Dry method gradient negative electrode sheet, preparation method thereof and battery

Abstract

This invention provides a dry-process gradient negative electrode sheet, its preparation method, and a battery. The preparation method includes: mixing a silicon-based material, a first conductive agent, and a first binder to obtain a bottom layer mixture; fiberizing a capacity-type negative electrode material, a second conductive agent, and a second binder to obtain an intermediate layer mixture; fiberizing a fast-charging negative electrode material, a third conductive agent, a third binder, and a solid electrolyte material to obtain a surface layer mixture; sequentially stacking the bottom layer mixture, intermediate layer mixture, and surface layer mixture on at least one side of a current collector, and then hot-pressing to obtain the dry-process gradient negative electrode sheet. This invention, through a dry process combined with a three-layer functional design, allows the binder to melt and flow simultaneously, the bonding network to fully extend, and the interlayer materials to diffuse into each other, ultimately achieving seamless fusion of the three layers at the microscale. This results in extremely low interlayer interface resistance, and the battery possesses high energy density, high rate capability, and long cycle performance.

Description

Technical Field

[0001] This invention belongs to the field of battery technology and relates to a dry gradient negative electrode sheet, its preparation method, and a battery. Background Technology

[0002] As the requirements for energy density and fast-charging performance of lithium-ion batteries become increasingly stringent, traditional wet coating processes are facing numerous challenges. Wet coating processes require the use of large amounts of toxic organic solvents (such as NMP), resulting in high energy consumption and significant pollution. Furthermore, during the drying process, the binder tends to migrate to the coating surface, leading to uneven binder distribution within the electrode and increased interfacial impedance. In addition, single-layer homogeneous structures are no longer sufficient to meet the multiple performance requirements of "high capacity, high rate capability, and long cycle life."

[0003] To address the above problems, the existing technology has made the following improvements:

[0004] (1) Wet multilayer coating: A gradient layered structure is constructed by multiple coatings, but there are problems such as poor interlayer bonding, solvent residue and secondary swelling.

[0005] (2) Dry electrode process: For example, the dry process technology of polytetrafluoroethylene (PTFE) fibrillation eliminates the solvent. However, most existing dry electrodes are single-layer homogeneous structures or simple double-layer composites, which fail to achieve refined functional gradient design. Moreover, the interlayer interface is mostly physically bonded, and the bonding force needs to be improved.

[0006] Therefore, there is a contradiction between the environmental friendliness of the process and the refinement of the structure in the existing technology. There is an urgent need to provide an electrode that can achieve solvent-free green manufacturing and construct a fine functional gradient structure with excellent interfacial bonding. Summary of the Invention

[0007] The purpose of this invention is to provide a dry gradient negative electrode sheet, its preparation method, and a battery. The preparation method combines a dry process with a three-layer functional design, which allows the binder to melt and flow, the bonding network to fully extend, and the interlayer materials to diffuse simultaneously, ultimately achieving seamless fusion of the three layers at the microscale. This results in extremely low interlayer interface resistance and enables the battery to have high energy density, high rate capability, and long cycle performance.

[0008] To achieve this objective, the present invention adopts the following technical solution:

[0009] In a first aspect, the present invention provides a method for preparing a dry gradient negative electrode sheet, the method comprising the following steps:

[0010] (1) The silicon-based material, the first conductive agent and the first binder are mixed to obtain the bottom mixture;

[0011] A mixture of a capacity-type negative electrode material, a second conductive agent, and a second binder is fibroinated to obtain an intermediate layer mixture.

[0012] The fast-charging negative electrode material, the third conductive agent, the third binder, and the solid electrolyte material are subjected to fibrous treatment to obtain a surface mixture;

[0013] Wherein, the first adhesive includes a polyethylene material, and the second and third adhesives each independently include a fiber-reducible adhesive;

[0014] (2) A bottom layer mixture, an intermediate layer mixture and a surface layer mixture are sequentially stacked on at least one side of the current collector to obtain a stacked negative electrode sheet. The stacked negative electrode sheet is hot-pressed to obtain the dry gradient negative electrode sheet.

