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

Abstract

A secondary battery, a preparation method for a negative electrode sheet, and an electric device. The secondary battery comprises a negative electrode sheet, wherein 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 an aqueous binder, the aqueous binder comprises a polymer with a weight-average molecular weight of greater than 1,000,000, and the polymer comprises a thioureido group.

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, 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 hydraulic, thermal, wind and solar power stations, and in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, etc.

[0003] How to further improve the initial efficiency and cycle stability of the 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 problems, and aims to provide a secondary battery with high initial efficiency and cycle stability.

[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, the polymer comprising a thiourea group.

[0007] With the aid of the high molecular weight of the polymer, the negative electrode film layer is rolled into a film and continuously forms an effective coating on the surface defects of the negative electrode active material during film formation and subsequent cycles, effectively improving the powder dropping problem during film formation, reducing lithium consumption during the cycle of the secondary battery, improving the initial efficiency and cycle life of the battery. Moreover, the thiourea group (-NH-C(=S)-NH-) on the polymer can form a large number of hydrogen bonds between the polymers, so that the binder forms a three-dimensional network structure wrapped around the material surface. Even if the hydrogen bonds are broken under external force, the network structure can also recover itself after the stress is released, inhibiting the volume change of the negative electrode active material during the cycle, further improving the cycle stability of the secondary battery while improving the rollable molding capability of the film layer.

[0008] In any embodiment, the weight average molecular weight of the polymer is 1.2 million to 4 million, which can be 2 million to 4 million.

[0009] The polymer with a weight average molecular weight in the above range can further improve the initial efficiency and cycle stability of the secondary battery.

[0010] In any embodiment, the polymer is an acrylic polymer, and the branched chain of the polymer comprises a polyether thiourea.

[0011] Acrylic polymer refers to a polymer obtained by homopolymerization of acrylic acid molecules and derivatives thereof or copolymerization with other monomers. The polyether thiourea is grafted on the acrylic polymer through amidation reaction, and the thiourea groups on the polyether thiourea form a hydrogen bond array, so that the polymer has excellent self-healing property. During the processes of rolling into a film and cyclic expansion, the hydrogen bonds can be broken and rearranged, and the three-dimensional network formed by the adhesive can continuously form effective wrapping for the materials in the film layer, thereby inhibiting the volume expansion of the pole piece, reducing the probability of exposure of defect sites of the negative active material and rupture of the solid electrolyte interface film (SEI), and improving the initial efficiency and cycle stability of the battery. Although there are a large number of thiourea groups capable of forming hydrogen bonds in the polyether thiourea, the flexibility of the polymer can be improved by virtue of the flexibility of the ether chain, thereby improving the ductility of the adhesive and the processability of the film layer during the process of rolling into a film.

[0012] In any embodiment, the polymer comprises a structural unit represented by Formula I,

[0013] wherein R1-R3 each independently comprises one or more of H, C 1-3 alkyl; Q comprises one or more of C 1-3 alkylene; R4 comprises one or more of C 1-4 alkoxy, n is any integer from 1 to 4, and x is an integer.

[0014] In any embodiment, Q comprises propylene, R4 comprises ethoxy, n is 2, and x is any integer from 1 to 5,000.

[0015] In any embodiment, the polymer further comprises at least one structural unit represented by Formula II;

[0016] wherein R5-R7 each independently comprises one or more of H, C 1-3 alkyl; and R8 comprises one or more of carboxyl, lithium carboxylate group, sodium carboxylate group, amide group, hydroxyl group, cyano group, and ester group.

[0017] The structural unit represented by Formula II comprises a large number of polar groups, which can provide sufficient cohesion in the negative electrode film and high adhesion strength between the negative electrode film and the current collector, thereby improving the cycle stability of the secondary battery. Moreover, the polar groups on the surface of the polymer can form strong surface adsorption to the negative active material, thereby reducing the probability of powder falling due to insufficient adhesion during the process of the negative electrode film, and improving the initial efficiency of the secondary battery.

[0018] In any embodiment, the molar proportion of the structural unit represented by Formula I is 1% to 10%, and optionally 4% to 10%, based on the total number of moles of the structural units in the polymer.

[0019] The polymer having the structural unit of Formula I in the above range of molar content has good self-healing property and processability of roll-pressing into a film, and thus the initial efficiency and cycle stability of the secondary battery are improved.

[0020] In any embodiment, the polymer comprises the structural unit of Formula II containing a cyano group, the structural unit of Formula II containing a carboxyl group or a hydroxyl group, and the structural unit of Formula II containing an ester group; wherein the molar content of the structural unit of Formula II containing a cyano group is 5% to 80%, 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 of Formula II containing a carboxyl group or a hydroxyl group is 10% to 80%, optionally 30% to 75%, based on the total moles of the structural units in the polymer; and / or the molar content of the structural unit of Formula II containing an ester group is 1% to 50%, based on the total moles of the structural units in the polymer.

[0021] The structural unit of Formula II containing a cyano group can improve the polarity of the polymer, and increase the adhesion between the film layer and the current collector; the structural unit of Formula II containing a carboxyl group or a hydroxyl group can improve the compatibility of the polymer with the aqueous solvent, so that the aqueous solvent can play the role of a lubricant in the process of forming the film layer, and thus the processability of roll-pressing the film layer is improved; and the structural unit of Formula II containing an ester group can improve the flexibility of the polymer, and increase the ductility and processability of roll-pressing the film layer.

[0022] The polymer having the above range of molar content based on the total moles of the structural units in the polymer can provide sufficient adhesion, strength and ductility, so that the high solid content of the negative electrode slurry can be roll-pressed into a film. When the slurry solid content is increased in the process of forming the negative electrode film layer, the thickness of the film layer can be reduced, which can solve the problems of cracking, warping and powdering of the negative electrode film layer in the process, improve the processability of the negative electrode film layer, and improve the initial efficiency and cycle stability of the secondary battery.

