Secondary battery, electrical device, artificial graphite, and preparation method therefor

Abstract

A secondary battery, an electrical device, artificial graphite, and a preparation method therefor. The secondary battery comprises a negative electrode plate. The negative electrode plate comprises a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector. The negative electrode film layer comprises artificial graphite. The artificial graphite comprises a surface portion and a central portion. The surface portion is continuously or discontinuously distributed on the central portion. An interplanar crystal spacing D002 of the surface portion is greater than an interplanar crystal spacing D002 of the central portion, the interplanar crystal spacing D002 being an interlayer spacing of the graphite (002) crystal plane.

Description

Secondary batteries, electrical devices, artificial graphite and their preparation methods

[0001] Cross-references to related applications

[0002] This disclosure is based on and claims priority to Chinese Patent Application No. 202410969163.4, filed on July 18, 2024, entitled "Secondary Battery, Electrical Device, Artificial Graphite and Preparation Method Thereof", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This disclosure relates to the field of battery technology, and in particular to a secondary battery, an electrical device, artificial graphite, and a method for preparing the same. Background Technology

[0004] In recent years, as the application scope of secondary batteries has become increasingly wide, they have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power plants, as well as in many fields such as power tools, electric bicycles, electric motorcycles, electric cars, and aerospace.

[0005] With the application and promotion of rechargeable batteries, people have higher and higher requirements for the initial coulombic efficiency and storage performance of rechargeable batteries. How to improve the initial coulombic efficiency and storage performance of batteries has become an urgent technical problem to be solved. Summary of the Invention

[0006] This disclosure is made in view of the above-mentioned problems, and its object is to provide a secondary battery, an electrical device, artificial graphite, and a method for preparing the same. The secondary battery has improved initial coulombic efficiency and storage performance.

[0007] To achieve the above objectives, a first aspect of this disclosure provides a secondary battery comprising a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector, the negative electrode film layer comprising artificial graphite, the artificial graphite including a surface portion and a central portion, the surface portion being continuously or discontinuously distributed in the central portion, the interplanar spacing D002 of the surface portion being greater than the interplanar spacing D002 of the central portion, wherein the interplanar spacing D002 is the interlayer spacing of the 002 crystal planes of the graphite. Providing a surface portion with a relatively large interplanar spacing outside the central portion of the graphite can increase the interlayer spacing of the graphite, reduce the diffusion resistance of active ions, thereby improving the stability of the negative electrode active material, and thus contributing to improved battery storage performance. Furthermore, the surface portion helps to reduce the specific surface area of ​​the artificial graphite, reducing side reactions between the artificial graphite and the electrolyte, thereby reducing the loss of active ions during the first cycle, and thus improving the first coulombic efficiency of the battery.

[0008] In some embodiments, the interplanar spacing D002 of the surface portion is 0.345 nm to 0.355 nm. By controlling the interplanar spacing D002 of the surface portion within the above range, it is further beneficial to improve the stability of the material and obtain a graphite material with high specific capacity.

[0009] In some embodiments, the interplanar spacing D002 at the center is 0.335 nm to 0.345 nm. By controlling the interplanar spacing D002 at the center within the above range, it is beneficial to improve the energy density of the battery.

[0010] In some implementations, the artificial graphite satisfies at least one of the following:

[0011] (1) The BET specific surface area of ​​artificial graphite is 0.6 m². 2 / g to 1.4m 2 / g;

[0012] (2) I of artificial graphite D / I G The value ranges from 0.05 to 0.15, where I D This indicates that the Raman spectrum is at 1350±50 cm⁻¹. -1 D peak intensity at I G This indicates that the Raman spectrum is at 1580±50 cm⁻¹. -1 The intensity of the G peak at that location;

[0013] (3) The volume average particle size Dv50 of artificial graphite is 12 μm to 22 μm;

[0014] (4) The compaction density of artificial graphite powder at 50000N is 1.93g / cc to 2.15g / cc;

[0015] (5) The specific capacity of artificial graphite is 355 mAh / g to 365 mAh / g.

[0016] In some embodiments, the specific surface area of ​​the artificial graphite is 0.85 m². 2 / g to 1.18m 2 / g. By controlling the BET specific surface area of ​​artificial graphite within the above range, it is beneficial to further improve the first coulombic efficiency of the battery.

[0017] In some implementations, the I of artificial graphite D / I G The value ranges from 0.067 to 0.087. By using I... D / I G Keeping the value within the above range is beneficial for secondary batteries to maintain high energy density and high capacity.

[0018] A second aspect of this disclosure provides an electrical device including a secondary battery as described in the first aspect.

[0019] This disclosure provides a third aspect of artificial graphite, comprising a surface portion and a central portion. The surface portion is continuously or discontinuously distributed in the central portion, and the interplanar spacing D002 of the surface portion is larger than that of the central portion. Providing a surface portion with a relatively larger interplanar spacing on the central portion can increase the interlamellar spacing of the graphite, reduce the diffusion resistance of active ions, thereby improving the cycle stability of the negative electrode active material and contributing to improved battery storage performance. Furthermore, the surface portion can reduce the specific surface area of ​​the artificial graphite, reducing side reactions between the artificial graphite and the electrolyte, thereby reducing the loss of active ions (e.g., active ions) during the first cycle, thus improving the initial coulombic efficiency of the battery.

[0020] In some embodiments, the interplanar spacing D002 of the surface portion is 0.345 nm to 0.355 nm. By controlling the interplanar spacing D002 of the surface portion within the above range, it is further beneficial to improve the stability of the material and obtain a graphite material with high specific capacity.

[0021] In some embodiments, the interplanar spacing D002 at the center is 0.335 nm to 0.345 nm. By controlling the interplanar spacing D002 at the center within the above range, it is beneficial to further improve the energy density of the battery.

[0022] In some implementations, the artificial graphite satisfies at least one of the following:

[0023] (1) The BET specific surface area of ​​artificial graphite is 0.6 m². 2 / g to 1.4m 2 / g;

[0024] (2) I of artificial graphite D / I G The value ranges from 0.05 to 0.15, where I D This indicates that the Raman spectrum is at 1350±50 cm⁻¹. -1 D peak intensity at I G This indicates that the Raman spectrum is at 1580±50 cm⁻¹. -1 The intensity of the G peak at that location.

[0025] This disclosure provides a method for preparing artificial graphite, comprising: mixing a binder and an artificial graphite precursor, and then granulating the mixture to obtain a first mixture; subjecting the first mixture to a first heat treatment to obtain a second mixture; mixing the second mixture with a carbon-containing additive to obtain a third mixture; and subjecting the third mixture to a second heat treatment to obtain artificial graphite, wherein the artificial graphite includes a surface portion and a central portion, the surface portion being continuously or discontinuously distributed in the central portion, and the interplanar spacing D002 of the surface portion being greater than the interplanar spacing D002 of the central portion, wherein the interplanar spacing D002 is the interlayer spacing of the 002 crystal plane of graphite.

