Lithium ion battery and preparation method and device of battery pole piece

Abstract

The invention provides a preparation method and device of a lithium ion battery and a battery pole piece, and particularly relates to the technical field of batteries. The lithium ion battery comprises a battery pole piece, a diaphragm and electrolyte, the battery pole piece comprises a current collector and electrode diaphragms arranged on the surface of at least one side of the current collector, and the electrode diaphragms comprise a first layer of electrode diaphragm and a second layer of electrode diaphragm which are sequentially arranged from the direction close to the current collector to the direction far away from the current collector; each layer of electrode diaphragm comprises an active material, a conductive agent and a binder, and the particle size D501 of the active material in the first layer of electrode diaphragm is greater than or equal to the particle size D502 of the active material in the second layer of electrode diaphragm; the content of the conductive agent in the first layer of electrode diaphragm is greater than that in the second layer of electrode diaphragm; and the porosity of the second layer of electrode membrane is greater than that of the first layer of electrode membrane. According to the invention, the double-layer pole piece is prepared by adopting a dry process, the integral wettability of the pole piece can be improved by controlling the particle size, thickness and pore of the upper and lower layer materials, and the dynamics and cycle performance are both improved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly to a lithium-ion battery, a preparation method and device for a battery electrode sheet. Background Art

[0002] With the rapid development of lithium-ion batteries, the applications of lithium-ion batteries are becoming more and more extensive, and the market requirements for them are also getting higher and higher. Higher energy density poses higher requirements for the design and preparation process of battery cells. The production of battery electrode sheets is the primary link in the manufacture of battery cells and is also one of the factors affecting the performance of battery cells. Currently, most battery electrode sheets are prepared by wet coating, and problems such as gel formation, coating cracking, and binder floating are likely to occur during the wet coating process. The dry electrode technology can solve the above problems, and has good performance, while reducing equipment, plant, and energy consumption costs. Therefore, the dry electrode process will be the key research direction in the field of battery preparation. Although the dry electrode process has various advantages, there are also some obvious defects. For example, when preparing thick electrode sheets, with the increase in the thickness of the electrode sheet, more binders need to be added, and the increase in the content of the binder will affect the kinetic transmission of the battery, thereby affecting the electrical performance of the battery cell.

[0003] Therefore, it is necessary to provide a lithium-ion battery, a preparation method and device for a battery electrode sheet to solve the above problems. Summary of the Invention

[0004] In view of the above disadvantages of the prior art, the present invention provides a lithium-ion battery, a preparation method and device for a battery electrode sheet to improve the problem of poor kinetics of thick electrode sheets prepared by the prior art.

[0005] To achieve the above object and other related objects, on the one hand, the present invention provides a lithium-ion battery, which includes: a current collector and an electrode film. The electrode film is disposed on at least one surface of the current collector. The electrode film includes a first-layer electrode film and a second-layer electrode film arranged in sequence from the direction close to the current collector to the direction away from the current collector; wherein, each layer of the electrode film includes an active material, a conductive agent, and a binder. The particle size D501 of the active material in the first-layer electrode film is greater than or equal to the particle size D502 of the active material in the second-layer electrode film; the content of the conductive agent in the first-layer electrode film is greater than the content of the conductive agent in the second-layer electrode film; the porosity of the second-layer electrode film is greater than the porosity of the first-layer electrode film.

[0006] In an example of the present invention, the total thickness of the electrode film is H, the thickness of the first-layer electrode film is H1, and the thickness of the second-layer electrode film is H2. Then the value range of H is 100 to 400 μm, and H1:H2 = (1:1) to (1:3).

[0007] In an example of the present invention, the ratio D501:D502 of the particle size D501 of the active material in the first-layer electrode film to the particle size D502 of the active material in the second-layer electrode film is (1:1) to (3:1).

[0008] In an example of the present invention, the battery electrode sheet is a positive electrode sheet. Each layer of electrode film of the positive electrode sheet includes a positive active material, a conductive agent, a first binder, and a second binder. The mass ratio of the positive active material, the conductive agent, the first binder, and the second binder in the first-layer electrode film of the positive electrode sheet is (90-98.6%):(0.4-5%):(0.5-2%):(0.5-3%); the mass ratio of the positive active material, the conductive agent, the first binder, and the second binder in the second-layer electrode film of the positive electrode sheet is (92-98.6%):(0.4-3%):(0.5-3%):(0.25-2%); and / or, the battery electrode sheet is a negative electrode sheet. Each layer of electrode film of the negative electrode sheet includes a negative active material, a conductive agent, a first binder, and a third binder. The mass ratio of the negative active material, the conductive agent, the first binder, and the third binder in the first-layer electrode film of the negative electrode sheet is (91.4-99%):(0.4-5%):(0.1-0.6%):(0.5-3%); the mass ratio of the negative active material, the conductive agent, the first binder, and the third binder in the second-layer electrode film of the negative electrode sheet is (92.4-98.5%):(0.4-3%):(0.1-0.6%):(1-4%).

[0009] In an example of the present invention, the mass ratio of the first binder in the first-layer electrode film to the first binder in the second-layer electrode film in the positive electrode sheet is 1:1, and the mass ratio of the second binder in the first-layer electrode film to the second binder in the second-layer electrode film is (1:1) to (2:1); the mass ratio of the first binder in the first-layer electrode film to the first binder in the second-layer electrode film in the negative electrode sheet is 1:1, and the mass ratio of the third binder in the first-layer electrode film to the third binder in the second-layer electrode film in the negative electrode sheet is (1:1) to (2:1).

[0010] In an example of the present invention, the first binder is selected from at least one of polytetrafluoroethylene, a copolymer of ethylene and tetrafluoroethylene, or a fluorinated ethylene propylene copolymer; the second binder is selected from at least one of polyvinylidene fluoride, a polyimide polymer, or an acrylate-acrylonitrile-acrylamide copolymer; the third binder is selected from at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, or an acrylate-acrylonitrile-acrylamide copolymer.

[0011] In an example of the present invention, the molecular weight of the first binder is 1,000,000 to 2,000,000, the particle size d1 < 15 μm, the molecular weight of the second binder is 800,000 to 1,200,000, and the particle size d2 is 15 to 30 μm.

[0012] In an example of the present invention, the tap density of the positive electrode sheet is 2.0 g / cm 3 to 3.7 g / cm 3 ; the tap density of the negative electrode sheet is 1.5 g / cm 3 to 3.0 g / cm 3 .

[0013] In an example of the present invention, a conductive coating is further provided on the surface of the current collector. The conductive coating includes a coating conductive agent, a coating binder, and a stabilizer, and the mass ratio of the coating conductive agent, the coating binder, and the stabilizer is (20 - 30%):(70 - 75%):(0 - 5%).

[0014] In an example of the present invention, the coating binder includes one or more of polyacrylic acid, styrene-butadiene rubber, carboxymethyl cellulose, polyvinyl alcohol, polyacrylamide, epoxy resin, and polyurethane. The stabilizer is selected from calcium hydroxide, and the single-sided coating thickness of the conductive coating is 0.5 to 2 μm.

[0015] On the other hand, the present invention provides a method for preparing a battery electrode sheet of a lithium-ion battery. The preparation method at least includes:

[0016] Powder premixing: According to the ratio of the first layer electrode film and the second layer electrode film of the battery electrode sheet, the active material, the conductive agent, and the binder are premixed at 0 - 20°C for 15 - 30 min to obtain a first mixture and a second mixture;

[0017] Pre-fibrillation: The first mixture and the second mixture are respectively kept at 60 - 140°C for 30 min for pre-fibrillation, and then continue to be dispersed for 15 - 60 min to obtain a third mixture and a fourth mixture;

[0018] First layer lamination: The third mixture is extruded into a film, thinned by roll pressing, and then laminated with the current collector to form a first layer electrode film on the current collector;

[0019] Second layer lamination: The fourth mixture is extruded into a film, thinned by roll pressing, and then laminated with the first layer electrode film to obtain a battery electrode sheet.

[0020] In an example of the present invention, the first layer lamination step includes: conveying the third mixture through a screw for mixing and extrusion through a die head to form an electrode diaphragm with a thickness of 2-7 mm, thinning it to a thickness of 0.5-2 mm by rolling and laminating it with a current collector to form a first layer electrode diaphragm, wherein the temperature of the screw is 140-230 °C.

