JPH09241411A - Porous membrane, its production, and lithium ion secondary cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a porous membrane which contains polyethylene and polypropylene, and is useful in the production of lithium ion secondary cells because it has specific temperature characteristics, shows excellent current-breaking properties as a battery separator, when used as a cell separator and stably works without ignition of fire by the external short-circuit. SOLUTION: This membrane is porous membrane containing (A) polyethylene and (B) polypropylene (for example, a simple porous membrane of a mixture of A and B or a porous membrane formed by alternately laminating a porous membrane of an A and B mixture and a porous membrane of B). When the porous membrane is impregnated with an electrolyte solution, electrodes are arranged onto both main faces of the porous membrane and an alternating voltage is applied to the electrodes to raise the temperature of the membrane at a rate of 10-50 deg.C/second by the resistance heat of the electrolyte solution, the maximum temperature to be reached is adjusted to be equal to or less then (the melting point of the component A +20 deg.C). This porous membrane is prepared, for example, by mixing component A of >=60% crystallinity with component B of <=70% crystallinity so that the proportion of component A becomes >=12wt.% and by monoaxially orienting the membrane of the mixture.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a porous membrane, a method for producing the same, and a lithium ion secondary battery (hereinafter also abbreviated as a lithium secondary battery).

[0002]

2. Description of the Related Art Various types of batteries have been put to practical use, and a separator is interposed between the two electrodes in order to prevent a short circuit between the positive and negative electrodes.

In recent years, lithium secondary batteries have been attracting attention as batteries for dealing with cordless electronic devices and the like because of their high energy density, high electromotive force, and low self-discharge.

As a lithium secondary battery, for example, a negative electrode material is metallic lithium, an alloy of lithium and a metal such as aluminum, a material such as carbon or graphite that adsorbs or stores lithium ions, or a lithium ion-doped conductive material. Those formed of a polymer or the like are known. The positive electrode material is, for example, fluorinated graphite generally represented by (CF _x ) _n , CoLiO ₂ , MnO ₂ , V ₂ O ₅ ,
CuO, Ag ₂ CrO _{4 and} other metal oxides, or TiO
₂ , formed of sulfides such as CuS.

In this lithium secondary battery, lithium as a negative electrode material has a strong reactivity, and ethylene carbonate, propylene carbonate, acetonitrile, γ-
Butyl lactone, 1,2-dimethoxyethane, LiPF _6, LiCF ₃ SO in an organic solvent such as tetrahydrofuran
_Since a non-aqueous electrolyte solution containing ₃ , LiClO ₄ , LiBF _4, etc. as an electrolyte is used, if an external short circuit occurs due to incorrect use of the battery, a current will flow between the positive electrode and negative electrode, and heat will be generated due to the resistance of the electrolytic solution. There is a risk that the inside of the battery will rise significantly and eventually cause a serious accident such as fire or explosion. Therefore, in order to prevent such accidents, various mechanisms are provided for safety measures in lithium secondary batteries.
For example, the current interrupting device is configured to forcibly cut off a part of the circuit by utilizing an increase in the internal pressure of the battery due to evaporation of the electrolyte when the temperature of the battery increases due to an external short circuit. Things. Also, the shutdown mechanism that the battery separator has is one of the safety mechanisms,
Various proposals have been made regarding this. For example, it is composed of a porous film of a mixture (alloy) of polyethylene having a melting point at the shutdown start design temperature and polypropylene having a melting point higher than polyethylene by about 30 ° C.
-206257) or a laminated porous membrane in which porous membranes of thermoplastic polymers having different melting points (specifically, a polyethylene porous membrane and a polypropylene porous membrane) are laminated (JP-A-4-181651, JP 62-1085
7) etc. In all of these, the pores of the porous membrane are blocked by the molten resin to increase the electrical resistance of the membrane (hereinafter simply referred to as resistance), thereby cutting off the electric current. By doing so, the shutdown starts at a low temperature, and the polypropylene having a high melting point does not melt at the time of melting of polyethylene, and acts to maintain the film shape of the separator, whereby a sufficient heat resistant temperature can be obtained. Incidentally, the shutdown characteristics in the separator made of such a porous film is usually formed by sandwiching both main surfaces of the separator impregnated with the electrolytic solution with electrodes, and after charging the cell into a dryer,
The electrode temperature and the resistance value between the electrodes are measured while the temperature of the cell is raised with a smooth gradient of about 0.01 to 0.1 ° C./sec, and evaluation is performed based on the relationship between the electrode temperature and the resistance value.

