JP2023040759A - Coating liquid, porous film, lithium ion battery, method of producing coating liquid, method of producing porous film, and method of producing lithium ion battery - Google Patents

Abstract

To improve the properties of porous films used in separators and the like of lithium ion batteries.SOLUTION: A porous film of the present invention is a porous film having a porous substrate and a coating film provided on a surface of the porous substrate. The coating film includes a first filler and a second filler. The first filler is cellulose containing a carboxyl group converted to a Na salt, and moreover is cellulose subjected to basic treatment (a SA-treated Ce salt or TEMPO oxidized cellulose). The second filler has at least one of materials selected from among nano silica, carbon nanotubes, talc, aluminum, aluminum hydroxide, boehmite and glass fibers. By using a SA-treated Ce salt and the like in this way, miniaturization due to repulsive force is advanced, so that mechanical strength and heat resistance of the porous film can be improved. Moreover, by adjusting the salt concentration of the SA-treated Ce salt, battery electrical characteristics (output characteristics, cycle characteristics) can be improved.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to a coating liquid for a porous film used for battery separators and the like, and relates to a coating liquid, a porous film, a lithium ion battery, a method for producing a coating liquid, a method for producing a porous film, and a lithium ion battery. It can be used for the manufacturing method of

Applications for secondary batteries are expanding from electronic devices to automobiles and large-scale power storage systems. Among them, lithium-ion batteries (secondary batteries), which can be made smaller and lighter and have high energy density, are attracting attention. ing.

For example, Patent Literature 1 discloses a non-aqueous binder for a lithium-ion battery electrode in which cellulose nanofibers and a thermoplastic fluorine-based resin are combined, wherein the cellulose nanofibers have a fiber diameter (diameter) of 0.5. 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 fiber length/cellulose nanofiber fiber diameter) of 2 or more and 100000 or less. .

WO2019/064583

As the capacity and output of lithium-ion batteries increase, further improvements in safety are required. The present inventors are engaged in research and development of porous films used for battery separators and the like, and are earnestly studying porous films with good characteristics.

As will be described later in detail, the separator provided between the positive electrode and the negative electrode of the battery has a plurality of fine pores through which lithium ions pass, and the lithium ions move between the positive electrode and the negative electrode through these pores. By doing so, charging and discharging can be repeated. This separator has a role of separating the positive electrode and the negative electrode to prevent short circuit. Also, when the temperature inside the battery rises for some reason, the micropores of the separator close, stopping the movement of lithium ions and stopping the battery function (shutdown function).

Thus, the separator plays a role of a battery safety device, and in order to improve safety, it is essential to improve the mechanical strength and heat resistance of the separator.

On the other hand, various factors are involved in the battery characteristics, and additives for improving the mechanical strength and heat resistance of the separator may rather deteriorate the battery characteristics.

Therefore, it is desired to study a technique for improving the battery characteristics while improving the mechanical strength and heat resistance of the separator.
Other problems and novel features will become apparent from the description of the specification and the accompanying drawings.

The coating liquid disclosed in the present application has a first filler and a second filler, the first filler is a cellulose fiber having a structure in which a primary hydroxyl group is oxidized to a carboxyl group, and the second The filler has at least one material selected from nanosilica, carbon nanotubes, talc, alumina, aluminum hydroxide, boehmite, and glass fiber.

The porous film disclosed in the present application has a porous substrate and a coating film provided on the surface of the porous substrate, and the coating film has a first filler and a second filler. The first filler is a cellulose fiber having a structure in which the primary hydroxyl group is oxidized to a carboxy group, and the second filler is nanosilica, carbon nanotube, talc, alumina, aluminum hydroxide, boehmite, and glass. It has at least one or more materials selected from fibers.

A lithium-ion battery disclosed in the present application includes a positive electrode, a negative electrode, a separator, and an electrolytic solution, and has the porous film as the separator.

The method for producing a coating liquid disclosed in the present application comprises (a) a step of introducing a carboxyl group into a cellulose fiber as a first filler, and (b) a step of adding sodium hydroxide to the carboxylated first filler. , have

The method for producing a porous film disclosed in the present application includes (a) the step of introducing a carboxyl group into a cellulose fiber as a first filler, and (b) the step of adding a hydroxide to the carboxylated first filler. , (c) after the step (b), by adding a material selected from nanosilica, carbon nanotubes, talc, alumina, aluminum hydroxide, boehmite, and glass fiber, which is the second filler, to prepare a coating liquid and (d) applying the coating liquid to the surface of a porous substrate to form a coating film.

The method for manufacturing a lithium-ion battery disclosed in the present application includes the steps of (a) forming a porous film as a separator, and (b) disposing the porous film between a positive electrode material and a negative electrode material. Then, the step (a) includes (a1) a step of introducing a carboxyl group into the cellulose fiber as the first filler, (a2) a step of adding a hydroxide to the carboxylated first filler, and (a3) ) After the step (a2), a coating liquid is formed by adding a material selected from nanosilica, carbon nanotubes, talc, alumina, aluminum hydroxide, boehmite, and glass fiber, which is the second filler. (a4) applying the coating liquid to the surface of the porous substrate to form a coating film;

The porous film coating liquid disclosed in the present application can improve the properties of the porous film.

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

According to the lithium ion battery disclosed in the present application, the characteristics of the lithium ion battery can be improved.

According to the method for producing a coating liquid for porous films disclosed in the present application, it is possible to produce a coating liquid for porous films with good properties.

According to the method for producing a porous film disclosed in the present application, a porous film with good properties can be produced.

According to the method for manufacturing a lithium ion battery disclosed in the present application, a lithium ion battery with good characteristics can be manufactured.

1 is a cross-sectional view schematically showing the configuration of the porous film of Embodiment 1. FIG. 1 is a diagram schematically showing the internal configuration of a lithium ion battery using the porous film of Embodiment 1. FIG. 1 is a diagram schematically showing a configuration example of a lithium-ion battery according to Embodiment 1; FIG. FIG. 4 is a diagram showing steps of preparing a dispersion of CeNF salt. It is a figure which shows the introduction|transduction process of the carboxy group to a cellulose fiber. It is a figure which shows cycling characteristics in 30 degreeC. It is a figure which shows cycling characteristics in 60 degreeC. FIG. 4 is a diagram showing the relationship between discharge rate and discharge voltage; FIG. 4 is a diagram showing the relationship between discharge rate and discharge voltage; FIG. 2 is a diagram schematically showing the structure of TEMPO-oxidized cellulose. It is a figure which shows cycling characteristics in 30 degreeC. It is a figure which shows cycling characteristics in 60 degreeC. FIG. 4 is a diagram showing the relationship between discharge rate and discharge voltage; FIG. 4 is a diagram showing the relationship between discharge rate and discharge capacity; It is a figure which shows cycling characteristics in 30 degreeC. It is a figure which shows cycling characteristics in 60 degreeC. FIG. 10 is a schematic diagram showing the configuration of a manufacturing apparatus according to Embodiment 3; It is a sectional view showing typically composition of a gravure coating device.

Hereinafter, embodiments will be described in detail based on examples and drawings. In addition, in all the drawings for describing the embodiments, members having the same functions are denoted by the same reference numerals, and repeated description thereof will be omitted.

(Embodiment 1)
The porous film of this embodiment and the method for producing the same will be described below. The porous film of the present embodiment can be used as a so-called battery separator.

[Structure description]
FIG. 1 is a cross-sectional view schematically showing the configuration of the porous film of this embodiment. As shown in FIG. 1, the porous film of the present embodiment is used as a battery separator SP, and is a substrate (porous substrate) S and a coating film (coating film) formed on the surface of the substrate S. Membrane) CF. The base material S used here can be used without particular limitation. As the substrate S, substrates that are usually used for porous films for lithium ion batteries are particularly preferable. The coating film CF includes a first filler (hydrophobized cellulose) and a second filler (inorganic filler). In FIG. 1, as an example, as will be described later, the first filler is SA-treated cellulose salt (sometimes referred to as SA-Ce salt), and the second filler is alumina (Al ₂ O ₃ ). The case of having and is exemplified.

FIG. 2 is a diagram schematically showing the internal configuration of a lithium ion battery using the porous film of the present embodiment, and FIG. 3 schematically shows a configuration example of the lithium ion battery of the present embodiment. It is a diagram. 2A shows the structure of the positive electrode, FIG. 2B shows the structure of the negative electrode, and FIG. 2C shows the structure of the electrode group. The battery of FIG. 3 is called a coin cell battery.

