JP6088166B2 - Base material for lithium ion secondary battery separator and lithium ion secondary battery separator - Google Patents

Description

  The present invention relates to a base material for a lithium ion secondary battery separator and a lithium ion secondary battery separator that can be suitably used for lithium ion secondary batteries such as lithium ion secondary batteries and lithium ion polymer secondary batteries.

  With the recent spread of portable electronic devices and higher performance, secondary batteries having high energy density are desired. As this type of battery, a lithium ion secondary battery using an organic electrolyte (non-aqueous electrolyte) has attracted attention. This lithium ion secondary battery has an energy density of about 3.7 V, which is about three times that of an alkaline secondary battery, which is a conventional secondary battery, and thus has a high energy density. Since a water-based electrolyte cannot be used, a non-aqueous electrolyte having sufficient oxidation-reduction resistance is used. Since non-aqueous electrolytes are flammable, there is a risk of ignition and the like, and careful attention is paid to safety in their use. There are several possible cases of exposure to fire and other hazards, but overcharging is particularly dangerous.

  In order to prevent overcharging, current non-aqueous secondary batteries are charged at a constant voltage and a constant current, and the battery is equipped with a precise IC (protection circuit). The cost required for this protection circuit is large, and it is a factor that increases the cost of non-aqueous secondary batteries.

  When overcharging is prevented by the protection circuit, it is naturally assumed that the protection circuit does not operate well, and it is difficult to say that it is intrinsically safe. The current non-aqueous secondary battery has a safety circuit / PTC element equipped and a separator with a thermal fuse function for the purpose of destroying the battery safely when overcharged. Has been made. However, even if equipped with the above-mentioned means, depending on the overcharge conditions, the safety during overcharge is not guaranteed, and in fact, non-aqueous secondary battery ignition accidents Is still happening.

  As the separator, a film-like porous film made of a polyolefin such as polyethylene is often used. When the temperature inside the battery reaches around 130 ° C., it melts and closes the micropores, thereby transferring lithium ions. Although there is a thermal fuse function (shutdown function) that cuts off the current and shuts off the current, it is suggested that if the temperature further increases due to some situation, the polyolefin itself melts and short-circuits, causing a thermal runaway. In view of this, a heat-resistant separator that does not melt and shrink even at temperatures close to 200 ° C. has been developed.

  As heat-resistant separators, 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-based fibers. References 1-3). On the other hand, an example in which various composite treatments are performed on a substrate such as a nonwoven fabric or a woven fabric to form a lithium ion secondary battery separator has also been reported. For example, a film-like porous film made of polyolefin is laminated on a nonwoven fabric substrate made of polyester fiber to form a composite, the filler particles are contained in the nonwoven fabric or woven fabric substrate, and the resin surface Examples have been reported in which heat resistance is imparted by performing a composite treatment by coating (see, for example, Patent Documents 4 to 6). However, the non-woven fabric used as the base material has large pores and low surface smoothness, so the surface variation when combined by surface coating is large, and composites such as filler particles and resins There was a quality problem such as being likely to drop off.

  In order to improve the denseness of the base material, the average fiber diameter of the synthetic resin short fibers constituting the base material is reduced, and a base material containing fibers having a specific fiber diameter and fiber length has been reported (for example, (See Patent Documents 7 and 8). However, depending on the conditions for producing the base material, the dispersibility of the fibers may deteriorate, and the uniformity of the base material may be impaired. Moreover, the base material which mix | blended the fibrillated fiber as a fiber which comprises a base material, and improved the denseness more is also reported (for example, refer patent documents 9-11). However, further strength improvement of the base material is desired.

JP 2003-123728 A JP 2007-317675 A JP 2006-19191 A JP 2005-293891 A JP 2005-536857 A JP 2007-157723 A JP 2009-230975 A JP 2011-82148 A JP 2012-3873 A JP 2011-187346 A International Publication No. 2011/46066 Pamphlet

  The subject of this invention is the base material for lithium ion secondary battery separators which was excellent in the workability at the time of manufacture, and was excellent in intensity | strength, uniformity, and the handleability in a post process, and this base material for lithium ion secondary battery separators It is providing the lithium ion secondary battery separator which uses this.

