Sodium-ion secondary battery, positive electrode slurry of sodium-ion secondary battery, and electric device

Abstract

Provided in the present application is a sodium-ion secondary battery. The sodium-ion secondary battery comprises a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on the positive electrode current collector, the positive electrode film layer comprising a positive electrode active material and a binder. The binder comprises a copolymer, and the copolymer comprises a first structural unit as represented by formula I and a second structural unit as represented by formula II, wherein R11, R12, R13, R14, R21, R22 and R23 are each independently selected from hydrogen and an unsubstituted or substituted C1-3 alkyl, with the substituent of the substituted C1-3 alkyl being selected from at least one of an ester group, a carboxyl salt, an acid anhydride, a hydroxyl, an amido and a carboxyl; and R24 is selected from hydrogen, an inorganic cation, and an unsubstituted or substituted C1-3 alkyl, with the substituent of the substituted C1-3 alkyl being selected from at least one of an ester group, a carboxyl salt, an acid anhydride, a hydroxyl, an amido and a carboxyl.

Description

Sodium-ion secondary batteries, sodium-ion secondary battery positive electrode slurry and electrical devices

[0001] Cross-references

[0002] This application incorporates Chinese Patent Application No. 202411448461.5, filed on October 16, 2024, entitled “Sodium-ion secondary battery, sodium-ion secondary battery positive electrode slurry and electrical device”, which is incorporated herein by reference in its entirety. Technical Field

[0003] This application relates to the field of sodium-ion battery technology, and in particular to a sodium-ion secondary battery positive electrode slurry and an electrical device for sodium-ion secondary batteries. Background Technology

[0004] Sodium-ion battery cathode materials primarily use sodium carbonate as the sodium source during production, but sodium carbonate has a high alkalinity. Common cathode systems mainly use polyvinylidene fluoride (PVDF) as a binder. While PVDF provides strong bonding strength and electrochemical stability, its alkali resistance is poor. Furthermore, PVDF undergoes an elimination reaction under alkaline sodium ion conditions, generating water. The resulting double bonds cause cross-linking between PVDF molecular chains, forming a gel that severely impacts battery processing performance and cell performance. Additionally, using PVDF as a binder results in poor flexibility of the cathode sheet. Moreover, the EU and other countries have implemented fluorine bans, explicitly restricting the use of fluorine. Therefore, there is a desire to find a fluorine-free or low-fluorine binder applicable to sodium-ion battery cathode systems, resulting in cathode sheets with improved flexibility. Summary of the Invention

[0005] This application is made in view of the above-mentioned problems, and its purpose is to provide a fluorine-free or low-fluorine binder that can be applied to the positive electrode system of sodium-ion batteries, so that the positive electrode sheet containing the binder has good flexibility.

[0006] The inventors have discovered that by adopting the technical solution of this application, the above-mentioned objectives can be achieved.

[0007] A first aspect of this application provides a sodium-ion secondary battery, the sodium-ion secondary battery comprising a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive electrode film layer disposed on the positive current collector, the positive electrode film layer comprising a positive electrode active material and a binder, the binder comprising a copolymer, the copolymer containing a first structural unit shown in Formula I and a second structural unit shown in Formula II.

[0008] Where R 11 R 12 R 13 R 14R 21 R 22 R 23 Each is independently selected from hydrogen, unsubstituted or substituted C. 1-3 Alkyl, the substituted C 1-3 The substituent group in the alkyl group is selected from at least one of ester group, carboxyl salt, acid anhydride, hydroxyl group, amide group, and carboxyl group;

[0009] R 24 Selected from hydrogen, inorganic cations, unsubstituted or substituted C 1-3 Alkyl, the substituted C 1-3 The substituent group in the alkyl group is selected from at least one of ester group, carboxyl salt, acid anhydride, hydroxyl group, amide group, and carboxyl group.

[0010] When using the sodium-ion secondary battery of this application, the use of fluorine is reduced, and the positive electrode has better flexibility.

[0011] In any implementation, R 11 R 12 R 13 R 14 Each is independently selected from hydrogen and unsubstituted C. 1-3 Alkyl; and / or the R 21 R 22 R 23 R 24 Each is independently selected from hydrogen and unsubstituted C. 1-3 alkyl.

[0012] Hydrogen, unsubstituted C 1-3 Alkyl groups are all saturated groups or segments. Choosing these groups avoids introducing easily oxidized and reduced groups into the system and prevents side reactions. In addition, weakly polar groups are beneficial to improving the flexibility of the binder.

[0013] In any embodiment, the copolymer further includes a third structural unit derived from one or more of maleic anhydride (MHA) monomers and glycidyl methacrylate (GMA) monomers.

[0014] The third structural unit includes polar groups and chain segments. Appropriate addition of these units can further increase the polarity of the binder to a certain extent, thereby increasing the adhesive force of the binder and improving the performance of the electrode.

[0015] In any implementation, the mass percentage of the first structural unit is 70%-80%, the mass percentage of the second structural unit is 20%-30%, and the mass percentage of the third structural unit is 0%-5%, calculated based on the total mass of the first structural unit, the second structural unit, and the third structural unit.

[0016] The first structural unit has the largest proportion, which avoids the binder containing too many polar groups, avoids the occurrence of side reactions, and is conducive to improving the flexibility of the binder and the electrode. The second and third structural units have appropriate proportions, and the polar groups or segments introduced by them can improve the bonding strength of the binder within a certain range, and avoid the problems of slurry gelation and poor electrode flexibility caused by excessive polarity.

[0017] In any implementation, the mass percentage of the third structural unit is 0.5%-2%, calculated based on the total mass of the first, second, and third structural units.

[0018] When the copolymer also includes a third structural unit comprising 0.5%-2% by mass, the introduced appropriate amount of polar groups or segments can improve the bonding strength of the binder within a certain range, and avoid the problems of slurry gelation and reduced electrode flexibility caused by excessive polarity, resulting in a positive electrode film layer with high bonding strength and low film resistance. In any embodiment, the copolymer includes one or more of ethylene-acrylate-maleic anhydride copolymer, ethylene-acrylate-glycidyl methacrylate copolymer, ethylene-acrylate copolymer, and ethylene-acrylate-methacrylic acid copolymer.

[0019] The first structural unit, as the main skeleton of the binder, primarily provides the binder's flexibility, and its weak polarity prevents subsequent side reactions in the battery. The second structural unit is a polar segment; an appropriate amount of polar segments can enhance the binder's bonding effect and improve the adhesion of the electrode. The introduction of the third structural unit, which contains polar groups, further increases the interaction between the binder and the current collector, thus improving the bonding strength. The interaction forces between the binder molecular chains also increase further, thus increasing the cohesive strength.

[0020] In any embodiment, the weight-average molecular weight of the copolymer is 50,000 to 500,000.

[0021] Within a certain range, a larger molecular weight increases the intermolecular forces of the adhesive and the interaction forces between the adhesive molecules and the current collector, which is beneficial for improving adhesion and cohesion.

[0022] In any embodiment, the copolymer comprises a first copolymer and a second copolymer, the first copolymer and the second copolymer being independently selected from one or more of ethylene-acrylate-maleic anhydride copolymer, ethylene-acrylate-glycidyl methacrylate copolymer, ethylene-acrylate copolymer, and ethylene-acrylate-methacrylic acid copolymer, the first copolymer having a weight-average molecular weight of 50,000 to 250,000, and the second copolymer having a weight-average molecular weight of 300,000 to 500,000.

[0023] When a copolymer comprises a first copolymer and a second copolymer, with the first copolymer having a weight-average molecular weight of 50,000-250,000 and the second copolymer having a weight-average molecular weight of 300,000-500,000, the combination of small molecules and macromolecules allows for a wider range of choices, enabling the simultaneous achievement of good slurry dispersion and bonding effects.

[0024] In any embodiment, the weight ratio of the first copolymer to the second copolymer is 1:5 to 5:1.

[0025] Low molecular weight polymers primarily function as dispersants, mainly dispersing high molecular weight polymers and slurries; high molecular weight polymers mainly act as binders, providing adhesion between the slurry and the electrode. A proper ratio of low and high molecular weight polymers ensures both strong overall binder adhesion and good slurry dispersion. In any embodiment, the copolymer content is 0.5%-2.5%, based on the total weight of the positive electrode film.

[0026] The appropriate amount of binder ensures good bonding and strong electrode adhesion. On the other hand, the high content of the main material does not affect the energy density, thus enabling the system to have both considerable bonding strength and cohesive strength, as well as high energy density.

[0027] In any embodiment, the adhesive further comprises one or more of the following: styrene-ethylene-butene-styrene block copolymer, styrene-isoprene-styrene copolymer, styrene-butadiene-styrene copolymer, hydrogenated styrene-isoprene-styrene copolymer, polyolefin elastomer, and ethylene-vinyl acetate copolymer.