As shown in FIG. 2A, the positive electrode 1 is composed of a current collector 1S and a positive electrode mixture layer 1M provided thereon. As shown in FIG. 2B, the negative electrode 2 is a current collector 2S and a negative electrode mixture layer 2M provided thereon. Then, as shown in FIG. 2C, the lithium ion battery has a positive electrode 1, a negative electrode 2, and a separator SP disposed therebetween. The layer 1M and the negative electrode mixture layer 2M are arranged to face each other so as to be 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 in a battery container (a bag made of a laminated film, a battery can, etc.) together with an electrolyte, and a positive electrode terminal (for example, a current collector 1S). part or the conductive portion electrically connected to the current collector 1S) and the negative electrode terminal (for example, part of the current collector 2S or the conductive portion electrically connected to the current collector 2S) are exposed. Sealed.

The battery (coin-type battery) shown in FIG. 3 has a can 6, and the can 6 accommodates an electrode group in which the above-described positive electrode 1 and negative electrode 2 are laminated via a separator SP. A current collector 1 S of the positive electrode 1 on the lower end surface of the electrode group is mounted on a can (battery can) 6 . The current collector 2S of the negative electrode 2 on the upper end face of the electrode group is arranged on the back side of the lid (battery cap) 7 . Here, a washer 8 is provided between the lid (battery cap) 7 and the current collector 2S of the negative electrode 2, and these are electrically connected. In addition, a heat-resistant gasket (fixing sealing material) is provided in the overlapping portion of the can 6 and the lid 7, and the electrolytic solution (not shown) and the like injected into the inside of the can 6 are sealed. . Note that although a coin-type battery has been described here, there is no limitation on the configuration of the battery, and for example, a cylindrical battery, a laminate-type battery, or a square-type battery can be used.

Thus, the lithium ion battery has a positive electrode 1, a negative electrode 2, a separator SP, and an electrolytic solution, and the separator SP is arranged between the positive electrode 1 and the negative electrode 2. The separator SP has many micropores. For example, during charging, that is, when a charger is connected between the positive electrode (bottom of the can 6) and the negative electrode (top of the lid 7), the lithium ions inserted in the positive electrode active material are desorbed and released into the electrolyte. released to The lithium ions released into the electrolyte move in the electrolyte, pass through the fine pores of the separator, and reach the negative electrode. Lithium ions that reach the negative electrode are inserted into the negative electrode active material that constitutes the negative electrode.

In this manner, charging and discharging can be repeated by moving lithium ions between the positive electrode and the negative electrode through micropores (not shown) provided in the substrate S shown in FIG.

Here, in the porous film of the present embodiment, as shown in FIG. 1, a coating film CF is provided on the surface of a substrate S provided with a large number of micropores. This coating film CF has SA-treated cellulose salt (SA-Ce salt) as a first filler, and alumina (Al ₂ O ₃ ) as a second filler.
And the SA-Ce salt is a cellulose fiber containing a carboxy group.

Thus, in the present 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 the micropores of the base material S, and the Gurley value (air permeability, [sec/100cc]) is 10 or more and 3000 or less, and air permeability is ensured.

The porous film used as the separator SP of the battery as shown in FIG. 3 preferably has a small shrinkage rate (specifically, the shrinkage rate in the width direction and in the direction orthogonal to the width direction) in a high-temperature environment. For example, as one index, the shrinkage ratio of the porous film at 200° C. is preferably 5% or less. According to studies by the inventors of the present application, the shrinkage ratio of the porous film can be reduced by increasing the thickness of each of the coating films CF shown in FIG. Specifically, the film thickness of the coating film CF shown in FIG. 1 is preferably 0.5 μm or more.

In particular, the use of the SA-Ce salt promotes micronization due to repulsive force, and can improve the mechanical strength and heat resistance of the porous film (separator). Further, by adjusting the salt concentration of the Ce salt of SA, the electrical characteristics (output characteristics, cycle characteristics (lifetime)) of the battery can be improved.

[Manufacturing method]
The manufacturing process of the porous film of the present embodiment will be described below, and the configurations of the porous film and the coating film will be clarified.
The manufacturing process of the porous film of the present embodiment has the following steps.

<<1: Preparatory process for base material (porous film before coating)>>
As the substrate S, a microporous membrane can be used. For example, a commercially available polyethylene microporous membrane can be used.

<<2: Coating liquid preparation process>>
A) Preparation of first filler In the present embodiment, cellulose fibers containing carboxyl groups (sometimes referred to as SA-Ce salt) are used as the first filler.

(Hydrophobicization)
Cellulose (Cellulose, Cell-OH, Ce) is a carbohydrate represented by (C ₁₂ H ₂₀ O ₁₀ ) _n . For example, it is represented by the following chemical structural formula (Formula 1). In this chemical structural formula, n representing the average repeating number is 1 or more, preferably 10 to 10,000, more preferably 50 to 2,000.

As shown in the following chemical structural formula (Formula 2), some of the plurality of hydroxyl groups of the carbohydrate represented by (C ₁₂ H ₂₀ O ₁₀ ) _n are groups having hydroxyl groups (for example, —CH ₂ OH A cellulose substituted with -R-OH (R represents a divalent hydrocarbon group) such as cellulose may also be used. Here, n indicating the average number of repetitions is a number of 1 or more, preferably 10 to 10,000, more preferably 50 to 2,000.

As can be seen from the chemical structural formulas (Chemical formulas 1 and 2), cellulose has hydroxyl groups (hydrophilic groups). A carboxy group is introduced into this using an esterifying agent (for example, a carboxylic acid compound). That is, some of the hydroxyl groups (--OH) of cellulose are esterified with a carboxylic acid compound (R--CO--OH (R represents a divalent hydrocarbon group)). In other words, the hydroxyl group (--OH) portion of cellulose is an ester bond (--O--CO--R, carboxy group (R represents a divalent hydrocarbon group)). It should be noted that not all the hydroxyl groups of cellulose need to be substituted, as long as some of them are substituted. An example of the esterification reaction of cellulose is shown by the following reaction formula. This carboxy group-containing cellulose is sometimes referred to as SA-Ce.

The esterification agent is not particularly limited as long as it has a composition capable of imparting a carboxyl group to the hydrophilic group of cellulose. For example, a carboxylic acid compound can be used. Among them, it is preferable to use compounds having two or more carboxy groups, acid anhydrides of compounds having two or more carboxy groups, and the like. Among compounds having two or more carboxy groups, it is preferable to use compounds having two carboxy groups (dicarboxylic acid compounds).

Compounds with two carboxyl groups include 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-2pentenedioic 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 dicarboxylic acid compounds such as ethanedioic acid (oxalic acid). Acid anhydrides of compounds having two carboxy groups include maleic anhydride, succinic anhydride, phthalic anhydride, glutaric anhydride, adipic anhydride, itaconic anhydride, pyromellitic anhydride, and 1,2-cyclohexanedicarboxylic anhydride. Dicarboxylic acid compounds such as acids and acid anhydrides of compounds containing a plurality of carboxy groups are included. Examples of acid anhydride derivatives of compounds having two carboxy groups include at least a portion of acid anhydrides of compounds having carboxy groups, such as dimethylmaleic anhydride, diethylmaleic anhydride, and diphenylmaleic anhydride. Examples thereof include those in which a hydrogen atom is substituted with a substituent (eg, an alkyl group, a phenyl group, etc.). Among these, maleic anhydride, succinic anhydride, and phthalic anhydride are preferable because they are easy to apply industrially and easy to gasify.

(basic treatment)
The Ce SA compound is dispersed in an aqueous solvent, and a base (eg, hydroxide) is added to form a Ce salt SA compound. Hydroxides are designated M(OH)m, where M is an m-valent metal. Specifically, sodium hydroxide (NaOH), calcium hydroxide (Ca(OH) ₂ ), potassium hydroxide (KOH), lithium hydroxide (LiOH), or the like can be used. From the viewpoint of ionization tendency, alkali metal hydroxides are preferable, and sodium hydroxide (NaOH) is particularly preferable because of its high versatility. Although the details will be described later, as a result of investigations by the inventors of the present application, it was found that when the carboxyl group of Ce of SA is converted to a Na salt, the characteristics of the battery, such as the cycle characteristics and the high-rate charge/discharge characteristics, can be improved. rice field.

