JP2012155941A - Separator for electrochemical element, and electrochemical element using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for an electrochemical element, which is excellent in anti-dendrite properties and winding properties, and an electrochemical element excellent in discharge capacity maintenance rate.SOLUTION: The separator for an electrochemical element is made of a wet-laid nonwoven fabric containing polytetrafluoroethylene particulates.

Description

  The present invention relates to a separator for an electrochemical element and an electrochemical element using the same.

  Conventionally, a microporous film made of a polyolefin resin has been used as a separator for a lithium secondary battery. In the case of a microporous film composed mainly of polyethylene, when the temperature inside the battery reaches around 130 ° C., it melts and closes the micropore, thereby preventing the movement of lithium ions and interrupting the current. Although there is a function (shutdown function), it is suggested that if the temperature further increases due to some situation, the separator itself may melt and short-circuit, causing a thermal runaway. Then, the nonwoven fabric separator which consists of synthetic fiber short fiber, and the nonwoven fabric separator containing a fibrillated fiber are proposed for the purpose of improving the safety | security of a battery. An example in which a porous substrate such as a nonwoven fabric is impregnated or coated with a slurry containing filler particles to form a separator for a lithium secondary battery is disclosed (for example, see Patent Documents 1 to 21).

  The nonwoven fabric separator made of synthetic short fibers has a problem that the gap between the fibers tends to be large and the lithium dendrite penetrates. Lithium dendrite refers to metallic lithium that deposits on the negative electrode surface when charging and discharging are repeated or overcharged. Lithium dendrite grows gradually, penetrates the separator, reaches the positive electrode, and may cause an internal short circuit. Nonwoven fabric separators containing fibrillated fibers have a problem that they tend to fluff due to rubbing during handling and winding. In the separators of Patent Documents 1 to 21, when the filler particle layer is thick, cracking or peeling occurs during handling or winding, which causes a problem in winding properties. Depending on the type and amount of the resin binder, the filler particles may be buried in the resin binder, or the resin binder may block the holes of the nonwoven fabric.

International Publication No. 2007/066768 Pamphlet International Publication No. 2008/143005 Pamphlet International Publication No. 2008/114727 Pamphlet International Publication No. 2009/096451 Pamphlet International Publication No. 2003/073534 (Special Publication 2006-504228) International Publication No. 2004/021476 (Special Table No. 2005-536857) International Publication No. 2004/021499 (Special Table 2005-536658) International Publication No. 2004/021469 Pamphlet (Special Table No. 2005-536858) International Publication No. 2004/021477 (Special Table 2005-536860) International Publication No. 2004/049471 pamphlet (Japanese translations of PCT publication No. 2006-507635) International Publication No. 2004/049472 pamphlet (Japanese Patent Publication No. 2006-507636) International Publication No. 2005/038959 Pamphlet (Special Publication 2007-508669) International Publication No. 2005/038946 Pamphlet (Special Table 2007-508670) International Publication No. 2005/038960 Pamphlet (Japanese Patent Publication No. 2007-509464) International Publication No. 2005/104269 Pamphlet (Special Table 2007-534123) International Publication No. 2006/136472 Pamphlet (Special Table 2008-544457) International Publication No. 2007/028662 Pamphlet (Special Table 2009-507353) JP 2008-208511 A JP 2009-230975 A JP 2005-293891 A JP 2007-157723 A

  An object of the present invention is to provide an electrochemical device separator excellent in dendrite resistance and winding property, and an electrochemical device excellent in discharge capacity retention rate.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have found a separator for an electrochemical element containing a wet nonwoven fabric containing polytetrafluoroethylene fine particles and an electrochemical element using the same.

  The separator for an electrochemical element of the present invention comprises a wet nonwoven fabric containing polytetrafluoroethylene fine particles, and therefore has strong bonds between fibers, small pores, and excellent dendrite resistance and winding properties. . Moreover, the electrochemical element which is excellent in discharge capacity maintenance factor is obtained by using this separator.

  In the separator for electrochemical devices of the present invention (hereinafter, sometimes referred to as “separator”), the wet nonwoven fabric containing polytetrafluoroethylene fine particles is a wet nonwoven fabric containing no pre-prepared polytetrafluoroethylene fine particles ( Hereinafter, a wet nonwoven fabric containing polytetrafluoroethylene fine particles (hereinafter referred to as “wet nonwoven fabric B”) is impregnated or coated with an aqueous dispersion of polytetrafluoroethylene fine particles on “wet nonwoven fabric A”) and dried. A wet non-woven fabric (hereinafter referred to as “wet non-woven fabric C”) containing polytetrafluoroethylene fine particles by wet papermaking a mixture of fibers and polytetrafluoroethylene fine particles. It can manufacture by the method to do. If necessary, the wet nonwoven fabrics A to C may be subjected to a calendar treatment, a thermal calendar treatment, a heat treatment, a hydrophilic treatment, or a composite treatment with a porous substrate such as another wet nonwoven fabric or a porous film. .

  You may add additives, such as surfactant, an antifoamer, a thickener, a paper strength enhancer, to a water dispersion liquid as needed. Examples of the paper strength enhancer include anionic polyacrylamide, cationic polyacrylamide, amphoteric polyacrylamide, and epoxy-modified polyamide. In order to impregnate the wet nonwoven fabric, for example, an impregnation machine such as a dip coater can be used. In order to coat the wet nonwoven fabric, for example, a coating machine such as a transfer roll coater, a reverse roll coater, a blade coater, an air doctor coater, a rod coater, a gravure coater, a die coater, or a notch bar coater can be used. As a drying method after impregnation or coating, a hot air dryer, a cylinder dryer, a Yankee dryer or the like may be used. If necessary, an infrared heater or a far infrared heater may be used in combination.

