JP6016466B2 - Lithium ion battery separator coating liquid and lithium ion battery separator - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a lithium ion battery separator coating liquid and a lithium ion battery separator obtained by coating such a coating liquid on a nonwoven fabric substrate.

  In a lithium ion battery (hereinafter sometimes abbreviated as “battery”), a separator for preventing contact between positive and negative electrodes is used.

  A porous film made of polyethylene or polypropylene conventionally used as a separator for lithium ion batteries (hereinafter sometimes abbreviated as “separator”) has low heat resistance and has serious safety problems. That is, in the battery using such a porous film as a separator, when local heat generation occurs due to an internal short circuit or the like, the separator around the heat generating portion contracts and the internal short circuit further expands, causing a runaway May generate significant events such as ignition and rupture.

  In order to solve such problems, a separator containing inorganic particles such as boehmite and a fibrous material has been proposed (for example, see Patent Document 1). Furthermore, a separator in which inorganic particles are contained in a nonwoven fabric substrate composed of two or more layers has been proposed (see, for example, Patent Document 2). As an embodiment of this technique, a fibrous material is contained in the coating liquid. (For example, see paragraphs 0036 to 0041 and 0086 of Patent Document 1 and paragraph 0084 of Patent Document 2).

  However, when manufacturing a separator by these techniques, the coating liquid is likely to be pushed into the substrate due to the strong dynamic pressure in the coating apparatus, and also due to the wind pressure from a large amount of hot air for drying at high speed. The coating liquid is likely to be pushed into the substrate. This tendency is particularly noticeable when a separator is to be manufactured at high speed. For this reason, a phenomenon occurs in which the pressed coating liquid oozes out from the side opposite to which the coating liquid is applied (hereinafter sometimes referred to as “coating liquid back-through”), and the roll of the coating apparatus is soiled. Since the dirt adhering to the roll is re-transferred to the separator and the surface thereof becomes non-uniform, it is extremely difficult to produce separators by these techniques at a high speed.

  Furthermore, when fibers are included in the coating liquid, the fibers adhere to the coating apparatus, and the coating liquid adhesion amount at the corresponding part becomes insufficient or excessive, and a uniform coating layer is obtained. There is a problem that a phenomenon (hereinafter sometimes referred to as “streak”) occurs.

JP 2008-4439 A JP 2008-4442 A

  An object of the present invention is to prevent lithium from penetrating from a nonwoven fabric base material when a coating layer containing inorganic particles and an organic polymer binder is provided on the nonwoven fabric base material, thereby improving the productivity of the separator. An object of the present invention is to provide a separator for ion battery separator and a separator with high productivity and few pinholes obtained by applying the coating liquid to a nonwoven fabric substrate.

In the present invention, as a result of earnest research to solve the above-mentioned problems, a coating solution for a lithium ion battery separator used for coating on a nonwoven fabric substrate is an inorganic pigment, an organic polymer binder, a fiber length of 0.10 mm to 1. 20 mm, seen including a non-fibrillated fibers of the following fiber diameter of 1.0 .mu.m, that the material of the non-fibrillated fibers is a thermoplastic resin, and, on at least one surface of the nonwoven substrate, an inorganic pigment, an organic polymer binder, fibers long 0.10Mm～1.20Mm, Ri Na provided a coating layer comprising a non-fibrillated fibers of the following fiber diameter of 1.0 .mu.m, a non-fibrillated fibers of a material separator for lithium ion secondary battery Ru der thermoplastic resin It has been found that the above problem can be solved.

  By using the coating liquid for a lithium ion battery separator of the present invention, when coated, there is no generation of back-through or pinholes in the coating liquid, and the roll of the coating apparatus becomes dirty due to back-through of the coating liquid, Since pinholes can be prevented from occurring, high-speed coating and high-speed drying are possible, and a lithium ion battery separator without pinholes can be manufactured with extremely high productivity.

  The coating solution for a lithium ion battery separator of the present invention (hereinafter sometimes abbreviated as “coating solution”) is a coating solution containing an inorganic pigment and an organic polymer binder, and further has a fiber length of 0.10 mm to 1.20 mm. And a non-fibrillated fiber having a fiber diameter of 1.0 μm or less (hereinafter sometimes abbreviated as “non-fibrillated fiber according to the present invention”). About the non-fibrillated fiber concerning this invention, a more preferable fiber diameter is 0.8 micrometer or less, and it is preferable that the lower limit of a fiber diameter is 0.1 micrometer. Even when the coating liquid of the present invention is coated on a nonwoven fabric substrate having a large pore size, the non-fibrillated fibers according to the present invention are entangled with the fibers of the nonwoven fabric substrate and the coating liquid is retained in the nonwoven fabric substrate. In this way, the strikethrough is suppressed. When the fiber diameter exceeds 1.0 μm, the number of fibers per mass decreases, and the effect of suppressing the back-through of the coating liquid does not appear.

