A dry-type lithium iron phosphate electrode sheet and a preparation method thereof

Abstract

The present invention belongs to the technical field of lithium-ion battery diaphragm, and particularly relates to a dry-process lithium iron phosphate electrode sheet and a preparation method thereof. The electrode sheet includes a diaphragm and a current collector. The diaphragm is composed of 90% - 98% by mass of lithium iron phosphate, 0.5% - 5% by mass of a conductive agent, 0.5% - 4.9% by mass of PTFE, and 0.1% - 1% by mass of a binder polymer. The thickness of the diaphragm is 300 - 420 μm, the tap density is 2.6 - 3.2 g·cm-3, and the peel strength is 0.5 - 1.25 N·cm-1. By improving the diaphragm components and processing technology, the present invention obtains a high-loading dry-process lithium iron phosphate electrode sheet, solves the problems of non-uniformity and cracking existing when the diaphragm is relatively thick, and has a high peel strength and battery performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery diaphragm, and particularly relates to a dry-process lithium iron phosphate electrode sheet and a preparation method thereof. Background Art

[0002] As a new generation of green high-energy batteries, lithium-ion batteries are widely used due to their advantages such as high voltage, large energy density, good cycling performance, low self-discharge, no memory effect, and wide operating temperature range. Among them, safety, cost, and high energy density are the keys to the large-scale application of lithium-ion batteries in electric vehicles and the like. Lithium iron phosphate (LiFePO 4 abbreviation LFP) has become a hot material in the current research of lithium battery cathodes because of its stable P-O bond, difficult to decompose, and its structure will not collapse even under high temperature or overcharge conditions. Its working voltage is stable, non-toxic and environmentally friendly, with stable structure, good safety, good thermal stability and extremely long cycle life. However, its relatively low energy density compared with ternary cathode materials limits its development in the power battery market.

[0003] At present, there are many methods to improve the energy density of LFP batteries, such as reducing the proportion of non-active components, increasing the compaction density of the electrode sheet, and increasing the thickness of the electrode sheet. Among them, a thick electrode sheet can greatly increase the loading of active materials, thereby improving the energy density of the battery. However, electrodes manufactured by the wet coating technology have problems such as cracks, delamination, and poor flexibility. Especially when preparing thick electrodes, these problems will be magnified. Therefore, the electrode thickness is greatly limited by the wet coating technology. The dry film-forming technology avoids the use of any solvents when preparing lithium battery electrode sheets, can easily control the electrode thickness and the uniformity of thick electrodes, and has unique advantages in preparing thick electrodes.

[0004] Chinese patent document CN117352666A discloses a dry-process electrode sheet, a preparation method and an application thereof. A coupling agent is introduced into the preparation process of the dry-process electrode sheet of a secondary battery, reducing the usage amount of the binder, increasing the proportion of the conductive carbon content, and optimizing some existing problems in the dry-process electrode preparation process, greatly improving the peel strength of the electrode sheet while reducing the film resistance. However, this patent has problems of technical complexity and uneven material dispersion. The characteristic of the dry-process electrode technology is the absence of solvent addition, but the addition of the coupling agent dispersion liquid in the above patent makes a solvent appear again in the electrode sheet preparation process. In addition, the viscosity of the coupling agent is very high, about 10 to 100 times that of water, and it is difficult to disperse the coupling agent evenly by dry mixing and stirring directly in the electrode material powder with a large specific surface area.

[0005] Chinese Patent Document CN112289976A discloses a cathode material layer, a preparation method thereof, a cathode sheet and a battery, including the following steps: 1) Mix a fibrillatable polymer powder, a carbon-coated cathode active material and a conductive carbon material, and then under the action of shear force, make the fibrillatable polymer powder draw into fibers to obtain a mixed material; 2) Perform hot pressing treatment on the mixed material on a current collector until a preset thickness is reached to obtain the cathode material layer. The use of a carbon-coated cathode active material improves the lubricity of the cathode material, enhances the uniformity of the mixed material, and effectively reduces the hardness of the cathode material, thereby improving the softness of the electrode sheet. It solves the problems of uneven mixing of the cathode material and cracking of the electrode sheet caused by a large cathode material ratio and high hardness during the dry preparation of the cathode material layer, and can also reduce the requirements for the roll shaft material of the roll press during the preparation of the dry cathode sheet, reducing equipment costs. However, the carbon coating content in this patent is between 2% and 15%, and it does not provide any capacity when present in the cathode film, resulting in problems of low mass and volume energy density. Summary of the Invention

