfy the above formula, the electrolyte can form an SEI film on the surface of the negative electrode active material and a CEI film on the surface of the positive electrode active material. The interfacial impedance of the two films is relatively low, resulting in good kinetic activity and low charge transfer impedance in the secondary battery, which is beneficial for the rapid migration of lithium ions, thereby improving the fast-charging performance of the secondary battery. Optionally, 1 ≤ b2 / b1 ≤ 40; for example, b2 / b1 can be 1, 2, 3, 5, 8, 10, 15, 20, 22, 25, 28, 30, 32, 35, 40, 42, 45, 48, 50, 52, 55, 58, or 60; or within any range of two of the above values.

[0080] In some implementations, 0.01 ≤ b1 ≤ 1.5.

[0081] When the negative electrode film-forming additive is within the above-mentioned range, it can form a dense and stable SEI film on the surface of the negative electrode active material, thereby providing sufficient protection for the negative electrode active material, improving the structural stability of the negative electrode active material, and thus ensuring the cycle stability of the secondary battery. Optionally, 0.1 ≤ b1 ≤ 1.2. For example, the mass percentage b1% of the negative electrode film-forming additive can be 0.01%, 0.05%, 0.1%, 0.5%, 0.8%, 1%, 1.2%, 1.3%, or 1.5%; or within any two of the above values.

[0082] In some implementations, 0.1 ≤ b2 ≤ 7.

[0083] When the positive electrode film-forming additive is within the above-mentioned range, it can form a dense and stable CEI film on the surface of the positive electrode active material, thereby providing sufficient protection for the positive electrode active material, improving the structural stability of the positive electrode active material, and thus ensuring the cycle stability of the secondary battery. Optionally, 0.5 ≤ b2 ≤ 5. For example, the mass percentage b2% of the positive electrode film-forming additive can be 0.1%, 0.5%, 0.8%, 1%, 1.2%, 1.3%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, or 7%; or within any two of the above values.

[0084] As an example of anode film-forming additives, anode film-forming additives may include one or more of boron-containing lithium salts, phosphorus-containing lithium salts, and sulfur-containing lithium salts.

[0085] Boron-containing lithium salts, as lithium salts with boron atoms as the central atom, can coordinate with alkoxy groups, ortho- and ortho-hydroxy groups, and carboxylic acids to form anionic complexes. These anionic complexes primarily exhibit large π-conjugated structures, with a relatively dispersed negative charge distribution of the central ion. The charge is delocalized, and the anionic radius is relatively large, making it difficult for the anion to form a strong ion pair with lithium ions in organic solvents, resulting in relatively good solubility. The more electron-withdrawing groups in the anionic complex, the more stable the anionic structure, and the higher the solubility of lithium ions in the electrolyte, which is beneficial for improving the electrolyte conductivity. Furthermore, boron-containing lithium salts can form a high-performance SEI film on the surface of the negative electrode active material. The SEI film is insoluble in organic solvents and can exist stably in organic electrolytes, effectively reducing the embedding of solvent molecules into the negative electrode active material. This ensures the structural stability of the negative electrode active material and thus improves the cycle performance of the secondary battery.

[0086] Phosphorus-containing lithium salt additives possess relatively large anionic groups and high ionic conductivity, which is beneficial for further improving the kinetic performance of the electrolyte. Furthermore, these additives can form a CEI film on the surface of the positive electrode active material. This CEI film exhibits high lithium-ion conductivity, significantly inhibiting the continuous decomposition of the electrolyte and reducing the dissolution of transition metal ions from the positive electrode active material, thereby improving the cycle performance of the secondary battery. In addition, phosphorus-containing lithium salt additives can also form an SEI film on the surface of the negative electrode active material, which has low interfacial impedance, thus significantly improving the battery's cycle performance.

[0087] Sulfur-containing lithium salt additives have good antioxidant properties, high thermal stability, are not sensitive to water in the electrolyte, and are not prone to side reactions. In addition, the relatively high conductivity of sulfur-containing lithium salt additives is beneficial to improving the migration rate of lithium ions, thereby improving the kinetic performance of the electrolyte.

[0088] As examples of boron-containing lithium salts, boron-containing lithium salts may include one or more of lithium tetrafluoroborate (LiBF4), lithium bis(C2O4)2 (LiBOB), and lithium bis(fluorooxalate)borate (LiBC2O4F2 (LiDFOB)). Further, boron-containing lithium salts may include compositions of lithium tetrafluoroborate LiBF4, lithium bis(C2O4)2 (LiBOB), and lithium bis(fluorooxalate)borate (LiBC2O4F2 (LiDFOB)).

