Hard carbon, negative pole piece, preparation method of negative pole piece, secondary battery containing negative pole piece and electric device

Abstract

The invention provides hard carbon, a negative pole piece, a preparation method of the negative pole piece, a secondary battery containing the negative pole piece and an electric device, in a nitrogen adsorption isotherm of the hard carbon measured at the temperature of 77K, the total nitrogen adsorption amount when the nitrogen relative pressure P / P0 ranges from 10 <-8 > to 0.035 is V1 cm < 3 > (STP) / g, the total nitrogen adsorption amount when the nitrogen relative pressure P / P0 ranges from 0.035 to 1 is V2 cm < 3 > (STP) / g, and the hard carbon meets the condition that V1 is larger than or equal to 20 and smaller than or equal to 150, p represents the real pressure of the nitrogen, and P0 represents the saturated vapor pressure of the nitrogen at the temperature of 77K. The hard carbon capacity and the first coulombic efficiency can be improved at the same time.

Description

This application is a divisional application based on the invention with application number 202180094968.3, application date December 31, 2021, applicant CATL, and invention title "Hard Carbon and its Preparation Method, Secondary Battery Containing the Same and Electrical Device". Technical Field

[0001] This application belongs to the field of battery technology, specifically relating to a hard carbon electrode, a negative electrode sheet and its preparation method, a secondary battery containing the same, and an electrical device thereof. Background Technology

[0002] In recent years, rechargeable batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and many other fields. With the application and promotion of rechargeable batteries, their energy density, lifespan, and rate performance have received increasing attention. Graphite is the most commonly used negative electrode active material in rechargeable batteries, but its theoretical specific capacity is only 372 mAh / g, limiting the potential for energy density improvement; at the same time, the small interlayer spacing of graphite also limits the improvement of rate performance. Hard carbon, as a novel negative electrode active material, can achieve rapid insertion and extraction of active ions during the charging and discharging process of rechargeable batteries, thus showing great development potential. However, hard carbon has low capacity and initial coulombic efficiency, limiting its contribution to improving the energy density, lifespan, and rate performance of rechargeable batteries. Summary of the Invention

[0003] The purpose of this application is to provide a hard carbon electrode, a negative electrode sheet, a method for preparing the same, a secondary battery containing the same, and an electrical device, which aims to simultaneously improve the capacity and initial coulombic efficiency of the hard carbon electrode.

[0004] The first aspect of this application provides a hard carbon, wherein the nitrogen relative pressure P / P0 in the nitrogen adsorption isotherm measured at 77K is within 10. -8 The total nitrogen adsorption value between 0.035 and 0.035 is V1 cm. 3 (STP) / g, the total amount of nitrogen adsorbed when the relative pressure of nitrogen P / P0 is between 0.035 and 1 is V² cm⁻¹. 3 (STP) / g, wherein the hard carbon satisfies: 20≤V1≤150, where P represents the actual pressure of nitrogen and P0 represents the saturated vapor pressure of nitrogen at 77K.

[0005] Compared to currently commercially available hard carbon, the hard carbon provided in this application simultaneously achieves higher capacity and first-time coulombic efficiency. Although the mechanism is not yet clear, the inventors speculate that one possible reason is that the hard carbon of this application has a moderate number of internal defects and a unique pore structure, resulting in a large number of active ion storage sites with high utilization. Therefore, the hard carbon structure of this application facilitates the insertion, storage, and extraction of active ions, thereby enabling the hard carbon of this application to simultaneously achieve high capacity and first-time coulombic efficiency.

[0006] In any embodiment of this application, 20 ≤ V1 ≤ 150. Optionally, 56.9 ≤ V1 ≤ 150. More preferably, 70.4 ≤ V1 ≤ 150. When V1 is within a suitable range, the hard carbon energy provided by this application can simultaneously have higher capacity and first-time coulombic efficiency.

[0007] In any embodiment of this application, 0.05 ≤ V2 / V1 ≤ 0.20. Optionally, 0.085 ≤ V2 / V1 ≤ 0.195. When V2 / V1 is within a suitable range, hard carbon can simultaneously have higher capacity and higher initial coulombic efficiency.

[0008] In any embodiment of this application, 0 < V2 ≤ 30. Optionally, 4 ≤ V2 ≤ 30. More preferably, 6 ≤ V2 ≤ 30. When V2 is within a suitable range, hard carbon can have higher capacity and higher initial coulombic efficiency, while also exhibiting excellent rate performance.

[0009] In any embodiment of this application, in the Raman spectrum of the hard carbon, I d / I g The value is 1.20-1.32, I d This indicates that the Raman displacement is within 1350 ± 50 cm. -1 The d-peak intensity within the range, I g This indicates that the Raman displacement is within 1580±50cm. -1 The g-peak intensity is within the range. At this point, the hard carbon structure has a moderate degree of order, resulting in higher capacity and higher first coulombic efficiency, as well as excellent rate performance.

[0010] In any embodiment of this application, in the X-ray diffraction spectrum of the hard carbon, the 2θ value corresponding to the 002 peak is between 22° and 24°.

[0011] In any embodiment of this application, the volumetric particle size Dv50 of the hard carbon is 2μm-15μm; optionally, it is 4μm-8μm.

[0012] In any embodiment of this application, the volumetric particle size Dv90 of the hard carbon is 5μm-25μm; optionally, it is 8μm-15μm.

[0013] When the volumetric particle size of hard carbon, Dv50 and / or Dv90, is within a suitable range, it is beneficial to improve the active ion and electron transport performance, thereby further improving the rate performance of secondary batteries.

[0014] In any embodiment of this application, the specific surface area of ​​the hard carbon is less than or equal to 5 m². 2 / g; optionally 0.5m 2 / g-5m 2 / g. When the specific surface area of ​​hard carbon is within a suitable range, it can simultaneously exhibit higher capacity and higher initial coulombic efficiency, as well as better rate performance. Furthermore, when the specific surface area of ​​hard carbon is within a suitable range, it also possesses strong bonding forces with the binder, thereby improving the cohesion and adhesion of the negative electrode sheet, reducing the volume expansion of the negative electrode sheet during cycling, and resulting in better cycle performance of the secondary battery.

[0015] In any embodiment of this application, the compacted density of the hard carbon powder under a force of 50,000 N is 0.96 g / cm³. 3 -1.05g / cm 3 When the compaction density of hard carbon powder is within a suitable range, it can increase the compaction density of the negative electrode sheet, thereby increasing the energy density of the secondary battery.

[0016] In any embodiment of this application, the tap density of the hard carbon is 0.80 g / cm³. 3 -0.95g / cm 3 When the tap density of hard carbon is within a suitable range, it can increase the compaction density of the negative electrode sheet, thereby increasing the energy density of the secondary battery.

[0017] A second aspect of this application provides a negative electrode sheet, which includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material, the negative electrode active material including hard carbon of the first aspect of this application.

[0018] In any embodiment of this application, the negative electrode film layer further includes other negative electrode active materials, including at least one of natural graphite, artificial graphite, soft carbon, silicon-based materials, tin-based materials, and lithium titanate.

[0019] The third aspect of this application provides a method for preparing a negative electrode sheet, comprising the following steps: coating a negative electrode slurry onto a negative electrode current collector, drying and cold pressing to obtain a negative electrode sheet, wherein the negative electrode slurry is dried and cold pressed to form a negative electrode film layer, wherein the negative electrode slurry is formed by dispersing a negative electrode active material, an optional conductive agent, an optional binder, and other optional additives in a solvent and stirring evenly, wherein the negative electrode active material includes hard carbon of the first aspect of this application.

[0020] The fourth aspect of this application provides a secondary battery including a negative electrode sheet, which is the negative electrode sheet of the second aspect of this application or a negative electrode sheet prepared according to the method of the third aspect of this application.

[0021] The fifth aspect of this application provides an electrical device that includes the secondary battery of the fourth aspect of this application.

[0022] The electrical device of this application includes the secondary battery provided in this application, and therefore has at least the same advantages as the secondary battery. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly described below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0024] Figure 1 Six common adsorption isotherm types for solid materials are shown.

[0025] Figure 2 This is a schematic diagram of one embodiment of the secondary battery of this application.

[0026] Figure 3 yes Figure 2 An exploded view of the implementation method of the secondary battery.

[0027] Figure 4 This is a schematic diagram of one embodiment of the battery module of this application.

[0028] Figure 5 This is a schematic diagram of one embodiment of the battery pack of this application.

[0029] Figure 6 yes Figure 5 An exploded view of an embodiment of the battery pack shown.

[0030] Figure 7 This is a schematic diagram of one embodiment of an electrical device that uses a secondary battery as a power source, as described in this application.

[0031] Figure 8 This is a scanning electron microscope (SEM) image of the hard carbon prepared in Example 2.

[0032] Figure 9 This is the nitrogen adsorption isotherm measured at 77K for the hard carbon prepared in Example 2.

