Secondary battery, preparation method for negative electrode plate, and electric device

Abstract

The present application provides a secondary battery, a preparation method for a negative electrode plate, and an electric device. The secondary battery comprises a negative electrode plate, the negative electrode plate comprises a negative electrode current collector and a negative electrode film layer provided on at least one side of the negative electrode current collector, the negative electrode film layer comprises an aqueous binder, and the aqueous binder comprises a polymer having a weight average molecular weight of greater than one million. By using a high-molecular-weight polymer as an aqueous binder in the negative electrode film layer, a negative electrode slurry can be formed into a film with the addition of a low content of a solvent by virtue of the adhesive force and strength of the high-molecular-weight polymer, which greatly reduces the content of the aqueous solvent in the negative electrode slurry, reduces the stress of the negative electrode film layer during drying, and makes a breakthrough in terms of the maximum areal density that can be achieved by the negative electrode film layer in the prior art, thereby improving the energy density of the secondary battery.

Description

Secondary battery, preparation method of negative electrode sheet, and electric device TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, and in particular to a secondary battery, a preparation method of a negative electrode sheet, and an electric device. BACKGROUND

[0002] In recent years, secondary batteries are widely used in energy storage power systems such as hydroelectric, thermal, wind, and solar power stations, and in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and the like.

[0003] How to further improve the energy density of a secondary battery is a technical problem that needs to be solved by those skilled in the art.

[0004] SUMMARY

[0005] The present application is made in view of the above-mentioned problem, and aims to provide a secondary battery with high energy density.

[0006] The first aspect of the present application provides a secondary battery, comprising a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode film layer arranged on at least one side of the negative electrode current collector, the negative electrode film layer comprising a water-based binder, the water-based binder comprising a polymer with a weight average molecular weight greater than 1 million.

[0007] By using a high-molecular-weight polymer as a water-based binder in the negative electrode film layer, the negative electrode slurry can be formed with low content of solvent, greatly reducing the content of the aqueous solvent in the negative electrode slurry, reducing the stress of the negative electrode film layer during drying, breaking through the maximum area density that can be achieved by the negative electrode film layer in the prior art, and further improving the energy density of the secondary battery. The high-molecular-weight water-based binder can also form a tight coating on the surface defects of the negative electrode active material, reduce lithium consumption during the cycling of the secondary battery, and improve the initial efficiency and cycle life of the battery.

[0008] In any embodiment, the weight average molecular weight of the polymer is 1.2 million to 4 million, and can be 1.5 million to 4 million.

[0009] The polymer with a weight average molecular weight in the above range can not only improve the energy density and cycle stability of the secondary battery, but also effectively control the material cost.

[0010] In any embodiment, the polymer comprises structural units derived from a monomer represented by Formula I and structural units derived from a monomer represented by Formula II.

[0011] wherein R1-R6 each independently comprises H, C1-3 One or more of alkyl groups; R7 includes one or more of carboxyl group, lithium carboxylate group, sodium carboxylate group, sulfonic acid group, lithium sulfonate group, sodium sulfonate group, amide group, and hydroxyl group.

[0012] The polymer contains a large number of polar groups, which can provide sufficient cohesion in the negative electrode film while providing high bonding strength between the negative electrode film and the current collector, which is beneficial to improving the cycle stability of the secondary battery. In addition, the polar groups on the polymer surface can form strong surface adsorption for the negative electrode active material, which helps to reduce the probability of powder loss due to insufficient bonding during the negative electrode film manufacturing process. At the same time, the high strength of the polymer ensures that the negative electrode slurry has sufficient viscoelasticity and flexibility to be extended during the rolling process, and will not produce brittle cracks during the rolling process, so that the negative electrode slurry can meet the requirements of rolling film formation.

[0013] In any embodiment, the polymer further comprises a structural unit derived from a monomer represented by formula III;

[0014] Among them, R8-R 10 Each independently includes H, C 1-3 One or more of alkyl groups; R 11 Including C 1-6 Alkyl, C 1-6 One or more of alkylhydroxyl groups.

[0015] The monomer represented by Formula III contains an ester group, which can further improve the flexibility of the polymer, thereby further increasing the limiting surface density of the secondary battery.

[0016] In any embodiment, R7 includes one or more of a carboxyl group, a lithium carboxylate group, and a sodium carboxylate group, and one or more of a sulfonic acid group, a lithium sulfonate group, and a sodium sulfonate group.

[0017] Experimental and computational results show that the inclusion of the above groups in the polymer is beneficial to inducing the formation of stable inorganic components in the solid electrolyte membrane (SEI), further improving the initial efficiency of the secondary battery.

[0018] In any embodiment, the monomer represented by formula I includes one or more of acrylonitrile and methacrylonitrile; and / or the monomer represented by formula II includes one or more of methacrylamide, acrylamide, propylene sulfonic acid, sodium propylene sulfonate, lithium propylene sulfonate, sodium acrylate, lithium acrylate, acrylic acid, and allyl alcohol; and / or the monomer represented by formula III includes one or more of ethylene glycol dimethacrylate, methyl methacrylate, ethyl methacrylate, methyl acrylate, hexyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, and methyl isocyanate.

[0019] In any embodiment, the molar content of the structural units derived from the monomer of formula I accounts for 5% to 80%, optionally 7% to 30%, based on the total moles of structural units in the polymer; and / or the molar content of the structural units derived from the monomer of formula II accounts for 10% to 95%, optionally 60% to 90%, based on the total moles of structural units in the polymer; and / or the molar content of the structural units derived from the monomer of formula III accounts for 0% to 30%, based on the total moles of structural units in the polymer.

[0020] The polymer with the molar content within the above range, based on the total moles of structural units in the polymer, can provide sufficient adhesion and strength to enable the negative electrode slurry to be rolled into a film, increase the solid content of the slurry during the formation of the negative electrode film layer of the secondary battery, and thus simultaneously improve the energy density and cycle stability of the secondary battery.

[0021] In any embodiment, the polymer comprises one or more of poly(acrylic acid-methyl acrylate-acrylonitrile-acrylsulfonic acid), poly(acrylic acid-ethyl acrylate-acrylonitrile-acrylsulfonic acid), poly(acrylic acid-methyl methacrylate-acrylonitrile-acrylsulfonic acid), poly(acrylic acid-acrylonitrile-acrylsulfonic acid) poly(acrylic acid-acrylonitrile), poly(acrylonitrile-acrylsulfonic acid).

[0022] In any embodiment, the viscosity of the glue solution prepared by dissolving the polymer in water with a polymer mass content of 2% is 1000 mPa·s to 10000 mPa·s, optionally 2000 mPa·s to 10000 mPa·s.

[0023] The viscosity of the glue solution prepared by dissolving the polymer in water with a polymer mass content of 2% is high, which enables the negative electrode slurry to be rolled into a film, increases the solid content of the slurry during the formation of the negative electrode film layer of the secondary battery, and thus simultaneously improves the energy density and cycle stability of the secondary battery.

[0024] In any embodiment, the swelling degree of the glue film prepared by the polymer in an ester solvent at 60°C is less than 10%.

[0025] The glue film prepared by the polymer has a low swelling degree, which can reduce the swelling in the electrolyte and is conducive to the improvement of the cycle life of the secondary battery.

