JP6408810B2 - Lithium secondary battery separator and method for producing lithium secondary battery separator - Google Patents

Description

  The present invention relates to a lithium secondary battery separator substrate and a lithium secondary battery separator that can be suitably used for lithium secondary batteries such as lithium ion secondary batteries and lithium ion polymer secondary batteries. Hereinafter, the “base material for a lithium secondary battery separator” may be referred to as a “base material”, and the “lithium secondary battery separator” may be referred to as a “separator”.

  With the recent spread of portable electronic devices and higher performance, secondary batteries having high energy density are desired. Non-aqueous secondary batteries typified by lithium secondary batteries such as lithium ion secondary batteries and lithium ion polymer secondary batteries using organic electrolytes (non-aqueous electrolytes) attract attention as batteries having high energy density. ing.

  Conventionally, a porous film is used as a separator for a lithium secondary battery. The porous film is a porous film made of polyolefin such as polyethylene and polypropylene, has low heat resistance, and has serious safety problems. That is, in the battery using such a porous film as a separator, when local heat generation occurs inside the battery due to an internal short circuit or the like, the separator around the heat generation site contracts and the internal short circuit further expands, Runaway fever may lead to serious events such as ignition and rupture. In order to solve this problem, a separator having high heat resistance is required.

  As a separator having high heat resistance, for example, a separator in which a nonwoven fabric made of glass fiber is laminated on a porous film made of polyolefin and bonded with a resin such as polyvinylidene fluoride has been proposed. (For example, refer to Patent Document 1). However, in the case of the separator of Patent Document 1, since the porous film and the glass nonwoven fabric are separately manufactured and then laminated, the separator becomes inevitably thick, and as a result, the field that can be used is limited, and the internal resistance There were problems such as poor battery characteristics.

  On the other hand, a heat-resistant separator using a nonwoven fabric instead of a porous film made of polyolefin has been proposed. For example, there are non-woven fabrics composed of polyester fibers, and non-woven fabrics in which aramid fibers, which are heat-resistant fibers, are blended with polyester fibers. However, compared to porous films, there are problems that the pore diameter is large and internal short-circuiting easily occurs. (For example, refer to Patent Documents 2 to 4).

  On the other hand, a composite separator is disclosed in which non-woven fabrics, woven fabrics, etc. are not used as separators as they are, but are used as a base material, and various materials are combined with the base material to provide heat resistance, a shutdown function, etc. ing. For example, composite separators in which a porous film is bonded to a base material, and composite separators in which a base material is impregnated and coated with filler particles, resin, gel electrolyte, solid electrolyte, etc. have been reported (for example, patents) References 5-7). However, in these documents, detailed examinations are made on composite materials such as filler particles, resins, and porous films (hereinafter, “composite materials” may be referred to as “composites”). However, no consideration has been given to the nonwoven fabric used as the substrate. The base material that has been used so far has large pores, so that the surface smoothness when combined by bonding, surface coating, impregnation, etc. is poor, and the composite product is easily peeled off or detached. there were. In addition, since the composite material is filled in the base material and the pores inside the base material are closed, the electrolyte solution retention of the separator is deteriorated and the internal resistance of the battery is increased.

  Also, a process of impregnating or applying a slurry containing filler particles, a process of impregnating or applying a slurry containing a resin, a process of laminating and integrating porous films, and impregnating or applying a solid electrolyte or gel electrolyte As a base material for a lithium secondary battery separator for producing a separator for a lithium secondary battery that is subjected to at least one composite treatment selected from the treatments to be performed, synthetic resin short fibers and fibrillated lyocell fibers (solvent-spun cellulose) A substrate for a separator for a lithium secondary battery made of a non-woven fabric containing a fiber) as an essential component is disclosed (for example, see Patent Document 8). The base material of Patent Document 8 is such that the fibrillated lyocell fiber is intertwined with the synthetic resin short fiber, so that the lithium secondary battery separator obtained by subjecting this base material to the composite treatment has a small surface variation. In addition, an effect is obtained that the composite is less likely to be peeled off or dropped off. In addition, the fibrillated lyocell fiber present on the surface of the base material for the lithium secondary battery separator is firmly bonded to the composite, thereby obtaining an effect that the composite can be further prevented from peeling and falling off. Yes.

However, it is required to further reduce the thickness of the separator in a trend where higher performance and higher energy density of lithium secondary batteries are required. For that purpose, it is necessary to make the substrate thinner. In general, the base material is thinned by applying heat calendering or calendering. However, if the base weight is kept as it is and the pressure is thinned, the voids in the base material become too small and the internal resistance of the battery decreases. May be higher. Therefore, it is necessary not only to make the substrate thin, but also to reduce the basis weight of the substrate, and a low basis weight thin film substrate having a basis weight of 10 g / m ² or less and a thickness of 15 μm or less is desired. ing. Patent Document 8 describes that the basis weight of the substrate is preferably 3.0 to 30.0 g / m ² , and the thickness of the substrate is preferably 4 to 45 μm. However, looking at the base material produced in the example of Patent Document 8, the thickness of the base material with the minimum basis weight of 8.0 g / m ² is 18 μm, and the base weight of the base material with the minimum thickness of 14 μm is The substrate of the low basic weight thin film which is 10.5 g / m < ² > and satisfy | fills the range desired in both basic weight and thickness is not obtained.

  Moreover, in the base material of low basic weight thin film, a base material may be damaged when handling a base material or when a composite treatment is performed. Furthermore, from the viewpoint of environmental sanitation, fire safety, and occupational health, it is desired that the combined treatment is an aqueous treatment. That is, as a slurry containing filler particles, a slurry containing a resin, and a coating liquid for impregnating or coating a solid electrolyte or a gel electrolyte, an aqueous slurry in which the main medium is water or an aqueous coating liquid is desired. Yes. In addition, the process of laminating and integrating the base material and the porous film can be performed by laminating and integrating by thermal lamination, but an adhesive can also be used. As the adhesive, a water-based adhesive is more preferable than an organic solvent-based adhesive. However, in the case of aqueous treatment, since the surface tension of the aqueous slurry or aqueous adhesive is high, there is a problem that the wet coating film formed on the substrate becomes non-uniform. This problem is particularly likely to occur with aqueous slurries. Therefore, in the aqueous treatment, it is preferable to apply a high tension in order to make the substrate flat. In particular, in order to perform the composite treatment at high speed, it is preferable to apply higher tension to the base material. In patent document 8, the mechanical strength of a base material is evaluated by puncture strength. Further, in Patent Document 8, the tensile strength is also measured, and the ratio (MDs / CDs) of the tensile strength (MDs) in the flow direction and the tensile strength (CDs) in the width direction of the base material is within a specific range. Although the effect of suppressing the generation of wrinkles at the time of compounding and the removal of the compounded product accompanying it is achieved, the absolute value of tensile strength has not been studied. As described above, when paying particular attention to the aqueous treatment, not only the ratio of the puncture strength and the tensile strength (MDs) in the flow direction of the substrate and the tensile strength (CDs) in the width direction, but also the absolute value of the tensile strength is increased. It is desirable to do.

  Furthermore, since the base material made of nonwoven fabric containing synthetic resin short fibers and fibrillated lyocell fibers as essential components is cellulose, the fibrillated lyocell fibers are cellulose. However, there is a problem that undulations (unevenness, undulation, etc.) are likely to occur on the substrate, and it is difficult to obtain a flat separator. In Patent Document 8, composite treatment using an organic solvent is performed, and this problem has not been studied. However, for the lithium secondary battery having a high energy density, the base weight of the base material is lowered and the thickness is reduced. When the thickness is reduced, this undulation is particularly likely to occur, and it has been difficult to obtain a thin and flat separator.

