Preparation method of carbon-coated graphite, carbon-coated graphite, negative pole piece and electrochemical energy storage device

Abstract

The invention provides a preparation method of carbon-coated graphite, the carbon-coated graphite, a negative pole piece and an electrochemical energy storage device, the preparation method comprises the following steps: (1) mixing graphite, furnace black and asphalt, and carrying out first carbonization in a protective atmosphere to obtain a graphite intermediate; (2) mixing acetylene method carbon black with the obtained graphite intermediate, and performing second carbonization in a protective atmosphere to obtain carbon-coated graphite; the oil absorption value of the furnace-process carbon black is greater than that of the acetylene-process carbon black; the temperature of the first carbonization is lower than that of the second carbonization. According to the preparation method, a modification strategy from inside to outside is adopted, so that the compactness and the structural integrity of a coating layer are greatly improved, defects on the surface of the material are remarkably reduced, and the conductivity of the carbon-coated material is also improved; therefore, the battery prepared from the carbon-coated graphite obtained by the preparation method has excellent high-temperature storage performance and cycling stability.

Description

Technical Field

[0001] This invention belongs to the field of battery technology and relates to a method for preparing carbon-coated graphite, and more particularly to a method for preparing carbon-coated graphite, carbon-coated graphite, negative electrode sheet and electrochemical energy storage device. Background Technology

[0002] With the complete shift of automotive start-stop and emergency backup power systems to lithium-ion batteries, the stringent operating conditions, such as high-current charging and discharging and high-temperature operation, place extremely high demands on the power characteristics and stability of anode materials. Graphite, as a core anode material, has become a key direction for optimizing battery performance due to its significantly improved conductivity and ion transport rate through carbon coating technology.

[0003] However, the asphalt coating technology currently widely used in industry has significant bottlenecks. Volatile components released during the high-temperature carbonization of asphalt form micropores and defects within the coating layer, leading to decreased density. These structural defects make the electrolyte easily permeable and induce continuous side reactions, accelerating the consumption of active lithium, ultimately causing deterioration in the battery's high-temperature storage performance and shortening its cycle life, making it difficult to meet the long-term use requirements under harsh operating conditions.

[0004] In summary, existing methods for preparing carbon-coated graphite all have certain drawbacks, including insufficient density of the carbon coating layer. This leads to inadequate high-temperature storage performance and poor cycle stability in the batteries, making it difficult to meet the increasingly stringent requirements for long-term battery life and reliability in start-stop and emergency power supplies. Therefore, it is crucial to develop a novel method for preparing carbon-coated graphite, carbon-coated graphite itself, a negative electrode sheet, and an electrochemical energy storage device. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a method for preparing carbon-coated graphite, carbon-coated graphite, a negative electrode sheet, and an electrochemical energy storage device. The preparation method employs an inside-out modification strategy, which not only significantly improves the density and structural integrity of the coating layer but also significantly reduces surface defects and enhances the conductivity of the carbon-coated material. Therefore, the battery prepared from the carbon-coated graphite obtained by the above method exhibits excellent high-temperature storage performance and cycle stability.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing carbon-coated graphite, the method comprising:

[0008] (1) After mixing graphite, furnace black and pitch, the first carbonization is carried out in a protective atmosphere to obtain graphite intermediate;

[0009] (2) After mixing acetylene carbon black with the graphite intermediate obtained in step (1), a second carbonization is carried out in a protective atmosphere to obtain carbon-coated graphite.

[0010] The oil absorption value of the furnace black in step (1) is greater than that of the acetylene black in step (2); the temperature of the first carbonization in step (1) is less than the temperature of the second carbonization in step (2).

[0011] In this invention, furnace black is produced by using gaseous hydrocarbons, liquid hydrocarbons, or mixtures thereof as raw materials, supplying an appropriate amount of air, and forming a closed turbulent system in a reactor. A portion of the raw material hydrocarbons and air are burned to generate high temperatures, while the other portion is cracked to produce furnace black. Furnace black is a black powdery substance produced by the incomplete combustion or pyrolysis of hydrocarbons in the gas phase under strictly controlled process conditions. Its main component is elemental carbon, and it contains small amounts of oxygen, hydrogen, sulfur, etc. The particles of furnace black are approximately spherical, with a particle size of 10μm to 500μm.

[0012] In this invention, the acetylene-based carbon black is produced by pyrolyzing high-purity acetylene (≥99%) refined from byproduct gas of naphtha pyrolysis using the calcium carbide method or naphtha pyrolysis at high temperatures. The reaction process is controlled by a water-cooled jacket. The acetylene-based carbon black is a black, extremely fine powder with a relative density of 1.95 and an apparent density of 0.2 g / cm³. 3 ~0.3g / cm 3 The average particle size is 30nm-45nm, and the specific surface area is 55m². 2 / g~70m 2 It has a carbon content of >99.5% and features low resistivity (≤3.0Ω·m), high conductivity and antistatic properties.

