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 natural graphite and amorphous carbon filled in 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; when the compaction density of the tablet of the negative electrode material is 1.5 to 2.0 g / cm<supgt;3< / supgt;, the orientation OI value of the tablet of the negative electrode material is y, and 4 < y ≤ 11. Applying this negative electrode material to a secondary battery can significantly improve the first Coulomb efficiency and cycle stability.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular, 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 advantages such as low self-discharge rate, high charge and discharge efficiency, no memory effect, and long cycle life. 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 graphite negative electrode materials have received extensive attention due to their high specific capacity, low charge and discharge platform, low cost, etc. However, natural graphite has high anisotropy and internal defects, and solvent co-insertion is likely to occur during the lithium-ion insertion process, resulting in a decrease in the stability of the SEI film, consuming additional lithium ions, and reducing the first Coulomb efficiency; and this high defect will cause graphite to experience uneven volume expansion. During the long-cycle process of the lithium-ion battery, the continuous expansion and contraction of the graphite structure may lead to further expansion of the defect area, and even cause graphite sheet layer peeling and crack formation, resulting in long-cycle capacity attenuation. 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 problems of low first Coulomb efficiency and insufficient cycle stability of the negative electrode material during charge and discharge in the prior art.

[0004] To achieve the above object, according to one aspect of the present invention, a negative electrode material is provided. The negative electrode material includes natural graphite and amorphous carbon filled in 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; when the compaction density of the pressed sheet of the negative electrode material is 1.5 to 2.0 g / cm 3 , the orientation OI value of the pressed sheet of the negative electrode material is y, 4 < y ≤ 11.

[0005] Further, the orientation OI value of the pressed sheet of the negative electrode material at different compaction densities is y, satisfying: y1 ≤ y ≤ y2;

[0006] wherein, y1 = 5.46x - 3.78, y2 = 8.9x - 7.76, 1.5 ≤ x ≤ 2.0, and x is the value corresponding to the compaction density of the pressed sheet of the negative electrode material.

[0007] Further, when the compaction density of the pressed sheet of the negative electrode material is 1.5 to 2.0 g / cm 3 , the orientation OI value of the pressed sheet of the negative electrode material is 5 to 10.

[0008] Further, the negative electrode material satisfies at least one of the following characteristics:

[0009] (1) The sphericity Sh(90%) of the negative electrode material is 0.90 - 0.95;

[0010] (2) The D50 volume average particle size of the negative electrode material is 8 - 18 μm;

[0011] (3) The equal - volume average particle diameter of the negative electrode material is 10 - 20 μm.

[0012] Furthermore, the shape factor of the negative electrode material is φ, and the sphericity of the particles is Sh(90%), satisfying: Δ = ∣φ - Sh(90%)∣, and Δ ≤ 0.08; where φ = D1 / D2, D1 is the D50 volume average particle size of the negative electrode material, with the unit of μm; D2 is the equal - volume average particle diameter of the negative electrode material, with the unit of μm.

[0013] Furthermore, the specific surface area of the negative electrode material is 2 - 5 m 2 / g.

[0014] Furthermore, the tap density of the negative electrode material is 0.9 - 1.3 g / cm 3 .

[0015] Furthermore, the powder compact density of the negative electrode material is 1.7 - 2.0 g / cm 3 .

[0016] 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.

[0017] 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.

[0018] Applying the technical solution of the present invention can improve the densification degree of the negative electrode material, reduce the anisotropy of the negative electrode material, and make the diffusion of lithium ions more uniform in different directions, which helps to improve the electrochemical performance of the negative electrode material; at the same time, by limiting the particle hardness of the negative electrode material to be 0.28 GPa - 0.4 GPa, the elastic modulus to be 7.0 GPa - 8.0 GPa, and the pressing orientation OI value to be y, where 4 < y ≤ 11, the structural stability of the negative electrode material can be enhanced, the volume expansion can be effectively controlled, ensuring that the negative electrode material can maintain structural stability during charge and discharge, while ensuring that there are unobstructed lithium - ion channels on its surface and inside, reducing the problems of sheet layer shedding and crack formation, thereby improving the cycle stability. Applying this negative electrode material to a secondary battery can make the initial Coulomb efficiency ≥ 94% and the capacity retention rate ≥ 92.5% after 400 cycles. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is the SEM image of the negative electrode material in Embodiment 1 of the present invention;

[0020] Figure 2 This is a coordinate diagram of the OI value of the tablet orientation of the negative electrode material at different compaction densities in the embodiments and comparative examples of the present invention;

[0021] Figure 3 This is a schematic structural diagram of a secondary battery during charging provided by an embodiment of the present invention;

[0022] Figure 4 This is a schematic structural diagram of a secondary battery during discharging provided by an embodiment of the present invention.

[0023] Description of reference numerals:

[0024] 100 - electrode assembly; 101 - positive electrode sheet; 102 - negative electrode sheet; 103 - separator. Detailed embodiments

[0025] 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. Those not specified in the embodiments are carried out according to conventional conditions or conditions recommended by the manufacturer. Those reagents or instruments not specified by the manufacturer can all be obtained as conventional products through commercial purchase.

[0026] As described in the background art of the present invention, in the prior art, there are problems of low initial Coulomb efficiency and insufficient cycle stability in the negative electrode material during charge and discharge. To solve the above problems, in a typical embodiment of the present invention, a negative electrode material is provided. The negative electrode material includes natural graphite and amorphous carbon filled in 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; when the compaction density of the tablet of the negative electrode material is 1.5 to 2.0 g / cm 3 , the OI value of the tablet orientation of the negative electrode material is y, and 4 < y ≤ 11.

[0027] The negative electrode material includes natural graphite and amorphous carbon filled in the pores of the natural graphite, which improves the densification degree of the negative electrode material. At the same time, due to the high disorder of the amorphous carbon, the filling of the amorphous carbon can reduce the anisotropy of the negative electrode material, making the diffusion of lithium ions more uniform in different directions, which helps to improve the electrochemical performance of the negative electrode material.

