Dry film forming method, dry electrode sheet and preparation method thereof, and battery

Abstract

The present invention relates to a dry film-forming method, a dry electrode pole piece, a preparation method thereof, and a battery. The dry film-forming method reduces dependence on mechanical shearing through pre-fiberization treatment of an adhesive, thereby reducing energy consumption. The method adopts graded rolling technology, first low-pressure molding, and then high-pressure densification, thereby improving the pressing effect. In addition, electrostatically assisted compounding and segmented electrification are adopted to increase charge density, improve interfacial contact through charge action, overcome the defect of insufficient bonding force, enhance the bonding strength between the dry electrode film and the current collector, and improve the electrode quality.

Description

Technical Field

[0001] The invention relates to a dry film-making method, a dry electrode pole piece and a preparation method thereof, and a battery. Background Art

[0002] With the development of science and technology, lithium batteries have been widely used due to their advantages of high operating voltage, long cycle life and high energy density. However, the electrodes of lithium batteries are generally made using a wet process. The environmental problems caused by this process are extremely serious. Therefore, a dry process was developed to prepare the electrodes.

[0003] Although the existing dry film-making technology has advantages such as environmental protection and no solvent residue, it still has defects in the actual industrialization process, such as uneven adhesive fiberization and weak interface bonding of the current collector. The thin electrode processing is not as expected, which restricts the large-scale application of this technology. Summary of the Invention

[0004] The technical problem to be solved by the present invention is: In order to overcome the above technical problems, the present invention provides a dry film forming method, a dry electrode plate and a preparation method thereof, and a battery.

[0005] The technical solution adopted by the present invention to solve the technical problem is: a dry film forming method, comprising the following steps:

[0006] a. Raw material pretreatment: Dry the active material to remove moisture from the active material, pre-disperse the conductive agent, and fiberize the adhesive;

[0007] b. Dry premixing: In a high-speed shear mixer, mix the active material and the conductive agent for 10 minutes at a mixing temperature of 30-50°C.

[0008] c. Dry mixing: Add a binder to the mixture of active material and conductive agent, with the binder accounting for 5-8%, mixing for 30 minutes at a temperature of 30-50°C, then add lithium powder to the mixture and mix again for 20 minutes to obtain a mixed dry powder;

[0009] d. Dry film forming: The mixed dry powder is pressed into a self-supporting film through a double-roll calender.

[0010] Furthermore, the active material includes a positive electrode active material and a negative electrode active material, the positive electrode active material includes at least one of lithium iron phosphate, lithium manganese phosphate or lithium iron manganese phosphate, the positive electrode active material is coated with a conductive layer, and the conductive layer includes aluminum oxide; the negative electrode active material includes at least one of a carbon-based material, a silicon-based material or lithium titanate.

[0011] Furthermore, the active material is in spherical or flake form, and the D50 particle size of the active material is 10-20 μm.

[0012] Furthermore, the conductive agent is carbon fiber, the adhesive is polytetrafluoroethylene, and the fiberization treatment method of the adhesive is: using a polytetrafluoroethylene emulsion with an average molecular weight of about 3 million and a particle size of 0.05-0.5 μm, the emulsion concentration is 60%, and after mixing it with a viscose fiber-forming carrier, a spinning solution is prepared. After spinning, the carrier is carbonized and removed at high temperature, and the polymer is sintered to continuously form fibers.

[0013] Furthermore, the specific steps of dry film formation include:

[0014] d1. Initial pressing: The mixed material is initially compacted by a double-roll calender to form a continuously transportable embryonic membrane. The roller spacing of the double-roll calender is 0.1-1m, the pressure is 5-15MPa, and the temperature is 80-120℃. The output form is a self-supporting loose membrane belt with a thickness of 0.5-2mm.

[0015] d2. Fine pressing: further compacted by a multi-stage roller press, the roller surface is plated with hard chrome, the roller surface parallelism is ≤1μm, the pressure is 50-200MPa, the line speed is 0.5-5m / min, the temperature is 100-150℃, and the output form is a dense electrode film with a thickness of 50-300μm and a porosity of 20-40%.

[0016] Furthermore, the pre-dispersion treatment step of the conductive agent includes:

[0017] a1. Surface modification: Using atomic layer deposition technology, a layer of aluminum oxide film is deposited on the surface of carbon nanotubes with a thickness of 2-5nm. Then, the carbon black surface is treated with argon and oxygen plasma.

