Graphite negative electrode material, secondary battery, and electronic device

Abstract

The invention provides a graphite negative electrode material, a secondary battery and an electronic device, the graphite negative electrode material comprises a first graphite material and a second graphite material, the hardness Cs < 1 > of the first graphite material satisfies C < s > < 1 > > = 35 Mpa, the hardness C < s2 > of the second graphite material satisfies C < s2 > < = 20 Mpa, based on the particle number of the graphite negative electrode material, the particle number ratio X of the first graphite material satisfies X > = 25.0%, and X > = 25.0%. The particle number proportion Y of the second graphite material is greater than or equal to 30.0%. The graphite negative electrode material provided by the invention can improve the thermal stability of the secondary battery.

Description

Technical Field

[0001] The present application relates to the field of electrochemical energy storage, and in particular to a graphite anode material, a secondary battery using the graphite anode material, and an electronic device using the secondary battery. Background Art

[0002] With the wide application of lithium-ion batteries in various fields, lithium-ion batteries may need to exhibit their due performance in different temperature ranges, which poses higher requirements for the thermal stability of lithium-ion batteries. For example, when the battery temperature may reach above 50°C, the lithium-ion battery can still work without bloating, smoking, or catching fire.

[0003] Currently, on the market, the thermal stability of batteries is mainly improved by increasing the thickness of the separator and adjusting the electrolyte, but these will all bring some deficiencies, such as deteriorating the rate performance of the battery, reducing the energy density or cycle performance of the battery. How to improve the thermal stability of the battery is also an important issue.

[0004] Therefore, it is necessary to propose a graphite anode material that can improve the thermal stability of lithium-ion batteries. Summary of the Invention

[0005] The present application provides a graphite anode material, a secondary battery, and an electronic device.

[0006] In the first aspect of the present application, a graphite anode material is provided, including a first graphite material and a second graphite material. The hardness Cs1 of the first graphite material satisfies: Cs1 ≥ 35 Mpa, and the hardness Cs2 of the second graphite material satisfies: Cs2 ≤ 20 Mpa. Based on the number of particles of the graphite anode material, the proportion X of the number of particles of the first graphite material satisfies: X ≥ 25.0%, and the proportion Y of the number of particles of the second graphite material satisfies: Y ≥ 30.0%.

[0007] In this application, graphite materials with different hardnesses and different proportions of particle numbers are used in combination, which can improve the uniformity of the current density distribution in the negative electrode active layer of the secondary battery, and can also improve the diffusion rate of lithium ions in the negative electrode sheet, thereby enhancing the thermal stability of the secondary battery. After cold pressing the negative electrode sheet, the first graphite material and the second graphite material with different hardnesses are used. The deformability of graphite material particles with different hardnesses is different. The second graphite material with relatively lower hardness deforms relatively more, which can fully increase the contact area between particles, reduce the pores of the overall graphite material particles, thereby improving the electronic conductivity between particles in the negative electrode active layer, enhancing the uniformity of the current density in the negative electrode sheet, reducing the formation of lithium dendrites, and improving the thermal stability of the secondary battery. At the same time, the first graphite material with relatively higher hardness in the graphite negative electrode material is not easily deformed during the cold pressing process of preparing the negative electrode sheet, which can keep a specific contact area between particles, and there are still certain gaps between the particles, so as to improve the diffusivity of lithium ions in the negative electrode sheet and reduce polarization. The combination of the first graphite material and the second graphite material in this application also facilitates the embedding of lithium ions into the central region inside the particles when the active material particles are lithiated, increasing the degree of lithium intercalation inside the particles, reducing the degree of lithium intercalation in the outer layer of the particles, slowing down the lithium ion extraction rate and reducing the formation of lithium dendrites.

[0008] Based on the first aspect, in some possible implementation manners, 60% ≥ X ≥ 38%. This is beneficial for further increasing the diffusion rate of lithium ions in the negative electrode sheet, reducing the polarization of the secondary battery, and improving the thermal stability of the secondary battery.

[0009] Based on the first aspect, in some possible implementation manners, 60% ≥ Y ≥ 35%. This is beneficial for ensuring sufficient contact between particles in the graphite negative electrode material, so as to improve the electronic conductivity between particles in the negative electrode active layer, enhance the uniformity of the current density in the negative electrode sheet, and thus improve the thermal stability of the secondary battery.

[0010] Based on the first aspect, in some possible implementation manners, the hardness of the first graphite material satisfies: 80 Mpa ≥ Cs1 ≥ 40 Mpa. During the cold pressing process of the negative electrode sheet, the graphite negative electrode material is maintained at a specific hardness, and there are certain pores between particles, increasing the diffusion rate of lithium ions in the negative electrode sheet, reducing the formation of lithium dendrites, and improving the thermal stability of the secondary battery.

[0011] Based on the first aspect, in some possible implementation manners, the hardness of the second graphite material satisfies: 15 Mpa ≥ Cs2 ≥ 5 Mpa. This is beneficial for increasing the contact area between particles, thereby improving the electronic conductivity of the negative electrode active layer, enhancing the current density uniformity of the negative electrode sheet, and thus improving the thermal stability of the secondary battery.

[0012] Based on the first aspect, in some possible implementation manners, the tap density P of the graphite anode material satisfies: 1.60 ≤ P ≤ 1.85X + 1.95Y. This is beneficial to further improve the energy density and thermal stability of the secondary battery.

[0013] Based on the first aspect, in some possible implementation manners, the specific capacity of the graphite anode material is Cap mAh / g, and 330 ≤ Cap ≤ 340X + 360Y. Under the condition that the secondary battery has good energy density, the thermal stability of the secondary battery is further improved.

[0014] Based on the first aspect, in some possible implementation manners, in the Raman spectrum of the graphite anode material, 0.10 ≤ Id / Ig ≤ 0.35, where Id is the peak intensity of the D peak at 1350 cm -1 ±5 cm -1 in the Raman spectrum of the graphite anode material, and Ig is the peak intensity of the G peak at 1575 cm -1 ±5 cm -1 in the Raman spectrum of the graphite anode material. Under the condition that the graphite anode material has good initial Coulomb efficiency and thermal stability, it is also beneficial to increase the diffusion rate of lithium ions on the particle surface, reduce the interfacial diffusion impedance of lithium ions intercalating / deintercalating from the graphite anode material, and is beneficial to improving the thermal stability of the secondary battery.

[0015] Based on the first aspect, in some possible implementation manners, the graphitization degree G of the graphite anode material satisfies: 90.0% ≤ G ≤ 95.0%. This is beneficial to keep the d002 crystal plane spacing within an appropriate range, increase the intercalation / deintercalation rate of lithium ions, and reduce the phenomenon of layer peeling caused by a relatively large crystal plane spacing, thereby improving the cycle performance and thermal stability of the secondary battery.

