Secondary battery and electric device

Abstract

The invention provides a secondary battery and an electric device. The secondary battery comprises a negative electrode piece, the negative electrode piece comprises a negative electrode current collector and a negative electrode film layer, the negative electrode film layer is provided with a first surface far away from the negative electrode current collector and a second surface opposite to the first surface, and the thickness of the negative electrode film layer is marked as H, a region from the second surface of the negative electrode film layer to a thickness range of 0.3 H is marked as a first region of the negative electrode film layer, the first region comprises a first negative electrode active material, and the first negative electrode active material comprises a first carbon-based material; a region from the first surface of the negative electrode film layer to a thickness range of 0.3 H is marked as a second region of the negative electrode film layer, the second region comprises a second negative electrode active material, and the second negative electrode active material comprises a second carbon-based material; the first carbon-based material comprises natural graphite; and the total pore area S2 of the inner region of the second carbon-based material is larger than the total pore area S1 of the outer region of the second carbon-based material.

Description

Technical Field

[0001] The present application relates to the technical field of batteries, and particularly to a secondary battery and an electrical device. Background Art

[0002] In recent years, secondary batteries have been widely used in energy storage power systems such as hydroelectric, thermal, wind, and solar power stations, as well as in multiple fields such as power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, and aerospace. With the increasingly wide application range of secondary batteries, severe challenges have been posed to the performance of secondary batteries. For example, for electric devices, secondary batteries are becoming increasingly miniaturized, capable of rapid charging and discharging, and having a longer service life. Therefore, it is required that secondary batteries have a higher energy density, cycle life, and better kinetic performance. Summary of the Invention

[0003] The present application is made in view of the above problems, and its purpose is to provide a secondary battery and an electrical device. The secondary battery has a high energy density and improved kinetic performance.

[0004] A first aspect of the present application provides a secondary battery, including a negative electrode plate. The negative electrode plate includes 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 has a first surface away from the negative electrode current collector and a second surface oppositely arranged with the first surface. The thickness of the negative electrode film layer is denoted as H. Among them, the region within the thickness range from the second surface of the negative electrode film layer to 0.3H is denoted as the first region of the negative electrode film layer. The first region is in contact with the surface of the negative electrode current collector and includes a first negative electrode active material. The first negative electrode active material includes a first carbon-based material. The region within the thickness range from the first surface of the negative electrode film layer to 0.3H is denoted as the second region of the negative electrode film layer. The second region includes a second negative electrode active material. The second negative electrode active material includes a second carbon-based material.

[0005] The first carbon-based material includes natural graphite.

[0006] The second carbon-based material includes an outer region and an inner region located inside the outer region. The outer region refers to the region formed by extending a distance of 2.5 μm from the particle surface of the second carbon-based material into the particle interior. In the cross-sectional view of the second carbon-based material, the total pore area of the outer region is denoted as S1, and the total pore area of the inner region is denoted as S2. Then, the second carbon-based material satisfies S2 > S1.

[0007] The first carbon-based material is natural graphite. Natural graphite has a relatively large pore structure, provides a relatively large reaction surface, is conducive to the reaction of active ions, and has a relatively high gram capacity. The total pore area S1 of the outer region of the second carbon-based material is smaller than the total pore area S2 of the inner region. This means that the structure of the outer region of the second carbon-based material is denser than that of the inner region, which improves the stability of the material. The arrangement of the negative electrode film layer of the negative electrode plate of the secondary battery of the present application in the above manner can give full play to the advantages of the first carbon-based material and the second carbon-based material, so that the secondary battery has an improved energy density and takes into account the cycle performance.

[0008] In some embodiments, the volume distribution particle size Dv50 of the first carbon-based material is greater than the volume distribution particle size Dv50 of the second carbon-based material. The second carbon-based material is mainly located in the second region away from the current collector, so a smaller volume distribution particle size is beneficial to further improve the kinetic performance of the secondary battery.

[0009] In some embodiments, the volume distribution particle size Dv50 of the first carbon-based material is ≥16 μm. Optionally, the volume distribution particle size Dv50 of the first carbon-based material is 16 μm-20 μm.

[0010] In some embodiments, the volume distribution particle size Dv50 of the second carbon-based material is ≤15 μm. Optionally, the volume distribution particle size Dv50 of the second carbon-based material is 10 μm-15 μm.

[0011] Controlling the volume distribution particle size Dv50 of the first carbon-based material and the second carbon-based material within the above range is beneficial to improving the transport performance of active ions, thereby further improving the kinetic performance of the secondary battery.

[0012] In some embodiments, at least a portion of the surface of the first carbon-based material has a coating layer; and / or at least a portion of the surface of the second carbon-based material has a coating layer. The carbon coating layer can further enhance active ion transport, which is beneficial to the kinetic performance of the secondary battery.

[0013] In some embodiments, the graphitization degree of the first carbon-based material is ≥95%, and optionally, the graphitization degree of the first carbon-based material is 95.0% to 98.0%; the graphitization degree of the second carbon-based material is ≥95% and the graphitization degree of the second carbon-based material is 95.5% to 98.5%. Both the first carbon-based material and the second carbon-based material have a high degree of graphitization, which, on the one hand, enables the negative electrode active material to have a higher gram capacity, and on the other hand, is conducive to improving the electron transmission in the negative electrode film layer, so that the secondary battery has improved kinetic performance and excellent energy density.

[0014] In some embodiments, the first carbon-based material has a lower degree of graphitization than the second carbon-based material.

[0015] Graphitization degree of the material.

[0016] In some embodiments, the second carbon-based material satisfies 1.5 ≤ S2 / S1 ≤ 500, optionally 2.4 ≤ S2 / S1 ≤ 450. When the second carbon-based material satisfies the above range of S2 / S1, the pore structure is mainly distributed in the internal region. Therefore, the side reaction between the second carbon-based material and the electrolyte is less, reducing the consumption of active ions and improving the cycling performance of the secondary battery.

[0017] In some embodiments, the specific surface area of the first carbon-based material is greater than that of the second carbon-based material. Optionally, the BET specific surface area of the first carbon-based material is ≤ 2.2 m 2 / g. More optionally, the BET specific surface area of the first carbon-based material is 1.8 m 2 / g - 2.2 m 2 / g. Optionally, the BET specific surface area of the second carbon-based material is ≤ 2.1 m 2 / g. More optionally, the BET specific surface area of the second carbon-based material is 1.3 m 2 / g - 2.0 m 2 / g. When the specific surface areas of the first carbon-based material and the second carbon-based material are within the above ranges, it is beneficial to balance the kinetic performance and the cycling performance. In particular, the relatively small specific surface area of the second carbon-based material mainly located in the second region of the negative electrode plate away from the current collector can relatively reduce the side reaction with the electrolyte, thus being beneficial to the cycling performance.

[0018] In some embodiments, based on the total weight of the negative electrode film layer, the amount of the second carbon-based material is ≥ 30 wt%, optionally 50 wt% - 70 wt%. When the mass ratio of the second carbon-based material in the negative electrode film layer is within the above range, the secondary battery can have excellent energy density while obtaining improved kinetic performance and balancing the cycling performance.

[0019] In some embodiments, the specific capacity of the first carbon-based material is ≥ 355 mAh / g. Optionally, the specific capacity of the first carbon-based material is 358 mAh / g - 367 mAh / g. When the specific capacity of the first carbon-based material is within the above range, the secondary battery can have a relatively high energy density.

[0020] In some embodiments, the second carbon-based material satisfies at least one of the following:

[0021] (1) The volume distribution particle size Dv90 of the second carbon-based material is ≤ 25 μm; optionally 18 μm - 25 μm.

[0022] (2) The particle size distribution of the second carbon-based material, [(Dv90)-(Dv10)] / (Dv50), is ≤ 1.30. Optionally, the particle size distribution of the second carbon-based material, [(Dv90)-(Dv10)] / (Dv50), is 1.05 - 1.25. When the particle size distribution of the first carbon-based material is within the above range, its particle packing performance is better, which is beneficial to improving the compaction density of the negative electrode film layer and the energy density of the secondary battery; in addition, it is also beneficial to form a reasonable pore structure between the particles of the negative electrode film layer, improving the transport performance of active ions and electrons, thereby improving the kinetic performance of the secondary battery.

[0023] (3) The specific capacity of the second carbon-based material is ≥ 358 mAh / g. Optionally, the specific capacity of the second carbon-based material is 360 mAh / g - 370 mAh / g. When the specific capacity of the second carbon-based material is within the above range, the secondary battery can have a relatively high energy density.

[0024] (4) The area of the pore structure in the outer region of the second carbon-based material is less than or equal to 0.15 μm 2 , and can be optionally less than or equal to 0.13 μm 2 . The second carbon-based material with this structure has a relatively dense outer region, which can reduce the entry of electrolyte into the particles, thereby reducing the side reactions with the electrolyte and being beneficial to the storage performance of the secondary battery.

[0025] (5) The inner region of the second carbon-based material includes one or more pore structures with a pore area of greater than or equal to 0.15 μm 2 . Optionally, the second carbon-based material includes one or more pore structures with a pore area of 0.15 - 2.0 μm 2 . By making the inner region of the second carbon-based material include pore structures of the above size, on the one hand, it can reserve sufficient and stable expansion space for the volume change of the first carbon-based material particles, and on the other hand, it can also improve the compaction density of the negative electrode film layer, thereby improving the energy density of the secondary battery.

[0026] In some embodiments, the first negative electrode active material and / or the second negative electrode active material further includes a silicon-based material. Optionally, the silicon-based material accounts for ≤ 10 wt% of the first negative electrode active material; and / or, the silicon-based material accounts for ≤ 10 wt% of the second negative electrode active material. Optionally, the mass ratio of the silicon-based material in the first negative electrode active material is greater than the mass ratio of the silicon-based material in the second negative electrode active material.

[0027] In some embodiments, the negative electrode film layer satisfies at least one of the following items:

[0028] (1) The compaction density of the negative electrode film layer is ≤ 1.85 g / cm3 Optionally, the compaction density of the negative electrode film layer is 1.55 g / cm 3 - 1.85 g / cm 3 .

[0029] (2) The areal density of the negative electrode film layer ≥ 6.0 mg / cm 2 Optionally, the areal density of the negative electrode film layer is 7.0 mg / cm 2- - 15.0 mg / cm 2 .

[0030] (3) The porosity of the negative electrode film layer is 18.0 - 36.7%, optionally, the porosity of the negative electrode film layer is 19.0 - 34.0%. This is beneficial for the negative electrode film layer to balance high capacity and a suitable pore structure, and further beneficial for the secondary battery to balance high energy density and good storage performance and kinetic performance.