  The average particle diameter of the polytetrafluoroethylene fine particles is preferably 0.01 to 6 μm, and more preferably 0.1 to 4 μm. When the polytetrafluoroethylene fine particles are not spherical, the diameter when converted to a sphere having the same volume is defined as the average particle diameter. If the average particle size is less than 0.01 μm, it may be difficult to obtain the effect of adding the polytetrafluoroethylene fine particles. If the average particle diameter is larger than 6 μm, it may be difficult to reduce the thickness of the separator or the pores of the separator may be blocked. The content of the polytetrafluoroethylene fine particles in the wet nonwoven fabric B or C is preferably 3 to 40% by mass, more preferably 5 to 30% by mass, and still more preferably 10 to 30% by mass. When the content is less than 3% by mass, it may be difficult to obtain the effect of adding the polytetrafluoroethylene fine particles. If the content is more than 40% by mass, the internal resistance may increase. The average particle diameter can be confirmed with a laser diffraction particle size distribution analyzer. The particle diameter when the mass ratio is 50% when measured with a diffraction particle size distribution analyzer, that is, D50 is defined as the average particle diameter.

  Wet non-woven fabrics A and C are obtained by wet paper making using a paper machine such as a circular paper machine, a long paper machine, an inclined paper machine, a short paper machine, an inclined short paper machine, or a combination paper machine thereof. It is done. Multi-layer papermaking may be used in which slurries are supplied to the nets of inclined paper machines and inclined short paper machines. If necessary, the wet nonwoven fabric A or C may be combined with electrostatic spinning. In the present invention, the wet nonwoven fabric A or C may be subjected to a calender treatment, a heat calender treatment, a heat treatment, a hydrophilization treatment, or the like as necessary.

  The separator of the present invention may be a single layer or a multilayer. Multi-layer is a laminate of two or more layers with the same constituent material, blending ratio, basis weight, thickness, etc., or two or more layers with different conditions such as constituent material, blending ratio, basis weight, etc. Refers to the laminated one. The latter is, for example, a laminate of two or more layers of wet nonwoven fabric A and wet nonwoven fabric B or C, a laminate of wet nonwoven fabric B or C on the front and back surfaces of wet nonwoven fabric A, and three layers. Or, wet nonwoven fabric A is laminated on the front and back surfaces of C to form three layers, wet nonwoven fabric B or C and two or more layers of porous film, wet nonwoven fabric A and B or C and porous film are laminated And the like. The multilayer separator is a method of laminating wet nonwoven fabrics B or C, wet nonwoven fabric A and wet nonwoven fabric B or C, wet nonwoven fabric B or C and porous film, wet nonwoven fabric A and B or C and porous film, They can be produced by a method of heat-bonding these by heat-pressure treatment, or a method of heat-bonding by sandwiching a hot-melt material between these layers. Two or more types of wet nonwoven fabrics A when producing a multilayer separator may be used, and two or more types of wet nonwoven fabrics B or C may be used, and two or more types of porous films may be used. Two or more types mean that one or more conditions such as constituent materials, blending ratio, basis weight, and thickness are different.

  The wet nonwoven fabrics A to C preferably contain fibrillated fibers. The fibrillated fiber is easy to carry polytetrafluoroethylene fine particles and improves the dendrite resistance. Examples of fibrillated fibers include fibers formed by fibrillating natural fibers, regenerated fibers, synthetic fibers, and the like. Examples of natural fibers include cellulose fibers derived from wood, cellulose fibers derived from non-wood such as hemp, cotton, and sugarcane, biocellulose fibers, wool, and silk. Examples of regenerated fibers include solvent-spun cellulose fibers and cupra fibers. Synthetic fibers include polypropylene, polyethylene, polymethylpentene, polyester, polyester derivatives, acrylic polymers, polyvinyl acetate, ethylene-vinyl acetate copolymers, polyvinyl chloride, polyvinylidene chloride, polyvinyl ether, polyvinyl ketone, poly Ether, polyvinyl alcohol, aliphatic polyamide, aromatic polyamide, wholly aromatic polyamide, wholly aromatic polyester, polyamideimide, polyimide, polyetheretherketone, polyphenylene sulfide, polybenzimidazole, poly-p-phenylenebenzobisthiazole, poly Examples thereof include fibers made of a resin such as -p-phenylenebenzobisoxazole and polytetrafluoroethylene.

  Among these, fibrillated fibers made of para-type wholly aromatic polyamide, wholly aromatic polyester, and solvent-spun cellulose are preferable because they have excellent ability to support polytetrafluoroethylene fine particles during coating. A fibrillated fiber made of wholly aromatic polyester is preferable because it is excellent in aggregating ability of polytetrafluoroethylene fine particles during papermaking. The degree of fibrillation is preferably 0 to 600 ml, more preferably 0 to 500 ml, and even more preferably 0 to 400 ml, according to JIS P8121. When the Canadian standard freeness is larger than 600 ml, the fiber diameter distribution is widened, and there may be formation of unevenness or thickness unevenness of the wet nonwoven fabric.

  When the degree of fibrillation exceeds a certain level, fibrillated fibers pass through the holes in the sieve plate used to measure Canadian standard freeness, resulting in abnormally high freeness and inaccurate freeness measurement. . In that case, modified freeness is adopted in the present invention. The modified freeness in the present invention was measured in accordance with JIS P811, except that an 80 mesh wire net having a wire diameter of 0.14 mm and an aperture of 0.18 mm was used as a sieve plate, and the sample concentration was 0.1%. Freeness. The modified freeness of the fibrillated fiber used in the present invention is preferably 0 to 400 ml, more preferably 0 to 300 ml. When the amount exceeds 400 ml, the ratio of the thick fiber diameter increases, so that there are cases where the separator and the thickness unevenness of the separator occur.

