A negative electrode material, a preparation method therefor, and an application thereof

Abstract

The application provides a negative electrode material and a preparation method and application thereof, the negative electrode material comprises modified graphite and a coating layer on the surface of the modified graphite; the coating layer comprises graphite and hard carbon; the thickness of the coating layer is 10nm-18nm, the content of the hard carbon in the coating layer is 1wt%-8wt%; and the modified graphite is obtained by sintering purified graphite in a nitrogen atmosphere. The application improves the pH of the purified graphite by sintering in a nitrogen atmosphere, avoids the discharge of acid liquid and does not cause the loss of battery cycle performance; the composite coating layer of graphite and hard carbon is formed on the surface of the modified graphite, the specific surface area of the negative electrode material is reduced, the high-temperature cycle, high-temperature storage performance, large-current charge-discharge performance of the negative electrode material are improved, the expansion rate of the electrode sheet, the expansion rate of the battery cell and the direct-current internal resistance at full charge are reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and relates to a negative electrode material, in particular to a negative electrode material and a preparation method and application thereof. BACKGROUND

[0002] In recent years, with the increasing demand for 3C digital, power battery energy storage equipment, the energy field, especially lithium ion batteries and supercapacitors, has attracted widespread attention. Lithium ion batteries are a kind of pollution-free green batteries developed successfully in the late 20th century, and have the advantages of high average discharge voltage, large volume capacity and mass capacity, long discharge time and light weight compared with traditional batteries.

[0003] The negative electrode material of the lithium ion battery widely used in the industry at present is a graphite material. At present, a large amount of strong acid such as HF, HNO3 and HCl is used to purify the graphite material. Although the purification of the graphite material is successfully realized, some strong acid remains in the purified graphite material, so that the pH is low. The residual strong acid in the graphite material is usually removed by water washing, but this way of pH improvement not only increases the production cost, but also produces a large amount of acid water which is difficult to handle.

[0004] At the same time, the use temperature of the conventional graphite negative electrode material is about room temperature 25℃, and when the use temperature is higher, especially when it is used in high temperature areas such as the equator, the graphite negative electrode material is easy to have a side reaction with the electrolyte in the process of lithium ion insertion and extraction, the electrolyte is quickly consumed, and gas is generated, which makes the lithium battery swell, the capacity rapidly decays, and the safety is greatly reduced. Therefore, if the use temperature range of the lithium ion battery is to be expanded, it is very important to develop a negative electrode material with good high temperature performance.

[0005] CN110817855A discloses a preparation method of modified natural graphite negative electrode material, and belongs to the technical field of lithium ion battery negative electrode material preparation. The preparation method of the application first purifies and oxidizes the natural graphite, separates and filters out the solid material, and then washes and dries it. Then the natural graphite is shaped to obtain spherical graphite powder, and then an organic coating agent is coated on the surface of the spherical graphite powder. After cooling to room temperature, the modified natural graphite is obtained by high temperature carbonization under inert atmosphere protection. However, the preparation method of the modified natural graphite negative electrode material can make the pH reach 5-7 by washing, which will produce a large amount of acid water and does not meet the environmental protection requirements.

[0006] CN107845794A discloses a preparation method of carbon nanotube / urea-formaldehyde resin carbon-coated spherical microcrystalline graphite negative electrode material. The invention uses spherical microcrystalline graphite as the core material, and a first coating layer is formed on the surface by coating carbon nanotubes. Then, a second coating layer is formed by secondary coating with urea-formaldehyde resin. The coated spherical microcrystalline graphite is subjected to carbonization treatment, and after cooling, it is crushed and sieved to obtain carbon nanotube / urea-formaldehyde resin carbon-coated spherical microcrystalline graphite negative electrode material. The invention improves the surface of spherical graphite, forms a uniform coating layer on the surface of graphite, and makes carbon nanotubes wrap around the surface of spherical graphite to form a network structure on the surface of spherical graphite, generating a ball-like structure, enhancing the surface strength, and fully utilizing the excellent performance of carbon nanotubes to improve the conductivity, discharge capacity, anti-aging performance and excellent rate characteristics of the negative electrode material. The first discharge capacity can reach 390 mAh / g. However, the carbon nanotube / urea-formaldehyde resin carbon-coated spherical microcrystalline graphite negative electrode material prepared in the invention is prone to side reactions with electrolyte at high temperature, and has low safety.

[0007] CN114068888A discloses a carbon-coated mixed-size particle graphite negative electrode material and a preparation method thereof. The carbon-coated mixed-size particle graphite negative electrode material comprises artificial graphite and natural graphite. The preparation method comprises natural graphite modification treatment and graphitization, fusion of natural graphite large particles and artificial graphite small particles, and preparation of carbon-coated mixed-size particle graphite negative electrode material by phenolic resin coating. The invention develops a graphite negative electrode material with high capacity and low cost, improves the electrochemical performance of the graphite negative electrode, reduces the cost of the battery, and improves the energy density of the battery. Similarly, the carbon-coated mixed-size particle graphite negative electrode material will react with electrolyte at high temperature, leading to rapid consumption of electrolyte and rapid capacity decay.

[0008] The currently disclosed graphite negative electrode materials have certain defects, and will react with electrolyte at high temperature, leading to rapid consumption of electrolyte and generation of gas, which in turn causes rapid decay of battery capacity and reduction of safety. The preparation method of the graphite negative electrode material also produces a large amount of acid water, which has high treatment cost and does not meet environmental protection requirements. Therefore, it is crucial to develop and design a new type of negative electrode material and its preparation method and application. SUMMARY

[0009] In view of the deficiencies of the prior art, the purpose of the present application is to provide a negative electrode material and a preparation method and application thereof, the present application improves the pH of the purified graphite through sintering in a nitrogen atmosphere, avoids the discharge of acid liquor and does not cause the loss of battery cycle performance; the composite coating layer of graphite and hard carbon is formed on the surface of the modified graphite, the specific surface area of the negative electrode material is reduced, the high-temperature cycle, high-temperature storage performance, large-current charge-discharge performance of the negative electrode material are improved, the expansion rate of the electrode sheet at full capacity, the expansion rate of the battery cell and the direct current internal resistance are reduced.

[0010] To achieve this purpose, the present application adopts the following technical solutions:

[0011] In a first aspect, a negative electrode material comprises modified graphite and a coating layer on the surface of the modified graphite; the coating layer comprises graphite and hard carbon;

[0012] The thickness of the coating layer is 10-18 nm, the content of hard carbon in the coating layer is 1-8 wt%, and the modified graphite is obtained by sintering the purified graphite in a nitrogen atmosphere.

