Negative electrode material, negative electrode sheet and secondary battery

Abstract

The present application relates to the technical field of secondary batteries. Disclosed are a negative electrode material, a negative electrode sheet and a secondary battery. The negative electrode material comprises natural graphite and amorphous carbon filled into pores of the natural graphite. The particle hardness of the negative electrode material is 0.28-0.4 GPa, and the elastic modulus thereof is 7.0-8.0 GPa. When the compaction density of a tablet of the negative electrode material is 1.5 g / cm3 to 2.0 g / cm3, the orientation OI value of the tablet of the negative electrode material is y, and 4<y≤11. When the negative electrode material is applied to a secondary battery, the initial coulombic efficiency and the cycling stability can be significantly improved.

Description

Negative electrode materials, negative electrode sheets and secondary batteries

[0001] Cross-reference to related applications

[0002] This application claims priority to Chinese Patent Application No. 202411850897.7, filed on December 16, 2024, entitled “A Negative Electrode Material, Negative Electrode Sheet and Secondary Battery”, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of secondary battery technology, and more specifically, to a negative electrode material, a negative electrode sheet, and a secondary battery. Background Technology

[0004] Lithium-ion batteries are widely used in 3C products, power devices, and energy storage equipment due to their advantages such as low self-discharge rate, high charge-discharge efficiency, no memory effect, and long cycle life. The anode material is a crucial component of lithium-ion batteries, and its performance directly affects the electrochemical performance of the battery. Natural graphite anode materials have attracted widespread attention due to their high specific capacity, low charge-discharge plateau, and low cost. However, natural graphite has high anisotropy and internal defects, which can easily lead to solvent co-intercalation during lithium-ion insertion, reducing the stability of the SEI film, consuming additional lithium ions, and lowering the initial coulombic efficiency. Furthermore, this high anisotropy and internal defects cause graphite to undergo uneven volume expansion. During long-cycle lithium-ion batteries, the continuous expansion and contraction of the graphite structure may further expand the defect area, even leading to graphite sheet detachment, crack formation, and long-cycle capacity decay. Summary of the Invention

[0005] The main objective of this application is to provide a negative electrode material, a negative electrode sheet, and a secondary battery to solve the problems of low initial coulombic efficiency and insufficient cycle stability of the negative electrode material in the prior art during charge and discharge.

[0006] To achieve the above objectives, according to one aspect of this application, a negative electrode material is provided, comprising natural graphite and amorphous carbon filling the pores of the natural graphite; the particle hardness of the negative electrode material is 0.28 GPa to 0.4 GPa, and the elastic modulus is 7.0 GPa to 8.0 GPa; the compacted density of the negative electrode material is 1.5 g / cm³. 3 ~2.0g / cm 3 The OI value of the negative electrode material is y, where 4 < y ≤ 11.

[0007] Furthermore, the OI value of the negative electrode material under different compaction densities is y, which satisfies: y1≤y≤y2;

[0008] Where y1=5.46x-3.78, y2=8.9x-7.76, 1.5≤x≤2.0, and x is the numerical value corresponding to the compaction density of the negative electrode material.

[0009] Furthermore, when the compaction density of the negative electrode material is 1.5 g / cm³ 3 ~2.0g / cm 3 The OI value of the negative electrode material during pressing is 5 to 10.

[0010] Furthermore, the negative electrode material satisfies at least one of the following characteristics:

[0011] (1) The sphericity Sh (90%) of the negative electrode material is 0.90 to 0.95;

[0012] (2) The D50 volume average particle size of the negative electrode material is 8μm~18μm;

[0013] (3) The average particle size of the negative electrode material is 10μm to 20μm.

[0014] Furthermore, the shape factor of the negative electrode material is The sphericity of the particles is Sh (90%), satisfying: And Δ≤0.08; where, D1 is the D50 volume average particle size of the negative electrode material, in μm; D2 is the equal volume average particle size of the negative electrode material, in μm.

[0015] Furthermore, the specific surface area of ​​the negative electrode material is 2m². 2 / g~5m 2 / g.

[0016] Furthermore, the tap density of the negative electrode material is 0.9 g / cm³. 3 ~1.3g / cm 3 .

[0017] Furthermore, the powder compaction density of the negative electrode material is 1.7 g / cm³. 3 ~2.0g / cm 3 .

[0018] A second aspect of this application provides a negative electrode sheet, which includes the negative electrode material provided in the first aspect.

[0019] A third aspect of this application provides a secondary battery, which includes the negative electrode provided in the second aspect.

[0020] By applying the technical solution of this application, the density of the anode material is improved, reducing its anisotropy and allowing for more uniform diffusion of lithium ions in different directions. This contributes to improving the electrochemical performance of the anode material. Simultaneously, by limiting the particle hardness of the anode material to 0.28 GPa–0.4 GPa, the elastic modulus to 7.0 GPa–8.0 GPa, and the OI value of the compression orientation y (4 < y ≤ 11), the structural stability of the anode material is enhanced, effectively controlling volume expansion and ensuring that the anode material maintains structural stability during charge and discharge. This also ensures unobstructed lithium-ion channels on its surface and internal structure, reducing issues such as sheet shedding and crack formation, thereby improving cycle stability. When this anode material is applied to a secondary battery, the initial coulombic efficiency is ≥94%, and the capacity retention after 400 cycles is ≥92.5%. Attached Figure Description

[0021] Figure 1 is a SEM image of the negative electrode material in Embodiment 1 of this application;

[0022] Figure 2 is a coordinate graph of the OI values ​​of the negative electrode material under different compaction densities in the embodiments and comparative examples of this application;

[0023] Figure 3 is a schematic diagram of the structure of a secondary battery provided in one embodiment of this application during charging;

[0024] Figure 4 is a schematic diagram of the structure of a secondary battery provided in one embodiment of this application during discharge.

[0025] Explanation of reference numerals in the attached figures: 100 - Electrode assembly; 101 - Positive electrode; 102 - Negative electrode; 103 - Separator. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0027] As described in the background section of this application, existing negative electrode materials suffer from low initial coulombic efficiency and insufficient cycle stability during charge and discharge. To address these issues, in a typical embodiment of this application, a negative electrode material is provided, comprising natural graphite and amorphous carbon filling the pores of the natural graphite; the particle hardness of the negative electrode material is 0.28 GPa to 0.4 GPa, and the elastic modulus is 7.0 GPa to 8.0 GPa; the compacted density of the negative electrode material is 1.5 to 2.0 g / cm³. 3 When the negative electrode material is pressed into an OI value of y, 4 < y ≤ 11.

[0028] The negative electrode material includes natural graphite and amorphous carbon filling the pores of natural graphite. This improves the density of the negative electrode material. At the same time, due to the high disorder of amorphous carbon, the anisotropy of the negative electrode material can be reduced by filling it with amorphous carbon, making the diffusion of lithium ions in different directions more uniform. This helps to improve the electrochemical performance of the negative electrode material.

[0029] The particle hardness and elastic modulus of the anode material are crucial to the electrochemical performance and cycle stability of secondary batteries, and also reflect the degree of internal compaction and amorphous carbon filling. Specifically, the particle hardness (HIT) of the anode material directly reflects the ability of graphite particles to resist elastic and plastic deformation under external forces. The elastic modulus (EIT) of the anode material reflects its ability to resist external forces during the elastic deformation stage, i.e., the stress required for the material to generate a unit strain under stress.

