JP4185748B2 - Nonwoven fabric and separator for lithium ion secondary battery - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to a nonwoven fabric and a separator for a lithium ion secondary battery.
[0002]
[Prior art]
  Nonwoven fabrics have various properties such as porosity, separability, filterability, shape retention, stretchability, and concealment, and are therefore applied to various uses.
[0003]
  For example, the applicant of the present application includes a high melting point polypropylene having a melting point of 166 ° C. or higher and a polymer having a melting point lower than that of the high melting point polypropylene, and the low melting point polymer constitutes at least a part of the fiber and has a fiber diameter of 5 μm or less. A non-woven fabric having excellent texture containing fibers was proposed (Japanese Patent Laid-Open No. 2000-160432). However, this nonwoven fabric is difficult to obtain sufficient mechanical strength.
[0004]
  In addition, when this nonwoven fabric is used as a separator for a lithium ion secondary battery, the separator is required to be electrically insulating, resistant to electrolytic solution, and thin, and at the same time due to an external short circuit of the battery, etc. A function to block the ion permeability by blocking the opening of the separator with heat in order to prevent the generation of flammable gas, rupture or ignition of the battery when the battery temperature rises significantly when an abnormal large current flows (Shutdown function) is also required, but the nonwoven fabric has an insufficient shutdown function, so it has been difficult to use as a separator for a lithium ion secondary battery.
[0005]
[Patent Document 1]
  JP 2000-160432 A
[0006]
[Problems to be solved by the invention]
  The present invention solves the problems as described above, and provides a nonwoven fabric excellent in mechanical strength containing ultrafine fibers, and a separator for a lithium ion secondary battery excellent in mechanical strength and shutdown performance. With the goal.
[0007]
[Means for Solving the Problems]
  The nonwoven fabric of the present invention has a “fiber diameter of 4 μm or less, andPolyethylene resin and polypropylene resin or polymethylpentene resinThe value (σ / d) obtained by dividing the standard deviation value (σ) of the fiber diameter distribution of the ultrafine fibers by the average fiber diameter (d) of the ultrafine fibers is 0.2. And saidPolyethylene resinThe mass percentage with respect to the whole ultrafine fiber exceeds 75% and is 95% or less, andPolyethylene resinIs a nonwoven fabric characterized by being fused. Since the nonwoven fabric of the present invention is mainly composed of ultrafine fibers having a fiber diameter of 4 μm or less, it can be a nonwoven fabric excellent in texture even if it is thin.Polyethylene resinSince the proportion of occupancy is high and the content of ultrafine fibers in the nonwoven fabric is also large,Polyethylene resinThe non-woven fabric having excellent mechanical strength can be obtained by fusing.
[0008]
  In addition, when this nonwoven fabric is used as a separator for a lithium ion secondary battery, even if there is abnormal heat generation due to overcurrent, the low melting point resin component can melt and block the micropores between the fibers, Since the shape of the nonwoven fabric can be held by the high melting point resin component having a higher melting point than the low melting point resin component, it has both a shutdown function and a shape holding property.
[0009]
  If the ultrafine fiber is not substantially fibrillated, it can be a thin nonwoven fabric with excellent texture. In addition, when used as a separator for a lithium ion secondary battery, even if it is thin, a short circuit does not easily occur, and the electrolyte can be uniformly distributed and held.
[0010]
  The ultrafine fiberMade of polyethylene resin and polypropylene resin or polymethylpentene resin,Because of its excellent chemical resistance, it is a non-woven fabric that can be applied to various applications. For example, even if it is used as a separator for a lithium ion secondary battery, it is not eroded by the electrolytic solution, so that it can exhibit a separating action for a long time.
[0011]
  The ultrafine fiber is made of polyethylene resin and polypropylene resin or polymethylpentene resin,Since it can be fused at a relatively low melting point, it is advantageous in terms of energy. In addition, when used as a separator for a lithium ion secondary battery, even if there is abnormal heat generation due to overcurrent, the polyethylene resin can be melted at a relatively low temperature to close the micropores between the fibers. Therefore, the shutdown function is excellent.
[0012]
  Although the nonwoven fabric of the present invention is mainly composed of ultrafine fibers having a fiber diameter of 4 μm or less, it is preferable that the tensile strength in at least one direction is excellent in mechanical strength of 50 N / 5 cm width or more.
