JP2016009579A - Separator for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator by using which a lithium ion secondary battery high in safety, less in deterioration with age of battery characteristics and capable of obtaining an extremely low internal resistance can be manufactured.SOLUTION: A separator for a lithium ion secondary battery is provided with a layer including an inorganic pigment in a non-woven fabric. In the inorganic pigment, the surface of a pigment particle comprises magnesium hydroxide, and the inside of the pigment particle is mainly composed of magnesium oxide.

Description

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

  In a lithium ion secondary 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, which is conventionally used as a separator for a lithium ion secondary battery (hereinafter sometimes abbreviated as “separator”), has low heat resistance and has serious safety problems. . That is, when a battery using such a porous film as a separator causes local heat generation inside the battery due to an internal short circuit or the like, the separator around the heat generation site contracts and the internal short circuit further expands. Runaway fever may lead to serious events such as ignition and rupture.

  For such a problem, a separator is proposed in which an inorganic pigment such as alumina and an organic polymer binder are coated on a nonwoven fabric made of highly heat-resistant fibers such as polyethylene terephthalate (PET) (for example, patent document). 1).

  However, a separator using an aluminum-based inorganic pigment for the coating layer has a problem that the decomposition of the electrolytic solution is accelerated and the deterioration of battery characteristics over time may be promoted. In order to avoid such a problem, a separator using a magnesium-based inorganic pigment such as magnesium oxide or magnesium hydroxide has been proposed as an inorganic pigment (see, for example, Patent Document 2).

  However, in recent years, the performance required for lithium ion secondary battery separators has been advanced, and in particular, the performance of extremely low internal resistance required to enable rapid charge / discharge has been achieved for either aluminum or magnesium. Even in the case of a separator using an inorganic pigment, it was difficult to say that it was sufficiently realized.

Special table 2009-507353 JP 2012-134024 A

  An object of the present invention relates to a separator for a lithium ion secondary battery, and to provide a separator in which a battery using the same is high, a battery characteristic is little deteriorated over time, and a battery having an extremely low internal resistance can be obtained. is there.

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

  A separator for a lithium ion secondary battery in which a layer containing an inorganic pigment is provided on a non-woven fabric, wherein the inorganic pigment is an inorganic pigment whose pigment particle surface is composed of magnesium hydroxide and whose pigment particles are mainly composed of magnesium oxide. A separator for a lithium ion secondary battery.

  By using the separator of the present invention, it is possible to produce a lithium ion secondary battery with high safety, small deterioration of battery characteristics over time, and extremely low internal resistance of the battery.

  The separator of the present invention is characterized in that a layer containing an inorganic pigment whose pigment particle surface is made of magnesium hydroxide and the inside of the pigment particle is mainly composed of magnesium oxide is provided on the nonwoven fabric. The separator according to the present invention is a conventionally proposed inorganic pigment in which the entire pigment particle is magnesium hydroxide in that the pigment particle surface is magnesium hydroxide and the inside of the pigment particle is mainly composed of magnesium oxide. And separators using inorganic pigments in which the entire pigment particles are magnesium oxide. The reason why low internal resistance can be obtained by using an inorganic pigment whose pigment particle surface is magnesium hydroxide and the inside of the pigment particle is mainly composed of magnesium oxide has not been investigated. It is presumed that the particles having the surface properties of magnesium particles have an advantageous effect on the internal resistance.