[0013] In the preparation method provided by the present invention, in step (1), the furnace black with a high oil absorption value (high oil absorption value requires furnace process, which helps carbon black dispersion, while acetylene carbon black with a high oil absorption value has poor dispersion) and pitch form a synergistic effect, that is: furnace black, with its complex three-dimensional chain dendritic structure and strong adsorption capacity, can not only improve the conductive network, but also effectively adsorb and fix pitch, so that it is uniformly wrapped on the graphite surface. It can also use the rich surface functional groups of furnace black to provide electrostatic repulsion, which significantly improves the dispersibility of graphite.

[0014] In the preparation method provided by the present invention, in step (2), acetylene-based carbon black with a low oil absorption value is used (a low oil absorption value requires acetylene method, which helps to reduce surface functional groups). This not only enables defect repair but also further improves the conductive network. Specifically, by utilizing the simple structure and excellent pore-filling ability of acetylene-based carbon black with a low oil absorption value, the micropores left in the first carbonization process are effectively filled, and a continuous and complete conductive network is constructed with the inner layer of furnace-processed carbon black with a high oil absorption value. In addition, since acetylene-based carbon black has fewer surface functional groups, combined with the second carbonization at a higher temperature, the surface defects of the final carbon coating layer are significantly reduced and the chemical properties are more stable, thereby effectively reducing the side reaction activity of carbon-coated graphite and electrolyte.

[0015] In summary, the preparation method provided by this invention first utilizes the synergistic effect of furnace black and pitch, which have high oil absorption values, to construct a uniform, dense, and conductive primary carbon coating layer in situ on the graphite matrix surface. Subsequently, through acetylene black with low oil absorption values ​​and combined with a second carbonization at a higher temperature, not only are the pores of the primary carbon coating layer filled, but an outer carbon coating layer with high density and few functional groups is also formed. The preparation method employs an inside-out modification strategy, which not only significantly improves the density and structural integrity of the coating layer, but also significantly reduces surface defects and improves the conductivity of the carbon coating material. Therefore, the battery prepared from carbon-coated graphite obtained by the above method has excellent high-temperature storage performance and cycle stability.

[0016] Preferably, the mixing in step (1) includes: initially mixing graphite and furnace black to obtain a preliminary mixture, and then mixing asphalt with the obtained preliminary mixture to obtain a mixed mixture.

[0017] Preferably, the graphite in step (1) includes any one or at least two of natural graphite, artificial graphite, and silicon-containing graphite. Typical but non-limiting combinations include combinations of natural graphite and artificial graphite, combinations of artificial graphite and silicon-containing graphite, combinations of natural graphite and silicon-containing graphite, or combinations of natural graphite, artificial graphite, and silicon-containing graphite.

[0018] Preferably, the graphite is in powder form.

[0019] Preferably, the oil absorption value D1 of the furnace black in step (1) is 200mL / 100g to 500mL / 100g, for example, it can be 200mL / 100g, 250mL / 100g, 300mL / 100g, 350mL / 100g, 400mL / 100g, 450mL / 100g or 500mL / 100g, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0020] Preferably, the furnace black in step (1) is in powder form.

[0021] Preferably, the softening point T of the asphalt in step (1) is... H The temperature range is 150℃ to 300℃, for example, it can be 150℃, 170℃, 190℃, 210℃, 230℃, 250℃, 270℃, 290℃ or 300℃, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0022] In the preparation method provided by the present invention, high-soft asphalt with a high softening point is used. This not only takes advantage of the low content of lightweight components in the asphalt matrix of high-soft asphalt and the fewer pores after carbonization, thus further reducing the porosity of the carbon coating layer in carbon-coated graphite; but also takes advantage of the high softening point of high-soft asphalt and the stronger structural strength of the carbon coating layer after carbonization, thus further improving the density and structural strength of the carbon coating layer.

[0023] Preferably, the asphalt in step (1) is in powder form.

[0024] Preferably, in the mixing process described in step (1), the mass ratio of graphite to furnace black is (10~50):(1~5), for example, it can be 10:1, 15:2, 20:3, 25:4, 30:5, 35:1, 40:2, 45:3, 50:4 or 50:5, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0025] Preferably, in the mixing process described in step (1), the mass ratio of graphite to bitumen is (10~50):(5~9), for example, it can be 10:5, 15:6, 20:7, 25:8, 30:9, 35:5, 40:6, 45:7, 50:8 or 50:9, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0026] Preferably, the oil absorption value D2 of the acetylene carbon black in step (2) is 50mL / 100g to 200mL / 100g, for example, it can be 50mL / 100g, 75mL / 100g, 100mL / 100g, 125mL / 100g, 150mL / 100g, 175mL / 100g or 200mL / 100g, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0027] Preferably, the acetylene-based carbon black in step (2) is in powder form.