[0013]
  Since the separator for lithium ion secondary batteries of the present invention is composed of the above-mentioned nonwoven fabric, it has both a shutdown function and shape retention.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
  The fibers constituting the nonwoven fabric of the present invention are mainly composed of ultrafine fibers having a fiber diameter of 4 μm or less so that the texture is excellent even if thin. When the fiber diameter of the ultrafine fiber exceeds 4 μm, it is difficult to form a thin nonwoven fabric excellent in texture. Therefore, the ultrafine fiber is preferably mainly 3 μm or less, more preferably 2 μm or less. Note that the lower limit of the fiber diameter of the ultrafine fiber is not particularly limited as long as it has a mechanical strength suitable for the intended use of the nonwoven fabric.
[0015]
  The “fiber diameter” in the present invention refers to the diameter when the cross-sectional shape of the fiber is circular, and when the cross-sectional shape of the fiber is non-circular, the diameter of the circle having the same cross-sectional area is the fiber. The diameter.
[0016]
  It is preferable that the fiber diameters between the ultrafine fibers are substantially the same so that the ultrafine fibers can form a uniform pore diameter. That is, the value (σ / d) obtained by dividing the standard deviation value (σ) of the fiber diameter distribution of the ultrafine fiber by the average fiber diameter (d) of the ultrafine fiber is 0.2 or less (preferably 0.18 or less). Is preferred. In addition, since the standard deviation value (σ) is 0 when the fiber diameters of all the ultrafine fibers are the same, the lower limit value of the value (σ / d) is 0. This “average fiber diameter (d)” was taken from an electron micrograph of a nonwoven fabric, and the fiber diameters of 100 or more (n) ultrafine fibers in this electron micrograph were measured, and the measured fiber diameters were averaged. Value. The “standard deviation value (σ)” of the ultrafine fiber is a value calculated from the following equation based on the measured fiber diameter (χ).
  Standard deviation = {(nΣχ²-(Σχ)²) / N (n-1)}^1/2
  Here, n means the number of measured ultrafine fibers, and χ means the fiber diameter of each ultrafine fiber.
[0017]
  In addition, when two or more ultrafine fiber groups exist, it is preferable that the said relationship is materialized about each ultrafine fiber group.
[0018]
  Further, it is preferable that the individual ultrafine fibers have substantially the same diameter in the fiber axis direction so that the ultrafine fibers can form a nonwoven fabric having a uniform pore diameter.
[0019]
  Such ultrafine fibers having substantially the same fiber diameter between the ultrafine fibers, or individual ultrafine fibers having substantially the same diameter in the fiber axis direction are, for example, regulated in the sea component at the spinneret portion. Then, the island component can be formed by removing the sea component of the sea-island fiber obtained by the composite spinning method in which the island component is extruded and combined. In addition, the method of removing the sea component of the sea-island fiber obtained by the method of spinning after mixing the resin constituting the island component and the resin constituting the sea component, which is generally referred to as a mixed spinning method, or by the melt blow method It is difficult to obtain ultrafine fibers having the same fiber diameter between the ultrafine fibers or individual ultrafine fibers having substantially the same diameter in the fiber axis direction.
[0020]
  The ultrafine fiber of the present invention comprises two types of resin components having different melting points, so that a low melting point resin component having a lower melting point is fused, and a high melting point resin component having a higher melting point than the low melting point resin component is used. A nonwoven fabric having excellent mechanical strength can be obtained due to the shape maintaining action. The melting point difference between the low melting point resin component and the high melting point resin component is such that the high melting point resin component is melted by the heat that is applied when the low melting point resin component is fused, and the mechanical strength of the nonwoven fabric is not lowered. It is preferably 10 ° C or higher, more preferably 20 ° C or higher.
[0021]
  The “melting point” in the present invention refers to a temperature that gives a maximum value of a melting endothermic curve obtained by using a differential scanning calorimeter at a temperature rising temperature of 10 ° C./min and raising the temperature from room temperature. When there are two or more maximum values, the highest temperature maximum value is taken as the melting point.
[0022]
  Although resin which comprises the ultrafine fiber of this invention is not specifically limited, For example, it can comprise from polyolefin-type resin, polyamide-type resin, or polyester-type resin. Among these, it is preferable that the ultrafine fiber is made of only a polyolefin resin because it is excellent in chemical resistance and can be applied to various applications. For example, even if it is used as a separator for a lithium ion secondary battery, it is not eroded by the electrolytic solution, so that it can exhibit a separating action for a long time.
