Secondary battery and electric device

Abstract

The present application relates to the technical field of batteries, and provides a secondary battery and an electric device. The secondary battery comprises a positive electrode sheet, a negative electrode sheet, and a separator; the positive electrode sheet and / or the negative electrode sheet comprises a binder; the binder comprises at least cyano, carboxyl, and hydroxyl groups in a molar ratio of 1:(250-5,000):(1-2).

Description

Secondary batteries and electrical equipment

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to the Chinese patent application filed with the China Patent Office on June 25, 2023, with application number 202310758314.7 and invention name “Secondary Batteries and Electrical Equipment”. The entire contents of the Chinese patent application are incorporated herein by reference. Technical Field

[0003] The present application relates to the field of battery technology, and in particular to a secondary battery and an electrical device. Background Art

[0004] Secondary batteries are widely used in various fields because they can be charged and discharged for thousands to tens of thousands of times and have strong practicality and economy.

[0005] Currently, in secondary batteries, the electrode binders commonly used have weak bonding strength with the electrode materials, which makes it impossible to effectively suppress the expansion of the electrodes, resulting in poor battery performance, such as low energy efficiency and poor cycle stability.

[0006] Summary of the Invention

[0007] The present application provides a secondary battery and an electrical device, which can be used to solve the technical problem that the current secondary battery has low energy efficiency and poor cycle stability due to the weak bonding force of the electrode binder, which cannot effectively suppress electrode expansion.

[0008] In a first aspect, an embodiment of the present application provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet and a separator, wherein the positive electrode sheet and / or the negative electrode sheet comprises a binder, wherein the binder comprises at least a cyano group, a carboxyl group and a hydroxyl group, and the molar ratio of the cyano group, the carboxyl group and the hydroxyl group is 1:(250~5000):(1~2).

[0009] Alternatively, in one embodiment, the binder comprises a compound having the structural formula I or II:

[0010] Wherein R1 includes H, Cl, F, Br, CH3 or CH3CH2; R2 includes H, Li, Na or K; R3 includes H, CH3, CH3CH2, Br or Cl; R4 includes H, CH3 or CH3CH2, m is 250 to 5000, and n is 250 to 5000.

[0011] Alternatively, in one embodiment, the structural formula of Formula I includes any one or more of the following:

[0012] The structural formula of Formula II includes any one or more of the following:

[0013] Alternatively, in one embodiment, the binder comprises a compound polymerized from a cyano group-containing monomer, a carboxylic acid-containing monomer, and a hydroxyl group-containing monomer;

[0014] The cyano group-containing monomer includes any one or more of acrylonitrile, methacrylonitrile, ethacrylonitrile, 2-chloroacrylonitrile and 3-bromo-2-methylacrylonitrile;

[0015] The carboxylic acid-containing monomer includes any one or more of lithium acrylate, lithium methacrylate, lithium ethacrylate, lithium 2-chloroacrylate and lithium 2-bromoacrylate;

[0016] The hydroxyl-containing monomer includes any one or more of 3-methacrylamidodopamine, 4-hydroxystyrene, 4-hydroxyphenylpropene and 4-hydroxyphenylbutene.

[0017] Optionally, in one embodiment, the number average molecular weight of the binder of formula I is 30,000 to 1,000,000.

[0018] Optionally, in one embodiment, the number average molecular weight of the binder of formula II is 30,000 to 1,000,000.

[0019] Optionally, in one embodiment, the positive electrode plate includes a positive electrode active material, and the positive electrode active material includes LiFePO4 or LiM x Fe 1-x PO4, wherein M includes one or more of Mn, Zr, Ti, Al, W, Nb, and Mg, and 0<x≤0.6.

[0020] Optionally, in one embodiment, the step of preparing the binder includes:

[0021] Mixing a cyano group-containing monomer, a carboxylic acid-containing monomer, a hydroxyl group-containing monomer and an initiator in a solvent to obtain a mixed solution;

[0022] The mixed solution is heat-treated in an inert environment to obtain the binder.

[0023] Alternatively, in one embodiment, the initiator includes any one or more of potassium persulfate, ammonium persulfate, sodium sulfite and sodium bisulfite.

[0024] In a second aspect, an embodiment of the present application provides an electric device, comprising the secondary battery provided in the first aspect of the present application, wherein the secondary battery serves as a power supply for the electric device.

