CN117015866A

CN117015866A - Electrochemical device and electronic device using safety coating

Info

Publication number: CN117015866A
Application number: CN202280010595.1A
Authority: CN
Inventors: 刘晓欠; 张青文; 韩冬冬; 陈梅锋
Original assignee: Ningde Amperex Technology Ltd
Current assignee: Ningde Amperex Technology Ltd
Priority date: 2022-03-30
Filing date: 2022-03-30
Publication date: 2023-11-07
Also published as: WO2023184227A1

Abstract

The present application relates to an electrochemical device, comprising: and the positive electrode comprises a positive electrode current collector, a protective layer and a positive electrode active material layer. The protective layer is arranged between the positive electrode current collector and the positive electrode active material layer, wherein Xg of the protective layer is dispersed in 50Xg of water at the temperature of 25-35 ℃, and is stirred at the stirring speed of 1200r/min by a stirrer with the diameter of a dispersion disc of 50-80 mm, and after being filtered and dried by a filter screen of a 100-mesh sieve, the weight of the protective layer on the filter screen is Wg, wherein W/X is less than or equal to 10%.

Description

Electrochemical device and electronic device using safety coating

Technical Field

The application relates to the field of energy storage, in particular to an electrochemical device and an electronic device using a safety coating.

Background

With the popularity of electronic products such as notebook computers, mobile phones, palm game players, tablet computers, etc., there is an increasing demand for safety and stability of electrochemical devices (e.g., lithium ion batteries). However, at present, safety accidents such as ignition and explosion caused by external impact or puncture still exist in the use process of the lithium ion battery, and the battery impedance is seriously increased at high temperature, so that the wide application of the lithium ion battery is limited. Therefore, a technical means capable of improving the safety performance and high temperature stability of the lithium ion battery is needed.

Disclosure of Invention

According to one aspect of the present application, the present application relates to an electrochemical device, comprising: and the positive electrode comprises a positive electrode current collector, a protective layer and a positive electrode active material layer. The protective layer is arranged between the positive electrode current collector and the positive electrode active material layer, wherein X g is taken and dispersed in 50X g water at the temperature of 25-35 ℃, and stirred at the stirring speed of 1200r/min by a stirrer with the diameter of a dispersion disc of 50-80 mm, and after being filtered and dried by a filter screen of a 100-mesh sieve, the weight of the protective layer on the filter screen is W g, wherein W/X is less than or equal to 10%. On the one hand, by arranging the protective layer between the positive electrode current collector and the positive electrode active material layer, short circuit between the positive electrode current collector and the negative electrode active material layer which is most dangerous in the process of being impacted by external force or punctured can be restrained, and the safety performance of the electrochemical device is improved; on the other hand, the whole protective layer which satisfies the condition that W/X is less than or equal to 10 percent tends to be hydrophilic rather than lipophilic, so that the protective layer can avoid the large-area loss of cohesive force due to swelling in oily electrolyte, thereby maintaining the cohesive effect and inhibiting the increase of the internal resistance of high-temperature storage.

In some embodiments, the protective layer has an infrared spectrum at 1400cm ^-1 Up to 1700cm ^-1 And/or having a characteristic peak in the range of 2100cm-1 to 2300 cm-1. In this case, the protective layer has a polar functional group such as carbonyl and/or cyano, which can enhance the adhesion to the positive electrode current collector and improve the high-temperature Chu Nazu growth rate of the electrochemical device.

In some embodiments, the resistance of the positive electrode is RΩ, 1.5+.R+. 5 when the electrochemical device is in a fully charged state. On the one hand, R is controlled to be more than or equal to 1.5 omega, so that the central through pin passing rate of the electrochemical device can be remarkably improved; on the other hand, controlling R to be 5 Ω or less can further improve the increase in internal resistance of the electrochemical device during high-temperature storage.

In some embodiments, the protective layer has a thickness T μm, T.gtoreq.0.5. The thickness T of the protective layer is more than or equal to 0.5 mu m, so that the short circuit between the positive electrode current collector and the negative electrode active material layer in the process of being impacted by external force or punctured can be effectively inhibited, and the safety of the electrochemical device is improved.

In some embodiments, the protective layer comprises a first active material, a first binder, and a first conductive agent.

In some embodiments, the protective layer further comprises a leveling agent.

In some embodiments, the first conductive agent is 0.5 to 15% by mass, the first active material is 60 to 98.5% by mass, and the first binder is 1 to 20% by mass, based on the mass of the protective layer.

In some embodiments, the leveling agent comprises at least one of a siloxane-based compound, a siloxane-based derivative, an oxy-olefin polymer, an acrylate-based polymer, an alcohol-based compound, an ether-based compound, or a fluorocarbon compound. In some embodiments, the leveling agent is 0.01% to 5% by mass based on the mass of the protective layer.

