JP2011081994A - Separator for high-temperature preservation characteristics storage device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a separator for a high-temperature preservation characteristics storage device which allows a storage device, having superior high-temperature preservation characteristics to be manufactured. <P>SOLUTION: A separator for a high-temperature preservation characteristics storage device has a diffusion coefficient D(Z) in a film thickness direction determined by a magnetic gradient NMR method where the coefficient satisfies the relation: D(Z)≥αT+β, where α=-2.5×10<SP>-12</SP>, β=1.2×10<SP>-10</SP>and T=air permeability (sec/100 (cc μm)). <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a separator for an electricity storage device having high temperature storage characteristics.

In recent years, power storage devices such as lithium ion secondary batteries and electric double layer capacitors (including those called lithium ion capacitors and non-aqueous lithium power storage elements) have been actively developed. In a power storage device, a microporous membrane (separator) is usually provided between the positive and negative electrodes. Such a separator has a function of preventing contact between positive and negative electrodes and transmitting ions.
Here, the separator is required to have a certain physical strength or more from the viewpoint of ensuring good safety of the electricity storage device. That is, pressure from the electrodes may be applied to the separator as the electricity storage device is charged / discharged, and the electrodes may break through the separator and cause a short circuit between the electrodes.
The separator is also required to have a low electrical resistance from the viewpoint of achieving high output of the electricity storage device.

  Under such circumstances, for example, Patent Document 1 proposes a microporous film using ultrahigh molecular weight polyethylene. Patent Document 2 discloses an inorganic particle-containing polyolefin microporous membrane.

Japanese Patent No. 2794179 International Publication No. 2006/0253323 Pamphlet

However, both of the microporous membranes described in Patent Documents 1 and 2 still have room for improvement from the viewpoint of high-temperature storage characteristics when used as a separator.
In view of the above circumstances, an object of the present invention is to provide a separator for an electricity storage device with high temperature storage characteristics that can produce an electricity storage device having excellent high temperature storage characteristics.

  The present inventors have used a microporous membrane satisfying a specific relational expression as a separator of an electricity storage device by a diffusion coefficient D (Z) in the thickness direction measured by a magnetic field gradient NMR method and an air permeability T of the membrane. The inventors have found that an electricity storage device having excellent high-temperature storage characteristics can be obtained, and completed the present invention.

That is, the present invention is as follows.
[1]
A separator for high-temperature storage characteristics electricity storage device, wherein a diffusion coefficient D (Z) in the film thickness direction measured by a magnetic field gradient NMR method satisfies D (Z) ≧ αT + β.
(Here, α = −2.5 × 10 ⁻¹² , β = 1.2 × 10 ⁻¹⁰ , T = air permeability (sec / 100 cc · μm)).
[2]
An electricity storage device comprising the separator for an electricity storage device with high temperature storage characteristics described in [1] above.

  ADVANTAGE OF THE INVENTION By this invention, the separator for high temperature storage characteristic electrical storage devices which can produce the electrical storage device which has the outstanding high temperature storage characteristic can be provided.

  Hereinafter, a mode for carrying out the present invention (hereinafter abbreviated as “the present embodiment”) will be described in detail. In addition, this invention is not limited to the following embodiment, It can implement by changing variously within the range of the summary.

In the separator for high temperature storage characteristics electricity storage device of the present embodiment, the diffusion coefficient D (Z) in the film thickness direction measured by the magnetic field gradient NMR method satisfies the relational expression D (Z) ≧ αT + β.
(Here, α = −2.5 × 10 ⁻¹² , β = 1.2 × 10 ⁻¹⁰ , T = air permeability (sec / 100 cc · μm)).

  When the diffusion coefficient D (Z) in the film thickness direction of the microporous film and the air permeability T of the microporous film satisfy the above specific relational expression, the inventors of the electricity storage device when used as a separator It has been found that the high temperature storage characteristics are remarkably improved. That is, by controlling each value so that the diffusion coefficient D (Z) measured by the magnetic field gradient NMR method and the air permeability T satisfy the above specific relational expression, an electricity storage device having excellent high-temperature storage characteristics can be obtained. It was found that it can be obtained. The reason for this is not clear, but by prescribing ion permeability and gas permeability, it is estimated that lithium dendrite growth is suppressed, internal short circuits are prevented at high temperatures, and high temperature storage characteristics are obtained. .

