KR101596491B1 - A electrochemical device for progressing cycle characteristic - Google Patents

Abstract

The electrochemical device according to the present invention comprises: (a) a porous substrate having pores; a first porous coating layer coated on one surface of the porous substrate and formed of a mixture containing inorganic particles and a binder polymer; A second porous coating layer formed of a mixture of inorganic particles and a binder polymer, wherein the porosity of the first porous coating layer is higher than the porosity of the second porous coating layer; (b) an anode arranged to face the first porous coating layer; And (c) a positive electrode facing the second porous coating layer. The electrochemical device of the present invention has improved thermal stability and improved cycle characteristics.

Description

TECHNICAL FIELD [0001] The present invention relates to an electrochemical device having improved cycle characteristics,

The present invention relates to a chemical device such as a lithium secondary battery, and more particularly, to an electrochemical device having a composite separator having a porous substrate coated with a porous coating layer containing a mixture of inorganic particles and a binder polymer.

Recently, interest in energy storage technology is increasing. As the application fields of cell phones, camcorders, notebook PCs and even electric vehicles are expanding, efforts for research and development of electrochemical devices are becoming more and more specified. Electrochemical devices have attracted the greatest attention in this respect, among which the development of rechargeable secondary batteries has become a focus of attention. In recent years, in order to improve the capacity density and specific energy in developing such batteries, And research and development on the design of the battery.

Among the currently applied secondary batteries, the lithium secondary battery developed in the early 1990s has advantages such as higher operating voltage and higher energy density than conventional batteries such as Ni-MH, Ni-Cd and sulfuric acid-lead batteries using an aqueous electrolyte solution . However, such a lithium ion battery has safety problems such as ignition and explosion when using an organic electrolytic solution, and it is disadvantageous in that it is difficult to manufacture. Recently, the lithium ion polymer battery is considered to be one of the next generation batteries by improving the weak point of the lithium ion battery. However, since the capacity of the battery is still relatively low as compared with the lithium ion battery and the discharge capacity at low temperature is insufficient, Is urgently required.

Such electrochemical devices are produced in many companies, but their safety characteristics are different from each other. Securing the electrochemical device is very important. The most important consideration is that the electrochemical device should not injure the user in case of malfunction. For this purpose, the safety standard strictly regulates the ignition and fuming in the electrochemical device. In the safety characteristics of the electrochemical device, there is a high possibility that the electrochemical device will be overheated to cause thermal runaway or explosion if the separator is penetrated. Particularly, the polyolefin-based porous substrate commonly used as a separator of an electrochemical device exhibits extreme heat shrinkage behavior at a temperature of 100 DEG C or higher owing to the characteristics of the manufacturing process including material properties and elongation, . ≪ / RTI >

In order to solve the safety problem of such an electrochemical device, Korean Patent Laid-Open Publication No. 2007-0019958 discloses a composite separator in which a porous coating layer composed of a mixture of inorganic particles and a binder polymer is coated on at least one surface of a porous substrate. In the composite separator, the inorganic particles in the porous coating layer coated on the porous substrate serve as a kind of spacer capable of maintaining the physical form of the porous coating layer. Accordingly, even when the electrochemical device is overheated, the heat shrinkage of the porous substrate is suppressed. In addition, interstitial volumes exist between the inorganic particles to form micropores.

On the other hand, the above-described composite separator is interposed between the positive electrode and the negative electrode to constitute an electrochemical device. As the number of times of use of the electrochemical device increases, a considerable amount of capacity deteriorates.

SUMMARY OF THE INVENTION The present invention provides an electrochemical device having improved thermal stability and improved cycle characteristics by introducing a porous coating layer into a porous substrate.

According to an aspect of the present invention, there is provided an electrochemical device comprising:

(a) a porous substrate having pores, a first porous coating layer coated on one surface of the porous substrate and formed of a mixture containing inorganic particles and a binder polymer, and a first porous coating layer coated on the other surface of the porous substrate, And a second porous coating layer formed on the first porous coating layer, wherein the porosity of the first porous coating layer is larger than the porosity of the second porous coating layer;

(b) an anode arranged to face the first porous coating layer; And

(c) a positive electrode facing the second porous coating layer.

In the electrochemical device according to the present invention, it is preferable that porosity of the first porous coating layer and the second porous coating layer satisfy the following formula (1).

In Equation (1), P ₁ is the porosity of the first porous coating layer, and P ₂ is the porosity of the second porous coating layer.

