Porous film and method for producing the same

Abstract

The present invention improves the characteristics of a porous film.　A porous film according to the present invention comprises a porous base material S and a coating film CF that is provided on the surface of the base material S; and the coating film CF has a thermal deformation of 5% or less, while containing a filler and an oxidized cellulose that has a structure wherein a primary hydroxyl group is oxidized by a carboxyl group. By providing the surface of the base material S with the coating film CF as described above, the mechanical strength and heat resistance of a porous film (separator) are able to be improved. Namely, by adding an oxidized cellulose as described above, alumina and the cellulose are complexed, and the heat resistance is thus able to be improved. In addition, a space between alumina and the base material S is able to be ensured by adding an oxidized cellulose, and the heat resistance is thus able to be improved, while maintaining the characteristics of a battery and without suppressing movement of Li ions in the battery.

Description

[Technical Field]

[0001] INDUSTRIAL APPLICABILITY The present invention can be suitably applied to a porous film used for battery separators and the like, and to a method for producing the porous film. [Background technology]

[0002] In recent years, the use of batteries such as lithium-ion batteries has become widespread in automobiles and infrastructure applications. Furthermore, as lithium-ion batteries continue to increase in capacity and power output, further improvements in safety are also required. Batteries such as lithium-ion batteries are separated between the positive and negative electrode materials by a porous film (porous insulator, porous resin molding) called a separator. The separator has multiple micropores, for example, large enough for lithium ions to pass through. The lithium ions move between the positive and negative electrode materials through these pores, enabling repeated charging and discharging. In this way, the separator serves to separate the positive and negative electrode materials and prevent short circuits.

[0003] Furthermore, if the temperature inside the battery becomes too high for some reason, the micropores in the separator close, stopping the movement of lithium ions and shutting down the battery. This function is called the "shutdown function."

[0004] In this way, the separator plays the role of a safety device for the battery, and in order to improve safety, it is essential to improve the mechanical strength and heat resistance of the separator.

[0005] For example, Patent Document 1 (JP 2016-183209 A) discloses a technique for forming a coating layer containing inorganic particles and a binder resin composition on at least one surface of a polyolefin resin porous film.

[0006] Furthermore, Patent Document 2 (JP 2017-068900 A) discloses a technique in which a coating liquid containing a filler and a resin binder is applied onto a polyolefin-based resin porous film, and then dried to form a coating layer. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-183209 [Patent Document 2] JP 2017-068900 A Summary of the Invention [Problem to be solved by the invention]

[0008] The present inventor has been engaged in research and development of porous films used for battery separators and the like, and has been making intensive studies on porous films with good properties. Endurance In order to improve thermal properties, we discovered a coating technology for porous films.

[0009] Other objects and novel features will become apparent from the description of this specification and the accompanying drawings. [Means for solving the problem]

[0010] The porous film disclosed in the present application is a porous film having a porous substrate and a coating film formed on the surface of the porous substrate, the coating film comprising oxidized cellulose having a structure in which primary hydroxyl groups are oxidized to carboxyl groups, and a filler, and having a thermal deformation of 5% or less. Examples of oxidized cellulose having a structure in which primary hydroxyl groups are oxidized to carboxyl groups include the oxidized cellulose shown below. Here, n, which indicates the average number of repetitions, is a number of 1 or more, preferably 10 to 10,000, and more preferably 50 to 2,000.

[0011] [ka] The method for producing a porous film disclosed in the present application includes the steps of: (a) preparing oxidized cellulose having a structure in which primary hydroxyl groups are oxidized to carboxyl groups; (b) forming a coating liquid by mixing the oxidized cellulose with an inorganic filler and a solvent; and (c) applying the coating liquid to the surface of a porous substrate to form a coating film. [Effects of the Invention]

[0012] According to the porous film disclosed in the present application, the properties of the porous film can be improved.

[0013] According to the method for producing a porous film disclosed in the present application, a porous film with excellent properties can be produced. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a cross-sectional view schematically showing the configuration of a porous film according to a first embodiment. [Figure 2] FIG. 2 is a diagram schematically illustrating an example of the configuration of a lithium ion battery using the porous film of the first embodiment. [Figure 3] FIG. 2 is a cross-sectional view schematically showing the configuration of a porous film of a comparative example. [Figure 4] FIG. 1 is a diagram showing a schematic diagram of the structure of oxidized cellulose, which has a structure in which primary hydroxyl groups are oxidized to carboxyl groups. [Figure 5] 1 is a diagram (photograph) showing the state of a sample before and after heating. [Figure 6] 1 is a SEM photograph showing the state of the samples of Example 1 and Comparative Example 2 before and after heating. [Figure 7] 1 is a graph showing the thermal shrinkage of samples after heating. [Figure 8] 1 is a graph showing the Gurley value of each sample. [Figure 9] 1 is a graph showing electric capacity during high-rate discharge. [Figure 10] 1 is a graph showing electric capacity during high-rate discharge. [Figure 11] FIG. 10 is a schematic diagram showing the configuration of a manufacturing apparatus (system) according to a second embodiment. [Figure 12] FIG. 1 is a cross-sectional view schematically showing the configuration of a gravure coating device. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, the embodiments will be described in detail with reference to examples and drawings. In all drawings for explaining the embodiments, the same reference numerals are used to designate components having the same functions, and repeated description thereof will be omitted.

[0016] (Embodiment 1) The porous film of this embodiment and its manufacturing method will be described below. The porous film of this embodiment can be used as a so-called battery separator.

