Negative pole piece and screening method thereof, battery and electric equipment

Abstract

The invention relates to a negative pole piece and a screening method thereof, a battery and electric equipment, the negative pole piece comprises a current collector and a negative active material layer arranged on the current collector, the negative active material layer contains negative active particles, and the negative active particles meet the relational expression: 0.7 < = R50 / Dv50 < = 0.95; wherein R50 is the equivalent spheroidized particle size of the negative electrode active particles, and Dv50 is the corresponding particle size when the cumulative particle size distribution percentage reaches 50% in particle size distribution measurement through a laser scattering method. By controlling the ratio of the equivalent spheroidized particle size to the median particle size of the negative electrode active particles within the range, the energy density and the dynamic performance of the negative electrode pole piece can be controlled to be in an equilibrium state, so that the battery with high energy density and long cycle life is obtained.

Description

Technical Field

[0001] The present application relates to the technical field of electrode materials, and in particular to a negative electrode sheet and a screening method thereof, a battery and an electrical device. Background Art

[0002] Currently, secondary batteries (such as lithium-ion batteries and sodium-ion batteries) are widely used in electric vehicles, portable mobile devices, aerospace and other fields due to their advantages such as high energy density, long life, no memory effect, and low self-discharge rate.

[0003] Secondary batteries nowadays are pursuing higher energy density and rate performance, and the compaction density of the negative electrode sheet has a great relationship with the energy density of lithium-ion batteries. Therefore, the electrode sheet needs to have a higher compaction density to ensure the energy density of the battery, but the increase in compaction density will lead to a loss in kinetics. Therefore, how to balance the compaction density and kinetic relationship of the negative electrode active particles in the negative electrode sheet has become the key. Summary of the invention

[0004] The purpose of this application is to provide a negative electrode sheet and a screening method thereof, a battery and an electrical device.

[0005] To achieve the purpose of this application, this application provides the following technical solutions:

[0006] In the first aspect, the present application provides a negative electrode plate, including a current collector and a negative electrode active material layer arranged on the current collector, the negative electrode active material layer comprising negative electrode active particles, the spherical regularity of the negative electrode active particles satisfying the relationship: 0.7≤R50 / Dv50≤0.95; wherein R50 is the equivalent spherical particle size of the negative electrode active particles, and Dv50 is the particle size corresponding to the cumulative particle size distribution percentage reaching 50% in the particle size distribution measurement by laser scattering method.

[0007] The present application controls the energy density and kinetic performance of the negative electrode sheet to a balanced state by controlling the ratio of the equivalent spherical particle size to the median particle size of the negative electrode active particles within the above range, thereby obtaining a battery with high energy density and long cycle life. When R50 / Dv50 exceeds the above range, the shape of the negative electrode active particles is more regular (closer to spherical), but the kinetic performance of the negative electrode sheet will also decrease, resulting in a decrease in the active ion transmission efficiency; when R50 / Dv50 is lower than the above range, the shape of the negative electrode active particles is irregular, and the compaction density that can be achieved by the negative electrode sheet will decrease, which is not conducive to the improvement of the battery energy density.

[0008] In one embodiment, the negative electrode active particles have pores, and the equivalent spherical particle size R50 of the negative electrode active particles satisfies the relationship: R50 = √(4π×(S1-S2) / N); wherein S1 is the area of ​​a preset area on a plane, S2 is the area of ​​the positive projection of the pores in the preset area on the plane, and N is the number of the negative electrode active particles in the preset area. The equivalent spherical particle size calculated by the above relationship in the present application will not count the pores inside the particles, and can avoid the impact of the internal pores in the particles on R50, thereby fully measuring the impact of the irregularity of the particle structure on the compaction density and rate performance of the material and the cycle performance. Therefore, the negative electrode active particles satisfy the spherical regularity calculated by the equivalent spherical particle size calculated by the above relationship, and the battery assembled with the negative electrode active particles expands less during the cycle, while making the battery have excellent high-temperature cycle and storage performance.

[0009] In one embodiment, the equivalent spherical particle size R50 of the negative electrode active particles satisfies: 1.4 μm ≤ R50 ≤ 4.75 μm. By controlling the equivalent spherical particle size of the negative electrode active particles within the above range and satisfying the above relationship, the Dv50 particle size of the negative electrode active particles can be controlled, thereby ensuring that the negative electrode sheet can have a suitable compaction density and battery capacity.

