A fast-charging negative electrode sheet and a preparation method and application thereof

Abstract

The application provides a fast-charging negative electrode sheet and a preparation method and application thereof. The fast-charging negative electrode sheet comprises a negative electrode current collector and a first active material layer and a second active material layer which are sequentially stacked on the surface of the negative electrode current collector. The first active material layer comprises a first particle size negative electrode active material, a second particle size negative electrode active material and carbon nanotubes. The second active material layer comprises a third particle size negative electrode active material and carbon nanofibers. The fast-charging negative electrode sheet uses conductive agents with different characteristics in the upper and lower active material layers to provide a reasonable distribution and fast electron / ion transmission channel. By changing the active material morphology and particle size distribution of the upper and lower active layers, the pore structure of the electrode sheet along the thickness direction is optimized, the effective contact area between the upper solid phase and the liquid electrolyte is enhanced, the consumption of Li + in the upper layer in the later charging stage is reduced, and the diffusion of Li to the lower layer is promoted.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and relates to a fast-charging type negative electrode sheet and a preparation method and application thereof. BACKGROUND

[0002] Electric vehicles driven by lithium ion batteries are one of the promising zero-emission transportation tools for solving air pollution and energy crisis problems. In recent years, with the rapid development of the global electric vehicle market, higher requirements and standards have been put forward for the performance of lithium ion batteries, and the demand for lithium ion batteries with high energy density and fast charging has increased.

[0003] However, the energy density of existing power lithium ion batteries is insufficient, which limits the cruising range of electric vehicles. In terms of electrode structure, by preparing a high-load thick electrode to increase the proportion of active materials, a promising solution for improving energy density is to increase the thickness of the electrode, which will significantly increase the migration distance of electric charge and resistance, limit the transmission dynamics of electrons and lithium ions, and unevenly distribute the current and lithium ion concentration along the thickness direction, thereby increasing polarization and reducing the utilization rate of active materials, accompanied by problems such as lithium precipitation on the surface of the graphite negative electrode during charging, which ultimately leads to deterioration of the rate performance and hinders the improvement of energy density.

[0004] CN113871568A discloses a lithium ion battery negative electrode sheet and a preparation method and application thereof, the negative electrode sheet includes a current collector, a first active material layer in contact with the current collector, and a second active material layer in contact with the first active material layer, wherein the active material in the first active material layer is graphite; the active material in the second active material layer is a binary mixed material of graphite and hard carbon.

[0005] CN108328614A discloses a fast-charging type lithium ion battery graphite negative electrode material and a preparation method thereof, the preparation method includes: performing crushing treatment and screening classification treatment on carbon-based materials to obtain primary particles; the D50 of the primary particles is 12-20 μm; the primary particles are mixed with pitch to obtain a powder; the powder is kneaded to obtain a kneaded material; the kneaded material is crushed and then carbonized in an inert atmosphere to obtain a carbonized material; the carbonized material is crushed, screened, and then subjected to graphitization treatment, followed by magnetic removal treatment and screening.

[0006] The negative electrode material or negative electrode sheet described in the above scheme has the problems of low material utilization rate and lithium precipitation on the surface of the graphite negative electrode after the battery is made, which greatly limits its application in lithium ion batteries. SUMMARY

[0007] The purpose of the present application is to provide a fast-charging type negative electrode sheet and its preparation method and application. The fast-charging type negative electrode sheet of the present application uses conductive agents with different properties in the upper and lower active material layers to provide a reasonable distribution and fast electron / ion transmission channel. By changing the active material morphology and particle size distribution of the upper and lower active layers, the pore structure of the electrode sheet along the thickness direction is optimized, the effective contact area between the solid phase and the liquid electrolyte of the upper layer is increased, the consumption of Li + in the upper layer in the later charging stage is reduced, and the diffusion to the lower layer is promoted.

[0008] To achieve the purpose of the present application, the following technical solutions are adopted:

[0009] In a first aspect, the present application provides a fast-charging type negative electrode sheet, which comprises a negative electrode current collector and a first active material layer and a second active material layer sequentially stacked on the surface of the negative electrode current collector. The first active material layer comprises a first particle size negative active material, a second particle size negative active material and carbon nanotubes, and the second active material layer comprises a third particle size negative active material and carbon nanofibers.

