Negative electrode active material and preparation method thereof, negative electrode sheet, secondary battery and device

Abstract

This application relates to a negative electrode active material and its preparation method, a negative electrode sheet, a secondary battery, and an electrical device. The negative electrode active material includes expanded graphite, porous carbon layers, and silicon particles. The expanded graphite includes multiple graphite layers, the porous carbon layers are distributed on at least one interlayer surface of the graphite layers, and the silicon particles are distributed at least in the pores of the porous carbon in the porous carbon layers.

Description

Technical Field

[0001] This application belongs to the field of battery technology, specifically relating to a negative electrode active material and its preparation method, a negative electrode sheet, a secondary battery and device. Background Technology

[0002] The statements herein are provided only as background information in connection with this application and do not necessarily constitute prior art.

[0003] Secondary batteries are widely used in various consumer electronics and electric vehicles due to their outstanding characteristics such as light weight, no pollution, and no memory effect. With the increasing market demand for high-capacity power batteries, anode materials with high theoretical specific capacity, such as silicon-based materials, have been developed.

[0004] However, while silicon-based materials possess high theoretical specific capacity, they exhibit significant expansion during charging and low initial discharge efficiency. Therefore, to obtain anode materials with more balanced performance, it is often necessary to combine silicon-based and carbon-based materials as anode active materials. However, the cycle stability of current silicon-carbon composite anode active materials remains relatively low. Summary of the Invention

[0005] Therefore, it is necessary to provide a negative electrode active material that can improve cycle stability, its preparation method, negative electrode sheet, secondary battery, and device.

[0006] In a first aspect, this application provides a negative electrode active material comprising expanded graphite, a porous carbon layer, and silicon particles, wherein the expanded graphite comprises a plurality of graphite layers, the porous carbon layers are distributed at least on the interlayer surface of one of the graphite layers, and the silicon particles are distributed at least in the pores of the porous carbon in the porous carbon layers.

[0007] Not wishing to be limited to any theory, the aforementioned negative electrode active material of this application, by pre-forming a porous carbon layer on the interlayer surface of expanded graphite, and then distributing silicon particles at least in the pores of the porous carbon in the porous carbon layer, provides storage space for the uniform deposition of silicon particles, allowing silicon particles to be uniformly dispersed in the interlayer of expanded graphite. This not only facilitates high loading of silicon particles, but the layered structure of expanded graphite also contributes to the internal structural enhancement and conductivity improvement of the negative electrode active material, and provides space for the expansion of silicon particles during cycling, thereby improving its stability during cycling.

[0008] The traditional method of directly depositing silicon particles in expanded graphite to prepare anode active materials has two main drawbacks. First, it is difficult to achieve a high loading of silicon particles. Second, the disordered filling state of silicon particles inside expanded graphite and the uneven grain size distribution of silicon particles can lead to uneven volume distribution of the prepared anode active material during cycling, resulting in uneven distribution and excessive consumption of active lithium, which in turn leads to poor cycling stability of the anode active material.

[0009] The aforementioned anode active material of this application retains the advantages of expanded graphite in enhancing mechanical strength and conductivity, while optimizing the distribution and filling method of silicon particles to obtain an anode active material with both high specific capacity and high cycle stability.

[0010] In any embodiment of this application, the porous carbon layer covers the surface of at least one of the graphite layers; optionally, the porous carbon layer covers the surface of each of the graphite layers.

[0011] In any embodiment of this application, the expanded graphite is oxidized expanded graphite;

[0012] Optionally, the oxygen-containing groups in the expanded graphite include at least one of hydroxyl, epoxy, and carboxyl groups.

[0013] In any embodiment of this application, at least one of the following conditions is satisfied:

[0014] (1a) The mass ratio of the porous carbon layer to the expanded graphite is (0.1-20):1, and can be (4-16):1;

[0015] (1b) The thickness of the porous carbon layer is 0.05 to 20 μm, and can be selected as 0.2 to 10 μm;

[0016] (1c) The BET specific surface area of ​​the porous carbon layer is 40–1500 m². 2 / g, selectable from 600 to 1500m 2 / g;

[0017] (1d) The average pore size of the porous carbon layer is 0.1–40 nm;

[0018] (1e) The porous carbon layer is doped with nitrogen atoms; optionally, the mass content of nitrogen atoms relative to the porous carbon layer is 0.1% to 20%.

[0019] In any embodiment of this application, at least one of the following conditions is satisfied:

[0020] (2a) The mass ratio of the silicon particles to the expanded graphite is (0.08 to 20):1, and can be (3 to 18):1;

[0021] (2b) The macroscopic average particle size of the silicon particles is 0.05 to 30 nm;

[0022] (2c) The mass ratio of the silicon particles to the porous carbon layer is (0.7-12):1, and can be selected as (0.7-1.5):1;

[0023] (2d) In the negative electrode active material, the mass content of the silicon particles is 6% to 65%;

[0024] (2e) The expanded graphite is oxidized expanded graphite; optionally, the oxygen-containing group in the expanded graphite includes at least one of hydroxyl, epoxy and carboxyl groups; optionally, the mass content of the oxygen-containing group in the expanded graphite is 0.5% to 38%.

[0025] In any embodiment of this application, the negative electrode active material further includes a coating layer that covers at least a portion of the outer surface of the expanded graphite;

[0026] Optionally, the coating layer comprises a carbon material;

[0027] Optionally, the coating layer has a mass content of 0.2% to 3% in the negative electrode active material;

[0028] Optionally, the thickness of the coating layer is 0.5 to 200 nm.

[0029] A second aspect of this application provides a method for preparing any of the above-mentioned negative electrode active materials, comprising the following steps:

[0030] Expanded graphite is mixed with a carbon source solution and subjected to a hydrothermal reaction. The solid product from the hydrothermal reaction is then calcined to obtain an intermediate product. The intermediate product comprises expanded graphite and a porous carbon layer distributed on the surface of the graphite layer within the expanded graphite.

[0031] Silicon particles are formed in the pores of the porous carbon in the intermediate product.

[0032] In any embodiment of this application, at least one of the following conditions is satisfied:

[0033] (3a) The carbon source in the carbon source solution includes at least one of glucose, asphalt, sucrose and phenolic resin;

[0034] (3b) The mass ratio of the carbon source in the carbon source solution to the expanded graphite is (1-200):1;

[0035] (3c) A nitrogen source is also added to the hydrothermal reaction; optionally, the nitrogen source includes at least one of pyrrole, melamine and acetonitrile;

[0036] (3d) The hydrothermal reaction is carried out at a temperature of 80–280°C for a time of 0.5–5 hours.

[0037] (3e) The calcination conditions include calcination at 400-1000℃ for 1-5 hours.

[0038] In any embodiment of this application, the step of forming silicon particles in the pores of the porous carbon of the intermediate product includes the following steps:

[0039] The intermediate product is passed through silane and a reducing gas, and chemical vapor deposition is performed under a protective atmosphere.

[0040] Optionally, the silane includes at least one of methylsilane and disilane;

[0041] Optionally, the temperature of the chemical vapor deposition is 400–1000°C;

[0042] Optionally, the chemical vapor deposition time is 1 to 5 hours.

