Coating fluid, porous film, and lithium-ion battery

Abstract

To improve a characteristic of a porous film used for a separator of a lithium ion battery.SOLUTION: A porous film (a separator) of the present invention, is a porous film having a porous base material S and a coating film CF provided to a front surface of the porous base material. The coating film includes an alkali silicate and a first filler. The first filler is formed by an inorganic particle, and contains 0.05 weight% or more of the alkali silicate to the inorganic particle. The inorganic particle contains a material selected from nano-silica, micro-silica, carbon nano-tube, talc, alumina, boehmite, an aluminum hydroxide, and a glass fiber, and includes a second filler (a hydrophilic group of cellulose is replaced to a hydrophobic group) of 0.05 weight% or more to the inorganic particle. By using the porous film, a battery characteristic such as a heat resistance, a cycle characteristic or the like can be improved.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a coating liquid for a porous film used for battery separators and the like, and can be used in a coating liquid, a porous film, and a lithium ion battery. [Background technology]

[0002] The use of secondary batteries has expanded from electronic devices to automobiles and large-scale energy storage systems, and lithium-ion batteries (secondary batteries) are attracting particular attention because they can be made small and lightweight and have high energy density.

[0003] For example, Patent Document 1 discloses a non-aqueous binder for electrodes of lithium ion batteries that is a composite of cellulose nanofibers and a thermoplastic fluorine-based resin, in which the cellulose nanofibers are cellulose with a fiber diameter of 0.002 μm or more and 1 μm or less, a fiber length of 0.5 μm or more and 10 mm or less, and an aspect ratio (cellulose nanofiber length / cellulose nanofiber diameter) of 2 or more and 100,000 or less. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2019 / 064583 Summary of the Invention [Problem to be solved by the invention]

[0005] As the capacity and power output of lithium-ion batteries increase, further improvements in safety are required. The present inventors have been engaged in research and development of porous films used for battery separators, etc., and have been actively studying porous films with good properties.

[0006] As will be explained in detail later, the separator placed between the positive and negative electrodes of the battery has multiple micropores large enough for lithium ions to pass through. The lithium ions move between the positive and negative electrodes through these pores, enabling repeated charging and discharging. The separator separates the positive and negative electrodes to prevent short circuits. 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 (shutdown function).

[0007] 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.

[0008] Therefore, it is desirable to investigate a technology that improves the mechanical strength and heat resistance of the separator while maintaining the battery characteristics.

[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] A coating liquid for a porous film disclosed in one embodiment of the present application contains inorganic particles and 0.05% by weight or more of an alkali silicate relative to the inorganic particles.

[0011] The porous film disclosed in one embodiment of the present application has a porous substrate and a coating film formed on the surface of the porous substrate, and the coating film contains inorganic particles and 0.05% by weight or more of an alkali silicate relative to the inorganic particles.

[0012] A lithium ion battery disclosed in one embodiment of the present application is a lithium ion battery including a positive electrode, a negative electrode, a separator, and an electrolyte, and has the porous film as the separator. [Effects of the Invention]

[0013] According to the coating liquid for a porous film disclosed in one embodiment of the present application, the properties of the porous film can be improved.

[0014] According to the porous film disclosed in one embodiment of the present application, the properties of the porous film can be improved.

[0015] According to the lithium ion battery disclosed in one embodiment of the present application, the characteristics of the lithium ion battery can be improved. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a cross-sectional view showing the configuration of a porous film according to a first embodiment. [Figure 2] 1 is a diagram schematically illustrating the internal configuration of a lithium ion battery using the porous film of the first embodiment. [Figure 3] 1 is a diagram schematically illustrating a configuration example of a lithium ion battery according to a first embodiment. [Figure 4] FIG. 1 is a diagram showing the steps for preparing a dispersion of hydrophobized cellulose nanofibers (CeNFs). [Figure 5] FIG. 1 shows the hydrophobization of cellulose. [Figure 6] FIG. 1 is a perspective view showing a coating step of a coating liquid using a bar coater. [Figure 7] FIG. 1 is a diagram showing the results of heat shrinkage test 1. [Figure 8] FIG. 1 is a diagram showing the results of heat shrinkage test 1. [Figure 9] FIG. 1 is a diagram showing the results of heat shrinkage test 1. [Figure 10] FIG. 1 is a diagram showing the results of heat shrinkage test 1. [Figure 11] FIG. 10 is a diagram showing the results of heat shrinkage test 2. [Figure 12] FIG. 1 is a diagram showing the results of SEM observation. [Figure 13] FIG. 1 is a diagram showing the results of SEM observation. [Figure 14] FIG. 1 is a diagram showing the results of SEM observation. [Figure 15] FIG. 1 is a diagram showing the results of SEM observation. [Figure 16] FIG. 1 is a diagram showing the results of SEM observation. [Figure 17] FIG. 1 is a diagram showing the results of SEM observation. [Figure 18] 1 is a graph showing the results of a puncture test of porous films (separators) 1, 2, and A. [Figure 19] 1 is a graph showing the Gurley values ​​of porous films (separators) 1, 2, and A. [Figure 20] FIG. 1 is a graph showing cycle characteristics at 30° C. [Figure 21] FIG. 1 is a graph showing cycle characteristics at 60° C. [Figure 22] 1 is a diagram (graph) showing the battery resistances of batteries 1, 2, A, and B. [Figure 23] FIG. 10 is a diagram showing the results of heat shrinkage test 2. [Figure 24] FIG. 1 is a graph showing cycle characteristics at 30° C. [Figure 25] FIG. 1 is a graph showing cycle characteristics at 60° C. [Figure 26] 10 is a diagram (graph) showing the battery resistances of batteries 3 and B. [Figure 27] FIG. 10 is a schematic diagram showing the configuration of a manufacturing apparatus according to a third embodiment. [Figure 28] FIG. 1 is a cross-sectional view schematically showing the configuration of a gravure coating device. DETAILED DESCRIPTION OF THE INVENTION

[0017] 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.

[0018] (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.

[0019] [Structure description] Fig. 1 is a cross-sectional view showing the configuration of a porous film of this embodiment. As shown in Fig. 1, the porous film of this embodiment is used as a battery separator SP and includes a substrate (porous substrate) S and a coating film (covering film) CF formed on the surface of the substrate S. As will be described later, the coating film CF includes inorganic particles as a first filler, SA-treated cellulose (sometimes referred to as SA-Ce or SACE) as a second filler, and an alkali silicate.

[0020] Fig. 2 is a diagram showing a schematic internal structure of a lithium ion battery using the porous film of this embodiment, and Fig. 3 is a diagram showing a schematic configuration example of the lithium ion battery of this embodiment. Fig. 2(A) shows the configuration of the positive electrode, Fig. 2(B) shows the configuration of the negative electrode, and Fig. 2(C) shows the configuration of the electrode group. The battery of Fig. 3 is called a coin battery.

[0021] As shown in FIG. 2(A), the positive electrode 1 comprises a current collector 1S and a positive electrode mixture layer 1M disposed thereon. As shown in FIG. 2(B), the negative electrode 2 comprises a current collector 2S and a negative electrode mixture layer 2M disposed thereon. As shown in FIG. 2(C), a lithium-ion battery includes a positive electrode 1, a negative electrode 2, and a separator SP disposed therebetween. The positive electrode 1 and the negative electrode 2 are disposed opposite each other so that the positive electrode mixture layer 1M and the negative electrode mixture layer 2M are in contact with the separator SP. A laminate (also referred to as an electrode group) of the positive electrode 1, the negative electrode 2, and the separator SP is housed together with an electrolyte in a battery container (such as a bag made of laminate film or a battery can) and sealed with the positive electrode terminal (e.g., a portion of the current collector 1S or a conductive portion electrically connected to the current collector 1S) and the negative electrode terminal (e.g., a portion of the current collector 2S or a conductive portion electrically connected to the current collector 2S) exposed.

[0022] The battery (coin-type battery) shown in FIG. 3 has a can 6, which houses an electrode group in which the aforementioned positive electrode 1 and negative electrode 2 are stacked with a separator SP interposed therebetween. The current collector 1S of the positive electrode 1 on the lower end surface of the electrode group is mounted on the can (battery can) 6. The current collector 2S of the negative electrode 2 on the upper end surface of the electrode group is disposed on the back side of a lid (battery cap) 7. Note that a washer 8 is provided between the lid (battery cap) 7 and the current collector 2S of the negative electrode 2, and they are electrically connected. In addition, a heat-resistant gasket (sealing material for fixing) is provided at the overlapping portion between the can 6 and the lid 7, sealing the electrolyte (not shown) injected inside the can 6. Note that although a coin-type battery has been described here, there are no limitations on the battery configuration; for example, a cylindrical battery or a laminated battery may be used.

