CN113088021A

CN113088021A - Color development gel and preparation method thereof, leakage detection method and battery cell

Info

Publication number: CN113088021A
Application number: CN202110342109.3A
Authority: CN
Inventors: 秦一鸣; 陈建鹏
Original assignee: Ningde Amperex Technology Ltd
Current assignee: Ningde Amperex Technology Ltd
Priority date: 2021-03-30
Filing date: 2021-03-30
Publication date: 2021-07-09

Abstract

The application provides a developing gel, a preparation method thereof, a leakage detection method and a battery cell. The color developing gel comprises a color developing agent, a cross-linking agent and a binder. The cross-linking agent comprises at least one of polyvinyl alcohol, polyacrylamide, polyethylene glycol or polyacrylic acid. The binder comprises at least one of agar, polyvinylidene fluoride, sodium alginate, modified starch, carboxymethyl cellulose, modified carboxymethyl cellulose or chitosan. The mass ratio of the color developing agent to the cross-linking agent to the binder is (0.01-1): (0.1-30): (0.1-5). According to the embodiment of the application, the cross-linking agent or the binder is physically cross-linked with the color developing agent to form the gelatinous color developing gel, the color developing gel is wrapped on the surface of the battery cell and is placed in a high-temperature and high-humidity leakage detection environment, the color developing gel can form a layer of protective film on the surface of the battery cell, and water drops condensed by water vapor cannot influence the color developing gel, so that the specific position of leakage can be accurately detected.

Description

Color development gel and preparation method thereof, leakage detection method and battery cell

Technical Field

The application relates to the field of electrochemical energy storage, in particular to a developing gel and a preparation method thereof, a leakage detection method and a battery cell.

Background

In the quality detection of the battery core of the lithium ion battery, the detection of leakage is an important aspect. At present, the common scheme for detecting the leakage of the battery core is to use a Charge Coupled Device (CCD) or a magnifying glass to observe the damage position of the battery core, or use litmus paper to wrap the battery core and to observe the color development condition of the litmus paper after standing in a high-temperature and high-humidity environment.

The CCD or the magnifier is low in efficiency in observing the damage position of the battery core, the damage position is not easy to find, and the correlation between damage and leakage is difficult to prove. After the electric core is wrapped by litmus test paper and placed still in a high-temperature and high-humidity environment, the color development condition of the litmus test paper is observed, water drops condensed by water vapor exist on the surface of the high-temperature and high-humidity electric core, the water drops can be dizzy on the test paper, and the accurate leakage position cannot be distinguished. Therefore, further improvements are desired.

Disclosure of Invention

Embodiments of the present application provide a color developing gel including a color developer, a crosslinking agent, and a binder. The cross-linking agent comprises at least one of polyvinyl alcohol, polyacrylamide, polyethylene glycol or polyacrylic acid. The binder comprises at least one of agar, polyvinylidene fluoride, sodium alginate, modified starch, carboxymethyl cellulose, modified carboxymethyl cellulose or chitosan. The mass ratio of the color developing agent to the cross-linking agent to the binder is (0.01-1): (0.1-30): (0.1-5).

In some embodiments, the color developer comprises litmus, thymol blue, methyl red, or bromocresol green. In some embodiments, the polyvinyl alcohol has a relative molecular weight of 2.5 to 30 million, a degree of polymerization greater than 1700, and a degree of alcoholysis greater than 99%. In some embodiments, the polyethylene glycol has a relative molecular weight greater than 300 ten thousand. In some embodiments, the polyacrylic acid has a relative molecular weight greater than 350 ten thousand. In some embodiments, the polyacrylamide has a relative molecular weight greater than 350 tens of thousands.

Another embodiment of the present application provides a method of preparing a developing gel, comprising: adding a cross-linking agent into deionized water, and mixing to obtain a first mixture; adding a binder and a color developing agent into the first mixture, and mixing at a first temperature until the binder and the color developing agent are completely dissolved to obtain a second mixture; transferring the second mixture into a mold, standing for a first time at a second temperature, and then thawing to obtain a chromogenic gel; the color developing agent, the cross-linking agent and the binding agent are in a mass ratio of (0.01-1): (0.1-30): (0.1-5).

In some embodiments, the first temperature is 90 ℃ to 95 ℃. In some embodiments, the second temperature is between-30 ℃ and-10 ℃ and the first time is between 15h and 25 h. In some embodiments, the method further comprises repeating the following steps a plurality of times after transferring the second mixture into the mold: standing at a second temperature for a first time, and then thawing. In some embodiments, the color developer comprises litmus, thymol blue, methyl red, or bromocresol green.