  The fiber length of the non-fibrillated fiber according to the present invention is 0.10 mm to 1.20 mm, preferably 0.10 mm to 0.50 mm. If the non-fibrillated fiber according to the present invention is too short, it will not be entangled with the fiber of the base material, and the suppression of the back-through which is the effect of the present invention will be insufficient. If the fiber length of the non-fibrillated fiber according to the present invention is too long, it will be easily caught on the surface of the coating apparatus, causing streaks. If the fiber length of the non-fibrillated fiber according to the present invention is too short, the effect of preventing the coating liquid from falling through becomes insufficient. In addition, when the fiber added to the coating liquid is a fibrillated fiber, the viscosity of the coating liquid increases and the coating operation becomes difficult, and the manufactured separator is closed by closing the pores of the formed coating layer. The internal resistance of the battery using is increased. The fiber diameter and fiber length of the non-fibrillated fiber according to the present invention are determined by observing the fiber with a scanning electron microscope and analyzing it by image analysis. The fiber diameter is the equivalent circular diameter, and the fiber length is the path length from one end of the fiber to the other end (the length when the fiber is straightened). Measurement is performed for 10 fibers each, and the median value of the measured values is defined as the fiber diameter and the fiber length.

  In the coating liquid of the present invention, the content of the non-fibrillated fibers according to the present invention is preferably 0.005% by mass to 0.050% by mass with respect to the entire coating liquid including the solvent and the dispersion medium. If the content is too low, the effect of adding the non-fibrillated fiber according to the present invention to the coating liquid of the present invention may not be sufficiently exhibited. Conversely, if the content is too high, defects such as streaks tend to occur during coating. The material of the non-fibrillated fiber according to the present invention is not particularly limited as long as it is a thermoplastic resin that does not affect the battery operation. Polyester such as polyethylene terephthalate (PET), 6-nylon, 6,6-nylon, etc. Examples include various synthetic fibers such as polyamide. On the other hand, glass fibers and whisker-like various inorganic fibers are too stiff and have a weak effect of suppressing the back-through of the coating liquid, and also tend to cause streaks during coating, and cannot be used as non-fibrillated fibers according to the present invention. .

  The inorganic pigment contained in the coating liquid of the present invention and the lithium ion battery separator is not particularly limited as long as it is suitable for use in the separator coating layer. Examples thereof include alumina such as α-alumina, β-alumina and γ-alumina, hydrated alumina such as boehmite, magnesium oxide, calcium oxide and the like. Among these, α-alumina or alumina hydrate is preferably used in terms of high stability to the electrolyte used in the lithium ion battery.

  The coating liquid of the present invention contains an organic polymer binder in order to increase the strength of the coating layer, and the organic polymer binder is not particularly limited as long as it is suitable for use in the separator coating layer. Examples thereof include (meth) acrylic acid ester polymers and styrene-butadiene polymers.

  The separator of the present invention is obtained by applying the coating liquid of the present invention to a nonwoven fabric substrate. In the production of the separator of the present invention, various coating apparatuses can be used as a method for coating the coating liquid on the nonwoven fabric substrate. As such a coating apparatus, various coaters such as a gravure coater, a die coater, a blade coater, a rod coater, and a roll coater can be used. Compared with conventional coating liquids, where it was difficult to use a coating apparatus that applies dynamic pressure in the depth direction during coating because the coating liquid will be exposed, productivity, etc. is more diverse. It is also an excellent feature of the coating liquid of the present invention that it can be selected from the viewpoint of the above.

  The nonwoven fabric substrate used in the separator of the present invention is not particularly limited, but in order to achieve the purpose of producing a separator having high heat resistance, the nonwoven fabric substrate is also preferably highly heat resistant. Is preferably composed of fibers having a high melting point. The fibers forming the nonwoven fabric substrate used in the separator of the present invention include polyolefins such as polypropylene and polyethylene, polyesters such as polyethylene terephthalate, polyethylene isophthalate and polyethylene naphthalate, acrylics such as polyacrylonitrile, 6,6-nylon, 6 Examples include various synthetic fibers such as polyamide such as nylon, various cellulose pulps such as wood pulp, hemp pulp and cotton pulp, and cellulose-based regenerated fibers such as rayon and lyocell. Among these, a nonwoven fabric base material mainly composed of polyester or polypropylene is preferred for reasons such as heat resistance and low hygroscopicity. Although the preferable fiber diameter of the fiber which forms a nonwoven fabric base material also depends on the physical property of the coating liquid to be used, it is preferable to exist in the range of 2-8 micrometers. The thickness of the nonwoven fabric substrate is preferably in the range of 15 to 30 μm. Moreover, if the porosity of the nonwoven fabric substrate obtained by {1- (basis weight ÷ fiber density) ÷ thickness} × 100 is too low, the internal resistance of the battery increases, and if it is too high, sufficient mechanical strength is obtained. From the absence, about 30% to 70% is preferable.

