CN113498558A

CN113498558A - Electrochemical device and electronic device

Info

Publication number: CN113498558A
Application number: CN202080016363.8A
Authority: CN
Inventors: 刘晓欠; 刘胜奇; 韩冬冬; 王可飞
Original assignee: Dongguan Amperex Technology Ltd
Current assignee: Dongguan Amperex Technology Ltd
Priority date: 2020-12-31
Filing date: 2020-12-31
Publication date: 2021-10-12
Anticipated expiration: 2040-12-31
Also published as: WO2022141508A1; CN113498558B

Abstract

The application provides an electrochemical device and an electronic device, which comprises a positive electrode, wherein the positive electrode comprises a current collector, a first material layer and a second material layer, the second material layer is arranged on at least one surface of the current collector, the first material layer is arranged between the current collector and the second material layer, the second material layer comprises a second active material, a second conductive agent and a second binder, and the specific surface area of the second active material is 0.05m²G to 20m²(ii)/g; the first material layer includes a first active material, a first conductive agent, and a first binder including a polymer obtained by polymerizing at least one monomer selected from acrylate and acrylamide. Through the design, the content of the second binder in the second material layer of the positive electrode is reduced, the relative content ratio of the second active material is increased, the positive electrode is applied to an electrochemical device,the energy density of the electrochemical device can be effectively improved.

Description

Electrochemical device and electronic device

Technical Field

The present application relates to the field of electrochemistry, and in particular, to an electrochemical device and an electronic device.

Background

The lithium ion battery has the advantages of high energy storage density, high open circuit voltage, low self-discharge rate, long cycle life, good safety performance and the like, and is widely applied to various fields of electric energy storage, mobile electronic equipment, electric automobiles, aerospace equipment and the like. As mobile electronic devices and electric vehicles enter a high-speed development stage, the market puts higher and higher requirements on energy density, safety, service life and the like of lithium ion batteries.

Lithium ion batteries generally include a positive electrode, a negative electrode, and a separator between the positive and negative electrodes. The active material layer is included in the positive electrode, and the process feasibility of the positive electrode and the performance (such as capacity, power, cycle and the like) of the lithium ion battery are influenced by the selection and the increase of the content of the binder in the conventional active material layer.

Disclosure of Invention

An object of the present application is to provide an electrochemical device and an electronic device to improve the energy density of the electrochemical device.

In the following, the present application is explained by taking a lithium ion battery as an example of an electrochemical device, but the electrochemical device of the present application is not limited to a lithium ion battery.

The specific technical scheme is as follows:

a first aspect of the present application provides an electrochemical device comprising a positive electrode including a current collector, a first material layer, and a second material layer, the current collector having at least one surface on which the first material layer is disposedThere is a second material layer, the first material layer is arranged between the current collector and the second material layer, the second material layer comprises a second active material, a second conductive agent and a second binder, the specific surface area of the second active material is 0.05m²G to 20m²(ii)/g; the first material layer includes a first active material, a first conductive agent, and a first binder including a polymer obtained by polymerizing at least one monomer selected from acrylate and acrylamide.

In one embodiment of the present application, the current collector is provided with a first material layer and a second material layer on at least one surface thereof, the first material layer being disposed between the current collector and the second material layer. The "surface" herein may be the entire region of the surface of the current collector or a partial region of the surface of the current collector, and the present application is not particularly limited as long as the object of the present application can be achieved.

In some embodiments of the present application, a first material layer and a second material layer are sequentially disposed on one surface of a current collector. In some embodiments of the present application, a first material layer and a second material layer are disposed on both surfaces of the current collector in sequence.

In the present application, the specific surface area of the second active material is 0.05m²G to 20m²(ii) in terms of/g. Without being bound by any theory, the specific surface area of the second active material is too small, e.g., less than 0.05m²The second active material cannot be in sufficient contact with the electrolyte, so that more oxide active sites cannot be provided, and the dynamics of the lithium ion battery are influenced; the specific surface area of the second active material is too large, for example, greater than 20m²The slurry is not easy to disperse uniformly, excessive active sites are caused, side reactions are increased, the stability of the battery cell is deteriorated, a larger proportion of the second binder is consumed, the binding force between the second material layer and the first material layer is reduced, and the internal resistance increase rate is increased.

In the present application, the first binder includes a polymer obtained by polymerizing at least one monomer of acrylate or acrylamide. The polymer is soaked in electrolyte for 24 hours at 85 ℃ or 7 days at 25 ℃, the swelling ratio is less than 5%, and the first material layer and the current collector or the second material layer can have good binding power.

The term "swelling" as used herein means that the polymer expands in volume in the electrolyte. Swelling ratio (volume after soaking-volume before soaking)/volume before soaking × 100%.

In summary, the use of the first adhesive in the first material layer improves the adhesive force of the first material layer, increases the adhesive force between the first material layer and the second material layer, and reduces the usage amount of the second adhesive in the second material layer. At the same time, controlling the specific surface area of the second active material within the range of the present application also allows the content of the second binder to be reduced. The reduction of the content of the second binder in the second material layer increases the relative content ratio of the second active material in the second active material layer, thereby effectively improving the energy density of the lithium ion battery.

In one embodiment of the present application, the content a of the first binder in the first material layer is 1 to 20 wt%. Without being limited to any theory, when the first binder content is too low, for example, less than 1 wt%, the first positive electrode active layer has poor film forming property, which is not favorable for coverage enhancement; when the content of the first binder is too high, for example, higher than 20 wt%, the first material layer shrinks to a greater extent during the drying film forming process due to the high binding power of the first binder, while the current collector substrate (e.g., aluminum foil) does not shrink substantially, and the difference between the two shrinkage rates is large, so that the first material layer falls off from the current collector substrate in a large area. By controlling the content of the first binder within the above content range, it is advantageous to improve the adhesion between the current collector and the first material layer.

