JP2015046230A - Separator for lithium ion secondary battery and method of manufacturing separator for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for lithium ion secondary battery having high heat resistance and a low internal resistance by compounding a nonwoven fabric, inorganic particles and an organic binder, and to provide a method of manufacturing a separator for lithium ion secondary battery.SOLUTION: In a separator for lithium ion secondary battery having high heat resistance produced by compounding a nonwoven fabric, inorganic particles and an organic binder, surface of the inorganic particles is coated with a hydrogenated styrene-butadiene rubber. A coating liquid containing inorganic particles and an organic binder is given, and the medium of the coating liquid is dried and removed. Subsequently, a process liquid produced by dissolving a hydrogenated styrene-butadiene rubber into hydrocarbon is given, and the hydrocarbon is dried and removed.

Description

  The present invention relates to a separator for a lithium ion secondary battery and a method for producing a separator for a lithium ion secondary battery.

  Lithium ion secondary batteries (hereinafter may be abbreviated as “batteries”) have a high energy density, and thus are attracting attention as power storage devices used in electric vehicles, electric storage systems, and the like. However, since a lithium ion secondary battery uses a flammable electrolyte solution or a negative electrode inside, there is a risk that an internal short circuit or the like triggers ignition. In particular, starting from small perforations in a separator for a lithium ion secondary battery (hereinafter sometimes abbreviated as “separator”), “short circuit” — “heat generation due to short circuit current” — “expansion of perforations due to contraction of separator” — An event (thermal runaway) in which the short-circuit current suddenly increases by repeating the cycle of “increasing short-circuit current” is known as a particularly dangerous event.

  As separators for lithium ion secondary batteries, porous films made of polyolefins such as polyethylene and polypropylene are widely used. However, separators made of these porous films tend to shrink when exposed to high temperatures, and are not preferable separators from the viewpoint of preventing thermal runaway. In order to suppress shrinkage when exposed to high temperatures, a separator provided with a coating layer mainly composed of heat-resistant particles on the surface of a porous film has been proposed, but heat shrinkage can be suppressed to a desirable level. Has not reached.

  As a separator with low shrinkage at high temperatures, a separator formed by forming a layer containing heat-resistant particles such as inorganic particles on a nonwoven fabric made of fibers having good heat resistance, such as polyethylene terephthalate, has been proposed. As an example, a separator having a porous inorganic coating on and in a nonwoven fabric, the inorganic coating having aluminum (Al), silicon (Si) and / or zirconium (Zr) oxide particles. It has been proposed (see, for example, Patent Document 1). This separator is composed only of non-flexible inorganic material, so damage such as powder falling or cracking due to impact or deformation is likely to occur. Since it is easy to produce a short circuit, it was hard to say that it was a useful separator.

  In order to avoid such problems, the surface or pores of a porous substrate coated with a mixture of inorganic particles and a binder polymer (organic binder) is coated with styrene-butadiene rubber. A separator has been proposed (see, for example, Patent Document 2). The separator obtained by this technology is an excellent separator in that it does not easily cause powder falling or cracking due to impact or deformation, and is difficult to cause an internal short circuit, but has a problem of high internal resistance. It was hard to say a separator.

JP 2005-536658 Gazette Special table 2008-521964

  An object of the present invention is to provide a lithium ion secondary battery separator having a high internal resistance and a low heat resistance, which is a composite of a nonwoven fabric, inorganic particles and an organic binder. Moreover, it is providing the manufacturing method of the separator for lithium ion secondary batteries for manufacturing the said separator for lithium ion secondary batteries.

  In order to solve the above problems, intensive research was conducted and the following means were found.

(1) A lithium ion secondary battery separator comprising a composite of a nonwoven fabric, inorganic particles and an organic binder, wherein the surface of the inorganic particles is coated with hydrogenated styrene-butadiene rubber. Secondary battery separator.