[0026] In the preparation method disclosed herein, by coating the surface of the second mixture with a carbon-containing additive before the second heat treatment, oxygen loss of the second mixture during the second heat treatment can be reduced, and defects on the surface / bulk phase of the second mixture can be repaired, resulting in a material with fewer defects and a lower BET specific surface area. Simultaneously, during the second heat treatment, the carbon-containing additive itself can rearrange to form a graphite material with a continuous lattice structure, thereby reducing side reactions between the obtained material and the electrolyte. The artificial graphite prepared by this method, when used as a negative electrode material, can improve the initial coulombic efficiency and storage performance of the battery.

[0027] In some embodiments, the artificial graphite precursor includes one or more of the following: calcined coke, petroleum-based non-needle coke, petroleum-based needle coke, coal-based non-needle coke, and coal-based needle coke. Using the above-mentioned artificial graphite precursors is beneficial for improving the energy density of the obtained artificial graphite.

[0028] In some embodiments, the precursor for artificial graphite includes calcined coke. Using calcined coke as a precursor for artificial graphite helps to further improve the energy density of the resulting artificial graphite.

[0029] In some embodiments, the carbon-containing additive includes at least one of solid asphalt, liquid asphalt, and polyacrylonitrile. The aforementioned carbon-containing additive is a material capable of high-temperature graphitization, and its use facilitates the formation of a surface portion with suitable interplanar spacing.

[0030] In some embodiments, the carbon-containing additive includes liquid asphalt. Using liquid asphalt as the carbon-containing additive facilitates uniform coating of the carbon-containing additive on the surface of the second mixture, thereby further reducing defects in the resulting material and lowering the BET specific surface area of ​​the resulting material.

[0031] In some embodiments, the coking value of the liquid bitumen is 10% to 20%. Liquid bitumen with a coking value of 10% to 20% has suitable heating stability, adhesion and aging properties for this disclosure, can effectively form a surface portion with a suitable interplanar spacing on the central surface, and is also beneficial for processing.

[0032] In some embodiments, the amount of carbon-containing additive added is less than 10% relative to the mass of the artificial graphite precursor. By controlling the amount of carbon-containing additive added within the above range, it is beneficial to maintain a high specific weight of the resulting artificial graphite.

[0033] In some embodiments, the amount of carbon-containing additive added is 0.5% to 6% relative to the mass of the artificial graphite precursor. By controlling the amount of carbon-containing additive added within the above range, it is beneficial for the artificial graphite to maintain a high specific capacity.

[0034] In some embodiments, the first heat treatment includes a pre-carbonization treatment at 1000°C to 1500°C. By performing the first heat treatment under these conditions, the organic matter in the precursor undergoes a carbonization reaction to form a carbonaceous structure, which is beneficial for the material to obtain higher electrical conductivity and better mechanical stability.

[0035] In some embodiments, the second heat treatment includes graphitization at 2800°C to 3200°C. Performing the second heat treatment under these conditions promotes the growth of graphite grains, ultimately yielding artificial graphite with suitable interplanar spacing, resulting in artificial graphite with high energy density. Attached Figure Description

[0036] Figure 1 is a schematic diagram of a battery cell according to an embodiment of the present disclosure.

[0037] Figure 2 is an exploded view of a battery cell according to an embodiment of the present disclosure shown in Figure 1.

[0038] Figure 3 is a schematic diagram of a battery module according to one embodiment of the present disclosure.

[0039] Figure 4 is a schematic diagram of a battery pack according to one embodiment of the present disclosure.

[0040] Figure 5 is an exploded view of a battery pack according to an embodiment of the present disclosure, as shown in Figure 4.

[0041] Figure 6 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of the present disclosure.

[0042] Figure 7 is a transmission electron microscope (TEM) image of the artificial graphite of Embodiment 1 of this disclosure.

[0043] Explanation of reference numerals in the attached diagram: 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed Implementation

[0044] The following detailed description, with appropriate reference to the accompanying drawings, provides a specific embodiment of the secondary battery, electrical device, artificial graphite, and its preparation method thereof. However, unnecessary details may be omitted. For example, detailed descriptions of well-known facts 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 disclosure and are not intended to limit the subject matter of the claims.

[0045] The "range" disclosed in this disclosure is defined by a lower limit and an upper limit, whereby a given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the 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 expected. Furthermore, if minimum range values ​​1 and 2 are listed, and if maximum range values ​​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 disclosure, 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.

[0046] Unless otherwise specified, all embodiments and optional embodiments of this disclosure can be combined to form new technical solutions.

[0047] Unless otherwise specified, all technical features and optional technical features of this disclosure can be combined to form new technical solutions.

[0048] Unless otherwise specified, all steps of this disclosure may be performed sequentially or randomly, preferably sequentially. For example, if a method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if it is mentioned that the method may also include step (c), it means 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.

[0049] Unless otherwise specified, the terminology used in this disclosure has the common meaning as commonly understood by those skilled in the art.

[0050] Unless otherwise specified, the values ​​of the parameters mentioned in this disclosure can be determined using various test methods commonly used in the art, for example, according to the test methods given in this disclosure.

[0051] Artificial graphite is a graphite material prepared through chemical or physical methods. Artificial graphite has advantages such as high powder compaction density and high specific capacity, therefore, it is widely used as a negative electrode material in secondary batteries.

[0052] The preparation process of artificial graphite introduces a large number of defects (e.g., defects caused by the crushing process), which makes artificial graphite, when used as a negative electrode material, prone to reacting with the electrolyte, thereby consuming the active ions inside the battery, reducing the battery's initial coulombic efficiency, and deteriorating the battery's storage performance.

[0053] Reports have suggested that after graphitization, graphite particles can be coated with asphalt and carbonized to form a layer of amorphous carbon on the surface of the graphite particles. This amorphous carbon can repair surface / bulk defects of the graphite particles.

[0054] The above methods can reduce the specific surface area of ​​artificial graphite to some extent, reduce side reactions between artificial graphite and electrolyte, and improve the initial coulombic efficiency and storage performance of the battery. However, the reduction in the specific surface area of ​​artificial graphite is limited, and the surface part is an amorphous carbon layer. The amorphous carbon layer has high activity and low capacity, and there are many side reactions between the surface part and electrolyte, which limits the improvement of the initial coulombic efficiency and storage performance of the battery.

[0055] Based on this, this disclosure proposes a secondary battery, an electrical device, artificial graphite, and a method for preparing the same. The secondary battery exhibits improved initial coulombic efficiency and storage performance. The invention and its optional embodiments are described in more detail below.

