A negative electrode sheet, a secondary battery, and an electric device

Abstract

The application discloses a negative pole piece, a secondary battery and an electric device, and belongs to the technical field of batteries. The application satisfies the following relationship by controlling the Dv50 particle size of the first negative pole active material and the second negative pole active material in the negative pole active material, the grading ratio (X) of the negative pole active material, the tensile strength of the negative pole piece and the content of the binder in the negative pole active material layer: 0.69 <= (A+B)*X / (N*C) <= 160. The adhesion and the conductivity of the negative pole piece can be effectively improved, the expansion and the shrinkage of the negative pole active material can be effectively inhibited, the stability of the negative pole active material is improved, the uniform distribution of the negative pole active material on the negative pole piece is realized, the migration and diffusion path of lithium ions in the negative pole active material is shortened, the impedance is reduced, and the cycle performance is improved.

Description

TECHNICAL FIELD

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

[0002] With the development of information industry and electronic technology, new requirements are put forward for battery power. Compared with other secondary battery systems, lithium ion batteries have the advantages of small self-discharge, high energy density, high working voltage, long cycle life and environmental friendliness, which meet the demand for green batteries and become a research hotspot in recent years.

[0003] Silicon-based negative electrode materials have attracted widespread attention of researchers due to their high specific capacity and high voltage platform. However, in the actual commercialization process, the problems such as consumption of electrolyte and loss of active lithium caused by volume expansion of silicon negative electrode during charging and discharging process will make the battery face problems such as poor cycle life, serious gas production during high temperature storage and increased impedance during use.

[0004] In view of this, the present application is proposed. SUMMARY

[0005] The purpose of the present application is to overcome the deficiencies in the prior art and provide a negative electrode sheet, a secondary battery and an electric device, which effectively improve the first coulomb efficiency of the secondary battery, reduce the impedance and improve the cycle performance.

[0006] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0007] To achieve the above-mentioned purpose, the first aspect of the present application provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer located on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer comprises a negative electrode active material and a first binder;

[0008] The negative electrode active material comprises a first negative electrode active material and a second negative electrode active material;

[0009] The negative electrode sheet satisfies 0.69≤(A+B)*X / (N*C)≤160, X=A / B;

[0010] A μm is the Dv50 particle size of the first negative electrode active material;

[0011] B μm is the Dv50 particle size of the second negative electrode active material;

[0012] N MPa is the tensile strength of the negative electrode sheet;

[0013] C % is the mass percentage content of the first binder in the negative electrode active material layer.

[0014] As an embodiment of the present application, at least one of the following (1) to (5) is satisfied:

[0015] (1) 0.4 ≤ A ≤ 8;

[0016] (2) 10 ≤ B ≤ 45;

[0017] (3) 0.01 ≤ X ≤ 0.8;

[0018] (4) 0.1 ≤ N ≤ 0.5;

[0019] (5) 0.05 ≤ C ≤ 10.

[0020] As an embodiment of the present application, the first binder includes at least one of polytetrafluoroethylene or a derivative thereof, polyvinylidene fluoride or a derivative thereof, carboxymethyl cellulose or a derivative thereof, nitrocellulose or a derivative thereof, styrene butadiene rubber or a derivative thereof, nitrile butadiene rubber or a derivative thereof, or polyacrylic acid or a derivative thereof.

[0021] As an embodiment of the present application, the first negative electrode active material has a mass percentage content of D in the negative electrode active material layer, 1% ≤ D ≤ 30%; and / or

[0022] the second negative electrode active material has a mass percentage content of E in the negative electrode active material layer, 60% ≤ E ≤ 98.9%.

[0023] As an embodiment of the present application, the negative electrode active material includes an inner core and a coating layer disposed on an outer surface of the inner core, the inner core includes a silicon-based material and graphite, and the coating layer includes a second binder and a conductive agent; a mass ratio of the silicon-based material, graphite, second binder, and conductive agent is (1-30):(69.4-95):(0.3-3):(0.1-1).

[0024] As an embodiment of the present application, the coating layer includes polyacrylic acid or a derivative thereof, styrene butadiene rubber or a derivative thereof, carboxymethyl cellulose or a derivative thereof, and a conductive agent; a mass ratio of the silicon-based material, graphite, polyacrylic acid or a derivative thereof, styrene butadiene rubber or a derivative thereof, carboxymethyl cellulose or a derivative thereof, and conductive agent is (1-30):(69.4-95):(0.1-1):(0.1-1):(0.1-1):(0.1-1).

[0025] As an embodiment of the present application, the silicon-based material includes at least one of a silicon-oxygen material and a silicon-carbon material.

[0026] As an embodiment of the present application, the conductive agent includes at least one of a carbon nanotube, carbon black, a carbon dot, graphene, and a carbon nanofiber.