[0023] Thus, a lithium-ion battery has a positive electrode 1, a negative electrode 2, a separator SP, and an electrolyte, with the separator SP disposed between the positive electrode 1 and the negative electrode 2. The separator SP has many micropores. For example, during charging, i.e., when a charger is connected between the positive electrode (bottom of the can 6) and the negative electrode (top of the lid 7), 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.

[0024] In this way, lithium ions move back and forth between the positive electrode and the negative electrode through the micropores (not shown) provided in the substrate S shown in FIG. 1, thereby enabling repeated charging and discharging.

[0025] 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. This coating film CF contains inorganic particles as a first filler, and SA-treated cellulose (sometimes referred to as SA-Ce) and alkali silicate as a second filler. SA treatment is a process for hydrophobizing some of the hydrophilic groups of cellulose.

[0026] As described above, in this embodiment, the mechanical strength and heat resistance of the porous film (separator) can be improved by providing the coating film on the surface of the substrate S. 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 200 or more and 3000 or less, ensuring breathability.

[0027] In particular, by adding an alkali silicate to the coating liquid, the electrical characteristics (output characteristics, cycle characteristics (life)) of the battery can be improved while also improving the mechanical strength and heat resistance of the porous film (separator).

[0028] [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.

[0029] The manufacturing process of the porous film of this embodiment includes the following steps.

[0030] <<1: Preparation process of the substrate (porous film before coating)>> The substrate S is not particularly limited and may be a substrate typically used for porous films for lithium ion batteries, and a microporous membrane may be used. For example, a commercially available polyethylene microporous membrane may be used.

[0031] <<2: Coating fluid preparation process>> 2-1) Preparation of the first filler In the present embodiment, inorganic particles (inorganic filler) are used as the first filler.

[0032] The inorganic particles are not particularly limited. For example, alumina, boehmite, aluminum hydroxide, nanosilica, microsilica, 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 shape of the alumina particles; 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. Alumina particles with different average particle sizes may be mixed. High-purity alumina is used. Even if impurity elements (e.g., Si, Fe, Na, Mg, Cu) are contained, it is preferred that the Si content be 400 ppm or less, Fe content be 300 ppm or less, Na content be 200 ppm or less, Mg content be 100 ppm or less, and Cu content be 100 ppm or less.

[0033] 2-2) Preparation of the second filler In this embodiment, hydrophobized (SA) cellulose (sometimes referred to as SA Ce) is used as the second filler.

[0034] (hydrophobization) Cellulose (Cellulose, Cell-OH, Ce) is (C 12 H 20 O 10 ) n For example, it can be represented by the following chemical structural formula (Chemical Formula 1): 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.

[0035] [ka] As shown in the following chemical structure (Chemical Formula 2), (C 12 H 20 O 10 ) nAlternatively, cellulose may be used in which some of the hydroxyl groups of a carbohydrate represented by the formula (I) have been substituted with groups having hydroxyl groups (for example, -R-OH (R represents a divalent hydrocarbon group) such as -CH2OH).

[0036] [ka] As can be seen from the chemical structural formulas (Chemical Formula 1 and Chemical Formula 2) above, cellulose has hydroxyl groups (hydrophilic groups). This is subjected to a hydrophobic treatment (lipophilic treatment) using a hydrophobizing agent (e.g., a carboxylic acid compound). That is, the hydroxyl groups (-OH) of cellulose are replaced with hydrophobic groups. Specifically, some of the hydroxyl groups of cellulose are esterified with a carboxylic acid compound (R-CO-OH). In other words, the hydroxyl groups (-OH) of cellulose are converted into ester bonds (-O-CO-R, carboxyl groups). Note that it is not necessary for all of the hydroxyl groups of cellulose to be replaced with hydrophobic groups; only some of them need to be replaced. An example of a cellulose esterification (hydrophobization) reaction is shown in the following reaction formula. Hydrophobized cellulose is sometimes referred to as SA-Ce.

[0037] [ka] The hydrophobizing agent is not particularly limited as long as it has a composition that can impart hydrophobic groups to the hydrophilic groups of cellulose, and for example, a carboxylic acid compound can be used. Among them, it is preferable to use a compound having two or more carboxyl groups, or an acid anhydride of a compound having two or more carboxyl groups. Among compounds having two or more carboxyl groups, it is preferable to use a compound having two carboxyl groups (dicarboxylic acid compound).

[0038] Examples of compounds having two carboxy groups include dicarboxylic acid compounds such as propanedioic acid (malonic acid), butanedioic acid (succinic acid), pentanedioic acid (glutaric acid), hexanedioic acid (adipic acid), 2-methylpropanedioic acid, 2-methylbutanedioic acid, 2-methylpentanedioic acid, 1,2-cyclohexanedicarboxylic acid, 2-butenedioic acid (maleic acid, fumaric acid), 2-pentenedioic acid, 2,4-hexadienedioic acid, 2-methyl-2-butenedioic acid, 2-methyl-2-pentenedioic acid, 2-methylidenebutanedioic acid (itaconic acid), benzene-1,2-dicarboxylic acid (phthalic acid), benzene-1,3-dicarboxylic acid (isophthalic acid), benzene-1,4-dicarboxylic acid (terephthalic acid), and ethanedioic acid (oxalic acid). Examples of acid anhydrides of compounds having two carboxy groups include dicarboxylic acid compounds such as maleic anhydride, succinic anhydride, phthalic anhydride, glutaric anhydride, adipic anhydride, itaconic anhydride, pyromellitic anhydride, and 1,2-cyclohexanedicarboxylic anhydride, as well as acid anhydrides of compounds containing multiple carboxy groups. Examples of derivatives of acid anhydrides of compounds having two carboxy groups include those in which at least some of the hydrogen atoms of the acid anhydrides of compounds having carboxy groups are substituted with substituents (e.g., alkyl groups, phenyl groups, etc.), such as dimethylmaleic anhydride, diethylmaleic anhydride, and diphenylmaleic anhydride. Among these, maleic anhydride, succinic anhydride, and phthalic anhydride are preferred because they are easily industrially applicable and easily gasified.

[0039] (Defibrillation processing) Cellulose can also be refined (nanofiberized) by defibration. Defibration (refining) can be done using chemical or mechanical methods. A combination of these methods can also be used. This type of defibration (refining) can produce cellulose with a fiber length (L) of 3 nm or more and 10 μm or less, and an aspect ratio (length L / diameter D) of 0.01 or more and 5000 or less. Cellulose fibers refined to nanometer size in this way are called cellulose nanofibers (CeNF).

[0040] The above-mentioned pulverization (nanization) of cellulose may be carried out before or after the hydrophobization treatment.

[0041] (Method for preparing hydrophobic CeNF dispersed in a solvent) The hydrophobic CeNF is preferably used in a state dispersed in a solvent in order to prevent aggregation and improve dispersibility in the coating liquid.

[0042] Figure 4 shows the process for preparing a dispersion of hydrophobic CeNFs. For example, as shown in Figure 4, cellulose (solid, e.g., powder) and succinic anhydride (solid, e.g., tablet form) are mixed at 100°C or higher. For example, they are mixed using a pressure kneader at 125°C for 20 minutes. The weight ratios of cellulose and succinic anhydride are, for example, 90 wt% (weight %, mass %) and 10 wt%.

[0043] The esterification reaction occurs by stirring under heating as described above, producing hydrophobic cellulose. After this, the mixture is washed with acetone or the like to remove unreacted succinic anhydride.

[0044] Next, the produced hydrophobic cellulose is dispersed in an aqueous solvent (water and / or alcohols, etc., here water (H2O)).

[0045] Fig. 5 is a diagram showing the hydrophobization of cellulose (hydrophobized Ce). As shown in Fig. 5, the hydrophobization replaces the hydroxyl groups (-OH) of cellulose with hydrophobic groups (-COOH).

[0046] FIG. 5 shows that eight of the ten hydroxyl groups (-OH) in cellulose have been substituted with hydrophobic groups (-COOH).

