JP2019087455A - Lithium ion battery separator and lithium ion battery - Google Patents

Abstract

To provide: a lithium ion battery separator which enables the reduction in the thickness of a lithium ion battery separator, which is superior in tensile strength and cuttability, and which is excellent in the internal resistance, the percentage of internal short circuit-originating defectives and cycle characteristics; and a lithium ion battery including the separator.SOLUTION: A lithium ion battery separator comprises: a heat resistant fiber having a modified freeness of 300 ml or less, measured according to JIS P8121 with the exception that a 80-mesh metal gauze of 0.14 mm in wire diameter and 0.18 mm in sieve opening is used as a sieve plate and the sample concentration is made 0.1%; and a synthetic resin short fiber. In the lithium ion battery separator, the heat resistant fiber is a fibrid including a para-oriented aromatic polyamide, of which the content is 1.0 mass% or more and 4.0 mass% or less.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a lithium ion battery separator (hereinafter sometimes abbreviated as "separator") and a lithium ion battery.

  With the spread of portable electronic devices in recent years and the enhancement of their performance, a secondary battery having a high energy density is desired. As this type of battery, a lithium ion battery using an organic electrolytic solution has attracted attention. This lithium ion battery has a high energy density because it can obtain about 3.7 V, which is about three times that of the conventional secondary battery alkaline secondary battery, as an average voltage, but like an alkaline secondary battery Because a water-based electrolyte can not be used, an organic electrolyte having sufficient resistance to oxidation and reduction is used. Since the organic electrolyte is flammable, there is a danger of ignition, etc., and safety is carefully taken in its use. Although there are multiple causes of exposure to danger such as ignition, overcharging is particularly dangerous.

  In order to prevent overcharging, constant-voltage and constant-current charging is performed in the current lithium ion battery, and the battery is equipped with a precise IC (protection circuit). The cost of the protection circuit is high, which is a factor that increases the cost of the lithium ion battery.

  In addition, when overcharging is prevented by the protection circuit, it is naturally assumed that the protection circuit does not work well, so it can not be said that it is intrinsically safe. The current lithium ion battery is equipped with means such as a safety valve, a PTC element, and a separator having a thermal fuse function in order to break the protection circuit during overcharge and safely destroy the battery when overcharged. There is. However, even when equipped with the above means, depending on the condition of overcharging, safety during overcharging can not be assuredly ensured, and in fact the ignition accident of lithium ion batteries is currently But it's happening.

  As a lithium ion battery separator, the porous film which consists of polyolefins, such as polyethylene or a polypropylene, is used abundantly. For a porous film made of polyolefin, when the temperature inside the battery reaches around 130 ° C, it melts and blocks the micropores, preventing the movement of lithium ions and blocking the current (shutdown function) There is. However, it has been suggested that, under some circumstances, if the temperature is further increased, the polyolefin itself may melt and short out, causing thermal runaway. Therefore, heat resistant separators that do not melt and shrink even at temperatures near 200 ° C. are currently being developed.

  As a heat resistant separator, the separator (for example, refer patent document 1) which consists of a polyester nonwoven fabric is disclosed. However, the separator made of the non-woven fabric has a large pore size of the non-woven fabric, a large leakage current, a poor electrolyte retention performance, and a high internal resistance of the separator.

  A lithium ion comprising a porous sheet comprising a non-woven fabric as a separator having high safety during overcharge, or a porous film comprising a porous organic polymer which contains the porous sheet and which swells in an electrolyte and holds the same A separator for a secondary battery is disclosed (see, for example, Patent Document 2). However, since the porous sheet made of the non-woven fabric of Patent Document 2 has a large void and a large number of through holes, internal short circuiting and self-discharge are likely to occur. It functions as a separator by providing a porous film including a porous film containing molecules, in the example, polyvinylidene fluoride. Therefore, in this case, the film thickness becomes as thick as 24 μm or more, and it is difficult to cope with the high capacity, and it is necessary to coagulate, wash and dry the porous polymer in order to provide the porous film. The

  Further, as a separator capable of preventing a short circuit at high temperature of the separator, a lithium ion secondary battery comprising a non-woven fabric containing heat resistant pulp fibers having a melting point or carbonization temperature of 300 ° C. or more and thermoplastic fibers having a melting point of 200 ° C. or more Separators (see, for example, Patent Document 3), heat resistance, electrolyte solution retention, internal short circuit prevention, winding property, low internal resistance of the separator, and at least a part of fibers as a separator that can have long life A separator for an electrochemical element comprising one or more types of fibrillated organic fibers having a diameter of 1 μm or less and one or more types of non-fibrillated organic fibers having a fineness of 0.5 dtex or less (for example, patent documents 4), reflow heat resistance, low internal resistance, high speed charge / discharge characteristics, melting point or thermal decomposition temperature of 250 ° C or higher For an electrochemical element comprising a non-woven fabric containing a fibrillated polymer at least a part of which has a fiber diameter of 1 μm or less and a weight-average fiber length of 0.2 mm to 2 mm, and an organic fiber having a fineness of 3.3 dtex or less A separator (see, for example, Patent Document 5) is disclosed.

