Negative electrode sheet, secondary battery, electric device, hard carbon material, and preparation method therefor

Abstract

A negative electrode sheet, a secondary battery, an electric device, a hard carbon material, and a preparation method therefor. The negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector; the negative electrode film layer comprises a hard carbon material; an X-ray diffraction pattern of the hard carbon material has a diffraction peak at the position where the diffraction angle 2θ ranges from 22° to 25°, wherein a line tangent to a first shoulder part on the left side of the diffraction peak and a first shoulder part on the right side of the diffraction peak is a straight line L, a line parallel to the straight line L and tangent to the diffraction peak is a tangent line M, and the intensity corresponding to the point of tangency of the tangent line M serves as B; if the intensity corresponding to an intersection point between a perpendicular line passing through the point of tangency and perpendicular to the horizontal axis serves as A, then 1≤B / A≤2.5; the saturated water vapor adsorption amount at 25° of the hard carbon material is ≥ 200 cm3 / g; and the methylene blue adsorption value of the hard carbon material is ≤ 10 mg / g.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present disclosure is presented based on Chinese Patent Application No. 202410543434.X, filed on April 30, 2024 and entitled “NEGATIVE ELECTRODE PLATE, SECONDARY BATTERY, ELECTRIC DEVICE, HARD CARBON MATERIAL, AND PREPARATION METHOD FOR HARD CARBON MATERIAL”, and claims priority to the Chinese patent application, the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present disclosure relates to the technical field of batteries, and in particular, to a negative electrode plate, a secondary battery, an electric device, a hard carbon material, and a preparation method for the hard carbon material. BACKGROUND

[0003] In recent years, secondary batteries have been widely applied in energy storage power systems such as hydropower, thermal power, wind power, and solar power stations, as well as in various fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, and aerospace. With the application and promotion of secondary batteries, the performance requirements for secondary batteries are becoming increasingly high.

[0004] As an important component of the secondary battery, the performance of the negative electrode active material affects the performance of the secondary battery to a certain extent. Hard carbon has attracted much attention as a negative electrode active material for secondary batteries. To further improve battery performance, there is an urgent need for a hard carbon material with high capacity. SUMMARY

[0005] The present disclosure is made in view of the above problems, and the objective of the present disclosure is to provide a negative electrode plate, a secondary battery, an electric device, a hard carbon material, and a preparation method for the hard carbon material. The hard carbon material has a high capacity.

[0006] To achieve the above objective, a first aspect of the present disclosure provides a negative electrode plate. The negative electrode plate includes a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector; the negative electrode film layer includes a hard carbon material. In an X-ray diffraction pattern of the hard carbon material, there is a diffraction peak at a position where a diffraction angle 20 is 22° to 25°, where a straight line L is tangent to a first shoulder on a left side of the diffraction peak and a first shoulder on a right side of the diffraction peak, and a tangent line M is parallel to the straight line L and tangent to the diffraction peak; if an intensity corresponding to a tangent point of the tangent line M is defined as B, and an intensity corresponding to an intersection point of the straight line L and a perpendicular line passing through the tangent point and perpendicular to a horizontal axis is defined as A, then 1 < B / A < 2.5. A saturated water vapor adsorption amount at 25 °C of the hard carbon material is > 200 cm3 / g, and a methylene blue adsorption value of the hard carbon material is < 10 mg / g.

[0007] In the present disclosure, by making the value of B / A fall within the above range, the adsorption capacity of the hard carbon material is increased. In addition, by making the water vapor adsorption amount and the methylene blue adsorption value fall within the above ranges, the electrolytic solution is prevented from entering the pore channel structure of the hard carbon material, and the pore filling capacity is increased, thereby enabling the hard carbon material to have a high capacity.

[0008] In some embodiments, the saturated water vapor adsorption amount at 25 °C of the hard carbon material is 350 cm3 / g to 390 cm3 / g.

[0009] In some embodiments, the methylene blue adsorption value of the hard carbon material is 0.1 mg / g to 4 mg / g. In this way, the pore filling capacity of the hard carbon material can be further increased.

[0010] In some embodiments, a compacted density of the hard carbon material is consecutively determined twice under a pressure of 5 tons, with a compacted density determined for a first time set as PD1, and a compacted density determined for a second time set as PD2, and the PD1 and the PD2 satisfy the following relationship: 0 < PD2 - PD1 < 0.03 g / cm3. Controlling the difference between PD2 and PD1 within the above range improves the structural stability of the hard carbon material, which is conducive to the transport of active ions during charging and discharging processes and is more conducive to improving the charge specific capacity and the initial coulombic efficiency of the battery.

[0011] In some embodiments, the hard carbon material includes a matrix and a carbon coating layer located on at least part of a surface of the matrix. In this way, it is conducive to improving the layer stacking and the pore structure in the hard carbon material, such that the numerical value of B / A, the water vapor adsorption amount, and the methylene blue adsorption value fall within the above ranges.

[0012] In some embodiments, the matrix includes a porous carbon skeleton and a phosphorus element within the porous carbon skeleton, and relative to the hard carbon material, a mass percentage of the phosphorus element is 0.5 wt% to 2.5 wt%. In this way, it is more conducive to improving the layer stacking and the pore structure.

[0013] In some embodiments, relative to the hard carbon material, a mass percentage of the coating layer is 1.5 wt% to 7.5 wt%. This is more conducive to forming a dense coating layer, such that the water vapor adsorption amount and the methylene blue adsorption value fall within the above ranges.

[0014] In some embodiments, in a potential range of 0 V to 2.5 V vs. Na / Na+, a charge specific capacity of the hard carbon material is 350 mAh / g or more.

[0015] In some embodiments, after the hard carbon material is sintered and heated at 1000 °C for 2h under an inert atmosphere, a surface oxygen content determined under a vacuum condition is set as X1, a surface oxygen content determined after exposure to air with a humidity of < 2% for 30 days is set as X2, and the X1 and the X2 satisfy X2 - X1 < 5 wt%. By keeping the numerical value of X2 - X1 within the above range, the surface activity of the hard carbon material is reduced, and the catalytic decomposition capability of the hard carbon material on the electrolytic solution is decreased, which, in turn, can reduce the amount of active Na that needs to be consumed during the formation of a solid electrolyte interphase (SEI) film. In this way, it is conducive to further improving the initial coulombic efficiency.

[0016] A second aspect of the present disclosure provides a secondary battery. The secondary battery includes the negative electrode plate according to the first aspect of the present disclosure.

[0017] In some embodiments, the secondary battery further includes a positive electrode plate. The positive electrode plate includes, as a positive electrode active material, at least one selected from a transition metal oxide, a polyanionic compound, and a Prussian blue compound.

[0018] A third aspect of the present disclosure provides an electric device. The electric device includes the secondary battery according to the second aspect of the present disclosure.

[0019] The electric device of the present disclosure includes the secondary battery according to the present disclosure and thus has at least the same advantages as the secondary battery.

[0020] A fourth aspect of the present disclosure provides a hard carbon material. In an X-ray diffraction pattern of the hard carbon material, there is a diffraction peak at a position where a diffraction angle 29 is 22° to 25°, where a straight line L is tangent to a first shoulder on a left side of the diffraction peak and a first shoulder on a right side of the diffraction peak, and a tangent line M is parallel to the straight line L and tangent to the diffraction peak; if an intensity corresponding to a tangent point of the tangent line M is defined as B, and an intensity corresponding to an intersection point of the straight line L and a perpendicular line passing through the tangent point and perpendicular to a horizontal axis is defined as A, then 1 < B / A < 2.5. A saturated water vapor adsorption amount at 25 °C of the hard carbon material is > 200 cm3 / g, and a methylene blue adsorption value of the hard carbon material is < 10 mg / g.

[0021] In the present disclosure, by making the value of B / A fall within the above range, the adsorption capacity of the hard carbon material is increased. In addition, by making the water vapor adsorption amount and the methylene blue adsorption value fall within the above ranges, the electrolytic solution is prevented from entering the pore channel structure of the hard carbon material, and the pore filling capacity is increased, thereby enabling the hard carbon material to have a high capacity.

[0022] In some embodiments, the saturated water vapor adsorption amount at 25 °C of the hard carbon material is 350 cm3 / g to 390 cm3 / g.

[0023] In some embodiments, the methylene blue adsorption value of the hard carbon material is 0.1 mg / g to 4 mg / g. In this way, the pore filling capacity of the hard carbon material can be further increased.

[0024] In some embodiments, a compacted density of the hard carbon material is consecutively determined twice under a pressure of 5 tons, with a compacted density determined for a first time set as PD1, and a compacted density determined for a second time set as PD2, and the PD1 and the PD2 satisfy the following relationship: 0 < PD2 - PD1 < 0.03 g / cm3. Controlling the difference between PD2 and PD1 within the above range improves the structural stability of the hard carbon material, which is conducive to the transport of active ions during charging and discharging processes and is more conducive to improving the charge specific capacity and the initial coulombic efficiency of the battery.