[0010] The present application prepares a double-layer structure fast-charging type negative electrode sheet by simply compounding the upper and lower active layers. The active material with different particle sizes is used to prepare an active lower layer (first active material layer) with high compaction density, which can form more conductive paths and further improve the electronic conductivity near the current collector end. The small particle size active material is used to prepare an active upper layer (second active material layer), which increases the effective contact area between the solid phase and the liquid electrolyte, is easily completely lithiated in the later charging stage, and can reduce the electrochemical reaction rate of the electrode upper layer, consume less Li + and promote more Li + diffusion to the lower layer. This is beneficial to reduce the concentration polarization in the graphite negative electrode, improve the utilization rate of the active material, increase the energy density of the battery, and improve the fast-charging performance.

[0011] The active lower layer uses carbon nanotube conductive agent with excellent electronic conductivity, which can form a continuous conductive network between the electrode active materials, reduce the contact resistance between the active layer and the current collector, and strengthen the electronic conduction. The active upper layer uses carbon nanofiber conductive agent which is conducive to the seepage of the electrolyte, improves the ion transmission capacity and local effective diffusion coefficient of the upper layer, and promotes the diffusion of Li + from the upper layer. The electrode with reasonable distribution and enhanced electronic / ion transmission capacity can improve the electrical performance of the battery.

[0012] Preferably, the negative active material comprises spherical artificial graphite.

[0013] Preferably, the particle size of the first particle size negative active material is 10-17 μm, for example: 10 μm, 11 μm, 12 μm, 15 μm or 17 μm, etc.

[0014] Preferably, the particle size of the second particle size negative active material is 3-10 μm, for example: 3 μm, 4 μm, 5 μm, 8 μm or 10 μm, etc.

[0015] Preferably, the particle size of the third particle size negative active material is 5-10 μm, for example: 5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm, etc.

[0016] Preferably, the thickness of the first active material layer is 50-100 μm, for example: 50 μm, 60 μm, 70 μm, 80 μm, 90 μm or 100 μm, etc.

[0017] Preferably, the thickness of the second active material layer is 30-80 μm, for example: 30 μm, 40 μm, 50 μm, 60 μm or 80 μm, etc.

[0018] Preferably, the mass fraction of the carbon nanotube is 1-10% based on 100% of the mass of the first active material layer, for example: 1%, 2%, 5%, 8% or 10%, etc., preferably 2-3%.

[0019] Preferably, the mass fraction of the carbon nanofiber is 1-10% based on 100% of the mass of the second active material layer, for example: 1%, 2%, 5%, 8% or 10%, etc., preferably 2-4%.

[0020] Preferably, the first active material layer and the second active material layer both comprise a binder.

[0021] Preferably, the binder comprises polyvinylidene fluoride and / or styrene butadiene rubber.

[0022] Preferably, the total mass fraction of the first particle size negative active material and the second particle size negative active material is 80-98% based on 100% of the mass of the first active material layer, for example: 80%, 85%, 90%, 96% or 98%, etc.

[0023] Preferably, the mass fraction of the binder is 1-10%, for example: 1%, 2%, 5%, 8% or 10%, etc.

[0024] Preferably, the mass fraction of the third particle size negative active material is 80-98% based on 100% of the mass of the second active material layer, for example: 80%, 85%, 90%, 96% or 98%, etc.

[0025] Preferably, the adhesive has a mass fraction of 1% to 10%, for example: 1%, 2%, 5%, 8% or 10%, etc.

[0026] Secondly, the present invention provides a method for preparing a fast-charging negative electrode sheet as described in the first aspect, the method comprising the following steps:

[0027] (1) A first slurry is obtained by mixing a first-size negative electrode active material, a second-size negative electrode active material, carbon nanotubes and binder with a solvent, and a second slurry is obtained by mixing a third-size negative electrode active material, carbon nanofibers and binder with a solvent.

[0028] (2) The first slurry is coated on the surface of the negative current collector, the second slurry is coated on the surface of the first slurry, and the fast-charging negative electrode sheet is obtained by drying.

[0029] Preferably, the solid content of the first slurry and the second slurry in step (1) is 30% to 70%, for example: 30%, 40%, 50%, 60% or 70%, etc.

[0030] Preferably, the drying temperature in step (2) is 110 to 130°C, for example: 110°C, 115°C, 120°C, 125°C or 130°C.

[0031] Preferably, the drying time is 0.5 to 2 hours, for example: 0.5 hours, 0.8 hours, 1 hour, 1.5 hours or 2 hours.

[0032] Thirdly, the present invention provides a lithium-ion battery comprising a fast-charging negative electrode as described in the first aspect.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] (1) The fast-charging negative electrode of this invention uses conductive agents with different properties in the upper and lower active material layers to provide a reasonable distribution and fast electron / ion transport channel; and by changing the morphology and particle size distribution of the active materials in the upper and lower active layers, the pore structure of the electrode along the thickness direction is optimized, while the effective contact area between the upper solid phase and the liquid electrolyte is enhanced, reducing the upper Li layer in the later stage of charging. + The energy density is reduced by absorbing the energy and allowing it to diffuse to the lower layers. This simple composite process between the upper and lower layers reduces polarization, thereby increasing energy density and improving fast-charging performance.

