Energy storage devices and separators for energy storage devices

Abstract

To provide a polyolefin microporous film capable of suppressing deterioration due to liquid shortage of a lithium ion battery, a lithium ion battery separator including them, and a lithium ion battery.SOLUTION: The present invention provides a polyolefin microporous film that in a liquid-absorptive test of a propylene carbonate, a liquid height of a longitudinal direction (MD) and a width direction (TD) is 1.0 mm or more and 50 mm or less, a leading extension elastic modulus of the MD and the TD is 3000 kgf / cm2 or large.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a polyolefin microporous membrane, a separator for lithium-ion batteries, and a lithium-ion battery. [Background technology]

[0002] Polyolefin microporous membranes (hereinafter sometimes simply abbreviated as "PO microporous membranes") are widely used for the separation of various substances, or as selective permeable separation membranes, isolation materials, etc. Their applications include, for example, microfiltration membranes; separators for batteries such as lithium-ion batteries, polymer electrolyte batteries, and fuel cells; separators for capacitors; and base materials for functional membranes that allow functional materials to be filled into the pores to create new functions. Among these, PO microporous membranes are particularly suitable as separators for lithium-ion batteries, which are widely used in mobile phones, smartphones, wearable devices, notebook personal computers (PCs), tablet PCs, digital cameras, and the like.

[0003] Conventionally, lithium-ion batteries have a power generation element in which a separator is interposed between the positive and negative electrodes, and the electrolyte is impregnated into this element. The separator separates the positive and negative electrodes and can function as a membrane that allows the electrolyte or ions to permeate. It has been proposed that improvements to the separator or negative electrode, including a PO microporous membrane, can improve battery characteristics such as cycle characteristics, energy density, energy capacity, and charge / discharge characteristics, and suppress the degradation of battery characteristics (Patent Documents 1-3).

[0004] For example, Patent Document 1 describes providing a non-aqueous electrolyte secondary battery that has excellent wettability between the separator and the non-aqueous electrolyte, as well as excellent battery capacity and charge / discharge characteristics, by using a hydrophilic or hydrophilically treated separator. Patent Document 2 investigates the white index (whiteness) and diethyl carbonate permeability of a separator containing a PO microporous membrane from the viewpoint of suppressing the deterioration of the battery characteristics of a secondary battery.

[0005] For example, Patent Document 3 proposes a method to facilitate battery manufacturing and improve battery cycle life and energy density by surface treatment of the negative electrode with a negative electrode active layer comprising a first layer containing Li metal and a second layer consisting of a temporary protective metal that can form an alloy with Li metal or can diffuse into Li metal. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 6-283153 [Patent Document 2] International Publication No. 2018 / 078707 [Patent Document 3] Special Publication No. 2003-515893 [Overview of the project] [Problems that the invention aims to solve]

[0007] In lithium-ion secondary batteries, under conditions such as charging at relatively low temperatures and relatively high currents, lithium (Li) can be deposited on the negative electrode plate, and metallic Li may grow from this negative electrode plate as dendrites (tree-like crystals). Furthermore, repeated dissolution and extraction reactions of Li ions occur in the battery, and each time a new electrode surface is formed, which can lead to severe reductive decomposition of the electrolyte. As the reductive decomposition of the electrolyte and dendrite formation progress due to the battery's charge-discharge cycle, the electrolyte in the battery is consumed, leading to electrolyte depletion, which tends to cause not only short circuits and overheating but also electrolyte depletion. This tendency is particularly pronounced in battery systems with charge-discharge mechanisms that facilitate Li deposition, or in battery systems equipped with negative electrodes that allow for high capacity.

[0008] However, the separators described in Patent Documents 1 and 2 and the negative electrode described in Patent Document 3 do not focus on the depletion of the battery electrolyte due to the formation of Li dendrites, and there is room for further improvement in suppressing battery degradation caused by electrolyte depletion.

[0009] Therefore, the present invention aims to provide a polyolefin microporous membrane that can suppress degradation due to electrolyte depletion in lithium-ion batteries, and a lithium-ion battery separator and lithium-ion battery containing the same. [Means for solving the problem]

[0010] The inventors of the present invention have elucidated the mechanism of electrolyte depletion in lithium-ion batteries caused by lithium (Li) dendrite formation, and have found that the above problems can be solved by specifying the liquid absorption height and tensile modulus in liquid absorption tests of polyolefin microporous membranes or separators for lithium-ion batteries, thereby completing the present invention. Examples of embodiments of the present invention are listed below. <1> In a liquid absorption test of propylene carbonate, the absorption height in the longitudinal direction (MD) and the width direction (TD) was 1.0 mm or more and 50 mm or less. The tensile modulus of MD and TD is 3000 kgf / cm². 2 That's all. Polyolefin microporous membrane. <2> The number of pores calculated by the gas-liquid method is 70 pores / μm 2 More than 500 pieces / μm 2 A polyolefin microporous membrane according to item 1, having the following porous layer. <3> The polyolefin microporous membrane as described in item 2, wherein the total pore volume of the porous layer with a pore diameter of 98 nm or less, as measured by a nitrogen gas adsorption test, is 20% or more and 85% or less of the total pore volume. <4> The polyolefin microporous membrane is a polyolefin microporous membrane according to any one of items 1 to 3, wherein in a compression test under the conditions of a temperature of 25°C, a pressure of 10 MPa, and a compression time of 3 minutes, the compressibility is 35% or less and the post-compression porosity is 35% or more. <5> The main component of the aforementioned polyolefin microporous membrane is polyethylene, and A polyolefin microporous membrane according to any one of items 1 to 4, wherein the crystal long period of the polyethylene, as measured by small-angle X-ray scattering (SAXS), is 30.0 nm or greater. <6> The main component of the aforementioned polyolefin microporous membrane is polyethylene, and A polyolefin microporous membrane according to any one of items 1 to 5, wherein the crystallite size of the polyethylene, as measured by wide-angle X-ray scattering (WAXS), is 35.0 nm or less. <7> In a liquid absorption test of propylene carbonate, the absorption height of MD and TD was 1.0 mm to 50 mm, and the tensile modulus of MD and TD was 3000 kgf / cm². 2 A lithium-ion battery comprising a polyolefin microporous membrane as a separator. <8> A lithium-ion battery as described in item 7, wherein an ion-conducting layer exists between the separator and the metal negative electrode. <9> The lithium-ion battery according to item 7 or 8, wherein the ion conducting layer is made of an inorganic material. <10> 1-4 below: 1. The Li ion concentration is 1.3 mol / L or higher; 2. Contains an electrolyte salt having a bisfluorosulfonylimide structure; 3. Includes carbonates in which hydrogen atoms are substituted with halogen atoms; and 4. The electrolyte is an ionic liquid; A lithium-ion battery according to any one of items 7 to 9, comprising an electrolyte characterized by at least one of the following. <11> A lithium-ion battery according to any one of items 7 to 10, comprising a polyolefin microporous membrane according to any one of items 1 to 6 as the separator. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a polyolefin microporous membrane that can suppress degradation due to electrolyte depletion in lithium-ion batteries, and a lithium-ion battery separator and lithium-ion battery containing the same. [Modes for carrying out the invention]

[0012] The following describes in detail embodiments for carrying out the present invention (hereinafter sometimes abbreviated as "embodiments"), but the present invention is not limited thereto, and various modifications are possible without departing from its essence.

[0013] In this specification, the flow direction of the film during film formation is defined as MD, and the direction intersecting MD at a 90-degree angle (90°) within the film plane is defined as TD. In this specification, the upper and lower limits of each numerical range can be combined arbitrarily. Furthermore, containing a specific component as a main component means that the content of that specific component is 50% by mass or more.

[0014] <Polyolefin microporous membrane> In one aspect of the present invention, a polyolefin microporous membrane (PO microporous membrane) is provided. The PO microporous membrane contains a polyolefin resin as its main component and exhibits excellent electrical insulation and ion permeability. Therefore, it can be used, for example, in lithium-ion batteries, specifically as a separator for lithium-ion secondary batteries or as a separator substrate.

[0015] A polyolefin microporous membrane according to the first embodiment of the present invention has the following characteristics: In the liquid absorption test of propylene carbonate, the absorption height of MD and TD is 1.0 mm or more and 50 mm or less; and The tensile modulus of MD and TD is 3000 kgf / cm². 2 That's all.

[0016] The polyolefin microporous membranes characterized as described above tend to suppress the degradation of lithium-ion batteries due to electrolyte depletion when used as separators for lithium-ion batteries. This tendency is particularly pronounced in lithium-ion batteries equipped with charge / discharge mechanisms that facilitate Li deposition or negative electrodes that enable high capacity, and consequently facilitates the improvement of battery characteristics in lithium-ion batteries that include negative electrodes made of Li metal.

[0017] According to the present invention, the following two patterns have been identified as mechanisms that make lithium-ion batteries more susceptible to electrolyte depletion due to Li dendrite formation: (a) Li dendrites cause irregularities to form on the negative electrode surface, especially on the Li metal negative electrode surface, increasing the specific surface area of ​​the negative electrode and increasing the consumption of electrolyte due to reductive decomposition. (i) Li dendrites formed on the negative electrode surface cause compressive deformation of the separator located between the positive and negative electrodes, causing the separator to expel the electrolyte (i.e., a phenomenon of electrolyte discharge occurs in the separator).

[0018] In the first embodiment, based on the above mechanisms (a) and (b), the affinity of a PO microporous membrane that can be used as a separator is improved with respect to the electrolyte and its compressibility, with respect to suppressing electrolyte depletion in lithium-ion batteries.

[0019] From the viewpoint of improving affinity with the electrolyte, PO microporous membranes have an absorption height of 1.0 mm to 50 mm in both MD and TD in propylene carbonate absorption tests. When a PO microporous membrane with an absorption height controlled within the numerical range of 1.0 mm to 50 mm is included in a lithium-ion battery as a separator, the electrolyte flows smoothly even if the electrolyte decreases due to consumption by reductive decomposition or compression deformation. Therefore, it is thought that an optimal amount of electrolyte is maintained in the pores of the separator, resulting in excellent cycle characteristics.

[0020] The liquid absorption test of propylene carbonate can be performed by the method described in the examples. From the viewpoint of further improving the affinity between the PO microporous membrane and the electrolyte, the liquid absorption height is preferably in the range of 1.0 mm to 50 mm, more preferably in the range of 2.0 mm to 45 mm, even more preferably in the range of 3.0 mm to 40 mm, and particularly preferably in the range of 4.0 mm to 30 mm for both MD and TD.

[0021] A lithium-ion battery may experience volume changes, causing the electrolyte held in the pores of the separator to flow. When the contact angle with respect to propylene carbonate is within the above numerical range, in response to volume changes as described above, the flow of the electrolyte occurs smoothly, and it is considered that an appropriate amount of electrolyte is retained in the pores of the separator, resulting in excellent cycle characteristics.

[0022] The liquid absorption height in the propylene carbonate liquid absorption test of the PO microporous membrane can be adjusted within the above numerical range, for example, by the inclusion of inorganic fillers or inorganic particles in the PO microporous membrane, the mixing of polyolefin (PO) and a polymer having a polar group in the manufacturing process of the PO microporous membrane, the surface treatment of the PO microporous membrane, etc.

[0023] From the viewpoint of improving compression resistance, the tensile elastic modulus in the MD and TD directions of the PO microporous membrane is 3000 kgf / cm 2 or more. When the PO microporous membrane with the MD and TD tensile elastic moduli controlled within the above numerical range is disposed between the positive and negative electrodes of a lithium-ion battery as a separator, even if Li dendrites precipitate on the electrode surface, it is difficult to be crushed, difficult to undergo compression deformation, and can suppress the ejection of the electrolyte.

[0024] The tensile elastic modulus of the PO microporous membrane can be measured by the method described in the examples. From the viewpoint of further improving the compression resistance of the PO microporous membrane, the tensile elastic moduli in the MD and TD directions of the PO microporous membrane are preferably in the range of 3,000 kgf / cm 2 or more and 15,000 kgf / cm 2 or less, more preferably in the range of 3,200 kgf / cm 2 or more and 12,000 kgf / cm 2 or less, still more preferably in the range of 3,500 kgf / cm 2 or more and 10,000 kgf / cm 2 or less, and even more preferably in the range of 3,800 kgf / cm 2 or more and 9,000 kgf / cm 2 or less.

[0025] The tensile modulus of the PO microporous membrane can be adjusted to within the above numerical range by, for example, selecting the raw material composition, controlling the proportion of plasticizer, biaxial stretching temperature, biaxial stretching ratio, heat-fixing temperature, stretching ratio during heat-fixing, and relaxation rate during heat-fixing.

[0026] From the viewpoint of further improving the compressibility of the PO microporous membrane, in a compression test under the conditions of a temperature of 25°C, a pressure of 10 MPa, and a compression time of 3 minutes, the compressibility is preferably 35% or less, more preferably in the range of 3% to 34%, even more preferably in the range of 5% to 33%, even more preferably in the range of 7% to 32%, particularly preferably in the range of 10% to 31%, and most preferably in the range of 11% to 30%. From a similar viewpoint, in a compression test under the conditions of a temperature of 25°C, a pressure of 10 MPa, and a compression time of 3 minutes, the post-compression porosity of the PO microporous membrane is preferably 35% or more, more preferably 36% or more, even more preferably 37% or more, even more preferably 38% or more, still still preferably 39% or more, and particularly preferably 40% or more.

[0027] If desired, an ion-conducting layer, an inorganic coating layer, or an organic (adhesive) layer may be formed on the surface of the PO microporous film.

[0028] Multiple PO microporous membranes can be laminated as desired, and a single PO microporous membrane can also be laminated with another porous membrane.

[0029] The preferred properties and components of the PO microporous membrane are described below.

[0030] (Pore ratio measured by nitrogen gas adsorption test) It is preferable that the PO microporous membrane has a single-point total pore volume of pores with a diameter of 98 nm or less, as measured by a nitrogen gas adsorption test, that is between 20% and 85% of the total pore volume. Furthermore, when the PO microporous membrane has a porous layer, it is also preferable that the single-point total pore volume of the porous layer with a diameter of 98 nm or less, as measured by a nitrogen gas adsorption test, is between 20% and 85% of the total pore volume.

