Composite graphite negative electrode material, preparation method thereof and secondary battery

Abstract

The invention provides a composite graphite negative electrode material, a preparation method thereof and a secondary battery. The composite graphite negative electrode material comprises modified porous graphite and a conductive filler filled in pores of the modified porous graphite. The structure of the modified porous graphite meets the condition that P * B * S / Dv50 is larger than or equal to 0.01 and smaller than or equal to 0.1, and the pore filling rate of the modified porous graphite is larger than or equal to 5%. Wherein P is the porosity of the modified porous graphite. B is the specific surface area of the modified porous graphite. S is the average pore size of the modified porous graphite. Dv50 is the volume median particle size of the modified porous graphite. The composite graphite negative electrode material provided by the invention can have relatively high fast charging performance and good high-temperature storage performance.

Description

Technical Field

[0001] The present application relates to the technical field of battery materials, and in particular to a composite graphite negative electrode material and a preparation method thereof, and a secondary battery. Background Art

[0002] Secondary batteries such as lithium-ion batteries have the advantages of high energy density, long cycle life, stable voltage platform, economy and environmental protection, and are widely used in consumer electronics, power tools and new energy vehicles. With the improvement of people's quality of life, the fast charging ability of lithium-ion batteries has gradually become the focus of consumers' attention. Among the main factors affecting the fast charging ability of lithium-ion batteries, the fast charging limit of the negative electrode material determines the upper limit of the fast charging ability of lithium-ion batteries. In order to improve the fast charging performance of lithium-ion battery negative electrode materials, people usually perform surface coating treatment on traditional graphite negative electrode materials to prepare fast-charging graphite negative electrode materials.

[0003] The Chinese patent (CN115566153A) provides a fast-charging graphite negative electrode material, in which the base material is modified and used as a coating agent to coat the graphite. After heat treatment, the modified base material forms an isotropic carbon coating layer on the graphite surface, which increases the diffusion rate of lithium ions on the graphite surface and reduces the accumulation effect of lithium ions on the graphite surface. Although this patent prepares a fast-charging graphite negative electrode material with high rate performance and high first coulomb efficiency, the capacity retention rate at 2C rate is only 38% of that at 0.2C, and the improvement effect is limited, which is difficult to meet the higher demands of consumers. Summary of the invention

[0004] The present application provides a composite graphite negative electrode material and a preparation method thereof, and a secondary battery, which can have both high fast charging performance and good high-temperature storage performance.

[0005] Specifically, the present application is implemented through the following technical solutions:

[0006] On the one hand, the present application provides a composite graphite negative electrode material, comprising:

[0007] Modified porous graphite and a conductive filler filled in the pores of the modified porous graphite;

[0008] The structure of the modified porous graphite satisfies the following relational formula (I), and the pore filling rate of the modified porous graphite is greater than or equal to 5%;

[0009] 0.01≤P×B×S / D v50 ≤0.1(I)

[0010] Wherein, P is the porosity of the modified porous graphite; B is the specific surface area of ​​the modified porous graphite; S is the average pore size of the modified porous graphite; D v50is the volume median particle size of the modified porous graphite.

[0011] Optionally, the structure of the modified porous graphite satisfies: 0.03≤P×B×S / D v50 ≤0.08, and the pore filling rate of the modified porous graphite is greater than or equal to 7%.

[0012] Optionally, the structure of the modified porous graphite satisfies: 0.04≤P×B×S / D v50 ≤0.06, and the pore filling rate of the modified porous graphite is greater than or equal to 9%.

[0013] Optionally, the porosity of the modified porous graphite is 5% to 20%;

[0014] And / or, the specific surface area of ​​the modified porous graphite is 3 to 7 m 2 / g;

[0015] And / or, the average pore size of the modified porous graphite is 0.1 to 1.2 μm;

[0016] and / or, the volume median particle size of the modified porous graphite is 9 to 16 μm;

[0017] And / or, the conductive filler includes at least one of graphene, carbon nanotubes, conductive carbon black, lithium ethylene monocarbonate, LiF, Li3P, Li2O, Li2CO3 and Al2O3.

[0018] On the other hand, the present application provides a method for preparing the composite graphite negative electrode material as described above, comprising:

[0019] After the graphite is broken up, it is placed in an oxidizing atmosphere for pore formation to obtain modified porous graphite that meets the preset structure;

[0020] The modified porous graphite and the conductive filler are added into a dispersant and stirred to obtain a mixed solution; the mixed solution is dried to obtain a composite graphite negative electrode material.

