Pole piece, secondary battery, electric device and binder

Abstract

The invention provides a pole piece, a secondary battery, an electric device and a binder. The pole piece comprises a current collector and a film layer arranged on at least one surface of the current collector, the film layer comprises an active material and a binder, the binder comprises a copolymer, the copolymer comprises a structural unit represented by a formula I and a structural unit represented by a formula II, R1 independently comprises H or-CH3, and R2 independently comprises H or-CH3. R2 is selected from substituted or unsubstituted C1-6 alkyl groups; r3 is selected from-(CH2) n-or-(CH2) n-C (= O)-, n is 1-6, and the cycle performance of the secondary battery adopting the pole piece is improved.

Description

Technical Field

[0001] This application relates to the field of battery technology, and more particularly to an electrode, a secondary battery, an electrical device, and an adhesive. Background Technology

[0002] In recent years, with the increasingly wide range of applications, rechargeable batteries 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. Due to the significant development of rechargeable batteries, higher requirements have been placed on their cycle performance.

[0003] As a crucial component of rechargeable batteries, the performance of the positive electrode significantly impacts the overall battery performance. Therefore, improving the cycle performance of rechargeable batteries through modifications to the positive electrode is a pressing technical challenge. Summary of the Invention

[0004] This application is made in view of the above-mentioned problems, and its purpose is to provide an electrode, a secondary battery, an electrical device and an adhesive, wherein the secondary battery using the electrode has improved cycle performance.

[0005] A first aspect of this application provides an electrode comprising a current collector and a film layer disposed on at least one surface of the current collector. The film layer comprises an active material and a binder. The binder comprises a copolymer comprising structural units represented by Formula I and structural units represented by Formula II, wherein R1 independently comprises H or -CH3; and R2 is selected from substituted or unsubstituted C. 1-6 Alkyl; R3 can be selected from -(CH2)n- or -(CH2)nC(=O)-, where n is 1 to 6.

[0006]

[0007] In this application, the structural unit represented by Formula I includes an ester group, and the structural unit represented by Formula II includes a carbonyl group and a hydroxyl group. Thus, the polar groups included in these structural units can form reversible hydrogen bonds with other components in the film layer, or these polar groups can form hydrogen bonds with different bond energies with each other, thereby giving the binder excellent adhesion. Simultaneously, the groups on the side chains of the two structural units can act as a dispersion regulator similar to that of amphiphilic copolymers, exhibiting excellent anti-swelling and anti-gelling properties. This improves the properties of the electrode including the binder, and consequently, enhances the cycle performance of the secondary battery including the electrode.

[0008] In some embodiments, the mass content of the binder, based on the total mass of the film layer, is greater than zero and less than or equal to 1.5 wt%. In this application, the structural unit represented by Formula I includes an ester group, and the structural unit represented by Formula II includes a carbonyl group and a hydroxyl group. Thus, the polar groups included in these structural units can form reversible hydrogen bonds with other components in the film layer, or these polar groups can form hydrogen bonds with different bond energies with each other. This effectively improves the adhesive strength (bonding force) of the binder, allowing a maximum of 1.5 wt% binder in the electrode to exert a considerable bonding effect, thereby increasing the energy density of the battery.

[0009] In some embodiments, the binder content is 0.6 wt% to 1.0 wt% based on the total mass of the membrane layer. Because the copolymers provided in this application have excellent adhesive strength, sufficient bonding can be achieved with a relatively low dosage under the same conditions, thereby contributing to improved energy density of the corresponding secondary battery.

[0010] In some embodiments, the adhesion strength of the electrode is from 5 N / m to 40 N / m. Within the range of adhesive dosage in this application, the electrode can achieve the above-mentioned adhesion strength, thus meeting the stability requirements of the electrode.

[0011] In some embodiments, the electrode exhibits a swelling degree of 10% to 22% after immersion in a carbonate solvent at 60°C for 7 days. Within this swelling range, the film layer does not detach from the current collector, maintaining the electrode's stability and thus contributing to the battery's cycle performance.

[0012] In some embodiments, the copolymer has a weight-average molecular weight of 750,000 to 1,000,000. By controlling the weight-average molecular weight of the copolymer within this range, the binder has a corresponding viscosity, thereby exhibiting excellent bonding properties and reducing the likelihood of the active material detaching from the current collector. Simultaneously, a suitable weight-average molecular weight allows the binder to dissolve completely in the solvent, resulting in a slurry with appropriate viscosity and improved anti-settling properties. This, in turn, gives the electrode good stability and processability.

[0013] In some embodiments, the mass percentage of the structural unit represented by Formula I in the copolymer is 80%-98%, and the mass percentage of the structural unit represented by Formula (II) is 2%-20%. By controlling the mass percentages of the structural units represented by Formula I and Formula (II) within the above ranges, it is beneficial for the polar groups to form reversible hydrogen bonds with other components in the film layer or for these polar groups to form hydrogen bonds with different bond energies with each other. This effectively improves the adhesive strength (adhesive force) of the binder and also improves the wettability of the electrolyte to the electrode to a certain extent, reducing the swelling degree of the electrode. Furthermore, the kinetics of the polymerization reaction are not significantly hindered within this content range, which is beneficial for synthesizing copolymers with a weight-average molecular weight within the preferred range. In addition, the combination of the structural units represented by Formula I and Formula II within the above proportion range can further reduce gelation abnormalities in the positive electrode during coating or cold pressing, resulting in improved processing performance of the electrode.

[0014] In some embodiments, R1 independently includes H or -CH3; R2 is selected from -CH3, -CH2CH3, and -(CH2)3CH3; R3 is selected from -(CH2)2-, -(CH2)3-, -(CH2)4-, and -(CH2)2-C(=O)-. The raw materials having the above structural units are simple and readily available, resulting in a reduced production cost for the obtained electrode.

[0015] In some embodiments, the electrode is a positive electrode.