In some embodiments, the first binder satisfies at least one of the following characteristics: (a) A polymer comprising at least one of acrylic acid, acrylamide, an acrylate, acrylonitrile, or an acrylate; (b) Comprises at least one of carboxymethyl cellulose salt or nitrile rubber.

In some embodiments, the first binder is an aqueous binder.

In some embodiments, the first binder has a weight average molecular weight of 20 to 200 tens of thousands.

In some embodiments, the first active material comprises at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium manganate, or lithium nickel cobalt manganate. In some embodiments, the first conductive agent comprises at least one of graphene, graphite fibers, carbon nanotubes, ketjen black, or conductive carbon.

In some embodiments, the positive electrode active material layer includes a second active material, a second binder, and a second conductive agent. In some embodiments, the second active material is 91.5 to 99% by mass, the second binder is 0.5 to 5% by mass, and the second conductive agent is 0.5 to 3.5% by mass, based on the mass of the positive electrode active material layer.

In some embodiments, the second active material comprises at least one of lithium cobalt oxide, lithium manganese oxide, or lithium nickel cobalt manganese oxide. In some embodiments, the second binder comprises at least one of polyacrylic acid, polyvinylidene fluoride, polytetrafluoroethylene-hexafluoropropylene, sodium polyacrylate, nitrile rubber, or polyacrylate. In some embodiments, the second conductive agent comprises at least one of graphene, graphite fibers, carbon nanotubes, ketjen black, or conductive carbon.

According to another aspect of the application, the application relates to an electronic device comprising an electrochemical device according to any of the foregoing embodiments.

Detailed Description

Hereinafter, the present application will be described in detail. It should be understood that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present application on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation. Thus, the description shown in the embodiments described in the specification is merely a specific example for the purpose of illustration and is not intended to show all technical aspects of the application, and it is to be understood that various alternative equivalents and variants may be made thereto at the time of filing the present application.

In the detailed description and claims, a list of items connected by the terms "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items a and B are listed, the phrase "at least one of a and B" means only a; only B; or A and B. In another example, if items A, B and C are listed, then the phrase "at least one of A, B and C" means only a; or only B; only C; a and B (excluding C); a and C (excluding B); b and C (excluding A); or A, B and C. Item a may comprise a single component or multiple components. Item B may comprise a single component or multiple components. Item C may comprise a single component or multiple components.

Additionally, amounts, ratios, and other numerical values are sometimes presented herein in a range format. It is to be understood that such range format is used for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited.

1. Electrochemical device

The present application relates to an electrochemical device, comprising: and the positive electrode comprises a positive electrode current collector, a protective layer and a positive electrode active material layer. The protective layer is disposed between the positive electrode current collector and the positive electrode active material layer. The protective layer satisfies: W/X is less than or equal to 10%, wherein X represents that the X g protective layer is dispersed in 50X g water at 25-35 ℃, W represents that after the X g protective layer is dispersed in the water under the conditions, the protective layer is stirred by a stirrer with a dispersion disc diameter of 50-80 mm at a stirring speed of 1200r/min, and the protective layer remained on the filter screen (unit is g) after the protective layer is filtered and dried by a filter screen of a 100-mesh sieve.

The protective layer can inhibit short circuit between the most dangerous positive electrode current collector and the negative electrode active material layer in the process of being impacted or punctured by external force, so that the safety performance of the electrochemical device is improved; meanwhile, the high-temperature storage internal resistance increase rate of the lithium ion battery can be controlled to be below 40% by satisfying the protective layer with W/X being 10% or less, wherein in some embodiments, the high-temperature storage refers to storage of the electrochemical device under a condition of being left at 85 ℃ for 6 hours. This is because the protective layer tends to be hydrophilic rather than lipophilic as a whole, so that the protective layer can be prevented from losing adhesion in a large area due to swelling in an oily electrolyte, thereby maintaining adhesion and suppressing an increase in the internal resistance of high-temperature storage. In some embodiments, W/X is less than or equal to 9%. In some embodiments, W/X is less than or equal to 8%. In some embodiments, W/X is less than or equal to 7%. In some embodiments, W/X is less than or equal to 6%. In some embodiments, W/X is less than or equal to 5%. In some embodiments, the value of W/X is 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc.

In some embodiments, X has a value of 20 to 80g.

In some embodiments, the protective layer has an infrared spectrum at 1400cm ^-1 Up to 1700cm ^-1 -and/or 2100cm ^-1 Up to 2300cm ^-1 Having characteristic peaks in the range. At this time, the protective layer has polar functional groups such as carbonyl and/or cyano, so that the adhesion of the protective layer to the positive electrode current collector can be enhanced, and the high-temperature storage internal resistance growth rate of the electrochemical device can be improved.

