Secondary battery and electric device

Abstract

A secondary battery and an electric device. The secondary battery comprises a negative electrode sheet; the negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer; the negative electrode film layer has a first surface away from the negative electrode current collector and a second surface arranged opposite to the first surface; the thickness of the negative electrode film layer is recorded as H; a region extending from the second surface to a thickness of 0.3 H of the negative electrode film layer is recorded 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 extending from the first surface to a thickness of 0.3 H of the negative electrode film layer is recorded 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 a total hole area S2 of an internal region of the second carbon-based material is greater than a total hole area S1 of an external region of the second carbon-based material.

Description

Secondary battery and power-consuming device

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This disclosure is based on the Chinese patent application with application number 202311641546.0, application date November 30, 2023, and invention name “A secondary battery and electrical device”, and claims the priority of the Chinese patent application. The entire content of the Chinese patent application is hereby introduced into the present disclosure as a reference. Technical Field

[0003] The present disclosure relates to the field of battery technology, and in particular to a secondary battery and an electrical device. Background Art

[0004] In recent years, secondary batteries have been widely used in energy storage systems such as hydropower, thermal, wind, and solar power plants, as well as in a variety of fields, including power tools, electric bicycles, electric motorcycles, electric vehicles, and aerospace. As the scope of secondary battery applications expands, so too does the performance of these batteries. For example, in electric devices, secondary batteries are becoming increasingly miniaturized, capable of rapid charge and discharge, and possessing a longer service life. Consequently, secondary batteries are required to have higher energy density, cycle life, and improved dynamic performance.

[0005] Summary of the Invention

[0006] The present disclosure is made in view of the above-mentioned problems, and its purpose is to provide a secondary battery and an electric device, wherein the secondary battery has high energy density and improved dynamic performance.

[0007] A first aspect of the present disclosure provides a secondary battery, comprising a negative electrode plate, the negative electrode plate comprising a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, the negative electrode film layer having a first surface away from the negative electrode current collector and a second surface disposed opposite the first surface, the thickness of the negative electrode film layer being denoted as H, wherein a region within a thickness range of 0.3H from the second surface of the negative electrode film layer is denoted as a first region of the negative electrode film layer, the first region being in contact with the surface of the negative electrode current collector and comprising a first negative electrode active material, the first negative electrode active material comprising a first carbon-based material; and a region within a thickness range of 0.3H from the first surface of the negative electrode film layer is denoted as a second region of the negative electrode film layer, the second region comprising a second negative electrode active material, the second negative electrode active material comprising a second carbon-based material;

[0008] The first carbon-based material includes natural graphite;

[0009] The second carbon-based material includes an external region and an internal region located inside the external region. The external region refers to a region extending 2.5 μm from the particle surface of the second carbon-based material to the interior of the particle. In the cross-sectional view of the second carbon-based material, the total pore area of ​​the external region is recorded as S1, and the total pore area of ​​the internal region is recorded as S2. Then the second carbon-based material satisfies S2>S1.

[0010] The first carbon-based material is natural graphite. Natural graphite has a relatively large pore structure, provides a larger reaction surface, is conducive to the reaction of active ions, and has a higher 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 negative electrode film layer of the negative electrode sheet of the secondary battery disclosed in the present invention is arranged in the above manner to 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.

[0011] In some embodiments, the volume distribution particle size Dv50 of the first carbon-based material is larger 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 the smaller volume distribution particle size is conducive to further improving the dynamic performance of the secondary battery.

[0012] In some embodiments, the volume distribution particle size Dv50 of the first carbon-based material is ≥16 μm. Alternatively, the volume distribution particle size Dv50 of the first carbon-based material is 16 μm-20 μm.

[0013] In some embodiments, the volume distribution particle size Dv50 of the second carbon-based material is ≤15 μm. Alternatively, the volume distribution particle size Dv50 of the second carbon-based material is 10 μm-15 μm.

[0014] 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.

[0015] In some embodiments, at least a portion of the surface of the first carbon-based material is coated; and / or at least a portion of the surface of the second carbon-based material is coated. The carbon coating can further enhance active ion transport, thereby improving the kinetic performance of the secondary battery.

[0016] In some embodiments, the first carbon-based material has a degree of graphitization of 95% or greater, and optionally, the first carbon-based material has a degree of graphitization of 95.0% to 98.0%; the second carbon-based material has a degree of graphitization of 95% or greater, and the second carbon-based material has a degree of graphitization of 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 specific capacity, and on the other hand, facilitates electron transport in the negative electrode film layer, thereby enabling the secondary battery to have improved kinetic performance and excellent energy density.

[0017] In some embodiments, the first carbon-based material has a lower degree of graphitization than the second carbon-based material.

[0018] In some embodiments, the second carbon-based material satisfies 1.5 ≤ S2 / S1 ≤ 500, and optionally, 2.4 ≤ S2 / S1 ≤ 450. When the second carbon-based material satisfies the above S2 / S1 range, the pore structure is primarily distributed in the inner region. As a result, the side reactions between the second carbon-based material and the electrolyte are reduced, active ion consumption is reduced, and the cycle performance of the secondary battery can be improved.

[0019] 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. Alternatively, the specific surface area of ​​the first carbon-based material is BET ≤ 2.2 m 2 / g. More optionally, the specific surface area of ​​the first carbon-based material is BET 1.8m 2 / g-2.2m 2 / g. Optionally, the specific surface area of ​​the second carbon-based material is BET ≤ 2.1m 2 / g. More optionally, the specific surface area of ​​the second carbon-based material is BET 1.3m 2 / g-2.0m 2 / g. When the specific surface areas of the first and second carbon-based materials are within the above range, they are beneficial for balancing kinetic and cycling performance. In particular, the smaller specific surface area of ​​the second carbon-based material, primarily located in the second region of the negative electrode sheet, away from the current collector, can relatively reduce side reactions with the electrolyte, thereby improving cycling performance.

[0020] In some embodiments, the amount of the second carbon-based material is ≥30 wt %, optionally 50 wt % to 70 wt %, based on the total weight of the negative electrode film layer. When the mass proportion of the second carbon-based material in the negative electrode film layer is within the above range, the secondary battery can achieve improved kinetic performance while having excellent energy density and taking into account cycling performance.

[0021] In some embodiments, the gram capacity of the first carbon-based material is ≥355 mAh / g. Alternatively, the gram capacity of the first carbon-based material is 358 mAh / g-367 mAh / g. When the gram capacity of the first carbon-based material is within the above range, the secondary battery can have a higher energy density.

[0022] In some embodiments, the second carbon-based material satisfies at least one of the following:

[0023] (1) The volume distribution particle size Dv90 of the second carbon-based material is ≤ 25 μm, which can be selected from 18 μm to 25 μm.

[0024] (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 stacking performance is good, which is beneficial to improving the compaction density of the negative electrode film layer and improving 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 transmission performance of active ions and electrons, thereby improving the kinetic performance of the secondary battery.

[0025] (3) The gram capacity of the second carbon-based material is ≥358 mAh / g. Optionally, the gram capacity of the second carbon-based material is 360 mAh / g to 370 mAh / g. When the gram capacity of the second carbon-based material is within the above range, the secondary battery can have a higher energy density.

[0026] (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 , can be selected to be less than or equal to 0.13μm 2 The outer region of the second carbon-based material having this structure is relatively dense, which can reduce the intrusion of electrolyte into the interior of the particles, thereby reducing side reactions with the electrolyte, which is beneficial to the storage performance of the secondary battery.

[0027] (5) The inner region of the second carbon-based material includes one or more pores with an area greater than or equal to 0.15 μm 2 Optionally, the second carbon-based material comprises one or more pores with an area of ​​0.15 to 2.0 μm. 2 By making the internal region of the second carbon-based material include a pore structure of the aforementioned size, on the one hand, sufficient and stable expansion space can be reserved for the volume change of the first carbon-based material particles, and on the other hand, the compaction density of the negative electrode film layer can be increased, thereby improving the energy density of the secondary battery.

