Composite insulating film, preparation method thereof and secondary battery

Abstract

The invention discloses a composite insulating film and a preparation method thereof and a secondary battery, the composite insulating film comprises a base film and a heat-absorbing flame-retardant layer, the heat-absorbing flame-retardant layer is arranged on at least one surface of the base film, and the heat-absorbing flame-retardant layer comprises the following raw materials by weight: 50-100 parts of an organic solvent, 20-30 parts of carbonate, 20-30 parts of urea, 10-20 parts of a binder, and 5-10 parts of a thickener. The composite insulating film provided by the invention can effectively absorb heat and prevent fire spreading when a battery is overheated, so that the problem of thermal runaway of a battery cell under extreme conditions is solved. Wherein the heat-absorbing flame-retardant layer can effectively absorb heat and prevent fire from spreading when the battery is overheated by reducing heat generated by thermal runaway and reducing the concentration of combustible gas to be less than an explosion lower limit, so that the problem of thermal runaway of the battery cell under an extreme condition is solved.

Description

Technical Field

[0001] The present invention relates to the field of secondary batteries, and in particular to a composite insulating film, a preparation method thereof, and a secondary battery. Background Art

[0002] As energy storage and electric vehicle technologies continue to advance, lithium-ion batteries, as their core components, are facing increasingly higher demands for energy density and charging speed. However, in actual use, lithium-ion batteries may encounter problems such as external impact, overcharging, and lithium deposition. These problems can cause the battery to short-circuit, leading to thermal runaway and potentially fire and explosion accidents. As energy density and fast charging requirements continue to increase, the consequences of thermal runaway are becoming increasingly serious.

[0003] While traditional Mylar film effectively wraps the entire battery cell, preventing direct contact between the cell and the metal casing, its functionality is relatively limited and lacks additional protective properties. Due to the poor thermal conductivity of polymer materials, the Mylar film hinders heat dissipation in the event of thermal runaway, resulting in poor safety performance.

[0004] Therefore, it is necessary to develop a composite insulating film that can effectively absorb heat and prevent the spread of fire when the battery overheats, thereby solving the problem of thermal runaway of the battery cell under extreme conditions. Summary of the Invention

[0005] The purpose of the present invention is to address the deficiencies of the existing technology and provide a composite insulating film that can effectively absorb heat and prevent the spread of fire when the battery overheats, thereby solving the problem of thermal runaway of the battery cell under extreme conditions.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A composite insulating film comprises a base film and a heat-absorbing flame-retardant layer, wherein the heat-absorbing flame-retardant layer is provided on at least one surface of the base film, and the heat-absorbing flame-retardant layer comprises the following raw materials in parts by weight:

[0008]

[0009] Preferably, the thickness of the heat-absorbing flame-retardant layer is 0.1-2 mm.

[0010] Preferably, the organic solvent is selected from at least one of dimethyl sulfoxide, ethanol, methanol, dichloromethane, tetrahydrofuran, 1,2-dichloroethane, N,N-dimethylformamide and 1,4-dioxane.

[0011] Preferably, the carbonate is selected from at least one of magnesium carbonate, calcium carbonate, sodium carbonate and ammonium carbonate.

[0012] Preferably, the binder is selected from at least one of oily polyvinylidene fluoride, polyimide and polyacrylic acid.

[0013] Preferably, the thickener is selected from at least one of sodium carboxymethyl cellulose, hydroxypropyl methyl cellulose, sodium alginate, polybutyl acrylate and polymethyl methacrylate.

[0014] Preferably, the base film is a Mylar film.

[0015] In addition, the present invention also provides a method for preparing a composite insulating film, comprising the following steps:

[0016] Step S1, adding carbonate, urea, a binder and a thickener to an organic solvent and stirring evenly to obtain a heat-absorbing flame-retardant layer slurry;

[0017] Step S2: uniformly coating the heat-absorbing flame-retardant layer slurry on at least one surface of the base film and drying the slurry to obtain the composite insulating film.

[0018] Preferably, in step S1, the stirring time is 0.5-4 h, and the speed is 100-1200 rpm.

[0019] Preferably, in step S2, the coating method is gravure roller coating or narrow slot extrusion.