Among these patent documents, in the separator of patent document 3, it is said that 30-80 mass% is preferable as a content rate of a heat resistant pulp fiber, 20 g / m < ² > of basic weight of the separator of an Example, thickness In the separator of Patent Document 4, the content of the fibrillated organic fiber or liquid crystalline polymer fiber is preferably 10% by mass or more and 70% by mass or less. The basis weight of the separator is as thick as 16 g / m ² or more and the thickness is 30 μm or more, and in the separator of Patent Document 5, the content of the fibrillated polymer is preferably 10% by mass or more. The basis weight of the separator was 18 g / m ² or more and the thickness was 55 μm or more, and it was difficult for any of the separators to cope with recent increase in capacity.

  In the separators disclosed in Patent Documents 3 to 5, the minimum value in the preferable content of heat resistant fibers such as heat resistant pulp fibers, fibrillated organic fibers, fibrillated polymers is 10% by mass. is there. However, since heat-resistant fibers are rigid and often have high strength, when the content is 10% by mass or more, it is difficult to crush the separator and to make the thickness of the separator thin. There was a problem of difficulty. When the separator is crushed by applying heat or load, there is a problem that the resistance of the separator becomes high, a problem that it is difficult to cut in the slit process when cut into a predetermined width and used as a separator, fused cutting in electrode lamination process There was a problem that was difficult to do. Furthermore, since the bonding strength between the heat-resistant fibers and between the heat-resistant fibers and the other fibers is low, the tensile strength of the separator is 10% by mass or more of the heat-resistant fibers. It was necessary to have a special consideration in the operation of the electrode lamination process.

JP 2003-123728 A JP 2007-317675 A Patent 2006-19191 gazette International Publication No. WO 01/93350 Brochure Japanese Patent Application Publication No. 2004-146137

  The object of the present invention is to provide a lithium ion battery separator which can reduce the thickness of the lithium ion battery separator, is excellent in tensile strength and cutability, has a low internal resistance, has a low internal short circuit defect rate, and has high cycle characteristics; It is an object of the present invention to provide a lithium ion battery comprising a separator.

  As a result of earnestly researching in order to solve the said subject, the following invention was discovered.

(1) A lithium ion battery separator comprising a heat resistant fiber having a modified freeness of 300 ml or less and a synthetic resin short fiber, wherein the heat resistant fiber is a fibrid composed of para aromatic polyamide, and the content thereof Is 1.0 mass% or more and 4.0 mass% or less.

  Modified freeness: Measured according to JIS P8121-2: 2012 except using 80 mesh wire mesh with 0.14 mm wire diameter and 0.18 mm mesh as a sieve plate and setting the sample concentration to 0.1%. value.

(2) The lithium ion battery separator according to (1), wherein the content of the synthetic resin short fibers is 90.0% by mass to 99.0% by mass with respect to all the fiber components.

(3) The lithium ion battery separator according to (1), wherein the content of the synthetic resin short fibers is 96.0% by mass to 99.0% by mass with respect to the total fiber components.

(4) A lithium ion battery comprising the lithium ion battery separator according to any one of the above (1) to (3).

  The lithium ion battery separator of the present invention can be reduced in thickness and is excellent in tensile strength and cutability. The lithium ion battery separator of the present invention can achieve the effects of low internal resistance, low internal short circuit failure rate, and high cycle characteristics.

It is an electron micrograph of the lithium ion secondary battery separator containing the fibrid which consists of para-type aromatic polyamide.

  The lithium ion battery separator of the present invention is a lithium ion battery separator comprising a heat resistant fiber having a modified freeness of 300 ml or less and a synthetic resin short fiber, and the heat resistant fiber comprises a para-based aromatic polyamide. It is a fibrid, and the content is 1.0% by mass or more and 4.0% by mass or less.

  The lithium ion battery in the present invention means a secondary battery in which lithium ions move between positive and negative electrodes in charge and discharge. For example, a lithium ion secondary battery and a lithium ion polymer secondary battery can be mentioned. The lithium ion battery includes a lithium ion secondary battery using a lithium storage material as a negative electrode active material, and a metal lithium secondary battery using metallic lithium as a negative electrode active material.

  A lithium ion battery contains a positive electrode, a separator and a negative electrode as members. And generally, it has the structure which laminated | stacked the positive electrode, the separator, and the negative electrode in this order. The electrolytic solution is absorbed in each of the positive electrode, the negative electrode and the separator. As a type of laminated structure, a cylindrical shape in which each member is laminated and then wound in a roll shape, and a wound flat shape in which two flat surfaces and curved end portions are formed by crushing the cylindrical shape, ninety nine Among the nine-folded separators, the ninety-nine-fold type in which electrodes cut out in a single leaf are inserted, the single-leaf stack type in which the separators cut out in a single leaf, and the electrodes cut out in a single leaf are laminated are exemplified. .