JP 2003-323878 A JP 2003-123728 A JP 2007-317675 A JP 2006-19191 A JP 2005-293891 A JP 2005-536857 A JP 2007-157723 A International Publication No. 2011/044606 Pamphlet

  An object of the present invention is to provide a fibrillated solvent spinning that is used to form a lithium secondary battery separator by being subjected to a composite treatment together with a composite such as a porous film, filler particles, a resin, a gel electrolyte, and a solid electrolyte. In the separator base material for lithium secondary batteries made of nonwoven fabric containing cellulose fiber and synthetic fiber, it is a low-basis-thin film base material, but it has low resistance and small surface variation when combined. An object of the present invention is to provide a base material that not only has a small amount of back-through of the liquid and can reduce peeling and dropping off of the composite, but also has high mechanical strength and is not easily damaged during the composite treatment. Another object of the present invention is to provide a lithium secondary battery separator base material that is less likely to cause undulations even when the composite treatment is an aqueous process. Further, by using this lithium secondary battery separator base material, And obtaining a flat separator for a lithium secondary battery.

  As a result of intensive studies to solve the above problems, the following means have been found.

(1) fibrillated Ri Do from solvent-spun cellulose fiber and nonwoven fabric containing synthetic fibers, 10-30 wt% of the full Iburiru of the solvent-spun cellulose fiber, the orientation crystallization of the average fiber diameter 2.0~3.5μm It consists of a nonwoven fabric containing 40 to 50% by mass of polyester short fibers and 30 to 40% by mass of polyester short fibers for unstretched binder having an average fiber diameter of 5.0 μm or less, and has a basis weight of 7.0 to 10.0 g / m ^2. in, and has a thickness of 10.0～15.0Myuemu, tensile strength in 450~700N / m der ruri lithium secondary battery separator base material, impregnating or coating a slurry containing filler particles Selected from a process for impregnating or applying a slurry containing a resin, a process for laminating and integrating a porous film, and a process for impregnating or applying a solid electrolyte or a gel electrolyte. A separator for a lithium secondary battery, wherein both are subjected to one composite treatment .
(2) Treatment for impregnating or applying a slurry containing filler particles to a separator base material for a lithium secondary battery, treatment for impregnating or applying a slurry containing a resin, and processing for laminating and integrating a porous film A method for producing a lithium secondary battery separator, wherein a separator for a lithium secondary battery is produced by performing at least one composite treatment selected from a treatment of impregnating or applying a solid electrolyte or a gel electrolyte,
A base material for a separator for a lithium secondary battery is composed of a nonwoven fabric containing a fibrillated solvent-spun cellulose fiber and a synthetic fiber, the fibrillated solvent-spun cellulose fiber is 10 to 30% by mass, and the average fiber diameter is 2.0 to 3 It consists of a nonwoven fabric containing 40 to 50% by mass of oriented crystallized polyester short fibers of 5 μm and 30 to 40% by mass of polyester short fibers for unstretched binder having an average fiber diameter of 5.0 μm or less, and has a basis weight of 7.0 to 7.0%. 10.0 g / m ² , a thickness of 10.0 to 15.0 μm, a tensile strength of 450 to 700 N / m,
Water is used as the main medium for the slurry containing filler particles, the slurry containing the resin, and the coating liquid for impregnating or coating the solid electrolyte or the gel electrolyte, and the substrate and the porous film are laminated and integrated. A method for producing a separator for a lithium secondary battery, wherein a water-based adhesive is used as an adhesive in the treatment.

The base material for a separator for a lithium secondary battery of the present invention is 10 to 30% by mass of fibrillated solvent-spun cellulose fiber and 40 to 50% by mass of oriented crystallized polyester short fiber having an average fiber diameter of 2.0 to 3.5 μm. %, A nonwoven fabric containing 30 to 40% by mass of polyester short fibers for unstretched binder having an average fiber diameter of 5.0 μm or less, a basis weight of 10 g / m ² or less, and a thickness of 15 μm or less. With this formulation, fibrillated solvent-spun cellulose fibers and finely oriented crystallized polyester short fibers are intertwined with each other, and the intersection of the fiber network is fixed with polyester short fibers for unstretched binder. The mechanical strength can be increased. As mechanical strength, not only puncture strength but also tensile strength can be increased. As a result, the handling property as a base material can be dramatically improved, and the base material can be prevented from being damaged when the base material is handled or combined. Moreover, although the base material for separators for lithium secondary batteries of this invention is low basic weight, it can be made thin without reducing the space | gap of a separator too much, and can reduce internal resistance. The lithium secondary battery separator of the present invention obtained by subjecting the base material for a lithium secondary battery separator of the present invention to a composite treatment such as bonding, impregnation, surface coating, etc. As the variation is reduced, the composite is less likely to be peeled off or dropped off. In addition, when the fibrillated solvent-spun cellulose fiber and the composite existing on the surface of the separator base material for a lithium secondary battery are firmly bonded, it is possible to further suppress the back-through, peeling and detachment of the coating liquid.

  Moreover, although the base material for separators for lithium secondary batteries of this invention is a low basic weight thin film, its tensile strength is high and it is hard to produce undulations. Therefore, high tension can be applied during the composite treatment. This makes it possible to perform a composite treatment on a substrate in a flat state, and the uniformity is high even in the case of an aqueous treatment in which it is difficult to obtain a uniform wet coating film if the substrate has low flatness. A wet coating film can be obtained. As a result, a thin and flat lithium secondary battery separator can be obtained.

  Hereinafter, the base material for a lithium secondary battery separator and the lithium secondary battery separator of the present invention will be described in detail.

  The base material for a lithium secondary battery separator of the present invention includes a process of impregnating or coating a slurry containing filler particles, a process of impregnating or applying a slurry containing a resin, and a porous film laminated and integrated. It is a base material for producing a separator by performing a compounding process such as a process, a process of impregnating or coating a solid electrolyte or a gel electrolyte, and a precursor sheet of a separator for a lithium secondary battery.

  The treatment of impregnating or coating the slurry containing filler particles is a preferred treatment among these composite treatments because a separator having high heat resistance is obtained, and the filler is more preferably an inorganic filler. From the viewpoint of environmental hygiene, fire safety, and occupational health, the combined treatment is preferably an aqueous treatment. The aqueous treatment is a treatment in which water is used as a main medium of a coating liquid for impregnating or coating a slurry containing filler particles, a slurry containing a resin, a solid electrolyte or a gel electrolyte. In addition, the process of laminating and integrating the base material and the porous film can be performed by laminating and integrating by thermal lamination, but an adhesive can also be used. The treatment using an aqueous adhesive instead of an organic solvent adhesive as an adhesive is also included in the aqueous treatment. Further, the treatment of impregnating or coating the slurry containing filler particles with water as the main medium is a particularly suitable composite treatment because the internal resistance of the separator becomes lower.

  The filler may be either inorganic or organic. Inorganic fillers include alumina, gibbsite, boehmite, magnesium oxide, magnesium hydroxide, silica, titanium oxide, barium titanate, zirconium oxide and other inorganic oxides, aluminum nitride and silicon nitride inorganic nitrides, aluminum compounds, zeolites And mica. Examples of the organic filler include polyethylene, polypropylene, polyacrylonitrile, polymethyl methacrylate, polyethylene oxide, polystyrene, polyvinylidene fluoride, ethylene-vinyl monomer copolymer, and polyolefin wax.