[0018] a2. Mechanical fusion: The material is fused using a fusion spheroidizer. The speed of the equipment is 2000-3000 rpm, the fusion time is 20-40 minutes, and the temperature is 80°C.

[0019] a3. Classification treatment: Use air flow classifier to separate large particles, control the D50 particle size to 1-5μm, and then use electrostatic separation method to disperse the particles.

[0020] A method for preparing a dry electrode sheet, comprising:

[0021] The dry-process membrane sheet prepared by the dry-process membrane preparation method as described above is compounded with a current collector to obtain a dry-process electrode sheet.

[0022] Furthermore, the step of compounding the dry process membrane and the current collector includes:

[0023] e1. Current collector preparation: Surface treatment of the current collector is performed, and surface oil stains are removed by plasma cleaning;

[0024] e2. Primary charging treatment: using corona discharge to charge the dry process membrane;

[0025] e3. Dry diaphragm activation: preheat the dry diaphragm to 80-100℃ by infrared preheating;

[0026] e4. Secondary charging treatment: Integrate corona electrodes on the roller press. Before pressing, make the membrane and current collector pass through the corona zone, so that the membrane is charged again and the current collector is charged for the first time.

[0027] e5. Hot pressing composite: The membrane and the charged surface of the current collector are hot pressed together by a continuous roller press. The hot pressing temperature is 120-180°C, the pressure is 20-50 MPa, the rolling time is 10-30 seconds, and the roller speed is 0.5-2 m / min.

[0028] e6. Film winding: Cool the pressure roller with cooling water to quickly cool it down to 20-25°C, and control the winding tension between 10-15N;

[0029] e7. Cutting: Cut into pole pieces according to battery specifications;

[0030] e8. Vacuum drying: Dry the electrode in a vacuum environment at a temperature of 120°C for 12 hours;

[0031] e9. Secondary rolling: The electrode is rolled for the second time, and the electrode density is controlled at 2.5-3.5 g / cm3.

[0032] A dry electrode plate is prepared by the method as described above.

[0033] A battery comprises the dry electrode plate described above.

[0034] The beneficial effect of the present invention is that the dry film preparation method reduces the dependence on mechanical shearing through the pre-fiberization treatment of the adhesive, thereby reducing energy consumption. It adopts graded rolling technology, first low-pressure molding, and then high-pressure densification, which improves the pressing effect. It also adopts electrostatic assisted compounding and segmented electrification to increase the charge density, improve the interface contact through the action of charge, overcome the defect of insufficient adhesion, improve the bonding strength between the dry electrode membrane and the current collector, and improve the electrode quality. DETAILED DESCRIPTION

[0035] A dry film forming method comprises the following steps:

[0036] a. Raw material pretreatment: Dry the active material to remove moisture from the active material, pre-disperse the conductive agent, and fiberize the adhesive;

[0037] b. Dry premixing: In a high-speed shear mixer, mix the active material and the conductive agent for 10 minutes at a mixing temperature of 30-50°C.

[0038] c. Dry mixing: Add a binder to the mixture of active material and conductive agent, with the binder accounting for 5-8%, mixing for 30 minutes at a temperature of 30-50°C, then add lithium powder to the mixture and mix again for 20 minutes to obtain a mixed dry powder;

[0039] d. Dry film forming: The mixed dry powder is pressed into a self-supporting film through a double-roll calender.

[0040] Furthermore, the active material includes a positive electrode active material and a negative electrode active material, the positive electrode active material includes at least one of lithium iron phosphate, lithium manganese phosphate or lithium iron manganese phosphate, the positive electrode active material is coated with a conductive layer, and the conductive layer includes aluminum oxide; the negative electrode active material includes at least one of a carbon-based material, a silicon-based material or lithium titanate.

[0041] Furthermore, the active material is in spherical or flake form, and the D50 particle size of the active material is 10-20 μm.

[0042] Furthermore, the conductive agent is carbon fiber, the adhesive is polytetrafluoroethylene, and the fiberization treatment method of the adhesive is: using a polytetrafluoroethylene emulsion with an average molecular weight of about 3 million and a particle size of 0.05-0.5 μm, the emulsion concentration is 60%, and after mixing it with a viscose fiber-forming carrier, a spinning solution is prepared. After spinning, the carrier is carbonized and removed at high temperature, and the polymer is sintered to continuously form fibers.