[0017] [Structure description] The porous film of this embodiment has a substrate (porous substrate) S and a coating film (covering film) CF formed on the surface of the substrate S.

[0018] Fig. 1 is a cross-sectional view schematically showing the configuration of a porous film of the present embodiment, and Fig. 2 is a diagram schematically showing an example of the configuration of a lithium ion battery using the porous film of the present embodiment.

[0019] As shown in FIG. 2, the cylindrical battery has a can 6, and this can 6 houses an electrode group in which strip-shaped positive electrode material 1 and negative electrode material 3 are wound with separator 5 interposed therebetween. A positive electrode current collector tab on the upper end surface of the electrode group is joined to a positive electrode cap. A negative electrode current collector tab on the lower end surface of the electrode group is joined to the bottom of the can 6. An insulating coating (not shown) is provided on the outer periphery of the can 6. An electrolyte (not shown) is poured into the can 6. Although a cylindrical battery has been described here, there are no limitations on the battery configuration, and for example, a laminated battery may be used.

[0020] Thus, a lithium-ion battery has a positive electrode material 1, a negative electrode material 3, a separator 5, and an electrolyte, with the separator 5 disposed between the positive electrode material 1 and the negative electrode material 3. The separator 5 has many micropores. For example, during charging, that is, when a charger is connected between the positive electrode (positive electrode cap) and the negative electrode (bottom of the can 6), lithium ions inserted into the positive electrode active material are desorbed and released into the electrolyte. The lithium ions released into the electrolyte move through the electrolyte, pass through the micropores in the separator, and reach the negative electrode. The lithium ions that reach the negative electrode are inserted into the negative electrode active material that constitutes the negative electrode.

[0021] In this way, lithium ions move back and forth between the positive electrode material and the negative electrode material through the micropores (not shown) provided in the substrate S shown in FIG. 1, thereby enabling repeated charging and discharging.

[0022] Here, as shown in FIG. 1, the porous film of this embodiment has a coating film CF formed on the surface of a substrate S having a large number of micropores. The substrate S used here is not particularly limited. As the substrate S, substrates typically used in porous films for lithium-ion batteries are particularly preferred. This coating film CF is composed of a first filler (oxidized cellulose having a structure in which primary hydroxyl groups are oxidized to carboxyl groups) and a second filler (inorganic filler). FIG. 1 illustrates an example in which the first filler is TEMPO-treated cellulose (sometimes referred to as TCe) and the second filler is alumina (Al2O3), and the following description will be based on this example.

[0023] Thus, in this embodiment, by providing the coating film on the surface of the substrate S, the mechanical strength and heat resistance of the porous film (separator) can be improved. The coating film CF is not formed so as to cover all of the micropores of the substrate S, and the Gurley value (air permeability, [sec / 100cc]) of the substrate S (porous film, separator) on which the coating film CF is formed is 10 or more and 3000 or less, ensuring breathability. Furthermore, with the porous film (separator) of this embodiment, as described below, thermal deformation can be reduced to 5% or less. The thermal deformation of the porous film is the thermal shrinkage rate calculated by the formula (Equation 1) described below when the porous film is heat-treated at 140°C for 1 hour, before and after the treatment. Furthermore, this thermal deformation condition is preferably met when the treatment temperature is 160°C, more preferably 180°C, and even more preferably 200°C.

[0024] On the other hand, when TEMPO-treated cellulose is not added to the coating film, the surface of the substrate S can be covered with alumina, but the heat resistance is insufficient (see Comparative Example 1 described later). FIG. 3 is a cross-sectional view schematically showing the structure of a porous film of the comparative example. In contrast, by adding TEMPO-treated cellulose as in the present embodiment, the alumina and cellulose are combined, thereby improving the heat resistance. Furthermore, the addition of TEMPO-treated cellulose can ensure a gap between the alumina and the substrate S, and the movement of Li ions in the battery is not inhibited, thereby improving the heat resistance while maintaining the battery characteristics.

[0025] [Production method explanation] The manufacturing process of the porous film of this embodiment will be described below, and the configurations of the porous film and the coating film will be made clearer.

[0026] The manufacturing process of the porous film of this embodiment includes the following steps.

[0027] (a: Preparation process of substrate (porous film before coating)) A microporous film can be used as the substrate S. For example, a commercially available polyethylene microporous film can be used. Alternatively, the substrate (microporous film) S may be formed by the following steps. Note that the manufacturing process of the substrate (microporous film) S can also be referred to in embodiment 2 (FIG. 11).

[0028] For example, polyolefin (resin) and a plasticizer are melt-kneaded using a kneader, extruded into a sheet using an extruder, and then stretched using a press or stretching machine to form a film (thin film).

[0029] The polyolefin used can be one that can be processed by conventional extrusion, injection, inflation, blow molding, etc. For example, homopolymers, copolymers, and multistage polymers of ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, etc. can be used as the polyolefin. Polyolefins selected from these homopolymers, copolymers, and multistage polymers can also be used alone or in combination. Representative examples of such polymers include low-density polyethylene, linear low-density polyethylene, medium-density polyethylene, high-density polyethylene, ultra-high molecular weight polyethylene, isotactic polypropylene, atactic polypropylene, ethylene-propylene random copolymer, polybutene, ethylene-propylene rubber, etc.

