KR20120035359A - A electrochemical device for progressing cycle characteristic - Google Patents

Abstract

PURPOSE: An electrochemical device is provided to prevent the short circuit of the both electrodes, because a pours coating layer restrains thermal compression of a porous substrate at overheat, and to reduce the effective ion transferring speed of a negative electrode side which has relatively severe side reactions. CONSTITUTION: An electrochemical device comprises: a composite separation film(10) comprising a porous substrate(1), a first porous coating layer(2), and a second porous coating layer(5), of which porosity is higher than the porosity of the first porous coating layer; a negative electrode(20) countering with the first porous coating layer; and a positive electrode(30) countering with the second porous coating layer. The first and the second porous coating layer is coated on one side and the other side of the porous substrate respectively, and is formed by a mixture comprising inorganic particles and a binder polymer.

Description

Electrochemical device with improved cycle characteristics {A ELECTROCHEMICAL DEVICE FOR PROGRESSING CYCLE CHARACTERISTIC}

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

Recently, interest in energy storage technology is increasing. As the field of application extends to the energy of mobile phones, camcorders, notebook PCs, and even electric vehicles, efforts for research and development of electrochemical devices are becoming more concrete. The electrochemical device is the most attracting field in this respect, and the development of a secondary battery capable of charging and discharging has been the focus of attention, and in recent years in the development of such a battery in order to improve the capacity density and specific energy The research and development of the design of the battery is progressing.

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 has been considered as one of the next generation batteries by improving the weakness of the lithium ion battery, but the capacity of the battery is still relatively lower than that of the lithium ion battery, and the discharge capacity is improved due to insufficient discharge capacity at low temperatures. This is urgently needed.

Such electrochemical devices are produced by many companies, but their safety characteristics are different, and securing the safety of the electrochemical devices is very important. The most important consideration is that an electrochemical device should not cause injury to the user in case of malfunction. For this purpose, safety standards strictly regulate the ignition and smoke in the electrochemical device. In the safety characteristics of the electrochemical device, there is a high possibility that an explosion occurs when the electrochemical device is overheated to cause thermal runaway or the separator penetrates. In particular, polyolefin-based porous substrates commonly used as separators for electrochemical devices exhibit extreme heat shrinkage behavior at temperatures of 100 degrees or more due to material characteristics and manufacturing process characteristics including stretching, and thus, a short circuit between the anode and the cathode. There is a problem that causes.

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

On the other hand, the above-described composite separator is interposed between the positive electrode and the negative electrode to constitute an electrochemical device, a phenomenon in which the capacity of the electrochemical device increases as the number of times of use is deteriorated.

The problem to be solved by the present invention is to provide an electrochemical device with excellent thermal stability and improved cycle characteristics by introducing a porous coating layer on a porous substrate.

In order to achieve the above object, the electrochemical device according to the present invention,

(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 coated on the other surface of the porous substrate and having inorganic particles and a binder polymer A composite separator having a second porous coating layer formed of a mixture including a porosity of the first porous coating layer greater than that of the second porous coating layer;

(b) a cathode provided to face the first porous coating layer; And

(c) an anode provided to face the second porous coating layer.

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 satisfies Equation 1 below.

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.

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

In Equation 2, P ₁ is the porosity of the first porous coating layer, 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, the thickness of the first porous coating layer and the second porous coating layer is preferably 1.0 to 10.0 ㎛, respectively.

In the electrochemical device according to the present invention, the average particle diameter of the inorganic particles embedded in the first porous coating layer is preferably larger than the average particle diameter of the inorganic particles embedded in the second porous coating layer, and is embedded in the first 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 embedded in the second porous coating layer.

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

In the electrochemical device according to the present invention, the anode is 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 mutually Independently any one selected from the group consisting of Al, Ni, Co, Fe, Mn, V, Cr, Ti, W, Ta, Mg and Mo, x, y and z independently of each other as atomic fraction of the oxide composition elements It is preferable that positive electrode active material particles, such as 0x <0.5, 0y <0.5, 0z <0.5, etc., are included alone or in mixture of two or more thereof. In addition, it is preferable that the negative electrode contains negative electrode active material particles such as natural graphite, artificial graphite, carbonaceous material, LTO, silicon (Si), tin (Sn), alone or a mixture of two or more thereof.