[0016] In some embodiments, the active material includes Li 1+x Fe 1-y A y P 1-z E z O4; where -0.1≤x≤0.1, 0≤y≤0.5, 0≤z≤0.1; A is selected from one or more of Mn, Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Ni, Co, Ga, Sn, Sb, Nb, and Ge; E is selected from one or more of B, Si, N, S, F, Cl, and Br. Typically, these lithium iron phosphate active materials have small volumetric particle sizes, resulting in large specific surface areas. To prevent lithium iron phosphate from detaching from the current collector surface, a relatively large amount of binder is usually required, which limits the improvement of battery energy density. However, since the binder in this application has relatively high adhesive strength, it can achieve a substantial bonding effect with a small amount. Therefore, its combination with lithium iron phosphate active materials is beneficial to improving battery energy density and cycle performance.

[0017] In some embodiments, the active material satisfies at least one of the following: (1) the volume distribution particle size of the active material is 0.9 μm ≤ Dv50 ≤ 100 μm; (2) the volume distribution particle size of the active material is Dv10 ≥ 10 μm. The binder used in the battery of this application has high bonding performance; therefore, the battery can still exhibit excellent cycle performance when used with the above-mentioned active material with the aforementioned volume distribution particle size and a relatively low amount of binder. Furthermore, the energy density of the battery is increased due to the use of a lower amount of binder.

[0018] A second aspect of this application provides a secondary battery, including the electrode provided in the first aspect.

[0019] A third aspect of this application provides an electrical device, including the secondary battery provided in the second aspect.

[0020] A fourth aspect of this application provides an adhesive comprising a copolymer, said copolymer comprising structural units represented by Formula I and structural units represented by Formula II.

[0021]

[0022] Wherein, R1 independently includes H or methyl; R2 can be selected from substituted or unsubstituted C. 1-6 Alkyl group; R3 can be -(CH2) n - or -(CH2)2-C(=O)-, where n is 1 to 6.

[0023] In this application, the structural unit represented by Formula I includes an ester group, while the structural unit represented by Formula II includes a carbonyl group and a hydroxyl group. Thus, the polar groups included in these structural units form hydrogen bonds with different bond energies. Furthermore, the groups on the side chains of the structural units represented by Formula I and Formula II can play a dispersing and regulating role similar to that of amphiphilic copolymers, thereby exhibiting high adhesion, low swelling degree, and high anti-gelling properties.

[0024] In some embodiments, the viscosity of the adhesive is between 2000 mPa·s and 6500 mPa·s. By keeping the viscosity of the adhesive within this range, a considerable bonding effect can be achieved using only a relatively small amount.

[0025] In some embodiments, the binder has a weight-average molecular weight of 750,000 to 1,000,000. By controlling the weight-average molecular weight of the copolymer within this range, the binder exhibits excellent bonding properties.

[0026] In some embodiments, the binder exhibits a swelling degree of 45% to 88% after immersion in a carbonate solvent at 25°C for 7 days. This swelling degree range results in lower swelling of the electrode (film) when applied to a battery, thereby contributing to electrode stability.

[0027] In some embodiments, the copolymer contains 80%-98% by mass of the structural unit represented by Formula (I) and 2%-20% by mass of the structural unit represented by Formula (II). By controlling the mass percentages of the structural units represented by Formula (I) and (II) within the above ranges, it is beneficial for the polar groups to form reversible hydrogen bonds with other components in the film layer or for these polar groups to form hydrogen bonds with different bond energies with each other. This effectively improves the adhesive strength (adhesion) of the binder and also improves the wettability of the electrolyte to the electrode to a certain extent, reducing the swelling degree of the electrode. Furthermore, within this content range, the polymerization kinetics are not significantly hindered, which is beneficial for synthesizing copolymers with a weight-average molecular weight within the preferred range.

[0028] In some implementations, R1 independently includes H or -CH3; R2 is selected from -CH3, -CH2CH3, and -(CH2)3CH3; R3 is selected from -(CH2)2-, -(CH2)3-, -(CH2)4-, and -(CH2)2-C(=O)-. 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] Figure 7 This is an infrared spectrum of the adhesive according to one embodiment of this application.

[0036] Explanation of reference numerals in the attached figures:

[0037] 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

[0038] The following detailed description, with appropriate reference to the accompanying drawings, specifically discloses embodiments of the electrode, secondary battery, power supply device, and adhesive 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.

[0039] 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.

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

[0041] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0042] 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.

[0043] Unless otherwise specified, the terms used in this application have the common meanings as commonly understood by those skilled in the art.

[0044] 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.

[0045] Binders are a crucial component of lithium-ion batteries, and commonly used binders are mostly in the form of copolymers. The swelling and anti-gel properties of these copolymers significantly impact battery processing and use. Ideally, binders should have low swelling to reduce the shedding of active material from the current collector. However, low swelling means poor solubility in solutions, such as electrolytes, leading to a higher likelihood of gelation during processing. To improve the gelation properties of binders, it's typically necessary to improve their dispersibility (e.g., by adding solvent-compatible groups or reducing molecular weight). However, increased dispersibility often leads to increased swelling; therefore, swelling and anti-gelation are seemingly contradictory properties in binders.

[0046] Therefore, seeking binders with low swelling and high anti-gelling properties while ensuring high adhesion is a research direction for improving the cycle life of secondary batteries.

[0047] In view of this, embodiments of this application provide an electrode, a secondary battery, an electrical device, and an adhesive, wherein the secondary battery using the electrode has improved cycle performance.

[0048] Polaroid

[0049] A first aspect of this application provides an electrode, the electrode comprising a current collector and a film layer disposed on at least one surface of the current collector, the film layer comprising an active material and a binder, the binder comprising a copolymer, wherein the copolymer comprises structural units represented by Formula I and structural units represented by Formula II.