The temperature in the step of adding the base is not particularly limited, but is preferably 5°C or higher and 80°C or lower, more preferably 10°C or higher and 60°C or lower.

(defibration treatment)
In addition, fibrillation treatment may be performed to make the cellulose finer (nano). The fibrillation treatment (refining treatment) includes a chemical treatment method, a mechanical treatment method, and the like. You may use the method which combined these. Through such fibrillation treatment (refinement treatment), cellulose having a fiber length (L) of 3 nm or more and 10 μm or less and an aspect ratio (length L/diameter R) of 0.01 or more and 5000 or less can be obtained. can. A cellulose fiber thus refined to a nanometer size is called a cellulose nanofiber (CeNF).

The cellulose refining (nanization) as described above may be performed before the introduction of the carboxy groups, or may be performed after the introduction of the carboxy groups.

(Method of SA CeNF salt dispersed in solvent)
The SA-CeNF salt is preferably used in a state of being dispersed in a solvent (dispersion medium) in order to prevent aggregation and improve dispersibility in the coating liquid.

FIG. 4 is a diagram showing the steps of preparing a dispersion of CeNF salt. For example, as shown in FIG. 4, cellulose (solid, eg, powder) and succinic anhydride (solid, eg, tablet) are mixed at 100° C. or higher. For example, it is mixed at 125° C. for 20 minutes using a pressure kneader. The weight of cellulose and succinic anhydride is, for example, 90 wt% (% by weight, mass %), 10 wt%.

Stirring in a heated state as described above causes an ester reaction to produce cellulose containing a carboxy group. Thereafter, washing is performed with acetone or the like in order to remove unreacted succinic anhydride.

Next, the produced cellulose containing carboxy groups is dispersed in an aqueous solvent (water and/or alcohols, here water (H ₂ O)), and a base is added (basic treatment is performed). For example, an aqueous sodium hydroxide solution is added to convert a carboxy group (--COOH) into a salt (--COONa). In other words, the carboxy group is converted to Na salt. An example of the reaction between cellulose containing a carboxy group and sodium hydroxide is shown by the following reaction formula.

The pH of the SA-Ce dispersion before the basic treatment is 2-6. Further, the pH of the SA-Ce salt dispersion after the basic treatment is higher and ranges from 5 to 10. In this manner, although the term "basic treatment" is used in this specification for convenience, the pH of the SA Ce salt dispersion may be in the neutral range. Here, all the carboxyl groups (--COOH) of SA-Ce do not have to be converted to Na salts, as long as some of the carboxyl groups (--COOH) are converted to Na salts.

That is, when there is a large amount of converted Na salt (high Na concentration), the fibrillation property due to the repulsive force is enhanced, the CeNF salt is made finer, and the mechanical strength and heat resistance can be improved. On the other hand, if the Na concentration is high, Na ions act on the active material of the electrode, inhibiting the insertion and extraction of Li ions, and can degrade the battery performance.

The Na concentration in the CeNF salt is preferably 5 mg/g to 30 mg/g. Further, the Na concentration in the coating film is preferably 0.05 mg/g to 5 mg/g.

FIG. 5 is a diagram showing carboxylation (SA-Ce) and basic treatment (SA-Ce salt) of cellulose. As shown in FIG. 5, hydroxyl groups (--OH) of cellulose are substituted with carboxy groups (--COOH), and the carboxy groups (--COOH) become Na salts (--COONa) by adding NaOH.

FIG. 5 shows that 8 out of 10 hydroxyl groups (--OH) of cellulose are replaced with carboxyl groups (--COOH), and 4 of them are Na salts (--COONa).

Here, the amount (mol) of carboxy groups (--COOH) in the 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 the sample, and its measurement is performed in accordance with "JIS-K0070 Acid value, saponification value, ester value, etc. of chemical products. Test method for elemental value, hydroxyl value and unsaponifiable matter”.

For example, the addition amount of NaOH ^{is, for example, 5.44×10 −4 mol} to 8.16×10 If it is ^-4 mol, 40 to 60% of the hydrophobic groups will be converted to Na salt. In this case, the Na (atomic weight 23) concentration is in the range of 12.5 mg/g to 18.8 mg/g, and the Na concentration in the SA CeNa salt is 12.5 mg/g to 18.8 mg/g. The addition amount of the SA-CeNa salt to the coating film is preferably 0.5 to 1.5 wt%, and the Na concentration in the coating film is 0.0625 mg/g to 0.282 mg/g.

Then, a refinement process (fibrillation process, nano-ization) is performed. For example, a treatment using a micronization device (masscolloider, bead mill) is performed to nanosize the cellulose in the SA Ce salt dispersion. Thereby, a dispersion liquid of SA-CeNF salt can be obtained.

As described above, carboxylated or basic-treated cellulose may be subjected to fibrillation treatment (miniaturization treatment), and cellulose may be fibrillated (miniaturization treatment) and then carboxylated or basic treatment. may

B) Mixing step (stirring treatment step)
A coating liquid is prepared by adding an inorganic filler (second filler) to the dispersion liquid of the SA-CeNF salt (first filler) and stirring the mixture. The amount of the SA-CeNF salt is preferably 0.3 wt% or more, more preferably 0.5 wt% or more, relative to the total solid content of the coating liquid.

As the stirring method, for example, a method in which a blade attached to a shaft is rotated by a motor or the like, a vibration method using ultrasonic waves, or the like can be used. In addition, in order to reduce entrainment of air bubbles in the coating liquid, preparation (mixing and stirring) of the coating liquid may be performed under reduced pressure.

Moreover, you may add an antifoamer to a coating liquid. When adding an antifoaming agent, it is preferably 0.001% by mass or more and 1% by mass or less with respect to the total amount of solid components in the coating liquid. By adding an antifoaming agent, air bubbles can be preferably suppressed, and the defoaming process can be simplified or the defoaming process becomes unnecessary.

Known antifoaming agents such as silicone antifoaming agents (e.g., polydimethylsiloxane), surfactant antifoaming agents (e.g., polyethylene glycol fatty acid), and alcohols (e.g., acetylene diol) can be applied. However, among these, the silicone-based antifoaming agent is preferable from the viewpoint that it is less likely to adversely affect the battery. The shape of the antifoaming agent is not particularly limited, and any type such as oil type, compound type, emulsion type, powder type, etc. can be used.

The inorganic filler (second filler) is not particularly limited. For example, alumina, aluminum hydroxide, boehmite, nanosilica, carbon nanotubes, talc, glass fiber, etc. can be used. In particular, it is preferable to use alumina from the viewpoint that chemical reaction with the electrolytic solution is unlikely to occur and physical properties and manufacturing technology are stable. The particle shape of alumina is not limited, and for example, spherical or flat particles can be used. The average particle size (diameter) of alumina is preferably 200 nm or more and 1000 nm or less, more preferably 400 nm or more and 900 nm or less. The average particle size can be determined by a laser diffraction scattering method. Further, as alumina, it is preferable to use one having an average particle diameter larger than the average pore diameter of the microporous membrane used for the substrate S. Alumina having different average particle diameters may also be mixed and used. In addition, alumina may contain impurity elements (eg, Si, Fe, Na, Mg, Cu), but Si is 400 ppm or less, Fe is 300 ppm or less, Na is 200 ppm or less, Mg is 100 ppm or less, Cu is preferably 100 ppm or less.

Other additives include thickeners (e.g., carboxymethyl cellulose), binders (e.g., acrylic resins, acrylic binders), dispersants (e.g., surfactants), antifoaming agents (e.g., silicone antifoaming agents). agent) and the like may be added.

Carboxymethyl cellulose is a water-soluble cellulose, and by adding it to the coating liquid, the viscosity increases and the coatability improves. Also, the addition of the acrylic resin improves the adhesiveness of the materials in the coating liquid.

By adding a surfactant, the wettability to the substrate S is improved. In particular, when using a base material made of polyethylene or polypropylene, the free energy is large, and it is preferable to add a surfactant. If the amount of the surfactant added is too small, the wettability is not sufficiently improved, and if it is too large, the surfactant may prevent pores in the substrate. % or more and 5 wt % or less. Surfactants that can be used at this time are not particularly limited, but examples thereof include anionic, cationic, amphoteric, and nonionic surfactants. Suitable surfactants include Tamol SN (TAMOL® SN), Tamol® LG available from Rhom and Haas Company, Triton (available from Rhom and Haas Company). TRITON® series, MARASPERSE® series marketed by GAF, and IGEPAL® series marketed by GAF, TERGITOL® marketed by the same company. series, and STRODEX® PK-90, commercially available from BASF, PLURONIC® F-68, commercially available from Marasperse. KARASPERSE TU (trademark), and the like.