  In the production of the nonwoven fabric substrate used in the present invention, the fiber can be formed into a sheet by various production methods such as a spunbond method, a melt blow method, an electrostatic spinning method, and a wet method. Among these, the wet method is preferable because a thin and dense structure can be obtained. As a method for bonding fibers, various methods such as a chemical bond method and a heat fusion method can be used. Among these, the thermal fusion method is preferable because a nonwoven fabric substrate having a smooth surface can be obtained.

  In addition to the inorganic pigment and the organic polymer binder, the coating liquid used to form the coating layer of the present invention includes various dispersing agents such as polyacrylic acid and sodium carboxymethyl cellulose, hydroxyethyl cellulose, sodium carboxymethyl cellulose, polyethylene oxide and the like. Various additives such as various thickeners, various wetting agents, preservatives and antifoaming agents can be blended as necessary.

The separator of the present invention is provided with a coating layer formed by applying the coating liquid of the present invention, preferably 5 to 20 g / m ² , as the coating amount after drying to the nonwoven fabric substrate. If the coating amount is too small, the pinhole may not be sufficiently filled. When the coating amount is too large, the internal resistance may become too high, and the thickness of the separator becomes thick. The thickness of the separator of the present invention is preferably 15 to 35 μm. If the thickness is too thick, the internal resistance may be too high, and if the thickness is too thin, safety may not be ensured. In producing the separator of the present invention, the coating liquid of the present invention may be applied only to one side of the nonwoven fabric substrate, or may be applied to both sides. Moreover, you may apply | coat twice or more in each surface.

  EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to these examples. In Examples,% and parts are all based on mass unless otherwise specified.

<Preparation of nonwoven substrate>
For single component binder with fineness of 0.1 dtex (average fiber diameter of 3.0 μm), 50 mass parts of oriented crystallized PET short fibers with fiber length of 3 mm, fineness of 0.2 dtex (average fiber diameter of 4.3 μm), fiber length of 3 mm 50 parts by mass of PET short fibers (softening point 120 ° C., melting point 230 ° C.) were dispersed in water by a pulper to prepare a uniform papermaking slurry having a concentration of 1% by mass. This slurry for paper making is made up by a wet type machine with a circular net type paper machine, and with a cylinder dryer at 135 ° C., the PET short fibers for binders, and the PET short fibers for binders and the oriented crystallized PET short The nonwoven fabric having a basis weight of 12 g / m ² was obtained by fusing the intersections of the fibers to develop the tensile strength. Furthermore, this nonwoven fabric was subjected to conditions of a hot roll temperature of 200 ° C., a linear pressure of 100 kN / m, and a processing speed of 30 m / min using a 1-nip heat calender consisting of a dielectric heating jacket roll (metal hot roll) and an elastic roll. Was subjected to thermal calendering to prepare a nonwoven fabric substrate having a thickness of 18 μm. The density of the used PET fiber is 1.38 g / cm ³ , and the porosity of this nonwoven fabric substrate is 52%.

<Preparation of coating liquid A>
Sodium carboxymethyl cellulose having a viscosity of 200 mPa · s at 25 ° C. in a 1% by mass aqueous solution of boehmite having a volume average particle size of 0.9 μm and a BET specific surface area of 5.5 m ² / g dispersed in 150 parts of water. 75 parts of a 2% salt aqueous solution was added and mixed with stirring, and 10 parts of a carboxy-modified styrene-butadiene copolymer resin emulsion (solid content concentration 50%) having a glass transition point of −18 ° C. and a volume average particle size of 0.2 μm was added and stirred. After mixing, 20 parts of 0.5% dispersion of PET fiber with a fiber diameter of 0.8 μm and fiber length of 0.30 mm was added and mixed. Finally, adjusted water was added to adjust the solid content concentration to 25%. Liquid A was prepared.

<Preparation of coating liquids B to M>
Coating liquids B to M were prepared in the same manner as the coating liquid A, except that the fibers to be added were changed as described in Table 1. For aramid and cellulose, which are fibrillated fibers, the diameter of the non-fibrillated portion of the fiber was used as the fiber diameter.

