Negative plate and battery

Abstract

The invention provides a negative electrode plate and a battery, the negative electrode plate comprises a negative electrode current collector and a negative electrode active layer arranged on at least one surface of the negative electrode current collector, the negative electrode active layer comprises a negative electrode active material, a first binder and a second binder, and the negative electrode active material comprises a silicon-based material and a graphite material; wherein the first binding agent comprises an acrylonitrile-acrylic acid copolymer, and the second binding agent comprises an acrylic ester monomer. According to the negative plate provided by the invention, under the synergistic effect of the first binder and the second binder, the expansion of the silicon-based material can be effectively inhibited, and the cycle, fast charge and safety performance of the battery can be improved under the condition that the energy density is not lost.

Description

Technical Field

[0001] The invention belongs to the technical field of batteries and relates to a negative electrode sheet and a battery. Background Art

[0002] As an important component of sustainable energy, battery technology has greatly promoted the sustainable development of society and the convenience and intelligence of human life. Among them, lithium-ion batteries have the advantages of high energy density and long cycle life, and have been widely used in mobile phones, laptops, power tools, new energy vehicles and energy storage. According to market research, mileage anxiety, service life, charging speed, and safety issues greatly affect the user experience of new energy vehicles, which shows that power batteries are in urgent need of targeted technological innovation. Therefore, how to invent a battery that takes into account high energy density, long cycle life, fast charging performance and high safety has become a research hotspot in the industry.

[0003] Research has found that the use of high-gram capacity silicon-based negative electrode materials is beneficial to improving battery energy density, but due to its special alloying lithium insertion mechanism, the silicon negative electrode will undergo severe volume expansion during the charging process (up to about 300%). High volume expansion will expose more electrochemically active sites, aggravate the vicious cycle of SEI film destruction-recombination-destruction, hinder electron / ion transmission, increase negative electrode polarization, and deteriorate the battery's fast charging capability and cycle stability. Under high-rate charging conditions, aging cells are prone to lithium precipitation, and the cell temperature continues to rise. When heat continues to accumulate and cannot be released, it will cause safety accidents, greatly affecting the safety performance of the battery.

[0004] How to improve the cycle, fast charging and safety performance of batteries without losing energy density has become a technical problem that needs to be urgently solved by technical personnel in this field. Summary of the invention

[0005] The present invention provides a negative electrode sheet, which can improve the cycle performance, fast charging performance and safety performance of a battery without losing energy density by selecting a first binder and a second binder of a specific type.

[0006] The present invention provides a battery, which comprises the negative electrode sheet and has excellent electrochemical performance and safety performance.

[0007] In one aspect, the present invention provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active layer comprises a negative electrode active material, a first binder and a second binder, and the negative electrode active material comprises a silicon-based material and a graphite material;

[0008] Among them, the first binder includes an acrylonitrile-acrylic copolymer, and the first binder has a characteristic peak attributed to the stretching vibration of -C≡N nitrile group at 2000-2500 cm in the infrared spectrum; -1 The second binder includes acrylate monomers, and the second binder has characteristic peaks attributed to -C=O and -C-O at 1600-1800 cm and / or 1000-1300 cm in the infrared spectrum.

[0009] For the negative electrode sheet as described above, the first binder further includes a first monomer, and the first monomer includes at least one of (meth)acrylamide, N-hydroxymethyl(meth)acrylamide, N,N-dimethylacrylamide, vinyl alcohol, ethylene glycol, maleic anhydride, itaconic acid, 2-acrylamido-2-methylpropanesulfonic acid, allyl sulfonic acid, p-styrenesulfonic acid, vinyl sulfonic acid, allyl sulfonic acid, 2-methylallyl sulfonic acid, ethyl sulfonic acid methacrylate, (meth)acrylic acid hydroxyethyl ester, (meth)acrylic acid hydroxypropyl ester, or dimethyldiallylammonium chloride; -1 and / or, -1 The acrylate monomers include at least one of butyl methacrylate, butyl acrylate, methyl methacrylate, methyl acrylate, ethyl methacrylate, ethyl acrylate, n-octyl methacrylate, n-octyl acrylate, isooctyl methacrylate, isooctyl acrylate, dodecyl methacrylate, hydroxyethyl methacrylate, hydroxyethyl acrylate, hydroxypropyl methacrylate, or hydroxypropyl acrylate;

[0010] For the negative electrode sheet as described above, the first binder further includes a first monomer, and the first monomer includes at least one of (meth)acrylamide, N-hydroxymethyl(meth)acrylamide, N,N-dimethylacrylamide, vinyl alcohol, ethylene glycol, maleic anhydride, itaconic acid, 2-acrylamido-2-methylpropanesulfonic acid, allyl sulfonic acid, p-styrenesulfonic acid, vinyl sulfonic acid, allyl sulfonic acid, 2-methylallyl sulfonic acid, ethyl sulfonic acid methacrylate, (meth)acrylic acid hydroxyethyl ester, (meth)acrylic acid hydroxypropyl ester, or dimethyldiallylammonium chloride;

[0011] and / or,

[0012] The acrylate monomers include at least one of butyl methacrylate, butyl acrylate, methyl methacrylate, methyl acrylate, ethyl methacrylate, ethyl acrylate, n-octyl methacrylate, n-octyl acrylate, isooctyl methacrylate, isooctyl acrylate, dodecyl methacrylate, hydroxyethyl methacrylate, hydroxyethyl acrylate, hydroxypropyl methacrylate, or hydroxypropyl acrylate;

[0013] and / or,

[0014] For the negative electrode sheet as described above, the second binder further includes a second monomer, and the second monomer includes at least one of butadiene, styrene, polyol, vinyl alkyl ether, propylene, phenyl ether, ethylene oxide, isophorone diisocyanate, hexamethylene diisocyanate, diphenylmethane diisocyanate, toluene diisocyanate, dopamine, vinylidene fluoride, or dimethylsiloxane.

[0015] For the negative electrode sheet as described above, the mass ratio of the first binder to the second binder is (0.17-2):1.

[0016] For the negative electrode sheet as described above, the first binder includes acrylonitrile monomer, and based on the mass of the first binder, the mass content of the acrylonitrile monomer is 5%-50%;

[0017] and / or, based on the mass of the second binder, the mass content of the acrylate monomers is 10%-70%.

[0018] The negative electrode sheet as described above, wherein the mass percentage of lithium element in the first binder is 3% to 8%;

[0019] and / or, the molecular weight of the first binder is 300,000 to 1,000,000;

[0020] and / or, the glass transition temperature of the first binder is 60 to 100 °C.

[0021] The negative electrode sheet as described above, wherein the glass transition temperature of the second binder is -40 to 40 °C;

[0022] and / or, the particle size Dv50 of the second binder is 100 to 250 nm.

[0023] The negative electrode sheet as described above, wherein the active layer further comprises a negative electrode active material, a conductive agent, and a dispersant;

[0024] Based on the total mass of the active layer, the mass percentage of the first binder is 0.2% to 5%, and the mass percentage of the second binder is 0.5% to 5%,

[0025] and / or, the mass percentage of the negative electrode active material is 84% to 99.1%, the mass percentage of the conductive agent is 0.1% to 3%, and the mass percentage of the dispersant is 0.1% to 3%.

