Negative electrode sheet, screening method thereof, battery, and electric device

Abstract

A negative electrode sheet and its screening method, battery, and electrical device are disclosed. The negative electrode sheet includes a current collector and a layer of negative electrode active material disposed on the current collector. The negative electrode active material layer contains negative electrode active particles, which satisfy the relationship: 0.7 ≤ R50 / Dv50 ≤ 0.95; where R50 is the equivalent spherical particle size of the negative electrode active particles, and Dv50 is the particle size corresponding to a cumulative particle size distribution percentage of 50% in particle size distribution measurement by laser scattering method. 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 sheet can be controlled to a balanced state, thereby obtaining a battery with high energy density and long cycle life.

Description

Technical Field

[0001] This application relates to the field of electrode material technology, specifically to a negative electrode sheet and its screening method, a battery, and an electrical device. Background Technology

[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] Modern rechargeable batteries are pursuing higher energy density and rate performance. The compaction density of the negative electrode is closely related to the energy density of lithium-ion batteries. Therefore, the electrode needs to have a high compaction density to ensure the energy density of the battery. However, increasing the compaction density will lead to a loss of kinetics. Therefore, balancing the compaction density and kinetic relationship of the negative electrode active particles in the negative electrode has become crucial. Summary of the Invention

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

[0005] To achieve the objectives of this application, the following technical solution is provided:

[0006] In a first aspect, this application provides a negative electrode sheet, including a current collector and a negative electrode active material layer disposed on the current collector, wherein the negative electrode active material layer has negative electrode active particles, and the sphericity 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 particle size distribution measurement by laser scattering method.

[0007] This application achieves a balance between the energy density and kinetic performance of the negative electrode by controlling the ratio of the equivalent spherical particle size to the median particle size within the aforementioned range, thereby obtaining a battery with high energy density and long cycle life. When R50 / Dv50 exceeds the aforementioned range, although the shape of the negative electrode active particles becomes more regular (closer to spherical), the kinetic performance of the negative electrode also decreases, leading to a reduction in active ion transport efficiency. When R50 / Dv50 is below the aforementioned range, the shape of the negative electrode active particles becomes irregular, and the compaction density achievable by the negative electrode decreases, which is detrimental to improving the battery's 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 following relationship: R50=√(4π×(S1-S2) / N); where S1 is the area of ​​a preset region on the plane, S2 is the area of ​​the orthogonal projection of the pores in the preset region onto the plane, and N is the number of negative electrode active particles in the preset region. This application calculates the equivalent spherical particle size using the above relationship, without considering the pores inside the particles, thus avoiding the influence of internal pores on R50. This allows for a full assessment of the impact of particle structure irregularity on material compaction density, rate performance, and cycle performance. Therefore, the negative electrode active particles satisfy the spherical regularity calculated using the above relationship for the equivalent spherical particle size, and the battery assembled using these negative electrode active particles exhibits less expansion during cycling, while also resulting in excellent high-temperature cycling and storage performance.

[0009] In one embodiment, the equivalent spherical particle size R50 of the negative electrode active particles satisfies: 1.4μm ≤ R50 ≤ 4.75μm. 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: 5μm ≤ Dv50 ≤ 20μm. Controlling the median particle size of the negative electrode active particles within the above range ensures that the negative electrode sheet has 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 negative electrode active particles account for 90% to 99.5% of the mass of the negative electrode active material layer.

[0013] Secondly, this 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 layer of negative electrode active material disposed on the current collector, the layer of negative electrode active material having negative electrode active particles; measuring the equivalent spheroidized 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 a cumulative particle size distribution percentage reaching 50% in particle size distribution measurement by laser scattering method; and screening out negative electrode sheets whose spheroidization regularity of the negative electrode active particles satisfies the relationship 0.7≤R50 / Dv50≤0.95.