[0050]

[0051] R1 independently includes H or methyl;

[0052] The choice of R2 includes substituted or unsubstituted C. 1-6 Alkyl group; R3 can be -(CH2) n -or-(CH2) n -C(=O)-, where n is 1 to 6. In this application, the structural unit represented by Formula I includes an ester group, and the structural unit represented by Formula II includes a carbonyl group and a hydroxyl group. Thus, the polar groups included in these structural units can form reversible hydrogen bonds with other components in the film layer, or these polar groups can form hydrogen bonds with different bond energies with each other, thereby giving the binder excellent adhesion. Simultaneously, the groups on the side chains of the two structural units can play a dispersing and regulating role similar to that of amphiphilic copolymers, exhibiting excellent anti-swelling and anti-gelling properties. This improves the properties of the electrode including the binder, and consequently, enhances the cycle performance of the secondary battery including the electrode.

[0053] The types of functional groups in the aforementioned adhesives can be determined using infrared spectroscopy. For example, the infrared spectrum of the material can be tested to identify its characteristic peaks, thereby determining the types of functional groups. Specifically, the material can be subjected to infrared spectral analysis using instruments and methods known in the art, such as an infrared spectrometer (e.g., the Nicolet IS10 Fourier transform infrared spectrometer) and tested according to the General Rules for Infrared Spectroscopic Analysis in GB / T 6040-2019.

[0054] In addition, theoretically, after disassembling the battery to obtain the electrode, a film layer is obtained. Then, based on the differences in material properties, the binder can be obtained by removing the active material and conductive agent in the film layer, and then the binder can be tested.

[0055] In some embodiments, the mass content of the binder, based on the total mass of the film, is greater than zero and less than or equal to 1.5 wt%. In this application, the structural unit represented by Formula I includes an ester group, and the structural unit represented by Formula II includes a carbonyl group and a hydroxyl group. Thus, the polar groups included in these structural units can form reversible hydrogen bonds with other components in the film, or these polar groups can form hydrogen bonds with different bond energies with each other. This effectively improves the adhesive strength (bonding force) of the binder, allowing the presence of at most 1.5 wt% binder in the electrode to exert a considerable bonding effect, thereby increasing the energy density.

[0056] In some embodiments, the binder content is 0.6 wt% to 1.0 wt% by weight of the total mass of the film layer. For example, the amount of binder added is 0.6 wt%, 0.65 wt%, 0.7 wt%, 0.75 wt%, 0.8 wt%, 0.85 wt%, 0.9 wt%, 0.95 wt%, or any range of two such values, but is not limited thereto. Because the copolymer provided in this application has excellent adhesive strength, sufficient bonding can be achieved with a relatively low dosage under the same conditions, thereby contributing to improved energy density of the corresponding secondary battery.

[0057] In some embodiments, the adhesion strength of the electrode is from 5 N / m to 40 N / m. Within the range of the amount of adhesive used in this application, the electrode can achieve the above-mentioned adhesion strength, thus meeting the stability requirements of the electrode.

[0058] In some embodiments, the swelling degree of the electrode after immersion in a carbonate solvent at 60°C for 7 days is 10% to 22%, such as 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, etc. The carbonate solvent is prepared by uniformly mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 3:7. Within this range of electrode swelling degree, the film layer will not detach from the current collector, maintaining the stability of the electrode and thus benefiting the cycle performance of the battery.

[0059] In some embodiments, the weight-average molecular weight of the copolymer is 750,000 to 1,000,000. In other embodiments, the weight-average molecular weight of the copolymer can be 800,000 to 950,000, 850,000 to 1,000,000, 900,000 to 1,000,000, 800,000 to 900,000, 850,000 to 950,000, or 900,000 to 950,000. By controlling the weight-average molecular weight of the copolymer within the above range, the binder has a corresponding viscosity, thereby exhibiting excellent bonding performance and reducing the probability of the active material detaching from the current collector; at the same time, a suitable weight-average molecular weight allows the binder to be completely dissolved in the solvent, the slurry has a suitable viscosity, and the slurry's anti-settling properties are improved; thus, the electrode has good stability and processability.

[0060] In this paper, the term "weight-average molecular weight" refers to the sum of the weight fractions of molecules with different molecular weights in a copolymer and the products of their corresponding molecular weights.

[0061] In this application, the weight-average molecular weight of the copolymer can be tested using methods known in the art, such as gel chromatography, for example, using a Waters 1525HPLC gel chromatograph.

[0062] In some embodiments, the mass percentage of the structural unit represented by Formula (I) is 80%-98%, and the mass percentage of the structural unit represented by Formula (II) is 2%-20%. In the copolymer, the mass percentage of the structural unit represented by Formula (I) is 85%-95%, and the mass percentage of the structural unit represented by Formula (II) is 5%-15%. Optionally, the mass percentage of the structural unit represented by Formula (I) can be 85%-94%, 86%-93%, 87%-92%, 85%-90%, etc., and the mass percentage of the structural unit represented by Formula (II) can be 6%-15%, 7%-14%, 8%-13%, 10%-15%, etc. By controlling the mass percentages of the structural units represented by Formula I and Formula II within the aforementioned range, it is beneficial for polar groups to form reversible hydrogen bonds with other components in the film layer, or for these polar groups to form hydrogen bonds with different bond energies with each other. This effectively improves the adhesive strength (bonding force) of the binder and also enhances the wettability of the electrolyte to the electrode to a certain extent, reducing the swelling degree of the electrode. Furthermore, within this content range, the kinetics of the polymerization reaction are not significantly hindered, which is beneficial for synthesizing copolymers with a weight-average molecular weight within the preferred range. In addition, the combination of structural units represented by Formula I and Formula II within the aforementioned proportion range can further reduce gelation anomalies in the positive electrode during coating or cold pressing, resulting in improved processing performance of the electrode.

