JP2014011041A - Separator for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for a lithium ion secondary battery, eliminating a drawback of a nonwoven fabric separator that is not suitable for use in a lithium ion secondary battery, causing no short circuit while reduced in thickness, and excellent in electrolyte retention characteristics and rate characteristics.SOLUTION: A nonwoven fabric which comprises a thermoplastic material having an average fiber diameter of 5-40 μm and has a basis weight of 2-20 g/mis subjected to thermal compression treatment to form a nonwoven fabric having a JIS standard (JIS Z 8741) glossiness at 60 degrees in the range of 15-30 and a thickness of 10-40 μm. Accordingly, the nonwoven fabric can be suitably used as a separator for a lithium ion secondary battery.

Description

  The present invention relates to a separator for a lithium ion secondary battery that is excellent in electrolytic solution retention and rate characteristics, can reduce the internal resistance of an electrochemical element, and can have a long life.

The most important characteristic required for separators used in various batteries is electrolyte retention. When this electrolyte solution retainability is low, the internal resistance of the electrochemical element becomes high, and as a result, problems such as insufficient capacity, voltage drop, and shortened life of the electrochemical element arise. For example, as a battery separator such as a lithium primary / secondary battery, Japanese Patent Laid-Open No. 3-105851 (Patent Document 1) states that “contains 1% by weight or more of ultrahigh molecular weight polyethylene having a weight average molecular weight of 7 × 10 ⁵ or more. And a weight average molecular weight / number average molecular weight of 10 to 300, a polyethylene composition having a thickness of 0.1 to 25 μm, a porosity of 40 to 95%, an average through-hole diameter of 0.001 to 0.1 μm, A lithium battery separator characterized by comprising a microporous membrane having a 10 mm width breaking strength of 0.5 kg or more is disclosed.

  However, since this type of separator has a very small pore size of submicron or less, when the viscosity of the electrolytic solution is high, the electrolytic solution is difficult to permeate the separator, resulting in poor battery assembly efficiency. In addition, since the pores are linearly formed in the thickness direction of the separator, the electrolyte holding capacity is slightly low, and the electrode expands and contracts with repeated charge and discharge, so the separator is pressed. There was a problem that the electrolyte held in the separator was pushed out and the capacity gradually decreased.

  For this reason, recently, it has been proposed to use a nonwoven fabric excellent in liquid absorbency as a separator. In the case of a nonwoven fabric, since each fiber is laminated | stacked comparatively disorderly in the thickness direction, a pore is not formed linearly but there exists an advantage which is excellent in electrolyte solution retainability. However, if the conventional nonwoven fabric is too thin, the positive and negative electrodes may be short-circuited. Conversely, if the thickness is increased, the positive and negative electrodes can be prevented from being short-circuited. As it was unsuitable.

JP-A-3-105851

  An object of the present invention is to provide a lithium ion secondary battery separator that eliminates the problems of the nonwoven fabric separator that the prior art has, and is thin but not short-circuited, and has excellent electrolyte retention and rate characteristics. is there.

As a result of intensive studies on the above problems, the present inventors have completed the present invention.
That is, in the present invention, a nonwoven fabric having a basis weight of ² to 20 g / m ² composed of fibers made of a thermoplastic material and having an average fiber diameter of 5 to 40 μm is subjected to a heat compression treatment, and a JIS standard at 60 degrees (JIS Z 8741). It is related with the separator for lithium ion secondary batteries which consists of a nonwoven fabric with the thickness which made the glossiness of 15-30 in the range of 10-40 micrometers.

  The separator for a lithium ion secondary battery of the present invention is thin, is safe without being short-circuited in spite of a high porosity, and has excellent electrolyte retention and rate characteristics.

It is a figure which shows the relationship between the glossiness of the JIS specification (JISZ8741) in 60 degree | times, the short circuit rate of a battery, and the initial stage discharge capacity of a battery. It is a figure which shows the relationship between the discharge rate of the battery produced in the Example, and discharge capacity.

