JP5274081B2 - Polyolefin microporous membrane - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a microporous film made of a polyolefin which excels in the impreganability with a fluid having a high dielectric constant and a high viscosity and is suitably used as a battery separator excellent in the productivity of batteries. <P>SOLUTION: The microporous film made of a polyolefin has a maximum pore diameter of 0.11-0.3 &mu;m and a surface fibril ratio at 300 nm from the surface layer of not less than 70%. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention is suitable for separators for electronic devices such as batteries, capacitors, capacitors and the like, and particularly relates to a microporous polyolefin membrane suitable as a separator for lithium ion batteries.

  Polyolefin microporous membranes are used in microfiltration membranes, separators for electronic devices such as batteries, capacitors and capacitors, and materials for fuel cells. Also, in the manufacturing process of a lithium ion battery, a spiral wound body composed of a positive electrode and a negative electrode coated with an electrode active material and two separators is inserted, and an electrolyte is injected from the upper part of the wound body. The step of impregnating the electrode and the separator with the electrolytic solution is included. Lithium ion battery electrolytes are generally blended with cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), and linear carbonates such as ethyl methyl carbonate to dissolve lithium salts as electrolytes. Is used.

  As a conventionally known polyolefin microporous membrane or battery, for example, Patent Document 1 includes that a microporous membrane having an average pore diameter of 0.15 μm or less and a ratio of the longest diameter to the shortest diameter in a specific range is included. A featured battery is disclosed. Patent Document 2 discloses a method for producing a pre-treatment microporous membrane in which a high-molecular-weight polyolefin film is heat-treated in a liquid to make it microporous, and the pre-treatment microporous membrane is compressed. ing.

Japanese Patent No. 3765396 Japanese Patent No. 3998860

  Here, with the recent increase in capacity of batteries, it is necessary to efficiently pack an active material in the wound body, so that the space in which the electrolyte in the wound body permeates tends to decrease. Furthermore, for the purpose of improving the performance and capacity of the battery, there is a tendency to use an electrolytic solution having a higher dielectric constant and higher viscosity, or an electrolytic solution having a higher electrolyte concentration. In this case, there may be a problem that productivity is lowered, such as a longer tact time in the liquid injection process in battery manufacturing, and a longer aging time until the separator is impregnated with the electrolyte even after the liquid injection. It was. That is, the separator is required to be more efficiently impregnated with the electrolytic solution. Patent Document 1 describes that the battery characteristics are improved when the average pore diameter and the ratio of the longest diameter to the shortest diameter of the holes are in a specific range, but are observed from the surface having a complicated communication structure. It merely defines the average pore diameter, and there is no description regarding the measurement method. For this reason, the disclosed technology alone still has room for improvement from the viewpoint of impregnation properties with respect to a high dielectric constant / high viscosity liquid. Moreover, since the maximum pore diameter estimated from the physical properties of the microporous film in Patent Document 2 is less than 0.1 μm, there is still room for improvement from the viewpoint of impregnation property with respect to a high dielectric constant / high viscosity liquid.

  In general, the relationship between the liquid penetrating the capillary and the capillary diameter is expressed by the Lucas-Washburn equation (Equation 1) (l: penetration depth, d: capillary diameter, γ: surface tension, θ: contact angle) , Η: liquid viscosity, t: time). According to the formula, the penetration depth is proportional to the square root of the capillary diameter and the square root of time, and inversely proportional to the square root of the liquid viscosity. Therefore, when this is applied to a microporous membrane, it is theoretically assumed that the larger pore diameter is superior in liquid permeation.

However, since the microporous membrane has a bent communication hole structure, it is greatly different from the through-hole structure used in the capillary model. In fact, it can be seen from the comparison between the present comparative examples 1, 2 and 3 and 4 that the small pore diameter membrane tends to be better than the large pore diameter membrane in terms of the electrolyte impregnation property.
In other words, in order to improve the electrolyte impregnation property of the microporous membrane, it was considered that not only the capillary diameter in the capillary model but also another measure should be taken.

  Under such circumstances, the present invention provides, for example, a polyolefin microporous membrane suitable as a battery separator that is excellent in impregnation even for a high dielectric constant / high viscosity liquid and excellent in battery productivity. It is the purpose.

  The present inventors have intensively studied to solve the above problems. As a result, it has been surprisingly found that a microporous membrane having a specific structure can solve the above-mentioned problems, and has led to the present invention.

That is, the present invention is as follows.
[1]
A polyolefin microporous membrane having a maximum pore size of 0.11 to 0.3 μm and a surface fibril ratio of 70% or more at 300 nm from the surface layer.
[2]
The polyolefin microporous film according to [1], wherein the fuse temperature is 150 ° C. or lower under a temperature rising condition of 2 ° C./min.
[3]
The polyolefin microporous membrane according to [1] or [2], wherein the polyolefin is polyethylene or a mixture of polyethylene and polypropylene.
[ 4 ]
The polyolefin microporous membrane according to any one of [1] to [3], wherein the ratio of the maximum pore size to the average pore size (maximum pore size / average pore size) is more than 1.4 and not more than 2.0.
[ 5 ]
The polyolefin microporous membrane according to any one of [1] to [4], which has an average pore diameter of 0.06 to less than 0.1 μm.

  ADVANTAGE OF THE INVENTION According to this invention, the polyolefin microporous film | membrane suitable as a battery separator which is excellent in the impregnation property and is excellent in the productivity of a battery is implement | achieved.

  The best mode for carrying out the present invention (hereinafter abbreviated as “embodiment”) will be described in detail below. In addition, this invention is not limited to the following embodiment, It can implement by changing variously within the range of the summary.