[0028] In some embodiments, the first negative electrode active material and / or the second negative electrode active material further include a silicon-based material. Optionally, the silicon-based material accounts for ≤10% by weight of the first negative electrode active material; and / or the silicon-based material accounts for ≤10% by weight of the second negative electrode active material. Optionally, the silicon-based material accounts for a greater proportion by weight of the first negative electrode active material than the silicon-based material accounts for a greater proportion by weight of the second negative electrode active material.

[0029] In some embodiments, the negative electrode film layer satisfies at least one of the following:

[0030] (1) The compaction density of the negative electrode film layer is ≤1.85g / cm 3 Optionally, the compaction density of the negative electrode film is 1.55 g / cm 3 -1.85g / cm 3 .

[0031] (2) The surface density of the negative electrode film layer is ≥6.0 mg / cm 2 Optionally, the surface density of the negative electrode film layer is 7.0 mg / cm 2- -15.0mg / cm 2 .

[0032] (3) The porosity of the negative electrode film layer is 18.0 to 36.7%, and optionally, the porosity of the negative electrode film layer is 19.0 to 34.0%. This is beneficial for the negative electrode film layer to have both high capacity and suitable pore structure, and further beneficial for the secondary battery to have both high energy density and good storage performance and dynamic performance.

[0033] (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.

[0034] A second aspect of the present disclosure further provides an electrical device comprising the secondary battery according to the first aspect of the present disclosure.

[0035] The electric device of the present disclosure includes the secondary battery provided by the present disclosure, and thus has at least the same advantages as the secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG1 is a schematic diagram of an embodiment of a negative electrode sheet of the present disclosure.

[0037] FIG2 is a schematic diagram of another embodiment of the negative electrode sheet disclosed herein.

[0038] FIG3 is a schematic diagram of another embodiment of the negative electrode sheet disclosed herein.

[0039] FIG4 is an ion polished cross-sectional (CP) image of an embodiment of the second carbon-based material disclosed herein.

[0040] FIG5 is a schematic diagram of a cross-sectional image of a particle of a second carbon-based material of the present disclosure.

[0041] FIG. 6 is a schematic diagram of a battery cell according to an embodiment of the present disclosure.

[0042] FIG. 7 is an exploded view of the battery cell according to the embodiment of the present disclosure shown in FIG. 6 .

[0043] FIG8 is a schematic diagram of a battery module according to an embodiment of the present disclosure.

[0044] FIG. 9 is a schematic diagram of a battery pack according to an embodiment of the present disclosure.

[0045] FIG. 10 is an exploded view of the battery pack shown in FIG. 9 according to an embodiment of the present disclosure.

[0046] FIG. 11 is a schematic diagram of an electric device using a secondary battery according to an embodiment of the present disclosure as a power source.

[0047] Explanation of the accompanying drawings: 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 battery cell; 51 shell; 52 electrode assembly; 53 top cover assembly; 10 negative electrode plate; 101 negative electrode current collector; 102 negative electrode film layer; 102a first surface; 102b second surface; 1021 first region; 1022 second region; 1023 middle region; 200 second carbon-based material; 201 external region; 202 internal region. DETAILED DESCRIPTION

[0048] Below, the embodiments of the secondary battery and electrical device disclosed in the present invention are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there may be cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy 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 disclosure and are not intended to limit the subject matter described in the claims.

[0049] " scope " disclosed in the present disclosure is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and selected lower limit and upper limit define the boundary of special scope.The scope that this mode limits can be to include end value or not include end value, and can be combined arbitrarily, and promptly any lower limit can form a scope with any upper limit combination.For example, if the scope of 60-120 and 80-110 is listed for specific parameter, it is understood that the scope of 60-110 and 80-120 is also expected.In addition, if the minimum range value 1 and 2 listed, and if the maximum range value 3,4 and 5 listed, then the following scope can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5.In the present disclosure, unless otherwise specified, numerical range " ab " represents the abbreviation of any real number combination between a and b, and wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0050] Unless otherwise specified, all embodiments and optional embodiments of the present disclosure can be combined with each other to form new technical solutions.

[0051] Unless otherwise specified, all technical features and optional technical features disclosed herein can be combined with each other to form a new technical solution.

[0052] Unless otherwise specified, all steps of the present disclosure may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0053] Natural graphite has a high specific capacity, but it has many surface defects and internal pores. During the charge-discharge cycle, it is prone to side reactions with the electrolyte and is prone to expansion, resulting in a poor cycle life. To this end, attempts have been made to use two different negative electrode active materials to form the upper and lower layers of the negative electrode film, respectively, with natural graphite placed in the lower layer and a material with better cycle performance, such as artificial graphite, placed in the upper layer. However, artificial graphite has a low specific capacity, resulting in insufficient energy density of secondary batteries.

[0054] Therefore, it is difficult for current secondary batteries to achieve both high energy density and good cycle performance.

[0055] Based on this, a first aspect of the present disclosure provides a secondary battery having high energy density and good cycle performance.

[0056] The term "secondary battery" referred to herein refers to a battery cell, a battery module, or a battery pack.

[0057] Typically, a secondary battery cell consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, active ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0058] [Negative electrode]

[0059] The secondary battery includes 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 arranged opposite to the first surface, the thickness of the negative electrode film layer is recorded as H, wherein the area within a thickness range from the second surface of the negative electrode film layer to 0.3H is recorded as the first area of ​​the negative electrode film layer, the first area 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 area within a thickness range from the first surface of the negative electrode film layer to 0.3H is recorded as It is 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 external region and an internal region located inside the external region, the external region refers to the region extending 2.5μm from the particle surface of the second carbon-based material to the interior of the particle, in the cross-sectional view of the second carbon-based material, the total pore area of ​​the external region is recorded as S1, and the total pore area of ​​the internal region is recorded as S2, then the second carbon-based material satisfies S2>S1.

[0060] Referring to Figures 1 to 3 , schematic diagrams of several different specific embodiments of the negative electrode sheet of the present disclosure are shown. As shown in Figures 1 to 3 , the negative electrode sheet 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 away from the negative electrode current collector 101 and a second surface 102b disposed opposite 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 located on a single side of the negative electrode current collector. The region within a thickness range of 0.3H from the second surface 102b of the negative electrode film layer 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 herein as the lower layer of the negative electrode film layer. The region within a thickness range of 0.3H from the first surface 102a of the negative electrode film layer 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 herein as the upper layer of the negative electrode film layer. The first region 1021 includes a first negative electrode active material, which includes a first carbon-based material, and the first carbon-based material has a pore structure. The second region 1022 includes a second negative electrode active material, which includes a second carbon-based material. Between the first region 1021 and the second region 1022, the region occupying a thickness range of 0.4H is recorded as the intermediate region 1023. It is easy to understand that according to Figures 1 to 3, the intermediate region 1023 includes at least one of the first carbon-based material and the second carbon-based material.

[0061] It should be understood that in the embodiments shown in Figures 1 to 3, the second surface 102b is in contact with the upper surface of the negative electrode current collector 101, but the structure of the negative electrode sheet of the present disclosure 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 does not directly contact the negative electrode current collector 101.

[0062] Specifically, Figures 1 to 3 show that a first carbon-based material and a second carbon-based material are sequentially coated on the negative electrode current collector 101. Figure 1 shows that the thickness of the two materials each accounts for approximately half of the thickness of the negative electrode film layer 102. Figure 2 shows that the thickness of the first carbon-based material accounts for approximately 70% of the thickness of the negative electrode film layer 102, while the thickness of the second carbon-based material accounts for approximately 30% of the thickness of the negative electrode film layer 102. Figure 3 shows a configuration opposite to Figure 2, that is, the thickness of the second carbon-based material accounts for approximately 70% of the thickness of the negative electrode film layer 102, while the thickness of the first carbon-based material accounts for approximately 30% of the thickness of the negative electrode film layer 102.

[0063] It should be understood that the diagrams shown in Figures 1 to 3 are schematic diagrams of ideal situations. The present disclosure does not particularly limit the thickness ratio of the first carbon-based material to the second carbon-based material. For example, the thickness ratio of the first carbon-based material to the second carbon-based material can be in the range of 3:7-7:3. Exemplarily, the thickness ratio of the first carbon-based material to the second carbon-based material is 4:6, 5:5, 6:4, etc., or a ratio between any two ratios.

