Negative electrode sheet and electrochemical device

Abstract

The present invention discloses a negative electrode sheet and an electrochemical device. In the negative electrode sheet, the layered hard carbon active material effectively reduces the internal resistance of the negative electrode active material layer by surface-to-surface contact instead of point-to-point contact of the normal form of the hard carbon active material, effectively improves the compression density of the negative electrode active material layer, reduces the porosity, and further improves the energy density of the lithium-ion battery. The diffusion of lithium ions in the layered hard carbon negative electrode is more excellent in kinetics and is advantageous for improving the rapid charging ability of the lithium-ion battery. At the same time, the low-expansion hard carbon active material gives better cycle performance to the lithium-ion battery. In addition, the preparation method according to the present invention is simple, easy to operate and control, low in cost, and suitable for industrial production.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary batteries, and specifically relates to a negative electrode sheet and an electrochemical device.

Background Art

[0002] Electrochemical devices such as lithium-ion batteries have remarkable features such as high energy density, long cycle life, no pollution, and no memory effect. As green energy, in order to meet the sustainable development strategy of the environment and energy, the application of electrochemical devices such as batteries has gradually spread from electronic products to large device fields such as electric vehicles. Thereby, higher requirements are also placed on the energy density of electrochemical devices such as batteries.

[0003] Currently, the commercialized negative electrode materials for lithium-ion batteries still mainly use graphite. Graphite has advantages such as high electrical conductivity and high stability. However, graphite has a low theoretical specific capacity, not only poor dynamic conditions, but also a high volume expansion rate in the case of rapid charging and discharging. Therefore, using graphite as the negative electrode material not only makes it difficult to further improve the energy density and cycle life of electrochemical devices such as batteries, but also brings safety risks to electrochemical devices such as batteries.

Summary of the Invention

[0004] In view of the above problems existing in the prior art, the present invention provides a negative electrode sheet, and since this negative electrode sheet has a high compression density, it can endow an electrochemical device with high energy density, long cycle life, excellent rate performance, and rapid charging performance.

[0005] The first aspect of the present invention provides a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active material layer located on at least one side surface of the negative electrode current collector. The negative electrode active material layer contains a negative electrode active material, and the negative electrode active material includes hard carbon particles and graphite particles. The hard carbon particles have a layered structure. Based on the number of hard carbon particles, the proportion of hard carbon particles with an aspect ratio of 3 to 7 is a%, and 30 ≤ a ≤ 70. The negative electrode active material layer composed of hard carbon particles with a specific aspect ratio can obtain a higher compression density after cold pressing, effectively shortening the transmission path of active ions, improving the solid-phase transmission rate of active ions inside the negative electrode active material, reducing the internal resistance of the electrochemical device, and thus improving the kinetic performance, cycle performance, rate performance, and rapid charging performance of the electrochemical device.

[0006] In some embodiments, 30 ≤ a ≤ 50. When the proportion of hard carbon particles with an aspect ratio of 3 to 7 is within this range, the electrochemical device has better cycle performance, rate performance, and rapid charging performance.

[0007] In any embodiment of the present invention, based on the number of hard carbon particles, the proportion of hard carbon particles with an aspect ratio of 2 to 3 is b%, and 20 ≤ b ≤ 60. When the proportion of hard carbon particles with an aspect ratio of 2 to 3 is within the above range, the negative electrode active material layer of the negative electrode sheet can have a high degree of compression and an appropriate porosity, increasing the energy density of the electrochemical device and improving the cycle performance, rate performance, and rapid charging performance of the electrochemical device.

[0008] In some embodiments, 20 ≤ b ≤ 50. When the proportion of hard carbon particles with an aspect ratio of 2 to 3 is within this range, the electrochemical device has better cycle performance, rate performance, and rapid charging performance.

[0009] In any embodiment of the present invention, when the hard carbon particles satisfy a + b ≥ 90, and the proportion of hard carbon particles with an aspect ratio of 2 to 3 and hard carbon particles with an aspect ratio of 3 to 7 in the hard carbon is within the above range, the electrochemical device can have a high energy density, a long cycle life, and excellent rapid charging performance.

[0010] In some embodiments, based on the number of hard carbon particles, the proportion of hard carbon particles with an aspect ratio of 3 to 7 is a%, and the proportion of hard carbon particles with an aspect ratio of 2 to 3 is b%. When the hard carbon particles satisfy a + b ≥ 95, the high energy density, long cycle life, and excellent rapid charging performance of the electrochemical device can be further improved.

[0011] In any embodiment of the present invention, based on the number of hard carbon particles, the proportion of hard carbon particles with an aspect ratio of 1 to 2 is c%, and the proportion of hard carbon particles with an aspect ratio greater than 7 is d%, satisfying 0.1 ≤ c ≤ 10 and 0.1 ≤ d ≤ 1. When the proportion of hard carbon particles with an aspect ratio of 1 to 2 and hard carbon particles with an aspect ratio greater than 7 is within the above appropriate range, it allows the negative electrode active material layer to have a high compression density and a high porosity, reduces the initial irreversible capacity, and can improve the cycle performance, rate performance, and rapid charging performance of the electrochemical device.

[0012] In any embodiment of the present invention, based on the number of hard carbon particles, the proportion of hard carbon particles with an aspect ratio of 2 to 3 is b%, the proportion of hard carbon particles with an aspect ratio of 1 to 2 is c%, and the proportion of hard carbon particles with an aspect ratio greater than 7 is d%, satisfying a + b + c + d = 100. By distributing the aspect ratio of the hard carbon within the above range, it is possible to more fully ensure that the hard carbon has a high compression density and an appropriate specific surface area, so that the negative electrode active material layer of the negative electrode sheet has a high compression density and an appropriate porosity, has a high energy density by the electrochemical device, and further shortens the transmission path of lithium ions in the negative electrode sheet, improves the rapid charging performance of the electrochemical device, reduces the internal resistance of the electrochemical device, reduces the first irreversible capacity of the electrochemical device, and can extend the cycle life of the electrochemical device.

[0013] In any embodiment of the present invention, the mass of the hard carbon particles is 85% to 99% of the mass of the negative electrode active material. When the mass of the hard carbon particles is within the above range, the compression density of the negative electrode active material layer can be further improved, and the electrochemical device can be given a relatively high energy density.

[0014] In any embodiment of the present invention, the X-ray diffraction pattern of the negative electrode active material includes a first diffraction peak and a second diffraction peak. The first diffraction peak is at 18° to 30°, the half-value width of the first diffraction peak is 4° to 12°, the second diffraction peak is at 26° to 27°, and the half-value width of the second diffraction peak is 0.1° to 0.4°.

[0015] In any embodiment of the present invention, the hard carbon particles contain pores, and the pore volume measured by the nitrogen gas - carbon dioxide adsorption - desorption method is 0.25 cc / g or more. The fact that the hard carbon active material has a larger pore volume means that it has a higher lithium storage capacity, can increase the capacity of the negative electrode active material, and can increase the energy density of the electrochemical device.

[0016] In any embodiment of the present invention, the particle size of the negative electrode active material satisfies 1 μm ≤ Dv10 ≤ 5 μm, 4 μm ≤ Dv50 ≤ 18 μm, and Dv99 ≤ 43 μm. Since the particle size of the negative electrode active material is within the above appropriate range, and by combining active materials with different particle sizes, after the active material layer is cold-pressed, it has a denser deposition, can obtain a higher compression density, and can further improve the energy density and cycle performance of the electrochemical device.

[0017] In any embodiment of the present invention, the specific surface area of the negative electrode active material is 1 m 2 / g to 30 m 2 / g. When the specific surface area of the negative electrode active material is within the above appropriate range, the negative electrode active material particles have an appropriate specific surface area, the area of the SEI film formed on the surface of the negative electrode sheet is appropriate, the consumption of irreversible lithium during the first charging process is reduced, and the electrochemical device can have good kinetic performance and a high energy density.

