Natural graphite, electrochemical devices, and methods for preparing natural graphite

Abstract

This invention provides natural graphite, an electrochemical device, and a method for preparing natural graphite, belonging to the field of secondary battery technology. The natural graphite includes a natural graphite core, in which doping elements, including P or N, are present. The gradient concentration ratio of the doping elements in the natural graphite core is 2.4 to 3.8. This gradient concentration ratio is the mass ratio of the doping elements in the first gradient region to the doping elements in the second gradient region of the natural graphite core. The natural graphite of this invention possesses the advantages of high capacity, low expansion, and high rate performance, effectively improving the storage and cycle performance of the assembled battery compared to existing natural graphite materials.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, specifically to a natural graphite, an electrochemical device, and a method for preparing natural graphite. Background Technology

[0002] Graphite materials are considered ideal anode materials for lithium-ion batteries due to their high stability, good conductivity, and wide availability. Natural and artificial graphite have always been the most widely used anode materials. However, artificial graphite requires a large amount of electricity (10,000 kWh / ton) for its key graphitization process, resulting in high carbon emissions. Natural graphite, on the other hand, has gained widespread attention due to its abundant resources, lack of need for graphitization, low carbon emissions (one-third that of artificial graphite), and lower cost.

[0003] Natural graphite possesses a rich porous structure, providing a large surface area for lithium insertion / extraction to wet the electrolyte. It also contains abundant functional groups, enabling the rapid desolvation of organic lithium in the electrolyte at the negative electrode interface and its participation in the lithium insertion / extraction process. These characteristics give natural graphite excellent kinetic properties, especially significant advantages in low-temperature kinetics.

[0004] However, the abundant porosity and functional groups in natural graphite also provide more reactive sites, making it prone to side reactions with the electrolyte and deteriorating the battery's cycle performance and high-temperature storage performance. Furthermore, due to its small interlayer spacing, natural graphite is prone to lattice expansion during lithium insertion / extraction, causing graphite sheets to easily peel off and detach, which further worsens the battery's cycle performance.

[0005] Therefore, it is necessary to design a natural graphite, an electrochemical device, and a method for preparing natural graphite to solve the above problems. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the present invention provides natural graphite, an electrochemical device, and a method for preparing natural graphite, in order to improve the technical problem of poor cycle performance and storage performance of natural graphite anode materials in the prior art.

[0007] To achieve the above and other related objectives, the present invention provides a natural graphite comprising a natural graphite core, wherein the natural graphite core is doped with a doping element, the doping element comprising P and / or N;

[0008] Wherein, the gradient concentration ratio of the dopant element in the natural graphite nucleus is 2.4 to 3.8; the gradient concentration ratio is the mass ratio of the dopant element in the first gradient region to the dopant element in the second gradient region in the natural graphite nucleus; the first gradient region is the region from the center of the natural graphite nucleus to a distance of R / 2 from the center, and the second gradient region is the region from a distance of R / 2 from the center to the surface of the natural graphite nucleus, where R is the radius of the natural graphite nucleus.

[0009] In one example of the present invention, the average particle size DV50 of the natural graphite nuclei is 5 to 20 μm.

[0010] In one example of the present invention, the mass of the dopant element is 1% to 1.5% of the mass of the natural graphite.

[0011] In one example of the present invention, the Young's modulus of the natural graphite is 25-32 GPa.

[0012] In one example of the present invention, the natural graphite contains pores, and the total pore volume of the pores with a pore size of 0.2 to 1.0 nm is 9.2 to 15.8 ml / g.

[0013] In one example of the present invention, the natural graphite further includes a coating layer attached to the surface of the natural graphite core, the coating layer comprising a carbon material, and the mass of the coating layer being 2% to 5% of the mass of the natural graphite core. Natural graphite core

[0014] The present invention also provides a method for preparing natural graphite according to any of the above examples, the method comprising:

[0015] Provides natural graphite precursors;

[0016] The first doping source and the natural graphite precursor are mixed and heated once under a first preset pressure environment; the first preset pressure is less than the standard atmospheric pressure.

[0017] The second doping source and the natural graphite precursor are subjected to secondary mixing and heating under a second preset pressure environment to obtain natural graphite; the second preset pressure is greater than the first preset pressure.

[0018] Wherein, the first doping source and the second doping source are compounds containing doping elements, the doping elements including P or N; the mass ratio of the first doping source to the second doping source satisfies the condition that the gradient concentration ratio of the doping elements in the natural graphite is 2.4 to 3.8; the gradient concentration ratio is the mass ratio of the doping elements in the first gradient region to the doping elements in the second gradient region of the natural graphite core; the first gradient region is the region from the center of the natural graphite core to a distance R / 2 from the center, and the second gradient region is the region from a distance R / 2 from the center to the surface of the natural graphite core, where R is the distance from the center of the natural graphite core to the surface.

[0019] In one example of the present invention, the step of providing a natural graphite precursor includes: shaping and classifying a natural graphite raw material to obtain a natural graphite precursor; the average particle size Dv50 of the natural graphite precursor is 15-25 μm, the particle size distribution width of the natural graphite precursor is less than or equal to 1.1, and the particle size distribution width is (Dv90-Dv10) / v50.

[0020] In one example of the present invention, the first preset gas pressure is less than 0.2 atm, the temperature of the first mixing and heating is 600-800°C, and the mass ratio of the natural graphite precursor to the first dopant source is 100:(0.1-0.5); the second preset gas pressure is less than 0.4 atm, the temperature of the second mixing and heating is 600-800°C, and the mass ratio of the natural graphite precursor to the second dopant source is 300:(0.1-0.3).

[0021] In one example of the present invention, after the secondary mixing and heating step, a step of quenching the natural graphite precursor is further included, comprising: cooling the natural graphite precursor with an inert gas within a preset temperature range; the inert gas flow rate is 40-80 L / min, the upper limit of the preset temperature range is 600-800°C, and the lower limit of the preset temperature range is 100-150°C.