  The separator for an electricity storage device with high temperature storage characteristics of the present embodiment satisfies a relational expression of D (Z) ≧ αT + β. If D (Z) and T do not satisfy the above relational expression, that is, if D (Z) <αT + β, sufficient high-temperature storage characteristics tend not to be ensured.

The numerical range of the diffusion coefficient D (Z) is not particularly limited as long as the above relational expression is satisfied, but is preferably 1.0 × 10 ⁻¹³ or more, more preferably 1.0 × 10 ⁻¹² or more, and the upper limit. Is preferably 1.0 × 10 ⁻⁸ or less, more preferably 1.0 × 10 ⁻⁹ or less. If the value of D (Z) is 1.0 × 10 ⁻¹³ or less, sufficient permeability tends not to be secured, and if it is 1.0 × 10 ⁻⁸ or more, sufficient film strength is obtained. There is no tendency.

  Examples of means for adjusting the value of the diffusion coefficient D (Z) to the specific range include adjustment of stretching conditions and heat setting / relaxation conditions. More specifically, in order to increase the value of the product of the diffusion coefficient, it is necessary to increase the draw ratio during the stretching process, to stretch during the heat setting / thermal relaxation process, and to increase the draw ratio. Furthermore, the total draw ratio may be adjusted so that the draw ratios in the MD direction and the TD direction are isotropic. On the other hand, in order to reduce the product value of the diffusion coefficient, the stretching ratio in the stretching process is reduced, the stretching ratio in the heat setting / thermal relaxation process is decreased, and the total stretching ratio is further reduced. And adjusting so that one of the draw ratios in the MD direction and the TD direction is larger than the other.

  Further, the numerical range of the air permeability T is not particularly limited as long as the above relational expression is satisfied, but is preferably 0.5 seconds / 100 cc · μm or more, more preferably 2 seconds / 100 cc · μm or more. Is preferably 50 seconds / 100 cc · μm or less, more preferably 30 seconds / 100 cc · μm or less, and even more preferably 20 seconds / 100 cc · μm or less. An air permeability of 0.5 sec / 100 cc · μm or more is preferable from the viewpoint of suppressing self-discharge of the electricity storage device. On the other hand, setting it to 50 seconds / 100 cc · μm or less is preferable from the viewpoint of obtaining good charge / discharge characteristics.

  Examples of the means for adjusting the numerical range of the air permeability T include a method of adjusting the temperature and magnification of a stretching process, a heat setting process, and a thermal relaxation process described later. More specifically, to increase T, the stretching ratio is increased in the stretching process, and the stretching ratio is increased by performing stretching in the heat setting / thermal relaxation process. To reduce the size, for example, the stretching ratio may be reduced in the stretching step. The magnetic field gradient NMR method can be performed according to the method described in the examples described later.

  The separator for high temperature storage characteristics electricity storage device (hereinafter, also simply referred to as “separator”) of the present embodiment is formed from a polyolefin resin composition containing a polyolefin resin. Examples of the polyolefin resin used in the present embodiment include polymers obtained by polymerizing monomers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene ( Homopolymers, copolymers, multistage polymers, etc.). These polymers can be used alone or in combination of two or more.

  Examples of the polyolefin resin include low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ultrahigh molecular weight polyethylene, isotactic polypropylene, atactic polypropylene, polybutene, and ethylene propylene rubber. May be.

  The separator of the present embodiment can be manufactured by, for example, a dry-stretching method. The dry-stretch method means a process in which pore formation is performed by stretching a non-porous precursor.

  The separator of the present embodiment can contain other known components. Examples of these other components include fillers, antistatic agents, anti-blocking materials, antioxidants, and lubricants.

