Method for preparing multi-layer dry-method thick electrode with multiple porosity gradients

Abstract

The invention relates to a preparation method of a multi-porosity gradient multi-layer dry method thick electrode, which comprises the following steps of: stirring and mixing raw materials under the condition that the temperature is less than or equal to 18 DEG C to obtain a premix; quickly shearing the premix and raising the temperature to be less than or equal to 120 DEG C to obtain fiberized powder; the powder is subjected to one-time double-roller hot pressing to form a film continuously, the formed film continuously passes through double rollers and hot pressing to be thinned and then is rolled, multiple layers of films are obtained, and each layer of film has different porosity; the multiple layers of diaphragms with different porosities and the current collector are subjected to continuous double-sided lamination through double-roller hot pressing; the temperature of the double rollers is controlled to be smaller than or equal to 180 DEG C, the pressure of the double rollers is controlled to be 10-200 T, the laminating speed is controlled to be 1-10 m / min, the gap of the double rollers is controlled to be smaller than or equal to 1000 microns, and the multi-layer dry-method thick electrode which is composed of multiple layers of membranes is obtained after rolling, the compaction density of each layer of membrane is different, and the material proportion of each layer of membrane is the same; the method has the advantages that only the compaction density of the pole piece needs to be regulated and controlled, the material ratio does not need to be adjusted, and the interface property is more stable after multiple times of film forming.

Description

Technical Field

[0001] The present application relates to, and specifically relates to a method for preparing a multi-porous gradient multi-layer dry thick electrode. Background Art

[0002] Lithium-ion batteries have the advantages of high energy density, long cycle life, high voltage, low self-discharge rate, etc., and have been widely used in fields such as 3C products, electric vehicles, and energy storage systems. In recent years, the rapid development of large-scale energy storage and electric vehicles has put forward higher requirements for the energy density of lithium-ion batteries: the energy density of lithium-ion batteries reaches 400 Wh·kg -1 in 2025, and the energy density reaches 500 Wh·kg -1 by 2030. At present, there are mainly three ways to improve the energy density of batteries from the material system: (1) developing new high specific capacity battery systems, such as lithium-sulfur batteries and lithium-air batteries. However, the new high specific capacity systems still face huge challenges and still require a long time to develop; (2) developing high-voltage cathodes, such as high-voltage lithium cobaltate, ternary layered cathodes, lithium-rich manganese-based cathodes, etc. High-voltage cathodes can significantly improve the energy density of batteries, but their poor cycle stability and extremely high requirements for electrolytes and binders limit their development; (3) thick electrode architecture design, that is, without changing the battery chemical system, increasing the content of active substances per unit volume of the battery, thereby increasing the energy density of the battery system. Compared with the previous two ways, the thick electrode architecture design has the advantage of being compatible with various electrode materials. Therefore, improving the energy density of batteries through thick electrode architecture design has received extensive attention.

[0003] Constructing a thick electrode can reduce the content of inactive substances at the battery device level, improve the energy density of the battery, and reduce the cost of the battery. However, the slow ion and electron transport and poor mechanical stability of traditional thick electrodes lead to poor rate performance and cycle performance. The charge transport in a porous electrode mainly has four steps: (1) the transport of lithium ions in the electrolyte; (2) the charge transport at the interface; (3) the diffusion of lithium ions in the electrode bulk; (4) the charge transport at the interface between the current collector and the electrode; therefore, the impedance in a thick electrode can be divided into corresponding four parts: electron impedance, charge transport impedance, bulk impedance of the electrolyte, and lithium ion transport impedance in the porous electrode; in a thin electrode battery, the charge transport impedance is the rate-determining step of the kinetic reaction of lithium-ion batteries, and as the electrode thickness increases, the lithium ion transport impedance in the porous electrode gradually becomes the rate-determining step of the kinetic behavior in lithium-ion batteries.