  Therefore, the fibrillated fiber in the present invention is preferably used if the Canadian standard freeness is in the range of 0 to 600 ml or the modified freeness is in the range of 0 to 400 ml. In general, the modified freeness value is larger than the Canadian standard freeness value. If the Canadian standard freeness value is greater than the modified freeness value, the fiber length of the fibrillated fiber is too short to slip through the Canadian standard freeness screen, so an accurate Canadian standard freeness value is obtained. It means that the water level is not shown. In the present invention, only one type of fibrillated fiber may be used, or two or more types may be used in combination. When two or more types are used in combination, fibrillated fibers having a Canadian freeness or modified freeness that is not a preferable value may be used as long as they do not cause formation unevenness or thickness unevenness of the wet nonwoven fabric.

  In the wet nonwoven fabrics A to C, the content of fibrillated fibers is preferably 5 to 80% by mass, more preferably 10 to 70% by mass, and still more preferably 15 to 60% by mass with respect to the total fibers. When the content is less than 5% by mass, the supporting ability of the polytetrafluoroethylene fine particles may be insufficient. When the content exceeds 80% by mass, the water resistance of the wet nonwoven fabric may be weakened, or the impregnation property and coating property of the aqueous dispersion of polytetrafluoroethylene fine particles may be insufficient.

  The wet nonwoven fabrics A to C may contain non-fibrillated fibers. When a non-fibrillated fiber is contained, the tensile strength and puncture strength of the separator can be increased, and the winding property of the separator can be increased. Examples of non-fibrillated fibers include organic fibers that are not fibrillated in the fiber group exemplified as fibrillated fibers, fibers composed of these derivatives, composite fibers composed of two or more kinds of resins, and inorganic fibers. Examples of the composite fiber include a core-sheath type, an eccentric type, a side-by-side type, a sea-island type, an orange type, a multiple bimetal type, and a split type.

  Examples of the inorganic fibers include silica / alumina fibers, alumina fibers, glass fibers, micro glass fibers, zirconia fibers, silicon nitride fibers, silicon carbide fibers, and the like.

  The fiber length of the non-fibrillated fiber is preferably 0.1 to 10 mm, and more preferably 0.3 to 6 mm. If the fiber length is less than 0.1 mm, it may fall off from the separator, and if it is longer than 10 mm, the fibers may rely on each other to cause formation unevenness or thickness unevenness. The average fiber diameter of the non-fibrillated fiber is preferably 0.1 to 15 μm, and more preferably 1 to 8 μm. When the average fiber diameter exceeds 15 μm, the number of fibers in the thickness direction decreases, so that a large through-hole or thickness spot may eventually occur in the separator or it may be difficult to reduce the thickness. When the average fiber diameter is less than 0.1 μm, stable production of the fibers becomes difficult. The non-fibrillated fiber may be a fiber (single fiber) made of a single resin, or may be a non-split composite fiber made of two or more resins. Moreover, the non-fibrillated fiber contained in the wet nonwoven fabric in this invention may be 1 type, and may be used in combination of 2 or more type. Examples of the composite fiber include a core-sheath type, an eccentric type, a side-by-side type, a sea-island type, an orange type, and a multiple bimetal type. The cross-sectional shape of the non-fibrillated fiber may be any of a circle, an ellipse, a triangle, a quadrangle, a polygon, and an indeterminate shape.

  In the wet nonwoven fabrics A to C, the content of non-fibrillated fibers is preferably 5 to 100% by mass, more preferably 10 to 90% by mass, and still more preferably 15 to 80% by mass with respect to the total fibers. If the content is less than 5% by mass, the tensile strength and puncture strength of the wet nonwoven fabric may be insufficient.

  In the present invention, among non-fibrillated fibers, synthetic short fibers having a theoretical flatness of 1.0 to 5.0 are preferred because they suppress the generation of pinholes and are excellent in dendrite resistance. If the theoretical flatness is greater than 5.0, the voids of the wet nonwoven fabric may be blocked. The theoretical flatness is the maximum length of the long axis of the synthetic short fiber cross section (hereinafter sometimes referred to as the long axis length) and the maximum length of the short axis (hereinafter sometimes referred to as the short axis length). It means the value divided by. Examples of synthetic short fibers having a theoretical flatness of 1.0 to 5.0 include ultrafine fibers obtained by dividing a split type composite fiber in which two component resins are adjacent to each other. Examples of the resin constituting the split-type composite fiber include polypropylene, polyethylene, polymethylpentene, polyethylene terephthalate, polyester derivatives, ethylene-vinyl alcohol copolymer, ethylene-vinyl acetate copolymer, and polyvinyl alcohol.

  Examples of the cross-sectional shape of the split type composite fiber include a radial type, a layered type, a comb type, a grid type, and a sea island type. Further, the radial mold may be a hollow mold in which a central portion viewed from a cross section is hollow. The split type composite fiber is preferably one that can be split by a method of stirring with a pulper or a mixer or a method of applying a high-pressure water stream. When the split type composite fiber is split by stirring with a pulper or a mixer, a dispersion aid or an antifoaming agent may be used as necessary. The average fiber diameter of the split composite fibers is preferably 3 to 18 μm, more preferably 3 to 16 μm, and even more preferably 6 to 16 μm. If it is less than 3 μm, it may be difficult to divide, and if it is thicker than 18 μm, the major axis of the cross-section of the ultrafine fiber obtained by the division becomes longer, so that the voids of the wet nonwoven fabric may be blocked. The average fiber diameter refers to a value obtained by converting the cross-sectional area into the diameter of a perfect circle having the same area.