[0021] In an example of the present invention, the second layer lamination step includes: conveying the fourth mixture through a screw for mixing and extrusion through a die head to form an electrode diaphragm with a thickness of 2-5 mm, thinning it to a thickness of 0.3-1 mm by rolling and laminating it with the first layer electrode diaphragm, and then regulating the thickness and compaction density of the overall electrode sheet by rolling to obtain a battery electrode sheet.

[0022] The present invention also provides a device for preparing a battery electrode sheet, which includes: a first electrode film forming mechanism, a second electrode film forming mechanism, a unwind mechanism and a rewind mechanism. Among them, the first electrode film forming mechanism: includes a first feeding device, a first screw extrusion mechanism, a first die head, a first thinning roller, a first lamination roller, and a first thickness monitoring device arranged in sequence along the advancing direction of the electrode sheet; the second electrode film forming mechanism includes a second feeding device, a second screw extrusion mechanism, a second die head, a second thinning roller, a second lamination roller, a thickness adjustment roller, and a second thickness detection device arranged in sequence along the advancing direction of the electrode sheet, and the second lamination roller is arranged at the rear end of the first thickness detection device; the unwind mechanism is arranged on one side of the first electrode film forming mechanism for releasing the current collector; the rewind mechanism is arranged at the ends of the first electrode film forming mechanism and the second electrode film forming mechanism for rewinding the formed battery electrode sheet.

[0023] In an example of the present invention, the thickness detection device includes a laser thickness detection mechanism, a ray thickness detection mechanism or an infrared thickness detection mechanism.

[0024] In an example of the present invention, the rolling pressure of the first thinning roller, the first lamination roller, the second thinning roller and the second lamination roller is 3-50 T, and the roller temperature is 80-200 °C; the rolling pressure of the thickness adjustment roller is 20-80 T, and the roller temperature is 45-150 °C; the roller speed ratio of the first thinning roller is 1:1-1:3; the roller speed ratio of the first lamination roller is 1:1-1:5; the roller speed ratio of the second thinning roller is 1:1-1:2, and the roller speed ratio of the second lamination roller is 1:1-1:3; the roller speed ratio of the thickness adjustment roller is 1:1.

[0025] The present invention uses a dry process to prepare a double-layer electrode sheet. By controlling the particle size, thickness and pore regulation of the upper and lower layers of materials, the infiltration performance of the overall electrode sheet can be improved, and then the kinetics and cycle performance of the electrode sheet can be enhanced. And the dry process reduces the equipment, energy consumption and factory building costs.

[0026] Using a binder with a smaller particle size can improve the dispersion uniformity, increase the bonding force and stability of the electrode sheet, extend the cycle life of the electrode sheet, and prevent the film from peeling off in the later stage of cycling. At the same time, a higher content of binder in the electrode film closer to the current collector and the current collector with a conductive coating increase the bonding force and can prevent the electrode film from falling off during lamination and in the later stage of cycling.

[0027] The content of the conductive agent in the first layer of the electrode film closer to the current collector is greater than that in the second layer of the electrode film farther from the current collector, which can form more conductive paths, reduce the electron transfer resistance of the electrode sheet, and thus improve the rate performance and cycle stability of the battery. Active materials with small particle sizes have a larger grain boundary area, shorter transport paths, and better rate performance. Large particle sizes have a higher tap density, and the best effect is achieved by combining layers of large and small particle sizes. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0029] Figure 1 It is a schematic structural diagram of the battery electrode sheet in an embodiment of the lithium-ion battery of the present invention;

[0030] Figure 2 It is a schematic structural diagram of the lithium-ion battery of the present invention in an embodiment;

[0031] Figure 3 It is a flowchart of the preparation method of the battery electrode sheet of the present invention in an embodiment;

[0032] Figure 4 It is a schematic structural diagram of the device for preparing the battery electrode sheet of the present invention in an embodiment.

[0033] REFERENCE NUMERALS

[0034] 100, lithium-ion battery; 110, housing; 111, bottom plate; 112, side plate; 120, cover plate; 121, positive electrode lead-out member; 122, negative electrode lead-out member; 130, battery electrode sheet; 131, current collector; 1311, conductive coating; 132, electrode film; 1321, first layer of electrode film; 1322, second layer of electrode film;

[0035] 200. Device; 210. First electrode film forming mechanism; 211. First feeding device; 212. First screw extrusion mechanism; 213. First die head; 214. First thinning roller; 215. First compounding roller; 216. First thickness detection device; 220. Second electrode film forming mechanism; 221. Second feeding device; 222. Second screw extrusion mechanism; 223. Second die head; 224. Second thinning roller; 225. Second compounding roller; 226. Thickness adjustment roller; 227. Second thickness detection device; 230. Unwinding mechanism; 240. Rewinding mechanism. Detailed implementation manners

[0036] The following uses specific specific examples to illustrate the implementation manners of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific implementation manners. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0037] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments, and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0038] Unless otherwise stated or there is a contradiction, the terms or phrases used herein have the following meanings:

[0039] As used herein, "a plurality of", "a variety of", "multiple times", etc., unless otherwise specified, mean greater than 2 or equal to 2 in quantity. For example, "one or more" means one or greater than or equal to two.

[0040] As used herein, "preferred", "better", "more preferably" are only used to describe embodiments or examples with better effects, and it should be understood that they do not constitute a limitation on the protection scope of the present invention. If "preferred" appears in multiple places in a technical solution, unless otherwise specified, and there is no contradiction or mutual restriction relationship, each "preferred" is independent of each other.

[0041] As used herein, "further", "even further", "especially", etc. are used for descriptive purposes and indicate differences in content, but should not be construed as a limitation on the protection scope of the present invention.

[0042] In this text, when it comes to numerical ranges, unless otherwise specified, the distribution of the selectable numerical values within the numerical range is considered continuous, and includes the two numerical endpoints of the numerical range (i.e., the minimum value and the maximum value), as well as each numerical value between these two numerical endpoints. When multiple numerical ranges are provided to describe features or characteristics, these numerical ranges can be combined.

[0043] Please refer to Figures 1 to 3 , the present invention provides a lithium-ion battery, a preparation method for a battery electrode sheet of the lithium-ion battery, and a device for preparing the battery electrode sheet. By controlling the particle size, thickness, and porosity of the upper and lower layer materials, the wettability of the overall electrode sheet is improved, thereby enhancing the kinetics and cycling performance of the dry thick electrode.

[0044] Please refer to Figure 1 and Figure 2 , on the one hand, the present invention provides a lithium-ion battery 100, which includes a battery electrode sheet 130, a separator, and an electrolyte. The battery electrode sheet 130 includes a positive electrode sheet and a negative electrode sheet with opposite polarities. The separator is disposed between the positive electrode sheet and the negative electrode sheet to play a role of isolation. The electrolyte infiltrates the positive electrode sheet and the negative electrode sheet and plays a role of conducting lithium ions during the charging and discharging process of the battery.

[0045] The battery electrode sheet 130 includes a current collector 131 and an electrode film 132. Among them, the electrode film 132 is disposed on at least one surface of the current collector 131. That is, the current collector 131 has a first surface and a second surface oppositely disposed along its thickness direction. The electrode film 132 can be disposed on any one of the first surface and the second surface, or can be disposed on both the first surface and the second surface. The material of the current collector 131 can be selected according to the type of the battery electrode sheet 130. For example, if the battery electrode sheet 130 is a positive electrode sheet, the current collector 131 can be selected as aluminum foil, and the thickness of the aluminum foil is 5 - 20 μm. Further, the thickness of the aluminum foil is 10 - 15 μm. More specifically, the thickness of the aluminum foil is 12 μm. The current collector 131 can also be selected as a composite current collector, which uses a polymer insulating resin material as the "sandwich" layer and deposits aluminum on its upper and lower surfaces. The polymer resin can be polyethylene terephthalate PET, polypropylene PP, polyimide PI, polystyrene PS, polyamide PA, etc. If the battery electrode sheet 130 is a negative electrode sheet, the current collector 131 can be copper foil, and the thickness of the copper foil is 4 - 15 μm. Further, the thickness of the copper foil is 5 - 10 μm. More specifically, the thickness of the copper foil is 8 μm. The current collector 110 can also be a composite current collector, which uses a polymer insulating resin material as the "sandwich" layer and deposits copper on the upper and lower surfaces. The polymer resin can be polyethylene terephthalate PET, polypropylene PP, polyimide PI, polystyrene PS, polyamide PA, etc.

