JP2012136704A - Microporous membrane made of polyethylene and battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a microporous membrane made of polyethylene which has excellent productivity and characteristics, also has excellent safety achieved by a perfect fuse effect, and is used for a battery, and to provide a battery using the microporous membrane.SOLUTION: The microporous membrane made of polyethylene is composed of a composition at least comprising: 10 to 95 wt.% of high density polyethylene having the viscosity-average molecular weight of 100,000 to 4,000,000; and 5 to 90 wt.% of polyethylene having a melting point of >125°C and ≤132°C.

Description

  The present invention is a polyethylene microporous membrane, in particular, polyethylene having excellent ion permeability, mechanical properties, and safety used for a battery using a carbon material capable of inserting lithium ions, lithium metal, lithium alloy, etc. as a negative electrode. The present invention relates to a microporous membrane and a battery using the same.

  With the remarkable development of information-related devices such as mobile phones, notebook personal computers, and PDAs in recent years, small and light batteries with high energy capacity are required. While various types of batteries are being researched, developed, and sold, lithium ion batteries are expanding the market, and polyethylene microporous membranes are used as separators used in such batteries.

  As separators for batteries such as this lithium ion battery, high ion permeability that can exhibit excellent battery characteristics, excellent mechanical characteristics that can be withstood during battery assembly and handling, temperature rise during overcharge, etc. Occasionally, excellent safety such as blocking the flow of ions and blocking current flow (fuse effect) is required. The above “fuse effect” will be described in more detail. When heat is applied to the polyethylene microporous membrane incorporated as a separator in the battery until it reaches its melting point, the pores are blocked to block the flow of ions and prevent the current from flowing. Heat generation can be suppressed and safety can be maintained. However, if the battery is heated very rapidly, there is a possibility that the flow of ions cannot be blocked due to a delay in the closing of the holes. Even in such a case, there has been a demand for a separator having a function of reliably blocking the flow of ions and blocking the flow of ions so that no current flows.

  As one means for solving the above problem, a method of adding polyethylene having a low melting point (95 to 125 ° C.) has been proposed (for example, JP-A-11-269289). However, if a material having a melting point that is too low is added, the pores are blocked, so that the heat setting temperature during the production of the microporous membrane cannot be increased, and as a result, a separator having a large heat shrinkage There is a problem of becoming.

  An object of the present invention is to provide a polyethylene microporous membrane used for a battery having excellent productivity and characteristics, and excellent safety realized by a complete fuse effect, and a battery using the polyethylene microporous film. .

In order to achieve the above object, the present invention provides (1) at least 10 to 95% by weight of high-density polyethylene having a viscosity average molecular weight of 100,000 to 4,000,000 and a melting point of more than 125 ° C and 132 ° C or less. A polyethylene microporous membrane characterized by containing ˜90% by weight, (2) polyethylene having a melting point of more than 125 ° C. and not more than 132 ° C. is composed of ethylene and an α-olefin having 4 or more carbon atoms. The polyethylene microporous membrane according to (1) above, which is a copolymer, (3) The density of polyethylene having a melting point of more than 125 ° C. and 132 ° C. or less is 0.86 to 0.95 g / cm ^3. And the ratio of the weight average molecular weight to the number average molecular weight (Mw / Mn) is in the range of 3.5 to 8.0, and the polyethylene microporous structure according to the above (1) or (2) film,( 4) The above (1) or (2), wherein the polyethylene having a melting point of more than 125 ° C. and not more than 132 ° C. has 0.3 or more terminal vinyl groups per 1000 carbon atoms in the polyethylene. The polyethylene microporous membrane according to any one of the above, (5) a battery separator comprising the polyethylene microporous membrane according to any one of (1) to (4), and (6) any one of (1) to (4) above. A battery using the polyethylene microporous membrane as described in 1 above as a separator.

  Since the polyethylene microporous membrane of the present invention has an excellent fuse effect, a battery with excellent safety can be produced.

  The present invention is described in detail below. The polyethylene microporous membrane of the present invention contains at least 10 to 95% by weight of high-density polyethylene having a viscosity average molecular weight of 100,000 to 4,000,000 and 5 to 90% by weight of polyethylene having a melting point of more than 125 and 132 ° C. or less. It consists of the composition which is characterized by the above-mentioned.