[0047] Here, the amount of hydrophobic groups (-COOH) in SA Ce can be calculated from the acid value. The acid value refers to the number of milligrams of potassium hydroxide required to neutralize the acidic components contained in 1 g of sample, and its measurement can be performed based on "JIS-K0070 Testing methods for acid value, saponification value, ester value, iodine value, hydroxyl value and unsaponifiable matter of chemical products." For example, SA Ce with an acid value of 76.5 mg / g (KOH, formula weight 56.11) has an acid value of 1.36 x 10 -3 It contains mol of hydrophobic groups (-COOH).

[0048] Next, a micronization process (defibration process, nanonization) is carried out. For example, a process using a micronization device (mass colloider, bead mill) is carried out to nanonize the cellulose in the dispersion of SA-Ce. This allows a dispersion of SA-CeNF to be obtained.

[0049] As described above, the hydrophobized cellulose may be subjected to a defibration treatment (refining treatment), or the cellulose may be subjected to a defibration treatment (refining treatment) and then subjected to a hydrophobization treatment.

[0050] 2-3) Preparation of alkali silicate The alkali silicate is preferably water glass. Water glass is an aqueous solution of silicate of an alkali metal or alkaline earth metal. For example, sodium silicate (sodium silicate, Na2O·nSiO2 (n=2 to 4)) or silicates containing Li, K, Rb, Ba, Ca, Mg, Sr, etc. instead of Na can be used. One type may be used alone, or two or more types may be used in combination.

[0051] Sodium silicate (NaO·nSiO2 (n=1-5)) can be formed by the thermal reaction of diatomaceous earth with caustic soda or by dissolving a heated melt of silica sand and soda ash in water, and many inexpensive products are available commercially. By adding this type of water glass, the amount of alumina added can be reduced, for example, to 90 wt% or less of the total solid content of the coating liquid, thereby reducing costs.

[0052] n, which represents the number of SiO2 atoms, is preferably 1 to 5, since binding strength is poor when the number is less than 1 or exceeds 5. From the viewpoint of particularly high binding strength, n is preferably 2 to 4. If binding strength is insufficient, peeling and cracking are likely to occur due to external factors such as changes in the volume of the electrode during charge and discharge and nail penetration tests.

[0053] Among the above alkali silicates, sodium silicate and potassium silicate have particularly excellent binding properties, and therefore can be used in an amount of 80 wt% or less of the total solid components of the coating liquid. On the other hand, lithium silicate does not have sufficient binding properties compared to sodium silicate and potassium silicate, but it can produce a battery with low resistance. From the viewpoint of excellent binding properties and low resistance, it is preferable to use a mixture of sodium silicate or potassium silicate and lithium silicate. If binding properties are important, it is recommended to use a larger amount of sodium silicate or potassium silicate, and if low resistance is important, it is recommended to use a larger amount of lithium silicate.

[0054] 2-4) Preparation of other additives Other additives that may be added include thickeners (e.g., carboxymethyl cellulose, xanthan gum, guar gum, alginic acid), binders (e.g., acrylic resin, acrylic binder, fluororesin), dispersants (e.g., surfactants, alcohols), etc.

[0055] Carboxymethylcellulose is a water-soluble cellulose salt, and adding it to a coating solution increases the viscosity and improves the coatability. Furthermore, adding an acrylic resin improves the adhesiveness of the materials in the coating solution. Specific examples of water-soluble cellulose salts include lithium carboxymethylcellulose, sodium carboxymethylcellulose, potassium carboxymethylcellulose, and ammonium carboxymethylcellulose. In this application, when simply referring to carboxymethylcellulose (CMC), this refers to sodium carboxymethylcellulose.

[0056] Adding a dispersing agent (surfactant, alcohols) improves wettability to the substrate S. In particular, when using a substrate made of polyethylene or polypropylene, the free energy is large and it is preferable to add a surfactant. The amount of surfactant added is preferably 0.001 wt% or more and 5 wt% or less of the solid components of the coating liquid. There are various types of surfactants, such as anionic surfactants, cationic surfactants, amphoteric surfactants, and nonionic surfactants, and although any of them can be used, nonionic surfactants are preferred because they cause less foaming.

[0057] Here, the solid components of the coating liquid mentioned above are the total amounts of the SA-CeNF (second filler), inorganic particles (first filler), thickener, binder, and dispersant contained in the coating liquid.

[0058] B) Mixing process (stirring process) A coating liquid is prepared by adding inorganic particles (first filler) and other additives to the dispersion of the SA-CeNF (second filler) described above, and further adding an alkali silicate, followed by stirring.

[0059] Although the amount of SA-CeNF (second filler) added is not particularly limited, it is preferable to add 0.05 wt% or more of the inorganic particles (first filler) from the viewpoints of coatability, cost, and battery performance. Furthermore, SA-CeNF is preferably added in an amount of 0.2 wt% to 10 wt%, more preferably 0.5 wt% to 5 wt%, based on the total solid content of the coating solution.

[0060] The amount of alkali silicate added is not particularly limited, but is preferably 0.05 wt% or more relative to the inorganic particles (first filler) from the viewpoints of coatability, cost, and battery performance. The amount of alkali silicate added is preferably 0.3 wt% or more and 20 wt% or less relative to the total solid content of the coating liquid, and more preferably 0.5 wt% or more and 15 wt% or less from the viewpoints of coatability, cost, and battery performance.

[0061] Examples of a stirring method that can be used include a method of rotating blades attached to a shaft using a motor or the like, and a vibration method using ultrasound, etc. In order to reduce the entrapment of air bubbles in the coating liquid, the preparation (mixing and stirring) of the coating liquid may be carried out under reduced pressure.

[0062] An antifoaming agent may also be added to the coating solution. When an antifoaming agent is added, it is preferably added in an amount of 0.001 wt% to 1 wt% of the total solid content of the coating solution. Adding an antifoaming agent effectively suppresses air bubbles, simplifying or eliminating the defoaming process.

[0063] As the defoaming agent, known agents such as silicone-based defoaming agents (e.g., polydimethylsiloxane), surfactant-based defoaming agents (e.g., polyethylene glycol fatty acid), and alcohols (e.g., acetylene diol) can be used, but among these, silicone-based defoaming agents are preferred from the viewpoint of being less likely to adversely affect the battery. Furthermore, the form of the defoaming agent is not particularly limited, and any type such as oil type, compound type, emulsion type, or powder type can be used.

[0064] <<3: Coating process onto substrate>> The coating liquid is applied to the surface of the substrate S described in <<1: Step of preparing substrate (porous film before coating)>>. There are no limitations on the coating method, and for example, a bar coater, lip coater, gravure coater, die coater, spray coater, screen coater, etc. can be used. After coating, the coating liquid is dried to form a coating film on the surface of the substrate S.

[0065] The drying method for the coating liquid is not particularly limited as long as it can remove the solvent or dispersion medium contained in the coating liquid. For example, a method of performing heat treatment at a temperature of 50°C or higher and 250°C or lower can be used. The heat treatment can be performed for 0.1 to 50 hours. The drying environment can be air, vacuum, rare gas, nitrogen, carbon dioxide, or hydrogen. When the drying environment contains carbon dioxide, the alkali silicate reacts with carbon dioxide to produce a carbonate compound (A2CO3) or a hydrogen carbonate compound (AHCO3) (A = at least one element selected from Li, K, Rb, Ba, Ca, Mg, and Sr). Therefore, when the environment contains carbon dioxide, the alkali silicate contained in the coating film can contain a carbonate compound (A2CO3) or a hydrogen carbonate compound (AHCO3) (A = at least one element selected from Li, K, Rb, Ba, Ca, Mg, and Sr).

[0066] A2CO3 or AHCO3 has the characteristic of having low solubility in water and decomposing to release carbon dioxide at voltages above 4.6 V. In other words, by including A2CO3 or AHCO3 in the coating film, the coating film becomes less hygroscopic and can be given the function of releasing carbon dioxide when overcharged.

[0067] The content of A2CO3 or AHCO3 in the coating film is preferably 1 wt% or more and 50 wt% or less. To adjust the content within this range, depending on the heat treatment temperature and heat treatment time, it is preferable that the ambient pressure be 0.001 to 100 MPa absolute and the carbon dioxide concentration be 400 ppm or more. More preferably, the ambient pressure is 0.01 to 50 MPa and the carbon dioxide concentration is 1000 ppm or more, and even more preferably, the ambient pressure is 0.1 to 10 MPa and the carbon dioxide concentration is 2000 ppm or more.