[0016] Based on the first aspect, in some possible implementation manners, the specific surface area S of the graphite anode material satisfies: 0.7 m 2 / g ≤ S ≤ 2.5 m 2 / g. This is beneficial to increase the surface active sites of the graphite anode material, make the graphite anode material have an appropriate number of positions for intercalating / deintercalating lithium ions, and reduce the excessive formation of the SEI film, which may cause the SEI film to decompose and generate gas at high temperature, so as to improve the rate performance and thermal stability of the secondary battery.

[0017] Based on the first aspect, in some possible implementation manners, the particle size of the graphite anode material satisfies: 1.5 ≤ Dv90 / Dv50 ≤ 2.6; the particle size distribution is appropriate, the processability of preparing the anode electrode sheet is strong, and some small particles fill the pores, which is beneficial to increasing the tap density of the anode electrode sheet and improving the storage performance and thermal stability of the secondary battery.

[0018] Based on the first aspect, in some possible implementation manners, the tap density TD of the graphite anode material satisfies: TD ≥ 0.80 g / cm 3 . It is beneficial to improve the processability of preparing the anode slurry, reduce abnormal viscosity during the process of preparing the slurry, such as slurry stratification and precipitation, so as to increase the coating weight during the process of preparing the anode sheet and improve the electrochemical performance of the secondary battery.

[0019] Based on the first aspect, in some possible implementation manners, the OI value of the graphite anode material satisfies: OI value ≤ 10. It is beneficial to improve the multidirectionality of the lithium insertion direction of the graphite anode material, increase the lithium insertion rate, and thus improve the rate performance and thermal stability of the secondary battery.

[0020] The second aspect of the present application provides a secondary battery, including a positive electrode sheet, a negative electrode sheet, and an electrolyte, and the negative electrode sheet includes the above-mentioned graphite anode material. Containing the above-mentioned graphite anode material can improve the thermal stability of the secondary battery.

[0021] Based on the second aspect, in some possible implementation manners, the compaction density PD of the negative electrode sheet satisfies: 1.40 g / cm 3 ≤ PD ≤ 1.70 g / cm 3 . The particles in the negative electrode active layer maintain good electronic contact with each other, reduce the polarization of the secondary battery, and are also beneficial to reducing side reactions caused by particle breakage during cold pressing, thereby improving the thermal stability of the secondary battery.

[0022] Based on the second aspect, in some possible implementation manners, the porosity K of the negative electrode sheet satisfies: 25% ≤ K ≤ 45%. It enables the negative electrode sheet to have an appropriate liquid retention capacity, is beneficial to improving the ability of the negative electrode sheet to store the electrolyte, improving the cycle performance of the secondary battery, and is also beneficial to reducing the swelling situation, reducing the phenomenon of gas swelling of the secondary battery during high-temperature storage and high-temperature cycling, improving the stability of the negative electrode sheet, and thus improving the rate performance and thermal stability of the secondary battery.

[0023] The third aspect of the present application provides an electronic device, and the electronic device includes the above-mentioned secondary battery. The secondary battery has excellent thermal stability, which is beneficial to improving the service life of the electronic device. Specific Embodiments

[0024] The technical solutions in the embodiments of the present application will be clearly and detailedly described below. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which this application belongs. The terms used in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0025] An embodiment of the present application provides a secondary battery, which includes a housing, an electrode assembly, and an electrolyte. The electrode assembly and the electrolyte are both located inside the housing.

[0026] The housing can be a packaging bag encapsulated with a packaging film (such as an aluminum-plastic film), for example, when the secondary battery is a soft-pack battery. In some other embodiments, the secondary battery can also be a steel-shell battery, an aluminum-shell battery, etc.

[0027] The electrode assembly includes a positive electrode tab, a negative electrode tab, and a separator. The separator is disposed between the positive electrode tab and the negative electrode tab. The electrode assembly can be a stacked structure, which is formed by laminating the positive electrode tab, the separator, and the negative electrode tab. In some other embodiments, the electrode assembly can also be a wound structure, which is formed by laminating and then winding the positive electrode tab, the separator, and the negative electrode tab.

[0028] Negative electrode tab

[0029] The negative electrode tab includes a negative current collector and a negative active layer located on the surface of the negative current collector. The negative current collector includes: copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or any combination thereof, 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.

[0030] In the negative electrode tab, the negative active layer includes a graphite negative electrode material. The graphite negative electrode material includes a first graphite material and a second graphite material. The hardness Cs1 of the first graphite material satisfies: Cs1 ≥ 35 Mpa, and the hardness Cs2 of the second graphite material satisfies: Cs2 ≤ 20 Mpa. Based on the number of particles of the graphite negative electrode material, the proportion X of the number of particles of the first graphite material satisfies: X ≥ 25.0%, and the proportion Y of the number of particles of the second graphite material satisfies: Y ≥ 30.0%.

[0031] In the present application, graphite materials with different hardnesses and different proportions of particle numbers are used in combination, which can improve the uniformity of the current density distribution in the negative active layer of the secondary battery, and can also improve the diffusion rate of lithium ions in the negative electrode sheet, thereby enhancing the thermal stability of the secondary battery. After cold pressing the negative electrode sheet, the first graphite material and the second graphite material with different hardnesses are used. The deformability of graphite material particles with different hardnesses is different. The second graphite material with relatively lower hardness deforms relatively more, which can fully increase the contact area between particles, reduce the pores of the overall graphite material particles, thereby enhancing the electronic conductivity between particles in the negative active layer, improving the uniformity of the current density in the negative electrode sheet, reducing the formation of lithium dendrites, and enhancing the thermal stability of the secondary battery. At the same time, the first graphite material with relatively higher hardness in the graphite negative electrode material is not easily deformed during the cold pressing process of preparing the negative electrode sheet, which can keep a specific contact area between particles, and there are still certain voids between the particles, so as to improve the diffusivity of lithium ions in the negative electrode sheet and reduce polarization. The combination of the first graphite material and the second graphite material in the present application also facilitates the insertion of lithium ions into the central region inside the particles when the active material particles are lithiated, increasing the degree of lithium insertion inside the particles, reducing the degree of lithium insertion in the outer layer of the particles, slowing down the lithium ion extraction rate and reducing the formation of lithium dendrites. And in combination with the present application, the proportion of the number of particles of the first graphite material and the second graphite material is designed to keep a good contact area between the particles, and there are still certain pores in the negative active layer after cold pressing, so that the negative active layer has good current density uniformity, improves the diffusion rate of lithium ions in the negative electrode sheet, reduces the formation of lithium dendrites, and enhances the thermal stability of the secondary battery.

[0032] If the hardness Cs1 of the first graphite material is small, during the cold pressing process, the first graphite material is relatively easy to deform, the pores in the negative active layer decrease, which will reduce the diffusion rate of lithium ions in the negative electrode sheet, is not conducive to reducing polarization, and will reduce the thermal stability of the secondary battery. If the proportion X of the number of particles of the first graphite material is small, such as less than 25.0%, the overall hardness of the graphite negative electrode material is small. After cold pressing the negative electrode sheet, the particles are in full contact with each other, which will reduce the diffusion rate of lithium ions in the negative electrode sheet, and the effect on improving the thermal stability of the secondary battery is not obvious.