[0026] The negative electrode sheet as described above, wherein the negative electrode active material comprises a silicon-based material and a graphite material;

[0027] Wherein, based on the total mass of the negative electrode active material, the mass percentage of the silicon-based material is ≥15%, and the mass percentage of the graphite material is ≤85%;

[0028] and / or, the particle size Dv50 of the silicon-based material is 5 to 14 μm, and the specific surface area is 0.8 to 5 m 2 / g;

[0029] and / or, the particle size Dv50 of the graphite material is 6 to 18 μm, and the specific surface area is 0.5 to 3 m 2 / g.

[0030] The negative electrode sheet as described above, wherein the conductive agent comprises single-walled carbon nanotubes;

[0031] Based on the total mass of the negative electrode active layer, the mass percentage of the single-walled carbon nanotubes is 0.05% - 1.5%;

[0032] And / or, the diameter of the single-walled carbon nanotubes is 0.5 to 3 nm, the tube length is 1 to 30 μm, and the aspect ratio of the single-walled carbon nanotubes is (1000 to 20000):1;

[0033] And / or, the distribution density of the single-walled carbon nanotubes is 5 per 50 μm 2 to 70 per 50 μm 2 .

[0034] For the negative electrode sheet as described above, the distribution density of the second binder is 200 per 5 μm 2 to 1000 per 5 μm 2 ;

[0035] And / or, the distribution density of the single-walled carbon nanotubes is A, and the distribution density of the second binder is B, satisfying: 7 ≤ B / A ≤ 70.

[0036] On the other hand, the present invention provides a battery including the negative electrode sheet as described above.

[0037] The negative electrode sheet provided by the present invention includes specific types of a first binder and a second binder. Among them, the first binder has a characteristic peak attributed to the stretching vibration of the -C≡N nitrile group. The presence of the nitrile group adds a strongly electronegative element with lone pair electrons to the molecular chain of the first binder. Under the action of an electric field, it will continuously undergo complexation / dissociation reactions with lithium ions, which is beneficial to the diffusion of lithium ions and improves the fast charging ability of the battery cell. At the same time, on the basis of using the first binder, further introducing the second binder helps to improve the flexibility of the electrode sheet, improve the edge powder dropping situation during electrode sheet cutting, and at the same time construct an elastic binding network. The elastic binding network can expand as the negative electrode active material expands and contract as the volume of the negative electrode active material shrinks, relieve the stress generated by volume expansion, significantly inhibit the expansion of the battery cell during the cycling process, and effectively improve the cycle life and safety performance. Under the synergistic effect of the first binder and the second binder, the cycle performance, fast charging performance, and safety performance of the negative electrode sheet are improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 is a scanning electron microscope image of the surface of a negative electrode sheet provided by the present invention;

[0039] Figure 2 is a schematic diagram of a normal temperature 1C / 1C charge-discharge cycle curve provided by the present invention;

[0040] Figure 3 is a schematic diagram of a normal temperature 2C / 2C charge-discharge cycle curve provided by the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0041] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0042] Silicon-based materials undergo significant volume expansion during lithiation. This expansion leads to an increase in mechanical stress of the material, thereby causing particle damage and damage to the electrode structure. This structural damage results in poor contact between the electrode and the electrolyte, increases the internal resistance of the battery, and reduces the cycle performance.

[0043] Moreover, due to volume expansion and contraction, the electrode structure of the silicon-based anode material changes significantly during charge and discharge. This change may lead to a decrease in the mechanical integrity of the electrode, affecting the overall performance and safety of the battery.

[0044] On the one hand, the present invention provides a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active layer provided on at least one surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material, a first binder, and a second binder. The negative electrode active material includes a silicon-based material and a graphite material.

[0045] Among them, the first binder includes an acrylonitrile-acrylic acid copolymer, and the first binder has a characteristic peak attributed to the stretching vibration of -C≡N nitrile groups at the position of 2000-2500 cm -1 in the infrared spectrum. The acrylonitrile-acrylic acid copolymer may refer to a copolymer of acrylonitrile monomer and acrylic acid monomer.

[0046] The second binder includes an acrylate monomer, and the second binder has characteristic peaks attributed to -C=O and -C-O at the position of 1600-1800 cm -1 and / or 1000-1300 cm -1 in the infrared spectrum.

[0047] The negative electrode sheet provided by the present invention has excellent cycle performance, fast charging performance, and safety performance. The negative electrode sheet of the present invention includes specific types of the first binder and the second binder. Among them, in the spectrum obtained by infrared testing, the first binder is at 2000-2500 cm -1There is a characteristic peak attributed to the stretching vibration of the -C≡N nitrile group at the position. The presence of the nitrile group adds a strongly electronegative element with lone pair electrons to the molecular chain of the first binder. Under the action of an electric field, it will continuously undergo complexation / dissociation reactions with lithium ions, which is beneficial to the diffusion of lithium ions and improves the fast charging ability of the battery cell. At the same time, on the basis of using the first binder, a second binder is further introduced. In the spectrum obtained by infrared testing, the second binder has characteristic peaks attributed to -C=O and -C-O at the 1600 - 1800 cm -1 position and 1000 - 1300 cm -1 respectively, indicating that the second binder contains acrylate monomers. The presence of acrylate monomers helps to improve the flexibility of the electrode sheet, improve the edge powder dropping situation during electrode sheet cutting, and at the same time construct an elastic bonding network. The elastic bonding network can expand as the negative active material expands and contract as the volume of the negative active material shrinks, relieving the stress generated by volume expansion. The expansion of the battery cell during the cycling process is significantly inhibited, and the cycling life and safety performance are effectively improved. At the same time, the presence of acrylate monomers also helps to enhance the affinity of the negative electrode side for the electrolyte, improve the liquid retention ability of the negative electrode side, extend the cycling life, and the affinity for the electrolyte can also accelerate the lithium ion transmission, contributing to the stable charging and discharging of the battery at high rates. Therefore, under the synergistic effect of the first binder and the second binder, the cycling performance, fast charging performance, and safety performance of the negative electrode sheet can be effectively improved.

[0048] In another specific embodiment, in addition to acrylonitrile monomer and acrylic acid monomer, the first binder may further include a first monomer during the copolymerization process. For example, the first monomer includes at least one of (meth)acrylamide, N-hydroxymethyl(meth)acrylamide, N,N-dimethylacrylamide, vinyl alcohol, ethylene glycol, maleic anhydride, itaconic acid, 2-acrylamido-2-methylpropanesulfonic acid, allylsulfonic acid, p-styrenesulfonic acid, vinylsulfonic acid, allylsulfonic acid, 2-methylallylsulfonic acid, ethylsulfonic acid methacrylate, (meth)acrylic acid hydroxyethyl ester, (meth)acrylic acid hydroxypropyl ester, or dimethyldiallylammonium chloride.