<Coating>
The nonwoven fabric substrate was coated using a reverse gravure coater as a coating apparatus at a line speed of 30 m / min so that the adhesion amount as a liquid was 47 g / m ² . The coated substrate was dried by blowing hot air at 90 ° C. with a floating air dryer directly connected to a reverse gravure coater to obtain a separator. As the evaluation of “coating liquid back-through”, the coating liquid was classified into the following three stages depending on the state of the coating liquid adhering to the guide roll of the coating apparatus and the floating air dryer.

○: Almost no coating liquid adheres to the inside of the guide roll or floating air dryer.
Δ: The coating liquid adheres inside the guide roll or floating air dryer, but is not retransferred to the separator.
X: The coating liquid which penetrated has adhered to the inside of a guide roll or a floating air dryer, and the nonuniformity of the surface by retransfer has arisen in the obtained separator.

Furthermore, the obtained “coating layer uniformity” was classified into the following three stages.
○: The coating layer has no defects and is uniform.
Δ: Nonuniformity is observed in the coating layer, but there is no streak.
X: There is a streak in the coating layer.

<Production of evaluation battery>
Each separator was used, lithium manganate as the positive electrode, mesocarbon microbeads as the negative electrode, and 1 mol / L diethyl carbonate / ethylene carbonate (volume ratio 7/3) mixed solvent solution of lithium hexafluorophosphate as the electrolyte. An evaluation battery with a design capacity of 30 mAh was produced.

<Evaluation of internal resistance>
For each battery manufactured, 60mA constant current charge → 4.2V constant voltage charge (1 hour) → constant current discharge at 60mA → 2.8V, the next cycle, 5 cycles of charge-discharge After performing, 60 mA constant current charge → 4.2 V constant voltage charge (1 hour) → 6 mA for 30 minutes constant current discharge (discharge amount 3 mAh) → Measure the voltage just before the end of discharge (voltage a) → 60 mA constant current charge → 4.2V constant voltage charge (1 hour) → 90 mA constant current discharge for 2 minutes (discharge amount 3 mAh) → voltage (voltage b) immediately before the end of discharge is measured, internal resistance Ω = (voltage a−voltage b) / The internal resistance was determined by the formula (90 mA-6 mA). The results are shown in Table 1.

○: Internal resistance 4Ω or less △: Internal resistance 4Ω or more and less than 5Ω ×: Internal resistance 5Ω or more

  As can be seen from the evaluation results of the coating liquids A to G, a non-fibrillated fiber having a fiber length of 0.10 mm to 1.20 mm and a fiber diameter of 1.0 μm or less is further added to the coating liquid containing the inorganic pigment and the organic polymer binder. As a result, the coating liquid can be prevented from seeping through, and the resulting coating liquid is excellent in uniformity of the coating layer and the internal resistance of the manufactured separator. In the coating liquids A to D and G having a fiber length of 0.50 mm or less, the coating layer uniformity is good. In the coating liquids A to F having a fiber diameter of 0.8 μm or less, the back-through of the coating liquid is good. In particular, the coating liquids A to D having a fiber length of 0.50 mm or less and a fiber diameter of 0.8 μm or less are very good in both coating back-through and coating layer uniformity.

  On the other hand, in the coating liquid H in which the fiber length of the fiber added to the coating liquid is long or the coating liquid I in which the fiber diameter is large, the coating layer uniformity is poor. In the coating liquid I having a large fiber diameter, the back-through of the coating liquid is also bad. When J has a short fiber length, the coating liquid does not show through. Coating liquids K and L in which the fibers added to the coating liquid are fibrillated fibers have poor coating layer uniformity and high internal resistance. The coating liquid M in which no fiber is added to the coating liquid has poor penetration of the coating liquid.

  The coating solution for a lithium ion battery separator and the lithium ion battery separator of the present invention can be used for producing a lithium ion battery having high safety and good internal resistance.

Claims (2)

A coating liquid for a lithium ion battery separator used for coating on a nonwoven fabric substrate, comprising an inorganic pigment, an organic polymer binder, a fiber length of 0.10 mm to 1.20 mm, and a fiber diameter of 1.0 μm or less. unrealized, lithium ion separator coating solution for a battery, wherein the material of the non-fibrillated fibers is a thermoplastic resin. On at least one surface of the nonwoven substrate, an inorganic pigment, an organic polymeric binder, fiber length 0.10Mm～1.20Mm, Ri Na provided a coating layer comprising a non-fibrillated fibers of the following fiber diameter of 1.0 .mu.m, a non fibrillating separator for a lithium ion battery material of fibers and said thermoplastic resin der Rukoto.