[0006] In order to solve the problems of cracks, delamination and poor flexibility in the electrode after using the wet coating technology to prepare a high-loading LFP electrode sheet, the present invention provides a lithium iron phosphate dry electrode sheet and a preparation method thereof, which can keep the film intact and uniform even in a relatively thick state.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] The present invention provides a lithium iron phosphate dry electrode sheet, including a film and a current collector. The film is composed of 90% - 98% by mass of lithium iron phosphate, 0.5% - 5% by mass of a conductive agent, 0.5% - 4.9% by mass of PTFE, and 0.1% - 1% by mass of a binder polymer; the thickness of the film is 300 - 420 μm, and the tap density is 2.6 - 3.2 g·cm -3 , and the peel strength is 0.5 - 1.25 N·cm -1 .

[0009] The present invention also provides a preparation method of a lithium iron phosphate dry electrode sheet, including the following steps:

[0010] (1) Predispersion: Mix lithium iron phosphate, a conductive agent, PTFE and a binder polymer uniformly at 10 - 19 °C to obtain a mixed material;

[0011] (2) Fibrillation: Put the mixed material into a high-speed shear mixer for stirring, and control the termination temperature of stirring at 60 - 90 °C to obtain a drawn agglomerate;

[0012] (3) Granulation: Put the agglomerates after wire drawing into a dry granulator, and obtain granules with a uniform particle size distribution through stirring;

[0013] (4) Roll pressing: Roll the granules into a diaphragm with the required thickness through multi-stage roll pressing, and then cut and wind the diaphragm according to the size of the current collector;

[0014] (5) Lamination: Roll and laminate the wound diaphragm and the current collector through a high-speed roll laminator at the same speed to obtain a dry lithium iron phosphate electrode sheet.

[0015] Preferably, the conductive agent in step (1) is one or a mixture of conductive agents such as conductive carbon black, conductive graphite, carbon fiber, carbon nanotube, graphene, etc.; Further preferably, the conductive agent is carbon black, with a median particle size of 100~1000nm and a specific surface area <100m 2 / g.

[0016] Preferably, the particle size of the lithium iron phosphate in step (1) is 1μm~20μm, the specific surface area is 5~10m 2 / g, the tapped density >0.8 g / cm 2 (GB / T5162), the initial discharge specific capacity >150mAh g -1 .

[0017] Preferably, in step (1), a high-speed mixer is used for mixing, a cooling medium is introduced into the jacket of the high-speed mixer equipment, the stirring linear speed is 15~25m / s, and the stirring time is 3-5 minutes.

[0018] Preferably, the adhesive polymer in step (1) is one or a mixture of two or more of adhesive polymers such as ethylene-ethyl acrylate copolymer, ethylene-acrylic acid copolymer, ethylene-methyl acrylate, ethylene-methyl methacrylate copolymer, ethylene-methacrylic acid copolymer, etc.; Further preferably, the adhesive polymer is ethylene-ethyl acrylate copolymer. The above-mentioned binder not only has strong adhesiveness, but also has certain flexibility and tensile strength. For the electrode sheet using the above-mentioned binder, compared with using polyacrylate, its toughness and environmental stress cracking resistance are both improved.

[0019] Preferably, in step (2), the stirring linear speed of the high-speed shearing machine is 20~50m / s, and the stirring is stopped when the temperature rises to 60~90°C. The obtained wire-drawn agglomerates are in the shape of "clay". Further preferably, the stirring linear speed is 40-45m / s, and the stirring termination temperature is 65-90°C.

[0020] Preferably, in step (3), the linear speed is set according to the particle size requirement. Further preferably, the linear speed of the dry granulator is 3~4 m / s, the stirring time is 3-5min, and the particle size D of the material after granulation50 is 1 to 2 mm.