[0013] In the present application, the coating layer with a thickness of 10-18 nm and a content of hard carbon of 1-8 wt% is coated on the surface of the modified graphite, thereby reducing the specific surface area of the negative electrode material, because the particle size of the modified graphite is increased after being coated with the coating layer, and the pores on the surface of the modified graphite particles are blocked by the coating layer, thereby causing the specific surface area to decrease; the reduction of the specific surface area can reduce the active sites on the surface of the modified graphite, thereby reducing the side reaction between the modified graphite and the electrolyte and improving the cycle life.

[0014] In the present application, the preparation method of the modified graphite is to sinter the purified graphite in a nitrogen atmosphere, thereby improving the pH of the purified graphite, avoiding the use of washing to improve the pH, and thereby avoiding the discharge of a large amount of acid liquor, saving production cost and accelerating production efficiency; compared with sintering in an oxygen atmosphere, sintering in a nitrogen atmosphere does not cause the increase of the specific surface area of the spherical graphite, and therefore does not cause the loss of battery cycle performance.

[0015] The coating layer comprises hard carbon and graphite, and the hard carbon coating can protect the graphite layers from directly contacting with the electrolyte, alleviate the side reaction between them, and at the same time, the hard carbon has many pores, which is beneficial to the entry and exit of lithium ions and can improve the kinetic performance; at the same time, when the coating layer is a simple hard carbon layer, the first efficiency is reduced and the voltage hysteresis phenomenon occurs.

[0016] The thickness of the coating layer is 10 nm to 18 nm, for example, can be 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, 17 nm or 18 nm, but not limited to the listed values, other values not listed in the range are also applicable; the thickness of the coating layer is within a reasonable range, which can ensure that the negative electrode material has excellent comprehensive performance, and the coating effect cannot be guaranteed when the thickness is too small, which is insufficient to block the direct contact of the core and the electrolyte, and the mitigation of the side reaction of the two is limited; at the same time, it cannot provide sufficient channels for the entry and exit of lithium ions to improve the large current charge and discharge performance; when the thickness of the coating layer is too large, it is not conducive to the entry and exit of lithium ions, which increases the impedance, and too much hard carbon content will cause the first efficiency to decrease and the voltage hysteresis phenomenon.

[0017] The content of hard carbon in the coating layer is 1wt% to 8wt%, for example, can be 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, 7wt% or 8wt%, but not limited to the listed values, other values not listed in the range are also applicable; the content of hard carbon in the coating layer is within a reasonable range, which can ensure that the negative electrode material has excellent comprehensive performance, and the content of hard carbon is too high, which will cause the voltage hysteresis phenomenon of the modified graphite to be serious, and the first efficiency of the battery to decrease; the content of hard carbon is too low, which will reduce the pore channels of the carbon layer and the lithium ion entry and exit channels, and the dynamic performance of the battery is insufficient.

[0018] Preferably, the carbon content of the modified graphite is not less than 99.95wt%, for example, can be 99.95wt%, 99.96wt%, 99.97wt%, 99.98wt% or 99.99wt%, but not limited to the listed values, other values not listed in the range are also applicable.

[0019] Preferably, the modified graphite is spherical graphite.

[0020] Preferably, the D50 particle size of the modified graphite is 5 to 25 μm, for example, can be 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm or 25 μm, but not limited to the listed values, other values not listed in the range are also applicable.

[0021] Preferably, the pH of the modified graphite is 6.5 to 7, for example, can be 6.6, 6.7, 6.8, 6.9 or 7, but not limited to the listed values, other values not listed in the range are also applicable.

[0022] Preferably, the thickness of the coating layer is 12-16 nm, for example, it can be 12 nm, 12.5 nm, 13 nm, 13.5 nm, 14 nm, 14.5 nm, 15 nm, 15.5 nm or 16 nm, but not limited to the listed values, other values not listed in the range are also applicable.

[0023] Preferably, the content of hard carbon in the coating layer is 2-5 wt%, for example, it can be 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt% or 5 wt%, but not limited to the listed values, other values not listed in the range are also applicable.

[0024] In a second aspect, the application provides a preparation method of the negative electrode material according to the first aspect, the preparation method comprising the following steps:

[0025] (1) sintering the purified graphite in a nitrogen atmosphere to obtain modified graphite;

[0026] (2) mixing the high molecular material, the solvent and the modified graphite obtained in step (1) to obtain a slurry, and drying the slurry to obtain a powder coated with the high molecular material on the surface;

[0027] (3) forming the powder coated with the high molecular material on the surface obtained in step (2) to obtain a graphite block, and graphitizing the graphite block to obtain the negative electrode material.

[0028] In the application, the modified graphite and the high molecular material are mixed to form a slurry, which is then directly dried to obtain a powder, and then the powder is formed into a block. In this way, the high molecular material can initially coat the modified graphite in the slurry, and the coating layer is more uniform and dense after the slurry is directly dried. Then, the forming process can densify the voids in the curled carbon layer of the graphite. Therefore, compared with the method of directly forming the mixture of the high molecular material and the modified graphite, the application can coat the surface of the modified graphite with a uniform and dense coating layer, which is beneficial to improving the comprehensive performance of the negative electrode material after graphitization.

[0029] Preferably, the solvent comprises deionized water and / or high-purity water.

[0030] Preferably, the rotation speed of the mixing is 200-600 rpm, for example, it can be 200 rpm, 300 rpm, 400 rpm, 500 rpm or 600 rpm, and the time is 20-60 min, for example, it can be 20 min, 30 min, 40 min, 50 min or 60 min, but not limited to the listed values, other values not listed in the range are also applicable.

[0031] Preferably, the carbon content of the purified graphite in step (1) is not less than 99.9 wt%, such as 99.9 wt%, 99.91 wt%, 99.92 wt%, 99.93 wt%, 99.94 wt%, 99.95 wt%, 99.96 wt%, 99.97 wt%, 99.98 wt%, or 99.99 wt%, but is not limited to the listed values, and other values not listed within the range are also applicable.

[0032] Preferably, the purified graphite in step (1) is spherical graphite.

[0033] Preferably, the D50 particle size of the purified graphite in step (1) is 5-25 μm, such as 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, or 25 μm, but is not limited to the listed values, and other values not listed within the range are also applicable.

