Graphite anode material with high conductivity and remote disordered carbon coating and its preparation and application

Abstract

The present invention belongs to the technical field of negative electrode materials for lithium secondary batteries, and specifically relates to a method for preparing a graphite negative electrode material with high conductivity and remote disordered carbon coating, comprising the following steps: step (1): pre-treating a graphite raw material in an atmosphere containing 10-40v% HF to obtain pre-treated graphite; step (2): mixing the pre-treated graphite, a resin and a transition metal source and performing a two-stage heat treatment; wherein the temperature of the first stage heat treatment is 450-600°C; the second stage heat treatment is performed under negative pressure conditions; and the temperature is 800-1250°C; step (3): pickling, washing and drying the product obtained in step (2) to obtain the graphite negative electrode material. The present invention also relates to a negative electrode obtained by the preparation method and its application in a lithium secondary battery. The present invention can improve the electrochemical properties of the obtained material through gas-solid pretreatment of HF vapor, two-stage heat treatment and joint control of parameters.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium battery electrode materials, and in particular relates to a lithium ion battery negative electrode material based on natural graphite and a preparation method thereof. Background Art

[0002] Lithium-ion batteries are typical representatives of new green rechargeable and dischargeable battery systems, and because of their excellent performance, they are widely used in many fields. The comprehensive performance of lithium-ion battery negative electrode materials is the key to achieving long cycle life and high energy density. Graphite materials are the key to the commercialization of lithium-ion batteries due to their advantages such as low lithium desorption potential, high theoretical lithium insertion capacity, good cycle stability, and good reversibility. However, the current commercial lithium-ion battery negative electrode materials are still mainly graphite, especially artificial graphite. The current artificial graphite is generally modified by asphalt-coated coke (petroleum coke, needle coke, etc.), and needs to undergo a high-temperature graphitization process above 2700°C, and the production process is energy-intensive. The output of natural graphite mines is abundant, but the current utilization rate in China is not high. The difficulty lies in the fact that the purification process of natural graphite has high equipment requirements, and the surface of natural graphite is rich in active groups, which leads to side reactions with the electrolyte and lithium source loss; in addition, the anisotropy of natural graphite needs to be optimized through special processes to improve its performance. Therefore, it is urgent to develop a new natural graphite negative electrode material and its preparation method to realize a new process for preparing high-performance graphite negative electrode materials under low energy consumption conditions. Summary of the invention

[0003] In view of the shortcomings of the prior art, a method for preparing a graphite negative electrode material with high conductivity and remote disordered carbon coating is provided, aiming to improve the capacity, rate and cycle stability of the negative electrode material.

[0004] The second object of the present invention is to provide a graphite negative electrode material with high conductivity and remote disordered carbon coating obtained by the preparation method.

[0005] The third object of the present invention is to provide the use of the graphite negative electrode material with high conductivity and remote disordered carbon coating obtained by the preparation method in a lithium secondary battery, and the prepared lithium secondary battery.

[0006] A method for preparing a graphite negative electrode material with high conductivity and remote disordered carbon coating, comprising the following steps:

[0007] Step (1): pretreating a graphite raw material in an atmosphere containing 10 to 40% by volume of HF to obtain pretreated graphite;

[0008] Step (2): pretreated graphite, resin and transition metal source are mixed and subjected to two-stage heat treatment; wherein the temperature of the first stage heat treatment is 450-600° C.; the second stage heat treatment is carried out under negative pressure and the temperature is 800-1250° C.;

[0009] Step (3): acid-washing, water-washing and drying the product obtained in step (2) to obtain the graphite negative electrode material.

[0010] In the prior art, natural graphite is usually pretreated by an acid-base method or a high-temperature heat treatment method, which has high energy consumption and uses a large amount of acid, alkali and water. In addition, since the microstructure of natural graphite is very regular and the interlayer spacing is small, it is difficult to completely remove the impurity elements in its structure, resulting in low purification efficiency and high purification cost. In addition, in order to improve the anisotropy of graphite, asphalt is often used for coating treatment, and the carbon layer obtained by the subsequent low-temperature heat treatment is highly amorphous, resulting in low initial Coulomb efficiency of the graphite material. To this end, the present invention provides the above-mentioned preparation method, which innovatively adopts HF atmosphere and its parameter conditions for gas-solid pretreatment, so that layer expansion and intercalation modification can be achieved, which is beneficial to repair structural defects, reduce abnormal active sites on the surface, and is more beneficial to the removal of silicon-containing impurities; more importantly, the product of the HF gas-solid pretreatment and the subsequent resin and transition metal source are subjected to the two-stage heat treatment together, and based on the combination of the two-stage heat treatment process, the structure of the gas-solid pretreated graphite can be maintained, and the conformal coating of the highly conductive carbon layer on its surface can be achieved, and a thin carbon layer with stable structure, close contact and controllable moderate graphitization can be formed, and the isotropic coating of the carbon layer on the surface of the pretreated graphite core can be achieved, so that a natural graphite negative electrode active material with excellent first-cycle coulomb efficiency, high cycle stability and rate characteristics can be unexpectedly obtained.