[0028] The aforementioned binders are all non-fluorinated, weakly polar binders. The introduction of saturated chains avoids subsequent side reactions in the battery and helps improve the flexibility of the electrode sheet.

[0029] The second aspect of this application provides a sodium-ion secondary battery positive electrode slurry.

[0030] It contains positive electrode active material, binder and solvent,

[0031] The adhesive comprises a copolymer containing a first structural unit of Formula I and a second structural unit of Formula II.

[0032] Where R 11 R 12 R 13 R 14 R 21 R 22 R 23 Each is independently selected from hydrogen, unsubstituted or substituted C. 1-3Alkyl, the substituted C 1-3 The substituent group in the alkyl group is selected from at least one of ester group, carboxyl salt, acid anhydride, hydroxyl group, amide group, and carboxyl group.

[0033] R 24 Selected from hydrogen, inorganic cations, unsubstituted or substituted C 1-3 Alkyl, the substituted C 1-3 The substituent group in the alkyl group is selected from at least one of ester group, carboxyl salt, acid anhydride, hydroxyl group, amide group, and carboxyl group.

[0034] The solvent includes solvent oil.

[0035] Because the binder in this application cannot be dissolved by conventional high-polarity NMP, a weakly polar solvent oil must be used as the solvent; and the solvent oil is less toxic than NMP, making it more environmentally friendly.

[0036] In any embodiment, the solvent oil includes D80 solvent oil.

[0037] Compared to high-boiling-point solvents, D80 has a relatively lower boiling point, allowing it to evaporate at lower temperatures and in a shorter time, thus improving efficiency. Compared to low-boiling-point solvents, D80 has a more moderate evaporation rate, preventing electrode cracking due to excessively rapid evaporation. Therefore, when the solvent oil includes D80 solvent oil, the operating time can be shortened, efficiency improved, and the resulting positive electrode exhibits better performance with no significant cracking.

[0038] A third aspect of this application provides an electrical device comprising a sodium-ion secondary battery as described in the first aspect of this application.

[0039] The fourth aspect of this application provides the use of copolymers as binders in secondary batteries, said copolymers containing structural units shown in Formula I and Formula II.

[0040] Where R 11 R 12 R 13 R 14 R 21 R 22 R 23 Each is independently selected from hydrogen, unsubstituted or substituted C. 1-3 Alkyl, the substituted C 1-3 The substituent group in the alkyl group is selected from at least one of ester group, carboxyl salt, acid anhydride, hydroxyl group, amide group, and carboxyl group.

[0041] R 24 Selected from hydrogen, inorganic cations, unsubstituted or substituted C 1-3 Alkyl, the substituted C 1-3The substituent group in the alkyl group is selected from at least one of ester group, carboxyl salt, acid anhydride, hydroxyl group, amide group, and carboxyl group. Attached Figure Description

[0042] Figure 1 is a schematic diagram of a secondary battery according to an embodiment of this application.

[0043] Figure 2 is an exploded view of a secondary battery according to an embodiment of this application shown in Figure 1.

[0044] Figure 3 is a schematic diagram of a battery module according to one embodiment of this application.

[0045] Figure 4 is a schematic diagram of a battery pack according to one embodiment of this application.

[0046] Figure 5 is an exploded view of the battery pack of one embodiment of this application shown in Figure 4.

[0047] Figure 6 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of this application.

[0048] Explanation of reference numerals in the attached diagram: 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Housing; 52 Electrode assembly; 53 Top cover assembly Detailed Implementation

[0049] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the secondary battery, its manufacturing method, and electrical device of this application. However, unnecessary details may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically 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 for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0050] 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.

[0051] In this application, when the term "about" is used to modify parameters such as weight-average molecular weight, temperature, and rate, it indicates that the parameter can vary by ±10%, and sometimes more preferably within ±5%.

[0052] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0053] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0054] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates 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.

[0055] 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.

[0056] 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).

[0057] Sodium-ion battery cathode materials primarily use sodium carbonate as the sodium source during production, but sodium carbonate has a high alkalinity. Common cathode systems mainly use polyvinylidene fluoride (PVDF) as a binder. While PVDF provides strong bonding strength and electrochemical stability, its alkali resistance is poor. Furthermore, PVDF undergoes an elimination reaction under alkaline sodium ion conditions, generating water. The resulting double bonds cause cross-linking between PVDF molecular chains, forming a gel that severely impacts battery processing performance and cell performance. Additionally, using PVDF as a binder results in poor flexibility of the cathode sheet. Moreover, the EU and other countries have implemented fluorine bans, explicitly restricting the use of fluorine. Therefore, there is a desire to find a fluorine-free or low-fluorine binder applicable to sodium-ion battery cathode systems, resulting in cathode sheets with improved flexibility.

[0058] Based on this, this application proposes a technical solution to solve the above-mentioned technical problems.

[0059] A first aspect of this application provides a sodium-ion secondary battery, the sodium-ion secondary battery comprising a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive electrode film layer disposed on the positive current collector, the positive electrode film layer comprising a positive electrode active material and a binder, the binder comprising a copolymer, the copolymer containing a first structural unit shown in Formula I and a second structural unit shown in Formula II.

[0060] Where R 11 R 12 R 13 R 14 R 21 R 22 R 23 Each is independently selected from hydrogen, unsubstituted or substituted C. 1-3 Alkyl, the substituted C 1-3 The substituent group in the alkyl group is selected from at least one of ester group, carboxyl salt, acid anhydride, hydroxyl group, amide group, and carboxyl group;

[0061] R 24 Selected from hydrogen, inorganic cations, unsubstituted or substituted C 1-3 Alkyl, the substituted C 1-3The substituent group in the alkyl group is selected from at least one of ester group, carboxyl salt, acid anhydride, hydroxyl group, amide group, and carboxyl group.

[0062] The first structural unit, serving as the main skeleton of the binder, primarily provides the binder's flexibility, and its weak polarity prevents subsequent side reactions in the battery. The second structural unit consists of polar segments; an appropriate amount of polar segments can enhance the binder's adhesion and improve the electrode's bonding strength. The selected substituents possess appropriate polarity and good stability, are not prone to side reactions, and do not adversely affect battery performance after contact or reaction with the electrolyte. When using the sodium-ion secondary battery of this application, since the copolymer is fluorine-free, compared to using PVDF as a binder, using a binder containing the polymer reduces the use of fluorine. Furthermore, due to the polymer's low glass transition temperature (below room temperature), the system is in a highly elastic state at room temperature, i.e., a state of good flexibility. Therefore, the electrode exhibits good flexibility at room temperature, resulting in a positive electrode with good flexibility. In contrast, PVDF has a high glass transition temperature and exhibits hard and brittle characteristics at room temperature, leading to poor electrode flexibility. Furthermore, from a production and processing perspective, the polymer's alkali resistance can solve the gelation problem caused by the elimination reaction of PVDF. At the same time, the polymer has relatively low requirements for environmental humidity, which can significantly improve the production and processing capabilities of sodium-ion cathode systems and reduce cathode production costs.

[0063] In this document, the term "binder" refers to a chemical compound, polymer, or mixture that forms a colloidal solution or colloidal dispersion in a dispersion medium.

[0064] In this document, the term "polymer" includes, on the one hand, an aggregate of chemically homogeneous macromolecules prepared by polymerization reactions, but differing in degree of polymerization, molar mass, and chain length. On the other hand, the term also includes derivatives of such aggregates of macromolecules formed by polymerization reactions, i.e., compounds that can be obtained through reactions of the functional groups in the aforementioned macromolecules, such as addition or substitution, and which may be chemically homogeneous or chemically heterogeneous.

[0065] In this document, the term "polymer" refers to a polymer comprising two or more monomer units.

[0066] In some embodiments, the dispersion medium of the binder is an aqueous solvent, such as water. That is, the binder is dissolved in an aqueous solvent.

[0067] In some embodiments, the dispersion medium for the binder is an oily solvent, examples of which include, but are not limited to, D80 solvent oil, dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, acetone, dimethyl carbonate, ethyl cellulose, and polycarbonate. That is, the binder is dissolved in an oily solvent.

[0068] In some embodiments, an adhesive is used to hold electrode materials and / or conductive agents in place and adhere them to a conductive metal component to form an electrode.

[0069] In this article, the term "inorganic cation" includes, but is not limited to, cations such as sodium ions, potassium ions, calcium ions, and magnesium ions.

[0070] In this document, the term "substituted" means that at least one hydrogen atom of the compound or chemical moiety is substituted by a substituent in another chemical moiety.