[0030] The particle hardness of the anode material affects its volume expansion and contraction. By limiting the particle hardness of the anode material to 0.28 GPa to 0.4 GPa, the graphite particles have sufficient mechanical strength to withstand the volume expansion and contraction generated during lithium-ion insertion and extraction, thus better resisting the mechanical stress during this process. This helps reduce particle breakage and shedding during cycling, thereby improving cycle stability. Secondly, it helps limit the expansion of internal pores in the particles, reducing the probability of electrolyte solvent molecules and lithium ions co-intercalating into the graphite layers, thereby reducing excessive consumption of the SEI film and improving the initial coulombic efficiency. In addition, during the charge-discharge cycle of the secondary battery, the graphite particles undergo repeated volume changes. When the particle hardness is 0.28 GPa to 0.4 GPa, it helps maintain the structural integrity of the anode material, reducing capacity decay caused by mechanical wear and structural damage, and extending the service life of the secondary battery. Typically, but not limitingly, the particle hardness of the anode material is 0.287 GPa, 0.288 GPa, 0.295 GPa, 0.308 GPa, 0.335 GPa, 0.356 GPa, 0.393 GPa, or a range of any two of the above values.

[0031] The elastic modulus of anode materials is related to their stress-strain properties. By limiting the elastic modulus of anode materials to 7.0 GPa–8.0 GPa, it is evident that the elastic deformation generated during lithium-ion insertion and extraction can be well controlled. This effectively mitigates volume changes during cycling, allowing the materials to better adapt to volume variations and reducing cracks and particle shedding caused by stress concentration, thus improving cycle stability. Secondly, it contributes to the stability of the SEI film during charge-discharge cycles, reducing film rupture caused by material expansion, thereby improving initial coulombic efficiency and cycle performance. Furthermore, the elastic modulus is related to the diffusion kinetics of lithium ions in the anode material. A suitable elastic modulus implies lower internal stress and smoother lithium-ion diffusion paths, which helps improve the lithium-ion transport rate, thereby enhancing the electrochemical performance of the secondary battery, such as charge-discharge rate and capacity. Typically, but not limitingly, the elastic modulus of the negative electrode material is 7.08 GPa, 7.15 GPa, 7.26 GPa, 7.45 GPa, 7.78 GPa, 7.9 GPa, 7.96 GPa, or a range consisting of any two of the above values.

[0032] The negative electrode material exhibits diffraction peaks on both the (004) and (110) crystal planes in its X-ray diffraction pattern. The OI value of the compacted negative electrode material is the ratio of the diffraction peak area on the (004) crystal plane to the diffraction peak area on the (110) crystal plane. In some embodiments, the compaction density of the negative electrode material is 1.5–2.0 g / cm³. 3 The OI value of the negative electrode material is y, where 4 < y ≤ 11. The OI value describes the degree of orderliness of the graphite sheet arrangement in the negative electrode material. By limiting the OI value to y, where 4 < y ≤ 11, it helps to improve the kinetic performance of the material. Since amorphous carbon has a high degree of disorder, its filling can reduce the OI value. This disorder helps prevent solvent co-intercalation in the electrolyte into the graphite layer, thereby improving the interfacial transport kinetics of lithium ions. This application, by limiting the OI value to the above range, helps to improve the initial coulombic efficiency and cycle stability of the negative electrode material. Typically, but not limitingly, the compaction density of the negative electrode material is 1.5 g / cm³. 3 -2.0g / cm 3 When the negative electrode material is pressed into a tablet, the OI value is 4.05, 4.48, 4.68, 5.4, 5.53, 5.92, 6.1, 6.31, 6.58, 6.62, 7.38, 7.94, 8.11, 8.46, 8.51, 8.67, 9.73 or a range of any two of the above values.

[0033] This application limits the particle hardness of the anode material to 0.28 GPa–0.4 GPa, the elastic modulus to 7.0 GPa–8.0 GPa, and the OI value of the pressed-off orientation to y, where 4 < y ≤ 11. This not only reflects the high degree of internal compaction and amorphous carbon filling in the particles but also achieves a balance between structural stability and lithium-ion transport kinetics, thus improving both initial coulombic efficiency and cycle stability. Specifically, on the one hand, it ensures the structural integrity of the anode material during cycling, thereby improving cycle stability; on the other hand, it optimizes the diffusion path of lithium ions in the anode material, which is beneficial to improving electrochemical performance, such as initial coulombic efficiency. This synergistic effect helps the anode material maintain appropriate elastic deformation during lithium-ion insertion and extraction, avoiding excessive plastic deformation, maintaining a more stable SEI film and better electrochemical performance, thereby improving both initial coulombic efficiency and cycle stability. TEM detection can distinguish between the graphite core and amorphous carbon.

[0034] The particle hardness and elastic modulus of the negative electrode material can be determined by indentation hardness testing, for example, using a nanoindenter. During the test, a hard indenter is pressed into the particle surface with a certain force, and then unloaded. The hardness value of the negative electrode material is calculated by measuring the depth and width of the indentation and the magnitude of the applied force. The elastic modulus is calculated by measuring the unloading process of the indentation on the material surface. At least 10 particles are tested for each sample, and the average value of the hardness and elastic modulus is taken after each test; this represents the particle hardness and elastic modulus of the negative electrode material.

[0035] According to the research in this application, applying the above-mentioned negative electrode material to a secondary battery can significantly improve the initial coulombic efficiency and cycle stability. This is because, on the one hand, the amorphous carbon within the pores of natural graphite in the negative electrode material of this application increases the compactness of the negative electrode material, reducing its anisotropy and making the diffusion of lithium ions more uniform in different directions, which helps to improve the electrochemical performance of the negative electrode material. On the other hand, by limiting the particle hardness of the negative electrode material to 0.28 GPa to 0.4 GPa, the elastic modulus to 7.0 GPa to 8.0 GPa, and the OI value of the pressed sheet orientation to y, where 4 < y ≤ 11, it not only indicates a high degree of compactness and a high degree of amorphous carbon filling within the particles, but also gives the negative electrode material appropriate mechanical strength and deformation capacity. This allows it to resist volume changes during charge and discharge and optimize lithium ion transport kinetics, thereby improving the stability of the SEI film and the overall performance of the secondary battery. Applying this negative electrode material to secondary batteries can achieve an initial coulombic efficiency of ≥94% and a capacity retention of ≥92.5% after 400 cycles, which helps to obtain more efficient, stable and long-life secondary batteries.

[0036] In some preferred embodiments, the OI value y of the negative electrode material under different compaction densities satisfies: y1≤y≤y2; where y1=5.46x-3.78, y2=8.9x-7.76, 1.5≤x≤2.0, and x is the numerical value corresponding to the compaction density of the negative electrode material; y2 and y1 are the upper and lower limits of the OI value of the compaction density, respectively. By limiting the OI value of the negative electrode material and the compaction density to meet the above requirements, the high isotropy of the negative electrode material can be guaranteed, the polarization effect during charging can be guaranteed to be small, the volume effect can be limited, and the formation of new SEI can be reduced. In addition to further improving the initial coulombic efficiency and cycle stability, the capacity can also be maximized, thereby improving the initial coulombic efficiency, capacity and cycle stability of the negative electrode material.

