Negative pole piece as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of lithium ion batteries, and particularly relates to a negative pole piece as well as a preparation method and application thereof. The negative pole piece comprises a current collector and active substance layers located on the surfaces of the two sides of the current collector; the current collector is provided with a first through hole; the active substance layer comprises a first active substance layer and a second active substance layer, the first active substance layer is located on the surface of the current collector, and the second active substance layer is located on the surface of the side, away from the current collector, of the first active substance layer; the first active material layer comprises single-particle graphite and is provided with a second through hole, and the second through hole is correspondingly communicated with the first through hole; the second active material layer includes secondary particle graphite. The negative pole piece with a specific structure is designed and synthesized, the single-particle graphite layer and the secondary-particle graphite layer are arranged on the surface of the current collector, and the current collector has a through hole structure penetrating through the current collector and the single-particle graphite layer, so that when the negative pole piece is applied to a lithium ion battery, the rate capability and the cycle performance of the battery can be remarkably improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and particularly relates to a negative electrode sheet, a preparation method thereof, and an application thereof. Background Art

[0002] With the booming development of the new energy industry, the requirements for the energy density, rate performance, and fast charging of lithium batteries are continuously increasing. Pore formation in the electrode sheet is an important technology in the manufacturing process of lithium batteries. Pore formation during the preparation of the electrode sheet enables the electrode sheet to absorb and store the electrolyte, which has a positive effect on improving the electrochemical performance, cycle stability of lithium batteries, and reducing battery costs. At present, the methods for pore formation in the electrode sheet mainly include mechanical pore formation, introducing pore-forming agents, and laser pore formation. Introducing pore-forming agents will reduce the proportion of active materials, and it is difficult to control the pore size in mechanical pore formation. Both of these methods will cause relatively large energy losses, and even cause local electrochemical imbalance due to too large pore size. Laser has the characteristics of high energy and high precision. Selecting a suitable laser can improve the effect of pore formation. However, at present, laser pore formation mainly focuses on the modification of pores on the coating surface, and fails to give full play to the advantages of laser pore formation technology, and has limited improvement effects on the rate performance, cycle performance and other performances of lithium batteries. Summary of the Invention

[0003] The purpose of the present invention is to provide a negative electrode sheet, a preparation method thereof, and an application thereof in view of the problem that the existing negative electrode sheet has limited improvement effects on the rate performance and cycle performance of lithium batteries.

[0004] In a first aspect, the present invention provides a negative electrode sheet, which includes a current collector and active material layers located on both surface sides of the current collector; the current collector is provided with first through holes; the active material layers include a first active material layer and a second active material layer. Among them, the first active material layer is located on the surface of the current collector, and the second active material layer is located on the surface of the first active material layer facing away from the current collector; the first active material layer includes single-particle graphite and is provided with second through holes, and the second through holes are correspondingly communicated with the first through holes; the second active material layer includes secondary-particle graphite.

[0005] In a preferred embodiment, the pore diameters of the first through holes and the second through holes are 4-10 μm, and the pore spacing is 10-100 mm.

[0006] In a preferred embodiment, the mass ratio of the first active material layer to the second active material layer is (3-9):1.

[0007] In a preferred embodiment, the content of single-particle graphite in the first active material layer is 80-95 wt%.

[0008] In a preferred embodiment, the content of the secondary particle type graphite in the second active material layer is 80 to 95 wt%.

[0009] In a preferred embodiment, the current collector is selected from copper foil and / or carbon-coated copper foil.

[0010] In a second aspect, the present invention provides a method for preparing a negative electrode sheet, and the method for preparing the negative electrode sheet includes the following steps:

[0011] S1. Mix single particle type graphite, a first conductive agent, a first binder, and water to obtain slurry A, mix secondary particle type graphite, a second conductive agent, a second binder, and water to obtain slurry B, coat slurry A on both side surfaces of the current collector and then perform a drying treatment to form a first active material layer on both side surfaces of the current collector, and then perform pulsed laser drilling to obtain a sheet with a porous structure, which has a through-hole structure penetrating the first active material layer and the current collector;

[0012] S2. Coat slurry B on the surface of the sheet with a porous structure obtained in step S1 and then perform a drying treatment to form a second active material layer on the surface of the first active material layer, and obtain the negative electrode sheet after rolling.