[0011] In the present invention, the graphite raw material may be any graphite raw material, for example, it may be at least one of natural graphite, recycled graphite waste, graphite crucible scraps, and the like.

[0012] The graphite raw material may be in natural form, or may be a graphite material obtained by pre-pelletizing treatment based on conventional means.

[0013] Preferably, the particle size of the graphite raw material is 5-24 μm;

[0014] Preferably, the median particle size D50 of the graphite raw material is 12-17 μm.

[0015] In the present invention, the combination of the HF atmosphere pretreatment and the two-stage heat treatment process and the joint control of the treatment parameters are the key to synergistically improving the electrochemical properties of the obtained negative electrode active material.

[0016] In the present invention, the HF-containing atmosphere is further allowed to contain at least one gas selected from water vapor, nitrogen, an inert gas, and hydrogen, preferably water vapor;

[0017] Preferably, the content of HF gas in the atmosphere is 30-40v%.

[0018] In the present invention, the pressure of the HF-containing atmosphere is 1.5 to 5 MPa; the temperature is preferably greater than or equal to 100° C., preferably 100 to 200° C.;

[0019] Preferably, the pretreatment time is 2 to 6 hours.

[0020] In the present invention, in step (1), the HF-containing atmosphere is introduced into the graphite raw material for pretreatment;

[0021] Alternatively, the graphite raw material and HF solution are placed in a closed container in a dry and wet partition, and the container is heated to vaporize the HF in the HF solution, and the graphite raw material is surrounded by the vaporized vapor for pretreatment. The dry and wet partition refers to that the graphite raw material and the HF solution are arranged in the same chamber, and the graphite is not in direct contact with the solution. For example, the solution can be set at the bottom of the container, and the graphite raw material is set in an area above the liquid surface of the solution, and the atmosphere of the solution and the graphite raw material are connected. In the present invention, there is no special requirement for the concentration of the HF solution, for example, it can be 20-40%. In the present invention, there is no special requirement for the temperature of the treatment process, as long as the HF can be vaporized, for example, it can be 100-200°C.

[0022] In the present invention, the graphite and resin pretreated in the HF atmosphere and the transition metal source are subjected to the two-stage heat treatment, which helps to achieve conformal encapsulation of the gas-phase pretreated graphite, improve the structure of the encapsulation layer, improve its stability, conductivity and anisotropy, and improve the overall electrochemical properties of the material.

[0023] In the present invention, the resin may be any polymer, for example, it may be one or more of phenolic resin, polyethylene, polylactic acid, nylon, and polypropylene.

[0024] Preferably, the transition metal source is a compound of a transition metal element;

[0025] Preferably, the transition metal element is at least one of iron, cobalt and nickel;

[0026] Preferably, the transition metal source is at least one of the oxalate, nitrate and chloride salts of the transition metal;

[0027] Preferably, the mass ratio of the graphite raw material to the resin is 1:0.05-0.2, preferably 1:0.05-0.1;

[0028] Preferably, the mass ratio of the graphite raw material to the transition metal source is 1:0.05-0.2.

[0029] In the present invention, the treated graphite raw material, resin and transition metal source can be mixed and pelletized before a second stage heat treatment. This helps to further improve the synergy of steps (1) and (2), helps to further achieve shape-conserving coating of the treated graphite, and helps to improve the electrochemical performance.

[0030] In the present invention, on the basis of the HF vapor infiltration pretreatment, the two-stage heat treatment process of the resin and the transition metal source is further combined to help achieve the conformal encapsulation of the carbon layer, improve the structure of the carbon layer, the appropriate degree of graphitization, and improve the electrochemical properties of the prepared material.

[0031] In the present invention, the two-stage heat treatment includes a first stage heat treatment performed under a protective atmosphere and a second stage heat treatment performed under negative pressure. The variable pressure two-stage heat treatment process is helpful to further control the structure of the obtained carbon layer and the conformal coating, and is helpful to obtain a high structural stability and highly conductive ultra-thin carbon layer with a localized graphitized structure.