[0018] In any embodiment of the present invention, the compression density of the negative electrode active material layer is 1.0 g / cm 3 to 1.7 g / cm 3 . Since the negative electrode active material layer of the present invention contains hard carbon particles having an appropriate diameter-thickness ratio, the stacking of the negative electrode active material particles in the negative electrode active material layer becomes denser, and because it can have a high compression density, the content of the negative electrode active material per unit volume can be increased, so that the electrochemical device has a higher energy density.

[0019] In any embodiment of the present invention, the porosity of the negative electrode active material layer is 10% to 40%. The layered hard carbon active material has more face-to-face contacts between particles and is stacked more densely, so the porosity of the negative electrode active material layer is made lower. When the porosity of the negative electrode active material layer is within the above appropriate range, not only can the internal resistance of the negative electrode sheet be reduced, but also the wettability of the negative electrode sheet with the electrolyte can be ensured, thereby improving the kinetic performance of the electrochemical device.

[0020] The second aspect of the present invention provides an electrochemical device including the negative electrode sheet of the first aspect.

[0021] In the present invention, hard carbon particles and graphite particles are incorporated into the negative electrode active material. By limiting the proportion of hard carbon particles having a layered structure and an aspect ratio of 3 to 7, the transmission path of active ions can be effectively shortened, the solid-phase transmission rate of active ions inside the negative electrode active material can be improved, the internal resistance of the electrochemical device can be reduced, and after the negative electrode active material layer is cold-pressed, a higher compression density can be obtained, and the kinetic performance, cycle performance, rate performance, and rapid charging performance of the electrochemical device can be improved.

Brief Description of the Drawings

[0022] To more clearly explain the technical solutions of the embodiments of the present invention, the drawings required in the embodiments of the present invention will be briefly described below. It is self-evident that the drawings described below are only some embodiments of the present invention.

Figure 1

Figure 2

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Modes for Carrying Out the Invention

[0023] To make the objectives, technical solutions, and beneficial technical effects of the present invention more clear, the present invention will be described in more detail below with reference to specific embodiments. It should be noted that the embodiments described in this specification are only for explaining the present invention and not for limiting the present invention.

[0024] For the sake of brevity, only some numerical ranges are explicitly disclosed in this specification. However, any lower limit can form a range that is not explicitly described in combination with any upper limit, and any lower limit can form a range that is not explicitly described in combination with other lower limits. Similarly, any upper limit can form a range that is not explicitly described in combination with any other upper limit. Also, although not explicitly described, each point or single numerical value between the endpoints of the range is included within this range. Therefore, each point or single numerical value can form a range that is not explicitly described in combination with any other point or single numerical value, or in combination with other lower or upper limits, using itself as the lower or upper limit.

[0025] In the description of this specification, unless otherwise specified, "above" and "below" include that number, and for the meaning of "plural" in "one or more", it is necessary to explain that it means two or more.

[0026] Unless otherwise specified, the terms used in the present invention have the common meanings generally understood by those skilled in the art. Unless otherwise specified, the values of each parameter referred to in the present invention can be measured using various measurement methods commonly used in the art (for example, they can be measured according to the methods described in the embodiments of the present invention).

[0027] A list of items connected by the terms "at least one of", "at least one kind of", "at least one of", or other similar terms means any combination of the listed items. For example, if item A and item B are listed, the expression "at least one of A and B" means only A, only B, or A and B. In other specific examples, if item A, item B, and item C are listed, the expression "at least one of A, B, and C" means only A, only B, only C, A and B (excluding C), A and C (excluding B), B and C (excluding A), or all of A, B, and C. Item A may include a single element or a plurality of elements. Item B may include a single element or a plurality of elements. Item C may include a single element or a plurality of elements.

[0028] The above invention content of the present invention is not intended to describe each disclosed embodiment or all implementation forms in the present invention. In the following description, exemplary embodiments will be more specifically described by taking examples. Throughout various parts of the present application, guidance is provided through a series of examples, and these examples can be used in various combinations. In each example, the enumeration is only a representative group and should not be construed as exhaustive.

[0029] The hardness of hard carbon is relatively high. After cold pressing, not only is it difficult to improve the compression density, but there is also a possibility of crushing the negative electrode current collector, increasing the risk of peeling off of the negative electrode active material layer, and ultimately causing a sharp increase in the internal resistance of an electrochemical device such as a battery and a significant decrease in the capacity retention rate. During the transportation or use of an electrochemical device such as a battery, the uncompacted hard carbon particles are also likely to crush the separator, resulting in an improvement in the voltage drop of the battery per unit time of the electrochemical device such as a battery. Therefore, when a hard carbon material is directly applied to an electrochemical device, not only is the effect of improving the energy density of the electrochemical device such as a battery very limited, but it also has an adverse impact on the cycle performance, capacity retention rate, and safety of the electrochemical device. Negative electrode sheet

[0030] The present invention provides a negative electrode sheet including a negative electrode current collector and a negative electrode active material layer located on at least one side surface of the negative electrode current collector. The negative electrode active material layer contains a negative electrode active material, and the negative electrode active material contains hard carbon particles and graphite particles. The hard carbon particles have a layered structure. Based on the number of hard carbon particles, the proportion of hard carbon particles with an aspect ratio of 3 to 7 is a%, and 30 ≤ a ≤ 70.

[0031] The aspect ratio of the hard carbon particles can represent the ratio of the major axis to the thickness of the hard carbon particles. The major axis is the longest diameter in the projection plane of the cross-sectional view of the hard carbon particles, and the thickness is the maximum thickness perpendicular to the major axis direction within the cross-section of the hard carbon particles.

[0032] Although not intended to be limited by any theory or interpretation, the inventors have found that, compared with spherical hard carbon particles and irregular-shaped hard carbon particles, the active material layer of hard carbon particles having a specific aspect ratio can obtain a higher compression density after cold pressing. A larger aspect ratio of the hard carbon particles means that the same volume of hard carbon particles can have a thinner layer structure. Thus, as shown in FIG. 1, the hard carbon particles can be stacked and arranged, and can have a high compression density after cold pressing. The layered hard carbon particles contact each other face to face, effectively shortening the transmission path of Li + and improving the solid-phase transmission rate inside the active material of Li + . This can reduce the internal resistance of the lithium-ion battery, improve its kinetic performance, and improve the cycle performance, rate performance, and rapid charging performance of the electrochemical device.

[0033] In some embodiments, a may be 70, 65, 60, 55, 50, 45, 40, 35, 30, or may be within the range consisting of any of the above numerical values.

[0034] In some embodiments, when 30 ≦ a ≦ 50 and the proportion of hard carbon particles with a diameter-to-thickness ratio of 3 to 7 is within this range, the electrochemical device has better cycle performance, rate performance, and rapid charging performance.

[0035] In some embodiments, based on the number of hard carbon particles, the proportion of hard carbon particles with a diameter-to-thickness ratio of 2 to 3 is b%, and 20 ≦ b ≦ 60. For example, b may be 20, 25, 30, 35, 40, 45, 50, 55, 60, or within the range consisting of any of the above numerical values.

[0036] Although not intended to be limited by any theory or interpretation, when the proportion of hard carbon particles with a diameter-to-thickness ratio of 2 to 3 is within the above range, it can be ensured that the hard carbon particles have a high compression density after cold pressing, and it can be ensured that the hard carbon particles have an appropriate specific surface area. Therefore, by applying such a hard carbon material to the negative electrode sheet, the negative electrode active material layer of the negative electrode sheet can be provided with a high degree of compression and an appropriate porosity, and the cycle performance, rate performance, and rapid charging performance of the electrochemical device can be improved.

[0037] In some embodiments, when 20 ≦ b ≦ 50 and the proportion of hard carbon particles with a diameter-to-thickness ratio of 2 to 3 is within this range, the electrochemical device has better cycle performance, rate performance, and rapid charging performance.