[0028] Preferably, in the mixing process described in step (2), the mass ratio of acetylene carbon black to graphite intermediate is (10~50):(1~5), for example, it can be 10:1, 15:2, 20:3, 25:4, 30:5, 35:1, 40:2, 45:3, 50:4 or 50:5, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0029] Preferably, in step (1), the temperature of the first carbonization is 300℃~500℃ and the time is 1h~3h.

[0030] In this invention, the temperature of the first carbonization in step (1) is 300℃~500℃, for example, it can be 300℃, 325℃, 350℃, 375℃, 400℃, 425℃, 450℃, 475℃ or 500℃, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0031] In this invention, the carbonization time in step (1) is 1h to 3h, for example, it can be 1.0h, 1.2h, 1.4h, 1.6h, 1.8h, 2.0h, 2.2h, 2.4h, 2.6h, 2.8h or 3.0h, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0032] Preferably, in step (2), the temperature of the second carbonization is 500℃~800℃ and the time is 1h~3h.

[0033] In this invention, the temperature of the second carbonization in step (2) is 500℃~800℃, for example, it can be 500℃, 550℃, 600℃, 650℃, 700℃, 750℃ or 800℃, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0034] In this invention, the carbonization time in step (2) is 1h to 3h, for example, it can be 1.0h, 1.2h, 1.4h, 1.6h, 1.8h, 2.0h, 2.2h, 2.4h, 2.6h, 2.8h or 3.0h, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0035] Preferably, step (1) further includes heating at a rate of 1℃ / min to 15℃ / min between mixing and the first carbonization. For example, the heating rate can be 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, 11℃ / min, 12℃ / min, 13℃ / min, 14℃ / min or 15℃ / min, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0036] Preferably, step (2) further includes heating at a rate of 1℃ / min to 15℃ / min between mixing and the second carbonization. For example, the rate can be 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, 11℃ / min, 12℃ / min, 13℃ / min, 14℃ / min or 15℃ / min, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0037] Preferably, the protective atmosphere described in step (1) and step (2) independently comprises nitrogen and / or an inert gas, respectively.

[0038] In a second aspect, the present invention provides carbon-coated graphite, wherein the carbon-coated graphite is obtained by the preparation method described in the first aspect.

[0039] Thirdly, the present invention provides a negative electrode sheet, the negative electrode sheet comprising the carbon-coated graphite described in the second aspect.

[0040] Fourthly, the present invention provides an electrochemical energy storage device, the electrochemical energy storage device comprising the negative electrode sheet described in the third aspect.

[0041] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] (1) In the preparation method provided by the present invention, in step (1), the furnace black with a high oil absorption value (high oil absorption value requires furnace process, which helps carbon black dispersion, while acetylene carbon black with a high oil absorption value has poor dispersion) and pitch form a synergistic effect, that is: furnace black, with its complex three-dimensional chain dendritic structure and strong adsorption capacity, can not only improve the conductive network, but also effectively adsorb and fix pitch, so that it is uniformly wrapped on the graphite surface. It can also use the rich surface functional groups of furnace black to provide electrostatic repulsion, which significantly improves the dispersibility of graphite.

[0044] (2) In the preparation method provided by the present invention, in step (2), acetylene-based carbon black with a low oil absorption value is used (the low oil absorption value requires the acetylene method, which helps to reduce surface functional groups). This not only enables defect repair but also further improves the conductive network. That is, by utilizing the characteristics of the low oil absorption value acetylene-based carbon black, which has a simple structure and excellent pore-filling ability, the micropores left in the first carbonization process are effectively filled, and a continuous and complete conductive network is constructed with the inner layer of furnace-processed carbon black with a high oil absorption value. In addition, since the acetylene-based carbon black has fewer surface functional groups, combined with the second carbonization at a higher temperature, the surface defects of the final carbon coating layer are significantly reduced and the chemical properties are more stable, thereby effectively reducing the side reaction activity of carbon-coated graphite and electrolyte.

[0045] (3) In the preparation method provided by the present invention, firstly, the synergistic effect of furnace black with high oil absorption value and pitch is used to construct a uniform, dense, and conductive primary carbon coating layer in situ on the surface of the graphite matrix; then, through acetylene black with low oil absorption value and combined with a second carbonization at a higher temperature, not only is the pore filling of the primary carbon coating layer achieved, but also an outer carbon coating layer with high density and few functional groups is formed; the preparation method adopts an inside-out modification strategy, which not only greatly improves the density and structural integrity of the coating layer, but also significantly reduces the defects on the material surface, and improves the conductivity of the carbon coating material; therefore, the battery prepared by the carbon-coated graphite obtained by the preparation method has excellent high-temperature storage performance and cycle stability. Detailed Implementation

[0046] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0047] The "range" disclosed in this invention 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 the specific range. This type of range definition can include or exclude endpoints; any endpoint can be independently included or excluded, and they can be arbitrarily combined, meaning 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 specific parameters, it is understood that ranges of 60~110 and 80~120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and maximum range values ​​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 invention, 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.

