KR101579575B1 - Lithium secondary battery with improved life and safety - Google Patents

Abstract

The present invention relates to an electrochemical device having improved durability and safety by designing different arrangement and electrode resistance of different types of separators and effectively preventing thermal shrinkage of the separator and having improved stability even in the event of electrode shortage, The heat generated in the central portion can be greatly suppressed, and the life characteristic can be improved ultimately.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to an electrochemical device having improved life and safety,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrochemical device having improved lifetime and safety, and more particularly, to an electrochemical device having improved lifetime and safety by using a heterogeneous separation membrane and an electrode having different resistance, Device.

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.

The electrochemical device is one of the most attracting fields in this respect, and the development of a rechargeable secondary battery has become a focus of attention.

Although electrochemical devices are produced in many companies, their safety characteristics are different, and securing of electrochemical devices is very important. The most important consideration is that the electrochemical device should not cause injury to the user in the event of malfunction. For this purpose, the firing and fuming in the electrochemical device are strictly regulated through the safety standard. 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 ° 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 Nos. 10-2006-72065, 10-2007-231 and the like disclose a porous substrate having a large number of pores, A separation membrane having a porous coating layer formed of a mixture of polymers is proposed. Korean Patent Laid-Open No. 10-2005-66652 discloses a separation membrane having a first separation membrane and a plurality of unit cells each having an anode and a cathode disposed on both sides of the first separation membrane, ; And a continuous single second separator interposed between each adjacent unit cell so as to correspond to each other in a stacked manner and to surround each of the unit cells, wherein the first separator and the second separator An electrochemical device using a porous substrate made of a material having a different melting point is disclosed. However, when the lithium secondary battery of the aforementioned patent publication can not prevent a short circuit in the unit cell due to heat shrinkage of the first separator and the temperature rises to the heat shrinkage occurrence temperature of the second separator due to excessive overcharging, There is a problem that explosion or ignition of the battery occurs.

In the pouch-type battery including the stack / folding type electrode assembly, which has been attracting attention, the cells are swelled due to the generation of a gas at a high temperature, and the electrodes and the separator are not firmly fixed. Especially, The safety problem becomes serious when the anode and cathode are short-circuited. In addition, when the pouch type battery is charged and discharged, heat is generated due to resistance. Heat transfer is not easily performed at the electrode of the unit cell disposed at the center layer rather than the unit cell disposed at the outer layer, and the heat generation is more severe. Therefore, the deterioration of the electrodes of the unit cells arranged in the center layer due to the high temperature is accelerated faster than that of the electrodes of the other unit cells in the vicinity, and battery performance is further deteriorated in the long term due to the difference in capacity and resistance between the electrode layers.

In the wound type jelly-roll battery, there is a problem that the separator at both ends of the electrode assembly shrinks to cause a short circuit between the positive electrode and the negative electrode. Also, in the jelly-roll battery, heat transfer in the central portion is not easily performed, have.

Disclosure of Invention Technical Problem [8] The present invention has been made to solve the above-mentioned technical problems, and it is an object of the present invention to suppress shrinkage of a separation membrane of an electrode assembly as much as possible and to solve a problem of heat generation at a central portion of an electrode assembly,

The present invention also provides an electrochemical device including such an electrode assembly.

According to an aspect of the present invention, there is provided a semiconductor device comprising: a first isolation layer and an outermost layer unit cell each having an anode and a cathode disposed on both sides of the first isolation layer; A middle layer unit cell having a second separation membrane and a low resistance anode and a low resistance cathode located on both sides of the second separation membrane; And a plurality of unit cells each having an anode and a cathode disposed on both sides of the second separation membrane and the second separation membrane, wherein the first separation membrane comprises a porous nonwoven base material and a plurality of inorganic particles and a binder polymer And a porous composite coating layer formed on the porous nonwoven fabric substrate and coated on at least one surface of the porous nonwoven fabric substrate, wherein the second separation membrane is a polyolefin film, the outermost layer unit cell is located at the outermost periphery, An outermost layer unit cell, an inner unit cell and a middle layer unit cell are stacked so as to be positioned in the center layer, and a single continuous second separator is interposed between adjacent stacked unit cells so as to correspond to each other, Wherein the low-resistance positive electrode and the low-resistance negative electrode each have a lower resistance than the peripheral positive electrode and the negative electrode, The electrode assembly includes:

