Dry-method membrane preparation method, dry-method electrode pole piece, preparation method of dry-method electrode pole piece and battery

Abstract

The invention relates to a dry-method membrane preparation method, a dry-method electrode pole piece, a preparation method thereof and a battery, the dry-method membrane preparation method reduces dependence on mechanical shearing through pre-fibration treatment of an adhesive so as to reduce energy consumption, adopts a graded rolling technology, firstly performs low-pressure forming and then performs high-pressure densification, improves the pressing effect, and improves the quality of the dry-method electrode pole piece. In addition, electrostatic auxiliary compounding is adopted, and a segmented electrifying mode is adopted, so that the charge density is improved, interface contact is improved through the charge effect, the defect of insufficient bonding force is overcome, the bonding strength of a dry-method electrode film and a current collector is improved, and the electrode quality is improved.

Description

Technical Field

[0001] The present invention relates to a dry film forming method, a dry electrode sheet, a preparation method thereof, and a battery. Background Art

[0002] With the development of technology, lithium batteries are widely used because of their advantages such as high working voltage, long cycle life, and high energy density. However, the electrode sheets of lithium batteries are generally prepared by a wet process, and the environmental problems caused by this process are extremely serious. Therefore, a dry process is developed to prepare electrode sheets.

[0003] Although the existing dry film forming technology has advantages such as environmental protection and no solvent residue, in the actual industrialization process, there are still defects, such as uneven fibrosis of the binder, weak bonding at the current collector interface, etc. The processing of thin electrodes fails to meet expectations, thus restricting 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 sheet, a preparation method thereof, and a battery.

[0005] The technical solution adopted by the present invention to solve its technical problems is: a dry film forming method, comprising the following steps: a. Raw material pretreatment: drying the active material to remove moisture in the active material, pre-dispersing the conductive agent, and fibrillating the binder; b. Dry pre-mixing: mixing the active material and the conductive agent in a high-speed shear mixer, with a mixing time of 10 min and a mixing temperature of 30 - 50 °C; c. Dry mixing: adding the binder to the mixture of the active material and the conductive agent, with the proportion of the binder being 5 - 8%, mixing for 30 min, and the mixing temperature being 30 - 50 °C, then adding lithium powder to the mixed material and mixing again for 20 min to obtain a mixed dry powder; d. Dry film forming: pressing the mixed dry powder into a self-supporting film through a double-roll calender.

[0006] Further, 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, and the positive electrode active material is coated with a conductive layer, and the conductive layer includes alumina; the negative electrode active material includes at least one of a carbon-based material, a silicon-based material, or lithium titanate.

[0007] Further, the active material is spherical or flaky, and the D50 particle size of the active material is 10 - 20 μm.

[0008] Further, the conductive agent is carbon fiber, and the binder is polytetrafluoroethylene. The fibrillation treatment method of the binder is as follows: Use a polytetrafluoroethylene emulsion with an average molecular weight of about 3 million and a particle size of 0.05 - 0.5 μm, with an emulsion concentration of 60%. After mixing with a viscose fiber-forming carrier, a spinning solution is prepared. After spinning, the carrier is carbonized at high temperature and removed, and the polymer is sintered to continuously form fibers.

[0009] Further, the specific steps of dry film formation include: d1. Initial pressing: The mixed material is preliminarily compacted by a double-roll calender to form a continuously conveyable embryo film. The roll spacing of the double-roll calender is 0.1 - 1 m, the pressure is 5 - 15 MPa, the temperature is 80 - 120 °C, and the output form is a self-supporting loose film strip with a film strip thickness of 0.5 - 2 mm. d2. Precision pressing: Further compaction is carried out by a multi-stage roll press. The roll surface is plated with hard chromium, the parallelism of the roll surface is ≤ 1 μm, the pressure is 50 - 200 MPa, the linear speed is 0.5 - 5 m / min, the temperature is 100 - 150 °C, and the output form is a dense electrode film with an electrode film thickness of 50 - 300 μm and a porosity of 20 - 40%.