[0049] In a specific embodiment, the acrylate monomers in the second binder may include at least one of (meth)acrylic acid alkyl ester and (meth)acrylic acid hydroxyalkyl ester.

[0050] Among them, the (meth)acrylic acid alkyl ester is selected from at least one of butyl methacrylate, butyl acrylate, methyl methacrylate, methyl acrylate, ethyl methacrylate, ethyl acrylate, n-octyl methacrylate, n-octyl acrylate, isooctyl methacrylate, isooctyl acrylate, and dodecyl methacrylate.

[0051] (Meth)acrylic hydroxyalkyl esters are selected from at least one of hydroxyethyl methacrylate, hydroxyethyl acrylate, hydroxypropyl methacrylate, and hydroxypropyl acrylate.

[0052] In another specific embodiment, in addition to acrylate monomers, the second binder may further include a second monomer. For example, the second monomer includes at least one of butadiene, styrene, polyol, vinyl alkyl ether, propylene, phenyl ether, ethylene oxide, isophorone diisocyanate, hexamethylene diisocyanate, diphenylmethane diisocyanate, toluene diisocyanate, dopamine, vinylidene fluoride, and dimethyl siloxane.

[0053] The mass ratio of the first binder to the second binder also has an important influence on the synergistic effect between the two.

[0054] In a specific embodiment, the mass ratio of the first binder to the second binder is (0.17 - 2):1.

[0055] Specifically, the mass ratio of the first binder to the second binder includes but is not limited to 0.17:1, 0.2:1, 0.4:1, 0.6:1, 0.8:1, 1.0:1, 1.2:1, 1.4:1, 1.6:1, 1.8:1, 2.0:1, or the range composed of any two of them.

[0056] When the mass ratio of the first binder to the second binder is within the above range, the acrylonitrile-acrylic copolymer has good chemical resistance and thermal stability. At the same time, the nitrile group in the first binder and the abundant active lithium can accelerate ion transport and improve the fast charging ability of the battery cell. The acrylate monomers in the second binder can provide good interfacial stability and construct an elastic bonding network. By adjusting their ratio, the overall mechanical properties of the active layer can be optimized, irreversible damage caused by the expansion and contraction of the electrode during cycling can be reduced, the structural stability of the electrode under different environmental conditions can be improved, and an appropriate ratio can optimize the electrochemical properties of the electrode and improve the fast charging ability and cycle life of the battery.

[0057] For the first binder and the second binder, the mass content of the constituent monomers of the copolymer also has an important influence on the properties of the first binder and the second binder.

[0058] In a specific embodiment, based on the mass of the first binder, the mass percentage content of acrylonitrile monomer is 5% - 50%; specifically, the mass percentage content of acrylonitrile monomer includes but is not limited to 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or the range composed of any two of them.

[0059] By adjusting the mass content of acrylonitrile monomer in the first binder, the chemical stability of the first binder material, the binding ability to the anode active material, and the dispersion ability of the anode active substance can be regulated. Moreover, an appropriate mass content of acrylonitrile monomer helps to optimize the ion conduction ability of the electrode, improve the fast charging ability and cycle life of the battery.

[0060] In another specific embodiment, based on the mass of the second binder, the mass content of acrylate monomers is 10% - 70%. Specifically, the mass content of acrylate monomers includes but is not limited to 10%, 20%, 30%, 40%, 50%, 60%, 70% or the range composed of any two of them.

[0061] When the mass percentage content of acrylate monomers in the second binder is within the above range, it helps to enhance the affinity of the anode side to the electrolyte, improve the liquid retention ability of the anode side. Having an affinity for the electrolyte can accelerate the lithium ion transmission, which helps the battery to stably charge and discharge at high rates. At the same time, when the mass percentage content of acrylate monomers in the second binder is within the above range, it can also construct an elastic and firm binding network for the electrode sheet, maintain the structural stability of the electrode sheet during the charge and discharge process, and extend the cycle life.

[0062] In another specific embodiment, the first binder is a lithiated binder. Lithiation treatment refers to the process of introducing lithium ions or lithium chemical groups into the copolymer, and lithium salts or other lithium compounds can be combined with the polymer through chemical reactions. Preferably, the lithiation treatment of the first binder can refer to the lithiation of acrylic acid to generate lithium acrylate.

[0063] As a doping element, the content of lithium element in the first binder will affect the electrochemical performance of the anode sheet.

[0064] In a specific embodiment, the mass percentage content of lithium element in the first binder is 3% - 8%. Specifically, the mass percentage content of lithium element in the first binder includes but is not limited to 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 8% or the range composed of any two of them.

[0065] The mass percentage content of lithium element in the first binder can be adjusted by controlling the doping amount of the lithium source during the preparation process.

[0066] An appropriate mass content of lithium element helps to improve the ionic conductivity of the binder itself. At the same time, it will also form a locally high-concentration lithium ion distribution around the anode active material, which helps to accelerate the lithium ion transmission, reduce the concentration polarization during the charge and discharge process of the battery, and ensure that the battery cell has excellent kinetic performance.

[0067] In another specific embodiment, the molecular weight of the first binder is 300,000 to 1,500,000; specifically, the molecular weight of the first binder includes but is not limited to 300,000, 400,000, 500,000, 600,000, 700,000, 800,000, 900,000, 1,000,000, 1,100,000, 1,300,000, 1,500,000 or the range formed between any two of them.

[0068] The molecular weight average molecular weight of the first binder in the present invention can be tested by gel permeation chromatography (GPC).

[0069] When the molecular weight of the first binder is controlled within the above range, the first binder has excellent adhesion, which is beneficial to stabilizing the structure of the negative electrode sheet, inhibiting the expansion of silicon materials, and can also promote the uniform dispersion of the negative electrode active material in the slurry, while maintaining good slurry viscosity stability and improving slurry coating processing. In addition, when the molecular weight of the first binder is within this range, the dosage of the first binder can be appropriately reduced, which is beneficial to improving the energy density of the battery cell and improving the kinetics on the negative electrode side. When the molecular weight of the first binder is less than 300,000, the thickening effect of the first binder on the slurry is not good, which is not conducive to the preparation of the negative electrode slurry, and the adhesion of the first binder is weak and insufficient to inhibit the expansion of the silicon negative electrode; when the molecular weight is greater than 1,500,000, the molecular weight of the first binder is prone to entanglement, resulting in poor slurry viscosity stability and not meeting the requirements of battery industrial production.

[0070] Furthermore, in a specific embodiment of the present invention, the glass transition temperature of the first binder is 60 to 100 °C. Specifically, the glass transition temperature of the first binder includes but is not limited to 60 °C, 65 °C, 70 °C, 75 °C, 80 °C, 85 °C, 90 °C, 95 °C, 100 °C or the range formed between any two of them.

[0071] The glass transition temperature refers to the temperature at which the first binder changes from a glassy state to a rubbery state, and common methods in the art can be used for detection, such as differential scanning calorimetry, dynamic mechanical analysis, thermomechanical analysis, etc.

