Lithium secondary battery, negative electrode sheet, and electric device

Abstract

The application provides a lithium secondary battery, a negative electrode sheet and an electric device. The lithium secondary battery comprises a negative electrode sheet, and the negative electrode sheet comprises a negative electrode current collector, a first negative electrode film layer and a second negative electrode film layer. The first negative electrode film layer is arranged on at least one side surface of the negative electrode current collector, and the second negative electrode film layer is arranged on the surface of the first negative electrode film layer away from the negative electrode current collector. The first negative electrode film layer comprises first graphite, and the second negative electrode film layer comprises second graphite with a carbon coating layer on the surface. The volume average particle size Dv50 of the first graphite is 13-15 mu m, and the gram capacity is less than or equal to 340 mAh / g. The volume average particle size Dv50 of the second graphite is 10-12 mu m, and the gram capacity is greater than or equal to 350 mAh / g. The lithium secondary battery has excellent cycle performance.

Description

Technical Field

[0001] This application relates to the field of lithium secondary battery technology, and in particular to a lithium secondary battery, a negative electrode sheet, and an electrical device. Background Technology

[0002] In recent years, with the increasingly wide application of lithium secondary batteries, they have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power plants, as well as in many fields such as power tools, electric bicycles, electric motorcycles and electric vehicles.

[0003] With the significant advancements in lithium-ion batteries, higher demands have been placed on their cycle performance. This is particularly true in energy storage systems, where superior cycle performance is typically required to improve the stability of the power supply. Therefore, developing a lithium-ion battery with excellent cycle performance is a key focus for those skilled in the art. Summary of the Invention

[0004] This application is made in view of the above-mentioned problems, and one of its objectives is to provide a lithium secondary battery, a negative electrode sheet, and an electrical device, wherein the lithium secondary battery has excellent cycle performance.

[0005] To achieve the above objectives, a first aspect of this application provides a lithium secondary battery, including a negative electrode sheet, the negative electrode sheet comprising a negative current collector, a first negative electrode film layer, and a second negative electrode film layer; the first negative electrode film layer is disposed on at least one side surface of the negative current collector, and the second negative electrode film layer is disposed on the surface of the first negative electrode film layer opposite to the negative current collector; the first negative electrode film layer comprises a first graphite, and the second negative electrode film layer comprises a second graphite having a carbon coating layer on its surface; the first graphite has a volume average particle size Dv50 of 13μm~15μm and a specific capacity less than or equal to 340mAh / g; the second graphite has a volume average particle size Dv50 of 10μm~12μm and a specific capacity greater than or equal to 350 mAh / g.

[0006] By using graphite material with larger particle size and lower specific capacity in the first negative electrode film layer, the graphite material has a smaller specific surface area, fewer reaction sites on its surface, and less consumption of active lithium, which is beneficial to improving the cycle stability and extending the cycle life of the lithium secondary battery. However, the lithium-ion diffusion path of large-particle-size, low-capacity graphite is longer, affecting the lithium-ion diffusion rate. Furthermore, due to its small specific surface area, the contact area with the electrolyte is smaller, reducing the lithium-ion transport rate. Therefore, the kinetic performance of large-particle-size, low-capacity graphite is often poor, and in the later stages of cell cycling, insufficient lithium intercalation capacity of the negative electrode may lead to a significant deterioration in battery performance, affecting the battery's cycle performance.

[0007] This application employs a dual-layer negative electrode film design. The outermost second negative electrode film layer utilizes a second graphite with smaller particle size and higher specific capacity. This second graphite possesses a large specific surface area and a relatively regular graphite crystal structure, providing more lithium-ion insertion sites while shortening the lithium-ion diffusion distance and accelerating the lithium insertion rate. The carbon coating layer on the surface of the second graphite accelerates lithium-ion transport, increasing the lithium-ion transport rate and thus improving graphite kinetics. By setting the aforementioned second negative electrode film layer outside the first negative electrode film layer, the lithium insertion capability of the negative electrode sheet can be effectively improved. This effectively alleviates the situation where, in the later stages of cycling of cells using only the first negative electrode film layer, the battery performance deteriorates sharply due to insufficient lithium insertion capability of large-particle graphite. The combination of the first and second negative electrode films enables the lithium-ion secondary battery to exhibit excellent cycle performance.

[0008] In any embodiment, the mass fraction of the carbon coating layer is less than or equal to 1.5% based on the total mass of the second graphite. This results in a smaller carbon coating layer on the surface of the second graphite, forming a micro-coating. This effectively improves the kinetic performance of the second graphite without causing a deterioration in the cycle performance of the lithium-ion battery due to excessive coating.

[0009] In any embodiment, the mass fraction of the carbon coating layer is 0.5% to 1.5% based on the total mass of the second graphite. This is beneficial for further improving the kinetic performance of the second graphite and enhancing the cycle performance of the lithium secondary battery.

[0010] In any embodiment, the carbon coating layer is made of amorphous carbon. This is beneficial for improving the kinetic properties of the second graphite; it also gives the negative electrode a better lithium intercalation capability and better kinetic performance.

[0011] In any embodiment, the specific capacity of the first graphite is 335 mAh / g to 340 mAh / g. This is beneficial for improving the cycle performance of the lithium secondary battery.

[0012] In any embodiment, the specific capacity of the second graphite is 350 mAh / g to 355 mAh / g. This is beneficial for improving the cycle performance of the lithium secondary battery while maintaining energy density.

[0013] In any embodiment, the mass ratio of the first graphite to the second graphite in the negative electrode sheet is 2:3 to 3:2. This is beneficial for improving the cycle performance of the lithium secondary battery while also considering its energy density.

[0014] In any embodiment, the compacted density of the first graphite powder under a pressure of 49000N is 1.76 g / cm³. 3~1.82 g / cm 3 .

[0015] In any embodiment, the compacted density of the second graphite powder under a pressure of 49000 N is 1.80 g / cm³. 3 ~1.86 g / cm 3 Thus, the first graphite has a larger particle size, lower specific capacity, and relatively lower powder compaction density; the second graphite has a smaller particle size, higher specific capacity, and relatively higher powder compaction density. Using the first and second graphite in a layered combination is beneficial for improving the cycle performance of lithium secondary batteries while also taking energy density into account.