[0025] The beneficial effects brought about by the embodiments of the present application are as follows:

[0026] According to the solution provided in the embodiment of the present application, the secondary battery includes a positive electrode sheet, a negative electrode sheet and a separator, and the positive electrode sheet and / or the negative electrode sheet include a binder, and the binder includes at least a cyano group, a carboxyl group and a hydroxyl group, and the molar ratio of the cyano group, the carboxyl group and the hydroxyl group is 1: (250-5000): (1-2). By giving the binder cyano groups, carboxyl groups and hydroxyl groups, wherein the cyano groups and the hydroxyl groups can form hydrogen bonds between the binder and the electrode material, and the carboxyl groups can generate electrostatic effects and complexation between the binder and the electrode material, the bonding force between the binder and the electrode material is improved. Moreover, by further setting the molar ratio of the cyano group, the carboxyl group and the hydroxyl group to 1: (250-5000): (1-2), the formed binder structure is more stable, thereby further improving the bonding force between the binder and the electrode material. As a result, the inhibitory effect on electrode expansion can be enhanced, thereby improving the energy efficiency, cycle stability and other performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for describing the embodiments. Obviously, the drawings described below are some embodiments of the present application. For those skilled in the art, other drawings can be obtained based on these drawings without inventive work. In the drawings:

[0028] FIG1 is a schematic flow chart of a method for preparing an adhesive provided in an embodiment of the present application. DETAILED DESCRIPTION

[0029] To make the purpose, technical solutions, and advantages of this application more clear, the technical solutions of this application will be clearly and completely described below in conjunction with the specific embodiments of this application and the corresponding drawings. Obviously, the embodiments described are only part of the embodiments of this application, not all of them. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0030] As described in the background of this application, commonly used electrode binders have weak adhesion to electrode materials. For example, polyvinylidene fluoride (PVDF), a widely used binder, lacks reactive groups on its molecular chain, resulting in weak binding and adhesion to electrode materials and thus inability to effectively suppress electrode expansion. This results in poor battery performance, such as low energy efficiency and poor cycle stability.

[0031] To this end, an embodiment of the present application provides a secondary battery. Specifically, the secondary battery may include a positive electrode sheet, a negative electrode sheet and a separator. The positive electrode sheet and / or the negative electrode sheet include a binder, and the binder includes at least a cyano group, a carboxyl group and a hydroxyl group. The molar ratio of the cyano group, the carboxyl group and the hydroxyl group is 1: (250~5000): (1~2).

[0032] The secondary battery may be a lithium-ion battery, a sodium-ion battery, or the like.

[0033] The positive electrode plate may include a current collector and a positive electrode material. The positive electrode material may further include a positive electrode active material, a conductive agent, and a binder. The positive electrode plate may be obtained by coating a mixture of a positive electrode active material, a conductive agent, and a binder on a current collector. In practical applications, the current collector for the positive electrode plate may be aluminum foil, and the positive electrode active material for the positive electrode plate may be selected according to the actual type of the secondary battery. For example, when the secondary battery is a lithium-ion battery, the positive electrode active material may include LiFePO4 or LiM x Fe 1-x PO4; wherein M may include one or more of Mn, Zr, Ti, Al, W, Nb, and Mg, and 0<x≤0.6.

[0034] Accordingly, the negative electrode plate may include a current collector and a negative electrode material. The negative electrode material may further include a negative electrode active material, a conductive agent, and a binder. The negative electrode plate may be obtained by coating a mixture of the negative electrode active material, the conductive agent, and the binder on a current collector. In practical applications, the current collector for the negative electrode plate may be copper foil, and the negative electrode active material for the negative electrode plate may be selected according to the actual type of secondary battery. For example, when the secondary battery is a lithium-ion battery, the negative electrode active material may include graphite.

[0035] In the above-mentioned positive electrode sheet and negative electrode sheet, the conductive agent may include any one or more of conductive carbon black, acetylene black, Ketjen black, carbon nanotubes, graphene, hard carbon, carbon fibers and carbon microspheres.

[0036] When the positive electrode sheet includes a binder, the binder can be used to bond the positive electrode active material, conductive agent, etc. to the current collector. Furthermore, the binder can act as a bond between electrode materials, which may include the positive electrode active material and / or conductive agent.

[0037] Accordingly, when the negative electrode sheet includes a binder, on the one hand, the binder can be used to bond the negative electrode active material, conductive agent, etc. to the current collector. On the other hand, the binder can also serve as a bond between the electrode materials; wherein the electrode materials may include the negative electrode active material and / or the conductive agent.

[0038] In the embodiments of the present application, the binder includes at least cyano, carboxyl, and hydroxyl groups, which can be understood as: the polymer corresponding to the binder contains cyano, carboxyl, and hydroxyl groups. The polymer corresponding to the binder can include one or more types of polymers. That is, when the binder includes multiple types of polymers, the binder can include a mixture of at least two polymers having adhesive properties.

[0039] In a specific implementation, the binder may comprise a compound formed by polymerizing a cyano-containing monomer, a carboxylic acid-containing monomer, and a hydroxyl-containing monomer. The cyano-containing monomer can impart a cyano group to the binder polymer, the hydroxyl-containing monomer can impart a hydroxyl group to the binder polymer, and the carboxylic acid-containing monomer can be used to connect the cyano-containing monomer and the hydroxyl-containing monomer, thereby polymerizing under certain conditions to obtain the corresponding polymer of the binder.