[0038] In some embodiments, based on the number of hard carbon particles, the proportion of hard carbon particles with an aspect ratio of 3 to 7 is a%, and the proportion of hard carbon particles with an aspect ratio of 2 to 3 is b%, and a + b ≥ 90 is satisfied. When the proportions of hard carbon particles with an aspect ratio of 2 to 3 and hard carbon particles with an aspect ratio of 3 to 7 in the hard carbon are within the above ranges, the compression density of the negative electrode active material layer can be increased, and the area of the SEI film formed on the surface of the negative electrode sheet can be made appropriate. Thereby, the negative electrode sheet of the present invention can be applied to an electrochemical device and can be allowed to have a high energy density, a long cycle life, and excellent rapid charging performance in the electrochemical device.

[0039] In some embodiments, based on the number of hard carbon particles, the proportion of hard carbon particles with an aspect ratio of 3 to 7 is a%, and the proportion of hard carbon particles with an aspect ratio of 2 to 3 is b%, and when a + b ≥ 95 is satisfied for the hard carbon particles, the cycle performance, rate performance, and rapid charging performance of the electrochemical device can be further improved.

[0040] In some embodiments, based on the number of hard carbon particles, the proportion of hard carbon particles with an aspect ratio of 1 to 2 is c%, and the proportion of hard carbon particles with an aspect ratio greater than 7 is d%, and 0.1 ≤ c ≤ 10, 0.1 ≤ d ≤ 1.

[0041] The main distribution range of the aspect ratio of the hard carbon particles in the normal form is 1 to 2. Since the rigidity of the hard carbon material is relatively strong and the contact between particles is mostly point-to-point contact, it is difficult to be densely laminated during the cold pressing process, leaving a relatively high porosity and reducing the energy density of the electrochemical device. If the aspect ratio of the hard carbon particles is too large, correspondingly, the specific surface area of the hard carbon particles also increases, and the area of the SEI film formed on the surface of the negative electrode sheet also increases, resulting in an increase in the first irreversible capacity. Therefore, the proportion of particles with a low aspect ratio and particles with an aspect ratio that is too large in the hard carbon active material should be reduced as much as possible. Although not intended to be limited by any theory or interpretation, when the proportion of hard carbon particles with an aspect ratio of 1 to 2 and hard carbon particles with an aspect ratio greater than 7 is within the above appropriate range, it is possible to allow the negative electrode active material layer to have a high compression density and a high porosity, and the first irreversible capacity can be reduced.

[0042] In some embodiments, based on the number of hard carbon particles, the proportion of hard carbon particles with an aspect ratio of 3 to 7 is a%, the proportion of hard carbon particles with an aspect ratio of 2 to 3 is b%, the proportion of hard carbon particles with an aspect ratio of 1 to 2 is c%, and the proportion of hard carbon particles with an aspect ratio greater than 7 is d%, satisfying 0.1 ≦ c ≦ 10, 0.1 ≦ d ≦ 1, and a + b + c + d = 100.

[0043] When the aspect ratio of the hard carbon is distributed within the above range, it is possible to more fully ensure that the hard carbon has a high compression density and an appropriate specific surface area. As a result, the negative electrode active material layer of the negative electrode sheet has a high compression density and an appropriate porosity. A negative electrode sheet with a high compression density not only allows the electrochemical device to have a higher energy density, but also can shorten the transmission path of lithium ions in the negative electrode sheet, thereby improving the rapid charging performance of the electrochemical device and reducing the internal resistance of the electrochemical device. Having an appropriate porosity in the negative electrode sheet can provide an appropriate area for the SEI film formed on the surface of the negative electrode sheet, thereby reducing the first irreversible capacity of the electrochemical device. Thus, by applying the negative electrode sheet of the present invention to an electrochemical device, the energy density of the electrochemical device can be significantly increased, the cycle life of the electrochemical device can be extended, and the electrochemical device can be provided with excellent rapid charging performance.

[0044] In some embodiments, pores are included inside the hard carbon particles, the pores satisfy a pore diameter of <2 nm, and the pore volume measured by the carbon dioxide adsorption-desorption method is 0.25 cc / g or more. In some embodiments, the pore volume of the hard carbon particles measured by the carbon dioxide adsorption-desorption method is 1 cc / g to 5 cc / g. At this time, the fact that the hard carbon active material has a larger pore volume means that to a certain extent, the hard carbon active material has a higher lithium storage capacity, increases the capacity of the negative electrode active material, and increases the energy density of the electrochemical device.

[0045] In some embodiments, the mass of the hard carbon particles is 85% to 99% of the mass of the negative electrode active material, and the mass of the graphite particles is 1% to 15% of the mass of the negative electrode active material. The negative electrode active material contains 95% hard carbon particles and 5% graphite particles. The inventors have found that when a small amount of graphite is blended into the negative electrode active material and the amount of the blended graphite is within the above appropriate range, the compression density of the negative electrode active material layer can be further improved, allowing the electrochemical device to have a relatively high energy density. Specifically, graphite has a graphene sheet stacking structure, and after being mixed with hard carbon, during cold pressing, the hard carbon can slide through the graphite layer. As a result, the layered hard carbon particles can be stacked in a more regular orientation, further increasing the compression density of the negative electrode active material layer and enhancing the energy density of the electrochemical device.

[0046] In some embodiments, the X-ray diffraction (XRD) pattern of the negative electrode active material can include a first diffraction peak and a second diffraction peak. The first diffraction peak is at 18° to 30°, and the half-value width of the first diffraction peak is 4° to 12°. The second diffraction peak is at 26° to 27°, and the half-value width of the second diffraction peak is 0.1° to 0.4°. When the XRD spectrum of the negative electrode active material satisfies the above conditions, it can be guaranteed that the negative electrode active material has an appropriate carbon microcrystalline structure and composition components.

[0047] In some embodiments, the graphite particles include natural graphite particles, artificial graphite particles, or a combination thereof.

[0048] Optionally, the artificial graphite particles may include mesocarbon microbead (MCMB)-based artificial graphite particles, petroleum coke-based artificial graphite particles, or a combination thereof.

[0049] In some embodiments, the particle size of the negative electrode active material can satisfy 1 μm ≤ Dv10 ≤ 5 μm, 4 μm ≤ Dv50 ≤ 18 μm, and Dv99 ≤ 43 μm. For example, Dv10 may be 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, or may be within the range consisting of any of the above numerical values. Dv50 may be 4 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, or may be within the range consisting of any of the above numerical values. Dv99 may be ≤ 43 μm, ≤ 40 μm, ≤ 38 μm, ≤ 35 μm, ≤ 32 μm, or ≤ 30 μm. When the particle size of the negative electrode active material is within the above appropriate range, by combining active materials with different particle sizes, the active material layer has a denser deposition after cold pressing, can obtain a higher compression density, and can further improve the energy density and cycle performance of the electrochemical device.

[0050] In some embodiments, the specific surface area of the negative electrode active material is 1 m 2 / g to 30 m 2 / g. For example, the specific surface area of the negative electrode active material may be 1 m 2 / g, 2 m2 / g, 3 m 2 / g, 4 m 2 / g, 5 m 2 / g, 6 m 2 / g, 7 m 2 / g, 8 m 2 / g, 9 m 2 / g, 10 m 2 / g, 15 m 2 / g, 20 m 2 / g, 30 m 2 / g, or may be within the range consisting of any of the above numerical values. When the specific surface area of the negative electrode active material is within the above appropriate range, the negative electrode active material particles have an appropriate specific surface area, the area of the SEI film formed on the surface of the negative electrode sheet is appropriate, and the consumption of irreversible lithium in the first charging process can be reduced. Thereby, the electrochemical device can have good kinetic performance and high energy density.