  The porous film is not particularly limited as long as it is a resin capable of forming a film, but polyolefin resins such as polyethylene resins and polypropylene resins are preferable. Polyethylene resins include not only ultra-low density polyethylene, low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, or ultra high density polyethylene alone, but also ethylene propylene copolymers. Or a mixture of a polyethylene-based resin and another polyolefin-based resin. Polypropylene resins include homopropylene (propylene homopolymer), or α-, such as propylene and ethylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene or 1-decene. Examples thereof include random copolymers with olefins and block copolymers.

The lithium secondary battery in the present invention means a lithium ion battery, a lithium ion polymer battery, or the like. 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.

  A phenomenon called “lithium dendrite” in which metallic lithium is deposited on the negative electrode surface when charging and discharging are repeated, this lithium dendrite gradually grows, penetrates the separator, reaches the positive electrode, and may cause an internal short circuit. is there. Even if the separator for lithium secondary batteries of the present invention is used for this lithium secondary battery, an effect that an internal short circuit hardly occurs is obtained.

  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 (LiPF ₆ ) and lithium tetrafluoroborate (LiBF ₄ ). Examples of the solid electrolyte include those obtained by dissolving a lithium salt in a gel polymer such as polyethylene glycol and derivatives thereof, acrylic resin, polysiloxane and derivatives thereof, and polyvinylidene fluoride. You may use the gel electrolyte which made these solid electrolytes contain an organic solvent and made it gelatinous.

  The base material for a separator for a lithium secondary battery of the present invention is a fibrillated solvent-spun cellulose fiber, an oriented crystallized polyester short fiber having an average fiber diameter of 2.0 to 3.5 μm, and an unstretched fiber having an average fiber diameter of 5.0 μm or less. Contains polyester staple fiber for binder.

  Solvent-spun cellulose fibers are different from so-called regenerated cellulose fibers in which cellulose is once chemically converted to cellulose derivatives and then returned to cellulose, as in conventional viscose rayon and copper ammonia rayon. A fiber in which cellulose is precipitated by dry and wet spinning of a spinning solution dissolved in amine oxide in water without change.

  Solvent-spun cellulose fibers are highly aligned in the fiber longitudinal direction compared to natural cellulose fibers, bacterial cellulose fibers, rayon fibers, etc., so when mechanical forces such as friction are applied in a wet state, It is easy to miniaturize, and thin and long fine fibers are produced. In order to firmly hold the electrolyte solution between the fine fibers, the fibrillated solvent-spun cellulose fiber is superior in liquid retention of the electrolyte solution compared to the refined product of natural cellulose fiber, bacterial cellulose fiber, and rayon fiber. The fibrillated fiber is not a film shape but a fiber shape having a portion finely divided mainly in a direction parallel to the fiber axis, and at least a portion thereof has a fiber diameter of 1 μm or less. The fibrillated fibers preferably have a length to width aspect ratio in the range of about 20 to about 100,000.

  In the present invention, it is preferable to use fibrillated solvent-spun cellulose fibers having a modified freeness of 80 to 130 ml. The modified drainage degree of the fibrillated solvent-spun cellulose fiber is more preferably 90 to 120 ml, and further preferably 95 to 115 ml. If the modified freeness is higher than 130 ml, when a thin film having a low basis weight is used, sufficient mechanical strength cannot be obtained, the variation in the surface becomes large, and the coating liquid is breached or the composite is peeled off or dropped off. It becomes easy. On the other hand, when the modified freeness is less than 80 ml, the average fiber length is shortened and the entanglement with the polyester fiber is lowered, so that the mechanical strength is lowered.

  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%. It is a value.

  In the case of solvent-spun cellulose fibers, as the fibrillation progresses, the fiber length becomes shorter. Especially when the sample concentration is low, the entanglement between the fibers decreases and it becomes difficult to form a fiber network. It itself slips through the holes in the sieve plate. In other words, in the case of fibrillated solvent-spun cellulose, an accurate freeness cannot be measured by the measuring method of JIS P8121. In more detail, natural cellulose fibers are in a state where many fine fibrils are torn apart from the trunk of the fiber as the degree of refinement progresses. Therefore, the fibers are easily entangled with each other through the fibrils, and a fiber network is easily formed. On the other hand, the solvent-spun cellulose fiber is easily finely divided in parallel to the long axis of the fiber by the refining treatment, and since the uniformity of the fiber diameter in each fiber after division is high, the shorter the average fiber length, It is considered that the fibers do not easily entangle with each other and it is difficult to form a fiber network. Therefore, in the present invention, in order to measure the exact freeness of the solvent-spun cellulose fiber, an 80-mesh wire mesh having a wire diameter of 0.14 mm and an opening of 0.18 mm is used as a sieve plate, and the sample concentration is 0.1%. The modified freeness measured according to JIS P8121 was used.

  A fibrillated solvent-spun cellulose fiber is prepared by a refiner, a beater, a mill, an attritor, a rotary blade homogenizer that applies shear force with a high-speed rotary blade, and a cylindrical inner blade that rotates at high speed. Double-cylindrical high-speed homogenizer that generates shearing force between the outer blades, ultrasonic crusher that is refined by impact by ultrasonic waves, and passes through a small-diameter orifice by applying a pressure difference of at least 20 MPa to the fiber suspension. And a high-pressure homogenizer that applies a shearing force and a cutting force to the fiber by causing a high speed and colliding with this to rapidly decelerate. Of these, refiners are particularly preferred.

  The length-weighted average fiber length of the fibrillated solvent-spun cellulose fiber is not particularly limited, but is preferably 0.70 to 1.25 mm, more preferably 0.80 to 1.10 mm, and further 0.90 to 1.05 mm. preferable. If the fiber length is shorter than 0.70 mm, the mechanical strength of the base material may be reduced. If the fiber length is longer than 1.25 mm, the variation in the surface may increase or the coating liquid may fall through or the composite may be peeled off or dropped off. May be more likely to occur.

The base material for a separator for a lithium secondary battery of the present invention contains 10 to 30% by mass of a fibrillated solvent-spun cellulose fiber. The content of the fibrillated solvent-spun cellulose fiber is more preferably 20 to 30% by mass. When the content of the fibrillated solvent-spun cellulose fiber is less than 10% by mass, when the low basis weight thin film is used, when the electrolytic solution has insufficient liquid retention and internal resistance increases, May occur. When the content of the fibrillated solvent-spun cellulose fiber exceeds 30% by mass, the content of the polyester short fiber is decreased, so that the mechanical strength and surface strength of the base material are lowered, and the base material is handled or combined. In some cases, the substrate may be damaged or fluffed. In addition, in the thickness adjustment by the calendar process or the heat calendar process, the fibrillated solvent-spun cellulose fills the gaps and the liquid retention property is lowered, so that the internal resistance is increased. Furthermore, when a separator is manufactured by applying an aqueous treatment to a low-basis-weight thin film substrate having a basis weight of 10 g / m ² or less and a thickness of 15 μm or less, undulations are likely to occur on the substrate surface, There arises a problem that a flat separator cannot be obtained.

The base material for a separator for a lithium secondary battery of the present invention contains 40 to 50% by mass of oriented crystallized polyester short fibers having an average fiber diameter of 2.0 to 3.5 μm. Since the adhesiveness between the oriented crystallized polyester short fiber and the polyester short fiber for unstretched binder is high, for example, even in a base material of a low basis weight thin film having a basis weight of 10 g / m ² or less and a thickness of 15 μm or less. The mechanical strength of the substrate can be increased. When the content of the oriented crystallized polyester short fiber is less than 40% by mass and exceeds 50% by mass, the mechanical strength of the base material necessary for the composite treatment becomes insufficient.

