Negative pole piece and electrochemical device

Abstract

The invention discloses a negative pole piece and an electrochemical device. The sheet layer hard carbon active material in the negative electrode plate replaces point-to-point contact of a hard carbon active material with a conventional morphology through surface-to-surface contact, so that the internal resistance of the negative electrode active material layer is effectively reduced, the compaction density of the negative electrode active material layer can be effectively improved, the porosity can be reduced, and the energy density of the lithium ion battery is further improved; the diffusion of lithium ions in the lamellar hard carbon negative electrode active material has more excellent dynamics, so that the rapid charging capacity of the lithium ion battery can be improved; meanwhile, the lithium ion battery is endowed with better cycle performance by the low-expansion hard carbon active material. In addition, the preparation method is simple, easy to operate and control, low in cost and suitable for industrial production.

Description

This application is a divisional application of the application with the application number 202280053779.6, the application date of June 22, 2022, and the invention title of "Negative Electrode Plate and Electrochemical Device". Technical Field

[0001] This application belongs to the technical field of secondary batteries, and specifically relates to a negative electrode plate and an electrochemical device. Background Art

[0002] Electrochemical devices such as lithium-ion batteries have outstanding characteristics such as high energy density, long cycle life, no pollution, and no memory effect. As a clean energy source, the application of electrochemical device batteries has gradually spread from electronic products to large-scale device fields such as electric vehicles to meet the sustainable development strategy of the environment and energy. Therefore, higher requirements are also put forward for the energy density of electrochemical device batteries.

[0003] Currently, the commercialized negative electrode material of lithium-ion batteries is still mainly graphite. Graphite has advantages such as high electrical conductivity and high stability. However, graphite not only has a low theoretical specific capacity and poor kinetic conditions, but also has a high volume expansion rate under fast charging and discharging conditions. Therefore, using graphite as the negative electrode material is not only difficult to further improve the energy density and cycle life of electrochemical device batteries, but also poses a safety hazard to electrochemical device batteries. Summary of the Invention

[0004] In view of the above problems existing in the prior art, this application provides a negative electrode plate with a high tap density, which can enable an electrochemical device to have a high energy density, a long cycle life, excellent rate performance, and fast charging performance.

[0005] In the first aspect of this application, a negative electrode plate is provided, which 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 negative electrode active material layer includes a negative electrode active material, and the negative electrode active material includes hard carbon particles and graphite particles. The hard carbon particles have a lamellar 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 hard carbon particles include micropores, and the micropore volume measured by the nitrogen-carbon dioxide adsorption-desorption method is greater than or equal to 0.25 cc / g. The negative electrode active material layer with hard carbon particles having a specific aspect ratio can obtain a higher tap density after cold pressing, which can effectively shorten the transmission path of active ions, improve the solid-phase transfer speed of active ions inside the negative electrode active material, and thus can reduce the internal impedance of the electrochemical device and improve the kinetic performance, cycle performance, rate performance, and fast charging performance of the electrochemical device. At the same time, the hard carbon active material contains a larger micropore volume, indicating that it has a higher lithium storage capacity, which can increase the capacity of the negative electrode active material and improve the energy density of the electrochemical device.

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

[0007] In any embodiment of the present application, 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 tap density and a suitable porosity, improving the energy density of the electrochemical device and enhancing the cycling performance, rate performance, and fast charging performance of the electrochemical device.

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

[0009] In any embodiment of the present application, the hard carbon particles satisfy: a + b ≥ 90. When the proportion of hard carbon particles with an aspect ratio of 2 to 3 and the proportion of hard carbon particles with an aspect ratio of 3 to 7 in the hard carbon are within the above range, the electrochemical device can have a high energy density, a 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 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%. The hard carbon particles satisfy: a + b ≥ 95. At this time, the high energy density, long cycle life, and excellent fast charging performance of the electrochemical device can be further improved.

[0011] In any embodiment of the present application, 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%, where 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 the proportion of hard carbon particles with an aspect ratio greater than 7 are within the above appropriate range, the negative electrode active material layer can be allowed to have a high tap density and a high porosity, and the irreversible capacity in the first cycle can be reduced, enhancing the cycling performance, rate performance, and fast charging performance of the electrochemical device.

[0012] In any embodiment of the present application, 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%. The hard carbon particles satisfy: a + b + c + d = 100. When the aspect ratio distribution of the hard carbon is within the above range, it can more fully ensure that the hard carbon has a high tap density and a suitable specific surface area, so that the negative electrode active material layer of the negative electrode sheet has a high tap density and a suitable porosity, enabling the electrochemical device to have a higher energy density. It can also shorten the transmission path of lithium ions in the negative electrode sheet, improve the fast charging performance of the electrochemical device, reduce the internal resistance of the electrochemical device, reduce the irreversible capacity of the first cycle of the electrochemical device, and extend the cycle life of the electrochemical device.

[0013] In any embodiment of the present application, the mass of the hard carbon particles accounts for 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, it can further improve the tap density of the negative electrode active material layer, enabling the electrochemical device to have a high energy density.

[0014] In any embodiment of the present application, 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 4° to 12°; the second diffraction peak is located between 26° and 27°, and the full width at half maximum of the second diffraction peak is 0.1° to 0.4°.

[0015] In any embodiment of the present application, the particle size of the negative electrode active material satisfies: 1μm ≤ Dv10 ≤ 5μm, 4μm ≤ Dv50 ≤ 18μm, Dv99 ≤ 43μm; when the particle size of the negative electrode active material is within the above suitable range, the combination of active materials with different particle sizes makes the active material layer have a more dense packing after cold pressing to obtain a higher tap density, which can further improve the energy density and cycle performance of the electrochemical device.

[0016] In any embodiment of the present application, the specific surface area of the negative electrode active material is 1m 2 / g to 30m 2 / g. When the specific surface area of the negative electrode active material is within the above suitable range, it can enable the negative electrode active material particles to have a suitable specific surface area, make the area of the SEI film formed on the surface of the negative electrode sheet appropriate, reduce the consumption of irreversible lithium during the first charging process, and enable the electrochemical device to have both good kinetic performance and high energy density.

[0017] In any embodiment of the present application, the tap density of the negative electrode active material layer is 1.0g / cm 3 to 1.7g / cm 3In the negative electrode active material layer of the present application, hard carbon particles with an appropriate aspect ratio are included. Therefore, the stacking of the negative electrode active material particles in the negative electrode active material layer is more dense, capable of having a high tap density, thereby increasing the content of the negative electrode active material per unit volume, and thus enhancing the energy density of the electrochemical device.