[0033] Figure 10This is the nitrogen adsorption isotherm measured at 77 K for the hard carbon prepared in Comparative Example 1.

[0034] The accompanying drawings are not necessarily drawn to scale. Detailed Implementation

[0035] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the hard carbon of this application, its preparation method, secondary batteries containing it, and electrical devices thereof. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0036] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0037] Unless otherwise specified, all embodiments and optional embodiments of this application may be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of this application.

[0038] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions, and such technical solutions shall be deemed to be included in the disclosure of this application.

[0039] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) 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 it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0040] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0041] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0042] Unless otherwise specified, in this application, the term "active ion" refers to ions that can be inserted and extracted back and forth between the positive and negative electrodes of a secondary battery, including but not limited to lithium ions, sodium ions, etc.

[0043] According to the International Union of Pure and Applied Chemistry (IUPAC), micropores are pores with a diameter < 2 nm, mesopores are pores with a diameter of 2 nm to 50 nm, and macropores are pores with a diameter > 50 nm. In the context of this application, the term "micropore" refers to a pore with a diameter < 2 nm; the term "mesopore" refers to a pore with a diameter of 2 nm to 50 nm; and the term "macropore" refers to a pore with a diameter > 50 nm. In the context of this application, the terms "larger pore" and "smaller pore" are relative concepts.

[0044] A secondary battery, also known as a rechargeable battery or accumulator, is a battery that can be recharged after discharge to reactivate its active materials and continue to be used. Typically, a secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. During charging and discharging, active ions repeatedly insert and extract between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits while allowing active ions to pass through. The electrolyte, situated between the positive and negative electrodes, conducts the active ions. Currently, secondary batteries are widely used in energy storage systems such as hydroelectric, thermal, wind, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric cars, military equipment, aerospace, and many other fields.

[0045] With the application and promotion of rechargeable batteries, their energy density, lifespan, and rate performance have received increasing attention. The performance of the negative electrode active material largely determines the energy density, lifespan, and safety of the rechargeable battery. Graphite (including natural and artificial graphite) is the most commonly used negative electrode active material in rechargeable batteries, but its theoretical specific capacity is only 372 mAh / g, leaving very limited room for energy density improvement. At the same time, the small interlayer spacing of graphite also limits the improvement of rate performance, making it unable to meet the actual needs of high-rate rechargeable batteries.

[0046] Hard carbon refers to carbon that is difficult to graphitize, even at temperatures above 2500℃. Hard carbon has a complex structure, exhibiting a graphitic microcrystalline structure with irregularly arranged crystallites, a small number of layers, and potential cross-linking at the edges. The hard carbon structure also includes amorphous regions, primarily comprising micropores, defects, sp3 hybridized carbon atoms, carbon chains, and various functional groups. Therefore, hard carbon structures may contain multiple active ion storage sites, such as on the surface of graphitic microcrystals, between graphitic microcrystal layers, at the edges of graphitic microcrystal layers, and within micropores.

[0047] Compared to graphite, hard carbon has a larger interlayer spacing and a richer microporous structure, which is beneficial for the storage, rapid insertion and extraction of active ions. This results in superior low-temperature performance, power performance, and safety performance in rechargeable batteries, especially in the field of power batteries, where hard carbon has unique advantages. However, most commercially available hard carbon batteries are currently low-capacity ordinary hard carbon batteries, with low capacity and initial coulombic efficiency. For example, the capacity is usually between 200 mAh / g and 280 mAh / g, and the initial coulombic efficiency is usually below 80%, which severely limits their practical applications.

[0048] Therefore, how to simultaneously improve the capacity and first coulomb efficiency of hard carbon remains a pressing technical challenge.

[0049] In view of this, a first aspect of the embodiments of this application provides a hard carbon that simultaneously achieves high capacity and initial coulombic efficiency, and enables the secondary battery to simultaneously possess high energy density, long service life and excellent rate performance.

[0050] In the nitrogen adsorption isotherm measured at 77K on hard carbon according to the first aspect of this application, the relative nitrogen pressure P / P0 is within 10. -8 The total nitrogen adsorption value between 0.035 and 0.035 is V1 cm. 3 (STP) / g, the total amount of nitrogen adsorbed when the relative pressure of nitrogen P / P0 is between 0.035 and 1 is V² cm⁻¹. 3 (STP) / g, wherein the hard carbon satisfies: V2 / V1≤0.20, and 20≤V1≤150, where P represents the actual pressure of nitrogen and P0 represents the saturated vapor pressure of nitrogen at 77K.

[0051] The K unit above is the standard Kelvin temperature unit, and 77K represents the temperature of liquid nitrogen.

[0052] When gas molecules move to the surface of a solid material, due to the interaction between the gas molecules and the molecules on the solid surface (e.g., van der Waals forces or chemical bonds), the gas molecules will temporarily remain on the solid surface, increasing the concentration of gas molecules on the solid surface. This phenomenon is called adsorption of gas molecules on the solid material surface. The gas can be called the adsorbate, and the solid material can be called the adsorbent. An adsorption isotherm is a curve showing the relationship between the concentrations of the adsorbate (e.g., gas) in the two phases when the adsorption process at a two-phase interface (e.g., the interface between a gas and a solid material) reaches equilibrium at a certain temperature. Different pore structures of solid materials result in different types of adsorption isotherms. Figure 1 Six common adsorption isotherm types for solid materials are shown.

[0053] In the nitrogen adsorption isotherm measured at 77K using the hard carbon of this application, the relative nitrogen pressure P / P0 is within 10. -8 The total nitrogen adsorption V1 cm between 0.035 and 0.035 3 (STP) / g and the total nitrogen adsorption volume V2 cm⁻² relative pressure P / P0 between 0.035 and 1. 3 (STP) / g satisfies V2 / V1≤0.20, therefore, the adsorption process of hard carbon in this application mainly occurs in the stage of relatively low pressure, that is, P / P0 is around 10. -8 Between 0.035 and 0.035. Meanwhile, the hard carbon of this application is used in nitrogen atmospheres with a relative pressure P / P0 of 10. -8 The total nitrogen adsorption V1 at 0.035 is at 20 cm⁻¹ 3 (STP) / g to 150cm 3 Between (STP) / g.

[0054] Compared to currently commercially available hard carbon, the hard carbon provided in this application simultaneously achieves higher capacity and first-time coulombic efficiency. Although the mechanism is not yet clear, the inventors speculate that one possible reason is that the hard carbon of this application has a moderate number of internal defects and a unique pore structure, resulting in a large number of active ion storage sites with high utilization. Therefore, the hard carbon structure of this application facilitates the insertion, storage, and extraction of active ions, thereby enabling the hard carbon of this application to simultaneously achieve high capacity and first-time coulombic efficiency.

[0055] Currently, in the nitrogen adsorption isotherm measured at 77K for commercially available hard carbon, the relative nitrogen pressure P / P0 is around 10. -8 The total nitrogen adsorption V1 cm between 0.035 and 0.035 3 (STP) / g and the total nitrogen adsorption volume V2 cm⁻² relative pressure P / P0 between 0.035 and 1. 3 The (STP) / g ratio does not satisfy V2 / V1≤0.20. Possible reasons include: commercially available hard carbon has numerous internal defects, an unreasonable pore structure, or few micropores, failing to provide sufficient active ion storage sites, and the utilization rate of the few available active ion storage sites is also low. Simultaneously, commercially available hard carbon may contain a high content of mesoporous or macroporous structures, resulting in a high proportion of electrolyte wetting area within the hard carbon. Further research by the inventors revealed that a higher V2 / V1 ratio in the adsorption isotherm corresponds to a higher proportion of electrolyte wetting area within the hard carbon, leading to a higher initial irreversible capacity loss and a lower initial coulombic efficiency.

[0056] Additionally, when hard carbon is in nitrogen with a relative pressure P / P0 of 10 -8 The total nitrogen adsorption at 0.035 is <20 cm⁻¹ 3 At (STP) / g, the pore structure of hard carbon is underdeveloped and the specific surface area is too low. Hard carbon cannot provide many active ion storage sites, resulting in a low specific capacity. At the same time, due to the underdeveloped pore structure of hard carbon, the active ions stored in the storage sites are not easy to escape, resulting in a low first coulombic efficiency of hard carbon.

[0057] When hard carbon is in nitrogen gas with a relative pressure P / P0 of 10 -8 The total nitrogen adsorption at 0.035 is >150 cm⁻¹ 3 At a concentration of (STP) / g, the hard carbon structure becomes brittle, making its internal pore structure prone to collapse and pore coalescence. This results in an excessively high proportion of mesoporous or macroporous structures within the hard carbon, which in turn facilitates the infiltration of large amounts of electrolyte. Therefore, although hard carbon can provide more active ion storage sites and a higher specific capacity, its complex structure prevents some of the active ions stored at these sites from being released, leading to a lower first coulombic efficiency.