  Many other materials can also be added to improve or enhance the properties of the separator. Examples of such other materials include (1) polyolefin or polyolefin oligomer having a melting point of less than 130 ° C., (2) calcium carbonate, zinc oxide, diatomaceous earth, talc, kaolin, synthetic silica, mica, clay, boron nitride. Mineral fillers including, but not limited to, silicon dioxide, titanium dioxide, barium sulfate, aluminum hydroxide, magnesium hydroxide, and mixtures thereof, and (3) ethylene propylene (EPR), ethylene propylene diene (EPDM) ), Styrene-butadiene (SBR), styrene isoprene (SIR), ethynylidene norbonene (ENB), epoxies and polyurethanes, and elastomers, including but not limited to, (4) ethoxylate alcohols, primary polymer carbo Wetting agents including but not limited to acids, glycols (eg, polypropylene glycol and polyethylene glycol), functionalized polyolefins, (5) silicon, fluoropolymers, Chemamide®, oleamide, stearamide, erucamide, calcium stearate, or Lubricants, including but not limited to other metal stearates, (6) flame retardants such as brominated flame retardants, ammonium phosphate, ammonium hydroxide, alumina trihydrate, and phosphate esters, (7) cross-linking Or a nucleating agent comprising a coupling agent, (8) a polymer processing agent, and (9) a beta nucleating agent for polypropylene, but US Pat. No. 6,602,593, incorporated herein by reference. Specific to the beta nucleating agent polypropylene disclosed in Excluded. Beta nucleating agent polypropylene is a cause to material produced in the beta crystals in polypropylene.), And the like.

  The porous film of the present embodiment may be used as a single layer or may be used after being laminated. In the case of a laminated product, at least one of the layers may be substantially the porous film layer of the present embodiment. As a lamination method, after melt coextrusion molding so as to form a desired layer, a method of obtaining a laminated porous film by stretching and forming a porous layer, or laminating and forming a laminated porous film after separately producing the desired layers, respectively. A method for obtaining a film can be used.

  Generally, the manufacturing process of the separator includes a step of extruding a non-porous precursor, a step of stretching the non-porous precursor, and a step of performing heat setting and / or heat relaxation. . The non-porous precursor may be annealed before stretching.

  Annealing can be carried out in one embodiment at a temperature between Tm-80 ° C. and Tm-10 ° C. (Tm means the melting point of the polymer). Some materials, such as those with high crystallinity after extrusion, such as polybutene, do not require annealing.

The stretching method at the time of stretching may be a uniaxial stretching method or a biaxial stretching method, and the biaxial stretching method may be a simultaneous biaxial stretching method or a sequential biaxial stretching method.
The stretching temperature is preferably not lower than the glass transition temperature and not higher than the melting point of the thermoplastic resin to be used, and more preferably not lower than the glass transition temperature and not higher than the melting point−10 ° C.
When the film is stretched in this temperature range, the original film can be stretched without breaking, and a porous structure in which the pores are highly connected tends to be obtained.
Furthermore, the film may be stretched once up to a desired stretch ratio, or may be stretched in multiple times, and the stretching temperature can be changed for each.

  Furthermore, heat treatment can be performed on the porous film made porous by the stretching. It is necessary to perform the heat treatment temperature in a temperature range where the shape does not substantially change such as the melting point or softening point of the thermoplastic resin used.

  In addition, you may perform a post-process with respect to the obtained separator after a heat setting process. Examples of such post-treatment include hydrophilic treatment with a surfactant or the like, and cross-linking treatment with ionizing radiation.

The puncture strength of the separator of the present embodiment is preferably 1.6 N / 20 μm or more, more preferably 2 N / 20 μm or more, and the upper limit is preferably 20 N / 20 μm or less, more preferably 10 N / 20 μm or less. . It is preferable that the puncture strength is 1.6 N / 20 μm or more from the viewpoint of suppressing membrane breakage due to the dropped active material or the like during battery winding. Moreover, it is preferable also from a viewpoint which can reduce the risk of a short circuit by the expansion and contraction of the electrode accompanying charging / discharging. On the other hand, it is preferable to set it as 20 N / 20 micrometers or less from a viewpoint which can reduce the width shrinkage by the orientation relaxation at the time of a heating.
The puncture strength can be adjusted by a method of adjusting the polyolefin molecular weight, the ratio of the polyolefin resin, the stretching temperature, and the stretching ratio.
Here, the puncture strength was determined by fixing the microporous membrane with a sample holder having a diameter of 11.3 mm using a handy compression tester KES-G5 (trademark) manufactured by Kato Tech, and then fixing the microporous membrane. The maximum puncture load (N) value measured by performing a puncture test in a 23 ± 2 ° C. atmosphere at a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm / sec at the center of the membrane.