[0068] Furthermore, by using an alkali silicate in which A2CO3 or AHCO3 powder has been mixed in advance in the coating solution, the heat treatment process in an environment containing carbon dioxide can be eliminated, but it is preferable to have the alkali silicate absorb carbon dioxide to produce A2CO3 or AHCO3 because this significantly reduces the hygroscopicity of the coating film. If the coating film is highly hygroscopic, it may absorb moisture from the atmosphere, degrading battery performance.

[0069] When cellulose is included as a second filler in the coating solution, A2CO3 or AHCO3 can be generated uniformly. This uniformity is particularly improved with cellulose in which hydrophilic groups have been replaced with hydrophobic groups, or with oxidized cellulose in which primary hydroxyl groups have been oxidized to carboxyl groups. By uniformly distributing A2CO3 or AHCO3 throughout the coating film, it is possible to improve hygroscopicity and evenly impart the ability to release carbon dioxide upon overcharging.

[0070] As described above, according to the present embodiment, by adding a filler and an alkali silicate to the coating liquid, it is possible to improve the electrical characteristics (output characteristics, cycle characteristics (life)) of the battery while improving the mechanical strength and heat resistance of the porous film (separator).

[0071] Examples of the coating liquid, porous film (separator) and battery using the same according to this embodiment will be described below.

[0072] [Example A] 1: Preparation process of the substrate (porous film before coating) As the substrate S, for example, a commercially available polyethylene microporous film (manufactured by CS TECH, average pore size 0.06, thickness 16 μm) was used.

[0073] 2: Coating liquid preparation process A) Adjustment of water glass As mentioned above, water glass is an aqueous solution of alkali silicate, and in this example, sodium silicate (Na2O·3SiO2 (ADEKA, ESX-2)) was used.

[0074] B) Adjustment of SA-CeNF Following the process shown in Figure 4, cellulose was hydrophobized with succinic anhydride (some of the -OH groups were converted to -COOH groups). The hydrophobized cellulose in the dispersion was then defibrated to nanosize the cellulose.

[0075] C) Mixing process Carboxymethyl cellulose (CMC), acrylic resin (binder), and octylphenol ethoxylate (Triton X) as a surfactant were added to the dispersion of SA-CeNF, followed by high-purity alumina (Sumitomo Chemical Co., Ltd., average particle size 700 nm). An aqueous solvent was then added to prepare the mixture. This mixture was stirred at 2000 rpm for 30 min using a planetary centrifugal mixer (Thinky Corporation, ARE310). Finally, water glass (sodium silicate) was added, and the mixture was stirred at 25 m / s for 1 min using a thin-film rotary mixer (Primix Corporation, FILMIX) to obtain a coating solution. The solid component ratios of each coating solution (cellulose, CMC, binder, surfactant, alumina, water glass (sodium silicate)) are defined as 100 wt%. Table 1 shows the solid component ratios for each coating solution. The surfactant was added from a 10 wt% solution containing 10 wt% solids. In addition, as Comparative Example A, a coating liquid (simulating a commercially available product) was also prepared to which no water glass (sodium silicate) or SA-CeNF dispersion was added.

[0076] [Table 1] In this embodiment, the ratio (concentration) of the solid components (cellulose, CMC, binder, surfactant, alumina, water glass) to the solvent is 40 wt%, and this ratio (concentration) can be adjusted to approximately 25 wt% to 50 wt%.

[0077] 3: Coating process onto the substrate (separator manufacturing process) The substrate (porous PE film) described in "1. Preparation process of substrate (porous film before coating)" was cut into a 50 mm square to prepare substrate (test piece) S. The above-mentioned coating liquid (either coating liquid 1, 2, or A) was applied to both sides of the surface of this substrate S using a bar coater BC, and the coated film was dried for about 5 minutes with cold air from a dryer (Figure 6). Figure 6 is a perspective view showing the coating liquid application process using a bar coater. The coating liquid was applied to the back side in the same way. D is the groove depth of the bar coater BC, and the application direction is the MD direction.

[0078] In this way, porous films (separators) 1, 2, and A each having a coating layer formed thereon were formed (see Table 1). As Comparative Example B, a battery B (described below) was produced using a porous film (separator) that was only a substrate without a coating layer formed thereon.

[0079] The above coating solutions 1, 2, and A and porous films (separators) 1, 2, and A were evaluated as follows.

[0080] 4: Evaluation of coating fluid and separator (Applicability) Coating Solutions 1 and 2 had good coatability without any problems such as clumping or repellency from the substrate.

[0081] (Gurley value) The time required for 100 ml of air to pass through the porous films (separators) 1, 2, and A was measured and used as the Gurley value. The number of test pieces (N) was set to 5, and the average value was calculated.

[0082] (Heat shrinkage test 1) The porous films (separators) 1 and A were left in a vacuum dryer at 180 to 220°C for 1 to 72 hours. The state of the film was observed before and after the application of a thermal load. The thermal shrinkage rate was calculated from the dimensional change of the film before and after the application of a thermal load. Note that since the substrate (PE porous film) used for coating was a dry separator manufactured by uniaxial stretching, the thermal shrinkage rate was calculated based on the dimensional change in the machine direction (MD). In addition, when the film melted and the dimensions could not be measured, the thermal shrinkage rate was taken as 100%.

[0083] (Heat shrinkage test 2) The porous film (separator) 2 and A were left in a vacuum dryer at 300°C for 1 hour. The state of the film was observed before and after the application of a thermal load. The thermal shrinkage rate was calculated from the dimensional change of the film before and after the application of a thermal load. Note that since the substrate (PE porous film) used for coating was a dry separator manufactured by uniaxial stretching, the thermal shrinkage rate was calculated based on the dimensional change in the machine direction (MD). In addition, when the film melted and the dimensions could not be measured, the thermal shrinkage rate was taken as 100%.

[0084] (SEM observation) The surfaces of the porous films (separators) 1, 2, and A were observed with an SEM (scanning electron microscope).

[0085] (Puncture test) The porous films (separators) 1, 2, and A were fixed in a jig, and a needle having a diameter of 1 mm was pierced at a test speed of 10 mm / min, and the maximum force (N) until the needle penetrated was measured.

[0086] 5: Battery construction The positive electrode slurry was prepared by mixing the positive electrode active material (NCM (lithium nickel cobalt manganese oxide), lithium nickel-cobalt-manganese oxide (Li(Ni 0.6 Co 0.2 Mn 0.2The mixture of cellulose acetate (C12), conductive agent (acetylene black), and binder (polyvinylidene fluoride (PVdF)) was mixed in a solid ratio of 94:3:3 wt% and kneaded using a planetary mixer (Thinky, Awatori Rentaro, 2000 rpm, 15 minutes) to form a slurry. This slurry was applied to a current collector (aluminum foil with a thickness of 15 μm) using an applicator, pre-dried at 80°C, rolled using a roll press, and dried under reduced pressure (160°C, 10 hours) to form a positive electrode (positive electrode mixture layer). The capacity density was 3.10 mAh / cm. 2 It was decided.

[0087] The negative electrode slurry was prepared by mixing the negative electrode active material (artificial graphite), conductive agent (acetylene black), thickener (carboxymethyl cellulose (CMC)), and binder (styrene butadiene rubber (SBR)) in a solids ratio of 96:1:1:2 wt% and kneading it using a planetary mixer (Thinky, Awatori Rentaro, 2000 rpm, 15 minutes). This slurry was applied to a current collector (10 μm thick copper foil) using an applicator, pre-dried at 80 °C, rolled using a roll press, and dried under reduced pressure (140 °C, 10 hours) to form a negative electrode (negative electrode mixture layer). The capacity density was 3.36 mAh / cm. 2 It was decided.

[0088] Using the porous films (separators) 1, 2, and A, the positive and negative electrodes, R2032 coin-type batteries (batteries 1, 2, and A) were fabricated (see Figure 3). 1 mol / L LiPF6 (EC:DEC = 50:50 vol%, +VC 1 wt%) was used as the electrolyte.

[0089] The above batteries 1, 2, A, and B were evaluated as follows.

[0090] 6: Battery evaluation (Cycle characteristics) The cycle characteristics of the coin-type batteries (Batteries 1, 2, and A) were investigated by charging them at a current of 0.1 C for 10 cycles at 30°C and a cutoff voltage of 4.2 to 2.5 V, followed by repeated charging and discharging at 0.5 C. The cycle characteristics were also investigated at 60°C.