[0025] In some embodiments, the hard carbon material includes a matrix and a carbon coating layer located on at least part of a surface of the matrix. In this way, it is conducive to improving the layer stacking and the pore structure in the hard carbon material, such that the numerical value of B / A, the water vapor adsorption amount, and the methylene blue adsorption value fall within the above ranges.

[0026] In some embodiments, the matrix includes a porous carbon skeleton and a phosphorus element within the porous carbon skeleton, and relative to the hard carbon material, a mass percentage of the phosphorus element is 0.5 wt% to 2.5 wt%. In this way, it is more conducive to improving the layer stacking and the pore structure.

[0027] In some embodiments, relative to the hard carbon material, a mass percentage of the coating layer is 1.5 wt% to 7.5 wt%. This is more conducive to forming a dense coating layer, such that the water vapor adsorption amount and the methylene blue adsorption value fall within the above ranges.

[0028] In some embodiments, in a potential range of 0 V to 2.5 V vs. Na / Na+, a charge specific capacity of the hard carbon material is 350 mAh / g or more.

[0029] In some embodiments, after the hard carbon material is sintered and heated at 1000 °C for 2h under an inert atmosphere, a surface oxygen content determined under a vacuum condition is set as X1, a surface oxygen content determined after exposure to air with a humidity of < 2% for 30 days is set as X2, and the X1 and the X2 satisfy X2 - X1 < 5 wt%. By keeping the numerical value of X2 - X1 within the above range, the surface activity of the hard carbon material is reduced, and the catalytic decomposition capability of the hard carbon material on the electrolytic solution is decreased, which, in turn, can reduce the amount of active Na that needs to be consumed during the formation of a solid electrolyte interphase (SEI) film. In this way, it is conducive to further improving the initial coulombic efficiency.

[0030] A fifth aspect of the present disclosure provides a preparation method for a hard carbon material. The method includes the following steps:

[0031] an impregnation step of impregnating a carbon source in a liquid containing a dopant for an impregnation time of 2 hours or more, where the dopant contains a phosphorus element or a zinc element as a doping element, and relative to a mass of the carbon source, a mass percentage of the phosphorus element is 9.5 wt% to 32.4 wt%, and a mass percentage of the zinc element is 14 wt% to 50 wt%;

[0032] a low-temperature heat treatment step of heating at 400 °C to 750 °C to obtain a matrix;

[0033] a kneading step of kneading a mixed solution, in which a resin-based polymer material and the matrix are dispersed, in a kneader, where a mass ratio of the resin-based polymer material to the matrix is (0.5-2):10; and

[0034] a carbonization step.

[0035] According to the preparation method of the present disclosure, the hard carbon material with high capacity according to the fourth aspect of the present disclosure can be obtained.

[0036] In some embodiments, the dopant includes at least one of phosphoric acid, a phosphate, polyphosphoric acid, and zinc chloride. Using the above dopant can improve the layer stacking and the pore structure.

[0037] In some embodiments, the resin-based polymer material includes at least one of an epoxy resin, a phenolic resin, an unsaturated polyester resin, and a furan resin. The resin-based polymer material is more conducive to the formation of a dense and uniform coating layer.

[0038] In some embodiments, in the kneading step, a kneading time is 0.5 hours or more. In this way, the resin-based polymer material is further facilitated to be distributed more uniformly on the surface of the matrix, thereby further facilitating the formation of a dense and uniform coating layer.

[0039] In some embodiments, in the kneading step, a solid content of the mixed solution is 55 wt% to 75 wt%. In this way, the resin-based polymer material is further facilitated to be more uniformly distributed on the surface of the matrix, thereby facilitating the formation of a dense and uniform coating layer.

[0040] In some embodiments, in the low-temperature heat treatment step, a temperature is raised to 400 °C to 750 °C at a heating rate of 1 °C / min to 20 °C / min, and a holding time is 1 hour to 12 hours. In this way, it is more conducive to improving the layer stacking and the pore structure.

[0041] In some embodiments, in the carbonization step, a temperature is raised to 1000 °C to 1800 °C at a heating rate of 2 °C / min to 20 °C / min. In this way, it is more conducive to controlling the pyrolysis process of the resin-based polymer material and reducing surface defects.

[0042] In some embodiments, in the carbonization step, under a condition of a pressure of 10 MPa or more, a temperature is raised to 1100 °C to 1600 °C at a heating rate of 2 °C / min to 10 °C / min and held. In this way, the escape of carbon free radicals formed by the thermal pyrolysis of the polymer material in the carbonization step can be reduced, allowing the carbon free radicals to fully repair defects prone to oxidation, thereby facilitating the improvement of the initial coulombic efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] FIG. 1 is a schematic diagram of a battery cell according to an embodiment of the present disclosure.

[0044] FIG. 2 is an exploded view of the battery cell shown in FIG. 1 according to an embodiment of the present disclosure.

[0045] FIG. 3 is a schematic diagram of a battery module according to an embodiment of the present disclosure.

[0046] FIG. 4 is a schematic diagram of a battery pack according to an embodiment of the present disclosure.

[0047] FIG. 5 is an exploded view of the battery pack shown in FIG. 4 according to an embodiment of the present disclosure.

[0048] FIG. 6 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present disclosure.

[0049] FIG. 7 is an XRD pattern of a hard carbon material in Example 1 according to the present disclosure.

[0050] Description of the reference numerals:

[0051] 1: battery pack; 2: upper case body; 3: lower case body; 4: battery module; 5: battery cell; 51: shell body; 52: electrode assembly; 53: top cover assembly DETAILED DESCRIPTION

[0052] Hereinafter, embodiments specifically disclosing a negative electrode plate, a secondary battery, an electric device, a hard carbon material, and a preparation method for the hard carbon material of the present disclosure will be described in detail with appropriate reference to the drawings. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of actually identical structures may be omitted. This is to avoid unnecessary lengthiness of the following descriptions and to facilitate understanding by those skilled in the art. Additionally, the drawings and the following descriptions are provided to enable those skilled in the art to fully understand the present disclosure and are not intended to limit the subject matter recited in the claims.

[0053] The “ranges” disclosed in the present disclosure are defined with lower and upper limits. A given range is defined by selecting a lower limit and an upper limit that delineate the boundaries of a particular range. Ranges defined in this manner are inclusive of the end values unless otherwise specified, and may be arbitrarily combined, which means that any lower limit may be combined with any upper limit to form a range. For example, if ranges of 60 to 120 and 80 to 110 are listed for a specific parameter, it is understood that ranges of 60 to 110 and 80 to 120 are also anticipated. Additionally, if the minimum range values listed are 1 and 2, and the maximum range values listed are 3, 4, and 5, then the following ranges can all be anticipated: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5. In the present disclosure, unless otherwise specified, the numerical range “a to b” indicates an abbreviated representation of any combination of real numbers between a and b, where both a and b are real numbers. For example, the numerical range “0 to 5” indicates that all real numbers between “0 and 5” are listed herein, and “0 to 5” is merely an abbreviated representation of a combination of these numerical values. Additionally, when stating that a parameter is an integer > 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or the like.

[0054] Unless otherwise specified, all embodiments and optional embodiments of the present disclosure may be combined with one another to form new technical solutions.

[0055] Unless otherwise specified, all technical features and optional technical features of the present disclosure may be combined with one another to form new technical solutions.

[0056] At present, research on negative electrode plates mostly focuses on improving the electrical performance of hard carbon materials included in the negative electrode film layer. To further improve battery performance, there is an urgent need for a negative electrode plate with high capacity.

[0057] Negative Electrode Plate

[0058] Based on this, a first aspect of the present disclosure provides a negative electrode plate. The negative electrode plate includes a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector; the negative electrode film layer includes a hard carbon material. In an X-ray diffraction pattern of the hard carbon material, there is a diffraction peak at a position where a diffraction angle 20 is 22° to 25°, where a straight line L is tangent to a first shoulder on a left side of the diffraction peak and a first shoulder on a right side of the diffraction peak, and a tangent line M is parallel to the straight line L and tangent to the diffraction peak; if an intensity corresponding to a tangent point of the tangent line M is defined as B, and an intensity corresponding to an intersection point of the straight line L and a perpendicular line passing through the tangent point and perpendicular to a horizontal axis is defined as A, then 1 < B / A < 2.5. A saturated water vapor adsorption amount at 25 °C of the hard carbon material is > 200 cm3 / g, and a methylene blue adsorption value of the hard carbon material is < 10 mg / g.