[0015] The binder for the bottom layer mixture of this invention is a polyethylene-based material, which has a high melt viscosity and forms a strong and tough three-dimensional network structure after hot pressing. This not only provides bonding strength far exceeding that of traditional binders but also effectively buffers the volume expansion stress of the bottom silicon particles. The middle layer mixture uses a high-capacity negative electrode material and a fiber-forming binder to ensure the overall capacity of the electrode and to form a three-dimensional network structure in advance. The surface layer mixture uses a fast-charging negative electrode material, a solid electrolyte material, and a fiber-forming binder. The solid electrolyte material further enhances interfacial kinetics, constructing a fast and stable electrochemical reaction interface. The electrode material enhances fast charging performance, and the fibrous binder constructs an efficient electron transport framework. The three-layer specific composition combined with the one-time hot pressing preparation process of this invention can obtain a dry gradient negative electrode sheet. Furthermore, since the bottom layer mixture, the middle layer mixture, and the surface layer mixture are not pre-rolled to prepare a self-supporting film, the melting and flow of polyethylene materials, the full extension of the fibrous binder network, and the mutual diffusion of interlayer materials occur simultaneously, ultimately achieving seamless fusion of the three layers at the microscale. No obvious boundary line can be observed in the three-layer structure, which not only improves the interlayer bonding force but also significantly reduces the charge transfer impedance of the electrode sheet.

[0016] It is understood that the bottom layer mixture, the intermediate layer mixture and the surface layer mixture described in this invention are fused together after hot pressing, without interfaces, thus constructing an integrated electrode with a smooth functional transition from the current collector to the outer surface.

[0017] Preferably, in step (1), the first adhesive comprises ultra-high molecular weight polyethylene (UHMWPE) with a viscosity-average molecular weight of more than 1 million, such as 1 million, 1.25 million, 1.5 million, 1.75 million or 2 million, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0018] This invention uses ultra-high molecular weight polyethylene (UHMWPE) as the bottom binder, which utilizes its extremely high molecular weight and melt viscosity to achieve melt flow and form a three-dimensional network structure.

[0019] Preferably, by mass, the first adhesive in the bottom mixture of step (1) is 45-75 parts, for example, 45, 55, 65 or 75 parts, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0020] In this invention, a large amount of first binder is used in the bottom mixture. If the amount of first binder added is too small, the bottom bonding strength will be insufficient, and it will not be able to effectively buffer the volume expansion of silicon particles. This will easily lead to the electrode pulverization and detachment during cycling, resulting in a decrease in battery cycle life. If the amount of first binder added is too large, the content of bottom active material (silicon) will be relatively reduced, which will sacrifice the overall energy density of the battery. At the same time, too much binder may also increase the resistance to lithium-ion transport.

[0021] Preferably, by mass, in the bottom mixture of step (1), the number of parts of the silicon-based material is 20-40 parts, for example, 20 parts, 25 parts, 30 parts, 35 parts or 40 parts, and the number of parts of the first conductive agent is 5-15 parts, for example, 5 parts, 7 parts, 9 parts, 11 parts, 13 parts or 15 parts, but not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0022] Preferably, the bottom mixture in step (1) comprises 100 parts by weight.

[0023] Preferably, the silicon-based material in step (1) is nano-silicon particles.

[0024] Preferably, the particle size D50 of the silicon-based material in step (1) is 50nm-200nm, for example, it can be 50nm, 100nm, 150nm or 200nm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0025] Preferably, in step (1), the first conductive agent, the second conductive agent, and the third conductive agent each independently comprise carbon black and / or carbon nanotubes.

[0026] Preferably, by mass, the solid electrolyte material in the surface mixture of step (1) is 5 to 10 parts, for example, 5, 6, 7, 8, 9 or 10 parts, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0027] The amount of solid electrolyte material added according to the present invention will affect its function and thus affect the surface interface dynamics, preferably within a specific range.

[0028] Preferably, by mass, in the surface mixture of step (1), the number of parts of the fast-charging negative electrode material is 80-90 parts, for example, 80 parts, 82 parts, 84 parts, 86 parts, 88 parts or 90 parts; the number of parts of the third conductive agent is 1-3 parts, for example, 1 part, 1.5 parts, 2 parts, 2.5 parts or 3 parts; and the number of parts of the third binder is 2-5 parts, for example, 2 parts, 3 parts, 4 parts or 5 parts, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0029] Preferably, the surface mixture in step (1) comprises 100 parts by mass.