[0043] Furthermore, the specific steps of dry film formation include:

[0044] d1. Initial pressing: The mixed material is initially compacted by a double-roll calender to form a continuously transportable embryonic membrane. The roller spacing of the double-roll calender is 0.1-1m, the pressure is 5-15MPa, and the temperature is 80-120℃. The output form is a self-supporting loose membrane belt with a thickness of 0.5-2mm.

[0045] d2. Fine pressing: further compacted by a multi-stage roller press, the roller surface is plated with hard chrome, the roller surface parallelism is ≤1μm, the pressure is 50-200MPa, the line speed is 0.5-5m / min, the temperature is 100-150℃, and the output form is a dense electrode film with a thickness of 50-300μm and a porosity of 20-40%.

[0046] Furthermore, the pre-dispersion treatment step of the conductive agent includes:

[0047] a1. Surface modification: Using atomic layer deposition technology, a layer of aluminum oxide film is deposited on the surface of carbon nanotubes with a thickness of 2-5nm. Then, the carbon black surface is treated with argon and oxygen plasma.

[0048] a2. Mechanical fusion: The material is fused using a fusion spheroidizer. The speed of the equipment is 2000-3000 rpm, the fusion time is 20-40 minutes, and the temperature is 80°C.

[0049] a3. Classification treatment: Use air flow classifier to separate large particles, control the D50 particle size to 1-5μm, and then use electrostatic separation method to disperse the particles.

[0050] A method for preparing a dry electrode sheet, comprising:

[0051] The dry-process membrane sheet prepared by the dry-process membrane preparation method as described above is compounded with a current collector to obtain a dry-process electrode sheet.

[0052] Furthermore, the step of compounding the dry process membrane and the current collector includes:

[0053] e1. Current collector preparation: Surface treatment of the current collector is performed, and surface oil stains are removed by plasma cleaning;

[0054] e2. Primary charging treatment: using corona discharge to charge the dry process membrane;

[0055] e3. Dry diaphragm activation: preheat the dry diaphragm to 80-100℃ by infrared preheating;

[0056] e4. Secondary charging treatment: Integrate corona electrodes on the roller press. Before pressing, make the membrane and current collector pass through the corona zone, so that the membrane is charged again and the current collector is charged for the first time.

[0057] e5. Hot pressing composite: The membrane and the charged surface of the current collector are hot pressed together by a continuous roller press. The hot pressing temperature is 120-180°C, the pressure is 20-50 MPa, the rolling time is 10-30 seconds, and the roller speed is 0.5-2 m / min.

[0058] e6. Film winding: Cool the pressure roller with cooling water to quickly cool it down to 20-25°C, and control the winding tension between 10-15N;

[0059] e7. Cutting: Cut into pole pieces according to battery specifications;

[0060] e8. Vacuum drying: Dry the electrode in a vacuum environment at a temperature of 120°C for 12 hours;

[0061] e9. Secondary rolling: The electrode is rolled for the second time, and the electrode density is controlled at 2.5-3.5 g / cm3.

[0062] Here, the adhesive is fibrillated in advance to form an adhesive network, avoiding local enrichment.

[0063] By pre-treating the conductive agent, agglomeration of the conductive agent during dry mixing is avoided.

[0064] The parameters of this dry film forming process are compared with those of traditional wet coating process:

[0065] 1. The dry film-making process does not require solvents and is directly pressed into film, while the traditional wet coating process requires slurry coating and then drying;

[0066] 2. The energy consumption of dry film making is low and no drying operation is required, while the energy consumption of traditional wet coating is high and energy is consumed for solvent evaporation;

[0067] 3. The electrode thickness of dry-process membrane can usually reach more than 200μm, while the electrode thickness of traditional wet coating is usually less than 150μm;

[0068] 4. The adhesive for dry-process membrane is mainly PTFE, and the PTFE content is generally less than 5%. The adhesive for traditional wet coating is generally PVDF, and NMP solvent is required;

[0069] 5. Dry film making is highly environmentally friendly and has no VOC emissions, while the traditional wet coating process is less environmentally friendly, the solvent has environmental pollution, and solvent recovery operations are required.

[0070] Example 1

[0071] 1. Tesla's 4860 battery uses this improved dry film-making process to produce electrode plates, with the thickness of the plates increased to 320μm and the energy density increased by about 21.2%.

[0072] 2. For supercapacitors, the electrode plates using this improved dry film-making process have a 21.6% reduction in interface impedance and a 34.1% improvement in rate performance.

