CN114270561A

CN114270561A - Electrochemical device and electronic device

Info

Publication number: CN114270561A
Application number: CN202180004994.2A
Authority: CN
Inventors: 李志愿; 李娅洁
Original assignee: Ningde Amperex Technology Ltd
Current assignee: Ningde Amperex Technology Ltd
Priority date: 2021-03-31
Filing date: 2021-03-31
Publication date: 2022-04-01
Anticipated expiration: 2041-03-31
Also published as: WO2022205110A1; CN114270561B

Abstract

The present application provides an electrochemical device and an electronic device. The electrochemical device includes an electrode including a current collector, a first coating layer, a second coating layer, and an active material layer, wherein the first coating layer is between the current collector and the second coating layer, and the second coating layer is between the first coating layer and the active material layer. The first coating includes a positive temperature coefficient material and a first conductive agent, and the second coating includes a second conductive agent, a binder, and a reinforcement. Embodiments of the present application prevent short circuits of electrochemical devices by providing a first coating layer and a second coating layer between a current collector and an active material layer, wherein a positive temperature coefficient material in the first coating layer increases in resistance at high temperatures, cutting off electron transport. And the second coating can protect the first coating and ensure the normal performance of the first coating.

Description

Electrochemical device and electronic device

Technical Field

The present application relates to the field of electrochemical energy storage, and more particularly to electrochemical devices and electronic devices.

Background

As electrochemical devices (e.g., lithium ion batteries) are developed and advanced, higher and higher requirements are placed on their safety performance. And a large amount of heat generation due to internal short circuit, overcharge, etc. is an important factor affecting the safety performance of the electrochemical device.

For this purpose, a Positive Temperature Coefficient (PTC) resistor may be generally added to the external circuit, and the electrochemical device is shut down by cutting off the current when the temperature of the electrochemical device rises; or the isolating membrane is subjected to structural or material modification, so that the closed pore temperature of the isolating membrane is reduced, the membrane breaking temperature is increased, and the thermal shrinkage is reduced to reduce the internal short circuit of the electrochemical device; or preparing a PTC mixed electrode or a PTC coated electrode. However, the PTC resistance sheet added in the external circuit has slow response and is not timely, and the PTC resistance sheet can only play a role when the side reaction generates more heat; regarding the structure or material modification of the isolating membrane, the isolating membrane material has limited heat resistance, the temperature difference between closed pores and membrane breaking is small, high-temperature membrane breaking cannot be inhibited, and the improvement on the safety performance of an electrochemical device is limited; in addition, the PTC mixed electrode needs to increase the amount of conductive agent and polymer, which affects the energy density of the electrochemical device, and the PTC coated electrode is difficult to process on the material layer surface, so that the coating uniformity is difficult to achieve, and the properties of the material are affected. Therefore, further improvements are still desired.

Disclosure of Invention

Some embodiments of the present application provide an electrochemical device including an electrode including a current collector, a first coating layer, a second coating layer, and an active material layer, wherein the first coating layer is between the current collector and the second coating layer, and the second coating layer is between the first coating layer and the active material layer. The first coating includes a positive temperature coefficient material and a first conductive agent, and the second coating includes a second conductive agent, a binder, and a reinforcement.

In some embodiments, the reinforcement comprises at least one of lithium iron phosphate, silicon dioxide, titanium dioxide, aluminum oxide, boehmite, magnesium oxide, zirconium oxide, titanium dioxide, silicon carbide, boron carbide, barium carbonate, potassium titanate, barium sulfate, vanadium trioxide, polyetheretherketone, polyamide, or cellulose powder.

In some embodiments, the reinforcement is present in an amount of 40 to 98% by mass, preferably 60 to 80% by mass, based on the total mass of the second coating.

In some embodiments, the reinforcement has a vickers hardness of 600 to 2000, preferably 800 to 1500.

In some embodiments, the particle sphericity of the reinforcement is in the range of 0.5 to 1, preferably 0.7 to 1.

In some embodiments, the reinforcement has a Dv50 of 0.05 μm to 2 μm, preferably 0.2 μm to 1 μm.

In some embodiments, the second coating has a thickness of 0.2 μm to 5 μm. In some embodiments, the mass ratio of the second conductive agent, the binder, and the reinforcement in the second coating layer is (1 to 20): 60 to 98. In some embodiments, the binder comprises at least one of polyvinyl alcohol, polyacrylic acid, polyethylene glycol, polyethylene oxide, carboxymethylcellulose salt, polyacrylamide, polymaleic anhydride, polyquaternary ammonium salt, starch, chitosan, pectin, polyacrylate, polyurethane, polyvinyl chloride, natural rubber emulsion, neoprene emulsion, butyronitrile emulsion, butylbenzene emulsion, or styrene-acrylic emulsion.

In some embodiments, the positive temperature coefficient material satisfies at least one of the following conditions: the melting point of the positive temperature coefficient material is 115 ℃ to 180 ℃; the positive temperature coefficient material comprises at least one of Polyethylene (PE), polypropylene (PP), polyvinyl chloride, polystyrene, polytetrafluoroethylene, polybutylene terephthalate, polyimide, polyvinyl alcohol, polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVDF), polyacrylonitrile, polyformaldehyde, ethylene-vinyl acetate copolymer or polyethylene terephthalate; the mass content of the positive temperature coefficient material in the first coating is 60-98%. In some embodiments, the first coating has a thickness of 0.5 μm to 12 μm.

