Graphite composite material and preparation method therefor, negative electrode active material, secondary battery, and electric device

Abstract

A graphite composite material and a preparation method therefor, a negative electrode active material, a secondary battery, and an electric device. The graphite composite material comprises an inner core and a coating layer located on the surface of the inner core, wherein the material of the inner core comprises graphite, and the material of the coating layer comprises a nitrogen-doped porous aluminum oxide-carbon composite material. The provided graphite composite material improves the initial efficiency of the graphite material, and improves the rate capability of a battery.

Description

Graphite composite material and preparation method thereof, negative electrode active material, secondary battery and electrical equipment

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on June 28, 2023, with application number 202310783638.6 and application name “Graphite composite material and preparation method thereof, negative electrode active material, secondary battery and electrical equipment”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the technical field of secondary batteries, and in particular to a composite material and a preparation method thereof, a negative electrode active material, a secondary battery and an electrical device. Background Art

[0003] Graphite is currently a widely used carbon anode material in lithium-ion batteries due to its advantages such as good conductivity, high crystallinity, and well-formed layered structure. However, during the initial charge and discharge process of lithium-ion batteries, a certain amount of active lithium ions is consumed due to the formation of an SEI membrane interface on the graphite anode side. This results in a low initial efficiency of the graphite anode, thus restricting further improvements in the energy density of lithium-ion batteries. Technical issues

[0004] The present application provides a graphite composite material, which can improve the initial efficiency of the graphite material and improve the rate performance of the battery. Technical Solutions

[0005] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0006] In a first aspect, the present application provides a graphite composite material, comprising a core and a coating layer located on the surface of the core; wherein the material of the core comprises graphite, and the material of the coating layer comprises a nitrogen-doped porous alumina-carbon composite material.

[0007] Optionally, in the graphite composite material, the mass proportion of the coating layer is 2 wt % to 10 wt %.

[0008] Optionally, the graphite includes at least one of spherical natural graphite, flake graphite, and artificial graphite.

[0009] Optionally, the material of the coating layer is further doped with non-metal atoms, and the non-metal atoms include at least one of sulfur and phosphorus.

[0010] In a second aspect, the present application provides a method for preparing a graphite composite material, comprising the following steps:

[0011] Dispersing graphite and aluminum carboxylate in a first organic solvent to obtain solution A;

[0012] dispersing the basic coupling agent in the second organic solvent to obtain solution B;

[0013] The solution A, the solution B, an oxidant and a nitrogen source are mixed, and then subjected to a solvothermal reaction to obtain an intermediate;

[0014] The intermediate is sintered under a protective atmosphere to obtain the graphite composite material.

[0015] Optionally, the graphite includes at least one of spherical natural graphite, flake graphite, and artificial graphite.

[0016] Optionally, the aluminum carboxylate salt includes at least one of aluminum citrate, aluminum stearate, dihydroxyamino aluminum acetate, aluminum acetate, aluminum acrylate, and aluminum oleate.

[0017] Optionally, the first organic solvent comprises polyethylene glycol.

[0018] Optionally, the mass ratio of the aluminum carboxylate salt to the graphite is 5:100 to 15:100.

[0019] Optionally, the mass percentage of the aluminum carboxylate salt to the first organic solvent is 0.5 wt % to 5 wt %.

[0020] Optionally, the basic coupling agent comprises an aluminum-based coupling agent containing an amino functional group.

[0021] Optionally, the alkaline coupling agent includes at least one of 5-aminoaluminum-3-carboxylic acid methyl ester, aluminum diisopropyldithiocarbamate, and dihydroxyaminoaluminum acetate.

[0022] Optionally, the second organic solvent includes at least one of cyclohexane, xylene, and acetone.

[0023] Optionally, the mass percentage of the basic coupling agent and the second organic solvent is 1 wt % to 10 wt %.

[0024] Optionally, the oxidant comprises hydrogen peroxide.

[0025] Optionally, the nitrogen source includes at least one of urea, melamine, pyrrole, and aniline.

[0026] Optionally, the mass ratio of the solution A, the solution B, the oxidant and the nitrogen source is 500:50:1:1 to 500:150:10:10.

[0027] Optionally, the solvothermal reaction is carried out at a pressure of 1 MPa to 5 MPa.

[0028] Optionally, the temperature of the solvent thermal reaction is 50° C. to 150° C., and the time of the solvent thermal reaction is 30 min to 300 min.

[0029] Optionally, the protective atmosphere includes at least one of nitrogen and inert gas.

[0030] Optionally, the heating rate of the sintering process is 1°C / min to 10°C / min.

[0031] Optionally, the sintering temperature is 600° C. to 1200° C., and the sintering time is 1 hour to 6 hours.

[0032] Optionally, gas is introduced during the sintering process, and the gas includes at least one of hydrogen sulfide, phosphine, ammonia, sulfur dioxide, and nitrogen dioxide.