[0073] In batteries, especially lithium-ion batteries, active materials are the core materials in the electrodes that directly participate in the electrochemical reactions. Their performance directly affects the battery's energy density, cycle life, and safety. Based on the electrode polarity, active materials can be divided into positive electrode active materials and negative electrode active materials.

[0074] The positive electrode active material is the cathode material. During discharge, a reduction reaction occurs, i.e., electrons are obtained. It is usually composed of lithium-containing transition metal oxides or phosphates and must meet the requirements of high voltage, high specific capacity and structural stability.

[0075] The mainstream positive electrode materials include lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese oxide, lithium iron phosphate, lithium manganese phosphate and lithium-rich manganese base.

[0076] Lithium cobalt oxide is characterized by high voltage, which can reach 3.7V, and mature technology, but the cost of cobalt is high and the thermal stability is poor. It is generally used in consumer electronic products such as mobile phones and laptops.

[0077] Lithium nickel cobalt manganese oxide is characterized by high energy density and the ability to balance performance and cost by adjusting the ratio of nickel, cobalt and manganese, such as Tesla car batteries.

[0078] Lithium nickel cobalt aluminum oxide is characterized by high nickel content, with nickel content >80%, and high energy density, but the risk of thermal runaway needs to be strictly controlled.

[0079] Lithium manganese oxide is characterized by low cost, high rate and good thermal stability, but has a low specific capacity and is generally used in power tools or hybrid vehicles.

[0080] The characteristics of lithium iron phosphate are high safety and long cycle life, with more than 2,000 cycles, but the voltage is relatively low, only reaching 3.2V, and the energy density is also low. It is generally used in energy storage systems or low-end electric vehicles.

[0081] Lithium manganese phosphate is characterized by high theoretical capacity but poor conductivity, and requires modification by nano-sizing or carbon coating.

[0082] The characteristic of lithium-rich manganese-based batteries is that they have an ultra-high specific capacity, which is greater than 250mAh / g, but the voltage will decay and the initial efficiency is low.

[0083] Here, aluminum oxide is coated on the surface of the positive electrode active material mainly to solve the problems of cation mixing and interface side reactions.

[0084] In fact, conductivity can be improved through nano-scaling and coating, such as BYD's blade battery technology.

[0085] The negative electrode active material is the anode material. During discharge, an oxidation reaction occurs, i.e., electrons are lost.

[0086] The mainstream materials of negative electrode active materials are graphite, silicon-based materials, lithium titanate, metallic lithium or amorphous carbon materials.

[0087] The characteristic of graphite is that its theoretical capacity is not high, only reaching 372mAh / g. It has low cost but slow charging speed and is generally used in commercial lithium batteries.

[0088] Silicon-based materials are characterized by a high theoretical capacity, which can reach 4200mAh / g, but their volume will expand, and they are generally used in high-end consumer electronics, such as mobile phones.

[0089] Lithium titanate is a zero-strain material with a cycle life of more than 100,000 times, but it has high voltage and low energy density and is generally used in energy storage devices.

[0090] The theoretical capacity of metallic lithium is 3860 mAh / g, but the dendrite problem is serious.

[0091] Amorphous carbon materials are suitable for sodium-ion batteries with a specific capacity of 200-300mAh / g and are generally used as nano-electrode negative electrodes.

[0092] Here, the expansion of silicon-based materials is generally suppressed by nano-sizing, porous structures and elastic adhesives, and dendrites are suppressed by solid electrolytes or artificial SEI films.

[0093] Spherical or flaky active material particles are used here mainly to improve the uniformity of mixing and avoid uneven dispersion caused by fibrous particles.

[0094] The D50 particle size is controlled at 10-20 μm mainly to achieve a narrow distribution, thereby reducing concentrated stress during roller pressing.

[0095] During the first charge and discharge process of the battery, there will be irreversible lithium loss, which will increase the battery's first coulombic efficiency and total energy density.

[0096] When the negative electrode is first embedded with lithium, the electrolyte is reduced on the surface to form a solid electrolyte interface, namely the SEI film, which consumes a large amount of lithium. For example, graphite loses 5-10%, and silicon-based materials lose even more, reaching 15-40%.

[0097] Oxygen release from the cathode material or dissolution of transition metals can also consume lithium.

[0098] Dendrites and “dead lithium” also lead to continuous loss of active lithium.