[0016] In any embodiment, the compaction density of the negative electrode sheet is greater than or equal to 1.5 g / cm³. 3 This helps to increase the energy density of the battery.

[0017] In any embodiment, the unilateral density of the first negative electrode film layer is greater than or equal to the unilateral density of the second negative electrode film layer. Thus, the combination of the first and second negative electrode film layers facilitates excellent cycle performance in the lithium-ion secondary battery.

[0018] In any embodiment, the one-sided density of the first negative electrode film layer is 4.7 mg / cm³. 2 ~5.8 mg / cm 2 .

[0019] In any embodiment, the one-sided density of the second negative electrode film is 3.5 mg / cm³. 2 ~4.7 mg / cm 2 Thus, the negative electrode sheet has a first negative electrode film layer and a second negative electrode film layer of appropriate thickness. The combination of the first negative electrode film layer and the second negative electrode film layer is beneficial to improving the cycle performance of lithium secondary batteries.

[0020] In any implementation, the mass fraction of the first graphite is 95% to 97% based on the total mass of the first negative electrode film layer.

[0021] In any embodiment, the mass fraction of the second graphite is 96% to 98% based on the total mass of the second negative electrode film.

[0022] In any embodiment, the first graphite and the second graphite each independently comprise one or more of single-particle graphite and secondary-particle graphite.

[0023] A second aspect of this application provides a negative electrode sheet, comprising a negative current collector, a first negative electrode film layer, and a second negative electrode film layer; the first negative electrode film layer is disposed on at least one side surface of the negative current collector, and the second negative electrode film layer is disposed on the surface of the first negative electrode film layer opposite to the negative current collector; the first negative electrode film layer comprises a first graphite, and the second negative electrode film layer comprises a second graphite having a carbon coating layer on its surface; the first graphite has a volume average particle size Dv50 of 13μm~15μm and a specific capacity of less than or equal to 340mAh / g; the second graphite has a volume average particle size Dv50 of 10μm~12μm and a specific capacity of greater than or equal to 350mAh / g.

[0024] By setting the second negative electrode film layer on the outside of the first negative electrode film layer, the lithium intercalation capability of the negative electrode sheet can be effectively improved; the situation where the battery performance of the cell using the first negative electrode film layer deteriorates sharply in the later stages of cycling due to insufficient lithium intercalation capability of large-particle graphite can be effectively alleviated. The combination of the first and second negative electrode films enables the lithium secondary battery to have excellent cycle performance.

[0025] A third aspect of this application provides an electrical device including a lithium secondary battery as described in the first aspect of this application.

[0026] Details of one or more embodiments of this application are set forth in the following drawings and description. Other features, objects, and advantages of this application will become apparent from the specification, drawings, and claims. Attached Figure Description

[0027] To better describe and illustrate the embodiments or examples provided in this application, reference may be made to one or more accompanying drawings. Additional details or examples used to describe the drawings should not be considered as limiting the scope of any of the disclosed applications, the currently described embodiments or examples, or the best mode of conduct of these applications as currently understood. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0028] Figure 1 This is a schematic diagram of a battery cell according to one embodiment of this application;

[0029] Figure 2 for Figure 1 An exploded view of a battery cell according to one embodiment of this application is shown.

[0030] Figure 3 This is a schematic diagram of the structure of the negative electrode sheet according to one embodiment of this application;

[0031] Figure 4 This is a schematic diagram of an electrical device that uses a lithium secondary battery as a power source according to an embodiment of this application.

[0032] Explanation of reference numerals in the attached figures:

[0033] 1. Negative electrode sheet; 11. Negative current collector; 12. First negative electrode film layer; 13. Second negative electrode film layer; 5. Battery cell; 51. Casing; 52. Electrode assembly; 53. Cover plate; 6. Electrical device. Detailed Implementation

[0034] The following describes embodiments of the lithium secondary battery, negative electrode, and power supply device of this application in detail with appropriate reference to the accompanying drawings. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0035] The "range" disclosed in this application can be defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints. Any endpoint can be independently included or excluded, and they can be combined arbitrarily; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60~120 and 80~110 are listed for a specific parameter, it is expected that ranges of 60~110 and 80~120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are also listed, then the following ranges are all expected: 1~3, 1~4, 1~5, 2~3, 2~4, and 2~5. In this application, unless otherwise stated, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0" and "5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is described as an integer ≥2, it is equivalent to listing integers such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For instance, when a parameter is described as an integer selected from "2~10", it is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0036] In this application, the terms "multiple" or "various" are used unless otherwise specified, referring to a quantity greater than or equal to 2. For example, "one or more" means one or more types.

[0037] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0038] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments. The term "implementation" as used herein has a similar understanding.

[0039] Those skilled in the art will understand that the order in which the steps are written in the methods of various embodiments or examples does not imply a strict execution order and does not constitute any limitation on the implementation process. The detailed execution order of each step should be determined by its function and possible internal logic. Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) can be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0040] In this application, open-ended technical features or solutions described using terms such as "containing," "including," or "comprising" do not exclude additional members beyond those listed unless otherwise specified. They can be considered as providing both closed-ended features or solutions comprised of the listed members and open-ended features or solutions that include additional members beyond the listed members. For example, if A includes a1, a2, and a3, it may also include other members or exclude additional members unless otherwise specified. This can be considered as providing both the feature or solution that "A consists of a1, a2, and a3" and the feature or solution that "A includes not only a1, a2, and a3, but also other members." In this application, unless otherwise specified, A (e.g., B) indicates that B is a non-limiting example of A, and it can be understood that A is not limited to B.

[0041] In this application, "optionally," "optionally," and "optional" mean that something is optional, that is, it means that it is selected from either "with" or "without." If there are multiple "optional" entries in a technical solution, unless otherwise specified and there are no contradictions or mutual constraints, each "optional" entry shall be independent.

[0042] Currently, with the significant development of lithium-ion batteries, higher requirements are being placed on their cycle performance. Especially in the field of energy storage systems, due to the specific application scenarios, lithium-ion batteries are typically required to have excellent cycle performance to ensure the stability of the power supply to the energy storage system. Based on this, this application provides a lithium-ion battery with excellent cycle performance.