In one embodiment of the present application, the polymer in the first binder further comprises acrylonitrile monomer.

In one embodiment of the present application, the mass percent of acrylonitrile is 30% to 70% and the sum of the mass percent of acrylate and acrylamide is 30% to 70% based on the total mass of the polymer. Preferably, the mass percent of acrylonitrile is 40% to 60% and the sum of the mass percent of acrylate and acrylamide is 40% to 60% based on the total mass of the polymer. By controlling the acrylonitrile, acrylate and acrylamide within the above-mentioned range of the total mass of the polymer, the first binder can be made to have better adhesion in use.

In the present application, the weight average molecular weight of the first binder is not particularly limited as long as the object of the present application can be achieved, and for example, the weight average molecular weight of the first binder is 100000 to 2000000. Without being limited to any theory, the weight average molecular weight of the first binder is too large, the thickening effect of the first binder is enhanced, the viscosity of the slurry is too high, the fluidity is poor, and the slurry of the first positive electrode active layer is easy to be coated; the weight average molecular weight of the first binder is too small, so that the viscosity of the slurry is too low, the film forming property of the slurry is poor, and the slurry of the insulating layer can be leaked to coat. By controlling the weight average molecular weight of the first binder within the above range, the first material layer slurry can form a film with uniform thickness on the surface of the current collector substrate, so that the first material layer is formed, and the binding force between the first material layer and the current collector can be improved.

In one embodiment of the present application, the second binder includes at least one of polyacrylic acid, polyvinylidene fluoride, polytetrafluoroethylene-hexafluoropropylene, sodium polyacrylate, nitrile rubber, or polyacrylate. Preferably, the second binder comprises at least one of polyacrylic acid, polyvinylidene fluoride, or nitrile rubber.

In the present application, the weight average molecular weight of the second binder is not particularly limited as long as the object of the present application can be achieved, and for example, the weight average molecular weight of the second binder is 100000 to 2000000.

In one embodiment of the present application, the second binder is present in the second material layer in an amount B of 0.5 to 10 wt%. Without being limited to any theory, when the content of the second binder is too low, for example, less than 0.5 wt%, the binding force of the second material layer is too low, the second material layer is easy to release, the cell performance is deteriorated, and the film forming property of the second material layer is poor, which is not beneficial to the improvement of the coverage; when the content of the second binder is too high, for example, higher than 10 wt%, the shrinkage degree of the second material layer in the drying film-forming process is different from the shrinkage degree of the first material layer, which easily causes the second material layer to fall off from the first material layer in a large area, the content of the second active material is reduced, the energy density of the battery cell is reduced, and meanwhile, the too high content of the second binder causes the increase of the ion diffusion resistance in the positive electrode, and the power performance of the lithium ion battery is deteriorated. By controlling the content of the second binder within the above content range, it is advantageous to improve the adhesion between the first material layer and the second material layer.

In one embodiment of the present application, the adhesion of the first material layer to the second material layer is 5N/m or more, indicating excellent adhesion between the first material layer and the second material layer.

In one embodiment of the present application, the second binder is present in the second material layer in an amount B of 0.5 to 3.99 wt%; the content A of the first binder in the first material layer and the content B of the second binder in the second material layer satisfy the following condition: b ═ n₁×A+n₂And-0.4. ltoreq. n₁≤-0.01、0.8％≤n₂Less than or equal to 4 percent. When the content of the first binder and the content of the second binder meet the relationship, the increase of the content of the first binder in the first material layer can reduce the using amount of the second binder in the second material layer, so that the relative content of the second active material in the second material layer is increased, and the energy density of the lithium ion battery is improved.

In one embodiment of the present application, the first material layer has a monolayer thickness of 0.01 μm to 20 μm, and when the thickness of the first material layer is too low, for example below 0.01 μm, the first material layer is too thin and the safety performance is affected; when the thickness of the first material layer is too high, for example, above 20 μm, the relative content of the first active material in the positive electrode decreases, affecting the energy density of the lithium ion battery.

In one embodiment of the present application, the monolayer thickness of the second material layer is 20 μm to 300 μm, and when the thickness of the second material layer is too low, for example, less than 20 μm, the energy density of the lithium ion battery is affected under a certain capacity, and the processing is not easy; when the thickness of the second material layer is too high, e.g. above 300 μm, the lithium ion battery dynamics are deteriorated.

In one embodiment of the present application, the resistance of the positive electrode after the electrochemical device is fully charged is 10 Ω or more, preferably 30 Ω to 100 Ω. The positive electrode resistance is controlled within the range, so that the internal resistance of the lithium ion battery in short circuit can be increased, the short-circuit current is reduced, and the temperature rise is reduced, thereby improving the safety of the lithium ion battery.

In one embodiment of the present application, the first conductive agent and the second conductive agent each independently comprise at least one of a lamellar, network, linear, or zero-dimensional conductive agent. Preferably, the first conductive agent and the second conductive agent each independently comprise at least one of graphene, reticulated graphite fibers, carbon nanotubes, ketjen black, graphite fibers, or nanoparticle conductive carbon. The first conductive agent is added into the first material layer, and the second conductive agent is added into the second material layer, so that the migration rate of lithium ions in the first active material and the second active material can be effectively improved, and the charge and discharge efficiency of the lithium ion battery is improved.

The method for preparing the first binder of the present application is not particularly limited, and a method known to those skilled in the art may be used, and for example, the following method may be used:

adding distilled water into a reaction kettle, starting stirring, introducing nitrogen to remove oxygen, adding at least one of the components such as acrylonitrile, acrylate, acrylamide and the like according to different mass ratios, heating to about 65 ℃ under an inert atmosphere, keeping the temperature constant, then adding an initiator to initiate reaction, and finishing the reaction after about 20 hours.