[0099] In this dry film preparation method, lithium powder is directly mixed into the negative electrode slurry to achieve pre-embedding of the lithium source. This can improve the initial efficiency of the silicon-based negative electrode from 70% to more than 85%, and can also reduce the continuous consumption of active lithium in the cycle. After compensating for the lithium loss, the amount of positive electrode material can be increased.

[0100] Conductive agents are important additives in lithium-ion battery electrodes, used to build an efficient electronic conductive network, reduce electrode internal resistance, and improve rate performance and cycling stability. Low-impedance conductive agents further enhance the battery's energy density and fast-charging capabilities by optimizing conductive pathways and reducing interfacial contact resistance.

[0101] The conductive agent made of carbon fiber has a fibrous structure, which enhances the mechanical strength of the electrode and is suitable for the manufacture of thick electrodes. It can reduce the resistance of the electrode transmission path, improve polarization problems, support high current charging and discharging, and avoid local "dead zones" caused by uneven conductivity.

[0102] Furthermore, the fibrous conductive agent is easier to disperse evenly in the dry mix.

[0103] Binders are key auxiliary materials in battery electrodes, used to adhere active materials, conductive agents, and other components to the current collector and provide mechanical stability. Their performance directly affects the cycle life, rate capability, and safety of the electrode.

[0104] In the dry film-forming method, polytetrafluoroethylene is used as an adhesive, which can form a network structure through fiberization and is resistant to chemical corrosion.

[0105] Polytetrafluoroethylene is a high-performance fluoropolymer with excellent chemical inertness, high temperature resistance and low surface energy. It has a melting point of 327°C and is insoluble in common solvents. However, it is difficult to process. In the battery dry electrode process, polytetrafluoroethylene needs to be fiberized to form a network structure to provide adhesion and mechanical strength.

[0106] In the process of preparing fibers by mixing polytetrafluoroethylene emulsion with viscose fiber-forming carrier, polytetrafluoroethylene with a molecular weight of 3 million has a high melt viscosity, and the emulsion particle size must be ensured to achieve uniform distribution and avoid clogging of the spinneret during spinning.

[0107] The carbonization process of the carrier at high temperature is:

[0108] 1. Low temperature stage: The temperature is 200-300℃, the viscose is pyrolyzed into carbonaceous residues, and the fiber structure is prevented from collapsing by slowly increasing the temperature;

[0109] 2. Medium temperature stage: The temperature is 300-500℃, which is the main stage of carbonization. Inert gas such as nitrogen or argon is introduced to avoid oxidation;

[0110] 3. In the high temperature stage, the temperature is 500-800℃, which completely removes carbon residues and the residual ash content is less than 1%.

[0111] The sintering temperature of polytetrafluoroethylene is 340-380°C, because the melting point of polytetrafluoroethylene is 327°C, and the decomposition temperature of polytetrafluoroethylene is 400°C, and the sintering temperature needs to be between the two.

[0112] The sintering time of polytetrafluoroethylene is 10-30 minutes, so that the polytetrafluoroethylene particles are melted to form a fiber structure, while avoiding excessive melting that causes fiber deformation.

[0113] When sintering polytetrafluoroethylene, slight tension needs to be applied to prevent shrinkage from causing uneven fiber diameter.

[0114] Here, a wet spinning process is used to pre-treat the adhesive for fiberization, and then the raw materials are mixed. Compared with using shearing equipment for fiberization, it can save a lot of energy, and the fiberization effect is better than the shearing effect.

[0115] In the dry film-forming step, a double-roll calender and a multi-stage roller press are used to achieve double pressing, thereby improving the pressing effect.

[0116] A twin-roll calender is used for the initial pressing process in order to form a continuous, self-supporting conveying precursor film tape.

[0117] The roller spacing is set at 0.1-1m in order to control the initial thickness between 0.5-2mm and retain the void structure with a porosity of 50-70%.

[0118] The pressure is set at 5-15MPa in order to achieve initial densification of the polytetrafluoroethylene and viscose mixture and avoid loss of carrier properties due to excessive compression.

[0119] The temperature is set to 80-120℃ in order to make the temperature slightly higher than the glass transition temperature of the viscose and improve the ductility. However, this temperature is actually much lower than the sintering temperature of polytetrafluoroethylene.

[0120] The loose film tape output form here has a film tape thickness of 0.5-2mm, a tensile strength of >0.5MPa, and a water content of <3%.

[0121] The precision pressing process is carried out using a multi-stage roller press in order to prepare an electrode membrane with high density and low porosity.

