JP5588722B2 - Polyolefin microporous membrane and lithium ion secondary battery - Google Patents

Description

  The present invention relates to a polyolefin microporous membrane and a lithium ion secondary battery.

In recent years, power storage devices such as lithium ion secondary batteries and electric double layer capacitors (including those called lithium ion capacitors and non-aqueous lithium power storage elements) have been actively developed. In a power storage device, a microporous membrane (separator) is usually provided between the positive and negative electrodes. Such a separator has a function of preventing contact between positive and negative electrodes and transmitting ions.
Here, the separator is required to have a certain physical strength or more from the viewpoint of ensuring good safety of the electricity storage device. Under such circumstances, Patent Documents 1 and 2 propose a microporous membrane made of a mixture of high density polyethylene and high molecular weight polypropylene.

JP 2002-194132 A JP 2002-105235 A

On the other hand, the separator may be required to achieve the high output characteristics of the electricity storage device. For example, in a lithium ion secondary battery for in-vehicle use, a polyolefin microporous film having a relatively large film thickness (a film thickness exceeding 20 μm) is used from the viewpoint of ensuring high safety. However, there is still room for improvement from the viewpoint of achieving high output characteristics.
The present invention provides a polyolefin microporous membrane capable of realizing a lithium ion secondary battery having both safety (evaluated by high-temperature storage test and nail penetration test) and high output characteristics (evaluated by high rate characteristics). For the purpose.

  As a result of intensive studies to achieve the above-mentioned object, the present inventors have found that a polyolefin microporous film having a specific film composition and film physical properties can solve the above-mentioned problems and complete the present invention. It came to.

That is, the present invention is as follows.
[1]
Polyolefin microporous membrane with continuous pores in the film thickness direction, with a film thickness of 21 μm or more and a porosity of 42% or more, and a heat shrinkage rate in the width direction at 120 ° C. of 4% or less. rate is Ri 5-20% by mass, the microporous polyolefin membrane viscosity average molecular weight of the polyolefin microporous membrane is 350,000 or less.
[2]
The polyolefin microporous membrane according to [1], which has an air permeability of 12 sec / 100 cc / μm or less.
[ 3 ]
The polyolefin microporous membrane according to [1] or [2] , wherein the thermal shrinkage in the width direction at 105 ° C. is 0.5% or less.
[ 4 ]
The lithium ion secondary battery formed using the polyolefin microporous film in any one of [1]-[ 3 ], a positive electrode, a negative electrode, and electrolyte solution.

  According to the present invention, there is provided a polyolefin microporous membrane capable of realizing a lithium ion secondary battery having both safety (evaluated by a high temperature storage test and a nail penetration test) and high output characteristics (evaluated by a high rate characteristic). can get.

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

The polyolefin microporous membrane of the present embodiment is a polyolefin microporous membrane having communication holes in the film thickness direction, a film thickness of 21 μm or more, and a porosity of 42% or more, and a heat shrinkage rate in the width direction at 120 ° C. Is 4% or less, and the content of polypropylene with respect to the whole polyolefin is 5 to 20% by mass.
In the present embodiment, the length direction of the polyolefin microporous film (the resin discharge direction during film formation) may hereinafter be abbreviated as “MD”. In addition, a direction orthogonal to MD is referred to as a width direction, and is hereinafter abbreviated as “TD”.

  The polyolefin microporous membrane of the present embodiment has the above-described structural characteristics, so it has excellent high rate characteristics, maintains a high capacity retention rate in a high-temperature storage test, and further provides high safety in a nail penetration test. The lithium ion secondary battery which has can be implement | achieved. The reason is not clear, but the TD thermal shrinkage at 120 ° C is below a certain value, so that deformation under high temperature environment is restricted, and when an external short circuit occurs, the expansion of the separator holes is suppressed and heat is generated. It is presumed that the safety during the nail penetration test was increased by slowing the speed. In addition, it has a high porosity in a state where thermal shrinkage is suppressed, so that even when a large current is passed, pore shrinkage is suppressed, voltage drop due to cell internal resistance is suppressed, and high output is maintained. It is done. Furthermore, by defining an appropriate propylene content, it is considered that self-discharge at a high temperature is suppressed and the capacity retention rate is high without impairing permeability.