In some embodiments, the first active material is 60% to 98.5% by mass based on the mass of the protective layer. In some embodiments, the mass percent of the first active material is 65% to 96% based on the mass of the protective layer. In some embodiments, the first active material is 70% to 95% by mass based on the mass of the protective layer. In some embodiments, the mass percent of the first active material is 75% to 95% based on the mass of the protective layer. In some embodiments, the mass percent of the first active material is 60%, 65%, 70%, 75%, 77%, 78%, 80%, 82%, 84%, 85%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 98.5% or a range between any two of the foregoing, based on the mass of the protective layer.

In some embodiments, the first active material comprises at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium manganate, or lithium nickel cobalt manganate.

In some embodiments, the first binder is 1% to 20% by mass based on the mass of the protective layer. In some embodiments, the first binder is 1%, 2%, 3%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20% by mass or a range between any two of the foregoing. By controlling the mass percentage of the first binder in the protective layer within the above range, a better binding force between the positive electrode active material layer, the protective layer and the current collector can be ensured, and loosening or even falling off of the protective layer and the positive electrode active material layer under abnormal conditions such as falling, needling, impact and the like can be reduced, so that the safety performance of the penetrating nail and the increase rate of high-temperature storage internal resistance of the electrochemical device can be improved.

In some embodiments, the first binder is an aqueous binder. In some embodiments, the first binder satisfies at least one of the following characteristics: (a) A polymer comprising at least one of acrylic acid, acrylamide, an acrylate, acrylonitrile, or an acrylate; (b) Comprises at least one of carboxymethyl cellulose salt or nitrile rubber. In some embodiments, the first binder is a polymer containing an acrylate salt, and at this time, the first binder has better hydrophilicity and oleophobicity, so that the protective layer has excellent adhesion and stability, and thus the penetrating nail safety performance and the high-temperature storage internal resistance increase rate of the electrochemical device can be improved.

In some embodiments, the first binder has a weight average molecular weight of 20 to 200 tens of thousands. In some embodiments, the first binder has a weight average molecular weight of 20 to 180 ten thousand. In some embodiments, the first binder has a weight average molecular weight of 20 to 160 tens of thousands. In some embodiments, the first binder has a weight average molecular weight of 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 90, 95, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200 or a range between any two of the foregoing. The weight average molecular weight of the first binder is in the above range, which can give consideration to both film forming property and adhesive force and improve uniformity and stability of the protective layer.

In some embodiments, the first conductive agent is 0.5% to 15% by mass based on the mass of the protective layer. In some embodiments, the first conductive agent is 0.5% to 10% by mass based on the mass of the protective layer. In some embodiments, the first conductive agent is 0.5% to 8% by mass based on the mass of the protective layer. In some embodiments, the first conductive agent is 1% to 7% by mass based on the mass of the protective layer. In some embodiments, the first conductive agent is 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.2%, 2.3%, 2.4%, 2.5%, 2.8%, 3%, 3.2%, 3.4%, 3.6%, 3.8%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15% or a range between any two of the foregoing based on the mass of the protective layer.

In some embodiments, the first conductive agent comprises at least one of graphene, graphite fibers, carbon nanotubes, ketjen black, or conductive carbon.

In some embodiments, the protective layer further comprises a leveling agent. In some embodiments, the leveling agent includes at least one of a siloxane-based compound, a siloxane-based derivative, an oxygen-containing olefin polymer, an acrylate-based polymer, an alcohol-based compound, an ether-based compound, or a fluorocarbon-based compound. In some embodiments, the mass percent of the leveling agent is 0.01% to 5% based on the mass of the protective layer. In some embodiments, the mass percent of the leveling agent is 0.03% to 4% based on the mass of the protective layer. In some embodiments, the mass percent of the leveling agent is 0.05% to 4% based on the mass of the protective layer. In some embodiments, the mass percent of the leveling agent is 0.08% to 4% based on the mass of the protective layer. In some embodiments, the mass percent of the leveling agent is 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5% or a range between any two of the foregoing values, based on the mass of the protective layer. The addition of the leveling agent is beneficial to forming a uniform and smooth protective layer, increasing the contact area of the protective layer and the current collector as well as the positive electrode active material layer, improving the conductivity and improving the internal resistance increase during high-temperature storage.

When the electrochemical device is in a full charge state, the resistance rΩ of the positive electrode. In some embodiments, R is 1 to 10. In some embodiments, R is 1.5 to 9. In some embodiments, R is 1.5 to 5. In some embodiments, R is 1, 1.5, 2, 2.1, 2.3, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, or a range between any two of the foregoing values. . When R is within this range, the electrochemical device can have a low rate of increase in high-temperature storage internal resistance while maintaining a high rate of penetration.