[0031] Nitrogen gas adsorption tests can be performed using the method described in the examples below. By defining the amount of adsorption at a nitrogen relative pressure (p / p0) of 0.98 as the total pore volume, the single-point method total pore volume (pore diameter 98 nm or less) can be derived. By comparing this value with the total pore volume of the microporous membrane, the ratio of the pore volume (pore diameter 98 nm or less) to the total pore volume can be determined. A higher ratio indicates that the microporous membrane is formed from smaller pores. From the viewpoint of making the microporous membrane more densified and efficiently delaying the growth of lithium dendrites and dendrites derived from foreign matter due to the elution of metallic foreign matter, the single-point method total pore volume (pore diameter 98 nm or less) is preferably 20% or more of the total pore volume, more preferably 25% or more, and even more preferably 30% or more. In the nitrogen gas adsorption test of a microporous membrane or porous layer using the single-point method, the lower limit of the pore diameter can exceed 0 nm in this art.

[0032] On the other hand, if the total pore volume measured by a single point method (pore diameter 98 nm or less) of a microporous membrane or porous layer exceeds 85% of the total pore volume, clogging of the separator can cause concentrated areas of increased resistance, which in turn promotes dendrite formation and leads to a decrease in the capacity maintenance cycle. From this perspective, it is preferable that the total pore volume measured by a single point method (pore diameter 98 nm or less) be 85% or less of the total pore volume, and more preferably 80% or less.

[0033] The ratio of the total pore volume by the single-point method (pore diameter 98 nm or less) to the total pore volume can be controlled, for example, by densifying the microporous membrane. More specifically, in the manufacturing process of the PO microporous membrane described later, the ratio can be controlled to remain within the range of the pore volume described above by using, for example, the following in combination: including high molecular weight PE of the molecular weight described later in the content described later; performing a stretching step before the extraction step; stretching under conditions of a stretching magnification of 50 times or more and / or a stretching temperature of 128°C or less; adjusting the stretching magnification in both the longitudinal and transverse directions in the stretching step to 5 times or more; adjusting the heat-fixing temperature to 115°C or more and 140°C or less during the heat-fixing (HS) step; adjusting the HS stretching magnification to 1.5 times or more and 2.2 times or less; and adjusting the HS relaxation magnification to 0.7 times or more and less than 0.95 times.

[0034] Furthermore, the pore volume ratio can also be controlled by the viscosity-average molecular weight of the PE used. By using polymer PE, the pore diameter is reduced during the stretching process, and the pores are less likely to be blocked during the heat-setting process. From this viewpoint, the viscosity-average molecular weight (Mv) of the PO microporous membrane is preferably 450,000 or more, more preferably 500,000 or more, and the content of PE with an Mw of 450,000 or more in the resin composition of the membrane is preferably 30% by mass or more, more preferably 40% by mass or more, and even more preferably 50% by mass or more. This content can be 100% by mass. In addition, in order for the above pore ratio not to exceed 85%, the Mv of the membrane is preferably 6,000,000 or less, more preferably 5,000,000 or less, and even more preferably 3,000,000 or less.

[0035] Furthermore, the pore ratio can be controlled to a predetermined range by the molecular structure of the PO used. Although the reason is not entirely clear, the pore ratio tends to fall within a predetermined range as the polydispersity of the raw material polymer (weight-average molecular weight Mw / number-average molecular weight Mn and z-average molecular weight Mz / weight-average molecular weight Mw) increases.

[0036] (Number of pores calculated by the gas-liquid method) The PO microporous membrane has a pore count B of 70 pores / μm, calculated by the gas-liquid method. 2 Preferably, the density is 500 particles / μm or higher. 2 The following is preferable. Furthermore, when the PO microporous membrane has a porous layer, the number of pores in the porous layer calculated by the gas-liquid method is 70 pores / μm 2 More than 500 pieces / μm 2 The following is also preferable.

[0037] The number of pores per cross-section (B) calculated by the gas-liquid method is set at 70 pores / μm, from the viewpoint of efficiently suppressing localized dendrite formation by diffusing lithium ions and metal ions associated with foreign matter elution within the microporous membrane or porous layer, and from the viewpoint of suppressing the increase in localized resistance areas and improving cycle characteristics. 2 The above is preferable, with 80 particles / μm 2 It is more preferable that the number be greater than or equal to 90 particles / μm 2 It is even more preferable that the value be greater than or equal to 100 particles / μm 2 It is even more preferable that the above is true, and 110 particles / μm 2 The above is particularly preferable. Furthermore, from the viewpoint that the growth of dendrites can be suppressed by the high strength of the microporous membrane or porous layer, the number of pores (B) is 500 pores / μm. 2 Preferably, the following: 350 particles / μm 2 The following is more preferable: The number of pores per cross-section (B) calculated by the gas-liquid method is adjusted to within the above range by adjusting the proportion of plasticizer in the total raw materials (i.e., the proportion of resin in the total raw materials), the heat-fixing temperature, etc., in the PO microporous membrane manufacturing process described later.

[0038] (Porosity) PO microporous membranes preferably have a porosity ε of 42% or higher. Porosity (ε) is sometimes referred to as "(pre-compression) porosity" below.

[0039] The porosity (ε) of the PO microporous membrane is preferably 42% or higher, more preferably 44% or higher, even more preferably 47% or higher, and particularly preferably 50% or higher, from the viewpoints of efficiently suppressing dendrite formation by reducing membrane resistance and improving ion diffusion, achieving a certain membrane strength and low air permeability, and having high ion conductivity and high power characteristics. Furthermore, from the viewpoints of suppressing dendrite growth by improving membrane strength and improving dielectric strength, the porosity (ε) is preferably 80% or lower, more preferably 70% or lower, even more preferably 65% ​​or lower, and particularly preferably 60% or lower.

[0040] (Average flow diameter measured by the half-dry method) The PO microporous membrane preferably has an average flow diameter d of 0.020 μm or more, and preferably 0.070 μm or less, as measured by the half-dry method. In this specification, the pore diameter of the PO microporous membrane refers to the average flow diameter measured using a palm porometer (Porous Materials, Inc.: CFP-1500AE) in accordance with the half-dry method.

[0041] By measuring the average flow diameter (d) using the half-dry method, the pore diameter of the narrowed portion of the pore structure in the microporous membrane can be determined. The average flow diameter d measured by the half-dry method is preferably 0.070 μm or less, more preferably 0.065 μm or less, even more preferably 0.060 μm or less, even more preferably 0.055 μm or less, and particularly preferably 0.050 μm or less, from the viewpoint of physically suppressing dendrite growth by densifying the microporous membrane and improving ion diffusion by increasing the number of pores per cross-section (B). Furthermore, from the viewpoint of preventing concentrated areas of increased resistance due to clogging, which would consequently promote dendrite formation, and from the viewpoint of improving cycle characteristics, the average flow diameter is preferably 0.020 μm or more, and more preferably 0.025 μm or more.

[0042] Means for controlling the pore size or porosity (ε) of the PO microporous membrane within the above numerical range include, for example, including high molecular weight PE of the molecular weight described later in the content described later in the method for manufacturing the PO microporous membrane described later; performing a stretching step before the extraction step; adjusting the stretching ratio in both the longitudinal and transverse directions in the stretching step to 5 times or more, and performing the stretching at a temperature of 128°C or lower; and adjusting the heat-fixing temperature to 115°C or higher and 140°C or lower during the heat-fixing (HS) step, adjusting the HS stretching ratio to 1.5 times or higher and 2.2 times or lower, and adjusting the HS relaxation ratio to 0.7 times or higher and less than 0.9 times, which can be used individually or in appropriate combinations.

[0043] (film thickness) The thickness of the PO microporous membrane is preferably 20 μm or less, more preferably 18 μm or less, even more preferably 16 μm or less, even more preferably 1 μm to 14 μm, particularly preferably 3 μm to 13 μm, and most preferably 5 μm to 12 μm, from the viewpoints of lithium-ion battery capacity, high ion permeability and good rate characteristics, delaying dendrite growth, and, when used for high-capacity batteries, contributing to improved battery capacity by reducing the volume occupied by the separator. The thickness of the PO microporous membrane is sometimes referred to as the "average thickness (before compression)" below.

[0044] The average film thickness (before compression) of the PO microporous film can be adjusted within the above numerical range by controlling the distance between the cast rolls, the cast clearance, the stretching ratio during the biaxial stretching process, the HS ratio, the HS temperature, etc., during the manufacturing process of the PO microporous film.

[0045] (Air permeability) The air permeability of the PO microporous membrane is considered to be equivalent to 100 cm² of air from the viewpoint of the output characteristics and cycle characteristics of lithium-ion batteries. 3 The per unit is preferably 200 seconds or less, more preferably 190 seconds or less. The lower limit of air permeability is 100 cm² of air, from the viewpoint of membrane strength. 3 The hit time is preferably 20 seconds or more.

[0046] (Puncture strength converted to basis weight, puncture strength, basis weight) The puncture strength of the PO microporous membrane, calculated based on basis weight, is 50 gf / (g / m²). 2 It is preferable that it be 50 gf / (g / m³). 2 A puncture strength equivalent to a base weight of ) or higher indicates a membrane structure that is highly durable and resistant to crushing under compressive stress. For example, during nail-piercing tests or pressure tests, PO microporous membranes used as separator substrates become less prone to rupture even with high porosity and low air permeability, thus improving the safety of lithium-ion batteries. The puncture strength equivalent to a base weight is measured by the method described in the examples. Along the TD of the membrane, the puncture strength not converted to a base weight (hereinafter simply referred to as puncture strength) is measured at a total of three points: two points 10% inward from both ends toward the center and one point in the center. The average value of these measurements is then divided by the base weight.

[0047] The advantages of controlling puncture strength are particularly pronounced when using electrodes that are prone to expansion and contraction within lithium-ion battery cells, and even more pronounced when using high-capacity electrodes used in automotive batteries, or silicon (Si)-containing negative electrodes. From this perspective, the basis weight equivalent puncture strength of PO microporous membrane is 40 gf / (g / m²). 2 )~150gf / (g / m 2 It is more preferable that it be 50 gf / (g / m³). 2 )~140gf / (g / m 2 It is even more preferable that it be 55 gf / (g / m³). 2 )~130gf / (g / m 2 It is even more preferable that it be 70 gf / (g / m³). 2 )~120gf / (g / m 2 It is especially preferable that it be )

[0048] From the same perspectives as above, as well as from the perspective of achieving a certain film strength and low air permeability, and from the perspective of improving short-circuit resistance, the basis weight of the PO microporous membrane is 1.0 g / m². 2 ~15g / m 2 It is preferable that it be within the range.

[0049] From the same viewpoints as above, from the viewpoint of safety when the lithium-ion battery is subjected to impact, and from the viewpoint of suppressing inter-electrode short circuits or voltage withstand failures caused by the separation of the separator due to foreign matter unintentionally mixed into the lithium-ion battery, the puncture strength of the PO microporous membrane is preferably 200 gf or more, more preferably 220 gf or more, even more preferably 250 gf or more, even more preferably 280 gf or more, and particularly preferably 300 gf or more. The upper limit of the puncture strength is not particularly limited, but can be determined according to the crystallinity of the membrane, the thermal shrinkage of the membrane, and the suppressed electrical resistance, and may be, for example, 900 gf or less, 850 gf or less, 700 gf or less, or 680 gf or less.

[0050] The puncture strength and basis weight puncture strength of the PO microporous membrane can be adjusted to within the numerical range described above by controlling, for example, the molecular weight and blending ratio of polymer raw materials such as polyolefins, the stretching ratio during the biaxial stretching process, the MD / TD stretching temperature during the biaxial stretching process, and the heat setting (HS) ratio during the manufacturing process of the PO microporous membrane.

[0051] (Components) Examples of PO microporous membranes include: porous membranes containing polyolefin resin; porous membranes containing polyketones, maleic anhydride-modified polyolefins, polycarbonates, polyethylene oxide, polyethylene terephthalate, polycycloolefins, polyethersulfones, polyamides, polyimides, polyimidamides, polyaramids, nylons, polytetrafluoroethylenes, and other resins in addition to polyolefin resin; woven fabrics made from polyolefin fibers; and nonwoven fabrics made from polyolefin fibers. Among these, microporous membranes containing polyolefin resin are preferred from the viewpoint of suppressing the decrease or increase in the electrical resistance of the membrane, as well as the compressive resistance and structural uniformity of the membrane, and microporous membranes containing polyethylene as the main component are more preferred.

[0052] (Ingredients included) The PO microporous membrane is formed from a resin composition containing a polyolefin resin. Optionally, the resin composition may further contain inorganic fillers or inorganic particles, resins other than polyolefins, etc.

[0053] From the viewpoint of improving the shutdown performance of lithium-ion batteries, the PO microporous membrane is preferably a porous membrane formed from a polyolefin resin composition in which polyolefin resin accounts for 50% to 100% by mass of the resin components constituting the porous membrane. More preferably, the proportion of polyolefin resin in the polyolefin resin composition is 60% to 100% by mass, even more preferably 70% to 100% by mass, and most preferably 95% to 100% by mass.

[0054] The polyolefin resin used is not particularly limited, and examples include polymers (e.g., homopolymers, copolymers, multi-stage polymers, etc.) obtained by polymerizing monomers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. These polymers can be used individually or in combination of two or more.

[0055] Furthermore, the polyolefin resin contained in the PO microporous membrane has a melting point preferably in the range of 120°C to 150°C, more preferably 125°C to 140°C, and / or a DSC 1st peak temperature preferably in the range of 136°C to 144°C, from the viewpoint of making the membrane rigid and improving its compressive resistance.

[0056] In particular, from the viewpoint of suppressing the decrease or increase in the electrical resistance of the membrane, and the compressive resistance and structural uniformity of the membrane, polyethylene, polypropylene, ethylene-propylene-polymers of other monomers, and mixtures thereof are preferred as polyolefin resins.

[0057] From the viewpoint of crystallinity, high strength, compressive resistance, and the ability to exhibit fuseability, a microporous membrane formed from a polyethylene composition in which polyethylene accounts for 50% to 100% by mass of the resin component constituting the microporous membrane is preferred. The proportion of polyethylene (PE) in the resin component constituting the microporous membrane is more preferably 60% to 100% by mass, even more preferably 70% to 100% by mass, even more preferably 80% to 100% by mass, and most preferably 90% to 100% by mass.