[0028] The particle hardness and elastic modulus of the negative electrode material are crucial for the electrochemical performance and cycling stability of secondary batteries, and can also reflect the degree of densification inside the particles and the degree of amorphous carbon filling. Among them, the particle hardness (HIT) of the negative electrode material directly reflects the ability of graphite particles to resist elastic and plastic deformation under the action of external forces. The elastic modulus (EIT) of the negative electrode material reflects the ability of the negative electrode material to resist external forces during the elastic deformation stage, that is, the stress required to produce a unit strain when the material is stressed.

[0029] The particle hardness of the negative electrode material affects the degree of its volume expansion and contraction. By limiting the particle hardness of the negative electrode 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 the lithium-ion insertion and extraction processes, that is, they can better resist the mechanical stress during this process, which helps to reduce the fragmentation and shedding of particles during cycling, thereby improving cycling stability; secondly, it helps to limit the expansion of pores inside the particles and reduce the probability of co-insertion of electrolyte solvent molecules and lithium ions into the graphite interlayer, thereby reducing the excessive consumption of the SEI film and improving the first Coulombic efficiency; in addition, during the charge-discharge cycle of secondary batteries, the graphite particles undergo repeated volume changes. When the particle hardness is 0.28 GPa to 0.4 GPa, it helps to maintain the structural integrity of the negative electrode material and reduce the capacity attenuation caused by mechanical wear and structural damage, thereby extending the service life of secondary batteries.

[0030] The elastic modulus of the negative electrode material is related to its stress-strain performance. By limiting the elastic modulus of the negative electrode material to 7.0 GPa to 8.0 GPa, this indicates that the elastic deformation generated during the lithium-ion insertion and extraction processes of the negative electrode material can be better controlled, effectively alleviating the volume change of the negative electrode material during cycling, enabling it to better adapt to volume changes, reducing cracks and particle shedding caused by stress concentration, and helping to improve cycling stability; secondly, it helps to stabilize the SEI film during the charge-discharge cycle and reduce film rupture caused by material expansion, thereby improving the first Coulombic efficiency and cycling performance; in addition, the elastic modulus is related to the diffusion kinetics of lithium ions in the negative electrode material. An appropriate elastic modulus means that the internal stress of the material is low, the diffusion path of lithium ions is more unobstructed, which helps to improve the transmission rate of lithium ions, thereby improving the electrochemical performance of secondary batteries, such as charge-discharge rate and capacity.

[0031] The negative electrode material has diffraction peaks of the (004) crystal plane and the (110) crystal plane in the X-ray diffraction pattern. The OI value of the tablet orientation of the negative electrode material is the ratio of the diffraction peak area of the tablet formed on the (004) crystal plane to the diffraction peak area of the (110) crystal plane. In some embodiments, when the compaction density of the tablet of the negative electrode material is 1.5 to 2.0 g / cm 3, the OI value of the tablet orientation of the negative electrode material is y, where 4 < y ≤ 11. The OI value of the tablet orientation is a parameter describing the degree of order in the arrangement of graphite layers in the negative electrode material. By limiting the OI value of the tablet orientation to y, where 4 < y ≤ 11, it helps to improve the kinetic performance of the material. Due to the high disorder of amorphous carbon, the filling of amorphous carbon can reduce the OI value of the tablet orientation. This disorder helps to prevent the co-insertion of solvents in the electrolyte into the graphite layer, thereby improving the interfacial transport kinetics of lithium ions. By limiting the OI value of the tablet orientation within the above range, the present invention helps to improve the first Coulombic efficiency and cycle stability of the negative electrode material.

[0032] The present invention limits the particle hardness of the negative electrode material to 0.28 GPa - 0.4 GPa, the elastic modulus to 7.0 GPa - 8.0 GPa, and the OI value of the tablet orientation to y, where 4 < y ≤ 11. This not only reflects a high degree of densification inside the particles and a high degree of amorphous carbon filling, but also achieves a balance between structural stability and lithium ion transport kinetics, taking into account the improvement of the first Coulombic efficiency and cycle stability. Specifically, on the one hand, it ensures the structural integrity of the negative electrode material during cycling, thus improving the cycle stability; on the other hand, it optimizes the diffusion path of lithium ions in the negative electrode material, which is beneficial to improving the electrochemical performance, such as the first Coulombic efficiency. This synergistic effect helps the negative electrode material to maintain a moderate elastic deformation during the lithium ion insertion and extraction process, avoid excessive plastic deformation, maintain a more stable SEI film and better electrochemical performance, thereby taking into account the improvement of the first Coulombic efficiency and cycle stability.

[0033] The particle hardness and elastic modulus of the negative electrode material can be determined by indentation hardness testing. For example, a nanoindenter can be used for measurement. During the test, a hard indenter is pressed into the surface of the particle with a certain force and then unloaded. By measuring the depth and width of the indentation, as well as the magnitude of the applied force, the hardness value of the negative electrode material is calculated. 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. After the test, the average values of the hardness value and the elastic modulus are taken respectively, which are the particle hardness and elastic modulus of the negative electrode material.

[0034] According to the research of the present invention, applying the above-mentioned negative electrode material to a secondary battery can significantly improve the first Coulombic efficiency and cycle stability. This is because, on the one hand, due to the amorphous carbon in the pores of natural graphite in the negative electrode material of the present application, the densification degree of the negative electrode material is improved, the anisotropy of the negative electrode material can be reduced, and the diffusion of lithium ions in different directions becomes more uniform, 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 - 0.4 GPa, the elastic modulus to 7.0 GPa - 8.0 GPa, and the OI value of the tablet orientation to y, where 4 < y ≤ 11, it not only indicates a high degree of densification inside the particles and a high degree of amorphous carbon filling, but also enables the negative electrode material to have appropriate mechanical strength and deformation ability, which can not only resist volume changes during charge and discharge, but also optimize the transport kinetics of lithium ions, thereby improving the stability of the SEI film and the overall performance of the secondary battery. Applying this negative electrode material to a secondary battery can make the first Coulombic efficiency ≥ 94% and the capacity retention rate ≥ 92.5% after 400 cycles, which helps to obtain a more efficient, stable and long-life secondary battery.