[0018] In any embodiment of the present application, the porosity of the negative electrode active material layer is 10% to 40%; there are more face-to-face contacts between the particles of the sheet-like hard carbon active material, and the more dense stacking results in a lower porosity of the negative electrode active material layer. When the porosity of the negative electrode active material layer is within the above suitable range, it can not only reduce the internal resistance of the negative electrode sheet, but also ensure the electrolyte infiltration performance of the negative electrode sheet, thereby enhancing the kinetic performance of the electrochemical device.

[0019] A second aspect of the present application provides an electrochemical device, including the negative electrode sheet of the first aspect.

[0020] In the present application, hard carbon particles and graphite particles are added to the negative electrode active material. The hard carbon particles have a sheet-like structure, and the proportion of hard carbon particles with an aspect ratio of 3 to 7 is defined, which can effectively shorten the transmission path of active ions, improve the solid-phase transfer speed of active ions inside the negative electrode active material, reduce the internal impedance of the electrochemical device, and the negative electrode active material layer can achieve a higher tap density after cold pressing, enhancing the kinetic performance, cycle performance, rate performance, and fast charging performance of the electrochemical device. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] To more clearly illustrate the technical solutions of the embodiments of the present application, the following briefly describes the drawings required for use in the embodiments of the present application; obviously, the drawings described below only relate to some embodiments of the present application.

[0022] Figure 1 is a schematic cross-sectional view of a negative electrode sheet according to an embodiment of the present application.

[0023] Figure 2 is a scanning electron microscope image (SEM image) of a cross-section of negative electrode active material particles according to an embodiment of the present application.

[0024] Figure 3 is a pore size distribution diagram of hard carbon particles in the negative electrode active material of Example 1 of the present application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] In order to make the invention purpose, technical solutions, and beneficial technical effects of the present application clearer, the following further details the present application with reference to embodiments. It should be understood that the embodiments described in this specification are only for explaining the present application and not for limiting the present application.

[0026] For simplicity, only some numerical ranges are explicitly disclosed in this document. However, any lower limit can be combined with any upper limit to form a range not explicitly recited; and any lower limit can be combined with other lower limits to form a range not explicitly recited, and similarly any upper limit can be combined with any other upper limit to form a range not explicitly recited. In addition, although not explicitly recited, each point or single value between the endpoints of the range is included in the range. Thus, each point or single value can be used as its own lower or upper limit and combined with any other point or single value or with other lower or upper limits to form a range not explicitly recited.

[0027] In the description of this application, it should be noted that unless otherwise specified, "above" and "below" include the recited number, and "one or more" means two or more in the phrase "one or more".

[0028] Unless otherwise specified, the terms used in this application have the well-known meanings commonly understood by those skilled in the art. Unless otherwise specified, the numerical values of the various parameters mentioned in this application can be measured by various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the embodiments of this application).

[0029] The list of items connected by the terms "at least one of", "at least one of", "at least one kind of" or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or 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 component or multiple components. Item B may include a single component or multiple components. Item C may include a single component or multiple components.

[0030] The above summary of the invention of this application does not intend to describe every disclosed embodiment or every implementation of this application. The following description more specifically exemplifies exemplary embodiments. Throughout the application, guidance is provided through a series of embodiments, which can be used in various combinations. In each instance, the listings are only representative groups and should not be construed as exhaustive.

[0031] Hard carbon has a relatively high hardness. After cold pressing, not only is it difficult to improve the compaction density, but it may also damage the negative electrode current collector, thereby increasing the risk of shedding of the negative electrode active material layer, ultimately leading to a sharp increase in the internal resistance of the electrochemical device battery and a significant decrease in the capacity retention rate. During the transportation or use of the electrochemical device battery, the uncompacted hard carbon particles are also extremely likely to damage the separator, resulting in an increase in the voltage drop of the battery per unit time of the electrochemical device. Therefore, the direct application of hard carbon materials to electrochemical devices not only has very limited improvement in the energy density of the electrochemical device battery, but also has a negative impact on the cycle performance, capacity retention rate, and safety performance of the electrochemical device. Negative electrode sheet

[0032] This application 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 of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, and the negative electrode active material includes hard carbon particles and graphite particles. The hard carbon particles have a lamellar 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 hard carbon particles include micropores, and the micropore volume measured by the nitrogen-carbon dioxide adsorption-desorption method is greater than or equal to 0.25 cc / g.

[0033] The aspect ratio of the hard carbon particles can represent the ratio of the major axis to the thickness of the hard carbon particles. Among them, the major axis is the longest diameter of the cross-sectional view of the hard carbon particles on the projection plane, and the thickness is the maximum thickness in the cross-section of the hard carbon particles perpendicular to the major axis direction.

[0034] Not intending to be limited by any theory or explanation, the inventors unexpectedly found that, compared with microspherical hard carbon particles and irregularly shaped hard carbon particles, the active material layer of hard carbon particles with a specific aspect ratio can achieve a higher compaction density after cold pressing. A large aspect ratio of the hard carbon particles means that the same volume of hard carbon particles can have a thinner lamellar structure. Thus, as Figure 1 shown, the hard carbon particles can be stacked, and after cold pressing, they can have a high compaction density. The lamellar hard carbon particles are in face-to-face contact, which can effectively shorten the transmission path of Li + , improve the solid-phase transfer speed of Li + inside the active material. Thereby, the internal impedance of the lithium-ion battery can be reduced, its kinetic performance can be improved, and the cycle performance, rate performance, and fast charging performance of the electrochemical device can be improved. At the same time, the hard carbon active material contains a larger micropore volume, indicating that it has a higher lithium storage capacity, which can increase the capacity of the negative electrode active material and the energy density of the electrochemical device.

[0035] In some embodiments, a can be 70, 65, 60, 55, 50, 45, 40, 35, 30 or within the range composed of any of the above values.

[0036] 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 cycling performance, rate performance, and fast charging performance.

[0037] In some embodiments, the micropore volume of the hard carbon particles measured by carbon dioxide adsorption-desorption method is 1 cc / g to 5 cc / g. At this time, the hard carbon active material contains a larger micropore volume, which to a certain extent means that it has a higher lithium storage capacity, improves the capacity of the negative electrode active material, and increases the energy density of the electrochemical device.

[0038] In some embodiments, the interior of the hard carbon particles includes micropores with a pore diameter < 2 nm.

[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 2 to 3 is b%, and 20 ≤ b ≤ 60. For example, b can be 20, 25, 30, 35, 40, 45, 50, 55, 60, or within the range composed of any of the above values.

[0040] Without intending to be limited by any theory or explanation, when the proportion of hard carbon particles with a diameter-to-thickness ratio of 2 to 3 is within the above range, it can ensure a high tap density of the hard carbon particles after cold pressing while ensuring an appropriate specific surface area of the hard carbon particles. Therefore, when such a hard carbon material is applied to the negative electrode sheet, the negative electrode active material layer of the negative electrode sheet can have a high tap density and an appropriate porosity, improving the cycling performance, rate performance, and fast charging performance of the electrochemical device.

