Negative pole piece, secondary battery, electric device and modified polyacrylic acid binder

Abstract

The invention provides a negative electrode plate, which comprises a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, and is characterized in that the negative electrode film layer comprises a modified polyacrylic acid binder, and the modified polyacrylic acid binder comprises a lithium carboxylate group, an amide group, a nitrile group and an ester group, in the modified polyacrylic acid binder, the mass content of the lithium element is 3.0%-8.9% by mass. The secondary battery provided by the invention has improved charging capability, cycle performance and initial coulombic efficiency.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a negative electrode sheet, a secondary battery, an electrical device, and a modified polyacrylic adhesive. Background Technology

[0002] In recent years, with the increasingly wide application of rechargeable batteries, they have been widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, aerospace, and many other fields. With the significant development of rechargeable batteries, higher requirements have been placed on their charging capacity, cycle performance, and initial coulombic efficiency.

[0003] Therefore, improving the charging capacity, cycle performance, and initial coulombic efficiency of secondary batteries has become an urgent technical problem to be solved. Summary of the Invention

[0004] This application is made in view of the above-mentioned problems, and its purpose is to provide a negative electrode sheet, a secondary battery, an electrical device, and a modified polyacrylic acid binder, thereby improving the charging capacity, cycle performance, and initial coulombic efficiency of the secondary battery.

[0005] To achieve the above objectives, this application provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector. The negative electrode film layer is characterized by comprising a modified polyacrylic acid binder, wherein the modified polyacrylic acid binder contains lithium carboxylate groups, amide groups, nitrile groups, and ester groups, and the lithium element content in the modified polyacrylic acid binder is 3.0%-8.9% by mass. By including a specific amount of lithium element in the modified polyacrylic acid binder, it is beneficial to improve the lithium-ion transport rate and compensate for the lithium-ion consumption during the SEI formation process, thereby improving the charging capacity, cycle performance, and initial coulombic efficiency of the secondary battery.

[0006] In some embodiments, the lithium content in the modified polyacrylic binder is 3.0% to 7.65% by mass. This further improves the charging capability, cycle performance, and initial coulombic efficiency of the secondary battery.

[0007] In some embodiments, the modified polyacrylic adhesive comprises structural units derived from monomers of formula 1, formula 2, formula 3, and formula 4.

[0008]

[0009] In the formula, R1, R2, R3, R4, R5, R6, R7, R8, R9, R 10 R 11 R 12 Each is independently selected from hydrogen or substituted or unsubstituted C1-C6 alkyl groups, R 13 The alkyl groups are either substituted or unsubstituted (C1-C20). This is beneficial for improving the charging capacity, cycle performance, and initial coulombic efficiency of secondary batteries. Furthermore, it also improves the adhesion of the binder and enhances the stability and flexibility of the slurry.

[0010] In some implementations, R1, R2, R3, R4, R5, R6, R7, R8, R9, R 10 R 11 R 12 Each is independently hydrogen, R 13 It is a C6-C12 alkyl group.

[0011] In some embodiments, the molar content of the structural units of the monomer derived from Formula 1 is greater than or equal to 40% and less than 100% relative to the total molar content of the structural units of the monomer derived from Formula 1, the monomer derived from Formula 2, the monomer derived from Formula 3, and the monomer derived from Formula 4. This is beneficial for improving lithium-ion transport efficiency and enhancing the charging capacity and cycle performance of the secondary battery.

[0012] In some embodiments, relative to the total molar content of the structural units of the monomers derived from Formula 1, Formula 2, Formula 3, and Formula 4, the molar content of the structural units of the monomers derived from Formula 2 is greater than 0 and less than or equal to 50%, the molar content of the structural units of the monomers derived from Formula 3 is greater than 0 and less than or equal to 50%, and the molar content of the structural units of the monomers derived from Formula 4 is greater than 0 and less than or equal to 50%. This is beneficial for improving the charging capacity of the secondary battery and enhancing the stability and flexibility of the slurry.

[0013] In some embodiments, the content of the modified polyacrylic binder is 0.5% to 3% by mass relative to the total weight of the negative electrode film. This helps to reduce the impedance of the secondary battery and improve its charging capacity and cycle performance.

[0014] In some embodiments, the modified polyacrylic acid adhesive has a weight-average molecular weight of 200,000 to 1,000,000. This is beneficial for improving the bonding performance and structural stability of the modified polyacrylic acid adhesive molecules.

[0015] In some embodiments, the negative electrode film layer further includes a carboxymethyl cellulose additive, and the mass content of the carboxymethyl cellulose additive is 0.1%-1.5% relative to the total mass of the negative electrode film layer. This helps to maintain the stability of the negative electrode slurry.

[0016] In some embodiments, the carboxymethyl cellulose additive contains 0-4% by mass of lithium. This is beneficial for further increasing the lithium content, thereby improving the charging capacity, cycle performance, and initial coulombic efficiency of the secondary battery.

[0017] The second aspect of this application also provides a secondary battery, which includes the negative electrode provided in the first aspect of this application.

[0018] In some embodiments, the secondary battery includes a positive electrode sheet, the positive electrode sheet includes a positive electrode active material, and the positive electrode active material includes a lithium phosphate.

[0019] A third aspect of this application also provides an electrical device, which includes a secondary battery according to the second aspect of this application.

[0020] A fourth aspect of this application also provides a modified polyacrylic acid binder comprising lithium carboxylate groups, amide groups, nitrile groups, and ester groups, wherein the lithium element content in the modified polyacrylic acid binder is 3.0%-8.9% by mass. By including a specific amount of lithium element in the modified polyacrylic acid binder, the lithium-ion transport efficiency is improved, and the lithium-ion consumption during SEI formation is supplemented, which is beneficial to improving the charging capacity, cycle performance, and first coulombic efficiency of the secondary battery.

[0021] In some embodiments, the lithium content in the modified polyacrylic binder is 3.0% to 7.65% by mass. This further improves the charging capability, cycle performance, and initial coulombic efficiency of the secondary battery.

[0022] In some embodiments, the modified polyacrylic adhesive comprises structural units derived from monomers of formula 1, formula 2, formula 3, and formula 4.

[0023]

[0024] In the formula, R1, R2, R3, R4, R5, R6, R7, R8, R9, R 10 R 11 R 12 Each is independently selected from hydrogen or substituted or unsubstituted C1-C6 alkyl groups, R 13The alkyl groups are either substituted or unsubstituted (C1-C20). This is beneficial for improving the charging capacity, cycle performance, and initial coulombic efficiency of secondary batteries. Furthermore, it also improves the adhesion of the binder and enhances the stability and flexibility of the slurry.