[0043] In any embodiment of this application, the product obtained by chemical vapor deposition is subjected to chemical vapor deposition by passing a gaseous feedstock, wherein the gaseous feedstock includes at least one of ethylene, acetylene, and propylene.

[0044] Optionally, the temperature of the chemical vapor deposition is 600–1200°C;

[0045] Optionally, the chemical vapor deposition time is 1 to 5 hours.

[0046] In any embodiment of this application, the expanded graphite is oxidized expanded graphite; the preparation method further includes the step of preparing the oxidized expanded graphite, comprising the following steps:

[0047] After soaking expanded graphite in acid solution and washing it until neutral, oxidized expanded graphite is obtained.

[0048] Optionally, the acid solution includes at least one of HNO3 and H2SO4.

[0049] In a third aspect, this application provides a negative electrode sheet comprising any of the aforementioned negative electrode active materials.

[0050] In a fourth aspect, this application provides a secondary battery comprising the aforementioned negative electrode sheet.

[0051] In a fifth aspect, this application provides an electrical device including the aforementioned secondary battery.

[0052] 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

[0053] To more clearly illustrate the technical solution of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without any creative effort.

[0054] Figure 1 This is a schematic diagram of the structure of the negative electrode active material according to one embodiment of this application;

[0055] Figure 2 This is a schematic diagram of the structure of the negative electrode active material according to another embodiment of this application;

[0056] Figure 3 This is a schematic diagram of a battery cell according to one embodiment of this application;

[0057] Figure 4 yes Figure 3 An exploded view of a battery cell according to one embodiment of this application is shown.

[0058] Figure 5 This is a schematic diagram of a battery module according to one embodiment of this application;

[0059] Figure 6 This is a schematic diagram of a battery pack according to one embodiment of this application;

[0060] Figure 7 for Figure 6 An exploded view of a battery pack according to one embodiment of this application is shown;

[0061] Figure 8 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application;

[0062] Explanation of reference numerals in the attached figures:

[0063] 1. Battery cell; 11. Casing; 12. Electrode assembly; 13. Cover plate; 2. Battery pack; 21. Upper casing; 22. Lower casing; 3. Battery module; 4. Electrical device; 5. Negative electrode active material; 511. Graphite layer; 52. Porous carbon layer; 53. Coating layer. Detailed Implementation

[0064] The embodiments of this application are hereby disclosed 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 actually identical structures may be omitted. This is to avoid making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0065] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0066] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0067] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may 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.

[0068] 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."

[0069] Unless otherwise stated, the terms used in this application have their common meanings as commonly understood by those skilled in the art. Unless otherwise stated, the values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application).

[0070] Secondary batteries

[0071] A rechargeable battery is a battery that can be recharged after it has been discharged, allowing the active materials to be reactivated and the battery to continue to be used.

[0072] Typically, a secondary battery consists of a positive electrode, a negative electrode, and an electrolyte. During the charging and discharging process, active ions move back and forth between the positive and negative electrodes, inserting and extracting. It can be understood that the active ions originate from the positive electrode active material of the positive electrode.

[0073] Negative electrode sheet

[0074] A negative electrode typically includes a negative current collector and a negative electrode film layer disposed on the negative current collector, the negative electrode film layer including a negative electrode active material.

[0075] One embodiment of this application provides a negative electrode active material, comprising expanded graphite, porous carbon layers, and silicon particles. The expanded graphite comprises multiple graphite layers. The porous carbon layers are distributed on at least the interlayer surfaces of one of the graphite layers, and the silicon particles are distributed at least in the pores of the porous carbon in the porous carbon layers.

[0076] Expanded graphite (EG) is a loose, porous, worm-like material obtained from natural graphite flakes through intercalation, washing, drying, and high-temperature expansion. Like graphite, expanded graphite has a layered structure with multiple graphite layers. Within each graphite layer, the carbon atoms have an sp[i] sq. 2 Hybridization, where each carbon atom is covalently bonded to the other three carbon atoms to form a hexagonal network structure; between the graphite layers, they are bonded by intermolecular forces.

[0077] In some embodiments, the structure of the aforementioned negative electrode active material can be confirmed by scanning transmission electron microscopy.

[0078] It is understandable that the graphite layers of expanded graphite include an intermediate graphite layer and an outer graphite layer according to their location. The outward-facing surface of the outer graphite layer is considered to be exposed and is not located between layers, i.e., the outer surface of the expanded graphite. The inward-facing surface of the outer graphite layer and the two surfaces of the intermediate graphite layer are all considered to be interlayer surfaces, i.e., "interlayer surfaces of graphite layers".

[0079] It is understandable that porous carbon layers are distributed not only on the interlayer surfaces of graphite layers, but also on the outer surface of expanded graphite.

[0080] Not wishing to be limited to any theory, the aforementioned negative electrode active material of this application, by pre-forming a porous carbon layer on the interlayer surface of expanded graphite, and then distributing silicon particles at least in the pores of the porous carbon in the porous carbon layer, provides storage space for the uniform deposition of silicon particles, allowing silicon particles to be uniformly dispersed in the interlayer of expanded graphite. This not only facilitates high loading of silicon particles, but the layered structure of expanded graphite also contributes to the internal structural enhancement and conductivity improvement of the negative electrode active material, and provides space for the expansion of silicon particles during cycling, thereby improving its stability during cycling.

[0081] The traditional method of directly depositing silicon particles in expanded graphite to prepare anode active materials has two main drawbacks. First, it is difficult to achieve a high loading of silicon particles. Second, the disordered filling state of silicon particles inside expanded graphite and the uneven grain size distribution of silicon particles can lead to uneven volume distribution of the prepared anode active material during cycling, resulting in uneven distribution and excessive consumption of active lithium, which in turn leads to poor cycling stability of the anode active material.

[0082] The aforementioned anode active material of this application retains the advantages of expanded graphite in enhancing mechanical strength and conductivity, while optimizing the distribution and filling method of silicon particles to obtain an anode active material with both high specific capacity and high cycle stability.

[0083] Furthermore, the application of the aforementioned negative electrode active material in this application to a secondary battery also improves the rate performance of the secondary battery.

[0084] In some embodiments, a porous carbon layer covers the surface of at least one graphite layer.

[0085] Optionally, a porous carbon layer may coat a portion of the expanded graphite or the surface of each graphite layer.

[0086] In some embodiments, the expanded graphite is oxidized expanded graphite. It is understood that oxidized expanded graphite contains oxygen-containing groups; optionally, the oxygen-containing groups in the expanded graphite include at least one of hydroxyl, epoxy, and carboxyl groups. Introducing oxygen-containing groups such as hydroxyl, epoxy, and carboxyl groups into the graphite layers of expanded graphite provides polar groups for the adsorption of carbon sources during the preparation of porous carbon layers. This allows the carbon source to adhere more strongly to the graphite layers through these oxygen-containing groups, promoting the directional and uniform growth of porous carbon on the graphite layer surface along the thickness direction.