[0089] When lithium tetrafluoroborate (LiBF4) is used in conjunction with organic solvents such as carbonates or additives in the electrolyte, the system formed by lithium tetrafluoroborate has a relatively low viscosity, which is beneficial for lithium-ion release and thus improves the conductivity of the electrolyte. The SEI film formed by lithium tetrafluoroborate has a more uniform thickness and better kinetic activity, resulting in lower charge transfer resistance in the secondary battery, thereby significantly improving the low-temperature performance of the secondary battery. The SEI film is not prone to thermal decomposition and its performance is more stable at high temperatures, thus significantly improving the high-temperature performance of the secondary battery.

[0090] Either lithium bis(oxalato)borate (LiBOB) or lithium difluorooxalato)borate (LiDFOB) can passivate the positive current collector in the positive electrode sheet, reducing the risk of side reactions that corrode the positive current collector and improving the structural stability of the positive electrode sheet. Furthermore, electrolytes containing either LiBOB or LiDFOB are less likely to generate acidic substances, further reducing the risk of corrosion of the positive current collector. LiBOB and LiDFOB exhibit good compatibility with the positive electrode active material, facilitating lithium ion migration; they can also form an effective SEI film on the surface of the negative electrode active material, improving its protective performance.

[0091] The combined use of lithium tetrafluoroborate (LiBF4), lithium dioxaborate (LiBOB), and lithium dioxaborate (LiDFOB) results in a SEI film with a more diverse composition and greater structural stability. Furthermore, while ensuring structural stability, it also guarantees a relatively low impedance value for the SEI film, thus ensuring the low-temperature performance of the secondary battery.

[0092] When the secondary battery of this application contains the above-mentioned boron-containing lithium salt, a structurally stable SEI film with a relatively low impedance value can be formed on the surface of the negative electrode active material, thereby improving the high and low temperature performance of the secondary battery.

[0093] As an example of a phosphorus-containing lithium salt, it may include one or more of lithium difluorophosphate (LiPO2F2), lithium fluorophosphate (Li2PO3F), and lithium phosphate (Li3PO4).

[0094] The aforementioned lithium phosphate salts are inorganic lithium phosphate salts, which can form Li-rich surfaces on the anode active material. x PO y F zThe SEI film, composed of LiF, exhibits low interfacial impedance, significantly improving the cycle performance of the secondary battery. Furthermore, during the initial charging process, it participates in positive electrode film formation, creating a stable and low-impedance CEI film. This CEI film effectively reduces electrolyte oxidative decomposition, for example, by minimizing side reactions between the carbonate solvent in the electrolyte and the surface of the positive electrode active material. This ensures the structural stability of the electrolyte, mitigates damage to the positive electrode active material, and improves the cycle performance of the secondary battery.

[0095] As an example of a sulfur-containing lithium salt, the sulfur-containing lithium salt may include one or more of lithium fluorosulfonate LiFSO3, lithium sulfate Li2SO4, and lithium aminosulfonate LiSO3NH2.

[0096] The aforementioned sulfur-containing lithium salts can form a dense and stable SEI film on the surface of the negative electrode active material, which can further improve the protection performance of the negative electrode active material.

[0097] In some embodiments, the positive electrode film-forming additive includes carbonate additives and / or sulfate additives.

[0098] The carbonate additives include one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), vinyl ethylene carbonate (VEC), and dioctyl carbonate (CC). These carbonate additives can form a dense and stable film, thus providing good protection for the active materials. Furthermore, these carbonate additives have a strong dielectric constant, which allows for greater dissolution of lithium salts, making it easier for lithium salts to dissociate into lithium ions and improving the conductivity of the electrolyte. In combination with the first organic solvent, they can better control the dielectric constant and viscosity of the electrolyte, thereby improving the ionic conductivity of the secondary battery and further enhancing the electrochemical window of the secondary battery.

[0099] The sulfate ester additives include cyclic sulfonate additives and / or sulfate hydrocarbon ester additives; further, the cyclic sulfonate additives include one or more of 1,3-propanesulfonate lactone (PS), propenesulfonate lactone (PES), and 3-fluoro-1,3-propanesulfonate lactone (FPS); the sulfate hydrocarbon ester additives include one or more of vinyl sulfate (DTD), diethyl sulfate (DES), and dimethyl sulfate (DMS). The above-mentioned sulfate ester additives can form a dense and stable film layer, thereby providing good protection for the active materials.

[0100] In some embodiments, the electrolyte may also include a lithium salt. The lithium salt includes one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0101] Lithium salts can be considered as complexes of lithium ions and anions, and they play a significant role in the electrolyte. The aforementioned lithium salts exhibit thermal stability and high electrical conductivity.

[0102] In some embodiments, the mass percentage of lithium salt relative to the total mass of the electrolyte is denoted as d%, 5% ≤ d ≤ 25%; further optionally, 10% ≤ d ≤ 20%.