[0027] As an embodiment of the present application, the second binder includes at least one of polytetrafluoroethylene or a derivative thereof, polyvinylidene fluoride or a derivative thereof, carboxymethyl cellulose or a derivative thereof, nitrocellulose or a derivative thereof, styrene butadiene rubber or a derivative thereof, nitrile butadiene rubber or a derivative thereof, polyacrylic acid or a derivative thereof.

[0028] A second aspect of the present application provides a secondary battery including the negative electrode tab described above.

[0029] A third aspect of the present application provides an electric device including the secondary battery described above.

[0030] The present application has the beneficial effect that: the present application can effectively improve the adhesion and conductivity of the negative electrode tab, effectively inhibit the expansion and shrinkage of the negative electrode active material, improve the ion conduction ability of the negative electrode tab, improve the dispersion effect of the binder, avoid the agglomeration phenomenon of the binder, improve the stability of the negative electrode active material, avoid the powdering phenomenon of the negative electrode tab, avoid the crushing of the negative electrode tab during the preparation of the negative electrode tab, improve the compaction density, promote the uniform distribution of the negative electrode active material on the negative electrode tab, shorten the migration and diffusion path of the active ion in the negative electrode active material, effectively improve the initial coulomb efficiency of the secondary battery, reduce the impedance, and improve the cycle performance. DETAILED DESCRIPTION

[0031] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of protection of the present application.

[0032] In the present application, the technical features described in an open manner include both the closed technical solutions consisting of the listed features and the open technical solutions containing the listed features.

[0033] In the present application, the numerical interval is involved, such as the above-mentioned numerical interval is considered to be continuous, and includes the minimum value and the maximum value of the range, and each value between the minimum value and the maximum value. Further, when the range is referred to as an integer, each integer between the minimum value and the maximum value of the range is included. In addition, when a plurality of ranges are provided to describe a feature or a characteristic, the ranges can be combined. In other words, unless otherwise indicated, all ranges disclosed herein should be understood to include any and all sub-ranges subsumed therein.

[0034] In the present application, the specific dispersion, stirring treatment method is not particularly limited.

[0035] The components and raw materials or instruments used in the embodiments and comparative examples of the present application are commercially available unless otherwise specified, and the components and raw materials used in each parallel experiment are the same.

[0036] In addition, the terms "first", "second" are only for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first", "second" can explicitly or implicitly include at least one of the features. In the description of the present application, the meaning of "a plurality of" is at least two, such as two, three, etc., unless otherwise specifically limited.

[0037] The inventors of the present application found that the existing dry negative electrode sheet is to mix the active material, the conductive agent and the adhesive agent into a fiberized powder, roll it into a self-supporting film, and then composite it with the current collector to form a dry electrode sheet. The adhesive agent reacts with lithium ions on the surface of the negative electrode to generate lithium fluoride, which weakens the adhesion effect and reduces the capacity of the secondary battery, seriously affecting the cycle stability of the battery. It is also difficult to inhibit the expansion of the silicon negative electrode, which leads to the collapse of the conductive network and the adhesion network of the electrode sheet during the charging and discharging process of the secondary battery cell due to the expansion of silicon, resulting in the pulverization of the electrode sheet and the failure of the secondary battery cell.

[0038] Therefore, based on the above problems, the embodiments of the present application provide a negative electrode sheet, which comprises a negative electrode current collector and a negative electrode active material layer located on at least one surface of the negative electrode current collector, the negative electrode active material layer comprising a negative electrode active material and a first adhesive agent.

[0039] The negative electrode active material comprises a first negative electrode active material and a second negative electrode active material.

[0040] The negative electrode sheet satisfies 0.69≤(A+B)*X / (N*C)≤160, for example, it can be 0.69, 0.7, 1, 2, 4, 5, 6, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160 or a range composed of any two of the above values, X=A / B.

[0041] A μm is the Dv50 particle size of the first negative active material;

[0042] B μm is the Dv50 particle size of the second negative active material;

[0043] N MPa is the tensile strength of the negative electrode tab;

[0044] C% is the mass percentage content of the first binder in the negative active material layer.

[0045] The inventors of the present application found that the performance of the negative electrode tab has a significant correlation with the Dv50 particle size of the negative active material, the grading ratio (X) of the negative active material, the tensile strength of the negative electrode tab, and the content of the binder in the negative active material layer. The present application controls the Dv50 particle size of the first negative active material and the second negative active material in the negative active material, the grading ratio (X) of the negative active material, the tensile strength of the negative electrode tab, and the content of the binder in the negative active material layer to satisfy the following relationship: 1.5≤(A+B)*X / (N*C)≤5, which can effectively improve the adhesion and conductivity of the negative electrode tab, effectively inhibit the expansion and shrinkage of the negative active material, improve the ion conductivity of the negative electrode tab, improve the dispersion effect of the binder, avoid agglomeration, reduce the amount of lithium fluoride generated on the surface of the negative electrode tab, improve the stability of the negative active material, avoid the phenomenon of tab powdering, promote the uniform distribution of the negative active material on the negative electrode tab, shorten the migration and diffusion path of the active ion in the negative active material, effectively improve the first coulomb efficiency of the secondary battery, reduce the impedance, and improve the cycle performance.

