Negative electrode sheet, secondary battery, and electric device

Abstract

The application discloses a negative electrode sheet, comprising: a negative electrode current collector, the surface of which is distributed with protruding carbon particles; a first negative electrode active layer, which is attached to at least one side surface of the negative electrode current collector; the first negative electrode active layer comprising first graphite particles and first silicon particles; a second negative electrode active layer, which is attached to the surface of the first negative electrode active layer; the second negative electrode active layer comprising second graphite particles and second silicon particles; wherein the D50 particle size of the first graphite particles is 4.5-16 mu m, the D50 particle size of the first silicon particles is 4.5-15.5 mu m; the D50 particle size of the second graphite particles is 11.2-18.9 mu m, the D50 particle size of the second silicon particles is 3.2-16 mu m, and the D50 particle size of the second graphite particles is greater than the D50 particle size of the second silicon particles. The application also discloses a secondary battery and an electric device. The negative electrode sheet provided by the application can improve the cycle stability of the battery under high-rate charging.

Description

TECHNICAL FIELD

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

[0002] In a lithium ion battery, a high-capacity silicon negative electrode material is the most potential negative electrode material, which is expected to gradually replace the existing graphite negative electrode. However, the lithium ion battery using the silicon negative electrode material inevitably has the following problems when fast charging: (1) under a large current condition, the rapid embedding of Li + in the negative electrode sheet will cause unstable interface reactions (such as lithium embedding and ohmic heat release) of the silicon material in the electrode sheet; (2) under a large current condition, the structure of the silicon-containing negative electrode sheet will irreversibly change, resulting in problems such as volume expansion and crushing of the silicon material, peeling of the active material layer of the electrode sheet, and the like, which seriously shortens the cycle life of the lithium ion battery. SUMMARY

[0003] The technical problem to be solved by the present application is to provide a negative electrode sheet which can improve the cycle stability of a battery under high-rate charging.

[0004] In order to solve the above technical problem, the present application provides the following technical solution:

[0005] The first aspect of the present application provides a negative electrode sheet, comprising:

[0006] a negative current collector having protruding carbon particles distributed on the surface thereof;

[0007] a first negative active layer attached to at least one side surface of the negative current collector; the first negative active layer comprising first graphite particles and first silicon particles;

[0008] a second negative active layer attached to the surface of the first negative active layer; the second negative active layer comprising second graphite particles and second silicon particles;

[0009] wherein the D50 particle size of the first graphite particles is 4.5-16 μm, and the D50 particle size of the first silicon particles is 4.5-15.5 μm;

[0010] the D50 particle size of the second graphite particles is 11.2-18.9 μm, and the D50 particle size of the second silicon particles is 3.2-16 μm, and the D50 particle size of the second graphite particles is greater than the D50 particle size of the second silicon particles.

[0011] Further, the ratio of the D50 particle size of the second graphite particles to the D50 particle size of the second silicon particles is 1:1.08-4.2.

[0012] Further, the negative current collector is selected from one of copper foil, carbon-coated copper foil, nickel-coated copper foil, zinc-coated copper foil, iron-coated copper foil, and titanium-coated copper foil.

[0013] Further, the carbon particles have a particle size of 0.02-15 μm.

[0014] Further, the carbon particles are obtained by spraying carbon powder on the negative current collector and curing at 180-500 °C for 1-2 h.

[0015] Further, the first negative active layer and the second negative active layer each further comprise a conductive agent and a binder.

[0016] In the first negative active layer, the mass ratio of the first graphite particles, the first silicon particles, the conductive agent, and the binder is 0.5-35:25-95:0.8-7.0:1.0-12.0.

[0017] In the second negative active layer, the mass ratio of the second graphite particles, the second silicon particles, the conductive agent, and the binder is 0.5-35:25-95:0.8-7.0:1.0-12.0.

[0018] Further, the content of the binder in the second negative active layer is higher than that in the first negative active layer.

[0019] Further, the total thickness of the first negative active layer and the second negative active layer is 0.032-0.180 mm.

[0020] Further, 0.015 mm≤ the thickness of the second negative active layer≤ the thickness of the first negative active layer.

[0021] Further, the compaction density of the first negative active layer and the second negative active layer is each 1.30-1.88 g / cm 3 .

[0022] Further, the second silicon particles are silicon-carbon composite particles coated with a carbon layer; and in the silicon-carbon composite particles, the silicon content is 22.5-74 wt%.

[0023] Further, the carbon layer is a modified amorphous carbon layer.