[0032] During the first heat treatment, the protective atmosphere is one or a mixture of argon, helium, hydrogen and nitrogen;

[0033] Preferably, the heating rate of the first heat treatment is 2-5°C / min; the time of the first heat treatment is 2-4 hours;

[0034] Preferably, during the second heat treatment process, the pressure is 10-100 Pa.

[0035] Preferably, the heating rate of the second heat treatment is 5-10°C / min, and the time of the second heat treatment is 0.5-2 hours;

[0036] Preferably, in step (3), the acid solution used for pickling is at least one of hydrochloric acid, sulfuric acid, nitric acid and hydrofluoric acid, and the concentration of the acid solution is preferably 0.1 to 2 M;

[0037] Preferably, washing with water until the filtrate is neutral;

[0038] Preferably, the washed product is dried, and then pulverized, sieved and demagnetized to obtain the graphite negative electrode material.

[0039] The preferred method for preparing the highly conductive remote disordered carbon-coated natural graphite negative electrode material of the present invention comprises the following steps:

[0040] Step (a): performing conventional spheroidization and granulation treatment on natural flake graphite to obtain natural graphite spherical particles with a particle size of 5-24 μm and a median particle size D50 of 12-17 μm;

[0041] Step (b): placing natural graphite spherical particles in hydrogen fluoride vapor and keeping the system airtight; the hydrogen fluoride vapor can be, for example, 10 to 40% by volume hydrogen fluoride-water vapor, the gas pressure in the system is 1.5 to 5 MPa, and the steam treatment time is 2 to 6 hours.

[0042] Step (c): the steam-treated graphite material, resin and transition metal salt are mixed evenly and pressed into pellets, and the pellets are placed in a protective atmosphere for a second-stage heat treatment. The resin can be one or more of phenolic resin, polyethylene, polylactic acid, nylon, polypropylene, etc.; the transition metal salt is an organic or inorganic salt of iron, cobalt, and nickel, preferably one or more of nickel oxalate, cobalt oxalate, iron oxalate, nickel nitrate, cobalt nitrate, and ferric chloride; the mass ratio of the natural graphite material to the resin is 1:0.05-0.2; the mass ratio of the natural graphite material to the metal salt is 1:0.05-0.2; the two-stage heat treatment is to heat the temperature to 450-600°C at a heating rate of 2-5°C / min, keep warm for 2-4 hours, then evacuate the heating system to make the vacuum degree in the system 10-100Pa, continue to heat the temperature to 800-1250°C at a heating rate of 5-10°C / min, and keep warm for 0.5-2 hours; the protective atmosphere is one or more of argon, helium, hydrogen, and nitrogen.

[0043] Step (d): The heat-treated material is subjected to acid washing and purification, washed with pure water until neutral, and dried. The acid washing is carried out at a temperature of 25 to 80° C. using 0.1 to 2M hydrochloric acid, nitric acid, sulfuric acid or other acid washing liquid to purify the obtained material for 2 to 4 hours.

[0044] Step (e): Powdering, screening and demagnetizing the reaction materials to obtain the highly conductive remote disordered carbon-coated natural graphite negative electrode material.

[0045] The present invention also includes the graphite negative electrode material with high conductivity and remote disordered carbon coating obtained by the preparation method.

[0046] The special preparation method described in the present invention can construct a material with a special phase and microstructure, and the material can unexpectedly exhibit excellent capacity, rate and cycle stability.

[0047] The negative electrode material of the present invention comprises a core and an outer shell, wherein the core is high-purity graphite and the outer shell is a highly conductive remote disordered pyrolytic carbon coating layer; the outer shell is anchored on the surface of the core, and the graphite in the core has high graphitization and anisotropy. The present invention has found that the highly conductive remote disordered carbon-coated natural graphite material has the advantages of large reversible capacity, excellent rate performance, high first efficiency and good cycle stability.

[0048] The particle size of the graphite negative electrode material with high conductivity and remote disordered carbon coating is micron-sized, i.e., 5-25 μm, and the median particle size D 50 At 12-18μm.

[0049] The mass content of the core graphite of the highly conductive remote disordered carbon-coated graphite negative electrode material is 90-98%.

[0050] The graphite negative electrode material with high conductivity and remote disordered carbon coating has a graphitization degree of 90-98% and a total specific surface area of ​​1.2-4m 2 / g.

