Multi-layer dry-method thick electrode with multiple active particle size gradients and preparation method of multi-layer dry-method thick electrode

Abstract

The invention relates to a multi-active particle size gradient multilayer dry-method thick electrode and a preparation method thereof, the structure of the dry-method thick electrode comprises a current collector, the current collector is provided with a first surface and a second surface which are oppositely arranged along the thickness direction, the first surface and the second surface are sequentially provided with a plurality of layers of dry-method electrode diaphragms from inside to outside, and the dry-method electrode diaphragms are arranged on the first surface and the second surface. The particle sizes of active substances in each layer of diaphragm of the multi-layer dry-method electrode diaphragm are different; the preparation method has the advantages that the electron / ion conductivity of the pole piece is improved, and the rate capability of the pole piece can also be improved.

Description

Technical Field

[0001] This application relates to the fields of primary and secondary batteries and supercapacitors, and particularly relates to a multi-layer dry thick electrode with a gradient of multi-active particle sizes and a preparation method thereof. Background Art

[0002] In recent years, due to their excellent electrochemical performance, lithium-ion batteries have become key energy carriers in key technical fields such as consumer electronics (3C products), new energy vehicles, and energy storage systems. With the rapid development of energy storage technologies, higher requirements have been put forward for the energy density of lithium-ion batteries. Although increasing the electrode thickness is a direct and effective means to improve the battery energy density, problems such as sluggish reaction kinetics and insufficient mechanical properties limit its development. However, the design of thick electrodes for lithium-ion batteries is a complex multi-objective optimization process that requires a comprehensive evaluation of multiple parameters including energy density, power density, cycle life, safety, electrochemical performance, and cost to successfully construct a thick electrode for a lithium battery with high energy density, high safety, and long life.

[0003] The research on thick electrodes involves complex engineering and scientific issues. First, it should have good mechanical properties to adapt to processes such as electrode stacking and winding. Second, an efficient ion and electron transport network should be constructed to accelerate the reaction kinetics of thick electrodes, reduce ohmic polarization and concentration polarization, and improve rate performance and cycle stability. Combining experimental methods and theoretical simulations to establish the structure-property relationship between the architecture design and electrochemical performance of thick electrodes is an important idea to promote the practical application of thick electrodes.

[0004] Therefore, to simultaneously improve the energy density and power density of lithium-ion batteries, a high-power thick electrode with an efficient lithium-ion transport channel needs to be constructed. Currently, the main strategy is to construct a thick electrode with a low tortuosity and straight through-channels to promote the lithium-ion transport in the thick electrode, reduce concentration polarization, and improve the battery performance.

[0005] ① Wet process: The electrode manufacturing process of wet coating has been relatively mature and has been used until now due to its advantages of simplicity, low cost, and scalability. However, it has certain limitations for the future development of thick electrodes: on the one hand, the thickness of the coated electrode is limited, and problems such as cracking and delamination are likely to occur during the drying process for thick electrodes, making it difficult to further increase the active material loading. On the other hand, the electrode material particles are randomly arranged, and the active particle sizes are relatively single, resulting in a high 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 thick electrodes for lithium-ion batteries.

[0006] ②Multi-layer coating: The slurries between the multi-layer coating layers have a certain degree of compatibility, which will affect the interfacial properties between the layers. At the same time, the solvent evaporation during the traditional wet drying process leads to negative behaviors such as severe binder migration. First, the greater the coating thickness, the more difficult it is for the existing binder system to

[0007] ensure that the electrode has sufficient toughness, and the electrode is more likely to crack during the drying process; second, due to the increase in solvent evaporation time, 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 leads to a decrease in the adhesion strength between the electrode film and the current collector. The multi-layer coating mainly includes sliding multi-layer coating, curtain multi-layer coating, and die extrusion multi-layer coating. The lithium-ion battery electrode generally adopts the third die extrusion multi-layer coating method. A uniform coating can only be obtained within a certain coating process window, that is, for a specific slurry system, there is a maximum possible coating speed at a certain coating film thickness. If the coating speed exceeds this value, the coating is unstable. At a certain coating speed, there is a minimum coating film thickness. If the thickness is smaller, the coating is unstable. Compared with single-layer coating, the process window range of double-layer coating is smaller. The main problems include: (1) The entrainment of air from the upstream direction is one of the main reasons for coating defects; (2) The mixing of the upper and lower layer slurries leads to coating instability. Therefore, the double-layer coating needs to optimize the coating process parameters and study the influence of the differences in the viscosity, surface tension, etc. of the two slurries on the coating quality.