[0034] Preferably, the pH value of the purified graphite in step (1) is 3-5, such as 3, 3.2, 3.4, 3.6, 3.8, 4, 4.2, 4.4, 4.6, 4.8, or 5, but is not limited to the listed values, and other values not listed within the range are also applicable.

[0035] Preferably, the graphite in step (1) is natural graphite, and the purification method is acid treatment.

[0036] Preferably, the acid in the acid treatment includes any one or a combination of at least two of hydrofluoric acid, nitric acid, hydrochloric acid, or sulfuric acid, and typical but non-limiting combinations include a combination of hydrofluoric acid and nitric acid, a combination of nitric acid and hydrochloric acid, a combination of hydrochloric acid and sulfuric acid, a combination of hydrofluoric acid, nitric acid, and hydrochloric acid, or a combination of hydrofluoric acid, nitric acid, hydrochloric acid, and sulfuric acid.

[0037] Preferably, step (1) further includes a temperature increase before the sintering.

[0038] Preferably, the temperature increase rate of the temperature increase in step (1) is 1-15 °C / min, such as 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, 11 °C / min, 12 °C / min, 13 °C / min, 14 °C / min, or 15 °C / min, but is not limited to the listed values, and other values not listed within the range are also applicable.

[0039] Preferably, the sintering of step (1) is performed at a temperature of 300°C to 800°C, for example, 300°C, 350°C, 400°C, 450°C, 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, or 800°C, but not limited to the listed values, other values within the range are also applicable; preferably, 400°C to 600°C, for example, 400°C, 420°C, 450°C, 480°C, 500°C, 520°C, 550°C, 580°C, or 600°C, but not limited to the listed values, other values within the range are also applicable.

[0040] Preferably, the sintering of step (1) is performed for a time period of 30 min to 10 h, for example, 30 min, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, or 10 h, but not limited to the listed values, other values within the range are also applicable; preferably, 2 h to 6 h, for example, 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 4.5 h, 5 h, 5.5 h, or 6 h, but not limited to the listed values, other values within the range are also applicable.

[0041] Preferably, the monomers of the polymeric material comprise styrenic compounds and hydrophobic monomers.

[0042] Preferably, the hydrophobic monomers comprise acrylic compounds and / or acrylonitrile compounds, preferably any one or a combination of at least two of methyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, or methacrylonitrile, typically but not limited to a combination of methyl methacrylate and butyl methacrylate, butyl methacrylate and 2-ethylhexyl methacrylate, 2-ethylhexyl methacrylate and methacrylonitrile, or methyl methacrylate, butyl methacrylate, and 2-ethylhexyl methacrylate.

[0043] Preferably, the styrenic compounds comprise any one or a combination of at least two of styrene, hydrocarbyl-substituted styrene, halogenated styrene, typically but not limited to a combination of styrene and hydrocarbyl-substituted styrene, halogenated styrene and hydrocarbyl-substituted styrene, or styrene and halogenated styrene, preferably styrene.

[0044] Preferably, the hydrocarbyl-substituted styrene comprises 3-methylstyrene and / or 4-methylstyrene.

[0045] Preferably, the halogenated styrene includes any one of 2-chloro-styrene, 3-chloro-styrene, 4-chloro-styrene, 2-fluoro-styrene, 3-fluoro-styrene or 4-fluoro-styrene or a combination of at least two of them, typically but not limitedly including a combination of 2-chloro-styrene and 3-chloro-styrene, a combination of 4-chloro-styrene and 2-fluoro-styrene, or a combination of 3-fluoro-styrene and 4-fluoro-styrene.

[0046] Preferably, the high molecular material is obtained by polymerization of the styrene-based compound, the hydrophobic monomer and the initiator by any one of a precipitation polymerization method, an emulsion polymerization method, a dispersion polymerization method or a suspension polymerization method.

[0047] Preferably, the styrene-based compound is in a portion of 0 to 75 parts by weight, but not including 0, for example, can be 1 part, 10 parts, 15 parts, 20 parts, 30 parts, 40 parts, 50 parts, 60 parts, 70 parts or 75 parts, but not limited to the listed values, other values not listed in the value range are also applicable.

[0048] Preferably, the hydrophobic monomer is in a portion of 24 to 99 parts by weight, for example, can be 24 parts, 30 parts, 40 parts, 50 parts, 60 parts, 70 parts, 80 parts or 90 parts, but not limited to the listed values, other values not listed in the value range are also applicable.

[0049] Preferably, the initiator is in a portion of 0.0025 to 2.0 parts by weight, for example, can be 0.0025 parts, 0.01 parts, 0.1 parts, 0.5 parts, 1 parts, 1.5 parts or 2.0 parts, but not limited to the listed values, other values not listed in the value range are also applicable.

[0050] Preferably, the initiator includes any one of azo compounds, sulfides or peroxides or a combination of at least two of them, exemplarily including any one of azobisisobutyronitrile, azobisisoheptyl nitrile, cumene hydroperoxide or tert-butyl hydroperoxide or a combination of at least two of them, typically but not limitedly including a combination of azobisisobutyronitrile and azobisisoheptyl nitrile, or a combination of cumene hydroperoxide and tert-butyl hydroperoxide.

[0051] Preferably, the sintering in step (1) further includes a scattering, a grading, a demagnetizing and a packaging procedure.

[0052] Preferably, the mass ratio of modified graphite to polymer material in step (2) is (1-15):1, for example, it can be 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, 10.5:1, 11:1, 11.5:1, 12:1, 12.5:1, 13:1, 13.5:1, 14:1, 14.5:1 or 15:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0053] In order to achieve the appropriate coating thickness, the mass ratio of modified graphite to polymer materials during the preparation process should also be within a reasonable range to achieve the corresponding coating effect.

[0054] Preferably, the solid content of the slurry, based on the mass percentage of the slurry described in step (2), is 10wt% to 60wt%, for example, it can be 10wt%, 15wt%, 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, 45wt%, 50wt%, 55wt%, or 60wt%, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0055] Preferably, the mixing in step (2) further includes mixing a dispersant.

[0056] In this invention, a dispersant is added to the slurry containing modified graphite, polymer materials and solvent to improve the dispersion stability of modified graphite in the slurry. Without the addition of a dispersant, the slurry is unstable and easily separates into liquid and solid phases, resulting in poor drying effect, reduced uniformity and density of coating, and easy occurrence of agglomeration or incomplete drying during the drying process.