[0103] When the mass percentage of lithium salt is within the above-mentioned range, it is beneficial for the rapid migration of lithium ions. Furthermore, the lithium salt can form an interfacial film on the surface of the active material together with other substances, thereby stabilizing the active material. Optionally, 10% ≤ d ≤ 20%; for example, the mass percentage of lithium salt d% can be 5%, 6%, 7%, 8%, 9%, 10%, 12%, 13%, 15%, 16%, 18%, 20%, 21%, 22%, 24%, or 25%; or within any two of the above value ranges.

[0104] The electrolyte of this application can be prepared using methods conventional in the art. For example, the additives, solvent, and electrolyte salt can be mixed evenly to obtain the electrolyte. There are no particular restrictions on the order in which the materials are added; for example, the additives and electrolyte salt can be added to the non-aqueous solvent and mixed evenly to obtain a non-aqueous electrolyte.

[0105] In this application, the components and their contents in the electrolyte can be determined according to methods known in the art. For example, they can be determined by gas chromatography-mass spectrometry (GC-MS), ion chromatography (IC), liquid chromatography (LC), nuclear magnetic resonance spectroscopy (NMR), etc.

[0106] It should be noted that during the electrolyte testing of this application, freshly prepared electrolyte can be used directly, or electrolyte can be obtained from a secondary battery. An exemplary method for obtaining electrolyte from a secondary battery includes the following steps: discharging the secondary battery to the discharge cutoff voltage (for safety, the battery is generally left fully discharged), followed by centrifugation. A suitable amount of the centrifuged liquid is then taken as the non-aqueous electrolyte. Alternatively, the non-aqueous electrolyte can be obtained directly from the secondary battery's filling port.

[0107] [Positive electrode plate]

[0108] In some embodiments, the positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector. For example, the positive current collector has two surfaces opposite each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0109] The positive electrode film layer includes a positive electrode active material, which may be a positive electrode active material known in the art for use in secondary batteries. For example, the positive electrode active material may include at least one of lithium transition metal oxides, lithium-containing phosphates with an olivine structure, and their respective modified compounds. Examples of lithium transition metal oxides may include 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 lithium-containing phosphates with an olivine structure may include at least one of lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, lithium manganese iron phosphate and carbon composites, and their respective modified compounds. This application is not limited to these materials, and other conventionally known materials that can be used as positive electrode active materials for secondary batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more.

[0110] In some embodiments, the positive electrode active material includes LiNi x Co y Mn z M 1-x-y-z O2 or LiNi a Co b Al c N 1-a-b-c O2, wherein M and N are each independently selected from any one of Co, Ni, Mn, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V, and Ti, and 0≤y≤1, 0≤x<1, 0≤z≤1, x+y+z≤1, 0≤a≤1, 0≤b≤1, 0≤c≤1, a+b+c≤1. When the positive electrode active material is used in conjunction with boron-containing lithium salts, the B atoms in the boron-containing lithium salts readily combine with the O atoms in the positive electrode active material, thereby reducing the charge transfer resistance of the positive electrode active material and reducing the diffusion resistance of lithium ions within the bulk phase of the positive electrode active material. Therefore, when the non-aqueous electrolyte contains appropriate amounts of lithium tetrafluoroborate and lithium difluorooxalate borate, low-cobalt or cobalt-free cathode active materials exhibit significantly improved lithium-ion diffusion rates. Lithium ions within the bulk phase of the low-cobalt or cobalt-free cathode active material can be promptly replenished to the surface, preventing excessive delithiation and thus stabilizing the crystal structure. Because the crystal structure of low-cobalt or cobalt-free cathode active materials is more stable, the probability of instability in the structural, chemical, or electrochemical properties of the cathode active material due to excessive delithiation on its surface is greatly reduced. For example, this can lead to irreversible distortion and increased lattice defects.

[0111] LiNi x Coy Mn z M 1-x-y-z O2 or LiNi a Co b Al c N 1-a-b-c O2 can be prepared according to conventional methods in the art. An exemplary preparation method is as follows: a lithium source, a nickel source, a cobalt source, a manganese source, an aluminum source, an M-element precursor, and an N-element precursor are mixed and then sintered. The sintering atmosphere can be an oxygen-containing atmosphere, such as an air atmosphere or an oxygen atmosphere. The O2 concentration in the sintering atmosphere is, for example, 70% to 100%. The sintering temperature and sintering time can be adjusted according to actual conditions.