The high density polyethylene used in the present invention has a density of 0.941 g / cm ³ or more and a viscosity average molecular weight of 100,000 to 4,000,000, preferably 150,000 to 2,000,000, more preferably 170,000 to 1,000,000, More preferably, it is 200,000 or more and less than 700,000. Here, when the viscosity average molecular weight is less than 100,000, the strength of the microporous membrane is insufficient. On the other hand, if it exceeds 4 million, kneading and molding at the time of producing the microporous membrane become difficult. The viscosity average molecular weight Mv is determined by measuring the intrinsic viscosity [η] in a decalin solution and using the following formula:
Polyethylene: [η] = 6.77 × 10 ⁻⁴ Mv ^0.67
Polypropylene: [η] = 1.10 × 10 ⁻⁴ Mv ^0.80
One polyethylene having a melting point of more than 125 ° C. and 132 ° C. or less used in the present invention is a copolymer of ethylene and an α-olefin. The α-olefin preferably has 4 or more carbon atoms. Such a copolymer can be produced by ionic polymerization using a Ziegler-type catalyst under a pressure of normal pressure to 100 kg / cm ² .

As the copolymer, polyethylene having a small structural nonuniformity can be used. The density of the copolymer at this time is preferably 0.86 to 0.95 g / cm ³ . The ratio of the weight average molecular weight to the number average molecular weight (Mw / Mn) is preferably in the range of 3.5 to 8.0. Such a copolymer can also be polymerized by a single site catalyst which is a homogeneous catalyst having one kind of active site. Examples of the single site catalyst include metallocene catalysts such as bis (cyclopentadienyl) zirconium chloride. Mw and Mn are measured by the following method;
[Measurement apparatus] Gel permeation chromatography (GPC) (manufactured by Waters: ALC / GPC 150 type)
[Measurement conditions] Column: Showa Denko AT-807S (1) and Tosoh GMH-HT6 (2) connected in series Mobile phase: Trichlorobenzene (TCB)
Column temperature: 140 ° C. Flow rate: 1.0 ml / min.
Sample preparation: A 20-30 mg sample is dissolved by heating at 140 ° C. in 20 ml of a TCB solution in which 0.1% by weight of 2,6-di-tert-butyl-4-methylphenol is dissolved [calculation method] A base line is drawn on the chromatogram, and the chromatogram is further divided into several equal parts to obtain the height Hi of each dividing point from the base line. Next, the molecular weight Mi at each dividing point is determined from a calibration curve using standard polystyrene with a known molecular weight. And obtain Mw and Mn from the following equations;
Mw = {Σ (Hi · Mi)} / (ΣHi)
Mn = (ΣHi) / {Σ (Hi / Mi)}

  Further, polyethylene having a melting point of more than 125 ° C. and not more than 132 ° C. and a copolymer of ethylene and an α-olefin having 4 or more carbon atoms and having a melting point of more than 125 ° C. and not more than 132 ° C. May have 0.3 or more terminal vinyl groups per 1000 carbon atoms in polyethylene. Such polyethylene can be polymerized by a chromium compound supported catalyst. Here, the chromium compound-supported catalyst is, for example, as shown in Japanese Patent Publication No. 1-17777.

When polymerized using such a catalyst, 0.3 or more terminal vinyl groups are observed per 1000 carbon atoms in the polymer, unlike when polymerized using a Ziegler type catalyst. The reason is not clear, but the more the terminal vinyl group is present, the lower the closing temperature of the pores of the microporous membrane, which is preferable. In addition, the number of terminal vinyl groups per 1000 carbon atoms in the polymer is derived by the following method; the IR spectrum of a sample obtained by heat-pressing the polymer and making it non-porous into a sheet is measured, and the vicinity of 910 cm ⁻¹ is measured. The height (absorbance: A) from the baseline of the peak indicating the terminal vinyl group is determined. Substituting this into the following equation to determine the number of terminal vinyl groups n per 1000 carbon atoms in the polymer:
n = 1.14 × A / (ρ × t)
Where ρ is the true density of the sample (g / cm ³ ),
t: sample thickness (mm)
The melting point in the present invention refers to the temperature at the endothermic peak top obtained by a differential scanning calorimeter (DSC).