[0031] (4) The thickness of the negative electrode film layer is ≥ 60 μm. Optionally, the thickness of the negative electrode film layer is 70 μm - 130 μm.

[0032] The second aspect of the present application further provides an electrical device, including the secondary battery of the first aspect of the present application.

[0033] The electrical device of the present application includes the secondary battery provided by the present application, and thus has at least the same advantages as the secondary battery. Description of the Drawings

[0034] Figure 1 is a schematic diagram of an embodiment of the negative electrode plate of the present application.

[0035] Figure 2 is a schematic diagram of another embodiment of the negative electrode plate of the present application.

[0036] Figure 3 is a schematic diagram of yet another embodiment of the negative electrode plate of the present application.

[0037] Figure 4 is an ion polishing cross-section (CP) diagram of an embodiment of the second carbon-based material of the present application.

[0038] Figure 5 is a schematic diagram of a cross-sectional image of particles of the second carbon-based material of the present application.

[0039] Figure 6 is a schematic diagram of a battery cell of an embodiment of the present application.

[0040] Figure 7 is Figure 6 an exploded view of the battery cell of an embodiment of the present application shown.

[0041] Figure 8 It is a schematic diagram of a battery module according to an embodiment of the present application.

[0042] Figure 9 It is a schematic diagram of a battery pack according to an embodiment of the present application.

[0043] Figure 10 is Figure 9 An exploded view of the battery pack according to an embodiment of the present application shown.

[0044] Figure 11 It is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of the present application.

[0045] Explanation of reference numerals:

[0046] 1 Battery pack; 2 Upper box body; 3 Lower box body; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly; 10 Negative electrode tab; 101 Negative current collector; 102 Negative electrode film layer; 102a First surface; 102b Second surface; 1021 First region; 1022 Second region; 1023 Intermediate region; 200 Second carbon-based material; 201 External region; 202 Internal region. Detailed implementation manners

[0047] Hereinafter, embodiments of the secondary battery and the electrical device of the present application specifically disclosed will be described in detail with appropriate reference to the drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually identical structures are omitted. This is to prevent the following description from becoming unnecessarily long and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter recited in the claims.

[0048] The "ranges" disclosed in this application are defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of a particular range. The ranges defined in this way can include or exclude the end values, and can be combined arbitrarily, 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, ranges of 60 - 110 and 80 - 120 are also contemplated. In addition, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, then the following ranges are all contemplated: 1 - 3, 1 - 4, 1 - 5, 2 - 3, 2 - 4, and 2 - 5. In this application, unless otherwise specified, the numerical range "a - b" represents an abbreviated representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0 - 5" means that all real numbers between "0 - 5" are fully listed herein, and "0 - 5" is only an abbreviated representation of these numerical combinations. Additionally, when stating that a certain parameter is an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, the integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0049] If there is no special instruction, all embodiments and optional embodiments of this application can be combined with each other to form new technical solutions.

[0050] If there is no special instruction, all technical features and optional technical features of this application can be combined with each other to form new technical solutions.

[0051] If there is no special instruction, all steps of this application can be carried out in sequence or randomly, preferably in sequence. For example, the method includes steps (a) and (b), which means that the method can include steps (a) and (b) carried out in sequence, or can also include steps (b) and (a) carried out in sequence. For example, it is mentioned that the method may further include step (c), which means that step (c) can be added to the method in any order. For example, the method can include steps (a), (b), and (c), or can also include steps (a), (c), and (b), or can also include steps (c), (a), and (b), etc.

[0052] Natural graphite has a relatively high specific capacity, but has more surface defects and more internal pores. During charge and discharge cycles, it is prone to side reactions with the electrolyte and is prone to swelling, resulting in poor cycle life. For this reason, attempts have been made to use two different negative electrode active materials to coat the upper and lower layers of the negative electrode film layer respectively, with natural graphite disposed in the lower layer, and at the same time, a material with better cycle performance, such as artificial graphite, is disposed in the upper layer. However, the specific capacity of artificial graphite is relatively low, resulting in insufficient energy density of the secondary battery.

[0053] Therefore, it is difficult for current secondary batteries to achieve both high energy density and good cycle performance.

[0054] Based on this, a first aspect of the present application provides a secondary battery with high energy density and good cycle performance.

[0055] The term "secondary battery" mentioned herein refers to a battery cell, a battery module, or a battery pack.

[0056] Generally, a secondary battery cell includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. During the charge and discharge process of the battery, active ions are embedded and extracted back and forth between the positive electrode sheet and the negative electrode sheet. The electrolyte plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is disposed between the positive electrode sheet and the negative electrode sheet, mainly to prevent short circuit between the positive and negative electrodes, and at the same time allows ions to pass through.

[0057] [Negative electrode sheet]

[0058] The secondary battery includes a negative electrode sheet, the negative electrode sheet includes 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 has 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 is denoted as H. Wherein, the region within the thickness range from the second surface of the negative electrode film layer to 0.3H is denoted as the first region of the negative electrode film layer. The first region is close to the negative electrode current collector and includes a first negative electrode active material. The first negative electrode active material includes a first carbon-based material. The region within the thickness range from the first surface of the negative electrode film layer to 0.3H is denoted as the second region of the negative electrode film layer. The second region is away from the surface of the negative electrode current collector and includes a second negative electrode active material. The second negative electrode active material includes a second carbon-based material. Wherein, the first carbon-based material includes natural graphite. The second carbon-based material includes an outer region and an inner region located inside the outer region. The outer region refers to the region formed by extending a distance of 2.5 μm from the particle surface of the second carbon-based material into the particle. In the cross-sectional view of the second carbon-based material, the total pore area of the outer region is denoted as S1, and the total pore area of the inner region is denoted as S2. Then, the second carbon-based material satisfies S2 > S1.

[0059] See Figures 1 to 3 , which shows schematic diagrams of several different specific embodiments of the negative electrode sheet of the present application. As Figures 1 to 3As shown, the negative electrode plate 10 includes a negative electrode current collector 101 and a negative electrode film layer 102 formed on at least one surface of the negative electrode current collector 101. The negative electrode film layer 102 has a first surface 102a facing away from the negative electrode current collector 101 and a second surface 102b disposed opposite to the first surface 102a. The thickness of the negative electrode film layer 102 is denoted as H. The thickness H of the negative electrode film layer refers to the thickness of the negative electrode film layer on one side of the negative electrode current collector. The region within the thickness range from the second surface 102b of the negative electrode film layer to 0.3H is denoted as the first region 1021 of the negative electrode film layer. The first region 1021 is close to the negative electrode current collector 101 and is also referred to as the lower layer of the negative electrode film layer in this text. The region within the thickness range from the first surface 102a of the negative electrode film layer to 0.3H is denoted as the second region 1022 of the negative electrode film layer. The second region 1022 is away from the negative electrode current collector 101 and is also referred to as the upper layer of the negative electrode film layer in this text. The first region 1021 includes a first negative electrode active material, and the first negative electrode active material includes a first carbon-based material having a pore structure. The second region 1022 includes a second negative electrode active material, and the second negative electrode active material includes a second carbon-based material. Between the first region 1021 and the second region 1022, the region occupying the thickness range of 0.4H is denoted as the intermediate region 1023. It is easy to understand that according to Figures 1 to 3 , the intermediate region 1023 contains at least one of the first carbon-based material and the second carbon-based material.

[0060] It should be understood that Figures 1 to 3 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 plate of the present application is not limited thereto. 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.

[0061] Specifically, Figures 1 to 3 shows that the first carbon-based material and the second carbon-based material are sequentially coated on the negative electrode current collector 101. Figure 1 shows that the thicknesses of both of them each occupy approximately one-half of the thickness of the negative electrode film layer 102. Figure 2 shows that the thickness of the first carbon-based material occupies approximately 70% of the thickness of the negative electrode film layer 102, while the thickness of the second carbon-based material occupies approximately 30% of the thickness of the negative electrode film layer 102. Figure 3 shows a composition opposite to Figure 2 , that is, the thickness of the second carbon-based material occupies approximately 70% of the thickness of the negative electrode film layer 102, while the thickness of the first carbon-based material occupies approximately 30% of the thickness of the negative electrode film layer 102.

[0062] It should be understood that Figures 1 to 3Shown is a schematic diagram of an ideal situation. The present application does not particularly limit the ratio of the thickness of the first carbon-based material to the second carbon-based material. For example, the two can be in the range of 3:7 - 7:3. Exemplarily, the ratio of the thickness of the first carbon-based material to the second carbon-based material is 4:6, 5:5, 6:4, etc., or the ratio between any two ratios.

[0063] It should also be understood that Figures 1 to 3 The middle region 1023 in shows an obvious boundary with the other two regions, but in fact, such an obvious interface does not exist. As mentioned above, within the range of the middle region 1023, both the first carbon-based material and the second carbon-based material may coexist. Similarly, there is no obvious interface between the coating containing the first carbon-based material and the coating containing the second carbon-based material.

[0064] In the present application, the total pore area S1 of the outer region of the second carbon-based material is smaller than the total pore area S2 of the inner region. This means that the structure of the outer region of the first carbon-based material is denser than that of the inner region. In addition, for the first carbon-based material in the present application, "S1 > S2" means that the second carbon-based material has a pore structure that can be directly observed from a cross-sectional image (such as a scanning electron microscope image with a magnification of 1000 times). That is, the pore area of the outer region in the main structure of the second carbon-based material is smaller.

[0065] The first carbon-based material and the second carbon-based material can be distinguished by a cross-section polisher. For example, the types of the first carbon-based material and the second carbon-based material can be distinguished by testing the cross-sectional morphology (CP) of the negative electrode sheet by ion polishing. As an example, the test method can be as follows: Cut the negative electrode sheet into a test sample of a certain size (such as 2 cm × 2 cm), fix the negative electrode sheet on the sample stage with paraffin; Install the sample stage in the sample holder and lock it for fixation, turn on the power of the argon ion cross-section polisher (such as the IB-09010CP type argon ion cross-section polisher of JEOL Company, Japan) and evacuate (such as 10-4 Pa), set the argon gas flow rate (such as 0.15 MPa), voltage (such as 8 KV), and polishing time (such as 2 h), adjust the sample stage to the swing mode and start polishing; Randomly select areas in the test sample for scanning test (such as referring to JY / T010-1996, using a scanning electron microscope for scanning), and obtain the cross-sectional morphology (CP) picture of the negative electrode sheet at a certain magnification (such as 1000 times), from which the first carbon-based material and the second carbon-based material can be distinguished. Figure 4 is the cross-sectional ion polishing (CP) diagram of the second carbon-based material in the present application. It can be seen from the figure that the outer region of the second carbon-based material particles is denser than the inner region, satisfying S2 > S1.