[0071] In some embodiments, the copolymer is unmodified, carboxyl-modified, hydroxyl-modified, ester-modified, anhydride-modified, carboxylic acid-modified, and / or sodium carboxylate-modified. In some embodiments, the copolymer is ester-modified and / or anhydride-modified.

[0072] When the copolymer is ester-modified and / or anhydride-modified, the presence of ester and / or anhydride polar groups further increases the interaction between the binder and the current collector, thus improving the bonding strength; the interaction force between the binder molecular chains also increases further, thus increasing the cohesive strength. Therefore, the positive electrode film has high bonding strength, cohesive strength and low film resistance.

[0073] In this article, the term "acrylate" refers to the general term for esters of acrylic acid and its homologues. Examples include, but are not limited to, methyl acrylate (MA), ethyl acrylate, methyl methacrylate, and ethyl methacrylate.

[0074] In some implementations, R 11 R 12 R 13 R 14 Each is independently selected from hydrogen and unsubstituted C. 1-3 Alkyl; and / or R 21 R 22 R 23 R 24 Each is independently selected from hydrogen and unsubstituted C. 1-3 alkyl.

[0075] Hydrogen, unsubstituted C 1-3 Alkyl groups are all saturated groups or segments. Choosing these groups avoids introducing easily oxidized and reduced groups into the system and prevents side reactions. In addition, weakly polar groups are beneficial to improving the flexibility of the binder.

[0076] In some implementations, R 11 R 12 R 13 R 14 All are hydrogen; and / or R 21 R22 R 23 R 24 Both are hydrogen.

[0077] Hydrogen is a saturated group, and choosing this group avoids the introduction of easily oxidized and reduced groups into the system, thus preventing side reactions; moreover, the weakly polar group is beneficial to improving the flexibility of the binder.

[0078] In some embodiments, the copolymer is a binary copolymer. In some embodiments, the copolymer includes an ethylene-acrylate binary copolymer. In some embodiments, the copolymer includes one or more of the following: ethylene-methyl acrylate binary copolymer, ethylene-ethyl acrylate binary copolymer, ethylene-propyl acrylate binary copolymer, ethylene-methyl methacrylate binary copolymer, and ethylene-ethyl methacrylate binary copolymer.

[0079] In some embodiments, the copolymer further includes a third structural unit derived from one or more of maleic anhydride monomers and glycidyl methacrylate monomers.

[0080] The third structural unit includes polar groups and chain segments. Appropriate addition of these units can further increase the polarity of the binder to a certain extent, thereby increasing the adhesive force of the binder and improving the performance of the electrode.

[0081] In some embodiments, the copolymer is a ternary or more monomeric copolymer. In some embodiments, the copolymer is a random copolymer. In some embodiments, the copolymer includes one or more of the following: ethylene-acrylate-maleic anhydride terpolymer, ethylene-acrylate-glycidyl methacrylate terpolymer, and ethylene-acrylate-methacrylic acid terpolymer. In some embodiments, the copolymer includes one or more of the following: ethylene-methyl acrylate-maleic anhydride terpolymer, ethylene-methyl acrylate-glycidyl methacrylate terpolymer, ethylene-methyl acrylate-glycidyl methacrylate terpolymer, ethylene-ethyl acrylate-maleic anhydride terpolymer, ethylene-ethyl acrylate-glycidyl methacrylate terpolymer, ethylene-ethyl acrylate-glycidyl methacrylate terpolymer, ethylene-propyl acrylate-glycidyl methacrylate copolymer, ethylene-propyl acrylate copolymer, ethylene-propyl acrylate-glycidyl methacrylate copolymer, ethylene-propyl acrylate-glycidyl methacrylate copolymer, ethylene-methyl acrylate-maleic anhydride copolymer, ethylene-methyl methacrylate-glycidyl methacrylate copolymer, ethylene-methyl methacrylate copolymer, ethylene-methyl methacrylate-glycidyl methacrylate copolymer, ethylene-ethyl methacrylate-glycidyl methacrylate copolymer, ethylene-ethyl methacrylate-glycidyl methacrylate copolymer, ethylene-ethyl methacrylate-glycidyl methacrylate copolymer.

[0082] In some embodiments, the copolymer further includes a third structural unit derived from one or more of maleic anhydride monomers and glycidyl methacrylate monomers.

[0083] Introducing a third structural unit containing polar groups further enhances the interaction between the binder and the current collector, thus increasing the bonding strength. The intermolecular forces between the binder molecular chains also increase, resulting in greater cohesive strength. When the copolymer further includes a third structural unit derived from one or more of maleic anhydride monomers and glycidyl methacrylate monomers, the positive electrode film exhibits higher bonding strength, cohesive strength, and lower film resistance.

[0084] In some embodiments, the copolymer includes ethylene-methyl acrylate binary copolymer, ethylene-ethyl acrylate binary copolymer, ethylene-propyl acrylate binary copolymer, ethylene-methyl methacrylate binary copolymer, ethylene-ethyl methacrylate binary copolymer, ethylene-methyl acrylate-maleic anhydride terpolymer, ethylene-methyl acrylate-glycidyl methacrylate terpolymer, ethylene-methyl acrylate-methacrylate terpolymer, ethylene-ethyl acrylate-maleic anhydride terpolymer, ethylene-ethyl acrylate-glycidyl methacrylate terpolymer, ethylene-ethyl acrylate-methacrylate terpolymer, ethylene- One or more of the following: propyl acrylate-glycidyl methacrylate copolymer, ethylene-propyl acrylate copolymer, ethylene-propyl acrylate-methacrylic acid copolymer, ethylene-methyl methacrylate-maleic anhydride copolymer, ethylene-methyl methacrylate-glycidyl methacrylate copolymer, ethylene-methyl methacrylate-methacrylic acid copolymer, ethylene-ethyl methacrylate-maleic anhydride copolymer, ethylene-ethyl methacrylate-glycidyl methacrylate copolymer, ethylene-ethyl methacrylate copolymer, ethylene-ethyl methacrylate-methacrylic acid copolymer.

[0085] In some implementations, the mass percentage of the first structural unit is 70%-80%, the mass percentage of the second structural unit is 20%-30%, and the mass percentage of the third structural unit is 0%-5%, calculated based on the total mass of the first, second, and third structural units.

[0086] The first structural unit has the largest proportion, which avoids the binder containing too many polar groups, avoids the occurrence of side reactions, and is conducive to improving the flexibility of the binder and the electrode. The second and third structural units have appropriate proportions, and the polar groups or segments introduced by them can improve the bonding strength of the binder within a certain range, and avoid the problems of slurry gelation and poor electrode flexibility caused by excessive polarity.

[0087] In some embodiments, the mass percentage of the first structural unit is 70%, 70.5%, 71%, 71.5%, 72%, 72.5%, 73%, 73.5%, 74%, 74.5%, 75%, 75.5%, 76%, 76.5%, 77%, 77.5%, 78%, 78.5%, 79%, 79.5%, or 80%, or a range consisting of any two of the above values ​​or a value within that range.

[0088] In some embodiments, the mass percentage of the second structural unit is 20%, 20.5%, 21%, 21.5%, 22%, 22.5%, 23%, 23.5%, 24%, 24.5%, 25%, 25.5%, 26%, 26.5%, 27%, 27.5%, 28%, 28.5%, 29%, 29.5%, or 30%, or a range consisting of any two of the above values ​​or a value within that range.

[0089] In some implementations, the mass percentage of the third structural unit is 0%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or a range consisting of any two of the above values ​​or a value within that range.

[0090] In some implementations, the mass percentage of the third structural unit is 0.5%-2%, calculated based on the total mass of the first, second, and third structural units.

[0091] When the copolymer also includes a third structural unit with a mass ratio of 0.5%-2%, an appropriate amount of polar groups or segments are introduced. The degree of modification is appropriately increased, which relatively increases the overall polarity of the copolymer. The interaction between the copolymer and the current collector and the main material is enhanced, thus improving the adhesion. At the same time, the interaction between molecular chains is also enhanced, so the cohesion is improved. This avoids the problems of slurry gelation and poor electrode flexibility caused by excessive polarity, thereby giving the positive electrode film layer high adhesion strength and low film resistance.

[0092] In some embodiments, the proportion of the third structural unit is 0.5%, 1%, 1.5%, 2%, or a range consisting of any two of the above values ​​or values ​​within that range. In some embodiments, the proportion of the third structural unit is 0.5%-1.5%. In some embodiments, the proportion of the third structural unit is 1.5%-2%.

[0093] When the proportion of the third structural unit is 0.5%-1.5%, the slightly lower degree of modification results in a lower polarity of the copolymer. The copolymer is still mainly weakly polar, and its molecular chain is soft, thus giving the positive electrode sheet good flexibility.