[0033] Optionally, the volume ratio of the gas to the protective atmosphere is 1:10 to 5:10.

[0034] In a third aspect, the present application provides a negative electrode active material, comprising the aforementioned graphite composite material; or, the negative electrode active material comprises the graphite composite material prepared by the aforementioned preparation method.

[0035] In a fourth aspect, the present application provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the negative electrode sheet comprises the aforementioned negative electrode active material.

[0036] In a fifth aspect, the present application provides an electrical device comprising the aforementioned secondary battery. Beneficial effects

[0037] 1. In the graphite composite material provided in this application, the surface of the graphite material is coated with a nitrogen-doped porous alumina-carbon composite material. The porous alumina does not consume lithium ions or form an SEI film during the charge and discharge process, thereby improving the initial efficiency of the graphite material.

[0038] 2. In the graphite composite material provided in the present application, the coating layer material is doped with nitrogen, which gives the coating layer material more active sites, thereby improving the electronic conductivity of the graphite material and improving the rate performance of the battery.

[0039] 3. In the graphite composite material provided in the present application, the coating layer material has a porous structure, which gives the graphite material a high specific surface area, improves the material's ability to absorb and retain liquid, and is beneficial to the transmission of ions.

[0040] 4. In the preparation method of the graphite composite material provided in this application, organic aluminum salt is used as the aluminum source, which has milder reaction conditions than inorganic aluminum salt, and has good coating uniformity. In addition, the organic aluminum salt forms a porous structure after carbonization, which is beneficial to improving the dynamic performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The following detailed description of the specific embodiments of the present application in conjunction with the accompanying drawings will make the technical solutions and other beneficial effects of the present application apparent.

[0042] FIG1 is a scanning electron microscope (SEM) image of the graphite composite material provided in this application.

[0043] Implementation Methods of the Application

[0044] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. The described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without making creative efforts are within the scope of protection of this application.

[0045] In the description of this application, it should be noted that, unless otherwise specified or limited, the term "and / or" herein is merely a description of an association relationship between associated objects, indicating that three possible relationships exist. For example, "A and / or B" can represent: A exists alone, A and B exist simultaneously, and B exists alone. Furthermore, the character " / " herein, unless otherwise specified, generally indicates that the associated objects are in an "or" relationship.

[0046] As described in the background art, when graphite is used as the negative electrode material of lithium-ion batteries, a SEI film is formed on the graphite surface during the first charge and discharge, which consumes some lithium ions, resulting in low initial efficiency of the battery and restricting further improvement of the energy density of lithium-ion batteries.

[0047] To solve this technical problem, the present application provides a graphite composite material that is both highly efficient and has both rate performance and high efficiency. The structure and preparation method of the graphite composite material are described in detail below.

[0048] The graphite composite material provided in the present application has a core-shell structure, comprising a core and a coating layer coated on the surface of the core, wherein the core material comprises graphite, and the coating material comprises a nitrogen-doped porous alumina-carbon composite material.

[0049] In this application, there is no limitation on the type of graphite in the core; for example, the core may be at least one of spherical natural graphite, flake graphite, and artificial graphite. Optionally, the graphite is artificial graphite. This application does not specifically limit the graphite particle size. Generally, the graphite D50 particle size can be controlled to be between 1 μm and 50 μm.

[0050] In the present application, the graphite may be either unmodified graphite or modified graphite, and the present application does not limit this. Modified graphite may be, for example, graphite doped with silicon material, graphite doped with conductive carbon material, and the like.

[0051] In the present application, the aluminum oxide-carbon composite material in the coating layer has a porous structure. The presence of these pores improves the wettability of the electrolyte to the graphite material and the liquid storage and retention capacity of the graphite material.

[0052] In the present application, the coating material is doped with a certain amount of nitrogen atoms. The presence of nitrogen atoms gives the coating material more active sites, and the presence of more active sites is conducive to electron transmission. Therefore, the doping of nitrogen atoms improves the electronic conductivity of the graphite material.

[0053] In the graphite composite material of the present application, the mass proportion of the coating layer needs to be controlled within a certain range. If the mass proportion is too low, it is impossible to achieve complete coating of the graphite particles; if the mass proportion is too high, it will lead to a low proportion of active substances in the graphite composite material, affecting the energy density of the battery. Generally, the mass proportion of the coating layer can be controlled to be between 2wt% and 10wt%, for example, it can be any value among 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt% or a range consisting of any two values ​​therein.

[0054] In some embodiments of the present application, the coating material is further doped with non-metallic atoms, including but not limited to at least one of sulfur and phosphorus. The doped non-metallic atoms can also increase the material's activity, further improving the electronic conductivity of the graphite material.