[0066] Further reference Figure 5, which shows a schematic cross-sectional image of the particles of the second carbon-based material 200 of the present application, and the cross-sectional image passes through the center of the particles of the second carbon-based material 200. As Figure 5 shown, the region formed by extending a distance of 2.5 μm from the surface of the particles of the second carbon-based material 200 into the particles is the outer region 201, and the region of the particles 200 other than the outer region 201 is the inner region 202.

[0067] In the electrode design of the present application, the first region (lower layer region) of the negative electrode film layer uses the first carbon-based material, and the second region (upper layer region) uses the second carbon-based material. The first carbon-based material is natural graphite, which has a high specific capacity and can contribute to the energy density of the secondary battery. In addition, natural graphite has a high powder compaction density, which can further improve the energy density of the battery. When the first carbon-based material is arranged in the first region of the negative electrode film layer, the energy density of the secondary battery can be improved, and the adverse effect on the battery cycle performance can be reduced to a certain extent. The total pore area S2 of the inner region of the particles of the second carbon-based material is larger than the total pore area S1 of the outer region. On the one hand, the second carbon-based material with such a structure has a high specific capacity and can improve the energy density of the battery. On the other hand, the pore structure with a smaller area in the outer region of the second carbon-based material is beneficial to the stability of the particles, reduces the side reaction with the electrolyte, and thus has a better cycle life.

[0068] Therefore, the second carbon-based material arranged in the second region of the negative electrode film layer is combined with the first carbon-based material arranged in the first region of the negative electrode film layer, so that the secondary battery not only improves the energy density, but also takes into account the cycle performance to a certain extent.

[0069] According to some embodiments, the second carbon-based material satisfies 1.5 ≤ S2 / S1 ≤ 500, optionally, 2.4 ≤ S2 / S1 ≤ 450. Exemplarily, S2 / S1 can have the following ranges: 2.2 ≤ S 2 / S 1 ≤ 400, 2.4 ≤ S 2 / S 1 ≤ 300, 2.5 ≤ S 2 / S 1 ≤ 250, 2.6 ≤ S 2 / S 1 ≤ 200, 2.8 ≤ S 2 / S 1 ≤ 150, or 3.0 ≤ S 2 / S 1≤100. When the second carbon-based material satisfies the above range of S2 / S1, it reflects that the structure of the outer region of the second carbon-based material particle is denser than the inner region. Particles with such a structure have an internal pore structure that can reserve a certain space for their expansion, thereby maintaining stability during the charge and discharge cycle, which is beneficial to the cycle performance of the secondary battery. In addition, the outer region of the second carbon-based material particle has fewer pores, and such a material has a higher gram capacity, which is also beneficial to the improvement of the energy density of the secondary battery.

[0070] In some embodiments, the volume distribution particle size Dv50 of the first carbon-based material is greater than the volume distribution particle size Dv50 of the second carbon-based material.

[0071] In some specific embodiments, the volume distribution particle size Dv50 of the first carbon-based material is ≥15 μm. Optionally, the volume distribution particle size Dv50 of the first carbon-based material is 16 μm to 20 μm. For example, the volume distribution particle size Dv50 of the second carbon-based material can be 16 μm, 17 μm, 17.5 μm, 18 μm, 18.5 μm, 19 μm, 20 μm, but is not limited thereto, and can also be a value between the ranges formed by any two values.

[0072] Controlling the volume distribution particle size of the first carbon-based material within the above range is beneficial to improving the transport performance of active ions in the first region of the negative electrode film layer, thereby improving the kinetic performance of the secondary battery.

[0073] In some embodiments, the volume distribution particle size Dv50 of the second carbon-based material is ≤13 μm. Optionally, the volume distribution particle size Dv50 of the second carbon-based material is 7 μm-12 μm. For example, the volume distribution particle size Dv50 of the second carbon-based material may be 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, but is not limited thereto, and may also be a value between the ranges formed by any two values.

[0074] The second carbon-based material has a smaller volume distribution particle size, may have a larger specific surface area, and can react with more active ions per unit time. When the second carbon-based material is arranged in the second region (i.e., the upper layer) of the negative electrode film layer, it is beneficial to improve the kinetic performance of the secondary battery.

[0075] Therefore, when the first carbon-based material and the second carbon-based material having the above-mentioned particle size range are arranged in the lower layer and the upper layer of the negative electrode film layer respectively, the secondary battery can have good energy density, improved kinetic performance, and take into account cycle performance.

[0076] In some embodiments, at least a portion of the surface of the first carbon-based material has a coating layer, optionally a carbon coating layer (such as an amorphous carbon coating layer). Exemplarily, more than 80% of the surface of the first carbon-based material, and even the entire surface, has a coating layer.

[0077] In some embodiments, the entire outer surface of the second carbon-based material has a coating layer, optionally a carbon coating layer (such as an amorphous carbon coating layer). Exemplarily, more than 80% of the surface of the second carbon-based material, and even the entire surface, has a coating layer.

[0078] When at least a portion of the surface of the first carbon-based material and / or the second carbon-based material has a coating layer, it is more conducive to the transport of active ions, thereby facilitating the further improvement of the kinetic performance of the secondary battery.

[0079] The first carbon-based material of the present application is natural graphite. Generally, the particles of natural graphite have relatively many pore structures throughout the region. For example, if the particles of natural graphite are also divided into an internal region and an external region (the region formed by extending 2.5 μm from the surface of the natural graphite particle into the particle is the external region), then both the internal region and the external region of the natural graphite have relatively many pore structures.

[0080] In some embodiments, the graphitization degree of the first carbon-based material is ≥ 95%, optionally 95.0% - 98.0%. Exemplarily, the graphitization degree of the first carbon-based material is 95%, 96%, 96.5%, 97%, 97.5%, 98%, etc., or a value within the range composed of any two values. In some embodiments, the graphitization degree of the second carbon-based material is ≥ 95%, optionally 95.5% - 98.5%. Exemplarily, the graphitization degree of the first carbon-based material is 95.5%, 96%, 96.5%, 97%, 97.5%, 98%, 98.5%, etc., or a value within the range composed of any two values. The high graphitization degree of the first carbon-based material and / or the second carbon-based material can provide excellent specific capacity and powder compression density, thereby effectively improving the energy density of the secondary battery.

[0081] In some embodiments, the graphitization degree of the first carbon-based material is less than the graphitization degree of the second carbon-based material.

[0082] In some embodiments, the specific surface area of the first carbon-based material is greater than the specific surface area of the second carbon-based material.

[0083] In some embodiments, the specific surface area of the first carbon-based material is ≤ 2.2 m 2 / g, optionally 1.8 m 2 / g - 2.2 m 2 / g. Exemplarily, the specific surface area of the first carbon-based material is 1.8 m2 / g, 1.9 m 2 / g, 2.0 m 2 / g, 2.1 m 2 / g, 2.2 m 2 / g, or a value between any two values. In some embodiments, the specific surface area of the second carbon-based material ≤ 2.1 m 2 / g, optionally 1.3 - 2.0 m 2 / g. Exemplarily, the specific surface area of the second carbon-based material is 1.3 m 2 / g, 1.4 m 2 / g, 1.5 m 2 / g, 1.6 m 2 / g, 1.7 m 2 / g, 1.8 m 2 / g, 1.9 m 2 / g, 2.0 m 2 / g, or a value between any two values.

[0084] The first carbon-based material is natural graphite, which usually has a relatively large specific surface area. By adjusting the first carbon-based material to have a relatively low specific surface area, the reaction activity of the material can be reduced, the occurrence of side reactions can be reduced, the consumption of active ions by the formation of the SEI film can be reduced, which is beneficial to the first Coulomb efficiency of the secondary battery, and the adverse effects on the cycle performance can be reduced.

[0085] In some embodiments, the mass percentage of the second carbon-based material in the negative electrode active material ≥ 30%, optionally 50% - 70%. Exemplarily, the mass percentage of the second carbon-based material in the negative electrode active material is 30%, 40%, 50%, 55%, 60%, 65%, 70%, etc., or a value between any two values. When the mass percentage of the second carbon-based material in the negative electrode active material is within the above range, the kinetic performance of the secondary battery can be effectively improved, and it is beneficial to the energy density, and the cycle performance can also be taken into account.

[0086] According to some embodiments, the negative electrode active material is composed of a first carbon-based material and a second carbon-based material. In the negative electrode active material, the mass percentage of the first carbon-based material is less than or equal to 70%, optionally 30% - 50%.

[0087] In the negative electrode active material, when the mass percentages of the first carbon-based material and the second carbon-based material are within the above ranges, they can jointly play their respective advantages, enabling the secondary battery to have excellent energy density while obtaining improved kinetic performance and taking into account the cycle performance.

[0088] According to some embodiments, the first carbon-based material further satisfies the specific capacity ≥ 355 mAh / g. Optionally, the specific capacity of the first carbon-based material is 358 mAh / g - 367 mAh / g. Exemplarily, the specific capacity of the first carbon-based material is 358 mAh / g, 360 mAh / g, 362 mAh / g, 365 mAh / g, 367 mAh / g, or a value between any two of these values. When the specific capacity of the first carbon-based material is within the above range, the secondary battery can have a relatively high energy density.

[0089] In some embodiments, the second carbon-based material further satisfies at least one of the following properties, so as to improve one or more of the following properties of the secondary battery: kinetic performance, cycling performance, energy density, etc.

[0090] The volume-based particle size Dv90 of the second carbon-based material ≤ 25 μm. Optionally, the volume-based particle size Dv90 of the second carbon-based material is 18 μm - 25 μm. Exemplarily, the volume-based particle size Dv90 of the second carbon-based material is 18 μm, 20 μm, 22 μm, 24 μm, 25 μm, or a value between any two values within the range formed by these two values. When the volume-based particle size Dv90 of the second carbon-based material is relatively small, it can further be beneficial to the kinetic performance of the secondary battery.

[0091] The particle size distribution [(Dv90) - (Dv10)] / (Dv50) of the second carbon-based material ≤ 1.30. Optionally, the particle size distribution [(Dv90) - (Dv10)] / (Dv50) of the second carbon-based material is 1.05 - 1.25. Exemplarily, the particle size distribution [(Dv90) - (Dv10)] / (Dv50) of the second carbon-based material is 1.05, 1.10, 1.15, 1.20, 1.25, 1.30, or a value between any two values within the range formed by these two values. When the particle size distribution of the second carbon-based material is within the above range, its particle packing performance is better, which is beneficial to improving the compaction density of the negative electrode film layer and the energy density of the secondary battery. In addition, the above particle size distribution range is also beneficial to forming a reasonable pore structure between the particles of the negative electrode film layer, improving the transport performance of active ions, and being beneficial to the kinetic performance of the secondary battery.

