Self-supporting dry-method electrode film and preparation method and application thereof

Abstract

The invention discloses a self-supporting dry-method electrode film and a preparation method and application thereof, and the method comprises the following steps: 1) uniformly mixing a conductive agent and a binder to obtain a mixture I; 2) uniformly mixing the mixture I and an active substance to obtain a mixture II; 3) placing the mixture II in a drying oven for uniform fibration to obtain a mixture III; and 4) carrying out step-by-step rolling on the mixture III to form a dry-method electrode film with a certain thickness. According to the preparation method, no organic solvent needs to be added, no toxic gas is discharged, a coating oven is also not needed, process equipment is simplified, and the process problem caused by homogenate coating is avoided. When the method is adopted to prepare the electrode by a dry method, the use amount of the binder is less, the formed self-supporting membrane can be directly used as the electrode, and the use of copper foil and aluminum foil current collectors is reduced, so that the energy density of the battery is improved, the cost of the battery is reduced, and the method has a wide application prospect in the field of lithium batteries.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a self-supporting dry electrode membrane and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries have accelerated the transition from oil- and gas-powered vehicles to pure electric vehicles, as well as the advent of a new era of smart grids and energy intelligence. However, for different income groups and environmental protection requirements, the manufacturing cost and pollution of lithium-ion batteries need to be further reduced, and safety performance needs to be improved.

[0003] At present, the mainstream production technology for key components of lithium-ion batteries is wet coating technology, which has the following disadvantages. (1) High energy consumption. According to statistics, wet coating and subsequent drying processes consume about 51% of the total energy for battery production. (2) High cost and environmental pollution: The electrode slurry mixing process requires the use of a large amount of N-methyl-2-pyrrolidone (NMP), which is very expensive (increasing costs) and toxic (polluting the environment). Therefore, during the drying process of large-scale production, a recovery device must be established to collect and reprocess the evaporated NMP. (3) Electrode stratification: During the solvent evaporation process, the binder and conductive agent will diffuse to the vicinity of the electrode surface due to capillary action and form agglomerates, while the active material will precipitate. This will lead to electrode stratification, thereby damaging the construction of the conductive network in the electrode and reducing the bonding strength between the active material and the current collector. (4) Limited electrode thickness: Electrodes manufactured by wet coating technology have problems such as cracks, stratification and poor flexibility, especially when thick electrodes are prepared. These problems will be magnified. Therefore, the electrode thickness is greatly limited by wet coating technology.

[0004] Dry electrode technology is a comprehensive upgrade of the traditional wet method and is more suitable for the needs of the new generation of batteries. In terms of the manufacturing process, dry electrodes have fewer steps, lower manufacturing costs and energy consumption, and environmentally friendly raw materials, which are more suitable for large-scale production; in terms of battery performance, dry batteries can achieve higher energy density, and the battery's electrical and mechanical properties are better; on the application side, dry batteries are more suitable for the manufacturing needs of new generation batteries such as solid-state batteries and 4680. Dry electrode technology does not use any solvents when preparing lithium batteries, which solves the problem of wet electrodes from the root.

[0005] CN 116705972 A discloses a dry electrode membrane and a preparation method thereof. The preparation method of the dry electrode membrane comprises the following steps: (1) mixing; (2) airflow milling; (3) fiberization: setting three heating gradients to raise the temperature in sequence, and fiberizing the powder obtained in step (2); (4) internal mixing; (5) open mixing: placing the obtained agglomerate in an open mixing mill for rolling to form a film, and preparing a dry electrode membrane. In the fiberization process of the dry electrode binder, the purpose of improving the electrochemical performance of the material is achieved by designing and applying the heating gradient. The process conditions in the present invention are simple, and the battery cycle performance and cycle efficiency of the prepared dry electrode membrane for lithium-ion batteries are improved.

[0006] CN 113871566 A discloses a dry electrode membrane, a preparation method and application thereof, the method comprising the following steps: 1) uniformly mixing an active substance, a conductive agent and a polymer membrane to obtain a mixture; 2) treating the above-mentioned mixed material to obtain a mixture; 3) heating and calendering the obtained mixture to obtain a dry electrode membrane. The method of the present invention can greatly reduce the process difficulty of dry preparation of electrode sheets, has low requirements on the equipment used for fiberization, and has more complete fiberization and better dispersion uniformity between components. When the method of the present invention is used to dry prepare electrodes, the amount of polymer (such as PTFE) used is less, which is beneficial to improving the energy density of the battery and facilitating the preparation of thick electrodes.

[0007] The existing dry-process electrode preparation technology has the problem of poor uniformity of the distribution of the components during mixing, resulting in large internal resistance of the positive / negative electrode sheets, which affects the rate performance and cycle performance of the battery. Most of them adopt the method of direct roller passing of powder materials. The self-supporting electrode film prepared in this way has poor mechanical properties and requires a high content of binder, which will reduce the energy density of the battery.

