Negative plate, preparation method thereof and battery

Abstract

The invention relates to the technical field of batteries, in particular to a negative plate, a preparation method thereof and a battery. The negative electrode plate comprises a current collector, a first negative electrode coating, a second negative electrode coating and a third negative electrode coating, wherein the interlayer spacing of the first carbon-based active material is smaller than the interlayer spacing of the second carbon-based active material; the interlayer spacing of the second carbon-based active material is smaller than the interlayer spacing of the third carbon-based active material. According to the negative plate, the overall diffusion rate of lithium ions can be greatly improved, the leap-type improvement of the charging performance of the lithium ion battery is facilitated, and the service life of the battery can be effectively prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a negative electrode sheet, a preparation method thereof, and a battery. Background Art

[0002] Lithium-ion batteries have the characteristics of high energy density, long cycle life, low cost and low environmental pollution, which makes them stand out among the types of clean energy. At present, the energy density of lithium-ion batteries has reached its limit. Under the background that the energy density will not be greatly improved, improving the fast charging performance of lithium-ion batteries has become a breakthrough direction. The biggest factor affecting the fast charging performance of lithium-ion batteries is the graphite negative electrode material. Due to the anisotropy of graphite structure, Li + It must be embedded from the end face of graphite and gradually diffuse into the interior of the particle. The diffusion path is long, and the small interlayer spacing of graphite (0.335nm) also causes Li + The diffusion rate is low, which limits its fast charging performance.

[0003] The existing technology improves the fast charging performance of graphite itself by modifying the graphite material; or designs the structure of the negative electrode such as double-layer coating to improve the fast charging performance of the battery. The active material coated on the upper layer is a material with high kinetics, which is beneficial to Li + The rapid diffusion of graphite reduces polarization and the occurrence of lithium precipitation in graphite. The active material coated on the lower layer is a high-capacity material, which is conducive to improving energy density. The reasonable design of the upper and lower materials is conducive to achieving a balance between fast charging performance and energy density of lithium-ion batteries, further expanding the application scenarios of lithium-ion batteries. However, simply modifying the graphite material is difficult to achieve the requirements for fast charging performance of batteries. The double-layer coating design has a large gap in the interlayer spacing between the upper and lower layers of active material, which will lead to a large gradient in the lithium ion diffusion rate, greater stress on the electrode, and cause the active material to fall off, resulting in a shortened service life of the negative electrode.

[0004] Therefore, it is necessary to take effective measures to modify the graphite material and improve the structural design of the negative electrode sheet to improve the fast charging performance and service life of the battery. Summary of the Invention

[0005] In view of this, the present invention aims to solve at least one of the technical problems in the related art to a certain extent. To this end, the present invention provides a negative electrode sheet and a preparation method thereof, and a battery having excellent fast charging performance and a long service life.

[0006] In order to solve the above technical problems, this application is implemented as follows:

[0007] According to one aspect of the present application, an embodiment of the present application provides a negative electrode sheet, the negative electrode sheet comprising:

[0008] current collector;

[0009] a first negative electrode coating disposed on at least one side of the current collector in a thickness direction, the first negative electrode coating comprising a first carbon-based active material;

[0010] a second negative electrode coating, disposed on a surface of the first negative electrode coating away from the current collector, the second negative electrode coating comprising a second carbon-based active material; and

[0011] a third negative electrode coating, disposed on a surface of the second negative electrode coating away from the first negative electrode coating, the third negative electrode coating comprising a third carbon-based active material;

[0012] wherein the interlayer spacing of the first carbon-based active material is smaller than the interlayer spacing of the second carbon-based active material;

[0013] The interlayer spacing of the second carbon-based active material is smaller than the interlayer spacing of the third carbon-based active material.

[0014] In some embodiments, the interlayer spacing of the first carbon-based active material is 0.33-0.34 nm.

[0015] In some embodiments, the interlayer spacing of the second carbon-based active material is 0.34-0.38 nm.

[0016] In some embodiments, the interlayer spacing of the third carbon-based active material is 0.37-0.40 nm.

[0017] In some embodiments, the first carbon-based active material includes graphite.

[0018] In some of these embodiments, the graphite includes secondary particle graphite.

[0019] In some embodiments, the average particle size D50 of the first carbon-based active material is 10 to 20 μm.

[0020] In some embodiments, the compacted density of the first carbon-based active material is 1.45-1.75 g / cm 3 .

[0021] In some embodiments, the first negative electrode coating further includes a first conductive agent, a first binder, and a first thickener.

[0022] In some embodiments, the mass ratio of the first carbon-based active material, the first conductive agent, the first binder and the first thickener is (96-97): (0.5-1.5): (0.5-2): (0.6-1.5).

[0023] In some embodiments, the thickness of the first negative electrode coating is 0.08 to 0.15 mm.

[0024] In some embodiments, the second carbon-based active material includes a first heteroatom-doped soft carbon.

[0025] In some embodiments, the first heteroatom includes at least one of a boron atom, a nitrogen atom, an oxygen atom, a phosphorus atom, a sulfur atom, a fluorine atom, or a chlorine atom.

[0026] In some embodiments, based on the total mass of the soft carbon doped with the first heteroatom, the mass fraction of the first heteroatom is 0.1-1%.

[0027] In some embodiments, the average particle size D50 of the second carbon-based active material is 10 to 20 μm.

[0028] In some embodiments, the compacted density of the second carbon-based active material is 1.45-1.75 g / cm 3 .

[0029] In some embodiments, the second negative electrode coating further includes a second conductive agent, a second binder, and a second thickener.

[0030] In some embodiments, the mass ratio of the second carbon-based active material, the second conductive agent, the second binder and the second thickener is (96.5-97.5): (0.5-1.5): (0.5-1.5): (0.4-1).

[0031] In some embodiments, the thickness of the second negative electrode coating is 0.04 to 0.15 mm.

[0032] In some embodiments, the third carbon-based active material includes a second heteroatom-doped hard carbon.

[0033] In some embodiments, the second heteroatom includes at least one of a boron atom, a nitrogen atom, an oxygen atom, a phosphorus atom, a sulfur atom, a fluorine atom, or a chlorine atom.

[0034] In some embodiments, the mass fraction of the second heteroatom is 1-3% based on the total mass of the hard carbon doped with the second heteroatom.

[0035] In some embodiments, the average particle size D50 of the third carbon-based active material is 10 to 20 μm.

[0036] In some embodiments, the compacted density of the third carbon-based active material is 1.45-1.75 g / cm 3 .

[0037] In some embodiments, the third negative electrode coating further includes a third conductive agent, a third binder, and a third thickener.

[0038] In some embodiments, the mass ratio of the third carbon-based active material, the third conductive agent, the third binder and the third thickener is (97.5-98): (0.5-1.5): (0.2-1): (0.2-0.8).

[0039] In some embodiments, the thickness of the third negative electrode coating is 0.04 to 0.15 mm.

[0040] According to another aspect of the present application, an embodiment of the present application provides a method for preparing a negative electrode sheet, comprising the following steps:

[0041] Coating a first negative electrode slurry on at least one side of the current collector to obtain a first negative electrode coating;

[0042] coating a second negative electrode slurry on the surface of the first negative electrode coating to obtain a second negative electrode coating;

[0043] coating a third negative electrode slurry on the surface of the second negative electrode coating to obtain a third negative electrode coating;

[0044] Drying the current collector coated with the first negative electrode coating, the second negative electrode coating, and the third negative electrode coating to obtain a negative electrode sheet;

[0045] Wherein, the first negative electrode slurry includes a first carbon-based active material;

[0046] The second negative electrode slurry includes a second carbon-based active material;

[0047] The third negative electrode slurry includes a third carbon-based active material;

[0048] The interlayer spacing of the first carbon-based active material is smaller than the interlayer spacing of the second carbon-based active material; and the interlayer spacing of the second carbon-based active material is smaller than the interlayer spacing of the third carbon-based active material.