[0021] Optionally, the weight ratio of the modified porous graphite to the conductive filler is (20-50): (1-2);

[0022] And / or, the temperature of the pore-forming treatment is 500-700° C., and the time of the pore-forming treatment is 2-5 hours;

[0023] And / or, the stirring temperature is 24-26° C., and the stirring time is 1-2 h;

[0024] and / or, the oxidizing atmosphere comprises at least one of air, oxygen and carbon dioxide;

[0025] And / or, the dispersant includes acetone; and / or, the graphite includes artificial graphite and / or natural graphite.

[0026] Optionally, drying the mixed solution includes:

[0027] The mixed solution is first pre-dried to a viscous state, and then dried under a vacuum state.

[0028] Optionally, performing the drying process under the vacuum state includes:

[0029] A pretreatment is first performed at a first temperature, and then a drying treatment is performed at a second temperature greater than the first temperature.

[0030] Optionally, the temperature of the pre-drying treatment is 50-70°C;

[0031] And / or, the first temperature is 24-26° C., and the pretreatment time is 1-3 hours;

[0032] And / or, the second temperature is 110-130° C., and the drying treatment time is 1-3 hours.

[0033] On the other hand, the present application also provides a secondary battery, including a negative electrode plate, wherein the negative electrode plate includes a negative electrode collector and a negative electrode active material arranged on the negative electrode collector, and the negative electrode active material includes any of the composite graphite negative electrode materials described above.

[0034] The technical solution provided by this application can achieve the following beneficial effects:

[0035] The present application provides a composite graphite negative electrode material and a preparation method thereof, and a secondary battery. The porosity, average pore size, specific surface area and volume median particle size are jointly controlled to be within an appropriate range, and a certain conductive filler is filled in the pores on the surface of the modified porous graphite. The conductive material filling can cover part of the inner surface area of ​​the pores, reduce the number of surface defects of the graphite particles, reduce the side reactions between the graphite and the electrolyte, and play a certain compensatory role. The combination of the two can make the modified porous graphite have both high fast charging performance and good high-temperature storage performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a schematic diagram of the structure of a composite graphite negative electrode material shown in an exemplary embodiment of the present application.

[0037] Figure numerals: 1. modified porous graphite; 2. pores; 3. conductive filler. DETAILED DESCRIPTION

[0038] In order to further understand the present application, the battery positive electrode composite material, preparation method and solid-state lithium-ion battery provided by the present application are described in detail below in combination with the embodiments and drawings. The protection scope of the present application is not limited to the following embodiments.

[0039] See also Figure 1 The present application provides a composite graphite negative electrode material, comprising: a modified porous graphite 1 and a conductive filler 3 filled in the pores 2 of the modified porous graphite 1. The structure of the modified porous graphite 1 satisfies the following relationship (I), and the pore filling rate of the modified porous graphite 1 is greater than or equal to 5%.

[0040] 0.01≤P×B×S / D v50 ≤0.1(I)

[0041] Wherein, P is the porosity of the modified porous graphite; B is the specific surface area of ​​the modified porous graphite; S is the average pore size of the modified porous graphite; D v50 is the volume median particle size of the modified porous graphite.

[0042] The lithium insertion behavior of graphite negative electrode can be roughly divided into the following four stages: (1) lithium ions pass through the solid electrolyte interface (SEI) membrane formed on the surface of graphite negative electrode to reach the surface of graphite particles; (2) lithium ions undergo charge transfer on the surface of graphite particles and embed into the pores of graphite particles; (3) lithium ions diffuse in the pores of graphite particles; (4) lithium ions diffuse between different graphite particles.

[0043] In order to reduce the charge transfer impedance of lithium ions on the surface of graphite particles and shorten the diffusion path of lithium ions in the pores of graphite particles, the purpose can be achieved by preparing modified porous graphite. The surface of modified porous graphite particles has a large number of pores, which can provide a larger internal surface area, increase the number of lithium-embedded active sites on the surface of graphite particles, and reduce the charge transfer impedance of graphite. In addition, the presence of a large number of pores can provide channels for the diffusion of lithium ions in graphite particles, shorten the diffusion path of lithium ions in graphite particles, and improve the fast charging ability of graphite.