According to another aspect of the present invention, there is provided an electrode assembly comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode and being wound in a jelly-roll type, wherein the separator comprises a porous nonwoven base material and a mixture of a plurality of inorganic particles and a binder polymer And a composite porous coating layer coated on at least one surface of the porous nonwoven substrate, and a second separator, which is a polyolefin-based film. The first separator is positioned at both ends of the first separator, There is provided an electrode assembly in which a separator is formed and the electrode of the winding start portion is a low resistance electrode having a reduced resistance than that of the adjacent electrode.

The porous substrate of the first separation membrane may be a polyester-based nonwoven fabric.

The inorganic particles may be high-permittivity inorganic particles having a dielectric constant of 5 or more, inorganic particles having lithium ion-transporting ability, or a mixture thereof.

The 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 <<y<1 _{Im), 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 and SiC.

The 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 <3), lithium aluminum titanium phosphate (Li _x Al _y Ti _z (PO ₄ ) ₃ , 0 <x <2, 0 <y <1, 0 <z <3), (LiAlTiP) _x O _y series glass <13), lithium lanthanum titanate (Li _x La _y TiO ₃ , 0 <x <2, 0 <y <3), lithium germanium thiophosphate (Li _x Ge _y P _z S _w , (Li _x N _y , 0 <x <4, 0 <y <2), SiS ₂ series glass (Li _x Si, 0 <y <1, 0 <z < _{y S z, 0 <x <} 3, 0 <y <2, 0 <z <4) and P ₂ S ₅ based glass _{(Li x P y S z,} 0 <x <3, 0 <y <3, 0 <z <7).

The binder polymer may be selected from the group consisting of polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-trichlorethylene, polymethylmethacrylate, poly Polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide, cellulose acetate, polyvinylpyrrolidone, , Cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethyl cellulose, Ethyl sucrose A mixture of one or more selected from the group consisting of cyanoethylsucrose, pullulan, and carboxyl methyl cellulose.

According to one aspect of the present invention, there is provided an electrochemical device comprising the above-described electrode assembly.

The electrochemical device may be a lithium secondary battery.

Since the heat shrinkage of the separator is effectively prevented, the electrochemical device of the present invention can have improved stability even when the electrode is short-circuited, and the heat generated at the central portion of the electrode assembly can be greatly suppressed and ultimately the lifetime characteristics can be improved.

1 is a schematic diagram of a stack / folding type electrode assembly according to an aspect of 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 interpreted 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 electrode assembly according to the present invention comprises a heterogeneous separator comprising a nonwoven fabric-based first separator and a polyolefin-based second separator, wherein the composite porous coating layer is formed on at least one side of the separator; And the central portion of the electrode assembly is designed to have a lower resistance than the peripheral portion.

When the electrode assembly according to an embodiment of the present invention is a stack / folding type, the electrode assembly includes a first separator and an outermost layer unit cell each having an anode and a cathode located on both sides of the first separator; A middle layer unit cell having a second separation membrane and a low resistance anode and a low resistance cathode located on both sides of the second separation membrane; And a plurality of unit cells each having an anode and a cathode disposed on both sides of the second separation membrane and the second separation membrane, wherein the first separation membrane comprises a porous nonwoven base material and a plurality of inorganic particles and a binder polymer Wherein the second separator is a polyolefin-based film, the outermost layer unit cell is located at an outermost part of the electrode assembly, and the center of the outermost layer unit cell is located at the center of the electrode assembly, The outermost layer unit cell, the inner unit cell and the middle layer unit cell are stacked so that the layer unit cell is located at the center layer of the electrode assembly, and between each adjacent unit cell so as to correspond to each other, And the low-resistance positive electrode and the low-resistance negative electrode are arranged so as to surround the respective unit cells, And has a lower resistance than the pole and cathode.

