JP5207569B2 - Lithium battery separator - Google Patents

Description

  The present invention relates to a separator optimal for lithium batteries such as lithium primary batteries, lithium ion secondary batteries, and lithium polymer secondary batteries.

  In recent years, the demand for high energy density secondary batteries has increased, and as a battery corresponding to this, lithium primary batteries, lithium ion secondary batteries, and the like using an organic solvent as an electrolyte have been developed. Since the battery uses an organic solvent as the electrolyte, safety is particularly important. Usually, a polyolefin microporous membrane is used as a separator, and when the battery reaches a certain temperature or higher, measures are taken such as melting and closing the microporous and shutting down the current flow. Metal containers such as iron cans and aluminum cans are used for packaging, but it is extremely difficult to completely prevent leakage.

  On the other hand, for example, in US Pat. No. 5,429,891, a material in which a polyvinylidene fluoride resin is crosslinked in the presence of an acrylate ester, di- or triallyl ester, di- or triglycidyl ester as a crosslinking monomer together with a plasticizer is prepared. Then, a solid polymer electrolyte impregnated with an electrolytic solution was proposed. When this solid electrolyte is used in a lithium ion secondary battery, there is no risk of leakage and excellent safety, and non-metallic materials can be used for packaging, and the degree of freedom in shape is increased.

  However, a battery using such a solid electrolyte cannot be manufactured in the manufacturing process of the conventional lithium ion secondary battery, and it is necessary to install a completely new unique manufacturing process. The battery characteristics, particularly the low temperature discharge characteristics, etc. were very low. Therefore, a battery that can use the conventional manufacturing process of a lithium ion secondary battery, has no anxiety about leakage of the electrolyte, and has excellent characteristics has been desired.

  On the other hand, for example, Japanese Patent Laid-Open No. 8-250127 has proposed a method of impregnating a porous film made of a vinylidene fluoride resin with an electrolytic solution and using the electrolytic solution-impregnated porous film for a diaphragm portion. In this case, the battery can be produced by a conventional lithium ion secondary battery production process, and the electrolyte solution forms a gel with the vinylidene fluoride resin microporous membrane, so that a battery that does not leak is obtained. However, since the strength of the vinylidene fluoride microporous film is low and it is easy to cut in the battery winding process, it is difficult to manufacture the battery in the conventional process. In addition, a battery made using the microporous membrane has a microporous blockage in the separator, although it is necessary to shut down the current flow at an internal temperature of 150 ° C. or lower. There was no function to shut down the current flow.

JP-A-6-76808 proposes a battery separator having a laminated structure of a polyolefin porous body and a fluororesin porous body. The intent is that when a polyolefin porous body separator is used in a lithium battery, complete melting or melt cracking occurs during shutdown, contact between electrodes occurs, and a short circuit is likely to occur, but lamination with a fluororesin porous body In the body, the fluororesin porous body has high heat resistance and withstands long-term use at 200 ° C., so that crack melting of the separator can be suppressed and insulation between electrodes can be maintained. From the spirit of the invention, a tetrafluoroethylene resin is preferably proposed as the fluororesin.
Further, JP-A-8-323910 discloses a laminated film of a porous film layer made of a resin that swells with a nonaqueous solvent and a porous film layer made of a high-melting crystalline resin. This is an attempt to shut down the flow of current by swelling a swellable resin with an electrolyte in a microporous film of a high melting point resin at 160 ° C. or higher to close the pores.

Problems to be solved by the invention

  The present invention is a battery manufactured by using a conventional manufacturing process of a lithium ion secondary battery, and can realize a battery that does not have to worry about leakage of an electrolyte, and has excellent shutdown characteristics. It is intended to provide a separator that blocks the flow of lithium ions when the temperature inside the battery rises and maintains the safety of the battery.