The initiator is not particularly limited as long as it can initiate polymerization of the monomers, and may be, for example, a 20% ammonium persulfate solution. The amount of the distilled water and the initiator added is not particularly limited as long as the polymerization of the added monomers is ensured. After the reaction, a basic solution was added to the precipitate of the reaction to neutralize it to a pH of 6.5 to 9. Then, the reaction product is filtered, washed, dried, crushed, sieved and the like.

The method for preparing the second binder of the present application is not particularly limited, and a method known to those skilled in the art may be used, and for example, the following method may be used:

and vacuumizing a reaction kettle containing a stirrer, vacuumizing, replacing oxygen with nitrogen, adding deionized water, vinylidene fluoride, an initiator, an emulsifier and a chain transfer agent into the reaction kettle, and carrying out polymerization reaction to obtain a second binder.

Among them, the initiator is not particularly limited and may be selected from, for example, diisopropylbenzene hydroperoxide; the emulsifier is not particularly limited and may be selected from, for example, perfluoroalkyl carboxylates; the chain transfer agent is not particularly limited and may be selected from, for example, isopropyl alcohol. As long as the object of the present application can be achieved.

In the positive electrode of the present application, the current collector is not particularly limited, and may be a current collector known in the art, such as an aluminum foil, an aluminum alloy foil, or a composite current collector. The first material layer includes a first active material, and the second material layer includes a second active material, and in the present application, the first active material and the second active material are not particularly limited, and active materials known in the art may be used, for example, the first active material may include at least one of lithium iron phosphate, lithium manganese iron phosphate, or lithium manganate, and the second active material may include at least one of lithium cobaltate, lithium nickel cobalt manganate (811, 622, 523, 111), lithium nickel cobalt aluminate, lithium iron phosphate, lithium manganese iron phosphate, or lithium manganate. In the present application, the thickness of the current collector is not particularly limited as long as the object of the present application can be achieved, and for example, the thickness of the current collector is 8 to 12 μm.

In the positive electrode of the present application, the current collector surface is provided with the first material layer and the second material layer, and the first material layer and the second material layer do not generally completely coat the surface of the current collector. The insulating layer may be provided on the current collector surface in a region where the first material layer and the second material layer are not provided according to actual needs by those skilled in the art, and is not particularly limited in the present application as long as the object of the present application can be achieved.

The negative electrode of the present application is not particularly limited as long as the object of the present application can be achieved. For example, the negative electrode typically includes a current collector and a material layer. In the negative electrode of the present invention, the current collector is not particularly limited, and a current collector known in the art, for example, a copper foil, a copper alloy foil, a composite current collector, or the like can be used. The material layer is not particularly limited, and active materials known in the art may be used. For example, at least one of artificial graphite, natural graphite, mesocarbon microbeads, silicon carbon, silicon oxy-compound, soft carbon, hard carbon, lithium titanate, niobium titanate, or the like may be included. In the negative electrode of the present application, the thicknesses of the current collector and the material layer are not particularly limited as long as the object of the present application can be achieved. For example, the thickness of the current collector is 4 μm to 10 μm, and the thickness of the material layer is 30 μm to 120 μm.

Optionally, the negative electrode may further comprise a conductive layer between the current collector and the material layer. The composition of the conductive layer is not particularly limited, and may be a conductive layer commonly used in the art. The conductive layer includes a conductive agent and a binder.

The above-mentioned conductive agent is not particularly limited as long as the object of the present application can be achieved. For example, the conductive agent may include at least one of conductive carbon black (Super P), Carbon Nanotubes (CNTs), carbon fibers, graphene, or the like. The above-mentioned binder is not particularly limited, and any binder known in the art may be used as long as the object of the present application can be achieved. For example, the binder may include at least one of styrene-butadiene rubber (SBR), polyvinyl alcohol (PVA), Polytetrafluoroethylene (PTFE), sodium carboxymethylcellulose (CMC-Na), or the like. For example, Styrene Butadiene Rubber (SBR) may be used as the binder.

The lithium ion battery further comprises an isolating membrane used for separating the positive electrode from the negative electrode, preventing short circuit inside the lithium ion battery, allowing electrolyte ions to freely pass through, and completing the effect of an electrochemical charging and discharging process. In the present application, the separator is not particularly limited as long as the object of the present application can be achieved.

For example, at least one of Polyolefin (PO) type separators mainly composed of Polyethylene (PE) and polypropylene (PP), polyester films (for example, polyethylene terephthalate (PET) films), cellulose films, polyimide films (PI), polyamide films (PA), spandex or aramid films, woven films, nonwoven films (nonwoven fabrics), microporous films, composite films, separator papers, roll-pressed films, and spun films.

For example, the release film may include a base material layer and a surface treatment layer. The substrate layer may be a non-woven fabric, a film or a composite film having a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate, polyimide, and the like. Optionally, a polypropylene porous film, a polyethylene porous film, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite film may be used. Optionally, a surface treatment layer is disposed on at least one surface of the substrate layer, and the surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic substance.

For example, the inorganic layer includes inorganic particles and a binder, and the inorganic particles are not particularly limited and may be, for example, at least one selected from the group consisting of alumina, silica, magnesia, titania, hafnia, tin oxide, ceria, nickel oxide, zinc oxide, calcium oxide, zirconia, yttria, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, barium sulfate, and the like. The binder is not particularly limited, and may be, for example, one or a combination of several selected from polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer contains a polymer, and the material of the polymer comprises at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, poly (vinylidene fluoride-hexafluoropropylene), and the like.

The lithium ion battery of the present application further includes an electrolyte, which may be one or more of a gel electrolyte, a solid electrolyte, and an electrolyte including a lithium salt and a non-aqueous solvent.