[0046] wherein the volume average particle size Dv50 of the first negative active material and the second negative active material is a meaning known in the art, which represents the particle size corresponding to the cumulative volume distribution percentage of 50%, and can be tested by instruments and methods known in the art, for example, a cross-section polisher (such as an IB-09010CP argon ion cross-section polisher of JEOL) is used to prepare a cross-section containing the first negative active material and the second negative active material; then an element distribution map in the cross-section is obtained by EDX or EDS element analysis combined with TEM or SEM (such as X-Max EDS of Oxford Instruments Group combined with Sigma-02-33 SEM of ZEISS) face scanning test; and the volume average particle size Dv50 of the first negative active material and the second negative active material is obtained according to the particle size distribution of the cross-section. More precisely, the volume average particle size Dv50 values of the negative active material at multiple (more than 3, such as 8, 10, 12, etc.) different positions on the cross-section can be tested, and the average value is taken as the volume average particle size Dv50 of the first negative active material and the second negative active material.

[0047] In one embodiment, 1.6≤(A+B)*X / (N*C)≤9.6, especially when (A+B)*X / (N*C) is controlled in this range, the impedance of the secondary battery can be further reduced, and the cycle performance of the secondary battery is improved.

[0048] In one embodiment, 0.4≤A≤8, for example, it can be 0.4, 0.5, 1, 2, 3, 4, 5, 6, 7, 8 or a range consisting of any two of the above values.

[0049] In one embodiment, 1≤A≤8.

[0050] In one embodiment, 10≤B≤45, for example, it can be 10, 12, 14, 15, 18, 20, 22, 24, 25, 26, 28, 30, 35, 40, 45 or a range consisting of any two of the above values.

[0051] In one embodiment, 10≤B≤30.

[0052] By controlling A and B in the above range, the wettability of the electrolyte to the negative active material can be effectively improved, and the electrolyte can be more fully contacted to form more active sites, and the volume expansion caused by lithium ion intercalation / deintercalation can be alleviated.

[0053] In one embodiment, 0.01≤X≤0.8, for example, it can be 0.01, 0.03, 0.033, 0.04, 0.05, 0.08, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 or a range consisting of any two of the above values, the particle porosity of the negative active material is maximized, the compaction density of the negative active material is effectively improved, the wettability of the electrolyte to the negative active material is improved, the interface side reaction is effectively reduced, the gas production of the secondary battery is reduced, the impedance of the secondary battery is effectively reduced, and the cycle performance of the secondary battery is improved.

[0054] In one embodiment, 0.033≤X≤0.8.

[0055] In one embodiment, 0.1≤N≤0.5, for example, it can be 0.1, 0.2, 0.3, 0.4, 0.5 or a range consisting of any two of the above values, by controlling the tensile strength of the negative electrode sheet in this range, the stability of the negative electrode sheet can be effectively improved, a good conductive network can be formed, the electronic transmission ability between the carbon-based material and the graphite is maintained, and the diffusion path of lithium ions is shortened, thereby effectively improving the cycle performance of the secondary battery.

[0056] The tensile strength of the negative electrode sheet is tested by a tensile tester.

[0057] Test method: speed 20 mm / min, interval 50 mm, sample width 4 mm, thickness P.

[0058] Calculation method: tensile strength: N = tensile force F / (0.004 * P).

[0059] In one of the embodiments, 0.1≤C≤10, for example, it can be 0.1, 0.2, 0.5, 0.8, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or a range consisting of any two of the above values. By controlling C in this range, the conductivity and processability of the negative electrode sheet can be effectively improved, the expansion and shrinkage of the negative active material can be effectively inhibited, and the cycle performance of the secondary battery can be effectively improved.

[0060] In one of the embodiments, the first binder includes at least one of polytetrafluoroethylene or a derivative thereof, polyvinylidene fluoride or a derivative thereof, carboxymethyl cellulose or a derivative thereof, nitrocellulose or a derivative thereof, butyl rubber or a derivative thereof, nitrile rubber or a derivative thereof, polyacrylic acid or a derivative thereof.

[0061] In one of the embodiments, the mass percentage content of the first negative active material in the negative active material layer is D, and 1%≤D≤30%. For example, it can be 1%, 2%, 3%, 5%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30% or a range consisting of any two of the above values.

