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

Abstract

The present disclosure provides 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; the hard carbon material comprises a core and an organic coating layer wrapping the core; and the core comprises porous carbon. The total pore volume A of the porous carbon satisfies: 0.2cm3 / g≤A≤0.6cm3 / g; and the methylene blue adsorption value Z of the hard carbon material is less than or equal to 8 mg / g.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present disclosure is based on the Chinese Patent Application No. 202410544841.2, filed on April 30, 2024, entitled "NEGATIVE ELECTRODE SHEET, SECONDARY BATTERY, ELECTRIC DEVICE, HARD CARBON MATERIAL, AND PREPARATION METHOD THEREFOR", and claims priority to this Chinese Patent Application. The entire content of this Chinese Patent Application 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 sheet, a secondary battery, an electric device, a hard carbon material, and a preparation method therefor. BACKGROUND

[0003] In recent years, secondary batteries have been widely used in energy storage power supply systems such as hydropower stations, thermal power stations, wind power stations, and solar power stations, and in multiple fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, and aerospace. With the application and popularization of secondary batteries, people have increasingly higher requirements for the energy density, cycling performance, and high-rate charging performance of secondary batteries. As an important component of secondary batteries, the negative electrode active material affects the performance of secondary batteries to a certain extent. Currently, hard carbon is commonly used as the negative electrode active material for secondary batteries. However, when hard carbon is used as the negative electrode active material, the charge specific capacity and first coulombic efficiency are low. SUMMARY

[0004] The present disclosure has been made in view of the above-mentioned issues, and aims to provide a negative electrode sheet, a secondary battery, an electric device, a hard carbon material, and a preparation method therefor. The negative electrode sheet, when used in a secondary battery, has improved charge specific capacity and first coulombic efficiency.

[0005] To achieve the above objective, a first aspect of the present disclosure provides a negative electrode sheet. The negative electrode sheet 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. The hard carbon material includes a core and an organic coating layer coating the core. The core includes porous carbon. A total pore volume A of the porous carbon satisfies: 0.2 cm3 / g < A < 0.6 cm3 / g. A methylene blue adsorption value Z of the hard carbon material is less than or equal to 8 mg / g.

[0006] In the present disclosure, by controlling the total pore volume of the porous carbon in the hard carbon material within the above range, the specific capacity of the hard carbon material can be improved while ensuring that the carbon skeleton of the hard carbon material has appropriate strength and stability, so that no collapse occurs during the intercalation and deintercalation of active ions, thereby improving the charge specific capacity and first coulombic efficiency of the secondary battery. In addition, by providing the organic coating layer on the outer surface of the porous carbon core, electrolyte solution molecules can be blocked from entering the pore structure of the porous carbon, which is beneficial to improving the charge specific capacity and first coulombic efficiency of the secondary battery. Further, the methylene blue adsorption value of the hard carbon material is within the above range, so that the organic coating layer has high denseness, can effectively isolate an electrolyte solution, and inhibit electrolyte solution molecules from entering the interior of the porous carbon, which is further beneficial to improving the charge specific capacity and first coulombic efficiency of the secondary battery.

[0007] In some embodiments, the methylene blue adsorption value Z of the hard carbon material is less than or equal to 4 mg / g. The smaller the methylene blue adsorption value, the better the coating effect. Thus, it is more beneficial to improving the charge specific capacity and first coulombic efficiency of the secondary battery.

[0008] In some embodiments, the porous carbon includes micropores having a pore size of 0.4 nm to 2.5 nm and mesopores having a pore size of 2.5 nm to 10 nm. A total pore volume X of the micropores and a total pore volume Y of the mesopores satisfy: 5.6% < X / Y < 33%, and X + Y = A. In some embodiments, the total pore volume X of the micropores and the total pore volume Y of the mesopores satisfy: 7 < X / Y < 16. Thus, the storage space inside the hard carbon material can be maximally utilized. Moreover, active ions can be reversibly intercalated and deintercalated. This is beneficial to the charge specific capacity and first coulombic efficiency of the secondary battery.

[0009] In some embodiments, the total pore volume X of the micropores and the total pore volume A of the porous carbon satisfy: 85% < X / A < 97%. Thus, more storage space for active ions can be provided, which is beneficial to improving the discharge specific capacity of the hard carbon material.

[0010] In some embodiments, the total pore volume Y of the mesopores and the total pore volume A of the porous carbon satisfy: 3% < Y / A < 15%. Thus, an appropriate number of transport channels for active ions can be provided, thereby improving the utilization rate of the micropores, and facilitating the improvement of the charge specific capacity and first coulombic efficiency of the secondary battery.

[0011] In some embodiments, the organic coating layer has a thickness of 1 nm to 90 nm. Thus, the kinetic performance of the secondary battery can be balanced with the charge specific capacity and first coulombic efficiency of the secondary battery.

[0012] In some embodiments, a mass of the organic coating layer in the hard carbon material is 0.5 wt% to 10 wt% of the porous carbon. Thus, the kinetic performance of the secondary battery can be balanced with the charge specific capacity and first coulombic efficiency of the secondary battery.

[0013] In some embodiments, the hard carbon material satisfies at least one of the following items: (1) The hard carbon material has a compacted density of 0.55 g / cm3 to 1.0 g / cm3 under 5 t. When the powder compacted density of the hard carbon material is within the above range, it is beneficial to forming a reasonable pore channel structure between particles of the negative electrode film layer, improving the transport performance of active ions and electrons, and thereby improving the kinetic performance of the secondary battery. (2) The hard carbon material has a tap density of 0.34 g / cm3 to 0.8 g / cm3. When the tap density of the hard carbon material is within the above range, it is beneficial to improving the compacted density of the negative electrode film layer and the energy density of the secondary battery. (3) The hard carbon material has a volume distribution particle size Dv50 of 3.0 pm to 7.9 pm. (4) The hard carbon material has a volume distribution particle size Dv90 of 8 pm to 15 pm. When the volume distribution particle sizes Dv50 and Dv90 of the hard carbon material particles are within the above ranges, it is beneficial to reducing the specific surface area of the hard carbon material and reducing the occurrence of side reactions, thereby improving the first coulombic efficiency of the secondary battery. At the same time, a bulk phase transport path of active ions can be shortened, thereby improving the kinetic performance of the secondary battery. (5) The hard carbon material has a BET specific surface area of 0.1 m2 / g to 0.8 m2 / g. When the specific surface area of the hard carbon material is within the above range, the consumption of active ions during the first charge can be reduced, which is beneficial to improving the first coulombic efficiency of the secondary battery.

[0014] A second aspect of the present disclosure provides a secondary battery. The secondary battery includes the negative electrode sheet according to the first aspect of the present disclosure.

[0015] In some embodiments, the secondary battery further includes a positive electrode sheet. The positive electrode sheet includes at least one selected from the group consisting of transition metal oxides, polyanionic compounds, and Prussian blue compounds as a positive electrode active material.

[0016] The secondary battery of the present disclosure has improved charge specific capacity and first coulombic efficiency.

[0017] 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.

[0018] The electric device of the present disclosure includes the secondary battery provided by the present disclosure, and thus has at least the same advantages as the secondary battery.

[0019] A fourth aspect of the present disclosure provides a hard carbon material. The hard carbon material includes a core and an organic coating layer coating the core. The core includes porous carbon. A total pore volume A of the porous carbon satisfies: 0.2 cm3 / g < A < 0.6 cm3 / g. A methylene blue adsorption value Z of the hard carbon material is less than or equal to 8 mg / g.

[0020] In the present disclosure, by controlling the total pore volume of the porous carbon in the hard carbon material within the above range, the specific capacity of the hard carbon material can be improved while ensuring that the carbon skeleton of the hard carbon material has appropriate strength and stability, so that no collapse occurs during the intercalation and deintercalation of active ions, thereby improving the charge specific capacity and first coulombic efficiency of the secondary battery. In addition, by providing the organic coating layer on the outer surface of the porous carbon core, electrolyte solution molecules can be blocked from entering the pore structure of the porous carbon, which is beneficial to improving the charge specific capacity and first coulombic efficiency of the secondary battery. Further, the methylene blue adsorption value of the hard carbon material is within the above range, so that the organic coating layer has high denseness, can effectively isolate an electrolyte solution, and inhibit electrolyte solution molecules from entering the interior of the porous carbon, which is further beneficial to improving the charge specific capacity and first coulombic efficiency of the secondary battery.

[0021] In some embodiments, the methylene blue adsorption value Z of the hard carbon material is less than or equal to 4 mg / g. The smaller the methylene blue adsorption value, the better the coating effect. Thus, it is more beneficial to improving the charge specific capacity and first coulombic efficiency of the secondary battery.

[0022] In some embodiments, the porous carbon includes micropores having a pore size of 0.4 nm to 2.5 nm and mesopores having a pore size of 2.5 nm to 10 nm. A total pore volume X of the micropores and a total pore volume Y of the mesopores satisfy: 5.6% < X / Y < 33%, and X + Y = A. In some embodiments, the total pore volume X of the micropores and the total pore volume Y of the mesopores satisfy: 7 < X / Y < 16. Thus, the storage space inside the hard carbon material can be maximally utilized. Moreover, active ions can be reversibly intercalated and deintercalated. This is beneficial to the charge specific capacity and first coulombic efficiency of the secondary battery.

[0023] In some embodiments, the total pore volume X of the micropores and the total pore volume A of the porous carbon satisfy: 85% < X / A < 97%. Thus, more storage space for active ions can be provided, which is beneficial to improving the discharge specific capacity of the hard carbon material.

[0024] In some embodiments, the total pore volume Y of the mesopores and the total pore volume A of the porous carbon satisfy: 3% < Y / A < 15%. Thus, an appropriate number of transport channels for active ions can be provided, thereby improving the utilization rate of the micropores, and facilitating the improvement of the charge specific capacity and first coulombic efficiency of the secondary battery.

[0025] In some embodiments, the organic coating layer has a thickness of 1 nm to 90 nm. Thus, the kinetic performance of the secondary battery can be balanced with the charge specific capacity and first coulombic efficiency of the secondary battery.

[0026] In some embodiments, a mass of the organic coating layer in the hard carbon material is 0.5 wt% to 10 wt% of the porous carbon. Thus, the kinetic performance of the secondary battery can be balanced with the charge specific capacity and first coulombic efficiency of the secondary battery.

[0027] In some embodiments, the hard carbon material satisfies at least one of the following items:

[0028] (1) The hard carbon material has a compacted density of 0.55 g / cm3 to 1.0 g / cm3 under 5 t. When the powder compacted density of the hard carbon material is within the above range, it is beneficial to forming a reasonable pore channel structure between particles of the negative electrode film layer, improving the transport performance of active ions and electrons, and thereby improving the kinetic performance of the secondary battery.

[0029] (2) The hard carbon material has a tap density of 0.34 g / cm3 to 0.8 g / cm3. When the tap density of the hard carbon material is within the above range, it is beneficial to improving the compacted density of the negative electrode film layer and the energy density of the secondary battery.

[0030] (3) The hard carbon material has a volume distribution particle size Dv50 of 3.0 gm to 7.9 gm.

[0031] (4) The hard carbon material has a volume distribution particle size Dv90 of 8 gm to 15 gm. When the volume distribution particle sizes Dv50 and Dv90 of the hard carbon material particles are within the above ranges, it is beneficial to reducing the specific surface area of the hard carbon material and reducing the occurrence of side reactions, thereby improving the first coulombic efficiency of the secondary battery. At the same time, a bulk phase transport path of active ions can be shortened, thereby improving the kinetic performance of the secondary battery.

