Secondary battery and electric device

Abstract

The application provides a secondary battery, comprising a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, the negative electrode film layer having a first surface away from the negative electrode current collector and a second surface arranged opposite to the first surface, the thickness of the negative electrode film layer is denoted as H, the region within the thickness range of 0.3H from the first surface of the negative electrode film layer is denoted as the first region of the negative electrode film layer, the region within the thickness range of 0.3H from the second surface of the negative electrode film layer is denoted as the second region of the negative electrode film layer, the first region comprises a first negative electrode active material, the second region comprises a second negative electrode active material, the first negative electrode active material comprises primary particles of a first artificial graphite, the powder OI value of the first negative electrode active material is 3-6, and the graphitization degree is 91%-93.5%, and the gram capacity of the second negative electrode active material is greater than that of the first negative electrode active material.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a secondary battery and a power consumption device. BACKGROUND

[0002] In recent years, with the application range of secondary batteries becoming more and more extensive, secondary batteries are widely used in energy storage power supply systems such as hydroelectric, thermal, wind and solar power stations, and in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, aerospace, etc. Due to the great development of secondary batteries, higher requirements are put forward for their energy density and cycle performance. SUMMARY

[0003] The present application is made in view of the above-mentioned problems, and aims to provide a secondary battery and a power consumption device. The secondary battery of the present application can improve the cycle performance while taking into account the energy density.

[0004] In order to achieve the above-mentioned purpose, the present application provides a secondary battery, comprising a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, the negative electrode film layer having a first surface away from the negative electrode current collector and a second surface disposed opposite to the first surface, the thickness of the negative electrode film layer being denoted as H, the region within the thickness range of 0.3H from the first surface of the negative electrode film layer being denoted as a first region of the negative electrode film layer, and the region within the thickness range of 0.3H from the second surface of the negative electrode film layer being denoted as a second region of the negative electrode film layer, the first region comprising a first negative electrode active material, the second region comprising a second negative electrode active material, the first negative electrode active material comprising primary particles of a first artificial graphite, the powder OI value of the first negative electrode active material being 3-6, and the graphitization degree being 91%-93.5%, the gram capacity of the second negative electrode active material being greater than that of the first negative electrode active material. By setting the negative electrode film layer as a double layer, the first region uses the above-mentioned first negative electrode active material, and the second region uses the above-mentioned second negative electrode active material, which is conducive to improving the cycle performance of the secondary battery while taking into account the energy density.

[0005] In some embodiments, the powder OI value of the first negative electrode active material is 3-3.8, and / or the graphitization degree is 91.0%-92.5%. Thereby, the structural stability of the SEI film is improved, and the cycle life of the secondary battery is improved.

[0006] In some embodiments, the I D / I G value of the first negative electrode active material is 0.08-0.17; wherein I D represents the D peak intensity at 1350±50 cm-1 in the Raman spectrum of the material, IG Indicates the intensity of the G peak at 1580±50 cm⁻¹ in the Raman spectrum of the material; I D / I G The value represents the ratio of the D peak intensity to the G peak intensity. This helps reduce defects in the first anode active material, decreases active lithium consumption during battery cycling and storage, and thus improves the battery's lifespan performance.

[0007] In some embodiments, the first negative electrode active material I D / I G The value is 0.1-0.15.

[0008] In some embodiments, the specific capacity of the second negative electrode active material is 359 mAh / g or higher. This is beneficial for increasing the specific capacity of the negative electrode film and improving the energy density of the secondary battery.

[0009] In some embodiments, the specific capacity of the first negative electrode active material is 335 mAh / g-350 mAh / g. This is beneficial for improving the cycle performance of the secondary battery while maintaining energy density.

[0010] In some embodiments, the specific capacity of the first negative electrode active material is 335 mAh / g-345 mAh / g; and / or, the specific capacity of the second negative electrode active material is 360 mAh / g-366 mAh / g. This is beneficial for improving the cycle performance of the secondary battery while maintaining energy density.

[0011] In some embodiments, the powder compaction density of the first negative electrode active material at 50,000 N is lower than that of the second negative electrode active material at 50,000 N. This is beneficial for improving the kinetic and cycle performance of the secondary battery while maintaining its energy density.

[0012] In some embodiments, the powder compaction density of the first negative electrode active material at 50,000 N is 1.65 g / cc to 1.83 g / cc; and / or, the powder compaction density of the second negative electrode active material at 50,000 N is 1.95 g / cc to 2.04 g / cc. This helps to reduce volume expansion during the cycling process of the secondary battery, thereby improving the battery's lifespan performance while maintaining its energy density.

[0013] In some embodiments, the powder compaction density of the first negative electrode active material at 50,000 N is 1.65 g / cc to 1.71 g / cc; and / or, the powder compaction density of the second negative electrode active material at 50,000 N is 1.96 to 2.02 g / cc.

[0014] In some embodiments, the degree of graphitization of the first negative electrode active material is less than that of the second negative electrode active material. Therefore, the secondary battery can improve cycle performance while maintaining energy density.

[0015] In some embodiments, the graphitization degree of the second negative electrode active material is 94.0%-96.0%. This is beneficial for improving the energy density of the secondary battery.

[0016] In some embodiments, the degree of graphitization of the second negative electrode active material is 94.2%-95.8%.

[0017] In some embodiments, the volume distribution particle size Dv50 of the first negative electrode active material is 9.0 μm-13.5 μm; and / or, the volume distribution particle size Dv50 of the second negative electrode active material is 14.5 μm-18.0 μm.

[0018] In some embodiments, the volume distribution particle size Dv50 of the first negative electrode active material is 12.5-13.5 μm; and / or, the volume distribution particle size Dv50 of the second negative electrode active material is 15.0 μm-17.5 μm.

[0019] In some embodiments, the specific surface area of ​​the first negative electrode active material is less than or equal to the specific surface area of ​​the second negative electrode active material.

[0020] In some embodiments, the specific surface area of ​​the first negative electrode active material is 0.8 m². 2 / g-1.8m 2 / g; and / or, the specific surface area of ​​the second negative electrode active material is 0.8m². 2 / g-2.1m 2 / g. This helps reduce the side reaction activity on the surface of the negative electrode active material, reduces the consumption of active ions by the SEI film formation, and thus improves the cycle performance and first coulombic efficiency of the secondary battery.

[0021] In some embodiments, the specific capacity of the negative electrode is 348.0 mAh / g-355.0 mAh / g. This is beneficial for the secondary battery to achieve a good balance between energy density and performance.

[0022] In some embodiments, the powder compaction density of the negative electrode film layer at a pressure of 50,000 N is 1.60 g / cc to 1.70 g / cc. This is beneficial for improving the kinetic and lifetime performance of the negative electrode film layer while also considering energy density.

[0023] In some embodiments, the areal density of the negative electrode film is 10.0 mg / cm³. 2 -20.0 mg / cm 2This allows secondary batteries to balance energy density with good storage and kinetic performance.

[0024] In some embodiments, the thickness of the negative electrode film is 70 μm-250 μm. This is beneficial for the secondary battery to have good storage and kinetic performance, while also maintaining energy density.

[0025] In some embodiments, the mass ratio of the first negative electrode active material to the second negative electrode active material in the negative electrode film layer is 3:7-6:4. This is beneficial for improving the cycle performance of the secondary battery while maintaining energy density.

[0026] In some embodiments, the second negative electrode active material comprises a second artificial graphite, wherein the La(110) of the second artificial graphite is 130nm-175nm, and the Lc(002) is 30nm-42nm. La(110) represents the crystallite size along the a-axis in the (110) crystal plane of the material, and Lc(002) represents the crystallite size along the c-axis in the (002) crystal plane of the material. This is beneficial for the secondary battery to have a high energy density.

[0027] In some embodiments, the La(110) of the second artificial graphite is 132nm-172nm, and the Lc(002) is 30nm-36nm. This is beneficial for the secondary battery to have a high energy density.

[0028] In some implementations, La(110) / Lc(002) is 3.5-5.5. This is beneficial for the secondary battery to have a high energy density.

[0029] In some embodiments, charge-discharge tests are performed on the coin cell prepared from the second artificial graphite under conditions of a delithiation rate of 0.1C and a lithium insertion rate of 0.05C, respectively, to obtain charge-discharge curves in the range of 0.005V-2.0V. In the discharge curve, a lithium insertion plateau exists in the voltage range of 0.005V-0.070V, and the discharge capacity corresponding to the lithium insertion plateau accounts for more than 43% of the total discharge capacity of the coin cell. This is beneficial for improving the energy density of the secondary battery.

[0030] In some embodiments, the discharge capacity corresponding to the lithium intercalation platform accounts for 43%-47% of the total discharge capacity of the coin cell.

[0031] In some embodiments, the second artificial graphite comprises secondary particles. This helps to reduce the volume expansion of the second artificial graphite, thereby improving the cycle life of the secondary battery.

[0032] In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the second negative electrode active material is 0.9-1.25. This is beneficial for improving the active ion and electron transport performance in the negative electrode film, thereby enhancing the kinetic performance of the secondary battery.

[0033] A second aspect of this application provides an electrical device that includes the secondary battery of the first aspect of this application. Since the electrical device of this application includes the secondary battery provided in this application, it has at least the same advantages as the secondary battery. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of one embodiment of the negative electrode sheet of this application.

[0035] Figure 2 This is a schematic diagram of a battery cell according to one embodiment of this application.