A lithium storage material is used for the negative electrode active material of the lithium ion battery. Examples of the lithium storage substance include carbon-based materials, silicon-based materials, composite oxides of transition metals and lithium, and the like. The carbon-based material is preferably used in that it has a good balance between the lithium storage capacity per mass and the difficulty of deterioration due to absorption and release of lithium. Examples of the carbon-based material include graphite such as natural graphite and artificial graphite; amorphous carbon such as hard carbon, soft carbon and mesoporous carbon; and nanocarbon materials such as carbon nanotubes and graphene. The silicon-based material is preferably used in view of the large lithium storage capacity per mass. Examples of silicon-based materials include silicon, silicon monoxide (SiO), and silicon dioxide (SiO ₂ ). Lithium titanate, which is a kind of composite oxide of transition metal and lithium, is preferably used in that it is difficult to cause deterioration due to absorption and release of lithium.

It is particularly preferable to use lithium titanate as the negative electrode active material used together with the lithium ion battery separator in the present invention. As an example of lithium titanate, spinel type lithium titanate (Li _{4 + X} Ti ₅ O ₁₂ ( _X changes between 0 and 3 depending on charge and discharge state)), and ramsdellite type lithium titanate (Li _{2 + X} Ti ₃ O ₇ ( _X changes between 0 and 2 depending on charge and discharge conditions). When lithium titanate is used as the negative electrode active material, a single type of lithium titanate may be used, or a mixture of two or more lithium titanates may be used.

  As a negative electrode of a lithium ion battery, the electrode which coated the negative electrode material containing the said negative electrode active material on metal foil is illustrated. In the negative electrode material, binders such as polyvinylidene fluoride and styrene-butadiene copolymer, conductive agents such as carbon black and nano carbon materials, dispersants, thickeners and the like can be mixed, if necessary. As a metal used for metal foil, copper, aluminum etc. are illustrated.

  Examples of positive electrode active materials of lithium ion batteries include composite oxides of transition metals and lithium, composite salts having an olivine structure of transition metals and lithium, sulfur, and the like. Examples of composite oxides of transition metals and lithium include composite oxides of one or more transition metals and lithium selected from cobalt, nickel and manganese. These composite oxides can further be complexed with typical metals such as aluminum and magnesium; transition metals such as titanium and chromium. Examples of the complex salt having an olivine structure of a transition metal and lithium include complex salts having an olivine structure of one or more transition metals selected from iron and manganese and lithium.

  As a positive electrode of a lithium ion battery, the electrode which coated the positive electrode material containing said positive electrode active material on metal foil is illustrated. In the positive electrode material, binders such as polyvinylidene fluoride and acrylic acid ester copolymer; conductive agents such as carbon black and nano carbon materials; dispersants; thickeners and the like can be mixed, if necessary. Aluminum etc. are illustrated as a metal used for metal foil.

As an electrolyte solution of a lithium ion battery, the solution which melt | dissolved lithium salt in the polar solvent, and the solution which melt | dissolved lithium salt in the ionic liquid are illustrated. As polar solvents used in the electrolyte of lithium secondary batteries, carbonates such as propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC); ethyl acetate, propyl acetate, Fatty acid esters such as ethyl propionate are exemplified. Examples of the lithium salt used in the electrolyte of a lithium secondary battery, lithium hexafluorophosphate (LiPF _6), lithium tetrafluoroborate (LiBF ₄₎ are exemplified. As the solid electrolyte, one in which a lithium salt is dissolved in a gel-like polymer such as polyethylene glycol and derivatives thereof, polymethacrylic acid derivatives, polysiloxane and derivatives thereof and polyvinylidene fluoride is used.

  In the present invention, the heat-resistant fiber having a modified freeness of 300 ml or less is a fibrid composed of a para-based aromatic polyamide polymer (hereinafter sometimes abbreviated as “fibrid”). A fibrid is a thin leaf-like or scale-like piece having minute fibrils, and is a fine heat-resistant material in which water molecules or water exist in the crystal structure in a non-crystalline state without the crystal structure of the fiber being firmly formed. Refers to sexual fibers. FIG. 1 is an electron micrograph of a lithium ion secondary battery separator containing a fibrid 1 composed of a para-based aromatic polyamide, and the fibrid 1 is in the form of a thin leaf.

  As the above-mentioned fibrid, a formed product obtained by introducing a fiber-forming high molecular weight polymer solution into an aqueous coagulation bath is recovered without drying, and is obtained by fibrillation such as beating as necessary. Be For example, non-crystals having a molecular orientation formed from a fibrid produced by mixing a polymer solution with the precipitant in a system in which shear force exists, or a polymer solution exhibiting optical anisotropy It is a substance with a water content and can be subjected to beating treatment as required.

  As the refining process, fibrids are refined by a refiner, a beater, a mill, an attritor, a rotary homogenizer which applies a shearing force by a high-speed rotary blade, and a shearing force between a high-speed rotating cylindrical inner blade and a fixed outer blade. Double cylinder type high speed homogenizer to generate, ultrasonic crusher to make it finer by ultrasonic impact, pressure difference is given to the fiber suspension to make it pass through small diameter orifice to make high speed, and it is made to collide and it is sudden It can obtain by processing using a high pressure homogenizer etc. which apply shear force and cutting force to a fiber by decelerating.