(1) In a base material for a lithium ion secondary battery separator to be a lithium ion secondary battery separator by providing a coating layer containing inorganic particles on at least one surface, a nonwoven fabric made of synthetic resin short fibers Yes, the average fiber diameter of the whole nonwoven fabric is 5.5 μm or less, and the average fiber diameter of the fibers constituting the surface of one side of the nonwoven fabric is different from the average fiber diameter of the fibers constituting the surface of the opposite side, The ratio R = D (W) / D (T) of the average fiber diameter D (W) of the surface having a large average fiber diameter and the average fiber diameter D (T) of the surface having a small average fiber diameter is 1.10 or more . A base material for a lithium ion secondary battery separator, wherein the base material is 58 or less ,
(2) The ratio R = D (W) / D (T) of the average fiber diameter D (W) of the surface having a large average fiber diameter and the average fiber diameter D (T) of the surface having a small average fiber diameter is 1.30. The base material for a lithium ion secondary battery separator according to (1), which is ˜2.50,
(3) The base material for a lithium ion secondary battery separator according to the above (1) or (2), comprising a synthetic resin ultrashort fiber having a fiber length of 1 mm or less,
(4) A lithium ion secondary battery in which a coating layer containing inorganic particles is provided on at least one surface of the lithium ion secondary battery separator substrate according to any one of (1) to (3). Separator,
I found.

  The lithium ion secondary battery separator base materials (1) to (3) of the present invention are provided with a coating layer containing inorganic particles on at least one surface, whereby the lithium ion secondary battery separator (4) and Become. A non-woven fabric composed of synthetic resin short fibers, the average fiber diameter of the entire non-woven fabric is 5.5 μm or less, and the average fiber diameter of the fibers constituting one surface of the non-woven fabric and the fibers constituting the surface of the opposite surface The average fiber diameter is different, and the ratio R = D (W) / D (T) of the average fiber diameter D (W) of the surface having a large average fiber diameter and the average fiber diameter D (T) of the surface having a small average fiber diameter is 1. The base material for lithium ion secondary battery separator (1) of the present invention which is 1.10 or more is a coating layer containing inorganic particles on the nonwoven fabric surface as compared with a conventional base material for lithium ion secondary battery separator When it is provided, the workability is good, and the strength, uniformity, and handleability in the subsequent process are excellent. Further, the ratio R = D (W) / D (T) of the average fiber diameter D (W) of the surface having a large average fiber diameter and the average fiber diameter D (T) of the surface having a small average fiber diameter is 1.30-2. The base material (2) for a lithium ion secondary battery separator of the present invention that is .50 has better workability when providing a coating layer containing inorganic particles on the surface of the nonwoven fabric, strength, uniformity, and post-process It is more excellent in handling property.

  The base material (3) for a lithium ion secondary battery separator of the present invention includes a synthetic resin ultrashort fiber having a fiber length of 1 mm or less, thereby providing a coating layer containing inorganic particles on the nonwoven fabric surface. Furthermore, the workability, strength, uniformity, and handleability in subsequent processes are further improved.

  Hereinafter, the base material for lithium ion secondary battery separators of the present invention (hereinafter sometimes abbreviated as “base material”) will be described in detail. The base material of the present invention is a nonwoven fabric composed of synthetic resin short fibers, the average fiber diameter of the entire nonwoven fabric is 5.5 μm or less, and is opposite to the average fiber diameter of the fibers constituting the surface of one surface of the nonwoven fabric. The ratio of the average fiber diameter D (W) of the surface having a large average fiber diameter to the average fiber diameter D (T) of the surface having a small average fiber diameter is different from the average fiber diameter of the fibers constituting the surface R = D ( W) / D (T) is 1.10 or more.