[0037] Specifically, when the compaction density of the negative electrode material is 1.5 g / cm³ 3 That is, when x is 1.5, y1 = 4.41, y2 = 5.59, and at this time, 4.41 ≤ y ≤ 5.59, the OI value of the tablet orientation is 4.41~5.59; when the compaction density of the negative electrode material is 2g / cm³ 3 That is, when x is 2, y1 = 7.14, y2 = 10.04. At this time, 7.14 ≤ y ≤ 10.04, and the OI value of the pressed sheet is 7.14 to 10.04, and so on. In the specific implementation of this application, the compaction density of the negative electrode material essentially refers to the compaction density of the negative electrode material formed under different pressures.

[0038] In some preferred embodiments, the OI value y of the negative electrode material under different compaction densities satisfies: y1≤y≤y2; where y1=5.46x-3.78, y2=8.88x-8.

[0039] In some preferred embodiments, the OI value of the negative electrode material is 5 to 10, preferably 5 to 9, which can further improve the initial coulombic efficiency and cycle stability of the negative electrode material.

[0040] The geometry of the negative electrode material particles has a certain impact on electrochemical performance and cycle stability. The particle geometry affects its contact area with the electrolyte and its mechanical stability during charge and discharge, thus influencing the charge-discharge performance and cycle stability of the secondary battery. In some embodiments, the shape factor of the negative electrode material is... The sphericity of the negative electrode material particles is Sh (90%), satisfying: And Δ≤0.08; where, D1 represents the D50 particle size of the negative electrode material, in μm; D2 represents the volumetric average particle size of the negative electrode material, in μm. Specifically, It is understandable that Δ is The absolute value of the difference between Sh (90%) and the negative electrode material. This is determined by limiting the shape factor of the negative electrode material. The sphericity Sh (90%) of the particles satisfies the above relationship, ensuring excellent interparticle contact while guaranteeing good mechanical stability, resisting volume expansion and contraction, thereby improving the charge-discharge performance and cycle stability of the secondary battery. In some preferred embodiments, Δ ≤ 0.06 can further enhance charge-discharge performance and cycle stability.

[0041] Sphericity refers to the degree to which particles approximate a sphere. The sphericity Sh(90%) of the negative electrode material refers to the sphericity of the particles when the volume cumulative distribution reaches 90% in the sphericity-volume cumulative distribution curve. The closer this ratio is to 1, the closer the particles are to a perfect sphere.

[0042] In some embodiments, the sphericity Sh (90%) of the negative electrode material is 0.90–0.95, meaning that most of the graphite particles have a high degree of sphericity. Besides contributing to further improvements in the initial coulombic efficiency and cycle stability of the secondary battery, this geometry helps form a denser structure during compaction, contributing to increased energy density. Furthermore, the isotropic nature of spherical graphite particles allows for the formation of more uniform lithium-ion transport paths and a more stable conductive network. Compared to irregularly shaped particles, spherical particles are more orderly arranged in the electrode, reducing electron and lithium-ion transport resistance within the secondary battery and improving its charge / discharge rate and capacity.

[0043] The D50 volume average particle size is the particle size corresponding to a cumulative volume distribution percentage of 50% in a sample, reflecting the average particle size of the negative electrode material. In some embodiments, the D50 volume average particle size of the negative electrode material is 8–18 μm. Equal volume average particle size refers to the average particle size of a sphere with the same average particle volume.

[0044] In some embodiments, the volumetric average particle size of the negative electrode material is 10–20 μm. By limiting the D50 volumetric average particle size and volumetric average particle size of the negative electrode material to a suitable range, in addition to helping to further improve the initial coulombic efficiency and cycle stability of the secondary battery, it can also shorten the diffusion path of lithium ions, reduce the transport resistance of lithium ions during insertion and extraction, thereby improving charge and discharge efficiency. Furthermore, it can reduce the porosity inside the electrode and increase the energy density of the secondary battery. In some preferred embodiments, the D50 volumetric average particle size of the negative electrode material is 10–18 μm, and the volumetric average particle size is 11–18 μm.

[0045] Specific surface area (SSA) refers to the total surface area per unit mass of material. In some embodiments, the specific surface area of ​​the negative electrode material is 2–5 m². 2 / g. By limiting the specific surface area of ​​the negative electrode material to a suitable range, in addition to further improving the initial coulombic efficiency and cycle performance, it can also provide a suitable diffusion path. This avoids the performance degradation caused by the electrolyte penetrating deep into the pores of the material, and also facilitates 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 graphite material surface, reducing the possibility of side reactions, such as electrolyte decomposition and solvent molecule co-intercalation.

[0046] Tap density refers to the degree of compactness of a material under physical vibration. In some embodiments, the tap density of the negative electrode material is 0.9–1.3 g / cm³. 3 By limiting the tap density of the negative electrode material within a suitable range, it helps to improve the compaction density of the material during electrode fabrication, reduce internal porosity, and enhance the utilization rate of the negative electrode material and the energy density of the secondary battery. Simultaneously, it reduces the contact area between the electrolyte and graphite particles, thereby reducing SEI film formation, lowering irreversible capacity during the first charge-discharge cycle, and improving the initial coulombic efficiency. It also helps to reduce volume expansion during cycling, lower mechanical stress between particles, and prevent crack formation, thus improving cycle stability. Furthermore, it helps maintain the integrity of the electrode structure, preventing active material from detaching from the current collector, further improving cycle performance. In some preferred embodiments, the tap density of the negative electrode material is 1–1.3 g / cm³. 3 .

[0047] Powder compaction density refers to the density of negative electrode material powder under a certain pressure (e.g., 2T). It is typically determined by compacting the material under pressure and then measuring the compacted volume and mass. Powder compaction density directly affects the energy density, initial coulombic efficiency, and cycle stability of the secondary battery. In some embodiments, the powder compaction density of the negative electrode material is 1.7–2.0 g / cm³. 3 By limiting the powder compaction density within a suitable range, graphite particles can be brought into closer contact, which helps to form a more uniform and stable SEI film, thereby further improving the initial coulombic efficiency. Simultaneously, reducing the relative movement between particles lowers the mechanical stress on the graphite particles during charge-discharge cycles, thus further improving cycle stability. Furthermore, it allows for a more uniform distribution of the negative electrode material, contributing to increased energy density.

[0048] The negative electrode material of this application can be used as a negative electrode material in secondary batteries, effectively improving the initial coulombic efficiency and cycle stability of the secondary battery. For example, in some embodiments, the initial coulombic efficiency of the negative electrode material is above 94%, the capacity retention rate after 400 cycles is ≥92.5%, and the capacity is ≥360mAh / g.