[0008] ③Dry electrode: The electrode sheets of traditional lithium-ion batteries use a wet coating process. In this process, the active material, conductive carbon, and binder are fully mixed and then uniformly dissolved in an organic solvent. The well-stirred slurry is coated on a metal current collector, and then through drying and roll pressing to reduce the thickness, a pole piece with a suitable thickness is obtained. Currently, dry electrodes are mainly divided into two categories: binder fibrillation method and electrostatic spraying, among which the binder fibrillation method is the mainstream. In the binder fibrillation method, the active material powder and the conductive agent are mixed and then the binder is added. Then, an external high shear force is applied to the dry mixture to fibrillate the binder and then bond the electrode film powder. Finally, the mixture is extruded to form a self-supporting film. The electrostatic spraying method uses high-pressure gas to premix the active material, conductive agent, and binder particles. Under the action of an electrostatic spray gun, the powder is negatively charged and sprayed onto a metal foil current collector with a positive charge. Then, the current collector carrying the binder is hot-pressed. 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. However, the ion and electron transport network of the dry thick electrode is relatively single, the reaction kinetics of the thick electrode is poor, the ohmic polarization and concentration polarization are relatively serious, resulting in a decline in rate performance and cycle stability. Problems such as slow reaction kinetics and insufficient mechanical properties limit its development. Summary of the Invention

[0009] In view of the above deficiencies of the prior art, the present application provides a multi-active particle size gradient multi-layer dry thick electrode that can improve the electron / ion conductivity of the pole piece and also improve its rate performance.

[0010] To solve the above technical problems, the technical solution adopted in the present application is: A multi-active particle size gradient multi-layer dry thick electrode, the structure of the electrode includes a current collector, the current collector has a first surface and a second surface oppositely arranged in the thickness direction, and the first surface and the second surface are sequentially provided with multi-layer dry electrode membranes from the inside to the outside, and the particle sizes of the active materials in each layer of the multi-layer dry electrode membranes are different.

[0011] With the above structure, in the present application, by preparing dry electrode membranes with different particle sizes of active materials on two surfaces of the current collector oppositely arranged in the thickness direction, a dry electrode membrane with a gradient change in active particle size is covered on the current collector. The active particle size gradient refers to the structural design of the electrode membrane in which the particle sizes of the active particles in each layer of the multi-layer electrode membranes are different. Generally, smaller particles provide a higher specific surface area, a shorter lithium diffusion path inside the particles, and promote more complete utilization of each particle. However, when small particles are stacked together, the small size results in a more tortuous inter-particle diffusion path, which may limit Li +Diffusion; on the contrary, larger particles have a lower specific surface area, which results in longer intra-particle lithium diffusion paths. However, when stacked together, their larger size leads to less tortuous inter-particle diffusion paths, which should be beneficial for Li + Diffusion; for the positive electrode, small particles increase the reaction specific surface area and shorten the solid-phase lithium diffusion path, which is beneficial to the performance; for the graphite negative electrode, small particles increase the specific surface area, thereby increasing the SEI passivation layer m to hinder Li + Diffusion; moreover, the lithium intercalation in graphite particles occurs at the edge planes of graphite flakes. The interstitial space between large graphite particles increases, and these edge planes are more likely to receive lithium in large-grained graphite, resulting in Li + being intercalated into graphite faster, and the performance of large-grained graphite is better; therefore, with the above structure of the present application, by regulating the gradient distribution of the sizes of multi-active particles in the electrode sheet, a structure with large particle sizes on the graphite separator side and small particle sizes on the current collector side of the negative electrode, and a structure with small particle sizes on the separator side and large particle sizes on the current collector side of the positive electrode are realized, thereby improving the electron / ion conductivity of the electrode sheet and its rate performance.

[0012] Furthermore, the multi-layer dry electrode film sheet includes at least two layers. When the dry electrode film sheet is a positive electrode film sheet, the particle size of the active material in each layer of the dry electrode film sheet gradually decreases from the side close to the current collector to the outside; when the dry electrode film sheet is a negative electrode film sheet, the particle size of the active material in each layer of the dry electrode film sheet gradually increases from the side close to the current collector to the outside.

[0013] Furthermore, the multi-layer dry electrode film sheet includes at least three layers. When the dry electrode film sheet is a positive electrode film sheet, the particle size of the active material in each layer of the dry electrode film sheet gradually decreases from the side close to the current collector to the outside; when the dry electrode film sheet is a negative electrode film sheet, the particle size of the active material in each layer of the dry electrode film sheet gradually increases from the side close to the current collector to the outside.

[0014] Furthermore, the current collector is a current collector with conductive layers provided on the first surface and the second surface, and the single-sided thickness of the conductive layer is 0.5 - 5 μm.

[0015] Furthermore, the particle sizes of the active materials in the dry electrode film sheets provided at symmetric positions on the first surface and the second surface are the same.