  The porosity of the separator of the present embodiment is preferably 30% or more, more preferably 35% or more, from the viewpoint of ensuring permeability. Further, from the viewpoint of film strength and self-discharge, it is preferably 90% or less, more preferably 80% or less. The porosity can be adjusted by a method for adjusting the magnification in the stretching step and / or a method for adjusting the temperature and magnification in the heat setting and thermal relaxation steps.

  The thickness of the separator of the present embodiment is preferably 2 μm or more, more preferably 5 μm or more, and the upper limit is preferably 100 μm or less, more preferably 60 μm or less, and still more preferably 50 μm or less. Setting the film thickness to 2 μm or more is preferable from the viewpoint of improving the mechanical strength. On the other hand, a thickness of 100 μm or less is preferable because the occupied volume of the separator is reduced, which tends to be advantageous in terms of increasing the capacity of the battery.

The electricity storage device of the present embodiment includes the above-described electricity storage device separator, a positive electrode, a negative electrode, and an electrolytic solution.
For example, the power storage device is prepared by preparing the separator as a vertically long separator having a width of 10 to 500 mm (preferably 80 to 500 mm) and a length of 200 to 4000 m (preferably 1000 to 4000 m). -Negative electrode-separator or negative electrode-separator-positive electrode-separator are stacked in this order, wound in a circular or flat spiral shape to obtain a wound body, the wound body is stored in a battery can, and an electrolyte solution is further added. It can be manufactured by injection.
The electricity storage device is manufactured through a process of laminating a flat plate in the order of positive electrode-separator-negative electrode-separator or negative electrode-separator-positive electrode-separator, laminating with a bag-like film, and injecting an electrolyte solution. You can also.

  Since the electricity storage device of this embodiment is excellent in high-temperature storage characteristics, it is particularly useful for electric vehicles and hybrid vehicles.

  In addition, about the measuring method of the various parameters mentioned above, unless there is particular notice, it measures according to the measuring method in the Example mentioned later.

  Next, the present embodiment will be described more specifically with reference to examples and comparative examples. However, the present embodiment is not limited to the following examples unless it exceeds the gist. In addition, the physical property in an Example was measured with the following method.

(1) Film thickness It measured at room temperature 23 degreeC using the micro thickness measuring device (type KBM by Toyo Seiki).

(2) Porosity A 10 cm × 10 cm square sample was cut from the microporous membrane, and its volume (cm ³ ) and mass (g) were determined. For the polypropylene microporous membrane, the membrane density was 0.91 (g / cm ³ ) Using the following formula.
Porosity = (1−mass / volume / 0.91) × 100
For the polyethylene microporous membrane, the membrane density was calculated as 0.95 (g / cm ³ ) using the following formula.
Porosity = (1−mass / volume / 0.95) × 100

(3) Air permeability It measured with the Gurley type air permeability meter (made by Toyo Seiki) based on JIS P-8117.