Another embodiment of the present application provides a liquid leakage detection method, including: coating the developing gel or the developing gel prepared by the method on the outer surface of the battery cell to be tested, and drying the developing gel; placing the to-be-detected cell coated with the developing gel in a preset environment for a preset time; and determining whether the electric core to be tested has leakage and the position of the leakage according to the color change of the developing gel.

In some embodiments, the predetermined environment is an environment with a temperature of 45 ℃ to 80 ℃ and a relative humidity of 55% to 95%, and the predetermined time is 0.5h to 1.5 h.

Another embodiment of the present application provides a battery cell, where the developing gel or the developing gel prepared according to the above method is present on an outer surface of the battery cell.

According to the embodiment of the application, the cross-linking agent or the binder is physically cross-linked with the color developing agent to form the gelatinous color developing gel, the color developing gel is wrapped on the surface of the battery cell and is placed in a high-temperature and high-humidity leakage detection environment, the color developing gel can form a layer of protective film on the surface of the battery cell, and water drops condensed by water vapor cannot influence the color developing gel, so that the specific position of leakage can be accurately detected.

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.

Embodiments of the present application provide a color developing gel including a color developer, a crosslinking agent, and a binder. In some embodiments, the crosslinking agent comprises at least one of polyvinyl alcohol, polyacrylamide, polyethylene glycol, or polyacrylic acid. The crosslinking agent may act as a physical crosslink to the color former. In some embodiments, the binder comprises at least one of agar, polyvinylidene fluoride, sodium alginate, modified starch, carboxymethyl cellulose, modified carboxymethyl cellulose, or chitosan. The binder can bind the cross-linking agent and the color developing agent together to form a stable gel structure. In some embodiments, a color developer is used to develop color to facilitate visual or microscopic observation of the leak site.

In some embodiments, the mass ratio of the color developing agent to the cross-linking agent to the binder is (0.01-1): (0.1-30): (0.1-5). If the content of the color developing agent is too small, the color development color is too light to be observed easily; if the content of the color-developing agent is too large, the color-developing action is not increased significantly, and the color-developing range is easily too large to make it difficult to determine the specific leaking position. In some embodiments, the developer in the color-developing gel is preferably present in an amount of 0.5% by mass. If the content of the cross-linking agent is too small, the cross-linking effect is too weak, which is not favorable for the stability of the color-developing gel structure; if the content of the crosslinking agent is too large, the rigidity of the color developing gel is likely to be too high, which is not favorable for shaping and bonding on the surface of the battery cell. If the content of the binder is too small, the stability of the structure of the developing gel is not facilitated; if the content of the binder is too large, the rigidity of the color developing gel is too high, which is not favorable for shaping and attaching on the surface of the battery core. In some embodiments, the mass content of the binder in the developing gel is preferably 1% to 3%. The developing gel is convenient to store, long in quality guarantee period and low in cost.

In some embodiments, the color developer comprises litmus, thymol blue, methyl red, or bromocresol green. If the color developing agent is litmus, the color developing gel is bluish purple, and the color developing gel at the corresponding position turns red when leakage is detected. If the color-developing agent used is thymol blue, the color-developing gel is blue, and the color-developing gel at the corresponding position becomes yellow when leakage is detected. If the color-developing agent used is methyl red, the color-developing gel is yellow, and the color-developing gel at the corresponding position becomes red when leakage is detected. If the color developing agent used is bromocresol green, the color developing gel is green, and the color developing gel at the corresponding position turns yellow when leakage is detected. The reason for the discoloration is that the electrolyte of the battery cell contains LiPF₆When water comes into contact, HF is discharged, the electrolyte is extremely toxic and acidic, and the pH value of the electrolyte is generally between 5 and 6.5.