In some embodiments herein, the lithium salt is selected from LiPF₆、LiBF₄、LiAsF₆、LiClO₄、LiB(C₆H₅)₄、LiCH₃SO₃、LiCF₃SO₃、LiN(SO₂CF₃)₂、LiC(SO₂CF₃)₃、LiSiF₆One or more of LiBOB and lithium difluoroborate. For example, the lithium salt may be LiPF₆Since it can give high ionic conductivity and improve cycle characteristics.

The non-aqueous solvent may be a carbonate compound, a carboxylate compound, an ether compound, other organic solvent, or a combination thereof.

The carbonate compound may be a chain carbonate compound, a cyclic carbonate compound, a fluoro carbonate compound, or a combination thereof.

Examples of the above chain carbonate compound are dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), Methyl Propyl Carbonate (MPC), Ethyl Propyl Carbonate (EPC), Methyl Ethyl Carbonate (MEC), and combinations thereof. Examples of the cyclic carbonate compound are Ethylene Carbonate (EC), Propylene Carbonate (PC), Butylene Carbonate (BC), Vinyl Ethylene Carbonate (VEC), and combinations thereof. Examples of the fluoro carbonate compound are fluoroethylene carbonate (FEC), 1, 2-difluoroethylene carbonate, 1, 2-trifluoroethylene carbonate, 1,2, 2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1, 2-difluoro-1-methylethylene carbonate, 1, 2-trifluoro-2-methylethylene carbonate, trifluoromethylethylene carbonate, and combinations thereof.

Examples of the above carboxylic acid ester compounds are methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ -butyrolactone, decalactone, valerolactone, mevalonolactone, caprolactone, and combinations thereof.

Examples of the above ether compounds are dibutyl ether, tetraglyme, diglyme, 1, 2-dimethoxyethane, 1, 2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, and combinations thereof.

Examples of such other organic solvents are dimethylsulfoxide, 1, 2-dioxolane, sulfolane, methyl sulfolane, 1, 3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, and phosphate esters and combinations thereof.

A second aspect of the present application provides an electronic device comprising the electrochemical device provided in the first aspect of the present application.

The electronic device of the present application is not particularly limited, and may be any electronic device known in the art. In some embodiments, the electronic device may include, but is not limited to, a notebook computer, a pen-input computer, a mobile computer, an electronic book player, a portable phone, a portable facsimile machine, a portable copier, a portable printer, a headphone, a video recorder, a liquid crystal television, a handheld cleaner, a portable CD player, a mini-disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable recorder, a radio, a backup power source, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a lighting fixture, a toy, a game machine, a clock, an electric tool, a flashlight, a camera, a large household battery, a lithium ion capacitor, and the like.

The process for preparing the electrochemical device is well known to those skilled in the art, and the present application is not particularly limited. For example, the electrochemical device may be manufactured by the following process: the positive electrode and the negative electrode are overlapped through a separator, and are wound, folded, and the like as needed, and then placed in a case, and an electrolyte is injected into the case and sealed. In addition, an overcurrent prevention element, a guide plate, or the like may be placed in the case as necessary to prevent a pressure rise and overcharge/discharge inside the electrochemical device.

An electrochemical device and an electronic device including a positive electrode including a current collector, a first material layer and a second material layer, the second material layer being disposed on at least one surface of the current collector, the first material layer being disposed between the current collector and the second material layer, the second material layer containing a second active material, a second conductive agent and a second binder, the second active material having a specific surface area of 0.05m²G to 20m²(ii)/g; the first material layer contains a first active material, a first conductive agent, and a first binder containing an acrylate or acryloyl groupA polymer obtained by polymerizing at least one monomer of amine. Through the design, the adhesive force of the first material layer to the second material layer in the anode is remarkably increased, the content of the second adhesive in the second material layer of the anode is reduced, the relative content of the second active material is increased, and the anode is applied to an electrochemical device, so that the energy density of the electrochemical device can be effectively improved.

Drawings

In order to illustrate the technical solutions of the present application and the prior art more clearly, the following briefly introduces examples and figures that need to be used in the prior art, it being obvious that the figures in the following description are only some examples of the present application.

FIG. 1 is a schematic diagram of a positive electrode structure according to an embodiment of the present application;

FIG. 2 is a schematic representation of a positive electrode structure of another embodiment of the present application;

fig. 3 is a graph showing the relationship between adhesion and stroke in the adhesion test.

Reference numerals: 10. current collector, 21, first material layer, 22, second material layer.

Detailed Description

In order to make the objects, technical solutions and advantages of the present application more apparent, the present application is further described in detail below with reference to the accompanying drawings and examples. It is to be understood that the embodiments described are only a few embodiments of the present application and not all embodiments. All other technical solutions obtained by a person of ordinary skill in the art based on the embodiments in the present application belong to the scope of protection of the present application.

In the embodiments of the present application, the present application is explained by taking a lithium ion battery as an example of an electrochemical device, but the electrochemical device of the present application is not limited to a lithium ion battery.

Fig. 1 shows a schematic structural diagram of a positive electrode according to an embodiment of the present application, where a first material layer 21 and a second material layer 22 are disposed on both a first surface and a second surface of a current collector 10, and an area of a region where the first material layer 21 and the second material layer 22 are disposed on the first surface of the current collector 10 is larger than that of the second surface of the current collector 10, and of course, an area of a region where the first material layer 21 and the second material layer 22 are disposed on the first surface of the current collector 10 may also be smaller than that of the second surface of the current collector 10; and the first material layers 21 are each disposed between the current collector 10 and the second material layer 22.

Fig. 2 shows a schematic structural view of a positive electrode according to another embodiment of the present application, in which a first material layer 21 and a second material layer 22 are disposed only on a first surface of a current collector 10. Of course, the first material layer 21 and the second material layer 22 may be provided on the second surface of the current collector 10.

Fig. 3 shows the relationship between the adhesion and the stroke in the adhesion test of the present application.

Examples

Hereinafter, embodiments of the present application will be described in more detail with reference to examples and comparative examples. Various tests and evaluations were carried out according to the following methods. Unless otherwise specified, "part" and "%" are based on mass.