[0064] It should also be understood that while the middle region 1023 in Figures 1 to 3 shows a clear boundary between the other two regions, in reality, no such clear interface exists. As previously mentioned, the first carbon-based material and the second carbon-based material may coexist within the middle region 1023. Similarly, there is no clear interface between the coating layer containing the first carbon-based material and the coating layer containing the second carbon-based material.

[0065] In the present disclosure, the total pore area S1 of the outer region of the second carbon-based material is less 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, the first carbon-based material in the present disclosure satisfies "S1>S2", which means that the second carbon-based material has a pore structure that can be directly observed from a cross-sectional image (e.g., 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.

[0066] The first carbon-based material and the second carbon-based material can be distinguished by a cross-section polishing instrument. For example, the first carbon-based material and the second carbon-based material can be distinguished by performing an ion polishing cross-section morphology (CP) test on the negative electrode. As an example, the test method may be: cutting the negative electrode piece into a sample to be tested of a certain size (for example, 2 cm × 2 cm), fixing the negative electrode piece on the sample stage with paraffin; placing the sample stage into the sample holder and locking it, turning on the power of the argon ion cross-section polisher (for example, the IB-09010CP argon ion cross-section polisher from Japan's JEOL company) and performing vacuuming (for example, 10-4 Pa), setting the argon flow rate (for example, 0.15 MPa) and voltage (for example, 8 KV) and polishing time (for example, 2 h), adjusting the sample stage to a rocking mode and starting polishing; randomly selecting an area in the sample to be tested for scanning testing (for example, referring to JY / T010-1996, scanning using a scanning electron microscope), and obtaining an ion polishing cross-sectional morphology (CP) image of the negative electrode piece at a certain magnification (for example, 1000 times), from which the first carbon-based material and the second carbon-based material can be distinguished. FIG4 is an ion polishing cross-section (CP) diagram of the second carbon-based material disclosed herein. It can be seen from the diagram that the outer region of the second carbon-based material particles is denser than the inner region, satisfying S2>S1.

[0067] Further referring to Figure 5 , there is shown a schematic diagram of a cross-sectional image of a particle of the second carbon-based material 200 of the present disclosure, wherein the cross-sectional image passes through the center of the particle of the second carbon-based material 200. As shown in Figure 5 , the region extending 2.5 μm from the particle surface of the second carbon-based material 200 toward the interior of the particle is the outer region 201, and the region of the particle 200 other than the outer region 201 is the inner region 202.

[0068] In the pole piece design disclosed in the present invention, the first region (lower region) of the negative electrode film layer adopts a first carbon-based material, and the second region (upper region) adopts a second carbon-based material. The first carbon-based material is natural graphite, which has a high gram 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 effects on the battery cycle performance can be reduced to a certain extent. The total pore area S2 of the internal region of the second carbon-based material particles is greater than the total pore area S1 of the external region. On the one hand, the second carbon-based material with such a structure has a high gram capacity and can improve the energy density of the battery. On the other hand, the smaller area of ​​the pore structure in the external region of the second carbon-based material is beneficial to the stability of the particles, reduces side reactions with the electrolyte, and thus has a better cycle life.

[0069] Thus, 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 has both improved energy density and, to a certain extent, cycle performance.

[0070] According to some embodiments, the second carbon-based material satisfies 1.5≤S2 / S1≤500, optionally, 2.4≤S2 / S1≤450. Exemplarily, S2 / S1 may have the following ranges: 2.2≤S2 / S1≤400, 2.4≤S2 / S1≤300, 2.5≤S2 / S1≤250, 2.6≤S2 / S1≤200, 2.8≤S2 / S1≤150, or 3.0≤S2 / S1≤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 particles has fewer pores, and such materials have a higher gram capacity, which is also beneficial to the improvement of the energy density of the secondary battery.

[0071] 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.

[0072] 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 may be 16 μm, 17 μm, 17.5 μm, 18 μm, 18.5 μm, 19 μm, or 20 μm, but is not limited thereto and may also be a value within a range consisting of any two values.

[0073] 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.

[0074] 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, or 13 μm, but is not limited thereto and may also be a value within a range consisting of any two values.

[0075] The second carbon-based material has a smaller volume distribution particle size and can have a larger specific surface area. It 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.

[0076] 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 dynamic performance, and take into account cycle performance.

[0077] 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, or even the entire surface, has the coating layer.

[0078] 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, or even the entire outer surface, has a coating layer.

[0079] When at least part 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 transmission of active ions, thereby facilitating further improvement of the kinetic performance of the secondary battery.

[0080] The first carbon-based material disclosed herein is natural graphite. Natural graphite particles typically have a relatively large porous structure throughout their entire area. For example, if the natural graphite particles are divided into an inner region and an outer region (the outer region is defined as the region extending 2.5 μm from the surface of the natural graphite particles to the interior of the particles), both the inner and outer regions of the natural graphite have relatively large porous structures.

[0081] In some embodiments, the degree of graphitization of the first carbon-based material is ≥95%, optionally 95.0% to 98.0%. Exemplarily, the degree of graphitization of the first carbon-based material is 95%, 96%, 96.5%, 97%, 97.5%, 98%, etc., or a value between the ranges consisting of any two numerical values. In some embodiments, the degree of graphitization of the second carbon-based material is ≥95%, optionally 95.5% to 98.5%. Exemplarily, the degree of graphitization of the first carbon-based material is 95.5%, 96%, 96.5%, 97%, 97.5%, 98%, 98.5%, etc., or a value between the ranges consisting of any two numerical values. The high degree of graphitization of the first carbon-based material and / or the second carbon-based material can provide excellent gram capacity and powder compression density, thereby effectively improving the energy density of the secondary battery.

[0082] In some embodiments, the first carbon-based material has a lower degree of graphitization than the second carbon-based material.

[0083] 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.

[0084] In some embodiments, the specific surface area of ​​the first carbon-based material is ≤ 2.2 m 2 / g, optional 1.8m 2 / g-2.2m 2 / g. Exemplarily, the specific surface area of ​​the first carbon-based material is 1.8m 2 / g, 1.9m 2 / g, 2.0m 2 / g, 2.1m 2 / g, 2.2m 2 / g, or a value between any two values. In some embodiments, the specific surface area of ​​the second carbon-based material is ≤2.1m 2 / g, optional range is 1.3~2.0m 2 / g. Exemplarily, the specific surface area of ​​the second carbon-based material is 1.3m 2 / g, 1.4m 2 / g, 1.5m 2 / g, 1.6m 2 / g, 1.7m 2 / g, 1.8m2 / g, 1.9m 2 / g, 2.0m 2 / g, or any value between two values.

[0085] The first carbon-based material is natural graphite, which typically has a large specific surface area. By adjusting the first carbon-based material to have a relatively low specific surface area, the reactivity of the material can be reduced, the occurrence of side reactions can be minimized, and the consumption of active ions during SEI film formation can be reduced, which is beneficial to the initial coulombic efficiency of the secondary battery and can reduce the adverse effects on cycle performance.

[0086] In some embodiments, the mass proportion of the second carbon-based material in the negative electrode active material is ≥30%, optionally 50% to 70%. Exemplarily, the mass proportion 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 proportion of the second carbon-based material in the negative electrode active material is within the above range, it can effectively improve the kinetic performance of the secondary battery, and is beneficial to energy density, while also taking into account cycle performance.

[0087] 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 proportion of the first carbon-based material is less than or equal to 70%, and optionally 30%-50%.

[0088] In the negative electrode active material, when the mass proportion of the first carbon-based material and the second carbon-based material is within the above range, they can jointly exert their respective advantages, so that the secondary battery has excellent energy density while obtaining improved dynamic performance and taking into account cycle performance.

[0089] According to some embodiments, the first carbon-based material further satisfies a gram capacity ≥ 355 mAh / g. Optionally, the gram capacity of the first carbon-based material is 358 mAh / g-367 mAh / g. Exemplarily, the gram 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 values. When the gram capacity of the first carbon-based material is within the above range, the secondary battery can have a higher energy density.

[0090] In some embodiments, the second carbon-based material further satisfies at least one of the following properties, thereby improving one or more of the following properties of the secondary battery: kinetic performance, cycle performance, energy density, etc.