[0020] In addition, the present invention also provides a secondary battery, comprising a bare cell and an insulating film coated on the surface of the bare cell, wherein the insulating film is the composite insulating film described above.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The composite insulating film provided by the present invention can effectively absorb heat and prevent the spread of fire when the battery is overheated, thereby solving the problem of thermal runaway of the battery cell under extreme conditions. Among them, the urea in the heat-absorbing flame-retardant layer sublimates and decomposes after being heated. This process can release inert gases. These gases can reduce the concentration of combustible gases or efficiently absorb the heat released by the battery cell under extreme conditions, thereby promoting the transformation of the molecular structure to a controllable state. By reducing the heat generation of thermal runaway and reducing the concentration of combustible gases to below the lower explosion limit, it can effectively absorb heat and prevent the spread of fire when the battery is overheated, thereby solving the problem of thermal runaway of the battery cell under extreme conditions. DETAILED DESCRIPTION

[0023] To make the technical solutions and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0024] According to a first aspect of the present application, the present application provides a composite insulating film, comprising a base film and a heat-absorbing flame-retardant layer, wherein the heat-absorbing flame-retardant layer is disposed on at least one surface of the base film, and the heat-absorbing flame-retardant layer comprises the following raw materials in parts by weight:

[0025]

[0026] When heated, urea decomposes into ammonia and cyanic acid, which reacts with carbonates to form flame-retardant cyanide salts. Simultaneously, it releases large amounts of N2 and CO2, inerting the flammable atmosphere and acting as a heat absorber and flame retardant. By reducing the heat generated by thermal runaway and lowering the concentration of combustible gases to below the lower explosion limit, it effectively absorbs heat and prevents the spread of fire when the battery overheats, thus resolving the problem of thermal runaway in battery cells under extreme conditions.

[0027] When the urea weight fraction is less than the specified range, insufficient cyanate is produced by thermal decomposition of the urea, which reacts with the carbonate to produce a flame-retardant cyanide salt and releases flame-retardant CO2 gas. When the urea weight fraction is greater than the specified range, the sufficient cyanide reacts with the aluminum at the thermal runaway temperature, further increasing the internal temperature of the battery cell. The cyanide and carbonate ratios are in a reasonable proportion; either excess or deficiency of the other affects the effectiveness of the other. The relationship between the two cannot be determined solely by the excess or deficiency of either component. In other words, the effect of a carbonate weight fraction greater than or less than the specified range is similar to that of the cyanide compound. Therefore, setting the ratio within a certain range ensures uniform and flawless coating of the mixed slurry onto the Mylar film while efficiently generating N2 and CO2 gases to inert the flammable gas atmosphere during thermal runaway, suppressing fire and explosion.

[0028] In some embodiments, the thickness of the heat-absorbing flame-retardant layer is 0.1-2 mm, for example, 0.1 mm, 0.5 mm, 1 mm, 1.5 mm or 2 mm.

[0029] Due to the limited space within the battery case, excessive thickness of the heat-absorbing flame-retardant layer can increase the cell group margin, leading to the risk of cycling failure later in the cycle due to expansion of the top shell. When the thickness of the heat-absorbing flame-retardant layer is too small, firstly, the coating is prone to leaks and large fluctuations in thickness control. Secondly, in the event of thermal runaway, sufficient N2 and CO2 gases cannot be released to inert the flammable gas atmosphere and suppress the occurrence of fire and explosion. Therefore, setting it within a certain range allows for better coating and can effectively suppress thermal runaway fires and explosions.

[0030] In some embodiments, the organic solvent is selected from at least one of dimethyl sulfoxide, ethanol, methanol, dichloromethane, tetrahydrofuran, 1,2-dichloroethane, N,N-dimethylformamide, and 1,4-dioxane. Preferably, the organic solvent is selected from at least one of dimethyl sulfoxide, N,N-dimethylformamide, and 1,4-dioxane.

[0031] In some embodiments, the carbonate is selected from at least one of magnesium carbonate, calcium carbonate, sodium carbonate, and ammonium carbonate.

[0032] In some embodiments, the binder is selected from at least one of oily polyvinylidene fluoride, polyimide, and polyacrylic acid.

[0033] In some embodiments, the thickener is selected from at least one of sodium carboxymethylcellulose, hydroxypropyl methylcellulose, sodium alginate, polybutyl acrylate, and polymethyl methacrylate.

[0034] In some embodiments, the base film is a Mylar film.

[0035] According to a second aspect of the present application, the present application provides a method for preparing a composite insulating film, comprising the following steps:

[0036] Step S1, adding carbonate, urea, a binder and a thickener to an organic solvent and stirring evenly to obtain a heat-absorbing flame-retardant layer slurry;

[0037] Step S2: uniformly coating the heat-absorbing flame-retardant layer slurry on at least one surface of the base film and drying the slurry to obtain a composite insulating film.