[0004] Therefore, to simultaneously improve the energy density and power density of lithium-ion batteries, it is necessary to construct a high-power thick electrode with efficient lithium-ion transport channels and a good electron conduction network. Currently, the main strategy of researchers is to construct a thick electrode with a low tortuosity and straight through-holes to promote the lithium-ion transport in the thick electrode, reduce the concentration polarization, and improve the battery performance. The current preparation process of such thick electrodes is as follows:

[0005] ① The electrode manufacturing process by wet coating has been relatively mature and has been used until now due to its advantages of simplicity, low cost, and scalability. However, there are certain limitations for the future development of thick electrodes. On the one hand, the thickness of the electrode processed by coating is limited, and problems such as cracking and delamination are more likely to occur during the drying process of the thick electrode, making it difficult to further increase the active material loading. On the other hand, the electrode material particles are randomly arranged, and the controllability of the pore structure is low, resulting in a higher tortuosity after the electrode thickness increases, which is not conducive to the rapid transport of lithium ions. Therefore, developing a suitable manufacturing process is an important way to achieve the precise design of the structure of the thick electrode of lithium-ion batteries.

[0006] ② Multi-layer electrode: A high loading amount will lead to an increase in the specific surface area. However, the solvent evaporation during the traditional wet drying process causes serious binder migration behavior. First, the greater the coating thickness, the more difficult it is for the existing binder system to ensure that the electrode has sufficient toughness, and the electrode is more likely to crack during the drying process. Second, as the solvent evaporation time increases, the probability of uneven spatial distribution of each component in the electrode also increases, which has a negative impact on the battery performance. The migration of the binder hinders the transport of the electrolyte in the electrode, thereby deteriorating the electrode resistance and the electrochemical performance of the corresponding high-energy density lithium-ion battery. In addition, the binder content near the interface between the electrode film and the current collector is relatively small, and the binder migration behavior reduces the adhesion strength between the electrode film and the current collector. Therefore, it is expected to form a thick electrode (for example, a thick positive electrode / cathode or negative electrode / anode) that can be adapted to the processing in the existing manufacturing process for an electrochemical cell unit or battery, providing a higher energy density to increase the storage capacity and / or reduce the size of the battery pack while maintaining a similar cycle life to other lithium-ion battery packs.

[0007] ③ Dry electrode: The traditional lithium-ion battery electrode sheet uses the wet coating process. In this process, the active material, conductive carbon, and binder are fully mixed and uniformly dissolved in an organic solvent. The well-stirred slurry is coated on a metal current collector and then dried and rolled to reduce the thickness to obtain a pole piece with a suitable thickness.

[0008] However, the wet coating process has certain drawbacks. A large amount of organic solvent N-methylpyrrolidone (NMP) is required in the slurry preparation process, which not only increases the cost of electrode production during the drying process, but also causes environmental pollution due to excessive evaporation of NMP. The dry electrode process aims to solve the above problems. Currently, dry electrodes are mainly divided into two categories: binder fibrillation method and electrostatic spraying method, among which binder fibrillation is the mainstream. The binder fibrillation method is to mix the active material powder and the conductive agent, then add the binder, and then apply an external high shear force to the dry mixture to fibrillate the binder and bond the electrode film powder, and finally extrude the mixture to form a self-supporting film. The electrostatic spraying method is to premix the active material, the conductive agent and the binder particles with high-pressure gas, make the powder negatively charged under the action of an electrostatic spray gun and spray it onto the metal foil current collector with a positive charge, and then thermally press the current collector carrying the binder. After the binder melts, it will adhere to other powders and be extruded into a self-supporting film. The electrostatic spraying method performs worse than the binder fibrillation method in terms of subsequent processability, adhesion stability, electrode flexibility and durability. Summary of the Invention

[0009] In view of the above deficiencies of the prior art, the present application provides a preparation method of a multi-porous gradient multi-layer dry thick electrode that only needs to adjust the compaction density of the electrode, does not need to adjust the material ratio, and has a more stable interfacial property after multiple film formations.