[0044] However, after in-depth research during the preparation of composite graphite negative electrode materials, the inventors found that the porosity P, average pore size S, specific surface area B and volume median particle size Dv50 of the modified porous graphite are the main influencing factors of fast charging performance and high temperature storage performance. The greater the porosity of the modified porous graphite particles, the larger the specific surface area, the more active lithium insertion sites on the particle surface, and the lower the charge transfer impedance. The pores of the modified porous graphite particles can also provide channels for lithium ions to quickly embed into the particles, improving the diffusion performance of lithium ions. In addition to porosity, the average pore size on the surface of the modified porous graphite particles also affects the electrochemical properties of the graphite negative electrode. Due to the surface tension of the electrolyte, when the average pore size on the surface of the modified porous graphite particles is small, the electrolyte wettability of the surface pores is poor, and the inner surface of the pores cannot provide more active lithium insertion sites because they cannot contact too much electrolyte, and lithium ions cannot pass through the pores to shorten the diffusion path, resulting in poor fast charging performance. When the average pore size on the surface of the modified porous graphite particles is large, the electrolyte can penetrate into the pores on the graphite surface and undergo electrochemical reactions with the inner surface of the pores, realizing charge transfer on the inner surface of the pores. At the same time, it can also shorten the diffusion path of lithium ions in the graphite, effectively improving the fast charging ability of the graphite. The larger the specific surface area of ​​the modified porous graphite, the more lithium-intercalation active sites on its surface, and the lower the charge transfer impedance of the modified porous graphite. The volume median particle size affects the diffusion path of lithium ions in the modified porous graphite particles. If the volume median particle size is too large, the path for lithium ions to be embedded in the graphite will be too long, which is not conducive to the rapid lithium deintercalation process of graphite. The lithium ion diffusion path in the modified porous graphite particles with a smaller volume median particle size is shorter, and the fast charging performance is better.

[0045] The inventors determined that there is a mutual restriction relationship between the porosity P, average pore size S, specific surface area B and volume median particle size Dv50, which cannot be too large or too small at the same time. When the porosity P of the modified porous graphite is too large, the average pore size S is too large, the specific surface area B is too large, or the volume median particle size Dv50 is too small, it will result in P×B×S / D v50 The upper limit exceeds 0.1, and the high-temperature storage performance of the modified porous graphite is poor. When the porosity P or the average pore size S or the specific surface area B of the graphite is too large, the exposed internal surface area of ​​the graphite is too large, that is, the contact area between the graphite and the electrolyte is large, resulting in a large number of defects on the graphite surface. During high-temperature storage, the side reactions between the graphite and the electrolyte are more intense, and the high-temperature storage performance deteriorates. When the volume median particle size Dv50 of the graphite is too small, it will also lead to an increase in the number of defects on the graphite surface and deteriorate the high-temperature storage performance. When the porosity P of the graphite is too small, the average pore size S is too small, the specific surface area B is too small, or the volume median particle size Dv50 is too large, it will result in P×B×S / D v50When the lower limit is lower than 0.01, the fast charging performance of the modified porous graphite is poor. When the porosity P of the modified porous graphite particles is too small, the number of lithium ion diffusion channels on the particle surface is small, the lithium ion diffusion impedance is high, and the fast charging performance is poor. When the specific surface area B of the graphite is too small, the number of lithium insertion active sites on the surface of the graphite particles is small, the charge transfer impedance of the graphite is high, and the fast charging ability is poor. When the volume median particle size Dv50 of the graphite is too large, the diffusion path of lithium ions in the graphite particles is too long, the lithium ion diffusion impedance is high, and the fast charging ability is poor.

[0046] Therefore, the present application controls the porosity, average pore size, specific surface area and volume median particle size in a suitable range by jointly controlling the porosity, average pore size, specific surface area and volume median particle size, and fills the pores on the surface of the modified porous graphite with a certain conductive filler. The conductive material filling can cover the inner surface area of ​​part of the pores, reduce the number of surface defects of the graphite particles, reduce the side reactions between graphite and the electrolyte, and play a certain compensatory role. The combination of the two can make the modified porous graphite have both high fast charging performance and good high temperature storage performance, so that the overall performance is better. In addition, during the process of graphite intercalation and delithiation, the conductive filler can also participate in the formation of the SEI film, effectively reducing the SEI film impedance of graphite and improving the fast charging performance of graphite. Finally, as an excellent electronic conductor, the conductive material can reduce the ohmic impedance of graphite, increase the electronic conduction rate of graphite, and further improve the fast charging performance of graphite.

[0047] In one embodiment, the structure of the modified porous graphite satisfies: 0.03≤P×B×S / Dv50≤0.08, and the pore filling rate of the modified porous graphite is greater than or equal to 7%. Thus, the relationship between the porosity P, average pore size S, specific surface area B and volume median particle size Dv50 of the modified porous graphite can be better balanced, so that the lithium-ion battery has both high fast charging performance and excellent storage performance.

[0048] In one embodiment, the structure of the modified porous graphite satisfies: 0.04≤P×B×S / D v50 ≤0.06, and the pore filling rate of the modified porous graphite is greater than or equal to 9%.

