Negative plate and lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a negative plate and a lithium ion secondary battery comprising the negative plate. The negative plate comprises a negative current collector and a negative active material layer; the negative electrode active material layer comprises a first negative electrode active material layer deviating from the negative electrode current collector and a second negative electrode active material layer close to the negative electrode current collector which are laminated in the thickness direction; the first negative electrode active material layer comprises a first graphite material, and the average particle size of the first graphite material is 10-20 [mu] m; the second negative electrode active material layer comprises a second graphite material, and the average particle size of the second graphite material is 5-16 [mu] m; when the battery comprising the negative plate is in a 0% SOC state, the vertical distance from the surface of the first graphite material to the contact surface of the first negative electrode active material layer and the second negative electrode active material layer is 0-35 [mu] m; and the vertical distance from the surface of the second graphite material to the negative electrode current collector is 0-26 [mu] m. Comprising the negative plate disclosed by the invention can give consideration to high energy density, rapid charging capability and relatively excellent cycling stability.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically to a negative electrode and a lithium-ion secondary battery including the negative electrode. Background Technology

[0002] Battery technology, as a crucial component of sustainable energy, has significantly promoted sustainable social development and made human life more convenient and intelligent. Lithium-ion batteries, in particular, boast advantages such as high energy density and long cycle life, and are widely used in mobile phones, laptops, power tools, new energy vehicles, and energy storage. Traditional negative electrode sheets are typically single-layer structures, where active materials, conductive agents, binders, and other components are uniformly dispersed in an electrode paste, with the component ratios remaining consistent from the bottom layer to the surface. Single-layer structures offer advantages such as simple coating processes and good consistency. However, batteries with single-layer negative electrode structures generally cannot effectively balance high energy density and fast-charging performance.

[0003] Therefore, power batteries that combine high energy density and fast charging performance have become a key research direction. Summary of the Invention

[0004] The purpose of this invention is to overcome the aforementioned problems in the prior art and to provide a negative electrode sheet and a lithium-ion secondary battery including the negative electrode sheet. The negative electrode sheet of this invention has a multi-layer coating structure, with graphite materials of different particle sizes in each coating layer, resulting in more uniform polarization throughout the negative electrode sheet. Furthermore, by adjusting the vertical distance from the surface of the first graphite material to the contact surface between the first and second negative electrode active material layers, and the vertical distance from the surface of the second graphite material to the negative electrode current collector, at 0% SOC, the dynamic performance of the negative electrode sheet is further improved. The lithium-ion secondary battery including the negative electrode sheet of this invention (hereinafter referred to as the battery) can achieve a balance of high energy density, fast charging capability, and superior cycle stability.

[0005] Batteries using traditional single-layer negative electrode structures typically cannot simultaneously achieve high energy density and fast-charging performance. The inventors of this invention discovered that the uniform composition of the single-layer structure leads to uneven electrolyte wetting and current density distribution from the bottom to the surface of the electrode, resulting in polarization within the electrode, with greater polarization at the surface and less at the bottom. This uneven polarization becomes more pronounced with increasing charge and discharge current, causing preferential lithium insertion / extraction in the surface active material and a more delayed process in the bottom active material. For the negative electrode, excessive polarization affects its kinetic performance, causing lithium plating at higher charge rates, thus impacting cycle life and fast-charging performance. Therefore, the inventors of this invention employ a multi-layer coating technology for the negative electrode (e.g., a double-layer coating technology) and, based on the polarization patterns of the surface and bottom layers, use graphite materials with different particle sizes in different coatings to mitigate the problem of excessive negative electrode polarization.

[0006] When the particle sizes of the first graphite material in the first negative electrode active material layer (located on the outermost layer) and the second graphite material in the second negative electrode active material layer (located on the innermost layer) are within a specific range, the first graphite material can leverage its superior kinetic performance, preferentially deintercalating and inserting lithium ions during high-rate charging and promptly transferring lithium ions to the bottom layer. The second graphite material can utilize its higher specific capacity, providing more lithium insertion sites during charging, thereby improving the overall capacity of the negative electrode and the battery energy density to some extent. However, simply limiting the average particle size of the first and second graphite materials does not yield ideal results in improving battery energy density and fast charging capability. This is because as lithium ions gradually transport from the surface of the negative electrode to the interior, their ion concentration gradually decreases, polarization gradually increases, and the reaction current decreases accordingly. The inventors of this invention, through extensive research, discovered that by simultaneously controlling the vertical distance d1 from the surface of the first graphite material to the contact surface between the first and second negative electrode active material layers and the vertical distance d2 from the surface of the second graphite material to the negative electrode current collector at 0% SOC, the battery can achieve a balance between high energy density, fast charging capability, and superior cycle stability. The reason for this is:

[0007] When the vertical distance from the surface of the first graphite material to the contact surface between the first and second negative electrode active material layers and the vertical distance from the surface of the second graphite material to the negative electrode current collector meet a specific range, the thicknesses of the first and second negative electrode active material layers are appropriate. This allows the first graphite material, with a specific particle size range, to fully contact the electrolyte and receive lithium ions conducted in the electrolyte, completing lithium intercalation at a relatively fast speed and preventing active lithium from depositing on the surface of the negative electrode. Subsequently, the second graphite material, with a specific particle size range, can receive lithium ions conducted from the surface layer and increase the overall capacity of the negative electrode with its larger specific capacity. At this point, neither insufficient contact between the active material and the electrolyte due to an excessively thick negative electrode active material layer, leading to lithium deposition, nor a reduction in battery energy density due to an excessively thin negative electrode active material layer, is avoided. Based on this, the inventors of this invention propose the following solution:

[0008] The first aspect of the present invention provides a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active material layer located on at least one side surface of the negative electrode current collector; the negative electrode active material layer comprises a first negative electrode active material layer and a second negative electrode active material layer stacked in the thickness direction of the negative electrode sheet, the first negative electrode active material layer being away from the negative electrode current collector, and the second negative electrode active material layer being close to the negative electrode current collector; the first negative electrode active material layer comprises a first graphite material, the average particle size of the first graphite material being 10μm-20μm; the second negative electrode active material layer comprises a second graphite material, the average particle size of the second graphite material being 5μm-16μm; when the battery including the negative electrode sheet is in a 0% SOC state, the vertical distance from the surface of the first graphite material to the contact surface between the first negative electrode active material layer and the second negative electrode active material layer is d1, d1 being 0μm-35μm; the vertical distance from the surface of the second graphite material to the negative electrode current collector is d2, d2 being 0μm-26μm.

[0009] A second aspect of the present invention provides a lithium-ion secondary battery, the lithium-ion secondary battery comprising the negative electrode sheet described in the first aspect of the present invention.

[0010] By employing the above technical solution, the present invention has at least the following advantages compared with the prior art:

[0011] (1) The negative electrode of the present invention has good dynamic performance and capacity utilization;

[0012] (2) The lithium-ion secondary battery including the negative electrode of the present invention can take into account high energy density, fast charging capability and better cycle stability.

[0013] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. Attached Figure Description

[0014] Figure 1 The diagram shown is a schematic diagram of the negative electrode sheet in an embodiment of the present invention.

[0015] Figure 2 The diagram shown is a schematic representation of the width of the groove in an embodiment of the present invention, wherein... Figure 2 (a)- Figure 2 (c) The two long sides of the central groove are straight lines. Figure 2 (d) The two long sides of the middle groove are curves.

[0016] Figure 3 The diagram shown is a schematic representation of the slot spacing in an embodiment of the present invention, wherein... Figure 3 (a) represents the case where two adjacent long sides are straight lines and parallel. Figure 3 (b) is the case where two adjacent long sides are straight lines and not parallel. Figure 3 (c) is the case where the two adjacent long sides are curves. Detailed Implementation

[0017] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0018] A first aspect of the present invention provides a negative electrode sheet, which may include a negative electrode current collector and a negative electrode active material layer located on at least one surface of the negative electrode current collector; the negative electrode active material layer may include a first negative electrode active material layer and a second negative electrode active material layer stacked in the thickness direction of the negative electrode sheet, wherein the first negative electrode active material layer is away from the negative electrode current collector, and the second negative electrode active material layer is close to the negative electrode current collector. Figure 1 The figure shows a schematic diagram of the negative electrode sheet in an embodiment of the present invention. As can be seen from the figure, the negative electrode sheet includes a negative electrode current collector 1 and a negative electrode active material layer 2 located on one side surface of the negative electrode current collector 1. The negative electrode active material layer 2 includes a first negative electrode active material layer 21 and a second negative electrode active material layer 22 stacked in the thickness direction of the negative electrode sheet. The first negative electrode active material layer 21 is away from the negative electrode current collector 1, and the second negative electrode active material layer 22 is close to the negative electrode current collector 1.

[0019] In this invention, the first negative electrode active material layer may include a first graphite material, the average particle size of which may be 10 μm-20 μm (e.g., 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm or 20 μm). The second negative electrode active material layer may include a second graphite material, the average particle size of which may be 5 μm-16 μm (e.g., 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm or 16 μm).

[0020] In this invention, when the battery including the negative electrode sheet is in a 0% SOC state, the vertical distance from the surface of the first graphite material to the contact surface between the first negative electrode active material layer and the second negative electrode active material layer is d1, and d1 can be 0μm-35μm (e.g., 0μm, 0.1μm, 0.5μm, 1μm, 5μm, 10μm, 15μm, 20μm, 25μm, 30μm or 35μm); the vertical distance from the surface of the second graphite material to the negative electrode current collector is d2, and d2 can be 0μm-26μm (e.g., 0μm, 0.1μm, 0.5μm, 1μm, 5μm, 10μm, 15μm, 20μm, 25μm or 26μm).

[0021] In one example, the average particle size of the first graphite material is 13 μm-16 μm.

[0022] In one example, the average particle size of the second graphite material is 6 μm-13 μm.

[0023] In one example, the average particle size of the second graphite material is 9 μm-12.1 μm.