[0023]
  As the ultrafine fiber made of only this polyolefin resin, which is suitable, for example, the high melting point resin component is made of polypropylene resin (for example, polypropylene, propylene copolymer, etc.), and the low melting point resin component is made of polyethylene resin (for example, Ultrafine fiber made of ultra high molecular weight polyethylene, high density polyethylene, medium density polyethylene, low density polyethylene, linear low density polyethylene, ethylene copolymer, etc.) or high melting point resin component is polymethylpentene resin (for example, poly Methylpentene, methylpentene copolymer, etc.), the low melting point resin component is made of polypropylene resin, or the high melting point resin component is made of high density polyethylene resin, and the low melting point resin component is linear low density polyethylene. Mention may be made of ultrafine fibers made of resin. Among these, the ultra-fine fiber in which the high melting point resin component is made of polypropylene resin or polymethylpentene resin and the low melting point resin component is made of polyethylene resin can be fused at a relatively low temperature. Especially when used as a separator for lithium ion secondary batteries, even if abnormal heat is generated due to overcurrent, the polyethylene resin melts at a relatively low temperature and closes micropores between fibers. It is a suitable combination because of its excellent shutdown function.
[0024]
  Such a low melting point resin component can be fused, and when used as a separator for a lithium ion secondary battery, the surface of the ultrafine fiber can be easily dissolved to close the micropores between the fibers. It usually occupies at least a part.
[0025]
  In the ultrafine fiber of the present invention, it is important that the mass percentage of the low melting point resin component having a lower melting point of the ultrafine fiber with respect to the entire ultrafine fiber is more than 75% and not more than 95%. If it is 75% or less, there is a tendency that it is difficult to improve the mechanical strength of the nonwoven fabric because the amount of the resin component that can participate in fusion is small, and when used as a separator for a lithium ion secondary battery, it melts due to heat generation. This is because the amount of the resin component is small, and it tends to be difficult to close the pores between the fibers, and when it exceeds 95%, the amount of the high melting point resin component becomes too small and the shape retention as a nonwoven fabric is reduced. This is because it tends to be difficult to maintain. More preferably, it is 77% to 93%, and still more preferably 79% to 91%.
[0026]
  It is preferable that the ultrafine fiber of the present invention is not substantially fibrillated. It is because it is excellent in texture and can be made thinner. Therefore, it is preferable that the nonwoven fabric of this invention is comprised only from the fiber which is not fibrillated. In particular, when used as a separator for a lithium ion secondary battery, an ultrafine fiber is substantially formed so that a short circuit does not easily occur even if it is thin, and an electrolyte can be uniformly distributed and held. It is preferable that it is not fibrillated, and it is preferable that it is composed only of fibers that are not fibrillated.
[0027]
  The term “substantially not fibrillated” means that a plurality of ultrafine fibers are not already combined ultrafine fibers. For example, fibers in a state where innumerable fibers branch from one fiber (for example, mechanical fibers). That can be divided in a splitting manner by using a beater or the like, or a fiber in which a plurality of fibers are already bonded and in a network state (for example, a fiber obtained by flash spinning). means.
[0028]
  The cross-sectional shape of the ultrafine fiber of the present invention is preferably circular so that the texture of the nonwoven fabric can be improved. Moreover, it is preferable that the ultrafine fibers are not in a bundle state but in a dispersed state in the nonwoven fabric so that the texture of the nonwoven fabric is excellent.
[0029]
  The ultrafine fiber of the present invention is preferably in a stretched state so that the nonwoven fabric is excellent in mechanical strength. This “stretched state” means that the film is stretched by a stretching process different from the spinning process (for example, a stretching process by a stretching yarn threading machine). The fiber formed by blowing hot air is not in the drawn state because the spinning process and the drawing process are the same. In addition, when generating ultrafine fibers from sea-island fibers, it is not necessary to go through a stretching process after generating ultrafine fibers. If sea-island fibers go through a stretching process before generating ultrafine fibers, ultrafine fibers Is in a stretched state.
