A dry electrode, a method of manufacturing the same, and a secondary battery

Abstract

The application provides a dry-method electrode, a preparation method thereof and a secondary battery, and particularly relates to the technical field of batteries. The preparation method comprises the following steps: pre-mixing an active main material and a conductive agent to obtain a first mixture; performing a freezing crushing treatment on a binder to obtain a binder powder; mixing the first mixture and the binder powder to obtain a second mixture; performing a kneading treatment on the second mixture to obtain an electrode sheet powder; rolling the electrode sheet powder into a film to obtain a dry-method film piece; and performing a hot rolling compounding of the dry-method film piece and a current collector to obtain the dry-method electrode. The preparation method can significantly improve the uniformity of powder dispersion and the strength of the dry-method film piece, reduce the anisotropy of the electrode sheet, and improve the initial efficiency and cycle life of the full battery.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a dry-process electrode, a preparation method thereof, and a secondary battery. Background Art

[0002] With the rapid development of new energy technologies, the power and energy storage industries, dominated by lithium-ion batteries, are constantly placing new demands on battery energy density and cost. To improve battery energy density, current key battery development directions include the development of high-capacity primary materials, high-performance auxiliary materials, and thicker electrodes. Thick electrodes are the simplest and most effective method for increasing battery energy density. However, the production of thick electrodes using conventional wet slurry and coating processes is prone to problems such as low coating thickness, poor peel strength, electrode cracking, and high resistance.

[0003] Compared to conventional wet-process electrodes, dry-process electrodes offer the following advantages: They require no solvents, drying ovens, or solvent recovery units, significantly reducing equipment investment and energy consumption. They also eliminate the problem of binder flotation, allowing for the production of thicker electrodes. The binder in dry-process electrodes is distributed in a fibrous pattern, providing more complete electrical contact between the conductive agent and the active material, improving electrode compaction density and rate performance. However, dry-process electrodes also suffer from issues such as uneven powder dispersion, low strength of the fiberized self-supporting film, weak adhesion between the dry-process film layer and the current collector, and poor initial efficiency of the full battery.

[0004] Therefore, it is necessary to provide a dry electrode, a preparation method thereof, and a secondary battery to solve the above problems. Summary of the Invention

[0005] In view of the above shortcomings of the prior art, the present invention provides a dry electrode and a preparation method thereof and a secondary battery to improve the problems of uneven dispersion of dry electrode powder, low tensile strength of dry membrane and poor initial efficiency of the whole battery.

[0006] To achieve the above-mentioned purpose and other related purposes, the present invention provides a method for preparing a dry electrode, which includes: a premixing step of premixing the active main material and the conductive agent to obtain a first mixture; a pretreatment step of freeze-crushing the binder to obtain a binder powder; a mixing step of mixing the first mixture and the binder powder to obtain a second mixture; a kneading step of kneading the second mixture to obtain a pole piece powder; a rolling step of rolling the pole piece powder into a film to obtain a dry membrane; and a compounding step of hot rolling the dry membrane and the current collector to obtain a dry electrode.

[0007] In one example of the present invention, the premixing step includes: mixing the active main material and the conductive agent at a linear speed of 100 to 200 m / s for 1 to 2 hours.

[0008] In one example of the present invention, in the pretreatment step, the working temperature of the freeze-crushing treatment is -100 to -20°C, and the particle size of the binder powder obtained after the freeze-crushing treatment is 20 to 100 μm.

[0009] In one example of the present invention, the mixing temperature of the mixing step is -50 to 10°C; and / or, the mixing step further includes a process of heat treating the second mixture, and the heat treatment temperature is 80 to 200°C.

[0010] In one example of the present invention, the kneading temperature of the kneading step is 100-200°C; and / or, the kneading step also includes a conveying process of the kneaded electrode powder, and during the conveying process, the kneaded electrode powder is kept warm at a temperature of 80-150°C.

[0011] In one example of the present invention, the rolling step uses a hot rolling process to roll the electrode powder into a film, wherein the hot rolling temperature is 100-300°C, the roller diameter is 160-360mm, and the rolling speed is 10-50m / min.

[0012] In one example of the present invention, after the rolling step, the dry membrane is further subjected to surface lithium replenishment, and the surface lithium replenishment includes sieving the lithium replenisher powder and attaching it to the surface of the dry membrane, and then hot pressing and compacting it on the surface of the dry membrane; wherein the hot pressing and compacting temperature is 80 to 150° C., and the porosity of the dry membrane after compaction is 30% to 45%.

[0013] In an example of the present invention, in the compounding step, the temperature of the hot rolling compounding is 100-200° C., and the porosity of the dry-process electrode obtained after compounding is 25%-35%.

[0014] Another aspect of the present invention provides a dry electrode, which is prepared using the preparation method described above.

[0015] The present invention also provides a secondary battery, which includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, and the positive electrode sheet and / or the negative electrode sheet are prepared using the preparation method of the present invention.

[0016] The preparation method of the dry electrode of the present invention first premixes the active main material and the conductive agent, freeze-crushes the binder into a small-particle binder powder, and then mixes the pretreated binder powder with the active main material and the conductive agent at low temperature, followed by kneading, roll-forming a film, and hot roll-compounding with a current collector to produce a dry electrode. Premixing the active main material and the conductive agent can reduce the agglomeration of the conductive agent and the active main material, while achieving uniform coating of the conductive agent on the surface of the main material and enhancing the electrical contact of the main material; freeze-crushing the binder into a small-particle powder can increase the specific surface area of ​​the binder, allowing it to fully contact the active main material and the conductive agent, improving dispersion uniformity, and making the dry electrode film high in strength and low in anisotropy, which is beneficial to improving the rate and cycle performance of the battery.