[0112] As examples, lithium sources include, but are not limited to, at least one of lithium oxide (Li₂O), lithium phosphate (Li₃PO₄), lithium dihydrogen phosphate (LiH₂PO₄), lithium acetate (CH₃COOLi), lithium hydroxide (LiOH), lithium carbonate (Li₂CO₃), and lithium nitrate (LiNO₃). As examples, nickel sources include, but are not limited to, at least one of nickel sulfate, nickel nitrate, nickel chloride, nickel oxalate, and nickel acetate. As examples, cobalt sources include, but are not limited to, at least one of cobalt sulfate, cobalt nitrate, cobalt chloride, cobalt oxalate, and cobalt acetate. As examples, manganese sources include, but are not limited to, at least one of manganese sulfate, manganese nitrate, manganese chloride, manganese oxalate, and manganese acetate. As examples, aluminum sources include, but are not limited to, at least one of aluminum sulfate, aluminum nitrate, aluminum chloride, aluminum oxalate, and aluminum acetate. As examples, precursors of element M include, but are not limited to, at least one of oxides, nitrate compounds, carbonate compounds, hydroxides, and acetate compounds of element M. As an example, precursors of nitrogen include, but are not limited to, at least one of ammonium fluoride, lithium fluoride, hydrogen fluoride, ammonium chloride, lithium chloride, hydrogen chloride, ammonium nitrate, ammonium nitrite, ammonium carbonate, ammonium bicarbonate, ammonium phosphate, phosphoric acid, ammonium sulfate, ammonium bisulfate, ammonium bisulfite, ammonium sulfite, ammonium hydrogen sulfide, hydrogen sulfide, lithium sulfide, ammonium sulfide, and elemental sulfur.

[0113] In some embodiments, based on the total mass of the positive electrode film, the molecular formula is LiNi. x Co y Mn z M 1-x-y-z O2 or LiNi a Co b Al c N 1-a-b-c The layered material containing O2 has a mass percentage of 80% to 99%. For example, the molecular formula LiNi x Co y Mn z M 1-x-y-z O2 or LiNi a Co b Alc N 1-a-b-c The mass percentage of the O2 layered material can be any range consisting of 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or higher. Optionally, the molecular formula is LiNi. x Co y Mn z M 1-x-y-z O2 or LiNi a Co b Al c N 1-a-b-c The mass percentage of layered O2 is 85% to 99%, 90% to 99%, 95% to 99%, 80% to 98%, 85% to 98%, 90% to 98%, 95% to 98%, 80% to 97%, 85% to 97%, 90% to 97%, or 95% to 97%.

[0114] In some embodiments, the positive electrode film may optionally include a positive electrode conductive agent. This application does not impose any particular limitation on the type of positive electrode conductive agent. As an example, the positive electrode conductive agent includes one or more combinations selected from superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, the mass percentage of the positive electrode conductive agent is less than 5% based on the total mass of the positive electrode film.

[0115] In some embodiments, the positive electrode film layer may optionally include a positive electrode binder. This application does not impose any particular limitation on the type of positive electrode binder. As an example, the positive electrode binder may include one or more combinations selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins. In some embodiments, the mass percentage of the positive electrode binder is less than 5% based on the total mass of the positive electrode film layer.

[0116] In some embodiments, the positive current collector may be a metal foil or a composite current collector. Examples of metal foils include aluminum foil or aluminum alloy foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. Examples of the metal material include one or more combinations selected from aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. The polymer substrate may include one or more combinations selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0117] The positive electrode film is typically formed by coating a positive electrode slurry onto a positive electrode current collector, followed by drying and cold pressing. The positive electrode slurry is usually formed by dispersing the positive electrode active material, optional conductive agent, optional binder, and any other components in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP), but is not limited to it.

[0118] [Negative electrode plate]

[0119] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0120] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0121] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, and lithium-aluminum alloys, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0122] In some embodiments, the negative electrode film layer may optionally include a negative electrode binder. This application does not impose any particular limitation on the type of negative electrode binder. As an example, the negative electrode binder may include one or more combinations selected from styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, waterborne acrylic resins (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS). In some embodiments, the mass percentage of the negative electrode binder is less than 5% based on the total mass of the negative electrode film layer.

[0123] In some embodiments, the negative electrode film layer may optionally include a negative electrode conductive agent. This application does not impose any particular limitation on the type of negative electrode conductive agent. As an example, the negative electrode conductive agent may include one or more combinations selected from superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, the mass percentage of the negative electrode conductive agent is less than 5% based on the total mass of the negative electrode film layer.

[0124] In some embodiments, the negative electrode film may optionally include other additives. As an example, other additives may include thickeners, such as sodium carboxymethyl cellulose (CMC-Na), PTC thermistor materials, etc. In some embodiments, the mass percentage of other additives is less than 2% based on the total mass of the negative electrode film.

[0125] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. Examples of metal foils include copper foil or copper alloy foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. Examples of the metal material include one or more combinations selected from copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. The polymer substrate may include one or more combinations selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0126] The negative electrode film is typically formed by coating a negative electrode slurry onto a negative electrode current collector, followed by drying and cold pressing. The negative electrode slurry is usually formed by dispersing the negative electrode active material, optional conductive agent, optional binder, and other optional additives in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP) or deionized water, but is not limited to these.