[0010] In one embodiment, the particle size Dv50 of the negative electrode active particles satisfies: 5 μm ≤ Dv50 ≤ 20 μm. Controlling the median particle size of the negative electrode active particles within the above range can ensure that the negative electrode sheet can have a suitable compaction density and battery capacity.

[0011] In one embodiment, the negative electrode active particles are one or more of graphite, soft carbon, hard carbon, silicon-based compounds, and lithium titanate.

[0012] In one embodiment, the mass proportion of the negative electrode active particles in the negative electrode active material layer is 90% to 99.5%.

[0013] In a second aspect, the present application also provides a method for screening negative electrode sheets, comprising: providing a negative electrode sheet, the negative electrode sheet comprising a current collector and a negative electrode active material layer disposed on the current collector, the negative electrode active material layer comprising negative electrode active particles, and measuring the equivalent spherical particle size R50 of the negative electrode active particles; measuring the Dv50 particle size of the negative electrode active particles, Dv50 being the particle size corresponding to when the cumulative particle size distribution percentage reaches 50% in a particle size distribution measurement by a laser scattering method; and screening out negative electrode sheets whose spherical regularity of the negative electrode active particles satisfies the relationship 0.7≤R50 / Dv50≤0.95.

[0014] The spherical regularity of the negative electrode active particles obtained by the above screening method can control the energy density and kinetic performance of the negative electrode active particles to a balanced state by controlling the ratio of the equivalent spherical particle size to the median particle size of the primary particles within the above range, thereby obtaining a battery with high energy density and long cycle life.

[0015] In one embodiment, measuring the equivalent spherical particle size R50 of the primary particles includes: measuring the equivalent spherical particle size R50 of the negative electrode active particles, including: making a cross section along a direction perpendicular to the current collector, and the cross section of the negative electrode plate obtained is a preset area, and the area of ​​the preset area is recorded as S1; projecting the negative electrode active particles in the preset area, and the projected area of ​​the pores in the negative electrode active particles is recorded as S2; calculating the equivalent spherical particle size R50 of the negative electrode active particles, R50=√(4π×(S1-S2) / N), where N is the number of the negative electrode active particles in the preset area.

[0016] The equivalent spherical particle size counted by the above screening method does not count the pores inside the particles, which can avoid the impact of the internal pores in the particles on R50, thereby fully measuring the impact of the irregularity of the particle structure on the material's compaction density, rate performance, and cycle performance.

[0017] In the third aspect, the present invention further discloses a battery, comprising an electrolyte, a positive electrode sheet, a separator and a negative electrode sheet as described in the first aspect above. The positive electrode sheet is at least partially immersed in the electrolyte; the separator is located on one side of the positive electrode sheet and is at least partially immersed in the electrolyte; the negative electrode sheet is arranged on the side of the separator away from the positive electrode sheet and is at least partially immersed in the electrolyte. It can be understood that all the technical effects of the negative electrode sheet as described in the first aspect above are not repeated here.

[0018] In a fourth aspect, the present invention further discloses an electrical device, comprising the battery as described in the third aspect above. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the implementation methods of the present application or the technical solutions in the prior art, the drawings required for use in the implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0020] Figure 1 is a schematic cross-sectional view of a negative electrode sheet of an embodiment;

[0021] Figure 2The cross-sectional view of a negative electrode sheet is obtained by photographing the negative electrode sheet using software for identifying the internal pores of the particles in one embodiment. DETAILED DESCRIPTION

[0022] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0023] It should be noted that when a component is said to be "fixed to" another component, it can be directly on the other component or there can be a central component. When a component is said to be "connected to" another component, it can be directly connected to the other component or there can be a central component at the same time.

[0024] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used in this application and in the specification are only for the purpose of describing specific embodiments and are not intended to limit this application. The term "and / or" used in this application includes any and all combinations of one or more of the related listed items.

[0025] In conjunction with the accompanying drawings, some embodiments of the present application are described in detail below. In the absence of conflict, the following embodiments and features in the embodiments can be combined with each other.