[0033] If the hardness C s2 of the second graphite material is relatively large, such as greater than 20 Mpa, during the cold pressing process, the second graphite material is not easily deformed, and the contact area between particles remains small, which will reduce the electronic conductivity between the particles in the negative electrode active layer, reduce the uniformity of the current density in the negative electrode sheet, and reduce the thermal stability of the secondary battery. If the proportion Y of the number of particles of the second graphite material is small, such as less than 30.0%, the content of the second graphite material in the graphite negative electrode material is small, making the contact area between particles relatively small, which is not conducive to improving the electronic conductivity of the negative electrode active layer and is not conducive to improving the thermal stability of the secondary battery.

[0034] In some embodiments, the hardness C s 1 of the first graphite material can be 35 Mpa, 38 Mpa, 40 Mpa, 45 Mpa, 50 Mpa, 55 Mpa, 60 Mpa, 65 Mpa, 70 Mpa, 75 Mpa, 80 Mpa, 85 Mpa or any value within the range composed of any two of the above values. In some embodiments, the hardness C s2 of the second graphite material can be 4 Mpa, 5 Mpa, 6 Mpa, 7 Mpa, 8 Mpa, 9 Mpa, 10 Mpa, 11 Mpa, 12 Mpa, 15 Mpa, 17 Mpa, 19 Mpa, 20 Mpa or any value within the range composed of any two of the above values.

[0035] In some embodiments, the proportion X of the number of particles of the first graphite material can be 25.0%, 30.0%, 35.0%, 38.0%, 40.0%, 45.0%, 50.0%, 55.0%, 60.0%, 65.0%, 70.0% or any value within the range composed of any two of the above values. The proportion Y of the number of particles of the second graphite material can be 30.0%, 35.0%, 40.0%, 45.0%, 50.0%, 55.0%, 60.0%, 70.0% or any value within the range composed of any two of the above values.

[0036] In some embodiments, 60% ≥ X ≥ 38%. When the proportion of the number of particles of the first graphite material is within the above range, it can further improve the diffusion rate of lithium ions in the negative electrode sheet, reduce the polarization of the secondary battery, and improve the thermal stability of the secondary battery.

[0037] In some embodiments, 60% ≥ Y ≥ 35%. When the proportion of the number of particles of the second graphite material is within the above range, it can make the contact between particles in the graphite negative electrode material sufficient, so as to improve the electronic conductivity between the particles in the negative electrode active layer, improve the uniformity of the current density in the negative electrode sheet, and thus improve the thermal stability of the secondary battery.

[0038] In some embodiments, 80 Mpa ≥ Cs1 ≥ 40 Mpa. The hardness of the first graphite material is within the above range, so that the hardness between the first graphite material and the second graphite material maintains a suitable matching degree, and during the cold pressing process of the negative electrode sheet, the graphite negative electrode material maintains a specific hardness, and there are certain pores between particles, improving the diffusion rate of lithium ions in the negative electrode sheet, reducing the formation of lithium dendrites, and improving the thermal stability of the secondary battery.

[0039] In some embodiments, 15 Mpa ≥ Cs2 ≥ 5 Mpa. The hardness of the second graphite material is within the above range. When cold pressing the negative electrode sheet, the second graphite material is locally deformed, which is beneficial to increasing the contact area between particles, thereby improving the electronic conductivity of the negative electrode active layer and the current density uniformity of the negative electrode sheet, and thus improving the thermal stability of the secondary battery.

[0040] In some embodiments, the tap density P of the graphite negative electrode material satisfies: 1.60 ≤ P ≤ 1.85X + 1.95Y. The number of particles of the first graphite material and the second graphite material affects the tap density of the graphite negative electrode material. When X and Y satisfy the above relationship with the tap density, the inventors found that the energy density and thermal stability of the secondary battery can be further improved.

[0041] In some embodiments, the specific capacity of the graphite negative electrode material is Cap mAh / g, 330 ≤ Cap ≤ 340X + 360Y. The number of particles of the first graphite material and the second graphite material affects the specific capacity of the graphite negative electrode material. When the above relationship is satisfied, under the condition that the secondary battery has good energy density, the thermal stability of the secondary battery is further improved.

[0042] In some embodiments, in the Raman spectrum of the graphite negative electrode material, 0.10 ≤ Id / Ig ≤ 0.35, where Id is the peak intensity of the D peak at 1350 cm -1 ±5 cm -1 in the Raman spectrum of the graphite negative electrode material, and Ig is the peak intensity of the G peak at 1575 cm -1 ±5 cm -1Peak intensity. The degree of defect of the graphite anode material is generally measured by Raman spectroscopy. The surface defect degree of the active material is represented by the intensity ratio Id / Ig of the D peak to the G peak in the Raman spectrum. The larger the Id / Ig, the more surface defects the active material has; the smaller the Id / Ig, the fewer surface defects and the higher the crystallinity of the active material. In the graphite anode material, when Id / Ig is within the above range, it not only endows the graphite anode material with good initial Coulomb efficiency and thermal stability, but also facilitates increasing the diffusion rate of lithium ions on the particle surface, reducing the interfacial diffusion impedance of lithium ions intercalating and deintercalating from the graphite anode material, and enhancing the thermal stability of the secondary battery. In some embodiments, Id / Ig can be 0.10, 0.15, 0.2, 0.25, 0.30, 0.35 or any value within the range composed of any two of the above values.

[0043] In some embodiments, the graphitization degree G of the graphite anode material satisfies: 90.0% ≤ G ≤ 95.0%. The graphitization degree G represents the development degree of graphite crystals. The higher the graphitization degree, the more complete the development of microcrystals and the smaller the layer spacing of the d002 crystal plane; the lower the graphitization degree, the larger the layer spacing of the d002 crystal plane. The d002 crystal plane of graphite is the crystal plane where active lithium ions intercalate and deintercalate, and the size of the d002 crystal plane spacing affects the intercalation and deintercalation rate of lithium ions in the anode active material. In this application, designing the graphitization degree G within the above range is conducive to keeping the d002 crystal plane spacing within a suitable range, increasing the intercalation and deintercalation rate of lithium ions, and reducing the phenomenon of layer peeling caused by a larger crystal plane spacing, thereby improving the cycling performance and thermal stability of the secondary battery. In some embodiments, G can be 90%, 91%, 92%, 93%, 94%, 95% or any value within the range composed of any two of the above values.

[0044] In some embodiments, the specific surface area S of the graphite anode material satisfies: 0.7 m 2 / g ≤ S ≤ 2.5 m 2 / g. When the specific surface area of the graphite anode material is within the above range, it is conducive to increasing the surface active sites of the graphite anode material, enabling the graphite anode material to have an appropriate number of positions for lithium ions to intercalate and deintercalate, and reducing the excessive formation of the SEI film, which is prone to decomposing and generating gas at high temperatures, so as to improve the rate performance and thermal stability of the secondary battery. In some embodiments, the specific surface area S can be 0.7 m 2 / g, 0.9 m 2 / g, 1.0 m 2 / g, 1.2 m 2 / g, 1.5 m 2 / g, 1.7 m 2 / g, 2.0 m 2 / g, 2.3 m 2 / g, 2.5 m 2 / g or any value within the range formed by any two of the above values.

