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

Abstract

Disclosed in the present disclosure are a negative electrode sheet, a secondary battery, an electric device, and 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, wherein the negative electrode film layer comprises a hard carbon material, and the hard carbon material comprises a core and a carbon coating layer that coats the core; the core comprises porous carbon; the total pore volume A of the porous carbon satisfies: 0.2 cm3 / g≤A≤0.6 cm3 / g; and the methylene blue adsorption capacity Z of the hard carbon material is less than or equal to 10 mg / g.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present disclosure is presented based on Chinese Patent Application No. 202410544868.1, filed on April 30, 2024 and entitled “NEGATIVE ELECTRODE PLATE, SECONDARY BATTERY, ELECTRIC DEVICE, HARD CARBON MATERIAL, AND PREPARATION METHOD FOR HARD CARBON MATERIAL”, and claims priority to the Chinese Patent Application, the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

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

[0003] In recent years, secondary batteries have been widely applied in energy storage power systems such as hydropower, thermal power, wind power, and solar power stations, as well as in various fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, and aerospace. With the application and promotion of secondary batteries, the requirements for the energy density, cycle performance, and large-rate charging performance of secondary batteries are becoming increasingly high. However, as an important component of the secondary battery, the performance of the negative electrode active material affects the performance of the secondary battery to a certain extent. At present, hard carbon is generally used as a negative electrode active material of a secondary battery. However, when the hard carbon is used as the negative electrode active material, the charge specific capacity and the initial coulombic efficiency are low. SUMMARY

[0004] The present disclosure is made in view of the above problems, and the objective of the present disclosure is to provide a negative electrode plate, a secondary battery, an electric device, a hard carbon material, and a preparation method for the hard carbon material. The negative electrode plate is used as a secondary battery, and has improved charge specific capacity and initial coulombic efficiency.

[0005] To achieve the above objective, a first aspect of the present disclosure provides a negative electrode plate. The negative electrode plate includes a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector. The negative electrode film layer includes a hard carbon material, the hard carbon material includes an inner core and a carbon coating layer coating the inner core, and the inner 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 <10 mg / g. In the present disclosure, maintaining the total pore volume of the porous carbon in the hard carbon material within the above range can enable the carbon skeleton of the hard carbon material to have appropriate strength and stability while the specific capacity of the hard carbon material is improved, without collapse during the intercalation and deintercalation of active ions, which is conducive to improving the charge specific capacity and the initial coulombic efficiency of the secondary battery. In addition, providing the carbon coating layer on the outer surface of the porous carbon inner core can block electrolytic solution molecules from entering the pore structure of the porous carbon, which is conducive to improving the charge specific capacity and the initial coulombic efficiency of the secondary battery. Further, the methylene blue adsorption value of the hard carbon material within the above range enables the density of the carbon coating layer to be high, which can effectively isolate the electrolytic solution and suppress electrolytic solution molecules from entering the interior of the porous carbon, thereby further helping to improve the charge specific capacity and the initial coulombic efficiency of the secondary battery.

[0006] In some embodiments, a methylene blue adsorption value Z of the hard carbon material is < 5 mg / g. The smaller the methylene blue adsorption value is, the better the coating effect is. In this way, the improvement in the charge specific capacity and the initial coulombic efficiency of the secondary battery is further facilitated.

[0007] In some embodiments, the porous carbon includes micropores with a pore size of 0.4 nm to 2.5 nm and mesopores with 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: 8 < X / Y < 13. Therefore, the utilization of storage space inside the hard carbon material can be maximized, and the reversible intercalation and deintercalation of active ions are enabled, which is conducive to improving the charge specific capacity and the initial coulombic efficiency of the secondary battery.

[0008] 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%. Therefore, more storage space for active ions can be provided, which is conducive to improving the discharge specific capacity of the hard carbon material.

[0009] 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%. Therefore, a suitable number of transport channels for active ions can be provided, thereby improving the utilization rate of micropores, and helping to improve the charge specific capacity and the initial coulombic efficiency of the secondary battery.

[0010] In some embodiments, ID / IG of the hard carbon material is 0.83 to 1.26, where ID represents a D peak intensity at 1350±50 cm-1 in a Raman spectrum, and IG represents a G peak intensity at 1580±50 cm-1 in the Raman spectrum. The ID / IG of the hard carbon material within the above range indicates that the surface coating layer has an appropriate degree of order and appropriate reactivity, enabling the transport of active ions.

[0011] In some embodiments, the hard carbon material satisfies at least one of the following:

[0012] (1) A compacted density of the hard carbon material at 5 t is 0.6 g / cm3 to 1.05 g / cm3. A powder compacted density of the hard carbon material within the above range helps to form a reasonable pore channel structure between the particles of the negative electrode film layer and improve the transport performance of active ions and electrons, thereby improving the kinetic performance of the secondary battery.

[0013] (2) A tap density of the hard carbon material is 0.4 g / cm3 to 0.85 g / cm3. The tap density of the hard carbon material within the above range helps to improve the compacted density of the negative electrode film layer, thereby increasing the energy density of the secondary battery.

[0014] (3) A volume distribution particle size Dv50 of the hard carbon material is 3.0 pm to 7.9 pm.

[0015] (4) A volume distribution particle size Dv90 of the hard carbon material is 8 pm to 15 pm. The volume distribution particle sizes Dv50 and Dv90 of the particles of the hard carbon material within the above ranges help to reduce the specific surface area of the hard carbon material and reduce side reactions, thereby improving the initial coulombic efficiency of the secondary battery. Meanwhile, the bulk phase transport path of active ions can also be shortened, thereby improving the kinetic performance of the secondary battery.

[0016] (5) A specific surface area BET of the hard carbon material is 0.1 m2 / g to 10 m2 / g. The specific surface area of the hard carbon material within the above range can reduce the consumption of active ions in the first charge, which is conducive to improving the initial coulombic efficiency of the secondary battery.

[0017] A second aspect of the present disclosure provides a secondary battery. The secondary battery includes the negative electrode plate according to the first aspect of the present disclosure.

[0018] In some embodiments, the secondary battery further includes a positive electrode plate. The positive electrode plate includes, as a positive electrode active material, at least one selected from a transition metal oxide, a polyanionic compound, and a Prussian blue compound.

[0019] The secondary battery in the present disclosure has improved charge specific capacity and initial coulombic efficiency.

[0020] 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.

[0021] The electric device of the present disclosure includes the secondary battery according to the present disclosure and thus has at least the same advantages as the secondary battery.

[0022] A fourth aspect of the present disclosure provides a hard carbon material. The hard carbon material includes an inner core and a carbon coating layer coating the inner core, and the inner 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 <10 mg / g. In the present disclosure, maintaining the total pore volume of the porous carbon in the hard carbon material within the above range can enable the carbon skeleton of the hard carbon material to have appropriate strength and stability while the specific capacity of the hard carbon material is improved, without collapse during the intercalation and deintercalation of active ions, which is conducive to improving the charge specific capacity and the initial coulombic efficiency of the secondary battery. In addition, providing the carbon coating layer on the outer surface of the porous carbon inner core can block electrolytic solution molecules from entering the pore structure of the porous carbon, which is conducive to improving the charge specific capacity and the initial coulombic efficiency of the secondary battery. Further, the methylene blue adsorption value of the hard carbon material within the above range enables the density of the carbon coating layer to be high, which can effectively isolate the electrolytic solution and suppress electrolytic solution molecules from entering the interior of the porous carbon, thereby further helping to improve the charge specific capacity and the initial coulombic efficiency of the secondary battery.

[0023] In some embodiments, a methylene blue adsorption value Z of the hard carbon material is < 5 mg / g. The smaller the methylene blue adsorption value is, the better the coating effect is. In this way, the improvement in the charge specific capacity and the initial coulombic efficiency of the secondary battery is further facilitated.

[0024] In some embodiments, the porous carbon includes micropores with a pore size of 0.4 nm to 2.5 nm and mesopores with 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: 8 < X / Y < 13. Therefore, the utilization of storage space inside the hard carbon material can be maximized, and the reversible intercalation and deintercalation of active ions are enabled, which is conducive to improving the charge specific capacity and the initial coulombic efficiency of the secondary battery.

[0025] 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%. Therefore, more storage space for active ions can be provided, which is conducive to improving the discharge specific capacity of the hard carbon material.

[0026] 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%. Therefore, a suitable number of transport channels for active ions can be provided, thereby improving the utilization rate of micropores, and helping to improve the charge specific capacity and the initial coulombic efficiency of the secondary battery.

[0027] In some embodiments, ID / IG of the hard carbon material is 0.83 to 1.26, where ID represents a D peak intensity at 1350±50 cm-1 in a Raman spectrum, and IG represents a G peak intensity at 1580±50 cm-1 in the Raman spectrum. The ID / IG of the hard carbon material within the above range indicates that the surface coating layer has an appropriate degree of order and appropriate reactivity, enabling the transport of active ions.

[0028] In some embodiments, the hard carbon material satisfies at least one of the following:

[0029] (1) A compacted density of the hard carbon material at 5 t is 0.6 g / cm3 to 1.05 g / cm3. A powder compacted density of the hard carbon material within the above range helps to form a reasonable pore channel structure between the particles of the negative electrode film layer and improve the transport performance of active ions and electrons, thereby improving the kinetic performance of the secondary battery.

[0030] (2) A tap density of the hard carbon material is 0.4 g / cm3 to 0.85 g / cm3. The tap density of the hard carbon material within the above range helps to improve the compacted density of the negative electrode film layer, thereby increasing the energy density of the secondary battery.

[0031] (3) A volume distribution particle size Dv50 of the hard carbon material is 3.0 pm to 7.9 pm.

[0032] (4) A volume distribution particle size Dv90 of the hard carbon material is 8 pm to 15 pm. The volume distribution particle sizes Dv50 and Dv90 of the particles of the hard carbon material within the above ranges help to reduce the specific surface area of the hard carbon material and reduce side reactions, thereby improving the initial coulombic efficiency of the secondary battery. Meanwhile, the bulk phase transport path of active ions can also be shortened, thereby improving the kinetic performance of the secondary battery.

[0033] (5) A specific surface area BET of the hard carbon material is 0.1 m2 / g to 10 m2 / g. The specific surface area of the hard carbon material within the above range can reduce the consumption of active ions in the first charge, which is conducive to improving the initial coulombic efficiency of the secondary battery.

[0034] A fifth aspect of the present disclosure further provides a preparation method for a hard carbon material. The method includes:

[0035] performing a pre-carbonization treatment on a hard carbon precursor to obtain a pre-carbonized body;

[0036] performing an activation treatment on the pre-carbonized body to obtain a porous carbon precursor;

[0037] performing a coating treatment on the porous carbon precursor to form a hard carbon precursor, where the coating treatment includes a kneading treatment of kneading a kneading mixture formed by the porous carbon precursor and a water-based solution of a coating raw material, and a curing treatment of curing a product of the kneading treatment, a solid content of the kneading mixture being 55 wt% to 75 wt%; and

[0038] performing a carbonization treatment on the hard carbon precursor to obtain the hard carbon material.