[0049] In one embodiment, the porosity of the modified porous graphite is 5% to 20%, for example, 5%, 8%, 12%, 16% or 20%, but not limited thereto. In one embodiment, the specific surface area of ​​the modified porous graphite is 3 to 7 m 2 / g, for example, 3m 2 / g, 4m 2 / g, 5m 2 / g, 6m 2 / g or 7m 2 / g, but not limited to this. In one embodiment, the average pore size of the modified porous graphite is 0.1-1.2μm, for example, it can be 0.1μm, 0.3μm, 0.6μm, 0.9μm or 1.2μm, but not limited to this. In one embodiment, the volume median particle size of the modified porous graphite is 9-16μm, for example, it can be 9μm, 11μm, 13μm, 15μm or 16μm, but not limited to this. In one embodiment, the conductive material includes at least one of graphene, carbon nanotubes, conductive carbon black, lithium ethylene carbonate (LEMC), LiF, Li3P, Li2O, Li2CO3 and Al2O3.

[0050] On the other hand, the present application provides a method for preparing the composite graphite negative electrode material as described above, comprising:

[0051] After the graphite is broken up, it is placed in an oxidizing atmosphere for pore formation to obtain modified porous graphite that meets the preset structure;

[0052] The modified porous graphite and the conductive filler are added into a dispersant and stirred to obtain a mixed solution; the mixed solution is dried to obtain a composite graphite negative electrode material.

[0053] In one embodiment, the weight ratio of the modified porous graphite to the conductive filler is (20-50):(1-2).

[0054] In one embodiment, the temperature of the pore-forming treatment is 500-700° C., for example, 500° C., 550° C., 600° C., 650° C. or 700° C., but not limited thereto. The time of the pore-forming treatment is 2-5 hours, for example, 2 hours, 3 hours, 4 hours or 5 hours, but not limited thereto.

[0055] In one embodiment, the stirring temperature is 24-26° C., for example, 24° C., 25° C. or 26° C., but not limited thereto. The stirring time is 1-2 h, for example, 1 h, 1.5 h or 2 h, but not limited thereto.

[0056] In one embodiment, the oxidizing atmosphere includes at least one of air, oxygen and carbon dioxide.

[0057] In one embodiment, the dispersant includes acetone; and / or the graphite includes artificial graphite and / or natural graphite.

[0058] In one embodiment, drying the mixed solution comprises:

[0059] The mixed solution is first pre-dried to a viscous state, and then dried under a vacuum state.

[0060] In one embodiment, the drying process under the vacuum state comprises:

[0061] A pretreatment is first performed at a first temperature, and then a drying treatment is performed at a second temperature greater than the first temperature.

[0062] In one embodiment, the temperature of the pre-drying treatment is 50-70°C, for example, it can be 50°C, 55°C, 60°C, 65°C or 70°C, but not limited to this. In one embodiment, the first temperature is 24-26°C, for example, it can be 24°C, 24.5°C, 25°C or 26°C, but not limited to this. The time of the pretreatment is 1-3h, for example, it can be 1h, 1.5h, 2h or 3h, but not limited to this. In one embodiment, the second temperature is 110-130°C, for example, it can be 110°C, 115°C, 120°C, 125°C or 130°C, but not limited to this. The time of the drying treatment is 1-3h, for example, it can be 1h, 1.5h, 2h or 3h, but not limited to this. In one embodiment, the air pressure under vacuum state is within 1KPa.

[0063] The present application also provides a secondary battery, comprising a negative electrode plate, wherein the negative electrode plate comprises a negative electrode collector and a negative electrode active material disposed on the negative electrode collector, wherein the negative electrode active material comprises any of the composite graphite negative electrode materials described above.

[0064] The negative electrode current collector includes but is not limited to copper foil, and can be selected from existing known technologies according to actual needs.

[0065] In one embodiment, the secondary battery further includes a positive electrode sheet, an electrolyte, and a separator.

[0066] Example 1

[0067] 1. Preparation of composite graphite negative electrode materials:

[0068] (1) Graphite was crushed to an average particle size of 9 μm, placed in a tubular furnace and introduced with air, heated to 600° C. at a rate of 10° C. / min, kept at that temperature for 2 h, and taken out after natural cooling to obtain modified porous graphite.

[0069] (2) The modified porous graphite and the conductive filler graphene were added into an acetone solution at a mass ratio of 40:1 with a solid content of 10%, and the mixture was stirred at room temperature at a speed of 1000 r / min for 1 h to obtain a mixed solution.

[0070] (3) The mixed solution was pre-dried at 60°C to a viscous state, and then the pre-dried material was transferred to a vacuum oven and pretreated at 25°C and 1 kPa for 2 h. The temperature was then raised to 120°C and vacuum dried for 2 h to obtain a composite graphite negative electrode material.

[0071] 2. Preparation of negative electrode sheet

[0072] (1) The mass ratio of the composite graphite negative electrode material, conductive carbon black, sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) in the negative electrode slurry is 93:2:2:3, and the solid content is 50%. The composite graphite negative electrode material is prepared by the above-mentioned composite graphite negative electrode material preparation method.