[0058] The hard carbon provided in this application is in nitrogen relative pressure P / P0 at 10. -8 The total nitrogen adsorption V1cm between 0.035 and 0.035 3 (STP) / g satisfies 20 ≤ V1 ≤ 150. Wherein, the higher V1 is, the more active ion storage sites the hard carbon provides, and the higher the capacity of the hard carbon. In some embodiments, optionally, the following values ​​are also satisfied: 30 ≤ V1 ≤ 150, 40 ≤ V1 ≤ 150, 50 ≤ V1 ≤ 150, 60 ≤ V1 ≤ 150, 70 ≤ V1 ≤ 150, 80 ≤ V1 ≤ 150, 90 ≤ V1 ≤ 150, 100 ≤ V1 ≤ 150, 110 ≤ V1 ≤ 150, 120 ≤ V1 ≤ 150, 30 ≤ V1 ≤ 140, 40 ≤ V1 ≤ 140, 50 ≤ V1 ≤ 140, 60 ≤ V1 ≤ 140, 70 ≤ V1 ≤ 140, 80 ≤ V1 ≤ 140, 90 ≤ V1 ≤ 140, 100 ≤ V1 ≤ 140, 110 ≤ V 1≤140, 120≤V1≤140, 30≤V1≤130, 40≤V1≤130, 50≤V1≤130, 60≤V1≤130, 70≤V1≤130, 80≤V1≤130, 90≤V1≤130, 100≤V1≤130, 110≤V1≤130, 30≤V1≤120, 40≤V1≤120, 50≤V1≤120, 60≤V1≤120, 70≤V1≤120, 80≤V1≤120, 90≤V1≤120, 100≤V1≤120, 110≤V1≤120. When V1 is within a suitable range, the hard carbon energy provided by this application can simultaneously possess higher capacity and initial coulombic efficiency.

[0059] The hard carbon provided in this application is in nitrogen relative pressure P / P0 at 10 -8 Total nitrogen adsorption V1cm between 0.035 and 0.035 3 (STP) / g and the total nitrogen adsorption volume V2 cm⁻² relative pressure P / P0 between 0.035 and 1. 3(STP) / g satisfies V2 / V1 ≤ 0.20. Within this range, hard carbon can possess a suitable pore structure, providing a large number of active ion storage sites with high utilization. Simultaneously, the internal pore structure of hard carbon is predominantly microporous, with a moderate content of mesoporous or macroporous structures, thereby reducing the electrolyte wetting area within the hard carbon. In some embodiments, V2 / V1 can be ≤ 0.19, ≤ 0.18, ≤ 0.17, ≤ 0.16, ≤ 0.15, ≤ 0.14, ≤ 0.13, ≤ 0.12, ≤ 0.11, or ≤ 0.10. However, V2 / V1 should not be too low, as this makes it difficult for active ions stored at the active ion storage sites to be released. In some embodiments, optionally, 0.05≤V2 / V1≤0.20, 0.06≤V2 / V1≤0.20, 0.08≤V2 / V1≤0.20, 0.10≤V2 / V1≤0.20, 0.12≤V2 / V1≤0.20, 0.14≤V2 / V1≤0.20, 0.16≤V2 / V1≤0.20, 0.05≤V2 / V1≤0.18, 0.06≤V2 / V1≤0.18, 0.08≤V2 / V1 ≤0.18, 0.10≤V2 / V1≤0.18, 0.12≤V2 / V1≤0.18, 0.14≤V2 / V1≤0.18, 0.16≤V2 / V1≤0.18, 0.05≤V2 / V1≤0.16, 0.06≤V2 / V1≤0.16, 0.08≤V2 / V1≤0.16, 0.10≤V2 / V1≤0.16, 0.12≤V2 / V1≤0.16, 0.14≤V2 / V1≤0.16. When V2 / V1 is within a suitable range, hard carbon can simultaneously exhibit higher capacity and higher initial coulombic efficiency.

[0060] In some embodiments, 0 < V2 ≤ 30. When V2 is higher, the proportion of the electrolyte-wetted area inside the hard carbon is higher, increasing the initial irreversible capacity loss of the hard carbon. Simultaneously, V2 should not be too low. When V2 is low, active ions have difficulty rapidly de-extracting and intercalating, potentially worsening the rate performance of the hard carbon. Optionally, 1 ≤ V2 ≤ 30, 2 ≤ V2 ≤ 30, 3 ≤ V2 ≤ 30, 4 ≤ V2 ≤ 30, 5 ≤ V2 ≤ 30, 6 ≤ V2 ≤ 30, 7 ≤ V2 ≤ 30, 8 ≤ V2 ≤ 30, 9 ≤ V2 ≤ 30, 10 ≤ V2 ≤ 30, 1 ≤ V2 ≤ 25, 2 ≤ V2 ≤ 25, 3 ≤ V2 ≤ 25, 4 ≤ V2 ≤ 25, 5 ≤ V2 ≤ 25 The following values ​​are allowed for V2: 6≤V2≤25, 7≤V2≤25, 8≤V2≤25, 9≤V2≤25, 10≤V2≤25, 1≤V2≤20, 2≤V2≤20, 3≤V2≤20, 4≤V2≤20, 5≤V2≤20, 6≤V2≤20, 7≤V2≤20, 8≤V2≤20, 9≤V2≤20, or 10≤V2≤20. When V2 is within a suitable range, hard carbon exhibits higher capacity and higher initial coulombic efficiency, while also possessing excellent rate performance.

[0061] In some embodiments, the hard carbon simultaneously satisfies: 0.08 ≤ V2 / V1 ≤ 0.20, 70 ≤ V1 ≤ 150, and 6 ≤ V2 ≤ 30. In this case, the hard carbon provides more active ion storage sites, resulting in higher capacity. Simultaneously, the hard carbon has a suitable pore structure, facilitating the insertion and storage of active ions without hindering their extraction, thus achieving high utilization of the active ion storage sites. Furthermore, the pore structure of the hard carbon is predominantly microporous, with a moderate content of mesoporous or macroporous structures, thereby reducing the electrolyte wetting area within the hard carbon. Therefore, the hard carbon exhibits higher capacity and higher initial coulombic efficiency, while also possessing excellent rate performance. Further, the hard carbon simultaneously satisfies: 0.08 ≤ V2 / V1 ≤ 0.20, 85 ≤ V1 ≤ 140, and 7 ≤ V2 ≤ 25.

[0062] In some embodiments, in the Raman spectrum of the hard carbon, I d / I g The value is 1.20-1.32, I d This indicates that the Raman displacement is within 1350 ± 50 cm. - The peak intensity of d within the range of 1, I g This indicates that the Raman displacement is within 1580±50cm. -1 The intensity of the g peak within the range, where the laser wavelength λ is 532 nm.

[0063] The d-peak originates from lattice defects in carbon atoms, while the g-peak originates from in-plane vibrations of sp2 carbon atoms. In hard carbon structures, the intensity of the d-peak is related to the number of defects in the hard carbon structure, and the intensity of the g-peak is related to the number of graphite crystallites in the hard carbon structure. Therefore, I d / I g It can characterize the degree of order in the structure of hard carbon. d / I g The smaller the carbon density, the higher the orderliness of the hard carbon structure, the higher the integrity of the carbon plane, and the higher the initial coulombic efficiency of the hard carbon, but the capacity decreases and the rate performance deteriorates. The hard carbon in this application satisfies requirement I. d / I g The value is 1.20-1.32. At this range, the order of the hard carbon structure is moderate, which gives hard carbon higher capacity and higher first coulombic efficiency, as well as excellent rate performance.

[0064] In some embodiments, the 2θ value corresponding to the 002 peak in the X-ray diffraction spectrum of the hard carbon is between 22° and 24°.

[0065] In some embodiments, the volumetric particle size Dv50 of the hard carbon is 2 μm-15 μm; optionally, it is 4 μm-8 μm.

[0066] In some embodiments, the volumetric particle size Dv90 of the hard carbon is 5 μm-25 μm; optionally, it is 8 μm-15 μm.

[0067] In some embodiments, the hard carbon simultaneously satisfies a volumetric particle size Dv50 of 4μm-8μm and a volumetric particle size Dv90 of 8μm-15μm.

[0068] When the volumetric particle size of hard carbon, Dv50 and / or Dv90, is within a suitable range, it is beneficial to improve the active ion and electron transport performance, thereby further improving the rate performance of secondary batteries.