[0024] In this invention, the average particle size of the first graphite material and the average particle size of the second graphite material can be obtained by conventional methods in the art. For example, by using a scanning electron microscope (SEM), at least 20 first graphite materials and 20 second graphite materials are randomly selected within the field of view of the electron microscope, and the average value is taken respectively to obtain the average particle size of the first graphite material and the average particle size of the second graphite material.

[0025] In one instance, d1 is 0μm-30μm.

[0026] In one instance, d2 is 0μm-20μm.

[0027] In this invention, d1 and d2 can be obtained by conventional methods in the art, such as using SEM. The battery is discharged to 0% SOC, the negative electrode is removed, and the negative electrode is cut open in the cross-sectional direction. Then it is observed under SEM. The minimum vertical distance from the surface of the first graphite material to the contact surface between the first negative electrode active material layer and the second negative electrode active material layer is measured as "the vertical distance from the surface of the first graphite material to the contact surface between the first negative electrode active material layer and the second negative electrode active material layer"; and the minimum vertical distance from the surface of the second graphite material to the surface of the negative electrode current collector is measured as "the vertical distance from the surface of the second graphite material to the negative electrode current collector".

[0028] In this invention, when the battery including the negative electrode sheet is in a 100% SOC state, the vertical distance from the surface of the first graphite material to the contact surface between the first negative electrode active material layer and the second negative electrode active material layer is d3, where d3 can be 0μm-40μm (e.g., 0μm, 0.1μm, 0.5μm, 1μm, 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, or 40μm). The vertical distance from the surface of the second graphite material to the negative electrode current collector is d4, where d4 can be 0μm-35μm (e.g., 0μm, 0.1μm, 0.5μm, 1μm, 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, or 35μm).

[0029] In one instance, d3 is 0μm-36μm.

[0030] In one instance, d4 is 0μm-30μm.

[0031] In this invention, d3 and d4 can be obtained by conventional methods in the art, such as using SEM. The battery is charged to 100% SOC, the negative electrode sheet is removed, and the negative electrode sheet is cut open in the cross-sectional direction. Then it is observed under SEM. The minimum vertical distance from the surface of the first graphite material to the contact surface between the first negative electrode active material layer and the second negative electrode active material layer is measured as "the vertical distance from the surface of the first graphite material to the contact surface between the first negative electrode active material layer and the second negative electrode active material layer"; and the minimum vertical distance from the surface of the second graphite material to the surface of the negative electrode current collector is measured as "the vertical distance from the surface of the second graphite material to the negative electrode current collector".

[0032] By further controlling the vertical distance from the surface of the first graphite material to the contact surface between the first and second negative electrode active material layers when the battery, including the negative electrode sheet, is at 100% SOC, and the vertical distance from the surface of the second graphite material to the negative electrode current collector, the thicknesses of the first and second negative electrode active material layers can be further kept within a suitable range. This ensures that the first graphite material with a specific particle size range in the first negative electrode active material layer is in full contact with the electrolyte and completes lithium intercalation quickly, avoiding lithium plating on the negative electrode sheet. Furthermore, this facilitates the capacity utilization of the second graphite material, improving the overall capacity of the negative electrode sheet.

[0033] In this invention, "battery in 0% SOC state" means that the battery is discharged to the lower limit voltage [e.g., 2.5V for ternary high-nickel materials (with a molar content of 80% or more of elemental nickel)]; "battery in 100% SOC state" means that the battery is charged to the upper limit voltage [e.g., 4.2V for ternary high-nickel materials (with a molar content of 80% or more of elemental nickel)].

[0034] In this invention, the first negative electrode active material layer may further include a first silicon-based material, and the second negative electrode active material layer may further include a second silicon-based material. The first silicon-based material and the second silicon-based material may each independently include at least one of silicon-carbon, silicon-oxygen, elemental silicon, and silicon alloys.

[0035] In this invention, the mass percentage of the first silicon-based material in the first silicon-based material and the first graphite material can be 0.01%-20%, for example, 0.01%, 0.1%, 0.5%, 0.8%, 1%, 3%, 5%, 7%, 9%, 12%, 15%, 18% or 20%.

[0036] In one example, the first silicon-based material accounts for 0.1%-15% of the mass of the first silicon-based material and the first graphite material.

[0037] In this invention, the mass percentage of the second silicon-based material in the second silicon-based material and the second graphite material is 0.01%-20%, for example, 0.01%, 0.1%, 0.5%, 0.8%, 1%, 3%, 5%, 7%, 9%, 12%, 15%, 18% or 20%.

[0038] In one example, the second silicon-based material accounts for 0.1%-15% of the mass of the second silicon-based material and the second graphite material.

[0039] Silicon-based materials can provide high specific capacity. By adjusting the mass content of silicon-based materials and graphite materials, it is beneficial to improve the energy density of the battery. At the same time, excessive silicon doping will not cause excessive cycle expansion of the battery and affect the cycle performance.

[0040] In this invention, the first silicon-based material and the second silicon-based material may each independently comprise silicon-carbon. The mass ratio of elemental silicon to elemental carbon in the silicon-carbon may be (0.82-1.22):1, for example, 0.82:1, 0.85:1, 0.9:1, 0.95:1, 1:1, 1.05:1, 1.1:1, 1.15:1, 1.2:1, or 1.22:1.

[0041] In one example, the mass ratio of elemental silicon to elemental carbon in the silicon-carbon is (0.9-1.1):1.

[0042] By controlling the mass ratio of elemental silicon to elemental carbon within a specific range, the carbon skeleton of silicon-carbon can be well coated, thereby effectively suppressing the expansion of silicon-carbon.

[0043] In this invention, the ratio of the average particle size of the first graphite material to that of the silicon carbon can be (1-3.3):1, for example, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1, 2.5:1, 2.6:1, 2.7:1, 2.8:1, 2.9:1, 3:1, 3.1:1, 3.2:1, or 3.3:1.

[0044] In one example, the ratio of the average particle size of the first graphite material to that of the silicon carbon is (1.3-2.7):1.

[0045] The inventors of this invention discovered that, while maintaining a specific mass ratio of elemental silicon to elemental carbon within a certain range, by adjusting the ratio of the average particle size of the first graphite material to the silicon carbon, the first graphite material and silicon carbon particles can form a mutually compatible environment, creating a favorable co-intercalation environment. This allows the silicon carbon particles to effectively fill the voids formed by the first graphite material, improving space utilization and lithium-ion transport efficiency. Simultaneously, the voids formed by the first graphite material can provide a certain buffer space for the volume expansion of the silicon carbon particles, which helps reduce the overall volume expansion of the negative electrode sheet. This allows the negative electrode sheet to maintain interface integrity during battery charging and discharging, thus improving the overall dynamic performance of the negative electrode sheet.

[0046] In this invention, the ratio of the average particle size of the second graphite material to that of the silicon carbon can be (0.5-2.6):1, for example, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1, 2.5:1 or 2.6:1.

[0047] In one example, the ratio of the average particle size of the second graphite material to that of the silicon carbon is (0.9-2):1.

[0048] The inventors of this invention discovered that, while maintaining a specific mass ratio of elemental silicon to elemental carbon within a certain range in silicon-carbon, by adjusting the ratio of the average particle size of the second graphite material to silicon-carbon, the second graphite material and silicon-carbon particles can form a mutually compatible environment, creating a favorable co-intercalation environment. This allows the silicon-carbon particles to effectively fill the voids formed by the second graphite material, improving space utilization and the compaction density of the negative electrode sheet. Simultaneously, the voids formed by the second graphite material can provide a certain buffer space for the volume expansion of silicon-carbon particles, which helps reduce the overall volume expansion of the negative electrode sheet. This allows the negative electrode sheet to maintain interface integrity during battery charging and discharging, thus improving the overall capacity utilization of the negative electrode sheet.

[0049] In this invention, the average particle size of the silicon carbon can be 6μm-12μm, for example, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm or 12μm.

[0050] In this invention, the average particle size of the silicon carbon can be obtained by conventional methods in the art, such as by randomly selecting at least 20 silicon carbon particles within the field of view of an electron microscope using SEM and taking the average value to obtain the average particle size of the silicon carbon.

[0051] In this invention, the first silicon-based material and the second silicon-based material may each independently comprise silicon oxide. The mass ratio of elemental silicon to elemental oxygen in the silicon oxide may be (0.8-1.2):1, for example, 0.8:1, 0.85:1, 0.9:1, 0.95:1, 1:1, 1.1:1, or 1.2:1.

[0052] In one example, the mass ratio of elemental silicon to elemental oxygen in the silicon-oxygen mixture is (0.9-1):1.

[0053] By controlling the mass ratio of elemental silicon to elemental oxygen in silicon-oxygen compounds within a specific range, the expansion of silicon-oxygen particles during the lithium insertion / extraction process can be suppressed, preventing the silicon-oxygen particles from breaking and maintaining their integrity.

[0054] In this invention, the ratio of the average particle size of the first graphite material to that of the silicon oxide can be (0.8-3):1, for example, 0.8:1, 0.9:1, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1, 2.5:1, 2.6:1, 2.7:1, 2.8:1, 2.86:1, or 3:1.

[0055] In one example, the ratio of the average particle size of the first graphite material to that of the silicon oxide is (0.8-2.86):1.

[0056] In one example, the ratio of the average particle size of the first graphite material to that of the silicon oxide is (1.1-2.3):1.

[0057] The inventors of this invention discovered that, while maintaining a specific mass ratio of elemental silicon to elemental oxygen in silicon-oxygen composites, by adjusting the ratio of the average particle size of the first graphite material to the silicon-oxygen composite, a mutually compatible environment can be formed between the first graphite material and the silicon-oxygen composite particles. This allows the silicon-oxygen composite particles to effectively fill the voids formed by the first graphite material, improving space utilization and lithium-ion transport efficiency. Simultaneously, the voids formed by the first graphite material can provide a certain buffer space for the volume expansion of the silicon-oxygen composite particles, which helps reduce the overall volume expansion of the negative electrode sheet. This allows the negative electrode sheet to maintain interface integrity during battery charging and discharging, thus improving the overall dynamic performance of the negative electrode sheet.