[0057] Preferably, the dispersant comprises CMC (carboxymethyl cellulose) and / or PVP (polyvinylpyrrolidone).

[0058] Preferably, the mixing in step (2) includes: after the dispersant and solvent are made into a colloid, they are mixed with the emulsion of polymeric material and the modified graphite obtained in step (1).

[0059] Preferably, the content of the dispersant, based on the mass percentage of the adhesive solution, is 0.01wt% to 0.3wt%, for example, it can be 0.01wt%, 0.05wt%, 0.1wt%, 0.15wt%, 0.2wt%, 0.25wt%, or 0.3wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 0.05wt% to 0.2wt%.

[0060] The dispersant described in this invention is added in an amount ranging from 0.01wt% to 0.3wt%, preferably from 0.05wt% to 0.2wt%, to achieve its dispersing effect without affecting the coating effect of the polymer material. If the amount of dispersant added is too small, the stability of the modified graphite in the slurry will decrease. If the amount of dispersant added is too large, it will lead to excessive slurry viscosity, making spray drying difficult, causing nozzle clogging, and affecting the spray drying effect.

[0061] Preferably, the solid content in the emulsion of the polymer material is 25wt% to 50wt%, for example, it can be 25wt%, 30wt%, 35wt%, 40wt%, 45wt% or 50wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0062] Preferably, the drying in step (2) includes spray drying.

[0063] The drying method described in this invention employs spray drying. Compared to the traditional method of direct drying at high temperatures, spray drying is equivalent to a secondary granulation process, enabling the polymer material to uniformly and densely coat the surface of the modified graphite particles. It also helps reduce the porosity of the modified graphite carbon layer during the molding process. Furthermore, compared to the traditional kneading method, which cannot coat the surface of the modified graphite particles with polymer material but only coats the agglomerated modified graphite, spray drying in this invention can coat the particle surface with polymer material and significantly improve the uniformity and density of the coating on the modified graphite particle surface, thereby enhancing the high-temperature cycling and high-temperature storage performance of the anode material.

[0064] Preferably, the inlet temperature of the spray dryer is 150℃ to 400℃, for example, 150℃, 200℃, 250℃, 300℃, 350℃ or 400℃, and the outlet temperature is 70℃ to 150℃, for example, 70℃, 80℃, 90℃, 100℃, 110℃, 120℃, 130℃, 140℃ or 150℃, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0065] If the inlet temperature of the spray dryer described in this invention is too high, the material is prone to agglomeration, which can damage the equipment. If the inlet temperature of the spray dryer is too low, the material cannot be completely dried, resulting in excessive moisture content, which affects the subsequent molding process.

[0066] Preferably, the molding process in step (3) is performed by cold isostatic pressing.

[0067] Preferably, the pressure of the cold isostatic pressing is 50MPa to 120MPa, for example, 50MPa, 60MPa, 70MPa, 80MPa, 90MPa, 100MPa, 110MPa or 120MPa, and the pressure holding time is 5s to 20min, for example, 5s, 30s, 1min, 5min, 10min, 15min or 20min, but not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0068] Preferably, the graphitization temperature in step (3) is 2500℃~3000℃, for example, it can be 2500℃, 2600℃, 2700℃, 2800℃, 2900℃ or 3000℃, and the graphitization time is 30min~6h, for example, it can be 30min, 1h, 2h, 3h, 4h, 5h or 6h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0069] Preferably, the graphitization process in step (2) further includes crushing, sieving, demagnetizing, and packaging steps.

[0070] As a preferred embodiment of the preparation method described in the second aspect of the present invention, the preparation method includes the following steps:

[0071] (1) In a nitrogen atmosphere, the acid-treated natural graphite is heated to 300℃~800℃ at a heating rate of 1℃ / min~15℃ / min and sintered for 30min~10h to obtain modified graphite.

[0072] The acid-treated natural graphite is spherical graphite with a carbon content of not less than 99.9 wt%, a D50 particle size of 5–25 μm, and a pH value of 3–5.

[0073] (2) After the dispersant and solvent are made into a colloid, it is mixed with an emulsion of polymer material with a solid content of 25wt% to 50wt% and the modified graphite obtained in step (1) to obtain a slurry with a solid content of 10wt% to 60wt%. The slurry is spray-dried at an inlet temperature of 150℃ to 400℃ and an outlet temperature of 70℃ to 150℃ to obtain a powder coated with polymer material.

[0074] The mass ratio of the modified graphite to the polymer material is (1-15):1; based on the mass of the adhesive solution, the content of the dispersant is 0.01wt% to 0.3wt%.

[0075] (3) Under a pressure of 50MPa to 120MPa, the powder coated with polymer material in step (2) is subjected to cold isostatic pressing for 5s to 20min to obtain a graphite block. The graphite block is graphitized at a temperature of 2500℃ to 3000℃ for 30min to 6h to obtain the negative electrode material.

[0076] Thirdly, the present invention provides a negative electrode sheet, the negative electrode sheet comprising the negative electrode material as described in the first aspect.

[0077] Fourthly, the present invention provides an electrochemical energy storage device, the electrochemical energy storage device comprising the negative electrode material described in the first aspect or the negative electrode sheet described in the third aspect.

[0078] The present invention does not specifically limit the types of electrochemical energy storage devices. For example, the electrochemical energy storage device includes one of lithium-ion batteries, sodium-ion batteries, supercapacitors, fuel cells, or solar cells.

[0079] Compared with the prior art, the present invention has the following beneficial effects:

[0080] (1) In this invention, the specific surface area of ​​the negative electrode material is reduced by coating the modified graphite with a coating layer with a thickness of 10nm to 18nm and a hard carbon content of 1wt% to 8wt%. This is because the particle size of the modified graphite increases after the coating layer coats the modified graphite, and the pores on the surface of the modified graphite particles are blocked by the coating layer, resulting in a decrease in specific surface area. The reduction in specific surface area can reduce the active sites on the surface of the modified graphite, thereby reducing the side reactions between the modified graphite and the electrolyte and improving the cycle life.

[0081] (2) The modified graphite preparation method in this invention increases the pH value of the purified graphite by sintering the purified graphite in a nitrogen atmosphere, avoiding the use of washing to increase the pH, thereby avoiding the discharge of a large amount of acid, saving production costs and accelerating production efficiency; compared with sintering in an oxygen atmosphere, sintering in a carbon dioxide atmosphere will not cause an increase in the specific surface area of ​​spherical graphite, and therefore will not cause a loss of battery cycle performance.

