Binder, positive electrode sheet, and battery

Abstract

The application provides a kind of binder, positive plate and battery, the binder includes nitrile structural unit, ether structural unit, nitrogen heterocyclic structural unit and sulfonic acid structural unit, the swelling degree of the binder is 20%~40%.The application can improve the adhesion, flexibility and anti-swelling performance of the binder, improve the flexibility of the electrode sheet using the binder, and improve the electrochemical performance of the battery, such as initial efficiency and cycle life.

Description

Technical Field

[0001] This application relates to the field of batteries, and more particularly to an adhesive, a positive electrode sheet, and a battery. Background Technology

[0002] Batteries are common electrochemical energy storage devices with wide applications. For example, lithium-ion batteries, with their advantages of high energy density and long cycle life, are widely used in many fields such as new energy vehicles and energy storage. Currently, PVDF is the main binder for the positive electrode of lithium-ion batteries. However, due to factors such as the presence of fluorine in PVDF, its application is limited as it is not environmentally friendly.

[0003] Currently, the main alternative to PVDF is the polyacrylonitrile modification route, which mainly uses acrylonitrile as the main component and obtains a multi-component copolymer through copolymerization with acrylate, and then uses this multi-component copolymer to replace PVDF.

[0004] However, copolymers modified by copolymerization of acrylonitrile and acrylate generally suffer from poor resistance to swelling and poor flexibility of electrode sheets using these multi-component copolymers as binders, which affect the electrochemical performance of batteries, such as initial efficiency and cycle life, and urgently need to be addressed. Summary of the Invention

[0005] This invention provides an adhesive, a positive electrode sheet, and a battery to at least solve the problems existing in the prior art, such as poor anti-swelling ability of the adhesive, poor flexibility of the electrode sheet, and poor initial efficiency and cycle life of the battery.

[0006] In one aspect, the present invention provides an adhesive comprising nitrile structural units, ether structural units, nitrogen heterocyclic structural units and sulfonic acid structural units, wherein the swelling degree of the adhesive is 20% to 40%.

[0007] According to one embodiment of the present invention, the weight-average molecular weight of the adhesive is 10 wt% to 40 wt%.

[0008] According to one embodiment of the present invention, the glass transition temperature of the adhesive is 0°C to 40°C.

[0009] According to one embodiment of the present invention, the particle size of the adhesive is 40 μm to 80 μm.

[0010] According to one embodiment of the present invention, the mass ratio of the nitrile structural unit, the ether structural unit, the nitrogen heterocyclic structural unit and the sulfonic acid structural unit is (30-50):(30-50):(20-30):(5-10).

[0011] According to one embodiment of the present invention, the nitrile structural unit includes one or more of acrylonitrile structural units and methacrylonitrile structural units; and / or, the ether structural unit includes an allyl ether structural unit, which includes one or more of allyl ethyl ether structural units, allyl propyl ether structural units, allyl butyl ether structural units, allyl hydroxyethyl ether structural units, and allyl propyl ether structural units.

[0012] According to one embodiment of the present invention, the sulfonic acid structural unit includes an alkenyl sulfonic acid structural unit, and the alkenyl sulfonic acid structural unit includes a C2-C14 vinyl sulfonic acid structural unit.

[0013] According to one embodiment of the present invention, the nitrogen heterocyclic structural unit contains a nitrogen heterocycle, which includes one or more of pyridyl, pyrazinyl, pyrimidinyl, quinolinyl, isoquinolinyl, and quinoxalinyl.

[0014] In one aspect, the present invention provides a positive electrode sheet comprising a positive electrode active layer, wherein the positive electrode active layer comprises the binder described above.

[0015] In one aspect, the present invention provides a battery comprising the above-described positive electrode.

[0016] This invention provides a binder, a positive electrode sheet, and a battery. The binder comprises nitrile structural units, ether structural units, nitrogen heterocyclic structural units, and sulfonic acid structural units. The swelling degree of the binder is 20%~40%. In the coexistence system of these structural units, the nitrile and sulfonic acid structural units can improve the adhesion of the binder, the ether structural units can improve the flexibility of the electrode sheet, and the nitrogen heterocyclic structural units can improve the swelling resistance of the binder. The interaction between the basic nitrogen heterocycle and the acidic sulfonic acid group forms an acid-base pair, which can synergistically play a role in anti-swelling. Based on this, this invention can simultaneously improve the adhesion, flexibility, and swelling resistance of the binder, improve the flexibility of the electrode sheet using this binder, reduce the brittleness of the electrode sheet, avoid problems such as powder shedding from the electrode sheet, and simultaneously improve the electrochemical performance of the battery, such as initial efficiency and cycle life. Detailed Implementation

[0017] To enable those skilled in the art to better understand the solutions of this invention, the following provides a more detailed description of this application. The specific embodiments listed below are merely descriptions of the principles and features of this invention; the examples are only for explaining the invention and are not intended to limit its scope. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this invention.

[0018] In related technologies, copolymers of acrylonitrile and acrylates are used to replace PVDF. However, copolymers of acrylonitrile and acrylates generally suffer from poor anti-swelling ability and poor flexibility of electrode sheets using this copolymer as a binder. Specifically, based on the long-term research of the inventors of this application, on the one hand, due to the high glass transition temperature of copolymers of acrylonitrile and acrylates, electrode sheets using this copolymer as a binder have poor toughness and are brittle. During the processing of electrode sheets (e.g., in the winding process of manufacturing wound cells), powdering and other phenomena are prone to occur, which cannot meet the requirements of battery manufacturing and processing, making it difficult to directly replace PVDF. On the other hand, acrylates have good affinity with electrolytes, and the introduction of acrylates will cause excessive swelling of the copolymer of acrylonitrile and acrylates, thereby causing a decline in the electrochemical performance of the battery, such as initial efficiency and cycle life. Therefore, developing a new fluorine-free binder with both good processing performance and electrochemical stability has become a key direction for overcoming technical bottlenecks.