The average fiber diameter of the fibers in the present invention was measured according to the following procedure.
1) A micrograph of the surface of the nonwoven fabric used for the base material for the lithium ion secondary battery separator of the present invention is taken at a magnification of 1000 times.
2) In the above photograph, a line is drawn in a direction (width direction, CD) perpendicular to the flow direction (machine direction, MD) of the nonwoven fabric, and the fiber diameter is measured with respect to the fiber that intersects the drawn line.
3) For the fibers to be measured, when the fibers intersect the line perpendicularly or obliquely, the fiber width in the direction orthogonal to the fiber axis is measured.
4) At least the fiber diameters of 50 or more fibers were measured and arithmetically averaged by measuring 1) to 3) for a plurality of micrographs.

  In the present invention, the average fiber diameter is measured for both surfaces of the nonwoven fabric, and the average value of the average fiber diameters on both surfaces is defined as the “average fiber diameter of the entire nonwoven fabric”.

  When a coating layer containing inorganic particles is provided on at least one surface of the substrate of the present invention, the coating solution containing inorganic particles needs to appropriately penetrate into the nonwoven fabric. When the coating liquid penetrates into the nonwoven fabric, the binding force at the interface between the coating layer and the nonwoven fabric becomes strong, and the strength of the coating layer becomes strong. Moreover, the intensity | strength of a base material improves because a coating liquid osmose | permeates moderately in the space | gap in a nonwoven fabric. However, when the average fiber diameter of the entire nonwoven fabric used for the base material for the lithium ion secondary battery separator of the present invention exceeds 5.5 μm, the density at which the fibers cross within the nonwoven fabric is reduced, and the strength is hardly exhibited. Become. Moreover, the coating liquid easily penetrates in the thickness direction of the nonwoven fabric, and the coating liquid penetrates the opposite surface to which the coating liquid is applied. As a result, the coating machine is soiled when the coating liquid is applied, the workability is remarkably deteriorated, and the penetrating coating liquid impairs the uniformity of the base material for the lithium ion secondary battery separator. The average fiber diameter of the whole nonwoven fabric used as the substrate of the present invention is 5.5 μm or less, more preferably in the range of 3.4 to 5.0 μm, still more preferably 3.9 to 5. The range is 0 μm.

  The nonwoven fabric used as the base material of the present invention is different in average fiber diameter of the fibers constituting the surface of the surface opposite to the average fiber diameter of the fibers constituting the surface of one surface, the average of the surface having a large average fiber diameter The ratio R = D (W) / D (T) of the fiber diameter D (W) and the average fiber diameter D (T) of the surface having a small average fiber diameter is 1.10 or more, and the ratio R is 1.30-2. .50 is more preferable.

  When the ratio R of the average fiber diameter D (W) of the surface having a large average fiber diameter and the average fiber diameter D (T) of the surface having a small average fiber diameter is less than 1.10, a coating layer containing inorganic particles When the coating is provided, the penetration of the coating liquid into the nonwoven fabric surface is difficult to proceed, and the liquid adhesion may be deteriorated, and the coating layer may not be uniformly adhered in the surface direction, and the uniformity may be impaired.

  The ratio R = D (W) / D (T) of the average fiber diameter D (W) of the surface having a large average fiber diameter of the substrate of the present invention and the average fiber diameter D (T) of the surface having a small average fiber diameter is: A range of 1.30 to 2.50 is more preferable. When the ratio R is 1.30 or more, the effect of improving the uniformity of the application of the coating liquid to the base material when the coating layer containing inorganic particles is provided becomes more prominent. However, when the ratio R exceeds 2.50, there is a difference in strength in the thickness direction between the surface having a large average fiber diameter and the surface having a small average fiber diameter, and there is unevenness in the surface direction of the base material for the lithium ion secondary battery separator. It may occur or curl may occur.

  Ratio R = D (W) / D of the average fiber diameter D (W) of the surface having a large average fiber diameter and the average fiber diameter D (T) of the surface having a small average fiber diameter used as the base material of the present invention. (T) can be adjusted by making a slurry of fibers with different average fiber diameters by making a nonwoven fabric using a multilayer papermaking method in which two or more layers of multi-layer webs are superimposed, and adjusting the thermal calendar conditions of the substrate. This can be achieved.