[0013] In a preferred embodiment, based on the solid mass in slurry A, the content of the single particle type graphite is 80 to 95 wt%, the content of the first conductive agent is 1 to 12 wt%, and the content of the first binder is 3 to 8 wt%.

[0014] In a preferred embodiment, based on the solid mass in slurry B, the content of the secondary particle type graphite is 80 to 95 wt%, the content of the second conductive agent is 1 to 12 wt%, and the content of the second binder is 3 to 8 wt%.

[0015] In a preferred embodiment, the pulse width of the pulsed laser is 0.5 to 5 ps, and the laser power is 1500 to 2000 W.

[0016] In a preferred embodiment, the aperture of the through-hole is 4 to 10 μm, and the hole pitch is 10 to 100 mm.

[0017] In a preferred embodiment, the mass ratio of the first active material layer to the second active material layer is (3 to 9):1.

[0018] In a preferred embodiment, the first conductive agent and the second conductive agent are each independently selected from at least one of conductive carbon black, graphene, and carbon fiber.

[0019] In a preferred embodiment, the first binder and the second binder are each independently selected from at least one of sodium carboxymethylcellulose, styrene-butadiene rubber, sodium alginate, and polyacrylic acid.

[0020] In a third aspect, the present invention provides a negative electrode sheet prepared by the above method.

[0021] In a fourth aspect, the present invention further provides an application of the above negative electrode sheet in a lithium-ion battery.

[0022] Beneficial effects:

[0023] The present invention designs and synthesizes a negative electrode sheet with a specific structure, which includes a current collector and a single-particle graphite active material layer and a secondary-particle graphite active material layer sequentially coated on the surface of the current collector, and has a through-hole structure penetrating the current collector and the single-particle graphite active material layer, so that the through-holes of the obtained negative electrode sheet and the active material layer present an I-shaped structure, having high mechanical strength. On the one hand, the double-layer coating of the single-particle graphite active material layer and the secondary-particle graphite active material layer can take into account the performance requirements in multiple aspects such as high energy density, fast charging ability, and cycle stability, while reducing the material cost. On the other hand, the through-hole structure of the electrode sheet can improve the liquid retention ability and the wettability of the electrolyte, shorten the diffusion and migration path of lithium ions during the charge and discharge process, and improve the charge and discharge performance of the battery at different rates and the cycle performance of the battery. Therefore, when the obtained negative electrode sheet is applied in a lithium-ion battery, it can significantly improve the rate performance and cycle performance of the battery. Description of the drawings

[0024] Figure 1 is a schematic cross-sectional structure diagram of the negative electrode sheet provided by the present invention. Detailed embodiments

[0025] Referring to Figure 1 , the negative electrode sheet provided by the present invention includes a current collector and active material layers located on both side surfaces of the current collector; the current collector is provided with a first through-hole; the active material layer includes a first active material layer and a second active material layer, wherein the first active material layer is located on the surface of the current collector, and the second active material layer is located on the side surface of the first active material layer facing away from the current collector; the first active material layer includes single-particle graphite and is provided with a second through-hole, and the second through-hole is correspondingly communicated with the first through-hole; the second active material layer includes secondary-particle graphite.

[0026] In the present invention, the single-particle graphite and the secondary-particle graphite can be obtained as commercially available products, or can be prepared according to existing methods or improved methods.

[0027] In the present invention, the aperture diameter of the first through-hole is preferably 4 to 10 μm, such as 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm or any value therebetween; the hole pitch is preferably 10 to 100 mm, such as 10 mm, 20 mm, 30 mm, 50 mm, 80 mm, 100 mm or any value therebetween. At this time, it is more beneficial to improve the mechanical strength and liquid retention capacity of the negative electrode sheet.

[0028] Further, the shapes of the first through-hole and the second through-hole are each independently preferably any one of circular, square, elliptical or other shapes.