[0036] Figure 3 yes Figure 2 An exploded view of a battery cell according to one embodiment of this application is shown.

[0037] Figure 4 This is a schematic diagram of a battery module according to one embodiment of this application.

[0038] Figure 5 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0039] Figure 6 yes Figure 5 An exploded view of a battery pack according to one embodiment of this application is shown.

[0040] Figure 7 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0041] Explanation of reference numerals in the attached figures:

[0042] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly; 10 Negative electrode sheet; 101 Negative current collector; 102 Negative electrode film; 102a First surface; 102b Second surface; 1021 First region; 1022 Second region; 1023 Intermediate region. Detailed Implementation

[0043] The following detailed description, with appropriate reference to the accompanying drawings, specifically discloses embodiments of the negative electrode, secondary battery, and electrical device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0044] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0045] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0046] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0047] Unless otherwise specified, the terms used in this application have the common meanings as commonly understood by those skilled in the art.

[0048] Unless otherwise specified, the values ​​of the parameters mentioned in this application can be determined using various testing methods commonly used in the art, for example, according to the testing methods given in this application.

[0049] Unless otherwise specified, in this application, the term "active ion" refers to ions that can be inserted and extracted back and forth between the positive and negative electrodes of a secondary battery, including but not limited to lithium ions.

[0050] Currently, to improve the energy density of rechargeable batteries, graphite materials with high specific capacity and high compaction density, such as natural graphite, are commonly used. However, high-specific-capacity graphite has a small interlayer spacing, affecting the battery's kinetic performance. Furthermore, during cycling, volume expansion leads to structural instability in the graphite, severely impacting the cycle performance of the rechargeable battery. Conversely, to improve cycle performance, graphite materials with low graphitization and larger interlayer spacing are typically used. However, low graphitization deteriorates the specific capacity and compaction density of the graphite material, affecting the battery's energy density. Therefore, it is currently difficult to simultaneously improve cycle performance and energy density.

[0051] Based on this, this application proposes a secondary battery and an electrical device, thereby enabling the secondary battery to improve cycle performance while maintaining energy density.

[0052] This application provides a secondary battery. The term "secondary battery" as used herein refers to a single battery cell, a battery module, or a battery pack. These are described below.

[0053] A single secondary battery cell includes a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0054] Negative electrode sheet

[0055] The secondary battery of this application includes a negative electrode sheet, which includes a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector. The negative electrode film layer has a first surface away from the negative current collector and a second surface disposed opposite to the first surface. The thickness of the negative electrode film layer is denoted as H. A region from the first surface of the negative electrode film layer to a thickness range of 0.3H is denoted as a first region of the negative electrode film layer, and a region from the second surface of the negative electrode film layer to a thickness range of 0.3H is denoted as a second region of the negative electrode film layer. The first region includes a first negative electrode active material, and the second region includes a second negative electrode active material. The first negative electrode active material includes primary particles of first artificial graphite. The powder OI value of the first negative electrode active material is 3-6, and the degree of graphitization is 91%-93.5%. The specific capacity of the second negative electrode active material is greater than that of the first negative electrode active material.

[0056] SeeFigure 1 The diagram shows a schematic representation of one embodiment of the negative electrode of this application. Figure 1 As shown, the negative electrode 10 includes a negative current collector 101 and a negative electrode film 102 formed on at least one surface of the negative current collector 101. The negative electrode film 102 has a first surface 102a away from the negative current collector 101 and a second surface 102b disposed opposite to the first surface 102a. The thickness of the negative electrode film 102 is denoted as H. The thickness H of the negative electrode film refers to the thickness of the negative electrode film located on one side of the negative current collector. The region from the first surface 102a of the negative electrode film to a thickness of 0.3H is denoted as the first region 1021 (upper layer) of the negative electrode film. The region from the second surface 102b of the negative electrode film to a thickness of 0.3H is denoted as the second region 1022 (lower layer) of the negative electrode film. The first region 1021 includes a first negative electrode active material, and the second region 1022 includes a second negative electrode active material. The region between the first region 1021 and the second region 1022 is denoted as the intermediate region 1023. It is readily understood that within the intermediate region 1023, there may be only the first negative electrode active material, only the second negative electrode active material, both the first and second negative electrode active materials, or other negative electrode active materials known in the art besides the first and second negative electrode active materials of this application.

[0057] It should be understood that, Figure 1 In the embodiment shown, the second surface 102b is in contact with the upper surface of the negative electrode current collector 101, but the structure of the negative electrode sheet in this application is not limited to this. For example, there may be an additional layer between the negative electrode film layer 102 and the negative electrode current collector 101. In this case, the second surface 102b is not in direct contact with the negative electrode current collector 101.

[0058] It should also be understood that, Figure 1 While the text clearly shows the boundaries between different areas, such clear interfaces may not exist in the actual product.

[0059] In this application, by setting the negative electrode film layer as a bilayer, the negative electrode sheet can still maintain a good pore structure, low tortuosity, short ion transport path, and high ion conduction efficiency even at a high compaction density. Furthermore, by using low-graphitization and highly isotropic graphite on the side away from the current collector (i.e., the first region 1021), the volume expansion during lithium insertion / extraction can be effectively reduced, thereby improving cycle performance. By using a second negative electrode active material with high specific capacity on the side closer to the current collector (i.e., the second region 1022), the energy density of the battery can be balanced.

[0060] In this application, the first region includes a first negative electrode active material comprising primary particles of first artificial graphite. The powder OI value of the first negative electrode active material is 3-6, and the degree of graphitization is 91%-93.5%. The inventors have discovered that by employing the aforementioned first negative electrode active material in the first region, a lower degree of graphitization is achieved, resulting in a larger interlayer spacing in the graphite material. Consequently, the volume expansion during lithium insertion / extraction is smaller, which is beneficial for improving the structural stability of the SEI film and enhancing cycle life. Simultaneously, the high isotropy of the graphite material effectively mitigates the volume expansion caused by lithium insertion / extraction, further enhancing the stability of the SEI film and improving cycle life. Furthermore, primary particles help reduce surface defects in the graphite, decreasing the consumption of active ions during secondary battery cycling, thereby improving the cycle life of the secondary battery.

[0061] For example, the powder OI value of the first negative electrode active material can be 3, 4, 5, 6, or a value within a range of any two of these values. For example, the degree of graphitization of the first negative electrode active material can be 91%, 91.5%, 92.0%, 92.5%, 93.0%, 93.5%, or a value within a range of any two of these values. In some alternative embodiments, the powder OI value of the first negative electrode active material is 3-3.8, and / or the degree of graphitization is 91.0%-92.5%.

[0062] In this application, the specific capacity of the second negative electrode active material is greater than that of the first negative electrode active material. In this application, the second negative electrode active material located in the second region has a higher specific capacity, which is beneficial for improving the specific capacity of the negative electrode film, thereby enabling the secondary battery to maintain energy density; the first negative electrode active material located in the first region can improve cycle performance.

[0063] In some embodiments, the first negative electrode active material I D / I G The value is 0.08-0.17; where I D This indicates that the Raman spectrum of the material is at 1350±50 cm⁻¹ -1 D peak intensity at I G This indicates that the Raman spectrum of the material is at 1580±50 cm⁻¹ -1 The intensity of peak G at point I; D / I G The value represents the ratio of the D peak intensity to the G peak intensity. This is achieved by adjusting the Ig of the first negative electrode active material. D / I G Values ​​within the aforementioned range are beneficial for reducing defects in the first anode active material, decreasing active lithium consumption during cycling and storage, and thus improving lifetime performance. For example, the I value of the first anode active material... D / IG The value can be a value between 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, or any two of these ranges. In some alternative embodiments, the I of the first negative electrode active material... D / I G The value is 0.1-0.15.

[0064] In some embodiments, the specific capacity of the first negative electrode active material is 335 mAh / g to 350 mAh / g. By keeping the specific capacity of the first negative electrode active material within this range, it is beneficial for the secondary battery to have good cycle performance while maintaining energy density. Exemplarily, the specific capacity of the first negative electrode active material can be 335 mAh / g, 340 mAh / g, 345 mAh / g, 350 mAh / g, or a value within a range of any two of these values. In some optional embodiments, the specific capacity of the first negative electrode active material can be 335 mAh / g to 345 mAh / g.

[0065] In some embodiments, the specific capacity of the second negative electrode active material is 359.0 mAh / g or higher. Having a high specific capacity in the negative electrode active material of the second region is beneficial for improving the energy density of the secondary battery. Exemplarily, the specific capacity of the second negative electrode active material is a value between 359 mAh / g, 360 mAh / g, 361 mAh / g, 362 mAh / g, 363 mAh / g, 364 mAh / g, 365 mAh / g, 366 mAh / g, 367 mAh / g, 368 mAh / g, 369 mAh / g, 370 mAh / g, or any two of these values. In some optional embodiments, the specific capacity of the second negative electrode active material is between 360 mAh / g and 366 mAh / g. Having the specific capacity of the second negative electrode active material within the above range is beneficial for improving the energy density of the secondary battery.

[0066] In some embodiments, the powder compaction density of the first negative electrode active material at 50,000 N is lower than that of the second negative electrode active material at 50,000 N. In this application, the second negative electrode active material located in the second region has a higher powder compaction density, which is beneficial to improving the energy density of the secondary battery. The first negative electrode active material located in the first region has a lower powder compaction density, which is beneficial to reducing the tortuosity of the negative electrode film and reducing the migration path of active ions, thereby improving the kinetic performance and cycle performance of the secondary battery.

