A binder and its application

Abstract

This application provides a binder, which is a polymer composed of at least the following components: (a) at least one flexible monomer; (b) at least one rigid monomer, wherein the rigid monomer comprises the following components: (b1) an olefin having a rigid group, and / or (b2) a polymerizable monomer having a strongly polar group; and (c) at least one monomer containing a hydrophilic group. The binder provided herein is particularly suitable for the preparation of negative electrode sheets using the roller pressing method.

Description

Technical Field

[0001] The present application relates to the technical field of secondary batteries, and in particular to a binder. In addition, the present application also relates to a negative electrode sheet containing the binder, and a secondary battery, battery module, battery pack, and electrical device containing the negative electrode sheet. Background Art

[0002] In recent years, the application range of secondary batteries has become increasingly broad. For example, they can be used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, as well as power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and other fields. As secondary batteries have achieved great development, higher requirements have been placed on their energy density.

[0003] Increasing the coating weight of active material on the electrode and the electrode compaction density is the best option for improving battery energy density. However, increasing the weight of active material can exacerbate the brittleness of the negative electrode, causing the electrode to fracture when bent by external forces. Therefore, the flexibility of the electrode still needs to be improved. Summary of the Invention

[0004] The present application is made in view of the above-mentioned problems, and its purpose is to improve the energy density of the secondary battery and ensure that the negative electrode sheet has good flexibility.

[0005] In order to achieve the above-mentioned objectives, the present application provides a flexible binder for a secondary battery, and a negative electrode sheet, a secondary battery, a battery module, a battery pack, and an electrical device comprising the flexible binder.

[0006] Therefore, the first aspect of the present application provides a negative electrode binder, which is a polymer, which is polymerized from at least the following components:

[0007] (a) at least one flexible monomer selected from C 4-18 Olefins, which may be butadiene, 1-octene, or isoprene;

[0008] (b) at least one rigid monomer, said rigid monomer comprising the following components

[0009] (b1) an olefin having a rigid group, wherein the rigid group is selected from C 5-20 Aryl, C 4-8 Saturated or unsaturated ring groups, C 5-30 The heterocyclic group,

[0010] Optionally, the rigid group is selected from phenyl, cyclohexenyl, six-membered cycloalkane, six-membered cycloketone, and six-membered cycloalcohol.

[0011] Component (b1) is optionally selected from styrene or limonene;

[0012] and / or

[0013] (b2) a polymerizable monomer having a strong polar group,

[0014] Component (b2) is optionally selected from acrylonitrile, acrylamide, acrylic acid, and acrylic acid salts;

[0015] (c) at least one hydrophilic monomer, wherein the hydrophilic monomer contains a hydrophilic group selected from carboxyl, hydroxyl, amine, aldehyde, alcohol, amino, and amide groups;

[0016] Component (c) is optionally selected from acrylic acid and its salts, ethylene glycol dimethacrylate, methacrylamide.

[0017] The negative electrode binder described in the present application may be a block copolymer.

[0018] In the negative electrode binder described in the first aspect of the present application, component (a) is the specific flexible monomer defined above, which can reduce the strong interaction force between the binder molecules, increase the molecular conformational changes of the binder itself, and enhance the flexibility of the binder itself, thereby improving the flexibility of the entire electrode sheet and greatly reducing the probability of breakage of the prepared thick negative electrode sheet during processing and use, thereby improving the processing yield and use safety of the lithium-ion battery, overcoming the problem of powder shedding of the negative electrode during charging and discharging, and extending the service life of the lithium-ion battery; component (b) is the specific rigid monomer defined above, which can make the molecules have strong interaction forces and can provide sufficient strength for the membrane; component (c) is the specific hydrophilic group monomer defined above, which makes the negative electrode binder hydrophilic, thereby being suitable for use in aqueous systems.

[0019] The inventors have discovered that the polymer binder formed from the aforementioned specific rigid component, specific flexible component, and specific hydrophilic component exhibits sufficient bonding properties and is suitable for aqueous systems. Furthermore, it possesses sufficient flexibility and strength, making it particularly suitable for use in the roll-pressing process for preparing negative electrode sheets. Using the binder described herein, when preparing negative electrode sheets using the roll-pressing process, the negative electrode sheets do not crack during the rolling process, and the resulting negative electrode film exhibits sufficient viscoelasticity and flexibility without residue remaining on the roller.

[0020] Optionally, the binder of the present application is not prepared by emulsion polymerization.

[0021] In any embodiment, the binder is polymerized from at least the following components:

[0022] (a) a flexible monomer selected from butadiene, 1-octene, and isoprene;

[0023] (b) a rigid monomer selected from the group consisting of: (b1) styrene or limonene, and / or, (b2) acrylonitrile, acrylamide;

[0024] (c) A hydrophilic monomer selected from acrylic acid, ethylene glycol dimethacrylate, and methacrylamide.

[0025] In any embodiment, based on the total molar amount of components (a)-(c), the molar proportion of component (a) ranges from 10% to 60%, optionally 30% to 55%; the molar proportion of component (b) ranges from 10% to 50%, optionally 15% to 45%; and the molar proportion of component (c) ranges from 10% to 30%, optionally 15% to 30%.

[0026] The above ratio range ensures that the formed negative electrode binder is suitable for the process of preparing negative electrode sheets by roller pressing, and the negative electrode sheets will not break and will not remain on the roller.

[0027] In any embodiment, the binder is a gel polymer. Gel polymers possess superior mechanical strength and flexibility compared to other polymer forms. On the one hand, they can provide more contact sites, enhancing interfacial adhesion and the required cohesion of the film layer. Furthermore, the flexibility of the gel polymer improves the flexibility of the electrode sheet, enabling ultra-thick coatings to prevent cracking and breakage of the electrode sheet, thereby improving the battery life and safety. Therefore, in this application, it is preferred that the binder be prepared as a gel polymer.

[0028] In any embodiment, the viscosity of the binder is 70,000 mPa·s to 500,000 mPa·s, optionally 100,000 mPa·s to 300,000 mPa·s, and more optionally 110,000 mPa·s to 250,000 mPa·s.