[0041] 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 cycling performance, rate performance, and fast charging performance.

[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%, and the proportion of hard carbon particles with a diameter-to-thickness ratio of 2 to 3 is b%. The hard carbon particles satisfy: a + b ≥ 90. When the proportions 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 are within the above ranges, it can improve the tap density of the negative electrode active material layer while making the area of the SEI film formed on the surface of the negative electrode sheet appropriate. Thus, when the negative electrode sheet of the present application is applied to an electrochemical device, it can allow the electrochemical device to have a high energy density, a long cycle life, and excellent fast charging performance.

[0043] 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%. The hard carbon particles satisfy: a + b ≥ 95, which can further improve the cycling performance, rate performance, and fast charging performance of the electrochemical device.

[0044] 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%. 0.1 ≤ c ≤ 10, 0.1 ≤ d ≤ 1.

[0045] The aspect ratio of conventional morphology hard carbon particles is mainly distributed in the range of 1 to 2. The hard carbon material has strong rigidity, and the contact between particles is mostly point-to-point, resulting in difficulty in dense stacking during the cold pressing process, leaving a high porosity and reducing the energy density of the electrochemical device. When 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 accordingly, resulting in an increase in the irreversible capacity in the first cycle. Therefore, the proportion of particles with a low aspect ratio and an overly large aspect ratio in the hard carbon active material should be reduced as much as possible. Without being bound by any theory or explanation, 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 suitable range, it can allow the negative electrode active material layer to have a high tap density and a high porosity, and reduce the irreversible capacity in the first cycle.

[0046] 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%. The hard carbon particles satisfy: 0.1 ≤ c ≤ 10, 0.1 ≤ d ≤ 1, a + b + c + d = 100.

[0047] When the aspect ratio distribution of the hard carbon is within the above range, it can more fully ensure that the hard carbon has a high tap density and a suitable specific surface area, so that the negative electrode active material layer of the negative electrode sheet has a high tap density and a suitable porosity. The negative electrode sheet with a high tap density can not only allow the electrochemical device to have a higher energy density, but also shorten the transmission path of lithium ions in the negative electrode sheet, thereby improving the fast charging performance of the electrochemical device and reducing the internal resistance of the electrochemical device. The appropriate porosity of the negative electrode sheet can also make the SEI film formed on the surface of the negative electrode sheet have a suitable area, thereby reducing the irreversible capacity in the first cycle of the electrochemical device. Therefore, when the negative electrode sheet is applied to the electrochemical device, it can significantly improve the energy density of the electrochemical device, extend the cycle life of the electrochemical device, and enable the electrochemical device to have excellent fast charging performance.

[0048] In some embodiments, the mass of the hard carbon particles accounts for 85% to 99% of the mass of the negative electrode active material, and the mass of the graphite particles accounts for 1% to 15% of the mass of the negative electrode active material. The negative electrode active material includes 95% hard carbon particles and 5% graphite particles. The inventors have found that blending a small amount of graphite in the negative electrode active material and the amount of the blended graphite being within the above suitable range can further improve the tap density of the negative electrode active material layer, thereby allowing the electrochemical device to have a higher energy density. Specifically, graphite has a stacked structure of graphene sheets. After being mixed with hard carbon, during the cold pressing process, the hard carbon can slip by means of the graphite sheets. Thus, the sheet-like hard carbon particles can be stacked together with a more regular orientation, thereby further increasing the tap density of the negative electrode active material layer and increasing the energy density of the electrochemical device.

[0049] In some embodiments, the X-ray diffraction (XRD) pattern of the negative electrode active material may include a first diffraction peak and a second diffraction peak. The first diffraction peak is between 18° and 30°, and the full width at half maximum of the first diffraction peak is between 4° and 12°. The second diffraction peak is between 26° and 27°, and the full width at half maximum of the second diffraction peak is between 0.1° and 0.4°. When the XRD pattern of the negative electrode active material meets the above conditions, it can ensure that the negative electrode active material has a suitable carbon microcrystalline structure and composition.

[0050] In some embodiments, the graphite particles include natural graphite particles, artificial graphite particles or a combination thereof.

[0051] Optionally, the artificial graphite particles may include mesocarbon microbead (MCMB)-type artificial graphite particles, petroleum coke-type artificial graphite particles or a combination thereof.

[0052] In some embodiments, the particle size of the negative electrode active material may satisfy: 1μm ≤ Dv10 ≤ 5μm, 4μm ≤ Dv50 ≤ 18μm, Dv99 ≤ 43μm. For example, Dv10 can be 1μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm or within the range composed of any of the above values; Dv50 can be 4μm, 6μm, 8μm, 10μm, 12μm, 14μm, 16μm, 18μm or within the range composed of any of the above values; Dv99 can be ≤ 43μm, ≤ 40μm, ≤ 38μm, ≤ 35μm, ≤ 32μm or ≤ 30μm. When the particle size of the negative electrode active material is within the above suitable range, the combination of active materials with different particle sizes makes the active material layer have a denser packing after cold pressing to obtain a higher tap density, which can further improve the energy density and cycle performance of the electrochemical device.

[0053] In some embodiments, the specific surface area of the negative electrode active material may be 1m 2 / g to 30 m 2 / g. For example, the specific surface area of the negative electrode active material can be 1 m 2 / g, 2 m 2 / 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 within the range composed of any of the above values. When the specific surface area of the negative electrode active material is within the above suitable range, the negative electrode active material particles can have a suitable specific surface area, and the area of the SEI film formed on the surface of the negative electrode sheet is appropriate, reducing the consumption of irreversible lithium during the first charging process. Thus, the electrochemical device can have both good kinetic performance and high energy density.

[0054] In some embodiments, the tap density of the negative electrode active material layer can be 1.0 g / cm 3 to 1.7 g / cm 3 . For example, the tap density of the negative electrode active material layer can 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 within the range composed of any of the above values. Optionally, the tap density of the negative electrode active material can be 1.3 g / cm 3 to 1.7 g / cm 3 . The negative electrode active material layer in this application includes hard carbon particles with an appropriate aspect ratio. Therefore, the negative electrode active material particles in the negative electrode active material layer are stacked more densely and can have a high tap density, thereby increasing the content of the negative electrode active material per unit volume and improving the energy density of the electrochemical device.

[0055] In some embodiments, the porosity of the negative electrode active material layer can be 10% to 40%. For example, the porosity of the negative electrode active material layer can be 10%, 15%, 20%, 25%, 30%, 35%, 40% or within the range composed of any of the above values.

[0056] Optionally, the porosity of the negative electrode active material may be 15% to 25%.