[0019] In view of this, embodiments of the present invention provide an adhesive comprising nitrile structural units, ether structural units, nitrogen heterocyclic structural units and sulfonic acid structural units, wherein the swelling degree of the adhesive is 20% to 40%.

[0020] According to the inventors' research, nitrile structural units, ether structural units, nitrogen heterocyclic structural units, and sulfonic acid structural units are simultaneously introduced into the binder, and the swelling degree of the binder is controlled at 20%~40%. In this system where these structural units coexist, the binder's adhesion, flexibility, and anti-swelling properties can be improved simultaneously. This improves the flexibility of the electrode sheet using this binder, reduces its brittleness, and avoids problems such as powder shedding from the electrode sheet. It also enhances the battery's initial efficiency and cycle life, among other electrochemical performance characteristics. The reasons for this are as follows:

[0021] (1) In the above-mentioned binder composition system, the nitrile structural unit has a polar group (cyano (-CN)), which can enhance the polarity of the binder and help the interaction between the binder and the electrode active material, as well as between the binder and the electrode current collector, thereby improving the adhesion between the electrode coating on the electrode sheet and the electrode current collector, as well as the adhesion between the electrode active material and other materials in the electrode coating, and improving the structural stability of the electrode sheet.

[0022] (2) In the above-mentioned binder composition system, the ether structural unit contains ether bonds. The presence of ether bonds makes the polymer chain of the binder more flexible, thereby improving the flexibility of the binder, which in turn improves the flexibility of the electrode sheet using the binder, reduces the brittleness of the electrode sheet, and avoids problems such as the electrode sheet falling off.

[0023] (3) In the above-mentioned binder composition system, the nitrogen heterocyclic structural unit has a stable ring structure, which can increase the steric hindrance of the binder, limit the penetration of electrolyte solvent molecules, and improve the stability of the binder, thereby improving the anti-swelling performance of the binder, so that the battery using the binder can have stable electrochemical performance; in addition, the interaction between the basic nitrogen heterocyclic and the acidic sulfonic acid group forms an acid-base pair, which can synergistically play an anti-swelling role, further improving the anti-swelling performance of the binder.

[0024] (4) In the above-mentioned binder composition system, the sulfonic acid group (-SO3H) in the sulfonic acid structural unit can form a strong intermolecular force with the electrode active material and the electrode current collector, thereby improving the adhesion between the electrode coating on the electrode sheet and the electrode current collector, as well as the adhesion between the electrode active material and other materials in the electrode coating. In addition, the sulfonic acid structural unit can also enhance the lithium ion transport capacity of the binder.

[0025] Therefore, in a system where these structural units coexist, the synergistic effect of these structural units can simultaneously improve the anti-swelling ability and adhesion of the binder, enhance the structural stability of the electrode sheet, and simultaneously reduce the glass transition temperature of the binder, improve the flexibility of the binder, and thus improve the flexibility of the electrode sheet using the binder, avoiding problems such as material shedding from the electrode sheet, thereby simultaneously improving the electrochemical performance of the battery, such as initial efficiency and cycle life.

[0026] Furthermore, the adhesive in this embodiment of the invention is a fluorine-free adhesive, which does not contain fluorine and has advantages such as being environmentally friendly. At the same time, the adhesive in this embodiment of the invention does not introduce acrylate structural units, thus avoiding the problem of poor swelling resistance of the adhesive caused by the high affinity of acrylates for the electrolyte.

[0027] In this embodiment of the invention, the binder exhibits good anti-swelling properties, preventing excessive swelling of the binder in the electrolyte and thus improving battery cycle life and other performance characteristics. The swelling degree A of the binder in the non-aqueous electrolyte is between 20% and 40%, for example, 20%, 25%, 30%, 35%, 40%, or any combination thereof. This suitable swelling degree maintains electrolyte wetting of the electrode sheets, shortens the lithium-ion diffusion path, reduces interfacial impedance, and improves the electrochemical performance of the battery. Simultaneously, it avoids excessive binder swelling and the resulting collapse of the binder network due to excessive insertion of electrolyte solvent molecules, which could lead to the separation of the electrode active material from the electrode current collector.

[0028] In this embodiment of the invention, the swelling degree A of the adhesive is A = (W2 - W1) / W1, where W1 is the mass of the adhesive before it is placed in the electrolyte, and W2 is the mass of the adhesive after it has been immersed in the electrolyte at 55°C for 48 hours. Specifically, the adhesive can be made into a film (such as a disc) sample, and then its swelling degree in a non-aqueous electrolyte can be tested. The composition of the non-aqueous electrolyte can be: solvents are EC, DEC, and EMC, with a volume ratio of EC, DEC, and EMC of 2:3:1, lithium salt is LiPF6, and the concentration of LiPF6 is 1 mol / L.

[0029] In this embodiment of the invention, the weight-average molecular weight of the binder is 10 wt / mol to 40 wt / mol (w represents the unit of ten thousand, that is, the molecular weight of the binder can be 100,000 to 400,000), for example, 10 wt / mol, 15 wt / mol, 20 wt / mol, 25 wt / mol, 30 wt / mol, 35 wt / mol, 40 wt / mol or any combination thereof. In this way, the molecular weight of the binder is not less than 10 wt / mol, which helps to improve the stability and anti-swelling properties of the binder. At the same time, the molecular weight of the binder is not higher than 40 wt / mol, which helps to improve the flexibility of the binder. By controlling the molecular weight of the binder within the range of 10 wt / mol to 40 wt / mol, it is beneficial to better balance the properties of the binder such as flexibility and anti-swelling properties, and to improve the adhesion, flexibility and anti-swelling properties of the binder, thereby enhancing the structural stability and flexibility of the electrode sheet.