[0091] The volume distribution particle size Dv90 of the second carbon-based material is ≤25μm. Optionally, the volume distribution particle size Dv90 of the second carbon-based material is 18μm-25μm. Exemplarily, the volume distribution particle size Dv90 of the second carbon-based material is 18μm, 20μm, 22μm, 24μm, 25μm, or a value between the ranges consisting of any two values. The smaller volume distribution particle size Dv90 of the second carbon-based material can further benefit the kinetic performance of the secondary battery.

[0092] The particle size distribution of the second carbon-based material [(Dv90)-(Dv10)] / (Dv50)] ≤ 1.30. Optionally, the particle size distribution of the second carbon-based material [(Dv90)-(Dv10)] / (Dv50)] is 1.05-1.25. Exemplarily, the particle size distribution of the second carbon-based material [(Dv90)-(Dv10)] / (Dv50)] is 1.05, 1.10, 1.15, 1.20, 1.25, 1.30, or a value between the ranges consisting of any two numerical values. When the particle size distribution of the second carbon-based material is within the above range, its particle stacking performance is better, which is beneficial to improving the compaction density of the negative electrode film layer and improving the energy density of the secondary battery. In addition, the above particle size distribution range is also beneficial to the formation of a reasonable pore structure between the particles of the negative electrode film layer, improving the transmission performance of active ions, and benefiting the kinetic performance of the secondary battery.

[0093] The gram capacity of the second carbon-based material is ≥358 mAh / g. Optionally, the gram capacity of the second carbon-based material is 360-370 mAh / g. Exemplarily, the gram 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 gram capacity of the second carbon-based material is within the above range, the energy density of the secondary battery can be improved.

[0094] 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 , can be selected to be less than or equal to 0.13μm 2 The outer region of the second carbon-based material having this structure is relatively dense, which can reduce the intrusion of electrolyte into the interior of the particles, thereby reducing side reactions with the electrolyte, which is beneficial to the storage performance of the secondary battery.

[0095] The inner region of the second carbon-based material includes one or more pores with an area of ​​≥0.15 μm 2 Optionally, the second carbon-based material includes one or more pores with an area of ​​0.15 μm 2 -2.0μm 2The internal region of the second carbon-based material includes a pore structure of the aforementioned size, which, on the one hand, can reserve sufficient and stable expansion space for the volume change of the second carbon-based material particles, and on the other hand, can also increase the compaction density of the negative electrode film layer, thereby improving the cycle performance and energy density of the secondary battery.

[0096] In some embodiments, the first negative electrode active material in the negative electrode film layer of the negative electrode sheet is composed of a first carbon-based material; and the second negative electrode active material is composed of a second carbon-based material.

[0097] In other embodiments, the negative electrode film layer further includes other negative electrode active materials known in the art.

[0098] According to some specific embodiments, the first negative electrode active material and / or the second negative electrode active material further comprises a silicon-based material. The silicon-based material can improve the pore structure in the negative electrode film layer, facilitating electrolyte infiltration and retention, thereby enhancing the dynamic performance of the secondary battery; it can also increase the negative electrode capacity, thereby further improving the energy density of the secondary battery.

[0099] In particular, the silicon-based material may be selected from at least one of elemental silicon, silicon-oxygen compounds, silicon-carbon compounds, silicon-nitrogen compounds, and silicon alloys.

[0100] When a silicon-based material is included, the mass proportion of the silicon-based material in the first negative electrode active material is ≤10%; and / or the mass proportion of the silicon-based material in the second negative electrode active material is ≤10%. Exemplary mass proportions of the silicon-based material in each negative electrode active material may be 3%-10%, 3%-8%, 5-10%, etc. Within the above ranges, the silicon-based material's high expansion properties will not adversely affect the cycle life of the secondary battery, while at the same time improving the capacity density of the secondary battery.

[0101] 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.

[0102] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0103] In some embodiments, the negative electrode film layer may further 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).

[0104] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0105] The present disclosure does not particularly limit the types and amounts of the above-mentioned conductive agents, binders and other additives, if any, and those skilled in the art can select and determine them according to actual needs.

[0106] 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, cycle performance, energy density, etc.

[0107] In some embodiments, the compaction density of the negative electrode film layer is ≤1.85 g / cm 3 Optionally, the compaction density of the negative electrode film is 1.55 g / cm 3 -1.85g / cm 3 For example, the compaction density of the negative electrode film is 1.55 g / cm 3 , 1.60g / cm 3 , 1.65g / cm 3 , 1.70g / cm 3 , 1.75g / cm 3 , 1.80g / cm 3 , 1.85g / cm 3 etc., but not limited thereto, and can also be a value between a range of any two values.

[0108] In some embodiments, the surface density of the negative electrode film layer is ≥6.0 mg / cm 2 Optionally, the surface density of the negative electrode film layer is 7.0 mg / cm 2 -15.0mg / cm 2 For example, the compaction 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 is not limited thereto and may also be a value within a range formed by any two values.

[0109] In some embodiments, the porosity of the negative electrode film is 18.0%-36.7%, and optionally, the porosity of the negative electrode film is 19.0%-34.0%. This helps the negative electrode film achieve both high capacity and a suitable pore structure, thereby facilitating a secondary battery with both high energy density and dynamic performance.

[0110] In some embodiments, 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. For example, 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 within a range consisting of any two values.

[0111] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0112] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0113] The negative electrode current collector has two surfaces that are opposite to each other in the thickness direction of the negative electrode current collector, and the negative electrode film layer is disposed on either or both of the two opposing surfaces of the negative electrode current collector. It should be noted that the various negative electrode film layer parameters (such as compaction density, surface density, porosity, OI value, thickness, etc.) given in this disclosure refer to the parameters of the negative electrode film layer on a single side of the negative electrode current collector. When the negative electrode film layer is disposed on both sides of the negative electrode current collector, the parameters of the negative electrode film layer on either side meet the requirements of this disclosure and are considered to fall within the scope of protection of this disclosure.

[0114] In the present disclosure, the negative electrode sheet may include other additional functional layers in addition to the negative electrode film layer. For example, in some embodiments, the negative electrode sheet further includes a conductive primer layer (e.g., composed of a conductive agent and a binder) sandwiched between the negative electrode current collector and the negative electrode film layer and disposed on the surface of the negative electrode current collector; in some embodiments, the negative electrode sheet further includes a protective layer covering the surface of the negative electrode film layer.

[0115] In the present disclosure, the ratio S2 / S1 of the total pore area S2 of the internal region of a material (such as a second carbon-based material, etc.) to the total pore area S1 of the external region is obtained by using a cross-section polisher (such as the IB-09010CP argon ion cross-section polisher of JEOL Corporation of Japan) to obtain the cross-section of the carbon-based material; then, referring to JY / T010-1996, a scanning electron microscope (such as the Sigma 300 scanning electron microscope of ZEISS Corporation of Germany) is used to scan the cross-section of the carbon-based material; finally, the pore area of ​​any one hole in the carbon-based material is obtained respectively through image processing software (such as AVIZO); and the total pore area S1 of the external region and the total pore area S2 of the internal region, and the value of S2 / S1 is obtained thereby. For example, samples can be obtained from different areas 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 sample to obtain cross sections using a cross-section polisher, and at least 10 particles (such as 10, 20, 50 or even more particles) are randomly selected from the scanning electron microscope images of each cross section. The total pore area S2' and the total pore area S1' of the inner region of each particle cross section are obtained using image processing software according to the above definition, and the S2' / S1' value of each particle cross section is obtained. The arithmetic average of the S2' / S1' of all the measured particle cross sections is calculated as the S2 / S1 value of the first carbon-based material.

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

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

[0118] In the present disclosure, the degree of graphitization of a material (e.g., the first carbon-based material, the second carbon-based material) has a well-known meaning in the art and can be tested using instruments and methods known in the art. For example, an X-ray diffractometer (e.g., Bruker D8 Discover) can be used for testing. The test can refer to JIS K 0131-1996 and JB / T 4220-2011 to obtain the average interlayer spacing d of the C(002) plane in the crystal structure of the material. 002 Then according to the formula g=(0.344-d 002) / (0.344-0.3354)×100% to calculate the degree of graphitization. In the above formula, d 002 It is the average interlayer spacing of the C(002) planes in the material's crystal structure expressed in nanometers (nm).