[0010] Further, the pre-dispersion treatment steps of the conductive agent include: a1. Surface modification: Using atomic layer deposition technology, deposit a layer of alumina film with a thickness of 2 - 5 nm on the surface of carbon nanotubes, and then treat the surface of carbon black with argon and oxygen plasma. a2. Mechanical fusion: Use a fusion spheroidizer to carry out fusion treatment on the material. The rotation speed of the equipment is 2000 - 3000 rpm, the fusion time is 20 - 40 min, and the temperature is 80 °C. a3. Classification treatment: Separate large particles through an air classifier, control the D50 particle size within 1 - 5 μm, and then carry out dispersion treatment by means of electrostatic separation.

[0011] A preparation method of a dry-process electrode sheet includes: Using the dry-process film obtained by the dry film formation method described above, the dry-process film is compounded with a current collector to obtain a dry-process electrode sheet.

[0012] Further, the compounding steps of the dry-process film and the current collector include: e1. Current collector preparation: The surface of the current collector is surface-treated, and the oil stain on the surface is removed by means of plasma cleaning. e2. Primary charging treatment: The dry-process film is charged by means of corona discharge. e3. Activation of the dry-process film: The dry-process film is preheated to 80 - 100 °C by means of infrared preheating. e4. Secondary charging treatment: Integrate a corona electrode on the roll press. Before pressing, both the diaphragm and the current collector pass through the corona area, so that the diaphragm is charged again and the current collector is charged for the first time. e5. Hot pressing and compounding: Through a continuous roll press, the charged surfaces of the diaphragm and the current collector are hot pressed and compounded. The hot pressing temperature is 120 - 180 °C, the pressure is 20 - 50 MPa, the roll pressing time is 10 - 30 s, and the roll speed is 0.5 - 2 m / min. e6. Diaphragm winding: Cool the pressure roller with cooling water to quickly cool the pressure roller to 20 - 25 °C, and control the winding tension between 10 - 15 N. e7. Slitting: Cut into electrode sheets according to the battery specifications. e8. Vacuum drying: Dry the electrode sheets in a vacuum environment at a temperature of 120 °C for 12 h. e9. Secondary roll pressing: Perform secondary roll pressing on the electrode sheets, and control the electrode density at 2.5 - 3.5 g / cm³.

[0013] A dry-process electrode sheet is prepared by the method as described above.

[0014] A battery includes the dry-process electrode sheet as described above.

[0015] The beneficial effects of the present invention are as follows: In this dry-film forming method, through the pre-fibrillation treatment of the binder, the dependence on mechanical shearing is reduced, thereby reducing energy consumption. The hierarchical roll pressing technology is adopted, with low-pressure forming first and then high-pressure densification, which improves the pressing effect. Electrostatic-assisted compounding is adopted, and the method of segmental power-on is used to increase the charge density, and the interface contact is improved through the charge effect, overcoming the defect of insufficient adhesion, enhancing the bonding strength between the dry-process electrode film and the current collector, and improving the electrode quality. Detailed implementation mode

[0016] A dry-film forming method includes the following steps: a. Raw material pretreatment: Dry the active material to remove the moisture in the active material, pre-disperse the conductive agent, and fibrillate the binder. b. Dry pre-mixing: In a high-speed shear mixer, mix the active material and the conductive agent for 10 min at a mixing temperature of 30 - 50 °C. c. Dry mixing: Add the binder to the mixture of the active material and the conductive agent. The proportion of the binder is 5 - 8%, mix for 30 min at a mixing temperature of 30 - 50 °C, and then add lithium powder to the mixed material and mix again for 20 min to obtain a mixed dry powder. d. Dry film formation: The mixed dry powder is pressed into a self-supporting film by a two-roll calender.

[0017] Further, the active material includes a positive active material and a negative active material. The positive active material includes at least one of lithium iron phosphate, lithium manganese phosphate, or lithium iron manganese phosphate. The positive active material is coated with a conductive layer, and the conductive layer includes aluminum oxide. The negative active material includes at least one of a carbon-based material, a silicon-based material, or lithium titanate.

