KR101117815B1 - Separator having ultrafine fibrous layer with high strength and secondary battery using the same - Google Patents

Abstract

The present invention relates to a separator having a high-strength ultra-fine fiber layer and a secondary battery using the same. More particularly, the present invention has a high shuttling function and excellent ion conductivity, and has excellent cycle characteristics when constructing a battery. The present invention relates to a separator having a high strength ultra-fine fiber layer having excellent adhesion to and a secondary battery using the same. To this end, the separation membrane having a high strength ultra-fine fiber layer according to the present invention is a separator in which a fiber layer is coated on one or both surfaces of the microporous membrane, and the fiber layer is made of ultra-fine fibers formed by electrospinning. Preferably, the fibrous layer is characterized in that it comprises an Ultra High Molecular Weight Polyethylene (Ultra High Molecular Weight Polyethylene) resin.

Description

Separator with high strength ultra-fine fiber layer and secondary battery using the same {SEPARATOR HAVING ULTRAFINE FIBROUS LAYER WITH HIGH STRENGTH AND SECONDARY BATTERY USING THE SAME}

The present invention relates to a separator having a high-strength ultra-fine fiber layer and a secondary battery using the same. More particularly, the present invention has a high shuttling function and excellent ion conductivity, and has excellent cycle characteristics when constructing a battery. The present invention relates to a separator having a high strength ultra-fine fiber layer having excellent adhesion to and a secondary battery using the same.

Recently, secondary batteries including lithium ion secondary batteries, lithium ion polymer batteries or supercapacitors (electric double layer capacitors and similar capacitors) have become thinner, lighter and lighter due to the demands of consumers due to the digitization and high performance of electronic products. The flow is changing with the development of batteries with high capacity due to high energy density. In addition, in order to cope with future energy and environmental problems, development of hybrid electric vehicles, electric vehicles and fuel cell vehicles has been actively conducted. The increase in size is required.

The secondary battery having such a high energy density has a relatively high operating temperature range, and the temperature increases when continuously used in a high rate charge / discharge state. Therefore, higher heat resistance and thermal stability than that required for ordinary separators are required. The separator is positioned between the cathode and the anode of the battery and insulated, and maintains the electrolyte to provide a passage for ion conduction, and when the temperature of the battery becomes too high, a part of the separator melts to block pores in order to block an electric current. SHUTDOWN FUNCTION). When the temperature rises further and the separator melts, a large hole is formed, which causes a short circuit between the anode and the cathode. This temperature is called SHORT CIRCUIT TEMPERATURE. In general, the membrane should have a low shut-down temperature and a higher short-circuit temperature.

For example, in the case of a polyethylene separation membrane, an abnormal heat generation of a battery may cause contraction at 150 ° C. or higher to expose an electrode part, which may cause a short circuit. Accordingly, since a shrinkage of about 20% is expected, a separator having a large area of 20% or more is used. There is usually no advantage in charging and discharging, and there is a problem of increasing weight of a battery and lowering volume efficiency. In particular, as the thickness of the separator becomes thinner, the short circuit temperature is lowered. Therefore, when a thinner separator is used to realize high energy density, a separator having high heat resistance is required. Therefore, it is very important to have both a shut-down function and heat resistance for high energy density and large sized secondary batteries. That is, there is a need for a separation membrane having excellent heat resistance, low thermal shrinkage, and excellent cycle performance.

In addition, lithium secondary batteries should be safe from external shocks. In the future, a hybrid electric vehicle (HYBRID ELECTRIC VEHICLE) or an electric vehicle (ELECTRIC VEHICLE) requires a more safe lithium secondary battery. In order to maintain safety under strong impact, the safety of the separator, a key component of the lithium secondary battery, This requires high strength separators. High-strength fibers using ultra high molecular weight polyethylene are high-tech fibers having a strength that is less than 5 times as much as 10 times more than ordinary fibers. Typical polyester fibers have a strength of 3-4 g / d, while high strength fibers made of ultra high molecular weight polyethylene have a strength of 30-37 g / d. Based on this high strength, ultra high molecular weight polyethylene fibers are widely used in the industry as high strength safety gloves, high strength piercing ropes, body armor and protective clothing, and other reinforcing dogs. In addition, for high capacity batteries, lithium has a very low molecular weight, high density, and can be integrated with energy, and thus, lithium secondary batteries have been proposed as one of the methods, and representative examples thereof include lithium ion batteries and lithium polymer batteries. Early lithium secondary batteries were manufactured using lithium metal or lithium alloy as a negative electrode. However, a secondary battery using lithium metal or lithium alloy as a negative electrode has a problem in that dendrites are formed on the negative electrode as cycles of charge and discharge are repeated, resulting in low cycle characteristics.