[0055] The graphite composite material provided in this application has the following advantages:

[0056] (1) Since aluminum oxide is relatively inert to lithium ions and will not react with the electrolyte to form a film layer containing SEI components such as lithium carbonate, coating the surface of the graphite material with an aluminum oxide layer can reduce the consumption of lithium ions caused by the formation of the SEI film during the charge and discharge process of the graphite negative electrode, thereby improving the initial efficiency of the graphite.

[0057] (2) Nitrogen atoms doped in the coating material can enhance the electronic conductivity of the material and reduce the impedance, thereby improving the rate performance of the battery.

[0058] (3) The carbon material in the coating layer is obtained by carbonizing organic compounds. It is an amorphous carbon with a porous structure, which is not only conducive to the transport of ions but also can improve the electronic conductivity of the graphite material.

[0059] (4) The porous structure can increase the specific surface area of ​​the material, which is beneficial to improving the wettability of the electrolyte to the graphite material and improving the liquid absorption and retention capacity of the graphite material; at the same time, the porous structure is conducive to the transmission of ions and is beneficial to improving the dynamic performance of the battery.

[0060] Optionally, the present application provides a method for preparing the above-mentioned graphite composite material, comprising the following steps:

[0061] S1. Dispersing graphite and aluminum carboxylate in a first organic solvent to obtain a solution A;

[0062] S2. dispersing the basic coupling agent in a second organic solvent to obtain a solution B;

[0063] S3. After mixing the solution A, the solution B, the oxidant and the nitrogen source, a solvothermal reaction is performed to obtain an intermediate;

[0064] S4. Sintering the intermediate under a protective atmosphere to obtain the graphite composite material.

[0065] In the above step S1, the graphite can be at least one of spherical natural graphite, flake graphite, and artificial graphite. Optionally, the graphite material is artificial graphite.

[0066] During the subsequent preparation process, aluminum carboxylates form a porous aluminum oxide coating on the graphite surface. This application uses organic aluminum carboxylates as an alternative to inorganic aluminum salts. Compared to inorganic aluminum salts, organic aluminum carboxylates have milder reaction conditions and can form an aluminum oxide precursor coating on the graphite particle surface during the subsequent solvent thermal reaction, resulting in a more uniform coating. Furthermore, organic aluminum carboxylates form a porous structure upon carbonization, which is beneficial for improving the battery's dynamic performance.

[0067] In the present application, the aluminum carboxylate salt includes but is not limited to at least one of aluminum citrate, aluminum stearate, dihydroxyamino aluminum acetate, aluminum acetate, aluminum acrylate, and aluminum oleate.

[0068] In the present application, the mass ratio of the aluminum carboxylate salt to the graphite is preferably 5:100 to 15:100, for example, it can be any value among 5:100, 6:100, 7:100, 8:100, 9:100, 10:100, 11:100, 12:100, 13:100, 14:100, 15:100, or a range consisting of any two values ​​thereof. If the mass ratio of the aluminum carboxylate salt is too low, a complete coating layer cannot be formed on the graphite surface; if the mass ratio of the aluminum carboxylate salt is too high, the coating layer formed will be too thick, which will reduce the proportion of active material in the graphite composite material and affect the energy density of the battery.

[0069] In the present application, the first organic solvent is mainly used to dissolve the aluminum carboxylate and disperse the graphite. Optionally, the first organic solvent is a solvent that is difficult to volatilize, so that the first organic solvent will not volatilize during subsequent drying and calcining, but will carbonize to form a porous carbon material during the calcining process, thereby improving the conductivity of the graphite composite material. Alternatively, in some embodiments, the first organic solvent is polyethylene glycol (PEG), which has good mutual solubility with other organic solvents and is not easy to volatilize.

[0070] In the present application, the mass percentage of the aluminum carboxylate salt and the first organic solvent is preferably 0.5 wt % to 5 wt %, for example, it can be any value among 0.5 wt %, 1 wt %, 1.5 wt %, 2 wt %, 2.5 wt %, 3 wt %, 3.5 wt %, 4 wt %, 4.5 wt %, 5 wt % or a range consisting of any two values ​​therein.

[0071] In the present application, when preparing solution A, the order of adding graphite and aluminum carboxylate salt is not limited. For example, the aluminum carboxylate salt can be first dissolved in the first organic solvent, and then the graphite is added and evenly dispersed. Alternatively, the graphite can be added first and evenly dispersed, and then the aluminum carboxylate salt is added.

[0072] In step S2 of the present application, the alkaline coupling agent has the function of improving the dispersibility of graphite in the solvent, thereby improving the uniformity of coating in the subsequent preparation process; and the function of the alkaline coupling agent is to react with the aluminum carboxylate to produce an aluminum oxide precursor.