[0029] When the viscosity of the binder is too high, exceeding the above-mentioned maximum value, it will cause difficulty in dispersing the slurry and fail to meet the processing requirements of the slurry; when the viscosity of the binder is too low, lower than the above-mentioned minimum value, it will cause insufficient adhesion of the slurry, and the electrode will easily shed powder and crack, causing the electrical performance to deteriorate, thereby affecting the service life of the battery.

[0030] In any embodiment, the binder has a weight average molecular weight of 300,000 to 1,000,000, optionally 400,000 to 900,000, and more optionally 500,000 to 800,000.

[0031] As the molecular weight of the binder increases, its bonding strength increases, and its mechanical strength improves. When the molecular weight is too low, the viscosity and mechanical strength of the binder will not meet the requirements. If the binder strength is too low, for example, less than 300,000, the adhesive properties of the binder will be insufficient, causing the electrode to shed or even break, affecting the electrical performance and service life of the battery cell. When the molecular weight is too high, for example, greater than 1,000,000, entanglement will occur between the molecular chains, making it difficult to disperse the slurry, and thus making the preparation process of the negative electrode sheet difficult.

[0032] In any embodiment, the solid content of the binder is 4% to 10%.

[0033] In any embodiment, the glass transition temperature Tg of the binder is in the range of 60-200°C, optionally 100-150°C, and more optionally 110-120°C.

[0034] If the binder's glass transition temperature is too high, for example, greater than 200°C, the movement of the binder's chain segments will be restricted, resulting in the electrode being too hard and brittle and prone to cracking, which in turn affects the battery's electrical performance and service life. If the binder's glass transition temperature is too low, for example, below 60°C, it will lead to reduced adhesion and cohesion, which in turn will cause the negative electrode powder to easily fall off during charging and discharging, or even the membrane to delaminate, posing a safety hazard to the battery cell.

[0035] In any embodiment, the pH value of the binder is in the range of 5.0-8.0. The pH value can be measured by potentiometric determination. If necessary, the pH value of the binder can be adjusted to a range of 5.0-8.0 using lithium hydroxide (LiOH) solution.

[0036] The pH value of the binder affects the performance of the battery. If the pH value of the binder is not controlled, the pH value may be too low, for example, less than 5.0. At this time, the binder contains a large amount of carboxyl groups (-COOH), which may cause the battery to be lithium-intercalated at low voltage, thereby making the battery's initial efficiency worse, thereby affecting the long-term cycle stability of the battery. The pH value may also be too high, for example, greater than 8.0. At this time, the binder contains a large amount of lithium carboxylate (-COOLi), which may cause the prepared electrode to be too brittle and easy to break, thereby increasing the risk of lithium plating, and the battery cell may also have safety hazards.

[0037] The second aspect of the present application provides a negative electrode plate, which includes the binder described in the first aspect of the present application.

[0038] In any embodiment, the binder accounts for 0.5-5% by weight in the negative electrode film, based on the weight of the negative electrode film.

[0039] In any embodiment, when the negative electrode sheet is wound around objects with diameters of 32 mm, 25 mm, and 16 mm, there are no cracks on the surface of the negative electrode sheet.

[0040] A third aspect of the present application provides a method for preparing a negative electrode sheet, comprising the following steps:

[0041] (1) mixing and kneading the negative electrode active material, the binder described in the first aspect of the present application, the conductive agent, and the solvent;

[0042] (2) the material containing the binder kneaded in step (1) is extruded through an extruder and granulated,

[0043] (3) Roll-pressing the particles obtained in step (2).

[0044] The negative electrode binder of the present application is particularly suitable for preparing negative electrode sheets by roller pressing. Since the negative electrode binder can provide sufficient flexibility, the film containing the negative electrode binder will not break during roller pressing, and it has appropriate bonding strength but will not remain on the roller.

[0045] The fourth aspect of the present application provides a secondary battery, which includes the negative electrode sheet described in the second aspect of the present application or the negative electrode sheet prepared according to the preparation method of the third aspect of the present application.

[0046] The negative electrode plate provided in the present application has good flexibility and the energy density of the secondary battery is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 Schematic diagram of the negative electrode plate forming method in one embodiment of the present application.

[0048] Figure 2 Schematic diagram of the negative electrode plate forming method in another embodiment of the present application.

[0049] Figure 3 This is a diagram showing the cracking of the negative electrode during the flexibility determination.

[0050] Figure 4 This is a picture showing that the negative electrode does not crack during the flexibility determination.

[0051] Figure 5 Schematic diagram of a secondary battery according to one embodiment of the present application.

[0052] Figure 6 yes Figure 5 FIG. 1 is an exploded view of a secondary battery according to an embodiment of the present application.

[0053] Figure 7 Schematic diagram of a battery module according to one embodiment of the present application.

[0054] Figure 8 Schematic diagram of a battery pack according to one embodiment of the present application.

[0055] Figure 9 yes Figure 8 An exploded view of a battery pack according to an embodiment of the present application is shown.

[0056] Figure 10 FIG. 1 is a schematic diagram of an electrical device using a secondary battery according to an embodiment of the present application as a power source.

[0057] Description of reference numerals:

[0058] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 secondary battery; 51 housing; 52 electrode assembly; 53 top cover assembly DETAILED DESCRIPTION

[0059] Below, with appropriate reference to the accompanying drawings, detailed descriptions are given of the embodiments of the negative electrode sheet and its manufacturing method, the positive electrode sheet, the secondary battery, the battery module, the battery pack and the electrical device of the present application. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0060] " range " disclosed in the application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-6. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0061] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0062] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0063] Unless otherwise specified, all steps of the present 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 may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0064] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0065] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0066] As described in the background technology, energy density is considered to be the biggest bottleneck restricting the current development of secondary batteries. Under the premise of not affecting the performance of the secondary battery, increasing the thickness of the electrode is currently a common means to improve the energy density of secondary batteries. However, if the thickness of the electrode is increased, the difficulty of electrode processing will also increase. Ultra-thick electrode sheets are prone to cracking or breaking during the winding process after drying. There is also the possibility of electrode sheet breakage during the use of the manufactured battery. The broken electrode sheet is likely to puncture the diaphragm and form a short circuit with the negative electrode, thereby affecting the service life of the lithium battery. In more serious cases, it may cause safety problems. Therefore, how to make the battery have an electrode sheet with good flexibility and improve the safety and service life of the battery has become a technical problem that needs to be solved urgently.