[0006]

Recently, the battery materials of lithium secondary batteries have been improved and the output capacity of the batteries has been increased, and it is expected that the capacity will be further increased in the future. To be done. For example, the positive electrode material Co is Ni
By changing to, the output capacity of the battery is improved by 20 to 30%.
Further, when a coke material is used for the negative electrode, it is possible to obtain nearly double the output capacity (provided that the constituent elements of the battery other than the negative electrode material need to be improved). However, when the output capacity of the battery increases, the current value flowing between the positive electrode and the negative electrode also increases when an external short circuit occurs, and the temperature rise due to resistance heat generation (heat generation due to the resistance of the electrolytic solution) during an external short circuit also increases. Therefore, as the separator, a separator having an excellent current cutoff characteristic (shutdown characteristic) that increases the resistance so that the current can be instantaneously cut off when an external short circuit occurs is required.

The present invention has been made in view of the above problems, and is a porous film containing polyethylene and polypropylene, and when polyethylene is melted when used as a battery separator, an electric current is immediately generated. An object of the present invention is to provide a porous film having a high resistance to the degree of blocking and a method for producing the same.

[0008]

[Means for Solving the Problems] In order to achieve the above-mentioned object, the inventors of the present invention have made diligent studies on a porous membrane made of polyethylene and polypropylene, and a porous membrane having the following temperature characteristics has an excellent current blocking property. It turned out to be a characteristic. That is, the porous membrane of the present invention is a porous membrane containing polyethylene and polypropylene, the electrodes are arranged on both main surfaces of the porous membrane impregnated with the electrolytic solution, and an AC voltage is applied to this electrode. The maximum temperature reached when the porous film was heated by resistance heating of the electrolytic solution at a rate of 10 to 50 ° C./sec was (the melting point of polyethylene + 20
C)) or less.

The temperature characteristics of the porous membrane of the present invention as described above were measured by the evaluation tester shown in FIG. In FIG. 1, reference numeral 10 is a porous film made of polyethylene and polypropylene impregnated with an electrolytic solution. This electrolytic solution is an electrolytic solution used in a general lithium secondary battery, and is the non-aqueous type electrolytic solution described in the prior art. Reference numeral 1 is an electrode, and 2 is an electrode that also serves as a mounting base for the porous film 10, and these electrodes are formed of rust-proof material such as platinum or stainless steel. 3 is a clip for fixing the porous film 10 to the upper surface of the electrode 2, 4a is a temperature sensor, 4 is a thermometer for outputting the temperature detected by the temperature sensor 4a, and 5 is an AC power supply.
When an AC voltage is applied between the electrodes 1 and 2 by the AC power supply 5, a current flows through the porous membrane 10 impregnated with the electrolytic solution, heat is generated due to the resistance of the electrolytic solution, and the temperature of the porous membrane 10 rises. Here, the power supply frequency is 1 kHz to 10 MHz,
The effective voltage value is in the range of 5 to 200 V, and is adjusted so that the rate of temperature increase due to electric resistance is 10 to 50 ° C./second (average of 2 to 5 seconds after the start of energization).