And since the said polyolefin microporous film has the above characteristics, it is especially suitable as a separator for high power density lithium ion secondary batteries.
The output density is a graph showing the relationship between the battery voltage and discharge current at 50% SOC (State Of Charge). The straight line between the discharge end voltage (3.0 V) and current-voltage characteristics of the battery is the discharge end voltage. From the current value (I) and the battery mass (Wt) when extrapolated up to
Output density (P) = (V × I) / Wt
“High power density” means a power density of 1000 W / kg or more.

  The polyolefin microporous membrane of the present embodiment (hereinafter sometimes simply referred to as “microporous membrane”) has communication holes in the film thickness direction, and has, for example, a three-dimensional network skeleton structure. The film thickness of the microporous membrane is 21 μm or more, preferably 23 μm or more. Although it does not specifically limit as an upper limit, It is preferable that it is 200 micrometers or less from a viewpoint of permeability, More preferably, it is 50 micrometers or less, More preferably, it is 35 micrometers or less. Setting the film thickness in the above range is preferable from the viewpoint of application to a high-power density battery with a relatively high calorific value or winding performance in a large battery winding machine.

The TD heat shrinkage rate at 120 ° C. of the microporous membrane is 4% or less, preferably 2.5% or less. The lower limit is preferably −0.5% or more, more preferably −0.1% or more.
Further, the TD heat shrinkage rate at 105 ° C. of the microporous membrane is preferably 1% or less, and more preferably 0.5% or less. The lower limit is preferably −0.5% or more, more preferably −0.1% or more.

Furthermore, it is desirable that the MD heat shrinkage rate is 10% or less at 100 ° C. from the viewpoint of preventing the generation of wrinkles due to winding during battery production. The lower limit is preferably −0.5% or more, more preferably −0.1% or more.

The porosity of the microporous membrane is 42% or more. Moreover, 90% or less is preferable, More preferably, it is 80% or less, More preferably, it is 50% or less. Setting the porosity to 42% or more is also preferable from the viewpoint of following the rapid movement of lithium ions at the high rate. On the other hand, 90% or less is preferable from the viewpoint of improving the film strength and suppressing self-discharge.
Further, the air permeability of the microporous membrane is preferably 0.1 sec / 100 cc / μm or more, more preferably 1 sec / 100 cc / μm or more, and further 3 sec / 100 cc / μm or more from the balance between the film thickness and the porosity. preferable. Further, from the viewpoint of permeability, it is preferably 20 sec / 100 cc / μm or less, and more preferably 12 sec / 100 cc / μm or less.

  The puncture strength (absolute strength) of the microporous membrane is preferably 3N or more, and more preferably 4N or more. A puncture strength of 3N or more is preferable from the viewpoint of reducing the occurrence of pinholes and cracks even when sharp portions such as electrode materials pierce the microporous film when used as a battery separator. Although there is no restriction | limiting in particular as an upper limit, It is preferable that it is 10 N or less.

  The withstand voltage of the microporous membrane is preferably 0.5 kV or more, more preferably 1 kV or more, and still more preferably 1.2 kV or more from the viewpoint of ensuring the safety of the electricity storage device. From the viewpoint of permeability, it is preferably 10 kV or less, more preferably 5 kV or less.

Examples of means for forming a microporous membrane having various characteristics as described above include a method of optimizing the polymer concentration and stretching ratio during extrusion, and stretching and relaxation operations after extraction.
Further, the aspect of the microporous film may be an aspect of a single layer or an aspect of a laminate.

Next, the method for producing the microporous membrane will be described. If the resulting microporous membrane satisfies the requirements of the present embodiment, the polymer species, solvent species, extrusion method, stretching method, extraction method, pore opening The method, heat setting / heat treatment method and the like are not limited at all.
As the method for producing the microporous membrane, a polymer material and a plasticizer, or a polymer material, a plasticizer, and an inorganic material are melt-kneaded and extruded, a stretching step, and a plasticizer (and an inorganic material if necessary). It is preferable to include an extraction step and a heat setting step from the viewpoint of appropriately controlling the balance between physical properties of permeability and heat shrinkage.