[0016] The present application also provides a preparation method for a multi-layer dry thick electrode with a gradient of multi-active particle sizes. The preparation steps include:

[0017] (1) Powder premixing: Under the condition of controlling the temperature (≤18 °C), the active material powders with different particle sizes, the binder, and the conductive agent are respectively stirred and mixed in proportion to obtain premixed materials with different active particle sizes;

[0018] (2) Powder fibrillation: Without controlling the temperature, the premixes with different active particle sizes obtained in step (1) are respectively subjected to rapid shear heating (≤120 °C), and after reaching the specified temperature, fibrillated powders with different active particle sizes are obtained;

[0019] (3) Film formation and thinning: The powders with different active particle sizes obtained by fibrillation in step (2) are subjected to one-time double-roll hot pressing for continuous film formation. Film formation: The temperature of the double rolls is controlled at ≤150 °C, the pressure of the double rolls is controlled at 5 - 20 T, the film formation speed is controlled at 1 - 10 m / min, the differential speed ratio of the double rolls is controlled at 1:0.9 - 1:3, and the gap between the double rolls is controlled at ≤300 μm; The formed film is continuously passed through the double rolls, hot pressed and thinned, and then wound up. Specifically for thinning: The temperature of the double rolls is controlled at ≤150 °C, the pressure of the double rolls is controlled at 5 - 50 T, the thinning speed is controlled at 1 - 10 m / min, the differential speed ratio of the double rolls is controlled at 1:0.9 - 1:3, and the gap between the double rolls is controlled at ≤200 μm; Films with different active particle sizes are obtained; Then dry electrode films are obtained;

[0020] (4) Laminating the film with the current collector: The films with different active particle sizes obtained in step (3) are laminated on both sides with the current collector by double-roll hot pressing in a certain size gradient. Among them, the temperature of the double rolls is controlled at ≤180 °C, the pressure of the double rolls is controlled at 10 - 200 T, the lamination speed is controlled at 1 - 10 m / min, the gap between the double rolls is controlled at ≤1000 μm, and a dry electrode roll is obtained after winding up.

[0021] Further, the active material powders with different particle sizes in step (1) include, but are not limited to, cathode active materials within the following particle size ranges: lithium iron phosphate 0.5 - 15 μm, lithium nickel cobalt manganese oxide 1 - 40 μm, lithium cobalt oxide 5 - 30 μm, etc., either as a single component or a mixture thereof.

[0022] Further, the active material powders with different particle sizes in step (1) include, but are not limited to, anode materials within the following particle size ranges: graphite 1 - 30 μm, silicon carbide 0.05 - 20 μm, etc., either as a single component or a mixture thereof.

[0023] Further, the binder in step (1) includes, but is not limited to, single components or mixtures thereof such as PTFE, PVDF, CMC, etc.

[0024] Further, the conductive agent in step (1) includes, but is not limited to, single components or mixtures thereof such as carbon black, carbon nanotubes, graphene, etc.

[0025] Further, the components in step (1) are in the following weight percentages specifically: active material: 80% - 97%; binder: 1% - 20%; conductive agent: 0% - 10%.

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

[0027] 1. The present application uses active materials with different particle sizes to fabricate dry film sheets of active materials with different particle sizes respectively, and the particle sizes of the active materials in the same dry film sheet are the same / nearly the same. Then, the dry film sheets of active materials with different particle sizes are made into a dry thick electrode according to a predetermined gradient of active material particle sizes (the particle size of each layer of dry film sheet is single and the porosity is consistent). The purpose is to obtain a dry thick electrode with a gradient of active material particle sizes (independent of porosity / with the same particle size); the present application directly regulates through the particle size of the active material (the formula of the dry film sheet with gradient size remains unchanged, and the particle size of the material of a single-layer dry film sheet remains unchanged). In the prior art, there is a method of regulating the porosity of the electrode sheet by controlling the mass ratio of Ketjen black, which does not involve the gradient change of multi-size active materials. The purpose of this prior art is to adjust the porosity of the sheet (Ketjen black used in its scheme is used as a conductive agent, not as an active material); there is also a method in the prior art that uses multi-size active materials. After mixing according to the ratio of active materials with different particle sizes, dry film sheets with different average particle sizes are fabricated, and then a pole piece with a higher compaction density (energy density) is fabricated, and then a dry thick electrode with different porosity gradients is obtained (the purpose is to regulate the porosity by the particle size difference ratio); compared with the above prior art schemes, the scheme of the present application has the following advantages: ① The particle sizes of the active materials in a single-layer dry film sheet are the same or similar, and the formed ion channels and electron channels are more stable; ② The fabrication of a single-layer dry film sheet is more convenient, without involving the ratio regulation of multi-component active materials, nor increasing the process of pre-mixed fibrillation; ③ The interfacial properties of the particle size gradient electrodes fabricated with the same materials and ratios are more stable, and the differences between the interfaces of electrodes with different active materials and ratios are greater, and side reactions (lithium deposition, collapse, etc.) are more likely to occur in the battery cell system.