[0030] In this embodiment of the invention, the binder is a multi-component copolymer system containing nitrile structural units, ether structural units, nitrogen heterocyclic structural units, and sulfonic acid structural units. Specifically, it can be a quaternary copolymer system composed of nitrile structural units, ether structural units, nitrogen heterocyclic structural units, and sulfonic acid structural units. While improving the adhesiveness and anti-swelling properties of the binder, it can also reduce the glass transition temperature of the binder and improve the flexibility of the binder. The reason for this is that the introduction of flexible ether structural units can reduce the glass transition temperature of the binder. With a lower glass transition temperature, the molecular chain mobility of the binder is enhanced, thus improving its flexibility.

[0031] In some embodiments, the glass transition temperature of the adhesive can be 0~40°C, for example, a range of 0°C, 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, 40°C or any two of these, which is beneficial to further improve the adhesiveness, flexibility and anti-swelling properties of the adhesive, and enhance the structural stability and flexibility of the electrode sheet.

[0032] The glass transition temperature of the adhesive can be determined by conventional testing methods for the glass transition temperature of polymers in this embodiment of the invention.

[0033] In related technologies, polyacrylonitrile binders are typically copolymers of acrylonitrile and acrylates. Their adhesion and flexibility need further improvement. Furthermore, due to the good affinity between the acrylate structure and the electrolyte, the binder has poor anti-swelling ability, severely affecting battery cycle life and other performance characteristics. In this invention, the binder is a copolymer of non-acrylate monomers and unsaturated nitrile monomers such as acrylonitrile. By simultaneously introducing nitrile structural units, ether structural units, nitrogen heterocyclic structural units, and sulfonic acid structural units into the binder, not only can the adhesion and flexibility of the binder be improved, but the introduction of the acrylate structure can also be avoided. This avoids the problem of poor anti-swelling ability caused by the good affinity between the acrylate structure and the electrolyte.

[0034] In this embodiment of the invention, the adhesive is granular, that is, the adhesive is polymer microparticles.

[0035] In this embodiment of the invention, the particle size of the binder can be 40μm to 80μm, that is, the particle size of the binder polymer particles is 40μm to 80μm. For example, it can be a range of 40μm, 50μm, 60μm, 70μm, 80μm or any two of them, which is beneficial to further improve the adhesiveness, flexibility and anti-swelling properties of the binder, and improve the structural stability and flexibility of the electrode sheet.

[0036] In this embodiment of the invention, the mass ratio of nitrile structural units, ether structural units, nitrogen heterocyclic structural units and sulfonic acid structural units is (30-50): (30-50): (20-30): (5-10).

[0037] Specifically, the aforementioned adhesive is a polymer adhesive, which can be a copolymer formed by copolymerizing monomeric raw materials including unsaturated nitrile monomers, unsaturated ether monomers, unsaturated nitrogen heterocyclic monomers, and unsaturated sulfonic acid monomers. These monomers polymerize through their respective unsaturated bonds, and after copolymerization, they form various structural units in the copolymer. That is, the adhesive includes nitrile structural units formed in the copolymer by copolymerizing unsaturated nitrile monomers, ether structural units formed in the copolymer by copolymerizing unsaturated ether monomers, nitrogen heterocyclic structural units formed in the copolymer by copolymerizing unsaturated nitrogen heterocyclic monomers, and sulfonic acid structural units formed in the copolymer by copolymerizing unsaturated sulfonic acid monomers.

[0038] In this embodiment of the invention, the nitrile structural unit may include one or more of acrylonitrile structural units and methacrylonitrile structural units, which can improve the structural stability of the electrode sheet. The reason for this is that the nitrile structural unit has a polar group (cyano (-CN)), which can enhance the polarity of the binder and facilitate the interaction between the binder and the electrode active material, as well as between the binder and the electrode current collector, thereby improving the adhesion between the electrode coating on the electrode sheet and the electrode current collector, as well as the adhesion between the electrode active material and other materials in the electrode coating.

[0039] Specifically, a nitrile structural unit is a structural unit formed by the polymerization of nitrile monomers containing unsaturated bonds (i.e., unsaturated nitrile monomers) through unsaturated bonds. Nitrile monomers containing unsaturated bonds may include nitrile monomers containing double bonds. For example, unsaturated nitrile monomers may include acrylonitrile and / or methacrylonitrile.

[0040] For example, the acrylonitrile monomer (CH2=CH-CN) contains carbon-carbon double bonds. After copolymerization, the acrylonitrile structural unit formed by the acrylonitrile has the structure shown in Equation 1-1:

[0041] -CH2-CH(CN)- Equation 1-1,

[0042] For example, the adhesive may contain repeating units formed of acrylonitrile structural units, the repeating unit structure having the structure shown in Formulas 1-2 below:

[0043] -[-CH2-CH(CN)-] n - Equation 1-2,

[0044] Where n represents the number of repeating units (-CH2-CH(CN)-) in Equation 1-2.

[0045] In this embodiment of the invention, the ether structural unit may include an allyl ether structural unit, which may include one or more of allyl ethyl ether, allyl propyl ether, allyl butyl ether, allyl hydroxyethyl ether, and allyl propyl ether. This can improve the flexibility of the binder, thereby enhancing the flexibility of the electrode sheet using the binder, reducing the brittleness of the electrode sheet, and preventing problems such as material shedding from the electrode sheet. The reason for this is that the ether structural unit contains ether bonds, and the presence of ether bonds makes the polymer chain of the binder more flexible.

[0046] Specifically, ether structural units are structural units formed by the polymerization of ether monomers containing unsaturated bonds (i.e., unsaturated ether monomers) through unsaturated bonds. Ether monomers containing unsaturated bonds may include ether monomers containing double bonds. For example, unsaturated ether monomers may include one or more of allyl ethyl ether, allyl hydroxyethyl ether, and allyl propyl ether.

