JP2002338730A - Polyethylene microporous membrane and battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a microporous polyethylene membrane used for a battery having excellent productivity, characteristics and excellent safety realized by a complete fuse effect, and a battery using the same. SOLUTION: At least the viscosity average molecular weight is 100,000-
4 million high-density polyethylene of 10 to 95% by weight and melting point of more than 125 ° C. and 132 ° C. or less of polyethylene 5
A polyethylene microporous membrane characterized by comprising a composition containing 90% by weight is used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microporous film made of polyethylene, and more particularly, to an ion-permeable material used for a battery using a carbon material, metal lithium, lithium alloy or the like into which lithium ions can be inserted as a negative electrode. The present invention relates to a microporous polyethylene membrane having excellent characteristics and safety, and a battery using the same.

[0002]

2. Description of the Related Art With the remarkable development of information-related devices such as portable telephones, notebook personal computers, and PDAs in recent years, small and lightweight batteries having a high energy capacity are required. While various types of batteries are being researched, developed, and sold, lithium-ion batteries, in particular, are expanding the market.
A polyethylene microporous membrane is used as a separator used therein.

[0003] As a separator for a battery such as a lithium ion battery, high ion permeability capable of exhibiting excellent battery characteristics, excellent mechanical properties that can withstand battery assembly and handling, and overcharging, etc. When the temperature rises, excellent safety, such as blocking the flow of ions and blocking the flow of current (fuse effect), is required. The “fuse effect” will be described in more detail.
When the microporous polyethylene membrane incorporated as a separator in the battery is heated to near its melting point, the pores are closed and the flow of ions is cut off, preventing current from flowing. Heat generation can be suppressed, and safety can be maintained. However, when heat is applied to the battery very rapidly, there is a possibility that the flow of ions cannot be cut off due to a delay in closing the holes. Even in such a case, there has been a demand for a separator having a function of reliably blocking the pores, blocking the flow of ions and stopping the flow of current.

As one means for solving the above problem, a method has been proposed in which polyethylene having a low melting point (95 to 125 ° C.) is added (for example, Japanese Patent Laid-Open No.
269289). However, if a substance having a too low melting point is added, the pores will be blocked, so that the heat setting (heat setting) temperature during the production of the microporous membrane cannot be increased, and as a result, a separator having a large heat shrinkage will not be obtained. Problem.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a microporous polyethylene membrane used in a battery having excellent productivity and characteristics and having excellent safety realized by a perfect fuse effect, and a polyethylene microporous membrane. It is to provide a used battery.

[0006]

In order to achieve the above object, the present invention provides (1) at least a compound having a viscosity average molecular weight of 1
100,000-4,000,000 high-density polyethylene 10-95% by weight
A polyethylene microporous membrane characterized by containing 5 to 90% by weight of polyethylene having a melting point of more than 125 ° C and 132 ° C or less;
The polyethylene having a temperature of 32 ° C. or less is a copolymer of ethylene and an α-olefin having 4 or more carbon atoms, wherein the polyethylene microporous membrane according to the above (1), When the density of the polyethylene at a temperature exceeding 132 ° C. 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. The microporous polyethylene membrane according to the above (1) or (2),
(4) The polyethylene according to (1) or (2), wherein the polyethylene having a melting point of more than 125 ° C. and 132 ° C. or less has at least 0.3 terminal vinyl groups per 1000 carbon atoms in the polyethylene. (5) a battery separator comprising the polyethylene microporous membrane according to any of (1) to (4); (6)
A battery using the microporous polyethylene membrane according to any one of the above (1) to (4) as a separator.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
The microporous polyethylene membrane of the present invention has a viscosity average molecular weight of at least 100,000 to 4,000,000.
The composition is characterized by comprising a composition containing 5 to 90% by weight of polyethylene having a melting point of more than 125 and 132 ° C. or less.

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 200,000.
10,000, more preferably 170,000 to 1,000,000, and still more preferably 200,000 or more and less than 700,000. Here, if the viscosity average molecular weight is less than 100,000, the strength of a microporous film becomes insufficient. On the other hand, if it exceeds 4,000,000, kneading and molding at the time of producing a microporous membrane become difficult. The viscosity average molecular weight Mv
Is the intrinsic viscosity [η] measured in a decalin solution and determined by the following equation: Polyethylene: [η] = 6.77 × 10 ⁻⁴ Mv ^0.67 Polypropylene: [η] = 1.10 × 10 ⁻⁴ Mv ^0.80 One of the polyethylenes 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 is prepared from a normal pressure to 100 kg / cm ² using a Ziegler type catalyst.
Can be produced by ionic polymerization under a pressure of