Hereinafter, the present invention will be described in detail.
The separator for a lithium ion secondary battery of the present invention is obtained by subjecting a nonwoven fabric having a basis weight of ² to 20 g / m ² made of a thermoplastic material having an average fiber diameter of 5 to 40 μm to heat compression treatment at 60 ° C. It consists of a nonwoven fabric having a thickness of 10 to 40 μm with a glossiness of JIS standard (JIS Z 8741) in the range of 15 to 30.

  The non-woven fabric before being subjected to the heat compression treatment is composed of fibers having an average fiber diameter of 5 to 40 μm made of a thermoplastic material. If the fiber diameter is larger than 40 μm, the nonwoven fabric becomes thick, which is not suitable. In addition, a nonwoven fabric having a large fiber diameter and a large volume has a problem that a large film portion is produced during the heat compression treatment, and the battery characteristics are deteriorated. On the other hand, when the fiber diameter is smaller than 5 μm, the strength of the nonwoven fabric is lowered.

The nonwoven fabric used in the present invention has a basis weight of ² to 20 g / m ² , preferably 4 to 10 g / m ² . If the basis weight is less than ² g / m ² , the non-woven fabric becomes thin, so if used as a separator, there is a problem that the battery is short-circuited, and if it exceeds 20 g / m ² , the non-woven fabric becomes thick and unsuitable as a separator.

  The fiber constituting the nonwoven fabric is not particularly limited as long as it is made of a thermoplastic material having an average fiber diameter of 5 to 40 μm, but polyethylene, polypropylene, polystyrene and other polyolefins, polyethylene terephthalate, polytrimethylene terephthalate, Various thermoplastic materials such as polyesters such as polybutylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, polyamide, polyimide, polyacrylonitrile, and polyvinyl alcohol can be used. These may be copolymers. These may be used alone or in combination of two or more.

  In the present invention, polyolefins such as polyethylene (PE) and polypropylene (PP) can be preferably used as the thermoplastic material, and in particular, composite fibers such as a core-sheath type composite fiber (core: PP, sheath: PE) of polyolefin. Is preferably used. In the core-sheath type composite fiber, the fibers can be easily fused to each other, and the fibers are fused by performing a heat compression treatment such as hot pressing or heat laminating, and the strength of the nonwoven fabric can be improved. It is also preferable to combine various fibers made of other thermoplastic materials as described above with the polyolefin core-sheath composite fiber.

  The composite fiber is not limited to the core-sheath type, and a side-by-side type, a split type, and the like are also preferably used. Moreover, the fiber which consists of each component which comprises the said composite fiber may be mixed. For example, when a mixture of fibers made of PP and fibers made of PE is used as the nonwoven fabric, an effect close to that obtained when a core (PP) -sheath (PE) type composite fiber is used can be obtained.

  The nonwoven fabric may contain other various fibers in addition to the fibers made of the thermoplastic material. The type of fiber is not particularly limited, and for example, cellulose, various fluororesins, and the like can be used. When other various fibers are included, the weight ratio of the fiber made of the thermoplastic material and the other various fibers is preferably 60:40 to 100: 0. When the weight ratio of the thermoplastic material is less than 60%, it is difficult to reduce the thickness during the heat compression treatment, which is not preferable.

  In the present invention, it is particularly preferable that the nonwoven fabric is composed of a core-sheath type composite fiber having a thermoplastic material as a sheath component or other fiber made of the core-sheath type composite fiber and a thermoplastic material.

  Regarding the method of heat compression treatment of the nonwoven fabric, there is no particular limitation on the type of the nonwoven fabric as long as it is a method of softening the fiber by applying heat to the nonwoven fabric and melting the part while applying pressure to reduce the thickness. , Methods such as hot pressing and heat laminating can be used.

  It is important that the glossiness of the JIS standard (JIS Z 8741) at 60 degrees of the nonwoven fabric is within a range of 15 to 30 by heat compression treatment. Preferably it is 17 or more, preferably 25 or less, and more preferably 23 or less. FIG. 1 shows the relationship between the glossiness of the JIS standard at 60 degrees, the short-circuit rate, and the initial discharge capacity. As shown in FIG. 1, the initial discharge capacity is maintained high when the glossiness of the JIS standard at 60 degrees is 15 to 23, but the initial discharge capacity decreases as the glossiness becomes higher than 23. The reason for this is that heat compression treatment under higher temperature and pressure conditions increases the number of fibers that melt, increases the area of the film-like portion that does not transmit ions as a whole nonwoven fabric, and reduces the initial discharge capacity. And since the area of this film-like part and glossiness are in a proportional relationship, a correlation as shown in FIG. 1 is shown.