  In the present invention, the composition of the inorganic pigment can be examined by analyzing the surface and cross section of the pigment particles observed in the cross section of the produced separator using an energy dispersive X-ray analyzer provided in the scanning electron microscope. it can. In the present invention, “the pigment particle surface is made of magnesium hydroxide” means that the atomic ratio of oxygen atom: magnesium atom determined by X-ray photoelectron spectroscopy is 1.8 to 2.1. Further, “the inside of the pigment particle is mainly composed of magnesium oxide” means that the atomic ratio of oxygen atom: magnesium atom determined by an energy dispersive X-ray analyzer is 0.9 to 1.6. In the X-ray photoelectron spectroscopy, the atomic composition in a region of several nm from the surface of the sample is measured, whereas in the energy dispersive X-ray analyzer, the atomic composition in a region of about 1 μm from the surface of the sample is measured. Therefore, by using both in combination, the atomic composition of the surface and the inside of the pigment particle can be measured. In magnesium oxide and magnesium hydroxide, stoichiometrically, the atomic ratio of oxygen atom: magnesium atom does not fall below 1.0 or exceed 2.0, but the analysis results by these measuring methods are as follows: Since an error of about 5% may be included, a low atomic ratio such as 0.9 or a high atomic ratio such as 2.1 may be observed.

An inorganic pigment in which the pigment particle surface is made of magnesium hydroxide and the inside of the pigment particle is mainly composed of magnesium oxide is obtained by placing the magnesium oxide particles in a dispersed state in water and hydrolyzing the magnesium oxide on the surface. It is done. In the hydrolysis of magnesium oxide, magnesium oxide is converted into magnesium hydroxide by a reaction of MgO + H ₂ O → Mg (OH) ₂ . However, when the magnesium oxide particles are left in a dispersed state in water for a long time, the inside of the pigment particles is highly converted to magnesium hydroxide, and the internal resistance reduction which is an effect of the present invention is a preferable level. Can't get in. In the present invention, the preferred range of the time for placing the magnesium oxide particles in a dispersed state in water varies depending on the type of the magnesium oxide particles and other materials added, but is generally in the range of 2 hours to 200 hours.

  In addition, as an inorganic pigment in which magnesium hydroxide and magnesium oxide are combined, in addition to the inorganic pigment obtained by placing the above magnesium oxide particles in a dispersed state in water, magnesium hydroxide is calcined at low temperature or for a short time. Examples thereof include inorganic pigments that are baked and partially dehydrated to form magnesium oxide. However, such an inorganic pigment has a high ratio of magnesium oxide on the surface of the pigment particles and a high ratio of magnesium hydroxide in the interior of the pigment particles, or the ratio of both does not change between the surface of the pigment particles and the inside. Therefore, a separator using such an inorganic pigment cannot obtain a battery having an extremely low internal resistance, which is an effect of the present invention.

  Although the nonwoven fabric used for the separator of this invention is not restrict | limited in particular, In order to achieve the objective of manufacturing a separator with high heat resistance, it is preferable that a nonwoven fabric is also high heat resistance. From such a viewpoint, it is preferable that the nonwoven fabric is composed of fibers having a high melting point. Examples of fibers forming the nonwoven fabric 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, and 6 nylon. Examples include various synthetic fibers such as polyamide, various cellulose pulps such as wood pulp, hemp pulp and cotton pulp, and cellulose-based regenerated fibers such as rayon and lyocell. Among these, non-woven fabrics mainly composed of polyester or polypropylene are preferred for reasons such as heat resistance and low hygroscopicity. Although the preferable fiber diameter of the fiber which forms a nonwoven fabric is dependent also on the physical property of a coating liquid, it is preferable to exist in the range of 1-8 micrometers. The thickness of the nonwoven fabric is preferably in the range of 8 to 25 μm.

  In the present invention, the layer containing the inorganic pigment (coating layer) preferably contains a binder, and high coating layer strength is obtained. Various organic polymers can be used as the binder. Examples include various organic materials such as styrene-butadiene copolymer elastomers, acrylonitrile-butadiene copolymer elastomers, (meth) acrylic acid ester polymer elastomers, styrene- (meth) acrylic acid ester polymer elastomers, and vinylidene fluoride polymers. Polymers.

  In the present invention, the coating liquid used to form the coating layer includes, in addition to the inorganic pigment and binder, 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.

  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 part are all based on mass unless otherwise specified. The coating amount is an absolutely dry coating amount.