[0047] For example, the allyl ethyl ether monomer (CH2=CH-CH2-O-CH2CH5) contains a carbon-carbon double bond, and after copolymerization, the allyl ethyl ether structural unit formed by the allyl ethyl ether monomer has the structure shown in Formulas 1-3 below:

[0048] -CH2-CH(CH2-O-C2H5)- Equation 1-3,

[0049] For example, the adhesive may contain repeating units formed of allyl ethyl ether structural units, the repeating unit structure having the structure shown in Formulas 1-4 below:

[0050] -[-CH2-CH(CH2-O-C2H5)-] n - Equation 1-4,

[0051] Where n represents the number of repeating units (-CH2-CH(CH2-O-C2H5)-) in Equation 1-4.

[0052] In this embodiment of the invention, the nitrogen heterocyclic structural unit includes a nitrogen heterocycle, which may include one or more of pyridyl, pyrazinyl, pyrimidinyl, quinolinyl, isoquinolinyl, and quinoxalinyl groups. This can improve the anti-swelling performance of the binder, enabling the battery using this binder to have stable electrochemical performance. The reason for this is that the nitrogen heterocyclic structural unit has a stable ring structure, which can increase the steric hindrance of the binder, limit the penetration of electrolyte solvent molecules, and improve the stability of the binder.

[0053] In some preferred embodiments, the nitrogen heterocyclic structural unit containing a pyridine group may include one or more of a 2-vinylpyridine structural unit, a 3-vinylpyridine structural unit, and a 4-vinylpyridine structural unit.

[0054] In some preferred embodiments, the nitrogen heterocyclic structural unit containing the pyrazine group may include a 2-vinylpyrazine structural unit.

[0055] In some preferred embodiments, the nitrogen heterocyclic structural unit containing a pyrimidine group may include a 2-vinylpyrimidine structural unit.

[0056] In some preferred embodiments, the nitrogen heterocyclic structural unit containing a quinoline group may include one or more of 2-vinylquinoline structural units, 3-vinylquinoline structural units, and 4-vinylquinoline structural units.

[0057] In some preferred embodiments, the nitrogen heterocyclic structural unit containing the isoquinoline group may include the 1-vinylisoquinoline structural unit.

[0058] In some preferred embodiments, the nitrogen heterocyclic structural unit containing the quinoxaline group may include the 2-vinylquinoxaline structural unit.

[0059] Specifically, a nitrogen heterocyclic structural unit is a structural unit formed by polymerizing nitrogen heterocyclic monomers containing unsaturated bonds (i.e., unsaturated nitrogen heterocyclic monomers) through unsaturated bonds. Nitrogen heterocyclic monomers containing unsaturated bonds may include nitrogen heterocyclic monomers containing double bonds. For example, unsaturated nitrogen heterocyclic monomers may include 2-vinylpyridine and / or 2-vinylquinoxaline.

[0060] For example, the 2-vinylpyridine monomer (CH2=CH-C5H4N) contains a carbon-carbon double bond, and after copolymerization, the 2-vinylpyridine structural unit formed by the 2-vinylpyridine monomer has the structure shown in Formulas 1-5 below:

[0061] -CH2-CH(C5H4N)- Equation 1-5,

[0062] For example, the adhesive may contain repeating units formed of 2-vinylpyridine structural units, the repeating unit structure having the structure shown in Formulas 1-6 below:

[0063] -[-CH2-CH(C5H4N)-] n - Equation 1-6,

[0064] Where n represents the number of repeating units (-CH2-CH(C5H4N)-) in Equations 1-6.

[0065] In this embodiment of the invention, the sulfonic acid structural unit may include an alkenyl sulfonic acid structural unit, which includes a C2-C14 vinyl sulfonic acid structural unit (i.e., the vinyl sulfonic acid structural unit has 2-14 carbon atoms). The number of carbon atoms in the vinyl sulfonic acid structural unit is, for example, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 or 14. This can improve the adhesion between the electrode coating and the electrode current collector on the electrode sheet, as well as the adhesion between materials such as the electrode active material in the electrode coating. The reason for this is that the sulfonic acid group (-SO3H) in the sulfonic acid structural unit can form a strong intermolecular force with the electrode active material and the electrode current collector.

[0066] Specifically, sulfonic acid structural units are structural units formed by the polymerization of sulfonic acid monomers containing unsaturated bonds (i.e., unsaturated sulfonic acid monomers) through unsaturated bonds. Sulfonic acid monomers containing unsaturated bonds may include sulfonic acid monomers containing double bonds. For example, unsaturated sulfonic acid monomers may include C2 vinyl sulfonic acid and / or C14 vinyl sulfonic acid.

[0067] For example, the C2 vinyl sulfonic acid monomer (CH2=CH-SO3H) contains a carbon-carbon double bond, and after copolymerization, the C2 vinyl sulfonic acid structural unit formed by the C2 vinyl sulfonic acid monomer has the structure shown in Formulas 1-7 below:

[0068] -CH2-CH(SO3H)- Equation 1-7,

[0069] For example, the adhesive may contain repeating units formed of C2 vinyl sulfonic acid structural units, the repeating unit structure having the structure shown in Formulas 1-8 below:

[0070] -[-CH2-CH(SO3H)-] n - Equation 1-8,

[0071] Wherein, represents the number of repeating units (-CH2-CH(SO3H)-) in Equations 1-8.

[0072] In this embodiment of the invention, the mass percentage of nitrile structural units in the binder is 30-50 parts, for example, 30 parts, 35 parts, 40 parts, 45 parts, 50 parts or any combination thereof, which is beneficial to further improve the adhesiveness, flexibility and anti-swelling properties of the binder, and enhance the structural stability and flexibility of the electrode sheet.

[0073] In this embodiment of the invention, the mass percentage of ether structural units in the binder is 30-50 parts, for example, 30 parts, 35 parts, 40 parts, 45 parts, 50 parts or any combination thereof. This is beneficial for further improving the adhesiveness, flexibility and anti-swelling properties of the binder, and enhancing the structural stability and flexibility of the electrode sheet.