When the energized state is continued, polyethylene is melted in the porous film 10 due to the temperature rise. In many cases, the melting of the polyethylene increases the resistance of the film, the current does not flow, the temperature rising rate decreases, and after reaching the maximum temperature, the temperature gradually decreases. Here, if the maximum temperature reached is (the melting point of polyethylene + 20 ° C.) or less, it can be interpreted that the electric resistance of the porous membrane is greatly increased immediately after the melting of polyethylene is started and the current is effectively cut off. . When a porous membrane having such temperature characteristics was incorporated into a battery as a separator, the shutdown was instantaneously initiated when an external short circuit occurred, and thereafter, the membrane shape was stably maintained and an excellent shutdown effect was obtained. . On the other hand, the highest temperature reached is (melting point of polyethylene +2
If the temperature is higher than 0 ° C), it takes time for the resistance of the porous film to increase significantly after the start of melting of polyethylene, and the temperature of the film becomes higher than (melting point of polyethylene + 20 ° C) due to the current flowing during this time. Can be interpreted as a thing. When such a porous membrane is incorporated into a battery as a separator, the start of shutdown when an external short circuit occurs is higher than that when a membrane whose maximum temperature is (melting point of polyethylene + 20 ° C.) or less is used. However, the film shape could not be stably maintained, and an excellent shutdown effect could not be obtained. In addition, unlike the above, in some cases, the temperature of the porous film continued to rise after the start of energization, and the polypropylene melted to form pinholes before the maximum temperature was reached. In addition, in the conventional method of charging in a dryer, the relationship between temperature and impedance was simply observed. On the other hand, in the method of the present invention described above, it is possible to confirm a phenomenon in which a hole is formed in the membrane, which cannot be predicted simply by passing a minute current such as measuring impedance, which is very important for the safety of the battery. .

Further, in the first method for producing a porous membrane of the present invention, polyethylene having a crystallinity of more than 60% and polypropylene having a crystallinity of more than 70% are contained in the polyethylene in an amount of 12% by weight or more. The film-like material obtained by mixing so as to be uniaxially stretched is made porous. According to such a manufacturing method, it is possible to rationally manufacture the porous film of the present invention having the temperature characteristics, that is, the porous film whose electric resistance increases to such an extent that the electric current can be cut off immediately after the melting of polyethylene begins. You can

Further, the second method for producing a porous membrane of the present invention is the first membrane-like product obtained by mixing polyethylene having a crystallinity of more than 60% and polypropylene having a crystallinity of more than 70%. And a second film-like material made of polypropylene are alternately laminated, and the film-like material having a laminated structure in which the total content of polyethylene is 12% by weight or more is uniaxially stretched to be made porous. . According to such a manufacturing method, the porous film of the present invention having the temperature characteristics can be rationally manufactured as in the first manufacturing method, and the obtained porous film is a polypropylene porous film. It is possible to obtain a porous film having a laminated structure including the film and maintaining the film shape more stable when polyethylene is melted.

The lithium ion secondary battery of the present invention is constructed by sandwiching a porous membrane of the present invention having the above-mentioned temperature characteristics impregnated with an electrolytic solution between a pair of electrodes. . With such a configuration, even if an external short circuit occurs, ignition or short circuit does not occur, and a lithium ion secondary battery that operates stably can be obtained.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The porous membrane of the present invention has the above-mentioned temperature characteristics, and the production method thereof is not particularly limited, but the following steps (work) are not concerned with environmental pollution.
An example of a simple manufacturing method will be described. This is a conventional method for producing porous membranes in this field, that is, a mixture of polyethylene and polypropylene (alloy).
In the method of obtaining a membranous substance of (1) and making the membranous substance porous by stretching, the polyethylene having a crystallinity of more than 60% (preferably 70% or more) and the polypropylene having a crystallinity of Is more than 70% (preferably 80% or more). Conventionally, polyethylene having a crystallinity of 40 to 60% and polypropylene having a crystallinity of 50 to 70% are generally used. The crystallinity is a value measured by a differential scanning calorimeter (DSC).