[0049] This application does not limit the specific preparation process of the negative electrode material, as long as the above parameters are met. In some embodiments, the preparation method of the above-mentioned negative electrode material includes the following steps:

[0050] S1, oxidize the graphite raw material to obtain graphite oxide;

[0051] S2, after mixing and coating graphite oxide and pitch in sequence and performing a first heat treatment, a composite is obtained;

[0052] S3, the complex is densified to obtain an intermediate product;

[0053] S4. The intermediate product is subjected to a second heat treatment to obtain the negative electrode material.

[0054] Specifically, in S1, the graphite raw material is oxidized, which can effectively improve the surface activity of the graphite raw material. This not only promotes the uniformity of the subsequent coating process, but also helps to improve the compatibility and stability of the material with the electrolyte, so as to achieve mechanical properties that match the lithium-ion intercalation / deintercalation.

[0055] The specific temperature, gas content, and time of the oxidation treatment can be adjusted according to the actual situation. For example, in some embodiments, the graphite raw material can be natural graphite, such as spherical graphite, with a D50 volume average particle size of 1-20 μm. This particle size range can ensure the uniformity and stability of the material in subsequent processing. The oxidation treatment can be carried out under conditions of 1%-10% oxygen volume content, at a temperature of 400-600°C, for a time of 2-3 hours.

[0056] In S2, graphite oxide is mixed and coated with asphalt. This can be understood as mixing graphite oxide and asphalt at room temperature. During the mixing and coating process, the asphalt and graphite oxide are mixed evenly to obtain a mixture. The mixture is then subjected to a first heat treatment, which allows the molten asphalt to fill the pores inside the graphite, resulting in a composite. Through mixing and coating and the first heat treatment, the density, compactness, and hardness of the material are improved.

[0057] The specific mixing ratio of graphite oxide and asphalt can be adjusted according to material properties and requirements. For example, in some embodiments, the mass ratio of graphite raw material to asphalt is 100:(1-30); the asphalt is selected from at least one of petroleum asphalt, coal tar pitch, and mesophase asphalt, and the D50 volume average particle size of the asphalt is 2-3 μm; the softening point of the asphalt is 100-300℃, and the mixing and coating time is 10-60 min; the first heat treatment can be carried out under a first inert atmosphere and at a temperature higher than the softening point of the asphalt; the first inert atmosphere is to prevent the material from oxidizing during the heat treatment process, and any inert gas can achieve this purpose, such as at least one of nitrogen, helium, and argon; the temperature of the first heat treatment is 200-600℃, and the time is 3-4 h.

[0058] In S3, the densification process includes molding and isostatic pressing. Specifically, the composite is first molded using a hydraulic press to obtain a molded product; then, the molded product is subjected to isostatic pressing to obtain an isostatically pressed product; finally, the isostatically pressed product is crushed to obtain an intermediate product. Densification effectively reduces the internal porosity of graphite, increases the material's density, enhances its structural compactness, and improves the pore-filling effect, thereby ensuring that the material's particle hardness and elastic modulus meet the aforementioned requirements.

[0059] The specific pressure, time, and number of cycles in the compression molding process can be adjusted according to the actual equipment capacity and material requirements. For example, in some embodiments, the compression molding conditions are: hydraulic press pressure of 10–40 MPa, holding time of 0–2 min, and slow reciprocating 2–4 times after depressurization for 0.5 min. The short holding time ensures that the material quickly forms a dense structure under high pressure, while avoiding equipment wear and reduced production efficiency caused by prolonged pressure holding. This is suitable for continuous production processes and can improve production efficiency and equipment utilization. Isostatic pressing can further improve the density and structural stability of the material, and the specific pressure and time can be optimized according to the material performance requirements and equipment capacity. For example, in some embodiments, the filling effect of asphalt inside graphite can be controlled by adjusting the pressure during isostatic pressing. For example, isostatic pressing can be performed using cold or warm isostatic pressing, with a pressure of 60–120 MPa and a holding time of 1–60 min.

[0060] In step S4, the intermediate product undergoes a second heat treatment under a second inert atmosphere to ensure the degree of graphitization of the material. After the second heat treatment, the material is obtained by dispersing, demagnetizing, and sieving. The second heat treatment converts the pitch into amorphous carbon, ensuring stable carbonization of the material at high temperatures and improving its degree of graphitization. Appropriate heat treatment temperature and time can affect the crystallinity and microstructure of graphite, thereby ensuring that its hardness and elastic modulus meet the above requirements.

[0061] 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 targets of the material. For example, in some embodiments, the temperature of the second heat treatment is higher than that of the first heat treatment, such as a temperature of 900–1600°C and a time of 10–24 hours. The second inert atmosphere is used to prevent oxidation of the material during the heat treatment process; any inert gas can achieve this purpose, such as at least one of nitrogen, helium, and argon.

[0062] A second aspect of this application provides a negative electrode sheet comprising the negative electrode material provided in the first aspect above.

[0063] The negative electrode sheet of this application includes a negative electrode current collector and a negative electrode material active layer disposed 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 be at least one of copper foil, nickel foil, stainless steel foil, titanium foil, or carbon-based current collector, or any composite current collector disclosed in the prior art, such as, but not limited to, current collectors formed by combining the aforementioned conductive foil and polymer substrate. Because it includes a negative electrode material with excellent performance, applying this negative electrode sheet to a secondary battery helps to improve the initial coulombic efficiency and cycle stability of the secondary battery.

[0064] The active layer of the negative electrode material also includes a binder to bond the negative electrode active material particles, thereby facilitating the formation of the film layer and improving the bonding force between the active layer of the negative electrode material and the negative electrode current collector. In some embodiments, the binder may include, but is not limited to, polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon, etc.

[0065] The active layer of the negative electrode material may further include a conductive material, which includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, carbon-based materials may include, but are not limited to, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, metal-based materials may include, but are not limited to, metal powders or metal fibers, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0066] In the specific preparation of the negative electrode sheet, the negative electrode material, conductive agent, and binder can be dispersed in an appropriate amount of solvent and thoroughly stirred to form a uniform negative electrode slurry. The negative electrode slurry is then uniformly coated onto the negative electrode current collector, and after drying, rolling, and slitting, the negative electrode sheet is obtained. In one specific embodiment, the active layer of the negative electrode material comprises, by mass percentage, 70%–99% negative electrode material, 0.5%–15% conductive agent, and 0.5%–15% binder.

[0067] The conductive agent may be selected from at least one of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, and graphene; the binder may be selected from at least one of carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyvinyl alcohol, and sodium polyacrylate.

[0068] A third aspect of this application provides a secondary battery comprising the negative electrode provided in the second aspect above. Due to the inclusion of the aforementioned high-performance negative electrode, the secondary battery exhibits excellent initial coulombic efficiency and cycle stability.

[0069] Specifically, the secondary battery includes a casing, an electrode assembly, and an electrolyte. Both the electrode assembly and the electrolyte are located inside the casing.

[0070] The outer casing can be a packaging bag encapsulated with a sealing film (such as aluminum-plastic film), for example, a pouch battery. In other embodiments, it can also be a steel-cased battery, an aluminum-cased battery, etc.