[0067] In some embodiments, the powder compaction density of the first negative electrode active material at 50,000 N is 1.65 g / cc to 1.83 g / cc; and / or, the powder compaction density of the second negative electrode active material at 50,000 N is 1.95 g / cc to 2.04 g / cc. By keeping the powder compaction density of the first negative electrode active material within the above range, it is beneficial to reduce volume expansion during the cycling process of the secondary battery, thereby improving the lifespan performance of the secondary battery; furthermore, keeping the powder compaction density of the second negative electrode active material within the above range is beneficial to improving the energy density of the secondary battery. For example, the powder compaction density of the first negative electrode active material at 50,000 N can be 1.65 g / cc, 1.68 g / cc, 1.70 g / cc, 1.72 g / cc, 1.75 g / cc, 1.78 g / cc, 1.80 g / cc, 1.83 g / cc, or a value within a range consisting of any two of these values. In some optional embodiments, the powder compaction density of the first negative electrode active material at 50,000 N is 1.65 g / cc to 1.71 g / cc. Exemplarily, the powder compaction density of the second negative electrode active material at 50,000 N can be 1.95 g / cc, 1.96 g / cc, 1.97 g / cc, 1.98 g / cc, 1.99 g / cc, 2.00 g / cc, 2.01 g / cc, 2.02 g / cc, 2.03 g / cc, 2.04 g / cc, or any two values ​​thereof. In some optional embodiments, the powder compaction density of the second negative electrode active material at 50,000 N is 1.96 to 2.02 g / cc.

[0068] In some embodiments, the degree of graphitization of the first negative electrode active material is less than that of the second negative electrode active material. In this application, the first negative electrode active material located in the first region has a lower degree of graphitization, thus resulting in a larger interlayer spacing, which is beneficial for reducing volume expansion during battery cycling and improving the cycle performance of the secondary battery. Furthermore, the second negative electrode active material located in the second region has a higher degree of graphitization, which is beneficial for increasing the specific capacity of the negative electrode film, thereby improving the energy density of the secondary battery. Therefore, the secondary battery can achieve both excellent cycle performance and high energy density.

[0069] In some embodiments, the graphitization degree of the second negative electrode active material is 94.0%-96.0%. By keeping the graphitization degree of the second negative electrode active material within this range, it is beneficial for the second negative electrode active material to have a high compaction density and specific capacity, thereby improving the energy density of the secondary battery. Exemplarily, the graphitization degree of the second negative electrode active material can be 94.0%, 94.5%, 95.0%, 95.5%, 96.0%, or a value between any two of these values. In some optional embodiments, the graphitization degree of the second negative electrode active material is 94.2%-95.8%.

[0070] In some embodiments, the volumetric particle size distribution Dv50 of the first negative electrode active material is 9.0 μm-13.5 μm; and / or, the volumetric particle size distribution Dv50 of the second negative electrode active material is 14.5 μm-18.0 μm. By ensuring that the volumetric particle size distribution Dv50 of the first and / or second negative electrode active materials is within the aforementioned range, it is beneficial to improve the cycle performance and kinetic performance of the secondary battery while maintaining energy density. Exemplarily, the volumetric particle size distribution Dv50 of the first negative electrode active material can be 9.0 μm, 9.5 μm, 10.0 μm, 10.5 μm, 11.0 μm, 11.5 μm, 12.0 μm, 12.5 μm, 13.0 μm, 13.5 μm, or a value within a range consisting of any two of these values. In some optional embodiments, the volumetric particle size distribution Dv50 of the first negative electrode active material is 12.5 μm-13.5 μm. Additionally, by way of example, the volume distribution particle size Dv50 of the second negative electrode active material can be 14.5 μm, 15.0 μm, 15.5 μm, 16.0 μm, 16.5 μm, 17.0 μm, 17.5 μm, 18.0 μm, or a value within a range consisting of any two of these values. In some alternative embodiments, the volume distribution particle size Dv50 of the second negative electrode active material is 15.0 μm-17.5 μm.

[0071] In some embodiments, the specific surface area of ​​the first negative electrode active material is less than or equal to the specific surface area of ​​the second negative electrode active material.

[0072] In some embodiments, the specific surface area of ​​the first negative electrode active material is 0.8-1.8 m². 2 / g; and / or, the specific surface area of ​​the second negative electrode active material is 0.8m². 2 / g-2.1m 2 / g. By ensuring the specific surface area of ​​the first negative electrode active material is within the aforementioned range, it is beneficial to reduce the surface side reaction activity of the negative electrode active material, thereby reducing the consumption of active ions during SEI film formation and improving the cycle performance of the secondary battery. Furthermore, by ensuring the specific surface area of ​​the second negative electrode active material is within the aforementioned range, it is beneficial to reduce the surface side reaction activity of the negative electrode active material, thereby reducing the consumption of active ions during SEI film formation and improving the initial coulombic efficiency and cycle performance of the secondary battery; on the other hand, it also possesses higher active ion transport performance, improving the kinetic performance of the secondary battery. For example, the specific surface area of ​​the first negative electrode active material can be 0.8m². 2 / g, 1.0m 2 / g, 1.2m 2 / g, 1.4m 2 / g, 1.6m 2 / g, 1.8m 2 / g or a value within a range of any two of these values. For example, the specific surface area of ​​the second negative electrode active material can be 0.8m². 2 / g, 1.0m 2 / g, 1.2m 2 / g, 1.4m 2 / g, 1.6m 2 / g, 1.8m 2 / g, 2.0m 2 / g、2.1m 2 / g or a range of values ​​between any two of them.

[0073] In some embodiments, the specific capacity of the negative electrode is between 348.0 mAh / g and 355.0 mAh / g. Maintaining the specific capacity of the negative electrode within this range is beneficial for improving the energy density of the secondary battery. For example, the specific capacity of the negative electrode can be 348.0 mAh / g, 350.0 mAh / g, 352.0 mAh / g, 355.0 mAh / g, or a value within a range of any two of these values.

[0074] In some embodiments, the powder compaction density of the negative electrode film layer at 50,000 N pressure is 1.60 g / cc to 1.70 g / cc. Maintaining the powder compaction density of the negative electrode film layer within this range helps maintain a better pore structure, reduces the tortuosity of the negative electrode film layer, and shortens the active ion transport path, thereby improving the kinetic and lifetime performance of the negative electrode film layer while also achieving high energy density. For example, the powder compaction density of the negative electrode film layer at 50,000 N pressure can be 1.60 g / cc, 1.62 g / cc, 1.64 g / cc, 1.66 g / cc, 1.68 g / cc, 1.70 g / cc, or a value within a range of any two of these values. In some optional embodiments, the powder compaction density of the negative electrode film layer at 50,000 N pressure is 1.62 g / cm³. 3 -1.68g / cm 3 .

[0075] In some embodiments, the areal density of the negative electrode film is 10.0 mg / cm³. 2 -20.0 mg / cm 2 By maintaining the areal density of the negative electrode film within the aforementioned range, it is beneficial for the negative electrode film to achieve a balance between high capacity, high ion and electron transport performance, and consequently, for the secondary battery to achieve a balance between high energy density and good storage and kinetic performance. For example, the areal density of the negative electrode film can be 10 mg / cm³. 2 12.0 mg / cm 2 15mg / cm 2 18.0 mg / cm 2 20mg / cm 2 Or a value between any two of these values. In some alternative embodiments, the areal density of the negative electrode film is 12.0 mg / cm³. 2 -18.5mg / cm 2 .

[0076] In some embodiments, the thickness of the negative electrode film is 70 μm-250 μm. By keeping the thickness of the negative electrode film within this range, it is beneficial for the negative electrode film to achieve both high capacity, high ion and electron transport performance, and consequently, for the secondary battery to achieve both high energy density and good storage and kinetic performance. Exemplarily, the thickness of the negative electrode film can be 70 μm, 80 μm, 90 μm, 95 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, 180 μm, 200 μm, 210 μm, 220 μm, 240 μm, 250 μm, or a value between any two of these values. In some optional embodiments, the thickness of the negative electrode film is 90 μm-220 μm.

[0077] In some embodiments, the mass ratio of the first negative electrode active material to the second negative electrode active material in the negative electrode film layer is 3:7-6:4. Maintaining the mass ratio of the first and second negative electrode active materials within this range is beneficial for the secondary battery to exhibit excellent cycle performance while maintaining energy density. For example, the mass ratio of the first and second negative electrode active materials can be 3:7, 4:6, 5:5, 6:4, or a ratio between any two of these values.

[0078] In some embodiments, the second negative electrode active material comprises a second artificial graphite, wherein the La(110) of the second artificial graphite is 130nm-175nm and the Lc(002) is 30nm-42nm. La(110) represents the crystallite size along the a-axis in the (110) crystal plane of the material, and Lc(002) represents the crystallite size along the c-axis in the (002) crystal plane of the material. An increase in the La of the second artificial graphite indicates an increase in the crystallite size, resulting in more lithium intercalation sites on the surface of the second artificial graphite, thus improving the specific capacity. Furthermore, the increased crystallite size can also reduce the number of crystal boundaries in the second artificial graphite, thereby increasing the number of lithium intercalation sites, which can reduce the adverse effects on specific capacity and powder compaction density, and further improve the specific capacity and powder compaction density of the second artificial graphite. When the Lc of the second artificial graphite is within the above range, it is beneficial to reduce the stacking of graphite crystals, thereby reducing the number of crystal boundaries between crystals, which is beneficial to increasing the number of lithium intercalation sites on the surface of the second artificial graphite, thereby improving the specific capacity of the second artificial graphite. Therefore, the second negative electrode active material located in the second region has high specific capacity and high compaction density, which is beneficial for the secondary battery to have high energy density.