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

First, upon overheating of the electrochemical device, the porous coating layer suppresses thermal shrinkage of the porous substrate, and prevents short circuit of both electrodes during thermal runaway.

Second, since the first porous coating layer having a larger porosity than the porosity of the second porous coating layer is opposed to the negative electrode, it is possible to reduce the actual ion transfer rate of the negative electrode surface where the side reaction is more severe than the positive electrode. Accordingly, the cycle characteristics of the electrochemical device are improved. In other words, the capacity of the composite separator in which the first porous coating layer and the second porous coating layer are formed with the same porosity is reduced, or the capacity deterioration is lower than that of the electrochemical device having the first porous coating layer having a relatively large porosity facing the anode.

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 or words used in the specification and claims should not be construed as having a conventional or dictionary meaning, and the inventors should properly explain the concept of terms in order to best explain their own invention. Based on the principle that can be defined, it should be interpreted as meaning and concept corresponding to the technical idea of the present invention.

Electrochemical device according to the present invention, (a) a porous substrate having pores, coated on one surface of the porous substrate and the first porous coating layer formed of a mixture containing inorganic particles and a binder polymer and the other surface of the porous substrate A composite separator coated with a second porous coating layer formed of a mixture including inorganic particles and a binder polymer, wherein the porosity of the first porous coating layer is greater than that of the second porous coating layer; (b) a cathode provided to face the first porous coating layer; And (c) an anode provided to face 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 separator 10 includes a first porous coating layer 2 and a second porous coating layer 5 respectively coated on both surfaces of the porous substrate 1 having pores. The first porous coating layer 2 and the second porous coating layer 5 respectively coated on both surfaces of the porous substrate 1 are each independently formed of a mixture including inorganic particles and a binder polymer. The inorganic particles present in the porous coating layers 2 and 5 serve as a kind of spacer that can maintain the physical form of the porous coating layers 2 and 5, so that the porous substrate 1 is heated when the electrochemical device is overheated. This prevents shrinkage or prevents short circuiting of both electrodes during thermal runaway. In addition, it is preferable that an interstitial volume exists between the inorganic particles to form fine pores. Meanwhile, the porosity of the first porous coating layer 2 is greater than that of the second porous coating layer 5.

Meanwhile, the cathode 20 is provided to face the first porous coating layer 2 of the composite separator 1 having the above-described structure. In addition, the anode 30 is provided to face the second porous coating layer 5 of the composite separator 1 having the above-described structure.

As such, the first porous coating layer 2 having larger porosity than the second porous coating layer 5 of the composite separator 10 is opposed to the negative electrode, so that the side reactions are substantially more ions on the negative electrode surface than the positive electrode. It is possible to reduce the delivery speed, and thus improve 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 in the above-described method employs a composite separator in which the first porous coating layer and the second porous coating layer have the same porosity, or have a relatively large porosity. 1 Less capacity deterioration than electrochemical devices with the porous coating layer facing the anode.

In the electrochemical device according to the present invention, the porosity of the first porous coating layer and the second porous coating layer preferably satisfies Equation 1 below.

&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. Here, the porosity of the first porous coating layer is preferably 0.50 to 0.80.

More preferably, the porosity of the first porous coating layer and the second porous coating layer satisfies Equation 2.

<Equation 2>

0.05 ≤ (P ₁ -P ₂ ) ≤ 0.2

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

In addition, the thickness of the first porous coating layer and the second porous coating layer is preferably 1.0 to 10.0 ㎛ each, the thickness 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 may be used. For example, a polyolefin-based porous membrane or a nonwoven fabric made of polyethylene terephthalate fiber, which is used as an electrochemical device, in particular, a separator of a lithium secondary battery, may be used. For example, the polyolefin-based porous membrane (membrane) is a polyolefin-based polymer such as polyethylene, polypropylene, polybutylene, polypentene, such as high density polyethylene, linear low density polyethylene, low density polyethylene, ultra high molecular weight polyethylene, or a mixture thereof. The polymer may be formed, and the nonwoven fabric may also be made of a fiber using a polyolefin-based polymer or a polymer having higher heat resistance. The thickness of the porous substrate is not particularly limited, but is preferably 1 to 100 μm, more preferably 5 to 50 μm, and the pore size and pore present in the porous substrate are also not particularly limited, but are 0.001 to 50 μm and 10, respectively. It is preferably from 95%. The porous substrate is preferably a polyolefin-based porous membrane.