[0067] Unexpectedly, the inventors discovered that a binder with a specific structure could solve this problem. Binders are used in secondary batteries to promote adhesion between the membrane and the current collector. However, the inventors unexpectedly discovered that a binder with a specific structure can ensure adhesion between the membrane and the current collector while also improving the flexibility of the electrode, thereby enhancing the safety and service life of the battery.

[0068] Therefore, the first aspect of the present application provides a negative electrode binder, which is a polymer, which is polymerized from at least the following components:

[0069] (a) at least one flexible monomer selected from C 4-18 Olefins, which may be butadiene, 1-octene, or isoprene;

[0070] (b) at least one rigid monomer, said rigid monomer comprising the following components

[0071] (b1) an olefin having a rigid group, wherein the rigid group is selected from C 5-20 Aryl, C 4-8 Saturated or unsaturated ring groups, C 5-30 The heterocyclic group,

[0072] Optionally, the rigid group is selected from phenyl, cyclohexenyl, six-membered cycloalkane, six-membered cycloketone, and six-membered cycloalcohol.

[0073] Component (b1) is optionally selected from styrene or limonene;

[0074] and / or

[0075] (b2) a polymerizable monomer having a strong polar group,

[0076] Component (b2) is optionally selected from acrylonitrile, acrylamide, acrylic acid, and acrylic acid salts;

[0077] (c) at least one hydrophilic monomer, wherein the hydrophilic monomer contains a hydrophilic group selected from carboxyl, hydroxyl, amine, aldehyde, alcohol, amino, and amide groups;

[0078] Component (c) is optionally selected from acrylic acid and its salts, ethylene glycol dimethacrylate, methacrylamide.

[0079] The term "monomer" means a compound in its simple, unpolymerized form having a relatively low molecular weight, such as acrylonitrile, styrene, methyl methacrylate, and the like.

[0080] In this application, C 4-18The olefin is an olefin having 4 to 18 carbon atoms and at least one double bond, which can be selected from 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene, styrene, o-, m- and p-methylstyrene, α-methylstyrene, β-methylstyrene, 2,4-dimethylstyrene, o-, m- and p-ethylstyrene, p-tert-butylstyrene, divinylbenzene and vinylnaphthalene.

[0081] In the present application, the polymerizable monomer having a strong polar group can be selected from succinonitrile, sebacate, fluorinated nitrile, chlorinated nitrile, acrylonitrile, methacrylonitrile, acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid, maleic acid, fumaric acid, citraconic acid, mesaconic acid, glutaconic acid, and itaconic acid.

[0082] The negative electrode binder described in the present application may be a block copolymer.

[0083] In the negative electrode binder described in the first aspect of the present application, component (a) is a flexible monomer, which can reduce the strong interaction force between the binder molecules, increase the molecular conformational changes of the binder itself, and enhance the flexibility of the binder itself, thereby improving the flexibility of the entire electrode sheet and greatly reducing the probability of breakage of the prepared thick negative electrode sheet during processing and use, thereby improving the processing yield and use safety of the lithium-ion battery, overcoming the problem of powder shedding of the negative electrode during charging and discharging, and extending the service life of the lithium-ion battery; component (b) is a rigid monomer, which can make the molecules have strong interaction forces and can provide sufficient strength for the membrane; component (c) is a monomer containing a hydrophilic group, which makes the negative electrode binder hydrophilic, thereby being suitable for use in an aqueous system.

[0084] The inventors have discovered that the polymer binder formed from the aforementioned specific rigid component, specific flexible component, and specific hydrophilic component exhibits sufficient bonding properties and is suitable for aqueous systems. Furthermore, it possesses sufficient flexibility and strength, making it particularly suitable for use in the roll-pressing process for preparing negative electrode sheets. Using the binder described herein, when preparing negative electrode sheets using the roll-pressing process, the negative electrode sheets do not crack during the rolling process, and the resulting negative electrode film exhibits sufficient viscoelasticity and flexibility without residue remaining on the roller.

[0085] In addition, the inventors of the present application also found that the strength of the negative electrode slurry prepared using the above binder is significantly increased.

[0086] The binder described herein can be synthesized by a method of sequential addition of anions. The method may comprise the following steps: using high-pressure nitrogen protection, for example, at a pressure of 0.3 to 0.5 MPa, carrying out polymerization on an anionic polymerization device (for example, HTSCP series, SCP2009). During the reaction, a nucleophilic reagent is used as an initiator. This application does not particularly limit the specific type of initiator, and any initiator commonly used in the art, for example, alkali metals and organic compounds thereof, can be used. The polymerization reaction of the binder can be an anionic polymerization reaction, in which ion pairs are present. After the initiator and reactant react, corresponding anionic active centers can be generated, and the anionic active centers can undergo a chain growth reaction in the next step, thereby polymerizing in the order of anions. Alternatively, monomers are added in the order of components (a), (b), (c) or in the order of components (b), (a), (c), and polymerization is carried out in sequence. Taking styrene-butadiene-acrylic acid as an example, the rigid monomer styrene is added first and reacted for 20 to 40 minutes, followed by the flexible monomer butadiene and reacted for 10 to 30 minutes. Finally, some hydrophilic monomers such as acrylic acid and acrylonitrile are added at about 30°C and reacted for 20 to 40 minutes. The reaction temperature is controlled at 25°C to 40°C. After the polymerization reaction is completed, polar substances such as alcohols can be used to terminate the reaction.

[0087] Optionally, the binder of the present application is not prepared by emulsion polymerization.

[0088] In some embodiments, the binder is polymerized from at least the following components:

[0089] (a) a flexible monomer selected from butadiene, 1-octene, and isoprene;

[0090] (b) a rigid monomer selected from the group consisting of: (b1) styrene or limonene, and / or, (b2) acrylonitrile, acrylamide;

[0091] (c) A hydrophilic monomer selected from acrylic acid, ethylene glycol dimethacrylate, and methacrylamide.

[0092] In some embodiments, based on the total molar amount of components (a)-(c), the molar proportion of component (a) ranges from 10% to 60%, optionally 30% to 55%; the molar proportion of component (b) ranges from 10% to 50%, optionally 15% to 45%; and the molar proportion of component (c) ranges from 10% to 30%, optionally 15% to 30%.