[0058] In this invention, the ratio of the average particle size of the second graphite material to that of the silicon oxide can be (0.41-2.5):1, for example, 0.41:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2:1, 2.1:1, 2.2:1, 2.28:1, 2.3:1, 2.4:1 or 2.5:1.

[0059] In one example, the ratio of the average particle size of the second graphite material to that of the silicon oxide is (0.41-2.28):1.

[0060] In one example, the ratio of the average particle size of the second graphite material to that of the silicon oxide is (0.7-1.8):1.

[0061] The inventors of this invention discovered that, while maintaining a specific mass ratio of elemental silicon to elemental oxygen in silicon-oxygen composites, by adjusting the ratio of the average particle size of the second graphite material to the silicon-oxygen composite, a mutually compatible environment can be formed between the second graphite material and the silicon-oxygen composite particles. This allows the silicon-oxygen composite particles to effectively fill the voids formed by the second graphite material, improving space utilization and the compaction density of the negative electrode sheet. Simultaneously, the voids formed by the second graphite material can provide a certain buffer space for the volume expansion of the silicon-oxygen composite particles, which helps reduce the overall volume expansion of the negative electrode sheet. This allows the negative electrode sheet to maintain interface integrity during battery charging and discharging, which is beneficial for improving the overall capacity utilization of the negative electrode sheet.

[0062] In this invention, the average particle size of the silicon oxide can be 7μm-15μm, for example, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm or 15μm.

[0063] In this invention, the average particle size of the silicon oxide can be obtained by conventional methods in the art, such as by randomly selecting at least 20 silicon oxide particles within the field of view of an electron microscope using SEM and taking the average value to obtain the average particle size of the silicon oxide.

[0064] In this invention, the mass ratio of elemental silicon to elemental carbon in the silicon-carbon and the mass ratio of elemental silicon to elemental oxygen in the silicon-oxygen can be obtained by conventional methods in the art. For example, by using an energy dispersive spectroscopy (EDS) instrument, at least 5 silicon-carbon particles are randomly selected within the field of view of the electron microscope, and the mass ratio of elemental silicon to elemental carbon in each silicon-carbon particle is tested and the average value is taken; at least 5 silicon-oxygen particles are randomly selected within the field of view of the electron microscope, and the mass ratio of elemental silicon to elemental oxygen in each silicon-oxygen particle is tested and the average value is taken.

[0065] In this invention, the first graphite material may include artificial graphite. The degree of graphitization of the artificial graphite may be 92%-94%, for example, 92%, 93%, or 94%. The second graphite material may include natural graphite, and the degree of graphitization of the natural graphite may be 95%-98%, for example, 95%, 96%, 97%, or 98%.

[0066] Graphite can be divided into artificial graphite and natural graphite. Natural graphite is extracted and processed directly from ores. Due to its high degree of graphitization and fewer internal structural defects, natural graphite has a high specific capacity. However, natural graphite is usually in single-particle form with a relatively large particle size, resulting in a lower specific surface area and a smaller contact area with the electrolyte, leading to relatively poor rate performance. Artificial graphite is produced by high-temperature graphitization of coke-like materials. It typically involves a secondary granulation process, combining primary particles with smaller particle sizes to form larger secondary particles. Therefore, artificial graphite has a relatively high specific surface area, promoting electrolyte wetting and facilitating lithium-ion diffusion within the particles. Furthermore, the lower degree of graphitization and larger interlayer spacing of artificial graphite also facilitate lithium-ion diffusion within the particles, further improving its rate performance. Therefore, placing artificial and natural graphite at different locations on the negative electrode allows for leveraging their respective advantages, enabling the battery to achieve high energy density, fast charging capability, and superior cycle stability.

[0067] In this invention, the degree of graphitization of the artificial graphite and the degree of graphitization of the natural graphite can be obtained by conventional methods in the art, such as X-ray diffraction (XRD) to measure the interlayer spacing d002 of the graphite crystal structure, and then the degree of graphitization can be calculated using the Franklin formula (i.e., degree of graphitization = (0.3440 - d002 / 0.0086 × 100%)).

[0068] In this invention, the specific surface area of ​​the artificial graphite can be 1.3 m². 2 / g-1.6m 2 / g, for example, 1.3m 2 / g, 1.4m 2 / g, 1.5m 2 / g or 1.6m 2 / g. The specific surface area of ​​the natural graphite can be 0.9m². 2 / g-1.2m 2 / g, for example, 0.9m 2 / g, 1m 2 / g, 1.1m 2 / g or 1.2m 2 / g.

[0069] In this invention, the outer surface of the negative electrode active material layer may have a plurality of recesses. The term "a plurality of" refers to the number of recesses being greater than or equal to 2.

[0070] In this invention, the depth of the recess can be 3μm-40μm, for example, 3μm, 5μm, 10μm, 20μm, 30μm or 40μm.

[0071] In this invention, the recess may include a hole and / or a groove.

[0072] In one example, the recess includes a hole.

[0073] In this invention, the aperture of the holes can be 50μm-500μm, for example, 50μm, 80μm, 100μm, 200μm, 300μm, 400μm or 500μm. The spacing between the holes can be 400μm-5500μm, for example, 400μm, 800μm, 1000μm, 2000μm, 3000μm, 4000μm, 5000μm or 5500μm.

[0074] In this invention, the aperture of the hole has a conventional meaning in the art. When the shape of the projection of the hole onto the negative electrode active material layer is a "regular circle," the aperture of the hole is the diameter of that regular circle; when the shape of the projection of the hole onto the negative electrode active material layer is a non-"regular circle" (e.g., an ellipse or an irregular curved polygon), the aperture of the hole is the diameter of an equivalent circle with an area equal to that of the non-"regular circle." The "aperture diameter" can be measured by conventional methods in the art, such as by SEM, selecting all or at least 10 holes in the electron microscope field of view, measuring the aperture diameter, and taking the average value.

[0075] In this invention, the spacing between the holes has a conventional meaning in the art. It refers to the shortest distance between the edges of two adjacent holes on the negative electrode active material layer. The "hole spacing" can be measured by conventional means in the art, such as by SEM, selecting all or at least 10 groups of adjacent holes in the electron microscope field, measuring the spacing, and taking the average value.

[0076] In one example, the recess includes a groove.

[0077] In this invention, the width of the groove can be 50μm-500μm, for example, 50μm, 80μm, 100μm, 200μm, 300μm, 400μm or 500μm. The spacing between the grooves can be 400μm-5500μm, for example, 400μm, 800μm, 1000μm, 2000μm, 3000μm, 4000μm, 5000μm or 5500μm.

[0078] In this invention, the width of the groove has a conventional meaning in the art. The projection of the groove onto the negative electrode active material layer includes two long sides, and the width of the groove refers to the average distance from one long side to the other along the length or width direction of the negative electrode sheet. For example... Figure 2 The diagram shown is a schematic representation of the width of the groove in an embodiment of the present invention, wherein... Figure 2(a)- Figure 2 In (c), the two long sides of the groove are straight lines. Figure 2 In (d), the two longer sides of the groove are curved. Figure 2 (a) and Figure 2 In (b), the two long sides are arranged parallel to each other. Therefore, in the width direction of the negative electrode sheet, the distance from any point on one long side to the other long side is equal. In this case, the width of the groove is the distance from any point on one long side to the other long side in the length or width direction of the negative electrode sheet. Figure 2 In (c), the two long sides of the groove are straight lines, but not parallel. Therefore, the distance from any point on one long side to the other long side is not equal. In this case, the width of the groove can be taken as the average value. That is, on one long side, based on the length of that side, 50 points are selected at equal intervals (i.e., the distance between each point is equal, which makes the calculation result more accurate), the width corresponding to each point is measured, and the average value is taken to obtain the width of the groove; Figure 2 In (d), the two long sides are curves. Therefore, the distance from any point on one long side to the other long side is not equal. In this case, the width of the groove can also be taken as the average value, that is, 50 points are randomly selected on one long side (because...). Figure 2 In (d), the two longer sides are curves, and there is no... Figure 2 (c) Given the relationship between the two long sides, 50 points can be randomly selected for measurement. The width of each point is measured, and the average value is taken to obtain the width of the groove. The "width of the groove" can be obtained by conventional methods in the art, such as SEM.

[0079] In this invention, the spacing of the slots has a conventional meaning in the art, referring to the average distance between the two adjacent long sides of two adjacent slots in the length or width direction of the negative electrode sheet. For example... Figure 3 The diagram shown is a schematic representation of the slot spacing in an embodiment of the present invention, wherein... Figure 3 (a) represents the case where two adjacent long sides are straight lines and parallel. Figure 3 (b) is the case where two adjacent long sides are straight lines and not parallel. Figure 3 (c) represents the case where two adjacent long sides are curves. Figure 3 In (a), adjacent long sides are straight lines and parallel to each other. Therefore, in the width direction, the distance from any point on one long side to the other long side is equal. In this case, the slot spacing is the distance D from any point on one long side to the other long side in the width direction. Figure 3In (b), two adjacent long sides are straight lines, but not parallel. Therefore, the distance from any point on one long side to the other long side is not equal. In this case, the spacing of the slots can be calculated by averaging the values. That is, on one long side, using the length of that side as a reference, 50 points are selected at equal intervals (i.e., the distance between each point is equal, which makes the calculation results more accurate), and the width corresponding to each point is measured. The average value is then used to obtain the spacing. Figure 3 In (c), two adjacent long sides are curves. Therefore, the distance from any point on one long side to the other long side is not equal. In this case, the spacing of the slots can also be taken as the average value, that is, 50 points are randomly selected on one long side (because...). Figure 3 (c) has two long sides that are curves, therefore it does not exist. Figure 3 (b) Given the relationship between the two long sides, 50 points can be randomly selected for measurement. The width corresponding to each point is measured, and the average value is taken to obtain the spacing. The "slot spacing" can be tested using conventional methods in the art, such as SEM.