[0087] Further, the mass content of oxygen-containing groups in expanded graphite is 0.5% to 38%; optionally, it is 3% to 37.5%; more preferably, it is 4% to 37.5%, or 17% to 37.5%. As an example, the mass content of oxygen-containing groups in expanded graphite can be 0.5%, 1%, 2%, 3%, 4%, 5%, 10%, 15%, 17%, 18%, 20%, 25%, 30%, 35%, 37.5%, or 38%.

[0088] In some embodiments, the mass ratio of porous carbon layer to expanded graphite is (0.1–20):1. Controlling the mass ratio of porous carbon layer to expanded graphite within the above range provides sufficient adhesion and storage space for subsequent silicon particle deposition, which is beneficial for uniform dispersion of silicon particles between expanded graphite layers and a high content loading.

[0089] As an example, the mass ratio of porous carbon layer to expanded graphite can be 0.1:1, 0.2:1, 0.5:1, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, 15:1, 16:1, 17:1, 18:1, 19:1, or 20:1. Optionally, the mass ratio of porous carbon layer to expanded graphite can be within the range formed by any two of the above values, for example, (4–16):1.

[0090] It is understood that "silicon particles" in this document include, but are not limited to, pure silicon particles, and may also be silicon-containing particles such as silicon-oxygen particles and silicon-carbon particles. In some embodiments, the mass ratio of silicon particles to the porous carbon layer is (0.7–12):1. As an example, the mass ratio of silicon particles to the porous carbon layer may be 0.7:1, 0.8:1, 0.9:1, 1:1, 1.2:1, 1.5:1, 2:1, 5:1, 6:1, 7:1, 10:1, or 12:1. The mass ratio of silicon particles to the porous carbon layer may be selected as (0.7–1.5):1, and more preferably (0.7–1.2):1. Further controlling the range of the mass ratio of silicon particles to the porous carbon layer allows silicon particles to be deposited as much as possible in the pores of the porous carbon layer, rather than being excessively deposited on the surface of the porous carbon layer, thereby improving the stability of the negative electrode active material and thus improving the cycle stability and rate performance of the battery in which it is applied.

[0091] In some embodiments, the thickness of the porous carbon layer is 0.05–20 μm. Controlling the thickness of the porous carbon layer within this range yields a structurally stable porous carbon layer. In some specific examples, the porous carbon layer can be formed in situ by growing a carbon source under hydrothermal conditions along the thickness direction of the graphite layer surface.

[0092] Understandably, the thickness of the porous carbon layer can be controlled by the mass ratio of carbon source to expanded graphite, the temperature of the hydrothermal reaction, and the time. The adhesion strength of the porous carbon layer to the surface of the graphite layer is related to the mass content of oxygen-containing groups in the expanded graphite.

[0093] As an example, the thickness of the porous carbon layer can be 0.05 μm, 0.1 μm, 0.15 μm, 0.2 μm, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, or 20 μm. Optionally, the thickness of the porous carbon layer can be within the range formed by any two of the above values, for example, 0.2 to 10 μm.

[0094] In some embodiments, the BET specific surface area of ​​the porous carbon layer is 40–1500 m². 2 / g, selectable from 600 to 1500m 2 / g. BET specific surface area refers to the specific surface area obtained using the BET adsorption test method. As an example, the BET specific surface area of ​​a porous carbon layer can be 40m². 2 / g, 100m 2 / g、160m 2 / g、170m 2 / g、200m 2 / g、300m 2 / g、400m 2 / g、500m2 / g、600m 2 / g、700m 2 / g、750m 2 / g、800m 2 / g、850m 2 / g、900m 2 / g、950m 2 / g, 1000m 2 / g、1100m 2 / g、1200m 2 / g、1300m 2 / g, 1400m 2 / g, 1500m 2 / g, or the range formed by any two of the above point values.

[0095] In some embodiments, the average pore size of the porous carbon layer is 0.1–40 nm. The average pore size is determined by magnifying the surface of the material, such as by a microscope photograph, and calculating the number of pores per unit length (e.g., 10 nm when the pore size is approximately 0.1–2 nm; or 100 nm when the pore size is approximately 2–40 nm) as the number of chambers. The average pore size is then calculated using the following equation: Average pore size = Unit length / Number of chambers.

[0096] In some embodiments, nitrogen atoms are doped into the porous carbon layer. This increases the defect concentration in the porous carbon layer, thereby increasing the lithium-ion transport rate of the negative electrode active material and further improving the rate performance of its application in secondary batteries.

[0097] Furthermore, the mass content of nitrogen atoms relative to the porous carbon layer is 0.1% to 20%. For example, the mass content of nitrogen atoms relative to the porous carbon layer can be 0.1%, 0.2%, 0.5%, 1%, 2%, 4%, 5%, 7%, 10%, 12%, 14%, 16%, 18%, or 20%. The mass content of nitrogen atoms relative to the porous carbon layer can be determined by elemental analysis.

[0098] In some embodiments, the mass ratio of silicon particles to expanded graphite is (0.08 to 20):1, optionally (3 to 18):1. As examples, the mass ratio of silicon particles to expanded graphite is 0.08:1, 0.2:1, 0.5:1, 0.7:1, 1:1, 2:1, 5:1, 6:1, 7:1, 8:1, 10:1, 11:1, 12:1, 13:1, 14:1, 15:1, 16:1, 18:1, 20:1, 22:1, or a range consisting of any two of the above values.

[0099] In some embodiments, the macroscopic average particle size of the silicon particles is 0.05–30 nm.

[0100] The macroscopic average particle size of silicon particles can be obtained by XRD testing of the above-mentioned negative electrode active material and by using the Scherrer formula based on the characteristic peaks of the silicon particles.

[0101] In some embodiments, the silicon particles include, but are not limited to, silicon particles, and may also include at least one of silicon-carbon, silicon-nitrogen particles, etc. In one specific example, the silicon particles are pure silicon particles.

[0102] In some embodiments, the mass content of silicon particles in the negative electrode active material is 6% to 65%; optionally, it is 6% to 50%, 10% to 50%, or 35% to 50%. As examples, the mass content of silicon particles can be 6%, 10%, 15%, 20%, 30%, 35%, 38%, 40%, 45%, 48%, 50%, 55%, 60%, 62%, or 65%. The mass content of silicon particles in the above-mentioned negative electrode active material can be obtained by ICP analysis.

[0103] In some embodiments, the mass content of expanded graphite in the negative electrode active material is 2% to 85%. As examples, the mass content of expanded graphite can be 2%, 5%, 6%, 7%, 10%, 20%, 30%, 35%, 40%, 50%, 55%, 60%, 70%, or 80%. The mass content of expanded graphite in the negative electrode active material can be obtained by: thoroughly soaking the negative electrode active material in DMC (dimethyl carbonate) for 5 hours, and then pulverizing it by ion polishing. The pulverized powder is then subjected to XRD testing to obtain diffraction peaks with a graphite structure. The mass content of expanded graphite can be calculated using the XRD internal standard method.