[0079] In some embodiments, the La(110) of the second artificial graphite is 132nm-172nm, and the Lc(002) is 30nm-36nm. By keeping the La(110) and Lc(002) of the second artificial graphite within the above ranges, it is beneficial to improve the specific capacity and compaction density of the second negative electrode active material, thereby contributing to a high energy density in the secondary battery.

[0080] In some embodiments, La(110) / Lc(002) is 3.5-5.5. By keeping La(110) / Lc(002) within this range, the graphite material exhibits good grain development and a low number of microcrystals, resulting in fewer interfaces between grains. This is beneficial for increasing the lithium intercalation sites on the surface of the artificial graphite, thereby improving the specific capacity of the negative electrode active material. Consequently, it is beneficial for increasing the specific capacity of the negative electrode film, thereby improving the energy density of the secondary battery. Exemplarily, La(110) / Lc(002) can be a value between 3.5, 3.8, 4.0, 4.2, 4.4, 4.6, 4.8, 5.0, 5.2, 5.5, or any two of these values. In some alternative embodiments, the La(110) / Lc(002) of the second artificial graphite is 4.5-5.5.

[0081] In some embodiments, charge-discharge tests are performed on the coin cell prepared from the second artificial graphite under conditions of a delithiation rate of 0.1C and a lithium insertion rate of 0.05C, respectively, to obtain charge-discharge curves in the range of 0.005V-2.0V. In the discharge curve, a lithium insertion plateau exists in the voltage range of 0.005V-0.070V, and the discharge capacity corresponding to the lithium insertion plateau accounts for more than 43% of the total discharge capacity of the coin cell.

[0082] In this application, the charge-discharge test is performed using the following method: Sample powder, conductive carbon black (SuperP), and binder polyvinylidene fluoride (PVDF) are mixed uniformly with solvent N-methylpyrrolidone (NMP) at a mass ratio of 91.6:1.8:6.6 to form a slurry; the prepared slurry is coated onto the surface of the negative electrode current collector copper foil and dried in an oven for later use; ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed at a volume ratio of 1:1:1 to obtain an organic solvent, and then LiPF6 is dissolved in the above organic solvent. An electrolyte with a concentration of 1 mol / L was prepared in a solvent. Then, using a lithium metal sheet as the counter electrode and a polyethylene (PE) film as the separator, the electrolyte was assembled into a CR2430 coin cell in an argon-protected glove box. After the coin cell was left to stand for 12 hours, it was discharged at 25°C with a constant current of 0.05C to 0.005V, left to stand for 10 minutes, and then discharged again with a constant current of 50 μA to 0.005V. After standing for 10 minutes, it was discharged again with a constant current of 10 μA to 0.005V. Then, it was charged at a constant current of 0.1C to 2.0V, and the charge-discharge curve of the material was recorded.

[0083] In this application, a coin cell prepared with the second negative electrode active material was subjected to the aforementioned charge-discharge test, resulting in charge-discharge curves. In the discharge curve, the discharge capacity corresponding to the lithium intercalation plateau within the voltage range of 0.005V to 0.070V accounted for more than 43% of the total discharge capacity of the coin cell. This indicates that the high specific capacity of the second negative electrode active material is beneficial for improving the energy density of the secondary battery. Although the mechanism is not yet clear, the inventors believe that during the lithium intercalation process of graphite materials, as the amount of lithium ion intercalation increases, graphite intercalation compounds of different orders gradually form, such as LiC24, LiC12, and LiC6. The formation of graphite intercalation compounds of different orders corresponds to lithium intercalation plateaus within different voltage ranges in the charge-discharge curves of the graphite materials. Since the theoretical capacities of graphite intercalation compounds of different orders differ—for example, the theoretical capacity of LiC6 is 372 mAh / g, while the theoretical specific capacity of LiCl2 is 186 mAh / g—the more LiC6 (i.e., the lithium intercalation platform corresponding to a voltage range of 0.005V-0.07V) is formed during lithium intercalation, the greater the corresponding capacity contribution, and thus the higher the specific capacity of the graphite material. Exemplarily, the proportion X can be 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, or a value within a range of any two of these values. In some embodiments, the proportion X of the discharge capacity corresponding to the lithium intercalation platform to the total discharge capacity of the coin cell is 43%-50%, optionally 43%-47%.

[0084] In some embodiments, the second artificial graphite includes secondary particles. This is beneficial for reducing the orientation degree of the second artificial graphite, increasing its isotropy, reducing its volume expansion, and helping to reduce the damage to the SEI film caused by volume expansion, thereby reducing the active ions consumed in the formation of a new SEI film and thus improving the cycle life of the secondary battery.

[0085] In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the second negative electrode active material is 0.9-1.25. This is beneficial for increasing the compaction density of the negative electrode film and improving the energy density of the secondary battery. Furthermore, it facilitates the formation of a reasonable pore structure between the particles in the negative electrode film, improving the transport performance of active ions and electrons in the negative electrode film, thereby enhancing the kinetic performance of the secondary battery. For example, the particle size distribution (Dv90-Dv10) / Dv50 of the second negative electrode active material is 0.90, 0.95, 1.00, 1.05, 1.10, 1.15, 1.20, 1.25, or a value within a range of any two of these values. In some optional embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the second negative electrode active material is 0.90-1.20.

[0086] In this application, the La(110) and Lc(002) of the material (such as second artificial graphite) can be determined using instruments and methods known in the art. For example, the negative electrode sheet removed from the secondary battery can be cleaned with an organic solvent such as DMC and then dried. The cleaned negative electrode sheet is then immersed in NMP and ultrasonically treated to separate the copper foil, thus obtaining the negative electrode material. After drying the negative electrode material, it is calcined at 350℃-500℃, washed with water multiple times, and dried at 80℃ to obtain the negative electrode active material. X-ray diffractometer (such as Bruker D8 Discover) is used for testing, and the testing can be referenced in JIS K 0131-1996 and JB / T4220-2011 to obtain the peak intensity and full width at half maximum (FWHM) of the diffraction peak corresponding to the (110) crystal plane and the peak intensity and full width at half maximum (FWHM) of the diffraction peak corresponding to the (002) crystal plane, and then calculated according to the Scherrer formula.

[0087] In this application, the I of the materials (first negative electrode active material, second negative electrode active material, etc.) D / I G The test can be performed using a Raman spectrometer. D This indicates that the Raman spectrum of the material is at 1350±50 cm⁻¹. -1 D peak intensity at I G This indicates that the Raman spectrum of the material is at 1580±50 cm⁻¹. -1 The intensity of the G peak at the location was measured. The test conditions were: excitation wavelength 532 nm, grating 600 lines, objective lens 50x, integration time 10 s, cumulative scan 3 times, area scan, obtaining the D and G peak intensities at 100 points, and calculating the I at 100 points. D / I G Remove the largest and smallest 30 I's. D / I G The average of the remaining 40 points is the material's I. D / I G The testing instrument can be a Horiba LabRAM HR800 Raman spectrometer.

[0088] In this application, the OI value of the powder of materials (first negative electrode active material, second negative electrode active material, etc.) refers to the graphite orientation degree of the material, describing the uniformity of crystal orientation in the graphite material particles. The powder OI value of the material is the ratio of the peak area of ​​surface (004) to the peak area of ​​surface (110) obtained by testing the material by X-ray diffraction (XRD). In this application, the powder OI value of the material can be determined using instruments and methods known in the art. For example, an X-ray diffractometer (such as a Bruker D8 Discover) can be used for testing, and the testing can be performed with reference to JIS K 0131-1996 and JB / T 4220-2011 to obtain the X-ray diffraction pattern of the powder sample. According to OI value = I 004 / I 110 The OI value of the powder in the sample was calculated. 004 I is the integrated area of ​​the diffraction peak of the crystalline carbon 004 crystal plane in the powder sample. 110 This represents the integrated area of ​​the diffraction peak on the 110 crystal plane of crystalline carbon in the powder sample. In the X-ray diffraction analysis of this application, a copper target can be used as the anode target, with CuKα rays as the radiation source, and the ray wavelength... The scanning 2θ angle range is 20°-80°, and the scanning rate is 4° / min.

[0089] In this application, the powder compaction density of the materials (first negative electrode active material, second negative electrode active material, etc.) is the mass per unit volume of the powder under specified conditions, which can be determined using instruments and methods known in the art. For example, it can be determined using an electronic pressure testing machine (e.g., a UTM7305 type electronic pressure testing machine) in accordance with GB / T 24533-2009. An exemplary test method is as follows: Weigh 1g of sample powder and add it to a container with a bottom area of ​​1.327cm². 2 In the mold, the pressure is increased to 50,000 N, held for 30 seconds, then the pressure is released and held for 10 seconds. The compaction density of the powder under 50,000 N pressure is then recorded and calculated.