[0026] This application provides a negative electrode plate, please refer to Figure 1 , including a current collector 20 and a negative electrode active material layer 10 arranged on the current collector 20, wherein the negative electrode active material layer 10 contains negative electrode active particles 11, and the spherical regularity of the negative electrode active particles 11 satisfies the relationship: 0.7≤R50 / Dv50≤0.95; wherein R50 is the equivalent spherical particle size of the negative electrode active particles 11, and Dv50 is the particle size corresponding to the cumulative particle size distribution percentage reaching 50% in the particle size distribution measurement by laser scattering method.

[0027] Optionally, the negative electrode active particles are the main material used to assemble the battery, and are used to store active ions (such as lithium ions or sodium ions) from the positive electrode when the battery is working. The material of the negative electrode active particles can be carbon-based materials, silicon-based materials or metal inorganic materials.

[0028] Optionally, the negative electrode active particles may include multiple primary particles. It should be explained that the primary particle should be the smallest active unit in the negative electrode plate. That is, a solid structure that can be observed by an optical microscope or an electron microscope. The primary particles in the negative electrode plate can be spherical or quasi-spherical structures. Secondary particles may also be included, that is, composed of multiple primary particles.

[0029] Optionally, R50 / Dv50 can be, but is not limited to, 0.7, 0.72, 0.74, 0.76, 0.78, 0.8, 0.82, 0.84, 0.86, 0.88, 0.9, 0.92, or 0.95.

[0030] R50 is the equivalent spherical particle size of the negative electrode active particles; preferably, R50 is the equivalent spherical particle size calculated after excluding the internal pores of the negative electrode active particles.

[0031] Dv50 can also refer to the particle size corresponding to when the cumulative particle size distribution percentage of a sample reaches 50%. Its physical meaning is that particles with a particle size larger than it account for 50%, and particles with a particle size smaller than it also account for 50%, so Dv50 is also called the median diameter or median particle size.

[0032] It should be noted that R50 / Dv50 can be used to indicate the spherical regularity of the negative electrode active particles. When the value of R50 / Dv50 is closer to 1, it indicates that the shape of the negative electrode active particles is closer to a sphere.

[0033] However, the relationship provided in this application is different from the existing calculation method of spherical regularity (the existing calculation method of spherical regularity is the ratio of the surface area of ​​a sphere of the same volume as the object to the surface area of ​​the object). This application can determine the spherical regularity of the negative electrode active particles by the ratio of the equivalent spherical particle size of the negative electrode active particles to their median particle size.

[0034] If the battery needs to have high energy density and rate performance, then the compaction density of the negative electrode sheet is closely related to the energy density of the battery. Therefore, the negative electrode sheet needs to have a higher compaction to ensure the energy density of the battery, but the increase in compaction will lead to a loss of kinetics. The actual negative electrode active particles are mostly irregularly shaped particles, and the regularity of their shape and the number of surface defects will be related to the kinetics, compaction density and cycle performance of the negative electrode sheet.

[0035] When the particles are more regular and have fewer surface defects, the compaction density of the negative electrode sheet will be higher and the cycle performance will be better; but when the negative electrode active particles are irregular in shape and the material has more surface defects, the material dynamics will be better, but the compaction density of the negative electrode sheet will be lower. Only when the negative electrode sheet has the appropriate compaction density and dynamics under the adapted conditions, the battery will have relatively excellent cycle performance and high energy density.

[0036] Therefore, by controlling the ratio of the equivalent spherical particle size to the median particle size of the negative electrode active particles within the above range, the energy density and kinetic performance of the negative electrode plate can be controlled to a balanced state, thereby obtaining a battery with high energy density and long cycle life. When R50 / Dv50 exceeds the above range, although the shape of the negative electrode active particles is more regular (closer to spherical), the kinetic performance of the negative electrode plate will also decrease, resulting in a decrease in the active ion transmission efficiency; when R50 / Dv50 is lower than the above range, the shape of the negative electrode active particles is irregular, and the compaction density that can be achieved by the negative electrode plate will decrease, which is not conducive to the improvement of the battery energy density.

[0037] Preferably, the negative electrode active particles satisfy the relationship: 0.75≤R50 / Dv50≤0.9.