[0091] (battery resistance) The battery was charged at a current of 0.1 C for 10 cycles under conditions of 30°C and a cutoff voltage of 4.2 to 2.5 V, then discharged at a predetermined rate (high-rate charge / discharge test), and then discharged at a predetermined current value for 10 seconds.The battery resistance was calculated from the relationship between the battery voltage and current value after 10 seconds.

[0092] 7:Result 7 to 10 are diagrams showing the results of Heat Shrinkage Test 1. In each figure, the upper part is a photograph showing the appearance of separator A and separator 1, and the lower part is a replica of the external shapes of separator A and separator 1. Fig. 7 shows the change after 1 hour at room temperature, Fig. 8 shows the change after 1 hour at 180°C, Fig. 9 shows the change after 3 hours at 200°C, and Fig. 10 shows the change after 72 hours at 220°C.

[0093] 11 is a diagram showing the results of heat shrinkage test 2. Here, the appearance of separator A and separator 2 is shown.

[0094] As shown in Figures 7 to 10, separator A, which uses coating solution A simulating a commercially available product, was found to have low heat resistance because the film completely melted after only one hour of heat load at 180°C (Figure 8). In contrast, separator 1, which uses coating solution 1 containing added water glass and cellulose, had a heat shrinkage rate of 0.5% after 72 hours of heat load at 220°C, demonstrating high heat resistance (Figure 10).

[0095] 11, the separator 2 using the coating solution 2 containing water glass but no cellulose had a heat shrinkage of 0.5% after applying a heat load at 300°C for 1 hour, demonstrating high heat resistance. Note that the separator A was completely melted at 180°C and could not be measured.

[0096] 12 to 17 are diagrams showing the results of SEM observation: Fig. 12 and Fig. 13 are SEM photographs of separator A, Fig. 14 and Fig. 15 are SEM photographs of separator 2, and Fig. 16 and Fig. 17 are SEM photographs of separator 1.

[0097] A comparison of Figures 12 to 17, particularly Figures 13, 15, and 17, reveals that separator A (Figure 13), which uses coating liquid A simulating a commercially available product, has alumina particles distributed relatively uniformly across its entire surface, whereas separators 1 and 2 (Figures 15 and 17), which use coating liquids 1 and 2 containing added water glass, show agglomeration of alumina particles. The particle size of the agglomerated particles is approximately 1 to 5 μm. Furthermore, SEM observation of separator 1, which uses coating liquid 1, revealed no sign of cellulose.

[0098] FIG. 18 is a graph showing the results of the puncture test of the porous films (separators) 1, 2, and A, and FIG. 19 is a graph showing the Gurley values ​​of the porous films (separators) 1, 2, and A.

[0099] As shown in Figure 18, the strength of the porous films (separators) 1 and 2 was found to be higher than that of separator A using coating solution A, which simulates a commercially available product, and no problems were found. In other words, it was found that adding water glass or the like to the coating solution did not cause any problems with strength. Furthermore, as shown in Figure 19, the Gurley value increased by less than 2% compared to separator A using coating solution A, which simulates a commercially available product, and no problems were found. In other words, it was found that adding water glass or the like to the coating solution did not clog the gaps (micropores) between the substrate and alumina particles, and breathability was maintained. It was also found that the addition of SA-CeNF improved the Gurley value.

[0100] Fig. 20 is a graph showing the cycle characteristics at 30° C., and Fig. 21 is a graph showing the cycle characteristics at 60° C. In Fig. 20 and Fig. 21, the horizontal axis represents the number of cycles (cycles) and the vertical axis represents the battery capacity retention rate (%).

[0101] 20 and 21, Battery 1 exhibited a better battery capacity retention rate than Battery A (Comparative Example A, commercially available coating liquid simulant). Battery 2 also exhibited a better battery capacity retention rate than Battery A (Comparative Example A, commercially available coating liquid simulant) in the cycle characteristics at 60°C.

[0102] In comparing Battery 1 and Battery 2, it was found that Battery 1 had a better battery capacity retention rate.

[0103] Fig. 22 is a diagram (graph) showing the battery resistances of batteries 1, 2, A, and B. In Fig. 22, the vertical axis represents the DC resistance (Ω) of the battery. As shown in Fig. 22, the battery resistances of batteries 1 and 2 are higher than that of battery B, which uses a porous film (separator) that is only a substrate without a coating layer, but the rate of increase is within 10%, which indicates that the battery resistance is within an acceptable range even when water glass or the like is added to the coating solution.

[0104] The reasons for this increase in battery resistance include factor 1) the addition of water glass or the like blocks the micropores in the separator, inhibiting the movement of Li ions, and factor 2) components in the coating solution dissolve into the electrolyte, preventing contact between the electrode and the electrolyte and reducing the effective electrode area.However, for batteries 1 and 2, the increase in battery resistance was slight, and it is thought that problems such as factors 1) and 2) above did not occur.

[0105] By using the above-mentioned Coating Liquids 1 and 2 as the coating liquid for the separator in this way, it is possible to improve battery characteristics such as heat resistance and cycle characteristics.

[0106] (Embodiment 2) In the first embodiment, SA Ce was used as the second filler in the coating liquid, but oxidized cellulose having a structure in which primary hydroxyl groups are oxidized to carboxyl groups, such as TEMPO-oxidized cellulose, may also be used. The following description takes TEMPO-oxidized cellulose as an example.

[0107] 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.

[0108] [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. The configuration of the porous film of this embodiment (FIG. 1) and the configuration of a lithium ion battery using this porous film (FIGS. 2 and 3) are the same as those of Embodiment 1 except that SA-Ce in Embodiment 1 is replaced with TEMPO-oxidized cellulose, and therefore detailed description thereof will be omitted.

[0109] [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.

[0110] The manufacturing process of the porous film of this embodiment includes the following steps.

[0111] <<1: Preparation process of the substrate (porous film before coating)>> The substrate S is not particularly limited and may be a substrate typically used for porous films for lithium ion batteries, and a microporous membrane may be used. For example, a commercially available polyethylene microporous membrane may be used.

[0112] <<2: Coating fluid preparation process>> 2-1) Preparation of the first filler In the present embodiment, inorganic particles (inorganic filler) are used as the first filler.

[0113] The inorganic particles are not particularly limited, and may be, for example, alumina, boehmite, aluminum hydroxide, nanosilica, microsilica, carbon nanotubes, talc, glass fiber, or the like, which are similar to those described in the first embodiment.

[0114] 2-2) Preparation of the second filler In this embodiment, TEMPO-treated cellulose is used as the second filler. 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."

[0115] Cellulose (Cellulose, Cell-OH) is (C 12 H 20 O 10 )n, and is represented by the aforementioned chemical structural formula (Chemical Formula 1), for example.

[0116] 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. Further treatment with alkali converts these groups to salts (carboxylates) of the C6-carboxyl groups. For example, when treated with alkali with sodium hydroxide solution, these groups are converted to the Na salts of the C6-carboxyl groups as shown below.

[0117] [ka] In the step of introducing carboxyl groups into the hydroxyl groups of cellulose (oxidation step), for example, TEMPO or sodium bromide is used as a catalyst and hypochlorous acid is used as an oxidizing agent to carry out the reaction in water. During the reaction, a basic solution, such as sodium hydroxide, is added in an amount that allows the desired pH to be maintained, and the reaction is carried out to obtain cellulose that has been subjected to a TEMPO oxidation treatment (the above (Chemical Formula 5)).

[0118] In TEMPO-treated cellulose (TEMPO-oxidized cellulose, TCe), the Na salt of the C6-carboxyl group ionizes in water, causing repulsion (electrostatic repulsion, osmotic pressure). Therefore, if the Na salts are densely distributed, they can be dispersed in a fine state. In the above chemical structural formula, only the left of the two glucose residues is shown as a COONa group, which is the substituent on the carbon atom at the C6 position. This is an example showing that some of the glucose residues have been oxidized to carboxyl groups, 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 their primary hydroxyl groups oxidized to carboxyl groups, and cases where neither glucose residue has their primary hydroxyl groups oxidized to carboxyl groups. It is sufficient for the oxidized cellulose as a whole to have a structure where some of the primary hydroxyl groups are oxidized to carboxyl groups. Furthermore, not all of the carboxyl groups need to be Na salts; some may be Na salts.