[0051] In some embodiments, the compression density of the negative electrode active material layer may be 1.0 g / cm 3 ~1.7 g / cm 3 For example, the compression density of the negative electrode active material layer may be 1.0 g / cm 3 、1.1 g / cm 3 、1.2 g / cm 3 、1.3 g / cm 3 、1.4 g / cm 3 、1.5 / cm 3 、1.6 g / cm 3 、1.7 g / cm 3 or may be within the range consisting of any of the above values. Optionally, the compression density of the negative electrode active material is 1.3 g / cm 3 ~1.7 g / cm 3 Since the negative electrode active material layer of the present invention contains hard carbon particles having an appropriate diameter-to-thickness ratio, the negative electrode active material particles in the negative electrode active material layer can be laminated more densely and can have a high compression density, and by increasing the content of the negative electrode active material per unit volume, the electrochemical device can have a higher energy density.

[0052] In some embodiments, the porosity of the negative electrode active material layer may be 10% to 40%. For example, the porosity of the negative electrode active material layer may be 10%, 15%, 20%, 25%, 30%, 35%, 40% or may be within the range consisting of any of the above values.

[0053] Optionally, the porosity of the negative electrode active material may be 15% to 25%.

[0054] Although not intended to be limited by any theory or interpretation, compared with the normal form of the hard carbon material, the layered hard carbon active material has more surface-to-surface contacts between particles and is laminated more densely, thereby reducing the porosity of the negative electrode active material layer. When the porosity of the negative electrode active material layer is within the above appropriate range, not only can the internal resistance of the negative electrode sheet be reduced, but also the wettability of the negative electrode sheet with the electrolyte can be ensured, thereby improving the kinetic performance of the electrochemical device. In some embodiments, the sheet resistance of the negative electrode sheet may be 2 mΩ to 50 mΩ. For example, the sheet resistance of the negative electrode sheet may be 2 mΩ, 5 mΩ, 8 mΩ, 10 mΩ, 15 mΩ, 20 mΩ, 25 mΩ, 30 mΩ, 35 mΩ, 40 mΩ, 45 mΩ, 50 mΩ, or may be within a range consisting of any of the above numerical values.

[0055] When the sheet resistance of the negative electrode sheet is within the above appropriate range, it can be guaranteed that the electrochemical device has low ohmic polarization, and by reducing the heat generation in the charge and discharge process of the electrochemical device, the long-term cycle performance and safety of the electrochemical device can be improved.

[0056] The hard carbon particles of the present invention can be obtained by various methods. As an example, the hard carbon particles are obtained by using the template method, including steps of mixing a layered inorganic template, a pore-forming agent, and a resin to obtain a mixture; curing the mixture at a pressure of 0 T to 5 T and a temperature of 25 °C to 200 °C for 0.1 h to 120 h; pyrolyzing the cured mixture at 700 °C to 1300 °C for 2 h, crushing and sieving it, and then treating it with an acid or alkali solution to remove the template to obtain the hard carbon particles. The layered inorganic template includes, but is not limited to, montmorillonite, macanite, two-dimensional silicon, and layered silicon dioxide. The pore-forming agent includes, but is not limited to, magnesium oxide, magnesium chloride, magnesium gluconate, zinc oxide, zinc chloride, zinc gluconate, zinc stearate, zinc borate, iron oxide, iron chloride, glucose, and sucrose. The resin includes, but is not limited to, phenol resin, furan resin, epoxy resin, polyester resin, bismaleimide, thermosetting polyimide, and cyanate. The mixing method may be powder mixing or solution mixing. When the mixing method is solution mixing, a solvent is selected according to the resin and the pore-forming agent, and the solvent may include, but is not limited to, deionized water, methanol, ethanol, acetone, dichloroethane, benzene, toluene, ethyl acetate, and tetrahydrofuran. After mixing the solution, the solvent may or may not be removed before curing.As a specific example, the hard carbon particles are obtained by completely dissolving 100 g of a thermosetting phenolic resin in 200 mL of ethanol, adding 100 g of micron-order layered silicon dioxide, stirring in an open environment for 24 h to volatilize the ethanol to obtain a mucous mixture, introducing the mucous mixture into a forming plate, setting the pressing pressure to 0.5 T, the pressing temperature to 200 °C, and the pressing time to 1 h, and obtaining a precursor material after the pressing is completed. Then, the precursor material is placed in a tubular oven, heated to 1100 °C at a heating rate of 3 °C / min in an argon gas atmosphere, and kept warm for 2 h to pyrolyze the precursor to obtain pyrolytic carbon. After crushing and sieving the pyrolytic carbon, the pyrolytic carbon is put into 1 L of 2 mol / L sodium hydroxide solution, stirred for 24 h, and then suction filtration is carried out. The suction filtration is repeated twice to ensure complete removal of the layered silicon dioxide template, and finally a layered hard carbon material is obtained.

[0057] The present invention does not limit the negative electrode current collector of the negative electrode sheet. A metal foil material or a porous metal plate, for example, a foil material or a porous plate of a metal such as copper, nickel, titanium, iron, or an alloy thereof may be used. As an example, the negative electrode current collector is a copper foil.

[0058] In some embodiments, the negative electrode current collector has two side surfaces facing each other in its thickness direction, and the negative electrode active material layer may be provided on one side surface of the negative electrode current collector or on two side surfaces of the negative electrode current collector. For example, the negative electrode current collector has two side surfaces facing each other in its thickness direction, and the negative electrode active material layer is provided on either one or both of the opposite sides of the negative electrode current collector.

[0059] In some embodiments, other negative electrode active materials other than hard carbon are not excluded from the negative electrode active material layer. The specific types of other negative electrode active materials are not particularly limited and can be selected as needed. As an example, other negative electrode active materials are soft carbon, silicon, silicon-carbon composite, SiO, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel-structured Li4Ti5O 12and includes, but is not limited to, at least one of Li-Al alloys.

[0060] In some embodiments, the negative electrode active material layer may optionally further include a binder. The binder may be at least one selected from the group consisting of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethyleneoxy-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, and nylon.

[0061] In some embodiments, the negative electrode active material layer may optionally further include a conductive agent. The conductive agent may be selected from carbon-based materials, metal-based materials, conductive polymers, and any combination of the above substances. As an example, the carbon-based material may be at least one selected from the group consisting of natural graphite, artificial graphite, superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The metal-based material may be selected from metal powders and metal fibers. The conductive polymer may include polyphenylene derivatives.

[0062] In some embodiments, the negative electrode active material layer may optionally further include other auxiliaries such as a thickener (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0063] In the present invention, the negative electrode sheet may be prepared according to the usual methods in this field. For example, hard carbon and optionally other negative electrode active materials, conductive agents, binders, and thickeners are dispersed in a solvent, which may be N-methylpyrrolidone (NMP) or deionized water, to form a uniform negative electrode slurry. The negative electrode slurry is coated on a negative electrode current collector, and through processes such as drying and cold pressing, a negative electrode sheet is obtained.

[0064] It should be noted that the parameters of each negative electrode active material layer provided in the present invention all refer to the parameter range on one side of the negative electrode active material layer. When the negative electrode active material layer is provided on both sides of the negative electrode current collector, if the parameters of the negative electrode active material layer on either side satisfy the present invention, it is considered to be within the protection scope of the present invention.

[0065] It should be noted that the negative electrode sheet in the present invention does not exclude other additional functional layers other than the negative electrode active material layer. For example, in some embodiments, the negative electrode sheet of the present invention is further included with a conductive undercoat layer (for example, composed of a conductive agent and a binder) sandwiched between the negative electrode current collector and the negative electrode active material layer and provided on the surface of the negative electrode current collector. In some other embodiments, the negative electrode sheet of the present invention further includes a protective layer covering the surface of the negative electrode active material layer.

[0066] In the present invention, the aspect ratio of the hard carbon particles is measured by methods and apparatuses known in the art. For example, a negative electrode sheet cut to a certain size with a conductive adhesive is attached to a silicon wafer carrier, one cross-section of the negative electrode sheet is polished by argon ion polishing to obtain a test piece, the morphological structure and element distribution of the polished cross-section are analyzed with a scanning electron microscope (SEM), the images of the hard carbon particles are selected with image processing software, and the aspect ratio of each hard carbon particle can be obtained by measuring the value of the major axis of each hard carbon particle in the cross-section and the maximum thickness perpendicular to the major axis direction.