[0057] Without being bound by any theory or explanation, compared with conventional morphology hard carbon materials, there are more face-to-face contacts between particles of the lamellar hard carbon active material, and the more dense stacking results in a lower porosity of the negative electrode active material layer. When the porosity of the negative electrode active material layer is within the above suitable range, it can not only reduce the internal resistance of the negative electrode sheet, but also ensure the electrolyte infiltration performance of the negative electrode sheet, thereby improving the kinetic performance of the electrochemical device.

[0058] 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 within the range composed of any of the above values.

[0059] When the sheet resistance of the negative electrode sheet is within the above suitable range, it can ensure that the electrochemical device has low ohmic polarization, reduce the heat generation during the charge and discharge process of the electrochemical device, and thus improve the long-term cycle performance and safety performance of the electrochemical device.

[0060] The hard carbon particles of the present application can be obtained in various ways. As an example, the hard carbon particles can be prepared by a template method through the following steps: mixing a lamellar 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; pyrolyzing the cured mixture at 700°C to 1300°C for 2h, crushing and screening, and then treating with an acid or alkali solution to remove the template to obtain hard carbon particles. Among them, the lamellar inorganic templates include but are not limited to: montmorillonite, mica, two-dimensional silicon, lamellar silica; the pore-forming agents include but are not limited to: magnesium oxide, magnesium chloride, magnesium gluconate, zinc oxide, zinc chloride, zinc gluconate, zinc stearate, zinc borate, iron oxide, iron chloride, glucose, sucrose; the resins include but are not limited to: phenolic resin, furan resin, epoxy resin, polyester resin, bismaleimide, thermosetting polyimide, cyanate ester. The mixing method can be powder intermixing or solution mixing. When the mixing method is solution mixing, the choice of solvent is determined by the resin and the pore-forming agent, and the solvent can include but are not limited to: deionized water, methanol, ethanol, acetone, dichloromethane, benzene, toluene, ethyl acetate, tetrahydrofuran. After solution mixing and before curing, the solvent can be removed or not removed. As a specific example, the hard carbon particles can be prepared through the following steps: completely dissolving 100g of thermosetting phenolic resin in 200mL of ethanol, then adding 100g of micron-scale lamellar silica, stirring in an open environment for 24h to volatilize the ethanol to obtain a viscous mixture; then introducing the viscous mixture into a mold plate, setting the molding pressure at 0.5T, the molding temperature at 200°C, and the molding time at 1h, and obtaining a precursor material after molding; placing the precursor material in a tube furnace, heating it to 1100°C at a heating rate of 3°C / min in an argon atmosphere, and holding for 2h to pyrolyze the precursor to obtain pyrolytic carbon, then crushing and screening the pyrolytic carbon, and then placing the pyrolytic carbon in 1L of 2mol / L sodium hydroxide solution, stirring for 24h and then filtering, repeating twice to ensure complete removal of the lamellar silica template, and finally obtaining a lamellar hard carbon material.

[0061] The present application does not limit the negative electrode current collector of the negative electrode sheet. A metal foil or a porous metal plate can be used, for example, a foil or a porous plate made of a metal such as copper, nickel, titanium, iron or their alloys. As an example, the negative electrode current collector is a copper foil.

[0062] In some embodiments, the negative electrode current collector has two opposite sides in its own thickness direction, and the negative electrode active material layer can be disposed on one side of the negative electrode current collector or on both sides at the same time. For example, the negative electrode current collector has two opposite sides in its own thickness direction, and the negative electrode active material layer is disposed on either one or both of the opposite sides of the negative electrode current collector.

[0063] In some embodiments, other negative electrode active materials in addition to hard carbon are not excluded from the negative electrode active material layer. The specific types of other negative electrode active materials are not specifically limited and can be selected according to requirements. As an example, other negative electrode active materials include, but are not limited to, soft carbon, silicon, silicon-carbon composite, SiO, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, and at least one of spinel-structured Li4Ti5O 12 , and at least one of Li-Al alloy.

[0064] In some embodiments, the negative electrode active material layer may also optionally include a binder. The binder can be selected from at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoroethylene, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0065] In some embodiments, the negative electrode active material layer may also optionally include a conductive agent. The conductive agent can be selected from carbon-based materials, metal-based materials, conductive polymers, or any combination of the above. As an example, the carbon-based materials can be selected from at least one of natural graphite, artificial graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The metal-based materials can be selected from metal powders and metal fibers. The conductive polymer may include polyphenylene derivatives.

[0066] In some embodiments, the negative electrode active material layer may also optionally include other additives, such as thickeners (such as sodium carboxymethyl cellulose (CMC-Na)), etc.

[0067] The negative electrode sheet in this application can be prepared according to conventional methods in the art. For example, hard carbon and optional other negative electrode active materials, conductive agents, binders, and thickeners are dispersed in a solvent, and the solvent can be N-methylpyrrolidone (NMP) or deionized water to form a uniform negative electrode slurry. The negative electrode slurry is coated on the negative electrode current collector and the negative electrode sheet is obtained through processes such as drying and cold pressing.

[0068] It should be noted that the parameters of each negative electrode active material layer given in this application refer to the parameter range of the single-sided negative electrode active material layer. When the negative electrode active material layer is disposed on both sides of the negative electrode current collector, the parameters of the negative electrode active material layer on any one side satisfying this application are considered to fall within the protection scope of this application.

[0069] In addition, the negative electrode plate in the present application does not exclude other additional functional layers in addition to the negative electrode active material layer. For example, in some embodiments, the negative electrode plate of the present application further includes a conductive bottom coating (for example, composed of a conductive agent and a binder) disposed on the surface of the negative electrode current collector and sandwiched between the negative electrode current collector and the negative electrode active material layer. In some other embodiments, the negative electrode plate of the present application further includes a protective layer covering the surface of the negative electrode active material layer.

[0070] In the present application, the aspect ratio of the hard carbon particles can be measured by methods and instruments known in the art. For example, a negative electrode plate cut to a certain size can be pasted on a silicon wafer carrier with conductive glue, and one cross-section of the negative electrode plate can be polished by argon ion polishing to obtain a specimen; the morphology structure and element distribution of the polished cross-section are analyzed by a scanning electron microscope (SEM), and the images of the hard carbon particles are screened out with image processing software, and the major axis value of each hard carbon particle in the cross-section and the maximum thickness in the direction perpendicular to the major axis are measured, so as to obtain the aspect ratio of each hard carbon particle.

[0071] In the present application, the XRD pattern can be measured by methods and instruments known in the art. For example, it can be obtained by performing XRD testing using a Bruker D8 ADVANCE X-ray powder diffractometer, wherein the radiation source for the XRD testing is a Cu Kα target, and the testing parameters can be set as follows: tube voltage is 40 kV, tube current is 40 mA, scanning step size is 0.00836°, the scanning duration for each scanning step is 0.3 s, and the 2θ range is from 5° to 80°.