[0038] In one embodiment, the negative electrode active particles have pores, and the equivalent spherical particle size R50 of the negative electrode active particles satisfies the relationship: R50 = √(4π×(S1-S2) / N); wherein S1 is the area of ​​a preset region on a plane, S2 is the area of ​​a positive projection of the pores in the preset region on the plane, and N is the number of negative electrode active particles in the preset region.

[0039] The equivalent spherical particle size R50 can be calculated with the assistance of SEM (scanning electron microscope) and particle internal pore recognition software (image processing software).

[0040] For example: provide 5 fully discharged lithium-ion batteries, disassemble the negative electrode sheet of each lithium-ion battery, and then soak it in DMC for 2 hours for cleaning and drying. Along the length direction of the negative electrode sheet, take any end as the head extension direction and divide it into sections at intervals of 5%, 20%, 35%, 50%, 65%, 80%, and 95% of the total length. Take 1cm×1cm square negative electrode sheet samples at 10%, 30%, 50%, 70%, and 90% of the film width direction in each section. The above-mentioned negative electrode sheet samples include a current collector and a negative electrode active material layer disposed on the current collector, and the negative electrode active material layer includes negative electrode active particles.

[0041] Then the cross-sectional porosity of the negative electrode sheet sample was measured. The measurement method was as follows: first, the negative electrode sheet sample was subjected to argon ion polishing (CP) cutting in a direction perpendicular to the current collector to obtain a cross section of the negative electrode active material layer. Then, the cross-sectional position to be tested (preset area) was selected. Then, a scanning electron microscope was used to take a photo of the preset area. The scanning electron microscope was set to a voltage of 5KV and an image magnification of 1000 times.

[0042] Then, the photo was processed by particle internal pore recognition software (image processing software): first, the image was set to grayscale, then the active material area of ​​the electrode was selected in the software, and the upper and lower bound values ​​of the tracking window were adjusted. The up & down values ​​were 110 & 40 respectively, and then the grayscale was separated. After that, the particle part in the negative electrode active material layer was displayed as a white area, and the pore part was a black area (please refer to Figure 2 ), and then the software automatically counts the area of ​​the pores in the negative electrode active particles occupied by the entire image.

[0043] It can be understood that the area of ​​the entire image occupied by the pores in the negative electrode active particles mentioned above is the projection area S2 of the pores in the negative electrode active particles in the preset region on the plane where the preset region is located.

[0044] Since the battery will have a lot of high SOC storage and high-temperature cycling processes during use, the battery needs to have good storage and high-temperature cycling performance. Therefore, the battery needs to have negative electrode active particles with regular shapes and few surface defects to improve the battery's storage and high-temperature cycling performance.

[0045] On the other hand, the battery will expand during the cycle process, and the battery expansion mainly comes from the expansion of lithium insertion in the negative electrode. Therefore, when the shape of the negative electrode active particles is more regular and there are fewer surface defects, the negative electrode active particles have less expansion space during the lithium insertion process. Therefore, there is a contradiction between the battery's electrical performance and the battery's expansion rate.

[0046] Therefore, the equivalent spherical particle size calculated by the above relationship in this application will not count the pores inside the particles, and can avoid the impact of the internal pores in the particles on R50, thereby fully measuring the impact of the irregularity of the particle structure on the material compaction density and rate performance and cycle performance. Therefore, the negative electrode active particles meet the spherical regularity calculated by the equivalent spherical particle size calculated by the above relationship, and the battery assembled with the negative electrode active particles has less expansion during the cycle process, and the high temperature cycle and storage performance of the battery are excellent.

[0047] In one embodiment, the equivalent spherical particle size R50 of the negative electrode active particles satisfies: 1.4 μm≤R50≤4.75 μm.

[0048] Optionally, R50 may be, but is not limited to, 1.4 μm, 1.6 μm, 1.8 μm, 2 μm, 2.3 μm, 2.6 μm, 2.9 μm, 3.2 μm, 3.6 μm, 4 μm, 4.5 μm, 4.75 μm. By controlling the equivalent spherical particle size of the negative electrode active particles within the above range and satisfying the above relationship, the Dv50 particle size of the negative electrode active particles can be controlled, thereby ensuring that the negative electrode sheet can have a suitable compaction density and battery capacity.