[0104] In some embodiments, the negative electrode active material further includes a coating layer. The coating layer covers at least a portion of the outer surface of the expanded graphite. The structure of the negative electrode active material with the coating layer can also be confirmed by scanning transmission electron microscopy.

[0105] Furthermore, the coating layer includes carbon material. As an example, the coating layer may be a carbon material layer, such as an amorphous carbon layer.

[0106] Furthermore, the mass content of the coating layer in the negative electrode active material is 0.2% to 3%. As an example, the mass content of the coating layer in the negative electrode active material can be 0.2%, 0.5%, 1%, 1.5%, 2%, 2.5%, or 3%.

[0107] Furthermore, the thickness of the coating layer is 0.5–200 nm.

[0108] In such Figure 1 In the example shown, the negative electrode active material includes expanded graphite, a porous carbon layer 52, and silicon particles (not shown). The expanded graphite includes multiple graphite layers 511, the porous carbon layer 52 is distributed on at least one interlayer surface of the graphite layer 511, and the silicon particles are distributed at least in the pores of the porous carbon in the porous carbon layer 52. Specifically, the porous carbon layer 52 covers the surface of each graphite layer 511.

[0109] In such Figure 2 In the example shown, with Figure 1 The difference shown is that it also includes a coating layer 53, which covers the outer surface of the expanded graphite; the coating may be continuous or partial.

[0110] On the other hand, this application also provides a method for preparing the above-mentioned negative electrode active material, including the following steps S20 to S40.

[0111] S20. The expanded graphite is mixed with the carbon source solution and subjected to a hydrothermal reaction. The solid product of the hydrothermal reaction is then calcined to obtain an intermediate product.

[0112] The intermediate products include expanded graphite and porous carbon layers distributed on the surface of the graphite layers in the expanded graphite.

[0113] In some embodiments, the expanded graphite is oxidized expanded graphite. Before step S20, the above preparation method further includes a step of preparing oxidized expanded graphite, including the following step S10.

[0114] S10. After soaking the expanded graphite in acid solution and washing it until neutral, oxidized expanded graphite is obtained.

[0115] Furthermore, the acid solution includes at least one of HNO3 and H2SO4.

[0116] As an example, the acid solution is a mixture of concentrated nitric acid and concentrated sulfuric acid in a volume ratio of 1:1.

[0117] It can be understood that the degree of oxidation can be controlled by controlling the soaking time and acid concentration in step S10.

[0118] In some embodiments, the carbon source in the carbon source solution includes at least one of glucose, asphalt, sucrose, and phenolic resin.

[0119] In some embodiments, the mass ratio of carbon source to expanded graphite in the carbon source solution is (1-200):1. It is understood that the mass ratio of porous carbon layer to expanded graphite can be controlled by controlling the mass ratio of carbon source to expanded graphite.

[0120] In some embodiments, a nitrogen source is also added to the hydrothermal reaction, thereby doping the porous carbon layer with nitrogen atoms.

[0121] Further, the nitrogen source includes at least one of pyrrole, melamine, and acetonitrile. Further, the mass ratio of nitrogen source to expanded graphite is (0.02–20):100; optionally, it is (0.1–20):100. It is understood that the mass content of nitrogen atoms in the porous carbon layer can be controlled by adjusting the mass ratio of nitrogen source to expanded graphite.

[0122] In some embodiments, the hydrothermal reaction temperature is 80–280°C, and the time is 0.5–5 hours. As examples, the hydrothermal reaction temperature can be 80°C, 100°C, 120°C, 150°C, 160°C, 180°C, 200°C, 240°C, 260°C, or 280°C, and the time can be 0.5 hours, 1 hour, 2 hours, 3 hours, 4 hours, or 5 hours. Further, the hydrothermal reaction temperature is 150–240°C, and the time is 1 hour to 3 hours.

[0123] In some embodiments, the calcination conditions include calcination at 400–1000°C for 1–5 hours. As an example, the calcination temperature can be 400°C, 500°C, 600°C, 700°C, 800°C, 900°C, or 1000°C; and the calcination time can be 1 hour, 2 hours, 3 hours, 4 hours, or 5 hours.

[0124] Furthermore, the calcination atmosphere can be an inert gas, nitrogen, air, or a vacuum atmosphere.

[0125] S40, Silicon particles are formed in the pores of the porous carbon in the intermediate product.

[0126] In some embodiments, S40 employs a chemical vapor deposition process; specifically, silane and a reducing gas are introduced into the intermediate product, and chemical vapor deposition is performed under a protective atmosphere. Thus, the silane is reduced to form silicon particles under the action of the reducing gas.

[0127] Furthermore, in S40, the temperature for chemical vapor deposition is 400–1000°C. Furthermore, in S40, the time for chemical vapor deposition is 1–5 hours.

[0128] As an example, in S40, the temperature of chemical vapor deposition can be 400℃, 500℃, 600℃, 700℃, 800℃, 900℃, or 1000℃; and the time of chemical vapor deposition can be 1h, 2h, 3h, 4h, or 5h.

[0129] Furthermore, the silane includes at least one of methylsilane and disilane.

[0130] Furthermore, the reducing gas includes acetylene. Further, the volume ratio of the reducing gas to silane is (0.02–1):1, and for example, it could be 0.02:1, 0.05:1, 0.1:1, 0.5:1, or 1:1. The reducing gas mainly serves to reduce silane to form silicon particles. Since the content of the reducing gas is relatively small compared to silane, it basically does not produce carbon particles, which can be coated with carbon in the subsequent step S50.

[0131] Furthermore, a protective gas can also be introduced into S40 as a carrier gas, which may include at least one of an inert gas and nitrogen.

[0132] In some embodiments, after S40, a step of coating the product obtained in S40 may be included, including step S50.

[0133] S50. The product obtained from the chemical vapor deposition in S40 is passed through a gaseous feedstock for chemical vapor deposition. The gaseous feedstock includes at least one of ethylene, acetylene, and propylene.

[0134] Carbon is thus formed by thermal decomposition of gaseous feedstock under chemical vapor deposition conditions.

[0135] Furthermore, the temperature for chemical vapor deposition in S50 is 600–1200 °C. Furthermore, the time for chemical vapor deposition in S50 is 1–5 h.

[0136] As an example, in S50, the temperature of chemical vapor deposition can be 600℃, 700℃, 800℃, 900℃, 1000℃, 1100℃, or 1200℃; and the time of chemical vapor deposition can be 1h, 2h, 3h, 4h, or 5h.

[0137] As an 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.

[0138] 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 may be formed by forming a metal material on the polymer material substrate. The metal material includes, but is not limited to, at least one of copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys; the polymer material substrate includes, but is not limited to, at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0139] In some embodiments, the aforementioned negative electrode film layer may contain, in addition to the aforementioned negative electrode active material, a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may also include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys.

[0140] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0141] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0142] In some embodiments, the negative electrode film layer may optionally include other additives, such as thickeners, for example, sodium carboxymethyl cellulose (CMC-Na).