[0093] The above ratio range ensures that the formed negative electrode binder is suitable for the process of preparing negative electrode sheets by roller pressing, and the negative electrode sheets will not break and will not remain on the roller.

[0094] In some embodiments, the binder is a gel polymer. Optionally, the binder is a gel polymer of medium strength, preferably high strength.

[0095] In this application, the binder is a gel polymer, which has the following advantages: (1) It can provide more contact sites, improving the interfacial bonding and the cohesive force required for the film layer; (2) The flexible gel polymer can improve the flexibility of the electrode sheet, so that the electrode sheet will not crack or break even in the case of ultra-thick coating, thereby improving the life and safety of the battery. Therefore, in this application, it is preferred to prepare the binder as a gel polymer.

[0096] In some embodiments, the viscosity of the binder is 70,000 mPa·s to 500,000 mPa·s.

[0097] Optionally, the viscosity of the binder is 100,000 mPa·s to 300,000 mPa·s, more preferably 110,000 mPa·s to 250,000 mPa·s.

[0098] When the viscosity of the binder is too high, exceeding the above-mentioned maximum value, it will cause difficulty in dispersing the slurry and fail to meet the processing requirements of the slurry; when the viscosity of the binder is too low, lower than the above-mentioned minimum value, it will cause insufficient adhesion of the slurry, and the electrode will easily shed powder and crack, causing the electrical performance to deteriorate, thereby affecting the service life of the battery.

[0099] The viscosity of the adhesive can be tested at 25°C using a rotation method according to standard GB / T10247-2008. The specific operation of this standard is as follows: Fill a beaker or sample container with the sample to be tested, making sure not to introduce bubbles. If necessary, vacuum to eliminate bubbles. If the sample is volatile or hygroscopic, the beaker or sample container must be sealed during the constant temperature process. Place the beaker or sample container with the prepared sample in a constant temperature bath and ensure that sufficient time is allowed to reach the specified temperature. Select a suitable rotor so that the reading is between 20% and 90% of the maximum range. For a 63# rotor, the following conditions should be met: 1000mPa·s≤viscosity≤10000mPa·s, 20%≤torque percentage≤90%. If the viscosity result is NG, use a 64# rotor instead, which should also meet the following results: 10000mPa·s<viscosity, 20%≤torque percentage≤90%. During the test, the rotor groove is flush with the slurry surface. Turn on the motor, wait for 5 minutes until the viscosity reading of the instrument stabilizes, then stop the motor, wait for the rotor to stop and then turn on the motor again to perform a second test, until the deviation of two consecutive test data relative to the average value is no more than 3%, and take the average of the two measured values.

[0100] Herein, the test may include the following steps:

[0101] Use a rotational viscometer and select the spindle according to the sample viscosity. Using the viscometer lift, slowly lower the viscometer, submerging the spindle in the slurry until the marking on the spindle is level with the liquid surface. Test temperature: 25°C, rotation speed: 12 rpm. Press the measurement button to begin the measurement. After 5 minutes, when the data remains stable, read the viscosity value.

[0102] In some embodiments, the binder has a weight average molecular weight of 300,000 to 1,000,000, optionally 400,000 to 900,000, and more optionally 500,000 to 800,000.

[0103] As the molecular weight of the binder increases, its bonding strength increases, and its mechanical strength improves. When the molecular weight is too small, for example, less than 300,000, the viscosity and mechanical strength of the binder will not meet the requirements. If the binder strength is too low, the adhesive performance of the binder will be insufficient, causing the electrode to shed or even break, affecting the electrical performance and service life of the battery cell. When the molecular weight is too large, for example, greater than 1,000,000, entanglement will occur between the molecular chains, making it difficult to disperse the slurry, thus complicating the preparation process of the negative electrode.

[0104] In some embodiments, the solid content of the binder is 4% to 10%.

[0105] Since the molecular weight of the binder is very large, if the solid content of the binder is greater than 10%, the preparation process of the binder is more difficult. If the solid content of the binder is less than 4%, the bonding performance of the binder is poor and the viscosity is small. When it is used to prepare the negative electrode sheet, the powder of the prepared negative electrode sheet is easy to fall off or even the membrane is easy to peel off during the charge and discharge process due to insufficient bonding force.

[0106] The solid content of the binder can be tested according to the following method:

[0107] Take the adhesive of mass M1 and put it into the oven, place it at 120℃ for 48 hours to dry, and the mass after drying is M2, solid content = M2 / M1×100%

[0108] In some embodiments, the glass transition temperature Tg of the binder ranges from 60-200°C, optionally from 100-150°C, and more optionally from 110-120°C.

[0109] If the binder's glass transition temperature is too high, for example, greater than 200°C, the movement of the binder's chain segments will be restricted, resulting in the electrode being too hard and brittle and prone to cracking, which in turn affects the battery's electrical performance and service life. If the binder's glass transition temperature is too low, for example, below 60°C, it will lead to reduced adhesion and cohesion, which in turn will cause the negative electrode to easily shed powder and delaminate, posing a safety hazard to the battery cell.

[0110] In some embodiments, the binder has a pH in the range of 5.0-8.0 as measured by potentiometry.

[0111] According to actual needs, the pH value of the binder can be adjusted to the range of 5.0-8.0 using LiOH solution.

[0112] The pH value of the binder can be tested according to the following method:

[0113] A glass electrode and a calomel electrode are immersed in the same solution to be tested to form a primary cell. The electromotive force of the cell is related to the pH value of the solution. The pH value of the solution can be obtained by measuring the electromotive force of the primary cell. The specific steps are as follows: (1) Soak the glass electrode according to the instructions of the acidometer. For the same sample, two buffer solutions with similar pH values ​​should be selected to calibrate the acidometer; (2) Dilute the sample to a viscosity of less than 20 Pa.s. Use a measuring cylinder to measure 50 mL of the sample and pour it into a beaker as the sample for pH measurement; (3) Place the beaker containing the sample in a constant temperature bath. After the temperature reaches a stable equilibrium, rinse the glass electrode with distilled water and wipe it dry. Then wash the electrode with the test solution and insert it into the sample for measurement; (4) In the measurement of three consecutive samples, if the difference between the three pH values ​​is greater than 0.2, three new samples should be taken and measured again until the difference in pH value is no greater than 0.2; (5) The result is the arithmetic mean of the pH values ​​of the three samples as the test result, and the value is rounded to one decimal place.