[0043] The first embodiment of this application provides a lithium secondary battery, which includes a negative electrode sheet, a negative current collector, a first negative electrode film, and a second negative electrode film. The first negative electrode film is disposed on at least one side surface of the negative current collector, and the second negative electrode film is disposed on the surface of the first negative electrode film facing away from the negative current collector. The first negative electrode film includes a first graphite, and the second negative electrode film includes a second graphite with a carbon coating layer on its surface. The volume average particle size Dv50 of the first graphite is 13μm to 15μm, and the specific capacity is less than or equal to 340mAh / g. The volume average particle size Dv50 of the second graphite is 10μm to 12μm, and the specific capacity is greater than or equal to 350mAh / g.

[0044] The aforementioned lithium-ion secondary battery utilizes graphite material with a larger particle size and lower specific capacity in the first negative electrode film. This larger particle size and lower specific capacity graphite material has a smaller specific surface area, fewer reaction sites on its surface, and less consumption of active lithium, which is beneficial for improving the cycle stability and extending the cycle life of the lithium-ion secondary battery. However, the lithium-ion diffusion path of large-particle-size, low-capacity graphite is longer, affecting the lithium-ion diffusion rate. Furthermore, due to its small specific surface area, the contact area with the electrolyte is smaller, reducing the lithium-ion transport rate. Therefore, the kinetic performance of large-particle-size, low-capacity graphite is often poor, and in the later stages of cell cycling, insufficient lithium intercalation capacity of the negative electrode may lead to a significant deterioration in battery performance, affecting the battery's cycle performance.

[0045] To address this, this application employs a dual-layer negative electrode film design. The inner first negative electrode film layer uses first graphite with a volume average particle size (Dv50) of 13 μm–15 μm and a specific capacity of less than or equal to 340 mAh / g. The outer second negative electrode film layer uses second graphite with a volume average particle size (Dv50) of 10 μm–12 μm, a specific capacity greater than or equal to 350 mAh / g, and a carbon coating on its surface. The smaller particle size and higher specific capacity of the second graphite in the second negative electrode film layer provide a larger specific surface area and a more regular graphite crystal structure, offering more lithium-ion insertion sites while shortening the lithium-ion diffusion distance and accelerating the lithium insertion rate. The carbon coating on the surface of the second graphite accelerates lithium-ion transport, increasing the lithium-ion transport rate and thus improving graphite kinetics.

[0046] By setting the aforementioned second negative electrode film layer outside the first negative electrode film layer, the lithium intercalation capability of the negative electrode sheet can be effectively improved; this effectively alleviates the situation where battery performance deteriorates sharply in the later stages of cell cycling due to insufficient lithium intercalation capability of large-particle graphite when only the first negative electrode film layer is used. The combination of the aforementioned first and second negative electrode film layers can effectively improve the cycle life of lithium-ion batteries, resulting in lithium-ion batteries with excellent cycle performance.

[0047] It should be noted that the volume average particle size Dv50 refers to the particle size corresponding to the cumulative volume fraction reaching 50% in the particle size distribution of the particle group. In other words, when the particle group is arranged in ascending order of particle size, the particle size corresponding to the cumulative particle volume accounting for half of the total volume is the volume average particle size Dv50. In this application, the carbon coating layer is part of the second graphite, and "the volume average particle size Dv50 of the second graphite" refers to the particle size of the graphite particles with the carbon coating layer. The specific capacity of graphite material refers to the amount of charge that a unit mass of graphite material can store and release during the charging and discharging process of a battery. It reflects the amount of electricity that each gram of graphite material can provide when used as a battery electrode material.

[0048] Understandably, the volume average particle size Dv50 of the first graphite can be 13 μm, 13.2 μm, 13.5 μm, 13.8 μm, 14 μm, 14.2 μm, 14.5 μm, 14.8 μm, 15 μm, or any value within the range formed by any two of the above values. The volume average particle size Dv50 of the second graphite can be 10 μm, 10.2 μm, 10.5 μm, 10.8 μm, 11 μm, 11.2 μm, 11.5 μm, 11.8 μm, 12 μm, or any value within the range formed by any two of the above values.

[0049] In some embodiments, the mass fraction of the carbon coating layer is less than or equal to 1.5% based on the total mass of the second graphite. The carbon coating layer on the surface of the second graphite is relatively small, forming a micro-coating. The mass fraction of the carbon coating layer on the surface of the second graphite is significantly lower than the coating amount in conventional fast-charging graphite (typically 10%~15%). Thus, while effectively improving the kinetic performance of the second graphite, the cycle performance of the lithium-ion battery is not deteriorated due to excessive coating.

[0050] Understandably, the mass fraction of the carbon coating layer, based on the total mass of the second graphite, can be, but is not limited to, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, and any value within the range formed by any two of the above values.

[0051] Alternatively, the mass fraction of the carbon coating layer is 0.5% to 1.5% based on the total mass of the second graphite. Controlling the mass fraction of the carbon coating layer within the above range is beneficial for further improving the kinetic performance of the second graphite and enhancing the cycle performance of the lithium secondary battery.

[0052] In some embodiments, the carbon coating material includes amorphous carbon. By using amorphous carbon material as the carbon coating layer on the surface of the second graphite in the second negative electrode film, it is beneficial to improve the kinetic performance of the second graphite; thus, the negative electrode sheet has better lithium intercalation capability and better kinetic performance.

[0053] In some embodiments, the specific capacity of the first graphite is 335 mAh / g to 340 mAh / g. Using a first graphite with the aforementioned specific capacity is beneficial for improving the cycle performance of the lithium-ion secondary battery. It is understood that the specific capacity of the first graphite can be, but is not limited to, 335 mAh / g, 336 mAh / g, 337 mAh / g, 338 mAh / g, 339 mAh / g, 340 mAh / g, or any value within the range formed by any two of the above values.

[0054] In some embodiments, the specific capacity of the second graphite is 350 mAh / g to 355 mAh / g. Using a second graphite with the aforementioned specific capacity is beneficial for improving the cycle performance of the lithium-ion secondary battery while maintaining energy density. It is understood that the specific capacity of the second graphite can be, but is not limited to, 350 mAh / g, 351 mAh / g, 352 mAh / g, 353 mAh / g, 354 mAh / g, 355 mAh / g, or any value within the range formed by any two of the above values.

