A negative electrode material, a negative electrode sheet and a secondary battery

Abstract

The present invention discloses a negative electrode material, a negative electrode sheet and a secondary battery, relating to the technical field of secondary batteries. The negative electrode material includes a core and a coating layer, the coating layer is located on the surface of the core, the core includes natural graphite and a first amorphous carbon filled in the pores of the natural graphite, and the coating layer includes a second amorphous carbon; the Raman spectrum of the negative electrode material has a D peak and a G peak, and the intensity ratio of the D peak to the G peak is I D / I G ; the core includes an inner layer region and an outer layer region located outside the inner layer region, and the outer layer region is adjacent to the coating layer; the average value of I D / I G in the inner layer region is K a , and the average value of the ratio of I D / I G between the inner layer region and the outer layer region is K; it satisfies: 0.4 ≤ K a ≤ 0.7, 0.5 ≤ K < 0.9. Applying this negative electrode material to a secondary battery can significantly improve the first Coulomb efficiency and reduce the swelling rate.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and more particularly, to a negative electrode material, a negative electrode sheet and a secondary battery. Background Art

[0002] Lithium-ion batteries are widely used in the fields of 3C, power devices and energy storage devices due to their low self-discharge rate, high charge and discharge efficiency, no memory effect, long cycle life and other advantages. Among them, the negative electrode material is an important part of the lithium-ion battery, and its performance directly affects the electrochemical performance of the lithium-ion battery. Natural negative electrode materials have received extensive attention due to their high specific capacity, low charge and discharge platform, low cost and other advantages. However, natural graphite has high anisotropy and internal defects. During the lithium-ion insertion process, the volume expansion of graphite mainly occurs in the thickness direction. Solvent molecules in the electrolyte will enter the graphite interlayer together with lithium ions. This process will further increase the expansion degree of graphite. At the same time, the insertion of solvent molecules not only increases the volume of graphite, but also may lead to an incomplete formation of the SEI film, affecting the first Coulombic efficiency of the battery. Summary of the Invention

[0003] The main object of the present invention is to provide a negative electrode material, a negative electrode sheet and a secondary battery to solve the problem that it is difficult for the negative electrode material in the prior art to balance the expansion rate and the first Coulombic efficiency.

[0004] To achieve the above object, according to one aspect of the present invention, there is provided a negative electrode material, which includes a core and a coating layer. The coating layer is located on the surface of the core. The core includes natural graphite and the first amorphous carbon filled in the pores of the natural graphite. The coating layer includes the second amorphous carbon. In the Raman spectrum of the negative electrode material, there are D peak and G peak, and the intensity ratio of the D peak to the G peak is I D / I G ; the core includes an inner layer region and an outer layer region located outside the inner layer region, and the outer layer region is adjacent to the coating layer; the average value of I D / I G in the inner layer region is K a , and the average value of the ratio of I D / I G between the inner layer region and the outer layer region is K; it satisfies: 0.4 ≤ K a ≤ 0.7, 0.5 ≤ K < 0.9.

[0005] Further, the pore area percentage of the core is , and it satisfies: 2% ≤ ≤ 5%.

[0006] Further, the average value of the ratio of the pore area percentage between the inner layer region and the outer layer region is A, and it satisfies: 1.2 ≤ A ≤ 2.0.

[0007] Furthermore, the D50 particle size of the negative electrode material is 5 to 20 μm.

[0008] Furthermore, the shape of the negative electrode material includes at least one of spherical, ellipsoidal, and quasi-spherical.

[0009] Furthermore, the specific surface area of the negative electrode material is 2 to 5 m 2 / g.

[0010] Furthermore, the tap density of the negative electrode material is 0.9 to 1.4 g / cm 3 .

[0011] Furthermore, the average pore diameter of the negative electrode material is 10 to 20 nm.

[0012] In the second aspect of the present invention, a negative electrode sheet is provided, and the negative electrode sheet includes the negative electrode material provided in the first aspect.

[0013] In the third aspect of the present invention, a secondary battery is provided, and the secondary battery includes the negative electrode sheet provided in the second aspect.

[0014] Applying the technical solution of the present invention, by filling the pores of natural graphite with the first amorphous carbon and coating the surface with the second amorphous carbon, the microstructure of the material is optimized, and the electrochemical performance of the material is improved. By defining that the average value of I D / I G in the inner layer region is K a , and the average value of the ratio of I D / I G between the inner layer region and the outer layer region is K; satisfying: 0.4 ≤ K a ≤ 0.7, 0.5 ≤ K < 0.9, it shows that the first amorphous carbon can achieve dense filling and realize a reasonable distribution of the D peak and the G peak, which helps to improve the lithium ion insertion and extraction efficiency, and at the same time effectively relieve the volume expansion during charge and discharge, thereby extending the service life of the secondary battery. Applying this negative electrode material to the secondary battery can make the initial Coulomb efficiency ≥ 94%, and the electrode sheet expansion rate ≤ 25.7% after 20 cycles. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Schematic diagram of the Raman Mapping area of the cross-section of the negative electrode material particles in an embodiment of the present invention;

[0016] Figure 2 SEM image of the cross-sectional morphology of the negative electrode material particles in another embodiment of the present invention.

[0017] Figure 3 Schematic diagram of the structure of the secondary battery provided in an embodiment of the present invention during charging;

[0018] Figure 4 Schematic diagram of the structure of a secondary battery provided by an embodiment of the present invention during discharge.

[0019] Description of reference numerals:

[0020] 100 - electrode assembly; 101 - positive electrode sheet; 102 - negative electrode sheet; 103 - separator. Detailed implementation manners

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. For those not specified in the embodiments, the conventional conditions or the conditions recommended by the manufacturer are followed. For reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0022] As described in the background art of the present invention, in the prior art, the negative electrode material has problems of high expansion rate and low initial Coulomb efficiency. To solve the above problems, in a typical embodiment of the present invention, a negative electrode material is provided. The negative electrode material includes a core and a coating layer. The coating layer is located on the surface of the core. The core includes natural graphite and the first amorphous carbon filled in the pores of the natural graphite. The coating layer includes the second amorphous carbon; in the Raman spectrum of the negative electrode material, there are D peak and G peak, and the intensity ratio of the D peak to the G peak is I D / I G ; the core includes an inner layer region and an outer layer region located outside the inner layer region. The outer layer region is adjacent to the coating layer; the average value of I D / I G in the inner layer region is K a , and the average value of the ratio of I D / I G between the inner layer region and the outer layer region is K; it satisfies: 0.4 ≤ K a ≤ 0.7, 0.5 ≤ K < 0.9.

[0023] The negative electrode material of the present invention has a multi-layer composite structure, including a core and a coating layer. Among them, the core includes natural graphite and the first amorphous carbon filled in the pores of the natural graphite. Through the filling of the first amorphous carbon in the core and the coating of the second amorphous carbon on the surface, the structure of the negative electrode material is significantly optimized. The coating layer includes the second amorphous carbon, which covers the surface of the core to form a protective film.

[0024] Specifically, on the one hand, the first amorphous carbon is a highly disordered carbon structure, which can fill the internal pores of natural graphite, effectively reduce the porosity, and increase the density of the particles; in addition, the filling of the first amorphous carbon can also reduce the anisotropy of the negative electrode material, making the diffusion of lithium ions more uniform in different directions, improving the diffusion rate of Li + between the graphite layers and reducing Li+ The structural strain generated during the embedding and de-embedding processes reduces the expansion rate of the material and improves the structural stability of the material. On the other hand, the coating of the second amorphous carbon helps to reduce the direct contact between natural graphite and the electrolyte, prevent co-embedding of solvents, and facilitate the formation of a stable and dense SEI film during the first charge and discharge processes, improving the electrochemical stability of the material and enhancing the first Coulombic efficiency.

[0025] In the Raman spectrum of the anode material, there are D peaks and G peaks. The diffraction peaks and the corresponding peak intensities can be obtained through Raman spectrum analysis. The D peak is located at approximately 1350 cm -1 , while the G peak is located at approximately 1580 cm -1 . The intensity of the D peak is related to the lattice defects, amorphous carbon, or disordered structure in the anode material, and the intensity of the G peak is related to the graphitization degree or orderliness of the graphite material. The intensity ratio of the D peak to the G peak (I D / I G ) is an important parameter for measuring the amorphous carbon distribution and graphitization degree inside the material, and it reflects the orderliness of the internal structure of the anode material particles and the changes in surface properties. The higher the I D / I G value, the more amorphous carbon or defect structures exist in the material. Conversely, it indicates a higher graphitization degree of the material and a more ordered structure.