[0114] The pH value of the binder affects battery performance. If the pH value of the binder is not controlled, it may be too low, for example, less than 5.0. In this case, the binder contains a large number of carboxyl groups (-COOH), which may cause lithium insertion in the battery cell, reducing the primary efficiency of the secondary battery and thus shortening the battery cell life.

[0115] The pH value may also be too high, for example, greater than 8.0. At this time, the binder contains a large amount of -COOLi, which may cause the prepared electrode to be too brittle and easy to break, thereby increasing the risk of lithium plating, and the battery cell may also have safety hazards.

[0116] The second aspect of the present application provides a negative electrode plate, which includes the binder described in the first aspect of the present application.

[0117] In some embodiments, the binder accounts for 0.5-5% by weight in the negative electrode film, based on the weight of the negative electrode film.

[0118] In some embodiments, when the negative electrode sheet is wound around objects with diameters of 32 mm, 25 mm, and 16 mm, there are no cracks on the surface of the negative electrode sheet.

[0119] A third aspect of the present application provides a method for preparing a negative electrode sheet, comprising the following steps:

[0120] (1) mixing and kneading the negative electrode active material, the binder described in the first aspect of the present application, the conductive agent, and the solvent;

[0121] (2) the material containing the binder kneaded in step (1) is extruded through an extruder and granulated,

[0122] (3) Roll-pressing the particles obtained in step (2).

[0123] The negative electrode binder of the present application is particularly suitable for preparing negative electrode sheets by roller pressing. Figure 1 and Figure 2 As shown, Figure 1 The figure shows a forming method of a negative electrode sheet in one embodiment, wherein the granular material is the granular material obtained in step (2), the granular material is rolled into a film by rollers A and B, and then combined with the current collector by roller C. Figure 2 Another negative electrode sheet forming method is shown, wherein the granular material is the granular material obtained in step (2), the granular material is rolled into a film by rollers A and B, and then combined with the current collector by roller C. The present application has no particular limitation on the forming method of the negative electrode sheet, as long as it is prepared by rolling. Because the negative electrode binder can provide sufficient flexibility, the film containing the negative electrode binder will not break during rolling, and it has appropriate bonding strength but will not remain on the roller.

[0124] The fourth aspect of the present application provides a secondary battery, which includes the negative electrode sheet described in the second aspect of the present application or the negative electrode sheet prepared according to the preparation method of the third aspect of the present application.

[0125] In one embodiment of the present application, a secondary battery is provided, comprising the above-mentioned binder or the above-mentioned negative electrode sheet.

[0126] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, active ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0127] [Positive electrode]

[0128] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes the positive electrode active material of the first aspect of the present application.

[0129] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0130] In some embodiments, the positive electrode 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 material base and a metal layer formed on at least one surface of the polymer material base. 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 material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0131] In some embodiments, the positive electrode active material may adopt the positive electrode active material for batteries that is well known in the art. As an example, the positive electrode active material may include at least one of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional 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 transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05 O2) and its modified compounds. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0132] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0133] In some embodiments, the positive electrode film layer may further include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0134] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0135] [Negative electrode]

[0136] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer arranged on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material and the binder described in the first aspect of the present application.

[0137] As an example, the negative electrode current collector has two surfaces opposite to each other in its 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.

[0138] 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 base layer and a metal layer formed on at least one surface of the polymer base material. 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 base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0139] In some embodiments, the negative electrode active material may be a negative electrode active material for a battery that is well 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, lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other traditional 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.

[0140] In some embodiments, the negative electrode film layer may further optionally include another binder. The other 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). Alternatively, the negative electrode film layer may include only the binder described in the first aspect of the present application.

[0141] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0142] In some embodiments, the negative electrode film layer may optionally further include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0143] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0144] [Electrolytes]

[0145] The electrolyte conducts ions between the positive and negative electrodes. This application does not specifically limit the type of electrolyte; it can be selected based on needs. For example, the electrolyte can be selected from at least one of a solid electrolyte and a liquid electrolyte (i.e., an electrolyte solution).

[0146] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0147] In some embodiments, the electrolyte salt may be selected from one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorobis(oxalatophosphate) (LiDFOP) and lithium tetrafluorooxalatophosphate (LiTFOP).

[0148] In some embodiments, the solvent can be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS) and diethyl sulfone (ESE).

[0149] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, or additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high-temperature performance, and additives that improve battery low-temperature performance.

[0150] [Isolation film]

[0151] In some embodiments, the secondary battery further includes a separator. This separator is disposed between the positive and negative electrode sheets to provide isolation. This application does not specifically limit the type of separator; any known porous separator with good chemical and mechanical stability may be used.

[0152] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven 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.

[0153] [Outer packaging]

[0154] In some embodiments, a secondary battery may include an outer packaging for encapsulating a positive electrode sheet, a negative electrode sheet, and an electrolyte. For example, the positive electrode sheet, the negative electrode sheet, and the separator may be stacked or wound to form a laminated or wound cell structure, which is encapsulated within the outer packaging. The electrolyte may be a liquid electrolyte, which is impregnated within the cell. The number of cells in a secondary battery may be one or more, and can be adjusted based on demand.

[0155] In one embodiment, the present application provides an electrode assembly. In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into the electrode assembly by a winding process or a lamination process. The outer packaging can be used to encapsulate the above-mentioned electrode assembly and electrolyte.

[0156] In some embodiments, the outer packaging of the secondary battery may be a soft bag, such as a pouch-type soft bag. The soft bag may be made of plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS). In some embodiments, the outer packaging of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell.

[0157] Method for preparing secondary battery

[0158] In one embodiment, the present application provides a method for preparing a secondary battery, wherein the negative electrode sheet described in the present application or the negative electrode sheet prepared according to the method described in the present application is used.