[0090] In this application, the volume distribution particle sizes Dv10, Dv50, and Dv90 of the materials (e.g., the first negative electrode active material, the second negative electrode active material, etc.) represent the particle sizes corresponding to a cumulative volume distribution percentage of 10%, 50%, and 90%, respectively, and can be determined using instruments and methods known in the art. For example, they can be determined using a laser particle size analyzer, referring to GB / T 19077-2016. The testing instrument can be a Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK.

[0091] In this application, the specific capacity of the materials (first negative electrode active material, second negative electrode active material, etc.) is the ratio of the electrical capacity that the active material can release to the mass of the active material, and can be tested using methods known in the art. An exemplary test method is as follows: The sample powder is mixed evenly with a conductive agent, a binder, and optionally other additives in a certain mass ratio with a solvent to form a slurry; the prepared slurry is coated on the surface of the negative electrode current collector copper foil and dried in an oven for later use; lithium salt is dissolved in an organic solvent to prepare an electrolyte; then, using a lithium metal sheet as the counter electrode and a polyethylene (PE) film as the separator, the CR2430 coin cell is assembled with the above electrolyte in an argon-protected glove box. After the obtained coin cells were allowed to stand for 12 hours, they were discharged at 25°C with a constant current of 0.15 mA to 0.005 V. After standing for 10 minutes, they were discharged again with a constant current of 50 μA to 0.005 V. After standing for 10 minutes, they were discharged again with a constant current of 10 μA to 0.005 V. Then, they were charged with a constant current of 0.3 mA to 2.0 V, and the first charge capacity of the coin cells was recorded. The ratio of the charge capacity to the sample mass is the specific capacity of the corresponding material (first negative electrode active material, second negative electrode active material, etc.).

[0092] In this application, the degree of graphitization of the materials (first negative electrode active material, second negative electrode active material, etc.) is the proportion of carbon elements in the material existing in the form of a graphite structure, which can be tested using instruments and methods known in the art. For example, an X-ray diffractometer (such as a Bruker D8 Discover) can be used for testing, and the testing can be referenced to JIS K 0131-1996 and JB / T 4220-2011 to obtain the average interlayer spacing d of the C(002) crystal plane in the material's crystal structure. 002 Then, according to the formula g = (0.344 - d) 002 The degree of graphitization is calculated as d / (0.344-0.3354)×100%. In the above formula, d... 002 It is the average interlayer spacing of the C(002) crystal plane in the material crystal structure, expressed in nanometers (nm).

[0093] In this application, the thickness of the negative electrode film layer has a meaning known in the art and can be tested using methods known in the art, such as a micrometer (e.g., a Mitutoyo 293-100 with an accuracy of 0.1 μm). The thickness range given in this application refers to the thickness range of the negative electrode film layer on one side of the negative electrode current collector.

[0094] In this application, the specific surface area of ​​the material (e.g., the first negative electrode active material, the second negative electrode active material, etc.) is the total area per unit mass of material, which can be determined using instruments and methods known in the art. For example, it can be measured using the nitrogen adsorption specific surface area analysis method according to GB / T 19587-2017, and calculated using the BET (Brunauer Emmett Teller) method. The testing instrument can be the Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA.

[0095] In this application, primary particles and secondary particles have meanings known in the art. Primary particles refer to non-agglomerated particles. Secondary particles refer to aggregated particles formed by the aggregation of two or more primary particles. Primary particles and secondary particles can be distinguished using scanning electron microscopy (SEM) images.

[0096] Preparation method of negative electrode sheet

[0097] The negative electrode sheet in this application can be prepared in the following manner: a first slurry containing a first negative electrode active material and a second slurry containing a second negative electrode active material are provided; the first slurry is coated on a negative electrode current collector, the second slurry is coated on the first slurry, and after drying and cold pressing, a negative electrode sheet is obtained.

[0098] The preparation method of the first negative electrode active material in this application includes the following steps:

[0099] Pretreatment steps: The petroleum coke is crushed and shaped to obtain a shaped material with a volumetric particle size distribution (Dv50) of 10μm-14μm; and

[0100] Graphitization step: The above-mentioned shaped material is subjected to a first graphitization treatment at 2400℃-2800℃ to obtain the first negative electrode active material.

[0101] This application uses petroleum coke as a raw material. Petroleum coke has high isotropy; therefore, the prepared first negative electrode active material also has high isotropy, which helps to reduce the volume expansion of the first negative electrode active material during lithium insertion / extraction and improves the cycle life of the secondary battery. For example, the petroleum coke can be petroleum needle coke.

[0102] In this application, the temperature of the first graphitization treatment is 2400℃-2800℃. By keeping the graphitization temperature within this range, the grain growth and development of the graphite material can be controlled, resulting in a graphite material with a larger interlayer spacing. This reduces the volume expansion of the first negative electrode active material during secondary battery cycling, maintains structural stability, and thus improves the lifespan performance of the secondary battery. Exemplarily, the temperature of the first graphitization treatment can be 2400℃, 2500℃, 2600℃, 2700℃, 2800℃, or a value within a range consisting of any two of these values.

[0103] In this application, the graphitization treatment time is 2h-48h. For example, when the graphitization temperature is 2600℃, the graphitization time can be 2h, 4h, 8h, 20h, 48h, or a value between any two of these values.

[0104] In some embodiments, the petroleum coke contains less than or equal to 0.6% sulfur and less than or equal to 15% volatile components. Here, volatile components refer to volatile organic compounds, such as alkanes, aromatics, and lipids. When the sulfur and volatile component content in the first raw material is within the above range, the defects caused by sulfur overflow during graphitization decrease, which helps reduce defects in the first negative electrode active material, improves the material's stability, and thus enhances the cycle life of the secondary battery.

[0105] In some embodiments, the shaping material is pre-carbonized before the graphitization step. The pre-carbonization step helps reduce volatile components in the binder, thereby reducing the impurity content in the first negative electrode active material and increasing its specific capacity and compaction density. This allows the secondary battery to improve cycle performance while maintaining energy density.

[0106] In some embodiments, the pre-carbonization is performed under a protective atmosphere, which may, exemplarily, be nitrogen or argon. In an alternative embodiment, the pre-carbonization is performed under a nitrogen atmosphere.

[0107] In some embodiments, the pre-carbonization temperature is 1000℃-1500℃. For example, the pre-carbonization temperature can be 1000℃, 1100℃, 1200℃, 1300℃, 1400℃, 1500℃, or any two of these values. By keeping the pre-carbonization temperature within the above range, volatile components in the raw material can be sufficiently removed, which helps reduce the number of impurities and defects in the artificial graphite, thereby increasing the specific capacity of the artificial graphite. Additionally, pre-carbonization can increase the density of the raw material, allowing for a larger furnace charge during graphitization. In an optional embodiment, the pre-carbonization temperature is 1100℃-1200℃.

[0108] In some embodiments, the pre-carbonization time is 1-4 hours. Exemplary pre-carbonization times can be 1 hour, 2 hours, 3 hours, 4 hours, or any range of two of these values.

[0109] The preparation method of the second negative electrode active material in this application includes the following steps:

[0110] Pretreatment step: The calcined needle coke is crushed and shaped to obtain shaped material; the calcined needle coke has a carbon content of more than 97%, a volatile component content of less than 2%, and a sulfur content of less than 0.5%;

[0111] Granulation step: The shaped material is mixed with the binder to obtain granulated material;

[0112] Graphitization step: The granulated material is subjected to a second graphitization treatment at 3000℃-3200℃ to obtain a second artificial graphite with La(110) of 130nm-175nm and Lc(002) of 30nm-42nm, which serves as the second negative electrode active material.

[0113] In this application, calcined needle coke is used as raw material. Calcined needle coke has a stronger needle-like structure and fewer interlocking structures. Using calcined needle coke as raw material helps to reduce the impurity content in the second artificial graphite, thereby improving the graphitization degree of the second artificial graphite.

[0114] In this application, the carbon content of the calcined needle coke is 97% or more, the volatile content is 2% or less, and the sulfur content is 0.5% or less. Here, volatile components refer to hydrocarbons with low molecular weight, such as light alkanes. With the sulfur content and volatile content of the calcined needle coke within the above range, the purity of the raw material is better, which is conducive to grain development during graphitization, reduces the number of defects during graphitization, and increases the graphitization degree of the second artificial graphite and the La(110) value of the artificial graphite. By making the La(110) of the second artificial graphite 130nm-175nm and the Lc(002) 30nm-42nm, the development of graphite grains in the material is better, which is conducive to the intercalation of active ions, thereby increasing the specific capacity and compaction density of the negative electrode film and improving the energy density of the secondary battery.

[0115] In this application, the graphitization temperature is 3000℃-3200℃. A graphitization temperature within this range is beneficial for the growth and development of graphite grains, increasing the degree of graphitization of the second artificial graphite, thereby increasing the specific capacity of the second artificial graphite and improving the energy density of the secondary battery. Exemplarily, the graphitization temperature can be 3000℃, 3050℃, 3100℃, 3150℃, 3200℃, or a value within a range consisting of any two of these values.

[0116] In this application, high-purity raw materials are used and the graphitization temperature is controlled at 3000℃-3200℃, which is beneficial for the development of the 110 crystal plane of graphite grains along the a-axis direction during the graphitization process, thereby increasing the La(110) value of artificial graphite. In addition, it is also beneficial to reduce the number of defects during the graphitization process, which is beneficial to improving the energy density of secondary batteries.

[0117] Those skilled in the art can adjust the graphitization time as needed. For example, an Atchison graphitization furnace can be used for graphitization treatment, and the graphitization time can be 2-8 days when the temperature is above 2800°C.