Here, the solid components of the coating liquid described above refer to the above-mentioned SA-CeNF salt (first filler), inorganic filler (second filler), thickener, binder, and dispersant contained in the coating liquid. is the total amount of agent.

The salt concentration (Na ion concentration when NaOH is added) in the coating liquid is preferably 1000 mg/L or more and 2000 mg/L or less. Moreover, the electric conductivity of the coating liquid is preferably 400 ms/m or more and 800 ms/m or less. The salt concentration in the coating liquid can be measured, for example, with an ion concentration measuring device (manufactured by Horiba Ltd.), and the electrical conductivity of the coating liquid can be measured with an electrical conductivity meter (manufactured by Horiba Ltd.). can.

<<3: Coating process to base material>>
The above coating solution is applied to the surface of the substrate S described in <<1: Substrate (porous film before coating) preparation step>>. Although the coating method is not limited, for example, a bar coater, lip coater, gravure coater and the like can be used. After coating, a coating film can be formed on the surface of the substrate S by drying the coating liquid.

As described above, according to the present embodiment, the characteristics of the porous film can be improved by adding the SA-CeNa salt to the coating liquid. In particular, the heat resistance of the porous film can be improved. In addition, by using a porous film coated with a coating liquid to which a CeNa salt of SA is added as a separator, battery characteristics (for example, output characteristics and cycle characteristics) can be improved.

Examples of the coating liquid, the porous film (separator), and the battery using the same according to the present embodiment are described below.

[Example A]
1: Substrate (porous film before coating) preparation step As the substrate S, for example, a commercially available polyethylene microporous membrane (manufactured by CS-TECH, average pore diameter 0.06 μm, thickness 16 μm) was used.

2: Coating liquid preparation process A) Preparation of SA-CeNFNa salt Carboxy groups were introduced into cellulose with succinic anhydride according to the process of Fig. 4 (part of -OH was changed to -COOH). The SA-modified cellulose dispersion had a pH of 4.0 and an acid value of 76.5 mg/g. Then, sodium hydroxide was added to convert the carboxy group to Na salt (part of --COOH was changed to --COONa). Then, defibration treatment was performed with a mass colloider to nanosize the cellulose. Table 1 shows the amount of sodium hydroxide added and the pH of the SA CeNFNa salt dispersion. The pH can be measured with a pH meter (manufactured by Horiba, Ltd.).

B) Stirring treatment After adding carboxymethyl cellulose (CMC), acrylic resin (binder), and octylphenol ethoxylate (Triton X) as a surfactant to the SA CeNFNa salt dispersion, high-purity alumina (manufactured by Sumitomo Chemical Co., Ltd. , average particle size of 700 nm). As a solvent, an aqueous solvent was further added to prepare a mixed solution. This mixed solution was stirred with a rotation-revolution stirrer (ARE310, manufactured by Thinky) at 2000 rpm for 30 minutes, and further stirred with a thin-film swirl stirrer (FILMIX, manufactured by Primix) at 25 m/s for 1 minute, and then coated. Got the working fluid. In the coating liquid, the ratio of solid components (cellulose, CMC, binder, surfactant, alumina) is 100 wt%, and Table 1 shows the solid component ratio and electrical conductivity (ms/m) of each coating liquid. show. This electrical conductivity has a correlation with the amount of Na added. In addition, as Comparative Example A, a coating liquid was also prepared in which the SA-CeNFNa salt dispersion liquid was not added.

In this example, the ratio (concentration) of the solid components (cellulose, CMC, binder, surfactant, alumina) to the solvent was 45 wt%, and this ratio (concentration) was about 20 to 60 wt%. It is possible to prepare

3: Coating process to base material (separator production process)
Any of the above coating liquids (coating liquids 1, 2, and A) is applied to the surface of the substrate (PE porous film) described in "1. Substrate (porous film before coating) preparation step". ) was coated on both sides with 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) 1, 2 and A each having a coating layer were formed (see Table 1). In addition, as Comparative Example B, a battery B, which will be described later, was produced using a porous film (separator) that was only a substrate without a coating layer formed thereon. In the coating liquids 1 and 2, there were no problems such as lumps or repelling of the substrate, and the coatability was good.

4: Fabrication of battery Positive electrode active material (NCM (lithium nickel cobalt manganese oxide, nickel-cobalt-lithium manganate (Li (Ni, Co, Mn) O ₂ )), conductive agent (acetylene black) and a binder (polyvinylidene fluoride (PVdF)) in a solid ratio of 94:3:3 wt%, and kneaded using a rotation-revolution mixer (manufactured by Thinky, Mixer, 2000 rpm, 15 minutes) to form a slurry. This slurry was applied onto a current collector (aluminum foil with a thickness of 15 μm) using an applicator, temporarily dried at 80° C., rolled by a roll press, and dried under reduced pressure (160° C., 10 hours). Thus, a positive electrode (positive electrode mixture layer) was formed with a capacity density of 3.10 mAh/cm ² .

As slurry for the negative electrode, a negative electrode active material (artificial graphite (graphite)), a conductive agent (acetylene black), a thickener (carboxymethyl cellulose (CMC)), and a binder (SBR (styrene-butadiene rubber)) at a solid ratio of 96: The ingredients were blended at a ratio of 1:1:2 wt %, and kneaded using a rotation-revolution mixer (manufactured by Thinky, Mixer, 2000 rpm, 15 minutes) to form a slurry. This slurry is applied to a current collector (10 μm thick copper foil) using an applicator, temporarily dried at 80° C., rolled by a roll press, and dried under reduced pressure (140° C., 10 hours) to form a negative electrode. (Negative electrode mixture layer) was formed. Note that the capacity density was set to 3.36 mAh/cm ² .

Using the above porous films (separators) 1, 2, A, positive electrodes, and negative electrodes, coin batteries (batteries 1, 2, A) were produced (see FIG. 3). In addition, 1 mol/L LiPF ₆ (EC:DEC=50:50 vol %, +VC 1 wt %) was used as an electrolytic solution.

5: Evaluation (Cycle characteristics)
Under the conditions of 30 ° C. and a cutoff voltage of 4.2 to 2.5 V, after 10 cycles of charging at a current of 0.1 C, by repeating charging and discharging at 0.5 C, coin batteries (batteries 1, 2, The cycle characteristics of A) were investigated. Also, the cycle characteristics were similarly examined under the condition of 60°C. FIG. 6 is a diagram showing cycle characteristics at 30.degree. C., and FIG. 7 is a diagram showing cycle characteristics at 60.degree. 6 and 7, the horizontal axis is the number of cycles, and the vertical axis is the battery capacity retention rate (%).

As shown in FIGS. 6 and 7, Battery 1 exhibited a better battery capacity retention rate than Battery A (Comparative Example A, simulated commercially available coating liquid). Battery 2 also showed a battery capacity retention rate equal to or better than that of battery A (comparative example A, simulated commercial coating liquid). In any battery, as the number of cycles increased, better battery capacity retention rate was exhibited than battery A (comparative example A, simulated commercial coating liquid).

Here, in comparison between Battery 1 and Battery 2, it was found that Battery 1 had a better battery capacity retention rate, and better cycle characteristics were obtained by adjusting the amount of Na.

Further, the Na concentration in the SA CeNa salt is 11.1 mg/g in the battery 1 and 22.3 mg/g in the battery 2, and the Na concentration in the SA CeNa salt is 5 mg/g to 30 mg/g. preferable. Furthermore, the Na concentration in the coating film is 0.05 mg/g in Battery 1 and 4.5 mg/g in Battery 2, and the Na concentration in the coating film is 0.05 mg/g to 5 mg/g. preferable.

(High rate charge/discharge characteristics)
Under the conditions of 30 ° C. and a cutoff voltage of 4.2 to 2.5 V, after 10 cycles of charging at a current of 0.1 C, discharging at 0.1 C to 6 C, and then repeating charging and discharging at 1 C, High rate discharge characteristics of coin cells (batteries 1, 2, A, B) were investigated.

FIG. 8 is a diagram showing the relationship between discharge rate and discharge voltage. The horizontal axis is the discharge rate (C rate, C), and the vertical axis is the average voltage (V). FIG. 9 is a diagram showing the relationship between discharge rate and discharge capacity. The horizontal axis is the discharge rate (C rate, C), and the vertical axis is the discharge capacity (mAh).