[0024] The modified amorphous carbon layer is obtained by heat-treating the silicon-carbon composite particles with 0.03-0.9 wt% hexafluorophosphate at 40-60 °C.

[0025] The second aspect of the present application provides a secondary battery comprising the negative electrode sheet described above.

[0026] The third aspect of the present application provides an electric device comprising the secondary battery described above.

[0027] Compared with the prior art, the present application has the following beneficial effects:

[0028] 1. In the negative electrode sheet of the present application, by arranging protruding carbon particles on the surface of the negative electrode current collector, the carbon particles improve the conductivity of the negative electrode sheet and strengthen the combination between the negative electrode current collector and the negative electrode active layer, thereby avoiding the separation of the entire negative electrode active layer and improving the strength of the electrode sheet structure, thus improving the stability of the electrode sheet under large-current charging conditions.

[0029] 2. In the negative electrode sheet of the present application, small-particle-size silicon material is combined with large-particle-size graphite as the second negative electrode active layer, and the pores of the large-particle-size graphite provide expansion space for the small-particle-size silicon material, thereby reducing the expansion influence of the silicon material during the charging and discharging process and improving the high-rate charging performance of the battery. In addition, the second negative electrode active layer can also play a protective role for the first negative electrode active layer, effectively slowing down the expansion of the first negative electrode active layer during the cycle process, improving the structural stability of the entire negative electrode sheet, and being beneficial to improving the cycle life of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 FIG. 1 is a structural schematic diagram of the negative electrode sheet according to an embodiment of the present application;

[0031] Figure 2 FIG. 2 is a structural schematic diagram of the negative electrode sheet according to another embodiment of the present application;

[0032] 1, negative electrode current collector; 2, carbon particles; 3, first negative electrode active layer; 31, first graphite particles; 32, first silicon particles; 4, second negative electrode active layer; 41, second graphite particles; 42, second silicon particles. DETAILED DESCRIPTION

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present application belongs. The terminology used in the description of the present application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0034] Please refer to Figure 1 The present application provides a silicon-containing negative electrode sheet capable of improving the cycle stability of a battery under high-rate charging, which comprises a negative electrode current collector 1, a first negative electrode active layer 3, and a second negative electrode active layer 4.

[0035] In the present application, the negative current collector can be made of a material with high conductivity commonly used in the art, for example, the negative current collector can be made of one of copper foil, carbon-coated copper foil, nickel-coated copper foil, zinc-coated copper foil, iron-coated copper foil, titanium-coated copper foil, etc. In some preferred embodiments, the negative current collector is made of copper foil or carbon-coated copper foil.

[0036] In the negative current collector 1 of the present application, the surface is distributed with protruding carbon particles 2. These carbon particles have excellent electrical conductivity, and their presence can promote ion transmission, thus improving the overall electrical conductivity of the negative electrode sheet, so that the electrode sheet can meet the charge and discharge requirements under large current. In addition, these carbon particles can also play an anchoring role, which can strengthen the bonding between the negative current collector and the negative active layer on its surface, thereby avoiding the entire negative active layer from being detached from the negative current collector, and improving the structural strength of the electrode sheet.

[0037] In the present application, the shape of the carbon particles is not limited, for example, it can be spherical, hemispherical, blocky, flaky, microporous on the surface, etc. In some embodiments of the present application, the particle size of the carbon particles is 0.02-15 μm, for example, 0.02 μm, 0.1 μm, 0.5 μm, 1 μm, 3 μm, 5 μm, 8 μm, 10 μm, 15 μm, or a range between any two of the above values.

[0038] In some embodiments of the present application, the protruding carbon particles are obtained by spraying carbon powder on the surface of the negative current collector and then performing high-temperature curing treatment. High-temperature treatment can make the carbon particles firmly adhere to the surface of the negative current collector. In some embodiments, the temperature of the high-temperature curing treatment can be 180-500°C, for example, 180°C, 200°C, 250°C, 300°C, 350°C, 400°C, 450°C, 500°C, or a range between any two of the above values; the time of the high-temperature curing treatment can be 1-2 h, for example, 1 h, 1.5 h, 2 h, or a range between any two of the above values.

[0039] In the present application, the negative active layer is composed of a first negative active layer and a second negative active layer, and the first negative active layer is attached to at least one side surface of the negative current collector. Figure 1 and 2 The cases where the first negative active layer is attached to the single-sided surface and the double-sided surface of the negative current collector are shown respectively.