  Furthermore, for the purpose of increasing the heat resistance of the separator, a polyolefin other than polyethylene, such as polypropylene, may be included. The content at this time is 1 to 30% by weight, more preferably 1 to 20% by weight, and further preferably 3 to 10% by weight. Moreover, although it is also possible to fill with an inorganic substance, it is preferable not to fill in this application.

  Next, a method for producing the polyethylene microporous membrane of the present invention will be described. First, polyethylene as a raw material is mixed, and this is melted and kneaded in a plasticizer at a temperature higher than its melting point and lower than its decomposition temperature in an extruder equipped with a die. This is extruded from a die lip and cast on a cooling roll to form a sheet having a thickness of several tens of μm to several mm to obtain a gel composition. The plasticizer here refers to an organic compound that can form a uniform solution with polyethylene at a temperature below the boiling point. Specific examples include liquid paraffin, decalin, xylene, dioctyl phthalate, dibutyl phthalate, stearyl alcohol, oleyl alcohol, decyl alcohol, nonyl alcohol, diphenyl ether, n-decane, and n-dodecane. preferable.

  The polyethylene concentration in the plasticizer is in the range of 10 to 70% by weight, preferably 25 to 50% by weight. If it exceeds 70% by weight, an appropriate porosity cannot be obtained, and if it is less than 10% by weight, the viscosity is lowered and continuous sheet molding becomes difficult. In addition, in order to prevent the oxidation of polyethylene at the time of heat-dissolution, it is preferable to add an antioxidant. The gel composition is heated and stretched to obtain a stretched film. The stretching temperature ranges from room temperature to the melting point of the polymer gel, preferably from 80 to 130 ° C, more preferably from 100 to 125 ° C.

  The stretching method is performed at a predetermined magnification by a tenter method, a roll method, an inflation method, a rolling method, or a combination of these methods. Although uniaxial stretching and biaxial stretching may be used, biaxial stretching is preferable, and in the case of biaxial stretching, longitudinal and transverse simultaneous stretching or sequential stretching may be performed. The draw ratio is 4 to 400 times, preferably 8 to 200 times, and more preferably 16 to 100 times in terms of area magnification. When the stretching ratio is less than 4 times, the strength as a separator is insufficient, and when it exceeds 400 times, stretching becomes difficult.

  Next, a microporous film is obtained by extracting a plasticizer from the stretched film. As an extraction method, there is extraction with an organic solvent. As the solvent at this time, methyl ethyl ketone, methylene chloride, hexane, diethyl ether or the like is used, and then dried by heating and air drying. When a low boiling point compound such as decalin is used as the plasticizer, it can be removed by heating and drying. In any case, it is desirable to carry out the process in a state where the film is constrained in order to prevent deterioration of physical properties due to film shrinkage.

  After the above, it is desirable to perform heat setting (heat setting) for the purpose of increasing the dimensional stability or reducing the thermal shrinkage rate. The temperature at this time is preferably in the range from the crystal dispersion temperature to the melting point. In the heat setting, stretching in the width direction is performed at the same time, and the stretching ratio at this time is 1.01 to 2.00 times, preferably 1.10 to 1.80 times. . Various properties of the separator manufactured as described above will be described below. The film thickness is in the range of 1-100 μm, preferably 5-50 μm. If the film thickness is less than 1 μm, the mechanical strength is insufficient, and if it exceeds 100 μm, the film becomes hard and difficult to wind as a battery, and it is disadvantageous in terms of battery capacity. The film thickness is measured with a dial gauge (manufactured by Ozaki Seisakusho: PEACOCK No. 25).