[0032] (5) The hard carbon material has a BET specific surface area of 0.1 m2 / g to 0.8 m2 / g. When the specific surface area of the hard carbon material is within the above range, the consumption of active ions during the first charge can be reduced, which is beneficial to improving the first coulombic efficiency of the secondary battery.

[0033] A fifth aspect of the present disclosure further provides a preparation method for a hard carbon material. The method includes:

[0034] subjecting a hard carbon precursor to a pre-carbonization treatment to obtain a pre-carbonized body; subjecting the pre-carbonized body to an activation treatment to obtain a porous carbon precursor; subjecting the porous carbon precursor to a carbonization treatment to obtain porous carbon; and subjecting the porous carbon to a coating treatment, where the coating treatment includes a kneading treatment of kneading a kneaded mixture formed by the porous carbon and an aqueous solution of a coating raw material, where the kneaded mixture has a solid content of 55 wt% to 75 wt%, and the aqueous solution of the coating raw material has a pH value of < 3.

[0035] In the present disclosure, by subjecting the hard carbon precursor to the precarbonization treatment and the activation pore-forming treatment, the porous carbon precursor is obtained, and the porous carbon precursor is subjected to the carbonization treatment to obtain the porous carbon. Then, the porous carbon and the aqueous solution of the coating raw material are subjected to the kneading treatment to form a hard carbon material having a coating layer. Under the above solid content and the pH condition of the aqueous solution of the coating raw material, the porous carbon and the coating raw material are kneaded, so that the two are more fully mixed and sheared, and the formed coating layer thus has good denseness. Therefore, electrolyte solution molecules can be blocked from entering the interior of the porous carbon, which is beneficial to improving the charge specific capacity and first coulombic efficiency of the secondary battery.

[0036] In some embodiments, the kneaded mixture has a solid content of 63 wt% to 67 wt%, and the aqueous solution of the coating raw material has a pH value of 1.2 to 1.8. Thus, a coating layer having an appropriate denseness can be formed, which can improve the charge specific capacity and first coulombic efficiency of the secondary battery without affecting the kinetic performance of the secondary battery.

[0037] In some embodiments, the coating raw material includes at least one of polyacrylic acid, a polyacrylate salt, and a copolymer of polyacrylic acid. Carboxyl groups on molecular chains of polyacrylic acid can react with oxygen-containing functional groups on a surface of the porous carbon to form carbon-oxygen covalent bonds. The carbon-oxygen covalent bonds enable the organic coating layer to be tightly connected to the porous carbon, forming an organic coating layer with high stability and high denseness. The organic coating layer can inhibit the electrolyte solution from entering the interior of the porous carbon, which is beneficial to improving the charge specific capacity and first coulombic efficiency of the secondary battery.

[0038] In some embodiments, a mass of the coating raw material is 0.5% to 10% of a mass of the porous carbon, and the kneading treatment is performed for at least 0.5 h. Thus, a coating layer having an appropriate denseness can be formed, which can improve the charge specific capacity and first coulombic efficiency of the secondary battery without affecting the kinetic performance of the secondary battery and without increasing the impedance of the secondary battery.

[0039] In some embodiments, the kneading treatment is performed in a twin-screw kneader, and the twin-screw kneader has a rotational speed of 10 rpm to 50 rpm. Under the above conditions, an organic coating layer with uniform coating can be obtained.

[0040] In some embodiments, the activation treatment includes treating the precarbonized body at a temperature of 700°C to 950°C under a predetermined mixed gas, the predetermined mixed gas includes carbon dioxide gas, water vapor, and an inert gas, and a volume ratio of the carbon dioxide gas to the water vapor in the predetermined mixed gas is greater than or equal to 2. The activation treatment performed at the above temperature can form an appropriate amount of mesopores and micropores, which is beneficial to improving the discharge specific capacity of the secondary battery.

[0041] In some embodiments, the carbon dioxide gas has a volume fraction of 5% to 20% in the predetermined mixed gas, and the water vapor has a volume fraction of 1% to 5% in the mixed gas. When the volume fraction of the carbon dioxide gas in the predetermined mixed gas is within the above range, an appropriate amount of micropores can be formed, which is beneficial to improving the discharge specific capacity of the secondary battery. When the volume fraction of the water vapor in the predetermined mixed gas is within the above range, an appropriate amount of mesopores can be formed, which is beneficial to improving the charge specific capacity and first coulombic efficiency of the secondary battery.

[0042] In some embodiments, the activation treatment includes impregnating the precarbonized body in a pore-forming solution for at least 2 h. The pore-forming solution includes a P-containing pore-forming solution or a Zn-containing pore-forming solution, where an addition amount of element P in the P-containing pore-forming solution is 9.5% to 32.4% of a mass of the porous carbon, and an addition amount of element Zn in the Zn-containing poreforming solution is 14% to 50% of the mass of the porous carbon. Thus, an appropriate amount of micropores and mesopores can be formed, which is beneficial to improving the discharge specific capacity, charge specific capacity, and first coulombic efficiency of the secondary battery.

[0043] In some embodiments, the impregnated mixture formed by the pre-carbonized body and the pore-forming solution has a solid content of 50 wt% to 70 wt%.

[0044] In some embodiments, the activation treatment further includes maintaining an impregnated mixture formed by the impregnated pre-carbonized body and the pore-forming solution at a temperature of 400°C to 750°C for 1 h to 12 h. Thus, the P-containing poreforming solution can chemically interact with the pre-carbonized body, and etching is performed by a chemical activation method, introducing a large number of pore structures into the skeleton structure of the pre-carbonized body.

[0045] In some embodiments, the pre-carbonization treatment is performed by raising a temperature to 400°C to 600°C at a heating rate of 1°C / min to 20°C / min, and maintaining the temperature for 1 h to 12 h. The pre-carbonization treatment performed under the above conditions is beneficial to forming a basic carbon skeleton structure, facilitating subsequent pore formation.

[0046] In some embodiments, the carbonization treatment is performed by raising a temperature to 1000°C to 1800°C at a heating rate of 2°C / min to 20°C / min, and maintaining the temperature for 1 h to 12 h. During the carbonization process, heteroatoms such as H and O are removed from the precursor, forming a stable hard carbon skeleton.

[0047] In some embodiments, the hard carbon precursor includes at least one of a biomass precursor and a synthetic polymer precursor. The synthetic polymer precursor includes a compound composed of elements C, H, and O. Using the above precursors to prepare the hard carbon material, the carbon formed after activation is relatively ordered, and an appropriate amount of micropores and mesopores can be formed. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] FIG. 1 is a schematic diagram of a battery cell according to an embodiment of the present disclosure;

[0049] FIG. 2 is an exploded view of the battery cell according to an embodiment of the present disclosure shown in FIG. 1;

[0050] FIG. 3 is a schematic diagram of a battery module according to an embodiment of the present disclosure;

[0051] FIG. 4 is a schematic diagram of a battery pack according to an embodiment of the present disclosure;

[0052] FIG. 5 is an exploded view of the battery pack according to an embodiment of the present disclosure shown in FIG. 4; and

[0053] FIG. 6 is a schematic diagram of an electric device using a secondary battery according to an embodiment of the present disclosure as a power source.

[0054] Description of reference signs: 1-battery pack; 2-upper box body; 3-lower box body; 4-battery module; 5-battery cell; 51-case; 52-electrode assembly; and 53-top cover assembly. DETAILED DESCRIPTION

[0055] Hereinafter, embodiments of a negative electrode sheet, a secondary battery, an electric device, a hard carbon material, and a preparation method therefor specifically disclosed in the present disclosure will be described in detail with appropriate reference to the accompanying drawings. However, there may be situations where unnecessary detailed descriptions are omitted. For example, there may be situations where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted, for the purpose of preventing the following descriptions from becoming unnecessarily lengthy, and helping to understand by those skilled in the art. In addition, the drawings and subsequent descriptions are provided for those skilled in the art to fully understand the present disclosure and are not intended to limit the subject matters defined in the claims.

[0056] Any "range" disclosed in the present disclosure is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit. The selected lower limit and upper limit define boundaries of the particular range. The range defined in this way may include or exclude end values, and may be arbitrarily combined, that is, any lower limit may be combined with any upper limit to form a new range. For example, if ranges 60 to 120 and 80 to 110 are listed for a particular parameter, it is to be understood that ranges of 60 to 110 and 80 to 120 are also expectable. Additionally, if the minimum range values 1 and 2 and the maximum range values 3, 4 and 5 are listed, the following ranges are all expectable: 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 stated, the numerical range "from a to b" represents 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" means that all real numbers between 0 and 5 are listed herein, and "0 to 5" is merely an abbreviated representation of the combination of these numbers. In addition, when a certain parameter is expressed as an integer >2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or the like.

[0057] Unless otherwise specified, all the embodiments and optional embodiments of the present disclosure may be combined with each other to form new technical solutions.

[0058] Unless otherwise specified, all technical features and optional technical features of the present disclosure may be combined with each other to form new technical solutions.

[0059] Unless otherwise specified, all steps in the present disclosure may be performed sequentially or randomly, and preferably, sequentially. For example, the method includes steps (a) and (b), meaning that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, "the method may further include step (c)" represents that step (c) may be added to the method in any order. For example, the method may include steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), or the like.

[0060] Unless otherwise specially stated, the terms used in the present disclosure have well-known meanings as commonly understood by those skilled in the art.

[0061] Unless otherwise specifically stated, the values of the parameters mentioned in the present disclosure may be determined by various test methods commonly used in the art. For example, they may be determined according to the test methods given in the present disclosure.

[0062] Unless otherwise specially stated, in the present disclosure, the term "active ions" refers to ions that can be intercalated and deintercalated back and forth between the positive electrode and the negative electrode of the secondary battery, including but not limited to lithium ions.

[0063] Currently, hard carbon is commonly used as the negative electrode active material for secondary batteries. However, when hard carbon is used as the negative electrode active material, both the charge specific capacity and first coulombic efficiency are low.

[0064] In view of this, the embodiments of the present disclosure provide a novel hard carbon material, which, when used as a negative electrode material for a secondary battery, has improved charge specific capacity and first coulombic efficiency.

[0065] Negative Electrode Sheet

[0066] A first aspect of the embodiments of the present disclosure provides an electrode electrode sheet. The negative electrode sheet 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. The hard carbon material includes a core and an organic coating layer coating the core. The core includes porous carbon. A total pore volume A of the porous carbon satisfies: 0.2 cm3 / g < A < 0.6 cm3 / g. A methylene blue adsorption value Z of the hard carbon material is less than or equal to 8 mg / g.

[0067] When the hard carbon material is used as a negative electrode active material for a secondary battery, active ions are first transported to defect sites in the hard carbon material for adsorption, and then transported to pores of the hard carbon material for filling. The active ions exist in the pores in the form of clusters, providing specific capacity. In the related art, the specific capacity of the hard carbon material is improved by forming more and larger pore structures. However, abundant pore structures lead to reduced carbon skeleton strength and poor stability of the hard carbon material. During charge-discharge cycles, the intercalation and deintercalation of active ions can easily cause the carbon skeleton of the hard carbon material to collapse, affecting the charge specific capacity and first coulombic efficiency. In addition, large-sized pore structures tend to allow electrolyte solution molecules to easily enter. The entry of electrolyte solution molecules into the pore structures not only occupies storage sites for active ions, but also consumes more active ions to form a solid electrolyte interface (SEI) film, further reducing the charge specific capacity and first coulombic efficiency.