[0092] The specific capacity of the second carbon-based material is ≥ 358 mAh / g. Optionally, the specific capacity of the second carbon-based material is 360 - 370 mAh / g. Exemplarily, the specific capacity of the second carbon-based material is 360 mAh / g, 362 mAh / g, 364 mAh / g, 366 mAh / g, 367 mAh / g, 368 mAh / g, 370 mAh / g, or a value between any two values. When the specific capacity of the second carbon-based material is within the above range, the energy density of the secondary battery can be improved.

[0093] The area of the pore structure in the outer region of the second carbon-based material is less than or equal to 0.15 μm 2 , and optionally less than or equal to 0.13 μm 2 . The second carbon-based material with such a structure has a relatively dense outer region, which can reduce the entry of the electrolyte into the particles, thereby reducing the side reactions with the electrolyte and being beneficial to the storage performance of the secondary battery.

[0094] The inner region of the second carbon-based material includes one or more pore structures with a pore area ≥ 0.15 μm 2 . Optionally, the second carbon-based material includes one or more pore structures with a pore area of 0.15 μm 2 -2.0 μm 2 . The inner region of the second carbon-based material includes pore structures of the above size. On the one hand, it can reserve sufficient and stable expansion space for the volume change of the second carbon-based material particles. On the other hand, it can also improve the compaction density of the negative electrode film layer, thereby improving the cycle performance and energy density of the secondary battery.

[0095] In some embodiments, the first negative electrode active material in the negative electrode film layer of the negative electrode plate is composed of a first carbon-based material; and the second negative electrode active material is composed of a second carbon-based material.

[0096] In other embodiments, the negative electrode film layer further includes other negative electrode active materials known in the art.

[0097] According to some specific embodiments, the first negative electrode active material and / or the second negative electrode active material further includes a silicon-based material. The silicon-based material can play a role in improving the pore structure in the negative electrode film layer, facilitating the infiltration and liquid retention of the electrolyte, and improving the kinetic performance of the secondary battery; at the same time, it can also increase the negative electrode capacity, thereby further improving the energy density of the secondary battery.

[0098] In particular, the silicon-based material can be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys.

[0099] When including the silicon-based material, the mass ratio of the silicon-based material in the first negative electrode active material ≤ 10%; and / or, the mass ratio of the silicon-based material in the second negative electrode active material ≤ 10%. The exemplary mass ratio of the silicon-based material in each negative electrode active material can be 3%-10%, 3%-8%, 5-10%, etc. Within the above range, the silicon-based material will not have an adverse impact on the cycle life of the secondary battery due to its high expansion characteristics, while being able to improve the energy density of the secondary battery.

[0100] In some embodiments, the mass proportion of the silicon-based material in the first negative electrode active material is greater than the mass proportion of the silicon-based material in the second negative electrode active material.

[0101] In some embodiments, the negative electrode film layer may further optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0102] In some embodiments, the negative electrode film layer may further optionally include a binder. The binder may 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), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0103] In some embodiments, the negative electrode film layer may further optionally include other additives, such as a thickening agent (such as sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0104] For the above-mentioned conductive agent, binder, and other additives, if any, the present application does not particularly limit their types and amounts, and those of ordinary skill in the art can select and determine them according to actual needs.

[0105] In some embodiments, the negative electrode film layer further satisfies at least one of the following properties, which is beneficial to one or more of the following properties of the secondary battery: kinetic performance, cycling performance, energy density, etc.

[0106] In some embodiments, the tap density of the negative electrode film layer is ≤ 1.85 g / cm 3 . Optionally, the tap density of the negative electrode film layer is 1.55 g / cm 3 - 1.85 g / cm 3 . Exemplarily, the tap density of the negative electrode film layer is 1.55 g / cm 3 , 1.60 g / cm 3 , 1.65 g / cm 3 , 1.70 g / cm 3 , 1.75 g / cm 3 , 1.80 g / cm 3 , 1.85 g / cm 3 and so on, but not limited thereto, and it may also be a value between any two numerical values.

[0107] In some embodiments, the areal density of the negative electrode film layer is ≥ 6.0 mg / cm 2 . Optionally, the areal density of the negative electrode film layer is 7.0 mg / cm 2 - 15.0 mg / cm2 Enumeratively, the tap density of the negative electrode film layer is 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0, etc., but not limited thereto, and it can also be a value between ranges composed of any two numerical values.

[0108] In some embodiments, the porosity of the negative electrode film layer is 18.0%-36.7%, and optionally, the porosity of the negative electrode film layer is 19.0%-34.0%. This is beneficial for the negative electrode film layer to balance high capacity and a suitable pore structure, and further beneficial for the secondary battery to balance high energy density and kinetic performance.

[0109] In some embodiments, the thickness of the negative electrode film layer ≥ 60 μm. Optionally, the thickness of the negative electrode film layer is 70 μm - 130 μm. Enumeratively, the thickness of the negative electrode film layer is 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, etc., or a value between ranges composed of any two numerical values.

[0110] In some embodiments, the negative electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, copper foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (such as copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0111] In some embodiments, the negative electrode plate can be prepared in the following manner: dispersing the above components for preparing the negative electrode plate, such as the negative electrode active material, conductive agent, binder, and any other components, in a solvent (such as deionized water) to form a negative electrode slurry; coating the negative electrode slurry on the negative electrode current collector, and after processes such as drying and cold pressing, the negative electrode plate can be obtained.

[0112] The negative electrode current collector has two opposite surfaces in its own thickness direction, and the negative electrode film layer is disposed on either one or both of the two opposite surfaces of the negative electrode current collector. It should be noted that each parameter of the negative electrode film layer given in this application (such as tap density, surface density, porosity, OI value, thickness, etc.) refers to the parameter of the negative electrode film layer on one side of the negative electrode current collector. When the negative electrode film layer is disposed on both sides of the negative electrode current collector, as long as the parameters of the negative electrode film layer on any one side meet the requirements of this application, it is considered to fall within the protection scope of this application.

[0113] In the present application, it is not excluded that in addition to the above-mentioned negative electrode film layer, the negative electrode sheet may further include other additional functional layers. For example, in some embodiments, the negative electrode sheet further includes a conductive bottom coating (such as composed of a conductive agent and a binder) disposed on the surface of the negative electrode current collector and sandwiched between the negative electrode current collector and the negative electrode film layer; in some embodiments, the negative electrode sheet further includes a protective layer covering the surface of the negative electrode film layer.

[0114] In the present application, the total pore area S of the internal region of the material (such as the second carbon-based material, etc.) 2 and the total pore area S of the external region 1 The ratio S2 / S1 is obtained by using a cross-section polishing instrument (such as the IB-09010CP type argon ion cross-section polishing instrument of JEOL Ltd., Japan) to obtain the cross-section of the carbon-based material; then referring to JY / T 010-1996, using a scanning electron microscope (such as the Sigma 300 type scanning electron microscope of ZEISS GmbH, Germany) to scan the cross-section of the carbon-based material; finally, using an image processing software (such as AVIZO) to respectively obtain the pore area of any one pore in the carbon-based material; and the total pore area S of the external region 1 and the total pore area S of the internal region 2 , and thus obtain the value of S2 / S1. Exemplarily, samples can be obtained from different regions of the negative electrode sheet in the secondary battery, and at least 5 positions (such as 5, 10, 15 or even more) are randomly selected from the samples to obtain cross-sections by using a cross-section polishing instrument, and at least 10 particles (such as 10, 20, 50 or even more particles) of cross-sections are randomly selected from the images of the scanning electron microscope of each cross-section. According to the above definition, the total pore area S2' of the internal region and the total pore area S1' of the external region of each particle cross-section are obtained by using an image processing software, and thus the value of S2' / S1' of each particle cross-section is obtained. Calculate the arithmetic mean of S2' / S1' of all measured particle cross-sections as the S2 / S1 value of the first carbon-based

[0115] material.

[0116] In the present application, the volume distribution particle sizes Dv10, Dv50, Dv90 of the material (such as the first carbon-based material, the second carbon-based material, etc.) have meanings well known in the art, which respectively represent the particle sizes corresponding to when the cumulative volume distribution percentage of the material reaches 10%, 50%, 90%, and can be measured by instruments and methods known in the art. For example, it can be measured by referring to GB / T 19077-2016 and using a laser particle size analyzer. The test instrument can be the Mastersizer 3000 type laser particle size analyzer of Malvern Instruments Ltd., UK.

[0117] In this application, whether there is a coating layer on the surface of the material (such as the first carbon-based material, the second carbon-based material, etc.) can be determined by transmission electron microscopy.

[0118] In this application, the graphitization degree of the material (such as the first carbon-based material, the second carbon-based material) has the meaning well-known in the art and can be tested by the instruments and methods known in the art. For example, it can be tested using an X-ray diffractometer (such as Bruker D8 Discover), and the test can refer to JIS K0131-1996 and JB / T 4220-2011 to obtain the average layer spacing d of the C(002) crystal plane in the crystal structure of the material. 002 , and then the graphitization degree can be calculated according to the formula g = (0.344 - d 002 ) / (0.344 - 0.3354) × 100%. In the above formula, d 002 is the average layer spacing of the C(002) crystal plane in the crystal structure of the material expressed in nanometers (nm).

[0119] In this application, the specific surface area of the material (such as the first carbon-based material, the second carbon-based material, etc.) has the meaning well-known in the art and can be measured by the instruments and methods known in the art. For example, it can be measured by referring to GB / T 19587-2017, using the nitrogen adsorption specific surface area analysis test method and calculated by the BET (Brunauer Emmett Teller) method. The test instrument can be the Tri-Star 3020 specific surface area and pore size analyzer of Micromeritics Company, USA.

[0120] In this application, the tapped density of the material (such as the first carbon-based material, the second carbon-based material, etc.) has the meaning well-known in the art and can be measured by the instruments and methods known in the art. For example, it can be measured by referring to GB / T 5162-2006 using a powder tapped density tester. The test instrument can be BT-301 of Dandong BETTER, and the test parameters are as follows: vibration frequency 250 ± 15 times / minute, amplitude 3 ± 0.2 mm, vibration times 5000 times, measuring cylinder 25 mL.