The porosity is 20 to 80%, preferably 30 to 55%. If the porosity is less than 20%, the ion permeability is insufficient, and if it is more than 80%, the mechanical strength is insufficient. The porosity is calculated by the following method;
Porosity = {1- (10000 × M / ρ) / (X × Y × T)} × 100
In the above formula, X, Y: vertical and horizontal (cm) of the sample
T: Sample thickness (μm), M: Sample weight (g)
ρ: True density of resin (g / cm ³ )
The air permeability is 10 to 2000 sec. / 100cc, preferably 30 to 1500 sec. / 100cc, more preferably 50 to 1000 sec. / 100cc. 2000 sec. If it exceeds / 100 cc, it is not preferable from the viewpoint of ion permeability. The air permeability is measured by the following method;
[Measuring apparatus] Gurley type air permeability meter according to JIS P-8117 At this time, pressure: 0.01276 atm, membrane area: 6.424 cm ² , permeated air amount: 100 cc

The puncture strength is 50 g or more, preferably 100 g or more, more preferably 150 g or more. If it is less than 50 g, the yield during battery production decreases, which is not preferable. The puncture strength is measured by the following method;
[Measurement device] Handy compression tester (Kato Tech: KES-G5)
At this time, the radius of curvature of the needle tip: 0.5 mm, the piercing speed: 2 mm / sec.
The pore diameter is 0.01 to 1 μm, preferably 0.03 to 0.7 μm, and more preferably 0.05 to 0.5 μm. If the pore size is smaller than 0.01 μm, the ion permeability is not sufficient, and if it is larger than 1 μm, there is a possibility of internal short circuit due to the deposited dendrites and active material particles, and it is also considered that ion blocking due to the fuse effect is delayed. . The pore diameter is measured by the following method;
[Measurement device] Mercury porosimeter (manufactured by Shimadzu Corporation: Autopore 9220)
[Measurement conditions] 0.1 to 0.2 g of a sample cut to a width of about 25 mm is folded and placed in a cell, and measurement is performed from an initial pressure of 20 kPa.
[Derivation of pore diameter] Draw a differential pore volume curve (horizontal axis: pore diameter, vertical axis: value obtained by dividing the volume change of injected mercury by the pore diameter change amount), and the pore diameter (mode diameter) at the peak top of the curve is the pore diameter of the separator As representative.

  Hereinafter, although an example is given and an embodiment of the present invention is further explained, the present invention is not limited at all by these.

(1) Separation material As raw material polyethylene, 80% by weight of high-density polyethylene having a viscosity average molecular weight of 280,000 and polyethylene polymerized using a metallocene catalyst (viscosity average molecular weight: 70,000, melting point: 127 ° C., copolymerization) The α-olefin used has a carbon number: 6, density of 0.94 g / cm ³ , Mw / Mn: 4.2) 20% by weight. 45 parts by weight of these polyethylenes are melt-kneaded at 240 ° C. together with 55 parts by weight of liquid paraffin and 0.3 parts by weight of an antioxidant, extruded from a T-die, and cast on a cooling roll to form a sheet. To. This is stretched 7 times in the winding direction and 7 times in the width direction in the range of 115 to 125 ° C. by a simultaneous biaxial stretching machine. Thereafter, liquid paraffin is extracted with methyl ethyl ketone and dried. Further, heat setting is performed in the range of 110 to 130 ° C. and the film is stretched 1.3 times in the width direction. The polyethylene microporous membrane thus prepared has a thickness of 25 μm and a porosity of 40%.

(2) Preparation of positive electrode 100 parts by weight of lithium cobalt composite oxide LiCoO ₂ as an active material, 2.5 parts by weight of flake graphite and acetylene black as a conductive agent, and 3.5% of polyvinylidene fluoride (PVDF) as a binder A slurry is prepared by dispersing parts by weight in N-methylpyrrolidone (NMP). This slurry is applied to both surfaces of a 20 μm thick aluminum foil serving as a positive electrode current collector with a die coater, dried at 130 ° C. for 3 minutes, and then compression molded with a roll press. At this time, the active material coating amount of the positive electrode is 250 g / m ² , and the bulk density of the active material is 3.00 g / cm ³ . This is cut to a width of about 40 mm to form a strip.