[0018] Further, the active material is spherical or flaky, and the D50 particle size of the active material is 10 - 20 μm.

[0019] Further, the conductive agent is carbon fiber, and the binder is polytetrafluoroethylene. The fibrillation treatment method of the binder is as follows: Use a polytetrafluoroethylene emulsion with an average molecular weight of about 3 million and a particle size of 0.05 - 0.5 μm, with an emulsion concentration of 60%. After mixing with a viscose fiber-forming carrier, a spinning solution is made. After spinning, the carrier is carbonized at high temperature and removed, and the polymer is sintered to continuously form fibers.

[0020] Further, the specific steps of dry film formation include: d1. Initial pressing: The mixed material is preliminarily compacted by a two-roll calender to form a continuously conveyable embryo film. The roll spacing of the two-roll calender is 0.1 - 1 m, the pressure is 5 - 15 MPa, the temperature is 80 - 120 °C, the output form is a self-supporting loose film belt, and the film belt thickness is 0.5 - 2 mm. d2. Fine pressing: Further compaction is carried out by a multi-stage roll press. The roll surface is plated with hard chromium, the parallelism of the roll surface is ≤1 μm, the pressure is 50 - 200 MPa, the linear speed is 0.5 - 5 m / min, the temperature is 100 - 150 °C, the output form is a dense electrode film, the thickness of the electrode film is 50 - 300 μm, and the porosity is 20 - 40%.

[0021] Further, the steps of pre-dispersing the conductive agent include: a1. Surface modification: Using atomic layer deposition technology, deposit a layer of aluminum oxide film with a thickness of 2 - 5 nm on the surface of carbon nanotubes, and then treat the surface of carbon black with argon and oxygen plasma. a2. Mechanical fusion: Use a fusion spheroidizer to carry out fusion treatment on the material. The rotation speed of the equipment is 2000 - 3000 rpm, the fusion time is 20 - 40 min, and the temperature is 80 °C. a3. Classification treatment: Separate large particles through an air classifier, control the D50 particle size within 1 - 5 μm, and then carry out dispersion treatment by means of electrostatic separation.

[0022] A preparation method of a dry-process electrode sheet includes: The dry film obtained by the dry film forming method described above is compounded with a current collector to obtain a dry electrode sheet.

[0023] Furthermore, the compounding step of the dry film and the current collector includes: e1. Current collector preparation: The surface of the current collector is surface-treated, and the oil stain on the surface is removed by plasma cleaning; e2. Primary charging treatment: The dry film is charged by corona discharge; e3. Dry film activation: The dry film is preheated to 80 - 100 °C by infrared preheating; e4. Secondary charging treatment: A corona electrode is integrated on the roll press. Before pressing, both the film and the current collector pass through the corona area, so that the film is charged again and the current collector is charged for the first time; e5. Hot pressing and compounding: Through a continuous roll press, the charged surfaces of the film and the current collector are hot-pressed and compounded. The hot pressing temperature is 120 - 180 °C, the pressure is 20 - 50 MPa, the roll pressing time is 10 - 30 s, and the roll speed is 0.5 - 2 m / min; e6. Film winding: The pressure roller is cooled by cooling water to quickly cool the pressure roller to 20 - 25 °C, and the winding tension is controlled between 10 - 15 N; e7. Slitting: Cut into electrode sheets according to the battery specifications; e8. Vacuum drying: The electrode sheets are dried in a vacuum environment at a temperature of 120 °C for 12 h; e9. Secondary roll pressing: The electrode sheets are subjected to secondary roll pressing, and the electrode density is controlled at 2.5 - 3.5 g / cm³.

[0024] Here, the binder is fibrillated in advance to form a bonding network, avoiding local enrichment.

[0025] By pre-treating the conductive agent, the agglomeration phenomenon of the conductive agent during dry mixing is avoided.