[0025] In some implementations, R1, R2, R3, R4, R5, R6, R7, R8, R9, R 10 R 11 R 12 Each is independently hydrogen, R 13 It is a C6-C12 alkyl group.

[0026] In some embodiments, the molar content of the structural units of the monomer derived from Formula 1 is greater than or equal to 40% and less than 100% relative to the total molar content of the structural units of the monomer derived from Formula 1, the monomer derived from Formula 2, the monomer derived from Formula 3, and the monomer derived from Formula 4. This is beneficial for improving lithium-ion transport efficiency and enhancing the charging capacity and cycle performance of the secondary battery.

[0027] In some embodiments, relative to the total molar content of the structural units of the monomers derived from Formula 1, Formula 2, Formula 3, and Formula 4, the molar content of the structural units of the monomers derived from Formula 2 is greater than 0 and less than or equal to 50%, the molar content of the structural units of the monomers derived from Formula 3 is greater than 0 and less than or equal to 50%, and the molar content of the structural units of the monomers derived from Formula 4 is greater than 0 and less than or equal to 50%. This is beneficial for improving the charging capacity of the secondary battery and enhancing the stability and flexibility of the slurry.

[0028] In some embodiments, the modified polyacrylic acid adhesive has a weight-average molecular weight of 200,000 to 1,000,000. This is beneficial for improving the bonding performance and structural stability of the modified polyacrylic acid adhesive molecules. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of a battery cell according to one embodiment of this application.

[0030] Figure 2 yes Figure 1 An exploded view of a battery cell according to one embodiment of this application is shown.

[0031] Figure 3 This is a schematic diagram of a battery module according to one embodiment of this application.

[0032] Figure 4 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0033] Figure 5 yes Figure 4 An exploded view of a battery pack according to one embodiment of this application is shown.

[0034] Figure 6 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0035] Explanation of reference numerals in the attached figures:

[0036] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly Detailed Implementation

[0037] The following detailed description, with appropriate reference to the accompanying drawings, specifically discloses embodiments of the negative electrode sheet, secondary battery, electrical device, and modified polyacrylic acid binder of this application. However, unnecessary detailed descriptions 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.

[0038] 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.

[0039] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0040] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0041] Unless otherwise specified, the terms used in this application have the common meanings as commonly understood by those skilled in the art.

[0042] Unless otherwise specified, the values ​​of the parameters mentioned in this application can be determined using various testing methods commonly used in the art, for example, according to the testing methods given in this application.

[0043] Unless otherwise specified, in this application, the term "active ion" refers to ions that can be inserted and extracted back and forth between the positive and negative electrodes of a secondary battery, including but not limited to lithium ions.

[0044] Currently, based on market demand, there is a need to further improve the charging capacity of secondary batteries, especially lithium iron phosphate batteries. In response, some reports have suggested using lithium polyacrylate in the negative electrode. While this improves the battery's charging capacity, the use of pure lithium acrylate as a binder results in poor processing performance during the preparation of the negative electrode slurry. The low binding force of the binder leads to poor stability of the negative electrode slurry, making it prone to sedimentation and unable to produce a qualified negative electrode sheet. Others have proposed using lithium carboxymethyl cellulose (CMC-Li) in the negative electrode film. However, CMC-Li has a low lithium content, limiting its improvement on the electrical performance of secondary batteries. Furthermore, increasing the lithium content in the negative electrode film by increasing the CMC content leads to excessively high viscosity of the negative electrode slurry, reduced solids content, and poor processing performance. Additionally, CMC-Li additives can coat the negative electrode active material, and increased CMC content leads to increased impedance in the negative electrode film, thus affecting the charging capacity and cycle performance of the secondary battery.

[0045] Based on this, this application proposes a negative electrode sheet, a modified polyacrylic acid binder, a secondary battery, and an electrical device, thereby improving the charging capacity, cycle performance, and initial coulombic efficiency of the secondary battery. The following is a detailed description.

[0046] Negative electrode sheet

[0047] This application proposes a negative electrode sheet, comprising a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, the negative electrode film layer comprising a modified polyacrylic acid binder, the modified polyacrylic acid binder comprising lithium carboxylate groups, amide groups, nitrile groups and ester groups, and wherein the lithium element content in the modified polyacrylic acid binder is 3.0%-8.9% by mass.

[0048] In this application, by including lithium carboxylate groups in the modified polyacrylic acid binder, thereby containing a specific amount of lithium, the number of lithium-ion transport channels can be increased, lithium-ion transport efficiency can be improved, and the battery's charging capability can be enhanced. Due to the accelerated lithium-ion transport rate, the battery impedance is reduced, thus improving cycle performance. Furthermore, by containing lithium, active lithium ions can be replenished in the secondary battery, and during the formation process, lithium-ion consumption during SEI formation can be compensated, thereby improving the initial coulombic efficiency of the secondary battery.

[0049] By including amide, nitrile, and ester groups in the modified polyacrylic binder, the amide groups give the binder a negative charge, causing different chain segments in the polymer to repel each other, allowing the polymer to extend in solution and enabling the chain segments to entangle with each other, thereby improving the adhesive strength of the binder and thus improving the viscosity and stability of the negative electrode slurry. The nitrile groups give the modified polyacrylic binder a strong polar group, increasing the interaction force between the binder and other substances and improving the polymer's adhesive strength. The ester groups have hydrophobic properties, forming a better affinity with the negative electrode active material and providing better encapsulation of the negative electrode active material, which is beneficial to improving the stability of the negative electrode slurry.

[0050] In the modified polyacrylic binder, the lithium content can be, for example, 3.0 wt%, 3.5 wt%, 4 wt%, 4.5 wt%, 5 wt%, 5.5 wt%, 6 wt%, 6.5 wt%, 7 wt%, 7.5 wt%, 7.55 wt%, 7.6 wt%, 7.65 wt%, 7.7 wt%, 7.8 wt%, 7.9 wt%, 8 wt%, 8.5 wt%, 8.9 wt%, or a range of any two of these values. Optionally, the lithium content is 3.0 wt% to 7.65 wt%, more preferably 4.5 wt% to 7.65 wt%. This further improves the charging capacity, cycle performance, and initial coulombic efficiency of the secondary battery.