[0118] In some embodiments, the softening point of the adhesive is 180℃-270℃. The softening point of the adhesive refers to the temperature at which the adhesive changes from a solid state to a softened state with a certain degree of fluidity. In this application, the softening point can be tested using instruments and methods known in the art, for example, it can be determined according to GB / T4507-2014. When the softening point of the adhesive is within the above range, the molecular weight and content of the polycyclic aromatic hydrocarbons in the adhesive are large, which is beneficial to improving the adhesiveness of the adhesive, reducing the amount of adhesive used in the granulation step, thereby increasing the specific capacity and compaction density of the second artificial graphite. Exemplarily, the softening point of the adhesive can be 180℃, 200℃, 220℃, 240℃, 260℃, 270℃, or a value between any two of these values.

[0119] In some embodiments, the coking value of the binder is 50%-70%. The coking value of the binder refers to the percentage of the mass of coke residue formed after heating, combustion, or other treatment of the binder under specific experimental conditions, relative to the mass of the original sample. In this application, the coking value can be tested using methods known in the art. For example, it can be determined with reference to GB / T 8727-2008. A coking value of the binder within the above range is beneficial for improving the specific capacity and compaction density of the second negative electrode active material, thereby increasing the energy density of the secondary battery.

[0120] In some embodiments, the mass ratio of the shaping material to the binder is 100:(6-12). A ratio of shaping material to binder within this range helps reduce the amount of residual carbon from the binder, increases the specific capacity and compaction density of the second negative electrode active material, and thus improves the energy density of the secondary battery.

[0121] In some embodiments, the granulated material is pre-carbonized before the graphitization step. This pre-carbonization step helps reduce volatile components in the binder, thereby reducing the impurity content in the second negative electrode active material, increasing the specific capacity and compaction density of the second negative electrode active material, and improving the energy density of the secondary battery.

[0122] In some embodiments, the pre-carbonization is performed under a protective atmosphere, which may, exemplarily, be nitrogen or argon. In an alternative embodiment, the pre-carbonization is performed under a nitrogen atmosphere.

[0123] In some embodiments, the pre-carbonization temperature is 1000-1500°C. For example, the pre-carbonization temperature can be 1000°C, 1100°C, 1200°C, 1300°C, 1400°C, 1500°C, or any two of these values. By keeping the pre-carbonization temperature within the above range, volatile components in the raw material can be sufficiently removed, which helps reduce the number of impurities and defects in the second artificial graphite, thereby increasing the specific capacity of the second artificial graphite. Additionally, the pre-carbonization treatment can increase the density of the raw material, allowing for a larger furnace charge during graphitization. In an optional embodiment, the pre-carbonization temperature is 1000°C-1100°C.

[0124] In some embodiments, the pre-carbonization time is 1-4 hours. Exemplary pre-carbonization times can be 1 hour, 2 hours, 3 hours, 4 hours, or any range of two of these values.

[0125] In some embodiments, the volumetric particle size Dv50 of the intermediate is 15 μm-18 μm. An intermediate particle size within this range is beneficial for increasing the particle size of the second artificial graphite, thereby increasing the specific capacity of the second artificial graphite and improving the energy density of the secondary battery.

[0126] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0127] In some embodiments, a first negative electrode active material, along with optional conductive agents, optional binders, and other optional additives, can be dispersed in a solvent (e.g., deionized water) to form a first slurry. A second negative electrode active material, along with optional conductive agents, optional binders, and other optional additives, can be dispersed in a solvent (e.g., deionized water) to form a second slurry. The binder can be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethyl methacrylate (PMAA), and carboxymethyl chitosan (CMCS). The conductive agent can be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, the negative electrode film layer may also optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0128] Positive electrode sheet

[0129] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including the positive electrode active material of the first aspect of this application.

[0130] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0131] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0132] In some embodiments, when the battery cell is a lithium-ion battery, the positive electrode active material may be a positive electrode active material known in the art for lithium-ion batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.1 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0133] During the charging and discharging process of a battery, Li undergoes insertion / extraction and consumption, resulting in varying molar Li content at different discharge states. In the examples of positive electrode active materials in this application, the molar Li content refers to the initial state of the material, i.e., before feeding. When the positive electrode active material is applied to the battery system, the molar Li content changes after charge-discharge cycles.

[0134] In the examples of positive electrode active materials in this application, the molar content of O is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of O will fluctuate.

[0135] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0136] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0137] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0138] electrolytes

[0139] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0140] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0141] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0142] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0143] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0144] Separating membrane

[0145] In some embodiments, the battery cell also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0146] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0147] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0148] In some embodiments, the battery cell may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0149] In some embodiments, the outer packaging of the battery cell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell can also be a flexible package, such as a pouch. The material of the flexible package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0150] This application does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 2 The example shown is a square-structured battery cell 5.

[0151] In some implementations, refer to Figure 3 The outer packaging may include a housing 51 and a top cover assembly 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can cover the opening to close the receiving cavity. A positive electrode sheet, a negative electrode sheet, and a separator can be formed into an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in a single battery cell 5 can be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0152] In some implementations, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0153] Figure 4 This is battery module 4, used as an example. (See reference...) Figure 4 In battery module 4, multiple battery cells 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple battery cells 5 can be fixed in place using fasteners.

[0154] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0155] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0156] Figure 5 and Figure 6 This is battery pack 1 as an example. (See reference...) Figure 5 and Figure 6 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0157] In addition, this application also provides an electrical device, which includes the secondary battery provided in this application. The secondary battery can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0158] As the electrical device, a single battery cell, a battery module, or a battery pack can be selected according to its usage requirements.

[0159] Figure 7This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0160] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0161] Example

[0162] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0163] Preparation of the first negative electrode active material

[0164] Material 1-1

[0165] Step 1: Crush and shape the petroleum coke (sulfur content 0.6%, volatile content 12%) to obtain a shaped material of primary particles with a volume distribution particle size Dv50 of 12.8μm;

[0166] Step 2: The above-mentioned shaped material is pre-carbonized at 1150℃ in a roller kiln to obtain an intermediate.

[0167] Step 3: Graphitize the above intermediate at 2450℃ to obtain material 1-1, which serves as the first negative electrode active material.

[0168] Materials 1-2 to 1-5 and materials 1-1' to 1-2'

[0169] The preparation methods of materials 1-2 to 1-5 and materials 1-1' to 1-2' are similar to those of material 1-1. The difference is that the types of raw materials, the particle size Dv50 of the shaping material, and the graphitization temperature in the preparation methods are adjusted according to the parameters shown in Table 1.

[0170] The above materials were tested using the following methods, and the results are shown in Table 1.

[0171] Test of graphitization degree of material:

[0172] Following the testing method in JIS K 0131-1996, a Bruker D8 Discover X-ray diffractometer was used for testing, with a copper target as the anode and CuKα rays as the radiation source. The wavelength of the rays was... The scanning angle range was 20°-80°, and the scanning rate was 4° / min. The average interlayer spacing d002 of the C(002) crystal planes in the material's crystal structure was obtained. Then, the degree of graphitization was calculated using the formula g = (0.344 - d002) / (0.344 - 0.3354) × 100%. In the above formula, d002 is the average interlayer spacing of the C(002) crystal planes in the material's crystal structure, expressed in nanometers (nm).

[0173] Test of powder OI value of material :

[0174] Following the testing method in JIS K 0131-1996, a Bruker D8 Discover X-ray diffractometer was used for testing, with a copper target as the anode and CuKα rays as the radiation source. The wavelength of the rays was... The X-ray diffraction pattern of the powder sample was obtained by scanning with a 2θ angle range of 20°-80° and a scanning rate of 4° / min. Based on the X-ray diffraction pattern, I0 was calculated. 004 and I 110 , where I 004 I is the integrated area of ​​the diffraction peak of the crystalline carbon 004 crystal plane in the powder sample. 110 This represents the integrated area of ​​the diffraction peak on the 110 crystal plane of crystalline carbon in the powder sample. Based on the OI value = I... 004 / I 110 The OI value of the powder in the sample was calculated.

[0175] Test of gram capacity of material :

[0176] The prepared sample powder was mixed with conductive carbon black (Super P), binder polyvinylidene fluoride (PVDF) at a mass ratio of 91.6:1.8:6.6 and solvent N-methylpyrrolidone (NMP) to form a slurry. The prepared slurry was coated onto the surface of the negative electrode current collector copper foil and dried in an oven for later use. Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed at a volume ratio of 1:1:1 to obtain an organic solvent. LiPF6 was then dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L. A lithium metal sheet was used as the counter electrode, and a 12 μm polyethylene (PE) film was used as the separator. The prepared electrolyte was then assembled into a CR2430 coin cell in an argon-protected glove box.

[0177] After the obtained coin cells were left to stand for 12 hours, they were first discharged at 25°C with a constant current of 0.15 mA to 0.005 V. After standing for 10 minutes, they were discharged again with a constant current of 50 μA to 0.005 V. After standing for 10 minutes, they were discharged again with a constant current of 10 μA to 0.005 V. The first discharge capacity of the coin cells was recorded. Then, they were charged with a constant current of 0.3 mA to 2.0 V. The first charge capacity of the coin cells was recorded. The ratio of the first charge capacity to the mass of the material is the material's specific capacity.