  The average fiber diameter of the oriented crystallized polyester short fibers is 2.0 to 3.5 μm. In order to increase the number of fibers per unit area and improve the surface smoothness of the substrate, the average fiber diameter of the oriented crystallized polyester short fibers is preferably as thin as possible. From the standpoint of single fiber strength, it is preferable to use melt-spun and drawn fibers, but fibers having an average fiber diameter of less than 2.0 μm are difficult to produce by melt spinning, and a large amount of inexpensive fibers are obtained. Is difficult. When the average fiber diameter exceeds 3.5 μm, the number of fibers decreases, so the mechanical strength of the substrate decreases. In the case of a low basis weight thin film base material, the maximum pore diameter of the base material is enlarged, and when the slurry containing filler particles and resin is subjected to surface coating treatment, it may be exposed.

  The base material for separators for lithium secondary batteries of this invention contains 30-40 mass% of polyester short fibers for unstretched binders having an average fiber diameter of 5.0 μm or less. When the content of the polyester short fiber for unstretched binder is less than 30% by mass, the adhesion between the fibrillated solvent-spun cellulose fiber and the polyester short fiber becomes sweet, and the fiber tends to fall off and fluff, etc. Decreases. When the content exceeds 40% by mass, the polyester short fibers for unstretched binder form a film, and the ionic conductivity is inhibited, whereby the internal resistance is increased and the discharge characteristics are decreased.

  The average fiber diameter of the polyester short fibers for unstretched binder is 5.0 μm or less. When the average fiber diameter exceeds 5.0 μm, the number of fibers per unit area is reduced, so that the adhesion between the fibrillated solvent-spun cellulose and the polyester short fibers is weakened, and the mechanical strength of the substrate is lowered. In the case of a low basis weight thin film base material, the maximum pore diameter of the base material is increased, and when the slurry containing filler particles and resin is subjected to surface coating treatment, it becomes easy to see through. On the other hand, the lower limit of the average fiber diameter is preferably 3.0 μm. When the average fiber diameter is less than 3.0 μm, the draw ratio becomes high, so that the crystallization of polyester short fibers proceeds and the adhesive strength as a binder decreases, so the number of fibers can be increased. The strength may decrease.

  In the present invention, the average fiber diameter is determined by measuring the fiber diameter in a scanning electron microscope image obtained by observing the cross section of the substrate. From the scanning electron microscope image, the fiber diameter in the thickness direction of the base material of 100 randomly selected fibers is measured, and the average value is defined as the average fiber diameter. The reason for using the fiber diameter in the thickness direction of the substrate is as follows. Generally, in a thin nonwoven fabric, the fibers are generally oriented in a direction perpendicular to the thickness direction of the substrate. On the other hand, the orientation of the fibers is indefinite on a plane perpendicular to the thickness direction of the substrate. Therefore, when the fiber diameter in a direction different from the thickness direction is measured, a fiber diameter that is significantly larger than the actual fiber diameter may be measured. Therefore, in the present invention, the fiber diameter in the thickness direction is measured.

  The fiber length of the oriented crystallized polyester short fibers and the polyester short fibers for unstretched binder is preferably 1 to 7 mm, more preferably 1.5 to 5 mm, and even more preferably 2 to 3 mm. When the fiber length is shorter than 1 mm, it may fall off from the base material. When the fiber length is longer than 7 mm, the fiber may be entangled and become lumpy, and the number of fibers decreases, so the mechanical strength of the base material decreases. There is a case.

  Examples of the resin of oriented crystallized polyester short fiber and unstretched binder polyester short fiber used in the present invention include, for example, polyethylene terephthalate resin, polybutylene terephthalate resin, polytrimethylene terephthalate resin, polyethylene naphthalate resin, polybutylene. Examples thereof include naphthalate resin, polyethylene isophthalate resin, and derivatives thereof. Among these, when used for a separator base material for a lithium secondary battery, a polyethylene terephthalate resin excellent in heat resistance and electrolyte solution resistance is preferable.

  The separator base material for a lithium secondary battery is a paper machine having one type of paper machine among paper types such as a circular net, a long net, a short net, and an inclined short net, and two or more types of same or different types. It can be manufactured by a method of making paper using a combination paper machine that combines paper making methods. In addition to the fiber raw material, a slurry, a thickener, an inorganic filler, an organic filler, an antifoaming agent, and the like can be appropriately added to the papermaking slurry, if necessary, and about 5 to 0.001% by mass. A papermaking slurry is prepared at a solid content concentration of. The paper slurry is further diluted to a predetermined concentration to make paper, and then dried.

  The base material for a lithium secondary battery separator obtained by papermaking is subjected to calendering, thermal calendering, heat treatment, and the like as necessary. Since the base material for a separator for a lithium secondary battery of the present invention uses polyester short fibers for an unstretched binder having an average fiber diameter of 5.0 μm or less, it is preferably subjected to a heat calendar treatment. In the heat calendering process, a temperature at which polyester short fibers for unstretched binder do not form a film (polyester for unstretched binder) passes between metal roll and metal roll, between metal roll and elastic roll, or between metal roll and cotton roll. It is preferable to apply pressure at a temperature 20 ° C. or more lower than the melting point of the short fiber. When the polyester short fiber for unstretched binder is formed into a film, it is not preferable because the internal resistance is increased, the discharge characteristics are lowered, and the mechanical strength of the substrate is lowered.

The basis weight of the base material for a separator for a lithium secondary battery is 10 g / m ² or less, and more preferably 10.0 g / m ² or less. The lower limit of the basis weight is preferably 5.0 g / ^{m 2,} more preferably ^{6.0g / m 2, 7.0g / m} 2 or more is more preferable. If it is less than 5.0 g / m ² , sufficient mechanical strength may not be obtained, and the substrate may be damaged when the substrate is handled or combined. When it exceeds 10 g / m ² , the thickness of the base material for a lithium secondary battery separator increases, and the internal resistance may increase or the discharge characteristics may decrease. The basis weight of the substrate of the present invention is a value measured according to JIS P8124.

  The thickness of the base material for a lithium secondary battery separator is 15 μm or less, and more preferably 15.0 μm or less. The lower limit of the thickness is preferably 6 μm, more preferably 8 μm, further preferably 10 μm, and particularly preferably 10.0 μm or more. If the thickness is less than 6 μm, sufficient mechanical strength may not be obtained, and internal resistance may increase. If it is thicker than 15 μm, the internal resistance of the lithium secondary battery becomes high and the discharge characteristics may be lowered. Moreover, since the thickness of the separator manufactured using this becomes thick, it may become impossible to respond to a high energy density. The thickness of the separator of the present invention means a value measured by a method defined in JIS B7502, that is, a value measured by an outer micrometer at a load of 5N.

  The tensile strength of the separator base material for a lithium secondary battery is preferably 450 N / m or more, and more preferably 500 N / m or more. If the tensile strength is less than 450 N / m, the substrate may be damaged during the handling or combination of the substrate. The higher the tensile strength, the better. However, when the tensile strength exceeds 700 N / m, in the case of the base material of the present invention, the polyester short fiber for unstretched binder is excessively formed, and the puncture strength Is likely to decrease, the resistance of the substrate may be increased, and the discharge characteristics may be decreased. Therefore, the tensile strength is preferably 700 N / m or less, and more preferably 650 N / m or less.

  The puncture strength of the lithium secondary battery separator base material is preferably 0.8 N or more. This puncture strength can be achieved by fibrillated solvent-spun cellulose fibers entangled with oriented crystallized polyester short fibers and bonded with polyester short fibers for unstretched binder to strengthen the bond between fibers. When the piercing strength of the substrate is less than 0.8N, internal short circuit defects are likely to occur when winding the wound material or heating and pressing the wound material even if the substrate is subjected to a composite treatment to form a separator for a lithium secondary battery. Become.