[0017] In addition, mixing the active main material, conductive agent and binder at low temperature can effectively inhibit the premature fiberization of the binder. At low temperature and high speed, the fine binder is in full contact with the active main material and conductive agent, and the mixing is more uniform. During the kneading process, the binder is fiberized under high shear force, forming a rich fiber network between the main material and the conductive agent, which significantly improves the adhesion strength of the dry membrane. At the same time, the small particle size of the binder can effectively reduce the mechanical anisotropy of the dry membrane, and the film forming processability is significantly improved. The fiberized powder is transported at high temperature to effectively improve the flexibility and looseness of the powder, and improve the dry film forming performance. The dry membrane is surface-replenished with lithium before thermally compounding with the current collector. The lithium replenisher adheres to the surface of the electrode, which can effectively improve the initial efficiency of the dry electrode. At the same time, the gas generated by the lithium replenisher placed on the surface has been discharged, which significantly improves the cycle performance of the entire battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0019] Figure 1 is a flowchart of a method for preparing a dry electrode according to an embodiment of the present invention;

[0020] Figure 2 is a flow chart of another embodiment of the method for preparing a dry electrode of the present invention;

[0021] Figure 3 Schematic diagram of the structure of a dry electrode according to an embodiment of the present invention;

[0022] Figure 4 Schematic diagram of the sample for dry electrode tensile test of the present invention.

[0023] Reference numerals

[0024] 1. Current collector; 11. Metal foil; 12. Carbon layer; 2. Dry membrane layer; 3. Lithium replenishment layer. DETAILED DESCRIPTION

[0025] The following describes the embodiments of the present invention through specific examples. Those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the following examples and the features in the examples can be combined with each other unless they conflict. It should also be understood that the terms used in the examples of the present invention are for the purpose of describing specific embodiments, not for the purpose of limiting the scope of protection of the present invention. The test methods for which specific conditions are not specified in the following examples are generally carried out under conventional conditions or under the conditions recommended by the manufacturers.

[0026] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this invention pertains. The terms used herein in the specification of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0027] Unless otherwise specified or incompatible herewith, the terms and phrases used herein shall have the following meanings:

[0028] As used herein, "plurality," "multiple," "multiple times," etc., unless otherwise specified, refer to a quantity greater than or equal to 2. For example, "one or more" means one or more than or equal to two.

[0029] Herein, the terms "preferred," "better," and "more preferred" are merely used to describe preferred implementations or examples and should not be construed as limiting the scope of protection of the present invention. If multiple "preferred" terms appear in a technical solution, each "preferred" term is considered independent unless otherwise specified and there are no contradictions or mutual constraints.

[0030] Herein, “further”, “further”, “particularly”, etc. are used for descriptive purposes to indicate differences in content, but should not be understood as limiting the scope of protection of the present invention.

[0031] When referring to a numerical range herein, unless otherwise specified, the distribution of the values ​​within the numerical range is considered continuous and includes the two numerical endpoints (i.e., the minimum and maximum values) of the numerical range, as well as every value between the two numerical endpoints. When multiple numerical ranges are provided to describe a feature or characteristic, these numerical ranges may be combined.

[0032] At present, dry-process electrodes have problems such as uneven powder dispersion, low strength of fibrous self-supporting membranes, weak adhesion between dry-process membrane layers and current collectors, and poor first efficiency of the entire battery. Patent CN114759158A provides a method for improving the adhesion of dry-process electrodes, which uses a porous current collector and a dry-process membrane for compounding. The rough surface of the porous current collector enhances the interfacial contact, thereby improving the strength of the dry-process electrode. However, the porous current collector has low tensile strength and is prone to breakage during thermal compounding. Patent CN114335409A uses carbon-coated foil as the current collector, and improves the adhesion of the coating by adding hot-melt adhesive to the carbon-coated layer. However, the hot-melt adhesive is difficult to disperse and evenly apply to the surface of the current collector, and also causes an increase in the interface resistance of the coating.

[0033] Therefore, the present invention provides a method for preparing a dry electrode, a dry electrode prepared by the method, and a secondary battery containing the dry electrode, which can improve the problems of uneven powder dispersion, large mechanical anisotropy of the electrode, low initial efficiency of the whole battery, and poor cyclability during the preparation process of the dry electrode.

[0034] See also Figure 1 The method for preparing a dry electrode of the present invention comprises at least the following steps:

[0035] S1, a premixing step of premixing the active main material and the conductive agent to prepare a first mixture;

[0036] S2, a pretreatment step, wherein the binder is subjected to a freeze-crushing process to obtain a binder powder;

[0037] S3, a mixing step of mixing the first mixed material and the binder powder to prepare a second mixed material;

[0038] S4, kneading step, kneading the second mixed material to obtain electrode powder;

[0039] S5, a rolling step, rolling the electrode powder into a film to produce a dry-process membrane;

[0040] S6, a compounding step, wherein the dry-process membrane and the current collector are hot-rolled to form a dry-process electrode.

[0041] The active main material in step S1 is the main active substance involved in the electrochemical reaction in the dry electrode. The selection of the active main material is related to the type of dry electrode and battery. For example, taking lithium-ion batteries as an example, when the dry electrode is a dry positive electrode, the active main material can be selected from one or more of lithium-rich positive electrode materials such as lithium cobaltate, lithium manganate, lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt manganate, etc. That is, the active main material can be selected from any one of the above-listed materials, for example, lithium iron phosphate, or lithium nickel cobalt manganate, etc. The active main material can also be selected from a combination of any two or more of the above-listed materials, for example, a combination of lithium cobaltate and lithium manganate, or a combination of lithium iron phosphate and lithium nickel cobalt manganate, or a combination of lithium iron phosphate, lithium manganese iron phosphate, and lithium nickel cobalt manganate, etc. When the dry electrode is a dry negative electrode, the active main material is selected from one or more of graphite, silicon-carbon negative electrode, lithium titanate, soft carbon, and hard carbon. The graphite is selected from artificial graphite and / or natural graphite. That is, the active main material can be selected from any one of the above-listed materials, for example, artificial graphite, or hard carbon, or soft carbon, etc. The active main material can also be selected from a combination of any two or more of the above-listed materials, for example, a combination of artificial graphite and hard carbon, or a combination of soft carbon and hard carbon, or a combination of artificial graphite, hard carbon, and soft carbon, etc. When the active main material is a combination of two or more materials, the ratio of the components in the combination is not limited and can be mixed in any ratio.