[0022] In one example of the present invention, the preparation method further includes the step of forming a coating layer on the surface of the natural graphite, which includes: mixing and sintering the natural graphite with a carbon source to form a coating layer on the surface of the natural graphite core.

[0023] The present invention also provides an electrochemical device comprising a positive electrode, a negative electrode, a separator, and an electrolyte. The negative electrode comprises a negative current collector and a negative active material layer, wherein the negative active material layer comprises natural graphite as described in any of the above examples, or natural graphite prepared by the preparation method described in any of the above examples.

[0024] This invention provides a natural graphite doped with P and / or N, wherein the gradient concentration ratio of the dopant elements in the natural graphite is limited to any value between 2.4 and 3.8. Based on the difference in doping concentration between the inner and outer layers, the natural graphite forms a particle structure that is hard inside and soft outside, reducing particle breakage caused by internal and external compression during charging and discharging, thus improving the expansion problem of the natural graphite and enhancing the storage and cycle performance of the battery. Furthermore, while ensuring the overall pore volume of the material, it reduces the surface pore structure, lowering the probability of side reactions between the natural graphite and the electrolyte, thereby improving the battery's cycle performance while maintaining its own high-rate performance. In summary, this natural graphite combines the advantages of high capacity, low expansion, and high-rate performance, effectively improving the storage and cycle performance of the assembled battery compared to existing natural graphite materials. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other embodiments can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a schematic diagram of the structure of natural graphite in one embodiment of the present invention;

[0027] Figure 2 This is a schematic flowchart of a method for preparing natural graphite in one embodiment of the present invention. Detailed Implementation

[0028] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features can be combined with each other. It should also be understood that the terminology used in the embodiments of the present invention is for describing specific implementation schemes and not for limiting the scope of protection of the present invention. Test methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions or according to the conditions recommended by the respective manufacturers.

[0029] In this specification, the average particle size (Dv50) can be defined as the particle size corresponding to 50% of the volumetric size in the particle size distribution curve, Dv10 can be defined as the particle size corresponding to 10% of the volumetric size in the particle size distribution curve, and Dv90 can be defined as the particle size corresponding to 90% of the volumetric size in the particle size distribution curve. Dv10, Dv50, and Dv90 can be measured, for example, by laser diffraction. Laser diffraction can typically measure particle sizes from the submicron range to several millimeters, thus providing highly reproducible and high-resolution results.

[0030] In this specification, Young's modulus represents the degree of change in particle length per unit cross-section when a powder is subjected to a certain tensile stress. Young's modulus E is defined as E = σ / ε, where σ represents the stress per unit area of ​​the material, and ε represents the strain per unit length. The method for measuring Young's modulus refers to GB / T 34186-2017.

[0031] The negative electrode material is one of the key materials determining the performance of a secondary battery, directly affecting its energy density, cycle life, rate performance, and safety. Natural graphite is widely used due to its high charge / discharge capacity, good charge / discharge platform, wide availability, and low cost. However, natural graphite has structural instability and excessive porosity, making its layers prone to detachment and breakage during charge / discharge. This exposes more surfaces that can react with the electrolyte, accelerating the reaction between natural graphite and the electrolyte. Consequently, lithium-ion batteries suffer from reduced charge / discharge efficiency, poor cycle performance, and lower safety, directly reducing the cycle life of the lithium-ion battery.

[0032] To improve the inherent technical defects of natural graphite as a negative electrode material, this application provides a natural graphite material that can further improve the charge and discharge capacity of natural graphite while retaining the rich porosity and point defect structure of natural graphite, and improve the problems of expansion of natural graphite and excessive side reactions with electrolyte.

[0033] In the first aspect, such as Figure 1 As shown, this invention provides a natural graphite comprising a natural graphite core, wherein the natural graphite core is doped with a dopant element, the dopant element including P and / or N. The gradient concentration ratio K of the dopant element in the natural graphite core is any value from 2.4 to 3.8, for example, the gradient concentration ratio K is 2.4, 2.5, 2.6, 2.8, 3.0, 3.1, 3.2, 3.4, 3.5, 3.6 or 3.8. The gradient concentration ratio K is the mass ratio of the dopant element in the first gradient region A1 to the dopant element in the second gradient region A2 of the natural graphite core; wherein, the first gradient region A1 is from the center of the natural graphite core (… Figure 1The first gradient region A1 is a spherical region extending from point O to a distance R / 2 from the center, and the second gradient region A2 is an annular region extending from a distance R / 2 from the center to the surface of the natural graphite nucleus, where R is the distance from the center of the natural graphite nucleus to the surface.

[0034] The gradient concentration ratio K of dopants in a natural graphite nucleus can be obtained by line scan measurement. Specifically, the mass ratio of dopants in the region from the center to half the radius of the natural graphite nucleus and the region from half the radius to the surface can be measured by line scan. For details, please refer to GB / T 17359-2023.

[0035] This natural graphite utilizes doping elements to form lattice defects in natural graphite particles, which can not only improve the electron cloud mobility in the negative electrode, but also increase lithium storage binding sites in natural graphite, reduce the lithium storage reaction barrier, improve the lithium ion migration speed of natural graphite, and enhance the rate performance and charge / discharge capacity of natural graphite. Meanwhile, based on the different doping concentrations inside and outside the natural graphite, firstly, it forms a particle structure that is hard inside and soft outside, making the internal volume change rate of natural graphite less than the external volume change rate. This reduces the excessive compression of the external structure by the internal expansion of natural graphite, enhances the overall structural strength of the natural graphite particles, and protects the surface structure, avoiding material cracking and increased side reactions caused by excessive compression of the natural graphite surface by internal expansion, thus improving the storage and cycle performance of the battery. Secondly, it can reduce the porosity and defect structure on the surface while ensuring that the overall pore volume of natural graphite particles does not decrease excessively, reducing the probability of side reactions between natural graphite and electrolyte, thereby improving its cycle performance while maintaining the good rate performance of natural graphite itself. Thirdly, it utilizes the characteristic that the element doping content inside the bulk phase of natural graphite is higher than that on the surface, so that natural graphite retains the high capacity brought by bulk doping while avoiding a low first charge and discharge efficiency.