[0059] The capacity of the hard carbon material includes adsorption capacity (the corresponding adsorption of active ions mainly occurs on the outer surface of stacked layers of the hard carbon material) and pore filling capacity (the corresponding filling of active ions mainly occurs in a pore structure that an electrolytic solution cannot enter). The ratio of B / A represents the average number of stacked layers of the hard carbon (refer to YINGHU Liu, et al. “MECHANISM OF LITHIUM INSERTION IN HARD CARBONS PREPARED BY PYROLYSIS OF EPOXY RESINS.” Carbon Vol. 34, No. 2, pp. 193-200, 1996); the smaller the ratio of B / A, the fewer the number of stacked layers, thus increasing the outer surface, which can provide adsorption sites and increase the adsorption capacity. In addition, the larger the water vapor adsorption amount and the smaller the methylene blue adsorption value, the more pore channel structures the electrolytic solution cannot enter; that is, the inner pore regions can store more active ions, and the pore filling capacity increases. The hard carbon material of the present disclosure is enabled to have a high capacity by making the B / A, the saturated water vapor adsorption amount at 25 °C, and the methylene blue adsorption value fall within the above ranges.

[0060] In the present disclosure, the X-ray diffraction pattern of the hard carbon material may be tested using an X-ray diffractometer with reference to JIS K 0131-1996. Illustratively, B / A is 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.2, 2.4, 2.5, or a value within a range defined by any two of the numerical values.

[0061] In the present disclosure, the saturated water vapor adsorption amount at 25 °C indicates the pore volume inside the hard carbon material capable of storing Na. The value can be determined by a dynamic water vapor adsorption (DVS) method. Specifically, after a sample of the hard carbon material is dried at 200 °C for 2 h, the sample is purged with a constant water vapor at a partial pressure of 3.14 kPa carried by nitrogen at a specific flow rate at 25 °C using a dynamic water vapor adsorption instrument, such that the sample undergoes adsorption at the water vapor partial pressure until equilibrium is reached, and the final adsorption value is the saturated water vapor adsorption amount at 25 °C of the sample.

[0062] Illustratively, the saturated water vapor adsorption amount at 25 °C is 200 cm3 / g, 210 cm3 / g, 220 cm3 / g, 250 cm3 / g, 280 cm3 / g, 300 cm3 / g, 320 cm3 / g, 350 cm3 / g, 380 cm3 / g, 400 cm3 / g, or a value within a range defined by any two of the numerical values. Preferably, the saturated water vapor adsorption amount at 25 °C is 350 cm3 / g to 390 cm3 / g.

[0063] In addition, in the present disclosure, the methylene blue adsorption value indicates the density of the surface coating of the material. The value may be determined with reference to GB / T 12496.10-1999. Specifically, the sample of the hard carbon material is mixed and reacted with a specific amount (in milliliters) of a methylene blue solution, left to stand for 30 min, and then filtered, and the absorbance of the filtrate is determined using a spectrophotometer. If the absorbance is lower than the absorbance of a standard solution at a specified concentration, then the consumed number of milliliters of methylene blue x a concentration of methylene blue in the methylene blue solution = a methylene blue adsorption mass, thereby obtaining a methylene blue adsorption value per unit mass of the hard carbon.

[0064] Illustratively, the methylene blue adsorption value may be 10 mg / g, 9 mg / g, 8 mg / g, 7 mg / g, 6 mg / g, 5 mg / g, 4 mg / g, 3 mg / g, 2 mg / g, 1 mg / g, 0.5 mg / g, 0.1 mg / g, or a value within a range defined by any two of the numerical values. Preferably, the methylene blue adsorption value is 0.1 mg / g to 4 mg / g.

[0065] In some embodiments, a compacted density of the hard carbon material is consecutively determined twice under a pressure of 5 tons, with a compacted density determined for a first time set as PD1, and a compacted density determined for a second time set as PD2, and the PD1 and the PD2 satisfy the following relationship: 0 < PD2 - PD1 < 0.03 g / cm3. The difference between PD2 and PD1 falling within the above range indicates that the structure of the hard carbon material is stable, which can reduce the possibility of structural collapse caused by the deintercalation of active ions that would block the ion transport path. This is conducive to the smooth transport of the active ions during charging and discharging processes and is more conducive to improving the charge specific capacity and the initial coulombic efficiency of the battery. A numerical value of PD2 - PD1 may be, for example, 0, 0.01 g / cm3, 0.02 g / cm3, or 0.03 g / cm3. The smaller the numerical value of PD2 - PD1, the more stable the sample structure, such that a stable structure can be maintained in a subsequent electrochemical process to achieve reversible intercalation and deintercalation of the active ions.

[0066] In the present disclosure, a compacted density of the hard carbon material has the meaning well-known in the art, and may be determined using instruments and methods known in the art. For example, the determination may be performed with reference to GB / T 24533-2009 using an electronic compression tester (e.g., UTM7305 electronic compression tester).

[0067] In some embodiments, the hard carbon material includes a matrix and a carbon coating layer located on at least part of a surface of the matrix. In this way, it is conducive to improving the layer stacking and the pore structure in the hard carbon material, such that the numerical value of B / A, the water vapor adsorption amount, and the methylene blue adsorption value fall within the above ranges.

[0068] In some embodiments, the matrix further includes a porous carbon skeleton and a phosphorus element within the porous carbon skeleton, and relative to the hard carbon material, a mass percentage of the phosphorus element is 0.5 wt% to 2.5 wt%. This is more conducive to improving the layer stacking and the pore structure. Illustratively, the mass percentage of the phosphorus element is 0.5 wt%, 0.75 wt%, 1.0 wt%, 1.25 wt%, 1.5 wt%, 1.75 wt%, 2.0 wt%, 2.25 wt%, 2.5 wt%, or a value within a range defined by any two of the numerical values.

[0069] In some embodiments, relative to the hard carbon material, a mass percentage of the coating layer is 1.5 wt% to 7.5 wt%. This is more conducive to forming a dense coating layer, such that the water vapor adsorption amount and the methylene blue adsorption value fall within the above ranges.

[0070] In some embodiments, in a potential range of 0 V to 2.5 V vs. Na / Na+, a charge specific capacity of the hard carbon material is 350 mAh / g or more. Illustratively, the charge specific capacity of the hard carbon material is 350 mAh / g, 370 mAh / g, 400 mAh / g, 420 mAh / g, 450 mAh / g, 470 mAh / g, 490 mAh / g, or a value within a range defined by any two of the numerical values.

[0071] In some embodiments, after the hard carbon material is sintered and heated at 1000 °C for 2h under an inert atmosphere, a surface oxygen content determined under a vacuum condition is set as X1, a surface oxygen content determined after exposure to air with a humidity of < 2% for 30 days is set as X2, and the X1 and the X2 satisfy X2 - X1 < 5 wt%.

[0072] Due to the high surface activity of the hard carbon material, the hard carbon material is highly susceptible to oxidation in air, thereby introducing a large number of oxygen-containing groups onto the surface. In the present disclosure, taking a surface oxygen content X1 determined under a vacuum condition as a benchmark, a surface oxygen content X2 determined after exposure to air with a humidity of < 2% for 30 days is compared with the surface oxygen content X1, and the surface activity level of the hard carbon material is characterized by an increase amount of X2 relative to X1 (that is, an increased amount of O oxidized due to the high surface activity of the hard carbon material). The higher the numerical value of X2 - X1, the higher the surface activity of the hard carbon material.

[0073] The hard carbon material of the present disclosure has X2 - X1 <5 wt%, which indicates that the surface activity of the hard carbon material is low, and the catalytic decomposition capability on an electrolytic solution is small, thereby reducing the amount of active Na that needs to be consumed during the formation of a solid electrolyte interphase (SEI) film, and improving the initial coulombic efficiency.

[0074] The surface oxygen content mentioned in the present disclosure indicates the oxygen element content on the surface of a solid material. The value may be determined by a conventional determination method in the art. For example, reference may be made to GB / T 33502-2017, and an X-ray photoelectron spectrometer (Thermo Fisher K-Alpha) may be adopted as a testing instrument. An Al K-Alpha is adopted as the excitation light source, the energy step is 1.0 eV, and the carbon peak is calibrated using standard C1s (284.6 eV).

[0075] The numerical value of X2 - X1, for example, may be 0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, or a value within a range defined by any two of the numerical values. Preferably, X2 - X1 of the hard carbon material is < 2.5 wt%. In this way, it indicates that the surface activity of the hard carbon material is lower, which is more conducive to improving the initial coulombic efficiency of the secondary battery.

[0076] As an example, the negative electrode current collector has two surfaces opposite to each other in its own thickness direction, and the negative electrode film layer is disposed on any one or both of the two opposite surfaces of the negative electrode current collector.

[0077] In some embodiments, a metal foil or a composite current collector may be used as the negative electrode current collector. For example, a copper foil or an aluminum foil may be adopted as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, or the like) on a polymer material substrate (such as a substrate made of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE)).