  The maximum pore diameter of the base material for a lithium secondary battery separator is preferably 8.0 μm or less. This maximum pore diameter can be achieved by blending the fibrillated solvent-spun cellulose fiber and melting the polyester fiber for unstretched binder. If the maximum pore diameter exceeds 8.0 μm, the coating liquid may be exposed to the surface or the surface smoothness may be deteriorated, and the composite may be easily peeled or dropped off, resulting in generation of fine pinholes. .

  The internal resistance of the lithium secondary battery separator correlates with the real component of the impedance measured by applying an alternating current by sandwiching the substrate infiltrated with the electrolyte with the electrode. The smaller the real number component of this impedance, the lower the internal resistance as a lithium secondary battery separator after the composite treatment is applied to the base material, and to obtain a lithium secondary battery with excellent discharge characteristics. Can do.

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

[Basis weight of base material]
The basis weight was measured in accordance with JIS P8124.

[Base material thickness]
The thickness was measured by the method defined in JIS B7502, that is, by an outer micrometer at 5N load.

Example 1
A refiner is used to treat solvent-spun cellulose fibers having an average fiber diameter of 10 μm and a fiber length of 4 mm, and 10 mass% of solvent-spun cellulose fibers having a modified freeness of 97 ml, an orientation of an average fiber diameter of 2.4 μm and a fiber length of 3 mm. 50% by mass of crystallized polyethylene terephthalate (PET) short fibers, an average fiber diameter of 4.4 μm, and a fiber length of 3 mm, 40% by mass of polyester fiber for unstretched binder are mixed together, disaggregated in water of a pulper, and an agitator ) To prepare a uniform papermaking slurry (concentration of 0.3% by mass). This papermaking slurry was layered with a slanted short net as the first layer and a circular net as the second layer, and the basis weight ratio of the slanted short net and the net was laminated at 50:50 to obtain a wet sheet, After drying at a Yankee dryer temperature of 130 ° C., a thermal calender treatment with a metal roll and an elastic roll with a surface temperature of 195 ° C. is performed, and a separator for a lithium secondary battery having a basis weight of 8.2 g / m ² and a thickness of 14.2 μm A substrate was obtained.

Example 2
The solvent-spun cellulose fiber having a modified freeness of 97 ml used in Example 1 was 20% by mass, the average fiber diameter was 2.4 μm, the oriented crystallized PET short fiber having a fiber length of 3 mm was 40% by mass, and the average fiber diameter was 4.4 μm. For a separator for a lithium secondary battery having a basis weight of 8.3 g / m ² and a thickness of 13.6 μm, in the same manner as in Example 1, except that the polyester fiber for unstretched binder having a fiber length of 3 mm is 40% by mass. A substrate was obtained.

Example 3
30 mass% of solvent-spun cellulose fibers having a modified freeness of 97 ml used in Example 1, 40 mass% of oriented crystallized PET short fibers having an average fiber diameter of 2.4 μm and a fiber length of 3 mm, and an average fiber diameter of 4.4 μm. For a separator for a lithium secondary battery having a basis weight of 8.4 g / m ² and a thickness of 13.7 μm, in the same manner as in Example 1, except that the polyester fiber for unstretched binder having a fiber length of 3 mm was 30% by mass. A substrate was obtained.

Example 4
The solvent-spun cellulose fiber having a modified freeness of 97 ml used in Example 1 was 20% by mass, the average fiber diameter was 3.2 μm, the oriented crystallized PET short fiber having a fiber length of 3 mm was 40% by mass, and the average fiber diameter was 4.4 μm. For a separator for a lithium secondary battery having a basis weight of 8.3 g / m ² and a thickness of 13.5 μm, in the same manner as in Example 1, except that the polyester fiber for unstretched binder having a fiber length of 3 mm is 40% by mass. A substrate was obtained.

Example 5
Using a refiner, solvent-spun cellulose fibers having an average fiber diameter of 10 μm and a fiber length of 4 mm are treated, and solvent-spun cellulose fibers having a modified freeness of 113 ml are 20% by mass, an average fiber diameter of 2.4 μm, and a fiber length of 3 mm. A basis weight of 8.4 g / min was obtained in the same manner as in Example 1 except that 40% by mass of the crystallized PET short fiber, 40% by mass of the unstretched binder polyester fiber having an average fiber diameter of 4.4 μm and a fiber length of 3 mm was used. A base material for a separator for a lithium secondary battery having m ² and a thickness of 13.4 μm was obtained.

Example 6
A modified solvent-spun cellulose fiber having a freeness of 97 ml was 20% by mass, an average fiber diameter of 2.4 μm, an oriented crystallized PET short fiber having a fiber length of 3 mm was 40% by mass, an average fiber diameter of 4.4 μm, and a fiber length. A base material for a separator for a lithium secondary battery having a basis weight of 7.1 g / m ² and a thickness of 11.6 μm in the same manner as in Example 1 except that 40% by mass of PET short fibers for unstretched binder of 3 mm was used. Got.

Example 7
A modified solvent-spun cellulose fiber having a freeness of 97 ml is 30% by mass, an average fiber diameter is 2.4 μm, an oriented crystallized PET short fiber having a fiber length of 3 mm is 40% by mass, an average fiber diameter is 4.4 μm, and a fiber length. A base material for a separator for a lithium secondary battery having a basis weight of 9.9 g / m ² and a thickness of 15.0 μm, in the same manner as in Example 1, except that 30% by mass of 3 mm PET short fibers for unstretched binder was used. Got.

Example 8
A modified solvent-spun cellulose fiber having a freeness of 97 ml was 20% by mass, an average fiber diameter of 2.4 μm, an oriented crystallized PET short fiber having a fiber length of 3 mm was 40% by mass, an average fiber diameter of 4.4 μm, and a fiber length. A base material for a separator for a lithium secondary battery having a basis weight of 7.0 g / m ² and a thickness of 10.1 μm in the same manner as in Example 1 except that 40% by mass of PET short fibers for 3 mm of unstretched binder was used. Got.

Example 9
A modified solvent-spun cellulose fiber having a freeness of 97 ml is 30% by mass, an average fiber diameter is 2.4 μm, an oriented crystallized PET short fiber having a fiber length of 3 mm is 40% by mass, an average fiber diameter is 4.4 μm, and a fiber length. A base material for a separator for a lithium secondary battery having a basis weight of 7.5 g / m ² and a thickness of 12.2 μm, in the same manner as in Example 1 except that 30% by mass of 3 mm PET short fibers for unstretched binder was used. Got.

Example 10
A modified solvent-spun cellulose fiber having a freeness of 97 ml is 10% by mass, an average fiber diameter is 2.4 μm, an oriented crystallized PET short fiber having a fiber length of 3 mm is 50% by mass, an average fiber diameter is 4.4 μm, and a fiber length. A base material for a separator for a lithium secondary battery having a basis weight of 9.0 g / m ² and a thickness of 15.0 μm in the same manner as in Example 1 except that 40% by mass of PET short fibers for 3 mm of unstretched binder was used. Got.

Example 11 (Reference Example)
A modified solvent-spun cellulose fiber having a freeness of 97 ml is 30% by mass, an average fiber diameter is 2.4 μm, an oriented crystallized PET short fiber having a fiber length of 3 mm is 40% by mass, an average fiber diameter is 4.4 μm, and a fiber length. A base material for a separator for a lithium secondary battery having a basis weight of 6.5 g / m ² and a thickness of 10.6 μm, in the same manner as in Example 1 except that 30% by mass of 3 mm PET short fibers for unstretched binder was used. Got.