[0042] The conductive agent in step S1 serves to improve the electronic conductivity. In order to ensure good charge and discharge performance of the battery, it serves to collect micro-current between the active main materials and between the active main material and the current collector, thereby reducing the contact resistance of the battery and accelerating the movement rate of electrons. The conductive agent can also improve the processability of the electrode, promote the infiltration of the electrolyte into the electrode, and effectively improve the migration rate of lithium ions in the battery material, thereby improving the charge and discharge efficiency of the battery and the service life of the lithium battery. The conductive agent includes but is not limited to one or more of carbon black, acetylene black, carbon nanotubes, carbon fibers, graphene, gold powder, and silver powder. That is, the conductive agent can be any one of the above-listed materials, or a combination of any two or more of the above-listed materials. For example, the conductive agent is carbon black, or carbon nanotubes, or a combination of carbon black and acetylene black, or a combination of carbon fibers, acetylene black, and graphene. It should be noted that when the conductive agent is a combination of multiple materials, the ratio of the components in the combination is not limited and can be mixed in any ratio.

[0043] The active material and the conductive agent are pre-mixed in step S1, which can fully disperse the conductive agent and the active material, reduce the agglomeration of the conductive agent and the active material, and realize uniform coating of the conductive agent on the surface of the active material at high speed, thereby enhancing the electrical contact of the active material and improving the conductivity thereof. In some embodiments, the specific steps of pre-mixing the active material and the conductive agent are as follows: the active material and the conductive agent are placed in a pre-mixing device, such as a high-speed disperser, and mixed at a linear speed of 100-200 m / s for 1-2 h, so as to fully mix the active material and the conductive agent. As an example, the linear speed can be 100 m / s, 150 m / s, or 200 m / s, etc. If the linear speed is too low during pre-mixing, the uniform dispersion of the active material and the conductive agent cannot be achieved, and if the linear speed is too high, the structure of the active material particles is easily damaged. Therefore, the linear speed during pre-mixing is set to 100-200 m / s and the pre-mixing time is set to 1-2 h, which can not only achieve the full mixing of the active material and the conductive agent, but also will not damage the material structure of the active material.

[0044] Referring to Figure 1 The binder in step S2 mainly functions to bond the active material and the conductive agent. The binder in the dry electrode is selected from a fiberizable binder, such as one or more of polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC), and polyvinylidene fluoride (PVDF), and is preferably PTFE. The F atoms in the PTFE molecule are symmetrical, the two elements of C-F are covalently combined, there are no free electrons in the molecule, and the entire molecule is neutral, which makes PTFE have excellent dielectric properties. In addition, the PTFE molecule has an inert fluorine-containing shell outside the molecule, which makes it have outstanding non-stick properties and a low friction coefficient. Moreover, the hexagonal crystal PTFE molecule has low cohesion between adjacent chains, and sliding along the chain axis (c-axis of the hexagonal system) is easier. When a shear load is applied to the PTFE crystal, crystal slip occurs along the c-axis, and the shape of the PTFE crystal changes, forming a nano-fiber structure with a high aspect ratio.

[0045] Since the PTFE binder powder has a large particle size, it is prone to fiberization in advance when dispersed at a temperature higher than room temperature, which leads to uneven dispersion of the binder, the active material, and the conductive agent, and affects the performance of the battery. In step S2, the PTFE material is first hardened and brittle at extremely low temperature by using a freeze-crushing technique, and then broken into small PTFE particles under the action of shearing. The small PTFE binder has a large specific surface area, which facilitates full contact with the active material and the conductive agent, improves the uniformity of dispersion, and produces a dry electrode film with high strength and small anisotropy, which is beneficial to improving the rate performance and cycle performance of the battery.

[0046] In some embodiments, the specific process of the step S2 is as follows: the binder is added into the freeze crusher, the working temperature is set to be -100℃ to -20℃, and the freeze crushing time is 30 to 60 minutes; the binder powder with a particle size of 20 to 100 μm is obtained after the freeze crushing treatment. In the freeze crushing treatment, the working temperature can be -100℃, -80℃, -50℃ or -20℃, and the treatment time can be 30 minutes, 40 minutes, 50 minutes or 60 minutes; the particle size of the binder powder obtained after the freeze crushing treatment can be 20 μm, 50 μm, 70 μm or 100 μm, etc. If the freeze crushing temperature is too high and exceeds the brittle temperature of the binder, it is difficult to crush the particles; if the freeze crushing temperature is too low, the mechanical units of the equipment can be damaged and metal chips can be introduced.

[0047] It should be noted that the ratio between the active main material, the conductive agent and the binder can be set according to the conventional ratio, and is not limited herein. The step S1 and the step S2 can be performed simultaneously or sequentially, i.e., the step S1 is performed first, and then the step S2 is performed; or the step S1 and the step S2 are exchanged in sequence.

[0048] Referring to Figure 1 , the step S3 is to mix the first mixture obtained in the step S1 and the binder powder obtained in the step S2 at a low temperature to obtain a second mixture. The mixing at a low temperature can effectively inhibit the binder from being prematurely fibrillated, and under the condition of a low temperature and a high speed, the fine binder powder can be fully contacted with the active main material and the conductive agent, and the mixing is more uniform. As an example, the temperature of the low-temperature mixing is -50℃ to 10℃, for example, -50℃, -30℃, -10℃ or 10℃, etc.; the time of the low-temperature mixing is 30 minutes to 60 minutes, for example, 30 minutes, 50 minutes or 60 minutes, etc.

[0049] Referring to Figure 2 , preferably, in some embodiments, the step S3 further includes a heat treatment process of the second mixture, i.e., the second mixture after the low-temperature mixing is subjected to a heat treatment at 80 to 200℃, and the heat treatment process is helpful for the softening and loosening of the binder and is beneficial to the subsequent fibrillation treatment (kneading treatment). In some embodiments, the temperature of the heat treatment can be 80℃, 120℃, 160℃ or 200℃, etc., and the heat treatment time can be 1 to 2 hours, for example, 1 hour, 1.5 hours or 2 hours, etc.