[0039] In some embodiments, a mass ratio of the coating raw material to the porous carbon precursor is 0.5:10 to 2:10, and the kneading treatment is performed for at least 0.5 h. The mass of the coating raw material within the above range enables the formation of a coating layer with an appropriate density, such that the charge specific capacity and the initial coulombic efficiency of the secondary battery can be improved without affecting the kinetic performance of the secondary battery and increasing the impedance of the secondary battery.

[0040] In some embodiments, the curing treatment is performed at 120 °C to 250 °C for at least 1 h. Curing under the above conditions can improve the degree of crosslinking of the carbon coating raw material, thereby improving the thermal stability of the carbon coating layer and reducing the volume shrinkage of the carbon coating layer in the subsequent carbonization process. This prevents the carbon coating layer from cracking in the subsequent carbonization process, thereby improving the density of the coating layer.

[0041] In some embodiments, the coating raw material includes a prepolymer of a thermosetting resin.

[0042] In some embodiments, the thermosetting resin includes at least one of a phenolic resin, an epoxy resin, an unsaturated polyester resin, and a furan resin. Using the thermosetting resin described above as the coating raw material of the carbon coating layer embraces the advantages of high uniformity, high density, and high coating integrity.

[0043] In some embodiments, the kneading treatment is performed in a twin-screw kneader, and a rotation speed of the twin-screw kneader is 10 rpm to 50 rpm.

[0044] In some embodiments, the activation treatment includes subjecting the pre-carbonized body to treatment at 700 °C to 950 °C under a preset mixed gas, where the preset mixed gas includes carbon dioxide gas, water vapor, and inert gas, and a volume ratio of the carbon dioxide gas to the water vapor in the preset mixed gas is > 2. The activation treatment being performed at the above temperature enables the formation of an appropriate number of mesopores and micropores, thereby helping to improve the discharge specific capacity of the secondary battery.

[0045] In some embodiments, a volume proportion of the carbon dioxide gas in the preset mixed gas is 5% to 20%, and a volume proportion of the water vapor in the mixed gas is 1% to 5%. The volume proportion of the carbon dioxide gas in the preset mixed gas within the above range enables the formation of an appropriate number of micropores, thereby helping to improve the discharge specific capacity of the secondary battery. The volume proportion of the water vapor in the preset mixed gas within the above range enables the formation of an appropriate number of mesopores, thereby helping to improve the charge specific capacity and the initial coulombic efficiency of the secondary battery.

[0046] In some embodiments, the activation treatment includes impregnating the pre-carbonized body in a P-containing pore-forming solution or a Zn-containing pore-forming solution for at least 2 h; an addition amount of an element P in the P-containing pore-forming solution is 9.5% to 32.4% of a mass of the pre-carbonized body, and an addition amount of an element Zn in the Zn-containing pore-forming solution is 14% to 50% of the mass of the pre-carbonized body. The addition amount of the element P or the element Zn in the pore-forming solution within the above range enables the formation of an appropriate number of micropores and mesopores, thereby helping to improve the discharge specific capacity, charge specific capacity, and initial coulombic efficiency of the secondary battery.

[0047] In some embodiments, a solid content of an impregnation mixture formed by the pre-carbonized body and the pore-forming solution is 50 wt% to 70 wt%. Maintaining the solid content of the impregnation mixture within the above range enables the formation of an appropriate number of micropores and mesopores, thereby helping to improve the discharge specific capacity, charge specific capacity, and initial coulombic efficiency of the secondary battery.

[0048] In some embodiments, the activation treatment further includes keeping an impregnation mixture formed by the immersed pre-carbonized body and the pore-forming solution at a temperature of 400 °C to 750 °C for 1 h to 12 h. Therefore, the P- or Zn-containing pore-forming solution and the pre-carbonized body can chemically react, and etching is performed in a chemical activation manner, thereby introducing a large number of pore structures into the skeleton structure of the pre-carbonized body.

[0049] In some embodiments, in the pre-carbonization treatment, a temperature is raised to 400 °C to 600 °C at a heating rate of 1 °C / min to 20 °C / min and held for 1 h to 12 h. The pre-carbonization treatment being performed under the above conditions facilitates the formation of the basic carbon skeleton structure, facilitating subsequent pore formation.

[0050] In some embodiments, in the carbonization treatment, a temperature is raised to 1000 °C to 1800 °C at a heating rate of 2 °C / min to 20 °C / min and held for 1 h to 12 h. In the carbonization process, the precursor loses heteroatoms such as H and O, forming a stable hard carbon skeleton, and the coating layer in the carbonization process is denser, which is conducive to improving the charge specific capacity and the initial coulombic efficiency of the secondary battery.

[0051] 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 C, H, and O elements. When the hard carbon material is prepared by using the above precursor, the carbon formed after activation is relatively ordered, and an appropriate number of micropores and mesopores can be formed. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] FIG. 1 is a schematic diagram of a battery cell according to an embodiment of the present disclosure.

[0053] FIG. 2 is an exploded view of the battery cell shown in FIG. 1 according to an embodiment of the present disclosure.

[0054] FIG. 3 is a schematic diagram of a battery module according to an embodiment of the present disclosure.

[0055] FIG. 4 is a schematic diagram of a battery pack according to an embodiment of the present disclosure.

[0056] FIG. 5 is an exploded view of the battery pack shown in FIG. 4 according to an embodiment of the present disclosure.

[0057] FIG. 6 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present disclosure.

[0058] Description of the reference numerals:

[0059] 1: battery pack; 2: upper case body; 3: lower case body; 4: battery module; 5: battery cell; 51: shell body; 52: electrode assembly; 53: top cover assembly. DETAILED DESCRIPTION

[0060] Hereinafter, embodiments specifically disclosing a negative electrode plate, a secondary battery, an electric device, a hard carbon material, and a preparation method for the hard carbon material of the present disclosure will be described in detail with appropriate reference to the drawings. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of actually identical structures may be omitted. This is to avoid unnecessary lengthiness of the following descriptions and to facilitate understanding by those skilled in the art. Additionally, the drawings and the following descriptions are provided to enable those skilled in the art to fully understand the present disclosure and are not intended to limit the subject matter recited in the claims.

[0061] The “ranges” disclosed in the present disclosure are defined with lower and upper limits. A given range is defined by selecting a lower limit and an upper limit that delineate the boundaries of a particular range. Ranges defined in this manner may include or exclude the end values and can be combined arbitrarily, which means that any lower limit may be combined with any upper limit to form a range. For example, if ranges of 60 to 120 and 80 to 110 are listed for a specific parameter, it is understood that ranges of 60 to 110 and 80 to 120 are also anticipated. Additionally, if the minimum range values listed are 1 and 2, and the maximum range values listed are 3, 4, and 5, then the following ranges can all be anticipated: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5. In the present disclosure, unless otherwise specified, the numerical range “a to b” indicates an abbreviated representation of any combination of real numbers between a and b, where both a and b are real numbers. For example, the numerical range “0 to 5” indicates that all real numbers between “0 and 5” are listed herein, and “0 to 5” is merely an abbreviated representation of a combination of these numerical values. Additionally, when stating that a parameter is an integer > 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or the like.

[0062] Unless otherwise specified, all embodiments and optional embodiments of the present disclosure may be combined with one another to form new technical solutions.

[0063] Unless otherwise specified, all technical features and optional technical features of the present disclosure may be combined with one another to form new technical solutions.

[0064] Unless otherwise specified, all steps of the present disclosure can be performed sequentially or randomly, preferably sequentially. For example, if a method includes steps (a) and (b), it indicates that the method may include steps (a) and (b) performed sequentially or steps (b) and (a) performed sequentially. For example, if the mentioned method may further include step (c), it indicates 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.

[0065] Unless otherwise specified, the terms used in the present disclosure have well-known meanings that are commonly understood by those skilled in the art.

[0066] Unless otherwise specified, the values of the parameters mentioned in the present disclosure can be measured by various test methods commonly used in the art. For example, they can be measured according to the test methods given in the present disclosure.

[0067] Unless otherwise specified, in the present disclosure, the term “active ion” refers to an ion that can be intercalated and deintercalated back and forth between the positive electrode and negative electrode of the secondary battery, including but not limited to, sodium ion, and the like.

[0068] At present, hard carbon is generally used as a negative electrode active material of a secondary battery. However, when the hard carbon is used as the negative electrode active material, the charge specific capacity and the initial coulombic efficiency are both low.

[0069] In view of this, the embodiments of the present disclosure provide a new hard carbon material as a negative electrode material of a secondary battery, which has improved charge specific capacity and initial coulombic efficiency.

[0070] Negative Electrode Plate

[0071] A first aspect of the embodiments of the present disclosure provides a negative electrode plate. The negative electrode plate includes a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector. The negative electrode film layer includes a hard carbon material, the hard carbon material includes an inner core and a carbon coating layer coating the inner core, and the inner 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 < 10 mg / g.

[0072] When the hard carbon material is used as a negative electrode active material of 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 a large number of large-sized pore structures. However, the large number of pore structures leads to decreased strength and poor stability of the carbon skeleton of the hard carbon material. During the charging and discharging cycle, the intercalation and deintercalation of active ions easily lead to the collapse of the carbon skeleton of the hard carbon material, affecting the charge specific capacity and the initial coulombic efficiency. In addition, the large-sized pore structures easily allow electrolytic solution molecules to enter. The electrolyte molecules entering the pore structures not only occupy the storage sites of active ions, but also consume more active ions to form a solid electrolyte interface (SEI) film, thereby further reducing the charge specific capacity and the initial coulombic efficiency.

[0073] Based on this, in the present disclosure, maintaining the total pore volume of the porous carbon in the hard carbon material within the above range can enable the carbon skeleton of the hard carbon material to have appropriate strength and stability while the specific capacity of the hard carbon material is improved, without collapse during the intercalation and deintercalation of active ions, which is conducive to improving the charge specific capacity and the initial coulombic efficiency of the secondary battery. In addition, providing the carbon coating layer on the outer surface of the porous carbon inner core can block electrolytic solution molecules from entering the pore structure of the porous carbon, which is conducive to improving the charge specific capacity and the initial coulombic efficiency of the secondary battery. Further, the methylene blue adsorption value of the hard carbon material within the above range enables the density of the carbon coating layer to be high, which can effectively isolate the electrolytic solution and suppress electrolytic solution molecules from entering the interior of the porous carbon, thereby further helping to improve the charge specific capacity and the initial coulombic efficiency of the secondary battery.