In some embodiments, the first conductive agent and the second conductive agent each independently comprise at least one of conductive carbon black, acetylene black, graphite, graphene, carbon nanotubes, carbon fibers, aluminum powder, nickel powder, or gold powder. In some embodiments, the first conductive agent in the first coating layer is present in an amount of 2% to 40% by mass.

Embodiments of the present application also provide an electronic device including the above electrochemical device.

Embodiments of the present application prevent a short circuit of an electrochemical device by providing a first coating layer and a second coating layer between a current collector and an active material layer, wherein the first coating layer includes a positive temperature coefficient material and a first conductive agent, and the positive temperature coefficient material increases in resistance at a high temperature, cutting off electron transfer. The second coating can protect the first coating, comprises a reinforcement, can weaken damage to the first coating during coating of the active material layer and/or cold pressing of the pole piece, prevents active substances in the active material layer from being embedded into the first coating and directly contacting with the current collector, enables the first coating to be kept complete, ensures that the first coating cuts off an electronic path at high temperature, and accordingly improves thermal runaway of the electrochemical device.

Detailed Description

The following examples are presented to enable those skilled in the art to more fully understand the present application and are not intended to limit the present application in any way.

Some embodiments of the present application provide an electrochemical device including an electrode including a current collector, a first coating layer, a second coating layer, and an active material layer, wherein the first coating layer is between the current collector and the second coating layer, and the second coating layer is between the first coating layer and the active material layer. It should be understood that the first coating, the second coating, and the active material layer may all be located on one or both sides of the current collector.

In some embodiments, the first coating includes a positive temperature coefficient material and a first conductive agent. The ptc material may exhibit a stepwise increase in resistance as the temperature increases. Therefore, when the temperature of the first coating layer rises due to short circuit or the like in the electrochemical device, the positive temperature coefficient material can be melted and expanded, electron transmission of the first coating layer is cut off, and the resistance of the first coating layer is increased, so that the protection effect is achieved.

In some embodiments, the second coating includes a second conductive agent, a binder, and a reinforcement. In some embodiments, the second coating layer can protect the first coating layer, and the reinforcement in the second coating layer can weaken damage to the first coating layer during the coating of the active material layer and/or the cold pressing of the pole piece, so that the first coating layer is kept intact, and the function of cutting off electronic paths of the first coating layer at high temperature is realized, thereby improving the thermal runaway of the electrochemical device. If the second coating layer does not exist, the edges and corners of the active material particles in the active material layer are easy to damage or even break through the first coating layer in the cold pressing process, so that the first coating layer is difficult to realize the function of cutting off electron transmission at high temperature. The presence of the reinforcement members in the second coating layer prevents the first coating layer from being pierced by the active material particles during cold pressing, thus protecting the first coating layer.

In some embodiments, the positive temperature coefficient material comprises a polymer. At high temperatures (e.g., greater than 100 ℃), the polymer expands and the conductive network between the particles of the first conductive agent is broken, thereby cutting off electron transport, so that the first coating has a PTC effect. In some embodiments, the positive temperature coefficient material comprises at least one of polyethylene, polypropylene, polyvinyl chloride, polystyrene, polytetrafluoroethylene, polybutylene terephthalate, polyimide, polyvinyl alcohol, polymethyl methacrylate, polyvinylidene fluoride, polyacrylonitrile, or polyethylene terephthalate. In some embodiments, the positive temperature coefficient material has a melting point of 115 ℃ to 180 ℃. The melting point of the positive temperature coefficient material is lower than 115 ℃ to influence the coating process of the pole piece, and the melting point is higher than 180 ℃ to hardly achieve the effect of improving the safety performance.

In some embodiments, the mass content of the positive temperature coefficient material is 60% to 98% based on the total mass of the first coating layer. In some embodiments, the first conductive agent is present in an amount of 2% to 40% by mass, based on the total mass of the first coating layer. If the mass content of the positive temperature coefficient material in the first coating layer is greater than 98%, such that the mass content of the first conductive agent is less than 2%, the greater resistance of the first coating layer itself may affect the performance of the electrochemical device. If the mass content of the positive temperature coefficient material in the first coating layer is less than 60%, the mass content of the first conductive agent is more than 40%, and the first coating layer hardly has the PTC effect. In some embodiments, the mass content of the positive temperature coefficient material in the first coating layer is 80% to 95%. At this time, the first coating layer has a strong PTC effect without adversely affecting the performance of the electrochemical device due to too weak electrical conductivity. In some embodiments, the first conductive agent comprises at least one of conductive carbon black, acetylene black, graphite, graphene, carbon nanotubes, carbon fibers, aluminum powder, nickel powder, or gold powder.

In some embodiments, the first coating has a thickness of 0.5 μm to 12 μm. If the thickness of the first coating layer is too small, it is difficult to achieve the PTC protection effect. If the thickness of the first coating layer is too large, the energy density of the electrochemical device is adversely affected.