[0049] When the equivalent spherical particle size of the negative electrode active particles is smaller than the above range, it means that the overall size of the negative electrode active particles in the negative electrode sheet is too small, which not only easily produces more side reactions, but also increases the difficulty of preparing the negative electrode active particles. The negative electrode active particles are easy to unite and difficult to subdivide. When the equivalent spherical particle size of the negative electrode active particles is larger than the above range, the particles are too large, the storage sites of the alkali metal ions are reduced, and the battery capacity is low.

[0050] In one embodiment, the particle size Dv50 of the negative electrode active particles satisfies: 5 μm≤Dv50≤20 μm.

[0051] Optionally, Dv50 may be, but is not limited to, 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, or 20 μm. Controlling the median particle size of the negative electrode active particles within the above range can ensure that the negative electrode sheet has a suitable compaction density and battery capacity.

[0052] When the median particle size of the negative electrode active particles is smaller than the above range, it means that the particle size of the negative electrode active particles in the negative electrode sheet is small as a whole, which not only easily produces more side reactions, but also increases the difficulty of preparing the negative electrode active particles. The negative electrode active particles are easy to unite and difficult to subdivide. When the median particle size of the negative electrode active particles is larger than the above range, the particles are too large, the storage sites of alkali metal ions are reduced, and the battery capacity is low.

[0053] In one embodiment, the negative electrode active particles are one or more of graphite, soft carbon, hard carbon, silicon-based compounds, and lithium titanate.

[0054] Optionally, the negative electrode current collector can be one or more of copper foil, porous copper foil, foam nickel / copper foil, galvanized copper foil, nickel-plated copper foil, carbon-coated copper foil, nickel foil, titanium foil, and carbon-containing porous copper foil. Preferably, copper foil, galvanized copper foil, nickel-plated copper foil, and carbon-coated copper foil are used.

[0055] Optionally, the negative electrode active material layer also includes a conductive agent and a binder. The negative electrode active particles, the conductive agent and the binder are added to a solvent and mixed to prepare a negative electrode slurry, which is then coated on a negative electrode current collector and dried to obtain a negative electrode sheet 100 .

[0056] Optionally, the conductive agent may be at least one of conductive carbon black, acetylene black, graphite, graphene, carbon micro-nano linear conductive material, and carbon micro-nano tubular conductive material.

[0057] Optionally, the binder can be a monomer, polymer, or copolymer of acrylonitrile, vinylidene fluoride, vinyl alcohol, carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, methacryloyl, acrylic acid, lithium acrylate, acrylamide, amide, imide, acrylate, styrene-butadiene rubber, sodium alginate, chitosan, ethylene glycol, or guar gum.

[0058] In one embodiment, the thickness of the negative electrode active material layer is 90 μm to 170 μm. Optionally, the thickness of the negative electrode active material layer may be, but is not limited to, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, or 170 μm. By controlling the thickness of the negative electrode active material layer within the above range, it is possible to ensure that the thickness of the negative electrode sheet is controlled within an appropriate range, and the phenomenon of the negative electrode sheet being too thick will not be caused.

[0059] In one embodiment, the mass proportion of the negative electrode active particles in the negative electrode active material layer is 90% to 99.5%. Optionally, the mass proportion of the negative electrode active particles in the negative electrode active material layer may be, but is not limited to, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 99.5%. By controlling the mass proportion of the negative electrode active particles in the negative electrode active material layer within the above range, it can be ensured that the negative electrode plate can have a higher energy storage density.

[0060] In one embodiment, the present application also provides a method for screening negative electrode sheets, comprising:

[0061] A negative electrode sheet is provided, the negative electrode sheet includes a current collector and a negative electrode active material layer arranged on the current collector, the negative electrode active material layer has negative electrode active particles, and the equivalent spherical particle size R50 of the negative electrode active particles is measured.

[0062] The Dv50 particle size of the negative electrode active particles was measured. Dv50 is the particle size corresponding to when the cumulative particle size distribution percentage reaches 50% in the particle size distribution measurement by the laser scattering method.

[0063] The negative electrode sheets whose spherical regularity of the negative electrode active particles satisfies the relationship 0.7≤R50 / Dv50≤0.95 are screened out.

[0064] The spherical regularity of the negative electrode active particles obtained by the above screening method can control the energy density and kinetic performance of the negative electrode plate to a balanced state by controlling the ratio of the equivalent spherical particle size to the median particle size of the primary particles within the above range, thereby obtaining a battery with high energy density and long cycle life.