[0143] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, conductive agent, binder and any other components, in a solvent such as water to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0144] Positive electrode sheet

[0145] In a secondary battery, the positive electrode typically includes a positive current collector and a positive electrode film layer disposed on the positive current collector, the positive electrode film layer including a positive active material.

[0146] The positive electrode current collector can be a metal foil or a composite current collector, wherein the composite current collector can be formed by depositing a metal material on a polymer substrate. As an example, the positive electrode current collector can be an aluminum foil.

[0147] The specific type of positive electrode active material can be any active material known in the art that can be used as the positive electrode of a secondary battery, and those skilled in the art can select it according to actual needs.

[0148] As an example, the positive electrode active material may include a lithium ion active material, and the lithium ion active material includes, but is not limited to, one or more of lithium transition metal oxides, lithium-containing phosphates having an olivine structure, and their respective modified compounds. Examples of the 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, lithium nickel cobalt aluminum oxide, and their modified compounds. Examples of the lithium-containing phosphates having an olivine structure may include, but are not limited to, one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite material of lithium manganese iron phosphate and carbon, and their modified compounds. These materials can all be obtained commercially.

[0149] As an example, the positive electrode active material may include a sodium ion active material, and the sodium ion active material may employ a positive electrode active material for sodium ion batteries well-known in the art. As an example, the sodium ion active material may include at least one of the following materials: Prussian blue (PBA) type, with the chemical formula (NaxMA[MB(CN)6]·zH2O, where MA and MB are transition metal ions, which is a compound composed of sodium, transition metal, and cyanide radical, such as Na4Fe2(CN)6, Na4Fe(CN)6, Na 1.72 MnFe2(CN)6, NaMnMn(CN)6, NaNiFe(CN)6, etc.; oxide type, with the chemical formula NaxMO2, 0 < x ≤ 1, where M is a transition metal element, which is composed of transition metal oxides. The variable-valence transition metals involved mainly include vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu). Among them, manganese and iron with relatively abundant resources are most commonly used. For example, NaCrO2, NaMnO2, NaMnO2, Na 0.61 Ti 0.48 Mn 0.52 O2, Na[Fe 0.5 Co 0.5 O2, NaMnO2, Na[Ni 0.25 Fe 0.5 Mn 0.25 O2, etc.; and polyanion compound type, with the chemical formula Na x M y [(XO m ) n- z ​M represents a metal ion with variable valence, and X represents elements such as P, S, and V. It is composed of sodium, transition metals, and anions. The main transition metals include iron, vanadium, and cobalt, while the main anions include phosphate, pyrophosphate, fluorophosphate, and sulfate, such as NaMnFe2(PO4)6, Na2MnP2O7, Na3V2(PO4)3, Na2Fe2(SO4)3, NaFePO4, Na3V2(PO4)2F3, and Na4Co3(PO4)2(P2O7).

[0150] In some embodiments, the modifying compounds for the above-mentioned materials may be those used for doping modification and / or surface coating modification of the materials.

[0151] Understandably, during the charging and discharging process of a battery, there is intercalation and deintercalation of active ions such as lithium (Li), and the content of active ions such as Li in the positive electrode varies depending on the state of discharge. Taking lithium-ion active materials as an example, unless otherwise specified, the Li content in the examples of positive electrode active materials in this application refers to the initial state of the material. When positive electrode active materials are applied to the positive electrode in a battery system, the Li content in the positive electrode active material usually changes after charge-discharge cycles. The Li content can be measured in 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 the state before it is added to the positive electrode slurry. It is understood that materials obtained by appropriate modification based on the listed positive electrode active materials are also within the scope of positive electrode materials. The aforementioned appropriate modification refers to an acceptable modification method for the positive electrode active material, and non-limiting examples include coating modification.

[0152] In the examples of positive electrode active materials listed 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 oxygen atom content will fluctuate. The oxygen atom content can be measured in molar content, but is not limited to this.

[0153] The positive electrode film layer may also optionally include binders, conductive agents, and other optional additives.

[0154] As an example, the conductive agent can be one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, Super P (SP), graphene, and carbon nanofibers.

[0155] As an example, the adhesive may be one or more of the following: styrene-butadiene rubber (SBR), water-based acrylic resin, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), and polyvinyl butyral (PVB).

[0156] In some embodiments, the positive electrode sheet can be prepared by dispersing the components used to prepare the positive electrode sheet, such as the positive electrode active material, conductive agent, binder, and any other components, in a solvent to form a positive electrode slurry; coating the positive electrode slurry onto a positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing, and other processes. The solvent includes, but is not limited to, N-methylpyrrolidone.

[0157] electrolytes

[0158] The electrolyte acts as a conductor of ions between the positive and negative electrodes. For example, the electrolyte can be liquid, gel, or completely solid.

[0159] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0160] In some embodiments, the electrolyte salt includes at least one of sodium salt, lithium salt, and potassium salt.

[0161] As an example, the sodium salt in the electrolyte salt may be selected from one or more of sodium perchlorate (NaClO4), sodium tetrafluoroborate (NaBF4), sodium hexafluorophosphate (NaPF6), sodium hexafluoroarsenate (NaAsF6), sodium trifluoroacetate (CF3COONa), sodium tetraphenylborate (NaB(C6H5)4), sodium trifluoromethanesulfonate (NaSO3CF3), sodium bis(fluorosulfonyl)imide (Na[(FSO2)2N]) or sodium bis(trifluoromethanesulfonyl)imide (Na[(CF3SO2)2N]).

[0162] As an example, the lithium salt in the electrolyte salt may be selected from 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).

[0163] In some embodiments, the solvent includes at least one of ether solvents, ester solvents, and sulfone solvents.

[0164] As an example, the ether solvent may include at least one of ethylene glycol dimethyl ether (DME), diethylene glycol dimethyl ether (DEGDME), triethylene glycol dimethyl ether (TRGDME), tetraethylene glycol dimethyl ether (TEGDME), and 1,3-dioxolane (DOL);

[0165] As an example, the ester solvent may include at least one of ethylene carbonate (EC), propylene carbonate (PC), butene carbonate (BC), vinylene carbonate (VC), fluoroethylene carbonate (FEC), methyl ethyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), γ-butyrolactone (BL), 1,3-propanesulfonate lactone (1,3-PS), methyl propionate (MP), methyl butyrate (MB), ethyl acetate (EA), ethyl propionate (EP), propyl propionate (PP), and ethyl butyrate (EB).

[0166] As an example, the sulfone solvent includes dimethyl sulfoxide (DMSO).

[0167] 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.

[0168] Separating membrane

[0169] In some embodiments, the secondary battery also includes a separator. The separator is disposed between the positive electrode and the negative electrode, serving as a barrier.

[0170] In some embodiments, the material of the separator may be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene and polyvinylidene fluoride.

[0171] The separator can be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer can be the same or different.

[0172] Unless otherwise specified, all of the above-mentioned raw materials can be obtained through commercial purchase.

[0173] The shape of the secondary battery in this application embodiment can be cylindrical, square, or other arbitrary shapes. For example... Figure 3 Here is a square-structured battery cell 1 as an example.