(2) After applying a coating liquid containing inorganic particles and an organic binder to the nonwoven fabric, the medium of the coating liquid is dried and removed, and then a treatment liquid in which hydrogenated styrene-butadiene rubber is dissolved in hydrocarbon is applied. After that, a method for producing a separator for a lithium ion secondary battery, wherein the hydrocarbon is removed by drying.

  Non-woven fabric, inorganic particles and organic binder are combined, and the surface of the inorganic particles is coated with hydrogenated styrene-butadiene rubber. The lithium ion secondary battery separator has high heat resistance and internal resistance. A lithium ion secondary battery separator can be obtained.

  The separator of the present invention is a separator for a lithium ion secondary battery in which a nonwoven fabric, inorganic particles, and an organic binder are combined, and the surface of the inorganic particles is coated with hydrogenated styrene-butadiene rubber. . The separator of the present invention is composed of a nonwoven fabric, inorganic particles and an organic binder, and the surface of the inorganic particles is not coated with hydrogenated styrene-butadiene rubber. It has excellent characteristics. Hereinafter, the reason why such an effect is obtained will be described.

  The main factor that increases the internal resistance of the lithium ion secondary battery is the concentration polarization of the electrolyte. The concentration polarization of the electrolyte means that lithium ion migration between the positive electrode and the negative electrode through the electrolyte solution penetrating into the separator is not smoothly performed, so in the case of discharge, the lithium ion concentration near the positive electrode is insufficient, In addition, there are a phenomenon in which the lithium ion concentration near the negative electrode becomes excessive, and in the case of charging, the lithium ion concentration in the vicinity of the positive electrode becomes excessive and the lithium ion concentration in the vicinity of the negative electrode becomes insufficient. In order to minimize concentration polarization and obtain a battery with low internal resistance, it is necessary to remove factors that hinder the movement of lithium ions between the positive electrode and the negative electrode.

  By the way, in the electrolyte solution of the lithium ion secondary battery, lithium ions are not bare ions but exist in a state solvated with molecules of the electrolyte solvent. Generally, when the affinity between the liquid phase and the solid phase is high, the frictional force generated between the liquid phase and the solid phase increases. In order for the solvated lithium ions to move smoothly in the electrolyte existing in the pores of the separator, the material constituting the walls of the pores is a material having a low affinity for the electrolyte solvent Must be configured.

  Since inorganic particles are compounds composed of a nonmetallic element having a high electronegativity and a metal element having a low electronegativity, the surface thereof is generally strongly polarized. On the other hand, the solvent used for the electrolyte solution is a cyclic carbonate such as propylene carbonate or ethylene carbonate in that it has a sufficiently high polarity to dissolve the lithium salt and has high electrochemical stability. A highly polar mixed solvent mainly composed of a chain carbonate such as ester, diethyl carbonate, dimethyl carbonate, or ethyl methyl carbonate is used.

  Since both the inorganic particles and the solvent used for the electrolytic solution are highly polar substances, they generally have a high affinity. In such a case, the movement of lithium ions solvated with the molecules of the solvent is hindered by a large frictional force with the wall surfaces of the pores in the separator. This is the reason why it is difficult to obtain a low internal resistance. The present invention pays attention to this point, and in a separator formed by combining a nonwoven fabric, inorganic particles and an organic binder, the surface of the inorganic particles has a low polarity, in other words, a hydrogenated styrene-butadiene having a low affinity with an electrolyte solvent. By coating with rubber, the friction force between the inorganic particles and lithium ions solvated with electrolyte solvent molecules is reduced, and as a result, a separator having a low internal resistance is obtained.