[0127] The negative electrode sheet does not exclude other additional functional layers besides the negative electrode film layer. For example, in some embodiments, the negative electrode sheet also includes a protective layer covering the surface of the negative electrode film layer.

[0128] [Isolation membrane]

[0129] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0130] In some embodiments, the separator includes a substrate material layer and a coating disposed on the surface of the substrate material layer; the substrate material of the substrate material layer includes one or more of polyethylene, polypropylene, poly(p-phenylene terephthalamide), polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, and polyamide; the coating includes a ceramic coating and / or a polymer coating.

[0131] The substrate material layer has good permeability to lithium ions, which is beneficial to the migration of lithium ions; the surface of the substrate material layer is coated, which can further improve the mechanical properties of the separator.

[0132] Optionally, the ceramic particles in the ceramic coating include one or more of SiO2, Al2O3, AlOOH, CaO, TiO2, MgO, ZnO, ZrO2, Mg(OH)2, and BaSO4.

[0133] Optionally, the polymer material of the polymer coating includes one or more of polyethylene (PE), polypropylene (PP), poly(p-phenylene terephthalamide) (PPTA), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN), polyimide (PI), and polyamide (PA). The polymer coating can be made of the same or different material as the substrate material layer, and the thicknesses of the polymer coating and the substrate material layer can be different. Optionally, the thickness of the polymer coating is less than the thickness of the substrate material layer.

[0134] In other embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0135] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0136] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0137] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0138] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. Figure 1 This is an example of a square-structured secondary battery 5.

[0139] In some embodiments, such as Figure 1 and Figure 2As shown, the outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 is used to cover the opening to close the receiving cavity. Positive electrode sheets, negative electrode sheets, and a separator may be formed into an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be adjusted according to requirements.

[0140] The method for preparing the secondary battery described in this application is well known. In some embodiments, a positive electrode, a separator, a negative electrode, and an electrolyte can be assembled to form a secondary battery. As an example, the positive electrode, separator, and negative electrode can be formed into an electrode assembly through a winding or stacking process. The electrode assembly is then placed in an outer packaging, dried, and injected with an electrolyte. After vacuum sealing, settling, formation, and shaping, a secondary battery is obtained.

[0141] In some embodiments of this application, the secondary battery according to this application can be assembled into a battery module. The number of secondary batteries contained in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module.

[0142] Figure 3 This is a schematic diagram of battery module 4 as an example. Figure 3 As shown, in battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0143] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0144] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0145] Figure 4 and Figure 5 This is a schematic diagram of battery pack 1 as an example. Figure 4 and Figure 5 As shown, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3. The upper body 2 covers the lower body 3, forming a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0146] Electrical appliances

[0147] Secondly, this application provides an electrical device, which includes at least one of the secondary battery, battery module, and battery pack described in this application. The secondary battery, battery module, and battery pack can be used as the power source for the electrical device or as the energy storage unit of the electrical device. The electrical device can be, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0148] Electrical devices can be equipped with secondary batteries, battery modules, or battery packs depending on their usage requirements.

[0149] Figure 6 This is a schematic diagram of an example electrical device. The electrical device 6 is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of this electrical device, a battery pack 1 or a battery module can be used.

[0150] Another example of an electrical device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use rechargeable batteries as their power source.

[0151] Example

[0152] The following embodiments describe the disclosure of this application in more detail. These embodiments are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of the disclosure of this application. 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 by conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available.

[0153] Example 1

[0154] 1. Preparation of positive electrode sheet

[0155] Aluminum foil with a thickness of 12μm was used as the positive electrode current collector.

[0156] LiNi, the positive electrode active material 0.65 Co 0.07 Mn 0.28Conductive agent carbon black and binder polyvinylidene fluoride (PVDF) are mixed in an appropriate amount of solvent NMP at a weight ratio of 97.5:1.4:1.1 to form a uniform positive electrode slurry. The positive electrode slurry is uniformly coated on the surface of the positive electrode current collector aluminum foil, and after drying and cold pressing, the positive electrode sheet is obtained.

[0157] 2. Preparation of negative electrode sheet

[0158] A copper foil with a thickness of 8μm was used as the negative electrode current collector.

[0159] The negative electrode active material graphite, the binder styrene-butadiene rubber (SBR), the thickener sodium carboxymethyl cellulose (CMC-Na), and the conductive agent carbon black (Super P) are mixed thoroughly in an appropriate amount of deionized water at a weight ratio of 96.2:1.8:1.2:0.8 to form a uniform negative electrode slurry. The negative electrode slurry is then uniformly coated onto the surface of the negative electrode current collector copper foil. After drying and cold pressing, the negative electrode sheet is obtained.