[0026] In any embodiment, the negative electrode film layer further comprises a negative electrode active material, and the mass content of the negative electrode active material accounts for 95% to 98%, based on the total mass of the negative electrode film layer; and / or the mass content of the water-based binder accounts for 1% to 4%, based on the total mass of the negative electrode film layer.

[0027] The secondary battery negative electrode film layer does not need to disperse the auxiliary agent and the amount of the binder is reduced, which is conducive to further increasing the active material loading of the negative electrode film layer and further improving the capacity and energy density of the secondary battery.

[0028] In any embodiment, the single-sided area density of the negative electrode film layer is 15 mg / cm 2 -30 mg / cm 2 ; and / or the compaction density of the negative electrode film layer is 1.55 g / cm 3 -1.65 g / cm 3 .

[0029] The secondary battery has a high area density, and the capacity and energy density are broken through.

[0030] In any embodiment, the OI value of the negative electrode film layer is 20-50; wherein the OI value of the negative electrode film layer is C OI =C 004 / C 110 , C 004 is the peak area of the 004 characteristic diffraction peak in the X-ray diffraction spectrum of the negative electrode film layer, and C 110 is the peak area of the 110 characteristic diffraction peak in the X-ray diffraction spectrum of the negative electrode film layer; and / or the adhesion strength between the negative electrode film layer and the negative electrode current collector is greater than or equal to 50 N / m, which can be optionally 100 N / m-200 N / m.

[0031] The negative electrode film layer with a high OI value is prone to lattice expansion in the longitudinal direction during full charging, rather than being extruded in the direction parallel to the current collector, thereby reducing the probability of wrinkling of the electrode sheet during full charging and improving the cycle stability of the secondary battery. Moreover, the secondary battery has a high adhesion strength, which is conducive to the synchronous improvement of the battery capacity retention rate.

[0032] The second aspect of the present application provides a preparation method of a negative electrode sheet, which comprises: providing a negative electrode slurry, and compounding the negative electrode slurry on a negative electrode current collector after rolling the negative electrode slurry into a film; and drying to form a negative electrode film layer arranged on the surface of the negative electrode current collector; wherein the negative electrode slurry comprises a water-based binder, and the water-based binder comprises a polymer with a weight average molecular weight greater than 1 million.

[0033] The preparation method can increase the solid content of the negative electrode slurry, reduce the water content that needs to be volatilized during the drying process of the slurry, effectively reduce the drying stress, and reduce the risk of cracking, while meeting the process requirements of thick electrode sheets, improving the area density of the negative electrode film layer, and breaking through the upper limit of the energy density of the secondary battery in the prior art. In addition, the preparation method can improve the coating effect of the binder on the surface of the negative electrode active material through the rolling forming method, reduce lithium consumption, and improve the initial efficiency and cycle stability of the secondary battery. In any embodiment, the solid content of the negative electrode slurry is 65%-90%.

[0034] The negative electrode slurry with the solid content in the above range can reduce the solvent content that needs to be evaporated on one hand, and reduce the cracking phenomenon of the negative electrode film layer, so that the area density of the negative electrode sheet is further improved; on the other hand, the slurry with high solid content can present a "dough-like" state, so that the roll pressing film forming of the negative electrode film layer can be realized. In any embodiment, the polymer comprises structural units derived from the monomer shown in formula I and structural units derived from the monomer shown in formula II;

[0035] wherein R1-R6 each independently comprises H, C 1-3 one or more of alkyl; R7 comprises one or more of carboxyl, lithium carboxylate group, sodium carboxylate group, sulfonic acid group, lithium sulfonate group, sodium sulfonate group, amide group, hydroxyl group.

[0036] The third aspect of the present application provides a power utilization device comprising the secondary battery of the first aspect of the present application or the secondary battery prepared by the preparation method of the second aspect of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0037] FIG. 1 is a schematic diagram of a secondary battery according to an embodiment of the present application;

[0038] FIG. 2 is an exploded view of the secondary battery according to an embodiment of the present application shown in FIG. 1;

[0039] FIG. 3 is a schematic diagram of a battery module according to an embodiment of the present application;

[0040] FIG. 4 is a schematic diagram of a battery pack according to an embodiment of the present application;

[0041] FIG. 5 is an exploded view of the battery pack according to an embodiment of the present application shown in FIG. 4;

[0042] FIG. 6 is a schematic diagram of a power utilization device using the secondary battery according to an embodiment of the present application as a power source.

[0043] REFERENCE SIGNS:

[0044] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 secondary battery; 51 shell; 52 electrode assembly; 53 cover plate. DETAILED DESCRIPTION

[0045] Hereinafter, specific embodiments of the secondary battery, the method of manufacturing a negative electrode sheet, and the electric device of the present application will be described in detail with appropriate reference to the accompanying drawings. However, there will be cases where unnecessary detailed description is omitted. For example, there will be cases where detailed description of matters known well, repeated description of substantially identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0046] The ranges disclosed herein are defined by their lower and upper limits. Ranges can be inclusive or exclusive of their endpoints, and are arbitrarily combinable. For example, if a range is listed as 60-120 and 80-110, it is understood that 60-110 and 80-120 are also contemplated. Also, where a minimum range value of 1 and 2 is listed, and a maximum range value of 3, 4, and 5 is listed, the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise indicated, a numerical range "a-b" is intended to mean any and all subcombinations of the values between a and b, inclusive of the values a and b. For example, the numerical range "0-5" is intended to mean that all real numbers between 0 and 5, inclusive of 0 and 5, have been listed herein. "0-5" is merely a shorthand for listing all of these numerical combinations. Also, when a parameter is stated to be an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0047] If not specifically stated, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0048] If not specifically stated, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0049] If not specified otherwise, all steps of the present application can be performed in sequence or randomly, preferably in sequence. For example, the method comprising steps (a) and (b) means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, the method can further comprise step (c) means that step (c) can be added to the method in any sequence, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0050] If not specified otherwise, the terms "comprising" and "including" as used in the present application are meant to be interpreted open-ended. For example, the terms "comprising" and "including" can mean that other components not listed can also be included or can mean that only the listed components are included.

[0051] If not specified otherwise, the term "or" in the present application is inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, any of the following satisfy the condition "A or B": A is true (or present) and B is false (or not present); A is false (or not present) and B is true (or present); or both A and B are true (or present).

[0052] Increasing the thickness of the film layer of the pole piece is an effective way to increase the capacity and energy density of the secondary battery. In the prior art, the negative electrode active material is uniformly coated on the surface of the negative electrode current collector after being dispersed in a large amount of aqueous solvent with a water-based binder. After the aqueous solvent in the slurry is volatilized and dried, the negative electrode film layer is formed. With the increase of the coating quality and the thickness of the film layer, the difficulty of volatilization of the aqueous solvent increases, and with the increase of the residual space in the film layer after the volatilization of the solvent, the slurry is prone to cracking under the action of drying stress during the drying process, so that the thickness of the negative electrode film layer cannot be further increased, and the energy density of the battery is difficult to further break through.