[0030] Preferably, the solid electrolyte material in step (1) includes a garnet-type solid electrolyte material, such as LLZO (lithium lanthanum zirconium oxide, Li7La3Zr2O). 12 ).

[0031] Preferably, the particle size D50 of the solid electrolyte material in step (1) is 50nm-200nm, for example, it can be 50nm, 80nm, 100nm, 125nm, 150nm, 175nm or 200nm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0032] Preferably, the fast-charging negative electrode material in step (1) includes soft carbon material.

[0033] Preferably, the particle size D50 of the fast-charging negative electrode material in step (1) is 5μm-15μm, for example, it can be 5μm, 7μm, 9μm, 11μm, 13μm or 15μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0034] Preferably, the fiberizable adhesive in step (1) comprises polytetrafluoroethylene.

[0035] Preferably, by mass, the amount of the capacity-type negative electrode material in the intermediate layer mixture in step (1) is 90-95 parts, for example, 90 parts, 91 parts, 92 parts, 93 parts, 94 parts or 95 parts, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0036] Preferably, by mass, the amount of the second conductive agent in the intermediate layer mixture of step (1) is 1 to 3 parts, for example, 1 part, 1.5 parts, 2 parts, 2.5 parts or 3 parts, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0037] Preferably, by mass, the second adhesive in the intermediate layer mixture of step (1) is 2 to 5 parts, for example, 2, 3, 4 or 5 parts, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0038] Preferably, the intermediate layer mixture in step (1) comprises 100 parts by mass.

[0039] Preferably, the capacity-type negative electrode material in step (1) comprises high-compact graphite material, wherein the compaction density of the high-compact graphite material is ≥1.7 g / cm³. 3 For example, it could be 1.7 g / cm³ 3 1.75g / cm 3 1.8g / cm 3 1.85g / cm 3 Or 1.9g / cm 3 However, this does not limit the listed values; other unlisted values ​​within the range are also applicable.

[0040] Preferably, the particle size D50 of the capacity-type negative electrode material in step (1) is 10μm-20μm, for example, it can be 10μm, 12μm, 14μm, 16μm, 18μm or 20μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0041] Preferably, the fiberization process in step (1) includes shearing in an air jet mill or a high-speed mixer.

[0042] Preferably, in the laminated negative electrode sheet described in step (2), the thickness ratio of the bottom layer mixture, the middle layer mixture, and the surface layer mixture is 1:(5~8):(1~2), for example, it can be 1:5:1, 1:6:1.5, 1:7:2 or 1:8:2, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0043] Preferably, the method of sequentially stacking the bottom layer mixture, the intermediate layer mixture and the top layer mixture in step (2) includes a co-extrusion method or an electrostatic spraying method.

[0044] Preferably, the hot pressing temperature in step (2) is 140℃-180℃, for example, it can be 140℃, 150℃, 160℃, 170℃ or 180℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0045] Preferably, the pressure of hot pressing in step (2) is 150MPa-300MPa, for example, it can be 150MPa, 200MPa, 250MPa or 300MPa, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0046] Preferably, the hot pressing time in step (2) is 4 min to 6 min, for example, it can be 4 min, 4.5 min, 5 min, 5.5 min or 6 min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0047] The hot pressing described in this invention is carried out at high temperature and high pressure, causing the layers to melt together. After hot pressing, the layers are held under pressure and cooled to room temperature (25-30°C).

[0048] In a second aspect, the present invention provides a dry gradient negative electrode sheet, which is prepared by the preparation method described in the first aspect.

[0049] Thirdly, the present invention provides a battery comprising a dry gradient negative electrode as described in the second aspect.