[0035] (2) The battery made from the fast-charging negative electrode sheet of the present invention can achieve an energy density of more than 308Wh / kg at 1C and a capacity retention rate of more than 97% at 4C. Detailed Implementation

[0036] The technical solutions of the present application are further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments are only used to help understand the present application and should not be regarded as specific limitations on the present application.

[0037] Example 1

[0038] The preparation method of the fast-charging type negative electrode sheet is as follows:

[0039] The 12 μm spherical artificial graphite, 4 μm spherical artificial graphite, carbon nanotubes and polyvinylidene fluoride are mixed in a mass ratio of 50:46:2:2 with N-methyl pyrrolidone to obtain a first slurry with a solid content of 40%, and the 6 μm spherical artificial graphite, carbon nanofiber and polyvinylidene fluoride are mixed in a mass ratio of 94:2:2:2 with N-methyl pyrrolidone to obtain a second slurry with a solid content of 40%;

[0040] (2) The first slurry is coated on the surface of the negative electrode current collector, and the second slurry is coated on the surface of the first slurry, and then dried at 120°C for 1 h to obtain the fast-charging type negative electrode sheet;

[0041] The thickness of the first active material layer of the positive electrode sheet is 80 μm, and the thickness of the second active material layer is 50 μm.

[0042] Example 2

[0043] The preparation method of the fast-charging type negative electrode sheet is as follows:

[0044] The 15 μm spherical artificial graphite, 6 μm spherical artificial graphite, carbon nanotubes and polyvinylidene fluoride are mixed in a mass ratio of 50:46:2:2 with N-methyl pyrrolidone to obtain a first slurry with a solid content of 40%, and the 8 μm spherical artificial graphite, carbon nanofiber and polyvinylidene fluoride are mixed in a mass ratio of 94:2:2:2 with N-methyl pyrrolidone to obtain a second slurry with a solid content of 40%;

[0045] (2) The first slurry is coated on the surface of the negative electrode current collector, and the second slurry is coated on the surface of the first slurry, and then dried at 120°C for 1 h to obtain the fast-charging type negative electrode sheet;

[0046] The thickness of the first active material layer of the positive electrode sheet is 75 μm, and the thickness of the second active material layer is 45 μm.

[0047] Example 3

[0048] The difference between this embodiment and Example 1 is that the mass ratio of carbon nanotubes in the first active material layer is 1.5%, and other conditions and parameters are the same as those of Example 1.

[0049] Example 4

[0050] The difference between this example and Example 1 is only that the mass ratio of carbon nanotubes in the first active material layer is 4%, and other conditions and parameters are exactly the same as those in Example 1.

[0051] Example 5

[0052] The difference between this example and Example 1 is only that the mass ratio of carbon nanofibers in the second active material layer is 1.5%, and other conditions and parameters are exactly the same as those in Example 1.

[0053] Example 6

[0054] The difference between this example and Example 1 is only that the mass ratio of carbon nanofibers in the second active material layer is 5%, and other conditions and parameters are exactly the same as those in Example 1.

[0055] Comparative Example 1

[0056] The difference between this comparative example and Example 1 is only that the second active material layer uses the same size particle diameter mixed negative electrode main material as the first active material layer, and other conditions and parameters are exactly the same as those in Example 1.

[0057] Comparative Example 2

[0058] The difference between this comparative example and Example 1 is only that the first active material layer and the second active material layer both use carbon nanotubes as the conductive agent, and other conditions and parameters are exactly the same as those in Example 1.

[0059] Comparative Example 3

[0060] The difference between this comparative example and Example 1 is only that the first active material layer and the second active material layer both use carbon nanofibers as the conductive agent, and other conditions and parameters are exactly the same as those in Example 1.

[0061] Performance test:

[0062] The positive active material (NCM811), the conductive agent (SP), and the binder (PVDF) are mixed and dispersed with the organic solvent NMP according to a mass ratio of 97:2:1 to obtain a positive electrode slurry. Then the slurry is coated on both sides of the positive current collector, and through cold pressing and die cutting, a positive electrode sheet is obtained. The fast-charging negative electrode sheet obtained in Examples 1-6 and Comparative Examples 1-3 and the positive electrode sheet are subjected to the processes of roll pressing, die cutting, lamination, drying, liquid injection, sealing, formation, capacity distribution, and standing to make soft package batteries for subsequent performance tests.