The reason why the porous film obtained by such a manufacturing method has the above-mentioned temperature characteristics is not clear, but the present inventor speculates as follows. That is,
Even if stretched under the same conditions, a material with high crystallinity is likely to be stretched, and when a film of polyethylene with crystallinity greater than 60% and polypropylene with crystallinity greater than 70% is stretched, conventional crystal Both polyethylene and polypropylene are stretched to a greater extent than when a film of polyethylene having a degree of crystallinity of 40 to 60% and polypropylene having a degree of crystallinity of 50 to 70% is stretched (especially polyethylene Be stretched.) Therefore, a porous film obtained by stretching a film of polyethylene having a crystallinity of more than 60% and polypropylene having a crystallinity of more than 70% of the present invention is flat as shown in FIG. 2 (a). Stretched polyethylene part 10
a is surrounded by the polypropylene portion 10b. On the other hand, a conventional porous membrane obtained by stretching a film of polyethylene having a crystallinity of 40 to 60% and polypropylene having a crystallinity of 50 to 70% is as shown in FIG. The substantially spherical polyethylene portion 10a is surrounded by the polypropylene portion 10b. Although the holes are omitted in the figure, the polyethylene portion 10a and the polypropylene 1
Holes are formed in both 0b. In both of these porous membranes, when polyethylene melts due to temperature rise and the pores of the polyethylene part 10a are closed, no current flows through the polyethylene part 10a and the polypropylene part 1
The electric current will flow as if sewing 0b.
In the porous membrane of the present invention (a), the polypropylene portion 10b
2 has a long current path, and in the conventional porous membrane of FIG. 2B, the current 1 flowing through the polypropylene portion 10b is 1
The current path of 2 becomes short. Therefore, in the porous membrane of the present invention of FIG. 2 (a), when the polyethylene is melted and the pores of the polyethylene portion 10a are closed, the resistance of the entire membrane is greatly increased, and the current interruption effect (shutdown effect) is obtained. ,
The temperature of the film once rises, and falls within the range where the maximum temperature does not exceed (melting point of polyethylene + 20 ° C.). On the other hand, in the conventional porous membrane of FIG. 2B, even if polyethylene melts and the pores of the polyethylene portion 10a are blocked, before the pores of the polyethylene portion 10a are blocked (polyethylene portion 1).
0a and the polypropylene part 10b) and the length of the current path is almost unchanged, and the increase in the resistance of the entire membrane at this point is small. At the time when the hole of 10b was closed, the resistance of the whole film was greatly increased, and the current cutoff effect (shutdown effect) was obtained. As a result, the temperature of the film was raised to a temperature exceeding (the melting point of polyethylene + 20 ° C). Will be reduced.

In the above, the mixture (alloy) of polyethylene and polypropylene is formed into a film (formed into a film) by a known method such as a T-die extrusion method or an inflation method. In addition, as a method for stretching a film of an alloy of polyethylene and polypropylene, a method such as roll stretching or tenter stretching in a uniaxial direction (first method), or uniaxial stretching in a low temperature region, A method of stretching again in the high temperature region in the same stretching direction as this stretching direction (second method), after uniaxially stretching in the low temperature region, the stretching direction is changed from the stretching direction of this stretching process to the high temperature region. (3rd method), after uniaxially stretching in the low temperature region, stretch in the high temperature region again in the same stretching direction as this stretching direction, and then change the stretching direction. Then, a method (fourth method) of performing re-stretching again is adopted. In the first method, in which stretching is performed once, the melting point of polyethylene is T _mb ℃.
Then, it is preferable to carry out in the temperature range of −20 ° C. to (T _mb −2) ° C. In addition, second to fourth stretching is performed twice or more.
In the low temperature region and the high temperature region in the method, when the melting point of polyethylene is T _mb ° C, the low temperature region is -20 ° C to (T _mb ° C.
−30) ° C., and the high temperature region is (T _mb −3)
It is preferably in the temperature range of 0) ° C to (T _mb -2) ° C. The reason for this is that by performing the respective stretching in these temperature regions, the film-like material can be efficiently stretched, and the pore size can be sufficiently enlarged, so that the preferable pore size (0.005
This is because a porous film having a porosity (20 to 80%) can be obtained with good reproducibility. Also, the fourth
When stretched by the method, more preferable results are obtained than in the first to third methods in that the pore diameter is enlarged and the pores are uniformly present in the film. The stretch ratio of the film-like material when stretched by these stretching methods, that is, the stretch ratio represented by the following formula (Formula 1) is generally 5 to 600%, preferably 20 to 300%. In the formula (Formula 1), L ₀ is the length of the film material before stretching, and L ₁ is the final length of the film material after stretching.