  Hydrogenated styrene-butadiene rubber is a styrene-butadiene copolymer synthesized by a solution polymerization method, in which hydrogen is allowed to act in the presence of a catalyst, so that almost all of the double bonds present in the butadiene unit become saturated bonds. This is a converted polymer. Hydrogenated styrene-butadiene rubber is not only different in chemical structure, but also has chemical and physical properties that differ greatly from styrene-butadiene rubber in that there is no restriction on main chain rotation due to double bonds. It is a polymer. Since hydrogenated styrene-butadiene rubber has very low polarity, aromatic hydrocarbons such as toluene and xylene, aliphatic hydrocarbons such as cyclohexane, methylcyclohexane, heptane, octane and decane, alicyclic hydrocarbons, etc. The non-polar solvent dissolves well, while polar solvents such as cyclic carbonates and chain carbonates hardly dissolve. Examples of commercially available hydrogenated styrene-butadiene rubbers include Dynalon (registered trademark) 1320P, 1321P, and 2324P manufactured by JSR Corporation.

  The composition of the inorganic particles used in the separator of the present invention is not particularly limited as long as it is not unstable with respect to the electrolyte solvent or has conductivity that causes a short circuit inside the battery. For example, metal carbonates such as magnesium carbonate and calcium carbonate; metal hydroxides such as magnesium hydroxide; metal oxides such as magnesium oxide, calcium oxide, aluminum oxide, zirconium oxide and zinc oxide; metal oxide water such as boehmite Non-metal oxides such as silicon oxides; various inorganics such as poorly soluble metal salts such as magnesium fluoride, calcium fluoride, aluminum fluoride, barium sulfate, complex oxides such as zeolite and kaolin, and hydrates thereof Particles can be used. From the viewpoint of particularly improving the safety of the battery, it is preferably inorganic particles that cause an endothermic decomposition reaction when the temperature exceeds the battery operating temperature, for example, 200 ° C., such as boehmite or Magnesium hydroxide is preferred.

  There is no restriction | limiting in particular also about the shape of the inorganic particle used for the separator of this invention. For example, the shape may be a substantially spherical shape, a rectangular parallelepiped shape, or a plate shape. However, from the viewpoint of highly forming the voids between the inorganic particles forming the pores in the separator serving as the lithium ion flow path, the anisotropic shape, for example, needle-like, fibrous, most The large surface preferably has a shape such as a flat plate that is not a quadrilateral. When dispersed in a medium, it may be inorganic particles in which primary particles are dispersed as they are, or may be inorganic particles in which primary particles are aggregated to form higher-order aggregates than secondary aggregates. Can also be used.

  In the separator of the present invention, an organic binder is used to bind inorganic particles. Specifically, for example, a polymer such as ethylene-vinyl acetate copolymer, a copolymer mainly composed of (meth) acrylic acid ester, styrene-butadiene rubber, polyvinyl alcohol, polyvinyl butyral, and polyurethane can be used. . These organic binders may be used alone or in combination of two or more. These resins are preferably those obtained by copolymerizing a small amount of polar monomers such as (meth) acrylonitrile, (meth) acrylic acid, crotonic acid and maleic anhydride in order to enhance the interaction with the inorganic particles. . Moreover, it is preferable that these polymers introduce | transduce the crosslinked structure in order to prevent melt | dissolution to electrolyte solution. Since the polymer into which the crosslinked structure is introduced cannot be made into a solution, it is usually added in the form of an aqueous dispersion such as latex. A latex of a copolymer mainly composed of styrene-butadiene or alkyl (meth) acrylate and copolymerized with a small amount of a polar monomer such as (meth) acrylic acid has a strong binding force to inorganic particles, and is particularly preferably used.

  The method for producing a separator for a lithium ion secondary battery of the present invention (hereinafter sometimes abbreviated as “manufacturing method”) provides a coating liquid containing inorganic particles and an organic binder, and then the medium of the coating liquid A non-woven fabric (hereinafter sometimes abbreviated as “coated substrate”) obtained by drying is applied with a treatment solution in which hydrogenated styrene-butadiene rubber is dissolved in hydrocarbon, and then the hydrocarbon of the treatment solution is added. Is removed by drying.