[0073] Alternatively, the alkaline coupling agent is an aluminum-based coupling agent, and optionally, the aluminum-based coupling agent is an amino-containing aluminum-based coupling agent. The amino group in the amino-containing aluminum-based coupling agent will be doped into the coating layer in the form of a nitrogen atom during the subsequent preparation process, which is beneficial to improving the electronic conductivity of the graphite composite material. In some embodiments, the alkaline coupling agent includes but is not limited to at least one of 5-aminoaluminum-3-carboxylate, aluminum diisopropyldithiocarbamate, and dihydroxyaminoaluminum acetate.

[0074] In the present application, the second organic solvent needs to be able to dissolve the alkaline coupling agent and have good mutual solubility with the first organic solvent. The second organic solvent that meets the requirements includes but is not limited to at least one of cyclohexane, xylene, and acetone.

[0075] In the present application, the mass percentage of the alkaline coupling agent and the second organic solvent is preferably 1 wt % to 10 wt %, for example, it can be any value among 1 wt %, 2 wt %, 3 wt %, 4 wt %, 5 wt %, 6 wt %, 7 wt %, 8 wt %, 9 wt %, 10 wt % or a range consisting of any two values ​​therein.

[0076] It should be pointed out that the present application separates the aluminum carboxylate salt and the alkaline coupling agent and prepares them into solution A and solution B respectively. The reason is that these two components are acidic and alkaline, respectively. If they are mixed together to prepare a solution, an acid-base reaction will occur. Therefore, they need to be dissolved separately to prepare a solution, which is also conducive to uniform dispersion of graphite.

[0077] In step S3 of the present application, after solution A, solution B, oxidant and nitrogen source are evenly mixed, an alumina precursor is formed through a solvent thermal reaction and coated on the surface of the graphite particles; at the same time, the nitrogen source forms nitrogen free radicals in the reaction and is doped into the alumina precursor; and the presence of the nitrogen source can increase the pore structure inside the coating layer, which is beneficial to improving the specific surface area and liquid absorption capacity of the material.

[0078] The oxidant is used to catalyze the acid-base reaction between the aluminum carboxylate salt and the alkaline coupling agent. The oxidant can be any commonly used oxidant in the art, and optionally, hydrogen peroxide. When hydrogen peroxide is used as the oxidant, it is reduced during the reaction, and the resulting product is water, thus preventing the introduction of impurities into the system.

[0079] In the present application, the nitrogen source may be selected from nitrogen-containing organic substances commonly used in the art, including but not limited to at least one of urea, melamine, pyrrole, and aniline.

[0080] In the present application, the mass ratio of solution A, solution B, oxidant and nitrogen source is preferably 500:50:1:1 to 500:150:10:10. This ratio can ensure that a complete coating layer with appropriate thickness is formed on the surface of the graphite particles.

[0081] In the present application, the solvent thermal reaction needs to be carried out in a high-pressure reaction vessel (e.g., a high-pressure reactor). Optionally, the reaction pressure is 1 MPa to 5 MPa, for example, it can be any value among 1 MPa, 2 MPa, 3 MPa, 4 MPa, 5 MPa, or a range consisting of any two values ​​therein.

[0082] Optionally, the solvothermal reaction temperature is 50° C. to 150° C., for example, any value selected from 50° C., 60° C., 70° C., 80° C., 90° C., 100° C., 110° C., 120° C., 130° C., 140° C., and 150° C., or a range consisting of any two thereof. The solvothermal reaction time is 30 min to 300 min, for example, any value selected from 30 min, 50 min, 80 min, 100 min, 120 min, 150 min, 180 min, 200 min, 220 min, 250 min, 270 min, and 300 min, or a range consisting of any two thereof.

[0083] After the solvent thermal reaction is completed, the reaction product is collected and dried to obtain an intermediate. In some embodiments of the present application, the product can be collected by filtering the reaction solution, centrifuging, etc. The collected product is then dried to remove the residual organic solvent (mainly the second organic solvent) for standby use. The drying method includes but is not limited to vacuum drying.

[0084] In step S4 of the present application, the intermediate is subjected to a high-temperature sintering treatment. During the sintering process, the alumina precursor on the surface of the graphite particles forms porous alumina, and the organic matter in the intermediate (including the first organic solvent, the aluminum carboxylate salt and the organic matter in the alkaline coupling agent) is carbonized to form a carbon material, and finally the graphite surface is coated with a nitrogen-doped porous alumina-carbon composite material.

[0085] The sintering reaction is carried out in a sintering furnace (e.g., a tubular furnace) under a protective atmosphere. Specifically, the intermediate is transferred to the tubular furnace, where a protective gas is first introduced to expel air from the tubes. The temperature is then raised to allow the sintering reaction to proceed. During the sintering process, the protective gas is continuously introduced into the tubes. The protective gas may include one or more of nitrogen and an inert gas, with argon being an option.

[0086] In the present application, the heating rate of the tube furnace is 1°C / min to 10°C / min, for example, it can be any value of 1°C / min, 2°C / min, 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min or a range consisting of any two values ​​therein.

