Negative electrode piece and electrochemical apparatus

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] This invention belongs to the technical field of secondary batteries, and more specifically, relates to negative electrode pieces and electrochemical devices. [Background technology]

[0002] Electrochemical devices such as lithium-ion batteries possess remarkable characteristics, including high energy density, long cycle life, no pollution, and no memory effect. As a form of green energy, and in line with sustainable development strategies for the environment and energy, the application of electrochemical devices such as batteries is gradually expanding from electronic products to large-scale equipment such as electric vehicles. This, in turn, demands higher energy density from these electrochemical devices.

[0003] Currently, commercially available lithium-ion battery anode materials are still primarily graphite. Graphite has advantages such as high electrical conductivity and high stability. However, graphite has a low theoretical specific capacity, poor dynamic conditions, and a high volume expansion rate during rapid charging and discharging. As a result, using graphite as an anode material not only makes it difficult to further improve the energy density and cycle life of electrochemical devices such as batteries, but also poses safety risks to such devices. [Overview of the project]

[0004] In view of the above-mentioned problems in the prior art, the present invention provides a negative electrode piece, which has a high compressible density, and thus enables electrochemical equipment to have high energy density, long cycle life, excellent rate performance, and rapid charging performance.

[0005] A first aspect of the present invention provides a negative electrode piece comprising a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector, wherein the negative electrode active material layer comprises a negative electrode active material comprising hard carbon particles and graphite particles, the hard carbon particles having a layered structure, and the percentage of hard carbon particles with a diameter-to-thickness ratio of 3 to 7 is a%, with 30 ≤ a ≤ 70, based on the number of hard carbon particles. A negative electrode active material layer of hard carbon particles having a specific diameter-to-thickness ratio can obtain a higher compression density after cold pressing, thereby effectively shortening the transmission path of active ions and improving the solid-phase transmission rate of active ions within the negative electrode active material, thereby reducing the internal resistance of the electrochemical apparatus and improving the dynamic performance, cycle performance, rate performance and rapid charging performance of the electrochemical apparatus.

[0006] 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 fast charging performance.

[0007] In any embodiment of the present invention, the proportion of hard carbon particles with a diameter-to-thickness ratio of 2 to 3 is b%, with respect to the number of hard carbon particles, where 20 ≤ b ≤ 60. When the proportion of hard carbon particles with a diameter-to-thickness ratio of 2 to 3 is within the above range, the negative electrode active material layer of the negative electrode piece can have high compressibility and appropriate porosity, thereby increasing the energy density of the electrochemical apparatus and improving the cycle performance, rate performance and rapid charging performance of the electrochemical apparatus.

[0008] 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 fast charging performance.

[0009] In any embodiment of the present invention, if the hard carbon particles satisfy a+b≧90 and the proportion of hard carbon particles with a diameter-to-thickness ratio of 2-3 and hard carbon particles with a diameter-to-thickness ratio of 3-7 is within the above range, the electrochemical device can have high energy density, long cycle life and excellent fast charging performance.

[0010] In some embodiments, based on the number of hard carbon particles, the proportion of hard carbon particles with a diameter-to-thickness ratio of 3 to 7 is a%, the proportion of hard carbon particles with a diameter-to-thickness ratio of 2 to 3 is b%, and the number of hard carbon particles satisfies a+b≧95, which can further improve the high energy density, long cycle life, and excellent fast-charging performance of the electrochemical device.

[0011] In any embodiment of the present invention, the proportion of hard carbon particles with a diameter-to-thickness ratio of 1 to 2 is c%, and the proportion of hard carbon particles with a diameter-to-thickness ratio greater than 7 is d%, with respect to the number of hard carbon particles, satisfying 0.1 ≤ c ≤ 10 and 0.1 ≤ d ≤ 1. When the proportions of hard carbon particles with a diameter-to-thickness ratio of 1 to 2 and hard carbon particles with a diameter-to-thickness ratio greater than 7 are within the above appropriate range, it is possible to allow the negative electrode active material layer to have a high compressive density and a high porosity, thereby reducing the initial irreversible capacity and improving 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 hard carbon within the above range, it can be more fully ensured that 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 an 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. 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. The particle size of the negative electrode active material is within the above appropriate range. 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 in 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-to-thickness ratio, the stacking of the negative electrode active material particles in the negative electrode active material layer becomes denser and can have a high compression density, so that the content of the negative electrode active material per unit volume can be increased, and 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 the wettability of the electrolyte of the negative electrode sheet can be ensured, thereby improving the kinetic performance of the electrochemical device.

[0020] A second aspect of the present invention provides an electrochemical apparatus comprising the negative electrode piece of the first aspect.

[0021] The present invention incorporates hard carbon particles and graphite particles into the negative electrode active material, with the hard carbon particles having a layered structure and limiting the proportion of hard carbon particles with a diameter-to-thickness ratio of 3 to 7. This effectively shortens the transmission path of active ions, improves the solid-phase transmission rate of active ions within the negative electrode active material, and reduces the internal resistance of the electrochemical apparatus. Furthermore, after cold pressing, the negative electrode active material layer achieves a higher compression density, improving the dynamic performance, cycle performance, rate performance, and rapid charging performance of the electrochemical apparatus. [Brief explanation of the drawing]

[0022] To more clearly explain the technical concepts of the embodiments of the present invention, the drawings required in the embodiments of the present invention will be briefly described below. It is obvious that the drawings described below represent only a few embodiments of the present invention. [Figure 1] Figure 1 is a schematic diagram of a cross-section of a negative electrode piece according to one embodiment of the present invention. [Figure 2] Figure 2 is a scanning electron microscope (SEM) image of a cross-section of negative electrode active material particles in one embodiment of the present invention. [Figure 3] Figure 3 is a pore size distribution diagram of hard carbon particles in the negative electrode active material of Example 1 of the present invention. [Modes for carrying out the invention]

[0023] To further clarify the object, technical solution, and beneficial technical effects of the present invention, the present invention will be described in more detail below with reference to specific examples. The examples described herein are for illustrative purposes only and are not intended to limit the present invention.