[0051] In some embodiments, the modified polyacrylic acid adhesive comprises structural units derived from the monomers shown in Formula 1, Formula 2, Formula 3, and Formula 4.

[0052]

[0053] In the formula, R1, R2, R3, R4, R5, R6, R7, R8, R9, R 10 R 11 R 12 Each is independently selected from hydrogen or substituted or unsubstituted C1-C6 alkyl groups, R 13It is a substituted or unsubstituted C1-C20 alkyl group.

[0054] Examples of C1-C6 alkyl groups include straight-chain or branched alkyl groups having 1-6 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, pentyl, and hexyl.

[0055] Examples of C1-C20 alkyl groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, pentyl, hexyl, 2-methylhexyl, heptyl, 2-methylheptyl, octyl, nonyl, decyl, undecyl, dodecyl, and other straight-chain or branched alkyl groups having 1-20 carbon atoms.

[0056] In this document, the term "substitution" means that at least one hydrogen atom of the above-mentioned group is substituted by a substituent, which can be independently selected from: hydroxyl, mercapto, amino, cyano, nitro, halogen atom (e.g., fluorine, chlorine, bromine, etc.), aryl (e.g., phenyl, naphthyl, etc.), heteroaryl (referring to an aryl group in which one or more carbon atoms are substituted by an oxygen atom, nitrogen atom, phosphorus atom or sulfur atom, such as 4-pyridyl, 2-imidazolyl, 3-pyrazolyl and isoquinolinyl), alkyl, alkoxy, alkenyl, alkynyl, etc.

[0057] In some implementations, R1, R2, R3, R4, R5, R6, R7, R8, R9, R 10 R 11 R 12 Each is independently hydrogen, R 13 It is a C6-C12 alkyl group.

[0058] Examples of C6-C12 alkyl groups include hexyl, 2-methylhexyl, heptyl, 2-methylheptyl, octyl, nonyl, decyl, undecyl, dodecyl, and other straight-chain or branched alkyl groups having 6-12 carbon atoms.

[0059] The inventors have discovered that structural units derived from the monomer shown in Formula 1 (lithium acrylate structural units) in modified polyacrylic acid binders can provide lithium-ion transport channels. Increasing the number of these structural units in the modified polyacrylic acid binder increases the number of lithium-ion transport channels, improves the lithium-ion transport rate, and thus enhances the charging capacity of the secondary battery. Furthermore, by introducing structural units derived from the monomer shown in Formula 2 (acrylonitrile structural units) into the modified polyacrylic acid binder, strong polar groups are introduced, enhancing the interaction between the binder and other substances, thereby strengthening the polymer's adhesion. Introducing structural units derived from the monomer shown in Formula 3 (acrylamide structural units) makes the binder negatively charged, generating repulsive forces between different chain segments in the polymer. This causes the polymer to extend in solution and allows the chain segments to entangle, improving the binder's adhesion, thereby improving the viscosity of the negative electrode slurry and enhancing its stability. By introducing structural units (acrylate structural units) derived from the monomer shown in Formula 4, the hydrophobic properties of the ester groups lead to better affinity with the negative electrode active material, resulting in better encapsulation of the negative electrode active material. This is beneficial for improving the stability of the prepared slurry and the coating quality of the slurry. In addition, the structural units derived from the monomer shown in Formula 4 can include longer side chain groups, which reduces the interaction between polymer molecular chain segments and makes it easier for single bond rotation to occur. This can increase the flexibility of the prepared slurry, reduce the stress shrinkage of the coating during coating, and reduce cracking and tab wrinkling problems.

[0060] In some embodiments, the molar content of the structural units of the monomer derived from Formula 1 is greater than or equal to 40% and less than 100% relative to the total molar content of the structural units of the monomers derived from Formula 1, Formula 2, Formula 3, and Formula 4. This increases the number of lithium-ion transport channels, thereby improving lithium-ion transport efficiency and enhancing the charging capability of the secondary battery. Exemplarily, the molar content of the structural units of the monomer derived from Formula 1 can be 40%, 50%, 60%, 70%, 80%, 90%, 95%, 99%, or a value within a range of any two of these values. Optionally, the molar content of the structural units of the monomer derived from Formula 1 is 40%-95%, and more preferably 50%-90%. The monomer shown in Formula 1 can be, for example, lithium acrylate.

[0061] In some embodiments, the molar content of the structural units of the monomer derived from Formula 2 is greater than 0 and less than or equal to 50% relative to the total molar content of the structural units of the monomers derived from Formula 1, Formula 2, Formula 3, and Formula 4. The structural units of the monomer derived from Formula 2 have strong polarity and can generate strong interaction forces with other substances, thus exhibiting good adhesion and enhancing the adhesive strength of the modified polyacrylic adhesive, enabling the modified polyacrylic adhesive to achieve both good lithium-ion transport rate and good adhesion. Exemplarily, the molar content of the structural units of the monomer derived from Formula 2 can be 1%, 5%, 10%, 15%, 20%, 30%, 40%, 50%, or a value within a range of any two of these values. Optionally, the molar content of the structural units of the monomer derived from Formula 2 is 1%-40%, more preferably 1%-30%. Acrylonitrile, for example, can be the monomer shown in Formula 2.

[0062] In some embodiments, the molar content of the structural units of the monomer derived from Formula 3 is greater than 0 and less than or equal to 50% relative to the total molar content of the structural units of the monomers derived from Formula 1, Formula 2, Formula 3, and Formula 4. The structural units of the monomer derived from Formula 3 cause the binder to carry a negative charge, generating repulsive forces between different chain segments in the polymer. This allows the polymer to extend in solution and enables the chain segments to entangle with each other, improving the adhesive strength of the binder. When the modified polyacrylic acid binder is used to prepare a negative electrode slurry, the stability of the negative electrode slurry can be improved. Exemplarily, the molar content of the structural units of the monomer derived from Formula 3 can be 1%, 5%, 10%, 15%, 20%, 30%, 40%, 50%, or a value within a range of any two of these values. Optionally, the molar content of the structural units of the monomer derived from Formula 3 is 1%-20%, and more preferably 1%-10%. Acrylamide, for example, can be a monomer of Formula 3.