[0160] 3. Separating membrane

[0161] Porous polyethylene (PE) membrane is used as the separator.

[0162] 4. Preparation of electrolyte

[0163] In an environment with a water content of less than 10 ppm, non-aqueous organic solvents ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate are mixed in a volume ratio of 1:1:1 to obtain the electrolyte solvent. Additives are then dissolved in the mixed solvent to prepare an electrolyte with a lithium salt concentration of 1 mol / L. The specific substances contained in the electrolyte are shown in the table below.

[0164] 5. Preparation of secondary batteries

[0165] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. The electrode assembly is then wound up. The electrode assembly is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a lithium-ion battery is obtained.

[0166] Example 2

[0167] Examples 2-1 to 2-8

[0168] The secondary battery was prepared in a similar manner to that in Example 1, except that the porosity ε% of the "separator membrane" was adjusted and 4≤(b*ε) / c≤240. For specific parameters, please refer to Tables 1 to 3.

[0169] Comparative Example

[0170] Comparative Example 1 and Comparative Example 2

[0171] The secondary battery was prepared in a similar manner to that in Example 1, except that the porosity ε% of the "separation membrane" was adjusted. Specific parameters are detailed in Tables 1 to 3.

[0172] Example 3

[0173] Examples 3-1 to 3-7

[0174] The secondary battery was prepared in a similar manner to that in Example 1, except that the mass percentage of the "negative electrode film-forming additive" was adjusted to b1%. Specific parameters are detailed in Tables 1 to 3.

[0175] Example 4

[0176] Examples 4-1 to 4-7

[0177] The secondary battery was prepared in a similar manner to that in Example 1, except that the mass percentage of the "positive electrode film-forming additive" was adjusted to b2%. Specific parameters are detailed in Tables 1 to 3.

[0178] Example 5

[0179] Examples 5-1 to 5-3

[0180] The secondary battery was prepared in a similar manner to that in Example 1, except that the mass percentage of the "first organic solvent" was adjusted to a%. Specific parameters are detailed in Tables 1 to 3.

[0181] Table 1

[0182]

[0183]

[0184] In Table 1, b1 = b11 + b12 + b13.

[0185] Table 2

[0186]

[0187] In Table 2, b2 = b21 + b22.

[0188] Table 3

[0189]

[0190] In Table 3, b = b1 + b2.

[0191] Test section

[0192] 1. Test method for the porosity ε% of the isolation membrane

[0193] According to GB / T24586, the gas displacement method is used for measurement. Porosity ε=(V1-V2) / V1*100%, where V1 is the apparent volume of the sample and V2 is the actual volume of the sample.

[0194] 2. Methods for testing the content of each component in the electrolyte

[0195] Freshly prepared electrolyte can be collected, or electrolyte can be obtained from a secondary battery. Then, the composition of the electrolyte can be determined using one or more of the following methods: gas chromatography-mass spectrometry (GC-MS), ion chromatography (IC), liquid chromatography (LC), nuclear magnetic resonance spectroscopy (NMR), etc.

[0196] Gas chromatography-mass spectrometry (GC-MS): In accordance with GB / T-9722-2006 / GB / T6041-2002, gas chromatography and mass spectrometry are coupled. After the components in the sample are separated by gas chromatography, the components are broken into ion fragments in mass spectrometry and separated according to the mass-to-charge ratio (m / z) to form a specific mass spectrum, which is used to obtain the qualitative analysis of each organic component in the electrolyte. Then, the organic components in the electrolyte are separated in the chromatographic column and the detection signal spectrum of each component is generated. The retention time is used for component qualitative analysis, and the peak area is corrected by standardization to achieve quantification, thus obtaining the quantitative test analysis of the organic components in the electrolyte.

[0197] Ion chromatography (IC): In accordance with JY / T-020, the anions of lithium salts and lithium salt additives in the electrolyte are detected and quantified by ion chromatography.

[0198] Nuclear magnetic resonance spectroscopy (NMR): According to JY / T 0578-2020, qualitative and quantitative analysis of the components in the electrolyte is obtained.

[0199] 3. Test method for the viscosity c (mPa·s) of electrolyte at 25℃

[0200] At a certain temperature, the shear force experienced by the rotor as it rotates continuously at a constant speed within the sample causes the spring to generate torque. This torque is proportional to the viscosity, thus yielding the viscosity value. Specifically, a Bollefeld (DV-2TLV) viscometer was used to test the viscosity of the finished electrolyte. The ambient temperature was controlled at 25℃, and the ambient humidity <80%. 30 mL of electrolyte was taken and kept at a constant temperature in a 25℃ water bath for at least 30 minutes. The rotor was placed in the sample cup, and the sample was added to approximately 0.3 cm from the rim. The connected viscometer was started, and a speed of 70 RPM was selected for testing. Ten data points were collected, and the average value was calculated.