[0050] Referring to Figure 1S4, the second mixture prepared in step S3 is subjected to kneading treatment to make it fiberize under high shear force, and the binder in the second mixture forms abundant fiber network between the active main material and the conductive agent, thereby significantly improving the adhesion strength of the dry film. Meanwhile, since the binder is subjected to the freeze-crushing treatment, the small particle size of the binder can effectively reduce the mechanical anisotropy of the dry film, thereby significantly improving the film-forming processability. In the present application, the temperature of the kneading treatment is 100-200°C, for example, 100°C, 140°C, 170°C or 200°C, etc.

[0051] Please refer to Figure 2 Preferably, after the kneading step, the process further includes a conveying process of the electrode sheet powder, i.e. conveying the electrode sheet powder obtained after the kneading to the next process. The electrode sheet powder is subjected to heat preservation treatment during the conveying process, which can effectively improve the flexibility and looseness of the powder and improve the dry film-forming performance. The temperature of the heat preservation treatment is 80-150°C, for example, 80°C, 100°C or 150°C, etc.

[0052] Please refer to Figure 1 In step S5, the electrode sheet powder prepared in step S4 is rolled into a film to prepare a dry film. Specifically, the rolling into a film is performed by using a hot rolling process, i.e. the electrode sheet powder is pressed into a continuous self-supporting film by a three-roller rolling machine, and then is subjected to multi-stage thinning by a plurality of rolling machines arranged in series to obtain a dry film with a desired thickness. The parameters of the hot rolling process are as follows: the rolling temperature is 100-300°C, the diameter of the rolling roller of the hot rolling equipment is Φ 160-360 mm, and the rolling speed is 10-50 m / min. In some embodiments, the rolling temperature can be 100°C, 200°C or 300°C, etc.; the diameter of the rolling roller can be 160 mm, 230 mm, 300 mm or 360 mm, etc.; and the rolling speed can be 10 m / min, 30 m / min or 50 m / min, etc.

[0053] Please refer to Figure 1 In step S6, the dry film prepared in step S5 is combined with a current collector to form a dry electrode. The current collector is a carbon-coated metal foil, for example, a carbon-coated copper foil or a carbon-coated aluminum foil. For a dry positive electrode, the carbon-coated aluminum foil is selected as the current collector; and for a dry negative electrode, the carbon-coated copper foil is selected as the current collector. Specifically, the dry film and the current collector are combined by hot rolling, and the temperature of the hot rolling is 100-200°C, for example, 100°C, 150°C or 200°C, etc. After the hot rolling, the dry electrode obtained has a porosity of 25%-35%, for example, 25%, 30% or 35%, etc.

[0054] Please refer to Figure 2Preferably, in some embodiments, the dry film electrode further comprises surface lithium supplementing of the dry film before the dry film and the current collector are combined, which can effectively improve the initial efficiency of the dry electrode, and the gas produced by the surface lithium supplementing agent has been removed, which can significantly improve the cycle performance of the full battery. The specific process of surface lithium supplementing is as follows: the lithium supplementing agent powder is attached to the surface of the dry film after being sieved, and then is compacted on the surface of the dry film by hot pressing. The lithium supplementing agent is selected according to the type of the dry electrode, and if the dry electrode is a dry positive electrode, the lithium supplementing agent includes but is not limited to Li5FeO4 and / or Li2NiO2; if the dry electrode is a dry negative electrode, the lithium supplementing agent includes but is not limited to stabilized lithium metal powder (SLMP). The temperature of the hot pressing compaction is 80-150 DEG C, for example, 80 DEG C, 120 DEG C or 150 DEG C, etc. The porosity of the dry film after compaction is 30-45%, for example, 30%, 40% or 45%, etc. The amount of the lithium supplementing agent added is set according to the type of the lithium supplementing agent and the performance of the active main material.

[0055] The preparation method of the dry electrode of the present application can significantly improve the uniformity of powder dispersion and the strength of the dry film, reduce the anisotropy of the electrode, and improve the initial efficiency and cycle life of the full battery.

[0056] Referring to Figure 3 The dry electrode of the present application is prepared by the above preparation method, which can be a dry positive electrode or a dry negative electrode. The dry electrode can be used in lithium ion batteries, sodium ion batteries and other secondary batteries.

[0057] Referring to Figure 3 In an embodiment, the dry electrode comprises a current collector 1 and a dry film layer 2, wherein the current collector 1 is a carbon-coated metal foil, which comprises a metal foil 11 and a carbon layer 12 coated on the metal foil 11; the current collector 1 has two opposite surfaces along the thickness direction, and the dry film layer 2 is arranged on at least one surface of the current collector 1, preferably, the dry film layer 2 is arranged on both surfaces of the current collector 1.

[0058] Referring to Figure 3 Preferably, the dry electrode further comprises a lithium supplementing layer 3, which is arranged on the dry film layer 2. The arrangement of the lithium supplementing layer 3 can effectively improve the initial efficiency of the dry electrode, and since the lithium supplementing layer 3 is compacted on the surface of the dry film layer 2 by hot pressing of the lithium supplementing agent, the gas produced by the lithium supplementing agent has been removed, which can significantly improve the cycle performance of the full battery.

[0059] The secondary battery of the present application comprises a positive electrode, a negative electrode, a separator and an electrolyte, wherein the separator is arranged between the positive electrode and the negative electrode to play a separating role; the electrolyte is soaked in the positive electrode and the negative electrode in the shell to play a role of conducting lithium ions; the positive electrode and / or the negative electrode is prepared by the preparation method described above.

[0060] The technical solutions of the present invention are described in detail below through several specific examples and comparative examples. Unless otherwise stated, the raw materials and reagents used in the following examples are commercially available or can be prepared by conventional methods in the art, and the instruments used in the examples are all commercially available.