More specifically, for example, a production method including the following steps (1) to (4) can be used.
(1) A kneading step of kneading a polyolefin composition containing a polyolefin, a plasticizer, and, if necessary, an inorganic material to form a kneaded product.
(2) A sheet forming step in which the kneaded product is extruded after the kneading step, formed into a sheet (whether it is a single layer or a laminated layer), and cooled and solidified.
(3) A stretching step of extracting a plasticizer or an inorganic material as necessary after the sheet forming step and further stretching the sheet in a direction of one axis or more.
(4) A post-processing step in which a plasticizer or an inorganic material is extracted as necessary after the stretching step, and further heat-treated.

  Examples of the polyolefin used in the step (1) include ethylene, propylene homopolymer, or ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, and norbornene. And a copolymer formed of at least two monomers selected from the group consisting of: These can be used alone or in combination of two or more. It is preferable to use a mixture because it makes it easy to control the fuse temperature and the short-circuit temperature. Hereinafter, polyethylene may be abbreviated as “PE” and polypropylene may be abbreviated as “PP”.

As the polyolefin, it is preferable to use high-density polyethylene from the viewpoint that heat fixation can be performed at a higher temperature while suppressing clogging of pores.
The proportion of such high-density polyethylene in the polyolefin is preferably 5% by mass or more, more preferably 10% by mass or more, and the upper limit is preferably 50% by mass or less, more preferably 40% by mass. It is as follows.

Further, as the polyolefin, it is preferable to use low molecular weight polyethylene having a viscosity average molecular weight (Mv) of 100,000 to 300,000 from the viewpoint of improving shutdown characteristics or improving the safety of a nail penetration test.
The proportion of such low molecular weight polyethylene in the polyolefin is preferably 30% by mass or more, more preferably 50% by mass or more, and the upper limit is preferably 100% by mass or less, more preferably 95% by mass. It is as follows.

Furthermore, as the polyolefin, polypropylene is used as an essential component.
The proportion of polypropylene in the polyolefin is preferably 5% by mass or more, and more preferably 8% by mass or more. On the other hand, the upper limit is preferably 20% by mass or less, and preferably 18% by mass or less.

  The total viscosity average molecular weight of the polyolefin is preferably 100,000 or more, more preferably 200,000 or more, and the upper limit is preferably 500,000 or less, more preferably 400,000 or less. It is preferable to set Mv to 100,000 or more from the viewpoint of expressing the film resistance at the time of melting. On the other hand, setting it to 500,000 or less is preferable from the viewpoint of facilitating the extrusion process.

The plasticizer is preferably a non-volatile solvent capable of forming a uniform solution at a temperature equal to or higher than the melting point of the polyolefin when mixed with the polyolefin. Moreover, it is preferable that it is a liquid at normal temperature.
Examples of the plasticizer include hydrocarbons such as liquid paraffin and paraffin wax; esters such as diethylhexyl phthalate and dibutyl phthalate; higher alcohols such as oleyl alcohol and stearyl alcohol; and the like.
In particular, when polyethylene is included as the polyolefin resin, the use of liquid paraffin as the plasticizer suppresses interfacial peeling between the polyolefin resin and the plasticizer, and from the viewpoint of carrying out uniform stretching or achieving high piercing strength preferable. Further, it is preferable to use diethylhexyl phthalate from the viewpoint of increasing the load when melt-extruding the kneaded product and improving the dispersibility of the inorganic material described later (to achieve a high-quality film).