[0073] (2) The preparation method of the negative electrode slurry is as follows: add solvent deionized water, conductive carbon black and 70% sodium carboxymethyl cellulose (CMC) powder to a stirring tank, stir at low speed for 2 hours to form a conductive glue; add composite graphite negative electrode material to the conductive glue, stir at low viscosity and high speed for 1 hour to form a first mixed solution. Add styrene-butadiene rubber (SBR) to the first mixed solution, stir at high viscosity and high speed for 1 hour to form a second mixed solution. Add ethanol and the remaining 30% sodium carboxymethyl cellulose (CMC) powder to the second mixed solution, stir at low viscosity and high speed for 2 hours to obtain a negative electrode slurry. The mass ratio of deionized water to ethanol in the slurry is 9:1. The negative electrode slurry is evenly coated on both sides of the negative electrode current collector copper foil, dried in an oven, and cold pressed and cut to obtain a negative electrode sheet.

[0074] 3. Preparation of positive electrode sheet

[0075] The positive electrode active material NCM811, the conductive agent carbon nanotubes, and the binder PVDF (polyvinylidene fluoride) are mixed in a mass ratio of 93:3:4, and the solvent NMP (N-methylpyrrolidone) is added, and the mixture is stirred evenly to obtain a positive electrode slurry; the positive electrode slurry is evenly coated on both sides of the positive electrode collector aluminum foil, dried in an oven, and then cold pressed and cut to obtain a positive electrode sheet.

[0076] 4. Preparation of electrolyte

[0077] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1, and then 1 mol / L of fully dried LiPF6 was added and mixed evenly to obtain an electrolyte.

[0078] 5. Preparation of lithium-ion batteries

[0079] The positive electrode sheet, isolation film (polyethylene film), and negative electrode sheet are stacked in sequence, with the isolation film being placed between the positive and negative electrode sheets to play an isolating role, and then wound to obtain a bare battery cell; the bare battery cell is placed in an outer packaging shell, and after drying, an electrolyte is injected, and a lithium-ion battery is obtained through vacuum packaging, standing, forming, shaping and other processes.

[0080] Example 2

[0081] The difference from Example 1 is that in the process of preparing the composite graphite negative electrode material, the graphite is crushed to an average particle size of 11 μm. The other experimental conditions are exactly the same as those in Example 1.

[0082] Example 3

[0083] The difference from Example 1 is that in the process of preparing the composite graphite negative electrode material, the graphite is crushed to an average particle size of 13 μm. The other experimental conditions are exactly the same as those in Example 1.

[0084] Example 4

[0085] The difference from Example 1 is that in the preparation process of the composite graphite negative electrode material, the graphite is crushed to an average particle size of 15 μm. The other experimental conditions are exactly the same as those in Example 1.

[0086] Example 5

[0087] The difference from Example 1 is that in the preparation process of the composite graphite negative electrode material, the holding time of graphite pore formation is 3 hours. The other experimental conditions are exactly the same as those in Example 1.

[0088] Example 6

[0089] The difference from Example 1 is that in the preparation process of the composite graphite negative electrode material, the holding time of graphite pore formation is 4 hours. The other experimental conditions are exactly the same as those in Example 1.

[0090] Example 7

[0091] The difference from Example 1 is that in the preparation process of the composite graphite negative electrode material, the holding time of graphite pore formation is 5 hours. The other experimental conditions are exactly the same as those in Example 1.

[0092] Example 8

[0093] The difference from Example 1 is that in the preparation process of the composite graphite negative electrode material, the graphene conductive filler is replaced by carbon nanotubes, and the other experimental conditions are exactly the same as those in Example 1.

[0094] Example 9

[0095] The difference from Example 1 is that in the preparation process of the composite graphite negative electrode material, the graphene conductive filler is replaced by LiF, and the other experimental conditions are exactly the same as those in Example 1.

[0096] Example 10

[0097] The difference from Example 1 is that in the preparation process of the composite graphite negative electrode material, the graphene conductive filler is replaced by Li2CO3, and the other experimental conditions are exactly the same as those in Example 1.

[0098] Comparative Example 1

[0099] 1. Preparation of composite graphite negative electrode materials:

[0100] (1) Graphite was crushed to an average particle size of 3 μm, placed in a tubular furnace and introduced with air, heated to 600° C. at a rate of 10° C. / min, kept at that temperature for 2 h, and taken out after natural cooling to obtain modified porous graphite.

[0101] (2) The modified porous graphite and the conductive filler graphene were added into an acetone solution at a mass ratio of 40:1 with a solid content of 10%, and the mixture was stirred at room temperature at a speed of 1000 r / min for 1 h to obtain a mixed solution.

[0102] (3) The mixed solution was pre-dried at 60°C to a viscous state, and then the pre-dried material was transferred to a vacuum oven and pretreated at 25°C and 1 kPa for 2 h. The temperature was then raised to 120°C and vacuum dried for 2 h to obtain a composite graphite negative electrode material.