[0030] In addition, because of the required performance of a high melting point and high strength, it is preferable to use a resin mainly composed of polyethylene as the base material S. Furthermore, in terms of shutdown properties, etc., it is preferable that polyethylene accounts for 50 mass% or more of the resin component. Furthermore, when an ultra-high molecular weight polyolefin having a molecular weight of 1 million or more is used, it is preferable that the amount of ultra-high molecular weight polyolefin is 50 mass parts or less per 100 mass parts of the kneaded material (resin and dispersion liquid), because if more than 50 mass parts of ultra-high molecular weight polyolefin is used, it becomes difficult to knead uniformly.

[0031] The plasticizer is added to the thermoplastic resin to improve flexibility and weather resistance. Furthermore, in this embodiment, the plasticizer is removed in a degreasing step described later, thereby forming holes in the resin molded body (film).

[0032] As the plasticizer, an organic solvent having a molecular weight of 100 to 1500 and a boiling point of 50 to 300°C can be used. Specifically, one or a mixture of several of the following can be used: linear or cyclic aliphatic hydrocarbons such as liquid paraffin, nonane, decane, decalin, paraxylene, undecane, and dodecane; mineral oil fractions having boiling points corresponding to these hydrocarbons; and phthalate esters such as dibutyl phthalate and dioctyl phthalate that are liquid at room temperature. Also usable are one or a mixture of several of the following: alcohols such as ethanol and methanol; nitrogen-based organic solvents such as NMP (N-methyl-2-pyrrolidone) and dimethylacetamide; ketones such as acetone and methyl ethyl ketone; and esters such as ethyl acetate and butyl acetate.

[0033] In the film (thin film) obtained by stretching the sheet-like kneaded material described above, the polyolefin and the plasticizer are in a phase-separated state. Specifically, the plasticizer becomes nano-sized islands. By removing these nano-sized plasticizers in the organic solvent treatment process described below, the plasticizer islands become pores, forming a porous thin film. The process of forming a separator in which a large number of fine pores are formed in the resin molded body by the plasticizer removal process is called the "wet method."

[0034] For example, the film (thin film) formed in the stretching step is immersed in an organic solvent, whereby the plasticizer in the film is extracted into the organic solvent and removed from the film (thin film).

[0035] As the organic solvent, methylene chloride, hexane, octane, cyclohexane, etc. can be used. Among them, methylene chloride is preferably used from the viewpoint of productivity.

[0036] Thereafter, the organic solvent on the surface of the film (thin film) is volatilized, and the film is heat-treated (thermal set) as necessary, whereby the substrate (microporous film) S can be obtained.

[0037] (b: Coating liquid preparation process) A) Preparation of oxidized cellulose (first filler) with a structure in which primary hydroxyl groups are oxidized to carboxyl groups In this embodiment, for example, TEMPO-treated cellulose is used. TEMPO treatment (TEMPO oxidation treatment) is a treatment by oxidation reaction using TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl) as a catalyst. For this reason, TEMPO-treated cellulose is sometimes called "TEMPO-oxidized cellulose."

[0038] Cellulose (Cell-OH) is (C 12 H 20 O 10 )n. For example, it can be represented by the following chemical structural formula (Chemical Formula 2): In this chemical structural formula, n, which indicates the average number of repetitions, is a number of 1 or more, preferably 10 to 10,000, and more preferably 50 to 2,000.

[0039] [ka] When cellulose is treated with TEMPO, the primary hydroxyl groups (-OH) of cellulose are regioselectively oxidized to C6-aldehyde groups and then to C6-carboxyl groups, which are then converted to salts (carboxylates) of the C6-carboxyl groups by alkali treatment, such as the Na salt of the C6-carboxyl groups when treated with alkali using sodium hydroxide solution, as shown below: Here, n, which indicates the average number of repeating units, is a number of 1 or more, preferably 10 to 10,000, and more preferably 50 to 2,000.

[0040] [ka] In TEMPO-treated cellulose (TEMPO-oxidized cellulose, TCe), the Na salts of the C6 carboxyl groups are arranged, and they ionize in water, causing repulsion (electrostatic repulsion, osmotic pressure). Therefore, if the Na salts are arranged at a high density, they can be dispersed in a very fine state. In the above chemical structural formula, only the left side of the two glucose residues is shown as a COONa group for the substituent at the carbon atom at the C6 position. This is an example showing that some of the glucose residues have a structure oxidized to a carboxyl group, but it does not mean that the cellulose is composed solely of this structural unit. In other words, the above chemical structural formula also includes cases where both glucose residues have a structure in which the primary hydroxyl groups are oxidized to a carboxyl group, and cases where neither glucose residue has a structure in which the primary hydroxyl groups are oxidized to a carboxyl group. As a whole, the oxidized cellulose only needs to have a structure in which some of the primary hydroxyl groups are oxidized to a carboxyl group. Furthermore, not all of the carboxyl groups need to be Na salts; cases in which some of them are Na salts are also included.

[0041] Figure 4 is a schematic diagram showing the structure of TEMPO-oxidized cellulose. In liquid, TEMPO-oxidized cellulose becomes fine cellulose with a width (minor axis, shorter length) W of 1000 nm or less and a length L of 500 μm or less, and more preferably a width W of 500 nm or less and a length L of 3 μm or less. Note that cellulose with a width W of about 4 nm and a length L of about 2 μm has also been confirmed.