  A synthetic short fiber having a theoretical flatness of 1.0 to 5.0 preferably has a minor axis length of 1 to 5 μm, more preferably 1 to 3 μm. When the minor axis length of the cross section is less than 1 μm, the flatness of the cross section becomes too large and the voids of the wet nonwoven fabric may be blocked, or the bonding of the intersections of the fibers may be insufficient. When the minor axis length of the cross section exceeds 5 μm, it may be difficult to reduce the thickness of the separator. The length of the synthetic short fiber having a theoretical flatness of 1.0 to 5.0 is preferably 0.5 to 10 mm, more preferably 1 to 6 mm, and further preferably 1 to 4 mm. When the length of the synthetic short fiber having a theoretical flatness of 1.0 to 5.0 is less than 0.5 mm, there is a case where the ratio of falling out of the screen and draining into the waste water during wet papermaking increases. If the length of the synthetic short fiber having a theoretical flatness of 1.0 to 5.0 is longer than 10 mm, the fine fibers may depend on each other to form a lump.

  The wet nonwoven fabrics A to C preferably contain an inorganic filler. Inorganic fillers include inorganic oxides such as alumina, gibbsite, boehmite, magnesium oxide, magnesium hydroxide, silica, titanium oxide, barium titanate, zirconium oxide, and inorganic nitrides such as aluminum nitride and silicon nitride. , Aluminum compounds, zeolite, mica and the like. Among these, magnesium oxide and magnesium hydroxide, which are excellent in the effect of suppressing deterioration of battery characteristics, are preferable.

  3-100 mass% is preferable, as for the content rate of the inorganic filler in the wet nonwoven fabric A, 5-80 mass% is more preferable, and 10-60 mass% is further more preferable. If the content is less than 1% by mass, the effect of adding an inorganic filler may be difficult to obtain. When there is more content than 100 mass%, the softness | flexibility of the wet nonwoven fabric A will be impaired, and it may become easy to crack and powder-off at the time of handling. In order to contain the inorganic filler in the wet nonwoven fabric A, a method of preparing a slurry by mixing the fibers constituting the wet nonwoven fabric and the inorganic filler and performing wet papermaking can be mentioned.

  1-60 mass% is preferable, as for the content rate of the inorganic filler in the wet nonwoven fabric B or C, 3-50 mass% is more preferable, and 5-40 mass% is further more preferable. If the content is less than 1% by mass, the effect of adding an inorganic filler may be difficult to obtain. When the content is more than 60% by mass, it may be difficult to reduce the thickness of the wet nonwoven fabric B or C. As a method of adding an inorganic filler to wet nonwoven fabric B or C, a method of impregnating or applying wet nonwoven fabric A by mixing an inorganic filler with an aqueous dispersion of polytetrafluoroethylene fine particles, fiber and inorganic filler, polytetrafluoroethylene There is a method of preparing a slurry in which fluoroethylene fine particles are mixed and performing wet papermaking.

  In order to prepare the wet nonwoven fabric C, it is preferable to add a flocculant as necessary when preparing a slurry in which fibers and polytetrafluoroethylene fine particles are mixed. By aggregating the fibers and the polytetrafluoroethylene fine particles, the outflow of the polytetrafluoroethylene fine particles to the waste water can be suppressed. As the flocculant, organic flocculants such as polyamine, polyacrylamide, sodium alginate, dicyanamide, inorganic systems such as sulfate band, ferric sulfate, polyferric sulfate, calcium sulfate, ferric chloride, polyaluminum chloride, etc. A flocculant is mentioned. The organic flocculant alone or the inorganic flocculant alone may be used, or the organic and inorganic flocculants may be used in combination. The organic flocculant may contain a salt such as sodium salt or potassium salt. In order to increase the acting force of the flocculant, the pH of the slurry may be adjusted. pH adjusters include sodium hydroxide, potassium hydroxide, lithium hydroxide, magnesium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, carbon dioxide, ammonia, citric acid, gluconic acid, succinic acid, lactic acid, fumaric acid Etc. The addition amount of the flocculant is preferably 0.01 to 10% by mass and more preferably 0.1 to 10% by mass with respect to the solid content of the slurry in terms of solid content. When the addition amount of the flocculant is less than 0.01% by mass, the aggregation of the slurry may be insufficient, and when it is more than 10% by mass, the drainage of the slurry may be deteriorated and papermaking may be difficult.

  10-60 micrometers is preferable, as for the thickness of the separator of this invention, 15-50 micrometers is more preferable, and 15-40 micrometers is further more preferable. When the thickness exceeds 60 μm, the electrical resistance of the separator may be increased. When the thickness is less than 10 μm, the strength of the separator becomes too weak and may be damaged when the separator is handled.

The density of the separator of the present invention is preferably 0.250~0.750g / cm ^3, more preferably 0.300~0.700g / cm ^3, more preferably 0.400~0.650g / cm ^3. If the density is less than 0.250 g / cm ³ , the coating liquid may be exposed, and if it exceeds 0.750 g / cm ³ , the electrical resistance of the separator may be increased.