[0061] Example 1

[0062] The dry-process electrode prepared in this embodiment is a dry-process positive electrode, and its active main material is NCM523 (LiNi 0.5 Co 0.2 Mn 0.3 ), the conductive agent is carbon black (SP) and carbon nanotubes (CNT), the binder is PTFE, and the mass percentage between the components is 95:2:1:2; the preparation method of the dry positive electrode is as follows:

[0063] (1) Premixing step: The active main material NCM523, the conductive agent SP and the CNT powder are premixed in a high-speed disperser of the premixing equipment. The dispersion line speed of the high-speed disperser is 150m / s and the dispersion time is 1h. After premixing, the surface of the NCM active main material is coated with SP and CNT to form a good electrical contact.

[0064] (2) Pretreatment step: Add the binder PTFE powder into a freeze crusher for freeze crushing. The operating temperature of the freeze crusher is -80°C, the freeze crushing time is 30 minutes, and the particle size of the crushed PTFE powder is D50 = 80 μm. The reduced particle size of the PTFE binder is more conducive to uniform contact with the main material and the conductive agent.

[0065] (3) Mixing step: The frozen crushed PTFE powder is mixed with the pre-mixed active main material NCM523 and the conductive agent SP and CNT at a low temperature, the low temperature mixing temperature is -10°C, and the low temperature mixing time is 30 minutes; under low temperature conditions, the PTFE powder is not prone to premature fiberization, and at the same time, high-speed stirring at low temperature can fully disperse and contact the active main material, conductive agent, and binder;

[0066] (4) Heat treatment step: The powder after low-temperature mixing is heat treated at a temperature of 120°C for 1 hour. Heat treatment helps soften and loosen the PTFE binder, which is beneficial for subsequent fiberization treatment.

[0067] (5) Kneading step: The heat-treated powder is placed in a kneader, and the PTFE is sheared at high speed at high temperature to fiberize it. The kneading temperature of the kneader is 150°C. High-temperature kneading can effectively improve the fiberization process of PTFE, so that a dense fiber network is formed between the NCM523 active main material and the conductive agent, which is conducive to dry film formation.

[0068] (6) Conveying step: The kneaded powder is conveyed to the lower hopper through a pipeline, and the conveying pipeline and the lower hopper are insulated to maintain the powder temperature at 90°C;

[0069] (7) Rolling step: The hot powder delivered to the lower hopper is pressed into a continuous self-supporting dry film by a three-roller press, and then multi-stage thinning is performed by a continuously arranged multi-roller press to obtain the NCM523 positive electrode dry film of the required thickness; wherein, the rolling film formation and multi-stage thinning are performed by hot rolling, and the rolling temperature is 150°C.

[0070] (8) Surface lithium replenishment: The lithium replenisher Li2NiO2 is evenly sprayed on the surface of the dry film through a high-speed vibrating screen, and then hot roller pressing is performed at a temperature of 100°C to compact the sprayed lithium replenisher particles and embed them on the surface of the dry film. The porosity of the dry film after compaction is 35%;

[0071] (9) Composite step: The dry film after surface lithium supplementation is double-sided hot composited with carbon-coated aluminum foil, the hot rolling temperature is 120°C, and the porosity of the electrode after hot rolling is 30%, thereby producing an NCM523 dry-process positive electrode.

[0072] Example 2

[0073] The difference between this embodiment and embodiment 1 is that during the freeze-crushing process in step (2), the operating temperature of the freeze-crushing machine is -50°C and the freeze-crushing time is 30 minutes.

[0074] Example 3

[0075] The difference between this embodiment and embodiment 1 is that: during the mixing in step (3), the low-temperature mixing temperature is 10° C. and the low-temperature mixing time is 30 minutes.

[0076] Example 4

[0077] The difference between this embodiment and embodiment 1 is that the heat treatment temperature in step (4) is 150° C. and the heat treatment time is 1 hour.

[0078] Example 5

[0079] The difference between this embodiment and embodiment 1 is that the kneading temperature in step (5) is 180°C.

[0080] Example 6

[0081] The difference between this embodiment and embodiment 1 is that the rolling temperature of the hot rolling in step (7) is 180°C.

[0082] Example 7

[0083] The difference between this embodiment and embodiment 1 is that the lithium replenishing agent used in step (8) for surface lithium replenishment is Li5FeO4.

[0084] Example 8

[0085] The difference between this embodiment and embodiment 1 is that the lithium replenishing agent used in step (8) for surface lithium replenishment is Li5FeO4, and the Li5FeO4 powder is evenly sprayed on the surface of the dry film by a high-speed vibrating screen, followed by hot roller pressing at a temperature of 120°C to compact the sprayed lithium replenishing agent particles and embed them on the surface of the dry film. The porosity of the compacted dry film is 30%.

[0086] Example 9

[0087] The difference between this embodiment and embodiment 1 is that: in step (9), the dry film after surface lithium supplementation is double-sided hot-compounded with the carbon-coated aluminum foil, the hot rolling temperature is 150° C., and the electrode porosity after hot rolling is 25%.

[0088] Example 10

[0089] The dry-process electrode prepared in this embodiment is a dry-process positive electrode, wherein the active main material is LiFePO4, the conductive agent is carbon black (SP) and carbon nanotubes (CNT), and the binder is PTFE. The mass percentage of each component is LiFePO4:SP:CNT:PTFE=94:2:1:3. The preparation method of the dry-process positive electrode is as follows:

[0090] (1) Premixing step: The active main material LiFePO4 and the conductive agent SP and CNT powder are premixed in a high-speed disperser of the premixing equipment. The dispersion line speed of the high-speed disperser is 200m / s and the dispersion time is 1h. After premixing, the surface of the LiFePO4 active main material is coated with SP and CNT to form a good electrical contact.

[0091] (2) Pretreatment step: Add the binder PTFE powder into a freeze crusher for freeze crushing. The operating temperature of the freeze crusher is -100°C, the freeze crushing time is 30 minutes, and the particle size of the crushed PTFE powder is D50 = 20 μm. The reduced particle size of the PTFE binder is more conducive to uniform contact with the main material and the conductive agent.