[0049] In some embodiments, the preparation of the first negative electrode slurry includes: uniformly mixing a first carbon-based active material, a first conductive agent, a first binder, and a first thickener in a solvent to obtain the first negative electrode slurry.

[0050] In some embodiments, the preparation of the second negative electrode slurry includes: uniformly mixing a second carbon-based active material, a second conductive agent, a second binder, and a second thickener in a solvent to obtain a second negative electrode slurry.

[0051] In some embodiments, the preparation of the third negative electrode slurry includes: uniformly mixing a third carbon-based active material, a third conductive agent, a third binder, and a third thickener in a solvent to obtain the third negative electrode slurry.

[0052] In some embodiments, the viscosity of the first negative electrode slurry is 4000-5000 mPa·s, and the solid content is 45-55%.

[0053] In some embodiments, the viscosity of the second negative electrode slurry is 3000-4000 mPa·s, and the solid content is 45-55%.

[0054] In some embodiments, the viscosity of the third negative electrode slurry is 2500-3500 mPa·s, and the solid content is 45-55%.

[0055] In some embodiments, the first negative electrode slurry is coated on the surface of the current collector, and after drying, a first negative electrode coating is formed on the surface of the current collector; the second negative electrode slurry is coated on the first negative electrode coating, and after drying, a second negative electrode coating is formed on the surface of the first negative electrode coating; the third negative electrode slurry is coated on the second negative electrode coating and dried; and the current collector coated with the first negative electrode coating, the second negative electrode coating, and the third negative electrode coating is roll-pressed to obtain a negative electrode sheet.

[0056] In some embodiments, the second carbon-based active material includes a first heteroatom-doped soft carbon, and the preparation method of the first heteroatom-doped soft carbon includes:

[0057] mixing a soft carbon source with a first heteroatom dopant and carbonizing the mixture to obtain first heteroatom-doped soft carbon;

[0058] The mass ratio of the soft carbon source to the first heteroatom dopant is 10:(1-5).

[0059] In some embodiments, the first heteroatom dopant includes at least one of borane, borax, boron tetrachloride, urea, ammonium chloride, ammonium sulfate, ammonium dihydrogen phosphate, elemental sulfur, thiourea, polyvinylidene fluoride, ammonium hexafluorophosphate, or hexafluorophosphoric acid.

[0060] In some embodiments, the carbonization temperature is 1200-1800° C., the carbonization time is 1-5 h, and the carbonization heating rate is 1-5° C. / min.

[0061] In some embodiments, the carbonization environment is an inert atmosphere.

[0062] In some embodiments, the third carbon-based active material includes a second heteroatom-doped hard carbon, and the preparation method of the second heteroatom-doped hard carbon includes:

[0063] mixing a hard carbon source with a second heteroatom dopant and carbonizing the mixture to obtain a second heteroatom-doped hard carbon;

[0064] The mass ratio of the hard carbon source to the second heteroatom dopant is 10:(1-5).

[0065] In some embodiments, the second heteroatom dopant includes at least one of borane, borax, boron tetrachloride, urea, ammonium chloride, ammonium sulfate, ammonium dihydrogen phosphate, elemental sulfur, thiourea, polyvinylidene fluoride, ammonium hexafluorophosphate, or hexafluorophosphoric acid.

[0066] In some embodiments, the carbonization temperature is 1200-1800° C., the carbonization time is 1-5 h, and the carbonization heating rate is 1-5° C. / min.

[0067] In some embodiments, the carbonization environment is an inert atmosphere.

[0068] According to another aspect of the present application, an embodiment of the present application provides a battery, including a negative electrode sheet, wherein the negative electrode sheet includes the aforementioned negative electrode sheet, or includes a negative electrode sheet prepared according to the aforementioned preparation method.

[0069] The implementation of the technical solution of the present invention has at least the following beneficial effects:

[0070] In the embodiment of the present application, in the negative electrode sheet provided, the interlayer spacing of the first carbon-based active material of the first negative electrode coating is the smallest, which can maintain a higher gram capacity; the interlayer spacing of the third carbon-based active material in the third negative electrode coating is the largest, which can maximize the diffusion rate of lithium ions and reduce the occurrence of polarization and lithium precipitation; the interlayer spacing of the second carbon-based active material in the second negative electrode coating is between the first carbon-based active material and the third carbon-based active material, which can reduce the diffusion gradient of lithium ions between the two, reduce the generation of electrode stress in subsequent cycles, and avoid the shedding of active materials on the electrode. Thus, during the charging process of the battery, lithium ions will begin to diffuse from the first carbon-based active material with the largest interlayer spacing, through the second carbon-based active material with a slightly smaller interlayer spacing, and finally to the third carbon-based active material with the smallest interlayer spacing, which can reduce the lithium ion diffusion rate gradient, reduce the generation of stress, and avoid the shedding of active materials; and the design of the above-mentioned negative electrode coating can gradually reduce the lithium ion diffusion rate, which is beneficial to reduce the occurrence of polarization during charging and discharging, and can avoid the occurrence of lithium precipitation from the anode electrode. Thereby, the overall diffusion rate of lithium ions can be greatly improved, which is conducive to achieving a leap-forward improvement in the charging performance of lithium-ion batteries and can effectively extend the service life of the batteries.

[0071] Additional aspects and advantages of the present application will be given in part in the following description and in part will become obvious from the following description or will be learned through practice of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 Shown is a schematic diagram of the negative electrode sheet structure provided in Example 1 of the present invention.

[0073] 1-current collector;

[0074] 2-first negative electrode coating;

[0075] 3-second negative electrode coating;

[0076] 4-Third negative electrode coating. DETAILED DESCRIPTION

[0077] The present application will be further described below with reference to specific examples. It should be understood that these examples are only used to illustrate the present application and are not intended to limit the scope of the present application.

[0078] The endpoints of the ranges and any values disclosed herein are not limited to the precise ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range or the individual point values, and the individual point values can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0079] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0080] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0081] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0082] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0083] In the related art, the fast charging capability of the battery is improved by modifying the graphite material or through a double-layer coating design. However, the above solutions still cannot effectively improve the fast charging capability of the battery, and there is also the defect of a short battery life.

[0084] In view of this, an embodiment of the present application provides a negative electrode sheet, the negative electrode sheet comprising:

[0085] current collector 1;

[0086] A first negative electrode coating 2 is provided on at least one side of the current collector 1 in the thickness direction, and the first negative electrode coating 2 includes a first carbon-based active material;

[0087] A second negative electrode coating 3 is provided on a surface of the first negative electrode coating 2 away from the current collector 1 , and the second negative electrode coating 3 includes a second carbon-based active material; and

[0088] a third negative electrode coating 4, disposed on a surface of the second negative electrode coating 3 away from the first negative electrode coating 2, the third negative electrode coating 4 comprising a third carbon-based active material;

[0089] wherein the interlayer spacing of the first carbon-based active material is smaller than the interlayer spacing of the second carbon-based active material;

[0090] The interlayer spacing of the second carbon-based active material is smaller than the interlayer spacing of the third carbon-based active material.

[0091] The provided negative electrode sheet has a multilayer structure, comprising a current collector 1, a first negative electrode coating 2, a second negative electrode coating 3, and a third negative electrode coating 4 stacked in sequence. Specifically, the first negative electrode coating 2 is disposed on at least one surface of the current collector 1, the second negative electrode coating 3 is disposed on the surface of the first negative electrode coating 2, and the third negative electrode coating 4 is disposed on the surface of the second negative electrode coating 3. The first negative electrode coating 2, the second negative electrode coating 3, and the third negative electrode coating 4 are all carbon-based material layers.

[0092] The phrase "the first negative electrode coating 2 is disposed on at least one surface of the current collector 1 along the thickness direction" means that the first negative electrode coating 2 can be disposed on one surface of the current collector 1 along its thickness direction, or on both surfaces of the current collector 1 along its thickness direction. The "surface" here can be the entire area of the current collector 1 or a portion of the current collector 1. For example, in this embodiment, the surface can be the entire area of the current collector 1. This is not particularly limited in this application, as long as the objectives of this application can be achieved.