[0014] By controlling the ratio of the equivalent spherical particle size to the median particle size of the primary particles within the above range, the energy density and kinetic performance of the negative electrode active particles can be balanced, 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 the direction perpendicular to the current collector, the cross-section of the resulting negative electrode sheet being a preset region, the area of ​​the preset region being denoted as S1; projecting the negative electrode active particles within the preset region, the projected area of ​​the pores in the negative electrode active particles being denoted 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 negative electrode active particles within the preset region.

[0016] The equivalent spherical particle size obtained by the above screening method does not include the internal pores of the particles. This avoids the impact of internal pores on R50, thus fully measuring the impact of particle structure irregularity on material compaction density, rate performance, and cycle performance.

[0017] Thirdly, the present invention also discloses a battery comprising an electrolyte, a positive electrode, a separator, and a negative electrode as described in the first aspect above. The positive electrode is at least partially immersed in the electrolyte; the separator is located on one side of the positive electrode and is at least partially immersed in the electrolyte; the negative electrode is disposed on the side of the separator opposite to the positive electrode and is at least partially immersed in the electrolyte. It is understood that the present invention possesses all the technical effects of the negative electrode as described in the first aspect above, and will not be repeated here.

[0018] Fourthly, the present invention also discloses an electrical device comprising a battery as described in the third aspect above. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0020] Figure 1 This is a schematic cross-sectional view of the negative electrode sheet in one embodiment;

[0021] Figure 2This is a cross-sectional view of the negative electrode sheet obtained by taking a picture using particle internal pore identification software, according to one embodiment. Detailed Implementation

[0022] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort 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 it can be in a middle component. When a component is said to be "connected" to another component, it can be directly connected to the other component or it may be in a middle component.

[0024] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. The terminology used in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used in this application includes any and all combinations of one or more of the associated listed items.

[0025] The following detailed description of some embodiments of this application is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0026] This application provides a negative electrode sheet; please refer to [reference needed]. Figure 1 The device includes a current collector 20 and a negative electrode active material layer 10 disposed on the current collector 20. The negative electrode active material layer 10 contains negative electrode active particles 11. The sphericity regularity of the negative electrode active particles 11 satisfies the following 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 negative electrode active particles can be made of carbon-based materials, silicon-based materials, or metallic inorganic materials.

[0028] Optionally, the negative electrode active particles may include multiple primary particles. It should be explained that a primary particle should be the smallest active unit in the negative electrode sheet, i.e., a solid structure observable by an optical microscope or electron microscope. Primary particles in the negative electrode sheet can be spherical or near-spherical structures. It may also include secondary particles, i.e., composed of multiple primary particles.

[0029] Optionally, R50 / Dv50 may 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 at which the cumulative particle size distribution percentage of a sample reaches 50%. Physically, it means that 50% of the particles are larger than Dv50, and 50% are smaller than Dv50. Therefore, Dv50 is also called the median diameter or median particle size.

[0032] It should be noted that R50 / Dv50 can be used to represent the sphericity 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 differs from the existing method for calculating sphericity regularity (the existing method for calculating sphericity 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 determines the sphericity regularity of negative electrode active particles by using the ratio of the equivalent spherical particle size to the median particle size of the negative electrode active particles.

[0034] If a battery needs to achieve high energy density and rate performance, then the compaction density of the negative electrode sheet is closely related to the battery's energy density. Therefore, the negative electrode sheet needs to have a high compaction density to ensure the battery's energy density, but an increase in compaction density can lead to a loss of kinetics. Most actual negative electrode active particles are irregularly shaped, and the regularity of their shape and the number of surface defects affect 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. However, when the shape of the negative electrode active particles is irregular and the material has more surface defects, the material kinetics will be better, but the compaction density of the negative electrode sheet will be lower. Only when the negative electrode sheet has suitable compaction density and kinetic performance under suitable conditions will the battery have 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 aforementioned range, this application can balance the energy density and kinetic performance of the negative electrode sheet, thereby obtaining a battery with high energy density and long cycle life. When R50 / Dv50 exceeds the aforementioned range, although the shape of the negative electrode active particles becomes more regular (closer to spherical), the kinetic performance of the negative electrode sheet will also decrease, leading to a reduction in active ion transport efficiency; when R50 / Dv50 is below the aforementioned range, the shape of the negative electrode active particles becomes irregular, and the compaction density achievable by the negative electrode sheet will decrease, which is not conducive to improving 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 following relationship: R50=√(4π×(S1-S2) / N); where S1 is the area of ​​a preset region on the plane, S2 is the area of ​​the pores in the preset region projected onto 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 aid of SEM (scanning electron microscope) and particle internal pore identification software (image processing software).