[0159] The preparation of a secondary battery may also include assembling the negative electrode sheet, positive electrode sheet, and electrolyte of the present application to form a secondary battery. In some embodiments, the positive electrode sheet, separator, and negative electrode sheet may be wound or stacked in sequence, with the separator positioned between the positive and negative electrode sheets to provide isolation, to produce a battery cell. The battery cell is then placed in an outer packaging, injected with electrolyte, and sealed to produce a secondary battery.

[0160] In some embodiments, the preparation of a secondary battery may further include the step of preparing a positive electrode sheet. For example, the positive electrode active material, conductive agent, and binder may be dispersed in a solvent (e.g., N-methylpyrrolidone, NMP) to form a uniform positive electrode slurry; the positive electrode slurry is then coated on a positive electrode current collector, and the positive electrode sheet is obtained after drying, cold pressing, and other processes.

[0161] In some embodiments, the preparation of a secondary battery includes the step of preparing a negative electrode sheet according to the method described in this application.

[0162] The present application has no particular restrictions on the shape of the secondary battery, which can be cylindrical, square or any other shape. For example, Figure 5 The secondary battery 5 is a square structure as an example.

[0163] In some embodiments, reference Figure 6 , the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0164] In some embodiments, secondary batteries can be assembled into a battery module. The number of secondary batteries contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0165] Figure 7 4 is an example of a battery module. Figure 7 In the battery module 4, the plurality of secondary batteries 5 may be arranged in sequence along the length of the battery module 4. Of course, they may also be arranged in any other manner. The plurality of secondary batteries 5 may further be fixed by fasteners.

[0166] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0167] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0168] Figure 8 and Figure 9 The battery pack 1 is used as an example. Figure 8 and Figure 9The battery pack 1 may include a battery box and multiple battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner in the battery box.

[0169] In addition, the present application also provides an electric device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as 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., but is not limited thereto.

[0170] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0171] Figure 10 This is an example of an electric device. This device is 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, a battery pack or battery module can be used.

[0172] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0173] Example

[0174] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0175] I. Binder, negative electrode slurry, negative electrode sheet Preparation

[0176] Example 1:

[0177] Step 1: Preparation of Styrene-Butadiene-Acrylic Acid Block Copolymer Binder

[0178] An HTSCP series SCP2009 anionic polymerization apparatus was used. 1000 ml of cyclohexane and 104.14 g of styrene were mixed uniformly at room temperature, 100 ml of tetrahydrofuran was added and mixed uniformly, and then 0.97 g (0.5% of the total monomer volume) of n-butyl lithium was added as an initiator. The reactants were then stirred and polymerized for 30 minutes, after which 54.09 g of butadiene was added and allowed to react for 25 minutes. The reaction system was then heated to 30°C, and 36.03 g of acrylic acid was added and allowed to react for 30 minutes, with the reaction temperature maintained at 30°C. The entire reaction process was carried out under high-pressure nitrogen protection. After completion of the polymerization reaction, the reaction was terminated with isopropyl alcohol.

[0179] The preparation scheme is as follows:

[0180]

[0181] The mean of x is 4020, the mean of y is 4020, and the mean of z is 2010.

[0182] After the binder was prepared, its pH was adjusted to 7.5 using LiOH solution.

[0183] The prepared adhesive is a high-strength gel polymer with a solid content of 5% and a weight-average molecular weight of 800,000.

[0184] Step 2: Preparation of negative electrode slurry

[0185] The bulk density of 975g is 0.85g / cm 3 100g of graphite powder and 5g of conductive carbon were added to a mixer and dry-mixed. The dry-mixed powder and 49ml of water were added to a kneader and kneaded and dispersed. 400g of the binder prepared in step 1 (solid content: 20g) was then added and kneaded. The kneaded material was extruded through a screw extruder to obtain slurry pellets with a solid content of 70%.

[0186] Step 3: Preparation of negative electrode

[0187] The slurry particles obtained in step 2 are spread on the Figure 2 The electrode film is formed by rolling the rollers A and B in the gap between the rollers A and B. Figure 2 The transfer roller B and laminating roller C shown in the figure roll together the electrode film and copper foil, and after drying, they form the final composite electrode sheet. During the preparation process, the equipment parameters were adjusted to produce three types of negative electrode sheets with thicknesses of 110μm, 130μm, and 150μm, respectively.

[0188] Example 2 and Comparative Example 2

[0189] Example 2 and Comparative Example 2 were prepared similarly to Example 1, with the differences shown in Table 1.

[0190] Example 3

[0191] Example 3 was prepared similarly to Example 1, except that an acrylic acid-butadiene-acrylonitrile block copolymer binder was prepared and used.

[0192] The preparation scheme of acrylic acid-butadiene-acrylonitrile block copolymer adhesive is as follows:

[0193]

[0194] The mean of x is 2565, the mean of y is 5130, and the mean of z is 2565.

[0195] The preparation method is as follows:

[0196] An HTSCP series SCP2009 anionic polymerization apparatus was used. 1000 ml of cyclohexane and 72.06 g of acrylic acid were mixed at room temperature. 100 ml of tetrahydrofuran was added and mixed thoroughly. Then, 1.16 g of n-butyl lithium was added as an initiator. The reaction mixture was stirred and polymerized for 30 minutes, after which 108.17 g of butadiene was added and allowed to react for 25 minutes. The reaction system was then heated to 30°C, and 53.05 g of acrylonitrile was added and allowed to react for 30 minutes, maintaining the reaction temperature at 30°C. The entire reaction was carried out under high-pressure nitrogen protection. After completion of the polymerization, the reaction was terminated with isopropyl alcohol.

[0197] Example 4 and Comparative Example 3

[0198] Example 4 and Comparative Example 3 were prepared similarly to Example 3, except for the differences shown in Table 1.

[0199] Example 5

[0200] Example 5 was prepared similarly to Example 1, except that a butadiene-styrene-acrylonitrile-acrylic acid block copolymer binder was prepared and used. The preparation method was as follows:

[0201] An HTSCP series SCP2009 anionic polymerization apparatus was used. 1000ml of cyclohexane and 54.09g of butadiene were mixed uniformly at room temperature. 100ml of tetrahydrofuran was added and mixed uniformly. Then, 1.10g of n-butyl lithium was added as an initiator. The reactants were stirred and polymerized for 30 minutes, followed by the addition of 104.14g of styrene and the reaction for 25 minutes. Subsequently, the reactants were stirred and polymerized for 30 minutes, followed by the addition of 26.53g of acrylonitrile and the reaction for 25 minutes. The reaction system was then heated to 30°C, and 36.03g of acrylic acid was added and the reaction was continued for 30 minutes at this temperature. The reaction temperature was maintained at 30°C. The entire reaction process was carried out under high-pressure nitrogen protection. After completion of the polymerization reaction, the reaction was terminated with isopropanol.