[0072] In the present application, the particle sizes Dv10, Dv50, and Dv99 of the negative electrode active material have meanings well-known in the art and can be measured by methods and instruments known in the art. For example, with reference to GB / T 19077-2016 Laser Diffraction Method for Particle Size Distribution, a laser particle size analyzer (such as Malvern Mastersizer 2000E from the UK) can be used for measurement.

[0073] In the present application, 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. For example, a specific surface area analyzer (such as TristarⅡ3020M) can be used to measure the specific surface area of the negative electrode active material by the nitrogen adsorption / desorption method.

[0074] In this application, the tap density of the negative electrode active material layer has the 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 sheets completely coated with the slurry and circular sheets not coated with the slurry with an area of S are punched out by a punching machine respectively, and the average masses W2 and W1 are obtained by weighing respectively, and the average thicknesses T2 and T1 are obtained by thickness measurement respectively. The tap density of the negative electrode sheet = (W2 - W1) / (T2 - T1) / S.

[0075] In this application, the porosity of the negative electrode active material layer has the 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 can be 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 sheet; the porosity of the negative electrode active material layer is tested with reference to the standard of GB / T 24586-2009 Determination of Apparent Density, True Density and Porosity of Iron Ores.

[0076] In this application, the sheet resistance of the negative electrode sheet has the meaning well-known in the art and can be measured by methods known in the art. For example, the negative electrode sheet can be cut into test pieces with a size of 60 mm × 80 mm, and the resistance of the sample is tested with a BER1100 multi-functional electrode sheet resistance meter to obtain the sheet resistance of the negative electrode sheet.

[0077] It should be noted that the above various parameter tests for the negative electrode active material layer or negative electrode active material particles can be sampled and tested during the preparation process of lithium-ion batteries, such as, or sampled and tested from the prepared lithium-ion batteries.

[0078] When the above test samples are sampled from the prepared lithium-ion batteries, as an example, the sampling can be carried out according to the following steps S10 - S30.

[0079] S10, perform a discharge treatment on the lithium-ion battery (for safety reasons, generally make the battery in a fully discharged state); after disassembling the battery, take out the negative electrode sheet, soak the negative electrode sheet with dimethyl carbonate (DMC) for a certain period of time (for example, 2 - 10 hours); then take out the negative electrode sheet and perform a drying treatment at a certain temperature and time (for example, 60 °C, 4 hours), and take out the negative electrode sheet after drying. At this time, samples can be taken from the dried negative electrode sheet to test the above various parameters related to the negative electrode active material layer of this application.

[0080] S20, bake the negative electrode sheet dried in step S10 at a certain temperature and time (for example, 400 °C, 2 hours), and select an area at random in the baked negative electrode sheet to sample the negative electrode active material (blade scraping for powder sampling can be used).

[0081] S30. Sieving the negative electrode active material collected in step S20 (for example, sieving with a 200-mesh sieve) to finally obtain a sample that can be used to test the parameters of the above-mentioned negative electrode active materials of this application. Electrochemical device

[0082] The second aspect of this application provides an electrochemical device, including any device in which an electrochemical reaction occurs to convert chemical energy and electrical energy into each other. Specific examples include, but are not limited to, lithium-ion batteries or sodium-ion batteries.

[0083] In some embodiments, the electrochemical device of this application includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte.

[0084] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be made into an electrode assembly by a winding process or a stacking process.

[0085] The electrochemical device of this application further includes an outer package for encapsulating the electrode assembly and the electrolyte. In some embodiments, the outer package can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc., or can be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, such as at least one of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS). [Negative electrode sheet]

[0086] The negative electrode sheet used in the electrochemical device of this application is the negative electrode sheet of the first aspect of this application. [Positive electrode sheet]

[0087] The material, composition, and manufacturing method of the positive electrode sheet used in the electrochemical device of this application can include any techniques known in the prior art.

[0088] 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 including a positive electrode active material. As an example, the positive electrode current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode active material layer is provided on any one or both of the two opposite surfaces of the positive electrode current collector.

[0089] In some embodiments, the positive electrode active material layer includes a positive electrode active material. The specific type of the positive electrode active material is not specifically limited and can be selected according to requirements. For example, the positive electrode active material can include one or several of lithium transition metal oxides, lithium-containing phosphates with an olivine structure, and their respective modified compounds. In the electrochemical device of this application, the above-mentioned modified compounds of the positive electrode active materials can be doping modification, surface coating modification, or doping and surface coating modification at the same time of the positive electrode active material.

[0090] 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 their modified compounds. As an example, the lithium-containing phosphate with 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 their modified compounds. These cathode active materials may be used alone or in combination of two or more.

[0091] In some embodiments, the cathode active material layer may further optionally include 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.

[0092] In some embodiments, the cathode active material layer may further optionally include a binder. As an example, the conductive agent may be selected from carbon-based materials, metal-based materials, conductive polymers, or any combination of the above. As an example, the carbon-based materials may be selected from at least one of natural graphite, artificial graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The metal-based materials may be selected from metal powders and metal fibers. The conductive polymer may include polyphenylene derivatives.

[0093] In some embodiments, the cathode current collector may be a metal foil or a composite current collector. As an example of the metal foil, the cathode current collector may be an 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 selected from one or more of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. As an example, the polymer material base layer may be selected from polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, polyethylene, etc.

[0094] The cathode electrode in this application can be prepared according to the conventional methods in the art. For example, the cathode active material layer is usually formed by coating a cathode slurry on a cathode current collector and then drying and cold pressing. The cathode slurry is usually formed by dispersing a cathode active material, an optional conductive agent, an optional binder, and any other components in a solvent and stirring evenly. The solvent may be N-methylpyrrolidone (NMP), but is not limited thereto.

[0095] The positive electrode sheet of the present application does not exclude other additional functional layers in addition to the positive electrode active material layer. For example, in some embodiments, the positive electrode sheet of the present application further includes a conductive bottom coating (for example, composed of a conductive agent and a binder) disposed on the surface of the positive electrode current collector and sandwiched between the positive electrode current collector and the positive electrode active material layer. In other embodiments, the positive electrode sheet of the present application further includes a protective layer covering the surface of the positive electrode active material layer. [Electrolyte]

[0096] The electrolyte functions to conduct active ions between the positive electrode sheet and the negative electrode sheet. The electrolyte that can be used in the electrochemical device of the present application can be the electrolytes known in the prior art.

[0097] In some embodiments, the electrolyte includes an organic solvent, a lithium salt, and an optional additive. The types of the organic solvent, the lithium salt, and the additive are not specifically limited and can be selected according to requirements.

[0098] In some embodiments, by way of 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 difluoro(dioxalato)phosphate), and LiTFOP (lithium tetrafluoro(oxalato)phosphate). The above lithium salts can be used alone or two or more of them can be used simultaneously.