[0028] 2. The regulation principle of this technical scheme of the present application is as follows: The active particle size gradient refers to the electrode structure design in which the particle sizes of the active particles in each layer are different in a multi-layer electrode. Generally, smaller particles provide a higher specific surface area, a shorter lithium diffusion path within the particles, and promote more complete utilization of each particle. However, when small particles are stacked together, the small size results in a more tortuous inter-particle diffusion path, which may limit Li + diffusion. On the contrary, larger particles have a lower specific surface area, which results in a longer lithium diffusion path within the particles. However, when stacked together, their larger size results in a less tortuous inter-particle diffusion path, which should be beneficial to Li + diffusion; for the positive electrode, small particles increase the reaction specific surface and shorten the solid-phase lithium diffusion path, which is beneficial to the performance. For the graphite negative electrode, small particles increase the specific surface area, thus increasing the SEI passivation layer m to hinder Li +Diffusion. Moreover, the lithium intercalation in graphite particles occurs at the edge planes of graphite flakes. The interstitial space between large graphite particles increases, and these edge planes in large-grained graphite are more likely to receive lithium, resulting in Li + being intercalated into graphite faster, and the large-grained graphite has better performance.

[0029] 3. Each active material used in this application has its own particle size range. For example, the cathode active materials: lithium iron phosphate (0.5 - 15 μm), lithium nickel cobalt manganese oxide (1 - 40 μm), lithium cobalt oxide (5 - 30 μm), etc., which are single components or their mixtures; the anode materials: graphite (1 - 30 μm), silicon carbide (0.05 - 20 μm), etc., which are single components or their mixtures; according to the relative size of the active size, they are divided into three grades: large, medium, and small. For example: 30 μm, 15 μm, 7 μm and 8 μm, 4 μm, 1 μm.

[0030] 4. From the basic principle of the battery, whether in the discharging or charging process, from the separator to the current collector in the thickness direction of the cathode or anode electrode sheet, the number of lithium ions transported (or the lithium ion current density, electrolyte current density) is decreasing. At the separator, the lithium ion current density is the largest. At the interface between the coating and the current collector, the lithium ion current density is 0; the smaller the particle size of the active material on the separator side of the cathode electrode sheet, the larger the surface area. The small particles increase the reaction specific surface area and shorten the solid-phase lithium diffusion path, which is beneficial to the performance. For the graphite anode, the small particles increase the specific surface area, thereby increasing the SEI passivation layer to hinder Li + diffusion. Moreover, the lithium intercalation in graphite particles occurs at the edge planes of graphite flakes. The interstitial space between large graphite particles increases, and these edge planes in large-grained graphite are more likely to receive lithium, resulting in Li + being intercalated into graphite faster, and the large-grained graphite has better performance; therefore, by regulating the gradient distribution of the multi-active particle sizes of the electrode sheet, realizing the structure with large particle sizes on the separator side of the graphite anode and small particle sizes on the current collector side, and controlling the structure with small particle sizes on the separator side of the cathode and large particle sizes on the current collector side, the electron / ion conductivity of the electrode sheet can be improved, and its rate performance can be enhanced.

[0031] 5. The mixing and fibrillation scheme in the preparation method of this application: The processes of premixing and fibrillation can be achieved by the cooperation of single or multiple devices such as air jet milling and twin-screw; Film formation, thinning and lamination: The processes of film formation, thinning and lamination can be achieved by the cooperation of single or multiple devices such as double steel belts, opposed rollers and multi-rollers; New technologies such as electrostatic spraying and 3D printing can also manufacture multi-layer electrodes; The wet double / multi-layer coating die heads (sliding multi-layer coating, curtain multi-layer coating and die head extrusion multi-layer coating) can also realize the production of multi-layer electrodes.

[0032] 6. The method of this application can achieve double-sided simultaneous lamination to produce a double-sided three-layer laminated electrode. This application only requires 3 laminations, and all 3 laminations are carried out continuously in the same process (lamination process), with stronger flexibility; while in the prior art, usually, the lamination between the film sheets is carried out first and then laminated with the electrode. Description of the Drawings

[0033] Figure 1 Dry thick electrode with multiple active particle size gradients (dry electrode film sheets with three different particle sizes are respectively covered on both surfaces in the thickness direction of the current collector).

[0034] Figure 2 Schematic diagram of a dry thick negative electrode with multiple active particle size gradients (Example 1).

[0035] Figure 3 Schematic diagram of a dry thick positive electrode with multiple active particle size gradients (Example 2).

[0036] Figure 4 Schematic diagram of a dry negative electrode film sheet with different active particle sizes.

[0037] Figure 5 Schematic diagram of a dry positive electrode film sheet with different active particle sizes.

[0038] Figure 6 SEM image of a dry electrode film sheet with small particle size (1μm)

[0039] Figure 7 SEM image of a dry electrode film sheet with medium particle size (4μm)

[0040] Figure 8 SEM image of a dry electrode film sheet with large particle size (8μm)

[0041] Figure 9 Schematic diagram before (a) / after (b) mixing and fibrillation.