[0122] The parallelism is set to be less than or equal to 1 μm mainly to ensure thickness uniformity.

[0123] The pressure is set to 50-200MPa. In fact, what is achieved here is graded pressurization. The pressure is increased from 50 to 100MPa, and then from 100MPa to 200MPa. This design is mainly to avoid sudden high pressure causing the membrane layer to break.

[0124] The line speed is set to 0.5-5m / min, mainly to balance efficiency and quality.

[0125] The temperature is set to 100-150°C mainly to soften the adhesive, promote densification, and activate the surface energy of the polytetrafluoroethylene particles.

[0126] The use of graded pressurization here can avoid sudden breakage of the membrane layer, thereby ensuring the continuity of membrane tape output and work efficiency.

[0127] In the initial pressing stage, polytetrafluoroethylene provides skeleton particles, and in the fine pressing stage, it deforms under pressure to fill the gaps and form a three-dimensional network structure.

[0128] In the initial pressing stage, the initial strength is given to the film strip. In the fine pressing stage, its thermoplastic flow promotes densification, which is subsequently removed by carbonization.

[0129] In fact, additives such as dispersants and lubricants can also be added during the dry film-forming step. Adding dispersants in the initial pressing stage can prevent agglomeration, and adding lubricants in the fine pressing stage can reduce the roller friction coefficient.

[0130] Preferably, in order to ensure continuity after sintering, the solid content of polytetrafluoroethylene is ≥40%.

[0131] Preferably, in order to balance the calenderability and the final porosity, the mass ratio of viscose to polytetrafluoroethylene is between 1:2 and 1:4.

[0132] During the pressing process, uneven roller temperature distribution will cause edge warping, which can be solved by increasing the roller surface heating zone; when the pressure is released too quickly or the moisture content is too high, transverse cracks will appear, which can be solved by gradient decompression after precision pressing; when the final pressure is insufficient or the temperature is too low, the porosity will exceed the standard, which can be solved by increasing the final roller pressure and roller temperature.

[0133] During the pre-dispersion treatment step of the conductive agent, a layer of aluminum oxide film is deposited on the surface of the carbon nanotubes. The precursors are trimethylaluminum and water, and the temperature is 150-200°C. The main mechanism of action is to form an aluminum oxide insulating layer, inhibit the electron tunneling effect between CNTs, reduce contact agglomeration, and enhance hydrogen bonding with polar substrates such as viscose.

[0134] The carbon black surface is treated with argon and oxygen plasma at a power of 100-200W for 5-10 minutes. The ratio of argon to oxygen is 4:1. The main mechanism of action is to introduce hydroxyl and carboxyl groups, increase surface energy, and improve dispersibility.

[0135] The mechanical fusion step is mainly to achieve uniform mixing and preliminary dispersion of the conductive agent and the carrier material.

[0136] A fusion spheroidizer is used here, and the shear force causes the alumina coating to mechanically interlock with the polytetrafluoroethylene particles, while the heat energy promotes the weak bonding between the carbon black surface groups and the CF2 groups of the polytetrafluoroethylene.

[0137] The purpose of the classification step is to obtain conductive composite particles with uniform particle size and eliminate residual agglomerates.

[0138] Here, a vortex airflow classifier is used to achieve this. The classifying wheel speed is set to 3000-6000rpm, and D50 is controlled at 1-5μm. This can be achieved by adjusting the feed rate and airflow pressure, and the yield is ≥90%.

[0139] During electrostatic separation, the voltage value is set to 10-30KV, the electrode spacing is 50-100mm, and the separation medium is dry air with a dew point less than 40℃.

[0140] The principle of sorting is: due to the difference in dielectric constants, carbon black and aluminum oxide are separated in the electric field, and the conductive particles are adsorbed and removed by the electrode.

[0141] Here, in the initial pressing stage, the conductive agent composite particles are extended synchronously with polytetrafluoroethylene and viscose at a temperature of 80-120°C to form the prototype of a conductive network; in the fine pressing stage, high pressure causes the carbon nanotubes to be oriented and reduce contact resistance.

[0142] In the composite step of the dry membrane and the current collector, the current collector preparation is actually the pretreatment of the current collector, the purpose of which is to ensure the cleanliness of the current collector surface and activate the chemical activity.