[0063] In some embodiments, R1 independently includes H or -CH3. In some embodiments, R2 is selected from -CH3, -CH2CH3, and -(CH2)3CH3. In some embodiments, R3 is selected from -(CH2)2-, -(CH2)3-, -(CH2)4-, and -(CH2)2-C(=O)-. The raw materials having the above structural units are simple and readily available, resulting in a lower production cost for the obtained electrode.

[0064] In some embodiments, the monomers corresponding to the structural units represented by formula (I) include one or more of methyl acrylate, ethyl acrylate, butyl acrylate, methyl 2-methacrylate, ethyl 2-methacrylate, and isooctyl methacrylate. These raw materials are readily available, resulting in electrode sheets with reduced production costs.

[0065] In some embodiments, the monomers corresponding to the structural units represented by formula (II) include one or more of hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, and 2-carboxyethyl acrylate.

[0066] In some implementations, the aforementioned electrode is a positive electrode.

[0067] Specifically, the positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material.

[0068] 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.

[0069] 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.).

[0070] In some embodiments, the positive electrode active material may be a known positive electrode active material for secondary batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.1 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, 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 manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0071] In some embodiments, the positive electrode active material used includes Li 1+x Fe 1-y A y P 1-z E z O4; where -0.1≤x≤0.1, 0≤y≤0.5, 0≤z≤0.1; where A is selected from one or more of Mn, Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Ni, Co, Ga, Sn, Sb, Nb, and Ge; and E is selected from one or more of B, Si, N, S, F, Cl, and Br. Typically, this type of lithium iron phosphate active material has a small volumetric particle size distribution, resulting in a large specific surface area. To suppress the detachment of lithium iron phosphate from the current collector surface, a relatively large amount of binder is usually required, which limits the improvement of battery energy density. However, since the binder in this application has relatively high adhesive strength, it can achieve considerable bonding effect with a small amount. Therefore, its combination with lithium iron phosphate active materials is beneficial to improving battery energy density and cycle performance.

[0072] In some embodiments, the active material satisfies at least one of the following: (1) the volume distribution particle size Dv50 of the active material satisfies: 0.9 μm ≤ Dv50 ≤ 100 μm; (2) the volume distribution particle size Dv10 of the active material ≥ 10 μm. The binder used in the battery of this application has high bonding performance; therefore, the battery can still exhibit excellent cycle performance when used with the aforementioned active material with a small volume distribution particle size and a relatively low amount of binder. Furthermore, the energy density of the battery is increased due to the use of a lower amount of binder.

[0073] In this application, the volumetric particle sizes Dv10 and Dv50 are the particle sizes corresponding to a cumulative volumetric distribution percentage of 10% and 50%, respectively, and can be determined using instruments and methods known in the art. For example, they can be determined using a laser particle size analyzer, referring to GB / T 19077-2016. The testing instrument can be a Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK.

[0074] In some embodiments, the positive electrode film layer may optionally further include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin. Theoretically, the copolymers of this application have good compatibility with these binders; simultaneously, they are chemically stable and will not react with other binders, thus providing a good basis for compounding with other binders.

[0075] In some embodiments, when the battery cell is a sodium-ion battery, the positive electrode active material may be a known positive electrode active material for sodium-ion batteries. As an example, the positive electrode active material may include sodium transition metal oxides, polyanionic compounds, Prussian blue compounds, etc.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] In some implementations, the aforementioned electrode is a negative electrode.

[0081] Specifically, the negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0082] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0083] 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.).

[0084] 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.

[0085] In some embodiments, the negative electrode film layer may optionally further include 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). Similarly, the copolymer of this application exhibits good compatibility with these binders; simultaneously, it is chemically stable and does not react with other binders, thus providing a good basis for compounding with other binders.

[0086] 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.

[0087] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0088] 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.

[0089] Secondary batteries

[0090] The second aspect of this application provides a secondary battery, which will be described below with appropriate reference to the accompanying drawings.

[0091] The term "secondary battery" used in this article refers to a single battery cell, a battery module, or a battery pack. These will be explained separately below.

[0092] Typically, a secondary battery cell includes the electrodes, electrolyte, and separator provided in the first aspect. The electrodes include a positive electrode and a negative electrode. During charging and discharging, active ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator is positioned between the positive and negative electrodes, primarily to prevent short circuits between the positive and negative electrodes while allowing ions to pass through.

[0093] electrolytes

[0094] 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.

[0095] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] Separating membrane

[0100] 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.

[0101] 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.

[0102] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0103] 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.

[0104] 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.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0110] 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.

[0111] 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.

[0112] Electrical appliances

[0113] A third aspect of 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.

[0114] As the electrical device, a single battery cell, a battery module, or a battery pack can be selected according to its usage requirements.

[0115] 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.

[0116] 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.

[0117] adhesive

[0118] A fourth aspect of this application provides an adhesive comprising a copolymer, wherein the copolymer comprises structural units represented by Formula I and structural units represented by Formula II;

[0119]

[0120] R1 independently includes H or methyl;

[0121] The choice of R2 includes substituted or unsubstituted C. 1-6 alkyl;

[0122] The choice of R3 includes -(CH2). n - or -(CH2)2-C(=O)-, where n is 1 to 6.

[0123] In this application, the structural unit represented by Formula I includes an ester group, while the structural unit represented by Formula II includes a carbonyl group and a hydroxyl group. Thus, the polar groups included in these structural units form hydrogen bonds with different bond energies, thereby exhibiting excellent adhesion. Furthermore, the groups on the side chains of the structural units represented by Formula I and Formula II can play a dispersing and regulating role similar to that of amphiphilic copolymers, resulting in the adhesive of this application having low swelling degree and high anti-gelling properties.

[0124] In some embodiments, the viscosity of the binder is between 2000 mPa·s and 6500 mPa·s. This viscosity range allows for considerable bonding with relatively small amounts and provides the subsequent slurry with a suitable viscosity, improving its resistance to settling.