[0042] Figure 10 Schematic diagram of the film formation and thinning process, a. fibrillated material; b. film formation; c. thinning; d. dry film roll

[0043] Figure 11 Schematic diagram of the lamination process, a. dry film roll; b. coated current collector / separator / electrode / electrolyte roll; c. film sheet lamination; d. dry electrode film roll.

[0044] Figure 12 Discharge (3C) performance - Example 1.

[0045] Figure 13 Charge (3C) performance - Example 2. Detailed Description of the Invention

[0046] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the embodiments and the 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 belong to the scope of protection of the present invention;

[0047] As shown in Figures 1-5 the figure, a multi-active particle size gradient multi-layer dry thick electrode prepared in an embodiment of the present application is shown. The structure of the electrode includes a current collector 1 (the structure located in the middle), and the current collector 1 has a first surface 101 and a second surface 102 that are oppositely arranged in the thickness direction. The first surface 101 and the second surface 102 are sequentially provided with multi-layers of dry electrode membranes 2 from the inside to the outside (starting from near the current collector, and then from both sides in the thickness direction to the outside is from the inside to the outside). The particle size of the active material in each layer of the multi-layer dry electrode membranes 2 is different: because each layer of the dry electrode membrane is prepared from an active material powder, a binder, and a conductive agent, by defining the particle size of the active material powder in each dry electrode membrane, the particle size of the active material of the prepared multi-layer dry electrode membranes is in a gradient distribution state, which improves the electron / ion conductivity of the electrode and its rate performance.

[0048] As an example, the multi-layer dry electrode membranes 2 described in the present application include at least two layers (that is, both the first surface and the second surface are covered with two layers). When the dry electrode membrane is a positive electrode membrane, the particle size of the active material in each layer of the dry electrode membrane gradually decreases from the side near the current collector to the outside; when the dry electrode membrane is a negative electrode membrane, the particle size of the active material in each layer of the dry electrode membrane gradually increases from the side near the current collector to the outside. That is, when preparing a negative electrode sheet, the particle size of the active material in the electrode sheet on the current collector side is small, and it gradually increases on the outside; when it is a positive electrode sheet, the particle size of the active material in the electrode sheet on the current collector side is large, and it gradually decreases on the outside.

[0049] As an example, the multi-layer dry electrode membranes 2 described in the present application include at least three layers (that is, both the first surface and the second surface are covered with three layers). When the dry electrode membrane is a positive electrode membrane, the particle size of the active material in each layer of the dry electrode membrane gradually decreases from the side near the current collector to the outside; when the dry electrode membrane is a positive electrode membrane, the particle size of the active material in each layer of the dry electrode membrane gradually increases from the side near the current collector to the outside. That is, when preparing a negative electrode sheet, the particle size of the active material in the electrode sheet on the current collector side is small, and it gradually increases on the outside; when it is a positive electrode sheet, the particle size of the active material in the electrode sheet on the current collector side is large, and it gradually decreases on the outside.

[0050] As an example, the current collector 1 described in this application is a current collector with conductive layers provided on the first surface 101 and the second surface 102 (the conductive layer material is a conventional conductive layer material for current collectors, which is very common in the electrode field and will not be specifically introduced). The single-sided thickness of the conductive layer is 0.5 - 5 μm, and in a specific example, a thickness of 1 μm can be adopted.

[0051] As an example, the particle sizes of the active materials in the dry electrode film sheets 2 provided on the symmetric positions of the first surface 101 and the second surface 102 described in this application are the same; that is, the particle sizes of the active materials in the dry electrode film sheets 2 provided at the corresponding positions on the first surface 101 and the second surface 102 on both sides of the thickness of the current collector 1 are the same.

[0052] Example 1

[0053] This example provides a method for preparing a multi-active particle size gradient multi-layer dry electrode sheet. Based on a specific implementation, the preparation method of this dry electrode sheet (dry graphite) is as follows: Graphite (particle size gradients of 7, 15, and 30 μm) is used as the negative electrode active material, a binder (PTFE:CMC = 90:10), and a conductive agent (the conductive agent is composed of the following components mixed in weight ratio: conductive graphite: carbon nanotube dry powder: graphene dry powder = 70:20:10). The mass ratios of the active material: binder: conductive agent are 96:3:1 respectively. The negative electrode active material and the conductive agent are added to a blender and stirred at 4000 rpm for 8 minutes to be mixed evenly. Then, it is slowly stirred at 200 rpm to cool down to 18 °C, and then the binder is added. The temperature is controlled < 18 °C, and it is stirred at 1000 rpm for 10 min to be mixed evenly. Then, the cooling system is turned off, and it is stirred at 4000 rpm, and the temperature rises to 70 °C naturally by shear to complete the fibrillation process, obtaining powders with three different particle sizes; Figure 9 It is a schematic diagram before (a) / after (b) mixing and fibrillation. Note: The positive / negative electrode materials here are not limited to positive / negative electrode materials such as graphite, silicon carbon, hard carbon, lithium-rich, and lithium nickel cobalt manganese oxide, but also include electrolyte powders. The auxiliary materials here are not limited to the above-mentioned conductive agent and binder, but also include other additives.