[0026] The parameter comparison between this dry film forming and traditional wet coating is as follows: 1. The process adopted for dry film forming does not require solvents and directly presses into a film, while traditional wet coating requires slurry coating and then drying operation; 2. The energy consumption of dry film forming is lower and no drying operation is required, while the energy consumption of traditional wet coating is higher and the evaporation of solvents requires energy; 3. The electrode thickness of dry film forming can usually reach more than 200 μm, while the electrode thickness of traditional wet coating is usually below 150 μm; 4. The binder for dry film forming is mainly PTFE, and the content of PTFE is generally less than 5%. While the binder for traditional wet coating is generally PVDF and requires NMP solvent. 5. Dry film forming has strong environmental friendliness and no VOC emissions. While the environmental friendliness of the traditional wet coating process is poor, the solvent causes environmental pollution and solvent recovery operations are also required.

[0027] Example 1 1. For Tesla's 4860 battery, the electrode pole piece using this improved dry film forming process has its thickness increased to 320 μm and the energy density increased by about 21.2%.

[0028] 2. For supercapacitors, the electrode pole piece using this improved dry film forming process has the interfacial impedance reduced by 21.6% and the rate performance improved by 34.1%.

[0029] In batteries, especially lithium-ion batteries, the active material is the core material directly participating in the electrochemical reaction in the electrode, and its performance directly affects the energy density, cycle life and safety of the battery. According to the electrode polarity, the active material can be divided into the positive electrode active material and the negative electrode active material.

[0030] The positive electrode active material is the cathode material. During discharge, a reduction reaction occurs, that is, it gains electrons. It is usually composed of lithium-containing transition metal oxides or phosphates and needs to meet the requirements of high voltage, high specific capacity and structural stability.

[0031] 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-based materials.

[0032] The characteristics of lithium cobalt oxide are high voltage, up to 3.7V, and mature process. However, the cobalt cost is high and the thermal stability is poor. It is generally used in consumer electronics products such as mobile phones and laptops.

[0033] The characteristics of lithium nickel cobalt manganese oxide are high energy density, and the performance and cost can be balanced by adjusting the ratio of nickel, cobalt and manganese, such as in Tesla car batteries.

[0034] The characteristics of lithium nickel cobalt aluminum oxide are high nickel content, where the nickel content > 80%, and high energy density. However, the risk of thermal runaway needs to be strictly controlled.

[0035] The characteristics of lithium manganese oxide are low cost, high rate and good thermal stability. However, the specific capacity is relatively low. It is generally used in power tools or hybrid vehicles.

[0036] The characteristics of lithium iron phosphate are high safety and long cycle life, with the number of cycles exceeding 2000 times. However, the voltage is 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.

[0037] Lithium manganese phosphate is characterized by a high theoretical capacity, but poor electrical conductivity, and needs to be nano-sized or carbon-coated for modification.

[0038] The characteristics of lithium-rich manganese-based materials are ultra-high specific capacity, with a specific capacity greater than 250 mAh / g, but the voltage will decay and the initial efficiency is relatively low.

[0039] Here, coating alumina on the surface of the cathode active material is mainly to solve the problems of cation mixing and interfacial side reactions.

[0040] Here, in fact, the electrical conductivity can also be improved through nano-sizing and carbon coating, such as BYD's blade battery technology.

[0041] The negative electrode active material is the anode material, and during discharge, an oxidation reaction occurs, that is, electrons are lost.

[0042] The mainstream materials of the negative electrode active material are graphite, silicon-based materials, lithium titanate, metallic lithium or amorphous carbon materials.

[0043] The characteristics of graphite are that its theoretical capacity is not high, only reaching 372 mAh / g, the cost is low but the charging speed is slow, and it is generally used in commercial lithium batteries.

[0044] The characteristics of silicon-based materials are that their theoretical capacity is relatively high, reaching 4200 mAh / g, but the volume will expand, and they are generally used in high-end consumer electronics, such as mobile phones.

[0045] Lithium titanate is a zero-strain material, and its cycle life can exceed 100,000 times, but the voltage is high and the energy density is low, and it is generally used in energy storage devices.

[0046] The theoretical capacity of metallic lithium is 3860 mAh / g, but the dendrite problem is serious.

[0047] Amorphous carbon materials are suitable for sodium-ion batteries, with a specific capacity of 200 - 300 mAh / g, and are generally used as the negative electrode of sodium-ion batteries.