[0029] In the present invention, the mass ratio of the first active material layer to the second active material layer is preferably (3 to 9):1, such as 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1 or any value therebetween. At this time, it is more beneficial to balance the performance requirements in multiple aspects such as high energy density, fast charging ability, cycle stability, etc., while reducing the material cost.

[0030] In the present invention, the content of single-particle graphite in the first active material layer is preferably 80 to 95 wt%, such as 80 wt%, 82 wt%, 85 wt%, 88 wt%, 90 wt%, 92 wt%, 95 wt% or any value therebetween. The content of secondary-particle graphite in the second active material layer is preferably 80 to 95 wt%, such as 80 wt%, 82 wt%, 85 wt%, 88 wt%, 90 wt%, 92 wt%, 95 wt% or any value therebetween. At this time, it is more beneficial to improve the electrical conductivity of the negative electrode sheet and the charge-discharge performance at different rates.

[0031] In the present invention, the current collector can be a conventional choice in the art, preferably copper foil and / or carbon-coated copper foil.

[0032] The method for preparing the negative electrode sheet provided by the present invention includes the following steps:

[0033] S1. Mix single-particle graphite, a first conductive agent, a first binder and water to obtain slurry A, mix secondary-particle graphite, a second conductive agent, a second binder and water to obtain slurry B, coat slurry A on both side surfaces of the current collector and then perform a drying treatment to form a first active material layer on both side surfaces of the current collector, and then perform pulsed laser drilling to obtain a porous electrode sheet, which has a through-hole structure penetrating the first active material layer and the current collector;

[0034] S2. Coat slurry B on the surface of the porous electrode sheet obtained in step S1 and then perform a drying treatment to form a second active material layer on the surface of the first active material layer, and obtain the negative electrode sheet after rolling.

[0035] In the present invention, based on the solid mass in the slurry A, the content of the single-particle graphite is preferably 80-95 wt%, such as 80 wt%, 82 wt%, 85 wt%, 88 wt%, 90 wt%, 92 wt%, 95 wt% or any value therebetween; the content of the first conductive agent is preferably 1-12 wt%, such as 1 wt%, 2 wt%, 5 wt%, 8 wt%, 10 wt%, 12 wt% or any value therebetween; the content of the first binder is preferably 3-8 wt%, such as 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt% or any value therebetween.

[0036] In the present invention, based on the solid mass in the slurry B, the content of the secondary-particle graphite is preferably 80-95 wt%, such as 80 wt%, 82 wt%, 85 wt%, 88 wt%, 90 wt%, 92 wt%, 95 wt% or any value therebetween; the content of the second conductive agent is preferably 1-12 wt%, such as 1 wt%, 2 wt%, 5 wt%, 8 wt%, 10 wt%, 12 wt% or any value therebetween; the content of the second binder is preferably 3-8 wt%, such as 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt% or any value therebetween.

[0037] In addition, the terms "first" and "second" are only for the purpose of convenient description and cannot be construed as specific limitations on the type or quantity of the defined technical features.

[0038] In the present invention, the ratio of the solid mass in the slurry A to the solid mass in the slurry B is preferably (3-9):1, such as 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1 or any value therebetween.

[0039] Specifically, in the process of preparing the negative electrode sheet, the present invention does not specifically limit the coating thickness of the slurry A and the slurry B. The main purpose is to control the ratio of the solid mass in the slurry A to the solid mass in the slurry B within the range of (3-9):1, and the coating thickness is mainly for uniform coating.

[0040] In the present invention, the method of pulsed laser drilling includes adjusting the tape running speed of the electrode sheet to match the path and energy of the pulsed laser, improving the drilling accuracy, so as to obtain a through-hole structure with uniform pore size and pore spacing.

[0041] Further, the pulse width of the pulsed laser is preferably 0.5 to 5 ps, such as 0.5 ps, 0.8 ps, 1 ps, 2 ps, 3 ps, 4 ps, 5 ps or any value therebetween; the laser power is preferably 1500 to 2000 W, such as 1500 W, 1600 W, 1700 W, 1800 W, 1900 W, 2000 W or any value therebetween. At this time, the pulsed laser has a short time, high energy and high precision, which is more conducive to improving the uniformity of the aperture and hole pitch of the through-hole structure of the electrode sheet and reducing the influence of thermal radiation on the active material.