[0087] In the present application, the temperature of the sintering reaction is 600°C to 1200°C, for example, it can be any value of 600°C, 650°C, 700°C, 750°C, 800°C, 850°C, 900°C, 950°C, 1000°C, 1050°C, 1100°C, 1150°C, 1200°C, or a range consisting of any two values ​​therein.

[0088] In the present application, the sintering reaction time is 1 h to 6 h, for example, it can be any value among 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, or a range consisting of any two values ​​therein.

[0089] Optionally, in some embodiments, a gas may be introduced during the sintering process. The gas may include, but is not limited to, at least one of hydrogen sulfide, phosphine, ammonia, sulfur dioxide, and nitrogen dioxide. During the sintering process, non-metallic atoms (N, S, P) in the gas may be doped into the coating layer, further improving the electronic conductivity of the material and reducing impedance. Furthermore, the introduced gas may easily form a porous structure on the surface of the material, thereby increasing the specific surface area of ​​the material.

[0090] In the present application, the gas can be mixed with the protective gas and introduced into the tube furnace together. In the mixed gas, optionally, the volume ratio of the gas is 1:10 to 5:10, for example, it can be any value of 1:10, 2:10, 3:10, 4:10, 5:10 or a range consisting of any two values ​​therein.

[0091] The present application also provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, wherein the separator is configured to separate the positive electrode sheet from the negative electrode sheet, wherein the negative electrode active material of the negative electrode sheet comprises at least the aforementioned graphite composite material.

[0092] The present application also provides an electrical device comprising the aforementioned secondary battery, and using the secondary battery as a power source for the electrical device. In some embodiments, the electrical device of the present application includes, but is not limited to, backup power supplies, motors, electric vehicles, electric motorcycles, power-assisted bicycles, bicycles, power tools, and large household batteries.

[0093] The present application is further described below with reference to the following examples and comparative examples, which are not to be construed as limiting the technical solutions of the present application.

[0094] Example 1

[0095] S1. 10 g of aluminum citrate was added to 1000 g of polyethylene glycol to prepare a 1 wt % homogeneous solution, followed by addition of 100 g of artificial graphite and uniform dispersion to obtain a solution A.

[0096] S2 5g of 5-aminoaluminum-3-carboxylate was added to 100g of cyclohexane organic solvent and dispersed uniformly to obtain a 5wt% solution B;

[0097] S3. Take 500g of solution A and 100g of solution B, add to the autoclave, and add 5g of hydrogen peroxide (concentration 30%), 5g of melamine, disperse evenly, and react at a temperature of 100 ° C and a pressure of 3 MPa for 90min, filter, and dry in vacuo at 80 ° C for 24h to obtain an intermediate;

[0098] S4. The intermediate was transferred to a tubular furnace. Argon was first introduced to expel the air from the tube. Then, the temperature was raised to 800°C at a rate of 5°C / min under a mixed gas (ammonia and argon, Vammonia:VArgon = 3:10, flow rate of 100 ml / min) and sintered for 3 h to obtain a graphite composite material.

[0099] Example 2

[0100] S1. 5 g of aluminum stearate was added to 1000 g of polyethylene glycol to prepare a 0.5 wt % homogeneous solution, followed by addition of 100 g of artificial graphite and uniform dispersion to obtain a solution A;

[0101] S2. 1 g of aluminum diisopropyldithiocarbamate was added to 100 g of xylene organic solvent and dispersed uniformly to obtain a 1 wt % solution B;

[0102] S3. Take 500g of solution A and 50g of solution B, add to the autoclave, and add 1g of hydrogen peroxide (concentration 30%), 1g of pyrrole, disperse evenly, and react at a temperature of 50 ° C and a pressure of 5 MPa for 300min, filter, and dry in vacuo at 80 ° C for 24h to obtain an intermediate;

[0103] S4. The intermediate was transferred to a tubular furnace. Argon was first introduced to expel the air from the tube. Then, the temperature was raised to 600°C at a rate of 1°C / min under a mixed gas (phosphine and argon, Vphosphine:Vargon = 1:10, flow rate of 100 ml / min) and sintered for 6 h to obtain a graphite composite material.

[0104] Example 3

[0105] S1. 15 g of aluminum acetate was added to 1000 g of polyethylene glycol to prepare a 0.15 wt % homogeneous solution, followed by addition of 100 g of artificial graphite and uniform dispersion to obtain a solution A.

