JP2007179758A - Separator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte battery separator capable of being formed into a thin-film and excelling in heat resistance and rupture resistance. <P>SOLUTION: This separator is made of a melt-blown nonwoven fabric formed out of a molten anisotropic polyester fiber, and having a fiber diameter of 1-12 μm, a basis weight not greater than 45 g/m<SP>2</SP>, and a thickness of 5-200 μm. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a separator for a nonaqueous battery or a nonaqueous capacitor having excellent heat resistance.

In recent years, various information terminal devices such as mobile phones and notebook personal computers, mobile devices have rapidly become smaller, lighter, thinner and popularized, hybrid vehicles have been popularized and expanded, and fuel cell vehicles have been popularized and put into practical use. Aiming at these demands, the demand for high energy density secondary batteries as power sources and auxiliary power sources is increasing.
In particular, lithium ion secondary batteries and capacitors using a non-aqueous electrolyte have a high operating voltage and a high energy density. This lithium ion secondary battery and capacitor generally have a predetermined non-electrode in a battery can also serving as a negative electrode terminal, with an electrode group having a separator having electrical insulation and liquid retention between a positive electrode and a negative electrode. The battery can be accommodated together with the water electrolyte, and the opening of the battery can is sealed with an insulating gasket with a sealing plate having a positive electrode terminal.

In particular, as a separator in a lithium ion secondary battery having a high energy density, a porous film of a polyolefin resin having low reactivity with an organic solvent is widely used. Examples of polyolefin resins include polyethylene and polypropylene, and it has been proposed to melt these polyolefin resins at about 100 ° C., close holes, and shut down the battery (see, for example, Patent Document 1). ).
However, when a polyolefin-based separator having a thin film is used, there is a problem that the separator is broken or avoided when it is wound together with an electrode or laminated to produce a battery.

  A safety measure called shutdown is the safety of lithium-ion batteries because unstable metals are present in the batteries, and there is a risk of short-circuiting, ignition, etc. Especially, molten metallic lithium is highly reactive. In order to ensure that the circuit is shut off before the temperature in the battery reaches the melting point of lithium, in addition to the shutdown, the separator itself melts and breaks (melts down) when the temperature rises further after the shutdown. It has been required to have a function of suppressing battery ignition and explosion, and to suppress thermal runaway during overcharge and heat storage tests.

In recent years, the safety of heat-resistant electrolytes such as ionic liquids, external short-circuit protection circuits, and polymer electrolytes such as polyethylene glycol, polymethacrylate, and polyacrylonitrile have increased. Therefore, a separator that is high and does not short-circuit has been demanded.
As a method of use, batteries such as hybrid cars used in the vicinity of the engine room of an automobile or electric double layer capacitors, etc., may be operated at a temperature higher than the melting point of conventional batteries using polyethylene or polypropylene separators. Therefore, a separator having further heat resistance and low resistance has been demanded.

JP-A-3-203160

  The object of the present invention has been made in view of the background and problems of the above-described prior art, and can provide a nonaqueous electrolyte battery separator that can cope with the thinning of the separator and has excellent heat resistance and burst resistance. It is in.

  As a result of intensive investigations to solve such problems, the present inventors have been able to achieve a thinner separator by using a melt-blown nonwoven fabric made of melt-anisotropic polyester fibers, and are excellent in heat resistance and burst resistance. It was found that separators for non-aqueous electrolyte batteries and capacitors were obtained, and the present invention was completed.

That is, the present invention is a separator composed of a melt-blown nonwoven fabric composed of melt anisotropic polyester fibers, having a fiber diameter of 1 to 12 μm, a basis weight of 45 g / m ² or less, and a thickness of 5 to 200 μm, and preferably comprises a nonwoven fabric The melt-anisotropic polyester is related to the above-mentioned separator whose main components are parahydroxybenzoic acid and 6-hydroxy-2-naphthoic acid.

  By using a melt-blown nonwoven fabric made of melt-anisotropic polyester fiber, it has excellent heat resistance and burst resistance, and keeps the shape of the separator even at high temperatures, and prevents explosive runaway such as melt fracture. A separator for a non-aqueous electrolyte battery and a capacitor excellent in safety can be obtained.