[0024] For the sake of brevity, only a limited number of numerical ranges are explicitly disclosed in this specification. However, any lower limit can be combined with any upper limit to form an unspecified range, and any lower limit can be combined with any other lower limit to form an unspecified range, and similarly, any upper limit can be combined with any other upper limit to form an unspecified range. Furthermore, although not explicitly stated, each point or single number between the endpoints of a range is included within this range. Thus, each point or single number, as its own lower or upper limit, can be combined with any other point or single number, or with any other lower or upper limit, to form an unspecified range.

[0025] In this specification, unless otherwise stated, "greater than or equal to" and "less than or equal to" include the numbers, and it is necessary to explain that "plural" in "one or more" means two or more types.

[0026] Unless otherwise specified, the terms used in this invention have the general meanings commonly understood by those skilled in the art. Unless otherwise specified, the values ​​of each parameter mentioned in this 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 this invention).

[0027] Item lists connected by the terms “at least one of,” “at least one of,” “at least one of,” or other similar terms mean 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 A only, B only, or A and B. In other specific examples, if items A, B, and C are listed, the expression “at least one of A, B, and C” means A only, B only, C only, 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 contain a single element or multiple elements. Item B may contain a single element or multiple elements. Item C may contain a single element or multiple elements.

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

[0029] Hard carbon is relatively hard, and after cold pressing, it not only does not easily improve its compressive density, but it can also crush the negative electrode current collector, increasing the risk of the negative electrode active material layer detaching. Ultimately, this leads to a sharp increase in the internal resistance of electrochemical devices such as batteries, and a significant decrease in capacity retention. During transport or use of electrochemical devices such as batteries, uncompressed hard carbon particles also easily crush the separator, increasing the voltage drop per unit time of the battery. Therefore, when hard carbon materials are directly applied to electrochemical devices, the effect of improving the energy density of the device is very limited, and it also negatively impacts the cycle performance, capacity retention, and safety of the electrochemical device. Negative electrode piece

[0030] The present invention provides a negative electrode piece comprising a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector, wherein the negative electrode active material layer comprises a negative electrode active material, the negative electrode active material comprises hard carbon particles and graphite particles, the hard carbon particles have a layered structure, and the proportion of hard carbon particles with a diameter-to-thickness ratio of 3 to 7 is a%, with 30 ≤ a ≤ 70, based on the number of hard carbon particles.

[0031] The diameter-to-thickness ratio of hard carbon particles can represent the ratio of the longest diameter to the thickness of the hard carbon particle. The longest diameter is the longest diameter in the projected plane of the hard carbon particle cross-section, and the thickness is the maximum thickness perpendicular to the longest diameter direction within the hard carbon particle cross-section.

[0032] While not intended to be limited to any particular theory or interpretation, the inventors have found that active material layers of hard carbon particles having a specific diameter-to-thickness ratio can achieve a higher compressive density after cold pressing compared to microspherical hard carbon particles and irregularly shaped hard carbon particles. A larger diameter-to-thickness ratio of hard carbon particles means that the same volume of hard carbon particles can have a thinner layer structure. As a result, as shown in Figure 1, the hard carbon particles can be stacked and arranged, achieving a high compressive density after cold pressing, and the layered hard carbon particles can contact each other surface-to-surface, thus enabling Li + Effectively shortens the transmission path, Li + By improving the solid-phase transmission rate within the active material, the internal resistance of lithium-ion batteries can be reduced, improving their dynamic performance and enhancing the cycle performance, rate performance, and rapid charging performance of electrochemical devices.

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

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

[0036] While not intended to be limited to any particular theory or interpretation, if the proportion of hard carbon particles with a diameter-to-thickness ratio of 2-3 falls within the above range, it can be ensured that the hard carbon particles will have a high compressive density after cold pressing, as well as an appropriate specific surface area. Therefore, by applying such hard carbon material to the anode piece, the anode active material layer of the anode piece can be given high compressibility and appropriate porosity, thereby improving the cycle performance, rate performance, and rapid charging performance of the electrochemical device.

[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 fast charging performance.

[0038] In some embodiments, based on the number of hard carbon particles, the proportion of hard carbon particles with a diameter-to-thickness ratio of 3 to 7 is a%, and the proportion of hard carbon particles with a diameter-to-thickness ratio of 2 to 3 is b%, satisfying a+b≧90. When the proportion of hard carbon particles with a diameter-to-thickness ratio of 2 to 3 and hard carbon particles with a diameter-to-thickness ratio of 3 to 7 in the hard carbon is within the above range, the compressive 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 piece can be appropriately adjusted. As a result, the negative electrode piece of the present invention can be applied to an electrochemical apparatus, allowing the electrochemical apparatus to have high energy density, long cycle life, and excellent rapid charging performance.

[0039] In some embodiments, based on the number of hard carbon particles, the proportion of hard carbon particles with a diameter-to-thickness ratio of 3 to 7 is a%, the proportion of hard carbon particles with a diameter-to-thickness ratio of 2 to 3 is b%, and the number of hard carbon particles satisfies a+b≧95, which can further improve the cycle performance, rate performance, and fast charging performance of the electrochemical apparatus.