[0008] Therefore, how to dry-process a positive and negative electrode sheet with low binder content and low equipment requirements has become an urgent problem that technicians in this field need to solve. Summary of the invention

[0009] In view of the deficiencies in the prior art, the object of the present invention is to provide a self-supporting dry electrode membrane and a preparation method and application thereof.

[0010] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0011] In a first aspect, the present invention provides a method for preparing a self-supporting dry electrode membrane, comprising the following steps:

[0012] 1) mixing the conductive agent and the binder uniformly to obtain a mixture I;

[0013] 2) Mixing the mixed material I and the active substance uniformly to obtain a mixed material II;

[0014] 3) placing the mixed material II in an oven for heating to homogenize and obtain mixed material III;

[0015] 4) The mixed material III is rolled step by step to form a self-supporting dry electrode film of a certain thickness.

[0016] Preferably, the self-supporting dry electrode membrane is prepared from raw materials in the following weight percentages: 93%-98% active material, 1%-3% conductive agent, and 0.5%-4% binder.

[0017] Preferably, the conductive agent is composed of one or more of Super P, acetylene black, graphene and Ketjen black and carbon nanotubes; the active material is one or at least two of hard carbon, soft carbon, artificial graphite, natural graphite and mesophase carbon microspheres; and the binder is polytetrafluoroethylene.

[0018] Preferably, the mixing equipment in step 1) is a mixer without any stirring paddle in the material tank, and the mixing speed and time are both controlled in a step-by-step manner, with each mixing speed being greater than 300 rpm and less than 1000 rpm, and each mixing speed time being greater than 100 s and less than 800 s.

[0019] Preferably, the mixing equipment in step 2) is a mixer without any stirring paddle in the material tank, and the mixing speed and time are both controlled in a step-by-step manner, with each mixing speed being greater than 500 rpm and less than 1500 rpm, and each mixing speed time being greater than 100 s and less than 800 s.

[0020] Preferably, in step 3), the heating temperature is 40° C.-120° C., and the heating time is 20 min-80 min.

[0021] Preferably, in the step 4), the mixed material III is first rolled to form a preliminary shape and then folded in half several times, and rolled in multiple directions at a rolling speed of 1rpm-5rpm. The rolling temperature is first high temperature and then room temperature, the high temperature is 60°C-120°C, and the rolling is reduced step by step in proportion. The roll gap reduction adjustment value parameter is 50%-3% of the previous roll gap thickness value, and the adjustment values ​​are performed in descending order to obtain an electrode film of target thickness.

[0022] Preferably, in step 1), the mixture I is in the original powdery mixed state of the conductive agent and the binder; in step 2), the mixture II is a fluffy flocculent mixture; and in step 3), the mixture III is a fluffy flocculent mixture.

[0023] Preferably, in step 4), the thickness of the self-supporting dry electrode film is 70 μm-500 μm.

[0024] The invention also provides a self-supporting dry electrode membrane prepared by the preparation method.

[0025] The self-supporting dry electrode membrane provided by the present invention can be used alone or combined with a current collector according to needs and then used, and the combination method is not limited.

[0026] The technical solution of the present invention has the following beneficial effects:

[0027] (1) The present invention is based on a dry process, completely removes the solvent, does not involve toxic substances, and has irreplaceable advantages for solvent-sensitive active materials. Compared with multi-layer wet coating technology, it reduces multiple drying steps, improves efficiency, and is more energy-saving and environmentally friendly.

[0028] (2) When the conductive agent, binder and active material are mixed, the present invention first selects a simple and efficient mixing device, and a mixer without any stirring paddles or ball milling beads or any other accessories other than the material in the material tank, so as to avoid damaging the material structure and particle size during mixing. Secondly, the mixing speed and time are both controlled in a step-by-step manner. The conductive agent and binder are first mixed evenly at a low speed, and then the conductive agent, binder and active material are mixed evenly at a higher speed to ensure uniform dispersion of each component. Finally, the binder is fibrillated at a high speed. The dry binder is fibrous and cross-linked to form a mesh skeleton of the dry functional layer. At present, there are still some problems with PTFE binder in the preparation of dry batteries. For example, the distribution of the binder fiber network during the fibrillation process will increase the contact resistance between the active particles, thereby causing the impedance of the overall electrode to be high. The present invention first mixes the conductive agent and binder evenly to ensure the uniformity of the conductive fiber network distribution in the overall space.