[0119] In the present disclosure, the specific surface area of ​​a material (e.g., a first carbon-based material, a second carbon-based material, etc.) is a well-known meaning in the art and can be measured using instruments and methods known in the art. For example, it can be measured using the nitrogen adsorption specific surface area analysis test method according to GB / T 19587-2017 and calculated using the BET (Brunauer Emmett Teller) method. The test instrument can be a Tri-Star 3020 specific surface area pore size analysis tester from Micromeritics, USA.

[0120] In the present disclosure, the tap density of a material (such as a first carbon-based material, a second carbon-based material, etc.) is a well-known meaning in the art and can be measured using instruments and methods known in the art. For example, GB / T 5162-2006 can be referred to and a powder tap density tester can be used for measurement. The test instrument can be Dandong Baxter BT-301, and the test parameters are as follows: vibration frequency 250 ± 15 times / minute, amplitude 3 ± 0.2 mm, vibration number 5000 times, and graduated cylinder 25 mL.

[0121] In the present disclosure, the gram capacity of a material (such as a first carbon-based material, a second carbon-based material, etc.) has a meaning well known in the art and can be tested using methods known in the art. An exemplary test method is as follows: the sample powder is mixed evenly with the conductive agent carbon black (Super P), the binder polyvinylidene fluoride (PVDF) in a mass ratio of 91.6:1.8:6.6 and the solvent N-methylpyrrolidone (NMP) to form a slurry; the prepared slurry is applied to the surface of the negative electrode current collector copper foil, dried in an oven and set aside; 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 LiPF6 is dissolved in the above organic solvent to a concentration of 1 mol / L The electrolyte was then added. A lithium metal sheet was used as the counter electrode, and a polyethylene (PE) film was used as the separator. The cells were assembled into CR2430 button cells in an argon-protected glove box with the electrolyte. After standing for 12 hours, the cells were discharged at 0.05C to 0.005V at 25°C. The cells were then allowed to stand for 10 minutes, and then discharged again at 50μA to 0.005V. The cells were allowed to stand for 10 minutes, and then discharged again at 10μA to 0.005V. The cells were then charged at 0.1C to 2V, and the charge capacity was recorded. The ratio of the charge capacity to the sample mass is the gram capacity of the corresponding material (e.g., the first carbon-based material, the second carbon-based material, etc.).

[0122] In the present disclosure, the compaction density of the negative electrode film layer is a well-known meaning in the art and can be tested using methods known in the art. The compaction density of the negative electrode film layer = the surface density of the negative electrode film layer / the thickness of the negative electrode film layer.

[0123] In the present disclosure, the areal density of the negative electrode film layer has a meaning well known in the art and can be tested using methods known in the art. For example, a negative electrode sheet coated on one side and cold pressed 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), punched into small discs with an area of ​​S1, weighed, and recorded as M1. Then, the negative electrode film layer of the weighed negative electrode sheet is wiped off, and the weight of the negative electrode current collector is weighed and recorded as M0. The areal density of the negative electrode sheet = (M1-M0) / S1.

[0124] In the present disclosure, the porosity of the negative electrode film layer has a well-known meaning in the art and can be tested using methods known in the art. For example, an exemplary test method is as follows: take a negative electrode sheet that has been coated on one side and cold-pressed (if it is a negative electrode sheet coated on both sides, the negative electrode film layer on one side can be wiped off first), punch it into small disc samples of a certain area, and calculate the apparent volume V1 of the negative electrode sheet; refer to GB / T24586-2009, use an inert gas (such as helium or nitrogen) as a medium, adopt a gas replacement method, and use a true density tester to measure the true volume V2 of the negative electrode sheet. The porosity of the negative electrode film layer = (V1-V2) / V1×100%. Multiple negative electrode sheet samples with good appearance and no powder falling on the edges (such as 30 pieces) can be taken for testing, and the results are averaged, thereby improving the accuracy of the test results. The testing instrument can be a Micromeritics AccuPyc II 1340 true density tester.

[0125] The thickness of the negative electrode film layer is well known in the art and can be measured using methods known in the art, such as using a micrometer (e.g., Mitutoyo 293-100, with an accuracy of 0.1 μm). The thickness ranges provided in this disclosure are for the thickness of the negative electrode film layer on one side of the negative electrode current collector. If the thickness of the negative electrode film layer on either side of the negative electrode current collector falls within the ranges provided in this disclosure, the present disclosure is satisfied.

[0126] In the X-ray diffraction analysis test disclosed herein, a copper target can be used as an anode target, and CuKα rays can be used as a radiation source. The wavelength of the rays is The scanning 2θ angle range was 20° to 80°, and the scanning rate was 4° / min.

[0127] It should be noted that the various parameter tests on the negative electrode active material or the negative electrode film layer can be performed by sampling and testing the prepared secondary battery according to the following steps.

[0128] Discharge the secondary battery (for safety reasons, the secondary battery is generally fully discharged). Disassemble the secondary battery, remove the negative electrode, and soak the negative electrode in dimethyl carbonate for a certain period of time (e.g., 2-10 hours). Then remove the negative electrode and dry it at a certain temperature and time (e.g., 60°C for more than 4 hours). After drying, remove the negative electrode. Samples can now be taken from the dried negative electrode to test the aforementioned parameters related to the negative electrode film, such as the surface density, compacted density, porosity, OI value, thickness, etc.

[0129] The dried negative electrode sheet is baked at a certain temperature and time (for example, 400°C for more than 2 hours), and a sample of the negative electrode active material is taken from any area of ​​the baked negative electrode sheet (a blade can be used for scraping powder sampling); the collected negative electrode active material is sieved (for example, sieved with a 200-mesh sieve) to finally obtain a sample that can be used to test the parameters of the above-mentioned negative electrode active materials.

[0130] In the present disclosure, the first carbon-based material and the second carbon-based material mentioned above can be obtained commercially, or can also be prepared by the following method of the present disclosure.

[0131] The first carbon-based material is natural graphite. Natural graphite generally refers to graphite formed naturally in nature, does not require graphitization, and generally has a relatively large number of pores within natural graphite particles. In some embodiments, the natural graphite includes one or more of flake graphite, natural spherical graphite, and microcrystalline graphite, and particularly includes natural spherical graphite.

[0132] "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 a desired particle size and morphology can be obtained by pre-treating flake graphite. Optionally, the pre-treatment includes crushing, classification, spheroidization, purification, and other processes.

[0133] In some embodiments, the first carbon-based material may be selected from natural spherical graphite, natural flake graphite, etc. In some embodiments, the raw material may be pretreated to obtain natural spherical graphite of desired particle size and morphology. Optionally, the pretreatment may include crushing, classification, spheroidization, purification, etc. to obtain the first carbon-based material having the above-mentioned properties.

[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 a portion of the surface of the first carbon-based material. The organic carbon source can be any carbon-containing material known in the art suitable for coating, such as coal tar, petroleum asphalt, phenolic resin, coconut shell, or the like. 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 having a plurality of pore structures; step 2, mixing the above raw material and the filling material uniformly in a predetermined proportion, then keeping it warm at a first temperature T1 for a first time t1, and cooling it to room temperature to obtain an intermediate; step 3, keeping the obtained intermediate warm at a second temperature T2 for a second time t2, and obtaining the second carbon-based material after the end.

[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 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 a desired particle size and morphology can be obtained by pre-treating flake graphite. Optionally, the pre-treatment includes crushing, classification, spheroidization, purification, and other processes.

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

[0139] In some embodiments, in step 1, the specific surface area of ​​the raw material may be ≥2.5m 2 / g, optional 2.5m 2 / g~10.0m 2 When the specific surface area of ​​the raw material is within the above range, it is beneficial to carry out subsequent filling processing and obtain the second carbon-based material with the required specific surface area, and it is also beneficial for the second carbon-based material to have both high capacity and high first coulombic efficiency. In addition, it is also beneficial for the second carbon-based material to have better kinetic properties.