[0094] When the proportion of the third structural unit is 1.5%-2%, the appropriately high degree of modification makes the overall polarity of the copolymer relatively high, and the interaction between it and the current collector and the main material is strong, so the adhesion is high; at the same time, the interaction between molecular chains is also strong, so the cohesion is high, thus making the positive electrode film layer have high adhesion strength and cohesion strength.

[0095] In some embodiments, the copolymer includes one or more of the following: ethylene-acrylate-maleic anhydride copolymer, ethylene-acrylate-glycidyl methacrylate copolymer, ethylene-acrylate copolymer, and ethylene-acrylate-methacrylic acid copolymer.

[0096] The first structural unit, as the main skeleton of the binder, primarily provides the binder's flexibility, and its weak polarity prevents subsequent side reactions in the battery. The second structural unit is a polar segment; an appropriate amount of polar segments can enhance the binder's bonding effect and improve the adhesion of the electrode. The introduction of the third structural unit, which contains polar groups, further increases the interaction between the binder and the current collector, thus improving the bonding strength. The interaction forces between the binder molecular chains also increase further, thus increasing the cohesive strength.

[0097] In some embodiments, the weight-average molecular weight of the copolymer is 50,000 to 500,000.

[0098] Within a certain range, a larger molecular weight increases the intermolecular forces of the adhesive and the interaction forces between the adhesive molecules and the current collector, which is beneficial for improving adhesion and cohesion.

[0099] In some embodiments, the weight-average molecular weight of the copolymer is 100,000 to 400,000. In some embodiments, the weight-average molecular weight of the copolymer is 50,000, 100,000, 150,000, 200,000, 250,000, 300,000, 350,000, 400,000, 450,000, or 500,000, or a range consisting of any two of the above values ​​or values ​​within that range.

[0100] In this paper, the term "weight-average molecular weight" refers to the sum of the products of the weight fraction of molecules of different molecular weights in a polymer and their corresponding molecular weights.

[0101] In some embodiments, the copolymer comprises a first copolymer and a second copolymer, the first copolymer and the second copolymer being independently selected from one or more of ethylene-acrylate-maleic anhydride copolymer, ethylene-acrylate-glycidyl methacrylate copolymer, ethylene-acrylate copolymer, and ethylene-acrylate-methacrylic acid copolymer, the first copolymer having a weight-average molecular weight of 50,000 to 250,000 and the second copolymer having a weight-average molecular weight of 300,000 to 500,000.

[0102] In some embodiments, the weight-average molecular weight of the first copolymer is 50,000 to 200,000. In some embodiments, the weight-average molecular weight of the first copolymer is 50,000 to 100,000. In some embodiments, the weight-average molecular weight of the first copolymer is 100,000 to 200,000. In some embodiments, the weight-average molecular weight of the first copolymer is 50,000, 100,000, 150,000, 200,000, 250,000, or a range consisting of any two of the above values ​​or values ​​within that range.

[0103] In some embodiments, the weight-average molecular weight of the second copolymer is 300,000 to 400,000. In some embodiments, the weight-average molecular weight of the second copolymer is 400,000 to 500,000. In some embodiments, the weight-average molecular weight of the second copolymer is 300,000, 350,000, 400,000, 450,000, or 500,000, or a range consisting of any two of the above values ​​or values ​​within that range.

[0104] When a copolymer comprises a first copolymer and a second copolymer, wherein the first copolymer and the second copolymer are independently selected from one or more of ethylene-acrylate-maleic anhydride copolymer, ethylene-acrylate-glycidyl methacrylate copolymer, ethylene-acrylate copolymer, and ethylene-acrylate-methacrylic acid copolymer, and the weight-average molecular weight of the first copolymer is 50,000-250,000 and the weight-average molecular weight of the second copolymer is 300,000-500,000, the lower molecular weight of the first copolymer can play a good dispersing role, and the higher molecular weight of the second copolymer can improve the adhesion, cohesion, and electrolyte resistance of the slurry. Through the compounding of small molecules and large molecules, the selection space is larger, and good slurry dispersion ability and adhesion effect can be achieved at the same time.

[0105] In some embodiments, the weight ratio of the first copolymer to the second copolymer is 1:5 to 5:1.

[0106] Low molecular weight polymers mainly act as dispersants, primarily dispersing high molecular weight polymers and slurries; high molecular weight polymers mainly act as binders, providing adhesion between the slurry and the electrode; a reasonable ratio of low molecular weight polymers and high molecular weight polymers can result in both strong overall binder adhesion and good slurry dispersion.

[0107] In some embodiments, the weight ratio of the first copolymer to the second copolymer is 1:4 to 4:1. In some embodiments, the weight ratio of the first copolymer to the second copolymer is 1:3 to 3:1. In some embodiments, the weight ratio of the first copolymer to the second copolymer is 1:2 to 2:1. In some embodiments, the weight ratio of the first copolymer to the second copolymer is 1:2 to 1:1. In some embodiments, the weight ratio of the first copolymer to the second copolymer is 1:5, 1:4, 1:3, 1:2, 1:1, 2:1, 3:1, 4:1, 5:1, or a range consisting of any two of the above ratios, or a ratio within that range.

[0108] Low molecular weight polymers primarily act as dispersants, mainly dispersing high molecular weight polymers and slurries; high molecular weight polymers mainly function as binders, providing adhesion between the slurry and the electrode. A proper ratio of low and high molecular weight polymers ensures both strong overall binder adhesion and good slurry dispersion. When the weight ratio of the first copolymer to the second copolymer is 1:2 to 1:1, the positive electrode film exhibits high bonding strength, cohesive strength, and low film resistance.

[0109] In some embodiments, the copolymer comprises a third copolymer selected from one or more of ethylene-acrylate-maleic anhydride copolymer, ethylene-acrylate-glycidyl methacrylate copolymer, ethylene-acrylate copolymer, and ethylene-acrylate-methacrylic acid copolymer, wherein the weight-average molecular weight of the third copolymer is 150,000 to 500,000. In some embodiments, the weight-average molecular weight of the third copolymer is 200,000 to 400,000. In some embodiments, the weight-average molecular weight of the third copolymer is 200,000 to 350,000. In some embodiments, the weight-average molecular weight of the third copolymer is 200,000 to 300,000. In some embodiments, the weight-average molecular weight of the third copolymer is 300,000 to 350,000. In some embodiments, the weight-average molecular weight of the third copolymer is 150,000, 200,000, 250,000, 300,000, 350,000, 400,000, 450,000, or 500,000, or a range consisting of any two of the above values ​​or values ​​within that range.

[0110] In some embodiments, the first copolymer, the second copolymer, and the third copolymer are independently selected from ethylene-methyl acrylate binary copolymers, ethylene-ethyl acrylate binary copolymers, ethylene-propyl acrylate binary copolymers, ethylene-methyl methacrylate binary copolymers, ethylene-ethyl methacrylate binary copolymers, ethylene-methyl acrylate-maleic anhydride terpolymers, ethylene-methyl acrylate-glycidyl methacrylate terpolymers, ethylene-methyl acrylate-methacrylate terpolymers, ethylene-ethyl acrylate-maleic anhydride terpolymers, ethylene-ethyl acrylate-glycidyl methacrylate terpolymers, and ethylene-ethyl acrylate-methacrylate terpolymers. One or more of the following: terpolymer, ethylene-propyl acrylate-glycidyl methacrylate copolymer, ethylene-propyl acrylate copolymer, ethylene-propyl acrylate-methacrylic acid copolymer, ethylene-methyl methacrylate-maleic anhydride copolymer, ethylene-methyl methacrylate-glycidyl methacrylate copolymer, ethylene-methyl methacrylate-methacrylic acid copolymer, ethylene-ethyl methacrylate-maleic anhydride copolymer, ethylene-ethyl methacrylate-glycidyl methacrylate copolymer, ethylene-ethyl methacrylate copolymer, ethylene-ethyl methacrylate-methacrylic acid copolymer.

[0111] In some embodiments, the copolymer comprises a first copolymer, a second copolymer, and a third copolymer.

[0112] In some embodiments, the copolymer comprises a first copolymer and a second copolymer, but does not contain a third copolymer.

[0113] In some embodiments, the copolymer comprises a third copolymer but does not comprise the first copolymer or the second copolymer.

[0114] In some embodiments, the copolymer content is 0.5%-2.5% based on the total weight of the positive electrode film.

[0115] The appropriate amount of binder ensures good bonding and strong electrode adhesion. On the other hand, the high content of the main material does not affect the energy density, thus enabling the system to have both considerable bonding strength and cohesive strength, as well as high energy density.