[0065] In one embodiment, calculating the equivalent spherical particle size R50 of the primary particles includes:

[0066] A cross section is made along a direction perpendicular to the current collector, and the cross section of the negative electrode sheet obtained is a preset area, and the area of ​​the preset area is recorded as S1.

[0067] The negative electrode active particles in the preset area are projected, and the projected area of ​​the pores in the negative electrode active particles is recorded as S2.

[0068] The equivalent spherical particle size R50 of the negative electrode active particles is calculated, R50=√(4π×(S1-S2) / N), where N is the number of negative electrode active particles in the preset area.

[0069] Specifically, the equivalent spherical particle size R50 of the primary particles can be calculated by the method provided in the above embodiment, which will not be described in detail here.

[0070] In one embodiment, the battery includes an electrolyte, a positive electrode sheet, a separator and the negative electrode sheet mentioned above. The positive electrode sheet is at least partially immersed in the electrolyte; the separator is located on one side of the positive electrode sheet and is at least partially immersed in the electrolyte, and the negative electrode sheet is arranged on the side of the separator away from the positive electrode sheet and is at least partially immersed in the electrolyte. The battery can be a lithium ion battery, a sodium ion battery, a potassium ion battery, an aluminum ion battery, etc. The battery provided in the present application has excellent battery rate performance, high energy density and long cycle life due to the use of the negative electrode sheet of the present application.

[0071] Optionally, the positive electrode material system in the positive electrode plate can be lithium iron phosphate, lithium cobalt oxide, ternary positive electrode material, etc.

[0072] The technical solution of the present invention will be further described below in conjunction with embodiments and drawings.

[0073] Example 1

[0074] Preparation of graphite particles: Natural flake graphite is crushed, spheroidized and purified to obtain spherical graphite, wherein the R50 of the spherical graphite is 12.30 μm, the Dv50 is 15.4 μm, and R50 / Dv50=0.799.

[0075] Example 2

[0076] Preparation of graphite particles: Natural flake graphite is crushed, spheroidized and purified to obtain spherical graphite, wherein the R50 of the spherical graphite is 13.46 μm, the Dv50 is 15.2 μm, and R50 / Dv50=0.886.

[0077] Example 3

[0078] Preparation of graphite particles: Natural flake graphite is crushed, spheroidized and purified to obtain spherical graphite, wherein the R50 of the spherical graphite is 8.89 μm, the Dv50 is 11.7 μm, and R50 / Dv50=0.76.

[0079] Comparative Example 1

[0080] Preparation of graphite particles: Natural flake graphite is crushed, spheroidized and purified to obtain spherical graphite, wherein the R50 of the spherical graphite is 14.2 μm, the Dv50 is 14.5 μm, and R50 / Dv50=0.977.

[0081] Comparative Example 2

[0082] Preparation of graphite particles: Natural flake graphite is crushed, spheroidized and purified to obtain spherical graphite, wherein the R50 of the spherical graphite is 13.60 μm, the Dv50 is 9.2 μm, and R50 / Dv50=0.676.

[0083] The negative electrode active particles provided in the above-mentioned Examples 1-3 and the negative electrode active particles provided in Comparative Examples 1-2 are respectively assembled into negative electrode sheets and lithium-ion secondary batteries according to the following methods:

[0084] 1) Preparation of positive electrode sheet: The positive electrode active material lithium iron phosphate (molecular formula LiFePO4), the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) are fully stirred and mixed in a proper amount of N-methylpyrrolidone (NMP) solvent at a weight ratio of 96:2:2 to form a uniform positive electrode slurry; the slurry is coated on the positive electrode current collector aluminum foil, dried, and cold pressed to obtain a positive electrode sheet.

[0085] 2) Preparation of negative electrode sheets: negative electrode active particles, conductive agent, thickener, binder and solvent are mixed in a solid content ratio of 96:1:1:2 to obtain negative electrode slurry; a die extrusion coater is used to coat the slurry on a negative electrode collector with a conductive coating, and the slurry is dried to obtain an unrolled negative electrode sheet; the sheet is then rolled by a roller press according to a designed compaction density to obtain a negative electrode sheet with a specific thickness.

[0086] 3) Isolation membrane: PE porous polymer film is used as the isolation membrane.

