Negative electrode sheet, screening method therefor, battery and electrical device

Abstract

A negative electrode sheet, a screening method therefor, a battery, and an electrical device. The negative electrode sheet (100) comprises a current collector (20) and a negative electrode active material layer (10) arranged on the current collector (20); the negative electrode active material layer (10) contains therein negative electrode active particles (11), the negative electrode active particles (11) satisfying the relational expression: 0.7≤R50 / Dv50≤0.95, where R50 is the equivalent spherical particle size of the negative electrode active particles (11), and Dv 50 is the corresponding particle size when the cumulative particle size distribution percentage in a particle size distribution measurement by means of a laser scattering method reaches 50%.

Description

Negative electrode sheet and screening method thereof, battery and electrical equipment

[0001] This disclosure claims priority to the Chinese patent application filed with the China Patent Office on June 29, 2023, with application number 202310791450.6, the entire contents of which are incorporated by reference into this disclosure. Technical Field

[0002] 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

[0003] 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.

[0004] Current secondary batteries are pursuing higher energy density and rate performance. The compaction density of the negative electrode sheet is closely related to the energy density of lithium-ion batteries. Therefore, the electrode sheet needs to have a higher compaction density to ensure the battery's energy density. However, increased compaction will lead to a loss of kinetics. Therefore, how to balance the compaction density and kinetics of the negative electrode active particles in the negative electrode sheet becomes the key.

[0005] Summary of the Invention

[0006] The purpose of this application is to provide a negative electrode sheet and its screening method, a battery and an electrical device.

[0007] To achieve the purpose of this application, this application provides the following technical solutions:

[0008] In the first aspect, the present application provides a negative electrode plate, comprising a current collector and a negative electrode active material layer arranged on the current collector, wherein the negative electrode active material layer comprises 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 when the cumulative particle size distribution percentage reaches 50% in the particle size distribution measurement by laser scattering method.

[0009] 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 becomes 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 improving the energy density of the battery.

[0010] In the 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 arranged 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 the 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.

[0011] The spherical regularity of the negative electrode active particles obtained by the above screening method can be controlled to a balanced state in terms of energy density and kinetic performance by controlling the ratio of the equivalent spherical particle size of the primary particles to the median particle size within the above range, thereby obtaining a battery with high energy density and long cycle life.

[0012] In a third aspect, the present invention further discloses a battery comprising an electrolyte, a positive electrode sheet, a separator, and the 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; and the negative electrode sheet is located on a side of the separator facing away from the positive electrode sheet and is at least partially immersed in the electrolyte. It is understood that all the technical effects of the negative electrode sheet described in the first aspect above are achieved and will not be further elaborated here.

[0013] 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

[0014] In order to more clearly illustrate the implementation methods of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the implementation methods or the description of the prior art. 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.

[0015] FIG1 is a schematic cross-sectional view of a negative electrode sheet according to an embodiment;

[0016] FIG2 is a cross-sectional view of a negative electrode sheet obtained by photographing using particle internal pore recognition software according to an embodiment. DETAILED DESCRIPTION

[0017] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0018] It should be noted that when a component is referred to as being "fixed to" another component, it may be directly on the other component or there may be an intermediate component. When a component is referred to as being "connected to" another component, it may be directly connected to the other component or there may be an intermediate component.

[0019] Unless otherwise defined, all technical and scientific terms used in this application have the same meanings as those commonly understood by those skilled in the art to which this application belongs. The terms used in this specification are for the purpose of describing specific embodiments only and are not intended to limit this application. The term "and / or" as used in this application includes any and all combinations of one or more of the relevant listed items.

[0020] The following describes some embodiments of the present application in detail with reference to the accompanying drawings. In the absence of conflict, the following embodiments and features therein may be combined with each other.

[0021] The present application provides a negative electrode plate 100, please refer to Figure 1, including a current collector 20 and a negative electrode active material layer 10 arranged on the current collector 20, 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.

[0022] 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.

[0023] Optionally, the negative electrode active particles may include multiple primary particles. It should be noted that a primary particle is the smallest active unit in the negative electrode plate. This refers to a solid structure observable through an optical microscope or electron microscope. The primary particles in the negative electrode plate may be spherical or quasi-spherical. Secondary particles may also be included, i.e., composed of multiple primary particles.