[0116] In some embodiments, the copolymer content is 1%-2%. In some embodiments, the copolymer content is 1%-1.5%. In some embodiments, the copolymer content is 1.5%-2%. In some embodiments, the copolymer content is 0.5%, 1%, 1.5%, 2%, 2.5%, or a range consisting of any two of the above values ​​or values ​​within that range.

[0117] When the copolymer content is 1.5%-2%, by selecting an appropriate binder content, the main material can be evenly dispersed and the mechanical properties of the electrode sheet—adhesion and bonding strength—can be improved. At the same time, the slurry dispersion ability can be improved, and the slurry is less likely to gel. As a result, the positive electrode film layer has high bonding strength and cohesive strength, as well as high energy density.

[0118] When the copolymer content is 1%-1.5%, by selecting an appropriately low binder content, the overall polarity is low, which makes the positive electrode sheet have good flexibility and low film resistance.

[0119] In some embodiments, the adhesive further comprises one or more of styrene-ethylene-butene-styrene block copolymers, styrene-isoprene-styrene copolymers, styrene-butadiene-styrene copolymers, hydrogenated styrene-isoprene-styrene copolymers, polyolefin elastomers, and ethylene-vinyl acetate copolymers. In some embodiments, the adhesive further comprises one or more of styrene-ethylene-butene-styrene block copolymers and polyolefin elastomers.

[0120] The aforementioned binders are all non-fluorinated, weakly polar binders. The introduction of saturated chains avoids subsequent side reactions in the battery and helps improve the flexibility of the electrode sheet.

[0121] In some implementations, the adhesive may also contain PVDF.

[0122] The second aspect of this application provides a sodium-ion secondary battery positive electrode slurry.

[0123] It contains positive electrode active material, binder and solvent,

[0124] The adhesive comprises a copolymer containing a first structural unit of Formula I and a second structural unit of Formula II.

[0125] Where R 11 R 12 R 13 R 14 R 21 R 22 R 23 Each is independently selected from hydrogen, unsubstituted or substituted C. 1-3 Alkyl, the substituted C 1-3 The substituent group in the alkyl group is selected from at least one of ester group, carboxyl salt, acid anhydride, hydroxyl group, amide group, and carboxyl group.

[0126] R 24 Selected from hydrogen, inorganic cations, unsubstituted or substituted C 1-3 Alkyl, the substituted C 1-3The substituent group in the alkyl group is selected from at least one of ester group, carboxyl salt, acid anhydride, hydroxyl group, amide group, and carboxyl group.

[0127] The solvent includes solvent oil.

[0128] Because the binder in this application cannot be dissolved by conventional high-polarity NMP, a weakly polar solvent oil must be used as the solvent; and the solvent oil is less toxic than NMP, making it more environmentally friendly.

[0129] In some embodiments, the solvent oil includes solvent oils containing aromatics or those not containing aromatics. In some embodiments, the solvent oil includes one or more of D20, D40, D60, D80, D100, D120, and D200 solvent oils.

[0130] D20, D40, D60, D80, D100, D120, and D200 are typical grades of solvent oils, which are related to the boiling point of the solvent oil and are common industry names.

[0131] In some embodiments, the solvent oil includes D80 solvent oil.

[0132] Compared to high-boiling-point solvents, D80 has a relatively lower boiling point, allowing it to evaporate at lower temperatures and in a shorter time, thus improving efficiency. Compared to low-boiling-point solvents, D80 has a more moderate evaporation rate, preventing electrode cracking due to excessively rapid evaporation. Therefore, when the solvent oil includes D80 solvent oil, the operating time can be shortened, efficiency improved, and the resulting positive electrode exhibits better performance with no significant cracking.

[0133] A third aspect of this application provides an electrical device comprising a sodium-ion secondary battery as described in the first aspect of this application.

[0134] The fourth aspect of this application provides the use of copolymers as binders in secondary batteries, said copolymers containing structural units shown in Formula I and Formula II.

[0135] Where R 11 R 12 R 13 R 14 R 21 R 22 R 23 Each is independently selected from hydrogen, unsubstituted or substituted C. 1-3 Alkyl, the substituted C 1-3 The substituent group in the alkyl group is selected from at least one of ester group, carboxyl salt, acid anhydride, hydroxyl group, amide group, and carboxyl group.

[0136] R 24Selected from hydrogen, inorganic cations, unsubstituted or substituted C 1-3 Alkyl, the substituted C 1-3 The substituent group in the alkyl group is selected from at least one of ester group, carboxyl salt, acid anhydride, hydroxyl group, amide group, and carboxyl group.

[0137] In addition, the secondary battery and power-consuming device of this application will be described below with appropriate reference to the accompanying drawings.

[0138] In one embodiment of this application, a secondary battery is provided.

[0139] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0140] [Positive electrode plate]

[0141] The positive electrode sheet includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector. The positive electrode film layer includes a positive electrode active material and a binder, the binder including the polymer described in this application.

[0142] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0143] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0144] In some embodiments, the positive electrode active material may include one or more of polyanionic compounds, sodium transition metal oxides, and Prussian blue compounds.

[0145] In sodium transition metal oxides, the transition metal can be one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. For example, sodium transition metal oxides are Na. yAO₂, where A is one or more of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu, and 0 < y ≤ 1. The polyanion-type compound includes one or more of sodium trifluorophosphate vanadate Na₃V₂(PO₄)₂F₃, sodium fluorophosphate vanadate NaVPO₄F, sodium vanadium phosphate Na₃V₂(PO₄)₃, Na₄Fe₃(PO₄)₂P₂O₇, NaFePO₄, Na₃V₂(PO₄)₃, disodium pyrophosphate Na₂Q₁P₂O₇ (Q₁ = Fe, Co, Mn), and mixed pyrophosphate Na₄Q₂₃(PO₄)₂P₂O₇ (Q₂ = Fe, Co, Mn, Ni). The Prussian blue-type compound is NazE 1 E 2 (CN)₆, where E 1 、E 2 is one or more of Fe, Mn, Co, Ni, Cu, Zn, Cr, Ti, V, Zr, and Ce, and 0 < z ≤ 2.

[0146] In some embodiments, the positive electrode film layer may further optionally include a conductive agent. As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0147] In some embodiments, the positive electrode sheet can be prepared by the following method: dispersing the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (such as D80 solvent oil) to form a positive electrode slurry; coating the positive electrode slurry on the positive electrode current collector, and after processes such as drying and cold pressing, the positive electrode sheet can be obtained.

[0148] [Negative electrode sheet]

[0149] The negative electrode sheet 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 the negative electrode film layer includes a negative electrode active material.

[0150] As an 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.

[0151] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0152] 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, and lithium titanate, 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.

[0153] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0154] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0155] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0156] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0157] Electrolyte

[0158] The electrolyte acts as a conductor of ions between the positive and negative electrodes. In this embodiment, the electrolyte is liquid and comprises an electrolyte salt and a solvent.

[0159] In some embodiments, the electrolyte salt may be selected from NaNO3, NaPF6, NaBF4, or NaClO4. - At least one of NaBOB, NaDFOB, NaSbF6, NaAsF6, NaN(SO2CF3)2, NaN(SO2C2F5)2, NaC(SO2CF3)3, and NaN(SO2F)2.

[0160] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0161] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0162] [Isolation membrane]

[0163] 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.

[0164] In some 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.

[0165] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0166] 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.

[0167] 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.

[0168] 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. For example, Figure 1 shows a square-structured secondary battery 5 as an example.

[0169] In some embodiments, referring to FIG2, the outer packaging 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, the bottom plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be placed over the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator can be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within 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 selected by those skilled in the art according to specific practical needs.

[0170] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0171] Figure 3 shows a battery module 4 as an example. Referring to Figure 3, in the battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other manner. Furthermore, the multiple secondary batteries 5 can be fixed in place using fasteners.

[0172] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0173] 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 one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0174] Figures 4 and 5 show a battery pack 1 as an example. Referring to Figures 4 and 5, 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 box 2 and a lower box 3, with the upper box 2 covering the lower box 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0175] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, 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.

[0176] As the electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0177] Figure 6 shows an example of an electrical device. This device 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 the secondary battery for this device, a battery pack or battery module can be used.

[0178] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0179] Example

[0180] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0181] I. Preparation Method

[0182] Example 1

[0183] (1) Preparation of the positive electrode sheet:

[0184] Preparation of the first copolymer

[0185] 1) Add the emulsifier sodium dodecyl sulfate to a beaker containing deionized water and stir at 500 rpm to dissolve it. Then add methyl acrylate and maleic anhydride monomer and stir at 800 rpm for 2 hours at 50 degrees Celsius.