[0053] [Secondary battery]

[0054] Based on this, the first aspect of the present application provides a secondary battery, comprising a negative electrode pole piece, the negative electrode pole piece comprising a negative electrode current collector and a negative electrode film layer arranged on at least one side of the negative electrode current collector, the negative electrode film layer comprising a water-based binder, the binder comprising a polymer with a weight average molecular weight greater than 1 million.

[0055] In this context, the term "binder" refers to a chemical compound, polymer or mixture that forms a colloidal solution or a colloidal dispersion in a dispersion medium.

[0056] In the present text, the term "polymer" comprises on the one hand a collection of macromolecules which are chemically uniform, but which differ in the degree of polymerization, molar mass and chain length, prepared by polymerization reactions. On the other hand, the term also comprises derivatives of such a collection of macromolecules formed by polymerization reactions, i.e. products which can be obtained by reaction, e.g. addition or substitution, of functional groups in the above macromolecules and which can be chemically uniform or chemically non-uniform.

[0057] In the present text, the term "weight average molecular weight" means the sum of the products of the weight fraction of the molecules of different molecular weight in a polymer and their corresponding molecular weights.

[0058] In the present text, the dispersion medium of water-based binders is an aqueous solvent. As an example, water-based solvents include water and mixtures thereof. Examples of water-based binders include, but are not limited to, polyacrylic polymers, emulsion-based polymers (such as styrene-acrylic emulsion SAE, styrene-butadiene emulsion SBR), polyacrylate polymers.

[0059] In some embodiments, the binder is used as a negative electrode binder for binding a negative electrode active material and / or a conductive agent to form an electrode.

[0060] In the present application, the test of the weight average molecular weight of the polymer can be performed by any known method in the art, for example, by using a gel chromatography, such as Waters 2695 Isocratic HPLC type gel chromatograph (differential refractive detector 2141). Specifically, the test method is to use a polystyrene solution sample with a mass fraction of 3.0% as a reference, and to select a matching water-soluble chromatographic column (water-based: PL aquagel-OH MIXED-H chromatographic column 7.5mm*300mm, 8μm). A 1.0% polymer glue solution is prepared with deionized water solvent, and the prepared solution is placed for one day for standby. During the test, the syringe is first used to suck tetrahydrofuran for flushing, and the operation is repeated several times. Then 5ml of experimental solution is sucked, and the air in the syringe is excluded, and the needle tip is wiped dry. Finally, the sample solution is slowly injected into the sample port. After the reading is stable, the data is obtained, and the weight average molecular weight is read.

[0061] In some embodiments, the weight average molecular weight of the polymer is 1.1 million, 1.2 million, 1.5 million, 2 million, 2.5 million, 3 million, 3.5 million, 4 million, 4.5 million, 5 million or any numerical range between any two of them.

[0062] By using a high molecular weight polymer as a water-based binder in the negative electrode film layer, the negative electrode slurry can be formed with the aid of the adhesion and strength of the high molecular weight polymer, with low content of solvent, greatly reducing the content of water-based solvent in the negative electrode slurry, reducing the stress of the negative electrode film layer during drying, breaking through the maximum surface density of the negative electrode film layer in the prior art, and further improving the energy density of the secondary battery. The high molecular weight water-based binder can also form a tight coating on the surface defects of the negative electrode active material, reduce lithium consumption during the cycling of the secondary battery, and improve the initial efficiency and cycle life of the battery.

[0063] In some embodiments, the weight average molecular weight of the polymer is 1.2 million to 4 million, and optionally 1.5 million to 4 million.

[0064] The polymer with a weight average molecular weight in the above range can not only improve the energy density and cycle stability of the secondary battery, but also effectively control the material cost.

[0065] In some embodiments, the polymer includes structural units derived from a monomer represented by Formula I and structural units derived from a monomer represented by Formula II.

[0066] wherein R1-R6 each independently includes one or more of H, C 1-3 alkyl; and R7 includes one or more of a carboxyl group, a lithium carboxylate group, a sodium carboxylate group, a sulfonic acid group, a lithium sulfonate group, a sodium sulfonate group, an amide group, and a hydroxyl group.

[0067] In this context, the term "C 1-3 alkyl" refers to an alkyl group containing 1-3 C. As an example, methyl, ethyl, propyl can be optionally included.

[0068] The polymer includes a large number of polar groups, which can provide sufficient cohesion in the negative electrode film layer while providing high adhesion strength between the negative electrode film layer and the current collector, which is beneficial to improve the cycle stability of the secondary battery. Moreover, the polar groups on the surface of the polymer can form strong surface adsorption on the negative electrode active material, which helps to reduce the probability of powder falling due to insufficient adhesion during the process of the negative electrode film layer. At the same time, the polymer has high strength, so that the negative electrode slurry has sufficient viscoelasticity and flexibility to be extended during the rolling process, and will not be brittle during the rolling process, so that the negative electrode slurry can meet the requirements of rolling film formation.

[0069] In some embodiments, R7 includes one or more of a carboxyl group, a lithium carboxylate group, a sodium carboxylate group, a sulfonic acid group, a lithium sulfonate group, and a sodium sulfonate group.

[0070] The above group is easy to form a coating on the surface defects of the negative electrode active material, thereby reducing lithium consumption and improving the initial efficiency of the secondary battery.

[0071] In some embodiments, R7 comprises one or more of carboxyl, lithium carboxylate group, sodium carboxylate group, and one or more of sulfonic acid group, lithium sulfonate group, sodium sulfonate group.

[0072] Experimental and calculation results show that the simultaneous inclusion of the above-mentioned groups in the polymer is conducive to inducing the formation of a stable inorganic component in the solid electrolyte membrane (SEI), further improving the initial efficiency of the secondary battery.

[0073] In some embodiments, the polymer further comprises structural units derived from a monomer represented by Formula III;

[0074] wherein, R8-R 10 each independently comprises one or more of H, C 1-3 alkyl, C 11 alkyl, C 1-6 alkyl, C 1-6 alkyl, C

[0075] The monomer represented by Formula III contains an ester group, which can further improve the flexibility of the polymer, so that the limit surface density of the secondary battery is further improved.

[0076] In some embodiments, the monomer represented by Formula I comprises one or more of acrylonitrile, methacrylonitrile; and the monomer represented by Formula II comprises one or more of methacrylamide, acrylamide, acrylic sulfonic acid, sodium acrylsulfonate, lithium acrylsulfonate, sodium acrylate, lithium acrylate, acrylic acid, propylene alcohol.

[0077] In some embodiments, the monomer represented by Formula II comprises (1) one or more of acrylic sulfonic acid, sodium acrylsulfonate, lithium acrylsulfonate, and (2) one or more of sodium acrylate, lithium acrylate, and acrylic acid.

[0078] In some embodiments, the monomer represented by Formula III comprises one or more of ethylene glycol dimethacrylate, methyl methacrylate, ethyl methacrylate, methyl acrylate, hexyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, and methyl isocyanate.

[0079] In some embodiments, the molar content of the structural units derived from the monomer represented by Formula I accounts for 5%-80%, optionally 7%-30%, based on the total moles of structural units in the polymer; and / or the molar content of the structural units derived from the monomer represented by Formula II accounts for 10%-95%, optionally 60%-90%, based on the total moles of structural units in the polymer; and / or the molar content of the structural units derived from the monomer represented by Formula III accounts for 0%-30%, based on the total moles of structural units in the polymer.