[0080] Setting recesses on the surface of the negative electrode active material layer can further improve the wettability of the electrolyte to the negative electrode sheet, and the specific size of the recesses can promote faster wettability of the electrolyte to the interior of the negative electrode, which is conducive to the diffusion and transport of lithium ions to the bottom layer of the negative electrode, reducing the polarization difference from the surface layer to the bottom layer of the negative electrode, thereby improving the fast charging performance and cycle performance of the negative electrode sheet.

[0081] In this invention, the areal density of the first negative electrode active material layer can be 4 mg / cm³. 2 -5mg / cm 2 For example, 4 mg / cm 2 4.2 mg / cm 2 4.5 mg / cm 2 4.8 mg / cm 2 Or 5mg / cm 2 The areal density of the second negative electrode active material layer can be 2.5 mg / cm³. 2 -3.5mg / cm 2 2.5 mg / cm 2 2.8 mg / cm 2 3mg / cm 2 3.2 mg / cm 2 Or 3.5 mg / cm 2 .

[0082] By adjusting the areal density of the first and second anode active material layers, the anode sheet can avoid excessive polarization and reduced fast-charging capability due to excessively high areal density, and also avoid insufficient energy density due to excessively low areal density. This allows the battery to balance energy density and fast-charging performance. Furthermore, the areal density of the first anode active material layer, located on the relatively surface, is higher than that of the second anode active material layer, located at the relatively bottom layer. This allows the active material in the first anode active material layer to accommodate as many lithium ions as possible during fast charging and conduct them to the bottom layer.

[0083] In one example, the areal density of the first negative electrode active material layer is greater than the areal density of the second negative electrode active material layer.

[0084] In this invention, the areal density of the first negative electrode active material layer and the areal density of the second negative electrode active material layer can be obtained by conventional testing methods in the art, such as punching the negative electrode sheet into a small disc and measuring its weight, with the total areal density being (weight of disc - weight of negative electrode current collector in disc) / disc area; subsequently, the thickness of the first active layer and the thickness of the second active layer can be measured by SEM of the cross-section of the negative electrode sheet, where the surface areal density is equal to the thickness of the first active layer / total thickness × total areal density.

[0085] In this invention, the porosity of the first negative electrode active material layer can be 30%-40%, for example, 30%, 32%, 35%, 38% or 40%. The porosity of the second negative electrode active material layer can be 25%-33%, for example, 25%, 28%, 30%, 32% or 33%.

[0086] In this invention, the compaction density of the negative electrode sheet can be 1.5 g / cm³. 3 -1.7g / cm 3 For example, 1.5g / cm³ 3 1.6g / cm 3 Or 1.7g / cm 3 .

[0087] By adjusting the compaction density of the negative electrode sheet and the porosity of the first and second negative electrode active material layers, the first negative electrode active material layer, located on the relatively surface, has a relatively high porosity. This facilitates the wettability of the electrolyte into the negative electrode sheet, thereby improving the rapid transport capability of lithium ions. Conversely, the second negative electrode active material layer, located at the relatively bottom layer, has a slightly lower porosity, which improves the space utilization of the bottom active material, thus increasing energy density. Furthermore, by adjusting the compaction density based on a specific porosity, the contact between the active material particles is ensured, promoting the formation of an effective and interconnected conductive network. Excessive compaction density will not lead to poor electrolyte wettability into the negative electrode sheet, hindering lithium ion transport and reducing the battery's fast-charging performance.

[0088] In this invention, the porosity of the first negative electrode active material layer and the porosity of the second negative electrode active material layer can be obtained by conventional testing methods in the art, such as mercury porosimetry.

[0089] In this invention, the compaction density of the negative electrode sheet can be obtained by conventional testing methods in the art, for example, compaction density = total areal density of the electrode sheet / electrode sheet thickness.

[0090] In this invention, the negative electrode current collector may include a metal layer and a base coating layer located on at least one side of the metal layer. The thickness of the base coating layer may be 0.5 μm to 2 μm, for example, 0.5 μm, 1 μm, 1.5 μm, or 2 μm. When the negative electrode current collector includes a base coating layer, the peel strength between the second negative electrode active material layer and the negative electrode current collector can be effectively improved, the adhesion between the second negative electrode active material layer and the negative electrode current collector can be enhanced, thereby improving the cycle performance of the battery. This invention does not limit the specific composition of the base coating layer; a base coating layer conventionally used in the art can be selected. For example, the base coating layer may include at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), conductive carbon black, graphene, and carbon nanotubes.

[0091] In one example, the metal layer comprises copper foil.

[0092] In this invention, the first negative electrode active material layer may further include a first binder, and the second negative electrode active material layer may further include a second binder. The first binder and the second binder each independently include at least one of styrene-butadiene rubber (SBR), polyacrylic acid, polyacrylate, polyvinyl alcohol, polyacrylonitrile, polyacrylamide, polymethacrylate, alginate, alginate, polytetrafluoroethylene (PTFE), and polyvinylidene fluoride (PVDF).

[0093] In one example, both the first and second adhesives comprise styrene-butadiene rubber (SBR). When both the first and second adhesives comprise SBR, the compatibility between the slurry of the first negative electrode active material layer and the slurry of the second negative electrode active material layer is optimal during coating. The two slurries will not experience edge shrinkage due to poor compatibility during coating, nor will there be mixing between the upper and lower layers. Furthermore, there will be no peeling between the first and second negative electrode active material layers after baking.

[0094] In this invention, the first negative electrode active material layer may further include a first conductive agent and a first thickener. The second negative electrode active material layer further includes a second thickener and a second conductive agent. The first conductive agent and the second conductive agent each independently include at least one selected from conductive carbon black (SP), Ketjen black, acetylene black, graphite conductive agents (KS-6, KS-15, SO, SEG-6), carbon fiber (VGCG), carbon nanotubes (CNT), and graphene. The first thickener and the second thickener each independently include at least one selected from carboxymethyl cellulose (CMC), lithium carboxymethyl cellulose (CMC-Li), and sodium carboxymethyl cellulose (CMC-Na).

[0095] In one example, based on the total weight of the first negative electrode active material layer, the weight content of the first negative electrode active material is 93%-99.7% (e.g., 93%, 94%, 95%, 96%, 97%, 98%, or 99.7%), the weight content of the first conductive agent is 0.1%-1% (e.g., 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1%), the weight content of the first binder is 0.1%-3% (e.g., 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, or 3%), and the weight content of the first thickener is 0.1%-3% (e.g., 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, or 3%).

[0096] In one example, based on the total weight of the second negative electrode active material layer, the weight content of the second negative electrode active material is 93%-99.7% (e.g., 93%, 94%, 95%, 96%, 97%, 98%, or 99.7%), the weight content of the second conductive agent is 0.1%-1% (e.g., 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1%), the weight content of the second binder is 0.1%-3% (e.g., 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, or 3%), and the weight content of the second thickener is 0.1%-3% (e.g., 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, or 3%).

[0097] In a second aspect of the present invention, a lithium ion secondary battery is provided, and the lithium ion secondary battery may include the negative electrode sheet described in the first aspect of the present invention.

[0098] In the present invention, the lithium ion secondary battery may further include a positive electrode sheet. The positive electrode sheet may include a positive electrode current collector and a positive electrode active material layer located on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material. The positive electrode active material may include a substance with the chemical formula Li a Ni x Co y Mn z A k O2, where 0.9 ≤ a ≤ 1.1 (for example, 0.9, 0.95, 1, 1.05 or 1.1), a 0.8 ≤ x ≤ 0.95 (for example, 0.8, 0.85, 0.9 or 0.95), 0 < y ≤ 0.2 (for example, 0.01, 0.05, 0.1, 0.15 or 0.2), 0 < z ≤ 0.2 (for example, 0.01, 0.05, 0.1, 0.15 or 0.2), 0 ≤ k ≤ 0.05 (for example, 0, 0.01, 0.02, 0.03, 0.04 or 0.05), and A is selected from at least one of Al, Zr, B, Y, Sr, W, Ti and Nb.

[0099] When the positive electrode active material satisfies the above conditions, the positive electrode sheet has a relatively high specific capacity, which is beneficial to improving the energy density of the battery.

[0100] In one example, 0.9 ≤ x ≤ 0.95.

[0101] In one example, the positive electrode active material has a layered structure.

[0102] In one example, the positive electrode active material includes single crystal particles and polycrystalline particles. When the positive electrode active material includes a combination of single crystal particles and polycrystalline particles, a better effect is achieved. The ion conduction performance of single crystal particles is poor, but the contact area with the electrolyte is small, the side reactions are few, and the structure is stable; while the ion conduction performance of polycrystalline particles is excellent, but the larger contact area with the electrolyte leads to more side reactions. Therefore, combining the two can integrate the advantages of single crystal particles and polycrystalline particles.

[0103] In one example, the mass ratio of the polycrystalline particles to the monocrystalline particles is (2.3-4):1 (e.g., 2.3:1, 2.5:1, 3:1, 3.5:1, or 4:1). When the cathode material meets the above conditions, the cathode sheet can have good cycle and rate performance. Excessive monocrystalline material will not lead to insufficient high-rate charge / discharge performance of the battery, nor will excessive polycrystalline material cause excessive side reactions at the cathode interface that affect cycle performance. When the cathode material meets a specific mass ratio of monocrystalline to polycrystalline material, it is beneficial to improve the cycle and rate performance of the battery.

[0104] In one example, the positive electrode active material layer further includes a positive electrode conductive agent and a positive electrode binder. The positive electrode conductive agent includes at least one selected from SP, Ketjen Black, acetylene black, graphite conductive agents (KS-6, KS-15, SO, SEG-6), carbon fiber (VGCG), carbon nanotubes (CNT), and graphene. The positive electrode binder includes at least one selected from PVDF, PVDF-HFP, polytetrafluoroethylene, polyacrylonitrile, polyimide, and perfluorosulfonic acid ionomer binders.

[0105] In this invention, the positive electrode sheet has at least one surface with a plurality of recesses and at least one surface with a plurality of protrusions. "A plurality of" means that the number of recesses is greater than or equal to 2 and the number of protrusions is greater than or equal to 2.