[0023] In any embodiment, the structural unit of Formula II is derived from one or more of acrylonitrile, methacrylonitrile, methacrylamide, acrylamide, acrylic sulfonic acid, sodium acrylsulfonate, lithium acrylsulfonate, sodium acrylate, lithium acrylate, acrylic acid, acrylic alcohol, ethylene glycol dimethacrylate, methyl methacrylate, ethyl methacrylate, methyl acrylate, hexyl acrylate, hydroxyethyl acrylate, and hydroxypropyl acrylate.

[0024] In any embodiment, the polymer comprises one or more of poly(acrylic acid-methyl acrylate-acrylonitrile)-graft-polyether sulfenamide, poly(acrylic acid-ethyl acrylate-acrylonitrile)-graft-polyether sulfenamide, poly(acrylic acid-methyl methacrylate-acrylonitrile)-graft-polyether sulfenamide, poly(acrylic acid-acrylonitrile)-graft-polyether sulfenamide, polyacrylic acid-graft-polyether sulfenamide.

[0025] In any embodiment, the polymer has a viscosity of 4000 mPa·s to 10000 mPa·s, optionally 5000 mPa·s to 10000 mPa·s, when dissolved in water to obtain a polymer mass content of 2%.

[0026] The viscosity of the polymer when dissolved in water to obtain a polymer mass content of 2% is high, so that the negative electrode slurry can be rolled into a film with a low content of water solvent, the solid content of the slurry is increased when the secondary battery negative electrode film layer is formed, the difficulty of solvent evaporation in the film layer is reduced, and the negative electrode slurry can be rolled into a film by the "dough kneading" method.

[0027] In any embodiment, the mass ratio of the water-based binder is 0.1% to 10%, optionally 1% to 3%, based on the total mass of the negative electrode film layer.

[0028] The binder in the secondary battery negative electrode film layer has a high molecular weight, can provide sufficient adhesion, and thus the amount of the binder can be reduced, which is conducive to further increasing the active material loading in the negative electrode film layer and further improving the capacity and energy density of the secondary battery.

[0029] In any embodiment, the negative electrode film layer further comprises a negative electrode active material, and the mass ratio of the negative electrode active material is 80% to 99%, optionally 95% to 99%, based on the total mass of the negative electrode film layer.

[0030] The secondary battery negative electrode film layer does not need a dispersing aid and the amount of the binder is reduced, which is conducive to further increasing the active material loading in the negative electrode film layer and further improving the capacity and energy density of the secondary battery.

[0031] In any embodiment, the single-sided surface density of the negative electrode film layer is 10 to 15 mg / cm 2 ; and / or the compaction density of the negative electrode film layer is 1.6 to 3.0 g / cm 3 .

[0032] The negative electrode film layer in the embodiments has a high compaction density, which is conducive to further improving the energy density of the battery.

[0033] In any embodiment, the OI value of the negative electrode film layer is 20 to 50; and the OI value of the negative electrode film layer is calculated according to the following formula:OI =C 004 / C 110 ,C 004 C is the peak area of the 004 characteristic diffraction peak in the X-ray diffraction pattern of the negative electrode film layer, C 110 C is the peak area of the 110 characteristic diffraction peak in the X-ray diffraction pattern of the negative electrode film layer.

[0034] 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.

[0035] In any embodiment, the bonding 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 100 N / m-200 N / m.

[0036] The secondary battery has high bonding strength, which is beneficial to further improving the cycle life of the battery.

[0037] The second aspect of the present application provides a preparation method of a negative electrode sheet, comprising: obtaining a negative electrode slurry, and rolling the negative electrode slurry into a film and then compositing on a negative electrode current collector; and drying to form a negative electrode film layer arranged on the surface of the negative electrode current collector; the negative electrode slurry comprises a water-based binder, the water-based binder comprises a polymer with a weight average molecular weight greater than 1 million, and the polymer comprises a thiourea group.

[0038] The preparation method can improve the coating effect of the binder on the surface of the negative electrode active material through the rolling forming mode, reduce lithium consumption, improve the binding effect of the three-dimensional network of the binder on the negative electrode active material during the cycle process, and comprehensively improve the initial efficiency and cycle stability of the secondary battery; the solid content of the negative electrode slurry can be improved, the water content that needs to be evaporated during the drying process of the slurry can be reduced, the drying stress can be effectively reduced, the cracking risk can be reduced, the process requirements of electrode sheets with different thicknesses can be met, the area density or the compaction density of the negative electrode film layer can be improved, and the upper limit of the energy density of the secondary battery in the prior art can be broken through.

[0039] In any embodiment, the solid content of the negative electrode slurry is 65%-90%, which can be 65%-80%.

[0040] The negative electrode slurry with the solid content in the above range can reduce the solvent content that needs to be evaporated, reduce the cracking and warping of the negative electrode film layer, and improve the forming quality of the negative electrode film layer; on the other hand, the slurry with high solid content can present a "dough-like" state, so that the rolling into a film of the negative electrode film layer can be realized.

[0041] In any embodiment, the polymer is an acrylic polymer, and the branched chains of the polymer include polyether thioureas.