  The proportion of the plasticizer in the polyolefin composition is preferably 30% by mass or more, more preferably 40% by mass or more, and the upper limit is preferably 80% by mass or less, preferably 70% by mass or less. It is. Setting the proportion of the plasticizer to 80% by mass or less is preferable from the viewpoint of maintaining high melt tension during melt molding and ensuring moldability. On the other hand, setting the proportion of the plasticizer to 30% by mass or more is preferable from the viewpoint of securing moldability and efficiently extending the lamellar crystals in the polyolefin crystal region. Here, the fact that the lamellar crystal is efficiently stretched means that the polyolefin chain is efficiently stretched without causing the polyolefin chain to be broken, and the formation of a uniform and fine pore structure, It can contribute to improvement of crystallinity.

The polyolefin composition may contain an inorganic material as necessary, but from the viewpoint of imparting shutdown performance, the proportion in the polyolefin composition is preferably 20% by mass or less, more preferably It is 10 mass% or less, More preferably, it is 5 mass% or less.
Examples of such inorganic materials include alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, iron oxide, and other oxide ceramics, silicon nitride, titanium nitride, boron nitride, and the like. Nitride ceramics, silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amicite, bentonite, asbestos And ceramics such as zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand, and glass fiber. These can be used alone or in combination of two or more. Among these, from the viewpoint of electrochemical stability, silica, alumina, and titanium are more preferable, and silica is particularly preferable.

  The polyolefin composition is further mixed with known additives such as metal soaps such as calcium stearate and zinc stearate, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments. It is also possible to use it.

Examples of the method for kneading the polyolefin composition include the following methods (a) and (b).
(A) A method in which a polyolefin and an inorganic material are put into a resin kneading apparatus such as an extruder or a kneader, and a plasticizer is further introduced and kneaded while being melted and kneaded by heating.
(B) A process of pre-kneading polyolefin, an inorganic material and a plasticizer in advance at a predetermined ratio using a Henschel mixer or the like, and then introducing the kneaded product into an extruder and introducing a plasticizer while heating and melting. Kneading method.

  During kneading, it is preferable to perform melt kneading while maintaining a nitrogen atmosphere. Moreover, the temperature at the time of melt kneading is preferably 160 ° C. or higher, and more preferably 180 ° C. or higher. Moreover, less than 300 degreeC is preferable and less than 240 degreeC is more preferable.

  The step (2) is, for example, a step of extruding the kneaded material into a sheet shape via a T-die or the like and bringing it into contact with a heat conductor to cool and solidify. As the heat conductor, metal, water, air, plasticizer itself, or the like can be used. Moreover, it is preferable to cool and solidify by sandwiching between rolls from the viewpoint of increasing the film strength of the sheet-like molded body and improving the surface smoothness of the sheet-like molded body.

  Sheet stretching methods include MD uniaxial stretching with a roll stretching machine, TD uniaxial stretching with a tenter, sequential biaxial stretching with a roll stretching machine and a tenter, or a combination of a tenter and a tenter, simultaneous biaxial tenter or inflation molding. Examples thereof include axial stretching. From the viewpoint of obtaining a more uniform film, simultaneous biaxial stretching is preferred. The total surface magnification is preferably 8 times or more, more preferably 15 times or more, and particularly preferably 30 times or more, from the viewpoint of film thickness uniformity and tensile elongation, and the balance between porosity and average pore diameter. When it is 30 times or more, a high-strength product is easily obtained. The stretching temperature is preferably 121 ° C. or higher from the viewpoint of imparting high permeability and high temperature and low shrinkage, and is preferably 135 ° C. or lower from the viewpoint of film strength.

Extraction can be performed by dipping in an extraction solvent or showering. The extraction solvent is preferably a poor solvent for polyolefin, a good solvent for plasticizers and inorganic materials, and a boiling point lower than the melting point of polyolefin. Examples of such extraction solvents include hydrocarbons such as n-hexane and cyclohexane, halogenated hydrocarbons such as methylene chloride, 1,1,1-trichloroethane, and fluorocarbons, alcohols such as ethanol and isopropanol, and acetone. And ketones such as 2-butanone and alkaline water. It selects from these and uses it individually or in mixture.
The inorganic material may be extracted in whole or in part in any of the entire steps, or may remain in the product. Moreover, there is no restriction | limiting in particular about the order of extraction, a method, and the frequency | count.