[0178] Test of volume distribution particle size of material :

[0179] Dv50 test: The test was conducted using a Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK, in accordance with GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method.

[0180] Test of powder compaction density of material :

[0181] A certain amount of powder is placed in a compaction mold, and then the mold is placed on a compaction density instrument. The pressure is set to 50000N. The thickness of the powder under this pressure (the thickness after depressurization) can be read on the instrument. The compaction density is calculated by ρ = m / v.

[0182] I D / I G Test of values

[0183] The Raman spectrometer was Horiba LabRAM HR800. The testing conditions were: excitation wavelength 532 nm, grating 600 lines, objective lens 50x, integration time 10 s, 3 integration scans, and area scanning. The intensities of the D and G peaks at 100 points were obtained, and the I values ​​at 100 points were calculated. D / I G Remove the largest and smallest 30 I's. D / I G The average of the remaining 40 points is the material's I. D / I G .

[0184] Table 1:

[0185]

[0186] Preparation of the second negative electrode active material

[0187] Material 2-1

[0188] Step 1: The calcined needle coke raw material (carbon content of 98.5%, volatile content of 1.0%, sulfur content of 0.5%) is mechanically crushed and shaped to obtain a shaped material with a Dv50 of 11μm;

[0189] Step 2: Using granulated asphalt with a softening point of 200℃ as the first binder, the above-mentioned shaping material is granulated together in a granulation kettle to obtain granulated material, wherein the mass ratio of the shaping material to the first binder is 100:8.

[0190] Step 3: The above granulated material is pre-carbonized at 1150°C for 2 hours under a nitrogen atmosphere to obtain an intermediate.

[0191] Step 4: The above intermediate is graphitized at a high temperature of 3020℃. The graphitized particles are sieved and demagnetized to obtain a first carbon material with La(110) of 131.7nm and Lc(002) of 32.5nm, which is used as the first negative electrode active material 1-1.

[0192] Materials 2-2 to 2-4 and materials 2-1' to 2-2'

[0193] The preparation methods of materials 2-2 to 1-4 and materials 2-1' to 2-2' are similar to those of material 2-1. The difference is that the types of raw materials and the graphitization temperature are adjusted so that the La(110), Lc(002), La / Lc, powder OI value, specific capacity, powder compaction density and Dv50 of materials 2-2 to 2-4 and materials 2-1' to 2-2' are the values ​​shown in Table 2.

[0194] For the above materials, the powder OI value, specific capacity, powder compaction density and Dv50 and other parameters were tested according to the method in the preparation of the first negative electrode active material. The results are shown in Table 2.

[0195] In addition, the above materials were tested according to the following methods, and the results are shown in Table 2.

[0196] Test of La(110), Lc(002), La / Lc of material:

[0197] Following the testing method in JIS K 0131-1996, a Bruker D8 Discover X-ray diffractometer was used for testing, with a copper target as the anode and CuKα rays as the radiation source. The wavelength of the rays was... The scanning angle range is 20°-80°, and the scanning rate is 4° / min. The peak intensity and full width at half maximum (FWHM) of the diffraction peaks corresponding to the (110) crystal plane and the (002) crystal plane are obtained. Then, La and Lc are calculated according to the following formula:

[0198] La calculation formula:

[0199] Where: K is the shape factor, taken as 1.84; λ is the wavelength (nm); β110 is the full width at half maximum (FWHM) of the 110 crystal plane corresponding to ~43° in the X-ray diffraction pattern; θ110 is the diffraction angle corresponding to the 110 crystal plane in the X-ray diffraction pattern.

[0200] Lc calculation formula:

[0201] Where: K is the shape factor, taken as 0.89; λ is the wavelength (nm); β002 is the full width at half maximum (FWHM) of the 002 crystal plane corresponding to ~26° in the X-ray diffraction pattern; θ002 is the diffraction angle corresponding to the 002 crystal plane in the X-ray diffraction pattern.

[0202] Proportion X of platform lithium intercalation capacity in the whole

[0203] The sample powder, conductive carbon black (Super P), and binder polyvinylidene fluoride (PvDF) were mixed uniformly with solvent N-methylpyrrolidone (NMP) at a mass ratio of 91.6:1.8:6.6 to prepare a slurry. The prepared slurry was coated onto the surface of the negative electrode current collector copper foil and dried in an oven for later use. Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed at a volume ratio of 1:1:1 to obtain an organic solvent. LiPF6 was then dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L. A lithium metal sheet was then used as the counter electrode. A polyethylene (PE) film was used as a separator, and the electrolyte was assembled into a CR2430 coin cell in an argon-protected glove box. After the coin cell was left to stand for 12 hours, it was first discharged at 25°C with a constant current of 0.15 mA to 0.005 V, left to stand for 10 minutes, then discharged again with a constant current of 50 μA to 0.005 V, left to stand for 10 minutes, and then discharged again with a constant current of 10 μA to 0.005 V. Afterwards, it was charged at a constant current of 0.3 mA to 2.0 V, and the relationship between the charge / discharge capacity and voltage was obtained, i.e., the charge / discharge curve. The lithium intercalation capacity of the lithium intercalation plateau appearing in the discharge curve between 0.005 V and 0.070 V was denoted as C1, and the total lithium intercalation capacity in the range of 0.005 V to 2.0 V was denoted as C2. The lithium intercalation capacity percentage of the plateau is X = C1 / C2*100%.

[0204] Table 2:

[0205]

[0206] Example 1

[0207] (1) Preparation of the first negative electrode active material slurry

[0208] The first negative electrode active material, conductive agent Super P, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber were thoroughly mixed in an appropriate amount of deionized water at a weight ratio of 96.6:0.80:1.0:1.6 to form a slurry of the first negative electrode active material.

[0209] (2) Preparation of the second negative electrode active material slurry

[0210] The above-mentioned second negative electrode active material, conductive agent Super P, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber are mixed thoroughly in an appropriate amount of deionized water at a weight ratio of 96.6:0.80:1.0:1.6 to form a slurry of the second negative electrode active material.

[0211] (3) Preparation of negative electrode sheet

[0212] Using a dual-cavity coating device, a first negative electrode active material slurry and a second negative electrode active material slurry are simultaneously extruded, with a mass ratio of 1:1. The second negative electrode active material slurry is coated onto the negative electrode current collector copper foil, specifically in the lower coating area, while the first negative electrode active material slurry is coated onto the upper coating area. After drying and cold pressing, the negative electrode sheet is obtained.

[0213] (4) Preparation of positive electrode sheet

[0214] Lithium iron phosphate, conductive carbon black, and PVDF were mixed in a mass ratio of 96.5:1.7:1.8, and then N-methylpyrrolidone solvent was added and stirred until homogeneous to obtain a positive electrode slurry. The positive electrode slurry was coated onto both surfaces of the positive electrode current collector aluminum foil, and after drying and cold pressing, a positive electrode sheet was obtained.

[0215] (5) Preparation of lithium-ion batteries

[0216] A polyethylene membrane is used as the separator. PVDF slurry is sprayed on both sides of the separator, and a ceramic coating of 1μm is sprayed on one side, with the ceramic coating side corresponding to the cathode electrode.

[0217] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. LiPF6 was then dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0218] The positive and negative electrode sheets prepared above are placed in sequence, with the separator placed between the positive and negative electrode sheets to provide isolation. Then, the electrode assembly is wound up to obtain the electrode assembly. The electrode assembly is placed in the outer packaging, dried, and then injected with electrolyte. After vacuum sealing, standing, formation, and shaping, a secondary battery is obtained.

[0219] Example 2

[0220] The battery preparation method in Example 2 is similar to that in Example 1, except that the types of the first and second negative electrode active materials are adjusted in the negative electrode preparation steps, as shown in Table 3.

[0221] Comparative Example 1

[0222] The battery preparation method of Comparative Example 1 is similar to that of Example 1, except that in the preparation step of the negative electrode sheet, the first negative electrode active material slurry is coated on the lower region and the second negative electrode active material slurry is coated on the upper region.

[0223] Comparative Example 2

[0224] The battery preparation method of Comparative Example 2 is similar to that of Example 1, except that in the preparation step of the negative electrode sheet, the first negative electrode active material 1-1 and the negative electrode active material 2-1 are mixed evenly to prepare a negative electrode slurry, and a single-layer negative electrode film is formed on the negative electrode current collector.

[0225] Comparative Example 3

[0226] The battery preparation method of Comparative Example 3 is similar to that of Example 1, except that in the preparation step of the negative electrode sheet, only the first negative electrode active material 1-1 is used to form a single layer of negative electrode film on the negative electrode current collector.

[0227] Comparative Example 4-5

[0228] The battery preparation methods of Comparative Examples 4-5 are similar to those of Example 1, except that the types of the first and second negative electrode active materials are adjusted in the negative electrode preparation steps, as shown in Table 3.

[0229] Performance testing

[0230] (1) Specific capacity of the negative electrode sheet

[0231] A coin cell was assembled using a negative electrode and a lithium sheet. After the resulting coin cell was left to stand for 12 hours, it was discharged at 25°C with a constant current of 0.05C to 0.005V. After standing for 10 minutes, it was discharged again with a constant current of 50μA to 0.005V. After standing for 10 minutes, it was discharged again with a constant current of 10μA to 0.005V. The resulting capacity is the lithium insertion capacity of the negative electrode. Then, it was charged at a constant current of 0.1C to 2V. The resulting capacity is the lithium removal capacity of the negative electrode.