[0201] 4. Cycle performance test of secondary batteries

[0202] At 45℃, the secondary battery was charged at a constant current of 1C to 4.3V, and then charged at a constant voltage until the current reached 0.05C. At this point, the secondary battery was fully charged, and the charging capacity was recorded, which is the first charge capacity. After the secondary battery was left to stand for 5 minutes, it was discharged at a constant current of 1C to 2.8V. This completes one charge-discharge cycle, and the discharge capacity was recorded, which is the first discharge capacity. The secondary battery was subjected to cyclic charge-discharge tests using the above method, and the discharge capacity after each cycle was recorded. The capacity retention rate (%) of the secondary battery after 600 cycles at 45℃ = discharge capacity after 600 cycles / discharge capacity of the first cycle × 100%.

[0203] 5. Fast charging performance of secondary batteries

[0204] At 25°C, the prepared secondary battery was charged at a constant current of 0.33C to the charging cutoff voltage of 4.4V, then charged at a constant voltage to a current of 0.05C, left to stand for 5 minutes, and then discharged at a constant current of 0.33C to the discharge cutoff voltage of 2.8V. Its actual capacity was recorded as C0.

[0205] Then, the secondary battery was sequentially charged at a constant current of 0.5C0, 1C0, 1.5C0, 2C0, 2.5C0, 3C0, 3.5C0, 4C0, and 4.5C0 until the full battery charging cutoff voltage of 4.4V or the negative terminal cutoff potential of 0V (whichever comes first). After each charging, it was discharged at 1C0 until the full battery discharge cutoff voltage of 2.8V. The state of charge (SOC) was recorded at different charging rates until 10%, 20%, 30%...80%. By plotting the negative electrode potential corresponding to the state of charge (SOC), rate-negative electrode potential curves are generated for different SOC states. Linear fitting yields the charging rate corresponding to a negative electrode potential of 0V at each SOC state. This charging rate is the charging window for that SOC state, denoted as C10%SOC, C20%SOC, C30%SOC, C40%SOC, C50%SOC, C60%SOC, C70%SOC, and C80%SOC. The charging time T from 10% SOC to 80% SOC is calculated using the formula (60 / C20%SOC + 60 / C30%SOC + 60 / C40%SOC + 60 / C50%SOC + 60 / C60%SOC + 60 / C70%SOC + 60 / C80%SOC) × 10%. A shorter charging time T indicates better fast-charging performance of the secondary battery.

[0206] Test Results

[0207] The effects of this application on improving the cycle performance and fast charging performance of secondary batteries are shown in Table 4.

[0208] Table 4

[0209]

[0210]

[0211] As shown in Table 4, the low porosity of the separator in Comparative Example 1 may result in a low migration rate of lithium ions during the process of passing through the separator, thus leading to poor fast charging capability of the secondary battery. By synergistically controlling the porosity of the separator and the mass percentage of film-forming additives, the solid-phase transport rate of lithium ions can be adjusted; by controlling the viscosity of the electrolyte, the liquid-phase transport rate of lithium ions can be adjusted. Compared to Comparative Example 2, Examples 1 to 2-8 simultaneously synergistically controlled the porosity of the separator, the mass percentage of film-forming additives, and the viscosity of the electrolyte to ensure 4 ≤ (b*ε) / c ≤ 240, especially 4 ≤ (b*ε) / c ≤ 180, which significantly improved the overall transport rate of lithium ions in both the solid and liquid phases, thereby improving the fast charging capability of the secondary battery. Furthermore, the additives in the electrolyte can form a film on the surface of the negative electrode active material, thereby improving the protective performance of the negative electrode active material and improving the cycle performance of the secondary battery.

[0212] Examples 3-1 to 3-7 show that by controlling the mass percentage b1% of the negative electrode film-forming additive, when 1≤b2 / b1≤60, especially when 1≤b2 / b1≤40, the protective ability of the negative electrode active material can be adjusted, thereby improving the cycle performance of the secondary battery.

[0213] Examples 4-1 to 4-7 show that by controlling the mass percentage b1% of the positive electrode film-forming additive, when 1≤b2 / b1≤60, especially when 1≤b2 / b1≤40, the protective ability of the positive electrode active material can be adjusted, thereby improving the cycle performance of the secondary battery.

[0214] Examples 5-1 and 5-3 show that by controlling the mass percentage a% of the first organic solvent, when 2≤c+2*a%≤8, the overall migration rate of lithium ions can be controlled, thereby improving the kinetic performance of the secondary battery.