In some embodiments, the protective layer has a thickness T μm, T.gtoreq.0.5. In some embodiments, T is 0.5 to 10. In some embodiments, T is 1 to 9. In some embodiments, T is 1.5 to 8.5. In some embodiments, T is 2 to 8. In some embodiments, T is 2.5 to 7.5. In some embodiments, T is 1.5 to 5. In some embodiments, T is 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10 or a range between any two of the foregoing values. The thickness of the protective layer is in the above range, so that short circuit between the positive electrode current collector and the negative electrode active material layer in the process of being impacted by external force or punctured can be effectively inhibited, and the safety of the electrochemical device is improved.

According to the application, the protective layer is arranged between the positive electrode current collector and the positive electrode active material layer, so that the occurrence of the short circuit between the positive electrode current collector and the negative electrode active material layer which is most dangerous in the process of being impacted or punctured by external force can be inhibited, and the safety performance of the electrochemical device is improved; meanwhile, the present application recognizes that when W/X is 10% or less by controlling the material of the protective layer (wherein X means X g protective layer is dispersed in 50X g water at 25 to 35 ℃, W means X g protective layer is dispersed in water under the above conditions, and then stirred with a stirrer having a dispersion disk diameter of 50 to 80mm at a stirring speed of 1200r/min, and the mass (in g) of the protective layer remaining on the screen after filtration and drying with a 100-mesh screen), the protective layer tends to be hydrophilic rather than lipophilic, the protective layer can be prevented from losing a large area of adhesive force due to swelling in an oily electrolyte, thereby maintaining the adhesive effect and suppressing the increase of the internal resistance in high-temperature storage. For example, it can control the increase rate of the internal resistance of the high-temperature storage to 40% or less.

The electrochemical device of the present application further includes a separator, an electrolyte, and a negative electrode.

In some embodiments, the electrochemical device of the present application comprises a primary or secondary battery. In some embodiments, the electrochemical device is a lithium secondary battery. In some embodiments, lithium secondary batteries include, but are not limited to: lithium metal secondary batteries, lithium ion secondary batteries, sodium ion batteries, lithium polymer secondary batteries, or lithium ion polymer secondary batteries.

2. Method for preparing the electrochemical device

The method of manufacturing the electrochemical device of the present application is described in detail below by taking a lithium ion battery as an example.

Preparation of the negative electrode: dispersing a negative electrode active substance (at least one of carbon material, silicon material or lithium titanate) and a negative electrode binder, and optionally a conductive material in a solvent system according to a certain mass ratio, fully stirring and uniformly mixing, coating the mixture on a negative electrode current collector, and drying and cold pressing the mixture to obtain the negative electrode.

Preparation of positive electrode: (1) Adding the first active material, the first conductive agent, the first binder, and optionally the leveling agent into a solvent, and uniformly mixing to obtain a slurry of the protective layer (hereinafter referred to as "first slurry"); (2) Coating the first slurry in the step (1) on a target area of the positive electrode current collector; (3) Drying the positive electrode current collector containing the first slurry obtained in the step (2) to remove the solvent, thereby obtaining a positive electrode current collector coated with a protective layer; (4) Dispersing the second active material, the second conductive agent and the second binder in a solvent system according to a certain mass ratio, and fully stirring and uniformly mixing to obtain slurry (hereinafter referred to as "second slurry") of the positive electrode active material; (5) Coating the second slurry on the target area of the positive electrode current collector coated with the protective layer, which is obtained in the step (3); (6) And (3) drying the positive electrode current collector containing the second slurry in the step (5) to remove the solvent, thereby obtaining the required positive electrode.

The types of the first active material, the first conductive agent, the first binder, the leveling agent, the second active material, the second conductive agent, and the second binder are as described above.

In some embodiments, examples of the solvent include, but are not limited to, N-methylpyrrolidone, acetone, or water. In some embodiments, the amount of solvent may be appropriately adjusted.

In some embodiments, the current collector has a thickness in the range of 3 micrometers to 20 micrometers, although the disclosure is not limited thereto. The current collector is not particularly limited as long as the current collector is conductive without causing adverse chemical changes in the fabricated battery. Examples of the current collector include copper, stainless steel, aluminum, nickel, titanium, or an alloy (e.g., co-nickel alloy), but the disclosure is not limited thereto. In some embodiments, fine irregularities (e.g., surface roughness) may be included on the surface of the current collector to enhance adhesion of the surface of the current collector to the active material. In some embodiments, the current collector may be used in a variety of forms, examples of which include a film, sheet, foil, mesh, porous structure, foam, or jeopardy, but the present disclosure is not limited thereto.