[0121] In this application, the specific capacity of the material (such as the first carbon-based material, the second carbon-based material, etc.) has a meaning well-known in the art and can be tested by methods known in the art. An exemplary test method is as follows: Mix the sample powder with conductive agent carbon black (Super P), binder polyvinylidene fluoride (PVDF) in a mass ratio of 91.6:1.8:6.6 with solvent N-methylpyrrolidone (NMP) to make a slurry; coat the prepared slurry on the surface of the negative electrode current collector copper foil, dry it in an oven and reserve it; Mix ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 1:1:1 to obtain an organic solvent, and then dissolve LiPF 6 in the above organic solvent to prepare an electrolyte with a concentration of 1 mol / L; then use a lithium metal sheet as the counter electrode, a polyethylene (PE) film as the separator, and assemble it with the above electrolyte into a CR2430 type button cell in an argon-protected glove box; After standing the obtained button cell for 12 h, at 25 °C, discharge it at a constant current of 0.05C to 0.005V, stand for 10 minutes, discharge it at a constant current of 50 μA to 0.005V again, stand for 10 minutes, and discharge it at a constant current of 10 μA to 0.005V; then charge it at a constant current of 0.1C to 2V and record the charge capacity. The ratio of the charge capacity to the sample mass is the specific capacity of the corresponding material (such as the first carbon-based material, the second carbon-based material, etc.).

[0122] In this application, the tap density of the negative electrode film layer has a meaning well-known in the art and can be tested by methods known in the art. Tap density of the negative electrode film layer = areal density of the negative electrode film layer / thickness of the negative electrode film layer.

[0123] In this application, the areal density of the negative electrode film layer has a meaning well-known in the art and can be tested by methods known in the art. For example, a single-sided coated and cold-pressed negative electrode sheet can be taken (if it is a double-sided coated negative electrode sheet, the negative electrode film layer on one side can be wiped off first), and it is punched into small round pieces with an area of S 1 and its weight is measured and recorded as M 1 . Then wipe off the negative electrode film layer of the above weighed negative electrode sheet and weigh the weight of the negative electrode current collector, which is recorded as M 0 . Areal density of the negative electrode sheet = (M 1 - M 0 ) / S 1 .

[0124] In this application, the porosity of the negative electrode film layer has a meaning well-known in the art and can be tested by methods known in the art. For example, an exemplary test method is as follows: Take a single-sided coated and cold-pressed negative electrode sheet (if it is a double-sided coated negative electrode sheet, the negative electrode film layer on one side can be wiped off first), punch it into small round piece samples with a certain area, and calculate the apparent volume V of the negative electrode sheet1 ; Refer to GB / T 24586-2009, use inert gas (such as helium or nitrogen) as the medium, adopt the gas displacement method, and use a true density tester to measure the true volume V of the negative electrode sheet 2 . Porosity of the negative electrode film layer = (V 1 - V 2 ) / V 1 × 100%. Multiple (such as 30 pieces) of negative electrode sheet samples with good appearance and no powder falling off at the edges can be selected for testing, and the results are averaged, thereby improving the accuracy of the test results. The test instrument can adopt a Micromeritics AccuPyc II 1340 type true density tester.

[0125] The thickness of the negative electrode film layer has the meaning well-known in the art, and can be tested by methods known in the art, such as using a micrometer (such as Mitutoyo 293-100 type, with an accuracy of 0.1 μm). The thickness range given in this application is the thickness range of the negative electrode film layer on one side of the negative electrode current collector. The thickness of the negative electrode film layer on either side of the negative electrode current collector within the range given in this application satisfies this application.

[0126] In the X-ray diffraction analysis test 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 range of the 2θ angle is 20° to 80°, and the scanning rate is 4° / min.

[0127] It should be noted that the various parameter tests for the negative electrode active material or the negative electrode film layer can be sampled and tested from the prepared secondary battery according to the following steps.

[0128] Perform a discharge treatment on the secondary battery (for safety reasons, generally make the secondary battery in a fully discharged state); disassemble the secondary battery and take out the negative electrode sheet, soak the negative electrode sheet with dimethyl carbonate for a certain time (such as 2h - 10h); then take out the negative electrode sheet and perform a drying treatment at a certain temperature and time (such as 60°C, for more than 4h), and take out the negative electrode sheet after drying. At this time, samples can be taken from the dried negative electrode sheet to test the above-mentioned parameters related to the negative electrode film layer, such as the surface density, tap density, porosity, OI value, thickness, etc. of the negative electrode film layer.

[0129] Bake the above-mentioned dried negative electrode sheet at a certain temperature and time (such as 400°C, for more than 2h), select an area at random in the baked negative electrode sheet, and sample the negative electrode active material (blade scraping and powder sampling can be used); perform a sieving treatment on the collected negative electrode active material (such as sieving with a 200-mesh sieve), and finally obtain a sample that can be used to test the above-mentioned parameters of the negative electrode active material.

[0130] In the present application, the above-mentioned first carbon-based material and second carbon-based material can be obtained through commercial purchase, or can also be prepared by the methods described below in the present application.

[0131] The first carbon-based material is natural graphite. Natural graphite generally refers to graphite naturally formed in nature, without the need for graphitization, and there are usually many pore structures inside natural graphite particles. In some embodiments, the natural graphite includes one or more of flake graphite, natural spherical graphite, and microcrystalline graphite, especially including natural spherical graphite.

[0132] "Natural spherical graphite" refers to natural graphite having a spherical or quasi-spherical shape, and not all natural graphite particles are controlled to be ideal spheres. In some embodiments, natural spherical graphite with the desired particle size and morphology can be obtained by pretreating flake graphite. Optionally, the above pretreatment includes processes such as crushing, classification, spheroidization, and purification.

[0133] In some embodiments, the first carbon-based material can be selected from natural spherical graphite, natural flake graphite, etc. In some embodiments, natural spherical graphite with the desired particle size and morphology can be obtained by pretreating the raw materials. Optionally, the above pretreatment includes processes such as crushing, classification, spheroidization, and purification to obtain the first carbon-based material with the above characteristics.

[0134] In some embodiments, the first carbon-based material is coated with an organic carbon source to form a carbon coating layer on at least part of the surface of the first carbon-based material. The organic carbon source can be a carbon-containing material known in the art suitable for coating, for example, it can include one or more of coal tar pitch, petroleum pitch, phenolic resin, coconut shell, etc. The carbonization temperature is 900°C to 1200°C.

[0135] In some embodiments, the preparation method of the second carbon-based material includes: Step 1, providing a raw material with a plurality of pore structures; Step 2, uniformly mixing the above raw material and a filling material in a predetermined ratio, and then keeping it at a first temperature T 1 for a first time t 1 , and after cooling to room temperature, an intermediate is obtained; Step 3, keeping the obtained intermediate at a second temperature T 2 for a second time t 2 , and after completion, the second carbon-based material is obtained.

[0136] In some embodiments, in Step 1, the raw material for preparing the second carbon-based material includes natural graphite. Optionally, the natural graphite includes one or more of flake graphite, natural spherical graphite, and microcrystalline graphite, especially including natural spherical graphite.

[0137] "Natural spherical graphite" refers to natural graphite with a spherical or quasi-spherical shape, and not all natural graphite particles are controlled to be ideal spheres. In some embodiments, natural spherical graphite with the desired particle size and morphology can be obtained by pretreating flake graphite. Optionally, the above pretreatment includes processes such as crushing, classification, spheroidization, and purification.

[0138] In some embodiments, in step 1, the volume distribution particle size Dv50 of the above raw material can be 6 μm to 13 μm.

[0139] In some embodiments, in step 1, the specific surface area of the above raw material can be ≥ 2.5 m 2 / g, optionally 2.5 m 2 / g to 10.0 m 2 / g. When the specific surface area of the raw material is within the above range, it is beneficial for subsequent filling treatment and obtaining the second carbon-based material with the desired specific surface area. It is also beneficial for the second carbon-based material to have both high capacity and high initial Coulomb efficiency. In addition, it is beneficial for the second carbon-based material to have better kinetic performance.

[0140] In some embodiments, in step 2, the softening point temperature of the above filling material is 90°C to 150°C. Optionally, the softening point temperature of the above filling material is 94°C to 146°C, 94°C to 142°C, 94°C to 138°C, 94°C to 134°C, 94°C to 130°C, 104°C to 146°C, 104°C to 142°C, 104°C to 138°C, 104°C to 134°C, 104°C to 130°C.

[0141] In some embodiments, in step 2, the volume distribution particle size Dv50 of the above filling material is less than or equal to 6 μm, optionally 1 μm to 6 μm, 1 μm to 5 μm, 2 μm to 5 μm, 3 μm to 5 μm. This is beneficial for the filling material to melt and fill into the pore structure of the raw material after heating, and is also beneficial for improving the dispersion uniformity of the filling material and the raw material.

[0142] In some embodiments, in step 2, the coking value of the above filling material is 15% to 40%, optionally 18% to 34%. In this application, the coking value of the filling material has the meaning well-known in the art and can be measured by instruments and methods known in the art. For example, it can be measured with reference to GB / T 8727-2008.

[0143] In some embodiments, in step 2, the above filling material includes one or more of coal tar pitch, petroleum pitch, high molecular compounds, and resins, optionally including one or more of coal tar pitch and petroleum pitch.

[0144] In some embodiments, in step 2, the mass ratio of the above-mentioned filling material to the above-mentioned raw material is (10 - 40):100, and may be optionally (10 - 30):100, (10 - 25):100, (10 - 20):100, (12 - 30):100, (14 - 28):100, (15 - 25):100.

[0145] In step 2, by adjusting one or more parameters such as the type, softening point, coking value, addition amount, etc. of the filling material within the above range, it is beneficial to adjust the number and / or size of pores in the outer region and the inner region of the second carbon-based material within a suitable range, and it is beneficial to adjust S2 / S1 of the second carbon-based material within a suitable range.

[0146] By adjusting parameters such as the type, softening point, coking value, addition amount, etc. of the filling material within the above range, after the filling material is heated and melted, its viscosity is not high, it maintains good fluidity, and at the same time it is not easy to bond the raw material particles, which can reduce the agglomeration of raw material particles during the subsequent preparation process. Thus, it can also reduce problems such as an increase in surface defects and an increase in surface active sites of the second carbon-based material particles due to the need to increase the depolymerization process.

[0147] In some embodiments, in step 2, after mixing the above-mentioned raw material and the above-mentioned filling material evenly according to a predetermined ratio, the temperature is raised to the first temperature T 1 The heating process can be a staged heating process.

[0148] In some embodiments, the above-mentioned staged heating process includes a first heating process, a second heating process, and a third heating process.

[0149] In some embodiments, the above-mentioned first heating process is to raise the temperature to 200°C - 250°C and hold the temperature at this temperature for 0.5 h - 3 h.

[0150] In some embodiments, the above-mentioned second heating process is to raise the temperature to 450°C - 550°C and hold the temperature at this temperature for 0 h - 2 h. When the holding time is 0 h, it means that when the temperature is raised to the range of 450°C - 550°C, no heat preservation treatment is carried out, but the temperature is continuously raised to the first temperature T 1 .