[0062] In one of the embodiments, the mass percentage content of the second negative active material in the negative active material layer is E, and 60%≤E≤98.9%. For example, it can be 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98%, 98.9% or a range consisting of any two of the above values.

[0063] In one embodiment, the negative active material includes an inner core and a coating layer arranged on the outer surface of the inner core, the inner core includes a silicon-based material and graphite, and the coating layer includes a second binder and a conductive agent; the mass ratio of the silicon-based material, graphite, second binder, and conductive agent is (1-30):(69.4-95):(0.3-3):(0.1-1). By coating the outer surface of the negative active material with the coating layer containing the binder and the conductive agent, the addition of the conductive agent can avoid the agglomeration of the conductive agent and the first binder, avoid uneven dispersion, improve the dispersion of the conductive agent and the first binder in the negative electrode sheet, effectively improve the stability of the negative active material, improve the conductivity and adhesion of the negative active material, effectively avoid the expansion and shrinkage of the negative active material, and improve the cycle stability of the secondary battery.

[0064] In one embodiment, the coating layer includes polyacrylic acid or a derivative thereof, styrene butadiene rubber or a derivative thereof, carboxymethyl cellulose or a derivative thereof, and a conductive agent; the mass ratio of the silicon-based material, graphite, polyacrylic acid or a derivative thereof, styrene butadiene rubber or a derivative thereof, carboxymethyl cellulose or a derivative thereof, and conductive agent is (1-30):(69.4-95):(0.1-1):(0.1-1):(0.1-1):(0.1-1). In particular, when this coating layer is used, the polyacrylic acid or a derivative thereof, styrene butadiene rubber or a derivative thereof, carboxymethyl cellulose or a derivative thereof, and conductive agent coated on the surface of the negative active material can improve its conductivity and adhesion, reduce the amount of the first binder added during subsequent mixing with the first binder, and avoid the agglomeration of the first binder and the conductive agent. This not only improves the initial efficiency of the negative electrode sheet, but also avoids the agglomeration problem during the mixing process. Moreover, because the surface of the active material is coated with the rigid adhesives such as polyacrylic acid, styrene butadiene rubber, and carboxymethyl cellulose, the dry granules and the granules are in closer contact under the combined action of the first binder, the ion transmission distance is shortened, the charging time of the battery is shortened, and the expansion of silicon is inhibited, the problem of sheet pulverization is solved, and the cycle stability of the battery is improved.

[0065] In one embodiment, the silicon-based material includes at least one of a silicon-oxygen material and a silicon-carbon material.

[0066] In one embodiment, the silicon-based material includes at least one of silicon, silicon-carbon composite, and SiO.

[0067] In one embodiment, the conductive agent includes at least one of carbon nanotubes, carbon black, carbon dots, graphene, and carbon nanofibers.

[0068] In one embodiment, the second binder includes at least one of polytetrafluoroethylene or a derivative thereof, polyvinylidene fluoride or a derivative thereof, carboxymethyl cellulose or a derivative thereof, nitrocellulose or a derivative thereof, styrene butadiene rubber or a derivative thereof, nitrile butadiene rubber or a derivative thereof, polyacrylic acid or a derivative thereof.

[0069] In one embodiment, the method for preparing the negative active material includes the following steps:

[0070] The silicon negative active material is mixed with graphite to obtain a mixture, the mixture is mixed with the second binder and the conductive agent to obtain a precursor, and the precursor is mixed with water to obtain a slurry with a solid content of 20-60%, and the slurry is atomized and granulated at 150-200°C, and ball-milled and sieved to obtain the negative active material and the second negative active material (by controlling the pore size of the atomizer in the atomization and granulation step and the ball-milling and sieving step, negative active materials with different Dv50 can be obtained).

[0071] In one embodiment, the secondary battery further includes a positive electrode tab.

[0072] In one embodiment, the positive electrode tab includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, and the positive active material layer includes a positive active material.

[0073] In one embodiment, the positive active material can be any known positive active material for a secondary battery. As a non-limiting example, the positive active material can include lithium-containing phosphates, lithium transition metal oxides, and modified compounds of each thereof. However, the present application is not limited to these materials or substances, and other conventional materials or substances that can be used as a positive active material for a secondary battery can also be used. These positive active materials can be used alone or in combination of two or more. As a non-limiting example of the lithium transition metal oxide, one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and modified compounds thereof can be used.

[0074] In one embodiment, the type of the positive current collector is not particularly limited, and it can be any known material suitable for use as a positive current collector. In one embodiment, the positive current collector includes aluminum, stainless steel, nickel plating, titanium, tantalum, and other metal materials, and carbon cloth, carbon paper, and other carbon materials.