[0021] Preferably, in step (4), the roll gap and roll speed difference of each stage of the roll press are adjusted, and the roll temperature is raised to 100 °C or above to maintain good fibrillation ability of PTFE. Further preferably, the roll press used is a three-stage roll press. The gap between the first and second rolls is 1600 - 1000 μm, preferably 1000 - 1200 μm; the gap between the second and third rolls is 1000 - 500 μm, preferably 500 - 800 μm; the gap between the third and fourth rolls is 500 - 50 μm, preferably 280 - 350 μm. Preferably, the rotational speed of the first roll is 10 - 18 rpm / min, preferably 12 - 14 rpm / min; the differential speed ratio of each stage of the roll is 1 - 1.5, and the roll temperature is 100 - 200 °C.

[0022] Preferably, in step (5), the linear speed of the same-speed roller can be adjusted according to the rotational speed of the differential roller. The rotational speed of the same-speed high-temperature roll press is 10 - 40 rpm / min, preferably 18 - 24 rpm / min; the roll temperature is 100 - 200 °C.

[0023] Preferably, in step (5), the current collector is one of battery-grade smooth aluminum foil, carbon-coated aluminum foil, perforated aluminum foil, or glue-coated aluminum foil; further preferably, the current collector is smooth aluminum foil.

[0024] Compared with the prior art, the present invention has at least the following beneficial effects or advantages:

[0025] (1) By improving the diaphragm composition and processing technology, the present invention obtains a high-loading lithium iron phosphate dry electrode sheet, solving the problems of non-uniformity and cracking existing when the diaphragm is relatively thick.

[0026] (2) In the present invention, the conductive carbon, PTFE, and binder polymer are dispersed into a fluffy state by high-speed blades, which is beneficial to improving the dispersibility of the conductive agent and preventing agglomeration. The conductive agent with good dispersibility can provide high conductivity; the mixture is high-speed blended at low temperature to avoid insufficient mixing caused by fibrillation of the binder during the mixing process.

[0027] (3) Through the three steps of fibrillation, granulation, and hot roll pressing, the present invention can achieve continuous feeding and prepare a diaphragm with excellent mechanical properties and high compaction density.

[0028] (4) By optimizing the diaphragm composition and adding the binder polymer, the present invention improves the peel strength between the diaphragm and the current collector. After hot roll pressing, the binder polymer can increase the peel force between the diaphragm and the aluminum foil, reduce the internal resistance of the lithium iron phosphate battery, increase the battery cycle life, and applying this method to prepare the dry electrode sheet is beneficial to improving the quality and performance of the battery and avoiding the problems of cracking and powder falling of the thick diaphragm. Description of the Drawings

[0029] Figure 1 SEM image of the material after fibrillation treatment in Example 1 of the present invention;

[0030] Figure 2 First charge-discharge curve of the electrode sheet in Example 1 of the present invention;

[0031] Figure 3 Cycling performance of the electrode sheet in the first 100 cycles in Example 1 of the present invention. Detailed implementation manners

[0032] The present invention will be further described below in conjunction with specific embodiments, but not limited thereto.

[0033] It should be noted that the experimental methods described in the following embodiments are all conventional methods unless otherwise specified; the reagents, materials and equipment can all be obtained from commercial channels unless otherwise specified. Among them, PTFE is produced by Shandong Dongyue Polymer Materials Co., Ltd., with the product number DF-213E. The lithium iron phosphate used has a D 50 = 1.1 μm, specific surface area of 0.9 m 2 / g, tapped density of 0.87 m 2 / g, and the first discharge specific capacity of 155 mAh / g; the conductive carbon black has a D 50 = 150 nm, specific surface area of 62 m 2 / g; the mass content of ethyl acrylate in the ethylene-ethyl acrylate copolymer is 22%.