[0106] In one example, one side surface of the positive electrode has a plurality of said recesses, and the other side surface has a plurality of said protrusions.

[0107] The recessed portion on one side of the positive electrode sheet and the protruding portion on the other side can be obtained through an embossing process. These recessed and protruding portions on both sides of the positive electrode sheet increase its specific surface area, promoting the diffusion and transport of lithium ions into the sheet, thereby reducing internal polarization, increasing the electrochemical reaction rate, and improving the power performance of the positive electrode and the cycle performance of the battery. Furthermore, when the negative electrode sheet has a recessed portion and the positive electrode sheet has both recessed and protruding portions on its two sides, the battery exhibits even better performance. This is because embossing on the negative electrode sheet can easily cause excessive compression of the particles in the embossed area, leading to breakage of the active material particles and a shortened cycle life. Conversely, drilling on the positive electrode sheet can easily consume the positive electrode active material, causing a decrease in battery capacity and energy density.

[0108] In this invention, the width of the recessed portion can be 0.5mm-50mm, for example, 0.5mm, 1mm, 5mm, 10mm, 20mm, 30mm, 40mm, or 50mm. The spacing between the recessed portions can be 1mm-10mm, for example, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, or 10mm. The depth of the recessed portion can be 10μm-80μm, for example, 10μm, 20μm, 30μm, 40μm, 50μm, 60μm, 70μm, or 80μm.

[0109] In this invention, the width of the recess, the spacing between the recesses, and the depth of the recess have conventional meanings in the art. The width of the recess refers to: when the orthographic projection of the recess onto the surface of the positive electrode is a regular circle, the width of the recess is the diameter of the circle; when the orthographic projection of the recess onto the surface of the positive electrode is an irregular circle, the width of the recess is the equivalent diameter of a regular circle with an area equal to that of the irregular circle. The spacing between the recesses refers to: the shortest distance between the orthographic projections of two adjacent recesses onto the surface of the positive electrode; the depth of the recess refers to the maximum vertical distance from any point within the recess to the surface of the positive electrode.

[0110] In this invention, the width of the orthographic projection of the protrusion onto the surface of the positive electrode sheet can be 0.5mm-50mm, for example, 0.5mm, 1mm, 5mm, 10mm, 20mm, 30mm, 40mm, or 50mm. The distance between the orthographic projections of two adjacent protrusions onto the surface of the positive electrode sheet can be 1mm-10mm, for example, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, or 10mm. The height of the protrusion can be 10μm-80μm, for example, 10μm, 20μm, 30μm, 40μm, 50μm, 60μm, 70μm, or 80μm.

[0111] In this invention, the height of the protrusion, the width of the orthographic projection of the protrusion onto the surface of the positive electrode, and the distance between the orthographic projections of two adjacent protrusions onto the surface of the positive electrode have conventional meanings in the art. The height of the protrusion refers to the maximum vertical distance from any point on the protrusion to the surface of the positive electrode. The width of the orthographic projection of the protrusion onto the surface of the positive electrode refers to the following: when the orthographic projection of the protrusion onto the surface of the positive electrode is a regular circle, the width of the protrusion is the diameter of the circle; when the orthographic projection of the protrusion onto the surface of the positive electrode is an irregular circle, the width of the protrusion is the equivalent diameter of a regular circle with an area equal to that of the irregular circle. The distance between the orthographic projections of two adjacent protrusions onto the surface of the positive electrode refers to the shortest distance between the orthographic projections of two adjacent protrusions onto the surface of the positive electrode.

[0112] Specific sizes of recesses and protrusions can improve the rate performance of a battery. If the volume of the recesses and / or protrusions is too small, the effect of the large specific surface area of ​​the positive electrode is not significant, and the lithium-ion transport effect cannot be effectively improved. If the volume of the recesses and / or protrusions is too large, it will reduce the adhesion strength between the positive electrode and the separator, and will instead increase the interfacial polarization between the positive electrode and the separator, which is also not conducive to lithium-ion transport.

[0113] In one example, the recessed portion is positioned on one side of the positive electrode surface and the protrusion on the other side corresponds one-to-one.

[0114] In one example, the CB value of the lithium-ion secondary battery is 1.1-1.2 (e.g., 1.1, 1.12, 1.15, 1.18, or 1.2). The CB value is the ratio of the theoretical capacity of the negative electrode to the theoretical capacity of the positive electrode. By controlling the CB value within the above range, the battery will not have insufficient fast charging capability due to a CB value that is too low, nor will it have low energy density due to a CB value that is too high.

[0115] In one example, the CB value of the lithium-ion secondary battery is 1.11-1.13, for example, 1.11, 1.12 or 1.13.

[0116] When the CB value is within a certain range, the battery will not have insufficient fast charging capability due to a CB value that is too low, nor will it have low energy density due to a CB value that is too high.

[0117] In this invention, the lithium-ion secondary battery may further include an electrolyte. The electrolyte may include a lithium salt and an organic solvent. The lithium salt includes at least one selected from lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, lithium difluorophosphate (LiPO2F2), lithium bis(oxalato)borate (LiBOB), lithium difluorooxalato)borate (LiDFOB), lithium tetrafluoroborate (LiBF4), lithium difluorodioxalato)borate (LiDFOP), and lithium bis(trifluoromethanesulfonyl)imide. The organic solvent may include at least one selected from ethylene carbonate (EC), fluoroethylene carbonate (FEC), propylene carbonate (PC), ethyl methyl carbonate (EMC), dimethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, diethyl carbonate, diethyl carbonate (DMC), ethyl formate, ethyl acetate (EA), ethyl propionate (EP), and propyl propionate (PP).

[0118] In one example, the lithium salt has a weight content of 10%-15% (e.g., 10%, 11%, 12%, 13%, 14% or 15%) based on the total weight of the electrolyte, and the organic solvent has a weight content of 85%-90% (e.g., 85%, 86%, 87%, 88%, 89% or 90%).

[0119] The present invention will be described in detail below through embodiments. The embodiments described herein are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0120] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0121] The following examples illustrate the present invention.

[0122] Example 1

[0123] Prepared according to the following method:

[0124] (1) Preparation of negative electrode sheet

[0125] Artificial graphite (93% graphitization, average particle size 15.2 μm, specific surface area 1.46 m²) was used. 2The following materials were mixed in a mass ratio of 87.3:9.7:0.3:0.9:0.3:1.5: silicon-carbon material (with a silicon to carbon mass ratio of 1:1 and an average particle size of 9.5 μm), lithium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, and conductive carbon black (SP). A uniformly dispersed mixture was obtained by high-speed stirring. Deionized water was used as a solvent to prepare the first negative electrode active slurry (solid content of 50 wt%). Natural graphite (graphitization degree of 98%, average particle size of 10.5 μm, specific surface area of ​​1.04 m²) was then added. 2 The following materials were mixed in a mass ratio of 87.3:9.7:0.3:0.9:0.3:1.5: silicon-carbon material (with a silicon-to-carbon mass ratio of 1:1 and an average particle size of 9.5 μm), lithium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, and conductive carbon black (SP) at a mass ratio of 87.3:9.7:0.3:0.9:0.3:1.5. The mixture was stirred at high speed to obtain a uniformly dispersed mixture. Deionized water was used as a solvent to prepare the second negative electrode active slurry (solid content of 50 wt%). The first and second negative electrode active slurries were simultaneously and uniformly sprayed onto the negative electrode current collector (a 5 μm thick copper foil) and a 1 μm thick base coating (containing conductive carbon black and SBR at a mass ratio of 87.3:9.7:0.3:0.9:0.3:1.5) on both sides of the outer surface of the copper foil. Two wet films are formed by mixing materials in a 1:1 ratio (the first negative electrode active slurry is on the surface, and the second negative electrode active slurry is on the bottom layer). After baking, a single-sided negative electrode sheet is obtained. Then, the other side is coated in the same way, baked, rolled, and then holes are made on the outer surfaces of both sides (the whole surface) using a laser (the depth of the holes is 22μm, the diameter of the holes is 215μm, and the spacing is 3.2mm) to obtain the negative electrode sheet. The average particle size ratio of artificial graphite (the first graphite material) to silicon carbon is 1.6:1, and the average particle size ratio of natural graphite (the second graphite material) to silicon carbon is 1.1:1. The thickness of the first negative electrode active material layer is 45.5μm, and the thickness of the second negative electrode active material layer is 30.6μm.

[0126] (2) Preparation of positive electrode sheet

[0127] The positive electrode active material (with a layered structure and polycrystalline particles, chemical formula LiNi) 0.92 Co 0.04 Mn 0.03 Al 0.01 O2, the chemical formula of single crystal particles is LiNi 0.92 Co 0.05 Mn 0.02 Al 0.01O2 (polycrystalline particles to single-crystal particles in a mass ratio of 3:1), polyvinylidene fluoride (PVDF) and conductive carbon black (SP) in a mass ratio of 98:1:1 were mixed and stirred at high speed to obtain a uniformly dispersed mixture. A positive electrode active material slurry (70 wt% solid content) was prepared using N-methylpyrrolidone (NMP) as a solvent. The positive electrode active material slurry was uniformly coated on both sides of an aluminum foil. After drying, rolling, and embossing (on the entire surface), the positive electrode sheet was obtained (the width of the recesses was 25 mm, the spacing was 6 mm, and the depth was 50 μm; the width of the projection of the protrusions onto the positive electrode surface was 25 mm, the spacing between two adjacent protrusions was 6 mm, and the height of the protrusions was 50 μm; the position of the recesses on one side of the positive electrode sheet corresponded one-to-one with the position of the protrusions on the other side).

[0128] (3) Preparation of electrolyte

[0129] The lithium salt in the electrolyte is lithium hexafluorophosphate with a concentration of 1 mol / L. Ethyl carbonate and diethyl carbonate are mixed at a mass ratio of 1:1, and then fluoroethylene carbonate (with a mass content of 10% in the electrolyte) is added.