[0051] The invention relates to a graphite negative electrode material with high conductivity and long-range disordered carbon coating. The surface carbon coating layer is a uniform mixture of graphitized structure and amorphous structure carbon material. The thickness of the carbon layer is 8-50nm and the degree of graphitization is 45-60%.

[0052] The present invention also provides an application of the graphite negative electrode material with high conductivity and remote disordered carbon coating obtained by the preparation method, which is used as a negative electrode active material of a lithium secondary battery.

[0053] The preferred application is to use it as a negative electrode active material, and to compound it with a conductive agent and a binder to prepare a negative electrode material. The conductive agent and the binder are all materials known in the industry.

[0054] In a further preferred application, the negative electrode material is placed on the surface of the negative electrode current collector to prepare the negative electrode. The negative electrode material of the present invention can be placed on the current collector by conventional methods, such as coating, to form the negative electrode. The current collector can be any material known in the industry.

[0055] In a further preferred application, the negative electrode, the positive electrode, the separator and the electrolyte are assembled into a lithium secondary battery.

[0056] A lithium secondary battery comprises the graphite negative electrode material with high conductivity and long-range disordered carbon coating obtained by the preparation method.

[0057] The lithium secondary battery, the negative electrode plate comprises the graphite negative electrode material.

[0058] Preferably, the lithium secondary battery is a lithium ion battery.

[0059] The beneficial effects of the technical solution of the present invention are:

[0060] (1) The present invention combines the gas-solid pretreatment of HF vapor with the two-stage heat treatment, and further based on the joint control of parameters, can achieve synergy, can effectively improve the purity of the graphite material and improve the performance of the prepared graphite material, so that during the charge and discharge process, the material has excellent isotropic performance, excellent conductivity, good structural stability, and can simultaneously have high first coulomb efficiency, high specific capacity, high rate charge and discharge characteristics and cycle stability.

[0061] (2) The highly conductive remotely disordered carbon-coated natural graphite negative electrode material has comprehensive electrochemical properties of high capacity, high first coulombic efficiency and good rate characteristics.

[0062] (3) The preparation process is environmentally friendly without high-temperature heat treatment.

[0063] (4) The main raw materials, natural flake graphite, resin and related auxiliary materials, are widely available and low in cost. The processes used, such as spheroidization, steam treatment, ball pressing, solid-liquid separation and heat treatment, are simple and easy to control, and can be easily scaled up for production, with good prospects for practical application. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Attached Figure 1 is a SEM image of the material obtained after hydrogen fluoride vapor treatment in Example 1;

[0065] Attached Figure 2 This is the SEM image of the final sample obtained in Example 1. DETAILED DESCRIPTION

[0066] The specific steps of the present invention are described below by way of examples. It should be understood that these examples are only for illustrating the present invention and are not intended to limit the scope of the present invention in any way. Various processes and methods not described in detail in the present invention are conventional methods known in the art.

[0067] Example 1

[0068] The natural flake graphite is subjected to conventional spheroidization and granulation treatment to obtain natural graphite spherical particles with a particle size of 5-24 μm and a median particle size D50 of 12 μm; the spheroidized natural graphite spherical particles (100 g) are placed in a polytetrafluoroethylene-lined reactor, and steam with a concentration of 20 vol% HF (the balance is water vapor) is blown therein, the steam temperature is 120° C., the pressure in the reactor is maintained at 2 MPa, and the treatment time is 4 h. The obtained powder material, phenolic resin (5 g), and nickel oxalate (5 g) are uniformly mixed and pressed into a mass, and the mass is placed in a protective atmosphere (argon) for a two-stage heat treatment, firstly heated to 500° C. at a heating rate of 5° C. / min, kept warm for 4 hours, and then the heating system is evacuated to make the vacuum degree in the system 50 Pa, and then heated to 800° C. at a heating rate of 10° C. / min, and kept warm for 2 hours. The obtained material was placed in a 2M hydrochloric acid solution with a liquid-to-solid ratio of 5:1, stirred and reacted at 30°C for 4 hours, then filtered and washed (washed with water until neutral, the same below), and the obtained slurry was placed in an oven at 120°C for drying. The dried material was powdered, sieved, and demagnetized to obtain the highly conductive remote disordered carbon-coated natural graphite negative electrode material.

[0069] The physical and chemical results of the material are: graphitization degree is 93%, total specific surface area is 1.5m 2 / g, the fixed carbon content is 99.98%, the thickness of the outer carbon layer is 12nm, and its graphitization degree is 48%.