[0035] In some preferred embodiments, the OI value y of the tablet orientation of the negative electrode material at different tablet 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 value corresponding to the compaction density of the negative electrode material; y2 and y1 are respectively the upper and lower limits of the OI value of the tablet orientation at different compaction densities. By limiting the OI value of the tablet orientation of the negative electrode material to satisfy the above requirements, it can ensure a high degree of isotropy of the negative electrode material, ensure a small polarization effect during the charging process, limit the volume effect, and reduce the formation of a new SEI. In addition to further improving the first Coulombic efficiency and cycle stability, it can also maximize the capacity, thereby taking into account the improvement of the first Coulombic efficiency, capacity and cycle stability of the negative electrode material.

[0036] Specifically, when the compaction density of the tablet of the negative electrode material is 1.5 g / cm 3 , that is, when x is 1.5, y1 = 4.41, y2 = 5.59. At this time, 4.41 ≤ y ≤ 5.59, and the OI value of the tablet orientation is 4.41 - 5.59; when the compaction density of the tablet of the negative electrode material is 2 g / cm 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 tablet orientation is 7.14 - 10.04, and so on. In the specific implementation process of the present invention, the compaction density of the tablet of the negative electrode material essentially refers to the compaction density of the tablet formed by the negative electrode material under different pressures.

[0037] In some preferred embodiments, the tablet orientation OI value y of the negative electrode material at different tablet compaction densities satisfies: y1 ≤ y ≤ y2; where y1 = 5.46x - 3.78 and y2 = 8.88x - 8.

[0038] In some preferred embodiments, the tablet orientation OI value of the negative electrode material is 5 to 10, preferably 5 to 9, which can further improve the first Coulombic efficiency and cycle stability of the negative electrode material.

[0039] The geometric morphology of the negative electrode material particles has a certain influence on the electrochemical performance and cycle stability. The geometric morphology of the particles will affect their contact area with the electrolyte and their mechanical stability during charge and discharge, thereby affecting the charge and discharge performance and cycle stability of the secondary battery. In some embodiments, the shape factor of the negative electrode material is φ, and the sphericity of the particles is Sh(90%), satisfying: Δ = ∣φ - Sh(90%)∣, and Δ ≤ 0.08; where φ = D1 / D2, D1 is the D50 particle size of the negative electrode material in μm; D2 is the volume-average diameter of the negative electrode material in μm. Specifically, Δ = ∣φ - Sh(90%)∣, and it can be understood that Δ is the absolute value of the difference between φ and Sh(90%). By limiting the shape factor φ of the negative electrode material and the sphericity Sh(90%) of the particles to satisfy the above relationship, excellent contact between particles can be ensured, and at the same time, the particles have good mechanical stability and can resist volume expansion and contraction, thereby improving the charge and discharge performance and cycle stability of the secondary battery. In some preferred embodiments, Δ ≤ 0.06, which can further improve the charge and discharge performance and cycle stability.

[0040] Sphericity refers to the degree to which a particle approaches a sphere. The sphericity Sh(90%) of the negative electrode material refers to the ratio of the maximum diameter to the minimum diameter of the particles among 90% of the particles. The closer this ratio is to 1, the closer the particles are to a perfect sphere. In some embodiments, the sphericity Sh(90%) of the negative electrode material is 0.90 to 0.95, which means that most of the graphite particles have a high sphericity. In addition to helping to further improve the first Coulombic efficiency and cycle stability of the secondary battery, this geometric morphology helps to form a denser structure during the compaction process, which helps to improve the energy density of the secondary battery. In addition, spherical graphite particles are isotropic, can form a more uniform lithium-ion transmission path and a more stable conductive network. Compared with irregularly shaped particles, spherical particles are arranged more orderly in the electrode, which can reduce the electron and lithium-ion transmission resistance inside the secondary battery and improve the charge and discharge rate and capacity of the secondary battery.

[0041] The D50 volume-average 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 anode material. In some embodiments, the D50 volume-average particle size of the anode material is 8 - 18 μm. The equivalent volume-average particle size refers to the average particle size of a sphere with the same average volume as the particles. In some embodiments, the equivalent volume-average particle size of the anode material is 10 - 20 μm. By limiting the D50 volume-average particle size and the equivalent volume-average particle size of the anode material within a suitable range, in addition to helping to further improve the first Coulombic efficiency and cycle stability of the secondary battery, it can also make the diffusion path of lithium ions shorter, reduce the transport resistance during the insertion and extraction of lithium ions, thereby improving the charge-discharge efficiency. In addition, it can reduce the internal pores of the electrode and increase the energy density of the secondary battery. In some preferred embodiments, the D50 volume-average particle size of the anode material is 10 - 18 μm, and the equivalent volume-average particle size is 11 - 18 μm.

[0042] The specific surface area (SSA) refers to the total surface area of a unit mass of the material. In some embodiments, the specific surface area of the anode material is 2 - 5 m 2 / g. By limiting the specific surface area of the anode material within a moderate range, in addition to further improving the first Coulombic efficiency and cycle performance, it can also provide a moderate diffusion path, which not only avoids the performance degradation caused by the electrolyte penetrating into the internal pores of the material, but 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 surface of the graphite material, reducing the possibility of side reactions such as electrolyte decomposition and co-insertion of solvent molecules.

[0043] The tapped density (Tap) refers to the degree of compaction of the material under physical vibration. In some embodiments, the tapped density of the anode material is 0.9 - 1.3 g / cm 3 . By limiting the tapped density of the anode material within a moderate range, it helps to increase the compaction density during the electrode manufacturing process, reduce the internal pores of the electrode, improve the utilization rate of the anode 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, reducing the irreversible capacity during the first charge and discharge, and improving the first Coulombic efficiency. It also helps to reduce the volume expansion during cycling, reduce the mechanical stress between particles, prevent the formation of cracks, and thus improve the cycle stability. In addition, it helps to maintain the integrity of the electrode structure, prevent the active material from falling off the current collector, and further improve the cycle performance. In some preferred embodiments, the tapped density of the anode material is 1 - 1.3 g / cm 3 .