[0056] Secondary batteries

[0057] The first aspect of this disclosure is to provide a secondary battery.

[0058] The term "secondary battery" used in this article refers to a single battery cell, a battery module, or a battery pack. These will be explained separately below.

[0059] The secondary battery disclosed herein includes a negative electrode sheet, the negative electrode sheet includes a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector, the negative electrode film layer includes artificial graphite, the artificial graphite includes a surface portion and a central portion, the surface portion is continuously or discontinuously distributed in the central portion, the interplanar spacing D002 of the surface portion is greater than the interplanar spacing D002 of the central portion, wherein the interplanar spacing D002 is the interlayer spacing of the 002 crystal plane of graphite.

[0060] In this disclosure, interplanar spacing refers to the distance between two lattices in the periodic configuration of a crystal, which is measured as follows: using Pt nanoparticles as a substrate, high-resolution testing is performed under bright field using a Thermo Scientific-Talos F200S G2 field emission transmission electron microscope (TEM): after adjusting the test interface to be clear, based on the difference in contrast between Pt nanoparticles and graphite particles, the graphite particles are located and the corresponding areas are selected. The fringe image is obtained by fast Fourier transform (FFT), and the fringe spacing is obtained by software measurement, which corresponds to the interplanar spacing.

[0061] By incorporating a surface portion with a relatively large interplanar spacing around the central portion of the graphite, the interlamellar spacing of the graphite can be increased, reducing the diffusion resistance of active ions and thus improving the stability of the negative electrode active material. This, in turn, is beneficial for enhancing the battery's storage performance. Furthermore, the arrangement of the surface portion helps reduce the specific surface area of ​​the artificial graphite, minimizing direct contact between the artificial graphite and the electrolyte. This reduces side reactions between the two, thereby minimizing the loss of active ions during the first cycle and improving the battery's initial coulombic efficiency.

[0062] In some embodiments, the thickness of the surface portion is less than or equal to 0.5 nm. Accordingly, when measuring the interplanar spacing of the surface portion, a region within the range of 0 to 0.5 nm at the contact point with the Pt nanoparticles is typically selected, and then a fringe image is obtained by Fourier transform. The interplanar spacing of the surface portion is then measured by software.

[0063] In some embodiments, the interplanar spacing D002 of the surface portion is from 0.345 nm to 0.355 nm, and optionally from 0.346 nm to 0.352 nm. For example, the interplanar spacing D002 of the surface portion is a value between 0.345 nm, 0.346 nm, 0.347 nm, 0.349 nm, 0.350 nm, 0.351 nm, 0.352 nm, 0.355 nm, or any combination of these values. By controlling the interplanar spacing D002 of the surface portion within the above range, it is further beneficial to improve the stability of the material and obtain a graphite material with high specific capacity.

[0064] In some embodiments, the interplanar spacing D002 at the center is between 0.335 nm and 0.345 nm. For example, the interplanar spacing D002 at the center is a value between 0.335 nm, 0.336 nm, 0.339 nm, 0.342 nm, 0.345 nm, or any combination of these values. Typically, in measuring the interplanar spacing at the center, samples are taken from a region extending approximately 50 nm or more from the sampling point on the surface towards the center. These samples are then subjected to a Fast Fourier Transform to obtain a fringe image, and the interplanar spacing at the center is measured using software.

[0065] By controlling the interplanar spacing D002 at the center within the aforementioned range, it is beneficial to improve the energy density of the battery.

[0066] In some embodiments, the mass ratio of the surface portion to the central portion is less than or equal to 2:100. The mass ratio of the surface portion to the central portion is a value within a range of 0.075:100, 0.45:100, 0.9:100, 2:100, or any combination of these values. Controlling the mass ratio of the surface portion to the central portion within the above range is beneficial for reducing the specific surface area of ​​the negative electrode active material, while also helping the negative electrode active material maintain a high specific capacity.

[0067] In some implementations, the artificial graphite satisfies at least one of the following:

[0068] (1) The BET specific surface area of ​​artificial graphite is 0.6 m². 2 / g to 1.4m 2 / g, optionally, the BET specific surface area is 0.85m². 2 / g to 1.2m 2 / g, optionally, the specific surface area of ​​the artificial graphite is 0.9m³. 2 / g to 1.2m 2 / g, optionally, the specific surface area of ​​the artificial graphite is 0.85m². 2 / g to 1.18m 2 / g, for example, the BET specific surface area of ​​artificial graphite is 0.6m². 2 / g, 0.7m 2 / g, 0.8m 2 / g, 0.85m 2 / g, 0.9m 2 / g, 1.0m 2 / g, 1.1m 2 / g, 1.2m 2 / g, 1.26m 2 / g, 1.3m 2 / g, 1.4m 2 / g or a value within a range of any two of these values. Controlling the BET specific surface area of ​​artificial graphite within the aforementioned range is beneficial for further improving the initial coulombic efficiency of the battery.

[0069] (2) I of artificial graphite D / I G The value ranges from 0.05 to 0.15, where I D This indicates that the Raman spectrum is at 1350±50 cm⁻¹. -1 D peak intensity at I G This indicates that the Raman spectrum is at 1580±50 cm⁻¹. -1 The intensity of the G peak at the location, optionally, I D / I G The value ranges from 0.067 to 0.096, with optional location I. D / I G The value ranges from 0.067 to 0.087, for example, I. D / I G A value between 0.05, 0.067, 0.081, 0.10, 0.15, or any combination of these values. By using I... D / I G Controlling the value within the above range is beneficial for the secondary battery to maintain a high energy density. (3) The volume average particle size Dv50 of artificial graphite is 12 μm to 22 μm, and optionally, the volume distribution average particle size Dv50 is 16.1 μm to 16.5 μm. For example, the volume average particle size Dv50 of artificial graphite is 12 μm, 14 μm, 16 μm, 16.1 μm, 16.5 μm, 18 μm, 20 μm, 22 μm or a value between any two of these values.

[0070] (4) The compacted density of artificial graphite powder at 50,000 N is between 1.93 g / cc and 2.15 g / cc, for example, the compacted density of artificial graphite powder at 50,000 N is 1.93 g / cc, 1.94 g / cc, 1.95 g / cc, 1.96 g / cc, 2.00 g / cc, 2.05 g / cc, 2.10 g / cc, 2.15 g / cc or a range of any two of these values.

[0071] (5) The specific capacity of the artificial graphite is from 355 mAh / g to 365 mAh / g, and optionally, the specific capacity is from 360.4 mAh / g to 362.1 mAh / g, for example, the specific capacity of the artificial graphite is 355 mAh / g, 356 mAh / g, 357 mAh / g, 358 mAh / g, 359 mAh / g, 360 mAh / g, 360.4 mAh / g, 361 mAh / g, 362 mAh / g, 362.1 mAh / g, 363 mAh / g, 364 mAh / g, 365 mAh / g or a value between any two of these values.