[0119] (Defibrillation processing) The TEMPO-oxidized cellulose may also be subjected to a defibration treatment to reduce the size of the cellulose (to nanosize). Defibration (refining) treatments include chemical and mechanical treatments. A combination of these methods may also be used. By such a defibration (refining) treatment, cellulose is obtained in a liquid, for example, with a width (minor axis, shorter length) W of 1000 nm or less and a length L of 500 μm or less, more preferably a width W of 500 nm or less and a length L of 3 μm or less. Cellulose with a width W of about 4 nm and a length L of about 2 μm has also been confirmed.

[0120] The above-mentioned pulverization (nanization) of cellulose may be carried out before or after the TEMPO oxidation treatment.

[0121] 2-3) Preparation of alkali silicate As in the first embodiment, it is preferable to use water glass as the alkali silicate.

[0122] 2-4) Preparation of other additives Other additives that can be used, as in the case of embodiment 1, include thickeners (e.g., carboxymethyl cellulose, xanthan gum, guar gum, alginic acid), binders (e.g., acrylic resin, acrylic binder, fluororesin), dispersants (e.g., surfactants, alcohols), etc.

[0123] B) Mixing process (stirring process) A coating liquid is prepared by adding inorganic particles (first filler) and other additives to the dispersion of the above-mentioned TEMPO-oxidized cellulose (second filler), and then adding an alkali silicate, followed by stirring.

[0124] The TEMPO-oxidized cellulose (second filler) is preferably added in an amount of 0.05 wt% or more relative to the inorganic particles (first filler). The amount of TEMPO-oxidized cellulose is preferably 0.3 wt% to 8 wt%, more preferably 0.5 wt% to 5 wt%, based on the total solid content of the coating solution.

[0125] The alkali silicate is preferably added in an amount of 0.05 wt% or more relative to the inorganic particles (first filler), and is preferably added in an amount of 0.3 wt% to 12.5 wt% or less, more preferably 0.5 wt% to 10 wt% or less, relative to the total amount of solid components in the coating liquid.

[0126] Examples of a stirring method that can be used include a method in which a blade attached to a shaft is rotated by a motor or the like, and a vibration method using ultrasound, etc. In order to reduce the entrapment of air bubbles in the coating liquid, the preparation (mixing and stirring) of the coating liquid may be carried out under reduced pressure.

[0127] <<3: Coating process onto substrate>> The coating liquid is applied to the surface of the substrate S described in <<1: Step of preparing substrate (porous film before coating)>>. There are no limitations on the coating method, and for example, a bar coater, lip coater, gravure coater, die coater, spray coater, screen coater, etc. can be used. After coating, the coating liquid is dried to form a coating film on the surface of the substrate S.

[0128] The drying method for the coating liquid is not particularly limited as long as it can remove the solvent or dispersion medium contained in the coating liquid. For example, a method of performing heat treatment at a temperature of 50°C or higher and 250°C or lower can be used. The heat treatment can be performed for 0.1 to 50 hours. The drying environment can be air, vacuum, rare gas, nitrogen, carbon dioxide, or hydrogen. When the drying environment contains carbon dioxide, the alkali silicate reacts with carbon dioxide to produce a carbonate compound (A2CO3) or a hydrogen carbonate compound (AHCO3) (A = at least one element selected from Li, K, Rb, Ba, Ca, Mg, and Sr). Therefore, when the environment contains carbon dioxide, the alkali silicate contained in the coating film can contain a carbonate compound (A2CO3) or a hydrogen carbonate compound (AHCO3) (A = at least one element selected from Li, K, Rb, Ba, Ca, Mg, and Sr).

[0129] A2CO3 or AHCO3 has the characteristic of having low solubility in water and decomposing to release carbon dioxide at voltages above 4.6 V. In other words, by including A2CO3 or AHCO3 in the coating film, the coating film becomes less hygroscopic and can be given the function of releasing carbon dioxide when overcharged.

[0130] The content of A2CO3 or AHCO3 in the coating film is preferably 1 wt% or more and 50 wt% or less. To adjust the content within this range, depending on the heat treatment temperature and heat treatment time, it is preferable that the ambient pressure be 0.001 to 100 MPa absolute and the carbon dioxide concentration be 400 ppm or more. More preferably, the ambient pressure is 0.01 to 50 MPa and the carbon dioxide concentration is 1000 ppm or more, and even more preferably, the ambient pressure is 0.1 to 10 MPa and the carbon dioxide concentration is 2000 ppm or more.

[0131] Furthermore, by using an alkali silicate in which A2CO3 or AHCO3 powder has been mixed in advance in the coating solution, the heat treatment process in an environment containing carbon dioxide can be eliminated, but it is preferable to have the alkali silicate absorb carbon dioxide to produce A2CO3 or AHCO3 because this significantly reduces the hygroscopicity of the coating film. If the coating film is highly hygroscopic, it may absorb moisture from the atmosphere, degrading battery performance.

[0132] When the coating solution contains cellulose as a second filler, A2CO3 or AHCO3 can be generated uniformly. This uniformity is particularly improved when the hydrophilic groups are replaced with hydrophobic groups or when the coating is treated with TEMPO. By uniformly distributing A2CO3 or AHCO3 in the coating film, it is possible to improve hygroscopicity and evenly impart the ability to release carbon dioxide upon overcharging.

[0133] As described above, according to the present embodiment, by adding a filler and an alkali silicate to the coating liquid, it is possible to improve the electrical characteristics (output characteristics, cycle characteristics (life)) of the battery while improving the mechanical strength and heat resistance of the porous film (separator).

[0134] Examples of the coating liquid, porous film (separator) and battery using the same according to this embodiment will be described below.

[0135] [Example B] 1: Preparation process of the substrate (porous film before coating) As the substrate S, for example, a commercially available polyethylene microporous film (manufactured by CS TECH, average pore size 0.06 μm, thickness 16 μm) was used.

[0136] 2: Coating liquid preparation process A) Adjustment of water glass As mentioned above, water glass is an aqueous solution of alkali silicate, and in this example, Na2O·3SiO2 (ADEKA, ESX-2) was used.

[0137] B) Preparation of TEMPO-oxidized cellulose 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 hardwood as the raw material.

[0138] C) Mixing process Carboxymethyl cellulose (CMC), acrylic resin (binder), and octylphenol ethoxylate (Triton X) as a surfactant were added to a dispersion of TEMPO-oxidized cellulose, followed by high-purity alumina (Sumitomo Chemical Co., Ltd., average particle size 670 nm). An aqueous solvent was further added to prepare the mixture. This mixture was stirred at 2000 rpm for 30 min using a planetary centrifugal mixer (Thinky Corporation, ARE310). Finally, water glass (sodium silicate) was added, and the mixture was stirred at 25 m / s for 1 min using a thin-film rotary mixer (Primix Corporation, FILMIX) to obtain a coating solution. The solid component ratios for each coating solution (cellulose, CMC, binder, surfactant, alumina, water glass (sodium silicate)) are defined as 100 wt%. The surfactant was added in a solution with a solid content of 10 wt% to achieve the above ratio. Furthermore, as Comparative Example A, a coating liquid (simulating a commercially available product) was also prepared to which no dispersion of water glass (sodium silicate) or TEMPO-oxidized cellulose was added.

[0139] [Table 2] In this embodiment, the ratio (concentration) of solid components (cellulose, CMC, binder, surfactant, alumina, water glass) to the solvent is 45 wt%, and this ratio (concentration) can be adjusted to approximately 20 wt% to 60 wt%.

[0140] 3: Coating process onto the substrate (separator manufacturing process) The above coating liquid (any of Coating Liquids 3, 4, 5, 6, 7, and 8) was applied to both sides of the surface of the substrate (porous film made of PE) described in "1. Preparation process of substrate (porous film before coating)" 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, porous films (separators) 3, 4, 5, 6, 7, and 8 on which coating layers were formed were formed (see Table 2).

[0141] 4: Evaluation of coating fluid and separator (Applicability) For Coating Liquid 4, the coating liquid solidified and the bar coater became unable to move, so the production of a porous film (separator) and a battery was abandoned. For Coating Liquids 7 and 8, the coating liquid did not wet and spread over the surface of the substrate (causing repelling), so the production of a porous film (separator) and a battery was abandoned. For the other coating liquids, there were no problems such as solidification of the coating liquid or repelling of the substrate, and the coating properties were good. Comparative Examples A and B are the same as those described in Embodiment 1 (Example A).