[0069] In some embodiments, the specific surface area of ​​the hard carbon is less than or equal to 5 m². 2 / g. Optionally, the specific surface area of ​​the hard carbon can be 0.1m². 2 / g-5m 2 / g, 0.5m 2 / g-5m 2 / g, 1m 2 / g-5m 2 / g, 1.5m 2 / g-5m 2 / g, 2m 2 / g-5m 2 / g, 2.5m 2 / g-5m 2 / g, 3m 2 / g-5m 2 / g, 3.5m 2 / g-5m 2 / g, 4m 2 / g-5m 2 / g, 0.5m 2 / g-4m 2 / g, 1m 2 / g-4m 2 / g, 1.5m 2 / g-4m 2 / g, 2m 2 / g-4m 2 / g, 2.5m 2 / g-4m 2 / g, 3m 2 / g-4m 2 / g, 0.5m 2 / g-3m 2 / g, 1m 2 / g-3m 2 / g, 1.5m 2 / g-3m 2 / g, or 2m 2 / g-3m 2 / g. A lower specific surface area helps reduce the surface activity of hard carbon and decrease the formation of the solid electrolyte interphase (SEI) film, thereby improving the initial coulombic efficiency of both hard carbon and the secondary battery. A higher specific surface area facilitates faster transport of active ions, thus improving the rate performance of the secondary battery. When the specific surface area of ​​hard carbon is within a suitable range, it can simultaneously exhibit higher capacity and initial coulombic efficiency, as well as better rate performance. Furthermore, when the specific surface area of ​​hard carbon is within a suitable range, it also exhibits strong bonding forces with the binder, thereby improving the cohesion and adhesion of the negative electrode, reducing the volume expansion of the negative electrode during cycling, and resulting in better cycle performance of the secondary battery.

[0070] In some embodiments, the compacted density of the hard carbon powder under a force of 50,000 N is 0.96 g / cm³. 3 -1.05g / cm 3 When the compaction density of hard carbon powder is within a suitable range, it can increase the compaction density of the negative electrode sheet, thereby increasing the energy density of the secondary battery.

[0071] In some embodiments, the tap density of the hard carbon is 0.80 g / cm³. 3 -0.95g / cm 3 When the tap density of hard carbon is within a suitable range, it can increase the compaction density of the negative electrode sheet, thereby increasing the energy density of the secondary battery.

[0072] In this application, the nitrogen adsorption test of hard carbon at 77K can be performed in accordance with GB / T 21650.2-2008 "Determination of Pore Size Distribution and Porosity of Solid Materials by Mercury Intrusion Porosimetry and Gas Adsorption Methods - Part 2: Analysis of Mesopores and Macropores by Gas Adsorption Method". For example, a surface area and pore size analyzer can be used for the determination, such as the ASAP 2460 surface area and pore size analyzer from Micromeritics, USA.

[0073] In this application, the volumetric particle sizes Dv50 and Dv90 of hard carbon have meanings known in the art, representing the particle size corresponding to a cumulative volume distribution percentage of 50% and 90%, respectively, and can be determined using instruments and methods known in the art. For example, they can be conveniently determined using a laser particle size analyzer, such as the Mastersizer 2000E laser particle size analyzer from Malvern Instruments Ltd., UK, in accordance with GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method.

[0074] In this application, the specific surface area of ​​hard carbon has a meaning known in the art and can be determined using instruments and methods known in the art. For example, it can be tested using the nitrogen adsorption specific surface area analysis method according to GB / T 19587-2017 and calculated using the BET (Brunauer Emmett Teller) method. The nitrogen adsorption specific surface area analysis can be performed using the ASAP 3020 surface area and pore size analyzer from Micromeritics, Inc., USA.

[0075] In this application, the compacted density of hard carbon powder has a meaning known in the art and can be determined using instruments and methods known in the art. For example, it can be determined using an electronic pressure testing machine (e.g., UTM7305 type) according to standard GB / T24533-2009. An exemplary test method is as follows: Weigh 1g of hard carbon powder and add it to a container with a bottom area of ​​1.327cm². 2 In the mold, the pressure is increased to 5000 kg (equivalent to 50000 N), held for 30 seconds, then depressurized and held for 10 seconds. The compaction density of hard carbon powder under a force of 50000 N is then recorded and calculated.

[0076] In this application, the tap density of hard carbon has a meaning known in the art and can be determined using instruments and methods known in the art. For example, it can be determined using a powder tap density tester (e.g., Dandong Baite BT-301) in accordance with GB / T 5162-2006. Preparation method of hard carbon

[0077] A second aspect of this application provides a method for preparing hard carbon, comprising the following steps: S10, providing a carbon source; S20, heat-treating the carbon source in an inert atmosphere at a first temperature T1 for t1 time to obtain a first intermediate product; S30, heat-treating the obtained first intermediate product in an air atmosphere at a second temperature T2 for t2 time to obtain a second intermediate product; S40, carbonizing the obtained second intermediate product in an inert atmosphere at a third temperature T3 for t3 time to obtain hard carbon. Wherein, in the nitrogen adsorption isotherm measured at 77K, the relative pressure of nitrogen P / P0 is within 10. -8 The total nitrogen adsorption value between 0.035 and 0.035 is V1 cm. 3 (STP) / g, the total amount of nitrogen adsorbed when the relative pressure of nitrogen P / P0 is between 0.035 and 1 is V² cm⁻¹. 3 (STP) / g, wherein the hard carbon satisfies: V2 / V1≤0.20, and 20≤V1≤150, where P represents the actual pressure of nitrogen and P0 represents the saturated vapor pressure of nitrogen at 77K.

[0078] The preparation method of the second aspect of the present application can prepare hard carbon according to any embodiment of the first aspect of the present application.

[0079] In some embodiments, T1 < T3, T2 < T3.

[0080] In some embodiments, T1 ≤ T2. Alternatively, T1 < T2.

[0081] The inventors were surprised to discover that the hard carbon obtained by simultaneously subjecting the carbon source to a low-temperature heat treatment process in an inert atmosphere, a secondary low-temperature heat treatment process in an air atmosphere, and a high-temperature carbonization process in an inert atmosphere can simultaneously possess both high capacity and high initial coulombic efficiency. Furthermore, compared to existing commercially available hard carbon, the hard carbon obtained through the preparation method of this application shows a significant improvement in both capacity and initial coulombic efficiency.

[0082] The method for preparing hard carbon provided in this application is simple and suitable for commercial production.

[0083] The method for preparing hard carbon provided in this application does not require the addition of any conductive agent or other additives. As a result, the hard carbon obtained by the method provided in this application has a lower heteroatom content, which can further reduce the irreversible consumption of active ions by heteroatoms.

[0084] In the method for preparing hard carbon provided in this application, heat-treating the carbon source in an inert atmosphere and at a lower first temperature can better stabilize the framework structure of the carbon source and provide a stable carbon skeleton structure for the subsequent pore-forming process of secondary low-temperature heat treatment and high-temperature carbonization treatment; performing secondary low-temperature heat treatment on the first intermediate product in an air atmosphere and at a lower second temperature can form a suitable pore structure in the carbon skeleton structure, which facilitates the secondary pore-forming process of subsequent high-temperature carbonization treatment; performing high-temperature carbonization treatment on the second intermediate product in an inert atmosphere and at a higher third temperature can improve the orderliness of the obtained hard carbon.

[0085] There is no particular limitation on the type of inert atmosphere. In some embodiments, the inert atmosphere is selected from one or more of nitrogen atmosphere and argon atmosphere.

[0086] In some embodiments, the first temperature T1 ≤ 300℃. A higher first temperature T1 can easily lead to the decomposition of a large portion of the framework structure formed after heat treatment of the carbon source, which is suitable for transforming into a suitable pore structure. This results in a lack of suitable framework structure to form a pore structure during subsequent heat treatment, manifested as smaller V1 and V2 values ​​on the nitrogen adsorption isotherm measured at 77K for hard carbon, indicating a lower capacity of hard carbon. Furthermore, the first temperature T1 should not be too low. A lower first temperature T1 results in a lower degree of cross-linking after heat treatment of the carbon source, leading to a more fragile and less stable framework structure. When low-temperature heat treatment is performed in an air atmosphere to introduce a pore structure, the fragile framework structure is prone to collapse, resulting in a reduction in the pore structure of hard carbon, manifested as a smaller V1 value on the nitrogen adsorption isotherm measured at 77K for hard carbon. Simultaneously, framework collapse causes pore merging, increasing the number and proportion of larger pores, further reducing the capacity and initial coulombic efficiency of hard carbon.

[0087] In some embodiments, the first temperature T1 can be 180℃-300℃, 200℃-300℃, 220℃-300℃, 240℃-300℃, 260℃-300℃, 280℃-300℃, 180℃-280℃, 200℃-280℃, 220℃-280℃, 240℃-280℃, 260℃-280℃, 180℃-260℃, 200℃-260℃, 220℃-260℃, 240℃-260℃, 180℃-240℃, 200℃-240℃, 220℃-240℃, or 180℃-220℃. When the first temperature T1 is within a suitable range, a larger number of skeleton structures suitable for introducing appropriate channel structures and stable and not easily collapsed can be preserved.

[0088] In some embodiments, the heat treatment time t1 is 4h-60h. Those skilled in the art can select a suitable heat treatment time within the above range based on the first temperature used; for example, the heat treatment time can be appropriately shortened when the first temperature is higher. The heat treatment time also varies slightly depending on the type of carbon source, and those skilled in the art can adjust it according to the actual situation.