[0040] By giving the binder cyano and hydroxyl groups, the binder can form hydrogen bonds with the electrode material. By giving the binder carboxyl groups, the binder and the electrode material can generate electrostatic interactions and complexation, thereby improving the adhesion to the electrode material. In addition, by controlling the molar ratio of cyano, carboxyl, and hydroxyl groups in the binder to 1: (250-5000): (1-2), the structure of the binder polymer can be made more stable, further improving the adhesion between the binder and the electrode material.

[0041] In some embodiments, the molar ratio of cyano group, carboxyl group and hydroxyl group can be 1:250:1, 1:800:1, 1:1500:1.5, 1:3000:1, 1:4500:2.0, or 1:5000:1.

[0042] In the embodiments of the present application, the molar ratio of cyano, carboxyl and hydroxyl groups in the binder can be obtained based on nuclear magnetic resonance measurement, which can be specifically as follows:

[0043] An internal standard substance is added to the binder, and a nuclear magnetic resonance (NMR) measurement is performed to obtain an NMR spectrum. The resonance peak areas attributable to protons on the cyano group, the carboxyl group, and the hydroxyl group are compared with the resonance peak areas attributable to protons on the designated groups of the internal standard substance. Furthermore, based on the comparison of the resonance peak areas, the molar ratio of cyano groups, carboxyl groups, and hydroxyl groups in the binder can be determined.

[0044] The selection criteria for the internal standard substance can include: (1) producing a single resonance peak that is at least 30 Hz apart from the sample peak; and (2) not interacting with any component in the sample. Based on this, the internal standard substance can be benzene, benzyl benzoate, or maleic acid.

[0045] It can be understood that the secondary battery provided by the embodiment of the present application includes a positive electrode sheet, a negative electrode sheet and a separator, and the positive electrode sheet and / or the negative electrode sheet include a binder, and the binder includes at least cyano, carboxyl and hydroxyl groups, and the molar ratio of the cyano, carboxyl and hydroxyl groups is 1: (250-5000): (1-2). By giving the binder cyano, carboxyl and hydroxyl groups, wherein the cyano and hydroxyl groups can form hydrogen bonds between the binder and the electrode material, and the carboxyl group can generate electrostatic effects and complexation between the binder and the electrode material, thereby improving the bonding force between the binder and the electrode material. Moreover, by further setting the molar ratio of cyano, carboxyl and hydroxyl groups to 1: (250-5000): (1-2), the formed binder structure is more stable, thereby further improving the bonding force between the binder and the electrode material. As a result, the inhibitory effect on electrode expansion can be improved, thereby improving the energy efficiency, cycle stability and other performance of the battery.

[0046] In one embodiment, the binder comprises a compound having the structural formula I:

[0047] Among them, R1 may include H, Cl, F, Br, CH3 or CH3CH2; R2 may include H, Li, Na or K; R3 may include H, CH3, CH3CH2, Br or Cl; R4 may include H, CH3 or CH3CH2; and n may be 250 to 5000.

[0048] In the embodiment of the present application, since the binder has the group:

[0049] The aforementioned groups also have antioxidant properties, which can inhibit interfacial side reactions and improve interfacial stability, thereby further suppressing electrode expansion and improving battery performance.

[0050] Specifically, the structural formula of Formula I may include any one or more of the following:

[0051] When the general structural formula of the adhesive is Formula I, an adhesive further having the structural formula of Formula I-a, Formula I-b, Formula I-c, Formula I-d or Formula I-e can achieve a better bonding effect.

[0052] When preparing the binder, the binder corresponding to Formula I can be obtained by polymerizing a cyano group-containing monomer corresponding to Formula I-1, a carboxylic acid-containing monomer corresponding to Formula I-2, and a hydroxyl group-containing monomer corresponding to Formula I-3:

[0053] The reaction equation for polymerizing the cyano group-containing monomer corresponding to Formula I-1, the carboxylic acid-containing monomer corresponding to Formula I-2, and the hydroxyl group-containing monomer corresponding to Formula I-3 to obtain the binder can be:

[0054] In a specific implementation, the cyano group-containing monomer may include any one or more of acrylonitrile, methacrylonitrile, ethacrylonitrile, 2-chloroacrylonitrile and 3-bromo-2-methylacrylonitrile; the carboxylic acid-containing monomer may include any one or more of lithium acrylate, lithium methacrylate, lithium ethacrylate, 2-chlorolithium acrylate and 2-bromolithium acrylate; the hydroxyl group-containing monomer may include any one or more of 4-hydroxystyrene, 4-hydroxyphenylpropene and 4-hydroxyphenylbutene.

[0055] In order to further improve the bonding effect of the binder of formula I on the electrode material, the number average molecular weight of the binder of formula I can be controlled to be 30,000 to 1,000,000.

[0056] In practice, gel permeation chromatography (GPC) can be used to determine the number average molecular weight of the binder polymer. Furthermore, based on the number average molecular weight and the molecular weights of the monomers in the binder polymer, the value of n in Formula I can be determined, thereby obtaining the molar ratio of cyano groups, carboxyl groups, and hydroxyl groups.

[0057] In addition to cyano, carboxyl and hydroxyl groups, the binder may further include amide groups, and the molar ratio of the amide groups to the cyano groups may be 1:1. The amide groups may be used to connect carboxylic acid-containing monomers.