As the above-mentioned copolymer, polyethylene having a small structural non-uniformity can be used. At this time, the copolymer preferably has a density of 0.86 to 0.95 g / cm ³ . In addition, 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 in which the active site is one kind of homogeneous catalyst. Single site catalysts include metallocene catalysts such as bis (cyclopentadienyl) zirconium chloride. In addition, Mw and Mn
Is measured by the following method: [Measurement device] Gel permeation chromatography (GPC) (manufactured by Waters: ALC / GPC 150)
Type) [Measurement conditions] Column: AT-807S manufactured by Showa Denko (1
) And Tosoh's GMH-HT6 (two) are connected in series. Mobile phase: trichlorobenzene (TCB) Column temperature: 140 ° C Flow rate: 1.0 ml / min. Sample preparation: 20 to 30 mg of sample is 0.1% by weight
Is dissolved in 20 ml of a TCB solution in which 2,6-di-t-butyl-4-methylphenol is dissolved by heating at 140 ° C. [Calculation method] A baseline is drawn on the chromatogram, and the chromatogram is further divided into several. The height Hi of each division point from the baseline is obtained by equal division. Next, the molecular weight Mi at each division point is determined from a calibration curve using standard polystyrene having a known molecular weight. Then, from the following equation, Mw and M
Mn = {(Hi · Mi)} / ({Hi) Mn = ({Hi) / {(Hi / Mi)}}

The melting point used in the present invention is 125 ° C.
Of polyethylene having a melting point of more than 125 ° C. and less than 132 ° C. of polyethylene having a melting point of more than 132 ° C. and a copolymer of ethylene and an α-olefin having 4 or more carbon atoms per 1000 carbon atoms in the polyethylene It may have a terminal vinyl group of 0.3 or more. Such polyethylene can be polymerized by a chromium compound-supported catalyst. Here, the chromium compound-supported catalyst is, for example, one as disclosed in Japanese Patent Publication No. 1-1277.

When polymerized using such a catalyst, unlike the case where polymerization is performed using a Ziegler-type catalyst, 0.3 or more terminal vinyl groups are observed per 1000 carbon atoms in the polymer. Although the reason is not clear, the more vinyl terminal groups are present, the lower the pore closing temperature of the microporous membrane is. Incidentally, the number of terminal vinyl groups per 1000 carbon atoms in the polymer is derived by the following method;
The polymer is heated and pressed to measure the IR spectrum of the sample which has been made nonporous into a sheet, and the height (absorbance: A) of the peak indicating the terminal vinyl group at around 910 cm ^-1 from the baseline is determined. This is substituted into the following equation to determine the number n of terminal vinyl groups per 1000 carbon atoms in the polymer; n = 1.14 × A / (ρ × t) where ρ: true density of the sample (g / Cm ³ ), t: thickness of sample (mm) The melting point in the present invention means a differential scanning calorimeter (DS
It refers to the temperature of the endothermic peak top obtained by C).

Further, polyolefins other than polyethylene for the purpose of increasing the heat resistance of the separator, etc.
For example, polypropylene may be contained. The content at this time is 1 to 30% by weight, more preferably 1 to 2% by weight.
0% by weight, more preferably 3 to 10% by weight. Although it is possible to fill with an inorganic substance, it is preferable not to fill with an inorganic substance in the present application.

Next, a method for producing the microporous polyethylene membrane of the present invention will be described. First, polyethylene as a raw material is mixed, and this is melted and kneaded in an extruder equipped with a die in a plasticizer at a temperature equal to or higher than its melting point and lower than the decomposition temperature. This is extruded from a die lip and cast on a cooling roll to form a sheet having a thickness of several tens μm to several mm to obtain a gel composition. The plasticizer as used herein refers to an organic compound capable of forming a uniform solution with polyethylene at a temperature equal to or lower than 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, n-dodecane and the like, and particularly liquid paraffin and decalin. preferable.

The polyethylene concentration in the plasticizer is 10 to 70.
%, Preferably in the range of 25 to 50% by weight. 7
If it exceeds 0% by weight, an appropriate porosity cannot be obtained,
If it is less than 10% by weight, the viscosity is reduced, and it is difficult to form a continuous sheet. In addition, at the time of heating and melting, it is preferable to add an antioxidant in order to prevent oxidation of polyethylene. The gel composition is heated and stretched to form a stretched film. The stretching temperature is in the range of room temperature to the melting point of the polymer gel, preferably 80 to 130 ° C, more preferably 100 to 12 ° C.
It is in the range of 5 ° 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. Uniaxial stretching or biaxial stretching may be used, but biaxial stretching is preferred. In the case of biaxial stretching, simultaneous vertical and horizontal stretching or sequential stretching may be used. The stretching ratio is in the range of 4 to 400 times, preferably 8 to 200 times, more preferably 16 to 100 times in area ratio. If the stretching ratio is less than 4 times, the strength as a separator is insufficient, and if it exceeds 400 times, stretching becomes difficult.