[0048] In this invention, "a combination of at least two" refers to a quantity greater than or equal to two, unless otherwise specified. For example, "any combination of one or at least two" means one or more or more items. It can be understood that when referring to "a combination of at least two," it refers to any suitable combination of multiple items, that is, a combination of "at least two" items carried out in a manner that does not conflict with and enables the implementation of this invention.

[0049] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0050] The term "embodiment" as used in this invention means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this invention can be combined with other embodiments.

[0051] Those skilled in the art will understand that the order in which the steps are written in the methods of the various embodiments does not imply a strict execution order. The detailed execution order of each step should be determined by its function and possible internal logic. Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, but are preferably performed 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, the method may also include step (c), meaning 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.

[0052] In this invention, open-ended technical features or solutions described using terms such as "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, A includes a1, a2, and a3. Unless otherwise specified, it may also include other members or exclude additional members. This can be considered as providing both technical features or solutions where "A is composed of a1, a2, and a3" or "A is selected from a1, a2, and a3," and technical features or solutions where "A includes not only a1, a2, and a3, but also other members."

[0053] In this invention, unless otherwise specified, the features or solutions corresponding to "and / or" include any one of two or more of the related listed items, as well as any and all combinations of the related listed items. These arbitrary and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. For example, "A and / or B" represents a group consisting of A, B, and "a combination of A and B". "Containing A and / or B" can mean "containing A, containing B, and containing A and B", or "containing A, containing B, or containing A and B", and can be appropriately understood according to the context.

[0054] In this invention, the terms "first aspect," "second aspect," "third aspect," "fourth aspect," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first," "second," "third," "fourth," etc., serve only as a non-exhaustive enumeration and should be understood not to constitute a closed limitation on the quantity.

[0055] In this invention, "optional" means that something is optional, that is, it refers to any one of the two parallel solutions of "having" or "not having". If there are multiple "optional" options in a technical solution, unless otherwise specified, and there are no contradictions or mutual constraints, then each "optional" option is independent.

[0056] In this invention, "room temperature" generally refers to 4℃~35℃, and can refer to 20℃±5℃. In some embodiments of this invention, room temperature refers to 20℃~30℃.

[0057] Example 1

[0058] This embodiment provides a method for preparing carbon-coated graphite, the method comprising:

[0059] (1) A primary mixture of graphite (natural graphite, powder) and furnace black (powder, produced by the French Erysz Group) with an oil absorption value D1 of 350 mL / 100 g was prepared to obtain a primary mixture, which was then mixed with a softening point T. H The asphalt (powdered, produced by China Petroleum & Chemical Corporation) at 230℃ was mixed with the obtained primary mixture to obtain a mixture. Then, in a nitrogen atmosphere, the temperature was increased to 400℃ at a rate of 5℃ / min, and the mixture was subjected to a first carbonization at 400℃ for 2 hours to obtain a graphite intermediate.

[0060] The mass ratio of graphite, furnace black and pitch is 30:3:7;

[0061] (2) After mixing acetylene carbon black (powder, produced by Jiaozuo Hexing Chemical Industry Co., Ltd.) with an oil absorption value D2 of 120 mL / 100 g with the graphite intermediate obtained in step (1), the mixture was heated to 650 °C at a rate of 10 °C / min in a nitrogen atmosphere and then subjected to a second carbonization at 650 °C for 2 h to obtain carbon-coated graphite.

[0062] The mass ratio of acetylene-based carbon black to graphite intermediate is 30:3;

[0063] The oil absorption value of the furnace black in step (1) is greater than that of the acetylene black in step (2).

[0064] Example 2

[0065] This embodiment provides a method for preparing carbon-coated graphite, the method comprising:

[0066] (1) A preliminary mixture of graphite (artificial graphite, powder) and furnace black (powder, produced by the French Erysz Group) with an oil absorption value D1 of 200 mL / 100 g was prepared to obtain a preliminary mixture, which was then mixed with a softening point T. HThe asphalt (powdered, produced by China Petroleum & Chemical Corporation) at 300℃ was mixed with the obtained primary mixture to obtain a mixture. Then, in an argon atmosphere, the temperature was increased to 500℃ at a rate of 8℃ / min, and the mixture was subjected to a first carbonization at 500℃ for 1 hour to obtain a graphite intermediate.

[0067] The mass ratio of graphite, furnace black and pitch is 50:1:5;

[0068] (2) After mixing acetylene carbon black (powder, produced by Jiaozuo Hexing Chemical Industry Co., Ltd.) with an oil absorption value D2 of 50 mL / 100 g with the graphite intermediate obtained in step (1), the mixture is heated to 800 °C at a rate of 15 °C / min in an argon atmosphere, and then subjected to a second carbonization at 800 °C for 1 h to obtain carbon-coated graphite.