[0058] WAXS measurement of polyolefin microporous membranes is described in detail in the examples, and the crystallite size obtained by WAXS measurement is thought to be related to the structure of polyethylene, which improves the structural uniformity and compressibility of the membrane and the reaction uniformity in non-aqueous secondary batteries. When the main component of the PO microporous membrane is polyethylene (PE), it is preferable that the MDND plane (110) crystallite size of the polyethylene contained as the main component of the PO microporous membrane is 35.0 nm or less. The MDND plane (110) crystallite size of polyethylene can be measured by X-ray diffraction (XRD) or wide-angle X-ray scattering (WAXS), as described in detail in the examples.

[0059] When the MDND (110) crystallite size of polyethylene is 35.0 nm or less, the polyolefin microporous film containing that polyethylene becomes rigid, and the film's compressive resistance tends to improve. This allows for both high power output and high cycle characteristics even when electrode volume changes occur during the use of non-aqueous secondary batteries. From this viewpoint, the MDND (110) crystallite size of polyethylene, the main component of the film, is more preferably 33.0 nm or less, and even more preferably 10.0 nm to 32.0 nm, or 15.0 nm to 31.0 nm, or 15.0 nm to 30.0 nm, or 15.0 nm to 25.0 nm. In particular, when the MDND (110) crystallite size of polyethylene is 25.0 nm or less, the rigidity of the film increases, and its compressive resistance improves.

[0060] The MDND (110) crystallite size of polyethylene, the main component of polyolefin microporous membranes, can be adjusted to within the numerical range described above by controlling, for example, the molecular weight of the polyolefin raw material, the molecular weight of the polyethylene raw material, the stretching ratio during the biaxial stretching process, the stretching ratio and temperature during the biaxial stretching process, and the stretching ratio and temperature during the biaxial stretching process during the manufacturing process of polyolefin microporous membranes.

[0061] While we do not wish to be constrained by theory, the crystal period of polyolefin microporous films is thought to correlate with the post-compression porosity of the film. From the viewpoint described above, the crystal period of polyolefin microporous films is preferably within the range of 30.0 nm or more, or 30.0 nm to 60.0 nm, or 31.0 nm to 55.0 nm, or 32.0 nm to 50.0 nm, or 33.0 nm to 50.0 nm, or 35.0 nm to 50.0 nm, or 37.0 nm to 50.0 nm.

[0062] The crystal long period of a polyolefin microporous film can be measured, for example, by small-angle X-ray scattering (SAXS). For example, in the manufacturing process of a polyolefin microporous film, the molecular weight of the polyolefin raw material, the molecular weight of the polyethylene raw material, the stretching ratio during the biaxial stretching process, the stretching ratio and temperature during the biaxial stretching process, and the stretching ratio and temperature during the biaxial stretching process can be adjusted to fall within the numerical range described above.

[0063] Furthermore, from the viewpoint of high strength, compressibility, and suppression of electrical resistance when a PO microporous film is formed as a separator substrate, the PE content in the polyolefin resin is preferably 30% by mass or more, more preferably 50% by mass or more, even more preferably 70% by mass or more, particularly preferably 80% by mass or more, preferably 100% by mass or less, more preferably 95% by mass or less, and even more preferably 95% by mass or less. It should also be noted that a PE content of 50% or more is preferable from the viewpoint of exhibiting fuse behavior with high responsiveness.

[0064] The viscosity-average molecular weight (Mv) of the polyolefin resin used as a raw material for the PO microporous membrane is preferably 30,000 to 6,000,000, more preferably 80,000 to 3,000,000, and even more preferably 150,000 to 2,000,000. An Mv of 30,000 or higher is preferable because it tends to result in high strength due to the entanglement of polymers. On the other hand, an Mv of 6,000,000 or lower is preferable from the viewpoint of improving moldability in the extrusion and stretching processes.

[0065] In one embodiment, when the PO microporous membrane contains polyethylene (PE), the Mv of at least one type of PE is preferably 600,000 or more, more preferably 700,000 or more, from the viewpoint of membrane orientation and rigidity, and the upper limit of the Mv of PE may be, for example, 2,000,000 or less. From the viewpoint of reduced fluidity when the membrane melts and short-circuit resistance during nail penetration tests, the proportion of PE with an Mv of 600,000 or more in the polyolefin resin constituting the PO microporous membrane is preferably 30% by mass or more, more preferably 50% by mass or more, even more preferably 60% by mass or more, particularly preferably 70% by mass or more, and may even be 100% by mass.

[0066] In another embodiment, the PO microporous membrane (PO microporous layer) preferably contains 45% by mass or more, more preferably 50% by mass or more, even more preferably 70% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more, of the PE raw material having an Mv of 500,000 to 6,000,000, based on the mass of the PO microporous membrane. When the PO microporous layer contains 45% by mass or more of the PE raw material having an Mv of 500,000 to 6,000,000, the crystals tend to be highly oriented during stretching, and the PO microporous layer tends to become smaller in pore size or denser. From a similar viewpoint, the PO raw material used in the production of the PO microporous layer or PO microporous membrane is preferably an ethylene homopolymer, and more preferably an ethylene homopolymer having an Mv of 500,000 to 6,000,000.

[0067] The polyolefin resin content in the PO microporous membrane (PO microporous layer) is 50% by mass or more, preferably 60% by mass or more, preferably 70% by mass or more, preferably 80% by mass or more, and may be 90% by mass or more, or 100% by mass or less, based on the mass of the PO microporous membrane (PO microporous layer).

[0068] Furthermore, as a polyolefin resin, for example, low-density polyethylene (density 0.910 g / cm³) is used. 3 More than 0.930g / cm 3 (less than), linear low-density polyethylene (density 0.910 g / cm³) 3 More than 0.940g / cm 3 (less than), medium-density polyethylene (density 0.930 g / cm³) 3 More than 0.942g / cm 3 (less than), high-density polyethylene (density 0.942 g / cm³) 3 (The above) Ultra-high molecular weight polyethylene (density 0.910 g / cm³) 3 More than 0.970g / cm 3 Examples include polyethylene (less than 350 lb), isotactic polypropylene, atactic polypropylene, polybutene, ethylene propylene rubber, etc. These can be used individually or in combination of two or more. In particular, using polyethylene alone, polypropylene alone, or a mixture of polyethylene and polypropylene is preferred from the viewpoint of obtaining a uniform film.

[0069] Furthermore, from the perspective of safety in lithium-ion batteries that use a PO microporous membrane as a separator, polyolefin resin is suitable at 0.930 g / cm³. 3 More than 0.942g / cm 3 It is preferable to use medium-density polyethylene (MDPE) having a density of less than 1,000,000, and for example, other types of PE besides high-density polyethylene (HDPE) may also be used. Furthermore, from the viewpoint of improving the safety of lithium-ion batteries even when the PO microporous membrane is a thin film, medium-density polyethylene with a viscosity-average molecular weight of less than 1,000,000, and medium-density polyethylene with a viscosity-average molecular weight of 1,000,000 or more and a density of 0.930 g / cm³ are preferred. 3More than 0.942g / cm 3 It is preferable that at least one type selected from ultra-high molecular weight polyethylenes of less than 50% by mass be included, more preferably 60% by mass or more, and even more preferably 70% by mass or more, based on the mass of the PO microporous membrane.

[0070] From the viewpoint of ensuring safety in high-temperature ranges (160°C or higher) where safety is difficult to ensure with PE, polyolefin resins preferably contain polypropylene (PP). As for polypropylene, propylene homopolymer is preferred from the viewpoint of heat resistance. From the viewpoint of further improving heat resistance, it is more preferable for the polyolefin resin to contain both polyethylene as the main component and polypropylene. Therefore, from the viewpoint of film-forming properties in the stretching process and film-breaking resistance, the proportion of PP raw material in the PO raw material is preferably greater than 0% by mass and 10% by mass or less.

[0071] When using PE and PP together, the polyethylene should be medium-density polyethylene with a viscosity-average molecular weight of less than 1 million, or polyethylene with a viscosity-average molecular weight of 1 million to 2 million and a density of 0.930 g / cm³. 3 More than 0.942g / cm 3 Using at least one type of ultra-high molecular weight polyethylene (MMO) is preferable from the viewpoint of balancing strength and permeability and maintaining an appropriate fuse temperature.

[0072] The viscosity-average molecular weight of the polyolefin resin (measured according to the measurement method described in the examples below) is preferably 50,000 or more, more preferably 100,000 or more, more preferably 500,000 or more, more preferably 700,000 or more, and more preferably 1,000,000 or more, with an upper limit of preferably 6,000,000 or less, preferably 3,000,000 or less, and preferably 1,900,000 or less. Setting the viscosity-average molecular weight to 50,000 or more is preferable from the viewpoint of maintaining high melt tension during melt molding to ensure good moldability, or from the viewpoint of imparting sufficient entanglement to the resin to increase the strength of the microporous film. On the other hand, setting the viscosity-average molecular weight to 6,000,000 or less is preferable from the viewpoint of achieving uniform melt kneading and improving the moldability of the sheet, especially the thickness moldability.

[0073] The polydispersity (Mw / Mn) of the polyethylene raw material is preferably between 4.0 and 10.0. The polydispersity (Mw / Mn) is measured according to the measurement method described in the examples below. Although the reason is not clear, setting the polydispersity of the raw polymer within the above range tends to result in a PO microporous membrane with a moderately high pore ratio and a more uniform pore structure. From this viewpoint, a polydispersity of 6.0 or higher is more preferable for the polyethylene raw material, and 7.0 or higher is even more preferable.

[0074] Furthermore, the ratio of the z-average molecular weight to the weight-average molecular weight (Mz / Mw) of the polyethylene raw material is preferably between 2.0 and 7.0. The Mz / Mw is measured according to the measurement method described in the examples below. Although the reason is not entirely clear, setting the Mz / Mw of the raw polymer within the above range tends to result in a moderately high pore ratio and a more uniform pore structure in the PO microporous membrane. From this viewpoint, the Mz / Mw of the polyethylene raw material is more preferably 4.0 or higher, and even more preferably 5.0 or higher.

[0075] The above resin composition may optionally contain inorganic fillers or inorganic particles; resins other than polyolefins having polar groups; antioxidants such as phenolic, phosphorus-based, or sulfur-based agents; metal soaps such as calcium stearate or zinc stearate; and various known additives such as ultraviolet absorbers, light stabilizers, antistatic agents, anti-fogging agents, and coloring pigments.

[0076] The inorganic fillers for PO microporous films are not particularly limited and include, for example, oxide-based ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitride-based ceramics such as silicon nitride, titanium nitride, and boron nitride; ceramics such as silicon carbide, calcium carbonate, aluminum sulfate, barium sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amethyst, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fibers. These can be used individually or in combination of two or more. Among these, from the viewpoint of electrochemical stability, at least one selected from the group consisting of silica, alumina, and barium sulfate is preferred as the inorganic particle.

[0077] When the PO microporous membrane contains an inorganic filler, the amount is preferably 1% to 50% by mass, more preferably 3% to 30% by mass, even more preferably 5% to 20% by mass, and particularly preferably 10% to 18% by mass. Including 1% or more by mass of inorganic filler increases its affinity for highly polar electrolytes, thereby improving the liquid absorption properties of propylene carbonate. Having 50% or less of inorganic filler prevents excessive pore size increase in the PO microporous membrane, thereby suppressing overcharging during charging.

[0078] Resins having polar groups other than polyolefins are not particularly limited, but include olefin copolymers such as ethylene-vinyl acetate copolymer and ethylene-acrylic acid-maleic anhydride copolymer; polyesters such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; polyvinyl acetates; polyvinyl alcohols; polyvinyl acetals; polyvinyl butyrals; fluorinated polybenzoxazoles; acrylic resins; methacrylic resins such as polymethyl methacrylate; polyacrylonitriles; acrylonitrile-butadiene-sterene copolymer and other acrylonitrile copolymers; styrene copolymers such as sterene-methacrylic acid copolymer and styrene-acrylonitrile copolymer; and polystyrenes. Examples include ionic polymers such as sodium sulfonate and sodium polyacrylate; acetal resins; polyamides such as nylon 66; gelatin; gum arabic; polycarbonates and polyester carbonates; cellulose resins; polyketones; polyethers; phenolic resins; urea resins; epoxy resins; unsaturated polyester resins; alkyd resins; melamine resins; polyurethanes; diaryl phthalate resins; polyphenylene oxides; polyphenylene sulfide polysulfones; polyphenyl sulfones; polyimides; bismaleimide triazine resins; fluorine-containing resins; polyimideamides; polyethersulfones; polyetherketones; and polyetherimides. From the viewpoint of affinity with the electrolyte, fluorine-containing resins, polycarbonates, polyketones, polyethers, and polyacrylonitriles are preferred.

[0079] When the PO microporous membrane contains a polymer with polar groups, the amount is preferably 1% to 50% by mass, more preferably 3% to 35% by mass, even more preferably 5% to 30% by mass, and particularly preferably 10% to 23% by mass. Including 1% or more by mass of a polymer with polar groups increases the affinity to highly polar electrolytes, thereby improving the liquid absorption of propylene carbonate. Having 50% or less of a polymer with polar groups provides the PO microporous membrane with appropriate strength.

[0080] (Method for manufacturing PO microporous membranes) The method for manufacturing a PO microporous membrane is not particularly limited, but for example: A mixing step (a) involves mixing a resin composition containing a polyolefin resin, a pore-forming material, and various additives as desired. The mixture obtained in step (a) is melted and kneaded and then extruded in extrusion step (b), The sheet forming process (c) involves forming the extruded material obtained in process (b) into a sheet, The sheet-like molded product obtained in step (c) is stretched at least once in at least one axial direction in a primary stretching step (d), The extraction step (e) involves extracting pore-forming material from the primary stretched film obtained in step (d), A method is provided which includes a heat-fixing step (f) in which the extracted film obtained in step (e) is heat-fixed (HS) at a predetermined temperature.

[0081] The above method for manufacturing a PO microporous membrane makes it possible to provide a PO microporous membrane that, when used as a separator for lithium-ion batteries, can highly balance lithium dendrite suppression ability, chemical short-circuit suppression ability due to metallic foreign matter, and high power output characteristics and cycle characteristics. In particular, the method of stretching to MD and TD in the primary stretching step (d), followed by extraction step (e), and then heat-fixing to TD in the heat-fixing step (f) tends to make it easier to adjust the pore ratio and pore diameter of the resulting PO microporous membrane within the numerical range described above. It should be noted that the method for manufacturing a PO microporous membrane in this embodiment is not limited to the above manufacturing method, and various modifications are possible without departing from its essence.