[0045] In some embodiments, the particle size of the graphite negative electrode material satisfies: 1.5 ≤ Dv90 / Dv50 ≤ 2.6. The smaller Dv90 / Dv50 is, the more concentrated the particle size distribution is; the larger Dv90 / Dv50 is, the more dispersed the particle size distribution is. When Dv90 / Dv50 is within the above range, the particle size is appropriately dispersed, the processability of preparing the negative electrode sheet is strong, and some small particles fill the pores, thereby facilitating the improvement of the compaction density of the negative electrode sheet and the storage performance and thermal stability of the secondary battery. In some embodiments, the ratio of Dv90 / Dv50 can be 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.5 or any value within the range formed by any two of the above values.

[0046] Dv50 is also called the "median particle size", which represents the particle size of the graphite negative electrode material that reaches 50% of the cumulative volume from the small particle size side in the particle size distribution based on volume, that is, the volume of the graphite negative electrode material particles smaller than this particle size accounts for 50% of the total volume of the graphite negative electrode material particles. Dv90 represents the particle size of the graphite negative electrode material that reaches 90% of the cumulative volume from the small particle size side in the particle size distribution based on volume, that is, the volume of the graphite negative electrode material particles smaller than this particle size accounts for 90% of the total volume of the graphite negative electrode material particles.

[0047] In some embodiments, the tapped density TD of the graphite negative electrode material satisfies: TD ≥ 0.80 g / cm 3 . When the tapped density of the graphite negative electrode material is within the above range, it is beneficial to improve the processability of preparing the negative electrode slurry and reduce viscosity abnormalities such as slurry stratification and precipitation during the preparation of the slurry, so as to increase the coating weight during the preparation of the negative electrode sheet and improve the electrochemical performance of the secondary battery. In some embodiments, the tapped density TD of the graphite negative electrode material can be 0.80 g / cm 3 , 0.90 g / cm 3 , 1.00 g / cm 3 , 1.10 g / cm 3 , 1.20 g / cm 3 or any value within the range formed by any two of the above values.

[0048] In some embodiments, the OI value of the graphite anode material satisfies: OI value ≤ 10. The crystal orientation degree of the graphite anode material is represented by the OI value, and the crystal orientation degree affects the lithium intercalation direction of lithium ions in the graphite anode material. When the OI value is within the above range, it is beneficial to improve the multi-directionality of the lithium intercalation direction of the graphite anode material, increase the lithium intercalation rate, and thus improve the rate performance and thermal stability of the secondary battery. In some embodiments, the OI value of the graphite anode material can be 5, 6, 7, 8, 9, 10 or any value within the range composed of any two of the above values.

[0049] In some embodiments, the compaction density PD of the negative electrode sheet satisfies: 1.40 g / cm 3 ≤ PD ≤ 1.70 g / cm 3 . When the compaction density PD of the negative electrode sheet is within the above range, good electronic contact is maintained between the particles in the negative electrode active layer, reducing the polarization of the secondary battery, and also facilitating the reduction of side reactions caused by particle breakage during cold pressing, thereby improving the thermal stability of the secondary battery. In some embodiments, the compaction density PD of the negative electrode sheet can be 1.40 g / cm 3 , 1.50 g / cm 3 , 1.60 g / cm 3 , 1.70 g / cm 3 or any value within the range composed of any two of the above values.

[0050] In some embodiments, the porosity K of the negative electrode sheet satisfies: 25% ≤ K ≤ 45%. When the porosity of the negative electrode sheet is within the above range, the negative electrode sheet has appropriate liquid retention ability, which is beneficial to improving the ability of the negative electrode sheet to store the electrolyte, improving the cycle performance of the secondary battery, and also facilitating the reduction of swelling and the reduction of the phenomenon of gas swelling in the secondary battery during high-temperature storage and high-temperature cycling, improving the stability of the negative electrode sheet, and thus improving the rate performance and thermal stability of the secondary battery. In some embodiments, the porosity K of the negative electrode sheet can be 25%, 30%, 35%, 40%, 45% or any value within the range composed of any two of the above values.

[0051] In some embodiments, the negative electrode active layer further includes a binder and a conductive agent. In some embodiments, the binder includes, 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 1,1-difluoroethylene, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin or nylon, etc.

[0052] In some embodiments, the conductive agent includes, but is not limited to: carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. In some embodiments, the carbon-based materials are selected from carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based materials are selected from metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0053] The first graphite material and the second graphite material are respectively selected from pitch coke and needle coke as raw materials.

[0054] The first graphite material is pitch coke, and the precursor of the pitch coke is selected from coal tar pitch. The carbon content in the coal tar pitch reaches more than 88%, so that coke with a high carbon content can be generated during the high-temperature pyrolysis process. The ash content is controlled below 2%, and the proportion of aromatic hydrocarbons is controlled above 35%, so as to facilitate the formation of a highly ordered carbon structure by aromatic hydrocarbons during the high-temperature pyrolysis process, which helps to improve the isotropy of the pitch coke.

[0055] This application also provides a method for preparing a graphite anode material, including:

[0056] Preheat the above-mentioned coal tar pitch and then put it into a high-temperature pyrolysis furnace for coking treatment. Heat it to a temperature T1 of 500°C to 900°C and keep it warm for a time h1 of 2h to 10h. During the high-temperature pyrolysis process, polycyclic aromatic hydrocarbons and other high-molecular compounds in the coal tar pitch will gradually carbonize to form a carbon structure. Continue to heat the coal tar pitch to a temperature T2 of 1000°C to 1400°C and keep it warm for 3h to 8h to further stabilize the coked carbon structure and obtain pitch coke. In some embodiments, the temperature T1 can be 500°C, 600°C, 700°C, 800°C, 900°C, or any value within the range composed of any two of the above values. The holding time h1 can be 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, or any value within the range composed of any two of the above values.

[0057] In some embodiments, the temperature T2 can be 1000°C, 1100°C, 1200°C, 1300°C, 1400°C, or any value within the range composed of any two of the above values.

[0058] The pitch coke is crushed to a Dv50 of 8.0 μm to 15.0 μm to obtain A coke powder, the needle coke is crushed to a Dv50 of 8.0 μm to 12.0 μm to obtain B coke powder, and the A coke powder and the B coke powder are mixed, wherein the proportion of the A coke powder is 10.0% to 90.0%. The mixed material is shaped and classified to further optimize the particle size distribution, and then graphitization treatment is carried out after completion. The graphitization temperature T3 is 2600 °C to 3000 °C, and the heat preservation time h2 is 10 h to 60 h. After graphitization is completed, demagnetization screening is carried out to obtain the graphite negative electrode material. In some embodiments, the temperature T3 can be 2600 °C, 2700 °C, 2800 °C, 2900 °C, 3000 °C or any value within the range formed by any two of the above values. The heat preservation time h2 can be 10 h, 20 h, 30 h, 40 h, 50 h, 60 h or any value within the range formed by any two of the above values.