[0074] In addition, the presence of the carbon coating layer improves the stability of the SEI film in the secondary battery, thereby achieving high charge specific capacity and initial coulombic efficiency.

[0075] It should be noted that since the molecular diameter of methylene blue is close to the size (about 1 nm) of an ester solvent commonly used in the electrolytic solution, the methylene blue adsorption value can be used to accurately characterize the region where the electrolytic solution can enter, that is, characterize the density of the carbon coating layer.

[0076] The smaller the methylene blue adsorption value is, the better the coating effect is. In some embodiments, a methylene blue adsorption value Z of the hard carbon material is < 5 mg / g.

[0077] Illustratively, the total pore volume A of the porous carbon is 0.2 cm3 / g, 0.25 cm3 / g, 0.3 cm3 / g, 0.32 cm3 / g, 0.35 cm3 / g, 0.38 cm3 / g, 0.4 cm3 / g, 0.41 cm3 / g, 0.42 cm3 / g, 0.43 cm3 / g, 0.44 cm3 / g, 0.45 cm3 / g, 0.46 cm3 / g, 0.47 cm3 / g, 0.48 cm3 / g, 0.5 cm3 / g, 0.55 cm3 / g, 0.6 cm3 / g, or any value within a range defined by any two of the values.

[0078] Illustratively, the methylene blue adsorption value Z of the hard carbon material may be 1.0 mg / g, 1.5 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, 6.0 mg / g, 7.0 mg / g, 8.0 mg / g, 9.0 mg / g, 10 mg / g, or any value within a range defined by any two of the values.

[0079] In some embodiments, the material of the carbon coating layer is a prepolymer of a thermosetting resin. Illustratively, the thermosetting resin may be, for example, at least one of a phenolic resin, an epoxy resin, an unsaturated polyester resin, and a furan resin. Using the prepolymer of the thermosetting resin described above as the carbon coating layer embraces the advantages of high uniformity, high density, and high coating integrity.

[0080] In some embodiments, the porous carbon includes micropores with a pore size of 0.4 nm to 2.5 nm and mesopores with 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: 8 < X / Y < 13. Illustratively, X / Y may be 5.7, 7.6, 8.0, 6.0, 10.4, 11.0, 12.1, 13, 15, 20, 25, 30, 33, or any value within a range defined by any two of the values.

[0081] Research shows that micropores with a pore size of 0.4 nm to 2.5 nm in the hard carbon material of the present disclosure can enable reversible intercalation and deintercalation of active ions; that is, an increase in the number of the micropores with a pore size of 0.4 nm to 2.5 nm in the hard carbon material can improve the discharge specific capacity of the hard carbon material. Mesopores with a pore size of 2.5 nm to 10 nm in the hard carbon material can serve as transport channels for active ions, such that the active ions can be effectively transported into the micropores, and the utilization rate of the micropores is improved, thereby improving the discharge specific capacity and the initial coulombic efficiency of the secondary battery.

[0082] 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 inner core within the above range can maximize the utilization of storage space inside the hard carbon material and enable reversible intercalation and deintercalation of active ions, which is conducive to improving the charge specific capacity and the initial coulombic efficiency of the secondary battery.

[0083] 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: 90% < X / A < 93%. Illustratively, X / A may be 85%, 86%, 87%, 90%, 91%, 93%, 95%, 97%, or any value within a range defined by any two of the values. The total pore volume X of the micropores and the total pore volume A of the porous carbon of the hard carbon material satisfying the above relationship can provide more storage space for active ions, which is conducive to improving the discharge specific capacity of the hard carbon material.

[0084] 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: 7% < Y / A < 10%. Illustratively, Y / A may be 3%, 5%, 7%, 8%, 10%, 12%, 15%, or any value within a range defined by any two of the values. The total pore volume Y of the mesopores and the total pore volume A of the porous carbon of the hard carbon material satisfying the above relationship can provide a suitable number of transport channels for active ions, thereby improving the utilization rate of micropores, and helping to improve the charge specific capacity and the initial coulombic efficiency of the secondary battery.

[0085] In some embodiments, ID / IG of the hard carbon material is 0.83 to 1.26, where ID represents a D peak intensity at 1350±50 cm-1 in a Raman spectrum, and IG represents a G peak intensity at 1580±50 cm-1 in the Raman spectrum. Illustratively, the ID / IG of the hard carbon material is 0.83, 0.9, 1.0, 1.1, 1.2, 1.26, or any value within a range defined by any two of the values. The ID / IG of the hard carbon material within the above range indicates that the surface coating layer has an appropriate degree of order and appropriate reactivity, enabling the transport of active ions.

[0086] In some embodiments, the hard carbon material satisfies at least one of the following:

[0087] (1) The compacted density of the hard carbon material at 5 t is 0.6 g / cm3 to 1.05 g / cm3. A powder compacted density of the hard carbon material within the above range helps to form a reasonable pore channel structure between the particles of the negative electrode film layer and improve the transport performance of active ions and electrons, thereby improving the kinetic performance of the secondary battery.

[0088] (2) The tap density of the hard carbon material is 0.4 g / cm3 to 0.85 g / cm3. The tap density of the hard carbon material within the above range helps to improve the compacted density of the negative electrode film layer, thereby increasing the energy density of the secondary battery.

[0089] (3) The volume distribution particle size Dv50 of the hard carbon material is 3.0 pm to 7.9 pm.

[0090] (4) The volume distribution particle size Dv90 of the hard carbon material is 8 pm to 15 pm. The volume distribution particle sizes Dv50 and Dv90 of the particles of the hard carbon material within the above ranges help to reduce the specific surface area of the hard carbon material and reduce side reactions, thereby improving the initial coulombic efficiency of the secondary battery. Meanwhile, the bulk phase transport path of active ions can also be shortened, thereby improving the kinetic performance of the secondary battery.

[0091] (5) The specific surface area BET of the hard carbon material is 0.1 m2 / g to 10 m2 / g. The specific surface area of the hard carbon material within the above range can reduce the consumption of active ions in the first charge, which is conducive to improving the initial coulombic efficiency of the secondary battery.

[0092] In the present disclosure, a methylene blue adsorption value of the hard carbon material has the meaning well-known in the art, and may be determined using instruments and methods known in the art. For example, the test may be performed with reference to the standard GB / T 12496.10-1999.

[0093] In the present disclosure, a pore volume of the porous carbon has the meaning well-known in the art, and may be determined using instruments and methods known in the art. For example, the test may be performed with reference to GB / T 21650.3, Part 3. A porous carbon powder is placed in a sample tube and degassed under vacuum at 200 °C for 12 h. The adsorption amount of the hard carbon material for nitrogen under different pressures is tested by an ASAP2460-physical adsorption analyzer, and adsorption and desorption isotherms are plotted. A pore shape is determined according to a shape of a hysteresis loop, a pore size distribution curve of micropores is fitted using a DFT model, and a pore volume X of micropores, a pore volume Y of mesopores, and a total pore volume A in the hard carbon material are calculated.

[0094] In the present disclosure, before testing the pore volume of the porous carbon, the hard carbon material is subjected to a pretreatment to damage the carbon coating layer, such that the gas subjected to the nitrogen adsorption-desorption test can enter the porous carbon. Exemplary treatment steps include the following: 2 g of hard carbon powder was added to a 250 mL beaker, 1 g of NaNO3 was added, 46 mL of concentrated sulfuric acid was slowly added, and the mixture was stirred well. The beaker was placed in an ice-water bath, and 6 g of KMnO4 was slowly added under stirring. The temperature of this process was maintained below 20 °C. The ice-water bath was removed after 5 min, and the temperature was raised to 35 °C and maintained for 30 min. Then, 92 mL of deionized water was added, and the mixture was stirred for 15 min. Finally, 80 mL of a 3% H2O2 solution at 60 °C was added to reduce excess KMnO4 until no obvious bubbles were present. Finally, filtration was performed, and a filter cake was collected and repeatedly washed with deionized water and absolute ethanol until the pH of the filtrate was > 6. The washed filter cake was dried in a vacuum oven at 80 °C for 24.

[0095] In the present disclosure, the ID / IG value of the hard carbon material may be measured using a Raman spectrometer, where ID represents a D peak intensity at 1350±50 cm-1 in a Raman spectrum of the material, and IG represents a G peak intensity at 1580±50 cm-1 in the Raman spectrum of the material. The measurement conditions are: an excitation wavelength of 532 nm, a grating of 600 grooves, an objective lens of 50 folds, an integration time of 10 s, a total of 3 times, and plane scanning. The D peak and G peak intensities of 100 points are acquired, and the ID / IG values of the 100 points are calculated. 30 maximum and 30 minimum ID / IG values are removed, and the average value of the remaining 40 points is the ID / IG of the material. The Horiba LabRAM HR800 Raman spectrometer may be used as the test instrument.

[0096] In the present disclosure, a compacted density of the hard carbon material has the meaning well-known in the art, and may be determined using instruments and methods known in the art. For example, the determination may be performed with reference to GB / T 24533-2009 using an electronic compression tester (e.g., UTM7305 electronic compression tester). An exemplary measurement method is as follows: 1 g of sample powder is placed into a mold with a base area of 1.327 cm2, the pressure is increased to 5 t and held for 30 s, then the pressure is released and held for 10 s, and subsequently the powder compacted density of the material under the pressure of 5 t is recorded and calculated.

[0097] In the present disclosure, a tap density of the hard carbon material has the meaning well-known in the art, and may be determined using instruments and methods known in the art. For example, the measurement can be performed with reference to the GB / T 5162-2006 using a powder tap density tester. The tester may be a Bettersize BT-301, with the following test parameters: vibration frequency of 250 ± 15 times / minute, amplitude of 3 ± 0.2 mm, total vibrations of 5000 times, and graduated cylinder volume of 25 mL.

[0098] In the present disclosure, the volume distribution particle sizes Dv50 and Dv90 of the hard carbon material have meanings well-known in the art, which respectively represent the corresponding particle sizes when the cumulative volume distribution percentages of the material reach 50% and 90%, and can be measured using instruments and methods known in the art. For example, the measurement may be performed with reference to GB / T 19077-2016 using a laser particle size analyzer. The test instrument may be the Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., United Kingdom.

[0099] In the present disclosure, a specific surface area BET of the hard carbon material has the meaning well-known in the art, and may be determined using instruments and methods known in the art. For example, the specific surface area of the material may be tested using the analysis and test method of specific surface area by nitrogen adsorption with reference to GB / T 19587-2017, and calculated by the BET (Brunauer Emmett Teller) method. The test instrument may be the Tri-Star 3020 specific surface area pore size analysis tester from Micromeritics, USA.

[0100] In the present disclosure, the microstructure of the hard carbon material can be observed by a scanning electron microscope or a transmission electron microscope.