[0082] [Mixing step (a)] Mixing step (a) is a step of mixing a resin composition containing a polyolefin resin, a pore-forming material, and various additives as desired. In mixing step (a), other components may be mixed with the resin composition as needed.

[0083] The pore-forming material can be any material as long as it is distinct from the PO resin material, and can be, for example, a plasticizer. As the plasticizer, a non-volatile solvent that can form a homogeneous solution above the melting point of the PO resin may be used, such as hydrocarbons such as liquid paraffin (LP) and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; or higher alcohols such as oleyl alcohol and stearyl alcohol.

[0084] The plasticizer content in the resin composition is preferably 60% to 90% by mass, more preferably 71% to 85% by mass, even more preferably 73% to 85% by mass, and particularly preferably 75% to 85% by mass. By adjusting the plasticizer content to 60% by mass or more, the number of pores in the film increases, ion diffusion increases, dendrite formation is efficiently suppressed, cycle characteristics are improved, and the melt viscosity of the resin composition decreases, suppressing melt fracture, which tends to improve film formation during extrusion. On the other hand, by adjusting the plasticizer content to 90% by mass or less, the elongation of the raw material during the film formation process can be suppressed.

[0085] In step (a), the resin composition containing PO may contain any additives. The additives are not particularly limited, but examples include resins or polymers other than polyolefin resins; inorganic fillers or inorganic particles; antioxidants such as phenolic compounds, phosphorus compounds, and sulfur compounds; metal soaps such as calcium stearate and zinc stearate; ultraviolet absorbers; light stabilizers; antistatic agents; antifogging agents; coloring pigments, etc.

[0086] From the viewpoint of adjusting the MD and TD absorption heights in the propylene carbonate liquid absorption test of the resulting PO microporous membrane to within the above numerical range, it is preferable to mix inorganic fillers such as silica and barium sulfate, and / or polar polymers such as polyketones, maleic anhydride-modified polyolefins, polycarbonates, and polyethylene oxide with the resin composition.

[0087] The total amount of these additives added is preferably more than 0 parts by mass and 50 parts by mass or less, more preferably 40 parts by mass or less, and even more preferably 35 parts by mass or less, per 100 parts by mass of polyolefin resin.

[0088] The mixing method in step (a) is not particularly limited, but for example, some of the raw materials may be pre-mixed using a Henschel mixer, ribbon blender, tumbler blender, etc., as needed. Among these, the method of mixing using a Henschel mixer is preferred.

[0089] [Extrusion process (b)] Extrusion step (b) is a step in which the resin composition obtained in step (a) is melted, kneaded, and extruded. In extrusion step (b), components other than polyolefin resin may be mixed with the resin composition as needed.

[0090] The method of melt-kneading in step (b) is not particularly limited, but for example, all raw materials, including the mixture mixed in step (a), can be melt-kneaded using a screw extruder such as a single-screw extruder or twin-screw extruder; a kneader; a mixer, etc. Among these, it is preferable to perform the melt-kneading using a screw with a twin-screw extruder. Furthermore, when performing melt-kneading, it is preferable to add the plasticizer in two or more stages, and when adding the additive in multiple stages, it is preferable to adjust the amount added in the first stage to be 80% by weight or less of the total amount added, from the viewpoint of suppressing aggregation of contained components and dispersing them uniformly. This is preferable from the viewpoint of improving the safety of the cell as a separator, as the resulting microporous membrane suppresses heat generation by shutting down over a large area.

[0091] When a pore-forming material is used in step (b), the temperature of the melt-mixing section is preferably less than 200°C from the viewpoint of uniformly mixing the resin composition. The lower limit of the temperature of the melt-mixing section is above the melting point of the polyolefin from the viewpoint of uniformly dissolving the polyolefin resin into the pore-forming material such as a plasticizer.

[0092] During the mixing process, although not particularly limited, it is preferable to first mix the raw material PO with an antioxidant at a predetermined concentration, then replace the surrounding atmosphere of the mixture with a nitrogen atmosphere, and perform melt mixing while maintaining the nitrogen atmosphere. The temperature during melt mixing is preferably 160°C or higher, more preferably 180°C or higher, and preferably less than 300°C.

[0093] In step (b), the kneaded material obtained after the kneading process is extruded by an extruder such as a T-die or annular die. At this time, it may be single-layer extrusion or co-extrusion. The conditions during extrusion are not particularly limited, and known methods can be used, for example. Furthermore, from the viewpoint of the film thickness of the resulting PO microporous film, it is preferable to control the (die) lip clearance and the like.

[0094] [Sheet forming process (c)] The sheet forming process (c) is a process of forming the extruded material obtained in the extrusion process (b) into a sheet. The sheet-like molded material obtained in the sheet forming process (c) may be a single layer or a laminate. The method of sheet forming is not particularly limited, but one example is a method of solidifying the extruded material by compression and cooling.

[0095] The compression cooling method is not particularly limited, but examples include a method of directly contacting the extruded material with a cooling medium such as cold air or cooling water; or a method of contacting the extruded material with a metal roll or press machine cooled with a refrigerant. Among these, the method of contacting the extruded material with a metal roll or press machine cooled with a refrigerant is preferred because it allows for easy control of the film thickness.

[0096] In the process of forming the molten material into a sheet after the melt-kneading step (b), it is preferable to set the temperature higher than the set temperature of the extruder. From the viewpoint of preventing thermal degradation of the polyolefin resin, the upper limit of the set temperature for sheet forming is preferably 300°C or less, and more preferably 260°C or less. For example, when continuously producing sheet-shaped molded products from an extruder, it is preferable that the set temperature of the process of forming the molten material into a sheet after the melt-kneading step, i.e., the path from the extruder outlet to the T-die, and the T-die, are set higher than the set temperature of the extrusion step, because it is possible to form the molten material into a sheet without the resin composition and the pore-forming material separating. Furthermore, from the viewpoint of the film thickness of the resulting PO microporous film, it is preferable to control the cast clearance and the like.

[0097] [Primary stretching process (d)] The primary stretching step (d) is a step in which the sheet-like molded product obtained in the sheet forming step (c) is stretched at least once in at least one axial direction. This stretching step (a stretching step performed before the next extraction step (e)) will be called "primary stretching," and the film obtained by primary stretching will be called "primary stretched film." In primary stretching, the sheet-like molded product can be stretched in at least one direction, and may be performed in both MD and TD directions, or in only one of MD or TD directions.

[0098] The primary stretching method is not particularly limited, but examples include uniaxial stretching using a roll stretcher; TD uniaxial stretching using a tenter; sequential biaxial stretching using a roll stretcher and a tenter, or a combination of multiple tenters; and simultaneous biaxial stretching using a simultaneous biaxial tenter or inflation molding. Among these, simultaneous biaxial stretching is preferred from the viewpoint of the physical stability of the resulting PO microporous film.

[0099] The stretching ratio of the MD and / or TD in the primary stretching is preferably 5 times or more, more preferably 6 times or more. When the stretching ratio of the MD and / or TD in the primary stretching is 5 times or more, the resulting PO microporous film tends to form dense fibrils, resulting in smaller pore sizes and improved strength. Alternatively, the stretching ratio of the MD and / or TD in the primary stretching is preferably 9 times or less, more preferably 8 times or less, or 7 times or less. When the stretching ratio of the MD and / or TD in the primary stretching is 9 times or less, breakage during stretching tends to be suppressed. When biaxial stretching is performed, either sequential stretching or simultaneous biaxial stretching may be used, but the stretching ratio in each axial direction is preferably 5 times or more and 9 times or less, more preferably 6 times or more and 8 times or less, or 6 times or more and 7 times or less.

[0100] The primary stretching temperature can be selected by referring to the raw material resin composition and concentration contained in the PO resin composition. The stretching temperature for MD and / or TD is preferably 110°C or higher, and more preferably 115°C or higher, from the viewpoint of reducing pore size and suppressing fracture. Furthermore, the stretching temperature for MD and / or TD is preferably 128°C or lower, more preferably 126°C or lower, even more preferably 124°C or lower, and particularly preferably 122°C or lower, from the viewpoint of increasing film strength or reducing pore size.

[0101] [Extraction step (e)] The extraction step (e) is a step of extracting pore-forming material from the primary stretched film obtained in the primary stretching step (d) to obtain an extracted film. Methods for removing pore-forming material include, for example, immersing the primary stretched film in an extraction solvent to extract the pore-forming material and then drying it thoroughly. The method for extracting pore-forming material may be either batch or continuous. Furthermore, it is preferable that the residual amount of pore-forming material, particularly plasticizer, in the porous film be less than 1% by mass.

[0102] When extracting pore-forming materials, it is preferable to use an extraction solvent that is a poor solvent for polyolefin resins and a good solvent for pore-forming materials or plasticizers, and whose boiling point is lower than the melting point of the polyolefin resin. Such extraction solvents are not particularly limited, but examples include hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; non-chlorinated halogenated solvents such as hydrofluoroethers and hydrofluorocarbons; alcohols such as ethanol and isopropanol; ethers such as diethyl ether and tetrahydrofuran; and ketones such as acetone and methyl ethyl ketone. These extraction solvents may be recovered and reused by operations such as distillation.

[0103] [Heat setting process (f)] The heat-fixing step (f) is a step in which the extracted film obtained in the extraction step (e) is heat-fixed at a predetermined temperature. The method of heat treatment in this step is not particularly limited, but one example is a heat-fixing method that involves stretching and relaxing operations using a tenter or roll stretcher.

[0104] The stretching operation in the heat-setting step (f) is an operation to stretch the PO microporous film in at least one of the MD and TD directions, and may be performed in both directions, or in only one of the MD or TD directions.

[0105] In the heat-setting process (f), the stretching ratios of MD and TD are preferably 1.5 times or more and 2.2 times or less, and more preferably 1.7 times or more and 2.1 times or less. From the viewpoint of developing film strength, the stretching ratios of MD and TD in process (f) are preferably 1.5 times or more, and from the viewpoint of suppressing breakage, they are preferably 2.2 times or less.

[0106] The stretching temperature in this stretching operation is preferably 115°C or higher, and more preferably 120°C or higher, from the viewpoint of suppressing breakage during stretching. Furthermore, the stretching temperature in the heat-setting process (f) is preferably 140°C or lower, more preferably 138°C or lower, more preferably 134°C or lower, more preferably 131°C or lower, more preferably 128°C or lower, or more preferably 125°C or lower, from the viewpoint of increasing the number of pores and achieving high porosity. In addition, when the stretching temperature is within the above numerical range, the pore size of the resulting PO microporous film tends to be easier to control.

[0107] The relaxation operation in the heat-setting process (f) is an operation to shrink the PO microporous film in at least one of the MD and TD directions, and may be performed in both directions, or in only one of the MD or TD directions. The relaxation ratio in the heat-setting process (f) is preferably 0.95 times or less, more preferably 0.90 times or less, and even more preferably 0.85 times or less. By having a relaxation ratio of 0.95 times or less in process (f), thermal shrinkage of the film tends to be suppressed. Furthermore, from the viewpoint of increasing the relaxation temperature, or from the viewpoint of increasing the number of pores and high porosity, the relaxation ratio is preferably 0.5 times or more, and more preferably 0.7 times or more. Here, "relaxation ratio" is the value obtained by dividing the dimensions of the film after the relaxation operation by the dimensions of the film before the relaxation operation, and if both MD and TD are relaxed, it is the value obtained by multiplying the relaxation ratio of MD by the relaxation ratio of TD. Relaxation ratio = (Dimensions of the membrane after relaxation (m)) / (Dimensions of the membrane before relaxation (m))

[0108] The relaxation temperature in this relaxation operation is preferably 115°C or higher, and more preferably 120°C or higher, from the viewpoint of suppressing fracture. Furthermore, from the viewpoint of increasing the number of pores and achieving high porosity, it is preferably 140°C or lower, more preferably 138°C or lower, more preferably 134°C or lower, more preferably 131°C or lower, more preferably 128°C or lower, or more preferably 125°C or lower. Moreover, when the relaxation temperature is within the above numerical range, the pore size of the resulting PO microporous film tends to be small and easily controlled uniformly.

[0109] The order of steps (a) to (f) above can be changed as desired, as long as it does not impair the effects of the present invention. After steps (a) to (f) above, the total stretching ratio of the PO microporous film is preferably in the range of 50 to 100 times, and more preferably in the range of 70 to 100 times. Here, "total stretching ratio" refers to the value obtained by multiplying the stretching ratio of MD and / or TD in the primary stretching step (d) by the stretching ratio and / or relaxation ratio in the heat setting step.

[0110] [Other processes] The method for manufacturing a PO microporous film may include steps other than those described in steps (a) to (f) above. These other steps are not particularly limited, but for example, in addition to the heat-setting step described above, a lamination step may be used to obtain a laminated PO microporous film by stacking multiple single-layer PO microporous films.

[0111] Furthermore, the method for manufacturing the PO microporous membrane may include surface treatment steps such as electron beam irradiation, plasma irradiation, F2O2 gas treatment, corona discharge treatment, UV treatment, UV ozone treatment, sulfur dioxide gas treatment, oxidation treatment with potassium dichromate solution or potassium permanganate solution, etching treatment with sodium treatment solution, application of surfactant, and chemical modification, from the viewpoint of controlling the absorption height in the propylene carbonate liquid absorption test described above. Among these, F2O2 gas treatment or plasma irradiation is preferred from the viewpoint of being excellent in treating the interior in the thickness direction of the PO microporous membrane. Moreover, a material for an inorganic porous layer or an organic (adhesive) layer may be coated on one or both sides of the PO microporous membrane to obtain a PO microporous membrane having at least one layer.

[0112] [Formation of an inorganic coating layer] From the viewpoints of safety, dimensional stability, and heat resistance, an inorganic coating layer can be provided on the surface of the PO microporous film. The inorganic coating layer is a layer containing inorganic components such as inorganic particles, and may optionally contain a binder resin to bind the inorganic particles together, a dispersant to disperse the inorganic particles in a solvent, and so on.

[0113] Examples of materials for inorganic particles contained in the inorganic coating layer include oxide ceramics such as alumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitride ceramics such as silicon nitride, titanium nitride, and boron nitride; ceramics such as silicon carbide, calcium carbonate, magnesium sulfate, aluminum sulfate, barium sulfate, aluminum hydroxide, aluminum hydroxide oxide (AlOOH, alumina monohydrate, or boehmite), potassium titanate, talc, kaolinite, decite, nacrite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amethyst, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fibers. Inorganic particles may be used individually or in combination.