In the electrochemical device according to the present invention, it is more preferable that the porosity of the first porous coating layer and the second porous coating layer satisfy the following formula (2).

In Equation (2), P ₁ is the porosity of the first porous coating layer, and P ₂ is the porosity of the second porous coating layer.

In the electrochemical device according to the present invention, the porosity of the first porous coating layer is preferably 0.50 to 0.80, and the thicknesses of the first porous coating layer and the second porous coating layer are preferably 1.0 to 10.0 탆.

In the electrochemical device according to the present invention, the average particle size of the inorganic particles contained in the first porous coating layer is preferably larger than the average particle size of the inorganic particles contained in the second porous coating layer, It is preferable that the weight ratio of the binder polymer / inorganic particles is smaller than the weight ratio of the binder polymer / inorganic particles contained in the second porous coating layer.

In the electrochemical device according to the present invention, the porous substrate is preferably a polyolefin-based porous film, and the thickness of the porous substrate is preferably 1 to 100 μm.

In the electrochemical device according to the present invention, the positive electrode may be made of LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiCoPO ₄ , LiFePO ₄ , LiNiMnCoO ₂ , LiNi _1x-yz Co _x M _{y y} M2 _z O ₂ X, y and z are independently selected from the group consisting of Al, Ni, Co, Fe, Mn, V, Cr, Ti, W, Ta, Mg and Mo, 0 x < 0.5, 0 y < 0.5, 0 z < 0.5), or a mixture of two or more thereof. The negative electrode preferably contains negative active material particles such as natural graphite, artificial graphite, carbonaceous material, LTO, silicon (Si), tin (Sn) or the like, or a mixture of two or more thereof.

The electrochemical device of the present invention exhibits the following characteristics.

First, when the electrochemical device is overheated, the porous coating layer suppresses the heat shrinkage of the porous substrate and prevents shorting of both electrodes when thermal runaway occurs.

Second, the first porous coating layer having a porosity higher than the porosity of the second porous coating layer is made to face the negative electrode, so that the actual ion transfer rate of the negative electrode surface having a relatively greater side reaction than the positive electrode can be reduced. The cycle characteristics of the electrochemical device are improved. That is, the composite separator in which the first porous coating layer and the second porous coating layer are formed with the same pore is employed, or the capacity degradation is smaller than that of the electrochemical device in which the first porous coating layer having a relatively large porosity is opposed to the positive electrode.

1 is an exploded cross-sectional view schematically showing the structure of an anode, a cathode, and a composite separator of an electrochemical device according to the present invention.

Hereinafter, the present invention will be described in detail. Prior to this, terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms, and the inventor should appropriately interpret the concepts of the terms appropriately It should be construed in accordance with the meaning and concept consistent with the technical idea of the present invention based on the principle that it can be defined.

The electrochemical device according to the present invention comprises: (a) a porous substrate having pores; a first porous coating layer coated on one surface of the porous substrate and formed of a mixture containing inorganic particles and a binder polymer; A second porous coating layer formed of a mixture of inorganic particles and a binder polymer, wherein the porosity of the first porous coating layer is higher than the porosity of the second porous coating layer; (b) an anode arranged to face the first porous coating layer; And (c) a positive electrode facing the second porous coating layer.

1 is an exploded cross-sectional view schematically showing the structure of an anode, a cathode, and a composite separator of an electrochemical device according to the present invention.

The composite membrane 10 has a first porous coating layer 2 and a second porous coating layer 5 coated on both sides of a porous substrate 1 having pores. The first porous coating layer 2 and the second porous coating layer 5 coated on both surfaces of the porous substrate 1 are formed of a mixture containing inorganic particles and a binder polymer independently of each other. The inorganic particles existing in the porous coating layers 2 and 5 serve as a kind of spacer capable of maintaining the physical form of the porous coating layers 2 and 5 so that when the electrochemical element is overheated, Thereby preventing shorting of both electrodes when thermal runaway occurs. In addition, interstitial volumes exist between the inorganic particles to form micropores. On the other hand, the porosity of the first porous coating layer 2 is larger than the porosity of the second porous coating layer 5.

On the other hand, the cathode 20 is provided so as to face the first porous coating layer 2 of the composite separator 1 having the above-described structure. Further, the anode 30 is provided so as to face the second porous coating layer 5 of the composite separator 1 having the above-described structure.