[0080] In some embodiments, the molar content of the structural units derived from the monomer of Formula I can be selected from 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or any numerical range between any two of the aforementioned values, based on the total moles of the structural units in the polymer.

[0081] In some embodiments, the molar content of the structural units derived from the monomer of Formula II can be selected from 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or any numerical range between any two of the aforementioned values, based on the total moles of the structural units in the polymer.

[0082] In some embodiments, the molar content of the structural units derived from the monomer of Formula III can be selected from 0%, 5%, 10%, 15%, 20%, 25%, 30%, or any numerical range between any two of the aforementioned values, based on the total moles of the structural units in the polymer.

[0083] The polymer having a molar content within the aforementioned range, based on the total moles of the structural units in the polymer, can provide sufficient adhesion and strength to enable the negative electrode slurry to be rolled into a film, increase the solid content of the slurry during the formation of the secondary battery negative electrode film layer, and thus simultaneously improve the energy density and cycle stability of the secondary battery.

[0084] In some embodiments, the polymer comprises one or more of poly(acrylic acid-methyl acrylate-acrylonitrile-acrylsulfonic acid), poly(acrylic acid-ethyl acrylate-acrylonitrile-acrylsulfonic acid), poly(acrylic acid-methyl methacrylate-acrylonitrile-acrylsulfonic acid), poly(acrylic acid-acrylonitrile-acrylsulfonic acid) poly(acrylic acid-acrylonitrile), poly(acrylonitrile-acrylsulfonic acid).

[0085] In some embodiments, the viscosity of the glue solution prepared by dissolving the polymer in water to obtain a polymer mass content of 2% is 1000 mPa·s to 10000 mPa·s, which can be selected from 2000 mPa·s to 10000 mPa·s.

[0086] The viscosity of the glue solution can be tested by any known method in the art. As an example, 50 g of the polymer and water solvent are weighed in a 500 ml beaker to prepare a glue solution with a mass fraction of 2%, and are stirred and dispersed using a Li Chen high-speed grinder at a speed of 800 r / min for 120 min, and then ultrasonic oscillation is performed for 30 min to remove air bubbles. At room temperature, the viscosity is tested using a Li Chen NDJ-5S rotary viscometer, a No. 3 rotor is inserted into the glue solution, the rotor liquid level mark is made to be level with the glue solution, the rotor is rotated at a speed of 12 r / min, and the viscosity data is read after 6 min.

[0087] In some embodiments, the viscosity of the glue solution prepared by dissolving the polymer in water with a polymer mass content of 2% can be selected from 1000 mPa s, 2000 mPa s, 3000 mPa s, 4000 mPa s, 5000 mPa s, 6000 mPa s, 7000 mPa s, 8000 mPa s, 9000 mPa s, 10000 mPa s, or any numerical range between any two of them.

[0088] The viscosity of the glue solution prepared by dissolving the polymer in water with a polymer mass content of 2% has high viscosity, so that the negative electrode slurry can be rolled into a film, the solid content of the slurry during the formation of the secondary battery negative electrode film layer is improved, and the energy density and cycle stability of the secondary battery are simultaneously improved.

[0089] In some embodiments, the swelling degree of the glue film prepared by the polymer in an ester solvent at 60°C is less than 10%.

[0090] In some embodiments, the swelling degree of the glue film prepared by the polymer in an ester solvent at 60°C can be selected from 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or any numerical range between any two of them.

[0091] The swelling degree of the glue film prepared by the polymer in an ester solvent at 60°C can be tested by a method known in the art. As an example, 50 g of the polymer is added to a deionized water solvent, and is diluted to a solid content of 2%, 600 ml of the glue solution is poured into a rectangular mold (20 x 30 cm 2 ) with a flat bottom, and after complete drying, it is cut into shapes with uniform sizes (2 x 2 cm 2 ), each is weighed, about 0.5 g of the sample is soaked in a mixed solvent prepared by mixing ethylene carbonate EC and dimethyl carbonate DMC in a volume ratio of 3:7, and is sealed and placed in a 60°C environment, after 72 hours, the solvent on the surface of the sample is wiped off, the sample is weighed and recorded, and the mass increase rate before and after soaking of the sample is calculated as the swelling degree of the glue film prepared by the polymer in an ester solvent at 60°C.

[0092] The polymer prepared adhesive film has low swelling degree, can reduce the swelling in electrolyte, and is beneficial to the improvement of the cycle life of the secondary battery.

[0093] In some embodiments, the negative electrode film layer further comprises a negative electrode active material, the mass content of the negative electrode active material is 95%-98% based on the total mass of the negative electrode film layer; and / or, the mass content of the aqueous binder is 1%-4% based on the total mass of the negative electrode film layer.

[0094] In some embodiments, the negative electrode film layer further comprises a negative electrode active material. The negative electrode active material can be any known negative electrode active material for a battery. As an example, the negative electrode active material can comprise at least one of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based material, tin-based material, lithium titanate, etc. The silicon-based material can be selected from at least one of elemental silicon, silicon oxide compound, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy. The tin-based material can be selected from at least one of elemental tin, tin oxide compound, and tin alloy. However, the application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries can also be used. These negative electrode active materials can be used alone or in combination of two or more.

[0095] In some embodiments, the mass content of the negative electrode active material can be selected as 95%, 96%, 97%, 98%, or any numerical range between any two of them, based on the total mass of the negative electrode film layer.

[0096] In some embodiments, the mass content of the aqueous binder can be selected as 1%, 2%, 3%, 4%, or any numerical range between any two of them, based on the total mass of the negative electrode film layer.

[0097] The secondary battery negative electrode film layer does not need a dispersing aid and the amount of binder is reduced, which is beneficial to further improving the active material loading in the negative electrode film layer and further improving the capacity and energy density of the secondary battery.

[0098] In some embodiments, the negative electrode film layer can further optionally comprise a conductive agent. The conductive agent can be selected from at least one of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0099] In some embodiments, the mass content of the conductive agent is 0.1%-2% based on the total mass of the negative electrode film layer.

[0100] In some embodiments, the mass content of the conductive agent can be selected as 0.1%, 0.5%, 1%, 1.5%, 2%, or any numerical range between any two of them, based on the total mass of the negative electrode film layer. In some embodiments, the single-sided surface density of the negative electrode film layer is 15 mg / cm2 - 30 mg / cm 2 .

[0101] In some embodiments, the single-sided area density of the negative electrode film layer can be selected from 15 mg / cm 2 , 16 mg / cm 2 , 17 mg / cm 2 , 18 mg / cm 2 , 19 mg / cm 2 , 20 mg / cm 2 , 21 mg / cm 2 , 22 mg / cm 2 , 23 mg / cm 2 , 24 mg / cm 2 , 25 mg / cm 2 , 26 mg / cm 2 , 27 mg / cm 2 , 28 mg / cm 2 , 29 mg / cm 2 , 30 mg / cm 2 or any range of values between any of the two.