[0092] (3) Mixing step: The frozen crushed PTFE powder is mixed with the pre-mixed active main material LiFePO4 and conductive agent SP and CNT at a low temperature, the low temperature mixing temperature is -50°C, and the low temperature mixing time is 30 minutes; under low temperature conditions, the PTFE powder is not prone to premature fiberization, and at the same time, high-speed stirring at low temperature can fully disperse and contact the active main material, conductive agent, and binder;

[0093] (4) Heat treatment step: The powder after low-temperature mixing is heat treated at a temperature of 200°C for 1 hour. The heat treatment helps soften and loosen the PTFE binder, which is beneficial for subsequent fiberization treatment.

[0094] (5) Kneading step: The heat-treated powder is placed in a kneader and the PTFE is sheared at high speed at high temperature to fiberize it. The kneading temperature of the kneader is 200°C. High-temperature kneading can effectively improve the fiberization process of PTFE, so that a dense fiber network is formed between the LiFePO4 active main material and the conductive agent, which is conducive to dry film formation.

[0095] (6) Conveying step: The kneaded powder is conveyed to the lower hopper through a pipeline, and the conveying pipeline and the lower hopper are insulated to maintain the powder temperature at 100°C;

[0096] (7) Rolling step: The hot powder delivered to the lower hopper is pressed into a continuous self-supporting dry film by a three-roller press, and then multi-stage thinning is performed by a continuously arranged multi-roller press to obtain a LiFePO4 positive electrode dry film of the required thickness; wherein, the rolling film formation and multi-stage thinning are performed by hot rolling, and the rolling temperature is 300°C.

[0097] (8) Surface lithium replenishment: The lithium replenisher Li2NiO2 is evenly sprayed on the surface of the dry film through a high-speed vibrating screen, and then hot roller pressing is performed at a temperature of 150°C to compact the sprayed lithium replenisher particles and embed them on the surface of the dry film. The porosity of the dry film after compaction is 45%;

[0098] (9) Composite step: The dry film after surface lithium supplementation is double-sided hot composited with carbon-coated aluminum foil, the hot rolling temperature is 100°C, and the porosity of the electrode after hot rolling is 35%, thereby preparing a LiFePO4 dry-process positive electrode.

[0099] Example 11

[0100] The difference between this embodiment and embodiment 1 is that the active main material is LiFePO4, and the mass percentage of the components is: LiFePO4:SP:CNT:PTFE=94:2:1:3.

[0101] Example 12

[0102] The dry-process electrode prepared in this embodiment is a dry-process negative electrode, wherein the active main material is artificial graphite, the conductive agent is carbon black (SP), and the binder is PTFE. The mass percentage of each component is artificial graphite:SP:PTFE=96:2:2. The preparation method of the dry-process negative electrode is as follows:

[0103] (1) Premixing step: the active main material artificial graphite and the conductive agent SP powder are premixed in a high-speed disperser of a premixing device, the dispersion linear speed of the high-speed disperser is 100 m / s, and the dispersion time is 2 h; after premixing, the artificial graphite is coated with SP to form good electrical contact;

[0104] (2) Pretreatment step: the binder PTFE powder is added to a frozen pulverizer for frozen crushing treatment, the working temperature of the frozen pulverizer is -20℃, the frozen crushing time is 30 min, and the particle size of the crushed PTFE powder is D50=100 μm; the particle size of the PTFE binder is reduced, which is more conducive to uniform contact with the main material and the conductive agent;

[0105] (3) Mixing step: the frozen crushed PTFE powder is mixed with the premixed active main material artificial graphite and the conductive agent SP at a low temperature, the low-temperature mixing temperature is -30℃, and the low-temperature mixing time is 30 min; under the low-temperature condition, the PTFE powder is not prone to premature fiberization, and high-speed stirring under the low temperature can make the active main material, the conductive agent and the binder fully dispersed and contacted;

[0106] (4) Heat treatment step: the powder after low-temperature mixing is subjected to heat treatment, the heat treatment temperature is 80℃, and the heat treatment time is 1 h; the heat treatment is helpful for softening and loosening of the PTFE binder, and is conducive to subsequent fiberization treatment;

[0107] (5) Kneading step: the powder after heat treatment is placed in a kneader, and the PTFE is subjected to high-speed shearing at a high temperature to make it fiberized, wherein the kneading temperature of the kneader is 100℃; the high-temperature kneading can effectively improve the fiberization process of the PTFE, so that a dense fiber network is formed between the artificial graphite and the conductive agent, which is helpful for dry film formation.

[0108] (6) Conveying step: the kneaded powder is conveyed to a hopper through a pipeline, and the pipeline and the hopper are subjected to heat preservation treatment to maintain the powder temperature at 80℃;

[0109] (7) Roll pressing step: the hot powder conveyed to the hopper is pressed into a continuous self-supporting dry film by a three-roller press, and then subjected to multi-stage thinning by a plurality of continuously arranged multi-roller presses to obtain an anode dry film with a required thickness; wherein the roll pressing and multi-stage thinning are performed by hot roll pressing, and the roll pressing temperature is 100℃.

[0110] (8) Surface lithium supplementing: the lithium supplementing agent metal lithium powder is uniformly sprayed on the surface of the dry film by a high-speed vibrating screen, and then subjected to hot roll pressing at a temperature of 80℃, so that the sprayed lithium supplementing agent particles are compacted and embedded in the surface of the dry film, and the porosity of the compacted dry film is 30%;

[0111] (9) Composite step: The dry film after surface lithium supplementation is double-sided hot composited with carbon-coated aluminum foil, the hot rolling temperature is 200°C, and the porosity of the electrode after hot rolling is 25%, thereby producing an artificial graphite dry-process negative electrode.

[0112] Example 13

[0113] The dry electrode prepared in this embodiment is a dry negative electrode. The main material of the dry electrode is artificial graphite, the conductive agent is carbon black, and the binder is PTFE. The mass percentage between the components is: artificial graphite: SP: PTFE = 96:2:2. The lithium supplement agent is metallic lithium powder. The preparation process is the same as that of Example 1.