[0042] A third aspect of the present application provides a power consuming device including the secondary battery of the first aspect or the secondary battery prepared from the negative electrode sheet prepared by the preparation method of the second aspect. BRIEF DESCRIPTION OF DRAWINGS

[0043] FIG. 1 is a schematic view of a secondary battery according to an embodiment of the present application;

[0044] FIG. 2 is an exploded view of the secondary battery according to an embodiment of the present application shown in FIG. 1;

[0045] FIG. 3 is a schematic view of a battery module according to an embodiment of the present application;

[0046] FIG. 4 is a schematic view of a battery pack according to an embodiment of the present application;

[0047] FIG. 5 is an exploded view of the battery pack according to an embodiment of the present application shown in FIG. 4;

[0048] FIG. 6 is a schematic view of a power consuming device using the secondary battery according to an embodiment of the present application as a power source.

[0049] EXPLANATION OF REFERENCE NUMERALS

[0050] 1: battery pack; 2: upper case; 3: lower case; 4: battery module; 5: secondary battery; 51: case; 52: electrode assembly; 53: cover plate. DETAILED DESCRIPTION

[0051] Hereinafter, embodiments of the secondary battery, the preparation method of the negative electrode sheet, and the power consuming device of the present application are specifically disclosed with appropriate reference to the accompanying drawings. However, there can be cases where unnecessary detailed description is omitted. For example, there can be cases where detailed description of matters well known in the art, repeated description of substantially the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the accompanying drawings and the following description are provided so that those skilled in the art can fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0052] The ranges disclosed herein are intended to be "open" ranges, i.e., the upper and lower limits of the range are not included. The ranges are also intended to include any and all sub-ranges of the range, i.e., all combinations of any two of the range limits, unless otherwise indicated. For example, a range of "1 to 10" is intended to include any number from 1 to 10, including the integers 1 and 10. Unless otherwise indicated, the use of "or" in the disclosed aspects herein is the inclusive, and not the exclusive use. Only the context, and not the number of times an item is used, can determine that it is the exclusive use. For example, the phrase "A uses B or C" means that A can use B, or A can use C, or A can use both B and C. Also, the use of the term "one" or "a" or "the" is intended to be singular as well as plural, unless only the singular form is used. For example, the phrase "one or more of A, B, and C" means that A, B, or C can be present, and that one of A, B, and C can be present, and that two of A, B, and C can be present, and that all of A, B, and C can be present.

[0053] All embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions, unless otherwise specified.

[0054] All technical features and optional technical features of the present application can be combined with each other to form new technical solutions, unless otherwise specified.

[0055] Unless otherwise specified, all steps of the present application can be performed in sequence or randomly, and preferably in sequence. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, the method also comprises step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0056] Unless otherwise specified, the "includes" and "contains" mentioned in the present application are open-ended, and can also be closed. For example, the "includes" and "contains" can mean that other components not listed can also be included or contained, or only the listed components can be included or contained.

[0057] If not specifically stated, the term "or" in this 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 exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0058] It is a common pursuit of those skilled in the art to further improve the initial efficiency and cycle stability of secondary batteries.

[0059] [Secondary battery]

[0060] Based on this, 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 disposed 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, the polymer comprising a thioureylene group.

[0061] 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.

[0062] In this context, the term "polymer" includes, on the one hand, a collection of macromolecules that are chemically uniform but differ in degree of polymerization, molar mass and chain length, which are prepared by polymerization reactions. On the other hand, the term also includes derivatives of such a collection of macromolecules formed by polymerization reactions, i.e. products that can be obtained by reaction, such as addition or substitution, of functional groups in the above-mentioned macromolecules and can be chemically uniform or chemically non-uniform.

[0063] In this context, the term "weight average molecular weight" refers to the sum of the products of the weight fraction of molecules with different molecular weights in a polymer and their corresponding molecular weights.

[0064] In this context, the dispersion medium of the water-based binder is an aqueous solvent. As an example, the aqueous solvent includes water and mixtures thereof. Examples of the water-based binder include, but are not limited to, polyacrylic polymers, emulsion-based polymers (such as styrene-acrylic emulsion SAE, styrene-butadiene emulsion SBR), polyacrylate polymers.

[0065] In some embodiments, the binder is used as a negative electrode binder to bind the negative electrode active material and / or the conductive agent to form an electrode.

[0066] In the present application, the weight average molecular weight of the polymer can be tested by any known method in the art, for example, by using a gel chromatography, such as a Waters 2695 Isocratic HPLC type gel chromatograph (differential refractive detector 2141). Specifically, the testing 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 left for one day for standby. During testing, tetrahydrofuran is first sucked into a syringe for flushing, and the operation is repeated several times. Then 5ml of the 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.

[0067] 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.

[0068] With the high molecular weight of the polymer, the negative electrode film layer can be rolled into a film, and the surface defects of the negative electrode active material are continuously covered during the film forming process and subsequent cycles, effectively improving the powder dropping problem during the film forming process, reducing the lithium consumption during the secondary battery cycle, improving the initial efficiency and cycle life of the battery. Moreover, the thiourea groups (-NH-C(=S)-NH-) on the polymer can form a large number of hydrogen bonds between the polymers, so that the binder forms a three-dimensional network structure wrapped around the material surface. Even if the hydrogen bonds are destroyed under external force, the network structure can be restored after the stress is released, inhibiting the volume change of the negative electrode active material during the cycle, further improving the cycle stability of the secondary battery while improving the rollable molding capability of the film layer.

[0069] In some embodiments, the weight average molecular weight of the polymer is 1.2 million to 4 million, and can be 2 million to 4 million.

[0070] The polymer with a weight average molecular weight in the above range can further improve the initial efficiency and cycle stability of the secondary battery.

[0071] In some embodiments, the polymer is an acrylic polymer, and the branched chain of the polymer comprises a polyether thiourea.