[0103] 2. Preparation of negative electrode sheet

[0104] (1) The mass ratio of the composite graphite negative electrode material, conductive carbon black, sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) in the negative electrode slurry is 93:2:2:3, and the solid content is 50%. The composite graphite negative electrode material is prepared by the above-mentioned composite graphite negative electrode material preparation method.

[0105] (2) The preparation method of the negative electrode slurry is as follows: add solvent deionized water, conductive carbon black and 70% sodium carboxymethyl cellulose (CMC) powder to a stirring tank, stir at low speed for 2 hours to form a conductive glue; add composite graphite negative electrode material to the conductive glue, stir at low viscosity and high speed for 1 hour to form a first mixed solution. Add styrene-butadiene rubber (SBR) to the first mixed solution, stir at high viscosity and high speed for 1 hour to form a second mixed solution. Add ethanol and the remaining 30% sodium carboxymethyl cellulose (CMC) powder to the second mixed solution, stir at low viscosity and high speed for 2 hours to obtain a negative electrode slurry. The mass ratio of deionized water to ethanol in the slurry is 9:1. The negative electrode slurry is evenly coated on both sides of the negative electrode current collector copper foil, dried in an oven, and cold pressed and cut to obtain a negative electrode sheet.

[0106] 3. Preparation of positive electrode sheet

[0107] The positive electrode active material NCM811, the conductive agent carbon nanotubes, and the binder PVDF (polyvinylidene fluoride) are mixed in a mass ratio of 93:3:4, and the solvent NMP (N-methylpyrrolidone) is added, and the mixture is stirred evenly to obtain a positive electrode slurry; the positive electrode slurry is evenly coated on both sides of the positive electrode collector aluminum foil, dried in an oven, and then cold pressed and cut to obtain a positive electrode sheet.

[0108] 4. Preparation of electrolyte

[0109] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1, and then 1 mol / L of fully dried LiPF6 was added and mixed evenly to obtain an electrolyte.

[0110] 5. Preparation of lithium-ion batteries

[0111] The positive electrode sheet, isolation film (polyethylene film), and negative electrode sheet are stacked in sequence, with the isolation film being placed between the positive and negative electrode sheets to play an isolating role, and then wound to obtain a bare battery cell; the bare battery cell is placed in an outer packaging shell, and after drying, an electrolyte is injected, and a lithium-ion battery is obtained through vacuum packaging, standing, forming, shaping and other processes.

[0112] Comparative Example 2

[0113] The difference from Comparative Example 1 is that in the preparation process of the composite graphite negative electrode material, the graphite is crushed to an average particle size of 20 μm. The other experimental conditions are exactly the same as those of Comparative Example 1.

[0114] Comparative Example 3

[0115] The difference from Comparative Example 1 is that, in the preparation process of the composite graphite negative electrode material, the insulation temperature of the graphite pore formation is 300° C. The other experimental conditions are exactly the same as those of Comparative Example 1.

[0116] Comparative Example 4

[0117] The difference from Comparative Example 1 is that, in the process of preparing the composite graphite negative electrode material, the insulation temperature of the graphite pore formation is 720° C. The other experimental conditions are exactly the same as those of Comparative Example 1.

[0118] Comparative Example 5

[0119] The difference from Comparative Example 1 is that no conductive filler graphene is added during the preparation of the composite graphite negative electrode material. The remaining experimental conditions are exactly the same as those of Comparative Example 1.

[0120] Test Example 1

[0121] The modified porous graphites prepared in the above Examples 1 to 10 and Comparative Examples 1 to 5 were subjected to the following structural parameter tests:

[0122] (1) Specific surface area B test of modified porous graphite: obtained by using a specific surface area tester (V-Sorb2800S). The modified porous graphite powder is placed in a U-shaped sample tube, placed in a liquid nitrogen environment, and the adsorbate gas is injected into the sample tube. The adsorption amount of the sample for adsorption analysis is determined based on the pressure or weight change before and after adsorption, and then the specific surface area is calculated using the BET adsorption isotherm equation.

[0123] (2) Test of average pore size S of modified porous graphite: obtained by using a nitrogen adsorption micropore size analyzer (3H-2000PM1). In a liquid nitrogen environment, the adsorption amount and adsorption-desorption isotherms at each partial pressure point were measured by injecting and pumping gas into the sample tube. The pore size parameters were then calculated using the BJH theory.

[0124] (3) Porosity P test of modified porous graphite: obtained by using a nitrogen adsorption micropore size analyzer (3H-2000PM1). In a liquid nitrogen environment, the adsorption amount and adsorption-desorption isotherms at each partial pressure point were measured by injecting and evacuating gas into the sample tube. The porosity parameters were then calculated using the BJH theory.