  On the other hand, from the viewpoint of the short-circuit rate, the short-circuit rate is 0% when the glossiness of JIS standard at 60 degrees is 27 or more, but as the glossiness becomes lower than 27, the short-circuit rate increases and the glossiness is increased. When it is lower than 15, the short-circuit rate increases significantly. From the balance between the short-circuit rate and the initial discharge capacity, it has been found that the JIS standard glossiness at 60 degrees is optimal in the range of 15-30.

  In this invention, in order to adjust the glossiness of the JIS standard (JIS Z8741) in 60 degree | times to the range of 15-30, the heat compression process of a nonwoven fabric is performed. The glossiness is appropriately adjusted depending on the temperature and pressure during the heat compression treatment. Usually, adjustment can be performed by lowering the pressure when raising the temperature and raising the pressure when lowering the temperature. For example, when the melting point of the thermoplastic material is low, a predetermined glossiness can be obtained by raising the pressure by lowering the temperature.

  The temperature of the heat compression treatment is appropriately determined depending on the type of thermoplastic material used for the fiber, but is usually in the range of 100 to 300 ° C, and preferably 100 to 160 ° C when the thermoplastic material is polyolefin. When using a composite fiber, a temperature range in which only the component to be fused melts and the other components do not melt is preferable. Further, the pressure is 0.5 to 3 MPa, preferably 1 to 1.5 MPa. At that time, it is also necessary to prevent the thickness of the nonwoven fabric after the heat compression treatment from being out of the range of 10 to 40 μm.

  The thickness of the nonwoven fabric after the heat compression treatment used as the separator of the present invention varies depending on the fiber diameter and thickness of the nonwoven fabric before the heat compression treatment and the heat compression treatment conditions, but it is necessary to be 40 μm or less. Those thicker than 40 μm cannot be used because they are not suitable as separators. In addition, Preferably it is 30 micrometers or less, More preferably, it is 25 micrometers or less. On the other hand, if the thickness of the nonwoven fabric after the heat compression treatment is too thin, the short-circuit rate is increased. Therefore, the thickness needs to be 10 μm or more, and preferably 15 μm or more.

  The porosity of the nonwoven fabric after the heat compression treatment can be appropriately adjusted depending on the type of fiber used, the heat compression treatment conditions, etc., but is preferably 30 to 80%, more preferably 50 to 80%, and further Preferably it is 60 to 80%. A separator having a higher porosity has better battery characteristics.

The nonwoven fabric having the requirements of ² to 20 g / m ² , thickness of 10 to 40 μm, and glossiness of 5 to 30 according to JIS standard (JIS Z 8741) at 60 degrees, obtained as described above. The strength of the non-woven fabric is improved and the pore diameter between the fibers is small because the fibers are thinly fused together. By reducing the hole diameter between the fibers, it is difficult for the electrodes to short-circuit, and safety can be improved.

Next, a lithium ion secondary battery provided with the separator of the present invention will be described.
A lithium ion secondary battery includes a negative electrode, a positive electrode, a separator, a solvent, and a non-aqueous electrolyte. Any separator that can be normally used in a lithium ion secondary battery can be used except that the separator of the present invention is used as the separator.

  The positive electrode and the negative electrode are generally composed of an active material, a binder polymer that binds the active material, and a current collector, and a conductive additive can be added for the purpose of improving the conductivity of the electrode.

Here, examples of the positive electrode active material include various lithium-containing transition metal oxides. However, the present invention is not particularly limited thereto, and any positive electrode active material used for a so-called 4V class lithium ion secondary battery can be used. May be used, and examples mainly include lithium-containing transition metal oxides. Examples of lithium-containing transition metal oxides include LiCoO ₂ , LiNiO ₂ , LiNiCoO ₂ , and LiMn ₂ O _4. .