[0072] Acrylic polymer refers to a polymer obtained by homopolymerization or copolymerization of acrylic acid molecules and derivatives thereof. The polyether thiourea is grafted onto the acrylic polymer through an amidation reaction, and the thiourea groups on the polyether thiourea form a hydrogen bond array, which enables the polymer to have excellent self-healing properties. During the rolling process and the cyclic expansion process, the hydrogen bonds can be broken and rearranged, and the three-dimensional network formed by the adhesive can continuously form an effective coating on the material in the film layer, thereby inhibiting the volume expansion of the pole piece, reducing the probability of exposure of defect sites of the negative active material and rupture of the solid electrolyte interface film (SEI), and improving the initial efficiency and cycle stability of the battery. Although there are a large number of thiourea groups capable of forming hydrogen bonds in the polyether thiourea, the flexibility of the polymer can be improved due to the flexibility of the ether chain, thereby improving the ductility and processability of the film layer during the rolling process.

[0073] In some embodiments, the polymer comprises a structural unit represented by Formula I,

[0074] wherein R1-R3 each independently comprises H, one or more of C 1-3 alkyl; Q comprises one or more of C 1-3 alkylene; R4 comprises one or more of C 1-4 alkoxy, n is any integer from 1-4, and x is any integer.

[0075] In this document, the term "C 1-3 alkyl" refers to an alkyl group containing 1-3 C. As examples, but not limitation, include methyl, ethyl, propyl.

[0076] In this document, the term "C 1-3 alkylene" refers to an alkylene group containing 1-3 C. As examples, but not limitation, include methylene, ethylene, propylene.

[0077] In this document, the term "C 1-4 alkoxy" generally consists of an alkyl or alkylene group containing 1-4 C and an oxygen atom. As examples, but not limitation, include methoxy (-CH2O-), ethoxy (-C2H4O-), propoxy (-C3H6O-), butoxy (-C4H8O-).

[0078] In some embodiments, n can be selected as 1, 2, 3, or 4.

[0079] In some embodiments, x can be selected as 100, 500, 1000, 2000, 3000, 4000, 5000, or any numerical range between any two of them.

[0080] In some embodiments, Q comprises propylene, R4 comprises ethoxy, n is 2, and x is any integer from 1-5000.

[0081] In some embodiments, the polymer further comprises at least one structural unit of Formula II.

[0082] wherein R5-R7 each independently comprises H, one or more of C 1-3 alkyl; and R8 comprises one or more of a carboxyl group, a lithium carboxylate group, a sodium carboxylate group, an amide group, a hydroxyl group, a cyano group, an ester group.

[0083] The structural unit of Formula II comprises 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, thereby improving the cycle stability of the secondary battery. Moreover, the polar groups on the surface of the polymer can form strong surface adsorption to the negative active material, thereby reducing the probability of powder falling due to insufficient adhesion during the process of forming the negative electrode film layer, and improving the initial efficiency of the secondary battery.

[0084] In some embodiments, the molar proportion of the structural unit of Formula I is 1%-10%, optionally 4%-10%, based on the total number of moles of structural units in the polymer.

[0085] In some embodiments, the molar proportion of the structural unit of Formula I is 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any numerical range between any two of them, based on the total number of moles of structural units in the polymer.

[0086] The polymer with the molar proportion of the structural unit of Formula I in the above range has good self-healing property and processability of roll forming into a film, thereby comprehensively improving the initial efficiency and cycle stability of the secondary battery.

[0087] In some embodiments, the polymer comprises the structural unit of Formula II containing a cyano group, the structural unit of Formula II containing a carboxyl group or a hydroxyl group, and the structural unit of Formula II containing an ester group.

[0088] The structural unit of Formula II containing a cyano group can improve the polarity of the polymer and increase the adhesion strength between the film layer and the current collector; the structural unit of Formula II containing a carboxyl group or a hydroxyl group can improve the compatibility of the polymer with the aqueous solvent, so that the aqueous solvent can play the role of a lubricant during the process of forming the film layer, thereby improving the processability of roll forming the film layer; and the structural unit of Formula II containing an ester group can improve the flexibility of the polymer and increase the ductility and processability of roll forming the film layer.

[0089] In some embodiments, the molar percentage of the structural unit of formula II containing a cyano group is 5% to 80%, optionally 7% to 30%, based on the total moles of structural units in the polymer; and / or the molar percentage of the structural unit of formula II containing a carboxyl group or a hydroxyl group is 10% to 80%, optionally 30% to 75%, based on the total moles of structural units in the polymer; and / or the molar percentage of the structural unit of formula II containing an ester group is 1% to 50%, based on the total moles of structural units in the polymer.

[0090] In some embodiments, the molar percentage of the structural unit of formula II containing a cyano group is 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 structural units in the polymer.

[0091] In some embodiments, the molar percentage of the structural unit of formula II containing a carboxyl group or a hydroxyl group is 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 structural units in the polymer.

[0092] In some embodiments, the molar percentage of the structural unit of formula II containing an ester group is 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or any numerical range between any two of the aforementioned values.

[0093] The polymer with a molar content within the aforementioned range, based on the total moles of structural units in the polymer, can provide sufficient adhesion, strength, and ductility to enable high solid content negative electrode slurry to be rolled into a film, and can reduce the thickness of the film layer while increasing the solid content of the slurry during the formation of the negative electrode film layer of the secondary battery, thereby solving the problems of cracking, warping, and powdering of the negative electrode film layer during processing, improving the processability of the negative electrode film layer, and improving the initial efficiency and cycle stability of the secondary battery.

[0094] In some embodiments, the structural unit of formula II is derived from one or more of acrylonitrile, methacrylonitrile, methacrylamide, acrylamide, acrylic sulfonic acid, sodium acrylsulfonate, lithium acrylsulfonate, sodium acrylate, lithium acrylate, acrylic acid, propylene alcohol, ethylene glycol dimethacrylate, methyl methacrylate, ethyl methacrylate, methyl acrylate, hexyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate.