[0082] (3) By using a specific polymer material as the coating source for modified graphite, the present invention can form a composite coating layer of graphite and hard carbon on the surface of modified graphite. The polymer material, after being treated at high temperature, forms a carbon layer structure in the form of single-layer particles and uniformly coats the surface of modified graphite. It can also enhance the bonding force between the negative electrode material particles and the current collector, thereby improving the high-temperature cycling and high-temperature storage performance of the negative electrode material, reducing the electrode expansion rate, cell expansion rate and DC internal resistance when fully charged, and improving the high-current charging and discharging performance.

[0083] (4) In the process of preparing the negative electrode material, the present invention directly dries the slurry including polymer materials and modified graphite to prepare powder, which can make the surface of modified graphite uniformly coated with polymer materials; and a dispersant is added to the slurry to ensure the stability of modified graphite in the slurry and avoid slurry stratification. Combined with the subsequent drying process, the present invention can make the polymer materials uniformly coated on the surface of modified graphite even when drying before molding, avoiding the problem of uneven coating caused by drying before molding.

[0084] (5) In this invention, the drying method adopted is spray drying, which enables the polymer material to be uniformly and densely coated on the surface of the modified graphite particles, rather than coated on the surface of the agglomerated modified graphite. This is beneficial to exert the effect of the coating layer and to reduce the porosity of the modified graphite carbon layer during the molding process. Therefore, the spray drying method described in this invention further enhances the performance of the coating layer of the negative electrode material, enabling the negative electrode material to have excellent comprehensive performance. Attached Figure Description

[0085] Figure 1 This is a SEM image of the modified graphite obtained in Example 1. Detailed Implementation

[0086] 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.

[0087] Example 1

[0088] This embodiment provides a negative electrode material, which includes modified graphite and a coating layer with a thickness of 14 nm on the surface of the modified graphite; the coating layer includes 95 wt% graphite and 5 wt% hard carbon.

[0089] This embodiment provides a method for preparing the aforementioned negative electrode material, the method comprising the following steps:

[0090] (1) In a nitrogen atmosphere, natural graphite treated with hydrofluoric acid was heated to 550°C for 3 hours at a heating rate of 8°C / min to obtain modified graphite.

[0091] The acid-treated natural graphite is spherical graphite with a carbon content of 99.92 wt%, a D50 particle size of 15.6 μm, and a pH value of 3.6.

[0092] (2) After CMC and solvent are made into a liquid, it is mixed with a polymer emulsion with a solid content of 40wt% and the modified graphite obtained in step (1) to obtain a slurry with a solid content of 35wt%. The slurry is spray-dried at an air inlet temperature of 300℃ and an air outlet temperature of 110℃ to obtain a powder coated with polymer material.

[0093] The monomers for polymerizing the polymer are styrene and methyl methacrylate; the mass ratio of the modified graphite to the polymer is 12:1; based on the mass of the adhesive, the CMC content is 0.1 wt%.

[0094] (3) Under a pressure of 80 MPa, the powder coated with polymer material in step (2) is subjected to cold isostatic pressing for 5 min to obtain a graphite block. The graphite block is graphitized at a temperature of 2700℃ for 2 h to obtain the negative electrode material.

[0095] The obtained modified graphite was tested using a scanning electron microscope, and the modified graphite was obtained. Figure 1 The SEM image shown reveals that the modified graphite is spherical.

[0096] Example 2

[0097] This embodiment provides a negative electrode material, which includes modified graphite and a coating layer with a thickness of 10 nm on the surface of the modified graphite; the coating layer includes 92 wt% graphite and 8 wt% hard carbon.

[0098] This embodiment provides a method for preparing the aforementioned negative electrode material, the method comprising the following steps:

[0099] (1) In a nitrogen atmosphere, natural graphite treated with nitric acid was heated to 800℃ and sintered for 30 min at a heating rate of 15℃ / min to obtain modified graphite.

[0100] The acid-treated natural graphite is spherical graphite with a carbon content of 99.9 wt%, a D50 particle size of 14.2 μm, and a pH value of 3.

[0101] (2) After PVP and solvent are made into a liquid, it is mixed with a polymer emulsion with a solid content of 50wt% and the modified graphite obtained in step (1) to obtain a slurry with a solid content of 60wt%. The slurry is spray-dried at an air inlet temperature of 400℃ and an air outlet temperature of 70℃ to obtain a powder coated with polymer material.

[0102] The monomers used to polymerize the polymer are 3-methylstyrene and butyl methacrylate; the mass ratio of the modified graphite to the polymer is 15:1; and the content of PVP is 0.01 wt% based on the mass of the adhesive solution.

[0103] (3) Under a pressure of 120 MPa, the powder coated with polymer material in step (2) is subjected to cold isostatic pressing for 5 s to obtain a graphite block. The graphite block is graphitized at a temperature of 2500℃ for 6 h to obtain the negative electrode material.

[0104] Example 3

[0105] This embodiment provides a negative electrode material, which includes modified graphite and a coating layer with a thickness of 18 nm on the surface of the modified graphite; the coating layer includes 99 wt% graphite and 1 wt% hard carbon.

[0106] This embodiment provides a method for preparing the aforementioned negative electrode material, the method comprising the following steps:

[0107] (1) In a nitrogen atmosphere, natural graphite treated with hydrochloric acid was heated to 300℃ and sintered for 10h at a heating rate of 1℃ / min to obtain modified graphite.

[0108] The acid-treated natural graphite is spherical graphite with a carbon content of 99.94 wt%, a D50 particle size of 16.8 μm, and a pH value of 5.

[0109] (2) After PVP and solvent are made into a liquid, it is mixed with a polymer emulsion with a solid content of 25wt% and the modified graphite obtained in step (1) to obtain a slurry with a solid content of 10wt%. The slurry is spray-dried at an air inlet temperature of 150°C and an air outlet temperature of 150°C to obtain a powder coated with polymer material.

[0110] The monomers used to polymerize the polymer are 2-chloro-styrene and 2-ethylhexyl methacrylate; the mass ratio of the modified graphite to the polymer is 10:1; and the content of PVP is 0.3 wt% based on the mass of the adhesive solution.

[0111] (3) Under a pressure of 50 MPa, the powder coated with polymer material in step (2) is subjected to cold isostatic pressing for 20 min to obtain a graphite block. The graphite block is graphitized at a temperature of 3000℃ for 30 min to obtain the negative electrode material.