[0151] In some embodiments, the above-mentioned third heating process is to raise the temperature to the above-mentioned first temperature T 1 and hold the temperature at this temperature for the first time t 1 .

[0152] During the staged heating process, the temperature is first raised to 200°C to 250°C. Since the heating temperature is higher than the softening point temperature of the filling material, at this time, the filling material is heated and melted and softened. Insulating for 0.5 h to 3 h can make it flow and fill into the pore structure of the raw material. Then the temperature is raised to 450°C to 550°C. At this time, the melted and softened filling material undergoes a carbonization reaction and gradually forms a semi-coke state, turning into a viscous liquid or solid, thereby preventing the filling material from entering all the pore structures of the raw material. Finally, the temperature is raised to the first temperature, and at this time, the filling material undergoes a carbonization reaction, whereby the pore structure occupied by the filling material can be effectively filled.

[0153] In some embodiments, in step 2, the temperature is raised to the above-mentioned first temperature T1 at a rate of 1°C / min to 10°C / min. For example, the heating rate can be 1°C / min, 2°C / min, 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min or any range composed of the above values. Optionally, the heating rate is 1.5°C / min to 8°C / min, 1.5°C / min to 6°C / min, 2°C / min to 6°C / min, 2°C / min to 5°C / min.

[0154] In some embodiments, the heating rate of the above-mentioned first heating process can be 1°C / min to 10°C / min, optionally 1.5°C / min to 8°C / min, 1.5°C / min to 6°C / min, 2°C / min to 6°C / min, 2°C / min to 5°C / min. In some embodiments, the heating rate of the above-mentioned second heating process can be 1°C / min to 10°C / min, optionally 2°C / min to 8°C / min. In some embodiments, the heating rate of the above-mentioned third heating process can be 1°C / min to 10°C / min, optionally 2°C / min to 8°C / min.

[0155] In some embodiments, in step 2, the above-mentioned first temperature T 1 is 800°C to 1200°C. For example, the first temperature T 1 can be 800°C, 850°C, 900°C, 950°C, 1000°C, 1050°C, 1100°C, 1200°C or any range composed of the above values. Optionally, the above-mentioned first temperature T 1 is 800°C to 1100°C, 850°C to 1100°C, 900°C to 1100°C, 850°C to 1000°C.

[0156] In some embodiments, in step 2, the above-mentioned first time t 1 is 1 h to 5 h. For example, the first time t 1It can be a range composed of any value such as 1 h, 1.5 h, 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 4.5 h, 5 h or above. Optionally, the above-mentioned first time t 1 is 2 h to 4 h.

[0157] In some embodiments, in step 2, the heat treatment can be carried out in equipment capable of programmed heating such as intermediate frequency furnaces, roller kilns, rotary kilns, pusher kilns, vertical granulation kettles, horizontal granulation kettles, vertical reaction kettles, horizontal reaction kettles or roller furnaces.

[0158] In some embodiments, in step 2, the heat treatment atmosphere can be a protective gas atmosphere. The above-mentioned protective gas can include one or more of nitrogen, argon, and helium.

[0159] In step 2, by adjusting one or more of the heating rate, the first temperature, the first time, the heating process, etc. within the above range, it is beneficial to adjust the number and / or size of pores in the outer region and the inner region of the second carbon-based material within a suitable range, and further beneficial to adjust the S of the second carbon-based material 2 / S 1 within a suitable range.

[0160] In some embodiments, in step 3, the above-mentioned second temperature T 2 is 1600 °C to 2800 °C. Optionally, the above-mentioned second temperature T 2 is 1600 °C to 2700 °C, 1600 °C to 2600 °C, 1600 °C to 2500 °C, 1600 °C to 2400 °C, 1800 °C to 2600 °C, 1800 °C to 2500 °C, 1800 °C to 2400 °C, 2000 °C to 2500 °C, 2000 °C to 2400 °C.

[0161] In some embodiments, in step 3, the above-mentioned second time t 2 is 1.5 h to 6 h. For example, the second time t 1 can be a range composed of any value such as 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 4.5 h, 5 h, 5.5 h, 6 h or above. Optionally, the above-mentioned second time t2 is 2 h to 5 h.

[0162] In some embodiments, in step 3, the above-mentioned heat treatment can be carried out in an intermediate frequency furnace, a box-type graphitization furnace, an Acheson graphitization furnace, a continuous graphitization furnace or an internal series graphitization furnace.

[0163] In some embodiments, in step 3, the intermediate frequency furnace, continuous graphitization heat treatment atmosphere can be a protective gas atmosphere. The above-mentioned protective gas can include one or more of nitrogen, argon, and helium.

[0164] In step 3, by adjusting one or more of the second temperature and the second time within the above ranges, it is beneficial to adjust the content of disordered carbon in the second carbon-based material within a suitable range, which is beneficial for the second carbon-based material to have a suitable degree of graphitization, interlayer spacing, and I D / I G etc.

[0165] In the above method for preparing the second carbon-based material, by adjusting one or more of the parameters of natural graphite, the parameters of the filling material, the heating rate, the first temperature, the first time, the heating process, the second temperature, the second time, etc. within the above ranges, it is beneficial to adjust the S 2 / S 1 parameters such as graphitization degree, specific capacity, specific surface area, particle size, powder compaction density, and tapped density of the second carbon-based material.

[0166] According to some embodiments, the second carbon-based material may have a coating layer. The preparation method further includes step 4: mixing an organic carbon source with the second carbon-based material prepared above and performing carbonization at a third temperature T3. The organic carbon source may be a carbon-containing material suitable for coating known in the art. For example, it may include one or more of coal tar pitch, petroleum pitch, phenolic resin, coconut shell, etc. In some embodiments, the second carbon-based material mixed with the organic carbon source is carbonized by holding at a third temperature T3 of 1150-1500 °C for 1-3 hours, so as to obtain a second carbon-based material having a carbon coating layer on at least part of its surface.

[0167] [Positive electrode plate]

[0168] The positive electrode plate includes a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector, and the positive electrode film layer includes the positive electrode active material of the first aspect of the present application.

[0169] As an example, the positive electrode current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode film layer is provided on any one or both of the two opposite surfaces of the positive electrode current collector.

[0170] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, aluminum foil may be used. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector may be formed by forming a metal material (such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0171] In some embodiments, when the battery cell is a lithium-ion battery, the positive electrode active material can be the positive electrode active material for lithium-ion batteries known in the art. As an example, the positive electrode active material can include at least one of the following materials: lithium-containing phosphates with olivine structure, lithium transition metal oxides, and their respective modified compounds. However, the present application is not limited to these materials, and other conventional materials that can be used as the positive electrode active material of the battery can also be used. These positive electrode active materials can be used alone or in combination of two or more. Among them, examples of lithium transition metal oxides can include but are not limited to lithium cobalt oxide (such as LiCoO 2 ), lithium nickel oxide (such as LiNiO 2 ), lithium manganese oxide (such as LiMnO 2 , LiMn 2 O 4 ), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 (which can also be abbreviated as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O 2 (which can also be abbreviated as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O 2 (which can also be abbreviated as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O 2 (which can also be abbreviated as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O 2 (which can also be abbreviated as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05 O 2 ) and their modified compounds, etc. At least one of them. Examples of lithium-containing phosphates with olivine structure can include but are not limited to lithium iron phosphate (such as LiFePO 4 (which can also be abbreviated as LFP)), composite materials of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO 4) At least one of the composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, and the composite material of lithium manganese iron phosphate and carbon.

[0172] In some embodiments, when the battery cell is a sodium-ion battery, the positive electrode active material can be the positive electrode active material for sodium-ion batteries well-known in the art. As an example, the positive electrode active material can include at least one of the following materials: sodium transition metal oxides, polyanion-type compounds, and Prussian blue compounds.

[0173] During the charge and discharge process of the battery, the deintercalation and consumption of Li will occur, and the molar content of Li is different when the battery is discharged to different states. In the listing of the positive electrode materials in this application, the molar content of Li is the initial state of the material, that is, the state before feeding. When the positive electrode material is applied to the battery system, after charge and discharge cycles, the molar content of Li will change.

[0174] In the listing of the positive electrode materials in this application, the molar content of O is only the theoretical state value. Lattice oxygen release will cause the molar content of oxygen to change, and the actual molar content of O will show fluctuations.

[0175] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0176] In some embodiments, the positive electrode film layer may further optionally include a conductive agent. As an example, the conductive agent can include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0177] In some embodiments, the positive electrode plate can be prepared in the following manner: Disperse the components for preparing the positive electrode plate, such as the positive electrode active material, conductive agent, binder, and any other components, in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; coat the positive electrode slurry on the positive electrode current collector, and after processes such as drying and cold pressing, the positive electrode plate can be obtained.

[0178] [Electrolyte]

[0179] The electrolyte plays a role in conducting ions between the positive electrode plate and the negative electrode plate. This application does not specifically limit the type of electrolyte, and it can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, or all-solid-state.

[0180] In some embodiments, the electrolyte uses an electrolytic solution. The electrolytic solution includes an electrolyte salt and a solvent.

[0181] 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 difluoro(oxalato)borate, lithium bis(oxalato)borate, lithium difluoro bis(oxalato)phosphate, and lithium tetrafluoro(oxalato)phosphate.

[0182] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene 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.

[0183] In some embodiments, the electrolyte may optionally further include additives. For example, the additives may include anode film-forming additives, cathode film-forming additives, and may also include additives that can improve certain battery performance, such as additives for improving battery overcharge performance, additives for improving battery high-temperature or low-temperature performance, and the like.

[0184] [Separator membrane]

[0185] In some embodiments, the battery cell further includes a separator membrane. The present application does not particularly limit the type of the separator membrane, and any well-known porous structure separator membrane with good chemical stability and mechanical stability can be selected.

[0186] In some embodiments, the material of the separator membrane may be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator membrane may be a single-layer film or a multi-layer composite film, without particular limitation. When the separator membrane is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0187] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator membrane can be made into an electrode assembly by a winding process or a stacking process.

[0188] In some embodiments, the battery cell may include an outer package. The outer package can be used to encapsulate the above electrode assembly and electrolyte.

[0189] In some embodiments, the outer package of the battery cell can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the battery cell can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0190] The present application has no particular limitation on the shape of the battery cell, which can be cylindrical, square or any other shape. For example, Figure 6 is a battery cell 5 with a square structure as an example.