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] (1) The binder of the bottom layer mixture of the present invention is selected from polyethylene materials, which have high melt viscosity and form a strong and tough three-dimensional network structure after hot pressing. It can not only provide a bonding force far exceeding that of traditional binders, but also effectively buffer the volume expansion stress of the bottom silicon particles. The middle layer mixture uses high-capacity negative electrode material and fibrous binder to ensure the overall capacity of the electrode and form a three-dimensional network structure in advance. The surface layer mixture uses fast-charging negative electrode material, solid electrolyte material and fibrous binder. The solid electrolyte material further improves the interfacial dynamics and constructs a fast and stable electrochemical reaction interface for fast charging. The novel negative electrode material enhances fast-charging performance, and the fibrous binder constructs an efficient electron transport framework. The three-layer specific composition combined with the one-time hot-pressing preparation process of this invention can produce a dry gradient negative electrode sheet. Furthermore, since the bottom layer mixture, the middle layer mixture, and the surface layer mixture are not pre-rolled to form a self-supporting film, the melting and flow of polyethylene materials, the full extension of the fibrous binder network, and the mutual diffusion of interlayer materials occur simultaneously, ultimately achieving seamless fusion of the three layers at the microscale. No obvious boundary line can be observed in the three-layer structure, which not only improves the interlayer bonding force but also significantly reduces the charge transfer impedance of the electrode sheet.

[0052] (2) The overall charge transfer impedance (Rct) of the dry gradient negative electrode obtained by the present invention is less than 15Ω, which is more than 30% lower than that of wet double-layer coating. At the same time, the battery achieves high energy density (>355Wh / kg), high rate (capacity retention rate of more than 88% after 3C charging) and long cycle (capacity retention rate of more than 84% after 1000 cycles), and does not require toxic solvents, reducing the energy consumption of electrode preparation by more than 40%. Detailed Implementation

[0053] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0054] Example 1

[0055] This embodiment provides a method for preparing a dry gradient negative electrode sheet, the method comprising the following steps:

[0056] (1) 20 parts of nano Si (particle size D50 is 100nm), 10 parts of conductive carbon black and 70 parts of UHMWPE (molecular weight is 2.5 million) were dry mixed evenly in a high-speed mixer to obtain the bottom mixture;

[0057] 94 portions of high-compact artificial graphite with a D50 of 15 μm (compacted density of 1.75 g / cm³) were subjected to high-compaction processing. 31 part conductive carbon black and 5 parts PTFE dry powder were subjected to air jet mill (8000 rpm, 3 min) to fully fibrillate them to obtain an intermediate layer mixture;

[0058] 85 parts of soft carbon material (particle size D50 of 10 μm), 8 parts of LLZO (particle size D50 of 150 nm), 2 parts of conductive carbon black and 5 parts of PTFE dry powder were passed through an air jet mill (8000 rpm, 3 min) to fully fibrillate them, and a surface mixture was obtained.

[0059] (2) The bottom layer mixture, the middle layer mixture and the surface layer mixture are uniformly deposited on the surface of an 8μm thick copper foil in sequence according to the thickness ratio of 1:6:1.5 through a three-layer co-extrusion die. The deposited electrode is hot-pressed for 5 minutes at 160℃ and 250MPa pressure, and then cooled to room temperature to obtain the dry gradient negative electrode.

[0060] Example 2

[0061] This embodiment provides a method for preparing a dry gradient negative electrode sheet, the method comprising the following steps:

[0062] (1) 40 parts of nano Si (particle size D50 of 200 nm), 15 parts of conductive carbon black and 45 parts of UHMWPE (molecular weight of 2 million) were dry-mixed evenly in a high-speed mixer to obtain the bottom mixture;

[0063] 95 parts of high-pressure compacted artificial graphite with a D50 of 10 μm (compacted density of 1.73 g / cm³) were used. 3 3 parts conductive carbon black and 2 parts PTFE dry powder were subjected to an air jet mill (10,000 rpm, 5 min) to fully fibrillate them, thus obtaining an intermediate layer mixture;

[0064] 89 parts of soft carbon material (particle size D50 of 5 μm), 5 parts of LLZO (particle size D50 of 50 nm), 1 part of conductive carbon black and 5 parts of PTFE dry powder were subjected to air jet mill (speed 10000 rpm, processing for 5 min) to fully fibrillate them and obtain surface mixture.