[0042] In the process of introducing carboxyl groups into the hydroxyl groups of a cellulose-based material (oxidation process), a reaction is carried out in water using, for example, TEMPO or sodium bromide as a catalyst and hypochlorous acid as an oxidizing agent. During the reaction, a neutralizing agent, such as sodium hydroxide, is added in an amount sufficient to maintain a desired pH, thereby obtaining TEMPO-oxidized cellulose. If a 100% neutralization rate is defined as a state in which the carboxyl groups on the cellulose surface are completely neutralized, a higher neutralization rate of TEMPO-oxidized cellulose for use in a coating solution is preferable from the perspective of repulsive defibration, while a lower neutralization rate tends to be preferable from the perspective of its impact on battery performance.

[0043] As mentioned above, the use of TEMPO-oxidized cellulose forms a dense network within the coating layer, preventing the peeling of alumina particles, which is important for maintaining heat resistance. Furthermore, the binder (resin) used to bond the coating layer to the film is compounded with cellulose, improving the heat resistance of the binder (resin).

[0044] Although the Na salt of the C6-carboxyl group is exemplified above, other counter ions (X + The counter ion is preferably an alkali metal ion, for example, K + Here, n, which indicates the average number of repetitions, is a number of 1 or more, preferably 10 to 10,000, and more preferably 50 to 2,000.

[0045] [ka] In the comparative examples described later, cellulose that has been subjected to a hydrophobic treatment (hydrophobized cellulose, SA Ce) was used. For example, cellulose (C 12 H 20 O 10 ) nSome of the hydroxyl groups can be substituted with hydrophobic groups (for example, -R-OH (R represents a divalent hydrocarbon group) such as -CH2OH) to make the polymer hydrophobic.

[0046] Furthermore, as the raw material for cellulose, those derived from plant fibers such as pulp or those derived from animal fibers such as sea squirts may be used.

[0047] B) Defibration processing The TEMPO-oxidized cellulose is supplied in a powder state, and it is preferable to further pulverize this powder before use. For example, the TEMPO-oxidized cellulose can be subjected to mechanical treatment using a stone mill, a head-on collision method, a ball mill, or the like.

[0048] The TEMPO-treated cellulose may be subjected to a defibration treatment (refining treatment), or the cellulose may be subjected to a defibration treatment (refining treatment) and then a TEMPO treatment. The defibration treatment (refining treatment) can produce TEMPO-oxidized cellulose having the width W and length L described above.

[0049] C) Mixing process (stirring process) The coating solution is prepared by mixing the aforementioned TEMPO-oxidized cellulose, inorganic filler (second filler), and solvent (dispersion medium). The TEMPO-oxidized cellulose content is preferably 0.3% by mass or more and 5% by mass or less, and more preferably 0.5% by mass or more and 2% by mass or less, based on the total solid content of the coating solution.

[0050] There are no limitations on the method for preparing the coating solution, but to prevent aggregation of the cellulose and ensure uniform mixing, the coating solution is prepared by mixing and stirring the TEMPO-oxidized cellulose, inorganic filler, and solvent. Stirring methods that can be used include, for example, rotating blades attached to a shaft using a motor or vibrating using ultrasound. The coating solution may be prepared (mixed and stirred) under reduced pressure to reduce the entrapment of air bubbles in the coating solution.

[0051] The inorganic filler (second filler) is not particularly limited. For example, alumina, nanosilica, carbon nanotubes, talc, glass fiber, etc. can be used. In particular, alumina is preferred because it is less likely to chemically react with the electrolyte and has stable physical properties and manufacturing techniques. There are no limitations on the particle shape of the alumina; for example, spherical or flat shapes can be used. The average particle size (diameter) of alumina is preferably 500 nm or more and 1000 nm or less. The average particle size can be determined by a laser diffraction scattering method. Furthermore, alumina having an average particle size larger than the average pore size of the microporous membrane used in the substrate S is preferably used. Alumina with different average particle sizes may also be mixed. Furthermore, alumina may contain impurity elements (e.g., Si, Fe, Na, Mg, Cu), and the amounts of Si, Fe, Na, Mg, and Cu are preferably 400 ppm or less, 300 ppm or less, 200 ppm or less, 100 ppm or less, and 100 ppm or less.

[0052] The solvent is not particularly limited, but in consideration of drying after application to the substrate, it is preferable to use water, ethanol, methanol, or the like, which has a boiling point of 100° C. or less.

[0053] Other additives that may be added include a thickener (for example, carboxymethyl cellulose), a binder (for example, acrylic resin, acrylic binder), a dispersant (for example, surfactant), and the like.

[0054] Carboxymethyl cellulose is a water-soluble cellulose, and adding it to a coating solution increases viscosity and improves coatability. In addition, adding an acrylic resin improves the adhesiveness of materials in the coating solution.

[0055] Adding a surfactant improves the wettability of the coating solution to the substrate S. In particular, when using a substrate made of polyethylene or polypropylene, the free energy is large, so it is preferable to add a surfactant. The amount of surfactant added is preferably 0.001% by mass or more and 5% by mass or less of the solid components of the coating solution.

[0056] Here, the solid components of the coating liquid mentioned above refer to the total amount of the TEMPO-oxidized cellulose, inorganic filler (second filler), thickener, binder, and dispersant contained in the coating liquid.

[0057] D) Coating process onto substrate The coating liquid is applied to the surface of the substrate S described in (a: Preparation step of substrate (porous film before coating)). There are no limitations on the coating method, and for example, a bar coater, lip coater, gravure coater, etc. can be used. After coating, the coating liquid is dried to form a coating film on the surface of the substrate S.