[0143] A plasma cleaning machine is used for cleaning. The gas in the machine is a mixture of argon and oxygen in a ratio of 7:3. The processing speed is 1-3m / min and the surface energy is greater than 72mN / m.

[0144] 4 hours after the plasma treatment is completed, it is necessary to proceed to the next process, mainly to prevent the surface energy from decaying.

[0145] A two-stage corona treatment method is used here, with an independent corona machine achieving a single-stage charging treatment with a voltage of 10-20KV and a single-side charge density of 0.5-1μC / cm², mainly to break the inertness of the PTFE surface.

[0146] Corona electrodes are integrated on the roller press, the inter-electrode spacing is set to 2-5mm, and the frequency is 5-10KHz. Double-sided treatment is used here to generate opposite charges on the diaphragm and the current collector, and the potential difference here is ≥200V.

[0147] Here, OH radicals are generated by corona discharge, forming C-OH polar groups on the surface of polytetrafluoroethylene.

[0148] The opposite charges increase the interfacial binding energy by more than 30%.

[0149] Between the primary charging treatment and the secondary charging treatment, preheating is performed by infrared preheating, mainly to enhance the activation effect, so that the polytetrafluoroethylene crystal area partially melts and enhances the secondary charging treatment effect.

[0150] During the hot pressing composite step, the temperature of the continuous roller press is set between 120-180°C, which is actually achieved by a temperature gradient, that is, the temperature rises from 120°C to 150°C, and then from 150°C to 180°C. Here, 120°C softens the amorphous area of ​​PTFE, while 180°C induces interface diffusion. The temperature realizes a gradient rising mode, causing the state of PTFE to gradually change.

[0151] The pressure of the continuous roller press is set between 20-50MPa, which is actually a linear increasing setting, that is, the pressure increases linearly from 20MPa to 50MPa. For every 10MPa increase in pressure, the porosity decreases by 8-12%.

[0152] The roller speed of the continuous roller press is set between 0.5-2m / min, mainly to match films of different thicknesses. When the film thickness is less than 50μm, 0.5m / min is used. The thickness is relatively larger, and the speed can be relatively faster.

[0153] During the diaphragm winding step, tension control is achieved through a magnetic powder brake and tension sensor closed-loop system to prevent shrinkage deformation.

[0154] Here, ultraviolet laser is used for pole piece cutting. The spot diameter of laser cutting is 50μm and the power is 200W.

[0155] Here, a multi-layer tunnel dryer is used for vacuum drying.

[0156] Here, a precision multi-roller machine is used to perform secondary rolling on the pole pieces, with a pressure of 80-100MPa and rolling at room temperature.

[0157] In the preparation method of this dry electrode pole piece, a two-stage corona collaborative operation is adopted to achieve synchronous charging of the diaphragm and the current collector, and the interface bonding energy is increased to 2.5J / m², while the traditional process is 1.2J / m²; gradient hot pressing technology is adopted to achieve three-dimensional parameter coupling control of temperature, pressure and speed, and a gradient distribution of porosity is achieved. Ultra-fast cooling and shaping are adopted, and a nanostructured water cooling channel design is adopted to avoid shrinkage cracking caused by PTFE recrystallization.

[0158] This method can significantly reduce the interface contact resistance through charged treatment and thermodynamic regulation, making the interface contact resistance less than 0.1Ω·cm² and increasing the cycle life by more than 20%.

[0159] A dry electrode plate is prepared by the method as described above.

[0160] A battery comprises the dry electrode plate described above.

[0161] Compared with the existing technology, this dry membrane preparation method reduces dependence on mechanical shearing through pre-fiberization treatment of the adhesive, thereby reducing energy consumption. It adopts graded roller pressing technology, first low-pressure molding, and then high-pressure densification, which improves the pressing effect. It also adopts electrostatic assisted compounding and segmented electrification to increase the charge density, improve interfacial contact through charge action, overcome the defect of insufficient adhesion, enhance the bonding strength between the dry electrode membrane and the current collector, and improve the electrode quality.

[0162] With the above-described preferred embodiments of the present invention as a guide, and with reference to the above description, relevant personnel are fully capable of making various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the contents of the specification and must be determined according to the scope of the claims.