[0069] The mass ratio of acetylene-based carbon black to graphite intermediate is 10:5;

[0070] The oil absorption value of the furnace black in step (1) is greater than that of the acetylene black in step (2).

[0071] Example 3

[0072] This embodiment provides a method for preparing carbon-coated graphite, the method comprising:

[0073] (1) A primary mixture of graphite (natural graphite, powder) and furnace black (powder, produced by the French Erysz Group) with an oil absorption value D1 of 500 mL / 100 g was prepared to obtain a primary mixture, which was then mixed with a softening point T. H The asphalt (powdered, produced by China Petroleum & Chemical Corporation) at 150℃ was mixed with the obtained primary mixture to obtain a mixture. Then, in an argon atmosphere, the temperature was increased to 300℃ at a rate of 1℃ / min, and the mixture was subjected to a first carbonization at 300℃ for 3 hours to obtain a graphite intermediate.

[0074] The mass ratio of graphite, furnace black and pitch is 10:5:9;

[0075] (2) After mixing acetylene carbon black (powder, produced by Jiaozuo Hexing Chemical Industry Co., Ltd.) with an oil absorption value D2 of 200 mL / 100 g with the graphite intermediate obtained in step (1), the mixture was heated to 500 °C at a rate of 1 °C / min in an argon atmosphere, and then subjected to a second carbonization at 500 °C for 3 h to obtain carbon-coated graphite.

[0076] The mass ratio of acetylene-based carbon black to graphite intermediate is 50:1;

[0077] The oil absorption value of the furnace black in step (1) is greater than that of the acetylene black in step (2).

[0078] Example 4

[0079] This embodiment provides a method for preparing carbon-coated graphite, except that graphite, furnace black and pitch are directly mixed in step (1);

[0080] In step (1), the initial mixture of graphite (natural graphite, powder) and furnace black (powder, produced by the French Eryszc Group) with an oil absorption value D1 of 350 mL / 100 g is mixed to obtain an initial mixture, which is then mixed with a softening point T. H The phrase "230℃ asphalt (powder, produced by China Petroleum & Chemical Corporation) and the resulting primary mixture yield a mixture" should be replaced with "primary mixed graphite (natural graphite, artificial graphite, silicon-containing graphite, powder), furnace black (powder, produced by Erythrope Group, France) with an oil absorption value D1 of 200mL / 100g~500mL / 100g and a softening point T..." H Except for the mixture obtained from asphalt (powdered, produced by China Petroleum & Chemical Corporation) at 150℃~300℃, everything else was the same as in Example 1.

[0081] Example 5

[0082] This embodiment provides a method for preparing carbon-coated graphite. Except for the oil absorption value of furnace black in step (1) being 150 mL / 100 g, the rest is the same as in Example 1.

[0083] Example 6

[0084] This embodiment provides a method for preparing carbon-coated graphite. Except for the oil absorption value of furnace black in step (1) being 600 mL / 100 g, the rest is the same as in Example 1.

[0085] Example 7

[0086] This embodiment provides a method for preparing carbon-coated graphite, except that the softening point T of the pitch in step (1) is... H Except for the temperature of 100°C, everything else is the same as in Example 1.

[0087] Example 8

[0088] This embodiment provides a method for preparing carbon-coated graphite, except that the softening point T of the pitch in step (1) is... H Except for the temperature of 400°C, everything else is the same as in Example 1.

[0089] Example 9

[0090] This embodiment provides a method for preparing carbon-coated graphite. Except for the mass ratio of graphite to furnace black in step (1) being 30:0.3, the rest is the same as in Example 1.

[0091] Example 10

[0092] This embodiment provides a method for preparing carbon-coated graphite. Except for the mass ratio of graphite to furnace black in step (1) being 30:30, the rest is the same as in Example 1.

[0093] Example 11

[0094] This embodiment provides a method for preparing carbon-coated graphite. Except for the first carbonization at 200°C in step (1), the rest is the same as in Example 1.

[0095] Example 12

[0096] This embodiment provides a method for preparing carbon-coated graphite. Except for the first carbonization at 600°C in step (1), the rest is the same as in Example 1.

[0097] Example 13

[0098] This embodiment provides a method for preparing carbon-coated graphite. Except for the oil absorption value D2 of acetylene carbon black in step (2), which is 30 mL / 100 g, the rest are the same as in Example 1.

[0099] Example 14

[0100] This embodiment provides a method for preparing carbon-coated graphite. Except for the oil absorption value D2 of acetylene carbon black in step (2), which is 280mL / 100g, the rest are the same as in Example 1.

[0101] Example 15

[0102] This embodiment provides a method for preparing carbon-coated graphite. Except for the second carbonization at 400°C in step (2), the rest is the same as in Example 1.

[0103] Example 16

[0104] This embodiment provides a method for preparing carbon-coated graphite. Except for the second carbonization at 1000°C in step (2), the rest is the same as in Example 1.