[0072] When the glass transition temperature (Tg) of the first binder is within the above range, a moderate Tg can provide a good balance of mechanical strength and flexibility, prevent the negative electrode sheet from cracking or peeling due to volume changes during charge and discharge cycles, and help maintain an appropriate porosity of the negative electrode sheet, build a smooth lithium ion transmission channel, and ensure that the binder maintains good adhesion and conductivity within the battery operating temperature range.

[0073] In another specific embodiment of the present invention, the glass transition temperature of the second binder is -40 to 40 °C; specifically, the glass transition temperature of the second binder includes but is not limited to -40 °C, -30 °C, -20 °C, -10 °C, 0 °C, 10 °C, 20 °C, 30 °C, 40 °C or the range formed by any two of them.

[0074] When the glass transition temperature of the second binder is within the above range, at room temperature and lower temperatures, the binder still maintains a certain flexibility, which helps to absorb the stress generated due to volume changes during charge and discharge. This can improve the cycle life and reliability of the battery. Moreover, it maintains good adhesion performance within a wide temperature range, ensuring a firm bond between the active material and the current collector, thereby improving the conductivity and structural integrity of the battery.

[0075] In a specific embodiment, the Dv50 particle size of the second binder is 100 to 250 nm.

[0076] Specifically, the Dv50 particle size means that 50% of the particle volume in the particle size distribution is less than or equal to this value. In other words, the Dv50 particle size is the median of the particle size distribution, meaning that half of the particle volume is less than Dv50 and the other half is greater than Dv50.

[0077] Specifically, the Dv50 particle size of the second binder includes but is not limited to 100 nm, 120 nm, 150 nm, 160 nm, 180 nm, 200 nm, 220 nm, 240 nm, 250 nm or the range formed by any two of them.

[0078] An appropriate particle size can provide sufficient bonding sites, ensure the strong bonding effect of the second binder on the electrode sheet, and maintain the structural stability of the negative electrode sheet during cyclic charge and discharge. In addition, within this particle size range, the dosage of the second binder can be appropriately reduced, which is beneficial to improving the energy density of the battery cell and improving the kinetics on the negative electrode side.

[0079] In a specific embodiment, the negative electrode active layer further includes a negative electrode active material, a conductive agent, and a dispersant;

[0080] Based on the total mass of the negative electrode active layer, the mass percentage content of the first binder is 0.2% to 5%, the mass percentage content of the second binder is 0.5% to 5%, the mass percentage content of the negative electrode active material is 84% to 99.1%, the mass percentage content of the conductive agent is 0.1% to 3%, and the mass percentage content of the dispersant is 0.1% to 3%.

[0081] Preferably, the mass percentage content of the first binder is 0.5% to 3%, and the mass percentage content of the second binder is 1.5% to 3%.

[0082] In the present invention, the negative electrode active material, conductive agent, and dispersant can be selected from common types in the art. For example, the conductive agent can include one or a combination of conductive carbon black, graphene, and carbon nanotubes; the binder can include one or a combination of styrene-butadiene rubber (SBR), poly(styrene-acrylate), polyacrylate, polyvinyl acetate, polymethacrylate, polyacrylic acid (PAA), polyacrylonitrile, polyacrylamide, polyvinylidene fluoride, etc.; the dispersant is sodium carboxymethyl cellulose and / or lithium carboxymethyl cellulose.

[0083] When the mass percentage contents of various components in the active layer are within the above ranges, it can enable the battery cell to exhibit a high energy density while ensuring that the electrode sheet has a firm bonding network and unobstructed electron / ion transport channels, guaranteeing the cycle performance, fast charging performance, and safety performance of the battery cell. Among them, a higher content of the negative electrode active material means that more materials can be used for the storage and release of lithium ions, enabling it to store more charges and improve the energy density of the battery cell; an appropriate content of the conductive agent can form a conductive network, reduce the internal resistance of the negative electrode sheet, and improve the rate performance and charge-discharge efficiency of the battery. An appropriate content of the binder helps the electrode sheet maintain good structural stability during charge and discharge, improving the cycle life and fast charging ability of the battery.

[0084] Further, in a specific embodiment, the negative electrode active material includes a silicon-based material and a graphite material;

[0085] Among them, based on the total mass of the negative electrode active material, the mass percentage content of the silicon-based material is 1% to 40%, and the mass percentage content of the graphite material is 60% to 99%.

[0086] The present invention does not limit the specific types of the silicon-based material and the graphite material, and common silicon-based materials and graphite materials in the art can be selected. For example, the silicon-based material can include at least one of silicon-carbon materials and silicon-oxygen materials; the graphite material can include at least one of artificial graphite and natural graphite.

[0087] When the mass percentage contents of the silicon-based material and the graphite material are within the above ranges, the silicon-based material has a very high theoretical specific capacity (about 4200 mAh / g), which is much higher than that of graphite (about 372 mAh / g). By ensuring that the content of the silicon-based material is not less than 15%, the overall capacity and energy density of the battery can be significantly improved. The graphite material has good cycle stability and conductivity, and can provide stable electrochemical performance. By limiting its content not to exceed 85%, while maintaining a good cycle life, the high-capacity characteristics of the silicon-based material can be fully utilized.

[0088] In a specific embodiment, the particle size Dv50 of the silicon-based material is 5 to 14 μm, and the specific surface area is 0.8 to 5 m 2 / g; Specifically, the Dv50 particle size of the silicon-based material includes, but is not limited to, 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, or the range formed between any two of them, and the specific surface area includes, but is not limited to, 0.8 m 2 / g, 1 m 2 / g, 2 m 2 / g, 3 m 2 / g, 4 m 2 / g, 5 m 2 / g, or the range formed between any two of them.

[0089] Preferably, the Dv50 particle size of the silicon-based material is 6 - 12 μm, and the specific surface area is 1 m 2 / g - 3 m 2 / g.

[0090] When the Dv50 particle size and the specific surface area of the silicon-based material are within the above ranges, the moderate particle size helps to better manage the volume expansion of silicon during charge and discharge, provides good mechanical stability, and reduces the damage to the electrode structure. The moderate specific surface area helps to improve the wettability of the electrolyte, promotes the transport of lithium ions, forms a stable solid electrolyte interface (SEI) layer, reduces side reactions, and improves the cycle life and fast charging ability of the battery.

[0091] In another specific embodiment, the Dv50 particle size of the graphite material is 6 - 18 μm, and the specific surface area is 0.5 - 3 m 2 / g. Specifically, the Dv50 particle size of the graphite material includes, but is not limited to, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, or the range formed between any two of them, and the specific surface area includes, but is not limited to, 0.5 m 2 / g, 1 m 2 / g, 1.5 m 2 / g, 2 m 2 / g, 2.5 m 2 / g, 3 m 2 / g, or the range formed between any two of them.

[0092] Preferably, the Dv50 particle size of the graphite material is 8 μm - 16 μm, and the specific surface area is 0.8 m 2 / g - 1.8 m 2 / g.