[0044] The powder compaction density refers to the density of the negative electrode material powder under a certain pressure (e.g., 2T). It is usually determined by compacting the material under a certain pressure and then measuring the volume and mass of the compacted material. The powder compaction density is directly related to 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 / cm 3 . By limiting the powder compaction density within an appropriate range, the graphite particles can be in closer contact, which helps to form a more uniform and stable SEI film, thereby further improving the initial Coulombic efficiency. At the same time, the relative movement between particles is reduced, and the mechanical stress borne by the graphite particles during the charge-discharge cycle is lowered, thus further improving the cycle stability. In addition, the distribution of the negative electrode material can be made more uniform, which helps to improve the energy density.

[0045] The negative electrode material of the present invention can be used as a negative electrode material in secondary batteries, effectively improving the initial Coulombic efficiency and cycle stability of secondary batteries. 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 ≥360 mAh / g.

[0046] 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:

[0047] S1, oxidizing the graphite raw material to obtain graphite oxide;

[0048] S2, mixing and coating the graphite oxide and pitch in sequence, and then performing a first heat treatment to obtain a composite;

[0049] S3, densifying the composite to obtain an intermediate product;

[0050] S4, performing a second heat treatment on the intermediate product to obtain the negative electrode material.

[0051] Specifically, in S1, oxidizing the graphite raw material can effectively improve the surface activity of the graphite raw material, which not only promotes the uniformity in 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 matching the lithium ion insertion / extraction.

[0052] 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. Natural graphite can be, for example, spherical graphite, and the D50 volume average particle size of the spherical graphite is 1 to 20 μm. This particle size range can ensure the uniformity and stability of the material during subsequent processing. The oxidation treatment can be carried out under the condition that the oxygen volume content is 1% to 10%, the temperature of the oxidation treatment is 400 to 600 °C, and the time is 2 to 3 h.

[0053] In S2, the oxidized graphite and pitch are mixed and coated. It can be understood that the oxidized graphite and pitch are mixed at room temperature. During the mixing and coating process, the pitch and oxidized graphite are evenly mixed to obtain a mixture. The mixture is subjected to a first heat treatment to make the molten pitch fill the pores inside the graphite, obtaining a composite body. Through the mixing and coating and the first heat treatment, it helps to improve the density, compactness, and hardness of the material.

[0054] The specific mixing ratio of the oxidized graphite and pitch can be adjusted according to the material characteristics and requirements. For example, in some embodiments, the mass ratio of the graphite raw material to the pitch is 100:(1 to 30); the pitch is selected from at least one of petroleum pitch, coal pitch, and mesophase pitch, and the D50 volume average particle size of the pitch is 2 to 3 μm; the softening point of the pitch is 100 to 300 °C, and the mixing and coating time is 10 to 60 min; the first heat treatment can be carried out under the condition of a first inert atmosphere and a temperature higher than the softening point of the pitch; the first 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 can include at least one of nitrogen, helium, and argon; the temperature of the first heat treatment is 200 to 600 °C, and the time is 3 to 4 h.

[0055] In S3, the densification treatment includes molding treatment and isostatic pressing treatment. Specifically, first, the composite body is molded by a hydraulic press to obtain a molded product; then the molded product is subjected to isostatic pressing treatment to obtain an isostatic pressing product, and finally the isostatic pressing product is crushed to obtain an intermediate product. Through the densification treatment, the pores inside the graphite can be effectively reduced, the density of the material can be increased, its structural densification can be enhanced, and the pore filling effect can be improved, so that the particle hardness and elastic modulus of the material meet the above requirements.

[0056] For the pressing process, the specific pressure, time, and number of cycles can be adjusted according to the actual equipment capacity and material requirements. For example, in some embodiments, the pressing conditions are as follows: the pressure of the hydraulic press is 10 - 40 MPa, the holding pressure time is 0 - 2 min, and after releasing pressure for 0.5 min, it reciprocates slowly 2 - 4 times. The short holding pressure 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 holding pressure. It is suitable for continuous production processes and can improve production efficiency and equipment utilization rate. Isostatic pressing can further enhance the density and structural stability of the material, and the specific pressure and time can be optimized according to the performance requirements of the material and equipment capacity. For example, in some embodiments, in isostatic pressing, the filling effect of asphalt inside graphite can be regulated by controlling the pressure. For example, isostatic pressing can adopt cold isostatic pressing or warm isostatic pressing, and the pressure of isostatic pressing is 60 - 120 MPa, and the holding pressure time is 1 - 60 min.

[0057] In S4, the intermediate product is subjected to a second heat treatment under 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, asphalt is converted into amorphous carbon, ensuring the stable carbonization of the material at high temperature and improving its graphitization degree. Appropriate heat treatment temperature and time can affect the crystallinity and microstructure of graphite, so that its hardness and elastic modulus meet the above requirements.

[0058] The specific temperature and time of the second heat treatment can be adjusted according to the conditions of the heat treatment device and the performance target of the material. For example, in some embodiments, the temperature of the second heat treatment is higher than that of the first heat treatment. For example, the temperature of the second heat treatment is 900 - 1600 °C, and the time is 10 - 24 h. Among them, the second 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.

[0059] 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 above.

[0060] 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, 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 cycle stability of the secondary battery.

[0061] The negative electrode material active layer further includes a binder for binding negative electrode active material particles to facilitate the formation of a film layer and at the same time 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 is not limited to polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin or nylon, etc.

[0062] The negative electrode material active layer may further include a conductive material, and the conductive material includes but is 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 are not 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 are not 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.

[0063] 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 slurry; the negative electrode slurry is uniformly coated on the negative electrode current collector, and after drying, rolling and slitting, a negative electrode sheet is obtained. In a specific embodiment, the negative electrode active layer includes 70% - 99% of the negative electrode material, 0.5% - 15% of the conductive agent, and 0.5% - 15% of the binder by mass percentage.

[0064] 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. 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.

[0065] Due to including the above-mentioned negative electrode sheet with excellent performance, the secondary battery has excellent initial Coulomb efficiency and cycle stability.

[0066] Specifically, the secondary battery includes a housing, an electrode assembly and an electrolyte. Both the electrode assembly and the electrolyte are located inside the housing.