[0040] For example: Five fully discharged lithium-ion batteries are provided. The negative electrode of each lithium-ion battery is disassembled, then soaked in DMC for 2 hours for cleaning and drying. Along the length direction of the negative electrode, with any end as the head, segments are taken at intervals of 5%, 20%, 35%, 50%, 65%, 80%, and 95% of the total length. For each segment, a 1cm × 1cm square negative electrode sample is taken transversely at positions of 10%, 30%, 50%, 70%, and 90% of the film width. The negative electrode sample includes a current collector and a layer of negative active material disposed on the current collector, the negative active material layer containing negative active particles.

[0041] Then, the cross-sectional porosity of the negative electrode sample was measured. The measurement method was as follows: the negative electrode sample was first argon-ion polished and cut (CP) along the direction perpendicular to the current collector to obtain the cross-section of the negative electrode active material layer. Then, the cross-sectional position to be tested (preset area) was selected, and then the preset area was photographed and scanned using a scanning electron microscope. The voltage of the scanning electron microscope was set to 5KV and the image magnification was 1000 times.

[0042] Then, the image was processed using 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 bounds of the tracking window were adjusted to 110 and 40 respectively. After grayscale separation, the particle part in the negative electrode active material layer was displayed as a white area, and the pore part as a black area (please refer to...). Figure 2 Then, the software automatically calculates the area of ​​the pores in the negative electrode active particles that make up the entire image.

[0043] It is understandable that the area of ​​the pores in the negative electrode active particles mentioned above that occupies the entire image is the projected area S2 of the pores in the negative electrode active particles in the preset area on the plane where the preset area is located.

[0044] Because batteries undergo significant high SOC storage and high-temperature cycling during use, they require excellent storage and high-temperature cycling performance. Therefore, batteries need negative electrode active particles with regular shapes and few surface defects to improve their storage and high-temperature cycling performance.

[0045] On the other hand, batteries expand during cycling, primarily due to lithium intercalation in the negative electrode. Therefore, when the active particles in the negative electrode have a relatively regular shape and few surface defects, the expansion space for these particles during lithium intercalation is limited. This creates a contradiction between the battery's electrical performance and its expansion rate.

[0046] Therefore, the equivalent spherical particle size calculated using the above-mentioned relationship does not account for the porosity within the particles, thus avoiding the impact of internal porosity on R50. This allows for a full assessment of the influence of particle structure irregularity on material compaction density, rate performance, and cycle performance. Consequently, the negative electrode active particles satisfy the spherical regularity calculated using the above-mentioned relationship for the equivalent spherical particle size, and batteries assembled with these negative electrode active particles exhibit minimal expansion during cycling, while also demonstrating excellent high-temperature cycling and storage performance.

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

[0048] Optionally, R50 can be, but is not limited to, 1.4μm, 1.6μm, 1.8μm, 2μm, 2.3μm, 2.6μm, 2.9μm, 3.2μm, 3.6μm, 4μm, 4.5μm, or 4.75μm. 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 has 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 indicates that the overall size of the negative electrode active particles in the negative electrode sheet is too small. This not only easily leads to more side reactions but also increases the difficulty of preparing the negative electrode active particles, making them prone to aggregation 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, reducing the storage sites for alkali metal ions and resulting in a lower battery capacity.