[0046] The electrode film 132 includes a first-layer electrode film 1321 and a second-layer electrode film 1322 which are sequentially arranged from the direction close to the current collector 131 to the direction far from the current collector 131. Each layer of the electrode film 132 includes an active material, a conductive agent, and a binder. Since the cracking of the electrode sheet becomes more obvious with the increase of the coating thickness in the wet coating process, it is difficult to coat a thick electrode sheet by wet coating. To meet the requirements of high energy density battery cells and obtain a thick electrode with excellent performance, the present invention uses a dry process to prepare the double-layer electrode film 132, and optimizes the performance of the electrode sheet by regulating the particle size, porosity, and conductive agent content of the materials in the upper and lower electrode films.

[0047] Specifically, the particle size D501 of the active material in the first-layer electrode film 1321 is greater than or equal to the particle size D502 of the active material in the second-layer electrode film 1322. Preferably, D501 > D502. Smaller particle sizes have a larger grain boundary area, shorter transport paths, and better rate performance. Larger particle sizes have a higher tap density. The best effect is achieved by combining the large and small particle size layers. Further, D501:D502 = 1:1 to 3:1. Optionally, D501:D502 = 1:1, or 2:1, or 3:1, etc.

[0048] The content of the conductive agent in the first-layer electrode film 1321 is greater than the content of the conductive agent in the second-layer electrode film 1322, which can form more conductive paths, reduce the electron transfer resistance of the electrode sheet, and thus improve the rate performance and cycle stability of the battery. The porosity of the second-layer electrode film 1322 is greater than the porosity of the first-layer electrode film 1321. The larger the porosity, the higher the volume fraction of the electrolyte phase, the more fully the electrolyte is infiltrated, and the greater the effective lithium ion conductivity. Creating a porosity gradient is conducive to the full infiltration of the electrolyte.

[0049] In an embodiment, the total thickness of the electrode film 132 is H, the thickness of the first-layer electrode film 1321 is H1, and the thickness of the second-layer electrode film 1322 is H2. Since the thicker the electrode sheet, the larger the Li + transport path and the lower the kinetic performance. Therefore, in this embodiment, the value range of the total thickness H of the electrode film is 100 to 400 μm. Optionally, H is 100 μm, 200 μm, 300 μm, or 400 μm, etc. Further, the thickness ratio H1:H2 of the first-layer electrode film 1321 and the second-layer electrode film 1322 is 1:1 to 1:3. Optionally, H1:H2 = 1:1, or 1:2, or 1:3, etc.

[0050] The active material in the electrode diaphragm 132 is the main substance participating in the electrochemical reaction. The conductive agent can improve the electronic conductivity and play a role in collecting microcurrents between the active materials and between the active material and the current collector to reduce the contact resistance of the battery and accelerate the electron movement rate, so as to ensure good charge and discharge performance of the battery. In addition, the conductive agent can also improve the processability of the electrode, promote the infiltration of the electrolyte into the electrode, and effectively increase the migration rate of lithium ions in the battery material, thereby improving the charge and discharge efficiency of the battery and the service life of the battery; the binder is used to bond the active material and the conductive agent. The selection of the active material, the conductive agent and the binder is related to the type of the battery electrode sheet 130.

[0051] In one embodiment, the battery electrode sheet 130 is a positive electrode sheet, and each layer of the electrode diaphragm 132 of the positive electrode sheet includes a positive active material, a conductive agent, a first binder and a second binder. The positive active material is selected from one or more of lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), nickel cobalt lithium, lithium iron manganese phosphate, lithium iron phosphate, ternary materials, where the ternary materials include but are not limited to LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.8 Co 0.15 Al 0.05 O2. That is, the active material can be any one of the materials listed above, or any combination of two or more of them, without limitation here. Of course, the active material can also be a positive electrode material not listed above.

[0052] The conductive agent includes but is not limited to carbon black, acetylene black, carbon nanotubes, carbon fibers, graphene, Ketjen black. The conductive agent can be any one of the materials listed above, or any combination of two or more of the materials listed above. For example, the conductive agent is carbon black; or, carbon nanotubes; or a combination of carbon black and acetylene black; or a combination of carbon fibers, acetylene black and graphene, and so on. It should be noted that when the conductive agent is a multi-component composition, the ratio between the components in the composition is not limited and can be mixed in any ratio.

[0053] The first binder includes one or more of polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene copolymer (ETFE), and fluorinated ethylene propylene copolymer (FEP). Preferably, the first binder is PTFE, which is fibrillated under the action of high shear force to form a rich fiber network between the active material and the conductive agent. Further, the molecular weight of the first binder is 1 million to 2 million, for example, 1 million, 1.2 million, 1.6 million, or 2 million, etc., and the relative standard density (SSG) is 2.1 to 2.3, for example, it can be 2.1, 2.2, or 2.3, etc. Further still, the particle size of the first binder is less than 15 μm. The second binder is selected from at least one of polyvinylidene fluoride (PVDF), polyimide (PI) polymers, or acrylate-acrylonitrile-acrylamide copolymer, that is, the second binder can be selected from any one or more of the substances listed above. For example, the second binder is PVDF, or a composition of PVDF and PI polymers, etc. Further, the molecular weight of the second binder is 800,000 to 1.2 million, for example, 800,000, or 1 million, or 1.2 million, etc. Further still, the particle size of the second binder is 15 to 30 μm, for example, 15 μm, 25 μm, or 30 μm, etc. Binders with large particle sizes will weaken the bonding effect, reduce the stability of the electrode membrane, and the large particles have poor dispersion uniformity. In this application, using binders with small particle sizes can achieve a more uniform distribution.

[0054] In this embodiment, in the first layer of the positive electrode sheet 1321, the mass ratio of the positive active material, the conductive agent, the first binder, and the second binder is (90 - 98.6%):(0.4 - 5%):(0.5 - 2%):(0.5 - 3%); in the second layer of the electrode sheet, the mass ratio of the positive active material, the conductive agent, the first binder, and the second binder is (92 - 98.6%):(0.4 - 3%):(0.5 - 3%):(0.25 - 2%). Further, the ratio of the mass content of the first binder in the first layer of the electrode sheet 121 to the mass content of the first binder in the second layer of the electrode sheet 122 is 1:1, and the ratio of the mass content of the second binder in the first layer of the electrode sheet 121 to the mass content of the second binder in the second layer of the electrode sheet 122 is (1:1) - (2:1). Those skilled in the art can select appropriate ratios within the above ranges according to actual needs. The tap density of the positive electrode sheet is 2.0 g / cm 3 ~3.7 g / cm 3 For example, 2.0 g / cm 3 、3.0 g / cm 3 or 3.7 g / cm 3 and so on.

[0055] In another embodiment, the battery electrode sheet 130 is a negative electrode sheet. Each layer of electrode film 132 of the negative electrode sheet includes a negative active material, a conductive agent, a first binder, and a third binder. Among them, the negative active material is selected from one or more mixtures of artificial graphite, natural graphite, mesophase carbon microspheres, soft carbon, hard carbon, silicon, silicon monoxide, silicon-carbon composite materials, etc. For example, the negative active material is artificial graphite, or a mixture of artificial graphite and silicon, etc. When the active material is a composition of two or more materials, the ratio between the components in the composition is not limited and can be mixed in any ratio. The conductive agent includes, but is not limited to, one or more of carbon black, acetylene black, carbon nanotubes, carbon fibers, graphene, Ketjen black. The type of the first binder in the negative electrode sheet refers to the first binder in the positive electrode sheet. The third binder is selected from at least one of sodium carboxymethylcellulose, lithium carboxymethylcellulose, and acrylate-acrylonitrile-acrylamide copolymer. Optionally, the third binder is sodium carboxymethylcellulose, or lithium carboxymethylcellulose, or a composition of acrylate-acrylonitrile-acrylamide and sodium carboxymethylcellulose, etc. Further, the molecular weight of the third binder is 800,000 - 1,200,000, for example, 800,000, or 1,000,000, or 1,200,000, etc. Further still, the particle size of the third binder is 15 - 30 μm, such as 15 μm, 25 μm, or 30 μm, etc.

[0056] In the first layer of electrode film 1321 of the negative electrode sheet, the mass ratio of the negative active material, the conductive agent, the first binder, and the third binder is (91.4 - 99%):(0.4 - 5%):(0.1 - 0.6%):(0.5 - 3%); in the second layer of electrode film 1322, the mass ratio of the negative active material, the conductive agent, the first binder, and the third binder is (92.4 - 98.5%):(0.4 - 3%):(0.1 - 0.6%):(1 - 4%). Further, the ratio of the mass content of the first binder in the first layer of electrode film 1321 to the mass content of the first binder in the second layer of electrode film 122 is 1:1, and the ratio of the mass content of the third binder in the first layer of electrode film 121 to the mass content of the third binder in the second layer of electrode film 122 is (1:1) - (2:1).