[0042] In the present invention, the aperture of the through-hole is preferably 4 to 10 μm, such as 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm or any value therebetween; the hole pitch is preferably 10 to 100 mm, such as 10 mm, 20 mm, 30 mm, 50 mm, 80 mm, 100 mm or any value therebetween.

[0043] Further, the shape of the through-hole is preferably any one of circular, square, oval or other shapes.

[0044] In the present invention, the specific types of the first conductive agent and the second conductive agent are not limited and can be conventional selections in the art, and are each independently preferably selected from at least one of conductive carbon black, graphene, and carbon fiber. The first conductive agent and the second conductive agent may be the same or different.

[0045] In the present invention, the specific types of the first binder and the second binder are not limited and can be conventional selections in the art, and are each independently preferably selected from at least one of sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), sodium alginate (SA), and polyacrylic acid (PAA). The first binder and the second binder may be the same or different.

[0046] The present invention will be described in detail below through specific examples.

[0047] Example 1 Preparation of Negative Electrode Sheet

[0048] S1. Single-grain graphite (purchased from Shanghai Shanshan Technology Co., Ltd., brand ESG-10), conductive carbon black (SP), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 95:1:1.5:2.5, and deionized water was added to mix evenly to obtain slurry A (solid content of 51wt%); secondary particle graphite (purchased from Huzhou Qiyuan Jincan New Energy Technology Co., Ltd., brand C30), conductive carbon black (SP), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 95:1:1.5:2.5, and deionized water was added to mix evenly to obtain slurry B (solid content of 51wt%);

[0049] S2. Slurry A is coated on a copper foil with a thickness of 6 μm by a double-sided coater, and the oven baking temperature is 60°C to form a first active material layer with a thickness of 165 μm on both sides of the copper foil, and then a pulse laser device is used for drilling. The laser path is adjusted by the laser galvanometer and laser emitter of the pulse laser device, and the pulse width is controlled to be 0.5 ps, and the laser power is 1500 W to obtain a pole piece with a through-hole structure. The through-hole shape is a circular hole, the aperture is 4 μm, and the hole spacing is 10 mm;

[0050] S3. Slurry B is applied to the electrode sheet with a through-hole structure by a double-sided coater to form a second active material layer with a thickness of 10 μm on the surface of the first active material layer (the mass ratio of the first active material layer to the second active material layer is 9:1), and finally the negative electrode sheet is obtained by rolling.

[0051] Example 2 Preparation of negative electrode sheet

[0052] S1. Single-grain graphite ESG-10, conductive carbon black (SP), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 90:3:3:4, and deionized water was added to mix evenly to obtain slurry A (solid content of 50wt%); secondary-grain graphite C30, conductive carbon black (SP), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 90:3:3:4, and deionized water was added to mix evenly to obtain slurry B (solid content of 50wt%);

[0053] S2. Slurry A is coated on a copper foil with a thickness of 6 μm by a double-sided coater, and the oven baking temperature is 61°C to form a first active material layer with a thickness of 155 μm on both sides of the copper foil, and then a pulse laser device is used for drilling. The laser path is adjusted by the laser galvanometer and laser emitter of the pulse laser device, and the pulse width is controlled to be 0.8 ps, and the laser power is 1800 W to obtain a pole piece with a through-hole structure. The through-hole shape is a circular hole, the aperture is 6 μm, and the hole spacing is 50 mm;

[0054] S3. Coating the paste B on the electrode sheet with a through-hole structure by a double-sided coater to form a second active material layer with a thickness of 15 μm on the surface of the first active material layer (the mass ratio of the first active material layer to the second active material layer is 6:1), and finally obtaining the negative electrode sheet after rolling.