The first porous coating layer 2 having relatively higher porosity than that of the second porous coating layer 5 of the composite separator 10 is opposed to the negative electrode so that the actual side of the negative electrode having a relatively greater side reaction than the positive electrode The transfer speed can be reduced, thereby improving the cycle characteristics of the electrochemical device. That is, the electrochemical device in which the composite separator 10 according to the present invention is disposed by the above-described method may be formed by employing a composite separator in which the first porous coating layer and the second porous coating layer are formed with the same pore, 1 capacity degradation is smaller than that of an electrochemical device in which a porous coating layer is opposed to a positive electrode.

In the electrochemical device according to the present invention, it is preferable that the porosity of the first porous coating layer and the second porous coating layer satisfy the following formula (1).

&Quot; (1) &quot;

_{0.03 ≤ (P 1 -P 2)} ≤ 0.3

In Equation (1), P ₁ is the porosity of the first porous coating layer, and P ₂ is the porosity of the second porous coating layer. Here, the porosity of the first porous coating layer is preferably 0.50 to 0.80.

It is more preferable that the porosity of the first porous coating layer and the second porous coating layer satisfy the following formula (2).

&Quot; (2) &quot;

0.05? (P ₁ -P ₂ )? 0.2

In Equation (2), P ₁ is the porosity of the first porous coating layer, and P ₂ is the porosity of the second porous coating layer.

The thicknesses of the first porous coating layer and the second porous coating layer are each preferably 1.0 to 10.0 탆, and the thicknesses of the first porous coating layer and the second porous coating layer may be the same or different.

In the electrochemical device according to the present invention, as the porous substrate, any porous substrate commonly used in an electrochemical device such as a porous membrane or a nonwoven fabric formed of various polymers can be used. For example, a polyolefin porous film used as a separator of an electrochemical device, particularly, a lithium secondary battery, or a nonwoven fabric made of polyethylene terephthalate fiber may be used. The material and the shape may be variously selected depending on the purpose. For example, the polyolefin-based porous membrane may be made of a polyolefin-based polymer such as polyethylene, polypropylene, polybutylene, or polypentene, such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, ultra-high molecular weight polyethylene, And the nonwoven fabric may be made of a polyolefin-based polymer or a fiber using a polymer having higher heat resistance. The thickness of the porous substrate is not particularly limited, but is preferably 1 to 100 占 퐉, more preferably 5 to 50 占 퐉, and the pore size and porosity present in the porous substrate are also not particularly limited, but 0.001 to 50 占 퐉 and 10 To 95%. The porous substrate is preferably a polyolefin-based porous film.

The inorganic particles contained in the first porous coating layer and the second porous coating layer are not particularly limited as long as they are electrochemically stable. That is, the inorganic particles usable in the present invention are not particularly limited as long as oxidation and / or reduction reaction does not occur in the operating voltage range of the applied electrochemical device (for example, 0 to 5 V based on Li / Li ⁺ ). Particularly, when inorganic particles having a high dielectric constant are used as the inorganic particles, the dissociation of the electrolyte salt, for example, the lithium salt in the liquid electrolyte, can be increased, and the ion conductivity of the electrolyte can be improved.

For the reasons stated above, it is preferable that the inorganic particles include high-permittivity inorganic particles having a dielectric constant of 5 or more, preferably 10 or more. Non-limiting examples of inorganic particles having a dielectric constant of 5 or more include BaTiO ₃ , Pb (Zr, Ti) O ₃ (PZT), Pb _{1 -x} La _x Zr _{1 -y} Ti _y O ₃ (PLZT, Pb (Mg _1/3 Nb _2/3 ) O ₃ -PbTiO ₃ (PMN-PT), hafnia (HfO ₂ ), SrTiO ₃ , SnO ₂ , CeO ₂ , MgO , NiO, CaO, ZnO, ZrO 2, Y 2 O 3, Al 2 O 3, TiO 2, SiC Or a mixture thereof.