[0101] As an example, the negative electrode current collector has two surfaces opposite to each other in its own thickness direction, and the negative electrode film layer is disposed on any one or both of the two opposite surfaces of the negative electrode current collector.

[0102] In some embodiments, a metal foil or a composite current collector may be used as the negative electrode 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 base layer. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, or the like) on a polymer material substrate (such as a substrate made of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE)).

[0103] In some embodiments, the negative electrode active material includes the hard carbon material according to the above embodiments or a hard carbon material prepared by the preparation method according to the above embodiments.

[0104] In some embodiments, the negative electrode film layer further optionally includes a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0105] In some embodiments, the negative electrode film layer further optionally includes a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, a carbon dot, a carbon nanotube, graphene, and a carbon nanofiber.

[0106] In some embodiments, the negative electrode film layer further optionally includes other auxiliary agents, such as a thickener (e.g., sodium carboxymethylcellulose (CMC-Na)).

[0107] In some embodiments, the negative electrode plate can be prepared in the following manner: dispersing the components described above for preparing the negative electrode plate, such as the negative electrode active material, the conductive agent, the binder, and any other components, in a solvent (such as deionized water) to form a negative electrode slurry; and coating the negative electrode current collector with the negative electrode slurry, and performing drying, cold pressing, and other processes, such that the negative electrode plate can be obtained.

[0108] Secondary battery

[0109] A second aspect of the embodiments of the present disclosure provides a secondary battery, and the secondary battery of the present disclosure will be described below with appropriate reference to the drawings.

[0110] The term “secondary battery” described herein refers to a battery cell, a battery module, or a battery pack. Description is provided separately below.

[0111] Typically, a secondary battery cell includes a positive electrode plate, the negative electrode plate of the above embodiments, an electrolyte, and a separator. During the charging and discharging processes of the battery, active ions are intercalated and deintercalated back and forth between the positive electrode plate and the negative electrode plate. The electrolyte conducts ions between the positive electrode plate and the negative electrode plate. The separator is disposed between the positive electrode plate and the negative electrode plate to primarily prevent the positive and negative electrodes from short-circuiting while allowing the passage of ions.

[0112] [Positive Electrode Plate]

[0113] The positive electrode plate includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, and the positive electrode film layer includes the positive electrode active material according to the first aspect of the present disclosure.

[0114] As an example, the positive electrode current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode film layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector.

[0115] In some embodiments, a metal foil or a composite current collector may be used as the positive electrode current collector. For example, as the metal foil, an aluminum foil may be used. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, or the like) on a polymer material substrate (such as a substrate made of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE)).

[0116] In some embodiments, the battery cell is a sodium-ion battery, and a positive electrode active material known in the art for sodium-ion batteries may be used as the positive electrode active material. As an example, the positive electrode active material may include sodium transition metal oxides, polyanionic compounds, Prussian blue compounds, and the like. However, the present disclosure is not limited to these materials, and other conventionally known materials that can be used as positive electrode active materials for sodium-ion batteries may also be used. For example, as an optional technical solution of the present disclosure, the transition metal in the sodium transition metal oxide may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. The sodium transition metal oxide is, for example, NaxMO2, where M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu, and 0 < x < 1.

[0117] As an optional technical solution of the present disclosure, the polyanionic compound may be a class of compounds having sodium ions, transition metal ions, and tetrahedral (YO4)n- anion units. The transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y may be at least one of P, S, and Si; and n represents the valence state of (YO4)n-. The polyanionic compound may also be a class of compounds having sodium ions, transition metal ions, tetrahedral (YO4)n- anion units, and halide anions. The transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y may be at least one of P, S, and Si, and n represents the valence state of (YO4)n-; and the halogen may be at least one of F, Cl, and Br.

[0118] The polyanionic compound may also be a class of compounds having sodium ions, tetrahedral (YO4)n- anion units, polyhedral units (ZOy)m+, and optional halide anions. Y may be at least one of P, S, and Si, and n represents the valence state of (YO4)n-; Z represents a transition metal, which may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce, and m represents the valence state of (ZOy)m+; and the halogen may be at least one of F, Cl, and Br. The polyanionic compound is, for example, at least one of NaFePO4, Na3V2(PO4)3, NaM’PO4F (where M’ is one or more of V, Fe, Mn, and Ni), and Na3(VOy)2(PO4)2F3-2y (0 < y < 1).

[0119] As an optional embodiment of the present disclosure, the chemical general formula of the polyanionic compound may be Nax-aAaVy-bMb(PO4)2-2c(DO4)2cFz-dQd, where the element A represents an alkali metal element doped to replace the element Na, the element M represents a metal element replacing the element V, the element D represents a doping element replacing the element P, the element Q represents a doping element replacing the element F, the element D includes at least one of Si and S, and the element Q includes at least one of Cl and O; 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, the A element includes at least one of K and Li; the M element includes at least one of Fe, Cr, Al, Sc, Ga, In, Ti, Zr, Mn, Zn, Ni, Cu, and Co.

[0120] As an optional embodiment of the present disclosure, the chemical general formula of the polyanionic compound may be NaxRy(PO4)2P2O7, where x = 3.5-4.5, y = 2.75-3.25, 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.

[0121] As an optional embodiment of the present disclosure, the chemical general formula of the polyanionic compound may be Na4+xR3-yP4-mO15 / 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.

[0122] The Prussian blue compound may be a class of compounds having sodium ions, transition metal ions, and cyanide ions (CN-). The transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. The Prussian blue compound is, for example, NaaMebMe’c(CN)6, where Me and Me’ are each independently at least one of Ni, Cu, Fe, Mn, Co, and Zn, 0 < a < 2, 0 < b < 1, and 0 < c < 1.

[0123] In other embodiments, the battery cell may also be a lithium-ion battery, and a positive electrode active material known in the art for lithium-ion batteries may be used as the positive electrode active material.

[0124] In the enumeration related to the positive electrode active material in the present disclosure, the molar content of oxygen is only a theoretical value. The molar content of oxygen will vary due to oxygen release from the crystal lattice, and actually, the molar content of oxygen will fluctuate.

[0125] In some embodiments, the positive electrode film layer further optionally includes a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene    fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene   terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylic resin.

[0126] In some embodiments, the positive electrode film layer further optionally includes a conductive agent. As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, a carbon dot, a carbon nanotube, graphene, and a carbon nanofiber.

[0127] In some embodiments, the positive electrode plate can be prepared in the following manner: dispersing the components described above for preparing the positive electrode plate, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; and coating the positive electrode current collector with the positive electrode slurry, and performing drying, cold pressing, and other processes, such that the positive electrode plate can be obtained.

[0128] [Electrolyte]

[0129] The electrolyte conducts ions between the positive electrode plate and the negative electrode plate. The present disclosure does not particularly limit the type of electrolyte, which may be selected according to needs. For example, the electrolyte may be liquid, gel, or all solid.

[0130] In some embodiments, the electrolyte is an electrolytic solution. The electrolytic solution includes an electrolyte salt and a solvent.

[0131] In some embodiments, when the battery cell is a sodium-ion battery, the electrolyte salt may be selected from at least one of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium perchlorate, sodium hexafluoroarsenate, sodium bis(fluorosulfonyl)imide,     sodium     bis(trifluoromethanesulfonyl)imide,     sodium trifluoromethanesulfonate, sodium difluoro(oxalato)borate, sodium bisoxalatoborate, sodium difluorophosphate,      sodium      difluorobisoxalatophosphate,      and      sodium tetrafluorooxalatophosphate.

[0132] In some embodiments, when the battery cell is a lithium-ion battery, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide,      lithium      bis(trifluoromethanesulfonyl)imide,      lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluoro(oxalato)borate, lithium bis(oxalato)borate, lithium difluorobis(oxalato)phosphate, and lithium tetrafluoro(oxalato)phosphate.

[0133] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, ethyl methyl sulfone, and diethyl sulfone.

[0134] In some embodiments, the electrolytic solution further optionally includes an additive. For example, the additive may include a negative electrode film-forming additive and a positive electrode film-forming additive, and may further include an additive capable of improving certain properties of the battery, such as an additive for improving the overcharge performance of the battery and an additive for improving the high- or low-temperature performance of the battery.

[0135] [Separator]

[0136] In some embodiments, the battery cell further includes a separator. The present disclosure does not particularly limit the type of separator, and any porous-structure separator known to have good chemical stability and mechanical stability may be selected.

[0137] In some embodiments, the separator may be made of a material selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, which is not specified. When the separator is a multi-layer composite film, the materials of the layers may be the same or different, which is not specified.

[0138] In some embodiments, the positive electrode plate, the negative electrode plate, and the separator may be manufactured into an electrode assembly through a winding process or a stacking process.

[0139] In some embodiments, the battery cell may include an outer packaging. The outer packaging may be configured to package the electrode assembly and electrolyte described above.

[0140] In some embodiments, the outer packaging of the battery cell may be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell may also be a soft pack, such as a pouch-type soft pack. The soft pack may be made of plastic, and examples of the plastic may include polypropylene, polybutylene terephthalate, polybutylene succinate, and the like.

[0141] The present disclosure does not particularly limit the shape of the battery cell, and it may have a cylindrical shape, a prismatic shape, or any other shape. For example, FIG. 1 shows a battery cell 5 having a prismatic structure as one example.

[0142] In some embodiments, referring to FIG. 2, the outer packaging may include a shell body 51 and a top cover assembly 53. The shell body 51 may include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates define, in an enclosing manner, an accommodating cavity. The shell body 51 is provided with an opening in communication with the accommodating cavity, and the top cover assembly 53 is capable of covering the opening to close the accommodating cavity. The positive electrode plate, the negative electrode plate, and the separator may be subjected to a winding process or a stacking process to form an electrode assembly 52. The electrode assembly 52 is packaged in the accommodating cavity. The electrolytic solution wets the electrode assembly 52. The number of the electrode assemblies 52 included in the battery cell 5 may be one or more, and those skilled in the art can select the number according to specific and actual needs.

[0143] In some embodiments, the battery cell may be assembled into a battery module. The number of battery cells included in the battery module may be one or more, and the specific number may be selected by those skilled in the art based on the use and capacity of the battery module.

[0144] FIG. 3 shows a battery module 4 as one example. Referring to FIG. 3, in the battery module 4, a plurality of battery cells 5 may be sequentially arranged along the length direction of the battery module 4. Certainly, the arrangement may also be in any other manner. Further, the plurality of battery cells 5 may be fixed by fasteners.

[0145] Optionally, the battery module 4 may further include a shell having an accommodating space in which the plurality of battery cells 5 are accommodated.

[0146] In some embodiments, the battery module described above may also be assembled into a battery pack. The number of battery modules included in the battery pack may be one or more, and the specific number may be selected by those skilled in the art based on the use and capacity of the battery pack.