The inorganic particles included in each of 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 that can be used in the present invention are not particularly limited as long as the oxidation and / or reduction reactions do not occur in the operating voltage range (for example, 0 to 5 V on the basis of Li / Li ⁺ ) of the applied electrochemical device. In particular, when inorganic particles having a high dielectric constant are used as the inorganic particles, the ionic conductivity of the electrolyte may be improved by contributing to an increase in the dissociation degree of the electrolyte salt, such as lithium salt, in the liquid electrolyte.

For the reasons described above, the inorganic particles preferably include high dielectric constant 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, where 0 <x <1, 0 <y <1), 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 mixtures thereof.

In addition, the inorganic particles may be inorganic particles having lithium ion transfer capability, that is, inorganic particles containing lithium elements but having a function of transferring lithium ions without storing lithium. Non-limiting examples of inorganic particles having a lithium ion transfer capacity include lithium phosphate (Li ₃ PO ₄ ), lithium titanium phosphate (Li _x Ti _y (PO ₄ ) ₃ , 0 <x <2, 0 <y <3), Lithium aluminum titanium phosphate (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 (LiAlTiP) _x O _y series glass such as O ₅ (0 <x <4, 0 <y <13), lithium lanthanum titanate (Li _x La _y TiO ₃ , 0 <x <2, 0 <y <3) , Li germanium thiophosphate such as Li _3.25 Ge _0.25 P _0.75 S ₄ (Li _x Ge _y P _z S _w , 0 <x <4, 0 <y <1, 0 <z <1, 0 <w <5 ), Lithium nitride such as Li ₃ N (Li _x N _y , 0 <x <4, 0 <y <2), SiS ₂ based glass such as Li ₃ PO ₄ -Li ₂ S-SiS ₂ (Li _x Si P ₂ S ₅ series glass (Li _x P _y S _z , 0 <x, such as _y S _z , 0 <x <3, 0 <y <2, 0 <z <4), LiI-Li ₂ SP ₂ S _5, etc. <3, 0 <y <3, 0 <z <7) or mixtures thereof.

In addition, the average particle diameter of the inorganic particles is not particularly limited, but in order to form a porous coating layer having a uniform thickness and an appropriate porosity, it is preferably in the range of 0.001 to 10 μm. When the thickness is less than 0.001 μm, the dispersibility may be decreased, and when the thickness is more than 10 μm, the thickness of the coating layer formed may be increased.

As the binder polymer included in each of the first porous coating layer and the second porous coating layer, it is preferable to use a polymer having a glass transition temperature (T _g ) of -200 to 200 ° C., which is the flexibility of the finally formed coating layer. And mechanical properties such as elasticity can be improved.

In addition, the binder polymer does not necessarily have an ion conducting ability, but when a polymer having an ion conducting ability is used, the performance of the electrochemical device may be further improved. Therefore, the binder polymer is preferably as high as possible dielectric constant. In fact, since the dissociation degree of the salt in the electrolyte depends on the dielectric constant of the solvent of the electrolyte, the higher the dielectric constant of the binder polymer, the higher the dissociation of the salt in the electrolyte. The dielectric constant of the binder polymer may be in the range of 1.0 to 100 (measurement frequency = 1 kHz), particularly preferably 10 or more.

In addition to the above-described function, the binder polymer may have a feature that can exhibit a high degree of swelling of the electrolyte by gelling upon impregnation of the liquid electrolyte. Accordingly, it is preferred to use polymers having a solubility index of 15 to 45 MPa ^1/2 , more preferred solubility indices in the range of 15 to 25 MPa ^1/2 and 30 to 45 MPa ^1/2 . Therefore, it is preferable to use hydrophilic polymers having more polar groups than hydrophobic polymers such as polyolefins. This is because when the solubility index is less than 15 MPa ^{1/2 and more} than 45 MPa ^1/2 , it is difficult to be swelled by a conventional battery liquid electrolyte.