In some embodiments, the second conductive agent comprises at least one of conductive carbon black, acetylene black, graphite, graphene, carbon nanotubes, carbon fibers, aluminum powder, nickel powder, or gold powder. In some embodiments, the binder comprises at least one of polyvinyl alcohol, polyacrylic acid, polyethylene glycol, polyethylene oxide, carboxymethylcellulose salt, polyacrylamide, polymaleic anhydride, polyquaternary ammonium salt, starch, chitosan, pectin, polyacrylate, polyurethane, polyvinyl chloride, natural rubber emulsion, neoprene emulsion, butyronitrile emulsion, butylbenzene emulsion, or styrene-acrylic emulsion. These binders may better bind the second conductive agent and the reinforcement together to form the second coating. In addition, the binder is of an aqueous solution type or an aqueous emulsion type, and prevents the second coating layer from being compatible with the active material layer during the coating process of the active material layer (using solvent NMP).

In some embodiments, the reinforcement comprises at least one of lithium iron phosphate, silicon dioxide, titanium dioxide, aluminum oxide, boehmite, magnesium oxide, zirconium oxide, titanium dioxide, silicon carbide, boron carbide, barium carbonate, potassium titanate, barium sulfate, vanadium trioxide, polyetheretherketone, polyamide, or powder. The second coating comprises the reinforcement, so that damage to the first coating during coating of the active material layer and cold pressing of the pole piece can be weakened, active substances in the active material layer are prevented from being embedded into the first coating and directly contacting with a current collector, the first coating is kept complete, an electronic path is cut off by the first coating at high temperature, and thermal runaway of the electrochemical device is improved.

In some embodiments, the reinforcement in the second coating is present in an amount of 40% to 98% by mass. If the mass content of the reinforcement in the second coating is too small, the protection that the reinforcement can play is relatively limited; if the mass content of the reinforcement in the second coating is too large, the electrical conductivity and the binding effect of the second coating may be affected, thereby affecting the performance of the electrochemical device. In some embodiments, the reinforcement in the second coating is present in an amount of 60% to 98% by mass. In some embodiments, the reinforcement in the second coating is present in an amount of 60% to 80% by mass. In some embodiments, the second conductive agent in the second coating layer is 1% to 20% by mass. In some embodiments, the binder in the second coating layer is present in an amount of 1% to 20% by mass. In some embodiments, the mass ratio of the second conductive agent, the binder, and the reinforcement in the second coating layer is (1 to 20): 60 to 98. Therefore, the content of each component in the second coating reaches a better balance to realize respective functions, the reinforcement can better protect the first coating, the adhesive can better adhere the second conductive agent and the reinforcement together, and the second conductive agent can endow the second coating with proper conductive performance.

In some embodiments, the reinforcement has a vickers hardness of 600 to 2000. If the Vickers hardness of the reinforcement is too small, it is easily destroyed by the particles in the active material layer; however, if the hardness of the reinforcement is extremely high, the high hardness of the reinforcement itself will rather cause the reinforcement to be embedded into the first coating during cold pressing, destroying the integrity of the first coating, affecting the PTC effect, and at the same time, the high hardness will also cause the active material particles in contact with the reinforcement to be broken, affecting the battery performance. In some embodiments, the reinforcement has a vickers hardness of 800 to 1500. In this case, the reinforcement can have a good protective effect.

In some embodiments, the particle sphericity of the reinforcement ranges from 0.5 to 1. If the sphericity is too small, for example, less than 0.5, the edges and corners of the reinforcement are too large, and the protective effect of the first coating layer is weak relative to the reinforcement of spherical particles. In some embodiments, the particle sphericity of the reinforcement ranges from 0.7 to 1. In this way, the adverse effect of the edges of the reinforcement on the protective effect can be substantially eliminated.

In some embodiments, the coverage of the second coating is more than 60%, so that a better protection effect can be achieved, if the coverage of the second coating is too small, the first coating which is not covered by the second coating can be in direct contact with the active material particles in the cold pressing process, and due to the polygonal structure of the active material particles, when the cold pressing process is not protected by the second coating, the first coating is embedded in the first coating, so that the first coating is damaged, and the PTC effect is influenced. In some embodiments, the second coating coverage is above 80%.

In some embodiments, the second coating has a thickness of 0.2 μm to 5 μm. If the thickness of the second coating is below 0.2 μm, the protective effect on the first coating is relatively limited; if the thickness of the second coating layer is greater than 5 μm, the resistance of the second coating layer is deteriorated on the one hand, and the energy density of the electrochemical device is lost on the other hand.

In some embodiments, the Dv50 of the reinforcement is 0.05 μm to 2 μm. Dv50 indicates the particle size at which the volume distribution of the particles reaches 50%. If the Dv50 of the reinforcement is too small, uniform dispersion of the reinforcement in the second coating is not favored; if the Dv50 of the reinforcement is too large, the thickness of the second coating will increase, affecting the energy density. In some embodiments, the Dv50 of the reinforcement is 0.2 μm to 1 μm. In this way, even dispersion of the reinforcement members can be ensured, and the thickness of the second coating layer does not have to be excessively affected.