[0093] As an example, the current collector 1 has two surfaces facing each other in its thickness direction, and the first negative electrode coating 2 is disposed on the two facing surfaces of the current collector 1. Furthermore, a second negative electrode coating 3 is formed on the surfaces of the first negative electrode coating 2 on both sides. Furthermore, a third negative electrode coating 4 is formed on the surfaces of the second negative electrode coating 3 on both sides. It is understood that in other embodiments, the first negative electrode coating 2 can also be laminated on either of the two surfaces of the current collector 1.

[0094] In the present application, in the negative electrode sheet, the material of the current collector 1 is not particularly limited.

[0095] In the present application, in the negative electrode sheet, the interlayer spacing of the first carbon-based active material of the first negative electrode coating 2 is the smallest, which can maintain a higher gram capacity; the interlayer spacing of the third carbon-based active material in the third negative electrode coating 4 is the largest, which can maximize the diffusion rate of lithium ions and reduce the occurrence of polarization and lithium precipitation; the interlayer spacing of the second carbon-based active material in the second negative electrode coating 3 is between the first carbon-based active material and the third carbon-based active material, which can reduce the diffusion gradient of lithium ions between the two, reduce the generation of electrode stress during subsequent cycles, and avoid the shedding of active materials on the electrode. Thus, during the charging process of the battery, lithium ions will begin to diffuse from the first carbon-based active material with the largest interlayer spacing, through the second carbon-based active material with a slightly smaller interlayer spacing, and finally to the third carbon-based active material with the smallest interlayer spacing, which can reduce the lithium ion diffusion rate gradient, reduce the generation of stress, and avoid the shedding of active materials; and the design of the above-mentioned negative electrode coating can gradually reduce the lithium ion diffusion rate, which is beneficial to reduce the occurrence of polarization during charging and discharging, and can avoid the occurrence of lithium precipitation on the anode electrode. Thereby, the overall diffusion rate of lithium ions can be greatly improved, which is conducive to achieving a leap-forward improvement in the charging performance of lithium-ion batteries and can effectively extend the service life of the batteries.

[0096] In some embodiments, the interlayer spacing of the first carbon-based active material is 0.33-0.34 nm. Preferably, the interlayer spacing of the first carbon-based active material is 0.335 nm.

[0097] In some embodiments, the interlayer spacing of the second carbon-based active material is 0.34-0.38 nm. As an example, the particle size of the second carbon-based active material can be 0.34 nm, 0.36 nm, 0.38 nm, etc., or any point within the above range, which is not specifically limited here.

[0098] In some embodiments, the interlayer spacing of the third carbon-based active material is 0.37-0.40 nm. As an example, the interlayer spacing of the third carbon-based active material can be 0.37 nm, 0.39 nm, 0.40 nm, etc., or any value within the above range, which is not specifically limited here.

[0099] The interlayer spacing of the carbon-based active material in the first negative electrode coating 2, the second negative electrode coating 3, and the third negative electrode coating 4 of the negative electrode sheet of the present application increases successively. It can also be understood that the interlayer spacing of the carbon-based active material gradually decreases along the direction of lithium ion diffusion, which can reduce the generation of stress. Furthermore, the large interlayer spacing is conducive to the rapid diffusion of lithium ions and improves the fast charging capability of the battery.

[0100] In some embodiments, the first carbon-based active material includes graphite.

[0101] In some embodiments, the graphite includes secondary graphite particles. Secondary graphite particles have good isotropy, allowing lithium ions to be embedded in all directions. Compared to single graphite particles, which can only embed in one direction, secondary graphite particles also have improved fast charging performance.

[0102] In some embodiments, the average particle size D50 of the first carbon-based active material is 10 to 20 μm. As an example, the average particle size D50 of the first carbon-based active material can be 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, etc., and can also be any point within the above range, which is not specifically limited here.

[0103] In some embodiments, the first carbon-based active material has a compacted density of 1.45-1.75 g / cm 3 As an example, the compacted density of the first carbon-based active material may be 1.45 g / cm 3 , 1.55g / cm 3 , 1.65g / cm 3 , 1.75g / cm 3 Of course, it can also be a certain point value within the above range, which is not specifically limited here.

[0104] In some embodiments, the first negative electrode coating 2 further includes a first conductive agent, a first binder, and a first thickener.

[0105] In some embodiments, the mass ratio of the first carbon-based active material, the first conductive agent, the first binder, and the first thickener is (96-97): (0.5-1.5): (0.5-2): (0.6-1.5). As an example, the mass ratio of the first carbon-based active material, the first conductive agent, the first binder, and the first thickener can be 96:0.5:0.5:0.6, 96.5:1:1:1, 97:1.5:2:1.5, etc. Of course, it can also be any ratio within the above range, and is not specifically limited here.

[0106] In some embodiments, the thickness of the first negative electrode coating 2 is 0.08-0.15 mm. As an example, the thickness of the first negative electrode coating 2 can be 0.08 mm, 0.1 mm, 0.12 mm, 0.14 mm, 0.15 mm, etc. Of course, it can also be a point value within the above range, and is not specifically limited here.

[0107] In some embodiments, the second carbon-based active material comprises soft carbon doped with a first heteroatom. By limiting the carbon source to soft carbon, a second carbon-based active material having a suitable interlayer spacing can be obtained after being doped with the first heteroatom.

[0108] In some embodiments, the soft carbon includes, but is not limited to, at least one of coal tar pitch, petroleum pitch, coal liquefaction pitch, ethylene tar pitch, natural pitch, anthracite, and mesocarbon microbeads. For example, the soft carbon may be coal tar pitch, petroleum pitch, or coal liquefaction pitch.

[0109] In some embodiments, the first heteroatom includes, but is not limited to, at least one of a boron atom, a nitrogen atom, an oxygen atom, a phosphorus atom, a sulfur atom, a fluorine atom, or a chlorine atom. For example, the first heteroatom may be a boron atom, a nitrogen atom, or an oxygen atom. Preferably, the first heteroatom is a boron atom, as the introduction of a boron atom into the carbon lattice can affect the arrangement of the carbon layers by changing the electronic structure between the layers, thereby changing the interlayer spacing.

[0110] In some embodiments, the first heteroatom dopant includes, but is not limited to, at least one of borane, borax, boron tetrachloride, urea, ammonium chloride, ammonium sulfate, ammonium dihydrogen phosphate, elemental sulfur, thiourea, polyvinylidene fluoride, ammonium hexafluorophosphate, or potassium hexafluorophosphate.

[0111] In some embodiments, the mass fraction of the first heteroatom is 0.1-1% based on the total mass of the soft carbon doped with the first heteroatom. As an example, the mass fraction of the first heteroatom can be 0.1%, 0.3%, 0.5%, 0.7%, 0.9%, 1%, etc. based on the total mass of the soft carbon doped with the first heteroatom. Of course, it can also be a point value within the above range, and is not specifically limited here.

[0112] In some embodiments, the average particle size D50 of the second carbon-based active material is 10 to 20 μm. As an example, the average particle size D50 of the second carbon-based active material can be 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, etc., and can also be any point within the above range, which is not specifically limited here.

[0113] In some embodiments, the compacted density of the second carbon-based active material is 1.45-1.75 g / cm 3As an example, the compacted density of the second carbon-based active material may be 1.45 g / cm 3 , 1.55g / cm 3 , 1.65g / cm 3 , 1.75g / cm 3 Of course, it can also be a certain point value within the above range, which is not specifically limited here.

[0114] In some embodiments, the second negative electrode coating layer 3 further includes a second conductive agent, a second binder, and a second thickener.

[0115] In some embodiments, the mass ratio of the second carbon-based active material, the second conductive agent, the second binder, and the second thickener is (96.5-97.5):(0.5-1.5):(0.5-1.5):(0.4-1). As an example, the mass ratio of the second carbon-based active material, the second conductive agent, the second binder, and the second thickener can be 96.5:0.5:0.5:0.4, 97:1:1:0.8, 97.5:1.5:1.5:1, etc. Of course, it can also be any ratio within the above range, and is not specifically limited here.