[0095] In some embodiments, the polymer comprises one or more of poly(acrylic acid-methyl acrylate-acrylonitrile)-graft-polyether sulfenamide, poly(acrylic acid-ethyl acrylate-acrylonitrile)-graft-polyether sulfenamide, poly(acrylic acid-methyl methacrylate-acrylonitrile)-graft-polyether sulfenamide, poly(acrylic acid-acrylonitrile)-graft-polyether sulfenamide, polyacrylic acid-graft-polyether sulfenamide.

[0096] In some embodiments, the polymer has a viscosity of 4000 mPa s to 10000 mPa s, optionally 5000 mPa s to 10000 mPa s, in a 2% polymer mass content polymer solution in water.

[0097] The viscosity of the polymer solution can be tested by any known method in the art. As an example, 50 g of polymer and water solvent are weighed in a 500 ml beaker to prepare a 2% mass fraction polymer solution, which is dispersed by stirring at a speed of 800 r / min for 120 min and then ultrasonic oscillation for 30 min to remove air bubbles. The viscosity is tested at room temperature using a Leybold NDJ-5S rotary viscometer, with a No. 3 rotor inserted into the polymer solution, the rotor liquid level mark and the polymer solution level being aligned, and the rotor rotating at a speed of 12 r / min. The viscosity data is read after 6 min.

[0098] In some embodiments, the viscosity of the polymer solution is optionally 4000 mPa s, 5000 mPa s, 6000 mPa s, 7000 mPa s, 7000 mPa s, 8000 mPa s, 9000 mPa s, 10000 mPa s, or any numerical range between any two of them, in a 2% polymer mass content polymer solution in water.

[0099] The high viscosity of the polymer solution in water allows the negative electrode slurry to be rolled into a film with a low content of water solvent, which increases the solid content of the slurry during the formation of the negative electrode film of the secondary battery, reduces the difficulty of solvent evaporation in the film, and allows the negative electrode slurry to be rolled into a film by the "dough kneading" method.

[0100] In some embodiments, the water-based binder has a mass fraction of 0.1% to 10%, optionally 1% to 3%, based on the total mass of the negative electrode film.

[0101] In some embodiments, the water-based binder has a mass fraction of 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any numerical range between any two of them, based on the total mass of the negative electrode film.

[0102] The binder in the negative electrode film layer has a high molecular weight, which can provide sufficient adhesion, thereby reducing the amount of the binder and facilitating further improvement of the active material loading in the negative electrode film layer and further improvement of the capacity and energy density of the secondary battery.

[0103] In some embodiments, the negative electrode film layer further comprises a negative electrode active material. The negative electrode active material can be any negative electrode active material known in the art for use in 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, and the like. 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. The negative electrode active material can be used alone or in combination of two or more.

[0104] In some embodiments, the mass fraction of the negative electrode active material is 80%-99%, and optionally 95%-99%, based on the total mass of the negative electrode film layer.

[0105] In some embodiments, the mass fraction of the negative electrode active material is 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, or any numerical range between any two of them, based on the total mass of the negative electrode film layer.

[0106] The secondary battery negative electrode film layer does not require a dispersing aid and the amount of the binder is reduced, which facilitates further improvement of the active material loading in the negative electrode film layer and further improvement of the capacity and energy density of the secondary battery.

[0107] In some embodiments, the negative electrode film layer can further optionally comprise a conductive agent.

[0108] 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.

[0109] In some embodiments, the mass fraction of the conductive agent is 0.1%-2%, based on the total mass of the negative electrode film layer.

[0110] In some embodiments, the mass fraction of the conductive agent is 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.

[0111] In some embodiments, the single-sided surface density of the negative electrode film layer is 10 mg / cm 2 -15 mg / cm2 .

[0112] In some embodiments, the single-sided area density of the negative electrode film layer can be selected from 10 mg / cm 2 , 11 mg / cm 2 , 12 mg / cm 2 , 13 mg / cm 2 , 14 mg / cm 2 , 15 mg / cm 2 , or any numerical range between any two of them.

[0113] In the present application, the area density of the negative electrode film layer is the meaning known in the art, which can be tested by the 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.

[0114] In some embodiments, the compaction density of the negative electrode film layer is 1.6-3.0 g / cm 3 .

[0115] In some embodiments, the compaction density of the negative electrode film layer can be selected from 1.6 g / cm 3 , 1.8 g / cm 3 , 2 g / cm 3 , 2.2 g / cm 3 , 2.4 g / cm 3 , 2.6 g / cm 3 , 2.8 g / cm 3 , 3.0 g / cm 3 .

[0116] In the present application, the compaction density of the negative electrode film layer is the meaning known in the art, which can be tested by the 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 known in the art, which can be tested by the methods known in the art, for example, by using a micrometer (e.g., Mitutoyo 293-100, accuracy of 0.1 μm).

[0117] The negative electrode film layer in the embodiments of the present application has a high compaction density, which is beneficial to further improve the energy density of the battery.

[0118] 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 C OI = C 004 / C 110 , C 004C is a peak area of a 004 characteristic diffraction peak in an X-ray diffraction pattern of the negative electrode film layer. 110 C is a peak area of a 004 characteristic diffraction peak in an X-ray diffraction pattern of the negative electrode film layer.

[0119] 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 1.5406 A, the scanning 2θ angle range is 20°-80°, and the scanning rate is 4° / min. 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 1.5406 A, the scanning 2θ angle range is 20°-80°, and the scanning rate is 4° / min.

[0120] 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 them.

[0121] 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 sheet wrinkling during full charging, and being beneficial to the improvement of the cycle stability of the secondary battery.

[0122] 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.

[0123] 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 them.