[0099] 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 organic solvents can be used alone or two or more of them can be used simultaneously. Optionally, two or more of the above organic solvents are used simultaneously.

[0100] In some embodiments, the additive may include a negative electrode film-forming additive, a positive electrode film-forming additive, and may also include additives that can improve certain battery performance, such as additives for improving the overcharge performance of the battery, additives for improving the high-temperature or low-temperature performance of the battery, etc.

[0101] As an example, the additive includes but is not limited to at least one of fluorinated ethylene 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), sulfonate cyclic quaternary ammonium salt, succinic anhydride, succinonitrile (SN), adiponitrile (AND), tris(trimethylsilyl) phosphate (TMSP), tris(trimethylsilyl) borate (TMSB).

[0102] The electrolyte can be prepared by conventional methods in the art. For example, an organic solvent, a lithium salt, and an optional additive can be mixed uniformly to obtain the electrolyte. There is no particular limitation on the addition order of each material. For example, the lithium salt and the optional additive are added to the organic solvent and mixed uniformly to obtain the electrolyte; or, the lithium salt is first added to the organic solvent, and then the optional additive is added to the organic solvent and mixed uniformly to obtain the electrolyte. [Separator film]

[0103] The separator film is disposed between the positive electrode sheet and the negative electrode sheet, mainly serving to prevent short circuit between the positive and negative electrodes, and at the same time allowing active ions to pass through. The present application does not particularly limit the type of the separator film, and any well-known porous structure separator film with good chemical stability and mechanical stability can be selected.

[0104] In some embodiments, the material of the separator film can be selected from one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, polyvinylidene fluoride, but not limited to these. Optionally, the material of the separator film can include polyethylene and / or polypropylene. The separator film can be a single-layer film or a multi-layer composite film. When the separator film is a multi-layer composite film, the materials of each layer are the same or different. In some embodiments, a ceramic coating or a metal oxide coating can also be provided on the separator film.

[0105] Although in the above description of the embodiments of the negative electrode sheet, mainly taking the lithium-ion battery as a specific example to illustrate the beneficial effects that can be achieved by the negative electrode sheet according to the present application, those skilled in the art can easily understand that the negative electrode active material layer of the negative electrode sheet according to the present application has a high tap density and a suitable porosity, so when applied to other types of electrochemical devices, the corresponding beneficial effects can also be achieved. Power-consuming device

[0106] A third aspect of the present application provides an electrical device, which includes the electrochemical device of the second aspect of the present application.

[0107] The electrical device of the present application is not particularly limited, and it can be any electrical device known in the prior art. In some embodiments, the electrical device may include, but is not limited to, laptop computers, pen input computers, mobile computers, e-book players, mobile phones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, liquid crystal TVs, portable cleaners, portable CD players, minidiscs, transceivers, electronic notebooks, calculators, memory cards, portable tape recorders, radios, backup power supplies, motors, automobiles, motorcycles, motorized bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium ion capacitors, etc. Example

[0108] The following examples more specifically describe the content disclosed in the present invention. These examples are only for illustrative purposes, because various modifications and changes within the scope of the present invention are obvious to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported 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 treatment, and the instruments used in the examples are all commercially available. Examples 1 to 21

[0109] Preparation of negative electrode sheet

[0110] A negative electrode active material, a binder of styrene-butadiene rubber, 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 then coated onto both sides of a negative electrode current collector, wherein the negative electrode current collector was a 6 μm thick copper foil with a single-side coating thickness of 50 μm. The negative electrode sheet was then dried at 85°C, cold pressed, cut, and slit, and then dried under vacuum at 120°C for 12 hours to obtain a negative electrode sheet. The mass percentage of lamellar hard carbon (w1%), the mass percentage of MCMB (w2%), a, b, c, d, Dv10, Dv50, and Dv99 of the negative electrode active material, the specific surface area of the negative electrode active material, the compacted 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 are shown in Tables 1, 2, and 3, respectively. Among them, the negative electrode active materials in Examples 1 to 8 are 95% hard carbon and 5% MCMB, the negative electrode active materials in Examples 9 to 16 are the same as those in Example 1 and have the same aspect ratio distribution as the hard carbon in Example 1, and the negative electrode active materials in Examples 17 to 21 have the same aspect ratio distribution as the hard carbon in Example 1.

[0111] Preparation of positive electrode sheet

[0112] The positive electrode active material lithium cobalt oxide, the conductive agent carbon black, and the binder PVDF were mixed in a mass ratio of 97:1.4:1.6, and an appropriate amount of solvent NMP was added and stirred to obtain a positive electrode slurry with a solid content of 72wt%. The positive electrode slurry was evenly coated on both sides of the positive electrode current collector aluminum foil, wherein the single-side coating thickness was 80μm; dried at 85°C, cold pressed, cut into pieces, and slit, and then dried under vacuum conditions at 85°C for 4 hours to obtain a positive electrode sheet.

[0113] Preparation of electrolyte

[0114] In a dry argon atmosphere 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; after thorough stirring, lithium salt LiPF6 was added and mixed evenly to obtain an electrolyte, wherein the mass content of LiPF6 was 12.5% based on the mass of the electrolyte, and 2% by mass of 1,3-propane sultone, 2% by mass of fluoroethylene carbonate, and 2% by mass of succinonitrile were added to the electrolyte.

[0115] Preparation of separator

[0116] Polyethylene (PE) with a thickness of 7 μm was used as the separator.

[0117] Preparation of lithium-ion battery

[0118] The positive electrode plate, separator, and negative electrode plate are stacked and wound in sequence to obtain an electrode assembly. The electrode assembly is placed in an outer package. After removing moisture at 80 °C, the above electrolyte is added. After processes such as encapsulation, static formation, degassing, and shaping, a lithium-ion battery is obtained. Comparative Examples 1 to 4

[0119] Adjust the type of negative active material. Based on the preparation processes of the negative electrode plate, positive electrode plate, electrolyte, separator, and lithium-ion battery in Examples 1 to 21, prepare the negative electrode plates, positive electrode plates, electrolytes, separators, and lithium-ion batteries of Comparative Examples 1 to 5.