[0102] In the present application, the area density of the negative electrode film layer is the meaning well known in the art, which can be tested by methods known in the art. For example, a negative electrode sheet coated on one side and cold-pressed can be punched into a small disc with an area of S1, weighed, and recorded as M1. Then the negative electrode film layer of the above weighed negative electrode sheet is wiped off, the weight of the negative electrode current collector is weighed, and recorded as M0. The area density of the negative electrode film layer = (M1-M0) / S1.

[0103] The secondary battery has a high area density, achieving a breakthrough in capacity and energy density.

[0104] In some embodiments, the compaction density of the negative electrode film layer can be 1.55 g / cm 3 - 1.65 g / cm 3 .

[0105] In the present application, the compaction density of the negative electrode film layer is the meaning well known in the art, which can be tested by methods known in the art. The compaction density of the negative electrode film layer = the area density of the negative electrode film layer / the thickness of the negative electrode film layer. The thickness of the negative electrode film layer is the meaning well known in the art, which can be tested by methods known in the art, for example, using a micrometer (e.g., Mitutoyo 293-100, accuracy of 0.1 μm).

[0106] In some embodiments, the compaction density of the negative electrode film layer can be selected from 1.55 g / cm 3 , 1.60 g / cm 31.65 g / cm3 3 or any numerical range between any two of the above values.

[0107] In some embodiments, the OI value of the negative electrode film layer is 20-50; wherein the OI value of the negative electrode film layer is C OI = C 004 / C 110 , C 004 is the peak area of the 004 characteristic diffraction peak in the X-ray diffraction pattern of the negative electrode film layer, and C 110 is the peak area of the 110 characteristic diffraction peak in the X-ray diffraction pattern of the negative electrode film layer.

[0108] In the present application, the OI value of the negative electrode film layer can be tested by instruments and methods known in the art. For example, an X-ray diffractometer (such as Bruker D8 Discover) can be used for testing, and the testing can refer to JIS K 0131-1996, JB / T 4220-2011 to obtain the X-ray diffraction pattern of the negative electrode sheet. In the X-ray diffraction analysis test of the present application, a copper target can be used as an anode target, CuKα ray is used as the radiation source, the wavelength of the ray is the scanning 2θ angle range is 20°-80°, and the scanning rate is 4° / min.

[0109] In some embodiments, the OI value of the negative electrode film layer can be 20, 25, 30, 35, 40, 45, 50 or any numerical range between any two of the above values.

[0110] The preparation method of rolling into a film can significantly improve the OI value of the negative electrode film layer. The negative electrode film layer with high OI value is prone to lattice expansion in the longitudinal direction during full charging, rather than being squeezed in the direction parallel to the current collector, thereby reducing the probability of wrinkling of the electrode sheet during full charging and being beneficial to the improvement of the cycle stability of the secondary battery.

[0111] In some embodiments, the adhesion strength between the negative electrode film layer and the negative electrode current collector is greater than or equal to 50 N / m, and can be 100 N / m-200 N / m.

[0112] In some embodiments, the adhesion strength between the negative electrode film layer and the negative electrode current collector can be 50 N / m, 60 N / m, 70 N / m, 80 N / m, 90 N / m, 100 N / m, 120 N / m, 130 N / m, 140 N / m, 150 N / m, 160 N / m, 170 N / m, 180 N / m, 190 N / m, 200 N / m or any numerical range between any two of the above values.

[0113] The adhesion strength between the negative electrode film layer and the negative electrode current collector can be tested by any known method in the art. As an example, referring to GB-T 2790-1995, the adhesion strength testing process of the examples and comparative examples of the present application is as follows: a piece of electrode sample with a width of 30 mm and a length of 100-160 mm is cut by a blade, and a special double-sided adhesive tape is attached to the steel plate, with a width of 20 mm and a length of 90-150 mm. The positive electrode film layer of the previously cut electrode sample is attached to the double-sided adhesive tape, and then a 2 kg roller is rolled three times in the same direction. A paper tape with a width equal to that of the electrode sample and a length of 250 mm is fixed to the electrode current collector, and is fixed with a corrugated adhesive. Turn on the power of the three-sense tensile testing machine (sensitivity: 1 N), and adjust the limit block to the appropriate position. The end of the steel plate without the electrode sample is fixed with the lower clamp. Fold the paper tape upwards and fix it with the upper clamp. Adjust the position of the upper clamp using the "up" and "down" buttons on the hand controller attached to the tensile testing machine. Then test and read the value. Divide the force when the electrode is balanced by the width of the adhesive tape to obtain the adhesion strength per unit length of the electrode, which represents the adhesion strength between the negative electrode film layer and the current collector.

[0114] The secondary battery has high adhesion strength, which is beneficial to the simultaneous improvement of the battery capacity retention rate.

[0115] In some embodiments, the negative electrode current collector can be a metal foil or a composite current collector. For example, as a metal foil, a copper foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base material. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0116] The second aspect of the present application provides a method for preparing a negative electrode tab, which includes: providing a negative electrode slurry, and compounding the negative electrode slurry on a negative electrode current collector after rolling the negative electrode slurry into a film; drying to form a negative electrode film layer arranged on the surface of the negative electrode current collector; the negative electrode slurry includes a water-based binder, and the water-based binder includes a polymer with a weight average molecular weight greater than 1 million.

[0117] The preparation method can not only increase the solid content of the negative electrode slurry, reduce the water content that needs to be evaporated in the slurry drying process, effectively reduce the drying stress, and reduce the risk of cracking, but also meet the process requirements of thick electrode sheets, improve the area density of the negative electrode film, and break through the upper limit of the energy density of the secondary battery in the prior art. In addition, the preparation method can improve the coating effect of the binder on the surface of the negative electrode active material through roll forming, reduce lithium consumption, and improve the initial efficiency and cycle stability of the secondary battery.

[0118] In some embodiments, the solid content of the negative electrode slurry is 65%-90%.

[0119] In some embodiments, the solid content of the negative electrode slurry can be selected as 65%, 70%, 75%, 80%, 85%, 90%, or any numerical range between any two of them.

[0120] The negative electrode slurry with the solid content in the above range can not only reduce the solvent content that needs to be evaporated, reduce the cracking phenomenon of the negative electrode film, and further improve the area density of the negative electrode sheet, but also enable the slurry with high solid content to present a "dough-like" state, so that the roll forming of the negative electrode film can be realized.

[0121] The third aspect of the present application provides a preparation method of an aqueous binder, which comprises the following steps: under polymerizable conditions, monomers represented by Formula I and monomers represented by Formula II are subjected to a polymerization reaction to prepare a polymer; the weight average molecular weight of the polymer is greater than 1 million,

[0122] wherein R1-R6 each independently comprises H, one or more of C 1-3 one or more of alkyl; R7 comprises one or more of carboxyl, lithium carboxylate group, sodium carboxylate group, sulfonic acid group, lithium sulfonate group, sodium sulfonate group, amide group, and hydroxyl group.

[0123] In some embodiments, the polymerization reaction comprises the following steps: raw materials comprising monomers represented by Formula I and Formula II are reacted under the initiation of an initiator at a reaction temperature of room temperature, a reaction pressure of 6-8 MPa, and in a non-reactive gas atmosphere for 0.5-1 hours; a chain extender is added, and after reaction for 0.3-1 hours, the temperature is increased to 50-80°C, monomers represented by Formula I are added dropwise, and after continuous reaction for 0.2-2 hours, a chain transfer agent is added, a pH adjuster is added to adjust the pH value to 6.2-8.0, and the polymer is obtained.