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

[0041] The main distribution range for the diameter-to-thickness ratio of hard carbon particles in their normal form is 1-2. Because hard carbon materials have relatively high rigidity and particle-to-particle contact is mostly point-to-point, they are not easily densely laminated during the cold pressing process, leaving a relatively high porosity and reducing the energy density of the electrochemical apparatus. If the diameter-to-thickness ratio of hard carbon particles is too large, the specific surface area of ​​the hard carbon particles also increases accordingly, and the area of ​​the SEI film formed on the surface of the negative electrode piece also increases, resulting in an increase in the initial irreversible capacity. For this reason, the proportion of particles with low diameter-to-thickness ratios and particles with excessively high diameter-to-thickness ratios in the hard carbon active material should be reduced as much as possible. Although not intended to be limited to any particular theory or interpretation, if the proportion of hard carbon particles with a diameter-to-thickness ratio of 1-2 and hard carbon particles with a diameter-to-thickness ratio greater than 7 is within the appropriate range described above, it is possible to allow for high compressive density and high porosity in the negative electrode active material layer, thereby reducing the initial irreversible capacity.

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

[0043] By ensuring that the diameter-to-thickness ratio of the hard carbon is distributed within the above range, it is more adequately guaranteed that the hard carbon has a high compressive density and an appropriate specific surface area, so that the negative electrode active material layer of the negative electrode piece has a high compressive density and an appropriate porosity. A negative electrode piece with a high compressive density not only allows the electrochemical device to have a higher energy density, but also shortens the transmission path of lithium ions in the negative electrode piece, 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 piece allows for an appropriate surface area of ​​the SEI film formed on the surface of the negative electrode piece, thereby reducing the initial irreversible capacity of the electrochemical device. As a result, by applying the negative electrode piece 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 given excellent rapid charging performance.

[0044] In some embodiments, hard carbon particles contain pores, the pore diameter of which is <2 nm, and the pore volume measured by carbon dioxide adsorption-desorption is 0.25 cc / g or greater. In some embodiments, the pore volume of hard carbon particles measured by carbon dioxide adsorption-desorption is 1 cc / g to 5 cc / g. In this case, having a larger pore volume in the hard carbon active material means, to some extent, that 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 anode active material, and the mass of the graphite particles is 1% to 15% of the mass of the anode active material. The anode active material contains 95% hard carbon particles and 5% graphite particles. The inventors have found that by incorporating a small amount of graphite into the anode active material, and when the amount of graphite is within the above appropriate range, the compressive density of the anode active material layer can be further improved, thereby allowing the electrochemical apparatus to have a relatively high energy density. Specifically, the graphite is in the form of a graphene sheet laminate structure, and after mixing with hard carbon, during cold pressing, the hard carbon can slide through the graphite layer. This allows the layered hard carbon particles to be laminated in a more regular orientation, further increasing the compressive density of the anode active material layer and thus increasing the energy density of the electrochemical apparatus.

[0046] In some embodiments, the X-ray diffraction (XRD) pattern of the anode active material may include a first diffraction peak and a second diffraction peak. The first diffraction peak is located at 18°–30°, and its full width at half maximum (FWHM) is 4°–12°. The second diffraction peak is located at 26°–27°, and its FWHM is 0.1°–0.4°. If the XRD spectrum of the anode active material satisfies the above conditions, it can be assured that the anode active material has a suitable carbon microcrystalline structure and compositional components.

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

[0048] Selectively, 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. <​In some embodiments, the compressed density of the negative electrode active material layer is 1.0 g / cm³. 3 ~1.7g / cm 3 This may also be the case. For example, the compressed density of the negative electrode active material layer may be 1.0 g / cm³. 3 , 1.1 g / cm³ 3 , 1.2 g / cm³ 3 1.3 g / cm³ 3 1.4 g / cm³ 3 , 1.5 / cm 3 1.6 g / cm³ 3 1.7 g / cm³ 3 It may be, or it may be within the range of any of the above values. Selectively, the compressible density of the negative electrode active material is 1.3 g / cm³. 3 ~1.7g / cm 3 This may also be the case. 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 stacked more densely, resulting in a higher compressive density and an increased content of negative electrode active material per unit volume, thereby enabling the electrochemical apparatus to 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 within any of the above ranges.

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

[0054] While not intended to be limited to any particular theory or interpretation, compared to conventional hard carbon materials, layered hard carbon active materials have more surface-to-surface contact between particles and are more densely laminated, resulting in a lower porosity of the anode active material layer. When the porosity of the anode active material layer is within the appropriate range described above, it not only reduces the internal resistance of the anode piece but also ensures the electrolyte wettability of the anode piece, thereby improving the dynamic performance of the electrochemical apparatus. In some embodiments, the sheet resistance of the negative electrode piece may be 2mΩ to 50mΩ. For example, the sheet resistance of the negative electrode piece may be 2mΩ, 5mΩ, 8mΩ, 10mΩ, 15mΩ, 20mΩ, 25mΩ, 30mΩ, 35mΩ, 40mΩ, 45mΩ, or 50mΩ, or it may be within the range of any of the above values.

[0055] When the sheet resistance of the negative electrode piece is within the appropriate range described above, it can be ensured that the electrochemical apparatus has low ohmic polarization, thereby reducing heat generation during the charging and discharging process of the electrochemical apparatus and improving its long-term cycle performance and safety.