[0125] In some embodiments, the weight-average molecular weight of the copolymer is 750,000 to 1,000,000. In some embodiments, the weight-average molecular weight of the copolymer is 800,000 to 1,000,000, for example, 800,000 to 900,000, 850,000 to 950,000, or 900,000 to 950,000. By controlling the weight-average molecular weight of the copolymer within the above range, the binder exhibits excellent bonding properties and dispersing effects.

[0126] In some embodiments, the mass percentage of the structural unit represented by Formula (I) in the copolymer is 80%-98%, and the mass percentage of the structural unit represented by Formula (II) is 2%-20%. In some embodiments, the mass percentage of the structural unit represented by Formula (I) is 85%-95%, and the mass percentage of the structural unit represented by Formula (II) is 5%-15%. Optionally, the mass percentage of the structural unit represented by Formula (I) can be 85%-94%, 86%-93%, 87%-92%, 85%-90%, etc., and the mass percentage of the structural unit represented by Formula (II) can be 6%-15%, 7%-14%, 8%-13%, 10%-15%, etc. By controlling the mass percentages of the structural units represented by Formula I and Formula II within the aforementioned ranges, it is beneficial for polar groups to form reversible hydrogen bonds with other components in the film layer, or for these polar groups to form hydrogen bonds with different bond energies with each other. This effectively improves the adhesive strength (bonding force) of the binder and also enhances the wettability of the electrolyte to the electrode to a certain extent, reducing the swelling degree of the electrode. Furthermore, within this content range, the kinetics of the polymerization reaction are not significantly hindered, which is conducive to synthesizing copolymers with a weight-average molecular weight within the preferred range.

[0127] Theoretically, the mass percentage of the aforementioned structural units can be determined using methods such as infrared spectroscopy. For example, a spectrum of a reference material with a known mass percentage can be obtained, and then the spectrum of the test sample can be compared with the reference spectrum to determine the intensity of characteristic peaks. The specific testing procedure is as follows: First, copolymers with mass percentages of structural units representing Formula I of 80%, 85%, 90%, 95%, and 98% are prepared. Infrared spectroscopy is performed on each copolymer. The intensity of the characteristic peaks is measured at 3500 cm⁻¹ in each infrared spectrum. -1 The hydroxyl peak around 1670 cm⁻¹ -1Integrate the carbonyl peaks on the left and right, calculate the ratio of the two integrals, and record it as 'a'. Plot the mass percentage of the structural unit represented by Equation I on the x-axis and 'a' on the y-axis, and perform linear fitting to obtain the fitting formula. This formula represents the relationship between the mass percentage (x) of the structural unit represented by Equation I and the infrared spectrum. For example, the fitting formula is a = -1.048x + 2.022.

[0128] Infrared spectroscopy was performed on the unknown sample, and the 3500 cm⁻¹ value in the obtained infrared spectrum was analyzed. -1 The hydroxyl peak around 1670 cm⁻¹ -1 Integrating the carbonyl peaks on the left and right sides, we obtain a. Substituting a into the formula, we calculate the result x. The result is the mass percentage of the structural unit represented by Equation I.

[0129] In some embodiments, the swelling degree of the binder after immersion in a carbonate solvent at 25°C for 7 days is 45% to 88%, such as 46, 47, 49, 50, 51, 52, 53, 54, 55, 57, 58, 59, 60, 61, 62, 63, 66, 68, 70, 71, 72, 73, 74, 75, 79, 80, 81, 82%, etc. The carbonate solvent is prepared by uniformly mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 3:7. The binder provided in this application has low solubility and low swelling degree in the electrolyte, thereby enabling the electrode to maintain good structural stability during long-term charge-discharge processes.

[0130] In some embodiments, R1 independently includes H or -CH3. In some embodiments, R2 is selected from -CH3, -CH2CH3, and -(CH2)3CH3. In some embodiments, R3 is selected from -(CH2)2-, -(CH2)3-, -(CH2)4-, and -(CH2)2-C(=O)-. The raw materials having the above structural units are simple and readily available, resulting in reduced production costs for the obtained electrode. Furthermore, the molecules with the above structures have chain segments of suitable length, which do not significantly hinder the polymerization reaction and are conducive to the formation of copolymers with suitable molecular weights.

[0131] In some embodiments, the monomers corresponding to the structural units represented by formula (I) include one or more of methyl acrylate, ethyl acrylate, butyl acrylate, methyl 2-methacrylate, ethyl 2-methacrylate, and isooctyl methacrylate. These raw materials are readily available, resulting in electrode sheets with reduced production costs.

[0132] In some embodiments, the monomers corresponding to the structural units represented by formula (II) include one or more of hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, and 2-carboxyethyl acrylate. These raw materials are readily available, resulting in electrode sheets with reduced production costs.

[0133] Methods for preparing adhesives

[0134] This application also provides a method for preparing the above-mentioned adhesive, which includes the following steps:

[0135] In the presence of an initiator, the monomers corresponding to the structural units represented by formula (I) and the monomers corresponding to the structural units represented by formula (II) are polymerized in a solvent.

[0136] In some embodiments, the initiator may include azo initiators, such as azobisisobutyronitrile, azobisisoheptanenitrile, azoisobutylcyanoformamide, dimethyl azobisisobutyrate, azodicarbonamide, etc.; and peroxide initiators, such as benzoyl peroxide, dicumyl peroxide, etc.

[0137] In some embodiments, the polymerization is carried out under an inert atmosphere, such as a nitrogen atmosphere.

[0138] In some implementations, the initiator and solvent are added to the reaction system in multiple steps, such as at least four times.

[0139] In some implementations, the adhesive can be prepared in the following ways:

[0140] The monomers corresponding to the structural unit represented by formula (I) and the monomers corresponding to the structural unit represented by formula (II) are added to the flask, followed by the addition of solvent and initiator, stirred evenly, and bubbled with inert gas to remove oxygen.

[0141] Heating to 80℃-85℃ and reacting for 0.5 hours removes polymerization inhibitors from the reactants.