[0147] FIGs. 4 and 5 show a battery pack 1 as one example. Referring to FIGs. 4 and 5, the battery pack 1 may include a battery case and a plurality of battery modules 4 disposed in the battery case. The battery case includes an upper case body 2 and a lower case body 3. The upper case body 2 is capable of covering the lower case body 3 to form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 may be arranged in any manner in the battery case.

[0148] Electric Device

[0149] 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 drawings.

[0150] The electric device mentioned in the embodiments of the present disclosure includes the secondary battery according to the present disclosure. The secondary battery may be used as a power source for the electric device or as an energy storage unit for the electric device. The electric device may include, but is not limited to, a mobile device (e.g., a mobile phone or a laptop computer), an electric vehicle (e.g., a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, or an electric truck), an electric train, a ship, or a satellite, an energy storage system, or the like.

[0151] As for the electric device, a battery cell, a battery module, or a battery pack may be selected according to usage requirements thereof.

[0152] FIG. 6 shows an electric device as one example. The electric device is a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, or the like. To meet the requirements of the electric device for high power and high energy density of the secondary battery, a battery pack or a battery module may be used.

[0153] As another example, the device may be a mobile phone, a tablet computer, a laptop computer, or the like. The device is generally required to be light and thin, and a battery cell can thus be used as a power source.

[0154] Hard Carbon Material

[0155] A fourth aspect of the present disclosure provides a hard carbon material included in the negative electrode plate according to the first aspect of the present disclosure. The hard carbon material includes an inner core and a carbon coating layer coating the inner core, and the inner 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 <10 mg / g. In the present disclosure, maintaining the total pore volume of the porous carbon in the hard carbon material within the above range can enable the carbon skeleton of the hard carbon material to have appropriate strength and stability while the specific capacity of the hard carbon material is improved, without collapse during the intercalation and deintercalation of active ions, which is conducive to improving the charge specific capacity and the initial coulombic efficiency of the secondary battery. In addition, providing the carbon coating layer on the outer surface of the porous carbon inner core can block electrolytic solution molecules from entering the pore structure of the porous carbon, which is conducive to improving the charge specific capacity and the initial coulombic efficiency of the secondary battery. Further, the methylene blue adsorption value of the hard carbon material within the above range enables the density of the carbon coating layer to be high, which can effectively isolate the electrolytic solution and suppress electrolytic solution molecules from entering the interior of the porous carbon, thereby further helping to improve the charge specific capacity and the initial coulombic efficiency of the secondary battery.

[0156] In some embodiments, a methylene blue adsorption value Z of the hard carbon material is < 5 mg / g. The smaller the methylene blue adsorption value is, the better the coating effect is. In this way, the improvement in the charge specific capacity and the initial coulombic efficiency of the secondary battery is further facilitated.

[0157] In some embodiments, the porous carbon includes micropores with a pore size of 0.4 nm to 2.5 nm and mesopores with 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: 8 < X / Y < 13. Therefore, the utilization of storage space inside the hard carbon material can be maximized, and the reversible intercalation and deintercalation of active ions are enabled, which is conducive to improving the charge specific capacity and the initial coulombic efficiency of the secondary battery.

[0158] 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%. Therefore, more storage space for active ions can be provided, which is conducive to improving the discharge specific capacity of the hard carbon material.

[0159] 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%. Therefore, a suitable number of transport channels for active ions can be provided, thereby improving the utilization rate of micropores, and helping to improve the charge specific capacity and the initial coulombic efficiency of the secondary battery.

[0160] In some embodiments, ID / IG of the hard carbon material is 0.83 to 1.26, where ID represents a D peak intensity at 1350±50 cm-1 in a Raman spectrum, and IG represents a G peak intensity at 1580±50 cm-1 in the Raman spectrum. The ID / IG of the hard carbon material within the above range indicates that the surface coating layer has an appropriate degree of order and appropriate reactivity, enabling the transport of active ions.

[0161] In some embodiments, the hard carbon material satisfies at least one of the following:

[0162] (1) The compacted density of the hard carbon material at 5 t is 0.6 g / cm3 to 1.05 g / cm3. A powder compacted density of the hard carbon material within the above range helps to form a reasonable pore channel structure between the particles of the negative electrode film layer and improve the transport performance of active ions and electrons, thereby improving the kinetic performance of the secondary battery.

[0163] (2) The tap density of the hard carbon material is 0.4 g / cm3 to 0.85 g / cm3. The tap density of the hard carbon material within the above range helps to improve the compacted density of the negative electrode film layer, thereby increasing the energy density of the secondary battery.

[0164] (3) The volume distribution particle size Dv50 of the hard carbon material is 3.0 pm to 7.9 pm.

[0165] (4) The volume distribution particle size Dv90 of the hard carbon material is 8 pm to 15 pm. The volume distribution particle sizes Dv50 and Dv90 of the particles of the hard carbon material within the above ranges help to reduce the specific surface area of the hard carbon material and reduce side reactions, thereby improving the initial coulombic efficiency of the secondary battery. Meanwhile, the bulk phase transport path of active ions can also be shortened, thereby improving the kinetic performance of the secondary battery.

[0166] (5) The specific surface area BET of the hard carbon material is 0.1 m2 / g to 10 m2 / g. The specific surface area of the hard carbon material within the above range can reduce the consumption of active ions in the first charge, which is conducive to improving the initial coulombic efficiency of the secondary battery.

[0167] Preparation Method for Hard Carbon Material

[0168] A fifth aspect of the embodiments of the present disclosure provides a preparation method for the hard carbon material described above. The method includes: performing a pre-carbonization treatment on a hard carbon precursor to obtain a pre-carbonized body; performing an activation treatment on the pre-carbonized body to obtain a porous carbon precursor; performing a coating treatment on the porous carbon precursor to form a hard carbon precursor, where the coating treatment includes a kneading treatment of kneading a kneading mixture formed by the porous carbon precursor and a water-based solution of a coating raw material, and a curing treatment of curing a product of the kneading treatment, a solid content of the kneading mixture being 55 wt% to 75 wt%; and performing a carbonization treatment on the hard carbon precursor to obtain the hard carbon material. 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 < 10 mg / g.

[0169] In the present disclosure, a porous carbon precursor is obtained by performing pre-carbonization and activation and pore formation treatments on a hard carbon precursor. Then, a kneading treatment is performed on the porous carbon precursor and a water-based solution of a coating raw material, and a curing treatment is performed on a product of the kneading treatment to form a hard carbon precursor having a coating layer. Subsequently, a carbonization treatment is performed on the hard carbon precursor having a coating layer to obtain a hard carbon material having a coating layer. Under the above solid content condition, the porous carbon precursor and the coating raw material are kneaded to enable more sufficient mixing and shearing thereof, and thus the formed coating layer exhibits better density. On this basis, the curing treatment is further performed to improve the thermal stability of the coating layer, such that the carbon coating layer does not crack in the subsequent carbonization process. Therefore, the coating layer in the present disclosure can block electrolytic solution molecules from entering the interior of the porous carbon, which is conducive to improving the charge specific capacity and the initial coulombic efficiency of the secondary battery.

[0170] Illustratively, the solid content of the kneading mixture is 55 wt%, 60 wt%, 62 wt%, 65 wt%, 68 wt%, 71 wt%, or 75 wt%.

[0171] In some embodiments, the solid content of the kneading mixture is 62 wt% to 68 wt%. The solid content of the kneading mixture within the above range helps to form a coating layer with an appropriate density, such that the charge specific capacity and the initial coulombic efficiency of the secondary battery can be improved without affecting the kinetic performance of the secondary battery.

[0172] In some embodiments, a mass ratio of the coating raw material to the porous carbon precursor is 0.5:10 to 2:10, and the kneading treatment is performed for at least 0.5 h. Optionally, the mass ratio of the coating raw material to the porous carbon precursor is 1.0:10 to 1.5:10; the kneading treatment is performed for at least 5 h to 7 h. The mass of the coating raw material within the above range enables the formation of a coating layer with an appropriate density, such that the charge specific capacity and the initial coulombic efficiency of the secondary battery can be improved without affecting the kinetic performance of the secondary battery and increasing the impedance of the secondary battery. In addition, a kneading time within the above range enables the coating raw material to be uniformly distributed on the surface of the porous carbon, thereby helping to form a carbon coating layer with high density, and further helping to improve the charge specific capacity and the initial coulombic efficiency of the secondary battery.

[0173] In some embodiments, the water-based solution may be an aqueous solution, an ethanol solution, or a mixed solution of an aqueous solution and an ethanol solution.

[0174] In some embodiments, the kneading process described above is performed at room temperature.

[0175] In some embodiments, the curing treatment is performed at 120 °C to 250 °C for at least 1 h. Optionally, a heating rate in the curing treatment may be 5 °C / min to 15 °C / min; a curing treatment temperature may be 160 °C to 200 °C, and a curing treatment time may be 5 h to 7 h. Illustratively, the curing treatment temperature is 180 °C, and the curing treatment time is 6 h. Curing under the above conditions can improve the degree of crosslinking of the carbon coating raw material, thereby improving the thermal stability of the carbon coating layer and reducing the volume shrinkage of the carbon coating layer in the subsequent carbonization process. This prevents the carbon coating layer from cracking in the subsequent carbonization process, thereby improving the density of the coating layer. The carbon coating layer with high density is conducive to improving the charge specific capacity and the initial coulombic efficiency of the secondary battery.

[0176] In some embodiments, the coating raw material includes a prepolymer of a thermosetting resin. Illustratively, the thermosetting resin includes at least one of a phenolic resin, an epoxy resin, an unsaturated polyester resin, and a furan resin. Using the prepolymer of the thermosetting resin described above as the coating raw material of the carbon coating layer embraces the advantages of high uniformity, high density, and high coating integrity.

[0177] In some embodiments, the coating raw material may further include compounds containing C, H, and O elements, such as sugars.

[0178] In some embodiments, the kneading treatment is performed in a twin-screw kneader, and a rotation speed of the twin-screw kneader is 10 rpm to 50 rpm. By kneading under the above conditions, a uniformly coated carbon coating layer can be obtained.

[0179] In some embodiments, the activation treatment includes treating the pre-carbonized body at 700 °C to 950 °C under a preset mixed gas. Optionally, the pre-carbonized body is subjected to treatment at 820 °C to 880 °C. Illustratively, an activation treatment temperature may be 700 °C, 850 °C, or 900 °C. The activation treatment being performed at the above temperature enables the formation of an appropriate number of mesopores and micropores, thereby helping to improve the discharge specific capacity of the secondary battery.