In some embodiments, when the positive electrode includes the above structure, the active material layer is a positive electrode active material layer, and includes a positive electrode active material. In some embodiments, the positive active material comprises at least one of lithium cobaltate, lithium iron phosphate, lithium manganese iron phosphate, sodium iron phosphate, lithium vanadium phosphate, sodium vanadium phosphate, lithium vanadyl phosphate, sodium vanadyl phosphate, lithium vanadate, lithium manganate, lithium nickelate, lithium nickel cobalt manganese, a lithium rich manganese based material, or lithium nickel cobalt aluminate. In some embodiments, the positive electrode active material layer may further include a conductive agent. In some embodiments, the conductive agent in the positive electrode active material layer may include at least one of conductive carbon black, ketjen black, flake graphite, graphene, carbon nanotubes, or carbon fibers. In some embodiments, the positive electrode active material layer may further include a binder, and the binder in the positive electrode active material layer may include at least one of carboxymethyl cellulose (CMC), polyacrylic acid, polyvinylpyrrolidone, polyaniline, polyimide, polyamideimide, polysiloxane, styrene-butadiene rubber, epoxy resin, polyester resin, polyurethane resin, or polyfluorene. In some embodiments, the mass ratio of the positive electrode active material, the conductive agent, and the binder in the positive electrode active material layer may be (80 to 99): (0.1 to 10). In some embodiments, the thickness of the positive electrode active material layer may be 10 μm to 500 μm. It should be understood that the above description is merely an example, and any other suitable material, thickness, and mass ratio may be employed for the positive electrode active material layer of the positive electrode.

In some embodiments, the current collector of the positive electrode may be an Al foil, but of course, other current collectors commonly used in the art may be used. In some embodiments, the thickness of the current collector of the positive electrode may be 1 μm to 200 μm. In some embodiments, the positive electrode active material layer may be coated only on a partial area of the current collector of the positive electrode.

In some embodiments, when the anode includes the above structure, the active material layer is an anode active material layer. In some embodiments, the negative active material layer includes a negative active material, which may include at least one of graphite, hard carbon, silicon, silica, or silicone. In some embodiments, a conductive agent and a binder may also be included in the negative active material layer. In some embodiments, the conductive agent in the negative active material layer may include at least one of conductive carbon black, ketjen black, flake graphite, graphene, carbon nanotubes, or carbon fibers. In some embodiments, the binder in the negative active material layer may include at least one of carboxymethyl cellulose (CMC), polyacrylic acid, polyvinyl pyrrolidone, polyaniline, polyimide, polyamideimide, polysiloxane, styrene-butadiene rubber, epoxy resin, polyester resin, polyurethane resin, or polyfluorene. In some embodiments, the mass ratio of the anode active material, the conductive agent, and the binder in the anode active material layer may be (80 to 98): (0.1 to 10). It will be appreciated that the above description is merely exemplary and that any other suitable materials and mass ratios may be employed. In some embodiments, the current collector of the negative electrode may employ at least one of a copper foil, a nickel foil, or a carbon-based current collector.

In some embodiments, the separator comprises at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, the polyethylene includes at least one selected from high density polyethylene, low density polyethylene, or ultra high molecular weight polyethylene. Particularly polyethylene and polypropylene, which have a good effect on preventing short circuits and can improve the stability of the battery through a shutdown effect. In some embodiments, the thickness of the isolation film is in the range of about 5 μm to 500 μm.

In some embodiments, the surface of the separator may further include a porous layer disposed on at least one surface of the substrate of the separator, the porous layer including inorganic particles selected from alumina (Al) and a binder₂O₃) Silicon oxide (SiO)₂) Magnesium oxide (MgO), titanium oxide (TiO)₂) Hafnium oxide (HfO)₂) Tin oxide (SnO)₂) Cerium oxide (CeO)₂) Nickel oxide (NiO), zinc oxide (ZnO), calcium oxide (CaO), zirconium oxide (ZrO)₂) Yttrium oxide (Y)₂O₃) At least one of silicon carbide (SiC), boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. In some embodiments, the pores of the separator film have a diameter in the range of about 0.01 μm to 1 μm. The binder of the porous layer is at least one selected from polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethylcellulose, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene and polyhexafluoropropylene. The porous layer on the surface of the isolating membrane can improve the heat resistance, the oxidation resistance and the electrolyte infiltration performance of the isolating membrane and enhance the adhesion between the isolating membrane and the pole piece.

In some embodiments of the present application, the electrode assembly of the electrochemical device is a wound electrode assembly, a stacked electrode assembly, or a folded electrode assembly. In some embodiments, the positive electrode and/or the negative electrode of the electrochemical device may be a multilayer structure formed by winding or stacking, or may be a single-layer structure in which a single-layer positive electrode, a single-layer negative electrode, and a separator are stacked.

In some embodiments, the electrochemical device comprises a lithium ion battery, but the application is not so limited. In some embodiments, the electrochemical device may further include an electrolyte. The electrolyte may be one or more of a gel electrolyte, a solid electrolyte, and an electrolytic solution including a lithium salt and a non-aqueous solvent. 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 or lithium difluoroborate. For example, LiPF is selected as lithium salt₆Because it has high ionic conductivity and can 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 chain carbonate compound are diethyl carbonate (DEC), dimethyl carbonate (DMC), 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), or a combination 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, or a combination thereof.