[0040] In the present application, the first negative electrode active layer 3 contains first graphite particles 31 and first silicon particles 32. Among them, the first graphite particles and the first silicon particles are graphite materials and silicon materials with conventional particle sizes. The D50 particle size of the first graphite particles is 4.5-16 μm, for example, 4.5 μm, 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 16 μm, or a range between any two of the above values. The D50 particle size of the first silicon particles is 4.5-15.5 μm, for example, 4.5 μm, 5 μm, 8 μm, 10 μm, 12 μm, 15.5 μm, or a range between any two of the above values.

[0041] In the present application, the second negative electrode active layer 4 is attached to the surface of the first negative electrode active layer 3, and the second negative electrode active layer 4 also contains second graphite particles 41 and second silicon particles 42. Unlike the first negative electrode active layer, the second negative electrode active layer uses small-particle silicon particles in combination with large-particle graphite particles, that is, the D50 particle size of the second graphite particles is greater than that of the second silicon particles. The purpose of such arrangement is to provide space for small-particle silicon materials to expand between the large-particle graphite particles, to reduce the impact of silicon particle expansion on the pole piece during high-current charging and discharging, to improve the stability of the pole piece structure, to ensure the integrity of the negative electrode active layer, and thus to improve the high-rate charging performance of the battery.

[0042] In the present application, the D50 particle size of the second graphite particles is 11.2-18.9 μm, for example, 11.2 μm, 12 μm, 14 μm, 15 μm, 16 μm, 18 μm, 18.9 μm, or a range between any two of the above values. The D50 particle size of the second silicon particles is 3.2-16 μm, for example, 3.2 μm, 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 15 μm, 16 μm, or a range between any two of the above values.

[0043] In some embodiments of the present application, the ratio of the D50 particle size of the second graphite particles to the D50 particle size of the second silicon particles is 1:1.08-4.2, for example, 1:1.08, 1:2, 1:3, 1:4, 1:4.2, or a range between any two of the above values.

[0044] In the negative pole piece of the present application, the second negative electrode active layer is arranged on the outside of the first active layer, which can play a role in protecting the first negative electrode active layer, effectively slowing down the expansion of the first negative electrode active layer during the cycle process, improving the structural stability of the whole negative pole piece, and being conducive to improving the cycle life of the battery.

[0045] In the present application, the first graphite particles and the second graphite particles can both use artificial graphite, modified natural graphite, or other graphite materials.

[0046] In some embodiments of the present application, the first silicon particles are silicon-carbon composite materials, such as carbon-coated silicon or carbon-coated silicon oxide SiOx x (0 < x < 0). The silicon-carbon composite materials can be micron-sized materials or nanometer-sized materials; and can be particulate materials or linear or other shaped materials.

[0047] In some embodiments of the present application, the second silicon particles are silicon-carbon composite materials, such as carbon-coated silicon or carbon-coated silicon oxide SiOx x (0 < x < 0). In some embodiments, the silicon content in the second silicon particles is 22.5-74wt%, such as 22.5wt%, 25wt%, 30wt%, 35wt%, 40wt%, 45wt%, 50wt%, 55wt%, 60wt%, 65wt%, 70wt%, 74wt%, or a range between any two of the above values.

[0048] In the present application, the carbon-coated layer of the second silicon particles can be obtained by a gas phase deposition method or a liquid phase coating method. In the gas phase deposition method, the carbon source includes, but is not limited to, at least one of methane, ethane, acetylene, butyne, propane, butane, etc. In the liquid phase coating method, the carbon source includes, but is not limited to, at least one of pitch, petroleum coke, needle coke, polybutadiene resin, epoxy resin, carboxymethyl cellulose, amino resin, phenolic resin, acrylate, polyurethane resin, silicone resin, acrylic acid, acrylamide, polyvinyl alcohol, polyimide, polyaniline, etc. The specific steps of the liquid phase coating method are as follows: by means of heating or other means, the carbon source is made into a liquid state, and is stirred at a uniform speed to obtain a coating liquid; then the silicon material is added to the coating liquid and stirred, and the obtained mixture is cooled and placed in a heating furnace for heat treatment, so as to obtain a carbon-coated layer on the surface of the silicon material. The heat treatment temperature ranges from 420-900℃, such as 420℃, 450℃, 500℃, 550℃, 600℃, 650℃, 700℃, 750℃, 800℃, 850℃, 900℃, or a range between any two of the above values; and the heat treatment time ranges from 2-15h, such as 2h, 4h, 5h, 8h, 10h, 12h, 15h, or a range between any two of the above values.