[0055] In some embodiments, the mass ratio of the first graphite and the second graphite in the negative electrode sheet is 2:3 to 3:2. A 1:1 ratio is also possible. Using the first and second graphite in a layered configuration according to the above mass ratio is beneficial for improving the cycle performance of the lithium-ion battery while maintaining its energy density.

[0056] Understandably, the mass ratio of the first graphite and the second graphite in the negative electrode sheet can be, but is not limited to, 2:3, 1.4:2, 3:4, 4:5, 1:1, 6:5, 3:2, and any ratio within the range formed by any two of the above ratios.

[0057] In some embodiments, the compacted density of the first graphite powder at a pressure of 49000 N is 1.76 g / cm³. 3 ~1.82 g / cm 3The compacted density of the second type of graphite powder under a pressure of 49000N is 1.80 g / cm³. 3 ~1.86 g / cm 3 Thus, the first graphite has a larger particle size, lower specific capacity, and relatively lower powder compaction density; the second graphite has a smaller particle size, higher specific capacity, and relatively higher powder compaction density. Using the first and second graphite in a layered combination is beneficial for improving the cycle performance of lithium secondary batteries while also taking energy density into account.

[0058] Powder compaction density refers to the mass of powder per unit volume after it has been compacted under specific conditions by applying pressure or external force. Powder compaction density can be measured using conventional methods, such as the vibrating table method and the press method.

[0059] Understandably, the compacted density of the first graphite powder can be 1.76 g / cm³. 3 1.77 g / cm 3 1.78 g / cm 3 1.79 g / cm 3 1.80 g / cm 3 1.81 g / cm 3 1.82 g / cm 3 And any value within the range formed by any two of the above values. The compacted density of the second graphite powder can be 1.80 g / cm³. 3 1.81 g / cm 3 1.82 g / cm 3 1.83 g / cm 3 1.84 g / cm 3 1.85 g / cm 3 1.86 g / cm 3 And any value within the range formed by any two of the above values.

[0060] In some embodiments, the compaction density of the negative electrode sheet is greater than or equal to 1.5 g / cm³. 3 The negative electrode sheet has a large compaction density, which is beneficial to improving the energy density of the battery.

[0061] In some embodiments, the lateral density of the first negative electrode film is greater than or equal to the lateral density of the second negative electrode film. Thus, the combination of the first and second negative electrode films described above facilitates excellent cycle performance in the lithium-ion secondary battery.

[0062] Furthermore, the unilateral density of the first negative electrode film layer is 4.7 mg / cm³. 2 ~5.8 mg / cm2 The single-sided density of the second negative electrode film is 3.5 mg / cm³. 2 ~4.7 mg / cm 2 Thus, the negative electrode sheet has a first negative electrode film layer and a second negative electrode film layer of appropriate thickness. The combination of the first negative electrode film layer and the second negative electrode film layer is beneficial to improving the cycle performance of lithium secondary batteries.

[0063] In some embodiments, the mass fraction of the first graphite is 95% to 97% based on the total mass of the first negative electrode film layer; and the mass fraction of the second graphite is 96% to 98% based on the total mass of the second negative electrode film layer. Thus, the first and second negative electrode film layers each contain appropriate amounts of the first and second graphite, respectively; this combination of the first and second negative electrode film layers is beneficial for improving the cycle performance of the lithium secondary battery.

[0064] In some embodiments, the first graphite and the second graphite each independently comprise one or more of single-particle graphite and secondary-particle graphite. That is, the first graphite may be entirely single-particle graphite, entirely secondary-particle graphite, or a mixture of single-particle and secondary-particle graphite. Similarly, the second graphite may be entirely single-particle graphite, entirely secondary-particle graphite, or a mixture of single-particle and secondary-particle graphite.

[0065] In some specific examples, the first graphite is made of single-particle graphite. The first graphite has a larger particle size, and the specific surface area of ​​large-particle graphite is relatively small. Its contact area with the electrolyte is smaller, which reduces the side reactions between the electrode and the electrolyte during battery charging and discharging. The formation and growth rate of the SEI film is slower, which is beneficial to improving the cycle performance and storage performance of lithium secondary batteries.

[0066] It should be noted that single-particle graphite refers to independent graphite particles obtained directly from raw materials through physical processes such as crushing, grinding, and grading, without secondary agglomeration or bonding. Secondary-particle graphite, on the other hand, is graphite particles prepared through a secondary granulation process. For example, using coal-based needle coke as raw material, primary-particle graphite is obtained through crushing and graphitization. Then, using pitch as a binder, secondary granulation is carried out in a reactor, followed by subsequent graphitization processes to obtain secondary-particle graphite.

[0067] A second embodiment of this application provides a negative electrode sheet, which includes a negative current collector, a first negative electrode film layer, and a second negative electrode film layer; wherein the first negative electrode film layer is disposed on at least one side surface of the negative current collector, and the second negative electrode film layer is disposed on the surface of the first negative electrode film layer opposite to the negative current collector; the first negative electrode film layer includes a first graphite, and the second negative electrode film layer includes a second graphite having a carbon coating layer on its surface; the volume average particle size Dv50 of the first graphite is 13μm~15μm, and the specific capacity is less than or equal to 340mAh / g; the volume average particle size Dv50 of the second graphite is 10μm~12μm, and the specific capacity is greater than or equal to 350 mAh / g.

[0068] By setting the second negative electrode film layer on the outside of the first negative electrode film layer, the lithium intercalation capability of the negative electrode sheet can be effectively improved; the situation where the battery performance of the cell using the first negative electrode film layer deteriorates sharply in the later stages of cycling due to insufficient lithium intercalation capability of large-particle graphite can be effectively alleviated. By combining the first and second negative electrode film layers, the cycle life of the lithium secondary battery can be effectively improved, resulting in a lithium secondary battery with excellent cycle performance.

[0069] The third embodiment of this application provides an electrical device, including the lithium secondary battery of the first embodiment of this application described above.

[0070] The lithium secondary battery, negative electrode, and power supply device of this application are described below with appropriate reference to the accompanying drawings.

[0071] In one embodiment of this application, a lithium secondary battery is provided.

[0072] Typically, a lithium-ion secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and extracting. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0073] The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector.