[0125] (4) Volume median particle size Dv50 of modified porous graphite: obtained by using a laser diffraction particle size analyzer (MS2000). The modified porous graphite sample particles are dispersed in a liquid medium at a concentration of 200-300 mg / L, and a monochromatic light beam is passed through. After the light is scattered by the particles, it is distributed at different angles. The regular multi-element detector receives the values ​​of the relevant scattering diagram at many angles and records these values ​​for analysis. Using the Rayleigh scattering formula, the scattering values ​​are calculated to obtain the ratio of the particle volume of each particle size level to the total volume, thereby obtaining the volume distribution of the particle size.

[0126] Test Example 2

[0127] The lithium ion batteries prepared in the above Examples 1 to 10 and Comparative Examples 1 to 5 were subjected to the following performance tests:

[0128] (1) Dynamic performance test: At 25°C, the lithium-ion battery was fully charged at a rate of 4C and fully discharged at a rate of 1C for 10 times, and then the lithium-ion battery was fully charged at a rate of 4C, and then the negative electrode was disassembled and the lithium deposition on the surface of the negative electrode was observed. Among them, the area of ​​lithium deposition on the surface of the negative electrode is less than 5%, which is considered to be slight lithium deposition, the area of ​​lithium deposition on the surface of the negative electrode is 5% to 40%, which is considered to be moderate lithium deposition, and the area of ​​lithium deposition on the surface of the negative electrode is greater than 40%, which is considered to be severe lithium deposition.

[0129] (2) Storage performance test: At 25°C, the lithium-ion battery was charged to 4.35V at a rate of 0.33C, and the initial capacity C0 was recorded. The lithium-ion battery was then transferred to a 60°C oven and allowed to stand for 50 days. After the standing period, the lithium-ion battery was taken out and discharged to 2.8V at a rate of 0.33C at 25°C, and then charged to 4.35V at a rate of 0.33C. The charging capacity was recorded as the recovery capacity C1, and C1 / C0 was calculated as the battery cell storage capacity recovery rate.

[0130] The above Examples 1 to 10 and Comparative Examples 1 to 5 were tested according to the above Test Example 1 and Test Example 2, respectively. The test results are shown in Table 1.

[0131] Table 1

[0132]

[0133]

[0134] It can be seen from the test results of Examples 1 to 10 that when the porosity P, average pore size S, specific surface area B and volume median particle size Dv50 of the modified porous graphite satisfy 0.02≤P×B×S / Dv50≤0.1, the kinetic properties and high-temperature storage properties of the modified porous graphite are both excellent. This is due to the interaction between the good particle size, pore structure and conductive filler of the modified porous graphite. The porous structure in the modified porous graphite provides abundant active lithium insertion sites and shortens the diffusion path of lithium ions in the graphite particles. The addition of conductive fillers can improve the electronic conductivity and SEI film structure of the graphite electrode, and the prepared battery has excellent comprehensive performance.

[0135] When the porosity P of the modified porous graphite is too large, the average pore size S is too large, the specific surface area B is too large, or the volume median particle size Dv50 is too small, resulting in P×B×S / Dv50 exceeding the upper limit, the high-temperature storage performance of the lithium-ion battery is poor. The reason is that when the porosity P of the graphite is too large, the average pore size S is too large, or the specific surface area B is too large, the number of defects on the graphite surface is large. During high-temperature storage, the graphite is prone to more side reactions with the electrolyte, resulting in deterioration of storage performance. The 60°C storage capacity recovery rate of Comparative Example 4 is only 82%. When the volume median particle size Dv50 of the modified porous graphite is too small, it will also lead to a larger specific surface area of ​​the graphite, a larger number of defects on the graphite surface, and deterioration of storage performance. The 60°C storage capacity recovery rate of Comparative Example 1 is only 86%.

[0136] When the porosity P of the modified porous graphite is too small, the average pore size S is too small, the specific surface area B is too small, or the volume median particle size Dv50 is too large, resulting in P×B×S / Dv50 exceeding the lower limit, the fast charging performance of the lithium-ion battery is poor. The reason is that when the porosity P of the graphite is too small, the average pore size S is too small, or the specific surface area B is too small, the number of lithium-embedded active sites on the graphite surface is small, which is not conducive to the charge transfer of lithium ions on the graphite surface, the charge transfer impedance is high, and the fast charging performance is poor. The kinetic performance test result of Comparative Example 3 is severe lithium precipitation. When the volume median particle size Dv50 of the graphite is too large, the diffusion path of lithium ions in the graphite particles is too long, which is not conducive to the rapid embedding of lithium ions, and the fast charging performance is poor. The kinetic performance test result of Comparative Example 2 is severe lithium precipitation.