Examples of carboxylate compounds are methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ -butyrolactone, decalactone, valerolactone, mevalonic lactone, caprolactone, methyl formate, or combinations thereof.

Examples of the ether compound are dibutyl ether, tetraglyme, diglyme, 1, 2-dimethoxyethane, 1, 2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, or a combination thereof.

Examples of 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 or combinations thereof.

In some embodiments of the present application, taking a lithium ion battery as an example, a positive electrode, a separator, and a negative electrode are sequentially wound or stacked to form an electrode member, and then the electrode member is placed in, for example, an aluminum plastic film for packaging, and an electrolyte is injected into the electrode member for formation and packaging, so as to form the lithium ion battery. And then, performing performance test on the prepared lithium ion battery.

Those skilled in the art will appreciate that the above-described methods of making electrochemical devices (e.g., lithium ion batteries) are merely examples. Other methods commonly used in the art may be employed without departing from the disclosure herein.

Embodiments of the present application also provide an electronic device including the electrochemical device described above. The electronic device of the embodiment 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.

In the following, some specific examples and comparative examples are listed to better illustrate the present application, wherein a lithium ion battery is taken as an example. For the sake of simplicity, the following only exemplifies that the positive electrode includes the above-described structure.

Example 1

Preparation of the positive electrode: an aluminum foil is used as a current collector of a positive electrode, slurry of a first coating is uniformly coated on the surface of the aluminum foil, and the slurry comprises 90 wt% of polyvinylidene fluoride (PVDF) and 10 wt% of conductive carbon black, and the first coating with the thickness of about 4 micrometers on one side is obtained after drying (ten-thousandth micrometer test). Subsequently, a second coat of a slurry having a composition of 85% by weight of boehmite, 8% by weight of conductive carbon black and 7% by weight of polyacrylic acid was applied on the first coat, and dried to give a second coat having a thickness of about 1 μm on one side (ten-thousandth scale test). And then coating a positive electrode active material layer on the second coating layer, specifically, dissolving a positive electrode active material lithium cobaltate, a conductive agent conductive carbon black and a binder polyacrylic acid in a weight ratio of 98.2: 0.5: 1.3 in an N-methyl pyrrolidone (NMP) solution to form slurry of the positive electrode active material layer, and coating the slurry on the second coating layer in an amount of 18.37mg/cm²And obtaining the positive active material layer, and drying, cold pressing and cutting to obtain the positive electrode.

Preparation of a negative electrode: graphite, sodium carboxymethyl cellulose (CMC) and styrene butadiene rubber serving as a binder are dissolved in deionized water according to the weight ratio of 97.8: 1.3: 0.9 to form negative electrode slurry. Copper foil with the thickness of 10 mu m is adopted as a current collector of the negative electrode, and the negative electrode slurry is coated on the current collector of the negative electrode, wherein the coating amount is 9.3mg/cm²And drying and cutting to obtain the cathode.

Preparing an isolating membrane: the base material of the isolating film is Polyethylene (PE) with the thickness of 8 mu mCoating ceramic layers of 2 μm alumina on both sides of the material, and finally coating ceramic layers of 2.5mg/cm on both sides²And (3) drying the binder polyvinylidene fluoride (PVDF).

Preparing an electrolyte: under the environment that the water content is less than 10ppm, LiPF₆Adding non-aqueous organic solvent (ethylene carbonate (EC):propylenecarbonate (PC): 50, weight ratio), LiPF₆The concentration of (A) is 1.15mol/L, and the electrolyte is obtained after uniform mixing.

Preparing a lithium ion battery: and sequentially stacking the anode, the isolating membrane and the cathode in sequence to enable the isolating membrane to be positioned between the anode and the cathode to play an isolating role, and winding to obtain the electrode assembly. And (3) placing the electrode assembly in an outer packaging aluminum-plastic film, dehydrating at 80 ℃, injecting the electrolyte, packaging, and performing technological processes such as formation, degassing, edge cutting and the like to obtain the lithium ion battery.

The examples and comparative examples were carried out by changing the parameters in addition to the procedure of example 1, and the specific changed parameters are shown in the following table.

The following describes a method of testing various parameters of the present application.

And (3) testing the normal temperature resistance:

taking 10 anodes with the length of 15cm and the width of 5cm to ensure that the pole pieces are flat and have no wrinkles, testing 12 different position resistances of a single pole piece as pole piece resistances along the vertical direction of the pole piece by using a meta-energy technology BER1200 pole piece resistance instrument in a room temperature environment, and taking the average value of the 10 pole piece resistances.

Testing of PTC response temperature:

the method comprises the steps of using a meta-energy science BER1200 pole piece resistor, placing the resistor in a blast oven, placing a 5 cm-5 cm positive electrode in a resistor test fixture, connecting a pole piece with a multi-path thermodetector, testing the actual temperature of the pole piece, raising the temperature of the blast oven from room temperature to 185 ℃ at the rate of 5 ℃/min, testing the resistance change of the pole piece between room temperature and 180 ℃, outputting one point every 3 seconds to obtain a temperature-resistance curve, obtaining a d (resistance)/d (temperature) -temperature curve by obtaining a derivative of the curve, taking the temperature corresponding to a first point with the derivative being more than or equal to 0.08 ohm/DEG C as PTC response temperature, and taking the average value of 5 pole pieces.