[0140] In some embodiments, in step 2, the softening point temperature of the filling material is 90° C. to 150° C. Optionally, the softening point temperature of the 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 filler material is less than or equal to 6 μm, and can be selected from 1 μm to 6 μm, 1 μm to 5 μm, 2 μm to 5 μm, and 3 μm to 5 μm. This facilitates the filler material to melt under heat and fill into the pore structure of the raw material, and also helps to improve the uniformity of the dispersion of the filler material and the raw material.

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

[0143] In some embodiments, in step 2, the filling material includes one or more of coal tar, petroleum asphalt, polymer compounds and resins, and may optionally include one or more of coal tar and petroleum asphalt.

[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 can 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, and addition amount of the filling material within the above-mentioned range, it is beneficial to adjust the number of pores and / or pore size in the outer and inner areas of the second carbon-based material within a suitable range, which is beneficial to adjust the S2 / S1 of the second carbon-based material within a suitable range.

[0146] By adjusting the type, softening point, coking value, addition amount and other parameters of the filling material within the above range, the viscosity of the filling material is not high after being heated and melted, and it maintains good fluidity. At the same time, it is not easy to adhere to the raw material particles, which can reduce the agglomeration of the raw material particles in the subsequent preparation process. This can also reduce the problems of increased surface defects of the second carbon-based material particles and increased surface active sites due to the need to add a depolymerization process.

[0147] In some embodiments, in step 2, the heating process of uniformly mixing the raw material and the filling material in a predetermined ratio and then heating the mixture to the first temperature T1 may be a staged heating process.

[0148] In some embodiments, the staged temperature increasing process includes a first temperature increasing process, a second temperature increasing process, and a third temperature increasing process.

[0149] In some embodiments, the first temperature rising process is to raise the temperature to 200° C. to 250° C. and keep the temperature at this temperature for 0.5 h to 3 h.

[0150] In some embodiments, the second temperature increase process is to increase the temperature to 450°C to 550°C and maintain the temperature at this temperature for 0 to 2 hours. When the temperature is maintained for 0 hours, it means that when the temperature is increased to the range of 450°C to 550°C, no temperature maintenance treatment is performed, and the temperature is continued to be increased to the first temperature T1.

[0151] In some embodiments, the third temperature increasing process is to increase the temperature to the first temperature T1 and keep the temperature at the first time t1.

[0152] In the staged heating process, the temperature is first raised to 200℃~250℃. Since the heating temperature is higher than the softening point of the filling material, the filling material is melted and softened by the heat. It can be kept warm for 0.5h~3h to allow it to flow and fill into the pore structure of the raw material; then the temperature is raised to 450℃~550℃, at which time the melted and softened filling material undergoes a carbonization reaction, gradually forming a semi-coke state and turning into a viscous liquid or solid, thereby preventing the filling material from entering the entire pore structure of the raw material; finally, the temperature is raised to the first temperature, at which time the filling material undergoes a carbonization reaction, thereby enabling the pore structure occupied by the filling material to be effectively filled.

[0153] In some embodiments, in step 2, the temperature is raised to the 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 thereof. Alternatively, 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, or 2°C / min to 5°C / min.

[0154] In some embodiments, the heating rate of the first heating process may 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, or 2°C / min to 5°C / min. In some embodiments, the heating rate of the second heating process may be 1°C / min to 10°C / min, optionally 2°C / min to 8°C / min. In some embodiments, the heating rate of the third heating process may be 1°C / min to 10°C / min, optionally 2°C / min to 8°C / min.

[0155] In some embodiments, in step 2, the first temperature T1 is 800° C. to 1200° C. For example, the first temperature T1 can be 800° C., 850° C., 900° C., 950° C., 1000° C., 1050° C., 1100° C., 1200° C., or any range thereof. Alternatively, the first temperature T1 is 800° C. to 1100° C., 850° C. to 1100° C., 900° C. to 1100° C., or 850° C. to 1000° C.

[0156] In some embodiments, in step 2, the first time t1 is 1 hour to 5 hours. For example, the first time t1 can be 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, or any range thereof. Optionally, the first time t1 is 2 hours to 4 hours.

[0157] In some embodiments, in step 2, the heat treatment can be carried out in a device capable of programmed temperature increase, such as a medium frequency furnace, a roller kiln, a rotary kiln, a push plate kiln, a vertical granulation kettle, a horizontal granulation kettle, a vertical reactor, a horizontal reactor or a drum furnace.

[0158] In some embodiments, in step 2, the heat treatment atmosphere may be a protective gas atmosphere, which may 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-mentioned range, it is beneficial to adjust the number of pores and / or the pore size in the outer area and the inner area of ​​the second carbon-based material within a suitable range, and thus it is beneficial to adjust the S2 / S1 of the second carbon-based material within a suitable range.

[0160] In some embodiments, in step 3, the second temperature T2 is 1600° C. to 2800° C. Optionally, the second temperature T2 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., or 2000° C. to 2400° C.

[0161] In some embodiments, in step 3, the second time t2 is 1.5 hours to 6 hours. For example, the second time t1 can be 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, 6 hours, or any range thereof. Optionally, the second time t2 is 2 hours to 5 hours.

[0162] In some embodiments, in step 3, the heat treatment may be performed in a medium frequency furnace, a box-type graphitization furnace, an Acheson graphitization furnace, a continuous graphitization furnace, or an inner-string graphitization furnace.

[0163] In some embodiments, in step 3, the medium frequency furnace and the continuous graphitization heat treatment atmosphere may be a protective gas atmosphere. The protective gas may 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 range, it is beneficial to adjust the content of disordered carbon in the second carbon-based material to be within a suitable range, which is beneficial for the second carbon-based material to have a suitable graphitization degree, interlayer spacing and I D / I G wait.

[0165] In the preparation method of the above-mentioned 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-mentioned range, it is beneficial to adjust the S2 / S1, graphitization degree, gram capacity, specific surface area, particle size, powder compaction density, tap density and other parameters 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 the second carbon-based material prepared above with an organic carbon source and carbonizing it 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, petroleum asphalt, phenolic resin, coconut shell, etc. In some embodiments, the second carbon-based material mixed with the organic carbon source is kept at a third temperature T3 of 1150 to 1500°C for 1-3 hours for carbonization, thereby obtaining a second carbon-based material having a carbon coating layer on at least part of its surface.

[0167] [Positive electrode]

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

[0169] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either 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, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer 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 may adopt the positive electrode active material for lithium-ion batteries known in the art. As an example, the positive electrode active material may include at least one of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, the present disclosure is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), 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 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05O2) and its modified compounds. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0172] In some embodiments, when the battery cell is a sodium-ion battery, the positive electrode active material may be a positive electrode active material known in the art for sodium-ion batteries. For example, the positive electrode active material may include at least one of the following materials: a sodium transition metal oxide, a polyanionic compound, or a Prussian blue-based compound.

[0173] The battery's charge and discharge processes involve the intercalation and deintercalation of lithium, leading to different molar Li contents at different discharge states. The molar Li contents listed in this disclosure for cathode materials refer to the initial state of the material, i.e., the state before addition. When the cathode material is used in a battery system, the molar Li content will change over the course of charge and discharge cycles.

[0174] In the list of positive electrode materials in this disclosure, the molar content of O is only a theoretical value. Lattice oxygen release will cause the molar content of oxygen to change, and the actual molar content of O will fluctuate.

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

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

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

[0178] [Electrolytes]

[0179] The electrolyte conducts ions between the positive and negative electrodes. This disclosure does not specifically limit the type of electrolyte, and the electrolyte can be selected based on needs. For example, the electrolyte can be liquid, gel, or solid.

[0180] In some embodiments, the electrolyte is an electrolyte solution comprising 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 difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0182] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl 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, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0183] In some embodiments, the electrolyte may further include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0184] [Isolation film]

[0185] In some embodiments, the battery cell further includes a separator. The present disclosure has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

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

[0187] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

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

[0189] In some embodiments, the outer packaging of the battery cell may be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the battery cell may be a soft shell, such as a pouch-type soft shell. The soft shell may be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0190] The present disclosure has no particular limitation on the shape of the battery cell, which may be cylindrical, square, or any other shape. For example, FIG6 shows a battery cell 5 with a square structure as an example.