[0067] The housing can be a packaging bag obtained by encapsulating with a packaging film (such as an aluminum-plastic film), for example, it is a soft-pack battery. In some other embodiments, it can also be a steel shell battery, an aluminum shell battery, etc.

[0068] Please refer to Figure 3 and Figure 4 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 crystal lattice of the positive electrode material (such as a lithiated intercalation compound) of the positive electrode sheet 101, pass through the electrolyte across the separator 103, reach the negative electrode sheet 102 and are inserted into the crystal lattice of the negative electrode material. During discharging, please refer to Figure 4 , active ions (such as lithium ions) are deinserted from the crystal lattice of the negative electrode material of the negative electrode sheet 102, pass through the electrolyte across the separator 103, reach the positive electrode sheet 101 and are embedded into the crystal lattice of the positive electrode material (such as a lithiated intercalation compound), generating electrons that travel from the negative electrode sheet 102 to the positive electrode sheet 101 through an external circuit, and the reverse movement of the electrons forms an electric current that can be used by an electrical appliance.

[0069] In some embodiments, the electrode assembly 100 may be a stacked structure, which is formed by alternately stacking the positive electrode sheet 101, the separator 103 and the negative electrode sheet 102 in sequence. In some other embodiments, the electrode assembly 100 may also be a wound structure, which is formed by winding the positive electrode sheet 101, the separator 103 and the negative electrode sheet 102 in sequence after stacking.

[0070] The positive electrode sheet 101 includes a positive electrode current collector and a positive electrode material active layer disposed on at least one surface of the positive electrode current collector. The positive electrode current collector may use aluminum foil or nickel foil, etc., or may 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 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 can reversibly intercalate and deintercalate 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 limited to at least one of lithium cobalt oxide (LiCoO2), lithium nickel cobalt manganese ternary material (NCM), lithium manganese oxide (LiMn2O4), lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O4) or lithium iron phosphate (LiFePO4).

[0071] The positive electrode material active layer further includes a binder for binding the positive 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 positive electrode material active layer and the positive electrode current collector. In some embodiments, the binder may include but is not 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, etc.

[0072] The positive electrode material active layer may further include a conductive material, and the conductive material includes but is 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 are not 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 are not 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.

[0073] The separator 103 includes a film layer having 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 film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite film, etc.

[0074] 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 is 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 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(trifluoromethylsulfonyl)imide (LiN(SO2CF3)2), lithium tris(trifluoromethylsulfonyl)methide (LiC(SO2CF3)3), lithium bis(oxalato)borate (LiBOB), and lithium difluorophosphate (LiPO2F2). For example, LiPF6 is selected as the lithium salt because it can provide 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 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.

[0075] When preparing a secondary battery, the positive electrode sheet, the separator, and the negative electrode sheet are wound or laminated to obtain an electrode core, the electrode core is encapsulated into a pre-stamped aluminum-plastic film, after the encapsulated battery is dried to remove moisture, the electrolytic solution is injected into the dried battery, and the battery is subjected to aging, formation, and secondary sealing to complete the preparation of the secondary battery.

[0076] 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.

[0077] Example 1

[0078] The preparation method of the negative electrode material in this embodiment includes the following steps:

[0079] S1. Keep 20 kg of graphite raw material (with a D50 volume average particle size of 16 μm) at a temperature of 500 °C and an oxygen volume content of 5% for 3 h. After cooling to room temperature, graphite oxide is obtained.

[0080] S2. Add the graphite oxide and 2.2 kg of petroleum asphalt (softening point: 250 °C) into a VC mixer and mix for 25 min. After uniform mixing, conduct heat treatment at 400 °C in a nitrogen atmosphere for 4 h. After cooling to room temperature, a composite is obtained.

[0081] S3. Conduct densification treatment on the composite. Perform molding treatment in a hydraulic press with a hydraulic press pressure of 30 MPa, a holding pressure time of 0.5 min, and reciprocate 2 times. Then conduct isostatic pressing densification treatment with a maximum isostatic pressing pressure of 70 MPa and a holding pressure of 3 min. After pulverization, an intermediate product is obtained.

[0082] S4. Carbonize the intermediate product at 1250 °C in a nitrogen atmosphere for 16 h. After carbonization, conduct dispersion, demagnetization, and screening to obtain the negative electrode material of this example.

[0083] Example 2

[0084] The preparation method of the negative electrode material of this example includes the following steps:

[0085] S1. Keep 20 kg of graphite raw material (with a D50 volume average particle size of 16 μm) at a temperature of 450 °C and an oxygen content of 6% for 3 h. After cooling to room temperature, graphite oxide is obtained.

[0086] S2. Add the graphite oxide and 2.2 kg of coal tar pitch (softening point: 180 °C) into a VC mixer and mix for 25 min. After uniform mixing, conduct heat treatment at 350 °C in a nitrogen atmosphere for 4 h. After cooling to room temperature, a composite is obtained.

[0087] S3. Conduct densification treatment on the composite. Perform molding in a hydraulic press with a hydraulic press pressure of 20 MPa, a holding pressure time of 0.5 min, and reciprocate 2 times. Then conduct isostatic pressing densification treatment with a maximum isostatic pressing pressure of 70 MPa and a holding pressure of 3 min. After pulverization, an intermediate product is obtained.

[0088] S4. Carbonize the intermediate product at 1250 °C in a nitrogen atmosphere for 14 h. After carbonization, conduct dispersion, demagnetization, and screening to obtain the negative electrode material of this example.

[0089] Example 3

[0090] The preparation method of the negative electrode material of this example includes the following steps:

[0091] S1. Keep 20 kg of graphite raw material (D50 volume average particle size is 16 μm) at 500 °C with an oxygen content of 6% for 3 h. After cooling to room temperature, graphite oxide is obtained;

[0092] S2. Add the graphite oxide and 1.6 kg of coal tar pitch (softening point 180 °C) to a VC mixer and mix for 25 min. After uniform mixing, heat-treat at 350 °C for 3 h under a nitrogen atmosphere. After cooling to room temperature, a composite is obtained;

[0093] S3. Compact the composite. Carry out molding in a hydraulic press with a hydraulic press pressure of 20 MPa, a holding pressure time of 0.5 min, reciprocating 2 times, and then carry out isostatic pressing densification treatment. The maximum isostatic pressing pressure is 60 MPa. After holding pressure for 3 min, it is crushed to obtain an intermediate product;

[0094] S4. Carbonize the intermediate product at 1250 °C for 12 h under a nitrogen atmosphere. After carbonization, it is dispersed, demagnetized, and screened to obtain the negative electrode material of this example.