[0142] Then the temperature is cooled to 60℃-80℃, and initiator and ethyl acetate are added dropwise to carry out the polymerization reaction.

[0143] Then, initiator and ethyl acetate were added to continue the chain propagation reaction;

[0144] Subsequently, an initiator and ethyl acetate were added directly to allow the residual monomers to react completely, thereby increasing the conversion rate of the polymerization reaction.

[0145] The reaction was stopped, the reactants were collected, and the solvent was removed by vacuum drying to obtain the target product (polymer).

[0146] Example

[0147] 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.

[0148] Example 1

[0149] Preparation of adhesive

[0150] (1) Add 180.0g of butyl acrylate and 20.0g of hydroxybutyl acrylate to a four-necked flask, along with 90g of ethyl acetate and 0.01g of azobisisobutyronitrile. Stir well and bubble with nitrogen for 1 hour to remove oxygen.

[0151] (2) Heat to 82°C and react for 0.5 h to remove the polymerization inhibitors in the reactants.

[0152] (3) Then adjust the temperature to 67°C, and add 0.02g of azobisisobutyronitrile and 40g of ethyl acetate dropwise to carry out the polymerization reaction.

[0153] (4) After the reaction has been going on for 3 hours, 0.04 g of azobisisobutyronitrile and 100 g of ethyl acetate are added dropwise to carry out the chain growth reaction and promote the increase of the molecular weight of the copolymer.

[0154] (5) After continuing the reaction for 3 hours, 0.02 g of azobisisobutyronitrile and 10 g of ethyl acetate were added directly, and then the reaction was carried out at 73 °C for 2 hours to allow the remaining monomers to react completely.

[0155] (6) Stop the reaction, collect the reactants, and dry them in a vacuum oven at 80°C to remove the solvent and obtain the binder.

[0156] Testing of adhesive parameters

[0157] (1) Weight-average molecular weight

[0158] At room temperature, 1 mg of the binder obtained in Example 1 was dissolved in 1 mL of N-methylpyrrolidone (NMP). After the sample was completely dissolved, it was filtered through a 0.2 μm ultrafiltration membrane and tested using a GPC gel permeation chromatograph (Waters 1525). The results are shown in Table 1 below.

[0159] (2) Viscosity

[0160] The adhesive obtained in Example 1 was placed in a Brookfield DV2T viscometer, and the viscosity value was measured at room temperature of 25°C and relative humidity of RH < 80%.

[0161] (3) Swelling

[0162] The adhesive obtained in Example 1 was cut into 1g samples and added to a test tube. Simultaneously, 40g of a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC), with an EC / DEC volume ratio of 3:7, was added. The test tube was placed in a 60°C incubator for one week. The sample was then removed, and any residual liquid was gently wiped away with lint-free paper. The mass was then measured to obtain mass data M. The degree of swelling of the adhesive was calculated using the formula: Swelling Degree = (M-1)*100%.

[0163] (4) Infrared characterization

[0164] The adhesive obtained in Example 1 was subjected to infrared spectroscopy testing, and the obtained infrared spectrum can be found in [reference needed]. Figure 7 ,from Figure 7 As can be seen from the infrared spectrum, the value is 1670 cm⁻¹. -1 There is a carbonyl characteristic peak nearby; 1500 cm⁻¹ -1 Characteristic peaks of carboxyl groups are present on both sides; 3500 cm⁻¹ -1 There are large, broad peaks of the OH stretching vibration on both sides; 2900 cm⁻¹ -1 There are CH stretching vibration peaks (methyl and methylene) on both sides.

[0165] (5) Anti-gel properties

[0166] The anti-gelling property of an adhesive is determined by the fluidity of the slurry containing that adhesive. Generally,

[0167] Typically, solvents, binders, active materials, and conductive agents are mixed evenly in a certain proportion to obtain a slurry. Then, the slurry is left to stand for 24 hours at 25°C in a drying room (2% humidity), and the anti-gelling properties of the binder are judged based on the slurry's flowability.

[0168] A coefficient of 5 indicates that the fluidity does not change much before and after standing, the slurry has excellent fluidity, the slurry viscosity is less than 3000 mPa·s, and it does not gel at all.

[0169] A coefficient of 4 indicates that the fluidity does not change much before and after standing, the slurry has good fluidity, the viscosity of the slurry is between 3000 mPa.s and 5000 mPa.s, and it does not gel at all.

[0170] A coefficient of 3 indicates that the fluidity does not change much before and after standing, the slurry has good fluidity, and the slurry viscosity is between 5000 mPa.s and 8000 mPa.s.

[0171] A coefficient of 2 indicates that the fluidity deteriorates after standing. The slurry has a certain fluidity and the viscosity is above 8000 mPa·s.

[0172] A coefficient of 1 indicates that the slurry cannot be formed, has lost its fluidity during the stirring process, and has gelled.

[0173] Preparation of positive electrode sheet

[0174] The positive electrode slurry was obtained by thoroughly mixing N-methylpyrrolidone, the binder obtained in Example 1, lithium iron phosphate (LiFePO4), and conductive carbon black in a weight ratio of 72:2:130:1.

[0175] A portion of the positive electrode slurry was subjected to the above-mentioned anti-gelling test, and the anti-gelling properties of the binder in Example 1 were judged based on the flowability of the positive electrode slurry.

[0176] The positive electrode slurry is uniformly coated onto the positive electrode current collector aluminum foil, and then dried, cold-pressed, and slit to obtain the positive electrode sheet.

[0177] Testing of positive electrode parameters

[0178] (1) Adhesion

[0179] The positive electrode sheet prepared in Example 1 was cut into 300mm*20mm strips. The strips were fixed to the test steel plate with double-sided tape, leaving a 50mm length for the tensile testing machine. The test steel plate with the strips fixed was first fixed on the 90-degree test fixture of the tensile testing machine, and the reserved end was fixed on the tensile sensor of the tensile testing machine. Then, the corresponding test mode of the electrode sheet was selected and the test was started. Three strips were tested for each group of samples, and the average value was taken.