[0071] Referring to Figures 3 and 4, the electrode assembly 100 includes a positive electrode 101, a negative electrode 102, and a separator 103, with the separator 103 disposed between the positive electrode 101 and the negative electrode 102. During charging, as shown in Figure 3, active ions (such as lithium ions) are extracted from the crystal lattice of the positive electrode material (such as a lithium-ion intercalated compound) of the positive electrode 101, pass through the separator 103 via the electrolyte, reach the negative electrode 102, and are inserted into the crystal lattice of the negative electrode material. During discharging, as shown in Figure 4, active ions (such as lithium ions) are extracted from the crystal lattice of the negative electrode material of the negative electrode 102, pass through the separator 103 via the electrolyte, reach the positive electrode 101, and are inserted into the crystal lattice of the positive electrode material (such as a lithium-ion intercalated compound). Electrons are generated and travel from the negative electrode 102 to the positive electrode 101 via an external circuit. The reverse movement of electrons forms an electric current, which can be used by electrical appliances.

[0072] In some embodiments, the electrode assembly 100 may be a stacked structure, which is formed by alternatingly stacking a positive electrode 101, a separator 103, and a negative electrode 102. In other embodiments, the electrode assembly 100 may also be a wound structure, which is formed by sequentially stacking and then winding the positive electrode 101, the separator 103, and the negative electrode 102.

[0073] The positive electrode 101 includes a positive current collector and a positive electrode material active layer disposed on at least one surface of the positive current collector. The positive current collector can be aluminum foil or nickel foil, or any composite current collector disclosed in the prior art, such as, but not limited to, current collectors formed by combining the aforementioned conductive foil and polymer substrate. The positive electrode material active layer includes a positive electrode active material, which includes a compound that reversibly inserts and extracts lithium ions (i.e., a lithiation intercalation compound). In some embodiments, the positive electrode active material may include a lithium transition metal composite oxide. This 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 is not limited to, lithium cobalt oxide (LiCoO2), lithium nickel manganese cobalt ternary materials (NCM), lithium manganese oxide (LiMn2O4), and lithium nickel manganese oxide (LiNi). 0.5 Mn 1.5 At least one of lithium iron phosphate (LiFePO4) or lithium iron phosphate (LiFePO4).

[0074] The positive electrode material active layer also includes an adhesive for bonding the positive electrode active material particles to facilitate the formation of the film layer, and also to 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 is not limited to, at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0075] The positive electrode material active layer may further include a conductive material, which includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, carbon-based materials may include, but are not limited to, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, metal-based materials may include, but are not limited to, metal powders or metal fibers, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0076] The separator 103 includes a membrane 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 may be a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane, etc.

[0077] The electrolyte serves to conduct ions between the positive electrode 101 and the negative electrode 102. The electrolyte can be in one or more states, including gel, solid, and liquid. In some embodiments, the electrolyte is a liquid electrolyte solution. The liquid electrolyte solution serves to conduct active ions between the positive electrode 101 and the negative electrode 102. In some embodiments, the liquid electrolyte solution includes a lithium salt and an organic solvent. The lithium salt may be selected from, but is not limited to, one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium tetraphenylborate (LiB(C6H5)4), lithium methanesulfonate (LiCH3SO3), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2), lithium tri(trifluoromethanesulfonyl)methyllithium (LiC(SO2CF3)3), lithium dioxolaneborate (LiBOB), and lithium difluorophosphate (LiPO2F2). For example, LiPF6 is selected as the lithium salt because it provides high ionic conductivity and improves cycling characteristics. The organic solvent may be a carbonate compound, a carboxylic acid ester compound, an ether compound, a nitrile compound, other organic solvents, or combinations thereof. Examples of carbonate compounds 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), butyl carbonate (BC), vinyl 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, trifluoromethylethylene carbonate, or combinations thereof.

[0078] In the preparation of secondary batteries, positive electrode sheets, separators and negative electrode sheets are wound or stacked to obtain battery cells. The battery cells are then encapsulated in pre-stamped aluminum-plastic films. After the encapsulated batteries are dried, electrolyte is injected into the dried batteries. The secondary batteries are then aged, formed and resealed to complete the preparation of the secondary batteries.

[0079] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0080] Example 1

[0081] The preparation method of the negative electrode material in this embodiment includes the following steps:

[0082] S1, 20 kg of graphite raw material (D50 volume average particle size of 16 μm) was kept at 500℃ and oxygen volume content of 5% for 3 h, and then cooled to room temperature to obtain graphite oxide.

[0083] S2, add graphite oxide and 2.2 kg of petroleum asphalt (softening point of 250℃) into a VC mixer and mix for 25 min. After uniform mixing, heat treat at 400℃ for 4 h in a nitrogen atmosphere. After cooling to room temperature, obtain the composite.

[0084] S3. The composite is densified by pressing in a hydraulic press at a pressure of 30 MPa for 0.5 min, repeated twice. Then, it is subjected to isostatic pressing densification at a maximum pressure of 70 MPa for 3 min. After pressing, it is crushed to obtain the intermediate product.

[0085] S4. The intermediate product is carbonized at 1250°C for 16 hours under a nitrogen atmosphere. After carbonization, it is dispersed, demagnetized and sieved to obtain the negative electrode material of this embodiment.

[0086] Example 2

[0087] The preparation method of the negative electrode material in this embodiment includes the following steps:

[0088] S1, 20 kg of graphite raw material (D50 volume average particle size of 16 μm) was kept at 450℃ for 3 h under the condition of oxygen volume content of 6%, and after cooling to room temperature, graphite oxide was obtained.

[0089] S2, add graphite oxide and 2.2 kg of coal tar pitch (softening point 180℃) to a VC mixer and mix for 25 min. After uniform mixing, heat treat at 350℃ for 4 h in a nitrogen atmosphere. After cooling to room temperature, the composite is obtained.

[0090] S3. The composite is densified by pressing in a hydraulic press at a pressure of 20 MPa for 0.5 min, repeated twice. Then, it is subjected to isostatic pressing densification at a maximum pressure of 70 MPa for 3 min. After pressing, it is crushed to obtain the intermediate product.

[0091] S4. The intermediate product is carbonized at 1250°C for 14 hours under a nitrogen atmosphere. After carbonization, it is dispersed, demagnetized and sieved to obtain the negative electrode material of this embodiment.

[0092] Example 3

[0093] The preparation method of the negative electrode material in this embodiment includes the following steps:

[0094] S1, 20 kg of graphite raw material (D50 volume average particle size of 16 μm) was kept at 500℃ and oxygen volume content of 6% for 3 h, and then cooled to room temperature to obtain graphite oxide.

[0095] S2, add graphite oxide and 1.6 kg of coal tar pitch (softening point 180℃) to a VC mixer and mix for 25 min. After uniform mixing, heat treat at 350℃ for 3 h under nitrogen atmosphere. After cooling to room temperature, obtain the composite.

[0096] S3. The composite is densified by pressing in a hydraulic press at a pressure of 20 MPa for 0.5 min, repeated twice. Then, it is subjected to isostatic pressing densification at a maximum pressure of 60 MPa for 3 min. After pressing, it is crushed to obtain the intermediate product.

[0097] S4. The intermediate product is carbonized at 1250°C for 12 hours under a nitrogen atmosphere. After carbonization, it is dispersed, demagnetized and sieved to obtain the negative electrode material of this embodiment.