(4) Diffusion coefficient D
Magnetic field gradient NMR measurement method in a state where an electrolyte solution in which a lithium salt LiN (SO ₂ CF ₃ ) ₂ (LiTFSI) is dissolved in a mixed solvent of ethylene carbonate and ethyl methyl carbonate is infiltrated and held inside the porous film. Thus, the diffusion coefficient D of lithium ions at 30 ° C. was determined. In the magnetic field gradient NMR measurement method, the observed peak height is E, the peak height when the magnetic field gradient pulse is not applied is E ₀ , the nuclear magnetic rotation ratio is γ (T ⁻¹ · s ⁻¹ ), and the magnetic field gradient intensity is When g (T · m ⁻¹ ), magnetic field gradient pulse application time is δ (s), diffusion waiting time is Δ (s), and self-diffusion coefficient is D (m ² · s ⁻¹ ), the following equation holds. .
Ln (E / E ₀ ) = D × γ ² × g ² × δ ² × (Δ−δ / 3)
From the above equation, D can be obtained by observing the change of the NMR peak by changing g, δ, and Δ. In practice, the bpp-led-DOSY method is used as the NMR sequence, and Δ and δ are fixed, g is changed from 0 to Ln (E / E ₀ ) ≦ −3 by 10 points or more, and Ln ( D was obtained from the slope of a straight line plotted with E / E ₀ ) as the Y axis and γ ² × g ² × δ ² × (Δ−δ / 3) as the X axis. The set values of Δ and δ are arbitrary, but the following conditions must be satisfied when the longitudinal relaxation time of the measurement target is T1 (s) and the lateral relaxation time is T2 (s).
10ms <Δ <T1
0.2 ms <δ <T2
In practice, the magnetic field gradient NMR measurement was carried out with Δ = 50 ms and δ with an arbitrary value in the range of 0.4 ms ≦ δ ≦ 3.2 ms. When the self-diffusion is inhibited due to the influence of the structure of the porous film, the above plot becomes a downwardly convex curve. In this case, the curve is in the range of Ln (E / E ₀ ) from 0 to −2. A straight line was approximated, and D was obtained from this slope.

(5) High temperature storage test a. Production of Positive Electrode 92.2% by mass of lithium cobalt composite oxide LiCoO ₂ as a positive electrode active material, 2.3% by mass of flake graphite and acetylene black as a conductive material, and polyvinylidene fluoride (PVDF) 3.2 as a binder, respectively. A slurry was prepared by dispersing mass% in N-methylpyrrolidone (NMP). This slurry was applied to an aluminum foil serving as a positive electrode current collector with a die coater, dried, and compression molded with a roll press to produce a positive electrode.
b. Preparation of Negative Electrode 96.9% by mass of artificial graphite as a negative electrode active material, 1.4% by mass of ammonium salt of carboxymethyl cellulose and 1.7% by mass of styrene-butadiene copolymer latex as a binder were dispersed in purified water to prepare a slurry. did. The slurry was applied to a copper foil serving as a negative electrode current collector with a die coater, dried, and compression molded with a roll press to produce a negative electrode.
c. Preparation of Nonaqueous Electrolytic Solution Prepared 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.
d. Battery assembly A negative electrode, a separator, a positive electrode, and a separator were stacked in this order and wound in a spiral shape to produce an electrode plate laminate. The electrode plate laminate is housed in a stainless steel container having an outer diameter of 18 mm and a height of 65 mm, and an aluminum tab derived from the positive electrode current collector is used as a container lid terminal portion, and a nickel tab derived from the negative electrode current collector Was welded to the container wall. Thereafter, vacuum drying was performed, and the above-described non-aqueous electrolyte was poured into a container in an argon box and sealed to assemble a battery.
e. Pre-treatment The assembled battery is charged at a constant current of 1 / 3C to a voltage of 4.2V, then charged at a constant voltage of 4.2V for 5 hours, and then discharged to a final voltage of 3.0V at a current of 1 / 3C. Went. Next, after constant current charging to a voltage of 4.2 V with a current value of 1 C, a constant voltage charge of 4.2 V was performed for 2 hours, and then discharging was performed to a final voltage of 3.0 V with a current of 1 C. Finally, 4.2V constant voltage charging was performed at a current value of 1C, and then 4.2V constant voltage charging was performed for 2 hours as a pretreatment.
f. The battery pretreated in the high temperature storage test e was put into an oven, heated from room temperature at 5 ° C./min, and then left at 80 ° C. for 10 days. Thereafter, the battery was taken out of the oven and allowed to cool to room temperature, and then discharged to 3.0 V at a current of 1 C, and the capacity retention rate relative to the amount of charge before charging the oven was calculated. The case where the capacity retention ratio was 85% or more was used as an index for high temperature storage characteristics, and ◎ was used when the capacity was 80% or more, and x when it was 80% or less.