  In order to produce a separator for a lithium ion secondary battery according to the present invention, inorganic particles whose surfaces are coated with hydrogenated styrene-butadiene rubber in advance are dispersed in a medium together with an organic binder and applied to a nonwoven fabric. There are the following problems. That is, inorganic particles whose surfaces are coated with hydrogenated styrene-butadiene rubber have a low affinity for a medium other than a non-polar organic solvent such as hydrocarbon, and therefore, an aqueous coating solution or a coating using a polar organic solvent as a medium. It is difficult to prepare a working solution. In addition, when a nonpolar organic solvent such as hydrocarbon is used as the medium, the hydrogenated styrene-butadiene rubber covering the inorganic particles is redissolved and the inorganic particles are exposed.

  On the other hand, if it is the manufacturing method of this invention which coat | covers the surface of an inorganic particle with hydrogenated styrene-butadiene rubber first after preparing a coated base material, an inorganic particle was disperse | distributed in water or a polar organic solvent. It is possible to apply the coating liquid to the nonwoven fabric by a conventionally known method, and a conventionally known latex or the like can also be used as the organic binder. Moreover, it is not necessary to perform a process in which the inorganic particles are exposed after the surface of the inorganic particles is coated by applying a treatment liquid in which hydrogenated styrene-butadiene rubber is dissolved in hydrocarbon.

  In the production method of the present invention, the solid content concentration of the treatment liquid in which hydrogenated styrene-butadiene rubber is dissolved in hydrocarbon is preferably 0.5 to 5% by mass. When the solid content concentration is too low, the surface of the inorganic particles is not sufficiently covered, and the effect of the present invention that the internal resistance is low may not be sufficiently obtained. When the solid content concentration is too high, pores in the separator may be clogged, and the internal resistance may be increased. A suitable amount of the treatment liquid applied to the coated substrate is an amount obtained when excess treatment liquid remaining on the surface that is not absorbed by the coated substrate is removed. The suitable application amount varies depending on the ratio of voids contained in the coated substrate, and the more applied amount is more suitable for the coated substrate having more voids. In the present invention, by defining the concentration of the treatment liquid as described above, when the excess treatment liquid that is not absorbed by the coated substrate and remains on the surface of the coated substrate is removed, The application amount of the added styrene-butadiene rubber can be automatically adjusted to a suitable application amount.

  In the production method of the present invention, as a medium used for a treatment liquid in which hydrogenated styrene-butadiene rubber is dissolved in hydrocarbon, aromatic hydrocarbons such as toluene and xylene, cyclohexane, methylcyclohexane, heptane, octane, decane, etc. Aliphatic hydrocarbons and alicyclic hydrocarbons are preferably used. Among these, aromatic hydrocarbons such as toluene and xylene have a strong solubility in hydrogenated styrene-butadiene rubber, and clog pores in the separator. It is preferably used because it can prepare a uniform treatment solution that does not contain undissolved substances that cause it. It should be noted that a small amount of polar solvent can be added to such a medium in order to prevent a small amount of water due to moisture in the air from causing processing spots. Examples of the polar solvent include acetone, methyl ethyl ketone, and ethyl acetate.

  In the present invention, the constituent material of the nonwoven fabric includes polyethylene terephthalate, polybutylene terephthalate and derivatives thereof, polyester such as aromatic polyester and wholly aromatic polyester; polyolefin; acrylic; polyacetal; polycarbonate; aliphatic polyketone, aromatic polyketone, etc. Polyketones of: Polyamides such as aliphatic polyamides, semi-aromatic polyamides, wholly aromatic polyamides, polyimides, polyamideimides, polyphenylene sulfides, polybenzimidazoles, polyether ether ketones, polyether sulfones, poly (para-phenylene benzobisthiazoles) Poly (para-phenylene-2,6-benzobisoxazole); fluororesin such as polyvinylidene fluoride and polytetrafluoroethylene; Fibers are exemplified made of a resin such as polyvinyl chloride; alcohol; polyurethane. The nonwoven fabric used in the present invention may contain two or more of these constituent materials.