[0017]

[Equation 1]

Further, in both the first method consisting of one stretching step and the second to fourth methods consisting of a plurality of stretching steps, the stretching speed in each step is generally 10 to 500.
0% / min, preferably 100-1000% / min
It is.

Although the porous film composed of a single layer of a porous film of an alloy of polyethylene and polypropylene has been described above, a porous film of an alloy of polyethylene and polypropylene (alloy layer) and a porous film of polypropylene which have been proposed hitherto. Also in a porous membrane of a type in which (polypropylene layer) is laminated, polyethylene having a crystallinity of more than 60% and polypropylene having a crystallinity of more than 70% should be used when obtaining an alloy layer of polyethylene and polypropylene. Thus, the porous film having the temperature characteristics to be heated of the present invention can be obtained. That is, a polyethylene alloy film having a crystallinity of more than 60% and a polypropylene alloy having a crystallinity of more than 70% and a polypropylene film are laminated and stretched by the same stretching method as described above. It becomes porous. The laminated structure here is a two-layer structure of an alloy layer and a polypropylene layer, a three-layer structure in which polypropylene layers are stacked on both main surfaces of the alloy layer, a three-layer structure in which an alloy layer is stacked on both main surfaces of the polypropylene layer, and an alloy layer. And a polypropylene layer may be alternately laminated to have a total number of layers of 4 or more.

Even in the case of obtaining a porous film composed of a single layer of a porous film of an alloy of polyethylene and polypropylene, a porous film of an alloy of polyethylene and polypropylene (alloy layer) and a porous film of polypropylene (polypropylene layer). Even in the case of obtaining a porous film of the type in which (1) and (2) are laminated, the film can be annealed before stretching the film. This annealing serves to increase the porosity during porosification. This annealing is (T _mb
It is preferably carried out in the temperature range of −30) ° C. to (T _mb −2) ° C., and is carried out for several seconds to several hours.

It has been confirmed by experiments that when the porous membrane of the present invention is produced by the above production method, the content of polyethylene in the entire membrane must be 12% by weight or more. This is probably because when the content of polyethylene is less than 12% by weight, the amount of the polyethylene portion existing in a flat and stretched shape in the film becomes too small and the current path is sufficiently lengthened. It is thought to be because it will not be done. Further, if the blending ratio of polyethylene with respect to the entire membrane exceeds 90% by weight, the shape of the membrane may not be maintained when polyethylene is melted, and the upper limit of the blending ratio of polyethylene with respect to the entire membrane is 90%.
It is preferred that the content be% by weight.

The pore size of the porous membrane of the present invention is generally 0.00
It is 5 to 1 μm, preferably 0.01 to 0.5 μm.
Porosity is generally 20-80%, preferably 30-70%
It is. Further, the kind of polyethylene constituting the porous membrane of the present invention is not particularly limited, and low density, medium density or high density polyethylene, and various polyethylene such as linear polyethylene can be used. . The type of polypropylene is also not particularly limited, but in order to obtain a high porosity, it is preferable to use isotactic polypropylene having an isotactic index of 90% or more, preferably 95% or more.

The porous film of the present invention is not limited to the separator of the lithium secondary battery, but of course, lithium (ion) 1
It can also be used as a separator for secondary batteries and other types of batteries.