As the inorganic particles, inorganic particles having a lithium ion transferring ability, that is, inorganic particles containing a lithium element but having a function of transferring lithium ions without storing lithium can be used. Non-limiting examples of inorganic particles having lithium ion transferring ability include lithium phosphate (Li ₃ PO ₄ ), lithium titanium phosphate (Li _x Ti _y (PO ₄ ) ₃ , 0 <x <2, 0 <y < (Li _x Al _y Ti _z (PO ₄ ) ₃ , 0 <x <2, 0 <y <1, 0 <z <3), 14Li ₂ O-9Al ₂ O ₃ -38TiO ₂ -39P ₂ O _5, including (LiAlTiP) _x O _y series glass (0 <x <4, 0 <y <13), lithium lanthanum titanate _{(Li x La y TiO 3,} 0 <x <2, 0 <y <3) (Li _x Ge _y P _z S _w , 0 <x <4, 0 <y <1, 0 <z <1, 0 <w <5) such as Li _3.25 Ge _0.25 P _0.75 S ₄ ), Li ₃ lithium nitride such as _{N (Li x N y, 0} <x <4, 0 <y <2), Li 3 PO 4 SiS 2 family, such as _{-Li 2 S-SiS 2 glass (} Li x Si _{y S z, 0 <x <} 3, 0 <y <2, 0 <z <4), LiI-Li 2 SP 2 S 5 , etc., such as P ₂ S ₅ based _{glass (Li x P y S z} , 0 <x <3, 0 <y <3, 0 <z <7), or a mixture thereof.

The average particle diameter of the inorganic particles is not particularly limited, but is preferably in the range of 0.001 to 10 mu m for the formation of the porous coating layer having a uniform thickness and the adequate porosity. If it is less than 0.001 mu m, the dispersibility may be deteriorated, and if it exceeds 10 mu m, the thickness of the formed coating layer may increase.

The binder polymer contained in each of the first porous coating layer and the second porous coating layer is preferably a polymer having a glass transition temperature (T _g ) of -200 to 200 ° C. This is because the flexibility of the finally formed coating layer And mechanical properties such as elasticity and the like can be improved.

In addition, the binder polymer does not necessarily have ion conductivity, but the performance of the electrochemical device can be further improved by using a polymer having ion conductivity. Therefore, it is preferable that the binder polymer has a high permittivity constant. In fact, since the dissociation degree of the salt in the electrolyte depends on the permittivity constant of the electrolyte solvent, the higher the permittivity constant of the binder polymer, the better the salt dissociation in the electrolyte. The dielectric constant of such a binder polymer may be in the range of 1.0 to 100 (measurement frequency = 1 kHz), preferably 10 or more.

In addition to the functions described above, the binder polymer may be characterized by being capable of exhibiting a high degree of swelling of the electrolyte due to gelation upon impregnation with a liquid electrolyte. Accordingly, it is preferable to use a polymer having a solubility index of 15 to 45 MPa ^1/2 , more preferably a solubility index of 15 to 25 MPa ^1/2 and a range of 30 to 45 MPa ^1/2 . Therefore, it is preferable to use hydrophilic polymers having many polar groups, rather than hydrophobic polymers such as polyolefins. If the solubility index is less than 15 MPa &lt; ^1/2 &gt; and more than 45 MPa &lt; ^1/2 &gt;, it is difficult to swell by a conventional liquid electrolyte for a battery.

Non-limiting examples of such a binder polymer include polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichlorethylene, polymethylmethacrylate, But are not limited to, polymethylmethacrylate, polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, Polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, and the like. ), Cyanoethylpolybio Cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methyl cellulose, and the like can be mentioned. These may be used alone or in combination of two or more thereof. Two or more species may be used in combination.

The weight ratio of the inorganic particles and the binder polymer contained in the first porous coating layer and the second porous coating layer is, for example, in the range of 50:50 to 99: 1, and more preferably 70:30 to 95: 5. When the content ratio of the inorganic particles to the binder polymer is less than 50:50, the content of the polymer is increased, and the pore size and porosity of the formed porous coating layer may be reduced. If the content of the inorganic particles exceeds 99 parts by weight, the fillerability of the formed porous coating layer may be weakened because the content of the binder polymer is small.

In the electrochemical device according to the present invention, the positive electrode and the negative electrode may be composed of a conventional positive electrode and a negative electrode used in an electrochemical device. Generally, the positive electrode and the negative electrode can be manufactured by coating a slurry containing a cathode active material particle and a slurry containing a cathode active material particle on a cathode current collector and a cathode current collector, respectively, and then drying the slurry. The positive electrode _{LiCoO 2, LiNiO 2, LiMn 2} O 4, LiCoPO 4, LiFePO 4, LiNiMnCoO 2, LiNi 1x-yz Co x M1 y M2 z O 2 (M1 and M2 are independently selected from Al, Ni, Co, Fe to each other, X, y and z are independently selected from the group consisting of Mn, V, Cr, Ti, W, Ta, Mg and Mo, 0 < z < 0.5), or a mixture of two or more of them. However, the present invention is not limited thereto. It is preferable that the negative electrode contains a negative electrode active material particle such as natural graphite, artificial graphite, carbonaceous material, LTO, silicon (Si), tin (Sn) or the like alone or a mixture of two or more thereof. It does not.