[0202] Example 6

[0203] Example 6 was prepared similarly to Example 1, except that a 1-octene-styrene-ethylene glycol dimethacrylate block copolymer binder was prepared and used. The preparation method was as follows:

[0204] An HTSCP series SCP2009 anionic polymerization apparatus was used. 1000 ml of cyclohexane and 112.21 g of 1-octene were mixed at room temperature. 100 ml of tetrahydrofuran was added and mixed thoroughly. Then, 1.58 g of n-butyl lithium was added as an initiator. The reactants were stirred and polymerized for 30 minutes. Afterwards, 104.14 g of styrene was added and allowed to react for 25 minutes. Subsequently, the reactants were stirred and polymerized for 30 minutes. Afterwards, 99.11 g of ethylene glycol dimethacrylate was added and allowed to react for 30 minutes. The reaction temperature was maintained at 30°C. The entire reaction was carried out under high-pressure nitrogen protection. After completion of the polymerization, the reaction was terminated with isopropyl alcohol.

[0205] Example 7

[0206] Example 7 was prepared similarly to Example 1, except that an isoprene-limonene-methacrylamide block copolymer binder was prepared and used. The preparation method was as follows:

[0207] An HTSCP series SCP2009 anionic polymerization apparatus was used. 1000 ml of cyclohexane and 68.11 g of isoprene were mixed at room temperature, 100 ml of tetrahydrofuran was added, and 1.23 g of n-butyl lithium was added as an initiator. The reaction mixture was stirred and polymerized for 30 minutes, after which 136.23 g of limonene was added and allowed to react for 25 minutes. The reaction system was then heated to 30°C, and 42.55 g of methacrylamide was added and allowed to react for 30 minutes at this temperature. The reaction was carried out under high-pressure nitrogen protection. After completion of the polymerization, the reaction was terminated with isopropyl alcohol.

[0208] Example 8

[0209] Example 8 was prepared similarly to Example 1, except that an isoprene-acrylamide-ethylene glycol dimethacrylate copolymer binder was prepared and used. The specific preparation method is as follows:

[0210] An HTSCP series SCP2009 anionic polymerization apparatus was used. 1000 ml of cyclohexane and 68.11 g of isoprene were mixed uniformly at room temperature. 100 ml of tetrahydrofuran was added and mixed uniformly. 0.70 g of n-butyl lithium was then added as an initiator. The reactants were stirred and polymerized for 30 minutes, followed by the addition of 35.54 g of acrylamide and a 25-minute reaction. Subsequently, the reactants were stirred and polymerized for 30 minutes, followed by the addition of 99.11 g of ethylene glycol dimethacrylate and a 30-minute reaction. The reaction temperature was maintained at 30°C. The entire reaction was carried out under high-pressure nitrogen protection. After completion of the polymerization, the reaction was terminated with isopropyl alcohol.

[0211] Example 9

[0212] Example 9 was prepared similarly to Example 1, except that a 1-octene-acrylic acid-methacrylamide copolymer binder was prepared and used. The specific preparation method is as follows:

[0213] An HTSCP series SCP2009 anionic polymerization apparatus was used. 1000 ml of cyclohexane and 112.21 g of 1-octene were mixed at room temperature. 100 ml of tetrahydrofuran was added and mixed thoroughly. Then, 0.70 g of n-butyl lithium was added as an initiator. The reaction mixture was stirred and polymerized for 30 minutes. Afterwards, 36.03 g of acrylic acid was added and allowed to react for 25 minutes. The reaction system was then heated to 30°C, and 36.03 g of methacrylamide was added and allowed to react for 30 minutes. The reaction temperature was maintained at 30°C. After completion of the polymerization, the reaction was terminated with isopropanol.

[0214] Comparative Example 1

[0215] Comparative Example 1 was prepared similarly to Example 1, except that styrene-butadiene rubber (SBR) was used as the binder and carboxymethyl cellulose (CMC) was added to the negative electrode slurry. The weight ratio of the materials added to the negative electrode slurry was graphite powder: conductive carbon: CMC: SBR = 97.5%: 0.5%: 1%: 1%.

[0216] In the above embodiments and comparative examples, the adhesives prepared in Examples 1, 3 and 5-9 are all high-strength gel polymers, the adhesives prepared in Examples 2 and 4 and Comparative Example 3 are medium-strength gel polymers, and the adhesives prepared in Comparative Examples 1-2 are low-strength gel polymers.

[0217] In Examples 1-9 and Comparative Examples 1-3, the amount of the added binder was 2 wt % based on the total weight of the materials (excluding the solvent) used to prepare the negative electrode slurry, wherein the amount of the added binder was calculated based on the solid content of the binder.

[0218] II. Performance Evaluation

[0219] 1. Determination of adhesive properties

[0220] (1) Determination of binder weight average molecular weight

[0221] Gel permeation chromatography (GPC) is used with a concentration detector (differential detector) to measure the elution time and concentration of each component. To determine the molecular weight of a sample using GPC, a series of standard samples of known molecular weights must first be used to establish a molecular weight versus elution time / volume calibration curve. The sample to be tested is then injected, and the relative molecular weight (Mw) is calculated based on the previously established calibration curve.

[0222] (2) Determination of adhesive viscosity

[0223] The test is performed using the rotation method according to the standard GB / T 10247-2008. The specific operation is as follows:

[0224] Use a rotational viscometer and select the rotor according to the viscosity of the sample. Use the viscometer lifting stand to slowly lower the viscometer and immerse the rotor in the slurry until the mark on the rotor is level with the liquid surface. Test temperature: 25°C, speed: 12 rpm. Press the measurement button to start the measurement. After 5 minutes, when the data remains stable, read the viscosity value.