[0010] To solve the above technical problems, the technical solution adopted in the present application is: a preparation method of a multi-porous gradient multi-layer dry thick electrode, comprising:

[0011] (1) Powder premixing: Under the condition of controlling the temperature ≤ 18°C, the active material powder, the binder and the conductive agent are stirred and mixed in proportion to obtain a premixed material;

[0012] (2) Powder fibrillation: Without controlling the temperature, the premixed material obtained in step (1) is rapidly sheared and heated to ≤ 120°C. After reaching the specified temperature (a temperature ≤ 120°C), the fibrillated powder material is obtained;

[0013] (3) Film formation and thinning: The fibrillated powder material obtained in step (2) is subjected to one-time roll-to-roll hot pressing for continuous film formation. The formed film is continuously passed through the rolls, thermally pressed and thinned, and then wound up to obtain multi-layer film sheets, and each layer of film sheet has a different porosity;

[0014] 4) Laminating the diaphragm with the current collector: Continuously double-side laminate the multi-layer diaphragms with different porosities obtained in step (3) and the current collector by roll-to-roll hot pressing; control the roll temperature at ≤180 °C, the roll pressure at 10 - 200 T, the lamination speed at 1 - 10 m / min, the roll gap at ≤1000 μm. After winding, a multi-layer dry thick electrode composed of multi-layer diaphragms with different compaction densities for each layer but the same material ratio for each layer is obtained.

[0015] Further, the active material powder includes, but is not limited to, cathode active materials: single components or mixtures thereof such as lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium cobalt oxide, etc.; anode materials: single components or mixtures thereof such as graphite, silicon carbon, etc.

[0016] Further, the binder includes, but is not limited to, single components or mixtures thereof such as PTFE (polytetrafluoroethylene), PVDF (polyvinylidene fluoride), CMC (carboxymethyl cellulose), etc.

[0017] Further, the conductive agent includes, but is not limited to, single components or mixtures thereof such as carbon black, carbon nanotubes, graphene, etc.

[0018] Further, the mass percentages of the active material powder, binder, and conductive agent are as follows: active material 80% - 97%, binder 1% - 20%, conductive agent 0% - 10%.

[0019] Further, the film formation in step (3) specifically includes: controlling the roll temperature at ≤150 °C, the roll pressure at 5 - 20 T, the film formation speed at 1 - 10 m / min, the roll differential ratio at 1:0.9 - 1:3, and the roll gap at ≤300 μm

[0020] Further, the thinning in step (3) specifically includes: controlling the roll temperature at ≤150 °C, the roll pressure at 5 - 50 T, the thinning speed at 1 - 10 m / min, the roll differential ratio at 1:0.9 - 1:3, and the roll gap at ≤200 μm.

[0021] Further, the double-side lamination means that multi-layer diaphragms with different porosities are provided on both surfaces oppositely arranged along the thickness direction of the current collector.

[0022] Furthermore, for the multi-layer diaphragms with different porosities, starting from the two surfaces oppositely arranged along the thickness direction of the current collector and moving outwards, their porosities gradually increase step by step.

[0023] Furthermore, the number of diaphragm layers arranged along the thickness direction of the current collector is the same, and the diaphragms symmetrically arranged along the thickness direction of the current collector have the same porosity.

[0024] Further, three diaphragms are provided in sequence from the inside to the outside along the thickness direction of the current collector, and the porosity of the three diaphragms on the same side is divided into three levels: large, medium, and small.

[0025] In step (3) of the present application, the obtained multi-layer diaphragms have different porosities; specifically, each layer of diaphragm is obtained by adopting the preparation process from step (1) to (3). The difference is that the porosity of each layer of diaphragm obtained by each time adopting the preparation process from step (1) to (3) is different; finally, through step (4), on the two opposite surfaces arranged in the thickness direction of the current collector, according to a certain porosity arrangement order, the multi-layer diaphragms with different porosities obtained by the preparation process from step (1) to (3) are compounded onto the current collector to obtain the final multi-layer dry thick electrode with a gradient distribution of porosity.