[0078] In some embodiments, the negative electrode film layer further optionally includes a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0079] In some embodiments, the negative electrode film layer further optionally includes a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, a carbon dot, a carbon nanotube, graphene, and a carbon nanofiber.

[0080] In some embodiments, the negative electrode film layer further optionally includes other auxiliary agents, such as a thickener (e.g., sodium carboxymethylcellulose (CMC-Na)).

[0081] In some embodiments, the negative electrode plate may be prepared with the following method: dispersing the components described above for preparing the negative electrode plate, such as the negative electrode active material including the hard carbon material, the conductive agent, the binder, and any other components, in a solvent (such as deionized water) to form a negative electrode slurry; and coating the negative electrode current collector with the negative electrode slurry, and performing drying, cold pressing, and other processes, such that the negative electrode plate can be obtained.

[0082] The negative electrode plate prepared using the hard carbon material described above has a high capacity.

[0083] Secondary Battery

[0084] A second aspect of the present disclosure provides a secondary battery, and the secondary battery of the present disclosure will be described below with appropriate reference to the drawings. The term “secondary battery” described herein refers to a battery cell, a battery module, or a battery pack. Description is provided separately below.

[0085] Typically, a secondary battery cell includes a positive electrode plate, the negative electrode plate of the above embodiments, an electrolyte, and a separator. During the charging and discharging processes of the battery, active ions, such as sodium ions, are intercalated and deintercalated back and forth between the positive electrode plate and the negative electrode plate. The electrolyte conducts ions between the positive electrode plate and the negative electrode plate. The separator is disposed between the positive electrode plate and the negative electrode plate to primarily prevent the positive and negative electrodes from short-circuiting while allowing the passage of ions.

[0086] [Positive Electrode Plate]

[0087] The positive electrode plate includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, and the positive electrode film layer includes a positive electrode active material.

[0088] As an example, the positive electrode current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode film layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector.

[0089] In some embodiments, a metal foil or a composite current collector may be used as the positive electrode current collector. For example, as the metal foil, an aluminum foil may be used. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, or the like) on a polymer material substrate (such as a substrate made of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE)).

[0090] In some embodiments, the battery cell is a sodium-ion battery, and a positive electrode active material known in the art for sodium-ion batteries may be used as the positive electrode active material. As an example, the positive electrode active material may include sodium transition metal oxides, polyanionic compounds, Prussian blue compounds, and the like. However, the present disclosure is not limited to these materials, and other conventionally known materials that can be used as positive electrode active materials for sodium-ion batteries may also be used. For example, as an optional technical solution of the present disclosure, the transition metal in the sodium transition metal oxide may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. The sodium transition metal oxide is, for example, NaxMO2, where M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu, and 0 < x < 1.

[0091] As an optional technical solution of the present disclosure, the polyanionic compound may be a class of compounds having sodium ions, transition metal ions, and tetrahedral (YO4)n- anion units. The transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y may be at least one of P, S, and Si; and n represents the valence state of (YO4)n-. The polyanionic compound may also be a class of compounds having sodium ions, transition metal ions, tetrahedral (YO4)n- anion units, and halide anions. The transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y may be at least one of P, S, and Si, and n represents the valence state of (YO4)n-; and the halogen may be at least one of F, Cl, and Br. The polyanionic compound may also be a class of compounds having sodium ions, tetrahedral (YO4)n- anion units, polyhedral units (ZOy)m+, and optional halide anions. Y may be at least one of P, S, and Si, and n represents the valence state of (YO4)n-; Z represents a transition metal, which may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce, and m represents the valence state of (ZOy)m+; and the halogen may be at least one of F, Cl, and Br. The polyanionic compound is, for example, at least one of NaFePO4, Na3V2(PO4)3, NaM’PO4F (where M’ is one or more of V, Fe, Mn, and Ni), and Na3(VOy)2(PO4)2F3-2y (0 < y < 1). The Prussian blue compound may be a class of compounds having sodium ions, transition metal ions, and cyanide ions (CN-). The transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. The Prussian blue compound is, for example, NaaMebMe’c(CN)6, where Me and Me’ are each independently at least one of Ni, Cu, Fe, Mn, Co, and Zn, 0 < a < 2, 0 < b < 1, and 0 < c < 1.

[0092] In some embodiments, when the battery cell is a lithium-ion battery, a positive electrode active material for use in lithium-ion batteries well known in the art may be used as the positive electrode active material.

[0093] The charging and discharging processes of the battery are accompanied by the intercalation and deintercalation and consumption of active ions (Na or K), and the molar content of Li or Na is different when the battery is discharged to different states. In the enumeration related to the positive electrode active material in the present disclosure, the molar content of Li or Na refers to the initial state of the material, that is, the state before material feeding; and when the positive electrode active material is applied in a battery system, the molar content of Li or Na will change after charging and discharging cycles. In the enumeration related to the positive electrode active material in the present disclosure, the molar content of oxygen is only a theoretical value. The molar content of oxygen will vary due to oxygen release from the crystal lattice, and actually, the molar content of oxygen will fluctuate.

[0094] In some embodiments, the positive electrode film layer further optionally includes a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene    fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene   fluoride-hexafluoropropylene-tetrafluoroethylene   terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylic resin.

[0095] In some embodiments, the positive electrode film layer further optionally includes a conductive agent. As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, a carbon dot, a carbon nanotube, graphene, and a carbon nanofiber.

[0096] In some embodiments, the positive electrode plate can be prepared in the following manner: dispersing the components described above for preparing the positive electrode plate, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; and coating the positive electrode current collector with the positive electrode slurry, and performing drying, cold pressing, and other processes, such that the positive electrode plate can be obtained.

[0097] [Electrolyte]

[0098] The electrolyte conducts ions between the positive electrode plate and the negative electrode plate. The present disclosure does not particularly limit the type of electrolyte, which may be selected according to needs. For example, the electrolyte may be liquid, gel, or all solid.

[0099] In some embodiments, the electrolyte is an electrolytic solution. The electrolytic solution includes an electrolyte salt and a solvent.

[0100] In some embodiments, when the battery cell is a sodium-ion battery, the electrolyte salt may be selected from at least one of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium perchlorate, sodium hexafluoroarsenate, sodium bis(fluorosulfonyl)imide,     sodium     bis(trifluoromethanesulfonyl)imide,     sodium trifluoromethanesulfonate, sodium difluorophosphate, sodium difluoro(oxalato)borate, sodium bisoxalatoborate, sodium difluorobisoxalatophosphate, and sodium tetrafluorooxalatophosphate.

[0101] In some embodiments, when the battery cell is a lithium-ion battery, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide,      lithium      bis(trifluoromethanesulfonyl)imide,      lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluoro(oxalato)borate, lithium bis(oxalato)borate, lithium difluorobis(oxalato)phosphate, and lithium tetrafluoro(oxalato)phosphate.

[0102] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, ethyl methyl sulfone, and diethyl sulfone.

[0103] In some embodiments, the electrolytic solution further optionally includes an additive. For example, the additive may include a negative electrode film-forming additive and a positive electrode film-forming additive, and may further include an additive capable of improving certain properties of the battery, such as an additive for improving the overcharge performance of the battery and an additive for improving the high- or low-temperature performance of the battery.

[0104] [Separator]

[0105] In some embodiments, the battery cell further includes a separator. The present disclosure does not particularly limit the type of separator, and any porous-structure separator known to have good chemical stability and mechanical stability may be selected.

[0106] In some embodiments, the separator may be made of a material selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, which is not specified. When the separator is a multi-layer composite film, the materials of the layers may be the same or different, which is not specified.

[0107] In some embodiments, the positive electrode plate, the negative electrode plate, and the separator may be manufactured into an electrode assembly through a winding process or a stacking process.

[0108] In some embodiments, the battery cell may include an outer packaging. The outer packaging may be configured to package the electrode assembly and electrolyte described above.

[0109] In some embodiments, the outer packaging of the battery cell may be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell may also be a soft pack, such as a pouch-type soft pack. The soft pack may be made of plastic, and examples of the plastic may include polypropylene, polybutylene terephthalate, polybutylene succinate, and the like.

[0110] The present disclosure does not particularly limit the shape of the battery cell, and it may have a cylindrical shape, a prismatic shape, or any other shape. For example, FIG. 1 shows a battery cell 5 having a prismatic structure as one example.

[0111] In some embodiments, referring to FIG. 2, the outer packaging may include a shell body 51 and a top cover assembly 53. The shell body 51 may include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates define, in an enclosing manner, an accommodating cavity. The shell body 51 is provided with an opening in communication with the accommodating cavity, and the top cover assembly 53 is capable of covering the opening to close the accommodating cavity. The positive electrode plate, the negative electrode plate, and the separator may be subjected to a winding process or a stacking process to form an electrode assembly 52. The electrode assembly 52 is packaged in the accommodating cavity. The electrolytic solution wets the electrode assembly 52. The number of the electrode assemblies 52 included in the battery cell 5 may be one or more, and those skilled in the art can select the number according to specific and actual needs.