[0215] Although this application has been described with reference to preferred embodiments, various modifications can be made thereto and components can be replaced with equivalents without departing from the scope of this application. In particular, the technical features mentioned in the various embodiments can be combined in any manner, provided there is no structural conflict. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A secondary battery, comprising: Positive electrode sheet, which includes positive electrode active material; Negative electrode sheet, which includes negative electrode active material; A separator is disposed between the positive electrode and the negative electrode; as well as An electrolyte comprising a first organic solvent and a film-forming additive, the film-forming additive being configured to form an interfacial film on the surface of the positive electrode active material and / or the negative electrode active material, wherein the first organic solvent comprises one or more of linear carbonate solvents, carboxylic acid ester solvents, and nitrile solvents; Wherein, the porosity of the isolation membrane is denoted as ε%, 30≤ε≤50; the mass percentage of the film-forming additive relative to the total mass of the electrolyte is denoted as b%; the viscosity of the electrolyte at 25°C is denoted as c, mPa·s, 2≤c≤5; The secondary battery satisfies: 4≤(b*ε) / c≤240; The mass percentage of the first organic solvent relative to the total mass of the electrolyte is denoted as a%; The secondary battery also satisfies: 2≤c+2*a%≤8; The film-forming additives include negative electrode film-forming additives and positive electrode film-forming additives; The negative electrode film-forming additive includes boron-containing lithium salt, phosphorus-containing lithium salt and sulfur-containing lithium salt. The boron-containing lithium salt includes lithium difluorooxalate borate LiDFOB and / or lithium tetrafluoroborate LiBF4. The phosphorus-containing lithium salt includes lithium difluorophosphate LiPO2F2 and / or lithium phosphate Li3PO4. The sulfur-containing lithium salt includes lithium fluorosulfonate LiFSO3. The positive electrode film-forming additive includes carbonate additives and sulfate additives. The carbonate additives include fluoroethylene carbonate (FEC) and / or vinylene carbonate (VC). The sulfate additives include one or more of vinyl sulfate (DTD), 1,3-propanesulfonate lactone (PS), and dimethyl sulfate (DMS). The negative electrode film-forming additive is configured to form an interfacial film on the surface of the negative electrode active material, and the mass percentage of the negative electrode film-forming additive relative to the total mass of the electrolyte is denoted as b1%. The positive electrode film-forming additive is configured to form an interfacial film on the surface of the positive electrode active material, and the mass percentage of the positive electrode film-forming additive relative to the total mass of the electrolyte is denoted as b2%. The secondary battery satisfies: 1≤b2 / b1≤60.

2. The secondary battery according to claim 1, wherein, The secondary battery also satisfies at least one of conditions (1) to (3): (1) 32≤ε≤48; （2）0.1≤b≤8； （3）2.5≤c≤4.5。 3. The secondary battery according to claim 1 or 2, wherein, 60≤a≤90。 4. The secondary battery according to any one of claims 1 to 2, wherein, The linear carbonate solvents include one or more of ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and methyl propyl carbonate (MPC). The carboxylic acid ester solvents include one or more of ethyl acetate (EA), methyl acetate (MA), ethyl propionate (EP), methyl formate (MF), ethyl butyrate (EB), butyl acetate (BA), methyl propionate (MP), methyl butyrate (MB), propyl butyrate (PB), and butyl butyrate (BB). The nitrile solvents include one or more of acetonitrile (AN), glutaronitrile (GLN), and adiponitrile (ADN).

5. The secondary battery according to claim 1, wherein, 0.01≤b1≤1.5; and / or 0.1≤b2≤7.

6. The secondary battery according to any one of claims 1 to 2, wherein, The electrolyte also includes lithium salts, which include one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The mass percentage of the lithium salt relative to the total mass of the electrolyte is denoted as d%, 5%≤d≤25%.

7. The secondary battery according to any one of claims 1 to 2, wherein, The separator includes a substrate material layer and a coating disposed on the surface of the substrate material layer. The substrate material of the substrate material layer includes one or more of polyethylene, polypropylene, poly(p-phenylene terephthalamide), polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, and polyamide; and / or the coating includes a ceramic coating and / or a polymer coating. The ceramic particles in the ceramic coating include one or more of SiO2, Al2O3, AlOOH, CaO, TiO2, MgO, ZnO, ZrO2, Mg(OH)2, and BaSO4; The polymer material of the polymer coating includes one or more of polyethylene (PE), polypropylene (PP), poly(p-phenylene terephthalamide) (PPTA), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN), polyimide (PI), and polyamide (PA).

8. A battery module comprising a secondary battery as described in any one of claims 1 to 7.

9. A battery pack comprising the battery module as claimed in claim 8.

10. An electrical device comprising a secondary battery as claimed in any one of claims 1 to 7, a battery module as claimed in claim 8, or a battery pack as claimed in claim 9.

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