[0034] Example 1

[0035] (1) 4.8 kg of lithium iron phosphate, 0.1 kg of conductive carbon black, 0.08 kg of PTFE and 0.02 kg of ethylene-ethyl acrylate copolymer are put into a high-speed blending device and stirred at a rotation speed of 20 m / s for 3 minutes at 15 °C until a loose state is obtained. When the bulk density is measured to be a certain value, a first mixture is obtained;

[0036] (2) The first mixture is stirred using a high-speed shearing machine with a linear velocity of 40 m / s, and the stirring termination temperature is controlled at 65 °C. The resulting second mixture is in a "clay" shape;

[0037] (3) The second mixture is granulated using a dry granulator with a rotation speed of 3 m / s for 3 minutes to obtain granulated material with a particle size of approximately 1.4 mm;

[0038] (4) It is rolled into a film by a three-stage roll press to obtain a film sheet. Adjust the gap between the first and second rolls to 1200 μm; the gap between the second and third rolls to 800 μm; the gap between the third and fourth rolls to 350 μm. Set the rotation speed of the first roll to 12 rpm / min, the differential speed ratio of each stage of rolls to 1.2, and the roll temperature to 100 °C. The surface of the obtained film sheet has no cracks and the thickness is 385 μm, and it can be wound up;

[0039] (5)The obtained film sheet is compounded with aluminum foil. The linear speed of the rolls at the same speed is 18 rpm / min, and the roll temperature is 150 °C to obtain the final electrode sheet.

[0040] From Figure 1 the SEM image of the fibrillated material (the second mixture), it can be seen that the PTFE after high-speed stirring has fibrillated, and a large number of fibers wrap the material, which helps to improve the mechanical properties in the subsequent film rolling process.

[0041] The electrode sheet prepared in Example 1 is assembled into a CR2032 lithium battery half-cell for testing. The separator is a multi-layer PE film, and the electrolyte is LiF 6 PO 4 (concentration 1.1 M) dissolved in a mixed solution of EC (ethylene carbonate) / DEC (diethyl carbonate) / DMC (dimethyl carbonate) with a volume ratio of 1:1:1. The amount of electrolyte used is generally 100 μL, the voltage test range is 2.25 - 3.75 V, and the charge and discharge tests are carried out at a rate of 0.1C. From the electrochemical data graph of the electrode sheet ( Figure 2 and Figure 3 ) we found that the initial charge / discharge specific capacity of the electrode sheet is 157.7 / 157.3 mAh / g, the initial coulomb efficiency is as high as 99.7%, and the capacity is still 138.3 mAh / g after 100 cycles, and the average capacity decay rate per cycle is only 0.12%, showing good electrochemical performance.

[0042] Example 2

[0043] (1)4.5 kg of lithium iron phosphate, 0.25 kg of conductive carbon black, 0.245 kg of PTFE and 0.005 kg of ethylene-acrylic ethyl ester copolymer are put into a high-speed blending equipment for stirring. The rotation speed is 20 m / s, and the stirring time is 3 min at 10 °C until it is in a loose state. The bulk density is measured to be a certain value to obtain the first mixture;

[0044] (2)The first mixture is stirred by a high-speed shear machine with a linear speed of 45 m / s, and the stirring termination temperature is controlled at 90 °C. The obtained second mixture is in a "clay" shape;

[0045] (3) Granulate the second mixture. The rotation speed of the granulator is 4 m / s and the time is 3 min to obtain granulated material with a particle size of approximately 1.1 mm.

[0046] (4) Calender into a film through a three-stage roll press to obtain a film sheet. Adjust the gap between the first and second rolls to 1000 μm; the gap between the second and third rolls to 500 μm; the gap between the third and fourth rolls to 280 μm. Set the rotation speed of the first roll to 12 rpm / min, the differential speed ratio of each stage of rolls to 1.2, and the roll temperature to 150 °C. The obtained film sheet has no cracks on the surface and a thickness of 324 μm, and can be wound up.

[0047] (5) Compound the obtained film sheet with aluminum foil. The linear speed of the same-speed rolls is 18 rpm / min and the roll temperature is 200 °C to obtain the final electrode sheet.

[0048] Example 3

[0049] (1) Put 4.9 kg of lithium iron phosphate, 0.025 kg of conductive carbon black, 0.025 kg of PTFE, and 0.05 kg of ethylene-ethyl acrylate copolymer into a high-speed blending device and stir at a rotation speed of 20 m / s for 3 min at 19 °C until it is in a loose state. Measure the bulk density to be a certain value to obtain the first mixture.

[0050] (2) Stir the first mixture using a high-speed shearer with a linear speed of 45 m / s and control the termination temperature of stirring at 90 °C. The obtained second mixture is in a "clay" shape.