[0024] 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.

[0025] 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.

[0026] Dv50 can also refer to the particle size corresponding to the 50% cumulative particle size distribution percentage of a sample. Its physical meaning is that 50% of the particles are larger than this particle size, and 50% of the particles are smaller than this particle size. Therefore, Dv50 is also called the median diameter or median particle size.

[0027] 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, the shape of the negative electrode active particles is closer to a sphere.

[0028] However, the relationship provided in this application differs from the existing method for calculating spherical regularity (which is calculated as 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 spherical regularity of the negative electrode active particles by the ratio of the equivalent spherical particle size to the median particle size.

[0029] If a battery requires high energy density and rate performance, 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 increased compaction will lead to a loss in kinetics. Actual negative electrode active particles are mostly irregularly shaped particles, and the regularity of their shape and the number of surface defects are related to the kinetics, compaction density, and cycling performance of the negative electrode sheet.

[0030] When the particles are more regular and have fewer surface defects, the negative electrode sheet's compaction density will be higher and its cycling performance will be better. However, when the negative electrode active particles are irregularly shaped and the material has more surface defects, the material's kinetics will be better, but the negative electrode sheet's compaction density will be lower. Only when the negative electrode sheet has the appropriate compaction density and kinetics under appropriate conditions will the battery have relatively good cycling performance and a high energy density.

[0031] 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 becomes 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 becomes irregular, and the compaction density that can be achieved by the negative electrode plate will decrease, which is not conducive to improving the battery energy density.

[0032] Preferably, the negative electrode active particles satisfy the relationship: 0.75≤R50 / Dv50≤0.9.

[0033] 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 negative electrode active particles in the preset area.

[0034] The equivalent spherical particle size R50 can be calculated with the assistance of SEM (scanning electron microscope) and particle internal pore identification software (image processing software).

[0035] For example: Provide 5 fully discharged lithium-ion batteries, disassemble the negative electrode sheets of each lithium-ion battery, and then soak them in DMC for 2 hours for cleaning and drying. Along the length direction of the negative electrode sheet, with either end as the head extension direction, the total length is divided into sections at intervals of 5%, 20%, 35%, 50%, 65%, 80%, and 95%. In each section, 1cm×1cm square negative electrode sheet samples are sampled at positions of 10%, 30%, 50%, 70%, and 90% of the membrane width in the horizontal direction. 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.

[0036] 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 perpendicular to the current collector to obtain the cross-sectional area of ​​the negative electrode active material layer. Then, the cross-sectional position to be tested (preset area) was selected, and then a scanning electron microscope was used to photograph and scan the preset area. The scanning electron microscope was set to a voltage of 5KV and an image magnification of 1000 times.

[0037] The photos are then processed using particle internal pore recognition software (image processing software): first, the image is set to a grayscale image, then the active material area of ​​the electrode is selected in the software, and the upper and lower bounds of the tracking window are adjusted to 110 and 40 respectively. Grayscale separation is then performed, and the particle portion of the negative electrode active material layer is displayed as a white area, and the pore portion as a black area (see Figure 2). The software then automatically counts the area of ​​the pores in the negative electrode active particles occupying the entire image.

[0038] 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.

[0039] Since batteries undergo a large number of high SOC storage and high-temperature cycling processes during use, they must possess excellent storage and high-temperature cycling performance. Therefore, batteries require negative electrode active particles with regular shapes and few surface defects to improve these performances.

[0040] On the other hand, batteries expand during cycling, primarily due to lithium insertion into the negative electrode. Therefore, when the negative electrode active particles have a regular shape and few surface defects, they have less room to expand during lithium insertion. This creates a conflict between the battery's electrical performance and its expansion rate.

[0041] Therefore, the equivalent spherical particle size calculated by the above relationship in this application 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. 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 such negative electrode active particles expands less during the cycle process, while also making the battery have excellent high-temperature cycle and storage performance.

[0042] In one embodiment, the equivalent spherical particle size R50 of the negative electrode active particles satisfies: 1.4 μm≤R50≤4.75 μm.

[0043] 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, or 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 has an appropriate compaction density and battery capacity.