[0105] Comparative Example 1

[0106] This comparative example provides a method for preparing carbon-coated graphite. Except for replacing the furnace black in step (1) with the same mass of acetylene black in step (2) and replacing the acetylene black in step (2) with the same mass of furnace black in step (1), the rest is the same as in Example 1.

[0107] Comparative Example 2

[0108] This comparative example provides a method for preparing carbon-coated graphite. Except for replacing the furnace black in step (1) with an equal mass of acetylene black in step (2), the rest is the same as in Example 1.

[0109] Comparative Example 3

[0110] This comparative example provides a method for preparing carbon-coated graphite. Except for replacing the acetylene carbon black in step (2) with the same mass of furnace black in step (1), the rest is the same as in Example 1.

[0111] Comparative Example 4

[0112] This comparative example provides a method for preparing carbon-coated graphite, which is the same as that in Example 1 except that step (2) is omitted.

[0113] The carbon-coated graphite provided in the above embodiments and comparative examples underwent initial efficiency testing. The testing method was as follows:

[0114] Carbon-coated graphite was used as the active material and mixed with polyvinylidene fluoride binder and acetylene black conductive agent in N-methylpyrrolidone solvent at a mass ratio of 96:2:2 to form a uniform slurry. The slurry was coated onto copper foil and dried under vacuum at 120°C for 12 hours to form a negative electrode sheet. A lithium metal sheet was used as the counter electrode, a Celgard 2400 polypropylene membrane was used as the separator, and a 1M LiPF6 EC / DEC / EMC solution (volume ratio 1:1:1) was used as the electrolyte. The cells were assembled into a CR2032 button cell in an argon-protected glove box.

[0115] In a 25°C constant temperature chamber, the obtained CR2032 button cell was first charged at a constant current of 0.1C to 0.005V (vs Li). + / Li), then constant voltage charging until the current drops to 0.01C, stand for 5 minutes, and then constant current discharge to 1.5V at 0.1C. The first efficiency of carbon-coated graphite is shown in Table 1.

[0116] The carbon-coated graphite provided in the above embodiments and comparative examples was tested using a powder resistivity tester. The powder resistivity of the carbon-coated graphite is shown in Table 1.

[0117] Table 1

[0118]

[0119] The method for preparing lithium batteries using carbon-coated graphite provided in the above embodiments and comparative examples is as follows: carbon-coated graphite, conductive carbon black, binder polyacrylic acid (PAA), and sodium carboxymethyl cellulose are mixed in deionized water at a mass ratio of 96:1:2:1 to prepare a slurry with a solid content of 45%. The obtained slurry is coated onto an 8 μm thick copper foil, dried at 85°C, and then pressed to a compaction density of 1.45 g / cm³ using a roller mill. 3 Then, it is punched and cut into the required electrode sheets;

[0120] Using lithium iron phosphate (LiFePO4) as the positive electrode (compacted density 2.4 g / cm³), polyethylene porous membrane as the separator, and injecting electrolyte (ensuring an injection coefficient of 1.2 g / Ah), the electrolyte is a 1 mol / L LiPF6 EC / EMC / DMC (volume ratio 1:1:1) solution, and then encapsulating it into a soft-pack battery.

[0121] The obtained pouch cells were subjected to a DC resistance (DCR) test. The test method was as follows: the pouch cells were charged at 25℃ with a constant current and constant voltage of 0.5C to 3.7V, with a cutoff current of 0.05C, and left to stand for 30 minutes to reach 100% SOC. Subsequently, a 1C constant current discharge pulse was applied for 18 seconds to adjust the battery SOC to 50%, and the cells were left to stand for 1 hour to ensure voltage and temperature stability. The batteries were then placed in a high and low temperature test chamber at -25℃ for 10 hours. After that, a 5A discharge pulse was applied for 18 seconds, and the voltage at the start and end of the pulse was recorded. The DC resistance (DCR) was calculated and is shown in Table 2.

[0122] The high-temperature storage performance of the obtained lithium battery was tested. The test method was as follows: the pouch battery was charged at 25℃ with a constant current of 1C to 3.7V, then switched to constant voltage charging to a current of 0.05C. After charging was completed, its initial thickness and initial capacity were measured. Subsequently, the pouch battery was stored in a constant temperature chamber at 60℃ for 7 days. After storage, the battery was removed and cooled to 25℃, and its thickness was measured again. Then, the battery was discharged at a constant current of 1C to 3.0V, and its remaining capacity was measured. Finally, the battery was fully charged again according to the initial charging method, and then discharged at 1C to 2.5V, and its recovered capacity was measured. The DCR growth rate and capacity recovery rate of the pouch battery after 30 days of storage at 60℃ are shown in Table 2.

[0123] The cycle stability of the obtained lithium battery was tested. The test method was as follows: the battery was placed in a high and low temperature chamber at 45℃, charged at a constant current of 1.5C to 3.7V, then charged at a constant voltage to a current of 0.05C, then left to stand for 5 minutes, and then discharged at a constant current of 1C to 2.5V. The cycle was repeated 1000 times. The capacity retention rate after 1000 cycles at 45℃ is shown in Table 2.