[0036] It should be noted that the type of natural graphite used in this invention is not limited, and natural graphite includes at least one of flake graphite and microcrystalline graphite. That is, natural graphite can be flake graphite or microcrystalline graphite, or a combination of flake graphite and microcrystalline graphite. Within the combination, the ratio of flake graphite to microcrystalline graphite is not limited, and they can be mixed in any proportion.

[0037] In some embodiments, the Young's modulus E of natural graphite is any value between 25 and 32 GPa, for example, the Young's modulus E of natural graphite can be 25 GPa, 26 GPa, 27 GPa, 28 GPa, 29 GPa, 30 GPa, 31 GPa, or 32 GPa. In this invention, the Young's modulus of natural graphite is close to that of artificial graphite, and its expansion problem in the negative electrode is significantly improved compared to unmodified natural graphite. When the Young's modulus of this natural graphite is within the above-mentioned suitable range, it can possess both strong structural strength and high compaction density.

[0038] In some embodiments, the natural graphite particles contain a porous structure; wherein the total pore volume of the porous structure with a pore size of 0.2–1.0 nm in the natural graphite particles is any value between 9.2 and 15.8 ml / g, for example, the pore volume can be 9.2 ml / g, 9.3 ml / g, 9.5 ml / g, 9.8 ml / g, 10 ml / g, 10.1 ml / g, 10.5 ml / g, 11 ml / g, 11.2 ml / g, 11.5 ml / g, 12 ml / g, 13 ml / g, 13.6 ml / g, 14 ml / g, 14.2 ml / g, 14.5 ml / g, 14.6 ml / g, 15 ml / g, 15.5 ml / g, or 15.8 ml / g. It should be noted that the method for measuring the pore volume in natural graphite refers to GB / T 19587-2017.

[0039] In some embodiments, the mass of the dopant element in the natural graphite is 1% to 1.5% of the mass of the natural graphite, for example, it can be 1%, 1.1%, 1.2%, 1.3%, 1.4% or 1.5%.

[0040] In some embodiments, the average particle size Dv50 of natural graphite is any value between 5 and 20 μm. For example, the average particle size Dv50 of natural graphite can be 5 μm, 7 μm, 8 μm, 10 μm, 12 μm, 13 μm, 13.6 μm, 13.7 μm, 14 μm, 14.5 μm, 14.8 μm, 15 μm, 15.8 μm, 16 μm, 17 μm, 18 μm, 19 μm, or 20 μm.

[0041] In some embodiments, natural graphite includes a natural graphite core and a coating layer. The natural graphite core is a natural graphite particle, and the coating layer is attached to the surface of the natural graphite core (i.e., the natural graphite particle). The coating layer includes a carbon material, and the mass of the coating layer is 2% to 5% of the mass of the natural graphite core, for example, 2%, 3%, 4%, or 5%. Within this range, the mass content of the coating layer relative to the natural graphite core can optimize both the coating effect and energy density of the natural graphite. If the mass content of the coating layer is too low, the coating effect on the natural graphite core will be poor; if the mass content of the coating layer is too high, over-coating will negatively impact the energy density of the natural graphite. The carbon material in the coating layer can be selected from at least one of amorphous carbon, graphene, and carbon nanotubes; optionally, the carbon material can be amorphous carbon.

[0042] In a second aspect, the present invention also provides a method for preparing natural graphite. For example... Figure 2 As shown, the preparation method of this natural graphite includes the following steps:

[0043] S1. Provides natural graphite precursors;

[0044] S2. The first doping source and the natural graphite precursor are mixed and heated once under a first preset pressure environment; the first preset pressure is less than the standard atmospheric pressure.

[0045] S3. The second doping source and the natural graphite precursor are placed in a second preset pressure environment for secondary mixing and heating to obtain natural graphite; the second preset pressure is greater than the first preset pressure.

[0046] This preparation method involves using a first preset gas pressure with a relatively high negative pressure during the first doping stage to incorporate most of the dopant elements from the first dopant source into the bulk phase near the core of the natural graphite precursor particles. During the second doping stage, a second preset gas pressure with a lower negative pressure is used to incorporate most of the dopant elements from the second dopant source into the bulk phase near the surface of the natural graphite precursor particles. Furthermore, by adjusting the mass ratio of the first and second dopant sources during both stages, the gradient concentration ratio of the dopant elements in the natural graphite is made to reach any value between 2.4 and 3.8, thereby completing the modification treatment of the natural graphite. The first and second dopant sources are compounds containing dopant elements, including P and / or N.

[0047] In some embodiments, step S1 includes shaping and grading the natural graphite raw material to make the natural graphite raw material spherical and uniform in particle size, thereby obtaining a natural graphite precursor; wherein the average particle size Dv50 of the obtained natural graphite precursor is 15-25 μm, and the particle size distribution width (Dv90-Dv10) / Dv50 of the natural graphite precursor is less than or equal to 1.1.

[0048] In step S1, the natural graphite raw material is spheroidized and its particle size reduced through shaping, resulting in an average particle size Dv50 of 15–25 μm for millimeter-sized natural graphite raw materials. Simultaneously, spheroidizing involves grinding down the edges and corners of the natural graphite raw material, making the particle shape closer to a sphere. The greater the degree of shaping, the closer the particles are to a sphere, thus increasing the ratio of the end face to the base face. Step S1 can employ equipment and methods known in the art to perform the shaping process on the natural graphite raw material, such as a shaping machine or other shaping equipment. In one example, a honeycomb mill is used for high-frequency shaping of the natural graphite raw material.