[0026] The anode material has multiple particles. The I D / I G in the inner layer region can be the arithmetic mean of the I D / I G in the inner layer region of multiple particles. The mean value of the ratio of I D / I G between the inner layer region and the outer layer region can be the arithmetic mean of the ratio of I D / I G between the inner layer region and the outer layer region of multiple particles. The mean value of I D / I G in the inner layer region is K a , and the mean value of the ratio of I D / I G between the inner layer region and the outer layer region is K. K a and K are used to characterize the overall structural characteristics of the anode material.

[0027] K a The magnitude of the value reflects the average distribution degree of amorphous carbon in the inner layer region and the structural defect state of this region. By limiting 0.4 ≤ K a≤0.7 can ensure that the inner region of the graphite particles contains an appropriate amount of amorphous carbon. This amorphous carbon fills the pores between the graphite layers, which can reduce the volume expansion of graphite during charge and discharge, improve the structural stability of the material, delay the exfoliation of graphite sheets, and thus improve the cycle life of the secondary battery; an appropriate range of K a value helps to form a more optimized SEI film on the surface and inside of the negative electrode material. The stability of the SEI film directly affects the initial Coulombic efficiency, and the amorphous carbon in the inner region can promote the formation of the SEI film, reduce the decomposition of the electrolyte, and thus improve the initial Coulombic efficiency.

[0028] By defining 0.5 ≤ K < 0.9, it shows that there are differences in the structures of the inner and outer regions of the particles. The content of amorphous carbon in the inner region is relatively high, while the content of amorphous carbon in the outer region is relatively low. This indicates that the pores in the inner region are filled with the first amorphous carbon, while the outer region maintains a certain degree of order. By optimizing the distribution of amorphous carbon in the inner and outer layers, the relative balance of the lithium-ion diffusion rates in the inner and outer regions can be ensured, avoiding performance problems caused by too fast or too slow diffusion rates in the internal region; secondly, reducing the loss of lithium ions during charge and discharge, improving the initial Coulombic efficiency. In addition, it helps to control the stress distribution inside the material, reduce the structural damage during charge and discharge, and reduce the risk of exfoliation of graphite sheets, thereby improving the structural stability and cycle life of the material.

[0029] According to the research of the present invention, applying the above negative electrode material to a secondary battery helps to balance the improvement of the initial Coulombic efficiency and the reduction of the swelling rate. This is because by filling the pores of natural graphite with the first amorphous carbon and coating the surface with the second amorphous carbon, the microstructure of the material is optimized; by defining the mean value of the ratio of I D / I G in the inner and outer regions of the core and the mean value of I D / I G in the inner region, this special structure combined with the limitation of I D / I G can ensure the stability of the overall structure of the material, with a small volume change during the lithium-ion insertion and extraction process, reduce the exfoliation and swelling of graphite sheets, and reduce the swelling rate of the electrode sheet during charge and discharge. At the same time, it can also ensure the uniform distribution of amorphous carbon on the surface and inside of the negative electrode material, form a stable SEI film, and the stable and uniform distribution of the SEI film reduces the direct contact between the electrolyte and graphite, thereby reducing the ineffective consumption of lithium ions during the first charge and discharge process and improving the initial Coulombic efficiency. Applying this negative electrode material to a secondary battery can make the initial Coulombic efficiency ≥ 93.5%, and further can achieve an initial Coulombic efficiency ≥ 94%. After 20 cycles, the swelling rate of the electrode sheet ≤ 27.3%, and further can achieve a swelling rate of the electrode sheet ≤ 25.9% after 20 cycles.

[0030] The present invention does not limit K a , the test method of K. Conventional methods in the art can be used, as long as the negative electrode material meets the requirements of the above structure and parameters. For example, in some embodiments, K a , K is obtained through the following steps:

[0031] S11, Select n particles in the negative electrode material, and select the i-th rectangular region on the inner core section of the i-th particle; wherein, the center of the i-th rectangular region is the center of the inner core section, 1 ≤ i ≤ n, n ≥ 15, and both i and n are positive integers;

[0032] S12, Divide the i-th rectangular region along the length direction into an inner rectangular region and an outer rectangular region; wherein, the inner rectangular region encompasses the center of the inner core section, and the outer rectangular region is located outside the inner rectangular region;

[0033] S13, Obtain the I D / I G of the inner rectangular region and the outer rectangular region in the i-th particle, and denote them as K ai and K bi respectively;

[0034] S14, Calculate the average value of K ai of the n particles to obtain the average value of I D / I G of the inner layer region, that is, K a ;

[0035] S15, Calculate the K i value of the i-th particle through K ai = K bi / K i ;

[0036] S16, Calculate the average value of K i of the n particles to obtain the average value of the ratio of I D / I G between the inner layer region and the outer layer region, that is, K.

[0037] Specifically, in S11, randomly select n particles from the negative electrode material, where n is a sufficiently large sample size, usually required to be a positive integer of ≥ 15, to ensure the statistical significance and representativeness of the analysis results. Then, the i-th particle can be cut using slicing or ion beam to expose the inner core section of the i-th particle, and an i-th rectangular region is selected on the inner core section. The center of the i-th rectangular region is set as the geometric center of the inner core section, that is, this region covers the center position of the particle. Such a selection ensures the representativeness of the analysis region and can reflect the characteristics of the particle core. The value range of i is a positive integer from 1 to m.

[0038] In S12, within the selected i-th rectangular region, it is further divided into three parts along the length direction: an inner rectangular region and two outer rectangular regions. The two outer rectangular regions are respectively located on both sides of the inner rectangular region, that is, the outer rectangular regions are adjacent to the edges of the inner rectangular region, corresponding to the structural characteristics of the inner layer region and the outer layer region in the particles. By comparing these two regions, the distribution of amorphous carbon can be analyzed.

[0039] In S13, using Raman spectroscopy technology, the inner rectangular region and the outer rectangular region of the i-th particle are scanned respectively to obtain the Raman spectroscopy data of these two regions; from the Raman spectroscopy, the intensities of the D peak and the G peak can be extracted, and then the I D / I G value can be calculated, which are respectively denoted as K ai (the I D / I G value of the inner layer region of the i-th particle) and K bi（ (the I D / I G value of the outer layer region of the i-th particle).

[0040] In S14, the I D / I G values of the inner rectangular regions of n particles are averaged to obtain the K a value, which represents the average I D / I G value of the inner layer region of the negative electrode material. The magnitude of the K a value reflects the average state of the relative content of amorphous carbon and the degree of structural disorder in the inner layer region.

[0041] In S15, for each particle, by calculating K i = K ai / K bi , the ratio of the I D / I G values between the inner rectangular region and the outer rectangular region in the i-th particle is obtained, that is, the K i value of the i-th particle. This ratio can reflect the difference in the structural characteristics between the inner layer region and the outer layer region in the i-th particle.

[0042] In S16, the K i values of n particles are averaged to obtain the K value, that is, the average value of the ratio of I D / I G between the inner rectangular region and the outer rectangular region. K reflects the average value of the ratio of I D / I G between the inner layer region and the outer layer region in the entire negative electrode material, and is used to represent the difference in the property characteristics between the inner and outer layer regions of the particles.

[0043] The setting of the i-th rectangular region is for more precise analysis of the distribution of amorphous carbon at different depths of the negative electrode material particles. In some embodiments, the width of the i-th rectangular region is 1 - 6 μm, ensuring that the width of the i-th rectangular region is small enough to finely detect changes in the internal structure of the material. In some embodiments, two endpoints in the length direction or two endpoints on the diagonal of the i-th rectangular region are located on the edge of the inner core section plane. In this way, it is ensured that the i-th rectangular region can span the entire inner core section plane, which can ensure that the obtained data is both accurate and representative, and helps to analyze the distribution of amorphous carbon at different depths.

[0044] The inner rectangular region represents the average state of the inner layer region of the particle, and the outer rectangular region represents the average state of the outer layer region of the particle. In some embodiments, the center of the inner rectangular region is the center of the inner core section plane, ensuring that the inner rectangular region is the inner layer region of the particle, which can more accurately evaluate the filling uniformity of amorphous carbon inside the particle. In some embodiments, the length of the inner rectangular region is equal to 1 / 2 of the length of the i-th rectangular region. This can ensure that the depth of the inner rectangular region is sufficient to reach the internal structure of the particle, rather than just the surface or near-surface region.