The test method and the test equipment are as follows:

first binder weight average molecular weight test:

the first binder in the first material layer was tested for weight average molecular weight using Gel Permeation Chromatography (GPC). In the present application, weight average molecular weight means molecular weight statistically averaged by mass.

Specific surface area (BET) test:

the specific surface area of the second active material was measured by a nitrogen adsorption method.

And (3) testing the adhesive force:

testing the adhesion between the first material layer and the second material layer by using a high-speed rail tensile machine and a 90-degree angle method: and cutting the positive pole piece provided with the first material layer part in the finished product lithium ion battery into a strip sample of 20mm multiplied by 60mm, wherein the length and width values of the sample can be proportionally adjusted according to actual conditions. Adhering a part of the sample to a steel plate from one end of the sample by double-sided adhesive along the length direction of the sample, wherein the adhering length is not less than 40 mm; then the steel plate is fixed at the corresponding position of a high-speed rail tensile machine, the other end which is not adhered to the steel plate is pulled, and a sample is placed into a chuck through a connector or directly and clamped, wherein the included angle of the pulled sample part and the steel plate is 90 degrees in space. The chuck pulls the sample at a speed of 5mm/min to separate the first material layer from the second material layer, and the average of the final measured pulling force in the plateau region is recorded as the adhesion between the first material layer and the second material layer. As shown in fig. 3, it is required that the ratio of the standard deviation of the above-mentioned bonding force data of the plateau region to the average value does not exceed 10%.

Thickness testing of the first material layer and the second material layer:

1) detaching the positive electrode coated with the first material layer and the second material layer from the finished lithium ion battery;

2) cutting the anode obtained in the step 1) along the thickness direction of the anode by using a plasma cutting technology to obtain cross sections of a first material layer and a second material layer;

3) observing the cross sections of the first material layer and the second material layer obtained in the step 2) under an SEM (electron microscope) (the length of the observed cross section is required to be not less than 2cm), respectively testing the single-side thicknesses of the first material layer and the second material layer under the SEM, testing at least 15 different points at intervals of 2 mm-3 mm at adjacent test points, and recording the thickness average value of all test positions of each layer as the thickness value of the corresponding layer.

And (3) testing the full charge internal resistance of the positive electrode:

1) the lithium ion battery is charged with a multiplying factor of 0.05C to a voltage of 4.45V (namely, full charge voltage) in a constant current, and then charged with a constant voltage of 4.45V to a current of 0.025C (cut-off current), so that the lithium ion battery reaches a full charge state

2) Disassembling the lithium ion battery to obtain a positive electrode;

3) soaking the positive electrode obtained in the step 2) in DMC (dimethyl carbonate) for 1h at 25 ℃, and then airing in a fume hood;

4) testing the resistance of the positive pole piece obtained in the step 3) by using a BER1200 model diaphragm resistance tester, wherein the interval between adjacent test points is 2mm to 3mm, at least 15 different points are tested, and the resistance mean value of all the test points is recorded as the diaphragm resistance of the positive pole piece. Wherein the parameters are as follows: pressure head area 153.94mm²Pressure 3.5t, hold time 50 s.

And (3) impact test:

under the environment of 20 +/-5 ℃, the lithium ion battery to be tested is subjected to constant current charging with the multiplying power of 0.05C until the voltage is 4.45V (namely full charge voltage), and then is subjected to constant voltage charging with the voltage of 4.45V until the current is 0.025C (cut-off current), so that the lithium ion battery reaches a full charge state, and the appearance of the lithium ion battery before testing is recorded. After a round bar with a diameter of 15.8 + -0.1 mm was placed in the center of the cell, an iron block weighing 9.1 + -0.46 kg dropped from a height of 610 + -25 mm onto the cell, during which the impacted sample was considered to pass the impact test without igniting and exploding.

Example 1

< preparation of first Binder >

Adding distilled water into a reaction kettle, starting stirring, introducing nitrogen to remove oxygen for 2 hours, adding the following monomers of acrylonitrile, sodium acrylate and acrylamide into the reaction kettle according to the mass ratio of 40:45:15, heating to 65 ℃ under an inert atmosphere, keeping constant temperature, adding 20% ammonium persulfate solution as an initiator to start reaction, taking out a precipitate after the reaction is carried out for 22 hours, adding alkali liquor to neutralize the pH value to 6.5. Wherein the mass ratio of distilled water, monomer and initiator is 89.5: 10: 0.5. And after the reaction, filtering, washing, drying, crushing, sieving and the like are carried out on the reaction product to obtain the first binder.

< preparation of second Binder >

Vacuumizing a reaction kettle containing a stirrer, vacuumizing the reaction kettle, replacing oxygen with nitrogen, adding deionized water, vinylidene fluoride, an initiator, namely diisopropylbenzene hydroperoxide, an emulsifier perfluoroalkyl carboxylate and a chain transfer agent, namely isopropanol into the reaction kettle until the pressure of the reaction kettle is 3.5MPa, wherein the deionized water is 7 times of the mass of the vinylidene fluoride monomer, the initiator accounts for 0.2 percent of the mass of the vinylidene fluoride monomer, the emulsifier accounts for 0.5 percent of the mass of the vinylidene fluoride monomer, and the chain transfer agent accounts for 5 percent of the mass of the vinylidene fluoride monomer. And then heating to 60 ℃, rotating the stirrer at a speed of 80r/min, starting a polymerization reaction, continuously adding the vinylidene fluoride monomer, keeping the pressure of the reaction kettle at 3.5MPa, stopping the reaction until the solid content of the emulsion in the reactor reaches 25%, recovering unreacted monomer, discharging the polymer emulsion, centrifuging, washing and drying to obtain the polyvinylidene fluoride.