[0089] In some embodiments, the second temperature T2 < 400°C. When the second temperature T2 is too high, the carbon framework material obtained in S20 reacts rapidly with air, resulting in insufficient air diffusion into the material particles. This leads to the pore structure formed during the pore-forming process being concentrated on the particle surface. Furthermore, excessive pore formation on the particle surface makes the pore structure prone to collapse and pore merging, resulting in a higher number and proportion of larger pores, which in turn reduces the capacity and initial coulombic efficiency of the hard carbon. Furthermore, the second temperature T2 should not be too low either. When the second temperature T2 is low, the carbon framework material obtained in S20 reacts slowly with air, resulting in weak pore-forming and thus lower capacity of the hard carbon.

[0090] In some embodiments, the second temperature T2 can be 200℃-395℃, 220℃-395℃, 240℃-395℃, 260℃-395℃, 280℃-395℃, 300℃-395℃, 320℃-395℃, 340℃-395℃, 360℃-395℃, 200℃-380℃, 220℃-380℃, 24℃-395 ...95℃, 220℃-380℃, 24℃-395℃, 24℃-395℃, 24℃-395℃, 200℃-380℃, 220℃-380℃, 24℃-395℃, 24℃-395℃, 24℃-395℃, 24℃-395℃, 24℃-395℃, 24℃-395℃, 24℃-395℃, 24℃-395℃, 24℃-395℃, 24℃-395℃, 24℃-395℃, 24℃-39 0℃-380℃, 260℃-380℃, 270℃-350℃, 270℃-380℃, 280℃-380℃, 300℃-380℃, 320℃-380℃, 340℃-380℃, 360℃-380℃, 290℃-380℃, 290℃-350℃, 200℃-360℃, 220℃-360℃, 24 0℃-360℃, 260℃-360℃, 280℃-360℃, 300℃-360℃, 320℃-360℃, 200℃-340℃, 220℃-340℃, 240℃-340℃, 260℃-340℃, 280℃-340℃, 300℃-340℃, 200℃-320℃, 220℃-320℃, 2 The temperature ranges are 40℃-320℃, 260℃-320℃, 280℃-320℃, 200℃-300℃, 220℃-300℃, 240℃-300℃, 260℃-300℃, 200℃-280℃, 220℃-280℃, 240℃-280℃, 200℃-260℃, 220℃-260℃, or 200℃-240℃. When the second temperature T2 is within a suitable range, the carbon framework material particles obtained in S20 can form a suitable and abundant pore structure from the inside out, improving the active ion and electron transport performance, thereby further improving the capacity and first coulombic efficiency of hard carbon.

[0091] In some embodiments, the heat treatment time t2 is 1h-12h. Those skilled in the art can select a suitable heat treatment time within the above range based on the second temperature used; for example, the heat treatment time can be appropriately shortened when the second temperature is higher. The heat treatment time also varies slightly depending on the type of carbon source, and those skilled in the art can adjust it according to the actual situation.

[0092] In some embodiments, optionally, when T2≤270℃, t2≥10h; further, when 200℃≤T2≤270℃, 10h≤t2≤12h.

[0093] In some embodiments, optionally, when 270℃ < T2 < 400℃, t2 ≤ 8h; further, when 270℃ < T2 < 400℃, 1h ≤ t2 ≤ 8h.

[0094] In some embodiments, 270℃≤T2≤380℃ and 1h≤t2≤4h. When both conditions are simultaneously within the given range, the capacity and initial coulombic efficiency of hard carbon can be further improved.

[0095] In some embodiments, the third temperature T3 is 1000℃-1600℃. When the third temperature T3 is low, a large number of extremely small pore structures in the second intermediate product structure obtained in S30 are retained. However, these extremely small pore structures have low strength and are easily destroyed during subsequent charge-discharge processes, leading to a decrease in the utilization rate of active ion storage sites, and consequently a decrease in both the capacity and initial coulombic efficiency of the hard carbon. Furthermore, the third temperature T3 should not be too high. When the third temperature T3 is high, the graphite microcrystalline structure formed during the carbonization process is prone to micro-movement, forcing adjustments to the formed pore structure, resulting in a significant reduction in the pore structure and a decrease in the capacity of the hard carbon.

[0096] In some embodiments, the third temperature T3 can be 1100℃-1600℃, 1200℃-1600℃, 1300℃-1600℃, 1400℃-1600℃, 1500℃-1600℃, 1000℃-1500℃, 1100℃-1500℃, 1200℃-1500℃, 1300℃-1500℃, 1400℃-1500℃. The temperature ranges from 1000℃ to 1400℃, 1100℃ to 1400℃, 1200℃ to 1400℃, 1300℃ to 1400℃, 1000℃ to 1300℃, 1000℃ to 1350℃, 1100℃ to 1300℃, 1200℃ to 1300℃, 1000℃ to 1200℃, 1100℃ to 1200℃, or 1000℃ to 1100℃. When the third temperature T3 is within a suitable range, it can better improve the ordering of hard carbon, enabling it to possess both high capacity and high initial coulombic efficiency.

[0097] In some embodiments, the heat treatment time t3 is 1h-12h. Those skilled in the art can select a suitable heat treatment time within the above range based on the third temperature used; for example, when the third temperature is higher, the heat treatment time can be appropriately shortened. The heat treatment time also varies slightly depending on the type of carbon source, and those skilled in the art can adjust it according to the actual situation.

[0098] In some embodiments, the heating rate in S20 may be 1℃ / min-10℃ / min. However, this application is not limited to this, and the heating rate may be adjusted according to actual conditions. For example, the heating rate may be 1℃ / min-9℃ / min, 1℃ / min-8℃ / min, 1℃ / min-7℃ / min, 1℃ / min-6℃ / min, 1℃ / min-5℃ / min, 1℃ / min-4℃ / min, 1℃ / min-3℃ / min, 1℃ / min-2℃ / min, 2℃ / min-10℃ / min, 3℃ / min-9℃ / min, 4℃ / min-8℃ / min, or 5℃ / min-7℃ / min.

[0099] In some embodiments, the heating rate in S30 may be 1℃ / min-5℃ / min. However, this application is not limited to this, and the heating rate may be adjusted according to actual conditions. For example, the heating rate may be 1℃ / min-4℃ / min, 1℃ / min-3℃ / min, 1℃ / min-2℃ / min, 2℃ / min-5℃ / min, 2℃ / min-4℃ / min, 2℃ / min-3℃ / min, 3℃ / min-5℃ / min, 3℃ / min-4℃ / min, or 4℃ / min-5℃ / min.

[0100] In some embodiments, the heating rate in S40 is 1℃ / min-10℃ / min. However, this application is not limited to this, and the heating rate can be adjusted according to actual conditions. For example, the heating rate can be 1℃ / min-9℃ / min, 1℃ / min-8℃ / min, 1℃ / min-7℃ / min, 1℃ / min-6℃ / min, 1℃ / min-5℃ / min, 1℃ / min-4℃ / min, 1℃ / min-3℃ / min, 1℃ / min-2℃ / min, 2℃ / min-10℃ / min, 3℃ / min-9℃ / min, 4℃ / min-8℃ / min, or 5℃ / min-7℃ / min.

[0101] In some embodiments, after S20 and before S30, the preparation method further includes: S21, crushing the first intermediate product obtained in S20, for example, crushing it to the mm level. At this time, the agglomerated first intermediate product can be crushed to ensure that the first intermediate product can fully contact the air during subsequent processing. Of course, in some embodiments, this step can be omitted.

[0102] In some embodiments, after S30 and before S40, the preparation method further includes: S31, crushing the second intermediate product obtained in S30. For example, in some embodiments, the volumetric particle size Dv50 of the crushed second intermediate product is 2 μm-15 μm, optionally 4 μm-8 μm. In some embodiments, the volumetric particle size Dv90 of the crushed second intermediate product is 5 μm-25 μm, optionally 8 μm-15 μm. In some embodiments, the volumetric particle size Dv50 of the crushed second intermediate product is 4 μm-8 μm and the volumetric particle size Dv90 is 8 μm-15 μm. Of course, in some embodiments, this step can be omitted.

[0103] In some embodiments, the preparation method further includes: S50, crushing the hard carbon obtained in S40. At this stage, the agglomerated hard carbon can be crushed to meet the required particle size, facilitating the preparation of the negative electrode slurry and negative electrode sheet. Of course, in some embodiments, this step can be omitted.

[0104] This application does not impose any particular limitation on the type of carbon source. In some embodiments, the carbon source includes one or more of polymers, resins, and biomass materials.

[0105] As an example, the polymer includes one or more of polyaniline and polypyrrole.

[0106] As an example, the resin includes one or more of phenolic resin and epoxy resin. Optionally, the phenolic resin includes one or more of phenol-formaldehyde resin, resorcinol-formaldehyde resin, p-resorcinol-formaldehyde resin, and phenol-furfural resin.