[0068] Based on this, in the present disclosure, by controlling the total pore volume of the porous carbon in the hard carbon material within the above range, the specific capacity of the hard carbon material can be improved while ensuring that the carbon skeleton of the hard carbon material has appropriate strength and stability. In this way, no collapse occurs during the intercalation and deintercalation of active ions, thereby improving the charge specific capacity and first coulombic efficiency of the secondary battery. In addition, by providing the organic coating layer on the outer surface of the porous carbon core, electrolyte solution molecules can be blocked from entering the pore structure of the porous carbon, which is beneficial to improving the charge specific capacity and first coulombic efficiency of the secondary battery. Further, the methylene blue adsorption value of the hard carbon material is within the above range, so that the organic coating layer has high denseness, can effectively isolate an electrolyte solution, and inhibit electrolyte solution molecules from entering the interior of the porous carbon, which is further beneficial to improving the charge specific capacity and first coulombic efficiency of the secondary battery.

[0069] In addition, the presence of the organic coating layer is equivalent to constructing an artificial organic SEI layer on a surface of the porous carbon, which improves the stability of the SEI film in the secondary battery and reduces the amount of active ions consumed for the subsequent formation of the SEI film, thereby achieving high charge specific capacity and first coulombic efficiency.

[0070] It needs to be stated that since the molecular diameter of methylene blue is close to the size of commonly used ester solvents in electrolyte solutions (about 1 nm), the methylene blue adsorption value can accurately characterize a region that the electrolyte solution can enter, that is, characterize the denseness of the organic coating layer.

[0071] The smaller the methylene blue adsorption value, the better the coating effect. In some embodiments, the methylene blue adsorption value Z of the hard carbon material is less than or equal to 4 mg / g.

[0072] Exemplarily, the total pore volume A of the porous carbon may be 0.2 cm3 / g, 0.25 cm3 / g, 0.3 cm3 / g, 0.32 cm3 / g, 0.35 cm3 / g, 0.37 cm3 / g, 0.38 cm3 / g, 0.39 cm3 / g, 0.4 cm3 / g, 0.45 cm3 / g, 0.5 cm3 / g, 0.55 cm3 / g, 0.6 cm3 / g, or any value within a range formed by any two of the above values.

[0073] Exemplarily, the methylene blue adsorption value Z of the hard carbon material may be 1.0 mg / g, 1.2 mg / g, 1.3 mg / g, 1.4 mg / g, 1.5 mg / g, 1.6 mg / g, 1.7 mg / g, 1.8 mg / g, 1.9 mg / g, 2.0 mg / g, 2.5 mg / g, 3.0 mg / g, 3.5 mg / g, 4.0 mg / g, 4.5 mg / g, 5.0 mg / g, 5.2 mg / g, 5.5 mg / g, 5.8 mg / g, 6.0 mg / g, 6.5 mg / g, 7.0 mg / g, 7.5 mg / g, 8.0 mg / g, or any value within a range formed by any two of the above values.

[0074] In some embodiments, the material of the organic coating layer includes at least one of polyacrylic acid, a copolymer of polyacrylic acid, and a polyacrylate salt. Exemplarily, the material of the organic coating layer may be polyacrylic acid, a copolymer of polyacrylic acid (such as a copolymer of polyacrylic acid with carboxymethyl cellulose segments, or a copolymer of polyacrylic acid with polyethylene oxide segments), sodium polyacrylate (PAANa), a composite of polyrotaxane (PR) and polyacrylic acid (PAA), or the like, but is not limited thereto. Using such an organic material as the coating layer has the advantages of resistance to electrolyte solution swelling and strong adhesion, and can form a coating layer similar to an SEI film on the surfaces of the porous carbon particles, thereby inhibiting the electrolyte solution from entering the internal pore structure of the porous carbon particles, and reducing the consumption of active ions for the subsequent formation of the SEI film.

[0075] In some embodiments, the porous carbon includes micropores having a pore size of 0.4 nm to 2.5 nm and mesopores having a pore size of 2.5 nm to 10 nm. A total pore volume X of the micropores and a total pore volume Y of the mesopores satisfy: 5.6% < X / Y < 33%, and X + Y = A. Optionally, the total pore volume X of the micropores and the total pore volume Y of the mesopores satisfy: 7 < X / Y < 16. Exemplarily, X / Y may be 5.6, 6.5, 7.0, 7.5, 8.0, 8.5, 10, 11.0, 12, 13, 13.5, 14, 14.5, 15, 20, 25, 30, or 33.

[0076] It has been found through research that in the hard carbon material of the present disclosure, the micropores having a pore size of 0.4 nm to 2.5 nm can enable reversible intercalation and deintercalation of active ions. That is, when the number of micropores having a pore size of 0.4 nm to 2.5 nm in the hard carbon material increases, the discharge specific capacity of the hard carbon material can be improved. In the hard carbon material, the mesopores having a pore size of 2.5 nm to 10 nm can serve as transport channels for active ions, allowing active ions to be transported into the micropores, thereby improving the utilization rate of the micropores and thus improving the discharge specific capacity and first coulombic efficiency of the secondary battery.

[0077] In the hard carbon material of the present disclosure, the ratio of the total pore volume X of the micropores to the total pore volume Y of the mesopores in the porous carbon core is within the above range, so that the storage space inside the hard carbon material can be maximally utilized, and active ions can be reversibly intercalated and deintercalated, which is beneficial to the charge specific capacity and first coulombic efficiency of the secondary battery.

[0078] In some embodiments, the total pore volume X of the micropores and the total pore volume A of the porous carbon satisfy: 85% < X / A < 97%. Optionally, the total pore volume X of the micropores and the total pore volume A of the porous carbon satisfy: 88 < X / A < 94. Exemplarily, X / A may be 85%, 86%, 87%, 88%, 90%, 93%, 94%, or 97%. When the total pore volume X of the micropores and the total pore volume A of the porous carbon of the hard carbon material satisfy the above relationship, more storage space for active ions can be provided, which is beneficial to improving the discharge specific capacity of the hard carbon material.

[0079] In some embodiments, the total pore volume Y of the mesopores and the total pore volume A of the porous carbon satisfy: 3% < Y / A < 15%. Optionally, the total pore volume Y of the mesopores and the total pore volume A of the porous carbon satisfy: 6 < Y / A < 12. Exemplarily, Y / A may be 3%, 5%, 6%, 8%, 10%, 12%, 13%, or 15%. When the total pore volume Y of the mesopores and the total pore volume A of the porous carbon of the hard carbon material satisfy the above relationship, an appropriate number of transport channels for active ions can be provided, thereby improving the utilization rate of the micropores, and facilitating the improvement of the charge specific capacity and first coulombic efficiency of the secondary battery.

[0080] In some embodiments, the organic coating layer has a thickness of 1 nm to 90 nm. Optionally, the organic coating layer has a thickness of 30 nm to 50 nm. Exemplarily, the organic coating layer has a thickness of 1 nm, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, or 90 nm. By controlling the thickness of the organic coating layer within the above ranges, the kinetic performance of the secondary battery and the charge specific capacity and first coulombic efficiency of the secondary battery can be balanced.

[0081] In some embodiments, a mass of the organic coating layer in the hard carbon material is 0.5 wt% to 10 wt% of the porous carbon. Exemplarily, the mass of the organic coating layer is 0.5%, 3%, 5%, 8%, or 10% of the porous carbon. By controlling the mass of the organic coating layer in the hard carbon material within the above ranges, the kinetic performance of the secondary battery and the charge specific capacity and first coulombic efficiency of the secondary battery can be balanced.

[0082] In some embodiments, the hard carbon material satisfies at least one of the following items:

[0083] (1) The hard carbon material has a compacted density of 0.55 g / cm3 to 1.0 g / cm3 under 5 t. When the powder compacted density of the hard carbon material is within the above range, it is beneficial to forming a reasonable pore channel structure between particles of the negative electrode film layer, improving the transport performance of active ions and electrons, and thereby improving the kinetic performance of the secondary battery.

[0084] (2) The hard carbon material has a tap density of 0.34 g / cm3 to 0.8 g / cm3. When the tap density of the hard carbon material is within the above range, it is beneficial to improving the compacted density of the negative electrode film layer and the energy density of the secondary battery.

[0085] (3) The hard carbon material has a volume distribution particle size Dv50 of 3.0 gm to 7.9 gm.

[0086] (4) The hard carbon material has a volume distribution particle size Dv90 of 8 gm to 15 gm. When the volume distribution particle sizes Dv50 and Dv90 of the hard carbon material particles are within the above ranges, it is beneficial to reducing the specific surface area of the hard carbon material and reducing the occurrence of side reactions, thereby improving the first coulombic efficiency of the secondary battery. At the same time, a bulk phase transport path of active ions can be shortened, thereby improving the kinetic performance of the secondary battery.

[0087] (5) The hard carbon material has a BET specific surface area of 0.1 m2 / g to 0.8 m2 / g. When the specific surface area of the hard carbon material is within the above range, the consumption of active ions during the first charge can be reduced, which is beneficial to improving the first coulombic efficiency of the secondary battery.

[0088] In the present disclosure, the methylene blue adsorption value of the hard carbon material has the meaning well known in the art, and can be measured using instruments and methods known in the art. For example, the test may be performed with reference to GB / T 12496.10-1999.

[0089] In the present disclosure, the pore volume of the porous carbon has the meaning well known in the art, and may be measured using instruments and methods known in the art. For example, the test may be performed with reference to GB / T 21650.3, Part 3. The porous carbon powder is placed in a sample tube, and degassed under vacuum at 200°C for 12 h. The adsorption amount of nitrogen of the hard carbon material at different pressures is measured using an ASAP 2460 physical adsorption analyzer, and adsorption and desorption isotherms are plotted. The shape of the pore is determined according to the shape of a hysteresis loop, the pore size distribution curve of the micropores is fitted using a DFT model, and the micropore volume X, mesopore volume Y, and total pore volume A of the hard carbon material are calculated.

[0090] In the present disclosure, before testing the pore volume of the porous carbon, a pretreatment is performed on the hard carbon material to break the organic coating layer, so that the gas used in the nitrogen adsorption-desorption test can enter the porous carbon. Exemplarily, the pretreatment steps include: adding 2 g of the hard carbon powder to a 250 ml beaker, adding 1 g of NaOH, and stirring uniformly. The beaker is placed in a water bath, heated to 80°C with stirring and maintained for 30 min. Then, the mixture is filtered, and a filter cake is collected and washed repeatedly with deionized water until a filtrate is neutral. The washed filter cake is dried in a vacuum oven at 80°C for 24.

[0091] In the present disclosure, the thickness of the organic coating layer in the hard carbon material may be measured by performing an ion-polished cross-section morphology (CP) test on the negative electrode sheet using a cross-section polisher. Specifically, the hard carbon material is cut into test samples with a specific size (for example, 2 cm x 2 cm), and the hard carbon material is fixed to a sample stage. The sample stage is installed into a sample holder and locked, an argon ion cross-section polisher (for example, the IB-09010 CP argon ion cross-section polisher from JEOL, Japan) is powered on, followed by vacuum pumping (for example, 10-7 Pa). The argon flow rate (for example, 0.12 MPa) and polishing time (for example, 90 min) are set, and the sample stage is adjusted to a rocking mode to start polishing. After the polishing is completed, energy-dispersive X-ray spectroscopy analysis is performed on the cross-section. The organic coating layer and the porous carbon can be distinguished by a difference in the content of element O, thereby obtaining the thickness of the organic coating layer.

