KR101402979B1 - Porous separator for secondary cell and its preparation method with meta aramide electrospinning and inorganic compound coating on polyolefin substrate - Google Patents

Abstract

A porous separator of the present invention manufactures a meta-aramide nano-nonwoven separator, coats the separator with inorganic compound particles, and improves thermal stability of the separator only by a thin thickness in order to solve a problem of low thermal stability of an existing polyolefin film-shaped separator, thereby improving the stability and output characteristics of a cell.

Description

TECHNICAL FIELD [0001] The present invention relates to a porous separator for a secondary battery and a method for producing the same,

The present invention relates to a porous separator for a secondary battery and a method of manufacturing the same, and more particularly, to a separator for a secondary battery and a method of manufacturing the same, To a porous separator for a secondary battery and a method for manufacturing the same.

In recent years, miniaturization, thinning, light weight, and the like of electronic devices have been made rapidly and rapidly, and batteries that supply electric power in accordance with this tendency are also required to have high performance.

A lithium secondary battery is the battery best suited to such a demand, and the lithium secondary battery comprises an anode, a cathode, an electrolyte, and a separator.

Here, the positive electrode active material applied to the positive electrode is a material capable of intercalating and deintercalating lithium, and is a complex of lithium cobalt oxide (LiCoO ₂ ), lithium manganese oxide (LiMn ₂ O ₄ ), lithium nickel cobalt oxide, lithium iron phosphate oxide Metal oxides are mainly used.

The negative electrode active material applied to the negative electrode is a lithium alloy, carbon, cokes, activated carbon, graphite, silicon (Si), tin (Sn), and the like capable of intercalating and deintercalating lithium ) And other metals and / or alloys are mainly used.

Further, as the non-aqueous electrolyte comprises a lithium salt and an organic solvent as an electrolytic solution, the lithium salt is _{LiClO 4, LiCF 3 SO 3,} LiAsF 6, LiBF 4, LiPF 6, LiSCN, LiC (CF 3 SO 2) 3, LiBOB Propylene carbonate (PC), dimethyl carbonate (DMC), dimethoxyethane (DME), diethoxyethane (DEE), 2-methyltetrahydrofuran (2-MeTHF ), Dimethylsulfoxide (DMSO), and the like are used respectively or in combination.

On the other hand, the separator is one of the four core materials composing the battery. It is located between the anode and the cathode of the battery and separates the lithium oxide, which is a highly active cathode material, directly from the cathode to prevent the explosion. And because it is located between two electrodes, it has pore structure so that lithium ions move smoothly between electrodes.

Here, the thinner the separator, the less the volume in the cell, and the greater the amount of electricity produced per unit volume. However, when the thickness of the separation membrane is made thinner than a specific thickness, the strength of the separation membrane is weakened. Therefore, the mechanical and thermal strength and durability of the separation membrane should also be focused. High-temperature storage and overcharging of the battery are related to the thermal strength of the separator, and safety problems due to nail penetration and foreign matter are related to durability.

On the other hand, polyolefin-based films such as polyethylene (PE) and polypropylene (PP) are mainly used as separation membranes used in lithium secondary batteries. Polyolefins are disadvantageous in that they have high heat shrinkage at a high temperature and are physically weak Have.

In order to improve the poor thermal stability of the polyolefin-based separator, attempts have been made to co-extrude a heat-resistant polymer having a high melting temperature with a polyolefin resin, or to use a heat-resistant polymer as a nonwoven fabric.

Aramid fibers, well known as heat-resistant polymers, are defined as 'fibers of a molecular structure in which at least 85% of amide bonds (-NHCO-) are bonded between aromatic rings', thereby distinguishing them from nylon fibers having the same amide bond . The aramid is divided into para-aramid and meta-aramid depending on the bonding form of the benzene ring. The para-aramid is used mainly as a material for the body armor because of its high strength properties. The meta-aramid has heat resistance Because of its excellent features, it is used for protective clothing including fire fighting clothing, battery insulating paper, industrial filters, industrial materials and construction. The meta-aramid refers to poly-meta-phenylene-terephthalamide in which an amide bond is bonded at the meta position of the benzene ring, and has the following molecular formula 1 structure.