[0048] Here, the expansion of silicon-based materials is generally inhibited through nano-sizing, porous structure and elastic binder, and dendrites are inhibited through solid-state electrolytes or artificial SEI films.

[0049] Here, spherical or flaky active material particles are used mainly to improve the mixing uniformity and avoid uneven dispersion caused by fibrous particles.

[0050] Controlling the D50 particle size within 10 - 20 μm is mainly to achieve a narrow distribution, thereby reducing the concentrated stress during rolling.

[0051] During the first charge and discharge process of the battery, there will be irreversible lithium loss, thereby improving the first Coulomb efficiency and total energy density of the battery.

[0052] When lithium is first intercalated into the negative electrode, the electrolyte is reduced on the surface to form a solid electrolyte interface, i.e., the SEI film, which consumes a large amount of lithium. For example, graphite loses 5-10%, and silicon-based materials lose more, reaching 15-40%.

[0053] The release of oxygen from the positive electrode material or the dissolution of transition metals also consumes lithium.

[0054] Dendrites and "dead lithium" also cause continuous loss of active lithium.

[0055] In this dry film-forming method, lithium powder is directly mixed into the negative electrode slurry to achieve the pre-embedding of the lithium source, which can improve the initial efficiency of the silicon-based negative electrode from 70% to over 85%. It can also reduce the continuous consumption of active lithium during cycling. After compensating for lithium loss, the amount of positive electrode material can be increased.

[0056] Conductive agents are important additives in lithium-ion battery electrodes, which are used to construct an efficient electronic conduction network, reduce the internal resistance of the electrode, improve the rate performance and cycling stability. Low-impedance conductive agents further enhance the energy density and fast charging ability of the battery by optimizing the conduction path and reducing the interfacial contact resistance.

[0057] The conductive agent using carbon fiber has a fibrous structure, which enhances the mechanical strength of the electrode, is suitable for the manufacture of thick electrodes, can reduce the resistance of the electrode transmission path, improve the polarization problem, support high-current charge and discharge, and avoid local "dead zones" caused by uneven conduction.

[0058] Moreover, the fibrous conductive agent is easier to be evenly dispersed in dry mixing.

[0059] Binder is a key auxiliary material in battery electrodes, which is used to adhere components such as active materials and conductive agents to the current collector and provide mechanical stability. Its performance directly affects the cycling life, rate performance and safety of the electrode.

[0060] In this dry film-forming method, polytetrafluoroethylene is used as the binder, which can form a network structure through fibrillation and is chemically corrosion-resistant.

[0061] Polytetrafluoroethylene is a high-performance fluoropolymer with excellent chemical inertness, high temperature resistance and low surface energy. Its melting point is 327°C and it is insoluble in common solvents. However, its processing is difficult. In the dry electrode process of the battery, polytetrafluoroethylene needs to form a network structure through fibrillation to provide adhesiveness and mechanical strength.

[0062] In the process of preparing fibers by mixing polytetrafluoroethylene emulsion with a fibrillating carrier of viscose filament, polytetrafluoroethylene with a molecular weight of 3 million has a high melt viscosity. It is necessary to ensure the emulsion particle size to achieve uniform distribution and avoid clogging the spinneret during spinning.

[0063] The carbonization process of the carrier at high temperature is as follows: 1. Low-temperature stage: The temperature is 200 - 300 °C. The viscose pyrolyzes into carbonaceous residues, and the fiber structure collapse is prevented by slowly raising the temperature. 2. Medium-temperature stage: The temperature is 300 - 500 °C. This is the main stage of carbonization. An inert gas, such as nitrogen or argon, is introduced to avoid oxidation. 3. High-temperature stage: The temperature is 500 - 800 °C. The carbon residues are completely removed, and the proportion of residual ash is < 1%.

[0064] The sintering temperature of polytetrafluoroethylene is 340 - 380 °C. Since the melting point of polytetrafluoroethylene is 327 °C and the decomposition temperature is 400 °C, the sintering temperature needs to be between the two.