[0070] According to GB / T 24533-2009, the heterogeneous carbon-coated graphite electrode was used as the working electrode, metal lithium was used as the negative electrode, 1 mol / L LiPF6 EC / EMC (volume ratio 1:1) was used as the electrolyte, and the PE-PP composite film was used as the diaphragm to assemble the CR2025 button cell in a dry glove box filled with argon. The electrochemical performance was tested at room temperature in the voltage range of 0.001-2.0V, and the charge and discharge test current density was 0.2C. The first reversible capacity was recorded to be 362mAh / g, the coulomb efficiency was 96%, and the capacity retention rate after 500 cycles was 99%. Under the condition of rapid charge and discharge at 2C, the reversible specific capacity was 323mAh / g.

[0071] Example 2

[0072] Compared with Example 1, the only difference is that the HF content in the HF vapor is 10 vol%, and the balance is water vapor. The other treatment processes, conditions and electrochemical determination methods are the same as those in Example 1.

[0073] The first reversible capacity at 0.2C is 361mAh / g, the coulombic efficiency is 95.7%, and the capacity retention rate after 500 cycles is 99%. Under 2C conditions, the reversible specific capacity is 322mAh / g.

[0074] Example 3

[0075] Compared with Example 1, the only difference is that the content of HF in the HF vapor is 30 vol%, and the balance is water vapor. The other treatment processes, conditions and electrochemical determination methods are the same as those in Example 1.

[0076] The first reversible capacity at 0.2C is 365mAh / g, the coulombic efficiency is 97%, and the capacity retention rate after 500 cycles is 99%. Under 2C conditions, the reversible specific capacity is 327mAh / g.

[0077] Example 4

[0078] Compared with Example 1, the only difference is that the content of HF in the HF vapor is 40 vol%, and the balance is water vapor. The other treatment processes, conditions and electrochemical determination methods are the same as those in Example 1.

[0079] The first reversible capacity at 0.2C is 368mAh / g, the coulombic efficiency is 97%, and the capacity retention rate after 500 cycles is 99%. Under 2C conditions, the reversible specific capacity is 328mAh / g.

[0080] Example 5

[0081] Compared with Example 1, the only difference is that the temperature of HF vapor is controlled to 150° C. and the pressure in the reactor is maintained at 3 MPa. Other treatment processes, conditions and electrochemical determination methods are the same as those in Example 1.

[0082] The first reversible capacity at 0.2C is 364mAh / g, the coulombic efficiency is 96.5%, and the capacity retention rate after 500 cycles is 99%. Under 2C conditions, the reversible specific capacity is 325mAh / g.

[0083] Example 6

[0084] Compared with Example 1, the only difference is that the temperature of HF vapor is controlled to 180° C. and the pressure in the reactor is maintained at 4 MPa. Other treatment processes, conditions and electrochemical determination methods are the same as those in Example 1.

[0085] The first reversible capacity at 0.2C is 366mAh / g, the coulombic efficiency is 96.8%, and the capacity retention rate after 500 cycles is 99%. Under 2C conditions, the reversible specific capacity is 328mAh / g.

[0086] Example 7

[0087] Compared with Example 1, the only difference is that the temperature of HF vapor is controlled to 200° C. and the pressure in the reactor is maintained at 5 MPa. Other treatment processes, conditions and electrochemical determination methods are the same as those in Example 1.

[0088] The first reversible capacity at 0.2C is 369mAh / g, the coulombic efficiency is 97.2%, and the capacity retention rate after 500 cycles is 99%. Under 2C conditions, the reversible specific capacity is 332mAh / g.

[0089] Example 8

[0090] Compared with Example 1, the only difference is that the amount of nickel oxalate is controlled to be 10 g, the pressure of the second stage calcination is 10 Pa, and the temperature of the second stage calcination is 1000° C. Other treatment processes, conditions and electrochemical determination methods are the same as those in Example 1.

[0091] The physical and chemical results of the material are: graphitization degree is 97%, total specific surface area is 1.6m 2 / g, the fixed carbon content is 99.98%, the thickness of the outer carbon layer is 20nm, and its graphitization degree is 53%.

[0092] The first reversible capacity at 0.2C is 362mAh / g, the coulombic efficiency is 97%, and the capacity retention rate after 500 cycles is 99%. Under 2C conditions, the reversible specific capacity is 329mAh / g.