  Further, by adding the synthetic resin ultrashort fiber having a fiber length of 1 mm or less (hereinafter sometimes abbreviated as “synthetic resin ultrashort fiber”) to the nonwoven fabric used as the base material of the present invention, The uniformity of the base material for a lithium ion secondary battery separator of the invention is improved. In particular, an excess of fibers having a small fiber diameter at the time of manufacturing the nonwoven fabric can be obtained by blending with a layer constituting the surface of the nonwoven fabric having a small average fiber diameter (hereinafter sometimes referred to as “layer having a small average fiber diameter”). The entanglement can be suppressed and a non-woven fabric excellent in uniformity can be provided, and the coating layer containing inorganic particles is a layer constituting the surface having a large average fiber diameter of the non-woven fabric (hereinafter referred to as “average fiber diameter”). In some cases, it is possible to prevent penetration of the coating liquid.

  In the base material of the present invention, the content of the synthetic resin ultrashort fiber is preferably 1 to 30% by mass, more preferably 5 to 20% by mass, based on the entire nonwoven fabric. If it is less than 1% by mass, the denseness and uniformity may not be changed as compared with the case where the synthetic resin ultrashort fibers are not contained. Although it may exceed 30% by mass, the improvement effect is saturated when it exceeds 30% by mass.

The basis weight of the base material of the present invention is preferably 6.0 to 30.0 g / m ² . If it exceeds 30.0 g / m ² , the base material alone occupies the majority of the separator, and it becomes difficult to obtain the effect of the composite. If it is less than 6.0 g / m ² , it will be difficult to obtain uniformity, and there will be a tendency for large variations to occur on the composite surface. More preferably, it is 8.0-20.0 g / m < ² >. The basis weight means a basis weight based on a method defined in JIS P 8124 (paper and paperboard—basis weight measurement method).

In the nonwoven fabric used as the base material of the present invention, when a layer having a large average fiber diameter and a layer having a small average fiber diameter are produced by a multilayer papermaking method, the weight ratio of each layer can be appropriately adjusted. it can. The weight per layer is preferably adjusted so as to be at least 3 g / m ² or more, more preferably 5 g / m ² or more, from the viewpoint of production efficiency and quality during the production of the nonwoven fabric.

  The synthetic resin short fibers may be heat-bonded short fibers (short fibers for binders) or non-heat-bonded short fibers. When used as a heat-sealing short fiber, a core-sheath type, an eccentric type, a side-by-side type, a sea-island type, an orange type, a multiple bimetal type composite fiber, or a single component type can be mentioned, but it is said that uniformity is obtained. In particular, the single component type is preferable.

  Synthetic resin short fibers and resins constituting the synthetic resin ultrashort fibers include polyolefin resins, polyester resins, polyvinyl acetate resins, ethylene-vinyl acetate copolymer resins, polyamide resins, acrylic resins, and polychlorinated resins. Vinyl resin, polyvinylidene chloride resin, polyvinyl ether resin, polyvinyl ketone resin, polyether resin, polyvinyl alcohol resin, diene resin, polyurethane resin, phenol resin, melamine resin, furan resin, Examples include urea resins, aniline resins, unsaturated polyester resins, alkyd resins, fluorine resins, and silicone resins. Of these, polyester resins, acrylic resins, and polyolefin resins are preferable from the viewpoint of strength characteristics. In particular, a polyester resin is preferable.

  Examples of the polyester resin include polyethylene terephthalate, polybutylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, and polyethylene isophthalate. These may be used alone or in combination of two or more. Among these, when used for a base material for a lithium ion secondary battery separator, a polyethylene terephthalate resin having excellent heat resistance is preferable.

  In the base material of the present invention, as a method for producing a nonwoven fabric, a method in which a fiber web is formed and the fibers in the fiber web are bonded, fused, and entangled can be used. The obtained nonwoven fabric may be used as it is or may be used as a laminate comprising a plurality of sheets. Examples of the method for producing the fiber web include a dry method such as a card method and an air array method, a wet method such as a papermaking method, a spunbond method, and a melt blow method. Among these, the web obtained by a wet method is homogeneous and dense, and can be suitably used as a base material for a lithium ion secondary battery separator. The wet method is a method in which fibers are dispersed in water to form a uniform papermaking slurry, and this papermaking slurry is obtained using a papermaking machine having at least one of a wire such as a circular net, a long net, and an inclined type to obtain a fiber web. is there.