[0225] (3) Determination of pH value of binder

[0226] A glass electrode and a calomel electrode are immersed in the same solution to be tested to form a primary cell. The electromotive force of the cell is related to the pH value of the solution. The pH value of the solution can be obtained by measuring the electromotive force of the primary cell. The specific steps are as follows: (1) Soak the glass electrode according to the instructions of the pH meter. For the same sample, two buffer solutions with similar pH values ​​should be selected to calibrate the pH meter; (2) Dilute the adhesive sample to a viscosity of less than 20 Pa.s. Use a measuring cylinder to measure 50 mL of the sample and pour it into a beaker as the sample for pH measurement; (3) Place the beaker containing the sample in a constant temperature bath. After the temperature reaches a stable equilibrium, rinse the glass electrode with distilled water and wipe it dry. Then wash the electrode with the test solution and insert it into the sample for measurement; (4) In the measurement of three consecutive samples, if the difference between the three pH values ​​is greater than 0.2, three new samples should be taken and measured again until the difference in pH value is no greater than 0.2; (5) The result is the arithmetic mean of the pH values ​​of the three samples as the test result, and the value is rounded to one decimal place.

[0227] (4) Determination of binder solid content

[0228] Take the adhesive of mass M1 and put it into an oven, dry it at 120℃ for 48 hours, and the mass after drying is M2.

[0229] Solid content = M2 / M1×100%.

[0230] (5) Determination of the glass transition temperature (Tg) of the binder

[0231] Tg was determined by differential scanning calorimetry. The specific procedure was as follows:

[0232] Take a 20mg sample and place it in a crucible. Open the DSC furnace (make sure it's at room temperature) and use tweezers to place the crucible containing the sample on the sample tray in the furnace. Place another identical empty crucible on the reference tray to serve as a reference. Close the furnace lid and measure the glass transition temperature of the sample.

[0233] 2. Negative electrode slurry strength test

[0234] The test was conducted using an Instron metal tensile testing machine. The specific steps included: in each embodiment and comparative example, the negative electrode slurry was roll-pressed to form a negative electrode active material layer with a thickness of 1 mm, which was then cut into test samples with a length of 100 mm, a width of 20 mm, and a thickness of 1 mm. The tensile strength (Rm) of the test sample was tested at 2 mm / min using a tensile testing machine. The test results are shown in Table 1.

[0235] 3. Determination of the flexibility of the negative electrode

[0236] The flexibility of the negative electrode sheets prepared in each embodiment and comparative example was tested using a winding method. The specific testing process is as follows:

[0237] The negative electrode sheets of three thicknesses prepared in the examples and comparative examples were respectively wound onto cylindrical rods with diameters of 32 mm, 25 mm, and 16 mm to observe whether they cracked. Figure 3 Indicates cracking. Figure 4 Indicates an uncracked condition.

[0238] The results are shown in the table below, where "√" indicates there are no cracks on the electrode surface and "×" indicates there are cracks on the electrode surface.

[0239] The rod diameter is small and the pole piece is not cracked, which means good flexibility.

[0240] The specific performance test results are shown in Table 1 below.

[0241]

[0242]

[0243] It can be seen from Table 1 that compared with Comparative Examples 1-3, Examples 1-9 to which the binder described in the present application is added achieve a negative electrode slurry with significantly higher strength (the strength range of the negative electrode slurry in Examples 1-9 is 0.20-0.35 MPa, while that in Comparative Examples 1-3 is less than 0.1 MPa), and achieve better electrode flexibility at a higher electrode thickness.

[0244] It can be seen from Comparative Example 1 in which a conventional binder, styrene-butadiene rubber, is added and other comparative examples and examples in which the binder of the present application is added that the addition of the binder of the present application can improve the flexibility of the negative electrode sheet.

[0245] From the comparison between Examples 1-2 and Comparative Example 2, and between Examples 3-4 and Comparative Example 3, it can be seen that the binder in a high-strength gel state and the binder with a large molecular weight, viscosity and Tg can achieve better flexibility of the negative electrode sheet.

[0246] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A binder, which is a polymer, is polymerized from at least the following components: (a) at least one flexible monomer selected from butadiene, 1-octene, and isoprene; (b) at least one rigid monomer, said rigid monomer comprising the following components (b1), component (b1) is selected from styrene or limonene; and / or (b2), component (b2) is selected from acrylonitrile or acrylamide; (c) at least one hydrophilic monomer, component (c) is selected from acrylic acid, ethylene glycol dimethacrylate or methacrylamide; wherein, Based on the total molar amount of components (a)-(c), the molar proportion of component (a) is in the range of 30%-55%, the molar proportion of component (b) is in the range of 15%-45%, and the molar proportion of component (c) is in the range of 15%-30%; wherein the weight average molecular weight of the binder is 500,000 to 800,000; Wherein, the glass transition temperature Tg of the binder is in the range of 110-120°C; Wherein, the pH value of the binder ranges from 5.0 to 8.

0.

2. The adhesive according to claim 1, wherein The binder is a gel polymer with medium or high strength.

3. The adhesive according to any one of claims 1 to 2, wherein The viscosity of the binder is 70,000 to 500,000 mPa·s.

4. The adhesive according to claim 3, wherein The viscosity of the binder is 100,000 to 300,000 mPa·s.

5. The adhesive according to claim 3, wherein The viscosity of the binder is 110,000 to 250,000 mPa·s.

6. The adhesive according to any one of claims 1 to 2, wherein The solid content of the binder is 4% to 10%. 7 . A negative electrode sheet comprising a negative electrode current collector and a negative electrode membrane, wherein the negative electrode membrane comprises any one of the binders described in claims 1 to 6 .

8. The negative electrode sheet according to claim 7, wherein when the negative electrode sheet is wound around objects with diameters of 32 mm, 25 mm, and 16 mm, there are no cracks on the surface of the negative electrode sheet. 9 . A secondary battery comprising the binder according to claim 1 or the negative electrode sheet according to claim 7 .

10. A battery module, characterized in that: The secondary battery according to claim 9 is included.

11. A battery pack, characterized in that: A battery module comprising the battery module according to claim 10.

12. An electrical device, characterized in that: The battery comprises any one selected from the group consisting of the secondary battery according to claim 9, the battery module according to claim 10, and the battery pack according to claim 11.

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