[0024]

【Example】

(Example 1) A polyethylene having a crystallinity of 70% and a polypropylene having a crystallinity of 80% were prepared, and these were mixed at a mixing ratio (polyethylene: polypropylene) of 6: 4 to form a film, and heat was applied ( Uniaxial stretching with a stretching ratio of 160% (at 115 ° C.) was performed to prepare a porous membrane having a thickness of 25 μm, an average pore diameter of 0.04 μm and a porosity of 45%. Then, this porous membrane was impregnated with an electrolytic solution (a mixed solution of LiBF ₄ molten propylene carbonate and DME (dimethoxyethane)), and both main surfaces were sandwiched by stainless electrodes. Then, the porous membrane having both main surfaces sandwiched by electrodes was mounted on the evaluation tester shown in FIG.
A sinusoidal AC voltage of Hz was applied with an effective value of 35V. Temperature rise rate at this time (average of 2 to 5 seconds after the start of energization) is 20 ° C
/ Sec. FIG. 3 is a diagram showing the temperature change characteristics of the porous film after the start of the application of the sinusoidal AC voltage. The maximum temperature was reached about 2 minutes after the start of the voltage application, and the temperature reached after about 5.5 minutes. Fell. The highest temperature reached is 125 ℃, which is (the melting point of polyethylene (125 ℃) + 20 ℃) 1
It was 45 ° C or lower. Then, this porous film was incorporated into a lithium ion secondary battery, and was overcharged (1C after fully charged).
After that, the battery was charged for 1 hour to 200% charge state), and an external short circuit test was performed. The lithium ion secondary battery here is
A positive electrode material obtained by applying a mixture of LiCoO ₂ as an active material, carbon as a conductive auxiliary agent, and N-methylpyrrolidone (NMP) to an aluminum foil and drying it, and graphite and NMP as an active material were obtained. LiPF ₆ was added to 1 liter of a solution prepared by mixing an equal amount (volume ratio 1: 1) of ethylene carbonate (EC) and DMC (dimethyl carbonate) in the porous film between the negative electrode material obtained by applying the mixture and drying. 1
It is an AA battery in which a plurality of laminated bodies sandwiching a material impregnated with a molly dissolved electrolytic solution are stacked and wound around a center pin, the wound product is housed in a negative electrode can, and sealed with a positive electrode lid. (The negative electrode can has a safety valve.) As a result of the external short-circuit test, the battery operated stably without any short circuit or ignition, and the battery tube wall temperature was 100 ° C. or lower. In addition, 1C [C] is a unit showing the magnitude of the charging / discharging current of the battery, and 1C is a current that changes the battery from a fully discharged state to a fully charged state (from a fully charged state to a completely discharged state) in 1 hour. Indicates the value (eg 1C is 7 for a 750mAh battery)
50mA, 2C is 750 × 2 = 1500mA, 0.5C
Is 750/2 = 350 mA).

Comparative Example 1 A porous membrane was obtained in the same manner as in Example 1 except that polyethylene having a crystallinity of 50% and polypropylene having a crystallinity of 60% were used. Then, the same test as in Example 1 was performed on this porous membrane. FIG. 4 is a diagram showing the temperature change characteristics of the porous film after the start of the application of the sinusoidal alternating voltage at this time, and the temperature reached 150 ° C. about 20 seconds after the start of the voltage application. The highest temperature can be expected to be higher, but when the voltage was continuously applied, the porous film melted and short-circuited between the positive and negative electrodes, so the safety device of the power supply actuated and the energization stopped, and further measurement was not possible. It became impossible. Then, this porous film was incorporated into a battery in the same manner as in Example 1 and an external short circuit test after overcharging was performed. As a result, 2 out of 10 batteries had a battery tube wall temperature of 12
The temperature rose to 0 ° C or higher.