[0054] The above-mentioned powders are respectively put into a one-time film-forming / thinning roll press. The film-forming roll gap is 150 μm, the thinning roll gap is 100 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. It is hot-rolled / thinned into a self-supporting film at 110 °C for the first time, obtaining dry negative electrode film sheets with three different particle sizes. Specifically, as shown in the appendix Figure 4 As shown, the areal density of the dry negative electrode film sheets with three different particle sizes is 18 mg / cm 2 , the thickness is about 120 μm, and the tap density is about 1.5 g / cm 3 ; specifically refer to Figure 10Schematic diagram of the film thinning process. Note: The film thinning here is not limited to horizontal or vertical, and the film thinning roller is not limited to a cold roller or a hot roller.

[0055] The dry anode films with different particle sizes mentioned above and the coated current collector (coated copper foil) were thermally roll-compounded at 130 °C in sequence. The coated current collector includes a copper foil and conductive layers provided on both sides of the copper foil. The single-sided thickness of the conductive layer is 1 μm. Specifically refer to Figure 11 As shown, it is a schematic diagram of the compounding process. Note: Here, b is not limited to the current collector. It can be multiple compoundings of the already compounded electrode, and the compounding is not limited to the compounding between the same type of electrode film and the current collector. It can be the compounding between different electrode films and the current collector, separator, or electrolyte roll.

[0056] The first compounding: Using the dry anode film with a particle size of 7 μm prepared above and the coated copper foil, with a thermal roll-compression roll gap of 200 μm and a roll-compression pressure of 20 T, a dry anode sheet with a thickness of about 220 μm was obtained, and the tap density was 1.7 g / cm 3 ;

[0057] The second compounding: Using the dry anode film with a particle size of 15 μm prepared above and the dry anode sheet after the first compounding, with a thermal roll-compression roll gap of 400 μm and a roll-compression pressure of 20 T, a dry anode sheet with a thickness of about 420 μm was obtained, and the tap density was 1.75 g / cm 3 ;

[0058] The third compounding: Using the dry anode film with a particle size of 30 μm prepared above and the dry anode sheet after the second compounding, with a thermal roll-compression roll gap of 600 μm and a roll-compression pressure of 20 T, a dry anode sheet with a thickness of about 620 μm was obtained, and the tap density was 1.77 g / cm 3 ;

[0059] Finally, the dry anode sheets with different particle size gradients (from the inside to the outside: 7 → 15 → 30 μm) with a tap density of 1.77 g / cm 3 , and a thickness of about 620 μm were cut; the schematic diagram of the structure of the obtained dry anode sheets is as Figure 2 shown.

[0060] Figure 12 is the multiple discharge (3C) performance of the anode sheet prepared in this example. Through experiments, it can be known that the performance of the dry anode sheets obtained in this Example 1 is as follows. By adjusting the distribution of the particle size gradient of the electrode sheet, an upper layer with a large particle size ratio structure and a lower layer with a small particle size structure are realized. From Figure 12 it can be intuitively seen that the 3C multiple discharge performance of Example 1 is better than that of Control Group 1. This is because the lithium insertion in the graphite particles occurs at the edge planes of the graphite flakes. The interstitial space between the large graphite particles increases, and these edge planes in the large particle graphite are more likely to receive lithium, resulting in Li+ Graphite can be embedded faster, and the rate performance of large-particle graphite is better. An electrode with a gradually decreasing active material particle size from the separator to the current collector can reduce the ionic resistance without compromising the electronic conductivity and volumetric energy density, regulate the gradient distribution of the multi-active particle sizes of the electrode sheet, achieve a structure with large particle sizes on the graphite separator side of the negative electrode and small particle sizes on the current collector side, improve the electronic / ionic conductivity of the electrode sheet, and enhance its rate performance.

[0061] Table 1 Performance of the negative electrode sheets obtained in Example 1 and Control Group 1

[0062]

[0063] Example 1: The particle sizes of the inner and outer layer film sheets with double-sided lamination are 7 → 15 → 30 μm respectively;

[0064] Control Group 1: The particle sizes of the inner and outer layer film sheets with double-sided lamination are both 15 μm.

[0065] Example 2

[0066] This example provides a preparation method for a multi-active particle size gradient multi-layer dry electrode sheet. Based on a specific implementation, the preparation method of this dry electrode sheet (dry positive electrode) is as follows: Lithium iron phosphate (with particle size gradients of 1, 4, and 8 μm) is used as the positive electrode active material, a binder (PTFE:PVDF = 95:5), and a conductive agent (conductive graphite: dry carbon nanotubes: dry graphene = 80:15:5). The mass ratios of the active material: binder: conductive agent are 95:4:1 respectively. Add the positive electrode active material and the conductive agent into a blender, stir at 4000 rpm for 10 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 stir at 4000 rpm to shear and heat up naturally to 85 °C to complete the fibrillation process, obtaining powders with three different particle sizes; Figure 9 Schematic diagrams before (a) / after (b) mixing and fibrillation. Note: The positive / negative electrode materials here are not limited to positive / negative electrode materials such as graphite, silicon carbon, hard carbon, lithium-rich, and lithium nickel cobalt manganese oxide, but also include electrolyte powders. The auxiliary materials here are not limited to the above-mentioned conductive agent and binder, but also include other additives.