[0092] In the present disclosure, the mass of the organic coating layer in the hard carbon material can be measured by thermogravimetric (TG) analysis. By measuring the weight loss of the hard carbon material in a temperature range of 150°C to 500°C under a nitrogen atmosphere, the content of the organic coating layer can be calculated.

[0093] In the present disclosure, the ID / IG value of the hard carbon material can be measured using a Raman spectrometer, where ID represents the intensity of the D peak at 1350±50 cm-1 in the Raman spectrum of the material, and IG represents the intensity of the G peak at 1580±50 cm-1 in the Raman spectrum of the material. The test conditions are as follows: an excitation wavelength of 532 nm, a grating of 600 grooves, a 50x objective lens, an integration time of 10 s, and an accumulation number of 3. Area scanning is performed to obtain the D peak and G peak intensities at 100 points. The ID / IG values at the 100 points are calculated, and the largest 30 and smallest 30 ID / IG values are removed. The average of the remaining 40 points is the ID / IG value of the material. A test instrument may be a Horiba LabRAM HR800 Raman spectrometer.

[0094] In the present disclosure, the compacted density of the hard carbon material has the meaning well known in the art, and may be measured using instruments and methods known in the art. For example, measurement may be made by using an electronic pressure testing machine (which may be, for example, a UTM7305 electronic pressure testing machine) with reference to the standard GB / T 24533-2009. An exemplary test method is as follows: 1 g of sample powder is weighed and added to a mold having a bottom area of 1.327 cm2, pressed to 5 t, held under pressure for 30 s, then the pressure is released and held for 10 s, and then the powder compacted density of the material under a pressure of 5 t is recorded and calculated.

[0095] In the present disclosure, the tap density of the hard carbon material has the meaning well known in the art, and may be measured using instruments and methods known in the art. For example, the measurement may be performed with reference to GB / T 5162-2006 using a powder tap density tester. The test instrument may be a Dandong Bettersize BT-301, and the test parameters are as follows: vibration frequency of 250 ± 15 times / min, amplitude of 3 ± 0.2 mm, vibration number of 5000 times, and a measuring cylinder of 25 mL.

[0096] In the present disclosure, the volume distribution particle sizes Dv50 and Dv90 of the hard carbon material have the meanings well known in the art, which respectively represent the particle sizes corresponding to the cumulative volume distribution percentages of the material reaching 50% and 90%, and may be measured using instruments and methods known in the art. For example, a laser particle size analyzer may be adopted for determination with reference to GB / T 19077-2016. The test instrument may be a Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK.

[0097] In the present disclosure, the BET specific surface area of the hard carbon material has the meaning well known in the art, and may be measured using instruments and methods known in the art. For example, the measurement may be performed with reference to GB / T 19587-2017 using a nitrogen adsorption specific surface area analysis test method, and calculated by the Brunauer Emmett Teller (BET) method. The test instrument may be a TriStar 3020 specific surface area and pore size analyzer from Micromeritics Instrument Corporation, USA.

[0098] In the present disclosure, the microstructure of the hard carbon material can be observed by a scanning electron microscope or a transmission electron microscope.

[0099] As an example, the negative electrode current collector has two opposite surfaces in its own thickness direction, and the negative electrode film layer is arranged on either or both of the two opposite surfaces of the negative electrode current collector.

[0100] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, a copper 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 substrate. 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 (for example, a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), or polyethylene (PE)).

[0101] In some embodiments, the negative electrode active material includes the hard carbon material provided in the above embodiments or a hard carbon material prepared by the preparation method according to the above embodiments.

[0102] In some embodiments, the negative electrode film layer further optionally includes a binder. The binder may be at least one selected from styrene-butadiene rubber (SBR), polyacrylic acid (PAA), polyacrylic acid sodium (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0103] In some embodiments, the negative electrode film layer further optionally includes a conductive agent. The conductive agent may be at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dot, carbon nanotube, graphene, and carbon nanofiber.

[0104] In some embodiments, the negative electrode film layer further optionally includes other adjuvants such as a thickener (for example, sodium carboxymethyl cellulose (CMC-Na)).

[0105] In some embodiments, the negative electrode sheet may be prepared by the following method: dispersing the above components for preparing the negative electrode sheet, such as a negative electrode active material, a conductive agent, a binder, and any other components, in a solvent (for example, deionized water) to form a negative electrode slurry; and coating the negative electrode slurry on the negative electrode current collector, followed by processes such as drying and cold pressing, to obtain the negative electrode sheet.

[0106] Secondary Battery

[0107] A second aspect of the embodiments of the present disclosure provides a secondary battery. The secondary battery of the present disclosure will be described below with appropriate reference to the accompanying drawings.

[0108] The term "secondary battery" as mentioned herein refers to a battery cell, a battery module, or a battery pack. Each will be described below.

[0109] Typically, a secondary battery cell includes a positive electrode sheet, the negative electrode sheet according to the above embodiments, an electrolyte, and a separator. During the charge and discharge process of the battery, active ions are intercalated into and deintercalated from the positive electrode sheet and the negative electrode sheet in a round-trip manner. The electrolyte plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet to mainly play a role in preventing a short circuit between a positive electrode and a negative electrode while allowing ions to pass through.

[0110] [Positive Electrode Sheet]

[0111] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector. The positive electrode film layer includes the positive electrode active material according to the first aspect of the present disclosure.

[0112] As an example, the positive electrode current collector has two opposite surfaces in its own thickness direction, and the positive electrode film layer is provided on either one or both of the two opposite surfaces of the positive electrode current collector.

[0113] In some embodiments, the positive electrode current collector may be a metal foil or a composite 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 (for example, a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), or polyethylene (PE)).

[0114] In some embodiments, the battery cell is a sodium-ion battery, and the positive electrode active material may be a positive electrode active material for sodium-ion batteries well known in the art. 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, in the sodium transition metal oxide, the transition metal 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.

[0115] As an optional technical approach 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. 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 halogen 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-. The halogen may be at least one of F, Cl, and Br.

[0116] The polyanionic compound may also be a class of compounds having sodium ions, tetrahedral (YO4)n- anion units, polyhedral (ZOy)m+ units, and optionally halogen 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+. 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 (where 0 < y < 1).

[0117] As an optional embodiment of the present disclosure, the polyanionic compound may have a general chemical formula of Nax-aAaVy-bMb(PO4)2-2c(DO4)2cFz-dQd, where element A represents an alkali metal element substituting for element Na, element M represents a metal element substituting for element V, element D represents a doping element substituting for element P, and element Q represents a doping element substituting for element F; element D includes at least one of Si and S, and element Q includes at least one of Cl and O; and 3.5 < x < 4.5, 0 < a < 0.15x, 0.8 < y < 1.1, 0 < b < 0.3y, 0 < c < 0.15, 0.8 < z < 1.1, and 0 < d < 0.2z. Optionally, element A includes at least one of K and Li. element M includes at least one of Fe, Cr, Al, Sc, Ga, In, Ti, Zr, Mn, Zn, Ni, Cu, and Co.

[0118] As an optional embodiment of the present disclosure, the polyanionic compound may have a general chemical formula of NaxRy(PO4)2P2O7, where x = 3.5 to 4.5, y = 2.75 to 3.25, and R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, and Pb.

[0119] As an optional embodiment of the present disclosure, the polyanionic compound may have a general chemical formula of Na4+xR3-yP4-mOi5 / C, where 0 < x < 0.5, 0 < y < 0.5, 0 < m < 0.2, and R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, and Pb.

[0120] 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, and 0 < a < 2, 0 < b < 1, and 0 < c < 1.

[0121] In other embodiments, the battery cell may also be a lithium-ion battery, and the positive electrode active material may be a positive electrode active material for lithium-ion batteries well known in the art.

[0122] In the examples of the positive electrode active material in the present disclosure, a molar content of oxygen is only a theoretical state value. Lattice oxygen release causes changes in the molar content of oxygen, and an actual molar content of oxygen fluctuates.

[0123] 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, tetrafluoroethylenehexafluoropropylene copolymer, and fluoroacrylate resin.

[0124] 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, carbon dot, carbon nanotube, graphene, and carbon nanofiber.

[0125] In some embodiments, the positive electrode sheet may be prepared by the following method: dispersing the above components for preparing the positive electrode sheet, such as a positive electrode active material, a conductive agent, a binder, and any other components, in a solvent (for example, N-methylpyrrolidone) to form a positive electrode slurry; and coating the positive electrode slurry onto the positive electrode current collector, followed by processes such as drying and cold pressing, to obtain the positive electrode sheet.

[0126] [Electrolyte]

[0127] The electrolyte plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet. The type of the electrolyte is not specifically limited in the present disclosure, which may be selected according to the requirements. For example, the electrolyte may be liquid, gelled, or all solid.

[0128] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0129] In some embodiments, when the battery cell is a sodium-ion battery, the electrolyte salt may be at least one selected from the group consisting of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium perchlorate, sodium hexafluoroarsenate,           sodium          bis(fluorosulfonyl)imide,           sodium bis(trifluoromethanesulfonyl)imide,     sodium     trifluoromethanesulfonate,     sodium difluoro(oxalato)borate, sodium bis(oxalato)borate, sodium difluorophosphate, sodium difluorobis(oxalato)phosphate, and sodium tetrafluoro(oxalato)phosphate.

[0130] In some embodiments, when the battery cell is a lithium-ion battery, the electrolyte salt may be at least one selected from the group consisting 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.

[0131] In some embodiments, the solvent may be at least one selected from 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, methyl ethyl sulfone, and diethyl sulfone.

[0132] In some embodiments, the electrolyte 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 that can improve particular performance of the battery, for example, an additive that improves the overcharging performance of the battery, an additive that improves high or low temperature performance of the battery, or the like.

[0133] [Separator]

[0134] In some embodiments, the battery cell further includes a separator. The type of the separator is not particularly limited in the present disclosure, and any well-known porous separator with good chemical stability and mechanical stability may be selected.

[0135] In some embodiments, the material of the separator may be at least one selected from glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multilayer composite film, which is not specially limited. When the separator is a multilayer composite film, the materials of the layers may be the same or different, and there is no particular limitation.

[0136] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be made into an electrode assembly through a winding process or a lamination process.

[0137] In some embodiments, the battery cell may include an outer package. The outer package may be used to encapsulate the above electrode assembly and electrolyte.

[0138] In some embodiments, the outer package of the battery cell may be a hard case, such as a hard plastic case, an aluminum case, a steel case, or the like. The outer package of the battery cell may also be a soft package, such as a pouch-type soft package. The material of the soft package may be plastic, and examples of plastic may include polypropylene, polybutylene terephthalate, polybutylene succinate, and the like.

[0139] The present disclosure has no particular limitation on the shape of the battery cell, which may be cylindrical, square, or any other shape. For example, FIG. 1 is a schematic diagram of a battery cell 5 having a square structure as an example.

[0140] In some embodiments, referring to FIG. 2, the outer package can include a case 51 and a top cover assembly 53. The case 51 may include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates form an accommodating cavity. An opening that communicates with the accommodating cavity is formed in the case 51, and the top cover assembly 53 can cover the opening to seal the accommodating cavity. The positive electrode sheet, the negative electrode sheet, and the separator may form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the accommodating cavity. The electrolyte solution infiltrates the electrode assembly 52. The number of the electrode assemblies 52 included in the battery cell 5 may be one or more, which may be selected by those skilled in the art according to the specific actual requirements.