  Examples of the heat treatment method include a heat setting method in which stretching and relaxation operations are performed using a tenter or a roll stretching machine. The relaxation operation is a reduction operation performed at a certain relaxation rate on the MD and / or TD of the film. The relaxation rate is a value obtained by dividing the MD dimension of the film after the relaxation operation by the MD dimension of the film before the operation, or a value obtained by dividing the TD dimension after the relaxation operation by the TD dimension of the film before the operation, or MD, TD When both are relaxed, it is a value obtained by multiplying the MD relaxation rate and the TD relaxation rate. The predetermined temperature is preferably 125 ° C. or higher and more preferably 127 ° C. or higher from the viewpoint of suppressing thermal shrinkage at 120 ° C. On the other hand, from the viewpoint of porosity and permeability, it is preferably less than 133 ° C. Further, from the viewpoint of heat shrinkage and permeability, it is preferably stretched 1.3 times or more in the heat treatment step, and more preferably 1.4 times or more. From the viewpoint of suppressing heat shrinkage, the predetermined relaxation rate is preferably 0.9 or less, and more preferably 0.8 or less. Moreover, it is preferable that it is 0.6 or more from a viewpoint of wrinkle generation | occurrence | production prevention, a porosity, and permeability | transmittance. The relaxation operation may be performed in both the MD and TD directions, but it is possible to reduce the thermal contraction rate not only in the operation direction but also in the operation and vertical direction even in the relaxation operation of only MD or TD.

In addition to the steps (1) to (4), as a method for producing the microporous membrane, a step of superimposing a plurality of single layer bodies can be employed as a step for obtaining a laminate. Further, a surface treatment process such as electron beam irradiation, plasma irradiation, surfactant coating, chemical modification, etc. may be employed.
Moreover, the master roll after the heat setting can be processed under a predetermined temperature (master roll aging operation), and then the master roll can be rewound. By this step, the residual stress of the polyolefin in the master roll is released. A preferable temperature for heat-treating the master roll is preferably 35 ° C. or higher, more preferably 45 ° C. or higher, and particularly preferably 60 ° C. or higher. From the viewpoint of maintaining permeability, 120 ° C. or lower is preferable. The heat treatment time is not limited, but a heat treatment time of 24 hours or longer is preferable because the effect is easily exhibited.

In the microporous membrane thus obtained, the proportion of polypropylene in the polyolefin is preferably 5% by mass or more, and more preferably 8% by mass or more. On the other hand, the upper limit is preferably 20% by mass or less, and preferably 18% by mass or less.
In addition, about the ratio for which a polypropylene accounts in polyolefin, the ratio in a raw material and the ratio in the microporous film obtained do not change substantially.

  Further, the viscosity average molecular weight of the microporous membrane is preferably 100,000 or more, more preferably 200,000 or more, and the upper limit is preferably 400,000 or less, more preferably 350,000 or less. It is preferable to set Mv to 100,000 or more from the viewpoint of expressing the film resistance at the time of melting. On the other hand, setting it to 400,000 or less is preferable from the viewpoint of reducing shrinkage stress and suppressing shrinkage at high temperature. Here, the Mv of the desired microporous membrane can be adjusted by the extrusion temperature, the screw rotation speed, and the like. For example, the Mv of the raw material can be lowered by increasing the screw rotation speed.

The separator made of the microporous membrane described above is suitable for applications requiring high power density characteristics and high safety, such as motorcycles, scooters, and automobiles, among lithium ion secondary batteries, and imparts battery characteristics higher than conventional ones. It becomes possible.
The measured values of the parameters described above are measured according to the measurement methods in the following examples unless otherwise specified.