[0093] When the particle size and specific surface area of the graphite material are within the above ranges, the moderate particle size range helps to improve the mechanical stability of the electrode, reduces the damage to the electrode structure caused by volume changes during charge and discharge cycles. Moreover, the moderate specific surface area helps to maintain good electrical conductivity, ensures the uniform distribution of current in the electrode, and improves the rate performance of the battery.

[0094] In a specific embodiment, the conductive agent includes single-walled carbon nanotubes; based on the total mass of the active material layer, the mass percentage of single-walled carbon nanotubes is 0.05% - 1.5%.

[0095] Single-walled carbon nanotubes (SWCNT) are hollow cylindrical nanomaterials formed by curling a single-layer graphene sheet. It is composed of a single layer of carbon atoms, forming a one-dimensional tubular structure.

[0096] The present invention does not limit the source of single-walled carbon nanotubes, which can be obtained through commercial channels or prepared by itself using methods such as chemical vapor deposition.

[0097] During the charge and discharge process of the silicon-based negative electrode sheet, the volume expansion is extremely likely to cause the interruption of electron transmission. The silicon material or graphite material that loses electrical contact can no longer play the role of lithium storage, which will lead to a rapid attenuation of the cell capacity. Using single-walled carbon nanotubes as the negative electrode conductive agent can build a long-range conductive network in the electrode sheet, ensuring that the silicon-based negative electrode still has good conductivity after volume expansion; in addition, the conductivity of single-walled carbon nanotubes is much higher than that of conventional carbon black materials, which means that even at a low dosage, single-walled carbon nanotubes can exhibit far better conductivity than conventional carbon black materials, which is beneficial to increasing the proportion of active substances on the negative electrode side and improving the energy density of the cell.

[0098] Among them, based on the total mass of the active material layer, the mass percentage of single-walled carbon nanotubes includes but is not limited to 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.5% or the range composed of any two of them.

[0099] It can be understood that in addition to single-walled carbon nanotubes, the conductive agent also includes other types of conductive agents. The present invention does not limit the specific types of other types of conductive agents, and common conductive agents in the art can be selected, such as acetylene black, conductive carbon black, multi-walled carbon nanotubes, graphene, etc.

[0100] When the mass percentage of single-walled carbon nanotubes is within the above range, it can not only form an efficient conductive network to ensure good contact between active materials, but also avoid problems such as agglomeration of single-walled carbon nanotubes, difficult slurry processing, and increased material costs caused by excessive addition.

[0101] In a specific embodiment, the diameter of the single-walled carbon nanotubes is 0.5 - 3 nm, the tube length is 1 - 30 μm, and the aspect ratio of the single-walled carbon nanotubes is (1000 - 20000):1.

[0102] Common detection methods in the art can be used to detect the size of single-walled carbon nanotubes, such as transmission electron microscopy, atomic force microscopy, dynamic light scattering, etc.

[0103] Specifically, the tube diameters of the single-walled carbon nanotubes include, but are not limited to, 0.5 nm, 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, or the ranges formed between any two of them. The tube lengths include, but are not limited to, 1 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, or the ranges formed between any two of them.

[0104] The aspect ratio of the single-walled carbon nanotubes refers to the ratio of the tube length to the tube diameter of the single-walled carbon nanotubes. In the present invention, the aspect ratios of the single-walled carbon nanotubes include, but are not limited to, 1000:1, 2000:1, 4000:1, 6000:1, 8000:1, 10000:1, 12000:1, 14000:1, 16000:1, 18000:1, 20000:1, or the ranges formed between any two of them.

[0105] When the single-walled carbon nanotubes meet the above conditions, the SWCNT can be uniformly dispersed in the slurry without agglomeration, and can be evenly distributed on the surface of the negative electrode active material to form a good conductive network, thereby improving the conductivity between the negative electrode particles and enhancing the cycle stability and rate performance of the battery. In addition, when meeting the above conditions, the SWCNT can exhibit high tensile strength, high elastic modulus, and excellent thermal conductivity, which is beneficial to suppressing the expansion of the silicon negative electrode and simultaneously achieving rapid heat conduction of the battery core to ensure the safety performance of the battery core.

[0106] In another specific embodiment, the distribution density of the single-walled carbon nanotubes is 10 per 50 μm 2 ~70 per 50 μm 2 .

[0107] In the present invention, the distribution density of the single-walled carbon nanotubes refers to the distribution of the single-walled carbon nanotubes on the surface of the active layer of the negative electrode sheet, which can be regarded as the average number of single-walled carbon nanotubes within any continuous 50 μm 2 area.

[0108] Any continuous 50 μm can be detected by common detection methods in the art 2The average number of single-walled carbon nanotubes within the area. In the present invention, the distribution densities of the single-walled carbon nanotubes and the second binder can be obtained through scanning electron microscopy (SEM) testing. The method is as follows: After fully discharging the prepared lithium-ion battery, disassemble it, take out the negative electrode sheet, soak it in dimethyl carbonate (DMC) for 12 hours, then take it out and dry it. Use an electron emission scanning electron microscope (S-3400N manufactured by Hitachi, Ltd.) to observe the surface of the negative electrode active material layer at a magnification of 5k. Select a square area of a specific area on the obtained image, and count the number of single-walled carbon nanotubes and the number of second binder particles (platelet-shaped gray dots) within this area, and then the distribution densities of the single-walled carbon nanotubes and the second binder particles in the negative electrode active material layer can be obtained.

[0109] Specifically, the distribution density of the single-walled carbon nanotubes includes but is not limited to 10 per 50 μm 2 , 15 per 50 μm 2 , 20 per 50 μm 2 , 25 per 50 μm 2 , 30 per 50 μm 2 , 35 per 50 μm 2 , 40 per 50 μm 2 , 45 per 50 μm 2 , 50 per 50 μm 2 , 55 per 50 μm 2 , 60 per 50 μm 2 , 65 per 50 μm 2 , 70 per 50 μm 2 or the range formed between any two of them.

[0110] When the distribution density of the single-walled carbon nanotubes is within the above range, it indicates that the SWCNT is evenly dispersed on the surface of the negative electrode active layer, which helps to build a good electron transport path and heat conduction network, effectively inhibits the expansion of the silicon-based negative electrode, and improves the fast charging ability, cycle life, and safety performance of the battery cell. When the distribution density of the single-walled carbon nanotubes is greater than 70 per 50 μm 2 , the content of SWCNT in the battery cell system is too high, which easily exacerbates the side reaction between the negative electrode active material and the electrolyte, deteriorates the high-temperature storage performance of the battery cell, and at the same time, the use of too much SWCNT will greatly increase the manufacturing cost of the battery cell, which is not conducive to popularization and use.

[0111] In another specific embodiment, the distribution density of the single-walled carbon nanotubes is 20 per 50 μm 2 ~50 per 50 μm 2 .

[0112] When the distribution density of the single-walled carbon nanotubes is within the above range, the balance between conductivity and cost can be better achieved.

[0113] Further, in a specific embodiment, the distribution density of the second binder is 200 per 5 μm 2 ~1000 per 5 μm 2 .

[0114] In the present invention, the distribution density of the second binder refers to the distribution of the second binder on the surface of the negative electrode active layer, and can be regarded as the average number of the second binder within any continuous 5 μm 2 area.