[0072] In this disclosure, the BET specific surface area of ​​artificial graphite is the total surface area per unit mass of material, which 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 testing instrument can be a Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA.

[0073] In this disclosure, the I of artificial graphite D / I G The Raman spectrum of the material is at 1350±50 cm⁻¹ -1 The intensity of peak D at 1580±50 cm⁻¹ corresponds to the Raman spectrum of the material. -1 The ratio of the G peak intensity at a given location can be used to characterize the degree of defect and graphitization in a material. It can be measured using a Raman spectrometer under the following conditions: excitation wavelength 532 nm, grating 600 lines, objective lens 50x, integration time 10 s, cumulative scan 3 times, area scan, obtaining the D and G peak intensities at 100 points, and calculating the I at 100 points. D / I G Remove the largest and smallest 30 I's. D / I G The average of the remaining 40 points is the material's I. D / I G The testing instrument can be a Horiba LabRAM HR800 Raman spectrometer.

[0074] In this disclosure, the compacted density of artificial graphite powder is the mass per unit volume of the powder material after being compressed under a certain pressure (e.g., 50,000 N), which 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., a UTM7305 type electronic pressure testing machine) in accordance with GB / T 24533-2009.

[0075] In this disclosure, the specific capacity of artificial graphite refers to the amount of electrical energy stored per unit mass of artificial graphite, which can be measured using instruments and methods known in the art. For specific details, please refer to the test methods described in the following embodiments.

[0076] In some embodiments, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

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

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

[0079] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0080] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0081] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material (artificial graphite), conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto a negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0082] Secondary batteries

[0083] The second aspect of this disclosure provides a secondary battery, which will be described below with appropriate reference to the accompanying drawings.

[0084] The term "secondary battery" used in this article refers to a single battery cell, a battery module, or a battery pack. These will be explained separately below.

[0085] Typically, a single secondary battery cell also includes a positive electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0086] Positive electrode sheet

[0087] 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, the positive electrode film layer including a positive electrode active material.

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

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

[0090] In some embodiments, when the battery cell is a lithium secondary battery, the positive electrode active material may be a positive electrode active material known in the art for lithium secondary batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this disclosure is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.1 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0091] In some embodiments, when the battery cell is a sodium secondary battery, the positive electrode active material may be a positive electrode active material known in the art for sodium secondary batteries. As examples, the positive electrode active material may include sodium transition metal oxides, polyanionic compounds, Prussian blue compounds, etc.

[0092] During the charging and discharging process of a battery, Li undergoes insertion / extraction and consumption, resulting in varying molar Li content at different discharge states. In the examples of positive electrode active materials in this disclosure, the molar Li content refers to the initial state of the material, i.e., the state before feeding. When the positive electrode active material is applied to the battery system, the molar Li content changes after charge-discharge cycles.

[0093] In the examples of positive electrode active materials in this disclosure, the molar content of O is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of O will fluctuate.

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

[0095] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0096] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0097] electrolytes

[0098] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This disclosure does not impose specific limitations on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, or entirely solid.

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

[0100] In some embodiments, the electrolyte salt may include 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 difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0101] In some embodiments, the solvent may include at least one selected from ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0102] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0103] Separating membrane

[0104] In some embodiments, the battery cell also includes a separator. This disclosure does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0105] In some embodiments, the material of the separator may include at least one selected from glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0106] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0107] In some embodiments, the battery cell may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0108] In some embodiments, the outer packaging of the battery cell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell can also be a flexible package, such as a pouch. The material of the flexible package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0109] This disclosure does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 shows a square battery cell 5 as an example.

[0110] In some embodiments, referring to FIG2, the outer packaging may include a housing 51 and a top cover assembly 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 top cover assembly 53 can cover the opening to close the receiving cavity. The positive electrode sheet, negative electrode sheet, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the battery cell 5 may be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0111] In some implementations, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0112] Figure 3 shows a battery module 4 as an example. Referring to Figure 3, in the battery module 4, multiple battery cells 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other manner. Furthermore, the multiple battery cells 5 can be fixed in place using fasteners.

[0113] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0114] 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 one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0115] Figures 4 and 5 show a battery pack 1 as an example. Referring to Figures 4 and 5, 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 box 2 and a lower box 3, with the upper box 2 covering the lower box 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0116] Electrical appliances

[0117] The second aspect of this disclosure provides an electrical device, which will be described below with appropriate reference to the accompanying drawings.

[0118] The electrical devices mentioned in the embodiments of this disclosure include the secondary batteries provided in this disclosure. The secondary battery can be used as the power source of the electrical device or as the energy storage unit of the electrical device. Electrical devices may include, but are 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.

[0119] As an electrical device, you can choose individual battery cells, battery modules, or battery packs according to your usage requirements.

[0120] Figure 6 shows an example of an electrical device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0121] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0122] Artificial graphite

[0123] The third aspect of this disclosure provides an artificial graphite, which includes a surface portion and a central portion, wherein the surface portion is continuously or discontinuously distributed in the central portion, and the interplanar spacing D002 of the surface portion is greater than the interplanar spacing D002 of the central portion.

[0124] By incorporating a surface portion with a relatively large interplanar spacing into the central part of the graphite, the interlayer spacing of the graphite can be increased, reducing the diffusion resistance of active ions and thus improving the cycle stability of the negative electrode active material, which is beneficial to improving the battery's storage performance. Furthermore, the surface portion can reduce the specific surface area of ​​the artificial graphite, minimizing side reactions between the artificial graphite and the electrolyte, thereby reducing the loss of active ions (e.g., active ions) during the first cycle and improving the battery's initial coulombic efficiency.

[0125] In some embodiments, the interplanar spacing D002 of the surface portion is 0.345 nm to 0.355 nm. By controlling the interplanar spacing D002 of the surface portion within the above range, it is beneficial to improve the stability of the material and obtain a graphite material with high specific capacity.

[0126] In some embodiments, the interplanar spacing D002 at the center is 0.335 nm to 0.345 nm. By controlling the interplanar spacing D002 at the center within the above range, it is beneficial to further improve the energy density of the battery.

[0127] In some implementations, the artificial graphite satisfies at least one of the following:

[0128] (1) The BET specific surface area of ​​artificial graphite is 0.6 m². 2 / g to 1.4m 2 / g, optionally, the BET specific surface area is 0.85m². 2 / g to 1.26m 2 / g, optionally, the specific surface area of ​​the artificial graphite is 0.9m³. 2 / g to 1.2m 2 / g, for example, the BET specific surface area of ​​artificial graphite is 0.6m². 2 / g, 0.7m 2 / g, 0.8m 2 / g, 0.85m 2 / g, 0.9m 2 / g, 1.0m 2 / g, 1.1m 2 / g, 1.2m 2 / g, 1.26m 2 / g, 1.3m 2 / g, 1.4m 2 / g or a value between any two of these values.