  The melt anisotropic polyester used in the present invention is a resin excellent in heat resistance and chemical resistance. The melt-anisotropic polyester referred to in the present invention is an aromatic polyester that exhibits optical anisotropy (liquid crystallinity) in the melt phase. For example, a sample is placed on a hot stage and heated in a nitrogen atmosphere, and the light transmitted through the sample is transmitted. It can be recognized by observing. The melt-anisotropic polyester is composed mainly of repeating structural units of aromatic diol, aromatic dicarboxylic acid, and aromatic hydroxycarboxylic acid. For example, those composed of combinations of repeating structural units shown below are preferable.

Among these, as the melt anisotropic polyester used in the present invention, a composition mainly composed of parahydroxybenzoic acid and 6-hydroxy-2-naphthoic acid, or parahydroxybenzoic acid and 6-hydroxy-2- A configuration mainly composed of naphthoic acid, terephthalic acid and biphenol is preferred.
In addition, the above-mentioned melt-anisotropic polyester has a range that does not hinder the function of the present invention, as required, such as colorants, inorganic fillers, antioxidants, ultraviolet absorbers, and other commonly used additives and thermoplastic elastomers. May be added.

  The melt anisotropic polyester nonwoven fabric of the present invention used for a separator is a melt blown nonwoven fabric obtained by a melt blow method. A known method can be adopted as the melt blow method. For example, a melted anisotropic polyester is ejected as a molten polymer from a plurality of nozzle holes arranged in a row, and an injection gas port provided adjacent to an orifice die. High temperature and high speed air is sprayed to make the discharged molten polymer into fine fibers, and then the fiber stream is collected on a conveyor net or the like as a collector to produce a nonwoven fabric.

  The fiber diameter of the melt blown nonwoven fabric made of the melt anisotropic polyester of the present invention used for the separator is required to be 1 to 12 μm. If the fiber diameter is less than 1 μm, the internal resistance of the battery becomes too large, and if it exceeds 12 μm, the risk of internal short circuit increases. Preferably it is 2-10 micrometers, More preferably, it is 3-6 micrometers.

The basis weight of the melt blown nonwoven fabric made of the melt anisotropic polyester of the present invention used for the separator is required to be 45 g / m ² or less. When the basis weight exceeds 45 g / m ² , the internal resistance increases. On the other hand the basis weight is insufficient becomes too the separator strength is sparse, reliability is lowered in the assembly, since the short is not preferable because the easily occur, it is preferable that 2 g / m ² or more. Preferably from 2 to 30 g / ^{m 2.}

  Furthermore, the thickness of the melt blown nonwoven fabric made of the melt anisotropic polyester of the present invention used for the separator needs to be 5 to 200 μm. When the thickness is less than 5 μm, the electrolytic solution holding ability is lowered, and when it exceeds 200 μm, the productivity is inferior. Preferably it is 7-100 micrometers, More preferably, it is 10-70 micrometers.

  The MD direction strength of the melt blown nonwoven fabric made of the melt anisotropic polyester of the present invention is preferably 10 N / 50 mm or more, and more preferably 20 N / 50 mm. If the strength in the MD direction is less than 10 N / 50 mm, the reliability at the time of producing the non-aqueous secondary battery and the capacitor is lowered.

In order to improve the accuracy of thickness and homogeneity, the separator of the present invention can be subjected to calendering and pressing as necessary to adjust the formability.
Further, the separator of the present invention can be used by improving the lyophilicity of the nonwoven fabric surface by graft polymerization, plasma treatment, corona treatment, surfactant coating or the like depending on the purpose.

The separator of the present invention may be a single melt blown nonwoven fabric made of a melt anisotropic polyester having the above physical properties, a plurality of laminates, or a conventional olefin-based sheet or polyvinylidene fluoride. Other materials such as silica gel may be coated.
The separator of the present invention thus obtained can be used as a separator for lithium-based primary and secondary batteries, and as a separator for electric double layer capacitors, etc., and keeps the shape of the separator even at high temperatures and melt fracture It can be used as a battery separator excellent in safety for preventing explosive thermal runaway such as (meltdown).

  EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited by an Example. In the present invention, the physical properties of the melt blown nonwoven fabric made of melt anisotropic polyester mean those measured by the following measuring method.

[Fiber diameter of the fibers constituting the nonwoven fabric μm]
Select 5 locations in the test piece, take 5 photos with an electron microscope, measure the diameter of 20 fibers per photo, and average 100 fiber diameters. It was.