  In the present invention, the fibrids are greatly shrunk when water present in the crystal structure is removed by heating or depressurization or the like, and the fiber network is strengthened, so that the strength characteristics of the separator can be improved.

  The modified freeness of the fibrid in the present invention is 0 to 300 ml, preferably 0 to 200 ml, more preferably 0 to 100 ml. When the modified freeness exceeds 300 ml, the fiber width of the fibrid is so large that it may be difficult to reduce the thickness of the separator, or the internal resistance may be increased. In addition, the formation (uneven density unevenness) of the separator may be deteriorated, the mechanical strength may be lowered, and the separator may be damaged when the battery is formed. In addition, the cutability of the separator may also deteriorate.

  In the present invention, the modified freeness is JIS P8121-2: 2012 except that the sample concentration is 0.1% using an 80-mesh wire mesh with a wire diameter of 0.14 mm and an opening of 0.18 mm as the sieve plate. Measured according to the

  The mass weighted average fiber length of the fibrids is preferably 0.30 mm or more and 1.00 mm or less. Moreover, it is preferable that the length weighted average fiber length of a fibrid is 0.10 mm or more and 0.50 mm or less. If the average fiber length is shorter than the preferred range, the fibrid may fall off the separator. When the average fiber length is longer than the preferable range, the formation of the separator may be deteriorated and the internal resistance of the separator may be increased.

  When the fibrids have the above-mentioned mass weighted average fiber length and length weighted average fiber length, even if the content of the fibrids contained in the separator is small, between the fibrids or between the fibrids and the synthetic resin short fibers, A dense network structure is formed, and a separator having a high tensile strength and a small thickness can be easily obtained without impairing the cutability.

  In the present invention, the mass-weighted average fiber length and the length-weighted average fiber length of fibrids are mass-weighted average fiber lengths measured in the projected fiber length (Proj) mode using KajaaniFiberLab V 3.5 (manufactured by Metso Automation). L (w) and length weighted average fiber length (L (l)).

  The average fiber width of the fibrids is preferably 3.0 μm to 40.0 μm, more preferably 5.0 μm to 35.0 μm, and still more preferably 10.0 μm to 30.0 μm. When the average fiber width exceeds 40.0 μm, the internal resistance of the separator is likely to be high, making it difficult to reduce the thickness or the cutting performance to be deteriorated. On the other hand, when the average fiber width is less than 3.0 μm, the processing time for refining the fibrid may be prolonged, and the productivity may be significantly reduced.

  In the present invention, the average fiber width of fibrids is a fiber width measured using KajaaniFiberLab V 3.5 (manufactured by Metso Automation).

  In the present invention, as the synthetic resin staple fiber, polyolefin, polyester, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyamide, acrylic, polyvinyl chloride, polyvinylidene chloride, polyvinyl ether, polyvinyl ketone, polyether, polyvinyl alcohol And nonfibrillated staple fibers (staples) made of synthetic resins such as dienes, polyurethanes, phenols, melamines, furans, ureas, anilines, unsaturated polyesters, fluorines, silicones, and derivatives thereof. By including the synthetic resin short fibers, the tensile strength and the piercing strength of the separator can be increased.

  The synthetic resin staple fiber may be a fiber (single fiber) made of a single resin, or a composite fiber made of two or more resins. Further, the synthetic resin staple fibers contained in the separator of the present invention may be used alone or in combination of two or more. As the composite fiber, core-sheath type, eccentric type, side-by-side type, sea-island type, orange type, multiple bimetal type can be mentioned.

  The denier of the synthetic resin staple fiber is preferably 0.01 dtex or more and 0.6 dtex or less, and more preferably 0.02 dtex or more and 0.3 dtex or less. When the fineness of the synthetic resin short fiber exceeds 0.6 dtex, the number of fibers in the thickness direction decreases, so the pore diameter distribution of the separator becomes wide, and as a result, the leakage current may increase. In addition, it may be difficult to reduce the thickness, and the strength characteristics may be easily deteriorated. When the denier of the synthetic resin staple fiber is less than 0.01 dtex, the fiber becomes very expensive, making stable production of the fiber difficult, and when producing a separator by a wet papermaking method, the dewatering property may be lowered. is there.

  As a fiber length of a synthetic short fiber, 1 mm or more and 10 mm or less are preferable, and 1 mm or more and 5 mm or less are more preferable. If the fiber length exceeds 10 mm, it may cause formation failure. On the other hand, when the fiber length is less than 1 mm, the mechanical strength of the separator is lowered, and the separator may be broken when forming the battery.

  The content of fibrids is 1.0% by mass or more and 4.0% by mass or less based on the total fiber components contained in the separator of the present invention. 2.0 mass% or more is more preferable, and 3.0 mass% or more is still more preferable. When the content of the fibrid exceeds 4.0% by mass, the internal resistance of the separator tends to be high, and particularly when the thickness is thin, the internal resistance increases more and the discharge characteristics are deteriorated. On the other hand, when the content of the fibrid is less than 1.0% by mass, although the tensile strength of the separator becomes strong, the improvement effect of the leak current of the separator and the internal short circuit defect rate becomes low.