[0067] In the present invention, the XRD spectrum is measured by methods and apparatuses known in the art. For example, it can be obtained by performing XRD measurement using a Bruker D8 ADVANCE X-ray powder diffractometer. The radiation source of the XRD measurement is set as a Cu Kα target, and the measurement parameters are set such that the tube voltage is 40 kV, the tube current is 40 mA, the scan step width is 0.00836°, the scan time for each scan step width is 0.3 s, and the 2θ range is 5° to 80°.

[0068] In the present invention, the particle diameters Dv10, Dv50, and Dv99 of the negative electrode active material have meanings well-known in the art and can be measured using methods and apparatuses known in the art. For example, they can be measured using a laser particle size analyzer (e.g., Malvern Mastersizer 2000E from the UK) in accordance with GB / T19077-2016 Laser Diffraction Method for Particle Size Distribution.

[0069] In the present invention, the specific surface area of the negative electrode active material has a meaning well-known in the art and can be measured by methods known in the art. The specific surface area of the negative electrode active material can be measured, for example, by the nitrogen adsorption / desorption method using a specific surface area analyzer (e.g., TristarII3020M).

[0070] In the present invention, the compression density of the negative electrode active material layer has a meaning well-known in the art and can be measured by methods known in the art. For example, after the negative electrode sheet is cold-pressed, several circular pieces with the entire surface coated with slurry and circular pieces without slurry coating, each with an area of S, are punched out using a punching machine and weighed to obtain the average masses W2 and W1, and their respective thicknesses are measured to obtain the average thicknesses T2 and T1. The compression density of the negative electrode sheet satisfies (W2 - W1) / (T2 - T1) / S.

[0071] In the present invention, the porosity of the negative electrode active material layer has a meaning well-known in the art and can be measured by methods known in the art. For example, the negative electrode sheet coated with the negative electrode active material is punched into circular test pieces. In each test piece, the volume of the negative electrode active material layer is determined by the area and thickness of the circular piece. The porosity of the negative electrode active material layer is measured in accordance with the measurement criteria for the apparent density, true density, and porosity of iron ore in GB / T24586-2009.

[0072] In the present invention, the sheet resistance of the negative electrode sheet has a meaning well-known in the art and can be measured by methods known in the art. For example, the negative electrode sheet is cut into test pieces with a size of 60 mm × 80 mm, and a resistance test is performed on the samples using a BER1100 multifunctional electrode sheet resistance measuring instrument to obtain the sheet resistance of the negative electrode sheet.

[0073] In addition, the measurement of various parameters for the above negative electrode active material layer or negative electrode active material particles may be performed, for example, by sampling and measuring during the preparation process of a lithium-ion battery, or by sampling and measuring from the prepared lithium-ion battery.

[0074] When the above measurement sample is sampled from the prepared lithium-ion battery, as an example, it may be sampled in the following steps S10 - S30.

[0075] In step S10, the lithium-ion battery is discharged (generally, the battery is fully discharged for safety), after the battery is disassembled, the negative electrode plate is taken out, the negative electrode plate is immersed in dimethyl carbonate (DMC) for a certain period of time (for example, 2 - 10 hours), then the negative electrode plate is taken out and dried at a certain temperature and time (for example, 60°C, 4 hours), and the negative electrode plate is taken out after drying. At this time, samples are taken from the dried negative electrode plate to measure each parameter related to the above negative electrode active material layer of the present invention.

[0076] In step S20, the negative electrode plate dried in step S10 is fired at a certain temperature and time (for example, 400°C, 2 hours), and an arbitrary region is selected from the fired negative electrode plate to sample the negative electrode active material (it can be sampled by scraping with a blade).

[0077] In step S30, the negative electrode active material collected in step S20 is sieved (for example, sieved with a 200-mesh sieve) to finally obtain a sample used to measure each of the above negative electrode active material parameters of the present invention.

[0078] Electrochemical device The second aspect of the present invention provides an electrochemical device, and the electrochemical device includes any device that causes an electrochemical reaction to convert chemical energy and electrical energy into each other. Specific examples thereof include, but are not limited to, a lithium-ion battery or a sodium-ion battery.

[0079] In some embodiments, the electrochemical device of the present invention includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolytic solution.

[0080] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be used to fabricate an electrode assembly by a winding process or a lamination process. The electrochemical device of the present invention further includes an outer package for sealing the electrode assembly and the electrolytic solution. In some embodiments, the outer package may be a rigid case such as a rigid plastic case, an aluminum case, or a steel case, or may be a soft pack such as a pouch-type soft pack. The material of the soft pack may be at least one of plastics such as polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0081] [Negative electrode sheet] The negative electrode sheet used in the electrochemical device of the present invention is the negative electrode sheet of the first aspect of the present invention.

[0082] [Positive electrode sheet] The material, structure, and manufacturing method of the positive electrode sheet used in the electrochemical device of the present invention may include any technology well-known in the prior art.

[0083] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector and containing a positive electrode active material. As an example, the positive electrode current collector has two surfaces facing each other in its thickness direction, and the positive electrode active material layer is provided on either one or both of the two opposing surfaces of the positive electrode current collector.

[0084] In some embodiments, the positive electrode active material layer contains a positive electrode active material, and the specific type of the positive electrode active material is not specifically limited and can be selected as needed. For example, the positive electrode active material may include one or more of lithium transition metal oxides, lithium-containing phosphates having an olivine structure, and modified compounds thereof. In the electrochemical device of the present invention, the modified compound of each of the above positive electrode active materials may be subjected to doping modification, surface coating modification, or doping and surface coating modification simultaneously with respect to the positive electrode active material.

[0085] As an example, the lithium transition metal oxide may include one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and modified compounds thereof. As an example, the lithium-containing phosphate having an olivine structure may include one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite material of lithium manganese iron phosphate and carbon, and modified compounds thereof. These positive electrode active materials may be used alone or in combination of two or more.

[0086] In some embodiments, the positive electrode active material layer optionally further contains a conductive agent. As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0087] In some embodiments, the positive electrode active material layer optionally further contains a binder. As an example, the conductive agent may be selected from carbon-based materials, metal-based materials, conductive polymers, and any combination of the above substances. As an example, the carbon-based material may be at least one selected from the group consisting of natural graphite, artificial graphite, superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The metal-based material may be selected from metal powders and metal fibers. The conductive polymer may include polyphenylene derivatives.

[0088] In some embodiments, the positive electrode current collector may be a metal foil sheet or a composite current collector. As an example of the metal foil sheet, the positive electrode current collector may be an aluminum foil. The composite current collector may include a polymer material-based layer and a metal material layer formed on at least one surface of the polymer material-based layer. As an example, the metal material may be one or more selected from the group consisting of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. As an example, the polymer material-based layer may be selected from polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, polyethylene, etc.

[0089] The positive electrode sheet in the present invention can be prepared according to the usual methods in this field. For example, the positive electrode active material layer is usually formed by applying a positive electrode slurry to a positive electrode current collector, drying, and cold pressing. The positive electrode slurry is usually formed by dispersing a positive electrode active material, an optional conductive agent, an optional binder, and any other components in a solvent and stirring uniformly. The solvent may be N-methylpyrrolidone (NMP), but is not limited thereto.

[0090] The positive electrode sheet of the present invention does not exclude other additional functional layers other than the positive electrode active material layer. For example, in some embodiments, the positive electrode sheet of the present invention further includes a conductive undercoat layer (for example, composed of a conductive agent and a binder) sandwiched between the positive electrode current collector and the positive electrode active material layer and provided on the surface of the positive electrode current collector. In some other embodiments, the positive electrode sheet of the present invention further includes a protective layer covering the surface of the positive electrode active material layer.

[0091] [Electrolyte solution] The electrolyte solution plays a role of transmitting active ions between the positive electrode sheet and the negative electrode sheet. The electrolyte solution used in the electrochemical device of the present invention may be an electrolyte solution known in the prior art.