[0065] (2) The bottom layer mixture, the middle layer mixture and the surface layer mixture are uniformly deposited on the surface of an 8μm thick copper foil in a thickness ratio of 1:5:1 through a three-layer co-extrusion die. The deposited electrode is hot-pressed for 4 minutes at 180℃ and 150MPa pressure, and then cooled to room temperature to obtain the dry gradient negative electrode.

[0066] Example 3

[0067] This embodiment provides a method for preparing a dry gradient negative electrode sheet, the method comprising the following steps:

[0068] (1) 20 parts of nano Si (particle size D50 is 50nm), 5 parts of conductive carbon black and 75 parts of UHMWPE (molecular weight is 2.5 million) were dry mixed evenly in a high-speed mixer to obtain the bottom mixture;

[0069] 92 portions of high-compact artificial graphite with a D50 of 20 μm (compacted density of 1.78 g / cm³) were subjected to high-compact compaction. 3 3 parts conductive carbon black and 5 parts PTFE dry powder were subjected to an air jet mill (10,000 rpm, 5 min) to fully fibrillate them, thus obtaining an intermediate layer mixture;

[0070] 85 parts of soft carbon material (particle size D50 of 15 μm), 10 parts of LLZO (particle size D50 of 50 nm-200 nm), 3 parts of conductive carbon black and 2 parts of PTFE dry powder were passed through an air jet mill (10,000 rpm, 5 min) to fully fibrillate it, and a surface mixture was obtained.

[0071] (2) The bottom layer mixture, the middle layer mixture and the surface layer mixture are uniformly deposited on the surface of an 8μm thick copper foil in a thickness ratio of 1:8:2 through a three-layer co-extrusion die. The deposited electrode is hot-pressed for 6 minutes at 140℃ and 300MPa pressure, and then cooled to room temperature to obtain the dry gradient negative electrode.

[0072] Example 4

[0073] This embodiment provides a method for preparing a dry gradient negative electrode sheet. The preparation method is the same as in Example 1, except that 50 parts of nano-Si (particle size D50 is 100nm), 15 parts of conductive carbon black and 35 parts of UHMWPE (molecular weight is 2.5 million) are dry mixed evenly in a high-speed mixer to obtain a bottom mixture.

[0074] Example 5

[0075] This embodiment provides a method for preparing a dry gradient negative electrode sheet. The preparation method is the same as in Example 1, except that 15 parts of nano-Si (particle size D50 is 100nm), 5 parts of conductive carbon black and 80 parts of UHMWPE (molecular weight is 2.5 million) are dry mixed evenly in a high-speed mixer to obtain a bottom mixture.

[0076] Example 6

[0077] This embodiment provides a method for preparing a dry gradient negative electrode sheet. The preparation method is the same as in Example 1, except that when preparing the surface mixture, LLZO is 3 parts and soft carbon material is 90 parts.

[0078] Example 7

[0079] This embodiment provides a method for preparing a dry gradient negative electrode sheet. Except for the preparation of the surface mixture, in which 15 parts of LLZO, 80 parts of soft carbon material, and 3 parts of PTFE dry powder are used, the preparation method is the same as in Example 1.

[0080] Comparative Example 1

[0081] This comparative example provides a method for preparing a negative electrode sheet, the method comprising the following steps:

[0082] High-compact artificial graphite with a D50 of 15 μm and a compaction density of 1.75 g / cm³ was used. 3 Conductive carbon black, CMC (carboxymethyl cellulose), and SBR (styrene-butadiene rubber) were added to deionized water at a mass ratio of 95.5:1.0:1.5:2.0 and stirred evenly to prepare a slurry with a solid content controlled at 45%. The slurry was coated on the surface of an 8μm thick copper foil, dried, and rolled to the same total thickness as in Example 1 to obtain the negative electrode sheet.

[0083] Comparative Example 2

[0084] This comparative example provides a method for preparing a negative electrode sheet. The preparation method is the same as in Example 1, except that the bottom layer mixture and the surface layer mixture are not prepared and the intermediate layer mixture is directly placed on the surface of an 8μm thick copper foil. The total thickness of the electrode sheet is controlled to be the same as in Example 1.