[0056] The hard carbon particles of the present invention can be obtained by various methods. For example, hard carbon particles may be prepared by the following steps: using a template method, mixing a layered inorganic template, a pore-forming agent, and a resin to obtain a mixture; curing the mixture at a pressure of 0T to 5T and a temperature of 25°C to 200°C for 0.1h to 120h; and thermally decomposing the cured mixture at 700°C to 1300°C for 2h, crushing and sieving it, then treating it with an acid or alkali solution to remove the template and obtain hard carbon particles. The layered inorganic template includes, but is not limited to, montmorillonite, makanite, 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 resins include, but are not limited to, phenolic resins, furan resins, epoxy resins, polyester resins, bismaleimides, thermosetting polyimides, and cyanates. The mixing method may be powder mixing or solution mixing. If the mixing method is solution mixing, the solvent may be selected according to the resin and porosizing 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 solutions, the solvent may or may not be removed before curing.As a specific example, hard carbon particles may be prepared by the following steps: completely dissolving 100g of thermosetting phenolic resin in 200mL of ethanol, adding 100g of micron-order layered silicon dioxide, stirring in an open environment for 24 hours to evaporate the ethanol and obtain a viscous mixture; introducing the viscous mixture into a molding plate, setting the press pressure to 0.5T, the press temperature to 200°C, and the press time to 1h, and obtaining a precursor material when the pressing is complete; and placing the precursor material in a tubular oven, heating it to 1100°C in an argon gas atmosphere at a heating rate of 3°C / min and maintaining the temperature for 2 hours to thermally decompose the precursor to obtain thermally decomposed carbon, crushing and sieving the thermally decomposed carbon, adding the thermally decomposed carbon to 1L of 2mol / L sodium hydroxide solution, stirring for 24 hours, and then performing suction filtration, repeating suction filtration twice to ensure complete removal of the layered silicon dioxide template, and finally obtaining layered hard carbon material.

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

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

[0059] In some embodiments, other anode active materials besides hard carbon are not excluded from the anode active material layer. The specific types of other anode active materials are not particularly limited and can be selected as needed. As an example, other anode active materials include soft carbon, silicon, silicon-carbon composites, SiO, Li-Sn alloys, Li-Sn-O alloys, Sn, SnO, SnO2, and Li4Ti5O with a spinel structure. 12This includes, but is not limited to, at least one of the following: and Li-Al alloys.

[0060] In some embodiments, the negative electrode active material layer may optionally further contain 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, ethylene oxy-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic (esterified) styrene-butadiene rubber, epoxy resin, and nylon.

[0061] In some embodiments, the negative electrode active material layer selectively further comprises a conductive agent. The conductive agent may be selected from carbon-based materials, metallic materials, conductive polymers, and any combination of the above materials. For 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 metallic 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 selectively further contain other additives such as a thickening agent (e.g., sodium carboxymethylcellulose (CMC-Na)).

[0063] In the present invention, the negative electrode pieces may be prepared according to conventional methods in the art. For example, hard carbon and other selectable negative electrode active materials, a conductive agent, a binder and a thickener are dispersed in a solvent, the solvent may be N-methylpyrrolidone (NMP) or deionized water to form a uniform negative electrode slurry, the negative electrode slurry is coated onto a negative electrode current collector, and the negative electrode pieces are obtained by processes such as drying and cold pressing.

[0064] Note that the parameters of each negative electrode active material layer provided in this invention all refer to the parameter range of one side of the negative electrode active material layer. When negative electrode active material layers are provided on both sides of the negative electrode current collector, it is considered that the protection range of this invention is maintained if the parameters of the negative electrode active material layer on either side satisfy the requirements of this invention.

[0065] It should be noted that the negative electrode piece 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 piece of the present invention further includes a conductive undercoat layer (e.g., consisting 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 piece of the present invention further includes a protective layer covering the surface of the negative electrode active material layer.

[0066] In this invention, the diameter-to-thickness ratio of hard carbon particles is measured by methods and apparatus known in the art. For example, a negative electrode piece cut to a certain size is attached to a silicon wafer carrier with a conductive adhesive, one cross-section of the negative electrode piece is polished by argon ion polishing to obtain a test piece, the morphological structure and elemental distribution of the polished cross-section are analyzed with a scanning electron microscope (SEM), images of hard carbon particles are selected with image processing software, and 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 are measured to obtain the diameter-to-thickness ratio of each hard carbon particle.

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

[0068] In the present invention, the particle sizes Dv10, Dv50, and Dv99 of the negative electrode active material are well known in the art and can be measured using methods and apparatus known in the art. For example, they can be measured using a laser particle size analyzer (e.g., Malvern Mastersizer 2000E, UK) according to the GB / T19077-2016 particle size distribution laser diffraction method.

[0069] In the present invention, the specific surface area of ​​the negative electrode active material is 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 nitrogen adsorption / desorption using a specific surface area analyzer (e.g., Tristar II 3020 M).

[0070] In the present invention, the compressive 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 piece is cold-pressed, several circular pieces with an area of ​​S, one with a slurry coating the entire surface and one without a slurry coating, are punched out using a punching machine, and each is weighed to obtain the average masses W2 and W1. The thickness of each is measured to obtain the average thicknesses T2 and T1, and the compressive density of the negative electrode piece = (W2-W1) / (T2-T1) / S is satisfied.

[0071] In this invention, the porosity of the negative electrode active material layer is of articulate significance and can be measured by methods known in the art. For example, a negative electrode piece coated with the negative electrode active material is punched out into a circular test piece, and 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 according to the measurement standards for apparent density, true density, and porosity of iron ore in GB / T24586-2009.

[0072] In this invention, the sheet resistance of the negative electrode piece is well known in the art and can be measured by methods known in the art. For example, the negative electrode piece can be sectioned to obtain a test piece measuring 60 mm × 80 mm, and the sheet resistance of the negative electrode piece can be obtained by performing a resistance test on the sample using the BER1100 multifunction electrode piece resistance meter.

[0073] Furthermore, the various parameters for the negative electrode active material layer or negative electrode active material particles described above may be measured by sampling during the lithium-ion battery preparation process, or by sampling from the prepared lithium-ion battery.

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

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

[0076] In step S20, the negative electrode piece dried in step S10 is fired at a constant temperature and time (e.g., 400°C for 2 hours), and the negative electrode active material is sampled from any region selected from the fired negative electrode piece (it can be sampled by scraping it 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 obtain a sample that will be used to measure each of the negative electrode active material parameters of the present invention.