[0115] Specifically, the distribution density of the second binder includes but is not limited to 200 per 5 μm 2 , 300 per 5 μm 2 , 400 per 5 μm 2 , 500 per 5 μm 2 , 600 per 5 μm 2 , 700 per 5 μm 2 , 800 per 5 μm 2 , 900 per 5 μm 2 , 1000 per 5 μm 2 or the range formed by any two of them.

[0116] When the distribution density of the second binder is within the above range, the uniform distribution of the second binder in the negative electrode active layer can be ensured, and a high-toughness and firm bonding network can be constructed, thereby suppressing the expansion of the silicon-based negative electrode. When the distribution density of the second binder is greater than 1000 per 5 μm 2 , too much of the second binder coats the surface of the active material, which will hinder electron / ion transport, increase the polarization on the negative electrode side, and affect the fast charging ability of the battery cell.

[0117] In a specific embodiment, the distribution density of the single-walled carbon nanotubes is A, and the distribution density of the second binder is B, satisfying: 7 ≤ B / A ≤ 70.

[0118] In the present invention, the unit of the distribution density A of the single-walled carbon nanotubes is per 50 μm 2 , while the unit of the distribution density B of the second binder is per 5 μm 2 . For example, when the distribution density of the single-walled carbon nanotubes is 70 per 50 μm 2 , the distribution density B of the second binder is 700 per 5 μm 2 , then B / A is 10.

[0119] Specifically, B / A includes but is not limited to 7, 10, 20, 30, 40, 50, 60, 70 or the range formed by any two of them.

[0120] When B / A is within the above range, the dosages of the second binder and SWCNT are appropriate and evenly distributed. The two work together synergistically to endow the negative electrode sheet with a firm bonding network, excellent electron transport path, and heat conduction network, significantly improving the fast charging ability, cycle life, and safety performance of the battery cell.

[0121] It can be understood that the negative electrode sheet further includes a negative electrode current collector. Materials commonly used in the art can be selected as the negative electrode current collector. In a specific embodiment, the negative electrode current collector is a copper foil with a thickness of 4.5 - 6 μm.

[0122] In another specific embodiment, the negative electrode sheet includes a negative electrode current collector and a bottom coating covering at least one side of the negative electrode current collector, and the active layer covers the outermost side of the negative electrode sheet away from the negative electrode current collector.

[0123] When the current collector includes a bottom coating, the peeling strength between the negative electrode paste and the current collector can be effectively improved, preventing the negative electrode active layer from peeling off the current collector due to excessive swelling during charge and discharge, thus effectively maintaining the structural integrity of the negative electrode sheet and enhancing the cycle performance of the battery.

[0124] The composition and thickness of the bottom coating can be selected according to actual needs. In a specific embodiment, the bottom coating contains a conductive agent, a binder, and a dispersant. Among them, the conductive agent can include one or a combination of conductive carbon black, graphene, and carbon nanotubes; the binder can include one or a combination of styrene-butadiene rubber (SBR), poly(styrene-acrylate), polyacrylate, polyvinyl acetate, polymethacrylate, polyacrylic acid (PAA), polyacrylonitrile, polyacrylamide, polyvinylidene fluoride, etc.; and the dispersant includes at least one of sodium carboxymethyl cellulose and lithium carboxymethyl cellulose.

[0125] In another specific embodiment, the thickness of the bottom coating is 0.5 - 2.0 μm.

[0126] Testing the negative electrode sheet including the negative electrode current collector, the bottom coating, and the active layer. In a specific embodiment, the peeling force of the negative electrode sheet is 12 N / m to 30 N / m, indicating good adhesion of the negative electrode sheet. The cohesive force of the negative electrode sheet is 15 N / m to 40 N / m, indicating good cohesive force of the negative electrode sheet.

[0127] On the other hand, the present invention provides a battery including the negative electrode sheet as described above.

[0128] Since the battery provided by the present invention includes the above negative electrode sheet, it has excellent cycle performance, fast charging performance, and safety performance.

[0129] It can be understood that the battery of the present invention further includes a positive electrode sheet, a separator, and an electrolyte.

[0130] Among them, the positive electrode sheet can be a conventional positive electrode sheet in the art. For example, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer located on one or both surfaces of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active substance, a positive electrode conductive agent, and a positive electrode binder. The positive electrode active substance includes a ternary material, and the chemical formula of the ternary material is Li a Ni x Co y Mn z A k O2, where 0.9 ≤ a ≤ 1.1, 0.5 ≤ x ≤ 0.95, 0 < y ≤ 0.2, 0 < z ≤ 0.3, 0 ≤ k ≤ 0.05, A is a doping element, and A includes one or more of Al, Mg, Ti, Zr, B, P, and Y. The positive electrode active substance can include a positive electrode active substance having a layered structure, and the above ternary material includes a single crystal structure and / or a polycrystalline structure.

[0131] The positive electrode conductive agent includes at least one of conductive carbon black (SP), Ketjen black, acetylene black, graphite conductive agent (KS-6, KS-15, S-O, SEG-6), carbon fiber (VGCG), carbon nanotube (CNT), and graphene.

[0132] The positive electrode binder includes one or more of PVDF, PVDF-HFP, polytetrafluoroethylene, polyacrylonitrile, and polyimide.

[0133] In a specific embodiment, based on the total weight of the positive electrode active material layer, the weight content of the positive electrode active substance is 80% - 99.8%, the weight content of the positive electrode conductive agent is 0.1% - 10%, and the weight content of the positive electrode binder is 0.1% - 10%.

[0134] The electrolyte can also be a conventional electrolyte in the art. For example, the electrolyte includes a lithium salt, an organic solvent, and an additive. The lithium salt includes one or more of lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, lithium difluorophosphate (LiPO2F2), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), lithium tetrafluoroborate (LiBF4), lithium difluoro(oxalato)phosphate (LiDFOP), and lithium bis(trifluoromethylsulfonyl)imide. The organic solvent includes one or more of ethylene carbonate (EC), fluoroethylene carbonate (FEC), propylene carbonate (PC), ethyl methyl carbonate (EMC), dimethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, diethyl carbonate (DMC), ethyl formate, ethyl acetate (EA), ethyl propionate (EP), and propyl propionate (PP). The additive includes one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene vinyl carbonate (VEC), methylene bis(sulfonic acid) methyl ester (MMDS), and 1,3-propane sultone (PS).

[0135] In a specific embodiment, based on the total weight of the electrolyte, the weight content of the lithium salt is 10% - 20%, the weight content of the organic solvent is 75% - 89.99%, and the weight content of the additive is 0.01% - 5%.

[0136] The separator can also be a conventional separator in the art. For example, the separator includes a base film and a porous layer provided on at least one surface of the base film. The base film can include at least one of polyethylene, polypropylene, polyvinylidene fluoride, a copolymer of vinylidene fluoride - hexafluoropropylene, polyethylene terephthalate, polyimide, and aramid. The porous layer includes inorganic particles and a first separator binder.