[0029] (3) Use carbon nanotube-based composite conductive agents. Carbon nanotubes have excellent conductivity and extremely high axial tensile strength, which is crucial for the film forming properties and mechanical strength of dry film formation. Increasing the tensile strength of the electrode film can reduce the amount of binder used and improve the calendering effect, thereby increasing the energy density of the battery. In addition, it also significantly improves the electrode conductivity, thereby reducing the polarization of the pole piece, which is conducive to the development and application of thick electrode high-capacity batteries and high-power batteries.

[0030] (4) The present invention regulates the mixing order and mixing method of each component, regulates the fiberization method, speed, temperature and time of the binder, as well as the rolling temperature, rolling direction and roll gap adjustment, thereby simplifying the process equipment, reducing the amount of binder used, improving the tensile strength and flexibility of the electrode membrane, and enhancing the operability of the continuous membrane making process.

[0031] (5) The method of the present invention can greatly reduce the process difficulty of dry-process preparation of electrode sheets, and has low requirements on the equipment used for fiberization. When high-speed dispersion is used, the stirring speed used is lower and the dispersion time is shorter. In addition, the fiberization is more complete and the dispersion uniformity between components is better, which is conducive to reducing costs and large-scale production of enterprises. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is an optical picture of a self-supporting dry electrode film according to an embodiment of the present invention.

[0033] Figure 2 This is an electron microscope picture of the self-supporting dry electrode membrane according to an embodiment of the present invention.

[0034] Figure 3 This is a picture of element distribution of the self-supporting dry electrode film according to an embodiment of the present invention.

[0035] Figure 4 This is an optical picture of the self-supporting dry electrode film of Comparative Example 1 of the present invention.

[0036] Figure 5 Optical image of the self-supporting dry electrode film of Comparative Example 2 of the present invention.

[0037] Figure 6 Optical image of the self-supporting dry electrode film of Comparative Example 3 of the present invention.

[0038] Figure 7 Optical image of the self-supporting dry electrode film of Comparative Example 4 of the present invention. DETAILED DESCRIPTION

[0039] The present invention will be described in further detail below in conjunction with specific implementation modes, but the implementation modes of the present invention are not limited thereto.

[0040] Example 1

[0041] The preparation method of the self-supporting dry electrode film in this embodiment is as follows:

[0042] Step 1: Place 2% conductive agent (the mass ratio of carbon nanotubes: Super P is 1:1) and 3% binder in a mixing tank, and mix according to the following procedure to obtain mixture I. The rotation speeds are 300 rpm, 600 rpm, 800 rpm, 600 rpm, 800 rpm, and 1000 rpm, and the corresponding times are 120 s, 180 s, 300 s, 300 s, 120 s, and 180 s, respectively. Before each mixing, the materials in the mixing tank must be stirred evenly before the next mixing.

[0043] Step 2: 95% active material graphite and 5% mixed material I are placed in a mixing tank, and mixed according to the following procedure to obtain mixed material II. The rotation speeds are 600rpm, 600rpm, 800rpm, 600rpm, 800rpm, 800rpm, 800rpm, 1000rpm, and 1200rpm, respectively, and the corresponding times are 120s, 180s, 300s, 300s, 120s, 180s, 300s, 120s, and 180s, respectively. Before each mixing, the materials in the mixing tank must be stirred evenly before the next mixing.

[0044] Step 3: Place the mixture II in an oven for homogenization. The oven temperature is set to 80° C. for 1 hour to obtain a mixture III.

[0045] Step 4: Roll the mixture III step by step. Initially, the mixture III is a fluffy flocculent mixture. Set the roller press temperature to 80°C, and reduce the roller gap from 2.5mm step by step. The first adjustment value is 50% of the previous roller gap thickness value. Each subsequent adjustment decreases by 5%. Fold the mixture III in half several times and roll it in multiple directions at a rolling speed of 2rpm. When the mixture III is rolled into a thick electrode film and is no longer a powder, adjust the rolling temperature to room temperature 25°C and roll it. Each time the roller gap decreases, the adjustment decreases by 3%. Roll it to the desired thickness of 200μm for a self-supporting dry electrode film.

[0046] Example 2

[0047] The difference from Example 1 is that the weight percentage ratio of the active material graphite, the conductive agent and the binder is 97:2:1, and the rest is the same as Example 1.

[0048] Example 3

[0049] The difference from Example 2 is that the fibers are uniformly fiberized, the oven temperature is set at 100° C., the drying time is 40 min, and the rest is the same as Example 2.

[0050] Comparative Example 1

[0051] The difference from Example 1 is that during the first mixing, the conductive agent, the binder and the active material are added to the mixing tank at the same time and mixed, and the rest is the same as Example 1.