[Microporous membrane structure]
The surface fibril ratio at 300 nm from the surface layer of the polyolefin microporous film of the present embodiment (hereinafter sometimes simply referred to as “microporous film”) may be simply abbreviated as “fibril ratio”. 70% or more, preferably 75%, more preferably 80% or more, and the upper limit is preferably 100% or less, more preferably 95% or less, and still more preferably 93% or less. A surface fibril ratio of 70% or more is preferable because the electrolyte solution impregnation property is improved.
Here, “electrolyte impregnation” in the present embodiment is an index evaluated by the permeability of the electrolytic solution and the electrolyte pouring property in the battery manufacturing process. In addition, the index is measured according to the measurement method in the examples described later.
In addition, as electrolyte solution in this Embodiment, For example, Preferably cyclic carbonates represented by ethylene carbonate, propylene carbonate, (gamma) -butyrolactone etc. are 20 mass% or more, More preferably, it is 30 mass% or more, More preferably A solvent containing 50% by mass or more of a lithium salt such as LiPF ₆ or LiBF ₄ is preferably dissolved in an amount of 0.5 mol / L or more. In addition, the viscosity (measurement temperature 30 ° C.) of the electrolytic solution in the present embodiment measured with a rotary viscometer is preferably 2.0 mPa · s or more, more preferably 2.5 mPa · s or more, and still more preferably. Is 3.0 mPa · s or more.

FIG. 1 is a schematic diagram of the surface fibril ratio. “Surface fibril ratio at 300 nm from the surface layer” means a ratio of how much fibrils constituting the microporous film in the vertical direction extending from the surface to 300 nm are observed when the microporous film is viewed in plan.
When the fibril ratio at a depth of 300 nm is high from the surface layer of the microporous membrane,
1) When the pore diameter of the microporous membrane is small and the fibril ratio of the outermost fibril is high,
2) When the pore size of the microporous membrane is large and not only the outermost layer but also the fibrils existing from the outermost layer to 300 nm from the outermost layer are densely present in the cross-sectional direction, the fibril ratio becomes high.
There are two. Among these, the aspect 2) is preferable from the viewpoint of impregnation with a high dielectric constant / high viscosity liquid. In general, the fibrils of the microporous membrane having a maximum pore size of less than 0.1 μm are contained almost 100% within 300 nm from the outermost surface.

The maximum pore size of the microporous membrane is 0.11 μm to 0.3 μm. Preferably they are 0.12 micrometer-0.25 micrometer, More preferably, they are 0.13 micrometer-0.2 micrometer. Setting the maximum pore size to 0.11 μm or more is preferable from the viewpoint of improving permeability and electrolyte impregnation. If it is 0.3 micrometer or less, it becomes the tendency which has sufficient mechanical strength, and since it becomes the tendency which is excellent in the short circuit resistance as a battery separator, it is preferable. The said index is measured according to the measuring method in the Example mentioned later.
The fibril ratio and the maximum pore diameter can be adjusted by the stretching temperature of the microporous membrane and the pressurizing step described later.

  The reason why the electrolyte solution impregnation is excellent when the surface fibril ratio is high is not necessarily clear, but is presumed as follows. Since the microporous membrane has a bent communication hole structure, it is greatly different from the through-hole structure used in the capillary model. When the electrolyte solution penetrates from the surface of the microporous membrane, the electrolyte solution contacts the fibril surface constituting the microporous membrane, and penetrates in the depth direction of the microporous membrane by the capillary force acting on the fibril surface in the pores. it is conceivable that. A conventional microporous membrane having a maximum pore size of 0.11 μm or more has a large pore size, so that the distance between the fibrils observed from the cross-sectional direction is large, and it takes time for the electrolyte to penetrate in the depth direction. Is presumed to have been inhibited. Since the microporous membrane of the present embodiment has a high surface fibril ratio, the distance between the fibrils is small (that is, the fibrils are close to each other), and it is assumed that the penetration of the electrolytic solution is promoted. Furthermore, since the pore diameter (corresponding to the capillary diameter) is large, it can be considered that it easily penetrates into high-polarity, high-viscosity liquids and electrolytes as indicated by the above-mentioned Lucas-Washburn equation.

The average pore size of the microporous membrane is preferably 0.06 μm to 0.15 μm, more preferably 0.07 μm to 0.13 μm, still more preferably 0.08 μm to less than 0.1 μm. If the average pore size is 0.06 μm or more, the permeability as a microporous membrane tends to be improved, and if it is 0.15 μm or less, it tends to be excellent in mechanical strength.
In addition, the said parameter | index is measured according to the measuring method in the Example mentioned later.

The ratio of the maximum pore diameter to the average pore diameter (hereinafter abbreviated as “maximum pore diameter / average pore diameter”) is preferably more than 0.14 and less than 2, more preferably 1.41 or more and 0.19 or less. Preferably they are 1.5 or more and 0.18 or less. If the maximum pore diameter / average pore diameter exceeds 0.14, it tends to be excellent in permeability, and if it is less than 0.2, it is preferable from the viewpoint of preventing deterioration of battery characteristics caused by uneven pore diameter distribution.
In addition, the said parameter | index is measured according to the measuring method in the Example mentioned later.

The fibril thickness observed from the surface of the microporous membrane is preferably 0.2 μm or more, more preferably 0.3 μm or more, and further preferably 0.4 μm or more. The fibril thickness can be measured, for example, by surface observation with a scanning electron microscope.
The average pore diameter, maximum pore diameter / average pore diameter, and fibril thickness can be adjusted by changing the ratio of the polyolefin and the plasticizer that forms a uniform solution above the melting point of the polyolefin, and the ratio of these to the inorganic particles. It is.