[0174] In some implementations, refer to Figure 4 The outer packaging may include a shell 11 and a cover plate 13. The shell 11 may include a bottom plate and side plates connected to the bottom plate, the bottom plate and side plates forming a receiving cavity. The shell 11 has an opening communicating with the receiving cavity, and the cover plate 13 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 12 by a winding process or a stacking process. The electrode assembly 12 is encapsulated within the receiving cavity. The electrolyte is immersed in the electrode assembly 12. The number of electrode assemblies 12 contained in the battery cell 1 can be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0175] Secondary batteries can be individual battery cells, battery modules, or battery packs.

[0176] Figure 5 This is battery module 3 as an example. (See reference...) Figure 5 In battery module 3, multiple battery cells 1 can be arranged sequentially along the length of battery module 3. Of course, they can also be arranged in any other way. Furthermore, these multiple battery cells 1 can be fixed in place using fasteners.

[0177] Optionally, the battery module 3 may also include a housing with a receiving space in which multiple battery cells 1 are received.

[0178] 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 can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0179] Figure 6 and Figure 7 This is battery pack 2 as an example. (See reference...) Figure 6 and Figure 7 The battery pack 2 may include a battery box and multiple battery modules 3 disposed within the battery box. The battery box includes an upper box 21 and a lower box 22, with the upper box 21 covering the lower box 22 to form a closed space for accommodating the battery modules 3. The multiple battery modules 3 can be arranged in any manner within the battery box.

[0180] In addition, the present invention also provides an electrical device comprising a secondary battery provided by the present invention. The secondary battery can be used as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices, electric vehicles, electric trains, ships and satellites, energy storage systems, etc. Mobile devices may include, for example, mobile phones, laptops, etc.; electric vehicles may include, for example, pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc., but are not limited to these.

[0181] As for the aforementioned electrical device, a secondary battery can be selected according to its usage requirements.

[0182] Figure 8 Here is an example of an electrical device 4. This electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc.

[0183] Another example device could be a mobile phone, tablet, laptop, etc.

[0184] To make the objectives, technical solutions, and advantages of this invention clearer and more concise, the invention is described using the following specific embodiments, but the invention is by no means limited to these embodiments. The embodiments described below are merely preferred embodiments of the invention and can be used to describe the invention, but should not be construed as limiting the scope of the invention. It should be noted that any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the protection scope of this invention.

[0185] To better illustrate the present invention, the following embodiments are provided for further explanation. The specific embodiments are as follows.

[0186] (I) Preparation of negative electrode active materials

[0187] Example 1

[0188] (1) After soaking the expanded graphite in a mixed acid solution of concentrated nitric acid and concentrated sulfuric acid with a volume ratio of 1:1 for 4 hours, it was washed until neutral to obtain oxidized expanded graphite.

[0189] (2) Oxidized expanded graphite was mixed with carbon source glucose and nitrogen source pyrrole to form a dispersion, and a hydrothermal reaction was carried out at a temperature of 200℃ for 2 hours.

[0190] (3) Centrifuge the hydrothermal product, take the precipitate and calcine it at 500℃ for 4 hours in an inert gas atmosphere to form porous carbon on the graphite interlayer surface of the expanded graphite.

[0191] (4) High-purity silane SiH4 is mixed with acetylene and nitrogen in a volume ratio of 1:0.1:1 and then fed into a chemical vapor deposition reaction system. The deposition temperature is controlled at 800℃ and the time is shown in Table 1. The silane is allowed to penetrate into the porous carbon channels of expanded graphite and complete diffusion and reduction to obtain silicon particles.

[0192] (5) Acetylene is introduced into the chemical vapor deposition reaction system of step (4), the deposition temperature is controlled at 800℃ and the time is 1h, and the product obtained in step (4) is carbon coated to obtain the negative electrode active material, namely carbon-coated carbon silicon composite expanded graphite material; wherein the mass content of carbon coating in the negative electrode active material is 1%.

[0193] The mass content of oxygen-containing groups in the oxidized expanded graphite in Example 1, the mass ratio of carbon source to oxidized expanded graphite, the mass ratio of nitrogen source to oxidized expanded graphite, the deposition time in step (4), and the parameters of the obtained negative electrode active material are detailed in Table 1.

[0194] It is understandable that the mass content of oxygen-containing groups in oxidized expanded graphite, the mass ratio of porous carbon layer to oxidized expanded graphite, and the mass ratio of silicon particles to oxidized expanded graphite can be calculated from the mass changes before and after.

[0195] Other embodiments

[0196] It is basically the same as Example 1, except that the preparation parameters and / or the parameters of the negative electrode active material in Table 1 are different.

[0197] It is understandable that the mass content of oxygen-containing groups in expanded graphite oxide can be controlled by the soaking time in step (1); the mass ratio A of porous carbon to expanded graphite oxide and the thickness of the porous carbon layer can be controlled by the mass ratio C of carbon source to expanded graphite oxide; the mass ratio B of silicon particles to expanded graphite oxide can be controlled by the deposition time of silicon particles in step (4); the doping content of nitrogen (relative to porous carbon) can be controlled by the ratio of nitrogen source to carbon source (which can be obtained by the mass ratio C of carbon source to expanded graphite oxide and the mass ratio D of nitrogen source to expanded graphite oxide).

[0198] Comparative Example 1

[0199] It is basically the same as Example 1, except that steps (1) to (3) in the example are omitted; that is, steps (4) and (5) are carried out directly using expanded graphite as raw material.

[0200] (II) Battery Manufacturing

[0201] (1) Preparation of positive electrode sheet

[0202] The positive electrode active material is a ternary material nickel-cobalt-manganese (NCM). 811The conductive agent acetylene black and the binder polyvinylidene fluoride (PVDF) are mixed evenly at a mass ratio of 97:2:1 and added to the solvent NMP to prepare a positive electrode slurry. The positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, dried at 85°C, cold pressed, and then die-cut and slit to produce a lithium-ion battery positive electrode sheet.

[0203] (2) Preparation of negative electrode sheet

[0204] Artificial graphite, the negative electrode active material prepared in the above embodiments or comparative examples, conductive agent acetylene black, thickener sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber (SBR) are added to solvent water in a mass ratio of 48:48:2:1:1 and mixed evenly to prepare a negative electrode slurry. The negative electrode slurry is evenly coated on the negative electrode current collector copper foil, dried at 85°C, and then cold-pressed to prepare a lithium-ion battery negative electrode sheet.

[0205] (3) Preparation of the separating membrane

[0206] A 12μm thick polyethylene microporous film was used as the porous separator substrate. Inorganic alumina powder, polyvinylpyrrolidone, and acetone solvent were mixed evenly in a weight ratio of 3:1.5:5.5 to prepare an inorganic slurry. The inorganic slurry was then coated on both sides of the substrate and dried to form an inorganic layer with a thickness of 5μm, thus obtaining the separator.

[0207] (4) Preparation of electrolyte

[0208] Lithium hexafluorophosphate was dissolved in a mixed solvent of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate (volume ratio of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate: 1:2:1) to obtain a lithium-ion battery electrolyte. The concentration of lithium hexafluorophosphate was 1 mol / L.