[0085] Comparative Example 3

[0086] This comparative example provides a method for preparing a negative electrode sheet, the method comprising the following steps: 94 parts of high-pressure compacted artificial graphite with a D50 of 15 μm (compacted density of 1.75 g / cm³) 3 1 part conductive carbon black and 5 parts PTFE dry powder were fully fibrillated by air jet mill (8000 rpm, 3 min) to obtain a mixture. The mixture was rolled to obtain a self-supporting film. Another self-supporting film was prepared by the same method. The two self-supporting films were stacked on the surface of an 8 μm thick copper foil and hot-pressed at 160°C and 5 MPa for 30 seconds to obtain the negative electrode sheet.

[0087] Comparative Example 4

[0088] This comparative example provides a method for preparing a negative electrode sheet. The preparation method is the same as in Example 1, except that the bottom layer mixture, the intermediate layer mixture and the surface layer mixture are rolled separately to obtain a bottom layer, an intermediate layer and a surface layer with a thickness ratio of 1:6:1.5, and then the bottom layer, the intermediate layer and the surface layer are stacked on the surface of an 8μm thick copper foil and hot-pressed at a temperature of 160°C and a pressure of 5MPa for 30 seconds.

[0089] Comparative Example 5

[0090] This comparative example provides a method for preparing a negative electrode sheet. The preparation method is the same as in Example 1, except that the PTFE dry powder is replaced by UHMWPE with a molecular weight of 2.5 million.

[0091] Comparative Example 6

[0092] This comparative example provides a method for preparing a negative electrode sheet. The preparation method is the same as in Example 1, except that UHMWPE is replaced by PTFE dry powder.

[0093] The negative electrode sheets obtained in the above embodiments and comparative examples were subjected to the following performance tests:

[0094] (1) SEM interface morphology observation: The cross section of the negative electrode sheet was polished by ion beam, and the interface bonding between the three layers was observed by scanning electron microscope.

[0095] (2) EIS impedance test: Assemble a CR2032 coin cell with the negative electrode sheet and perform EIS test on an electrochemical workstation. The frequency range is 0.1Hz-100kHz and the amplitude is 5mV. The charge transfer impedance Rct is obtained by fitting.

[0096] (3) Peel strength test: Use a universal testing machine to test the 180° peel strength between the negative electrode coating and the copper foil.

[0097] (4) Full battery performance test: The negative electrode sheet and the high nickel positive electrode are matched and assembled into an 18650 battery. The initial discharge capacity at 0.2C, the capacity retention rate at 3C rate discharge, and the capacity retention rate after 1000 cycles at 1C / 1C are tested. The charging and discharging conditions are: the charging and discharging voltage range is 2.5V-4.2V, and the test is carried out at 25±2℃.

[0098] The test results are shown in Table 1 below:

[0099] Table 1

[0100]

[0101] As can be seen from Table 1 above:

[0102] As shown in Example 1 and Comparative Example 1, the electrode obtained by the dry process of the present invention has excellent interlayer fusion with no obvious boundaries, high peel strength, low charge transfer impedance, and excellent electrochemical performance. As shown in Example 1 and Comparative Example 2, Comparative Example 2 is a single-layer structure without the gradient design of the present invention, which leads to a decrease in the overall electrochemical performance of the battery. As shown in Example 1 and Comparative Example 3, although Comparative Example 3 is a double-layer structure, the composition of the double-layer structure is the same, and it also lacks the gradient design of the present invention, resulting in a decrease in performance compared to Example 1. As shown in Example 1 and Comparative Example 4, compared with the traditional dry preparation process, the present invention does not roll the bottom layer mixture, intermediate layer mixture, and surface layer mixture before hot pressing, and performs hot pressing under high temperature and high pressure. Pressure can enable the layers to fuse together, improve the bonding strength between the layers, reduce charge transfer impedance, and thus improve the electrochemical performance of the battery. As shown in Example 1 and Comparative Examples 5-6, the bottom layer mixture of the present invention uses a polyethylene binder, and the middle layer mixture and the surface layer mixture use a fibrous binder, which can promote the fusion of the layers and the extension of the bonding network, thereby promoting interface fusion. As shown in Example 1 and Examples 4-5, the amount of binder added to the bottom layer mixture of the present invention affects its function, and is preferably within a specific range. As shown in Example 1 and Examples 6-7, the amount of solid electrolyte material added to the surface layer mixture of the present invention affects charge transfer impedance, etc., and is preferably added within a specific range.