[0075] The form of the positive electrode current collector is not particularly limited. When the positive electrode current collector is a metal material, the form of the positive electrode current collector can be a metal foil, a metal cylinder, a metal belt roll, a metal plate, a metal foil, a metal plate mesh, a punched metal, a foamed metal, or the like. When the positive electrode current collector is a carbon material, the form of the positive electrode current collector can include, but is not limited to, a carbon plate, a carbon film, a carbon cylinder, or the like.

[0076] In one embodiment, the positive electrode active material layer further includes a positive electrode conductive agent and a positive electrode binder.

[0077] In one embodiment, the type of the positive electrode conductive agent is not limited and any known positive electrode conductive agent can be used.

[0078] In one embodiment, the positive electrode conductive agent includes at least one of a super carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, carbon nanofibers, or the like.

[0079] In one embodiment, the positive electrode binder is not limited.

[0080] In one embodiment, the positive electrode binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, a fluorine-containing acrylate resin, polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitrocellulose, styrene butadiene rubber, nitrile rubber, fluororubber, isoprene rubber, polybutadiene rubber, ethylene-propylene rubber, or the like.

[0081] In the secondary battery mentioned in the present application, a separator is generally provided between the positive electrode and the negative electrode in order to prevent a short circuit. The material and shape of the separator are not particularly limited as long as the effects of the present application are not significantly impaired.

[0082] In one embodiment, the separator includes a substance in a porous sheet or nonwoven fabric form having excellent liquid retention, or the like. The material of the resin or glass fiber separator includes, but is not limited to, polyolefin, aromatic polyamide, polytetrafluoroethylene, polyether sulfone, or the like.

[0083] In one embodiment, the polyolefin is polyethylene or polypropylene. In some embodiments, the polyolefin is polypropylene. The material of the above-mentioned separator can be used alone or in any combination.

[0084] In one embodiment, the secondary battery can include an outer package that can be used to package the above-mentioned electrode assembly and electrolyte.

[0085] In one embodiment, the electrolyte is not particularly limited in kind. The electrolyte includes an electrolyte salt and an organic solvent, and the specific kind of the electrolyte salt and the organic solvent is not particularly limited and can be selected as needed. The electrolyte can further include an additive, and the kind of the additive is not particularly limited and can be a film-forming additive for the positive electrode and / or the negative electrode or an additive capable of improving certain performance of the battery, such as an additive capable of improving high- or low-temperature performance of the battery.

[0086] In one embodiment, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, or the like. 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 the plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, or the like can be listed.

[0087] The shape of the secondary battery is not particularly limited in the present application, and the secondary battery can be cylindrical, square, or any other shape.

[0088] One embodiment of the present application provides a power consumption device including the above-described secondary battery.

[0089] For example, the power consumption device can include a mobile device (such as a mobile phone, a notebook computer, or the like), 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, or the like), an electric train, a ship and a satellite, an energy storage system, or the like, but is not limited thereto.

[0090] The following examples are provided to facilitate an understanding of the present application. The provision of the examples is not intended to limit the scope of the claims.

[0091] Example 1

[0092] A method for preparing a secondary battery includes the following steps:

[0093] (1) Preparation of a negative electrode active material:

[0094] 1. The silicon negative electrode material is mixed with graphite to obtain a mixture, and the mixture is mixed with polyacrylic acid, carboxymethyl cellulose, butadiene rubber, and carbon nanotubes to obtain a precursor. The precursor is mixed with water to prepare a slurry with a solid content of 40%, and the slurry is atomized and granulated at 180°C, and ball-milled and sieved to obtain a first negative electrode active material and a second negative electrode active material (by controlling the pore size of the atomizer for atomization and granulation and the ball-milling and sieving steps, the first negative electrode active material and the second negative electrode active material with different Dv50 can be obtained);

[0095] The mass ratio of the silicon negative electrode material, graphite, polyacrylic acid, carboxymethyl cellulose, butadiene rubber, and carbon nanotubes is 5:93:0.5:0.5:0.5:0.5.

[0096] (2) Preparation of the negative electrode tab: the first negative electrode active material, the second negative electrode active material, and polytetrafluoroethylene are placed in a stirring barrel in a mass ratio of 15:80:5, and stirring is performed according to the following stirring steps:

[0097] First, stirring is performed at 15°C, with a reverse rotation (stirring paddle) of 300 rpm and a forward rotation (barrel) of 45 rpm, for a time of 120 s;

[0098] Then, stirring is performed at 15°C, with a reverse rotation (stirring paddle) of 2500 rpm and a forward rotation (barrel) of 45 rpm, for a time of 120 s;

[0099] Then, stirring is performed at 65°C, with a reverse rotation (stirring paddle) of 5000 rpm and a forward rotation (barrel) of 45 rpm, for a time of 900 s;

[0100] The stirred material is made into a film sheet at 120°C by using a differential roller gap of 0.7 between the upper and lower rollers (the thickness of the film sheet can be controlled by adjusting the number of times of this step), and the thickness of the film sheet is controlled to be 100 μm;

[0101] The film sheet is compounded onto a 8 μm copper foil current collector by using a roller gap with the same speed between the upper and lower rollers at 120°C.