1 is a cross-sectional view schematically illustrating the structure of a stack / folding type electrode assembly according to an embodiment of the present invention.

1, the electrode assembly 10 includes unit cells each having a cathode 11a and a cathode 15a disposed on both sides of a first separator 13a 'and a first separator 13a' Unit cell 17a and a unit cell having a low-resistance cathode 11a 'and a low-resistance anode 15a' located on both sides of the second separation membrane 13a and the second separation membrane 13a, And the remaining cells constitute the inner unit cell 17b having the cathode 11a and the anode 15a located on both sides of the second separation membrane 13a and the second separation membrane 13a.

1, a unit cell is a full cell structure in which one cathode 11a and a cathode 11a 'are disposed on both sides of a separation membrane 13a and 13a', and a unit cell is disposed on both sides of the separation membrane A structure of a pull cell in which one anode and a cathode are located, or a unit cell having various structures such as a structure of a bipolar cell in which a separator is disposed on both sides of an anode or a cathode and a cathode or an anode is disposed on each separator, It does not.

In the electrode assembly 10 of FIG. 1, the unit cells 17a, 17b and 17c are present in a laminated form, and the unit cells 17a including the first separator 13a ' And the unit cells 17c including the second separation membrane 13a and the low-resistance electrodes 11a 'and 15a' are laminated so as to form a center layer. At this time, a single continuous second separation membrane 19a disposed to surround the respective unit cells 17a, 17b and 17c is disposed between adjacent unit cells 17a, 17b and 17c to correspond to each other For example, in a folding manner or in a zigzag fashion to perform a separating function between the respective unit cells 17a, 17b and 17c.

When the electrode assembly according to an embodiment of the present invention is of a winding type, the electrode assembly includes an anode, a cathode, and a separator interposed between the anode and the cathode, and the separator includes a porous nonwoven substrate, And a binder polymer, and is coated on at least one surface of the porous nonwoven substrate, and a second separation membrane, which is a polyolefin-based film, and the first separation membrane is positioned at both ends of the separation membrane A second separator is formed therebetween, and a low resistance electrode having a resistance lower than that of the adjacent portion is formed at the winding start portion of the electrode assembly. The connection portion between the first separation membrane and the second separation membrane may be in the form of overlapping, and an adhesive or the like may be applied, if necessary.

The 'winding start portion' means a certain region including the end portion where the winding starts.

The separation membrane of the present invention comprises a first separation membrane and a second separation membrane.

The first separation membrane includes a porous substrate formed of a nonwoven fabric, and a composite porous coating layer formed of a mixture of a plurality of inorganic particles and a binder polymer and coated on at least one surface of the porous substrate.

As the porous substrate of the first separation membrane, a nonwoven fabric formed from a polyester such as polyethylene terephthalate or polybutylene terephthalate may be used, and thus it is not particularly limited.

The thickness, pore size and porosity of the nonwoven substrate are not particularly limited, but preferably the thickness is 1 to 100 占 퐉 or 5 to 50 占 퐉, the pore sizes are 0.01 to 50 占 퐉 or 0.1 to 20 占 퐉, 5 to 95%.

Since the nonwoven porous substrate has a low heat shrinkage, when the separator is used as a separator, an electrode short circuit is prevented even if the temperature rises rapidly due to excessive overcharging of the electrochemical device, thereby preventing explosion and ignition of the electrochemical device. However, since the nonwoven fabric separator has a disadvantage in that the strength thereof is weak, short-circuiting may occur due to formation of lithium dendrite upon penetration of the nail or overcharging. To prevent this, a composite porous coating layer is formed on the nonwoven fabric substrate.