[0065] The sintering time of polytetrafluoroethylene is 10 - 30 min, which enables the polytetrafluoroethylene particles to melt and form a fiber structure, while avoiding fiber deformation caused by excessive melting.

[0066] When sintering polytetrafluoroethylene, a slight tension needs to be applied to prevent uneven fiber diameters caused by shrinkage.

[0067] Here, the wet spinning process is adopted to pre-treat the adhesives into fibrils, and then the raw materials are mixed. Compared with using shearing equipment for fibrillation, it can save a large amount of energy consumption, and the fibrillation effect is better than that of shearing.

[0068] In the dry film-forming step, a double-roll calender and a multi-stage roll press are used to achieve double pressing, thereby improving the pressing effect.

[0069] The double-roll calender is used for the initial pressing process, aiming to form a continuous and self-supporting precursor film belt for conveying.

[0070] The roll spacing is set at 0.1 - 1 m to control the initial thickness between 0.5 - 2 mm, retain the void structure, and the porosity is 50 - 70%.

[0071] The pressure is set at 5 - 15 MPa to preliminarily densify the polytetrafluoroethylene and viscose mixture, and avoid the loss of the carrier's fluidity caused by excessive compression.

[0072] The temperature is set at 80 - 120 °C to make the temperature slightly higher than the glass transition temperature of the viscose, improving the ductility, and this temperature is actually much lower than the sintering temperature of polytetrafluoroethylene.

[0073] In the form of the loose film belt output here, the film belt thickness is 0.5 - 2 mm, the tensile strength > 0.5 MPa, and the water content < 3%.

[0074] The multi-stage roller press is used for the fine pressing process to prepare an electrode film with high density and low porosity.

[0075] The parallelism is set to be less than or equal to 1 μm, mainly to ensure the thickness uniformity.

[0076] The pressure is set to 50 - 200 MPa. In fact, a stepped pressure increase is realized here. The pressure is increased from 50 MPa to 100 MPa, and then from 100 MPa to 200 MPa. This design is mainly to avoid the film layer breaking due to sudden high pressure.

[0077] The linear velocity is set to 0.5 - 5 m / min, mainly to balance efficiency and quality.

[0078] The temperature is set to 100 - 150 °C, mainly to soften the viscose, promote densification, and simultaneously activate the surface energy of polytetrafluoroethylene particles.

[0079] Here, the form of stepped pressure increase is adopted, which can avoid the sudden breakage of the film layer, thus ensuring the continuity of the film belt output and guaranteeing the working efficiency.

[0080] In the initial pressing stage, polytetrafluoroethylene provides the framework particles. In the fine pressing stage, it deforms under pressure to fill the voids and forms a three-dimensional network structure.

[0081] In the initial pressing stage, the film belt is given an initial strength. In the fine pressing stage, its thermoplastic flow promotes densification, and it is removed by subsequent carbonization.

[0082] In fact, in the dry film-forming step, additives such as dispersants and lubricants can also be added. Adding a dispersant in the initial pressing stage can prevent agglomeration. In the fine pressing stage, adding a lubricant can reduce the rolling friction coefficient.

[0083] Preferably, to ensure the continuity after sintering, the solid content of polytetrafluoroethylene ≥ 40%.

[0084] Preferably, to balance the rollability and the final porosity, the mass ratio of viscose to polytetrafluoroethylene is between 1:2 and 1:4.

[0085] During the pressing process, when the temperature distribution of the roller press is uneven, it will cause edge warping, which can be solved by increasing the heating zones on the roller surface; when the pressure is released too quickly or the moisture content is too high, it will cause the appearance of transverse cracks, which can be solved by performing gradient pressure reduction after fine pressing; when the final pressure is insufficient or the temperature is too low, it will cause the porosity to exceed the standard, which can be solved by increasing the final-stage roller pressing and the roller temperature.

[0086] In the pre-dispersion treatment step of the conductive agent, a layer of alumina 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 is to form an alumina insulating layer, inhibit the electron tunneling effect between CNTs, reduce contact agglomeration, and enhance the hydrogen bond binding with polar matrices such as viscose.

