Method for manufacturing insulating film for secondary batteries

Abstract

To provide an insulating film for secondary batteries that exhibits desired insulating properties and heat resistance, does not significantly increase film thickness, and contributes to higher energy density of the battery, and a manufacturing method thereof.SOLUTION: An insulating film 1 for a secondary batteries includes a base film 10 made of a microporous membrane made of polyolefin, and a metal oxide 20 contained in the base film 10. In the insulating film 1, the metal oxide 10 is accommodated in at least some micropores 10p and is present on the inner walls of the micropores 10p. A method of manufacturing an insulating film for secondary batteries is a method in which a solution containing an alkyl compound corresponding to a metal of a metal oxide and / or a partial hydrolyzate of an alkyl compound is sprayed onto a base film, followed by drying.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for manufacturing an insulating film for a secondary battery, and more particularly, to an insulating film for a secondary battery that can be suitably used as a separator for a lithium-ion secondary battery, exhibits good insulating properties and heat resistance, can be made thinner, and can promote high energy density of the battery. Mu The present invention relates to a manufacturing method. Mu

Background Art

[0002] Conventionally, in order to improve the heat resistance of a separator for a lithium-ion battery and prevent deterioration of the separator due to high energy density of the battery to improve safety, a separator in which an inorganic particle layer formed using a specific binder is laminated on at least one of the front and back surfaces of a polyolefin porous film has been proposed (for example, Patent Document 1).

[0003] In addition, a separator for a lithium-ion battery has been proposed, which includes a polyolefin film containing colloidal inorganic particles having a predetermined particle size, and a microporous inorganic surface layer formed of an aqueous dispersion containing predetermined fumed inorganic particles and covering at least a part of this film (Patent Document 2).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in the case of the separator described in Patent Document 1, as is clear from the fact that inorganic particle layers are stacked, there is no intention to impregnate the inorganic particles into the polyolefin porous membrane. Therefore, the thickness of the separator film increases to a considerable extent, which ultimately reduces the energy density of the battery.

[0006] On the other hand, in the separator described in Patent Document 2, the film thickness increases by the amount of the microporous inorganic surface layer covering the polyolefin film, and depending on the particle size of the inorganic particles, they may not be able to enter the pores of the polyolefin film, which also leads to an increase in the separator film thickness. Furthermore, while the presence of moisture is generally restricted in the raw material management and processing steps during lithium-ion battery manufacturing, this conventional surface treatment method requires extensive drying processes and management of residual moisture in subsequent steps, raising concerns about cumbersome processes, inconsistent product quality, and reduced manufacturing efficiency.

[0007] This invention has been made in view of the problems of the prior art, and its objective is to provide an insulating film for secondary batteries that exhibits desired insulation and heat resistance, does not significantly increase the film thickness, and contributes to increasing the energy density of the battery. Mu The objective is to provide a manufacturing method. [Means for solving the problem]

[0008] The inventors of this invention have conducted extensive research to achieve the above objective and have found that this objective can be achieved by using a precursor solution of a metal oxide and appropriately arranging the metal oxide within the micropores of a microporous membrane, thereby completing the present invention. In other words, the gist of the present invention is as follows. (1) An insulating film for secondary batteries, comprising a polyolefin substrate film having numerous micropores, wherein at least some of the micropores contain a metal oxide, Insulating film for secondary batteries in which the above metal oxide exists on the inner wall of the above micropores In manufacturing, A method for producing an insulating film for secondary batteries, comprising spraying a non-aqueous solution containing an alkyl compound corresponding to the metal of the above-mentioned metal oxide and / or a partial hydrolysate of the alkyl compound onto the above-mentioned substrate film and drying the drying at room temperature to 80°C. (2) A method for producing an insulating film for a secondary battery as described in (1) above, wherein the alkyl compound corresponding to the metal of the above metal oxide is an alkylaluminum compound. [Effects of the Invention]