Isolation film: in some embodiments, a porous polymeric film of polyethylene (abbreviated PE) is used as the separator. In some embodiments, the material of the isolation film may include fiberglass, polyester, polyethylene, polypropylene, polytetrafluoroethylene, or a combination thereof. In some embodiments, the pores in the separator have diameters in the range of 0.01 microns to 1 micron, and the thickness of the separator is in the range of 5 microns to 500 microns.

Electrolyte solution: in some embodiments, the electrolyte includes an organic solvent, a lithium salt, and an additive.

In some embodiments, the organic solvent includes at least one of ethylene carbonate (abbreviated EC), propylene carbonate (abbreviated PC), diethyl carbonate (abbreviated DEC), ethylmethyl carbonate (abbreviated EMC), dimethyl carbonate (abbreviated DMC), propylene carbonate, ethyl acetate, ethyl propionate, or propyl propionate.

In some embodiments, the lithium salt includes at least one of an organic lithium salt or an inorganic lithium salt.

In some embodiments, the lithium salt comprises lithium hexafluorophosphate (LiPF ₆ ) Lithium tetrafluoroborate (LiBF) ₄ ) Lithium difluorophosphate (LiPO) ₂ F ₂ ) Lithium bis (trifluoromethanesulfonyl) imide LiN (CF) ₃ SO ₂ ) ₂ (LiTFSI), lithium bis (fluorosulfonyl) imide Li (N (SO) ₂ F) ₂ ) (LiLSI), lithium bisoxalato borate LiB (C) ₂ O ₄ ) ₂ (LiBOB) or lithium difluorooxalato borate LiBF ₂ (C ₂ O ₄ ) At least one of (LiDFOB).

In some embodiments, the lithium salt is present in an amount of 8% to 30% based on the mass of the electrolyte. In some embodiments, the lithium salt content is 8%, 9%, 10%, 11%, 12%, 15%, 18%, 20%, 23%, 25%, 28%, 30%, or a range between any two of the foregoing.

In some embodiments, the additive comprises at least one of fluoroethylene carbonate (FEC), vinylene Carbonate (VC), vinyl Ethylene Carbonate (VEC), 1, 3-Propane Sultone (PS), ethylene sulfate (DTD), succinonitrile (SN), adiponitrile (ADN), 1,3, 6-Hexanetrinitrile (HTCN), succinic Anhydride (SA).

And stacking the positive electrode, the isolating film and the negative electrode in sequence, enabling the isolating film to be positioned in the middle of the positive electrode and the negative electrode to play a role of isolation, and winding to obtain the bare cell. And placing the wound bare cell in an outer package, injecting electrolyte, packaging, and performing technological processes such as formation, degassing, trimming and the like to obtain the lithium ion battery.

3. Electronic device

The present application provides an electronic device comprising an electrochemical device according to the foregoing.

According to some embodiments of the application, the electronic device includes, but is not limited to: notebook computers, pen-input computers, mobile computers, electronic book players, portable telephones, portable fax machines, portable copiers, portable printers, headsets, video recorders, liquid crystal televisions, hand-held cleaners, portable CD-players, mini-compact discs, transceivers, electronic notebooks, calculators, memory cards, portable audio recorders, radios, stand-by power supplies, motors, automobiles, motorcycles, mopeds, bicycles, lighting fixtures, toys, game machines, watches, electric tools, flash lamps, cameras, household large-sized batteries or lithium/sodium ion capacitors, and the like.

4. Detailed description of the preferred embodiments

The present application will be described in further detail with reference to examples. However, it should be understood that the following embodiments are merely examples, and the embodiment modes of the present application are not limited thereto.

Examples 1 to 33 and comparative examples 1 to 2

Manufacturing of positive electrode

Step (1): adding the first active material, the first conductive agent, the first binder, and optionally the leveling agent into water, and uniformly mixing to obtain a slurry of the protective layer (hereinafter referred to as "first slurry");

step (2), coating the first slurry in the step (1) on a target area of the positive electrode current collector;

step (3), drying the positive electrode current collector containing the first slurry obtained in the step (2) to remove the solvent, thereby obtaining a positive electrode current collector coated with a protective layer;

dispersing a second active substance (lithium cobaltate, 97.3 mass percent), a second conductive agent (conductive carbon (trade name Super P) with the mass percent of 0.6 percent and carbon nano tube (abbreviated as CNT) with the mass percent of 0.5 percent) and a second binder (polyvinylidene fluoride (abbreviated as PVDF) with the mass percent of 1.6 percent) in an N-methylpyrrolidone solvent system, and fully stirring and uniformly mixing to obtain a slurry (hereinafter referred to as a second slurry) of the positive electrode active substance;

step (5) coating the second slurry on the target area of the positive electrode current collector coated with the protective layer, which is obtained in the step (3);

and (6) drying the positive electrode current collector containing the second slurry in the step (5) to remove the solvent, thereby obtaining the required positive electrode.