[0058] In the case where the binder includes cyano, carboxyl, hydroxyl and amide groups, in one embodiment, the binder comprises a compound having the structural formula II:

[0059] Wherein, R1, R2 and R3 in Formula II can refer to Formula I and are not described in detail here. m can be 250 to 5000.

[0060] In the embodiment of the present application, since the binder has the group:

[0061] The aforementioned groups also have antioxidant properties, which can inhibit interfacial side reactions and improve interfacial stability, thereby further suppressing electrode expansion and improving battery performance.

[0062] Specifically, the structural formula of Formula II may include any one or more of the following:

[0063] When the general structural formula of the adhesive is Formula II, an adhesive further having the structural formula of Formula II-a, Formula II-b, Formula II-c, Formula II-d or Formula II-e can play a better bonding role.

[0064] The binder corresponding to Formula I can be obtained by polymerizing a cyano group-containing monomer corresponding to Formula I-1, a carboxylic acid-containing monomer corresponding to Formula I-2, and a hydroxyl group-containing monomer corresponding to the following Formula II-1:

[0065] The reaction equation for polymerizing the cyano group-containing monomer corresponding to Formula I-1, the carboxylic acid-containing monomer corresponding to Formula I-2, and the hydroxyl group-containing monomer corresponding to Formula II-1 to obtain the binder can be:

[0066] In a specific implementation, the cyanide-containing monomer may include any one or more of acrylonitrile, methacrylonitrile, ethacrylonitrile, 2-chloroacrylonitrile and 3-bromo-2-methylacrylonitrile; the carboxylic acid-containing monomer may include any one or more of lithium acrylate, lithium methacrylate, lithium ethacrylate, 2-chlorolithium acrylate and 2-bromolithium acrylate; the hydroxyl-containing monomer may include 3-methacrylamidodopamine.

[0067] In order to further improve the bonding effect of the binder of formula II on the electrode material, the number average molecular weight of the binder of formula II can be controlled to be 30,000 to 1,000,000.

[0068] In practice, gel permeation chromatography (GPC) can be used to determine the number average molecular weight of the binder polymer. Furthermore, based on the number average molecular weight and the molecular weights of the monomers in the binder polymer, the value of m in Formula II can be determined, thereby obtaining the molar ratio of cyano groups, carboxyl groups, and hydroxyl groups.

[0069] The binder with the structure of Formula I can be considered a first-class binder, and the binder with the structure of Formula II can be considered a second-class binder. Therefore, in practical applications, the binder in the positive electrode sheet and / or the negative electrode sheet can include the first-class binder and / or the second-class binder. The first-class binder can include multiple sub-binders based on the selection of R1, R2, R3, R4, and n. The second-class binder can also include multiple sub-binders based on the selection of R1, R2, R3, and m.

[0070] In one embodiment, as shown in FIG1 , the steps for preparing the binder provided in any of the above embodiments of the present application include:

[0071] Step 101 : uniformly mix a cyano group-containing monomer, a carboxylic acid-containing monomer, a hydroxyl group-containing monomer, and an initiator in a solvent to obtain a mixed solution.

[0072] The solvent may be deionized water. The initiator may include any one or more of potassium persulfate, ammonium persulfate, sodium sulfite and sodium bisulfite.

[0073] In order to obtain an adhesive with a stable structure and good bonding performance, in a specific implementation, the added amounts of the cyano group-containing monomer, the carboxylic acid-containing monomer, the hydroxyl group-containing monomer, the initiator and the solvent can be: 1-50 parts by weight of the cyano group-containing monomer, 50 parts by weight of the carboxylic acid-containing monomer, 1-5 parts by weight of the hydroxyl group-containing monomer, 0.1-0.5 parts by weight of the initiator and 100-400 parts by weight of the solvent.

[0074] Step 102: heat-treating the mixed solution in an inert environment to obtain a binder.

[0075] The inert environment can be formed by using inert gases such as nitrogen and argon.

[0076] In order to allow the initiator to successfully initiate the polymerization reaction, in a specific implementation, the mixed solution is subjected to heat treatment, which can be: initiating the polymerization at a temperature of 20° C.-80° C.

[0077] It can be understood that the above scheme can produce a binder containing cyano, carboxyl, and hydroxyl groups. This binder can form hydrogen bonds with electrode materials and generate electrostatic interactions and complexation with the electrode materials, thereby improving the bonding strength between the electrode materials. Therefore, when this binder is used as an electrode binder in secondary batteries, it can enhance the effect of suppressing electrode expansion, thereby improving battery performance such as energy efficiency and cycle stability.

[0078] When the hydroxyl-containing monomer in step 101 specifically includes any one or more of 3-methacrylamidodopamine, 4-hydroxystyrene, 4-hydroxyphenylpropene, and 4-hydroxyphenylbutene, the resulting binder also contains antioxidant groups, which can further inhibit interfacial side reactions and improve interfacial stability. Therefore, when this binder is used as an electrode binder in a secondary battery, it can further inhibit electrode expansion, thereby further improving battery performance.