[0063] In some embodiments, the molar content of the structural units of the monomer derived from Formula 4 is greater than 0 and less than or equal to 50% relative to the total molar content of the structural units of the monomers derived from Formula 1, Formula 2, Formula 3, and Formula 4. The ester groups in the structural units of the monomer derived from Formula 4 have hydrophobic properties, forming a better affinity with the negative electrode active material and providing better encapsulation of the negative electrode active material, thus acting as a stabilizer. When the modified polyacrylic acid binder is used to prepare the negative electrode slurry, it can improve the stability of the negative electrode slurry. Furthermore, the structural units of the monomer derived from Formula 4 may include longer side chain groups, reducing the interaction between polymer molecular chain segments and making internal rotation of single bonds easier to occur. This increases the flexibility of the negative electrode slurry, reduces stress shrinkage of the coating during coating, and reduces cracking and tab wrinkling problems. For example, the molar content of the structural units of the monomer derived from Formula 4 can be 1%, 5%, 10%, 15%, 20%, 30%, 40%, 50%, or any two of these values ​​within a range. Optionally, the molar content of the structural units of the monomer derived from Formula 4 can be 1%-40%, and more preferably 1%-25%.

[0064] In this application, the monomer shown in Formula 4 can be, for example, an acrylate. The acrylate includes C1-C20 alkyl esters of acrylic acid. C1-C20 alkyl refers to a straight-chain or branched alkyl group having 1-20 carbon atoms, such as 1-15 carbon atoms, 1-10 carbon atoms, or 1-5 carbon atoms. Exemplarily, acrylates include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, and butyl acrylate, but are not limited thereto.

[0065] In some embodiments, the weight-average molecular weight of the modified polyacrylic adhesive is between 200,000 and 1,000,000. In this application, the weight-average molecular weight of the modified polyacrylic adhesive falls within this range, which is beneficial for improving the bonding performance and structural stability of the modified polyacrylic adhesive molecules. Exemplarily, the weight-average molecular weight of the modified polyacrylic adhesive can be a value between 200,000, 300,000, 400,000, 500,000, 600,000, 700,000, 800,000, 900,000, 1,000,000, and any two of these values.

[0066] In some embodiments, the content of the modified polyacrylic binder is 0.5% to 3% by mass relative to the total weight of the negative electrode film. The content of the modified polyacrylic binder within this range is beneficial for improving the lithium-ion transport rate and electron transport rate of the negative electrode sheet, reducing the impedance of the secondary battery, and improving the charging capacity and cycle performance of the secondary battery. Exemplarily, the content of the modified polyacrylic binder relative to the total weight of the negative electrode film can be 0.5% by mass, 0.8% by mass, 1.0% by mass, 1.2% by mass, 1.4% by mass, 1.5% by mass, 1.8% by mass, 2.0% by mass, 2.5% by mass, 3.0% by mass, or any two of these values. Optionally, the content of the modified polyacrylic binder relative to the total weight of the negative electrode film is 0.5% to 2.3% by mass, or alternatively, 0.5% to 1.8% by mass.

[0067] In some embodiments, the negative electrode film layer further includes carboxymethyl cellulose (CMC) additives, such as CMC-Na or CMC-Li. In some embodiments, the mass percentage of the carboxymethyl cellulose additive relative to the total weight of the negative electrode film layer is 0.1% to 1.5% by mass. By keeping the content of the carboxymethyl cellulose additive within the above range, a gel can be formed in water, increasing the consistency of the slurry and facilitating the dispersion of the negative electrode active material, preventing particle sedimentation, and thus helping to maintain the stability of the negative electrode slurry. Exemplarily, the mass percentage of the carboxymethyl cellulose additive relative to the total weight of the negative electrode film layer can be 0.1%, 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, or a value between any two of these values.

[0068] In some embodiments, the carboxymethyl cellulose additive contains 0-4% by mass of lithium. The carboxymethyl cellulose additive has abundant carboxyl groups, which can increase the number of lithium binding sites in the negative electrode film, thereby increasing the number of lithium-ion transport channels and the lithium-ion transport rate, thus improving the charging capacity of the secondary battery. Furthermore, the lithium-bound carboxymethyl cellulose additive contains a relatively high amount of lithium, which can act as an active lithium-ion supplement, replenishing active lithium ions during the use of the secondary battery, thereby increasing the initial charge-discharge efficiency and cycle performance of the secondary battery. When the lithium content in the carboxymethyl cellulose additive is within the above-mentioned range, it is beneficial to improve the charging capacity and initial coulombic efficiency of the secondary battery. For example, in the carboxymethyl cellulose additive, the lithium content is 0% by mass, 0.5% by mass, 1.0% by mass, 1.5% by mass, 2.0% by mass, 2.5% by mass, 2.7% by mass, 2.9% by mass, 3.09% by mass, 3.2% by mass, 3.5% by mass, 3.7% by mass, 3.9% by mass, 4.0% by mass, or a value between any two of these values.

[0069] For example, the preparation method of the above-mentioned modified polyacrylic adhesive can be adopted by the following method, which includes the following steps:

[0070] Synthesis steps: The monomers shown in Formula 1', Formula 2, Formula 3, and Formula 4 are mixed, and an initiator is added to react and obtain a multi-component copolymer.

[0071]

[0072] In the formula, R1, R2, R3, R4, R5, R6, R7, R8, R9, R 10 R 11 R 12 Each is independently selected from hydrogen or substituted or unsubstituted C1-C6 alkyl groups, R 13 These are substituted or unsubstituted C1-C20 alkyl groups. The definitions of the groups and substituents here are the same as above; and

[0073] Lithification step: Lithium hydroxide is added to the multi-component copolymer to neutralize the carboxyl groups of the multi-component copolymer, and finally a modified polyacrylic acid binder with a pH of 7.0-8.5 is obtained. The mass percentage of lithium in the modified polyacrylic acid binder is 3.0%-8.9% by mass.

[0074] In the above method, monomers shown in Formula 1', Formula 2, Formula 3, and Formula 4 are used as raw materials to synthesize multi-component copolymers. The monomer shown in Formula 1' provides lithium binding sites, increases the number of lithium-ion migration channels, and improves the lithium-ion migration rate, thereby enhancing the charging capacity of the secondary battery. The monomer shown in Formula 2 introduces strongly polar groups into the modified polyacrylic acid binder, enhancing the interaction between the binder and other substances, thus strengthening the polymer's adhesion. The monomer shown in Formula 3 makes the binder negatively charged, generating repulsive forces between different chain segments in the polymer, causing the polymer to extend in solution and allowing the chain segments to entangle, improving the binder's adhesion and thus improving the viscosity and stability of the negative electrode slurry. The ester group in the monomer shown in Formula 4 has hydrophobic properties, forming a better affinity with the negative electrode active material and providing better encapsulation of the negative electrode active material. This makes the prepared modified polyacrylic acid binder act as a stabilizer, stabilizing the slurry in subsequent battery preparation. Furthermore, the structural units derived from the monomers shown in Formula 4 may include longer side chain groups, which reduces the interaction between polymer molecular chain segments and makes it easier for single bond rotation to occur. This can increase the flexibility of the prepared slurry, reduce the stress shrinkage of the coating during coating, and reduce cracking and tab wrinkling problems.