[0095] Example 4

[0096] The preparation method of the negative electrode material of this example includes the following steps:

[0097] S1. Keep 20 kg of graphite raw material (D50 volume average particle size is 14.5 μm) at 500 °C with an oxygen content of 5% for 3 h. After cooling to room temperature, graphite oxide is obtained;

[0098] S2. Add the graphite oxide and 2.2 kg of petroleum asphalt (softening point 250 °C) to a VC mixer and mix for 25 min. After uniform mixing, heat-treat at 400 °C for 4 h under a nitrogen atmosphere. After cooling to room temperature, a composite is obtained;

[0099] S3. Compact the composite. Carry out molding in a hydraulic press with a hydraulic press pressure of 30 MPa, a holding pressure time of 0.5 min, reciprocating 2 times, and carry out isostatic pressing densification treatment. The maximum isostatic pressing pressure is 70 MPa. After holding pressure for 3 min, it is crushed to obtain an intermediate product;

[0100] S4. Carbonize the intermediate product at 1250 °C for 16 h under a nitrogen atmosphere. After carbonization, it is dispersed, demagnetized, and screened to obtain the negative electrode material of this example.

[0101] Example 5

[0102] The preparation method of the negative electrode material of this example includes the following steps:

[0103] S1. Keep 20 kg of graphite raw material (D50 volume average particle size is 10 μm) at a temperature of 550 °C and an oxygen content of 5% for 3 h. After cooling to room temperature, graphite oxide is obtained;

[0104] S2. Add the graphite oxide and 2.2 kg of petroleum pitch (softening point 250 °C) to a VC mixer and mix for 25 min. After uniform mixing, perform heat treatment at 400 °C under a nitrogen atmosphere for 4 h. After cooling to room temperature, a composite is obtained;

[0105] S3. Compact the composite, perform molding in a hydraulic press. The pressure of the hydraulic press is 30 MPa, the pressure holding time is 0.5 min, repeat 2 times, and then perform isostatic pressing densification treatment. The maximum isostatic pressing pressure is 70 MPa, and after pressure holding for 3 min, it is crushed to obtain an intermediate product;

[0106] S4. Carbonize the intermediate product at 1250 °C for 16 h under a nitrogen atmosphere. After carbonization, it is dispersed, demagnetized, and screened to obtain the negative electrode material of this example.

[0107] Example 6

[0108] The preparation method of the negative electrode material in this example includes the following steps:

[0109] S1. Perform molding on 20 kg of graphite raw material (D50 volume average particle size is 16 μm) in a hydraulic press. The pressure of the hydraulic press is 40 MPa, the pressure holding time is 0.5 min, repeat 3 times. After molding, it is crushed to an average particle size of 16 μm, and then perform isostatic pressing densification treatment. The maximum isostatic pressing pressure is 80 MPa, and after pressure holding for 3 min, it is crushed to obtain an intermediate product;

[0110] S2. Add the intermediate product and 2.2 kg of pitch (softening point 250 °C) to a VC mixer and mix for 25 min. After uniform mixing, carbonize at 1250 °C for 16 h under a nitrogen atmosphere. After carbonization, it is dispersed, demagnetized, and screened to obtain the negative electrode material of this comparative example.

[0111] Example 7

[0112] The preparation method of the negative electrode material in this example includes the following steps:

[0113] S1. Perform molding on 20 kg of graphite raw material (D50 volume average particle size is 16 μm) in a hydraulic press. The pressure of the hydraulic press is 30 MPa, the pressure holding time is 0.5 min, repeat 3 times. After molding, it is crushed to an average particle size of 16 μm, and then perform isostatic pressing densification treatment. The maximum isostatic pressing pressure is 100 MPa, and after pressure holding for 3 min, it is crushed to obtain an intermediate product;

[0114] S2. Add the intermediate product and 2.2 kg of pitch (softening point 250 °C) to a VC mixer and mix for 25 min. After uniform mixing, carbonize at 1250 °C for 18 h under a nitrogen atmosphere. After carbonization, break up, demagnetize, and screen to obtain the negative electrode material of this comparative example.

[0115] Comparative Example 1

[0116] The preparation method of the negative electrode material of this comparative example includes the following steps:

[0117] Add 20 kg of graphite raw material (D50 volume average particle size of 16 μm) and 2.2 kg of pitch (softening point 250 °C) to a VC mixer and mix for 25 min. After mixing, carbonize at 1250 °C for 16 h under a nitrogen atmosphere. After carbonization, break up, demagnetize, and screen to obtain the negative electrode material of this comparative example.

[0118] Comparative Example 2

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

[0120] Add 20 kg of graphite raw material (D50 volume average particle size of 10 μm) and 2.2 kg of pitch (softening point 250 °C) to a VC mixer and mix for 25 min. After mixing, carbonize at 1250 °C for 16 h under a nitrogen atmosphere. After carbonization, break up, demagnetize, and screen to obtain the negative electrode material of this comparative example.

[0121] Comparative Example 3

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

[0123] S1. Add 20 kg of graphite raw material (D50 volume average particle size of 16 μm) and 1.4 kg of petroleum pitch (softening point 120 °C) to a VC mixer and mix for 25 min. After uniform mixing, heat-treat at 400 °C for 4 h under a nitrogen atmosphere. After cooling to room temperature, obtain a composite body;

[0124] S2. Compact the composite body, perform molding in a hydraulic press. The pressure of the hydraulic press is 20 MPa, the holding time is 0.5 min, reciprocate 2 times, and then perform isostatic pressing densification treatment. The maximum pressure of isostatic pressing is 50 MPa, and after holding pressure for 3 min, crush to obtain an intermediate product;

[0125] S3. Carbonize the intermediate product at 1250 °C for 14 h under a nitrogen atmosphere. After carbonization, break up, demagnetize, and screen to obtain the negative electrode material of this comparative example.