[0142] (Heat shrinkage test 2) The porous film (separator) 3 was left in a vacuum dryer at 300°C for 1 hour. The state of the film was observed before and after the application of a heat load. Note that the substrate (PE porous film) used for coating was a dry separator manufactured by uniaxial stretching, so the heat shrinkage rate was calculated based on the dimensional change in the machine direction (MD). In addition, when the film melted and the dimensions could not be measured, the heat shrinkage rate was taken as 100%.

[0143] 5: Battery fabrication and evaluation Similar to Example A of Embodiment 1, batteries (batteries 3, 5, and 6) were fabricated, and the cycle characteristics and battery resistance were evaluated.

[0144] 6:Result Fig. 23 shows the results of Heat Shrinkage Test 2. As shown in Fig. 23, the separator 3 using Coating Solution 3 containing water glass and TEMPO-oxidized cellulose had a heat shrinkage rate of 0.5%, demonstrating high heat resistance. Furthermore, it was found that this separator 3 had heat resistance comparable to that of the separator 1 in Heat Shrinkage Test 2 described above.

[0145] Fig. 24 is a graph showing the cycle characteristics at 30° C., and Fig. 25 is a graph showing the cycle characteristics at 60° C. In Figs. 24 and 25, the horizontal axis represents the number of cycles (cycles) and the vertical axis represents the battery capacity retention rate (%).

[0146] 24 and 25, Battery 3 exhibited a better battery capacity retention rate than Battery A (Comparative Example A, a commercially available coating liquid simulant). Battery 3 also exhibited a better battery capacity retention rate than Battery 6, which did not contain water glass (FIG. 25).

[0147] Fig. 26 is a diagram (graph) showing the battery resistances of batteries 3 and B. In Fig. 26, the vertical axis represents the DC resistance (Ω) of the battery. As shown in Fig. 26, the battery resistance of battery 3 is higher than that of battery B, which uses a porous film (separator) that is only a substrate without a coating layer, but the rate of increase is within 10%, which indicates that the battery resistance is within an acceptable range even when water glass or the like is added to the coating liquid.

[0148] As mentioned above, the causes of this increase in battery resistance include factor 1) the addition of water glass or the like blocks the micropores in the separator, inhibiting the movement of Li ions, and factor 2) components in the coating solution dissolve into the electrolyte, preventing contact between the electrode and the electrolyte and thereby reducing the effective electrode area.However, for batteries 1 and 2, the increase in battery resistance was slight, and it is thought that problems such as factors 1) and 2) above did not occur.

[0149] By using the coating liquid 3 as the coating liquid for the separator in this way, it is possible to improve battery characteristics such as heat resistance and cycle characteristics.

[0150] (summary) From the results of the above examples, from the viewpoint of coatability, the amount of water glass added must be less than 15.5 wt% of the total amount of solid components in the coating solution or coating film, and preferably 10.5 wt% or less. Furthermore, from the viewpoints of coatability and cost, the amount of water glass added must be less than 19.4 wt% of the amount of inorganic particles in the coating solution or coating film, and more preferably 12.4 wt% or less. It is believed that this amount of water glass added will hardly change depending on whether cellulose is added or the type of cellulose added.

[0151] Furthermore, based on the results of the above examples and other experiments, it is more preferable that the amount of cellulose added is 0.1 wt % to 5 wt % relative to the total amount of solid components in the coating liquid or coating film.

[0152] Furthermore, from the above Examples A and B, it is clear that by using the above coating liquid, the thermal shrinkage rate can be suppressed to 5% or less, more preferably 3% or less.

[0153] Furthermore, the addition of water glass improves the viscosity of the coating solution, allowing the amount of CMC, which acts as a thickener, to be reduced, for example, to 3 wt% or less of the total amount of solid components in the coating solution and coating film. In particular, since CMC is water-soluble and contains Na ions, reducing the amount of CMC added is preferable from the viewpoint of suppressing deterioration of battery performance.

[0154] (Embodiment 3) In the examples of the first and second embodiments, a commercially available polyethylene microporous film was used as the substrate S, but the polyethylene microporous film can be formed as follows.

[0155] 27 is a schematic diagram showing the configuration of a manufacturing apparatus according to this embodiment. In this embodiment, a separator manufacturing process using the manufacturing apparatus will be described.

[0156] 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. 27, 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.

[0157] 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.

[0158] 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.

[0159] 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.

[0160] 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.

[0161] 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. 28 is installed between the second transverse stretching device S7 and the winding device S8. FIG. 28 is a cross-sectional view that schematically shows the 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.

[0162] 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.

[0163] In this way, high-performance separators can be efficiently manufactured using the apparatus shown in FIGS.

[0164] As described above, 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).

[0165] 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.

[0166] 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. Also, in terms of shutdown properties, it is preferable that polyethylene accounts for 50 wt% or more of the resin components. Furthermore, when using an ultra-high molecular weight polyolefin having a molecular weight of 1 million or more, it is preferable that the amount of ultra-high molecular weight polyolefin is 50 parts by mass or less per 100 parts by mass of the kneaded material (resin and dispersion liquid), because if more than 50 parts by mass of the ultra-high molecular weight polyolefin exceeds 50 parts by mass, it becomes difficult to knead uniformly.

[0167] 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).

[0168] The plasticizer may be an organic solvent having a molecular weight of 100 to 1500 and a boiling point of 50 to 300°C. Specifically, one or a mixture of several of the following may 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. Alternatively, one or a mixture of several of the following may be used: 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.

[0169] 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."

[0170] 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).

[0171] 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.

[0172] 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.

[0173] (Application example) In the above, the Na salt of the C6-carboxyl group shown in (Chemical Formula 4) is exemplified, but other counter ions (X +The counter ion is preferably an alkali metal ion, for example, K + etc.

[0174] [ka] Furthermore, the raw material for cellulose may be derived from plant fibers such as pulp or animal fibers such as sea squirts.

[0175] (Fourth embodiment) In the first and second embodiments, the second filler is used, but if the drying process is performed in an environment containing carbon dioxide, the second filler is not necessarily required.

[0176] 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.

[0177] [Structure description] The porous film of this embodiment has a substrate (porous substrate) S and a coating film (coating film) CF formed on the surface of the substrate S. Here, the configuration of the porous film of this embodiment (FIG. 1) and the configuration of the lithium ion battery using this porous film (FIGS. 2 and 3) are the same as those of the first embodiment except that SA-Ce as in the first embodiment is not contained, and instead A2CO3 or AHCO3 (A = Li, Na, K, Rb) is contained, and therefore detailed description thereof will be omitted.

[0178] [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.

[0179] The manufacturing process of the porous film of this embodiment includes the following steps.

[0180] <<1: Preparation process of the substrate (porous film before coating)>> The substrate S is not particularly limited and may be a substrate typically used for porous films for lithium ion batteries, and a microporous membrane may be used. For example, a commercially available polyethylene microporous membrane or polypropylene microporous membrane may be used.

[0181] <<2: Coating fluid preparation process>> 2-1) Preparation of the first filler In the present embodiment, inorganic particles (inorganic filler) are used as the first filler.

[0182] The inorganic particles are not particularly limited, and may be, for example, alumina, boehmite, aluminum hydroxide, nanosilica, microsilica, carbon nanotubes, talc, glass fiber, or the like, which are similar to those described in the first embodiment.

[0183] 2-2) Preparation of the second filler In this embodiment, the second filler is not used.

[0184] 2-3) Preparation of alkali silicate As in the first embodiment, it is preferable to use water glass as the alkali silicate.

[0185] 2-4) Preparation of other additives Other additives that can be used, as in the case of embodiment 1, include thickeners (e.g., carboxymethyl cellulose, xanthan gum, guar gum, alginic acid), binders (e.g., acrylic resin, acrylic binder, fluororesin), dispersants (e.g., surfactants, alcohols), etc.

[0186] B) Mixing process (stirring process) The inorganic particles (first filler) described above and, if necessary, additives such as a surfactant and an acrylic binder are added, and then an alkali silicate is added and stirred to prepare a coating liquid.

[0187] The alkali silicate is preferably added in an amount of 0.05 wt% or more relative to the inorganic particles (first filler), and is preferably added in an amount of 0.3 wt% to 5 wt% and more preferably 0.5 wt% to 1.5 wt% relative to the total amount of solid components in the coating liquid.