[0137] In a specific embodiment, the thickness of the separator is 3 μm - 12 μm.

[0138] The present invention is not limited to the type of battery either. For example, the battery can be selected from square aluminum - shell, soft - package, cylindrical batteries, etc.

[0139] The following provides a detailed introduction to the negative electrode sheet provided by the present invention through specific examples.

[0140] Example 1

[0141] The preparation process of the negative electrode sheet provided in this example includes the following steps:

[0142] 78.7 parts by weight of graphite, 17.3 parts by weight of silicon - carbon material, 1.5 parts by weight of a first binder (lithiated acrylonitrile, acrylic acid, acrylamide copolymer), 1.5 parts by weight of a second binder (butyl acrylate, butadiene, styrene copolymer), 0.7 parts by weight of a conductive agent (single - walled carbon nanotubes), and 0.3 parts by weight of a dispersant (lithium carboxymethyl cellulose) are subjected to high - speed stirring to obtain a uniformly dispersed mixture. The mixture is made into a negative electrode active material slurry using water as a solvent, and the solid content in the slurry is 50 wt%. The slurry is uniformly coated on both sides of the negative electrode current collector copper foil, and after drying and rolling, a negative electrode sheet is obtained.

[0143] The specific parameters of each component and the negative electrode sheet are shown in Table 1.

[0144] The negative electrode sheet provided in Example 1 is detected by scanning electron microscopy, Figure 1 is a scanning electron microscopy detection photo of the surface of the negative electrode sheet, and from Figure 1 it can be observed that the second binder in the shape of platelets and single - walled carbon nanotubes are distributed between particles, forming a conductive network.

[0145] Figure 2 is the normal - temperature 1C / 1C charge - discharge cycle curve of Example 1, Figure 3 is the normal - temperature 2C / 2C charge - discharge cycle curve of Example 1.

[0146] The preparation methods of the negative electrode sheets of Examples 2 - 25 and Comparative Examples 1 and 2 are basically the same as that of Example 1, and some components and parameters of some negative electrode sheets will be adjusted. The specific contents are shown in Table 1.

[0147] The preparation methods of the negative electrode sheets of Examples 26 - 33 are basically the same as that of Example 1, and some components and parameters of some negative electrode sheets will be adjusted. The specific contents are shown in Table 2.

[0148] Test Example

[0149] The batteries were assembled and tested in sequence using the negative electrode sheets provided in all the examples and comparative examples according to the following method. The specific test results are shown in Table 3.

[0150] The method for assembling the battery includes:

[0151] 1) Preparation of the positive electrode sheet: The positive electrode active material, binder, and conductive agent were mixed and uniformly dispersed by high - speed stirring. The mixture was made into a positive electrode active material slurry using N - methylpyrrolidone as the solvent, and the solid content in the slurry was 70 wt%. The slurry was evenly coated on both sides of the current collector, and after drying and rolling, the positive electrode sheet was obtained.

[0152] Among them, the positive electrode active material layer includes 29.1 parts by weight of single - crystal LiNi 0.9 Co 0.04 Mn 0.04 Al 0.02 O2, 67.9 parts by weight of polycrystalline LiNi 0.92 Co 0.02 Mn 0.04 Al 0.02 O2, 1 part by weight of binder polyvinylidene fluoride (PVDF), 1 part by weight of conductive agent conductive carbon black, and 1 part by weight of carbon nanotubes.

[0153] 2) Assembly of the battery: The positive electrode sheet, separator, and negative electrode sheet were wound into a core, and after welding, casing, encapsulation, electrolyte injection, formation, and sorting, a lithium - ion battery was made.

[0154] The method for testing the battery is as follows:

[0155] 1. Cycling performance test

[0156] At 25 °C, within the charge-discharge window of 4.25 V to 2.5 V, 1C / 1C charge-discharge cycles were carried out. The test process is as follows: First, charge at a constant current of 1C to 4.25 V, then charge at a constant voltage with a cut-off current of 0.05C, and finally discharge at a constant current of 1C to 2.5 V. Conduct cyclic tests in this way. Record the thickness of the battery cell at the first discharge as H1. Record the number of cycles when the ratio of the discharge capacity to the first discharge capacity (capacity retention rate) reaches 90%. At the same time, record the thickness of the fully charged battery cell H2 at the end of the cycle. The battery cell cycle expansion rate = [(H2 - H1) / H1] × 100%.

[0157] 2. Fast charging performance test

[0158] At 25 °C, within the charge-discharge window of 4.25 V to 2.5 V, 2C / 2C charge-discharge cycles were carried out. The test process is as follows: First, charge at a constant current of 2C to 4.25 V, then charge at a constant voltage with a cut-off current of 0.05C, and finally discharge at a constant current of 2C to 2.5 V. Conduct cyclic tests in this way. Record the thickness of the battery cell at the first discharge as H3. Record the number of cycles when the ratio of the discharge capacity to the first discharge capacity (capacity retention rate) reaches 90% in Table 2. At the same time, record the thickness of the fully charged battery cell H4 at the end of the cycle. The battery cell cycle expansion rate = [(H4 - H3) / H3] × 100%.

[0159] 3. Lithium plating situation test

[0160] At room temperature, charge the battery cell nC (n = 1, 2, 3, 4) to 4.25 V and then charge at a constant voltage with a cut-off current of 0.05C, and let it stand for 30 min; discharge at 1C to 2.5 V and let it stand for 30 min; conduct such charge-discharge cycles 20 times, and finally charge the battery cell nC at a constant current and constant voltage to 4.25 V to end the test. Then, dissect the fully charged battery cell to observe whether lithium plating occurs on the negative electrode side under different DC charging rates. The maximum test rate is 4C. The results are expressed by the lithium plating window and the lithium plating situation. For example, if lithium plating starts at 4C, the result is expressed as "lithium plating at 4C", and if there is no lithium plating at 4C, the result is expressed as "no lithium plating at 4C".

[0161] (4) Overcharge test

[0162] At 25°C, the battery is charged at a constant current of 1C until 4.25V, then charged at a constant voltage with a cut-off current of 0.05C. After standing for 2h, the battery is charged at a constant current of 1C until the voltage reaches 5.46V or the time reaches 1h, and then the test stops; observe for 1h or until the maximum temperature on the battery surface drops to the peak temperature of 10°C or below, and then stop the test. During the test, observe whether the sample battery catches fire, explodes, or leaks. If none of these occur, it means the test is passed. If at least one of these three situations occurs, it means the test is not passed. A total of 3 battery samples are tested, and the result is expressed as "the number of samples passing the test / 3". For example, "1 / 3" means only 1 out of 3 battery samples passes the test.

[0163] Table 1

[0164]

[0165]

[0166] Table 1 - continued

[0167]

[0168]

[0169] Table 2

[0170]

[0171] Table 3

[0172]

[0173]

[0174] After testing:

[0175] 1. Referring to Table 1 and Table 3, compared with Comparative Example 1 and Comparative Example 2, when the negative electrode sheet includes both the first binder and the second binder in Examples 1 - 9, the cycle performance of the battery is significantly improved (the number of cycles at 1C and 2C increases significantly, and the swelling rate of the cell thickness at 1C and 2C decreases significantly). No lithium plating occurs in the battery, and all overcharge tests are passed. Therefore, the technical solution of the present application can effectively improve the cycle performance, fast charging performance, and safety performance of the battery.