The inorganic particles in the porous coating layer formed on one side or both sides of the first separation membrane serve as a kind of spacer capable of maintaining the physical form of the porous coating layer, thereby suppressing heat shrinkage of the porous substrate upon overheating of the electrochemical device, And has a function of preventing contact between the anode and the cathode even when it is melted. Accordingly, the first separation membrane having the porous coating layer contributes to the improvement of the safety of the electrochemical device.

As the inorganic particles used for forming the porous coating layer, oxidation and / or reduction reactions usually take place in the inorganic particles used as the inorganic particles, that is, in the operating voltage range of the electrochemical device (for example, 0 to 5 V based on Li / Li ⁺ ) Inorganic particles can be used. Particularly, when inorganic particles having lithium ion transfer ability are used, the ion conductivity in the electrochemical device can be increased to improve the performance.

When inorganic particles having a high dielectric constant are used as the inorganic particles, dissociation of an electrolyte salt, for example, a lithium salt, in the liquid electrolyte is also increased, and ion conductivity of the electrolyte can be improved.

For the above reasons, it is preferable that the inorganic particles include high-permittivity inorganic particles having a dielectric constant of 5 or more, preferably 10 or more, inorganic particles having lithium ion-transporting ability, or a mixture thereof. Nonlimiting 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.

Particularly, the above-mentioned 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 < Inorganic particles such as Pb (Mg _1/3 Nb _2/3 ) O ₃ -PbTiO ₃ (PMN-PT) and HfO ₂ show not only high-k characteristics with a dielectric constant of 100 or more, And a piezoelectricity is generated so that a potential difference is generated between both sides by generating a charge when being stretched or compressed so as to prevent the occurrence of an internal short circuit between the two electrodes due to an external impact so as to improve the safety of the electrochemical device can do. Further, when the above-mentioned high-permittivity inorganic particles and inorganic particles having lithium ion transferring ability are mixed, their synergistic effect can be doubled.

In the present invention, the inorganic particles having a lithium ion transferring ability refer to inorganic particles containing a lithium element but not lithium and having a function of transferring lithium ions. The inorganic particles having lithium ion transferring ability are contained in the particle structure Since lithium ions can be transferred and transferred due to a kind of defect present, the lithium ion conductivity in the battery is improved, thereby improving battery performance.

Examples of the 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 (LiAlTiP) x O} y series glass (glass) (0 <x < 4, 0 <y <13), lithium lanthanum titanate _{(Li x La y TiO 3,} 0 <x <2, such as 0 < _{y <3), Li 3 .25} Ge 0 .25 P 0 .75 S 4 Mani lithium germanium thiophosphate Titanium (Li _x Ge _y P _z S _w, such as 0 <x <4, 0 < y <1, 0 (Li _x N _y , 0 <x <4, 0 <y <2) such as Li ₃ N and Li ₃ PO ₄ -Li ₂ S-SiS ₂ SiS ₂ based glass, P ₂ S ₅ based glass, such as _{(Li x Si y S z,} 0 <x <3, 0 <y <2, 0 <z <4), LiI-Li 2 SP 2 S 5 , such as ( Li _x P _y S _z , 0 <x <3, 0 <y <3, 0 <z <7), or mixtures thereof.

In the electrochemical device of the present invention, the size of the inorganic particles of the porous coating layer formed on the first separator is not limited, but is preferably 0.001 to 10 탆 for the formation of a coating layer having a uniform thickness and proper porosity. If it is less than 0.001 m, the dispersibility of the inorganic particles may be deteriorated. If it is more than 10 m, the thickness of the porous coating layer may increase and mechanical properties may be deteriorated. Also, due to an excessively large pore size, The probability of happening increases.

As the binder polymer used for forming the porous coating layer of the first separation membrane, a polymer conventionally used for forming a porous coating layer in the art can be used. Particularly, it is preferable to use a polymer having a glass transition temperature (T _g ) of -200 to 200 ° C because it can improve the mechanical properties such as flexibility and elasticity of the finally formed porous coating layer . Such a binder polymer faithfully performs a binder function to connect and stably fix inorganic particles, thereby contributing to prevention of deterioration of the mechanical properties of the separation membrane into which the porous coating layer is introduced.