[0092] In some embodiments, the electrolyte solution includes an organic solvent, a lithium salt, and optional additives, and the types of the organic solvent, the lithium salt, and the additives are not specifically limited and can be selected as needed.

[0093] In some embodiments, as an example, the lithium salt includes at least one of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluoro(oxalato)borate), LiBOB (lithium bis(oxalato)borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluorobis(oxalato)phosphate), and LiTFOP (lithium tetrafluoro(oxalato)phosphate), but is not limited thereto. The above lithium salts may be used alone or in combination of two or more.

[0094] In some embodiments, by way of example, the organic solvent includes, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4 - butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS), and diethyl sulfone (ESE). The above - mentioned organic solvents may be used alone or in combination of two or more. Optionally, the above - mentioned organic solvents may be used in combination of two or more.

[0095] In some embodiments, the additive may include a negative electrode film - forming additive and a positive electrode film - forming additive, and may further include additives that can improve specific performance of the battery, such as additives that improve the overcharge performance of the battery, and additives that improve the high - temperature performance or low - temperature performance of the battery.

[0096] By way of example, the additive includes, but is not limited to, at least one of fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), ethylene sulfate (DTD), propylene sulfate, ethylene sulfite (ES), 1,3 - propane sultone (PS), 1,3 - propene sultone (PST), sulfonic acid ester cyclic quaternary ammonium salt, succinic anhydride, succinonitrile (SN), adiponitrile (AND), tris(trimethylsilyl) phosphate (TMSP), and tris(trimethylsilyl) borate (TMSB).

[0097] The electrolyte can be prepared according to the conventional methods in this field. For example, an organic solvent, a lithium salt, and an optional additive can be uniformly mixed to obtain the electrolyte. The addition procedure of each substance is not particularly limited. For example, the lithium salt and the optional additive can be put into the organic solvent and uniformly mixed to obtain the electrolyte. Or, first put the lithium salt into the organic solvent, then put the optional additive into the organic solvent and uniformly mix to obtain the electrolyte.

[0098] [Separator] The separator is provided between the positive electrode sheet and the negative electrode sheet, mainly playing a role in preventing the short circuit between the positive and negative electrodes and allowing active ions to pass through. The present invention has no particular limitation on the type of the separator, and any well-known separator with a porous structure having good chemical stability and mechanical stability can be selected.

[0099] In some embodiments, the separator may be one or more selected from the group consisting of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride, but is not limited thereto. Optionally, the material of the separator may include polyethylene and / or polypropylene. The separator may be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer may be the same or different. In some embodiments, the separator may be provided with a ceramic coating layer or a metal oxide coating layer.

[0100] In the above description of the embodiment of the negative electrode sheet, the beneficial effects that can be achieved by the negative electrode sheet according to the present invention are mainly described by taking a lithium-ion battery as a specific example. However, since the negative electrode active material layer of the negative electrode sheet according to the present invention has a high compression density and an appropriate porosity, when applied to other types of electrochemical devices, the corresponding beneficial effects can also be achieved.

[0101] Power consumption device The third aspect of the present invention provides a power consumption device including the electrochemical device of the second aspect of the present invention.

[0102] The power consumption device of the present invention is not particularly limited, and may be any power consumption device known in the prior art. In some embodiments, the power consumption device may include, but is not limited to, a notebook computer, a pen input computer, a mobile computer, an e-book player, a mobile phone, a mobile fax, a mobile copy, a mobile printer, a headphone stereo, a video movie, a liquid crystal television, a handy cleaner, a portable CD, a mini disk, a transceiver, an electronic organizer, a calculator, a memory card, a portable tape recorder, a radio, a backup power source, a motor, an automobile, a motorcycle, a motorized bicycle, a bicycle, a lighting fixture, a toy, a game device, a clock, a power tool, a strobe, a camera, a large household storage battery, and a lithium ion capacitor, etc.

[0103] [Examples] The following examples will more specifically illustrate the content disclosed in the present application. However, these examples are for illustrative purposes only, and various modifications and changes within the scope of the disclosure of the present application will be apparent to those skilled in the art. Unless otherwise specified, all parts, percentages, and ratios described in the following examples are based on mass. All reagents used in the examples are commercially available or synthesized according to conventional methods, and can be used directly without further treatment. Also, the devices used in the examples are commercially available.

[0104] Examples 1 to 21 Preparation of negative electrode sheet The negative electrode active material, styrene butadiene rubber as a binder, and sodium carboxymethyl cellulose (CMC-Na) were dissolved in deionized water at a mass ratio of 97:1.5:1.5 to obtain a negative electrode slurry with a solid content of 40 wt%. The negative electrode slurry was applied to both sides of the negative electrode current collector, the negative electrode current collector was a copper foil with a thickness of 6 μm, and the single-sided coating thickness was 50 μm. After drying at 85°C, cold pressing, slicing, and cutting, it was dried under vacuum conditions at 120°C for 12 hours to obtain a negative electrode sheet. Based on the mass of the negative electrode active material, the mass percentage w1% of the layered hard carbon, the mass percentage w2% of the MCMB, a, b, c, d, the Dv10, Dv50, Dv99 of the negative electrode active material, the specific surface area of the negative electrode active material, the compression density of the negative electrode active material layer, the porosity of the negative electrode active material layer, and the sheet resistance of the negative electrode sheet were as shown in Table 1, Table 2, and Table 3, respectively. The negative electrode active materials in Examples 1 to 8 were 95% hard carbon and 5% MCMB. The negative electrode active materials in Examples 9 to 16 were the same as those in Example 1 and had the same diameter-to-thickness ratio distribution as the hard carbon in Example 1. The negative electrode active materials in Examples 17 to 21 had the same diameter-to-thickness ratio distribution as the hard carbon in Example 1.

[0105] Preparation of positive electrode sheet The positive electrode active material lithium cobaltate, carbon black as a conductive agent, and PVDF as a binder were mixed at a mass ratio of 97:1.4:1.6, an appropriate amount of the solvent NMP was added, and the mixture was stirred uniformly to obtain a positive electrode slurry with a solid content of 72 wt%. The positive electrode slurry was uniformly coated on both sides of the aluminum foil, which was the positive electrode current collector, and the single-sided coating thickness was 80 μm. After drying at 85°C, cold pressing, slicing, and cutting, it was dried under vacuum conditions at 85°C for 4 hours to obtain a positive electrode sheet.

[0106] Preparation of electrolyte In a dried argon atmosphere glove box, ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) were mixed at a mass ratio of EC:PC:DEC = 1:1:1 and stirred well. Then, lithium salt LiPF6 was added and mixed uniformly to obtain an electrolyte solution. Based on the mass of the electrolyte solution, the mass content of LiPF6 was set to 12.5%. To the electrolyte solution, 1,3 - propane sultone with a mass content of 2%, fluoroethylene carbonate with a mass content of 2%, and succinonitrile with a mass content of 2% were added.

[0107] Preparation of separator As the separator, polyethylene (PE) with a thickness of 7 μm was used.

[0108] Preparation of lithium-ion battery The positive electrode sheet, separator, and negative electrode sheet were laminated and wound in this order to obtain an electrode assembly. The electrode assembly was placed in an outer package, and after removing moisture at 80°C, the above - mentioned electrolyte solution was added. Through processes such as sealing, static formation, degassing, and shaping, a lithium - ion battery was obtained.

[0109] Comparative Examples 1 - 4 The type of negative electrode active material was adjusted, and according to the preparation processes of the negative electrode sheets, positive electrode sheets, electrolyte solutions, separators, and lithium - ion batteries in Examples 1 - 21, the negative electrode sheets, positive electrode sheets, electrolyte solutions, separators, and lithium - ion batteries of Comparative Examples 1 - 5 were prepared. The negative electrode active material of Comparative Example 1 was MCMB, the negative electrode active material of Comparative Example 2 was flaky graphite, the negative electrode active material of Comparative Example 3 was 95% normal - form hard carbon and 5% MCMB, and the negative electrode active material of Comparative Example 4 was 95% micro - spherical hard carbon particles and 5% MCMB.