[0052] Comparative Example 2

[0053] The difference from Example 1 is that the mixing speed and time are not controlled in a step-by-step manner during mixing, that is, step 1 is directly mixed at a speed of 600 rpm for 1200 s, and step 2 is directly mixed at a speed of 800 rpm for 1800 s. The rest is the same as Example 1.

[0054] Comparative Example 3

[0055] The difference from Example 2 is that 2% of the conductive agent is Super P, that is, the conductive agent does not contain carbon nanotubes, and the rest is the same as Example 2.

[0056] Comparative Example 4

[0057] The difference from Example 1 is that in step 4, the mixed material III is not folded in half, but is directly rolled step by step in one direction.

[0058] Table 1 Performance test results of the electrode membranes of the embodiments and comparative examples after assembling the battery

[0059]

[0060] It can be seen from the optical images of Examples 1-3 and Comparative Examples 1-4 that the surface of the self-supporting dry electrode membrane obtained in the examples is uniform, smooth and flat. However, due to the uneven mixing of materials in Comparative Example 1, there are black spots on the membrane surface and part of the conductive agent agglomerates. Comparative Example 2 also has uneven material mixing, and the material mixing and preliminary fiberization of the binder are carried out simultaneously, resulting in an uneven membrane surface with a large number of black spots. Comparative Example 3 has a low binder content and no carbon nanotubes are used in the conductive agent, which makes it difficult to roll the powder into a film. Comparative Example 4 does not fold the mixture in half, but directly rolls it step by step in one direction, resulting in poor tensile strength and flexibility of the electrode membrane, which is very easy to break, affecting subsequent processing performance.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them. Although the present invention has been described in detail with reference to the above embodiments, it should be understood by those skilled in the art that the technical solutions described in the above embodiments can still be modified, or some of the technical features can be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a self-supporting dry electrode membrane, characterized in that: The steps include: 1) mixing the conductive agent and the binder uniformly to obtain a mixture I; 2) Mixing the mixed material I and the active substance uniformly to obtain a mixed material II; 3) placing the mixed material II in an oven for heating to homogenize and obtain mixed material III; 4) The mixed material III is rolled step by step to form a self-supporting dry electrode film of a certain thickness.

2. The method for preparing a self-supporting dry electrode membrane according to claim 1, characterized in that: The self-supporting dry electrode membrane is prepared from the following raw materials in percentage by weight: 93%-98% of active material, 1%-3% of conductive agent, and 0.5%-4% of binder.

3. The method for preparing a self-supporting dry electrode membrane according to claim 1, characterized in that: The conductive agent is composed of one or more of Super P, acetylene black, graphene and Ketjen black and carbon nanotubes; the active material is one or at least two of hard carbon, soft carbon, artificial graphite, natural graphite and mesophase carbon microspheres; and the binder is polytetrafluoroethylene.

4. The method for preparing a self-supporting dry electrode membrane according to claim 1, characterized in that: The mixing equipment in step 1) is a mixer without any stirring paddle in the material tank, and the mixing speed and time are both controlled in a step-by-step manner, with each mixing speed being greater than 300 rpm and less than 1000 rpm, and each mixing speed time being greater than 100 s and less than 800 s.

5. The method for preparing a self-supporting dry electrode membrane according to claim 1, characterized in that: The mixing equipment in step 2) is a mixer without any stirring paddle in the material tank, and the mixing speed and time are both controlled in a step-by-step manner, with each mixing speed being greater than 500 rpm and less than 1500 rpm, and each mixing speed time being greater than 100 s and less than 800 s.

6. The method for preparing a self-supporting dry electrode membrane according to claim 1, characterized in that: In the step 3), the heating temperature is 40° C.-120° C., and the heating time is 20 min-80 min.

7. The method for preparing a self-supporting dry electrode membrane according to claim 1, characterized in that: In the step 4), the mixed material III is first rolled to form a preliminary shape and then folded in half several times, and rolled in multiple directions at a rolling speed of 1rpm-5rpm. The rolling temperature is first high temperature and then room temperature, and the high temperature is 60°C-120°C. The rolling is reduced step by step in proportion, and the roll gap reduction adjustment value parameter is 50%-3% of the previous roll gap thickness value. The adjustment values ​​are made in descending order to obtain an electrode film of target thickness.

8. The method for preparing a self-supporting dry electrode membrane according to claim 1, characterized in that: In the step 1), the mixture I is in the original powdery mixed state of the conductive agent and the binder; in the step 2), the mixture II is a fluffy flocculent mixture; and in the step 3), the mixture III is a fluffy flocculent mixture.

9. The method for preparing a self-supporting dry electrode membrane according to claim 1, characterized in that: In the step 4), the thickness of the self-supporting dry electrode film is 70 μm-500 μm.

10. A self-supporting dry electrode membrane prepared according to the preparation method according to any one of claims 1 to 9.

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