[0124] In some embodiments, the raw materials further comprise monomers represented by Formula III.

[0125] In some embodiments, the initiator comprises a persulfate initiator, which can be selected as one or more of potassium persulfate, sodium persulfate, and ammonium persulfate.

[0126] In some embodiments, the chain transfer agent includes one or more of cyclohexane, isopropyl alcohol, methanol, and acetone.

[0127] In some embodiments, the chain extender includes butadiene.

[0128] In some embodiments, the amount of initiator is 0.15% to 1% of the total mass of the monomers. As an example, it can be 0.25%, 0.5%, 0.75%, or any value range between any two of them.

[0129] In some embodiments, the pH adjuster includes one or more of LiOH-H2O, NaOH-H2O, lithium carbonate, sodium carbonate, sodium bicarbonate.

[0130] A fourth aspect of the present application provides a power consuming device, which includes the secondary battery of any of the embodiments or the secondary battery prepared from the negative electrode sheet prepared by the preparation method of any of the embodiments.

[0131] [Positive electrode sheet]

[0132] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, the positive electrode film layer including a positive electrode active material, a conductive agent, and, in some embodiments, a binder or, in some embodiments, a binder prepared by the preparation method.

[0133] As an example, the positive electrode current collector has two surfaces opposite in the thickness direction of itself, and the positive electrode film layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector.

[0134] In some embodiments, the positive electrode current collector can adopt a metal foil or a composite current collector. For example, as the metal foil, an aluminum foil can be adopted. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0135] In some embodiments, the positive active material can employ a positive active material for a battery known in the art. As an example, the positive active material can include at least one of a lithium-containing phosphate of an olivine structure, a lithium transition metal oxide, and a modified compound of each thereof. However, the present application is not limited to these materials, and other conventional materials that can be used as a positive active material for a battery can also be used. These positive active materials can be used alone only one or in combination of two or more. Among them, examples of the lithium transition metal oxide can include, but are not limited to, lithium cobalt oxide (e.g., LiCoO2), lithium nickel oxide (e.g., LiNiO2), lithium manganese oxide (e.g., LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (e.g., LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(also can be simply referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2(also can be simply referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2(also can be simply referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2(also can be simply referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2(also can be simply referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (e.g., LiNi 0.85 Co 0.15 Al 0.05 O2), and a modified compound thereof, etc. Examples of the lithium-containing phosphate of an olivine structure can include, but are not limited to, at least one of lithium iron phosphate (e.g., LiFePO4(also can be simply referred to as LFP)), a composite of lithium iron phosphate and carbon, lithium manganese phosphate (e.g., LiMnPO4), a composite of lithium manganese phosphate and carbon, lithium manganese iron phosphate, and a composite of lithium manganese iron phosphate and carbon.

[0136] In some embodiments, the positive electrode film layer can further optionally include a conductive agent. As an example, the conductive agent can include at least one of super P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0137] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on a positive electrode current collector; and drying, cold-pressing, or the like.

[0138] [Electrolyte]

[0139] The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The type of electrolyte is not particularly limited in the present application and can be selected as needed. For example, the electrolyte can be liquid, gel, or all-solid.

[0140] In some embodiments, the electrolyte is an electrolytic solution. The electrolytic solution includes an electrolyte salt and a solvent.

[0141] In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluoro oxalate borate, lithium di-oxalate borate, lithium difluoro di-oxalate phosphate, and lithium tetrafluoro oxalate phosphate.

[0142] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclobutane sulfone, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0143] In some embodiments, the electrolytic solution can further optionally include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include an additive capable of improving certain properties of the battery, such as an additive for improving overcharge performance of the battery, an additive for improving high-temperature or low-temperature performance of the battery, etc.

[0144] In some embodiments, the secondary battery can include an outer package. The outer package can be used to package the above-mentioned electrode assembly and electrolyte.

[0145] In some embodiments, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, and as plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, etc. can be listed.

[0146] The shape of the secondary battery is not particularly limited, and can be cylindrical, square, or any other shape. For example, FIG. 1 is a square structure of a secondary battery 5 as an example.

[0147] In some embodiments, referring to FIG. 2, the outer package can include a housing 51 and a cover plate 53. The housing 51 can include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be arranged on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet, and the separator can form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is packaged in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, which can be selected by a person skilled in the art according to specific actual needs.

[0148] In some embodiments, the secondary battery can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, which can be selected by a person skilled in the art according to the application and capacity of the battery module.

[0149] FIG. 3 is a battery module 4 as an example. Referring to FIG. 3, in the battery module 4, a plurality of secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, other arrangements can also be used. Further, the plurality of secondary batteries 5 can be fixed by fasteners.

[0150] Optionally, the battery module 4 can also include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0151] In some embodiments, the above battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, which can be selected by a person skilled in the art according to the application and capacity of the battery pack.

[0152] FIGS. 4 and 5 are a battery pack 1 as an example. Referring to FIGS. 4 and 5, the battery pack 1 can include a battery box and a plurality of battery modules 4 arranged in the battery box. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 can be arranged on the lower box body 3 to form a closed space for receiving the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0153] In addition, the application also provides a power utilization device comprising at least one of the secondary battery, the battery module, or the battery pack provided by the application. The secondary battery, the battery module, or the battery pack can be used as a power supply of the power utilization device, and can also be used as an energy storage unit of the power utilization device. The power utilization device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0154] As the power utilization device, the secondary battery, the battery module, or the battery pack can be selected according to the use requirement thereof.

[0155] FIG. 6 is a power utilization device as an example. The power utilization device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the requirement of the power utilization device for high power and high energy density of the secondary battery, a battery pack or a battery module can be used.

[0156] The device as another example can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thinning, and a secondary battery can be used as a power supply.

[0157] Embodiment

[0158] Hereinafter, the embodiments of the application are described. The embodiments described below are exemplary and are only used to explain the application, and cannot be understood as a limitation of the application. If the specific technology or condition is not indicated in the embodiments, the technology or condition described in the literature in the art or according to the product instruction is used. If the reagent or instrument used is not indicated by the manufacturer, it is a conventional product that can be obtained by market purchase.

[0159] I. Preparation method

[0160] Embodiment 1

[0161] 1) Preparation of the binder

[0162] The acrylic acid, methyl acrylate, acrylonitrile and acrylic sulfonic acid were mixed in a certain molar ratio so that the total amount of the above monomers was 750 mol, and then ammonium persulfate with a mass of 0.5% of the total mass of the planned monomers was added as an initiator. Then the reactants were stirred for 30 min, and then 1 mol of butadiene was added and reacted for 25 min. Then the reaction system was heated to 70°C, and 120 mol of acrylonitrile was added dropwise at this temperature, and then the reaction was continued for 30 min. The reaction temperature was controlled at 70°C, and the whole reaction process was carried out under nitrogen protection, and the pressure was 7 MPa. After the completion of the polymerization reaction, isopropyl alcohol was used to terminate the reaction, and finally LiOH·H2O was used to adjust the pH to 7.2, and a water-based binder poly(acrylic acid-methyl acrylate-acrylonitrile-acrylic sulfonic acid) was obtained, wherein acrylonitrile: acrylic acid: methyl acrylate: acrylic sulfonic acid = 1:3:3:3. It can be understood that part of the structural units derived from acrylic acid are converted into structural units derived from lithium acrylate. The weight average molecular weight of the binder is 2 million.