[0191] In some embodiments, referring to Figure 7 , the outer package may include a housing 51 and a top cover assembly 53. Among them, the housing 51 may include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates enclose to form a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can be covered on the opening to close the receiving cavity. The positive electrode plate, the negative electrode plate and the separator can be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. The number of electrode assemblies 52 included in the battery cell 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0192] In some embodiments, the battery cells can be assembled into a battery module. The number of battery cells included in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0193] Figure 8 is a battery module 4 as an example. Referring to Figure 8 , in the battery module 4, a plurality of battery cells 5 can be arranged in sequence along the length direction of the battery module 4. Of course, they can also be arranged in any other way. Further, the plurality of battery cells 5 can be fixed by fasteners.

[0194] Optionally, the battery module 4 may further include a housing having a receiving space, and a plurality of battery cells 5 are received in the receiving space.

[0195] In some embodiments, the above battery module can be further assembled into a battery pack. The number of battery modules included in the battery pack can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0196] Figure 9 and Figure 10 is a battery pack 1 as an example. Referring to Figure 9 and Figure 10 , the battery pack 1 may include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can be covered on the lower box body 3 and form a closed space for receiving the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any way.

[0197] In addition, the second aspect of the present application further provides an electrical device, and the electrical device includes the secondary battery provided by the present application. The secondary battery can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships, satellites, energy storage systems, etc., but is not limited thereto.

[0198] As the electrical device, battery cells, battery modules or battery packs can be selected according to its usage requirements.

[0199] Figure 11 This is an example of an electrical device. The electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the electrical device for the secondary battery, battery packs or battery modules can be used.

[0200] Another example of the device can be a mobile phone, a tablet computer, a laptop computer, etc. This device usually requires thinness and lightness, and battery cells can be used as the power source.

[0201] Embodiment

[0202] Hereinafter, embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application and should not be construed as a limitation of the present application. For those embodiments where specific technologies or conditions are not indicated, the technologies or conditions described in the literature in the art or according to the product specifications are followed. For reagents or instruments whose manufacturers are not indicated, they are all conventional products that can be obtained through commercial purchase.

[0203] All materials used in the embodiments and comparative examples of the present application can be obtained through commercial purchase or can be prepared by the following processes.

[0204] Preparation of Material 1-1:

[0205] Natural spherical graphite is provided, and the natural spherical graphite is uniformly mixed with petroleum pitch at a mass ratio of 100:10. The softening point of the petroleum pitch is 250 °C, and low-temperature heat treatment is carried out at 1120 °C for 2 hours to obtain natural graphite with a carbon coating layer, that is, Material 1-1, as the first carbon-based material. Among them, Material 1-1 satisfies: volume distribution particle size Dv50 = 17.5 μm, specific surface area BET = 1.96 m 2 / g, graphitization degree = 96.0%, and specific capacity = 366.5 mAh / g.

[0206] Preparation of Materials 1-2 to 1-4:

[0207] The preparation methods of Materials 1-2 to 1-4 are similar to that of Material 1-1, with the difference being that the particle size of the spherical natural graphite used is different from that in Material 1-1, such that the volume-based particle size Dv50 of Materials 1-1 to 1-4 is in the range of 16 μm to 22 μm. Specifically, it is as follows:

[0208] Table 1.

[0209]

[0210] Preparation of Materials 1-5 to 1-6:

[0211] The preparation methods of Materials 1-5 to 1-6 are similar to that of Material 1-1, with the difference being that the carbonization temperature is adjusted according to the following table, such that the graphitization degree of Materials 1-5 to 1-6 is in the range of 95-98%. Specifically, it is as follows:

[0212] Table 2.

[0213] Material Number Carbonization Temperature Graphitization Degree of the First Carbon-Based Material 1-5 900℃ 95.0％ 1-6 1200℃ 97.5％

[0214] Preparation of Material 2-1:

[0215] The flake graphite is mechanically crushed, classified, spheroidized, and purified to obtain natural spherical graphite, wherein the volume-based particle size Dv50 of the natural spherical graphite is 10.5 μm. The obtained natural spherical graphite is mixed with the filler petroleum pitch at a mass ratio of 100:22, and the softening point of the petroleum pitch is 115°C. Then, the mixed material is placed in an equipment with programmable temperature rise, and the temperature is continuously raised to 650°C (the first treatment temperature), held for 2 h, and after cooling to room temperature, an intermediate is obtained. The obtained intermediate is placed in a graphitization furnace and heat-treated at 2390°C (the second treatment temperature), and after demagnetization and screening, the resulting material is mixed with petroleum pitch and heat-treated at 1150°C for 2.5 hours to obtain Material 2-1 as the second carbon-based material. Material 2-1 satisfies: S2 / S1 = 8.1, volume-based particle size Dv50 = 11.2 μm, and graphitization degree of 96.5%.

[0216] Preparation of Materials 2-2 to 2-5:

[0217] The preparation methods of Materials 2-2 to 2-5 are similar to that of Material 2-1, with the difference being that the mass ratio of the natural spherical graphite to the filler and the first treatment temperature are adjusted according to the following table to obtain Materials 2-2 to 2-5. Specifically, it is as follows:

[0218] Table 3.

[0219]

[0220] Preparation of Material 2-6:

[0221] The preparation method of Materials 2-6 is similar to that of Material 1, with the differences being: adjusting the ratio of natural spherical graphite to the filler to 100:7, and adjusting the first treatment temperature T1 to 1200 °C, such that S2 / S1 of the material is 0.82.

[0222] Preparation of Materials 2-7 to 2-9:

[0223] The preparation methods of Materials 2-7 to 2-9 are similar to that of Material 1, with the differences being: adjusting the above raw materials, the volume distribution particle size Dv50 of natural spherical graphite, such that it is in the range of 5.5 μm - 11 μm, so that the volume distribution particle size Dv50 of Materials 2-7 to 2-9 is in the range of 6 μm - 12 μm. Specifically as follows:

[0224] Table 4.

[0225]

[0226]

[0227] Preparation of Materials 2-10 to 2-11:

[0228] The preparation methods of Materials 2-10 to 2-11 are similar to that of Material 2-1, with the differences being: adjusting the second treatment temperature and the mass ratio of natural spherical graphite to the filler according to the following table, such that the graphitization degree of Materials 2-10 to 2-11 is in the range of 95.5% - 98.0%. Specifically as follows:

[0229] Table 5.

[0230]

[0231] Example 1

[0232] Preparation of the Secondary Battery:

[0233] 1. Negative electrode sheet: The first negative electrode active material (Material 1-1, as the first carbon-based material) and the second negative electrode active material (Material 2-1, as the second carbon-based material)) are respectively and fully stirred and mixed with conductive agent carbon black (Super P), thickening agent sodium carboxymethylcellulose, and binder styrene-butadiene rubber in an appropriate amount of solvent deionized water according to a weight ratio of 96.4:1:1.2:1.4 to form the first negative electrode slurry and the second. The first negative electrode slurry and the second negative electrode slurry are sequentially coated on two surfaces of the negative electrode current collector copper foil in an equal mass ratio, and after drying and cold pressing, a negative electrode sheet is obtained.

[0234] 2. Positive electrode sheet: LiNi 0.5 Co 0.2 Mn 0.3 O 2(NCM523) is mixed with conductive agent carbon black (Super P) and binder polyvinylidene fluoride in a weight ratio of 96:2:2, and an appropriate amount of solvent N-methylpyrrolidone NMP is added and stirred evenly to obtain the positive electrode paste. The positive electrode paste is coated on both surfaces of the positive electrode current collector aluminum foil, and after drying and cold pressing, the positive electrode plate is obtained.

[0235] 3. Electrolyte: Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then LiPF 6 is dissolved in the above organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0236] 4. Separator: A polypropylene membrane is used.

[0237] 5. Preparation of secondary battery: The positive electrode plate and the negative electrode plate prepared above are placed in order, and the separator is placed in the middle of the positive electrode plate and the negative electrode plate to play a separation role, and then wound to obtain an electrode assembly; the electrode assembly is placed in an outer package, dried and then injected with electrolyte, and after vacuum packaging, standing, forming, shaping and other processes, a secondary battery is obtained.

[0238] Examples 2 to 5

[0239] The battery preparation methods of Examples 2 to 5 are similar to those of Example 1, and the differences are: different materials 2-2 to 2-5 are selected for the second carbon-based material, and the details are shown in Table 8.

[0240] Comparative Example 1

[0241] A secondary battery is assembled in a similar manner to the preparation method of Example 1, and the difference is: a material 2-6 with S2 less than S1 is selected for the second carbon-based material, and the details are shown in Table 8.

[0242] Performance Test

[0243] 1. Material testing

[0244] S2 / S1 of the second carbon-based material is obtained by the following method.

[0245] The binder for sample preparation is mixed evenly with the second carbon-based material powder and then coated on the copper foil, dried at 60 °C for 30 min and reserved; 5 test samples with a size of 6 mm × 6 mm are cut at 5 different positions and pasted on the sample stage of a CP type argon ion cross-section polishing instrument; the samples are cut with a plasma beam to obtain the cross-sections of the samples. The testing instrument can be an IB-09010CP type argon ion cross-section polishing instrument of JEOL Company, Japan.

[0246] The cross-sections of each sample of the first carbon-based material were scanned using a scanning electron microscope, and scanning images were obtained by arbitrarily selecting regions in each sample cross-section. The test can refer to JY / T 010-1996. The test instrument can be a Sigma 300 type scanning electron microscope from ZEISS, Germany.

[0247] Twenty cross-sections of particles of the first carbon-based material were randomly selected from the scanning images. The region formed by extending 0.25 μm from the surface of the particles of the first carbon-based material towards the inside of the particles was denoted as the outer region, and the region inside the outer region was denoted as the inner region. An image processing software was used to obtain the total pore area S1' of the outer region of each particle cross-section, and the pore area S2' of the inner region of the first carbon-based material. The value of S2' / S1' was calculated; and the arithmetic mean of S2' / S1' of all 20 particles was calculated as the value of S2 / S1 of the first carbon-based material. The image processing software can be AVIZO.

[0248] 2. Morphology test of the second carbon-based material

[0249] The negative electrode sheets of the above-mentioned examples and comparative examples were taken and cut into test samples of 2 cm × 2 cm. The test samples were fixed on the sample stage using paraffin; the sample stage was loaded into the sample holder and locked for fixation. The power of the argon ion cross-section polishing instrument (IB-09010CP type argon ion cross-section polishing instrument from JEOL, Japan) was turned on and vacuum was pumped to 10 -4 Pa, the argon gas flow rate was set to 0.15 MPa, the voltage was set to 8 KV, and the polishing time was set to 2 h. The sample stage was adjusted to the swing mode to start polishing; regions were randomly selected in the test samples for scanning tests (refer to JY / T 010-1996, scanning electron microscope (Sigma300 from ZEISS), and images of the ion-polished cross-section morphology (CP) of the second carbon-based material were obtained at a magnification of 1000 times, as Figure 4 shown. It can be observed from Figure 4 that the second carbon-based material has a certain number of pores at the position near the center of the particles, while the region near the surface of the particles has a denser structure.