In order to solve the problems caused by the formation of the dendrite is a lithium ion battery. The lithium ion battery is composed of a negative electrode active material, a positive electrode active material, an organic electrolyte solution, and a polyolefin separator. The separator serves to prevent internal short circuit caused by contact between the positive electrode and the negative electrode of the lithium ion battery and to permeate ions. Currently, the separator is a separator made of polyethylene or polypropylene.

Since the polyethylene or polypropylene separator does not have an affinity with the electrolyte, leakage of the liquid electrolyte occurs, so that the case is sealed and used in a metal can to ensure safety. Therefore, the battery is heavy, there is a risk of leakage and explosion due to the electrolyte solution filled in the metal can flow out, and when overcharged, dendrites are generated, the electrolyte is decomposed to generate gas, and a protective circuit is required. Since the separator is rolled and used in a circular battery case, it is difficult to manufacture other types of batteries other than the cylindrical battery, the manufacturing process is somewhat complicated, the manufacturing cost is very high, and the production of large and high capacity batteries is difficult.

The battery which improved the problem of such a lithium ion battery is a lithium polymer battery. The lithium polymer battery is used by adding a polymer electrolyte instead of a separator and a liquid electrolyte inserted between the positive electrode and the negative electrode, thereby solving the liquid leakage problem by not using the liquid electrolyte, and reducing the risk of explosion. By using an aluminum pouch, the weight is reduced, and various types of batteries, such as thinning and thinning of the battery, can be manufactured by using plasticity peculiar to a polymer. The polymer electrolyte used in the lithium ion polymer battery includes a gel polymer electrolyte or a plasticized polymer electrolyte. The liquid electrolyte is maintained in the polymer matrix porous structure, and thus has a sufficient ion conductivity of 10 ^-3 Scm ^-1 at room temperature. Since the thermoplastic melts at high temperatures, the battery may be shorted. That is, there is no SHUTDOWN FUNCTION, which is the main function of the separator, and the mechanical properties are weak.

In order to solve this problem, there is a method of coating a polymer electrolyte solution on a polyolefin separator that is conventionally used in lithium ion batteries. A separator is placed between the anode and the cathode and rolled into a uniform shape and inserted into an aluminum pouch. A battery comprising a monomer, a catalyst, a solvent, and a lithium salt is added thereto, then sealed and heat is added thereto to crosslink the polymer chain. Such a battery has a very simple manufacturing method, good mechanical properties because it uses a separator of a conventional lithium ion battery, and has excellent electrochemical characteristics such as high ion conductivity and low interfacial resistance. However, since a method of inducing crosslinking by the reaction between monomers and a catalyst in a state where the battery is completely assembled, reactive monomers of all monomers may remain without participating in the reaction. It may worsen the performance of the battery.

In Japanese Patent Laid-Open Publication No. 2006-92848 and Japanese Patent Laid-Open Publication No. 2006-92847, a polyolefin porous membrane supported on a reactive polymer containing an epoxy resin curing agent is laminated on an electrode, pressed, and the laminate is immersed in an electrolyte solution. And a method of crosslinking the reactive polymer with an epoxy curing agent. However, after rolling the positive electrode, the negative electrode and the separator, there is a problem that the manufacturing process takes a long time because the liquid electrolyte impregnation rate is very slow in the portion where the liquid electrolyte is impregnated. The reason why the impregnation takes a long time is that the liquid electrolyte cannot be impregnated quickly because the porosity of the separator used is only about 40%.

In addition, Korean Patent No. 10-0470314 discloses polyvinylidene fluoride by electrospinning (ELECTROSPINNING) to a polyolefin porous membrane in order to increase the injection rate of the electrolyte, and to prepare a separator having excellent absorption and mechanical strength of the electrolyte solution and excellent binding properties with the electrode. A composite membrane incorporating a superfine fiber layer of a poly (vinylidene fluoride) homopolymer or copolymer has been proposed. However, there is a problem that the high strength required in a high capacity, large area battery such as for automobiles does not have.