[0191] In some embodiments, referring to Figure 7, the outer packaging may include a shell 51 and a top cover assembly 53. Among them, the shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the top cover assembly 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can form an electrode assembly 52 through a winding process or a lamination 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 contained 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, battery cells may be assembled into a battery module. The battery module may contain one or more battery cells. The specific number may be selected by those skilled in the art based on the application and capacity of the battery module.

[0193] Figure 8 shows an example battery module 4. Referring to Figure 8 , within the battery module 4, multiple battery cells 5 may be arranged sequentially along the length of the battery module 4. Of course, they may also be arranged in any other manner. Furthermore, the multiple battery cells 5 may be secured together using fasteners.

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

[0195] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0196] Figures 9 and 10 illustrate an example battery pack 1. Referring to Figures 9 and 10 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be placed over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0197] In addition, the second aspect of the present disclosure further provides an electrical device, which includes a secondary battery provided by the present disclosure. The secondary battery can be used as a power source for the electrical device, and can also be used as an energy storage unit for the electrical device. The electrical device can 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 and satellites, energy storage systems, etc., but is not limited thereto.

[0198] As the electrical device, a battery cell, a battery module or a battery pack can be selected according to its usage requirements.

[0199] Figure 11 shows an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0200] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be lightweight and thin, and may use a battery cell as a power source.

[0201] Example

[0202] The following examples are provided. The examples described below are illustrative and are intended only to explain the present disclosure and are not to be construed as limiting the present disclosure. Where specific techniques or conditions are not specified in the examples, the methods were performed according to those described in the literature in the art or according to the product specifications. Reagents or instruments used without manufacturer's indication are all commercially available conventional products.

[0203] The materials used in the examples and comparative examples of the present disclosure can be obtained commercially or prepared by the following process.

[0204] Preparation of material 1-1:

[0205] Provide natural spherical graphite, mix the natural spherical graphite and petroleum asphalt in a mass ratio of 100:10, the softening point of the petroleum asphalt is 250°C, and perform low-temperature heat treatment at 1120°C for 2 hours to obtain natural graphite with a carbon coating layer, namely material 1-1, as the first carbon-based material. Wherein, material 1-1 meets the following requirements: volume distribution particle size Dv50 = 17.5 μm, specific surface area BET = 1.96 m 2 / g, graphitization degree = 96.0%, gram 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 those of material 1-1, except that the particle size of the spherical natural graphite used is different from that in material 1-1, so that the volume distribution particle size Dv50 of materials 1-1 to 1-4 is in the range of 16μm to 22μm. The details are as follows:

[0208] Table 1.

[0209] Preparation of materials 1-5 to 1-6:

[0210] The preparation method of materials 1-5 to 1-6 is similar to that of material 1-1, except that the carbonization temperature is adjusted according to the table below so that the graphitization degree of materials 1-5 to 1-6 is within the range of 95-98%. The details are as follows:

[0211] Table 2.

[0212] Preparation of Material 2-1:

[0213] The flake graphite was mechanically crushed, classified, spheroidized, and purified to obtain natural spherical graphite, wherein the volume distribution particle size of the natural spherical graphite was Dv50 = 10.5 μm. The obtained natural spherical graphite was mixed with petroleum asphalt filler at a mass ratio of 100:22, with the petroleum asphalt having a softening point of 115°C. The mixed material was then placed in a programmable temperature device and continuously heated to 650°C (first treatment temperature), held at this temperature for 2 hours, and then cooled to room temperature to obtain an intermediate. The obtained intermediate was placed in a graphitization furnace and heat treated at 2390°C (second treatment temperature). After completion, it was demagnetized and sieved. The resulting material was mixed with asphalt and heat treated at 1150°C for 2.5 hours to obtain material 2-1 as the second carbon-based material. Material 2-1 met the following requirements: S2 / S1 = 8.1, volume distribution particle size Dv50 = 11.2 μm, and degree of graphitization of 96.5%.

[0214] Preparation of materials 2-2 to 2-5:

[0215] The preparation methods of materials 2-2 to 2-5 are similar to those of material 2-1, except that the mass ratio of natural spherical graphite to filler and the first treatment temperature are adjusted according to the table below to obtain materials 2-2 to 2-5. The details are as follows:

[0216] Table 3.

[0217] Preparation of materials 2-6:

[0218] The preparation method of material 2-6 is similar to that of material 1, except that the ratio of natural spherical graphite to filler is adjusted to 100:7, and the first processing temperature T1 is adjusted to 1200° C., so that material S2 / S1=0.82.

[0219] Preparation of materials 2-7 to 2-9:

[0220] The preparation methods of materials 2-7 to 2-9 are similar to those of material 1, except that the volume distribution particle size Dv50 of the above raw materials and natural spherical graphite is adjusted to be within the range of 5.5 μm-11 μm, so that the volume distribution particle size Dv50 of materials 2-7 to 2-9 is within the range of 6 μm-12 μm. The specific preparation methods are as follows:

[0221] Table 4.

[0222] Preparation of materials 2-10 to 2-11:

[0223] The preparation method of materials 2-10 to 2-11 is similar to that of material 2-1, except that the second treatment temperature and the mass ratio of natural spherical graphite to filler are adjusted according to the table below, so that the graphitization degree of materials 2-10 to 2-11 is within the range of 95.5% to 98.0%. The details are as follows:

[0224] Table 5.

[0225] Example 1

[0226] Preparation of secondary batteries:

[0227] 1. Negative Electrode Sheet: The first negative electrode active material (Material 1-1, serving as the first carbon-based material) and the second negative electrode active material (Material 2-1, serving as the second carbon-based material) were mixed with a conductive agent, carbon black (Super P), a thickener, sodium carboxymethyl cellulose, and a binder, styrene-butadiene rubber, in an appropriate amount of deionized water at a weight ratio of 96.4:1:1.2:1.4, respectively, to form a first negative electrode slurry and a second negative electrode slurry. The first and second negative electrode slurries were sequentially coated on both surfaces of the negative electrode current collector copper foil in equal weight ratios. After drying and cold pressing, a negative electrode sheet was obtained.

[0228] 2. Positive electrode: LiNi 0.5 Co 0.2 Mn 0.3 O2 (NCM523) is mixed with carbon black (Super P), a conductive agent, and polyvinylidene fluoride (PVDF) as a binder in a weight ratio of 96:2:2. An appropriate amount of N-methylpyrrolidone (NMP) is added as a solvent and stirred evenly to obtain a positive electrode slurry. The positive electrode slurry is then coated on both surfaces of the positive electrode current collector aluminum foil. After drying and cold pressing, a positive electrode sheet is obtained.

[0229] 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 LiPF6 is dissolved in the above organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0230] 4. Isolation film: polypropylene film.

[0231] 5. Preparation of secondary batteries: Place the positive electrode and negative electrode sheets prepared above in order, with the separator placed between the positive and negative electrode sheets to act as an isolate, and then wind them to obtain an electrode assembly; place the electrode assembly in an outer package, inject the electrolyte after drying, and obtain a secondary battery through vacuum packaging, standing, formation, shaping and other processes.

[0232] Examples 2 to 5

[0233] The battery preparation methods of Examples 2 to 5 are similar to those of Example 1, except that different materials 2-2 to 2-5 are selected as the second carbon-based material. See Table 8 for details.

[0234] Comparative Example 1

[0235] The secondary battery was assembled similarly to the preparation method of Example 1, except that the second carbon-based material selected was material 2-6 in which S2 was smaller than S1. See Table 8 for details.

[0236] Performance Testing

[0237] 1. Material testing

[0238] The S2 / S1 of the second carbon-based material is obtained by testing using the following method.

[0239] Mix the sample preparation binder and the second carbon-based material powder evenly, then apply the mixture to a copper foil and dry at 60°C for 30 minutes. Five samples to be tested, each measuring 6 mm x 6 mm, were cut at five different locations and attached to the sample stage of a CP-type argon ion cross-section polisher. A plasma beam was used to cut the samples to obtain a cross section. The testing instrument can be the IB-09010CP-type argon ion cross-section polisher from JEOL, Japan.