[0129] (2) I of artificial graphite D / I G The value ranges from 0.05 to 0.15, where I D This indicates that the Raman spectrum is at 1350±50 cm⁻¹. -1D peak intensity at I G This indicates that the Raman spectrum is at 1580±50 cm⁻¹. -1 The intensity of the G peak at the location, optionally, I D / I G The value ranges from 0.067 to 0.096, and optionally, I D / I G The value ranges from 0.067 to 0.087, for example, I. D / I G A value between 0.05, 0.067, 0.081, 0.10, 0.15, or any combination of these values.

[0130] (3) The volume average particle size Dv50 of the artificial graphite is from 12 μm to 22 μm, and optionally, the volume average particle size Dv50 is from 16.1 μm to 16.5 μm, for example, the volume average particle size Dv50 of the artificial graphite is 12 μm, 14 μm, 16 μm, 16.1 μm, 16.5 μm, 18 μm, 20 μm, 22 μm or a value between any two of these values.

[0131] (4) The compacted density of artificial graphite powder at 50,000 N is between 1.93 g / cc and 2.15 g / cc, for example, the compacted density of artificial graphite powder at 50,000 N is 1.93 g / cc, 1.94 g / cc, 1.95 g / cc, 1.96 g / cc, 2.00 g / cc, 2.05 g / cc, 2.10 g / cc, 2.15 g / cc or a range of any two of these values.

[0132] (5) The specific capacity of the artificial graphite is from 355 mAh / g to 365 mAh / g, and optionally, the specific capacity is from 360.4 mAh / g to 362.1 mAh / g, for example, the specific capacity of the artificial graphite is 355 mAh / g, 356 mAh / g, 357 mAh / g, 358 mAh / g, 359 mAh / g, 360 mAh / g, 360.4 mAh / g, 361 mAh / g, 362 mAh / g, 362.1 mAh / g, 363 mAh / g, 364 mAh / g, 365 mAh / g or a value between any two of these values.

[0133] Preparation method of artificial graphite

[0134] The fourth aspect of this disclosure provides a method for preparing artificial graphite, the method comprising steps (1) to (4).

[0135] Step (1): Mix the binder and the artificial graphite precursor, and then granulate to obtain the first mixture.

[0136] In this disclosure, the term "artificial graphite precursor" refers to the raw material used in the synthesis of artificial graphite. This disclosure does not specifically limit the type of artificial graphite precursor; artificial graphite precursors known in the art can be used. For example, artificial graphite precursors include one or more of calcined coke, petroleum-based non-needle coke, petroleum-based needle coke, coal-based non-needle coke, and coal-based needle coke. Using the above-mentioned artificial graphite precursors is beneficial for improving the energy density of the obtained artificial graphite.

[0137] In some embodiments, the precursor for artificial graphite includes calcined coke. Using calcined coke as a precursor for artificial graphite helps to further improve the energy density of the resulting artificial graphite.

[0138] In some embodiments, the artificial graphite precursor includes calcined coke. Calcinated coke includes coke formed by calcining the aforementioned petroleum-based non-needle coke, petroleum-based needle coke, coal-based non-needle coke, and coal-based needle coke.

[0139] In some embodiments, the calcined coke is pulverized to facilitate mixing with the binder.

[0140] In some embodiments, the pulverized calcined coke is shaped and graded to obtain an artificial graphite precursor with a suitable volume average particle size DV50. This disclosure does not specifically limit the method of shaping and grading; methods known in the art can be used.

[0141] This disclosure does not specifically limit the type of adhesive; adhesives known in the art can be used. For example, adhesives may include solid asphalt, liquid asphalt, etc.

[0142] In this disclosure, solid asphalt refers to asphalt that is solid at room temperature, and liquid asphalt refers to asphalt that is liquid at room temperature.

[0143] This disclosure does not specify the method of granulation, and any granulation method known in the art can be used.

[0144] Step (2): Perform a first heat treatment on the first mixture to obtain a second mixture.

[0145] In some embodiments, the first heat treatment includes a pre-carbonization treatment at 1000°C to 1500°C for, for example, 2 to 4 hours. By performing the first heat treatment under these conditions, the organic matter in the precursor undergoes a carbonization reaction to form a carbonaceous structure, which is beneficial for the material to obtain higher electrical conductivity and better mechanical stability.

[0146] Step (3): Mix the second mixture and the carbon-containing additive to obtain the third mixture.

[0147] This disclosure does not specify the type of carbon-containing additive; any carbon-containing substance known in the art may be used.

[0148] The purpose of mixing the second mixture (e.g., carbonized particles) with the carbon-containing additive is to coat the surface of the second mixture with the carbon-containing additive. This allows the carbon material in the carbon-containing additive to transfer to the surface / bulk phase of the second mixture during the subsequent second heat treatment, repairing surface / bulk phase defects, reducing the BET specific surface area of ​​the artificial graphite, and decreasing direct contact between the resulting material and the electrolyte. This reduces side reactions between the two, thereby minimizing the loss of active ions during the first cycle and improving the first coulombic efficiency of the battery. Furthermore, during the second heat treatment, the carbon-containing additive itself can rearrange to form a graphite material with a continuous lattice structure, which is beneficial for maintaining a high specific capacity of the graphite material.

[0149] In some embodiments, the carbon-containing additive includes at least one of solid asphalt, liquid asphalt, and polyacrylonitrile. The aforementioned carbon-containing additive is a material capable of high-temperature graphitization, and its use facilitates the formation of a surface portion with suitable interplanar spacing.

[0150] In some embodiments, the carbon-containing additive includes liquid bitumen. Liquid bitumen is easier to mix with the second mixture, thereby facilitating the uniform coating of the carbon-containing additive on the surface of the second mixture, resulting in a continuous distribution of the surface portion on the central portion, which further reduces defects in the resulting material and lowers the BET specific surface area of ​​the resulting material.

[0151] In some implementations, the coking value of liquid asphalt is between 10% and 20%. For example, the coking value of liquid asphalt is a value between 10%, 11%, 12%, 13%, 14%, 15%, or any combination of these values. The "coking value" of asphalt refers to the content of combustible substances in the asphalt, primarily the content of hydrocarbons with a high hydrogen content. During the processing, storage, transportation, and use of asphalt, the higher the content of combustible substances, the worse its stability, and the easier it is to oxidize and decompose, thus causing damage to the asphalt during use and resulting in the loss of its original properties. The formula for calculating the coking value of asphalt is: Coking value (%) = (Initial mass - Residual mass) / Initial mass × 100%; where the initial mass refers to the mass of the original asphalt after drying to normal weight; the residual mass refers to the mass remaining after heating the dried asphalt to 180°C under specific conditions. It can usually be determined by conventional methods such as thermogravimetric analysis and infrared spectroscopy. Commercially available asphalt with different coking values ​​is available. In this disclosure, liquid asphalt with a coking value of 10% to 20% has appropriate heating stability, adhesion and aging properties, and can effectively form a surface portion with a suitable interplanar spacing on the central surface, while also being conducive to processing.