[0030]
  Such ultrafine fibers are mainly composed of ultrafine fibers (50 mass% or more) so that they can be nonwoven fabrics having excellent fusion strength and excellent mechanical strength due to their large content. It is preferable to contain 60 mass% or more in the nonwoven fabric constituting fiber, more preferably 70 mass% or more, more preferably 80 mass% or more, and more preferably 90 mass% or more. Preferably, it consists of 100 mass% extra fine fiber.
[0031]
  Such an ultrafine fiber of the present invention is a base for extruding an island component when a sea-island type fiber is spun by a conventional compound spinning method. For example, the resin component is a core-sheath type, an eccentric type, or a sea-island type. When using what can be placed or spinning sea-island fiber by the conventional compound spinning method, the sea-island fiber is supplied to the die that extrudes the island component with a mixture of high-melting resin component and low-melting resin component. Can be obtained by spinning and removing sea components.
[0032]
  The nonwoven fabric of the present invention is mainly composed of the ultrafine fibers as described above, but may contain fibers other than the ultrafine fibers as long as the uniform dispersibility of the ultrafine fibers is not impaired. Examples of fibers other than ultrafine fibers include, for example, (1) thick fibers having a fiber diameter exceeding 4 μm, (2) fibers having a fiber diameter of 4 μm or less, and comprising only one kind of resin component, and (3) fiber diameter of 4 μm. (4) Fibers having a fiber diameter of 4 μm or less and 2 resin components, but having a mass percentage of 75% or less of the low-melting-point resin component based on the total ultrafine fibers (5) Although the fiber diameter is 4 μm or less and consists of two types of resin components, the mass percentage of the low melting point resin component with respect to the entire ultrafine fiber is more than 95% and less than 100%.
[0033]
  The resin constituting the fiber other than the ultrafine fiber is not particularly limited, but can be composed of the same resin as the ultrafine fiber (for example, polyolefin resin, polyamide resin, or polyester resin). For the same reason, it is preferably composed of only a polyolefin resin. The preferred polyolefin-based resin can be the same resin as the ultrafine fiber, and preferably contains a polyethylene-based resin having a relatively low melting point.
[0034]
  The fiber length of the nonwoven fabric constituting fiber of the present invention is not particularly limited. However, the shorter the fiber length, the higher the degree of freedom of the fibers, and the more uniformly dispersible, so the fiber length is 0.5 to 30 mm. And is more preferably 1 to 20 mm. Moreover, it is preferable that it is the cut | disconnected fiber. “Fiber length” refers to the length obtained by JIS L 1015 (chemical fiber staple test method) B method (corrected staple diagram method).
[0035]
  The nonwoven fabric of the present invention has excellent mechanical strength even though the texture is excellent and thin because the low melting point resin component of the ultrafine fiber as described above is fused. Such fusion can be carried out using, for example, a Yankee dryer, a can dryer, a hot air circulation dryer, and the like, with a temperature higher than the softening temperature of the low melting point resin component and lower than the melting point of the high melting point resin component. it can. The “softening temperature” refers to a temperature that gives a starting point of a melting endothermic curve obtained by using a differential calorimeter and raising the temperature from room temperature at a temperature rising rate of 10 ° C./min.
[0036]
  The nonwoven fabric of the present invention preferably has a tensile strength in at least one direction of 50 N / 5 cm width or more, more preferably 60 N / 5 cm width or more, and 75 N / 5 cm width so as to have excellent mechanical strength. The above is more preferable. The tensile strength may be a value in any direction, but in general, the length direction of the nonwoven fabric, that is, the production (flow) direction of the nonwoven fabric preferably satisfies the above value. “Tensile strength” is a non-woven fabric sample cut to a width of 5 cm and fixed between chucks (distance between chucks: 10 cm) of a tensile strength tester (Orientec, Tensilon UTM-III-100). It refers to the force required to pull the nonwoven fabric sample in the longitudinal direction at 300 mm / min and break the nonwoven fabric sample.
[0037]
  The nonwoven fabric of the present invention can be thin because it is mainly composed of the ultrafine fibers as described above. More specifically, the thickness can be 3 μm to 100 μm. In particular, when the nonwoven fabric is used as a separator for a lithium ion secondary battery, the thickness is preferably 4 μm to 60 μm. In addition, this "thickness" means the thickness measured with the outside micrometer (0-25 mm) prescribed | regulated to JISB7502: 1994.