[0112] In some embodiments, the battery cell may be assembled into a battery module. The number of battery cells included in the battery module may be one or more, and the specific number may be selected by those skilled in the art based on the use and capacity of the battery module.

[0113] FIG. 3 shows a battery module 4 as one example. Referring to FIG. 3, in the battery module 4, a plurality of battery cells 5 may be sequentially arranged along the length direction of the battery module 4. Certainly, the arrangement may also be in any other manner. Further, the plurality of battery cells 5 may be fixed by fasteners.

[0114] Optionally, the battery module 4 may further include a shell having an accommodating space in which the plurality of battery cells 5 are accommodated.

[0115] In some embodiments, the battery module described above may also be assembled into a battery pack. The number of battery modules included in the battery pack may be one or more, and the specific number may be selected by those skilled in the art based on the use and capacity of the battery pack.

[0116] FIGs. 4 and 5 show a battery pack 1 as one example. Referring to FIGs. 4 and 5, the battery pack 1 may include a battery case and a plurality of battery modules 4 disposed in the battery case. The battery case includes an upper case body 2 and a lower case body 3. The upper case body 2 is capable of covering the lower case body 3 to form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 may be arranged in any manner in the battery case.

[0117] Electric Device

[0118] A third aspect of the embodiments of the present disclosure further provides an electric device. The electric device of the present disclosure will be described below with appropriate reference to the drawings.

[0119] The electric device mentioned in the embodiments of the present disclosure includes the secondary battery according to the present disclosure. The secondary battery may be used as a power source for the electric device or as an energy storage unit for the electric device. The electric device may include, but is not limited to, a mobile device (e.g., a mobile phone or a laptop computer), an electric vehicle (e.g., a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, or an electric truck), an electric train, a ship, or a satellite, an energy storage system, or the like.

[0120] As for the electric device, a battery cell, a battery module, or a battery pack may be selected according to usage requirements thereof.

[0121] FIG. 6 shows an electric device as one example. The electric device is a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, or the like. To meet the requirements of the electric device for high power and high energy density of the secondary battery, a battery pack or a battery module may be used.

[0122] As another example, the device may be a mobile phone, a tablet computer, a laptop computer, or the like. The device is generally required to be light and thin, and a battery cell can thus be used as a power source.

[0123] Hard Carbon Material

[0124] A fourth aspect of the present disclosure provides a hard carbon material. In an X-ray diffraction pattern of the hard carbon material, there is a diffraction peak at a position where a diffraction angle 20 is 22° to 25°, where a straight line L is tangent to a first shoulder on a left side of the diffraction peak and a first shoulder on a right side of the diffraction peak, and a tangent line M is parallel to the straight line L and tangent to the diffraction peak; if an intensity corresponding to a tangent point of the tangent line M is defined as B, and an intensity corresponding to an intersection point of the straight line L and a perpendicular line passing through the tangent point and perpendicular to a horizontal axis is defined as A, then 1 < B / A < 2.5. A saturated water vapor adsorption amount at 25 °C of the hard carbon material is > 200 cm3 / g, and a methylene blue adsorption value of the hard carbon material is < 10 mg / g.

[0125] In the present disclosure, by making the value of B / A fall within the above range, the adsorption capacity of the hard carbon material is increased. In addition, by making the water vapor adsorption amount and the methylene blue adsorption value fall within the above ranges, the electrolytic solution is prevented from entering the pore channel structure of the hard carbon material, and the pore filling capacity is increased, thereby enabling the hard carbon material to have a high capacity.

[0126] In some embodiments, the saturated water vapor adsorption amount at 25 °C of the hard carbon material is 350 cm3 / g to 390 cm3 / g.

[0127] In some embodiments, the methylene blue adsorption value of the hard carbon material is 0.1 mg / g to 4 mg / g. In this way, the pore filling capacity of the hard carbon material can be further increased.

[0128] In some embodiments, a compacted density of the hard carbon material is consecutively determined twice under a pressure of 5 tons, with a compacted density determined for a first time set as PD1, and a compacted density determined for a second time set as PD2, and the PD1 and the PD2 satisfy the following relationship: 0 < PD2 - PD1 < 0.03 g / cm3. Controlling the difference between PD2 and PD1 within the above range improves the structural stability of the hard carbon material, which is conducive to the transport of active ions during charging and discharging processes and is more conducive to improving the charge specific capacity and the initial coulombic efficiency of the battery.

[0129] In some embodiments, the hard carbon material includes a matrix and a carbon coating layer located on at least part of a surface of the matrix. In this way, it is conducive to improving the layer stacking and the pore structure in the hard carbon material, such that the numerical value of B / A, the water vapor adsorption amount, and the methylene blue adsorption value fall within the above ranges.

[0130] In some embodiments, the matrix includes a porous carbon skeleton and a phosphorus element within the porous carbon skeleton, and relative to the hard carbon material, a mass percentage of the phosphorus element is 0.5 wt% to 2.5 wt%. In this way, it is more conducive to improving the layer stacking and the pore structure.

[0131] In some embodiments, relative to the hard carbon material, a mass percentage of the coating layer is 1.5 wt% to 7.5 wt%. This is more conducive to forming a dense coating layer, such that the water vapor adsorption amount and the methylene blue adsorption value fall within the above ranges.

[0132] In some embodiments, in a potential range of 0 V to 2.5 V vs. Na / Na+, a charge specific capacity of the hard carbon material is 350 mAh / g or more.

[0133] In some embodiments, after the hard carbon material is sintered and heated at 1000 °C for 2h under an inert atmosphere, a surface oxygen content determined under a vacuum condition is set as X1, a surface oxygen content determined after exposure to air with a humidity of < 2% for 30 days is set as X2, and the X1 and the X2 satisfy X2 - X1 < 5 wt%. By keeping the numerical value of X2 - X1 within the above range, the surface activity of the hard carbon material is reduced, and the catalytic decomposition capability of the hard carbon material on the electrolytic solution is decreased, which, in turn, can reduce the amount of active Na that needs to be consumed during the formation of a solid electrolyte interphase (SEI) film. In this way, it is conducive to further improving the initial coulombic efficiency.

[0134] Preparation Method for Hard Carbon Material

[0135] A fifth aspect of the present disclosure provides a preparation method for the hard carbon material according to the fourth aspect of the present disclosure. The method includes the following steps: an impregnation step of impregnating a carbon source in a liquid containing a dopant for an impregnation time of 2 hours or more, where the dopant contains a phosphorus element or a zinc element as a doping element, and relative to a mass of the carbon source, a mass percentage of the phosphorus element is 9.5 wt% to 32.4 wt%, and a mass percentage of the zinc element is 14 wt% to 50 wt%; a low-temperature heat treatment step of heating at 400 °C to 750 °C to obtain a matrix; a kneading step of kneading a mixed solution, in which a resin-based polymer material and the matrix are dispersed, in a kneader, where a mass ratio of the resin-based polymer material to the matrix is (0.5-2):10; and a carbonization step.

[0136] The above steps are separately described below.

[0137] (1) Impregnation step

[0138] The carbon source is fully impregnated in the liquid containing the dopant, such that an activation reaction is uniformly performed in the subsequent low-temperature heat treatment step.

[0139] In the impregnation step, the carbon source includes a carbon-hydrogen-oxygen polymer, which may be, for example, any one or more of a phenolic resin, an epoxy resin, an unsaturated polyester resin, and a furan resin. The carbon source has structural designability and a low impurity content, which is more conducive to improving the layer stacking and the pore structure.

[0140] The dopant contains the phosphorus element or the zinc element, and includes, for example, at least one of phosphoric acid, a phosphate, polyphosphoric acid, and zinc chloride. In some embodiments, relative to the carbon source, the mass percentage of the phosphorus element is, for example, 9.5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 32.4 wt%, or within a range defined by any two of the values. Relative to the carbon source, the mass percentage of the zinc element is 14 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, or within a range defined by any two of the values. In the present disclosure, by forming covalent bonds such as P-C, P-O, Zn-C, and Zn-O during the subsequent low-temperature heat treatment, the above doping elements are doped and embedded into a carbon skeleton structure. Because the sizes of P atoms and Zn atoms are relatively large, the dopant can inhibit the stacking of layers and inhibit the increase in B / A, thereby keeping B / A in the range of 1 to 2.5. In addition, by introducing the above doping elements, an appropriate amount of pore structures can be introduced, such that a saturated water vapor adsorption amount at 25 °C is > 200 cm3 / g, and a methylene blue adsorption value of the hard carbon material is < 10 mg / g.