[0045] In some embodiments, the percentage of the pore area of the inner core is , satisfying: 2% ≤ ≤ 5%. By controlling the proportion of the inner core pore area between 2% and 5%, the volume change of the negative electrode material during charge and discharge can be reduced, preventing the particles from cracking due to excessive expansion, thereby improving the structural stability of the material. In addition to further enhancing the first Coulomb efficiency and reducing the expansion rate, it also helps to construct a continuous conductive network, reducing the resistance of electron and lithium ion transmission, and improving the efficiency and safety of the secondary battery.

[0046] The average value of the ratio of the percentage of the pore area of the inner layer region to that of the outer layer region is A, satisfying: 1.2 ≤ A ≤ 2.0. By defining the average value of the ratio of the proportion of the pore area of the inner layer region to that of the outer layer region, it helps to construct a more uniform pore distribution inside the negative electrode material, which is beneficial to the uniform insertion and extraction of lithium ions, reduces local stress, and further improves the structural stability and the first Coulomb efficiency. In addition, the appropriate pore area and uniform pore distribution can promote the more efficient diffusion of lithium ions between the graphite layers, shortening the charge and discharge time and increasing the power density of the secondary battery.

[0047] In the present invention, the exposed surface obtained by cutting the negative electrode material particles can be called a section plane, and the internal structural characteristics of the material can be shown through the section plane. The section plane can be cut through the center of the particle and perpendicular to the length direction or the width direction of the particle. The section plane can be used for microstructure analysis, such as scanning electron microscopy (SEM), Raman spectrometer, etc.

[0048] The present invention is not limited , to the test method of A, conventional methods in the art can be used, as long as the negative electrode material meets the requirements of the above structure and parameters. In some embodiments, , A is obtained through the following steps:

[0049] S21, Select m particles from the negative electrode material, and divide the inner core section of the j-th particle into a central region and an edge region; wherein, the edge region is located on the outer periphery of the central region, 1 ≤ j ≤ n, m ≥ 20, and both j and m are positive integers;

[0050] S22, Obtain the pore area ratio of the j-th particle, and the pore area ratios of the central region and the edge region, and denote them as , and ;

[0051] S23, Calculate the i average value of m particles to obtain the pore area percentage of the inner core ;

[0052] S24, Calculate the A of the j-th particle through A j = aj / bj ; j value of the j-th particle;

[0053] S25, Calculate the average value of A of m particles j to obtain the average value of the ratio of the pore area percentages of the inner layer region and the outer layer region, that is, A.

[0054] Specifically, in S21, randomly select m particles from the negative electrode material. m is a sufficiently large sample size to ensure the representativeness of the statistical sample. Usually, m is a positive integer ≥ 20. The j-th particle can be cut using a slicing method or an ion beam to expose the inner core section of the j-th particle, and it is divided into two parts: a central region and an edge region. The edge region is located on the outer periphery of the central region. This division helps us analyze the pore distribution in different regions, thereby evaluating the consistency of the internal structure of the material. The value range of j is a positive integer from 1 to m.

[0055] In S22, SEM (scanning electron microscope) images can be used. Through software tools, the pore area on the inner core section of the particle is identified and measured. Dividing the pore area by the total area of the inner core section of the entire particle can obtain the pore area ratio of the j-th particle, that is, j, which reflects the pore distribution of the entire particle. Similarly, identify and measure the pore area of the central region, and divide the pore area of the central region by the total area of the central region to obtain the proportion of the pore area of the central region of the j-th particle, that is aj , which reflects the pore distribution of the central region; identify and measure the pore area of the edge region, and divide the pore area of the edge region by the total area of the edge region to obtain the proportion of the pore area of the edge region of the j-th particle, that is bj , which reflects the pore distribution of the edge region. It can be understood that the pore area of the j-th particle is equal to the sum of the pore areas of its central region and edge region, and the total area of the j-th particle is equal to the sum of the areas of its central region and edge region.

[0056] In S23, for m particles j perform an average calculation to obtain the percentage of the pore area of the kernel, that is , which reflects the proportion of the pore area of the kernel in the entire negative electrode material.

[0057] In S24, for each particle, through A j = aj / bj calculate to obtain the ratio of the percentage of the pore area of the central region to the percentage of the pore area of the edge region of the j-th particle, that is the A j value of the j-th particle. This ratio can reflect the difference in the pore structure characteristics between the inner layer region and the outer layer region of the j-th particle.

[0058] In S25, perform an average calculation on the A j of m particles to obtain the A value, that is, the average value of the ratio of the percentage of the pore area of the central region to the percentage of the pore area of the edge region. A reflects the average value of the ratio of the percentage of the pore area of the inner layer region to the percentage of the pore area of the outer layer region in the entire negative electrode material, and is used to represent the difference in the property characteristics between the inner and outer layer regions of the particles.

[0059] The central region represents the average state of the inner layer region of the particle, and the edge region represents the average state of the outer layer region of the particle. In some embodiments, the central region is circular or elliptical, the center of the central region is the center of the kernel section plane, and the definition of the central region enables the analysis range to cover the central part of the kernel section plane of the particle.

[0060] In some embodiments, the major axis of the central region is equal to 1 / 2 of the length of the transverse median line of the kernel cross-section, and the minor axis of the central region is equal to 1 / 2 of the length of the longitudinal median line of the kernel cross-section. The kernel cross-section has two mutually perpendicular transverse and longitudinal median lines passing through the center of the kernel cross-section, and the major and minor axes of the central region are 1 / 2 of the transverse and longitudinal median lines respectively.

[0061] The D50 particle size is the particle size corresponding to when the cumulative volume distribution percentage in the sample reaches 50%, which reflects the average particle size of the negative electrode material. In some embodiments, the D50 particle size of the negative electrode material is 5 - 20 μm. By limiting the D50 particle size of the negative electrode material within an appropriate range, in addition to helping to further improve the first Coulombic efficiency of the secondary battery and reduce the swelling rate, it can also make the diffusion path of lithium ions shorter, reduce the transport resistance of lithium ions during the insertion and extraction processes, thereby improving the charge and discharge efficiency. In addition, it can also reduce the internal pores of the electrode and improve the energy density of the secondary battery.

[0062] A regular particle shape helps to improve the cycle stability and lifespan of the secondary battery. In some embodiments, the shape of the negative electrode material includes at least one of spherical, ellipsoidal, and quasi-spherical. The shape of the negative electrode material essentially refers to the particle shape of the negative electrode material. By limiting the shape of the negative electrode material, it helps to form a more uniform electron and lithium ion transport path, reduce local stress concentration, and improve the power density of the secondary battery.

[0063] The specific surface area (SSA) refers to the total surface area of a unit mass of material. In some embodiments, the specific surface area of the negative electrode material is 2 - 5 m 2 / g. By limiting the specific surface area of the negative electrode material within a moderate range, in addition to further improving the first Coulombic efficiency and reducing the swelling rate, it can also provide a moderate diffusion path, avoiding the performance degradation caused by the electrolyte penetrating into the internal pores of the material, and being beneficial to the rapid and uniform diffusion of lithium ions during charge and discharge, thereby improving the rate performance and cycle stability of the secondary battery. In addition, it can reduce the contact between the electrolyte and the surface of the graphite material and reduce the possibility of side reactions such as electrolyte decomposition and co-insertion of solvent molecules.

[0064] The tapped density (Tap) refers to the degree of compaction of the material under physical vibration. In some embodiments, the tapped density of the negative electrode material is 0.9 - 1.4 g / cm 3By limiting the tap density of the negative electrode material within a moderate range, it helps to improve the compaction density of the material during the electrode manufacturing process, reduce the pores inside the electrode, enhance the utilization rate of the negative electrode material and the energy density of the secondary battery. At the same time, it can reduce the contact area between the electrolyte and the graphite particles, thereby reducing the formation of the SEI film, lowering the irreversible capacity during the first charge and discharge, and further improving the first Coulomb efficiency. It also helps to reduce the volume expansion during the cycling process, lower the mechanical stress between the particles, and prevent the formation of cracks. In addition, it helps to maintain the integrity of the electrode structure, prevent the active material from falling off the current collector, and contribute to improving the cycling performance.

[0065] In some embodiments, the average pore diameter of the negative electrode material is 10 - 20 nm. By limiting the average pore diameter of the negative electrode material within the above range, firstly, it helps lithium ions to be more uniformly inserted into the graphite layer during the first charge and discharge process, which is conducive to the formation of a stable and dense SEI film, and can further improve the first Coulomb efficiency. Secondly, restricting the average pore diameter size can reduce the volume change of the material during the lithium ion insertion and extraction process, further reducing the expansion rate of the graphite particles during the charge and discharge process, helping to improve the stability of the material structure, reducing the stress accumulation inside the secondary battery, and extending the service life of the secondary battery. In addition, lithium ions can diffuse relatively easily and quickly between the graphite layers, which helps to improve the charge and discharge rate and shorten the charge and discharge time.