  The separator of the present invention may be blended with heat resistant fibers having a modified freeness of 300 ml or less and fibers other than synthetic resin short fibers. For example, cellulose fiber, pulp or fibrillated cellulose fiber, fibrid made of synthetic resin, pulp made of synthetic resin, inorganic fiber can be mentioned. Examples of inorganic fibers include glass, alumina, silica, ceramics and rock wool. The cellulose fiber may be either natural cellulose or regenerated cellulose.

  90.0 mass% or more is preferable, as for the content rate of a synthetic resin short fiber with respect to the total fiber component contained in the separator of this invention, 92.0 mass% or more is more preferable, 94.0 mass% or more is still more preferable. More than 95.0% by mass is particularly preferable. Moreover, 99.0 mass% or less is preferable, 98.0 mass% or less is more preferable, and 97.0 mass% or less is still more preferable. If the synthetic resin short fiber is more than 99.0% by mass, although the tensile strength of the separator is increased, the improvement effect of the leakage current and the internal short circuit failure rate may be lowered. On the other hand, when the synthetic resin short fiber is less than 90.0 mass%, when the basis weight of the separator is lowered, the mechanical strength is lowered, and the separator may be broken when forming the battery.

  In the present invention, the most preferable separators are those in which the fiber component is a heat-resistant fiber having a modified freeness of 300 ml or less and a synthetic resin staple fiber. In that case, the content of synthetic resin short fibers is 96.0% by mass or more and 99.0% by mass or less based on the total fiber components contained in the separator of the present invention. And 98.0 mass% or less is more preferable, and 97.0 mass% or less is still more preferable. Further, it is particularly preferable that the content of the heat-resistant fiber having a modified freeness of 300 ml or less is 3.5% by mass or less and the content of the synthetic resin short fiber is 96.5% by mass or more.

  10 micrometers or more are preferable, as for the thickness of the separator of this invention, 11 micrometers or more are more preferable, and 12 micrometers or more are still more preferable. Moreover, 20 micrometers or less are preferable, 18 micrometers or less are more preferable, and 15 micrometers or less are still more preferable. Even when the thickness of the separator is in the above range, the separator of the present invention can suppress the internal resistance to a low level and maintain the tensile strength required in the step of laminating the electrodes. There is no loss of workability in the process. If the thickness of the separator exceeds 20 μm, the internal resistance may be too high. In addition, the battery may not be able to have a high capacity. If the thickness of the separator is less than 9 μm, the strength of the separator becomes too weak, and there is a risk that the separator may be damaged during handling or when the battery is formed.

Density of the lithium ion battery separator of the present invention, 0.40 g / cm ³ or more preferably, 0.45 g / cm ³ or more, and also is preferably not more than ^{0.75g / cm 3, 0.70g / cm} 3 The following are more preferable. If the density is less than 0.40 g / cm ³ , the strength of the separator becomes too weak and there is a risk of breakage when handling the separator or forming a battery, and if it exceeds 0.75 g / cm ³ , The internal resistance of the separator may be too high.

  The lithium ion battery separator of the present invention is preferably a wet non-woven fabric produced by a wet sheet-forming method. In the wet paper-making method, fibers are dispersed in water to make a uniform paper-making slurry, and the paper-making slurry is scooped up by a paper machine to make a wet nonwoven fabric. Examples of the paper machine include a cylinder paper machine, a fourdrinier paper machine, an inclined type paper machine, an inclined short mesh paper machine, and a composite machine of these. In the process of producing a wet non-woven fabric, a hydroentanglement process may be performed as necessary. As processing of the wet non-woven fabric, heat treatment, calendering, heat calendering or the like may be performed.

  EXAMPLES The present invention will be described by way of examples, which should not be construed as limiting the present invention. In the examples, percentages (%) and parts are all based on mass unless otherwise noted.

Example 1
<Preparation of Separator>
Unstretched PET-based synthetic resin short for single component type binder with 56.5 parts by mass of oriented crystallized polyethylene terephthalate (PET) synthetic resin short fiber with a denier of 0.06 dtex and a fiber length of 3 mm and a denier of 0.2 dtex with a fiber length of 3 mm Fibrid consisting of 40.0 parts by mass of fiber (softening point 120 ° C., melting point 230 ° C.) and para-aromatic polyamide as heat resistant fiber previously defibrated and beaten using a high speed homogenizer and having a modified freeness of 88 ml Is dispersed in water with a pulper to prepare a uniform papermaking slurry with a concentration of 0.1% by mass, and a wet paper web is obtained using an inclined papermaking machine, and has a surface temperature of 135 ° C. It was dried by a cylinder drier to obtain a sheet. The resulting sheet was produced using a steel roll with one of the rolls plated with chromium, a resin roll with hardness Shore D 92, and a thermal calender with a surface temperature of 195 ° C and a linear pressure of 100 kN / m for the steel roll. And calendering to prepare a separator having a basis weight of 10 g / m ² and a thickness of 15 μm.