[0124] 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 test 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 a 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.

[0125] The secondary battery has high adhesion strength, which is beneficial to further improving the cycle life of the battery.

[0126] 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 layer. 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 layer (such as a base layer of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0127] The second aspect of the present application provides a method for preparing a negative electrode tab, which includes: obtaining a negative electrode slurry, and rolling the negative electrode slurry into a film and then compounding it on a negative electrode current collector; and 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, the water-based binder includes a polymer with a weight average molecular weight greater than 1 million, and the polymer includes a thiourea group.

[0128] In some embodiments, the rolling into a film includes multi-stage rolling, and the film is compounded on the negative electrode current collector after rolling.

[0129] Different from the technical solution in the prior art that the slurry is dried into a film and then cold-pressed to prepare a film layer, the preparation method can improve the coating effect of the binder on the surface of the negative active material through roll forming, reduce lithium consumption, improve the binding effect of the three-dimensional network of the binder on the negative active material during the cycle process, and comprehensively improve the initial efficiency and cycle stability of the secondary battery; the preparation method can also improve 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, reduce the risk of cracking, meet the process requirements of negative electrode sheets of different thicknesses, improve the area density or the compaction density of the negative electrode film layer, and break through the upper limit of the energy density of the secondary battery in the prior art.

[0130] In some embodiments, the solid content of the negative electrode slurry is 65%-90%, and can be 65%-80%.

[0131] In some embodiments, the solid content of the negative electrode slurry can be 65%, 70%, 75%, 80%, 85%, 90%, or any numerical range between any two of them.

[0132] The negative electrode slurry with the solid content in the above range can reduce the solvent content that needs to be evaporated on the one hand, reduce the cracking and warping of the negative electrode film layer, and improve the forming quality of the negative electrode film layer; on the other hand, the slurry with high solid content can present a "dough-like" state, so that the roll forming of the negative electrode film layer can be realized.

[0133] The third aspect of the present application provides a preparation method of an aqueous binder, which comprises the following steps: under polymerizable conditions, polymerizing an acrylic monomer to prepare a first product; and grafting the first product with a polyether thiourea to obtain a polymer, wherein the weight average molecular weight of the polymer is greater than 1 million.

[0134] In some embodiments, the polymerization reaction comprises the following steps: reacting raw materials including an acrylic monomer under the initiation of an initiator at a reaction pressure of 6-8 MPa, a reaction temperature of room temperature, and in a non-reactive gas atmosphere for 0.5-1 hour; adding a chain extender, and after reacting for 0.3-1 hour, increasing the temperature to 50-80°C, dropping the monomer, and continuing to react for 0.2-2 hours, adding a chain transfer agent, and adding a pH adjuster to adjust the pH value to 6.2-8.0 to obtain the polymer.

[0135] In some embodiments, the initiator includes a persulfate initiator, which can be one or more of potassium persulfate, sodium persulfate, and ammonium persulfate.

[0136] In some embodiments, the chain transfer agent includes one or more of cyclohexane, isopropyl alcohol, methanol, and acetone.

[0137] In some embodiments, the chain extender includes butadiene.

[0138] 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 selected from 0.25%, 0.5%, 0.75%, or any numerical range between any two of them.

[0139] In some embodiments, the pH adjuster includes one or more of LiOH-H2O, NaOH-H2O, lithium carbonate, sodium carbonate, sodium bicarbonate.

[0140] In some embodiments, the reaction temperature of the grafting reaction is 80°C to 150°C, and the reaction time is 2h to 15h.

[0141] 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.

[0142] [Positive electrode sheet]

[0143] 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.

[0144] As an example, the positive electrode current collector has two surfaces opposite in the thickness direction thereof, and the positive electrode film layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector.

[0145] 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 high molecular material base layer and a metal layer formed on at least one surface of the high molecular 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, silver alloy, etc.) on a high molecular material base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0146] In some embodiments, the positive electrode active material can employ a positive electrode active material for a battery known in the art. As an example, the positive electrode 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 electrode active material for a battery can also be used. These positive electrode 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.

[0147] 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.

[0148] 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.

[0149] [Electrolyte]

[0150] 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.

[0151] In some embodiments, the electrolyte is an electrolytic solution. The electrolytic solution includes an electrolyte salt and a solvent.

[0152] 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.

[0153] 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.

[0154] In some embodiments, the electrolytic solution can optionally further 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.

[0155] 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.

[0156] 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.

[0157] 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.

[0158] 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.

[0159] 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.

[0160] 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.

[0161] 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.

[0162] 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.

[0163] 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.

[0164] 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.

[0165] As the power utilization device, the secondary battery, the battery module, or the battery pack can be selected according to the use requirement thereof.

[0166] 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, the battery pack or the battery module can be used.

[0167] The device as another example can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thinning, and the secondary battery can be used as a power supply.

[0168] Embodiment

[0169] 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.

[0170] I. Preparation method

[0171] Embodiment 1

[0172] 1) Preparation of the binder

[0173] The acrylic acid, methyl acrylate and acrylonitrile 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 a certain amount 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 the first product poly(acrylic acid-methyl acrylate-acrylonitrile) was obtained, wherein the acrylic acid:methyl acrylate:acrylonitrile = 3:3:1. It can be understood that part of the structural units derived from acrylic acid are converted into structural units derived from lithium acrylate.

[0174] Polyether thiourea was added to the first product, heated to 120°C, and reacted for 4h to introduce side chains through amidation reaction. The reaction scheme of the first product with polyether thiourea is shown in the following figure. After the reaction, the water-based adhesive polymer was obtained, and the mass content of the polymer was 2 million, wherein Q is propylene, R4 is -O-CH2-CH2-, and n is 2. The weight average molecular weight of the polyether thiourea added is 30,000.