[0106] S2. 10 g of dihydroxyaluminum aminoacetate was added to 100 g of acetone organic solvent and dispersed uniformly to obtain a 10 wt % solution B;

[0107] S3. Take 500g of solution A and 150g of solution B, add to the autoclave, and add 10g of hydrogen peroxide (concentration 30%), 10g of urea, disperse evenly, and react at a temperature of 150°C and a pressure of 1 MPa for 30min, filter, and dry in vacuo at 80°C for 24h to obtain an intermediate;

[0108] S4. The intermediate was transferred to a tubular furnace. Argon was first introduced to expel the air from the tube. Then, the temperature was raised to 1200°C at a rate of 10°C / min in a mixed gas atmosphere (sulfur dioxide and argon, V sulfur dioxide:V argon = 5:10, flow rate 100 ml / min) and sintered for 1 h to obtain a graphite composite material.

[0109] Comparative Example 1

[0110] S1. 100 g of aluminum chloride was added to 500 g of deionized water, followed by 3 g of methyl 5-aminoaluminum-3-carboxylate and 1000 g of artificial graphite, and the mixture was mixed uniformly; 30 g of aqueous ammonia was then added, and the mixture was transferred to an autoclave, reacted at a temperature of 100 ° C and a pressure of 3 MPa for 60 min, filtered, and dried in vacuo at 80 ° C for 24 h to obtain a precursor material;

[0111] S2. Transfer the precursor material to a tubular furnace. First, introduce argon to expel the air in the tube. Then introduce a mixed gas (consisting of ammonia and argon, Vammonia:VArgon = 3:10, flow rate 10 ml / min). Raise the temperature to 800°C at a heating rate of 5°C / min and sinter for 3 h to obtain a graphite composite material.

[0112] Comparative Example 2

[0113] The difference between Comparative Example 2 and Example 1 is that in step S2, when preparing solution B, 5-aminoaluminum-3-carboxylic acid methyl ester is not added.

[0114] Comparative Example 3

[0115] The difference between Comparative Example 3 and Comparative Example 1 is that in step S4, during the sintering process, the gas introduced into the tube furnace is pure argon, and no ammonia is introduced.

[0116] Comparative Example 4

[0117] The difference between Comparative Example 4 and Example 1 is that no melamine nitrogen source is added in step S3.

[0118] Performance Testing

[0119] 1.SEM testing

[0120] The graphite composite material prepared in Example 1 was subjected to SEM testing, and the test results are shown in FIG1 .

[0121] As can be seen from FIG1 , the graphite composite material prepared in Example 1 is in a granular form, has a small amount of porous structure on the surface, and has a particle size D50 ranging from 8 μm to 15 μm.

[0122] 2.Physical and chemical properties and button battery testing

[0123] 2.1 Physical and chemical performance test

[0124] The OI value, specific surface area, and tap density of the graphite composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 4 were measured according to the methods specified in GBT-24533-2019, "Graphite Anode Materials for Lithium-Ion Batteries." The test results are shown in Table 1.

[0125] 2.2 Snap-on assembly

[0126] The graphite composite materials in Examples 1 to 3 and Comparative Examples 1 to 4 were used as negative electrode materials for lithium-ion batteries and assembled into button batteries. The specific preparation method of the negative electrode materials was as follows:

[0127] The negative electrode sheet was prepared by mixing graphite composite, CMC, SBR, SP, and H2O in a mass ratio of 95:2.5:1.5:1:150. A lithium sheet served as the positive electrode, and the electrolyte used was LiPF6 / EC+DEC (LiPF6 as the electrolyte, a mixture of EC and DEC in a 1:1 volume ratio as the solvent, and an electrolyte concentration of 1.3 mol / L). The separator was a composite film of polyethylene (PE), polypropylene (PP), and polyethylene (PEP). The button cell was assembled in an argon-filled glove box.

[0128] 2.3 Power-off performance test

[0129] The electrochemical performance of the button cell was tested on a Wuhan Blue Power CT2001A battery tester with a charge and discharge voltage range of 0.005 V to 2.0 V and a charge and discharge rate of 0.1 C. The first discharge capacity and first efficiency of the button cell, as well as the rate performance (2 C / 0.1 C) were tested. The test results are shown in Table 1.

[0130] Table 1 Test results of the physicochemical properties and electrical conductivity of the graphite composite materials in the examples and comparative examples.

[0131] Please refer to Table 1. In Example 1, since the graphite surface is coated with a porous alumina-carbon coating layer, the porous structure makes the specific surface area of ​​the graphite material larger, reaching 2.5m 2 / g; Due to the coating of alumina layer, the tap density of the material is improved to 1.19g / cm 3; and the OI value of the powder is low, only 2.4, which is conducive to ion diffusion and rate performance. In addition, the presence of the coating layer can improve the initial efficiency and capacity of graphite and improve the rate performance of the battery. In this embodiment, the first discharge capacity of the deduction reached 361.2mAh / g, the first efficiency reached 96.2%, and the rate performance at 2C / 0.1C reached 92.1%.

[0132] Examples 2 and 3 showed similar results to Example 1. The first discharge capacity of the battery was above 359 mAh / g, the first efficiency was above 95.5%, and the rate performance at 2C / 0.1C was above 91%, demonstrating excellent electrochemical performance.