< preparation of Positive electrode >

Mixing a first active material lithium iron phosphate, a first binder, a first conductive agent nanoparticle conductive carbon and a carbon nano tube according to a mass ratio of 95.8: 3.0: 0.7: 0.5, adding N-methylpyrrolidone (NMP) serving as a solvent, preparing slurry with the solid content of 35%, and uniformly stirring. Uniformly coating the slurry on a current collector aluminum foil with the thickness of 10 mu m, and drying at 90 ℃ to obtain a first material layer with the thickness of 5 mu m; wherein the first binder has a weight average molecular weight of 500000;

mixing a second active material Lithium Cobaltate (LCO), a second binder polyvinylidene fluoride (PVDF), a second conductive agent conductive carbon black and the carbon nano tube according to the mass ratio of 97.3: 1.6: 0.6: 0.5, adding N-methyl pyrrolidone (NMP) as a solvent, preparing slurry with the solid content of 75%, and uniformly stirring. Uniformly coating the slurry on the first material layer, and drying at 90 deg.C to obtain a second material layer with a thickness of 85 μm, wherein the specific surface area of the second active material is 2.0m²/g；

And then repeating the steps on the other surface of the positive electrode to obtain the positive electrode with the first material layer and the second positive electrode layer coated on both sides. Cutting the positive electrode into a size of 74mm multiplied by 867mm, and welding a lug for later use.

< preparation of negative electrode >

Mixing active material graphite, styrene-butadiene polymer and sodium carboxymethylcellulose according to a weight ratio of 97.5: 1.3: 1.2, adding deionized water as a solvent, blending to obtain slurry with a solid content of 70%, and uniformly stirring. And uniformly coating the slurry on a copper foil of a negative current collector, drying at 110 ℃, and cold-pressing to obtain the negative electrode with a single-side coating material layer with the material layer thickness of 150 mu m.

And after the steps are finished, the steps are also finished on the back surface of the cathode by adopting the same method, and the cathode with the double-sided coating is obtained. After the coating is completed, the negative electrode is cut into sheets of 76mm × 851mm in specification and the tab is welded for standby.

< preparation of electrolyte solution >

Mixing organic solvents ethylene carbonate, ethyl methyl carbonate and diethyl carbonate at a mass ratio of EC: EMC: DEC: 30: 50: 20 in a dry argon atmosphere to obtain an organic solution, and then adding lithium salt lithium hexafluorophosphate into the organic solvent to dissolve and uniformly mix the organic solution to obtain the electrolyte with the lithium salt concentration of 1.15 Mol/L.

< preparation of separator >

Alumina and polyvinylidene fluoride were mixed in a mass ratio of 90: 10 and dissolved in deionized water to form a ceramic slurry with 50% solids. The ceramic slurry was then uniformly coated on one side of a porous substrate (polyethylene, thickness 7 μm, average pore diameter 0.073 μm, porosity 26%) by a gravure coating method, and dried to obtain a two-layer structure of a ceramic coating layer and the porous substrate, the ceramic coating layer having a thickness of 50 μm.

Polyvinylidene fluoride (PVDF) was mixed with polyacrylate in a mass ratio of 96: 4 and dissolved in deionized water to form a polymer slurry with 50% solids. And then uniformly coating the polymer slurry on two surfaces of the ceramic coating and porous substrate double-layer structure by adopting a dimple coating method, and drying to obtain the isolating membrane, wherein the thickness of a single-layer coating formed by the polymer slurry is 2 mu m.

< preparation of lithium ion Battery >

And (3) stacking the prepared positive electrode, the prepared isolating membrane and the prepared negative electrode in sequence to enable the isolating membrane to be positioned between the positive electrode and the negative electrode to play an isolating role, and winding to obtain the electrode assembly. And (3) putting the electrode assembly into an aluminum-plastic film packaging bag, dehydrating at 80 ℃, injecting the prepared electrolyte, and performing vacuum packaging, standing, formation, shaping and other processes to obtain the lithium ion battery.

Example 2

Except that at<Preparation of the Positive electrode>In (2), the specific surface area of the second active material is controlled to 0.05m²The same as example 1 except for the fact of being used as the reaction solution/g.

Example 3

Except that at<Preparation of the Positive electrode>In (2), the specific surface area of the second active material is controlled to 0.1m²The same as example 1 except for the fact of being used as the reaction solution/g.

Example 4

Except that at<Preparation of the Positive electrode>In (2), the specific surface area of the second active material is controlled to be 4.0m²The same as example 1 except for the fact of being used as the reaction solution/g.

Example 5

Except that at<Preparation of the Positive electrode>In (2), the specific surface area of the second active material is controlled to be 8.0m²The same as example 1 except for the fact of being used as the reaction solution/g.

Example 6

Except that at<Preparation of the Positive electrode>Wherein the specific surface area of the second active material is controlled to be 15m²The same as example 1 except for the fact of being used as the reaction solution/g.

Example 7

Except that at<Preparation of the Positive electrode>In (2), the specific surface area of the second active material is controlled to 20m²The same as example 1 except for the fact of being used as the reaction solution/g.

Example 8

The procedure of example 1 was repeated, except that sodium polyacrylate was used as the binder in < preparation of first binder >.

Example 9

The procedure was as in example 1, except that acrylamide and sodium acrylate were used in a mass ratio of 40:60 as the monomers in < preparation of first binder >.

Example 10

The procedure of example 1 was repeated, except that acrylonitrile and acrylamide were used in a mass ratio of 40:60 in < preparation of first binder >.

Example 11

The procedure of example 1 was repeated, except that acrylonitrile and sodium acrylate were used in a mass ratio of 40:60 in < preparation of first binder >.

Example 12

The procedure was as in example 1 except that acrylonitrile, sodium acrylate and acrylamide were used in a mass ratio of 30:60:10 as the monomers in < preparation of first binder >.