[0093] Example 9

[0094] Compared with Example 1, the only difference is that the amount of nickel oxalate is controlled to be 20 g, the pressure of the second stage calcination is 100 Pa, and the temperature of the second stage calcination is 1250° C. Other treatment processes, conditions and electrochemical determination methods are the same as those in Example 1.

[0095] The physical and chemical results of the material are: graphitization degree is 96%, total specific surface area is 2.2m 2 / g, the fixed carbon content is 99.99%, the thickness of the outer carbon layer is 21nm, and its graphitization degree is 58%.

[0096] The first reversible capacity at 0.2C is 361mAh / g, the coulombic efficiency is 95%, and the capacity retention rate after 500 cycles is 98%. Under 2C conditions, the reversible specific capacity is 328mAh / g.

[0097] Comparative Example 1:

[0098] Compared with Example 1, the difference is that the HF vapor treatment is not performed, but it is immersed in an HF solution, and the steps are:

[0099] The spherical natural graphite spherical particles (same as in Example 1, 100g) are placed in a polytetrafluoroethylene lined reactor, and a hydrofluoric acid aqueous solution (the graphite is immersed in a hydrofluoric acid aqueous solution, wherein the HF content is the same as in Example 1) is added thereto, the liquid-solid ratio is 3:1, the pressure in the reactor is maintained at normal pressure, and the stirring treatment is carried out for 4 hours. After the obtained powder material is dried, it is mixed evenly with phenolic resin (5g) and nickel oxalate (5g) and pressed into a ball, and the ball is placed in a protective atmosphere (argon) for a two-stage heat treatment, firstly heating to 500°C at a heating rate of 5°C / min, and heat preservation for 4 hours, and then vacuuming the heating system to make the vacuum degree in the system 50Pa, and continuing to heat to 800°C at a heating rate of 10°C / min, and heat preservation for 2 hours. The obtained material is placed in a hydrochloric acid solution with a concentration of 2M, and the liquid-solid ratio is 5:1. After stirring and reacting for 4 hours at 30°C, it is filtered and washed, and the obtained slurry is placed in a 120°C oven for drying. The dry materials can be powdered, screened and demagnetized.

[0100] The first reversible capacity at a charge and discharge test current density of 0.2C is 182mAh / g, the coulombic efficiency is 81%, and the capacity retention rate after 500 cycles is 48%. Under fast charge and discharge conditions at 2C, its reversible specific capacity is 84mAh / g.

[0101] Comparative Example 2:

[0102] Compared with Comparative Example 1, the only difference is that the pretreatment stage is carried out under high pressure, and the steps are:

[0103] The spherical natural graphite spherical particles (same as in Example 1, 100g) are placed in a polytetrafluoroethylene lined reactor, and a hydrofluoric acid aqueous solution (the graphite is immersed in a hydrofluoric acid aqueous solution, wherein the HF content is the same as in Example 1) is added thereto, the liquid-solid ratio is 3:1, and argon is filled into the reactor to make the system pressure 2MPa, and the stirring treatment is carried out for 4h. After the obtained powder material is dried, it is mixed evenly with phenolic resin (5g) and nickel oxalate (5g) and pressed into a ball, and the ball is placed in a protective atmosphere (argon) for a two-stage heat treatment, firstly, the temperature is raised to 500°C at a heating rate of 5°C / min, and the temperature is kept for 4 hours, and then the heating system is evacuated to make the vacuum degree in the system 50Pa, and the temperature is continued to be raised to 800°C at a heating rate of 10°C / min, and the temperature is kept for 2 hours. The obtained material is placed in a 2M hydrochloric acid solution with a liquid-to-solid ratio of 5:1, stirred and reacted at 30°C for 4 hours, then filtered and washed, and the obtained slurry is placed in an oven at 120°C for drying. The dried material can be powdered, sieved, and demagnetized.

[0104] The first reversible capacity at a charge and discharge test current density of 0.2C is 185mAh / g, the coulombic efficiency is 81.5%, and the capacity retention rate after 500 cycles is 49%. Under fast charge and discharge conditions at 2C, its reversible specific capacity is 86mAh / g.

[0105] Comparative Example 3:

[0106] Compared with Example 1, the only difference is that the content of HF vapor is not controlled within the range required by the present invention, for example, the content of hydrogen fluoride vapor is 50v%. Other processing processes, conditions and electrochemical determination methods are the same as those in Example 1.

[0107] The first reversible capacity at 0.2C is 232mAh / g, the coulombic efficiency is 72%, and the capacity retention rate after 500 cycles is 63%. Under 2C conditions, the reversible specific capacity is 123mAh / g.