[0066] In some embodiments, the average particle diameter of the core is 5 - 20 μm; the thickness of the coating layer is 2 - 100 nm. It can be adjusted according to actual needs.

[0067] In some embodiments, the mass ratio of the core to the coating layer is 100:(4 - 10), such as 100:4, 100:5, 100:6, 100:7, 100:8, 100:9, 100:10 or the range composed of any two of them; the mass ratio of the first amorphous carbon to the second amorphous carbon is (5 - 10):3, such as 5:3, 6:3, 7:3, 8:3, 9:3, 10:3 or the range composed of any two of them.

[0068] The negative electrode material of the present invention can be used as a negative electrode material in secondary batteries, which can effectively improve the first Coulomb efficiency of the secondary battery and reduce the expansion rate. For example, in some embodiments, the first Coulomb efficiency of the negative electrode material is ≥93.5%, and further, the first Coulomb efficiency ≥94% can be achieved. After 20 cycles, the expansion rate of the electrode sheet is ≤27.3%, and further, the expansion rate of the electrode sheet ≤25.9% can be achieved after 20 cycles, and the capacity is ≥362 mAh / g.

[0069] The present invention does not limit the specific preparation process of the negative electrode material, as long as the above parameters are satisfied. In some embodiments, the preparation method of the above negative electrode material includes the following steps:

[0070] S1. After successively performing first mixing and coating and first heat treatment on graphite raw materials and the first part of pitch, a composite is obtained.

[0071] S2. The composite is subjected to isostatic pressing densification treatment, and then after being crushed, a first intermediate product is obtained.

[0072] S3. The first intermediate product and the second part of pitch are subjected to second mixing and coating to obtain a secondary coated product.

[0073] S4. The secondary coated product is subjected to molding treatment to obtain a second intermediate product.

[0074] S5. The second intermediate product is subjected to second heat treatment to obtain the negative electrode material.

[0075] In S1, when the graphite raw materials and the first part of pitch are mixed and coated, it can be understood that the graphite raw materials and the pitch are mixed under the temperature condition of the softening point of the pitch. During the first mixing and coating process, the first part of pitch is in a molten state and is mixed evenly with the graphite raw materials to obtain a primary coated product; the primary coated product is subjected to first heat treatment, so that the molten first part of pitch fills the pores inside the graphite to obtain a composite. Through mixing and coating and first heat treatment, it helps to improve the densification of the material. The graphite raw materials can be spherical graphite with a D50 particle size of 1 - 20 μm.

[0076] The specific mixing ratio of the graphite raw materials and the first part of pitch can be adjusted according to material characteristics and requirements. The first part of pitch is selected from at least one of petroleum pitch, coal pitch, and mesophase pitch, and the D50 particle size of the pitch is 2 - 3 mm; the softening point of the first part of pitch is 100 - 300 °C. It can be understood that the temperature of mixing and coating is 100 - 300 °C, and the mixing and coating time is 10 - 60 min; the first heat treatment can be carried out under the condition of a first inert atmosphere and at a temperature 100 - 200 °C higher than the softening point of the pitch; the first inert atmosphere is to prevent the oxidation of the material during the heat treatment process, and any inert gas can achieve this purpose. For example, it can include at least one of nitrogen, helium, and argon; the temperature of the first heat treatment is 200 - 600 °C, and the time is 3 - 4 h.

[0077] In S2, the composite is subjected to isostatic pressing densification treatment to obtain an isostatic pressing product, and then the isostatic pressing product is crushed to obtain a first intermediate product. Through isostatic pressing densification treatment, further internal filling is realized, which can effectively reduce the pores inside the graphite, increase the density of the material, enhance its structural densification, and improve the pore filling effect.

[0078] The specific pressure and time of the isostatic pressing treatment can be optimized according to the performance requirements of the material and the equipment capabilities. For example, in some embodiments, during the isostatic pressing treatment, the filling effect of the pitch inside the graphite can be regulated by controlling the pressure. For example, the isostatic pressing treatment can adopt cold isostatic pressing treatment or warm isostatic pressing treatment. The pressure of the isostatic pressing treatment is 60-120 MPa, and the pressure holding time is 1-60 min.

[0079] In S3, the first intermediate product and the second part of the pitch are subjected to a second mixing and coating. It can be understood that the first intermediate product and the second part of the pitch are mixed under the temperature condition of the pitch softening point. During the second mixing and coating process, the second part of the pitch is in a molten state and is uniformly mixed with the first intermediate product, so that the molten second part of the pitch coats the surface of the first intermediate product to obtain a secondary coated product.

[0080] In some embodiments, the mass ratio of the graphite raw material to the pitch is 100:(8-12), such as 100:8, 100:9, 100:10, 100:11, 100:12 or the range composed of any two of them. The mass of the pitch refers to the sum of the masses of the first part of the pitch and the second part of the pitch. The mass ratio of the first part of the pitch to the second part of the pitch is (5-10):3, such as 5:3, 6:3, 7:3, 8:3, 9:3, 10:3 or the range composed of any two of them.

[0081] In S4, a hydraulic press can be used to press the secondary coated product to obtain a second intermediate product. The specific pressure, time, and number of cycles of the pressing treatment can be adjusted according to the actual equipment capabilities and material requirements. For example, in some embodiments, the pressing conditions are: the pressure of the hydraulic press is 10-40 MPa, the pressure holding time is 0-2 min, and after releasing the pressure for 0.5 min, it reciprocates slowly 2-4 times. The short pressure holding time can ensure that the material quickly forms a dense structure under high pressure, while avoiding equipment wear and reduced production efficiency caused by long-term pressure holding. It is suitable for continuous production processes and can improve production efficiency and equipment utilization rate.

[0082] In S5, the second intermediate product is subjected to a second heat treatment in a second inert atmosphere to ensure the graphitization degree of the material. After the second heat treatment, the negative electrode material is obtained after being broken up, demagnetized, and sieved. Through the second heat treatment, the first part of the pitch filled into the pores inside the graphite is converted into the first amorphous carbon, and the second part of the pitch coated on the surface is converted into the second amorphous carbon, obtaining the negative electrode material with the above special structure. Through the second heat treatment, it is ensured that the material is stably carbonized at high temperature and its graphitization degree is improved. Appropriate heat treatment temperature and time can affect the crystallinity and microstructure of the graphite, so that the I D / I G meets the above requirements.

[0083] The specific temperature and time of the second heat treatment can be adjusted according to the conditions of the heat treatment apparatus and the performance target of the material. For example, in some embodiments, the temperature of the second heat treatment is greater than that of the first heat treatment. For example, the temperature of the second heat treatment is 900 - 1500 °C, and the time is 1 - 24 h. Among them, the second inert atmosphere is to prevent the oxidation of the material during the heat treatment, and any inert gas can achieve this purpose. For example, it may include at least one of nitrogen, helium, and argon.

[0084] In a second aspect of the present invention, a negative electrode sheet is provided, which includes the negative electrode material provided in the first aspect above.

[0085] The negative electrode sheet of the present invention includes a negative electrode current collector and a negative electrode material active layer provided on at least one surface of the negative electrode current collector. The negative electrode material active layer includes the negative electrode material provided in the first aspect. The negative electrode current collector can use at least one of copper foil, nickel foil, stainless steel foil, titanium foil, or carbon-based current collector, etc., and can also be a composite current collector disclosed in any prior art, such as but not limited to the current collector formed by combining the aforementioned conductive foil and polymer substrate. Since it includes a negative electrode material with excellent performance, applying this negative electrode sheet to a secondary battery helps to improve the first Coulomb efficiency and reduce the swelling rate of the secondary battery.

[0086] The negative electrode material active layer further includes a binder, which is used to bond the negative electrode active material particles to facilitate the formation of a film layer, and at the same time can also improve the bonding force between the negative electrode material active layer and the negative electrode current collector. In some embodiments, the binder may include but not be limited to polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoroethylene, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon, etc.