[0074] In this embodiment of the invention, the mass percentage of nitrogen heterocyclic structural units in the binder is 20-30 parts, for example, 20 parts, 23 parts, 25 parts, 28 parts, 30 parts, or any combination thereof. This is beneficial for further improving the adhesiveness, flexibility, and anti-swelling properties of the binder, thereby enhancing the structural stability and flexibility of the electrode sheet.

[0075] In this embodiment of the invention, the mass ratio of sulfonic acid structural units in the binder is 5-10 parts, for example, 5 parts, 6 parts, 7 parts, 8 parts, 9 parts, 10 parts or any two of them. This is beneficial to further improve the adhesiveness, flexibility and anti-swelling properties of the binder, and enhance the structural stability and flexibility of the electrode sheet.

[0076] Generally, the mass fraction of each structural unit in an adhesive is basically equal to the mass fraction of the monomer used to form that structural unit during the preparation of the adhesive. For example, if the adhesive is a copolymer of unsaturated nitrile monomers, unsaturated ether monomers, unsaturated nitrogen heterocyclic monomers, and unsaturated sulfonic acid monomers, then the mass fraction of the nitrile structural units in the adhesive is basically equal to the mass fraction of the unsaturated nitrile monomers.

[0077] In this embodiment of the invention, a method for preparing an adhesive is provided, comprising the following steps: polymerizing (copolymerizing) monomer raw materials including unsaturated nitrile monomers, unsaturated ether monomers, unsaturated nitrogen heterocyclic monomers and unsaturated sulfonic acid monomers to obtain the above-mentioned adhesive.

[0078] Specifically, the above polymerization reaction can be emulsion polymerization, which is a process in which monomer raw materials form an emulsion in a dispersion medium (solvent) under the action of an emulsifier, and the polymerization is initiated by an initiator. That is, the above monomer raw materials undergo a polymerization reaction under the action of an emulsifier and an initiator to obtain a binder.

[0079] Specifically, the monomer raw materials may include unsaturated nitrile monomers, unsaturated ether monomers, unsaturated nitrogen heterocyclic monomers, and unsaturated sulfonic acid monomers. The mass parts of the unsaturated nitrile monomers may be 30-50 parts, for example, 30 parts, 35 parts, 40 parts, 45 parts, 50 parts, or any combination thereof. The mass parts of the unsaturated ether monomers may be 30-50 parts, for example, 30 parts, 35 parts, 40 parts, 45 parts, 50 parts, or any combination thereof. The mass parts of the unsaturated nitrogen heterocyclic monomers may be 20-30 parts, for example, 20 parts, 23 parts, 25 parts, 28 parts, 30 parts, or any combination thereof. The mass parts of the unsaturated sulfonic acid monomers may be 5-10 parts, for example, 5 parts, 6 parts, 7 parts, 8 parts, 9 parts, 10 parts, or any combination thereof.

[0080] Specifically, the emulsifier may include one or more of the following: sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, sodium dodecyl sulfonate, sodium octadecyl sulfate, sodium dioctyl succinate sulfonate, sodium fatty alcohol ether sulfate, sulfonates of ethoxylated fatty acid methyl esters, sodium α-alkenyl sulfonate, sodium secondary alkyl sulfonate, alcohol ether carboxylates, alcohol ether phosphates, lauryl ether phosphates, and isooctyl ether phosphates.

[0081] Specifically, the dispersion medium (solvent) includes water, and in practice, deionized water can be used.

[0082] Specifically, the initiator may include one or more of sodium persulfate, ammonium persulfate, and potassium persulfate.

[0083] In some embodiments, the process of polymerizing monomer raw materials including unsaturated nitrile monomers, unsaturated ether monomers, unsaturated nitrogen heterocyclic monomers and unsaturated sulfonic acid monomers may include: mixing monomer raw materials, emulsifiers and water to obtain a pre-emulsion; then mixing the pre-emulsion with an initiator and then carrying out a polymerization reaction to obtain a binder.

[0084] In practice, the pre-emulsifier can be added dropwise to a solution containing an initiator and a dispersion medium (solvent). Specifically, the pre-emulsifier can be added dropwise to a solution containing an initiator and a dispersion medium. During the dropwise addition of the pre-emulsifier, the temperature of the solution containing the initiator and dispersion medium can be controlled between 60℃ and 80℃, for example, a range of 60℃, 65℃, 70℃, 75℃, 80℃, or any combination thereof. The dropwise addition time of the pre-emulsifier can be between 2h and 4h, for example, a range of 2h, 2.5h, 3h, 3.5h, 4h, or any combination thereof.

[0085] In this embodiment of the invention, the preemulsifier can be added to the above solution by conventional dropwise addition of liquid reagents, and there are no particular limitations on this.

[0086] In some embodiments, the polymerization temperature of the monomer feedstock comprising unsaturated nitrile monomers, unsaturated ether monomers, unsaturated nitrogen heterocyclic monomers, and unsaturated sulfonic acid monomers is 60°C to 80°C, for example, 60°C, 65°C, 70°C, 75°C, or 80°C; the reaction time is 4-8 hours, for example, 4 hours, 5 hours, 6 hours, 7 hours, or 8 hours, or any combination thereof.

[0087] Specifically, the preparation process of the above-mentioned binder may also include: after the polymerization reaction is completed, drying the obtained liquid (generally an emulsion), specifically by drying and powdering to obtain the binder.

[0088] In practice, after the polymerization reaction is complete, the obtained polymerization reaction system can be cooled to room temperature, then filtered to obtain an emulsion; the emulsion is then dried to obtain a binder.

[0089] Specifically, the above emulsion can be dried using conventional drying processes, such as demulsification drying, spray drying, fluidized bed drying, flash drying, and microwave drying.