[0209] (5) Preparation of lithium-ion batteries

[0210] The above-mentioned positive electrode, negative electrode, and separator are wound together to obtain a bare battery. Then, through processes such as encapsulation, electrolyte injection, formation, and degassing, various lithium-ion batteries are produced. This battery has an N / P ratio of 1 and a designed rated capacity of 100 Ah.

[0211] (III) Electrochemical Testing

[0212] (1) Cyclic performance test

[0213] The lithium-ion batteries prepared in the above embodiments and comparative examples are used as examples. At 25°C, they are charged to 4.3V with a constant current of 1C, then charged to 0.05C with a constant voltage of 4.3V, and then discharged to 2.5V with a constant current of 1C to obtain the discharge specific capacity of the first cycle (Cd1); this charging and discharging is repeated until the nth cycle, and the discharge specific capacity after n cycles is denoted as Cdn.

[0214] Capacity retention rate = discharge specific capacity after n cycles (Cdn) / discharge specific capacity in the first cycle (Cd1). The number of cycles corresponding to the capacity retention rate in Table 1 is 500 cycles.

[0215] (2) Specific capacity test of negative electrode sheet

[0216] The negative electrode sheets prepared in the above embodiments and comparative examples are used as examples. At 25°C, the negative electrode sheets were fabricated into coin cells and charged at a constant current rate of 0.1C to 0.05V, then charged at a constant voltage rate to a current of 0.05C. After standing for 5 minutes, they were discharged at a constant current rate of 0.1C to 2V, and the discharge capacity was recorded. Dividing this discharge capacity by the total mass of the negative electrode active material in the negative electrode sheet yields the 0.1C g / g capacity.

[0217] (3) Ratio Performance Test

[0218] Capacity testing of secondary batteries

[0219] The lithium-ion batteries prepared in the above embodiments and comparative examples are used as examples. At 25°C, each secondary battery is charged at a constant current of 1C to 4.3V, then charged at a constant voltage to a current of 0.05C, left to stand for 5 minutes, and then discharged at a constant current of 1C to 2.5V. The discharge capacity at this time is recorded as the 1C discharge capacity.

[0220] The lithium-ion batteries prepared in the above embodiments and comparative examples are used as examples. At 25°C, the secondary battery is charged at a constant current rate of 0.33C to 4.3V, then charged at a constant voltage rate to a current of 0.05C, left to stand for 5 minutes, and then discharged at a constant current rate of 0.33C to 2.5V. The discharge capacity at this time is recorded, which is the 0.33C discharge capacity.

[0221] The rate performance of a battery is expressed as 1C / 0.33C (%), where 1C / 0.33C (%) = 1C discharge capacity / 0.33C discharge capacity × 100%. The larger this value, the better its rate performance.

[0222] Table 1 shows some parameters and battery performance results of the above embodiments and comparative examples.

[0223] Table 1

[0224]

[0225]

[0226] In Table 1:

[0227] The mass content of oxygen-containing groups, i.e. the mass content of oxygen-containing groups in oxidized expanded graphite, can be calculated by the mass change before and after step (1).

[0228] The mass ratio A, i.e., the mass ratio of porous carbon to expanded oxide graphite, can be obtained by subtracting the mass of expanded oxide graphite used in step (2) from the mass of the product obtained in step (3), and then calculating the ratio of the mass of porous carbon to the mass of expanded oxide graphite. When a nitrogen source is added in step (2), the mass of the porous carbon obtained includes the mass of the doped nitrogen atoms; in this case, the mass ratio of porous carbon to expanded oxide graphite refers to the mass ratio of nitrogen-doped porous carbon to expanded oxide graphite.

[0229] The nitrogen doping content relative to porous carbon is obtained by subtracting the mass of the oxidized expanded graphite used in step (2) from the mass of the product obtained in step (3). The mass of nitrogen atoms is then obtained by elemental analysis and other methods. The ratio of the mass of nitrogen atoms to the mass of porous carbon is the nitrogen doping content relative to porous carbon.

[0230] The thickness of the porous carbon layer was obtained by performing CP cross-section analysis on the negative electrode active material.

[0231] The BET specific surface area of ​​porous carbon layers can be obtained by testing the BET specific surface area.

[0232] The average pore size of the porous carbon layer is determined by magnifying the surface of the material using a microscope or similar method, calculating the number of pores per unit length (e.g., 10 nm when the pore size is in the range of approximately 0.1–2 nm; or 100 nm when the pore size is in the range of approximately 2–40 nm) as the number of chambers, and then calculating the average pore size using the following equation: Average pore size = Unit length / Number of chambers.

[0233] The mass ratio B, which is the mass ratio of silicon particles to expanded oxidized graphite, can be obtained by comparing the mass difference between the product obtained in step (4) and the expanded oxidized graphite with porous carbon deposited in step (4), and then by calculating the ratio with the mass of the expanded oxidized graphite used.

[0234] Comparative Example 1 omits steps (1) to (3) of forming porous carbon on the interlayer surface of expanded graphite, and directly uses the expanded graphite raw material from step (1) to perform step (4) of depositing silicon particles and step (5) of carbon coating. The silicon particles are not firmly deposited on the expanded graphite, making it difficult to achieve a high loading of silicon particles. Furthermore, the disordered filling state of silicon particles inside the expanded graphite and the uneven grain size distribution of silicon particles will lead to uneven volume distribution of the prepared negative electrode active material during cycling, and cause uneven distribution and excessive consumption of active lithium, resulting in poor cycle stability and rate performance of the negative electrode active material. In addition, the specific capacity is also low.

[0235] Each embodiment pre-forms a porous carbon layer on the interlayer surface of expanded graphite, and then deposits silicon particles in the porous carbon channels of the porous carbon layer. In this way, the porous carbon channels provide storage space for the uniform deposition of silicon particles, so that the silicon particles can be uniformly dispersed in the interlayer of expanded graphite. This is not only conducive to the high loading of silicon particles, but also the layered structure of expanded graphite is conducive to the internal structural enhancement and conductivity improvement of the negative electrode active material. It also provides space for the expansion of silicon particles during cycling, thereby improving its stability and rate performance during cycling.

[0236] Comparing Examples 1-5, it can be seen that the total amount of negative electrode active material in Example 2 is relatively small, therefore its specific capacity is relatively low. Comparing the mass of porous carbon and silicon particles in Example 4, it can be seen that the mass of silicon particles is much greater than the mass of porous carbon. Therefore, in addition to being deposited within the pores of porous carbon, some silicon particles are also deposited on the surface of porous carbon, thus reducing its cycle retention rate and rate performance to a certain extent.

[0237] Comparing Examples 6 to 9, the difference lies in the amount of nitrogen source added during the preparation process. Specifically, when the amount of expanded graphite added remains unchanged, the mass ratio of nitrogen source to expanded graphite is different, as shown in Table 1, which in turn results in different nitrogen content relative to porous carbon doping in the prepared negative electrode active material. In Example 9, the amount of nitrogen source added is 0, and the cycle retention rate and rate performance of its battery are relatively low.

