KR20180081228A - Electrode Assembly Comprising Electrode Having Different Porosity Depending on Position of Unit-cell - Google Patents

Abstract

The present invention relates to an electrode assembly having a structure where a plurality of unit cells are stacked. Unit cells located on the outer portion of the unit-cell stacked structure have electrode assembly layers coated on an electrode plate with relatively low porosities compared to unit cells located on the inner portion of the structure. An objective of the present invention is to improve the impregnability of an electrolyte solution in an electrode disposed inside the electrode assembly, thereby reducing the manufacturing time of a cell.

Description

[0001] The present invention relates to an electrode assembly including an electrode having different porosity depending on the position of a unit cell,

The present invention relates to an electrode assembly including an electrode having a different porosity depending on the position of a unit cell.

As the technology development and demand for mobile devices have increased, the demand for secondary batteries has increased sharply as an energy source. Among such secondary batteries, lithium secondary batteries having high energy density and operating potential, long cycle life, Have been commercialized and widely used.

In addition, as the interest in environmental issues grows, researches on electric vehicles and hybrid electric vehicles that can replace fossil fuel-based vehicles such as gasoline vehicles and diesel vehicles, which are one of the main causes of air pollution, . Although nickel-metal hydride secondary batteries are mainly used as power sources for such electric vehicles and hybrid electric vehicles, researches using lithium secondary batteries having high energy density and discharge voltage are being actively carried out, and they are in the commercialization stage.

Such a secondary battery is classified into a cylindrical battery and a prismatic battery in which an electrode assembly is embedded in a cylindrical or rectangular metal can according to the shape of the battery case, and a pouch type battery in which an electrode assembly is embedded in a pouch-shaped case of an aluminum laminate sheet do.

Also, the secondary battery is classified according to the structure of the electrode assembly composed of the positive electrode, the negative electrode, and the separation membrane. Representatively, the secondary battery is composed of long-sheet type anodes and cathodes, A stacked (stacked) electrode assembly in which a plurality of positive electrodes and negative electrodes cut in units of a predetermined size are sequentially stacked with a separator interposed therebetween, a stacked (stacked) electrode assembly in which a predetermined unit of positive and negative electrodes are stacked, A bicell or a full cell, which is a unit cell stacked in one state, is placed on a separation film, and then a stacked-folding type electrode assembly or a bicell or full cell Stacked electrode assemblies having a structure in which they are stacked with a separator interposed therebetween.

In recent years, a secondary battery including an electrode assembly including a bi-cell or a pull cell having a high structural applicability has been attracting attention in recent years in response to various types of devices as well as a simple manufacturing process and a low manufacturing cost.

However, in the case of the unit cell located inside the stacked (stacked) electrode assembly, the stacked-folded electrode assembly, or the lamination-stacked electrode assembly, the unit cells located outside and the electrode It is difficult to impregnate the electrolytic solution relatively due to the plate and the separator, and it takes a long time to impregnate the unit cells.

If the unit cell located inside the electrode assembly is not properly impregnated into the electrolyte solution, a region where a SEI (Solid Electrolyte Interface) film is not formed is present on the active material surface of the electrode plate. In the case of the negative electrode plate, Resulting in a significant problem in performance and safety, such as a reduction in cycle time and a micro short-circuit caused by lithium dendrites.

Therefore, there is a high need for a technique capable of improving the impregnability of the electrolyte solution up to the unit cells located inside the electrode assembly.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems of the prior art and the technical problems required from the past.

Specifically, it is an object of the present invention to provide an electrode assembly that improves the electrolyte impregnability of an electrode located inside the electrode assembly, and can shorten the cell fabrication time.

According to an aspect of the present invention,

The electrode assembly of a structure in which a plurality of unit cells are laminated is characterized in that the unit cells located at the outer side in the unit cell lamination structure have a relatively low porosity of the electrode assembly layer coated on the electrode plate .