[0152] In some embodiments, the amount of carbon-containing additive added is less than 10% relative to the mass of the synthetic graphite precursor. Optionally, the amount added is 0.5% to 6%, 0.5% to 3%, 4% to 6%, 1% to 4%, or 2% to 4%. For example, the amount of carbon-containing additive added is a value between 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or any combination of these values. By controlling the amount of carbon-containing additive added within the above ranges, it is beneficial to maintain a high specific weight of the resulting synthetic graphite.

[0153] In some embodiments, the amount of residual carbon on the surface after secondary heat treatment can be used as the mass of the middle surface portion of the artificial graphite, wherein the amount of residual carbon on the surface can be calculated by the following formula: residual carbon on the surface = amount of liquid asphalt added * coking value of liquid asphalt.

[0154] In some embodiments, the carbon-containing additive includes liquid bitumen with a coking value of 15%, and the amount of carbon-containing additive added is 0.5% to 3%. The surface residual carbon content is calculated using the following formula: Surface residual carbon content = Amount of liquid bitumen added * 15%, resulting in a surface residual carbon content of 0.075% to 0.45%, and the BET specific surface area of ​​the obtained artificial graphite is 0.90 m². 2 / g to 1.2m 2 / g.

[0155] In some embodiments, the carbon-containing additive includes liquid bitumen with a coking value of 15%, and the amount of carbon-containing additive added is 3% to 6%. The surface residual carbon content is calculated using the following formula: Surface residual carbon content = Amount of liquid bitumen added * 15%, resulting in a surface residual carbon content of 0.45% to 0.9%, and the specific surface area of ​​the resulting artificial graphite is 0.65 m². 2 / g to 1.05m 2 / g.

[0156] Step (4): The third mixture is subjected to a second heat treatment to obtain artificial graphite, wherein the artificial graphite includes a central part and a surface part disposed on at least a portion of the surface of the central part, and the interplanar spacing D002 of the surface part is greater than the interplanar spacing D002 of the central part.

[0157] In some embodiments, the second heat treatment includes a graphitization process, which enables the carbon material containing carbon additives to rearrange to form a graphite material with a continuous graphite lattice.

[0158] In some embodiments, the second heat treatment includes graphitization at 2800°C to 3200°C for, for example, 5 to 10 days. Performing the second heat treatment under these conditions promotes the growth of graphite grains, ultimately yielding artificial graphite with suitable interplanar spacing, resulting in artificial graphite with high energy density.

[0159] In the preparation method disclosed herein, by coating the surface of the second mixture with a carbon-containing additive before the second heat treatment, oxygen loss of the second mixture during the second heat treatment can be reduced, and defects on the surface / bulk phase of the second mixture can be repaired. This results in a material with fewer defects and a lower BET specific surface area, thereby reducing side reactions between the obtained material and the electrolyte. The artificial graphite prepared by this method, when used as a negative electrode material, can improve the initial coulombic efficiency and storage performance of the battery. Simultaneously, during the second heat treatment, the carbon-containing additive itself can rearrange to form a graphite material with a continuous lattice structure, which is beneficial for maintaining a high specific capacity of the graphite material.

[0160] Example

[0161] The following describes embodiments of this disclosure. The embodiments described below are exemplary and are only used to explain this disclosure, and should not be construed as limiting this disclosure. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0162] Example 1

[0163] Preparation of artificial graphite

[0164] The pulverized calcined coke was shaped and graded to obtain an artificial graphite precursor (precursor I) with a volume average particle size DV50 of 11.0 μm.

[0165] Precursor I is mixed with binder (solid asphalt with a softening point of 200°C) at a mass ratio of 100:10, and then granulated to obtain the first mixture (granulated product).

[0166] The granulated product was subjected to a first heat treatment (pre-carbonization treatment) at 1150℃ for 2 hours to obtain a second mixture (precursor II) with a volume average particle size DV50 of 16.1 μm.

[0167] Precursor II was mixed with carbon-containing additives (liquid asphalt with a coking value of 15%) at a mass ratio of 100:2 to obtain a third mixture.

[0168] The third mixture was subjected to a second heat treatment (graphitization) at 3000℃ for 7 days, and then demagnetized and sieved to obtain artificial graphite.

[0169] Determination of parameters of artificial graphite

[0170] (1) Testing of the interplanar spacing D002 of the surface / center portion:

[0171] After the artificial graphite prepared in Example 1 was subjected to focused ion beam (FIB) thinning pretreatment, Pt nanoparticles were used as a substrate, and high-resolution testing was performed using a Thermo Scientific-Talos F200S G2 field emission transmission electron microscope (TEM). During the test, samples were taken from the area in contact with Pt (as surface data), and samples were taken from the area extending about 50 nm from the sampling point to the center (as center data). The instrument automatically performed fast Fourier transform (FFT) to obtain fringe images, and the lattice spacing of the surface part and the interplanar spacing of the center part were obtained by software measurement. The test results can be seen in Figure 7.

[0172] Figure 7(1) is a TEM image of the surface portion of the artificial graphite obtained in Example 1, and Figure 7(2) is the result of Fourier transform of Figure 7(1). It can be seen that the interplanar spacing of the 002 crystal plane of the surface portion is 0.350 nm.

[0173] Figure 7(3) is a TEM image of the central part of the artificial graphite in Example 1, and Figure 7(4) is the result of Fourier transform of Figure 7(3). It can be seen that the interplanar spacing of the 002 crystal plane in the central part is about 0.342 nm.

[0174] (2) Test of BET specific surface area:

[0175] Referring to GB / T 19587-2017, the nitrogen adsorption specific surface area analysis (using a Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA) was used for testing, and the BET method was used for calculation. The BET specific surface area of ​​the artificial graphite prepared in Example 1 was found to be 1.02 m². 2 / g.

[0176] (3) Test of volume average particle size Dv50:

[0177] Referring to GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method, the average particle size of the artificial graphite prepared in Example 1 was tested to be 16.3 μm.

[0178] (4) Test of powder compaction density at 50000N:

[0179] Weigh 1g of the artificial graphite from Example 1 and add it to a substrate with a bottom area of ​​1.327cm². 2 In the mold, the pressure was increased to 5000 kg (equivalent to 50000 N), held for 30 seconds, then depressurized and held for 10 seconds. The compaction density of the artificial graphite powder under 50000 N pressure in Example 1 was recorded and calculated to be 1.96 g / cc.