As shown in FIG. 9, all the batteries (batteries 1, 2, A, and B) had similar discharge capacities, but as shown in FIG. showed a better average discharge voltage than Battery A (Comparative Example A, simulated commercially available coating liquid) and Battery B (Comparative Example B, without coating film).

Also, in comparison between Battery 1 and Battery 2, Battery 1 tended to have a higher average discharge voltage than Battery 2, especially at a discharge rate of 2C or higher.

As described above, it was found that although the battery characteristics are improved by carboxylating cellulose to Na salt, there is a more suitable range for the addition amount as described above.

(Embodiment 2)
In Embodiment 1, SA-Ce salt was used as the first filler of the coating liquid, but oxidized cellulose having a structure in which primary hydroxyl groups are oxidized to carboxyl groups may be used.

The porous film of this embodiment and the method for producing the same will be described below. The porous film of the present embodiment can be used as a so-called battery separator.

[Structure description]
The porous film of the present 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 the present embodiment (FIG. 1) and the configuration of the lithium-ion battery using this porous film (FIGS. 2 and 3) are similar to the SA-Ce Since the salt is the same as in Embodiment 1 except that the salt becomes oxidized cellulose having a structure in which primary hydroxyl groups are oxidized to carboxyl groups, detailed description thereof will be omitted.

[Manufacturing method]
The manufacturing process of the porous film of the present embodiment will be described below, and the configurations of the porous film and the coating film will be clarified.
The manufacturing process of the porous film of the present embodiment has the following steps.

<<1: Preparatory process for base material (porous film before coating)>>
As the substrate S, a microporous membrane can be used. For example, a commercially available polyethylene microporous membrane can be used.

<<2: Coating liquid preparation process>>
A) Preparation of First Filler In the present embodiment, an example of using TEMPO-treated cellulose as oxidized cellulose having a structure in which primary hydroxyl groups are oxidized to carboxyl groups will be described below. TEMPO treatment (TEMPO oxidation treatment) is a treatment by an oxidation reaction using TEMPO (2,2,6,6-tetramethylpiperidine1-oxyl) as a catalyst. be. For this reason, TEMPO-treated cellulose is sometimes referred to as "TEMPO-oxidized cellulose."

Cellulose (Cell-OH) is a carbohydrate represented by (C ₁₂ H ₂₀ O ₁₀ ) _n and is represented, for example, by the aforementioned chemical structural formula (Formula 1).

When cellulose is subjected to TEMPO treatment, -OH, which is the primary hydroxyl group of cellulose, is regioselectively oxidized to C6-aldehyde group and then to C6-carboxyl group, and further treated with alkali. A salt of C6-carboxyl group (carboxylate), for example, when alkali treated with sodium hydroxide solution, it is converted to Na salt of C6-carboxyl group as follows.

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, and the reaction is carried out in water. During the reaction, a basic solution such as sodium hydroxide is added in an amount capable of maintaining an arbitrary pH, and the reaction is carried out to obtain cellulose subjected to TEMPO oxidation treatment (the above (chemical formula 5)). be able to.

In the oxidation by TEMPO treatment, not all the primary hydroxyl groups at the C6 position of the glucose unit are oxidized, but the primary hydroxyl groups at the C6 position exposed on the surface of the cellulose crystal structure are selectively oxidized. be. As a result, the primary hydroxyl group of one of the two glucose units is oxidized, as shown in the chemical structural formula (Formula 5) above. Of course, since it is a chemical reaction, it is not limited to oxidizing the primary hydroxyl group at the C6 position exposed on the surface of the crystal structure of cellulose. The above-mentioned expression "By adding sodium hydroxide and allowing it to react, cellulose subjected to TEMPO oxidation treatment ((Chem. 5) above) can be obtained" is represented by the chemical structural formula (Chem. 1). It means that the sodium salt represented by the chemical structural formula (Chem. 5) is obtained in at least part of the plurality of units of cellulose.

In the case of the TEMPO-treated cellulose (TEMPO-oxidized cellulose, TCe), the Na salt of the C6-carboxy group is ionized in water, so that the repulsive force (electrostatic repulsive force , osmotic pressure) works. Therefore, when the Na salt is arranged at a high density, it can be dispersed in a fine state.
FIG. 10 is a diagram schematically showing the structure of TEMPO-oxidized cellulose.

In TEMPO-oxidized cellulose, the amount of regioselective oxidation of —OH (that is, the amount of —COOH via the C6-aldehyde group) can be adjusted by adjusting the amount of oxidizing agent (for example, hypochlorous acid) added. Furthermore, by adjusting the amount of basic solution added, the amount of Na (amount of C6-carboxyl group converted to Na salt) can be adjusted.

Therefore, even in TEMPO-oxidized cellulose, the degree of oxidation (the amount of -COOH) can be calculated from the acid value, and the amount of substitution with Na salt can be calculated by adding a basic solution (here, NaOH aqueous solution) It can be calculated by quantity.

For example, with respect to TEMPO-oxidized cellulose containing 2.18×10 ⁻³ mol of carboxy groups (—COOH), the amount of NaOH added is, for example, 8.73×10 ⁻⁴ mol to 13.1×10 If it is ^-4 mol, 40 to 60% of the hydrophobic groups will be converted to Na salt. In this case, the Na (atomic weight 23) concentration is in the range of 20.1 mg/g to 30.1 mg/g, and the Na concentration in TCe is 20.1 mg/g to 30.1 mg/g. The addition amount of the SA-CeNa salt to the coating film is preferably 0.5 to 1.5 wt%, and the Na concentration in the coating film is 0.100 mg/g to 0.45 mg/g.

(defibration treatment)
Further, the TEMPO-oxidized cellulose may be defibrated to make the cellulose fine (nano). The fibrillation treatment (refining treatment) includes a chemical treatment method, a mechanical treatment method, and the like. You may use the method which combined these. By such fibrillation treatment (refinement treatment), in liquid, for example, width (minor diameter, length of shorter side) W is 1000 nm or less, length L is 500 μm or less, more preferably width W However, it becomes fine cellulose of 500 nm or less and length L of 3 μm or less. It is also confirmed that the width W is about 4 nm and the length L is about 2 μm.

The cellulose refining (nanization) as described above may be performed before the TEMPO oxidation treatment, or may be performed after the TEMPO oxidation treatment.

B) Mixing step (stirring treatment step)
A coating liquid is prepared by adding an inorganic filler (second filler) to the dispersion liquid of TCeNF (first filler) described above and stirring the mixture. TCeNF (first filler) is preferably 0.3 wt% or more and 5 wt% or less, more preferably 0.5 wt% or more and 1.5 wt% or less, relative to the total solid content of the coating liquid. preferable.

As the stirring method, for example, a method in which a blade attached to a shaft is rotated by a motor or the like, a vibration method using ultrasonic waves, or the like can be used. In addition, in order to reduce entrainment of air bubbles in the coating liquid, preparation (mixing and stirring) of the coating liquid may be performed under reduced pressure.

The inorganic filler (second filler) is not particularly limited, and the same materials as those described in Embodiment 1 can be used.

As other additives, thickeners (eg, carboxymethylcellulose), binders (eg, acrylic resins, acrylic binders), dispersants (eg, surfactants), and the like may be added. Materials similar to those described in the first embodiment can also be used for these materials.

Here, the solid component of the coating liquid described above is the total of the TCeNF (first filler), inorganic filler (second filler), thickener, binder, and dispersant contained in the coating liquid. quantity.

<<3: Coating process to base material>>
The above coating solution is applied to the surface of the substrate S described in <<1: Substrate (porous film before coating) preparation step>>. Although the coating method is not limited, for example, a bar coater, lip coater, gravure coater and the like can be used. After coating, a coating film can be formed on the surface of the substrate S by drying the coating liquid.

As described above, according to the present embodiment, the properties of the porous film can be improved by adding TCeNF (TEMPO-oxidized cellulose nanofibers) to the coating liquid. In particular, the heat resistance of the porous film can be improved. Moreover, by using a porous film coated with a coating liquid containing TCeNF as a separator, battery characteristics (for example, output characteristics and cycle characteristics) can be improved.

Examples of the coating liquid, the porous film (separator), and the battery using the same according to the present embodiment are described below.

[Example B]
1: Substrate (porous film before coating) preparation step As the substrate S, for example, a commercially available polyethylene microporous membrane (manufactured by CS-TECH, average pore diameter 0.06 μm, thickness 16 μm) was used.