The number of the unit cells constituting the electrode assembly increases as the demand for high capacity of the battery increases. However, when the electrode assembly is packed in the battery case and the electrolyte solution is injected, the impregnation speed is low due to the high viscosity of the electrolyte, It is difficult for the electrolyte solution to reach the unit cell located on the inner side of the electrode assembly.

In particular, when the electrolyte does not reach the inner unit cell, an unreacted active material region is formed on the electrode plate constituting the unit cell, thereby generating lithium dendrite.

However, in the electrode assembly according to the present invention, the electrode porosity of the unit cells located in the inner region is higher than that of the unit cell located in the outer region, and the electrolyte impregnability of the electrode located in the inner region can be improved overall, It is possible to reduce the occurrence of the drip and ensure the safety of the battery.

The electrode assembly can be applied to any structure as long as the unit cells are stacked, but the electrode assembly can be a stacked structure, a stacked-folded structure, or a lamination-stacked structure.

The unit cells may be a bi-cell, a pull cell, or a combination of a bi-cell and a pull cell. The bi-cell may be an A-type bi-cell in which an anode, a cathode, and an anode are sequentially stacked with a separator interposed therebetween, One or more of the C-type bi-cells in which the cathodes are sequentially stacked with the separator interposed therebetween may be used.

The structure and effect of the specific electrode assembly for accomplishing this will be described in detail with reference to the following non-limiting examples.

In one specific example, the total number of unit cells constituting the electrode assembly is n (4? N? 50);

The unit cells located at the outer portion are the outermost unit cells or the combination of the outermost unit cells and the unit cells adjacent to the outermost unit cells and are in a range of 10% to 60% of the total number of unit cells ;

The unit cells located at the inner portion may be unit cells other than the unit cells located at the outer portion of the unit cells.

If the number of unit cells located at the outer portion is less than 10% of the total number of unit cells, the electrode assembly layer having a high porosity occupies most of the electrode assembly. At this time, in order to keep the energy amount of the electrode assembly equal to that of the conventional electrode assembly, the amount of the electrode assembly is increased more than before, so that the area or thickness of the electrode assembly increases and the energy density of the battery may be lowered.

If the number of unit cells located at the outer portion exceeds 60% of the total number of unit cells, it means that the unit cell having a low porosity exceeds 60% of the electrode assembly. Therefore, The impregnating ability of the electrolyte solution may become insufficient.

The difference between the average porosity of the unit cells located at the outer portion and the average porosity of the unit cells located at the inner portion may be in the range of 0.1% to 20%, and more specifically, in the range of 1% to 10%. When the difference in average porosity is less than 0.1%, the difference in porosity between the inner region and the outer region is not large, so that it is difficult to affect the electrolyte impregnability of the inner partial battery cell, and when the difference is more than 20% The amount of the electrode mixture becomes insufficient, so that the energy density and output characteristics of the battery may be deteriorated.

(0? K? 13, wherein k is a range that is smaller than n and satisfies the condition of 10% to 60%) adjacent to the outermost unit cell and the outermost unit cell in order ), The porosity may be the same from the outermost unit cell to the k-th unit cell, as the case may be.

In another specific example, the unit cells located at the outer portion are sequentially disposed in the outermost unit cell and the outermost unit cell, k (0? K? 13, where k is smaller than n, 60%), the porosity may be sequentially increased from the outermost unit cell to the k-th unit cell.

The impregnability of the electrolytic solution can be lowered as the unit cells are classified closer to the center of the electrode assembly, and the porosity from the outermost unit cell to the k-th unit cell is sequentially The impregnability of the electrode assembly can be further improved.

The unit cells located at the k-th position from the outermost unit cell in the electrode assembly are all two, regardless of the structure of the electrode assembly, except for the unit cells located at the center when the unit cells are stacked in an odd number. The two unit cells may have the same or different porosity, but if the two unit cells are set to have lower porosity than the adjacent (k + 1) th unit cell, there is no problem in manufacturing.