[0049] In some embodiments of the present application, the carbon-coated layer of the second silicon particles is an amorphous carbon layer, and is further preferably a modified amorphous carbon layer. The modification method is as follows: the second silicon particles coated with an amorphous carbon layer are co-heated with a hexafluorophosphate salt in a solution, so that the phosphate salt is attached to the amorphous carbon layer to obtain a modified amorphous carbon layer. The hexafluorophosphate salt includes, but is not limited to, at least one of ammonium hexafluorophosphate, lithium hexafluorophosphate, sodium hexafluorophosphate, aluminum hexafluorophosphate, magnesium hexafluorophosphate, and the like, and the concentration thereof is in the range of 0.03-0.9wt%, such as 0.03wt%, 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, or a range between any two of the above values. The co-heating temperature is in the range of 40-60℃, such as 40℃, 45℃, 50℃, 55℃, 60℃, or a range between any two of the above values. The co-heating time is in the range of 0.25-6h, such as 0.25h, 0.5h, 1h, 2h, 3h, 4h, 5h, 6h, or a range between any two of the above values.

[0050] In the present application, the amorphous carbon layer is modified by using a hexafluorophosphate salt, thereby introducing a phosphorus-oxygen-fluorine bond on the surface of the silicon-carbon material. The phosphorus-oxygen-fluorine bond can significantly reduce the interfacial impedance of the silicon-carbon material in the SEI film formation, promote lithium ion migration and charge transfer, and ensure that the silicon-carbon material also has excellent rate performance under large current. Therefore, through the modification process, the SEI film is stabilized while the electrochemical performance of the silicon-carbon material is significantly improved.

[0051] It can be understood that, in the first negative electrode active layer and the second negative electrode active layer in the present application, in addition to graphite and silicon, a conductive agent and a binder are also included. In the first negative electrode active layer, the mass ratio of the first graphite particles, the first silicon particles, the conductive agent, and the binder is 0.5-35:25-95:0.8-7.0:1.0-12.0. In the second negative electrode active layer, the mass ratio of the second graphite particles, the second silicon particles, the conductive agent, and the binder is 0.5-35:25-95:0.8-7.0:1.0-12.0.

[0052] In the present application, the binder includes, but is not limited to, at least one of acrylonitrile, vinylidene fluoride, sodium carboxymethyl cellulose, methacrylamide, acrylic acid, acrylamide, amide, imide, acrylate, styrene butadiene rubber, ethylene vinyl alcohol, sodium alginate, chitosan, ethylene glycol, and the like monomers, their polymers or copolymers. In some preferred embodiments, the binder is a mixture of sodium carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR) in different proportions (1:2, 1:3, 2:3, 1:1, etc.).

[0053] In the present application, the conductive agent includes, but is not limited to, at least one of conductive carbon black, acetylene black, graphite, graphene, micro-nano fibrous conductive substances (carbon nanofibers), micro-nano tubular conductive substances (such as single-arm carbon nanotubes, multi-arm carbon nanotubes), and the like.

[0054] In some embodiments of the present application, the content of the binder in the second negative electrode active layer is higher than the content of the binder in the first negative electrode active layer. The purpose of such arrangement is that the binder has a tendency to diffuse to a low concentration direction under high temperature conditions. Therefore, during the subsequent high-temperature treatment of the electrode piece (such as the electrode piece drying process), the high-temperature heating promotes the high-content binder in the second negative electrode active layer to migrate to the underlying first negative electrode active layer, thereby reducing the influence of the interfacial separation between the first negative electrode active layer and the second negative electrode active layer, promoting better fusion between the layers; and can make the interfacial adhesion between the two layers higher and more tenacious, while being able to alleviate the structural stress caused by the volume expansion of the first silicon particles and the second silicon particles in the two layers.

[0055] In some embodiments of the present application, the thickness of the negative electrode active layer is 0.032-0.180 mm, such as 0.032 mm, 0.05 mm, 0.08 mm, 0.10 mm, 0.12 mm, 0.15 mm, 0.18 mm, etc. In some preferred embodiments, the thickness of the negative electrode active layer is 0.055-0.130 mm.

[0056] In some embodiments of the present application, h1 represents the thickness of the first negative electrode active layer, and h2 represents the thickness of the first negative electrode active layer, and h1 and h2 satisfy the following relationship: 0.015 mm≤h2≤h1.

[0057] In some embodiments of the present application, the compaction density of the negative electrode active layer is 1.30-1.88 g / cm 3 , such as 1.30 g / cm 3 , 1.4 g / cm 3 , 1.50 g / cm 3 , 1.60 g / cm 3 , 1.70 g / cm 3 , 1.80 g / cm 3 , 1.88 g / cm 3 , etc. In some preferred embodiments, the compaction density of the negative electrode active layer is 1.45-1.60 g / cm 3 .