[0038]
  Although the fabric weight of the nonwoven fabric of this invention is not specifically limited, By containing the above ultrafine fibers, it can be excellent in texture even with a low fabric weight. More specifically, the basis weight is 4 g / m.²~ 60g / m²Can be. The “weight per unit area” means the basis weight obtained based on the method specified in JIS P 8124 (paper and paperboard—basis weight measurement method).
[0039]
  The texture of the nonwoven fabric of the present invention is excellent, but “texture index” can be cited as an index of the texture of the nonwoven fabric. If the value of this formation index is within 0.15, the formation is excellent. A more preferable formation index is 0.10 or less. The “geometric index” is a value obtained by the method disclosed in Japanese Patent Laid-Open No. 2001-50902. That is, the value obtained as follows.
(1) Light is applied to the nonwoven fabric from the light source, and the reflected light reflected in a predetermined region of the nonwoven fabric is received by the light receiving element, and the luminance information is acquired.
(2) A predetermined region of the nonwoven fabric is equally divided into image sizes of 3 mm square, 6 mm square, 12 mm square, and 24 mm square to obtain four divided patterns.
(3) The luminance value of each section equally divided for each obtained division pattern is calculated based on the luminance information.
(4) Based on the luminance value of each section, the average luminance (X) for each division pattern is calculated.
(5) A standard deviation (σ) for each division pattern is obtained.
(6) The coefficient of variation (CV) for each division pattern is calculated by the following equation.
  Coefficient of variation (CV) = (σ / X) × 100
  Here, σ indicates a standard deviation for each divided pattern, and X indicates a luminance average for each divided pattern.
(7) A coordinate group obtained as a result of taking the logarithm of each image size as the X coordinate and the coefficient of variation corresponding to the image size as the Y coordinate is regressed to a linear line by the least square method, and the inclination is calculated. The absolute value of is the formation index.
[0040]
  It is preferable that the nonwoven fabric of the present invention is substantially fixed only by fusion of fibers (particularly, a low melting point resin component of ultrafine fibers). This is because the texture is excellent by being fixed only by fiber fusion. For example, when fibers are fixed by entanglement other than fusion, penetration from the front surface to the back surface of the nonwoven fabric is caused by an action (for example, fluid flow such as water flow, needle, etc.) to entangle the fibers. Holes tend to be formed and the texture tends to deteriorate. However, if the fibers are fixed only by fusion, the fiber arrangement is not disturbed, and the texture is excellent.
[0041]
  In addition, when manufacturing a nonwoven fabric, fibers may get entangled. For example, when a fiber web is formed by a card machine or a fiber web is formed by a wet method, the fiber web can be maintained to some extent, so that the fibers are more or less intertwined. However, since this entanglement does not disturb the texture of the nonwoven fabric, it can be considered that it is not substantially entangled. Thus, “substantially only fiber fusion” means that the fibers are fixed only by fusion after the fiber web is formed. From another viewpoint, this state is a state in which the fibers constituting the nonwoven fabric are substantially two-dimensionally arranged.
[0042]
  The nonwoven fabric of the present invention preferably has a single layer structure. This is because not only the thickness of the nonwoven fabric can be reduced but also the production of the nonwoven fabric can be simplified and manufactured at a low cost. Even when a single-layered nonwoven fabric is used as a separator for a lithium ion secondary battery, a sufficient amount of low-melting point resin component is present, so that it has sufficient shutdown performance.
[0043]
  Although the nonwoven fabric of the present invention is thin, it has excellent texture and excellent mechanical strength, and therefore can be applied to various applications. For example, it can be used for separators for lithium ion secondary batteries, separators or supports for lithium polymer batteries, separators for alkaline batteries, separators for electric double layer capacitors, wiping materials, printing substrates, degreasing papers, and the like. . In particular, when used as a separator for a lithium ion secondary battery, a separator for a lithium polymer battery, or a support, it has an excellent effect of being excellent in shutdown performance.
[0044]
  Such a nonwoven fabric of this invention can be manufactured as follows, for example.
[0045]
  First, the ultrafine fibers as described above are prepared. If necessary, prepare fibers other than ultrafine fibers.