[0175] 2) Preparation of negative electrode sheet

[0176] 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 water-based adhesive prepared above, and stir with deionized water for 0.5 hours to obtain a negative electrode slurry. The solid content of the slurry is 66%.

[0177] 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.

[0178] The single-sided surface density of the negative electrode sheet is 10.1 mg / cm 2 , and the compacted density is 1.6 g / cm 3 .

[0179] 3) Preparation of positive electrode sheet

[0180] 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 97:2:1, they are coated on an Al foil, dried, and cold-pressed to obtain a positive electrode sheet.

[0181] The single-sided surface density of the positive electrode sheet is 21.8 mg / cm 2 , and the compacted density is 2.6 g / cm3 .

[0182] 4) separator

[0183] The polypropylene film is used as the separator.

[0184] 5) Preparation of electrolyte

[0185] In an argon atmosphere glove box (H2O <0.1 ppm, O2 <0.1 ppm), organic solvent ethylene carbonate (EC) / ethyl methyl carbonate (EMC) is mixed uniformly at a volume ratio of 3 / 7, LiPF6 lithium salt is dissolved in the organic solvent, and the mixture is stirred uniformly to prepare a 1M LiPF6 EC / EMC solution to obtain the electrolyte.

[0186] 6) Preparation of battery

[0187] The single-layer coated positive electrode sheet, the separator, and the double-layer coated negative electrode sheet are stacked in the order of "positive electrode sheet-separator-negative electrode sheet-separator-positive electrode sheet", the separator is placed between the positive and negative electrodes to play a separating role, the bare battery cell is placed in the outer package, the prepared electrolyte is injected and packaged, liquid injection, formation, and exhaust processes are performed, and a secondary battery is obtained.

[0188] Examples 2-6

[0189] The preparation method of Examples 2-6 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.

[0190] In Example 2, the amount of initiator is 0.75% of the total mass of monomers, and the weight average molecular weight of the polymer is 1.5 million.

[0191] 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.

[0192] 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.

[0193] 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.

[0194] In Example 6, the amount of initiator is 0.2% of the total mass of monomers, and the reaction time is increased from 20 min to 30 min after the addition of butadiene, and the weight average molecular weight of the polymer is 3.5 million.

[0195] Examples 7-8

[0196] The preparation method of Example 7-8 is basically the same as that of Example 1, except that the input mass of polyether thiourea is adjusted.

[0197] Example 9

[0198] The preparation method of Example 9 is basically the same as that of Example 1, except that the types and molar ratios of monomers input in the polymer preparation process are adjusted, and the input ratio of acrylonitrile to acrylic acid is 2:5.

[0199] Comparative Example 1

[0200] The negative electrode binder component in Comparative Example 1 is the same as that in Example 1, and the weight average molecular weight is 700,000. The preparation method of the negative electrode sheet in Comparative Example 1 is as follows:

[0201] Take 92 parts of artificial graphite, 1 part of Super P, and add it to the stirring tank, mix for 30 min, then add 4 parts of butadiene styrene emulsion SBR, 2 parts of carboxymethyl cellulose sodium CMC-Na, 1 part of negative electrode binder, and deionized water is stirred for 1 hour to obtain a negative electrode slurry, and the solid content of the slurry is 55%.

[0202] The slurry is coated on the composite current collector by fixed-width slot extrusion, and dried at 90°C for 5 minutes to obtain a negative electrode sheet. The single-sided surface density of the negative electrode sheet is 10 mg / cm 2 .

[0203] Comparative Example 2

[0204] The negative electrode binder in Comparative Example 2 is the same as that in Comparative Example 1. The preparation method of the negative electrode sheet in Comparative Example 2 is as follows:

[0205] Take 96 parts of artificial graphite, 1 part of Super P, and add it to the stirring tank, mix for 30 min, then 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 66%.

[0206] 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. The single-sided design surface density of the negative electrode sheet is 10 mg / cm 2 .

[0207] Table 1

[0208] II. Performance Test

[0209] 1. First Effect Test Method

[0210] 25℃, the battery is charged to 3.65V at 1 / 3C constant current, then charged to 0.05C at 3.65V constant voltage, and then discharged to 2.0V at 1 / 3C. The ratio of the first discharge capacity to the first charge capacity is taken as the initial efficiency of the battery.

[0211] 2. Cycle stability test method

[0212] The battery capacity retention test process is as follows: at 25℃, the battery is charged to 3.65V at 1 / 3C constant current, then charged to 0.05C at 3.65V constant voltage, and then discharged to 2.3V at 1 / 3C. The obtained capacity is recorded as the initial capacity Co. The above steps are repeated for the same battery, and the discharge capacity Cn of the battery after the nth cycle is recorded. The battery capacity retention Pn after each cycle is (Cn / C0) x 100%, and the cycle number when the cycle capacity retention decays to 80% is taken as the cycle number.

[0213] III. Analysis of test results of each embodiment and comparative example

[0214] The batteries of each embodiment and comparative example are prepared according to the above method, and each performance parameter is measured. The results are shown in Table 2.

[0215] Table 2

[0216] The OI value of the negative electrode film layer in Example 1 is 25; the adhesion strength between the negative electrode film layer and the negative electrode current collector is 117N / m, and the adhesion strength between the negative electrode film layer and the negative electrode current collector in the example is all greater than 50N / m. The OI value of the negative electrode film layer in Comparative Example 1 is 12, and the adhesion strength between the negative electrode film layer and the negative electrode current collector is 29N / m. The adhesive 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.