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

[0051] Optionally, Dv50 can be, but is not limited to, 5μm, 6μm, 8μm, 10μm, 12μm, 14μm, 16μm, 18μm, or 20μm. Controlling the median particle size of the negative electrode active particles within the above range ensures 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 indicates that the overall particle size of the negative electrode active particles in the negative electrode sheet is too small. This not only easily leads to more side reactions but also increases the difficulty of preparing the negative electrode active particles, making them prone to aggregation 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, reducing the storage sites for alkali metal ions and resulting in a lower battery capacity.

[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 the following: copper foil, porous copper foil, foamed 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, it is copper foil, galvanized copper foil, nickel-plated copper foil, or carbon-coated copper foil.

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

[0056] Optionally, the conductive agent can be at least one of conductive carbon black, acetylene black, graphite, graphene, carbon micro / nano-wire conductive materials, and carbon micro / nano-tube conductive materials.

[0057] Optionally, the binder may be a monomer, polymer, or copolymer of acrylonitrile, vinylidene fluoride, vinyl alcohol, carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, methacrylamide, acrylic acid, lithium acrylate, acrylamide, 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 can 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 can be ensured that the thickness of the negative electrode sheet is controlled within a suitable range, preventing the negative electrode sheet from becoming excessively thick.

[0059] In one embodiment, the mass percentage of negative electrode active particles in the negative electrode active material layer is 90% to 99.5%. Optionally, the mass percentage of 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%, or 99.5%. By controlling the mass percentage of negative electrode active particles in the negative electrode active material layer within the above range, it can be ensured that the negative electrode sheet has a high energy storage density.

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

[0061] A negative electrode sheet is provided, which includes a current collector and a negative electrode active material layer disposed 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 is measured. 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.

[0063] Anode sheets with a sphericity regularity of negative electrode active particles satisfying the relationship 0.7≤R50 / Dv50≤0.95 were selected.

[0064] By controlling the ratio of the equivalent spherical particle size to the median particle size of the primary particles within the above range, the energy density and kinetic performance of the negative electrode sheet can be balanced, thereby obtaining a battery with high energy density and long cycle life.

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

[0066] A cross-section is made along the direction perpendicular to the current collector. The cross-section of the resulting negative electrode sheet is the preset region, and the area of ​​the preset region is denoted as S1.

[0067] Project the negative electrode active particles within the preset area, and denote the projected area of ​​the pores in the negative electrode active particles as S2.

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

[0069] Specifically, the equivalent spheroidized particle size R50 of a primary particle can be calculated using the method provided in the above embodiments, and will not be elaborated here.

[0070] In one embodiment, the battery includes an electrolyte, a positive electrode, a separator, and a negative electrode as described above. The positive electrode is at least partially immersed in the electrolyte; the separator is located on one side of the positive electrode and is at least partially immersed in the electrolyte; the negative electrode is disposed on the side of the separator opposite to the positive electrode and is at least partially immersed in the electrolyte. The battery can be a lithium-ion battery, sodium-ion battery, potassium-ion battery, aluminum-ion battery, etc. The battery provided in this application, due to the use of the negative electrode of this application, exhibits excellent rate performance, high energy density, and long cycle life.

[0071] Optionally, the cathode material system in the cathode sheet can be lithium iron phosphate, lithium cobalt oxide, ternary cathode material, etc.

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

[0073] Example 1

[0074] Graphite particle preparation: Natural flake graphite was crushed, spheroidized, and purified to obtain near-spherical graphite. The near-spherical graphite had an R50 of 12.30 μm, a Dv50 of 15.4 μm, and an R50 / Dv50 ratio of 0.799.

[0075] Example 2

[0076] Graphite particle preparation: Natural flake graphite was crushed, spheroidized, and purified to obtain near-spherical graphite. The near-spherical graphite had an R50 of 13.46 μm, a Dv50 of 15.2 μm, and an R50 / Dv50 ratio of 0.886.

[0077] Example 3

[0078] Graphite particle preparation: Natural flake graphite was crushed, spheroidized, and purified to obtain near-spherical graphite. The near-spherical graphite had an R50 of 8.89 μm, a Dv50 of 11.7 μm, and an R50 / Dv50 ratio of 0.76.