Molecular formula 1. Molecular structure of meta-aramid

The meta-aramid is the first high heat resistant aramid fiber that can be used at 350 ℃ for a short period of time and 210 ℃ for continuous use. When exposed to temperatures above this temperature, it is carbonized without melting or burning like other fibers. Above all, unlike other products that have been flame retarded or refractory treated, they do not emit toxic gases or harmful substances even when carbonized, and thus have excellent properties as eco-friendly fibers.

In addition, since the molecules constituting the fibers themselves have a very rigid molecular structure, the meta-aramid is not only strong in its inherent strength, but also improves the strength as the molecules are easily oriented in the fiber axis direction in the spinning stage .

As another method for improving physical properties such as heat resistance stability of the secondary battery separator membrane, when a nonwoven fabric is used as a separator material, it is in the form of a fibrous mat made by chemical, physical or mechanical connection of natural or synthetic fibers. High porosity and a large melting point. Although these nonwoven fabrics have been used in nickel-cadmium batteries and the like, they have not been applied to lithium secondary batteries because they are difficult to prevent short-circuiting of the battery due to the structure having a relatively large pores and the rough surface even though they have excellent mechanical strength.

In recent years, polyester nonwoven fabric membranes have been studied in order to improve safety and lifetime. High temperature safety using high melting point of polyester and life improving effect due to uniform pore structure have been reported.

Electrospinning is a technique for producing ultrafine fibers and porous webs, that is, nonwoven fabrics having a diameter of several nanometers to several micrometers, using an electrostatic spraying phenomenon of a high viscosity fluid such as a polymer, It is a device that can emit ultra fine fibers by pulling.

The polymer solution at the tip of a vertically positioned capillary (nozzle) in an electrospinning device forms an equilibrium between gravity and surface tension, forming a hemispherical droplet which, when applied to an electric field, causes charge or dipole orientation So that a force opposite to the surface tension is generated due to charge or dipole repulsion. This force causes the hemispherical surface of the solution suspended at the tip of the capillary to expand into a conical shape known as the Taylor cone. At some critical electric field strength (Vc), this repulsive electrostatic force overcomes the surface tension, Jet is emitted from the tail cone tip. In the case of a solution with a low viscosity, the jet collapses into fine droplets, resulting in spraying. However, in the case of a solution having a high viscosity such as a polymer, the jet is not collapsed, the solvent is evaporated as it flows toward the collecting plate toward the air, and the charged continuous polymer fibers are accumulated on the collecting plate. This phenomenon is called electrospinning . The reason why very thin fibers are produced by electrospinning is because the jet is thinned by the elongation of the jet and the spraying phenomenon in the course of flying towards the collector plate. Because of this small diameter, the electrospun fiber will have a larger surface area and volume, and more moisture can be absorbed than other fibers of larger diameter. The ultrafine fiber web thus produced is ultra thin and light in weight and has a very high volume to surface ratio and a high porosity as compared with conventional fibers. Therefore, it is possible to manufacture the structure so that liquid does not come in from the outside of the membrane. Therefore, studies are being conducted to apply the electrospinning phenomenon of polymers to various fields such as the manufacture of ultrafine high performance filters, porous supports for tissue engineering, and chemical sensors.

Further, as the battery has become larger, the problem of wettability with electrolyte has become more serious. If the affinity of the electrolyte solution and the separator membrane is low, the lithium ion transfer capacity in the membrane is deteriorated, leading to a phenomenon in which the output characteristics of the battery are deteriorated. In particular, when the electrolytic solution component has a polarity such as ethylene carbonate (EC) or propylene carbonate (PC), it exhibits poor wettability to the non-polar polyolefin-based separator. Therefore, The problem of aspect appears more clearly.

Therefore, techniques for modifying the surface of the separator to improve the affinity with the electrolyte have been developed. In particular, the inorganic composite separator or the ceramic separator is manufactured by connecting inorganic particles of a very small size with a small amount of a binder. Show high wettability to electrolytes due to their high hydrophilicity and large specific surface area, and excellent wettability in electrolytes such as EC and PC improves battery life and performance. These ceramic separators have very good thermal stability and hardly shrink even at high temperatures.