[0038] In some embodiments, in step S1, the stirring time is 0.5-4h, for example, it can be 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h or 4h; the speed is 100-1200rpm, for example, it can be 100rpm, 200rpm, 400rpm, 600rpm, 800rpm, 1000rpm or 1200rpm.

[0039] In some embodiments, in step S2, the coating method is gravure roller coating or narrow slot extrusion.

[0040] According to the third aspect of the present application, the present application also provides a secondary battery, comprising a bare cell, an electrolyte, and a shell encapsulating the bare cell and the electrolyte, wherein the outer surface of the bare cell is covered with the above-mentioned composite insulating film.

[0041] In some embodiments, the bare cell includes a positive electrode sheet, a negative electrode sheet, and a separator spaced between the positive electrode sheet and the negative electrode sheet. The bare cell can be manufactured by winding or laminating.

[0042] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material. The positive electrode active material may include but is not limited to a chemical formula such as Li a Ni x Co y M z O 2-b N b (wherein 0.95≤a≤1.2, x>0, y≥0, z≥0, and x+y+z=1, 0≤b≤1, M is selected from a combination of one or more of Mn and Al, and N is selected from a combination of one or more of F, P, and S), the positive electrode active material may also be, but is not limited to, LiCoO2, LiNiO2, LiVO2, LiCrO2, LiMn2O4, LiCoMnO4, Li2NiMn3O8, LiNi 0.5 Mn 1.5 The positive electrode active material may be a combination of one or more of O4, LiCoPO4, LiMnPO4, LiFePO4, LiNiPO4, LiCoFSO4, CuS2, FeS2, MoS2, NiS, TiS2, etc. The positive electrode active material may also be subjected to a modification treatment. Methods for modifying the positive electrode active material should be known to those skilled in the art. For example, the positive electrode active material may be modified by coating, doping, etc. The materials used for the modification treatment may include, but are not limited to, a combination of one or more of Al, B, P, Zr, Si, Ti, Ge, Sn, Mg, Ce, W, etc. The positive electrode current collector is generally a structure or part that collects current. The positive electrode current collector may be any material suitable for use as a positive electrode current collector for lithium-ion batteries in the art. For example, the positive electrode current collector may include, but is not limited to, metal foil, and more specifically, may include, but is not limited to, aluminum foil.

[0043] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the surface of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material. The negative electrode active material may include, but is not limited to, graphite, soft carbon, hard carbon, carbon fiber, mesocarbon microbeads, silicon-based materials, tin-based materials, lithium titanate, or other metals capable of forming alloys with lithium. Graphite may be selected from one or more of artificial graphite, natural graphite, and modified graphite; silicon-based materials may be selected from one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, and silicon alloys; and tin-based materials may be selected from one or more of elemental tin, tin oxide compounds, and tin alloys. The negative electrode current collector is typically a structure or component that collects current. The negative electrode current collector may be any material suitable for use as a negative electrode current collector in lithium-ion batteries. For example, the negative electrode current collector may include, but is not limited to, metal foil, and more specifically, copper foil.

[0044] The electrolyte includes an organic solvent, an electrolyte lithium salt, and additives. The electrolyte lithium salt can be LiPF6 and / or LiBOB, as used in high-temperature electrolytes; at least one of LiBF4, LiBOB, and LiPF6, as used in low-temperature electrolytes; at least one of LiBF4, LiBOB, LiPF6, and LiTFSI, as used in overcharge-preventing electrolytes; or at least one of LiClO4, LiAsF6, LiCF3SO3, and LiN(CF3SO2)2. The organic solvent can be a cyclic carbonate, including PC and EC; a chain carbonate, including DFC, DMC, or EMC; or a carboxylic acid ester, including MF, MA, EA, and MP. Additives include, but are not limited to, at least one of a film-forming additive, a conductive additive, a flame retardant additive, an overcharge prevention additive, an additive for controlling the H2O and HF content in the electrolyte, an additive for improving low-temperature performance, and a multifunctional additive.