In some embodiments, the polyvinyl alcohol has a relative molecular weight of 2.5 to 30 million, a degree of polymerization greater than 1700, and a degree of alcoholysis greater than 99%. If the relative molecular weight of the polyvinyl alcohol is too small, the crosslinking effect is not fully exerted; if the relative molecular weight of the polyvinyl alcohol is too large, uniform dispersion of the crosslinking agent is not facilitated. In addition, if the degree of polymerization of polyvinyl alcohol is too small, it is difficult to achieve a corresponding molecular weight. If the degree of alcoholysis of the polyvinyl alcohol is too low, the structural stability of the crosslinking agent is adversely affected. In some embodiments, the polyethylene glycol has a relative molecular weight greater than 300 ten thousand. If the relative molecular weight of the polyethylene glycol is too small, it is not favorable for the crosslinking action to be fully exerted. In some embodiments, the polyacrylic acid has a relative molecular weight greater than 350 ten thousand. If the relative molecular weight of polyacrylic acid is too small, it is not favorable for the crosslinking action to be sufficiently exerted. In some embodiments, the polyacrylamide has a relative molecular weight greater than 350 tens of thousands. If the relative molecular weight of polyacrylamide is too small, it is not favorable for the crosslinking action to be fully exerted.

Some embodiments of the present application provide a method of preparing a developing gel, the method comprising: adding a cross-linking agent into deionized water, and mixing to obtain a first mixture; adding a binder and a color developing agent into the first mixture, and mixing at a first temperature until the binder and the color developing agent are completely dissolved to obtain a second mixture; the second mixture is transferred to a mold, allowed to stand at a second temperature for a first time, and then thawed to provide a chromogenic gel. In some embodiments, the mold may be a glass mold. In some embodiments, the crosslinking agent comprises at least one of polyvinyl alcohol, polyacrylamide, polyethylene glycol, or polyacrylic acid. In some embodiments, the binder comprises at least one of agar, polyvinylidene fluoride, sodium alginate, modified starch, carboxymethyl cellulose, modified carboxymethyl cellulose, or chitosan. In some embodiments, the mass ratio of the color developing agent to the cross-linking agent to the binder is (0.01-1): (0.1-30): (0.1-5). Some parameters regarding the developing gel in the method for preparing the developing gel may be referred to the above description regarding the developing gel, and will not be repeated here.

In some embodiments, the first temperature is 90 ℃ to 95 ℃. If the first temperature is too low, rapid uniform dispersion of the materials is not favored. If the first temperature is too high, it is liable to cause structural damage of various raw materials used. In some embodiments, the second temperature is between-30 ℃ and-10 ℃ and the first time is between 15h and 25 h. The developing gel is formed at the freezing temperature, and the formed developing gel has a cross-linked network structure, so that the developing gel has excellent properties, such as high water content and high mechanical strength.

In some embodiments, the method further comprises repeating the following steps a plurality of times after transferring the second mixture into the mold: standing at a second temperature for a first time, and then thawing. By repeatedly freezing and unfreezing, the formed colored gel has higher strength and crosslinking degree. In some embodiments, the developing gel may be soaked in deionized water for use prior to use.

Some embodiments of the present application provide a method of leak detection, comprising: coating the developing gel or the developing gel prepared by the method on the outer surface of the battery cell to be tested, and drying the developing gel; placing the to-be-detected cell coated with the developing gel in a preset environment for a preset time; and determining whether the electric core to be tested has leakage and the position of the leakage according to the color change of the developing gel. In some embodiments, the developing gel is coated on the surface of the battery core, and the developing gel can form a layer of protective film on the surface of the battery core, so that the problem that the liquid leakage position cannot be accurately determined due to the fact that the test paper is affected by water drops condensed by water vapor when the test paper is used for detection is avoided.

The color development gel is stable in structure and shape, after the color development gel is coated on the battery core, drying is carried out (for example, drying is carried out at 20-60 ℃), hydrogel is remained on the surface of the battery core after drying, when the battery core is placed in a high-temperature and high-humidity environment, and after the battery core is placed for a period of time, the corresponding leakage position of the battery core can change color, and the leakage position of the battery core can be accurately judged. Because of the inorganic salt LiPF in the electrolyte₆When water comes into contact, HF is discharged, the electrolyte is extremely toxic and acidic, and the pH value of the electrolyte is generally between 5 and 6.5.

In some embodiments, the predetermined environment is an environment with a temperature of 45 ℃ to 80 ℃ and a relative humidity of 55% to 95%, and the predetermined time is 0.5h to 1.5 h. At high temperatures, electrolyte in a damaged cell tends to swell and leak. Under this high humidity environment, lithium hexafluorophosphate in the electrolyte meets moisture to form HF, which in turn causes a color change of the developing gel.

Some embodiments of the present application provide a battery cell having an outer surface on which the above-described developing gel or the developing gel prepared according to the above-described method is present. Through coating the color development gel on the outer surface of the battery cell, the accurate detection of the leakage of the battery cell can be facilitated, and the problem that the leakage position cannot be accurately determined due to the fact that the test paper is influenced by water drops condensed by vapor when the test paper is used for detection is avoided.