[0126] Comparative Example 4

[0127] The preparation method of the negative electrode material of this example includes the following steps:

[0128] S1. Press 20 kg of graphite raw materials (with a D50 volume average particle size of 16 μm) in a hydraulic press. The pressure of the hydraulic press is 30 MPa, keep the pressure for 0.5 min, reciprocate 3 times. After pressing, crush it to an average particle size of 16 μm, and then carry out isostatic pressing densification treatment. The maximum pressure of isostatic pressing is 120 MPa. After keeping the pressure for 3 min, crush it to obtain an intermediate product;

[0129] S2. Add the intermediate product and 2.2 kg of asphalt (softening point 180 °C) to a VC mixer and mix for 25 min. After uniform mixing, carbonize at 1250 °C for 14 h under a nitrogen atmosphere. After carbonization, disperse, demagnetize and screen to obtain the negative electrode material of this comparative example.

[0130] Test Example

[0131] 1. Test of particle hardness (HIT) and elastic modulus (EIT)

[0132] Use an Anton Paar NHT2 nanoindentation instrument to test the particle hardness (HIT) and elastic modulus (EIT) of the negative electrode material. Place the negative electrode material on the sample stage of the indentation instrument, adopt the static displacement mode, use a Berkovich triangular pyramid indenter, the indenter contacts the sample and records at a set rate until the set indentation depth is 1200 nm. After reaching the set depth, keep the load time for 15 s, and then unload the load at the same rate. Among them, the loading and unloading rate is 5 mN / min, and the Poisson's ratio is 0.3. Calculate the particle hardness according to the indentation depth and the load magnitude, and calculate the elastic modulus according to the fitting line slope of 40 - 98% of the unloading curve. Measure 10 particles for each sample, and take the average value after testing.

[0133] 2. Test of tablet orientation

[0134] Mix the negative electrode material, carboxymethyl cellulose solution, and styrene-butadiene rubber solution in a mass ratio of 19.3:25:0.8. The concentration of styrene-butadiene rubber in the styrene-butadiene rubber solution is 50 wt%, and the concentration of carboxymethyl cellulose in the carboxymethyl cellulose solution is 1.2 wt%. Use a high-speed disperser to disperse evenly to obtain a slurry; evenly coat the slurry on the surface of the aluminum foil, and then place it in a constant-temperature drying oven at 80 - 95 °C and bake for more than 6 h until the slurry is completely dry to obtain the sample to be tested; after grinding the sample to be tested through a 200-mesh sieve, use a tablet press to apply pressure to the sample to be tested at pressures of 0.5 T (ton), 1 T (ton), and 2 T (ton) respectively to obtain sheet-like samples (tablets); place the tablets under a thickness gauge to obtain the initial thickness of the tablets after 10 s, then place the tablets in a constant-temperature environment (25 ± 3 °C) and let them stand for 16 h, and use a micrometer to measure the thickness of the tablets after rebound (after standing for 16 h). Calculate the compaction density of the tablets under the corresponding pressure according to the mass, initial thickness, and thickness after rebound of the tablets; use an X-ray diffractometer to analyze the tablets to obtain the crystal structure and orientation; by analyzing the X-ray diffraction pattern, obtain the crystal plane orientation and the corresponding peak area, and calculate the OI value 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 compaction density of the tablets, and y represents the OI value of the tablet orientation, Figure 2 where X represents the compaction density of the tablets and Y represents the OI value of the tablet orientation.

[0135] 3. Particle Size Test

[0136] Use a Malvern 3000 laser particle size analyzer to measure the D50 volume average particle size (D1) and the equal volume average particle diameter D2 of the negative electrode material respectively. Add the sample, a small amount of dispersant (a mixture of ethanol, pure water, and a low-foam surfactant), and pure water to a 50 mL beaker, 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.

[0137] 4. Sphericity Sh(90%) Test

[0138] Use a SYMPATEC QICPIC dynamic particle image analyzer to measure the sphericity of the negative electrode material. Sh(90%) refers to the ratio of the maximum diameter to the minimum diameter of the particles among 90% of the particles.

[0139] 5. Specific Surface Area SSA Test

[0140] The specific surface area of the anode material was tested using a JW-DX dynamic specific surface area measuring instrument. 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 measured. A mixed gas with hydrogen as the carrier and nitrogen as the adsorption 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 would be physically adsorbed by the sample until adsorption saturation. At this time, the proportion of nitrogen in the mixed gas would change. During the adsorption process, a high-precision thermal conductivity detection instrument would complete the detection and calculation work.

[0141] 6. Tap density test of the tapped density

[0142] The tapped density of the anode material was tested using a Quantachrome Dual Autotap instrument. 100 mL of the anode material sample was placed in a graduated cylinder. After mechanical vibration 1000 times, the sample mass and the volume after tapping were obtained, and the tapped density (g / cm³) = sample mass / volume after tapping was calculated.

[0143] 7. Powder compaction density test

[0144] 1.0 ± 0.05 g of the anode material was introduced into a metal sleeve, and the sleeve loaded with the sample was placed in the center of a compaction density instrument CARVER 4350.22. Pressure was slowly applied to 2 T (tons), and then the pressure application was stopped. The stopwatch was started. After maintaining the pressure for 30 s, the pressure was quickly removed. The sample was taken out of the metal mold cavity and placed on the horizontal workbench of a thickness gauge. The thickness after compaction was measured using a thickness gauge, and the powder compaction density (g / cm³) = sample mass / volume after compaction was calculated.

[0145] 8. First Coulombic efficiency test

[0146] The anode 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, and the solid content was controlled to be 50% to obtain the anode slurry; the anode slurry was coated on a copper foil current collector, and after vacuum drying at 95 °C, rolling, and pressurization, an anode sheet was obtained; a lithium metal sheet was used as the counter electrode, and a button cell was assembled in a glove box filled with argon.

[0147] The button cell was subjected to charge-discharge tests 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, and the first Coulombic efficiency = first cycle discharge capacity / first cycle charge capacity was calculated.