Means for solving the problem

[Means for Solving the Problems]
As a result of intensive studies to solve the above problems, the present inventors have found the following inventions.
(1) A three-layer structure in which both surface layers are microporous layers having a melting point of 145 ° C. or lower made of a copolymer containing vinylidene fluoride, and the intermediate layers are microporous layers having a melting point of 140 ° C. or lower made of polyolefin. A lithium battery separator comprising a microporous membrane, wherein the interlayer peel strength is 2 g / cm or more, and the continuous micropores in the surface layer and the intermediate layer are not blocked.
(2) The lithium battery separator according to (1), wherein the microporous film is a three-layer laminated film of surface layer-intermediate layer-surface layer, and each layer is laminated without using an adhesive.
(3) A copolymer microporous membrane containing vinylidene fluoride and a polyethylene microporous membrane are prepared in advance, and after superposing these three sheets, they are stretched under pressure at a temperature 10 to 20 ° C. lower than the melting point of both polymers. The lithium battery separator according to (1) or (2), wherein the separator is laminated.
(4) The separator for a lithium battery according to any one of (1) to (3), wherein the microporous layer forming the surface layer is crosslinked and the gel fraction thereof is 10% or more.
(5) The lithium battery separator according to any one of (1) to (4), wherein both the surface layer and the intermediate layer have a porosity of 30 to 70%.
(6) The separator for lithium batteries according to any one of (1) to (5), wherein the total thickness of the separator is 10 to 50 μm and the thickness of the intermediate layer is 5 to 40 μm.
(7) A method for producing a lithium battery separator according to any one of (1) to (6), wherein a copolymer microporous membrane and a polyethylene microporous membrane containing vinylidene fluoride are prepared in advance. A method for producing a separator for a lithium battery, wherein the sheets are superposed and then stretched under pressure at a temperature lower by 10 to 20 ° C. than the melting point of both polymers.
(8) A method for producing a lithium battery separator according to any one of (1) to (2) and (4) to (6), wherein after mixing each polymer and solvent to be a surface layer and an intermediate layer, A method for producing a separator for a lithium battery, characterized in that a three-layer sheet is extruded and then cooled and phase-separated and then the solvent is removed and then stretched or stretched and then the solvent is removed to obtain a three-layer microporous membrane .

The present invention will be described in detail below.
In the three-layer microporous membrane according to the present invention, it is important that a microporous layer made of a copolymer containing vinylidene fluoride has a polyethylene microporous layer sandwiched between intermediate layers. In the process of manufacturing a lithium ion secondary battery, the separator is sandwiched between a positive electrode and a negative electrode, wound in a coil shape, mounted in the battery, and an electrolyte is injected. The electrolyte permeates into the micropores of the separator and forms a conduction path for lithium ions, but the copolymer containing vinylidene fluoride on the surface layer has the effect of absorbing a certain amount of the permeated electrolyte, so that The liquid is held in the separator and does not easily flow out. However, lithium ion conductivity is sufficiently exhibited. Furthermore, since the copolymer layer containing vinylidene fluoride swollen with the electrolyte solution is in direct contact with the positive electrode and the negative electrode inside the battery, compared with the case where a separator made of a conventional polyolefin microporous film is used, the electrode layer is separated from the separator. Is advantageous in terms of battery characteristics.

  In the intermediate polyolefin microporous layer, the electrolytic solution is not absorbed and swelled by the polymer and exists in the microporous layer. However, the electrolytic solution does not leak because it is sealed by the surface gel layer. On the other hand, when the temperature in the battery rises, the pores of the intermediate polyolefin microporous layer are blocked and the current is shut down, so that the safety of the battery is guaranteed. When there is no polyolefin microporous layer having a melting point of 140 ° C. or lower in the intermediate layer, the shutdown mechanism does not work and it is difficult to maintain the safety of the battery.

  Polyolefin used for the intermediate microporous layer is polyethylene, poly (ethylene-propylene) copolymer, etc. having a melting point of 140 ° C. or lower, or polypropylene, polybutene 1, poly (ethylene-propylene) copolymer, poly ( Examples thereof include a mixture with propylene-butene 1) copolymer and the like. When the melting point is higher than 140 ° C., the shutdown temperature of the separator is increased, which is not preferable for battery safety.

Examples of the copolymer containing vinylidene fluoride include poly (vinylidene fluoride-hexafluoropropylene) copolymer, poly (vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene) copolymer, poly (tetra Fluoroethylene-vinylidene fluoride) copolymer and the like.
The layer made of a copolymer containing vinylidene fluoride in the surface layer is more preferably crosslinked. This is because it is preferable to prevent this because it changes its shape such as remarkably swelling or dissolving at high temperature by absorbing the electrolytic solution. As a simple and industrially excellent crosslinking method, there is a method of irradiating an electron beam, γ-ray or the like, and a method by electron beam irradiation is particularly excellent. The degree of crosslinking is preferably such that the gel fraction of the copolymer containing vinylidene fluoride is 10% or more. The gel fraction can be measured by dissolving in a good solvent of a copolymer containing vinylidene fluoride and measuring the undissolved fraction. The good solvent varies depending on the type of polymer, but the vinylidene fluoride polymer usually includes N-methylpyrrolidone, dimethylformamide, tetrahydrofuran and the like.