[0074] As a non-limiting example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive active material layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0075] 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 polymeric material substrate and a metal layer formed on at least one surface of the polymeric material substrate. The composite current collector can be obtained by forming a metal material on the polymeric material substrate. Non-limiting examples of the metal material in the positive electrode current collector may include one or more of aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. Non-limiting examples of the polymeric material substrate in the positive electrode current collector may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0076] In some embodiments, the positive electrode active material may comprise a positive electrode active material known in the art for use in batteries.

[0077] As a non-limiting example, the positive electrode active material of a lithium secondary battery may include one or more of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as positive electrode active materials of batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Among them, examples of lithium transition metal oxides may include, but are not limited to, one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, and their modified compounds. Non-limiting examples of lithium phosphates with an olivine structure may include, but are not limited to, one or more of lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites. Non-limiting examples of lithium cobalt oxides may include LiCoO2; non-limiting examples of lithium nickel oxides may include LiNiO2; non-limiting examples of lithium manganese oxides may include LiMnO2, LiMn2O4, etc.; non-limiting examples of lithium nickel cobalt manganese oxides may include LiNi 1 / 3Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 Examples of lithium nickel cobalt aluminum oxides include LiNi, etc. 0.85 Co 0.1 Al 0.05 O2.

[0078] Understandably, lithium (Li) is intercalated and deintercalated during the charging and discharging process of a battery, and the Li content in the positive electrode varies depending on the state of discharge. Unless otherwise specified, the Li content in the examples of positive electrode materials listed in this application refers to the initial state of the material. When a positive electrode material is applied to a positive electrode in a battery system, the Li content in the positive electrode material typically changes after charge-discharge cycles. The Li content can be measured using molar content, but is not limited to this. Regarding "Li content refers to the initial state of the material," the initial state of the material refers to its state before being added to the positive electrode slurry. It is understood that new materials obtained by appropriately modifying the listed positive electrode materials are also within the scope of positive electrode materials. The aforementioned appropriate modification refers to acceptable modification methods for the positive electrode material; non-limiting examples include coating modification.

[0079] In the examples of cathode materials in this application, the oxygen (O) content is only a theoretical value. Lattice oxygen release will cause changes in the molar content of oxygen, and the actual O content will fluctuate. The O content can be measured in molar content, but is not limited to this.

[0080] In some embodiments, the positive electrode active material layer may optionally include a binder. As a non-limiting example, the binder may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins. The binder accounts for 0% to 20% by weight of the positive electrode active material layer, based on the total weight of the positive electrode active material layer.

[0081] In some embodiments, the positive electrode active material layer may optionally include a conductive agent. As a non-limiting example, the conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The conductive agent accounts for 0% to 20% by weight of the positive electrode active material layer, based on the total weight of the positive electrode active material layer.

[0082] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry, wherein the solid content of the positive electrode slurry is 40wt%~80wt%, the viscosity at room temperature is adjusted to 5000mPa·s~25000mPa·s, the positive electrode slurry is coated on both sides of the positive current collector, dried and then cold-pressed by a cold rolling mill to form the positive electrode sheet.

[0083] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0084] As a non-limiting example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0085] 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 material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be obtained by forming a metal material on the polymer material substrate. Non-limiting examples of the metal material in the negative electrode current collector may include one or more of copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. Non-limiting examples of the polymer material substrate in the negative electrode current collector may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0086] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may include one or more 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).

[0087] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0088] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0089] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder, and any other components, in a solvent (a non-limiting example of a solvent is deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto at least one surface of the negative electrode current collector, and then obtaining the negative electrode sheet after processes such as drying and cold pressing. The surface of the negative electrode current collector coated with the negative electrode slurry can be a single surface of the negative electrode current collector or both surfaces of the negative electrode current collector.

[0090] Electrolytes function to conduct ions between the positive and negative electrode plates. This application does not impose any particular restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or completely solid.

[0091] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0092] In some embodiments, the electrolyte salt of the lithium secondary battery may include one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP).

[0093] In some embodiments, the solvent may include one or more of the following: fluoroethylene carbonate (FEC), ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl carbonate (BC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0094] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0095] In some embodiments, the additives in the electrolyte may include, but are not limited to, one or more of fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoromethyl ethylene carbonate (TFPC), etc.

[0096] In some embodiments, the lithium secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0097] In some embodiments, the material of the separator may include one or more of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may 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 may be the same or different, without particular limitation.

[0098] In some embodiments, the thickness of the separator is 6 μm to 40 μm, and optionally 12 μm to 20 μm.

[0099] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0100] In some embodiments, the lithium secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0101] In some embodiments, the outer packaging of the lithium secondary battery can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the lithium secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; further, non-limiting examples of plastic may include one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0102] A lithium secondary battery includes at least one battery cell. A lithium secondary battery may include one or more battery cells.

[0103] In this application, unless otherwise specified, "cell battery" refers to the basic unit capable of converting chemical energy into electrical energy, and generally includes at least a positive electrode, a negative electrode, and an electrolyte. During the charging and discharging process of the battery, active ions move back and forth between the positive and negative electrode plates, inserting and extracting. The electrolyte acts as a conductor for the active ions between the positive and negative electrode plates.

[0104] This application does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 The example shown is a square-structured battery cell 5.

[0105] In some implementations, refer to Figure 2 The outer packaging may include a housing 51 and a cover plate 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be placed over the opening to close the receiving cavity. The positive electrode sheet, negative electrode sheet, and separator can be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in a single battery cell 5 can be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0106] In some embodiments, the battery cells 5 can be assembled into a battery module, and the number of battery cells 5 contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0107] In the battery module, multiple battery cells 5 can be arranged sequentially along the length of the battery module. Of course, they can also be arranged in any other manner. Furthermore, these multiple battery cells 5 can be secured with fasteners.

[0108] Optionally, the battery module may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0109] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0110] The battery pack may include a battery box and multiple battery modules disposed within the battery box. The battery box includes an upper body and a lower body, with the upper body covering the lower body to form a closed space for accommodating the battery modules. The multiple battery modules can be arranged in any manner within the battery box.