[0090] In this embodiment of the invention, a positive electrode sheet is provided, including a positive electrode active layer, wherein the positive electrode active layer includes the above-mentioned binder or a binder prepared by the above-mentioned binder preparation method.

[0091] In this embodiment of the invention, the positive electrode sheet mainly includes a positive current collector and a positive active layer. The positive active layer mainly includes a conductive agent, a binder, an electrode active material, etc. The positive active layer can be disposed on one side of the positive current collector, or positive active layers can be disposed on both opposite sides in the thickness direction of the positive current collector.

[0092] In other words, the binder in this embodiment of the invention can be used to prepare the positive electrode sheet. When the binder is applied to the positive electrode sheet, it is more compatible with the positive electrode system and the preparation process of the positive electrode sheet, has good processing performance, and can improve the adhesion between materials such as the positive electrode active material in the positive electrode active layer, as well as the adhesion between the positive electrode active layer and the positive electrode current collector, thereby improving the structural stability of the positive electrode sheet. At the same time, the binder has good anti-swelling ability and can also improve the flexibility of the positive electrode sheet, avoiding problems such as material shedding from the positive electrode sheet.

[0093] This invention also provides a battery including the above-described positive electrode sheet, which has advantages corresponding to the above-described positive electrode sheet, and will not be described in detail here.

[0094] Specifically, the aforementioned battery can be a lithium-ion battery, specifically a nickel-cobalt-manganese ternary system battery.

[0095] Generally, a battery includes a cell and a casing that encapsulates the cell. Electrolyte is injected into the cell within the casing. The cell includes a positive electrode, a negative electrode, and a separator located between the positive and negative electrodes. The cell can be a stacked cell, meaning it is composed of alternating layers of positive electrode, separator, and negative electrode; or it can be a wound cell, meaning it is composed of positive electrode, separator, and negative electrode layers stacked sequentially and then wound together.

[0096] In this embodiment of the invention, the positive electrode sheet may include a positive current collector and a positive active layer located on at least one side surface of the positive current collector. Specifically, the positive active layer may be provided on one side surface of the positive current collector, or the positive active layers may be provided on both sides of the positive current collector in the thickness direction (i.e., the two surfaces of the positive current collector). The positive active material is present in the positive active layer.

[0097] Generally, the positive electrode active layer may include a positive electrode active material (positive electrode active substance), a conductive agent, and the aforementioned binder. The positive electrode active material and the conductive agent can be conventional materials in the art. For example, the positive electrode active material may include one or more of lithium nickel oxide, lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, and positive electrode ternary materials. The positive electrode ternary material may include nickel cobalt manganese ternary material (NCM) and / or nickel cobalt aluminum ternary material (NCA). Specifically, NCM may include NCM811. The conductive agent may include one or more of conductive carbon black, carbon nanotubes (CNT), acetylene black, graphene, Ketjen black, and carbon fiber.

[0098] Generally, in the positive electrode active layer, the mass percentage of the positive electrode active material (i.e., the ratio of the mass of the positive electrode active material to the total mass of the positive electrode active layer) can be 70% to 99%, for example, 70%, 75%, 80%, 85%, 90%, 93%, 95%, 97%, 99%, or any combination thereof; the mass percentage of the conductive agent can be 0.5% to 15%, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 8%, 10%, 13%, 15%, or any combination thereof; and the mass percentage of the binder can be 0.5% to 15%, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 8%, 10%, 13%, 15%, or any combination thereof.

[0099] The embodiments of the present invention may employ conventional positive current collectors in the art, for example, positive current collectors may include aluminum foil.

[0100] In this embodiment of the invention, the positive electrode sheet can be prepared by conventional methods in the art, such as by coating. Specifically, the positive electrode active material, conductive agent, binder, and other components used to form the positive electrode active layer can be dispersed in a first solvent, such as N-methylpyrrolidone (NMP), to prepare a positive electrode slurry. This slurry is then coated onto the surface of the positive electrode current collector, and after drying, rolling, and other processes, the positive electrode sheet is obtained. The coating, drying, and rolling processes involved are conventional operations for preparing positive electrode sheets using the coating method, and are not particularly limited thereto.

[0101] In this embodiment of the invention, conventional negative electrode sheets in the art can be used, and there are no particular limitations. For example, the negative electrode sheet may include a negative current collector and a negative active layer located on at least one side surface of the negative current collector. Specifically, the negative active layer may be provided on one side surface of the negative current collector, or negative active layers may be provided on both opposite sides of the negative current collector in the thickness direction.

[0102] Specifically, the negative electrode active layer may include a negative electrode active material, a conductive agent, and a binder, all of which can be conventional materials in the art. For example, the negative electrode active material may include graphite, which may include artificial graphite and / or natural graphite; the conductive agent may include one or more of conductive carbon black, carbon nanotubes (CNT), acetylene black, graphene, Ketjen black, and carbon fiber; the binder may include one or more of sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyvinyl alcohol, and sodium polyacrylate.

[0103] The embodiments of the present invention may employ conventional negative electrode current collectors in the art, for example, negative electrode current collectors include copper foil.

[0104] In this embodiment of the invention, the negative electrode sheet can be prepared by conventional methods in the art, such as by coating. Specifically, the components used to form the negative electrode active layer, such as the negative electrode active material, conductive agent, and binder, can be dispersed in a solvent, such as water, to prepare a negative electrode slurry. This slurry is then coated onto the surface of the negative electrode current collector, and after drying, rolling, and other processes, the negative electrode sheet is obtained. The coating, drying, and rolling processes involved are conventional operations for preparing negative electrode sheets using the coating method, and are not particularly limited thereto.

[0105] In this embodiment of the invention, the battery cell can be packaged using conventional housing materials in the art, such as flexible packaging materials like aluminum-plastic film, but is not limited thereto.