Non-limiting examples of such binder polymers include polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethylmethacryl Polymethylmethacrylate, polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, ethylene vinyl acetate copolymer (polyethylene-co-vinyl acetate) , Polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan ), Cyanoethylpolybi Cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methyl cellulose, and the like, each of which may be used alone or among them. Two or more kinds can be mixed and used.

The weight ratio of the inorganic particles and the binder polymer included in the first porous coating layer and the second porous coating layer, respectively, is preferably 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 pore size and porosity of the porous coating layer formed by increasing the content of the polymer may be reduced. When the content of the inorganic particles exceeds 99 parts by weight, since the binder polymer content is small, the peeling resistance of the porous coating layer formed may be weakened.

In the electrochemical device according to the present invention, the positive electrode and the negative electrode may be composed of conventional positive and negative electrodes used in the electrochemical device. In general, the positive electrode and the negative electrode may be prepared by applying a slurry including the positive electrode active material particles and a slurry including the negative electrode active material particles to the positive electrode current collector and the negative electrode current collector, respectively, and then drying them. The anode is 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 of each other Al, Ni, Co, Fe, Mn, V, Cr, Ti, W, Ta, Mg and Mo, and any one selected from the group consisting of x, y and z independently of each other as the atomic fraction of the elements of the oxide composition 0 x <0.5, 0 y <0.5, It is preferred that the positive electrode active material particles, such as 0 z <0.5), alone or in combination of two or more thereof, but are not limited thereto. In addition, the negative electrode preferably includes negative electrode active material particles such as natural graphite, artificial graphite, carbonaceous material, LTO, silicon (Si), tin (Sn), or the like, alone or in combination of two or more thereof. It doesn't work.

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

First, a binder polymer is dissolved in a solvent to prepare a binder polymer solution, and inorganic particles are added and dispersed to prepare a slurry. As the solvent, the solubility index is similar to that of the binder polymer to be used, and the boiling point is preferably low. This is to facilitate uniform mixing and subsequent solvent removal. Non-limiting examples of solvents that can be used include acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone ( N-methyl-2-pyrrolidone, NMP), cyclohexane, water or a mixture thereof. After adding the inorganic particles to the binder polymer solution, it is preferable to carry out the crushing of the inorganic particles. In this case, the crushing time is suitably 1 to 20 hours, and the particle size of the crushed inorganic particles is preferably 0.001 to 10 μm as described above. As the shredding 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 a humidity condition of 10-80% 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 greater than that of the second porous coating layer.

The method of coating the slurry on the porous substrate may use a conventional coating method known in the art, for example, die coating, roll coating and the like. There are various methods of forming the porosity of the first porous coating layer larger than that of the second porous coating layer. For example, it can be achieved by varying the average particle diameter of the inorganic particles in the slurry respectively applied to both sides of the porous substrate. That is, the average particle diameter of the inorganic particles dispersed in the slurry for forming the first porous coating layer may be controlled to be larger than the average particle diameter of the inorganic particles dispersed in the slurry for forming the second porous coating layer. The average particle diameter of the inorganic particles embedded in the first porous coating layer formed therefrom is larger than the average particle diameter of the inorganic particles embedded in the second porous coating layer. As another method, it can be achieved by varying the content ratio of the binder polymer and the inorganic particles in the slurry respectively applied to both sides of the porous substrate. That is, the content ratio of the binder polymer to the inorganic particles in the slurry for the first porous coating layer may be controlled to be smaller than the content ratio of the binder polymer to the inorganic particles in the slurry for the second porous coating layer. The weight ratio of the binder polymer / inorganic particles embedded in the first porous coating layer formed therefrom is smaller than the weight ratio of the binder polymer / inorganic particles embedded in the second porous coating layer.

The composite separator thus prepared is interposed between the positive electrode and the negative electrode, and the negative electrode is disposed to face the first porous coating layer, and the positive electrode is disposed to face the second porous coating layer. The composite separator and the positive electrode may be physically in contact with each other, but may be laminated with each other as necessary.