2: Preparation step of coating liquid A) Preparation of TEMPO-oxidized cellulose TEMPO-oxidized cellulose was prepared. This TEMPO-oxidized cellulose was powdery with an average particle size of about 10 μm, and was produced using softwood-derived pulp as a raw material. Table 2 shows the amount of sodium hydroxide added and the pH of the TEMPO oxidized cellulose dispersion.

B) Stirring treatment After adding carboxymethyl cellulose (CMC), acrylic resin (binder), and octylphenol ethoxylate (Triton X) as a surfactant to the TEMPO oxidized cellulose dispersion, high-purity alumina (manufactured by Sumitomo Chemical Co., Ltd., average particle diameter of 700 nm) was added. As a solvent, an aqueous solvent was further added to prepare a mixed solution. This mixed solution was stirred with a rotation-revolution stirrer (ARE310, manufactured by Thinky) at 2000 rpm for 30 minutes, and further stirred with a thin-film swirl stirrer (FILMIX, manufactured by Primix) at 25 m/s for 1 minute, and then coated. Got the working fluid. In the coating liquid, the ratio of solid components (cellulose, CMC, binder, surfactant, alumina) is 100 wt%, and Table 2 shows the solid component ratio and electrical conductivity (ms/m) of each coating liquid. show. This electrical conductivity has a correlation with the amount of Na added. In addition, as Comparative Example A, a coating liquid was also prepared to which no TEMPO-oxidized cellulose dispersion was added.

In this example, the ratio (concentration) of the solid components (cellulose, CMC, binder, surfactant, alumina) to the solvent was 45 wt%, and this ratio (concentration) was about 20 to 60 wt%. It is possible to prepare

3: Coating process to base material (separator production process)
The above coating liquids (coating liquids 3, 4, 5, and A) are applied to the surface of the substrate (PE porous film) described in "1. Substrate (porous film before coating) preparation step". Either) was coated on both sides with 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 and A each having a coating layer were formed (see Table 2). In coating liquids 3, 4 and 5. There were no defects such as lumps or repelling of the base material, and the coatability was good. In addition, as Comparative Example B, a battery B, which will be described later, was produced using a porous film (separator) that was only a substrate without a coating layer formed thereon.

4: Fabrication of Battery Batteries (Batteries 3, 4, 5, A, and B) were fabricated in the same manner as in Examples of Embodiment 1, and cycle characteristics and high-rate charge/discharge characteristics were evaluated.

(Cycle characteristics)
FIG. 11 is a diagram showing cycle characteristics at 30.degree. C., and FIG. 12 is a diagram showing cycle characteristics at 60.degree. 11 and 12, the horizontal axis is the number of cycles, and the vertical axis is the battery capacity retention rate (%).

As shown in FIG. 11, all of the batteries (batteries 3, 4, and 5) exhibited better battery capacity retention rates than battery A (comparative example A, simulated commercial coating liquid). As shown in FIG. 12, Batteries 4 and 5 exhibited a better battery capacity retention rate than Battery A (Comparative Example A, simulated commercially available coating liquid). Battery 3 exhibited the same battery capacity retention rate as Battery A (Comparative Example A, simulated commercial coating liquid).

In particular, Batteries 4 and 5 exhibited better battery capacity retention rates than Battery A (Comparative Example A, simulated commercial coating liquid) as the number of cycles increased.

Here, in comparing batteries 3, 4, and 5, batteries 4 and 5 to which NaOH was added had better battery capacity retention rates than battery 3 to which NaOH was not added. Furthermore, it was found that Battery 5 had a better battery capacity retention rate than Battery 4 in terms of cycle characteristics at 30°C, but the battery capacity retention rate was reversed in terms of cycle characteristics at 60°C. In particular, in a high temperature range exceeding 30° C., it can be seen that better cycle characteristics can be obtained by adjusting the amount of Na.

The Na concentration in TCe is 9 mg/g in Battery 4 and 18 mg/g in Battery 5, and the Na concentration in TCe is preferably 5 mg/g to 30 mg/g. Further, the Na concentration in the coating film is 0.2 mg/g in Battery 4 and 12 mg/g in Battery 5, and the Na concentration in the coating film is preferably 0.1 mg/g to 15 mg/g. .

(High rate charge/discharge characteristics)
FIG. 13 is a diagram showing the relationship between discharge rate and discharge voltage. The horizontal axis is the discharge rate (C rate, C), and the vertical axis is the average voltage (V). FIG. 14 is a diagram showing the relationship between discharge rate and discharge capacity. The horizontal axis is the discharge rate (C rate, C), and the vertical axis is the discharge capacity (mAh).

As shown in FIG. 14, all the batteries (batteries 3, 4, 5, A, and B) had similar discharge capacities, but as shown in FIG. , 5 showed a better average discharge voltage than Battery A (Comparative Example A, simulated commercially available coating liquid) and Battery B (Comparative Example B, without coating film).

In addition, when comparing batteries 4 and 5, battery 4 tended to have a higher average discharge voltage than battery 5, especially at a discharge rate of 2C or higher.

As described above, it was found that although the battery characteristics are improved by carboxylating cellulose to Na salt, there is a more suitable range for the addition amount as described above.

[Example C]
Next, the relationship between the diameter (fineness) of TCeNF and battery characteristics was investigated.

1: Substrate (porous film before coating) preparation step As the substrate S, for example, a commercially available polyethylene microporous membrane (manufactured by CS-TECH, average pore diameter 0.06 μm, thickness 16 μm) was used.

2: Preparation step of coating liquid A) Preparation of TEMPO-oxidized cellulose TEMPO-oxidized cellulose was prepared. This TEMPO-oxidized cellulose was powdery with an average particle size of about 10 μm, and was produced using softwood-derived pulp as a raw material. Here, defibration treatment was performed on the TEMPO-oxidized cellulose. In sample 6, cellulose was miniaturized (nanized) by masscolloider treatment, and in sample 7, cellulose was miniaturized (nanized) by bead mill treatment. The bead mill treatment has a higher fibrillating ability than the masscolloid treatment, and can make the cellulose finer. In Examples A and B, the cellulose is made finer (nanized) by masscolloider treatment.

B) Stirring Treatment After adding carboxymethyl cellulose (CMC), acrylic resin (binder), and octylphenol ethoxylate (0.1 wt%) as a surfactant to the TEMPO oxidized cellulose dispersion, high-purity alumina (Sumitomo Chemical Co., Ltd. ) was introduced. As a solvent, an aqueous solvent was further added to prepare a mixed solution. This mixed solution was stirred with a rotation-revolution stirrer (ARE310, manufactured by Thinky) at 2000 rpm for 30 minutes, and further stirred with a thin-film swirl stirrer (FILMIX, manufactured by Primix) at 25 m/s for 1 minute, and then coated. Got the working fluid. In the coating liquid, the ratio of solid components (cellulose, CMC, binder, surfactant, alumina) was set to 100 wt %, and mixed in the same ratio as sample 5 in Table 2. The electrical conductivity (ms/m) of both samples 6 and 7 was 498 ms/s. In addition, as Comparative Example A, a coating liquid was also prepared to which no basic hydrophobized cellulose dispersion was added.

3: Coating process to base material (separator production process)
Any one of the above coating liquids (coating liquids 6, 7, and A) is applied to the surface of the substrate (PE porous film) described in "1. Substrate (porous film before coating) preparation step". ) was coated on both sides with 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) 6, 7 and A each having a coating layer were formed (see Table 2).

4: Fabrication of Battery Batteries (Batteries 6, 7, and A) were fabricated in the same manner as in Examples of Embodiment 1, and cycle characteristics and high-rate charge/discharge characteristics were evaluated.

(Cycle characteristics)
FIG. 15 is a diagram showing cycle characteristics at 30.degree. C., and FIG. 16 is a diagram showing cycle characteristics at 60.degree. 15 and 16, the horizontal axis is the number of cycles, and the vertical axis is the battery capacity retention rate (%).

As shown in FIGS. 15 and 16, both batteries (batteries 6 and 7) exhibited a better battery capacity maintenance rate than battery A (comparative example A, simulated commercial coating liquid).

In particular, in a high temperature range exceeding 30° C., better cycle characteristics were obtained in the battery 7 using the coating liquid 7 that was finely fibrillated.

(Embodiment 3)
In the examples of Embodiments 1 and 2, a commercially available polyethylene microporous membrane was used as the substrate S, but the polyethylene microporous membrane can be formed as follows.