[0075] The lithiation step uses lithium hydroxide to neutralize the acrylic acid units in the above-mentioned multi-component copolymer to obtain lithium acrylate units, so that the prepared modified polyacrylic acid binder contains lithium elements, thereby increasing the active lithium ions participating in the charge and discharge process in the secondary battery, effectively improving the first charge and discharge efficiency of the secondary battery. In addition, it increases the number of lithium ion transfer channels in the secondary battery, thereby improving the lithium ion migration rate and improving the charging capability of the secondary battery.

[0076] In some embodiments, relative to the total molar amount of the monomers shown in Formula 1', Formula 2, Formula 3, and Formula 4, the amount of monomer shown in Formula 1' added is greater than or equal to 40% and less than 100%, the amount of monomer shown in Formula 2 added is greater than 0 and less than or equal to 50%, the amount of monomer shown in Formula 3 added is greater than 0 and less than or equal to 50%, and the amount of monomer shown in Formula 4 added is greater than 0 and less than or equal to 50%. This ensures that the molar content of the monomer shown in Formula 1' is within the aforementioned range, resulting in a multi-component copolymer containing a large number of carboxylic acid groups. This provides numerous lithium binding sites, improves lithium-ion transport channels, thereby enhancing lithium-ion transport efficiency and improving the charging capability of the secondary battery. Exemplarily, the amount of monomer shown in Formula 1' added can be 40%, 50%, 60%, 70%, 80%, 90%, 95%, 99%, or a value within a range of any two of these values. For example, the amount of monomer added as shown in Formula 2 can be 1%, 5%, 10%, 15%, 20%, 30%, 40%, 50%, or a value within a range of any two values ​​therein. For example, the amount of monomer added as shown in Formula 3 can be 1%, 5%, 10%, 15%, 20%, 30%, 40%, 50%, or a value within a range of any two values ​​therein. For example, the amount of monomer added as shown in Formula 4 can be 1%, 5%, 10%, 15%, 20%, 30%, 40%, 50%, or a value within a range of any two values ​​therein.

[0077] In some embodiments, the reaction time in the synthesis step is 4-24 hours. By keeping the reaction time within this range, sufficient reaction between the monomers can be achieved, resulting in a multi-component copolymer with good adhesion. Exemplarily, the reaction time in the synthesis step can be 4 hours, 6 hours, 8 hours, 10 hours, 15 hours, 20 hours, 24 hours, or a range consisting of any two of these values. In some optional embodiments, the reaction time in the synthesis step of the modified polyacrylic adhesive is 6-10 hours.

[0078] In some embodiments, the reaction temperature in the synthesis step is 40℃-100℃. Setting the reaction temperature within this range facilitates the complete reaction of the polymer monomers to form the polymer, thereby improving the conversion rate of the polymer monomers. Exemplarily, the reaction temperature in the synthesis step can be 40℃, 50℃, 60℃, 70℃, 80℃, 90℃, 100℃, or a value between any two of these values. In some embodiments, the reaction temperature in the synthesis step of the modified polyacrylic acid adhesive is 50℃-80℃.

[0079] In some embodiments, the initiator comprises a persulfate, optionally ammonium persulfate. In some embodiments, the amount of the initiator added is 0.05% relative to the total weight of the reactants.

[0080] In some embodiments, the weight-average molecular weight of the modified polyacrylic adhesive is between 200,000 and 1,000,000. A weight-average molecular weight within this range is beneficial for improving the bonding performance and structural stability of the polymer molecules. Exemplarily, the weight-average molecular weight of the modified polyacrylic adhesive can be a value between 200,000, 400,000, 500,000, 600,000, 700,000, 800,000, 900,000, 1,000,000, and any two values ​​thereof.

[0081] In some embodiments, the negative electrode film layer may further include an additional binder in addition to the modified polyacrylic binder. The additional 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).

[0082] In addition to the aforementioned binder, the negative electrode film also includes a negative electrode active material. In some embodiments, the negative electrode active material may be a battery-grade negative electrode active material known in the art. 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 battery negative electrode active materials may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0083] 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.

[0084] The negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the opposite surfaces of the negative electrode current collector. 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 material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0085] 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.

[0086] In this application, weight-average molecular weight (MAM) has its common meaning in the art and is the average molecular weight obtained by multiplying the mass of each molecular chain in the polymer by its mole fraction in the polymer. MAM can be determined using methods known in the art, with exemplary testing methods as follows: for example, gel permeation chromatography, such as using a Waters 2695 Isocratic HPLC gel permeation chromatograph (differential refractive index detector 2141). In some embodiments, the testing method uses a 3.0% polystyrene solution sample as a reference, selecting a matched chromatographic column (oil-based: Styragel HT5DMF7.8*300mm + Styragel HT4). A 3.0% polymer gel solution is prepared using purified N-methylpyrrolidone (NMP) solvent, and the prepared solution is allowed to stand for one day for later use. During testing, tetrahydrofuran is first drawn into a syringe for rinsing, repeated several times. Then, 5 ml of the experimental solution is drawn, air is expelled from the syringe, and the needle tip is dried. Finally, the sample solution is slowly injected into the injection port. After the reading stabilizes, the data is acquired, and the weight-average molecular weight is read.

[0087] In this application, viscosity is a physical quantity that measures the magnitude of fluid viscosity. Viscosity can be determined using methods known in the art. An exemplary test method is as follows: The viscosity of the sample is tested using a DV-2TLV Borlefeld viscometer. The test rotor is selected according to the different viscosities corresponding to different rotors. The detection temperature is set to 25±1℃, 12r / min, and the value at the 6th minute is taken as the viscosity value.

[0088] Secondary batteries

[0089] This application also provides a secondary battery. The term "secondary battery" as used herein refers to a single battery cell, a battery module, or a battery pack. These are described below.