[0067] Put the mixed powder into a one-time film-forming / thinning roll press respectively. 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-time hot roll pressing / thinning at 120 °C to form a self-supporting film, obtaining a dry positive electrode film with a surface density of 20 mg / cm 2 The thickness is about 90 μm, and the tap density is about 2.22 g / cm3 ; Specifically refer to Figure 10 which is a schematic diagram of the film thinning process. Note: The film thinning here is not limited to horizontal or vertical, and the film thinning roller is not limited to a cold roller or a hot roller; The specific structural schematic diagrams of three specific dry-process cathode films are as Figure 5 shown. Figure 6 is the SEM image (1μm) of the dry-process electrode film with small particle size, Figure 7 is the SEM image (4μm) of the dry-process electrode film with medium particle size, Figure 8 is the SEM image (8μm) of the dry-process electrode film with large particle size.

[0068] The above dry-process cathode films with different particle sizes are thermally roll-compounded with the coated current collector (coated aluminum foil) at 130°C for multiple times. The coated current collector includes aluminum foil and conductive layers provided on both sides of the aluminum foil. The single-side thickness of the conductive layer is 1μm; Specifically refer to Figure 11 shown, which is a schematic diagram of the compounding process. Note: Here, b is not limited to the current collector. It can be multiple compoundings of the already compounded electrode, and the compounding is not limited to the compounding between the same type of electrode film and the current collector. It can be the compounding between different electrode films and the current collector, separator, or electrolyte roll.

[0069] The first compounding: Using the above-prepared dry-process cathode film with a particle size of 8μm and the coated aluminum foil, with a thermal roll-pressing roll gap of 130μm and a roll-pressing pressure of 30T, a dry-process cathode sheet with a thickness of about 160μm and a tap density of 2.7g / cm 3 is obtained;

[0070] The second compounding: Using the above-prepared dry-process cathode film with a particle size of 4μm and the dry-process cathode sheet after the first compounding, with a thermal roll-pressing roll gap of 280μm and a roll-pressing pressure of 30T, a dry-process cathode sheet with a thickness of about 300μm and a tap density of 2.78g / cm 3 is obtained;

[0071] The third compounding: Using the above-prepared dry-process cathode film with a particle size of 1μm and the dry-process cathode sheet after the second compounding, with a thermal roll-pressing roll gap of 410μm and a roll-pressing pressure of 30T, a dry-process cathode sheet with a thickness of about 440μm and a tap density of 2.8g / cm 3 is obtained;

[0072] Finally, the dry-process cathode sheets with different particle size gradients (from inside to outside: 8→4→1μm) with a tap density of 2.8g / cm 3 and a thickness of about 440μm are cut; The structural schematic diagram of the obtained cathode sheet is as Figure 3 shown.

[0073] Figure 13This is the 3C charge performance of the dry-process positive electrode sheet obtained in Example 2. The performance of the dry-process positive electrode sheet obtained in Example 2 is as follows: The particle size gradient distribution of the electrode sheet is regulated to achieve a large particle size ratio structure in the upper layer and a small particle size structure in the lower layer. Starting from Figure 13 It can be intuitively seen from

[0074] Table 2 Performance of the negative electrode sheets obtained in Example 21 and Control Group 2

[0075]

[0076] Example 2: The particle sizes of the inner and outer layer films for double-sided lamination are 8→4→1 μm respectively;

[0077] Control Group 2: The particle sizes of the inner and outer layer films for double-sided lamination are both 4 μm.

[0078] Therefore, through the performance comparison of the electrode sheets obtained from the above examples and the control group, it can be seen that this method of the present application has the following advantages: ① The active material particle sizes of the single-layer dry-process film are similar, and the formed ion channels and electron channels are more stable; ② The production of the single-layer dry-process film is more convenient, without involving the ratio regulation of multi-component active materials, nor adding the process of premixed fibrillation; ③ The interfacial properties of the particle size gradient electrodes made with the same materials and ratios are more stable, and the differences in the interfaces of electrodes with different active materials and ratios are greater, and side reactions (such as lithium deposition, collapse, etc.) are more likely to occur in the battery cell system.

Claims

1. A multi-layer dry thick electrode with multiple active particle size gradients, characterized in that: The structure of the electrode includes a current collector having a first surface and a second surface arranged opposite to each other along the thickness direction, and the first surface and the second surface are sequentially provided with multiple layers of dry electrode membranes from the inside to the outside, and the particle size of the active material in each layer of the multi-layer dry electrode membrane is different.