However, such a ceramic separator exhibits excellent electrolyte wettability and thermal stability, but has disadvantages in that it does not show adequate mechanical properties, particularly flexibility, for a battery assembly process including a winding.

In the present invention, in order to improve low thermal stability of a polyolefin substrate, a metha-aramid nano-nonwoven fabric separator is prepared, and an inorganic material layer capable of improving electrolyte wettability is firmly bonded by coating inorganic particles on the nano- , A porous separator for a secondary battery capable of improving battery stability and output characteristics, and a method for manufacturing the same.

The present invention provides a method for producing a metha-aramid nano-sized nonwoven fabric separator, comprising: electrospinning a polyolefin substrate with a spinning solution having meta-aramid dissolved in an appropriate organic solvent to form a meta-aramid nano- And coating an inorganic slurry on the meta-aramid nano-nonwoven fabric separating membrane to form an inorganic coating layer. The present invention also provides a method for manufacturing a porous separator for a secondary battery.

According to a preferred embodiment of the present invention, the meta-aramid has a weight average molecular weight (Mw) of 3,000 to 500,000, and the organic solvent is dimethylformamide (DMF) or dimethylacetamide (DMAc).

According to another preferred embodiment of the present invention, the electrospinning is characterized by using a bottom-up electrospinning method.

According to another preferred embodiment of the invention, the inorganic material in the slurry is the weight ratio of inorganic binder and 95: and characterized in that 5 to 50: 50, wherein the inorganic substance is _{SiO 2, Al 2 O 3,} TiO 2, Li ₃ PO ₄ , zeolite, MgO, CaO, BaTiO ₃ , Li ₂ O, LiF, LiOH, Li ₃ N, BaO, Na ₂ O, Li ₂ CO ₃ , CaCO ₃ , LiAlO ₂ , SiO, SnO, SnO ₂ , PbO ₂ , ZnO, P ₂ O ₅ , CuO, MoO, V ₂ O ₅ , B ₂ O ₃ , Si ₃ N ₄ , CeO ₂ , Mn ₃ O ₄ , Sn ₂ P ₂ O ₇ , Sn ₂ B ₂ O ₅ Sn ₂ BPO _6, and mixtures thereof. The binder may be at least one selected from the group consisting of meta-aramid, polymethyl methacrylate (PMMA), polyvinyl alcohol (PVA), and carboxymethyl cellulose (CMC ). ≪ / RTI >

The present invention also relates to a polyolefin substrate; A meta-aramid nano-fabric separator formed on one surface of the polyolefin substrate through electrospinning; And an inorganic coating layer formed on one side of the meta-aramid nano-fabric separator. The present invention also provides a porous separator for a secondary battery.

Since the porous separator for a secondary battery of the present invention is strongly bonded to the inorganic coating layer by electrospunmeta nano nonwoven fabric, the separator of the conventional polyolefin-type film and the separator of the polyolefin- .

1 is a schematic view of a porous separator of the present invention.
2 is a schematic view of a method for producing the porous separation membrane of the present invention.
3 is a schematic view of a method of manufacturing a downwardly porous separator of the present invention.

In order to accomplish the above object, the present invention provides a method for manufacturing a membrane-electrode assembly, which comprises applying a nanofiber separator membrane on a polyolefin substrate by electrospinning a meta-aramid, coating the inorganic particle on the membrane separator, And to improve the heat stability.

The thickness of the polyolefin substrate is preferably 5 to 50 μm and the porosity (porosity (%)) = 1- (apparent density of the membrane / resin density) x 100] is preferably 30 to 80%. The tensile strength was 700 kg / cm 2 or more in the mechanical direction (MD), 150 kg / cm 2 or more in the transverse direction (CD), the piercing strength was 200 g or more per mil (1 mil = 25.4 탆) It is particularly suitable for use in an electrochemical device having a physical property of less than 10% for 1 hour, an average pore size of 0.005 to 3 占 퐉 and an electrical characteristic of electric resistance of 10,000? / Cm2 or more at 130 to 185 占 폚.