[Characteristics of microporous membrane]
The film thickness is preferably 3 μm or more from the viewpoint of maintaining electrical insulation when used as a separator, and preferably 60 μm or less from the viewpoint of ensuring transparency. More preferably, it is 5-40 micrometers, More preferably, it is 10-30 micrometers.
The porosity is preferably 20% or more from the viewpoint of maintaining gas and ion permeability, and preferably 80% or less from the viewpoint of ensuring mechanical strength. More preferably, it is 30-70%, More preferably, it is 30-60%.
The air permeability is preferably 3 seconds / 20 μm or more in terms of a film thickness of 20 μm from the viewpoint of maintaining electrical insulation, and preferably 500 seconds / 20 μm or less from the viewpoint of maintaining gas and ion permeability. More preferably, it is 10-300 seconds / 20 micrometers, More preferably, it is 50-200 seconds / 20 micrometers or less.
The puncture strength is preferably 1 N / 20 μm or more in terms of film thickness 20 μm from the viewpoint of mechanical strength, and preferably 10 N / 20 μm or less from the viewpoint of preventing an increase in thermal shrinkage due to excessive stretching orientation. More preferably, it is 2-6N / 20micrometer, More preferably, it is 3-5.5N / 20micrometer.
The fuse temperature of the polyolefin microporous film is preferably 150 ° C. or lower under a temperature rising condition of 2 ° C./min from the viewpoint of safety at the time of battery temperature rising. More preferably, it is 145 degrees C or less, More preferably, it is 140 degrees C or less. Assuming the usage environment of the battery, the temperature is preferably 100 ° C. or higher, more preferably 120 ° C. or higher, and still more preferably 130 ° C. or higher.
The short-circuit temperature of the polyolefin microporous membrane is preferably higher than 160 ° C., more preferably higher than 170 ° C. under the temperature rising condition of 2 ° C./min, from the viewpoint of safety and heat resistance when the battery is heated. More preferably, it is higher than 180 degreeC.

The electrolyte impregnation property of the microporous membrane can be evaluated by an electrolyte penetration test and an electrolyte injection test. Further, the microporous membrane tends to be excellent in liquid permeability other than the electrolytic solution.
The penetration time in the electrolyte permeability test of the microporous membrane is preferably 60 seconds or less, more preferably 30 seconds or less, and even more preferably 10 seconds or less from the viewpoint of improving battery characteristics. It is preferable that the electrolytic solution permeation time is 60 seconds or less because it leads to excellent affinity with a high dielectric constant / high viscosity electrolytic solution.
Further, the electrolytic solution penetration area ratio obtained in the electrolytic solution pouring test is preferably 30% or more, more preferably 50% or more, and most preferably 70% or more, with 100% being the case where it penetrates the entire separator. . The penetration area of 30% or more is preferable from the viewpoint of improving battery productivity because it leads to shortening both the liquid injection step in battery production and the time for the electrolytic solution to penetrate the entire separator after the step.
In addition, about each parameter regarding the characteristic of the microporous film mentioned above, it is a value measured according to the measuring method in the Example mentioned later. In addition, for each of these parameters, changing the mass ratio of the polyolefin and the plasticizer that forms a uniform solution above the melting point of the polyolefin, and the mass ratio of them to the inorganic particles, the stretching ratio and the stretching temperature in the stretching process It can be adjusted by adjusting.

When the microporous membrane of the present embodiment is used as a battery separator, for example, a battery may be created by the following method.
First, the microporous membrane is formed into a vertically long shape having a width of 10 mm to 100 mm and a length of 200 mm to 2000 mm. The separator is stacked in the order of positive electrode-separator-negative electrode-separator, or negative electrode-separator-positive electrode-separator, and wound into a circular or flat spiral shape. Further, the wound body is housed in a battery can and an electrolyte is injected.

[Production method of microporous membrane]
The method for producing the microporous membrane is not particularly limited. For example, a method including a step of kneading a polyolefin and a plasticizer that forms a uniform solution at a temperature equal to or higher than the melting point of the polyolefin (hereinafter referred to as a phase separation method) A method (hereinafter referred to as “stretching and opening method”) including a step of forming pores by stretching a nonporous sheet in the longitudinal direction (hereinafter referred to as MD), and a step of swelling a polyolefin nonporous film with the plasticizer. And the like. Among these, a phase separation method is preferable from the viewpoint of having a large pore diameter and excellent mechanical strength, and a method using an inorganic fine powder is more preferable.

Hereinafter, the microporous membrane manufacturing process using the inorganic fine powder of the phase separation method will be described in detail. The method for producing a polyolefin microporous membrane by a phase separation method using inorganic fine powder includes the following steps (a) to (e):
(A) Step of melt-kneading a mixture containing at least a polyolefin resin, a plasticizer that forms a homogeneous solution with the resin and a melting point or higher, and inorganic fine powder, and then extruding, cooling and solidifying into a sheet (made by phase separation) Membrane process),
(B) a step of extracting the plasticizer and the inorganic filler (extraction step),
(C) Step of stretching at least uniaxially (stretching step),
(D) heat setting step,
(E) Step of pressurizing in the film thickness direction (pressurizing step)
It is preferable to contain.

The order of the steps (a) to (c) is as follows: (a) step → (b) step → (c) step, (a) step → (c) step → (b) step → (c) step, (a) Any one of the process → (c) process → (b) process can be selected. Among these, the (a) step → (b) step → (c) step is more preferable. (C) The process may be carried out in several stages.
Further, if the step (d) is after the step (a), an effect of reducing thermal shrinkage is obtained, and the number of times is not particularly limited, but the step (d) is at least one after the steps (b) and (c). It is preferable to perform the rotation once because heat shrinkage can be further reduced.
Furthermore, the step (e) is preferably performed at least once after the steps (a) to (d) from the viewpoint of further improving the electrolyte solution impregnation property.