[0163] 2) Preparation of negative electrode sheet

[0164] Take 97 parts of artificial graphite, 1 part of Super P, and add it to the stirring tank, mix for 30 min, add 2 parts of the binder prepared above, and stir for 0.5 hours with 30 parts of deionized water to obtain a negative electrode slurry. The solid content of the slurry is 77%.

[0165] The obtained negative electrode slurry is gradually thinned by multi-stage rolling, and the thinned film is pressed on the composite current collector by rolling force, and dried at 90°C for 5 minutes to obtain a negative electrode sheet.

[0166] The single-sided surface density of the negative electrode sheet is 18.5 mg / cm 2 , and the compacted density is 1.65 g / cm 3 .

[0167] 3) Preparation of positive electrode sheet

[0168] After the positive electrode active material lithium iron phosphate, the conductive agent acetylene black, and the binder PVDF are fully stirred and uniformly mixed in NMP according to the ratio of 96:2:2, they are coated on an Al foil, dried, and cold-pressed to obtain a positive electrode sheet.

[0169] The single-sided surface density of the positive electrode sheet is 26 mg / cm 2 , and the compacted density is 2.6 g / cm 3 .

[0170] 4) Isolation film

[0171] A polypropylene film is used as the isolation film.

[0172] 5) Preparation of electrolyte

[0173] In an argon atmosphere glove box (H2O <0.1 ppm, O2 <0.1 ppm), mix the organic solvent ethylene carbonate (EC) / ethyl methyl carbonate (EMC) uniformly according to a volume ratio of 3 / 7, add LiPF6 lithium salt dissolved in the organic solvent, stir uniformly, and configure a 1M LiPF6 EC / EMC solution to obtain an electrolyte.

[0174] 6) Battery preparation

[0175] Stack the positive electrode sheet, the separator, and the negative electrode sheet in order, with the separator in the middle of the positive and negative electrodes to play a separating role, and roll them up. Place the bare battery cell in an outer package, inject the prepared electrolyte, and perform packaging, liquid injection, formation, and exhaust processes to obtain a secondary battery.

[0176] Examples 2-5

[0177] The preparation method of Example 2-5 is basically the same as that of Example 1, except that the parameters for preparing the polymer are adjusted in Example 2-5, thereby adjusting the weight average molecular weight of the polymer.

[0178] In Example 2, the amount of initiator is 0.25% of the total mass of monomers, and the weight average molecular weight of the polymer is 1.5 million.

[0179] In Example 3, the amount of initiator is 0.25% of the total mass of monomers, and the weight average molecular weight of the polymer is 3 million.

[0180] In Example 4, the reaction time is reduced from 30 min to 15 min after the second addition of acrylonitrile, and the weight average molecular weight of the polymer is 1.2 million.

[0181] In Example 5, the reaction time is increased from 20 min to 40 min after the addition of butadiene, and the weight average molecular weight of the polymer is 4 million.

[0182] Examples 6-8

[0183] The preparation method of Examples 6-8 is basically the same as that of Example 1, except that the types and molar ratios of monomers used in the preparation of the polymer are adjusted, as shown in Table 1.

[0184] Comparative Example 1

[0185] In Comparative Example 1, the components of the negative electrode binder are the same as in Example 1, and the weight average molecular weight is 700,000. In Comparative Example 1, the preparation method of the negative electrode sheet is as follows:

[0186] Take 92 parts of artificial graphite, 1 part of Super P, and add them to a stirring tank. After mixing for 30 min, add 4 parts of styrene-butadiene emulsion SBR, 2 parts of carboxymethyl cellulose sodium CMC-Na, 1 part of negative electrode binder, and deionized water. Stir for 1 hour to obtain a negative electrode slurry, and the solid content of the slurry is 55%.

[0187] The slurry was extrusion coated on the composite current collector through a fixed-width slit, and dried at 90°C for 5 minutes to obtain a negative electrode sheet. The single-sided surface density of the negative electrode sheet was 12 mg / cm 2 .

[0188] Comparative Example 2

[0189] The negative electrode binder in Comparative Example 2 was the same as in Comparative Example 1. The preparation method of the negative electrode sheet in Comparative Example 2 was as follows:

[0190] Take 96 parts of artificial graphite, 1 part of Super P into the stirring tank, mix for 30 min, add 2 parts of negative electrode binder and deionized water and stir for 0.5 hours to obtain a negative electrode slurry, and the solid content of the slurry is 77%.

[0191] The obtained negative electrode slurry was gradually thinned by multi-stage rolling, and the thinned film was pressed on the composite current collector by rolling force, and dried at 90°C for 5 minutes to obtain a negative electrode sheet. The single-sided design surface density of the negative electrode sheet was 18.5 mg / cm 2 .

[0192] Table 1

[0193] II. Performance Test

[0194] 1. Initial efficiency test method

[0195] At 25°C, the battery was charged at 1 / 3C constant current to 3.65V, then charged at 3.65V constant voltage to 0.05C current, rested for 5 min, and then discharged at 1 / 3C to 2.0V. The ratio of the first discharge capacity to the first charge capacity was taken as the initial efficiency of the battery.

[0196] 2. Volume energy density test method

[0197] At 25°C, the battery was charged at 1 / 3C constant current to 3.65V, then charged at 3.65V constant voltage to 0.05C current, rested for 5 min, and then discharged at 1 / 3C to 2.0V. The first discharge capacity was recorded as C, in Ah, the discharge platform was U, in V, the volume of the square box was V, in L, and the volume energy density calculation formula was: u = (C x U) / V

[0198] 3. Cycle stability test method

[0199] The battery capacity retention test process is as follows: at 25℃, the battery is charged at 1 / 3C constant current to 3.65V, then charged at 3.65V constant voltage to the current of 0.05C, and then rested for 5min, and then discharged at 1 / 3C to 2.3V, and the obtained capacity is recorded as the initial capacity C0. The above steps are repeated for the same battery, and the discharge capacity Cn of the battery after the nth cycle is recorded at the same time, then the battery capacity retention Pn=(Cn / C0)×100% after each cycle, and the battery capacity retention and cycle number curve is obtained with P1, P2……P700 as the vertical coordinate and the corresponding cycle number as the horizontal coordinate. In the test process, the first cycle corresponds to n=1, the second cycle corresponds to n=2, and the 700th cycle corresponds to n=700. The battery capacity retention data is the data measured after 700 cycles under the above test conditions, i.e. the value of P700.

[0200] III. Analysis of test results of each embodiment and comparative example

[0201] The batteries of each embodiment and comparative example were prepared according to the above method, and each performance parameter was measured, and the results are shown in Table 2.