[0078] Electrochemical apparatus A second aspect of the present invention provides an electrochemical apparatus, which includes any apparatus that causes an electrochemical reaction to convert chemical energy and electrical energy into each other, specific examples of which include, but are not limited to, lithium-ion batteries or sodium-ion batteries.

[0079] In some embodiments, the electrochemical apparatus of the present invention includes a positive electrode piece, a negative electrode piece, a separator, and an electrolyte.

[0080] In some embodiments, the positive electrode piece, negative electrode piece, and separator can be fabricated into an electrode assembly by a winding process or a lamination process. The electrochemical apparatus of the present invention further includes an outer casing for sealing the electrode assembly and electrolyte. In some embodiments, the outer casing may be a rigid case such as a rigid plastic case, an aluminum case, or a steel case, or 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 piece] The negative electrode piece used in the electrochemical apparatus of the present invention is the negative electrode piece according to the first embodiment of the present invention.

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

[0083] The positive electrode piece 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, which contains positive electrode active material. For example, the positive electrode current collector has two opposing surfaces in its thickness direction, and the positive electrode active material layer is provided on one or both of the two opposing surfaces of the positive electrode current collector.

[0084] In some embodiments, the positive electrode active material layer comprises a positive electrode active material, the specific type of which is not specifically limited and can be selected as needed. For example, the positive electrode active material may comprise one or more of lithium transition metal oxides, lithium-containing phosphates with an olivine structure, and their modified compounds. In the electrochemical apparatus of the present invention, the modified compounds of each of the above positive electrode active materials may be obtained by doping modification, surface coating modification, or simultaneous doping and surface coating modification of the positive electrode active material.

[0085] As an example, the lithium transition metal oxide may include one or more of the following: 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 their modified compounds. As an example, the lithium-containing phosphate with an olivine structure may include one or more of the following: lithium iron phosphate, composite materials of lithium iron phosphate and carbon, lithium manganese phosphate, composite materials of lithium manganese phosphate and carbon, lithium iron manganese phosphate, composite materials of lithium iron manganese phosphate and carbon, and their modified compounds. These positive electrode active materials may be used individually or in combination of two or more.

[0086] In some embodiments, the positive electrode active material layer selectively further comprises a conductive agent. For 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 comprises a binder. For example, the conductive agent may be selected from carbon-based materials, metallic materials, conductive polymers, and any combination of the above materials. For 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 metallic 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 a metal foil sheet, the positive electrode current collector may be aluminum foil. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material may be one or more selected from the group consisting of aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. As an example, the polymer material base layer may be selected from polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, polyethylene, and the like.

[0089] The positive electrode pieces in the present invention can be prepared according to conventional methods in the art. For example, the positive electrode active material layer is usually obtained by coating a positive electrode slurry onto a positive electrode current collector, drying it, and cold pressing it. The positive electrode slurry is usually obtained by dispersing the positive electrode active material, a selectable conductive agent, a selectable binder, and any other components in a solvent and stirring them uniformly. The solvent may be, but is not limited to, N-methylpyrrolidone (NMP).

[0090] The positive electrode piece of the present invention does not exclude any additional functional layers other than the positive electrode active material layer. For example, in some embodiments, the positive electrode piece of the present invention further includes a conductive undercoat layer (e.g., consisting 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 piece of the present invention further includes a protective layer covering the surface of the positive electrode active material layer.

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

[0092] In some embodiments, the electrolyte comprises an organic solvent, a lithium salt, and a selectable additive, the types of which are not specifically limited and can be selected as needed.

[0093] In some embodiments, for example, the lithium salt includes, but is not limited to, 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). The above lithium salts may be used individually or in combination of two or more.

[0094] In some embodiments, the organic solvent may include, 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 organic solvents may be used individually or in combination of two or more. Optionally, two or more of the above organic solvents may be used in combination.

[0095] In some embodiments, the additives may include negative electrode film-forming additives and positive electrode film-forming additives, 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] As an example, the additives include, but are not limited to, at least one of the following: fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinylethylene carbonate (VEC), ethylene sulfate (DTD), propylene sulfate, ethylene sulfite (ES), 1,3-propanesultone (PS), 1,3-propensultone (PST), cyclic quaternary ammonium sulfonic acid salts, succinic anhydride, succinonitrile (SN), adiponitrile (AND), tris(trimethylsilyl) phosphate (TMSP), and tris(trimethylsilyl) borate (TMSB).

[0097] The electrolyte can be prepared according to the usual methods in this art. For example, an electrolyte can be obtained by homogeneously mixing an organic solvent, a lithium salt, and a selectable additive. The procedure for adding each substance is not particularly limited, but for example, the lithium salt and the selectable additive can be added to the organic solvent and homogeneously mixed to obtain the electrolyte. Alternatively, the lithium salt can be added to the organic solvent first, and then the selectable additive can be added to the organic solvent and homogeneously mixed to obtain the electrolyte.

[0098] [Separator] The separator is placed between the positive and negative electrode pieces and primarily serves to prevent short circuits between the positive and negative electrodes while allowing active ions to pass through. The present invention is not particularly limited in terms of the type of separator, and any well-known porous separator with good chemical 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, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride, but is not limited thereto. Selectively, the material of the separator may include polyethylene and / or polypropylene. The separator may be a single-layer film or a multilayer composite film. If the separator is a multilayer 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 examples of negative electrode pieces, the beneficial effects that can be achieved by the negative electrode piece according to the present invention were mainly explained using lithium-ion batteries as specific examples. However, since the negative electrode active material layer of the negative electrode piece according to the present invention has a high compressive density and appropriate porosity, the corresponding beneficial effects can be achieved when applied to other types of electrochemical devices as well.