The above-described electrochemical device can be manufactured by the following method, but is not limited thereto.

First, a binder polymer solution is prepared by dissolving a binder polymer in a solvent, and inorganic particles are added and dispersed to prepare a slurry. It is preferable that the solvent has a solubility index similar to that of the binder polymer to be used and a low boiling point. This is to facilitate uniform mixing and subsequent solvent removal. Non-limiting examples of usable solvents include acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone ( N-methyl-2-pyrrolidone, NMP), cyclohexane, water or a mixture thereof. It is preferable to add inorganic particles to the binder polymer solution and then crush the inorganic particles. In this case, the crushing time is preferably 1 to 20 hours, and the particle size of the crushed inorganic particles is preferably 0.001 to 10 mu m as described above. As the crushing method, a conventional method can be used, and a ball mill method is particularly preferable.

The slurry is then coated on both sides of the porous substrate under 10 to 80% humidity conditions and dried to form a first porous coating layer and a second porous coating layer. At this time, the porosity of the first porous coating layer is formed larger than that of the second porous coating layer.

As a method of coating the slurry on the porous substrate, a conventional coating method known in the art can be used, and for example, die coating, roll coating and the like can be used. There are various methods for forming the porosity of the first porous coating layer larger than the porosity of the second porous coating layer. For example, by varying the average particle size of the inorganic particles in the slurry applied to both surfaces of the porous substrate. That is, the average particle size of the inorganic particles dispersed in the slurry for forming the first porous coating layer can be controlled by adjusting the average particle size of the inorganic particles dispersed in the slurry for forming the second porous coating layer. The average particle diameter of the inorganic particles contained in the first porous coating layer formed therefrom is larger than the average particle diameter of the inorganic particles contained in the second porous coating layer. Another method can be achieved by making the content ratios of the binder polymer and the inorganic particles in the slurry applied to both surfaces of the porous substrate different from each other. That is, the content ratio of the binder polymer to the inorganic particles in the slurry for forming the first porous coating layer can be controlled by adjusting the content ratio of the binder polymer to the inorganic particles in the slurry for forming the second porous coating layer. The weight ratio of the binder polymer / inorganic particles contained in the first porous coating layer formed therefrom is smaller than the weight ratio of the binder polymer / inorganic particles contained in the second porous coating layer.

The thus-prepared composite separator is interposed between the anode and the cathode. The cathode is opposed to the first porous coating layer, and the anode is disposed to face the second porous coating layer. It is sufficient if the composite separator and both electrodes are physically in contact with each other, but they can be laminated to each other as needed.

The electrochemical device of the present invention includes all devices that perform an electrochemical reaction, and specific examples thereof include capacitors such as all kinds of primary, secondary cells, fuel cells, solar cells, and super capacitors . Particularly, a lithium secondary battery including a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery is preferable.

The electrochemical device can be manufactured according to a conventional method known in the art. For example, the electrode structure made of the positive electrode, the negative electrode, and the composite separator may be sealed in a casing such as a pouch, and then an electrolyte may be injected .

As salts of the structure, such as, A ⁺ is Li ^+, Na ^+, include alkali metal cation or an ion composed of a combination thereof, such as K ⁺ and B ^{- -} electrolyte that may be used are A ⁺ B is PF ₆ ^-, BF _{^{4 -, Cl -, Br -}} , I -, ClO 4 -, AsF 6 -, CH 3 CO 2 -, CF 3 SO 3 -, N (CF 3 SO 2) 2 -, C (CF 2 SO 2) 3 ^- , anion such as anion, or a combination thereof, is preferably selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (NMP), ethylmethyl carbonate (EMC), gamma-butyrolactone (gamma -butyrolactone), or a mixture of these solvents. , And the like, but the present invention is not limited thereto.

The electrolyte injection may be performed at an appropriate stage of the battery manufacturing process, depending on the manufacturing process and required properties of the final product. That is, it can be applied before assembling the cell or at the final stage of assembling the cell.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to examples. However, the embodiments according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to enable those skilled in the art to more fully understand the present invention.