[0045] Example 1

[0046] Preparation of composite insulating film:

[0047] Step S1: adding 30 parts of calcium carbonate, 30 parts of urea, 10 parts of oily polyvinylidene fluoride, and 5 parts of sodium carboxymethyl cellulose to 100 parts of dimethyl sulfoxide, and stirring at a stirring speed of 600 rpm for 3 hours in a biaxial stirring apparatus to obtain a heat-absorbing flame-retardant layer slurry;

[0048] Step S2: evenly coating the heat-absorbing flame-retardant layer slurry on both sides of the base film by gravure roller coating, and drying to obtain the composite insulating film; wherein the thickness of the heat-absorbing flame-retardant layer is 1 mm.

[0049] Preparation of positive electrode:

[0050] By weight percentage, 96.7% LiCoO2, 2.5% carboxymethyl cellulose, 0.5% conductive agent SP and 0.3% dispersant PVP are evenly dispersed in N-methylpyrrolidone through a double planetary mixer, and evenly coated on a 9μm aluminum foil on an extrusion coater. After drying, rolling to a certain thickness, and slitting to a certain width, the positive electrode sheet is obtained.

[0051] Preparation of negative electrode sheet:

[0052] By weight percentage, 97.7% graphite, 1.2% polystyrene butadiene copolymer, and 1.1% lithium carboxymethyl cellulose are evenly dispersed in deionized water using a double planetary mixer, and evenly coated on a 5μm copper foil using an extrusion coater. After drying, rolling to a certain thickness, and slitting to a certain width, the negative electrode sheet is obtained.

[0053] Preparation of the battery:

[0054] The positive electrode sheet, negative electrode sheet and separator prepared above are welded with pole ears, glued, wound, cut and hot pressed on a winding machine to obtain bare battery cells, and the composite insulating film is wrapped around the bare battery cells; the packaged battery cells are injected with electrolyte and pre-sealed, and then left to stand at high temperature and room temperature for a certain period of time before hot pressing and forming, and after forming, the air bag is removed and the battery cells are sealed again.

[0055] Example 2

[0056] Different from Example 1, the preparation of the composite insulating film in this example is as follows:

[0057] Step S1: adding 20 parts of calcium carbonate, 30 parts of urea, 20 parts of viscous polyvinylidene fluoride, and 5 parts of sodium carboxymethyl cellulose to 100 parts of N,N-dimethylformamide, and stirring at a stirring speed of 1000 rpm for 0.5 h in a three-roll mill to obtain a heat-absorbing flame-retardant layer slurry;

[0058] Step S2: evenly coating the heat-absorbing flame-retardant layer slurry on both sides of the base film by gravure roller coating, and drying to obtain the composite insulating film.

[0059] The rest is the same as in Example 1 and will not be described again here.

[0060] Example 3

[0061] The difference from Example 1 is that the weight portion of urea in this example is 20 parts.

[0062] The rest is the same as in Example 1 and will not be described again here.

[0063] Example 4

[0064] Different from Example 1, the thickness of the heat-absorbing flame-retardant layer in this embodiment is 0.1 mm.

[0065] The rest is the same as in Example 1 and will not be described again here.

[0066] Example 5

[0067] Different from Example 1, the thickness of the heat-absorbing flame-retardant layer in this embodiment is 2 mm.

[0068] The rest is the same as in Example 1 and will not be described again here.

[0069] Example 6

[0070] Different from Example 1, the carbonate in this embodiment is selected from ammonium carbonate.

[0071] The rest is the same as in Example 1 and will not be described again here.

[0072] Comparative Example 1

[0073] Different from Example 1, this comparative example uses a conventional Mylar film instead of the composite insulating film provided in this application.

[0074] The rest is the same as in Example 1 and will not be described again here.

[0075] Comparative Example 2

[0076] The difference from Example 1 is that the weight portion of urea in this comparative example is 10 parts.

[0077] The rest is the same as in Example 1 and will not be described again here.

[0078] Comparative Example 3

[0079] The difference from Example 1 is that the weight portion of urea in this comparative example is 40 parts.

[0080] The rest is the same as in Example 1 and will not be described again here.

[0081] Comparative Example 4

[0082] The difference from Example 1 is that the weight portion of carbonate in this comparative example is 10 parts.

[0083] The rest is the same as in Example 1 and will not be described again here.

[0084] Comparative Example 5

[0085] The difference from Example 1 is that the weight portion of carbonate in this comparative example is 40 parts.

[0086] The rest is the same as in Example 1 and will not be described again here.

[0087] Comparative Example 6

[0088] Different from Example 1, the thickness of the heat-absorbing flame-retardant layer in this comparative example is 4 mm.

[0089] The rest is the same as in Example 1 and will not be described again here.