[Non-woven fabric weight g / m ² ]
A test piece of 100 × 100 mm was taken from the sample length direction, the mass in a moisture equilibrium state was measured, and calculated per 1 m ² .

[Nonwoven fabric thickness μm]
A 100 × 100 mm test piece was taken from the sample length direction and measured with a dial thickness gauge.

[Production of lithium-ion batteries]
The positive electrode has a current collector made of a rectangular aluminum foil having a thickness of 100 μm, a solid electrolyte of polyethylene oxide in a mixture of an active material powder of lithium cobaltate, a conductive material such as acetylene black, and a binder on one side; A mixture of an electrolytic solution composed of a propylene carbonate solution of lithium fluorophosphate was applied by a doctor blade and cured by hot pressing to obtain a positive electrode plate. The negative electrode has a mixture made of a mixture of a solid electrolyte of carbon powder and polyethylene oxide and an electrolyte solution of a propylene carbonate solution of lithium 6-fluorophosphate on one surface of a current collector made of a rectangular copper foil having a thickness of 100 μm. It consists of what was bound to copper foil and hardened by hot pressing like the positive electrode. By sticking the separator thus formed on the active material surfaces of the positive electrode and the negative electrode, a pole group of a lithium ion polymer battery is completed. A lithium ion polymer battery is completed by wrapping this pole group with an aluminum laminate.
The dimensions of the completed battery were 54 × 75 × 0.57 mm and the capacity was 40 mAh.

[Performance evaluation of lithium-ion battery]
Using the obtained lithium ion battery, the first discharge capacity and the 20th discharge capacity were measured under the following conditions 1) to 3), and the 20th discharge capacity was divided by the first discharge capacity. (Percentage) was determined as the discharge capacity maintenance rate.
1) Charging / discharging Charging conditions: 0.1 C (4 mA)
Discharge condition: 0.1 C (4 mA)
2) Capacity Planned capacity: 40 mAh
Effective capacity: 40 mAh (first discharge capacity)
3) Charging / discharging test temperature 100 ° C

[Example 1]
A melt anisotropic polyester produced from a copolymer of parahydroxybenzoic acid and 6-hydroxy-2-naphthoic acid and having an average fiber diameter of 5 μm, a basis weight of 6 g / m ² , a thickness of 20 μm, and an MD direction strength of 15 N / 50 mm. A battery was prepared using the melt blown nonwoven fabric as a separator. The evaluation results as a separator are shown in Table 1.

[Example 2]
In the same manner as in Example 1, produced from a copolymer of parahydroxybenzoic acid and 6-hydroxy-2-naphthoic acid, having an average fiber diameter of 8 μm, a basis weight of 9 g / m ² , a thickness of 30 μm, and an MD strength of 20 N / 50 mm. A battery was fabricated using a melt blown nonwoven fabric made of melt anisotropic polyester as a separator. The evaluation results as a separator are shown in Table 1.

[Comparative Example 1]
A battery was produced using a melt blown nonwoven fabric made of polypropylene having an average fiber diameter of 4 μm, a basis weight of 10 g / m ² , a thickness of 40 μm, and a strength in the MD direction of 4 N / 50 mm as a separator. The evaluation results as a separator are shown in Table 1.

  As shown in Table 1, the battery using the melt-blown nonwoven fabric composed of the melt-anisotropic polyester fibers of Examples 1 and 2 as a separator had an excellent discharge capacity maintenance rate of 90% or more. On the other hand, as in Comparative Example 1, a battery using a melt blown nonwoven fabric made of polypropylene as a separator was short-circuited by repeating charging and discharging five times, and was far from practical.

The separator made of the melt blown nonwoven fabric composed of the melt anisotropic polyester fiber of the present invention can be used as a separator for lithium-based primary and secondary batteries, and a separator for electric double layer capacitors, etc. It can be used as a battery separator excellent in safety that keeps the shape and prevents explosive thermal runaway such as melt fracture (meltdown).

Claims (2)

A separator made of a melt-blown nonwoven fabric composed of melt anisotropic polyester fibers, having a fiber diameter of 1 to 12 μm, a basis weight of 45 g / m ² or less, and a thickness of 5 to 200 μm. The separator according to claim 1, wherein the melt-anisotropic polyester constituting the nonwoven fabric is composed mainly of parahydroxybenzoic acid and 6-hydroxy-2-naphthoic acid.