[0026] Advantages and beneficial effects of the present application:

[0027] 1. Through the dry composite process of the multi-layer diaphragms with different porosities and the current collector in the present application, a pole piece product with a gradient distribution of different porosities is obtained; the gradient distribution of porosity refers to a layered structure with two or more layers in the thickness direction of the electrode, and the porosity of each layer is different; from the basic principle of the battery, whether in the discharge or charge process, from the separator to the current collector in the thickness direction of the positive or negative pole piece, the number of lithium ions transported (or the lithium ion current density, electrolyte current density) is decreasing, while the number of electrons transported (or the electron current density, electrode current density) is increasing; at the separator, the lithium ion current density is the largest, and the electron current density is 0; at the interface between the coating and the current collector, the lithium ion current density is 0, and the electron current density is the largest; therefore, the larger the porosity of the pole piece, the smaller the pore tortuosity, the higher the effective diffusion coefficient and effective conductivity of lithium ions, and the smaller the ion impedance; however, too high porosity will reduce the volumetric energy density of the battery and also reduce the electronic conductivity of the electrode; and the design of the gradient distribution of porosity in the thickness direction of the electrode can optimize the ion transport performance on the basis of ensuring the total porosity remains unchanged; therefore, by regulating the gradient distribution of the porous structure of the pole piece in the present application, a high-porosity structure in the upper layer and a high-compaction density structure in the lower layer are realized, perfectly taking into account high energy density and super fast charging; an electrode with a gradually decreasing porosity from the separator to the current collector can reduce the ion resistance without damaging the electronic conductivity and volumetric energy density, thereby improving the rate performance.

[0028] 2. The electrode products with different porosity distribution gradients obtained by the preparation method of the present application, wherein the preparation of multiple layers of membranes with different porosities is to use active materials of the same particle size to make dry membranes with different porosities respectively, and the porosity of the same dry membrane is the same / similar, and then the dry membranes with different porosities are made into dry thick electrodes according to a predetermined porosity gradient (the porosity of each layer of dry membrane is single), in order to obtain dry thick electrodes with porosity gradient; the porosity of the electrode / membrane is regulated by compaction, and the ion channels between the electrode / membrane materials are regulated, which is a macroscopic regulation (principle: the larger the porosity, the smaller the pore tortuosity, the higher the effective diffusion coefficient and effective conductivity of lithium ions, and the smaller the ionic impedance; however, too high a porosity will reduce the volume energy density of the battery and the electronic conductivity of the electrode; and the porosity in the thickness direction of the electrode The porosity distribution gradient design can optimize the ion transmission performance on the basis of ensuring that the total porosity remains unchanged); therefore, the method of the present application does not require changes in the ratio of each layer of the membrane, nor does it require the use of active material ratios of different particle sizes for regulation. The membrane formula and the particle size of the active material of each layer of the present application remain unchanged, and the porosity of the single-layer dry membrane remains unchanged; therefore, the electrode prepared by this method has the following advantages: the same material can be continuously formed into a film at one time to produce multiple membranes with different porosity gradients. Different membranes only need to adjust the compaction density of the electrode, and there is no need to adjust the material ratio for multiple film formation; the interface properties of the porosity gradient electrode produced by the present application using the same material and ratio are more stable, while the electrode interface using different active substances and ratios in the prior art is more different, and side reactions (lithium precipitation, collapse, etc.) are more likely to occur in the battery system.

[0029] 3. In the process of compounding multiple layers of membranes with different porosities on both sides of the current collector, the first layer of membrane directly in contact with both sides of the current collector is compounded first, and then the second layer, the third layer... and more layers of membranes are sequentially laminated on the laminated structure obtained in the previous process as an adhesion layer; while the prior art mostly laminates dry membranes with different gradients first and then laminates them with the current collector, which is essentially different from the process. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Dry electrode sheets with multi-porosity gradients (a, b, c, and d are dry electrode membranes with different porosities, respectively).

[0031] Figure 2 Schematic diagram of a dry electrode sheet with a multi-porosity gradient.

[0032] Figure 3 Schematic diagram of a dry electrode membrane with large pores (large pores here are relative, and are compared with the porosity of other membranes attached to the same current collector).

[0033] Figure 4Schematic diagram of a dry electrode diaphragm with medium pores (here, the medium pores are relative, representing a relative comparison of the porosity with other diaphragms attached to the same current collector).

[0034] Figure 5 Schematic diagram of a dry electrode diaphragm with small pores (here, the small pores are relative, representing a relative comparison of the porosity with other diaphragms attached to the same current collector).

[0035] Figure 6 SEM image of the dry electrode diaphragm with small pores (Example 2, 15%).

[0036] Figure 7 SEM image of the dry electrode diaphragm with medium pores (Example 2, 20%).