[0055] Preparation of the negative electrode sheet in Example 3

[0056] S1. Mix single-particle graphite ESG-10, conductive carbon black (SP), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) in a mass ratio of 83:10:2:5, and add deionized water and mix evenly to obtain paste A (solid content is 48 wt%); mix secondary-particle graphite C30, conductive carbon black (SP), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) in a mass ratio of 83:10:2:5, and add deionized water and mix evenly to obtain paste B (solid content is 48 wt%);

[0057] S2. Coating paste A on a copper foil with a thickness of 6 μm by a double-sided coater, and the oven baking temperature is 63 °C to form a first active material layer with a thickness of 135 μm on both surfaces of the copper foil. Then, use a pulsed laser device to punch holes, adjust the laser path through the laser galvanometer and laser emitter of the pulsed laser device, control the pulse width to be 5 ps, and the laser power to be 2000 W to obtain an electrode sheet with a through-hole structure. The through-hole shape is a round hole, the aperture is 10 μm, and the hole spacing is 100 mm;

[0058] S2. Coating the paste B on the electrode sheet with a through-hole structure by a double-sided coater to form a second active material layer with a thickness of 30 μm on the surface of the first active material layer (the mass ratio of the first active material layer to the second active material layer is 3:1), and finally obtaining the negative electrode sheet after rolling.

[0059] Preparation of the negative electrode sheet in Example 4

[0060] Prepare the negative electrode sheet according to the method of Example 1. The difference is that in step S2, control the pulse width to be 20 ps and the laser power to be 2500 W to obtain an electrode sheet with a through-hole structure. The through-hole shape is a round hole, the aperture is 20 μm, and the hole spacing is 150 mm. The other conditions are the same as those in Example 1, and thus the negative electrode sheet is obtained.

[0061] Preparation of the negative electrode sheet in Example 5

[0062] The negative electrode sheet was prepared according to the method of Example 1, except that in step S2, the pulse width was controlled to be 0.1 ps and the laser power was 1000 W, to obtain a sheet with a through-hole structure. The through-holes were circular in shape, with a pore diameter of 1 μm and a pore spacing of 5 mm. Other conditions were the same as those in Example 1, and thus the negative electrode sheet was obtained.

[0063] Preparation of the reference negative electrode sheet for Comparative Example 1

[0064] Single-particle graphite ESG-10, conductive carbon black (SP), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 95:1:1.5:2.5, and deionized water was added and mixed evenly to obtain Slurry A (solid content: 51 wt%). Slurry A was coated on copper foil by a double-sided coater, and the oven baking temperature was 60 °C to form a first active material layer with a thickness of 180 μm on both surfaces of the copper foil. Finally, the reference negative electrode sheet was obtained after rolling.

[0065] Preparation of the reference negative electrode sheet for Comparative Example 2

[0066] Single-particle graphite ESG-10, conductive carbon black (SP), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 95:1:1.5:2.5, and deionized water was added and mixed evenly to obtain Slurry A (solid content: 51 wt%). Slurry A was coated on copper foil with a thickness of 6 μm by a double-sided coater, and the oven baking temperature was 60 °C to form a first active material layer with a thickness of 180 μm on both surfaces of the copper foil. Then, a pulsed laser device was used for punching. The laser path was adjusted by the laser galvanometer and laser emitter of the pulsed laser device. The pulse width was controlled to be 0.5 ps and the laser power was 1500 W to obtain a sheet with a through-hole structure. The through-holes were circular in shape, with a pore diameter of 4 μm and a pore spacing of 10 mm. Finally, the reference negative electrode sheet was obtained after rolling.

[0067] Preparation of the reference negative electrode sheet for Comparative Example 3

[0068] S1. Single-particle graphite ESG-10, conductive carbon black (SP), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 95:1:1.5:2.5, and deionized water was added and mixed evenly to obtain Slurry A (solid content: 51 wt%); secondary-particle graphite C30, conductive carbon black (SP), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 95:1:1.5:2.5, and deionized water was added and mixed evenly to obtain Slurry B (solid content: 51 wt%);

[0069] S2. Coating slurry A on a copper foil with a thickness of 6 μm using a double-sided coater, and baking it in an oven at 60 °C to form a first active material layer with a thickness of 165 μm on both sides of the copper foil; then coating slurry B on the electrode sheet with a through-hole structure using a double-sided coater to form a second active material layer with a thickness of 10 μm on the surface of the first active material layer (the mass ratio of the first active material layer to the second active material layer is 9:1). Finally, the reference negative electrode sheet is obtained after rolling.