[0044] When the equivalent spherical particle size of the negative electrode active particles is smaller than the above range, the overall size of the negative electrode active particles in the negative electrode sheet is too small, which not only easily leads to more side reactions but also increases the difficulty of preparing the negative electrode active particles. The negative electrode active particles tend to agglomerate and are difficult to separate. 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 for alkali metal ions are reduced, and the battery capacity is relatively low.

[0045] In one embodiment, the particle size Dv50 of the negative electrode active particles satisfies: 5 μm≤Dv50≤20 μm.

[0046] 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.

[0047] When the median particle size of the negative electrode active particles is smaller than the above range, the overall particle size of the negative electrode active particles in the negative electrode sheet is too small, which not only easily leads to more side reactions but also increases the difficulty of preparing the negative electrode active particles. The negative electrode active particles tend to agglomerate and are difficult to separate. 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 for alkali metal ions are reduced, and the battery capacity is relatively low.

[0048] In one embodiment, the negative electrode active particles are one or more of graphite, soft carbon, hard carbon, silicon-based compounds, and lithium titanate.

[0049] 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, or carbon-coated copper foil is used.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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, the thickness of the negative electrode sheet can be kept within an appropriate range, preventing the negative electrode sheet from being too thick.

[0054] In one embodiment, the mass percentage of the negative electrode active particles in the negative electrode active material layer is 90% to 99.5%. Alternatively, the mass percentage 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%, or 99.5%. By controlling the mass percentage of the negative electrode active particles in the negative electrode active material layer within the above range, the negative electrode sheet can be ensured to have a high energy storage density.

[0055] In one embodiment, the present application further provides a method for screening negative electrode sheets, comprising:

[0056] A negative electrode plate 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 contains negative electrode active particles, and the equivalent spherical particle size R50 of the negative electrode active particles is measured.

[0057] 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.

[0058] 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.

[0059] 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 of the primary particles to the median particle size within the above range, thereby obtaining a battery with high energy density and long cycle life.

[0060] In one embodiment, calculating the equivalent spherical particle size R50 of the primary particles includes:

[0061] A cross section is made along the direction perpendicular to the current collector, and the cross section of the negative electrode obtained is the preset area, and the area of ​​the preset area is recorded as S1.

[0062] 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 recorded as S2.

[0063] 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.

[0064] 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.

[0065] In one embodiment, a battery includes an electrolyte, a positive electrode sheet, a separator, and the negative electrode sheet described 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 located on the side of the separator facing away from the positive electrode sheet and is at least partially immersed in the electrolyte. The battery may be a lithium-ion battery, a sodium-ion battery, a potassium-ion battery, an aluminum-ion battery, or the like. The battery provided in this application has excellent rate performance, high energy density, and long cycle life due to the use of the negative electrode sheet described in this application.

[0066] Optionally, the positive electrode material system in the positive electrode plate can be lithium iron phosphate, lithium cobalt oxide, ternary positive electrode material, etc.

[0067] The technical solution of the present invention will be further described below in conjunction with embodiments and drawings.

[0068] Example 1

[0069] Graphite particle preparation: Natural flake graphite is crushed and spheroidized to obtain quasi-spherical graphite. The quasi-spherical graphite has an R50 of 12.30 μm, a Dv50 of 15.4 μm, and an R50 / Dv50 ratio of 0.799.

[0070] Example 2

[0071] Graphite particle preparation: Natural flake graphite is crushed and spheroidized to obtain quasi-spherical graphite. The quasi-spherical graphite has an R50 of 13.46 μm, a Dv50 of 15.2 μm, and an R50 / Dv50 ratio of 0.886.

[0072] Example 3

[0073] Graphite particle preparation: Natural flake graphite is crushed and spheroidized to obtain quasi-spherical graphite. The quasi-spherical graphite has an R50 of 8.89 μm, a Dv50 of 11.7 μm, and an R50 / Dv50 ratio of 0.76.

[0074] Comparative Example 1

[0075] Graphite particle preparation: Natural flake graphite is crushed and spheroidized to obtain quasi-spherical graphite. The quasi-spherical graphite has an R50 of 14.2 μm, a Dv50 of 14.5 μm, and an R50 / Dv50 of 0.977.