[0141] In some embodiments, the impregnation time is 2 hours or more, for example, 2 hours to 24 hours. Setting the impregnation time within the above range can allow the dopant to be uniformly distributed in the carbon source, improve the stacking of layers, and control the B / A value. Moreover, the carbon source is allowed to be uniformly activated to introduce an appropriate amount of pore structures, such that the adsorption amount and the adsorption value fall within the specific ranges described above.

[0142] (2) Low-temperature heat treatment step

[0143] In the present disclosure, according to the low-temperature heat treatment, on the one hand, the dopant etches a raw material by means of chemical activation to introduce pore structures into a carbon skeleton structure; on the other hand, the dopant and raw material molecules undergo a chemical reaction to form covalent bonds such as P-C, P-O, Zn-C, and Zn-O, and P atoms / Zn atoms are doped into the carbon skeleton structure. Because the sizes of the P atoms / Zn atoms are larger than the size of carbon atoms, the P atoms / Zn atoms can start to play a role in inhibiting the layer stacking in the initial stage of layer formation.

[0144] The temperature of the low-temperature heat treatment is 400 °C to 750 °C. Illustratively, the temperature may be 400 °C, 450 °C, 500 °C, 550 °C, 600 °C, 650 °C, 700 °C, 750 °C, or within a range defined by any two of the values. Setting the temperature within the above range is more conducive to obtaining a sufficient activation effect, introducing an appropriate amount of pore structures, controlling the layer stacking, and reducing the B / A value.

[0145] In some embodiments, the temperature is raised to 400 °C to 750 °C at a rate of 1 °C / min to 20 °C / min in an N2 or Ar atmosphere, and the temperature is maintained for 1 h to 12 h. In this way, it is more conducive to improving the layer stacking and the pore structure. Illustratively, the heating rate of the low-temperature heat treatment step may be 1 °C / min, 2 °C / min, 4 °C / min, 6 °C / min, 8 °C / min, 10 °C / min, 12 °C / min, 15 °C / min, 18 °C / min, 20 °C / min, or within a range defined by any two of the values.

[0146] In some embodiments, after the low-temperature heat treatment, a crushing treatment is performed to reduce the particle size, and then washing is performed to remove P-containing impurities and the like after the reaction.

[0147] (3) Kneading step

[0148] According to the kneading step, a kneading raw material (resin-based polymer material) is uniformly coated on the surface of the matrix to form a dense organic coating layer.

[0149] In some embodiments, in the kneading step, a kneading time is 0.5 hours or more, thereby facilitating a more uniform distribution of the resin-based polymer material on the surface of the matrix. Illustratively, the kneading time is 0.5 hours, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 8 hours, 9 hours, 10 hours, or within a range defined by any two of the values. Preferably, the kneading time is 3 hours to 9 hours.

[0150] In some embodiments, the resin-based polymer material includes at least one of an epoxy resin, a phenolic resin, an unsaturated polyester resin, and a furan resin. The resin-based polymer material is more conducive to the formation of a dense and uniform coating layer.

[0151] In addition, in some embodiments, in this step, the mixed solution, in which the matrix and the resin-based polymer material are dispersed, is kneaded, and a solid content of the mixed solution is 55 wt% to 75 wt%. Illustratively, the solid content is 55 wt%, 60 wt%, 65 wt%, 70 wt%, 75 wt%, or within a range defined by any two of the values. The solid content of the mixed solution refers to the content of the matrix and the resin-based polymer material in the mixed solution. Setting the solid content within the above range can allow the resin-based polymer material to be distributed more uniformly on the surface of the hard carbon, which is more conducive to the formation of a dense coating layer, thereby controlling the methylene blue adsorption value to 10 mg / g or less.

[0152] In addition, the mass ratio of the resin-based polymer material to the matrix is (0.5-2):10, which is more conducive to the formation of a dense and uniform coating layer with an appropriate thickness and the formation of an appropriate amount of pore structures, thereby controlling the methylene blue adsorption value. Illustratively, the mass ratio of the resin-based polymer material to the matrix of the present disclosure is 0.5:10, 0.7:10, 1:10, 1.2:10, 1.5:10, 1.7:10, 2:10, or within a range defined by any two of the values.

[0153] In some embodiments, the kneading is performed by a twin-screw kneader at a rotational speed of 10 rpm to 50 rpm.

[0154] (4) Carbonization step

[0155] In some embodiments, in the carbonization step, a temperature is raised to 1000 °C to 1800 °C at a heating rate of 2 °C / min to 20 °C / min. This is more conducive to controlling the pyrolysis process of the resin-based polymer material and reducing surface defects. Illustratively, the heating rate of the carbonization step may be 2 °C / min, 4 °C / min, 6 °C / min, 8 °C / min, 10 °C / min, 12 °C / min, 15 °C / min, 18 °C / min, 20 °C / min, or within a range defined by any two of the values. Illustratively, the temperature of the carbonization is 1000 °C, 1200 °C, 1400 °C, 1600 °C, 1800 °C, or within a range defined by any two of the values. In the carbonization step, when the dopant containing the phosphorus element is used, at least part of the phosphorus element incorporated into the matrix is retained; when the dopant containing the zinc element is used, the zinc element incorporated into the matrix is substantially all volatilized.

[0156] In some embodiments, in the carbonization step, under a condition of a pressure of 10 MPa or more, a temperature is raised to 1100 °C to 1600 °C at a heating rate of 2 °C / min to 10 °C / min and held.

[0157] Through the above hot-pressing carbonization, excess H and O in the matrix carbon structure can be removed to form a suitable Na-storing skeleton structure; additionally, the organic matter of the coating layer is thermally pyrolyzed into carbon free radicals to repair the surface defects. By means of pressurization, the escape of the carbon free radicals formed by the pyrolysis of the organic matter of the coating layer can be reduced, allowing the carbon free radicals to remain at surface defect sites for a longer time to repair the surface. The completion rate of the repair is increased, thereby reducing the content of surface defects and further reducing the surface activity.

[0158] In this step, the hot-pressing carbonization is performed under a condition of a pressure of 10 MPa or more. For example, the pressure may be 10 MPa, 20 MPa, 30 MPa, 35 MPa, 40 MPa, 50 MPa, or 60 MPa. Preferably, the pressure is 30 MPa or more. In this way, the escape of the carbon free radicals formed by the pyrolysis of the organic matter of the coating layer can be reduced, allowing the carbon free radicals to stay at the surface defect sites for a relatively long time, thereby improving the efficiency of surface defect repair.

[0159] Examples

[0160] The examples of the present disclosure are illustrated hereinafter. The examples described below are illustrative and are merely used to explain the present disclosure, and they should not be construed as limiting the present disclosure. The examples without techniques or conditions specified therein are implemented according to techniques or conditions described in the literature in the art or according to product instructions. Reagents or instruments used herein without specified manufacturers are all commercially available conventional products.

[0161] Example 1

[0162] Preparation of Hard Carbon Material:

[0163] 1) Impregnation

[0164] In a reaction kettle, 1000 g of phenolic resin (CAS number 9003-35-4, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.) serving as a carbon source was added to a dopant solution (807 g of phosphoric acid, 85 wt%), and impregnation was performed for 6 hours. Relative to the carbon source, a mass percentage of element P is 21.7 wt%.

[0165] 2) Low-temperature heat treatment

[0166] The liquid in step 1) was heated to 600 °C at a heating rate of 5 °C / min under an N2 atmosphere in a tube furnace, and the temperature was kept for 8 hours. Afterward, crushing was performed, followed by washing (to remove P-containing impurities after reaction), to obtain a matrix.

[0167] 3) Kneading

[0168] Phenolic resin (CAS number 9003-35-4, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.), serving as a kneading raw material, and the matrix were mixed at a mass ratio of 1:10 and dispersed into an ethanol solution to obtain a mixed solution, where a solid content of the mixed solution was 65 wt%. Then, the mixed solution was added into a twin-screw kneader, and kneading was performed at a rotational speed of 40 rpm for a kneading time of 6 hours; filtering and discharging were performed, and then vacuum drying was performed at 80 °C for 12 h to obtain a matrix having a coating layer.

[0169] 4) Carbonization

[0170] The matrix having the coating layer was heated to 1250 °C at a heating rate of 2 °C / min under an N2 atmosphere, and the temperature was kept for 4 hours to obtain a hard carbon material.

[0171] Preparation of Negative Electrode Plate:

[0172] The hard carbon material, a binder styrene-butadiene rubber (SBR), a thickener sodium carboxymethylcellulose (CMC-Na), and a conductive agent carbon black were fully stirred and mixed in an appropriate amount of solvent deionized water at a mass ratio of 96.2:1.8:1.2:0.8, to form a uniform negative electrode slurry.