(3) Production of negative electrode 90 parts by weight of graphitized mesophase pitch carbon fiber (MCF) as active material and 10 parts by weight of flake graphite, 1.4 parts by weight of ammonium salt of carboxymethyl cellulose as binder and styrene-butadiene copolymer A slurry is prepared by dispersing 1.8 parts by weight of latex in purified water. This slurry is applied to both sides of a 12 μm thick copper foil serving as a negative electrode current collector with a die coater, dried at 120 ° C. for 3 minutes, and then compression molded with a roll press. At this time, the active material coating amount of the negative electrode is set to 106 g / m ² and the bulk density of the active material is set to 1.35 g / cm ³ . This is cut to a width of about 40 mm to form a strip.
(4) Adjustment of Nonaqueous Electrolyte Solution It is adjusted by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1: 2 (volume ratio) to a concentration of 1.0 mol / liter.

(5) Battery assembly The above-mentioned microporous membrane separator, strip-shaped positive electrode, and strip-shaped negative electrode are stacked in the order of the strip-shaped negative electrode, the separator, the strip-shaped positive electrode, and the separator and wound in a spiral manner to produce an electrode plate laminate. This electrode plate laminate is pressed into a flat plate shape, and then stored in an aluminum container. The aluminum lead is led out from the positive electrode current collector to the battery lid, and the nickel lead is led out from the negative electrode current collector to the bottom of the container. Weld. Further, the non-aqueous electrolyte described above is injected into the container and sealed. The lithium ion battery thus manufactured has a size of 6.3 mm in length (thickness), 30 mm in width, 48 mm in height, and a nominal discharge capacity of 620 mAh.

(6) Battery evaluation The lithium-ion battery assembled as described above was charged to a battery voltage of 4.2 V at a current value of 310 mA (0.5 C) in an atmosphere at 25 ° C., and further maintained at 4.2 V. The first charge after battery preparation was performed for a total of 6 hours by a method of starting to reduce the value from 310 mA. The current value immediately before the end of charging was almost zero. And it was left to stand in 25 degreeC atmosphere for 1 week. Thereafter, the battery is charged to a battery voltage of 4.2 V at a current value of 620 mA in an atmosphere of 25 ° C., and further charged for a total of 3 hours by starting to reduce the current value from 620 mA so as to hold 4.2 V. The cycle of discharging to a battery voltage of 3.0 V at a current value of 620 mA was repeated 10 times. Furthermore, after making 4.2V charge state by the same method as described above, an overcharge test was performed. The current value was 620 mA (1.0 C), and the voltage value (maximum charging voltage value) at which the current was reduced was 10 V.

  As raw material polyethylene, 20% by weight of high-density polyethylene having a viscosity average molecular weight of 2 million and polyethylene polymerized using a chromium compound-supported catalyst (viscosity average molecular weight: 400,000, melting point: 132 ° C., 1000 in the polymer) The number of terminal vinyl groups per carbon atom: 0.5) 80% by weight is used in the same manner as in Example 1.

Comparative Example 1

Examples 1 and 2 are the same as the raw material polyethylene except that high-density polyethylene having a viscosity average molecular weight of 280,000 is used alone.
[Results of Overcharge Test] In Examples 1 and 2, heat generation was suppressed by the fuse effect, whereas in Comparative Example 1, the fuse effect was not sufficient and heat generation was observed.

Claims (6)

  And comprising at least 10 to 95% by weight of high-density polyethylene having a viscosity average molecular weight of 100,000 to 4,000,000 and 5 to 90% by weight of polyethylene having a melting point exceeding 125 ° C and not more than 132 ° C. Polyethylene microporous membrane characterized.   The polyethylene microporous membrane according to claim 1, wherein the polyethylene having a melting point of more than 125 ° C and not more than 132 ° C is a copolymer of ethylene and an α-olefin having 4 or more carbon atoms. The density of polyethylene having a melting point of more than 125 ° C. and not more than 132 ° C. is 0.86 to 0.95 g / cm ³ , and the ratio of weight average molecular weight to number average molecular weight (Mw / Mn) is 3.5 to 8 The polyethylene microporous membrane according to claim 1 or 2, which is in a range of 0.0.   3. The polyethylene microfiber according to claim 1, wherein the polyethylene having a melting point of more than 125 ° C. and not more than 132 ° C. has 0.3 or more terminal vinyl groups per 1000 carbon atoms in the polyethylene. Porous membrane.   A battery separator comprising the polyethylene microporous membrane according to claim 1.   A battery using the polyethylene microporous membrane according to claim 1 as a separator.