The electrochemical device of the present invention includes all devices that undergo an electrochemical reaction, and specific examples include capacitors such as all kinds of primary, secondary cells, fuel cells, solar cells, or supercapacitor elements. . 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 may be manufactured according to a conventional method known in the art, for example, by injecting an electrolyte solution after encapsulating the electrode structure of the positive electrode, the negative electrode, and the composite separator in an exterior material such as a pouch. Can be prepared.

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 ^- is an anion, or a salt containing an ion composed of a combination of propylene carbonate (PC) such as, ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethylsulfoxide Said, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethylmethylcarbonate (EMC), gamma butyrolactone (γ-butyrolactone) or these Some are dissolved or dissociated in an organic solvent consisting of a mixture of, but are not limited thereto.

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

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to examples. However, embodiments according to the present invention can be modified in many different forms, the scope of the present invention should not be construed as limited to the embodiments described below. Embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Example 1

Preparation of Composite Membranes

PVdF-CTFE (polyvinylidene fluoride-chlorotrifluoroethylene copolymer) and Cyanoethylpullulan (cyanoethyl pullulan) were added to acetone in a weight ratio of 10: 2, respectively, and dissolved at 50 ° C. for at least 12 hours. Was prepared. Inorganic particles in which Al ₂ O ₃ powder and BaTiO ₃ powder were mixed in a weight ratio of 9: 1 were added to the polymer solution prepared so that the polymer / inorganic particles = 5/95 weight ratio, and the ball mill method was used for 12 hours or more. The first slurry was prepared by crushing and dispersing the inorganic particles. In addition, a second slurry in which the weight ratio of the polymer / inorganic particles in the polymer solution was changed to 10/90 was prepared. The particle diameter of the inorganic particles of the first slurry and the second slurry thus prepared was 600 nm on average.

The first slurry and the second slurry were slot coated on both sides of a polyethylene porous membrane (Asahisa ND209) having a thickness of 9 µm, respectively, and then the solvent was dried. The porosities of the porous coating layers formed on both sides of the porous membrane were 0.58 (first porous coating layer) and 0.47 (second porous coating layer), respectively. In addition, the thicknesses of the first porous coating layer and the second porous coating layer were 3 μm, 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 then its weight was measured to calculate the measured density. The solids density of the slurry for forming the porous coating layer was theoretically calculated, and then the porosity was calculated by the following equation.

Porosity of Porous Coating Layer = 1-[(Measurement Density of Slurry of Porous Coating Layer) / (Solid Density of Slurry)]

Manufacture of batteries

The composite separator prepared by the above method was sandwiched between the positive electrode and the negative electrode and then wound and assembled. In this case, the first porous coating layer was disposed to face the negative electrode coated with the negative electrode active material particles made of graphite on the copper (Cu) thin film, and the second porous coating layer was disposed to face the positive electrode coated with the positive electrode active material particles made of LiCoO ₂ . .

Into the assembled battery, a nonaqueous electrolyte obtained by dissolving 1 mole of lithium hexafluorophosphate (LiPF6) in an organic solvent of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 1/2 (volume ratio) was injected. The battery was prepared.

Example 2

Preparation of Composite Membranes

PVdF-CTFE (polyvinylidene fluoride-chlorotrifluoroethylene copolymer) and Cyanoethylpullulan (cyanoethyl pullulan) were added to acetone in a weight ratio of 10: 2, respectively, and dissolved at 50 ° C. for at least 12 hours. Was prepared. Inorganic particles in which Al ₂ O ₃ powder and BaTiO ₃ powder were mixed in a weight ratio of 9: 1 were added to a polymer solution prepared so that polymer / inorganic particles = 10/90 weight ratio, and the averages were different using a ball mill method. A first slurry and a second slurry in which inorganic particles having particle diameters 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 polyethylene porous membrane (Asahisa ND209) having a thickness of 9 µm, respectively, and then the solvent was dried. The porosities of the porous coating layers formed on both sides of the porous membrane were 0.64 (first porous coating layer) and 0.48 (second porous coating layer), respectively. In addition, the thicknesses of the first porous coating layer and the second porous coating layer were 3 μm, respectively.