[0079] Comparative Example 1

[0080] Graphite particle preparation: Natural flake graphite was crushed, spheroidized, and purified to obtain near-spherical graphite. The near-spherical graphite had an R50 of 14.2 μm, a Dv50 of 14.5 μm, and an R50 / Dv50 ratio of 0.977.

[0081] Comparative Example 2

[0082] Graphite particle preparation: Natural flake graphite was crushed, spheroidized, and purified to obtain near-spherical graphite. The near-spherical graphite had an R50 of 13.60 μm, a Dv50 of 9.2 μm, and an R50 / Dv50 ratio of 0.676.

[0083] The negative electrode active particles provided in Examples 1-3 and the negative electrode active particles provided in Comparative Examples 1-2 were assembled into negative electrode sheets and lithium-ion secondary batteries respectively 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 mixed in an appropriate amount of N-methylpyrrolidone (NMP) solvent at a weight ratio of 96:2:2 to form a uniform positive electrode slurry; this slurry is coated on the positive electrode current collector aluminum foil, dried, and cold-pressed to obtain the positive electrode sheet.

[0085] 2) Preparation of negative electrode sheet: Negative electrode active particles, conductive agent, thickener, and binder are mixed with solvent in a solid content ratio of 96:1:1:2 to obtain negative electrode slurry; the slurry is coated onto the negative electrode current collector with conductive coating using a die extrusion coating machine and dried to obtain unrolled negative electrode sheet; then the electrode sheet is rolled according to the designed compaction density using a rolling press to obtain a negative electrode sheet with a specific thickness.

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

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

[0088] 5) Preparation of lithium-ion batteries: The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. Then, they are wound to obtain a bare battery. The bare battery is placed in an outer packaging foil, and the prepared electrolyte is injected into the dried battery. After vacuum sealing, settling, formation, and shaping, the preparation of the lithium-ion battery is completed.

[0089] The lithium-ion batteries prepared in Examples 1-3 and Comparative Examples 1-2 were subjected to lithium plating tests, and the screening method is as follows:

[0090] Ten lithium-ion batteries from each of the embodiments and comparative examples were taken. First, the batteries were placed at 25°C for 1 hour. Then, two batteries were grouped together and charged to 3.65V at constant power at 1P, 1.1P, 1.2P, 1.4P, 1.6P, 1.8P, 2P, 2.1P, and 2.2P, respectively. After resting for 30 minutes, they were discharged to 2.5V at constant power at 1P, 1.1P, 1.2P, 1.4P, 1.6P, 1.8P, 2P, 2.1P, and 2.2P, respectively. After resting for 30 minutes, the batteries were charged and discharged 10 times. Then, they were charged to 3.65V at constant power at 1P, 1.1P, 1.2P, 1.4P, 1.6P, 1.8P, 2P, 2.1P, and 2.2P, respectively. The batteries were then disassembled, and the lithium plating on the surface of the negative electrode was observed.

[0091] Lithium plating degree determination: It is determined based on the state of the negative electrode after full charge disassembly. If the negative electrode is golden yellow and the area of ​​silver-gray is less than 2%, it is determined that there is no lithium plating. If the negative electrode is golden yellow and the area of ​​silver-gray is ≥ 2%, it is determined that there is lithium plating. The minimum power at which lithium plating occurs is recorded as the lithium plating rate.

[0092] Understandably, a lower lithium plating rate indicates poorer kinetic performance of the negative electrode.

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

[0094] Five lithium-ion batteries from each of the examples and comparative examples were selected, and the average value was taken. The lithium-ion batteries were repeatedly charged and discharged using the following steps, and the discharge capacity retention rate of the lithium-ion batteries was calculated.