The electrochemical element in the present invention is a lithium battery, lithium secondary battery, polyacene battery, organic radical battery, electric double layer capacitor, lithium ion capacitor, hybrid capacitor, redox capacitor, aluminum electrolytic capacitor, conductive polymer aluminum electrolytic capacitor It means an electrochemical element having a power storage function and a rectifying function. The lithium secondary battery means a lithium ion battery or a lithium ion polymer battery. Examples of the negative electrode active material of the lithium secondary battery include carbon materials such as graphite and coke, metallic lithium, aluminum, silica, tin, nickel, and an alloy of lithium and lithium, SiO, SnO, Fe Metal oxides such as ₂ O ₃ , WO ₂ , Nb ₂ O ₅ , Li _4/3 Ti _5/3 O ₄ , and nitrides such as Li _0.4 CoN are used. As the positive electrode active material, lithium cobaltate, lithium manganate, lithium nickelate, lithium titanate, lithium nickel manganese oxide, or lithium iron phosphate is used. Further, the lithium iron phosphate may be a composite with one or more metals selected from manganese, chromium, cobalt, copper, nickel, vanadium, molybdenum, titanium, zinc, aluminum, gallium, magnesium, boron, and niobium.

  As an electrolytic solution for a lithium secondary battery, a solution obtained by dissolving a lithium salt in an organic solvent such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, dimethoxyethane, dimethoxymethane, or a mixed solvent thereof is used. Examples of the lithium salt include lithium hexafluorophosphate and lithium tetrafluoroborate. As solid electrolyte, what melt | dissolved lithium salt in gel-like polymers, such as polyethyleneglycol, its derivative (s), polymethacrylic acid derivative, polysiloxane, its derivative (s), polyvinylidene fluoride, is used.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to a present Example.

  Table 1 shows the fibers, inorganic fillers, and polyolefin resins used in Examples and Comparative Examples of the present invention. In Table 1, S8 is a core-sheath type composite fiber in which the core part is made of polypropylene and the sheath part is made of high-density polyethylene, and S9 is a core-sheath type composite fiber in which the core part is made of polyethylene terephthalate and the sheath part is made of modified polyester. The cross-sectional area ratio between the core part and the sheath part of S8 and S9 is 50:50. S11 is a 16-partition split type composite fiber in which polypropylene and ethylene-vinyl alcohol copolymer are adjacent to each other, and S12 is a 12-partition type split type in which polyethylene terephthalate and high-density polyethylene are adjacent to each other. The composite fiber, S13, means a 16-part split type composite fiber in which polyethylene terephthalate and ethylene-vinyl alcohol copolymer are adjacent to each other. PTFE of P1 and P2 means polytetrafluoroethylene. CSF means Canadian standard freeness. Table 2 shows the blending ratio of the papermaking slurry. The raw material symbols in Table 2 correspond to the symbols in Table 1. The blending ratio of P1 and P2 of the slurry 20 to 27 is a numerical value converted into the solid content of P1 and P2.

<Preparation of slurries 1-15>
Predetermined fibers were weighed so that the blending ratio of the slurries 1 to 15 was reached, and these fibers were dispersed together in water with a pulper to prepare slurries 1 to 15. A dispersion aid, a thickener, and an antifoaming agent were added as necessary. In the slurries 2, 3, 7, 8, 10, 11, 12, 14, and 15, it was confirmed that the split composite fibers were almost divided.

<Preparation of slurry 16>
F2, S2, S5, S10, S11, and M1 were weighed so that the mixing ratio of the slurry 16 was obtained. F2 and M1 were dispersed together in water with a pulper to prepare a slurry in which an aggregate of M1 and F2 was formed. Separately, a slurry in which S2, S5, S10, and S11 were dispersed together in water with a pulper was prepared, and the slurry containing the aggregate was mixed and stirred to prepare slurry 16. It was confirmed that the split type composite fiber of S11 was almost split.

<Preparation of slurry 17>
F2, S2, S5, S10, S11, and M2 were weighed so that the blending ratio of the slurry 17 was obtained. F2 and M2 were dispersed together in water with a pulper to prepare a slurry in which aggregates of F2 and M2 were formed. Separately, a slurry in which S2, S5, S10, and S11 were dispersed together in water with a pulper was prepared, and the slurry containing the aggregate was mixed and stirred to prepare slurry 17. It was confirmed that the split type composite fiber of S11 was almost split.

<Preparation of slurry 18>
F2, S2, S5, S10, S11, and M3 were weighed so that the mixing ratio of the slurry 18 was obtained. F2 and M3 were dispersed together in water with a pulper to prepare a slurry in which aggregates of F2 and M3 were formed. Separately, a slurry in which S2, S5, S10, and S11 were dispersed together in water with a pulper was prepared, and the slurry containing the aggregate was mixed and stirred to prepare slurry 18. It was confirmed that the split type composite fiber of S11 was almost split.

<Preparation of slurry 19>
F6, S13, and M4 were weighed so that the blending ratio of the slurry 19 was obtained. F6 and M4 are dispersed together in water using a pulper, and a polyacrylamide flocculant (manufactured by Ciba Specialty Chemicals, trade name: Hydrocol (registered trademark) HC-880) is solid with respect to the total mass of F6 and M4. 1% by mass was added per minute and stirred to prepare a slurry in which aggregates of F6 and M4 were formed. Separately, a slurry in which S13 was dispersed in water with a pulper was prepared, and the slurry 19 was mixed and stirred into a slurry containing aggregates to prepare slurry 19. It was confirmed that the split type composite fiber of S13 was almost split.

<Preparation of slurry 20>
F2, S3, S10, S11, M1, and P1 were weighed so that the mixing ratio of the slurry 20 was obtained. M1 was added to the aqueous dispersion of P1 and stirred to prepare a uniformly dispersed slurry. This slurry was mixed and stirred in a slurry in which F2 was dispersed in water with a pulper to prepare a slurry in which aggregates of F2, M1, and P1 were formed. Next, a slurry was prepared by dispersing S3, S10, and S11 together in a water using a pulper, and the slurry containing the aggregate was mixed and stirred to prepare slurry 20. It was confirmed that the split type composite fiber of S11 was almost split.