Resistance test at 150 ℃:

the method comprises the steps of using a meta-energy science BER1200 pole piece resistance meter, placing the resistance meter in a blast oven, placing a 5 cm-5 cm positive pole in a resistance meter test fixture, connecting a pole piece with a multi-channel thermodetector and testing the actual temperature of the pole piece, raising the temperature of the blast oven from room temperature to 185 ℃ at the temperature raising rate of 5 ℃/min, testing the resistance change of the pole piece between room temperature and 180 ℃, outputting one point every 3 seconds to obtain a temperature-resistance curve, calculating the resistance average value at 150 ℃ when the temperature is 150 +/-0.3 ℃, and taking the test average value of 5 pole pieces.

Resistance test at 180 ℃:

the method comprises the steps of using a meta-energy science BER1200 pole piece resistance meter, placing the resistance meter in a blast oven, placing a 5 cm-5 cm positive pole in a resistance meter test fixture, connecting a pole piece with a multi-channel thermodetector and testing the actual temperature of the pole piece, raising the temperature of the blast oven from room temperature to 185 ℃ at the temperature raising rate of 5 ℃/min, testing the resistance change of the pole piece between room temperature and 180 ℃, outputting one point every 3 seconds to obtain a temperature-resistance curve, calculating the resistance average value when the temperature is 180 +/-0.3 ℃ to be 180 ℃ lower resistance, and taking the test average value of 5 pole pieces.

Testing a lithium ion battery hot box:

the lithium ion batteries are stored for 1 hour in a thermal shock box at 130 ℃ and 150 ℃ in a state of charge (SOC) of 0.5 CC to be stopped or immediately stopped after thermal runaway of 10 lithium ion batteries in each group, the voltage change of the lithium ion batteries and the surface temperature change of the lithium ion batteries are collected, the experimental phenomenon is recorded, and the test failure is considered to be the smoke generation, the ignition and the explosion of the lithium ion batteries.

And (3) overcharge test of the lithium ion battery:

the method comprises the steps of collecting the voltage change of the lithium ion battery and the surface temperature change of the lithium ion battery by using 3C CC-5V, 5V CV 2h, 3C CC-6V and 6V CV 2h, recording the experimental phenomenon, considering the smoking, ignition and explosion of the lithium ion battery as test failures, and testing 10 lithium ion batteries under each group of test conditions.

Vickers hardness test method:

taking the reinforcing body particles, testing the hardness of 3 positions of each particle by using an HX-1000 microhardness tester, taking the average value as the Vickers hardness, and testing 12 particles and taking the average value of the Vickers hardness.

The particle size test method comprises the following steps:

the enhancement particles were taken and the Dv50 data was taken using a Mastersizer 3000 to measure the particle size distribution and averaged over 3 tests.

The sphericity testing method comprises the following steps:

the enhancement particles were taken and tested for sphericity using the VISION218-D particle image workstation, and the average was taken 3 times.

The coverage test method comprises the following steps:

taking the positive electrode, removing the current collector and the first coating, cutting the positive electrode into a size of 10mm by 10mm, observing the second coating surface by using KEYENCE VHX5000, wherein the magnification is 500 times, calculating the coverage degree by an automatic area measuring mode, and taking 12 sheets to calculate the average value.

Tables 1 and 2 show respective parameters and evaluation results of examples 1 to 6 and comparative examples 1 to 3, respectively. The kind and/or melting point of the ptc materials of examples 2 to 6 are different from those of example 1, and the other parameters are the same as those of example 1. In comparative example 1, the positive active material layer was directly coated on the positive current collector without the first coating layer and the second coating layer; in comparative example 2, the positive electrode active material layer was coated directly over the first coating layer without the second coating layer; in comparative example 3, there were no reinforcement particles in the second coating layer, only 50 wt% conductive carbon black and 50 wt% polyacrylic acid.

TABLE 1

As can be seen from comparing examples 1 to 6 and comparative example 1, the positive electrode has no PTC effect in the absence of the first coating layer and the second coating layer in the positive electrode. As can be seen by comparing examples 1 to 6 and comparative example 2, the positive electrode has no PTC effect in the absence of the second coating layer in the positive electrode. As can be seen by comparing examples 1 to 6 and comparative example 3, the positive electrode has no PTC effect and no significant increase in resistance at high temperatures when there are no reinforcement particles in the second coating layer of the positive electrode.