[0141] In some embodiments, the battery cell can be assembled into a battery module, the number of battery cells contained in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0142] FIG. 3 shows a battery module 4 as an example. Referring to FIG. 3, in the battery module 4, the plurality of battery cells 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. Further, the plurality of battery cells 5 may be fixed by fasteners.

[0143] Optionally, the battery module 4 may further include a housing having an accommodating space, and the plurality of battery cells 5 are accommodated in the accommodating space.

[0144] In some embodiments, the battery module may also be assembled into a battery pack, the number of the 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 according to the application and capacity of the battery pack.

[0145] FIG. 4 and FIG. 5 show a battery pack 1 as an example. Referring to FIG. 4 and FIG. 5, the battery pack 1 may include a battery box and multiple battery modules 4 arranged in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 to form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 may be arranged in the battery box in any way.

[0146] Electric Device

[0147] A third aspect of the embodiments of the present disclosure further provides an electric device. The secondary battery of the present disclosure will be described below with appropriate reference to the accompanying drawings.

[0148] The electric device mentioned in the embodiments of the present disclosure includes the secondary battery provided by the present disclosure. The secondary battery may be used as a power source for the electric device, or may be used as an energy storage unit of the electric device. The electric device may include, but is not limited to, a mobile device (e.g., a mobile phone, a laptop computer, etc.), an electric vehicle (e.g., an all-electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship, a satellite, an energy storage system, and the like.

[0149] As an electric device, the battery cell, the battery module, or the battery pack may be selected according to its usage requirements.

[0150] FIG. 6 shows an electric device as an example. The electric device is an allelectric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the requirements of the electric device for high power and high energy density of secondary batteries, a battery pack or a battery module may be adopted.

[0151] As another example, the device may be a mobile phone, a tablet computer, a laptop computer, or the like. Such a device is generally required to be light and thin, and a battery cell may be used as a power source.

[0152] Hard Carbon Material

[0153] A fourth aspect of the present disclosure provides the hard carbon material included in the negative electrode sheet according to the first aspect of the present disclosure. The hard carbon material includes the hard carbon material includes a core and an organic coating layer coating the core, where the core includes porous carbon. A total pore volume A of the porous carbon satisfies: 0.2 cm3 / g < A < 0.6 cm3 / g. A methylene blue adsorption value Z of the hard carbon material is less than or equal to 8 mg / g.

[0154] In the present disclosure, by controlling the total pore volume of the porous carbon in the hard carbon material within the above range, the specific capacity of the hard carbon material can be improved while ensuring that the carbon skeleton of the hard carbon material has appropriate strength and stability, so that no collapse occurs during the intercalation and deintercalation of active ions, thereby improving the charge specific capacity and first coulombic efficiency of the secondary battery. In addition, by providing the organic coating layer on the outer surface of the porous carbon core, electrolyte solution molecules can be blocked from entering the pore structure of the porous carbon, which is beneficial to improving the charge specific capacity and first coulombic efficiency of the secondary battery. Further, the methylene blue adsorption value of the hard carbon material is within the above range, so that the organic coating layer has high denseness, can effectively isolate an electrolyte solution, and inhibit electrolyte solution molecules from entering the interior of the porous carbon, which is further beneficial to improving the charge specific capacity and first coulombic efficiency of the secondary battery.

[0155] In some embodiments, the methylene blue adsorption value Z of the hard carbon material is less than or equal to 4 mg / g. The smaller the methylene blue adsorption value, the better the coating effect. Thus, it is more beneficial to improving the charge specific capacity and first coulombic efficiency of the secondary battery.

[0156] In some embodiments, the porous carbon includes micropores having a pore size of 0.4 nm to 2.5 nm and mesopores having a pore size of 2.5 nm to 10 nm. A total pore volume X of the micropores and a total pore volume Y of the mesopores satisfy: 5.6% < X / Y < 33%, and X + Y = A. In some embodiments, the total pore volume X of the micropores and the total pore volume Y of the mesopores satisfy: 7 < X / Y < 16. Thus, the storage space inside the hard carbon material can be maximally utilized. Moreover, active ions can be reversibly intercalated and deintercalated. This is beneficial to the charge specific capacity and first coulombic efficiency of the secondary battery.

[0157] In some embodiments, the total pore volume X of the micropores and the total pore volume A of the porous carbon satisfy: 85% < X / A < 97%. Thus, more storage space for active ions can be provided, which is beneficial to improving the discharge specific capacity of the hard carbon material.

[0158] In some embodiments, the total pore volume Y of the mesopores and the total pore volume A of the porous carbon satisfy: 3% < Y / A < 15%. Thus, an appropriate number of transport channels for active ions can be provided, thereby improving the utilization rate of the micropores, and facilitating the improvement of the charge specific capacity and first coulombic efficiency of the secondary battery.

[0159] In some embodiments, the organic coating layer has a thickness of 1 nm to 90 nm. Thus, the kinetic performance of the secondary battery can be balanced with the charge specific capacity and first coulombic efficiency of the secondary battery.

[0160] In some embodiments, a mass of the organic coating layer in the hard carbon material is 0.5 wt% to 10 wt% of the porous carbon. Thus, the kinetic performance of the secondary battery can be balanced with the charge specific capacity and first coulombic efficiency of the secondary battery.

[0161] In some embodiments, the hard carbon material satisfies at least one of the following items: (1) The hard carbon material has a compacted density of 0.55 g / cm3 to 1.0 g / cm3 under 5 t. When the powder compacted density of the hard carbon material is within the above range, it is beneficial to forming a reasonable pore channel structure between particles of the negative electrode film layer, improving the transport performance of active ions and electrons, and thereby improving the kinetic performance of the secondary battery. (2) The hard carbon material has a tap density of 0.34 g / cm3 to 0.8 g / cm3. When the tap density of the hard carbon material is within the above range, it is beneficial to improving the compacted density of the negative electrode film layer and the energy density of the secondary battery. (3) The hard carbon material has a volume distribution particle size Dv50 of 3.0 pm to 7.9 pm. (4) The hard carbon material has a volume distribution particle size Dv90 of 8 pm to 15 pm. When the volume distribution particle sizes Dv50 and Dv90 of the hard carbon material particles are within the above ranges, it is beneficial to reducing the specific surface area of the hard carbon material and reducing the occurrence of side reactions, thereby improving the first coulombic efficiency of the secondary battery. At the same time, a bulk phase transport path of active ions can be shortened, thereby improving the kinetic performance of the secondary battery. (5) The hard carbon material has a BET specific surface area of 0.1 m2 / g to 0.8 m2 / g. When the specific surface area of the hard carbon material is within the above range, the consumption of active ions during the first charge can be reduced, which is beneficial to improving the first coulombic efficiency of the secondary battery.

[0162] Preparation Method for Hard Carbon Material

[0163] A fifth aspect of the embodiments of the present disclosure provides a preparation method for the above hard carbon material. The method includes: subjecting a hard carbon precursor to a pre-carbonization treatment to obtain a pre-carbonized body; subjecting the precarbonized body to an activation treatment to obtain a porous carbon precursor; subjecting the porous carbon precursor to a carbonization treatment to obtain porous carbon; and subjecting the porous carbon to a coating treatment, where the coating treatment includes a kneading treatment of kneading a kneaded mixture formed by the porous carbon and an aqueous solution of a coating raw material, where the kneaded mixture has a solid content of 55 wt% to 75 wt%, and the aqueous solution of the coating raw material has a pH value of < 3. A total pore volume A of the formed porous carbon satisfies: 0.2 cm3 / g < A < 0.6 cm3 / g. A methylene blue adsorption value Z of the formed hard carbon material is less than or equal to 8 mg / g.

[0164] In the present disclosure, by subjecting the hard carbon precursor to the precarbonization treatment and the activation pore-forming treatment, the porous carbon precursor is obtained, and the porous carbon precursor is subjected to the carbonization treatment to obtain the porous carbon. Then, the porous carbon and the aqueous solution of the coating raw material are subjected to the kneading treatment to form a hard carbon material having a coating layer. Under the above solid content and the pH condition of the aqueous solution of the coating raw material, the porous carbon and the coating raw material are kneaded, so that the two are more fully mixed and sheared, and the formed coating layer thus has good denseness. Therefore, electrolyte solution molecules can be blocked from entering the interior of the porous carbon, which is beneficial to improving the charge specific capacity and first coulombic efficiency of the secondary battery.

[0165] Exemplarily, the kneaded mixture has a solid content of 55 wt%, 60 wt%, 62 wt%, 65 wt%, 68 wt%, 71 wt%, or 75 wt%.

[0166] In some embodiments, the kneaded mixture has a solid content of 63 wt% to 67 wt%, and the aqueous solution of the coating raw material has a pH value of 1.2 to 1.8. When the solid content of the kneaded mixture is within the above range, it is beneficial to forming a coating layer having an appropriate denseness, which can improve the charge specific capacity and first coulombic efficiency of the secondary battery without affecting the kinetic performance of the secondary battery. When the pH value of the aqueous solution of the coating raw material during the kneading process is within the above range, a coating layer having an appropriate denseness can be rapidly formed, which is beneficial to improving the charge specific capacity and first coulombic efficiency of the secondary battery.

[0167] In some embodiments, the coating raw material includes at least polyacrylic acid. Exemplarily, the coating raw material includes at least one of polyacrylic acid, a copolymer of polyacrylic acid, sodium polyacrylate, and a composite of polyrotaxane and polyacrylic acid. During the kneading process, carboxyl groups on molecular chains of polyacrylic acid can react with oxygen-containing functional groups on a surface of the porous carbon to form carbonoxygen covalent bonds. The carbon-oxygen covalent bonds enable the organic coating layer to be tightly connected to the porous carbon, forming an organic coating layer with high stability and high denseness. The organic coating layer can inhibit the electrolyte solution from entering the interior of the porous carbon, which is beneficial to improving the charge specific capacity and first coulombic efficiency of the secondary battery. In addition, since polyacrylic acid does not swell in the electrolyte solution, it can achieve the effect of isolating the electrolyte solution.

[0168] In some embodiments, a mass of the coating raw material is 0.5% to 10% of a mass of the porous carbon, and the kneading treatment is performed for at least 0.5 h. Optionally, the kneading treatment is performed for 9 h to 12 h. Exemplarily, the mass of the coating raw material is 5% of the mass of the porous carbon, and the kneading treatment time is 12 h. When the mass of the coating raw material is within the above range, a coating layer having an appropriate denseness can be formed, which can improve the charge specific capacity and first coulombic efficiency of the secondary battery without affecting the kinetic performance of the secondary battery and without increasing the impedance of the secondary battery.

[0169] In some embodiments, the kneading treatment is performed in a twin-screw kneader, and the twin-screw kneader has a rotational speed of 10 rpm to 50 rpm. Under the above conditions, an organic coating layer with uniform coating can be obtained.

[0170] In some embodiments, the aqueous solution may be a water solution, or may be an ethanol solution or an isopropanol solution, or may be a mixed solution of at least two of a water solution, an ethanol solution, and an isopropanol solution.

[0171] In some embodiments, the above kneading process is performed at room temperature.