  As the negative electrode active material, a material mainly composed of a carbon material that can be doped / undoped with lithium ions can be used. Here, examples of the carbon material include polyacrylonitrile, phenol resin, phenol novolac resin, a sintered organic polymer such as cellulose, artificial graphite, and natural graphite.

  The positive electrode preferably contains a conductive additive, and artificial graphite, carbon black (acetylene black), nickel powder and the like are suitably used. On the other hand, the conductive auxiliary agent is unnecessary in the negative electrode, but may be contained.

  Binder polymers include polyvinylidene fluoride (PVdF), PVdF copolymer resins such as copolymers of vinylidene fluoride and hexafluoropropylene (HFP), perfluoromethyl vinyl ether (PFMV), and tetrafluoroethylene, polytetrafluoroethylene. Fluoropolymers such as fluoroethylene and fluororubber, hydrocarbon polymers such as styrene-butadiene copolymer and styrene-acrylonitrile copolymer, carboxymethylcellulose, polyimide resin, etc. can be used, but are not limited to these is not. These may be used alone or in combination of two or more.

  As for the current collector, a material excellent in oxidation resistance is used for the positive electrode, and a material excellent in reduction resistance is used for the negative electrode. Specifically, examples of the positive electrode current collector include aluminum and stainless steel, and examples of the negative electrode current collector include copper, nickel, and stainless steel. Moreover, about a shape, a foil shape and a mesh shape can be used. In particular, it is preferable to use an aluminum foil as the positive electrode current collector and a copper foil as the negative electrode current collector.

  The blending ratio of the active material, binder polymer, and conductive additive is preferably in the range of 3 to 30 parts by weight of the binder polymer with respect to 100 parts by weight of the active material. You can do it.

  As the non-aqueous electrolyte used in the lithium ion secondary battery, an electrolytic solution in which a lithium salt is dissolved in a solvent is used. The solvent to be used is not particularly limited as long as it is a polar organic solvent having 10 or less carbon atoms generally used in lithium ion secondary batteries. For example, propylene carbonate, ethylene carbonate, butylene carbonate, diethyl carbonate, Mention may be made of methyl ethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, sulfolane, acetonitrile and the like or mixtures thereof.

  Examples of the lithium salt dissolved in the solvent include lithium perchlorate, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium trifluorosulfonate, lithium perfluoromethylsulfonylimide, lithium Examples include fluoroethylsulfonylimide. These may be mixed. The concentration of the dissolved lithium salt is preferably in the range of 0.2 to 2 M / L.

  There is no limitation in particular as a manufacturing method of the lithium ion battery provided with the separator of this invention, You may employ | adopt any manufacturing method of a well-known lithium ion secondary battery. Specifically, a method is generally used in which a joined body in which a positive electrode and a negative electrode are joined via a separator of the present invention is put in an exterior, and a nonaqueous electrolyte is injected and then sealed. Here, a vacuum injection method is preferably used for the injection of the non-aqueous electrolyte, but is not particularly limited thereto. In addition, the joined body may be impregnated with a non-aqueous electrolyte before being put in the exterior.

  In a so-called film-clad battery in which the exterior is a pack made of an aluminum laminate film, it is preferable that the electrode and the separator are bonded and integrated. Adhesion between the separator and the electrode is mainly performed by a thermocompression bonding method, and this may be performed in a dry state not containing a non-aqueous electrolyte or in a wet state containing a non-aqueous electrolyte. In addition, when the adhesiveness between the separator and the electrode is good, it is possible to manufacture without undergoing the thermocompression bonding step.

  The shape of the lithium ion secondary battery thus obtained is not particularly limited, and may be any shape such as a cylindrical shape, a flat shape such as a square shape, and a button shape. Examples of the exterior include a pack made of a steel can, an aluminum can, and an aluminum laminate film, but are not particularly limited thereto.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples. Moreover, each value in an Example was calculated | required with the following method.