[0059] The graphite negative electrode material, conductive carbon black, binder styrene-butadiene rubber (abbreviated as SBR), and thickener sodium carboxymethyl cellulose (abbreviated as CMC) are mixed according to a certain ratio to obtain a mixture. Among them, based on the mass of the mixture, the mass proportion of the conductive carbon black is 1.5%, the mass proportion of the graphite negative electrode material is 90% to 97%, the mass proportion of the binder styrene-butadiene rubber is 1.2% to 2.2%, and the mass proportion of the thickener sodium carboxymethyl cellulose is 0.5% to 1.5%. Then, it is fully stirred and mixed in an appropriate amount of deionized water solvent to form a uniform negative electrode slurry. This slurry is coated on the current collector Cu foil, dried, and cold-pressed to obtain the negative electrode sheet.

[0060] Separator

[0061] There are no particular limitations on the material and shape of the separator used in the secondary battery of the present application, and it can be any technology disclosed in the prior art. In some embodiments, the separator includes a polymer or inorganic substance formed of a material stable to the electrolyte of the present application.

[0062] For example, the separator may include a base material layer and a surface treatment layer. The base material layer is a non-woven fabric, film, or composite film having a porous structure, and the material of the base material layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, 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 can be selected.

[0063] A surface treatment layer is provided on at least one surface of the base material layer. The surface treatment layer can be a polymer layer, an inorganic layer, or a layer formed by mixing polymers and inorganic substances. The inorganic layer includes inorganic particles and a binder. The inorganic particles are selected from at least one of alumina, silica, magnesia, titania, hafnium dioxide, tin dioxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconia, yttria, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder is selected from at least one of polyvinylidene fluoride, copolymer of vinylidene fluoride - hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyethylene alkoxide, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer contains a polymer, and the material of the polymer is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyethylene alkoxide, polyvinylidene fluoride, poly(vinylidene fluoride - hexafluoropropylene).

[0064] Electrolyte

[0065] According to some embodiments of the present application, the electrolyte includes an organic solvent, a lithium salt, and an optional additive. The organic solvent in the electrolyte of the present application can be any organic solvent known in the prior art that can be used as a solvent for the electrolyte. There is no limitation on the electrolyte used in the electrolyte according to the present application, and it can be any electrolyte known in the prior art. The additive of the electrolyte according to the present application can be any additive known in the prior art that can be used as an electrolyte additive. In some embodiments, the organic solvent includes, but is not limited to: ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate, or ethyl propionate. In some embodiments, the organic solvent includes an ether solvent, such as including at least one of 1,3 - dioxolane (DOL) and ethylene glycol dimethyl ether (DME). In some embodiments, the lithium salt includes at least one of an organic lithium salt or an inorganic lithium salt. In some embodiments, the lithium salt includes, but is not limited to: lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ), lithium difluorophosphate (LiPO 2 F 2 ), lithium bis(trifluoromethanesulfonyl)imide LiN(CF 3 SO 2 ), 2 (LiTF S I), lithium bis(fluorosulfonyl)imide Li(N(SO 2 F) 2 )(LiF SI), lithium bis(oxalato)borate LiB(C 2 O 4 ), 2(LiB OB) or lithium difluorooxalate borate LiBF 2 (C 2 O 4 )(LiDFOB). In some embodiments, the additive includes at least one of fluoroethylene carbonate and adiponitrile.

[0066] Positive electrode sheet

[0067] The positive electrode sheet includes a positive electrode current collector and a positive electrode active layer provided on the positive electrode current collector. The positive electrode current collector can be made of aluminum foil, nickel foil, etc., or can 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. The positive electrode active layer contains a positive electrode active material, and the positive electrode active material includes a compound that can reversibly intercalate and deintercalate lithium ions (i.e., lithiated intercalation compound). In some embodiments, the positive electrode active material can include lithium transition metal composite oxides. 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 can include but not limited to at least one of lithium cobaltate, lithium nickel manganese cobaltate, lithium nickel manganese aluminate, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel-type lithium manganate, spinel-type lithium nickel manganate, and lithium titanate.

[0068] The positive electrode active layer also contains a binder for bonding the positive electrode active material particles to facilitate the formation of a film layer and at the same time improve the bonding force between the positive electrode active layer and the positive electrode current collector. In some embodiments, the binder can include but not limited to at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoroethylene, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon, etc.

[0069] The positive electrode active layer may also contain a conductive material, and the conductive material includes but not limited to carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, the carbon-based materials can include but 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 can include but not limited to metal powder or metal fiber, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer can be a polyphenylene derivative.

[0070] The above-mentioned secondary battery is applied to an electronic device to power the load in the electronic device. Moreover, the graphite negative electrode material in the above-mentioned secondary battery has excellent thermal stability, which is conducive to improving the service life of the electronic device. Among them, the electronic device may include but is not limited to a laptop computer, a pen-input computer, a mobile computer, an e-book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, an electric tool, a flashlight, a camera, a large household battery and a lithium-ion capacitor, etc.

[0071] The present application is described below by specific examples and comparative examples. Those skilled in the art should understand that the preparation methods described in the present application are only examples, and any other suitable preparation methods are within the scope of the present application.

[0072] Example 1

[0073] Preparation of graphite negative electrode materials:

[0074] Asphalt coke and needle coke are used as raw materials for negative electrode materials, wherein the precursor of asphalt coke is selected from coal tar pitch, the carbon content of coal tar pitch is 90.5%, the ash content is 1.5%, and the proportion of aromatic hydrocarbons is 40.0%. The coal tar pitch is preheated and placed in a high-temperature cracking furnace for coking treatment, heated to T1 of 600°C and kept warm for 5h. After the coking reaction is completed, continue to heat to T2 of 1100°C, keep warm for 4h, and obtain asphalt coke. The asphalt coke is crushed to Dv50 of 9.5μm to obtain A coke powder, and then the needle coke is crushed to Dv50 of 10.0μm to obtain B coke powder, and the A coke powder and the B coke powder are mixed to obtain a mixed material, wherein the proportion of A coke powder in the mixed material is 30.0%. The mixed mixed material is shaped and graded, and the particle size distribution of the particles is further optimized, and then graphitized, the graphitization temperature T3 is 2900°C, and the holding time is 25h. After graphitization is completed, demagnetization and screening are performed to obtain graphite negative electrode materials.

[0075] Preparation of negative electrode sheet: Mix the above-mentioned graphite negative electrode material, conductive carbon black, binder styrene butadiene rubber (abbreviated as SBR), and thickener sodium carboxymethyl cellulose (abbreviated as CMC) in a mass ratio of 96.0%:1.5%:1.5%:1.0%, and then add an appropriate amount of deionized water solvent, stir and mix thoroughly to form a uniform negative electrode slurry, apply the slurry on the current collector Cu foil, dry and cold press to obtain the negative electrode sheet.