<Production of non-woven fabric>
50 parts by mass of oriented crystallized polyethylene terephthalate (PET) short fibers having a fineness of 0.1 dtex (average fiber diameter of 3.0 μm) and a fiber length of 3 mm, a fineness of 0.2 dtex (average fiber diameter of 4.3 μm), and a single fiber length of 3 mm 50 parts by mass of a PET short fiber for a one-component binder (softening point 120 ° C., melting point 230 ° C.) was dispersed in water by a pulper to prepare a uniform papermaking slurry having a concentration of 1% by mass. This slurry for papermaking is made up by a circular net type paper machine, and the PET short fibers for binders and the intersections of PET short fibers for binders and oriented crystallized PET short fibers are bonded by a cylinder dryer at 135 ° C. It was made to express strength and it was set as the nonwoven fabric of 12 g / m < ² > of fabric weights. 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 thermal calender consisting of a dielectric heating jacket roll (metal hot roll) and an elastic roll. To produce a nonwoven fabric having a thickness of 18 μm.

<Coating liquid A>
In 80 parts of water, 0.4 part of a dispersant comprising a maleic acid-acrylic acid copolymer is dissolved, and 100 parts of magnesium hydroxide having a volume average particle diameter of 0.7 μm and a specific surface area of 11 m ² / g is added. Dispersion was performed for 30 minutes by a stator type disperser. Next, 200 parts (1.0 part as a solid content) of a 0.5% aqueous solution of carboxymethylcellulose sodium salt having a viscosity of 7000 mPa · s at 25 ° C. in a 1 mass% aqueous solution, and a styrene-butadiene latex (solid content ratio: 48 mass) %) 3 parts (1.5 parts as a solid content) was added and stirred for 15 minutes, and water was added to prepare a coating liquid A having a solid content rate of 25 mass%.

<Coating fluid B>
In 80 parts of water, 0.4 part of a dispersant composed of a maleic acid-acrylic acid copolymer is dissolved, and 100 parts of magnesium oxide having a volume average particle diameter of 0.8 μm and a specific surface area of 10 m ² / g is added to the rotor-stator. Dispersed for 30 minutes with a mold disperser. Next, 200 parts (1.0 part as a solid content) of a 0.5% aqueous solution of carboxymethylcellulose sodium salt having a viscosity of 7000 mPa · s at 25 ° C. in a 1 mass% aqueous solution, and a styrene-butadiene latex (solid content ratio: 48 mass) %) 3 parts (1.5 parts as a solid content) was added and stirred for 15 minutes, and water was added to prepare a coating liquid B having a solid content rate of 25 mass%.

<Coating liquid C>
In 80 parts of water, 0.4 part of a dispersant comprising a maleic acid-acrylic acid copolymer is dissolved, and 100 parts of partially calcined magnesium hydroxide having a volume average particle diameter of 0.8 μm and a specific surface area of 10 m ² / g is added. The dispersion was performed for 30 minutes by a rotor-stator type disperser. Next, 200 parts (1.0 part as a solid content) of a 0.5% aqueous solution of carboxymethylcellulose sodium salt having a viscosity of 7000 mPa · s at 25 ° C. in a 1 mass% aqueous solution, and a styrene-butadiene latex (solid content ratio: 48 mass) %) 3 parts (1.5 parts as a solid content) was added and stirred for 15 minutes, and water was added to prepare a coating liquid C having a solid content of 25% by mass.

<Preparation of separator A1>
After 10 minutes from the preparation of the coating liquid A, it was coated on the non-woven fabric using a rod coater so that the absolute dry coating amount was 8 g / m ^2, and dried by a hot air dryer. A separator A having a thickness of 25 μm was produced.

<Preparation of separators A2, B1-B3, C1-C2>
Separators A2, B1 to B3, C1 to C1 are the same as separator A1, except that the type of coating liquid applied to each surface and the time from preparation of the coating liquid to coating are changed as shown in Table 1. C2 was produced.