[0130] (4) Battery fabrication

[0131] The negative electrode sheet and separator (polyethylene film) prepared in step (1) and the positive electrode sheet prepared in step (2) are wound together to obtain a core. After welding, encapsulation, electrolyte injection, formation and sorting, a battery is obtained. The CB value is 1.12, d1 is 0-30μm, d2 is 0-20μm, d3 is 0-36μm and d4 is 0-30μm.

[0132] Example 2

[0133] The process was carried out in accordance with Example 1, except that the preparation of the positive and negative electrode sheets was as follows:

[0134] (1) Preparation of negative electrode sheet

[0135] Artificial graphite (92% graphitization, average particle size 13.1 μm, specific surface area 1.55 m²) was used. 2 The following materials were mixed in a mass ratio of 96.9:0.1:0.3:0.9:0.3:1.5: silicon-carbon material (with a silicon to carbon mass ratio of 0.9:1 and an average particle size of 10 μm), lithium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, and conductive carbon black (SP). A uniformly dispersed mixture was obtained by high-speed stirring. Deionized water was used as a solvent to prepare the first negative electrode active slurry (solid content of 50 wt%). Natural graphite (graphitization degree of 96%, average particle size of 9.2 μm, specific surface area of ​​1.15 m²) was then added. 2The following materials were mixed in a mass ratio of 96.9:0.1:0.3:0.9:0.3:1.5: silicon-carbon material (with a silicon to carbon mass ratio of 0.9:1 and an average particle size of 10 μm), lithium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, and conductive carbon black (SP) to obtain a uniformly dispersed mixture by high-speed stirring. A second negative electrode active slurry (solid content of 50 wt%) was prepared using deionized water as a solvent. The first and second negative electrode active slurries were simultaneously and uniformly sprayed onto a 5 μm thick copper foil and a 1 μm thick base coating (containing conductive carbon black and SBR in a mass ratio of 96.9:0.1:0.3:0.9:0.3:1.5) on both sides of the outer surface of the copper foil. A 1:1 mixture is applied to the substrate (the first negative electrode active slurry is on the surface, and the second negative electrode active slurry is on the bottom layer) to form two wet films. After baking, a single-sided negative electrode sheet is obtained. Then, the other side is coated in the same way, baked, rolled, and then holes are made on the outer surfaces of both sides (the entire surface) using a laser (the depth of the holes is 3μm, the diameter of the holes is 53μm, and the spacing is 402μm) to obtain the negative electrode sheet. The average particle size ratio of artificial graphite (the first graphite material) to silicon carbon is 1.31:1, and the average particle size ratio of natural graphite (the second graphite material) to silicon carbon is 0.92:1. The thickness of the first negative electrode active material layer is 43.7μm, and the thickness of the second negative electrode active material layer is 29.7μm.

[0136] (2) Preparation of positive electrode sheet

[0137] The positive electrode active material (with a layered structure and polycrystalline particles, chemical formula LiNi) 0.92 Co 0.04 Mn 0.03 Al 0.01 O2, the chemical formula of single crystal particles is LiNi 0.92 Co 0.05 Mn 0.02 Al 0.01 O2 (polycrystalline particles to monocrystalline particles in a mass ratio of 2.3:1), polyvinylidene fluoride (PVDF), and conductive carbon black (SP) in a mass ratio of 98:1:1 were mixed and stirred at high speed to obtain a uniformly dispersed mixture. A positive electrode active material slurry (70 wt% solid content) was prepared using N-methylpyrrolidone (NMP) as a solvent. The positive electrode active material slurry was uniformly coated on both sides of an aluminum foil, and after drying, rolling, and embossing (recesses 0.5 mm wide, spacing...) The diameter of the protrusion is 1 mm and the depth is 10 μm; the width of the protrusion projected onto the surface of the positive electrode is 0.5 mm, the distance between the projections of two adjacent protrusions onto the surface of the positive electrode is 1 mm, and the height of the protrusion is 10 μm; the position of the recess on one side of the surface of the positive electrode corresponds one-to-one with the position of the protrusion on the other side, thus obtaining the positive electrode, wherein the CB value is 1.12, d1 is 0-30 μm, d2 is 0-20 μm, d3 is 0-36 μm, and d4 is 0-30 μm.

[0138] Example 3

[0139] The process was carried out in accordance with Example 1, except that the preparation of the positive and negative electrode sheets was as follows:

[0140] (1) Preparation of negative electrode sheet

[0141] Artificial graphite (94% graphitization, average particle size 16 μm, specific surface area 1.37 m²) was used. 2 The following materials were mixed in a mass ratio of 82.45:14.55:0.3:0.9:0.3:1.5: silicon-carbon material (with a silicon to carbon mass ratio of 1.1:1 and an average particle size of 6.1 μm), lithium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, and conductive carbon black (SP). A uniformly dispersed mixture was obtained by high-speed stirring. Deionized water was used as the solvent to prepare the first negative electrode active slurry (solid content of 50 wt%). Natural graphite (graphitization degree of 95%, average particle size of 12.1 μm, specific surface area of ​​0.97 m²) was then added. 2 The following materials were mixed in a mass ratio of 82.45:14.55:0.3:0.9:0.3:1.5: silicon-carbon material (with a silicon-to-carbon mass ratio of 1.1:1 and an average particle size of 6.1 μm), lithium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, and conductive carbon black (SP) at a mass ratio of 82.45:14.55:0.3:0.9:0.3:1.5. The mixture was stirred at high speed to obtain a uniformly dispersed mixture. Deionized water was used as a solvent to prepare the second negative electrode active slurry (solid content of 50 wt%). The first and second negative electrode active slurries were simultaneously and uniformly sprayed onto the negative electrode current collector (a 5 μm thick copper foil) and a 1 μm thick base coating (containing conductive carbon black and SBR at a mass ratio of 82.45:14.55:0.3:0.9:0.3:1.5). Two wet films are formed by mixing the first and second negative electrode active materials (1:1 ratio) on a substrate (the first negative electrode active material is on the surface, and the second negative electrode active material is on the bottom layer). After baking, a single-sided negative electrode sheet is obtained. The other side is then coated in the same way, baked, rolled, and then holes are made on the outer surfaces of both sides (the entire surface) using a laser (the depth of the holes is 40 μm, the diameter of the holes is 499 μm, and the spacing is 5.1 mm) to obtain the negative electrode sheet. The average particle size ratio of artificial graphite (the first graphite material) to silicon carbon is 2.62:1, and the average particle size ratio of natural graphite (the second graphite material) to silicon carbon is 1.98:1. The thickness of the first negative electrode active material layer is 46.2 μm, and the thickness of the second negative electrode active material layer is 32.5 μm.

[0142] (2) Preparation of positive electrode sheet

[0143] The positive electrode active material (with a layered structure and polycrystalline particles, chemical formula LiNi) 0.92 Co 0.04 Mn 0.03 Al0.01 O2, the chemical formula of single crystal particles is LiNi 0.92 Co 0.05 Mn 0.02 Al 0.01 O2 (polycrystalline particles to monocrystalline particles in a mass ratio of 4:1), polyvinylidene fluoride (PVDF), and conductive carbon black (SP) in a mass ratio of 98:1:1 were mixed and stirred at high speed to obtain a uniformly dispersed mixture. A positive electrode active material slurry (70 wt% solid content) was prepared using N-methylpyrrolidone (NMP) as a solvent. The positive electrode active material slurry was uniformly coated on both sides of an aluminum foil, and then dried, rolled, and embossed (the width of the recesses was 50 mm, and the spacing was 1 mm). The diameter of the protrusion is 0 mm, the depth is 80 μm; the width of the protrusion projected onto the surface of the positive electrode is 50 mm, the distance between the projections of two adjacent protrusions onto the surface of the positive electrode is 10 mm, and the height of the protrusion is 80 μm; the position of the recess on one side of the surface of the positive electrode corresponds one-to-one with the position of the protrusion on the other side, thus obtaining the positive electrode, wherein the CB value is 1.12, d1 is 0-30 μm, d2 is 0-20 μm, d3 is 0-36 μm, and d4 is 0-30 μm.

[0144] Example 4 group

[0145] This set of examples is used to verify the impact of changes in the "average particle size of the first graphite material".

[0146] This set of embodiments is based on Embodiment 1, except that the average particle size of the first graphite material is changed, as follows:

[0147] Example 4a: The artificial graphite had an average particle size of 10.3 μm and a specific surface area of ​​1.58 m². 2 / g, the average particle size ratio of artificial graphite (first graphite material) to silicon carbon is 1.08:1 (at the same time, in order to control d1 to 0-30μm, d2 to 0-20μm, d3 to 0-36μm, d4 to 0-30μm, the thickness of the first negative electrode active material layer is adjusted to 40.5μm).

[0148] Example 4b: The artificial graphite had an average particle size of 10 μm and a specific surface area of ​​1.31 m². 2 / g, the average particle size ratio of artificial graphite (first graphite material) to silicon carbon is 2.11:1 (at the same time, in order to control d1 to 0-30μm, d2 to 0-20μm, d3 to 0-36μm, d4 to 0-30μm, the thickness of the first negative electrode active material layer is adjusted to 50.3μm).

[0149] Example 5 group

[0150] This set of examples is used to verify the impact of changes in the "average particle size of the second graphite material".

[0151] This set of embodiments is based on Embodiment 1, except that the average particle size of the second graphite material is changed, as follows:

[0152] Example 5a: The natural graphite had an average particle size of 5.2 μm and a specific surface area of ​​1.19 m². 2 / g, the average particle size ratio of natural graphite (second graphite material) to silicon carbon is 0.55:1 (at the same time, in order to control d1 to 0-30μm, d2 to 0-20μm, d3 to 0-36μm, d4 to 0-30μm, the thickness of the second negative electrode active material layer is adjusted to 26μm).