In another specific example, in order to uniformly improve the electrolyte impregnability of the unit cells located in the inner region, the unit cells in the inner region may have the same porosity.

In another specific example, in the unit cells located at the inner portion, the porosity of the unit cell located at the center is the highest, and the porosity of the unit cell is sequentially decreased corresponding to the distance from the unit cell located at the center .

Since the electrolyte solution is generally injected from the end of the battery case for smooth sealing of the battery and impregnated in the order of the outermost unit cell and the outermost unit cell in close order, The area farthest from the outermost unit cell of the assembly is farthest apart, so that a region which is not impregnable with the electrolyte can be formed wider or the time for impregnation with the electrolyte can be further increased. That is, since the unit cells are not easily impregnated into the electrolytic solution as the distance from the closest outermost unit cells increases, the porosity of the unit cells is adjusted to be closer to the central unit cell, It is possible to further enhance the electrolyte impregnability of the assembly.

The porosity can be controlled by the rolling rate after the electrode assembly is coated on the electrode plate. In this case, the same amount of electrode mixture is used for the electrode plates of each unit cell, and the porosity is adjusted by adjusting the thickness , The porosity can be designed for each unit cell and / or electrode, and the quality of the unit cell included in the electrode assembly can be uniformly produced.

The present invention also provides a secondary battery in which an electrode assembly is embedded in a battery case together with a nonaqueous electrolyte solution. The secondary battery is not particularly limited, but specific examples thereof include a high energy density, a discharge voltage, Lithium secondary batteries such as Li-ion secondary batteries, Li-polymer secondary batteries, Li-ion polymer secondary batteries, and the like.

Generally, a lithium secondary battery refers to a battery composed of a positive electrode, a negative electrode, a separation membrane, and a lithium salt-containing non-aqueous electrolyte, which are the configurations of the prior electrode assembly.

The positive electrode is prepared, for example, by coating a mixture of a positive electrode active material, a conductive material and a binder on a positive electrode current collector and / or an extended current collector, and then drying the resultant. Optionally, do.

The cathode current collector and / or the elongated current collector are generally made to have a thickness of 3 to 500 micrometers. The positive electrode current collector and the elongate current collector are not particularly limited as long as they have high conductivity without causing a chemical change in the battery, and examples thereof include stainless steel, aluminum, nickel, titanium, A surface treated with carbon, nickel, titanium, or silver on the surface of stainless steel may be used. The anode current collector and the elongate current collector may have various shapes such as a film, a sheet, a foil, a net, a porous body, a foam, a nonwoven fabric, or the like by forming fine irregularities on the surface thereof to increase the adhesive force of the cathode active material.

The cathode active material may be a layered compound such as lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), or a compound substituted with one or more transition metals; Lithium manganese oxides such as Li _{1 + x} Mn _{2 -x} O ₄ (where x is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ and the like; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ and Cu ₂ V ₂ O ₇ ; A Ni-site type lithium nickel oxide expressed by the formula LiNi _1-x M _x O ₂ (where M = Co, Mn, Al, Cu, Fe, Mg, B or Ga and x = 0.01 to 0.3); Formula _{LiMn 2-x M x O 2} ( where, M = Co, Ni, Fe , Cr, and Zn, or Ta, x = 0.01 ~ 0.1 Im) or Li ₂ Mn ₃ MO ₈ (where, M = Fe, Co, Ni, Cu, or Zn); LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with an alkaline earth metal ion; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ , and the like. However, the present invention is not limited to these.

The conductive material is usually added in an amount of 1 to 30% by weight based on the total weight of the mixture including the cathode active material. Such a conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, for example, graphite such as natural graphite or artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The binder is a component which assists in bonding of the active material and the conductive material and bonding to the current collector, and is usually added in an amount of 1 to 30% by weight based on the total weight of the mixture containing the cathode active material. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber, fluorine rubber, various copolymers and the like.