[0090] Typically, a secondary battery cell includes a positive electrode, a negative electrode as described in this application, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and extracting. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0091] Positive electrode sheet

[0092] 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, which can be any positive electrode active material known in the art and is not particularly limited.

[0093] In some embodiments, the positive electrode active material includes at least one of the following materials: lithium phosphates with an olivine structure and their modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium phosphates with an olivine structure include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium iron manganese phosphate, and lithium iron manganese phosphate and carbon composites.

[0094] 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.

[0095] 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.).

[0096] During the charging and discharging process of a battery, Li undergoes insertion / extraction and consumption, resulting in varying molar Li content at different discharge states. In the examples of positive electrode active materials in this application, the molar Li content refers to the initial state of the material, i.e., before feeding. When the positive electrode active material is applied to the battery system, the molar Li content changes after charge-discharge cycles.

[0097] In the examples of positive electrode active materials in this application, the molar content of O is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of O will fluctuate.

[0098] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0099] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0100] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0101] electrolytes

[0102] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0103] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0104] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0105] 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.

[0106] 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.

[0107] Separating membrane

[0108] In some embodiments, the battery cell 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.

[0109] 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.

[0110] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0111] In some embodiments, the battery cell may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0112] In some embodiments, the outer packaging of the battery cell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell can also be a flexible package, such as a pouch. The material of the flexible package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0113] This application does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 The example shown is a square-structured battery cell 5.

[0114] In some implementations, refer to Figure 2The outer packaging may include a housing 51 and a top cover assembly 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can cover the opening to close the receiving cavity. A positive electrode sheet, a negative electrode sheet, and a separator can be formed into an electrode assembly 52 through 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 number of electrode assemblies 52 contained in a single battery cell 5 can be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0115] In some implementations, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0116] Figure 3 This is battery module 4, used as an example. (See reference...) Figure 3 In battery module 4, multiple battery cells 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple battery cells 5 can be fixed in place using fasteners.

[0117] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0118] 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.

[0119] Figure 4 and Figure 5 This is battery pack 1 as an example. (See reference...) Figure 4 and Figure 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 body 2 and a lower body 3, with the upper body 2 covering the lower body 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.

[0120] In addition, this application also provides an electrical device, which includes the secondary battery provided in this application. The secondary battery 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.

[0121] As the electrical device, a single battery cell, a battery module, or a battery pack can be selected according to its usage requirements.

[0122] Figure 6 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. 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.

[0123] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0124] Example

[0125] 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.

[0126] Preparation of modified polyacrylic acid binders

[0127] Adhesive 1-1

[0128] Weigh 1023.57 g (13.14 mol) of acrylic acid monomer of formula 1', 23.2 g (0.44 mol) of acrylonitrile monomer of formula 2, 20.76 g (0.29 mol) of acrylamide monomer of formula 3, and 134.5 g (0.73 mol) of n-octyl acrylate monomer of formula 4 and add them to a three-necked flask equipped with a reflux condenser and a stirrer. Add 10.8 kg of deionized water, and then add ammonium persulfate initiator. The amount of initiator is 0.5 g of the total weight of the monomers. 0.05% was reacted at 80℃ with a constant rotation speed of 500 rpm for 8 hours. After the reaction was completed, lithium hydroxide was added for neutralization, and the pH was adjusted to 7-8 to obtain a modified polyacrylic acid adhesive solution with a solid content of 10% and a viscosity of 6350 mPa·s. A small amount of the above modified polyacrylic acid adhesive solution was taken, dried, and then measured by inductively coupled plasma spectrometry (ICP). The mass percentage of lithium in the modified acrylic adhesive was 7.65%.

[0129] Adhesive 1-2 to 1-4

[0130] The preparation methods of adhesives 1-2 to 1-4 are similar to those of adhesive 1-1. The difference is that by adjusting the amount of acrylic acid, acrylonitrile, acrylamide and octyl acrylate added, the molar percentage content of the monomers shown in Formula 1, Formula 2, Formula 3 and Formula 4 in the modified polyacrylic acid adhesive, as well as the lithium content in the modified polyacrylic acid adhesive, are as shown in Table 1.

[0131] Adhesive 1-5 to 1-8

[0132] The preparation methods of adhesives 1-5 to 1-8 are similar to those of adhesive 1-1. The difference is that the types of polymer monomers are adjusted according to the monomers shown in Table 1, and the amount of monomers added is also adjusted so that the molar percentage content of the monomers shown in Formula 1, Formula 2, Formula 3 and Formula 4, as well as the lithium content in the modified polyacrylic adhesive, have the values ​​shown in Table 1.

[0133] Adhesive 1-1'

[0134] The preparation of adhesive 1-1' is similar to that of adhesive 1-1, except that the amount of polymer monomer added is adjusted in the preparation step of the adhesive so that the mass content of Li element in the modified polyacrylic adhesive is 1.96%.

[0135] Adhesive 1-2'

[0136] 1200g of acrylic acid was weighed and added to a three-necked flask equipped with a reflux condenser and a stirrer. 3.6kg of deionized water was added, followed by ammonium persulfate initiator (0.05% of the total monomer weight). The mixture was reacted at 80℃ with a rotation speed of 500 rpm for 8 hours. After the reaction was complete, lithium hydroxide was added for neutralization, and the pH was adjusted to 7-8, yielding a lithium polyacrylate binder solution with a solid content of 25% and a viscosity of 549 mPa·s. A small amount of the above lithium polyacrylate binder solution was dried and analyzed by inductively coupled plasma optical emission spectrometry (ICP). The mass percentage of lithium in the modified acrylic binder was determined to be 8.99%.

[0137] The viscosity of the prepared adhesive slurry was tested according to the following method, and the results are recorded in Table 1.

[0138] Viscosity test:

[0139] The viscosity of the adhesive slurry was tested using a DV-2TLV Bollerfei viscometer. The rotor was selected according to the different viscosities corresponding to different rotors. The test temperature was 25±1℃, the rotation speed was 12r / min, and the value at the 6th minute was taken as the viscosity value.

[0140] Table 1

[0141]

[0142] Example 1

[0143] Preparation of negative electrode sheet

[0144] Artificial graphite, silicon carbide, conductive carbon, SBR binder, the modified polyacrylic acid binder 1-1 prepared above, and sodium carboxymethyl cellulose additive are thoroughly mixed in a deionized water solvent system at a weight ratio of 96.5:0.5:1:1:1. The mixture is then coated onto Cu foil to form a negative electrode film, dried, and cold-pressed to obtain the negative electrode sheet.