[0114] Comparative Example 1

[0115] The difference between this comparative example and Example 1 is that the dispersion linear velocity during premixing in step (1) is 50 m / s and the dispersion time is 1 h.

[0116] Comparative Example 2

[0117] The difference between this comparative example and Example 1 is that the dispersion linear velocity during premixing in step (1) is 250 m / s and the dispersion time is 1 h.

[0118] Comparative Example 3

[0119] The difference between this comparative example and Example 1 is that during the pretreatment in step (2), the operating temperature of the freezing crusher is -10°C, the freezing crushing time is 30 minutes, and the particle size D50 of the crushed PTFE powder is 250 μm.

[0120] Comparative Example 4

[0121] The difference between this comparative example and Example 1 is that the mixing temperature during low-temperature mixing in step (3) is 20° C. and the mixing time is 30 min.

[0122] Comparative Example 5

[0123] The difference between this comparative example and Example 1 is that the conveying process in step (6) is carried out at room temperature.

[0124] Comparative Example 6

[0125] The difference between this comparative example and Example 1 is that: the NCM523 active main material and the conductive agent powder are first premixed, and then PTFE powder is added for fiberization treatment; the fiberized powder is formed into a self-supporting film and thinned by the same method as in Example 1 to obtain an electrode dry film, and the prepared dry film is directly hot-rolled and composited with carbon-coated aluminum foil to obtain an NCM523 dry-process positive electrode.

[0126] Comparative Example 7

[0127] The active main material of the dry electrode of the present comparative example is LiFePO4, and the mass percentage of each component is: LiFePO4: SP: CNT: PTFE = 94: 2: 1: 3, and the preparation process is the same as that of Comparative Example 6.

[0128] Comparative Example 8

[0129] The main material of the dry electrode of the present comparative example is artificial graphite, the conductive agent is carbon black, and the binder is PTFE, and the mass percentage of each component is: artificial graphite: SP: PTFE = 96: 2: 2, and the preparation process is the same as that of Comparative Example 6.

[0130] The dry electrodes prepared in Examples 1 to 13 and Comparative Examples 1 to 8 were tested for performance, and the test method is shown below, and the test results are shown in Table 1.

[0131] (1) Tensile strength test:

[0132] According to the requirements, 5 samples were cut along the longitudinal direction and the transverse direction respectively, and the shape and size of the sample are shown in Figure 4 According to the national standard GB 6672, the thickness of the sample was measured, and the thickness of three points in the sample was measured, and the arithmetic mean value was taken. The sample was clamped so that the longitudinal axis of the sample coincided with the center line of the upper and lower clamps of the testing machine, and the tension was appropriate. The testing machine was started at a specified speed ((250 ± 50) mm / min) for tensile test, and after the sample was broken, the load and elongation of the distance between the marks were read to calculate the tensile strength.

[0133] (2) Test of electrode resistance:

[0134] The electrode resistance tester was used to test the resistance of the dry electrode, and the dry electrode to be tested was placed between the two electrodes of the electrode resistance tester to start the test, and the software automatically read the thickness, resistance, resistivity, conductivity and other data of the electrode.

[0135] (3) Adhesion test:

[0136] The hot-composite electrode was cut into 20*100mm, and the side to be tested was adhered to the stainless steel plate with double-sided tape, and the pressure roller was used to compact it to completely adhere to the electrode. One end of the stainless steel plate was fixed to the lower clamp of the tensile testing machine, and the curved end of the sample was fixed to the upper clamp. The sample was tested at a tensile speed of 50mm / min.

[0137] The dry electrodes prepared in Examples 1 to 13 and Comparative Examples 1 to 8 were assembled into battery cells, and the DCR (direct current impedance) performance and cycle performance of the battery cells were tested, and the assembly process is as follows:

[0138] Examples 1 to 11 prepared dry-process positive electrodes, and Examples 12 and 13 prepared dry-process negative electrodes. When assembling the battery cells, the negative electrodes of Examples 1 to 11 were graphite negative electrodes prepared using a conventional wet slurry process; the positive electrodes of Examples 12 and 13 were LFP positive electrodes prepared using a conventional wet slurry process. The negative electrodes of the assembled battery cells of Comparative Examples 1 to 7 were graphite negative electrodes prepared using a conventional wet slurry process, and the positive electrode of the assembled battery cell of Comparative Example 8 was an LFP positive electrode prepared using a conventional wet slurry process.

[0139] (4)DCR test

[0140] Charge the cell with 1 / 3C constant current to 4.2V, charge it with constant voltage to 0.05C, let it sit for 30min, discharge it with 1 / 3C0 to 3V, let it sit for 30min, and record the discharge capacity C0; charge it with 1 / 3C0 constant current to 4.2V, charge it with constant voltage to 0.05C0, let it sit for 30min, discharge it with 1 / 3C0 for 180min, let it sit for 10min, and record the end-of-storage voltage V0. Discharge it with 2C for 30s, and record the end-of-discharge voltage V1. Discharge DCR = (V0-V1) / I.

[0141] (5) Cycle test:

[0142] Charge the cell with 1 / 3C constant current to 4.2V, charge it with constant voltage to 0.05C, let it sit for 30 minutes, discharge it with 1 / 3C0 to 3V, let it sit for 30 minutes, record the discharge capacity C0, charge it with 1 / 3C0 constant current to 4.2V, charge it with constant voltage to 0.05C0, let it sit for 30 minutes, discharge it with 1 / 3C0 to 3V, let it sit for 30 minutes, record the discharge capacity C1, and record the discharge capacity Cn at the end of each cycle in this way. The cycle capacity retention rate is Cn / C1*100%. Record the number of cycles N when the retention rate is 80%.