[0133] In Comparative Example 1, the aluminum salt used is an inorganic aluminum salt aluminum chloride. Aluminum chloride and ammonia water react under high temperature and high pressure to form aluminum hydroxide coated on the surface of graphite, which is then sintered to obtain an aluminum oxide coating. The surface of the coating does not have a porous structure. Therefore, compared with Comparative Example 1, the specific surface area of ​​the graphite material has decreased significantly to 2.0 m 2 / g; and the powder OI value has increased significantly, reaching 3.7. In terms of electrochemical performance, the graphite composite material of Comparative Example 1 has no porous structure on the surface, which affects its ability to absorb and retain liquid, reducing the rate performance of the battery. It also has a certain impact on the first efficiency and capacity of the battery. From the results, the first discharge capacity of the battery is 355.5mAh / g, the first efficiency is 94.4%, and the rate performance at 2C / 0.1C is only 87.6%.

[0134] During the preparation of the graphite composite material, the coupling agent can improve the dispersibility of the graphite, which is conducive to the formation of a more uniform coating layer. At the same time, the nitrogen atoms contained in the coupling agent can improve the electronic conductivity of the material. In Comparative Example 2, no coupling agent was added to the graphite composite material during the preparation process, so a uniform coating layer could not be formed on the graphite surface, which resulted in a significant decrease in the first effect, capacity and rate performance of the graphite composite material of Comparative Example 2. The first discharge capacity of the deduction was 352.3mAh / g, the first efficiency was 92.2%, and the rate performance at 2C / 0.1C was only 84.7%.

[0135] Compared with Example 1, Comparative Example 3 did not introduce ammonia, resulting in poor electronic conductivity of the material shell and reduced rate performance, with a rate performance of only 78.5% at 2C / 0.1C. Comparative Example 4 did not add a melamine nitrogen source, resulting in poor electronic conductivity of the material core and fewer pores in the core, reducing its specific surface area and, consequently, its rate performance, with a rate performance of only 85.3% at 2C / 0.1C.

[0136] 3. Soft pack battery test

[0137] 3.1 Soft-pack battery preparation

[0138] The graphite composite materials in Examples 1 to 3 and Comparative Examples 1 to 4 were slurried and coated to prepare negative electrode sheets, NCM622 was used as the positive electrode, LiPF6 / EC+DEC+PC was used as the electrolyte (the solute was LiPF6, the concentration was 1.2 mol / L, the solvent volume ratio EC:DEC:PC=1:1:1), and Celgard2400 membrane was used as the separator to prepare a 5Ah soft-pack battery.

[0139] 3.2 Electrochemical performance test

[0140] Liquid absorption capacity of the negative electrode: Use a 1mL burette and draw VmL of electrolyte, add one drop on the surface of the above negative electrode, and time until the electrolyte is completely absorbed. Record the time t and calculate the liquid absorption rate V / t of the electrode.

[0141] Cycle performance: charge and discharge current 1.0C / 1.0C, voltage range 2.8V to 4.2V, cycle number 500 times.

[0142] Rate performance: constant current ratio under 2C charging conditions, that is, constant current capacity / (constant current capacity + constant voltage capacity).

[0143] The above test results are shown in Table 2.

[0144] Table 2 Performance test results of soft pack batteries

[0145] Please refer to Table 2. In Examples 1 to 3, since the coating layer on the surface of the graphite has a porous structure, the specific surface area of ​​the graphite is increased, and the negative electrode sheet exhibits a very high liquid absorption rate. At the same time, the soft-pack batteries of Examples 1 to 3 also have good cycle performance, and the cycle retention rate after 500 cycles is above 94.5%. In addition, since nitrogen doping is carried out by introducing ammonia gas during the sintering process, the active sites of the material are increased, the electronic conductivity of the material is improved, the impedance is reduced, and ultimately the constant current ratio of the graphite composite material is improved. Therefore, the 2C constant current ratio of the soft-pack batteries of Examples 1 to 3 is above 93.5%.

[0146] In Comparative Example 1, the graphite coating lacks a porous structure, resulting in a very slow liquid absorption rate for the negative electrode. This low absorption rate also affects the battery's cycling performance. After 500 cycles, the soft-pack battery in Comparative Example 1 achieved a cycle retention rate of only 87.9% and a 2C constant current ratio of only 89.3%.

[0147] In Comparative Example 2, the absence of a coupling agent during the preparation of the graphite composite material compromised the dispersion of the graphite, ultimately affecting the coating effect. The results show that compared to Examples 1 to 3, the cycle retention rate and 2C constant current ratio of the soft-pack battery in Comparative Example 2 declined somewhat. After 500 cycles, the cycle retention rate was only 89.3%, and the 2C constant current ratio was only 88.2%.