[0076] Comparative Example 2

[0077] Graphite particle preparation: Natural flake graphite is crushed and spheroidized to obtain quasi-spherical graphite. The quasi-spherical graphite has an R50 of 13.60 μm, a Dv50 of 9.2 μm, and an R50 / Dv50 ratio of 0.676.

[0078] 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:

[0079] 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 thoroughly stirred and 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 a positive electrode sheet.

[0080] 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; the slurry is coated on the negative electrode collector with a conductive coating using a die head extrusion coater and dried to obtain an unrolled negative electrode sheet, which is then rolled using a roller press according to the designed compaction density to obtain a negative electrode sheet with a specific thickness.

[0081] 3) Isolation membrane: PE porous polymer film is used as the isolation membrane.

[0082] 4) Electrolyte: Ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in a volume ratio of 3:7, and then fully dried lithium salt LiPF6 was 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) was additionally added to prepare the electrolyte.

[0083] 5) Preparation of lithium-ion batteries: The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator placed between the positive and negative electrode sheets to act as an isolation layer, and then wound to obtain a bare cell; the bare cell is placed in an outer packaging foil, and the prepared electrolyte is injected into the dried battery. After vacuum packaging, standing, formation, shaping and other processes, the preparation of the lithium-ion battery is completed.

[0084] The lithium ion batteries of Examples 1-3 and Comparative Examples 1-2 prepared above were subjected to lithium deposition tests, and the screening method was as follows:

[0085] 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 power of 1P, 1.1P, 1.2P, 1.4P, 1.6P, 1.8P, 2P, 2.1P, and 2.2P respectively; allowed to stand for 30 minutes, and then discharged to 2.5V at constant power of 1P, 1.1P, 1.2P, 1.4P, 1.6P, 1.8P, 2P, 2.1P, and 2.2P respectively. After standing for 30 minutes and 10 charge and discharge cycles, the batteries were charged to 3.65V at constant power of 1P, 1.1P, 1.2P, 1.4P, 1.6P, 1.8P, 2P, 2.1P, and 2.2P respectively according to the group, and then the batteries were disassembled to observe the lithium deposition on the surface of the negative electrode.

[0086] Determination of the degree of lithium plating: The degree of lithium plating 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 less than 2%, it is determined that there is no lithium plating. When the negative electrode as a whole appears golden yellow and the area displayed as silver-gray is ≥2%, it is determined that lithium plating has occurred. The minimum power at which lithium plating occurs is recorded as the lithium plating rate.

[0087] It is understandable that the smaller the lithium plating rate is, the worse the kinetic performance of the negative electrode is.

[0088] 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:

[0089] Five lithium-ion batteries were collected from each group of all the examples and comparative examples, 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.

[0090] 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 switched. 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.

[0091] Capacity retention rate after 3000 cycles = (discharge capacity at the 3000th cycle / discharge capacity at the second cycle) × 100%

[0092] 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. 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. The powder compaction density after 30s is taken as the powder compaction density.

[0093] 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, and thus the battery using the negative electrode sheet has a higher energy density.

[0094] The test results are shown in Table 1 below:

[0095] Table 1

[0096] 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 such 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 plates provided in this application have relatively excellent performance.

[0097] 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 described in the accompanying drawings. They are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, they cannot be understood as limitations on the present application.

[0098] The above disclosure is only a preferred embodiment of the present application, and certainly cannot be used to limit the scope of the claims of the present application. Ordinary technicians in this field can understand that all or part of the processes of the above embodiment and equivalent changes made in accordance with the claims of the present application are still within the scope covered by the present application.

Claims

1. A negative electrode sheet, wherein: 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, wherein: 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 ​​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 or 2, wherein: 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 any one of claims 1 to 3, wherein: The particle size Dv50 of the negative electrode active particles satisfies: 5 μm≤Dv50≤20 μm.

5. The negative electrode sheet according to any one of claims 1 to 4, wherein: 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 any one of claims 1 to 5, wherein: 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, wherein: 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, wherein 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; 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 the negative electrode active particles in the preset area.

9. A battery, wherein: 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, wherein: Comprising the battery as claimed in claim 9.

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