[0112] Example 4

[0113] This embodiment provides a negative electrode material and a method for preparing the negative electrode material. Except for step (2), in which the adhesive, polymer emulsion and modified graphite are mixed and kneaded, and then step (3) is carried out directly without the spray drying step described in step (2), so that the obtained negative electrode material changes accordingly, the rest of the preparation method is the same as in Example 1.

[0114] Example 5

[0115] This embodiment provides a negative electrode material and a method for preparing the negative electrode material. The preparation method is the same as in Example 1, except that the spray drying in step (2) is replaced by direct drying in a drying oven at 300°C, so that the obtained negative electrode material changes accordingly.

[0116] Example 6

[0117] This embodiment provides a negative electrode material and a method for preparing the negative electrode material. Except for omitting the dispersant CMC in step (2) to change the resulting negative electrode material, the preparation method is the same as in Example 1.

[0118] Example 7

[0119] This embodiment provides a negative electrode material and a method for preparing the negative electrode material. Except for the dispersant content in step (2) being 0.005 wt%, which causes a corresponding change in the obtained negative electrode material, the preparation method is the same as in Example 1.

[0120] Example 8

[0121] This embodiment provides a negative electrode material and a method for preparing the negative electrode material. Except for the content of the dispersant in step (2) being 0.35 wt%, which changes the obtained negative electrode material accordingly, the preparation method is the same as in Example 1.

[0122] Example 9

[0123] This embodiment provides a negative electrode material and a method for preparing the negative electrode material. Except for the sintering temperature of the purified graphite in step (1) being 150°C, which causes a corresponding change in the obtained negative electrode material, the preparation method is the same as in Example 1.

[0124] Example 10

[0125] This embodiment provides a negative electrode material and a method for preparing the negative electrode material. Except for the sintering temperature of the purified graphite in step (1) being 1000℃, which causes a corresponding change in the obtained negative electrode material, the preparation method is the same as in Example 1.

[0126] Example 11

[0127] This embodiment provides a negative electrode material and a method for preparing the negative electrode material. Except for the sintering time of the purified graphite in step (1) being 15 min, which causes a corresponding change in the obtained negative electrode material, the preparation method is the same as in Example 1.

[0128] Example 12

[0129] This embodiment provides a negative electrode material and a method for preparing the negative electrode material. Except for the sintering time of the purified graphite in step (1) being 15 hours, which causes a corresponding change in the obtained negative electrode material, the preparation method is the same as in Example 1.

[0130] Comparative Example 1

[0131] This embodiment provides a negative electrode material and a method for preparing the negative electrode material. Except for step (2), in which the mass ratio of modified graphite to polymer material is 20:1, so that the coating thickness of the obtained negative electrode material is 5nm, the rest of the preparation method is the same as in Example 1.

[0132] Comparative Example 2

[0133] This embodiment provides a negative electrode material and a method for preparing the negative electrode material. Except for step (2), in which the mass ratio of modified graphite to polymer material is 0.6:1, so that the coating thickness of the obtained negative electrode material is 25nm, the rest of the preparation method is the same as in Example 1.

[0134] Comparative Example 3

[0135] This comparative example provides a negative electrode material and a method for preparing the negative electrode material. Except for replacing the nitrogen atmosphere in step (1) with an oxygen atmosphere to change the resulting negative electrode material, the preparation method is the same as in Example 1.

[0136] The modified graphite obtained in the above examples and comparative examples was tested for carbon content, particle size range and pH value. The test results are shown in Table 1.

[0137] The carbon content test method is as follows: Take 3g of modified graphite and sinter it in a muffle furnace at 900℃ for 7min. The residual mass is the impurity mass. The mass lost during sintering divided by the total mass of modified graphite before sintering is the carbon content.

[0138] The particle size range test method is as follows: the modified graphite is tested by wet method using a Malvern 3000 particle size analyzer to obtain the particle size of the modified graphite;

[0139] The pH test method is as follows: the pH of the modified graphite is obtained by using a Wantong 914 pH meter according to GB / T 24533-2019.

[0140] Table 1

[0141] Carbon content (wt%) D50 particle size (pm) pH Example 1 99.96 17.4 6.8 Example 2 99.95 17.5 6.8 Example 3 99.95 17.6 6.9 Example 4 99.97 17.8 6.8 Example 5 99.98 17.5 6.7 Example 6 99.95 17.6 6.9 Example 7 99.96 17.3 6.5 Example 8 99.95 17.5 6.7 Example 9 99.97 17.5 4.6 Example 10 99.95 17.4 6.7 Example 11 99.97 17.6 4.8 Example 12 99.96 17.4 6.9 Comparative Example 1 99.95 17.7 6.8 Comparative Example 2 99.96 17.4 6.7 Comparative Example 3 99.97 17.8 6.9

[0142] The negative electrode materials obtained in the above embodiments and comparative examples were compounded with S66 binder, CMC, and conductive agent SP in a mass ratio of 96.7:1.8:0.5:1 to form a negative electrode sheet. This negative electrode sheet was then assembled with a lithium iron phosphate positive electrode sheet, a PE separator, and a YE-PO5 electrolyte according to the general process for lithium-ion battery manufacturing. The lithium iron phosphate positive electrode sheet consisted of lithium iron phosphate, SP, CNT, and PVDF in a mass ratio of 95:1.5:1.0:2.5. The battery was then charged at a constant current to 3.7V and discharged at a constant current to 2V. The capacity of the first discharge was tested. The electrode sheet expansion rate at full charge, the capacity retention rate after 1000 cycles at 1C, the cell expansion rate, the capacity retention rate after 1000 cycles at 45℃ at 1C, and the capacity retention rate and capacity recovery rate after 7 days of storage at 60℃ were also tested. The test results are shown in Tables 2 and 3.

[0143] Table 2

[0144]

[0145]

[0146] Table 2

[0147]

[0148]

[0149] From Tables 1-3, we can obtain:

[0150] (1) The modified graphite in Examples 1-3 has a smaller particle size and therefore a smaller specific surface area, and the pH value is also improved. The preparation method of the modified graphite in the invention improves the pH value of the purified graphite by sintering the purified graphite in a nitrogen atmosphere, avoiding the use of washing to improve the pH, thereby avoiding the discharge of a large amount of acid, saving production costs and speeding up production efficiency. Compared with sintering in an oxygen atmosphere, sintering in a carbon dioxide atmosphere will not cause an increase in the specific surface area of ​​spherical graphite, and therefore will not cause a loss of battery cycle performance. The battery assembled with the negative electrode material in Examples 1-3 has excellent high-temperature cycle and high-temperature storage performance, low electrode expansion rate and cell expansion rate when fully charged, and excellent high-current charge and discharge performance.