[0107] As an example, the biomass material includes one or more of starch, glucose, fructose, maltose, sucrose, cellulose, hemicellulose, and lignin. Optionally, the starch includes one or more of cereal starch, tuber starch, and legume starch. Examples of cereal starch may include, but are not limited to, one or more of corn starch, rice starch, millet starch, sorghum starch, wheat starch, oat starch, buckwheat starch, and rye starch; examples of tuber starch may include, but are not limited to, one or more of cassava starch, potato starch, sweet potato starch, yam starch, and taro starch; and examples of legume starch may include, but are not limited to, one or more of mung bean starch, broad bean starch, pea starch, and cowpea starch.

[0108] Optionally, the carbon source is selected from biomass materials. Further, the carbon source is selected from one or more of starch, glucose, fructose, maltose, sucrose, cellulose, hemicellulose, and lignin. The aforementioned biomass materials have fewer types of heteroatoms (atoms other than O and H), higher carbon content, and lower cost, thus avoiding the influence of heteroatoms on the hard carbon preparation process. Simultaneously, the aforementioned biomass materials can cross-link at lower temperatures, which is beneficial for forming a more stable framework structure, thereby obtaining hard carbon with a suitable pore structure.

[0109] In this application, the biomass material can be obtained commercially or extracted from plants.

[0110] In some embodiments, the method for preparing hard carbon includes the following steps: providing a carbon source, wherein the carbon source is a biomass material, optionally starch; heat-treating the carbon source in an inert atmosphere at 300°C or less for 4-60 hours to obtain a first intermediate product; heat-treating the obtained first intermediate product in an air atmosphere at 400°C or less for 1-12 hours to obtain a second intermediate product; and carbonizing the obtained second intermediate product in an inert atmosphere at 1000°C-1600°C for 1-12 hours to obtain hard carbon.

[0111] In some embodiments, the method for preparing hard carbon includes the following steps: providing a carbon source, wherein the carbon source is a biomass material, optionally starch; heat-treating the carbon source in an inert atmosphere at 300°C or less for 4-60 hours to obtain a first intermediate product; heat-treating the obtained first intermediate product in an air atmosphere at 400°C or less for 1-12 hours to obtain a second intermediate product; crushing the obtained second intermediate product to a volumetric particle size Dv50 of 2μm-15μm and / or Dv90 of 5μm-25μm, and then carbonizing it in an inert atmosphere at 1000°C-1600°C for 1-12 hours to obtain hard carbon. Secondary batteries

[0112] A third aspect of this application provides a secondary battery. This application does not particularly limit the type of secondary battery; for example, the secondary battery can be a lithium-ion battery, a sodium-ion battery, etc., and specifically, a sodium-ion secondary battery. Typically, a secondary battery includes a positive electrode, a negative electrode, and an electrolyte. During the charging and discharging process of the secondary battery, active ions repeatedly insert and extract between the positive and negative electrode, and the electrolyte serves to conduct these active ions between the positive and negative electrode. [Negative electrode plate]

[0113] In some embodiments, the negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector. For example, the negative current collector has two surfaces opposite each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative current collector.

[0114] In some embodiments, the negative electrode film layer includes at least one of the hard carbon described in any embodiment of the first aspect of the present application and the hard carbon prepared by the method described in any embodiment of the second aspect of the present application.

[0115] In some embodiments, the negative electrode film layer may further include other negative electrode active materials besides the hard carbon described above. In some embodiments, the other negative electrode active materials include, but are not limited to, at least one of natural graphite, artificial graphite, soft carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based materials may include at least one of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy materials. The tin-based materials may include at least one of elemental tin, tin oxide, and tin alloy materials. This application is not limited to these materials, and other conventionally known materials that can be used as negative electrode active materials for secondary batteries may also be used.

[0116] In some embodiments, the negative electrode film layer may optionally include a negative electrode conductive agent. This application does not impose any particular limitation on the type of negative electrode conductive agent. As an example, the negative electrode conductive agent may include at least one of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, based on the total mass of the negative electrode film layer, the mass percentage content of the negative electrode conductive agent is ≤5%.

[0117] In some embodiments, the negative electrode film layer may optionally include a negative electrode binder. This application does not impose any particular limitation on the type of negative electrode binder. As an example, the negative electrode binder may include at least one of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, waterborne acrylic resins (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS). In some embodiments, the mass percentage of the negative electrode binder is ≤5% based on the total mass of the negative electrode film layer.

[0118] In some embodiments, the negative electrode film layer may optionally include other additives. As an example, other additives may include thickeners, such as sodium carboxymethyl cellulose (CMC-Na), PTC thermistor materials, etc. In some embodiments, the mass percentage of the other additives is ≤2% based on the total mass of the negative electrode film layer.

[0119] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, copper foil may be used. The composite current collector may include a polymer substrate and a metal material layer formed on at least one surface of the polymer substrate. As an example, the metal material may be selected from at least one of copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. As an example, the polymer substrate may be selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0120] The negative electrode film layer is typically formed by coating a negative electrode slurry onto a negative electrode current collector, followed by drying and cold pressing. The negative electrode slurry is typically formed by dispersing the negative electrode active material, optional conductive agent, optional binder, and other optional additives in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP) or deionized water, but is not limited to these.

[0121] The negative electrode sheet does not exclude other additional functional layers besides the negative electrode film layer. For example, in some embodiments, the negative electrode sheet of this application further includes a conductive undercoat layer (e.g., composed of a conductive agent and an adhesive) sandwiched between the negative electrode current collector and the negative electrode film layer and disposed on the surface of the negative electrode current collector. In other embodiments, the negative electrode sheet of this application further includes a protective layer covering the surface of the negative electrode film layer. [Positive electrode plate]

[0122] In some embodiments, the positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector. For example, the positive current collector has two surfaces opposite each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0123] The positive electrode current collector can be a metal foil or a composite current collector. As an example of a metal foil, aluminum foil can be used. The composite current collector may include a polymer material substrate and a metal material layer formed on at least one surface of the polymer material substrate. As an example, the metal material can be selected from one or more of aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. As an example, the polymer material substrate can be selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0124] The positive electrode film typically comprises a positive electrode active material, an optional binder, and an optional conductive agent. The positive electrode film is typically formed by coating a positive electrode slurry onto the positive electrode current collector, followed by drying and cold pressing. The positive electrode slurry is typically formed by dispersing the positive electrode active material, an optional conductive agent, an optional binder, and any other components in a solvent and stirring until homogeneous. The solvent may be N-methylpyrrolidone (NMP), but is not limited to it. As an example, the binder used for the positive electrode film may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins. As an example, the conductive agent used for the positive electrode film includes one or more of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0125] The positive electrode active material may be a positive electrode active material known in the art for use in secondary batteries.

[0126] When the secondary battery of this application is a lithium-ion battery, the positive electrode active material may include one or more of lithium transition metal oxides, lithium-containing phosphates with an olivine structure, and their respective modified compounds. Examples of lithium transition metal oxides may include, but are not limited to, one or more of lithium cobalt oxides, lithium nickel oxides, lithium manganese oxides, lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, lithium nickel cobalt manganese oxides, lithium nickel cobalt aluminum oxides, and their respective modified compounds. Examples of lithium-containing phosphates with an olivine structure may include, but are not limited to, lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, lithium manganese iron phosphate and carbon composites, and their respective modified compounds. This application is not limited to these materials, and other conventionally known materials that can be used as positive electrode active materials for lithium-ion batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more.

[0127] In some embodiments, to further improve the energy density of the secondary battery, the positive electrode active material for the lithium-ion battery may include one or more of the lithium transition metal oxides and their modified compounds shown in Formula 1.

[0128] Li a Ni b Co c M d O e A f Formula 1,

[0129] In Formula 1, 0.8≤a≤1.2, 0.5≤b<1, 0<c<1, 0<d<1, 1≤e≤2, 0≤f≤1, M is selected from one or more of Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti and B, and A is selected from one or more of N, F, S and Cl.

[0130] As an example, positive electrode active materials for lithium-ion batteries may include LiCoO2, LiNiO2, LiMnO2, LiMn2O4, and LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), LiNi 0.85 Co 0.15 Al 0.05 One or more of O2, LiFePO4, and LiMnPO4.

[0131] When the secondary battery of this application is a sodium-ion battery, particularly a sodium-ion secondary battery, the positive electrode active material may include one or more of the following: sodium-containing transition metal oxides, polyanionic materials (such as phosphates, fluorophosphates, pyrophosphates, sulfates, etc.), and Prussian blue materials. This application is not limited to these materials; other conventionally known materials that can be used as positive electrode active materials for sodium-ion batteries may also be used. These positive electrode active materials may be used individually or in combination of two or more.