[0049] In step S1, after shaping the natural graphite raw material, it needs to be further graded to remove large and small particles from the shaped natural graphite raw material, thereby obtaining a natural graphite precursor with a particle size distribution width (Dv90-Dv10) / Dv50 less than or equal to 1.1. In step S1, graded materials and methods known in the art can be used for the grading process, such as grading sieves, gravity classifiers, and centrifugal classifiers.

[0050] Furthermore, it should be noted that the natural graphite raw material is selected from at least one of flake graphite and microcrystalline graphite. That is, the natural graphite raw material can be flake graphite or microcrystalline graphite, or a combination of flake graphite and microcrystalline graphite. Within the combination, there is no restriction on the ratio of flake graphite and microcrystalline graphite, and they can be mixed in any proportion.

[0051] In some embodiments, in step S2, the natural graphite precursor and the first dopant source are mixed and heated once under a first preset pressure to dope the dopant element in the first dopant source into the bulk phase near the center of the natural graphite precursor particles. The first preset pressure is less than 0.2 atm; the mass ratio of the natural graphite precursor to the first dopant source is 100:(0.1 to 0.5), for example, 100:0.1, 100:0.2, 100:0.3, 100:0.4, or 100:0.5; the temperature for the first mixing heating is any value between 600℃ and 800℃, for example, 600℃, 620℃, 640℃, 650℃, 660℃, 680℃, 700℃, 740℃, 760℃, 780℃, or 800℃. In addition, in some examples, the mixing and heating process also includes further shaping of the natural graphite precursor. For example, the shaping frequency is 100Hz to 140Hz, such as 100Hz, 120Hz or 140Hz, and the shaping time is 0.5h to 2h, such as 0.5h, 1h, 1.5h or 2h.

[0052] In some embodiments, in step S3, the natural graphite precursor and the second dopant source are mixed and subjected to a second preset gas pressure for secondary mixing and heating, so as to incorporate the dopant element in the second dopant source into the bulk phase near the surface of the natural graphite precursor particles. The second preset gas pressure is greater than the first preset gas pressure and less than 0.4 atm; the mass ratio of the natural graphite precursor to the second dopant source is 300:(0.1 to 0.3), for example, 300:0.1, 300:0.2, or 300:0.3; the temperature of the secondary mixing and heating is any value between 600℃ and 800℃, for example, 600℃, 620℃, 640℃, 650℃, 660℃, 680℃, 700℃, 740℃, 760℃, 780℃, or 800℃; in one example, the temperature of the primary mixing and heating is the same as that of the secondary mixing and heating. Excessive temperatures in the primary and secondary mixing heating processes can increase the graphitization degree of natural graphite, damaging its original properties. Conversely, excessively low temperatures can lead to insufficient heat, preventing the achievement of target mechanical properties during subsequent cold quenching. Furthermore, in some examples, the secondary mixing heating process includes further shaping of the natural graphite precursor. For instance, the shaping frequency is 100Hz–140Hz, such as 100Hz, 120Hz, or 140Hz, and the hot-forming time is 0.5h–2h, such as 0.5h, 1h, 1.5h, or 2h.

[0053] In one example, steps S2 and S3 include: placing the natural graphite precursor in an air jet mill, heating the air jet mill to the primary mixing heating temperature, and evacuating the air jet mill to bring the internal pressure to a first preset pressure; then pumping the first dopant source solution into the air jet mill at a mass ratio of 100:(0.1-0.5) for the natural graphite precursor to the first dopant source, while simultaneously performing a first high-frequency thermal shaping of the natural graphite precursor powder in the air jet mill for 1 to 3 hours, and continuously extracting the reaction gas; when the internal pressure of the air jet mill reaches a second preset pressure, adjusting the temperature inside the air jet mill to the secondary mixing heating temperature; then pumping the second dopant source solution into the air jet mill at a mass ratio of 300:(0.1-0.3) for the natural graphite precursor to the second dopant source, while simultaneously performing a second high-frequency thermal shaping of the natural graphite precursor powder in the air jet mill for 1 to 3 hours, and continuously extracting the reaction gas. In addition, in steps S2 and S3, the ratio of the amount of the first dopant source and the second dopant source needs to be adjusted so that the gradient concentration ratio of the dopant elements in the natural graphite is 2.4 to 3.8; and the mass of the dopant elements is 1% to 1.5% of the mass of the natural graphite.

[0054] It should be noted that the first and second doping sources can be compounds containing dopant elements. When the doping element is P, the first and second doping sources can be, for example, phosphorus pentoxide, phosphorus trioxide, ammonium phosphate, ammonium dihydrogen phosphate, or diammonium hydrogen phosphate; optionally, the first and second doping sources are diammonium hydrogen phosphate. When the doping element is N, the first and second doping sources can be, for example, urea, melamine, or ammonium bicarbonate; optionally, the first and second doping sources are ammonium bicarbonate.

[0055] In some embodiments, after the secondary mixing and heating in step S3, the preparation method of the present invention further includes a cold extraction treatment step for the natural graphite precursor. Specifically, this includes continuously introducing inert gas into an air jet mill to cool the natural graphite precursor using the flowing, low-temperature inert gas, thereby reducing the temperature of the natural graphite precursor from the upper limit of a preset temperature range to the lower limit. The cold extraction treatment step can remove impurities from the natural graphite, improve its purity and crystallinity, repair defects in the natural graphite particles, and reduce stress concentration within the natural graphite. This further enhances the structural strength and Young's modulus of the natural graphite based on the doping modification.