[0070] Preparation of the reference negative electrode sheet for Comparative Example 4

[0071] Punching holes in the copper foil using a pulsed laser device, adjusting the laser path through the laser galvanometer and laser emitter of the pulsed laser device, controlling the pulse width to be 0.5 ps and the laser power to be 1500 W to obtain a copper foil with a through-hole structure. The shape of the through-hole is a round hole, the aperture is 4 μm, and the pitch of the holes is 10 mm.

[0072] Mix single-particle graphite ESG-10, conductive carbon black (SP), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) in a mass ratio of 95:1:1.5:2.5, and add deionized water and mix evenly to obtain slurry A (solid content: 51 wt%); mix secondary-particle graphite C30, conductive carbon black (SP), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) in a mass ratio of 95:1:1.5:2.5, and add deionized water and mix evenly to obtain slurry B (solid content: 51 wt%).

[0073] Coating slurry A on a copper foil with a thickness of 6 μm using a double-sided coater, and baking it in an oven at 60 °C to form a first active material layer with a thickness of 165 μm on both sides of the copper foil; then coating slurry B on the electrode sheet with a through-hole structure using a double-sided coater to form a second active material layer with a thickness of 10 μm on the surface of the first active material layer (the mass ratio of the first active material layer to the second active material layer is 9:1). Finally, the reference negative electrode sheet is obtained after rolling.

[0074] Test Example

[0075] Assemble the negative electrode sheets prepared in the above examples and comparative examples into soft-pack batteries according to the following method, and test the cycle performance and rate performance. The obtained results are shown in Table 1.

[0076] (1) Preparation of the positive electrode sheet: Lithium iron phosphate, CNT (carbon nanotube), conductive carbon black, and polyvinylidene fluoride binder are mixed in a mass ratio of 95.6:1.3:1:2.1, and NMP (N-methylpyrrolidone) solvent is added. The mixture is stirred under a vacuum mixer until the system becomes homogeneous to obtain the positive electrode paste. The positive electrode paste is evenly coated on the surface of the positive electrode current collector aluminum foil and dried. After drying, it is roll-pressed to obtain the positive electrode sheet.

[0077] Preparation of the electrolyte: Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain a mixed organic solvent. Then, the fully dried lithium salt LiPF 6 is dissolved in the mixed organic solvent to prepare an electrolyte with a lithium salt concentration of 1 mol / L.

[0078] Preparation of the separator: A polypropylene membrane is selected as the separator.

[0079] The above-mentioned positive electrode sheet, separator, and negative electrode sheets prepared in the examples and comparative examples are assembled in sequence, with the separator placed between the positive and negative electrode sheets to play a role in isolation. Then, it is wound to obtain a bare battery cell. The bare battery cell is placed in an outer packaging shell, dried, and then injected with the electrolyte. After vacuum packaging, standing, forming, shaping, and other processes, a soft-pack battery is assembled.

[0080] (2) 500-cycle performance test: At 25 °C, it is charged at a constant current and constant voltage of 3C to 3.65V, with a cut-off current of 0.05C, and left standing for 30 min. Then, it is discharged at a constant current of 1C to 2.5V. This is taken as one cycle. After repeating 500 cycles, the discharge capacity after the 500th cycle is divided by the discharge capacity after the first cycle, which is recorded as the 500-cycle capacity retention rate.

[0081] (3) Rate performance test:

[0082] At 25 °C, it is charged at a constant current and constant voltage of 0.5C to 3.65V, with a cut-off current of 0.05C, and left standing for 10 min. Then, it is discharged at a constant current of 0.5C to 2.5V and left standing for 10 min. The discharge capacity at 0.5C is recorded.

[0083] At 25 °C, it is charged at a constant current and constant voltage of 0.5C to 3.65V, with a cut-off current of 0.05C, and left standing for 10 min. Then, it is discharged at a constant current of 3C to 2.5V and left standing for 10 min. The discharge capacity at 3C is recorded, and the capacity retention rate is calculated by dividing the discharge capacity at 3C by the discharge capacity at 0.5C.