FIG. 17 is a schematic diagram showing the configuration of the manufacturing apparatus of this embodiment. In the present embodiment, a separator manufacturing process using the manufacturing apparatus described above will be described.

For example, a plasticizer (liquid paraffin) and polyolefin (for example, polyethylene) are put into the raw material supply section of the twin-screw kneading extruder (S1) in FIG. 17, and the plasticizer and polyolefin are kneaded in the kneading section. The kneading conditions are, for example, 180° C. for 12 minutes, and the rotation speed of the shaft is 100 rpm.

The kneaded material (molten resin) is conveyed from the discharge part to the T-die S2, and the molten resin is cooled in the original fabric cooling device S3 while being extruded through the slit of the T-die S2, thereby forming a thin resin molding. .

Next, the thin film-shaped resin molding is stretched in the longitudinal direction by the first longitudinal stretching device S4, and further stretched in the lateral direction by the first lateral stretching device S5.

The stretched thin film is then immersed in an organic solvent (eg, methylene chloride) in extraction tank S6. In the stretched thin film, the polyolefin (eg, polyethylene) and the plasticizer (paraffin) are phase-separated. Specifically, the plasticizer (paraffin) becomes nano-sized islands. This nano-sized plasticizer (paraffin) is removed (degreased) with an organic solvent (eg, methylene chloride) in the extraction tank S6. Thereby, a porous thin film can be formed.

After that, the thin film is dried and heat-set while being stretched in the lateral direction by the second lateral stretching device S7 to relax the internal stress during the stretching. Next, the thin film conveyed from the second transverse stretching device S7 is wound up by the winding device S8.

Thus, a porous thin film (substrate of Embodiment 1) can be produced. Here, for example, a gravure coating device (S7') shown in FIG. 18 is incorporated between the second lateral stretching device S7 and the winding device S8. FIG. 18 is a cross-sectional view schematically showing the configuration of a gravure coating apparatus. This gravure coating device has two gravure rolls R. As shown in FIG. The gravure roll R has, for example, a plurality of oblique concave portions, and is arranged so that a part thereof is immersed in the coating liquid CL, and is rotated to hold the coating liquid in the oblique concave portions. The substrate S is coated with the coating liquid CL in this state.

By using the coating liquid CL described in Embodiment 1 as the coating liquid CL, coating films can be formed on both surfaces of the substrate. In addition, a drying device for the coating liquid can be appropriately incorporated as necessary.

As described above, the apparatus shown in FIGS. 17 and 18 can be used to efficiently manufacture high-performance separators.

As described above, the polyolefin (resin) and the plasticizer are melt-kneaded using a kneader, extruded into a sheet shape using an extruder, and then the kneaded product is stretched using a press or stretching machine. Forms a film (thin film).

Polyolefins that can be processed by ordinary extrusion, injection, inflation, blow molding, and the like are used. For example, polyolefins that can be used include homopolymers and copolymers of ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene, multistage polymers, and the like. Polyolefins selected from the group of these homopolymers, copolymers, and multi-stage polymers can also be used singly or in combination. Representative examples of the polymer 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, and polybutene. , ethylene propylene rubber and the like.

As the base material S, it is particularly preferable to use a resin containing polyethylene as a main component because it has a high melting point and requires high strength. Moreover, from the viewpoint of shutdown property, it is preferable that polyethylene accounts for 50 wt % or more of the resin component. Further, in the case of using an ultra-high molecular weight polyolefin having a molecular weight of 1,000,000 or more, if the ultra-high-molecular-weight polyolefin exceeds 50 parts by mass with respect to 100 parts by mass of the kneaded product (resin and dispersion), uniform kneading becomes difficult. Therefore, it is preferably 50 parts by mass or less.

Plasticizers improve flexibility and weather resistance in addition to thermoplastic resins. Furthermore, in the present embodiment, pores can be provided in the resin molded body (film) by removing the plasticizer in a degreasing step, which will be described later.

As the plasticizer, an organic solvent having a molecular weight of 100 to 1500 and a boiling point of 50°C to 300°C can be used. Specifically, liquid paraffin, nonane, decane, decalin, paraxylene, undecane, dodecane chain or cyclic aliphatic hydrocarbons, mineral oil fractions with boiling points corresponding to them, and dibutyl phthalate, dioctyl phthalate One or a mixture of several phthalates that are liquid at room temperature can be used. Among 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, One type or a mixture of several types can be used.

In the film (thin film) obtained by stretching the sheet-like kneaded material, the polyolefin and the plasticizer are in a phase-separated state. Specifically, the plasticizer becomes nano-sized islands. By removing the nano-sized plasticizer in the organic solvent treatment step, which will be described later, the island-shaped plasticizer portions become pores and a porous thin film is formed. A process of forming a separator in which a large number of fine pores are formed in a resin molding by removing a plasticizer is called a "wet method".

For example, by immersing the film (thin film) formed in the above stretching step in an organic solvent, the plasticizer in the film is extracted into the organic solvent and removed from the film (thin film).

As an organic solvent, methylene chloride, hexane, octane, cyclohexane, etc. can be used. Among them, it is preferable to use methylene chloride from the viewpoint of productivity.

Thereafter, the organic solvent on the surface of the film (thin film) is volatilized, and heat treatment (heat setting) is performed as necessary to obtain the substrate (microporous film) S.

[Example D]
The correlation between viscosity and coatability of heat-resistant coating liquids containing cellulose nanofibers was evaluated. Although 45%, 40%, and the like are indicated below as the solid content ratio of the coating liquid, they all mean % by mass.

(Raw material) The type of cellulose nanofiber was fixed only to Ce of SA, and coating solutions with different viscosities were produced by diluting with water. The solid content ratios of the coating liquids were diluted with water to 45%, 40%, 35% and 30%, respectively.

(Preparation of Coating Liquid) The raw materials of the coating liquid were put into a planetary mixer so as to have the above solid content ratio, and mixed for 60 minutes. The viscosity of the produced coating liquid at room temperature (25 ° C.) is 800 mPa s for 45%, 650 mPa s for 40%, 580 mPa s for 35%, and 420 mPa s for 30%. Met. The viscosity of the coating liquid is measured with a Zahn cup (manufactured by Allgood, No. 2, No. 3) that can be easily measured, and after confirming that there is a viscosity difference, a B-type viscometer (Brookfield DV-2T) was used to measure viscosity.

(Coating process)
Coatability was evaluated using a bar coater (Daiichi Rika Co., Ltd. No. 2) using the manufactured coating liquids with different viscosities. The model number of the bar coater was selected so that the coating film thickness was 4 μm on one side. In order to evaluate coatability, the coating solution was applied to the surface of a PE separator (manufactured by CS-TECH) using the aforementioned bar coater. When the solid content concentration was 45% (viscosity 800 mPa·s) and 40% (viscosity 650 mPa·s), the coatability was poor, and the coating film thickness increased to 25 μm and 20 μm, respectively. On the other hand, it was confirmed that when the solid content concentration was 35% (viscosity 580 mPa·s) and 30% (viscosity 420 mPa·s), the coating film thickness could be adjusted to about 4 µm.

Next, each of the trial coating liquids having different viscosities was fed using a general-purpose roller pump and coated on a PE separator using a gravure coater. It was confirmed that under conditions of 45% and 40% solid concentration, fluidity was poor and it was difficult to pump the liquid. In addition, separation of the coating liquid occurred in the chamber for storing the coating liquid, and the coating film thickness fluctuated during the operation. It was confirmed that separation of the coating liquid also occurred in the tube of the roller pump used for conveying the coating liquid. Furthermore, when the film is brought into contact with the gravure roll, the resistance of the roll increases instantaneously, the roll stops momentarily, and the coating thickness becomes thicker in some areas, causing poor drying in those areas. , problems such as staining of the transport roll on the downstream side and deterioration of the winding shape occurred. As for the coating film thickness, it was difficult to adjust the coating film thickness to 4 μm or less as with the bar coater.

On the other hand, when the solid content concentration was 35% and 30%, the above coating problems were not observed, and the coating could be performed without problems. However, when the solid content ratio is 35% (viscosity of 580 mPa s), if the continuous operation is performed for 1 hour, the solid and liquid components of the coating liquid will separate in the chamber and the hose of the roller pump, resulting in fluidity. It was confirmed that the decrease in In the gravure roll method, the coating liquid circulates between the chamber and the coating tank. It is presumed that the solid content concentration of the working solution increased and the fluidity decreased. On the other hand, at a solid content ratio of 30% (viscosity of 420 mPa·s), no decrease in fluidity was observed even after continuous operation for 1 hour.