[0108] Comparative Example 4:

[0109] Compared with Example 1, the only difference is that the content of HF vapor is not controlled within the range required by the present invention, for example, the content of hydrogen fluoride vapor is 5v%. Other processing processes, conditions and electrochemical determination methods are the same as those in Example 1.

[0110] The first reversible capacity at 0.2C is 225mAh / g, the coulombic efficiency is 67%, and the capacity retention rate after 500 cycles is 48%. Under 2C conditions, the reversible specific capacity is 112mAh / g.

[0111] Comparative Example 5:

[0112] Compared with Example 1, the only difference is that nickel oxalate is not added, and the other treatment processes, conditions and electrochemical determination methods are the same as Example 1.

[0113] The first reversible capacity at a charge and discharge test current density of 0.2C is 272mAh / g, the coulombic efficiency is 82%, and the capacity retention rate after 500 cycles is 68%. Under fast charge and discharge conditions at 2C, its reversible specific capacity is 146mAh / g.

[0114] Comparative Example 6:

[0115] Compared with Example 1, the only difference is that no resin material is added, and the other processing processes, conditions and electrochemical measurement methods are the same as Example 1.

[0116] The first reversible capacity at a charge and discharge test current density of 0.2C is 212mAh / g, the coulombic efficiency is 83%, and the capacity retention rate after 500 cycles is 55%. Under fast charge and discharge conditions at 2C, its reversible specific capacity is 126mAh / g.

[0117] Comparative Example 7:

[0118] Compared with Example 1, the only difference is that the heat treatment process does not adopt a two-stage method, and the steps are as follows: the obtained powder material, phenolic resin (5g), and nickel oxalate (5g) are mixed evenly and then pressed into a pellet, and the pellet is placed in a protective atmosphere (argon) for heat treatment, that is, the temperature is increased to 800°C at a heating rate of 10°C / min and kept at this temperature for 6 hours. The other treatment processes, conditions, and electrochemical determination methods are the same as those in Example 1.

[0119] The first reversible capacity at a charge and discharge test current density of 0.2C is 262mAh / g, the coulombic efficiency is 81%, and the capacity retention rate after 500 cycles is 56%. Under fast charge and discharge conditions at 2C, its reversible specific capacity is 146mAh / g.

[0120] Comparative Example 8

[0121] Compared with Example 1, the only difference is that the second heat treatment stage is not controlled under the negative pressure. The other treatment processes, conditions and electrochemical determination methods are the same as those in Example 1.

[0122] The first reversible capacity at a charge and discharge test current density of 0.2C is 328mAh / g, the coulombic efficiency is 87%, and the capacity retention rate after 500 cycles is 72%. Under fast charge and discharge conditions at 2C, its reversible specific capacity is 191mAh / g.

Claims

1. A method for preparing a graphite negative electrode material having high conductivity and remote disordered carbon coating, characterized in that: The following steps are involved: Step (1): pretreating a graphite raw material in an atmosphere containing 10-40 vol% HF to obtain pretreated graphite; Step (2): mixing the pretreated graphite, the resin and the transition metal source and subjecting the mixture to a two-stage heat treatment; wherein the temperature of the first stage heat treatment is 450-600°C; the second stage heat treatment is carried out under negative pressure and at a temperature of 800-1250°C; the mass ratio of the graphite raw material to the resin is 1:0.05-0.2; the mass ratio of the graphite raw material to the transition metal source is 1:0.05-0.2; Step (3): The product obtained in step (2) is acid-washed, water-washed, and dried to obtain the graphite negative electrode material.

2. The method for preparing a graphite negative electrode material having a high conductivity and a long-range disordered carbon coating as claimed in claim 1, characterized in that: The graphite raw material is at least one of natural graphite, recycled graphite waste and graphite crucible scraps.

3. The method for preparing a graphite negative electrode material having a high conductivity and remote disordered carbon coating as claimed in claim 1, characterized in that: The particle size of the graphite raw material is 5-24 μm.

4. The method for preparing a graphite negative electrode material having a high conductivity and a remote disordered carbon coating as claimed in claim 1, characterized in that: The median particle size D50 of the graphite raw material is 12-17 μm.

5. The method for preparing a graphite negative electrode material having high conductivity and remote disordered carbon coating according to claim 1, characterized in that: The HF-containing atmosphere further contains at least one gas selected from water vapor, nitrogen, an inert gas and hydrogen.

6. The method for preparing a graphite negative electrode material having high conductivity and remote disordered carbon coating as claimed in claim 5, characterized in that: In the atmosphere, the content of HF gas is 30-40v%.