[0180] (5)I D / I G The test:

[0181] The tests were performed using a Horiba LabRAM HR800 Raman spectrometer. The test conditions were: excitation wavelength 532 nm, grating 600 lines, objective lens 50x, integration time 10 s, 3 integration cycles, and a surface scan. 100 points of D peaks were obtained (Raman spectra at 1350 ± 50 cm⁻¹). -1 Intensity I D G peak (Raman spectrum at 1580±50 cm⁻¹) -1 Intensity I G Calculate I for 100 points D / I G Remove the largest and smallest 30 I's. D / I G The average value of the remaining 40 points, 0.075, is the I value of the artificial graphite obtained in Example 1. D / I G .

[0182] Preparation of button cells

[0183] The artificial graphite obtained in Example 1, conductive carbon black (Super P), thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber were mixed thoroughly in an appropriate amount of deionized water at a mass ratio of 96.4:1:1.2:1.4 to form a negative electrode slurry.

[0184] The prepared slurry was coated onto the surface of the negative electrode current collector copper foil and dried in a vacuum drying oven for later use. A lithium metal sheet was used as the counter electrode and a 12μm polyethylene (PE) film was used as the separator. The slurry and the electrolyte prepared above were assembled into a button cell in an argon-protected glove box.

[0185] Gram capacity test

[0186] At 25°C, the coin cell prepared in Example 1 was first discharged to 0.005V with a constant current of 0.15mA, allowed to stand for 5 minutes, and then discharged to 0.005V with a constant current of 10μA. The first discharge capacity of the coin cell was recorded. After that, it was charged to 2.0V with a constant current of 0.3mA, and the first charging capacity of the coin cell was recorded. The ratio of the charging capacity to the mass of the artificial graphite is the specific capacity of the artificial graphite.

[0187] Preparation of secondary batteries

[0188] (1) Preparation of the positive electrode sheet:

[0189] Lithium iron phosphate, conductive carbon black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 96:2:2, and then N-methylpyrrolidone solvent was added and stirred until homogeneous to obtain a positive electrode slurry. The positive electrode slurry was coated onto both surfaces of the positive electrode current collector aluminum foil, and after drying and cold pressing, a positive electrode sheet was obtained.

[0190] (2) Preparation of negative electrode sheet:

[0191] Artificial graphite, conductive carbon black (Super P), thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber were mixed in a mass ratio of 96.4:1:1.2:1.4 in an appropriate amount of deionized water to form a negative electrode slurry. The negative electrode slurry was then coated onto the negative electrode current collector copper foil by extrusion coating. After drying and cold pressing, the negative electrode sheet was obtained.

[0192] (3) Separating membrane:

[0193] A 12μm polyethylene film was used as the separator.

[0194] (4) Preparation of electrolyte:

[0195] 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. LiPF6 was then dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0196] (5) Assembly of secondary batteries:

[0197] The positive and negative electrode sheets prepared above are arranged in order, with the separator placed between the positive and negative electrode sheets to provide isolation. Then, the electrode assembly is wound up. The electrode assembly is placed in an outer packaging, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a secondary battery is obtained.

[0198] Testing the performance of secondary batteries

[0199] (1) Test of the initial coulombic efficiency of the secondary battery:

[0200] The secondary battery prepared in Example 1 was formed at 45°C and charged at a constant current of 0.02C for 10 hours (the charging capacity C0 at this time was recorded). At 25°C, the secondary battery was discharged at a constant current of 0.2C to 2.0V, and the discharge capacity D0 at this time was recorded. It was then charged at a constant current of 0.33C to 3.8V, and then charged at a constant voltage until the current reached 0.05C, and the charging capacity C1 at this time was recorded. It was then discharged at a constant current of 0.33C to 2.5V, allowed to stand for 5 minutes, discharged at a constant current of 0.33C to 2.0V, and then discharged at a constant current of 0.1C to 2.0V, and the discharge capacity D1 at this time was recorded.

[0201] The initial coulombic efficiency (%) of a secondary battery = D1 / (C0-D0+C1).

[0202] (2) Testing of secondary battery storage performance:

[0203] At 25°C, the secondary battery prepared in Example 1 was charged to 3.8V with a constant current of 1C, and then charged with a constant voltage to a current of 0.05C. After standing for 5 minutes, the secondary battery was discharged to 2.0V with a constant current of 1C. The discharge capacity at this time was recorded, which is the discharge capacity before storage.

[0204] At 25°C, the prepared secondary battery was charged at a constant current of 1C to 3.8V, and then charged at a constant voltage to a current of 0.05C. The secondary battery was then stored in a 60°C constant temperature chamber for 200 days, and then discharged at a constant current of 1C to 2.0V. The discharge capacity at this point was recorded, which is the discharge capacity after storage.

[0205] The capacity retention rate (%) of a secondary battery stored at 60℃ for 200 days = discharge capacity after storage / discharge capacity before storage × 100%.

[0206] Example 2

[0207] Artificial graphite was prepared using a similar method to Example 1, and coin cells and secondary cells were assembled. The only difference was that precursor II was mixed with liquid asphalt with a coking value of 15% at a mass ratio of 100:4.

[0208] The artificial graphite, button cell, and secondary cell obtained in Example 2 were tested using the same test method as in Example 1. The test results are shown in Tables 1-2 and 1-3.

[0209] Example 3

[0210] Artificial graphite was prepared using a similar method to Example 1, and coin cells and secondary cells were assembled. The only difference was that precursor II was mixed with liquid asphalt with a coking value of 15% at a mass ratio of 100:1.

[0211] The artificial graphite, button cell, and secondary cell obtained in Example 3 were tested using the same test method as in Example 1. The test results are shown in Tables 1-2 and 1-3.

[0212] Comparative Example 1

[0213] Artificial graphite was prepared using a similar method to Example 1, and coin cells and secondary cells were assembled. The only difference was that the precursor II was directly graphitized at 3000°C for 7 days, and then demagnetized and sieved to obtain artificial graphite.

[0214] The artificial graphite, button cell, and secondary cell obtained in Comparative Example 1 were tested using the same test method as in Example 1. The test results are shown in Tables 1-2 and 1-3.

[0215] Comparative Example 2

[0216] Artificial graphite was prepared using a similar method to Comparative Example 1, and coin cells and secondary cells were assembled. The only difference was that precursor II was graphitized at 3000℃ for 7 days, and the graphitized product after demagnetization and sieving was coated with liquid asphalt with a coking value of 15% at a mass ratio of 100:2. The coated material was then carbonized at 1150℃ for 2 hours to obtain artificial graphite with an amorphous carbon layer on the surface.