[0009] According to the present invention, by using a metal oxide precursor solution and appropriately positioning this metal oxide within the micropores of a microporous film, the desired insulation and heat resistance are achieved, and the film thickness does not increase significantly, contributing to the high energy density of the battery. Mu We can provide a manufacturing method. [Brief explanation of the drawing]

[0010] [Figure 1] This is a partially enlarged cross-sectional view showing the schematic shape of the insulating film for secondary batteries of the present invention. [Figure 2] This is a TEM photograph of the insulating film of Example 1. [Figure 3] This is a photograph showing the results of scanning transmission electron microscopy / energy dispersive X-ray spectroscopy (STEM / EDX) analysis of the insulating film of Example 1. [Figure 4] This graph shows the XPS analysis results of the insulating film from Example 1. [Modes for carrying out the invention]

[0011] The insulating film for secondary batteries of the present invention will be described below. As described above, the insulating film for secondary batteries of the present invention consists of a polyolefin base film having a large number of micropores. The insulating film for secondary batteries of the present invention contains a metal oxide in at least some of the micropores and is present on the inner wall of the micropores.

[0012] <Structure, etc.> Figure 1 is a partially enlarged cross-sectional view showing one embodiment of the insulating film for secondary batteries of the present invention, and shows the insulating film cut by a plane parallel to its thickness direction. In the figure, the insulating film 1 of the present embodiment includes a base film 10 which is a microporous film having a plurality of micropores 10p, and metal oxides 20 are present on the inner walls of the micropores 10p. Typically, there is almost no gap between the inner walls of the micropores and the metal oxides 20, and the metal oxides are also in close contact with each other. The micropores 10p may also be continuous in a random direction such as a direction perpendicular to the plane of the paper, but that state is not shown in the figure.

[0013] Here, most of the plurality of micropores 10p present in the base film 10 communicate with other micropores (not shown), and as such a path formed by such micropore-micropore, there are extremely many paths that lead from the front surface 10f to the back surface 10b of the base film 10 (hereinafter sometimes referred to as "through paths"). Such through paths often meander or wind around between the front and back surfaces of the base film 10 or between the micropores, but may also communicate linearly.

[0014] If such a through path is formed, when this insulating film is used as a separator for a battery, the flow of the electrolytic solution can be ensured. However, in the insulating film 1 of the present embodiment, it is not necessary for all of the micropores 10p to form the above-mentioned through paths, and there may be paths that are open only to one of the front surface 10f and the back surface 10b.

[0015] In this insulating film , metal oxides 20 are present on the inner walls of the micropores 10p. In other words, the metal oxides 20 are in close contact with the inner walls of the micropores 10p and are present in a layer covering all or part, preferably most of the regions of the inner wall surface. Due to the presence of such metal oxides, it is possible to achieve the manifestation of uniform insulating properties and heat resistance of the entire insulating film. Also, since the content of the metal oxides can be increased without almost increasing the thickness of the base film 10, not only can the insulating properties and heat resistance be improved, but also the energy density of the battery to be used can be improved Conventionally, when forming an oxide layer on a film surface, it was necessary to control the density of the oxide layer to allow the electrolyte to penetrate into it. Furthermore, if the wettability between the electrolyte and the film was insufficient, a special process was required to allow the oxide to penetrate (infiltrate). However, according to the present invention, such time and effort can be eliminated, and oxide penetration can be easily achieved.

[0016] In the insulating film 1, it is preferable that the occupancy rate of the metal oxide relative to the micropores is 5 to 20%, expressed as the cross-sectional area of ​​the metal oxide portion 20 / the cross-sectional area of ​​the micropore 10p × 100 (%) in cross-sectional observation. This occupancy rate of 5-20% allows for better overall insulation and heat resistance of the film, resulting in more appropriate electrolyte flow.

[0017] In the present invention, the content of metal oxides in the insulating film is preferably 1 to 5% by mass relative to the total weight of the insulating film. By having a metal oxide content of 1-5% by mass, the desired insulation and heat resistance effects of the film as a whole are better achieved, and the electrolyte flow becomes more appropriate.