  As a method for producing a nonwoven fabric from a fibrous web, a hydroentanglement method, a needle punch method, a binder adhesion method, or the like can be used. In particular, when the wet method is used with emphasis on uniformity, it is preferable to bond the heat-bonded short fibers by performing a binder bonding method. A uniform nonwoven fabric is formed from a uniform web by the binder bonding method. It is preferable to apply pressure to the wet nonwoven fabric produced in this way with a calender to adjust the thickness or make the thickness uniform. However, it is preferable to apply pressure at a temperature at which the heat-bonded short fibers do not form a film (a temperature lower by 20 ° C. or more than the melting point of the heat-bonded short fibers).

  In the base material for a lithium ion secondary battery separator of the present invention, the fiber length of the synthetic resin short fibers other than the synthetic resin ultrashort fibers is preferably more than 2 mm and not more than 7 mm, more preferably 3 to 6 mm, and more preferably 3 to 5 mm. Is more preferable. When the fiber length exceeds 7 mm, there is a problem that dispersion of fibers in the production of the nonwoven fabric may be difficult due to the balance with the average fiber diameter, resulting in poor formation and the like, making it impossible to form a good fiber web. May occur. On the other hand, when the fiber length of the synthetic resin short fiber is 2 mm or less, the strength necessary for the base material may not be exhibited.

  In the base material for a lithium ion secondary battery separator of the present invention, the fineness of the synthetic resin short fiber is preferably 0.007 to 1.3 dtex, more preferably 0.02 to 1.1 dtex, and 0.04 to 0.8 dtex. Is more preferable. When the fineness of the synthetic resin short fibers exceeds 1.3 dtex, the number of fibers in the thickness direction decreases, and thus the required denseness of the separator base material may not be ensured. When the fineness of the synthetic resin short fiber is less than 0.007 dtex, stable production of the fiber becomes difficult.

  The lithium ion secondary battery separator of the present invention is formed by providing a coating layer containing inorganic particles on at least one surface of the base material for a lithium ion secondary battery separator of the present invention. The coating layer provided on the substrate contains inorganic particles, and optionally contains a binder resin. As the inorganic particles, alumina such as α-alumina, β-alumina and γ-alumina, alumina hydrate such as boehmite, magnesium oxide, calcium oxide and the like can be used. Among these, α-alumina or alumina hydrate is preferably used in terms of high stability to the electrolyte used in the lithium ion battery. As the binder resin, various synthetic resins such as styrene-butadiene resin, acrylic ester resin, methacrylic ester resin, and fluororesin such as polyvinylidene fluoride can be used.

  In the present invention, the coating liquid used to form the coating layer includes, in addition to the inorganic particles and the binder resin, various dispersants such as polyacrylic acid and sodium carboxymethyl cellulose, hydroxyethyl cellulose, sodium carboxymethyl cellulose, Various thickeners such as polyethylene oxide, various additives such as various wetting agents, preservatives and antifoaming agents can be blended as necessary. Among these additives, agents such as thickeners and wetting agents can be suitably used for adjusting the degree of penetration of the coating liquid in the present invention.

  In the present invention, there is no particular limitation on the coating method when the coating layer is provided on the substrate, for example, conventionally known air doctor coater, blade coater, knife coater, rod coater, squeeze coater, impregnation coater, gravure Examples thereof include a coater, a kiss roll coater, a die coater, a reverse roll coater, a transfer roll coater, and a spray coater.

In the present invention, the coating amount of the coating layer containing inorganic particles provided on the substrate is preferably 1.0 to 20.0 g / m ^{2, and} more preferably 4.0 to 15.0 g / m ^2. Is more preferable. When the adhesion amount of the coating layer is less than 1.0 g / m ² , the surface of the nonwoven fabric cannot be sufficiently covered, the pore diameter becomes large, a short circuit occurs, etc., and good battery characteristics do not appear. There is. On the other hand, when the adhesion amount of the coating layer exceeds 20.0 g / m ² , it may be difficult to reduce the thickness of the separator.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to a present Example. All parts and percentages in the examples are based on mass unless otherwise specified.