[0111] In addition, this application also provides a negative electrode sheet, which is used to form an electrode assembly with a positive electrode sheet and a separator. For example, Figure 3 This application provides an example of a negative electrode sheet 1, which includes a negative electrode current collector 11. A first negative electrode film layer 12 is provided on both sides of the negative electrode current collector 11, and a second negative electrode film layer 13 is provided on the surface of the first negative electrode film layer 12 facing away from the negative electrode current collector 11. The first negative electrode film layer 12 includes first graphite, and the second negative electrode film layer 13 includes second graphite with a carbon coating layer on its surface.

[0112] This application also provides an electrical device, which includes at least one of the lithium secondary battery, battery module, or battery pack provided in this application. The lithium secondary battery, battery module, or battery pack can be used as a power source for the electrical device or as an energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0113] As an electrical device, lithium secondary batteries, battery modules, or battery packs can be selected according to their usage requirements.

[0114] Figure 4 Here is an example of an electrical device 6. This electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the lithium secondary battery for this electrical device, a battery pack or battery module can be used.

[0115] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a lithium-ion battery as their power source.

[0116] The following are some examples.

[0117] To make the technical problems, technical solutions, and beneficial effects solved by this application clearer, the application will be further described in detail below with reference to embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit this application or its applications. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0118] Where specific techniques or conditions are not specified in the examples, they shall be performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0119] Example 1:

[0120] (1) Preparation of positive electrode sheet

[0121] Lithium iron phosphate (LFP) as the positive electrode active material, Super P as the conductive agent, and PVDF as the binder were mixed at a mass ratio of 97:1:2. N-methylpyrrolidone as the solvent was added, and the mixture was stirred until homogeneous to obtain a positive electrode slurry. The positive electrode slurry was then uniformly coated onto both sides of an aluminum foil used as the positive electrode current collector. After drying, cold pressing, and slitting, the positive electrode sheet was obtained. The compacted density of the positive electrode sheet was 2.5 g / cm³. 3 The density of one side is 4.62 g / cm³. 2 .

[0122] (2) Preparation of negative electrode sheet

[0123] First graphite, conductive agent Super P, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber were mixed in a mass ratio of 96:1:1.2:1.8, and then deionized water was added as solvent. The mixture was stirred until homogeneous to obtain negative electrode slurry 1. Negative electrode slurry 1 was uniformly coated on both sides of the negative electrode current collector copper foil, dried, and cold-pressed to obtain the first negative electrode film layer. The single-sided density of the first negative electrode film layer was 4.7 mg / cm³. 2 .

[0124] A mixture of second graphite, conductive agent Super P, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber at a mass ratio of 96:1:1.2:1.8 was added, and the mixture was stirred until homogeneous to obtain negative electrode slurry 2. Negative electrode slurry 2 was uniformly coated onto the first negative electrode film layer on both sides of the copper foil used as a negative electrode current collector. The mixture was dried and cold-pressed to obtain the second negative electrode film layer. After slitting, the negative electrode sheet was obtained. The single-sided density of the second negative electrode film layer was 3.5 mg / cm³. 2 .

[0125] The first type of graphite has a volume average particle size (Dv50) of 13 μm, a specific capacity of 340 mAh / g, and a powder compaction density of 1.76 g / cm³. 3 The first type of graphite consists of single-particle graphite. The second type of graphite has a volume average particle size (Dv50) of 10 μm, a specific capacity of 350 mAh / g, and a powder compaction density of 1.82 g / cm³. 3 The mass fraction of the amorphous carbon coating on the surface of the second graphite is 1.5%, and the second graphite is a single-particle graphite. The compaction density of the negative electrode sheet is 1.8 g / cm³. 3 .

[0126] (3) Separating membrane

[0127] A polyethylene film with a thickness of 7μm was used as the separator.

[0128] (4) Electrolyte

[0129] In an argon atmosphere glove box with a water content of <10 ppm, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Dry lithium salt LiPF6 was added to prepare an electrolyte with a concentration of 1 mol / L. Sodium dithionite additive with a mass content of 0.3% was added to the above solution.

[0130] (5) Battery assembly

[0131] The prepared positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. After winding, a bare cell is obtained. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. Following vacuum sealing, settling, formation, and shaping processes, a lithium secondary battery is obtained. A rigid shell with dimensions of 148 mm × 28.5 mm × 97.5 mm is selected for the outer packaging.

[0132] Example 2:

[0133] This embodiment is basically the same as Embodiment 1, except that the mass fraction of the amorphous carbon coating layer in the second graphite is 1%; correspondingly, the powder compaction density of the second graphite is 1.83 g / cm³. 3 .

[0134] Example 3:

[0135] This embodiment is basically the same as Embodiment 1, except that the mass fraction of the amorphous carbon coating layer in the second graphite is 0.5%; correspondingly, the powder compaction density of the second graphite is 1.83 g / cm³. 3 .

[0136] Example 4:

[0137] This embodiment is basically the same as Embodiment 1, except that the mass fraction of the amorphous carbon coating layer in the second graphite is 2%; correspondingly, the powder compaction density of the second graphite is 1.81 g / cm³. 3 .

[0138] Example 5:

[0139] This embodiment is basically the same as Embodiment 1, except that the mass fraction of the amorphous carbon coating layer in the second graphite is 0.1%; correspondingly, the powder compaction density of the second graphite is 1.84 g / cm³. 3 .

[0140] Example 6:

[0141] This embodiment is basically the same as Embodiment 1, except that the specific capacity of the first graphite is 335 mAh / g, and correspondingly, the volume average particle size Dv50 of the first graphite is 11 μm, and the powder compaction density is 1.76 g / cm³. 3 The specific capacity of the second graphite is 355 mAh / g, and correspondingly, the volume average particle size (Dv50) of the second graphite is 12 μm, and the powder compaction density of the second graphite is 1.88 g / cm³. 3 .

[0142] Example 7:

[0143] This embodiment is basically the same as Embodiment 1, except that the density of the first negative electrode film layer on one side is 5.8 mg / cm³. 2 The single-sided density of the second negative electrode film is 4.7 mg / cm³. 2 .

[0144] Example 8:

[0145] This embodiment is basically the same as Embodiment 1, except that the mass ratio of the first graphite, conductive agent SuperP, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber in the first negative electrode film layer is 97:0.8:0.8:1.4; and the mass ratio of the second graphite, conductive agent SuperP, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber in the second negative electrode film layer is 98:0.5:0.5:1.