(A) Phase separation film-forming step The polyolefin used in this step may be a kind of polyolefin or a polyolefin composition. Examples of the polyolefin include polyethylene, polypropylene, polymethylpentene, polybutene and the like, and two or more of these may be blended. From the viewpoint of improving the permeability, mechanical strength, and heat resistance, it is preferable to use a mixture of polyethylene and polypropylene.

The types of polyethylene, high density polyethylene such as density exceeding 0.94 g / cm ^3, density polyethylene in the range density of 0.93~0.94g / cm ^3, density of from 0.93 g / cm ³ Low low density polyethylene, linear low density polyethylene, etc. are mentioned. From the viewpoint of improving the mechanical strength, high-density polyethylene or medium-density polyethylene is preferably used, and these may be used alone or as a mixture. Polyethylene can be used alone or in a blend of ultra high molecular weight polyethylene and high molecular weight polyethylene. The viscosity average molecular weight (Mv) of the ultrahigh molecular weight polyethylene is preferably 500,000 or more from the viewpoint of further improving the mechanical strength of the polyolefin microporous film, and is preferably 3 million or less because the moldability may be impaired. More preferably, it is 600,000 to 2.5 million. The Mv of the high molecular weight polyethylene is preferably 30,000 or more in order to improve the mechanical strength, and is preferably less than 500,000 from the viewpoint of good shutdown characteristics. More preferably, it is 150,000-400,000, More preferably, it is 300,000 or less. The blend ratio of ultra high molecular weight polyethylene to the entire polyethylene is preferably 10 to 90%, more preferably 20 to 80%, and still more preferably 60% or less.

  As a kind of polypropylene, a propylene homopolymer, an ethylene propylene random copolymer, or an ethylene propylene block copolymer can be used. Of these, homopolypropylene is preferably used. In the case of a copolymer, the ethylene content in the copolymer is preferably 1 mol% or less from the viewpoint of lowering the crystallinity of polypropylene and preventing the permeability of the microporous membrane from decreasing. The Mv of the polypropylene to be used is preferably 100,000 or more in order to improve the heat resistance of the resulting microporous membrane, and is preferably less than 1 million from the viewpoint of preventing poor dispersion when blended. More preferably, it is 200,000-800,000, More preferably, it is 400,000-800,000.

  A plasticizer refers to a non-volatile solvent that can form a uniform solution at or above the melting point of polyolefin and polyolefin. For example, di-2-ethylhexyl phthalate (DOP), di-n-octyl phthalate, diheptyl phthalate, diphthalic acid phthalates such as dibutyl, organic acid esters such as adipic acid esters and glyceric acid esters, Examples thereof include phosphate esters such as trioctyl phosphate, liquid paraffin, solid wax, mineral oil and the like. Of these, phthalates are preferred in view of compatibility with polyolefins. These may be used alone or as a mixture.

  Silica, alumina, titania, zirconia, calcium carbonate, barium carbonate, etc. can be used as the inorganic fine powder. Of these, silica is preferable from the viewpoint of uniformity in melt kneading. It is preferable to mix and granulate polyolefin, a plasticizer, and inorganic fine powder with a Henschel mixer or the like from the viewpoint of uniformly dispersing the inorganic fine powder.

The ratio of the polyolefin resin to the total amount of the polyolefin resin, the plasticizer, and the inorganic fine powder is preferably 10% by mass or more from the viewpoint of moldability during film formation, and 60% by mass from the viewpoint of the permeability of the microporous film. It is preferable. More preferably, it is 20-50 mass%, More preferably, it is 40 mass% or less. Further, the ratio of the plasticizer to the total amount of the plasticizer and the inorganic fine powder is preferably 50% by mass or more from the viewpoint of preventing the deterioration of the quality due to the aggregation of the inorganic fine powder, and 80% or less from the viewpoint of imparting an appropriate pore size and permeability. Is preferred. More preferably, it is 60-75 mass%.
In addition to polyolefins, inorganic fine powders, and plasticizers, various additives such as antioxidants, antistatic agents, ultraviolet absorbers, lubricants, and antiblocking agents should be added as long as the effects of the present embodiment are not impaired. Can do.

  Examples of a method for forming the kneaded material obtained by melt kneading into a sheet shape include a method of solidifying the melt by cooling. Examples of the cooling method include a method of directly contacting a cooling medium such as cold air or cooling water, a method of contacting a roll or press machine cooled by a refrigerant, and the like. A method of contacting with a roll cooled by a refrigerant or a press is preferable in terms of excellent thickness control.

(B) Extraction step In the extraction step, the plasticizer and inorganic fine powder are extracted and removed with a solvent. The solvent for extracting the plasticizer is preferably a poor solvent for the polyolefin constituting the film, a good solvent for the plasticizer, and a boiling point lower than the melting point of the polyolefin constituting the film. Examples of such an extraction solvent include hydrocarbons such as n-hexane and cyclohexane, halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane, and non-chlorine-based solvents such as hydrofluoroether and hydrofluorocarbon. Examples thereof include halogenated solvents, alcohols such as ethanol and isopropanol, ethers such as diethyl ether and tetrahydrofuran, and ketones such as acetone and methyl ethyl ketone. It selects from these suitably, and it uses individually or in mixture. Of these, methylene chloride and methyl ethyl ketone are preferred.