[0250] 3. Battery performance test

[0251] (1) Kinetic performance test of secondary batteries

[0252] At 25 °C, the secondary batteries prepared in the examples and comparative examples were charged at a constant current of xC (an initial charging rate that can be determined empirically and will not cause lithium plating on the surface of the negative electrode tab, such as 1.5C) until fully charged, then discharged at 1C until fully discharged and repeated 10 times. After that, the battery was charged at xC until fully charged, and then the negative electrode tab was disassembled and the lithium plating situation on the surface of the negative electrode tab was observed. If there is no lithium plating on the negative electrode surface, the charging rate xC was incremented in steps of 0.05C and tested again until lithium plating occurred on the negative electrode surface, and the test was stopped. At this time, the charging rate (x - 0.05)×C is the maximum charging rate of the battery.

[0253] (2) Energy density test

[0254] At 25 °C, the secondary battery was charged at a constant current of 1 / 3C to 4.25V, then charged at a constant voltage of 4.25V until the current was 0.05C, left standing for 5 min, and then discharged at a constant current of 1 / 3C to 2.8V. Record the discharge energy of the battery at this time. The discharge energy of the battery divided by the mass of the battery is the mass energy density of the battery, with the unit of Wh / kg.

[0255] (3) Cycle performance test of secondary battery

[0256] At 45 °C, the secondary battery prepared above was charged at a constant current of 1C to the upper cut-off voltage (corresponding to 100% SOC), then charged at a constant voltage until the current was 0.05C. After standing for 5 min, the secondary battery was discharged at a constant current of 1C to the lower cut-off voltage (corresponding to 0% SOC), and the discharge capacity at this time was recorded, which is the discharge capacity of the first cycle. The secondary battery was subjected to cycle charge and discharge tests according to the above method, and the discharge capacity after each cycle was recorded.

[0257] The capacity retention rate (%) of the secondary battery after 900 cycles at 45 °C = discharge capacity after 900 cycles / discharge capacity of the first cycle × 100%.

[0258] The test results of the secondary batteries prepared in Examples 1 - 5 and Comparative Example 1 are shown in Table 6 below.

[0259] Table 6.

[0260]

[0261]

[0262] From Table 6 above, it can be seen that as the S2 / S1 value gradually increases, the charge rate of the secondary battery gradually decreases, the cycle retention rate gradually increases, and the energy density is less affected. The smaller the S2 / S1 value, the more pore structures the material has on the surface, so the consumption of active lithium will also increase, and the cycle will slightly deteriorate. However, when the S2 / S1 value is too small (Comparative Example 1), it indicates that the surface has a rich pore structure, which will significantly increase the side reaction, so the cycle performance will be significantly deteriorated.

[0263] Embodiments 6 to 8

[0264] The battery preparation methods of Examples 6-8 are similar to those of Example 1, except that different materials 1-2 to 1-4 are selected as the first carbon-based material.

[0265] Examples 9 to 11

[0266] The battery preparation methods of Examples 9 to 11 are similar to those of Example 1, except that different materials 2-7 to 2-9 are selected as the second carbon-based material.

[0267] The material parameters and test results of the secondary batteries prepared in Example 1 and Examples 6 to 11 are shown in Table 7 below.

[0268] Table 7.

[0269]

[0270] From Table 7 above, it can be seen that as the particle size of the first carbon-based material or the second carbon-based material gradually increases, the energy density of the secondary battery gradually increases, the cycle retention rate is slightly improved, but the kinetics deteriorates. When the particle size increases, the material gram capacity and powder compaction density are both improved to a certain extent, and the consumption of lithium ions is reduced, so the energy density and cycle performance of the secondary battery are improved. However, when the particle size is large, it is not conducive to the migration and diffusion of lithium ions, and affects the kinetic performance of the secondary battery.

[0271] Embodiments 12 to 13

[0272] The battery preparation method of Examples 12-13 is similar to that of Example 1, except that different materials 1-5 to 1-6 are selected as the first carbon-based material.

[0273] Embodiments 14 to 15

[0274] The battery preparation methods of Examples 14 to 15 are similar to those of Example 1, except that different materials 2-10 to 2-11 are selected as the second carbon-based material.

[0275] The material parameters and test results of the secondary batteries prepared in Example 1 and Examples 12 to 15 are shown in Table 8 below.

[0276] Table 8.

[0277]

[0278] As can be seen from Table 8 above, the change in graphitization degree mainly affects the energy density of secondary batteries and has little effect on kinetic performance and cycle stability. The higher the graphitization, the higher the specific capacity of the material and the higher the powder tap density, so the energy density of the battery cell is higher.

[0279] It should be noted that this application is not limited to the above embodiments. The above embodiments are only examples, and embodiments with the same structure and the same effect as the technical idea within the technical solution scope of this application are included in the technical scope of this application. In addition, within the scope of not departing from the gist of this application, various modifications that those skilled in the art can think of to the embodiments, and other ways constructed by combining some of the constituent elements in the embodiments are also included in the scope of this application.

Claims

1. A secondary battery, characterized in that, it includes a negative electrode plate, and the negative electrode plate includes 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 has a first surface far from the negative electrode current collector and a second surface disposed opposite to the first surface, and the thickness of the negative electrode film layer is denoted as H, wherein, the region within a thickness range from the second surface of the negative electrode film layer to 0.3H is denoted as the first region of the negative electrode film layer, the surface of the first region close to the negative electrode current collector includes a first negative electrode active material, and the first negative electrode active material includes a first carbon-based material; the region within a thickness range from the first surface of the negative electrode film layer to 0.3H is denoted as the second region of the negative electrode film layer, the surface of the second region far from the negative electrode current collector includes a second negative electrode active material, and the second negative electrode active material includes a second carbon-based material; the first carbon-based material includes natural graphite; the second carbon-based material includes an outer region and an inner region located inside the outer region. The outer region refers to the region formed by extending 2.5 μm from the particle surface of the second carbon-based material into the particle interior. In the cross-sectional view of the second carbon-based material, the total pore area of the outer region is denoted as S1, and the total pore area of the inner region is denoted as S2, then the second carbon-based material satisfies S2 > S1.

2. The secondary battery according to claim 1, wherein, the volume distribution particle size Dv50 of the first carbon-based material is greater than the volume distribution particle size Dv50 of the second carbon-based material.

3. The secondary battery according to claim 1 or 2, wherein, the volume distribution particle size Dv50 of the first carbon-based material is ≥ 15 μm; optionally 16 μm - 20 μm.

4. The secondary battery according to any one of claims 1 - 3, wherein, the volume distribution particle size Dv50 of the second carbon-based material is ≤ 13 μm; optionally 7 μm - 12 μm.

5. The secondary battery according to any one of claims 1 - 4, wherein, at least part of the surface of the first carbon-based material has a coating layer; and / or, at least part of the surface of the second carbon-based material has a coating layer.

6. The secondary battery according to any one of claims 1 - 5, wherein, the graphitization degree of the first carbon-based material is ≥ 95%, optionally 95.0% - 98.0%; and / or, the graphitization degree of the second carbon-based material is ≥ 95%; the graphitization degree of the second carbon-based material is 95.5% - 98.5%.

7. The secondary battery according to any one of claims 1 - 6, wherein, the graphitization degree of the first carbon-based material is less than the graphitization degree of the second carbon-based material.

8. The secondary battery according to any one of claims 1 - 7, wherein, the second carbon-based material satisfies 1.5 ≤ S2 / S1 ≤ 500; optionally, 2.4 ≤ S2 / S1 ≤ 450.

9. The secondary battery according to any one of claims 1 - 8, wherein, the specific surface area of the first carbon-based material is greater than the specific surface area of the second carbon-based material; Optionally, the specific surface area of the first carbon-based material is ≤ 2.2 m 2 / g, more optionally 1.8 m 2 / g - 2.2 m 2 / g; and / or, Optionally, the specific surface area of the second carbon-based material is ≤ 2.1 m 2 / g, more preferably 1.3 m 2 / g - 2.0 m 2 / g.

10. The secondary battery according to any one of claims 1 to 9, wherein, the mass ratio of the second carbon-based material in the negative electrode active material is ≥ 30%, and may be optionally 50% - 70%.

11. The secondary battery according to any one of claims 1 to 10, wherein, the specific capacity of the first carbon-based material is ≥ 355 mAh / g, and may be optionally 358 mAh / g - 367 mAh / g.

12. The secondary battery according to any one of claims 1 to 11, wherein, the second carbon-based material satisfies at least one of the following items: (1) The volume distribution particle size Dv90 of the second carbon-based material is ≤ 25 μm; it may be optionally 18 μm - 25 μm; (2) The second carbon-based material satisfies: [(Dv90) - (Dv10)] / (Dv50) ≤ 1.30; it may be optionally 1.05 - 1.25; (3) The specific capacity of the second carbon-based material is ≥ 358 mAh / g, and may be optionally 360 mAh / g - 370 mAh / g; (4) The area of the pore structure in the outer region of the second carbon-based material is less than or equal to 0.15 μm 2 , optionally less than or equal to 0.13 μm 2 ; (5) The internal region of the second carbon-based material includes one or more pore structures with an area greater than or equal to 0.15 μm 2 , optionally including one or more pore structures with an area of 0.15 μm 2 - 2.0 μm 2 .

13. The secondary battery according to any one of claims 1 to 12, wherein, the first negative electrode active material and / or the second negative electrode active material further includes a silicon-based material; Optionally, the mass ratio of the silicon-based material in the first negative electrode active material is ≤ 10% wt%; Optionally, the mass ratio of the silicon-based material in the second negative electrode active material is ≤ 10 wt%; Optionally, the mass ratio of the silicon-based material in the first negative electrode active material is greater than the mass ratio of the silicon-based material in the second negative electrode active material.

14. The secondary battery according to any one of claims 1 to 13, wherein, the negative electrode film layer satisfies at least one of the following items: (1) The compaction density of the negative electrode film layer is ≤ 1.85 g / cm 3 , optionally 1.55 g / cm 3 -1.85 g / cm 3 ; (2) The areal density of the negative electrode film layer ≥ 6.0 mg / cm 2 , optionally 7.0 mg / cm 2 - 15.0 mg / cm 2 ; (3) The porosity of the negative electrode film layer is 18.0% - 36.7%, and may be optionally 19.0% - 34.0%; (4) The thickness of the negative electrode film layer is ≥ 60 μm, and may be optionally 70 μm - 130 μm.

15. An electrical device, characterized in that, it includes the secondary battery according to any one of claims 1 to 14.

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