[0217] As can be seen from the comparison of the example and the comparative example, the secondary battery includes a negative electrode sheet, the negative electrode sheet includes 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 includes a water-based binder, the water-based binder includes a polymer with a weight average molecular weight greater than 1 million, and the polymer includes a thiourea group, which can realize the simultaneous improvement of the initial efficiency and cycle stability of the secondary battery.

[0218] 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, and the polymer comprises a thioureylene group.

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 2 million to 4 million.

3. The secondary battery according to claim 1 or 2, characterized by The polymer is an acrylic polymer, and the branched chain of the polymer comprises a polyether thiourea.

4. The secondary battery according to any one of claims 1 to 3, characterized by, The polymer comprises structural units of the formula I, wherein R1-R3 each independently include H, C 1-3 one or more of alkyl; Q includes C 1-3 one or more of alkylene; R4 includes C 1-4 one or more of alkoxy, n is any integer from 1-4, and x is an integer.

5. The secondary battery according to claim 4, characterized by Q comprises a propylene group, R4 comprises an ethoxy group, n is 2, and x is any integer in a range of 1 to 5,000.

6. The secondary battery according to any one of claims 1 to 5, characterized by, The polymer further comprises at least one structural unit of formula II; wherein R5-R7 each independently include H, C 1-3 one or more of alkyl; R8 includes one or more of carboxyl, lithium carboxylate group, sodium carboxylate group, amide group, hydroxyl, cyano, ester group.

7. The secondary battery according to any one of claims 1 to 6, characterized by The molar percentage of the structural unit shown in Formula I is 1% to 10% based on the total moles of the structural units in the polymer, and is optionally 4% to 10%.

8. The secondary battery according to any one of claims 1 to 7, characterized by, The polymer comprises one or more of a structural unit shown in Formula II containing a cyano group, a structural unit shown in Formula II containing a carboxyl group or a hydroxyl group, and a structural unit shown in Formula II containing an ester group. The molar percentage of the structural unit shown in Formula II containing a cyano group is 5% to 80% based on the total moles of the structural units in the polymer, and is optionally 7% to 30%; and / or The molar percentage of the structural unit shown in Formula II containing a carboxyl group or a hydroxyl group is 10% to 80% based on the total moles of the structural units in the polymer; and is optionally 30% to 75%; and / or The molar percentage of the structural unit shown in Formula II containing an ester group is 1% to 50% based on the total moles of the structural units in the polymer.

9. The secondary battery according to any one of claims 1 to 8, characterized by, The structural unit shown in Formula II is derived from one or more of acrylonitrile, methacrylonitrile, methacrylamide, acrylamide, acrylic sulfonic acid, sodium acrylsulfonate, lithium acrylsulfonate, sodium acrylate, lithium acrylate, acrylic acid, acrylic alcohol, ethylene glycol dimethacrylate, methyl methacrylate, ethyl methacrylate, methyl acrylate, hexyl acrylate, hydroxyethyl acrylate, and hydroxypropyl acrylate.

10. The secondary battery according to any one of claims 1 to 9, characterized by The polymer comprises one or more of poly(acrylic acid-methyl acrylate-acrylonitrile)-graft-polyether thiourea, poly(acrylic acid-ethyl acrylate-acrylonitrile)-graft-polyether thiourea, poly(acrylic acid-methyl methacrylate-acrylonitrile)-graft-polyether thiourea, poly(acrylic acid-acrylonitrile)-graft-polyether thiourea, and polyacrylic acid-graft-polyether thiourea.

11. The secondary battery according to any one of claims 1 to 10, characterized by The polymer has a viscosity of 4,000 mPa·s to 10,000 mPa·s, and is optionally 5,000 mPa·s to 10,000 mPa·s, when the polymer is dissolved in water to obtain a polymer solution with a polymer mass content of 2%.

12. The secondary battery according to any one of claims 1 to 11, characterized by The water-based binder has a mass percentage of 0.1% to 10% based on the total mass of the negative electrode film layer, and is optionally 1% to 3%.

13. The secondary battery according to any one of claims 1 to 12, characterized by The negative electrode film layer further comprises a negative electrode active material, and the mass percentage of the negative electrode active material is 80% to 99% based on the total mass of the negative electrode film layer, and is optionally 95% to 99%.

14. The secondary battery according to any one of claims 1 to 13, characterized by, The negative electrode film layer further comprises a negative electrode active material, and the mass percentage of the negative electrode active material is 95% to 99% based on the total mass of the negative electrode film layer, and the mass percentage of the water-based binder is less than or equal to 3%.

15. The secondary battery according to any one of claims 1 to 14, characterized by The single-sided surface density of the negative electrode film layer is 10-15 mg / cm 2 ; and / or the compaction density of the negative electrode film layer is 1.6-3.0 g / cm 3 .

16. The secondary battery according to any one of claims 1 to 15, 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.

17. The secondary battery according to any one of claims 1 to 16, characterized by 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 100 N / m-200 N / m.

18. A method for producing a negative electrode sheet, characterized by Comprise: A negative electrode slurry is obtained, and the negative electrode slurry is rolled into a film and then combined with a negative electrode current collector; after drying, a negative electrode film layer arranged on the surface of the negative electrode current collector is formed; the negative electrode slurry comprises a water-based binder, the water-based binder comprises a polymer with a weight average molecular weight greater than 1 million, and the polymer comprises a thiourea group.

19. The method of claim 18, wherein, The solid content of the negative electrode slurry is 65%-90%, which can be 65%-80%.

20. The method of manufacturing according to claim 18 or 19, wherein, The polymer is an acrylic polymer, and the branched chain of the polymer comprises a polyether thiourea.

21. An electrical device, comprising: The electric device comprises the secondary battery of any one of claims 1-17 or the secondary battery comprising the negative electrode pole piece prepared by the preparation method of any one of claims 18-20.

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