[0116] In some embodiments, the thickness of the second negative electrode coating 3 is 0.04-0.15 mm. As an example, the thickness of the second negative electrode coating 3 can be 0.04 mm, 0.06 mm, 0.08 mm, 0.1 mm, 0.12 mm, 0.14 mm, 0.15 mm, etc. Of course, it can also be a point value within the above range, and is not specifically limited here.

[0117] In some embodiments, the third carbon-based active material includes hard carbon doped with a second heteroatom. By limiting the carbon source to hard carbon, a third carbon-based active material with a suitable interlayer spacing can be obtained after being doped with the second heteroatom.

[0118] In some embodiments, the hard carbon includes, but is not limited to, at least one of coconut shell, cotton, wood, glucose, lignin, cellulose, phenolic resin, polyaniline, or polyacrylonitrile. As an example, the hard carbon can be coconut shell, cotton, or glucose.

[0119] In some embodiments, the second heteroatom includes, but is not limited to, at least one of a boron atom, a nitrogen atom, an oxygen atom, a phosphorus atom, a sulfur atom, a fluorine atom, or a chlorine atom. For example, the second heteroatom may be a boron atom, a nitrogen atom, or an oxygen atom. Preferably, the second heteroatom is a boron atom, as the introduction of a boron atom into the carbon lattice can affect the arrangement of the carbon layers by changing the electronic structure between the layers, thereby changing the interlayer spacing.

[0120] In some embodiments, the second heteroatom dopant includes, but is not limited to, at least one of borane, borax, boron tetrachloride, urea, ammonium chloride, ammonium sulfate, ammonium dihydrogen phosphate, elemental sulfur, thiourea, polyvinylidene fluoride, ammonium hexafluorophosphate, or potassium hexafluorophosphate.

[0121] In some embodiments, the mass fraction of the second heteroatom is 1-3% based on the total mass of the hard carbon doped with the second heteroatom. For example, the mass fraction of the second heteroatom can be 1%, 2%, 3%, etc., based on the total mass of the hard carbon doped with the second heteroatom. Of course, it can also be any value within the above range, and is not specifically limited here.

[0122] In some embodiments, the heteroatoms doped in the same electrode are preferably of the same type, i.e., the heteroatom dopant is preferably of the same type. If two or more heteroatoms are used for doping, the interlayer spacing of the third carbon-based active material (heteroatom-doped hard carbon) must be greater than that of the second carbon-based active material (heteroatom-doped soft carbon).

[0123] In some embodiments, the average particle size D50 of the third carbon-based active material is 10 to 20 μm. As an example, the average particle size D50 of the third carbon-based active material can be 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, etc., and can also be any point value within the above range, which is not specifically limited here.

[0124] In some embodiments, the compacted density of the third carbon-based active material is 1.45-1.75 g / cm 3 As an example, the compacted density of the second carbon-based active material may be 1.45 g / cm 3 , 1.55g / cm 3 , 1.65g / cm 3 , 1.75g / cm 3 Of course, it can also be a certain point value within the above range, which is not specifically limited here.

[0125] In some embodiments, the third negative electrode coating 4 further includes a third conductive agent, a third binder, and a third thickener.

[0126] In some embodiments, the mass ratio of the third carbon-based active material, the third conductive agent, the third binder, and the third thickener is (97.5-98): (0.5-1.5): (0.2-1): (0.2-0.8). As an example, the mass ratio of the third carbon-based active material, the third conductive agent, the third binder, and the third thickener can be 97.5:0.5:0.2:0.2, 97.8:1:0.5:0.5, 98:1.5:1:0.8, etc. Of course, it can also be any ratio within the above range, and is not specifically limited here.

[0127] In some embodiments, the thickness of the third negative electrode coating 4 is 0.04-0.15 mm. As an example, the thickness of the third negative electrode coating 4 can be 0.04 mm, 0.06 mm, 0.08 mm, 0.1 mm, 0.12 mm, 0.14 mm, 0.15 mm, etc. Of course, it can also be a point value within the above range, and is not specifically limited here.

[0128] It should be noted that the first conductive agent, the second conductive agent, and the third conductive agent each independently include, but are not limited to, at least one of conductive graphite, conductive carbon black, conductive carbon fiber, carbon nanotubes, or graphene. The conductive carbon black includes acetylene black, Ketjen black, and the like. The conductive carbon fiber includes vapor-grown carbon fiber.

[0129] It should also be noted that the first binder, the second binder, and the third binder each independently include, but are not limited to, at least one of polyvinylidene fluoride, sodium carboxymethyl cellulose, styrene-butadiene rubber, sodium alginate, or polyacrylic acid. By selecting the type of binder and the binder ratio of each negative electrode coating, the adverse effects of binder floating during the drying process can be reduced, and the possibility of separation between the coatings during subsequent battery cycling can be reduced.

[0130] It should also be noted that the first thickener, the second thickener and the third thickener each independently include, but are not limited to, at least one of sodium carboxymethyl cellulose, hydroxypropyl methylcellulose or polyvinyl pyrrolidone.

[0131] It is understandable that the mass ratio of the first carbon-based active material, the first conductive agent, the first binder and the first thickener, the mass ratio of the second carbon-based active material, the second conductive agent, the second binder and the second thickener, and the mass ratio of the third carbon-based active material, the third conductive agent, the third binder and the third thickener are related to the electrochemical performance of the corresponding battery, and further affect the structural stability of the negative electrode sheet. By controlling the ratio of each substance in the first negative electrode coating 2, the second negative electrode coating 3, and the third negative electrode coating 4 within the above range, the interaction between the materials is fully utilized, and the cycle performance, fast charging performance and structural stability of the negative electrode sheet are effectively improved.

[0132] It can also be understood that the surface thickness of the coating will affect the electrical properties of the battery (such as energy density, cycle performance and rate performance), preparation cost and safety performance. If the thickness of the first negative electrode coating 2, the second negative electrode coating 3 and the third negative electrode coating 4 is too large, the electron transmission distance increases, the electronic resistance increases, and the rate performance decreases, which has an adverse effect on the electrical performance of the battery and further increases the difficulty of thermal management of the battery. However, if the thickness of the first negative electrode coating 2, the second negative electrode coating 3 and the third negative electrode coating 4 is too low, the structural stability of the positive electrode sheet will not be significantly improved, which will have an adverse effect on the safety and long-term cycle stability of the battery.

[0133] In some embodiments, the current collector 1 includes, but is not limited to, at least one of copper foil, nickel foam, nickel mesh, or composite copper foil. As an example, the current collector 1 can be copper foil or nickel foam.

[0134] Thus, based on the above, a negative electrode sheet is provided. In the negative electrode sheet, the interlayer spacing of the first carbon-based active material of the first negative electrode coating 2 is the smallest, which can maintain a higher gram capacity; the interlayer spacing of the third carbon-based active material in the third negative electrode coating 4 is the largest, which can maximize the diffusion rate of lithium ions and reduce the occurrence of polarization and lithium precipitation; the interlayer spacing of the second carbon-based active material in the second negative electrode coating 3 is between the first carbon-based active material and the third carbon-based active material, which can reduce the diffusion gradient of lithium ions between the two, reduce the generation of electrode stress during subsequent cycles, and avoid the shedding of active materials on the electrode. Therefore, during the charging process of the battery, lithium ions will start to diffuse from the first carbon-based active material with the largest interlayer spacing, pass through the second carbon-based active material with a slightly smaller interlayer spacing, and finally the third carbon-based active material with the smallest interlayer spacing, which can reduce the lithium ion diffusion rate gradient, reduce the generation of stress, and avoid the shedding of active materials; and the design of the above-mentioned negative electrode coating can gradually reduce the lithium ion diffusion rate, which is conducive to reducing the occurrence of polarization during charging and discharging, and can avoid the occurrence of lithium precipitation on the anode electrode. Thereby, the overall diffusion rate of lithium ions can be greatly improved, which is conducive to achieving a leap-forward improvement in the charging performance of lithium-ion batteries and can effectively extend the service life of the batteries.