[0180] (2) Swelling

[0181] Scrape off the film layer from the obtained positive electrode sheet, weigh it, and record it as M; then add it to a test tube, along with sufficient amounts of a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC), where EC / DEC (volume ratio 3:7). Place the test tube in a 60℃ thermostat for one week. Remove it, remove any residual liquid with lint-free paper, and weigh it again, recording the weight as M1. Calculate the degree of swelling using the formula: Degree of Swelling = (M1 - M) / M * 100%.

[0182] Battery manufacturing

[0183] (1) Preparation of negative electrode sheet

[0184] The anode material is graphite, and the conductive agent is conductive carbon black; the binder is a 1:1 mixture of styrene-butadiene rubber and sodium carboxymethyl cellulose. The ratio of graphite material: conductive carbon black: binder is 96:2:2.

[0185] (2) Preparation of electrolyte

[0186] Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 3:7 to obtain an organic solvent, wherein EC / DEC (volume ratio 3:7). LiPF6 was then dissolved in the above organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0187] (3) Separating membrane

[0188] A 12μm thick polyethylene diaphragm (Shenzhen ZTE New Material Technology Co., Ltd.) is used.

[0189] (4) Assembly of secondary batteries

[0190] The prepared positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. They are then wound to obtain a bare cell, tabs are welded onto the bare cell, and the cell is placed in an aluminum casing. It is then baked at 80°C to remove water, followed by the injection of electrolyte and sealing to obtain a non-charged battery. This non-charged battery then undergoes a series of processes including settling, hot and cold pressing, formation, shaping, and capacity testing to produce a secondary battery.

[0191] Battery cycle performance testing

[0192] At 60℃, the lithium-ion battery was charged at a constant current of 1 / 3C to 3.8V, then charged at a constant voltage of 3.8V to a current of 0.05C, left to stand for 5 minutes, and then discharged at 1 / 3C to 2.0V. The discharge capacity C0 was recorded.

[0193] Repeat the above steps 1000 times, and record the discharge capacity C1000 of the lithium-ion battery after the 1000th cycle. The capacity retention rate P1000 = C1000 / C0 × 100%.

[0194] Example 2

[0195] The adhesive was prepared in a similar manner to that in Example 1, except that 180 g of butyl acrylate and 20 g of 2-carboxyethyl acrylate were added to a four-necked flask.

[0196] The obtained binder was then tested using the same method as in Example 1, and the positive electrode sheet was prepared, tested, and a secondary battery was prepared and tested for cycle performance.

[0197] The corresponding results are shown in Table 1 below.

[0198] Comparative Example 1

[0199] The adhesive was prepared in a similar manner to that in Example 1, except that 180 g of butyl acrylate and 20 g of isooctyl acrylate were added to a four-necked flask.

[0200] The obtained binder was then tested using the same method as in Example 1, and the positive electrode sheet was prepared, tested, and a secondary battery was prepared and tested for cycle performance.

[0201] The corresponding results are shown in Table 1 below.

[0202] Comparative Example 2

[0203] The adhesive was prepared in a manner similar to that in Example 1, except that 180 g of 2-carboxyethyl acrylate and 20 g of hydroxybutyl acrylate were added to a four-necked flask.

[0204] The obtained binder was then tested using the same method as in Example 1, and the positive electrode sheet was prepared, tested, and a secondary battery was prepared and tested for cycle performance.

[0205] Comparative Example 3

[0206] The positive electrode slurry was obtained by thoroughly mixing N-methylpyrrolidone, polyvinylidene fluoride (PVDF, Mw=91W), lithium iron phosphate and conductive carbon black in a weight ratio of 72:2:130:1.

[0207] Then, the positive electrode and the secondary battery were prepared in the same manner as in Example 1. Subsequently, corresponding tests were conducted.

[0208] The corresponding results are shown in Table 1 below.

[0209] Table 1

[0210]

[0211] As shown in Table 1, the copolymers in Examples 1 and 2 exhibited low swelling, relatively high anti-gel coefficients, and high adhesion, indicating that monomer B, regardless of its hydroxyl or carboxyl structure, could provide good modification. Furthermore, in Comparative Example 1, replacing monomer B with a nonpolar alkane structure drastically worsened the copolymer's swelling performance and adhesion, and also reduced cycle performance, demonstrating that using two type A monomers was ineffective and highlighting the importance of polar groups such as hydroxyl and carboxyl groups. In Comparative Example 2, replacing both monomers A and B with monomers containing hydroxyl and carboxyl groups resulted in a sharp decrease in the molecular weight of the copolymer under the same conditions, indicating that a large number of polar groups hindered the polymerization reaction, and consequently, the swelling and cycle performance of the electrode decreased. In conclusion, the binder obtained using monomers A and B within the scope of this application can achieve both improved adhesion and swelling in the electrode, thereby enhancing the battery's cycle performance.

[0212] Example 3

[0213] The adhesive was prepared using the same method as in Example 1.

[0214] Subsequently, in the positive electrode sheet, N-methylpyrrolidone, the binder obtained above, lithium iron phosphate and conductive carbon black are thoroughly mixed in a weight ratio of 72:1.3:127.7:1 to obtain the positive electrode slurry.

[0215] Subsequently, the parameters of the positive electrode were tested, and a secondary battery was fabricated and its cycle performance was tested.

[0216] The test results are shown in Table 2.

[0217] Example 4

[0218] The adhesive was prepared using the same method as in Example 1.

[0219] Subsequently, in the positive electrode sheet, N-methylpyrrolidone, the binder obtained above, lithium iron phosphate and conductive carbon black are thoroughly mixed in a weight ratio of 72:0.8:128.2:1 to obtain the positive electrode slurry.

[0220] Subsequently, the parameters of the positive electrode were tested, and a secondary battery was prepared and its cycle performance was tested.