Example 2
59.0 parts by mass of an orientation crystallized PET synthetic resin short fiber having a fineness of 0.06 dtex and a fiber length of 3 mm, and an undrawn PET synthetic resin short fiber for a single component type binder having a fineness of 0.2 dtex and a fiber length of 3 mm A separator having a basis weight of 10 g / m ² and a thickness of 15 μm was produced in the same manner as in Example 1 except that 0 parts by mass and 1.0 part by mass of a fibrid of para-based aromatic polyamide were used.

Example 3
56.0 parts by mass of an orientation-crystallized synthetic resin short fiber having a fineness of 0.06 dtex and a fiber length of 3 mm, and a non-stretched synthetic synthetic resin short fiber for a single component type binder having a fineness of 0.2 dtex and a fiber length of 3 mm A separator having a basis weight of 8 g / m ² and a thickness of 11 μm was produced in the same manner as in Example 1 except that 0 parts by mass and a fibrid consisting of para-based aromatic polyamide were changed to 4.0 parts by mass.

Example 4
Unstretched PET synthetic resin short fibers for single component type binders of 56.0 parts by mass of oriented crystallized PET synthetic resin short fibers with a fineness of 0.06 dtex and a fiber length of 3 mm (softened (softened) Example 1 and Example 1 except that 40.0 parts by mass of a point 120 ° C., melting point 230 ° C.) and 260 ml of modified freeness using a high-speed homogenizer were used as the 4.0 parts by mass of a fibrid of para-based aromatic polyamide. In the same manner, a separator having a basis weight of 10 g / m ² and a thickness of 15 μm was produced.

Comparative Example 1
Unstretched PET synthetic resin short fibers for single component type binders with a denier of 0.06 dtex and a fiber length of 3 mm and an oriented crystallized PET synthetic resin short fiber of 60.0 parts by mass, a fineness of 0.2 dtex and a fiber length of 3 mm A separator having a basis weight of 10 g / m ² and a thickness of 15 μm was produced in the same manner as in Example 1 except that the point 120 ° C. and the melting point 230 ° C. were changed to 40.0 parts by mass.

Comparative example 2
Unstretched PET synthetic resin short fibers for single component type binders of 55.9 parts by mass of oriented crystallized PET synthetic resin short fibers with a fineness of 0.06 dtex and a fiber length of 3 mm (softened (softened) 40.0 parts by mass of a point 120 ° C. and a melting point of 230 ° C., and 4.1 parts by mass of a fibrid of 88 ml of modified freeness used in Example 1 to make the thickness of the separator uniform. A separator having a basis weight of 8 g / m ² and a thickness of 11 μm was produced in the same manner as in Example 1 except that the temperature was 195 ° C. and the linear pressure was a thermal calendering condition of 105 kN / m.

Comparative example 3
Unstretched PET synthetic resin short fibers for single component type binders with a denier of 0.06 dtex and a fiber length of 3 mm and an orientation-crystallized synthetic synthetic resin short fiber of 50.0 parts by mass, a fineness of 0.2 dtex and a fiber length of 3 mm 40.0 parts by mass of a point 120 ° C. and a melting point of 230 ° C., 10.0 parts by mass of heat resistant fibers having a modified freeness of 88 ml used in Example 1 to make the thickness of the separator uniform. A separator having a basis weight of 8 g / m ² and a thickness of 11 μm was produced in the same manner as in Example 1 except that the surface temperature was set at 195 ° C. and the linear pressure was 150 kN / m.

Example 5
A separator having a basis weight of 7 g / m ² and a thickness of 10 μm was produced in the same manner as in Example 1 except that the linear pressure under the heat calendering conditions was set to 95 kN / m using the paper making slurry of Example 3.

Comparative example 4
Using the paper making slurry of Comparative Example 1, a separator having a basis weight of 8 g / m ² and a thickness of 11 μm was produced in the same manner as in Example 1.

Example 6
50.0 parts by mass of an orientation-crystallized synthetic resin short fiber with a fineness of 0.06 dtex and a fiber length of 3 mm, and a non-stretched synthetic synthetic resin short fiber with a fineness of 0.2 dtex and a single fiber length of 3 mm .0 parts by mass, 4.0 parts by mass of the fibrid used in Example 1, and a refiner to refine a solvent-spun cellulose fiber having an average fiber diameter of 10.4 .mu.m and a fiber length of 4 mm, and beat it to 90 ml of modified freeness A separator having a basis weight of 10 g / m ² and a thickness of 15 μm was produced in the same manner as in Example 1 except that the amount of the solvent-spun cellulose fiber obtained was 6.0 parts by mass.

Example 7
49.0 parts by weight of an orientation-crystallized synthetic resin short fiber with a fineness of 0.06 dtex and a fiber length of 3 mm, and a non-stretched synthetic synthetic resin short fiber with a fineness of 0.2 dtex and a single fiber length of 3 mm .0 parts by mass, 4.0 parts by mass of the fibrid used in Example 1, and 7.0 parts by mass of solvent-spun cellulose fibers beaten to 90 ml of modified freeness used in Example 6 In the same manner as in Example 1, a separator having a basis weight of 10 g / m ² and a thickness of 15 μm was produced.