[0148] 9. Cycle stability test

[0149] Mix large single crystal lithium nickel cobalt manganese oxide (NCM523) with conductive carbon black and PVDF in a mass ratio of 94:3.0:3.0, dissolve them in N-methylpyrrolidone, and control the solid content to be 50% to obtain the positive electrode slurry; coat the positive electrode slurry on an aluminum foil current collector, and after vacuum drying at 95°C, rolling, and pressurizing, obtain the positive electrode sheet;

[0150] Dissolve the negative electrode materials, carboxymethyl cellulose, styrene-butadiene rubber, and conductive carbon black of the examples and comparative examples in N-methylpyrrolidone respectively according to a mass ratio of 95:1.5:2.1:1.2, control the solid content to be 50%, and obtain the negative electrode slurry; coat the negative electrode slurry on a copper foil current collector, and after vacuum drying at 95°C, rolling, and pressurizing, obtain the negative electrode sheet;

[0151] Assemble the positive electrode sheet, separator, and negative electrode sheet into a lithium-ion battery, and inject the electrolyte to obtain a soft-pack battery of about 40 mAh; among them, the electrolyte is 1 mol / L LiPF6 / ethylene carbonate (EC) + propylene carbonate (PC) + diethyl carbonate (DEC) + EMC (volume ratio 1:0.3:1:1), and the separator is a three-layer composite separator of PP / PE / PP.

[0152] Use the above soft-pack battery to test the cycling performance of the material. Constant current charge at a charging rate of 1C to 4.20V, then charge at a constant voltage until the current gradually decreases to 0.05C, and then discharge at a discharge rate of 1C to 2.75V. Repeat this charge-discharge cycle 400 times, and measure the discharge capacity Q1 at the first cycle and the discharge capacity Q400 at the 400th cycle. Calculate the 400-week capacity retention rate according to 400-week capacity retention rate = Q400 / Q1×100%.

[0153] The test results are shown in Table 1 and Table 2.

[0154] Table 1

[0155]

[0156] Table 2

[0157]

[0158] In Table 2, D1 is the D50 volume average particle size of the negative electrode material, and D2 is the equal volume average particle diameter of the negative electrode material.

[0159] According to Table 1, Table 2 and Figure 1 、 Figure 2It can be seen that the anode materials of Examples 1 to 7 all satisfy that the particle hardness is 0.28 GPa to 0.4 GPa, the elastic modulus is 7.0 GPa to 8.0 GPa, and the OI value of the tablet pressing orientation is y, where 4 < y ≤ 11. However, the anode materials of Comparative Example 1 do not satisfy that the particle hardness is 0.28 GPa to 0.4 GPa, the elastic modulus is 7.0 GPa to 8.0 GPa, and the OI value of the tablet pressing orientation is y, where 4 < y ≤ 11. The anode materials of Comparative Example 2 do not satisfy that the elastic modulus is 7.0 GPa to 8.0 GPa. The anode materials of Comparative Example 3 do not satisfy that the particle hardness is 0.28 GPa to 0.4 GPa. The anode materials of Comparative Example 4 do not satisfy the range of the OI value of the tablet pressing orientation when the compaction density is 1.61 g / cm 3

[0160] At , the initial Coulomb efficiency of Examples 1 to 7 is higher than that of Comparative Examples 1 to 4, the swelling performance is improved, the capacity retention rate after 400 cycles is high, and the overall electrical performance is good. It can be seen that by making the material satisfy that the particle hardness is 0.28 GPa to 0.4 GPa, the elastic modulus is 7.0 GPa to 8.0 GPa, and the OI value of the tablet pressing orientation is y, where 4 < y ≤ 11, the initial Coulomb efficiency and cycle stability of the present invention can be significantly improved.

[0161] Furthermore, compared with Examples 6 to 7, on the basis that the anode materials of Examples 1 to 5 satisfy that the particle hardness is 0.28 GPa to 0.4 GPa, the elastic modulus is 7.0 GPa to 8.0 GPa, and the OI value of the tablet pressing orientation is y, where 4 < y ≤ 11, the anode materials of Examples 1 to 5 further satisfy the following relationship: the OI value of the tablet pressing orientation is within the range of y1 ≤ y ≤ y2 at different tablet pressing compaction densities, and y1 = 5.46x - 3.78, y2 = 8.9x - 7.76. The densification degree of the anode material is further improved, and the structural stability is also further improved, so that the initial Coulomb efficiency of the anode materials of Examples 1 to 5 is all above 94%, the swelling performance is improved, and the capacity retention rate after 400 cycles is all ≥ 92.5%, which is better than that of Examples 6 to 7.

[0162] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. 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 includes natural graphite and amorphous carbon filled in 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; when the tablet compaction density of the negative electrode material is 1.5 to 2.0 g / cm 3 ³, the orientation OI value of the tablet of the negative electrode material is y, where 4 < y ≤ 11; And the OI value y of the tablet orientation of the negative electrode material at different compaction densities satisfies the following conditions: y1 ≤ y ≤ y2; wherein, y1 = 5.46x - 3.78, y2 = 8.9x - 7.76, 1.5 ≤ x ≤ 2.0, and x is the value corresponding to the compaction density of the tablet of the negative electrode material.

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

3. 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 8 to 18 μm; (3) The equal volume average particle diameter of the negative electrode material is 10 to 20 μm.

4. The negative electrode material according to claim 3, wherein The shape factor φ of the negative electrode material and the sphericity Sh(90%) of the particles satisfy the following relationship: Δ = ∣φ - Sh(90%)∣, and Δ ≤ 0.08; wherein, φ = D1 / D2, D1 is the D50 volume average particle size of the negative electrode material, in μm; D2 is the equal volume average particle diameter of the negative electrode material, in μm.

5. 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.

6. The negative electrode material according to claim 1, wherein The tap density of the negative electrode material is 0.9 to 1.3 g / cm 3 .

7. The negative electrode material according to claim 1, characterized in that, The tap density of the powder of the negative electrode material is 1.7~2.0 g / cm 3 .

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

9. A secondary battery, characterized in that, The secondary battery includes the negative electrode sheet according to claim 8.

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