[0221] The test results are shown in Table 2.

[0222] Table 2

[0223]

[0224] As can be seen from the data in Table 2, as the amount of binder added decreases, the electrode adhesion also decreases to a certain extent. When the amount added is 0.6wt%, the electrode adhesion can reach 11N / m, which can meet the electrode forming requirements.

[0225] Examples 5-13

[0226] The adhesive was prepared using a method similar to that in Example 1, with the differences shown in Table 3 below.

[0227] Then, the binders obtained in Examples 5-13 were tested in the same way as in Example 1, positive electrode sheets were prepared and tested, and secondary batteries were prepared and tested for cycle performance.

[0228] The corresponding results are shown in Table 3 below.

[0229] Table 3

[0230]

[0231] As can be seen from the data in Table 3, within the molecular weight range of this application, the higher the molecular weight, the better the adhesion of the electrode. This is because, when the molecular structures of copolymers are similar, the molecular weight of the copolymer plays a major role in the adhesion. Simultaneously, within the molecular weight range of this application, the binder can be well dispersed without entanglement, resulting in good processability of the slurry. Furthermore, within the mass ratio range of monomer A and monomer B in this application, the resulting binder achieves a balance between molecular weight, swelling degree, and anti-gelling properties, thereby realizing superior performance.

[0232] Meanwhile, within the ratio of monomer A to monomer B in this application, the resulting binder contains an appropriate amount of polar groups, thereby giving the copolymer an amphiphilic effect and maintaining a balance between swelling and antigelling properties.

[0233] Furthermore, the alkanes on monomer A can also affect the final performance of the copolymer. Shorter and longer alkanes may affect the regularity of the copolymer and the intermolecular interactions, resulting in a decrease in adhesion and anti-swelling properties, and affecting the cycle performance of the battery.

[0234] 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. An electrode sheet, characterized in that, The electrode includes a current collector and a film layer disposed on at least one surface of the current collector. The film layer includes an active material and a binder. The binder includes a copolymer, and the copolymer includes structural units represented by Formula I and structural units represented by Formula II. Wherein, R1 independently includes H or -CH3; The choice of R2 includes substituted or unsubstituted C. 1-6 alkyl; The choice of R3 includes -(CH2). n -or-(CH2) n -C(=O)-, where n is 1 to 6.

2. The electrode sheet according to claim 1, characterized in that, The mass content of the adhesive is greater than zero and less than or equal to 1.5 wt% based on the total mass of the film layer.

3. The electrode sheet according to claim 1 or 2, characterized in that, The adhesive content is 0.6 wt% to 1.0 wt% based on the total mass of the film layer.

4. The electrode sheet according to any one of claims 1 to 3, characterized in that, The adhesion strength of the electrode is from 5 N / m to 40 N / m.

5. The electrode sheet according to any one of claims 1 to 4, characterized in that, The electrode exhibits a swelling degree of 10% to 22% after being immersed in a carbonate solvent at 60°C for 7 days.

6. The electrode sheet according to any one of claims 1 to 5, characterized in that, The copolymer has a weight-average molecular weight of 750,000 to 1,000,000.

7. The electrode sheet according to any one of claims 1 to 6, characterized in that, In the copolymer, the mass percentage of the structural unit represented by Formula I is 80%-98%, and the mass percentage of the structural unit represented by Formula (II) is 2%-20%.

8. The electrode sheet according to any one of claims 1 to 7, characterized in that, R1 independently includes H or -CH3; The choices of R2 include -CH3, -CH2CH3, and -(CH2)3CH3; The choices of R3 include -(CH2)2-, -(CH2)3-, -(CH2)4-, and -(CH2)2-C(=O)-.

9. The electrode sheet according to any one of claims 1 to 8, characterized in that, The electrode is a positive electrode.

10. The electrode sheet according to claim 9, characterized in that, The active material includes Li 1+x Fe 1-y A y P 1-z E z O4; Where -0.1≤x≤0.1, 0≤y≤0.5, 0≤z≤0.1; The choice of A includes one or more of Mn, Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Ni, Co, Ga, Sn, Sb, Nb, and Ge; The choice of E includes one or more of B, Si, N, S, F, Cl, and Br.

11. The electrode according to claim 10, characterized in that, The active material satisfies at least one of the following conditions: (1) The volume distribution particle size Dv50 of the active material satisfies: 0.9μm≤Dv50≤100μm; (2) The volume distribution particle size Dv10 of the active material is ≥10μm.

12. A secondary battery, characterized in that, The electrode includes any one of claims 1 to 11.

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

14. An adhesive, characterized in that, The copolymer includes structural units represented by Formula I and structural units represented by Formula II. R1 independently includes H or methyl; The choice of R2 includes substituted or unsubstituted C. 1-6 alkyl; The choice of R3 includes -(CH2). n - or -(CH2)2-C(=O)-, where n is 1 to 6.

15. The adhesive according to claim 14, characterized in that, The viscosity of the adhesive is between 2000 mPa·s and 6500 mPa·s.

16. The adhesive according to claim 14 or 15, characterized in that, The weight-average molecular weight of the adhesive is between 750,000 and 1,000,000.

17. The adhesive according to any one of claims 14 to 16, characterized in that, The adhesive exhibits a swelling degree of 45% to 88% after being immersed in a carbonate solvent at 25°C for 7 days.

18. The adhesive according to any one of claims 14 to 17, characterized in that, In the copolymer, the mass percentage of the structural unit represented by Formula I is 80%-98%, and the mass percentage of the structural unit represented by Formula (II) is 2%-20%.

19. The adhesive according to any one of claims 14 to 18, characterized in that, R1 independently includes H or -CH3; The choices of R2 include -CH3, -CH2CH3, and -(CH2)3CH3; The choices of R3 include -(CH2)2-, -(CH2)3-, -(CH2)4-, and -(CH2)2-C(=O)-.

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