Example 1

Preparation of composite membrane

PVdF-CTFE (polyvinylidene fluoride-chlorotrifluoroethylene copolymer) and Cyanoethylpullulan (cyanoethylpullulan) were added to acetone at a weight ratio of 10: 2, respectively, and dissolved at 50 DEG C for about 12 hours or longer to obtain a polymer solution . Al ₂ O ₃ powder and BaTiO ₃ powder at a weight ratio of 9: 1 were added to a polymer solution prepared so as to have a polymer / inorganic particle = 5/95 weight ratio, and the mixture was ball milled for 12 hours or longer The inorganic particles were crushed and dispersed to prepare a first slurry. Further, a second slurry was prepared by changing the weight ratio of the polymer / inorganic particles in the polymer solution to 10/90. The average particle size of the inorganic particles of the first slurry and the second slurry thus produced was 600 nm on average.

The first slurry and the second slurry were slot-coated on both sides of a 9 占 퐉 -thick polyethylene porous film (Asahi Company ND209), and then the solvent was dried. The porosity of the porous coating layer formed on both surfaces of the porous membrane was 0.58 (first porous coating layer) and 0.47 (second porous coating layer). The thicknesses of the first porous coating layer and the second porous coating layer were 3 mu m, respectively.

The porosity of the first and second porous coating layers was determined by the following method.

The formed porous coating layer was peeled off a predetermined area using an adhesive tape, and the weight was measured to calculate the actual density. After the solid content density of the slurry for forming the porous coating layer was calculated theoretically, porosity was calculated by the following equation.

Porosity of the porous coating layer = 1 - [(solid content density of slurry: actual density of porous coating layer) / (solid content density of slurry)]

Manufacture of batteries

The composite membrane prepared by the above-described method was sandwiched between the positive electrode and the negative electrode, and then wound and assembled. At this time, the first porous coating layer was disposed so as to face the negative electrode coated with the graphite anode active material particles in the copper (Cu) thin film, and the second porous coating layer was opposed to the positive electrode coated with the LiCoO ₂ particles .

A nonaqueous electrolytic solution obtained by dissolving lithium hexafluorophosphate (LiPF6) 1 mole in an organic solvent containing ethylene carbonate (EC) / ethylmethyl carbonate (EMC) = 1/2 (volume ratio) was injected into the assembled battery, A battery was prepared.

Example 2

Preparation of composite membrane

PVdF-CTFE (polyvinylidene fluoride-chlorotrifluoroethylene copolymer) and Cyanoethylpullulan (cyanoethylpullulan) were added to acetone at a weight ratio of 10: 2, respectively, and dissolved at 50 DEG C for about 12 hours or longer to obtain a polymer solution . Al ₂ O ₃ powder and BaTiO ₃ powder in a weight ratio of 9: 1 were added to a polymer solution prepared so as to have a ratio of polymer / inorganic particles = 10/90 by weight, A first slurry and a second slurry in which inorganic particles having a particle size were dispersed were prepared. The average particle diameter of the inorganic particles in the first slurry was 1400 nm and the average particle diameter of the inorganic particles in the second slurry was 600 nm.

The first slurry and the second slurry were slot-coated on both sides of a 9 占 퐉 -thick polyethylene porous film (Asahi Company ND209), and then the solvent was dried. The porosity of the porous coating layer formed on both surfaces of the porous membrane was 0.64 (the first porous coating layer) and 0.48 (the second porous coating layer). The thicknesses of the first porous coating layer and the second porous coating layer were 3 mu m, respectively.

A lithium secondary battery was prepared in the same manner as in Example 1 using the composite separator prepared by the above-mentioned method.

Comparative Example 1

 The procedure of Example 1 was repeated except that a slurry in which the weight ratio of polymer / inorganic particles was changed to 7.5 / 92.5 was prepared, and then the polyethylene porous membrane was immersed in the slurry and dried to produce a composite membrane.

The porosities of the first porous coating layer and the second porous coating layer formed on both surfaces of the porous membrane were all 0.50. The thicknesses of the first porous coating layer and the second porous coating layer were 3 mu m, respectively.

Comparative Example 2

Except that the second porous coating layer was disposed so as to face the negative electrode coated with the graphite negative electrode active material particles on the copper (Cu) thin film and the first porous coating layer was arranged so as to face the positive electrode coated with the positive electrode active material particles made of LiCoO ₂ , A lithium secondary battery was produced in the same manner as in Example 1.