[0124] Table 2

[0125]

[0126] From Table 1 and Table 2, we can obtain:

[0127] (1) The carbon-coated graphite prepared by the preparation method of carbon-coated graphite provided in Examples 1 to 3 exhibits high initial efficiency and low powder resistivity; the soft-pack battery prepared by the obtained carbon-coated graphite exhibits low DC resistance, excellent high-temperature storage performance in high-temperature storage performance test, and excellent cycle stability in cycle stability test.

[0128] (2) By comparing Example 1 and Example 4, it can be seen that in this invention, graphite is first mixed with furnace black with high oil absorption value, and then mixed with asphalt. Compared with directly mixing graphite with furnace black with high oil absorption value and asphalt, carbon-coated graphite and battery exhibit better comprehensive performance. This is because premixing can make full use of the electrostatic repulsion generated by the rich functional groups on the surface of furnace black, so that it is evenly dispersed on the graphite surface. If it is directly mixed with viscous asphalt, the carbon black is easily wrapped and agglomerated, and cannot be evenly spread, resulting in uneven distribution of conductive network and poor uniformity of coating layer, thereby affecting battery performance.

[0129] (3) By comparing Example 1 with Examples 5 and 6, it can be seen that in this invention, when the oil absorption value of the furnace black in step (1) is greater than that of the acetylene carbon black in step (2), when the oil absorption value of the furnace black in step (1) is 200mL / 100g~500mL / 100g, the carbon-coated graphite and the battery exhibit better comprehensive performance. This is because when the oil absorption value is too low, the chain branch structure is not well developed and it cannot effectively adsorb pitch and construct a perfect conductive network; when the oil absorption value is too high, the carbon black is difficult to disperse and easily agglomerates, and the surface area is too large, which increases the side reaction; when the oil absorption value of the furnace black in step (1) is within this range, it is beneficial to obtain a more uniform and dense carbon coating layer.

[0130] (4) By comparing Example 1 with Examples 7 and 8, it can be seen that in this invention, when the softening point of the asphalt in step (1) is 150℃~300℃, the carbon-coated graphite and battery exhibit better comprehensive performance. This is because when the softening point is too low, there are more light components in the asphalt, and the amount of volatiles released during carbonization is large, which easily leads to a loose and porous coating layer; when the softening point is too high, the fluidity is poor and it is difficult to uniformly wet the graphite; a suitable softening point can ensure a high residual carbon rate, form a dense and high-strength carbon layer, and improve high-temperature storage and cycle performance.

[0131] (5) By comparing Example 1 with Examples 9 and 10, it can be seen that in this invention, when the mass ratio of graphite to furnace black in step (1) is (10~50):(1~5), the carbon-coated graphite and the battery exhibit better overall performance. This is because when the carbon black content is too low, a continuous and effective conductive network and sufficient asphalt adsorption sites cannot be formed; when the carbon black content is too high, agglomeration is likely to occur, resulting in an overly thick and brittle coating layer, which reduces the battery energy density and increases the risk of peeling. When the mass ratio of graphite to furnace black in step (1) is within this range, conductivity and coating integrity can be better balanced.

[0132] (6) By comparing Example 1 with Examples 11 and 12, it can be seen that in this invention, when the temperature of the first carbonization in step (1) is 300℃~500℃, the carbon-coated graphite and the battery exhibit better comprehensive performance. This is because when the temperature is too low, the asphalt is easily not fully cured and the intermediate structure is unstable; when the temperature is too high, it will cause violent pyrolysis to generate internal stress cracks and destroy the coating.

[0133] (7) By comparing Example 1 with Examples 13 and 14, it can be seen that in this invention, when the oil absorption value of the furnace black in step (1) is greater than that of the acetylene black in step (2), when the oil absorption value D2 of the acetylene black in step (2) is 50mL / 100g~200mL / 100g, the carbon-coated graphite and the battery exhibit better overall performance. This is because when the oil absorption value is too low, an effective auxiliary conductive path cannot be formed; when the oil absorption value is too high, the structure is too complex and the micropore filling ability is poor. When the oil absorption value D2 of the acetylene black in step (2) is in this range, the acetylene black with low oil absorption value can accurately fill the micropores left in the first carbonization, improve the density of the coating layer, and form a complementary continuous conductive network with the inner carbon black.

[0134] (8) By comparing Example 1 with Examples 15 and 16, it can be seen that in the present invention, when the temperature of the second carbonization in step (2) is 500℃~800℃, the carbon-coated graphite and the battery exhibit better comprehensive performance. This is because when the temperature of the second carbonization is in this range, it can ensure that the asphalt is completely carbonized to form a dense carbon layer with high conductivity and low defects, and at the same time, it can effectively repair the surface defects of the carbon coating layer and improve chemical stability.