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

(1) Viscosity average molecular weight (Mv)
In order to prevent deterioration of the sample, 2,6-di-t-butyl-4-methylphenol is dissolved in decahydronaphthalene to a concentration of 0.1 w%, and this (hereinafter abbreviated as DHN) is used as the sample solvent. . The sample is dissolved in DHN at 150 ° C. to a concentration of 0.1 w%. 10 ml of the prepared sample solution is sampled, and the number of seconds passing through the marked line (t) at 135 ° C. is measured with a Canon Fenceke viscometer (SO100). Further, after heating the DHN to 0.99 ° C., and 10ml collected to measure the number of seconds passing between marked lines of the viscometer in the same manner _{(t B).} The intrinsic viscosity [η] was calculated by the following conversion formula using the obtained passing seconds t and t _B.
[Η] = ((1.651 t / t _B −0.651) ^0.5 −1) /0.0834
The viscosity average molecular weight (Mv) was calculated from the determined [η].
Mv of the raw material polyethylene, the raw material polyolefin mixture, and the microporous membrane was calculated by the following equation.
[Η] = 6.77 × 10 ⁻⁴ Mv ^0.67
For the raw material polypropylene, Mv was calculated by the following formula.
[Η] = 1.10 × 10 ⁻⁴ Mv ^0.80

(2) Film thickness (μm)
The measurement was performed at a room temperature of 23 ± 2 ° C. using a micro thickness measuring instrument manufactured by Toyo Seiki, KBM (trademark).

(3) Porosity (%)
A sample of 10 cm × 10 cm square was cut out from the microporous membrane, its volume (cm ³ ) and mass (g) were determined, and calculated from these and the film density (g / cm ³ ) using the following formula.
Porosity = (volume−mass / film density) / volume × 100
The film density was calculated from the fraction of the composition with polyethylene = 0.95 and polypropylene = 0.91. In addition, the density | concentration calculated | required by the density gradient tube method of JISK-7112 can be used for various film densities.

(4) Air permeability (second)
Based on JIS P-8117, it measured with the Gurley type air permeability meter (Toyo Seiki Co., Ltd. product, G-B2 (trademark)).

(5) Puncture strength (N)
By using a KES-G5 (trademark) handy compression tester manufactured by Kato Tech, the needle tip curvature radius is 0.5 mm and the needle stick speed is 2 mm / sec. The puncture load (N) was measured and used as the puncture strength.

(6) 100 ° C. MD heat shrinkage (%), 120 ° C. TD heat shrinkage (%), 105 ° C. TD heat shrinkage (%)
A sample obtained by cutting a polyolefin microporous membrane 100 mm in MD and 100 mm in TD was left in an oven at a predetermined temperature (100 ° C., 120 ° C., or 105 ° C.) for 1 hour. At this time, the sample was sandwiched between two sheets of paper so that the hot air did not directly hit the sample. After taking out the sample from the oven and cooling, the length of the MD (mm) or the length of the TD (mm) is measured, and the thermal shrinkage calculated by the following formula is the thermal shrinkage at a predetermined temperature (100 The thermal shrinkage rate (° C. MD) (%), 120 ° C. TD thermal shrinkage rate (%), or 105 ° C. TD thermal shrinkage rate (%)). In addition, about what cannot secure sample length, the longest possible sample was used in the range which enters into 100 mm x 100 mm.
MD thermal shrinkage (%) = (100−MD length after heating (mm)) / 100 × 100
TD heat shrinkage rate (%) = (100−length of TD after heating (mm)) / 100 × 100

(7) Withstand voltage (kV)
A microporous membrane was sandwiched between aluminum electrodes having a diameter of 3 cm, a load of 15 g was applied, and this was connected to a withstand voltage measuring machine (TOS9201) manufactured by Kikusui Electronics Corporation to carry out the measurement. The measurement condition is that an alternating voltage (60 Hz) is applied at a speed of 1.0 kV / sec, and the short-circuited voltage value is taken as the withstand voltage measurement value of the microporous membrane.