[0153] Example 5b: The average particle size of the natural graphite was 16 μm, and the specific surface area was 0.91 m². 2 / g, the average particle size ratio of natural graphite (second graphite material) to silicon carbon is 1.68:1 (at the same time, in order to control d1 to 0-30μm, d2 to 0-20μm, d3 to 0-36μm, d4 to 0-30μm, the thickness of the second negative electrode active material layer is adjusted to 36.4μm).

[0154] Example 6 group

[0155] This set of examples is used to verify the impact of changes to "d1 and d3 or d2 and d4".

[0156] This set of embodiments refers to Embodiment 1, except that d1 and d3 or d2 and d4 are changed by adjusting the thickness of the first negative electrode active material layer or the thickness of the second negative electrode active material layer, as detailed below:

[0157] In Example 6a, the thickness of the first negative electrode active material layer was adjusted to 50.5 μm, d1 was 0-35 μm, d2 was 0-20 μm, d3 was 0-40 μm, and d4 was 0-30 μm.

[0158] In Example 6b, the thickness of the second negative electrode active material layer was adjusted to 37.2 μm, with d1 being 0-30 μm, d2 being 0-26 μm, d3 being 0-36 μm, and d4 being 0-35 μm.

[0159] Example 7 group

[0160] This set of embodiments is used to verify the impact of changes to the "first graphite material and / or the second graphite material".

[0161] This set of embodiments is based on Embodiment 1, except that the first graphite material and / or the second graphite material are changed, as follows:

[0162] Example 7a: The artificial graphite was replaced with natural graphite (98% graphitization) with the same average particle size.

[0163] Example 7b: Natural graphite was replaced with artificial graphite (93% graphitization) with the same average particle size.

[0164] Example 7c: The artificial graphite was replaced with natural graphite (98% graphitization) with the same average particle size, and the natural graphite was replaced with artificial graphite (93% graphitization) with the same average particle size.

[0165] Example 8 group

[0166] This set of embodiments is used to verify the impact of changes in "the mass ratio of the first silicon-based material in the first silicon-based material and the first graphite material and the mass ratio of the second silicon-based material in the second silicon-based material and the second graphite material".

[0167] This set of embodiments refers to Embodiment 1, except that the mass ratio of the first silicon-based material in the first silicon-based material and the first graphite material, and the mass ratio of the second silicon-based material in the second silicon-based material and the second graphite material are as follows:

[0168] Example 8a: Artificial graphite (graphitization degree 93%, average particle size 15.2 μm, specific surface area 1.46 m²) 2 / g), silicon-carbon material (the mass ratio of elemental silicon to elemental carbon in the silicon-carbon material is 1:1, and the average particle size is 9.5μm), lithium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, and conductive carbon black (SP) in a mass ratio of 96.99:0.01:0.3:0.9:0.3:1.5; natural graphite (graphitization degree is 98%, average particle size is 10.5μm, specific surface area is 1.04m²). 2 / g), silicon-carbon material (the mass ratio of elemental silicon to elemental carbon in the silicon-carbon material is 1:1, and the average particle size is 9.5μm), lithium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid and conductive carbon black (SP) in a mass ratio of 96.99:0.01:0.3:0.9:0.3:1.5;

[0169] Example 8b: Artificial graphite (graphitization degree 93%, average particle size 15.2 μm, specific surface area 1.46 m²) 2 The composition of silicon-carbon material (with a silicon to carbon mass ratio of 1:1 and an average particle size of 9.5 μm), lithium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, and conductive carbon black (SP) is in a mass ratio of 77.6:19.4:0.3:0.9:0.3:1.5; natural graphite (98% graphitization, average particle size of 10.5 μm, and specific surface area of ​​1.04 m²) is also present. 2 / g), silicon-carbon material (the mass ratio of elemental silicon to elemental carbon in the silicon-carbon material is 1:1, and the average particle size is 9.5μm), lithium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid and conductive carbon black (SP) in a mass ratio of 77.6:19.4:0.3:0.9:0.3:1.5.

[0170] Example 9 group

[0171] This set of embodiments is used to verify the impact of changes to the "first silicon-based material and the second silicon-based material".

[0172] This set of embodiments refers to Embodiments 1, 2 and 3 respectively, except that the first silicon-based material and the second silicon-based material are changed, as follows:

[0173] Example 9a was carried out with reference to Example 1, except that silicon carbon was replaced with the same mass of silicon-oxygen material, wherein the mass ratio of elemental silicon to elemental oxygen in the silicon-oxygen material was 0.96:1, the average particle size was 10.2 μm, the average particle size ratio of artificial graphite (first graphite material) to silicon-oxygen was 1.5:1, and the average particle size ratio of natural graphite (second graphite material) to silicon-oxygen was 1:1;

[0174] Example 9b was carried out with reference to Example 2, except that silicon carbon was replaced with the same mass of silicon-oxygen material, wherein the mass ratio of elemental silicon to elemental oxygen in the silicon-oxygen material was 0.9:1, the average particle size was 11.8 μm, the average particle size ratio of artificial graphite (first graphite material) to silicon-oxygen was 1.1:1, and the average particle size ratio of natural graphite (second graphite material) to silicon-oxygen was 0.8:1;

[0175] Example 9c was carried out with reference to Example 3, except that silicon carbon was replaced with the same mass of silicon-oxygen material, wherein the mass ratio of elemental silicon to elemental oxygen in the silicon-oxygen material was 1:1, the average particle size was 7 μm, the average particle size ratio of artificial graphite (first graphite material) to silicon-oxygen was 2.3:1, and the average particle size ratio of natural graphite (second graphite material) to silicon-oxygen was 1.7:1.

[0176] Example 10 group

[0177] This set of examples is used to verify the impact of changing the "mass ratio of elemental silicon to elemental carbon in silicon-carbon materials".

[0178] This set of embodiments is based on Embodiment 1, except that the mass ratio of elemental silicon to elemental carbon in the silicon-carbon material is changed, as follows:

[0179] In Example 10a, the mass ratio of elemental silicon to elemental carbon in the silicon-carbon material is 0.82:1;

[0180] In Example 10b, the mass ratio of elemental silicon to elemental carbon in the silicon-carbon material is 1.22:1.

[0181] Example 11 group

[0182] This set of examples is used to verify the impact of changing the "mass ratio of elemental silicon to elemental oxygen in silicon-oxygen materials".

[0183] This set of embodiments is based on Embodiment 1, except that the mass ratio of elemental silicon to elemental oxygen in the silicon-oxygen material is changed, as follows:

[0184] In Example 11a, the mass ratio of elemental silicon to elemental oxygen in the silicon-carbon material is 0.8:1;

[0185] In Example 11b, the mass ratio of elemental silicon to elemental oxygen in the silicon-carbon material is 1.2:1.

[0186] Example 12

[0187] Used to verify the effects of changes to the "first adhesive and second adhesive".

[0188] The procedure was carried out in accordance with Example 1, except that the first adhesive was replaced with polyacrylic acid of the same mass, and the second adhesive was replaced with polyacrylic acid of the same mass.

[0189] Example 13

[0190] This was used to verify the impact of changes to the "positive electrode active material".

[0191] The procedure was carried out in accordance with Example 1, except that the positive electrode active material was replaced with single-crystal particles of the same mass, wherein the chemical formula of the single-crystal particles was LiNi. 0.6 Co 0.18 Mn 0.21 Al 0.01 O2.

[0192] Example 14

[0193] This is used to verify the effects of changes to the "concave portion of the negative electrode surface and the concave and convex portions of the positive electrode surface".

[0194] The procedure is carried out in accordance with Example 1, except that no recesses are made on the surface of the negative electrode, and no embossing is performed on the surface of the positive electrode.

[0195] Example 15

[0196] Used to verify the impact of changes to the "CB value".

[0197] The same procedure was followed as in Example 1, except that the CB value was adjusted by changing the areal density of the positive and negative electrodes to make the CB value 1.16.

[0198] The above embodiments satisfy the following condition: the areal density of the first negative electrode active material layer is 4 mg / cm³. 2 -5mg / cm 2 The areal density of the second negative electrode active material layer is 2.5 mg / cm³. 2 -3.5mg / cm 2 The porosity of the first negative electrode active material layer is 30%-40%, and the porosity of the second negative electrode active material layer is 25%-33%; the compaction density of the negative electrode sheet is 1.5 g / cm³. 3 -1.7g / cm 3 .

[0199] Comparative Example 1

[0200] The procedure was carried out in accordance with Example 1, except that the negative electrode was prepared as follows:

[0201] Artificial graphite (93% graphitization, average particle size 15.2 μm, specific surface area 1.46 m²) was used. 2 The following materials were mixed in a mass ratio of 87.3:9.7:0.3:0.9:0.3:1.5: silicon-carbon material (with a mass ratio of elemental silicon to elemental carbon of 1:1 and an average particle size of 9.5 μm), lithium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, and conductive carbon black (SP). The mixture was stirred at high speed to obtain a uniformly dispersed mixture. Deionized water was used as a solvent to prepare a negative electrode active slurry (with a solid content of 50 wt%). The negative electrode active slurry was uniformly sprayed onto a negative electrode current collector (a copper foil with a thickness of 5 μm and a base coating with a thickness of 1 μm on both sides of the outer surface of the copper foil, which included conductive carbon black and SBR mixed in a mass ratio of 1:1). After baking and rolling, holes were made on both sides of the outer surface (the entire surface) using a laser (the depth of the holes was 22 μm, the diameter of the holes was 215 μm, and the spacing was 3.2 mm) to obtain the negative electrode sheet.

[0202] Comparative Example 2

[0203] The procedure was carried out in accordance with Example 1, except that the negative electrode was prepared as follows:

[0204] Natural graphite (98% graphitization, average particle size 10.5 μm, specific surface area 1.04 m²) was used. 2The following materials were mixed in a mass ratio of 87.3:9.7:0.3:0.9:0.3:1.5: silicon-carbon material (with a mass ratio of elemental silicon to elemental carbon of 1:1 and an average particle size of 9.5 μm), lithium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, and conductive carbon black (SP). The mixture was stirred at high speed to obtain a uniformly dispersed mixture. Deionized water was used as a solvent to prepare a negative electrode active slurry (with a solid content of 50 wt%). The negative electrode active slurry was uniformly sprayed onto a negative electrode current collector (a copper foil with a thickness of 5 μm and a base coating with a thickness of 1 μm on both sides of the outer surface of the copper foil, which included conductive carbon black and SBR mixed in a mass ratio of 1:1). After baking and rolling, holes were made on both sides of the outer surface (the entire surface) using a laser (the depth of the holes was 22 μm, the diameter of the holes was 215 μm, and the spacing was 3.2 mm) to obtain the negative electrode sheet.