[0076] Preparation of the positive electrode plate: Lithium iron phosphate (chemical formula: LiFePO 4 ) is selected as the positive electrode active material, and it is fully stirred and mixed with conductive agent acetylene black and binder polyvinylidene fluoride (abbreviated as PVDF) in a weight ratio of 96.3:2.2:1.5 in an appropriate amount of N-methylpyrrolidone (abbreviated as NMP) solvent to form a uniform positive electrode paste; this paste is coated on the current collector Al foil, dried and cold-pressed to obtain the positive electrode plate.

[0077] Preparation of the electrolyte: In a dry argon atmosphere glove box, ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed according to a mass ratio of EC:EMC:DEC = 1:3:3:3. Then, 1,3-propane sultone is added, dissolved and fully stirred, and then lithium salt LiPF 6 is added. After mixing evenly, the electrolyte is obtained. Among them, the mass percentage content of LiPF 6 is 11.0%, and the mass percentage content of 1,3-propane sultone is 2.9%. The mass percentage content of each substance is calculated based on the mass of the electrolyte, and the balance is non-aqueous organic solvent.

[0078] Separator: An 8-μm PE porous polymer film is selected as the separator.

[0079] Assembly of the secondary battery: Stack the above-mentioned positive electrode plate, separator, and negative electrode plate in sequence, so that the separator is in the middle of the positive electrode plate and the negative electrode plate to play a role of isolation, and wind it to obtain an electrode assembly. After welding the electrode tabs, place the electrode assembly in an aluminum-plastic film packaging bag, dry it, inject the electrolyte, and obtain a lithium-ion secondary battery through processes such as vacuum packaging, standing, formation (formation upper limit voltage is 4.5 V, formation temperature is 70 °C, formation standing time is 2 hours), degassing, and trimming.

[0080] Examples 2 to 10

[0081] The differences between Examples 2 to 10 and Example 1 are that the composition of pitch coke, the proportion of different coke powders A, and the treatment process are adjusted, and the remaining steps are the same as those in Example 1. The specific preparation parameters can be referred to in Tables 1 and 2.

[0082] Examples 11 to 16

[0083] The differences between Examples 11 to 16 and Example 6 are that the proportion of particles with different hardness in the graphite negative electrode material is adjusted, and the remaining steps are the same as those in Example 6. The specific preparation parameters can be referred to in Table 3.

[0084] Examples 17 to 22

[0085] Examples 17 to 22 are different from Example 15 in that the proportion of particles with different hardnesses in the graphite anode material is adjusted, and the remaining steps are the same as those in Example 15. The specific preparation parameters can be referred to in Table 4.

[0086] Examples 23 to 36

[0087] Examples 23 to 36 are different from Example 21 in that the crushing size, fine powder control content, graphitization temperature and time in the preparation of the graphite anode material are changed, and the specific surface area BET, particle size distribution, and graphitization degree G of the graphite anode material are adjusted. The remaining steps are the same as those in Example 21. The specific preparation parameters can be referred to in Table 5.

[0088] Examples 37 to 49

[0089] Examples 37 to 49 are different from Example 35 in that the heat treatment temperature or shaping degree in the preparation of the graphite anode material is changed, and the Id / Ig, OI value, and tap density TD of the graphite anode material are adjusted. The remaining steps are the same as those in Example 35. The specific preparation parameters can be referred to in Table 6.

[0090] Examples 50 to 55

[0091] Examples 50 to 55 are different from Example 48 in that the coating conditions in the preparation of the negative electrode sheet are changed, and the compaction density PD and porosity K of the negative electrode sheet are adjusted. The remaining steps are the same as those in Example 48. The specific preparation parameters can be referred to in Table 7.

[0092] Comparative Examples 1 to 8

[0093] Comparative Examples 1 to 8 are different from Example 1 in that the composition of the pitch coke, the proportion of different coke powders A, and the treatment process are adjusted. The remaining steps are the same as those in Example 1. The specific preparation parameters can be referred to in Tables 1 and 2.

[0094] The prepared graphite anode materials and assembled lithium-ion batteries in each example and comparative example are tested.

[0095] Testing method:

[0096] (1) Testing of the particle hardness and particle number ratio of the graphite anode material

[0097] Tested by a single-particle mechanical property testing system, which generally consists of an optical microscope, a pressure measurement system, a displacement measurement system, and a control system. Through high-precision displacement and pressure control, the pressure F and displacement S curves after the indenter is loaded onto a single particle can be collected. The sample base and the indenter are both made of high-hardness alloy steel to ensure that the base and the indenter do not deform, and both the base and the indenter are flat. During the test, the active material particles of the graphite anode material are placed between the two planes, and then the indenter starts to move downward slowly at a set displacement speed. After just contacting the active particles, the force sensor starts to display the pressure F, and at the same time, the displacement starts to be calculated from 0 μm. The indenter continues to press the active material particles downward, and the change curve of the pressure F and the displacement S can be collected. At first, the curve is an oblique line. When a plateau region appears in the curve, it indicates that the particles have undergone the first collapse and rupture. The inflection point is the point where the collapse occurs. The abscissa corresponding to this point is the first collapse pressure F, and the ordinate corresponding to this point is the displacement S at the first collapse. The test of the particle hardness is calculated by dividing the pressure F by the equivalent area of the particle, where the equivalent area is Π×Dv50 2 , so the hardness Cs = F / (Π×Dv50 2 ), and the unit of hardness is Mpa. F and Dv50 are converted according to the actual situation. Requirements for selecting test particles: The size D of the selected active material particles for testing should satisfy Dv50 - 1.0 μm ≤ D ≤ Dv50 + 1.0 μm, as close as possible to the Dv50 size. The number of active material particles for testing is 100. Record the measured hardness values, and the proportion of the number of particles with hardness values in a certain range can be calculated.

[0098] (2) Test of the OI value of the graphite anode material

[0099] Test the (004) plane diffraction line pattern and the (110) plane diffraction line pattern in the X-ray diffraction pattern of the graphite anode material according to the national mechanical industry standard JB / T 4220-2011 "Method for Determining the Lattice Parameters of Artificial Graphite" of the People's Republic of China. The test conditions are as follows: The X-ray uses CuKα radiation, and the CuKα radiation is removed by a filter or a monochromator. The working voltage of the X-ray tube is (30 - 35) kV, and the working current is (15 - 20) mA. The scanning speed of the counter is 1 / 4 (°) / min. When recording the 004 diffraction line pattern, the scanning range of the diffraction angle 2θ is 53° - 57°. When recording the 110 diffraction line pattern, the scanning range of the diffraction angle 2θ is 75° - 79°. The peak area obtained from the (004) plane diffraction line pattern is denoted as C004. The peak area obtained from the (110) plane diffraction line pattern is denoted as C110. Calculate the ratio of C004 / C110 of the graphite anode material, which is the OI value of the graphite anode material.