[0102] (3) Preparation of the positive electrode tab: the positive electrode active material lithium cobaltate is mixed with PVDF and carbon black SP (the mass ratio of lithium cobaltate, PVDF, and carbon black SP is 8:1:1) and NMP to prepare a positive electrode slurry with a solid content of 70%, and the positive electrode slurry is coated on a 9 um aluminum foil to form a positive electrode tab, wherein the area density is 19.48 mg / cm 2 .

[0103] (4) Separator: the base film is a 16 μm PE separator.

[0104] (5) Electrolyte: EC and DMC are mixed in a volume ratio of 1:1, and then lithium hexafluorophosphate is added in a glove box to prepare an electrolyte with a concentration of 1 mol / L.

[0105] (6) Assembly of the secondary battery: the positive electrode tab, the separator, and the negative electrode tab are stacked in order, with the separator between the positive and negative electrode tabs, and then wound, heat-pressed, and shaped, and the tabs are welded to obtain a bare cell, which is placed in an outer packaging aluminum plastic film and baked in an oven at 85±10°C for 24 h, and the electrolyte prepared above is injected into the dried battery, and then the battery is left to stand, formed, and divided to obtain a secondary battery.

[0106] The parameters of Example 1 are shown in Table 1.

[0107] Examples 2-5

[0108] Examples 2-5 differ from Example 1 in that the mass ratio of the first negative electrode active material, the second negative electrode active material, and polytetrafluoroethylene is changed, thereby changing the value of C.

[0109] In Example 2, the first negative electrode active material, the second negative electrode active material, and polytetrafluoroethylene are in a mass ratio of 15:84.9:0.1.

[0110] In Example 3, the first negative electrode active material, the second negative electrode active material, and polytetrafluoroethylene are in a mass ratio of 15:83:2.

[0111] In Example 4, the first negative electrode active material, the second negative electrode active material, and polytetrafluoroethylene are in a mass ratio of 15:77:8.

[0112] In Example 5, the first negative electrode active material, the second negative electrode active material, and polytetrafluoroethylene are in a mass ratio of 15:75:10.

[0113] Examples 6-13

[0114] Examples 6-13 differ from Example 1 in that the first negative electrode active material and the second negative electrode active material having different Dv50 are obtained by controlling the pore size of the atomizer for atomization granulation and the ball milling and sieving steps, thereby changing A and / or B.

[0115] Examples 14-17

[0116] Examples 14-17 differ from Example 1 in that the value of N is adjusted by changing the thickness of the membrane.

[0117] Examples 18-21

[0118] Examples 18-21 differ from Example 1 in that the mass ratio of the first negative electrode active material, the second negative electrode active material, and polytetrafluoroethylene is changed.

[0119] In Example 18, the first negative electrode active material, the second negative electrode active material, and polytetrafluoroethylene are in a mass ratio of 1:94:5.

[0120] In Example 19, the first negative electrode active material, the second negative electrode active material, and polytetrafluoroethylene are in a mass ratio of 10:85:5.

[0121] In Example 20, the first negative electrode active material, the second negative electrode active material, and polytetrafluoroethylene are in a mass ratio of 20:75:5.

[0122] The first negative active material, the second negative active material, and the polytetrafluoroethylene in Example 21 are in a mass ratio of 30:65:5.

[0123] Examples 22-28

[0124] Examples 22-26 differ from Example 1 in that the mass ratio of the silicon negative active material, graphite, polyacrylic acid, carboxymethyl cellulose, butadiene-styrene rubber, and carbon nanotube is changed.

[0125] The mass ratio of the silicon negative active material, graphite, polyacrylic acid, carboxymethyl cellulose, butadiene-styrene rubber, and carbon nanotube in Example 22 is 1:95:1:1:1:1.

[0126] The mass ratio of the silicon negative active material, graphite, polyacrylic acid, carboxymethyl cellulose, butadiene-styrene rubber, and carbon nanotube in Example 23 is 2:94.8:0.8:0.8:0.8:0.8.

[0127] The mass ratio of the silicon negative active material, graphite, polyacrylic acid, carboxymethyl cellulose, butadiene-styrene rubber, and carbon nanotube in Example 24 is 10:88.4:0.4:0.4:0.4:0.4.

[0128] The mass ratio of the silicon negative active material, graphite, polyacrylic acid, carboxymethyl cellulose, butadiene-styrene rubber, and carbon nanotube in Example 25 is 20:79.2:0.2:0.2:0.2:0.2.

[0129] The mass ratio of the silicon negative active material, graphite, polyacrylic acid, carboxymethyl cellulose, butadiene-styrene rubber, and carbon nanotube in Example 26 is 30:69.6:0.1:0.1:0.1:0.1.