Japanese Patent Laid-Open No. 2006-92848 Japanese Patent Publication No. 2006-92847 Republic of Korea Patent No. 10-0470314

The present invention has been made to solve the above problems, an object of the present invention has a high strength and excellent ion conductivity while having a closed function (SHUTDOWN FUNCTION), excellent cycle characteristics when constructing a battery and excellent It is an object of the present invention to provide a separator having a high strength ultrafine fiber layer having excellent adhesiveness and a secondary battery using the same.

These and other objects and advantages of the present invention will become more apparent from the following description of a preferred embodiment thereof.

The object is achieved by a separation membrane having a high strength ultra-fine fiber layer, characterized in that the fiber layer is coated on one or both sides of the microporous membrane, the fiber layer is made of ultra-fine fibers formed by electrospinning.

Here, the fiber layer is characterized in that it comprises an ultra high molecular weight polyethylene (Ultra High Molecular Weight Polyethylene) resin.

Preferably, the weight average molecular weight of the ultra high molecular weight polyethylene resin is characterized in that 1,000,000 ~ 5,000,000.

Preferably, the microporous membrane is characterized in that it comprises a polyolefin polymer resin having a weight average molecular weight of 300,000 ~ 600,000.

Preferably, the molecular weight distribution (Mw / Mn) of the polyolefin polymer is characterized in that 4 to 8.

Preferably, the electrospinning is characterized in that it is conventional electro-spinning, electro-blowing, melt-blown radiation or flash-spinning.

Preferably, the diameter of the ultra-fine fibers of the fiber layer is 1-3000nm, the pore size of the ultra-fine fibers is 1-5000nm, the porosity of the ultra-fine fibers is characterized in that 30-95%.

Preferably, the ultrahigh molecular weight polyethylene fibrous content is more than 0 to 95% by weight based on the polymer component of the separator.

In addition, the above object is achieved by a secondary battery comprising a separator and an electrolyte having two different electrodes and the high strength ultrafine fiber layer interposed between the two electrodes.

According to the present invention, while having a shut-down function (SHUTDOWN FUNCTION) and has high strength and excellent ion conductivity, it has the effect of excellent cycle characteristics and excellent adhesion to the electrode when the battery configuration.

1 is a schematic view of an electrospinning apparatus.

Hereinafter, the present invention will be described in detail with reference to embodiments and drawings of the present invention. These examples are only presented by way of example only to more specifically describe the present invention, it will be apparent to those skilled in the art that the scope of the present invention is not limited by these examples. .

According to the present invention, an ultra-fine molecular layer of ultra-high molecular weight polyethylene prepared by the method of electrospinning and a porous polyolefin membrane are bonded to each other to provide an integrated polyolefin separator. In addition, in the present invention, the formation of a high strength ultrafine fiber layer extends the electrospinning concept to form a superfine fiber by a high melt electric field or air spray as a conventional melt-blown spinning or flash-spinning and variations thereof. Manufacturing methods are also possible. For example, electro-blowing is also possible. Therefore, electrospinning in the present invention includes all these methods.

The separation membrane having a high strength ultra-fine fiber layer according to the present invention is characterized by adopting a very simple and simple process of removing the solvent and forming pores while forming the ultra-fine fiber layer by using the electrospinning method.

1 shows a schematic view of an electrospinning apparatus. The electrospinning apparatus comprises a barrel (1) for storing an ultra high molecular weight polyethylene resin solution and a liquid paraffin, a metering pump (2) for discharging a polymer solution at a constant speed, and a radiation nozzle (4) to which the high voltage generator (3) is connected. Include. The polymer solution discharged through the metering pump (2) is discharged as ultra-fine fibers while passing through the spinning nozzle (4) charged by the high voltage generator (3), the grounded collector plate of the conveyor type moving at a constant speed (6) The polyolefin porous membrane located above is collected. This ultra-fine fiber web is ultra thin, ultra light, has a very high surface area to volume ratio, and high porosity compared to conventional fibers.