  In order to prepare a three-layer microporous film, for example, a copolymer microporous film containing vinylidene fluoride and a polyolefin microporous film are prepared in advance, and then superposed, stretched, pressure-bonded, and the like. A method for individually producing the microporous membrane is not particularly limited, and a known stretch opening method or phase separation method can be applied. For example, a method described in JP-A-3-215535, JP-B-61-38207 is applicable. The method described in JP-A-54-16382, the method described in JP-A-54-16382, and the like can be used. The interlayer adhesive strength of the three-layer microporous membrane is important for preventing troubles in the battery manufacturing process, and the interlayer peel strength is preferably 2 g / cm or more.

  However, it is not preferable to use an adhesive to increase the peel strength because it may block the communication hole of the membrane. Even when interlayer adhesion is performed by heat or pressure, it is important that the continuous micropores in the surface layer and the inner layer are not blocked. As a method of making a laminated film having high peel strength without impairing the communication holes, for example, a copolymer microporous film containing vinylidene fluoride and a polyethylene microporous film are prepared in advance, and after superposing these three sheets, both polymers There is a method of stretching under pressure at a temperature lower by 10 to 20 ° C. than the melting point, and as stretching under high temperature and pressure, roll stretching or the like is preferable.

It is also possible to prepare a three-layer microporous membrane by coextrusion using phase separation. In this case, after mixing each polymer and solvent (plasticizer), the three-layer sheet is extruded, and after cooling phase separation, the solvent (plasticizer) is removed and then stretched, or after stretching, the solvent is removed. Thus, a three-layer microporous film is obtained.
In either case of the coextrusion method or the lamination method, it is preferable that the difference in melting point of each polymer is small in order to obtain the peel strength between layers.

  The thickness of the separator used for the lithium secondary battery is preferably 100 μm or less, more preferably 10 to 50 μm. When the thickness is less than 10 μm, the inter-electrode insulation is not guaranteed, and when the thickness is more than 100 μm, the electrical resistance of the separator increases, which is not preferable. The thickness of the surface layer and the intermediate layer is not particularly limited, but the thickness of the intermediate polyethylene layer is preferably 5 μm or more in order to sufficiently exhibit the shutdown function, and 40 μm or less in order to keep the electric resistance as low as possible. preferable. In order to maintain the planar stability of the three-layer microporous membrane, the thicknesses of both surface layers are preferably equal and plane-symmetric.

  A three-layer microporous membrane requires sufficient tensile strength and elastic modulus to be stably wound without being cut or stretched in the battery winding process. Is effectively achieved. The porosity of the three-layer microporous membrane is important for determining the electrical characteristics of the separator, and both the outer layer and the intermediate layer are preferably about 30 to 70%.

Embodiment of the present invention

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited thereto. Various physical properties used in the present invention were measured based on the following test methods.
(1) Melting point (° C)
The sample was attached to a measurement container, heated at a rate of 10 ° C./min, and the endothermic peak temperature was measured. For the measurement, DSC220C manufactured by Seiko Instruments Inc. was used.
(2) Peel strength (g / cm)
Based on JIS P-8117, it measured by the T-type peeling test.

(3) Gel fraction (%)
About 1 g of the porous membrane sample was vacuum-dried at 50 ° C., and then the weight was measured to determine the weight before dissolution (Wx). The sample was cut to a size of about 1 cm square and placed in a glass sample bottle, and 100 ml of N-methylpyrrolidone was added. Subsequently, the mixture was stirred for 24 hours while being heated to 80 ° C., and then filtered using a glass fiber filter paper having a particle retention capacity of 0.7 μm. Subsequently, after washing with 20 ml of N-methylpyrrolidone, the operation of filtration was performed twice, followed by washing twice with 20 ml of ethanol, followed by vacuum drying at 50 ° C. The weight of each filter was measured, and the dissolution residual weight (Wz) was determined by subtracting from the weight of the filter only measured in advance. The gel fraction was calculated from the following formula.
Gel fraction (%) = 100 × Wz / Wx
(4) Shutdown temperature (° C)
FIG. 1 shows a schematic diagram of a shutdown temperature measuring apparatus. 1 is a separator, 2A and 2B are nickel foils having a thickness of 10 μm, and 3A and 3B are glass plates. 4 is an electrical resistance measuring device (LCR meter AG-4411 manufactured by Ando Electric Co., Ltd.), which is connected to the nickel foils 2A and 2B. A thermocouple 5 is connected to the thermometer 6. A data collector 7 is connected to the electric resistance device 4 and the thermometer 6. 8 is an oven which heats the separator.