[0063] (1) Energy density test: after the prepared soft package battery was weighed, it was charged at 1C constant current and constant voltage to 4.2V, and then discharged at 1C constant current to 2.75V. The discharge energy was recorded, and the 1C energy density was calculated.

[0064] (2) Rate test: the battery was charged at 1C constant current and constant voltage to 4.2V, and then discharged at 1C, 2C and 4C to 2.75V, respectively. The discharge capacity was recorded, and the capacity retention rate was calculated. The test results are shown in Table 1:

[0065] Table 1

[0066] 1C energy density (Wh / kg) 4C capacity retention rate (%) Example 1 312 97 Example 2 308 98 Example 3 298 95 Example 4 305 93 Example 5 301 95 Example 6 296 94 Comparative Example 1 290 85 Comparative Example 2 283 87 Comparative Example 3 287 90

[0067] As can be seen from Table 1, according to Examples 1-2, the 1C energy density of the battery made of the fast-charging negative electrode sheet of the application can reach more than 308Wh / kg, and the capacity retention rate at 4C can reach more than 97%.

[0068] As can be seen from the comparison of Example 1 and Examples 3-4, in the first active material layer of the fast-charging negative electrode sheet of the application, the mass fraction of carbon nanotubes will affect its performance. If the mass fraction of carbon nanotubes in the first active material layer is controlled to be 2-3%, the performance of the fast-charging negative electrode sheet prepared is better. If the addition amount of carbon nanotubes is too low, the electron conduction of the electrode sheet can be enhanced, but the effect is limited, and the rate performance of the battery is improved. With the increase of the addition amount, the rate performance is further improved. If the addition amount is too high, it will affect the distribution of each component in the electrode sheet, reduce the effective utilization rate of the active material, and cause the pore blockage of the electrode sheet, resulting in the decrease of the capacity retention rate at high rate and the reduction of the rate performance.

[0069] As can be seen from the comparison of Example 1 and Examples 5-6, in the second active material layer of the fast-charging negative electrode sheet of the application, the mass fraction of carbon nanofibers will affect its performance. If the mass fraction of carbon nanofibers in the second active material layer is controlled to be 2-4%, the performance of the fast-charging negative electrode sheet prepared is better. If the addition amount of carbon nanofibers is too low, it is helpful for the seepage of electrolyte, and has a certain promoting effect on the transmission and diffusion of lithium ions, and the rate performance of the battery is poor. With the increase of the addition amount, the rate performance is further improved. If the addition amount is too high, it will affect the distribution of each component in the electrode sheet, reduce the utilization rate of the active material, and too much carbon nanofibers will block the pores, reduce the porosity, cause the decrease of the capacity retention rate at high rate, reduce the rate performance, and hinder the improvement of the energy density.

[0070] From the comparison of Example 1 and Comparative Example 1, it can be seen that the active material with mixed particle sizes is used in the present application to prepare the active lower layer with high tap density, which can form more conductive paths and further improve the electronic conductivity near the end of the current collector; the small particle size active material is used to prepare the active upper layer, which increases the effective contact area between the solid phase and the liquid phase electrolyte, is easily completely lithiated at the later stage of charging, and can reduce the electrochemical reaction rate of the upper layer of the electrode, consume less Li + and promote more Li + diffusion to the lower layer. This is beneficial to reduce the concentration polarization in the graphite negative electrode, improve the utilization rate of the active material, improve the battery energy density, and improve the fast charging performance.

[0071] From the comparison of Example 1 and Comparative Examples 2-3, it can be seen that the carbon nanotube conductive agent with excellent electronic conductivity is used in the active lower layer of the fast-charging negative electrode sheet of the present application, which can form a continuous conductive network between the electrode active materials, reduce the contact impedance between the active layer and the current collector, and strengthen the electronic conduction; the carbon nanofiber conductive agent that helps electrolyte seepage is used in the active upper layer, which improves the ion transport capacity and local effective diffusion coefficient of the upper layer, and promotes the diffusion of Li + from the upper layer. The electrode with reasonable distribution and enhanced electronic / ionic transport capacity can improve the battery performance.

[0072] The applicant declares that the above description is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto. It should be understood by those skilled in the art that any changes or replacements within the technical scope disclosed by the present application can be easily thought of by those skilled in the art, and all fall within the protection scope and disclosure scope of the present application.