[0101] power consumption equipment A third aspect of the present invention provides a power consumption device including an electrochemical apparatus according to 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 portable fax machine, a portable copier, a portable printer, a headphone stereo, a video camcorder, an LCD television, a handheld vacuum cleaner, a portable CD player, a MiniDisc player, a transceiver, an electronic organizer, a calculator, a memory card, a portable tape recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a moped, a bicycle, lighting fixtures, toys, game consoles, a clock, a power tool, a strobe light, a camera, a large household storage battery, and a lithium-ion capacitor.

[0103] [Examples] The following examples illustrate the contents disclosed herein in more detail, but these examples are for illustrative purposes only, and various modifications and changes within the scope of the disclosure herein will be obvious to those skilled in the art. Unless otherwise specified, all parts, percentages, and ratios described in the following examples are based on mass, and all reagents used in the examples are commercially available or synthesized according to conventional methods and can be used directly without further processing. The apparatus used in the examples is also commercially available.

[0104] Examples 1-21 Preparation of negative electrode piece The negative electrode active material, styrene-butadiene rubber (a binder), and sodium carboxymethylcellulose (CMC-Na) were dissolved in deionized water in 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 two sides of a negative electrode current collector, forming a 6 μm thick copper foil for the negative electrode current collector, with a single-sided coating thickness of 50 μm. After drying at 85°C, cold pressing, sectioning, and cutting, the material was dried under vacuum conditions at 120°C for 12 hours to obtain a negative electrode piece. Based on the mass of the negative electrode active material, the mass percentage w1% of layered hard carbon, the mass percentage w2% of MCMB, a, b, c, d, Dv10, Dv50, Dv99 of the negative electrode active material, the specific surface area of ​​the negative electrode active material, the compressive 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 piece are as shown in Tables 1, 2, and 3, respectively. The negative electrode active material in Examples 1 to 8 was 95% hard carbon and 5% MCMB. The negative electrode active material in Examples 9 to 16 was the same as in Example 1 and had the same diameter-to-thickness ratio distribution as the hard carbon in Example 1. The negative electrode active material in Examples 17 to 21 also had the same diameter-to-thickness ratio distribution as the hard carbon in Example 1.

[0105] Preparation of positive electrode pieces Lithium cobalt oxide (the positive electrode active material), carbon black (the conductive agent), and PVDF (the binder) were mixed in a mass ratio of 97:1.4:1.6. An appropriate amount of NMP (a solvent) was added, and the mixture was uniformly stirred to obtain a positive electrode slurry with a solid content of 72 wt%. The positive electrode slurry was uniformly coated onto two sides of an aluminum foil, which served as the positive electrode current collector, with a single-sided coating thickness of 80 μm. After drying at 85°C, cold pressing, sectioning, and cutting, the material was dried under vacuum conditions at 85°C for 4 hours to obtain a positive electrode piece.

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

[0107] Preparation of separators A 7μm thick polyethylene (PE) was used as the separator.

[0108] Preparation of lithium-ion batteries The positive electrode piece, separator, and negative electrode piece were stacked and wound in this order to obtain an electrode assembly. The electrode assembly was placed in an outer casing, moisture was removed at 80°C, the above-mentioned electrolyte was added, and after processes such as sealing, static formation, degassing, and shaping were carried out to obtain a lithium-ion battery.

[0109] Comparative Examples 1-4 The type of negative electrode active material was adjusted, and the negative electrode pieces, positive electrode pieces, electrolyte, separator, and lithium-ion batteries of Comparative Examples 1 to 5 were prepared according to the preparation process for the negative electrode pieces, positive electrode pieces, electrolyte, separator, and lithium-ion batteries in Examples 1 to 21. The negative electrode active material of Comparative Example 1 was MCMB, the negative electrode active material of Comparative Example 2 was flake graphite, the negative electrode active material of Comparative Example 3 was 95% conventional hard carbon and 5% MCMB, and the negative electrode active material of Comparative Example 4 was 95% microspherical hard carbon particles and 5% MCMB.

[0110] Comparative Examples 5-6 Following the preparation processes for the negative electrode pieces, positive electrode pieces, electrolyte, separator, and lithium-ion batteries in Examples 1 to 21, the values ​​of a, b, c, and d of the hard carbon material were adjusted as shown in Table 1, and the negative electrode pieces, positive electrode pieces, electrolyte, separator, and lithium-ion batteries of Comparative Examples 5 and 6 were prepared. Measuring part

[0111] Measurement of the negative electrode (1) Measurement of the diameter-to-thickness ratio of hard carbon particles Measurement equipment: Scanning electron microscope JSM-6360LV and its associated energy-dispersive X-ray spectrometer. A fully discharged lithium-ion battery was removed, disassembled, and the negative electrode piece was extracted. It was immersed in DMC for 20 minutes, then sequentially washed with DMC and acetone, and finally dried in an oven at 80°C for 12 hours. The dried negative electrode piece was cut into 0.5 cm wide pieces to form the negative electrode sample, and the sample was attached to a silicon wafer carrier with a 1 cm wide conductive adhesive. The sample was polished with argon ions (operating parameters: acceleration voltage 8 kV, polishing time 4 hours), and the cross-section of one end of the negative electrode piece was polished to obtain a test specimen. The morphological structure and elemental distribution of the polished cross-section were analyzed using a scanning electron microscope, and images of hard carbon particles were selected using image processing software (Multiphase). The diameter-to-thickness ratio of each hard carbon particle was obtained by measuring the major axis value and the maximum thickness perpendicular to the major axis direction of each hard carbon particle in the cross-section. For each example or comparative example of the negative electrode piece, 10 SEM images were processed (see Figure 2), and statistical values ​​a, b, c, and d were obtained.