[0240] Each sample cross section of the first carbon-based material was scanned using a scanning electron microscope, and a scanned image was obtained from an arbitrarily selected region within each sample cross section. The test may refer to JY / T010-1996. The testing instrument may be a Sigma 300 scanning electron microscope from ZEISS, Germany.

[0241] Randomly select cross sections of 20 particles of the first carbon-based material from the scanned image. The area formed by extending 0.25 μm from the particle surface of the first carbon-based material to the interior of the particle is recorded as the external area, and the area inside the external area is recorded as the internal area. Use image processing software to obtain the total pore area S1' of the external area of ​​each particle cross section and the pore area S2' of the internal area of ​​the first carbon-based material, and calculate the value of S2' / S1'; and calculate the arithmetic average of S2' / S1' of all 20 particles as the value of S2 / S1 of the first carbon-based material. The image processing software can be AVIZO.

[0242] 2. Second carbon-based material morphology test

[0243] Take the negative electrode sheets of the above embodiments and comparative examples and cut them into 2 cm × 2 cm samples to be tested. Use paraffin to fix the samples to be tested on the sample table. Put the sample table into the sample holder and lock it. Turn on the power of the argon ion cross-section polisher (IB-09010CP argon ion cross-section polisher produced by JEOL, Japan) and evacuate to 10 -4 Pa, set the argon flow rate to 0.15 MPa, the voltage to 8 kV, and the polishing time to 2 h, adjust the sample stage to the swing mode and start polishing; randomly select an area in the sample to be tested for scanning test (refer to JY / T010-1996, scanning electron microscope (Sigma 300 of ZEISS Company), and obtain the ion polishing cross-sectional morphology (CP) image of the second carbon-based material at a magnification of 1000 times, as shown in Figure 4. It can be observed from Figure 4 that the second carbon-based material has a certain number of pores near the center of the particle, while the structure of the area near the surface of the particle is denser.

[0244] 3. Battery performance test

[0245] (1) Dynamic performance test of secondary batteries

[0246] At 25°C, the secondary batteries prepared in the Examples and Comparative Examples were charged to full capacity at a constant current of xC (a starting charge rate that does not cause lithium deposition on the surface of the negative electrode can be determined empirically, such as 1.5C). This was then repeated 10 times at a full discharge rate of 1C. The batteries were then fully charged at xC, the negative electrode was removed, and lithium deposition on the surface of the negative electrode was observed. If no lithium deposition occurred on the negative electrode surface, the charge rate xC was increased by 0.05C and the test was repeated until lithium deposition occurred on the negative electrode surface. The test was then stopped. The charge rate at this point, (x-0.05)×C, was the maximum charge rate of the battery.

[0247] (2) Energy density test

[0248] At 25°C, charge the secondary battery at a constant current of 1 / 3C to 4.25V. Then, charge it at a constant voltage at 4.25V to a current of 0.05C. Let it rest for 5 minutes, and then discharge it at a constant current of 1 / 3C to 2.8V. Record the battery discharge energy at this time. The battery discharge energy divided by the battery mass is the battery's mass energy density, expressed in Wh / kg.

[0249] (3) Cycle performance test of secondary batteries

[0250] At 45°C, the prepared secondary battery was charged at a constant current of 1C to the upper cutoff voltage (corresponding to 100% SOC), then charged at a constant voltage to a current of 0.05C. After standing for 5 minutes, the secondary battery was discharged at a constant current of 1C to the lower cutoff voltage (corresponding to 0% SOC). The discharge capacity at this point was recorded as the discharge capacity of the first cycle. The secondary battery was subjected to cyclic charge and discharge tests according to the above method, and the discharge capacity after each cycle was recorded.

[0251] Capacity retention rate (%) of the secondary battery after 900 cycles at 45° C. = discharge capacity after 900 cycles / discharge capacity at the first cycle×100%.

[0252] The test results of the secondary batteries prepared in Examples 1 to 5 and Comparative Example 1 are shown in Table 6 below.

[0253] Table 6.

[0254] 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 effect on energy density is small. The smaller the S2 / S1 value, the more pore structure the material has on the surface, so the consumption of active lithium will also increase, and the cycle will deteriorate slightly. However, when the S2 / S1 value is too small (Comparative Example 1), it shows that the surface has a rich pore structure, which will significantly increase side reactions and therefore significantly deteriorate the cycle performance.

[0255] Examples 6 to 8

[0256] 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.

[0257] Examples 9 to 11

[0258] 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.

[0259] 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.

[0260] Table 7.

[0261] As shown in Table 7 above, 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 slightly improves, but the kinetics deteriorates. When the particle size increases, the material's 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 the kinetic performance of the secondary battery is affected.

[0262] Examples 12-13

[0263] 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.

[0264] Examples 14-15

[0265] 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.

[0266] 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.

[0267] Table 8.

[0268] Table 8 shows that changes in graphitization primarily affect the energy density of secondary batteries, with minimal impact on kinetic performance and cycling stability. Higher graphitization leads to higher gram capacity and higher powder compaction density, resulting in higher cell energy density.

[0269] It should be noted that the present disclosure is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and within the scope of the technical solution of the present disclosure, embodiments having substantially the same structure as the technical concept and exerting the same effects are all included in the technical scope of the present disclosure. In addition, within the scope of the present disclosure, various modifications that can be imagined by those skilled in the art to the embodiments, and other methods constructed by combining some of the constituent elements of the embodiments are also included in the scope of the present disclosure.

Claims

1. A secondary battery, comprising a negative electrode plate, wherein the negative electrode plate comprises 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 arranged opposite to the first surface. The thickness of the negative electrode film layer is denoted as H. in, The region from the second surface of the negative electrode film layer to a thickness range of 0.3H is recorded as the first region of the negative electrode film layer, the first region is close to the surface of the negative electrode current collector and includes a first negative electrode active material, and the first negative electrode active material includes a first carbon-based material; The region from the first surface of the negative electrode film layer to a thickness of 0.3H is recorded as the second region of the negative electrode film layer, the second region is far away from the surface of the negative electrode current collector and 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 external region and an internal region located inside the external region, wherein the external region refers to a region extending 2.5 μm from the particle surface of the second carbon-based material to the inside of the particle. In the cross-sectional view of the second carbon-based material, the total pore area of ​​the external region is denoted as S1, and the total pore area of ​​the internal 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 to 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 to 4, wherein: 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.

6. The secondary battery according to any one of claims 1 to 5, wherein The graphitization degree of the first carbon-based material is ≥ 95%, which may be 95.0% to 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% to 98.5%.

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

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

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

11. The secondary battery according to any one of claims 1 to 10, wherein The gram capacity of the first carbon-based material is ≥355 mAh / g, and can 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: (1) The volume distribution particle size Dv90 of the second carbon-based material is ≤25 μm; it can be selected to be 18 μm-25 μm; (2) The second carbon-based material satisfies: [(Dv90)-(Dv10)] / (Dv50)]≤1.30; optionally 1.05-1.25; (3) The gram capacity of the second carbon-based material is ≥358 mAh / g, and can be 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 , can be selected to be less than or equal to 0.13μm 2 ; (5) The inner region of the second carbon-based material includes one or more areas greater than or equal to 0.15 μm 2 The pore structure may include one or more pores with an area of ​​0.15 μm 2 -2.0μm 2 pore structure.

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 comprises a silicon-based material; Optionally, the mass proportion of the silicon-based material in the first negative electrode active material is ≤10%wt%; Optionally, the mass proportion of the silicon-based material in the second negative electrode active material is ≤10wt%; Optionally, 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.

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: (1) The compaction density of the negative electrode film layer is ≤1.85 g / cm 3 , optionally 1.55 g / cm 3 -1.85g / cm 3 ; (2) The surface density of the negative electrode film layer is ≥6.0 mg / cm 2 , optionally 7.0 mg / cm 2 -15.0mg / cm 2 ; (3) The porosity of the negative electrode film layer is 18.0%-36.7%, optionally 19.0%-34.0%; (4) The thickness of the negative electrode film layer is ≥60 μm, and can be optionally 70 μm-130 μm.

15. An electrical device comprising the secondary battery according to any one of claims 1 to 14.

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