<Manufacture of non-woven fabric>
The nonwoven fabrics used as the base materials for the lithium ion secondary battery separators of Examples 1 to 18 and Comparative Examples 1 to 3 were prepared by preparing a fiber-mixed slurry shown in Table 1, and making a two-layer paper made of a circular net and an inclined wire. A wet method is used to create a non-woven fabric having a basis weight of 12 g / m ² and a width of 50 cm by bonding a polyethylene terephthalate (PET) short fiber for binder with a cylinder dryer at 120 ° C. Produced. Next, a calendar process was performed at a roll temperature of 180 ° C. to produce a nonwoven fabric having a basis weight of 12 g / m ² and a thickness of 17 μm.

<Manufacture of lithium ion secondary battery separator>
100 parts of boehmite having a volume average particle diameter of 2.3 μm and a specific surface area of 3 m ² / g are sufficiently mixed with 120 parts of a 0.3% aqueous solution of carboxymethylcellulose sodium salt having a viscosity of 200 mPa · s at 25 ° C. Then, 300 parts of a 0.5% aqueous solution of carboxymethyl cellulose sodium salt having a viscosity of 7000 mPa · s at 25 ° C. of the 1% by mass aqueous solution and carboxy-modified styrene having a glass transition point of 5 ° C. and a volume average particle size of 0.2 μm A coating solution was prepared by mixing and stirring 10 parts of a butadiene resin (SBR) emulsion (solid content concentration 50%). Using a gravure coater, this coating solution was applied to the surface of the nonwoven fabric described in Table 2 and dried so that the dry coating amount was 15 g / m ^2, and Examples 1 to 18 and Comparative Examples 1 to 3 lithium ion secondary battery separators were produced. In the column “Coating surface”, “Large” indicates that the surface is coated with a large average fiber diameter, and “Small” indicates that the surface is coated with a small average fiber diameter. “Same” indicates that the average fiber diameters on both sides of the nonwoven fabric are the same. In the case of “same”, the type of the paper machine on the side provided with the coating layer was also described. “Both” indicates that a coating layer of 7.5 g / m ² per side was applied to both sides of the substrate.

<Evaluation>
The following evaluation was performed about the base material for lithium ion secondary battery separators and lithium ion secondary battery separator which were obtained by the Example and the comparative example, and the result was shown in Table 2.

Strength:
The separators of the examples and comparative examples were cut into a strip shape having a width of 50 mm. The test piece was mounted on a 40 mmφ fixed frame installed on a tabletop material testing machine (trade name: STA-1150, manufactured by Orientec Co., Ltd.), and the tip was rounded (curvature: 1.6) with a diameter of 1.0 mm. The metal needle (manufactured by Orientec Co., Ltd.) was lowered at a constant speed of 50 mm / min. The maximum load (g) at this time was measured and used as the puncture strength. Measure puncture strength at 5 or more locations for one sample, and the minimum puncture strength among all measured values is excellent in strength if it is 50 g or more, practically usable if it is 30 g or more and less than 50 g, less than 30 g If so, it is considered weak in strength.

Coating layer strength:
The separators of the examples and comparative examples were folded with a crease so that the coating layer was on the inside, and then folded back and expanded, and the appearance of cracks in the coating layer at the crease portion was visually evaluated by the following index Went. Practically, it was judged that it could be used if it was “3” or more.
“5”: No damage is observed in the coating layer at the crease portion.
“4”: Small cracks are observed in several places in the crease portion.
“3”: Cracks are observed in several places in the crease portion.
“2”: Many cracks are observed in the crease portion.
“1”: The coating layer is peeled off from the nonwoven fabric surface around the crease.