[0135] Furthermore, by limiting the carbon source of the second and third carbon-based active materials and doping them with heteroatoms, the interlayer spacing of the carbon source is expanded, which facilitates the rapid diffusion of lithium ions and further improves the fast charging performance of the battery. In addition, by selecting the type of binder and the binder ratio of each negative electrode coating, the adverse effects of the binder floating during the drying process of the electrode can be reduced, and the possibility of shedding between the coatings during subsequent battery cycles can be reduced.

[0136] Based on the same inventive concept, an embodiment of the present application provides a method for preparing a negative electrode sheet, comprising the following steps:

[0137] Coating a first negative electrode slurry on at least one side of the current collector 1 to obtain a first negative electrode coating 2;

[0138] Coating a second negative electrode slurry on the surface of the first negative electrode coating 2 to obtain a second negative electrode coating 3;

[0139] coating a third negative electrode slurry on the surface of the second negative electrode coating 3 to obtain a third negative electrode coating 4;

[0140] The current collector 1 coated with the first negative electrode coating 2, the second negative electrode coating 3, and the third negative electrode coating 4 is dried to obtain a negative electrode sheet;

[0141] Wherein, the first negative electrode slurry includes a first carbon-based active material;

[0142] The second negative electrode slurry includes a second carbon-based active material;

[0143] The third negative electrode slurry includes a third carbon-based active material;

[0144] The interlayer spacing of the first carbon-based active material is smaller than the interlayer spacing of the second carbon-based active material; and the interlayer spacing of the second carbon-based active material is smaller than the interlayer spacing of the third carbon-based active material.

[0145] It should be understood that all the features and advantages described above for the “negative electrode sheet” are also applicable to the “method for preparing the negative electrode sheet” and will not be described in detail here.

[0146] In some embodiments, the preparation of the first negative electrode slurry includes: uniformly mixing a first carbon-based active material, a first conductive agent, a first binder, and a first thickener in a solvent to obtain the first negative electrode slurry.

[0147] In some embodiments, the preparation of the second negative electrode slurry includes: uniformly mixing a second carbon-based active material, a second conductive agent, a second binder, and a second thickener in a solvent to obtain the second negative electrode slurry.

[0148] In some embodiments, the preparation of the third negative electrode slurry includes: uniformly mixing a third carbon-based active material, a third conductive agent, a third binder, and a third thickener in a solvent to obtain the third negative electrode slurry.

[0149] In some embodiments, the viscosity of the first negative electrode slurry is 4000-5000 mPa·s and the solids content is 45-55%. As an example, the viscosity of the first negative electrode slurry can be 4000 mPa·s, 4500 mPa·s, 5000 mPa·s, etc., and of course it can also be a value within the above range, and is not specifically limited here. The solids content of the first negative electrode slurry can be 45%, 50%, 55%, etc., and of course it can also be a value within the above range, and is not specifically limited here.

[0150] In some embodiments, the viscosity of the second negative electrode slurry is 3000-4000 mPa·s and the solids content is 45-55%. As an example, the viscosity of the second negative electrode slurry can be 3000 mPa·s, 3500 mPa·s, 4000 mPa·s, etc., and of course it can also be a value within the above range, and is not specifically limited here. The solids content of the second negative electrode slurry can be 45%, 50%, 55%, etc., and of course it can also be a value within the above range, and is not specifically limited here.

[0151] In some embodiments, the viscosity of the third negative electrode slurry is 2500-3500 mPa·s and the solids content is 45-55%. As an example, the viscosity of the third negative electrode slurry can be 2500 mPa·s, 3000 mPa·s, 3500 mPa·s, etc., and of course it can also be a value within the above range, and is not specifically limited here. The solids content of the third negative electrode slurry can be 45%, 50%, 55%, etc., and of course it can also be a value within the above range, and is not specifically limited here.

[0152] In some embodiments, the solvents in the first negative electrode slurry, the second negative electrode slurry, and the third negative electrode slurry are each independently selected from water.

[0153] In some embodiments, a first negative electrode slurry is coated on the surface of a current collector 1, and after drying, a first negative electrode coating 2 is formed on the surface of the current collector 1; a second negative electrode slurry is coated on the first negative electrode coating 2, and after drying, a second negative electrode coating 3 is formed on the surface of the first negative electrode coating 2; a third negative electrode slurry is coated on the second negative electrode coating 3 and dried; and the current collector 1 coated with the first negative electrode coating 2, the second negative electrode coating 3, and the third negative electrode coating 4 is roll-pressed to obtain a negative electrode sheet.

[0154] In some embodiments, the second carbon-based active material includes a first heteroatom-doped soft carbon, and the preparation method of the first heteroatom-doped soft carbon includes:

[0155] mixing a soft carbon source with a first heteroatom dopant and carbonizing the mixture to obtain first heteroatom-doped soft carbon;

[0156] The mass ratio of the soft carbon source to the first heteroatom dopant is 10:(1-5).

[0157] In some embodiments, the mass ratio of the soft carbon source to the first heteroatom dopant can be 10:1, 10:2, 10:3, 10:5, etc., or any other ratio within the aforementioned range, without specific limitation herein. The mass ratio of the soft carbon source to the first heteroatom dopant affects the interlayer spacing of the doped soft carbon. By limiting the mass ratio of the soft carbon source to the first heteroatom dopant to within the aforementioned range, heteroatom-doped soft carbon with a suitable interlayer spacing can be obtained.

[0158] In some embodiments, the first heteroatom dopant includes at least one of borane, borax, boron tetrachloride, urea, ammonium chloride, ammonium sulfate, ammonium dihydrogen phosphate, elemental sulfur, thiourea, polyvinylidene fluoride, ammonium hexafluorophosphate, or hexafluorophosphoric acid. As an example, the first heteroatom dopant can be borane, borax, or ammonium chloride.

[0159] In some embodiments, the carbonization temperature is 1200-1800°C, the carbonization time is 1-5h, and the carbonization heating rate is 1-5°C / min. As an example, the carbonization temperature can be 1200°C, 1400°C, 1600°C, 1800°C, etc., and of course it can also be a point value within the above range, which is not specifically limited here. The carbonization time can be 1h, 3h, 5h, etc., and of course it can also be a point value within the above range, which is not specifically limited here. The carbonization heating rate can be 1°C / min, 3°C / min, 5°C / min, etc., and of course it can also be a point value within the above range, which is not specifically limited here.

[0160] In some embodiments, the carbonization environment is an inert atmosphere. For example, the carbonization environment may be a nitrogen atmosphere.

[0161] In some embodiments, the third carbon-based active material includes a second heteroatom-doped hard carbon, and the preparation method of the second heteroatom-doped hard carbon includes:

[0162] mixing a hard carbon source with a second heteroatom dopant and carbonizing the mixture to obtain a second heteroatom-doped hard carbon;

[0163] The mass ratio of the hard carbon source to the second heteroatom dopant is 10:(5-10).

[0164] In some embodiments, the mass ratio of the hard carbon source to the second heteroatom dopant can be 10:5, 10:6, 10:8, 10:10, or other ratios. Alternatively, the mass ratio can be within the aforementioned range and is not specifically limited herein. The mass ratio of the hard carbon source to the second heteroatom dopant affects the interlayer spacing of the doped soft carbon. By limiting the mass ratio of the hard carbon source to the second heteroatom dopant to within the aforementioned range, heteroatom-doped hard carbon with a suitable interlayer spacing can be obtained.

[0165] In some embodiments, the second heteroatom dopant includes at least one of borane, borax, boron tetrachloride, urea, ammonium chloride, ammonium sulfate, ammonium dihydrogen phosphate, elemental sulfur, thiourea, polyvinylidene fluoride, ammonium hexafluorophosphate, or hexafluorophosphoric acid. As an example, the first heteroatom dopant can be borane, borax, or ammonium chloride.