[0058] As described above, according to the present embodiment, the heat resistance of the porous film can be improved by adding TEMPO-oxidized cellulose to the coating solution. Furthermore, the battery characteristics can be improved by using a porous film having a TEMPO-oxidized cellulose-containing coating formed thereon as a separator.

[0059] (Example) Examples of the porous film (separator) of this embodiment and a battery using the same will be described below.

[0060] [Example A] 1: Formation process of the substrate (porous film before coating) 30 parts by mass of ultra-high molecular weight polyethylene (Mitsui Hi-Zex Million 030S (Mitsui Chemicals)) and 70 parts by mass of liquid paraffin (P-350P (Moresco)) were melted and mixed in a kneader (tabletop twin-screw kneader), and then extruded into a sheet from a T-die. The kneading temperature was 180°C, and the kneading time was 12 minutes. The kneader (tabletop twin-screw kneader) is a device that kneads the input raw materials using a shaft with two intermeshing screws, and the rotation speed of the shaft (screw) was 80 rpm.

[0061] The kneaded mixture was then processed using a press machine, and the edges of the pressed sheet were held with pins (clips) in a stretching machine and simultaneously biaxially stretched to form a film. The film thickness was approximately 25 μm. Here, simultaneous biaxial stretching refers to simultaneously stretching in a first direction (longitudinal direction, MD) and a second direction (transverse direction, TD) intersecting the first direction. The stretching conditions in the first direction (longitudinal direction, MD) were a stretching temperature of 110°C, a stretch ratio of 6, and a stretching speed of 3000 mm / min. The stretching conditions in the second direction (transverse direction, TD) were a stretching temperature of 110°C, a stretch ratio of 7, and a stretching speed of 3000 mm / min.

[0062] The film was then immersed in methylene chloride to remove the liquid paraffin. The film was then stretched again in a transverse stretching device to obtain a substrate with micropores (a porous PE film before coating). The thickness of the substrate was approximately 16 μm.

[0063] 2: Coating liquid preparation process A) Preparation of cellulose (first filler) TEMPO-oxidized cellulose (Na salt) was prepared. This TEMPO-oxidized cellulose was in powder form with an average particle size of approximately 10 μm and was produced using pulp derived from coniferous trees as the raw material.

[0064] Hydrophobized cellulose (SA cellulose) was prepared as a comparative example of cellulose. Specifically, Ceolus FD101 (manufactured by Asahi Kasei Chemicals Corporation) with an average particle size of 50 μm was added to a kneader (a benchtop twin-screw kneader) with succinic anhydride (SA) as an additive at a mass ratio of CeNF:succinic anhydride (SA) = 86.5:13.5, and the mixture was kneaded at 130°C for 15 minutes. Unreacted succinic anhydride (SA) remained in the kneaded mixture, so the mixture was washed with acetone and dried to prepare hydrophobized cellulose.

[0065] B) Defibration processing Two parts by mass of TEMPO-oxidized cellulose powder (40 g) was added to water and passed through a defibration treatment device (Masscolloider, manufactured by Masuko Sangyo Co., Ltd.) 10 times to obtain a dispersion of defibrated TEMPO-oxidized cellulose. A dispersion was also prepared in the same manner for hydrophobic cellulose. This defibration treatment results in the cellulose being refined (nanofiberized). Therefore, defibrated TEMPO-oxidized cellulose can also be called TEMPO-oxidized cellulose nanofibers. Hydrophobic cellulose was also defibrated in the same manner.

[0066] C) Mixing process A dispersion of defibrated TEMPO-oxidized cellulose, carboxymethyl cellulose (CMC), acrylic resin (binder), octylphenol ethoxylate (0.1% by mass) as a surfactant, and water were mixed, and then high-purity alumina (Sumitomo Chemical Co., Ltd.) was added. This mixture was stirred at 1000-2000 rpm for 1-60 minutes using a mixer (Thinky Corporation, ARE310) to obtain a coating solution. <2> The coating solution was prepared with a solvent (water) so that the ratio of solid components (cellulose, CMC, binder, surfactant, alumina) was 40 mass %. The same procedure was repeated for the dispersion of defibrated hydrophobic cellulose to obtain a coating solution. <3> In addition, a coating solution containing high-purity alumina, carboxymethyl cellulose, acrylic resin, surfactant, and water, to which no cellulose dispersion was added, was prepared. <1> This coating liquid was prepared. <1> The solid content of each coating solution is shown in Table 1.

[0067] [Table 1] In this example, the proportion of the solid components (cellulose, CMC, binder, surfactant, alumina) was set to 40 mass %, but this proportion can be adjusted to within the range of 20 mass % to 45 mass %.

[0068] 3: Coating process onto the substrate The above coating liquid (coating liquid) is applied to the surface of the substrate (porous film made of PE) described in "1. Formation process of substrate (porous film before coating)". <1> <2> <3> The substrate was coated with either one of the coating solutions (either of the above) on one or both sides using a bar coater and dried at 80°C for 1 hour. The coating thickness was 4 μm on one side (8 μm on both sides). In this way, a porous film (separator) with a coating layer formed thereon was formed. Table 2 shows the combinations of the coating solution used and the coating conditions (one or both sides). As a comparative example, only a substrate without a coating layer formed thereon was prepared.

[0069] [Table 2] 4: Evaluation The obtained porous film (separator) was cut into a 50 mm×50 mm square so as to be perpendicular to the MD direction and the TD direction.