[0172] In some embodiments, the activation treatment includes treating the precarbonized body at a temperature of 700°C to 950°C under a predetermined mixed gas. Optionally, the pre-carbonized body is treated at a temperature of 830°C to 870°C. Exemplarily, the activation treatment temperature may be 700°C, 850°C, or 900°C. The activation treatment performed at the above temperature can form an appropriate amount of mesopores and micropores, which is beneficial to improving the discharge specific capacity of the secondary battery.

[0173] In some embodiments, the predetermined mixed gas includes carbon dioxide gas, water vapor, and an inert gas, and a volume ratio of the carbon dioxide gas to the water vapor in the predetermined mixed gas is greater than or equal to 2. Optionally, the volume ratio of the carbon dioxide gas to the water vapor in the predetermined mixed gas is 4 to 6. Exemplarily, the volume ratio of the carbon dioxide gas to the water vapor in the predetermined mixed gas may be 2, 3, 4, 5, 6, 8, or 10. When the volume ratio of the carbon dioxide gas to the water vapor in the predetermined mixed gas is within the above range, an appropriate amount of mesopores and micropores can be formed, which is beneficial to improving the discharge specific capacity of the secondary battery.

[0174] In some embodiments, the inert gas in the above mixed gas includes helium, neon, argon, or the like.

[0175] In some embodiments, the carbon dioxide gas has a volume fraction of 5% to 20% in the predetermined mixed gas. Optionally, the carbon dioxide gas has a volume fraction of 8% to 12% in the predetermined mixed gas. Exemplarily, the carbon dioxide gas may have a volume fraction of 4%, 5%, 6%, 10%, 15%, or 20% in the predetermined mixed gas. When the volume fraction of the carbon dioxide gas in the predetermined mixed gas is within the above range, an appropriate amount of micropores can be formed, which is beneficial to improving the discharge specific capacity of the secondary battery.

[0176] In some embodiments, the water vapor has a volume fraction of 1% to 5% in the mixed gas. Optionally, the water vapor has a volume fraction of 1.5% to 2.5% in the mixed gas. Exemplarily, the water vapor may have a volume fraction of 1%, 1.5%, 2%, 2.5%, 3%, 4%, or 5% in the mixed gas. When the volume fraction of the water vapor in the predetermined mixed gas is within the above range, an appropriate amount of mesopores can be formed, which is beneficial to improving the charge specific capacity and first coulombic efficiency of the secondary battery.

[0177] In some embodiments, the activation treatment includes impregnating the precarbonized body in a pore-forming solution for at least 2 h. The pore-forming solution includes a P-containing pore-forming solution or a Zn-containing pore-forming solution, where an addition amount of element P in the P-containing pore-forming solution is 9.5% to 32.4% of a mass of the porous carbon. Optionally, the addition amount of element P in the P-containing pore-forming solution is 18% to 24% of the mass of the porous carbon. An addition amount of element Zn in the Zn-containing pore-forming solution is 14% to 50% of the mass of the porous carbon. Optionally, the addition amount of element Zn in the Zn-containing pore-forming solution is 25% to 35% of the mass of the porous carbon. Exemplarily, the addition amount of element P in the pore-forming solution may be 9.5%, 12%, 15%, 18%, 20%, 24%, 27%, 30%, or 32% of the mass of the porous carbon. Exemplarily, the addition amount of element Zn in the pore-forming solution may be 14%, 20%, 25%, 30%, 35%, 40%, 45%, or 50% of the mass of the porous carbon. When the addition amount of element P or element Zn in the pore-forming solution is within the above ranges, an appropriate amount of micropores and mesopores can be formed, which is beneficial to improving the discharge specific capacity, charge specific capacity, and first coulombic efficiency of the secondary battery. In addition, by impregnating the pre-carbonized body in the pore-forming solution for the above time, an appropriate amount of micropores and mesopores can be formed, which is beneficial to improving the discharge specific capacity, charge specific capacity, and first coulombic efficiency of the secondary battery.

[0178] Exemplarily, the P-containing pore-forming solution may include, for example, at least one of a phosphoric acid solution, a polyphosphoric acid solution, and a phosphate ester solution.

[0179] Exemplarily, the Zn-containing pore-forming solution may be, for example, an aqueous zinc chloride solution.

[0180] In some embodiments, the impregnated mixture formed by the pre-carbonized body and the pore-forming solution has a solid content of 50 wt% to 70 wt%. Exemplarily, the impregnated mixture formed by the pre-carbonized body and the pore-forming solution may have a solid content of 50 wt%, 55 wt%, 60 wt%, 65 wt%, or 70 wt%. By controlling the solid content of the impregnated mixture within the above range, an appropriate amount of micropores and mesopores can be formed, which is beneficial to improving the discharge specific capacity, charge specific capacity, and first coulombic efficiency of the secondary battery.

[0181] In some embodiments, the activation treatment further includes maintaining an impregnated mixture formed by the impregnated pre-carbonized body and the pore-forming solution at a temperature of 400°C to 750°C for 1 h to 12 h. Optionally, the temperature is raised at a heating rate of 1°C / min to 20°C / min under a nitrogen gas or inert gas atmosphere. Exemplarily, the heat treatment process is performed by raising a temperature to 600°C at a heating rate of 5°C / min under a nitrogen gas atmosphere, and maintaining the temperature for 2 h. Maintaining the temperature at 400°C to 750°C for 1 h to 12 h enables the P-containing pore-forming solution to chemically interact with the pre-carbonized body, and etching is performed by a chemical activation method, introducing a large number of pore structures into the skeleton structure of the pre-carbonized body.

[0182] In some embodiments, the pre-carbonization treatment is performed by raising a temperature to 400°C to 600°C at a heating rate of 1°C / min to 20°C / min, and maintaining the temperature for 1 h to 12 h. Exemplarily, the pre-carbonization temperature may be 550°C, and the pre-carbonization treatment time may be 6 h. The pre-carbonization treatment performed under the above conditions is beneficial to forming a basic carbon skeleton structure, facilitating subsequent pore formation.

[0183] In some embodiments, the carbonization treatment is performed by raising a temperature to 1000°C to 1800°C at a heating rate of 2°C / min to 20°C / min, and maintaining the temperature for 1 h to 12 h. Exemplarily, the carbonization temperature may be 1250°C, and the carbonization treatment time may be 10 h. During the carbonization process, heteroatoms such as H and O are removed from the precursor, forming a stable hard carbon skeleton.

[0184] In some embodiments, the above pre-carbonization process and carbonization process are performed under a nitrogen or inert atmosphere. Optionally, the inert atmosphere is argon.

[0185] In some embodiments, after the pre-carbonization treatment, the method further includes a pulverization treatment. The pulverization treatment is performed by ball milling. The pulverization treatment can pulverize the pre-carbonized body to a suitable particle size, facilitating the subsequent activation treatment.

[0186] In some embodiments, the hard carbon precursor includes at least one of a biomass precursor and a synthetic polymer precursor. The synthetic polymer precursor includes a compound composed of elements C, H, and O. Exemplarily, the biomass precursor may be coconut shell, pine wood powder, walnut shell, giant reed, straw, or the like. The synthetic polymer precursor may be phenolic resin, epoxy resin, unsaturated polyester resin, or the like. Using the above precursors to prepare the hard carbon material, the carbon formed after activation is relatively ordered, and an appropriate amount of micropores and mesopores can be formed.

[0187] In some embodiments, when the hard carbon precursor is a biomass precursor, after the pulverization treatment, the method further includes a demineralization treatment. The demineralization treatment is performed by soaking in an acidic aqueous solution at room temperature to 95°C for 1 h to 12 h, and repeating 1 to 5 times. The demineralization treatment can remove ash content such as minerals and metal oxides from the pre-carbonized body, improving the purity of the pre-carbonized body.

[0188] In some embodiments, after the activation treatment step, the method further includes a demineralization treatment. The demineralization treatment is performed by soaking in an acidic aqueous solution at room temperature to 95°C for 1 h to 12 h, and repeating 1 to 5 times. The demineralization treatment is used to remove residual element P or element Zn from the porous carbon precursor.

[0189] Examples

[0190] Examples of the present disclosure will be described below. The examples described below are exemplary and only intended to explain the present disclosure, and cannot be construed as limitations on the present disclosure. If the specific technologies or conditions are not specified in the examples, the technologies or conditions described in the literature in the art or the product manual shall be followed. The used reagents or instruments without manufacturers indicated are all conventional products that may be purchased in the market.

[0191] Example 1

[0192] Preparation of Hard Carbon Material:

[0193] 1 . Pre-carbonization treatment: Under a nitrogen gas flow, a hard carbon precursor of pine wood powder was placed in a tube furnace, and a temperature was raised to 550°C at a heating rate of 10°C / min and maintained for 6 h to obtain a pre-carbonized body. The flow rate of nitrogen gas was 500 mL / min.

[0194] 2. Pulverization treatment: The pre-carbonized body obtained above was subjected to a ball milling and pulverization treatment using zirconia grinding balls and a zirconia grinding jar, with a mass ratio of the pre-carbonized body to the grinding balls of 1:3. The ball mill was operated at a rotational speed of 800 rpm for 8 h. After removing the grinding balls, a pre-carbonized body having a volume distribution particle size Dv50 of 5.4 pm and a volume distribution particle size Dv90 of 11.3 pm was obtained.

[0195] 3. Demineralization treatment: The pulverized pre-carbonized body obtained in step 2) above was washed with an aqueous hydrochloric acid solution at a pickling temperature of 60°C for 3 h, and the washing process was repeated 4 times. Then, the mixture was filtered, and a filter cake was collected and washed repeatedly with deionized water and absolute ethanol until the pH of a filtrate was greater than 6. The washed filter cake was dried in a vacuum oven at 80°C for 24 h.

[0196] 4. Activation treatment: The demineralized pre-carbonized body obtained in step 3) above was subjected to an activation treatment at 700°C under a predetermined mixed gas to obtain a porous carbon precursor. The predetermined mixed gas included carbon dioxide gas, water vapor, and an inert gas, with a carbon dioxide gas fraction of 12% and a water vapor fraction of 3% in the predetermined mixed gas.

[0197] 5. Carbonization treatment: Under a nitrogen gas flow, the porous carbon precursor prepared in step 4) above was placed in a tube furnace, and a temperature was raised to 1250°C at a heating rate of 10°C / min and maintained for 10 h to obtain porous carbon.

[0198] 6. Coating treatment: An aqueous polyacrylic acid solution with a pH value of 1.5 was prepared. The porous carbon prepared in step 5) above and the prepared aqueous acrylic acid solution were mixed uniformly to form a kneaded mixture. The kneaded mixture was placed in a twin-screw kneader and kneaded for 6 h to obtain a hard carbon material. The twinscrew kneader was operated at a rotational speed of 40 rpm, the kneaded mixture had a solid content of 65 wt%, and the mass of polyacrylic acid in the kneaded mixture was 5% of the mass of the porous carbon.

[0199] Tests Related to Hard Carbon Material:

[0200] 1) Pore Volume Test

[0201] First, a pretreatment was performed on the hard carbon material to obtain a porous carbon material. Specifically, 2 g of the hard carbon powder was added to a 250 mL beaker, and 1 g of NaOH was added and stirred uniformly. The beaker was placed in a water bath, heated to 80°C with stirring and maintained for 30 min. Then, the mixture was filtered, and a filter cake was collected and washed repeatedly with deionized water until a filtrate was neutral. The washed filter cake was dried in a vacuum oven at 80°C for 24 h.