The following examples were carried out in the pilot device representing the normal uniaxial stretching line comprising the extruder, the crystal forming part (ie the variable speed roll equipped with the temperature control), and the stretching frame as the main components. . The remaining time on the roll during crystal formation is about 30 seconds for all samples. The polypropylene resin was Exxon's Escorene PP 4352FI, the beta nucleating agent was NJ Star NU-100, and 0.2 wt% resin was used. Other conditions are shown below.
[Example 1]
Polypropylene resin was extruded at 220 ° C. in a 2.5 inch extruder and the molten polymer was cooled by blown air. Next, the extruded thin film was annealed at 125 ° C. for 2 minutes, then cold-stretched to 20% at room temperature, then uniaxially stretched to 150%, and then heat-treated at 130 ° C. for 2 minutes. Table 1 shows the physical properties and evaluation results of the obtained separator.

[Example 2]
The polypropylene resin was extruded at 220 ° C. with a 2.5 inch extruder and the molten polymer was cooled by blown air. Next, the extruded thin film was annealed at 127 ° C. for 2 minutes, then cold-stretched to 20% at room temperature, then uniaxially stretched to 300%, and then heat-treated at 130 ° C. for 2 minutes. Table 1 shows the physical properties and evaluation results of the obtained separator.

[Comparative Example 1]
47.5% by mass of homopolymer polyethylene with a viscosity average molecular weight (Mv) of 700,000, 47.5% by mass of homopolymer polyethylene with an Mv of 300,000, 5% by mass of polypropylene with an Mv of 400,000 (PP blend amount 5% by mass) ) Was dry blended using a tumbler blender to obtain a polymer mixture. The obtained mixture of polymers and the like was substituted with nitrogen and then fed to the twin-screw extruder with a feeder under a nitrogen atmosphere. Further, liquid paraffin (kinematic viscosity at 37.78 ° C .: 7.59 × 10 ⁻⁵ m ² / s) was injected into the extruder cylinder by a plunger pump.
Feeder so that the liquid paraffin content ratio in the total mixture to be melt-kneaded and extruded is 66% by mass (that is, the polymer concentration (sometimes abbreviated as “PC”) is 34% by mass). And the pump was adjusted.
Subsequently, the melt-kneaded product was extruded and cast on a cooling roll controlled at a surface temperature of 25 ° C. through a T-die, to obtain a gel sheet having a thickness of 1300 μm.
Next, it led to the simultaneous biaxial tenter stretching machine, and biaxial stretching was performed. The set stretching conditions were an MD magnification of 7.0 times, a TD magnification of 6.4 times, and a preset temperature of 125 ° C.
Next, the solution was introduced into a methyl ethyl ketone tank and sufficiently immersed in methyl ethyl ketone to extract and remove liquid paraffin, and then the methyl ethyl ketone was removed by drying.
Next, it was led to a TD tenter and heat fixed. A separator was obtained with a heat setting temperature of 125 ° C. and a TD relaxation rate of 0.80. Table 1 shows the physical properties and evaluation results of the obtained separator.

[Comparative Example 2]
A separator was obtained in the same manner as in Comparative Example 1 except that the gel sheet thickness was 1150 μm, the biaxial stretching temperature was 122 ° C., the HS temperature was 122 ° C., and the HS magnification was 1.5 times. Table 1 shows the physical properties and evaluation results of the obtained separator.

  As is clear from the results in Table 1, the batteries using the separators of Examples 1 and 2 in which the diffusion coefficient D (Z) in the film thickness direction measured by the magnetic field gradient NMR method satisfies D (Z) ≧ αT + β. All exhibited excellent high-temperature storage characteristics.

  ADVANTAGE OF THE INVENTION By this invention, the separator for high temperature storage characteristic electrical storage devices which can produce the electrical storage device which has the outstanding high temperature storage characteristic can be provided.

Claims (2)

A separator for high-temperature storage characteristics electricity storage device, wherein a diffusion coefficient D (Z) in the film thickness direction measured by a magnetic field gradient NMR method satisfies D (Z) ≧ αT + β.
(Here, α = −2.5 × 10 ⁻¹² , β = 1.2 × 10 ⁻¹⁰ , T = air permeability (sec / 100 cc · μm)).   An electrical storage device comprising the separator for an electrical storage device with high temperature storage characteristics according to claim 1.