High-strength ultra-fine fiber layer formation by the electrospinning method according to the present invention can be seen in Figure 1 because the pore structure is formed by the gap between the accumulated ultra-fine fibers and fibers can be obtained uniform pores.

Meanwhile, in the lithium secondary battery, a large amount of gas is generated inside the battery during the first charging after the battery is sealed. Such gas generation causes bubbles to be generated between the electrode and the polymer electrolyte layer, resulting in a drastic decrease in battery performance due to poor contact, but does not cause a problem due to gas generation in the heat resistant ultrafine fiber layer of the separator having a high strength ultrafine fiber layer according to the present invention. Do not.

The polyolefin porous membrane of the separator having a high strength ultra-fine fiber layer according to the present invention includes a separator and a nonwoven fabric made of a polyolefin resin including polyethylene (PE), polypropylene (PP), copolymers thereof, and the like. Preferably, the polyolefin-based porous membrane includes a polyolefin polymer resin having a weight average molecular weight of 300,000 to 600,000, and the molecular weight distribution (Mw / Mn) of the polyolefin polymer resin is 4 to 8. If the molecular weight distribution (Mw / Mn) is less than 4, it is difficult to obtain a polyolefin polymer resin without special treatment, and if it exceeds 8, it causes a decrease in permeability of the final polyolefin microporous membrane. In addition, the polyolefin porous membrane has a melting point of 100-180 ° C., preferably 120-150 ° C., for a shut down function, and a pore size of the polyolefin porous membrane is 1-5000 nm, and porosity is 30-80%, preferably 40-60%.

In addition, the ultra-fine fiber layer including the ultra-high molecular weight polyethylene resin of the separator having a high-strength ultra-fine fiber layer according to the present invention may have a higher strength than the single-layer separator consisting of only a polyolefin membrane layer. In the present invention, the ultra-high molecular weight polyethylene resin is formed on one side or both sides of the polyolefin porous membrane by forming a heat-resistant ultra-fine fiber layer by using ultra-high molecular weight polyethylene resin on the one or both sides of the polyolefin porous membrane. On the other hand, the average diameter of the fiber has a great influence on the porosity and pore size distribution of the ultrafine fiber layer. The smaller the fiber diameter, the smaller the pore size and the smaller the pore size distribution. In addition, as the diameter of the fiber is smaller, the specific surface area of the fiber is increased, thereby increasing the electrolyte retention capacity, thereby reducing the possibility of electrolyte leakage. Therefore, in the present invention, the diameter of the ultrafine fibers of the high strength ultrafine fiber layer is 1-3000 nm, preferably 1-1000 nm, more preferably 50-800 nm. In addition, the pore size of the ultrafine fibers of the high strength ultrafine fiber layer is 1-5000 nm, preferably 1-3000 nm, and more preferably maintained at 1-1000 nm may have excellent electrolyte solution holding ability without leakage of the electrolyte solution.

In addition, the content of the ultrahigh molecular weight polyethylene fiber phase according to the present invention is preferably more than 0 to 95% by weight based on the polymer component of the separator. That is, if there is a 1 m ² polyolefin separator, the content of the ultrahigh molecular weight polyethylene fiber formed thereon is preferably greater than 0 to 95 wt% based on the polymer component of the separator.

In addition, the porosity of the ultra-fine fibers of the high-strength ultra-fine fiber layer according to the present invention should not be less than the porosity of the polyolefin porous membrane can maintain a high ion conductivity of the polyolefin separator membrane laminated with a high-strength fiber layer, thereby excellent in battery construction Cycle characteristics can be obtained. Therefore, the porosity of the ultrafine fibers of the high strength ultrafine fiber layer is 30-95%, preferably 40-90%.

In addition, the weight average molecular weight of the ultrahigh molecular weight polyethylene resin is preferably 1,000,000 ~ 5,000,000.

In addition, since the strength of the polyolefin separation membrane is generally 10-25 kgf / mm ² , the thickness of the high-strength ultrafine fiber layer according to the present invention is not particularly determined as long as the strength can be maintained at 30 g / d or more. Thickness. Preferably it is 1-20 micrometers, More preferably, it is 1-10 micrometers.

In addition, the secondary battery according to the present invention is characterized in that it comprises a separator and an electrolyte having two different electrodes and the above-described high strength ultra-fine fiber layer interposed between these two electrodes.