[0132] As an example, positive electrode active materials for sodium-ion batteries may include NaFeO2, NaCoO2, NaCrO2, NaMnO2, NaNiO2, and NaNi 1 / 2 Ti 1 / 2 O2, NaNi 1 / 2 Mn 1 / 2 O2, Na 2 / 3 Fe 1 / 3 Mn 2 / 3 O2, NaNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, NaFePO4, NaMnPO4, NaCoPO4, Prussian blue materials, with the general formula A a M b(PO4) c O x Y 3-x Materials (where A is selected from H) + Li + Na + K + and NH4 + M is one or more of the following: M is a transition metal cation, which may be one or more of V, Ti, Mn, Fe, Co, Ni, Cu and Zn; Y is a halide anion, which may be one or more of F, Cl and Br; and 0 < a ≤ 4, 0 < b ≤ 2, 1 ≤ c ≤ 3, 0 ≤ x ≤ 2.

[0133] In this application, the modified compounds of the above-mentioned positive electrode active materials can be used to dope or surface-coat the positive electrode active materials.

[0134] The positive electrode sheet of this application does not exclude other additional functional layers besides the positive electrode film layer. For example, in some embodiments, the positive electrode sheet of this application further includes a conductive undercoating layer (e.g., composed of a conductive agent and a binder) sandwiched between the positive current collector and the positive electrode film layer and disposed on the surface of the positive current collector. In other embodiments, the positive electrode sheet of this application further includes a protective layer covering the surface of the positive electrode film layer. [Electrolytes]

[0135] This application does not impose any particular limitation on the type of electrolyte, which can be selected according to actual needs. For example, the electrolyte can be selected from at least one of solid electrolytes and liquid electrolytes (i.e., electrolyte solutions).

[0136] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0137] The type of electrolyte salt is not specifically limited and can be selected according to actual needs.

[0138] When the secondary battery of this application is a lithium-ion battery, as an example, the electrolyte salt may include at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP).

[0139] When the secondary battery of this application is a sodium-ion battery, particularly a sodium-ion secondary battery, the electrolyte salt may include one or more of the following: sodium hexafluorophosphate (NaPF6), sodium tetrafluoroborate (NaBF4), sodium perchlorate (NaClO4), sodium hexafluoroarsenate (NaAsF6), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium trifluoromethanesulfonate (NaTFS), sodium difluorooxalate borate (NaDFOB), sodium dioxalate borate (NaBOB), sodium difluorophosphate (NaPO2F2), sodium difluorodioxalate phosphate (NaDFOP), and sodium tetrafluorooxalate phosphate (NaTFOP).

[0140] The type of solvent is not specifically limited and can be selected according to actual needs. In some embodiments, as an example, the solvent may include one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl ester carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0141] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature performance, and additives that improve battery low-temperature power performance. [Isolation membrane]

[0142] Secondary batteries using electrolytes, and some secondary batteries using solid electrolytes, also include a separator. The separator is disposed between the positive electrode and the negative electrode, serving as a barrier. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0143] In some embodiments, the material of the separator can be selected from one or more of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer may be the same or different.

[0144] In some embodiments, the positive electrode, the separator, and the negative electrode can be fabricated into an electrode assembly using a winding process or a stacking process.

[0145] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0146] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0147] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. Figure 2 This is an example of a square-structured secondary battery 5.

[0148] In some embodiments, such as Figure 3 As shown, the outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 is used to cover the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be adjusted according to requirements.

[0149] The method for preparing the secondary battery described in this application is well known. In some embodiments, a positive electrode, a separator, a negative electrode, and an electrolyte can be assembled to form a secondary battery. As an example, the positive electrode, separator, and negative electrode can be formed into an electrode assembly through a winding or stacking process. The electrode assembly is then placed in an outer packaging, dried, and injected with an electrolyte. After vacuum sealing, settling, formation, and shaping, a secondary battery is obtained.

[0150] In some embodiments of this application, the secondary battery according to this application can be assembled into a battery module. The number of secondary batteries contained in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module.

[0151] Figure 4 This is a schematic diagram of battery module 4 as an example. Figure 4As shown, in battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0152] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0153] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0154] Figure 5 and Figure 6 This is a schematic diagram of battery pack 1 as an example. Figure 5 and Figure 6 As shown, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3. The upper body 2 covers the lower body 3, forming a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box. Electrical appliances

[0155] This application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack described in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device can be, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0156] The electrical device can be equipped with a secondary battery, battery module, or battery pack according to its usage requirements.

[0157] Figure 7 This is a schematic diagram of an example electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the device's requirements for high power and high energy density, a battery pack or battery module can be used.

[0158] Another example of an electrical device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use rechargeable batteries as their power source. Example

[0159] The following embodiments describe the disclosure of this application in more detail. These embodiments are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of the disclosure of this application. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on mass, and all reagents used in the embodiments are commercially available or synthesized by conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available. Example 1

[0160] Corn starch, used as a carbon source, was placed in a tube furnace and heated to 300℃ (first temperature T1) at 5℃ / min under a nitrogen atmosphere, and then held at that temperature for 18h (heat treatment time t1) to obtain a first intermediate product. The nitrogen atmosphere was then changed to an air atmosphere, and the temperature was increased to 300℃ (second temperature T2) at 3℃ / min, and then held at that temperature for 2h (heat treatment time t2) to obtain a second intermediate product. The obtained second intermediate product was crushed to a volumetric particle size Dv50 of 4μm-8μm and a Dv90 of 8μm-15μm, and then heated to 1100℃ (third temperature T3) at 5℃ / min under a nitrogen atmosphere, and then held at that temperature for 12h (heat treatment time t3) to obtain hard carbon. Example 2-20

[0161] The preparation method of hard carbon is similar to that in Example 1, except that the preparation process parameters of hard carbon are adjusted, as detailed in Table 1. Comparative Example 1

[0162] Corn starch, used as a carbon source, was placed in a tube furnace and heated to 240°C at 3°C / min under a nitrogen atmosphere, and then held for 36 h to obtain a first intermediate product. The first intermediate product was crushed to a volumetric particle size Dv50 of 4μm-8μm and a Dv90 of 8μm-15μm, and then heated to 1200°C at 5°C / min under a nitrogen atmosphere and held for 12 h to obtain hard carbon. Comparative Example 2

[0163] Corn starch, used as a carbon source, was placed in a tube furnace and heated to 240°C at 3°C / min under an air atmosphere, and then held for 36 h to obtain a first intermediate product. The first intermediate product was crushed to a volumetric particle size Dv50 of 4μm-8μm and a Dv90 of 8μm-15μm, and then heated to 1200°C at 5°C / min under a nitrogen atmosphere and held for 12 h to obtain hard carbon. Comparative Example 3

[0164] Corn starch, used as a carbon source, was placed in a tube furnace and heated to 300°C at 3°C / min under an air atmosphere and held for 2 hours to obtain a first intermediate product. The first intermediate product was crushed to a volumetric particle size Dv50 of 4μm-8μm and a Dv90 of 8μm-15μm, and then heated to 1200°C at 5°C / min under a nitrogen atmosphere and held for 12 hours to obtain hard carbon. Comparative Examples 4-6

[0165] The preparation method of hard carbon is similar to that in Example 1, except that the preparation process parameters of hard carbon are adjusted, as detailed in Table 1.

[0166] The hard carbon prepared in each embodiment and comparative example was thoroughly mixed with styrene-butadiene rubber (SBR) as a binder, sodium carboxymethyl cellulose (CMC-Na) as a thickener, and carbon black as a conductive agent in a mass ratio of 96.2:1.8:1.2:0.8 in an appropriate amount of deionized water to form a uniform negative electrode slurry. The negative electrode slurry was uniformly coated onto the surface of the copper foil current collector and dried in an oven 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. NaPF6 was then dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L. A CR2430 coin cell was then assembled in an argon-protected glove box using a sodium metal sheet as the counter electrode and a polyethylene (PE) film as the separator.

[0167] At 25°C, the coin cells prepared in each example and comparative example were first discharged to 0V at a constant current density of 10mA / g, and the first discharge capacity of the coin cells was recorded; then, they were charged to 2.0V at a constant current density of 10mA / g, and the first charge capacity of the coin cells was recorded.

[0168] The specific capacity of hard carbon (mAh / g) = the first charge capacity of the button cell / the mass of the hard carbon.

[0169] The initial coulombic efficiency (%) of hard carbon is equal to the first charge capacity of the coin cell / the first discharge capacity of the coin cell × 100%.

[0170] The test results of Examples 1-20 and Comparative Examples 1-6 are shown in Table 2.

[0171] Figure 8 This is a scanning electron microscope (SEM) image of the hard carbon prepared in Example 2, from... Figure 8 It can be seen that the hard carbon obtained by the preparation method of this application has a regular morphology and uniform size.

[0172] Figure 9 and Figure 10The figures are nitrogen adsorption isotherms measured at 77 K for the hard carbon prepared in Example 2 and Comparative Example 1, respectively.