[0098] Example 4

[0099] The preparation method of the negative electrode material in this embodiment includes the following steps:

[0100] S1, 20 kg of graphite raw material (D50 volume average particle size of 14.5 μm) was kept at 500℃ for 3 h under the condition of oxygen volume content of 5%, and then cooled to room temperature to obtain graphite oxide.

[0101] S2, add graphite oxide and 2.2 kg of petroleum asphalt (softening point 250℃) to a VC mixer and mix for 25 min. After uniform mixing, heat treat at 400℃ for 4 h under nitrogen atmosphere. After cooling to room temperature, obtain the composite.

[0102] S3. The composite is densified by pressing in a hydraulic press at a pressure of 30 MPa for 0.5 min, repeated twice, followed by isostatic pressing at a maximum pressure of 70 MPa for 3 min. After pressing, it is crushed to obtain the intermediate product.

[0103] S4. The intermediate product is carbonized at 1250°C for 16 hours under a nitrogen atmosphere. After carbonization, it is dispersed, demagnetized and sieved to obtain the negative electrode material of this embodiment.

[0104] Example 5

[0105] The preparation method of the negative electrode material in this embodiment includes the following steps:

[0106] S1, 20 kg of graphite raw material (D50 volume average particle size of 10 μm) was kept at 550℃ for 3 h under the condition of oxygen volume content of 5%, and after cooling to room temperature, graphite oxide was obtained.

[0107] S2, add graphite oxide and 2.2 kg of petroleum asphalt (softening point 250℃) to a VC mixer and mix for 25 min. After uniform mixing, heat treat at 400℃ for 4 h under nitrogen atmosphere. After cooling to room temperature, obtain the composite.

[0108] S3. The composite is densified by pressing in a hydraulic press at a pressure of 30 MPa for 0.5 min, repeated twice. Then, it is subjected to isostatic pressing densification at a maximum pressure of 70 MPa for 3 min. After pressing, it is crushed to obtain the intermediate product.

[0109] S4. The intermediate product is carbonized at 1250°C for 16 hours under a nitrogen atmosphere. After carbonization, it is dispersed, demagnetized and sieved to obtain the negative electrode material of this embodiment.

[0110] Example 6

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

[0112] S1, 20 kg of graphite raw material (D50 volume average particle size of 16 μm) is pressed in a hydraulic press at a pressure of 40 MPa for 0.5 min, repeated 3 times. After pressing, it is crushed to an average particle size of 16 μm, and then subjected to isostatic pressing densification treatment at a maximum pressure of 80 MPa for 3 min. After crushing, an intermediate product is obtained.

[0113] S2, the intermediate product and 2.2 kg of asphalt (softening point 250℃) are added to a VC mixer and mixed for 25 min. After uniform mixing, the mixture is carbonized at 1250℃ for 16 h in a nitrogen atmosphere. After carbonization, the material is dispersed, demagnetized and sieved to obtain the negative electrode material of this embodiment.

[0114] Example 7

[0115] The preparation method of the negative electrode material in this embodiment includes the following steps:

[0116] S1, 20 kg of graphite raw material (D50 volume average particle size of 16 μm) is pressed in a hydraulic press at a pressure of 30 MPa for 0.5 min, repeated 3 times. After pressing, it is crushed to an average particle size of 16 μm, and then subjected to isostatic pressing densification treatment at a maximum pressure of 100 MPa for 3 min. After crushing, an intermediate product is obtained.

[0117] S2, the intermediate product and 2.2 kg of asphalt (softening point 250℃) are added to a VC mixer and mixed for 25 min. After uniform mixing, the mixture is carbonized at 1250℃ for 18 h in a nitrogen atmosphere. After carbonization, the material is dispersed, demagnetized and sieved to obtain the negative electrode material of this embodiment.

[0118] Comparative Example 1

[0119] The preparation method of the comparative example negative electrode material includes the following steps:

[0120] 20 kg of graphite raw material (D50 volume average particle size of 16 μm) and 2.2 kg of asphalt (softening point of 250℃) were added to a VC mixer and mixed for 25 min. After mixing, the mixture was carbonized at 1250℃ for 16 h under a nitrogen atmosphere. After carbonization, the anode material of this comparative example was obtained by dispersing, demagnetizing and sieving.

[0121] Comparative Example 2

[0122] The preparation method of the comparative example negative electrode material includes the following steps:

[0123] 20 kg of graphite raw material (D50 volume average particle size of 10 μm) and 2.2 kg of asphalt (softening point of 250℃) were added to a VC mixer and mixed for 25 min. After mixing, the mixture was carbonized at 1250℃ for 16 h under a nitrogen atmosphere. After carbonization, the anode material of this comparative example was obtained by dispersing, demagnetizing and sieving.

[0124] Comparative Example 3

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

[0126] S1. 20 kg of graphite raw material (D50 volume average particle size of 16 μm) and 1.4 kg of petroleum asphalt (softening point of 120℃) were added to a VC mixer and mixed for 25 min. After uniform mixing, the mixture was heat-treated at 400℃ for 4 h under a nitrogen atmosphere and cooled to room temperature to obtain the composite.

[0127] S2, the composite is densified by pressing in a hydraulic press at a pressure of 20 MPa for 0.5 min, repeated twice, and then isostatic pressing is performed at a maximum pressure of 50 MPa for 3 min. After pressing, it is crushed to obtain the intermediate product.

[0128] S3. The intermediate product was carbonized at 1250°C for 14 hours under a nitrogen atmosphere. After carbonization, it was dispersed, demagnetized and sieved to obtain the negative electrode material of this comparative example.

[0129] Comparative Example 4

[0130] The preparation method of the comparative example negative electrode material includes the following steps:

[0131] S1, 20 kg of graphite raw material (D50 volume average particle size of 16 μm) is pressed in a hydraulic press at a pressure of 30 MPa for 0.5 min, repeated 3 times. After pressing, it is crushed to an average particle size of 16 μm, and then subjected to isostatic pressing densification treatment at a maximum pressure of 120 MPa for 3 min. After crushing, an intermediate product is obtained.

[0132] S2, the intermediate product and 2.2 kg of asphalt (softening point 180℃) were added to a VC mixer and mixed for 25 min. After uniform mixing, the mixture was carbonized at 1250℃ for 14 h under a nitrogen atmosphere. After carbonization, the anode material of this comparative example was obtained by dispersing, demagnetizing and sieving.

[0133] Test case

[0134] 1. Particle hardness (HIT) and elastic modulus (EIT) tests

[0135] The particle hardness (HIT) and elastic modulus (EIT) of the anode material were tested using an Anton Paar NHT2 nanoindenter. The anode material was embedded in epoxy resin, ground and polished, and then placed on the sample stage of the indenter. Static displacement mode was used, with a glass triangular pyramid indenter positioned close to the particle center; the indentation should not approach the particle edge. The indentation depth was set to 1200 nm, the loading and unloading rate to 5 mN / min, the holding time to 15 s, and the Poisson's ratio to 0.3. Hardness was calculated based on the indentation depth and load magnitude, and elastic modulus was calculated based on the slope of the fitted line from 40% to 98% of the unloading curve. Ten particles were tested for each sample, and the average value was taken. Outlier handling: Box plot analysis was performed on the tested data, and outliers (values ​​exceeding Q1-1.5IQR (interquartile range) or Q3+1.5IQR) were removed.