[0143] Table 1: Performance comparison of dry-process electrodes prepared in Examples 1 to 13 and Comparative Examples 1 to 8

[0144]

[0145] Comparing Examples 1 to 9 with Comparative Example 6, Examples 10 to 11 with Comparative Example 7, and Examples 12 to 13 with Comparative Example 8, the results show that the tensile strength and bonding strength of the dry-process electrodes prepared by the preparation method of the present invention are greater than those of the dry-process electrodes prepared by the conventional method, and the electrode sheet resistance and the DCR value of the battery cell are both lower than those of the dry-process electrodes prepared by the conventional method; the cycle performance of the battery cell is better than that of the battery cell prepared by the conventional method; this indicates that the preparation method of the present invention can significantly improve the powder dispersion uniformity and dry film strength, reduce the anisotropy of the electrode sheet, and improve the first efficiency and cycle life of the full battery.

[0146] By comparing Example 1 with Comparative Examples 1 and 2, it can be concluded that: in the pre-mixing step of the active main material and the conductive agent, if the dispersion line speed is too small, the active main material and the conductive agent cannot be fully dispersed, resulting in agglomeration and uneven dispersion, causing the electrode resistance and internal resistance to increase; if the dispersion line speed is too large, the active main material particles are likely to break, causing failure of the main material structure, and thus affecting the cycle performance of the battery cell.

[0147] By comparing Example 1 and Comparative Example 3, it can be concluded that in the pretreatment step, the temperature of the freeze-crushing is too high, and the PTFE is difficult to be crushed into small particles, which affects its uniform dispersion and film-forming uniformity.

[0148] By comparing Example 1 and Comparative Example 4, it can be concluded that when mixing at low temperature, if the temperature is too high, the PTFE binder is prone to premature fiberization, adhesion of the conductive agent causes local agglomeration, and affects the uniformity of film formation.

[0149] By comparing Example 1 and Comparative Example 5, it can be concluded that if the powder is not subjected to heat preservation treatment during transportation, the fiberized powder will be cooled quickly and the film-forming stability will be deteriorated.

[0150] When preparing a dry electrode, the present invention first pre-mixes the active main material and the conductive agent at high speed, which can fully disperse the conductive agent and the active main material and reduce the agglomeration of the conductive agent and the active main material. At the same time, the high-speed dispersion can achieve uniform coating of the conductive agent on the surface of the active main material, thereby enhancing the electrical contact of the active main material. The binder is frozen and crushed to obtain small particles of the binder. The fine binder has a large specific surface area and is easy to fully contact with the active main material and the conductive agent, thereby improving the dispersion uniformity. The obtained dry electrode film has high strength and small anisotropy, which is beneficial to improving the rate performance and cycle performance of the battery. Mixing under low temperature conditions can effectively inhibit the premature fiberization of the binder, and the fine particles under low temperature and high speed can effectively prevent the binder from being fibrillated. The small binder is in full contact with the active main material and the conductive agent, and the mixing is more uniform; when kneaded at high temperature, the binder is fiberized under high shear force, forming a rich fiber network between the main material and the conductive agent, which significantly improves the adhesion strength of the dry film. At the same time, the fine binder can effectively reduce the mechanical anisotropy of the dry film, and the film forming processability is significantly improved; the fiberized powder is transported at high temperature, which can effectively improve the flexibility and looseness of the powder and improve the dry film forming performance; before the dry film is thermally composited with the current collector, the surface lithium replenishment is performed, so that the lithium replenisher is attached to the electrode surface, which can effectively improve the dry electrode first effect. At the same time, the gas generated by the lithium replenisher placed on the surface has been discharged, which significantly improves the full battery cycle performance. Therefore, the present invention effectively overcomes some practical problems in the prior art and has high utilization value and use significance.

[0151] The above embodiments are only illustrative of the principles of the present application and its efficacy, and are not intended to limit the present application. Any modification or change made by any person skilled in the art without departing from the spirit and scope of the present application shall be covered by the claims of the present application.

Claims

1. A method for preparing a dry electrode, characterized in that: include: a premixing step of premixing the active main material and the conductive agent to prepare a first mixed material; In a pretreatment step, the binder is subjected to a freeze-crushing treatment to obtain a binder powder; a mixing step of mixing the first mixed material and the binder powder to obtain a second mixed material; a kneading step of kneading the second mixed material to obtain a pole piece powder; a rolling step of rolling the electrode powder into a film to produce a dry-process membrane; a compounding step of hot rolling-combining the dry-process membrane and the current collector to obtain a dry-process electrode; Wherein, the mixing temperature of the mixing step is -50~10°C; After the rolling step, the dry membrane is further subjected to surface lithium replenishment. The surface lithium replenishment includes sieving lithium replenisher powder and attaching it to the surface of the dry membrane, and then hot pressing and compacting it on the surface of the dry membrane. The hot pressing and compacting temperature is 80-150° C., and the porosity of the compacted dry membrane is 30%-45%.

2. The preparation method according to claim 1, wherein The premixing step includes mixing the active main material and the conductive agent at a linear speed of 100-200 m / s for 1-2 hours.

3. The preparation method according to claim 1, wherein In the pretreatment step, the working temperature of the freeze-crushing treatment is -100 to -20°C, and the particle size of the binder powder obtained after the freeze-crushing treatment is 20 to 100 μm.

4. The preparation method according to claim 1, characterized in that The mixing step further includes a process of heat treating the second mixed material, wherein the heat treatment temperature is 80-200°C.

5. The preparation method according to claim 1, characterized in that The kneading temperature of the kneading step is 100~200℃; and / or, the kneading step also includes a transportation process for the kneaded electrode powder, and the kneaded electrode powder is kept warm during the transportation process, and the insulation temperature is 80~150℃.

6. The preparation method according to claim 1, characterized in that The rolling step adopts a hot rolling process to roll the electrode powder into a film, wherein the hot rolling temperature is 100-300° C., the roller diameter is 160-360 mm, and the rolling speed is 10-50 m / min.

7. The preparation method according to any one of claims 1 to 6, characterized in that In the compounding step, the temperature of the hot rolling compounding is 100-200° C., and the porosity of the dry-process electrode obtained after compounding is 25%-35%.

8. A dry electrode, characterized in that: The dry electrode is prepared by the preparation method according to any one of claims 1 to 7.

9. A secondary battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet and / or the negative electrode sheet are prepared by the preparation method according to any one of claims 1 to 7.

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