When coating was performed at a high speed of 40 m/min or more, the viscosity increased in about 30 minutes at a solid content concentration of 35% (viscosity of 580 mPa·s), resulting in poor coating. On the other hand, when the solid content concentration was 30% (viscosity of 420 mPa·s), even if continuous operation was performed for 1 hour, no coating failure occurred, and a separator with uniform coating could be obtained.

From the above results, when using a gravure coater, the viscosity of the coating liquid is preferably 420 mPa·s or less at 25°C. Although the tube pump used for conveying the coating liquid is used in this embodiment, it is possible to stably convey the liquid by changing this to a Mono pump. However, separation of the coating liquid occurs in the feeding path of the coating liquid. Problems such as increase cannot be solved. Furthermore, it is possible to solve the above-mentioned problems by changing the gravure method to a die coater method and changing the feeding of the coating liquid to a mono pump. Moreover, it is also difficult to stably apply a thin film of about several μm required for a separator film for lithium ion batteries. Furthermore, compared with the gravure method, maintenance and management of the die are required, leading to an increase in cost. From the above results, in the patent of the present application, the viscosity of the heat-resistant coating liquid is preferably 400 mPa·s or less.

As a result of evaluating the heat resistance of the separator coated on both sides with a coating liquid having a solid content concentration of 30%, the thermal shrinkage rate at 200 ° C. is 0.5% in both the machine direction (direction perpendicular to the width direction) and the width direction. , and a sample with excellent heat resistance could be obtained.

(Application example)
In the above, the Na salt of the C6-carboxyl group shown in (Chem. 5) was exemplified, but compounds having other counter ions (X ⁺ ) such as the following may be used as cellulose. The counter ion is preferably an alkali metal ion such as K ⁺ and the like.

As in the case described using the chemical structural formula (Chemical Formula 5) described above, oxidation by TEMPO treatment does not oxidize all the primary hydroxyl groups at the C6 position of the glucose unit, and the crystal structure of cellulose The primary hydroxyl group at the C6 position exposed on the surface is selectively oxidized. As a result, the primary hydroxyl group of one of the two glucose units is oxidized, as shown in the chemical structural formula (Formula 5) above. Of course, since it is a chemical reaction, it is not limited to oxidizing the primary hydroxyl group at the C6 position exposed on the surface of the crystal structure of cellulose. Therefore, the above-mentioned expression "using a compound having another counter ion (X ⁺ ) as cellulose" means that at least a part of the plurality of units of cellulose represented by the chemical structural formula (Chemical Formula 1) has the chemical structure It means that the compound having the counter ion (X ⁺ ) represented by the formula (Chem. 6) is used as the TEMPO-oxidized cellulose.

As the raw material of cellulose, those derived from plant fibers such as pulp or those derived from animal fibers such as sea squirt may be used.

Moreover, in the above description, the lithium ion battery is exemplified, but a metal lithium battery, a lithium polymer battery, an air lithium ion battery, or the like may also be used. Moreover, you may use as a separator of nonaqueous electrolyte secondary batteries, such as a sodium ion battery, a potassium ion battery, a calcium ion battery, a magnesium ion battery, and an aluminum ion battery. These batteries refer to battery systems in which ions (carriers) responsible for electric conduction are replaced with cations such as sodium, calcium, magnesium, and aluminum instead of lithium.

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

1 positive electrode 1M positive electrode mixture layer 1S current collector 2 negative electrode 2M negative electrode mixture layer 2S current collector 6 can (battery can)
7 lid (battery cap)
8 Washer CF Coating film CL Coating liquid R Gravure roll S Base material S1 Twin-screw kneading extruder S2 Die S3 Raw fabric cooling device S4 First longitudinal stretching device S5 First lateral stretching device S6 Extraction tank S7 Second lateral stretching device S7' Gravure coating device S8 Winding device SP Separator

Claims (19)

A porous film coating liquid comprising a first filler, a second filler, and a solvent,
The first filler is a cellulose fiber having a structure in which a primary hydroxyl group is oxidized to a carboxyl group,
A coating liquid for a porous film, wherein the second filler contains at least one material selected from nanosilica, carbon nanotubes, talc, alumina, aluminum hydroxide, boehmite, and glass fiber. In the coating liquid according to claim 1,
A coating liquid for a porous film, wherein the first filler is TEMPO-oxidized cellulose represented by the following chemical formula.

In the coating liquid according to claim 1 or 2,
A coating liquid for a porous film, having a Na ion concentration of 1000 mg/L or more and 2000 mg/L or less. In the coating liquid according to claim 1 or 2,
A coating liquid for a porous film, having an electrical conductivity of 400 mS/m or more and 800 mS/m or less. In the coating liquid according to any one of claims 1 to 4,
A coating liquid for porous films having a viscosity of 400 mPa·s or less at 25°C. A porous film having a porous substrate and a coating film provided on the surface of the porous substrate,
The coating film has a first filler and a second filler,
The first filler is a cellulose fiber having a structure in which a primary hydroxyl group is oxidized to a carboxyl group,
The porous film, wherein the second filler comprises at least one material selected from nanosilica, carbon nanotubes, talc, alumina, aluminum hydroxide, boehmite, and glass fiber. In the porous film according to claim 6,
The porous film, wherein the first filler is TEMPO-oxidized cellulose represented by the following chemical formula.

In the porous film according to claim 6,
The porous film, wherein the Na concentration in the coating film is 0.05 mg/g or more and 5 mg/g or less. In the porous film according to claim 7,
The porous film, wherein the Na concentration in the coating film is 0.1 mg/g or more and 15 mg/g or less. 7. The porous film according to claim 6, wherein said coating film has a thickness of 0.5 [mu]m or more, and a heat shrinkage rate of 5% or less before and after standing in a 200[deg.] C. environment. A lithium ion battery comprising a positive electrode, a negative electrode, a separator, and an electrolytic solution,
A lithium ion battery comprising the porous film according to any one of claims 6 to 10 as the separator. A method for producing a coating liquid for a porous film containing a first filler, a second filler, and a solvent,
(a) introducing a carboxyl group into the cellulose as the first filler;
(b) adding sodium hydroxide to the carboxylated first filler;
(c) after step (b), adding a second filler material selected from nanosilica, carbon nanotubes, talc, alumina, aluminum hydroxide, boehmite, and glass fiber; A method for producing a coating liquid for quality films. In the method for producing a coating liquid for a porous film according to claim 12,
The pH in the step (a) is 2 to 6,
A method for producing a coating liquid for a porous film, wherein the pH in the step (b) is 5 to 10. (a) introducing a carboxyl group into the cellulose as the first filler;
(b) adding sodium hydroxide to the carboxylated first filler;
(c) After the step (b), a coating liquid is formed by adding a material selected from nanosilica, carbon nanotubes, talc, alumina, aluminum hydroxide, boehmite, and glass fiber as a second filler. the process of
(d) applying the coating liquid to the surface of the porous substrate to form a coating film;
A method for producing a porous film. In the method for producing a porous film according to claim 14,
The pH in the step (a) is 2 to 6,
The method for producing a porous film, wherein the pH in the step (b) is 5-10. (a) forming a porous film as a separator;
(b) disposing the porous film between a positive electrode material and a negative electrode material;
has
The step (a) is
(a1) introducing a carboxyl group into the cellulose as the first filler;
(a2) adding sodium hydroxide to the carboxylated first filler;
(a3) forming a coating liquid by adding a material selected from nanosilica, carbon nanotubes, talc, alumina and glass fiber as a second filler after the step (a2);
(a4) applying the coating liquid to the surface of the porous substrate to form a coating film;
A method for manufacturing a lithium ion battery. In the method for manufacturing a lithium ion battery according to claim 16,
The pH in the step (a1) is 2 to 6,
The method for producing a lithium ion battery, wherein the pH in the step (a2) is 5-10. In the method for manufacturing a lithium ion battery according to claim 17,
The method for producing a lithium ion battery, wherein in the step (a2), the cellulose becomes Na salt. In the method for manufacturing a lithium ion battery according to claim 17,
A method for producing a lithium ion battery, wherein the steps (a1) and (a2) are steps of forming TEMPO oxidized cellulose represented by the following chemical formula.

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