[0087] The surface of carbon black is treated with argon and oxygen plasma. The power is 100 - 200 W, the time is 5 - 10 min, and the ratio of argon to oxygen is 4:1. The main mechanism is to introduce hydroxyl and carboxyl groups, increase the surface energy, and improve the dispersibility.

[0088] The mechanical fusion step is mainly to achieve the uniform mixing and preliminary dispersion of the conductive agent and the carrier material.

[0089] Here, a fusion spheroidizer is used. The shear force makes the alumina coating and the polytetrafluoroethylene particles mechanically interlock, and the thermal energy promotes the weak bonding of the surface groups of carbon black with the CF 2 groups of polytetrafluoroethylene.

[0090] The purpose of the classification treatment step is to obtain conductive composite particles with uniform particle size and eliminate residual agglomerates.

[0091] Here, a vortex air classifier is used to achieve this. The rotational speed of the classification wheel is set at 3000 - 6000 rpm, the D50 is controlled at 1 - 5 μm, which can be achieved by adjusting the feeding speed and the air flow pressure, and the yield is ≥ 90%.

[0092] In electrostatic separation, the voltage value is set at 10 - 30 KV, the electrode spacing is 50 - 100 mm, and the separation medium is dry air with a dew point less than 40 °C.

[0093] The separation principle is: due to the difference in dielectric constant between carbon black and alumina, the trajectories are separated in the electric field, and the conductive particles are adsorbed and removed by the electrode plate.

[0094] Here, in the initial pressing stage, the conductive agent composite particles are synchronously extended with polytetrafluoroethylene and viscose at a temperature of 80 - 120 °C to form a prototype of the conductive network; in the fine pressing stage, the high pressure makes the carbon nanotubes align directionally, reducing the contact resistance.

[0095] In the step of compounding the dry film and the current collector, the preparation of the current collector is actually the pretreatment of the current collector, aiming to ensure the cleanliness of the current collector surface and activate the chemical activity.

[0096] Here, a plasma cleaner is used for cleaning. The gas in the machine is a mixed gas of argon and oxygen, and the ratio of the two is 7:3. The treatment speed is 1 - 3 m / min, and the surface energy is greater than 72 mN / m.

[0097] Four hours after the plasma treatment is completed, it is necessary to enter the next process, mainly to prevent the attenuation of surface energy.

[0098] Here, a two-stage corona treatment method is adopted. An independent corona machine is used to achieve the first charging treatment. The voltage is 10 - 20 KV, and the single-sided charge density of the diaphragm is 0.5 - 1 μC / cm². This is mainly to break the surface inertness of polytetrafluoroethylene.

[0099] The corona electrode is integrated on the roller press. The electrode spacing is set to 2 - 5 mm, and the frequency is 5 - 10 KHz. Double-sided treatment is adopted here to make the diaphragm and the current collector generate opposite charges, and the potential difference here is ≥ 200 V.

[0100] Here, ·OH free radicals are generated through corona discharge, and C - OH polar groups are formed on the surface of polytetrafluoroethylene.

[0101] Opposite charges increase the interfacial binding energy by more than 30%.

[0102] Between the first charging treatment and the second charging treatment, preheating is carried out by infrared preheating method, mainly to improve the activation effect, make the crystalline region of polytetrafluoroethylene partially melt, and improve the effect of the second charging treatment.

[0103] In the hot pressing and compounding step, the temperature of the continuous roller press is set between 120 - 180 °C. In fact, it is also the realization of 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 region of polytetrafluoroethylene, and 180 °C induces interfacial diffusion. The temperature realizes a gradient rising mode, making the state of polytetrafluoroethylene change gradually.

[0104] The pressure of the continuous roller press is set between 20 - 50 MPa. In fact, it is a linearly increasing setting here, that is, the pressure linearly increases from 20 MPa to 50 MPa. For every 10 MPa increase in pressure, the porosity decreases by 8 - 12%.