[0166] In some embodiments, the carbonization temperature is 1200-1800°C, the carbonization time is 1-5h, and the carbonization heating rate is 1-5°C / min. For example, the carbonization temperature can be 1200°C, 1400°C, 1600°C, 1800°C, etc., and of course it can also be a point value within the above range, which is not specifically limited here. The carbonization time can be 1h, 3h, 5h, etc., and of course it can also be a point value within the above range, which is not specifically limited here. The carbonization heating rate can be 1°C / min, 3°C / min, 5°C / min, etc., and of course it can also be a point value within the above range, which is not specifically limited here.

[0167] In some embodiments, the carbonization environment is an inert atmosphere. For example, the carbonization environment may be a nitrogen atmosphere.

[0168] It should be noted that when a soft carbon source or a hard carbon source is doped with heteroatoms, the carbonization temperature, carbonization time, and carbonization heating rate all have an impact on the interlayer spacing of the final material. For example, as the carbonization temperature increases, the degree of graphitization of the hard carbon / soft carbon increases, and the interlayer spacing decreases. By regulating the carbonization temperature, carbonization time, and carbonization heating rate, it is possible to ensure that the interlayer spacing of the hard carbon material doped with heteroatoms of the third carbon-based active substance is greater than the interlayer spacing of the soft carbon material doped with heteroatoms of the second carbon-based active substance, so that the diffusion rate of lithium ions meets the condition of gradually decreasing.

[0169] Thus, based on the above scheme, in the negative electrode sheet, the interlayer spacing of the first carbon-based active material of the first negative electrode coating 2 is the smallest, which can maintain a higher gram capacity; the interlayer spacing of the third carbon-based active material in the third negative electrode coating 4 is the largest, which can maximize the diffusion rate of lithium ions and reduce the occurrence of polarization and lithium precipitation; the interlayer spacing of the second carbon-based active material in the second negative electrode coating 3 is between the first carbon-based active material and the third carbon-based active material, which can reduce the diffusion gradient of lithium ions between the two, reduce the generation of electrode stress during subsequent cycles, and avoid the shedding of active materials on the electrode. Therefore, during the charging process of the battery, lithium ions will begin to diffuse from the first carbon-based active material with the largest interlayer spacing, through the second carbon-based active material with a slightly smaller interlayer spacing, and finally to the third carbon-based active material with the smallest interlayer spacing, which can reduce the lithium ion diffusion rate gradient, reduce the generation of stress, and avoid the shedding of active materials; and the design of the above negative electrode coating can gradually reduce the lithium ion diffusion rate, which is beneficial to reduce the occurrence of polarization during charging and discharging, and can avoid the occurrence of lithium precipitation on the anode electrode. Thereby, the overall diffusion rate of lithium ions can be greatly improved, which is conducive to achieving a leap-forward improvement in the charging performance of lithium-ion batteries and can effectively extend the service life of the batteries.

[0170] Furthermore, this application improves the fast-charging performance of the battery by limiting the carbon source of the second carbon-based active material and the third carbon-based active material, doping them with heteroatoms, and expanding the interlayer spacing of the carbon source by controlling the doping process, thereby facilitating the rapid diffusion of lithium ions. In addition, by selecting the type of binder and the binder ratio of each negative electrode coating, the adverse effects of the binder floating on the electrode during the drying process can be reduced, and the possibility of shedding between the coatings during subsequent battery cycles can be reduced.

[0171] Based on the same inventive concept, an embodiment of the present application provides a battery, including a negative electrode sheet, wherein the negative electrode sheet includes the aforementioned negative electrode sheet, or includes a negative electrode sheet prepared according to the aforementioned preparation method.

[0172] Since the battery includes the negative electrode sheet provided in the embodiment of the present application, it has relatively excellent fast charging performance and structural stability.

[0173] In some embodiments, the battery may be a lithium-ion battery. The battery stack type may be, for example, a wound or laminated battery, and the battery structure may be, for example, a prismatic (aluminum, steel, etc.) battery, a soft-pack battery, or a cylindrical battery, without limitation.

[0174] In some embodiments, the battery further comprises a positive electrode sheet, an electrolyte, and a separator. That is, the battery comprises a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator.

[0175] In this embodiment, there is no limitation on the materials and structures of the positive electrode sheet, the positive current collector, the conductive agent, the binder in the positive active material layer, and the like. Positive electrode sheet structures and components known in the art that can be used for secondary batteries can be selected.

[0176] In this embodiment, there is no limitation on the specific material or type of the separator, and any separator known in the art that can be used in secondary batteries can be selected.

[0177] It should also be noted that the battery of the present application is not limited to the specific material or type of the electrolyte, and any components and types that are known in the art and can be used for secondary batteries can be selected as long as the purpose of the present application can be achieved.

[0178] Since the battery provided by the embodiment of the present invention adopts all the technical solutions of the above embodiments, it has at least all the beneficial effects brought by the technical solutions of the above embodiments, which will not be described one by one here.

[0179] Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents, materials, or instruments used that do not specify the manufacturer are all conventional products that can be purchased commercially.

[0180] Example 1

[0181] S1: Primary artificial graphite particles prepared from needle coke are granulated to obtain secondary graphite particles. The particle size D50 of the secondary graphite particles is 11.5 μm and the compacted density is 1.54 g / cm 3 .

[0182] S2: 10g of a mixture of petroleum asphalt and natural asphalt (5g:5g) was ground and mixed with 4g of urea. The mixture was then heated to 1500°C at a rate of 3°C / min under an inert atmosphere. The mixture was kept at this temperature for 3h and cooled to obtain nitrogen-doped soft carbon. The particle size D50 of the nitrogen-doped soft carbon was 13.5μm and the compacted density was 1.46g / cm 3 .

[0183] S3: 10g of a mixture of phenolic resin and polyacrylonitrile (8g:2g) was ground and mixed with 8g of urea. The mixture was then heated to 1700°C at a rate of 5°C under an inert atmosphere. The mixture was kept at this temperature for 2 hours and cooled to obtain nitrogen-doped hard carbon. The particle size D50 of the sulfur-doped hard carbon was 12.7μm and the compacted density was 1.64g / cm 3 .

[0184] S4: Dry-mix the secondary graphite particles and conductive carbon black. Then, add styrene-butadiene rubber, sodium carboxymethyl cellulose, and water, respectively, and mix thoroughly to form a first negative electrode slurry. The weight ratio of the secondary graphite particles, conductive carbon black, binder, and thickener is 96.2%:1.1%:1.1%:1.6%. The resulting first negative electrode slurry has a viscosity of 4600 mPa·s and a solids content of 51.1%.

[0185] S5: Dry-mix the heteroatom-doped soft carbon and conductive carbon black. Then, add styrene-butadiene rubber, sodium carboxymethyl cellulose, and water, respectively, and mix thoroughly to form a second negative electrode slurry. The weight ratio of the heteroatom-doped soft carbon, conductive carbon black, binder, and thickener is 96.7%:1.2%:0.7%:1.4%. The resulting second negative electrode slurry has a viscosity of 3450 mPa·s and a solids content of 51.5%.

[0186] S6: Heteroatom-doped hard carbon and conductive carbon black are dry-mixed. Then, styrene-butadiene rubber, sodium carboxymethyl cellulose, and solvent are added and stirred uniformly to form a third negative electrode slurry. The weight ratio of the heteroatom-doped hard carbon, conductive carbon black, binder, and thickener is 97.5%:1.0%:0.7%:0.8%. The resulting third negative electrode slurry has a viscosity of 3440 mPa·s and a solids content of 52.3%.

[0187] S7: Using a three-layer coating machine, apply a first negative electrode slurry on the surface of the current collector to obtain a first negative electrode coating, apply a second negative electrode slurry on the surface of the first negative electrode slurry to obtain a second negative electrode coating, apply a third negative electrode slurry on the surface of the second negative electrode slurry to obtain a third negative electrode coating, and dry it to obtain a negative electrode sheet.