The filler is optionally used as a component for suppressing the expansion of the anode, and is not particularly limited as long as it is a fibrous material without causing a chemical change in the battery. Examples of the filler include olefin polymers such as polyethylene and polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

The negative electrode is manufactured by applying and drying a negative electrode active material on the negative electrode current collector and / or the extended current collector, and may optionally further include the components as described above.

The cathode current collector and / or the extension current collector are generally made to a thickness of 3 to 500 micrometers. The negative electrode current collector and / or the elongated current collector are not particularly limited as long as they have electrical conductivity without causing chemical changes in the battery, and examples thereof include copper, stainless steel, aluminum, nickel, titanium, Surface treated with carbon, nickel, titanium, silver or the like on the surface of copper or stainless steel, aluminum-cadmium alloy, or the like can be used. In addition, like the positive electrode collector, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams and nonwoven fabrics.

Examples of the negative electrode active material include carbon such as non-graphitized carbon and graphite carbon; _{Li x Fe 2 O 3 (0≤x≤1} ), Li x WO 2 (0≤x≤1), Sn x Me 1-x Me 'y O z (Me: Mn, Fe, Pb, Ge; Me' : Metal complex oxides such as Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, Halogen, 0 < x &lt; Lithium metal; Lithium alloy; Silicon-based alloys; Tin alloy; _{SnO, SnO 2, PbO, PbO} 2, Pb 2 O 3, Pb 3 O 4, Sb 2 O 3, Sb 2 O 4, Sb 2 O 5, GeO, GeO 2, Bi 2 O 3, Bi 2 O 4, and Bi ₂ O ₅ ; Conductive polymers such as polyacetylene; Li-Co-Ni-based materials and the like can be used.

The separation membrane is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the membrane is generally 0.01 to 10 micrometers, and the thickness is generally 5 to 300 micrometers. Such separation membranes include, for example, olefinic polymers such as polypropylene, which are chemically resistant and hydrophobic; A sheet or nonwoven fabric made of glass fiber, polyethylene or the like is used. When a solid electrolyte such as a polymer is used as an electrolyte, the solid electrolyte may also serve as a separation membrane.

The electrolytic solution may be a non-aqueous electrolytic solution containing a lithium salt, and is composed of a non-aqueous electrolytic solution and a lithium salt. As the non-aqueous electrolyte, non-aqueous organic solvents, organic solid electrolytes, inorganic solid electrolytes, and the like are used, but the present invention is not limited thereto.

Examples of the non-aqueous organic solvent include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butylolactone, Tetrahydrofuran, tetrahydrofuran, tetrahydrofuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, Examples of the organic solvent include methyl acetate, phosphoric acid triester, trimethoxy methane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, Propylenic organic solvents such as methylmethyl, ethylpropionate and the like can be used.

Examples of the organic solid electrolyte include a polymer electrolyte such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, Polymers containing ionic dissociation groups, and the like can be used.

Examples of the inorganic solid electrolyte include Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides and sulfates of Li such as Li ₄ SiO ₄ -LiI-LiOH and Li ₃ PO ₄ -Li ₂ S-SiS ₂ can be used.

The lithium salt is a material that is readily soluble in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO 4, LiBF 4, LiB 10 Cl 10, LiPF 6, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF _{6, LiSbF 6, LiAlCl 4,} CH 3 SO 3 Li, CF 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide have.

For the purpose of improving the charge-discharge characteristics and the flame retardancy, the nonaqueous electrolytic solution is preferably a solution prepared by dissolving or dispersing in a solvent such as pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, hexaphosphoric triamide, N, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride and the like may be added have. In some cases, a halogen-containing solvent such as carbon tetrachloride, ethylene trifluoride or the like may be further added to impart nonflammability. In order to improve the high-temperature storage characteristics, carbon dioxide gas may be further added. FEC (Fluoro-Ethylene Carbonate ), PRS (Propene sultone), and the like.