<Preparation of slurry 21>
F10, S2, S10, M2, and P1 were weighed so that the mixing ratio of the slurry 21 was obtained. M2 was added to the aqueous dispersion of P1 and stirred to prepare a uniformly dispersed slurry. This slurry was mixed and stirred in a slurry in which F10 was dispersed in water with a pulper to prepare a slurry in which aggregates of F10, M2, and P1 were formed. Next, a slurry in which S2 and S10 were dispersed in water with a pulper was prepared, and a slurry 21 was prepared by mixing and stirring the slurry containing aggregates.

<Preparation of slurry 22>
F1, S1, S2, S10, M3, and P1 were weighed so that the mixing ratio of the slurry 22 was obtained. M3 was added to the aqueous P1 dispersion and stirred to prepare a uniformly dispersed slurry. This slurry was mixed and stirred in a slurry in which F1 was dispersed in water with a pulper to prepare a slurry in which aggregates of F1, M3, and P1 were formed. Next, a slurry in which S1, S2, and S10 were dispersed together in a water with a pulper was prepared, and the slurry containing the aggregate was mixed and stirred to prepare slurry 22.

<Preparation of slurry 23>
F6, S3, M4, and P2 were weighed so that the mixing ratio of the slurry 23 was obtained. M4 was added to the aqueous dispersion of P2 and stirred to prepare a uniformly dispersed slurry. While this slurry was mixed and stirred in a slurry in which F6 was dispersed in water with a pulper, a polyacrylamide flocculant (manufactured by Ciba Specialty Chemicals, trade name: Hydrocor (registered trademark) HC-880) was added to F6, M4. , 1% by mass as a solid content was added to the total mass of P2, and stirred to prepare a slurry in which aggregates of F6, M4, and P2 were formed. Separately, a slurry in which S3 was dispersed in water with a pulper was prepared, and the slurry containing the aggregate was mixed and stirred to prepare slurry 23.

<Preparation of slurry 24>
S2, S3, S4, and P2 were weighed so that the blending ratio of the slurry 24 was obtained. S2, S3, and S4 were dispersed together in water using a pulper, and P2 was mixed and stirred therein to prepare slurry 24 in which aggregates of S2, S3, S4, and P2 were formed.

<Preparation of slurry 25>
S3, S10, S13, and P2 were weighed so that the blending ratio of the slurry 25 was obtained. S3, S10, and S13 were dispersed together in water using a pulper, and P2 was mixed and stirred therein to prepare slurry 25 in which aggregates of S3, S10, S13, and P2 were formed. It was confirmed that the split type composite fiber of S13 was almost split.

<Preparation of slurry 26>
F2, S3, S10, S11, and P1 were weighed so that the mixing ratio of the slurry 26 was obtained. F2 was dispersed in water with a pulper, and P1 was mixed therein and stirred to prepare a slurry in which an aggregate of F2 and P1 was formed. Separately, a slurry was prepared by dispersing S3, S10, and S11 together in a water with a pulper, and the slurry containing the aggregate was mixed and stirred to prepare slurry 26. It was confirmed that the split type composite fiber of S11 was almost split.

<Preparation of slurry 27>
F3, S2, S3, S10, and P1 were weighed so that the blending ratio of the slurry 27 was obtained. F3 was dispersed in water with a pulper, and P1 was mixed therein and stirred to prepare a slurry in which an aggregate of F3 and P1 was formed. Separately, a slurry in which S2, S3, and S10 were dispersed in water with a pulper was prepared, and the slurry 27 was prepared by mixing and stirring into a slurry containing aggregates.

<Preparation of wet nonwoven fabric A1>
Slurry 1 was fed to an inclined paper machine, wet papermaking, dried at Yankee dryer 140 ° C., and then heat calendered at 200 ° C. and linear pressure 300 N / cm to prepare wet nonwoven fabric A1.

<Preparation of wet nonwoven fabric A2>
Slurry 2 was fed to an inclined paper machine, wet-made, and dried at Yankee dryer 130 ° C. to prepare wet nonwoven fabric A2.

<Production of wet nonwoven fabrics A3, A4, A7, A8, A10 to A12, A14, A15 to A19>
Wet nonwoven fabrics A3, A4, A7, A8, A10 to A12, A14, A15 to A19 in the same manner as wet nonwoven fabric A2, except that slurries 3, 4, 7, 8, 10-12, 14, 15-19 were used. Was made.

<Preparation of wet nonwoven fabric A5, A6, A13>
Slurries 5, 6, and 13 are fed to an inclined paper machine and wet-made, and after drying at a Yankee dryer temperature of 130 ° C, the front and back surfaces of the wet nonwoven fabric are brought into contact with a roll heated to 200 ° C and heat-treated. Wet nonwoven fabrics A5, A6 and A13 were prepared.

<Production of wet nonwoven fabric A9>
A wet nonwoven fabric A9 was produced in the same manner as the wet nonwoven fabric A1 except that the slurry 9 was used.

<Production of wet nonwoven fabric C1>
The slurry 20 was fed to an inclined paper machine and wet-made, and dried at a Yankee dryer temperature of 130 ° C., and then calendered to adjust the thickness to produce a wet nonwoven fabric C1.

<Production of wet nonwoven fabrics C2 to C8>
Except for using the slurries 21 to 27, wet nonwoven fabrics C2 to C8 were prepared in the same manner as the wet nonwoven fabric C1.