[0087] The negative electrode material active layer may further include a conductive material, and the conductive material includes but not limited to carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, the carbon-based materials may include but not be limited to natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based materials may include but not be limited to metal powder or metal fiber, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0088] When specifically preparing the negative electrode sheet, the negative electrode material, the conductive agent, and the binder can be dispersed in an appropriate amount of solvent, and fully stirred and mixed to form a uniform negative electrode paste; the negative electrode paste is uniformly coated on the negative electrode current collector, and after drying, rolling, and slitting, the negative electrode sheet is obtained. In a specific embodiment, the negative electrode active layer includes 70% to 99% of the negative electrode material, 0.5% to 15% of the conductive agent, and 0.5% to 15% of the binder by mass percentage.

[0089] Among them, the conductive agent can be selected from at least one of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, graphene; the binder can be selected from at least one of carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyvinyl alcohol, sodium polyacrylate.

[0090] In the third aspect of the present invention, a secondary battery is provided, and the secondary battery includes the negative electrode sheet provided in the second aspect above.

[0091] Due to including the above-mentioned negative electrode sheet with excellent performance, the secondary battery has excellent initial Coulomb efficiency and low expansion rate.

[0092] Specifically, the secondary battery includes a housing, an electrode assembly, and an electrolyte. The electrode assembly and the electrolyte are both located inside the housing.

[0093] The housing can be a packaging bag obtained by encapsulating with a packaging film (such as an aluminum-plastic film), for example, a soft-pack battery. In some other embodiments, it can also be a steel-shell battery, an aluminum-shell battery, etc.

[0094] Please refer to Figure 3 and Figure 4 , the electrode assembly 100 includes a positive electrode sheet 101, a negative electrode sheet 102, and a separator 103, and the separator 103 is disposed between the positive electrode sheet 101 and the negative electrode sheet 102. During charging, please refer to Figure 3 , active ions (such as lithium ions) are deintercalated from the lattice of the positive electrode material (such as a lithiated intercalation compound) of the positive electrode sheet 101, pass through the electrolyte and through the separator 103, reach the negative electrode sheet 102 and are inserted into the lattice of the negative electrode material. During discharging, please refer to Figure 4 , active ions (such as lithium ions) are deinserted from the lattice of the negative electrode material of the negative electrode sheet 102, pass through the electrolyte and through the separator 103, reach the positive electrode sheet 101 and are embedded into the lattice of the positive electrode material (such as a lithiated intercalation compound), generating electrons that reach the positive electrode sheet 101 from the negative electrode sheet 102 through the external circuit, and the reverse movement of the electrons forms a current, which can be used by electrical appliances.

[0095] In some embodiments, the electrode assembly 100 may be a laminated structure, which is formed by alternately laminating a positive electrode sheet 101, a separator 103, and a negative electrode sheet 102 in sequence. In other embodiments, the electrode assembly 100 may also be a wound structure, which is formed by laminating a positive electrode sheet 101, a separator 103, and a negative electrode sheet 102 in sequence and then winding them.

[0096] The positive electrode sheet 101 includes a positive electrode current collector and a positive electrode material active layer provided on at least one surface of the positive electrode current collector. The positive electrode current collector may use aluminum foil, nickel foil, etc., or may be a composite current collector disclosed in any prior art, such as but not limited to the current collector formed by combining the aforementioned conductive foil and a polymer substrate. The positive electrode material active layer includes a positive electrode active material, and the positive electrode active material includes a compound that reversibly intercalates and deintercalates lithium ions (i.e., a lithiated intercalation compound). In some embodiments, the positive electrode active material may include a lithium transition metal composite oxide. The lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, manganese, and nickel. In some embodiments, the positive electrode active material may include but not be limited to lithium cobalt oxide (LiCoO 2 ), lithium nickel manganese cobalt ternary material (NCM), lithium manganese oxide (LiMn 2 O 4 ), lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O 4 ), or lithium iron phosphate (LiFePO 4 ), or at least one of them.

[0097] The positive electrode material active layer further includes an adhesive for bonding the positive electrode active material particles to facilitate the formation of a film layer and at the same time improve the bonding force between the positive electrode material active layer and the positive electrode current collector. In some embodiments, the adhesive may include but not be limited to at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoroethylene, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0098] The positive electrode material active layer may further include a conductive material, and the conductive material includes but not limited to carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, the carbon-based materials may include but not be limited to natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based materials may include but not be limited to metal powder or metal fiber, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0099] The separator 103 includes a film layer with a porous structure, and its material includes but is not limited to at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, the separator 103 can be a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric, or a polypropylene-polyethylene-polypropylene porous composite film, etc.

[0100] The electrolyte functions to conduct ions between the positive electrode sheet 101 and the negative electrode sheet 102. The state of the electrolyte can be one or more of gel state, solid state, and liquid state. In some embodiments, the electrolyte uses an electrolytic solution. The electrolytic solution functions to conduct active ions between the positive electrode sheet 101 and the negative electrode sheet 102. In some embodiments, the electrolytic solution includes a lithium salt and an organic solvent. The lithium salt can be selected from but is not limited to lithium hexafluorophosphate (LiPF 6 ) 4 , lithium tetrafluoroborate (LiBF 6 ), lithium hexafluoroarsenate (LiAsF 4 ), lithium perchlorate (LiClO 6 ), lithium tetraphenylborate (LiB(C 5 H 4 ) 3 ), lithium methanesulfonate (LiCH 3 SO 3 ), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiCF 3 SO 2 ), lithium bis(trifluoromethylsulfonyl)imide (LiN(SO 3 CF 2 )) 2 , lithium tris(trifluoromethylsulfonyl)methide (LiC(SO 3 CF 3 )) 2 , lithium difluoro(oxalato)borate (LiBOB), and lithium difluorophosphate (LiPO 2 F 6, because it can give a high ionic conductivity and improve the cycling performance. The organic solvent can be a carbonate compound, a carboxylate compound, an ether compound, a nitrile compound, other organic solvents, or a combination thereof. Examples of the carbonate compound include but are not limited to diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene ethylene carbonate (VEC), fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethyl ethylene carbonate, or a combination thereof.

[0101] When preparing a secondary battery, a positive electrode sheet, a separator, and a negative electrode sheet are wound or laminated to obtain an electrode core, and the electrode core is encapsulated in a pre-stamped aluminum-plastic film. After the encapsulated battery is dried to remove moisture, an electrolyte is injected into the dried battery. After the battery is left standing, formed, and secondarily sealed, the preparation of the secondary battery is completed.

[0102] The following further describes the present application in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by the present application.

[0103] Example 1

[0104] The preparation method of the negative electrode material in this example includes the following steps:

[0105] S1, Add 20 kg of graphite raw material (D50 particle size is 16 μm) and 1.6 kg of asphalt (softening point 250 °C) to a VC mixer and mix for 25 min. After uniform mixing, perform low-temperature heat treatment at 400 °C for 4 h under a nitrogen protection atmosphere. After cooling to room temperature, obtain a composite body;

[0106] S2, Perform isostatic pressing densification treatment on the composite body. The maximum isostatic pressure is 60 MPa. After maintaining the pressure for 15 min, crush the material to about 16 μm to obtain a first intermediate product;

[0107] S3, Add the first intermediate product and 0.6 kg of asphalt (softening point 250 °C) to a VC mixer and mix for 25 min to obtain a secondary coating;

[0108] S4, Press the secondary coating on a hydraulic press. The pressure of the hydraulic press is 30 MPa, maintain the pressure for 0.5 min, and reciprocate 3 times to obtain a second intermediate product;

[0109] S5. Subject the second intermediate product to high-temperature heat treatment, carbonize it at 1250 °C for 16 h under a nitrogen protection atmosphere, and after carbonization, obtain the negative electrode material of this example through dispersion, demagnetization, and screening.

[0110] Example 2

[0111] The preparation method of the negative electrode material of this example includes the following steps:

[0112] S1. Add 20 kg of graphite raw material (D50 particle size is 16 μm) and 1.6 kg of asphalt (softening point 180 °C) to a VC mixer and mix for 25 min. After uniform mixing, perform low-temperature heat treatment at 350 °C for 4 h under a nitrogen protection atmosphere. After cooling to room temperature, obtain a composite body.

[0113] S2. Perform isostatic pressing densification treatment on the composite body. The maximum isostatic pressing pressure is 50 MPa. After maintaining the pressure for 15 min, crush the material to about 16 μm to obtain a first intermediate product.

[0114] S3. Add the first intermediate product and 0.6 kg of asphalt (softening point 180 °C) to a VC mixer and mix for 30 min to obtain a secondary coating.

[0115] S4. Press the secondary coating on a hydraulic press. The pressure of the hydraulic press is 30 MPa, maintain the pressure for 0.5 min, and reciprocate 3 times to obtain a second intermediate product.