The electrospinning device includes a spinning liquid main tank for storing a spinning solution, a metering pump for supplying a spinning solution in a fixed amount, a nozzle block for combining a plurality of tubular nozzles composed of a plurality of pins in block form and discharging the spinning solution in a fiber form, A collector for accumulating short fibers emitted at a position corresponding to the nozzle block, a voltage generating device for generating a high voltage, and a spinning solution discharging device connected to the top of the nozzle block.

Hereinafter, a method for preparing a nanofiber nonwoven fabric by electrospinning the meta-aramid of the present invention will be described.

First, a spinning solution is prepared by dissolving meta-aramid in an appropriate organic solvent, and the meta-aramid spinning solution is stored in the spinning solution main tank, metered by a separate metering pump, and supplied to the spinning solution dropping device in a fixed amount.

The meta-aramid has a weight average molecular weight of 3,000 to 500,000, and includes a meta-oriented synthetic aromatic polyamide. When the weight average molecular weight is less than 3,000, the physical properties of the fiber are poor. When the weight average molecular weight exceeds 500,000, the processability is poor.

The meta-aramid polymer may comprise a polyamide homopolymer, a copolymer and mixtures thereof predominantly aromatic, wherein at least 85% of the amide (-CONH-) bonds are attached directly to the two aromatic rings. The ring may be unsubstituted or substituted. The polymer becomes a meta-aramid when the two rings or radicals are meta-oriented along the molecular chain with respect to each other. Preferably, the copolymer contains up to 10% other diamines substituted for the primary diamine used to form the polymer, or up to 10% other diacids substituted for the primary diacid chloride used to form the polymer It has a diacid chloride. Preferred meta-aramids are poly (metaphenylene isophthalamide) (MPD-I) and copolymers thereof. One such meta-aramid fiber is < RTI ID = 0.0 > a < / RTI > children. Nomex < (R) > aramid fibers available from EI du Pont de Nemours and Company, but meta-aramid fibers are available from Teijin Ltd., Tokyo, Japan Tejinconex (registered trademark); New Star (TM) meta-aramid available from Yantai Spandex Co. Ltd, Shandong, China; And Chinfunex (registered trademark) Aramid 1313, available from Guangdong Charming Chemical Co., Ltd., Shinshu, Guangdong, China.

The organic solvent is not particularly limited as long as it can sufficiently dissolve the polymer and is applicable to the electrospinning process. In addition, when the nano-nonwoven fabric separator is prepared by electrospinning, the organic solvent is almost removed, Can also be used. For example, propylene carbonate, butylene carbonate, 1,4-butyrolactone, diethyl carbonate, dimethyl carbonate, 1,2-dimethoxyethane, 1,3-dimethyl-2-imidazolidinone, N-dimethylformamide, N, N-dimethylacetamide, N-methyl-2-pyrrolidone, polyethylene sulfolane, tetraethylene glycol dimethyl ether, acetone, alcohol or mixtures thereof (DMF) or dimethylacetamide (DMAc) may be preferably used as the solvent, and more preferably, dimethylformamide (DMF) or dimethylacetamide (DMAc) may be used.

In this way, the spinning solution supplied into the spinning solution dropping device is conveyed to the spinning solution supply pipe of the nozzle block which has a discontinuous high voltage while passing through the spinning solution dropping device and in which the stirrer is installed. The spinning liquid drop device also blocks the flow of spinning liquid and prevents electricity from flowing into the spinning liquid main tank.

Subsequently, the spinning solution transferred to the spinning liquid supply pipe is discharged to the collector through the multi-tubular nozzle to form a nano-sized nonwoven fabric. At this time, the nano-nonwoven fabric electrospun from the tubular nozzle is widely spread by the air injected from the nozzle for supplying air, and is collected on the collector, thereby widening the collecting area and making the density uniform. The excess flushing liquid which is not fibrous in the multistage nozzle is collected in the overflow removing nozzle and moved to the flushing liquid supply plate through the temporary storage plate of the overflow liquid.