  More specifically, as shown in FIG. 2, the separator 1 is stacked on the nickel foil 2A, and is fixed to the nickel foil 2A with Teflon tape in the vertical direction. The separator 1 is impregnated with a 1 mol / liter lithium borofluoride solution (solvent: propylene carbonate / ethylene carbonate / γ-butyllactone = 1/1/2) as an electrolytic solution. As shown in FIG. 3, a Teflon tape is bonded onto the nickel foil 2B, and masking is performed leaving a 15 mm × 10 mm window portion at the center of the foil 2B.

The nickel foil 2A and the nickel foil 2B are overlapped so as to sandwich the separator 1, and two nickel foils are sandwiched by the glass plates 3A and 3B from both sides thereof. At this time, the window portion of the foil 2B and the separator 1 come to face each other. Two glass plates are fixed by pinching with a commercially available double clip. The thermocouple 5 is fixed to the glass plate with Teflon tape.
Temperature and electric resistance are continuously measured with such an apparatus. The temperature is raised from 25 ° C. to 200 ° C. at a rate of 2 ° C./min, and the electric resistance value is measured at an alternating current of 1 kHz. Here, the shutdown temperature is defined as the temperature at which the electrical resistance value of the separator reaches 10 ³ Ω.

Poly (vinylidene fluoride-hexafluoropropylene) copolymer having a melting point of 139 ° C. (KYNAR 2800 manufactured by Elf Atchem Japan) 32.4% by volume, hydrophobic silica fine powder having a specific surface area of 110 m ² / g, 14.2% by volume, dibutyl A sheet of 50 μm thickness was prepared by mixing 4.4 vol% phthalate and 48.5 vol% diethylhexyl phthalate with a Hensyl mixer and extruding the mixture using a sheet manufacturing apparatus with a T-die attached to a twin screw extruder. Got. The sheet is immersed in methylene chloride to extract and remove dibutyl phthalate and diethylhexyl phthalate, and further immersed in a 20% by weight aqueous caustic soda solution to extract and remove silica, followed by washing with water and drying to obtain poly (vinylidene fluoride). An iodohexafluoropropylene) copolymer microporous membrane was obtained. The melting point of this film was 139 ° C.

  Next, 34% by weight of high-density polyethylene having a weight average molecular weight of 250,000, a melting point of 135 ° C., 19% by weight of hydrophilic silica fine powder, and 47% by weight of diethylhexyl phthalate were similarly mixed with a Hensyl mixer, A sheet having a thickness of 50 μm was extruded, and diethylhexyl phthalate and silica were extracted and removed with an aqueous solution of methylene chloride and caustic soda to obtain a polyethylene microporous film. The melting point of this film was 135 ° C.

  The two types of microporous membranes thus obtained were stacked in three layers using a poly (vinylidene fluoride-hexafluoropropylene) copolymer membrane on both surface layers and a polyethylene microporous membrane on the intermediate layer, and were stretched under tension with a roll stretching machine. The film was stretched 4 times in the length direction and then stretched 2 times in the width direction to obtain a three-layer laminated film (stretched film) having a thickness of 25 μm. The thickness of each layer constituting the stretched film is 9 μm / 8 μm / 8 μm, respectively. When the stretched film was peeled and separated, and the porosity was measured, the porosity was 50% / 55% / 45%, respectively.

The laminated film is immersed in an electrolytic solution (a solution of LiBF ₄ dissolved at a concentration of 1.5 mol / liter in a 1: 1: 2 mixed solvent of ethylene carbonate / propylene carbonate / γ-butyllactone) at room temperature and held in the solution. After that, the excess electrolytic solution adhering to the surface was lifted off. The three-layer membrane thus obtained became an electrolyte-impregnated membrane that contained the electrolyte and did not ooze out. In addition, elution of the polymer into the electrolytic solution was observed after being immersed in the electrolytic solution for a long time, but no elution was detected. This electrolyte-impregnated membrane was sandwiched between stainless sheets, applied between electrodes, the resistance component was measured, and the ionic conductivity was calculated from the real impedance intercept of the Cole-Cole plot. The ionic conductivity of the three-layer laminated film at room temperature was 1.2 mS / cm. Further, the shutdown temperature of the three-layer laminated film was 136 ° C., and the current was cut off at the shutdown temperature or higher.

  The three-layer laminated film prepared in Example 1 was irradiated with an electron beam at an irradiation dose of 15 Mrad to form a crosslinked film. When the surface layer and the intermediate layer of the crosslinked film were separated and separated, and the respective gel fractions were measured, the gel fraction of the poly (vinylidene fluoride-hexafluoropropylene) copolymer microporous layer on the surface layer was 60%, and the intermediate layer The gel fraction of the polyethylene microporous membrane was 70%.