[0116] S5. Subject the second intermediate product to high-temperature heat treatment, carbonize it at 1150 °C for 12 h under a nitrogen protection atmosphere, and after carbonization, obtain the negative electrode material of this example through dispersion, demagnetization, and screening.

[0117] Example 3

[0118] The preparation method of the negative electrode material of this example includes the following steps:

[0119] S1. Add 20 kg of graphite raw material (D50 particle size is 16 μm) and 1.0 kg of asphalt (softening point 180 °C) to a VC mixer and mix for 25 min. After uniform mixing, perform low-temperature heat treatment at 350 °C for 4 h under a nitrogen protection atmosphere. After cooling to room temperature, obtain a composite body.

[0120] S2. Perform isostatic pressing densification treatment on the composite body. The maximum isostatic pressing pressure is 50 MPa. After maintaining the pressure for 15 min, crush the material to about 16 μm to obtain a first intermediate product.

[0121] S3. Add the first intermediate product and 0.6 kg of asphalt (softening point 180 °C) to a VC mixer and mix for 25 min to obtain a first intermediate product.

[0122] S4. Press the first intermediate product in a hydraulic press at a pressure of 25 MPa for 0.5 min with 3 reciprocations to obtain the second intermediate product.

[0123] S5. Heat-treat the second intermediate product at high temperature and carbonize it at 1150 °C for 10 h under a nitrogen protection atmosphere. After carbonization, it is dispersed, demagnetized, and screened to obtain the negative electrode material of this example.

[0124] Example 4

[0125] The preparation method of the negative electrode material in this example includes the following steps:

[0126] S1. Add 20 kg of graphite raw material (D50 particle size of 14.5 μm) and 1.6 kg of asphalt (softening point of 250 °C) to a VC mixer and mix for 25 min. After uniform mixing, perform low-temperature heat treatment at 400 °C for 4 h under a nitrogen protection atmosphere. After cooling to room temperature, obtain a composite body.

[0127] S2. Perform isostatic pressing densification treatment on the composite body. The maximum isostatic pressing pressure is 70 MPa. After holding the pressure for 15 min, crush the material to about 14.5 μm to obtain the first intermediate product.

[0128] S3. Add the first intermediate product and 0.6 kg of asphalt (softening point of 250 °C) to a VC mixer and mix for 30 min to obtain a secondary coating.

[0129] S4. Press the secondary coating in a hydraulic press at a pressure of 30 MPa for 0.5 min with 3 reciprocations. After rough crushing, obtain the second intermediate product.

[0130] S5. Heat-treat the second intermediate product at high temperature and carbonize it at 1250 °C for 16 h under a nitrogen protection atmosphere. After carbonization, it is dispersed, demagnetized, and screened to obtain the negative electrode material of this example.

[0131] Example 5

[0132] The preparation method of the negative electrode material in this example includes the following steps:

[0133] S1. Add 20 kg of graphite raw material (D50 particle size of 12 μm) and 1.6 kg of asphalt (softening point of 250 °C) to a VC mixer and mix for 25 min. After uniform mixing, perform low-temperature heat treatment at 400 °C for 4 h under a nitrogen protection atmosphere. After cooling to room temperature, obtain a composite body.

[0134] S2. Perform isostatic pressing densification treatment on the composite body. The maximum isostatic pressing pressure is 60 MPa. After holding the pressure for 15 min, crush the material to about 12 μm to obtain the first intermediate product.

[0135] S3. Add 0.6 kg of pitch (softening point 250 °C) and the first intermediate product to a VC mixer and mix for 30 min to obtain a secondary coating;

[0136] S4. Compress the secondary coating on a hydraulic press. The pressure of the hydraulic press is 30 MPa, the pressure holding time is 0.5 min, and it reciprocates 3 times. After rough crushing, a second intermediate product is obtained;

[0137] S5. Conduct high-temperature heat treatment on the second intermediate product. Under a nitrogen protection atmosphere, carbonize at 1250 °C for 14 h. After carbonization, through dispersion, demagnetization, and screening, the negative electrode material of this example is obtained.

[0138] Example 6

[0139] The preparation method of the negative electrode material in this example includes the following steps:

[0140] S1. Add 20 kg of graphite raw material (D50 particle size is 10 μm) and 1.6 kg of pitch (softening point 250 °C) to a VC mixer and mix for 25 min. After uniform mixing, conduct low-temperature heat treatment at 400 °C for 4 h under a nitrogen protection atmosphere. After cooling to room temperature, a composite body is obtained;

[0141] S2. Conduct isostatic pressing densification treatment on the composite body. The maximum pressure of isostatic pressing is 60 MPa. After pressure holding for 15 min, crush the material to about 10 μm to obtain a first intermediate product;

[0142] S3. Add 0.6 kg of pitch (softening point 250 °C) and the first intermediate product to a VC mixer and mix for 25 min to obtain a secondary coating;

[0143] S4. Compress the secondary coating on a hydraulic press. The pressure of the hydraulic press is 30 MPa, the pressure holding time is 0.5 min, and it reciprocates 3 times. After rough crushing, a second intermediate product is obtained;

[0144] S5. Conduct high-temperature heat treatment on the second intermediate product. Under a nitrogen protection atmosphere, carbonize at 1250 °C for 18 h. After carbonization, through dispersion, demagnetization, and screening, the negative electrode material of this example is obtained.

[0145] Example 7

[0146] The preparation method of the negative electrode material in this example includes the following steps:

[0147] S1. Add 20 kg of graphite raw material (D50 particle size is 16 μm) and 0.7 kg of pitch (softening point 180 °C) to a VC mixer and mix for 25 min. After uniform mixing, conduct low-temperature heat treatment at 350 °C for 4 h under a nitrogen protection atmosphere. After cooling to room temperature, a composite body is obtained;

[0148] S2. Isostatically compact the composite. The maximum isostatic pressure is 50 MPa. After maintaining the pressure for 15 min, crush the material to about 16 μm to obtain the first intermediate product.

[0149] S3. Add 0.3 kg of asphalt (softening point 180 °C) to the first intermediate product and mix them in a VC mixer for 25 min to obtain the first intermediate product.

[0150] S4. Press the first intermediate product on a hydraulic press. The pressure of the hydraulic press is 25 MPa. Maintain the pressure for 0.5 min and reciprocate 3 times to obtain the second intermediate product.

[0151] S5. Perform high-temperature heat treatment on the second intermediate product. Under a nitrogen protection atmosphere, carbonize it at 1150 °C for 12 h. After carbonization, disperse, demagnetize, and screen it to obtain the negative electrode material of this example.

[0152] Example 8

[0153] The preparation method of the negative electrode material in this example includes the following steps:

[0154] S1. Add 20 kg of graphite raw material (D50 particle size is 16 μm) and 2.0 kg of asphalt (softening point 180 °C) to a VC mixer and mix for 25 min. After uniform mixing, perform low-temperature heat treatment at 350 °C for 4 h under a nitrogen protection atmosphere. After cooling to room temperature, obtain the composite.

[0155] S2. Isostatically compact the composite. The maximum isostatic pressure is 50 MPa. After maintaining the pressure for 15 min, crush the material to about 16 μm to obtain the first intermediate product.

[0156] S3. Add 1.0 kg of asphalt (softening point 180 °C) to the first intermediate product and mix them in a VC mixer for 25 min to obtain the first intermediate product.

[0157] S4. Press the first intermediate product on a hydraulic press. The pressure of the hydraulic press is 25 MPa. Maintain the pressure for 0.5 min and reciprocate 3 times to obtain the second intermediate product.

[0158] S5. Perform high-temperature heat treatment on the second intermediate product. Under a nitrogen protection atmosphere, carbonize it at 1250 °C for 18 h. After carbonization, disperse, demagnetize, and screen it to obtain the negative electrode material of this example.

[0159] Example 9

[0160] S1. Add 20 kg of graphite raw material (D50 particle size is 16 μm) and 2.5 kg of asphalt (softening point 180 °C) to a VC mixer and mix for 25 min. After uniform mixing, perform low-temperature heat treatment at 350 °C for 4 h under a nitrogen protection atmosphere. After cooling to room temperature, obtain the composite.

[0161] S2. Isostatically compact the composite. The maximum isostatic pressure is 50 MPa. After holding the pressure for 15 min, crush the material to about 16 μm to obtain the first intermediate product.

[0162] S3. Add 1.5 kg of asphalt (softening point 180 °C) to the first intermediate product and mix them in a VC mixer for 25 min to obtain the first intermediate product.