[0095] First, in an environment of 25℃, the first charge and discharge cycle was performed. Constant power charging was performed at a charging power of 1P until the upper limit voltage of 3.65V was reached, then constant voltage charging was switched. Then constant power discharging was performed at a discharging 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 in the 3000th cycle / Discharge capacity in the second cycle) × 100%

[0097] Powder compaction density test: The compaction density tester (model LD43.305) was used. 1g of negative electrode powder was placed into the mold and compacted at a displacement speed of 10mm / min and a force of 9800N. The compaction was held for 30s and the compaction density of the powder after 30s was taken as the powder compaction density.

[0098] It is understandable that the higher the compaction density of the negative electrode powder, the higher the compaction density of the negative electrode sheet when it is made from the negative electrode powder, and thus 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] The test results from the above embodiments and comparative examples directly demonstrate that controlling the sphericity regularity of the negative electrode active particles within the range provided in this application results in batteries with these negative electrode active particles exhibiting higher cycle performance than the comparative examples. This is because the energy density and kinetics of the negative electrode sheets in Examples 1-3 can be controlled to a balanced state, thus improving battery performance. However, the data from Comparative Examples 1 and 2 show that when the sphericity regularity exceeds the range provided in this application, battery cycle performance decreases, and excessively extreme situations arise in powder compaction density and lithium plating rate P. Therefore, the negative electrode sheet provided in this application possesses superior performance.

[0103] In the description of the embodiments of this application, it should be noted that the orientation or positional relationship of the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and other indicators are based on the orientation or positional relationship of the drawings, and are only for the convenience of describing this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0104] The above-disclosed embodiments are merely preferred embodiments of this application and should not be construed as limiting the scope of the claims. Those skilled in the art will understand that all or part of the processes for implementing the above embodiments and equivalent variations made in accordance with the claims of this application are still within the scope of this application.

Claims

1. A negative electrode sheet, characterized in that, The device includes a current collector and a negative electrode active material layer disposed on the current collector. The negative electrode active material layer contains negative electrode active particles, and the sphericity regularity of the negative electrode active particles satisfies the following relationship: 0.7≤R50 / Dv50≤0.95; where 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 particle size distribution measurement by laser scattering method. The negative electrode active particles have pores, and the equivalent spherical particle size R50 of the negative electrode active particles satisfies the following relationship: Wherein, S1 is the area of ​​a preset region on the plane, S2 is the area of ​​the orthogonal projection of the pores in the preset region onto the plane, and N is the number of negative electrode active particles in the preset region.

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

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

4. 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.

5. The negative electrode sheet according to claim 1, characterized in that, The negative electrode active particles account for 90% to 99.5% of the mass of the negative electrode active material layer.

6. A method for screening negative electrode sheets, characterized in that, include: A negative electrode sheet is provided, the negative electrode sheet including a current collector and a negative electrode active material layer disposed on the current collector, the negative electrode active material layer having negative electrode active particles, and the equivalent spherical particle size R50 of the negative electrode active particles is measured. The Dv50 particle size of the negative electrode active particles is measured. 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. The negative electrode sheets with a sphericity regularity of the negative electrode active particles satisfying the relationship 0.7≤R50 / Dv50≤0.95 are selected. Measuring the equivalent spheroidized particle size R50 of the negative electrode active particles includes: A cross-section is made along the direction perpendicular to the current collector, and the cross-section of the resulting negative electrode sheet is a preset region, the area of ​​which is denoted as S1; The negative electrode active particles within the preset area are projected, and the projected area of ​​the pores in the negative electrode active particles is denoted as S2. Calculate the equivalent spheroidized particle size R50 of the negative electrode active particles. N is the number of negative electrode active particles in the preset area.

7. A battery, characterized in that, include: Electrolyte The positive electrode sheet is at least partially immersed in the electrolyte; A separator, located on one side of the positive electrode, and at least partially immersed in the electrolyte, and The negative electrode sheet according to any one of claims 1-5, wherein the negative electrode sheet is disposed on the side of the diaphragm opposite to the positive electrode sheet and is at least partially immersed in the electrolyte.

8. An electrical appliance, characterized in that, Includes the battery as described in claim 7.

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