  In the same manner as in Example 1, the cross-linked three-layer laminated film was immersed in the electrolytic solution at room temperature and held in the liquid. As a result, the electrolytic solution was impregnated with the crosslinked film. When the electrolyte solution on the surface was lifted up and wiped off, an impregnated film that did not ooze out while containing the electrolyte solution was obtained. After immersing in the electrolytic solution for a long time, the elution of the polymer into the electrolytic solution was checked. The crosslinked membrane had an ionic conductivity at room temperature of 1.5 mS / cm. Further, the shutdown temperature of the crosslinked film was 136 ° C., and the current was cut off when the shutdown temperature was exceeded.

Comparative Example 1

  34% by weight of high density polyethylene having a weight average molecular weight of 250,000, melting point of 135 ° C., 19% by weight of hydrophilic silica fine powder, and 47% by weight of diethyl hexyl phthalate were mixed with a Hensyl mixer, and the thickness was 150 μm with a 30 mmΦ twin screw extruder A film-like molded body was obtained, and diethylhexyl phthalate was extracted and removed with methylene chloride and aqueous caustic soda solution, then stretched 4 times in the length direction with a roll stretching machine, and then stretched 2 times in the width direction with a tenter to obtain a thickness of 25 μm. A polyethylene microporous membrane having a porosity of 50% was obtained.

  In the same manner as in Example 1, the polyethylene microporous membrane was immersed in an electrolytic solution at room temperature and held in the solution. When the electrolyte solution on the surface was lifted up and wiped off, the electrolyte solution remaining in the abdomen was almost adhered and almost absent. Moreover, the ionic conductivity at room temperature could not be measured. Moreover, when it pulled up out of electrolyte solution and was left without wiping off, electrolyte solution was not hold | maintained at the film | membrane, but fell or fell.

Effect of the invention

  The separator composed of the three-layer laminated microporous membrane according to the present invention can be manufactured by using the conventional manufacturing process of a lithium ion secondary battery as it is, and the separator has a good electrolyte holding property. Since the current is shut down by shutting down at a temperature of ℃ or less, it is possible to provide a very safe battery.

  The schematic of the measuring device of the shutdown temperature used by the present invention.   The partial view of the measuring device of the shutdown temperature used by the present invention.   The partial view of the measuring device of the shutdown temperature used by the present invention.

Claims (8)

A three-layered microporous membrane characterized in that both surface layers are microporous layers made of a copolymer containing vinylidene fluoride and having a melting point of 145 ° C or lower, and the intermediate layers are microporous layers made of polyolefin and having a melting point of 140 ° C or lower. A separator for a lithium battery, comprising: a peel strength between layers of 2 g / cm or more, and the continuous micropores in the surface layer and the intermediate layer are not blocked.
The separator for a lithium battery according to claim 1, wherein the microporous film is a three-layer laminated film of surface layer-intermediate layer-surface layer, and each layer is laminated without using an adhesive.
A copolymer microporous membrane and a polyethylene microporous membrane containing vinylidene fluoride are prepared in advance, and after superposing these three sheets, they are stretched and laminated under pressure at a temperature lower by 10 to 20 ° C. than the melting point of both polymers. The lithium battery separator according to claim 1, wherein the separator is a lithium battery separator.
The separator for a lithium battery according to any one of claims 1 to 3, wherein the microporous layer forming the surface layer is cross-linked and has a gel fraction of 10% or more.
The lithium battery separator according to any one of claims 1 to 4, wherein both the surface layer and the intermediate layer have a porosity of 30 to 70%.
The separator for a lithium battery according to any one of claims 1 to 5, wherein a total thickness of the separator is 10 to 50 µm, and a thickness of the intermediate layer is 5 to 40 µm.
It is a manufacturing method of the separator for lithium batteries in any one of Claims 1-6, Comprising: The copolymer microporous film and polyethylene microporous film containing a vinylidene fluoride were produced previously, and these 3 sheets were piled up. Then, the manufacturing method of the separator for lithium batteries characterized by extending | stretching under pressure at the temperature 10-20 degreeC lower than melting | fusing point of both polymers.
  It is a manufacturing method of the separator for lithium batteries in any one of Claims 1-2, 4-6, Comprising: After mixing each polymer and solvent used as a surface layer and an intermediate | middle layer, a three-layer sheet | seat is extruded and cooling phase separation is carried out. A method for producing a separator for a lithium battery, comprising: removing the solvent and then stretching, or stretching, or removing the solvent to obtain a three-layer microporous membrane.

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