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. Actually, the dissociation degree of the salt in the electrolytic solution depends on the permittivity constant of the solvent of the electrolyte. Therefore, the higher the permittivity constant of the polymer, the better the salt dissociation degree 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. In this way, and a solubility parameter of 15 to 45 MPa ^1/2 in the polymer preferably is more preferably 15 to 25 MPa1 / 2, and 30 to 45 MPa ^1/2 range. Therefore, hydrophilic polymers having many polar groups are preferred over hydrophobic polymers such as polyolefins. If the solubility is more than ^1/2 and less than 15 MPa ^1/2 45MPa, it is difficult to be impregnated with (swelling) by conventional liquid electrolyte batteries.

Non-limiting examples of the binder polymer include polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-trichloroethylene, polymethylmethacrylate polymethylmethacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide, cellulose acetate cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, and the like. ), Siah Ethyl sucrose (cyanoethylsucrose), and the like pullulan (pullulan), carboxymethyl cellulose (carboxyl methyl cellulose).

When a gelable polymer is used as a binder polymer component of the first separation membrane, the gel electrolyte is formed by reacting the injected electrolyte with the binder polymer after gelation.

The composition ratio of the inorganic particles and the binder polymer in the porous coating layer of the first separation membrane is preferably in the range of, for example, 50:50 to 99: 1, 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 binder polymer is increased, and the improvement of the thermal stability of the separation membrane may be deteriorated. In addition, the pore size and porosity due to the reduction of the void space formed between the inorganic particles may be reduced, resulting in deterioration of the final cell performance. If the content of the inorganic particles exceeds 99 parts by weight, the fillerability of the porous coating layer may be weakened because the content of the binder polymer is too small. The thickness of the porous coating layer composed of the inorganic particles and the binder polymer is not particularly limited, but is preferably in the range of 0.01 to 20 mu m.

The pore size and porosity of the first separation membrane are not particularly limited, but the pore size is preferably in the range of 0.001 to 10 mu m, and the porosity is preferably in the range of 10 to 90%. The pore size and porosity mainly depend on the size of the inorganic particles. For example, when inorganic particles having a particle size of 1 탆 or less are used, pores formed are also about 1 탆 or less. Such a pore structure is filled with an electrolyte solution to be injected at a later stage, and the filled electrolyte serves as an ion transfer function. When the pore size and porosity are 0.001 탆 or less and 10% or less, respectively, it can act as a resistive layer. If the pore size and porosity exceed 10 탆 and 90% respectively, mechanical properties may be deteriorated.

In the separator of the present invention, the inorganic porous particles and the binder polymer may further include other additives as components constituting the composite porous coating layer.

According to the present invention, a separation membrane having a composite porous coating layer containing inorganic particles and a binder polymer can be produced by a conventional method, and a preferable production method is shown below, but the present invention is not limited thereto.

First, a binder polymer solution is dissolved in a solvent to prepare a binder polymer solution.

Next, inorganic particles are added to the solution of the binder polymer and dispersed. 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 disintegration time is preferably 1 to 20 hours, and the particle size of the disintegrated inorganic particles is preferably 0.001 to 10 占 퐉 as described above. As the crushing method, a conventional method can be used, and a ball mill method is particularly preferable.

Then, the solution of the binder polymer in which the inorganic particles are dispersed is coated on the porous substrate under a humidity condition of 10 to 80% and dried.

A method of coating a solution of a binder polymer in which inorganic particles are dispersed on a porous substrate may be a conventional coating method known in the art. For example, a dip coating, a die coating, a roll coating, Various methods such as coating, comma coating, or a combination thereof can be used.