[0056] In some embodiments, the upper limit of the preset temperature range for cold extraction is the temperature at which the natural graphite precursor is heated during secondary mixing, to simplify the preparation process. The upper limit of the preset temperature range can be any value between 600°C and 800°C, for example, 600°C, 620°C, 640°C, 650°C, 660°C, 680°C, 700°C, 740°C, 760°C, 780°C, or 800°C; the lower limit of the preset temperature range for cold extraction is the temperature of the inert gas, and the lower limit of the lower limit is any value between 100°C and 150°C, for example, 100°C, 110°C, 120°C, 130°C, 140°C, or 150°C.

[0057] During the cold extraction process, the flow rate of the inert gas introduced into the air jet mill affects the cooling rate of the natural graphite precursor. In some embodiments, the inert gas flow rate can be any value between 40 and 80 L / min, for example, 40 L / min, 45 L / min, 50 L / min, 55 L / min, 60 L / min, 65 L / min, 70 L / min, 75 L / min, or 80 L / min. Furthermore, in some embodiments, the inert gas is one or a combination of at least two of argon, nitrogen, hydrogen, and helium. Optionally, the inert gas is a mixture of nitrogen, argon, and hydrogen.

[0058] In some embodiments, after the secondary mixing and sintering, step S3 further includes rinsing and drying the natural graphite. Specifically, the natural graphite is rinsed with deionized water until the pH of the rinse solution is neutral; then the natural graphite is dried. The drying method can be one or more of the following: vacuum drying, spray drying, freeze drying, and filtration drying.

[0059] In some embodiments, the preparation method of the present invention further includes the step of forming a coating layer on the surface of the natural graphite. This step includes: using natural graphite particles as a natural graphite core, and mixing and sintering the natural graphite particles with a carbon source to form a coating layer on the outside of the natural graphite core. Specifically, the natural graphite particles and the carbon source are uniformly mixed at a mass ratio of (20-50):1, and the uniformly mixed natural graphite particles and carbon source are sintered at a carbonization temperature, so that the carbon source carbonizes on the surface of the natural graphite particles to form a coating layer. The carbon source is selected from at least one of coal tar pitch, petroleum pitch, phenolic resin, and ethylene tar. The carbonization sintering temperature is any temperature between 600 and 1300°C, for example, 600°C, 700°C, 800°C, 900°C, 1000°C, 1100°C, 1200°C, or 1300°C.

[0060] In a third aspect, the present invention also provides an electrochemical device, which can be a solid-state lithium-ion secondary battery or a liquid lithium-ion secondary battery. Taking a liquid lithium-ion secondary battery as an example, the electrochemical device includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte. The negative electrode includes a negative current collector and a negative active material layer disposed on the negative current collector. The negative active material layer includes a negative electrode material, a negative electrode conductive agent, a negative electrode thickener, and a negative electrode binder. The negative electrode material includes natural graphite as described in any of the above embodiments, or natural graphite prepared by the preparation method described in any of the above embodiments. The positive electrode includes a positive current collector and a positive active material layer disposed on the positive current collector. The positive active material layer includes a positive electrode material, a positive electrode conductive agent, and a positive electrode binder. Among them, the positive and negative electrode materials can intercalate and deintercalate lithium ions to achieve energy storage and release, the electrolyte is the carrier for lithium ions to be transported between the positive and negative electrodes, and the separator can be permeated by lithium ions but is not conductive, thereby separating the positive and negative electrodes to prevent short circuits.

[0061] It should be noted that the preparation of the positive electrode, negative electrode, separator, and electrolyte, as well as the assembly of the electrochemical device, can be carried out using conventional methods in this field. The preparation methods for the electrochemical device are described below with examples:

[0062] (1) Preparation of positive electrode sheet: The positive electrode material, positive electrode conductive agent, and positive electrode binder are mixed in a weight ratio of (90 to 99):(1 to 10):(1 to 10), optionally 95:2.5:2.5. N-methylpyrrolidone (NMP) solvent is added, and the mixture is thoroughly stirred to obtain a positive electrode slurry. The mixture is stirred under vacuum until it becomes homogeneous and transparent. The positive electrode slurry is then uniformly coated onto a positive electrode current collector aluminum foil. After the aluminum foil is dried at room temperature, it is transferred to an oven for drying. The foil is then cold-pressed and slit to obtain the positive electrode sheet. The positive electrode material is selected from one or more of lithium cobalt oxide, ternary materials, lithium phosphate, and spinel materials. Specifically, ternary materials include, but are not limited to, lithium nickel cobalt manganese oxide (LiNi). x Co y Mn z O2 (x+y+z=1); lithium phosphates include, but are not limited to, lithium manganese iron phosphate, lithium iron phosphate, lithium manganese phosphate, etc.; spinel materials can be nickel manganese spinel materials. Positive electrode conductive agents can be selected from at least one of the following conductive materials: carbon black (Super P), acetylene black, carbon nanotubes (CNT), graphene, and carbon nanofibers (VGCF); positive electrode binders can be selected from at least one of the following: polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE), for example, PVDF can be used as the positive electrode binder.

[0063] (2) Preparation of negative electrode sheet: The negative electrode material, negative electrode conductive agent, negative electrode binder and thickener are mixed in a mass ratio of (90 to 99):(0.5 to 10):(1 to 3):(0.5 to 3), deionized water is added, and the mixture is thoroughly stirred under vacuum to obtain a negative electrode slurry; the negative electrode slurry is uniformly coated on both sides of the negative electrode current collector copper foil; after drying at room temperature, it is transferred to an oven for drying, and then the negative electrode sheet is obtained through cold pressing, slitting and other processes. Among them, the negative electrode conductive agent can be selected from at least one of conductive materials such as carbon black (Super P), acetylene black, carbon nanotubes (CNT), graphene, and carbon nanofibers (VGCF). The negative electrode binder is selected from at least one of the binding materials such as polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), and styrene-butadiene rubber (SBR), for example, the negative electrode binder is PAA and SBR, and the mass ratio of PAA to SBR is 1.3:0.5. The thickener is selected from carboxymethyl cellulose, which can be sodium carboxymethyl cellulose (CMC-Na) or lithium carboxymethyl cellulose (CMC-Li).