  Table 3 shows the physical properties of wet nonwoven fabrics A1 to A19 and C1 to C8.

<Preparation of coating solution>
An aqueous dispersion containing polytetrafluoroethylene fine particles and inorganic filler was prepared so as to have the blending ratio shown in Table 4, and used as a coating liquid. P1 and P2 in Table 4 correspond to P1 and P2 in Table 1, and the blending ratio of P1 and P2 is a numerical value converted into the solid content of P1 and P2.

<Preparation of separator>

(Examples 1 to 60)
According to the combination of wet nonwoven fabric and coating liquid shown in Tables 5 and 6, wet nonwoven fabrics A1 to A19 are coated with an aqueous dispersion containing polytetrafluoroethylene fine particles, dried at 120 ° C., calendered, and then processed into a thickness. It adjusted and produced the separator of Examples 1-60. The adhesion rates of the polytetrafluoroethylene fine particles and the inorganic filler to the wet nonwoven fabrics A1 to A19 are shown in Tables 5 and 6.

(Examples 61-68)
Wet nonwoven fabrics C1 to C8 were used as separators of Examples 61 to 68. Table 6 shows the content ratios of the polytetrafluoroethylene fine particles and the inorganic filler with respect to the wet nonwoven fabrics C1 to C8.

(Comparative Examples 1-19)
Wet nonwoven fabrics A1 and A9 were used as separators of Comparative Examples 1 and 9, respectively. Wet nonwoven fabrics A2 to A8 and A10 to A19 were calendered to adjust the thickness, and separators of Comparative Examples 2 to 8 and 10 to 19 were obtained.

(Comparative Example 20)
M1 is 95% by mass and styrene butadiene latex is in a proportion of 5% by mass and dispersed in water so that the total solid content concentration is 20% by mass, and this is applied to the wet nonwoven fabric A2 and dried at 120 ° C. The thickness was adjusted by calendering, and the separator of Comparative Example 20 was produced.

(Comparative Example 21)
M1 is 95% by mass and styrene butadiene latex is in a proportion of 5% by mass and dispersed in water so that the total solid content concentration is 20% by mass, and this is applied to wet nonwoven fabric A9, dried at 120 ° C., The separator was subjected to calendering to adjust the thickness, and a separator of Comparative Example 21 was produced.

(Comparative Example 22)
M1 is dispersed in N-methyl-2-pyrrolidone at a ratio of 95% by mass and polyvinylidene fluoride in a proportion of 5% by mass so that the total solid concentration is 10% by mass, and this is applied to wet nonwoven fabric A2. After drying at 120 ° C., the thickness was adjusted by calendering to produce a separator of Comparative Example 22.

(Comparative Example 23)
Compared to a non-woven fabric with a thickness of 35 μm, which is made of polyester fiber, impregnated with an aqueous ethanol solution containing porous alumina with an average particle size of 1.0 μm in a wet non-woven fabric with a thickness of 22 μm and a density of 0.681 g / cm ^3. The separator of Example 23 was obtained.

[Evaluation]
The following evaluation was performed about the lithium secondary battery produced using the separator of an Example and a comparative example, and this separator, and the result was shown to Tables 7-9.

<Thickness>
The thickness was measured according to JIS P8118 and the average value was shown.

<Density>
The basis weight of the separator was measured in accordance with JIS P8124, and the value obtained by dividing the basis weight by the thickness and multiplying by 100 was defined as the density.

<Fuzzy>
When the surface of the separator is rubbed with a finger, there is almost no fluffing and there is almost no fiber attached to the finger, there is fiber fluffing, and when there is a slight amount of fiber attached to the finger, Δ, fiber fluffing is severe The case where there was much adhesion of the fiber to a finger was set as x.

<Peeling>
When the surface of the separator is rubbed with a finger, there is almost no dropout of the inorganic filler, and there is almost no inorganic filler attached to the finger. The case where the inorganic filler was severely dropped and the inorganic filler adhered to the finger was marked as x. In the case of a separator that does not contain an inorganic filler, “−” is written because it is not subject to evaluation.

<Windability>
The separator was cut to a width of 50 mm and a length of 500 mm. The positive electrode and the negative electrode were cut to a width of 45 mm and a length of 490 mm. The separator, the negative electrode, the separator, and the positive electrode were laminated one by one in this order, and this was wound on a winding machine. At this time, when the separator can be wound without being cut or broken, ○, when it can be wound by adjusting the tension of the winding machine to be weak, Δ, when the separator is cut or broken, or the coating layer The case where cracking or peeling occurred was indicated as x. For the positive electrode, use is made of a slurry in which an active material lithium cobaltate, a conductive auxiliary agent acetylene black, and a binder polyvinylidene fluoride mixed in a mass ratio of 90: 5: 5 are applied to both sides of an aluminum current collector. It was. The negative electrode used was a slurry in which 97% by mass of natural graphite and 3% by mass of polyvinylidene fluoride were mixed and applied to both surfaces of a copper foil current collector.

<Dendrite resistance>
A metal lithium foil was placed on one side of the separator and a positive electrode was placed on the opposite side of the separator and laminated, and an electrolyte was injected to prepare 100 laminate cells. The battery was charged at a constant current of up to 3.6 V at 0.5 mA / cm ² and further overcharged by applying 3.6 V for 24 hours. The case where an abnormal current flowed during this overcharge was regarded as an internal short circuit, the overcharge was stopped, the laminate cell was opened, and the generation state of lithium dendrite was confirmed. Table 2 shows the dendrite resistance as a percentage of cells where lithium dendrite was generated due to overcharge and penetrated the separator. It means that it is excellent in dendrite resistance, so that this ratio is small. The positive electrode is the same as that described in the evaluation of <winding property>. As the electrolytic solution, a mixed solution in which 1 mol / l of LiPF ₆ was dissolved was used. The mixed solution is ethylene carbonate and diethyl carbonate in a mass ratio of 3: 7.