Example 13

The procedure was as in example 1 except that acrylonitrile, sodium acrylate and acrylamide were used in a mass ratio of 30:10:60 as the monomers in < preparation of first binder >.

Example 14

The procedure was as in example 1 except that acrylonitrile, sodium acrylate and acrylamide were used in a mass ratio of 50:40:10 as the monomers in < preparation of first binder >.

Example 15

The procedure was as in example 1 except that acrylonitrile, sodium acrylate and acrylamide were used in a mass ratio of 60:10:30 in < preparation of first binder >.

Example 16

The procedure was as in example 1 except that acrylonitrile, sodium acrylate and acrylamide were used in a mass ratio of 70:20:10 as the monomers in < preparation of first binder >.

Example 17

The procedure was as in example 1, except that in < preparation of positive electrode >, the mass ratio of the first active material to the first binder was 97.8: 1.0.

Example 18

The procedure was as in example 1, except that in < preparation of positive electrode >, the mass ratio of the first active material to the first binder was 93.8: 5.0.

Example 19

The procedure was as in example 1, except that in < preparation of positive electrode >, the mass ratio of the first active material to the first binder was 88.8: 10.0.

Example 20

The procedure was as in example 1, except that in < preparation of positive electrode >, the mass ratio of the first active material to the first binder was 83.8: 15.0.

Example 21

The procedure was as in example 1, except that in < preparation of positive electrode >, the mass ratio of the first active material to the first binder was 78.8: 20.0.

Example 22

The same as example 1 was repeated, except that in < preparation of positive electrode >, the weight average molecular weight of the first binder was 100000.

Example 23

Except that in < preparation of cathode >, the weight average molecular weight of the first binder was 800000, the same as example 1 was used.

Example 24

Except that in < preparation of positive electrode >, the weight average molecular weight of the first binder was 1000000, the same as example 1.

Example 25

The same as example 1 was repeated, except that in < preparation of cathode >, the weight average molecular weight of the first binder was 1400000.

Example 26

The same as example 1 was repeated, except that in < preparation of positive electrode >, the weight average molecular weight of the first binder was 2000000.

Example 27

The procedure of example 1 was repeated, except that nitrile rubber was used as the second binder in < preparation of positive electrode >.

Example 28

The same as example 1 was repeated, except that in < preparation of positive electrode >, the mass ratio of the second active material to the second binder was 98.4: 0.5.

Example 29

The procedure was as in example 1, except that in < preparation of positive electrode >, the mass ratio of the second active material to the second binder was 93.9: 5.0.

Example 30

The same as example 1 was repeated, except that in < preparation of positive electrode >, the mass ratio of the second active material to the second binder was 88.9: 10.

Example 31

The procedure was carried out in example 1 except that in < preparation of positive electrode >, the mass ratio of lithium iron phosphate to the first binder was 97.8:1.0, the mass ratio of lithium cobaltate to the second binder was 96.3:2.6, n1 ═ 0.4, and n2 ═ 3%.

Example 32

The procedure was carried out in the same manner as in example 31 except that in < preparation of positive electrode >, the mass ratio of lithium iron phosphate to the first binder was 96.8:2.0, and the mass ratio of lithium cobaltate to the second binder was 96.7: 2.2.

Example 33

The procedure of example 1 was repeated, except that the mass ratio of lithium cobaltate to the second binder in < preparation of positive electrode > was 97.1:1.8, n1 ═ 0.2, and n2 ═ 2.4%.

Example 34

The procedure was carried out in example 33 except that in < preparation of positive electrode >, the mass ratio of lithium iron phosphate to the first binder was 94.8:4.0, and the mass ratio of lithium cobaltate to the second binder was 97.3: 1.6.

Example 35

The procedure was carried out in example 33 except that in < preparation of positive electrode >, the mass ratio of lithium iron phosphate to the first binder was 93.8:5.0, and the mass ratio of lithium cobaltate to the second binder was 97.5: 1.4.

Example 36

The procedure was carried out in example 1 except that in < preparation of positive electrode >, the mass ratio of lithium iron phosphate to the first binder was 82.8:16, the mass ratio of lithium cobaltate to the second binder was 98.26:0.64, n1 was-0.01, and n2 was 0.8%.

Example 37

The procedure was carried out in example 36 except that in < preparation of positive electrode >, the mass ratio of lithium iron phosphate to the first binder was 80.8:18, and the mass ratio of lithium cobaltate to the second binder was 98.28: 0.62.

Example 38

The procedure was carried out in example 36 except that in < preparation of positive electrode >, the mass ratio of lithium iron phosphate to the first binder was 78.8:20, and the mass ratio of lithium cobaltate to the second binder was 98.3: 0.6.

Example 39

The procedure was as in example 1, except that in < preparation of positive electrode >, the mass ratio of lithium iron phosphate to the first binder was 97.8:1.0, the mass ratio of lithium cobaltate to the second binder was 94.91:3.99, n1 ═ 0.01, and n2 ═ 4.0%.

Example 40

The procedure was carried out in the same manner as in example 39 except that in < preparation of positive electrode >, the mass ratio of lithium iron phosphate to the first binder was 78.8:20, and the mass ratio of lithium cobaltate to the second binder was 95.1: 3.8.

Comparative example 1

Except that at<Preparation of the Positive electrode>In (2), the specific surface area of the second active material is 40m²The same as example 1 except for the fact of being used as the reaction solution/g.

Comparative example 2

The procedure of example 1 was repeated, except that polyvinylidene fluoride was used as the binder in < preparation of first binder >.

Comparative example 3

The same as example 1 except that in < preparation of positive electrode >, the weight average molecular weight of the first binder was 2500000.

The preparation parameters and test results of the respective examples and comparative examples are shown in the following tables 1 to 3.