[0106] The electrolyte in this embodiment of the invention can be a conventional electrolyte in the art. For example, the electrolyte is a non-aqueous electrolyte, which may specifically include organic solvents, additives and electrolyte salts. Organic solvents include one or more of ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and propylene carbonate (PC). Additives include, for example, fluoroethylene carbonate (FEC) and / or vinylene carbonate (VC). Electrolyte salts may include lithium salts, such as lithium hexafluorophosphate (LiPF6), but are not limited thereto.

[0107] In this embodiment of the invention, the separator is used to separate the positive electrode and the negative electrode to prevent the positive electrode and the negative electrode from short-circuiting due to contact. Conventional separators in the art can be used in this embodiment of the invention, and there are no special limitations on this.

[0108] The embodiments of the present invention can assemble components such as positive electrode sheets, separators and negative electrode sheets into batteries using conventional methods in the art. For example, positive electrode sheets, separators and negative electrode sheets can be stacked alternately to obtain a stacked battery cell, or positive electrode sheets, separators and negative electrode sheets can be stacked sequentially and then wound to form a wound battery cell. Then the battery cell is placed in a casing and subjected to conventional processes such as electrolyte injection and formation to obtain a battery.

[0109] The present invention will be further described below through specific embodiments.

[0110] In the following examples and comparative examples, the method for testing the flexibility of the positive electrode sheet is as follows: After winding the positive electrode sheet with a winding needle of different particle size, observe the corresponding winding needle diameter when there are cracks and powder falling off the surface of the positive electrode sheet. The larger the winding needle diameter, the worse the flexibility of the positive electrode sheet. The unit of winding needle diameter is mm. The results are shown in Table 3.

[0111] In the following examples and comparative examples, the swelling degree A of the positive electrode binder was measured through the following process: 20g of binder was dissolved in 380g of N-methylpyrrolidone (NMP) solution to prepare a solution with a solid content of 5%. The prepared solution was poured into a polytetrafluoroethylene petri dish and dried in an 80℃ drying oven for 48h. After drying, the solid binder film (film) was removed and cut into discs with a diameter of 17mm. The mass of the discs was weighed and recorded as W1. The discs were placed in a sealed glass bottle containing electrolyte and kept at a constant temperature of 55℃ for 48h. At this point, the discs were saturated with adsorption (i.e., further soaking would not increase the mass of the discs). The discs were then removed, their surface liquid was wiped dry, and the mass of the discs at this point was recorded as W2. The swelling degree of the positive electrode binder can be calculated using the following formula: A=(W2-W1) / W1×100%, where A is the swelling degree of the binder (%), W1 is the mass of the binder before absorbing the electrolyte (g), and W2 is the mass of the binder after absorbing the electrolyte (g). The results are shown in Table 3.

[0112] In the following examples and comparative examples, the initial efficiency (first-time efficiency) test process of the prepared lithium-ion batteries is as follows: The battery is charged to 4.5V at 0.5C at 25°C, and the voltage is kept constant at 4.5V with a cutoff current of 0.05C. The charging capacity Q1 is recorded. Then the battery is discharged to 3V at a constant current of 0.5C, and the discharge capacity Q2 is recorded. The initial efficiency of the battery is calculated as the initial discharge capacity Q2 / initial charging capacity Q1, that is, the initial efficiency of the battery = Q2 / Q1 × 100%. The results are shown in Table 3.

[0113] In the following examples and comparative examples, the cycle performance (capacity retention rate after 500 cycles at 1C) of the prepared lithium-ion batteries was tested as follows: The battery was charged to 4.5V at 1C constant current at 25°C, then charged at constant voltage with a cutoff current of 0.05mA, and then discharged to 3V at 1C constant current. This constitutes one cycle. After repeating this process 500 times, the capacity retention rate of the battery after 500 cycles was calculated according to the following formula: Capacity retention rate of the battery after 500 cycles at 1C = (Discharge capacity after 500 cycles / Initial discharge capacity) × 100%. The results are shown in Table 3.

[0114] The present invention will be further described below through specific embodiments.

[0115] Example 1

[0116] 1) Preparation of positive electrode binder (quaternary copolymer)

[0117] According to the weight, 30 parts acrylonitrile, 40 parts allyl butyl ether, 25 parts 2-vinylpyridine, 5 parts C2 vinyl sulfonic acid, 1 part sodium dodecyl sulfate, and 150 parts deionized water are mixed and stirred evenly to obtain a pre-emulsion.

[0118] Add 80 parts of deionized water and 1 part of ammonium persulfate to the reaction vessel, control the temperature of the solution in the reactor to 70°C, and then add the above pre-emulsion dropwise to the solution in the reaction vessel at a uniform rate for 3 hours. Then raise the temperature to 75°C and carry out the polymerization reaction for 6 hours. After the reaction is completed, cool to room temperature, filter to obtain the emulsion, and spray dry to obtain the binder.

[0119] 2) Preparation of positive electrode sheet

[0120] By weight, 95 parts of positive electrode active material NCM811, 3 parts of acetylene black, 2 parts of positive electrode binder (quaternary copolymer) and 100 parts of N-methylpyrrolidone were mixed evenly using a planetary mixer to obtain a positive electrode slurry; the above positive electrode slurry was coated on the opposite sides of an aluminum foil using a coating machine, and the positive electrode sheet was obtained after baking and rolling.

[0121] 3) Preparation of negative electrode sheet

[0122] According to the weight parts, 95 parts of graphite, 3 parts of styrene-butadiene rubber binder, 2 parts of conductive carbon black and 100 parts of deionized water are mixed evenly using a planetary mixer to obtain a negative electrode slurry. The slurry is then coated on the opposite sides of the copper foil using a coating machine. After baking and rolling, a negative electrode sheet is obtained.