When the nano-sized nonwoven fabric is to be manufactured, the speed of the air in the air supply nozzle is preferably 0.05 to 50 m / sec, more preferably 1 to 30 m / sec. When the velocity of the air is less than 0.05 m / sec, the spreading property of the nanofiber nonwoven fabric collected in the collector is low and the collecting area is not greatly improved. When the air velocity exceeds 50 m / second, And the more serious problem is adhered to the collector in the form of coarse tuft, not in the form of fiber, so that the ability to form nano-nonwoven fabric is remarkably deteriorated.

In addition, the spinning solution which is excessively supplied to the top of the nozzle block is forcibly transferred to the spinning liquid main tank by the spinning solution discharging device.

In order to promote the formation of the nonwoven fabric by the electric force, a voltage of 1 kV or more, preferably 20 kV or more, generated in the voltage generator is applied to the conductive plate and the collector provided at the lower end of the nozzle block. It is more advantageous in terms of productivity to use an endless belt as the collector. It is preferable that the collector reciprocates a predetermined distance to the left and right to uniform the density of the nonwoven fabric.

The nonwoven fabric formed on the collector is wound on the winding roller through the web supporting roller to complete the nonwoven fabric manufacturing process.

The manufacturing apparatus can increase the collecting area to uniform the density of the nano-woven fabric, effectively prevent the droplet phenomenon, improve the quality of the non-woven fabric, increase the fiber-forming effect by the electric force, Mass production is possible. In addition, the width and thickness of the nonwoven fabric can be freely changed and adjusted by arranging the nozzles composed of a plurality of pins in block form.

The electrospinning apparatus may be any of a bottom-up type, a top-down type, a horizontal type, and a combination type. It is more preferable to use a bottom-up type electrospinning apparatus as shown in FIG.

After the meta-aramid nano-nonwoven fabric separator was attached to the polyolefin substrate through the electrospinning method as described above, inorganic slurry prepared by adding inorganic particles and binder resin to acetone was coated on the metha-aramide nano-nonwoven fabric separator to prepare a porous separator do.

The inorganic particles are _{SiO 2, Al 2 O 3,} TiO 2, Li 3 PO 4, _{zeolites, MgO, CaO, BaTiO 3,} Li 2 O, LiF, LiOH, Li 3 N, BaO, Na 2 O, Li 2 CO _{3, CaCO 3, LiAlO 2,} SiO, SnO, SnO 2, PbO 2, ZnO, P 2 O 5, CuO, MoO, V 2 O 5, B 2 O 3, Si 3 N 4, CeO 2, Mn 3 O ₄ , Sn ₂ P ₂ O ₇ , Sn ₂ B ₂ O ₅ , Sn ₂ BPO _6, and mixtures thereof, and is particularly preferably SiO ₂ or Al ₂ O ₃ .

The binder resin is made of meta-aramid, polymethyl methacrylate (PMMA), polyvinyl alcohol (PVA) and carboxymethyl cellulose (CMC), and the inorganic particles are adhered on the nano-sized nonwoven fabric separator.

The coating method may be various coating methods such as chemical vapor deposition (CVD), physical vapor deposition (PVD), spray coating, dip coating, spin coating and casting. , In particular a coating by a casting method.

The porous separator for a secondary battery having improved thermal stability by coating inorganic particles on the polyolefin substrate of the present invention after electrospinning the nano-sized nonwoven fabric on the polyolefin substrate according to the present invention will be described in more detail below . It will be apparent, however, to those skilled in the art that the scope of the invention is not limited to the disclosed embodiments and modifications or improvements made therefrom.

(1) Heat shrinkage measurement

A porous membrane sample of 5 cm x 2.5 cm was placed between two slide glasses and clamped with a clip. The sample was allowed to stand at 150 ° C for 30 minutes and the shrinkage ratio was calculated.

Example 1

A methacrylamide having a weight average molecular weight (Mw) of 50,000 was dissolved in a DMAc solvent to prepare a spinning solution. The spinning solution was sprayed onto a polyolefin substrate (Celgard 2400) having a thickness of 10 탆 by a distance of 40 cm between the electrode and the collector, , A spinning liquid flow rate of 0.1 mL / h, a temperature of 22 DEG C, and a humidity of 20% to form a 3-m thick meta-aramid nano-fabric separator membrane.