[0136] 2. Tablet orientation test

[0137] The negative electrode material, carboxymethyl cellulose solution, and styrene-butadiene rubber solution were mixed at a mass ratio of 19.3:25:0.8, wherein the concentration of styrene-butadiene rubber in the styrene-butadiene rubber solution was 50 wt%, and the concentration of carboxymethyl cellulose in the carboxymethyl cellulose solution was 1.2 wt%. The slurry was uniformly dispersed using a high-speed disperser. The slurry was then uniformly coated onto the surface of aluminum foil and baked in a constant-temperature drying oven at 80–95℃ for at least 6 hours until completely dry, yielding the sample to be tested. The sample was then ground through a 200-mesh sieve and pressurized with a tablet press at pressures of 0.5T, 1T, and 2T to obtain sheet-like samples (tablets). The tablets were placed under a thickness gauge to obtain the initial thickness after 10 seconds. The tablets were then placed in a constant-temperature environment (25±3℃) and allowed to stand for 16 hours. The thickness of the tablet after rebound (after 16 hours of standing) was measured using a micrometer. The compaction density of the tablet under the corresponding pressure was calculated based on the tablet mass, initial thickness, and thickness after rebound. X'pert Pro was used to measure the compaction density. X-ray diffraction was used to analyze the compressed tablet to obtain its crystal structure and orientation. The crystal plane orientation and corresponding peak area were obtained by analyzing the X-ray diffraction pattern. The OI value was calculated according to the tablet orientation OI value = I004 / I110, where I004 is the diffraction peak area of ​​the (004) crystal plane in the X-ray diffraction pattern, and I110 is the diffraction peak area of ​​the (110) crystal plane in the X-ray diffraction pattern. The test results are shown in Figure 2 and Table 1. In Table 1, x represents the tablet compaction density, and y represents the tablet orientation OI value. In Figure 2, X represents the tablet compaction density, and Y represents the tablet orientation OI value.

[0138] 3. Particle size test

[0139] The D50 volume average particle size (D1) and equal volume average particle size (D2) of the negative electrode material were tested using a Malvern 3000 laser particle size analyzer. The sample, a small amount of dispersant (a mixture of ethanol, pure water, and a low-foaming surfactant), and pure water were added to a 50 mL beaker. The mixture was stirred thoroughly with a glass rod to ensure uniform dispersion. The sample was then transferred to the sample cell of the Malvern 3000 laser particle size analyzer, and the pump speed was set to 2400–2500 r / min with a frequency of 19.5 Hz for particle size testing.

[0140] 4. Sphericity Sh (90%) test

[0141] The sphericity of the anode material was tested using SYMPATEC's QICPIC dynamic particle image analyzer. Sh(90%) refers to the sphericity of the anode material particles when the volume cumulative distribution reaches 90% in the sphericity-volume cumulative distribution curve.

[0142] 5. Specific surface area SSA test

[0143] The specific surface area of ​​the negative electrode material was measured using a JW-DX dynamic specific surface area analyzer. Based on the relevant theories of physical adsorption, and using the continuous flow method proposed by Nelsen and Eggertsen as the structure, the specific surface area of ​​the solid was determined. A mixed gas of hydrogen as the carrier and nitrogen as the adsorbent gas was introduced into the sample tube. When the sample tube was immersed in liquid nitrogen to reach a low-temperature environment, the nitrogen in the mixed gas was physically adsorbed by the sample until adsorption saturation. At this point, the proportion of nitrogen in the mixed gas changed. During the adsorption process, a high-precision thermal conductivity detector performed the detection and calculation.

[0144] 6. Tap density test

[0145] The tap density of the anode material was tested using a Quanta Dual Autotap instrument. A 100 mL sample of the anode material was placed in a graduated cylinder and mechanically vibrated 1000 times. The sample mass and volume after tapping were then recorded. The tap density (g / cm³) was calculated accordingly. 3 The tapped density is calculated as (sample mass / tapped volume).

[0146] 7. Powder compaction density test

[0147] 1.0 ± 0.05 g of negative electrode material was introduced into a metal sleeve, and the sleeve containing the sample was placed in the center of a CARVER 4350.22 compaction density meter. Pressure was slowly applied up to 2 T (tons), then stopped. The second hand was started, and the pressure was maintained for 30 seconds before being quickly released. The sample was removed from the metal mold cavity and placed on the horizontal worktable of a thickness gauge. The compacted height was measured using a thickness gauge. The compaction density was determined according to the powder compaction density (g / cm³). 3 The compacted density of powder is calculated as (sample mass / compacted volume).

[0148] 8. First Coulomb efficiency test

[0149] The negative electrode materials, carboxymethyl cellulose, and styrene-butadiene rubber of the examples and comparative examples were dissolved in pure water at a mass ratio of 96.5:1.5:2, respectively, 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 at 95°C, rolling, and pressurizing, a negative electrode sheet was obtained. A coin cell was assembled using a lithium metal sheet as the counter electrode in an argon-filled glove box. The electrolyte was purchased from Guotai Huarong Chemical New Materials Co., Ltd., and the model was LB5315C.

[0150] The coin cell was charged and discharged at a current density of 0.1C, within a range of 0.001 to 1.5V, to obtain the initial reversible specific capacity, the first charge capacity, and the first discharge capacity. The initial coulombic efficiency was calculated as follows: initial coulombic efficiency = first discharge capacity / first charge capacity.

[0151] 9. Cyclic stability test

[0152] Large single-crystal lithium nickel cobalt manganese oxide (NCM523) was mixed with conductive carbon black and PVDF at a mass ratio of 94:3.0:3.0 and dissolved in N-methylpyrrolidone, with the solid content controlled at 50%, to obtain a positive electrode slurry. The positive electrode slurry was coated onto an aluminum foil current collector, and after vacuum drying at 95°C, rolling, and pressurizing, a positive electrode sheet was obtained.

[0153] The negative electrode materials, carboxymethyl cellulose, styrene-butadiene rubber, and conductive carbon black of the examples and comparative examples were dissolved in N-methylpyrrolidone at a mass ratio of 95:1.5:2.1:1.2, respectively, 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 at 95°C, rolling, and pressing, a negative electrode sheet was obtained.

[0154] The positive electrode, separator, and negative electrode are assembled into a lithium-ion battery, and an electrolyte is injected to obtain a soft-pack battery with a capacity of about 40mAh. The electrolyte is a 1mol / L LiPF6 solution, and the solvent is ethylene carbonate (EC) + propylene carbonate (PC) + diethyl carbonate (DEC) + EMC (volume ratio 1:0.3:1:1). The separator is a PP / PE / PP three-layer composite separator.

[0155] The aforementioned pouch cell was used to test the cycle performance of the material. It was charged at a constant current rate of 1C to 4.20V, then charged at a constant voltage rate with the current gradually decreasing to 0.05C, and finally discharged at a discharge rate of 1C to 2.75V. This charge-discharge cycle was repeated 400 times. The discharge capacity Q1 at the first cycle and the discharge capacity Q400 at the 400th cycle were measured. The 400-cycle capacity retention rate was calculated as Q400 / Q1 × 100%.