<Discharge capacity maintenance rate>
After winding the wound element produced in the evaluation of <winding property> in a battery can, an electrolytic solution was injected and sealed to prepare a lithium ion secondary battery. The lithium ion battery was charged at a constant current of up to 4.2 V at 25 ° C. and 100 mA, and further charged and charged at 4.2 V for 3 hours, and then the discharge capacity when discharged to 3.0 V at 100 mA was measured. Discharge capacity. Subsequently, charging / discharging was repeated at 60 ° C. and 100 mA for 1000 hours, and the ratio of the discharge capacity after 1000 hours to the initial discharge capacity was calculated to obtain the discharge capacity retention rate. The larger the discharge capacity maintenance rate, the better.

  Since the separators for electrochemical elements of Examples 1 to 68 contained polytetrafluoroethylene fine particles, fluff was hardly generated and the winding property was excellent. Moreover, compared with the separator for electrochemical elements of Comparative Examples 1-19 which is a wet nonwoven fabric which does not contain polytetrafluoroethylene fine particles, it was excellent in dendrite resistance and discharge capacity maintenance rate.

  Since the separators for electrochemical devices of Examples 21 to 64, 67, and 68 contained fibrillated fibers, they were excellent in dendrite resistance. The separators for electrochemical elements of Examples 21 to 60 contain polytetrafluoroethylene fine particles because they contain any of fibrillated solvent-spun cellulose fibers, fibrillated wholly aromatic polyamide fibers, and fibrillated wholly aromatic polyester fibers. The carrying ability of the fine particles was excellent when the coating liquid was applied. Since the separators for electrochemical elements of Examples 61, 63, 67, and 68 contain fibrillated wholly aromatic polyamide fibers or fibrillated wholly aromatic polyester fibers, polytetrafluoroethylene fine particles without using a flocculant And aggregates could be formed. Since the separators for electrochemical elements of Examples 11 to 15 contain synthetic short fibers having a theoretical flatness of 1.0 to 5.0, the separators for electrochemical elements of Examples 16 to 20 that do not contain the fibers are included. It was excellent in dendrite resistance.

  For electrochemical elements of Examples 2 to 5, 7 to 10, 12 to 15, 17 to 20, 22 to 25, 27 to 30, 35 to 38, 40 to 43, 46 to 49, 51 to 55, 61 to 64 Since the separator contains an inorganic filler, the discharge capacity retention rate was higher than that of a separator for an electrochemical device not containing an inorganic filler. For electrochemical elements of Examples 3 to 5, 8 to 10, 13 to 15, 18 to 20, 23 to 25, 28 to 30, 36 to 38, 41 to 43, 47 to 49, 52 to 54, 62 to 64 Since the separator contained magnesium oxide or magnesium hydroxide, the discharge capacity retention rate was higher and excellent.

  On the other hand, the separators for electrochemical elements of Comparative Examples 1 to 5, 12, and 15 were less prone to fluff and had good winding properties, but they did not contain polytetrafluoroethylene fine particles, so that dendrite resistance and discharge capacity maintenance were maintained. The rate was bad.

  Since the separators for electrochemical elements of Comparative Examples 6 to 11, 13, and 14 did not contain polytetrafluoroethylene fine particles, the fluff was severe, the winding property was poor, the dendrite resistance and the discharge capacity retention rate were poor.

  Since the separators for electrochemical devices of Comparative Examples 16 to 18 did not contain polytetrafluoroethylene fine particles, the fuzz was severe, the inorganic filler was severely peeled off, and the winding property was poor.

  The separator for an electrochemical element of Comparative Example 19 had good fuzzing and winding properties, but did not contain polytetrafluoroethylene fine particles, so that the inorganic filler was severely peeled off and the discharge capacity retention rate was poor.

  The separators for electrochemical elements of Comparative Examples 20, 21, and 23 were less prone to fluff, excellent in dendrite resistance, and had a high discharge capacity retention rate, but did not contain polytetrafluoroethylene fine particles, so that the inorganic filler peeled off However, the winding property was bad.

  The separator for an electrochemical element of Comparative Example 22 was less prone to fluff and was excellent in dendrite resistance, but because it did not contain polytetrafluoroethylene fine particles, the inorganic filler was severely peeled off, the winding property was poor, and the discharge The capacity maintenance rate was bad.

  The separator for electrochemical devices 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. Moreover, it can also be used as a base material for laminating and combining other porous base materials and materials on the separator for electrochemical devices of the present invention.

Claims (7)

  A separator for an electrochemical element comprising a wet nonwoven fabric containing polytetrafluoroethylene fine particles.   The separator for an electrochemical element according to claim 1, wherein the wet nonwoven fabric contains fibrillated fibers.   The separator for an electrochemical element according to claim 2, wherein the fibrillated fiber is at least one selected from para-type wholly aromatic polyamide fiber, wholly aromatic polyester fiber, and solvent-spun cellulose fiber.   The separator for an electrochemical element according to claim 1, wherein the wet nonwoven fabric contains synthetic short fibers having a theoretical flatness of 1.0 to 5.0.   The separator for an electrochemical element according to claim 1, wherein the wet nonwoven fabric contains an inorganic filler.   The separator for an electrochemical element according to claim 5, wherein the inorganic filler is magnesium oxide or magnesium hydroxide.   The electrochemical element which comprises the separator for electrochemical elements of any one of Claims 1-6.