Comparative Example 3

Except that the second porous coating layer was disposed so as to face the negative electrode coated with the graphite negative electrode active material particles on the copper (Cu) thin film and the first porous coating layer was arranged so as to face the positive electrode coated with the positive electrode active material particles made of LiCoO ₂ , A lithium secondary battery was produced in the same manner as in Example 2.

Capacity evaluation of the battery

Cycle characteristics of the cells of the examples and comparative examples were measured and are shown in Table 1 below.

Referring to the results of Table 1, the cycle characteristics of the batteries of Examples 1 and 2, in which the first porous coating layer having a relatively higher porosity than that of the second porous coating layer was opposed to the negative electrode, It can be seen that the cell of Comparative Example 1 having the same pore size and the first porous coating layer having a relatively higher porosity than the second porous coating layer were improved in cycle characteristics than the batteries of Comparative Examples 2 and 3 which were opposed to the anode.

Claims (16)

(a) a porous substrate having pores, a first porous coating layer coated on one surface of the porous substrate and formed of a mixture containing inorganic particles and a binder polymer, and a first porous coating layer coated on the other surface of the porous substrate, Wherein the porosity of the first porous coating layer is greater than the porosity of the second porous coating layer and the porosity of the first porous coating layer and the second porous coating layer satisfy the following formula 1;
&Quot; (1) &quot;
_{0.03 ≤ (P 1 -P 2)} ≤ 0.3
In Equation (1), P ₁ is the porosity of the first porous coating layer, P ₂ is the porosity of the second porous coating layer,
(b) an anode arranged to face the first porous coating layer; And
(c) an anode provided so as to face the second porous coating layer. delete The method according to claim 1,
Wherein the porosity of the first porous coating layer and the second porous coating layer satisfy the following formula (2).
&Quot; (2) &quot;
0.05? (P ₁ -P ₂ )? 0.2
In Equation (2), P ₁ is the porosity of the first porous coating layer, and P ₂ is the porosity of the second porous coating layer. The method according to claim 1,
Wherein the porosity of the first porous coating layer is 0.50 to 0.80. The method according to claim 1,
Wherein the thicknesses of the first porous coating layer and the second porous coating layer are respectively 1.0 to 10.0 탆 independently of each other. The method according to claim 1,
Wherein the porous substrate is a polyolefin-based porous film. The method according to claim 1,
Wherein the porous substrate has a thickness of 1 to 100 mu m. The method according to claim 1,
Wherein the solubility index of the binder polymer is 15 to 45 MPa &lt; ^{1/2 &} gt ;. The method according to claim 1,
The binder polymer may be at least one selected from the group consisting of polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichlorethylene, polymethylmethacrylate, Polybutylene terephthalate, polybutyl acrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide (polyethylene) oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpyrrolidone, Polyvinyl alcohol (cyanoethylpolyviny a binder polymer selected from the group consisting of lanolin, lanolin, lanolin, cyanoethylcellulose, cyanoethylsucrose, pullulan, and carboxyl methyl cellulose; And the mixture is a mixture. The method according to claim 1,
Wherein an average particle diameter of the inorganic particles is 0.001 to 10 mu m. The method according to claim 1,
Wherein the average particle size of the inorganic particles contained in the first porous coating layer is larger than the average particle size of the inorganic particles contained in the second porous coating layer. The method according to claim 1,
Wherein the weight ratio of the inorganic particles to the binder polymer is 50:50 to 99: 1. The method according to claim 1,
Wherein the weight ratio of the binder polymer / inorganic particles contained in the first porous coating layer is less than the weight ratio of the binder polymer / inorganic particles contained in the second porous coating layer. The method according to claim 1,
The positive electrode may be made of at least one selected from the group consisting of LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiCoPO ₄ , LiFePO ₄ , LiNiMnCoO ₂ and LiNi _1x-yz Co _x M1 _y M2 _z O ₂ Y, and z are independently selected from the group consisting of Mn, V, Cr, Ti, W, Ta, Mg and Mo, , 0 < z < 0.5), or a mixture of two or more of the foregoing cathode active material particles. The method according to claim 1,
Wherein the negative electrode comprises any one of negative active material particles selected from the group consisting of a carbonaceous material, LTO, silicon (Si), and tin (Sn), or a mixture of two or more thereof. The method according to claim 1,
Wherein the electrochemical device is a lithium secondary battery.

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