[0156] The test results are shown in Tables 1 and 2.

[0157] Table 1

[0158] Table 2

[0159] In Table 2, D1 is the D50 volume average particle size of the negative electrode material, and D2 is the equal volume average particle size of the negative electrode material.

[0160] According to Tables 1 and 2 and Figures 1 and 2, the negative electrode materials of Examples 1 to 7 all meet the requirements of particle hardness of 0.28 GPa to 0.4 GPa, elastic modulus of 7.0 GPa to 8.0 GPa, and OI value of y for compaction orientation, where 4 < y ≤ 11. However, the negative electrode materials of Comparative Example 1 do not meet these requirements. The negative electrode material of Comparative Document 2 does not meet the requirement of elastic modulus of 7.0 GPa to 8.0 GPa, the negative electrode material of Comparative Example 3 does not meet the requirement of particle hardness of 0.28 GPa to 0.4 GPa, and the negative electrode material of Comparative Example 4 has a compaction density of 1.61 g / cm³. 3 When the OI value for tablet orientation does not meet the specified range, Examples 1-7 show higher initial coulombic efficiency, improved expansion performance, higher capacity retention at 400 cycles, and better overall electrical performance compared to Comparative Examples 1-4. Therefore, it can be seen that by ensuring the material meets the following requirements—particle hardness of 0.28 GPa to 0.4 GPa, elastic modulus of 7.0 GPa to 8.0 GPa, and tablet orientation OI value y, where 4 < y ≤ 11—this application can significantly improve initial coulombic efficiency and cycle stability.

[0161] Furthermore, compared to Examples 6-7, the negative electrode materials of Examples 1-5, while satisfying the requirements of particle hardness of 0.28 GPa-0.4 GPa, elastic modulus of 7.0 GPa-8.0 GPa, and OI value of y, where 4 < y ≤ 11, further satisfy the following relationship: the OI value of the tablet orientation is within the range of y1 ≤ y ≤ y2 under different tablet compaction densities, and y1 = 5.46x - 3.78, y2 = 8.9x - 7.76. The compaction degree of the negative electrode material is further improved, and the structural stability is also further improved. As a result, the initial coulombic efficiency of the negative electrode materials of Examples 1-5 is above 94%, the expansion performance is improved, and the capacity retention rate at 400 cycles is ≥ 92.5%, which is better than that of Examples 6-7.

[0162] Furthermore, compared to Example 3, the negative electrode materials of Examples 1, 2, and 4, while satisfying the requirements of particle hardness of 0.28 GPa to 0.4 GPa, elastic modulus of 7.0 GPa to 8.0 GPa, OI value of y (4 < y ≤ 11), and OI value of y1 ≤ y ≤ y2 under different compaction densities (y1 = 5.46x - 3.78, y2 = 8.9x - 7.76), further satisfy the following: OI value of 5 to 9. The filling rate and structural stability of the negative electrode material are further improved, and the volume expansion of the material is further alleviated. As a result, the initial coulombic efficiency of the negative electrode materials of Examples 1, 2, and 4 is above 94.84%, and the capacity retention rate at 400 cycles is ≥ 93.98%, which is better than that of Example 3.

[0163] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A negative electrode material, characterized in that, The negative electrode material comprises natural graphite and amorphous carbon filling the pores of the natural graphite; the particle hardness of the negative electrode material is 0.28 GPa to 0.4 GPa, and the elastic modulus is 7.0 GPa to 8.0 GPa; the compacted density of the negative electrode material is 1.5 g / cm³. 3 ~2.0g / cm 3 When the negative electrode material is pressed into a sheet, the OI value is y, where 4 < y ≤ 11.

2. The negative electrode material according to claim 1, characterized in that, The OI value y of the negative electrode material under different compaction densities satisfies the following condition: y1≤y≤y2; Where y1=5.46x-3.78, y2=8.9x-7.76, 1.5≤x≤2.0, and x is the numerical value corresponding to the compaction density of the negative electrode material.

3. The negative electrode material according to claim 1, characterized in that, When the compaction density of the negative electrode material is 1.5 g / cm³ 3 ~2.0g / cm 3 The OI value y of the negative electrode material is 5 to 10.

4. The negative electrode material according to claim 1 or 2, characterized in that, The negative electrode material satisfies at least one of the following characteristics: (1) The sphericity Sh (90%) of the negative electrode material is 0.90 to 0.95; (2) The D50 volume average particle size of the negative electrode material is 8μm to 18μm; (3) The average particle size of the negative electrode material is 10 μm to 20 μm.

5. The negative electrode material according to claim 4, characterized in that, The shape factor of the negative electrode material The sphericity Sh (90%) of the particles satisfies the following relationship: And Δ≤0.08; where, D1 is the D50 volume average particle size of the negative electrode material, in μm; D2 is the equal volume average particle size of the negative electrode material, in μm.

6. The negative electrode material according to claim 1 or 2, characterized in that, The specific surface area of ​​the negative electrode material is 2 g / cm³. 3 ~5m 2 / g.

7. The negative electrode material according to claim 1 or 2, characterized in that, The tap density of the negative electrode material is 0.9 g / cm³. 3 ~1.3g / cm 3 .

8. The negative electrode material according to claim 1 or 2, characterized in that, The compacted density of the negative electrode material powder is 1.7 g / cm³. 3 ~2.0g / cm 3 .

9. The negative electrode material according to claim 1, characterized in that, The particle hardness of the negative electrode material is 0.287 GPa, 0.288 GPa, 0.295 GPa, 0.308 GPa, 0.335 GPa, 0.356 GPa, 0.393 GPa, or any two of the above values ​​within a range. And / or, the elastic modulus of the negative electrode material is 7.08 GPa, 7.15 GPa, 7.26 GPa, 7.45 GPa, 7.78 GPa, 7.9 GPa, 7.96 GPa, or a range consisting of any two of the above values.

10. The negative electrode material according to claim 1, characterized in that, The compaction density of the negative electrode material is 1.5 g / cm³. 3 -2.0g / cm 3 The OI value of the negative electrode material is 4.05, 4.48, 4.68, 5.4, 5.53, 5.92, 6.1, 6.31, 6.58, 6.62, 7.38, 7.94, 8.11, 8.46, 8.51, 8.67, 9.73 or a range of any two of the above values.

11. The negative electrode material according to claim 1, characterized in that, The sphericity Sh (90%) of the negative electrode material is 0.90 to 0.95; And / or, the D50 volume average particle size of the negative electrode material is 8μm to 18μm.

12. The negative electrode material according to claim 1, characterized in that, The average particle size of the negative electrode material is 10 μm to 20 μm.

13. The negative electrode material according to claim 1, characterized in that, The negative electrode material has a D50 volume average particle size of 10μm to 18μm and an equal volume average particle size of 11μm to 18μm.

14. A negative electrode sheet, characterized in that, The negative electrode sheet comprises the negative electrode material according to any one of claims 1 to 13.

15. A secondary battery, characterized in that, The secondary battery includes the negative electrode sheet as described in claim 14.

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