[0046]
  Next, a fiber web is formed from the prepared fibers (particularly ultrafine fibers). The method for forming the fiber web is not particularly limited, but can be formed by a dry method (for example, a card method, an air lay method, a spun bond method, a melt blow method, etc.) or a wet method. Among these, it is preferable to form by a wet method having excellent uniform dispersibility of fibers (particularly ultrafine fibers). This wet method can be formed by a conventionally known method, for example, a horizontal long net method, an inclined wire type short net method, a circular net method, or a long net / circular net combination method.
[0047]
  Subsequently, the nonwoven fabric is manufactured by fusing the low melting point resin component of the ultrafine fibers constituting the fiber web. The nonwoven fabric of the present invention is preferably fixed only by fusion. This is because if the fixing is performed only by fusion, the arrangement of the fibers is not disturbed, and it is easy to manufacture a nonwoven fabric with excellent texture.
[0048]
  The fusion of the low melting point resin component of the ultrafine fibers constituting the fiber web may be performed under no pressure, may be performed under pressure, or after the low melting point resin component is melted under no pressure. Pressurization (preferably immediately pressurization) may be performed. In any case, it is preferable that the heating temperature be a temperature equal to or higher than the softening temperature of the low melting point resin component and a temperature lower than the melting point of the high melting point resin component. Moreover, the pressure in the case of pressurizing should just be a pressure which can form the nonwoven fabric which has required mechanical strength, and is not specifically limited. This pressure can be appropriately set by repeating the experiment.
[0049]
  The nonwoven fabric of the present invention can be produced, for example, as described above, but the nonwoven fabric having a formation index of 0.15 or less may be one that is not fibrillated as an ultrafine fiber constituting the nonwoven fabric, or has a fiber length. Manufactured by using these together, such as using ultra-fine fibers with a length of about 1 to 20 mm, forming a fiber web by a wet method, or fixing the fibers together by fusion (no entanglement treatment). can do.
[0050]
  The nonwoven fabric having a tensile strength in at least one direction of the present invention of 50 N / 5 cm width or more adjusts the moving speed of the net for making up the fiber and the slurry flow rate, and the fiber orientation is in a state close to one direction, It can be produced by using these in combination, such as increasing the degree of fusion or increasing the fiber length.
[0051]
  EXAMPLES Hereinafter, the present invention will be specifically described by way of examples, but these do not limit the scope of the present invention.
[0052]
【Example】
  Example 1
  Sea-island composite fiber obtained by a composite spinning method (25 fineness: 1.65 dtex, cut), in which 25 island components in a state where polypropylene and high-density polyethylene are mixed are present in the sea component composed of poly-L-lactic acid. Fiber length: 2 mm) was prepared.
[0053]
  Next, this sea-island type composite fiber was immersed in a bath composed of a 10 mass% sodium hydroxide aqueous solution at a temperature of 80 ° C. for 30 minutes to remove poly-L-lactic acid, which is a sea component of the sea-island type fiber, and high density polyethylene. Polypropylene-high density polyethylene mixed ultrafine fiber occupying a part of the fiber surface (fiber diameter: 2 μm, ρ / d: 0.083, mass percentage of the entire high density polyethylene ultrafine fiber: 80%, melting point of polypropylene: 172 ° C. The melting point of the high-density polyethylene was 135 ° C., the cut fiber length was 2 mm, the fiber was not fibrillated, the stretched state, the cross-sectional shape: a circle, and substantially the same diameter in the fiber axis direction.
[0054]
  Subsequently, 100% of this polypropylene-high density polyethylene mixed ultrafine fiber was used, and a fiber web was formed from the dispersed slurry by a wet method (horizontal long net method). In this fiber web, the polypropylene-high density polyethylene mixed ultrafine fibers were not in a bundle but in a dispersed state.
[0055]
  Next, this fiber web is supplied to a hot air circulation dryer set at a temperature of 128 ° C., dried and fused with the high density polyethylene component of the polypropylene-high density polyethylene mixed ultrafine fiber, and then a roll at a temperature of 60 ° C. The thickness is adjusted with a press machine, and the basis weight is 10 g / m.²And a non-woven fabric (tensile strength in the length direction: 60 N / 5 cm width, formation index: 0.06) having a thickness of 25 μm and consisting essentially of a single layer structure and having fibers arranged substantially two-dimensionally. .