As the second separation membrane of the present invention, a film formed from a polyolefin resin commonly used in an electrochemical device may be used. Non-limiting examples of the polyolefin resin include, but are not limited to, polyolefin resins formed from polyethylene, polypropylene, derivatives thereof, etc., such as high density polyethylene, linear low density polyethylene, low density polyethylene and ultra high molecular weight polyethylene. The thickness, pore size and porosity of the porous substrate of the above-mentioned polyolefin film are not particularly limited, but the thickness is 1 to 100 占 퐉 or 2 to 30 占 퐉, the pore size and the porosity are 0.1 to 50 占 퐉 and 10 to 95% Lt; / RTI &gt;

When the electrode assembly is folded / stacked, the second separation membrane is interposed between the anode and the cathode and is laminated with the electrodes to form a unit cell excluding the outermost layer unit cell. The second separation membrane is disposed between the unit cells so as to surround adjacent unit cells so as to correspond to each other, and also functions as a separation membrane between the unit cells.

Alternatively, when the electrode assembly is of the winding type, the first separator is positioned at both ends and the second separator is positioned between the first separators, and is interposed between the anode and the cathode.

The electrochemical device includes all devices that perform an electrochemical reaction, and specific examples include capacitors such as all kinds of primary, secondary cells, fuel cells, solar cells, or super capacitor devices. 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 electrode to be applied together with the separator of the present invention can be produced by applying the electrode active material slurry to a current collector according to a conventional method known in the art.

The positive electrode active material and the negative electrode active material used for the electrode may be a conventional electrode active material that can be used for the positive electrode and the negative electrode of the conventional electrochemical device. As a non-limiting example of the cathode active material among the electrode active materials, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or a lithium composite oxide in combination thereof is preferably used. Non-limiting examples of the negative electrode active material are lithium metal or lithium alloy, lithium adsorbent materials such as carbon, petroleum coke, activated carbon, graphite or other carbon materials. Non-limiting examples of the positive current collector include aluminum, nickel, or a combination thereof. Examples of the negative current collector include copper, gold, nickel, or a copper alloy or a combination thereof Foil to be manufactured, and the like.

However, the electrode disposed at the center layer of the stack / folding type electrode assembly and the winding start portion of the electrode of the wound type electrode assembly are made to have a low resistance.

As used herein, the term 'low resistance electrode' refers to an electrode having a relatively low resistance as compared to other electrodes or other electrode portions in the electrode assembly.

The low resistance electrode can be manufactured by applying the electrode active material to the electrode current collector with a relatively small loading amount.

Alternatively, the low resistance electrode can be manufactured by using an active material having a relatively low resistance. When LiMO ₄ (M = V ₂ , FeP), LiMO ₂ (M = Mn, Mo, W), LiV ₆ O ₁₃ , LiTiS ₂ , LiWO ₂ or a mixture thereof is used as the cathode active material, non-limiting examples of the cathode active material is _{LiMO 2 (M = Co, Mn} , Ni, Ni 1/3 Co 1/3 Mn 1/3, Cr, V), LiMO 4 (M = CoMn, NiV, CoV, CoP, MnP, NiP, Mn ₂₎ or a mixture thereof and the like. When Si, Al, or the like is used as the negative electrode active material, non-limiting examples of the low-resistance negative electrode active material include natural graphite, artificial graphite, coal tar pitch, petroleum pitch, based material prepared by heat-treating an organic material as a raw material.

In addition, the low-resistance electrode can be manufactured by dispersing the conductive material used in the electrode material mixture slurry more uniformly or by changing the kind of conductive material or binder polymer used in the electrode material mixture.

As described above, in the stack / folding type electrode assembly, the low-resistance electrode is laminated so as to be located at the center layer of the electrode assembly, and the low-resistance electrode in the wound type electrode assembly is wound around the winding start portion, An electrode can be manufactured so that a low resistance electrode is present.

The electrolyte which can be used in the electrochemical device of the present invention is a salt having a structure such as A ⁺ B ^- , wherein A ⁺ includes an alkali metal cation such as Li ⁺ , Na ⁺ , K ^{+ -} it 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, 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 (DMP), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethylmethyl carbonate (EMC), gamma butyrolactone -Butyrolactone), or a mixture thereof, 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. Embodiments of the invention are provided to more fully describe the present invention to those skilled in the art.