The inorganic fine powder is extracted after the plasticizer extraction. As the extraction solvent, an alkaline aqueous solution such as sodium hydroxide or potassium hydroxide can be used.
As an extraction method, the extraction may be performed by dipping in an extraction solvent or showering, and then sufficiently drying.

(C) Stretching step The stretching step is a step of stretching at least in a uniaxial direction, and includes uniaxial stretching by a roll stretching machine, sequential biaxial stretching by a combination of a roll stretching machine and a tenter, simultaneous biaxial stretching by a simultaneous biaxial tenter, and the like. It is done. Among these, sequential biaxial stretching by a combination of a roll stretching machine and a tenter is preferable because the pore size is large and the strength is high.

  The draw ratio (surface ratio) is preferably 3 times or more from the viewpoint of improving the mechanical strength, and preferably 20 times or less from the viewpoint of preventing an excessive increase in permeability. More preferably, it is 6 to 15 times. Since the microporous film obtained at the time of stretching has high strength, it is preferable to stretch two sheets obtained in the step (b).

(D) Heat setting step Heat setting is to reduce the thermal shrinkage of the microporous membrane before treatment by performing low-magnification drawing and / or relaxation operation in a predetermined temperature atmosphere with a tenter, a roll drawing machine or the like. . Low magnification stretching is an area magnification of 3.0 times or less.
The stretch ratio in the low-stretch stretching is preferably 1.0 to 3.0 times, more preferably 1.5 to 2.0 times the MD and / or TD of the pre-treatment microporous film. Excessive stretching is not preferable because the possibility of film breakage increases.

  The temperature during stretching is preferably 100 ° C. or higher, more preferably 110 ° C. or higher, and still more preferably 120 ° C. or higher from the viewpoint of preventing thermal shrinkage. In order to prevent the pre-treatment microporous membrane from being melted by heating, it is preferably 135 ° C. or lower, more preferably 133 ° C. or lower, and still more preferably 130 ° C. or lower.

The relaxation operation is an operation for slightly returning the MD and / or TD dimensions of the pre-treatment microporous membrane. The relaxation ratio with respect to the film size at the time of stretching is preferably 1.0 times or less, more preferably 0.95 times or less from the viewpoint of reducing thermal shrinkage. Moreover, in order to prevent wrinkle generation | occurrence | production by excessive relaxation, 0.65 times or more are preferable, More preferably, it is 0.7 times or more.
The temperature during relaxation is preferably 120 ° C. or higher, more preferably 125 ° C. or higher, from the viewpoint of reducing thermal shrinkage. Moreover, in order to prevent the permeability | transmittance fall of a film | membrane, 140 degrees C or less is preferable, More preferably, it is 138 degrees C or less.

(E) Pressurization step The pressurization step is a step of pressurizing the pre-treatment microporous membrane having pores in the film thickness direction in order to improve the electrolyte impregnation property. The pre-treatment microporous membrane having substantially void portions means that the porosity is 20% or more, and preferably has curved communication holes. Pressurization in the present embodiment means applying pressure such as compression or rolling.
In the present embodiment, the pre-treatment microporous membrane refers to a membrane before pressure treatment.

  The pressurization method is not particularly limited as long as the pre-treatment microporous membrane can be pressurized in the film thickness direction, but a batch type compression press machine and a double belt capable of pressurization with a sample sandwiched between a pair of belts Examples thereof include a press machine and a calendar press machine capable of pressing at least once while being sandwiched between a pair of rolls. Among these, a calendar press is preferable from the viewpoint of maintenance of the apparatus.

  The roll material of the calender press machine can be selected from metal, resin, etc., depending on the material of the microporous film before treatment to be pressurized. From the viewpoint of preventing the roll from slipping and damaging the surface of the pre-treatment microporous membrane, the at least one roll is preferably a resin roll. The hardness of the resin roll can be adjusted as appropriate depending on the applied pressure and pressurization time, but is measured with a type D durometer in accordance with JIS-K7215 from the viewpoint of performing pressurization in the film thickness direction with high productivity. The hardness is preferably 100 or less, more preferably 90 or less, and still more preferably 80 or less.

  The deformation rate due to pressurization in the film thickness direction is not particularly limited because it can be appropriately changed depending on the porosity of the pre-treatment microporous film to be pressurized, but is preferably 60% or less in order to prevent a decrease in permeability due to excessive deformation. More preferably, it is 40% or less, more preferably 30% or less, and most preferably 20% or less. From the viewpoint of improving the electrolytic solution impregnation property, it is preferably 1% or more, more preferably 3% or more, and further preferably 5% or more. The deformation rate in the film thickness direction is a numerical value obtained by dividing the film thickness reduced by pressurization by the film thickness before pressurization.

  The area change accompanying pressurization in the film thickness direction is preferably 15% or less, more preferably 10%, in order to prevent deterioration of the electrolyte solution impregnation property or increase in heat shrinkage due to the structural change accompanying the area change. Hereinafter, it is more preferably 5% or less, most preferably 3% or less. The area change is a numerical value obtained by dividing the film area increased by pressurization by the film area before pressurization.

The pressure applied to the pre-treatment microporous membrane is not particularly limited as long as a desired deformation rate can be obtained, but from the viewpoint of obtaining an appropriate deformation rate and preventing breakage of the pre-treatment microporous membrane due to excessive deformation. The linear pressure is preferably 1500 N / cm or less, more preferably 1000 to 50 N / cm, still more preferably 700 to 100 N / cm, and most preferably 500 to 200 N / cm. The surface pressure is preferably 3000 N / cm ² or less, more preferably 2000 to 100 N / cm ² , still more preferably 1500 to 200 N / cm ² , and most preferably 1000 to 300 N / cm ² .