[0103] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a dry gradient negative electrode sheet, characterized in that, The preparation method includes the following steps: (1) The silicon-based material, the first conductive agent and the first binder are mixed to obtain the bottom mixture; A mixture of a capacity-type negative electrode material, a second conductive agent, and a second binder is fibroinated to obtain an intermediate layer mixture. The fast-charging negative electrode material, the third conductive agent, the third binder, and the solid electrolyte material are subjected to fibrous treatment to obtain a surface mixture; Wherein, the first adhesive includes a polyethylene material, and the second and third adhesives each independently include a fiber-reducible adhesive; (2) A bottom layer mixture, an intermediate layer mixture and a surface layer mixture are sequentially stacked on at least one side of the current collector to obtain a stacked negative electrode sheet. The stacked negative electrode sheet is hot-pressed to obtain the dry gradient negative electrode sheet.

2. The preparation method according to claim 1, characterized in that, Step (1) The first adhesive comprises ultra-high molecular weight polyethylene with a viscosity-average molecular weight of more than 1 million; And / or, by mass, the first adhesive in the bottom mixture of step (1) is 45-75 parts; And / or, by mass, in the bottom mixture of step (1), the amount of silicon-based material is 20-40 parts and the amount of the first conductive agent is 5-15 parts.

3. The preparation method according to claim 1 or 2, characterized in that, By mass, the solid electrolyte material in the surface mixture of step (1) is 5-10 parts; And / or, by mass, in the surface mixture of step (1), the fast-charging negative electrode material is 80-90 parts, the third conductive agent is 1-3 parts, and the third binder is 2-5 parts.

4. The preparation method according to claim 1 or 2, characterized in that, The solid electrolyte material mentioned in step (1) includes a garnet-type solid electrolyte material; And / or, the particle size D50 of the solid electrolyte material in step (1) is 50nm-200nm; And / or, the fast-charging negative electrode material in step (1) includes soft carbon material; And / or, the particle size D50 of the fast-charging negative electrode material in step (1) is 5μm-15μm; And / or, the fiberizable adhesive of step (1) includes polytetrafluoroethylene.

5. The preparation method according to claim 1 or 2, characterized in that, By mass, the amount of the capacity-type negative electrode material in the intermediate layer mixture in step (1) is 90-95 parts; And / or, by mass, the second conductive agent in the intermediate layer mixture of step (1) is 1 to 3 parts; And / or, by mass, the second adhesive in the intermediate layer mixture of step (1) is 2 to 5 parts.

6. The preparation method according to claim 1 or 2, characterized in that, The capacity-type negative electrode material in step (1) includes high-compact graphite material, wherein the compaction density of the high-compact graphite material is ≥1.7 g / cm³. 3 ; And / or, the particle size D50 of the capacity-type negative electrode material in step (1) is 10μm-20μm.

7. The preparation method according to claim 1 or 2, characterized in that, In step (2), the thickness ratio of the bottom layer mixture, the middle layer mixture, and the surface layer mixture in the stacked negative electrode sheet is 1:(5~8):(1~2); And / or, the method of sequentially stacking the bottom layer mixture, the intermediate layer mixture and the top layer mixture in step (2) includes a co-extrusion method or an electrostatic spraying method.

8. The preparation method according to claim 1 or 2, characterized in that, The hot pressing temperature in step (2) is 140℃-180℃; And / or, the pressure of the hot pressing in step (2) is 150MPa-300MPa; And / or, the hot pressing time in step (2) is 4 min-6 min.

9. A dry-process gradient negative electrode sheet, characterized in that, The dry gradient negative electrode sheet is prepared by the preparation method according to any one of claims 1-8.

10. A battery, characterized in that, The battery includes the dry-process gradient negative electrode as described in claim 9.

Images