[0037] Figure 8 SEM image of the dry electrode diaphragm with large pores (Example 2, 25%).

[0038] Figure 9 Schematic diagram of the mixture before (a) / after (b) fibrillation.

[0039] Figure 10 Schematic diagram of the film thinning process.

[0040] Figure 11 Schematic diagram of the lamination process.

[0041] Figure 12 Performance of Example 1 under fast charge (3C).

[0042] Figure 13 Performance of Example 2 under fast discharge (3C). Detailed implementation manners

[0043] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the embodiments and the accompanying drawings. Obviously, the described embodiments are only preferred embodiments, rather than all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention;

[0044] Example 1

[0045] This embodiment provides a method for preparing a multi-porous gradient multi-layer dry electrode. Based on a specific implementation, the method for preparing this dry electrode (graphite) is as follows: Graphite is used as the negative electrode active material, with a binder (PTFE:CMC weight ratio = 90:10) and a conductive agent (conductive graphite: dry carbon nanotube: dry graphene weight ratio = 70:20:10). The mass ratio of the active material: binder: conductive agent is 96:3:1. Add the negative electrode active material and the conductive agent into a blender, stir at 4000 rpm for 8 minutes to mix evenly, slow down the stirring to 200 rpm to cool down to 18 °C, then add the binder, control the temperature < 18 °C, stir at 1000 rpm for 10 min to mix evenly, then turn off the cooling system, and complete the fibrillation process when the natural shear heating reaches 70 °C at 4000 rpm; as Figure 9 shown, it is a schematic diagram before (a) / after (b) mixing and fibrillation.

[0046] Put the mixed powder into a one-time film-forming / thinning roll press. The film-forming roll gap is 180 μm, the thinning roll gap is 150 μm, the film-forming differential speed is 1:2, the thinning differential speed is 1:1, the film-forming pressure is 5T, and the thinning pressure is 10T. Perform one-time hot roll pressing / thinning at 110 °C to form a self-supporting film, obtaining a dry negative electrode film with a surface density of 18 mg / cm2, a thickness of approximately 140 μm, a compaction density of approximately 1.29 g / cm3, and a porosity of 40% (as Figure 3 shown, it can be called a schematic diagram of a dry electrode film with large pores); for the specific process flow, refer to Figure 10 shown.

[0047] Similarly, control the film-forming roll gap to be 150 μm, the thinning roll gap to be 100 μm, the film-forming differential speed to be 1:2, the thinning differential speed to be 1:1, the film-forming pressure to be 5T, and the thinning pressure to be 10T. Perform one-time hot roll pressing / thinning at 110 °C to form a self-supporting film, obtaining a dry negative electrode film with a surface density of 18 mg / cm2, a thickness of approximately 120 μm, a compaction density of approximately 1.5 g / cm3, and a porosity of 30%, as Figure 4 shown, it can be called a schematic diagram of a dry electrode film with medium pores.

[0048] Similarly, control the film-forming roll gap to be 120 μm, the thinning roll gap to be 80 μm, the film-forming differential speed to be 1:2, the thinning differential speed to be 1:1, the film-forming pressure to be 5T, and the thinning pressure to be 10T. Perform one-time hot roll pressing / thinning at 110 °C to form a self-supporting film, obtaining a dry negative electrode film with a surface density of 18 mg / cm2, a thickness of approximately 100 μm, a compaction density of approximately 1.8 g / cm3, and a porosity of 25%, as Figure 5 shown, it can be called a schematic diagram of a dry electrode film with small pores.

[0049] The dry anode films with different porosities described above are thermally roll-compounded with a coated current collector (coated copper foil) at 130 °C multiple times. The coated current collector includes a copper foil and conductive layers provided on both sides of the copper foil (the conductive layer here is the general conductive layer of the carbon-coated / current collector, and the conventional conductive layer of the electrode sheet), and the single-sided thickness of the conductive layer is 1 μm; for the specific process flow, refer to the appendix Figure 11 as shown.