[0151] (2) By comparing Example 1 and Example 4, it can be seen that replacing spray drying with the traditional kneading method makes it impossible for the particle surface to fully contact the polymer material, resulting in poor coating effect and failure to uniformly and densely coat the polymer material on the particle surface. Therefore, the overall performance of the obtained negative electrode material decreases.

[0152] (3) By comparing Example 1 and Example 5, it can be seen that when spray drying is replaced with conventional drying at the same temperature, the slurry is prone to clumping during the drying process, which makes it impossible to uniformly coat the surface of the graphite material particles with polymer materials. However, the present invention, through spray drying, not only has a good drying effect, but also makes the surface of the particles uniformly coated with polymer materials, improving the uniformity and density of the coating, and can improve the performance of the graphitized negative electrode material.

[0153] (4) By comparing Example 1 with Examples 6 to 8, it can be seen that the addition of dispersant in this invention can improve the dispersion stability of graphite material in slurry, improve the coating effect, and will not affect the effect of subsequent spray drying process. Furthermore, the amount of dispersant added should be within a reasonable range in order to enable it to disperse graphite material.

[0154] (5) By comparing Example 1 with Examples 9 and 10, it can be seen that the sintering temperature in this invention will affect the performance of modified graphite, thereby affecting the performance of lithium-ion batteries prepared by the negative electrode material. When the sintering temperature is too high, the kinetic performance of modified graphite will decrease, energy consumption will increase, and cost will rise. When the sintering temperature is too low, the polymer coating layer will not be sufficiently carbonized, residual acid will not be fully removed, pH value will be low, and cycle performance will decrease.

[0155] (6) By comparing Example 1 with Examples 11 and 12, it can be seen that the sintering time in this invention will affect the performance of modified graphite, thereby affecting the performance of lithium-ion batteries prepared by negative electrode materials. When the sintering time is too long, it will lead to increased energy consumption and increased cost. When the sintering time is too short, it will lead to insufficient carbonization of polymer coating, insufficient removal of residual acid, low pH value, and decreased cycle performance.

[0156] (7) By comparing Example 1 with Comparative Examples 1 and 2, it can be seen that if the coating layer of the present invention is too thick, the graphite capacity will be too low, the first efficiency will be too low and the voltage hysteresis will be severe; if the coating layer is too thin, the specific surface area will be too large and the cycle performance will be reduced.

[0157] (8) By comparing Example 1 and Comparative Example 3, it can be seen that the sintering atmosphere in this invention will affect the performance of modified graphite, thereby affecting the performance of lithium-ion batteries prepared by negative electrode materials. Nitrogen can remove residual acid on the graphite surface without affecting the specific surface area, while oxygen is prone to excessive oxidation, too many openings, graphite capacity decreases, specific surface area is too large, and cycle performance decreases.

[0158] In summary, this invention reduces the specific surface area of ​​the negative electrode material by coating the modified graphite with a coating layer of 10nm-18nm thickness and a hard carbon content of 1wt%-8wt%. This is because the coating layer increases the particle size of the modified graphite, blocking the pores on the surface of the modified graphite particles, thus leading to a decrease in specific surface area. The reduction in specific surface area reduces the number of active sites on the modified graphite surface, thereby reducing side reactions between the modified graphite and the electrolyte and improving cycle life. The modified graphite preparation method in this invention involves processing purified graphite under a nitrogen atmosphere... Sintering increases the pH value of purified graphite, avoiding the need for washing to raise the pH and thus preventing the discharge of large amounts of acid, saving production costs and accelerating production efficiency. Compared to sintering in an oxygen atmosphere, sintering in a carbon dioxide atmosphere does not increase the specific surface area of ​​spherical graphite, therefore avoiding loss of battery cycle performance. This invention uses a specific polymer material as the coating source for modified graphite, enabling the formation of a composite coating layer of graphite and hard carbon on the surface of the modified graphite. The polymer material, after high-temperature treatment, forms a uniform carbon layer structure in the form of single-layer particles. The modified graphite is coated on its surface, enhancing the bonding force between the negative electrode material particles and the current collector. This improves the high-temperature cycling and high-temperature storage performance of the negative electrode material, reduces the electrode expansion rate, cell expansion rate, and DC internal resistance at full charge, and enhances high-current charge-discharge performance. In the preparation of the negative electrode material, the slurry comprising polymer materials and modified graphite is directly dried to prepare powder, ensuring a uniform coating of polymer materials on the surface of the modified graphite. Furthermore, a dispersant is added to the slurry to ensure the stability of the modified graphite within the slurry, preventing slurry stratification and subsequent drying processes. In combination, the present invention allows for drying before molding, which enables the polymer material to be uniformly coated on the surface of the modified graphite, avoiding the problem of uneven coating caused by drying before molding. The drying method used in the present invention is spray drying, which enables the polymer material to be uniformly and densely coated on the surface of the modified graphite particles, rather than coating the surface of agglomerated modified graphite. This is beneficial to the effect of the coating layer and helps to reduce the porosity of the modified graphite carbon layer during molding. Therefore, the spray drying described in the present invention further improves the performance of the coating layer of the negative electrode material, giving the negative electrode material excellent comprehensive performance.

[0159] 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 negative electrode material, characterized in that, The negative electrode material includes modified graphite and a coating layer on the surface of the modified graphite; the coating layer includes graphite and hard carbon. The thickness of the coating layer is 10nm~18nm, and the content of hard carbon in the coating layer is 1wt%~8wt%; the modified graphite is obtained by sintering acid-treated natural graphite at 300℃~800℃ in a nitrogen atmosphere for 30min~10h. The acid-treated natural graphite has a D50 particle size of 5~25μm and a pH value of 3~5. The negative electrode material uses a polymer material as the coating source for modified graphite. After high-temperature treatment, the polymer material forms a composite coating layer of graphite and hard carbon on the surface of the modified graphite. The monomers of the polymer material include styrene compounds and hydrophobic monomers; the hydrophobic monomers include acrylic compounds and / or acrylonitrile compounds; The mass ratio of the modified graphite to the polymer material is (1~15):

1.