Table 1 below specifically shows the difference in protective layer in the positive electrodes in examples 1 to 33 and comparative examples 1 to 2.

TABLE 1

The positive electrode active material layers, positive electrode current collectors, and the like of the positive electrodes in examples 1 to 33 and comparative examples 1 to 2 were prepared by the foregoing processes, except for the above-described differences.

Fabrication of electrochemical device

The positive electrode of the electrochemical device is fabricated as described above.

And (3) a negative electrode: the preparation method comprises the steps of fully and uniformly stirring active substances of artificial graphite, a conductive agent of acetylene black, a binder of styrene-butadiene rubber (SBR for short) and a thickener of sodium carboxymethylcellulose (CMC for short) in water according to the mass ratio of 95:2:2:1, coating the mixture on a Cu foil, drying and cold pressing to obtain the negative electrode.

Electrolyte solution: at the water content<In a 10ppm argon glove box, ethylene Carbonate (EC), diethyl carbonate (DEC) and Propylene Carbonate (PC) are uniformly mixed according to the mass ratio of 2:6:2, and then fully dried lithium salt LiPF ₆ After dissolving in the above solvent, 1, 3-propane sultone, fluoroethylene carbonate and adiponitrile are added. After mixing, liPF ₆ The content of (2) C, C and PC is 12.5%, 1, 3-propane sultone is 1.5%, fluoroethylene carbonate is 3%, adiponitrile is 2%, and the rest is EC, DEC and PC, wherein the content of each substance is based on the total weight of the electrolyte.

Isolation film: a porous polymeric film of Polyethylene (PE) was used as a separator.

And stacking the anode, the isolating film and the cathode in sequence, enabling the isolating film to be positioned in the middle of the anode and the cathode to play a role of isolation, winding, placing in an outer package, injecting the prepared electrolyte, packaging, and carrying out processes such as formation, degassing, trimming and the like to obtain the lithium ion battery.

Performance test method

Internal resistance of lithium ion battery

And using a resistor to test the alternating current internal resistance of the lithium ion battery by adopting sine and 1000Hz frequency waves.

Internal resistance increasing rate of lithium ion battery

Storage condition 1 (85 ℃ C. For 6 h):

in an environment of 25+/-3 ℃, the lithium ion battery is charged to 4.45V at a constant current of 0.5C, then is charged to 0.025C at a constant voltage of 4.45V, and the initial internal resistance of the lithium ion battery is tested to be IMP0. And (3) putting the lithium ion battery into a high-temperature furnace at 85+/-3 ℃ for 6 hours, taking out, and after the temperature of the lithium ion battery is reduced to 25+/-3 ℃, testing the internal resistance of the lithium ion battery as IMP6 hours, wherein the IMP growth rate of the lithium ion battery at 85 ℃ is (IMP 6h-IMP 0)/IMP 0 multiplied by 100 percent after the lithium ion battery is placed for 6 hours.

Storage condition 2 (30 days at 60 ℃):

in an environment of 25+/-3 ℃, the lithium ion battery is charged to 4.45V at a constant current of 0.5C, then is charged to 0.025C at a constant voltage of 4.45V, and the initial internal resistance of the lithium ion battery is tested to be IMP1. And (3) placing the lithium ion battery in a high-temperature furnace at 60+/-3 ℃ for 30d, taking the lithium ion battery out of the high-temperature furnace, and after the temperature of the lithium ion battery is reduced to 25+/-3 ℃, testing the internal resistance of the lithium ion battery to be IMP30d, and keeping the lithium ion battery at 60 ℃ for 30 days until the IMP growth rate is (IMP 30d-IMP 1)/IMP 1 multiplied by 100 percent.

Rate of passage of central through-nails

And (3) charging the lithium ion battery to be tested to 4.45V at a constant current of 0.05C, then charging the lithium ion battery to a constant 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 recording the appearance of the lithium ion battery before the test. And (3) carrying out a penetrating nail test on the battery in an environment of 25+/-3 ℃, wherein the diameter of a steel nail is 4mm, the penetrating speed is 30mm/s, the penetrating nail is positioned at the geometric center of the lithium ion battery, the test is stopped after the surface temperature of the battery is reduced to 50 ℃ for 3.5min (minutes), 10 lithium ion batteries are used as a group, the state of the lithium ion batteries in the test process is observed, and the passing number of the lithium ion batteries is confirmed by taking the non-combustion and non-explosion of the lithium ion batteries as passing standards.