[0120] Among them, the negative active material of Comparative Example 1 is MCMB; the negative active material of Comparative Example 2 is flake graphite; the negative active material of Comparative Example 3 is 95% conventional morphology hard carbon and 5% MCMB; the negative active material of Comparative Example 4 is 95% microspherical hard carbon particles and 5% MCMB. Comparative Examples 5 to 6

[0121] Based on the preparation processes of the negative electrode plate, positive electrode plate, electrolyte, separator, and lithium-ion battery in Examples 1 to 21, adjust the values of a, b, c, and d of the hard carbon material as shown in Table 1, and prepare the negative electrode plates, positive electrode plates, electrolytes, separators, and lithium-ion batteries of Comparative Examples 5 and 6. Test Part Testing of negative electrode sheet

[0122] (1) Aspect ratio test of hard carbon particles

[0123] Testing instrument: Scanning electron microscope JSM-6360LV and its supporting X-ray energy spectrometer

[0124] Take the lithium-ion battery after full charge and discharge, disassemble it and take out the negative electrode plate; soak 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 plate into 0.5 cm specimens of the negative electrode, paste the specimens on a 1 cm silicon wafer carrier of the negative electrode with conductive glue; use argon ion polishing (working parameters: acceleration voltage 8 kV, polishing duration 4 h) to polish one end of the cross-section of the negative electrode plate to obtain specimens; analyze the cross-sectional morphology structure and element distribution of the polished specimens through a scanning electron microscope, use image processing software (Multiphase) to screen the images of hard carbon particles, and measure the major axis value of each hard carbon particle in the cross-section and the maximum thickness perpendicular to the major axis direction, so as to obtain the aspect ratio of each hard carbon particle. For the negative electrode plates in each example or comparative example, process 10 SEM images respectively (see Figure 2) Values of a, b, c, and d are obtained through statistics.

[0125] (2) XRD Test of Anode Active Material

[0126] Testing Instrument: Bruker D8 ADVANCE X-ray Powder Diffractometer

[0127] Take a fully discharged lithium-ion battery, disassemble the anode plate; after cleaning and drying, use a scraper to process the anode active material layer to obtain the anode active material layer powder; place the anode active material layer powder in a tube furnace, keep it at 400 °C in an argon atmosphere for 4 h to remove the binder adhered to the surface of the anode active material layer powder, thereby obtaining the anode active material powder; test the anode active material powder with an X-ray powder diffractometer to obtain the XRD test pattern of the anode active material. Among them, the radiation source for the XRD test is a Cu Kα target, and the test parameters can be set as follows: tube voltage is 40 kV, tube current is 40 mA, scanning step size is 0.00836 °, the scanning duration for each scanning step is 0.3 s, and the 2θ range is from 5 ° to 80 °.

[0128] (3) Particle Size Test of Anode Active Material

[0129] Testing Instrument: Bruker D8 Advance

[0130] Refer to the steps in the XRD test of the anode active material to obtain the anode active material powder; disperse the anode active material powder in ethanol, and after ultrasonic treatment for 30 minutes, obtain the ethanol dispersion of the anode active material; add the ethanol dispersion of the anode active material into a Malvern particle size analyzer to test Dv10, Dv50, and Dv99 of the anode active material particles.

[0131] (4) Specific Surface Area Test of Anode Active Material

[0132] Testing Instrument: Specific Surface Area Analyzer TristarⅡ3020M

[0133] Refer to the steps in the XRD test of the anode active material to obtain the anode active material powder; dry the anode active material powder in a vacuum drying oven; measure the specific surface area of the anode active material with a specific surface area analyzer.

[0134] (5) Compaction Density Test of Anode Active Material Layer

[0135] Take a fully discharged lithium-ion battery and disassemble the negative electrode sheet. After cleaning and drying, measure 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. After washing off the negative electrode active material layer with a solvent, dry it and measure the mass W2 of the negative electrode current collector and the thickness T2 of the negative electrode current collector. Calculate the tap density of the negative electrode active material layer through the following formulas 1 to 3. W0 = (W1 - W2) / 2 Formula 1 T0 = (T1 - T2) / 2 Formula 2 Tap density = W0 / (T0×S) Formula 3

[0136] Among them, 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.

[0137] (6) Porosity test of the negative electrode active material layer

[0138] Testing instrument: True density tester (AccuPycⅡ1340)

[0139] Cut the negative electrode sheet coated with the negative electrode active material into circular test pieces. In each test piece, the volume of the negative electrode active material layer is about 0.35 cm 3 ; Refer to the standard of GB / T24586-2009 Determination of Apparent Density, True Density and Porosity of Iron Ores to test the porosity of the negative electrode active material layer.

[0140] (7) Specific capacity test of the negative electrode active material

[0141] Assemble the negative electrode sheet, positive electrode sheet and lithium sheet into a button cell; discharge at 0.05C to 5.0 mV, discharge at 50 μA to 5.0 mV, discharge at 10 μA to 5.0 mV, and charge at 0.1C to 2.0 V, and record the first charging capacity of the button cell at this time. The specific capacity of the negative electrode active material = first charging capacity (mAh) / mass (g) of the negative electrode active material.

[0142] (8) Pore size distribution test of the negative electrode active material

[0143] Testing instrument: ASAP2460 - Physical adsorption analyzer. Refer to the steps in the XRD test of the negative electrode active material to obtain the negative electrode active material powder; place the sample after drying and degassing treatment in liquid nitrogen, adjust different test pressures, measure the adsorption amount of nitrogen respectively, and draw the adsorption and desorption isotherms. Determine the pore shape according to the shape of the hysteresis loop, calculate the pore distribution and pore volume according to different pore models, use the BJH model to fit the pore size distribution curve of mesopores and macropores, and use the DFT model to fit the pore size distribution curve of micropores. Testing of lithium-ion battery

[0144] (1) Energy density test of lithium-ion batteries

[0145] For each example or comparative example, 5 lithium-ion batteries were taken for energy density test. The specific test steps are as follows.

[0146] In an environment of 25°C, the first charge and discharge were carried out. Constant current and constant voltage charging were carried out at a charging current of 0.5C until the upper limit voltage was 4.48V, and then constant current discharge was carried out at a discharge current of 0.2C until the discharge cut-off voltage (3V). Calculate the energy density M of the lithium-ion batteries in each example and comparative example. i The percentage A% relative to the energy density M1 of Comparative Example 1 was used as the energy density parameter A of the lithium-ion batteries in each example and comparative example, where A = M i / M1.

[0147] (2) Rate performance test of lithium-ion batteries

[0148] For each example or comparative example, 5 lithium-ion batteries were taken for rate performance test. The specific test steps were as follows: The lithium-ion batteries were placed in an environment of 25°C and left standing for 1 hour; the battery was charged at a constant current (CC section) at a charging rate of I = 1C. After charging to 4.48V, it was switched to constant voltage charging (CV section). Charging stopped when the charging current was lower than 0.05C. After standing for 5 minutes, the battery was discharged at a rate of 0.2C until 3V, and then left standing for 5 minutes. This was the first charge and discharge cycle. The charging capacity (average value) of each charging stage was statistically calculated, and the proportion of the CC section capacity was calculated. Sequentially adjust I to 0.2C, 0.5C, 1C, 2C, 3C, and perform the 2nd to 6th charge and discharge cycles according to the process of the first charge and discharge cycle. Calculate the proportion of the CC section capacity at a charging rate of 3C according to Equation 4. Proportion of CC section capacity = [CC section charging capacity / (CC section charging capacity + CV section charging capacity)] × 100% Equation 4

[0149] (3) Cycle performance test of lithium-ion batteries

[0150] For each example or comparative example, 5 lithium-ion batteries were taken for cycle performance test. The specific test steps are as follows.