[0173] The uniformly stirred negative electrode slurry was coated on a single side of a Cu foil using a double-sided coater. After the completion of coating, drying, cold pressing, and punching were sequentially performed to prepare a negative electrode plate.

[0174] Preparation of Coin Cell:

[0175] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed at a volume ratio of 1:1:1 to obtain an organic solvent, and sodium hexafluorophosphate (NaPF6) was dissolved in the organic solvent to formulate an electrolytic solution having a concentration of 1 mol / L. Then, a metal Na sheet was used as a counter electrode, and a glass fiber was used as a separator; a CR2430 coin cell was assembled with the prepared negative electrode plate in an argon-protected glove box.

[0176] Examples 2 to 7

[0177] Batteries were prepared in the same manner as in Example 1, except that the preparation conditions were changed according to Table 1.

[0178] Comparative Examples 1 to 3

[0179] Batteries were prepared in the same manner as in Example 1, except that the preparation conditions were changed according to Table 1.

[0180] Tests Related to Hard Carbon Material:

[0181] Test of B / A

[0182] The hard carbon materials prepared in the above examples and comparative examples were tested using a Bruker D8 Discover X-ray diffractometer. The hard carbon material was prepared using a flat plate sample preparation method, with CuKa ray as a radiation source, a copper target as an anode target, a voltage of 40 KV, a current of 40 mA, an anti-scatter slit of 1 mm, a scanning 20 angle range of 20° to 80°, a step size of 0.01671°, a duration of 0.24 s per step, and a scanning rate of 4° / min. Specifically, in the obtained X-ray diffraction pattern, there is a diffraction peak at the position where the diffraction angle 20 is 22° to 25°. A straight line L, tangent to both a first shoulder on the left side of the diffraction peak and a first shoulder on the right side of the diffraction peak, is drawn, and a tangent line M, parallel to the straight line L and tangent to the diffraction peak, is drawn; an intensity corresponding to a tangent point is defined as B; a perpendicular line, passing through the tangent point and perpendicular to a horizontal axis, is drawn, and an intensity corresponding to an intersection point of the perpendicular line and the straight line L is defined as A.

[0183] The above determination was performed on the hard carbon material of Example 1, and the X-ray diffraction pattern of the hard carbon material is shown in FIG. 7. In FIG. 7, B / A = 1.42.

[0184] Saturated water vapor adsorption amount at 25 °C

[0185] After samples of the hard carbon materials prepared in the above examples and comparative examples were dried at 200 °C for 2 h, the samples were purged with constant water vapor at a partial pressure of 3.14 kPa carried by nitrogen at a flow rate of 100 mL / min at 25 °C using a dynamic water vapor adsorption instrument, such that the samples underwent adsorption at the water vapor partial pressure until equilibrium was reached, and the final adsorption value was the saturated water vapor adsorption amount of the sample at 25 °C in cm3 / g.

[0186] Methylene blue adsorption value

[0187] After samples of the hard carbon materials prepared in the above examples and comparative examples were dried at 200 °C for 2 h, 100 mg of the samples was placed into a 100 mL Erlenmeyer flask, and 10 mL of methylene blue solution (methylene blue concentration: 1.5 g / L) was added using a burette. After the hard carbon samples were completely wetted, the hard carbon samples were immediately placed on an electric shaker and shaken for 20 min, and then filtered with a neutral qualitative filter paper. The filtrate was placed into a cuvette with an optical path of 1 cm, and the absorbance was determined at a wavelength of 665 nm using a spectrophotometer. In comparison with the absorbance of a standard copper sulfate color filter solution (4.000 g of copper sulfate pentahydrate was weighed and dissolved in 1000 mL of deionized water), the number of milliliters of a consumed methylene blue test solution multiplied by the concentration of methylene blue and divided by a mass of 0.1 g of hard carbon is the methylene blue adsorption value of the hard carbon in mg / g (referring to GB / T 12496.10-1999).

[0188] PD2 - PD1

[0189] The hard carbon materials prepared in the above examples and comparative examples were determined using a UTM7305 electronic compression tester with reference to GB / T 24533-2009. Specifically, 1 g of sample was weighed and added to a mold with a bottom area of 1.327 cm2. A pressure of 5 tons was applied to the sample, and the pressure was maintained for 30 s. Then the pressure was released, and the mold was kept for 10 s. A powder compacted density PD1 of the sample under the pressure of 5 tons was recorded and calculated. Next, a pressure of 5 tons was applied to the sample again, and the same operation as above was performed to obtain a powder compacted density PD2 of the sample under the pressure of 5 tons.

[0190] Determination of element P content in hard carbon material

[0191] The hard carbon materials prepared in the above examples and comparative examples were digested according to a national standard method (LY / T 1232-1999 Determination of total phosphorus in forest soil). All contained phosphorus compounds were oxidized into orthophosphate, transferred to a 50 mL volumetric flask, and made up to volume. After uniform mixing, the resulting mixture was used as a solution to be tested. The solution was diluted 10 times with deionized water (0.5 mL of the solution to be tested was added with 4.5 mL of deionized water), introduced into a nebulizer via an autosampler, and carried into a plasma torch reaching a temperature of 6000 K to 10000 K by a carrier gas through a spray chamber. Components in the sample were atomized, ionized, and excited. When these excited particles returned to a ground state, a certain amount of energy was released (manifested as a spectrum of a certain wavelength). By determining the specific spectral line and intensity of the element P and comparing with a standard P solution, quantitative analysis of the element P in the sample could be performed. To prepare the standard P solution, 0 mL, 0.1 mL, 0.2 mL, 0.4 mL, 0.8 mL, 1.6 mL, and 10 mL of a solution with a PO43- concentration of 50 mg / L were added to 50 mL volumetric flasks, respectively. A control solution with the same background ionic strength as the sample was then added and made up to volume with deionized water. The selected testing wavelengths were 214.914 nm and 178.222 nm.

[0192] Battery Performance Test

[0193] Specific capacity and initial coulombic efficiency

[0194] At 25 °C, the coin cells prepared in the above examples and comparative examples were first discharged at a constant current with a current density of 10 mA / g to 0 V, and an initial discharge capacity of the coin cells was recorded. Thereafter, the coin cells were charged at a constant current with a current density of 10 mA / g to 2.5 V, and an initial charge capacity of the coin cells was recorded. The mass of the hard carbon material in the negative electrode plate was calculated according to the coating weight and the coating area of the slurry during the preparation of the electrode plate described above. Initial charge specific capacity = initial charge capacity / mass of hard carbon material; initial discharge specific capacity = initial discharge capacity / mass of hard carbon material; and initial coulombic efficiency (%) of coin cell = initial charge specific capacity of coin cell / initial discharge specific capacity of coin cell x 100%.

[0195] Test results of B / A, methylene blue adsorption value, water vapor adsorption amount, PD2 - PD1, and specific capacity of the hard carbon materials prepared in the above Examples 1 to 7 and Comparative Examples 1 to 3 are shown in Table 2. Table 1: Impregnation step Low-temperature heat treatment step Kneading step Content of element P of dopant relative to mass of carbon source (wt%) Impregnation time (hour) Temperature (°C) Mass ratio of resin-based polymer to matrix Kneading time (hour) Solid content of mixed solution (wt%) Example 1 21.7 6 600 1:10 6 65 Example 2 31.2 6 600 1.6:10 6 65 Example 3 21.7 2 600 1:10 6 60 Example 4 12.2 6 500 0.75:10 6 65 Example 5 21.7 6 600 2:10 12 65 Example 6 21.7 6 600 0.5:10 0.5 75 Example 7 21.7 6 700 1.2:10 6 60 Comparative Example 1 4.1 6 400 0.6:10 3 55 Comparative Example 2 21.7 0.5 300 1:10 6 65 Comparative Example 3 21.7 6 600 0.3:10 6 65 Table 2 Serial number B / A Saturated water vapor adsorptio n amount at 25 °C (cm3 / g) Methylene blue adsorption value (mg / g) PD2-PD 1 (g / cm3) Content of element P relative to hard carbon material (wt%) Discharge specific capacity (mAh / g) Charge specific capacit y (mAh / g ) Initial coulombic efficiency Example 1 1.42 379 1.1 0.025 1.518 463 420 90.71% Example 2 1 396 8.1 0.029 2.150 485 401 82.68% Example 3 2 350 4.8 0.017 1.518 434 371 85.48% Example 4 2.5 314 1.3 0.015 0.866 396 354 89.39% Example 5 1.42 200 0.2 0.027 1.474 390 356 91.28% Example 6 1.42 374 9.1 0.026 1.561 459 365 79.52% Example 7 1.02 399 10.00 0.038 1.507 488 378 77.46% Comparati ve Example 1 2.67 256 0.4 0.003 0.291 302 275 91.06% Comparati ve Example 2 2.71 173 0.1 0.001 1.518 271 249 91.88% Comparati ve Example 3 1.42 371 15.2 0.027 1.583 457 324 70.90%

[0196] As can be seen from Table 2, by enabling the hard carbon material to have 1 < B / A < 2.5, the saturated water vapor adsorption amount at 25 °C > 200 cm3 / g, and the methylene blue adsorption value < 10 mg / g, the charge specific capacity can be significantly improved. In addition, by controlling the numerical value of PD2 - PD1 within the range of 1 g / cm3 to 0.03 g / cm3, an excellent initial coulombic efficiency can be further obtained.