[0050] First compounding: Using the dry anode film with a porosity of 25% and the coated copper foil (current collector) prepared above, with a thermal roll-compounding roll gap of 190 μm and a roll-compounding pressure of 20 T, a dry anode sheet with a thickness of about 200 μm is obtained, and the compaction density is 1.88 g / cm 3 ;

[0051] Second compounding: Using the dry anode film with a porosity of 30% and the dry anode sheet after the first compounding, with a thermal roll-compounding roll gap of 400 μm and a roll-compounding pressure of 20 T, a dry anode sheet with a thickness of about 420 μm is obtained, and the compaction density is 1.75 g / cm 3 ;

[0052] Third compounding: Using the dry anode film with a porosity of 35% and the dry anode sheet after the second compounding, with a thermal roll-compounding roll gap of 630 μm and a roll-compounding pressure of 20 T, a dry anode sheet with a thickness of about 660 μm is obtained, and the compaction density is 1.66 g / cm 3 ;

[0053] Finally, the dry anode sheets with different pore gradient (porosity from the inside to the outside: 25% → 30% → 40%, specifically refer to 3 as shown, which is a schematic diagram of a dry electrode sheet with a multi-pore gradient) with a compaction density of 1.66 g / cm Figure 2 and a thickness of about 660 μm are cut into pieces.

[0054] As Figure 1 shown is a schematic diagram of the cross-sectional structure layer of a dry electrode sheet with a multi-pore gradient, where a, b, c, and d respectively represent dry electrode membrane sheets with different porosities (four layers), and the middle is the current collector structure.

[0055] Figure 12 For the multiple charge (3C) performance of the anode sheet prepared in Example 1, the performance of the dry anode sheet obtained in this Example 1 is as follows. By adjusting the distribution of the pore rate gradient of the electrode sheet, a large-porosity structure on the outer layer (separator side) and a small-porosity structure on the inner layer (current collector side) are realized. From Figure 12It can be intuitively seen that the 3C fast charging performance of Example 1 is better than that of Control Group 1. The performance of the dry electrode sheet obtained in this example is shown in Table 1 below. By regulating the gradient distribution of the porous structure of the electrode sheet, a high-porosity structure in the upper layer and a high-compaction-density structure in the lower layer are achieved, perfectly balancing high energy density and super fast charging. An electrode with a gradually decreasing porosity from the separator to the current collector can reduce the ionic resistance without compromising the electronic conductivity and volumetric energy density, thereby improving the rate performance.

[0056] Table 1 Performance of the dry electrode sheet obtained in Example 1

[0057]

[0058] Example 1: Double-sided lamination, with the porosity of the inner and outer layer films being 25% → 30% → 35% respectively, and a total of 6 layers on both sides;

[0059] Control Group: Double-sided lamination, with the porosity of each inner and outer layer film being 30%, and a total of 6 layers on both sides, and each layer is the same.

[0060] Example 2

[0061] This example provides a preparation method for a multi-porosity gradient multi-layer dry electrode sheet. Based on a specific implementation, the preparation method of this dry electrode sheet (dry cathode) is as follows: Using high-nickel ternary as the cathode active material (here, high-nickel ternary with a particle size of 10 μm is used), binder (PTFE:PVDF = 95:5), and conductive agent (conductive graphite: carbon nanotube dry powder: graphene dry powder = 80:15:5), the mass ratios of the active material: binder: conductive agent are 95:4:1 respectively. Add the cathode active material and the conductive agent into a blender, stir at 4000 rpm for 10 minutes to mix evenly, slow down the stirring speed to 200 rpm to cool down to 18 °C, then add the binder, control the temperature < 18 °C, stir at 1000 rpm for 10 min to mix evenly, then turn off the cooling system, and stir at 4000 rpm to shear and heat up naturally to 85 °C to complete the fibrillation process.

[0062] Put the mixed powder into a one-step film-forming / thinning roll press. The film-forming roll gap is 120 μm, the thinning roll gap is 80 μm, the film-forming differential speed is 1:2, the thinning differential speed is 1:1, the film-forming pressure is 5T, and the thinning pressure is 10T. Perform one-step hot roll pressing / thinning at 120 °C to form a self-supporting film, and obtain a dry cathode film with a surface density of 30 mg / cm 2 , a thickness of about 90 μm, a compaction density of about 3.33 g / cm 3 , and a porosity of 15%. Specifically, as shown in the appendix Figure 6 This is the SEM image of the dry electrode film sheet with small pores here.