[0018] Furthermore, in the present invention, the metal oxide 20 can be unevenly distributed on the surface 10f side or the back surface 10b side of the insulating film 1. This uneven distribution allows for, for example, increasing the amount of metal oxide on the side facing the positive electrode or the side facing the negative electrode, thereby efficiently improving heat resistance and suppressing short circuits caused by dendrite formation.

[0019] <Material etc.> Next, we will explain the materials and other aspects of the insulating film for secondary batteries described above. The base film is made of polyolefin and has numerous micropores. Here, typical examples of polyolefins used to form the base film include polyethylene and polypropylene.

[0020] When the insulating film for secondary batteries of the present invention is used as a separator for lithium secondary batteries, the thickness of the base film is preferably 5 to 25 μm. By having a base film thickness of 5 to 25 μm, deformation of the film during the impregnation and drying process of the metal oxide precursor solution can be better prevented, and the uniformity of the metal oxide dispersion state inside can be better maintained. Similarly, when used as a separator, micropores with a diameter of approximately 1 μm or less are preferred, but the material is not limited to this.

[0021] As a substrate film that is a microporous polyolefin film as described above, for example, it can be produced by stretching a polyolefin film prepared by a conventionally known method in one or more directions. Such base films are available on the market; for example, "Celgard" (manufactured by Asahi Kasei Corporation, product name) can be used.

[0022] Next, as the metal oxide, a material that possesses both insulating and heat-resistant properties is preferable, and specifically, aluminum oxide can be mentioned. In this invention, the above-mentioned aluminum oxide can be produced using a solution containing an alkylaluminum compound and / or a partial hydrolysate of an alkylaluminum compound, as described later.

[0023] <Method for manufacturing insulating film for secondary batteries> Next, the method for manufacturing an insulating film for secondary batteries according to the present invention will be described. The manufacturing method of the present invention is a method for manufacturing the insulating film for secondary batteries of the present invention described above, and is a manufacturing method that utilizes a solution containing an alkyl compound corresponding to the metal of the above metal oxide and / or a partial hydrolysate of the alkyl compound and a non-aqueous solvent, preferably the non-aqueous solvent of this solution. In this manufacturing method, the above solution is sprayed onto a base film and dried, with the drying carried out at room temperature to 80°C. While spraying and drying can be performed simultaneously, drying can also be performed after spraying. Simultaneous spraying and drying reduces the surface deposition layer, making it easier to prevent the film from becoming thick; however, drying after spraying can also effectively prevent the film from becoming thick.

[0024] Furthermore, in the manufacturing method of the present invention, it is preferable to spray and dry a non-aqueous solvent of the solution following the spraying and drying of the solution. It is desirable to perform this continuous process of spraying and drying the solution and non-aqueous solvent at least once, as such spraying and drying of the non-aqueous solvent can promote the filling of metal oxides into the micropores of the substrate film. Furthermore, drying of the non-aqueous solvent can be carried out at the same temperature as the above solution.

[0025] Here, examples of alkyl compounds corresponding to the metal in metal oxides include alkylaluminum compounds, specifically trimethylaluminum (hereinafter sometimes abbreviated as "TMAL"), triethylaluminum, tri-n-propylaluminum, triisopropylaluminum, tri-n-butylaluminum, triisobutylaluminum, tri-t-butylaluminum, tri-n-pentylaluminum, tri-n-hexylaluminum, tri-n-heptylaluminum, tri-n-octylaluminum, and the like. Furthermore, examples of non-aqueous solvents include cyclic amides such as N-methyl-2-pyrrolidone (hereinafter sometimes abbreviated as "NMP"), 1,3-dimethylimidazolidinone, and 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone; aliphatic hydrocarbons such as n-hexane, octane, and n-decane; alicyclic hydrocarbons such as cyclopentane, cyclohexane, methylcyclohexane, and ethylcyclohexane; aromatic hydrocarbons such as benzene, toluene, xylene, and cumene; hydrocarbon solvents such as mineral spirits, solvent naphtha, kerosene, and petroleum ether; ethers such as diethyl ether, tetrahydrofuran, diisopropyl ether, dioxane, di-n-butyl ether, dialkylethylene glycol, dialkyldiethylene glycol, and dialkyltriethylene glycol; and glyme, diglyme, and triglyme solvents. Furthermore, as a non-aqueous solvent, the above compounds can preferably be used individually or in appropriate mixtures. These non-aqueous solvents can be selected in various ways depending on the manufacturing and usage conditions. For example, amide solvents such as N-methyl-2-pyrrolidone are preferred because they are commonly used in the manufacture of electrodes, aromatic hydrocarbon solvents such as toluene and xylene are preferred because they are widely used industrially, and ether solvents such as tetrahydrofuran and glyme can be preferably used because they coordinate with and stabilize the partially hydrolyzed alkylaluminum products.