[0114] Examples of binder resins include conjugated diene polymers, acrylic polymers, polyvinyl alcohol-based resins, and fluoropolymer resins. The binder resin can also be in the form of latex and may contain water or an aqueous solvent.

[0115] Dispersants are substances that adsorb to the surface of inorganic particles in a slurry and stabilize the inorganic particles through electrostatic repulsion, etc. Examples include polycarboxylates, sulfonates, polyoxyethers, and surfactants.

[0116] The inorganic coating layer can be formed, for example, by applying and drying a slurry of the components described above onto the surface of a PO microporous film.

[0117] [Formation of the adhesive layer] To prevent deformation or swelling due to gas generation in laminate-type batteries, which have recently been increasingly adopted for automotive batteries to increase energy density, an adhesive layer containing a thermoplastic resin can be provided on the surface of the PO microporous membrane.

[0118] The thermoplastic resin included in the adhesive layer is not particularly limited and includes, for example, polyolefins such as polyethylene or polypropylene; fluorine-containing resins such as polyvinylidene fluoride and polytetrafluoroethylene; fluorine-containing rubbers such as vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, and ethylene-tetrafluoroethylene copolymer; styrene-butadiene copolymer and its hydride, acrylonitrile-butadiene copolymer and its hydride, acrylonitrile-butadiene-styrene Examples include rubbers such as styrene copolymers and their hydrogenates, (meth)acrylic acid ester copolymers, styrene-acrylic acid ester copolymers, acrylonitrile-acrylic acid ester copolymers, ethylene propylene rubber, polyvinyl alcohol, and polyvinyl acetate; cellulose derivatives such as ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, and carboxymethyl cellulose; and resins with a melting point and / or glass transition temperature of 180°C or higher, such as polyphenylene ether, polysulfone, polyethersulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyamide, and polyester.

[0119] Furthermore, after the heat setting process (f), the lamination process, or the surface treatment process, the master roll on which the PO microporous film is wound can be subjected to an aging process under predetermined temperature conditions, and then the master roll can be reversed. This tends to make it easier to obtain a PO microporous film with higher thermal stability than the PO microporous film before rewinding. When aging and rewinding the master roll, the temperature during the aging process is not particularly limited, but is preferably 35°C or higher, more preferably 45°C or higher, and even more preferably 60°C or higher. Also, from the viewpoint of maintaining the permeability of the PO microporous film, the temperature during the aging process of the master roll is preferably 120°C or lower. The time required for the aging process is not particularly limited, but 24 hours or more is preferable because the above effects are more likely to manifest.

[0120] <Lithium-ion battery separator> The separator for lithium-ion batteries may be a single-layer separator made of the PO microporous film described above, or a laminated separator including a substrate such as a PO microporous film and at least one layer disposed on the substrate. Generally, the at least one layer included in the laminated separator may have properties such as insulating, adhesive, thermoplastic, or inorganic porous, and may be formed as a single film or as a pattern formed by dot coating, stripe coating, etc.

[0121] From the viewpoint of improving Li ion diffusion in lithium-ion batteries, flattening the Li deposition on the electrode surface, and suppressing battery electrolyte depletion, the separator preferably comprises an ion-conducting layer disposed on at least a portion of at least one side of the PO microporous membrane. The ion-conducting layer may be organic or inorganic, but it is more preferable that it be made of inorganic material. The structure and formation method of the inorganic ion-conducting layer may be the same as that of the inorganic coating layer described above.

[0122] In addition to the inorganic coating layer configuration described above, materials for the ion-conducting layer made of inorganic materials include lithium silicate, lithium borate, lithium aluminate, lithium phosphate, lithium nitride, lithium phosphorus-oxynitride, lithium silisulfide, lithium germanosulfide, lithium lanthanum oxide, lithium tantalum oxide, lithium niobium oxide, lithium titanium oxide, lithium fluoride, lithium borosulfide, lithium aluminosulfide, lithium phosphate sulfide, graphene oxide, and glass selected from the group consisting of combinations thereof.

[0123] Examples of ion-conducting layers made of organic materials are not particularly limited, but include polyesters such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; polyvinyl acetates; polyvinyl alcohols; polyvinyl acetals; polyvinyl butyrals; fluorinated polybenzoxazoles; acrylic resins; methacrylic resins such as polymethyl methacrylate; polyacrylonitriles; acrylonitrile-butadiene-sterene copolymers; styrene copolymers such as sterene-methacrylic acid copolymers and styrene-acrylonitrile copolymers; and ion-conducting layers such as sodium polystyrene sulfonate and sodium polyacrylate. Examples include polypolymers; acetal resins; polyamides such as nylon 66; gelatin; gum arabic; polycarbonates, polyester carbonates; cellulose resins; polyketones; polyethers; phenolic resins; urea resins; epoxy resins; unsaturated polyester resins; alkyd resins; melamine resins; polyurethanes; diaryl phthalate resins; polyphenylene oxides; polyphenylene sulfide polysulfones; polyphenyl sulfones; polyimides; bismaleimide triazine resins; fluorine-containing resins; polyimideamides; polyethersulfones; polyetherketones; polyetherimides, etc. From the viewpoint of affinity with the electrolyte, fluorine-containing resins, polycarbonates, polyketones, polyethers, and polyacrylonitriles are preferred.

[0124] Furthermore, from the viewpoint of planarizing the Li deposition on the electrode surface, it is also preferable to position the ion-conducting layer of the separator so as to be in contact with or facing the negative electrode surface (or Li deposition surface) of the lithium-ion battery, thereby eliminating the Li ion concentration distribution and controlling the Li deposition morphology (suppressing Li dendrite formation).

[0125] <Lithium-ion battery> In another aspect of the present invention, a lithium-ion battery is provided. Generally, a lithium-ion battery consists of a positive electrode, a negative electrode, a liquid, gel, or solid electrolyte, a separator, and an outer casing. Examples of lithium-ion batteries include non-aqueous electrolyte lithium-ion secondary batteries, non-aqueous gel secondary batteries, non-aqueous solid secondary batteries, lithium-ion capacitors, electric double-layer capacitors, and the like.

[0126] A lithium-ion battery may comprise a separator, a positive electrode plate, a negative electrode plate, and a non-aqueous electrolyte (containing a non-aqueous solvent and a metal salt dissolved therein). Specifically, for example, a positive electrode plate containing a transition metal oxide capable of intercalating and releasing lithium ions, and a negative electrode plate capable of intercalating and releasing lithium ions are wound or stacked facing each other via a separator, the non-aqueous electrolyte is retained, and the battery is housed in a device container or device casing.

[0127] In the lithium-ion battery according to the second embodiment of the present invention, the liquid absorption height of MD and TD in a propylene carbonate liquid absorption test is 1.0 mm or more and 50 mm or less, and the tensile modulus of MD and TD is 3000 kgf / cm². 2 The lithium-ion battery according to the second embodiment is characterized by having a PO microporous membrane as a separator. Furthermore, in the lithium-ion battery according to the second embodiment, it is preferable that the ion conducting layer is located between the separator and the metal anode. The lithium-ion battery characterized in the above can be made high-capacity by having a metal anode and can suppress degradation due to electrolyte depletion.

[0128] In the second embodiment, based on (a) and (b) described above as mechanisms that make lithium-ion batteries prone to electrolyte depletion, the affinity and compressibility of the PO microporous membrane usable as a separator with the electrolyte can be improved, and the Li ion diffusion can be increased by the ion conductive layer present between the separator and the metal anode, thereby planarizing the Li deposition.

[0129] The affinity of PO microporous membranes, which can be used as separators, for electrolytes has been determined by the fact that the absorption height of MD and TD in propylene carbonate liquid absorption tests is between 1.0 mm and 50 mm, and the compressive resistance is determined by the tensile modulus of MD and TD being 3000 kgf / cm². 2 The above is used to specify the details. The separator according to the second embodiment may be a single-layer separator made of a PO microporous film according to the first embodiment, or a multilayer separator comprising a substrate such as a PO microporous film and at least one layer such as an inorganic coating layer, adhesive layer, or ion conductive layer as described above.

[0130] The ion-conducting layer between the separator and the metal anode is preferably made of inorganic material, from the viewpoint of further increasing Li ion diffusion and planarizing the Li deposition. The structure and formation method of the ion-conducting layer according to the second embodiment may be the same as the ion-conducting layer that can be formed on the PO microporous film according to the first embodiment, as described above.

[0131] The positive electrode is described below. As the positive electrode active material, for example, lithium composite metal oxides such as lithium nickelate, lithium manganeseate, or lithium cobaltate, or lithium composite metal phosphates such as lithium iron phosphate can be used. The positive electrode active material is mixed with a conductive agent and a binder, and the positive electrode paste is applied to a positive electrode current collector such as aluminum foil and dried, then rolled to a predetermined thickness and cut to a predetermined size to form a positive electrode plate. Here, as the conductive agent, a metal powder that is stable under positive electrode potential, for example, carbon black such as acetylene black or graphite material can be used. As the binder, a material that is stable under positive electrode potential, for example, polyvinylidene fluoride, modified acrylic rubber, or polytetrafluoroethylene can be used.

[0132] The metal anode is described below. As the anode active material, lithium (Li) metal, copper (Cu) foil, or a metal material capable of intercalating lithium can be used, and Li metal is preferred from the viewpoint of compatibility with the separator. In the second embodiment, an ion conductive layer is provided in the separator, and an ion conductive layer can also be provided in the metal anode.

[0133] It is preferable to provide an ion-conducting layer on the surface of the negative electrode facing the separator. The ion-conducting layer may be made of organic or inorganic material, but it is more preferable to be made of inorganic material. Examples of the composition of the ion-conducting layer include those similar to the ion-conducting layer on the separator described above, but alumina, titanium oxide, lithium phosphate, lithium nitride, and lithium silicate are particularly preferred.

[0134] Any known method can be used to form an ion-conducting layer on a metal anode. Examples include physical vapor deposition, chemical vapor deposition, extrusion, and electroplating. Suitable physical or chemical vapor deposition methods, but not limited to these, include thermal evaporation (e.g., resistance heating, induction heating, radiation heating, electron beam heating, etc.), sputtering (e.g., diode, DC magnetron, RF, RF magnetron, pulse, dual magnetron, AC, MF, and reaction sputtering, etc.), plasma chemical vapor deposition, laser chemical vapor deposition, ion plating, cathode arc method, jet vapor deposition, and laser ablation. Alternatively, a method may be used in which the metal anode is treated with a gas or liquid to form an ion-conducting layer by a chemical reaction, or a method may be used to form an ion-conducting layer by a reaction during battery charging and discharging.

[0135] Non-aqueous electrolytes are described below. Non-aqueous electrolytes generally consist of a non-aqueous solvent and metal salts such as lithium salts, sodium salts, and calcium salts dissolved in it.

[0136] In the second embodiment, from the viewpoint of planarizing the Li precipitate by stabilizing (low-resisting) the reduction product, the following 1-4: 1. The Li ion concentration is 1.3 mol / L or higher; 2. Contains an electrolyte salt having a bisfluorosulfonylimide structure; 3. Includes carbonates in which hydrogen atoms are substituted with halogen atoms; and 4. The electrolyte is an ionic liquid; An electrolyte characterized by at least one of the above is preferred.

[0137] The Li ion concentration in the non-aqueous electrolyte is preferably 1.3 mol / L or higher, more preferably 1.8 to 5.0 mol / L, and even more preferably 2.8 to 3.5 mol / L. A Li ion concentration of 1.3 mol / L or higher suppresses the generation of high-resistance reduction products by increasing the amount of solvated solvent molecules, maintaining a flat metal deposition surface and reducing the specific surface area, thereby suppressing electrolyte consumption. A Li ion concentration of 5.0 mol / L or lower ensures sufficient ionic conductivity.

[0138] Examples of electrolyte salts used in non-aqueous electrolytes include LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB 10 Cl 10Examples include LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, LiSCN, LiC(CF3SO2)3, (CF3SO2)2NLi, (FSO2)2NLi, lithium chloroborane, lithium lower aliphatic carboxylate, lithium tetraphenyl lithium borate, lithium imide, lithium bistrifluoromethanesulfonylimide represented by (CF3SO2)2NLi (LiTFSI, Lithium bis(trifluoromethane sulfonyl)imide), and lithium bisfluorosulfonylimide represented by (FSO2)2NLi (LiFSI, Lithium bis(fluorosulfonyl)imide). Among these, those having a bisfluorosulfonylimide structure such as lithium bistrifluoromethanesulfonylimide, which has high chemical stability, are preferred from the viewpoint of being less likely to create voids without electrolyte that inhibit ion permeation due to gas generation.

[0139] Non-aqueous electrolyte solvents include carbonate solvents (e.g., propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, butylene carbonate, vinylene carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butyl ethyl carbonate, dipropyl carbonate, etc.), halogenated carbonate solvents in which some or more hydrogen atoms of the carbonate solvent are substituted with halogen atoms (e.g., fluoroethylene carbonate, difluoroethylene carbonate, etc.), lactone solvents (e.g., γ-butyrolactone, etc.), ester solvents (e.g., methyl acetate, ethyl acetate, etc.), and chain ether solvents (e.g., 1,2-dimethoxyethane, dimethyl Examples of aprotic solvents include ethers (e.g., diethyl ether), cyclic ether solvents (e.g., tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane, 4-methyldioxolane), ketone solvents (e.g., cyclopentanone), sulfolane solvents (e.g., sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane), sulfoxide solvents (e.g., dimethyl sulfoxide), nitrile solvents (e.g., acetonitrile, propionitrile, benzonitrile), amide solvents (e.g., N,N-dimethylformamide, N,N-dimethylacetamide), urethane solvents (e.g., 3-methyl-1,3-oxazolidine-2-one), and polyoxyalkylene glycol solvents (e.g., diethylene glycol). In particular, from the viewpoint of facilitating the formation of low-resistance reduction products, suppressing the non-uniformity of ion permeation to maintain a flat metal deposition surface, and reducing electrolyte consumption by reducing the specific surface area, the non-aqueous electrolyte solvent preferably contains halogenated carbonate solvents, and more preferably contains fluoroethylene carbonate or difluoroethylene carbonate.

[0140] If the non-aqueous electrolyte solvent contains a halogenated carbonate solvent, the content of the halogenated carbonate solvent is preferably 5% to 50% by volume relative to the total volume of the non-aqueous electrolyte solvent.