  In the present invention, the nonwoven fabric preferably contains 50% by mass or more of fibers having a diameter of 3.5 μm or less. This can more reliably prevent the in-plane coating amount distribution from becoming uneven. The thickness of the nonwoven fabric is preferably 10 μm or more, more preferably 15 μm or more. This can prevent the distribution of the coating liquid from becoming non-uniform. On the other hand, when the nonwoven fabric is too thick, the separator is too thick, so the thickness of the nonwoven fabric is preferably 30 μm or less, more preferably 25 μm or less.

In the present invention, there is no particular limitation on the method for applying a coating liquid containing inorganic particles and an organic binder to a nonwoven fabric, and the method for applying a treatment liquid in which hydrogenated styrene-butadiene rubber is dissolved in hydrocarbon to a coated substrate. An air doctor coater, blade coater, knife coater, rod coater, squeeze coater, impregnation coater, gravure coater, kiss roll coater, die coater, reverse roll coater, transfer roll coater, spray coater and the like can be used. In the present invention, the application amount of the coating liquid containing inorganic particles and the organic binder to the nonwoven fabric is preferably 3.0 to 30.0 g / m ² as the application amount after the medium is removed by drying. 20 g / m ² is more preferable. When the applied amount is too small, a good battery may not be obtained because the pore size of the obtained separator is too large and an internal short circuit occurs. On the other hand, if the applied amount is too large, the internal resistance may increase.

  In the present invention, a drying method used for drying and removing a medium of a coating liquid containing inorganic particles and an organic binder, or drying and removing a medium used for a treatment liquid in which hydrogenated styrene-butadiene rubber is dissolved in hydrocarbon, The drying method is not particularly limited, and various drying methods such as an air dryer, an infrared dryer, microwave drying, and vacuum drying can be used. Among these, drying with an air dryer is preferably used since the drying speed is high and the productivity is high.

  Examples of the present invention will be described below.

Example 1
[Nonwoven fabric]
It consists of 70 parts by mass of stretched crystallized polyethylene terephthalate staples with a fineness of 0.1 dtex and a cut length of 3 mm, and 30 parts by mass of non-stretched polyethylene terephthalate staples with a fineness of 0.2 dtex and a cut length of 3 mm. A wet papermaking nonwoven fabric having a basis weight of 10 g / m ² and a thickness of 18 μm was prepared by adjusting the thickness while fusing.

[Preparation of coating solution]
Carboxymethyl cellulose having a mass composition ratio of solid content of alumina hydrate (boehmite): acrylic ester latex: sodium salt of maleic acid-acrylic acid copolymer: viscosity of 1% by mass aqueous solution is 7000 mPa · sec = 40: A coating solution having a solid content concentration of 40% by mass, which was 2: 0.4: 0.2, was prepared.

[Preparation of treatment solution]
Toluene was added to 5 g of hydrogenated styrene-butadiene copolymer (manufactured by JSR Corporation, Dynalon (registered trademark) 1320P) and 25 g of methyl ethyl ketone to make the total amount 500 g, and the mixture was completely stirred and dissolved to give a solid content concentration of 1% by mass. A liquid was prepared.

[Preparation of coated substrate]
The coating liquid is applied to the nonwoven fabric using a wire bar so that the applied amount after drying is 8 g / m ^2, and water as a medium is removed by drying using a hot air dryer. A coated substrate was prepared.

[Preparation of separator]
The coated substrate was passed through the treatment liquid to give a treatment liquid. The surface of the coated substrate to which the treatment liquid has been applied is gently wiped with filter paper to remove excess treatment liquid adhering to the surface, and then the toluene and methyl ethyl ketone, which are media, are removed by drying using a hot air dryer. Thus, the separator of Example 1 was produced.

Example 2
A separator of Example 2 was produced in the same manner as in Example 1 except that the applied amount of the coating liquid on the coated substrate after drying was 10 g / m ² .

Comparative Example 1
The coated substrate of Example 1 without applying the treatment liquid was used as the separator of Comparative Example 1 as it was.