[0130] The mass ratio of the silicon negative active material, graphite, polyacrylic acid, carboxymethyl cellulose, butadiene-styrene rubber, and carbon nanotube in Example 27 is 20:79.2:0.8:0.0:0.0:0.0

[0131] The mass ratio of the silicon negative active material, graphite, polyacrylic acid, carboxymethyl cellulose, butadiene-styrene rubber, and carbon nanotube in Example 28 is 20:79.2:0.0:0.8:0.0:0.0

[0132] The mass ratio of the silicon negative active material, graphite, polyacrylic acid, carboxymethyl cellulose, butadiene-styrene rubber, and carbon nanotube in Example 29 is 20:79.2:0.4:0.2:0.2:0, and the carbon nanotube, which is a conductive agent, is dry-stirred in the subsequent negative electrode sheet preparation step.

[0133] Comparative Example 1

[0134] Comparative Example 1 differs from Example 1 in that the first negative electrode active material and the second negative electrode active material with different Dv50 are obtained by controlling the pore size of the atomizer for atomization granulation and the ball milling and sieving steps, and A and / or B are changed.

[0135] Comparative Example 2

[0136] Comparative Example 2 differs from Example 1 in that the mass ratio of the first negative electrode active material, the second negative electrode active material and polytetrafluoroethylene is changed.

[0137] In Comparative Example 2, the mass ratio of the first negative electrode active material, the second negative electrode active material and polytetrafluoroethylene is 15:84.95:0.05.

[0138] Comparative Example 3

[0139] Comparative Example 3 differs from Example 1 in that Comparative Example 1 does not contain the first negative electrode active material.

[0140] In Comparative Example 3, the mass ratio of the second negative electrode active material and polytetrafluoroethylene is 95:5.

[0141] Comparative Example 4

[0142] Comparative Example 4 differs from Example 1 in that Comparative Example 4 does not contain the second negative electrode active material.

[0143] In Comparative Example 4, the mass ratio of the first negative electrode active material, the second negative electrode active material and polytetrafluoroethylene is 95:5.

[0144] Table 1

[0145]

[0146] Performance Test

[0147] Cycle Test Method & DCR Test Method: Environment: Oven configuration: temperature 25℃, specific steps as follows:

[0148] Step 1: 0.2C discharge to 3.0V, stand for 5min;

[0149] Step 2: 0.5C charge to 4.5V, cut-off rate 0.02C, stand for 5min;

[0150] Step 3: 1A constant current discharge, time 500ms, stand for 5min; Step 4: 0.2C discharge to 3.0V, stand for 5min;

[0151] Step 5: 0.5C charge to 4.5V, cut-off rate 0.02C, stand for 5min;

[0152] Step 6: 0.2C discharge to 3.0V, rest for 5min, record this discharge capacity as Cap0;

[0153] Step 7: 1.5C charge to 4.5V, cut-off ratio 0.02C, rest for 5min;

[0154] Step 8: 0.7C discharge to 3.0V, rest for 5min;

[0155] Step 9: 0.5C charge to 3.0V, cut-off ratio 0.02C, rest for 5min;

[0156] Step 10: 1A constant current discharge, time 500ms, rest for 5min, record DCR(1A-500ms);

[0157] Repeat steps 7-8 for 800 times, the capacity of each cycle is Cap1.

[0158] Calculate the capacity retention rate of each cycle Cap1 / Cap.

[0159] Table 2

[0160]

[0161]

[0162] As can be seen from Table 2, by controlling the Dv50 particle size of the first negative active material, the Dv50 particle size of the second negative active material, the grading ratio (X) of the negative active material, the tensile strength of the negative electrode plate and the content of the binder in the negative active material layer to satisfy the following relationship: 0.69≤(A+B)*X / (N*C)≤160, the adhesion and conductivity of the negative electrode plate can be effectively improved, the expansion and shrinkage of the negative active material can be effectively inhibited, the ion conduction ability of the negative electrode plate can be improved, the dispersion effect of the binder can be improved, the agglomeration phenomenon of the binder can be avoided, the amount of lithium fluoride generated on the surface of the negative electrode plate can be reduced, the stability of the negative active material can be improved, the powdering phenomenon of the negative electrode plate can be avoided, the negative active material can be prevented from being broken during the preparation of the negative electrode plate, the compaction density can be improved, the uniform distribution of the negative active material on the negative electrode plate can be promoted, the migration and diffusion path of the active ion in the negative active material can be shortened, the first coulombic efficiency of the secondary battery can be effectively improved, the impedance can be reduced, and the cycle performance can be improved.