[0146] Example 9:

[0147] This embodiment is basically the same as Embodiment 1, except that the first graphite is secondary particle graphite.

[0148] Comparative Example 1:

[0149] This comparative example is basically the same as Example 1, except that in step (2): the first graphite, conductive agent SuperP, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber are mixed in a mass ratio of 96:1:1.2:1.8, and then deionized water is added as a solvent and stirred evenly to obtain a negative electrode slurry; the negative electrode slurry is uniformly coated on both sides of the negative electrode current collector copper foil, dried, and cold-pressed to form a negative electrode film layer; after slitting, a negative electrode sheet is obtained. The single-sided density of the negative electrode film layer is 8.2 mg / cm³. 2 .

[0150] Comparative Example 2:

[0151] This comparative example is basically the same as Example 1, except that in step (2): the second graphite, conductive agent SuperP, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber are mixed in a mass ratio of 96:1:1.2:1.8, and then deionized water is added as a solvent and stirred evenly to obtain a negative electrode slurry; the negative electrode slurry is evenly coated on both sides of the negative electrode current collector copper foil, dried, and cold-pressed to form a negative electrode film layer; after slitting, a negative electrode sheet is obtained. The density of one side of the negative electrode film layer is 8.2 mg / cm³. 2 .

[0152] Comparative Example 3:

[0153] This comparative example is basically the same as Example 1, except that in step (2):

[0154] First graphite, conductive agent Super P, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber were mixed in a mass ratio of 96:1:1.2:1.8, and then deionized water was added as solvent. The mixture was stirred until homogeneous to obtain negative electrode slurry 1. Negative electrode slurry 1 was uniformly coated onto both sides of the negative electrode current collector copper foil, dried, and cold-pressed to obtain the first negative electrode film layer. The single-sided density of the first negative electrode film layer was 4.7 mg / cm³. 2 .

[0155] A mixture of second graphite, conductive agent Super P, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber at a mass ratio of 96:1:1.2:1.8 was added, and the mixture was stirred until homogeneous to obtain negative electrode slurry 2. Negative electrode slurry 2 was uniformly coated onto the first negative electrode film layer on both sides of the copper foil used as a negative electrode current collector. The mixture was dried and cold-pressed to obtain the second negative electrode film layer. After slitting, the negative electrode sheet was obtained. The single-sided density of the second negative electrode film layer was 3.5 mg / cm³. 2 .

[0156] The first type of graphite has a volume average particle size (Dv50) of 10 μm, a specific capacity of 350 mAh / g, and a powder compaction density of 1.86 g / cm³. 3 The mass fraction of the amorphous carbon coating layer on the surface of the first graphite is 1.5%; the volume average particle size (Dv50) of the second graphite is 13 μm, the specific capacity is 340 mAh / g, and the powder compaction density is 1.76 g / cm³. 3 That is, compared with Example 1, the first graphite and the second graphite are interchanged.

[0157] Test method:

[0158] (1) Battery cycle performance test

[0159] Step 1: Charge the battery cell at 25 ℃ at 0.33C to 3.65V, then charge it at 3.65V constant voltage to 0.05C; let it stand for 5 minutes; discharge it at 0.33C to 2.5V, and record the capacity at this time as C0 (this step is the actual initial capacity test).

[0160] Step 2: Charge the battery at 0.5C to 3.65V, and continue constant voltage charging at this charging cutoff voltage until the current is 0.05C; let it stand for 10 minutes; discharge at 0.5C to 2.5V, and continue constant voltage discharge at this discharge cutoff voltage until the current is 0.05C, until the capacity decays to less than or equal to 80% of the initial capacity, and record the number of cycles at this point.

[0161] (2) Volume average particle size Dv50 test

[0162] Particle size was measured using a laser particle size analyzer.

[0163] Preprocessing:

[0164] 1. Disassemble a 0% SOC battery cell, take an appropriate amount of negative electrode sheet, scrape off the first and second negative electrode film layers and store them separately. First, soak them in dimethyl carbonate (DMC) for 24 h, and then place the powder in an oven to dry at 60 ℃ for 4 h. Place the dried negative electrode sheet in a tube furnace and calcine at 550 ℃ for 6 h under argon protection to obtain the first graphite sample and the second graphite sample with the binder removed.

[0165] 2. Take two clean beakers, add appropriate amounts of the first graphite sample and the second graphite sample to each, add dispersant, and sonicate at 120 W / 5 min to ensure that the samples are completely dispersed in the dispersant.

[0166] Test: The two samples were tested separately. After the sample was poured into the injection tower, it was circulated to the test optical path system with the solution. The particle size distribution characteristics of the particles could be obtained by receiving and measuring the energy distribution of the scattered light under the irradiation of the laser beam (shading degree: 8%~12%).

[0167] Where Dv50 indicates that the diameter of 50% of the particles in the particle size distribution is smaller than this value.

[0168] (3) Calculation of electrode surface density and compaction density

[0169] Take the electrode sheets from the disassembled battery, first centrifuge to remove the electrolyte, then dry at 120℃ for 2 hours; use a punching machine to cut a piece with an area of ​​S mm. 2 The electrode was removed, and then weighed, with the weight recorded as W1 g. After weighing, it was soaked in a water / ethanol solution to remove the membranes on both sides of the current collector. After drying, the current collector was weighed, with the weight recorded as W2 g.

[0170] One-sided lateral density = (W1-W2) / 2S, unit is g / mm 2 .

[0171] Compacted density = surface density / (extruded electrode thickness - current collector thickness).

[0172] (4) Graphite capacity test

[0173] The graphite sample, styrene-butadiene rubber (SBR) binder, sodium carboxymethyl cellulose (CMC) thickener, and carbon black conductive agent were mixed thoroughly in an appropriate amount of deionized water at a mass ratio of 96.2:1.8:1.2:0.8 to form a uniform negative electrode slurry. The negative electrode slurry was then uniformly coated on both sides of the copper foil of the negative electrode current collector and dried in an oven for later use.

[0174] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. LiPF6 was then dissolved in the organic solvent to prepare an electrolyte with a LiPF6 concentration of 1 mol / L.