[0058] The negative electrode sheet of the present application can be prepared by a conventional electrode sheet preparation process. In some embodiments, the negative electrode sheet is prepared by the following method: first, mix the first graphite particles, the first silicon particles, the conductive agent, and the binder, add water to make a slurry, and obtain a first negative electrode slurry; then, prepare a second negative electrode slurry in the same way. Next, sequentially coat the first negative electrode slurry and the second negative electrode slurry on one side or both sides of the negative electrode current collector by extrusion coating, and then go through the processes of rolling, slicing, slitting, and drying to obtain the negative electrode sheet. In some embodiments, the pressure of rolling can be 0.05-0.55 MPa, the temperature of drying can be 80-180°C, and the time of drying can be 4-20 h.

[0059] The present application further provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, wherein the separator is arranged to separate the positive electrode sheet from the negative electrode sheet. In specific implementation, the above-mentioned positive electrode sheet, negative electrode sheet, separator, and electrolyte are assembled into a secondary battery. The secondary battery can be a lithium ion battery, a sodium ion battery, or other batteries.

[0060] Taking a lithium ion battery as an example, the positive electrode active material in the positive electrode sheet can be at least one of lithium cobaltate, lithium nickelate, lithium manganate, lithium nickel-manganese oxide, lithium nickel-cobalt-manganese oxide, lithium nickel-cobalt-aluminum oxide, lithium manganese phosphate, lithium iron-manganese phosphate, and lithium iron phosphate.

[0061] The type of the separator is not particularly limited and can be any separator material used in existing batteries, such as polyethylene, polypropylene, polyvinylidene fluoride, non-woven fabric, their multi-layer composite films, and modified separators such as ceramic-modified, PVDF-modified separators, but is not limited to these.

[0062] The present application will be further described below in conjunction with the accompanying drawings and specific embodiments, so that those skilled in the art can better understand the present application and implement it. However, the embodiments are not intended to limit the present application.

[0063] In the following examples, the experimental methods are conventional methods unless otherwise specified, and the materials, reagents, etc. used are commercially available unless otherwise specified.

[0064] Example 1

[0065] 1. Preparation of the negative electrode sheet

[0066] (1) Sprinkle carbon powder with a particle size of 0.02-15 μm on the surface of a nickel-plated copper foil, and solidify at 180°C for 1 h to obtain a negative electrode current collector with raised carbon particles on the surface.

[0067] (2) The first graphite particles, the first silicon particles, the conductive agent and the binder are mixed according to a mass ratio of 6.7:89.8:1.2:2.3, deionized water is added to make a slurry, and a first negative electrode slurry is obtained; the second graphite particles, the second silicon particles, the conductive agent and the binder are mixed according to a mass ratio of 6.7:89.8:1.0:2.5, deionized water is added to make a slurry, and a second negative electrode slurry is obtained. Wherein:

[0068] The first graphite particles are modified natural graphite with a D50 particle size of 10.6 μm, and the second graphite particles are modified natural graphite with a D50 particle size of 13.4 μm;

[0069] The first silicon particles are carbon-coated micron silicon material with a D50 particle size of 5.3 μm;

[0070] The second silicon particles are silicon-carbon material with a D50 particle size of 4.9 μm and a carbon-coated layer, which contains 38.4 wt% of silicon; the carbon-coated layer is a modified amorphous carbon layer, wherein the preparation method of the modified amorphous carbon layer is: depositing an amorphous carbon layer on the surface of the silicon particles by acetylene gas phase deposition at 700 °C, and then co-heating with 0.18 wt% of ammonium hexafluorophosphate solution at 55 °C for 1 h to obtain the modified amorphous carbon layer.

[0071] The binder is obtained by mixing carboxymethyl cellulose sodium and butadiene rubber according to a mass ratio of 1:1;

[0072] The conductive agent is carbon nanotube.

[0073] (3) The first negative electrode slurry and the second negative electrode slurry are successively extrusion coated on one side of the above-mentioned negative electrode current collector to form a first negative electrode active layer and a second negative electrode active layer; after rolling, slicing and striping treatment, 155 °C drying for 12 h, a negative electrode sheet is obtained. The thickness of the first negative electrode active layer is 0.05 mm, and the thickness of the second negative electrode active layer is 0.022 mm.