[0051] (3) Granulate the second mixture. The rotation speed of the granulator is 4 m / s and the time is 3 min to obtain granulated material with a particle size of approximately 1.1 mm.

[0052] (4) Calender into a film through a three-stage roll press to obtain a film sheet. Adjust the gap between the first and second rolls to 1000 μm; the gap between the second and third rolls to 500 μm; the gap between the third and fourth rolls to 280 μm. Set the rotation speed of the first roll to 14 rpm / min, the differential speed ratio of each stage of rolls to 1.4, and the roll temperature to 200 °C. The obtained film sheet has no cracks on the surface and a thickness of 410 μm, and can be wound up.

[0053] (5) Compound the obtained film sheet with aluminum foil. The linear speed of the same-speed rolls is 24 rpm / min and the roll temperature is 100 °C to obtain the final electrode sheet.

[0054] Example 4

[0055] (1) Put 4.8 kg of lithium iron phosphate, 0.1 kg of conductive carbon black, 0.08 kg of PTFE, and 0.02 kg of ethylene-ethyl acrylate copolymer into a high-speed blending device and stir at a rotational speed of 20 m / s for 3 minutes at 15 °C until it becomes loose. Measure the bulk density to a certain value to obtain the first mixture;

[0056] (2) Stir the first mixture using a high-speed shearing machine with a linear velocity of 45 m / s and control the termination temperature of stirring at 90 °C. The resulting second mixture is in a "clay" shape;

[0057] (3) Granulate the second mixture with a granulator rotational speed of 4 m / s for 3 minutes to obtain granulated material with a particle size of approximately 1.1 mm;

[0058] (4) Roll it into a film through a three-stage roll press to obtain a film sheet. Adjust the gap between the first and second rolls to 1000 μm; the gap between the second and third rolls to 400 μm; the gap between the third and fourth rolls to 280 μm. Set the rotational speed of the first roll to 12 rpm / min, the differential speed ratio of each stage of the roll to 1.2, and the roll temperature to 100 °C. The resulting film sheet has no cracks on the surface and a thickness of 315 μm and can be wound up;

[0059] (5) Compound the obtained film sheet with aluminum foil at a co-rotating roll linear velocity of 18 rpm / min and a roll temperature of 150 °C to obtain the final electrode sheet.

[0060] Example 5

[0061] (1) Put 4.8 kg of lithium iron phosphate, 0.1 kg of conductive carbon black, 0.08 kg of PTFE, and 0.02 kg of ethylene-ethyl acrylate copolymer into a high-speed blending device and stir at a rotational speed of 20 m / s for 3 minutes at 15 °C until it becomes loose. Measure the bulk density to a certain value to obtain the first mixture;

[0062] (2) Stir the first mixture using a high-speed shearing machine with a linear velocity of 45 m / s and control the termination temperature of stirring at 90 °C. The resulting second mixture is in a "clay" shape;

[0063] (3) Granulate the second mixture with a granulator rotational speed of 4 m / s for 3 minutes to obtain granulated material with a particle size of approximately 1.1 mm;

[0064] (4) Calender into a film through a three-stage roll press to obtain a film sheet; adjust the gap between the first and second rolls to 1000 μm; the gap between the second and third rolls to 400 μm; the gap between the third and fourth rolls to 280 μm. Set the rotational speed of the first roll to 14 rpm / min, the differential speed ratio of each stage of rolls to 1.4, and the roll temperature to 100 °C. The surface of the obtained film sheet has no cracks and the thickness is 357 μm, and it can be wound up;

[0065] (5) Compound the obtained film sheet with aluminum foil, with the same-speed roll linear speed of 24 rpm / min and the roll temperature of 150 °C to obtain the final electrode sheet.