[0057] In one embodiment, the tap density of the negative electrode sheet is 1.5 g / cm 3 ~3 g / cm 3 , optionally, the tap density is 1.5 g / cm 3 、2.5 g / cm 3 、3 g / cm 3 etc. Further, the tap density of the first layer of electrode film 1321 of the negative electrode sheet is 1.4 g / cm 3 ~1.7 g / cm 3, for example, 1.5 g / cm 3 ; the compaction density of the second electrode film 1322 of the negative electrode tab is 1.1 g / cm 3 to 1.5 g / cm 3 , for example, 1.3 g / cm 3 and so on.

[0058] Please refer to Figure 1 , in an embodiment, a conductive coating 1311 is further provided on the current collector 131. The conductive coating 1311 is disposed between the current collector 131 and the electrode film 132, which can effectively improve the adhesion of the film, increase the bonding force between the electrode film 132 and the current collector 131, reduce the risk of film layer peeling off during the lamination process, and at the same time reduce the contact resistance, increase the conductivity, and improve the dynamic performance of the tab. In an example, the conductive coating 1311 includes a coating conductive agent, a coating binder, and a stabilizer. Among them, the coating conductive agent is selected from one or more of carbon black, acetylene black, carbon nanotubes, carbon fibers, graphene, and Ketjen black. For example, conductive carbon black, or a composition of acetylene black and carbon nanotubes, or graphene, and so on; the coating binder is selected from one or more of polyacrylic acid (PAA), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyvinyl alcohol, polyacrylamide, epoxy resin, and polyurethane. For example, the binder is PAA, or a composition of SBR and CMC, and so on; the stabilizer can be selected from calcium hydroxide. When coating the conductive coating on the current collector 131, first mix the coating conductive agent, the coating binder, and the stabilizer in a mass ratio of (20-30%):(70-75%):(0-5%), for example, 25%:72%:3%; or, 30%:70%:0%; or 20%:75%:5%, and so on, and then add a solvent to prepare a slurry with a solid content of 5-15%, and then evenly coat it on the surface of the current collector 131. Among them, the thickness of the single-sided coating needs to meet the thickness after drying of 0.5-2 μm, for example, 0.5 μm, 1 μm, 1.5 μm, or 2 μm, and so on.

[0059] In some embodiments, the electrolyte includes a lithium salt and an organic solvent. As an example, the lithium salt can be selected from one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium bis(oxalato)borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluoro bis(oxalato)phosphate (LiDFOP), and lithium tetrafluoro bis(oxalato)phosphate (LiTFOP). The organic solvent can be selected from one or more of fluoroethylene carbonate (FEC), ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), and ethyl butyrate (EB).

[0060] In some embodiments, additives can also be added to the electrolyte. For example, the electrolyte can include anode film-forming additives, cathode film-forming additives, additives for improving the overcharge performance of the battery, additives for improving the high-temperature performance of the battery, additives for improving the low-temperature performance of the battery, and the like.

[0061] The separator separates the positive electrode sheet from the negative electrode sheet, preventing internal short circuit of the battery, while allowing active ions to pass through the separator and move between the positive and negative electrodes. In the lithium-ion battery of the present application, the type of the separator is not particularly limited, and any well-known porous structure separator with good chemical stability and mechanical stability can be selected. As an example, the material of the separator can be selected from one or more of polyethylene (PE) films, polypropylene (PP) films, polyvinylidene fluoride films, and multi-layer composite films containing one or more of them. The separator can be a single-layer separator or a multi-layer composite separator, without particular limitation. When the separator is a multi-layer composite separator, the materials of each layer can be the same or different, without particular limitation.

[0062] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be made into an electrode assembly by a winding process or a stacking process.

[0063] Please refer to Figure 1 and Figure 2, the lithium-ion battery 100 further includes a housing, which can be used to encapsulate the electrode assembly and the electrolyte as described above. In some embodiments, the housing of the lithium-ion battery 100 can be a hard case, such as a hard plastic case, an aluminum case, a steel case, etc.; it can also be a soft package, such as an aluminum-plastic film, etc.

[0064] This application does not particularly limit the shape of the lithium-ion battery 100, which can be cylindrical, square or any other shape. Figure 2 The lithium-ion battery 100 with a square structure is shown. In this embodiment, the housing of the battery includes a housing body 110 and a cover plate 120. Among them, the housing body 110 includes a bottom plate 111 and side plates 112 connected to the bottom plate 111, and the bottom plate 111 and the side plates 112 enclose a receiving cavity. The housing body 110 has an opening communicating with the receiving cavity, and the cover plate 120 can be covered on the opening to close the receiving cavity. A positive electrode lead-out member 121 and a negative electrode lead-out member 122 are provided on the cover plate 120. The electrode assembly formed by winding or laminating the positive electrode plate, the negative electrode plate and the separator is encapsulated in the receiving cavity, and its positive electrode side is electrically connected to the positive electrode lead-out member 121, and the negative electrode side is electrically connected to the negative electrode lead-out member 122. The electrolyte infiltrates the electrode assembly. The number of electrode assemblies included in the lithium-ion battery 100 can be one or more, and those skilled in the art can select according to specific actual needs.

[0065] Please refer to Figure 1 and Figure 3 , on the other hand, the present invention provides a method for preparing the above-mentioned battery electrode plate 130, which at least includes the following steps:

[0066] S1. Premixing of powders. According to the ratio of the first-layer electrode film 1321 and the second-layer electrode film 1322 of the battery electrode plate 130, the active material, the conductive agent and the binder are premixed at 0-20°C for 15-30 minutes to obtain a first mixture and a second mixture;

[0067] S2. Prefibrillation. The first mixture and the second mixture are respectively kept at 60-140°C for 30 minutes for prefibrillation, and then continue to be dispersed for 15-60 minutes to obtain a third mixture and a fourth mixture;

[0068] S3. First-layer lamination. The third mixture is extruded into a film, and after being rolled and thinned, it is laminated with the current collector 110 to form the first-layer electrode film 1321 on the current collector 131;

[0069] S4. Second-layer lamination. The fourth mixture is extruded into a film, and after being rolled and thinned, it is laminated with the first-layer electrode film 1321 to obtain the battery electrode plate 130.

[0070] Please refer to Figure 3, in step S1, the selection of the active material, conductive agent, and binder is related to the type of the battery electrode sheet 130. For specific details, please refer to the detailed description above. The ratio between the components is related to the type of the battery electrode sheet: when the battery electrode sheet is a positive electrode sheet, the mass ratio between the positive active material, conductive agent, first binder, and second binder in the first layer of electrode film 1321 is (90-98.6%):(0.4-5%):(0.5-2%):(0.5-3%); in the second layer of electrode film, the mass ratio between the positive active material, conductive agent, first binder, and second binder is (92-98.6%):(0.4-3%):(0.5-3):(0.25-2%). Preferably, the ratio of the mass content of the first binder in the first layer of electrode film 1321 to the mass content of the first binder in the second layer of electrode film 1322 is 1:1, and the ratio of the mass content of the second binder in the first layer of electrode film 1321 to the mass content of the second binder in the second layer of electrode film 1322 is (1:1)-(2:1). When the battery electrode sheet 130 is a negative electrode sheet, in the first layer of electrode film 1321, the mass ratio between the negative active material, conductive agent, first binder, and third binder is (91.4-99%):(0.4-5%):(0.1-0.6%):(0.5-3%); in the second layer of electrode film 1322, the mass ratio between the negative active material, conductive agent, first binder, and third binder is (92.4-98.5%):(0.4-3%):(0.1-0.6%):(1-4%). The specific ratio can be selected within the above range according to actual requirements.

[0071] In step S1, the above-mentioned active material, conductive agent, and binder are mixed evenly according to a certain ratio. Here, there is no limitation on the specific mixing method, and any method in the art that can mix the components evenly can be used. As an example: the positive active material, conductive agent, first binder, and second binder or the negative active material, conductive agent, first binder, and third binder are placed in a premixing device, such as a high-speed disperser, according to a certain ratio, and premixed at a low temperature of 0°C-20°C for 15-30 minutes to make them fully dispersed. The premixing temperature and premixing time can be any value within the above range. For example, the premixing temperature can be 0°C, 10°C, or 20°C, etc., and the premixing time can be 15 minutes, 20 minutes, 25 minutes, or 30 minutes, etc. Mixing under low-temperature conditions can effectively inhibit the premature fibrillation of polytetrafluoroethylene, and at low temperature and high speed, the fine binder powder can fully contact with the active material and conductive agent, and the mixing is more uniform.