[0238] In Examples 10 and 11, the mass of silicon particles in Example 10 is much greater than that of porous carbon. Therefore, in addition to being deposited in the pores of porous carbon, some silicon particles are also deposited on the surface of porous carbon, which reduces its cycle retention rate and rate performance to a certain extent.

[0239] Compared with Example 1, the difference in Example 12 is that the expanded graphite was not subjected to the oxidation treatment in step (1), but directly carried out steps (2) to (5). Therefore, the porous carbon deposited on the expanded graphite was not firm. With repeated charging and discharging of the battery, the stability of the negative electrode active material decreased, which in turn led to a decrease in the cycle performance of the battery.

[0240] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0241] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims, and the specification and drawings can be used to interpret the content of the claims.

Claims

1. A negative electrode active material, characterized in that, It includes expanded graphite, porous carbon layers, and silicon particles. The expanded graphite includes multiple graphite layers, the porous carbon layers are distributed on at least one interlayer surface of the graphite layers, and the silicon particles are distributed at least in the pores of the porous carbon in the porous carbon layers.

2. The negative electrode active material as described in claim 1, characterized in that, The porous carbon layer covers the surface of at least one of the graphite layers.

3. The negative electrode active material as described in claim 2, characterized in that, The porous carbon layer covers the surface of each of the graphite layers.

4. The negative electrode active material as described in claim 1, characterized in that, The expanded graphite is oxidized expanded graphite.

5. The negative electrode active material as described in claim 4, characterized in that, The oxygen-containing groups in the expanded graphite include at least one of hydroxyl, epoxy, and carboxyl groups.

6. The negative electrode active material as described in claim 1, characterized in that, At least one of the following conditions must be met: (1a) The mass ratio of the porous carbon layer to the expanded graphite is (0.1~20):1; (1b) The thickness of the porous carbon layer is 0.05~20μm; (1c) The BET specific surface area of ​​the porous carbon layer is 40~1500 m². 2 / g; (1d) The average pore size of the porous carbon layer is 0.1~40 nm; (1e) The porous carbon layer is doped with nitrogen atoms.

7. The negative electrode active material as described in claim 6, characterized in that, At least one of the following conditions must be met: (1) The mass ratio of the porous carbon layer to the expanded graphite is (4~16):1; (2) The thickness of the porous carbon layer is 0.2~10μm; (3) The BET specific surface area of ​​the porous carbon layer is 600~1500 m². 2 / g; (4) The mass content of nitrogen atoms relative to the porous carbon layer is 0.1% to 20%.

8. The negative electrode active material according to any one of claims 1 to 7, characterized in that, At least one of the following conditions must be met: (2a) The mass ratio of the silicon particles to the expanded graphite is (0.08~20):1; (2b) The macroscopic average particle size of the silicon particles is 0.05 nm to 30 nm; (2c) The mass ratio of the silicon particles to the porous carbon layer is (0.7~12):1; (2d) In the negative electrode active material, the mass content of the silicon particles is 6%~65%.

9. The negative electrode active material as described in claim 8, characterized in that, At least one of the following conditions must be met: (1) The mass ratio of the silicon particles to the expanded graphite is (3~18):1; (2) The mass ratio of the silicon particles to the porous carbon layer is (0.7~1.5):

1.

10. The negative electrode active material according to any one of claims 1 to 7, characterized in that, The negative electrode active material further includes a coating layer that covers at least a portion of the outer surface of the expanded graphite.

11. The negative electrode active material as described in claim 10, characterized in that, At least one of the following conditions must be met: (1) The coating layer comprises carbon material; (2) The coating layer has a mass content of 0.2% to 3% in the negative electrode active material; (3) The thickness of the coating layer is 0.5~200nm.

12. The method for preparing the negative electrode active material according to any one of claims 1 to 11, characterized in that, Includes the following steps: Expanded graphite is mixed with a carbon source solution and subjected to a hydrothermal reaction. The solid product from the hydrothermal reaction is then calcined to obtain an intermediate product. The intermediate product comprises expanded graphite and a porous carbon layer distributed on the surface of the graphite layer within the expanded graphite. Silicon particles are formed in the pores of the porous carbon in the intermediate product.

13. The method for preparing the negative electrode active material as described in claim 12, characterized in that, At least one of the following conditions must be met: (3a) The carbon source in the carbon source solution includes at least one of glucose, asphalt, sucrose and phenolic resin; (3b) The mass ratio of the carbon source in the carbon source solution to the expanded graphite is (1~200):1; (3c) A nitrogen source is also added to the hydrothermal reaction described above; (3d) The hydrothermal reaction temperature is 80~280℃ and the time is 0.5~5h; (3e) The calcination conditions include: calcination at 400~1000℃ for 1~5 hours; (3f) The expanded graphite is oxidized expanded graphite.

14. The method for preparing the negative electrode active material as described in claim 13, characterized in that, At least one of the following conditions must be met: (1) When a nitrogen source is added to the hydrothermal reaction, the nitrogen source includes at least one of pyrrole, melamine and acetonitrile; (2) The oxygen-containing groups in the expanded graphite include at least one of hydroxyl, epoxy and carboxyl groups; (3) The mass content of oxygen-containing groups in the expanded graphite is 0.5%~38%.

15. The method for preparing the negative electrode active material as described in claim 12, characterized in that, The step of forming silicon particles in the pores of the porous carbon in the intermediate product includes the following steps: The intermediate product is introduced with silane and a reducing gas, and chemical vapor deposition is performed under a protective atmosphere.

16. The method for preparing the negative electrode active material as described in claim 15, characterized in that, At least one of the following conditions must be met: (1) The silane includes at least one of methylsilane and disilane; (2) The temperature of the chemical vapor deposition is 400~1000℃; (3) The chemical vapor deposition time is 1~5h.

17. The method for preparing the negative electrode active material as described in claim 15, characterized in that, The product obtained by chemical vapor deposition is passed through a gaseous feedstock for further chemical vapor deposition, wherein the gaseous feedstock includes at least one of ethylene, acetylene, and propylene.

18. The method for preparing the negative electrode active material as described in claim 17, characterized in that, At least one of the following conditions must be met: (1) The temperature of the chemical vapor deposition is 600~1200℃; (2) The chemical vapor deposition time is 1~5h.

19. The method for preparing the negative electrode active material according to any one of claims 12 to 18, characterized in that, The expanded graphite is oxidized expanded graphite; the preparation method further includes the step of preparing the oxidized expanded graphite, comprising the following steps: Expanded graphite is soaked in acid solution and then washed until neutral to obtain oxidized expanded graphite.

20. The method for preparing the negative electrode active material as described in claim 19, characterized in that, The acid solution includes at least one of HNO3 and H2SO4.

21. A negative electrode sheet, characterized in that, The negative electrode sheet comprises the negative electrode active material as described in any one of claims 1 to 11.

22. A secondary battery, characterized in that, Includes the negative electrode sheet as described in claim 21.

23. An electrical appliance, characterized in that, Includes the secondary battery as described in claim 22.

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