[0026] In the manufacturing method of the present invention, the above solution functions as a precursor solution for a metal oxide. Such solutions, particularly those containing alkylaluminum compounds and / or partial hydrolysates of alkylaluminum compounds, can be prepared according to the method described in Japanese Patent Publication No. 6756634.

[0027] The solution is preferably one with an aluminum concentration of 0.5 to 4% by mass. If the aluminum concentration is less than 0.5% by mass, the desired metal oxide may not be obtained inside the film, and if it exceeds 4% by mass, a large amount of metal oxide may accumulate on the coated surface. Furthermore, the turbidity of this solution is preferably 0 to 20.9 NTU, and the viscosity is preferably 1.743 to 2.790 mPa·s. By setting the turbidity and viscosity within the above ranges, the precipitation of solvent-insoluble gels or solids can be more effectively prevented.

[0028] In the manufacturing method of the present invention, by continuously spraying the above-mentioned non-aqueous solution, preferably with a non-aqueous solvent, onto a base film, typically the surface of the base film, the metal oxide penetrates deeply and widely into the film through the micropores without leaving much residue on the surface of the base film, thereby achieving close adhesion between the metal oxide and the inner walls of the micropores. Furthermore, even if drying (including drying with a non-aqueous solvent) is performed after the spraying described above, almost no metal oxide film is formed on the surface of the substrate film. [Examples]

[0029] The insulating film for secondary batteries of the present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0030] (Example 1) <Raw materials> Celgard2400 (manufactured by Asahi Kasei Corporation, trade name) was used as the substrate film, which consists of a microporous membrane made of polyolefin. The shape and dimensions of this substrate film were 25 × 25 mm square, with a thickness of 20 μm, and the micropore opening size was 50 to 250 nm. Furthermore, a 1% Al-MAO / NMP solution containing 1% Al was used as the solution of trimethylaluminum (TMAL) partial hydrolysate (methylaluminoxane (MAO)) in NMP (N-methyl-2-pyrrolidone) solvent, i.e., the MAO / NMP solution. In this case, NMP (N-methyl-2-pyrrolidone) was manufactured by Kishida Chemical Co., Ltd., and TMAL (TRIMETHYLALUMINIUM) was manufactured by Lake Materials. The turbidity of the 1% Al-MAO / NMP solution was 20.9 NTU, and its viscosity was 2.790 mPa·s. The turbidity of the NMP solution was 10.0 NTU, and its viscosity was 1.824 mPa·s.

[0031] <Manufacturing operations> The above-mentioned base film was placed on a glass substrate, and after heating the base film to 50°C, 1 ml of the above-mentioned 1% Al-MAO / NMP solution was sprayed onto the surface (top side) at a supply rate of 10 ml / h using a carrier gas (nitrogen gas) at a flow rate of 8 l / min, and simultaneously dried to obtain the insulating film for secondary batteries of this example.

[0032] (Example 2) Following the spraying and drying of the 1% Al-MAO / NMP solution in Example 1, 1 ml of NMP solvent was sprayed and dried in the same manner as the MAO / NMP solution to obtain the insulating film for secondary batteries of this example.