(8) High rate characteristic evaluation, high temperature storage test evaluation, nail penetration test evaluation a. Production of Positive Electrode 92.2% by mass of lithium cobalt composite oxide LiCoO ₂ as a positive electrode active material, 2.3% by mass of flake graphite and acetylene black as a conductive material, and polyvinylidene fluoride (PVDF) 3.2 as a binder, respectively. A slurry was prepared by dispersing mass% in N-methylpyrrolidone (NMP). This slurry was applied to an aluminum foil serving as a positive electrode current collector with a die coater, dried, and compression molded with a roll press to produce a positive electrode.
b. Preparation of Negative Electrode 96.9% by mass of artificial graphite as a negative electrode active material, 1.4% by mass of ammonium salt of carboxymethyl cellulose and 1.7% by mass of styrene-butadiene copolymer latex as a binder were dispersed in purified water to prepare a slurry. did. The slurry was applied to a copper foil serving as a negative electrode current collector with a die coater, dried, and compression molded with a roll press to produce a negative electrode.
c. Preparation of Nonaqueous Electrolytic Solution Prepared by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1: 2 (volume ratio) to a concentration of 1.0 mol / liter.
d. High rate characteristics evaluation Polyolefin microporous membrane is cut into a circle of 18 mmφ, positive electrode and negative electrode are cut into a circle of 16 mmφ, and the positive electrode, polyolefin microporous membrane, and negative electrode are stacked in this order so that the active material surfaces of the positive electrode and negative electrode face each other. Stored in a container. The container and the lid were insulated, and the container was in contact with the negative electrode copper foil and the lid was in contact with the positive electrode aluminum foil. The non-aqueous electrolyte was poured into this container and sealed. After standing at room temperature for 1 day, the battery voltage was charged to 4.2V at a current value of 3mA (0.5C) in a 25 ° C atmosphere, and the current value was reduced from 3mA so as to maintain 4.2V after reaching the battery voltage. By the method of starting, the first charge after making a battery for a total of 6 hours was performed. Subsequently, the battery was discharged to a battery voltage of 3.0 V at a current value of 3 mA (0.5 C).
Charging to a battery voltage of 3.6V at a current value of 1.1A (1.0C) in an atmosphere of 25 ° C., and further reducing the current value from 1.1A so as to maintain 3.6V. The battery was charged for 3 hours. Next, the battery was discharged to a battery voltage of 2.0 V at a current value of 1.1 A (1.0 C) or 5.5 A (5 C) to obtain a 1 C discharge capacity or a 5 C discharge capacity.
The ratio of 5C discharge capacity to 1C discharge capacity is defined as capacity maintenance rate (%). When the maintenance rate at this time is 85% or more, ◎, when it is 80% or more and less than 85%, when it is less than 80% X was used as an index of high rate characteristics.
e. High temperature storage test evaluation, nail penetration test evaluation The electrode plate laminate was prepared by stacking the negative electrode, the polyolefin microporous membrane, the positive electrode, and the polyolefin microporous membrane in this order and winding them in a spiral shape. The electrode plate laminate is housed in a stainless steel container having an outer diameter of 18 mm and a height of 65 mm, and an aluminum tab derived from the positive electrode current collector is used as a container lid terminal portion, and a nickel tab derived from the negative electrode current collector Was welded to the container wall. Thereafter, vacuum drying was performed, and the battery was assembled by injecting and sealing the non-aqueous electrolyte into the container in an argon box.
The assembled battery was charged at a constant current of 1 / 3C to a voltage of 4.2V, then charged at a constant voltage of 4.2V for 5 hours, and then discharged at a current of 1 / 3C to a final voltage of 3.0V. It was. Next, after constant current charging to a voltage of 4.2 V with a current value of 1 C, a constant voltage charge of 4.2 V was performed for 2 hours, and then discharging was performed to a final voltage of 3.0 V with a current of 1 C. Finally, after constant voltage charging of 4.2 V with a current value of 1 C, constant voltage charging of 4.2 V was performed for 2 hours. In this way, a pretreated battery was obtained.
The pretreated battery was placed in an oven, heated from room temperature at 5 ° C./min, and then allowed to stand at 80 ° C. for 10 days. Thereafter, the battery was taken out of the oven and allowed to cool to room temperature, and then discharged to 3.0 V at a current of 1 C, and the capacity retention rate with respect to the amount of charge before charging the oven was calculated. When the capacity retention ratio was 90% or more, ◎, when 85% or more and less than 90%, and when less than 85%, × were used as indicators of high temperature storage characteristics.
Further, with respect to the battery after the pretreatment, in a room temperature of 23 ± 2 ° C., an iron nail having a diameter of 2.7 mm is placed outside the case and 3 mm from the end of the laminated body at a speed of 5 mm / second. Punctured to a depth of 2 mm. The temperature reached after 30 seconds was measured with a thermocouple attached to the side of the battery away from the nail penetration position. When the temperature at this time was 50 ° C. or less, ◎, when it exceeded 50 ° C. and below 60 ° C., and when it exceeded 60 ° C. were used as indicators of the characteristics of the nail penetration test.