Example  1: Stack / Folding type  Manufacture of electrode assembly

Preparation of first separation membrane

PVdF-CTFE (polyvinylidene fluoride-chlorotrifluoroethylene copolymer) was added to acetone in a weight ratio of about 5 wt% and dissolved at 50 DEG C for about 12 hours or longer to prepare a polymer solution. Al ₂ O ₃ powder and BaTiO ₃ powder were added to the prepared polymer solution so that the weight ratio of polymer / inorganic powder = 20/80 was added at a weight ratio of 9: 1 and the mixture was subjected to ball milling for 12 hours or longer, Was crushed and dispersed in a size of 300 nm to prepare a slurry.

The slurry thus prepared was coated on a polyethylene terephthalate nonwoven fabric having a thickness of 20 占 퐉 by a dip coating method. The coating thickness was adjusted to about 2 탆. As a result of measurement with a porosimeter, the pore size in the porous coating layer coated on polyethylene terephthalate was about 0.3 μm and the porosity was about 55%.

The second separator

A polyethylene film was prepared as a second separation membrane.

Manufacture of anode A

92 wt% of a lithium cobalt composite oxide as cathode active material particles, 4 wt% of carbon black as a conductive material, and 4 wt% of PVDF as a binder were added to N-methyl-2-pyrrolidone (NMP) To prepare an active material slurry. The positive electrode active material slurry was applied to an aluminum (Al) thin film of a positive electrode current collector having a thickness of 20 μm and dried to produce a positive electrode, followed by roll pressing.

Anode B ( Low resistance  Anode)

A positive electrode was manufactured by the same method as that for producing the positive electrode A except that the positive electrode active material slurry was applied to the aluminum thin film of the positive electrode current collector in a smaller amount than that of the positive electrode A,

Manufacture of cathode A

(N-methyl-2-pyrrolidone) as a negative electrode active material, polyvinylidene fluoride (PVdF) as a binder and carbon black as a conductive material were respectively 96 wt%, 3 wt% and 1 wt% (NMP) to prepare an anode active material slurry. The negative electrode active material slurry was applied to a copper (Cu) thin film as an anode current collector having a thickness of 10 μm and dried to prepare a negative electrode, followed by roll pressing.

Cathode B ( Low resistance  Cathode)

A negative electrode, which was a low resistance negative electrode, was prepared by preparing a negative electrode in the same manner as in the production of the negative electrode A except that the negative electrode active material slurry was applied to the copper thin film of the negative electrode current collector in a smaller amount than the above described negative electrode A.

Manufacture of batteries

The outermost layer unit cells were fabricated with the first separator interposed between the anode A and the cathode A manufactured by the above-described method, and the middle layer unit cell was fabricated with the first separator interposed between the anode B and the cathode B. Also, inner unit cells were fabricated with a second separator interposed between the anode A and the cathode A. The outermost layer unit cells were disposed at the outermost portion of the electrode assembly, and the unit cells were stacked and folded in a stacking and folding manner so that the center layer unit cell was the center layer of the electrode assembly. Then, the electrode assembly was manufactured and housed in the square can. Then, a lithium secondary battery was prepared by injecting an electrolyte (ethylene carbonate (EC) / ethylmethyl carbonate (EMC) = 1/2 (volume ratio) and 1 mol of lithium hexafluorophosphate (LiPF ₆ )).

Comparative Example  One

A cell was prepared in the same manner as in Example 1, except that a polyethylene porous film was used as the porous substrate of the first separation membrane.

Comparative Example  2

A battery was prepared in the same manner as in Example 1, except that a separator was prepared using a polyethylene porous film as a porous substrate of the first separator and used as a separator in all the unit cells.

Comparative Example  3

A battery was prepared in the same manner as in Comparative Example 2, except that a polyethylene terephthalate nonwoven fabric was used as the porous base material.