[0064] (3) Electrolyte preparation: In an argon atmosphere glove box with a water content of <10ppm, ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are uniformly mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1mol / L.

[0065] (4) Preparation of membrane: The membrane is selected from conventional types in the art, such as PE porous membrane. The thickness of the membrane is 9 to 18 μm, the air permeability is 180 s / 100 mL to 380 s / 100 mL, and the porosity is 30% to 50%.

[0066] (5) Battery assembly: Battery assembly is carried out according to conventional methods. For example, after preparation, the negative electrode, separator, and positive electrode are stacked in sequence and placed in an aluminum-plastic film to obtain a dry cell. The dry cell is then baked to remove water. The prepared electrolyte is injected into the dry cell and sealed to obtain the finished lithium-ion battery.

[0067] The technical solution of the present invention will be described in detail below through several specific embodiments and comparative examples. Unless otherwise stated, the raw materials and reagents used in the following embodiments are commercially available products or can be prepared by conventional methods in the art.

[0068] Example 1

[0069] This embodiment provides a natural graphite, comprising a natural graphite core and a coating layer covering the outside of the natural graphite core. The natural graphite is doped with phosphorus (P), with a P concentration gradient ratio of 3.1, and the mass of P is 2.1% of the natural graphite's mass content. The coating layer comprises amorphous carbon, and its mass is 4% of the mass of the natural graphite core particles. The preparation method of this natural graphite is as follows:

[0070] S1. The natural graphite raw material (flake graphite) that has been washed and floated by water is shaped at a high frequency of 90Hz for 6 hours using a honeycomb mill to reduce the size of the millimeter-sized natural graphite raw material to a natural graphite precursor with an average particle size Dv50 of 20μm. The graded treatment removes large and small particles from the natural graphite precursor, so that the particle size distribution width of the natural graphite precursor is 1.0.

[0071] S2. Place the natural graphite precursor in an air jet mill, heat the air jet mill to the primary mixing heating temperature of 600℃, and evacuate the air jet mill to bring the air pressure inside the air jet mill to below the first preset air pressure of 0.2 atm. Then, pump the first doping source aqueous solution (the first doping source aqueous solution is a diammonium hydrogen phosphate aqueous solution, and the mass percentage of diammonium hydrogen phosphate in the diammonium hydrogen phosphate aqueous solution is 5%) into the air jet mill at a mass ratio of 20:1 between the natural graphite precursor and the first doping source aqueous solution. At the same time, perform high-frequency thermal shaping of the natural graphite precursor powder in the air jet mill at a frequency of 120Hz for 2 hours, and continuously extract the ammonia gas generated by the reaction of diammonium hydrogen phosphate.

[0072] S3. When the gas pressure inside the air jet mill reaches the second preset gas pressure of 0.4 atm, maintain the temperature inside the air jet mill up to the secondary mixing and heating temperature of 600℃. Pump the second doping source aqueous solution (the second doping source aqueous solution is a diammonium hydrogen phosphate aqueous solution, and the mass percentage of diammonium hydrogen phosphate in the diammonium hydrogen phosphate aqueous solution is 2%) into the air jet mill at a mass ratio of 30:1 for natural graphite precursor to second doping source aqueous solution. At the same time, perform high-frequency thermal shaping of natural graphite precursor powder at a frequency of 120Hz for 2 hours in the air jet mill, and continuously extract the ammonia gas generated by the reaction of diammonium hydrogen phosphate. Introduce inert gas at a temperature of 120℃ from the top of the air jet mill at a flow rate of 60L / min. After maintaining the gas flow for 2 hours, stop the gas flow. After the air jet mill cools to room temperature, take out natural graphite with an average particle size of 13μm.

[0073] S4. Use deionized water to clean the natural graphite until the pH of the rinse solution is neutral, and dry the natural graphite powder. Use the dried natural graphite particles as the natural graphite core, and then mix the natural graphite powder with asphalt and sinter and carbonize it at 800℃ to form an amorphous carbon coating layer on the surface of the natural graphite particles. The mass of the coating layer is 4% of the mass of the natural graphite, and the negative electrode material can be obtained.

[0074] Example 2

[0075] This embodiment provides natural graphite with the same system as in Example 1. The difference between this embodiment and Example 1 is that the mass percentage of diammonium hydrogen phosphate in the diammonium hydrogen phosphate aqueous solution used in step S2 is 3.6%; and the mass percentage of diammonium hydrogen phosphate in the diammonium hydrogen phosphate aqueous solution used in step S3 is 2%.

[0076] Example 3

[0077] This embodiment provides natural graphite with the same system as in Example 1. The difference between this embodiment and Example 1 is that the mass percentage of diammonium hydrogen phosphate in the diammonium hydrogen phosphate aqueous solution used in step S2 is 10%; and the mass percentage of diammonium hydrogen phosphate in the diammonium hydrogen phosphate aqueous solution used in step S3 is 2%.

[0078] Example 4

[0079] This embodiment provides natural graphite with the same system as in Example 1. The difference between this embodiment and Example 1 is that the mass percentage of diammonium hydrogen phosphate in the diammonium hydrogen phosphate aqueous solution used in step S2 is 5%; and the mass percentage of diammonium hydrogen phosphate in the diammonium hydrogen phosphate aqueous solution used in step S3 is 3%.

[0080] Example 5

[0081] This embodiment provides natural graphite with the same system as in Example 1. The difference between this embodiment and Example 1 is that the mass percentage of diammonium hydrogen phosphate in the diammonium hydrogen phosphate aqueous solution used in step S2 is 10%; and the mass percentage of diammonium hydrogen phosphate in the diammonium hydrogen phosphate aqueous solution used in step S3 is 2%.