[0112] (2) XRD measurement of negative electrode active material Measurement device: Bruker D8 ADVANCE X-ray powder diffractometer A fully discharged lithium-ion battery was taken, disassembled, and the negative electrode piece was removed. After washing and drying, the negative electrode active material layer was processed with a blade to obtain negative electrode active material layer powder. The negative electrode active material layer powder was placed in a tubular oven and kept at 400°C in an argon gas atmosphere for 4 hours to remove the binder adhering to the surface of the negative electrode active material layer powder, thereby obtaining the negative electrode active material powder. The negative electrode active material powder was measured using an X-ray powder diffractometer, and an XRD measurement graph of the negative electrode active material was obtained. The radiation source for the XRD measurement was a Cu Kα target, and the measurement parameters were set to a tube voltage of 40kV, a tube current of 40mA, a scan step width of 0.00836°, a scan time of 0.3s per scan step width, and a 2θ range of 5° to 80°.

[0113] (3) Measurement of particle size of negative electrode active material particles Measurement device: Bruker D8 Advance The negative electrode active material powder was obtained according to the steps for XRD measurement of the negative electrode active material. The negative electrode active material powder was dispersed in ethanol and sonicated for 30 minutes to obtain an ethanol dispersion of the negative electrode active material. The ethanol dispersion of the negative electrode active material was placed in a Malvern particle size analyzer and the Dv10, Dv50, and Dv99 values ​​of the negative electrode active material particles were measured.

[0114] (4) Measurement of the specific surface area of ​​the negative electrode active material Measuring device: Specific surface area analyzer TristarII3020M The negative electrode active material powder was obtained according to the steps for measuring the XRD of the negative electrode active material. The negative electrode active material powder was dried in a vacuum oven, and the specific surface area of ​​the negative electrode active material was measured using a specific surface area analyzer.

[0115] (5) Measurement of the compression density of the negative electrode active material layer A fully discharged lithium-ion battery was taken, disassembled, and the negative electrode piece was removed. After cleaning and drying, the area S of the negative electrode active material layer on one side, the mass W1 of the negative electrode piece, and the thickness T1 of the negative electrode piece were measured. After washing the negative electrode active material layer with a solvent and drying it, the mass W2 and thickness T2 of the negative electrode current collector were measured. The compressed density of the negative electrode active material layer was calculated using the following equations 1 to 3.

[0116] [Formula 1] JPEG0007897964000001.jpg9153 [Formula 2] JPEG0007897964000002.jpg8153 [Formula 3] JPEG0007897964000003.jpg8153 W0 represents the mass of the negative electrode active material layer on one side, and T0 represents the thickness of the negative electrode active material layer on one side.

[0117] (6) Measurement of the porosity of the negative electrode active material layer Measuring device: True density measuring device (AccuPycII1340) A negative electrode piece coated with the negative electrode active material was punched out into a circular test piece, and in each test piece, the volume of the negative electrode active material layer was approximately 0.35 cm³. 3 The porosity of the negative electrode active material layer was measured according to the GB / T24586-2009 criteria for the apparent density, true density, and porosity of iron ore.

[0118] (7) Measurement of the volume per gram of negative electrode active material Lithium sheets, consisting of negative and positive electrode pieces, were assembled into a coin cell battery. The battery was discharged to 5.0mV at 0.05C, then discharged to 5.0mV at 50μA, then discharged to 5.0mV at 10μA, and finally charged to 2.0V at 0.1C. The initial charge capacity of the coin cell battery at this time was recorded. The 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 Measurement device: ASAP2460 - Physical adsorption analyzer. The negative electrode active material powder was obtained according to the steps for XRD measurement of the negative electrode active material. After drying and degassing, the sample was placed in liquid nitrogen, and the amount of adsorption to nitrogen gas was measured at different measurement pressures while adjusting the measurement pressure, and adsorption and desorption isotherms were plotted. The shape of the pores was determined by the shape of the hysteresis loop, and the pore distribution and pore volume were calculated using different pore models. The pore size distribution curves for mesopores and macropores were fitted using the BJH model, and the pore size distribution curve for micropores was fitted using the DFT model.

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

[0121] (2) Measurement of the rate performance of lithium-ion batteries For each example or comparative example, five lithium-ion batteries were taken and rate performance measurements were performed. The specific measurement steps were as follows: the lithium-ion batteries were left in a 25°C environment for 1 hour, then the batteries were charged with a constant current at a charge rate of I=1C (CC stage) until they reached 4.48V, then charged with a constant voltage (CV stage), and charging was stopped when the charging current was lower than 0.05C and left for 5 minutes. The batteries were then discharged at a rate of 0.2C until they reached 3V, and left for 5 minutes. This constituted one target charge-discharge cycle. The charge capacity (average value) at each charging stage was statistically calculated, and the percentage of the capacity at the CC stage was calculated. I was sequentially adjusted to 0.2C, 0.5C, 1C, 2C, and 3C, and 2 to 6 target charge-discharge cycles were performed following the process of one target charge-discharge cycle. The percentage of the capacity at the CC stage at a charge rate of 3C was calculated using Equation 4. [Formula 4] JPEG0007897964000004.jpg5168

[0122] (3) Measurement of the cycle performance of lithium-ion batteries For each example or comparative example, five lithium-ion batteries were taken and their cycle performance was measured. The specific evaluation steps are as follows: At 25°C, a lithium-ion battery was charged to 4.48V at a rate of 1C, then continuously charged at a constant voltage until the charge cutoff current, and discharged to 3V at a rate of 1C. This constituted one charge-discharge cycle. The initial charge capacity, initial discharge capacity, and thickness of the fully charged lithium-ion battery after the first cycle were recorded. Subsequently, the charge-discharge cycle was continued, and the discharge capacity and thickness of the fully charged lithium-ion battery after 400 cycles were recorded. [Formula 5] JPEG0007897964000005.jpg7168 [Formula 6] JPEG0007897964000006.jpg6168 [Formula 7] JPEG0007897964000007.jpg6168 [Formula 8] JPEG0007897964000008.jpg18168