[0105] The roller speed of the continuous roller press is set between 0.5 - 2 m / min, mainly to match membranes of different thicknesses. When the membrane thickness is less than 50 μm, 0.5 m / min is used. When the thickness is relatively larger, the speed can be relatively faster.

[0106] In the diaphragm winding step, tension control is achieved through the closed-loop system of the magnetic powder brake and the tension sensor to prevent cold shrinkage deformation.

[0107] Here, an ultraviolet laser is used for pole piece cutting. The spot diameter of the laser cutting is 50 μm, and the power is 200 W.

[0108] Here, a multi-layer tunnel dryer is used for vacuum drying.

[0109] 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.

[0110] In the preparation method of the dry electrode pole piece, a double-stage corona collaborative operation is adopted to achieve synchronous charging of the membrane and the current collector, and the interface binding 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.

[0111] 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%.

[0112] A dry electrode plate is prepared by the method as described above.

[0113] A battery comprises the dry electrode plate as described above.

[0114] Compared with the existing technology, this dry film-making method reduces the dependence on mechanical shearing through 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 interfacial contact through charge action, overcome the defect of insufficient adhesion, and improve the bonding strength between the dry electrode film and the current collector, thereby improving the electrode quality.

[0115] Based on the above ideal embodiments of the present invention, the relevant staff can make various changes and modifications without departing from the technical concept of the present invention through the above description. The technical scope of the present invention is not limited to the contents of the specification, and its technical scope 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: drying the active material to remove moisture from the active material, pre-dispersing the conductive agent, and fiberizing 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, the proportion of the binder is 5-8%, the mixing time is 30 minutes, the mixing temperature is 30-50°C, and then add lithium powder to the mixture and mix again, the mixing time is 20 minutes, and a mixed dry powder is obtained; d. Dry film forming: The mixed dry powder is pressed into a self-supporting film through a double-roll calender.

2. The dry film forming method according to claim 1, characterized in that: 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, characterized in that: 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, characterized in that: The conductive agent is carbon fiber, the adhesive is polytetrafluoroethylene, and the fiberization treatment method of the adhesive is: use 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 it is mixed with a viscose fiber-forming carrier to prepare a spinning solution. After spinning, the carrier is carbonized and removed at high temperature, and the polymer is sintered to continuously form fibers.

5. The dry film forming method according to claim 1, characterized in that: The specific steps of dry film formation include: d1. Initial pressing: The mixed material is initially compacted by a double-roll calender to form an embryonic membrane that can be continuously transported. The roller spacing of the double-roll calender is 0.1-1m, the pressure is 5-15MPa, the temperature is 80-120℃, and 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%.

6. The dry film forming method according to claim 1, characterized in that: 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. The thickness of the film is 2-5nm, and then the carbon black surface is treated with argon and oxygen plasma; a2. Mechanical fusion: The materials are fused by a fusion spheroidizer. The speed of the equipment is 2000-3000rpm, the fusion time is 20-40min, and the temperature is 80℃. a3. Classification treatment: Separate large particles through airflow classifier, control the D50 particle size to 1-5μm, and then use electrostatic separation method for dispersion treatment.

7. A method for preparing a dry electrode sheet, characterized in that: include: A dry film sheet is prepared by the dry film preparation method according to any one of claims 1 to 6, and the dry film sheet is compounded with a current collector to obtain a dry electrode sheet.

8. The method for preparing a dry electrode sheet according to claim 7, characterized in that: The steps of compounding the dry membrane and the current collector include: e1. Current collector preparation: Surface treatment is performed on the current collector surface, and the oil stains on the surface are removed by plasma cleaning; e2. Primary charging treatment: using corona discharge to charge the dry film; 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, both the membrane and the 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 and composited by a continuous roller press. The hot pressing temperature is 120-180°C, the pressure is 20-50MPa, the rolling time is 10-30s, and the roller speed is 0.5-2m / min. e6. Film winding: Cool the pressure roller with cooling water to quickly cool the pressure roller 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.

9. A dry electrode plate, characterized in that: The dry electrode plate is prepared by the method according to claim 7 or 8.

10. A battery, characterized in that: The battery comprises the dry electrode sheet according to claim 9.