[0100] (3) Specific surface area

[0101] The test method for specific surface area refers to GB / T 19587-2017. The specific process is to weigh 1-8 g of the graphite anode material sample (the minimum sample weight should cover at least 1 / 3 of the volume of the sphere) and place it in a 1 / 2-inch long tube with a spherical bubble (the diameter of the spherical part is 12 mm). After pretreatment at 200 °C for 2 h, it is placed in the test equipment Tri Star 3030 (manufactured by Micromeritics, USA) for testing. The adsorption gas used is N2 (purity: 99.999%), the test is carried out under the condition of 77 K, and the specific surface area is tested by the BET calculation method.

[0102] (4) Test method for the tapped density of the graphite anode material

[0103] Weigh 5 g of the graphite anode material powder, put it all into a graduated cylinder, and then fix the graduated cylinder containing the powder on the instrument and vibrate it. After vibration, visually estimate the volume according to the height of the powder surface, and then calculate the tapped density. The test equipment used is BT-301 from Dandong BETOP.

[0104] (5) Raman spectrum test of the graphite anode material

[0105] Select an area of 100 μm × 100 μm on the negative electrode active layer, and use a laser confocal Raman spectrometer (Raman, HR Evolution, HORIBA Scientific Instruments Division) to scan the particles within this area to obtain the D peak and G peak of all particles within this area. Use Lab Spec software to process the data to obtain the peak intensities of the D peak and G peak of each particle, which are Id and Ig respectively. Take 0.02 as the step size to count the frequency of Id / Ig to obtain a normal distribution graph, and calculate the average value of Id / Ig, which is the intensity ratio Id / Ig of the D peak to the G peak of the graphite anode material. The laser wavelength of the Raman spectrometer can be in the range of 532 nm to 785 nm.

[0106] D peak: Generally around 1350 cm -1 nearby, caused by the symmetric stretching vibration radial breathing mode of sp 2 carbon atoms in the aromatic ring (structural defects);

[0107] G peak: Appears around 1575 cm -1 nearby, caused by the stretching vibration between sp 2 carbon atoms, which corresponds to the vibration of the E2g optical phonon at the center of the Brillouin zone (in-plane vibration of carbon atoms).

[0108] (6) Test for the compression density of the graphite anode material

[0109] The test standard for powder compaction refers to GB / T 24533-2009 "Graphite Anode Materials for Lithium-Ion Batteries". The specific test method is to weigh 1.0000±0.0500 g of the graphite anode material sample and place it in the test mold (CARVER #3619 (13 mm)), then place the sample in the test equipment. The test equipment is the Sansi Zongheng UTM7305 with test tonnages of 0.3 t, 0.5 t, 0.75 t, 1.0 t, 1.5 t, 2.0 t, 2.5 t, 3.0 t, 4.0 t, 5.0 t, the pressure increase rate is 10 mm / min, the pressure holding time is 30 s, the pressure relief rate is 30 mm / min, and the pressure relief holding time is 10 s. The powder compaction density described in this article is the compaction density measured when the pressure is relieved at 5 t. The calculation formula for the compaction density is: Compaction density = mass of graphite anode material / force-bearing area of graphite anode material / thickness of the sample.

[0110] (7) Particle size test

[0111] The particle size test method of the particles refers to GB / T 19077-2016. The specific process is to weigh 1 g of the graphite anode material sample, mix it evenly with 20 mL of deionized water and a small amount of dispersant, place it in an ultrasonic device and ultrasonic for 5 min, then pour the solution into the injection system Hydro 2000SM for testing. The test equipment used is the Mastersizer3000 produced by Malvern Company. During the test, when the laser beam passes through the dispersed particle sample, the particle size measurement is completed by measuring the intensity of the scattered light. Then the data is used for analysis and calculation to form the particle size distribution of the scattered spectrum diagram. The refractive index of the particles used in the test is 1.8, and one sample is tested three times. The final particle size is the average value of the three tests.

[0112] (8) Porosity test of the negative electrode plate

[0113] Analysis principle: Gas replacement method, applying the Archimedes principle of gas displacement (density = mass / volume), using the Boyle's law (PV = nRT) of inert gas with a small molecular diameter under certain conditions to accurately measure the true volume of the material to be tested, so as to obtain its true density and porosity.

[0114] Test: Cut the negative electrode plate into small squares of 10 mm×10 mm, weigh the sample mass, then place it in a true density tester (model AccuPycⅡ1340), seal the test system, introduce nitrogen according to the procedure, detect the gas pressure in the sample chamber and the expansion chamber, and then calculate the true volume V2 according to Boyle's law (PV = nRT). The apparent volume V1 = S×H (S is the surface area of the sample, H is the thickness of the sample), and then calculate the porosity K, K = (V1 - V2) / V1×100%.

[0115] (9) Compaction Density Test of Negative Electrode Plate

[0116] Compaction density = mass / volume × 100%. Take the negative electrode plate, then use a punching machine to punch small round pieces with a radius of 1 cm. The punched small round pieces are double-sided. Measure the double-sided thickness L cm of them, weigh the mass m g of the small round pieces. Then soak and clean the small round pieces with soapy water to remove the active material layer, leaving only the clean copper foil of the small round pieces. Measure the copper foil thickness L1 and weigh the copper foil weight m1 g. Then according to the formula PD = (m - m1) / ((3.14×1²)×(L - L1)), repeat the operation 5 times, and take the average value to obtain the compaction density PD of the negative electrode plate.

[0117] (10) Specific Capacity Test

[0118] The specific capacity test can refer to the standard: GB / T 24533-2019. The graphite negative electrode active material is made into a negative electrode plate through mixing, coating, rolling, slitting, and drying. Use a lithium sheet as the positive electrode and assemble it into a button cell for testing. Calculate the specific capacity of the material by testing the capacity of the button cell. Among them, the active negative electrode material can also be extracted from the battery cell (the battery cell is allowed to have overcharge and overdischarge). After the battery cell is slowly discharged, extract the negative electrode plate, and then process it at 300 °C to extract the active material. Then assemble a button cell to test its specific capacity.

[0119] (11) Graphitization Degree Test

[0120] Use high-purity silicon powder as the standard sample. Mix the graphite negative electrode material sample and the silicon standard sample according to a weight ratio of 5:1. Measure the 002 peak of the graphite negative electrode material and the 111 peak of silicon. Calibrate the obtained 002 peak. Indirectly calculate the graphitization degree through the calibrated 002 crystal plane spacing d002. The calculation formula is g = (0.344 - d002) / ((0.344 - 0.3354)). 0.3440 represents the layer spacing of completely ungraphitized carbon, and 0.3354 represents the layer spacing of ideal graphite, with the unit of nm for both.