Example 12 (Reference Example)
A modified solvent-spun cellulose fiber having a freeness of 97 ml is 30% by mass, an average fiber diameter is 2.4 μm, an oriented crystallized PET short fiber having a fiber length of 3 mm is 40% by mass, an average fiber diameter is 4.4 μm, and a fiber length. A base material for a separator for a lithium secondary battery having a basis weight of 7.2 g / m ² and a thickness of 9.8 μm, in the same manner as in Example 1 except that 30% by mass of 3 mm PET short fibers for unstretched binder was used. Got.

Example 13 (Reference Example)
A modified solvent-spun cellulose fiber having a freeness of 97 ml is 10% by mass, an average fiber diameter is 2.4 μm, an oriented crystallized PET short fiber having a fiber length of 3 mm is 50% by mass, an average fiber diameter is 4.4 μm, and a fiber length. A base material for a separator for a lithium secondary battery having a basis weight of 8.8 g / m ² and a thickness of 13.1 μm, in the same manner as in Example 1, except that 40% by mass of PET short fibers for unstretched binder of 3 mm was used. Got.

Comparative Example 1
35 mass% of solvent-spun cellulose fibers having a modified freeness of 97 ml used in Example 1, 35 mass% of oriented crystallized PET short fibers having an average fiber diameter of 2.4 μm and a fiber length of 3 mm, and an average fiber diameter of 4.4 μm. For a separator for a lithium secondary battery having a basis weight of 8.4 g / m ² and a thickness of 14.2 μm, in the same manner as in Example 1, except that the polyester fiber for unstretched binder having a fiber length of 3 mm is 30% by mass. A substrate was obtained.

Comparative Example 2
5 mass% of solvent-spun cellulose fibers having a modified freeness of 97 ml used in Example 1, 50 mass% of oriented crystallized PET short fibers having an average fiber diameter of 2.4 μm and a fiber length of 3 mm, and an average fiber diameter of 4.4 μm. For a separator for a lithium secondary battery having a basis weight of 8.2 g / m ² and a thickness of 13.5 μm, in the same manner as in Example 1 except that the polyester fiber for unstretched binder having a fiber length of 3 mm was 45% by mass. A substrate was obtained.

Comparative Example 3
30 mass% of solvent-spun cellulose fibers having a modified freeness of 97 ml used in Example 1, 30 mass% of oriented crystallized PET short fibers having an average fiber diameter of 2.4 μm and a fiber length of 3 mm, and an average fiber diameter of 4.4 μm. For a separator for a lithium secondary battery having a basis weight of 8.3 g / m ² and a thickness of 13.5 μm, in the same manner as in Example 1, except that the polyester fiber for unstretched binder having a fiber length of 3 mm is 40% by mass. A substrate was obtained.

Comparative Example 4
10 mass% of solvent-spun cellulose fibers with a modified freeness of 97 ml used in Example 1, an average fiber diameter of 2.4 μm, 60% by mass of oriented crystallized PET short fibers with a fiber length of 3 mm, and an average fiber diameter of 4.4 μm. For a separator for a lithium secondary battery having a basis weight of 8.2 g / m ² and a thickness of 13.7 μm, in the same manner as in Example 1, except that the polyester fiber for unstretched binder having a fiber length of 3 mm was 30% by mass. A substrate was obtained.

Comparative Example 5
10 mass% of solvent-spun cellulose fibers having a modified freeness of 97 ml used in Example 1, an average fiber diameter of 2.4 μm, an oriented crystallized PET short fiber of 3 mm in fiber length of 40 mass%, and an average fiber diameter of 4.4 μm. For a separator for a lithium secondary battery having a basis weight of 8.1 g / m ² and a thickness of 13.5 μm, in the same manner as in Example 1 except that the polyester fiber for unstretched binder having a fiber length of 3 mm was 50% by mass. A substrate was obtained.

Comparative Example 6
30 mass% of solvent-spun cellulose fibers having a modified freeness of 97 ml used in Example 1, 50 mass% of oriented crystallized PET short fibers having an average fiber diameter of 2.4 μm and a fiber length of 3 mm, and an average fiber diameter of 4.4 μm. For a separator for a lithium secondary battery having a basis weight of 8.2 g / m ² and a thickness of 13.2 μm, in the same manner as in Example 1, except that the polyester fiber for unstretched binder having a fiber length of 3 mm was 20% by mass. A substrate was obtained.

Comparative Example 7
The solvent-spun cellulose fiber having a modified freeness of 97 ml used in Example 1 was 20% by mass, the average fiber diameter was 5.3 μm, the oriented crystallized PET short fiber having a fiber length of 3 mm was 40% by mass, and the average fiber diameter was 4.4 μm. For a separator for a lithium secondary battery having a basis weight of 8.1 g / m ² and a thickness of 13.5 μm, in the same manner as in Example 1, except that the polyester fiber for unstretched binder having a fiber length of 3 mm was 40% by mass. A substrate was obtained.

Comparative Example 8
The solvent-spun cellulose fiber having a modified freeness of 97 ml used in Example 1 was 20% by mass, the average fiber diameter was 2.4 μm, the oriented crystallized PET short fiber having a fiber length of 3 mm was 40% by mass, and the average fiber diameter was 6.5 μm. For a separator for a lithium secondary battery having a basis weight of 8.3 g / m ² and a thickness of 13.6 μm, in the same manner as in Example 1, except that the polyester fiber for unstretched binder having a fiber length of 3 mm is 40% by mass. A substrate was obtained.

Comparative Example 9
70 mass% of solvent-spun cellulose fibers having a modified freeness of 97 ml used in Example 1, 15 mass% of oriented crystallized PET short fibers having an average fiber diameter of 2.4 μm and a fiber length of 3 mm, and an average fiber diameter of 3.2 μm. Lithium having a basis weight of 8.4 g / m ² and a thickness of 13.1 μm was obtained in the same manner as in Example 1 except that 15% by mass of oriented crystallized PET short fibers having a fiber length of 3 mm was calendered at room temperature. A base material for a secondary battery separator was obtained.

Comparative Example 10
40% by mass of oriented crystallized PET short fibers having an average fiber diameter of 2.4 μm and a fiber length of 3 mm used in Example 1, 20% by mass of oriented crystallized PET short fibers having an average fiber diameter of 3.2 μm and a fiber length of 3 mm, Lithium having a basis weight of 8.0 g / m ² and a thickness of 13.0 μm in the same manner as in Example 1 except that the polyester fiber for unstretched binder having an average fiber diameter of 4.4 μm and a fiber length of 3 mm was 40% by mass. A base material for a secondary battery separator was obtained.

Comparative Example 11
A modified solvent-spun cellulose fiber having a freeness of 97 ml is 30% by mass, an average fiber diameter is 2.4 μm, an oriented crystallized PET short fiber having a fiber length of 3 mm is 40% by mass, an average fiber diameter is 4.4 μm, and a fiber length. A base material for a separator for a lithium secondary battery having a basis weight of 10.5 g / m ² and a thickness of 15.0 μm, in the same manner as in Example 1, except that 30% by mass of 3 mm PET short fibers for unstretched binder was used. Got.

Comparative Example 12
A modified solvent-spun cellulose fiber having a freeness of 97 ml is 30% by mass, an average fiber diameter is 2.4 μm, an oriented crystallized PET short fiber having a fiber length of 3 mm is 40% by mass, an average fiber diameter is 4.4 μm, and a fiber length. A base material for a separator for a lithium secondary battery having a basis weight of 8.5 g / m ² and a thickness of 15.5 μm, in the same manner as in Example 1, except that 30% by mass of PET short fibers for unstretched binder of 3 mm was used. Got.