[0180] In some embodiments, the preset mixed gas includes carbon dioxide gas, water vapor, and inert gas, and a volume ratio of the carbon dioxide gas to the water vapor in the preset mixed gas is > 2. Optionally, a volume ratio of the carbon dioxide gas to the water vapor in the preset mixed gas is 2.25 to 7.5. Illustratively, the volume ratio of the carbon dioxide gas to the water vapor in the preset mixed gas may be 2, 4, 6, 8, 10, or 12. The volume ratio of the carbon dioxide gas to the water vapor in the preset mixed gas within the above range enables the formation of an appropriate number of mesopores and micropores, thereby helping to improve the discharge specific capacity of the secondary battery.

[0181] In some embodiments, the inert gas in the mixed gas described above includes helium, neon, argon, and the like.

[0182] In some embodiments, a volume proportion of the carbon dioxide gas in the preset mixed gas is 5% to 20%; optionally, the volume proportion of the carbon dioxide gas in the preset mixed gas is 9% to 15%. Illustratively, the volume proportion of the carbon dioxide gas in the preset mixed gas may be 6%, 9%, 10%, 12%, 15%, 18%, or 20%. The volume proportion of the carbon dioxide gas in the preset mixed gas within the above range enables the formation of an appropriate number of micropores, thereby helping to improve the discharge specific capacity of the secondary battery.

[0183] In some embodiments, a volume proportion of the water vapor in the mixed gas is 1% to 5%; optionally, the volume proportion of the water vapor in the mixed gas is 2% to 4%. Illustratively, the volume proportion of the water vapor in the mixed gas may be 1%, 2%, 3%, 4%, or 5%. The volume proportion of the water vapor in the preset mixed gas within the above range enables the formation of an appropriate number of mesopores, thereby helping to improve the charge specific capacity and the initial coulombic efficiency of the secondary battery.

[0184] In some embodiments, the activation treatment includes impregnating the pre-carbonized 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 the element P in the P-containing pore-forming solution is 9.5%-32.4% of a mass of the porous carbon, and an addition amount of the element Zn in the Zn-containing pore-forming solution is 14%-50% of the mass of the porous carbon. Illustratively, the addition amount of the element P in the P-containing pore-forming solution may be 9.5%, 10%, 15%, 19%, 25%, 30%, or 32.4% of the mass of the porous carbon. Illustratively, the addition amount of the Zn element in the Zn-containing pore-forming solution may be 14%, 20%, 25%, 30%, 35%, 40%, 45%, or 50% of the mass of the porous carbon. The addition amount of the element P or the element Zn in the pore-forming solution within the above range enables the formation of an appropriate number of micropores and mesopores, thereby helping to improve the discharge specific capacity, charge specific capacity, and initial coulombic efficiency of the secondary battery. In addition, impregnating the pre-carbonized body in the pore-forming solution for the above time enables the formation of an appropriate number of micropores and mesopores, thereby helping to improve the discharge specific capacity, charge specific capacity, and initial coulombic efficiency of the secondary battery.

[0185] Illustratively, the P-containing pore-forming solution may include, for example, at least one of phosphoric acid, polyphosphoric acid, and phosphoric acid ester.

[0186] Illustratively, the Zn-containing pore-forming solution may be, for example, zinc chloride.

[0187] In some embodiments, a solid content of an impregnation mixture formed by the pre-carbonized body and the pore-forming solution is 50 wt% to 70 wt%. Illustratively, the solid content of the impregnation mixture formed by the pre-carbonized body and the pore-forming solution may be 50 wt%, 55 wt%, 60 wt%, 65 wt%, or 70 wt%. Maintaining the solid content of the impregnation mixture within the above range enables the formation of an appropriate number of micropores and mesopores, thereby helping to improve the discharge specific capacity, charge specific capacity, and initial coulombic efficiency of the secondary battery.

[0188] In some embodiments, the activation treatment further includes keeping an impregnation mixture formed by the immersed 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 the introduction of nitrogen or inert gas. Illustratively, in the heat treatment process, nitrogen is introduced, and the temperature is raised to 600 °C at a heating rate of 5 °C / min and maintained for 2 h. Maintaining the temperature at 400 °C to 750 °C for 1 h to 12 h enables the P- or Zn-containing pore-forming solution and the pre-carbonized body to chemically react, and etching is performed in a chemical activation manner, thereby introducing a large number of pore structures into the skeleton structure of the pre-carbonized body.

[0189] In some embodiments, in the pre-carbonization treatment, a temperature is raised to 400 °C to 600 °C at a heating rate of 1 °C / min to 20 °C / min and held for 1 h to 12 h. Illustratively, a pre-carbonization temperature may be 500 °C, and a pre-carbonization treatment time may be 2 h. The pre-carbonization treatment being performed under the above conditions facilitates the formation of the basic carbon skeleton structure, facilitating subsequent pore formation.

[0190] In some embodiments, in the carbonization treatment, a temperature is raised to 1000 °C to 1800 °C at a heating rate of 2 °C / min to 20 °C / min and held for 1 h to 12 h. Illustratively, a carbonization temperature may be 1200 °C, and a carbonization treatment time may be 12 h. In the carbonization process, the precursor loses heteroatoms such as H and O, forming a stable hard carbon skeleton, and the coating layer in the carbonization process is denser, which is conducive to improving the charge specific capacity and the initial coulombic efficiency of the secondary battery.

[0191] 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.

[0192] In some embodiments, after the pre-carbonization treatment, a pulverization treatment is further included. The pulverization treatment was performed by means of ball milling. The pulverization treatment can pulverize the pre-carbonized body to a suitable particle size for subsequent activation treatment.

[0193] 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 C, H, and O elements. Illustratively, the biomass precursor may be coconut shells, walnut shells, giant reed, straws, or the like; the synthetic polymer precursor may be a phenolic resin, an epoxy resin, an unsaturated polyester resin, or the like. When the hard carbon material is prepared by using the above precursor, the carbon formed after activation is relatively ordered, and an appropriate number of micropores and mesopores can be formed.

[0194] In some embodiments, when the hard carbon precursor is a biomass precursor, after the pulverization treatment, a demineralization treatment is further included. The demineralization treatment is carried out by soaking in an acidic aqueous solution at a temperature ranging from room temperature to 95 °C for 1 h to 12 h, with the process repeated 1 to 5 times. The demineralization treatment can remove ash components such as minerals and metal oxides from the pre-carbonized body, thereby improving the purity of the pre-carbonized body.

[0195] Examples

[0196] The examples of the present disclosure are illustrated hereinafter. The examples described below are illustrative and are merely used to explain the present disclosure, and they should not be construed as limiting the present disclosure. The examples without techniques or conditions specified therein are implemented according to techniques or conditions described in the literature in the art or according to product instructions. Reagents or instruments used herein without specified manufacturers are all commercially available conventional products.

[0197] Example 1

[0198] Preparation of Hard Carbon Material:

[0199] 1) Pre-carbonization treatment: When nitrogen was introduced, a hard carbon precursor coconut shell was placed in a tube furnace, heated to 500 °C at a heating rate of 5 °C / min, and maintained for 2 h to obtain a pre-carbonized body, where the flow rate of the nitrogen was 500 mL / min.

[0200] 2) Crushing treatment: The obtained pre-carbonized body was subjected to a ball milling crushing treatment by using zirconium oxide ball mill beads and a zirconium oxide ball mill jar, where a mass ratio of the pre-carbonized body to the ball mill beads was 1:3, a rotation speed of a ball mill was 800 rpm, and a ball milling time was 8 h; the ball mill beads were removed from the pre-carbonized body to obtain a pre-carbonized body with a volume distribution particle size Dv50 of 4.9 um and a volume distribution particle size Dv90 of 10.3 um.

[0201] 3) Demineralization treatment: The pre-carbonized body after the crushing treatment in the above step 2) was washed in an aqueous hydrochloric acid solution, where the acid washing was carried out in a temperature range of 50 °C for 4 h, and the washing process was repeated 3 times; then, filtration was performed, and a filter cake was collected and washed repeatedly with deionized water and absolute ethanol until the pH of the filtrate was > 6; the washed filter cake was dried in a vacuum oven at 80 °C for 24.

[0202] 4) Activation treatment: The pre-carbonized body after the demineralization treatment in the above step 3) was subjected to an activation treatment at 700 °C under a preset mixed gas to obtain a porous carbon precursor, where the preset mixed gas includes carbon dioxide gas, water vapor, and inert gas, a proportion of the carbon dioxide gas in the preset mixed gas was 12%, and a proportion of the water vapor in the preset mixed gas was 3%.

[0203] 5) Coating treatment: The porous carbon prepared in the above step 4) and an aqueous solution of a coating raw material phenolic resin were uniformly mixed to form a kneading mixture, and the kneading mixture was placed in a twin-screw kneader for kneading for 6 h, where a rotation speed of the twin-screw kneader was 30 rpm, a solid content of the kneading mixture was 65 wt%, and a mass ratio of the coating raw material phenolic resin to the porous carbon precursor in the kneading mixture was 1.2:10; when nitrogen was introduced, the above kneaded product was placed in a tube furnace, heated to 180 °C at a heating rate of 5 °C / min, and maintained at 180 °C for 6 h to carry out a curing treatment, thereby obtaining a hard carbon precursor, where the flow rate of the nitrogen was 500 mL / min.

[0204] 6) Carbonization treatment: When nitrogen was introduced, the hard carbon precursor prepared in the above step 5) was placed in a tube furnace, heated to 1200 °C at a heating rate of 5 °C / min, and maintained for 12 h to obtain a hard carbon material.

[0205] Tests Related to Hard Carbon Material:

[0206] 1) Pore volume test

[0207] First, the hard carbon material was pretreated to obtain a porous carbon material. Specifically, 2 g of hard carbon powder was added to a 250 mL beaker, 1 g of NaNO3 was added, 46 mL of concentrated sulfuric acid was slowly added, and the mixture was stirred well. The beaker was placed in an ice-water bath, and 6 g of KMnO4 was slowly added under stirring. The temperature of this process was maintained below 20 °C. The ice-water bath was removed after 5 min, and the temperature was raised to 35 °C and maintained for 30 min. Then, 92 mL of deionized water was added, and the mixture was stirred for 15 min. Finally, 80 mL of a 3% H2O2 solution at 60 °C was added to reduce excess KMnO4 until no obvious bubbles were present. Finally, filtration was performed, and a filter cake was collected and repeatedly washed with deionized water and absolute ethanol until the pH of the filtrate was > 6. The washed filter cake was dried in a vacuum oven at 80 °C for 24 to obtain a porous carbon material.

[0208] Next, a porous carbon material powder was placed in a sample tube and degassed under vacuum at 200 °C for 12 h. The adsorption amount of the hard carbon material for nitrogen under different pressures is tested by an ASAP2460-physical adsorption analyzer, and adsorption and desorption isotherms are plotted. A pore shape was determined according to a shape of a hysteresis loop, a pore size distribution curve of micropores was fitted using a DFT model, and a pore volume X of micropores, a pore volume Y of mesopores, and a total pore volume A in the hard carbon material were calculated.