[0202] Table 2

[0203] The volume energy density of the secondary battery in Example 1 is 426Wh / L, and the OI value of the negative electrode film layer is 23; the volume energy density of the secondary battery in Comparative Example 1 is 418Wh / L, and the OI value of the negative electrode film layer is 13; the gel strength in Comparative Example 2 is low, which cannot meet the process requirements of roll thinning, and the negative electrode film layer cracks during the thinning process.

[0204] As can be seen from the comparison of the embodiments and comparative examples, the secondary battery comprises a negative electrode sheet, the negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer arranged on at least one side of the negative electrode current collector, the negative electrode film layer comprises a water-based binder, and the water-based binder comprises a polymer with a weight average molecular weight greater than 1 million, which can improve the energy density of the battery while simultaneously improving the initial efficiency and cycle stability of the secondary battery.

[0205] As can be seen from the comparison of Example 7 and Example 8, the polymer comprising a sulfonic acid group simultaneously helps to further improve the initial efficiency and cycle stability of the battery.

[0206] Note that the present application is not limited to the above-described embodiments. The above-described embodiments are merely examples, and embodiments having substantially the same configuration, function, and effect as the technical idea of the present application are included in the technical scope of the present application. Furthermore, other modes constructed by applying various modifications to the embodiments, or by combining part of the configurations of the embodiments, which can be conceived by those skilled in the art, without departing from the spirit of the present application, are also included in the scope of the present application.

Claims

1. A secondary battery characterized by comprising: The negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer arranged on at least one side of the negative electrode current collector, wherein the negative electrode film layer comprises a water-based binder, and the water-based binder comprises a polymer with a weight average molecular weight greater than 1 million.

2. The secondary battery according to claim 1, characterized by The weight average molecular weight of the polymer is 1.2 million to 4 million, and is optionally 1.5 million to 4 million.

3. The secondary battery according to claim 1 or 2, characterized by The polymer includes structural units derived from a monomer represented by Formula I and structural units derived from a monomer represented by Formula II. wherein R1-R6 each independently include H, C 1-3 one or more of alkyl; R7 includes one or more of carboxyl, lithium carboxylate group, sodium carboxylate group, sulfonic acid group, lithium sulfonate group, sodium sulfonate group, amide group, hydroxyl group.

4. The secondary battery according to any one of claims 1 to 3, characterized by The polymer further comprises structural units derived from a monomer of formula III: wherein R8-R 10 each independently comprises H, C 1-3 one or more of alkyl, C 11 comprises C 1-6 one or more of alkyl, C 1-6 one or more of alkyl, C 5. The secondary battery according to any one of claims 1 to 4, characterized by R7 comprises one or more of a carboxyl group, a lithium carboxylate group, a sodium carboxylate group, and one or more of a sulfonic acid group, a lithium sulfonate group, and a sodium sulfonate group.

6. The secondary battery according to any one of claims 1 to 5, characterized in that, the monomer represented by formula I comprises one or more of acrylonitrile and methacrylonitrile; and / or the monomer represented by formula II comprises one or more of methacrylamide, acrylamide, acrylic sulfonic acid, sodium acrylic sulfonate, lithium acrylic sulfonate, sodium acrylate, lithium acrylate, acrylic acid, and acrylic alcohol; and / or the monomer represented by formula III comprises one or more of ethylene glycol dimethacrylate, methyl methacrylate, ethyl methacrylate, methyl acrylate, hexyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, and methyl isocyanate.

7. The secondary battery according to any one of claims 1 to 6, characterized in that, the molar content of the structural unit derived from the monomer represented by formula I accounts for 5% to 80%, and is optionally 7% to 30%, based on the total moles of the structural units in the polymer; and / or the molar content of the structural unit derived from the monomer represented by formula II accounts for 10% to 95%, and is optionally 60% to 90%, based on the total moles of the structural units in the polymer; and / or the molar content of the structural unit derived from the monomer represented by formula III accounts for 0% to 30%, based on the total moles of the structural units in the polymer.

8. The secondary battery according to any one of claims 1 to 7, characterized by, The polymer comprises one or more of poly(acrylic acid-methyl acrylate-acrylonitrile-acrylic sulfonic acid), poly(acrylic acid-ethyl acrylate-acrylonitrile-acrylic sulfonic acid), poly(acrylic acid-methyl methacrylate-acrylonitrile-acrylic sulfonic acid), poly(acrylic acid-acrylonitrile-acrylic sulfonic acid), poly(acrylic acid-acrylonitrile), and poly(acrylonitrile-acrylic sulfonic acid).

9. The secondary battery according to any one of claims 1 to 8, characterized by, The polymer has a viscosity of 1000 mPa·s to 10000 mPa·s, and is optionally 2000 mPa·s to 10000 mPa·s, when the polymer is dissolved in water to prepare a glue solution with a polymer mass content of 2%.

10. The secondary battery according to any one of claims 1 to 9, characterized by, The glue film prepared from the polymer has a swelling degree of less than 10% in an ester solvent at 60°C.

11. The secondary battery according to any one of claims 1 to 10, characterized by The negative electrode film layer further comprises a negative electrode active material, and the mass content of the negative electrode active material is 95% to 98%, based on the total mass of the negative electrode film layer; and / or the mass content of the water-based binder is 1% to 4%, based on the total mass of the negative electrode film layer.

12. The secondary battery according to any one of claims 1 to 11, characterized in that, The single-sided surface density of the negative electrode film layer is 15 mg / cm 2 - 30 mg / cm 2 ; and / or the compaction density of the negative electrode film layer is 1.55 g / cm 3 - 1.65 g / cm 3 .

13. The secondary battery according to any one of claims 1 to 12, characterized by The OI value of the negative electrode film layer is 20-50; wherein, the OI value of the negative electrode film layer C OI = C 004 / C 110 , C 004 is the peak area of the 004 characteristic diffraction peak in the X-ray diffraction spectrum of the negative electrode film layer, C 110 is the peak area of the 110 characteristic diffraction peak in the X-ray diffraction spectrum of the negative electrode film layer; the adhesive strength between the negative electrode film layer and the negative electrode current collector is greater than or equal to 50 N / m, and is optionally 100 N / m to 200 N / m.

14. A method for producing a negative electrode sheet, characterized by The secondary battery comprises: A negative electrode slurry is provided, and after the negative electrode slurry is rolled into a film, the film is combined with a negative electrode current collector; after drying, a negative electrode film layer is formed on the surface of the negative electrode current collector; the negative electrode slurry includes a water-based binder, and the water-based binder includes a polymer with a weight average molecular weight greater than 1 million. The solid content of the negative electrode slurry is 65%-90%.

15. The method of claim 14, wherein, The electric device includes the secondary battery of any one of claims 1-13 or a secondary battery including the negative electrode sheet prepared by the preparation method of any one of claims 14-16.

16. The production method according to claim 14 or 15, characterized by, The polymer includes structural units derived from a monomer represented by Formula I and structural units derived from a monomer represented by Formula II. wherein R1-R6 each independently include H, C 1-3 one or more of alkyl; R7 includes one or more of carboxyl, lithium carboxylate group, sodium carboxylate group, sulfonic acid group, lithium sulfonate group, sodium sulfonate group, amide group, hydroxyl group.

17. An electrical device, comprising: ​

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