[0070] The obtained porous film (separator) was then left for 1 hour in a drying oven (AS ONE, AVO-250NB) heated to 100-200°C, and the thermal shrinkage (thermal deformation) was calculated from the dimensional change of the porous film before and after heating using the following formula (1). The changes in the porous film before and after heating were examined by visual observation and SEM. The thermal shrinkage (thermal deformation) was also calculated from the dimensional change of the porous film before and after drying using the following formula (1).

[0071] The Gurley value of the resulting porous film (separator) was also measured.

[0072]

number

[0073] Example 1 Coating fluid <2> The sample (porous film, separator) was prepared by coating both sides of the substrate with the above mixture, and evaluated. The coating thickness was 4 μm on one side (8 μm on both sides).

[0074] Example 2 Coating fluid <2> The samples (porous film, separator) were formed and evaluated by coating one side of the substrate with the above mixture. The coating thickness was only 4 μm on one side.

[0075] (Comparative Example 1) The substrate described in "1. Process for forming substrate (porous film before coating)" was evaluated without being coated.

[0076] (Comparative Example 2) Coating fluid <1> The sample (porous film, separator) was prepared by coating both sides of the substrate with the above mixture, and evaluated. The coating thickness was 4 μm on one side (8 μm on both sides).

[0077] (Comparative Example 3) Coating fluid <3> The sample (porous film, separator) was prepared by coating both sides of the substrate with the above mixture, and evaluated. The coating thickness was 4 μm on one side (8 μm on both sides).

[0078] 5:Result 5 is a diagram (photograph) showing the state of the sample before and after heating. For each example and comparative example, photographs are shown of the initial state (unheated state), after heating at 160°C, and after heating at 200°C.

[0079] In the uncoated Comparative Example 1, the sample was melted by heating at 160°C and 200°C, and only a small amount of film component remained.

[0080] Coating liquid consisting of high-purity alumina, carboxymethyl cellulose, acrylic resin, surfactant and water <1> In Comparative Example 2 using the above, thermal deformation was observed when heated at 160°C and 200°C.

[0081] Coating fluid containing hydrophobic cellulose <3> In Comparative Example 3 using the above, the thermal deformation due to heating was improved compared to Comparative Example 2, but breakage was confirmed when heated at 200°C.

[0082] In contrast, the coating solution containing TEMPO-oxidized cellulose <2> In Example 1, which used the material, almost no thermal deformation due to heating was observed, and it was found that the material had high heat resistance.

[0083] In addition, as shown in Example 2, the coating solution containing TEMPO-oxidized cellulose <2> When using the coating agent, it was found that the coating did not melt as in Comparative Example 1, even when it was coated on only one side, and the heat resistance was improved.

[0084] Figure 6 shows SEM photographs of the samples of Example 1 and Comparative Example 2 before and after heating. The photographs show the initial (unheated) state and the state after heating at 200°C for Example 1 and Comparative Example 2. Observation was performed using an SEM (Carl Zeiss, SUPER 55VP) at an accelerating voltage of 3 kV and a magnification of 10,000.

[0085] In Comparative Example 2, the alumina was buried after heating at 200° C., whereas in Example 1, alumina particles were still visible.

[0086] FIG. 7 is a graph showing the heat shrinkage of samples after heating. For each example and comparative example, the heat shrinkage after heating at 120°C, 140°C, 160°C, 180°C, and 200°C is shown. The horizontal axis of the graph represents heating temperature, and the vertical axis represents heat shrinkage [%]. At each temperature on the horizontal axis, the graphs are arranged in the order of Example 1, Example 2, Comparative Example 1, Comparative Example 2, and Comparative Example 3 from left to right. However, at 120°C, data for only the comparative examples is shown. Furthermore, a heat shrinkage of Max (20%) means that the heat shrinkage is 20% or more.

[0087] As shown in FIG. 7, it was found that in Example 1, the thermal shrinkage rate was smaller than in Comparative Examples 1 to 3, and the thermal deformation was 5% or less.

[0088] Fig. 8 is a graph showing the Gurley values ​​of each sample. As shown in Fig. 8, the Gurley value (air permeability, [sec / 100cc]) of the substrate without a coating layer (Comparative Example 1) was about 260, and the Gurley values ​​of Examples 1 and 2 were equivalent to this. From this, it can be seen that the coating solution containing TEMPO-oxidized cellulose <2> It was found that the coating layer using cellulose has pores and does not hinder the movement of Li ions. <1> The coating layer using the above (Comparative Example 2) showed a poor Gurley value.

[0089] Thus, in Example 1, an improvement in heat resistance was confirmed. This is thought to be due to the aforementioned composite of alumina and cellulose. Furthermore, in Example 1, it was confirmed that the Gurley value was similar to that of the substrate before coating, which is thought to be because the addition of TEMPO-treated cellulose ensured a gap between the alumina and the substrate S. As a result, as shown in Example B below, it is thought that the movement of Li ions in the battery is not inhibited, and the heat resistance can be improved while maintaining the battery characteristics.

[0090] [Example B] 1: Formation process of the substrate (porous film before coating) A substrate having micropores (a porous PE film before coating) was formed in the same manner as in Example A. The thickness of the substrate was about 20 μm.