Claims

1. A fast-charging type negative electrode sheet, characterized by, The fast-charging negative electrode sheet comprises a negative electrode current collector and first and second active material layers sequentially stacked on the surface of the negative electrode current collector, the first active material layer comprises a first particle size negative electrode active material, a second particle size negative electrode active material and carbon nanotubes, and the second active material layer comprises a third particle size negative electrode active material and carbon nanofibers. The negative electrode active material comprises spherical artificial graphite, the particle size of the first particle size negative electrode active material is 11-17 μm, the particle size of the second particle size negative electrode active material is 3-10 μm, and the particle size of the third particle size negative electrode active material is 5-10 μm. 2.The fast-charging negative electrode sheet of claim 1, wherein the carbon-based material is selected from the group consisting of carbon black, acetylene black, ketjen black, super-P, and a mixture thereof. The thickness of the first active material layer is 50-100 μm. 3.The fast-charging negative electrode sheet of claim 1, wherein the carbon-based material is selected from the group consisting of carbon black, acetylene black, ketjen black, super-P, and a mixture thereof. The thickness of the second active material layer is 30-80 μm.

4. The fast-charging negative electrode sheet as described in claim 1, characterized in that, The mass fraction of the carbon nanotubes is 1-10% based on 100% of the mass of the first active material layer. 5.The fast-charging negative electrode sheet of claim 1, wherein the carbon-based material is selected from the group consisting of carbon black, acetylene black, ketjen black, super-P, and a mixture thereof. The mass fraction of the carbon nanotubes is 2-3% based on 100% of the mass of the first active material layer. 6.The fast-charging negative electrode sheet of claim 1, wherein the carbon-based material is selected from the group consisting of carbon black, acetylene black, ketjen black, super-P, and a mixture thereof. The mass fraction of the carbon nanofibers is 1-10% based on 100% of the mass of the second active material layer. 7.The fast-charging negative electrode sheet of claim 1, wherein the carbon-based material is selected from the group consisting of carbon black, acetylene black, ketjen black, super-P, and a mixture thereof. The mass fraction of the carbon nanofibers is 2-4% based on 100% of the mass of the second active material layer. 8.The fast-charging negative electrode sheet of claim 1, wherein the carbon-based material is selected from the group consisting of carbon black, acetylene black, ketjen black, super-P, and a mixture thereof. The first and second active material layers both comprise a binder. 9.The fast-charging negative electrode sheet of claim 8, wherein the carbon-based material is selected from the group consisting of carbon black, acetylene black, ketjen black, super-P, and a mixture thereof. The binder comprises polyvinylidene fluoride and / or styrene-butadiene rubber. 10.The fast-charging negative electrode sheet of claim 8, wherein the carbon-based material is selected from the group consisting of carbon black, acetylene black, ketjen black, super-P, and a mixture thereof. The total mass fraction of the first and second particle size negative electrode active materials is 80-98% based on 100% of the mass of the first active material layer.

11. The fast-charging negative electrode sheet of claim 10, wherein the carbon-based material is selected from the group consisting of carbon black, acetylene black, ketjen black, super-P, and a mixture thereof. The mass fraction of the binder is 1-10% based on 100% of the mass of the first active material layer.

12. The fast-charging negative electrode sheet of claim 8, wherein the carbon-based material is selected from the group consisting of carbon black, acetylene black, ketjen black, super-P, and a mixture thereof. The mass fraction of the third particle size negative electrode active material is 80-98% based on 100% of the mass of the second active material layer.

13. The fast-charging negative electrode sheet of claim 12, wherein the carbon-based material is selected from the group consisting of carbon black, acetylene black, ketjen black, super-P, and a mixture thereof. The mass fraction of the binder is 1-10% based on 100% of the mass of the second active material layer.

14. A method of producing the fast-charging negative electrode sheet according to any one of claims 1 to 13, characterized by, The preparation method comprises the following steps: (1) mixing a first particle size negative electrode active material, a second particle size negative electrode active material, carbon nanotubes and a binder with a solvent to obtain a first slurry, and mixing a third particle size negative electrode active material, carbon nanofibers and a binder with a solvent to obtain a second slurry; (2) coating the first slurry on the surface of a negative electrode current collector, coating the second slurry on the surface of the first slurry, and drying to obtain the fast-charging negative electrode sheet.

15. The production method according to claim 14, wherein The solid content of the first and second slurries in step (1) is 30-70%.

16. The production method according to claim 14, wherein The temperature of the drying in step (2) is 110-130 °C.

17. The production method according to claim 16, wherein The time of the drying is 0.5-2 h.

18. A lithium-ion battery, characterized by, The lithium ion battery comprises the fast-charging negative electrode sheet according to any one of claims 1-13.