[0197] It should be noted that the present disclosure is not limited to the embodiments described above. The embodiments described above are merely examples, and any embodiments having a structure substantially identical to the technical concept and exerting the same functional effects within the scope of the technical solutions of the present disclosure are all included within the technical scope of the present disclosure. Furthermore, without departing from the spirit of the present disclosure, various modifications that can be conceived by those skilled in the art to the embodiments, as well as other embodiments formed by combining some of the constituent elements of the embodiments, are also included within the scope of the present disclosure.

Claims

1. A negative electrode plate, comprising a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, wherein the negative electrode film layer comprises a hard carbon material;in an X-ray diffraction pattern of the hard carbon material, there is a diffraction peak at a position where a diffraction angle 20 is 22° to 25°, wherein a straight line L is tangent to a first shoulder on a left side of the diffraction peak and a first shoulder on a right side of the diffraction peak, and a tangent line M is parallel to the straight line L and tangent to the diffraction peak; if an intensity corresponding to a tangent point of the tangent line M is defined as B, and an intensity corresponding to an intersection point of the straight line L and a perpendicular line passing through the tangent point and perpendicular to a horizontal axis is defined as A, then 1 < B / A < 2.5; anda saturated water vapor adsorption amount at 25 °C of the hard carbon material is > 200 cm3 / g, and a methylene blue adsorption value of the hard carbon material is < 10 mg / g.

2. The negative electrode plate according to claim 1, wherein the saturated water vapor adsorption amount at 25 °C of the hard carbon material is 350 cm3 / g to 390 cm3 / g.

3. The negative electrode plate according to claim 1 or 2, wherein the methylene blue adsorption value of the hard carbon material is 0.1 mg / g to 4 mg / g.

4. The negative electrode plate according to any one of claims 1 to 3, wherein a compacted density of the hard carbon material is consecutively determined twice under a pressure of 5 tons, with a compacted density determined for a first time set as PD1, and a compacted density determined for a second time set as PD2, and the PD1 and the PD2 satisfy the following relationship:0 < PD2 - PD1 < 0.03 g / cm3.

5. The negative electrode plate according to any one of claims 1 to 4, wherein the hard carbon material comprises a matrix and a carbon coating layer located on at least part of a surface of the matrix.

6. The negative electrode plate according to any one of claims 1 to 5, wherein the matrix comprises a porous carbon skeleton and a phosphorus element within the porous carbon skeleton, and relative to the hard carbon material, a mass percentage of the phosphorus element is 0.5 wt% to 2.5 wt%.

7. The negative electrode plate according to any one of claims 1 to 6, wherein relative to the hard carbon material, a mass percentage of the coating layer is 1.5 wt% to 7.5 wt%.

8. The negative electrode plate according to any one of claims 1 to 7, wherein in a potential range of 0 V to 2.5 V vs. Na / Na+, a charge specific capacity of the hard carbon material is 350 mAh / g or more.

9. The negative electrode plate according to any one of claims 1 to 8, wherein after the hard carbon material is sintered and heated at 1000 °C for 2h under an inert atmosphere, a surface oxygen content determined under a vacuum condition is set as X1, a surface oxygen content determined after exposure to air with a humidity of < 2% for 30 days is set as X2, and the X1 and the X2 satisfy X2 - X1 < 5 wt%.

10. A secondary battery, wherein the secondary battery comprises the negative electrode plate according to any one of claims 1 to 9.

11. The secondary battery according to claim 10, wherein the secondary battery further comprises a positive electrode plate, and the positive electrode plate comprises, as a positive electrode active material, at least one selected from a transition metal oxide, a polyanionic compound, and a Prussian blue compound.

12. An electric device, comprising the secondary battery according to claim 10 or 11.

13. A hard carbon material, wherein in an X-ray diffraction pattern of the hard carbon material, there is a diffraction peak at a position where a diffraction angle 20 is 22° to 25°, wherein a straight line L is tangent to a first shoulder on a left side of the diffraction peak and a first shoulder on a right side of the diffraction peak, and a tangent line M is parallel to the straight line L and tangent to the diffraction peak; if an intensity corresponding to a tangent point of the tangent line M is defined as B, and an intensity corresponding to an intersection point of the straight line L and a perpendicular line passing through the tangent point and perpendicular to a horizontal axis is defined as A, then 1 < B / A < 2.5; anda saturated water vapor adsorption amount at 25 °C of the hard carbon material is > 200 cm3 / g, and a methylene blue adsorption value of the hard carbon material is < 10 mg / g.

14. The hard carbon material according to claim 13, wherein the saturated water vapor adsorption amount at 25 °C of the hard carbon material is 350 cm3 / g to 390 cm3 / g.

15. The hard carbon material according to claim 13 or 14, wherein the methylene blue adsorption value of the hard carbon material is 0.1 mg / g to 4 mg / g.

16. The hard carbon material according to any one of claims 13 to 15, wherein a compacted density of the hard carbon material is consecutively determined twice under a pressure of 5 tons, with a compacted density determined for a first time set as PD1, and a compacted density determined for a second time set as PD2, and the PD1 and the PD2 satisfy the following relationship:0 < PD2 - PD1 < 0.03 g / cm3.

17. The hard carbon material according to any one of claims 13 to 16, wherein the hard carbon material comprises a matrix and a carbon coating layer located on at least part of a surface of the matrix.

18. The hard carbon material according to any one of claims 13 to 17, wherein the matrix comprises a porous carbon skeleton and a phosphorus element within the porous carbon skeleton, and relative to the hard carbon material, a mass percentage of the phosphorus element is 0.5 wt% to 2.5 wt%.

19. The hard carbon material according to any one of claims 13 to 18, wherein relative to the hard carbon material, a mass percentage of the coating layer is 1.5 wt% to 7.5 wt%.

20. The hard carbon material according to any one of claims 13 to 19, wherein in a potential range of 0 V to 2.5 V vs. Na / Na+, a charge specific capacity of the hard carbon material is 350 mAh / g or more.

21. The hard carbon material according to any one of claims 13 to 20, wherein after the hard carbon material is sintered and heated at 1000 °C for 2h under an inert atmosphere, a surface oxygen content determined under a vacuum condition is set as X1, a surface oxygen content determined after exposure to air with a humidity of < 2% for 30 days is set as X2, and the X1 and the X2 satisfy X2 - X1 < 5 wt%.

22. A preparation method for a hard carbon material, comprising the following steps:an impregnation step of impregnating a carbon source in a liquid containing a dopant for an impregnation time of 2 hours or more, wherein the dopant contains a phosphorus element or a zinc element as a doping element, and relative to a mass of the carbon source, a mass percentage of the phosphorus element is 9.5 wt% to 32.4 wt%, and a mass percentage of the zinc element is 14 wt% to 50 wt%;a low-temperature heat treatment step of heating at 400 °C to 750 °C to obtain a matrix;a kneading step of kneading a mixed solution, in which a resin-based polymer materialand the matrix are dispersed, in a kneader, wherein a mass ratio of the resin-based polymer material to the matrix is (0.5-2):10; anda carbonization step.

23. The preparation method according to claim 22, wherein the dopant comprises at least one of phosphoric acid, a phosphate, polyphosphoric acid, and zinc chloride.

24. The preparation method according to claim 22 or 23, wherein the resin-based polymer material comprises at least one of an epoxy resin, a phenolic resin, an unsaturated polyester resin, and a furan resin.

25. The preparation method according to any one of claims 22 to 24, wherein in the kneading step, a kneading time is 0.5 hours or more.

26. The preparation method according to any one of claims 22 to 25, wherein in the kneading step, a solid content of the mixed solution is 55 wt% to 75 wt%.

27. The preparation method according to any one of claims 22 to 26, wherein in the low-temperature heat treatment step, a temperature is raised to 400 °C to 750 °C at a heating rate of 1 °C / min to 20 °C / min, and a holding time is 1 hour to 12 hours.

28. The preparation method according to any one of claims 22 to 27, wherein in the carbonization step, a temperature is raised to 1000 °C to 1800 °C at a heating rate of 2 °C / min to 20 °C / min.

29. The preparation method according to claim 28, wherein in the carbonization step, under a condition of a pressure of 10 MPa or more, a temperature is raised to 1100 °C to 1600 °C at a heating rate of 2 °C / min to 10 °C / min and held.

Images