KR101645470B1 - Electrode Assembly for Secondary Battery of Improved Low-Temperature Property and Secondary Battery Containing the Same - Google Patents

Abstract

The present invention includes a plurality of unit cells each including a positive electrode coated with a positive electrode mixture on a positive electrode collector, a negative electrode coated with a negative electrode mixture on a negative electrode collector, and a separator interposed between the positive electrode and the negative electrode Wherein at least one of the unit cells is a first unit cell and the remaining unit cells are a second unit cell and the second unit cell has a loading amount of the electrode mixture applied on the current collector relative to the first unit cell Wherein the electrode assembly includes a plurality of electrode assemblies.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode assembly having improved low-temperature characteristics, and a secondary battery including the secondary battery.

The present invention relates to an electrode assembly having improved low temperature characteristics and a secondary battery including the same.

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, are being conducted. 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.

On the other hand, in order to use a lithium secondary battery as a power source for an electric vehicle or a hybrid electric vehicle, it is required to have performance capable of operating under harsh conditions in a mobile phone, a notebook computer, a PDA, and the like. In general, the vehicle must be able to operate at low temperatures, such as during winter, and a representative example of such requirements is good output characteristics at low temperatures.

Therefore, in the past, attempts have been made to improve the low-temperature characteristics of a lithium secondary battery using a new cathode active material and an electrolyte solution. However, in spite of such attempts, development has not been made in a secondary battery exhibiting a desired low temperature characteristic. Particularly, the problem of reducing the capacity and output of the battery due to the decrease in the electrochemical reaction rate at a low temperature is very serious.

Therefore, there is a high need for a lithium secondary battery technology that has a high capacity under low temperature conditions and excellent output characteristics.

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.

The inventors of the present application have conducted intensive research and various experiments and have found that when the electrode assembly includes a plurality of unit cells having different loading amounts of the electrode mixture as described later, The present invention has been accomplished on the basis of these findings.

Therefore, the electrode assembly according to the present invention comprises a positive electrode having a positive electrode mixture coated on a positive electrode collector, a negative electrode having a negative electrode mixture coated on the negative electrode collector, and a separator interposed between the positive electrode and the negative electrode wherein at least one of the unit cells is a first unit cell and the remaining unit cells are a second unit cell, and the second unit cell includes a plurality of unit cells, 1 unit cell.

In the electrode assembly having the above structure, relatively few electrode assemblies are loaded in the second unit cell, so that the electric resistance is smaller than that of the first unit cell, When the temperature of the first unit cell is increased, the electrochemical reaction of the first unit cell is promoted. Therefore, when the temperature of the first unit cell is increased, There is an effect that the output characteristics and the capacity at low temperatures are improved.

In this regard, the general formula for electrical resistance, calorific value, and heat transfer is as follows.

V = IR (1)

^{Q = IVt = I 2 Rt (} 2)

Qc = kA? T / L (3)

In the above equations, V is the voltage, I is the current, R is the electrical resistance, Q is the caloric value, t is the time, Qc is the heat transfer to conduction, k is the thermal conductivity coefficient, A is the cross- The temperature difference between two objects occurring, and L the heat transfer distance.

As can be seen from the above equations (1) and (2), the calorific power is proportional to the square of the current and the resistance, and thus is more influenced by the magnitude of the current than the resistance. The electric resistance is relatively small, The larger second unit cell has a larger calorific value than that of the first unit cell, whereby the temperature of the second unit cell rises faster than that of the first unit cell, and the temperature difference? T between the unit cells becomes larger.

Referring to Equation (3), if the temperature difference? T between the unit cells is large, the heat transfer amount Qc between the unit cells is large. Thus, in the case of the electrode assembly according to the present invention, There is an effect that active heat transfer occurs between the unit cells as compared with the case of evenly loading, and therefore the output characteristics and capacity at low temperatures are improved.

Hereinafter, the configuration of the electrode assembly according to the present invention will be described in more detail.

In one specific example, the number of the second unit cells may be 35% to 90%, and in particular, 40% to 80% of the total number of unit cells.

If the number of the second unit cells is less than 35% of the total number of unit cells, there is a problem that the calorific value of the second unit cell is not sufficient to activate the electrochemical reaction of the first unit cell, The amount of the electrode assembly to be loaded in the first unit cell to obtain the same battery capacity at a constant volume becomes excessively large, and the rate characteristic deteriorates, which is not preferable.

The electrode assembly may be a cathode mix and / or a cathode mix. In one specific example, the first unit cell may have a relatively larger loading amount of at least one of the cathode mixture and the anode mixture than the second unit cell. have.

In one example, when the electrode mix is a cathode mix, the loading amount of the cathode mix of the second unit cell may be 50% to 95% of the loading amount of the cathode mix of the first unit cell on a weight basis, May range from 65% to 90%.

If the loading amount of the positive electrode mixture in the second unit cell is less than 50% of the loading amount of the positive electrode mixture in the first unit cell on the basis of weight, The electric resistance of the second unit cell is similar to that of the first unit cell and the magnitudes of the currents are similar to each other. As a result, There is a problem that the amount of heat generated by the unit cell is not sufficient to activate the first unit cell.

In another example, when the electrode mixture is an anode mixture, the loading amount of the anode mixture of the second unit cell may be 50% to 95% of the loading amount of the anode mixture of the first unit cell on a weight basis, May range from 65% to 90%.