[0091] 2: Coating liquid preparation process Coating solutions a to e were prepared in the same manner as in Example A. The solid component ratios of the coating solutions are shown in Table 3. Here too, the coating solutions were prepared with a solvent (water) so that the ratio of solid components (cellulose, CMC, binder, surfactant, alumina) was 40 mass %. In Example B, a surfactant was added to the coating solution. Triton X (nonionic surfactant) manufactured by Nacalai Tesque, Inc. was used as the surfactant.

[0092] [Table 3] 3: Coating process onto the substrate In the same manner as in Example A, the coating solutions a to c were applied to both sides of the surface of the substrate (porous PE film) using a bar coater, and then dried at 80°C for 1 hour to form samples (porous film, separator). The coating thickness (total thickness of both sides), coating amount (mg / cm 2 ) are as shown in Table 4. Here, the sample using coating liquid a is shown as Coat a, the sample using coating liquid b is shown as Coat b, and the sample using coating liquid c is shown as Coat c. As a comparative example, a substrate without any coating layer (Uncoated) was prepared.

[0093] [Table 4] 4: Evaluation The Gurley values ​​of the obtained samples (porous film, separator) were measured, and the results are shown in Table 4.

[0094] Furthermore, the wettability of coating solutions a to e to the substrate was examined. Coating solutions a, b, and c had good wettability to the substrate. Coating solutions d and e had poor wettability to the substrate.

[0095] In addition, test batteries were fabricated using Coats a to c, and their characteristics were evaluated. A laminate in which a positive electrode, a separator, and a negative electrode were laminated in this order was housed in a case, and after injecting an electrolyte into the inside, the case was sealed with a part of the electrode exposed, and a test battery (lithium ion secondary battery) was fabricated. For the positive electrode, a positive electrode plate (Al foil) was coated with LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 The battery was prepared by coating a cathode mixture containing O2 (NCM111) on the cathode and drying it. The anode was prepared by coating a cathode mixture containing graphite on a Cu foil and drying it. The electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF6) at a concentration of 1.0 mol / L in a mixed solvent of EC (ethylene carbonate) and DEC (diethyl carbonate) in a volume ratio of 1:1, with 1% by mass of VC (vinylene carbonate) added as an additive. The battery capacity was approximately 2 mAh / cell.

[0096] The test battery was charged to 4.2 V at 30°C under CV 1 h conditions and then discharged to -2.8 V, a cycle of 50 times, and the battery capacity after each charge / discharge cycle was evaluated. To compare the performance differences between the coated samples over a short cycle period, the samples were charged and discharged at C-rates of 0.1 to 5 C for cycles 1 to 20. Figures 9 and 10 are graphs showing the electrical capacity during high-rate discharge. The vertical axis represents battery capacity, and the horizontal axis represents cycles. The vertical axis on the right represents C-rate. Figure 9 shows the measurement results for the positive electrode surface, and Figure 10 shows the measurement results for the negative electrode surface.

[0097] As shown in FIGS. 9 and 10, Coats a to c using Coating Liquids a, b, and c exhibited battery characteristics comparable to those of the substrate alone (Uncoated) without any coating layer formed thereon.

[0098] Furthermore, detailed analysis of the results revealed that in the measurement results for the positive electrode surface (Fig. 9), the battery capacity was greatest in the order of Coat c, Coat a, Uncoated, and Coat b, while in the measurement results for the negative electrode surface (Fig. 10), the battery capacity was greatest in the order of Coat c, Uncoated, Coat a, and Coat b. This revealed that, of Coats a to c, Coat b was the most useful as a separator.

[0099] In the coating solution b used for this coat b, the TEMPO-oxidized cellulose (TCe) content was 0.93 mass% relative to the total amount of solid components, and 1 mass% relative to the alumina. Thus, the TEMPO-oxidized cellulose (TCe) content is preferably 0.5 to 1.5 mass%, more preferably 0.7 to 1.3 mass%, relative to the alumina.

[0100] Furthermore, in coating solution b, the ratio of carboxymethyl cellulose (CMC) to TEMPO-oxidized cellulose (TCe) is 3: 1. Thus, when the ratio of carboxymethyl cellulose (CMC) to TEMPO-oxidized cellulose (TCe) is 1:a, it is preferable that a is less than 1, and more preferably a is 0.2 or more and 0.4 or less.

[0101] [Example C] In this example, TEMPO-oxidized cellulose with different neutralization rates was prepared, and similarly to Example A, "B) defibration treatment" and "C) stirring treatment" were performed, followed by coating on a substrate.

[0102] Here, sodium hydroxide was used as the neutralizing agent, and TEMPO-oxidized cellulose with a neutralization rate of 100% and a neutralization rate of 50% was prepared.

[0103] The heat shrinkage (thermal deformation) of substrates coated with coating solutions using TEMPO-oxidized cellulose with different neutralization rates was calculated in the same manner as in Example A. The heat shrinkage was 4% for the substrate with a neutralization rate of 100%, and 5% for the substrate with a neutralization rate of 50%. This indicates a tendency for the heat shrinkage to be smaller and the heat resistance to be higher as the neutralization rate increases. However, even for the substrate with a neutralization rate of 50%, the heat shrinkage was only about 5%. Considering the results of Examples A and B above as well as other factors, it is believed that the substrate with a neutralization rate of 50% also has sufficient heat resistance and is effective for use as a coating solution for separators.

[0104] (Embodiment 2) 11 is a schematic diagram showing the configuration of a manufacturing apparatus (system) according to this embodiment. In this embodiment, a separator manufacturing process using the manufacturing apparatus (system) will be described.