As can be seen from comparative examples 1 to 6, the positive electrode having the first coating layer can be made to have the PTC effect using the positive temperature coefficient material having the melting point of 115 ℃ to 180 ℃. In addition, as the melting point of the PTC material increases, the PTC response temperature tends to increase because the higher the melting point of the PTC material, the higher the cut-off temperature of the corresponding conductive network, and the higher the PTC response temperature. In addition, the resistances at 150 ℃ and 180 ℃ tend to decrease. Further, when the temperature of the positive electrode exceeds the PTC response temperature, the resistance of the positive electrode exhibits an increase several times. In addition, if the melting point of the positive temperature coefficient material is too low, an irreversible PTC effect may be triggered in the film coating process, which causes too high resistance of the electrode sheet and affects the performance of the lithium ion battery. And the melting point of the positive temperature coefficient material is too high, the corresponding cut-off temperature of the conductive network is higher, which is not beneficial to protecting the lithium ion battery early.

Tables 3 and 4 show the respective parameters and evaluation results of examples 1 and 7 to 39. The mass contents of the positive temperature coefficient materials in the first coating layers of examples 7 to 11 are different from those in example 1. The thickness of the first coating layer of examples 12 to 15 was different from that of example 1. Examples 16 to 19 differ from example 1 in the type of reinforcement particles and in the Vickers hardness of the particles. The mass content of the reinforcement particles in the second coating of examples 20 to 23 is different from that of example 1. The second coating of examples 24 to 27 differs from example 1 in the sphericity of the reinforcement particles. The coverage of the second coating of examples 28 to 31 is different from that of example 1. The Dv50 of the reinforcement particles in the second coating of examples 32 to 35 differs from example 1. The thickness of the second coating layer of examples 36 to 39 is different from that of example 1.

TABLE 4

As can be seen from comparison of examples 1 and 7 to 11, when the mass content of the ptc material in the first coating layer is 60% to 98%, the room-temperature resistance of the positive electrode starts to be stable with the increase of the mass content of the ptc material in the first coating layer, and then tends to increase; the PTC response temperature remains unchanged; the resistance at 150 ℃ and 180 ℃ tends to increase. If the mass content of the positive temperature coefficient material is increased, the content of the first conductive agent needs to be reduced, and the normal temperature resistance of the pole piece is increased subsequently. In addition, the mass content of the positive temperature coefficient material is reduced, the conductive network formed by more first conductive agent is difficult to break at high temperature, and the PTC effect is weakened, namely the increase multiple of the resistance at 180 ℃ relative to the resistance at 150 ℃ is reduced.

As can be seen from comparison of examples 1, 12 to 15, the room-temperature resistance of the positive electrode tends to increase as the thickness of the first coating layer increases; the PTC response temperature remains unchanged; the resistance at 150 ℃ and 180 ℃ tends to increase. The thickness of the first coating is increased, the cutting degree of the conductive network at high temperature is higher, and the PTC effect is more obvious. On the other hand, the thickness of the first coating layer is increased, and the overall thickness of the pole piece is increased, which has an influence on the energy density of the electrochemical device. However, if the thickness of the first coating layer is too small, the PTC effect is weakened.

As can be seen by comparing examples 1, 16 to 19, PTC effect can be obtained with reinforcement particles of suitable hardness. In addition, the resistance of the positive electrode at 150 ℃ and 180 ℃ varies depending on the reinforcement used. The reinforcement body is used for resisting the damage of the main material to the first coating layer in the cold pressing process and keeping the integrity of the first coating layer, so that the reinforcement body needs to have high hardness to bear the force of the active material in the cold pressing process and reduce the damage of the active material to the first coating layer in the cold pressing process, but if the hardness of the reinforcement body is extremely high, the high hardness of the reinforcement body can be embedded into the first coating layer in the cold pressing process to damage the integrity of the first coating layer to influence the PTC effect, and meanwhile, the high hardness can also cause the active material particles contacted with the reinforcement body to be broken to influence the performance.

As can be seen from comparison of examples 1 and 20 to 23, as the mass content of the reinforcement in the second coating layer increases, the room-temperature resistance of the positive electrode tends to increase, because the mass content of the reinforcement in the second coating layer increases, the mass content of the second conductive agent needs to decrease, and the room-temperature resistance increases; the PTC response temperature remains unchanged; the resistance at 150 ℃ and 180 ℃ tends to increase. The mass content of the reinforcing body particles is increased, the protection degree of the first coating is increased, the damage to the first coating in the cold pressing process is weakened, and the integrity of the first coating is enhanced. However, if the mass content of the reinforcement is too small, the PTC effect is reduced.

As can be seen from comparing examples 1, 24 to 27, the reinforcement acts to resist damage to the first coating by the active material during cold pressing, maintaining the integrity of the first coating, while the reinforcement is unable to damage the first coating by itself during cold pressing, so that the sphericity of the reinforcement is increased and the angularity is reduced, the damage to the first coating during cold pressing is reduced, the integrity of the first coating is increased and the PTC effect is increased.

As can be seen from comparing examples 1, 28 to 31, the second coating layer acts to resist damage to the first coating layer by the active material during cold pressing, and to maintain the integrity of the first coating layer, and therefore the second coating layer needs to have a high degree of coverage to withstand the force of the active material during cold pressing, to reduce damage to the first coating layer by the active material during cold pressing, to increase the integrity of the first coating layer, and to enhance the PTC effect. If the coverage is too small, the PTC effect is reduced.