[0163] S4. Press the first intermediate product in a hydraulic press. The pressure of the hydraulic press is 25 MPa. Hold the pressure for 0.5 min and reciprocate 3 times to obtain the second intermediate product.

[0164] S5. Perform high-temperature heat treatment on the second intermediate product. Under a nitrogen protection atmosphere, carbonize it at 1250 °C for 18 h. After carbonization, disperse, demagnetize, and screen it to obtain the negative electrode material of this example.

[0165] Comparative Example 1

[0166] The preparation method of the negative electrode material in this comparative example includes the following steps:

[0167] Add 20 kg of graphite raw material (D50 particle size is 16 μm) and 2.2 kg of asphalt (softening point 250 °C) to a VC mixer and mix for 25 min. After mixing, under a nitrogen protection atmosphere, carbonize at 1250 °C for 16 h. After carbonization, disperse, demagnetize, and screen it to obtain the negative electrode material of this comparative example.

[0168] Comparative Example 2

[0169] The preparation method of the negative electrode material in this comparative example includes the following steps:

[0170] Add 20 kg of graphite raw material (D50 particle size is 16 μm), 2.25 kg of asphalt (softening point 250 °C), and 40 g of graphene powder to a VC mixer and mix for 25 min. Then perform heat treatment at 400 °C to make the asphalt and graphene powder uniformly and tightly adhere to the surface of the graphite particles. After that, under a nitrogen protection atmosphere, carbonize at 1250 °C for 16 h. After carbonization, disperse, demagnetize, and screen it to obtain the negative electrode material of this comparative example.

[0171] Comparative Example 3

[0172] The preparation method of the negative electrode material in this comparative example includes the following steps:

[0173] S1. Isostatically compact 20 kg of graphite raw material (D50 particle size is 16 μm). The maximum isostatic pressure is 60 MPa. Hold the pressure for 15 min. After crushing the material, press it in a hydraulic press. The pressure of the hydraulic press is 30 MPa. Hold the pressure for 0.5 min and reciprocate 3 times, then crush it to about 16 μm to obtain the densified product.

[0174] S2. Add the densified product and 2.2 kg of asphalt (softening point 250 °C) to a VC mixer and mix for 25 min. Then, under a nitrogen protection atmosphere, carbonize at 1250 °C for 16 h. After carbonization, break up, demagnetize, and screen to obtain the negative electrode material of this comparative example.

[0175] Test Example

[0176] 1. Cross-sectional morphology test

[0177] Mill the particle sample of the negative electrode material using an ion milling machine (HITACHI E3500) to observe the cross-section of the particles. Place it under a high-power electron microscope (HITACHI S4800) to observe the cross-section of the particles. The magnification of a single particle is 2.5kX - 9.0kX to ensure that the cross-section presents a complete single particle.

[0178] Select at least 20 particles and cut through the center of each particle to expose the cross-section. Take the intersection of the horizontal median line and the vertical median line of the cross-section of the inner core of a single particle as the center of the ellipse. The major axis of the ellipse is 1 / 2 of the length of the horizontal median line, and the minor axis is 1 / 2 of the length of the vertical median line. Divide the cross-section of the inner core into a central region (a1) and an edge region (b1) by the ellipse, as Figure 2 shown.

[0179] Use the Aztec Feature software of the SEM electron microscope 0XFORD Instruments to statistically calculate and calculate the ratio of the pore area of the cross-section of the inner core of a single particle , the ratio of the pore area of the central region , the ratio of the pore area of the edge region and the ratio of the ratio of the pore area of the central region and the edge region (A j ), where = the pore area of the central region of the cross-section of the jth particle / the area of the central region * 100%, = the pore area of the edge region of the cross-section of the jth particle / the area of the edge region * 100%, = the pore area of the cross-section of the inner core of the jth particle / the area of the cross-section of the inner core * 100%, A j = / ;

[0180] Calculate the percentage of the pore area of the cross-section of the inner core of the entire negative electrode material ( ) and the average value of the ratio of the pore area of the central region and the edge region to the cross-sectional area (A) according to the following formula:

[0181]

[0182]

[0183] Among them, m≥20, 1≤j≤m, and j and m are positive integers.

[0184] 2. I D / I G Test

[0185] Use a confocal Raman microscope of the InVia model to test the Raman scattering spectrum of the anode material, and use a laser wavelength of 532 nm for the test. The D peak position of the material is around 1350 cm -1 nearby, and the G peak position is around 1580 cm -1 nearby. I D / I G The value is the intensity ratio of the D / G peaks. The schematic diagram of the Raman Mapping area of a single particle cross-section is as Figure 1 shown.

[0186] Select at least 15 particles, cut through the particle centers respectively to expose the cross-sections; select a rectangular area with a width of 1 - 6 μm and a length equal to the minor axis of the inner core cross-section on the inner core cross-section of each particle, perform Raman Mapping on this rectangular area, and divide the Mapping area into an inner rectangular area a and an outer rectangular area b. By performing peak fitting on the Raman scattering spectrum of each point, determine the characteristic peaks of the material. The D peak position is around 1350 cm -1 nearby, and the G peak position is around 1580 cm -1 nearby. Calculate the intensity ratio (I D / I G ) value of the D peak and the G peak in the inner rectangular area a and the outer rectangular area b of each particle. K ai is the I D / I G of the inner rectangular area of the i-th particle, and K bi is the I D / I G of the outer rectangular area of the i-th particle;

[0187] Calculate the mean value (K D / I G ) of the I a / I D / I G of the inner layer area and the mean value of the ratio of the I

[0188]

[0189] K i =K ai / K bi

[0190]

[0191] Among them, n≥15, 1≤i≤n, and i and n are positive integers.

[0192] 3. Specific surface area SSA test

[0193] The specific surface area of the negative electrode material is tested by using a JW-DX dynamic specific surface area measuring instrument. Based on the relevant theories of physical adsorption and with the continuous flow method proposed by Nelsen and Eggertsen as the structure, the specific surface area of the solid is measured. A mixed gas with hydrogen as the carrier and nitrogen as the adsorption gas is introduced into the sample tube. When the sample tube is immersed in liquid nitrogen to reach a low-temperature environment, the nitrogen in the mixed gas will be physically adsorbed by the sample until adsorption saturation. At this time, the proportion of nitrogen in the mixed gas will change. During the adsorption process, a high-precision thermal conductivity detection instrument will complete the detection and calculation work.

[0194] 4. D50 particle size test

[0195] The D50 particle size of the negative electrode material is tested using a Malvern 3000 laser particle size analyzer. In a 50 mL beaker, add the sample, a small amount of dispersant (a mixed solution of ethanol, pure water, and a low-foam surfactant), and pure water. Stir well with a glass rod to disperse the sample evenly. Transfer the sample to the sample cell of the Malvern 3000 laser particle size analyzer, and set the pump rotation speed of the device to 2400 - 2500 r / min and the frequency to 19.5 Hz for particle size testing.

[0196] 5. Tap density Tap test

[0197] The tap density of the negative electrode material is tested using a Quantachrome Dual Autotap device. Place 100 mL of the negative electrode material sample in a graduated cylinder. After mechanical vibration 1000 times, obtain the sample mass and the volume after tapping. Calculate the tap density (g / cm³) = sample mass / volume after tapping.

[0198] 6. Average pore diameter d test

[0199] The average pore diameter d of the negative electrode material is tested using a BSD-660M A6M. The static volumetric method is adopted. Degas at 300 °C for 300 min, and use nitrogen (77.3 K) as the adsorbate for testing. Then measure the amount of nitrogen adsorbed at different pressures. By analyzing the adsorption isotherm, the average pore diameter d of the negative electrode material can be determined.

[0200] 7. Electrochemical performance test

[0201] The negative electrode materials, carboxymethyl cellulose, and styrene-butadiene rubber of the examples and comparative examples were respectively dissolved in pure water at a mass ratio of 96.5:1.5:2, and the solid content was controlled to be 50% to obtain a negative electrode slurry; the negative electrode slurry was coated on a copper foil current collector, and after vacuum drying at 95 °C, rolling, and pressurization, a negative electrode sheet was obtained; a lithium metal sheet was used as a counter electrode, and a button cell was assembled in a glove box filled with argon.

[0202] The button cell was subjected to charge-discharge testing at a current density of 0.1 C in the charge-discharge range of 0.01 - 1.5 V to obtain the first reversible specific capacity, the first-cycle charge capacity, and the first-cycle discharge capacity. The first Coulombic efficiency (first efficiency) was calculated according to the first Coulombic efficiency = first-cycle discharge capacity / first-cycle charge capacity.