[0082] Example 6

[0083] This embodiment provides natural graphite of the same system as in Embodiment 1. The difference between this embodiment and Embodiment 1 is that the temperature of the inert gas introduced into the air jet mill in step S3 is adjusted to 100°C.

[0084] Example 7

[0085] This embodiment provides natural graphite of the same system as in Embodiment 1. The difference between this embodiment and Embodiment 1 is that the temperature of the inert gas introduced into the air jet mill in step S3 is adjusted to 150°C.

[0086] Example 8

[0087] This embodiment provides a natural graphite, comprising a natural graphite core and a coating layer covering the outside of the natural graphite core. The natural graphite is doped with nitrogen (N), with a nitrogen concentration gradient ratio of 2.9, and the mass of N is 2.1% of the natural graphite mass content. The coating layer comprises amorphous carbon, and the mass of the coating layer is 4% of the mass of the natural graphite core particles. This embodiment differs from Embodiment 1 in that, in steps S2 and S3, the first and second dopant source aqueous solutions added during the phase flow milling are ammonium bicarbonate solutions.

[0088] Comparative Example 1

[0089] Comparative Example 1 provides a natural graphite, which is a commercially available unmodified natural graphite with a particle size of 12 μm.

[0090] Comparative Example 2

[0091] Comparative Example 2 provides natural graphite with the same system as Example 1. The difference between this comparative example and Example 1 is that no first dopant source is added in step S2.

[0092] Comparative Example 3

[0093] Comparative Example 2 provides natural graphite with the same system as Example 1. The difference between this comparative example and Example 1 is that no second doping source is added in step S3.

[0094] The natural graphite prepared in Examples 1 to 8 and Comparative Examples 1 to 3 was used as anode material in lithium-ion batteries. The lithium-ion batteries assembled in Examples 1 to 8 and Comparative Examples 1 to 3 were subjected to first charge-discharge efficiency test, fast charge time test, cycle performance test and storage performance test to verify the improvement effect of the anode material on battery capacity, fast charge, cycle and storage performance. The test results are shown in Table 1.

[0095] The assembly process of lithium-ion batteries is as follows:

[0096] Positive electrode preparation: The positive electrode material NCM622, positive electrode conductive agent SP and positive electrode binder PVDF are mixed in a mass ratio of 97:1.8:1.2 (100 parts by mass in total), and 82 parts by mass of NMP are added. The mixture is stirred thoroughly to obtain a positive electrode slurry. The positive electrode slurry is coated on the positive electrode current collector aluminum foil, and then dried, rolled, and cut to form a positive electrode sheet.

[0097] Negative electrode preparation: The natural graphite, negative electrode conductive agent SP, negative electrode binder (PAA and SBR, with a mass ratio of 1.3:0.5) and thickener carboxymethyl cellulose CMC from the above examples or comparative examples are mixed in a mass ratio of 97.2:0.5:1.8:0.5 (100 parts by mass in total), and 82 parts by mass of deionized water are added. The mixture is stirred thoroughly to obtain a negative electrode slurry. The negative electrode slurry is then uniformly coated onto copper foil. After drying, rolling, cutting and other processes, the negative electrode is made.

[0098] Membrane preparation: A polyethylene porous film with a thickness of 11 μm was used as the membrane. The membrane had an air permeability of 230 s / 100 mL and a porosity of 40%.

[0099] Electrolyte preparation: The electrolyte used is a commercially available electrolyte (manufacturer: Xinya Shanshan New Material Technology (Quzhou) Co., Ltd., model: E3).

[0100] Battery assembly: The negative electrode, separator, and positive electrode are stacked sequentially and placed in an aluminum-plastic film to obtain a dry cell. The dry cell is then baked to remove moisture. The prepared electrolyte is injected into the dry cell and packaged to obtain a finished soft-pack lithium-ion battery with a capacity of 1.2Ah.

[0101] Battery performance test:

[0102] Battery discharge capacity and initial coulombic efficiency tests: At room temperature of 25°C, the battery was charged at a constant current rate of 1C to 4.35V, and the charging capacity C1 was recorded during this process; then, the battery was continued to be charged at a constant voltage until the current rate decreased to 0.05C, and the charging capacity C2 was recorded during this process. The charging capacity of the battery was calculated as C1 + C2; for the fully charged battery, it was discharged at a constant current rate of 1C to 2.8V, and the battery discharge capacity was recorded as C3; the initial coulombic efficiency of the battery was calculated as: Initial coulombic efficiency = C3 / (C1 + C2).

[0103] Battery fast charging time test: At room temperature of 25℃, the battery was directly charged to 8% SOC at a current rate of 0.33C. Then, based on the measured three-electrode windows of the battery cell, the charging windows for 10%, 20%, 30%, 40%, 50%, 60%, 70%, and 80% were tested as C1, C2, C3, C4, C5, C6, C7, and C8, respectively. A step charge method was used to charge to 80%, i.e., 8%-10% using C1, 10%-20% using C2, and so on. The charging time from 8% to 80% SOC was recorded as a standard for measuring fast charging capability. The calculation formula is: T (8％-80％) =(0.02 / c1+0.1 / c2+0.1 / c3+0.1 / c4+0.1 / c5+0.1 / c6+0.1 / c7+0.1 / c8)×60.

[0104] Battery cycle life test: At room temperature of 25°C, the battery is charged and discharged at a current rate of 1C / 1C within the test voltage range of 2.8V (discharge cut-off voltage) to 4.35V (charge cut-off voltage) until the capacity is less than or equal to 80% of the initial capacity. The number of cycles is then recorded.

[0105] Battery storage performance testing involves using a 0.33C current to calibrate the lithium-ion battery at room temperature (25°C), denoted as C0. The battery cells are then stored at a high temperature (60°C). Every 7 days, the cells are removed and their capacity is tested at room temperature, denoted as C1, C2, ..., Cn. The number of days until Cn is less than or equal to 80% of C0 is used as the standard for measuring storage capacity and is recorded as the storage days.