As can be seen from comparison of examples 1 and 32 to 35, when the reinforcing body particles are large, the thickness of the second coating layer is increased, which increases the thickness of the pole piece and affects the energy density, and meanwhile, the large particles of the reinforcing body decrease the density of the second coating layer, so that the effect of resisting the damage of the active material to the first coating layer in the cold pressing process is weakened, and the integrity of the first coating layer is affected; if the reinforcing body particles are difficult to disperse when being too small, the reinforcing bodies in the second coating are unevenly distributed, the effect of resisting the damage of the active materials to the first coating in the cold pressing process is weakened, and the integrity of the first coating is influenced.

As can be seen from comparison of examples 1, 36 to 39, the room temperature resistance of the positive electrode tends to increase as the thickness of the second coating layer increases; the PTC response temperature remains unchanged; the resistance at 150 ℃ and 180 ℃ tends to increase. The thickness of the second coating is increased, the protection degree of the first coating is enhanced, the damage to the first coating in the cold pressing process is weakened, and the integrity of the first coating is enhanced. Of course, as the thickness of the second coating layer increases, the energy density of the electrochemical device may be reduced. If the thickness of the second coating layer is too small, its protective effect is reduced; if the thickness of the second coating is too large, the normal temperature resistance of the second coating is also increased, which is not beneficial to the improvement of the electrical performance of the lithium ion battery.

The above description is only a preferred embodiment of the application and is illustrative of the principles of the technology employed. It will be appreciated by those skilled in the art that the scope of the disclosure herein is not limited to the particular combination of features described above, but also encompasses other combinations of features described above or equivalents thereof. For example, the above features and the technical features having similar functions disclosed in the present application are mutually replaced to form the technical solution.

Claims

1. An electrochemical device comprising an electrode including a current collector, a first coating layer, a second coating layer, and an active material layer,

wherein the first coating is positioned between the current collector and the second coating, the second coating is positioned between the first coating and the active material layer, the first coating comprises a positive temperature coefficient material and a first conductive agent, and the second coating comprises a second conductive agent, a binder, and a reinforcement.

2. The electrochemical device of claim 1, wherein the reinforcement comprises at least one of lithium iron phosphate, silica, titania, alumina, boehmite, magnesia, zirconia, titania, silicon carbide, boron carbide, barium carbonate, potassium titanate, barium sulfate, vanadium trioxide, polyetheretherketone, polyamide, or cellulose powder.

3. The electrochemical device of claim 1, wherein the reinforcement member satisfies at least one of the following conditions:

the mass content of the reinforcement in the second coating is 40% to 98%;

the vickers hardness of the reinforcement is 600 to 2000;

the particle sphericity of the reinforcement is in the range of 0.5 to 1;

the reinforcement has a Dv50 of 0.05 μm to 2 μm.

4. The electrochemical device of claim 1, wherein the reinforcement member satisfies at least one of the following conditions:

the mass content of the reinforcement in the second coating layer is 60% to 80%;

the Vickers hardness of the reinforcement is 800 to 1500;

the particle sphericity of the reinforcement is in the range of 0.7 to 1;

the reinforcement has a Dv50 of 0.2 μm to 1 μm.

5. The electrochemical device of claim 1, wherein the second coating has a thickness of 0.2 μm to 5 μm and a second coating coverage of 60% or more.

6. The electrochemical device according to claim 1, wherein a mass ratio of the second conductive agent, the binder, and the reinforcement in the second coating layer is (1 to 20): (1 to 20): (60 to 98).

7. The electrochemical device of claim 1, wherein the binder comprises at least one of polyvinyl alcohol, polyacrylic acid, polyethylene glycol, polyethylene oxide, carboxymethyl cellulose salt, polyacrylamide, polymaleic anhydride, polyquaternary ammonium salt, starch, chitosan, pectin, polyacrylate, polyurethane, polyvinyl chloride, natural rubber emulsion, neoprene emulsion, butyronitrile emulsion, butylbenzene emulsion, or styrene-acrylic emulsion.

8. The electrochemical device of claim 1, wherein the positive temperature coefficient material satisfies at least one of the following conditions:

the melting point of the positive temperature coefficient material is 115-180 ℃;

the positive temperature coefficient material comprises at least one of polyethylene, polypropylene, polyvinyl chloride, polystyrene, polytetrafluoroethylene, polybutylene terephthalate, polyimide, polyvinyl alcohol, polymethyl methacrylate, polyvinylidene fluoride, polyacrylonitrile, polyformaldehyde, ethylene-vinyl acetate copolymer or polyethylene glycol terephthalate;

the mass content of the positive temperature coefficient material in the first coating layer is 60-98%.

9. The electrochemical device of claim 1, wherein the first coating layer has a thickness of 0.5 μ ι η to 12 μ ι η.

10. The electrochemical device according to claim 1, wherein the first conductive agent and the second conductive agent each independently comprise at least one of conductive carbon black, acetylene black, graphite, graphene, carbon nanotubes, carbon fibers, aluminum powder, nickel powder, or gold powder.

11. The electrochemical device according to claim 1, wherein the first conductive agent in the first coating layer is contained in an amount of 2 to 40% by mass.

12. An electronic device comprising the electrochemical device according to any one of claims 1 to 11.