[0186] 2) Add the above liquid to the reactor that has been purged with ethylene gas, add the initiator benzoyl peroxide (the amount of initiator added is 0.5% of the total mass of ethylene, methyl acrylate and maleic anhydride monomer), introduce ethylene gas (ethylene:methyl acrylate:maleic anhydride monomer (mass ratio) = 73:25.5:1.5), heat and stir, and keep the pressure of the system constant. When the temperature rises to 70 degrees Celsius, maintain the temperature and pressure for 4 hours.

[0187] 3) After the reaction is complete, the reaction liquid is introduced into a methanol solution containing a trace amount of hydroquinone for demulsification. When the precipitate no longer increases, it is filtered, and the upper filter cake is dried statically to finally obtain an ethylene-methyl acrylate-maleic anhydride terpolymer with a weight average molecular weight of approximately 10W. The mass ratio of the first structural unit: the second structural unit: the third structural unit is 73:25.5:1.5, and the mass percentage of the third structural unit is 1.5%.

[0188] The preparation of the second copolymer differs from that of the first copolymer mainly in that the amount of ethylene, methyl acrylate and maleic anhydride monomers is increased, resulting in an ethylene-methyl acrylate-maleic anhydride terpolymer with a weight-average molecular weight of approximately 40W. The mass ratio of the first structural unit to the second structural unit to the third structural unit is 73:25.5:1.5, and the mass percentage of the third structural unit is 1.5%.

[0189] Sodium iron phosphate, conductive agent Super P, first copolymer, and second copolymer were mixed in a weight ratio of 95.5:3.0:0.5:1.0. D80 solvent oil was added as a solvent, and the mixture was stirred until homogeneous to obtain a positive electrode slurry with a solid content of 60 wt%. The positive electrode slurry was then mixed at a concentration of 300 mg / 1540.25 mm. 2 The material is uniformly coated onto an aluminum foil current collector and dried at high temperature, then cold-pressed to obtain a positive electrode sheet.

[0190] (2) Preparation of negative electrode sheet:

[0191] Hard carbon, conductive carbon black (conductive agent), styrene-butadiene rubber (SBR) (binder), and sodium carboxymethyl cellulose (CMC-Na) (thickener) were mixed evenly in a weight percentage of 95:1.0:2.0:2.0, and deionized water was added. After stirring and dispersing, a negative electrode slurry was obtained. The negative electrode slurry was then mixed at a concentration of 211 mg / 1540 mm. 2 The negative electrode sheet is obtained by coating the substrate Cu foil, drying, cold pressing, slitting, and sheet forming.

[0192] (3) Battery fabrication:

[0193] The positive electrode, separator, and negative electrode are stacked in sequence. The separator must be able to isolate the anode and cathode. The bare cell is obtained by winding. The bare cell is placed in the outer packaging, electrolyte is injected, and after processes such as encapsulation, formation, and degassing, a sodium-ion battery is finally obtained.

[0194] Example 2

[0195] The main difference between Example 2 and Example 1 is that solvent oil D60 is used instead of solvent oil D80.

[0196] Example 3

[0197] The main difference between Example 3 and Example 1 is that solvent oil D100 is used instead of solvent oil D80.

[0198] Examples 4-7

[0199] Compared with Example 1, Examples 4-7 differ mainly in that the weight percentages of the first copolymer and the second copolymer were changed, and the weight percentage of the positive electrode active material was changed accordingly.

[0200] Example 8

[0201] The main difference between Example 8 and Example 1 is that the first copolymer and the second copolymer are different.

[0202] Preparation of the first copolymer

[0203] 1) Add the emulsifier sodium dodecyl sulfate to a beaker containing deionized water and stir at 500 rpm to dissolve it. Then add methyl acrylate and glycidyl methacrylate monomers and stir at 800 rpm for 2 hours at 50 degrees Celsius.

[0204] 2) Add the above liquid to a reactor that has been purged with ethylene gas, add benzoyl peroxide as an initiator (the amount of initiator added is 0.5% of the total mass of ethylene, methyl acrylate, and glycidyl methacrylate monomers), introduce ethylene gas (ethylene:methyl acrylate:glycidyl methacrylate monomer (mass ratio) = 73:25.5:1.5), heat and stir, and keep the pressure of the system constant. When the temperature rises to 70 degrees Celsius, maintain the temperature and pressure for 4 hours.

[0205] 3) After the reaction is complete, the reaction liquid is introduced into a methanol solution containing a trace amount of hydroquinone for demulsification. When the precipitate no longer increases, it is filtered, and the upper filter cake is dried statically to finally obtain an ethylene-methyl acrylate-glycidyl methacrylate terpolymer with a weight average molecular weight of approximately 10W. The mass ratio of the first structural unit to the second structural unit to the third structural unit is 73:25.5:1.5, and the mass percentage of the third structural unit is 1.5%.

[0206] The preparation of the second copolymer differs from that of the first copolymer mainly in that the amount of ethylene, methyl acrylate, and glycidyl methacrylate monomers added is increased, resulting in an ethylene-methyl acrylate-glycidyl methacrylate terpolymer with a weight-average molecular weight of approximately 40W. The mass ratio of the first structural unit to the second structural unit to the third structural unit is 73:25.5:1.5, and the mass percentage of the third structural unit is 1.5%.

[0207] Example 9

[0208] The main difference between Example 9 and Example 8 is that solvent oil D60 is used instead of solvent oil D80.

[0209] Example 10

[0210] The main difference between Example 10 and Example 8 is that solvent oil D100 is used instead of solvent oil D80.

[0211] Examples 11-14

[0212] The main difference between Examples 11-14 and Example 8 is that the weight percentages of the first copolymer and the second copolymer were changed, and the weight percentage of the positive electrode active material was changed accordingly.

[0213] Example 15

[0214] The main difference between Example 15 and Example 1 is that the first copolymer and the second copolymer are different.

[0215] Preparation of the first copolymer

[0216] 1) Add the emulsifier sodium dodecyl sulfate to a beaker containing deionized water and stir at 500 rpm to dissolve it. Then add methyl acrylate monomer and stir at 800 rpm for 2 hours at 50 degrees Celsius.

[0217] 2) Add the above liquid to the reactor that has been purged with ethylene gas, add the initiator benzoyl peroxide (the amount of initiator added is 0.5% of the total mass of ethylene and methyl acrylate monomers), introduce ethylene gas (ethylene:methyl acrylate monomer (mass ratio) = 73:27), heat and stir, and keep the pressure of the system constant. When the temperature rises to 70 degrees Celsius, maintain the temperature and pressure for 4 hours.

[0218] 3) After the reaction is complete, the reaction liquid is introduced into a methanol solution containing a trace amount of hydroquinone for demulsification. When the precipitate no longer increases, it is filtered, and the upper filter cake is dried statically to finally obtain an ethylene-methyl acrylate binary copolymer with a weight average molecular weight of approximately 10W. The mass ratio of the first structural unit to the second structural unit is 73:27, and it does not contain a third structural unit.

[0219] The preparation of the second copolymer differs from that of the first copolymer mainly in that the amount of ethylene and methyl acrylate monomers added is increased, so that the final ethylene-methyl acrylate binary copolymer is obtained with a weight-average molecular weight of about 40W, wherein the mass ratio of the first structural unit to the second structural unit is 73:27, and the third structural unit is not included.

[0220] Example 16

[0221] The main difference between Example 16 and Example 1 is that the first copolymer and the second copolymer are different.

[0222] Preparation of the first copolymer

[0223] 1) Add the emulsifier sodium dodecyl sulfate to a beaker containing deionized water and stir at 500 rpm to dissolve it. Then add methyl acrylate and methacrylic acid (MAA) monomer and stir at 800 rpm for 2 hours at 50 degrees Celsius.

[0224] 2) Add the above liquid to the reactor that has been purged with ethylene gas, add the initiator benzoyl peroxide (the amount of initiator added is 0.5% of the total mass of ethylene, methyl acrylate and methacrylic acid monomer), introduce ethylene gas (ethylene:methyl acrylate:methacrylic acid monomer (mass ratio) = 73:25.5:1.5), heat and stir, and keep the pressure of the system constant. When the temperature rises to 70 degrees Celsius, maintain the temperature and pressure for 4 hours.

[0225] 3) After the reaction is complete, the reaction liquid is introduced into a methanol solution containing trace amounts of hydroquinone for demulsification. When the precipitate no longer increases, it is filtered, and the upper filter cake is dried statically to finally obtain an ethylene-methyl acrylate-methacrylic acid terpolymer with a weight-average molecular weight of approximately 10W. The mass ratio of the first structural unit to the second structural unit to the third structural unit is 73:25.5:1.5, and the mass percentage of the third structural unit is 1.5%.