[0151] At 25°C, the lithium-ion battery was charged at a rate of 1C to 4.48V and continued to be charged at a constant voltage until the charging cut-off current; it was discharged at a rate of 1C to 3V. This was one charge and discharge cycle. Record the first charging capacity, the first discharge capacity, and the thickness of the fully charged lithium-ion battery in the first cycle. Then continue the charge and discharge cycle, and record the discharge capacity and the thickness of the fully charged lithium-ion battery in the 400th cycle.

[0152] Initial Coulomb efficiency (%) = (First discharge capacity / First charging capacity) × 100%;

[0153] The percentage of irreversible capacity in the first week (%) = [(the first charge capacity - the first discharge capacity) / the first charge capacity] × 100%

[0154] The capacity retention rate (%) = (the discharge capacity of the 400th cycle / the discharge capacity of the first cycle) × 100%;

[0155] The thickness expansion rate (%) = [(the thickness of the fully charged lithium-ion battery in the 400th cycle - the thickness of the fully charged lithium-ion battery in the first cycle) / the thickness of the fully charged lithium-ion battery in the first cycle] × 100%.

[0156] (4) Self-discharge rate test of lithium-ion batteries

[0157] For each example or comparative example, 5 lithium-ion batteries were taken for the self-discharge rate test. The specific test steps are as follows.

[0158] Take lithium-ion batteries with a state of charge (SOC) of 80%, test the initial open-circuit voltage of the lithium-ion battery, and record it as V1; after standing at 25°C for 48 hours, test the open-circuit voltage of the lithium-ion battery again and record it as V2.

[0159] The self-discharge rate of the lithium-ion battery = (V1 - V2) / 48.

[0160] (5) DC impedance DCR test of lithium-ion batteries

[0161] In an environment of 25°C, charge the lithium-ion battery with a current of 0.5C until the voltage reaches 4.4V, then switch to constant-voltage charging until the current is 0.05C. Then discharge with a current of 0.1C for 2 hours, after standing for 1h, discharge with a current of 0.1C (I1) for 10s, record the discharge voltage V3 in the last 1s, and then discharge with a current of 1C (I2) for 1s, record the discharge voltage V4 in the last 1s, then DCR = (V3 - V4) / (I2 - I1).

[0162] The settings and test results of the examples and comparative examples are shown in Tables 1 to 3.

[0163] Table 1

[0164] Table 2

[0165] Table 3

[0166] As can be seen from Table 1, the negative electrode active materials of Comparative Example 1 and Comparative Example 5 are both spherical or quasi-spherical carbon material particles, and their aspect ratio distributions are relatively concentrated; under similar cold pressing processes, the tap densities of the hard carbon-based negative electrode active materials are lower than those of the graphite-based negative electrode active materials. Using the lamellar hard carbon in the present application, since the hard carbon particles are more densely packed after cold pressing, the negative electrode active material layer has a higher tap density and a lower porosity. The lithium-ion battery has a relatively high self-discharge rate, indicating that the lamellar hard carbon with a suitable aspect ratio distribution has a flatter surface of the active material layer after cold pressing, is not easily damaged to the separator, and the degree of internal physical micro-short circuit is greatly reduced. Moreover, the lamellar hard carbon particles are in face-to-face contact, which can effectively shorten the + transport path of Li + and improve the solid-phase transfer rate of Li

[0167] in the active material, thereby reducing the internal impedance of the lithium-ion battery and improving its kinetic performance. As a result, the fast charging ability of the lithium-ion battery is effectively improved, the lithium-ion battery has better cycle performance and rate performance, and the energy density of the lithium-ion battery is further increased.

[0168] As can be seen from Example 1 and Examples 9 to 16, when the particle size of the negative electrode active material is relatively large, it will increase the irreversible capacity in the first cycle of the lithium-ion battery and affect the cycle performance of the lithium-ion battery.

[0169] The above is only the specific implementation manner of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present application can easily think of various equivalent modifications or substitutions, and these modifications or substitutions should all be covered within the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.

Claims

1. An electrochemical device, comprising a negative electrode tab, wherein, The negative electrode sheet 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 negative electrode active material layer includes a negative electrode active material, and the negative electrode active material includes hard carbon particles and graphite particles. The hard carbon particles have a lamellar structure, and based on the number of the hard carbon particles, the proportion of the hard carbon particles with an aspect ratio of 3 to 7 is a%, and 30 ≤ a ≤ 70; The hard carbon particles include micropores, and the micropore volume measured by the nitrogen-carbon dioxide adsorption-desorption method is greater than or equal to 0.25 cc / g.

2. The electrochemical device according to claim 1, wherein, Based on the number of the hard carbon particles, the proportion of the hard carbon particles with an aspect ratio of 2 to 3 is b%, and 20 ≤ b ≤ 60.

3. The electrochemical device according to claim 2, wherein, The hard carbon particles satisfy: a + b ≥ 90.

4. The electrochemical device according to claim 1, wherein, Based on the number of the hard carbon particles, the proportion of the hard carbon particles with an aspect ratio of 1 to 2 is c%, and 0.1 ≤ c ≤ 10.

5. The electrochemical device according to claim 1, wherein, Based on the number of the hard carbon particles, the proportion of the hard carbon particles with an aspect ratio greater than 7 is d%, and 0.1 ≤ d ≤ 1.

6. The electrochemical device according to claim 1, wherein, The mass of the hard carbon particles accounts for 85% to 99% of the mass of the negative electrode active material.

7. The electrochemical device according to claim 1, wherein, 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 4° to 12°; the second diffraction peak is located between 26° and 27°, and the full width at half maximum of the second diffraction peak is 0.1° to 0.4°.

8. The electrochemical device according to claim 1, wherein, The negative electrode active material satisfies at least one of the following: (1) The particle size of the negative electrode active material satisfies: 1 μm ≤ Dv10 ≤ 5 μm, 4 μm ≤ Dv50 ≤ 18 μm, Dv99 ≤ 43 μm; (2) The specific surface area of the negative electrode active material is 1 m 2 / g to 30 m 2 / g.

9. The electrochemical device according to claim 1, wherein, The negative electrode active material layer satisfies at least one of the following: (3) The tap density of the negative electrode active material layer is 1.0 g / cm 3 to 1.7 g / cm 3 ; (4) The porosity of the negative electrode active material layer is 10% to 40%.

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

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