[0205] Comparative Example 3

[0206] The same procedure was followed as in Example 1, except that d1 and d3 were changed by adjusting the thickness of the first negative electrode active material layer. Specifically, the thickness of the first negative electrode active material layer was adjusted to 55 μm, d1 was 0-38.6 μm, d2 was 0-20 μm, d3 was 0-43.7 μm, and d4 was 0-30 μm.

[0207] Comparative Example 4

[0208] The same procedure was followed as in Example 1, except that d2 and d4 were changed by adjusting the thickness of the second negative electrode active material layer. Specifically, the thickness of the second negative electrode active material layer was adjusted to 40 μm, d1 was 0-30 μm, d2 was 0-28.7 μm, d3 was 0-36 μm, and d4 was 0-39 μm.

[0209] Test case

[0210] (1) Room temperature cycling test

[0211] The batteries prepared in the examples and comparative examples were subjected to room temperature cycling tests. The specific test methods are as follows:

[0212] At 25℃, within a charge / discharge window of 4.2V to 2.5V, 1C / 1C charge / discharge cycles were performed. The test process was as follows: first, 1C constant current charging to 4.2V, then constant voltage charging with a cutoff current of 0.05C, and finally 1C constant current discharging to 2.5V. This cycle was repeated. The number of times the ratio of discharge capacity to the first discharge capacity (capacity retention rate) reached 80% is recorded in Table 1.

[0213] The ratio of the battery thickness at 50% SOC at the end of the cycle to the thickness in the initial state (cycle expansion rate) is recorded in Table 1.

[0214] (2) Fast charging performance test

[0215] The batteries prepared in the examples and comparative examples were subjected to fast charging performance tests. The specific test methods are as follows:

[0216] At room temperature, the battery was charged to 4.2V at nC (n = 1, 2, 3, 4, and 5), then constant voltage charging was applied with a cutoff current of 0.05C, followed by resting for 30 minutes. The battery was then discharged to 2.5V at 1C and rested for 30 minutes. This charge-discharge cycle was repeated 20 times. After fully charging the battery, it was dissected to observe whether lithium plating occurred on the negative electrode side. The maximum rate at which lithium plating did not occur was the battery's fast-charging window. The results are recorded in Table 1. The temperature change of the battery during charging was recorded, and the maximum value was the battery's highest temperature. The lower the highest temperature, the better the battery's fast-charging performance. The results are recorded in Table 1.

[0217] (3) Energy density test

[0218] The energy density of the batteries prepared in the examples and comparative examples was tested, and the specific testing methods are as follows:

[0219] At 25°C, the battery is charged at a constant current and constant voltage of 0.33C to the upper limit voltage (4.2V), and then discharged at 0.33C to the lower limit voltage (2.5V). The energy released by the battery can be measured at this time. The battery gravimetric energy density (WED) is calculated as follows: WED = energy / battery weight. The results are recorded in Table 1.

[0220] Table 1

[0221]

[0222]

[0223] As shown in Table 1, the battery prepared using the negative electrode sheet of the present invention, compared with the comparative example, has a longer cycle count, lower cycle expansion rate, higher fast charging window, lower maximum temperature, and higher energy density. The battery incorporating the negative electrode sheet of the present invention can achieve a balance between high energy density, fast charging capability, and superior cycle stability.

[0224] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A negative electrode sheet characterized by comprising: The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer located on at least one side surface of the negative electrode current collector; the negative electrode active material layer comprises a first negative electrode active material layer and a second negative electrode active material layer stacked in the thickness direction of the negative electrode sheet, the first negative electrode active material layer faces away from the negative electrode current collector, and the second negative electrode active material layer is close to the negative electrode current collector; The first negative electrode active material layer comprises a first graphite material, and the average particle size of the first graphite material is 10 μm-20 μm; the second negative electrode active material layer comprises a second graphite material, and the average particle size of the second graphite material is 5 μm-16 μm; When the battery comprising the negative electrode sheet is in a 0% SOC state, the vertical distance from the surface of the first graphite material to the contact surface of the first negative electrode active material layer and the second negative electrode active material layer is d1, and d1 is 0 μm-35 μm; the vertical distance from the surface of the second graphite material to the negative electrode current collector is d2, and d2 is 0 μm-26 μm.

2. The negative electrode sheet according to claim 1, wherein The average particle size of the first graphite material is 13 μm-16 μm, and the average particle size of the second graphite material is 6 μm-13 μm; And / or, d1 is 0 μm-30 μm; d2 is 0 μm-20 μm; And / or, when the battery comprising the negative electrode sheet is in a 100% SOC state, the vertical distance from the surface of the first graphite material to the contact surface of the first negative electrode active material layer and the second negative electrode active material layer is d3, and d3 is 0 μm-40 μm; the vertical distance from the surface of the second graphite material to the negative electrode current collector is d4, and d4 is 0 μm-35 μm; Preferably, d3 is 0 μm-36 μm, and d4 is 0 μm-30 μm.

3. The negative electrode sheet according to claim 1 or 2, wherein The first negative electrode active material layer further comprises a first silicon-based material, and the second negative electrode active material layer further comprises a second silicon-based material; Preferably, the mass ratio of the first silicon-based material to the first graphite material is 0.01%-20%; more preferably, 0.1%-15%. Preferably, the mass ratio of the second silicon-based material to the second graphite material is 0.01%-20%; more preferably, 0.1%-15%.

4. The negative electrode sheet according to claim 3, wherein The first silicon-based material and the second silicon-based material each independently comprise silicon-carbon; the mass ratio of elemental silicon to elemental carbon in the silicon-carbon is (0.8-1.3):1; preferably, (0.9-1.1):1; Preferably, the ratio of the average particle size of the first graphite material to the silicon-carbon is (1-3.3):1; more preferably, (1.3-2.7):1; Preferably, the ratio of the average particle size of the second graphite material to the silicon-carbon is (0.5-2.6):1; more preferably, (0.9-2):1; Preferably, the average particle size of the silicon-carbon is 6 μm-12 μm.

5. The negative electrode sheet according to claim 3, wherein The first silicon-based material and the second silicon-based material each independently comprise silicon-oxygen; the mass ratio of elemental silicon to elemental oxygen in the silicon-oxygen is (0.8-1.2):1; Preferably, the ratio of the average particle size of the first graphite material to the silicon oxide is (0.8-3):1; Preferably, the ratio of the average particle size of the second graphite material to the silicon oxide is (0.41-2.5):1; Preferably, the average particle size of the silicon oxide is 7-15 μm.

6. The negative electrode sheet according to claim 1 or 2, wherein The outer surface of the negative electrode active material layer has a plurality of recesses, and the depth of the recesses is 3-40 μm; Preferably, the recesses comprise holes, the hole diameter of the holes is 50-500 μm, and the spacing of the holes is 400-5500 μm; Preferably, the recesses comprise grooves, the width of the grooves is 50-500 μm, and the spacing of the grooves is 400-5500 μm.

7. The negative electrode sheet according to claim 1 or 2, wherein The first graphite material comprises artificial graphite, and the graphitization degree of the artificial graphite is 92-94%; And / or, the second graphite material comprises natural graphite, and the graphitization degree of the natural graphite is 95-98%.

8. The negative electrode sheet according to claim 1 or 2, wherein The first negative electrode active material layer has an area density of 4 mg / cm 2 -5 mg / cm 2 ; And / or, the porosity of the first negative electrode active material layer is 30-40%; and / or the face density of the second negative electrode active material layer is 2.5 mg / cm 2 - 3.5 mg / cm 2 ; And / or, the porosity of the second negative electrode active material layer is 25-33%; and / or the compacted density of the negative electrode sheet is 1.5 g / cm 3 -1.7 g / cm 3 .

9. The negative electrode sheet according to claim 1 or 2, wherein The negative electrode current collector comprises a metal layer and a primer layer on at least one side surface of the metal layer, and the thickness of the primer layer is 0.5-2 μm.

10. A lithium-ion secondary battery, characterized by comprising: The lithium ion secondary battery comprises the negative electrode sheet according to any one of claims 1-9.

11. The lithium-ion secondary battery according to claim 10, wherein The lithium ion secondary battery further includes a positive electrode sheet including a positive electrode active material including a substance of a chemical formula of Li a Ni x Co y Mn z A k O2, 0.9≤a≤1.1, 0.8≤x≤0.95, 0<y≤0.2, 0<z≤0.2, 0≤k≤0.05, A is selected from at least one of Al, Zr, B, Y, Sr, W, Ti, and Nb; Preferably, 0.9≤x≤0.95; Preferably, the positive electrode active material comprises single-crystal particles and polycrystal particles; More preferably, the mass ratio of the polycrystal particles to the single-crystal particles is (2.3-4):

1.

12. The lithium-ion secondary battery according to claim 11, wherein The positive electrode sheet has a plurality of recesses on at least one side surface thereof; The width of the recesses is 0.5-50 mm, the spacing of the recesses is 1-10 mm, and the depth of the recesses is 10-80 μm; And / or, the positive electrode sheet has a plurality of protrusions on at least one side surface thereof; The width of the positive projection of the protrusions on the surface of the positive electrode sheet is 0.5-50 mm, the spacing of the positive projections of adjacent two protrusions on the surface of the positive electrode sheet is 1-10 mm, and the height of the protrusions is 10-80 μm.

13. The lithium-ion secondary battery according to any one of claims 10 to 12, wherein, The CB value of the lithium ion secondary battery is 1.1-1.2.

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