[0148] Comparative Example 3 did not introduce ammonia during the sintering process, and Comparative Example 4 did not add a nitrogen source during the solvothermal reaction. This not only resulted in fewer pores on the surface and in the core of the material, affecting the material's specific surface area and liquid absorption rate, but also caused the nitrogen doping concentration in the coating layer to decrease, affecting the material's electronic conductivity, increasing the battery impedance, and ultimately reducing the battery's rate performance and cycling performance. The results show that the soft-pack batteries of Comparative Examples 3 and 4 achieved cycle retention rates below 88% and 2C constant current ratios below 87% after 500 cycles, which are significant decreases compared to Examples 1 to 3.

[0149] In summary, although the present application has been disclosed as above with optional embodiments, the above optional embodiments are not intended to limit the present application. Ordinary technicians in this field can make various changes and modifications without departing from the spirit and scope of the present application. Therefore, the scope of protection of the present application shall be based on the scope defined by the claims.

Claims

1. A graphite composite material, wherein: It comprises an inner core and a coating layer located on the surface of the inner core; wherein the material of the inner core comprises graphite, and the material of the coating layer comprises a nitrogen-doped porous alumina-carbon composite material.

2. The graphite composite material according to claim 1, wherein In the graphite composite material, the coating layer accounts for 2wt% to 10wt% by weight.

3. The graphite composite material according to claim 1, wherein The graphite includes at least one of spherical natural graphite, flake graphite, and artificial graphite.

4. The graphite composite material according to claim 1, wherein The material of the coating layer is also doped with non-metal atoms, and the non-metal atoms include at least one of sulfur and phosphorus.

5. A method for preparing a graphite composite material, wherein: The following steps are involved: Dispersing graphite and aluminum carboxylate in a first organic solvent to obtain solution A; dispersing a basic coupling agent in a second organic solvent to obtain a solution B; The solution A, the solution B, the oxidant and the nitrogen source are mixed, and then subjected to a solvothermal reaction to obtain an intermediate; The intermediate is sintered under a protective atmosphere to obtain the graphite composite material.

6. The method for preparing the graphite composite material according to claim 5, wherein: The graphite includes at least one of spherical natural graphite, flake graphite, and artificial graphite; and / or, The aluminum carboxylate salt comprises at least one of aluminum citrate, aluminum stearate, dihydroxyamino aluminum acetate, aluminum acetate, aluminum acrylate, and aluminum oleate; and / or, The first organic solvent includes polyethylene glycol.

7. The method for preparing the graphite composite material according to claim 5, wherein: The mass ratio of the aluminum carboxylate salt to the graphite is 5:100 to 15:

100.

8. The method for preparing the graphite composite material according to claim 5, wherein: The mass percentage of the aluminum carboxylate salt and the first organic solvent is 0.5 wt % to 5 wt %.

9. The method for preparing the graphite composite material according to claim 5, wherein: The alkaline coupling agent includes an aluminum-based coupling agent containing an amino functional group; and / or, The alkaline coupling agent includes at least one of 5-aminoaluminum-3-carboxylic acid methyl ester, aluminum diisopropyldithiocarbamate, and dihydroxyaminoaluminum acetate; and / or, The second organic solvent includes at least one of cyclohexane, xylene and acetone.

10. The method for preparing the graphite composite material according to claim 5, wherein: The mass percentage of the basic coupling agent and the second organic solvent is 1 wt % to 10 wt %.

11. The method for preparing a graphite composite material according to claim 5, wherein: The oxidant comprises hydrogen peroxide; and / or, The nitrogen source includes at least one of urea, melamine, pyrrole and aniline; and / or, The mass ratio of the solution A, the solution B, the oxidant and the nitrogen source is 500:50:1:1 to 500:150:10:10; and / or, The solvothermal reaction is carried out at a pressure of 1 MPa to 5 MPa; and / or, The temperature of the solvent thermal reaction is 50° C. to 150° C., and the time of the solvent thermal reaction is 30 min to 300 min.

12. The method for preparing a graphite composite material according to claim 5, wherein: The protective atmosphere comprises at least one of nitrogen and an inert gas; and / or, The heating rate of the sintering process is 1°C / min to 10°C / min; and / or, The sintering temperature is 600° C. to 1200° C., and the sintering time is 1 hour to 6 hours; and / or, During the sintering process, a gas is introduced, wherein the gas includes at least one of hydrogen sulfide, phosphine, ammonia, sulfur dioxide and nitrogen dioxide; and / or, The volume ratio of the gas to the protective atmosphere is 1:10 to 5:

10.

13. A negative electrode active material, wherein: The invention comprises the graphite composite material as claimed in claim 1; or comprises the graphite composite material prepared by the preparation method as claimed in any one of claims 5.

14. A secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein: The negative electrode sheet includes the negative electrode active material as claimed in claim 13.

15. An electrical device, wherein: Comprising the secondary battery as claimed in claim 14.

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