[0137] When the macropores of the modified porous graphite are not filled with conductive substances, the fast charging performance and high-temperature storage performance of the graphite electrode are poor. The reason is that when the macropores on the surface of the modified porous graphite are not filled with conductive substances, the inner surface of the macropores is completely exposed to the electrolyte, which leads to the aggravation of the side reactions of graphite and electrolyte, and the deterioration of high-temperature storage performance. The 60°C storage capacity recovery rate of Comparative Example 5 is only 85%. In addition, when the macropores on the surface of the modified porous graphite are not filled with conductive substances, the electronic conductivity and SEI film of the graphite are not improved, the ohmic impedance and SEI film impedance are high, the fast charging performance is poor, and the kinetic performance test results of Comparative Example 5 are severe lithium precipitation. Therefore, the modified porous graphite negative electrode material provided in the present application can solve the problem of poor fast charging performance of traditional graphite negative electrode materials.

[0138] It should be noted that the technical solutions or technical features described in the above embodiments can be combined or supplemented with each other without causing conflicts. The scope of protection of this application is not limited to the precise structures described in the above embodiments and shown in the drawings; all modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of this application should be included in the scope of protection of this application.

Claims

1. A composite graphite negative electrode material, characterized in that: include: Modified porous graphite and a conductive filler filled in the pores of the modified porous graphite; The structure of the modified porous graphite satisfies the following relational formula (I), and the pore filling rate of the modified porous graphite is greater than or equal to 5%; 0.01≤P×B×S / D v50 ≤0.1(I) Wherein, P is the porosity of the modified porous graphite; B is the specific surface area of ​​the modified porous graphite; S is the average pore size of the modified porous graphite; D v50 is the volume median particle size of the modified porous graphite.

2. The composite graphite negative electrode material according to claim 1, characterized in that: The structure of the modified porous graphite satisfies: 0.03≤P×B×S / D v50 ≤0.08, and the pore filling rate of the modified porous graphite is greater than or equal to 7%.

3. The composite graphite negative electrode material according to claim 2, characterized in that: The structure of the modified porous graphite satisfies: 0.04≤P×B×S / D v50 ≤0.06, and the pore filling rate of the modified porous graphite is greater than or equal to 9%.

4. The composite graphite negative electrode material according to claim 3, characterized in that: The porosity of the modified porous graphite is 5% to 20%; And / or, the specific surface area of ​​the modified porous graphite is 3 to 7 m 2 / g; And / or, the average pore size of the modified porous graphite is 0.1 to 1.2 μm; and / or, the volume median particle size of the modified porous graphite is 9 to 16 μm; And / or, the conductive filler includes at least one of graphene, carbon nanotubes, conductive carbon black, lithium ethylene monocarbonate, LiF, Li3P, Li2O, Li2CO3 and Al2O3.

5. A method for preparing the composite graphite negative electrode material according to any one of claims 1 to 4, characterized in that: include: After the graphite is broken up, it is placed in an oxidizing atmosphere for pore formation to obtain modified porous graphite that meets the preset structure; The modified porous graphite and the conductive filler are added into a dispersant and stirred to obtain a mixed solution; the mixed solution is dried to obtain a composite graphite negative electrode material.

6. The method for preparing the composite graphite negative electrode material according to claim 5, characterized in that: The weight ratio of the modified porous graphite to the conductive filler is (20-50): (1-2); And / or, the temperature of the pore-forming treatment is 500-700° C., and the time of the pore-forming treatment is 2-5 hours; And / or, the stirring temperature is 24-26° C., and the stirring time is 1-2 h; and / or, the oxidizing atmosphere comprises at least one of air, oxygen and carbon dioxide; And / or, the dispersant includes acetone; and / or, the graphite includes artificial graphite and / or natural graphite.

7. The method for preparing the composite graphite negative electrode material according to claim 5, characterized in that: The mixed solution is dried, comprising: The mixed solution is first pre-dried to a viscous state, and then dried under a vacuum state.

8. The method for preparing the composite graphite negative electrode material according to claim 7, characterized in that: The drying process under the vacuum state comprises: A pretreatment is first performed at a first temperature, and then a drying treatment is performed at a second temperature greater than the first temperature.

9. The method for preparing the composite graphite negative electrode material according to claim 8, characterized in that: The temperature of the pre-drying treatment is 50-70°C; And / or, the first temperature is 24-26° C., and the pretreatment time is 1-3 hours; And / or, the second temperature is 110-130° C., and the drying treatment time is 1-3 hours.

10. A secondary battery, comprising a negative electrode plate, wherein the negative electrode plate comprises a negative electrode current collector and a negative electrode active material disposed on the negative electrode current collector, characterized in that: The negative electrode active material comprises the composite graphite negative electrode material as described in any one of claims 1 to 4 above.

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