[0123] (4) Measurement of the self-discharge rate of lithium-ion batteries For each example or comparative example, five lithium-ion batteries were taken and their self-discharge rates were measured. The specific evaluation steps are as follows. A lithium-ion battery with 80% state of charge (SOC) was taken, its initial open-circuit voltage was measured and recorded as V1, and after being left 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] JPEG0007897964000009.jpg7150

[0124] (5) Measurement of the DC resistance (DCR) of a lithium-ion battery In a 25°C environment, the lithium-ion battery was charged to a voltage of 4.4V with a current of 0.5C, and then charged at a constant voltage until the current was reduced to 0.05C. It was then discharged at a current of 0.1C for 2 hours, allowed to stand for 1 hour, discharged again at a current of 0.1C (I1) for 10 seconds, the discharge voltage V3 for the last 1 second was recorded, and then discharged again at a current of 1C (I2) for 1 second, the discharge voltage V4 for the last 1 second was recorded, satisfying the DCR = (V3-V4) / (I2-I1). Details of the settings and measurement results for the examples and comparative examples are shown in Tables 1 to 3.

[0125] [Table 1]

[0126] [Table 2]

[0127] [Table 3]

[0128] Table 1 shows that the negative electrode active materials of Comparative Example 1 and Comparative Example 5 are both spherical or nearly spherical carbon material particles, and their diameter-to-thickness ratio distribution is relatively concentrated. It can be seen that in similar cold pressing processes, the compressive density of the hard carbon-based negative electrode active materials is lower than that of the graphite-based negative electrode active materials. When layered hard carbon is used in the present invention, the hard carbon particles are more densely stacked after cold pressing, so the negative electrode active material layer has a higher compressive density and a lower porosity. Since lithium-ion batteries have a relatively high self-discharge rate, layered hard carbon with an appropriate diameter-to-thickness ratio distribution has a flatter surface of the active material layer after cold pressing, making it less likely to damage the separator, and the degree of internal physical micro-short circuits is significantly reduced. The layered hard carbon particles come into surface contact with each other, thus Li + Effectively shortens the transmission path, Li + By improving the solid-phase transmission rate within the active material, the internal resistance of lithium-ion batteries can be reduced, improving their dynamic performance. This effectively enhances the rapid charging capability of lithium-ion batteries, giving them relatively superior cycle performance and rate performance, and further increasing their energy density.

[0129] Examples 1 and 9-16 show that a relatively large particle size of the negative electrode active material improves the initial irreversible capacity of the lithium-ion battery and thus affects the cycle performance of the lithium-ion battery.

[0130] Examples 1 and 17-21 show that when the mass ratio of graphite particles and hard carbon particles is appropriate, the negative electrode piece can have both a relatively high hard carbon content and a relatively high compressive density, and the corresponding energy density of the lithium-ion battery is also higher. Hard carbon not only has a low rate of volume expansion during the processes of lithium release and lithium storage, but it can also limit the volume expansion during the charge and discharge processes of graphite. In lithium-ion batteries corresponding to examples with a high hard carbon content, the smaller the rate of thickness expansion after 400 cycles, the better the capacity retention rate after 400 cycles.

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

Claims

1. It is a negative electrode piece, It includes a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector, The aforementioned negative electrode active material layer contains a negative electrode active material. The negative electrode active material comprises 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 a diameter-to-thickness ratio of 3 to 7 is a%, where 30 ≤ a ≤ 70. 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%, where 20 ≤ b ≤ 60. The diameter-to-thickness ratio of the hard carbon particle represents the ratio of the major axis to the thickness of the hard carbon particle, where the major axis is the longest diameter in the projection plane of the cross-sectional view of the hard carbon particle, and the thickness is the maximum thickness perpendicular to the major axis direction within the cross-section of the hard carbon particle. The compressive density of the negative electrode active material layer is 1.0 g / cm³ to 1.7 g / cm³. Negative pole piece.

2. The negative electrode piece according to claim 1, wherein the hard carbon particles satisfy a + b ≥ 90.

3. The negative electrode piece according to claim 1, wherein, based on the number of hard carbon particles, the proportion of hard carbon particles with a diameter-to-thickness ratio of 1 to 2 is c%, the proportion of hard carbon particles with a diameter-to-thickness ratio greater than 7 is d%, and 0.1 ≤ c ≤ 10 and 0.1 ≤ d ≤ 1.

4. The anode piece according to claim 1, wherein the mass of the hard carbon particles is 85% to 99% of the mass of the anode active material.

5. The negative electrode piece according to claim 1, wherein the particle size of the negative electrode active material satisfies 1 μm ≤ Dv10 ≤ 5 μm, 4 μm ≤ Dv50 ≤ 18 μm, and Dv99 ≤ 43 μm.

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 located between 18° and 30°, and the full width at half maximum of the first diffraction peak is between 4° and 12°. The negative electrode piece according to claim 1, wherein the second diffraction peak is located at 26° to 27°, and the full width at half maximum of the second diffraction peak is 0.1° to 0.4°.

7. The negative electrode piece according to claim 1, wherein the porosity of the negative electrode active material layer is 10% to 40%.

8. The negative electrode piece according to Claim 1, wherein the proportion of hard carbon particles having a diameter-to-thickness ratio of 3 to 7, based on the number of hard carbon particles, is a%, and 30 ≤ a ≤ 50.

9. The negative electrode piece according to Claim 1, wherein the proportion of hard carbon particles having a diameter-to-thickness ratio of 2 to 3 is b%, and 20 ≤ b ≤ 50, based on the number of hard carbon particles.

10. The negative electrode piece according to claim 1, wherein the hard carbon particles contain pores, and the pore volume measured by the nitrogen gas-carbon dioxide adsorption-desorption method is 1 cc / g to 5 cc / g.

11. An electrochemical apparatus comprising a negative electrode piece according to any one of claims 1 to 10.

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