Positive electrode resistance in full charge state

1) Constant current charging to 4.45V at a multiplying power of 0.05C, and then constant voltage charging to 0.025C (cut-off current) at a constant voltage of 4.45V to enable the lithium ion battery to reach a full charge state;

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

3) Placing the positive electrode obtained in the step 2) in an environment with the humidity of 5-15% for 30min, and then sealing and transferring to a resistance test site;

4) Testing the resistance of the positive electrode obtained in 3) by using a BER1200 type diaphragm resistance tester, wherein the intervals between adjacent test points are 2mm to 3mm, at least 15 different points are tested, the average value of the resistance of all the test points is recorded as the positive electrode resistance R in a full charge state, and the test parameters are as follows: area of ram 153.94mm ² Pressure 3.5t, hold time 50s.

Thickness of protective layer

1) The positive electrode coated with the protective layer was removed from the lithium ion battery in an environment of (25.+ -.3). Degree.C. Wiping the electrolyte remained on the surface of the positive electrode by using dust-free paper;

2) Cutting the positive electrode coated with the protective layer under plasma to obtain the cross section of the positive electrode;

3) The cross section of the positive electrode obtained in 2) is observed under a scanning electron microscope SEM, the thickness T μm of the protective layer is tested, adjacent test points are separated by 2mm to 3mm, at least 15 different points are tested, and the average value of all the test points is recorded as the thickness T μm of the protective layer.

Table 2 below shows the properties of examples 1 to 33 and comparative examples 1 to 2.

TABLE 2

1. The presence or absence of the protective layer, the thickness (T, μm) of the protective layer, the full charge positive electrode resistance (R, Ω) and the W/X (W: residual weight after dissolution and filtration of the protective layer; X: weight before dissolution (unit g)) of the protective layer were examined for their influence on the performance of the electrochemical device

As is clear from the foregoing tables 1 and 2, examples 1 to 33 in which the protective layer satisfies W/x.ltoreq.10% have significantly improved high-temperature storage internal resistance increase rate (40% or less in the condition of 6 hours at 85 ℃ or 3 days at 60 ℃) in lithium ion batteries as compared with comparative example 1 in which no protective layer and comparative example 2 in which W/X is much larger than 10%, probably because the protective layer satisfying W/x.ltoreq.10% tends to be hydrophilic rather than lipophilic as a whole, and thus can suppress the large-area loss of cohesive force of the protective layer due to swelling in an oily electrolyte, thereby maintaining the cohesive effect and suppressing the increase of the high-temperature storage internal resistance. Meanwhile, as can be seen from a comparison of example 15 and example 16, when the W/X of the protective layer becomes smaller (i.e., the water solubility of the binder is better), the effect of improving the increase rate of the internal resistance in high-temperature storage is more remarkable while improving the passage rate of the center pin of the electrochemical device.

In addition, it can be found from examples 1 to 33 that the center feed-through passing rate (throughput/total tested) of examples 1 to 32 satisfying R.gtoreq.1.5 is significantly better than that of example 33. Therefore, the added protective layer controls the full charge positive electrode resistance R to be more than or equal to 1.5, and the passing rate of the central through nail of the lithium ion battery can be obviously improved. Meanwhile, the high-temperature storage internal resistance increase rate of examples 1 to 31 satisfying R.ltoreq.5 is also significantly superior to that of example 32. Therefore, by controlling R to be more than or equal to 1.5 and less than or equal to 5, the lithium ion battery with higher through-pin passing rate and lower high-temperature storage internal resistance increasing rate can be obtained.

In addition, the thickness T of the protective layer is more than or equal to 0.5 mu m, so that the passing rate of the central penetrating nail of the lithium ion battery can be effectively improved, and the improvement effect of the high-temperature storage internal resistance growth rate is more obvious.

2. The influence of the composition of the protective layer on the performance of the electrochemical device was examined

2.1 conductive Agents

The conductive agent used for the protective layer in embodiments 1 to 33 of the present application includes at least one of carbon nanotube (abbreviated as CNT), conductive carbon (abbreviated as SP), ketjen black, graphene (abbreviated as GN), and graphite fiber. However, it should be understood that the conductive agent used for the protective layer of the present application is not limited to the kind exemplified in the specific examples, and may contain the like thereof.

2.2 adhesive

The binder used for the protective layer in embodiments 1 to 33 of the present application may include at least one of acrylonitrile, acrylate, acrylamide polymer, polyacrylic acid, sodium carboxymethyl cellulose, sodium polyacrylate, polyacrylonitrile, or nitrile rubber. However, it should be understood that the binder used for the protective layer of the present application is not limited to the kind exemplified in the specific examples, and may include a polymer formed of at least one of acrylic acid, acrylamide, acrylate, acrylonitrile, or acrylate, or the like.

For example, examples 17 to 20 show that when the binder comprises at least one selected from the group consisting of sodium polyacrylate, polyacrylonitrile, and nitrile rubber, it is possible to improve the rate of increase in the internal resistance in high-temperature storage while improving the rate of passage of the center pin of the electrochemical device.