[0066] Comparative Example 1

[0067] (1) Put 4.8 kg of lithium iron phosphate, 0.1 kg of conductive carbon black, 0.08 kg of PTFE, and 0.02 kg of ethylene-ethyl acrylate copolymer into a high-speed blending device for stirring, with a rotational speed of 20 m / s and a stirring time of 3 min, without temperature control, to obtain the first mixture;

[0068] (2) Stir the first mixture using a high-speed shearing machine, with a linear speed of 40 m / s, and control the termination temperature of stirring at 65 °C. The obtained second mixture is in a "clay" shape;

[0069] (3) Granulate the second mixture, with the rotational speed of the granulator being 3 m / s and the time being 3 min, to obtain granulated material with a particle size of approximately 1.6 mm;

[0070] (4) Calender into a film through a three-stage roll press to obtain a film sheet; adjust the gap between the first and second rolls to 1200 μm; the gap between the second and third rolls to 800 μm; the gap between the third and fourth rolls to 350 μm. Set the rotational speed of the first roll to 12 rpm / min, the differential speed ratio of each stage of rolls to 1.2, and the roll temperature to 100 °C. The surface of the obtained film sheet has transverse stripes and is discontinuous, with a thickness of 607 μm and cannot be wound up;

[0071] (5) Compound the obtained film sheet with aluminum foil, with the same-speed roll linear speed of 18 rpm / min and the roll temperature of 150 °C to obtain the final electrode sheet.

[0072] Comparative Example 2

[0073] (1) Put 4.8 kg of lithium iron phosphate, 0.1 kg of conductive carbon black, 0.08 kg of PTFE, and 0.02 kg of ethylene-ethyl acrylate copolymer into a high-speed blending device for stirring, with a rotational speed of 20 m / s, stir at 15 °C for 3 min until it is in a loose state, and measure the bulk density to be a certain value to obtain the first mixture;

[0074] (2) Stir the first mixture using a high-speed shear mixer at a linear velocity of 45 m / s, and control the final stirring temperature at 90 °C. The resulting second mixture is in a "clay" shape.

[0075] (3) Directly roll the second mixture in a "clay" shape into a film using a three-stage roll press to obtain a film sheet. Adjust the gap between the first and second rolls to 1000 μm, the gap between the second and third rolls to 500 μm, and the gap between the third and fourth rolls to 280 μm. Set the rotational speed of the first roll to 14 rpm / min, the differential speed ratio of each stage of rolls to 1.4, and the roll temperature to 100 °C. Since the second mixture after fibrillation is too agglomerated, it cannot be fed to form a film.

[0076] Comparative Example 3

[0077] (1) Put 4.8 kg of lithium iron phosphate, 0.1 kg of conductive carbon black, 0.08 kg of PTFE, and 0.02 kg of ethylene-ethyl acrylate copolymer into a high-speed blending device and stir at a rotational speed of 20 m / s for 3 minutes at 15 °C until it reaches a loose state. Measure the bulk density to a certain value to obtain the first mixture.

[0078] (2) Stir the first mixture using a high-speed shear mixer at a linear velocity of 40 m / s, and control the final stirring temperature at 65 °C. The resulting second mixture is in a "clay" shape.

[0079] (3) Evenly spread the second mixture on a smooth metal plate, manually roll it into a film using a stainless steel roller, fold it multiple times, and continuously roll it multiple times until it cannot be thinned. The resulting film sheet is prone to breakage, with a thickness of 865 μm.

[0080] (5) Composite the obtained film sheet with aluminum foil at a co-rotating roll linear velocity of 18 rpm / min and a roll temperature of 150 °C to obtain the final electrode sheet.

[0081] Comparative Example 4

[0082] (1) Put 4.8 kg of lithium iron phosphate, 0.1 kg of conductive carbon black, and 0.1 kg of PTFE into a high-speed blending device and stir at a rotational speed of 20 m / s for 3 minutes at 15 °C until it reaches a loose state. Measure the bulk density to a certain value to obtain the first mixture.

[0083] (2) Stir the first mixture using a high-speed shear mixer at a linear velocity of 40 m / s, and control the final stirring temperature at 65 °C. The resulting second mixture is in a "clay" shape.

[0084] (3) Granulate the second mixture using a granulator at a rotational speed of 3 m / s for 3 minutes to obtain granulated material with a particle size of approximately 1.4 mm.

[0085] (4)The film is calendared into a film sheet by a three-stage roll press. Adjust the gap between the first and second rollers to 1200 μm, the gap between the second and third rollers to 800 μm, and the gap between the third and fourth rollers to 350 μm. Set the rotational speed of the first roller to 12 rpm / min, the differential speed ratio of each stage of rollers to 1.2, and the roller temperature to 100 °C. The obtained film sheet has fine cracks on the surface and a thickness of 399 μm, and can be wound up.