[0145] Preparation of positive electrode sheet

[0146] Based on solid content, the positive electrode active material LiFePO4:PVDF:conductive carbon:dispersant sodium dodecyl sulfate is mixed thoroughly in an N-methylpyrrolidone solvent system at a weight ratio of 96:2.5:1:0.5. After being coated onto Al foil by extrusion coating or transfer coating, the mixture is dried and cold-pressed to obtain the positive electrode sheet.

[0147] Preparation of secondary batteries

[0148] PE porous polymer film is used as the separator.

[0149] Lithium salt LiPF6 was added to a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a mass ratio of 35:65 and mixed thoroughly to obtain an electrolyte. The molar concentration of LiPF6 in the electrolyte was 1 mol / L.

[0150] The positive electrode, separator, and negative electrode are wound in sequence and according to size to form a cell. After encapsulation, electrolyte injection, and formation processes, a secondary battery is obtained.

[0151] Examples 2-8

[0152] The preparation methods of the batteries in Examples 2-8 are similar to those in Example 1, except that in the preparation step of the negative electrode sheet, different binders are selected according to Table 2 below, and the amount of binder added is adjusted.

[0153] Comparative Example 1

[0154] The preparation method of the battery in Comparative Example 1 is similar to that in Example 1, except that in the preparation step of the negative electrode sheet, binder 1-1' is selected as a modified polyacrylic acid binder according to Table 2 below.

[0155] Comparative Example 2

[0156] Comparative Example 2 prepared a secondary battery using a method similar to that of Example 1, except that in the negative electrode preparation step, binders 1-2' were selected as binders according to Table 2 below. Because the viscosity of binders 1-2' was too low, the distribution of the negative electrode active material in the negative electrode film layer formed after the negative electrode slurry was coated on the Cu foil was uneven, making it impossible to prepare a usable negative electrode. Therefore, a secondary battery could not be prepared for battery performance testing.

[0157] The secondary batteries prepared in the above embodiments and comparative examples were subjected to performance tests according to the following methods, and the results are shown in Table 2.

[0158] Performance testing

[0159] 1. First Coulomb efficiency

[0160] At 45℃, the secondary battery was charged at a constant current of 0.02C for 10 hours, and the charging capacity C1 was recorded. After resting at 25℃ for 5 minutes, the secondary battery was charged at a constant current of 1 / 3C until the voltage reached 3.65V. Then, it was charged at a constant voltage of 3.65V until the current reached 0.05C, and the charging capacity C2 was recorded. After resting for 5 minutes, the battery was discharged at a constant current of 1 / 3C until the voltage reached 2.0V, and the discharge capacity D0 was recorded. Calculate the initial coulombic efficiency of the secondary battery according to the following supply:

[0161] First Coulomb efficiency (%) = D0 / (C1+C2).

[0162] 2. Cyclic performance

[0163] The cycle performance of a secondary battery is characterized by the number of cycles at 80% SOH. The test method for the number of cycles at 80% SOH is as follows: At 25°C, the secondary battery is charged at a constant current of 1 / 3C until the voltage reaches 3.65V. Then, it is charged at a constant voltage of 3.65V until the current reaches 0.05C. After resting for 5 minutes, the battery is discharged at a constant current of 1 / 3C until the voltage reaches 2.0V. This constitutes one charge-discharge cycle, and the discharge capacity of this cycle is C1. After performing the charge-discharge test according to the above method, the cycle ends when the discharge capacity decays to 80% of the initial value C1. The number of cycles when the capacity decays to 80% is recorded as the "80% SOH cycle number".

[0164] The higher the number of 80% SOH cycles, the better the cycle performance and the longer the cycle life of the battery cell.

[0165] 3. Ratio Performance

[0166] The rate performance of the secondary battery was characterized by testing at 25°C within a 10%–80% SOC equivalent charging window. The test method is as follows:

[0167] At 25℃, the secondary battery was charged at a constant current of 1 / 3C until the charging cutoff voltage of 3.65V. Then, it was charged at a constant voltage of 3.65V until the current reached 0.05C. After resting for 5 minutes, it was discharged at a constant current of 1 / 3C until the discharge cutoff voltage reached 2V. Its actual capacity was recorded as C0. The secondary battery was then sequentially charged at constant currents of 0.5C0, 1C0, 1.5C0, 2C0, 2.5C0, 3C0, 3.5C0, 4C0, and 4.5C0 until the full battery charging cutoff voltage of 3.65V or the negative terminal cutoff potential (whichever comes first). After each charging, it was discharged at 1C0 until the full battery discharge cutoff voltage of 2V. The SOC (State of Charge) was recorded at different charging rates until 10%, 20%, 30%...80%. The negative electrode potential corresponding to the state of charge (SOC) is plotted, and the rate-negative electrode potential curves are drawn for different SOC states. After linear fitting, the charging rate corresponding to the negative electrode potential of 0V under different SOC states is obtained. This charging rate is the charging window for the corresponding SOC state, which is denoted as C10%SOC, C20%SOC, C30%SOC, C40%SOC, C50%SOC, C60%SOC, C70%SOC and C80%SOC respectively. The equivalent charging window of 10%~80%SOC is the average value of the charging window of 10%SOC, 20%SOC, 30%SOC, 40%SOC, 50%SOC, 60%SOC, 70%SOC and 80%SOC.

[0168] A larger equivalent charging window value indicates that the battery can be charged and discharged at a higher rate under set temperature and SOC conditions, and the anode (negative electrode) is less likely to reach the 0V lithium plating potential, thus improving the battery's rate performance. The larger the equivalent charging window, or equivalent rate value, of a battery cell, the better its rate performance.

[0169] Table 2

[0170]

[0171] In the table, " / " indicates data that was not measured.

[0172] Based on the above results, compared with Comparative Example 1, the modified polyacrylic acid binder in the examples, with a lithium content of 3.0%-8.9% by mass, resulted in improved charging performance, cycle performance, and initial coulombic effect of the secondary battery, achieving excellent technical results. In Comparative Example 2, due to the low viscosity of the binder, the solid content of the slurry needed to be increased to meet the coating requirements. However, excessively high solid content would lead to uneven coating, a thick film, and electrode cracking, preventing the formation of a usable negative electrode and thus making it impossible to measure the performance data of the secondary battery.