[0105] For example, a plasticizer (liquid paraffin) and a polyolefin (e.g., polyethylene) are fed into the raw material supply section of the twin-screw kneading extruder (S1) in Fig. 11, and the plasticizer and polyolefin are kneaded in the kneading section. The kneading conditions are, for example, 180°C, 12 minutes, and a shaft rotation speed of 100 rpm.

[0106] The kneaded material (molten resin) is conveyed from the discharge section to the T-die S2, and the molten resin is extruded from the slit of the T-die S2 while being cooled in the raw material cooling device S3, thereby forming a thin-film resin molded body.

[0107] Next, the thin-film resin molded body is stretched in the longitudinal direction by a first longitudinal stretching device S4, and further stretched in the transverse direction by a first transverse stretching device S5.

[0108] The stretched thin film is then immersed in an organic solvent (e.g., methylene chloride) in extraction tank S6. In the stretched thin film, the polyolefin (e.g., polyethylene) and the plasticizer (paraffin) are phase-separated. Specifically, the plasticizer (paraffin) forms nano-sized islands. This nano-sized plasticizer (paraffin) is removed (degreased) by the organic solvent (e.g., methylene chloride) in extraction tank S6. This allows the formation of a porous thin film.

[0109] Thereafter, the thin film is further stretched in the transverse direction in the second transverse stretching device S7, while being dried and heat-set to relieve internal stress generated during stretching. Next, the thin film transported from the second transverse stretching device S7 is wound up by a winding device S8.

[0110] In this manner, a porous thin film (substrate of the first embodiment) can be produced. Here, for example, a gravure coating device (S7') shown in FIG. 12 is installed between the second transverse stretching device S7 and the winding device S8. FIG. 12 is a cross-sectional view showing a schematic configuration of the gravure coating device. This gravure coating device has two gravure rolls R. The gravure rolls R have, for example, a plurality of oblique recesses, and are arranged so that some of them are immersed in the coating liquid CL. By rotating the gravure rolls R, the coating liquid CL is applied to the substrate S while the coating liquid is held in the oblique recesses.

[0111] By using the coating liquid CL described in embodiment 1 as this coating liquid CL, it is possible to form a coating film on both sides of the substrate. If necessary, a drying device for the coating liquid or the like can be appropriately incorporated.

[0112] In this way, high-performance separators can be produced efficiently using the apparatus shown in FIGS.

[0113] The invention made by the inventor has been specifically described above based on the embodiments and examples, but it goes without saying that the present invention is not limited to the above embodiments or examples, and various modifications are possible within the scope of the gist of the invention. [Explanation of symbols]

[0114] 1. Cathode material 3. Anode material 5 Porous film (separator) 6 cans CF coating film CL coating liquid R Gravure Roll S base material S1 Twin-screw Extruder S2 T-die S3 Fabric cooling device (CAST device) S4 Longitudinal stretching device (MD device) S5 1st lateral stretching device (1st TD device) S6 extraction tank S7 2nd lateral stretching device (2nd TD device) S7' Gravure Coating Machine S8 Winder

Claims

1. A porous film having a porous substrate and a coating film provided on the surface of the porous substrate, the coating film comprises oxidized cellulose having a structure in which a primary hydroxyl group is oxidized to a carboxyl group, an inorganic filler, and carboxymethyl cellulose; the ratio of the carboxymethyl cellulose to the oxidized cellulose is 1:a, where a is 0.2 or more and 0.4 or less; the inorganic filler is alumina, A porous film having a thermal deformation of 5% or less.

2. The porous film according to claim 1, A porous film, wherein the amount of the oxidized cellulose added is 0.5% by mass or more and 1.5% by mass or less relative to the alumina.

3. The porous film according to claim 1, The porous film, wherein the porous substrate is a polyolefin containing 50% by mass or more of polyethylene.

4. The porous film according to claim 1, The coating film is a porous film containing a resin as a binder.

5. The porous film according to claim 4, The coating film is a porous film containing a surfactant.

6. The porous film according to claim 1, A porous film, wherein the oxidized cellulose has a width of 1000 nm or less.

7. A method for producing a porous film, comprising: (a) preparing oxidized cellulose having a structure in which primary hydroxyl groups are oxidized to carboxyl groups; (b) forming a coating liquid by mixing the oxidized cellulose, an inorganic filler, carboxymethyl cellulose, and a solvent; (c) applying the coating liquid to the surface of a porous substrate to form a coating film; and the inorganic filler is alumina, the ratio of the carboxymethyl cellulose to the oxidized cellulose is 1:a, where a is 0.2 or more and 0.4 or less; The thermal deformation of the porous film is 5% or less, The method for producing a porous film, wherein the oxidized cellulose is the following compound: 【Transformation 5】

8. The method for producing a porous film according to claim 7, A method for producing a porous film, wherein the amount of oxidized cellulose added is 0.5% by mass or more and 1.5% by mass or less relative to the alumina.

9. The method for producing a porous film according to claim 7, The method for producing a porous film, wherein the porous substrate is a polyolefin containing 50% by mass or more of polyethylene.

10. The method for producing a porous film according to claim 7, The method for producing a porous film, wherein the coating liquid contains a resin as a binder.

11. The method for producing a porous film according to claim 10, The method for producing a porous film, wherein the coating liquid contains a surfactant.

12. The method for producing a porous film according to claim 7, The method for producing a porous film, wherein the oxidized cellulose has a width of 1000 nm or less.

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