[0175] A CR2430 coin cell was assembled in an argon-protected glove box using a lithium metal sheet as the counter electrode and a polyethylene (PE) film as the separator. At 25 °C, the prepared coin cell was first discharged at a constant current of 0.15 mA to 0.005 V, allowed to stand for 5 min, and then discharged at a constant current of 10 μA to 0.005 V. The first discharge capacity of the coin cell was recorded. Subsequently, it was charged at a constant current of 0.3 mA to 2.0 V, and the charging capacity of the coin cell was recorded. The ratio of the charging capacity of the coin cell to the mass of the graphite sample is the specific capacity of the graphite.

[0176] (5) Powder compaction density test

[0177] Referring to GB / T 24533-2009, weigh 1g of negative electrode active material powder and add it to a container with a bottom area of ​​1.327cm². 2 In the mold, pressure is applied to 49000N, held for 30s, then depressurized and held for 10s. The compaction density of the negative electrode active material under 49000N pressure is determined by an electronic pressure testing machine (such as UTM7305 electronic pressure testing machine).

[0178] The parameters and performance test data of the lithium secondary batteries in the above embodiments and comparative examples are shown in Tables 1, 2, and 3. Note: The cycle count data in the tables are rounded to the nearest hundred.

[0179] Table 1

[0180]

[0181] Table 2

[0182]

[0183] Table 3

[0184]

[0185] As shown in Tables 1, 2, and 3, the lithium secondary batteries of the various embodiments of this application exhibit excellent cycle performance. In Comparative Example 1, no second negative electrode layer is provided; in Comparative Example 2, no first negative electrode layer is provided; and in Comparative Example 3, the types of graphite particles in the first and second negative electrode layers are interchanged. The cycle performance of the lithium secondary batteries in Comparative Examples 1 to 3 is significantly lower than that of the embodiments.

[0186] The description of the various embodiments above tends to emphasize the differences between the various embodiments. The similarities or similarities between them can be referred to, and for the sake of brevity, they will not be repeated here.

[0187] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A lithium secondary battery, characterized in that, The device includes a negative electrode sheet, which comprises a negative current collector, a first negative electrode film layer, and a second negative electrode film layer. The first negative electrode film layer is disposed on at least one side surface of the negative current collector, and the second negative electrode film layer is disposed on the surface of the first negative electrode film layer opposite to the negative current collector. The first negative electrode film layer comprises a first graphite, and the second negative electrode film layer comprises a second graphite having a carbon coating layer on its surface. The first graphite has a volume average particle size Dv50 of 13 μm to 15 μm and a specific capacity of less than or equal to 340 mAh / g. The second graphite has a volume average particle size Dv50 of 10 μm to 12 μm and a specific capacity of greater than or equal to 350 mAh / g.

2. The lithium secondary battery according to claim 1, characterized in that, Based on the total mass of the second graphite, the mass fraction of the carbon coating layer is less than or equal to 1.5%.

3. The lithium secondary battery according to claim 2, characterized in that, Based on the total mass of the second graphite, the mass fraction of the carbon coating layer is 0.5% to 1.5%.

4. The lithium secondary battery according to any one of claims 1 to 3, characterized in that, The carbon coating material includes amorphous carbon.

5. The lithium secondary battery according to any one of claims 1 to 4, characterized in that, The specific capacity of the first graphite is 335 mAh / g to 340 mAh / g.

6. The lithium secondary battery according to any one of claims 1 to 5, characterized in that, The specific capacity of the second graphite is 350 mAh / g to 355 mAh / g.

7. The lithium secondary battery according to any one of claims 1 to 6, characterized in that, The mass ratio of the first graphite to the second graphite in the negative electrode sheet is 2:3 to 3:

2.

8. The lithium secondary battery according to any one of claims 1 to 7, characterized in that, The compacted density of the first graphite powder under a pressure of 49000N is 1.76 g / cm³. 3 ~1.82 g / cm 3 .

9. The lithium secondary battery according to any one of claims 1 to 8, characterized in that, The compacted density of the second graphite powder under a pressure of 49000N is 1.80 g / cm³. 3 ~1.86 g / cm 3 .

10. The lithium secondary battery according to any one of claims 1 to 9, characterized in that, The compaction density of the negative electrode sheet is greater than or equal to 1.5 g / cm³. 3 .

11. The lithium secondary battery according to any one of claims 1 to 10, characterized in that, The density of one side of the first negative electrode film is greater than or equal to the density of one side of the second negative electrode film.

12. The lithium secondary battery according to any one of claims 1 to 11, characterized in that, The density of the first negative electrode film layer on one side is 4.7 mg / cm³. 2 ~5.8 mg / cm 2 .

13. The lithium secondary battery according to any one of claims 1 to 12, characterized in that, The single-sided density of the second negative electrode film is 3.5 mg / cm³. 2 ~4.7 mg / cm 2 .

14. The lithium secondary battery according to any one of claims 1 to 13, characterized in that, Based on the total mass of the first negative electrode film layer, the mass fraction of the first graphite is 95%~97%.

15. The lithium secondary battery according to any one of claims 1 to 14, characterized in that, Based on the total mass of the second negative electrode film, the mass fraction of the second graphite is 96%~98%.

16. The lithium secondary battery according to any one of claims 1 to 15, characterized in that, The first graphite and the second graphite each independently comprise one or more of single-particle graphite and secondary-particle graphite.

17. A negative electrode sheet, characterized in that, The device includes a negative electrode current collector, a first negative electrode film layer, and a second negative electrode film layer. The first negative electrode film layer is disposed on at least one side surface of the negative electrode current collector, and the second negative electrode film layer is disposed on the surface of the first negative electrode film layer opposite to the negative electrode current collector. The first negative electrode film layer includes a first graphite, and the second negative electrode film layer includes a second graphite with a carbon coating layer on its surface. The first graphite has a volume average particle size Dv50 of 13 μm to 15 μm and a specific capacity of less than or equal to 340 mAh / g. The second graphite has a volume average particle size Dv50 of 10 μm to 12 μm and a specific capacity of greater than or equal to 350 mAh / g.

18. The negative electrode sheet according to claim 17, characterized in that, The negative electrode sheet is the negative electrode sheet used in the lithium secondary battery according to any one of claims 2 to 16.

19. An electrical appliance, characterized in that, The lithium secondary battery includes any one of claims 1 to 16.

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