Example 1

95% by weight and 5% by weight of ultra high molecular weight polyethylene and liquid paraffin, respectively, to produce high strength ultrafine fibers by electrospinning the surface of the membrane extruded and cast at a ratio of 60% by weight and 40% by weight of high density polyethylene and liquid paraffin, respectively. The polymer resin solution stirred at the ratio was put in the barrel of the electrospinning apparatus as shown in Fig. 1, and the polymer solution was discharged at a rate of 500 mu l / min using a metering pump. At this time, a charge of 17 kV was applied to the spinning nozzle using a high voltage generator, and an ultra-high molecular weight polyethylene ultrafine fiber layer having a thickness of 50 µm was laminated on the surface of the separator. The lamination amount at this time was 12.5 g / m <^2> .

The separator containing the ultrahigh molecular weight polyethylene ultrafine fiber layer was then prepared as a final separator by coaxial stretching, extraction, drying and heat setting.

[Example 2]

The final separation membrane was manufactured in the same manner as in Example 1, except that the deposition rate of the metering pump was controlled to 25 g / m ² to stack the ultrahigh molecular weight polyethylene ultrafine fiber layer on the surface of the separator.

Example 3

The final separation membrane was manufactured in the same manner as in Example 1, except that the deposition rate of the metering pump was controlled to produce 37.5 g / m ² of the laminated amount of the ultra high molecular weight polyethylene ultrafine fiber layer on the surface of the separator.

Comparative Example 1

A final separator was prepared in the same manner as in Example 1, except that the ultrahigh molecular weight polyethylene ultrafine fiber layer was not prepared by electrospinning on the separator surface.

Break stress of the separator according to the Examples and Comparative Examples was measured and the results are shown in Table 1.

Break Stress (kgf / mm ² ) Example 1 16 Example 2 18 Example 3 22 Comparative Example 1 15

According to Table 1, in the case of the embodiments according to the present invention can be seen as a result of increased strength compared to the comparative example.

The separator according to the present invention is particularly useful for electrochemical devices used in hybrid electric vehicles, electric vehicles, fuel cell vehicles, etc., which require high strength.

1: barrel 2: metering pump
3: high voltage generator 4: radiation nozzle
5: charged ultrafine fiber 6: grounded metal current collector

Claims (9)

delete In the separation membrane coated with a fiber layer containing ultra high molecular weight polyethylene (Ultra High Molecular Weight Polyethylene) resin on one side or both sides of the microporous membrane comprising a polyolefin polymer resin,
The fiber layer is made of ultra-fine fibers formed by electrospinning (electro-spinning), characterized in that the stacking amount of the fiber layer consisting of the ultra-fine fibers is 12.5 ~ 37.5 g / ㎡, the membrane having a high strength ultra-fine fiber layer. The method of claim 2,
Separation membrane having a high strength ultra-fine fiber layer, characterized in that the weight average molecular weight of the ultra-high molecular weight polyethylene resin contained in the fiber layer is 1,000,000 ~ 5,000,000. The method of claim 2,
Separation membrane having a high strength ultra-fine fiber layer, characterized in that the weight average molecular weight of the polyolefin polymer resin contained in the microporous membrane is 300,000 ~ 600,000. The method of claim 4, wherein
Separation membrane having a high strength ultra-fine fiber layer, characterized in that the molecular weight distribution (Mw / Mn) of the polyolefin polymer is 4 to 8. The method of claim 2,
The electrospinning is characterized in that the high-strength ultrafine fibrous layer, characterized in that the conventional electro-spinning, electro-blowing, melt-blown spinning or flash-spinning. Separator. The method of claim 2,
The diameter of the ultra-fine fibers of the fiber layer is 1-3000nm, the pore size of the ultra-fine fibers is 1-5000nm, the porosity of the ultra-fine fibers is 30-95%, separation membrane having a high strength ultra-fine fiber layer. The method of claim 2,
The ultra-high molecular weight polyethylene fibrous content is greater than 0 to 95% by weight based on the polymer component of the separator, the separation membrane having a high strength ultra-fine fiber layer. In a secondary battery,
A secondary battery comprising a separator and an electrolyte having a high strength ultra-fine fiber layer according to any one of claims 2 to 8 interposed between two different electrodes and these two electrodes.

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