[0110] Comparative Examples 5 - 6 According to the preparation processes of the negative electrode sheets, positive electrode sheets, electrolyte solutions, separators, and lithium - ion batteries in Examples 1 - 21, as shown in Table 1, the values of a, b, c, and d of the hard - carbon material were adjusted, and the negative electrode sheets, positive electrode sheets, electrolyte solutions, separators, and lithium - ion batteries of Comparative Examples 5 and 6 were prepared. Measurement section

[0111] Measurement of negative electrode sheet (1) Measurement of diameter-thickness ratio of hard carbon particles Measuring device: Scanning electron microscope JSM - 6360LV and the associated energy - dispersive X - ray spectrometer. Take a fully - discharged lithium - ion battery, disassemble it, take out the negative electrode sheet after disassembly, immerse it in DMC for 20 min, wash it successively with DMC and acetone, then place it in an oven and dry it at 80 °C for 12 h. Cut the dried negative electrode sheet into negative electrode sheets with a width of 0.5 cm, use them as samples of the negative electrode, and attach the samples to a silicon wafer carrier with a conductive adhesive with a width of 1 cm. Polish with argon ions (operation parameters: acceleration voltage 8 kV, polishing time 4 h), polish the cross - section at one end of the negative electrode sheet to obtain a test piece. Analyze the morphological structure and element distribution of the polished cross - section with a scanning electron microscope, select the images of hard carbon particles with image - processing software (Multiphase), and measure the major axis value and the maximum thickness perpendicular to the major axis direction of each hard carbon particle in the cross - section to obtain the aspect ratio of each hard carbon particle. For the negative electrode sheets in each example or comparative example, process 10 SEM images respectively (see Figure 2), and statistically obtain the values of a, b, c, and d.

[0112] (2) XRD measurement of negative electrode active material Measuring device: Bruker D8 ADVANCE X - ray powder diffractometer Take a fully - discharged lithium - ion battery, disassemble it to take out the negative electrode sheet, wash and dry it, then process the negative electrode active material layer with a blade to obtain negative electrode active material layer powder. Put the negative electrode active material layer powder into a tubular oven, keep it warm at 400 °C in an argon gas atmosphere for 4 h, and remove the binder attached to the surface of the negative electrode active material layer powder to obtain negative electrode active material powder. Measure the negative electrode active material powder with an X - ray powder diffractometer to obtain an XRD measurement graph of the negative electrode active material. The radiation source for XRD measurement is a Cu Kα target, and the measurement parameters are set as follows: tube voltage is 40 kV, tube current is 40 mA, scan step width is 0.00836 °, scan time for each scan step width is 0.3 s, and the 2θ range is 5 ° - 80 °.

[0113] (3) Measurement of particle diameter of negative electrode active material particles Measuring device: Bruker D8 Advance According to the steps in the XRD measurement of the negative electrode active material, negative electrode active material powder was obtained. The negative electrode active material powder was dispersed in ethanol and sonicated for 30 minutes, and then an ethanol dispersion of the negative electrode active material was obtained. The ethanol dispersion of the negative electrode active material was put into a Malvern particle size measuring device, and Dv10, Dv50, and Dv99 of the negative electrode active material particles were measured.

[0114] (4) Measurement of specific surface area of negative electrode active material Measuring device: Specific surface area analyzer TristarII3020M According to the steps in the XRD measurement of the negative electrode active material, negative electrode active material powder was obtained. The negative electrode active material powder was put into a vacuum oven for drying, and the specific surface area of the negative electrode active material was measured with a specific surface area analyzer.

[0115] (5) Measurement of compression density of negative electrode active material layer A fully discharged lithium-ion battery was taken, disassembled to take out the negative electrode sheet, washed and dried, and then the area S of the single-sided negative electrode active material layer, the mass W1 of the negative electrode sheet, and the thickness T1 of the negative electrode sheet were measured. After washing away the negative electrode active material layer with a solvent and drying, the mass W2 and thickness T2 of the negative electrode current collector were measured. The compression density of the negative electrode active material layer was calculated by the following formulas 1 to 3.

[0116] [Formula 1] JPEG2025519824000002.jpg9153 [Formula 2] JPEG2025519824000003.jpg8153 [Formula 3] JPEG2025519824000004.jpg8153 W0 represents the mass of the single-sided negative electrode active material layer, and T0 represents the thickness of the single-sided negative electrode active material layer.

[0117] (6) Measurement of porosity of negative electrode active material layer Measuring device: True density measuring device (AccuPycII1340) The negative electrode sheet coated with the negative electrode active material was punched into circular sheet test pieces. In each test piece, the volume of the negative electrode active material layer was about 0.35 cm 3 and the porosity of the negative electrode active material layer was measured according to the measurement standards of the apparent density, true density and porosity of iron ore in GB / T24586-2009.

[0118] (7) Measurement of capacity per gram of negative electrode active material The lithium sheets, which are the negative electrode sheet and the positive electrode sheet, were assembled into a coin cell battery, discharged at 0.05C to 5.0 mV, discharged at 50 μA to 5.0 mV, discharged at 10 μA to 5.0 mV, charged at 0.1C to 2.0 V, and the initial charge capacity of the coin cell battery at this time was recorded. Capacity per gram of negative electrode active material = initial charge capacity (mAh) / mass of negative electrode active material (g).

[0119] (8) Measurement of pore size distribution of negative electrode active material Measuring device: ASAP2460 - Physical adsorption analyzer. According to the steps in the XRD measurement of the negative electrode active material, the negative electrode active material powder was obtained. After the drying and degassing treatment, the sample was put into liquid nitrogen, and while adjusting the measurement pressure, the adsorption amount of nitrogen gas was measured at different measurement pressures respectively, and the adsorption and desorption isotherms were drawn. The pore shape was determined by the shape of the hysteresis loop, the pore distribution and pore volume were calculated with different pore models, the pore size distribution curve of mesopores and macropores was fitted with the BJH model, and the pore size distribution curve of micropores was fitted with the DFT model.

[0120] Measurement of lithium-ion battery (1) Measurement of energy density of lithium-ion battery For each example or comparative example, five lithium-ion batteries were taken respectively and their energy densities were measured. The specific measurement steps are as follows. In an environment of 25°C, the first charge and discharge were carried out. Constant current charging and constant voltage charging were carried out with a charging current of 0.5C until the upper limit voltage reached 4.48V, and then constant current discharge was carried out with a discharge current of 0.2C until the discharge cut-off voltage (3V) was reached. The energy density M of the lithium-ion batteries in each example and comparative examplei The percentage A% of the energy density M1 of Comparative Example 1 was calculated as parameter A of the energy density of the lithium-ion battery in each Example and Comparative Example, where A = M i / M1.