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

Comparative Example 1

 A slurry in which the weight ratio of the polymer / inorganic particles was changed to 7.5 / 92.5 was prepared, and then, except that the composite porous membrane was prepared by immersing the polyethylene porous membrane in the slurry and drying the same, the same procedure as in Example 1 was carried out.

The porosity of the first porous coating layer and the second porous coating layer respectively formed on both surfaces of the porous membrane was 0.50. In addition, the thicknesses of the first porous coating layer and the second porous coating layer were 3 μm, respectively.

Comparative Example 2

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

Comparative Example 3

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

Battery capacity evaluation

Cycle characteristics of the batteries of Examples and Comparative Examples were measured and 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 the second porous coating layer were opposite to the negative electrode, the first porous coating layer and the second porous coating layer It can be seen that the cycle characteristics of the battery of Comparative Example 1 formed with the same porosity and the cells of Comparative Examples 2 and 3 in which the first porous coating layer having a larger porosity than the second porous coating layer are opposed to the positive electrode are improved.

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 coated on the other surface of the porous substrate and having inorganic particles and a binder polymer A composite separator having a second porous coating layer formed of a mixture including a porosity of the first porous coating layer greater than that of the second porous coating layer;
(b) a cathode provided to face the first porous coating layer; And
(c) an electrochemical device comprising an anode provided to face the second porous coating layer. The method of claim 1,
An electrochemical device, characterized in that the porosity of the first porous coating layer and the second porous coating layer satisfies the following equation (1).
<Equation 1>
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. The method of claim 2,
The porosity of the first porous coating layer and the second porous coating layer is characterized by satisfying the following equation (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, P ₂ is the porosity of the second porous coating layer. The method of claim 2,
The porosity of the first porous coating layer is an electrochemical device, characterized in that 0.50 to 0.80. The method of claim 1,
The thickness of the first porous coating layer and the second porous coating layer is an electrochemical device, characterized in that each independently 1.0 to 10.0 ㎛. The method of claim 1,
The porous substrate is an electrochemical device, characterized in that the polyolefin-based porous membrane. The method of claim 1,
The thickness of the porous substrate is an electrochemical device, characterized in that 1 to 100 ㎛. The method of claim 1,
Solubility index of the binder polymer is an electrochemical device, characterized in that 15 to 45 Mpa ^1/2 . The method of claim 1,
The binder polymer may be polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethylmethacrylate, Polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, ethylene vinyl acetate copolymer, polyethylene oxide oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethyl Polyvinyl alcohol (cyanoethylpolyviny) lalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, and carboxyl methyl cellulose, any one binder polymer selected from the group consisting of two or more thereof Electrochemical device, characterized in that the mixture. The method of claim 1,
The average particle diameter of the inorganic particles is an electrochemical device, characterized in that 0.001 to 10 ㎛. The method of claim 1,
And an average particle diameter of the inorganic particles embedded in the first porous coating layer is greater than an average particle diameter of the inorganic particles embedded in the second porous coating layer. The method of claim 1,
Electrochemical device, characterized in that the weight ratio of the inorganic particles and the binder polymer is 50:50 to 99: 1. The method of claim 1,
The weight ratio of the binder polymer / inorganic particles embedded in the first porous coating layer is smaller than the weight ratio of the binder polymer / inorganic particles embedded in the second porous coating layer. The method of claim 1,
The anode is 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 (M1 and M2 are independently of each other Al, Ni, Co, Fe , Mn, V, Cr, Ti, W, Ta, Mg and Mo, and x, y and z are each independently the atomic fraction of the elements of the oxide composition 0 x <0.5, 0 y <0.5 , 0 z <0.5), any one of the positive electrode active material particles selected from the group consisting of or a mixture of two or more thereof. The method of claim 1,
The negative electrode is characterized in that the negative electrode active material particles selected from the group consisting of natural graphite, artificial graphite, carbonaceous material, LTO, silicon (Si) and tin (Sn), or a mixture of two or more thereof. Chemical element. The method of claim 1,
The electrochemical device is an electrochemical device, characterized in that the lithium secondary battery.

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