[0072] Please refer to Figure 3, step S2 is to perform pre-fibrillation treatment on the first mixture and the second mixture obtained in step S1 under high shear force, so that a rich fiber network is formed between the binder, the active material and the conductive agent, significantly improving the adhesion strength of the electrode membrane. The specific process of pre-fibrillation is as follows: First, the first mixture and the second mixture obtained in step S1 are respectively kept at 60-140°C for 30 minutes for pre-fibrillation, so that the polytetrafluoroethylene is softened and loosened, which is beneficial to subsequent fibrillation treatment. Then, they are dispersed at high speed for 15-60 minutes to fibrillate the polytetrafluoroethylene under the action of shear force to form a rich network structure. Among them, the pre-fibrillation temperature can be 60°C, 100°C or 140°C, etc., and the time for high-speed dispersion treatment can be 15 minutes, 25 minutes, 40 minutes or 60 minutes, etc.

[0073] Please refer to Figure 1 and Figure 3 , step S3 is to extrude and mold the third mixture prepared in step S2 to obtain the first-layer electrode membrane 1321. Specifically, the third mixture is placed in a screw extruder and formed into a membrane through screw mixing and conveying and die extrusion. Among them, the screw extruder can be a single-screw extruder or a twin-screw extruder, and the die of the screw extruder can be selected as a coat-hanger type or a plunger type, which is not limited here. The extrusion temperature of the screw is 140-230°C, such as 140°C, 200°C or 230°C, etc., and the thickness of the extruded and formed membrane is 2-7 mm, such as 2 mm, 5 mm or 7 mm, etc. Then, the membrane is thinned to 0.5-2 mm by roll pressing with a pair of rollers. For example, 0.5 mm, 1 mm, 1.5 mm or 2 mm, etc., and then thermally roll-compounded with the current collector 131 to form the first-layer electrode membrane 1321. It should be noted that the selection of the current collector 131 in this step is related to the type of the battery electrode 130. For specific details, please refer to the description of the battery electrode 130 above, which will not be elaborated here.

[0074] Please refer to Figure 1 and Figure 3, in step S4, the fourth mixture prepared in step S2 is extruded and formed to laminate the second electrode film 1322 on the first electrode film 1321 to obtain a complete battery electrode sheet 130. Specifically, the fourth mixture is placed in a screw extruder and formed into a film through screw mixing and conveying and die extrusion. The screw extruder can be a single-screw extruder or a twin-screw extruder. The die of the screw extruder can be a coat-hanger type or a plunger type, which is not limited herein. The extrusion temperature of the screw is 140-230 °C, such as 140 °C, 200 °C or 230 °C, etc. The thickness of the extruded and formed film is 2-5 mm, such as 2 mm, 3 mm or 5 mm, etc. Then, the film is thinned to 0.3-1 mm by roll pressing with a pair of rollers. For example, 0.3 mm, 0.5 mm or 0.8 mm, etc., and then thermally roll-pressed and laminated with the first electrode film 1321. Further, after the second electrode film 1322 is laminated with the first electrode film 1321, the thickness of the overall battery electrode sheet can be adjusted by roll pressing to meet the requirements of the compaction density.

[0075] Please refer to Figure 4 , the present invention also provides a device for preparing a battery electrode sheet. The device 200 includes a first electrode film forming mechanism 210, a second electrode film forming mechanism 220, a unwind mechanism 230 and a winding mechanism 240. Among them, the first electrode film forming mechanism 210 is used to press the first electrode film 121, the second electrode film forming mechanism is used to press the second electrode film 122, the unwind mechanism 230 releases the current collector 131, and the winding mechanism 240 is used to wind and collect the formed battery electrode sheet 130.

[0076] Specifically, the first electrode film forming mechanism 210 includes a first feeding device 211, a first screw extrusion mechanism 212, a first die head 213, a first thinning roller 214, a first composite roller 215, and a first thickness monitoring device 216 arranged in sequence along the advancing direction of the pole piece. Among them, the first feeding device 211 is used to hold the prepared first mixture and inject the first mixture into the first screw extrusion mechanism 212 through this feeding device; the first screw extrusion mechanism 212 can be a single-screw extrusion mechanism or a twin-screw extrusion mechanism, and the screw elements inside it can be one or more of screw elements, kneading blocks, toothed disks, etc. The first die head 213 is arranged at the end of the first screw extrusion mechanism 212 and is used to extrude and form the mixture. The first die head 213 can be selected as a coat-hanger type or a plunger type, and during extrusion molding, the die head is heat-insulated, and the heat-insulation temperature is 140 - 230 °C. As an example, the first screw extrusion mechanism 212 can be selected as a lip-adjustable coat-hanger type plunger extruder. The first thinning roller 214 includes two relatively arranged pressing rollers, respectively denoted as the first pressing roller 2141 and the second pressing roller 2142. The first pressing roller 2141 and the second pressing roller 2142 are respectively arranged on the upper and lower sides of the pole piece, and the extruded film can be thinned to the required thickness through the extrusion of the two pressing rollers. The first composite roller 215 includes two relatively arranged pressing rollers, respectively denoted as the third pressing roller 2151 and the fourth pressing roller 2152. The third pressing roller 2151 and the fourth pressing roller 2152 are respectively arranged on the upper and lower sides of the pole piece, and the film thinned by the first thinning roller 214 is compounded with the current collector 110 through the extrusion of the two pressing rollers. The first thickness detection device 216 is used to detect the thickness of the pole piece after the first compounding to feedback and adjust the extrusion speed and the rolling speed. The first thickness detection device 216 can be a conventional mechanism in the art that can detect the thickness of the pole piece, for example, a laser thickness detection mechanism, or a ray thickness detection mechanism, or an infrared thickness detection mechanism, etc. It should be noted that during the preparation process, the rolling pressure of the first pressing roller 2141 and the second pressing roller 2142 is 3 - 50 T, the roller speed ratio is 1:1 - 1:3, and the roller temperature is maintained at 80 - 200 °C. The rolling pressure of the third pressing roller 2151 and the fourth pressing roller 2152 is 3 - 30 T, the roller speed ratio is 1:1 - 1:5, and the roller temperature is maintained at 80 - 200 °C.

[0077] The second electrode film forming mechanism 220 includes a second feeding device 221, a second screw extrusion mechanism 222, a second die head 223, a second thinning roller 224, a second composite roller 225, a thickness adjustment roller 226, and a second thickness detection device 227, which are arranged in sequence along the advancing direction of the electrode sheet. Among them, the second feeding device 221 is used to hold the prepared second mixture and inject the second mixture into the second screw extrusion mechanism 222 through this feeding device; the second screw extrusion mechanism 222 is the same as the first screw extrusion mechanism 212 and can be a single-screw extrusion mechanism or a twin-screw extrusion mechanism. The second die head 223 is arranged at the end of the second screw extrusion mechanism 222 and is used to extrude and form the mixture. The second die head 223 can be selected as a coat hanger type or a plunger type, and during the extrusion molding, the die head is heat-insulated, and the heat-insulation temperature is 140 - 230 °C. The second thinning roller 224 includes two relatively arranged pressing rollers, which are respectively denoted as the fifth pressing roller 2241 and the sixth pressing roller 2242. The fifth pressing roller 2241 and the second pressing roller 2242 are respectively arranged on the upper and lower sides of the electrode sheet, and the extruded film sheet can be thinned to the required thickness by the extrusion of the two pressing rollers. The second composite roller 225 includes two relatively arranged pressing rollers, which are respectively denoted as the seventh pressing roller 2251 and the eighth pressing roller 2252. The seventh pressing roller 2251 and the eighth pressing roller 2252 are respectively arranged on the upper and lower sides of the electrode sheet, and the film sheet thinned by the second thinning roller 224 is compounded with the first layer of electrode film sheet 121 through the extrusion of the two pressing rollers. Therefore, the second composite roller 225 is arranged at the rear end of the first thickness detection device 216. The thickness adjustment roller 226 includes two pressing rollers respectively arranged on the upper and lower sides of the electrode sheet, which are respectively denoted as the ninth pressing roller 2261 and the tenth pressing roller 2262. After the first layer of electrode film sheet 121 and the second layer of electrode film sheet 122 are compounded, the thickness of the overall electrode sheet can be adjusted by the ninth pressing roller 2261 and the tenth pressing roller 2262 to achieve the required compaction density. The second thickness detection device 227 is used to detect the thickness of the electrode sheet to feedback and adjust the extrusion speed and the rolling speed. The second thickness detection device 227 is the same as the first thickness detection device 216 and can be a conventional mechanism in the art that can detect the thickness of the electrode sheet, for example, a laser thickness detection mechanism, or a ray thickness detection mechanism, or an infrared thickness detection mechanism, etc. It should be noted that during the preparation process, the rolling pressure of the fifth pressing roller 2241 and the sixth pressing roller 2242 is 3 - 50T, the roller speed ratio is 1:1 - 1:3, and the roller temperature is maintained at 80 - 200 °C; the rolling pressure of the seventh pressing roller 2251 and the eighth pressing roller 2252 is 3 - 30T, the roller speed ratio is 1:1 - 1:5, and the roller temperature is maintained at 80 - 200 °C; the rolling pressure of the ninth pressing roller 2261 and the tenth pressing roller 2262 is 20 - 80T, the roller speed ratio is 1:1, and the roller temperature is maintained at 45 - 150 °C.