7. The method for preparing a graphite negative electrode material having high conductivity and remote disordered carbon coating according to claim 1, characterized in that: The pressure of the HF-containing atmosphere is 1.5-5 MPa; the temperature is greater than or equal to 100°C.

8. The method for preparing a graphite negative electrode material having high conductivity and remote disordered carbon coating as claimed in claim 7, characterized in that: The temperature of the HF-containing atmosphere is 100-200°C.

9. The method for preparing a graphite negative electrode material having high conductivity and remote disordered carbon coating according to claim 1, characterized in that: The pretreatment time is 2~6h.

10. The method for preparing a graphite negative electrode material having high conductivity and remote disordered carbon coating according to claim 1, characterized in that: In step (1), the HF-containing atmosphere is introduced into the graphite raw material for pretreatment; Alternatively, the graphite raw material and the HF solution are placed in a sealed container in a dry and wet manner, and the container is heated to vaporize the HF in the HF solution, and the graphite raw material is surrounded by the vaporized steam for pretreatment.

11. The method for preparing a graphite negative electrode material having high conductivity and remote disordered carbon coating according to claim 1, characterized in that: The resin is one or more of phenolic resin, polyethylene, polylactic acid, nylon and polypropylene.

12. The method for preparing a graphite negative electrode material having high conductivity and remote disordered carbon coating according to claim 1, characterized in that: The transition metal source is a compound of transition metal elements.

13. The method for preparing a graphite negative electrode material having high conductivity and remote disordered carbon coating according to claim 12, characterized in that: The transition metal element is at least one of iron, cobalt and nickel.

14. The method for preparing a graphite negative electrode material having high conductivity and remote disordered carbon coating according to claim 13, characterized in that: The transition metal source is at least one of the oxalate, nitrate and chloride salts of the transition metal.

15. The method for preparing a graphite negative electrode material having high conductivity and remote disordered carbon coating according to claim 1, characterized in that: The processed graphite raw material, resin and transition metal source are mixed and pelletized and then subjected to a second stage heat treatment.

16. The method for preparing a graphite negative electrode material having high conductivity and remote disordered carbon coating according to claim 1, characterized in that: The first stage of heat treatment is carried out under a protective atmosphere, wherein the protective atmosphere is one or a mixture of argon, helium, hydrogen and nitrogen.

17. The method for preparing a graphite negative electrode material having high conductivity and remote disordered carbon coating according to claim 1, characterized in that: The heating rate of the first heat treatment is 2-5°C / min; the time of the first heat treatment is 2-4 hours.

18. The method for preparing a graphite negative electrode material having high conductivity and remote disordered carbon coating according to claim 1, characterized in that: The second heat treatment is carried out under negative pressure.

19. The method for preparing a graphite negative electrode material having high conductivity and remote disordered carbon coating according to claim 1, characterized in that: The pressure of the second heat treatment is 10-100Pa.

20. The method for preparing a graphite negative electrode material with high conductivity and remote disordered carbon coating as claimed in claim 1, characterized in that: The heating rate of the second heat treatment is 5-10°C / min, and the time of the second heat treatment is 0.5-2 hours.

21. The method for preparing a graphite negative electrode material having high conductivity and remote disordered carbon coating as claimed in claim 1, characterized in that: In step (3), the acid solution used for pickling is at least one of hydrochloric acid, sulfuric acid, nitric acid, and hydrofluoric acid, and the concentration of the acid solution is 0.1~2M.

22. The method for preparing a graphite negative electrode material having high conductivity and remote disordered carbon coating as claimed in claim 1, characterized in that: In step (3), the filtrate is washed with water until it is neutral.

23. The method for preparing a graphite negative electrode material having high conductivity and remote disordered carbon coating as claimed in claim 1, characterized in that: In step (3), the washed product is dried, and then pulverized, sieved, and demagnetized to obtain the graphite negative electrode material.

24. A graphite negative electrode material having high conductivity and remote disordered carbon coating obtained by the preparation method according to any one of claims 1 to 23.

25. An application of the graphite negative electrode material with high conductivity and remote disordered carbon coating obtained by the preparation method according to any one of claims 1 to 23, characterized in that: It is used as a negative electrode active material for lithium secondary batteries.

26. A lithium secondary battery, characterized in that: It comprises the graphite negative electrode material with high conductivity and remote disordered carbon coating obtained by the preparation method described in any one of claims 1 to 23.

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