[0173] Examples 9-11

[0174] The preparation methods of Examples 9-11 are similar to those of Example 1, except that 1-2 are selected as binders, and the content of binder and sodium carboxymethyl cellulose in the negative electrode film layer are adjusted according to Table 3.

[0175] Examples 12-14

[0176] The preparation methods of Examples 12-14 are similar to those of Example 1, except that binder 1-2 is selected as the binder, and lithium carboxymethyl cellulose with a lithium content of 3.09% is used to replace the additive sodium carboxymethyl cellulose. The contents of the binder and lithium carboxymethyl cellulose are adjusted according to Table 3.

[0177] Table 3

[0178]

[0179] Based on the above results, it can be seen that the content of modified polyacrylic acid binder in Examples 9-11 is in the range of 0.5%-3% relative to the negative electrode film. In Examples 12-14, lithium-containing carboxymethyl cellulose additives are added to the negative electrode film. Compared with Comparative Example 1, by adjusting the content of modified polyacrylic acid binder or adding other lithium-containing binders, the lithium content in the negative electrode film is increased, thereby enabling the secondary battery to achieve good charging performance, cycle performance and first coulombic effect, and obtain excellent technical results.

[0180] 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 negative electrode sheet, comprising a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, characterized in that, The negative electrode film layer includes a modified polyacrylic acid binder, which contains lithium carboxylate groups, amide groups, nitrile groups, and ester groups. In the modified polyacrylic adhesive, the lithium content is 3.0%-8.9% by mass.

2. The negative electrode sheet according to claim 1, characterized in that, In the modified polyacrylic adhesive, the lithium content is 3.0%-7.65% by mass.

3. The negative electrode sheet according to claim 1 or 2, characterized in that, The modified polyacrylic acid adhesive comprises structural units derived from the monomers shown in Formula 1, Formula 2, Formula 3, and Formula 4. In the formula, R1, R2, R3, R4, R5, R6, R7, R8, R9, R 10 R 11 R 12 Each is independently selected from hydrogen or substituted or unsubstituted C1-C6 alkyl groups, R 13 It is a substituted or unsubstituted C1-C20 alkyl group.

4. The negative electrode sheet according to claim 3, characterized in that, R1, R2, R3, R4, R5, R6, R7, R8, R9, R 10 R 11 R 12 Each is independently hydrogen, R 13 It is a C6-C12 alkyl group.

5. The negative electrode sheet according to claim 3 or 4, characterized in that, Relative to the total molar content of the structural units of the monomers derived from Formula 1, Formula 2, Formula 3, and Formula 4, the molar content of the structural units of the monomers derived from Formula 1 is greater than or equal to 40% and less than 100%.

6. The negative electrode sheet according to any one of claims 3-5, characterized in that, Relative to the total molar content of the structural units of the monomers derived from Formula 1, Formula 2, Formula 3, and Formula 4, the molar content of the structural units of the monomers derived from Formula 2 is greater than 0 and less than or equal to 50%, the molar content of the structural units of the monomers derived from Formula 3 is greater than 0 and less than or equal to 50%, and the molar content of the structural units of the monomers derived from Formula 4 is greater than 0 and less than or equal to 50%.

7. The negative electrode sheet according to any one of claims 1-6, characterized in that, The modified polyacrylic acid adhesive has a weight-average molecular weight of 200,000 to 1,000,000.

8. The negative electrode sheet according to any one of claims 1-7, characterized in that, The content of the modified polyacrylic binder is 0.5%-3% by mass relative to the total weight of the negative electrode film.

9. The negative electrode sheet according to any one of claims 1-8, characterized in that, The negative electrode film also includes carboxymethyl acrylate additives. Furthermore, the mass content of the carboxymethyl cellulose additive is 0.1%-1.5% relative to the total mass of the negative electrode film.

10. The negative electrode sheet according to claim 9, characterized in that, The carboxymethyl cellulose additive contains 0%-4% by mass of lithium.

11. A secondary battery, characterized in that, The secondary battery includes the negative electrode sheet as described in claims 1-10.

12. The secondary battery according to claim 11, characterized in that, The secondary battery includes a positive electrode sheet, the positive electrode sheet includes a positive electrode active material, and the positive electrode active material includes lithium phosphate.

13. An electrical appliance, characterized in that, Includes the secondary battery as described in claim 11 or 12.

14. A modified polyacrylic acid adhesive, characterized in that, The modified polyacrylic adhesive contains lithium carboxylate groups, amide groups, nitrile groups, and ester groups, and the lithium content in the modified polyacrylic adhesive is 3.0%-8.9% by mass.

15. The modified polyacrylic adhesive according to claim 14, characterized in that, In the modified polyacrylic adhesive, the lithium content is 3.0%-7.65% by mass.

16. The modified polyacrylic adhesive according to claim 14 or 15, characterized in that, The modified polyacrylic acid adhesive comprises structural units derived from the monomers shown in Formula 1, Formula 2, Formula 3, and Formula 4. In the formula, R1, R2, R3, R4, R5, R6, R7, R8, R9, R 10 R 11 R 12 Each is independently selected from hydrogen or substituted or unsubstituted C1-C6 alkyl groups, R 13 It is a substituted or unsubstituted C1-C20 alkyl group.

17. The modified polyacrylic adhesive according to claim 16, characterized in that, R1, R2, R3, R4, R5, R6, R7, R8, R9, R 10 R 11 R 12 Each is independently hydrogen, R 13 It is a C6-C12 alkyl group.

18. The modified polyacrylic adhesive according to claim 16 or 17, characterized in that, Relative to the total molar content of the structural units of the monomers derived from Formula 1, Formula 2, Formula 3, and Formula 4, the molar content of the structural units of the monomers derived from Formula 1 is greater than or equal to 40% and less than 100%.

19. The modified polyacrylic adhesive according to any one of claims 16-18, characterized in that, Relative to the total molar content of the structural units of the monomers derived from Formula 1, Formula 2, Formula 3, and Formula 4, the molar content of the structural units of the monomers derived from Formula 2 is greater than 0 and less than or equal to 50%, the molar content of the structural units of the monomers derived from Formula 3 is greater than 0 and less than or equal to 50%, and the molar content of the structural units of the monomers derived from Formula 4 is greater than 0 and less than or equal to 50%.

20. The modified polyacrylic adhesive according to any one of claims 14-19, characterized in that, The modified polyacrylic acid adhesive has a weight-average molecular weight of 200,000 to 1,000,000.

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