  Using the substrates for separators for lithium secondary batteries of Examples and Comparative Examples, the evaluation of the substrates, the production of the separators and the evaluation of the separators were performed, and the results are shown in Tables 1 to 6.

[Tensile strength of substrate]
About the produced base material, the tensile strength of the vertical direction was measured according to JISP8113 using the desktop type | mold material testing machine (Orientec Co., LTD., Brand name: STA-1150). . The size of the test piece was 250 mm in the vertical direction, 50 mm in width, the interval between the two grippers was 100 mm, and the tensile speed was 300 mm / min.

[Puncture strength of substrate]
About the produced base material, a metal needle of 1 mm in diameter with a curvature of 1.6 at the tip is placed on a desktop material testing machine (Orientec Co., Ltd., trade name: STA-1150). It was mounted and lowered until it penetrated at a constant speed of 1 mm / s perpendicular to the sample surface. The maximum load (N) at this time was measured, and the puncture strength was measured.

[Maximum pore diameter of substrate]
About the produced base material, it measured according to JIS K3832, ASTM F316-86, and ASTM E1294-89 using PMI company palm porometer (Perm-Porometer) CFP-1500A, and measured the maximum pore diameter.

[Substrate impedance]
The prepared substrate was immersed in an electrolytic solution (1M-LiPF ₆ / ethylene carbonate (EC) + diethyl carbonate (DEC) + dimethyl carbonate (DMC) (1: 1: 1, vol ratio)), and then two abbreviations. A real component of AC impedance at 200 kHz was measured using an LCR meter (Instec Corporation, apparatus name: LCR-821) sandwiched between cylindrical copper electrodes (diameter 25 mm).

[Abrasion resistance of substrate]
The produced base material was cut into a width of 20 mm and a length of 250 mm, and it was applied to a Gakushin type friction fastness tester (Color Fastness Rubbing Tester, TESTER SANGYO Co., LTD., Trade name: AB-301). After setting the test black cloth (KURABO INDUSTRIES LTD., KURABO COMBED BROAD CLOTH H-444) on the wear jig, the wear jig is loaded on the base material. The load was a minimum load of 200 gf (1.96 N), and the occurrence of raising (fluff) by rubbing 5 reciprocations was visually observed at a speed of 30 reciprocations per minute at a distance of 120 mm, and evaluation was performed according to the following criteria. If it was “Δ” or more, no problem in the process occurred when handling the substrate or during the composite treatment.

○: Fiber wrinkles due to fluff hardly adhered to the black cloth.
(Triangle | delta): The fiber wrinkles by a fluff adhered to the black cloth only slightly.
X: Fiber wrinkles due to fluff adhered to the black cloth.

[Preparation of Separator A]
Plate boehmite (average particle size: 1 μm, aspect ratio: 10), 1000 g of N-methylpyrrolidone, and 375 g of polyvinylidene fluoride are placed in a container, and a stirrer (trade name: Three one motor, Shinto Kagaku Co., Ltd.). (Shinto Scientific Co., Ltd.) for 1 hour and dispersed to obtain a uniform slurry. After coating the slurry on one side of each surface of the substrate, the slurry is passed through a gap having a predetermined interval, and then dried at 120 ° C. with an explosion-proof dryer, and the thickness per side is 3 μm. A separator A having a porous film was obtained.

[Preparation of Separator B]
Plate boehmite (average particle size: 1 μm, aspect ratio: 10), 1000 g of water, 2000 g of water, 5 g of sodium polyacrylate are placed in a container and dispersed by stirring for 1 hour with a disperser equipped with a saw blade. It was. To this, 1000 g of a 1% by mass aqueous solution of sodium carboxymethylcellulose having a viscosity of 8000 mPa · s of the 1% by mass solution was added and further stirred for 15 minutes to obtain an aqueous slurry. After surface-coating this aqueous slurry on one side of the substrate, it is passed through a gap adjusted so that the coating amount after drying is 9 g / m ^2, and then hot air at 120 ° C. is applied in a dryer. It was applied and dried to obtain separator B.

[Coating properties]
About the produced separator A, thickness measurement of arbitrary 10 places was performed, and it evaluated by the following evaluation. The thickness means a value measured by a method defined in JIS B7502, that is, a value measured by an outer micrometer at 5N load.

○: The difference in thickness is 0.8 μm or less.
(Triangle | delta): The difference of thickness exceeds 0.8 micrometer and is 1 micrometer or less.
X: The thickness difference exceeds 1 μm.

[Existence of dropout]
The produced separator A was cut into a strip of 50 mm width × 300 mm, the state of the porous film when wound around a polytetrafluoroethylene rod having a diameter of 10 mm was visually confirmed, and evaluated according to the following criteria.

○: No change in the state of the porous membrane.
(Triangle | delta): Although the crack has spread over the whole thickness of the porous film, peeling has not arisen.
X: Peeling has occurred.

[Cut off]
In the manufacturing process of the separator A, the state of the substrate when the substrate was applied was observed and evaluated according to the following criteria.

○: The substrate was not cut, torn or cracked, and could be applied without any problem.
X: Cutting, tearing, or cracking of the base material frequently occurred and hindered coating.

[Back-through]
In the manufacturing process of the separator A, the strike-through was evaluated according to the following criteria.

○: The coating liquid does not show through at all.
Δ: Slightly overlooked, but there is no problem such as sticking the back of the equipment to the roll of the coating apparatus.
X: It broke through, and the back surface of the base material adhered to the roll, causing problems such as inability to perform smooth coating.

[Forming relief]
In separator B, the formation of undulations was evaluated according to the following criteria.

○: Unevenness is not formed at all.
(Triangle | delta): Although the undulation is formed, when it pinches | interposes between glass plates, it will planarize and there will be no trouble in contact | adherence with an electrode.
X: Unevenness is formed, and even if it is sandwiched between glass plates, it is not flattened, and there is a problem in close contact with the electrode.

The base material for separators for lithium secondary batteries of Examples 1 to 13 is 40 to 30 mass% of fibrillated solvent-spun cellulose fibers and 40 oriented crystallized polyester short fibers having an average fiber diameter of 2.0 to 3.5 μm. It consists of a nonwoven fabric containing 30 to 40% by mass of polyester short fibers for unstretched binder having an average fiber diameter of 5.0 μm or less, and a basis weight of 10 g / m ² or less and a thickness of 15 μm or less. Therefore, in the base materials of Examples other than Example 11 and Example 12 other than Example 12 where the basis weight is the lowest and the thickness is the lowest, while the mechanical strength is strong, there is almost no flaking or fluffing of the fibers, even though it is a low basis weight thin film. In the separator production, the substrate was not cut or torn. In addition, the separator has excellent coatability and the adhesion between the porous film and the substrate after coating is strong, so the porous film does not fall off, and the coating liquid does not show through the screen to the extent that hinders the coating process. It was. Furthermore, there was little undulation after coating the aqueous slurry containing filler particles. Moreover, the real component of the impedance could be kept low.

The base material for separators for lithium secondary batteries of Examples 1 to 5 is a low basis weight thin film base material of 8 g / m ² and 13 to 14 μm. Since 10-30% by mass of the fibrillated solvent-spun cellulose is contained, the fibers are easily entangled with each other, so that a fiber network is easily formed. . In particular, the puncture strength of the base material was stronger than the base material composed only of the polyester short fibers of Comparative Example 10, and the base material was not cut or torn during the production of the separator. Moreover, even if the base material for separators for lithium secondary batteries of this invention is such a low basic weight thin film, it was hard to produce the relief | undulation after apply | coating the aqueous slurry containing a filler particle.