[0226] The preparation of the second copolymer differs from that of the first copolymer mainly in that the amount of ethylene, methyl acrylate and methacrylic acid monomers is increased, so that the final ethylene-methyl acrylate-methacrylic acid terpolymer is obtained with a weight average molecular weight of about 40W, wherein the mass ratio of the first structural unit: the second structural unit: the third structural unit is 73:25.5:1.5, and the mass percentage of the third structural unit is 1.5%.

[0227] Example 17

[0228] The main difference between Example 17 and Example 1 is that the first copolymer and the second copolymer are different.

[0229] Preparation of the first copolymer

[0230] 1) Add the emulsifier sodium dodecyl sulfate to a beaker containing deionized water and stir at 500 rpm to dissolve it. Then add methyl acrylate and maleic anhydride monomer and stir at 800 rpm for 2 hours at 50°C.

[0231] 2) Add the above liquid to the reactor that has been purged with ethylene gas, add the initiator benzoyl peroxide (the amount of initiator added is 0.5% of the total mass of ethylene, methyl acrylate and maleic anhydride monomer), introduce ethylene gas (ethylene:methyl acrylate:maleic anhydride monomer (mass ratio) = 74:25.5:0.5), heat and stir, and keep the pressure of the system constant. When the temperature rises to 70°C, maintain the temperature and pressure for 4 hours.

[0232] 3) After the reaction is complete, the reaction liquid is introduced into a methanol solution containing a trace amount of hydroquinone for demulsification. When the precipitate no longer increases, it is filtered, and the upper filter cake is dried statically to finally obtain an ethylene-methyl acrylate-maleic anhydride terpolymer with a weight average molecular weight of approximately 10W. The mass ratio of the first structural unit to the second structural unit to the third structural unit is 74:25.5:0.5, and the mass percentage of the third structural unit is 0.5%.

[0233] The preparation of the second copolymer differs from that of the first copolymer mainly in that the amount of ethylene, methyl acrylate and maleic anhydride monomers is increased, resulting in an ethylene-methyl acrylate-maleic anhydride terpolymer with a weight-average molecular weight of approximately 40W. The mass ratio of the first structural unit to the second structural unit to the third structural unit is 74:25.5:0.5, and the mass percentage of the third structural unit is 0.5%.

[0234] Example 18

[0235] The main difference between Example 18 and Example 1 is that the first copolymer and the second copolymer are different.

[0236] Preparation of the first copolymer

[0237] 1) Add the emulsifier sodium dodecyl sulfate to a beaker containing deionized water and stir at 500 rpm to dissolve it. Then add methyl acrylate and maleic anhydride monomer and stir at 800 rpm for 2 hours at 50°C.

[0238] 2) Add the above liquid to the reactor that has been purged with ethylene gas, add the initiator benzoyl peroxide (the amount of initiator added is 0.5% of the total mass of ethylene, methyl acrylate and maleic anhydride monomers), introduce ethylene gas (ethylene:methyl acrylate:maleic anhydride monomer (mass ratio) = 72.5:25.5:2), heat and stir, and keep the pressure of the system constant. When the temperature rises to 70℃, maintain the temperature and pressure for 4 hours.

[0239] 3) After the reaction is complete, the reaction liquid is introduced into a methanol solution containing a trace amount of hydroquinone for demulsification. When the precipitate no longer increases, it is filtered, and the upper filter cake is dried statically to finally obtain an ethylene-methyl acrylate-maleic anhydride terpolymer with a weight average molecular weight of approximately 10W. The mass ratio of the first structural unit: the second structural unit: the third structural unit is 72.5:25.5:2, and the mass percentage of the third structural unit is 2%.

[0240] The preparation of the second copolymer differs from that of the first copolymer mainly in that the amount of ethylene, methyl acrylate and maleic anhydride monomers is increased, so that the final ethylene-methyl acrylate-maleic anhydride terpolymer is obtained with a weight-average molecular weight of about 40W, wherein the mass ratio of the first structural unit: the second structural unit: the third structural unit is 72.5:25.5:2, and the mass percentage of the third structural unit is 2%.

[0241] Example 19

[0242] The main difference between Example 19 and Example 1 is that the weight-average molecular weights of the first copolymer and the second copolymer are different (the mass ratio of the first structural unit: the second structural unit: the third structural unit remains unchanged). By changing the amount of ethylene, methyl acrylate and maleic anhydride monomers, the weight-average molecular weight of the first copolymer is about 5W and the weight-average molecular weight of the second copolymer is about 50W.

[0243] Example 20

[0244] The main difference between Example 20 and Example 1 is that the weight-average molecular weights of the first copolymer and the second copolymer are different (the mass ratio of the first structural unit: the second structural unit: the third structural unit remains unchanged). By changing the amount of ethylene, methyl acrylate and maleic anhydride monomers, the weight-average molecular weight of the first copolymer is about 20W and the weight-average molecular weight of the second copolymer is about 30W.

[0245] Example 21

[0246] The main difference between Example 21 and Example 1 is that the third copolymer is used instead of the first copolymer and the second copolymer.

[0247] Preparation of the third copolymer

[0248] 1) Add the emulsifier sodium dodecyl sulfate to a beaker containing deionized water and stir at 500 rpm to dissolve it. Then add methyl acrylate and maleic anhydride monomer and stir at 800 rpm for 2 hours at 50 degrees Celsius.

[0249] 2) Add the above liquid to the reactor that has been purged with ethylene gas, add the initiator benzoyl peroxide (the amount of initiator added is 0.5% of the total mass of ethylene, methyl acrylate and maleic anhydride monomer), introduce ethylene gas (ethylene:methyl acrylate:maleic anhydride monomer (mass ratio) = 73:25.5:1.5), heat and stir, and keep the pressure of the system constant. When the temperature rises to 70 degrees, keep the temperature and pressure and react for 4 hours.

[0250] 3) After the reaction is complete, the reaction liquid is introduced into a methanol solution containing a trace amount of hydroquinone for demulsification. When the precipitate no longer increases, it is filtered, and the upper filter cake is dried statically to finally obtain an ethylene-methyl acrylate-maleic anhydride terpolymer with a weight average molecular weight of approximately 35W. The mass ratio of the first structural unit to the second structural unit to the third structural unit is 73:25.5:1.5, and the mass percentage of the third structural unit is 1.5%.

[0251] Example 22

[0252] The main difference between Example 22 and Example 21 is that SEBS (Baling Petrochemical YH-511) is used in addition.

[0253] Example 23

[0254] The main difference between Example 23 and Example 21 is that an additional polyolefin elastomer (Mitsui Chemicals anhydride-grafted POE-MH7510) is used.

[0255] Comparative Example 1

[0256] The main difference between Comparative Example 1 and Example 1 is that PVDF (Solvay 5130) was used instead of the first copolymer and the second copolymer, and NMP solvent was used instead of solvent oil D80 solvent oil.

[0257] II. Performance Testing

[0258] 1) Electrode flexibility

[0259] Take a 20mm*100mm (longitudinal) sample of the prepared positive electrode sheet and take the sample along the electrode sheet rolling direction; place the pre-folded experimental electrode sheet on the experimental table plane and roll it with a 2kg cylindrical roller. After each rolling, observe whether the electrode sheet is transparent. When the electrode sheet is transparent, record the corresponding number of rolling times. The number of rolling times represents the flexibility of the electrode sheet.

[0260] 2) Adhesion force

[0261] Cut the prepared positive electrode sheet into test specimens with dimensions of 20mm*100mm for later use; attach one side of double-sided tape (3M) to the surface of the steel plate electrode sheet, and the other side to the side of the electrode sheet to be tested, and press it with a pressure roller to make it completely adhered to the electrode sheet; bend one end of the current collector in the opposite direction with a bending angle of 180°; use a high-speed rail tensile testing machine to test, fix one end of the steel plate to the lower clamp of the tensile testing machine, fix the bent end of the current collector to the upper clamp, adjust the angle of the current collector to ensure that the upper and lower ends are in a vertical position, and then stretch the specimen at a speed of 50mm / min until the current collector is completely peeled off from the current collector surface, record the displacement and force during the process, take the force when the force is balanced as the bonding force of the electrode sheet, and divide this force by the adhesion length of the specimen as the bonding strength.

[0262] 3) Cohesion

[0263] Cut the prepared positive electrode sheet into test specimens with dimensions of 20mm*100mm for later use; attach one side of double-sided tape (3M) to the surface of the steel plate electrode sheet, and the other side to the side of the electrode sheet to be tested, and press it with a pressure roller to make it completely adhere to the electrode sheet; attach cohesive force test tape to the other side of the electrode sheet and press it with a pressure roller; bend one end of the cohesive force test tape in the opposite direction with a bending angle of 180°; use a high-speed rail tensile testing machine to test, fix one end of the steel plate to the lower clamp of the tensile testing machine, fix the bent end of the current collector to the upper clamp, adjust the angle of the current collector to ensure that the upper and lower ends are in a vertical position, and then stretch the specimen at a speed of 50mm / min until the current collector is completely peeled off from the current collector surface, record the displacement and force during the process, take the force when the force is balanced as the cohesive force of the electrode sheet, and divide this force by the adhesion length of the specimen as the cohesive strength.