[0033] <Performance Evaluation> [Observation inside micropores] TEM observation was performed on the insulating film for secondary batteries mentioned above. The results are shown in Figure 2. Figure 2 is a photograph showing the results of observing the insulating film of Example 1, which was thinned by cutting it along a plane parallel to its thickness, using a transmission electron microscope (TEM). The contrast in the TEM image is due to the difference in electron beam absorption between carbon and aluminum oxide, the main components of the insulating film. Specifically, the aluminum oxide that is in close contact with the inner wall of the micropores parallel to the observation direction has a greater thickness in the depth direction (electron beam transmission direction), resulting in a stronger contrast.

[0034] As shown in Figure 2, in the insulating film of Example 1, which falls within the scope of the present invention, it is clear that aluminum oxide adheres closely to the inner walls of the micropores in a cross-section parallel to the thickness direction of the film, and that the void space of the micropores remains with a relatively large volume. Therefore, the film as a whole can exhibit good insulating properties and heat resistance, and when this insulating film is used as a separator, good flow of the electrolyte can also be ensured.

[0035] [Confirmation of metal oxides (composition)] The insulating film of Example 1 was analyzed using a scanning transmission electron microscope / energy dispersive X-ray spectroscopy (STEM / EDX), and the results are shown in Figure 3. In the STEM images, the contrast was stronger around the micropores in the film, and high concentrations of Al and O were detected in these regions by STEM / EDX analysis. Furthermore, the composition ratio of Al to O was close to 0.4, which is the composition of aluminum oxide. From this point as well, it is clear that aluminum oxide is in close contact with the inner walls of the micropores inside the insulating film.

[0036] The official name and brief description of the aforementioned STEM / EDX are as follows: ·STEM:Scanning Transmission Electron Microscope An electron probe focused to a sub-nanometer beam diameter is scanned across the sample surface, and an image is obtained by detecting electrons that have passed through the sample. ·EDX:Energy Dispersive X-ray Spectroscope By detecting characteristic X-rays generated by electron beam irradiation and spectrally analyzing their energy, elemental and compositional analyses can be performed. · STEM / EDX By performing EDX analysis while scanning with an electron beam using STEM, it becomes possible to perform elemental analysis over a wide range of areas, and this is what we will do.

[0037] [Location (depth) of metal oxides] XPS analysis was performed on the insulating film of Example 1, and the results are shown in Figure 4. The official name and brief description of XPS were as follows: ·XPS:X-ray Photoelectron Spectroscopy By irradiating a material with soft X-rays, capturing the emitted photoelectrons, and performing energy analysis, quantitative and constant-volume analysis and chemical bonding state analysis can be performed. Depth profiling can be performed by sputter etching with Ar ions.

[0038] As shown in Figure 4, in the insulating film of Example 1, it is clear that aluminum oxide is located at a depth of 1 μm or more from the film surface, indicating that it has significantly penetrated into the micropores.

[0039] (Increased film thickness) In each example of the insulating film, the thickness increase from the base film was approximately 1% or less in Example 1 and approximately 1% or less in Example 2, indicating that there was almost no increase in thickness. [Explanation of symbols]

[0040] 1. Insulating film 10. Base film 10p micropore 10f surface of the base film 10b Back surface of the base film 20 Metal Oxides

Claims

1. An insulating film for secondary batteries, comprising a polyolefin substrate film having numerous micropores, wherein at least some of the micropores contain a metal oxide, In manufacturing an insulating film for secondary batteries in which the above metal oxide is present on the inner wall of the above micropores, A method for producing an insulating film for secondary batteries, comprising spraying a non-aqueous solution containing an alkyl compound corresponding to the metal of the above-mentioned metal oxide and / or a partial hydrolysate of the alkyl compound onto the above-mentioned base film and drying the solution at room temperature to 80°C.

2. A method for producing an insulating film for a secondary battery according to claim 1, wherein the alkyl compound corresponding to the metal of the above metal oxide is an alkylaluminum compound.

Images