On the other hand, the base material for separators for lithium secondary batteries of Comparative Examples 1 to 10 is also a low basis weight thin film base material of 8 g / m ² and 13 to 14 μm. Comparative Example 1 and Comparative Example 2 are cases where the content of fibrillated solvent-spun cellulose fibers deviated from 10 to 30% by mass. In Comparative Example 1 exceeding 30% by mass, the tensile strength and puncture strength were lowered, and the substrate was cut and the fibers were dropped. Furthermore, the undulation after applying the aqueous slurry containing the filler particles was likely to occur. Further, in Comparative Example 2 of less than 10% by mass, the maximum pore diameter was increased, so that back-through occurred and the polyester short fibers for unstretched binder increased, and the real component of the impedance was increased. In addition, the coatability and dropout were inferior.

  Comparative Example 3 and Comparative Example 4 are cases where the content of oriented crystallized polyester short fibers deviated from 40 to 50% by mass. In Comparative Example 3 of less than 40% by mass, the tensile strength and the puncture strength were lowered, so that the substrate was cut. Further, in Comparative Example 4 exceeding 50% by mass, the maximum pore diameter was increased, so that a slight breakthrough occurred.

  Comparative Example 5 and Comparative Example 6 are cases where the content of polyester short fibers for unstretched binder deviates from 30 to 40% by mass. In Comparative Example 5 exceeding 40% by mass, the maximum pore diameter was increased, so that a back-through occurred. Moreover, the real number component of the impedance was increased, and the coatability was lowered. On the other hand, in Comparative Example 6 of less than 30% by mass, the tensile strength and the puncture strength were lowered, and therefore, the fibers dropped from the base material and the base material was cut.

  Comparative Example 7 is a case where the average fiber diameter of the oriented crystallized polyester short fibers exceeds 3.5 μm, but since the piercing strength is reduced and the maximum pore diameter is enlarged, slight penetration occurs, Decreased.

  Comparative Example 8 is a case where the average fiber diameter of the polyester short fibers for unstretched binder exceeds 5.0 μm, but the number of fibers of the polyester short fibers for unstretched binder is decreased, so that the tensile strength and puncture strength are decreased. However, since the maximum pore diameter was also increased, the coatability was lowered, the fibers were detached from the base material, and the base material was cut or breached.

  Comparative Example 9 was a composition that did not contain polyester short fibers for unstretched binders, but the tensile strength and puncture strength were extremely low, the fibers dropped off from the substrate, and the substrate was frequently cut. Furthermore, the undulation after applying the aqueous slurry containing the filler particles was likely to occur.

  Comparative Example 10 was a formulation that did not contain fibrillated solvent-spun cellulose fibers, but the maximum pore diameter was large and show-through occurred. Moreover, coating property fell and the porous film fell off.

  From the comparison of Examples 1 to 3, as the blended amount of fibrillated solvent-spun cellulose increases, the tensile strength and puncture strength decrease somewhat, but cutting of the base material, dropping of fibers from the base material and fluffing occur. There wasn't. Also, the coating property was good, the porous film did not fall off, and the real component of impedance was reduced.

  From the comparison between Example 2 and Example 4, the separator for lithium secondary battery of Example 4 in which the oriented crystallized polyester short fibers were slightly thickened had an enlarged maximum pore diameter. Dropout did not occur. From the comparison between Example 2 and Example 5, in the base material of Example 5 in which fibrillation of solvent-spun cellulose was suppressed, even if the modified freeness was 113 ml, no show-through occurred and the coating property was improved. There was no problem, and the porous membrane did not fall off.

When Examples 1-13 and Comparative Examples 11-12 are compared, in Example 11 whose basic weight is less than 7.0 g / m < ² >, compared with Examples 1-10, tensile strength and puncture strength fall. A trend was seen and the substrate was cut. In Comparative Example 11 in which the basis weight exceeded 10.0 g / m ² , the real number component of the impedance increased as compared with Examples 1 to 10. When this base material is used, it is estimated that the discharge characteristic of a battery falls. In Example 12 having a thickness of less than 10.0 μm, the real number component of the impedance increased as compared with Examples 1 to 10. When this base material is used, it is estimated that the discharge performance of a battery falls. In addition, the tensile strength and the puncture strength were lowered, and the substrate was cut. In Comparative Example 12 having a thickness exceeding 15.0 μm, the fusion of the polyester short fibers for unstretched binder became sweeter than in Examples 1 to 10, and the tensile strength was slightly reduced and the puncture strength was also reduced. Cutting and fiber fluffing occurred. In Examples 11 and 12 having a tensile strength of less than 450 N / m, the substrate was cut as compared with Examples 1 to 10. In Example 13 in which the tensile strength was greater than 700 N / m, compared to Examples 1 to 10, since the calender pressure of the thermal calendar process was increased, the puncture strength was decreased and the real component of the impedance was increased. . When this base material is used, it is estimated that discharge performance falls. From these results, it is preferable that the basis weight is 7.0 to 10.0 g / m ² , the thickness is 10.0 to 15.0 μm, and the tensile strength is 450 N / m or more, It can be seen that the tensile strength is more preferably 700 N / m or less.

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

Claims (2)

Fibrillated Ri Do from solvent-spun cellulose fiber and nonwoven fabric containing synthetic fibers, 10-30 wt% of the full Iburiru of the solvent-spun cellulose fibers, oriented crystallized polyester having an average fiber diameter 2.0~3.5μm short fibers 40 to 50% by mass, a nonwoven fabric containing 30 to 40% by mass of polyester short fibers for unstretched binder having an average fiber diameter of 5.0 μm or less, the basis weight is 7.0 to 10.0 g / m ² , and has a thickness of 10.0～15.0Myuemu, the tensile strength 450~700N / m der ruri lithium secondary battery separator base material, a process for impregnating or coating a slurry containing the filler particles, At least one selected from a treatment of impregnating or applying a slurry containing a resin, a treatment of laminating and integrating porous films, and a treatment of impregnating or applying a solid electrolyte or gel electrolyte A separator for a lithium secondary battery, characterized by being subjected to two composite treatments . A treatment for impregnating or coating a slurry containing filler particles on a separator substrate for a lithium secondary battery, a treatment for impregnating or applying a slurry containing a resin, a treatment for laminating and integrating a porous film, a solid electrolyte Or a method for producing a lithium secondary battery separator, wherein a separator for a lithium secondary battery is produced by performing at least one composite treatment selected from a treatment of impregnating or applying a gel electrolyte,
A base material for a separator for a lithium secondary battery is composed of a nonwoven fabric containing a fibrillated solvent-spun cellulose fiber and a synthetic fiber, the fibrillated solvent-spun cellulose fiber is 10 to 30% by mass, and the average fiber diameter is 2.0 to 3 It consists of a nonwoven fabric containing 40 to 50% by mass of oriented crystallized polyester short fibers of 5 μm and 30 to 40% by mass of polyester short fibers for unstretched binder having an average fiber diameter of 5.0 μm or less, and the basis weight is 7.0 to 7.0%. 10.0 g / m ² And a thickness of 10.0 to 15.0 μm, a tensile strength of 450 to 700 N / m,
Water is used as the main medium for the slurry containing filler particles, the slurry containing the resin, and the coating liquid for impregnating or coating the solid electrolyte or the gel electrolyte, and the substrate and the porous film are laminated and integrated. A method for producing a separator for a lithium secondary battery, wherein a water-based adhesive is used as an adhesive in the treatment.