[0078] The unwinding mechanism 230 is arranged on one side of the first electrode film forming mechanism 210 for releasing the current collector 110; the winding mechanism 240 is arranged at the ends of the first electrode film forming mechanism 210 and the second electrode film forming mechanism 220 for winding the formed battery electrode sheet 100. The unwinding mechanism 230 and the winding mechanism 240 can adopt conventional devices in the art and are not limited herein.

[0079] By using the above device, the preparation of double-layer electrodes by the dry process can be realized, and the thickness of each layer of electrode film can be detected by the thickness detection device, and the extrusion speed and the rolling speed can be adjusted according to the detection results to adjust the thickness of the electrode sheet so as to meet the requirements of the thickness and the compaction density of the electrode sheet.

[0080] The technical solution of the present invention will be described in detail below through several specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are all commercially available products, or can be prepared by conventional methods in the art, and the instruments used in the examples can be commercially purchased.

[0081] Example 1

[0082] Please refer to Figures 1 to 4 and Table 1. This example provides a lithium-ion battery, which includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. The preparation method process is as follows:

[0083] (1) Preparation of the positive electrode sheet: The positive electrode active material LiNi 0.6 Co 0.2 Mn 0.2 O2:SP:first binder PTFE:second binder PVDF are premixed at a mass ratio of 93.8%:2.5%:1.2%:2.5% at 10°C for 20 min, then kept at 100°C for 30 min and then dispersed at high speed for 30 min. A first dry electrode film with a thickness of 5 mm is extruded at 150°C through a single-screw extruder and a die head. The first dry electrode film is rolled and thinned to 1 mm by the first pressing roller 2141 and the second pressing roller 2142, and then rolled and compounded with the current collector 131 by the third pressing roller 2151 and the fourth pressing roller 2152;

[0084] The positive electrode active material LiNi 0.6 Co 0.2 Mn 0.2O2: SP: The first binder is PTFE, the second binder is PVDF, and their mass ratio is 95.8%: 1.5%: 1.2%: 1.5%. They are premixed at 10°C for 20 min, then kept at 100°C for 30 min and dispersed at high speed for 30 min. Through a single-screw extruder and a die head, they are extruded at 150°C to form a 3-mm second dry electrode film. The second dry electrode film is rolled and thinned to 0.5 mm by the fifth roller 2241 and the sixth roller 2242, and then pressed and compounded with the electrode sheet containing the first-layer electrode film sheet by the seventh roller 2251 and the eighth roller 2252 to obtain the positive electrode sheet. Among them, the thickness of the first-layer electrode film sheet of the positive electrode sheet is 80 μm, and the thickness of the second-layer electrode film sheet is 80 μm; the particle size of the second binder is 20 μm, the D50 of the active material particle size in the first-layer electrode film sheet 1321 is 9.98 μm, and the D50 of the active material particle size in the second-layer electrode film sheet 1322 is 6.65 μm. The overall compaction density of the positive electrode sheet is 3.4 g / cm 3 .

[0085] The above current collector 131 is an aluminum foil with a thickness of 12 μm, and a conductive coating 1311 is coated on the aluminum foil. The mass ratio of the coating conductive agent: the coating binder: the stabilizer is 25%: 70%: 5%, the solid content of the slurry is 10%, and the coating thickness is 1 μm.

[0086] (2) Negative electrode preparation: The preparation process is the same as that of the positive electrode sheet, the difference is that: the ratio of the first-layer electrode film sheet 1321 is negative active material artificial graphite: conductive agent SP: the first binder PTFE: the third binder CMC-Na = 96.95: 0.6: 0.15: 2.3; the ratio of the second-layer electrode film sheet 1322 is negative active material artificial graphite: conductive agent SP: the first binder PTFE: the third binder CMC-Na = 97.85: 0.2: 0.15: 1.8. The current collector 131 is selected as a copper foil with a thickness of 8 μm, and a conductive coating 1311 is coated on the copper foil, which is the same as that on the positive electrode sheet. The compaction density of the negative electrode sheet is 1.55 g / cm 3 .

[0087] (3) Separator: A polypropylene film (PP) with a thickness of 12 μm is used as the separator.

[0088] (4) Electrolyte preparation: In an argon atmosphere glove box with a water content < 10 ppm, battery-grade ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), and ethyl acetate (EA) are mixed in a mass ratio of 1: 1: 2: 6 to form an organic solvent, and the lithium salt LiPF6 is added in a proportion of 1 mol / L of LiPF6 and mixed evenly to obtain the electrolyte.

[0089] (5) Battery assembly: The positive electrode plate, separator, and negative electrode plate are wound so that the separator is in the middle of the positive and negative electrodes to play a role in isolation, forming an electrode assembly. Then it is hot-pressed into a shell, transferred to a vacuum oven for drying at 120 °C, the prepared electrolyte is injected, and then formation, aging, and sealing nail welding are carried out. Finally, a prismatic battery (i.e., a lithium-ion battery) with a capacity of 100 Ah is prepared.

[0090] Example 2

[0091] The difference between this example and Example 1 is that the thickness of the first electrode film 1321 of the positive electrode plate is 100 μm.

[0092] Example 3

[0093] The difference between this example and Example 1 is that the thickness of the first electrode film 1321 of the positive electrode plate is 130 μm, and the thickness of the second electrode film 1322 is 130 μm.

[0094] Example 4

[0095] The difference between this example and Example 1 is that the thickness of the first electrode film 1321 of the positive electrode plate is 160 μm, and the thickness of the second electrode film 1322 is 160 μm.

[0096] Example 5

[0097] The difference between this example and Example 1 is that the thickness of the first electrode film 1321 of the positive electrode plate is 67 μm, and the thickness of the second electrode film 1322 is 133 μm.

[0098] Example 6

[0099] The difference between this example and Example 1 is that the thickness of the first electrode film 1321 of the positive electrode plate is 50 μm, the thickness of the second electrode film 1322 is 150 μm, and the thickness ratio of the first electrode film 1321 to the second electrode film 1322 is 1:3.

[0100] Example 7

[0101] The difference between this example and Example 1 is that the thickness of the first electrode film 1321 of the positive electrode plate is 100 μm, the thickness of the second electrode film 1322 is 100 μm, the thickness ratio of the first electrode film 1321 to the second electrode film 1322 is 1:1, the particle size of the second binder is 20 μm, and the D50 of the positive active material in both the first electrode film 1321 and the second electrode film 1322 is 12.25 μm.

[0102] Example 8

[0103] The differences between this embodiment and Embodiment 1 are as follows: the thickness of the first electrode film 1321 of the positive electrode plate is 100 μm, the thickness of the second electrode film 1322 is 100 μm, the thickness ratio of the first electrode film 1321 to the second electrode film 1322 is 1:1, the particle size of the second binder is 20 μm, the particle size D50 of the positive electrode active material of the first electrode film 121 is 13.6 μm, and the particle size D50 of the positive electrode active material of the second electrode film 122 is 10.2 μm.