[0264] 4) Diaphragm resistance

[0265] Cut the prepared positive electrode sheet into a suitable size; place it in a film resistance meter (Yuaneng Technology BER series electrode resistance meter) for testing, record the film resistance, and take the average value of three measurements as the film resistance value of the electrode sheet.

[0266] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0267] Batteries for each embodiment and comparative example were prepared according to the above method, and various performance parameters were measured. The binder parameters are shown in Table 1, and the performance test results are shown in Table 2.

[0268] Table 2: Performance Test Results

[0269] Based on the above results, it can be seen that the sodium-ion secondary batteries in Examples 1-23 all include a positive electrode sheet. The positive electrode sheet includes a positive current collector and a positive electrode film layer disposed on the positive current collector. The positive electrode film layer includes a positive electrode active material and a binder. The binder includes a copolymer, and the copolymer contains a first structural unit shown in Formula I and a second structural unit shown in Formula II.

[0270] Where R 11 R 12 R 13 R 14 R 21 R 22 R 23 Each is independently selected from hydrogen, unsubstituted or substituted C. 1-3 Alkyl, the substituted C 1-3 The substituent group in the alkyl group is selected from at least one of ester group, carboxyl salt, acid anhydride, hydroxyl group, amide group, and carboxyl group;

[0271] R 24 Selected from hydrogen, inorganic cations, unsubstituted or substituted C 1-3 Alkyl, the substituted C 1-3 The substituent group in the alkyl group is selected from at least one of ester group, carboxyl salt, acid anhydride, hydroxyl group, amide group, and carboxyl group.

[0272] As can be seen from the comparison between Examples 1-23 and Comparative Example 1, the use of this sodium-ion secondary battery reduces the use of fluorine and the positive electrode has better flexibility.

[0273] A comparison of Examples 1-3, 8-10 and Example 15 shows that when the copolymer also includes a third structural unit with a mass percentage of 0.5%-2%, the positive electrode film has higher bonding strength, cohesive strength and lower film resistance.

[0274] As can be seen from the comparison between Examples 1 and 5 and Example 4, when the content of copolymer is 1.5%-2%, the positive electrode film layer has high bonding strength and cohesive strength.

[0275] As can be seen from the comparison of Examples 1, 6 and 7, when the weight ratio of the first copolymer to the second copolymer is 1:2 to 1:1, the positive electrode film has higher bonding strength, cohesive strength and lower film resistance.

[0276] As can be seen from the comparison between Examples 1 and 8 and Examples 15 and 16, when the copolymer is ester-modified and / or anhydride-modified, the positive electrode film has higher bonding strength, cohesive strength and lower film resistance.

[0277] A comparison of Examples 1 and 18 with Example 17 shows that when the mass percentage of the third structural unit is 1.5%-2%, the positive electrode film layer has high bonding strength and cohesive strength.

[0278] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A sodium-ion secondary battery comprising a positive electrode sheet, the positive electrode sheet comprising a positive electrode current collector and a positive electrode film layer disposed on the positive electrode current collector, the positive electrode film layer comprising a positive electrode active material and a binder, the binder comprising a copolymer, the copolymer containing a first structural unit represented by Formula I and a second structural unit represented by Formula II, Formula I Formula II Where R 11 R 12 R 13 R 14 R 21 R 22 R 23 Each is independently selected from hydrogen, unsubstituted or substituted C. 1-3 Alkyl, the substituted C 1-3 The substituent group in the alkyl group is selected from at least one of ester group, carboxyl salt, acid anhydride, hydroxyl group, amide group, and carboxyl group; R 24 Selected from hydrogen, inorganic cations, unsubstituted or substituted C 1-3 Alkyl, the substituted C 1-3 The substituent group in the alkyl group is selected from at least one of ester group, carboxyl salt, acid anhydride, hydroxyl group, amide group, and carboxyl group.

2. The sodium-ion secondary battery according to claim 1, wherein R 11 R 12 R 13 R 14 Each is independently selected from hydrogen and unsubstituted C. 1-3 Alkyl; and / or, the R 21 R 22 R 23 R 24 Each is independently selected from hydrogen and unsubstituted C. 1-3 alkyl.

3. The sodium-ion secondary battery according to claim 1 or 2, wherein the copolymer further comprises a third structural unit, the third structural unit being derived from one or more of maleic anhydride monomers and glycidyl methacrylate monomers.

4. The sodium-ion secondary battery according to any one of claims 1-3, wherein the mass percentage of the first structural unit is 70%-80%, the mass percentage of the second structural unit is 20%-30%, and the mass percentage of the third structural unit is 0%-5%, calculated based on the total mass of the first structural unit, the second structural unit, and the third structural unit.

5. The sodium-ion secondary battery according to any one of claims 1-4, wherein the mass percentage of the third structural unit is 0.5%-2%, calculated based on the total mass of the first structural unit, the second structural unit, and the third structural unit.

6. The sodium-ion secondary battery according to any one of claims 1-5, wherein the copolymer comprises one or more of ethylene-acrylate-maleic anhydride copolymer, ethylene-acrylate-glycidyl methacrylate copolymer, ethylene-acrylate copolymer, and ethylene-acrylate-methacrylic acid copolymer.

7. The sodium-ion secondary battery according to any one of claims 1-6, wherein the weight-average molecular weight of the copolymer is 50,000 to 500,000.

8. The sodium-ion secondary battery according to any one of claims 1-7, wherein the copolymer comprises a first copolymer and a second copolymer, the first copolymer and the second copolymer being independently selected from one or more of ethylene-acrylate-maleic anhydride copolymer, ethylene-acrylate-glycidyl methacrylate copolymer, ethylene-acrylate copolymer, and ethylene-acrylate-methacrylic acid copolymer, wherein the weight-average molecular weight of the first copolymer is 50,000 to 250,000, and the weight-average molecular weight of the second copolymer is 300,000 to 500,000.

9. The sodium-ion secondary battery according to claim 8, wherein the weight ratio of the first copolymer to the second copolymer is 1:5 to 5:

1.

10. The sodium-ion secondary battery according to any one of claims 1-9, wherein the content of said copolymer is 0.5%-2.5% based on the total weight of said positive electrode film.

11. The sodium-ion secondary battery according to any one of claims 1-10, wherein the binder further comprises one or more of styrene-ethylene-butene-styrene block copolymer, styrene-isoprene-styrene copolymer, styrene-butadiene-styrene copolymer, hydrogenated styrene-isoprene-styrene copolymer, polyolefin elastomer, and ethylene-vinyl acetate copolymer.

12. A sodium-ion secondary battery positive electrode slurry, comprising a positive electrode active material, a binder, and a solvent. The binder comprises a copolymer containing a first structural unit represented by Formula I and a second structural unit represented by Formula II, Where R 11 R 12 R 13 R 14 R 21 R 22 R 23 Each is independently selected from hydrogen, unsubstituted or substituted C. 1-3 Alkyl, the substituted C 1-3 The substituent group in the alkyl group is selected from at least one of ester group, carboxyl salt, acid anhydride, hydroxyl group, amide group, and carboxyl group. R 24 selected from the group consisting of hydrogen, inorganic cation, unsubstituted or substituted C 1-3 alkyl, the substituents in the substituted C 1-3 alkyl are selected from the group consisting of ester group, carboxylate salt, acid anhydride, hydroxyl group, amide group, at least one of carboxyl groups, The solvent includes solvent oil.

13. The sodium-ion secondary battery positive electrode slurry according to claim 12, wherein the solvent oil comprises D80 solvent oil.

14. An electrical device comprising a sodium-ion secondary battery according to any one of claims 1-11.

15. Use of a copolymer as a binder in a secondary battery, the copolymer containing structural units represented by Formula I and structural units represented by Formula II, ###0001### Formula I ###0002### Formula II Where R 11 R 12 R 13 R 14 R 21 R 22 R 23 Each is independently selected from hydrogen, unsubstituted or substituted C. 1-3 Alkyl, the substituted C 1-3 The substituent group in the alkyl group is selected from at least one of ester group, carboxyl salt, acid anhydride, hydroxyl group, amide group, and carboxyl group; R 24 Selected from hydrogen, inorganic cations, unsubstituted or substituted C 1-3 Alkyl, the substituted C 1-3 The substituent group in the alkyl group is selected from at least one of ester group, carboxyl salt, acid anhydride, hydroxyl group, amide group, and carboxyl group.

Images