<Production of evaluation battery>
Lithium manganate as the positive electrode, mesocarbon microbeads as the negative electrode, 1 mol / L diethyl carbonate / ethylene carbonate (volume ratio 7/3) mixed solvent solution of lithium hexafluorophosphate as the electrolyte, and separators A1 to A2, B1 as separators A battery for evaluation having a design capacity of 30 mAh using -B3 and C1-C2 was produced. In addition, the coating surface of the separator was made to oppose the negative electrode.

<Evaluation of internal resistance>
About each produced battery, 60mA constant current charge-> 4.2V constant voltage charge (1 hour)-> constant current discharge at 60mA-> When it becomes 2.8V, 5 cycles of acclimatization charge-discharge are performed in the next cycle sequence. Then, 60 mA constant current charge → 4.2 V constant voltage charge (1 hour) → 30 mA constant current discharge at 6 mA (discharge amount 3 mAh) → Measure the voltage just before the end of discharge (voltage a) → 60 mA constant current charge → 4 .2V constant voltage charging (1 hour) → constant current discharge at 90 mA for 2 minutes (discharge amount 3 mAh) → measurement of voltage (voltage b) just before the end of discharge, 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.

<Atomic composition of inorganic pigment>
About each produced separator, the atomic ratio of oxygen / magnesium on the pigment particle surface of an inorganic pigment was measured with an X-ray photoelectron spectrometer (device name: PHI5400, ULVAC-PHI Co., Ltd.). The atomic ratio was analyzed by an energy dispersive X-ray analyzer (device name: 6743A, Thermo Scientific Co., Ltd.) provided in a scanning electron microscope (device name: JSM-6610, JEOL Ltd.). The results are shown in Table 1.

  As shown in Table 1, a separator for a lithium ion secondary battery in which a layer containing an inorganic pigment is provided on a non-woven fabric, the inorganic pigment is made of magnesium hydroxide on the pigment particle surface, and the inside of the pigment particle is oxidized. By using an inorganic pigment mainly composed of magnesium, the internal resistance of the battery can be lowered. That is, the internal resistances of the separators A1 and A2, both of which are made of magnesium hydroxide on the pigment particle surface and inside, are 4.4Ω, the pigment particle surface is made of a mixture of magnesium hydroxide and magnesium oxide, The internal resistance of the separator B1 mainly composed of magnesium oxide is 4.3Ω, and the internal resistance of the separator C1 made of a mixture of magnesium hydroxide and magnesium oxide both on the surface and inside of the pigment particles is 4.4Ω. The internal resistance of the separator C2 whose surface is made of magnesium hydroxide and whose pigment particles are mainly composed of magnesium oxide was 4.3Ω. In contrast, the internal resistance of separators B2 and B3, in which the pigment particle surface is made of magnesium hydroxide and the inside of the pigment particle is mainly composed of magnesium oxide, is as low as 3.9Ω.

  Moreover, since the separator of this invention provides the layer containing an inorganic pigment in the nonwoven fabric which consists of fiber with high heat resistance, such as a polyethylene terephthalate (PET), it has high heat resistance, and the safety | security of the battery manufactured using this High nature.

  Moreover, since the separator of the present invention uses a magnesium-based inorganic pigment, it does not promote the decomposition of the electrolytic solution, and the deterioration of battery characteristics over time is small.

  INDUSTRIAL APPLICABILITY The present invention can be used for manufacturing a separator that provides a battery with high battery safety, low aging deterioration of battery characteristics, and extremely low internal resistance.

Claims (1)

  A separator for a lithium ion secondary battery in which a layer containing an inorganic pigment is provided on a non-woven fabric, wherein the inorganic pigment is an inorganic pigment whose pigment particle surface is composed of magnesium hydroxide and whose pigment particles are mainly composed of magnesium oxide. A separator for a lithium ion secondary battery.