[0079] It will be appreciated that when preparing a binder having Formula I-a, the cyano-containing monomer in step 101 may be acrylonitrile, the carboxylic acid-containing monomer may be lithium acrylate, and the hydroxyl-containing monomer may be 4-hydroxystyrene. The remaining steps for preparing the binder having Formula I-a can be referred to above and are not further described here.

[0080] When preparing a binder having Formula I-b, the cyano group-containing monomer in step 101 can be 2-chloroacrylonitrile, the carboxylic acid-containing monomer can be sodium methacrylate, and the hydroxyl group-containing monomer can be 4-hydroxystyrene. The remaining steps for preparing the binder having Formula I-b can be referred to above and are not repeated here.

[0081] When preparing a binder having Formula I-c, the cyano group-containing monomer in step 101 can be methacrylonitrile, the carboxylic acid-containing monomer can be potassium 2-bromoacrylate, and the hydroxyl group-containing monomer can be 4-hydroxyphenylpropene. The remaining steps for preparing the binder having Formula I-c can be referred to above and are not repeated here.

[0082] When preparing a binder having Formula I-d, the cyano group-containing monomer in step 101 can be 2-bromoacrylonitrile, the carboxylic acid-containing monomer can be lithium acrylate, and the hydroxyl group-containing monomer can be 4-hydroxystyrene. The remaining steps for preparing the binder having Formula I-d can be referred to above and are not repeated here.

[0083] When preparing a binder having Formula I-e, the cyano-containing monomer in step 101 can be methacrylonitrile, the carboxylic acid-containing monomer can be lithium acrylate, and the hydroxyl-containing monomer can be 4-hydroxyphenylpropene. The remaining steps for preparing the binder having Formula I-e can be referred to above and are not repeated here.

[0084] When preparing a binder having Formula II-a, the cyano-containing monomer in step 101 can be acrylonitrile, the carboxylic acid-containing monomer can be lithium acrylate, and the hydroxyl-containing monomer can be 3-methacrylamido-dopamine. The remaining steps for preparing the binder having Formula II-a can be referred to above and are not repeated here.

[0085] When preparing a binder having Formula II-b, the cyano-containing monomer in step 101 can be 2-chloroacrylonitrile, the carboxylic acid-containing monomer can be sodium methacrylate, and the hydroxyl-containing monomer can be 3-methacrylamido-dopamine. The remaining steps for preparing the binder having Formula II-b can be referred to above and are not repeated here.

[0086] When preparing a binder having Formula II-c, the cyano-containing monomer in step 101 can be methacrylonitrile, the carboxylic acid-containing monomer can be potassium 2-bromoacrylate, and the hydroxyl-containing monomer can be 3-methacrylamido-dopamine. The remaining steps for preparing the binder having Formula II-c can be referred to above and are not repeated here.

[0087] When preparing a binder having Formula II-d, the cyano-containing monomer in step 101 can be 2-bromoacrylonitrile, the carboxylic acid-containing monomer can be lithium acrylate, and the hydroxyl-containing monomer can be 3-methacrylamido-dopamine. The remaining steps for preparing the binder having Formula II-d can be referred to above and are not repeated here.

[0088] When preparing a binder having Formula II-e, the cyano-containing monomer in step 101 can be methacrylonitrile, the carboxylic acid-containing monomer can be lithium acrylate, and the hydroxyl-containing monomer can be 3-methacrylamido-dopamine. The remaining steps for preparing the binder having Formula II-e can be referred to above and are not repeated here.

[0089] Based on the secondary battery provided in the above-mentioned embodiment of the present application, the embodiment of the present application further provides an electric device, in which the secondary battery serves as a power supply for the electric device.

[0090] Among them, electrical equipment may include vehicles such as cars and ships, as well as terminal devices such as computers, mobile phones, and cameras.

[0091] It can be understood that when the secondary battery provided in the embodiments of the present application is used as a power source for electrical devices, the binder in the secondary battery includes cyano, carboxyl, and hydroxyl groups, which enable the binder to form hydrogen bonds with the electrode material and generate electrostatic interactions and complexation with the electrode material, thereby improving the bonding strength between the binder and the electrode material. This can enhance the effect of suppressing electrode expansion and improve battery performance, such as energy efficiency and cycle stability, thereby increasing the endurance and service life of the electrical device.

[0092] To facilitate the description of the technical effects of the secondary battery provided by the embodiments of the present application, the following description is made in conjunction with specific embodiments, comparative examples, and corresponding test data. In the following embodiments and comparative examples, the secondary battery is a lithium-ion battery.

[0093] Example 1

[0094] A secondary battery is provided, and the preparation steps are as follows:

[0095] 1) Preparation of binder:

[0096] In 400 parts by weight of deionized water, 50 parts by weight of lithium acrylate, 50 parts by weight of acrylonitrile and 5 parts by weight of 3-methacrylamide dopamine were added, and then 0.2 parts by weight of ammonium persulfate was added.