Example 8
57.0 parts by mass of an orientation crystallized PET synthetic resin short fiber having a fineness of 0.06 dtex and a fiber length of 3 mm, and an undrawn PET synthetic resin short fiber for a single component type binder having a fineness of 0.2 dtex and a fiber length of 3 mm A separator having a basis weight of 7 g / m ² and a thickness of 10 μm was produced in the same manner as in Example 1 except that 0 parts by mass and the fibrid used in Example 1 were 3.0 parts by mass.

Example 9
58.0 parts by mass of an orientation crystallized PET synthetic resin short fiber having a fineness of 0.06 dtex and a fiber length of 3 mm, and an undrawn PET synthetic resin short fiber for a single component type binder having a fineness of 0.2 dtex and a fiber length of 3 mm A separator having a basis weight of 7 g / m ² and a thickness of 10 μm was produced in the same manner as in Example 1 except that 0 part by mass and the fibrid used in Example 1 were changed to 2.0 parts by mass.

Comparative example 5
A separator having a basis weight of 7 g / m ² and a thickness of 10 μm was produced in the same manner as in Example 1 except that the linear pressure under the heat calendering conditions was set to 90 kN / m using the paper making slurry of Comparative Example 1.

Comparative example 6
Unstretched PET synthetic resin short fibers for single component type binders of 56.0 parts by mass of oriented crystallized PET synthetic resin short fibers with a fineness of 0.06 dtex and a fiber length of 3 mm (softened (softened) 40.0 parts by mass of a point 120 ° C. and a melting point of 230 ° C.), and 4.0 parts by mass of a fibrid of a para-based aromatic polyamide having a freeness of 310 ml with a domestic mixer A separator having a basis weight of 8 g / m ² and a thickness of 11 μm was produced in the same manner as in Example 1 except that the pressure was 105 kN / m.

  The following physical properties were measured and evaluated for the lithium ion battery separators of Examples and Comparative Examples, and the results are shown in Tables 1 and 2.

<Basic weight of separator>
The basis weight of the separator was measured in accordance with JIS P8124.

<Thickness of Separator>
The thickness at the time of 5 N load was measured using the outside micrometer specified in JIS B7502.

<Evaluation of tensile strength>
A sample piece of 250 mm in the flow direction and 50 mm in the width direction is cut out for each separator so that the long side is in the flow direction, and a bench-type material tester (trade name STA-1150, manufactured by Orientec Co., Ltd.) is used. According to P8113, a tensile test was performed at a tensile speed of 200 mm / min. The maximum value of tensile stress was taken as tensile strength. Generally, high tensile strength is preferable, and when it is low, it is necessary to precisely control the tension applied to the separator at the time of battery production, and there is a problem that a device necessary for this control becomes large. In the case of ○ or 、, practical use is possible if the tension applied to the separator is precisely controlled.

○: tensile strength 800 N / m or more Δ: tensile strength 400 N / m or more and less than 800 N / m ×: tensile strength less than 400 N

<Cutting of separators>
Each of the separators having a width of 120 mm was cut by melting using a heat seal device, and the melted cross section was visually observed and evaluated according to the following evaluation criteria.

○: There is no fluff on the melted cross section.
Δ: Slight fluff is observed in the melted cross section.
X: Fluffs are noticeable on the melted cross section.

<Fabrication of evaluation battery>
As a negative electrode active material, 95% by mass of lithium titanate represented by Li ₄ Ti ₅ O ₁₂ having a spinel structure with an average particle diameter of 0.7 μm and a Li storage potential of 1.55 V as a negative electrode active material, as a conductive material A slurry was prepared by mixing 2.5% by mass of acetylene black and 2.5% by mass of polyvinylidene fluoride, and dispersing this in N-methyl-2-pyrrolidone, and coated on both sides of a 15 μm thick aluminum foil. After rolling and rolling, it was vacuum dried at 150 ° C. for 2 hours to prepare a 100 μm-thick negative electrode for a lithium ion secondary battery, which was used as a negative electrode.

As a positive electrode active material, 90% by mass of lithium cobalt oxide (LiCoO ₂ ) powder, 3% by mass of acetylene black, 3% by mass of graphite and 4% by mass of polyvinylidene fluoride are mixed, and this is added to N-methyl-2-pyrrolidone A dispersed slurry was prepared. The slurry is applied to both sides of a current collector made of an aluminum foil having a thickness of 15 μm, rolled, and vacuum dried at 150 ° C. for 2 hours to prepare a positive electrode for a lithium ion secondary battery having a thickness of 100 μm. As the positive electrode.