In some embodiments, the cell includes a positive pole piece, a negative pole piece, and a separator disposed between the positive and negative pole pieces. In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer on the positive electrode current collector. In some embodiments, the positive active material layer is located on one or both sides of the positive current collector. In some embodiments, the positive electrode active material layer includes a positive electrode active material including at least one of lithium cobaltate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminate, or lithium manganese oxide. 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 (80-99): (0.1-10): (0.1-10). In some embodiments, the thickness of the positive electrode active material layer may be 10 μm to 200 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 positive current collector may be an Al foil, but other current collectors commonly used in the art may also be used. In some embodiments, the thickness of the positive electrode current collector may be 1 μm to 100 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, the negative electrode tab includes a negative electrode current collector and a negative active material layer on the negative electrode current collector. In some embodiments, the negative active material layer is located on one or both sides of the negative current collector. 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 negative active material, the conductive agent and the binder in the negative active material layer may be (80-98): (0.1-10): (0.1-10). 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 negative electrode current collector 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 separator is in the range of about 5 μm to 50 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 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 cells may be wound electrode assemblies, stacked electrode assemblies, or folded electrode assemblies. In some embodiments, the positive electrode sheet and/or the negative electrode sheet may be a multilayer structure formed by winding or stacking, or may be a single-layer structure formed by stacking a single-layer positive electrode, a single-layer negative electrode, and a separator.

In some embodiments, the cells are cells of a lithium ion battery, although the application is not so limited. In some embodiments, the cell may further include an electrolyte. The electrolyte may be at least one 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、LiSiF₆At least one 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 lithium ion batteries are examples only. Other methods commonly used in the art may be employed without departing from the disclosure herein.

The embodiment of the application also provides an electronic device comprising the battery cell. 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.

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. A chromogenic gel comprising:

a color-developing agent;

a crosslinking agent; and

a binder;

the color developing agent, the cross-linking agent and the binding agent are mixed according to a mixing ratio of (0.01-1): (0.1-30): (0.1-5).

2. The color-developing gel of claim 1, wherein the color-developer comprises litmus, thymol blue, methyl red, or bromocresol green.

3. The developing gel of claim 1, wherein the cross-linking agent satisfies at least one of the following conditions:

the relative molecular weight of the polyvinyl alcohol is 2.5-30 ten thousand, the degree of polymerization is larger than 1700, and the degree of alcoholysis is larger than 99%;

the relative molecular weight of the polyethylene glycol is more than 300 ten thousand;

the polyacrylic acid has a relative molecular weight of greater than 350 ten thousand;

the relative molecular weight of the polyacrylamide is more than 350 ten thousand.

4. A method of making a color developing gel comprising:

adding a cross-linking agent into deionized water, and mixing to obtain a first mixture;

adding a binder and a color developing agent into the first mixture, and mixing at a first temperature until the binder and the color developing agent are completely dissolved to obtain a second mixture;

transferring the second mixture to a mold, standing at a second temperature for a first time, and then thawing to obtain the chromogenic gel;

5. The method of claim 4, wherein the first temperature is 90 ℃ to 95 ℃.

6. The method of claim 4, wherein the second temperature is-30 ℃ to-10 ℃ and the first time is 15h to 25 h.

7. The method of claim 4, further comprising repeating the following steps a plurality of times after transferring the second mixture into the mold: standing at a second temperature for a first time, and then thawing.

8. The method of claim 4, wherein the color developer comprises litmus, thymol blue, methyl red, or bromocresol green.

9. A method of leak detection, comprising:

coating the developing gel according to any one of claims 1 to 3 or the developing gel prepared by the method according to any one of claims 4 to 8 on the outer surface of a cell to be tested, and drying the developing gel;

placing the to-be-detected electric core coated with the developing gel in a preset environment for a preset time;

and determining whether the electric core to be tested has leakage and the position of the leakage according to the color change of the developing gel.

10. The leakage detection method according to claim 9, wherein the predetermined environment is an environment having a temperature of 45 ℃ to 80 ℃ and a relative humidity of 55% to 95%, and the predetermined time is 0.5h to 1.5 h.

11. An electrical core, characterized in that the developing gel according to any of claims 1 to 3 or prepared according to the method of any of claims 4 to 8 is present on the outer surface of the electrical core.