[0074] 2. Preparation of lithium ion secondary battery

[0075] The positive electrode sheet (containing 92 wt% of lithium nickel cobalt manganese oxide), the polypropylene separator and the negative electrode sheet are successively wound to obtain a bare cell, the tabs are ultrasonically welded, and then the bare cell is placed into a battery shell, dried to remove water, injected with electrolyte, and the battery shell is sealed to obtain a lithium ion secondary battery. The preparation method of the electrolyte is: mixing ethylene carbonate (EC), methyl ethyl carbonate (EMC) and diethyl carbonate (DEC) according to a volume ratio of 1:1:1, then dissolving the fully dried lithium salt LiPF6 in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0076] Example 2

[0077] Example 2 differs from Example 1 in that:

[0078] (1) the proportions of the first graphite particles, the first silicon particles, the conductive agent, and the binder are 6.7:89.8:1.6:2.1, and the proportions of the second graphite particles, the second silicon particles, the conductive agent, and the binder are 6.7:89.6:1.2:2.5;

[0079] (2) the modified amorphous carbon layer is obtained by co-heating the silicon particles on which the amorphous carbon layer is deposited and a 0.32 wt% ammonium hexafluorophosphate solution at 55°C;

[0080] (3) the time for drying the electrode plate is 14 h.

[0081] Example 3

[0082] Example 3 differs from Example 1 in that:

[0083] (1) the proportions of the first graphite particles, the first silicon particles, the conductive agent, and the binder are 6.7:92.9:1.0:2.0, and the proportions of the second graphite particles, the second silicon particles, the conductive agent, and the binder are 6.7:92.9:1.0:2.5;

[0084] (2) the modified amorphous carbon layer is obtained by co-heating the silicon particles on which the amorphous carbon layer is deposited and a 0.45 wt% ammonium hexafluorophosphate solution at 55°C.

[0085] Example 4

[0086] Example 4 differs from Example 1 in that:

[0087] (1) the proportions of the first graphite particles, the first silicon particles, the conductive agent, and the binder are 18.9:75.8:1.2:4.0, and the proportions of the second graphite particles, the second silicon particles, the conductive agent, and the binder are 18.9:75.8:1.2:4.5;

[0088] (2) the first silicon particles are carbon-coated micron-sized silicon monoxide material with a D50 particle size of 8.3 μm;

[0089] (3) the second silicon particles are silicon-carbon material with a carbon-coated layer with a D50 particle size of 5.7 μm, which contains 46.8 wt% silicon; the carbon-coated layer is obtained by vapor deposition of a carbon source at 650°C; the modified amorphous carbon layer is obtained by co-heating the silicon particles on which the amorphous carbon layer is deposited and an ammonium hexafluorophosphate solution at 50°C;

[0090] (4) the second graphite particles are modified natural graphite with a D50 particle size of 13.9 μm;

[0091] (5) the thickness of the first negative electrode active layer is 0.075 mm, and the thickness of the second negative electrode active layer is 0.028 mm;

[0092] (6) The time for drying the electrode sheet was 13 h.

[0093] (7) The positive electrode sheet contained 95.5 wt% of lithium nickel cobalt manganese oxide.

[0094] Example 5

[0095] Example 5 differs from Example 4 in that:

[0096] (1) The proportions of the first graphite particles, the first silicon particles, the conductive agent, and the binder were 18.9:75.8:1.2:3.5, and the proportions of the second graphite particles, the second silicon particles, the conductive agent, and the binder were 18.9:75.8:1.2:4.0;

[0097] (2) The second silicon particles contained 42.2 wt% of silicon; the modified amorphous carbon layer was obtained by co-heating silicon particles on which an amorphous carbon layer was deposited with 0.32 wt% of an ammonium hexafluorophosphate solution;

[0098] (3) The time for drying the electrode sheet was 16 h.

[0099] Example 6

[0100] Example 6 differs from Example 4 in that:

[0101] (1) The proportions of the first graphite particles, the first silicon particles, the conductive agent, and the binder were 18.9:75.8:1.2:3.2, and the proportions of the second graphite particles, the second silicon particles, the conductive agent, and the binder were 18.9:75.8:1.2:3.5;

[0102] (2) The second silicon particles contained 43.6 wt% of silicon; the modified amorphous carbon layer was obtained by co-heating silicon particles on which an amorphous carbon layer was deposited with 0.45 wt% of an ammonium hexafluorophosphate solution;

[0103] (3) The time for drying the electrode sheet was 12 h.

[0104] Comparative Example 1

[0105] Comparative Example 1 differs from Example 2 in that there is no second negative electrode active layer.