(1) Average fiber diameter:
The surface of the fiber assembly was photographed (magnification: 7000 times) with a scanning electron microscope (SU-1500, manufactured by Hitachi, Ltd.). Select 10 photos randomly, measure the diameter of all the fibers in the photo, and calculate the average value of all the fiber diameters contained in the 10 photos as the average fiber diameter of the fibers constituting the nonwoven fabric. did.
(2) Glossiness:
The glossiness at 60 degrees of the nonwoven fabric was measured with a glossiness meter (PG-IIM manufactured by Nippon Denshoku Industries Co., Ltd.). For the TD direction and the MD direction of the nonwoven fabric, measurement was performed randomly at 5 locations each, and the average value of a total of 10 measurements was taken as the glossiness of the nonwoven fabric.

<Preparation of positive electrode>
25.5 g of lithium cobaltate (LiCoO ₂ ) powder, 1.5 g of acetylene black, 3 g of PVdF, and 27 g of N-methyl-pyrrolidone (NMP) were mixed uniformly to produce a positive electrode paste. The obtained paste was applied onto an aluminum foil having a thickness of 20 μm and dried. This was punched out to a diameter of 15 mm and then pressed to produce a positive electrode.

<Production of negative electrode>
15 g of artificial graphite, 0.16 g of acetylene black, 0.8 g of PVdF, and 11.2 g of N-methyl-pyrrolidone (NMP) were applied on a copper foil having a thickness of 20 μm and dried. This was punched out to a diameter of 15 mm and then pressed to produce a negative electrode.

<Preparation of non-aqueous electrolyte solution>
To a solvent obtained by mixing ethylene carbonate (EC) and dimethyl carbonate (DMC) at a volume ratio of 3: 7, lithium hexafluorophosphate (LiPF ₆ ) is added so that the electrolyte concentration becomes 1.2M. In addition, it was dissolved to prepare a non-aqueous electrolyte solution.

[Example 1]
(Preparation of sample A)
A core-sheath fiber (core: PP, sheath: PE) having an average fiber diameter of 15 μm and a cellulose fiber having an average fiber diameter of 5 μm are made by a wet papermaking method at a weight ratio of 85:15, a film thickness of 34 μm, and a basis weight. A nonwoven fabric of 8 g / m ² was obtained. This nonwoven fabric was heat-compressed at 135 ° C. and 2 MPa by a heat laminating method to obtain Sample A. The film thickness of Sample A was 23 μm, the basis weight was 8 g / m ² , and the glossiness of the JIS standard (JIS Z 8741) at 60 degrees was 15.
Although sample A had a short-circuit rate of 50%, the initial discharge capacity was as extremely high as 140 mAh / g, and it can be sufficiently used as a separator (FIG. 1).

[Example 2]
(Preparation of sample B)
A core-sheath fiber (core: PP, sheath: PE) having an average fiber diameter of 15 μm and a cellulose fiber having an average fiber diameter of 5 μm are made by a wet papermaking method at a weight ratio of 85:15, a film thickness of 34 μm, and a basis weight. A nonwoven fabric of 8 g / m ² was obtained. This nonwoven fabric was heat-compressed at 135 ° C. and 2.3 MPa by a heat laminating method to obtain Sample B. The film thickness of Sample B was 20 μm, the basis weight was 8 g / m ² , and the glossiness of the JIS standard (JIS Z 8741) at 60 degrees was 22.
Sample B has a short-circuit rate of about 18% and an initial discharge capacity as extremely high as 140 mAh / g, and can be suitably used as a separator (FIG. 1).

[Example 3]
(Preparation of sample C)
A core-sheath fiber (core: PP, sheath: PE) having an average fiber diameter of 15 μm and a cellulose fiber having an average fiber diameter of 5 μm are made by a wet papermaking method at a weight ratio of 85:15, a film thickness of 34 μm, and a basis weight. A nonwoven fabric of 8 g / m ² was obtained. This nonwoven fabric was heat-compressed at 135 ° C. and 2.5 MPa by a heat laminating method to obtain Sample C. Sample C had a film thickness of 17 μm, a basis weight of 8 g / m ² , and a glossiness of 27 according to JIS standard (JIS Z 8741) at 60 degrees.
Sample C has a short rate of 0% and an initial discharge capacity of 120 mAh / g, which is sufficiently high, and can be suitably used as a separator (FIG. 1).