(Example 2) Polyethylene having a crystallinity of 70% and polypropylene having a crystallinity of 80% were prepared, and these were mixed at a mixing ratio (polyethylene: polypropylene) of 5: 5 to form a film (film). To obtain a first film-like material (film). Further, a second film-shaped material (film) made of polypropylene alone was separately prepared. And
The second membranous material was superposed on both main surfaces of the first membranous material (film), and the laminate was heated (at 115 ° C.) and uniaxially stretched at a stretching ratio of 160% to obtain a thickness of A 25 μm laminated porous membrane was prepared. The thickness ratio of the three layers is 1: 1: 1
And the blending ratio of polyethylene per membrane is 16.
7% by weight. Then, the same test as in Example 1 was performed on this laminated porous membrane. FIG. 5 is a diagram showing the temperature change characteristics of the porous film after the start of the application of the sinusoidal alternating voltage at this time, and the maximum temperature was reached about 11 minutes after the start of the voltage application. The highest temperature reached was 127 ° C., which was below (the melting point of polyethylene (125 ° C.) + 20 ° C.), which was 145 ° C. Subsequently, as in Example 1, the porous film was incorporated into a lithium ion secondary battery and subjected to an external short circuit test. As a result, no short circuit or ignition occurred, the battery operated stably, and the battery tube wall temperature was 100 ° C. It was below.

(Comparative Example 2) The same as Example 2 except that polyethylene having a crystallinity of 50% and polypropylene having a crystallinity of 60% were used as raw materials for the first film-like material (film). Thus, a laminated porous film having a three-layer structure was obtained. Then, the same test as in Example 1 was performed on this porous membrane. FIG. 6 is a diagram showing the temperature change characteristics of the porous film after the start of the application of the sinusoidal AC voltage at this time, and the maximum attainable temperature is 157 ° C. It was higher than 145 ° C. When the porous membrane was incorporated into a lithium-ion secondary battery and an external short-circuit test was conducted in the same manner as in Example 1, two out of ten batteries had a battery tube wall temperature of 120 ° C. or higher.

(Comparative Example 3) The thickness ratio of the three layers was set to 2: 1 :.
2 (the thickness of the layer obtained by making the first membrane-like material in the middle porous is small, and the thickness of the layer obtained by making the second membrane-like material outside both sides large) A laminated porous membrane having a laminated structure of a three-layer structure was obtained in the same manner as in Example 2 except that the content of polyethylene was 10% by weight based on the whole membrane. FIG. 7 is a diagram showing the temperature change characteristics of the porous film after the start of the application of the sinusoidal alternating voltage at this time, and the maximum attainable temperature is 1
At 60 ° C, (melting point of polyethylene (125 ° C) + 20
C.) of 145 ° C. Subsequently, as in Example 1, the porous membrane was incorporated into a lithium ion secondary battery and subjected to an external short-circuit test. Two out of ten batteries had a battery tube wall temperature of 120 ° C. or higher. did.

[0029]

As described above, according to the present invention, a porous membrane composed of polyethylene and polypropylene, which has an excellent current interruption effect (shutdown effect) when used as a battery separator, that is, polyethylene It is possible to provide a porous film that shuts down after melting so that the resistance can be instantaneously interrupted after melting.
Further, it is possible to provide a method for producing a porous membrane which can easily and rationally produce the porous membrane of the present invention. Further, it is possible to provide a lithium-ion secondary battery that operates stably without causing ignition or short even if an external short circuit occurs.

[Brief description of drawings]

FIG. 1 is a side view showing a schematic configuration of an evaluation tester for evaluating temperature characteristics of a porous film.

FIG. 2 is a cross-sectional perspective view schematically showing the film structure of a porous film of an alloy of polyethylene and polypropylene according to the present invention.

FIG. 3 is a diagram showing changes in temperature of the porous film after the start of applying a sinusoidal AC voltage to the porous film according to Example 1.

FIG. 4 is a diagram showing a temperature change of a porous film according to a comparative example 1 after a sinusoidal AC voltage is applied to the porous film.

FIG. 5 is a diagram showing changes in temperature of the porous film after the start of applying a sinusoidal AC voltage to the porous film according to Example 2.