Terminals are connected to the current collectors of the positive and negative electrodes, respectively, and the positive electrode, the separator, the negative electrode and the separator are laminated in this order, and then the laminate is made to be perpendicular to the longitudinal direction of the separator. It was rolled up. Subsequently, the rolled product was heat-pressed at 90 ° C. to produce a flat electrode group having dimensions of 70 × 100 mm and a thickness of 3.0 mm. Subsequently, a pack (bag-like exterior material) made of a laminate film having a thickness of 0.1 mm and made of an aluminum foil having a thickness of 40 μm, in which polyethylene films are laminated on both sides, is prepared. The electrode group was housed so that the positive electrode terminal and the negative electrode terminal extended to the outside from the opening of the package, and vacuum drying was performed at 80 ° C. for 24 hours. Next, the above-mentioned electrode group is housed, and in the bag-like exterior material, a mixed solvent (volume ratio 25: 75) of ethylene carbonate (EC) and γ-butyrolactone (BL) as an electrolyte is used as an electrolyte of 1.5 mol. After injecting a solution of 1.5 L of lithium tetrafluoroborate (1.5 M-LiBF ₄ / EC + BL (25:75, vol ratio)), the opening of the bag-like exterior material is completely sealed by heat sealing. A lithium ion secondary battery was produced.

<Evaluation of resistance>
The prepared separator was immersed in an electrolytic solution (1M-LiPF ₆ / EC + diethyl carbonate (DEC) + dimethyl carbonate (DMC) (1: 1: 1, vol ratio)) and then sandwiched between two substantially cylindrical copper electrodes. The resistance component of the alternating current impedance at 200 kHz was measured using an LCR meter (manufactured by Instec, device name: LCR-821).

<Internal short circuit defect rate>
After preparing an electrode group by interposing and winding the produced separator between the electrodes which consist of aluminum foils, the presence or absence of a short (short circuit) does not impregnate in electrolyte solution, but investigates conduction between electrodes with a tester. It was confirmed. The internal short circuit failure rate (%) was calculated from the number of shorts with respect to the total number of electrode groups by inspecting 200 electrode groups.

<Cycle characteristics>
Each lithium ion secondary battery is subjected to charge / discharge cycle test at 1C rate under 45 ° C environment, and the discharge capacity at 1000th cycle is measured, and the discharge capacity maintenance ratio (%) against that at 5 cycles is the cycle characteristic Was calculated.

As shown in Table 1, the separators prepared in Examples 1 to 9 contain fibrids composed of para-based aromatic polyamide having a modified freeness of 300 ml or less and synthetic resin short fibers, and all the separators contained in the separators The content of fibrid is 1.0% by mass or more and 4.0% by mass or less with respect to the fiber component. The separators of Examples 1 to 9 maintain tensile strength even when the basis weight is lowered to 7 g / m ² by the fibrid made of para-aromatic polyamide and the synthetic resin short fibers firmly forming a network of fibers. We were able to. Moreover, it was excellent in cutability. Furthermore, by setting the content of the fibrid to 1.0 mass% or more and 4.0 mass% or less, the internal short circuit failure rate was small and the cycle characteristics could be maintained while suppressing the impedance which is the resistance component low. .

  The separators of Examples 6 and 7 contain fibers other than fibrids and synthetic resin staple fibers. From the comparison of Example 6 and Example 7, the separator of Example 6 in which the content of the synthetic resin short fiber is 90.0 mass% or more was superior in the strength characteristics.

  Since the separators of Comparative Examples 1, 4 and 5 do not contain fibrids, there was a tendency that the internal short circuit failure rate tends to occur when the basis weight is lowered.

  The separators of Comparative Examples 2 and 3 contain a fibrid with a modified freeness of 300 ml or less and a synthetic resin staple fiber, and the content of the fibrid is 4.0% by mass with respect to the total fiber component contained in the separator. Is a separator that exceeds. Therefore, in the evaluation of cutability, a lot of fluff remained on the edge surface. Furthermore, when adjusting a separator to predetermined | prescribed thickness by a heat calendar process, it was necessary to raise a linear pressure, and it turned out that it is difficult to make a separator thin. As a result, the impedance which is a resistance component deteriorated. In the case of Comparative Example 3 in which the content of fibrids was 10% by mass, the strength characteristics slightly deteriorated.

  The separator of Comparative Example 6 had poor cuttability because the modified freeness of the fibrid exceeded 300 ml, and slight fuzz was observed on the edge surface. In addition, the resistance component deteriorated significantly.

  The lithium ion battery separator of the present invention can be suitably used for lithium ion secondary batteries, such as lithium ion secondary batteries and lithium ion polymer secondary batteries.

1 Fibrid

Claims (4)

A lithium ion battery separator comprising a heat resistant fiber having a modified freeness of 300 ml or less and a synthetic resin short fiber, wherein the heat resistant fiber is a fibrid comprising a para-based aromatic polyamide, and the content thereof is 1. A lithium ion battery separator characterized by having 0 mass% or more and 4.0 mass% or less.
Modified freeness: Measured according to JIS P8121-2: 2012 except using 80 mesh wire mesh with 0.14 mm wire diameter and 0.18 mm mesh as a sieve plate and setting the sample concentration to 0.1%. value.   The lithium ion battery separator according to claim 1, wherein the content of the synthetic resin short fibers is 90.0% by mass or more and 99.0% by mass or less based on the total fiber components.   The lithium ion battery separator according to claim 1, wherein the content of the synthetic resin short fibers is 96.0% by mass to 99.0% by mass with respect to the total fiber component.   A lithium ion battery comprising the lithium ion battery separator according to any one of claims 1 to 3.