[0106] Comparative Example 2

[0107] Comparative Example 2 differs from Example 2 in that, in the second negative electrode active layer, the first silicon material is a silicon-carbon material containing a carbon coating layer having a D50 particle size of 10.2 pm, and the second graphite is modified natural graphite having a D50 particle size of 10.6 pm.

[0108] Comparative Example 3

[0109] The difference between Comparative Example 3 and Example 2 is that no ammonium hexafluorophosphate is introduced and the silicon-carbon material is co-heated at 55°C without forming a modified amorphous carbon layer.

[0110] Comparative Example 4

[0111] The difference between Comparative Example 4 and Example 2 is that there are no protruding carbon particles on the surface of the negative current collector.

[0112] 1. The peel force of the electrode sheet and the expansion of the negative electrode sheet at 100% SOC

[0113] (1) The peel force of the electrode sheet was measured using a peel force tester for each of the examples and comparative examples.

[0114] (2) The thickness of the silicon-containing negative electrode sheet after rolling was measured using a micrometer. The silicon-containing negative electrode sheet was obtained by disassembling the batteries of the examples and comparative examples at 100% SOC, and the thickness of the silicon-containing negative electrode sheet at 100% SOC was measured using a micrometer.

[0115] The negative electrode sheet expansion rate = (battery electrode sheet thickness at 100% SOC - silicon negative electrode sheet thickness after rolling) / negative electrode sheet thickness after rolling.

[0116] 2. Electrochemical performance test

[0117] (1) 1.5C CC Rate (1.5C charge rate) test: At room temperature 25°C, the initial voltage is 2.8V and the cutoff voltage is 4.2V. The batteries of each example and comparative example are first charged at a low rate of 0.5C to 4.2V, and then charged at a constant voltage of 4.2V until the current decreases to 0.05C. The charge amount 1 at this time is recorded. After discharging at 0.5C to 2.8V, the battery is charged at a high rate of 1.5C to 4.2V. The charge amount 2 at this time is recorded, and the CC Rate = charge amount 2 / charge amount 1*100% is calculated.

[0118] (2) 2.0C CC Rate (2.0C charge rate) test: At room temperature 25°C, the initial voltage is 2.8V and the cutoff voltage is 4.2V. The batteries of each example and comparative example are first charged at a low rate of 0.5C to 4.2V, and then charged at a constant voltage of 4.2V until the current decreases to 0.05C. The charge amount 3 at this time is recorded. After discharging at 0.5C to 2.8V, the battery is charged at a high rate of 2.0C to 4.2V. The charge amount 4 at this time is recorded, and the CC Rate = charge amount 4 / charge amount 3*100% is calculated.

[0119] (3) 1.5C / 0.5C high-rate charging and low-rate discharging cycle test: At room temperature of 25℃, the starting voltage is 2.8V and the cutoff voltage is 4.2V. The batteries of each embodiment and comparative example are charged to 4.2V at 1.5C, charged at 4.2V constant voltage until the current decreases to 0.05C, discharged at 0.5C to 2.8V, charged to 4.2V at 1.5C again, charged at 4.2V constant voltage until the current decreases to 0.05C. The batteries are charged and discharged in this way, and the capacity retention rate of the batteries after 500 cycles and after 800 cycles is recorded.

[0120] Table 1

[0121] Peeling force / N Swelling ratio Example 1 0.23 0.363 Example 2 0.26 0.356 Example 3 0.28 0.340 Example 4 0.37 0.356 Example 5 0.25 0.342 Example 6 0.30 0.351 Comparative Example 1 0.23 0.405 Comparative Example 2 0.28 0.367 Comparative Example 3 0.33 0.383 Comparative Example 4 0.16 0.346

[0122] Please refer to Table 1. In Comparative Example 4, the electrode sheet has a lower peeling force because there are no protruding carbon particles on the surface of the negative electrode current collector. In the other embodiments and comparative examples, the negative electrode current collector has protruding carbon particles, which act as anchors and strengthen the bond between the negative electrode current collector and the negative electrode active layer, thus improving the peeling strength of the electrode sheet.

[0123] Furthermore, in the negative electrode of Comparative Example 1, the expansion rate of the electrode is relatively high because no second negative electrode active layer is provided on the first negative electrode active layer. In contrast, in other embodiments and comparative examples, a second negative electrode active layer is provided on the surface of the first negative electrode active layer. The second negative electrode active layer can protect the first negative electrode active layer and effectively alleviate the expansion of the negative electrode material during cycling, thus reducing the expansion rate of the electrode.