TABLE 1 preparation parameters and test results for examples 1-21 and comparative examples 1-2

TABLE 2 preparation parameters and test results for examples 22-30 and comparative example 3

TABLE 3 preparation parameters and test results for examples 31-40

As can be seen from examples 1 to 7 and comparative example 1, in the lithium ion battery of the present application in which the specific surface area of the second active material is within the range defined in the present application, the adhesion between the first material layer and the second material layer is significantly improved, and in particular, in example 3, the adhesion between the first material layer and the second material layer is more than 35N/m, so that the content of the second binder in the second material layer is reduced, the relative content of the second active material in the second material layer is increased, and the improvement of the energy density of the lithium ion battery is facilitated; and the nail penetration test passing rate is obviously improved, and the safety of the lithium ion battery can be effectively improved.

As can be seen from examples 8 to 21 and comparative example 2, in the lithium ion battery in which the first material layer contains the first binder of the present application and the content of the first binder is within the range defined in the present application, the adhesion between the first material layer and the second material layer is significantly improved, especially in examples 14 to 15 and examples 18 to 21, the adhesion between the first material layer and the second material layer is more than 30N/m and as high as 55N/m, so that the content of the second binder in the second material layer is reduced, the relative content of the second active material in the second material layer is increased, and the energy density of the lithium ion battery is improved; and the nail penetration test passing rate is obviously improved, and the safety of the lithium ion battery can be effectively improved.

As can be seen from examples 22 to 26 and comparative example 3, in the case of the lithium ion battery of the present application in which the second binder has the same composition and the same content, the adhesion between the first material layer and the second material layer is significantly improved in the case of the lithium ion battery of the present application in which the first binder has the weight average molecular weight within the range defined in the present application, and particularly in examples 25 to 26 in which the adhesion between the first material layer and the second material layer is higher than 40N/m, the content of the second binder in the second material layer is reduced, the relative content of the second active material in the second material layer is increased, and the improvement of the energy density of the lithium ion battery is facilitated; and the nail penetration test passing rate is obviously improved, and the safety of the lithium ion battery can be effectively improved.

As can be seen from examples 27 to 30, controlling the content of the second binder within the range of the present application can effectively improve the adhesion between the first material layer and the second material layer, thereby improving the energy density and safety of the lithium ion battery.

From examples 31 to 40, it can be seen that when n1 and n2 have the same coefficient value, the increase of the first binder content a increases the binding force between the first material layer and the second material layer, and simultaneously decreases the second binder content B in the second material layer, so that the relative content of the second active material in the second material layer increases, which is beneficial to the improvement of the energy density of the lithium ion battery; and the nail penetration test passing rate is obviously improved, and the safety of the lithium ion battery can be effectively improved.

The above description is only exemplary of the present application and should not be taken as limiting the present application, as any modification, equivalent replacement, or improvement made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. An electrochemical device comprising a positive electrode comprising a current collector, a first material layer, and a second material layer, the second material layer disposed on at least one surface of the current collector, the first material layer disposed between the current collector and the second material layer; the second material layer comprises a second active material, a second conductive agent and a second binder; wherein the specific surface area of the second active material is 0.05m²G to 20m²(ii)/g; the first material layer includes a first active material, a first conductive agent, and a first binder including a polymer obtained by polymerizing at least one monomer selected from acrylate and acrylamide.

2. The electrochemical device according to claim 1, wherein a content a of the first binder in the first material layer is 1 to 20 wt%.

3. The electrochemical device of claim 1, wherein the polymer in the first binder further comprises acrylonitrile monomer.

4. The electrochemical device according to claim 3, wherein the acrylonitrile is contained in an amount of 30 to 70% by mass and the sum of the acrylate and the acrylamide is contained in an amount of 30 to 70% by mass, based on the total mass of the polymer.

5. The electrochemical device according to claim 1, wherein the first binder has a weight average molecular weight of 100000 to 2000000.

6. The electrochemical device of claim 1, wherein the second binder comprises at least one of polyacrylic acid, polyvinylidene fluoride, polytetrafluoroethylene-hexafluoropropylene, sodium polyacrylate, nitrile rubber, or polyacrylate.

7. The electrochemical device according to claim 2, wherein the second binder is contained in the second material layer in an amount B of 0.5 to 10 wt%.

8. The electrochemical device according to claim 1, wherein the adhesion between the first material layer and the second material layer is 5N/m or more.

9. The electrochemical device according to claim 7, wherein the second binder is present in the second material layer in an amount B of 0.5 to 3.99 wt%;

the content A of the first binder in the first material layer and the content B of the second binder in the second material layer satisfy the following condition:

b is n1 xA + n2, n1 is equal to or more than-0.4 and equal to or more than-0.01, and n2 is equal to or more than 0.8 and equal to or more than 4%.

10. The electrochemical device of claim 1, wherein the first material layer has a monolayer thickness of 0.01 μ ι η to 20 μ ι η and the second material layer has a monolayer thickness of 20 μ ι η to 300 μ ι η.

11. The electrochemical device according to claim 1, wherein the resistance of the positive electrode after the electrochemical device is fully charged is 10 Ω or more.

12. The electrochemical device of claim 1, wherein the first and second conductive agents each independently comprise at least one of a sheet-like, mesh-like, wire-like, or zero-dimensional conductive agent.

13. The electrochemical device of claim 1 or 3, wherein the electrochemical device satisfies at least one of the following characteristics:

(a) the mass percentage of the acrylonitrile is 40 to 60 percent, and the sum of the mass percentages of the acrylate and the acrylamide is 40 to 60 percent, based on the total mass of the polymer;

(b) the second binder comprises at least one of polyacrylic acid, polyvinylidene fluoride or nitrile rubber;

(c) the first and second conductive agents each independently comprise at least one of graphene, reticulated graphite fibers, carbon nanotubes, ketjen black, graphite fibers, or nanoparticle conductive carbon.

14. An electronic device comprising the electrochemical device of any one of claims 1 to 13.