[0217] The artificial graphite obtained in Comparative Example 2 was tested using the same testing method as in Example 1. When the lattice spacing of the surface portion was tested using the same method as in Example 1, samples were taken from the area in contact with Pt. After a fast Fourier transform, no obvious stripes were obtained. Therefore, the surface portion of the artificial graphite obtained in Comparative Example 2 was determined to be amorphous carbon.

[0218] Subsequently, the button cell and secondary cell obtained in Comparative Example 2 were tested using the same test method as in Example 1. The test results are shown in Tables 1-2 and 1-3.

[0219] Table 1-1

[0220] Table 1-2

[0221] Table 1-3

[0222] In Table 1-1, " / " indicates that no related items have been added.

[0223] The data in Tables 1-1 to 1-3 show that, compared with Comparative Example 1 (no surface portion in artificial graphite) and Comparative Example 2 (amorphous carbon surface portion), Examples 1 to 3 can improve the first coulombic efficiency and storage performance of the battery.

[0224] Examples 4-6

[0225] Artificial graphite was prepared using a similar method to that in Example 1, and coin cells and secondary cells were assembled. The only difference was that the types of carbon-containing additives were adjusted according to Table 2-1 below when preparing artificial graphite.

[0226] In addition, referring to the test method of Example 1, the artificial graphite, button cell and secondary cell of Examples 4-6 were tested, and the test results are shown in Table 2-2.

[0227] Table 2-1

[0228] Table 2-2

[0229] The data in Tables 1-1, 1-2, 1-3, 2-1, and 2-2 show that using the carbon-containing additives specified in this disclosure is beneficial for reducing the BET specific surface area of ​​artificial graphite, thereby further improving the initial coulombic efficiency and storage performance of secondary batteries.

[0230] It should be noted that this disclosure is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same essential structure and achieving the same effect as the technical concept within the scope of this disclosure are included in the technical scope of this disclosure. 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, are also included in the scope of this disclosure without departing from the spirit of this disclosure.

Claims

1. A secondary battery comprising a negative electrode tab including a negative electrode current collector and a negative electrode film layer on at least one surface of the negative electrode current collector, the negative electrode film layer including artificial graphite, the artificial graphite including a surface portion and a center portion, the surface portion being continuously or discontinuously distributed in the center portion, a crystal plane spacing D002 of the surface portion being greater than a crystal plane spacing D002 of the center portion, wherein the crystal plane spacing D002 is an interlayer spacing of a graphite 002 crystal plane.

2. The secondary battery according to claim 1, wherein the crystal plane spacing D002 of the surface portion is 0.345 nm to 0.355 nm.

3. The secondary battery according to claim 1 or 2, wherein the crystal plane spacing D002 of the center portion is 0.335 nm to 0.345 nm.

4. The secondary battery according to any one of claims 1 to 3, wherein the artificial graphite satisfies at least one of the following: (1) the artificial graphite has a BET specific surface area of 0.6 m 2 / g to 1.4 m 2 / g; (2) the artificial graphite has an I D / I G value of 0.05 to 0.15, wherein I D represents the D peak intensity of the Raman spectrum at 1350 ± 50 cm -1 -1, and I G represents the G peak intensity of the Raman spectrum at 1580 ± 50 cm -1 -1; (3) a volume average particle size Dv50 of the artificial graphite is 12 μm to 22 μm; (4) a powder compaction density of the artificial graphite at 50000 N is 1.93 g / cc to 2.15 g / cc; (5) a gravimetric capacity of the artificial graphite is 355 mAh / g to 365 mAh / g.

5. The secondary battery according to any one of claims 1 to 4, wherein The artificial graphite has a BET specific surface area of 0.85 m 2 / g to 1.18 m 2 / g.

6. The secondary battery according to any one of claims 1 to 5, wherein The artificial graphite has I D / I G a value of 0.067 to 0.

087. 7.An electric device comprising the secondary battery according to any one of claims 1 to 6. 8.Artificial graphite including a surface portion and a center portion, the surface portion being continuously or discontinuously distributed in the center portion, a crystal plane spacing D002 of the surface portion being greater than a crystal plane spacing D002 of the center portion, wherein the crystal plane spacing D002 is an interlayer spacing of a graphite 002 crystal plane.

9. The artificial graphite according to claim 8, wherein, the crystal plane spacing D002 of the surface portion is 0.345 nm to 0.355 nm.

10. Artificial graphite according to claim 8 or 9, wherein the crystal plane spacing D002 of the center portion is 0.335 nm to 0.345 nm.

11. Artificial graphite according to any one of claims 8 to 10, wherein, the artificial graphite satisfies at least one of the following: (1) the artificial graphite has a BET specific surface area of 0.6 m 2 / g to 1.4 m 2 / g; (2) the I D / I G value is 0.05 to 0.15, wherein I D represents the D peak intensity of the Raman spectrum at 1350 ± 50 cm -1 -1, and I G represents the G peak intensity of the Raman spectrum at 1580 ± 50 cm -1 -1. 12.A method of producing artificial graphite, comprising: mixing a binder and artificial graphite precursor, and then performing granulation to obtain a first mixture; performing first heat treatment on the first mixture to obtain a second mixture; mixing the second mixture and a carbon-containing additive to obtain a third mixture; performing second heat treatment on the third mixture to obtain artificial graphite, the artificial graphite including a surface portion and a center portion, the surface portion being continuously or discontinuously distributed in the center portion, a crystal plane spacing D002 of the surface portion being greater than a crystal plane spacing D002 of the center portion, wherein the crystal plane spacing D002 is an interlayer spacing of a graphite 002 crystal plane.

13. The method of making according to claim 12, wherein, the artificial graphite precursor includes one or more of calcined coke, petroleum-based non-pitch-based coke, petroleum-based pitch-based coke, coal-based non-pitch-based coke, and coal-based pitch-based coke.

14. The production method according to claim 12 or 13, wherein the artificial graphite precursor includes calcined coke.

15. The production process according to any one of claims 12 to 14, wherein, the carbon-containing additive includes at least one of solid pitch, liquid pitch, and polyacrylonitrile.

16. The production method according to any one of claims 12 to 15, wherein, the carbon-containing additive includes liquid pitch.

17. The production method according to any one of claims 12 to 16, wherein, a coking value of the liquid pitch is 10% to 20%.

18. The production method according to any one of claims 12 to 17, wherein, an amount of the carbon-containing additive added is 10% or less compared to a mass of the artificial graphite precursor.

19. The production process according to any one of claims 12 to 18, wherein, an amount of the carbon-containing additive added is 0.5% to 6% compared to a mass of the artificial graphite precursor.

20. The production process according to any one of claims 12 to 19, wherein, The first heat treatment includes a pre-carbonization treatment at 1000°C to 1500°C.

21. The production process according to any one of claims 12 to 20, wherein, The second heat treatment includes a graphitization treatment at 2800°C to 3200°C.

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