[0063] Similarly, control the film-forming roll gap to be 150 μm, the thinning roll gap to be 100 μm, the film-forming differential speed to be 1:2, the thinning differential speed to be 1:1, the film-forming pressure to be 5 T, and the thinning pressure to be 10 T. Perform one-time hot rolling / thinning at 120 °C to form a self-supporting film, obtaining a dry cathode film with a surface density of 30 mg / cm 2 , a thickness of approximately 120 μm, and a tap density of approximately 2.5 g / cm 3 , with a porosity of 20%; specifically, as shown in the appendix Figure 7 is the SEM image of the dry electrode film sheet with medium pores here.

[0064] Similarly, control the film-forming roll gap to be 180 μm, the thinning roll gap to be 130 μm, the film-forming differential speed to be 1:2, the thinning differential speed to be 1:1, the film-forming pressure to be 5 T, and the thinning pressure to be 10 T. Perform one-time hot rolling / thinning at 120 °C to form a self-supporting film, obtaining a dry cathode film with a surface density of 30 mg / cm 2 , a thickness of approximately 150 μm, and a tap density of approximately 2.0 g / cm 3 , with a porosity of 25%; specifically, as shown in the appendix Figure 8 is the SEM image of the dry electrode film sheet with large pores here.

[0065] Thermally roll and laminate the above dry cathode films with different porosities with a coated current collector (coated aluminum foil) multiple times at 130 °C. The coated current collector includes aluminum foil and conductive layers provided on both sides of the aluminum foil, and the single-sided thickness of the conductive layer is 1 μm.

[0066] First lamination: Use the dry cathode film with a porosity of 15% prepared above and the coated aluminum foil. At a hot rolling roll gap of 150 μm and a rolling pressure of 30 T, a dry cathode sheet with a thickness of approximately 170 μm and a tap density of 3.7 g / cm 3 is obtained;

[0067] Second lamination: Use the dry cathode film with a porosity of 20% prepared above and the dry cathode sheet after the first lamination. At a hot rolling roll gap of 340 μm and a rolling pressure of 30 T, a dry cathode sheet with a thickness of approximately 370 μm and a tap density of 3.3 g / cm 3 is obtained;

[0068] Third lamination: Use the dry cathode film with a porosity of 25% prepared above and the dry cathode sheet after the second lamination. At a hot rolling roll gap of 570 μm and a rolling pressure of 30 T, a dry cathode sheet with a thickness of approximately 600 μm and a tap density of 3.0 g / cm 3 is obtained;

[0069] Finally, with a tap density of 3.0 g / cm 3, the dry cathode sheets with different pore gradients (from inside to outside: 15% → 20% → 25%) and a thickness of about 600 μm were cut into pieces.

[0070] Figure 13 For the multiple charge (3C) performance of the electrode sheet obtained in Example 2, the performance of the dry cathode sheet obtained in this Example 2 is as follows. By regulating the distribution of the pore rate gradient of the electrode sheet, a large pore rate structure on the outer layer (separator side) and a small pore rate structure on the inner layer (current collector side) are realized. It can be Figure 13 intuitively seen from that the 3C multiple charge performance of Example 2 is better than that of Control Group 2. The performance of the dry electrode sheet obtained in this Example is as follows. By regulating the gradient distribution of the porous structure of the electrode sheet, a high pore rate structure on the upper layer and a high compaction density structure on the lower layer are realized, perfectly taking into account high energy density and super fast charging. An electrode with a gradually decreasing pore rate from the separator to the current collector can reduce the ionic resistance without damaging the

[0071] electron conductivity and the volume energy density, thereby improving the rate performance.

[0072] Specifically, refer to Table 2 below:

[0073] Table 2 Performance of the dry electrode sheet obtained in Example 2

[0074]

[0075] Example 2: Double-sided lamination, the pore rates of the inner to outer membrane sheets are 15% → 20% → 25% respectively, with a total of 6 layers on both sides; Control Group 2: Double-sided lamination, the pore rates of the inner to outer membrane sheets are all 20%, with a total of 6 layers on both sides, and each layer is the same.