[0087] 4) Electrolyte: Ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed in a volume ratio of 3:7, and then fully dried lithium salt LiPF6 is dissolved in a mixed organic solvent at a ratio of 1 mol / L. Based on the above basic electrolyte, 2 wt% of fluoroethylene carbonate (FEC) is additionally added to prepare the electrolyte.

[0088] 5) Preparation of lithium-ion batteries: stack the positive electrode sheet, the separator, and the negative electrode sheet in order, so that the separator is between the positive electrode sheet and the negative electrode sheet to play an isolating role, and then wind them to obtain a bare cell; place the bare cell in an outer packaging foil, inject the above-prepared electrolyte into the dried battery, and complete the preparation of the lithium-ion battery after vacuum packaging, standing, forming, shaping and other processes.

[0089] The lithium ion batteries of Examples 1-3 and Comparative Examples 1-2 prepared above were subjected to lithium precipitation tests, and the screening method was as follows:

[0090] Ten lithium-ion batteries from each group of all the embodiments and comparative examples were taken. The batteries were first placed at 25° C. for 1 hour, and then two batteries were grouped together and charged to 3.65V at constant powers of 1P, 1.1P, 1.2P, 1.4P, 1.6P, 1.8P, 2P, 2.1P, and 2.2P respectively; they were allowed to stand for 30 minutes, and then discharged to 2.5V at constant powers of 1P, 1.1P, 1.2P, 1.4P, 1.6P, 1.8P, 2P, 2.1P, and 2.2P respectively, and allowed to stand for 30 minutes. After 10 charge and discharge cycles, the batteries were charged to 3.65V at constant powers of 1P, 1.1P, 1.2P, 1.4P, 1.6P, 1.8P, 2P, 2.1P, and 2.2P respectively according to the groups, and then the batteries were disassembled to observe the lithium precipitation on the surface of the negative electrode sheet.

[0091] Determination of the degree of lithium deposition: It is determined based on the state of the negative electrode when it is fully charged and disassembled. When the negative electrode as a whole appears golden yellow and the area displayed as silver-gray is <2%, it is determined that there is no lithium deposition; when the negative electrode as a whole appears golden yellow and the area displayed as silver-gray is ≥2%, it is determined that there is lithium deposition, and the minimum power at which lithium deposition occurs is recorded as the lithium deposition rate.

[0092] It is understandable that the smaller the lithium plating rate is, the worse the kinetic performance of the negative electrode is.

[0093] The lithium ion batteries of Examples 1-3 and Comparative Examples 1-2 prepared above were subjected to cycle performance tests, and the screening method was as follows:

[0094] Five lithium ion batteries of each group of all embodiments and comparative examples were selected and the average value was calculated. The lithium ion batteries were repeatedly charged and discharged through the following steps, and the discharge capacity retention rate of the lithium ion batteries was calculated.

[0095] First, in an environment of 25°C, the first charge and discharge were carried out. Constant-power charging was performed at a charging power of 1P until the upper limit voltage of 3.65V was reached, and then constant-voltage charging was performed. Then, constant-power discharge was performed at a discharge power of 1P until the final voltage was 2.5V. This was repeated twice, and the discharge capacity of the second cycle was recorded. Then, 3000 charge and discharge cycles were performed, and the discharge capacity of the 3000th cycle was recorded.

[0096] Capacity retention rate after 3000 cycles = (discharge capacity at the 3000th cycle / discharge capacity at the second cycle) × 100%

[0097] Powder compaction density test: Use a compaction density meter (model: LD43.305) to test, take 1g of negative electrode powder and put it into the mold, and perform a powder compaction test on the powder at a displacement speed of 10mm / min and a force of 9800N. Maintain the pressure for 30S, and take the powder compaction density after 30s as the powder compaction density.

[0098] It can be understood that the greater the compaction density of the negative electrode powder, the higher the compaction density of the negative electrode sheet when the negative electrode powder is used to prepare the negative electrode sheet, so that the battery using the negative electrode sheet has a higher energy density.