[0188] Examples of a stirring method that can be used include a method in which a blade attached to a shaft is rotated by a motor or the like, and a vibration method using ultrasound, etc. In order to reduce the entrapment of air bubbles in the coating liquid, the preparation (mixing and stirring) of the coating liquid may be carried out under reduced pressure.

[0189] <<3: Coating process onto substrate>> The coating liquid is applied to the surface of the substrate S described in <<1: Substrate (porous film before coating) preparation step>>. There are no limitations on the coating method, and for example, a bar coater, lip coater, gravure coater, die coater, spray coater, screen coater, etc. can be used. After coating, the coating liquid is dried in an environment containing carbon dioxide, thereby forming a coating film on the surface of the substrate S.

[0190] In this embodiment, since a second filler like that in embodiments 1 and 2 is not included, when the substrate S is dried in an environment that does not contain carbon dioxide, the coating film CF clogs 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 likely to be 3500 or more, resulting in poor breathability (for example, when dried in an argon environment, the Gurley value was 3504 sec / 100cc). Therefore, the drying environment must be one that contains carbon dioxide. Drying in an environment that contains carbon dioxide causes the alkali silicate and carbon dioxide to react with each other to produce a carbonate compound (A2CO3) or a hydrogen carbonate compound (AHCO3) (A = at least one element selected from Li, K, Rb, Ba, Ca, Mg, and Sr) and silica (SiO2). Due to the generated A2CO3 or AHCO3, and SiO2, 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 200 or more and 3000 or less, ensuring breathability.

[0191] As described above, according to the present embodiment, by drying in an environment containing carbon dioxide, and adding the first filler and alkali silicate to the coating liquid without using the second filler, it is possible to improve the electrical characteristics (output characteristics, cycle characteristics (life)) of the battery while also improving the mechanical strength and heat resistance of the porous film (separator).

[0192] Examples of the coating liquid, porous film (separator) and battery using the same according to this embodiment will be described below.

[0193] [Example C] 1: Preparation process of the substrate (porous film before coating) As the substrate S, for example, a commercially available polyethylene microporous film (manufactured by CS TECH, average pore size 0.06 μm, thickness 16 μm) was used.

[0194] 2: Coating liquid preparation process A) Adjustment of water glass As mentioned above, water glass is an aqueous solution of alkali silicate, and in this example, a mixture of Li2O·3.5SiO2 (Lithium silicate 35, manufactured by Nippon Chemical Industry Co., Ltd.) and Na2O·3SiO2 (ESX-2, manufactured by ADEKA) in a solid mass ratio of 1:1 was used.

[0195] B) Mixing process To the water containing dissolved carboxymethyl cellulose (CMC), acrylic resin (binder) and octylphenol ethoxylate (Triton X) as a surfactant were added, followed by high-purity alumina (Sumitomo Chemical Co., Ltd., average particle size 700 nm). An aqueous solvent was then added to prepare the mixture. This mixture was stirred at 2000 rpm for 30 minutes using a planetary centrifugal mixer (Thinky Corporation, ARE310). Finally, water glass (a mixture of lithium silicate and sodium silicate) was added, and the mixture was stirred at 25 m / s for 1 minute using a thin-film rotary mixer (Primix Corporation, FILMIX) to obtain the coating solution. In the coating liquid, the ratio of solid components (CMC, binder, surfactant, alumina, water glass (lithium silicate + sodium silicate)) was 100 wt%, with CMC 0.9 wt%, binder 3 wt%, surfactant 0.1 wt%, alumina 85 wt%, and water glass (lithium silicate + sodium silicate) 11 wt%. Note that the surfactant was added in a solution with a solid content of 10 wt% to achieve the above ratio.

[0196] In this embodiment, the ratio (concentration) of the solid components (CMC, binder, surfactant, alumina, water glass) to the solvent is 45 wt%, and this ratio (concentration) can be adjusted to approximately 15 to 65 wt%.

[0197] 3: Coating process onto the substrate (separator manufacturing process) The above coating liquid was applied to both sides of the surface of the substrate (porous PE film) described in "1. Preparation process of substrate (porous film before coating)" using a gravure roll, and dried at 80°C for 1 hour in a carbon dioxide atmosphere of 0.1 MPa. 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.

[0198] 4: Evaluation of coating fluid and separator (Applicability) This coating solution had good applicability without causing any problems such as solidification of the coating solution or cissing of the substrate.

[0199] 5: Gurley value of separator The time it took for 100 ml of air to pass through the porous film (separator) was measured and used as the Gurley value. The Gurley value was calculated by averaging the values ​​for five test pieces.

[0200] 6:Result The Gurley value was 289 sec / 100 cc, indicating that breathability was ensured.

[0201] (summary) From the results of the above examples, when a second filler is not used, the Gurley value can be set to 200 or more and 3000 or less by drying (heat treating) the separator in an environment containing carbon dioxide. This is thought to be because drying in an environment containing carbon dioxide causes the alkali silicate and carbon dioxide to react, producing A2CO3 or AHCO3 and SiO2. It is thought that the coating film CF is not formed to cover all of the micropores of the substrate S due to the produced A2CO3 or AHCO3 and SiO2. Although lithium ion batteries have been exemplified above, metal lithium batteries, lithium polymer batteries, air lithium ion batteries, etc. may also be used. Furthermore, the separator may be used as a separator for non-aqueous electrolyte secondary batteries such as sodium ion batteries, potassium ion batteries, calcium ion batteries, magnesium ion batteries, and aluminum ion batteries. These batteries refer to battery systems in which the ions (carriers) responsible for electrical conduction are replaced by cations such as sodium, calcium, magnesium, and aluminum, instead of lithium.

[0202] 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]

[0203] 1 positive electrode 1M Positive electrode mixture layer 1S current collector 2 negative electrode 2M negative electrode mixture layer 2S current collector 6 cans (battery cans) 7 Lid (battery cap) 8 washers CF coating film CL coating liquid R Gravure Roll S base material S1 Twin-screw Extruder S2 Die S3 Fabric cooling device S4 First longitudinal stretching device S5 1st horizontal stretching device S6 extraction tank S7 2nd horizontal stretching device S7' Gravure Coating Machine S8 Winder SP Separator

Claims

1. A coating liquid for a porous film of a separator of a nonaqueous electrolyte secondary battery, the coating liquid comprising an alkali silicate and a first filler, the first filler is made of inorganic particles, The alkali silicate is contained in an amount of 0.05% by weight or more relative to the inorganic particles, The second filler is 0.05% by weight or more relative to the inorganic particles, A coating liquid for a porous film, wherein the second filler is a cellulose filler in which an OH group is substituted with a COOH group.

2. The coating liquid according to claim 1, The coating liquid for a porous film, wherein the inorganic particles are a material selected from nanosilica, microsilica, carbon nanotubes, talc, alumina, boehmite, aluminum hydroxide, and glass fibers.

3. The coating liquid according to claim 2, A coating liquid for a porous film, wherein the second filler is TEMPO-oxidized cellulose shown below. 【Chemistry 6】

4. The coating liquid according to claim 1, A coating liquid for a porous film, wherein the second filler is 12.4% by weight or less relative to the inorganic particles.

5. A porous film for a separator of a non-aqueous electrolyte secondary battery, comprising a porous substrate and a coating film provided on a surface of the porous substrate, the coating film includes an alkali silicate and a first filler, the first filler is made of inorganic particles, The alkali silicate is contained in an amount of 0.05% by weight or more relative to the inorganic particles, the coating film contains a second filler in an amount of 0.05% by weight or more relative to the inorganic particles; A porous film, wherein the second filler is a cellulose filler in which an OH group is substituted with a COOH group.

6. The porous film according to claim 5, A porous film, wherein the inorganic particles are a material selected from nanosilica, carbon nanotubes, talc, alumina, and glass fibers.

7. The porous film according to claim 6, A porous film, wherein the second filler is TEMPO-oxidized cellulose as shown below. 【Chemistry 7】

8. The porous film according to claim 6, The coating film is A ２ CO ３ or AHCO ３ (A=Li, Na, K, Rb).

9. The porous film according to claim 8, The above A ２ CO ３ or the AHCO ３ is 1% by weight or more and 50% by weight or less relative to the coating film.

10. The porous film according to claim 5, A porous film, wherein the second filler is 12.4% by weight or less relative to the inorganic particles.

11. A lithium ion battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, A lithium ion battery having the porous film according to any one of claims 5 to 10 as the separator.

Images