[0141] Ionic liquids can also be used as non-aqueous electrolytes. Ionic liquids are preferred because they have high ionic conductivity, suppress heterogeneity by increasing ion diffusion, maintain a flat metal deposition surface, and reduce electrolyte consumption by reducing the specific surface area. Ionic liquids are composed of cations and anions.

[0142] Ionic liquids are classified according to the cation species into imidazolium, ammonium, pyrrolidinium, piperidinium, pyridinium, morpholinium, phosphonium, and sulfonium systems. Examples of cations that make up imidazolium ionic liquids include alkylimidazolium cations such as 1-butyl-3-methylimidazolium (BMI).

[0143] Cationic cations that make up ammonium-based ionic liquids include, for example, tetraamylammonium and alkylammonium cations such as N,N,N-trimethyl-N-propylammonium. Cationic cations that make up pyrrolidinium-based ionic liquids include, for example, alkylpyrrolidinium cations such as N-methyl-N-propylpyrrolidinium (Py13) and 1-butyl-1-methylpyrrolidinium. Cationic cations that make up piperidinium-based ionic liquids include, for example, alkylpiperidinium cations such as N-methyl-N-propylpiperidinium (PP13) and 1-butyl-1-methylpiperidinium. Cationic cations that make up pyridinium-based ionic liquids include, for example, alkylpyridinium cations such as 1-butylpyridinium and 1-butyl-4-methylpyridinium. Cationic cations that make up morpholinium-based ionic liquids include, for example, alkylmorpholinium cations such as 4-ethyl-4-methylmorpholinium. Cationic cations that make up phosphonium-based ionic liquids include, for example, alkylphosphonium cations such as tetrabutylphosphonium and tributylmethylphosphonium. Examples of cations that constitute sulfonium-based ionic liquids include alkylsulfonium cations such as trimethylsulfonium and tributylsulfonium. Examples of anions that pair with these cations include bis(trifluoromethanesulfonyl)imide (TFSI), bis(fluorosulfonyl)imide, tetrafluoroborate (BF4), hexafluorophosphate (PF6), bis(pentafluoroethanesulfonyl)imide (BETI), trifluoromethanesulfonate (triflate), acetate, dimethylphosphate, dicyanamide, and trifluoro(trifluoromethyl)borate. These ionic liquids may be used individually or in combination.

[0144] The electrolyte salt may be included in a non-aqueous solvent or ionic liquid. The electrolyte salt can be one that can be uniformly dispersed in the solvent. A lithium salt can be used if its cation consists of lithium and the anion described above. Examples include, but are not limited to, lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), lithium tetrafluoroborate (LiBF4), lithium hexafluorophosphate (LiPF6), and lithium triflate. These electrolyte salts may be used individually or in combination.

[0145] Ether-based solvents consist of a solvated electrolyte salt and a solvated ionic liquid. As an example of an ether-based solvent, there is a known glyme (RO(CH2CH2O)) which exhibits properties similar to those of an ionic liquid. n -R'{wherein R and R' are saturated hydrocarbons and n is an integer} is a general term for symmetric glycol diethers. From the viewpoint of ionic conductivity, tetraglyme (tetraethylene dimethyl glycol, G4), triglyme (triethylene glycol dimethyl ether, G3), pentaglyme (pentaethylene glycol dimethyl ether, G5), and hexaglyme (hexaethylene glycol dimethyl ether, G6) can be preferably used. In addition, crown ether ((-CH2-CH2-O)) can be used as an ether solvent. n Macrocyclic ethers represented by {wherein n is an integer} can be used. Specifically, 12-crown-4, 15-crown-5, 18-crown-6, dibenzo-18-crown-6, etc., can be preferably used, but are not limited to these. These ether solvents may be used individually or in combination. Tetraglyme and triglyme are preferred because they can form complex structures with solvated electrolyte salts.

[0146] Lithium salts such as LiFSI, LiTFSI, and LiBETI can be used as solvating electrolyte salts, but are not limited to these. As non-aqueous solvents, mixtures of ether-based solvents and solvating electrolyte salts may be used individually or in combination.

[0147] Unless otherwise specified, the various parameters mentioned above are measured in accordance with the measurement methods described in the examples below. [Examples]

[0148] Next, this embodiment will be described in more detail with reference to examples and comparative examples, but this embodiment is not limited to the following examples unless it exceeds the gist of the embodiment. The physical properties in the examples were measured by the following methods. Unless otherwise specified, measurements were taken in an environment of 23°C and 40% humidity.

[0149] (1) Total pore volume and pore ratio using the single-point method (1a) Total pore volume by single-point method (also known as: volume adsorption by single-point method) (pore diameter 98 nm) (cm 3 / g) A nitrogen gas adsorption test was conducted using the constant volume method based on JIS Z8831-2:2010 Pore Distribution and Pore Characteristics of Powders (Solids), "Part 2: Measurement Method for Mesopores and Macropores by Gas Adsorption". Specifically, the amount of gas adsorbed in standard conditions at temperature and pressure (STP) with a relative nitrogen pressure (p / p0) of 0.98 was defined as the volume equivalent of the liquid volume of the adsorbate, and the single-point volume adsorption amount (pore diameter 98 nm) was derived. At this time, a relative pressure (p / p0) of 0.98 is expressed as rp = rk + t, assuming a cylindrical pore and its pore radius rp, and corresponds to a pore diameter of 98.8 nm. Here, rk is the radius of curvature of the adsorbate condensed in the pore, and is given by the following equation. rk = -0.953 / ln(p / p0). Furthermore, t is the average thickness of the multilayer nitrogen adsorption film at relative pressure obtained from experimentally or theoretically determined reference adsorption isotherms, and is expressed by the following formula. t = 0.354 * [-5 / ln(p / p0)]^1 / 3

[0150] (1b) Pore percentage (pore diameter 98 nm or less) (%) Based on the total pore volume (pore diameter 98 nm or less) A obtained by the single-point method and the resin raw material density, the porosity consisting of pores with a diameter of 98 nm or less was calculated using the following formula. Porosity P (%) consisting of pores with a diameter of 98 nm or less = A / (A + 1 / resin raw material density) × 100 The pore ratio was calculated by determining the proportion of the above porosity to the total porosity of the membrane, as described later. Pore ​​density (pore diameter 98 nm or less) (%) = (P / total porosity of the film) × 100

[0151] (2) Density (g / cm 3 ) The density of the sample was measured using the density gradient tube method (23°C) in accordance with JIS K7112:1999.

[0152] (3) Film thickness (μm) The thickness of the PO microporous film was measured at room temperature of 23±2℃ using a microthickness gauge, KBM (trademark), manufactured by Toyo Seiki Co., Ltd. However, the average film thickness (μm) of the microporous film (before compression) and the film thickness (μm) of the porous film (before compression) were measured as follows.

[0153] [Average film thickness of microporous membrane (before compression) (μm)] The thickness was measured using a micro-thickness gauge (Type KBM, terminal diameter Φ5mm) manufactured by Toyo Seiki at an ambient temperature of 23±2℃. To measure the thickness, a 10cm × 10cm sample of the microporous film was first collected. Multiple films were then stacked to create a thickness of 15μm or more, and the thickness was measured at nine points. The average value was then taken and divided by the number of stacked films to determine the thickness of a single film.

[0154] (4) Porosity of the entire membrane (%) A 10cm x 10cm square sample is cut from the microporous membrane, and its volume (cm³) is measured. 3 Calculate the mass (g) and density (g / cm³), and then compare them with the density (g / cm³). 3 The porosity was calculated using the following formula. Porosity (%) = (Volume - Mass / Density) / Volume × 100

[0155] (5) Air permeability (sec / 100cm 3 ) The air permeability was defined as the air resistance in accordance with JIS P-8117. The air permeability of microporous membranes was measured in accordance with JIS P-8117, using a Gurley-type air permeability meter, G-B2 (trademark), manufactured by Toyo Seiki Co., Ltd., under conditions of 23°C and 40% humidity. The air permeability of multilayer porous membranes or the air permeability resistance of PO microporous membranes was measured and defined as the air permeability.

[0156] However, separate from (4) and (5) above, the porosity (%) (before compression) and the air permeability (seconds / 100cm) (before compression) are also required. 3 The following measurements were taken for the ) measurement and compression press test.

[0157] [(Uncompressed) porosity (%)] A sample measuring 3cm x 3cm, 1cm x 1cm, 5cm x 5cm, or 10cm x 10cm is cut from the polyolefin microporous membrane, and its volume (cm³) is determined from the measurement results of the aforementioned film thickness. 3 Calculate the mass (g) and density (g / cm³), and then compare them with the density (g / cm³). 3 The calculation was performed using the following formula. Porosity (%) = (Volume - Mass / Density of mixed composition) / Volume × 100 The density of the mixed composition was calculated using the density of the polyolefin resin and other components used, along with their respective mixing ratios. Furthermore, the porosity of a multilayer porous membrane (before compression) can be calculated using the following formula. Porosity of a multilayer porous film = (Porosity of the polyolefin resin microporous film used as the base material) × (Average film thickness of the polyolefin resin microporous film used as the base material) ÷ ​​(Total thickness of the multilayer porous film) + (Porosity of the coating layer) × (Thickness of the coating layer) ÷ (Total thickness of the multilayer porous film) Here, the porosity of the coating layer was calculated assuming a porosity of 50% for the multilayer porous film. If the porosity of the coating layer is not 50%, the porosity of the coating layer can be calculated as needed, similar to the porosity of the polyolefin microporous film, using the same formula as above. Specifically, the thickness of the coating layer is measured by direct observation with a SEM or by the change in film thickness before and after coating. The volume of a coating layer sample of a specific area is determined, and the porosity of the coating layer is calculated using the mass-average density of the coating components calculated from the material ratio of the constituent components of the coating layer.

[0158] [Air permeability (before compression) (seconds / 100cm)] 3 )] The air permeability was measured using the "EGO2" air permeability meter manufactured by Asahi Seiko Co., Ltd. The air permeability measurement is obtained by taking measurements at three points along the width direction (TD) of the membrane: two points 10% inward from both ends towards the center, and one point in the center. The average of these measurements is then calculated.

[0159] [Compression press test] A 0.8 mm thick rubber cushioning material, a 0.1 mm thick PET film, two microporous membranes, the aforementioned PET film, and the aforementioned cushioning material were laminated in that order. The resulting laminate was left to stand, and a compression test was performed by applying pressure to one side of the cushioning material surface of the laminate. The microporous membranes used were 5 × 5 cm square, and the average film thickness (average of 9 points), basis weight, and air permeability were measured before use in the press test. The porosity before the compression press test was also calculated from the basis weight and average film thickness. The compression test was performed using a press machine at a temperature of 25°C and a compression time of 3 minutes under a pressure of 10 MPa. After unloading, the microporous membrane was removed from the laminate one hour later, and the average film thickness (average of 9 points) and air permeability after compression were measured. The porosity after compression was calculated from the basis weight and the average film thickness after compression.

[0160] (6) Weight (g / m 2 ), piercing strength (gf) and area weight conversion piercing strength (gf / (g / m 2 )) Basis is measured per unit area (1 m²). 2This is the weight (g) of the polyolefin microporous membrane per unit area (1m x 1m). After sampling a 1m x 1m area, the weight was measured using a Shimadzu Corporation electronic balance (AUW120D). If sampling a 1m x 1m area is not possible, the weight was measured by cutting out an appropriate area and then measured per unit area (1m). 2 The weight was converted to grams per unit.

[0161] A microporous membrane was fixed using a Kato Tech KES-G5 (trademark) handy compression tester with a sample holder having an opening diameter of 11.3 mm. Next, a puncture test was performed on the central part of the fixed microporous membrane with a needle tip radius of curvature of 0.5 mm and a puncture speed of 2 mm / sec under room temperature of 23°C and 40% humidity. The puncture strength (gf) was measured as the maximum puncture load. The measured values ​​for the puncture test were taken at three points along the TD of the membrane: two points 10% inward from both ends toward the center, and one point in the center. The average value of these measurements was then calculated. The puncture strength converted to base weight is calculated using the following formula. Penetration strength converted to basis weight [gf / (g / m 2 )]=Piercing strength [gf] / Weight [g / m 2 ] Here, regarding the puncture strength and basis weight equivalent puncture strength of a multilayer porous film having at least one layer on a polyolefin microporous film substrate, the properties were evaluated using the puncture strength and basis weight equivalent puncture strength of the polyolefin microporous film substrate, from the viewpoint of evaluating the strength of the resin and the strength per basis weight.