Next, a plasticizer is extracted from the stretched film to obtain a microporous film. As an extraction method, there is an extraction with an organic solvent. As a solvent at this time, methyl ethyl ketone, methylene chloride, hexane, diethyl ether or the like is used, and thereafter, drying is performed 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, in order to prevent a decrease in physical properties due to shrinkage of the film, it is desirable to perform the process with the film restrained.

After the above, it is desirable to perform heat setting (heat setting) for the purpose of increasing the dimensional stability and reducing the heat shrinkage. At this time,
It is preferable to carry out in a range from the crystal dispersion temperature to the melting point. At the time of heat fixation, stretching in the width direction is performed at the same time, and the stretching ratio at this time is in the range of 1.01 to 2.00, preferably 1.10 to 1.80. . The physical properties of the separator manufactured as described above will be described below. The film thickness is in the range of 1 to 100 μm, preferably 5 to 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 be wound as a battery, and the battery capacity is disadvantageous. In addition, the film thickness is measured with a dial gauge (manufactured by Ozaki Seisakusho: PEACK No. 25).

The porosity is 20 to 80%, preferably 30 to 80%.
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 length of the sample (cm) T: sample thickness (μm), M: sample weight (g) ρ: true density of resin (g / cm ³ ) The degree is 10 to 2000 sec. / 100cc, preferably 30 to 1500 sec. / 100 cc, more preferably 50 to 1000 sec. / 100cc. 20
00sec. If it exceeds / 100 cc, it is not preferable from the viewpoint of ion permeability. The air permeability is measured by the following method: [Measurement device] Gurley-type air permeability meter according to JIS P-8117 At this time, pressure: 0.01276 atm, membrane area: 6.
424cm ² , permeated air volume: 100cc

The piercing strength is 50 g or more, preferably 100 g.
g or more, more preferably 150 g or more. If the amount 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 (Katotech: K
ES-G5) At this time, the needle tip curvature radius: 0.5 mm, the piercing speed: 2
mm / sec. The pore size is 0.01-1 μm, preferably 0.03-0.7.
μm, and more preferably 0.05 to 0.5 μm.
If the pore diameter is smaller than 0.01 μm, the ion permeability is not sufficient. If the pore diameter is larger than 1 μm, there is a possibility that precipitated dendrite or internal short-circuit due to active material particles may occur, and it may be considered that the blocking of ions due to the fuse effect is delayed. .
The pore size is measured by the following method: [Measurement device] Mercury porosimeter (manufactured by Shimadzu Corporation: Autopore 9220) [Measurement conditions] 0.1 to 0.1 mm cut sample
0.2 g is folded and put into the cell, and the initial pressure is 20 kPa
Measure from. [Derivation of pore size] Draw a differential pore volume curve (horizontal axis: pore size, vertical axis: value obtained by dividing the change in volume of the injected mercury volume by the change in pore size), and calculate the pore size (mode diameter) at the peak top of the curve by the pore size of the separator. As a representative.

[0020]

EXAMPLES Hereinafter, embodiments of the present invention will be further described with reference to examples, but the present invention is not limited thereto.

[0021]

Example 1 (1) Preparation of Separator 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: 7
10,000, melting point: 127 ° C., carbon number of copolymerized α-olefin: 6, density: 0.94 g / cm ³ , Mw / Mn:
4.2) Use 20% by weight. These polyethylene 45
55 parts by weight of liquid paraffin, 0.3 part by weight
The mixture is melt-kneaded at 240 ° C. with a weight part of an antioxidant, extruded from a T-die, and cast on a cooling roll to form a sheet. This is done by a simultaneous biaxial stretching machine.
The film is stretched 7 times in the winding direction and 7 times in the width direction within a range of 115 to 125 ° C. Thereafter, liquid paraffin is extracted with methyl ethyl ketone and dried. 110-130 ° C
, And stretched 1.3 times in the width direction. The microporous polyethylene membrane thus produced has a thickness of 25 μm and a porosity of 40%.

(2) Preparation of Positive Electrode LiCoO ₂ Lithium-Cobalt Composite Oxide as Active Material
100 parts by weight, 2.5 parts by weight of flaky graphite and acetylene black as conductive agents, and 3.5 parts by weight of polyvinylidene fluoride (PVDF) as a binder in N-methylpyrrolidone (NMP). Is prepared. This slurry is applied to both sides 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 by a roll press. At this time, the active material coating amount of the positive electrode was 250 g /
m ² , and the bulk density of the active material is adjusted to 3.00 g / cm ³ . This is cut to a width of about 40 mm to form a band.