  The pressurization temperature is preferably 0 ° C. or more from the viewpoint of production equipment, and the structure of the microporous membrane before treatment may be deformed not only in the film thickness direction but also in the film surface direction, and the electrolyte impregnation property may be reduced. 100 degrees C or less is preferable. More preferably, it is 15-80 degreeC, More preferably, it is 25-60 degreeC, Most preferably, it is 25-50 degreeC.

  In the pressurizing step, it is also preferable to install a static eliminating device or a dust removing device from the viewpoint of improving the quality after pressurization. Static electricity removal is preferably performed on both the pre-treatment microporous membrane and the microporous membrane before and after pressurization, and when using a resin roll at the time of pressurization, static electricity generated on the roll is also preferably removed. Moreover, it is preferable as an effective dust removal measure that the dust removing apparatus is attached to remove dust from the pre-processing microporous film and / or the pressure roll. This makes it possible to prevent dust from adhering to the pre-treatment microporous membrane and / or roll and damaging the pre-treatment microporous membrane surface in the pressurizing step.

  Since the microporous membrane of the present embodiment has a large pore diameter, it has excellent permeability, can improve battery characteristics, and is excellent in impregnation with a high dielectric constant and high viscosity electrolyte. Therefore, when the microporous membrane of the present embodiment is used as a lithium ion battery separator, it is possible to contribute to improving battery performance and productivity. In addition, in electronic devices other than lithium ion batteries, it is possible to contribute to performance improvement and productivity because of excellent electrolyte impregnation properties. Moreover, since it has the outstanding permeability | transmittance with respect to a high-viscosity liquid and a high surface tension liquid also as a filtration membrane use, it becomes possible to contribute to a performance improvement.

  Next, the present embodiment will be described more specifically with reference to examples and comparative examples. However, the present embodiment is not limited to the following examples unless it exceeds the gist. In addition, the physical property in an Example was measured with the following method.

(1) Surface fibril ratio (%)
Using Scand Probe Microscope (SPM) Dimension (trademark) 3000 manufactured by Didital Instruments / Veeco Metrology Group, Inc. and NanoScope (registered trademark) III version5.31R1 manufactured by DiditalInstruments / Veeco Metrology Group, Inc. as measurement analysis software The concavo-convex image was observed in the tapping mode. The measurement sample is placed horizontally with respect to the sample table by applying a commercially available water-soluble correction solution on the sample table attached to the measurement device, and then placing a polyolefin microporous film cut out in a 10 mm square on the sample table. What was left to stand for 24 hours was used.
The measurement was performed under the conditions of a scan size of 20 μm, a scan rate of 0.3 Hz, a scan angle of 90 °, and a drive frequency of 305 kHz. The probe is a single crystal Si probe for tapping mode (trade name NANOSENSORS) (cantilever length is 130 μm, short needle height is 10 to 15 μm, probe pole radius is 5 to 10 nm, probe 1/2 opening angle is 18 ° lateral, 25 ° front, 10 ° back).
Next, the concavo-convex image obtained by the measurement was analyzed by the Bearing function attached to the analysis software. On the histogram, the depth (Hist depth) at which the histogram% (Hist%) exceeds 0 was defined as the outermost surface, and the histogram area (Bearing area%) up to a depth of 300 nm was defined as the surface fibril ratio. In addition, analysis was performed using the grain size function as necessary, and fibril images existing from the depth defined as the outermost surface to a depth of 300 nm were obtained by the analysis using the bearing function (FIGS. 2 and 3).

(2) Viscosity average molecular weight The viscosity average molecular weight of the polyethylene and polyolefin microporous membrane was measured at a measurement temperature of 135 ° C. using decalin as a solvent, and was calculated from the viscosity [η] by the following formula.
[Η] = 6.77 × 10 ⁻⁴ Mv ^0.67 (Chiang's formula)
For polypropylene, Mv was calculated by the following formula.
[Η] = 1.10 × 10 ⁻⁴ Mv ^0.80

(3) Density (g / cm ³ )
Based on ASTM-D1505, it measured by the density gradient tube method (23 degreeC).

(4) Film thickness (μm)
The measurement was performed at a room temperature of 23 ° C. using a micro thickness measuring instrument manufactured by Toyo Seiki, KBM (trademark).

(5) Porosity (%)
A sample of 10 cm × 10 cm square was cut out from the microporous membrane, its volume (cm ³ ) and mass (g) were determined, and calculated from these and the film density (g / cm ³ ) using the following formula.
Porosity = (volume−mass / film density) / volume × 100
The film density was calculated as 0.95 (g / cm ³ ).

(6) Air permeability (sec)
Based on JIS P-8117, it measured with the Gurley type air permeability meter by the Toyo Seiki Co., Ltd. and G-B2 (trademark).

(7) Puncture strength (N / 20μm)
Using a handy compression tester KES-G5 (trademark) manufactured by Kato Tech, the microporous membrane was fixed with a sample holder having a diameter of 11.3 mm at the opening. Next, the center of the fixed microporous membrane is subjected to a piercing test in a 25 ° C. atmosphere at a needle radius of curvature of 0.5 mm and a piercing speed of 2 mm / sec. (N) was obtained. By multiplying this by 20 (μm) / film thickness (μm), a 20 μm film thickness equivalent puncture strength (N / 20 μm) was calculated.

(8) Maximum pore size (μm)
It calculated based on the bubble point (kPa) in ethanol based on ASTM E-128-61. The surface tension of ethanol was calculated as 24.5 mN / m. Moreover, when the bubble point was 980 kPa or more, it was set to less than 0.07 μm.