[0086] (5)The obtained film sheet is laminated with aluminum foil at a co-rotating roller linear speed of 18 rpm / min and a roller temperature of 150 °C to obtain the final electrode sheet.

[0087] Test Example

[0088] The film sheets obtained in Examples 1-5 and Comparative Examples 1-4 were subjected to performance tests, including film sheet thickness, compaction density, and peel strength tests. The peel strength between the film sheet and aluminum foil was measured with reference to the ASTM D3330 standard; compaction density = film sheet mass / film sheet volume. The results are shown in Table 1.

[0089] Table 1. Performance data of the film sheets obtained in Examples 1-5 and Comparative Examples 1-4

[0090]

[0091] As can be seen from Table 1, the film sheets prepared by the three steps of fibrillation, granulation, and hot roll pressing exhibit excellent mechanical properties and high compaction density, and the peel strength between the film sheet and aluminum foil is greatly enhanced after adding the adhesive polymer. The electrode sheet prepared by the method of the present invention can improve the energy density of the battery and reduce the cost of the battery, and has great commercial potential in the field of lithium iron phosphate batteries.

Claims

1. A lithium iron phosphate dry electrode sheet, characterized in that: The invention comprises a membrane and a current collector, wherein the membrane is composed of 90% to 98% by mass of lithium iron phosphate, 0.5% to 5% by mass of a conductive agent, 0.5% to 4.9% by mass of PTFE, and 0.1% to 1% by mass of an adhesive polymer; the membrane has a thickness of 300 to 420 μm and a compacted density of 2.6 to 3.2 g·cm -3 , peel strength is 0.5~1.25N·cm -1 , the adhesive polymer is a mixture of one or more of ethylene-ethyl acrylate copolymer, ethylene acrylic acid copolymer, ethylene methyl methacrylate copolymer, and ethylene-methacrylic acid copolymer; The method for preparing the lithium iron phosphate dry-process electrode sheet comprises the following steps: (1) Pre-dispersion: mixing lithium iron phosphate, conductive agent, PTFE and adhesive polymer at 10-19° C. to obtain a mixture; (2) Fibrillation: The mixed material is placed in a high-speed shearing machine for stirring, and the stirring termination temperature is controlled at 60-90°C to obtain a drawing agglomerate; (3) Granulation: The agglomerates after drawing are placed into a dry granulator and stirred to obtain granules with uniform particle size distribution; (4) Rolling: The pellets are rolled into a membrane of the required thickness through multi-stage rolling, and then the membrane is cut and rolled according to the size of the current collector; (5) Compounding: The rolled membrane and the current collector are rolled and compounded by a high-temperature roller press at the same speed to obtain a lithium iron phosphate dry-process electrode sheet.

2. The lithium iron phosphate dry electrode sheet according to claim 1, characterized in that: The conductive agent is a mixture of one or more of conductive carbon black, conductive graphite, carbon fiber, carbon nanotube, and graphene.

3. The lithium iron phosphate dry-process electrode sheet according to claim 1, characterized in that: The adhesive polymer is ethylene-ethyl acrylate copolymer.

4. The lithium iron phosphate dry-process electrode sheet according to claim 1, characterized in that: The mixing in step (1) has a stirring line speed of 15-25 m / s and a stirring time of 3-5 minutes.

5. The lithium iron phosphate dry-process electrode sheet according to claim 1, characterized in that: The stirring linear velocity of step (2) is 20-50 m / s.

6. The lithium iron phosphate dry-process electrode sheet according to claim 1, characterized in that: In step (3), the linear speed of the dry granulator is 3-4 m / s, and the stirring time is 3-5 min.

7. The lithium iron phosphate dry-process electrode sheet according to claim 1, characterized in that: In step (4), the roller temperature is 100-200° C.; and the differential speed ratio of each roller of the multi-stage roller press is 1-1.

5.

8. The lithium iron phosphate dry-process electrode sheet according to claim 1, characterized in that: In step (5), the roller speed of the high-temperature roller press is 10-40 rpm / min, and the roller temperature is 100-200°C.

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