Processability:
In the examples and comparative examples, the working efficiency when coating the coating liquid on the surface of the nonwoven fabric used as the substrate using a gravure coater was evaluated by the following index as workability. Practically, it was judged that it could be used if it was “3” or more.
“5”: The coating liquid does not penetrate the nonwoven fabric, and the coating roll of the coater is not soiled.
“4”: The coating liquid does not penetrate the nonwoven fabric, but sometimes it is necessary to clean the coating roll of the coater.
“3”: The coating solution sometimes penetrates the nonwoven fabric. Operation is possible while washing the coater's backing roll.
“2”: The coating liquid penetrates part of the nonwoven fabric, and the coating roll of the coater partially becomes white. Occasionally stop and roll cleaning is required.
“1”: The coating liquid penetrates through the nonwoven fabric, the coating roll of the coater becomes white, and the operability is considerably poor. Stop frequently and need roll cleaning.

Uniformity:
The uniformity of the separators of the examples and comparative examples was visually evaluated with respect to the separator having an area of 150 mm width and 1000 mm length by the following index. Practically, it was judged that it could be used if it was “3” or more.
“4”: Both sides of the separator are excellent in uniformity.
“3”: Very small protrusions and irregularities are seen on both sides of the separator.
“2”: Small protrusions and irregularities are seen on both sides of the separator.
“1”: Clear protrusions and irregularities are seen on both sides of the separator.

Handleability:
The handling properties of the separators of the examples and comparative examples were evaluated by visualizing the curl degrees of the four corners of the separator by using the following indices, with an A4 size substrate placed on a horizontal table.
Practically, it was judged that it could be used if it was “2” or more.
“4”: No separator curl.
“3”: The separator is slightly curled, but no curling correction is required.
“2”: The separator is curled, but can be used if curl correction is performed.
“1”: The separator is rounded, and curling correction is difficult.

  By comparing Example 1 and Comparative Example 1, when the average fiber diameter of the whole nonwoven fabric exceeds 5.5 μm, the strength of the separator also decreases, and when providing a coating layer containing inorganic particles, It can be seen that as a result of the coating liquid penetrating the base material too early, the coating liquid penetrates the base material, and the uniformity of the lithium ion secondary battery separator is impaired. Moreover, it turns out that the workability at the time of coating is low, and the working efficiency of a coating process is reduced.

  By comparing Example 1 with Comparative Examples 2 and 3, the average fiber diameter of the whole nonwoven fabric is 5.5 μm or less, and the average fiber diameter of the fibers constituting the surface of one surface of the nonwoven fabric is the opposite surface. The ratio of the average fiber diameter D (W) of the surface having a large average fiber diameter to the average fiber diameter D (T) of the surface having a small average fiber diameter is R = D (W) / When D (T) is 1.10 or more, when a coating layer containing inorganic particles is provided, the application of the coating liquid is improved, and the penetration of the coating liquid into the substrate is uniform. It can be seen that the uniformity of the separator is also improved.

  The lithium ion secondary battery separator obtained in Comparative Example 2 has the same average fiber diameter of fibers constituting the surface of one surface of the nonwoven fabric as that of the fibers constituting the surface of the opposite surface, There is no difference in the average fiber diameter in the thickness direction of the nonwoven fabric. As a result, when providing a coating layer containing inorganic particles, it is difficult for the coating liquid to penetrate the nonwoven fabric surface, the coating layer strength is low, and the uniformity of the coating layer is low. It can be seen that the uniformity is reduced compared to the examples.

  The lithium ion secondary battery separator obtained in Comparative Example 3 is different in the average fiber diameter of the fibers constituting the surface of one side of the nonwoven fabric and the average fiber diameter of the fibers constituting the surface of the opposite side. The ratio R = D (W) / D (T) of the average fiber diameter D (W) of the surface having a large fiber diameter and the average fiber diameter D (T) of the surface having a small average fiber diameter is less than 1.10, It can be seen that the penetration of the coating solution into the nonwoven fabric surface is low, and the coating layer strength is also reduced as compared to the examples.