In one specific _{example, LiPF 6, LiClO 4, LiBF} 4, LiN (SO 2 CF 3) 2 , such as a lithium salt, a highly dielectric solvent of DEC, DMC or EMC Fig solvent cyclic carbonate and a low viscosity of the EC or PC of And then adding it to a mixed solvent of linear carbonate to prepare a lithium salt-containing non-aqueous electrolyte.

As described above, in the electrode assembly according to the present invention, the electrode porosity of the unit cells located in the inner region is higher than that of the unit cell located in the outer region, and the electrolyte impregnability of the electrode located in the inner region can be improved. It is possible to reduce the lithium dendrite and secure the safety and cycle characteristics of the battery.

1 is a schematic diagram of a conventional stack-folding electrode assembly;
2 is a schematic diagram of an electrode assembly according to one embodiment of the present invention;
3 is a schematic view of an electrode assembly according to another embodiment of the present invention;
4 is a schematic diagram of an electrode assembly according to another embodiment of the present invention;
5 is a schematic diagram of an electrode assembly according to another embodiment of the present invention;
6 is a schematic diagram of an electrode assembly according to another embodiment of the present invention;
7 is a schematic diagram of an electrode assembly according to another embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings, but the present invention is not limited by the scope of the present invention.

1 schematically shows a general structure of a conventional stack-folding type electrode assembly.

1, the electrode assembly 100 includes unit cells of a cathode-separator-anode-separator-cathode structure and unit cells of an anode-separator-cathode-separator-anode structure, The separation film 150 interposed between the unit cells encloses the respective side surfaces of the unit cells 111, 112, 113, 114, 115, 116, 117 in which no electrode terminal is formed, The electrodes forming the cells all have the same porosity. Meanwhile, the direct structure of the unit cells is not shown for simplification of the drawings.

2 schematically shows the structure of an electrode assembly according to one embodiment of the present invention.

2, the electrode assembly 200 includes unit cells 211, 212, 221 and 222 located at the outer parts 210 and 220 and unit cells 231, 232 and 233 located at the inner part 220. [ And a separation film 250. The unit cell located at the outer portion is formed by a combination of the outermost unit cells 211 and 221 and the unit cells 212 and 222 adjacent to the outermost unit cells And the electrode mixture layer coated on the electrode plate has a relatively lower porosity than the unit cells 231, 232, and 233 located in the inner region.

3 schematically shows a structure of an electrode assembly according to another embodiment of the present invention.

3, the electrode assembly 300 includes unit cells 311, 312, 313, and 314 located in the outer region 310 and unit cells 331, 332, and 333 located in the inner region 330, And a separation film 350. The unit cells located at the outer portion have the same porosity and lower porosity than the unit cells 331, 332, and 333 located at the inner portion.

4 schematically shows a structure of an electrode assembly according to another embodiment of the present invention.

Referring to FIG. 4, the unit cells 411, 412, 421, and 422 located at the outer portions 410 and 420 of the electrode assembly 400 sequentially have higher porosity as they are positioned closer to the center of the electrode assembly. Specifically, the unit cell 412 has a higher porosity than the unit cell 411, and the unit cell 422 has a higher porosity than the unit cell 421.

5 schematically shows a structure of an electrode assembly according to another embodiment of the present invention.

Referring to FIG. 5, unit cells 531, 532 and 533 located in the inner region 530 of the electrode assembly 500 are connected to the unit cells 511, 512, 521 and 522 located in the outer regions 510 and 520 ), And the unit cells 531, 532, and 533 located at the inner portion have the same porosity.

6 schematically shows the structure of an electrode assembly according to another embodiment of the present invention.