[0090] The following performance tests were performed on the lithium-ion batteries prepared in the examples and comparative examples:

[0091] (1) Charge the cell at 0.5C to 100% SOC. Measure the cell thickness and calculate the cell expansion rate. The expansion rate calculation formula is: expansion rate = (full-charge cell thickness - initial cell thickness) / initial cell thickness.

[0092] (2) Use a heating sheet that can cover the large surface of the battery cell to heat the two large surfaces of the battery cell with a heating power of 500W. Continue heating until the battery cell experiences thermal runaway and then stop heating.

[0093] (3) Collect the gas generated before the thermal runaway fire, and then conduct ignition flammability test on the gas.

[0094] The above performance test results are shown in Table 1 below.

[0095] Table 1

[0096]

[0097]

[0098] From the comparison of the experimental data in Table 1 above, it can be seen that the composite insulating film provided in the present application can effectively absorb heat and prevent the spread of fire when the battery is overheated, thereby solving the problem of thermal runaway of the battery cell under extreme conditions.

[0099] From the comparison of the test results of Example 1 and Comparative Example 1, it can be seen that when the conventional Mylar film is used, the battery cannot be prevented from catching fire when thermal runaway occurs, and the safety performance is poor.

[0100] By comparing the test results of Examples 1-3 and Comparative Examples 2-5, it can be seen that when the weight ratio of urea is less than the range, the amount of cyanic acid produced by the thermal decomposition of urea will be insufficient, and it will react with carbonate to produce flame-retardant cyanide compound salts and release flame-retardant CO2 gas. When the weight ratio of urea is greater than the range, sufficient cyanic acid will further react with aluminum at the thermal runaway temperature, exacerbating the increase in the internal temperature of the battery cell.

[0101] Comparing the test results of Examples 1, 4-5, and Comparative Example 6 shows that when the thickness of the heat-absorbing flame-retardant layer is too great, the electrode sheet expands after full charge, causing cell top crusting and excessive cell expansion. Therefore, setting the thickness within a certain range effectively suppresses thermal runaway explosions and fires, while also ensuring that cell top crusting is less likely to occur during cycling.

[0102] Based on the disclosure and teachings of the above description, those skilled in the art will be able to make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the above specific embodiments. Any obvious improvements, substitutions, or modifications made by those skilled in the art based on the present invention fall within the scope of protection of the present invention. In addition, although certain specific terms are used in this description, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. A composite insulating film, characterized in that: The heat-absorbing flame-retardant layer comprises a base film and a heat-absorbing flame-retardant layer, wherein the heat-absorbing flame-retardant layer is arranged on at least one surface of the base film, and the heat-absorbing flame-retardant layer comprises the following raw materials in parts by weight:

2. The composite insulating film according to claim 1, wherein The thickness of the heat-absorbing flame-retardant layer is 0.1-2 mm.

3. The composite insulating film according to claim 1, wherein The organic solvent is selected from at least one of dimethyl sulfoxide, ethanol, methanol, dichloromethane, tetrahydrofuran, 1,2-dichloroethane, N,N-dimethylformamide and 1,4-dioxane.

4. The composite insulating film according to claim 1, wherein The carbonate is selected from at least one of magnesium carbonate, calcium carbonate, sodium carbonate and ammonium carbonate.

5. The composite insulating film according to claim 1, wherein The binder is selected from at least one of oily polyvinylidene fluoride, polyimide and polyacrylic acid. The composite insulating film according to claim 1 , wherein: The thickener is selected from at least one of sodium carboxymethyl cellulose, hydroxypropyl methyl cellulose, sodium alginate, polybutyl acrylate and polymethyl methacrylate.

7. The composite insulating film according to claim 1, wherein: The base film is a Mylar film.

8. A method for preparing a composite insulating film according to any one of claims 1 to 7, characterized in that: The following steps are involved: Step S1, adding carbonate, urea, a binder and a thickener to an organic solvent and stirring evenly to obtain a heat-absorbing flame-retardant layer slurry; Step S2: uniformly coating the heat-absorbing flame-retardant layer slurry on at least one surface of the base film and drying the slurry to obtain the composite insulating film.

9. The method for preparing a composite insulating film according to claim 8, wherein: In step S1, the stirring time is 0.5-4h, and the speed is 100-1200rpm.

10. A secondary battery comprising a bare cell and an insulating film covering a surface of the bare cell, characterized in that: The insulating film is the composite insulating film according to any one of claims 1 to 7.

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