[0099] The test results are shown in Table 1 below:

[0100] Table 1

[0101]

[0102] It can be directly seen from the test results of the above embodiments and comparative examples that the spherical regularity of the negative electrode active particles is controlled within the range provided in this application, and the battery cycle performance made with the negative electrode active particles is higher than that of the comparative example. This is because the energy density and kinetics of the negative electrode plates in Examples 1-3 can be controlled to a balanced state, so the battery performance is improved. It can be seen from the data results of Comparative Examples 1 and 2 that when the spherical regularity exceeds the range provided in this application, the battery cycle performance decreases, and the powder compaction density and lithium plating rate P are too extreme. Therefore, the negative electrode plate provided in this application has relatively excellent performance.

[0103] In the description of the embodiments of the present application, it should be noted that the orientation or positional relationship of terms such as "center", "up", "down", "left", "right", "vertical", "horizontal", "inside" and "outside" are based on the orientation or positional relationship of the accompanying drawings, which are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present application.

[0104] What is disclosed above is only a preferred embodiment of the present application, and it certainly cannot be used to limit the scope of the claims of the present application. A person of ordinary skill in the art can understand that all or part of the processes of the above embodiment and equivalent changes made according to the claims of the present application still fall within the scope covered by the present application.

Claims

1. A negative electrode plate, characterized in that: It includes a current collector and a negative electrode active material layer arranged on the current collector, wherein the negative electrode active material layer has negative electrode active particles, and the spherical regularity of the negative electrode active particles satisfies the relationship: 0.7≤R50 / Dv50≤0.95; wherein R50 is the equivalent spherical particle size of the negative electrode active particles, and Dv50 is the particle size corresponding to the cumulative particle size distribution percentage reaching 50% in the particle size distribution measurement by laser scattering method.

2. The negative electrode sheet according to claim 1, characterized in that: The negative electrode active particles have pores, and the equivalent spherical particle size R50 of the negative electrode active particles satisfies the relationship: Wherein, S1 is the area of ​​a preset region on a plane, S2 is the area of ​​the positive projection of the pores in the preset region on the plane, and N is the number of the negative electrode active particles in the preset region.

3. The negative electrode sheet according to claim 1, characterized in that: The equivalent spherical particle size R50 of the negative electrode active particles satisfies: 1.4 μm≤R50≤4.75 μm.

4. The negative electrode sheet according to claim 1, characterized in that: The particle size Dv50 of the negative electrode active particles satisfies: 5 μm≤Dv50≤20 μm.

5. The negative electrode sheet according to claim 1, characterized in that: The negative electrode active particles are one or more of graphite, soft carbon, hard carbon, silicon-based compounds, and lithium titanate.

6. The negative electrode sheet according to claim 1, characterized in that: The mass proportion of the negative electrode active particles in the negative electrode active material layer is 90% to 99.5%.

7. A method for screening negative electrode sheets, characterized in that: include: Providing a negative electrode plate, the negative electrode plate comprising a current collector and a negative electrode active material layer disposed on the current collector, the negative electrode active material layer comprising negative electrode active particles, and measuring an equivalent spherical particle size R50 of the negative electrode active particles; Measuring the Dv50 particle size of the negative electrode active particles, where Dv50 is the particle size corresponding to when the cumulative particle size distribution percentage reaches 50% in the particle size distribution measurement by laser scattering method; The negative electrode sheets whose spherical regularity of the negative electrode active particles satisfies the relationship 0.7≤R50 / Dv50≤0.95 are screened out.

8. The screening method according to claim 7, characterized in that Measuring the equivalent spherical particle size R50 of the negative electrode active particles includes: A cross section is made along a direction perpendicular to the current collector, and the cross section of the negative electrode sheet obtained is a preset area, and the area of ​​the preset area is recorded as S1; Projecting the negative electrode active particles in the preset area, the projected area of ​​the pores in the negative electrode active particles is recorded as S2; Calculate the equivalent spherical particle size R50 of the negative electrode active particles, N is the number of the negative electrode active particles in the preset area.

9. A battery, characterized in that: include: Electrolyte, A positive electrode sheet, at least partially immersed in the electrolyte; a diaphragm, located on one side of the positive electrode plate and at least partially immersed in the electrolyte, and The negative electrode plate according to any one of claims 1 to 6, wherein the negative electrode plate is arranged on a side of the diaphragm away from the positive electrode plate and is at least partially immersed in the electrolyte.

10. An electrical device, characterized in that: Comprising the battery as claimed in claim 9.

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