[0162] (7) Number of pores (pores / μm) of porous membrane or porous layer determined by the gas-liquid method 2 ) It is known that the fluid inside a capillary follows a Knudsen flow when the mean free path of the fluid is greater than the pore diameter of the capillary, and a Poiseuille flow when it is smaller. Therefore, we assume that the airflow in the permeability measurement of a porous membrane or porous layer follows a Knudsen flow, and the water flow in the permeability measurement of a porous membrane or porous layer follows a Poiseuille flow. In this case, the average pore size d (μm) of the porous film and the curvature ratio τ a (Dimensionless) is the air permeation rate constant R gas(m 3 / (m 2 ·sec·Pa)), the water permeation rate constant R liq (m 3 / (m 2 ·sec·Pa)), the molecular velocity ν (m / sec) of air, the viscosity η (Pa·sec) of water, the standard pressure P s (= 101325 Pa), the porosity ε (%), and the film thickness L (μm) were obtained using the following equation. d = 2ν × (R liq / R gas ) × (16η / 3Ps) × 10 6 τ a = (d × (ε / 100) × ν / (3L × P s × R gas )) 1 / 2 Here, R gas was obtained using the following equation from the air permeability (sec). R gas = 0.0001 / (air permeability × (6.424 × 10 -4 ) × (0.01276 × 101325)) Also, R liq was obtained using the following equation from the water permeability (cm 3 / (cm 2 ·sec·Pa)). Obtained. R liq = water permeability / 100 The water permeability was obtained as follows. A porous membrane or porous layer previously immersed in ethanol was set in a liquid permeation cell made of stainless steel with a diameter of 41 mm. After washing the ethanol on the membrane or layer with water, water was permeated at a differential pressure of about 50000 Pa, and the water permeation amount (cm 3 ) after 120 seconds elapsed was used to calculate the water permeation amount per unit time, unit pressure, and unit area, which was taken as the water permeability. Also, ν was obtained using the following equation from the gas constant R (= 8.314), the absolute temperature T (K), the circumference ratio π, and the average molecular weight M of air (= 2.896 × 10 -2 kg / mol). ν = ((8R × T) / (π × M)) 1 / 2 Furthermore, the number of pores B (pieces / μm 2The value was obtained from the following formula. B = 4 × (ε / 100) / (π × d 2 ×τ a )

[0163] (8) Pore diameter (8a) Average flow diameter (μm) by half-dry method Following the half-dry method, the average pore size (μm) was measured using a palm porometer (Porous Materials, Inc.: CFP-1500AE). Perfluoropolyester (product name "Galwick," surface tension 15.6 dyn / cm) manufactured by the same company was used as the immersion solution. The applied pressure and air permeability were measured for both the dry and wet curves. The average pore size dHD (μm) was calculated from the pressure PHD (Pa) at which the half-moon curve of the dry curve intersected the wet curve, using the following formula, and was defined as the average flow diameter. dHD = 2860 × γ / PHD

[0164] (9) Viscosity average molecular weight (Mv) Based on ASTM-D4020, the intrinsic viscosity [η] (dl / g) at 135°C in decalin solvent was determined. For PO microporous membranes and polyethylene raw materials, Mv was calculated using the following formula. [η] = 6.77 × 10 -4 Mv 0.67 For polypropylene raw materials, Mv was calculated using the following formula. [η] = 1.10 × 10 -4 Mv 0.80

[0165] (10) Tensile modulus of MD and TD (MPa) For the measurement of MD and TD, MD samples (MD 120mm × TD 10mm) and TD samples (MD 10mm × TD 120mm) were cut out. Under ambient temperature of 23±2℃ and humidity of 40±2%, the tensile modulus of MD and TD of the samples were measured in accordance with JIS K7127 using a Shimadzu Corporation Autograph AG-A tensile testing machine. The samples were set with a chuck distance of 50mm, and stretched at a tensile speed of 200mm / min until the chuck distance reached 60mm, i.e., the strain reached 20.0%. The tensile modulus (MPa) was determined from the slope of the resulting stress-strain curve from 1.0% to 4.0% strain.

[0166] (11) Crystal structure analysis The crystal long period in polyolefin microporous films was determined by small-angle X-ray scattering measurements using the transmission method with a Rigaku NANOPIX detector. CuKα rays were irradiated onto the sample, and scattering was detected by a HyPix-6000 semiconductor detector. The sample-detector distance was 1312 mm, and measurements were performed under the conditions of 40 kV, 30 mA. A point focus optical system was employed, with slit diameters of 1st slit: φ=0.55 mm, 2nd slit: open, and guard slit: φ=0.35 mm. The sample was set so that the sample surface and the direction of X-ray incidence were perpendicular.

[0167] Furthermore, the crystallite size of the polyethylene MDND plane (110) in the polyolefin microporous membrane was determined by transmission wide-angle X-ray scattering measurements using a Rigaku NANOPIX system. CuKα rays were irradiated onto the sample, and scattering was detected using an imaging plate. The sample-detector distance was 110 mm, and the measurements were performed under the conditions of 40 kV, 30 mA. A point focus optical system was employed, with slit diameters of 1st slit: φ=1.2 mm and guard slit: φ=0.35 mm. The sample was set so that the sample surface and the X-ray incidence direction formed an angle of 11.0°. At this time, the X-ray incidence direction and the sample's MD were perpendicular.

[0168] (crystal long period [nm]) A SAXS profile I(q) was obtained from the X-ray scattering pattern obtained from HyPix-6000 by ring averaging. In the linear-linear plot of the obtained one-dimensional profile I(q), 0.1 nm -1 < q < 0.6nm -1 A linear baseline was drawn within the range, and a Gaussian function was used for fitting. The position with the maximum intensity was identified as the peak position q, which originates from the crystal's long period. m The crystal period was calculated from Equation 4. d = 2π / q m formula 4 {where d(nm): crystal long period q m (mm -1 ): Lamellar-derived peak positions in the SAXS profile}

[0169] (crystallite size) The obtained XRD profile was separated into three peaks in the range from 2θ = 10.0° to 2θ = 30.0°: orthorhombic (110) plane diffraction peak, orthorhombic (200) plane diffraction peak, and amorphous peak. The crystallite size was calculated from the full width at half maximum (FMAX) of the (110) plane diffraction peak according to Scherrer's equation (Equation 1). The (110) plane diffraction peak and (200) plane diffraction peak were approximated by the Voigt function, and the amorphous peak was approximated by the Gauss function. The peak position of the amorphous peak was fixed at 2θ = 19.6° and the FMAX was fixed at 6.3°, while the peak position and FMAX of the crystalline peak were not fixed during peak separation. The crystallite size was calculated from the FMAX of the (110) plane diffraction peak calculated by peak separation using Scherrer's equation (Equation 1). D(110) = Kλ / (βcosθ) Equation 1 {In the formula, D(110): crystallite size (nm)} K: 0.9 (constant) λ: Wavelength of X-ray (nm) β:(β1 2 -β2 2 ) 0.5 β1: Full width at half maximum (rad) of the peak (hkl) calculated as a result of peak separation. β2: Total width at half maximum (rad) of the incident beam. θ: Bragg angle}

[0170] (12) Liquid absorption test of propylene carbonate Strips of MD (50mm MD x 10mm TD) and TD (10mm MD x 50mm TD) samples were cut from the microporous membrane. Next, a horizontal rod of a predetermined height was placed on a glass water tank containing propylene carbonate, and the ends of the strip-shaped samples were pinned to the horizontal rod so that the longer side was parallel to the vertical direction. Then, the horizontal rod was lowered so that the ends of the sample pieces were immersed in the propylene carbonate by 10mm. After 60 minutes of immersion, the maximum height (mm) from the liquid surface at which the propylene carbonate rose through the microporous membrane was measured, and this measurement was taken as the absorption height of the propylene carbonate for MD and TD. The measurements were performed in a 25°C environment, and the average value of three measurements for both MD and TD was adopted.

[0171] (13) Battery Test

[0172] <Cycle Testing> Each battery assembled in the examples and comparative examples described below was charged at 25°C to a battery voltage of 4.3V with a current of 3mA (approximately 0.5C), and then the current was gradually reduced from 3mA to maintain the 4.3V voltage. This process lasted for a total of approximately 3 hours, after which the initial charge was performed. Subsequently, the battery was discharged to a battery voltage of 3.0V with a current of 3mA, and this cycle was repeated. The cycle characteristics were then evaluated using the number of cycles at which the capacity retention rate first fell below 80% of the initial capacity (capacity in the first cycle), according to the following criteria. Evaluation Criteria for Cycle Characteristics A: Over 300 cycles B: 200 cycles or more, less than 300 cycles C: 100 cycles or more, less than 200 cycles D: 50 cycles or more, less than 100 cycles E: 1 cycle or more, less than 50 cycles

[0173] <Initial Overcharge Test> Aside from the cell used for cycle testing, a cell assembled using the method described in the example was subjected to constant current (CC)-constant voltage (CV) charging at 4.3V with a set current value of 0.1C (Cut Off condition: convergence current value of 0.03mA), and the normal charge amount (i) was measured. A new cell was prepared separately from the cell used to measure the normal charge level (i), and the overloaded battery (ii) was measured by performing CC-CV charging at 4.3V with a set current value of 20mA / cm2 (Cut Off condition: 25mAh or convergence current value of 0.03mA). The values ​​in (ii)-(i) were evaluated as the overcharge values ​​due to the dendrite short circuit, according to the following criteria. [Rating Rank] A: Less than 0.05mAh B: 0.05mAh or more and less than 0.1mAh C: 0.1mAh or more and less than 0.5mAh D: 0.5mAh or more and less than 1.0mAh E: 1.0mAh or more and 20.0mAh or less

[0174] a. Fabrication of polyolefin microporous membranes Polyolefin microporous membranes a1 to a12 were fabricated under the following conditions. The properties of the polyolefin microporous membranes are shown in Table 1.

[0175] [a1] A polyolefin microporous membrane was prepared using the following procedure. The resin raw material consisted of 93 parts by mass of high-density polyethylene with a melting point of 135°C and a viscosity-average molecular weight of 900,000, and 7 parts by mass of isotactic polypropylene with a melting point of 161°C and a viscosity-average molecular weight of 400,000. An appropriate amount of pentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] was added to the aforementioned resin composition as an antioxidant and mixed in a Henschel mixer. The resulting mixture was supplied to the feed port of a twin-screw co-direction screw extruder via a feeder. Liquid paraffin (LP) was added to the twin-screw extruder cylinder in two separate additions via side feed so that the liquid paraffin content in the total mixture (100 parts by mass) to be melt-kneaded and extruded was 77.0 parts by mass. The set temperatures were 160°C for the kneading section and 200°C for the T-die. Subsequently, the molten mixture was extruded from the T-die into a sheet shape and cooled with a cooling roll controlled to a surface temperature of 90°C to obtain a sheet-shaped molded product.

[0176] The obtained sheet-like molded material was guided into a simultaneous biaxial stretching machine to obtain a primary stretched film (primary stretching step). The set stretching conditions were an MD stretching ratio of 7x, a TD stretching ratio of 6.4x, and a biaxial stretching temperature of 121°C for both MD and TD. Next, the obtained primary stretched film was guided into a methylene chloride bath and thoroughly immersed to extract and remove the liquid paraffin, which is a plasticizer. After that, the methylene chloride was dried off to obtain an extracted film.

[0177] Next, the extracted film was guided to a TD uniaxial tenter for heat fixation. As part of the heat fixation process, after stretching under conditions of a TD stretching temperature of 125°C and a TD stretching ratio of 1.92 times, a relaxation operation was performed under conditions of a relaxation temperature of 125°C and a relaxation ratio of 0.78 times to obtain a polyolefin microporous film a1.

[0178] [a2] After preparing a polyolefin microporous membrane in the same manner as a1, the polyolefin microporous membrane was treated with a mixed gas in which the partial pressure ratio of the concentrations of F2, O2, and N2 gases was adjusted to obtain polyolefin microporous membrane a2. The mixed gas of O2 and N2 to be mixed with F2 was mixed in a ratio of 2:8, similar to the composition of air. The partial pressure of F2 and the mixed gas of O2 and N2 was adjusted to 1:9, and the mixture was mixed in a buffer tank to produce a mixed treatment gas. The prepared polyolefin microporous membrane was placed in the reaction chamber, and the mixed treatment gas was added. The concentration of the mixed treatment gas was adjusted to a pressure condition of 30 kPa using a gas valve between the buffer tank and the vacuum reaction chamber, and the reaction time was adjusted to 20 minutes.

[0179] [a3] Polyolefin microporous membrane a3 was prepared in the same manner as a1, except that the composition of the resin raw materials was 70 parts by mass of high-density polyethylene with a melting point of 135°C and a viscosity-average molecular weight of 900,000, 20 parts by mass of polyketone with a melting point of 220°C and a melt index of 3.0 g / 10 min at a temperature of 240°C and a load of 2.16 kg as measured according to ASTM D1238, and 10 parts by mass of maleic anhydride-modified polyethylene with a melting point of 131°C and a viscosity-average molecular weight of 70,000, and the amount ratio of liquid paraffin (LP) in the total extruded mixture (100 parts by mass) was set to 65.0 parts by mass.

[0180] [a4] Polyolefin microporous membrane a4 was prepared in the same manner as a1, except that the resin raw material composition was 70 parts by mass of high-density polyethylene with a melting point of 135°C and a viscosity-average molecular weight of 900,000, and 30 parts by mass of high-density polyethylene with a melting point of 135°C and a viscosity-average molecular weight of 250,000, the amount ratio of liquid paraffin (LP) in the total extruded mixture (100 parts by mass) was 75.0 parts by mass, the biaxial stretching temperature was 126°C, and as a heat setting step, after stretching under conditions of a TD stretching ratio of 1.90 times, a relaxation operation was performed under conditions of a relaxation temperature of 136°C and a relaxation ratio of 0.84 times.

[0181] [a5] Polyolefin microporous membrane a5 was prepared in the same manner as a1, except that the resin raw material composition was 40 parts by mass of high-density polyethylene with a melting point of 135°C and a viscosity-average molecular weight of 700,000, 40 parts by mass of high-density polyethylene with a melting point of 135°C and a viscosity-average molecular weight of 250,000, 5 parts by mass of isotactic polypropylene with a melting point of 161°C and a viscosity-average molecular weight of 400,000, and 15 parts by weight of silica "DM10C" (trademark, manufactured by Tokuyama Corporation, hydrophobic treatment with dimethyldichlorosilane) with an average primary particle size of 15 nm, the amount ratio of liquid paraffin (LP) in the total extruded mixture (100 parts by mass) was 70.0 parts by mass, the biaxial stretching temperature was 127°C, and as a heat setting step, after stretching under conditions of stretching temperature 129°C and TD stretching ratio of 1.90 times, a relaxation operation was performed under conditions of relaxation temperature 136°C and relaxation ratio of 0.79 times.

[0182] [a6] Polyolefin microporous membrane a6 was prepared in the same manner as a1, except that the resin raw material composition was 30 parts by mass of high-density polyethylene with a melting point of 135°C and a viscosity-average molecular weight of 900,000, and 70 parts by mass of high-density polyethylene with a melting point of 135°C and a viscosity-average molecular weight of 250,000, the amount ratio of liquid paraffin (LP) in the total extruded mixture (100 parts by mass) was 71.0 parts by mass, the biaxial stretching temperature was 128°C, and as a heat setting step, after stretching under conditions of stretching temperature of 128°C and TD stretching ratio of 1.70 times, a relaxation operation was performed under conditions of relaxation temperature of 128°C and relaxation ratio of 0.94 times.

[0183] [a7] After fabricating a polyolefin microporous film using the same method as in a4, the fabricated polyolefin microporous film was plasma-treated to obtain polyolefin microporous film a7. The plasma treatment was performed under atmospheric pressure using a dielectric barrier discharge type electrode. 300 ml of nitrogen per minute was injected as the carrier gas, and 1 ml of oxygen per minute was injected as the reaction gas. A plasma discharge was generated with a power of 3.6 kW and a voltage of 12 kV, bringing both sides of the polyolefin microporous film into simultaneous contact with the plasma for 3 seconds. The distance between the electrode and the microporous film during the plasma discharge was fixed at 3 mm.