(9) Average pore size (half dry method) (μm)
Measurement was performed using ethanol in accordance with ASTM F-316-86.

(10) Electrolyte permeability When a mixed liquid of 20% by mass of ethylene carbonate and 80% by mass of propylene carbonate is used as the electrolytic solution, and a drop is dropped with a dropper from a height of 30 mm, the mixed liquid penetrates the membrane and becomes transparent Was measured. The case where it became transparent within 5 seconds was marked with ◎, the case within 30 seconds with ◯, the case where slight penetration was observed within 30 to 180 seconds, and the case where it did not penetrate within 180 seconds with x.

(11) Liquid permeability It was carried out in the same manner as (10) except that a wet index standard solution (manufactured by Nacalai Tesque) having a surface tension (hereinafter referred to as γ) of 42 mN / m was used as the liquid. The case where it penetrated within 5 minutes and became transparent, the case where it penetrated within 8 minutes, Δ, and the case where it did not penetrate within 8 minutes were marked with ×.

(12) Electrolyte injection property a. Production of positive electrode 92.2% by mass of lithium cobalt composite oxide LiCoO ₂ as an active material, 2.3% by mass of flake graphite and acetylene black as a conductive agent, and 3.2% of polyvinylidene fluoride (PVDF) as a binder % Was dispersed in N-methylpyrrolidone (NMP) to prepare a slurry. This slurry was applied to both surfaces of an aluminum foil having a thickness of 20 μm serving as a positive electrode current collector with a die coater, dried at 130 ° C. for 3 minutes, and then compression molded with a roll press. At this time, the active material application amount of the positive electrode was 250 g / m ² , and the active material bulk density was 3.00 g / cm ³ .
b. Production of Negative Electrode 96.9% by mass of artificial graphite as an active material, 1.4% by mass of ammonium salt of carboxymethyl cellulose and 1.7% by mass of styrene-butadiene copolymer latex as a binder were dispersed in purified water to prepare a slurry. . This slurry was applied to both surfaces of a 12 μm thick copper foil serving as a negative electrode current collector with a die coater, dried at 120 ° C. for 3 minutes, and then compression molded with a roll press. At this time, the active material application amount of the negative electrode was set to 106 g / m ² , and the active material bulk density was set to 1.35 g / cm ³ .
c. Preparation of Nonaqueous Electrolytic Solution A solution obtained by dissolving LiBF ₄ as an electrolyte at a concentration of 0.5 mol / L in a mixed solvent of ethylene carbonate: propylene carbonate: γ-butyrolactone = 1: 1: 2 (volume ratio) was prepared.
d. Evaluation of liquid injection property The positive electrode prepared in the previous item a was cut in the vertical direction by 96 mm and the horizontal direction by 40 mm, the negative electrode prepared in the previous item b was cut in the vertical direction by 98 mm and the horizontal direction by 42 mm, and the microporous film was cut in MD to 100 mm TD Cut to 44 mm size. Next, a laminated body was created by superimposing the negative electrode, the separator, and the positive electrode in this order from the lower side so that the central portions coincide. In a state where a load of 58.8 N (6.0 kg) was uniformly applied to the entire laminate, the pressure was reduced to 5 torr, and then 5 ml of the electrolytic solution prepared in c was poured around the laminate. After standing in this state for 10 minutes, the pressure was returned to normal pressure, the excess electrolyte solution was wiped off, and the laminate was disassembled. The case where the area where the electrolyte solution permeated the separator area was 90% or more was rated as “◯”, the case where it was 70% or more as Δ, and the case where it was less than 70% as “X”.

[Example 1]
Mv 1 million, ultra high molecular weight polyethylene 19.2 mass% with melting point 134 ° C, Mv 250,000, high density polyethylene 12.8 mass% with melting point 136 ° C, dioctyl phthalate (DOP) 48 wt%, fine powder silica 20 wt% After granulation, the mixture was melt-kneaded with a twin screw extruder equipped with a T-die at the tip, extruded, rolled with rolls heated from both sides, and formed into a sheet having a thickness of 110 μm. From the molded product, DOP was extracted with methylene chloride and finely divided silica was extracted with sodium hydroxide to prepare a microporous membrane. Two microporous membranes were stacked and stretched 4.5 times in the longitudinal direction at 115 ° C., then stretched 2.1 times in the transverse direction at 120 ° C., and finally heat treated at 137 ° C. before treatment. A polyolefin microporous membrane was obtained.
Next, the polyolefin microporous membrane before treatment is composed of a resin roll having a diameter of 100 mm, which is a durometer hardness of type D measured in accordance with JIS-K7215, and a metal roll having a temperature of 200 mm and adjusted to 40 ° C. A microporous membrane made of polyolefin was obtained by continuous pressurization at a line pressure of 327 N / cm and a line speed of 10 m / min. The physical properties of the obtained microporous membrane are shown in Table 1, and a fibril image existing at a depth of 300 nm from the outermost surface is shown in FIG.