[0106] Table 1: Battery performance test results of anode materials prepared in Examples 1 to 8 and Comparative Examples 1 to 3

[0107]

[0108] Comparing the test results of Examples 1 to 8 and Comparative Example 1, it can be seen that, compared with unmodified natural graphite anode materials, this invention, by limiting the doping concentration ratio of doping elements in natural graphite to 2.4 to 3.8, comprehensively improves the mechanical strength and electrochemical performance of natural graphite. Specifically, while retaining the high capacity of natural graphite, it increases the Young's modulus of natural graphite to near the level of artificial graphite, thereby suppressing the charge-discharge expansion of natural graphite and further improving its rate performance and electrical stability. This results in batteries equipped with embodiments of this invention exhibiting superior discharge capacity, first-charge efficiency, fast-charge performance, cycle performance, and storage performance.

[0109] Comparing the test results of Examples 1 to 8 and Comparative Examples 2 and 3, it can be seen that, compared with natural graphite that is only doped and modified near the surface or inside the bulk phase, the embodiments of the present invention, by limiting the concentration ratio of dopants inside and outside the natural graphite to form a particle structure that is hard inside and soft outside, can relatively reduce the surface pore structure while retaining the rich pore structure of natural graphite, and significantly improve the Young's modulus of natural graphite. This effectively improves the surface collapse problem caused by the expansion of natural graphite, and improves the cycle, fast charging and storage performance of the battery while maintaining the high first efficiency of the battery.

[0110] Comparing the test results of Examples 1, 6, and 7, it can be seen that the Young's modulus of natural graphite can be further adjusted by controlling the inert gas temperature during cold extraction. The Young's modulus of natural graphite increases relatively as the cold extraction cooling temperature decreases.

[0111] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A natural graphite characterized in that, The natural graphite core is doped with a doping element, and the doping element includes P and / or N. The gradient concentration ratio of the doping element in the natural graphite core is 2.4-3.8, and the Young's modulus of the natural graphite is 25-32 GPa. The gradient concentration ratio is the mass ratio of the doping element in a first gradient region to the doping element in a second gradient region in the natural graphite core. The first gradient region is a region from the center of the natural graphite core to a position at a distance of R / 2 from the center, and the second gradient region is a region from the position at a distance of R / 2 from the center to the surface of the natural graphite core, and R is the distance from the center to the surface of the natural graphite core.

2. The natural graphite according to claim 1, characterized by, The average particle size Dv50 of the natural graphite core is 5-20 μm, and / or the mass of the doping element is 1%-1.5% of the mass of the natural graphite.

3. The natural graphite according to claim 1, characterized by, The natural graphite contains pores, and the total pore volume of the pores with a pore size of 0.2-1.0 nm is 9.2-15.8 ml / g.

4. The natural graphite according to claim 1, characterized by, The natural graphite core is further provided with a coating layer attached to the surface of the natural graphite core, and the coating layer includes a carbon material. The mass of the coating layer is 2%-5% of the mass of the natural graphite core.

5. A method for producing natural graphite, characterized by, The method comprises the following steps: A natural graphite precursor is provided. A first doping source is mixed with the natural graphite precursor under a first preset gas pressure environment for a first mixing and heating process. The mass ratio of the natural graphite precursor to the first doping source is 100:(0.1-0.5), and the first preset gas pressure is less than the standard atmospheric pressure. A second doping source is mixed with the natural graphite precursor after the first mixing and heating process under a second preset gas pressure environment for a second mixing and heating process to obtain natural graphite. The mass ratio of the natural graphite precursor after the first mixing and heating process to the second doping source is 300:(0.1-0.3), and the second preset gas pressure is greater than the first preset gas pressure and less than the standard atmospheric pressure. The first doping source and the second doping source are compounds containing a doping element, and the doping element independently includes P and / or N. The mass ratio of the first doping source to the second doping source satisfies the condition that the gradient concentration ratio of the doping element in the natural graphite is 2.4-3.

8. The gradient concentration ratio is the mass ratio of the doping element in a first gradient region to the doping element in a second gradient region in the natural graphite particle. The first gradient region is a region from the center of the natural graphite particle to a position at a distance of R / 2 from the center, and the second gradient region is a region from the position at a distance of R / 2 from the center to the surface of the natural graphite particle, and R is the distance from the center to the surface of the natural graphite particle.

6. The production method according to claim 5, wherein The step of providing a natural graphite precursor comprises the following steps: The natural graphite raw material is shaped and classified to obtain a natural graphite precursor. The average particle size Dv50 of the natural graphite precursor is 15-25 μm, and the particle size distribution width of the natural graphite precursor is less than or equal to 1.

1. The particle size distribution width is (Dv90-Dv10) / Dv50.

7. The preparation method according to claim 5, characterized in that, The first preset air pressure is less than 0.2 atm, and the temperature of the first mixed heating is 600-800℃; the second preset air pressure is less than 0.4 atm, and the temperature of the second mixed heating is 600-800℃.

8. The preparation method according to claim 5, characterized in that, After the step of the second mixed heating, a step of quenching treatment of the natural graphite precursor is further included, which comprises: In a preset temperature range, the natural graphite precursor is subjected to cooling treatment by using inert gas; the inert gas flow is 40-80 L / min, the upper limit of the temperature of the preset temperature range is 600-800℃, and the lower limit of the temperature of the preset temperature range is 100-150℃.

9. The preparation method according to claim 5, characterized in that, A step of forming a coating layer on the surface of the natural graphite is further included, which comprises: The natural graphite is mixed and sintered with a carbon source to form a coating layer on the surface of the natural graphite particles.

10. An electrochemical device, characterized by, The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer, and the negative electrode active material layer comprises the natural graphite according to any one of claims 1-4 or the natural graphite prepared by the preparation method according to any one of claims 5-9.

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