FIG. 6 is a diagram showing changes in temperature of the porous film according to Comparative Example 2 after the start of application of a sinusoidal alternating voltage to the porous film.

FIG. 7 is a diagram showing a temperature change of the porous film according to Comparative Example 3 after the start of application of a sinusoidal AC voltage to the porous film.

[Explanation of symbols]

 1 Electrode 2 Electrode that also serves as a mounting base for the porous membrane 3 Clip for fixing the porous membrane to the upper surface of the electrode 4a Temperature sensor 4 Thermometer that outputs the temperature detected by the temperature sensor 5 AC power supply 10 Electrolyte Impregnated porous membrane 10a Polyethylene part 10b Polypropylene part 11,12 Current

Claims (12)

[Claims] 1. A porous membrane comprising polyethylene and polypropylene, wherein electrodes are arranged on both main surfaces of a porous membrane impregnated with an electrolytic solution, and an alternating voltage is applied to the electrodes to form a porous membrane. The membrane is heated to 10 to 5 by resistance heating of the electrolyte.
A porous membrane having a maximum ultimate temperature (melting point of polyethylene + 20 ° C.) or less when heated at a rate of 0 ° C./sec. 2. The porous membrane according to claim 1, wherein the porous membrane is a single body of a mixture of polyethylene and polypropylene. 3. The porous membrane having a laminated structure in which a first porous membrane made of a mixture of polyethylene and polypropylene and a second porous membrane made of polypropylene are alternately laminated to each other. Porous membrane. 4. The method for producing a porous membrane according to claim 1, wherein the polyethylene having a crystallinity of more than 60% and the polypropylene having a crystallinity of more than 70% have a blending ratio of 12 of polyethylene. A method for producing a porous film, which comprises uniaxially stretching a film-like material obtained by mixing so as to be at least wt% to make it porous. 5. When the melting point of polyethylene is T _mb ° C,
The film-like material is uniaxially stretched in a low temperature region of -20 ° C to (T _mb -30) ° C, and then stretched in the high temperature region of (T _mb -30) ° C to (T _mb -2) ° C. The method for producing a porous film according to claim 4, wherein the porous film is stretched in the same or different direction as the stretching direction in the temperature region to make it porous. 6. The stretching direction in the low temperature region and the stretching direction in the high temperature region are the same direction, and the stretching direction of the film after stretching in the high temperature region is different from the stretching direction in the low temperature region. The method for producing a porous film according to claim 5, wherein the porous film is stretched again. 7. The method for producing a porous membrane according to claim 4, wherein the upper limit of the blending ratio of polyethylene based on the entire membrane is 90% by weight. 8. The method for producing a porous film according to claim 1, wherein the polyethylene having a crystallinity of more than 60% and the polypropylene having a crystallinity of more than 70% are mixed. Membrane and polypropylene second
The method for producing a porous membrane in which the film-like material having a laminated structure in which the total content of polyethylene is 12% by weight or more is uniaxially stretched. 9. When the melting point of polyethylene is T _mb ° C,
The film having a laminated structure is uniaxially stretched in the low temperature region of −20 ° C. to (T _mb −30) ° C., and then (T _mb −30) ° C. to (T _mb −).
2. The method for producing a porous membrane according to claim 8, wherein the porous film is stretched in the high temperature region of 2 ° C. in the same direction as or different from the stretching direction in the low temperature region. 10. The stretching direction in the low temperature region and the stretching direction in the high temperature region are the same direction, and after stretching in the high temperature region, the film having a laminated structure is stretched in the low temperature region. 10. The method for producing a porous film according to claim 9, wherein the porous film is stretched again in a direction different from the above. 11. The method for producing a porous membrane according to claim 8, wherein the upper limit of the blending ratio of polyethylene is 90% by weight based on the entire laminated structure. 12. A lithium ion secondary battery in which the porous membrane according to claim 1 impregnated with an electrolytic solution is sandwiched between a pair of electrodes.