[Comparative Example 1]
(Preparation of sample D)
A core-sheath fiber (core: PP, sheath: PE) having an average fiber diameter of 10 μm and a cellulose fiber having an average fiber diameter of 5 μm are made by a wet papermaking method at a weight ratio of 90:10, a film thickness of 20 μm, and a basis weight. A 4 g / m ² nonwoven fabric was obtained. This nonwoven fabric was subjected to heat compression treatment at 135 ° C. and 1 MPa by a heat laminating method to obtain Sample D. The film thickness of Sample D was 18 μm, the basis weight was 4 g / m ² , and the glossiness of the JIS standard (JIS Z 8741) at 60 degrees was 4.
Sample D had a short rate of 100% (FIG. 1).

[Comparative Example 2]
(Preparation of sample E)
A core-sheath fiber (core: PP, sheath: PE) having an average fiber diameter of 15 μm and a cellulose fiber having an average fiber diameter of 5 μm are made by wet papermaking at a weight ratio of 85:15, and the film thickness is 30 μm and the basis weight. A nonwoven fabric of 6 g / m ² was obtained. This nonwoven fabric was heat-compressed at 135 ° C. and 1.5 MPa by a heat laminating method to obtain Sample E. Sample E had a film thickness of 22 μm, a basis weight of 6 g / m ² , and a glossiness of 11 according to JIS standard (JIS Z 8741) at 60 degrees.
Sample E had a short rate of 100% (FIG. 1).

[Comparative Example 3]
(Preparation of sample F)
A core-sheath fiber (core: PP, sheath: PE) having an average fiber diameter of 15 μm and cellulose fiber having an average fiber diameter of 5 μm are made by a wet papermaking method with a weight ratio of 85:15, a film thickness of 34 μm, and a basis weight of 8 g. A nonwoven fabric of / m ² was obtained. This nonwoven fabric was heat-compressed at 135 ° C. and 3 MPa by a heat laminating method to obtain Sample F. The film thickness was 12 μm, the basis weight was 7 g / m ² , and the glossiness of the JIS standard (JIS Z 8741) at 60 degrees was 33.
Sample F has an initial discharge capacity of about 62 mAh / g although the short-circuit rate is 0%, which is insufficient as a separator (FIG. 1).

<Production of battery, battery test>
Samples A to F were used as separators as non-woven fabrics after heat compression treatment with different glossiness according to JIS standard (JIS Z 8741) at 60 degrees, and 2032 type coin type batteries were manufactured (N = 5 for each). The battery was evaluated by a constant current (0.1 C) / constant voltage charge with a cut-off voltage of 4.15V. FIG. 1 shows the relationship between the glossiness and short-circuit rate of JIS standard (JIS Z 8741) at 60 degrees and the glossiness and discharge capacity (0.1 C) of JIS standard (JIS Z 8741) at 60 degrees. FIG. 2 shows the relationship between the discharge rate and the discharge capacity for the batteries using Sample A (Example 1) and Sample F (Comparative Example 3) as separators.

Claims (5)

A nonwoven fabric having a basis weight of ² to 20 g / m ² composed of fibers of an average fiber diameter of 5 to 40 μm made of a thermoplastic material is subjected to heat compression treatment, and the glossiness of JIS standard (JIS Z 8741) at 60 degrees is 15 A separator for a lithium ion secondary battery comprising a nonwoven fabric having a thickness in the range of ˜30 and having a thickness of 10 to 40 μm.   The lithium ion secondary battery according to claim 1, wherein the thermoplastic material is at least one thermoplastic material selected from the group consisting of polyolefin, polyester, polyamide, polyimide, polyacrylonitrile, and polyvinyl alcohol. Separator for use.   The nonwoven fabric is comprised from the core-sheath-type composite fiber which uses a thermoplastic material as a sheath component, or the other fiber which consists of this core-sheath-type composite fiber and a thermoplastic material, The Claim 1 or 2 characterized by the above-mentioned. Separator for lithium ion secondary battery.   The separator for a lithium ion secondary battery according to claim 3, wherein the core component of the core-sheath composite fiber is polypropylene and the sheath component is polyethylene.   The separator for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the heat compression treatment is performed at 100 to 300 ° C under a condition of 0.5 to 3 MPa.

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