[0209] 2) Methylene blue adsorption value test

[0210] A methylene blue adsorption value test was performed according to the following steps:

[0211] (i) 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. (ii) Preparation of methylene blue solution: 1.0 g of dry methylene blue was weighed and dissolved in a buffer solution at a temperature of 60 ± 1 °C. After complete dissolution, the mixture was cooled to room temperature and filtered into a 1000 mL volumetric flask. The filter residue was washed with a 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. (iii) Sample treatment: A certain amount of ground and dried hard carbon material was weighed and fully mixed with the prepared methylene blue solution of known concentration for absorption. The mixture was shaken at room temperature for a certain time to allow the hard carbon material to fully adsorb methylene blue. (iv) Filtration: Filtration was performed using a medium-speed qualitative filter paper with a diameter of 12.5 cm to separate the unadsorbed methylene blue solution. (v) Determination of absorbance: The filtrate was taken and the absorbance was determined at the maximum absorption wavelength (665 nm) using an ultraviolet spectrophotometer. (vi) Comparison and calculation: The absorbance was compared with that of a copper sulfate standard color solution (an aqueous solution with a mass fraction of 0.4%), and the amount of the added methylene blue solution was adjusted until the difference between measured absorbance readings of the sample filtrate and the copper sulfate standard color solution did not exceed ±0.02. (vii) The concentration of methylene blue in the solution was obtained according to a standard curve, and a mass of adsorbed methylene blue per gram of the hard carbon material was calculated to obtain a Z value.

[0212] Preparation of Coin-Type Half-Cell:

[0213] The hard carbon material prepared in Example 1, a binder styrene-butadiene rubber (SBR), a thickener sodium carboxymethylcellulose (CMC-Na), and a conductive agent carbon black were fully stirred and mixed in an appropriate amount of solvent deionized water at a mass ratio of 96.2:1.8:1.2:0.8, to form a uniform negative electrode slurry. The surface of a negative electrode current collector copper foil was uniformly coated with the negative electrode slurry, and the copper foil was dried in an oven and sliced for later use. Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, NaPF6 was dissolved in the above organic solvent to prepare an electrolytic solution with a concentration of 1 mol / L. Then, a CR2430 coin-type half-cell was assembled in an argon-protected glove box with a sodium metal sheet as a counter electrode and a glass fiber film as a separator.

[0214] Performance Test of Coin-Type Half-Cell

[0215] At 25 °C, the coin-type half-cell prepared above was first discharged at a constant current with a current density of 10 mA / g to 0 V, and a first-cycle discharge specific capacity (i.e., discharge specific capacity) of the coin-type half-cell was recorded. Thereafter, the coin-type half-cell was charged at a constant current with a current density of 10 mA / g to 2.0 V, and a first-cycle charge specific capacity (i.e., charge specific capacity) of the coin-type half-cell was recorded.

[0216] Initial coulombic efficiency (%) = first-cycle charge specific capacity / first-cycle discharge specific capacity x 100%.

[0217] Examples 2 to 11

[0218] Hard carbon materials were prepared according to a method similar to that of Example 1, and each assembled into a coin-type half-cell. The only difference was that when the hard carbon materials were prepared, the process conditions of the activation treatment and the coating 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, as detailed in Table 1.

[0219] Comparative Example 1

[0220] A hard carbon material was prepared according to a method similar to that of Example 1, and assembled into a coin-type half-cell. The only difference was that when the hard carbon material was prepared, the process conditions of the activation treatment and the coating treatment were adjusted according to Table 1 below to obtain a hard carbon material with a total pore volume A of 0.437 cm3 / g and a methylene blue adsorption value of greater than 10 mg / g.

[0221] Comparative Example 2

[0222] A hard carbon material was prepared according to a method similar to that of Example 1, and assembled into a coin-type half-cell. The only difference was that when the hard carbon material was prepared, the process conditions of the activation treatment and the coating treatment were adjusted according to Table 1 below to obtain a hard carbon material with a total pore volume A of less than 0.2 cm3 / g and a methylene blue adsorption value of less than 10 mg / g.

[0223] Comparative Example 3

[0224] A hard carbon material was prepared according to a method similar to that of Example 1, and assembled into a coin-type half-cell. The only difference was that when the hard carbon material was prepared, the process conditions of the activation treatment and the coating treatment were adjusted according to Table 1 below to obtain a hard carbon material with a total pore volume A of less than 0.2 cm3 / g and a methylene blue adsorption value of greater than 10 mg / g.

[0225] The process parameters for preparing the hard carbon materials in Examples 1 to 11 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 coin-type half-cell are shown in Table 2 below. Table 1: Item Activation treatment Coating treatment Activation temperatu re (°C) Proportio n of carbon dioxide in preset gas (%) Proportio n of water vapor in preset gas (%) Volum e ratio of carbon dioxid e to water vapor Mass ratio of coatin g raw material to porous carbon precurso r Kneadin g time (h) Solid conte nt (wt%) Curing temperatu re (°C) Curin g time (h) Example 1 700 12 3 4 1.2: 10 6 65 180 6 Example 2 850 12 3 4 1.2: 10 6 65 180 6 Example 3 900 12 3 4 1.2: 10 6 65 180 6 Example 4 850 6 3 2 1.2: 10 6 65 180 6 Example 5 850 12 3 4 0.8: 10 6 65 180 6 Example 6 850 12 3 4 2.0: 10 6 65 180 6 Example 7 850 12 3 4 1.2: 10 2 65 180 6 Example 8 850 12 3 4 1.2: 10 6 71 180 6 Example 9 850 12 3 4 1.2: 10 6 65 140 6 Example 10 850 12 3 4 1.2: 10 6 65 250 6 Example 11 850 12 3 4 1.2: 10 6 65 180 3 Comparati ve Example 1 850 12 3 4 1.2: 10 6 65 180 0.5 Comparati ve Example 2 550 12 3 4 1.2: 10 6 65 180 6 Comparati ve Example 3 550 12 3 4 1.2: 10 0.1 65 180 6 Table 2: Item Hard carbon material parameter Coin-type half-cell performance A(cm3 / g) X / A(%) Y / A(%) X / Y Z(mg / g) Discharge specific capacity Charge specific capacity Initial coulombic efficiency (mAh / g) (mAh / g) (%) Example 1 0.271 89.67 10.33 8.7 2.4 351 312 88.89 Example 2 0.457 91.90 8.10 11.4 1.7 487 439 90.14 Example 3 0.476 86.13 13.87 6.2 4.2 482 420 87.14 Example 4 0.422 85.07 14.93 5.7 2.1 433 387 89.38 Example 5 0.443 91.65 8.35 11.0 6.2 475 401 84.42 Example 6 0.407 92.38 7.62 12.1 1.5 436 389 89.22 Example 7 0.425 91.29 8.71 10.5 7.0 458 381 83.19 Example 8 0.435 91.26 8.74 10.4 8.3 464 375 80.82 Example 9 0.429 91.61 8.39 10.9 9.8 460 368 80.00 Example 10 0.443 91.87 8.13 11.3 9.6 479 384 80.17 Example 11 0.423 91.25 8.75 10.4 9.1 452 364 80.53 Comparative Example 1 0.437 91.53 8.47 10.8 13.9 471 364 77.28 Comparative Example 2 0.154 87.01 12.99 6.7 4.8 327 277 84.71 Comparative Example 3 0.121 86.78 13.22 6.6 14.4 314 240 76.43

[0226] As can be seen from Tables 1 and 2 above, in Examples 1 to 11, by keeping the total pore volume of the porous carbon between 0.2 cm3 / g and 0.6 cm3 / g and keeping the methylene blue adsorption value of the hard carbon material less than 10 mg / g, the initial coulombic efficiency and the charge specific capacity are significantly improved, while a high discharge specific capacity is achieved. In Comparative Example 1, the methylene blue adsorption value of the hard carbon material is greater than 10 mg / g; although the discharge specific capacity of the hard carbon material is relatively high, the initial coulombic efficiency and the charge specific capacity are relatively low. In Comparative Example 2, the total pore volume of the porous carbon is less than 0.2 cm3 / g, resulting in a relatively low discharge specific capacity. In Comparative Example 3, since the total pore volume of the porous carbon is less than 0.2 cm3 / g, and the methylene blue adsorption value of the hard carbon material is greater than 10 mg / g, the discharge specific capacity, the charge specific capacity, and the initial coulombic efficiency are all low.

[0227] Example 12

[0228] Preparation of Hard Carbon Material:

[0229] 1) Pre-carbonization treatment: When nitrogen was introduced, a hard carbon precursor coconut shell was placed in a tube furnace, heated to 500 °C at a heating rate of 5 °C / min, and maintained for 2 h to obtain a pre-carbonized body, where the flow rate of the nitrogen was 500 mL / min.

[0230] 2) Crushing treatment: The obtained pre-carbonized body was subjected to a ball milling crushing treatment by using zirconium oxide ball mill beads and a zirconium oxide ball mill jar, where a mass ratio of the pre-carbonized body to the ball mill beads was 1:3, a rotation speed of a ball mill was 800 rpm, and a ball milling time was 8 h; the ball mill beads were removed from the pre-carbonized body to obtain a pre-carbonized body with a volume distribution particle size Dv50 of 4.9 um and a volume distribution particle size Dv90 of 10.3 um.

[0231] 3) Activation treatment: The pre-carbonized body after the crushing treatment in the above step 2) was soaked in a pore-forming solution aqueous phosphoric acid solution for 6 h, and then placed in a tube furnace; when nitrogen was introduced, the temperature was raised to 600 °C at a heating rate of 5 °C / min and maintained for 2 h, and the flow rate of the nitrogen was 500 mL / min, to obtain a porous carbon precursor, where an addition amount of the element P in the pore-forming solution aqueous phosphoric acid solution was 10.8% of a mass of the pre-carbonized body; a solid content of an impregnation mixture formed by the pre-carbonized body and the pore-forming solution was 65%.

[0232] 4) Demineralization treatment: The porous carbon precursor in the above step 3) was washed in an aqueous hydrochloric acid solution, where the acid washing was carried out in a temperature range of 50 °C for 4 h, and the washing process was repeated 3 times; then, filtration was performed, and a filter cake was collected and washed repeatedly with deionized water and absolute ethanol until the pH of the filtrate was > 6; the washed filter cake was dried in a vacuum oven at 80 °C for 24.