2. The multi-layer dry thick electrode with multiple active particle size gradients according to claim 1, characterized in that: The multilayer dry electrode membrane comprises at least two layers. When the dry electrode membrane is a positive electrode membrane, the particle size of the active material in each layer of the dry electrode membrane gradually decreases from the side close to the current collector to the outside; when the dry electrode membrane is a negative electrode membrane, the particle size of the active material in each layer of the dry electrode membrane gradually increases from the side close to the current collector to the outside.

3. The multi-layer dry-process thick electrode with multiple active particle size gradients according to claim 1, characterized in that: The multilayer dry electrode membrane comprises at least three layers. When the dry electrode membrane is a positive electrode membrane, the particle size of the active material in each layer of the dry electrode membrane gradually decreases from the side close to the current collector to the outside; when the dry electrode membrane is a negative electrode membrane, the particle size of the active material in each layer of the dry electrode membrane gradually increases from the side close to the current collector to the outside.

4. The multi-layer dry thick electrode with multiple active particle size gradients according to claim 1, characterized in that: The current collector is a current collector with a conductive layer disposed on the first surface and the second surface, and the single-side thickness of the conductive layer is 0.5-5 μm.

5. The multi-layer dry thick electrode with multiple active particle size gradients according to claim 1, characterized in that: The particle sizes of the active materials in the dry electrode membranes symmetrically arranged on the first surface and the second surface are the same.

6. A method for preparing a multi-layer dry thick electrode with multiple active particle size gradients according to any one of claims 1 to 5, characterized in that: The preparation steps include: (1) Powder premixing: Under the condition of controlling the temperature to ≤18°C, active material powders of different particle sizes, binders and conductive agents are stirred and mixed in proportion to obtain premixes of different active particle sizes; (2) Powder fiberization: without controlling the temperature, the premixes of different active particle sizes obtained in step (1) are subjected to rapid shearing and heating to ≤120° C., and after reaching the specified temperature, the premixes of different active particle sizes are fiberized to obtain powders; (3) Film formation and thinning: The powder of different active particle sizes obtained by fiberization in step (2) is subjected to continuous film formation by hot pressing with rollers, wherein the temperature of the rollers is controlled at ≤150°C, the pressure of the rollers is controlled at 5-20T, the film forming speed is controlled at 1-10m / min, the differential speed ratio of the rollers is controlled at 1:0.9-1:3, and the gap between the rollers is controlled at ≤300μm; the film after film formation is continuously passed through the rollers, and is rolled up after hot pressing and thinning, wherein the thinning is specifically: the temperature of the rollers is controlled at ≤150°C, the pressure of the rollers is controlled at 5-50T, the thinning speed is controlled at 1-10m / min, the differential speed ratio of the rollers is controlled at 1:0.9-1:3, and the gap between the rollers is controlled at ≤200μm; and membranes of different active particle sizes are obtained; and then a dry electrode membrane is obtained; (4) Laminating the membrane with the current collector: The membrane with different active particle sizes obtained in step (3) is laminated on both sides with the current collector in a certain size gradient by hot pressing with two rollers, wherein the temperature of the two rollers is controlled at ≤180°C, the pressure of the two rollers is controlled at 10-200T, the laminating speed is controlled at 1-10m / min, and the gap between the two rollers is controlled at ≤1000μm. After winding, a dry electrode coil is obtained.

7. The method for preparing a multi-layer dry thick electrode with multiple active particle size gradients according to claim 6, characterized in that: The active material powders of different particle sizes described in step (1) include but are not limited to positive electrode active materials in the following particle size ranges: single components of lithium iron phosphate 0.5-15 μm, lithium nickel cobalt manganese oxide 1-40 μm, lithium cobalt oxide 5-30 μm or mixtures thereof.

8. The method for preparing a multi-layer dry thick electrode with multiple active particle size gradients according to claim 6, characterized in that: The active material powders of different particle sizes described in step (1) include but are not limited to the following particle size ranges for negative electrode materials: single components or mixtures thereof in graphite 1-30 μm, silicon carbon 0.05-20 μm.

9. The method for preparing a multi-layer dry thick electrode with multiple active particle size gradients according to claim 6, characterized in that: The binder described in step (1) includes but is not limited to single components of PTFE, PVDF, CMC or mixtures thereof.

10. The method for preparing a multi-layer dry thick electrode with multiple active particle size gradients according to claim 6, characterized in that: The conductive agent described in step (1) includes but is not limited to single components of carbon black, carbon nanotubes, and graphene or a mixture thereof.

11. The method for preparing a multi-layer dry thick electrode with multiple active particle size gradients according to claim 6, characterized in that: The components in step (1) are specifically in the following weight percentages: active material: 80%-97%; binder: 1%-20%; conductive agent: 0%-10%.

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