[0097] Under nitrogen protection, polymerization was initiated at 70° C. to obtain a binder with a solid content of 20.1%.

[0098] The structural formula of the binder is:

[0099] Testing (using gel permeation chromatography) shows that the number average molecular weight of the binder is approximately 218,500, and m in the above structural formula is approximately 2,800. The molar ratio of cyano, carboxyl and hydroxyl in the binder is approximately 1:2,800:2.

[0100] 2) Preparation of positive electrode sheet:

[0101] Lithium iron phosphate (LiFePO4) with a mass ratio of 96:2.5:1.5, the binder prepared in step 1) and conductive carbon black are mixed evenly, dissolved in NMP, and stirred evenly to obtain a positive electrode slurry. The positive electrode slurry is evenly coated on both sides of the positive electrode current collector aluminum foil, and after drying, cold pressing, slitting and cutting, a positive electrode sheet is obtained.

[0102] 3) Preparation of negative electrode sheet:

[0103] Graphite, conductive carbon black, thickener CMC, and binder SBR in a mass ratio of 96.2:1.2:1.2:1.4 were mixed evenly, deionized water was added, and the mixture was stirred evenly to obtain a negative electrode slurry. The negative electrode slurry was evenly coated on both sides of the negative electrode current collector copper foil. After drying, cold pressing, slitting, and cutting, the negative electrode sheet was obtained.

[0104] 4) Assemble into battery:

[0105] The positive electrode sheet, the negative electrode sheet and the separator (PP separator) are wound to obtain a core, which is placed in a square aluminum shell, and an electrolyte is injected (ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are mixed in a mass ratio of 30:40:30 to obtain a non-aqueous organic solvent, 1 mol / L LiPF6 is dissolved in the above non-aqueous organic solvent, and mixed evenly to obtain an electrolyte) to obtain a square lithium-ion battery.

[0106] Example 2

[0107] The difference from Example 1 is that the binder is prepared according to the formula in Table 1, and the structural formula of the obtained binder is:

[0108] Wherein, m is approximately 4900, and the number average molecular weight of the binder is approximately 382500.

[0109] Example 3

[0110] The difference from Example 1 is that the adhesive is prepared according to the formula in Table 1, and the structural formula of the obtained adhesive is:

[0111] Wherein, m is approximately 3600, and the number average molecular weight of the binder is approximately 280,000.

[0112] Example 4

[0113] The difference from Example 1 is that the binder is prepared according to the formula in Table 1, and the structural formula of the obtained binder is:

[0114] Wherein, m is approximately 300, and the number average molecular weight of the binder is approximately 32,000.

[0115] Example 5

[0116] The difference from Example 1 is that the binder is prepared according to the formula in Table 1, and the structural formula of the obtained binder is:

[0117] Wherein, m is approximately 2300, and the number average molecular weight of the binder is approximately 430,000.

[0118] Example 6

[0119] The difference from Example 1 is that the binder is prepared according to the formula in Table 1, and the structural formula of the obtained binder is:

[0120] Here, m is approximately 1500, and the number average molecular weight of the binder is approximately 117,000.

[0121] Example 7

[0122] The difference from Example 1 is that the binder is prepared according to the formula in Table 1, and the structural formula of the obtained binder is:

[0123] Here, m is approximately 900, and the number average molecular weight of the binder is approximately 70,000.

[0124] Example 8

[0125] The difference from Example 1 is that the binder is prepared according to the formula in Table 1, and the structural formula of the obtained binder is:

[0126] Wherein, n is about 3500, and the number average molecular weight of the binder is about 273000.

[0127] Example 9

[0128] The difference from Example 1 is that the binder is prepared according to the formula in Table 1, and the structural formula of the obtained binder is:

[0129] Wherein, n is approximately 4700, and the number average molecular weight of the binder is approximately 366000.

[0130] Example 10

[0131] The difference from Example 1 is that the binder is prepared according to the formula in Table 1, and the structural formula of the obtained binder is:

[0132] Wherein, n is approximately 4100, and the number average molecular weight of the binder is approximately 320,000.

[0133] Example 11

[0134] The difference from Example 1 is that the binder is prepared according to the formula in Table 1, and the structural formula of the obtained binder is:

[0135] Wherein, n is about 280, and the number average molecular weight of the binder is about 30,000.

[0136] Example 12

[0137] The difference from Example 1 is that the binder is prepared according to the formula in Table 1, and the structural formula of the obtained binder is:

[0138] Wherein, n is approximately 1900, and the number average molecular weight of the binder is approximately 360,000.

[0139] Example 13

[0140] The difference from Example 1 is that the binder is prepared according to the formula in Table 1, and the structural formula of the obtained binder is:

[0141] Wherein, n is about 1600, and the number average molecular weight of the binder is about 125,000.

[0142] Example 14

[0143] The difference from Example 1 is that the binder is prepared according to the formula in Table 1, and the structural formula of the obtained binder is:

[0144] Wherein, n is approximately 1000, and the number average molecular weight of the binder is approximately 78000.