A slurry prepared by adding Al ₂ O ₃ inorganic particles having a size of 0.5 μm and poly (methyl methacrylate) (PMMA) (LG IG840) as a binder to acetone at a weight ratio of 9: 1 was applied to the above- Coat the separator with a casting method to a thickness of 5 탆.

The heat shrinkage ratio was evaluated using the porous separator for secondary battery manufactured by the above method, and it is shown in Table 1 below.

Example 2

A porous separator for a secondary battery was prepared in the same manner as in Example 1 except that Al ₂ O ₃ inorganic particles having a size of 0.5 μm and polymethyl methacrylate (PMMA, LG IG 840) were mixed at an 8: 2 weight ratio. The performance was evaluated by the method.

Comparative Example 1

The performance of the separator was evaluated by using a polyolefin film (Celgard 2400) having a thickness of 18 탆 which was not subjected to any further treatment.

Comparative Example 2

A porous separator for a secondary battery was prepared by directly coating an inorganic slurry of Example 1 with a thickness of 5 탆 on a polyolefin substrate (Celgard 2400) having a thickness of 13 탆 and evaluating the performance of the separator by the same method as in Example.

Example 1 Example 2 Comparative Example 1 Comparative Example 2 Heat shrinkage (%) 2 3 42 12 Thickness (㎛) 18 18 18 18

As described above, the porous separator for a secondary battery provided by the present invention has improved heat stability compared to a separator in which an inorganic coating layer is firmly bonded to a general polyolefin film and a polyolefin film by a nonwoven fabric layer by electrospinning .

The present invention is not limited to the above embodiments and is protected by the claims.

1: Fluid Main tank 2: Metering pump
3: Fluid drop device 4: Nozzle block
5: nozzle 6: nanofiber
7: Collector 8: Collector support roller
9: voltage generating device 10: nozzle block left-right reciprocating device
11: Stirrer 12: Fluid discharging device
13: transfer tube 14: nano-woven fabric supporting roller
15: Nano-nonwoven fabric 16: Nano-nonwoven fabric winding roller

Claims (7)

A step of electrospinning a spinning solution in which a meta-aramid is dissolved in an organic solvent on a polyolefin substrate to form a meta-aramid nano-fabric separator; And
And coating an inorganic slurry on the meta-aramid nano-fabric separator to form an inorganic coating layer.
The method according to claim 1,
Wherein the meta-aramid has a weight average molecular weight (Mw) of 3,000 to 500,000, and the organic solvent is dimethylformamide (DMF) or dimethylacetamide (DMAc).
The method according to claim 1,
Wherein the electrospinning uses a bottom-up electrospinning method.
The method according to claim 1,
Wherein the weight ratio of the inorganic material and the binder in the inorganic slurry is 95: 5 to 50:50.
The method according to claim 1,
The inorganic substance is _{SiO 2, Al 2 O 3,} TiO 2, Li 3 PO 4, _{zeolites, MgO, CaO, BaTiO 3,} Li 2 O, LiF, LiOH, Li 3 N, BaO, Na 2 O, Li 2 CO 3 , CaCO ₃ , LiAlO ₂ , SiO, SnO, SnO ₂ , PbO ₂ , ZnO, P ₂ O ₅ , CuO, MoO, V ₂ O ₅ , B ₂ O ₃ , Si ₃ N ₄ , CeO ₂ , Mn ₃ O _{4 , Sn 2 P 2 O 7,} Sn 2 B 2 O 5, Sn 2 BPO 6 and a method of manufacturing a secondary battery porous separator, characterized in that any one selected from the group consisting of each mixture.
5. The method of claim 4,
Wherein the binder is any one selected from the group consisting of meta-aramid, polymethyl methacrylate (PMMA), polyvinyl alcohol (PVA), and carboxymethyl cellulose (CMC).
Polyolefin substrates;
A meta-aramid nano-fabric separator formed on one surface of the polyolefin substrate through electrospinning; And
And an inorganic coating layer formed on one side of the meta-aramid nano-fabric separator.

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