[0176] Furthermore, compared with Examples 12 and 13, and Examples 1, 10, and 11, when the mass ratio of the first binder to the second binder is controlled to be (0.17 - 2):1, the cycle performance of the battery is also improved to a certain extent (the number of cycles at 1C and 2C increases, and the swelling rate of the cell thickness at 1C and 2C decreases), and the safety and fast charging performance are improved to a certain extent (no lithium plating occurs in the battery or lithium plating only occurs at 4C, and all overcharge tests are passed).

[0177] Further, compared with Embodiments 16 and 17, and Embodiments 1, 14 and 15, when the content of lithium element in the first binder is controlled within the corresponding range, the cycle performance, safety and fast charging performance of the battery are also improved to a certain extent.

[0178] Further, it can be seen from Embodiments 18-25 that when the content of acrylate monomer and the content of acrylate monomer are controlled within the corresponding range, the number of cycles of the battery at 1C is significantly increased and the lithium deposition situation is significantly reduced. When the molecular weight of the first binder, the glass transition temperature of the first binder, the molecular weight of the second binder and the glass transition temperature of the second binder are respectively controlled within the corresponding range, compared with the data outside the range, the cycle performance, safety and fast charging performance of the battery are all improved to a certain extent.

[0179] 2. Referring to Table 2 and Table 3, compared with Embodiments 28, 29, 32 and 33, for the remaining embodiments in Embodiments 26-33, when the content of silicon-based material, the content of graphite material, the content of carbon nanotubes, the distribution density of carbon nanotubes, the distribution density of the second binder, and the ratio of the distribution density of the second binder to the distribution density of carbon nanotubes in the negative electrode sheet are all adjusted to the appropriate range, the battery can maintain better cycle performance, safety and fast charging performance.

[0180] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A negative electrode sheet, characterized in that: The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least one surface of the negative electrode current collector, the negative electrode active layer includes a negative electrode active material, a first binder and a second binder, and the negative electrode active material includes a silicon-based material and a graphite material; Wherein, the first binder comprises acrylonitrile-acrylic acid copolymer, and the first binder has a wavelength of 2000-2500cm in the infrared spectrum. -1 There is a characteristic peak at the position attributed to the stretching vibration of the -C≡N nitrile group; The second binder includes an acrylate monomer, and the second binder has a wavelength of 1600-1800 cm in the infrared spectrum. -1 Position and / or 1000-1300cm -1 There are characteristic peaks attributed to -C=O and -CO.

2. The negative electrode sheet according to claim 1, characterized in that: The first binder further includes a first monomer, the first monomer including at least one of (meth)acrylamide, N-hydroxymethyl (meth)acrylamide, N,N-dimethylacrylamide, vinyl alcohol, ethylene glycol, maleic anhydride, itaconic acid, 2-acrylamido-2-methylpropanesulfonic acid, allyl sulfonic acid, p-styrenesulfonic acid, vinyl sulfonic acid, allyl sulfonic acid, 2-methylallyl sulfonic acid, ethyl methacrylate sulfonic acid, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, or dimethyldiallylammonium chloride; and / or, The acrylic acid ester monomers include at least one of butyl methacrylate, butyl acrylate, methyl methacrylate, methyl acrylate, ethyl methacrylate, ethyl acrylate, n-octyl methacrylate, n-octyl acrylate, isooctyl methacrylate, isooctyl acrylate, dodecyl methacrylate, hydroxyethyl methacrylate, hydroxyethyl acrylate, hydroxypropyl methacrylate, and hydroxypropyl acrylate; and / or, The second binder also includes a second monomer, which includes at least one of butadiene, styrene, polyol, vinyl alkyl ether, propylene, phenylene ether, ethylene oxide, isophorone diisocyanate, hexamethylene diisocyanate, diphenylmethane diisocyanate, toluene diisocyanate, dopamine, vinylidene fluoride, and dimethylsiloxane.

3. The negative electrode sheet according to claim 1, characterized in that: The mass ratio of the first binder to the second binder is (0.17-2):

1.

4. The negative electrode sheet according to claim 2, characterized in that: The first binder includes acrylonitrile monomer, and based on the mass of the first binder, the mass percentage of the acrylonitrile monomer is 5% to 50%; and / or, The second adhesive comprises an acrylate monomer, and based on the mass of the second adhesive, the mass percentage of the acrylate monomer is 10% to 70%.

5. The negative electrode sheet according to any one of claims 1 to 4, characterized in that: The first binder is a lithiated binder, and the mass percentage of lithium element in the first binder is 3% to 8%; and / or, the molecular weight of the first binder is 300,000 to 1.5 million; and / or, the glass transition temperature of the first binder is 60 to 100° C.; The glass transition temperature of the second binder is -40 to 40°C; And / or, the particle size Dv50 of the second binder is 100-250 nm.

6. The negative electrode sheet according to any one of claims 1 to 4, characterized in that: The negative electrode active layer also includes a conductive agent and a dispersant; Based on the total mass of the negative electrode active layer, the mass percentage of the first binder is 0.2% to 5%, the mass percentage of the second binder is 0.5% to 5%, the mass percentage of the negative electrode active material is 84% ​​to 99.1%, the mass percentage of the conductive agent is 0.1% to 3%, and the mass percentage of the dispersant is 0.1% to 3%.

7. The negative electrode sheet according to claim 6, characterized in that: Based on the total mass of the negative electrode active material, the mass percentage of the silicon-based material is 1% to 40%, and the mass percentage of the graphite material is 60% to 99%; And / or, the particle size Dv50 of the silicon-based material is 5 to 14 μm, and the specific surface area is 0.8 to 5 m 2 / g; And / or, the particle size Dv50 of the graphite material is 6 to 18 μm, and the specific surface area is 0.5 to 3 m 2 / g.

8. The negative electrode sheet according to claim 6, characterized in that: The conductive agent includes single-walled carbon nanotubes; Based on the total mass of the negative electrode active layer, the mass percentage of the single-walled carbon nanotubes is 0.05%-1.5%; And / or, the diameter of the single-walled carbon nanotube is 0.5-3 nm, the length is 1-30 μm, and the aspect ratio of the single-walled carbon nanotube is (1000-20000):1; And / or, the distribution density of the single-walled carbon nanotubes is 10 / 50 μm 2 ~70 / 50μm 2 .

9. The negative electrode sheet according to claim 8, characterized in that: The distribution density of the second binder is 200 pieces / 5 μm 2 ~1000 pieces / 5μm 2 ; And / or, the distribution density of the single-walled carbon nanotubes is A, and the distribution density of the second binder is B, satisfying: 7≤B / A≤70.

10. A battery, characterized in that: A negative electrode sheet comprising any one of claims 1 to 9.

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