2. The negative electrode material according to claim 1, characterized in that, The modified graphite has a carbon content of not less than 99.95 wt%.

3. The negative electrode material according to claim 1, characterized in that, The modified graphite is spherical graphite.

4. The negative electrode material according to claim 1, characterized in that, The modified graphite has a D50 particle size of 5~25μm.

5. The negative electrode material according to claim 1, characterized in that, The modified graphite has a pH of 6.5 to 7.

6. The negative electrode material according to claim 1, characterized in that, The thickness of the coating layer is 12nm~16nm.

7. The negative electrode material according to claim 1, characterized in that, The hard carbon content in the coating layer is 2wt%~5wt%.

8. A method for preparing the negative electrode material according to any one of claims 1 to 7, characterized in that, The preparation method includes the following steps: (1) In a nitrogen atmosphere, the acid-treated natural graphite is heated to 300℃~800℃ and sintered for 30min~10h to obtain modified graphite; The acid-treated natural graphite has a D50 particle size of 5~25μm and a pH value of 3~5. (2) A slurry is obtained by mixing polymer materials, solvent and modified graphite obtained in step (1), and the slurry is dried to obtain a powder coated with polymer materials. The mass ratio of the modified graphite to the polymer material is (1~15):1; the monomers of the polymer material include styrene compounds and hydrophobic monomers; the hydrophobic monomers include acrylic compounds and / or acrylonitrile compounds; (3) The powder with polymer material coated on the surface obtained in the molding process (2) is used to obtain a graphite block. The graphite block is graphitized at a temperature of 2500℃~3000℃ to obtain the negative electrode material.

9. The preparation method according to claim 8, characterized in that, The carbon content in the natural graphite after acid treatment in step (1) is not less than 99.9 wt%.

10. The preparation method according to claim 8, characterized in that, The natural graphite after acid treatment in step (1) is spherical graphite.

11. The preparation method according to claim 8, characterized in that, The acid used in the acid treatment includes any one or a combination of at least two of hydrofluoric acid, nitric acid, hydrochloric acid, or sulfuric acid.

12. The preparation method according to claim 8, characterized in that, The heating rate in step (1) is 1℃ / min to 15℃ / min.

13. The preparation method according to claim 8, characterized in that, The sintering temperature in step (1) is 400℃~600℃.

14. The preparation method according to claim 8, characterized in that, The sintering time in step (1) is 2h~6h.

15. The preparation method according to claim 8, characterized in that, The hydrophobic monomer is any one or a combination of at least two of methyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, or methacrylonitrile.

16. The preparation method according to claim 8, characterized in that, The styrene compounds include any one or a combination of at least two of styrene, hydrocarbon-substituted styrene, and halostyrene.

17. The preparation method according to claim 16, characterized in that, The hydrocarbon-substituted styrene includes 3-methylstyrene and / or 4-methylstyrene.

18. The preparation method according to claim 16, characterized in that, The halostyrene includes any one or a combination of at least two of 2-chlorostyrene, 3-chlorostyrene, 4-chlorostyrene, 2-fluorostyrene, 3-fluorostyrene or 4-fluorostyrene.

19. The preparation method according to claim 8, characterized in that, The solid content of the slurry is 10wt%~60wt%, based on the mass percentage of the slurry described in step (2).

20. The preparation method according to claim 8, characterized in that, The mixing in step (2) also includes the mixing of dispersants.

21. The preparation method according to claim 20, characterized in that, The dispersant includes CMC and / or PVP.

22. The preparation method according to claim 20, characterized in that, The mixing in step (2) includes: after the dispersant and solvent are made into a colloid, they are mixed with the emulsion of polymeric material and the modified graphite obtained in step (1).

23. The preparation method according to claim 22, characterized in that, The content of the dispersant is 0.01wt%~0.3wt% based on the mass percentage of the adhesive solution.

24. The preparation method according to claim 23, characterized in that, The content of the dispersant is 0.05wt%~0.2wt% based on the mass percentage of the adhesive solution.

25. The preparation method according to claim 22, characterized in that, The emulsion of the polymer material has a solid content of 25wt% to 50wt%.

26. The preparation method according to claim 8, characterized in that, The drying in step (2) includes spray drying.

27. The preparation method according to claim 26, characterized in that, The inlet temperature of the spray dryer is 150℃~400℃, and the outlet temperature is 70℃~150℃.

28. The preparation method according to claim 8, characterized in that, The molding process described in step (3) is cold isostatic pressing.

29. The preparation method according to claim 28, characterized in that, The pressure of the cold isostatic pressing is 50MPa~120MPa, and the holding time is 5s~20min.

30. The preparation method according to claim 8, characterized in that, The graphitization time is 30 min to 6 h.

31. The preparation method according to claim 8, characterized in that, The preparation method includes the following steps: (1) In a nitrogen atmosphere, the acid-treated natural graphite is heated to 300℃~800℃ at a heating rate of 1℃ / min~15℃ / min and sintered for 30min~10h to obtain modified graphite. The acid-treated natural graphite is spherical graphite with a carbon content of not less than 99.9 wt%, a D50 particle size of 5~25 μm, and a pH value of 3~5. (2) After the dispersant and solvent are made into a colloid, it is mixed with an emulsion of polymer material with a solid content of 25wt%~50wt% and the modified graphite obtained in step (1) to obtain a slurry with a solid content of 10wt%~60wt%. The slurry is spray-dried at an inlet temperature of 150℃~400℃ and an outlet temperature of 70℃~150℃ to obtain a powder coated with polymer material. The mass ratio of the modified graphite to the polymer material is (1~15):1; based on the mass of the adhesive solution, the content of the dispersant is 0.01wt%~0.3wt%; (3) Under a pressure of 50MPa~120MPa, the powder coated with polymer material in step (2) is subjected to cold isostatic pressing for 5s~20min to obtain a graphite block. The graphite block is graphitized at a temperature of 2500℃~3000℃ for 30min~6h to obtain the negative electrode material.

32. A negative electrode sheet, characterized in that, The negative electrode sheet comprises the negative electrode material as described in any one of claims 1 to 7.

33. An electrochemical energy storage device, characterized in that, The electrochemical energy storage device includes the negative electrode material according to any one of claims 1 to 7 or the negative electrode sheet according to claim 32.

34. The electrochemical energy storage device according to claim 33, characterized in that, The electrochemical energy storage device is selected from one of lithium-ion batteries, sodium-ion batteries, supercapacitors, fuel cells, or solar cells.

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