Claims

1. A preparation method of a multi-porous gradient multi-layer dry thick electrode, characterized in that: Including: (1) Powder premixing: Under the condition of controlling the temperature ≤ 18°C, the active material powder, binder, and conductive agent are stirred and mixed in proportion to obtain a premix; (2) Powder fibrillation: Without controlling the temperature, the premix obtained in step (1) is rapidly sheared and heated to ≤ 120°C. After reaching the specified temperature, fibrillated powder is obtained; (3) Film forming and thinning: The fibrillated powder obtained in step (2) is subjected to one-time roll-to-roll hot pressing for continuous film forming. The formed film is continuously passed through the rolls, hot pressed and thinned, and then wound up to obtain a multi-layer film. Each layer of the film has a different porosity; (4) Laminating the film with the current collector: The multi-layer films with different porosities obtained in step (3) are continuously double-sided laminated with the current collector by roll-to-roll hot pressing; the roll temperature is controlled at ≤ 180°C, the roll pressure is controlled at 10 - 200T, the lamination speed is controlled at 1 - 10m / min, the roll gap is controlled at ≤ 1000μm. After winding up, a multi-layer dry thick electrode composed of multi-layer films is obtained, where the compaction density of each layer of the film is different but the material ratio of each layer of the film is the same.

2. The preparation method of the multi-porous gradient multi-layer dry thick electrode according to claim 1, wherein: The active material powder includes but is not limited to cathode active materials: single components or mixtures thereof such as lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium cobalt oxide, etc.; anode materials: single components or mixtures thereof such as graphite, silicon carbon, etc.

3. The preparation method of the multi-porous gradient multi-layer dry electrode according to claim 1, characterized in that: The binder includes but is not limited to single components or mixtures thereof in PTFE, PVDF, CMC.

4. The preparation method of the multi-porous gradient multi-layer dry electrode according to claim 1, wherein: The conductive agent includes but is not limited to single components or mixtures thereof in carbon black, carbon nanotubes, graphene.

5. The preparation method of the multi-porous gradient multi-layer dry electrode according to claim 1, characterized in that: The mass percentages of the active material powder, binder, and conductive agent are as follows: active material 80% - 97%, binder 1% - 20%, conductive agent 0% - 10%.

6. The preparation method of the multi-porous gradient multi-layer dry electrode according to claim 1, characterized in that: In the film forming described in step (3), the specific procedure includes: the roll temperature is controlled at ≤ 150°C, the roll pressure is controlled at 5 - 20T, the film forming speed is controlled at 1 - 10m / min, the roll differential ratio is controlled at 1:0.9 - 1:3, and the roll gap is controlled at ≤ 300μm.

7. The preparation method of the multi-porous gradient multi-layer dry electrode according to claim 1, characterized in that: In the thinning described in step (3), the specific procedure includes: the roll temperature is controlled at ≤ 150°C, the roll pressure is controlled at 5 - 50T, the thinning speed is controlled at 1 - 10m / min, the roll differential ratio is controlled at 1:0.9 - 1:3, and the roll gap is controlled at ≤ 200μm.

8. The preparation method of the multi-porous gradient multi-layer dry electrode according to claim 1, characterized in that: The double-sided lamination means that multi-layer films with different porosities are arranged on two surfaces oppositely arranged along the thickness direction of the current collector.

9. The preparation method of the multi-porous gradient multi-layer dry thick electrode according to claim 8, characterized in that: For the multi-layer films with different porosities, starting from the two surfaces oppositely arranged along the thickness direction of the current collector and outward, their porosities gradually increase step by step.

10. The preparation method of the multi-porous gradient multi-layer dry electrode according to claim 9, characterized in that: The number of layers of the films arranged on the two surfaces oppositely arranged along the thickness direction of the current collector is the same, and the films symmetrically arranged along the thickness direction of the current collector have the same porosity.

11. The preparation method of the multi-porous gradient multi-layer dry thick electrode according to claim 10, characterized in that: The films arranged successively from the inside to the outside along the thickness direction of the current collector are all arranged in three layers, and the porosities of the three layers of films on the same side are divided into three grades: large, medium, and small.

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