[0203] The measurement of the expansion rate of the electrode sheet was carried out using the battery electrode sheet thickness change measurement device and system disclosed in the patent document CN209991940U. The negative electrode materials, carboxymethyl cellulose, and styrene-butadiene rubber were uniformly mixed at a mass percentage of 96.5:1.5:2, and the solid content was controlled at 50% to obtain a negative electrode slurry; the negative electrode slurry was coated on a copper foil current collector, and after vacuum drying and rolling, a negative electrode sheet with a compaction density of 1.60 g / cm 3 was obtained, and the initial thickness d 1 of the negative electrode sheet was measured; the positive electrode active material lithium cobaltate, conductive carbon black, and polyvinylidene fluoride were mixed evenly at a mass ratio of 96.5:2:1.5 and then coated on an aluminum foil (single-sided) to obtain a positive electrode sheet. The above-prepared positive electrode sheet and negative electrode sheet were loaded into a self-made three-electrode test device for testing, and this three-electrode test device could in-situ record the thickness change of the electrode sheet.

[0204] The following charge-discharge regime was used for testing: in the first week, it was charged at a constant rate of 0.01 C for 30 min, charged at a constant rate of 0.05 C for 30 min, and then charged at a constant rate of 0.1 C until 4.2 V. After the voltage reached the upper limit of 4.2 V, it was charged at a constant voltage, and the current gradually decreased to 0.01 C to end the charge, and then discharged at 0.1 C to 3 V; in the second week, it was charged at a constant rate of 0.2 C until 4.2 V; after the voltage reached 4.2 V, the current gradually decreased to 0.01 C to end the charge, and then discharged at a constant rate of 0.2 C to 3 V; in the 3rd - 20th weeks, it was charged at a constant rate of 0.5 C until 4.2 V, after the voltage reached 4.2 V, it was charged at a constant voltage, the current decreased to 0.01 C to end the charge, and then discharged at a constant rate of 0.5 C to 3 V; in the last half week, it was charged at a constant rate of 0.5 C until 4.2 V, after the voltage reached 4.2 V, it was charged at a constant voltage, the current decreased to 0.01 C to end, the battery was disassembled, and the thickness d 2 of the negative electrode sheet after 20 weeks of cycling was measured. According to the electrode sheet expansion rate = (d 2 -d 1 ) / d 1 ×100% to calculate the expansion rate.

[0205] Table 1

[0206]

[0207] In Table 1, d is the average pore diameter of the negative electrode material.

[0208] According to Table 1, among the negative electrode materials of Examples 1 to 9, amorphous carbon is filled inside the graphite, and all satisfy 0.4 ≤ K a ≤ 0.7, 0.5 ≤ K < 0.9, while Comparative Example 1 and Comparative Example 3 do not satisfy 0.5 ≤ K ≤ 0.9, 0.4 ≤ K a ≤ 0.7, and the negative electrode material of Comparative Example 2 only satisfies 0.5 ≤ K ≤ 0.9, but does not satisfy 0.4 ≤ K a ≤ 0.7. The first Coulombic efficiency of the negative electrode materials of Examples 1 to 9 is higher than that of Comparative Examples 1 to 3, the swelling rate is reduced, and the capacity is high. It can be seen from this that by making the negative electrode material satisfy 0.4 ≤ K a ≤ 0.7, 0.5 ≤ K < 0.9, the structural swelling caused by Li + entering and leaving the graphite interlayer can be reduced, the structural stability of the material can be improved, and the first Coulombic efficiency, swelling performance, and capacity are improved.

[0209] Furthermore, compared with Examples 7, 8, and 9, the negative electrode materials of Examples 1 to 6, while satisfying 0.5 ≤ K ≤ 0.9, 0.4 ≤ K a ≤ 0.7, make the percentage of pore area of the inner core satisfy: 2% ≤ ≤ 5%, and further make the average value A of the ratio of the percentage of pore area of the inner layer region to the outer layer region satisfy 1.2 ≤ A ≤ 2.0, which can further improve the first Coulombic efficiency, swelling performance, and capacity. The negative electrode material of Example 9 does not satisfy 2% ≤ ≤ 5%, and the improvement of the electrochemical performance of the negative electrode material is limited, and the performance is worse than that of Examples 1 to 6. The negative electrode materials of Example 7 do not satisfy 1.2 ≤ A ≤ 2.0, and the negative electrode material of Example 8 does not satisfy 2% ≤ ≤ 5%, and the improvement of the electrochemical performance of the negative electrode material is limited, and the performance is worse than that of Examples 1 to 6. The negative electrode materials of Example 9 do not satisfy 2% ≤ ≤ 5%, 1.2 ≤ A ≤ 2.0, and are worse than Examples 7 - 8.

[0210] Examples 1 to 6 use steps such as batch coating, low-temperature heat treatment, pressing treatment, and high-temperature heat treatment, so that amorphous carbon is filled inside the particles, the internal defects of the particles are improved, the pores in the inner layer region and the outer layer region are filled, especially the pores in the outer layer region are filled, so that the material satisfies 0.5 ≤ K ≤ 0.9, 0.4 ≤ Ka ≤ 0.7, 1.2 ≤ A ≤ 2.0, 2% ≤ ≤ 5%. The graphite negative electrode material has the advantages of high density and low expansion. When applied to secondary lithium-ion batteries, it exhibits excellent electrochemical performance. In Comparative Example 1, the amorphous carbon could not be filled into the interior of the graphite, neither satisfying 0.5 ≤ K ≤ 0.9, 0.4 ≤ K a ≤ 0.7 nor satisfying 1.2 ≤ A ≤ 2.0, 2% ≤ ≤ 5%, resulting in poor initial Coulombic efficiency, capacity, and expansion performance of the negative electrode material; in Comparative Example 2, a small amount of graphene was introduced. Although it satisfied 0.5 ≤ K ≤ 0.9, it did not satisfy 0.4 ≤ K a ≤ 0.7 and 1.2 ≤ A ≤ 2.0, 2% ≤ ≤ 5%. The initial Coulombic efficiency, capacity, and expansion performance were improved compared to Comparative Example 1, but the improvement effect was limited. In Comparative Example 3, a densification technique was used to reduce the internal pores of the graphite before coating with pitch, but the amorphous carbon could not be filled into the interior of the graphite to the maximum extent, such that although the negative electrode material satisfied 1.2 ≤ A ≤ 2.0, 2% ≤ ≤ 5%, it did not satisfy 0.5 ≤ K ≤ 0.9, 0.4 ≤ K a ≤ 0.7. The initial Coulombic efficiency and expansion performance were improved compared to Comparative Example 1 and Comparative Example 2, but the capacity decreased compared to Comparative Example 2, and the improvement degree was limited.

[0211] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A negative electrode material, characterized in that: The negative electrode material comprises a core and a coating layer, wherein the coating layer is located on the surface of the core, the core comprises natural graphite and a first amorphous carbon filled in the pores of the natural graphite, and the coating layer comprises a second amorphous carbon; The Raman spectrum of the negative electrode material has a D peak and a G peak, and the intensity ratio of the D peak to the G peak is 1 D / I G ; The inner core includes an inner layer region and an outer layer region located outside the inner layer region, and the outer layer region is adjacent to the cladding layer; The inner region I D / I G The mean value is K a , the inner region and the outer region I D / I G The mean of the ratio is K; Satisfy: 0.4≤K a ≤0.7,0.5≤K<0.9; The pore area percentage of the core is , meet: 2% ≤ ≤5%; The average value of the ratio of the pore area percentage of the inner layer region to the outer layer region is A, which satisfies: 1.2≤A≤2.

0.

2. The negative electrode material according to claim 1, characterized in that The D50 particle size of the negative electrode material is 5 μm to 20 μm.

3. The negative electrode material according to claim 1, characterized in that The shape of the negative electrode material includes at least one of a sphere, an ellipsoid, and a quasi-spherical shape.

4. The negative electrode material according to claim 1, characterized in that The specific surface area of ​​the negative electrode material is 2 to 5 m 2 / g.

5. The negative electrode material according to claim 1, characterized in that The tap density of the negative electrode material is 0.9-1.4 g / cm 3 .

6. The negative electrode material according to claim 1, characterized in that The average pore size of the negative electrode material is 10-20 nm.

7. A negative electrode sheet, characterized in that: The negative electrode sheet comprises the negative electrode material according to any one of claims 1 to 6.

8. A secondary battery, characterized in that: The secondary battery comprises the negative electrode sheet according to claim 7.

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