2.3 leveling agent

The leveling agent used for the protective layer in embodiments 1 to 33 of the present application may include at least one of an oxyalkylene polymer, ethanol, a siloxane-based compound, an acrylate polymer, diethyl ether, or ethanol. However, it should be understood that the leveling agent used for the protective layer of the present application is not limited to the kind exemplified in the specific examples, and may contain the analogues thereof.

In summary, the electrochemical device of the present application can have improved high-temperature storage internal resistance growth rate and higher central piercing pin passing rate.

Reference throughout this specification to "some embodiments," "one embodiment," "another example," "an example," "a particular example," or "a partial example" means that at least one embodiment or example in the present application includes the particular feature, structure, material, or characteristic described in the embodiment or example. Thus, descriptions appearing throughout the specification, for example: "in some embodiments," "in an embodiment," "in one embodiment," "in another example," "in one example," "in a particular example," or "example," which do not necessarily reference the same embodiments or examples in the application. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments or examples.

Although illustrative embodiments have been shown and described, it will be understood by those skilled in the art that the foregoing embodiments are not to be construed as limiting the application, and that changes, substitutions and alterations may be made herein without departing from the spirit, principles and scope of the application.

Claims

An electrochemical device, comprising: the positive electrode comprises a positive electrode current collector, a protective layer and a positive electrode active material layer, wherein the protective layer is arranged between the positive electrode current collector and the positive electrode active material layer, xg of the protective layer is dispersed in 50Xg of water at 25-35 ℃, a stirrer with the diameter of a dispersion disc of 50-80 mm is used for stirring at the stirring speed of 1200r/min, a filter screen of a 100-mesh sieve is used for filtering and drying, and the weight of the protective layer on the filter screen is Wg, wherein W/X is less than or equal to 10%.
The electrochemical device of claim 1, wherein the protective layer has an infrared spectrum at 1400cm ^-1 Up to 1700cm ^-1 And/or 2100cm ^-1 Up to 2300cm ^-1 Having characteristic peaks in the range.
The electrochemical device of claim 1, wherein at least one of the following conditions is satisfied:

(i) When the electrochemical device is in a full charge state, the resistance of the positive electrode is RΩ, R is more than or equal to 1.5 and less than or equal to 5;

(ii) The thickness of the protective layer is T mu m, and T is more than or equal to 0.5.
The electrochemical device of claim 1, wherein the protective layer comprises a first active material, a first binder, and a first conductive agent.
The electrochemical device of claim 4, wherein at least one of the following conditions is satisfied:

(iii) The protective layer also comprises a leveling agent;

(iv) The first binder is an aqueous binder;

(v) The first conductive agent is 0.5 to 15% by mass, the first active material is 60 to 98.5% by mass, and the first binder is 1 to 20% by mass, based on the mass of the protective layer.
The electrochemical device according to claim 5, wherein the leveling agent comprises at least one of a siloxane-based compound, a siloxane-based derivative, an oxyalkylene-containing polymer, an acrylate-based polymer, an alcohol-based compound, an ether-based compound, or a fluorocarbon, and the leveling agent is 0.01 to 5% by mass based on the mass of the protective layer.
The electrochemical device of claim 4, wherein the first binder satisfies at least one of the following characteristics:

(a) A polymer comprising at least one of acrylic acid, acrylamide, an acrylate, acrylonitrile, or an acrylate;

(b) Comprises at least one of a carboxymethyl cellulose salt or a nitrile rubber;

(c) The first binder has a weight average molecular weight of 20 to 200 tens of thousands.
The electrochemical device of claim 4, wherein the first active material comprises at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium manganate, or lithium nickel cobalt manganate; the first conductive agent comprises at least one of graphene, graphite fibers, carbon nanotubes, ketjen black, or conductive carbon.
The electrochemical device of claim 1, wherein the positive electrode active material layer comprises a second active material, a second binder, and a second conductive agent, satisfying at least one of the following characteristics:

(d) The second active material is 91.5 to 99% by mass, the second binder is 0.5 to 5% by mass, and the second conductive agent is 0.5 to 3.5% by mass, based on the mass of the positive electrode active material layer;

(e) The second active material comprises at least one of lithium cobaltate, lithium manganate or lithium nickel cobalt manganate;

(f) The second binder comprises at least one of polyacrylic acid, polyvinylidene fluoride, polytetrafluoroethylene-hexafluoropropylene, sodium polyacrylate, nitrile rubber or polyacrylate;

(g) The second conductive agent comprises at least one of graphene, graphite fibers, carbon nanotubes, ketjen black, or conductive carbon.
An electronic device comprising the electrochemical device according to any one of claims 1-9.