[0123] 4) Preparation of lithium-ion batteries

[0124] A lithium-ion coin cell is assembled sequentially from a negative electrode, a separator, a positive electrode, and another separator. After processes such as electrolyte injection and formation, a lithium-ion battery is obtained. The electrolyte composition is as follows: the solvents are EC, DEC, and EMC, with a volume ratio of 2:3:1. The lithium salt in the electrolyte is LiPF6, and the concentration of LiPF6 in the electrolyte is 1 mol / L.

[0125] Examples 2 to 11: The difference from Example 1 is that the formulation of the prepared positive electrode binder is different, while the rest of the steps are the same as in Example 1. The specific differences are shown in Table 1 and Table 2.

[0126] Examples 12 to 15 differ from Example 1 in that the amount of initiator added in the reaction is different, while the rest of the steps are the same as in Example 1. The specific differences are shown in Table 1 and Table 2.

[0127] Examples 16-20: The difference from Example 1 is that the formulation of the prepared positive electrode binder is different, while the rest of the steps are the same as in Example 1. The specific differences are shown in Table 1 and Table 2.

[0128] Comparative Example 1: The difference from Example 1 is that PVDF was used instead of the quaternary copolymer in Example 1 as the positive electrode binder, while the remaining steps and conditions were the same as in Example 1.

[0129] Comparative Example 2: The copolymer of acrylonitrile (55 parts) with butyl acrylate (40 parts) and acrylic acid (5 parts) was used instead of the copolymer of Example 1 as the positive electrode binder, and the remaining steps and conditions were the same as those of Example 1.

[0130] Comparative Example 3: The difference from Example 1 is that no ether monomers are added during the preparation of the positive electrode binder, as shown in Tables 1 and 2. Except for the differences shown in Tables 1 and 2, the other conditions and steps are the same as in Example 1.

[0131] Comparative Example 4: The difference from Example 1 is that no nitrogen heterocyclic monomer is added during the preparation of the positive electrode binder, as shown in Tables 1 and 2. Except for the differences shown in Tables 1 and 2, the other conditions and steps are the same as in Example 1.

[0132] Comparative Example 5: The difference from Example 1 is that no sulfonic acid monomers are added during the preparation of the positive electrode binder, as shown in Tables 1 and 2. Except for the differences shown in Tables 1 and 2, the other conditions and steps are the same as in Example 1.

[0133] The weight-average molecular weight, glass transition temperature, swelling degree of the positive electrode binder, flexibility of the positive electrode sheet, initial efficiency of the battery, and capacity retention rate of the battery in each embodiment and comparative example obtained from the test are summarized in Table 3.

[0134] Table 1. Feeding data for the examples and comparative examples.

[0135]

[0136] Table 2. Monomer types in the examples and comparative examples

[0137]

[0138] Table 3 Performance Test Data

[0139]

[0140] As can be seen from Tables 1-3, compared with Comparative Examples 1-5, the binders in Examples 1-20 include nitrile structural units, ether structural units, nitrogen heterocyclic structural units, and sulfonic acid structural units. The swelling degree of the binder is 20%-40%, which can improve the adhesion, flexibility, and swelling resistance of the binder, improve the flexibility of the electrode sheet using the binder, reduce the brittleness of the electrode sheet, and avoid problems such as powder shedding from the electrode sheet, thereby improving the electrochemical performance of the battery, such as initial efficiency and cycle life.

[0141] The electrochemical performance of the positive electrode binder, the flexibility of the positive electrode sheet, the first efficiency of the battery, and the cycle life are basically the same as or even better than those of Comparative Example 1, indicating that the present invention can basically replace PVDF.

[0142] Meanwhile, compared to the PVDF in Comparative Example 1, the binders in Examples 1 to 20 do not contain fluorine, which is beneficial to environmental protection.

[0143] Compared to Examples 12 and 15, the binders of Examples 1 to 11, 13, 14 and 16 to 20 have a weight-average molecular weight of 10 wt% to 40 wt%. While improving the adhesion, flexibility and anti-swelling properties of the binder and the flexibility of the electrode sheet using the binder, they can further improve the electrochemical performance of the battery, such as the first efficiency and cycle life.

[0144] Finally, it should be noted that other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein, and is not limited to what has been described above. Various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. An adhesive, characterized in that, The adhesive comprises nitrile structural units, ether structural units, nitrogen heterocyclic structural units, and sulfonic acid structural units; the nitrile structural units include one or more of acrylonitrile structural units and methacrylonitrile structural units; the ether structural units include allyl ether structural units; the sulfonic acid structural units include alkenyl sulfonic acid structural units; the nitrogen heterocyclic structural units include nitrogen heterocycles containing double bonds, and the nitrogen heterocycles include one or more of pyridyl, pyrazinyl, pyrimidinyl, quinolinyl, isoquinolinyl, and quinoxalinyl; the mass ratio of the nitrile structural units, the ether structural units, the nitrogen heterocyclic structural units, and the sulfonic acid structural units is (30-50):(30-50):(20-30):(5-10); the swelling degree of the adhesive is 20%~40%.

2. The adhesive according to claim 1, characterized in that, The weight-average molecular weight of the binder is 100,000 g / mol to 400,000 g / mol.

3. The adhesive according to claim 1, characterized in that, The glass transition temperature of the adhesive is 0℃~40℃.

4. The adhesive according to claim 1, characterized in that, The particle size of the binder is 40μm~80μm.

5. The adhesive according to claim 1, characterized in that, The allyl ether structural unit includes one or more of the following: allyl ethyl ether structural unit, allyl propyl ether structural unit, allyl butyl ether structural unit, and allyl hydroxyethyl ether structural unit.

6. The adhesive according to claim 1, characterized in that, The alkenyl sulfonic acid structural units include C2-C14 vinyl sulfonic acid structural units.

7. A positive electrode plate, characterized in that, It includes a positive electrode active layer, wherein the positive electrode active layer includes the binder according to any one of claims 1-6.

8. A battery, characterized in that, Includes the positive electrode sheet as described in claim 7.

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