[0188] Examples 2 to 6

[0189] The differences between Examples 2 to 6 and Example 1 are detailed in Tables 1 to 6.

[0190] Table 1

[0191]

[0192] Table 2

[0193]

[0194]

[0195] Table 3

[0196]

[0197]

[0198] Table 4

[0199]

[0200] Table 5

[0201]

[0202]

[0203] Table 6

[0204]

[0205] Comparative Example 1

[0206] The difference between Comparative Example 1 and Example 1 is that Comparative Example 1 only contains the first negative electrode coating layer.

[0207] Comparative Example 2

[0208] The difference between Comparative Example 2 and Example 1 is that Comparative Example 2 does not contain the second negative electrode coating, and the rest is the same as Example 1.

[0209] Comparative Example 3

[0210] The difference between Comparative Example 3 and Example 1 is that Comparative Example 3 does not contain the third negative electrode coating, and the rest is the same as Example 1.

[0211] Comparative Example 4

[0212] The difference between Comparative Example 4 and Example 1 is that in Comparative Example 4, a mixture of petroleum asphalt and natural asphalt is directly used as the carbon-based active material in the second negative electrode coating.

[0213] Comparative Example 5

[0214] The difference between Comparative Example 5 and Example 1 is that in Comparative Example 5, a mixture of phenolic resin and polyacrylonitrile is directly used as the carbon-based active material in the third negative electrode coating.

[0215] Comparative Example 6

[0216] The difference between Comparative Example 6 and Example 1 is that in Comparative Example 6, a mixture of petroleum asphalt and natural asphalt is directly used as the carbon-based active material in the second negative electrode coating; and a mixture of phenolic resin and polyacrylonitrile is directly used as the carbon-based active material in the third negative electrode coating.

[0217] Performance Testing

[0218] 1. Battery preparation

[0219] The negative electrode sheets obtained in the examples and comparative examples and the lithium iron phosphate positive electrode sheets were assembled into 2.8 Ah soft-pack batteries.

[0220] Preparation of positive electrode sheet: The positive electrode sheet used in the present invention is an electrode sheet made of 98wt.% active material lithium iron phosphate and 2wt.% PVDF. Lithium iron phosphate and PVDF are added to a certain amount of NMP according to the above mass ratio, and continuously stirred to prepare a uniform slurry. The slurry is evenly coated on a blank aluminum foil using a coating machine and dried at 80°C. The dried positive electrode sheet is rolled using a roller press.

[0221] Isolation film: PE-based film (thickness 10 μm) was used as the isolation film.

[0222] Electrolyte: In a glove box filled with inert gas, ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate were mixed in a mass ratio of 1:1:1 to obtain an organic solvent, and then lithium salt LiPF6 was dissolved in the organic solvent. The concentration of the lithium salt was 1 mol / L to obtain an electrolyte.

[0223] Battery assembly: The above-mentioned positive electrode sheet, the negative electrode sheet of the embodiment and the comparative example as the negative electrode, the separator, and the electrolyte are assembled into a soft-pack battery with a rated capacity of 2.8 Ah.

[0224] 2. Test the electrochemical performance of the battery

[0225] Lithium plating test (10C direct charge 80% SOC): Discharge a 2.8Ah soft-pack battery to 2.5V, then charge it at 10C for 4.8 minutes, i.e., to 80% SOC. The battery is then disassembled and the anode plate is observed for lithium plating. (Note: Lithium plating refers to the phenomenon where lithium ions are not embedded in the negative electrode material but remain on the surface of the anode plate. It is specifically manifested as an off-white portion on the surface of the anode plate. Normal plates without lithium plating are golden yellow and do not have any off-white portion. There are two degrees of lithium plating: mild lithium plating and severe lithium plating. Mild lithium plating refers to the presence of only a very small off-white area on the plate, and the off-white area accounts for approximately less than 20% of the total area of the plate. Severe lithium plating refers to the presence of an off-white area on the plate that accounts for more than 20% of the total area of the plate.)

[0226] Cycling test (4C charge and discharge for 100 cycles): The 2.8Ah soft-pack battery was discharged to 2.5V and then cycled for 100 cycles at 4C, with a charge upper limit voltage of 3.65V and a discharge lower limit voltage of 2.5V. After cycling, the soft-pack battery was disassembled and the anode electrode was inspected for lithium deposition and active material shedding. (Note: Active material shedding that accounts for less than 10% of the total electrode area is considered mild shedding; active material shedding that accounts for more than 10% of the total electrode area is considered severe shedding.)

[0227] The specific test results are shown in Table 7.

[0228] Table 7

[0229]

[0230]

[0231] It can be seen from the test results in Table 7 that the batteries prepared from the negative electrode sheets of Examples 1 to 6 did not exhibit lithium deposition after the lithium deposition test (10C direct charge 80% SOC), while the batteries prepared from the negative electrode sheets of Comparative Examples 1 and 2 exhibited severe lithium deposition and slight lithium deposition, respectively. The above test results appear because at a high current density of 10C, the migration rate of lithium ions in the material is less than the migration rate of lithium ions in the electrolyte, resulting in the precipitation of lithium ions on the graphite surface, thus leading to the occurrence of lithium deposition. The fundamental reason is that the insufficient interlayer spacing limits the migration rate of lithium ions. The more severe the lithium deposition phenomenon is, the greater the restriction of the interlayer spacing of the material on the migration of lithium ions. The above test results indicate that the negative electrode sheet designed in this application has excellent fast charging performance.

[0232] Furthermore, the batteries prepared from the negative electrode sheets of Examples 1 to 6 did not experience active material shedding or lithium precipitation in the cycle test (4C charge and discharge for 100 cycles), while the active material in Comparative Example 1 did not shed, but the lithium precipitation was severe; the active material in Comparative Example 2 shed severely, but the lithium precipitation was mild; the active material in Comparative Example 3 did not shed, but the lithium precipitation was severe; the active material in Comparative Example 4 shed slightly and the lithium precipitation was mild; the active material in Comparative Example 5 did not shed, but the lithium precipitation was severe; the active material in Comparative Example 6 did not shed, but the lithium precipitation was severe. In general, the occurrence of active material shedding is due to the fact that the gradient distribution of lithium concentration during fast charging causes uneven stress between components, which in turn makes the active material likely to shed. When the lithium concentration difference between different components is smaller, the stress distribution generated between different components is more uniform, and the less likely it is to cause stress-induced shedding between components, that is, shedding between active materials. The more severe the shedding phenomenon is, the greater the lithium concentration difference between the components, that is, the greater the difference in the interlayer spacing between the components. The test results show that the negative electrode sheet designed in this application can have a longer service life.

[0233] Parts of the present invention that are not described in detail are well known to those skilled in the art.

[0234] The basic principles of the present invention have been described above in conjunction with specific embodiments. However, it should be noted that the advantages, strengths, and effects mentioned in the present invention are merely illustrative and non-limiting, and should not be construed as necessarily possessed by each embodiment of the present invention. Furthermore, the specific details disclosed above are provided for illustrative purposes and to facilitate understanding, and are not intended to be limiting. These details do not necessarily limit the present invention to being implemented using these specific details.

[0235] It should be noted that the terms "and / or" or " / " used herein are merely a description of an association relationship between associated objects, indicating that three relationships may exist. For example, A and / or B may represent three situations: A exists alone, A and B exist at the same time, and B exists alone. The singular forms "a," "the," and "the" used in the embodiments of the present invention and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0236] In the detailed description and claims, a list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single element or multiple elements. Item B may contain a single element or multiple elements. Item C may contain a single element or multiple elements.