(3) Preparation of Negative Electrode 90 parts by weight of graphitized mesophase pitch carbon fiber (MCF) as active material, 10 parts by weight of flake graphite, 1.4 parts by weight of ammonium salt of carboxymethyl cellulose as binder and styrene-butadiene A slurry is prepared by dispersing 1.8 parts by weight of the copolymer latex in purified water. The slurry is applied to both surfaces of a copper foil having a thickness of 12 μm as a negative electrode current collector by a die coater, dried at 120 ° C. for 3 minutes, and then compression-molded by a roll press. At this time, the active material application amount of the negative electrode was 10
The active material has a bulk density of 6 g / m ² and a bulk density of 1.35 g / cm ³ . This is cut to a width of about 40 mm to form a band. (4) Preparation of non-aqueous electrolyte Ethylene carbonate: Ethyl methyl carbonate =
LiPF ₆ as a solute in a 1: 2 (volume ratio) mixed solvent
Is dissolved and adjusted to a concentration of 1.0 mol / liter.

(5) Battery assembly The above microporous membrane separator, strip-shaped positive electrode and strip-shaped negative electrode were
The strip-shaped negative electrode, the separator, the strip-shaped positive electrode, and the separator are sequentially stacked and spirally wound to form an electrode plate laminate. After pressing this electrode plate laminate into a flat plate shape, it is housed in an aluminum container, the aluminum lead is led out from the positive electrode current collector, and the nickel lead is pulled out from the negative electrode current collector, and the nickel lead is drawn out from the negative electrode current collector. Weld. Further, the above-mentioned non-aqueous electrolyte 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, and 48 mm in height, and has a nominal discharge capacity of 6 mm.
20 mAh.

(6) Battery evaluation Lithium-ion battery assembled at 25 ° C.
Under an atmosphere, the battery is charged to a battery voltage of 4.2 V with a current value of 310 mA (0.5 C), and the current value is started to be reduced from 310 mA while maintaining 4.2 V.
The first charge after the production of the battery was performed for a total of 6 hours. The current value immediately before the end of charging was almost zero. And 25
It was left for 1 week in an atmosphere of ° C. Thereafter, the battery was charged to a battery voltage of 4.2 V at a current value of 620 mA in an atmosphere of 25 ° C.
A cycle of charging for a total of 3 hours in a manner of starting to squeeze from 0 mA and discharging to a battery voltage of 3.0 V at a current value of 620 mA was repeated 10 times. Further, after the battery was charged to 4.2 V in the same manner as described above, an overcharge test was performed. The current value is 620 mA (1.0 C)
And the voltage value at which the current is reduced (the maximum charging voltage value) is 10 V
And

[0026]

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

[0027]

Comparative Example 1 The procedure was the same as in Examples 1 and 2, except that high-density polyethylene having a viscosity average molecular weight of 280,000 was used alone as a raw material polyethylene. [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.

[0028]

As described above, since the polyethylene microporous membrane of the present invention has an excellent fuse effect, a battery excellent in safety can be manufactured.

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) H01M 10/40 H01M 10/40 Z F term (reference) 4F074 AA17A AA18 AB01 AB03 AD04 AG20 CB31 CB34 CC02X DA13 DA49 4J002 BB03W BB03X BB05X EA046 EC066 EH146 GQ00 5H021 EE04 HH01 HH05 HH06 HH07 5H029 AJ11 AJ12 AK03 AL07 AM03 AM07 EJ14 HJ00 HJ01 HJ08 HJ14

Claims (6)

[Claims] At least a viscosity average molecular weight of at least 100,000
4 million high-density polyethylene of 10 to 95% by weight and melting point of more than 125 ° C. and 132 ° C. or less of polyethylene 5
A microporous polyethylene membrane, comprising a composition containing 90% by weight. 2. The polyethylene fine particle according to claim 1, wherein the polyethylene having a melting point of more than 125 ° C. and 132 ° C. or less is a copolymer of ethylene and an α-olefin having 4 or more carbon atoms. Porous membrane. 3. The polyethylene having a melting point of more than 125 ° C. and 132 ° C. or less has a density of 0.86 to 0.95 g / cm ³ and a ratio of a weight average molecular weight to a number average molecular weight (Mw / Mn). The polyethylene microporous membrane according to claim 1 or 2, wherein the thickness is in the range of 3.5 to 8.0. 4. A polyethylene having a melting point of not less than 125 ° C. and not more than 132 ° C., wherein the polyethylene has at least 0.3 terminal vinyl groups per 1,000 carbon atoms in the polyethylene. The microporous polyethylene membrane as described in the above. 5. A battery separator comprising the polyethylene microporous membrane according to claim 1. 6. A battery using the microporous polyethylene membrane according to claim 1 as a separator.