[0163] As can be seen from Comparative Examples 2 and 12 and other examples, by controlling 1.6≤(A+B)*X / (N*C)≤9.6, the impedance of the secondary battery can be further reduced, and the cycle performance of the secondary battery can be improved.

[0164] As can be seen from Comparative Examples 1-5, by controlling 2≤C≤8, the cycle performance is further improved, and the impedance is reduced.

[0165] As can be seen from Comparative Example 1, Examples 6-8, by controlling 2≤A≤4, the cycle performance is further improved and the impedance is reduced.

[0166] As can be seen from Comparative Example 1, Examples 9-11, by controlling 20≤B≤25, the cycle performance is further improved and the impedance is reduced.

[0167] As can be seen from Comparative Example 1, Examples 14-17, by controlling 0.2≤N≤0.4, the cycle performance is further improved and the impedance is reduced.

[0168] As can be seen from Comparative Example 1, Examples 18-21, by controlling 10≤D≤20, 75≤E≤85, the cycle performance is further improved and the impedance is reduced.

[0169] As can be seen from Comparative Example 1, Examples 24-26, by controlling the mass ratio of the silicon-based material, graphite, second binder, and conductive agent as (2-20):(79.2-94.8):(0.6-2.4):(0.0.2-0.8), the cycle performance is further improved and the impedance is reduced.

[0170] As can be seen from Comparative Example 1, Examples 27-29, by controlling the mass ratio of the silicon-based material, graphite, second binder, and conductive agent as (1-30):(69.4-95):(0.3-3):(0.1-1), the cycle performance is further improved and the impedance is reduced.

[0171] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application and not to limit the protection scope of the present application. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present application.

Claims

1. A negative electrode sheet, characterized by, The negative electrode active material layer comprises a negative electrode active material and a first binder; The negative electrode active material comprises a first negative electrode active material and a second negative electrode active material; the negative electrode active material comprises an inner core and a coating layer arranged on the outer surface of the inner core, the inner core comprises a silicon-based material and graphite, and the coating layer comprises a second binder and a conductive agent; The negative electrode sheet satisfies: 0.69≤(A+B)*X / (N*C)≤160, X=A / B; 0.01≤X≤0.8; A μm is the Dv50 particle size of the first negative electrode active material; 0.4≤A≤8； B μm is the Dv50 particle size of the second negative electrode active material; 10≤B≤45; N MPa is the tensile strength of the negative electrode sheet; 0.1≤N≤0.5； C % is the mass percentage content of the first binder in the negative electrode active material layer; 0.05≤C≤10.

2. The negative electrode sheet according to claim 1, characterized by The first binder comprises at least one of polytetrafluoroethylene or a derivative thereof, polyvinylidene fluoride or a derivative thereof, carboxymethyl cellulose or a derivative thereof, nitrocellulose or a derivative thereof, styrene butadiene rubber or a derivative thereof, nitrile butadiene rubber or a derivative thereof, and polyacrylic acid or a derivative thereof.

3. The negative electrode sheet according to claim 1, characterized by The mass percentage content of the first negative electrode active material in the negative electrode active material layer is D, 1 %≤D≤30 %; and / or The mass percentage content of the second negative electrode active material in the negative electrode active material layer is E, 60 %≤E≤98.9 %.

4. The negative electrode sheet according to claim 1, wherein The mass ratio of the silicon-based material, graphite, second binder, and conductive agent is (1-30):(69.4-95):(0.3-3):(0.1-1).

5. The negative electrode sheet according to claim 4, characterized by The coating layer comprises polyacrylic acid or a derivative thereof, styrene butadiene rubber or a derivative thereof, carboxymethyl cellulose or a derivative thereof, and a conductive agent; the mass ratio of the silicon-based material, graphite, polyacrylic acid or a derivative thereof, styrene butadiene rubber or a derivative thereof, carboxymethyl cellulose or a derivative thereof, and conductive agent is (1-30):(69.4-95):(0.1-1):(0.1-1):(0.1-1):(0.1-1).

6. The negative electrode sheet according to claim 4, wherein The silicon-based material comprises at least one of silicon-oxygen material, silicon-carbon material, and nano-silicon.

7. The negative electrode sheet according to claim 4, wherein The conductive agent comprises at least one of carbon nanotube, carbon black, carbon dot, graphene, and carbon nanofiber.

8. The negative electrode sheet according to claim 4, wherein The second binder comprises at least one of polytetrafluoroethylene or a derivative thereof, polyvinylidene fluoride or a derivative thereof, carboxymethyl cellulose or a derivative thereof, nitrocellulose or a derivative thereof, styrene butadiene rubber or a derivative thereof, nitrile butadiene rubber or a derivative thereof, and polyacrylic acid or a derivative thereof.

9. A secondary battery characterized by comprising: The negative electrode sheet comprises any one of claims 1-8.

10. An electrical device, characterized by The secondary battery comprises claim 9.

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