[0202] Next, the porous carbon material powder was placed in a sample tube, and degassed under vacuum at 200°C for 12 h. The adsorption amount of nitrogen of the hard carbon material at different pressures was measured using an ASAP 2460 physical adsorption analyzer, and adsorption and desorption isotherms were plotted. The shape of the pore was determined according to the shape of a hysteresis loop, the pore size distribution curve of the micropores was fitted using the DFT model, and the micropore volume X, mesopore volume Y, and total pore volume A of the hard carbon material were calculated.

[0203] 2) Methylene Blue Adsorption Value Test

[0204] The methylene blue adsorption value test was performed according to the following steps:

[0205] © Preparation of buffer solution: 3.6 g of potassium dihydrogen phosphate and 14.3 g of disodium hydrogen phosphate were weighed and dissolved in 1000 mL of water. © Preparation of methylene blue solution: 1.0 g of dried methylene blue was weighed and dissolved in a buffer solution at a temperature of 60 ± 1°C. After complete dissolution, the solution was cooled to room temperature and filtered into a 1000 mL volumetric flask. Filter residue was washed with the buffer solution in portions, and finally diluted to the mark with the buffer solution to prepare a methylene blue solution with a concentration of 1000 mg / L. © Sample treatment: a specific amount of the ground and dried hard carbon material was weighed and fully mixed and absorbed with the prepared methylene blue solution of known concentration. The mixture was shaken at room temperature for a period of time to allow the hard carbon material to fully adsorb methylene blue. © Filtration: the mixture was filtered using medium-speed qualitative filter paper with a diameter of 12.5 cm to separate the unadsorbed methylene blue solution. © Absorbance measurement: the absorbance of the filtrate was measured using an ultraviolet-visible spectrophotometer at the maximum absorption wavelength (665 nm). © Comparison and calculation: the absorbance was compared with that of a standard copper sulfate color solution (an aqueous solution with a mass fraction of 0.4%), and the amount of the methylene blue solution added was adjusted until a difference between the absorbance readings of the sample filtrate and the standard copper sulfate color solution was within ±0.02. ® The concentration of methylene blue in the solution was obtained according to a standard curve, and the mass of methylene blue adsorbed per gram of the hard carbon material was calculated to obtain the Z value.

[0206] Preparation of Button-Type Half Battery

[0207] The hard carbon material prepared in Example 1, a binder of styrene-butadiene rubber (SBR), a thickener of sodium carboxymethyl cellulose (CMC-Na), and a conductive agent of carbon black were mixed at a mass ratio of 96.2:1.8:1.2:0.8 in an appropriate amount of a solvent of deionized water with sufficient stirring to form a uniform negative electrode slurry. The negative electrode slurry was uniformly applied onto a surface of a copper foil as a negative electrode current collector, dried in an oven, and then cut into pieces for later use. 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 then NaPF6 was dissolved in the above organic solvent to prepare an electrolyte solution with a concentration of 1 mol / L. Subsequently, using a metallic sodium sheet as a counter electrode and a glass fiber film as a separator, a CR2430-type button battery was assembled in a glove box under an argon atmosphere.

[0208] Performance Test of Button-Type Half Battery

[0209] At 25°C, the button-type half battery prepared above was first discharged at a constant current to 0 V at a current density of 10 mA / g, and the first-cycle discharge specific capacity (i.e., the discharge specific capacity) of the button-type half battery was recorded. Subsequently, the battery was charged at a constant current to 2.0 V at a current density of 10 mA / g, and the first-cycle charge specific capacity (i.e., the charge specific capacity) of the button-type battery was recorded.

[0210] First coulombic efficiency (%) = first-cycle charge capacity / first-cycle discharge specific capacity x 100%.

[0211] Examples 2 to 13

[0212] Hard carbon materials were prepared and assembled into button-type half batteries according to methods similar to that of Example 1, except that during the preparation of the hard carbon materials, process conditions of the activation treatment and the kneading treatment were adjusted according to Table 1 below, to obtain hard carbon materials with different total pore volumes A and different methylene blue adsorption values Z. Details are shown in Table 1.

[0213] Comparative Example 1

[0214] A hard carbon material was prepared and assembled into a button-type half battery according to a method similar to that of Example 1, except that during the preparation of the hard carbon material, process conditions of the activation treatment and the coating treatment were adjusted according to Table 1 below, to obtain a hard carbon material having a total pore volume A of less than 0.2 cm3 / g.

[0215] Comparative Example 2

[0216] A hard carbon material was prepared and assembled into a button-type half battery according to a method similar to that of Example 1, except that during the preparation of the hard carbon material, process conditions of the activation treatment and the coating treatment were adjusted according to Table 1 below, to obtain a hard carbon material having a methylene blue adsorption value of greater than 8 mg / g.

[0217] Comparative Example 3

[0218] A hard carbon material was prepared and assembled into a button-type half battery according to a method similar to that of Example 1, except that during the preparation of the hard carbon material, process conditions of the activation treatment and the coating treatment were adjusted according to Table 1 below, to obtain a hard carbon material having a total pore 5 volume A of less than 0.2 cm3 / g and a methylene blue adsorption value of greater than 8 mg / g.

[0219] The process parameters for preparing the hard carbon materials of Examples 1 to 13 and Comparative Examples 1 to 3 are shown in Table 1 below, and the performance parameters of the prepared hard carbon materials and the test results of the button-type half batteries are shown in Table 2 below. 10

[0220] Table 1: Item Activation treatment Coating treatment Activation temperature (°C) Volume fraction of carbon dioxide in the predetermined mixed gas (%) Volume fraction of water vapor in the predetermined mixed gas (%) Volume ratio of carbon dioxide to water vapor Addition amount of coating raw material as percentage of porous carbon mass (%) Kneading time (h) Solid content (wt%) pH Example 1 750 10 2 5.0 5 12 65 1.5 Example 2 850 10 2 5.0 5 12 65 1.5 Example 3 950 10 2 5.0 5 12 65 1.5 Example 4 850 5 2 2.5 5 12 65 1.5 Example 5 850 20 2 10.0 5 12 65 1.5 Example 6 850 10 1 10.0 5 12 65 1.5 Example 7 850 10 4 2.5 5 12 65 1.5 Example 8 850 10 2 5.0 2 12 65 1.5 Example 9 850 10 2 5.0 10 12 65 1.5 Example 10 850 10 2 5.0 5 5 65 1.5 Example 11 850 10 2 5.0 5 12 58 1.5 Example 12 850 10 2 5.0 5 12 71 1.5 Example 13 850 10 2 5.0 5 12 65 2.5 Comparative Example 1 550 10 2 5.0 5 12 65 1.5 Comparative Example 2 850 10 2 5.0 5 0.3 65 1.5 Comparative Example 3 650 10 2 5.0 5 12 65 6

[0221] Table 2: Item Parameters of hard carbon material Performance of button-type half battery A (cm3 / g) X / A (%) Y / A (%) X / Y Z (mg / g) Discharge specific capacity (mAh / g) Charge specific capacity (mAh / g) First coulombic efficiency (%) Example 1 0.247 91.49 8.51 10.8 1.8 300.2 269.7 89.8 Example 2 0.416 93.84 6.16 15.2 1.2 416.6 379.6 91.1 Example 3 0.542 87.82 12.18 7.2 3.7 355.0 310.2 87.4 Example 4 0.384 86.80 13.20 6.6 1.5 370.4 334.6 90.3 Example 5 0.350 96.63 3.37 28.7 1.4 372.2 310.4 83.4 Example 6 0.376 95.33 4.67 20.4 1.7 381.2 324.6 85.1 Example 7 0.399 88.43 11.57 7.6 1.9 379.6 335.4 88.4 Example 8 0.403 93.53 6.47 14.5 4.6 406.3 346.7 85.3 Example 9 0.370 94.25 5.75 16.4 1.3 360.4 300.9 83.5 Example 10 0.387 93.13 6.87 13.5 5.2 391.8 329.4 84.1 Example 11 0.402 93.23 6.77 13.8 5.8 395.2 328.5 83.1 Example 12 0.396 93.02 6.98 13.3 6.9 394.9 324.2 82.1 Example 13 0.390 93.43 6.57 14.2 7.5 393.5 318.1 80.9 Comparative Example 1 0.185 87.30 12.70 6.9 3.9 287.4 250.8 87.3 Comparative Example 2 0.381 89.20 10.80 8.3 9.4 374.9 282.9 75.5 Comparative Example 3 0.139 90.80 9.20 9.9 10.8 264.6 191.1 72.2

[0222] It can be seen from Tables 1 and 2 above that in Examples 1 to 13, by controlling the total pore volume of the porous carbon between 0.2 cm3 / g and 0.6 cm3 / g, and the methylene 5 blue adsorption value of the hard carbon material to be less than 8 mg / g, the first coulombic efficiency and charge specific capacity were significantly improved, while a high discharge specific capacity was also achieved. In Comparative Example 1, since the total pore volume of the porous carbon was less than 0.2 cm3 / g, the discharge specific capacity was relatively low. In Comparative Example 2, since the methylene blue adsorption value of the hard carbon 10 material was greater than 8 mg / g, although the discharge specific capacity was relatively high, the first coulombic efficiency and charge specific capacity were relatively low. In Comparative Example 3, since the total pore volume of the porous carbon was less than 0.2 cm3 / g and the methylene blue adsorption value of the hard carbon material was greater than 8 mg / g, the discharge specific capacity, charge specific capacity, and first coulombic efficiency were all relatively low. Within an appropriate range of the total pore volume and methylene blue adsorption value, the pore volume ratio X / Y of micropores to mesopores within an appropriate range, for example, 5.6 to 33, particularly within the range of 7 to 16, can further balance the discharge specific capacity, charge specific capacity, and first coulombic efficiency.

[0223] Example 14

[0224] Preparation of Hard Carbon Material:

[0225] 1 . Pre-carbonization treatment: Under a nitrogen gas flow, a hard carbon precursor of epoxy resin was placed in a tube furnace, and a temperature was raised to 550°C at a heating rate of 10°C / min and maintained for 6 h to obtain a pre-carbonized body. The flow rate of nitrogen gas was 500 mL / min.

[0226] 2. Pulverization treatment: The pre-carbonized body obtained above was subjected to a ball milling and pulverization treatment using zirconia grinding balls and a zirconia grinding jar, with a mass ratio of the pre-carbonized body to the grinding balls of 1:3. The ball mill was operated at a rotational speed of 800 rpm for 8 h. After removing the grinding balls, a pre-carbonized body having a volume distribution particle size Dv50 of 5.1 pm and a volume distribution particle size Dv90 of 11 pm was obtained.

[0227] 3. Activation treatment: The pulverized pre-carbonized body obtained in step 2) above was immersed in a pore-forming solution of an aqueous phosphoric acid solution for 6 h, and then placed in a tube furnace. Under a nitrogen gas flow, a temperature was raised to 600°C at a heating rate of 5°C / min and maintained for 2 h, with a nitrogen flow rate of 500 mL / min, to obtain a porous carbon precursor. The addition amount of element P was 12.2% of the mass of the porous carbon. The impregnated mixture formed by the pre-carbonized body and the pore-forming solution had a solid content of 65%.

[0228] 4. Demineralization treatment: The porous carbon precursor obtained in step 4) above was washed with an aqueous hydrochloric acid solution at a pickling temperature of 60°C for 3 h, and the washing process was repeated 4 times. Then, the mixture was filtered, and a filter cake was collected and washed repeatedly with deionized water and absolute ethanol until the pH of the filtrate was greater than 6. The washed filter cake was dried in a vacuum oven at 80°C for 24 h.