[Example 2]
Average dispersion particle size prepared by dry method 0.25 μm, 95 vol% cumulative particle size 0.45 μm, 5 vol% cumulative particle size 0.15 μm, dispersed particle size ratio 3.0, oil absorption 240 ml / 100 g, primary particle size 12 nm 20% by mass of silica fine powder, Mv 1 million, ultra high molecular weight polyethylene having a melting point of 134 ° C. 4% by mass, Mv 250,000, high density polyethylene having a melting point of 136 ° C. 17% by mass, Mv 150,000 linear low density polyethylene After 11 mass% dioctyl phthalate (DOP) was mixed and granulated, it was melt kneaded in a twin screw extruder equipped with a T die, extruded, rolled with rolls heated from both sides, and a thickness of 90 μm It was molded into a sheet shape. From the molded product, DOP was extracted with methylene chloride and finely divided silica was extracted with sodium hydroxide to prepare a microporous membrane. Two microporous membranes are stacked and stretched 4.5 times in the longitudinal direction at 110 ° C., then stretched 2.0 times in the transverse direction at 125 ° C., and heat-treated at 135 ° C. to heat the polyolefin microporous before treatment. A membrane was obtained. Next, the polyolefin microporous membrane before treatment is composed of a resin roll having a diameter of 100 mm, which is a durometer hardness of type D measured in accordance with JIS-K7215, and a metal roll having a temperature of 200 mm and adjusted to 40 ° C. Using a roll press machine, a polyolefin microporous membrane was obtained by continuously pressurizing at a line pressure of 245 N / cm and a line speed of 10 m / min. Table 1 shows the physical properties of the obtained microporous membrane. Table 1 shows the physical properties of the obtained microporous membrane.

[Comparative Examples 1-2]
Except for the compression process, the same procedure as in Example 1 was performed. Table 1 shows the physical properties of the obtained microporous membrane. A fibril image of the microporous membrane obtained in Comparative Example 1 existing at a depth of 300 nm from the outermost surface is shown in FIG.

[Comparative Example 3]
40% by mass of a high-density polyethylene having an Mv of 250,000 and a melting point of 136 ° C., and 60% by mass of an ultrahigh molecular weight polyethylene having an Mv of 700,000 and a melting point of 135 ° C. were dry-blended using a tumbler blender. 1% by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant is added to 99% by mass of the obtained polymer mixture, and the tumbler is again added. The mixture was obtained by dry blending using a blender. The atmosphere in the feeder and the biaxial stretching machine was replaced with nitrogen, and the resulting mixture was fed to the biaxial extruder through the feeder and liquid paraffin (kinematic viscosity at 37.78 ° C. 7.59 × 10 ⁻⁵ m ² / s). ) Was injected into the extruder cylinder by a plunger pump.
The feeder and the pump were adjusted so that the resin concentration in the entire mixture extruded by melt kneading was 35% by mass.
Subsequently, the melt-kneaded product was extruded from a T-die and cooled and solidified to obtain a 1800 μm sheet.
Next, this sheet was guided to a simultaneous biaxial tenter stretching machine, and biaxial stretching was performed. The stretching conditions were an MD magnification of 7.0 times, a TD magnification of 6.4 times, and a set temperature of 123 ° C.
Thereafter, the sheet was introduced into a methylene chloride bath and sufficiently immersed in methylene chloride to extract and remove liquid paraffin, and then methylene chloride was removed by drying. Further, this sheet was guided to a TD tenter, and stretched at a low magnification at a temperature of 120 ° C. and a magnification of 1.5 times, and heat fixed at a temperature of 127 ° C. and a relaxation rate of 0.85 times to obtain a microporous membrane. It was. Table 1 shows the physical properties of the obtained microporous membrane.

[Comparative Example 4]
40% by mass of a high-density polyethylene having an Mv of 250,000 and a melting point of 136 ° C., and 60% by mass of an ultrahigh molecular weight polyethylene having an Mv of 700,000 and a melting point of 135 ° C. were dry-blended using a tumbler blender. 1% by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant is added to 99% by mass of the obtained polymer mixture, and the tumbler is again added. The mixture was obtained by dry blending using a blender. The atmosphere in the feeder and the biaxial stretching machine was replaced with nitrogen, and the resulting mixture was fed to the biaxial extruder through the feeder and liquid paraffin (kinematic viscosity at 37.78 ° C. 7.59 × 10 ⁻⁵ m ² / s). ) Was injected into the extruder cylinder by a plunger pump.
The feeder and the pump were adjusted so that the resin concentration in the entire mixture extruded by melt kneading was 40% by mass.
Subsequently, the melt-kneaded product was extruded from a T-die and cooled and solidified to obtain a 1800 μm sheet.
Next, this sheet was guided to a simultaneous biaxial tenter stretching machine, and biaxial stretching was performed. The stretching conditions were an MD magnification of 7.0 times, a TD magnification of 6.4 times, and a set temperature of 123 ° C.
Thereafter, this sheet was introduced into a methylene chloride bath and sufficiently immersed in methylene chloride to extract and remove liquid paraffin, and then methylene chloride was removed by drying to obtain a microporous membrane. Table 1 shows the physical properties of the obtained microporous membrane.

  The polyolefin microporous membrane of the present invention is suitable, for example, as a separator for a lithium ion battery.

Schematic about surface fibril rate Fibril image present at a depth of 300 nm from the outermost surface in Example 1 Fibril image present at a depth of 300 nm from the outermost surface in Comparative Example 1

Claims (5)

  A polyolefin microporous membrane having a maximum pore size of 0.11 to 0.3 μm and a surface fibril ratio of 70% or more at 300 nm from the surface layer. The polyolefin microporous film according to claim 1, wherein the fuse temperature is 150 ° C. or lower under a temperature rising condition of 2 ° C./min. The polyolefin microporous membrane according to claim 1 or 2, wherein the polyolefin is polyethylene or a mixture of polyethylene and polypropylene. The polyolefin microporous membrane according to any one of claims 1 to 3, wherein a ratio of the maximum pore diameter to the average pore diameter (maximum pore diameter / average pore diameter) exceeds 1.4 and is 2.0 or less. The polyolefin microporous membrane according to any one of claims 1 to 4 , having an average pore diameter of 0.06 to less than 0.1 µm.

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