[0104] Example 9

[0105] The differences between this embodiment and Embodiment 1 are as follows: the thickness of the first electrode film 1321 of the positive electrode plate is 100 μm, the thickness of the second electrode film 1322 is 100 μm, the thickness ratio of the first electrode film 1321 to the second electrode film 122 is 1:1, the particle size of the second binder is 15 μm, the particle size D50 of the positive electrode active material of the first electrode film 1321 is 9.98 μm, and the particle size D50 of the positive electrode active material of the second electrode film 1322 is 6.65 μm.

[0106] Example 10

[0107] The differences between this embodiment and Embodiment 1 are as follows: the thickness of the first electrode film 1321 of the positive electrode plate is 100 μm, the thickness of the second electrode film 1322 is 100 μm, the thickness ratio of the first electrode film 1321 to the second electrode film 1322 is 1:1, the particle size of the second binder is 30 μm, the particle size D50 of the positive electrode active material of the first electrode film 1321 is 10.2 μm, and the particle size D50 of the positive electrode active material of the second electrode film 1322 is 10.2 μm

[0108] Comparative Example 1

[0109] The differences between this comparative example and Embodiment 1 are as follows: in the first electrode film 1321 and the second electrode film 1322 of the positive electrode plate, the positive electrode active material LiNi 0.6 Co 0.2 Mn 0.2 O2: SP: the first binder PTFE: the second binder PVDF are all 94.8%: 2%: 1.2%: 2%, and the thicknesses of both the first electrode film 121 and the second electrode film 122 are 100 μm.

[0110] Comparative Example 2

[0111] The differences between this comparative example and Embodiment 1 are as follows: the particle size of the second binder is 50 μm.

[0112] The lithium-ion batteries prepared in Examples 1 to 10 and Comparative Examples 1 to 2 were subjected to performance tests. The test methods are as follows, and the test results are shown in Table 1.

[0113] (1) Adhesion test:

[0114] The prepared electrode sheet was cut into 20*100 mm, and the side to be tested was bonded to the stainless steel plate with double-sided tape and compacted with a roller to make it fully fit with the electrode sheet. One end of the stainless steel plate was fixed to the lower fixture of the tensile machine, and the bent end of the specimen was fixed to the upper fixture. The tensile speed was 50 mm / min.

[0115] (2) Rate performance test:

[0116] The battery cell was charged at a constant current of 1 / 3C to 4.2 V, then charged at a constant voltage to 0.05C, and left standing for 30 min. It was then discharged at 1 / 3C0 to 3 V and left standing for 30 min. The discharge capacity C0 was recorded. Then it was charged at a constant current of 1 / 3C0 to 4.2 V, charged at a constant voltage to 0.05C0, left standing for 30 min, discharged at 1 / 3C0 to 3 V, and left standing for 30 min. The discharge capacity C1 was recorded. Then it was charged at a constant current of 1 / 3C0 to 4.2 V, charged at a constant voltage to 0.05C0, left standing for 30 min, discharged at 3C to 3 V, and left standing for 30 min. The discharge capacity Cn was recorded. The 3C capacity retention rate was Cn / C1*100%.

[0117] (3) DCR (DC resistance) test:

[0118] The battery cell was charged at a constant current of 1 / 3C to 4.2 V, then charged at a constant voltage to 0.05C, and left standing for 30 min. It was then discharged at 1 / 3C0 to 3 V and left standing for 30 min. The discharge capacity C0 was recorded. Then it was charged at a constant current of 1 / 3C0 to 4.2 V, charged at a constant voltage to 0.05C0, left standing for 30 min, discharged at 1 / 3C0 for 180 min, and left standing for 10 min. The voltage V0 at the end of the standing was recorded. It was then discharged at 2C for 30 s, and the voltage V1 at the end of the discharge was recorded. The discharge DCR = (V0 - V1) / I.

[0119] (4) Membrane resistance test:

[0120] The electrode sheet was cut into 5*5 cm, and the membrane resistance was tested by Yuaneng BER2500. Six data were tested in each group, and the average value was obtained to get the membrane resistance.

[0121] (5) Initial efficiency and cycle test:

[0122] The battery cell is charged at a constant current of 1 / 3C to 4.2V, then charged at a constant voltage to 0.05C, and the charging capacity C0 is recorded. After standing for 30 minutes, it is discharged at 1 / 3C0 to 3V, then stands for 30 minutes, and the discharge capacity D0 is recorded. It is charged at 1 / 3D0 at a constant current to 4.2V, then charged at a constant voltage to 0.05D0, stands for 30 minutes, is discharged at 1 / 3D0 to 3V, and stands for 30 minutes, and the discharge capacity D1 is recorded. Record the discharge capacity Dn at the end of each cycle in this way. The cycle capacity retention rate is Dn / D1*100%, record the number of cycles N when the retention rate is 80%, and the initial efficiency is D0 / C0*100%.

[0123] Table 1: Parameters of the positive electrode sheets of Examples 1 to 10 and Comparative Examples 1 to 2

[0124]

[0125] Table 2: Performance test results of the batteries of Examples 1 to 10 and Comparative Examples 1 to 2

[0126]

[0127]

[0128] By comparing Examples 1 to 10 and Comparative Examples 1 to 2, it can be seen that the adhesion between the electrode film and the current collector, the initial efficiency, the rate performance, and the cycle performance have been significantly improved, and the film resistance and DCR value have decreased.

[0129] By comparing Examples 1 to 4, it can be concluded that: when other conditions remain the same, as the thickness of the electrode sheet increases, the resistance and DCR value of the film gradually increase. This is because the increase in the thickness of the electrode sheet leads to the growth of the output path of Li + and the decline of the kinetic performance, resulting in the decline of the initial efficiency, rate performance, and cycle performance.

[0130] By comparing Example 2, Example 5, and Example 6, it can be concluded that: keeping the total thickness of the electrode film unchanged and reducing the ratio of the thickness of the first-layer electrode film to the thickness of the second-layer electrode film, the initial efficiency and cycle performance of the battery first increase and then decrease.

[0131] By comparing Example 2, Example 7, and Example 8, it can be concluded that: on the premise that other conditions remain unchanged, increasing the size of the active material particles in the electrode film, the resistance and DCR value of the film increase, and the initial efficiency, rate performance, and cycle performance of the battery will decrease accordingly. However, when the particle sizes of the active materials in the double-layer electrode film are combined with large and small sizes, the rate performance and cycle performance are better. This is because the small particle size has a larger grain boundary area and a shorter transmission path, resulting in better rate performance, and the large particle size has a higher compaction density. The best effect can be achieved by combining the large and small particle size layers.

[0132] Comparing Comparative Example 2, Example 9 and Example 10, it can be concluded that: under the condition that other conditions remain unchanged, as the particle size of the second binder particles decreases, the initial efficiency, rate performance and cycle performance of the battery gradually improve, while the film resistance and DCR value gradually increase. This is because: binders with smaller particle sizes can improve the dispersion uniformity, increase the binding force and stability of the electrode sheet, improve the cycle life of the electrode sheet, and avoid film peeling in the later stage of cycling.

[0133] Comparing Example 2, Comparative Example 1 and Comparative Example 2, it can be concluded that the binding force between the electrode film of the positive electrode sheet prepared in Example 2 and the current collector is greater than that of Comparative Example 1 and Comparative Example 2, and the various performances of Example 2 are also better than those of Comparative Example 1 and Comparative Example 2. This is because: in Comparative Example 1, the content of the binder in the lower-layer electrode film is equal to the content of the binder in the upper-layer electrode film, and the binder particles in Comparative Example 2 are larger, resulting in a decrease in the binding force between the electrode film and the current collector, making the electrode prone to falling off during compounding or cycling, thus affecting the cycle performance of the battery.

[0134] The present invention uses a dry process to prepare a double-layer electrode sheet, and by controlling the particle size, thickness and pore regulation of the upper and lower layer active materials, the overall wetting performance of the electrode sheet is improved, and the kinetics and cycle performance of the battery are enhanced. Using binders with smaller particle sizes can improve the dispersion uniformity, increase the binding force and stability of the electrode sheet, improve the cycle life of the electrode sheet, and avoid film peeling in the later stage of cycling; at the same time, a higher bottom binder and a substrate with a conductive coating are used to increase the binding force and avoid film layer peeling off in the later stage of compounding and cycling. The content of the conductive agent in the first-layer electrode film is greater than that in the second-layer electrode film, which can form more conductive paths, the electron transfer resistance of the electrode sheet is low, and the rate performance and cycle stability of the battery are improved. Therefore, the present invention effectively overcomes some practical problems in the prior art and thus has high utilization value and practical significance.