[0229] 5. Carbonization treatment: Under a nitrogen gas flow, the demineralized porous carbon precursor obtained in step 4) above was placed in a tube furnace, and a temperature was raised to 1250°C at a heating rate of 10°C / min and maintained for 10 h to obtain porous carbon.

[0230] 6. Coating treatment: An aqueous polyacrylic acid solution with a pH value of 1.5 was prepared. The porous carbon prepared in step 5) above and the prepared aqueous acrylic acid solution were mixed uniformly to form a kneaded mixture. The kneaded mixture was placed in a twin-screw kneader and kneaded for 6 h to obtain a hard carbon material. The twinscrew kneader was operated at a rotational speed of 40 rpm, the kneaded mixture had a solid content of 65 wt%, and the mass of polyacrylic acid in the kneaded mixture was 5% of the mass of the porous carbon.

[0231] A button-type half battery was assembled according to a method similar to that of Example 1.

[0232] Examples 15 to 18

[0233] Hard carbon materials were prepared and assembled into button-type half batteries according to methods similar to that of Example 14, except that during the preparation of the hard carbon materials, process conditions of the activation treatment were adjusted according to Table 3 below, to obtain hard carbon materials with different total pore volumes A of the porous carbon. Details are shown in Table 3.

[0234] The process parameters for preparing the hard carbon materials of Examples 14 to 18 are shown in Table 3 below, and the performance parameters of the prepared hard carbon materials and the test results of the button-type half batteries are shown in Table 4 below.

[0235] Table 3: Item Activation treatment Addition amount of element P in pore-forming solution as percentage of mass of porous carbon (%) Impregnation time (h) Example 14 12.2 6 Example 15 21.7 6 Example 16 27.1 6 Example 17 21.7 4 Example 18 21.7 1

[0236] Table 4: Item Parameters of hard carbon material Performance of button-type half battery A (cm3 / g) X / A (%) Y / A (%) X / Y Z (mg / g) Discharge specific capacity (mAh / g) Charge specific capacity (mAh / g) First coulombic efficiency (%) Example 14 0.284 89.14 10.86 8.2 2.1 319.4 281.1 88.0 Example 15 0.392 89.35 10.65 8.4 1.6 376.9 339.6 90.1 Example 16 0.457 86.79 13.21 6.6 3.5 365.2 320.7 87.8 Example 17 0.386 88.42 11.58 7.6 2.0 376.5 332.2 88.2 Example 18 0.345 88.07 11.93 7.4 3.9 347.8 300.7 86.5

[0237] It can be seen from Tables 3 and 4 above that in Examples 14 to 18, by reasonably adjusting the phosphoric acid content in the pore-forming solution or the impregnation time, 5 the pore structure of the hard carbon material could be regulated, such that the total pore volume of the porous carbon was between 0.2 cm3 / g and 0.6 cm3 / g, and the methylene blue adsorption value of the hard carbon material was less than 8 mg / g. As a result, the first coulombic efficiency and charge specific capacity were significantly improved, while a high discharge specific capacity was also achieved. 10

[0238] It needs to be stated that the present disclosure is not limited to the above embodiments. The above described embodiments are merely exemplary, and embodiments having substantially the same technical idea and the same effects within the scope of the technical solution of the present disclosure are all included in the technical scope of the present disclosure. In addition, other embodiments constructed by applying various modifications 15 conceivable to those skilled in the art to the embodiments and combining some of the constituent elements of the embodiments without departing from the scope of the essence of the present disclosure are also included in the scope of the present disclosure.

Claims

1. A negative electrode sheet, 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, the hard carbon material comprises a core and an organic coating layer coating the core, and the core comprises porous carbon;a total pore volume A of the porous carbon satisfies: 0.2 cm3 / g < A < 0.6 cm3 / g; anda methylene blue adsorption value Z of the hard carbon material is less than or equal to 8 mg / g.

2. The negative electrode sheet according to claim 1, wherein the methylene blue adsorption value Z of the hard carbon material is less than or equal to 4 mg / g.

3. The negative electrode sheet according to claim 1 or 2, wherein the porous carbon comprises micropores having a pore size of 0.4 nm to 2.5 nm and mesopores having a pore size of 2.5 nm to 10 nm; and a total pore volume X of the micropores and a total pore volume Y of the mesopores satisfy: 5.6% < X / Y < 33%, and X + Y = A.

4. The negative electrode sheet according to claim 3, wherein the total pore volume X of the micropores and the total pore volume Y of the mesopores satisfy: 7 < X / Y < 16.

5. The negative electrode sheet according to claim 3 or 4, wherein the total pore volume X of the micropores and the total pore volume A of the porous carbon satisfy: 85% < X / A < 97%.

6. The negative electrode sheet according to any one of claims 3 to 5, wherein the total pore volume Y of the mesopores and the total pore volume A of the porous carbon satisfy: 3% < Y / A < 15%.

7. The negative electrode sheet according to any one of claims 1 to 6, wherein the organic coating layer has a thickness of 1 nm to 90 nm.

8. The negative electrode sheet according to any one of claims 1 to 7, wherein a mass of the organic coating layer in the hard carbon material is 0.5 wt% to 10 wt% of the porous carbon.

9. The negative electrode sheet according to any one of claims 1 to 8, wherein the hard carbon material satisfies at least one of the following items:(1) the hard carbon material has a compacted density of 0.55 g / cm3 to 1.0 g / cm3 under 5 t;(2) the hard carbon material has a tap density of 0.34 g / cm3 to 0.8 g / cm3;(3) the hard carbon material has a volume distribution particle size Dv50 of 3.0 pm to 7.9gm;(4) the hard carbon material has a volume distribution particle size Dv90 of 8 gm to 15 gm; and(5) the hard carbon material has a BET specific surface area of 0.1 m2 / g to 0.8 m2 / g.

10. A secondary battery, comprising the negative electrode sheet according to any one of claims 1 to 9.

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

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

13. A hard carbon material, comprising a core and an organic coating layer coating the core, wherein the core comprises porous carbon;a total pore volume A of the porous carbon satisfies: 0.2 cm3 / g < A < 0.6 cm3 / g; anda methylene blue adsorption value Z of the hard carbon material is less than or equal to 8 mg / g.

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

15. The hard carbon material according to claim 13 or 14, wherein the porous carbon comprises micropores having a pore size of 0.4 nm to 2.5 nm and mesopores having a pore size of 2.5 nm to 10 nm; and a total pore volume X of the micropores and a total pore volume Y of the mesopores satisfy: 5.6% < X / Y < 33%, and X + Y = A.

16. The hard carbon material according to claim 15, wherein the total pore volume X of the micropores and the total pore volume Y of the mesopores satisfy: 7 < X / Y < 16.

17. The hard carbon material according to claim 15 or 16, wherein the total pore volume X of the micropores and the total pore volume A of the porous carbon satisfy: 85% < X / A < 97%.

18. The hard carbon material according to any one of claims 15 to 17, wherein the total pore volume Y of the mesopores and the total pore volume A of the porous carbon satisfy: 3% < Y / A < 15%.

19. The hard carbon material according to any one of claims 13 to 18, wherein the organic coating layer has a thickness of 1 nm to 90 nm.

20. The hard carbon material according to any one of claims 13 to 19, wherein a mass of the organic coating layer in the hard carbon material is 0.5 wt% to 10 wt% of the porous carbon.

21. The hard carbon material according to any one of claims 13 to 20, wherein the hard carbon material satisfies at least one of the following items:(1) the hard carbon material has a compacted density of 0.55 g / cm3 to 1.0 g / cm3 under 5 t;(2) the hard carbon material has a tap density of 0.34 g / cm3 to 0.8 g / cm3;(3) the hard carbon material has a volume distribution particle size Dv50 of 3.0 gm to 7.9 gm;(4) the hard carbon material has a volume distribution particle size Dv90 of 8 gm to 15 gm; and(5) the hard carbon material has a BET specific surface area of 0.1 m2 / g to 0.8 m2 / g.

22. A preparation method for a hard carbon material, the method comprising:subjecting a hard carbon precursor to a pre-carbonization treatment to obtain a precarbonized body;subjecting the pre-carbonized body to an activation treatment to obtain a porous carbon precursor;subjecting the porous carbon precursor to a carbonization treatment to obtain porous carbon; andsubjecting the porous carbon to a coating treatment, wherein the coating treatment comprises a kneading treatment of kneading a kneaded mixture formed by the porous carbon and an aqueous solution of a coating raw material, wherein the kneaded mixture has a solid content of 55 wt% to 75 wt%, and the aqueous solution of the coating raw material has a pH value of < 3.

23. The preparation method according to claim 22, wherein the kneaded mixture has a solid content of 63 wt% to 67 wt%, and the aqueous solution of the coating raw material has a pH value of 1.2 to 1.8.

24. The preparation method according to claim 22 or 23, wherein the coating raw material comprises at least one of polyacrylic acid, a polyacrylate salt, and a copolymer of polyacrylic acid.

25. The preparation method according to any one of claims 22 to 24, wherein a mass of the coating raw material is 0.5% to 10% of a mass of the porous carbon, and the kneading treatment is performed for at least 0.5 h.

26. The preparation method according to any one of claims 22 to 25, wherein the kneading treatment is performed in a twin-screw kneader, and the twin-screw kneader has a rotational speed of 10 rpm to 50 rpm.

27. The preparation method according to any one of claims 22 to 26, wherein the activation treatment comprises treating the pre-carbonized body at a temperature of 700°C to 950°C under a predetermined mixed gas, the predetermined mixed gas comprises carbon dioxide gas, water vapor, and an inert gas, and a volume ratio of the carbon dioxide gas to the water vapor in the predetermined mixed gas is greater than or equal to 2.

28. The preparation method according to claim 27, wherein the carbon dioxide gas has a volume fraction of 5% to 20% in the predetermined mixed gas, and the water vapor has a volume fraction of 1% to 5% in the mixed gas.

29. The preparation method according to any one of claims 22 to 28, wherein the activation treatment comprises impregnating the pre-carbonized body in a pore-forming solution for at least 2 h;the pore-forming solution comprises a P-containing pore-forming solution or a Zn-containing pore-forming solution, wherein an addition amount of element P in the P-containing pore-forming solution is 9.5% to 32.4% of the mass of the porous carbon, and an addition amount of element Zn in the Zn-containing pore-forming solution is 14% to 50% of the mass of the porous carbon.

30. The preparation method according to claim 29, wherein an impregnated mixture formed by the pre-carbonized body and the pore-forming solution has a solid content of 50 wt% to 70 wt%.

31. The preparation method according to claim 29 or 30, wherein the activation treatment further comprises maintaining the impregnated mixture formed by the impregnated precarbonized body and the pore-forming solution at a temperature of 400°C to 750°C for 1 h to 12 h.

32. The preparation method according to any one of claims 22 to 31, wherein the precarbonization treatment is performed by raising a temperature to 400°C to 600°C at a heating rate of 1°C / min to 20°C / min, and maintaining the temperature for 1 h to 12 h.

33. The preparation method according to any one of claims 22 to 32, wherein the carbonization treatment is performed by raising a temperature to 1000°C to 1800°C at a heating rate of 2°C / min to 20°C / min, and maintaining the temperature for 1 h to 12 h.

34. The preparation method according to any one of claims 22 to 33, wherein the hard carbon precursor comprises at least one of a biomass precursor and a synthetic polymer precursor; andthe synthetic polymer precursor comprises a compound composed of elements C, H, and O.

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