[0121] (12) Thermal Box Passing Temperature Test

[0122] First, fully charge the secondary battery to 100% SOC (3.60 V), then place it in the thermal box and heat it at a heating rate of 5 °C / min. After heating to T °C, keep it warm for 2 h. During the heating and warming process, the battery does not smoke or catch fire. However, at T + 1 °C, the battery smokes or catches fire during the heating and warming process. Then it can be considered that the thermal box temperature of the battery is T °C.

[0123] Table 1

[0124]

[0125]

[0126] Table 2

[0127]

[0128]

[0129] Combining the above Table 1 and Table 2, compared with Comparative Examples 1 to 8, in Examples 1 to 10, when the first graphite material and the second graphite material in the graphite negative electrode material respectively meet the designed hardness range and particle content, the thermal stability of the secondary battery is improved. In the above examples, the combination of the first graphite material and the second graphite material, on the one hand, improves the uniformity of the current density distribution in the negative electrode active layer of the negative electrode sheet, thereby reducing or even avoiding the formation of lithium dendrites and reducing the lithium dendrites formed at the current concentration points. On the other hand, it improves the lithium ion diffusivity in the negative electrode active layer of the negative electrode sheet. Good lithium ion diffusivity will reduce the polarization of the secondary battery, so that when lithium ions are intercalated into the graphite negative electrode material particles, the lithium ions can be intercalated into the deeper inner central region of the particles, reducing the degree of lithium ion intercalation in the outer layer of the particles. In this way, the lithium ion deintercalation speed will be slowed down and the formed lithium dendrites will be reduced.

[0130] Table 3

[0131]

[0132] In Table 3, compared with Example 6, in Examples 11 to 16, when the tap density P of the graphite negative electrode material and the particle numbers X and Y satisfy 1.60 ≤ P ≤ 1.85X + 1.95Y, the thermal box passing temperature of the secondary battery is significantly increased, and the thermal stability of the secondary battery is improved.

[0133] Table 4

[0134]

[0135] In Table 4, compared with Example 15, in Examples 17 to 22, when the specific capacity of the graphite negative electrode material and the particle numbers X and Y satisfy 330 ≤ Cap ≤ 340X + 360Y, the thermal box passing temperature of the secondary battery is significantly increased.

[0136] Table 5

[0137]

[0138]

[0139] In Table 5, in Examples 23 to 36, when the graphitization degree G, specific surface area S, and Dv90 / Dv50 of the graphite negative electrode material all meet their respective suitable ranges, they all further improve the thermal stability of the secondary battery.

[0140] Table 6

[0141] Number Id / Ig <![CDATA[TD (g / cm 3 )]]> OI value Hot box passing temperature (°C) Example 35 0.40 0.72 13 144.5 Example 37 0.41 0.85 8 145.8 Example 38 0.08 0.90 10 146.0 Example 39 0.21 0.75 9 145.8 Example 40 0.25 0.70 9 145.8 Example 41 0.22 0.95 14 145.9 Example 42 0.45 0.70 8 145.3 Example 43 0.50 0.77 10 145.4 Example 44 0.30 0.65 12 145.4 Example 45 0.35 0.78 11 145.3 Example 46 0.07 1.10 11 145.2 Example 47 0.45 1.12 13 145.2 Example 48 0.25 1.15 6 146.9 Example 49 0.29 1.13 7 146.7

[0142] In Table 6, in Examples 37 to 49, when the Id / Ig, tap density TD, and OI value of the graphite negative electrode material all meet their respective suitable ranges, they all further improve the thermal stability of the secondary battery.

[0143] Table 7

[0144]

[0145]

[0146] In Table 7, in Examples 50 to 55, the compaction density PD and porosity K of the negative electrode sheet are adjusted. When the compaction density PD and porosity K both meet their respective suitable ranges, they both further improve the thermal stability of the secondary battery.

[0147] The above-disclosed is only the preferred embodiment of the present application. Of course, the present application cannot be limited by this. Therefore, the equivalent changes made according to the present application still fall within the scope covered by the present application.

Claims

1. A graphite negative electrode material, characterized in that: Comprising a first graphite material and a second graphite material, the hardness Cs1 of the first graphite material satisfies: Cs1≥35Mpa, the hardness Cs2 of the second graphite material satisfies: Cs2≤20Mpa, based on the number of particles of the graphite negative electrode material, the number of particles of the first graphite material accounts for X and satisfies: X≥25.0%, and the number of particles of the second graphite material accounts for Y and satisfies: Y≥30.0%.

2. The graphite negative electrode material according to claim 1, characterized in that The graphite negative electrode material satisfies at least one of the following conditions: (1)60％≥X≥38％； (2)60％≥Y≥35％。 3. The graphite negative electrode material according to claim 1, characterized in that The graphite negative electrode material satisfies at least one of the following conditions: (1) The hardness of the first graphite material satisfies: 80Mpa ≥ Cs 1 ≥ 40Mpa; (2) The hardness of the second graphite material satisfies: 15Mpa≥Cs2≥5Mpa.

4. The graphite negative electrode material according to any one of claims 1 to 3, characterized in that The graphite negative electrode material satisfies at least one of the following conditions: (1) The compaction density P of the graphite negative electrode material satisfies: 1.60≤P≤1.85X+1.95Y; (2) The gram capacity of the graphite negative electrode material is Cap mAh / g, 330≤Cap≤340X+360Y.

5. The graphite negative electrode material according to any one of claims 1 to 3, characterized in that In the Raman spectrum of the graphite negative electrode material, 0.10≤Id / Ig≤0.35, Id is the D peak 1350cm in the Raman spectrum of the graphite negative electrode material -1 ±5cm -1 The peak intensity of Ig is the peak of G at 1575 cm in the Raman spectrum of the negative electrode material of graphite. -1 ±5cm -1 The peak strength.

6. The graphite negative electrode material according to any one of claims 1 to 3, characterized in that The graphite negative electrode material satisfies at least one of the following conditions: (1) The graphitization degree G of the graphite negative electrode material satisfies: 90.0%≤G≤95.0%; (2) The specific surface area S of the graphite negative electrode material satisfies: 0.7 m 2 / g≤S≤2.5m 2 / g; (3) The particle size of the graphite negative electrode material satisfies: 1.5≤Dv90 / Dv50≤2.6; (4) The tap density TD of the graphite negative electrode material satisfies: TD ≥ 0.80 g / cm 3 .

7. The graphite negative electrode material according to any one of claims 1 to 3, characterized in that The OI value of the graphite negative electrode material satisfies: OI value ≤10.

8. A secondary battery, characterized in that: It comprises a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the negative electrode sheet comprises the graphite negative electrode material according to any one of claims 1 to 7.

9. The secondary battery according to claim 8, characterized in that: The negative electrode sheet satisfies at least one of the following conditions: (1) The compaction density PD of the negative electrode sheet satisfies: 1.40 g / cm 3 ≤PD≤1.70g / cm 3 ; (2) The porosity K of the negative electrode sheet satisfies: 25%≤K≤45%.

10. An electronic device, characterized in that: The electronic device includes the secondary battery according to claim 8 or 9.

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