[0145] Example 15

[0146] The difference from Example 1 is that the binder is prepared according to the formula in Table 1, and the structural formula of the obtained binder is:

[0147] Wherein, n is approximately 2200, and the number average molecular weight of the binder is approximately 172000.

[0148] Example 16

[0149] The difference from Example 1 is that the positive electrode material is LiMn 0.1 Fe 0.9 PO4.

[0150] Example 17

[0151] The difference from Example 1 is that the positive electrode material is LiTi 0.3 Fe 0.7 PO4.

[0152] Example 18

[0153] The difference from Example 1 is that the positive electrode material is LiAl 0.5 Fe 0.5 PO4

[0154] Comparative Example 1

[0155] The difference from Example 1 is that 3-methacrylamide dopamine is not added. The number average molecular weight of the binder is about 195,000, and the molar ratio of cyano group, carboxyl group and hydroxyl group in the binder is about 1:1:0.

[0156] Comparative Example 2

[0157] The difference from Example 1 is that lithium acrylate is not added, the number average molecular weight of the binder is about 43,000, and the molar ratio of cyano group, carboxyl group and hydroxyl group in the binder is about 5:0:1.

[0158] Comparative Example 3

[0159] The difference from Example 1 is that no acrylonitrile is added, the number average molecular weight of the binder is about 85,000, and the molar ratio of cyano group, carboxyl group and hydroxyl group in the binder is about 0:4:1.

[0160] Comparative Example 4

[0161] The difference from Example 8 is that lithium acrylate is not added, the number average molecular weight of the binder is about 21,000, and the molar ratio of cyano group, carboxyl group and hydroxyl group in the binder is about 10:0:1.

[0162] Comparative Example 5

[0163] The difference from Example 8 is that acrylonitrile is not added. The number average molecular weight of the binder is about 50,000, and the molar ratio of cyano group, carboxyl group and hydroxyl group in the binder is about 0:10:1.

[0164] Comparative Example 6

[0165] The difference from Example 1 is that the binder is prepared according to the formula in Table 1, and the structural formula of the obtained binder is:

[0166] Wherein, m is approximately 6300, and the number average molecular weight of the binder is approximately 490000.

[0167] Comparative Example 7

[0168] The difference from Example 8 is that the binder is prepared according to the formula in Table 1, and the structural formula of the obtained binder is:

[0169] Wherein, n is about 130, and the number average molecular weight of the binder is about 10,000.

[0170] Table 1: The formula composition of lithium ion batteries in each embodiment and each comparative example

[0171] The batteries obtained in each embodiment and each comparative example were subjected to relevant tests, and the test process is as follows:

[0172] Energy density test: At 25°C, the battery was charged at a constant current and constant voltage (CCCV) of 1C to a cut-off voltage of 3.65V and a cut-off current of 0.05C. It was then discharged at a current of 1C to a cut-off voltage of 2.5V. The discharge energy (E0) was recorded and the corresponding battery weight (W0) was measured. Battery gravimetric energy density = battery discharge energy (E0) / battery weight (W0).

[0173] Cycle performance test: At 25°C, place the battery aside for 10 minutes, charge it to 3.65V at a constant current rate of 1C, then charge it at a constant voltage until the current is less than or equal to 0.05C, then place it aside for 10 minutes, and then discharge it to 2.5V at a constant current rate of 1C. This is one charge and discharge cycle, and the discharge capacity this time is recorded as the discharge capacity of the battery in the first cycle. Continue to perform 500 cycle charge and discharge tests on the battery according to the above method, and record the discharge capacity of each cycle. The capacity retention rate (%) of the battery after 500 cycles at 25°C and 1C / 1C = (discharge capacity of the 500th cycle / discharge capacity of the first cycle) × 100%.

[0174] Storage test: At 25°C, charge and discharge the battery at a charge / discharge rate of 1C / 1C over a voltage range of 2.5-3.65V, and record the initial discharge specific capacity. The battery is then stored at 25°C for 120 days, and the charge and discharge test is repeated, with the discharge specific capacity recorded. Storage capacity retention = (discharge specific capacity after 120 days / initial discharge specific capacity) × 100%.

[0175] Energy efficiency test: At 25°C, the battery was charged at a constant current and constant voltage (CCCV) current of 0.5C to a termination voltage of 3.65V. The battery was then discharged at a current of 0.5C to a termination voltage of 2.5V. At 25°C, the battery was fully charged again to a maximum voltage of 3.65V, and the charge energy (E0) was recorded. The battery was then discharged at a current of 0.5C to a termination voltage of 2.5V, and the discharge energy (E1) was recorded. Energy efficiency = (E1 / E0) × 100%.

[0176] 500-cycle positive electrode sheet expansion rate test: Measure the thickness of the positive electrode sheet before winding, recorded as T0. After 500 cycles at a 1C rate, fully discharge the battery and disassemble it to obtain the cycled positive electrode sheet. Measure the thickness of the positive electrode sheet after the cycle, recorded as T1. Positive electrode sheet expansion rate = 100% * (T1 - T0) / T0.