Claims

1. A dry film forming method, characterized in that: The following steps are involved: a. Raw material pretreatment: Dry the active material to remove moisture from the active material, pre-disperse the conductive agent, and fiberize the adhesive; b. Dry premixing: In a high-speed shear mixer, mix the active material and the conductive agent for 10 minutes at a mixing temperature of 30-50°C. c. Dry mixing: Add a binder to the mixture of active material and conductive agent, with the binder accounting for 5-8%, mixing for 30 minutes at a temperature of 30-50°C, then add lithium powder to the mixture and mix again for 20 minutes to obtain a mixed dry powder; d. Dry film forming: The mixed dry powder is pressed into a self-supporting film through a double-roll calender; The specific steps of dry film formation include: d1. Initial pressing: The mixed material is initially compacted by a double-roll calender to form a continuously transportable embryonic membrane. The roller spacing of the double-roll calender is 0.1-1m, the pressure is 5-15MPa, and the temperature is 80-120℃. The output form is a self-supporting loose membrane belt with a thickness of 0.5-2mm. d2. Fine pressing: further compacted by a multi-stage roller press, the roller surface is plated with hard chrome, the roller surface parallelism is ≤1μm, the pressure is 50-200MPa, the line speed is 0.5-5m / min, the temperature is 100-150℃, and the output form is a dense electrode film with a thickness of 50-300μm and a porosity of 20-40%; The pre-dispersion treatment steps of the conductive agent include: a1. Surface modification: Using atomic layer deposition technology, a layer of aluminum oxide film is deposited on the surface of carbon nanotubes with a thickness of 2-5nm. Then, the carbon black surface is treated with argon and oxygen plasma. a2. Mechanical fusion: The material is fused using a fusion spheroidizer. The speed of the equipment is 2000-3000 rpm, the fusion time is 20-40 minutes, and the temperature is 80°C. a3. Classification treatment: Use air flow classifier to separate large particles, control the D50 particle size to 1-5μm, and then use electrostatic separation method to disperse the particles.

2. The dry film forming method according to claim 1, wherein The active material includes a positive electrode active material and a negative electrode active material, the positive electrode active material includes at least one of lithium iron phosphate, lithium manganese phosphate or lithium iron manganese phosphate, the positive electrode active material is coated with a conductive layer, and the conductive layer includes aluminum oxide; the negative electrode active material includes at least one of a carbon-based material, a silicon-based material or lithium titanate.

3. The dry film forming method according to claim 1, wherein: The active material is in spherical or flake form, and the D50 particle size of the active material is 10-20 μm.

4. The dry film forming method according to claim 1, wherein The adhesive is polytetrafluoroethylene, and the fiberization treatment method of the adhesive is: use a polytetrafluoroethylene emulsion with an average molecular weight of 3 million and a particle size of 0.05-0.5 μm, with an emulsion concentration of 60%, mix it with a viscose fiber-forming carrier to make a spinning solution, and after spinning, carbonize and remove the carrier at high temperature, and the polymer is sintered to continuously form fibers.

5. A method for preparing a dry electrode sheet, characterized in that: include: A dry film sheet prepared by the dry film forming method according to any one of claims 1 to 4 is compounded with a current collector to obtain a dry electrode sheet.

6. The method for preparing a dry electrode sheet according to claim 5, wherein: The steps of compounding the dry membrane and the current collector include: e1. Current collector preparation: Surface treatment of the current collector is performed, and surface oil stains are removed by plasma cleaning; e2. Primary charging treatment: using corona discharge to charge the dry process membrane; e3. Dry diaphragm activation: preheat the dry diaphragm to 80-100℃ by infrared preheating; e4. Secondary charging treatment: Integrate corona electrodes on the roller press. Before pressing, make the membrane and current collector pass through the corona zone, so that the membrane is charged again and the current collector is charged for the first time. e5. Hot pressing composite: The membrane and the charged surface of the current collector are hot pressed together by a continuous roller press. The hot pressing temperature is 120-180°C, the pressure is 20-50 MPa, the rolling time is 10-30 seconds, and the roller speed is 0.5-2 m / min. e6. Film winding: Cool the pressure roller with cooling water to quickly cool it down to 20-25°C, and control the winding tension between 10-15N; e7. Cutting: Cut into pole pieces according to battery specifications; e8. Vacuum drying: Dry the electrode in a vacuum environment at a temperature of 120°C for 12 hours; e9. Secondary rolling: The electrode is rolled for the second time, and the electrode density is controlled at 2.5-3.5 g / cm3.

7. A dry electrode plate, characterized in that: The dry electrode plate is prepared by the method according to claim 5 or 6.

8. A battery, characterized in that: The battery comprises the dry electrode sheet according to claim 7.