Comparative Example  4

A battery was prepared in the same manner as in Example 1 except that the electrode A and the electrode B were used as the electrodes of the central layer unit cell.

Hot box ( Hot Box ) Experiment

The lithium secondary batteries of Example 1 and Comparative Examples 1 to 4 were allowed to stand at a temperature of 160 DEG C for 1 hour to evaluate the state of the battery.

Overcharge experiment

The batteries of Example 1 and Comparative Examples 1 to 4 were charged under the conditions of 6V / 1A, 10V / 1A and 12V / 1A, and then the state of the battery was evaluated.

Claims (9)

An outermost layer unit cell including a first separator and an anode and a cathode disposed on both sides of the first separator; A middle layer unit cell having a second separation membrane and a low resistance anode and a low resistance cathode located on both sides of the second separation membrane; And a plurality of unit cells each comprising an inner unit cell including a second separator and an anode and a cathode located on both sides of the second separator,
The first separation membrane is a separation membrane comprising a porous nonwoven substrate and a composite porous coating layer formed of a mixture of a plurality of inorganic particles and a binder polymer and coated on at least one surface of the porous nonwoven substrate,
The second separation membrane is a polyolefin-based film,
The outermost layer unit cell is located at the outermost layer, the outermost layer unit cell, the inner unit cell and the middle layer unit cell are stacked so that the middle layer unit cell is located at the center layer,
A second single separation membrane is interposed between adjacent unit cells stacked so as to correspond to each other so as to surround each unit cell,
Wherein the low resistance positive electrode and the low resistance negative electrode each have a lower resistance than the peripheral positive electrode and the negative electrode.
An electrode assembly comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode and wound in a jelly-
The separation membrane comprises a first separation membrane comprising a porous nonwoven substrate and a composite porous coating layer formed of a mixture of a plurality of inorganic particles and a binder polymer and coated on at least one surface of the porous nonwoven substrate, and a second separation membrane being a polyolefin-based film ,
Wherein the first separation membrane is located at both ends and a second separation membrane is formed therebetween,
Wherein the electrode of the winding start portion is a low resistance electrode having a reduced resistance than the adjacent electrode.
3. The method according to claim 1 or 2,
Wherein the porous substrate of the first separation membrane is a polyester nonwoven fabric.
3. The method according to claim 1 or 2,
Wherein the inorganic particles are high-permittivity inorganic particles having a dielectric constant of 5 or more, inorganic particles having lithium ion-transporting ability, or a mixture thereof.
5. The method of claim 4,
The 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, <y <1 _{Im), 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, and SiC.
5. The method of claim 4,
The 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 <3), lithium aluminum titanium phosphate (Li _x Al _y Ti _z (PO ₄ ) ₃ , 0 <x <2, 0 <y <1, 0 <z <3), (LiAlTiP) _x O _y series glass <13), lithium lanthanum titanate (Li _x La _y TiO ₃ , 0 <x <2, 0 <y <3), lithium germanium thiophosphate (Li _x Ge _y P _z S _w , (Li _x N _y , 0 <x <4, 0 <y <2), SiS ₂ series glass (Li _x Si, 0 <y <1, 0 <z < _{y S z, 0 <x <} 3, 0 <y <2, 0 <z <4) and P ₂ S ₅ based glass _{(Li x P y S z,} 0 <x <3, 0 <y <3, 0 < z < 7).
3. The method according to claim 1 or 2,
The binder polymer may be selected from the group consisting of polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-trichlorethylene, polymethylmethacrylate, poly Polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide, cellulose acetate, polyvinylpyrrolidone, , Cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethyl cellulose, Ethyl sucrose Wherein the electrode assembly is one or a mixture of two or more selected from the group consisting of cyanoethylsucrose, pullulan, and carboxyl methyl cellulose.
An electrochemical device comprising an electrode assembly according to any one of claims 1 to 3.
9. The method of claim 8,
Wherein the electrochemical device is a lithium secondary battery.

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