[0233] 5) Coating treatment: The porous carbon precursor after the demineralization treatment in the above step 4) and an aqueous solution of a coating raw material phenolic resin were uniformly mixed to form a kneading mixture, and the kneading mixture was placed in a twin-screw kneader for kneading for 6 h, where a rotation speed of the twin-screw kneader was 30 rpm, a solid content of the kneading mixture was 65 wt%, and a mass ratio of the coating raw material phenolic resin to the porous carbon precursor in the kneading mixture was 1.2:10; when nitrogen was introduced, the above kneaded product was placed in a tube furnace, heated to 180 °C at a heating rate of 5 °C / min, and maintained at 180 °C for 6 h to carry out a curing treatment, thereby obtaining a hard carbon precursor, where the flow rate of the nitrogen was 500 mL / min.

[0234] 6) Carbonization treatment: When nitrogen was introduced, the hard carbon precursor prepared in the above step 5) was placed in a tube furnace, heated to 1200 °C at a heating rate of 5 °C / min, and maintained for 12 h to obtain a hard carbon material.

[0235] The coin-type half-cell was assembled according to a method similar to that of Example 1.

[0236] Examples 13 to 16

[0237] Hard carbon materials were prepared according to a method similar to that of Example 12, and each assembled into a coin-type half-cell. The only difference was that when the hard carbon materials were prepared, the 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, as detailed in Table 3.

[0238] The process parameters for preparing the hard carbon materials in Examples 12 to 16 are shown in Table 3 below, and the performance parameters of the prepared hard carbon materials and the test results of the coin-type half-cell are shown in Table 4 below. Table 3: Item Activation treatment Coating treatment Percentage of addition amount of P element in pore-forming solution relative to mass of pre-carbonized body (%) Impregnation time (h) Mass ratio of coating raw material to porous carbon precursor Kneading time (h) Solid content (wt%) Curing temperature (°C) Curing time (h) Example 12 10.8 6 1.2: 10 6 65 180 6 Example 13 19.0 6 1.2: 10 6 65 180 6 Example 29.8 6 1.2: 10 6 65 180 6 14 Example 15 19.0 2 1.2: 10 6 65 180 6 Example 16 19.0 0.5 1.2: 10 6 65 180 6 Table 4: Item Hard carbon material parameter Coin-type half-cell performance A(cm3 / g) X / A(%) Y / A(%) X / Y Z(mg / g) Discharge specific capacity (mAh / g) Charge specific capacity (mAh / g) Initial coulombic efficiency (%) Example 12 0.327 88.38 11.62 7.6 2.5 381 340 89.24 Example 13 0.410 89.76 10.24 8.8 2.4 435 384 88.28 Example 14 0.468 85.68 14.32 6.0 9.2 472 385 81.57 Example 15 0.367 87.19 12.81 6.8 2.3 405 359 88.64 Example 16 0.301 87.04 12.96 6.7 2.0 370 326 88.11

[0239] As can be seen from Tables 3 and 4 above, in Examples 12 to 16, by keeping the total pore volume of the porous carbon between 0.2 cm3 / g and 0.6 cm3 / g and keeping the methylene blue adsorption value of the hard carbon material less than 10 mg / g, the initial coulombic efficiency and the charge specific capacity are significantly improved, while a high discharge specific capacity is achieved.

[0240] It should be noted that the present disclosure is not limited to the embodiments described above. The embodiments described above are merely examples, and any embodiments having a structure substantially identical to the technical concept and exerting the same functional effects within the scope of the technical solutions of the present disclosure are all included within the technical scope of the present disclosure. Furthermore, without departing from the spirit of the present disclosure, various modifications that can be conceived by those skilled in the art to the embodiments, as well as other embodiments formed by combining some of the constituent elements of the embodiments, are also included within the scope of the present disclosure.

Claims

1. A negative electrode plate, comprising a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, wherein the negative electrode film layer comprises a hard carbon material, and the hard carbon material comprises an inner core and a carbon coating layer coating the inner core; the inner core comprises 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 < 10 mg / g.

2. The negative electrode plate according to claim 1, wherein the methylene blue adsorption value Z of the hard carbon material is < 5 mg / g.

3. The negative electrode plate according to claim 1 or 2, wherein the porous carbon comprises micropores with a pore size of 0.4 nm to 2.5 nm and mesopores with 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.

4. The negative electrode plate according to claim 3, wherein the total pore volume X of the micropores and the total pore volume Y of the mesopores satisfy: 8 < X / Y < 13.

5. The negative electrode plate 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 plate 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 plate according to any one of claims 1 to 6, wherein ID / IG of the hard carbon material is 0.83 to 1.26,wherein ID represents a D peak intensity at 1350±50 cm-1 in a Raman spectrum, and IG represents a G peak intensity at 1580±50 cm-1 in the Raman spectrum.

8. The negative electrode plate according to any one of claims 1 to 7, wherein the hard carbon material satisfies at least one of the following:(1) a compacted density of the hard carbon material at 5 t being 0.6 g / cm3 to 1.05 g / cm3;(2) a tap density of the hard carbon material being 0.4 g / cm3 to 0.85 g / cm3;(3) a volume distribution particle size Dv50 of the hard carbon material being 3.0 gm to 7.9 gm;(4) a volume distribution particle size Dv90 of the hard carbon material being 8 gm to 15 gm; and(5) a specific surface area BET of the hard carbon material being 0.1 m2 / g to 10 m2 / g.

9. A secondary battery, wherein the secondary battery comprises the negative electrode plate according to any one of claims 1 to 8.

10. The secondary battery according to claim 9, wherein the secondary battery further comprises a positive electrode plate, and the positive electrode plate comprises, as a positive electrode active material, at least one selected from a transition metal oxide, a polyanionic compound, and a Prussian blue compound.

11. An electric device, comprising the secondary battery according to claim 9 or 10.

12. A hard carbon material, comprising an inner core and a carbon coating layer coating the inner core, wherein the inner core comprises 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 < 10 mg / g.

13. The hard carbon material according to claim 12, wherein the methylene blue adsorption value Z of the hard carbon material is < 5 mg / g.

14. The hard carbon material according to claim 12 or 13, wherein the porous carbon comprises micropores with a pore size of 0.4 nm to 2.5 nm and mesopores with 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.

15. The hard carbon material according to claim 14, wherein the total pore volume X of the micropores and the total pore volume Y of the mesopores satisfy: 8 < X / Y < 13.

16. The hard carbon material according to claim 14 or 15, wherein the total pore volume X of the micropores and the total pore volume A of the porous carbon satisfy: 85% < X / A < 97%.

17. The hard carbon material according to any one of claims 14 to 16, wherein the total pore volume Y of the mesopores and the total pore volume A of the porous carbon satisfy: 3% < Y / A < 15%.

18. The hard carbon material according to any one of claims 12 to 17, wherein ID / IG of the hard carbon material is 0.83 to 1.26,wherein ID represents a D peak intensity at 1350±50 cm-1 in a Raman spectrum, and IG represents a G peak intensity at 1580±50 cm-1 in the Raman spectrum.

19. The hard carbon material according to any one of claims 12 to 18, wherein the hard carbon material satisfies at least one of the following:(1) a compacted density of the hard carbon material at 5 t being 0.6 g / cm3 to 1.05 g / cm3;(2) a tap density of the hard carbon material being 0.4 g / cm3 to 0.85 g / cm3;(3) a volume distribution particle size Dv50 of the hard carbon material being 3.0 gm to 7.9 gm;(4) a volume distribution particle size Dv90 of the hard carbon material being 8 gm to 15 gm; and(5) a specific surface area BET of the hard carbon material being 0.1 m2 / g to 10 m2 / g.

20. A preparation method for a hard carbon material, wherein the method comprises:performing a pre-carbonization treatment on a hard carbon precursor to obtain a pre-carbonized body;performing an activation treatment on the pre-carbonized body to obtain a porous carbon precursor;performing a coating treatment on the porous carbon precursor to form a hard carbon precursor, wherein the coating treatment comprises a kneading treatment of kneading a kneading mixture formed by the porous carbon precursor and a water-based solution of a coating raw material, and a curing treatment of curing a product of the kneading treatment, a solid content of the kneading mixture being 55 wt% to 75 wt%; andperforming a carbonization treatment on the hard carbon precursor to obtain the hard carbon material.

21. The preparation method according to claim 20, wherein a mass ratio of the coatingraw material to the porous carbon precursor is 0.5:10 to 2:10, and the kneading treatment is performed for at least 0.5 h.

22. The preparation method according to claim 20 or 21, wherein the curing treatment is performed at 120 °C to 250 °C for at least 1 h.

23. The preparation method according to any one of claims 20 to 22, wherein the coating raw material comprises a prepolymer of a thermosetting resin.

24. The preparation method according to claim 23, wherein the thermosetting resin comprises at least one of a phenolic resin, an epoxy resin, an unsaturated polyester resin, and a furan resin.

25. The preparation method according to any one of claims 20 to 24, wherein the kneading treatment is performed in a twin-screw kneader, and a rotation speed of the twin-screw kneader is 10 rpm to 50 rpm.

26. The preparation method according to any one of claims 20 to 25, wherein the activation treatment comprises subjecting the pre-carbonized body to treatment at 700 °C to 950 °C under a preset mixed gas, wherein the preset mixed gas comprises carbon dioxide gas, water vapor, and inert gas, and a volume ratio of the carbon dioxide gas to the water vapor in the preset mixed gas is > 2.

27. The preparation method according to claim 26, wherein a volume proportion of the carbon dioxide gas in the preset mixed gas is 5% to 20%, and a volume proportion of the water vapor in the mixed gas is 1% to 5%.

28. The preparation method according to any one of claims 20 to 27, wherein the activation treatment comprises impregnating the pre-carbonized body in a P-containing pore-forming solution or a Zn-containing pore-forming solution for at least 2 h; an addition amount of an element P in the P-containing pore-forming solution is 9.5% to 32.4% of a mass of the pre-carbonized body, and an addition amount of an element Zn in the Zn-containing pore-forming solution is 14% to 50% of the mass of the pre-carbonized body.

29. The preparation method according to claim 28, wherein a solid content of an impregnation mixture formed by the pre-carbonized body and the pore-forming solution is 50 wt% to 70 wt%.

30. The preparation method according to claim 28 or 29, wherein the activation treatment further comprises keeping an impregnation mixture formed by the immersedpre-carbonized body and the pore-forming solution at a temperature of 400 °C to 750 °C for 1 h to 12 h.

31. The preparation method according to any one of claims 20 to 30, wherein in the pre-carbonization treatment, a temperature is raised to 400 °C to 600 °C at a heating rate of 1 °C / min to 20 °C / min and held for 1 h to 12 h.

32. The preparation method according to any one of claims 20 to 31, wherein in the carbonization treatment, a temperature is raised to 1000 °C to 1800 °C at a heating rate of 2 °C / min to 20 °C / min and held for 1 h to 12 h.

33. The preparation method according to any one of claims 20 to 32, wherein the hard carbon precursor comprises at least one of a biomass precursor and a synthetic polymer precursor;the synthetic polymer precursor comprises a compound composed of C, H, and O elements.

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