[0173] like Figure 9 As shown, the nitrogen adsorption isotherm of the hard carbon prepared in Example 2, measured at 77 K, satisfies... Figure 1 In the type I adsorption isotherm, the relative pressure of nitrogen P / P0 is 10. -8 The total nitrogen adsorption amount between 0.035 and 117.1 cm⁻¹ 3 (STP) / g, the total nitrogen adsorption amount for nitrogen with relative pressures P / P0 between 0.035 and 1 is 15.5 cm³. 3 (STP) / g, and satisfying V2 / V1≤0.20. As shown in Table 2, the hard carbon prepared in Example 2 has a specific capacity of 403 mAh / g and an initial coulombic efficiency of 85.4%. Although the mechanism is not yet clear, the inventors speculate that one possible reason is that the hard carbon prepared in Example 2 has a suitable and well-developed pore structure, providing more active ion storage sites, thus resulting in a higher specific capacity; simultaneously, due to the well-developed pore structure of the hard carbon, the active ions stored in the active ion storage sites are easily released, thus the initial coulombic efficiency of the hard carbon is also higher.

[0174] Comparative Example 1 involves subjecting corn starch to a low-temperature heat treatment process in an inert atmosphere, followed by direct high-temperature carbonization in an inert atmosphere. For example... Figure 10 As shown, the nitrogen adsorption isotherm measured at 77 K for the hard carbon prepared in Comparative Example 1 does not satisfy the requirements. Figure 1 The type I adsorption isotherm in the text is not similar to the type I adsorption isotherm in the text. Figure 1 The type II adsorption isotherm of the prepared hard carbon at 77 K also failed to satisfy the conditions V2 / V1≤0.20 and 20≤V1≤150. Table 2 shows that the specific capacity of the hard carbon prepared in Comparative Example 1 was only 251 mAh / g, and the initial coulombic efficiency was only 77.7%. This may be because the hard carbon prepared in Comparative Example 1 has fewer microporous structures, failing to provide sufficient active ion storage sites, and the utilization rate of the few available sites is also low. Furthermore, the hard carbon prepared in Comparative Example 1 has an excessively high proportion of larger pores (e.g., mesopores or macropores). When the hard carbon is used to prepare a secondary battery, the electrolyte wetting area inside the hard carbon is too large, further reducing the utilization rate of the active ion storage sites.

[0175] In Comparative Examples 2 and 3, corn starch was subjected to a low-temperature heat treatment in air followed by high-temperature carbonization in an inert atmosphere. The resulting hard carbon also failed to simultaneously exhibit both high specific capacity and high initial coulombic efficiency. This may be because the hard carbon prepared in Comparative Examples 2 and 3 has fewer micropores, limiting the number of active ion storage sites and resulting in low utilization of the limited number available. Furthermore, the hard carbon prepared in Comparative Examples 2 and 3 has an excessively high proportion of large pores. When the hard carbon is used to fabricate a secondary battery, the electrolyte-wetting area within the hard carbon is too large, further reducing the utilization rate of the active ion storage sites.

[0176] Comparative Examples 4-6 involve simultaneously subjecting corn starch to low-temperature heat treatment in an inert atmosphere, secondary low-temperature heat treatment in air, and high-temperature carbonization in an inert atmosphere. However, the heat treatment temperature and time are not reasonable, resulting in the adsorption isotherms of the prepared hard carbon failing to simultaneously satisfy V2 / V1≤0.20 and 20≤V1≤150. Consequently, the prepared hard carbon cannot simultaneously possess both high specific capacity and high initial coulombic efficiency.

[0177] In Comparative Example 4, the heat treatment time in air atmosphere during the preparation of hard carbon was too long. This resulted in excessive pore formation in the hard carbon, making its pore structure prone to collapse and pore merging. The number and proportion of larger pores within the hard carbon increased. When the hard carbon was used to fabricate a secondary battery, the increased proportion of the electrolyte-wetting area inside the hard carbon reduced the utilization rate of active ion storage sites, making it difficult for the hard carbon to simultaneously achieve both high specific capacity and high initial coulombic efficiency.

[0178] Comparative Example 5 used an excessively low third temperature (T3) during the preparation of hard carbon, resulting in a large number of extremely small pore structures in the prepared hard carbon. These extremely small pore structures have low strength and are easily damaged or collapsed, leading to pore merging. This increased the number and proportion of larger pores within the hard carbon. When the hard carbon was used to prepare a secondary battery, the proportion of the electrolyte-wetting area inside the hard carbon increased, reducing the utilization rate of active ion storage sites. Consequently, the hard carbon struggled to achieve high specific capacity and high initial coulombic efficiency.

[0179] Comparative Example 6 used an excessively high third temperature T3 when preparing hard carbon. The graphite microcrystalline structure formed during the carbonization process was prone to micro-movement, which forced the hard carbon pore structure to be adjusted and the number of pores to be greatly reduced. As a result, hard carbon could not simultaneously have high specific capacity and high first coulombic efficiency.

[0180] The test results from Examples 1-20 show that when the nitrogen adsorption isotherm of hard carbon at 77 K simultaneously satisfies V2 / V1≤0.20 and 20≤V1≤150, the hard carbon exhibits both high capacity and high initial coulombic efficiency. The test results from Examples 1-20 also show that when the nitrogen adsorption isotherm of hard carbon at 77 K further simultaneously satisfies 0.08≤V2 / V1≤0.20, 70≤V1≤150, and 6≤V2≤30, the capacity and initial coulombic efficiency of the hard carbon are further improved.

[0181] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application. Table 2

Claims

1. A hard carbon, wherein the nitrogen relative pressure P / P0 in the nitrogen adsorption isotherm measured at 77 K is greater than or equal to 10. -8 The total nitrogen adsorption value between 0.035 and 0.035 is V1 cm. 3 (STP) / g, the total amount of nitrogen adsorbed when the relative pressure of nitrogen P / P0 is between 0.035 and 1 is V² cm⁻¹. 3 (STP) / g, wherein the hard carbon satisfies: V2 / V1≤0.20, where, P represents the actual pressure of nitrogen gas, and P0 represents the saturated vapor pressure of nitrogen gas at a temperature of 77K.

2. The hard carbon according to claim 1, wherein, 20≤V1≤150, optionally, 56.9≤V1≤150, more optionally, 70.4≤V1≤150.

3. The hard carbon according to claim 1 or 2, wherein, 0.05≤V2 / V1≤0.20, optionally, 0.085≤V2 / V1≤0.

195.

4. The hard carbon according to any one of claims 1-3, wherein, 0 < V2 ≤ 30, optionally, 4 ≤ V2 ≤ 30, and more optionally, 6 ≤ V2 ≤ 30.

5. The hard carbon according to any one of claims 1-4, wherein, In the Raman spectrum of the hard carbon, I d / I g The value is 1.20-1.32, I d This indicates that the Raman displacement is within 1350 ± 50 cm. -1 The d-peak intensity within the range, I g This indicates that the Raman displacement is within 1580±50cm. -1 The intensity of the g peak within the range.

6. The hard carbon according to any one of claims 1-5, wherein, In the X-ray diffraction spectrum of the hard carbon, the 2θ value corresponding to the 002 peak is between 22° and 24°.

7. The hard carbon according to any one of claims 1-6, wherein, The hard carbon satisfies at least one of the following conditions (1) to (5): (1) The volumetric particle size Dv50 of the hard carbon is 2μm-15μm, and optionally 4μm-8μm; (2) The volumetric particle size Dv90 of the hard carbon is 5μm-25μm, and optionally 8μm-15μm; (3) The specific surface area of ​​the hard carbon is less than or equal to 5 m². 2 / g, optionally 0.5m 2 / g-5m 2 / g; (4) The compacted density of the hard carbon powder under a force of 50,000 N is 0.96 g / cm³. 3 -1.05g / cm 3 ; (5) The tap density of the hard carbon is 0.80 g / cm³. 3 -0.95g / cm 3 .

8. A negative electrode sheet, comprising a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer comprising a negative electrode active material, the negative electrode active material comprising hard carbon according to any one of claims 1-7.

9. The negative electrode sheet according to claim 8, wherein, The negative electrode film layer also includes other negative electrode active materials, including at least one of natural graphite, artificial graphite, soft carbon, silicon-based materials, tin-based materials, and lithium titanate.

10. A method for preparing a negative electrode sheet, comprising the following steps: coating a negative electrode slurry onto a negative electrode current collector, drying and cold pressing to obtain a negative electrode sheet, wherein the negative electrode slurry is dried and cold pressed to form a negative electrode film layer, wherein the negative electrode slurry is formed by dispersing a negative electrode active material, an optional conductive agent, an optional binder, and other optional additives in a solvent and stirring evenly, wherein the negative electrode active material includes hard carbon according to any one of claims 1-7.

11. A secondary battery, comprising a negative electrode sheet, wherein the negative electrode sheet is a negative electrode sheet according to any one of claims 8-9, or a negative electrode sheet prepared according to the method of claim 10.

12. An electrical device comprising a secondary battery according to claim 11.

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