[0121] (2) Measurement of rate performance of lithium-ion battery For each Example or Comparative Example, five lithium-ion batteries were taken respectively and rate performance measurement was carried out. The specific measurement steps were as follows: the lithium-ion battery was left standing in a 25 °C environment for 1 hour, the battery was charged at a constant current (CC stage) at a charging rate of I = 1C, after charging to 4.48V, it was charged at a constant voltage (CV stage), and when the charging current was lower than 0.05C, the charging was stopped and left standing for 5 minutes. Further, the battery was discharged to 3V at a rate of 0.2C and left standing for 5 minutes, and this was taken as one target charge-discharge cycle. The charging capacity (average value) of each stage of charging was statistically analyzed, and the ratio of the capacity in the CC stage was calculated. Sequentially, I was adjusted to 0.2C, 0.5C, 1C, 2C, 3C, and the second to sixth target charge-discharge cycles were carried out according to the process of one target charge-discharge cycle. The ratio of the capacity in the CC stage at a charging rate of 3C was calculated by Equation 4. [Equation 4] JPEG2025519824000005.jpg5168

[0122] (3) Measurement of cycle performance of lithium-ion battery For each Example or Comparative Example, five lithium-ion batteries were taken respectively and cycle performance was measured. The specific evaluation steps were as follows. At 25 °C, the lithium-ion battery was charged to 4.48V at a rate of 1C, and then continuously charged at a constant voltage until the charging cut-off current. It was discharged to 3V at a rate of 1C, and this was taken as one charge-discharge cycle. The initial charging capacity, the initial discharge capacity, and the thickness of the fully charged lithium-ion battery in the first cycle were recorded. Thereafter, the charge-discharge cycle was continued, and the discharge capacity at the 400th cycle and the thickness of the fully charged lithium-ion battery were recorded. [Equation 5] JPEG2025519824000006.jpg7168 [Equation 6] JPEG2025519824000007.jpg6168 [Formula 7] JPEG2025519824000008.jpg6168 [Formula 8] JPEG2025519824000009.jpg18168

[0123] (4) Measurement of self-discharge rate of lithium-ion battery For each example or comparative example, five lithium-ion batteries were taken respectively and the self-discharge rate was measured. The specific evaluation steps are as follows. A lithium-ion battery with a state of charge (SOC) of 80% was taken, the initial open-circuit voltage of the lithium-ion battery was measured and recorded as V1, and after standing at 25°C for 48 hours, the open-circuit voltage of the lithium-ion battery was measured again and recorded as V2. [Formula 9] JPEG2025519824000010.jpg7150

[0124] (5) Measurement of the direct current resistance DCR of the lithium-ion battery In an environment of 25°C, the lithium-ion battery was charged at a current of 0.5C until the voltage reached 4.4V, and then charged at a constant voltage until the current reached 0.05C. Then, it was discharged at a current of 0.1C for 2 hours, stood for 1 hour, and then discharged at a current of 0.1C (I1) for 10s, and the discharge voltage V3 in the last 1s was recorded. And it was discharged at a current of 1C (I2) for 1s, and the discharge voltage V4 in the last 1s was recorded, satisfying DCR = (V3 - V4) / (I2 - I1). The details of the settings and measurement results of the examples and comparative examples are as shown in Tables 1 to 3.

[0125]

Table 1

[0126]

Table 2

[0127]

Table 3

[0128] From Table 1, it can be seen that the negative electrode active materials of Comparative Example 1 and Comparative Example 5 are both spherical or substantially spherical carbon material particles, and the distribution of their aspect ratios is relatively concentrated. In a similar cold pressing process, the compression density of the hard carbon-based negative electrode active material is lower than that of the graphite-based negative electrode active material. When using the layered hard carbon in the present invention, since the hard carbon particles are laminated more densely after cold pressing, the negative electrode active material layer has a higher compression density and a lower porosity. Since the lithium-ion battery has a relatively high self-discharge rate, the layered hard carbon having an appropriate aspect ratio distribution has a flatter surface of the active material layer after cold pressing, is less likely to damage the separator, and the degree of internal physical short circuit is significantly reduced. The layered hard carbon particles contact face to face, effectively shortening the transmission path of Li + , and improving the solid-phase transmission rate inside the active material of Li + . As a result, the internal resistance of the lithium-ion battery can be reduced, and the kinetic performance can be improved, effectively improving the rapid charging ability of the lithium-ion battery, endowing the lithium-ion battery with relatively excellent cycle performance and rate performance, and further increasing the energy density of the lithium-ion battery.

[0129] From Examples 1 and 9 to 16, it can be seen that when the particle size of the negative electrode active material is relatively large, it affects the cycle performance of the lithium-ion battery in order to improve the first irreversible capacity of the lithium-ion battery.

[0130] From Examples 1, 17 to 21, it can be seen that when the mass ratios of graphite particles and hard carbon particles are appropriate, the negative electrode sheet can have a relatively high hard carbon content and a relatively high compression density, and the energy density of the corresponding lithium-ion battery will also be higher. Hard carbon not only has a low volume expansion rate during the processes of lithium release and lithium occlusion, but also can limit the volume expansion of graphite during charge and discharge. The lithium-ion battery corresponding to the example with a high hard carbon content will have a better capacity retention rate after 400 cycles as the thickness expansion rate after 400 cycles is smaller.

[0131] The above description is merely specific embodiments of the present application, and the protection scope of the present application is not limited thereto. Those skilled in the art can easily conceive various equivalent modifications or substitutions within the technical scope disclosed in the present application, and all of these modifications or substitutions should be included within the protection scope of the present application. Therefore, the protection scope of the present application should be based on the claims.

Claims

1. A negative electrode plate, comprising: a negative electrode current collector and a negative electrode active material layer located on at least one side surface of the negative electrode current collector, wherein the negative electrode active material layer contains a negative electrode active material, the negative electrode active material contains hard carbon particles and graphite particles, the hard carbon particles have a layered structure, and based on the number of the hard carbon particles, the proportion of the hard carbon particles having an aspect ratio of 3 to 7 is a%, and 30 ≤ a ≤ 70. The negative electrode plate.

2. Based on the number of the hard carbon particles, the proportion of the hard carbon particles having an aspect ratio of 2 to 3 is b%, and 20 ≤ b ≤ 60. The negative electrode plate according to claim 1.

3. The hard carbon particles satisfy a + b ≥ 90. The negative electrode plate according to claim 2.

4. Based on the number of the hard carbon particles, the proportion of the hard carbon particles having an aspect ratio of 1 to 2 is c%, and the proportion of the hard carbon particles having an aspect ratio greater than 7 is d%, and 0.1 ≤ c ≤ 10, 0.1 ≤ d ≤ 1. The negative electrode plate according to claim 1.

5. The mass of the hard carbon particles is 85% to 99% of the mass of the negative electrode active material. The negative electrode plate according to claim 1.

6. The X-ray diffraction pattern of the negative electrode active material includes a first diffraction peak and a second diffraction peak, the first diffraction peak is at 18° to 30°, and the half-value width of the first diffraction peak is 4° to 12°, the second diffraction peak is at 26° to 27°, and the half-value width of the second diffraction peak is 0.1° to 0.4°. The negative electrode plate according to claim 1.

7. The hard carbon particles contain pores, and the pore volume measured by the nitrogen gas / carbon dioxide adsorption-desorption method is 0.25 cc / g or more. The negative electrode plate according to claim 1.

8. The negative electrode active material (1) satisfies 1 μm ≤ Dv10 ≤ 5 μm, 4 μm ≤ Dv50 ≤ 18 μm, and Dv99 ≤ 43 μm for the particle diameter of the negative electrode active material, and (2) The specific surface area of the negative electrode active material is 1 m ２ / g to 30 m ２ / g, The negative electrode plate according to claim 1, satisfying at least one of the above.

9. The negative electrode active material layer (3) The compression density of the negative electrode active material layer is 1.0 g / cm ３ ~1.7 g / cm ３ and (4) has a porosity of 10% to 40%, The negative electrode plate according to claim 1, satisfying at least one of the above.

10. The negative electrode plate (5) Based on the number of the hard carbon particles, the proportion of the hard carbon particles having an aspect ratio of 3 to 7 is a%, and 30 ≤ a ≤ 50. (6) Based on the number of the hard carbon particles, the ratio of the hard carbon particles having an aspect ratio of 2 to 3 is b%, and 20 ≤ b ≤ 50; (7) Based on the number of the hard carbon particles, the ratio of the hard carbon particles having an aspect ratio of 3 to 7 is a%, the ratio of the hard carbon particles having an aspect ratio of 2 to 3 is b%, and the hard carbon particles satisfy a + b ≥ 95; and (8) The pore volume of the hard carbon particles measured by the nitrogen gas - carbon dioxide adsorption - desorption method is 1 cc / g to 5 cc / g. The negative electrode sheet according to any one of claims 1 to 9, satisfying at least one of the above.

11. An electrochemical device including the negative electrode sheet according to any one of claims 1 to 10.

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