[0135] (9) By comparing Example 1 with Comparative Examples 1 to 4, it can be seen that in the preparation method provided by the present invention, in step (1), the furnace black with a high oil absorption value (high oil absorption value requires furnace process, which helps carbon black dispersion, while acetylene carbon black with a high oil absorption value has poor dispersion) and pitch have formed a synergistic effect, that is: furnace black, with its complex three-dimensional chain dendritic structure and strong adsorption capacity, can not only improve the conductive network, but also effectively adsorb and fix pitch, so that it is uniformly wrapped on the graphite surface, and can also use the rich surface functional groups of furnace black to provide electrostatic repulsion, which significantly improves the dispersibility of graphite;

[0136] In the preparation method provided by the present invention, in step (2), acetylene-based carbon black with a low oil absorption value is used (a low oil absorption value requires acetylene method, which helps to reduce surface functional groups). This not only enables defect repair but also further improves the conductive network. Specifically, by utilizing the simple structure and excellent pore-filling ability of acetylene-based carbon black with a low oil absorption value, the micropores left in the first carbonization process are effectively filled, and a continuous and complete conductive network is constructed with the inner layer of furnace-processed carbon black with a high oil absorption value. In addition, since acetylene-based carbon black has fewer surface functional groups, combined with the second carbonization at a higher temperature, the surface defects of the final carbon coating layer are significantly reduced and the chemical properties are more stable, thereby effectively reducing the side reaction activity of carbon-coated graphite and electrolyte.

[0137] In summary, the preparation method provided by this invention first utilizes the synergistic effect of furnace black and pitch, which have high oil absorption values, to construct a uniform, dense, and conductive primary carbon coating layer in situ on the graphite matrix surface. Subsequently, through acetylene black with low oil absorption values ​​and combined with a second carbonization at a higher temperature, not only are the pores of the primary carbon coating layer filled, but an outer carbon coating layer with high density and few functional groups is also formed. The preparation method employs an inside-out modification strategy, which not only significantly improves the density and structural integrity of the coating layer, but also significantly reduces surface defects and improves the conductivity of the carbon coating material. Therefore, the battery prepared from carbon-coated graphite obtained by the above method has excellent high-temperature storage performance and cycle stability.

[0138] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing carbon-coated graphite, characterized in that, The preparation method includes: (1) After mixing graphite, furnace black and pitch, the first carbonization is carried out in a protective atmosphere to obtain graphite intermediate; (2) After mixing acetylene carbon black with the graphite intermediate obtained in step (1), a second carbonization is carried out in a protective atmosphere to obtain carbon-coated graphite. The oil absorption value of the furnace black in step (1) is greater than that of the acetylene black in step (2); the temperature of the first carbonization in step (1) is less than the temperature of the second carbonization in step (2).

2. The preparation method according to claim 1, characterized in that, The mixing in step (1) includes: initially mixing graphite and furnace black to obtain a preliminary mixture, and then mixing asphalt with the obtained preliminary mixture to obtain a mixed mixture.

3. The preparation method according to claim 1, characterized in that, The graphite in step (1) includes any one or a combination of at least two of the following: natural graphite, artificial graphite, and silicon-containing graphite; And / or, the oil absorption value D1 of the furnace black in step (1) is 200mL / 100g~500mL / 100g; And / or, the softening point T of the asphalt in step (1) H The temperature range is 150℃ to 300℃.

4. The preparation method according to claim 1, characterized in that, In the mixing process described in step (1), the mass ratio of graphite to furnace black is (10~50):(1~5); And / or, in the mixing described in step (1), the mass ratio of graphite to bitumen is (10~50):(5~9).

5. The preparation method according to claim 1, characterized in that, The oil absorption value D2 of the acetylene carbon black in step (2) is 50mL / 100g~200mL / 100g.

6. The preparation method according to claim 1, characterized in that, In the mixing process described in step (2), the mass ratio of acetylene carbon black to graphite intermediate is (10~50):(1~5).

7. The preparation method according to any one of claims 1 to 6, characterized in that, Step (1) The temperature of the first carbonization is 300℃~500℃, and the time is 1h~3h; And / or, in step (2), the temperature of the second carbonization is 500℃~800℃ and the time is 1h~3h; And / or, between the mixing and the first carbonization in step (1), the process further includes: heating at a rate of 1°C / min to 15°C / min; And / or, between the mixing and the second carbonization in step (2), the temperature is increased at a rate of 1°C / min to 15°C / min.

8. A carbon-coated graphite, characterized in that, The carbon-coated graphite is obtained by the preparation method according to any one of claims 1 to 7.

9. A negative electrode sheet, characterized in that, The negative electrode sheet comprises the carbon-coated graphite as described in claim 8.

10. An electrochemical energy storage device, characterized in that, The electrochemical energy storage device includes the negative electrode sheet as described in claim 9.

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