[0056]
  The nonwoven fabric was allowed to stand for 3 minutes in a hot air oven set at a temperature of 135 ° C. and then observed with an electron microscope. As a result, pores between the fibers were blocked (see FIG. 1). Therefore, it was found that the shutdown function works effectively when used as a separator for a lithium ion secondary battery.
[0057]
  (Comparative Example 1)
  Polypropylene-high density polyethylene mixed ultrafine fiber in which high density polyethylene occupies a part of the fiber surface (fiber diameter: 2 μm, ρ / d: 0.083, mass percentage of the entire high density polyethylene ultrafine fiber: 65%, melting point of polypropylene 172 ° C., melting point of high-density polyethylene: 135 ° C., cut fiber length of 2 mm, non-fibrillated, stretched state, cross-sectional shape: circular, having substantially the same diameter in the fiber axis direction) Except for the above, the basis weight was 10 g / m in the same manner as in Example 1.²And a non-woven fabric (tensile strength in the length direction: 45 N / 5 cm width, formation index: 0.06) having a thickness of 25 μm and a substantially single layer structure in which the fibers are substantially two-dimensionally arranged. .
[0058]
  The nonwoven fabric was allowed to stand for 3 minutes in a hot air oven set at a temperature of 135 ° C. and then observed with an electron microscope. As a result, a portion where pores between fibers were not blocked remained (see FIG. 2). Therefore, it was found that the shutdown function is weak when used as a separator for a lithium ion secondary battery.
[0059]
  (Comparative Example 2)
  High-density polyethylene microfiber (fiber diameter: 2 μm, ρ / d: 0.083, mass percentage of high-density polyethylene to the entire ultrafine fiber: 100%, melting point of high-density polyethylene: 135 ° C., cut fiber length of 2 mm, fibrillated No stretched state, cross-sectional shape: circular, having substantially the same diameter in the fiber axis direction) except that 100% was used, and the basis weight was 10 g / m.²A non-woven fabric having a thickness of 25 μm and a substantially single layer structure in which fibers are substantially two-dimensionally arranged (tensile strength in the length direction: 75 N / 5 cm width, formation index: 0.06) was produced. .
[0060]
  When this nonwoven fabric was allowed to stand for 3 minutes in a hot air oven set at a temperature of 135 ° C., it contracted and the nonwoven fabric form could not be maintained.
[0061]
  (Comparative Example 3)
  The basis weight is 10g / m²A polyethylene meltblown nonwoven fabric (tensile strength in the length direction: 15 N / 5 cm width, formation index: 1.2) having a single layer structure and a thickness of 25 μm was produced.
[0062]
  When this nonwoven fabric was allowed to stand for 3 minutes in a hot air oven set at a temperature of 135 ° C., it contracted and the nonwoven fabric form could not be maintained. In addition, the strength is weak, it is difficult to withstand the tension when the battery is configured, and there is a risk of causing a short circuit due to cutting or the like.
[0063]
【The invention's effect】
  Even if the nonwoven fabric of the present invention is thin, it can be a nonwoven fabric with a uniform texture, and it is a nonwoven fabric having excellent mechanical strength.
[0064]
  The separator for a lithium ion secondary battery of the present invention has both a shutdown function and shape retention.
[Brief description of the drawings]
1 is an electron micrograph after leaving the nonwoven fabric of Example 1 for 3 minutes in a hot air oven set at a temperature of 135 ° C. FIG.
FIG. 2 is an electron micrograph after the nonwoven fabric of Comparative Example 1 is left in a hot air oven set at a temperature of 135 ° C. for 3 minutes.

Claims (4)

Non- woven fabric mainly composed of ultrafine fibers composed of polyethylene resin and polypropylene resin or polymethylpentene resin , and having a standard deviation value (σ) of the fiber diameter distribution of the ultrafine fibers. The value (σ / d) divided by the average fiber diameter (d) is 0.2 or less, the mass percentage of the polyethylene resin to the whole ultrafine fiber is more than 75% and 95% or less, and the polyethylene A non-woven fabric characterized in that a base resin is fused.   The nonwoven fabric according to claim 1, wherein the ultrafine fibers are not substantially fibrillated. The nonwoven fabric according to claim 1 or 2 , wherein a tensile strength in at least one direction is 50 N / 5 cm width or more. The separator for lithium ion secondary batteries which consists of a nonwoven fabric in any one of Claims 1-3 .

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