Comparative Example 2
The coated base material of Example 2 to which no treatment liquid was applied was used as a separator of Comparative Example 2 as it was.

Comparative Example 3
As the treatment liquid, an aqueous dispersion obtained by diluting a commercially available styrene-butadiene rubber latex with water (solid content concentration 1% by mass) was used instead of the methylated ketone-toluene mixed solvent solution of the hydrogenated styrene-butadiene copolymer. A separator of Comparative Example 3 was produced in the same manner as Example 1.

[Production of evaluation battery]
The positive electrode active material is lithium manganate, the negative electrode active material is mesophase carbon microbeads, the electrolyte is a solution of 1 mol of LiPF ₆ per liter in an EC: DEC (30:70 vol%) mixed solvent, and each separator is implemented Pouch-type lithium ion secondary batteries having a design capacity of 30 mAh, which are separators of Examples and Comparative Examples, were produced.

[Charging the evaluation battery]
The battery for evaluation was charged with a constant current at 30 mA. After the voltage between the positive and negative electrodes reached 4.2 V, constant voltage charging was performed at this voltage. The charging was terminated when the charging current decreased to 3 mA.

[Measurement of internal resistance]
Each test battery was charged, a constant current discharge of 30 mA, was recorded discharge starting 6 minutes after the voltage E _{30 (V).} Subsequently, after charging each evaluation battery again, constant current discharge was performed at ₆₀ mA, and voltage E ₆₀ (V) 3 minutes after the start of discharge was recorded. The internal resistance R (Ω) = (E ₃₀ −E ₆₀ ) /0.03 (A) was calculated. The results are shown in Table 1.

  As shown in Table 1, a separator for a lithium ion secondary battery in which a nonwoven fabric, inorganic particles and an organic binder are combined, and the surface of the inorganic particles is coated with hydrogenated styrene-butadiene rubber. Separators (Examples 1 and 2) are separators other than the present invention in which the surface of inorganic particles is not coated (Comparative Examples 1 and 2), and separators in which the surface of inorganic particles is coated with styrene-butadiene rubber (Comparison) Compared with Example 3), the internal resistance is low.

When Example 1 and Comparative Examples 1 and 2 in which the coating amounts of the inorganic particles and the organic binder are the same as 8 g / m ² are compared, the surface of the inorganic particles is coated with hydrogenated styrene-butadiene rubber. While the internal resistance of the separator of Example 1 is 4.2Ω, the internal resistance of the separator of Comparative Example 1 in which the surface of the inorganic particles is not coated is as high as 4.4Ω, and the surface of the inorganic particles is styrene-butadiene. The internal resistance of the separator of Comparative Example 3 coated with rubber is even higher at 4.6Ω. Further, when Example 2 and Comparative Example 2 in which the coating amount of the inorganic particles and the organic binder is the same as 10 g / m ² are compared, the surface of the inorganic particles is coated with hydrogenated styrene-butadiene rubber. While the internal resistance of the separator of Example 2 is 4.3Ω, the internal resistance of the separator of Comparative Example 2 in which the surface of the inorganic particles is not coated is as high as 4.6Ω.

  The separator for a lithium ion secondary battery and the separator for a lithium ion secondary battery of the present invention can be used as a separator for a lithium ion secondary battery or a lithium polymer secondary battery and a method for manufacturing the separator.

Claims (2)

  A lithium ion secondary battery separator comprising a nonwoven fabric, inorganic particles and an organic binder combined, wherein the surface of the inorganic particles is coated with hydrogenated styrene-butadiene rubber. Separator.   After applying a coating liquid containing inorganic particles and an organic binder to the nonwoven fabric, the medium of the coating liquid is dried and removed, and then a treatment liquid in which hydrogenated styrene-butadiene rubber is dissolved in hydrocarbon is applied, A method for producing a separator for a lithium ion secondary battery, wherein the hydrocarbon is removed by drying.