[0177] The energy density, cycle performance, storage performance and energy efficiency test results of the batteries of each embodiment and each comparative example are shown in Table 2:

[0178] Table 2 Performance test results of batteries in various embodiments and comparative examples

[0179] Comparing the test results of Example 1 with those of Comparative Examples 1, 2, and 3, and comparing the test results of Example 8 with those of Comparative Examples 4 and 5, it can be seen that since the binder of the batteries in Examples 1 and 8 is obtained by polymerizing cyano-containing monomers, carboxylic acid-containing monomers, and hydroxyl-containing monomers, the batteries prepared thereby have better cycle performance and higher capacity retention. Comparing the test results of Example 1 with those of Comparative Example 6, and comparing the test results of Example 8 with those of Comparative Example 7, it can be seen that by controlling the molar ratio of cyano, carboxyl, and hydroxyl in the binder to 1:(250-5000):(1-2), the batteries prepared thereby have better cycle performance and higher capacity retention. In addition, the batteries obtained in Examples 1 to 18 all showed excellent results in terms of cycle capacity retention, capacity retention, and energy efficiency. Thus, it is explained that the secondary battery provided in the embodiment of the present application, by giving the binder of the electrode cyano group, carboxyl group and hydroxyl group, enables the binder to form hydrogen bonds with the electrode material, and can generate electrostatic effects and complexation with the electrode material, thereby improving the bonding force between the binder and the electrode material, thereby enhancing the effect of inhibiting electrode expansion, and thus improving the energy efficiency, cycle stability and other performance of the battery.

[0180] It should also be noted that the terms "comprises," "includes," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, commodity, or apparatus that includes a series of elements includes not only those elements but also other elements not explicitly listed, or includes elements inherent to such process, method, commodity, or apparatus. In the absence of further limitations, an element defined by the phrase "comprises a ..." does not exclude the presence of other identical elements in the process, method, commodity, or apparatus that includes the element.

[0181] The foregoing is merely an embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various changes and variations. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application should all be included within the scope of the claims of the present application.

Claims

1. A secondary battery, comprising a positive electrode sheet, a negative electrode sheet and a separator, wherein the positive electrode sheet and / or the negative electrode sheet comprises a binder, wherein the binder comprises at least a cyano group, a carboxyl group and a hydroxyl group, and the molar ratio of the cyano group, the carboxyl group and the hydroxyl group is 1:(250-5000):(1-2).

2. The secondary battery according to claim 1, wherein The binder comprises a compound having a structural formula of Formula I or Formula II: Wherein R1 includes H, Cl, F, Br, CH3 or CH3CH2; R2 includes H, Li, Na or K; R3 includes H, CH3, CH3CH2, Br or Cl; R4 includes H, CH3 or CH3CH2, m is 250 to 5000, and n is 250 to 5000.

3. The secondary battery according to claim 2, wherein: The structural formula of Formula I includes any one or more of the following: The structural formula of Formula II includes any one or more of the following:

4. The secondary battery according to claim 1, wherein The binder comprises a compound polymerized from a cyano-containing monomer, a carboxylic acid-containing monomer, and a hydroxyl-containing monomer; The cyano group-containing monomer includes any one or more of acrylonitrile, methacrylonitrile, ethacrylonitrile, 2-chloroacrylonitrile and 3-bromo-2-methacrylonitrile; The carboxylic acid-containing monomer includes any one or more of lithium acrylate, lithium methacrylate, lithium ethacrylate, lithium 2-chloroacrylate and lithium 2-bromoacrylate; The hydroxyl-containing monomer includes any one or more of 3-methacrylamide dopamine, 4-hydroxystyrene, 4-hydroxyphenylpropylene and 4-hydroxyphenylbutylene.

5. The secondary battery according to claim 2, wherein The number average molecular weight of the binder of formula I is 30,000 to 1,000,000.

6. The secondary battery according to claim 2, wherein The number average molecular weight of the binder of formula II is 30,000 to 1,000,000.

7. The secondary battery according to claim 1, wherein The positive electrode sheet includes a positive electrode active material, and the positive electrode active material includes LiFePO4 or LiM x Fe 1-x PO4, wherein M includes one or more of Mn, Zr, Ti, Al, W, Nb, Mg, and 0<x≤0.

6.

8. The secondary battery according to any one of claims 1 to 7, wherein The preparation steps of the binder include: Mixing cyano-containing monomers, carboxylic acid-containing monomers, hydroxyl-containing monomers and an initiator uniformly in a solvent to obtain a mixed solution; The mixed solution is heat-treated in an inert environment to obtain the binder.

9. The secondary battery according to claim 8, wherein The initiator includes any one or more of potassium persulfate, ammonium persulfate, sodium sulfite and sodium bisulfite.

10. An electric device, comprising the secondary battery according to any one of claims 1 to 9, wherein the secondary battery serves as a power supply for the electric device.

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