[0124] Table 2

[0125]

[0126] Please see Figure 2 Under 1.5C charge-discharge conditions, except for Comparative Example 3, the charge-discharge efficiencies of the other comparative examples were not significantly different from those of the embodiments. Comparative Example 3 exhibited lower charge-discharge efficiency because a modified amorphous carbon layer was not formed on the surface of the silicon particles. The modified amorphous carbon layer, due to the introduction of phosphorus-oxygen-fluorine bonds, can significantly reduce the interfacial impedance of silicon-carbon materials in the SEI film formation, promote lithium-ion migration and charge transfer, and ensure excellent rate performance of silicon-carbon materials even under high current, thus improving the charge-discharge efficiency of the battery.

[0127] Under 2.0C charge-discharge conditions, Comparative Examples 1-4 showed a significant difference in charge-discharge efficiency compared to Examples 1-6. This indicates that the batteries of Examples 1-6 exhibited superior high-rate charge-discharge performance.

[0128] Under the condition of 1.5C / 0.5C high rate charging, after 500 cycles, the capacity retention rate of Comparative Example 1 is 87.2%, and the capacity retention rates of other comparative examples are all less than 85%; in contrast, the capacity retention rates of Examples 1-6 are all more than 87%. After 800 cycles, the capacity retention rates of each comparative example are all less than Examples 1-6. This shows that the batteries of Examples 1-6 have better cycle stability under the condition of high rate charging and discharging.

[0129] The above examples are only preferred examples for fully illustrating the present application, and the protection scope of the present application is not limited thereto. Any equivalent replacement or transformation made by the person skilled in the art based on the present application is within the protection scope of the present application. The protection scope of the present application is subject to the claims.

Claims

1. A negative electrode sheet characterized by comprising: The negative electrode current collector has protruding carbon particles distributed on its surface; the carbon particles have a particle size of 0.02-15 μm; the carbon particles are obtained by spraying carbon powder on the negative electrode current collector and curing at 180-500 °C for 1-2 h; The first negative electrode active layer is attached to at least one side surface of the negative electrode current collector; the first negative electrode active layer contains first graphite particles and first silicon particles; The second negative electrode active layer is attached to the surface of the first negative electrode active layer; the second negative electrode active layer contains second graphite particles and second silicon particles; the second silicon particles are silicon-carbon composite particles coated with a carbon layer; the carbon layer is a modified amorphous carbon layer; the modified amorphous carbon layer is obtained by heat treating the silicon-carbon composite particles with 0.03-0.9 wt% hexafluorophosphate at 40-60 °C; The D50 particle size of the first graphite particles is 4.5-16 μm, and the D50 particle size of the first silicon particles is 4.5-15.5 μm; The D50 particle size of the second graphite particles is 11.2-18.9 μm, and the D50 particle size of the second silicon particles is 3.2-16 μm; the D50 particle size of the second graphite particles is greater than the D50 particle size of the second silicon particles. The ratio of the D50 particle size of the second silicon particles to the D50 particle size of the second graphite particles is 1:(1.08-4.2).

2. The negative electrode sheet according to claim 1, wherein The negative electrode current collector is selected from one of a copper foil, a carbon-coated copper foil, a nickel-coated copper foil, a zinc-coated copper foil, an iron-coated copper foil, and a titanium-coated copper foil.

3. The negative electrode sheet according to claim 1, wherein The first negative electrode active layer and the second negative electrode active layer each further contain a conductive agent and a binder; 4. The negative electrode sheet according to claim 1, wherein In the first negative electrode active layer, the mass ratio of the first graphite particles, the first silicon particles, the conductive agent, and the binder is 0.5-35:25-95:0.8-7.0:1.0-12.0; In the second negative electrode active layer, the mass ratio of the second graphite particles, the second silicon particles, the conductive agent, and the binder is 0.5-35:25-95:0.8-7.0:1.0-12.

0. The content of the binder in the second negative electrode active layer is higher than the content of the binder in the first negative electrode active layer.

5. The negative electrode sheet according to claim 4, wherein The total thickness of the first negative electrode active layer and the second negative electrode active layer is 0.032-0.180 mm; 6. The negative electrode sheet according to claim 1, wherein And / or, 0.015 mm≤the thickness of the second negative electrode active layer≤the thickness of the first negative electrode active layer; In the silicon-carbon composite particles, the silicon content is 22.5-74 wt%. and / or the compaction density of the first negative electrode active layer and the second negative electrode active layer is 1.30 to 1.88 g / cm 3 .

7. The negative electrode sheet according to claim 1, wherein The negative electrode sheet of any one of claims 1-7.

8. A secondary battery characterized by comprising: The secondary battery of claim 8.

9. An electric device, characterized by ​

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