[0237] Finally, it should be noted that 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 aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A negative electrode sheet, characterized in that: The negative electrode sheet comprises: current collector; a first negative electrode coating disposed on at least one side of the current collector in a thickness direction, the first negative electrode coating comprising a first carbon-based active material; a second negative electrode coating, disposed on a surface of the first negative electrode coating away from the current collector, the second negative electrode coating comprising a second carbon-based active material; and a third negative electrode coating, disposed on a surface of the second negative electrode coating away from the first negative electrode coating, the third negative electrode coating comprising a third carbon-based active material; wherein the interlayer spacing of the first carbon-based active material is smaller than the interlayer spacing of the second carbon-based active material; The interlayer spacing of the second carbon-based active material is smaller than the interlayer spacing of the third carbon-based active material.

2. The negative electrode sheet according to claim 1, characterized in that: The interlayer spacing of the first carbon-based active material is 0.33 to 0.34 nm; and / or, the interlayer spacing of the second carbon-based active material is 0.34 to 0.38 nm; And / or, the interlayer spacing of the third carbon-based active material is 0.37-0.40 nm.

3. The negative electrode sheet according to claim 1, characterized in that: The first negative electrode coating satisfies at least one of the following characteristics (1) to (5): (1) The first carbon-based active material includes graphite; Preferably, the graphite comprises secondary particle graphite; (2) the average particle size D50 of the first carbon-based active material is 10 to 20 μm; (3) The compacted density of the first carbon-based active material is 1.45-1.75 g / cm 3 ; (4) The first negative electrode coating further includes a first conductive agent, a first binder and a first thickener; Preferably, the mass ratio of the first carbon-based active material, the first conductive agent, the first binder and the first thickener is (96-97): (0.5-1.5): (0.5-2): (0.6-1.5); (5) The thickness of the first negative electrode coating layer is 0.08 to 0.15 mm.

4. The negative electrode sheet according to claim 1, characterized in that: The second negative electrode coating satisfies at least one of the following characteristics (1) to (5): (1) The second carbon-based active material includes a first heteroatom-doped soft carbon; Preferably, the first heteroatom includes at least one of a boron atom, a nitrogen atom, an oxygen atom, a phosphorus atom, a sulfur atom, a fluorine atom or a chlorine atom; Preferably, based on the total mass of the soft carbon doped with the first heteroatom, the mass fraction of the first heteroatom is 0.1 to 1%; (2) the average particle size D50 of the second carbon-based active material is 10 to 20 μm; (3) The compacted density of the second carbon-based active material is 1.45-1.75 g / cm 3 ; (4) The second negative electrode coating further includes a second conductive agent, a second binder and a second thickener; Preferably, the mass ratio of the second carbon-based active material, the second conductive agent, the second binder and the second thickener is (96.5-97.5): (0.5-1.5): (0.5-1.5): (0.4-1); (5) The thickness of the second negative electrode coating layer is 0.04 to 0.15 mm.

5. The negative electrode sheet according to claim 1, characterized in that: The third negative electrode coating satisfies at least one of the following characteristics (1) to (5): (1) The third carbon-based active material includes a second heteroatom-doped hard carbon; Preferably, the second heteroatom includes at least one of a boron atom, a nitrogen atom, an oxygen atom, a phosphorus atom, a sulfur atom, a fluorine atom or a chlorine atom; Preferably, based on the total mass of the hard carbon doped with the second heteroatom, the mass fraction of the second heteroatom is 1 to 3%; (2) the average particle size D50 of the third carbon-based active material is 10 to 20 μm; (3) The compacted density of the third carbon-based active material is 1.45-1.75 g / cm 3 ; (4) The third negative electrode coating further includes a third conductive agent, a third binder and a third thickener; Preferably, the mass ratio of the third carbon-based active material, the third conductive agent, the third binder and the third thickener is (97.5-98): (0.5-1.5): (0.2-1): (0.2-0.8); (5) The thickness of the third negative electrode coating is 0.04 to 0.15 mm.

6. A method for preparing a negative electrode sheet, characterized in that: The following steps are involved: Coating a first negative electrode slurry on at least one side of the current collector to obtain a first negative electrode coating; coating a second negative electrode slurry on the surface of the first negative electrode coating to obtain a second negative electrode coating; coating a third negative electrode slurry on the surface of the second negative electrode coating to obtain a third negative electrode coating; Drying the current collector coated with the first negative electrode coating, the second negative electrode coating, and the third negative electrode coating to obtain a negative electrode sheet; Wherein, the first negative electrode slurry includes a first carbon-based active material; The second negative electrode slurry includes a second carbon-based active material; The third negative electrode slurry includes a third carbon-based active material; The interlayer spacing of the first carbon-based active material is smaller than the interlayer spacing of the second carbon-based active material; and the interlayer spacing of the second carbon-based active material is smaller than the interlayer spacing of the third carbon-based active material.

7. The method for preparing a negative electrode sheet according to claim 6, wherein: The preparation of the first negative electrode slurry comprises: uniformly mixing a first carbon-based active material, a first conductive agent, a first binder and a first thickener in a solvent to obtain a first negative electrode slurry; The preparation of the second negative electrode slurry includes: uniformly mixing a second carbon-based active material, a second conductive agent, a second binder, and a second thickener in a solvent to obtain a second negative electrode slurry; The preparation of the third negative electrode slurry includes: uniformly mixing a third carbon-based active material, a third conductive agent, a third binder and a third thickener in a solvent to obtain a third negative electrode slurry; Preferably, the viscosity of the first negative electrode slurry is 4000-5000 mPa·s, and the solid content is 45-55%; Preferably, the viscosity of the second negative electrode slurry is 3000-4000 mPa·s, and the solid content is 45-55%; Preferably, the viscosity of the third negative electrode slurry is 2500-3500 mPa·s, and the solid content is 45-55%; Preferably, the first negative electrode slurry is coated on the surface of the current collector, and after drying, a first negative electrode coating is formed on the surface of the current collector; the second negative electrode slurry is coated on the first negative electrode coating, and after drying, a second negative electrode coating is formed on the surface of the first negative electrode coating; the third negative electrode slurry is coated on the second negative electrode coating and dried; and the current collector coated with the first negative electrode coating, the second negative electrode coating, and the third negative electrode coating is roll-pressed to obtain a negative electrode sheet.

8. The method for preparing a negative electrode sheet according to claim 6, wherein: The second carbon-based active material includes a first heteroatom-doped soft carbon, and the preparation method of the first heteroatom-doped soft carbon includes: mixing a soft carbon source with a first heteroatom dopant and carbonizing the mixture to obtain first heteroatom-doped soft carbon; The mass ratio of the soft carbon source to the first heteroatom dopant is 10:(1-5); Preferably, the first heteroatom dopant includes at least one of borane, borax, boron tetrachloride, urea, ammonium chloride, ammonium sulfate, ammonium dihydrogen phosphate, elemental sulfur, thiourea, polyvinylidene fluoride, ammonium hexafluorophosphate or hexafluorophosphoric acid; Preferably, the carbonization temperature is 1200-1800°C, the carbonization time is 1-5 hours, and the carbonization heating rate is 1-5°C / min; Preferably, the carbonization environment is an inert atmosphere.

9. The method for preparing a negative electrode sheet according to claim 6, wherein: The third carbon-based active material includes a second heteroatom-doped hard carbon, and the preparation method of the second heteroatom-doped hard carbon includes: mixing a hard carbon source with a second heteroatom dopant and carbonizing the mixture to obtain a second heteroatom-doped hard carbon; The mass ratio of the hard carbon source to the second heteroatom dopant is 10:(1-5); Preferably, the second heteroatom dopant includes at least one of borane, borax, boron tetrachloride, urea, ammonium chloride, ammonium sulfate, ammonium dihydrogen phosphate, elemental sulfur, thiourea, polyvinylidene fluoride, ammonium hexafluorophosphate or hexafluorophosphoric acid; Preferably, the carbonization temperature is 1200-1800°C, the carbonization time is 1-5 hours, and the carbonization heating rate is 1-5°C / min; Preferably, the carbonization environment is an inert atmosphere.

10. A battery comprising a negative electrode sheet, characterized in that: The negative electrode sheet includes the negative electrode sheet according to any one of claims 1 to 5, or includes the negative electrode sheet prepared by the preparation method according to any one of claims 6 to 9.

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