[Example 1]
Using a tumbler blender, 95% by mass of a homopolymer (low molecular weight polyethylene) having an Mv of 250,000 and 5% by mass of a polypropylene having an Mv of 400,000 were dry blended. 1% by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant was added to 99% by mass of the obtained pure polymer mixture, and the tumbler was again added. A dry blend was performed using a blender to obtain a polymer mixture. The obtained mixture of polymers and the like was substituted with nitrogen and then fed to the twin-screw extruder with a feeder under a nitrogen atmosphere. Further, liquid paraffin (kinematic viscosity at 37.78 ° C .: 7.59 × 10 ⁻⁵ m ² / s) was injected into the extruder cylinder by a plunger pump.
Feeder so that the liquid paraffin content ratio in the total mixture to be melt-kneaded and extruded is 50% by mass (that is, the polymer concentration (sometimes abbreviated as “PC”) is 50% by mass). And the pump was adjusted. The melt kneading conditions were a set temperature of 200 ° C., a screw rotation speed of 240 rpm, and a discharge rate of 12 kg / h.
Subsequently, the melt-kneaded material was extruded and cast on a cooling roll controlled at a surface temperature of 25 ° C. through a T-die, thereby obtaining a gel sheet having an original film thickness of 1750 μm.
Next, it led to the simultaneous biaxial tenter stretching machine, and biaxial stretching was performed. The set stretching conditions are an MD magnification of 7.0 times, a TD magnification of 6.4 times (that is, 7 × 6.4 times), and a biaxial stretching temperature of 124 ° C.
Next, the solution was introduced into a methyl ethyl ketone tank and sufficiently immersed in methyl ethyl ketone to extract and remove liquid paraffin, and then the methyl ethyl ketone was removed by drying.
Next, it is guided to a TD tenter to perform heat setting (sometimes abbreviated as “HS”), HS is performed at a heat setting temperature of 131 ° C. and a draw ratio of 1.6 times, and then a relaxation operation of 0.85 times is performed. (In other words, the HS relaxation rate was 0.85 times) to obtain a polyolefin microporous membrane. The results of evaluating various characteristics of the obtained microporous membrane are shown in Table 1 below.

[Examples 2-7, Comparative Examples 1-8]
A microporous membrane was obtained in the same manner as in Example 1 except for the conditions shown in Table 1 below. The PE with Mv 700,000 is high density polyethylene.
Various characteristics of the obtained microporous membrane were evaluated. The results are shown in Tables 1 and 2 below.

  The polyolefin microporous membrane of the present invention is suitably used as a separator for high power density lithium ion batteries.

Claims (4)

Polyolefin microporous membrane with continuous pores in the film thickness direction, with a film thickness of 21 μm or more and a porosity of 42% or more, and a heat shrinkage rate in the width direction at 120 ° C. of 4% or less. rate is Ri 5-20% by mass, the microporous polyolefin membrane viscosity average molecular weight of the polyolefin microporous membrane is 350,000 or less.   The polyolefin microporous membrane according to claim 1, wherein the air permeability is 12 sec / 100 cc / μm or less. The polyolefin microporous membrane according to claim 1 or 2 , wherein the thermal shrinkage in the width direction at 105 ° C is 0.5% or less. The lithium ion secondary battery formed using the polyolefin microporous film in any one of Claims 1-3 , a positive electrode, a negative electrode, and electrolyte solution.