  By comparing Examples 1-2, 9 and Examples 3-8, the ratio of the average fiber diameter D (W) of the surface having a large average fiber diameter to the average fiber diameter D (T) of the surface having a small average fiber diameter It can be seen that, when R is 1.3 to 2.5, it is possible to provide a base material for a lithium ion secondary battery separator that is more excellent in strength, coating layer strength, and working efficiency of the coating process.

  When Example 5 is compared with Examples 10 to 12, in the base material having the same average fiber diameter of the entire nonwoven fabric, the ratio R is increased by increasing the average fiber diameter D (W) of the surface having a large average fiber diameter. The ratio can be adjusted by adjusting the average fiber diameter D (T) of the surface having a small average fiber diameter while keeping the average fiber diameter D (W) of the surface having a large average fiber diameter constant. It is possible to adjust R. In any method, the average fiber diameter of the entire nonwoven fabric is 5.5 μm or less, and the ratio R of the average fiber diameter D (W) of the surface having a large average fiber diameter and the average fiber diameter D (T) of the surface having a small average fiber diameter is R. By adjusting so that = D (W) / D (T) is 1.10 or more, it is possible to provide a separator excellent in strength, uniformity, and handleability in subsequent steps.

  From the comparison between Example 5 and Examples 13 and 14, and the comparison between Example 7 and Examples 15 and 16, the strength was improved by adding a synthetic resin ultrashort fiber having a fiber length of 1 mm or less. As a result of improving the denseness of the layer having a small fiber diameter, when providing a coating layer containing inorganic particles, penetration of the coating liquid can be suppressed, and workability during coating can be improved. . Moreover, the uniformity of the whole nonwoven fabric improved, and the handleability also improved.

  From the comparison between Example 7 and Example 17, the average fiber diameter of the whole nonwoven fabric is 5.5 μm or less, and the average fiber diameter of the fibers constituting the surface of one surface of the nonwoven fabric and the surface of the opposite surface are configured. The ratio of the average fiber diameter D (W) of the surface having a large average fiber diameter and the average fiber diameter D (T) of the surface having a small average fiber diameter is R = D (W) / D (T). However, even if a coating layer containing inorganic particles is provided on the surface having a large average fiber diameter of the substrate of 1.10 or more, the coating containing inorganic particles on the surface having a small average fiber diameter is provided. It can be seen that a base material for a lithium ion secondary battery separator excellent in strength, coating layer strength, working efficiency of the coating process, and appearance can be provided even if a working layer is provided. In addition, it is better to provide a coating layer containing inorganic particles on the surface with a large average fiber diameter. The lithium ion secondary battery is more excellent in strength, coating layer strength, coating process work efficiency, and appearance. A substrate for a separator can be provided.

  From the comparison between Example 7 and Example 18, by providing the coating layers on both surfaces of the base material, a lithium ion secondary battery separator that is superior in strength, coating layer strength, working efficiency of the coating process, and appearance. It can be seen that a base material can be provided.

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

Claims (4)

In the base material for a lithium ion secondary battery separator to be a lithium ion secondary battery separator by providing a coating layer containing inorganic particles on at least one surface, the nonwoven fabric is a nonwoven fabric made of synthetic resin short fibers. The overall average fiber diameter is 5.5 μm or less, the average fiber diameter of the fibers constituting the surface of one side of the nonwoven fabric is different from the average fiber diameter of the fibers constituting the surface of the opposite side, and the average fiber diameter The ratio R = D (W) / D (T) of the average fiber diameter D (W) of the large surface and the average fiber diameter D (T) of the small average fiber surface is 1.10 or more and 2.58 or less . A base material for a lithium ion secondary battery separator.   The ratio R = D (W) / D (T) of the average fiber diameter D (W) of the surface having a large average fiber diameter and the average fiber diameter D (T) of the surface having a small average fiber diameter is 1.30-2. The base material for a lithium ion secondary battery separator according to claim 1, wherein the base material is 50.   The base material for lithium ion secondary battery separators according to claim 1 or 2, comprising a synthetic resin ultrashort fiber having a fiber length of 1 mm or less.   The lithium ion secondary battery separator which provides the coating layer containing an inorganic type particle | grain on the at least one surface of the base material for lithium ion secondary battery separators in any one of Claims 1-3.