[0232] (2) Compacted density of the negative electrode sheet

[0233] The electrode is cut into small round pieces with a diameter of 13mm. The mass and thickness of the small round pieces are measured by a balance and a micrometer. The electrode compaction is calculated according to the formula PD (compaction) = m (mass) / V (volume).

[0234] (3) Energy density

[0235] At 25℃, the secondary battery was charged at a constant current of 1 / 3C to 3.65V, then charged at a constant voltage of 3.65V to a current of 0.05C, allowed to stand for 5 minutes, and then discharged at a constant current of 1 / 3C to 2.5V. The battery discharge energy was recorded at this point. The battery discharge energy divided by the battery weight is the battery's gravimetric energy density, expressed in Wh / kg.

[0236] (4) Cycle performance test of secondary batteries

[0237] At 45℃, the secondary battery was charged at a constant current of 1C to the upper limit of 3.65V (corresponding to 100% SOC), then charged at a constant voltage to a current of 0.05C. After resting for 5 minutes, the secondary battery was discharged at a constant current of 1C to 2.5V (corresponding to 0% SOC), and the discharge capacity at this point was recorded, which is the discharge capacity of the first cycle. The secondary battery was subjected to a cyclic charge-discharge test using the above method, and the discharge capacity after each cycle was recorded.

[0238] The capacity retention rate (%) of a secondary battery after 1000 cycles at 45℃ = discharge capacity after 1000 cycles / discharge capacity of the first cycle × 100%.

[0239] The performance of the secondary batteries prepared in the above embodiments and comparative examples was tested, and the results are shown in Table 3.

[0240] Table 3

[0241]

[0242] As can be seen from the data in Table 3, compared with Comparative Examples 1 and 2, the secondary batteries prepared in Examples 1-2 of this application employ a double-layer design for the negative electrode. The first region uses a first negative electrode active material with low graphitization and high isotropy, while the second region uses a second negative electrode active material with high energy density. Therefore, the secondary battery can improve cycle performance while maintaining energy density. The secondary battery in Comparative Example 3, using a negative electrode prepared with a first negative electrode active material, exhibits good cycle performance but low energy density. In contrast, Examples 1-2 of this application employ a double-layer design, which can improve the cycle performance of the secondary battery while also increasing energy density.

[0243] Example 3-12

[0244] The battery preparation methods in Examples 3-12 are similar to those in Example 1, except that the types and mass ratios of the first and second negative electrode active materials are different.

[0245] The performance of the secondary batteries prepared in the above embodiments was tested, and the results are shown in Table 4.

[0246] Table 4

[0247]

[0248]

[0249] As can be seen from the data in Table 4, the secondary batteries prepared in the embodiments of this application can improve cycle performance while maintaining energy density.

[0250] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A secondary battery, comprising a negative electrode, characterized in that, The negative electrode sheet includes a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector. The negative electrode film layer has a first surface away from the negative current collector and a second surface disposed opposite to the first surface. The thickness of the negative electrode film layer is denoted as H. The region from the first surface of the negative electrode film layer to a thickness range of 0.3H is denoted as the first region of the negative electrode film layer, and the region from the second surface of the negative electrode film layer to a thickness range of 0.3H is denoted as the second region of the negative electrode film layer. The first region includes a first negative electrode active material, and the second region includes a second negative electrode active material. The first negative electrode active material comprises primary particles of first artificial graphite. The powder OI value of the first negative electrode active material is 3-6, and the degree of graphitization is 91%-93.5%. The powder OI value is the ratio of the peak area of ​​surface (004) to the peak area of ​​surface (001) obtained by X-ray diffraction (XRD) testing of the material. The specific capacity of the second negative electrode active material is greater than that of the first negative electrode active material. The specific capacity of the first negative electrode active material is 335 mAh / g - 350 mAh / g.

2. The secondary battery according to claim 1, characterized in that, The powder OI value of the first negative electrode active material is 3-3.8, and / or the degree of graphitization is 91.0%-92.5%.

3. The secondary battery according to claim 1 or 2, characterized in that, The first negative electrode active material I D / I G The value is 0.08-0.17; Among them, I D This indicates that the Raman spectrum of the material is at 1350±50 cm⁻¹ -1 The intensity of peak D at I G This indicates that the Raman spectrum of the material is at 1580±50 cm⁻¹ -1 The intensity of peak G at point I; D / I G The value represents the ratio of the D peak intensity to the G peak intensity.

4. The secondary battery according to claim 3, characterized in that, The first negative electrode active material I D / I G The value is 0.1-0.

15.

5. The secondary battery according to any one of claims 1-4, characterized in that, The specific capacity of the second negative electrode active material is above 359.0 mAh / g.

6. The secondary battery according to claim 5, characterized in that, The specific capacity of the first negative electrode active material is 335 mAh / g - 345 mAh / g; and / or The specific capacity of the second negative electrode active material is 360mAh / g-366mAh / g.

7. The secondary battery according to claim 1 or 2, characterized in that, The compacted powder density of the first negative electrode active material at 50000N is less than that of the second negative electrode active material at 50000N.

8. The secondary battery according to claim 7, characterized in that, The first negative electrode active material has a powder compaction density of 1.65 g / cc - 1.83 g / cc at 50000 N; and / or, The second negative electrode active material has a powder compaction density of 1.95 g / cc - 2.04 g / cc under a pressure of 50,000 N.

9. The secondary battery according to claim 8, characterized in that, The first negative electrode active material has a powder compaction density of 1.65 g / cc - 1.71 g / cc at 50000 N; and / or, The second negative electrode active material has a powder compaction density of 1.96-2.02 g / cc under a pressure of 50,000 N.

10. The secondary battery according to claim 1 or 2, characterized in that, The degree of graphitization of the first negative electrode active material is less than that of the second negative electrode active material.

11. The secondary battery according to claim 10, characterized in that, The degree of graphitization of the second negative electrode active material is 94.0%-96.0%.

12. The secondary battery according to claim 11, characterized in that, The graphitization degree of the second negative electrode active material is 94.2%-95.8%.

13. The secondary battery according to claim 1 or 2, characterized in that, The volume distribution particle size Dv50 of the first negative electrode active material is 9.0 μm - 13.5 μm; and / or, The volume distribution particle size Dv50 of the second negative electrode active material is 14.5 μm - 18.0 μm.

14. The secondary battery according to claim 13, characterized in that, The volume distribution particle size Dv50 of the first negative electrode active material is 12.5-13.5 μm; and / or, The volume distribution particle size Dv50 of the second negative electrode active material is 15.0 μm - 17.5 μm.

15. The secondary battery according to claim 1 or 2, characterized in that, The specific surface area of ​​the first negative electrode active material is less than or equal to the specific surface area of ​​the second negative electrode active material.

16. The secondary battery according to claim 15, characterized in that, The specific surface area of ​​the first negative electrode active material is 0.8 m². 2 / g -1.8m 2 / g; and / or, The specific surface area of ​​the second negative electrode active material is 0.8 m². 2 / g -2.1m 2 / g.

17. The secondary battery according to claim 1 or 2, characterized in that, The secondary battery satisfies one or more of the following conditions: The specific capacity of the negative electrode sheet is 348.0 mAh / g - 355.0 mAh / g; The powder compaction density of the negative electrode film layer under a pressure of 50,000 N is 1.60 g / cc - 1.70 g / cc; The areal density of the negative electrode film is 10.0 mg / cm³. 2 -20.0 mg / cm 2 ; The thickness of the negative electrode film is 70μm-250μm.

18. The secondary battery according to claim 1 or 2, characterized in that, In the negative electrode film layer, the mass ratio of the first negative electrode active material to the second negative electrode active material is 3:7-6:

4.

19. The secondary battery according to claim 1 or 2, characterized in that, The second negative electrode active material includes a second artificial graphite, wherein the La(110) of the second artificial graphite is 130nm-175nm, and the Lc(002) is 30nm-42nm. La(110) represents the crystallite size along the a-axis in the (110) crystal plane of the material, and Lc(002) represents the crystallite size along the c-axis in the (002) crystal plane of the material.

20. The secondary battery according to claim 19, characterized in that, The second artificial graphite has a La(110) of 132nm-172nm and an Lc(002) of 30nm-36nm.

21. The secondary battery according to claim 19 or 20, characterized in that, The ratio of La(110) / Lc(002) is 3.5-5.

5.

22. The secondary battery according to claim 19 or 20, characterized in that, Charge-discharge tests were conducted on the coin cell prepared from the second artificial graphite under conditions of 0.1C delithiation rate and 0.05C lithium insertion rate, respectively, to obtain charge-discharge curves in the range of 0.005V-2.0V. In the discharge curve, a lithium insertion plateau exists in the voltage range of 0.005V-0.070V, and the discharge capacity corresponding to the lithium insertion plateau accounts for more than 43% of the total discharge capacity of the coin cell.

23. The secondary battery according to claim 22, characterized in that, The discharge capacity corresponding to the lithium intercalation platform accounts for 43%-47% of the total discharge capacity of the coin cell.

24. The secondary battery according to claim 19 or 20, characterized in that, The second artificial graphite comprises secondary particles.

25. The secondary battery according to claim 19 or 20, characterized in that, The particle size distribution (Dv90-Dv10) / Dv50 of the second negative electrode active material is 0.9-1.

25.

26. An electrical appliance, characterized in that, The secondary battery includes any one of claims 1-25.

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