[0135] The above embodiments merely illustrate the principles and effects of the present invention, rather than limiting the present invention. Any person familiar with this technology can modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes completed by those with ordinary knowledge in the technical field without departing from the spirit and technical idea disclosed by the present invention should still be covered by the claims of the present invention.

Claims

1. A lithium-ion battery, comprising a battery electrode sheet, a separator and an electrolyte, characterized in that, The battery electrode sheet includes: A current collector; An electrode film disposed on at least one surface of the current collector; the electrode film includes a first-layer electrode film and a second-layer electrode film arranged in sequence from the side close to the current collector to the side far from the current collector; Wherein, each layer of the electrode film includes an active material, a conductive agent, and a binder. The particle size D501 of the active material in the first-layer electrode film is greater than or equal to the particle size D502 of the active material in the second-layer electrode film; the content of the conductive agent in the first-layer electrode film is greater than that in the second-layer electrode film; the porosity of the second-layer electrode film is greater than that of the first-layer electrode film.

2. The lithium-ion battery according to claim 1, wherein The thickness of the electrode film is H, the thickness of the first-layer electrode film is H1, and the thickness of the second-layer electrode film is H2. Then the value range of H is 100 to 400 μm, and H1:H2 = (1:1) to (1:3).

3. The lithium-ion battery according to claim 1, characterized in that, The ratio D501:D502 of the particle size D501 of the active material in the first-layer electrode film to the particle size D502 of the active material in the second-layer electrode film is (1:1) to (3:1).

4. The lithium ion battery according to claim 1, characterized in that, The battery electrode sheet is a positive electrode sheet. The electrode film of the positive electrode sheet includes a positive active material, a conductive agent, a first binder, and a second binder. The mass ratio of the positive active material, the conductive agent, the first binder, and the second binder in the first-layer electrode film of the positive electrode sheet is (90 - 98.6%):(0.4 - 5%):(0.5 - 2%):(0.5 - 3%); the mass ratio of the positive active material, the conductive agent, the first binder, and the second binder in the second-layer electrode film of the positive electrode sheet is (92 - 98.6%):(0.4 - 3%):(0.5 - 3%):(0.25 - 2%); and / or, the battery electrode sheet is a negative electrode sheet. The electrode film of the negative electrode sheet includes a negative active material, a conductive agent, a first binder, and a third binder. The mass ratio of the negative active material, the conductive agent, the first binder, and the third binder in the first-layer electrode film of the negative electrode sheet is (91.4 - 99%):(0.4 - 5%):(0.1 - 0.6%):(0.5 - 3%); the mass ratio of the negative active material, the conductive agent, the first binder, and the third binder in the second-layer electrode film of the negative electrode sheet is (92.4 - 98.5%):(0.4 - 3%):(0.1 - 0.6%):(1 - 4%).

5. The lithium ion battery according to claim 4, characterized in that, The mass ratio of the first binder in the first-layer electrode film to the first binder in the second-layer electrode film in the positive electrode sheet is 1:

1. The mass ratio of the second binder in the first-layer electrode film to the second binder in the second-layer electrode film in the positive electrode sheet is (1:1) to (2:1); the mass ratio of the first binder in the first-layer electrode film to the first binder in the second-layer electrode film in the negative electrode sheet is 1:

1. The mass ratio of the third binder in the first-layer electrode film to the third binder in the second-layer electrode film in the negative electrode sheet is (1:1) to (2:1).

6. The lithium-ion battery according to claim 4, characterized in that, The first binder is selected from at least one of polytetrafluoroethylene, a copolymer of ethylene and tetrafluoroethylene, or fluorinated ethylene propylene copolymer; the second binder is selected from at least one of polyvinylidene fluoride, polyimide polymers, or acrylate-acrylonitrile-acrylamide copolymer; the third binder is selected from at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, or acrylate-acrylonitrile-acrylamide copolymer.

7. The lithium ion battery according to claim 4, characterized in that, The molecular weight of the first binder is 1 million to 2 million, and the particle size d1 < 15 μm. The molecular weight of the second binder is 0.8 million to 1.2 million, and the particle size d2 is 15 to 30 μm.

8. The lithium ion battery according to claim 4, wherein, The compaction density of the positive electrode sheet is 2.0 g / cm 3 to 3.7 g / cm 3 ; The compaction density of the negative electrode sheet is 1.5 g / cm 3 to 3.0 g / cm 3 .

9. The lithium ion battery according to claim 1, characterized in that, A conductive coating is further provided on the surface of the current collector. The conductive coating includes a coating conductive agent, a coating binder, and a stabilizer, and the mass ratio of the coating conductive agent, the coating binder, and the stabilizer is (20 - 30%):(70 - 75%):(0 - 5%).

10. The lithium ion battery according to claim 9, wherein, The coating binder includes one or several of polyacrylic acid, styrene-butadiene rubber, carboxymethyl cellulose, polyvinyl alcohol, polyacrylamide, epoxy resin, polyurethane, etc. The stabilizer is selected from calcium hydroxide, and the single-sided coating thickness of the conductive coating is 0.5 to 2 μm.

11. A method for preparing a battery electrode of the lithium-ion battery according to claim 1, characterized in that, It includes the following steps: Powder premixing: According to the ratio of the first layer electrode film and the second layer electrode film of the battery electrode sheet, the active material, the conductive agent, and the binder are premixed at 0 - 20 °C for 15 - 30 min to obtain a first mixture and a second mixture. Pre-fibrillation: The first mixture and the second mixture are respectively kept warm at 60 - 140 °C for 30 min for pre-fibrillation, and then continue to be dispersed for 15 - 60 min to obtain a third mixture and a fourth mixture. First layer lamination: The third mixture is extruded into a film, thinned by roll pressing, and then laminated with the current collector to form the first layer electrode film on the current collector. Second layer lamination: The fourth mixture is extruded into a film, thinned by roll pressing, and then laminated with the first layer electrode film to obtain the battery electrode sheet.

12. The preparation method according to claim 11, characterized in that, The first layer lamination step includes: The third mixture is mixed and conveyed by a screw, extruded through a die head to form an electrode film with a thickness of 2 - 7 mm, thinned to a thickness of 0.5 - 2 mm by roll pressing, and laminated with the current collector to form the first layer electrode film, where the screw temperature is 140 - 230 °C.

13. The preparation method according to claim 12, characterized in that, The second layer lamination step includes: The fourth mixture is mixed and conveyed by a screw, extruded through a die head to form an electrode film with a thickness of 2 - 5 mm, thinned to a thickness of 0.3 - 1 mm by roll pressing, and laminated with the first layer electrode film, and then the thickness and compaction density of the overall electrode sheet are adjusted by roll pressing to obtain the battery electrode sheet.

14. An apparatus for preparing a battery electrode sheet of the lithium-ion battery according to claim 1, characterized in that, It includes: The first electrode film forming mechanism: It includes a first feeding device, a first screw extrusion mechanism, a first die head, a first thinning roller, a first lamination roller, and a first thickness detection device arranged in sequence along the advancing direction of the electrode sheet. Second electrode film forming mechanism: It includes a second feeding device, a second screw extrusion mechanism, a second die head, a second thinning roller, a second composite roller, a thickness adjustment roller, and a second thickness detection device arranged in sequence along the advancing direction of the electrode sheet. Moreover, the second composite roller is arranged at the rear end of the first thickness detection device; Unwinding mechanism, arranged on one side of the first electrode film forming mechanism, for releasing the current collector; Rewinding mechanism, arranged at the ends of the first electrode film forming mechanism and the second electrode film forming mechanism, for rewinding the formed battery electrode sheet.

15. The device according to claim 14, characterized in that, The thickness detection device is selected from a laser thickness detection mechanism, a ray thickness detection mechanism, or an infrared thickness detection mechanism.

16. The device according to claim 14, characterized in that The rolling pressure of the first thinning roller, the first composite roller, the second thinning roller, and the second composite roller is 3 - 50T, and the roller temperature is 80 - 200°C; The rolling pressure of the thickness adjustment roller is 20 - 80T, and the roller temperature is 45 - 150°C; the roller speed ratio of the first thinning roller is 1:1 - 1:3; the roller speed ratio of the first composite roller is 1:1 - 1:5; the roller speed ratio of the second thinning roller is 1:1 - 1:2, and the roller speed ratio of the second composite roller is 1:1 - 1:3; the roller speed ratio of the thickness adjustment roller is 1:1.

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