Referring to FIG. 6, the unit cells 631, 632 and 633 located in the inner region 630 of the electrode assembly 600 are connected to the unit cells 611, 612, 613 and 614 located at the outer region 610 The unit cells 611, 612, 613, and 614 located at the outer portion all have the same porosity and the unit cells 631, 632, and 633 located at the inner portion have the same porosity.

7 is a schematic view showing the structure of an electrode assembly according to another embodiment of the present invention.

Referring to FIG. 7, in the battery cells constituting the electrode assembly 700, the center cell unit 741 has the highest porosity and the unit cells 731 and 732 closest to the unit cell 741 ) Is the next highest in porosity. As described above, the porosity of the unit cells 711 and 712 located at the outermost periphery of the electrode assembly is lowered as the positions of the unit cells 741 located farther from the center are decreased.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (17)

The electrode assembly of a structure in which a plurality of unit cells are laminated is characterized in that the unit cells located at the outer side in the unit cell lamination structure have a relatively low porosity of the electrode assembly layer coated on the electrode plate Wherein the electrode assembly comprises: The electrode assembly of claim 1, wherein the electrode assembly comprises a stacked structure, a stacked-folded structure, or a lamination-stacked structure. The electrode assembly according to claim 1, wherein the unit cells are formed of a combination of a bi-cell, a pull cell, or a bi-cell and a pull cell. [6] The method of claim 3, wherein the bi-cell is an A-type bi-cell in which an anode, a cathode, and an anode are sequentially stacked with a separator interposed therebetween, or a C-bi- cell in which a cathode, an anode, and a cathode are sequentially stacked with a separator interposed therebetween The electrode assembly comprising: The method according to claim 1,
The total number of unit cells constituting the electrode assembly is n (4? N? 50);
The unit cells located at the outer portion are the outermost unit cells or the combination of the outermost unit cells and the unit cells adjacent to the outermost unit cells and are in a range of 10% to 60% of the total number of unit cells ;
Wherein the unit cells located at the inner portion are unit cells other than the unit cells located at the outer portion of all the unit cells. The electrode assembly according to claim 5, wherein a difference between an average porosity of the unit cells located at the outer portion and an average porosity of the unit cells located at the inner portion is in a range of 0.1% to 20%. The electrode assembly according to claim 5, wherein a difference between an average porosity of the unit cells located at the outer portion and an average porosity of the unit cells located at the inner portion is in a range of 1% to 10%. 6. The method of claim 5,
(0? K? 13, wherein k is a range that is smaller than n and satisfies the condition of 10% to 60%) adjacent to the outermost unit cell and the outermost unit cell in order Lt; RTI ID = 0.0 &gt; 1, &lt; / RTI &gt;
And the porosities from the outermost unit cell to the k-th unit cell are all the same. 6. The method of claim 5,
(0? K? 13, wherein k is a range that is smaller than n and satisfies the condition of 10% to 60%) adjacent to the outermost unit cell and the outermost unit cell in order Lt; RTI ID = 0.0 &gt; 1, &lt; / RTI &gt;
And the porosity of the electrode assembly is sequentially increased from the outermost unit cell to the k-th unit cell. 6. The electrode assembly of claim 5, wherein the unit cells located in the inner region have the same porosity. The method according to claim 5, wherein the unit cells located at the inner portion have the highest porosity and the porosity of the unit cells gradually decreases corresponding to the distance from the center of the unit cells located at the center . The electrode assembly according to claim 1, wherein the porosity of the electrode assembly is controlled by the rolling rate when the electrode assembly is coated on the electrode plate and then rolled. The secondary battery according to any one of claims 1 to 11, wherein the electrode assembly is embedded in the battery case together with the non-aqueous electrolyte. A battery module comprising a secondary battery according to claim 13 as a unit cell. A battery pack comprising the battery module according to claim 14. A device comprising a battery pack according to claim 15. 17. The system of claim 16, wherein the device is a computer, a mobile phone, a power tool, an electric vehicle (EV), a hybrid electric vehicle, a plug- Lt; / RTI &gt; system.

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