[0084] At 25 °C, charge at a constant current of 0.5C with constant voltage up to 3.65V, cut-off current 0.05C, rest for 10 min, then discharge at a constant current of 5C to 2.5V, rest for 10 min, record the discharge capacity at 5C, and calculate the capacity retention rate by dividing the discharge capacity at 5C by the discharge capacity at 0.5C.

[0085] At 25 °C, charge at a constant current of 0.5C with constant voltage up to 3.65V, cut-off current 0.05C, rest for 10 min, then discharge at a constant current of 6C to 2.5V, rest for 10 min, record the discharge capacity at 6C, and calculate the capacity retention rate by dividing the discharge capacity at 6C by the discharge capacity at 0.5C.

[0086] Table 1

[0087]

[0088] From the results in Table 1, it can be seen that compared with Comparative Examples 1-4, the negative electrode sheets provided in Examples 1-5 of the present invention have better room temperature cycling performance and high rate performance.

[0089] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention without departing from the principle and spirit of the present invention.

Claims

1. A negative electrode plate, characterized in that: The negative electrode plate includes a current collector and an active material layer located on both side surfaces of the current collector; the current collector is provided with a first through hole; the active material layer includes a first active material layer and a second active material layer, wherein the first active material layer is located on the surface of the current collector, and the second active material layer is located on the side of the first active material layer away from the current collector; the first active material layer includes single-particle graphite and is provided with a second through hole, and the second through hole is connected to the first through hole correspondingly; the second active material layer includes secondary particle graphite.

2. The negative electrode sheet according to claim 1, characterized in that: The diameter of the first through hole and the second through hole is 4-10 μm, and the distance between the holes is 10-100 mm.

3. The negative electrode sheet according to claim 1, characterized in that: The mass ratio of the first active material layer to the second active material layer is (3-9):

1.

4. The negative electrode sheet according to claim 1, characterized in that: The content of single-particle graphite in the first active material layer is 80-95wt%; preferably, the content of secondary-particle graphite in the second active material layer is 80-95wt%; preferably, the current collector is selected from copper foil and / or carbon-coated copper foil.

5. A method for preparing a negative electrode sheet, characterized in that: The method for preparing the negative electrode sheet comprises the following steps: S1. Mixing single-grain graphite, a first conductive agent, a first binder and water to obtain slurry A, mixing secondary-grain graphite, a second conductive agent, a second binder and water to obtain slurry B, coating slurry A on both sides of the current collector and drying the slurry to form a first active material layer on both sides of the current collector, and then drilling with a pulsed laser to obtain a pole piece with a porous structure, which has a through-hole structure penetrating the first active material layer and the current collector; S2. Apply slurry B on the surface of the electrode sheet with a porous structure obtained in step S1 and then dry it to form a second active material layer on the surface of the first active material layer. After rolling, the negative electrode sheet is obtained.

6. The method for preparing a negative electrode sheet according to claim 5, characterized in that: Based on the solid mass in the slurry A, the content of the single-grain graphite is 80-95wt%, the content of the first conductive agent is 1-12wt%, and the content of the first binder is 3-8wt%; Preferably, based on the solid mass in the slurry B, the content of the secondary particle-type graphite is 80-95wt%, the content of the second conductive agent is 1-12wt%, and the content of the second binder is 3-8wt%.

7. The method for preparing a negative electrode sheet according to claim 5, characterized in that: The pulse width of the pulse laser is 0.5-5ps, and the laser power is 1500-2000W; Preferably, the through holes have a diameter of 4 to 10 μm and a hole spacing of 10 to 100 mm; Preferably, the mass ratio of the first active material layer to the second active material layer is (3-9):

1.

8. The method for preparing a negative electrode sheet according to claim 5, characterized in that: The first conductive agent and the second conductive agent are each independently selected from at least one of conductive carbon black, graphene, and carbon fiber; Preferably, the first binder and the second binder are each independently selected from at least one of sodium carboxymethyl cellulose, styrene-butadiene rubber, sodium alginate and polyacrylic acid.

9. A negative electrode sheet prepared by the method according to any one of claims 5 to 8.

10. Use of the negative electrode sheet according to any one of claims 1 to 4 and claim 9 in a lithium-ion battery.

Images