[0184] [a8] The composition of the resin raw materials was 47.5 parts by mass of high-density polyethylene with a melting point of 135°C and a viscosity-average molecular weight of 700,000, 47.5 parts by mass of high-density polyethylene with a melting point of 135°C and a viscosity-average molecular weight of 250,000, and 5 parts by mass of isotactic polypropylene with a melting point of 161°C and a viscosity-average molecular weight of 400,000. The amount ratio of liquid paraffin (LP) in the total extruded mixture (100 parts by mass) was 68.0 parts by mass, the biaxial stretching temperature was 120°C, and as a heat setting step, a stretching operation was performed under conditions of stretching temperature of 130°C and TD stretching ratio of 1.75 times, followed by a relaxation operation under conditions of relaxation temperature of 131°C and relaxation ratio of 0.86 times. A polyolefin microporous membrane prepared in the same manner as a1 was then treated with F2 and a mixed gas of O2 and N2 in the same manner as a2 to obtain a polyolefin microporous membrane a8.

[0185] [a9] Polyolefin microporous membrane a9 was prepared in the same manner as a1, except that the resin raw material composition was 70 parts by mass of high-density polyethylene with a melting point of 135°C and viscosity-average molecular weight of 700,000, 23 parts by mass of high-density polyethylene with a melting point of 135°C and viscosity-average molecular weight of 250,000, and 7 parts by mass of isotactic polypropylene with a melting point of 161°C, viscosity-average molecular weight of 400,000 and molecular weight distribution of 6.0, the amount ratio of liquid paraffin (LP) in the total extruded mixture (100 parts by mass) was 71.0 parts by mass, the biaxial stretching temperature was 125°C, and as a heat setting step, a stretching operation was performed under conditions of stretching temperature of 130°C and TD stretching ratio of 1.90 times, followed by a relaxation operation under conditions of relaxation temperature of 128°C and relaxation ratio of 0.83 times.

[0186] [a10] Polyolefin microporous membrane a10 was obtained in the same manner as a8, except that treatment was not performed with F2 and a mixed gas of O2 and N2.

[0187] [a11] Polypropylene (PP) resin with a melt index of 0.25 g / 10 min at a temperature of 190°C and a load of 2.16 kg, as measured according to ASTM D1238, was melted in a 2.5-inch extruder and fed to an annular die using a gear pump. The die temperature was set to 260°C, and the molten polymer was cooled by blown air before being wound onto a roll. Similarly, high-density polyethylene (PE) resin with a melt index of 0.38 g / 10 min at a temperature of 190°C and a load of 2.16 kg was melted in a 2.5-inch extruder and fed to an annular die using a gear pump. The die temperature was set to 230°C, and the molten polymer was cooled by blown air before being wound onto a roll. The polypropylene and polyethylene precursors (raw film) wound onto the rolls each had a thickness of 5 μm, and then the PP and PE precursors were bonded together to form a PP / PE / PP structure, obtaining a raw film with a PP / PE / PP three-layer structure. The raw film was annealed at 125°C for 20 minutes. The annealed film was then cold-stretched to 15% at room temperature, then hot-stretched to 1.5 times MD at 115°C, relaxed to 0.69 times MD at 125°C, and then treated with F2, O2, and N2 gas in the same manner as a2 to obtain polyolefin microporous film a11.

[0188] [a12] The resin raw material composition consists of 70 parts by mass of ultra-high molecular weight polyethylene with a melting point of 134°C and an average viscosity molecular weight of 4.5 million, 30 parts by mass of polyethylene wax with a melting point of 113°C and an average viscosity molecular weight of 1000, and further, with an average pore size of 0.1 μm and a BET specific surface area of ​​11.6 m², so that it accounts for 36 volume percent of the total volume. 2Calcium carbonate was added at a concentration of / g. An appropriate amount of pentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] was added to the resin composition as an antioxidant and mixed in a Henschel mixer. The resulting mixture was supplied to the feed port of a twin-screw extruder via a feeder and extruded into a sheet from a T-die at a set temperature of 200°C. A rolled sheet was prepared by rolling this sheet using a pair of rolls at a surface temperature of 150°C. Subsequently, the calcium carbonate was removed by immersing the rolled sheet in hydrochloric acid (4 mol / L) containing 0.5 wt% nonionic surfactant, and then it was stretched 7.0 times in MD, and then heat-set at 123°C with the magnification fixed to obtain a polyolefin microporous film a12.

[0189] b. Fabrication of the positive electrode Positive electrode b1 was fabricated under the following conditions. Lithium nickel manganese cobalt composite oxide (Li[Ni) is used as the positive electrode active material. 1 / 3 Mn 1 / 3 Co 1 / 3 A slurry was prepared by dispersing 91.2 parts by mass of 2.3 parts by mass each of flake graphite and acetylene black as conductive materials, and 4.2 parts by mass of polyvinylidene fluoride (PVdF) as a resin binder in N-methylpyrrolidone (NMP). This slurry was then applied to one side of a 20 μm thick aluminum foil, which would serve as the positive electrode current collector, using a die coater, until the positive electrode active material was coated at a rate of 120 g / m². 2 The material was applied in this manner. After drying at 130°C for 3 minutes, a roll press was used to measure the bulk density of the positive electrode active material to 2.90 g / cm³. 3 It was compressed and molded to form the positive electrode. This was then used to create a surface with an area of ​​2.00 cm². 2 It was punched out in a circular shape.

[0190] c. Fabrication of the negative electrode Negative electrodes c1 and c2 were fabricated under the following conditions.

[0191] [c1] Thickness 0.5 mm, Area 2.00 cm² 2Circular metallic lithium (Honjo Metals) was placed in a SUS glove box under a nitrogen atmosphere, and high-purity nitrogen gas (99.99%) was constantly supplied. The metallic lithium was left standing at a temperature of 60 °C for 2 hours to fabricate a metallic lithium negative electrode c1 with lithium nitride formed on its surface.

[0192] [c2] Circular metallic lithium (Honjo Metals) with a thickness of 0.5 mm and an area of 2.00 cm 2 was used as the negative electrode c2.

[0193] d. Preparation of non-aqueous electrolyte Electrolytes d1 to d8 were prepared under the following conditions.

[0194] [d1] LiPF6 was dissolved as a solute in a mixed solvent of ethylene carbonate:ethyl methyl carbonate = 1:2 (volume ratio) to a concentration of 2.0 mol / L to prepare electrolyte d1.

[0195] [d2] LiPF6 was dissolved as a solute in a mixed solvent of ethylene carbonate:ethyl methyl carbonate = 1:2 (volume ratio) to a concentration of 1.3 mol / L to prepare electrolyte d2.

[0196] [d3] LiPF6 was dissolved as a solute in a mixed solvent of fluoroethylene carbonate:ethyl methyl carbonate = 1:2 (volume ratio) to a concentration of 1.0 mol / L to prepare electrolyte d3.

[0197] [d4] LiTFSI was dissolved as a solute in a mixed solvent of ethylene carbonate:ethyl methyl carbonate = 1:2 (volume ratio) to a concentration of 1.0 mol / L to prepare electrolyte d4.

[0198] [d5] Electrolyte d5 was prepared by dissolving LiPF6 as a solute in a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1:2 to a concentration of 3.0 mol / L.

[0199] [d6] An electrolyte solution was prepared by dissolving LiPF6 in the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) to a concentration of 1.0 mol / L, thereby obtaining electrolyte solution d6.

[0200] [d7] Tetraglyme (G4) and LiTFSI were placed in a beaker in a molar ratio of 1:1 and mixed until a homogeneous solvent was obtained to prepare electrolyte d7.

[0201] [d8] Electrolyte d8 was prepared by dissolving LiPF6 as a solute to a concentration of 1.0 mol / L in a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1:2.

[0202] e. Battery assembly <Examples 1-11 and Comparative Examples 1-4> The combinations of the polyolefin microporous membrane, positive electrode, negative electrode, and electrolyte prepared in steps a to b above were changed as shown in Table 2, and batteries of Examples 1 to 11 and Comparative Examples 1 to 4 were prepared under the following conditions. In an argon box, the negative electrode, separator, and positive electrode were stacked from bottom to top so that the positive electrode active material and the negative electrode faced each other. This stack was then placed in a stainless steel container with a lid that insulated the container body and lid, so that the lithium foil of the negative electrode and the aluminum foil of the positive electrode were in contact with the container body and lid, respectively, to obtain a cell. This cell was dried under reduced pressure at 25°C for 10 hours. After that, a non-aqueous electrolyte was injected into the container in an argon box and sealed to obtain a battery. Table 2 shows the combinations of polyolefin microporous membrane, positive electrode, negative electrode, and electrolyte for Examples 1-11, Example 12, and Comparative Examples 1-4. When negative electrode c1 was used, the battery was assembled with the lithium nitride formation surface facing the polyolefin microporous membrane side.

[0203] <Example 12> Polyvinylidene fluoride resin particles (melting point 140 °C, particle size 0.2 μm) were dissolved in acetone to a concentration of 8.0 wt% to prepare a coating solution. The prepared coating solution was applied to one side of a polyolefin microporous membrane prepared in the same manner as a1 above using a bar coater, and dried at 40 °C for 1 hour to obtain a separator with PVdF coated on one side. The coating amount of the PVdF layer was 3.2 g / m 2 2. The obtained PVdF-coated separator, the positive electrode b1, the negative electrode c2, and the electrolyte d2 were used to assemble a battery in the same manner as in Examples 1 to 11 and Comparative Examples 1 to 4. The PVdF-coated surface of the separator was arranged to face the negative electrode.

[0204] The configurations and battery evaluation results of Examples 1 to 12 and Comparative Examples 1 to 4 are shown in Table 2.

[0205]

Table1-1

[0206]

Table1-2

[0207]

Table2-1

[0208]

Table2-2

[0209]

Table2-3

Claims

1. A polyolefin microporous membrane used in a lithium-ion battery separator, wherein an ion-conducting layer exists between the separator and the metal negative electrode, In a liquid absorption test of propylene carbonate, the liquid absorption height in the longitudinal direction (MD) and the width direction (TD) was 1.0 mm or more and 50 mm or less. The tensile modulus of MD and TD is 3000 kgf / cm² or higher. The air permeability is between 20 seconds and 200 seconds per 100 cm³ of air. In a compression test conducted under the conditions of a temperature of 25°C, a pressure of 10 MPa, and a compression time of 3 minutes, the post-compression porosity was 35% or higher. The absorption height is calculated by cutting strips of MD 50 mm × TD 10 mm and TD 10 mm respectively from the polyolefin microporous membrane, then placing a horizontal rod on a propylene carbonate water tank at 25°C, securing the ends of the strip-shaped samples to the horizontal rod so that the longer side is parallel to the vertical, then lowering the horizontal rod to immerse the ends of the strip-shaped samples by 10 mm in the propylene carbonate, and during 60 minutes after immersion, measuring the maximum height from the liquid surface at the position where the propylene carbonate has risen above the strip-shaped samples three times for both the MD and TD strip-shaped samples, and adopting the average value. The tensile modulus is calculated from the slope of the strain from 1.0% to 4.0% in the stress-strain curve obtained by cutting an MD sample of MD 120 mm × TD 10 mm and a TD sample of MD 10 mm × TD 120 mm from the polyolefin microporous membrane, setting the MD sample of MD 120 mm × TD 10 mm and the TD sample of MD 10 mm × TD 120 mm in accordance with JIS K7127 using a tensile testing machine under ambient temperature of 23 ± 2 °C and humidity of 40 ± 2%, stretching the MD sample of MD 120 mm × TD 10 mm and the TD sample of MD 10 mm × TD 120 mm with a distance of 50 mm between the chucks, and obtaining a stress-strain curve.

2. A separator for a lithium-ion battery, wherein an inorganic coating layer, an organic layer, or an adhesive layer is formed on the surface of the polyolefin microporous film described in Claim 1.

3. The following steps: (1) A step of preparing a stainless steel metal container with a lid, insulated from the container body and the lid, and a non-aqueous electrolyte. (2) A step of preparing a polyolefin microporous membrane as a separator, wherein in a liquid absorption test of propylene carbonate, the liquid absorption height in the longitudinal direction (MD) and width direction (TD) is 1.0 mm or more and 50 mm or less, the tensile modulus of MD and TD is 3000 kgf / cm² or more, the air permeability is 20 seconds or more and 200 seconds or less per 100 cm³ of air, and the post-compression porosity is 35% or more in a compression test under the conditions of a temperature of 25°C, a pressure of 10 MPa, and a compression time of 3 minutes. (3) A step of preparing a metal negative electrode in which an ion-conducting layer exists on at least one side, (4) Steps to prepare the positive electrode, (5) A step of preparing a laminate in which the ion-conducting layer of the metal anode and the separator are facing each other, and the metal anode, separator, and cathode are stacked in that order, and (6) A cell is manufactured by placing the laminate in a stainless steel container with a lid, insulated from the container body and the lid, such that the metal negative electrode and positive electrode are in contact with the container body and the lid, respectively; the cell is dried under reduced pressure at 25°C for 10 hours; and then a non-aqueous electrolyte is injected into the container and sealed to manufacture a battery. Includes, The absorption height is calculated by cutting strips of MD 50 mm × TD 10 mm and TD 10 mm respectively from the polyolefin microporous membrane, then placing a horizontal rod on a propylene carbonate water tank at 25°C, securing the ends of the strip-shaped samples to the horizontal rod so that the longer side is parallel to the vertical, then lowering the horizontal rod to immerse the ends of the strip-shaped samples by 10 mm in the propylene carbonate, and during 60 minutes after immersion, measuring the maximum height from the liquid surface at the position where the propylene carbonate has risen above the strip-shaped samples three times for both the MD and TD strip-shaped samples, and adopting the average value. The tensile modulus is calculated from the slope of the strain from 1.0% to 4.0% in the stress-strain curve obtained by cutting an MD sample of MD 120 mm × TD 10 mm and a TD sample of MD 10 mm × TD 120 mm from the polyolefin microporous membrane, setting the MD sample of MD 120 mm × TD 10 mm and the TD sample of MD 10 mm × TD 120 mm in accordance with JIS K7127 using a tensile testing machine under ambient temperature of 23 ± 2 °C and humidity of 40 ± 2%, stretching the samples at a tensile speed of 200 mm / min until the distance between the chucks reaches 60 mm, and obtaining a stress-strain curve. The tensile modulus is calculated from the slope of the strain from 1.0% to 4.0% in the stress-strain curve. A method for manufacturing a lithium-ion battery in which an ion-conducting layer exists between the separator and the metal negative electrode.

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