In one specific example, the positive electrode mixture of the second unit cell and the positive electrode mixture of the first unit cell may have the same composition with each other, and in another example, they may have different compositions. Also, the negative electrode mixture of the second unit cell and the negative electrode mixture of the first unit cell may have the same composition, and in another example, they may have different compositions.

In the case where the composition of the positive electrode mixture in the second unit cell and the first unit cell are the same or the compositions of the negative electrode mixture are the same, only one type of positive electrode mixture or negative electrode mixture can be produced, Cost reduction is possible.

On the other hand, when the composition of the positive electrode mixture is different or the composition of the negative electrode mixture is different in the second unit cell and the first unit cell, the positive electrode material mixture or the negative electrode material mixture having a relatively low- It is possible to further improve the low-temperature characteristics of the electrode assembly as a whole.

The arrangement of the first unit cell and the second unit cell in the electrode assembly is not limited. Specifically, the first unit cell and the second unit cell may be alternately arranged in at least a part of the electrode assembly, Two unit cells can be arranged on the outermost side of the electrode assembly.

In addition, the type of the electrode assembly is not limited, and a plurality of unit cells of the electrode assembly may be stacked and folded by a separator, stacked sequentially, And may be a jelly roll type structure in which an anode, a cathode, and a separator are wound together.

The jelly roll type electrode assembly is manufactured by coating an electrode active material or the like on a metal foil used as a current collector, drying and pressing the electrode active material, cutting the electrode into a band shape having a desired width and length, separating the anode and the cathode using a separator, And is suitable for a cylindrical battery. However, when applied to a square or pouch type battery, there is a disadvantage in peeling of an electrode active material and low space utilization.

On the other hand, the stacked electrode assembly has a structure in which a plurality of cathode and anode unit bodies are sequentially stacked, and it is easy to obtain a rectangular shape. However, when the manufacturing process is troublesome and impact is applied, the electrode is pushed and short- There are disadvantages.

On the other hand, the stack and folding type electrode assemblies have a structure in which a full cell of a negative electrode / separator / positive electrode structure having a constant unit size or a bi-cell of a negative electrode (positive electrode) / separator / positive electrode (negative electrode) / separator / and a bicell is folded by using a continuous long separator. The structure of the jelly roll type electrode assembly and the stacked electrode assembly is complemented by the structure.

The negative electrode constituting the unit cell is manufactured, for example, by applying a negative electrode mixture containing an anode active material, a conductive material and a binder to both surfaces of a negative electrode collector, followed by drying and pressing. If necessary, Other additives may be added.

The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. Examples of the negative electrode current collector include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel The surface of which is surface-treated with carbon, nickel, titanium, silver or the like, and aluminum-cadmium alloy may be used. In particular, copper may be used. The current collector may have fine irregularities on the surface thereof to increase the adhesive force of the negative electrode active material, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric are possible.

The negative electrode active material may include, for example, carbon such as non-graphitized carbon or 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 < 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, but the present invention is not limited thereto.

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 anode 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 that assists in binding of the active material to the conductive material and bonding to the current collector, and is usually added in an amount of 1 to 30 wt% based on the total weight of the negative electrode material mixture. 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 butylene rubber, fluorine rubber, various copolymers and the like.

The filler is optionally used as a component for suppressing the expansion of the negative electrode, and is not particularly limited as long as it is a fibrous material without causing 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.

On the other hand, the positive electrode can be prepared by applying, drying and pressing a positive electrode active material on the positive electrode current collector, and optionally a positive electrode material mixture containing a conductive material, a binder and a filler as described above.

The anode current collector is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery. For example, the anode current collector may be made of stainless steel, aluminum, nickel, titanium, Carbon, nickel, titanium or silver, and the like may be used. In addition, like the negative electrode collector, fine unevenness can be formed on the surface to enhance the bonding force of the positive electrode active material, and it can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The cathode active material may be, for example, 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 separation membrane is interposed between the cathode and the anode, 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 present invention provides a secondary battery comprising the electrode assembly, wherein the secondary battery is a structure in which a lithium salt-containing nonaqueous electrolyte is impregnated in the electrode assembly according to the present invention.

The lithium salt-containing nonaqueous electrolyte is composed of a nonaqueous electrolyte and a lithium salt. Examples of the nonaqueous electrolyte include nonaqueous organic solvents, organic solid electrolytes, inorganic solid electrolytes, and the like.

Examples of the non-aqueous organic solvent include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma -Butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile , Nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, Tetrahydrofuran derivatives, ether, methyl pyrophosphate, ethyl propionate 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, A polymer containing an ionic dissociation group and the like may 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 substance that is soluble in the nonaqueous electrolyte and includes, 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 ₂ ) ₂ NLi, chloroborane lithium, lower aliphatic carboxylate lithium, lithium tetraphenylborate, and imide.

The nonaqueous electrolyte may contain, for the purpose of improving charge / discharge characteristics, flame retardancy, etc., for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, 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, halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride 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 a preferred _{embodiment, 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 nonaqueous electrolyte.

The present invention also provides a battery pack including the secondary battery as a unit battery.

The battery pack can be used as a power source for a device requiring high output and high capacity under low temperature conditions. Specific examples of the device include a small device such as a computer, a mobile phone, a power tool, A power tool for receiving and moving the power tool; An electric vehicle including an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and the like; An electric motorcycle including an electric bike (E-bike) and an electric scooter (E-scooter); An electric golf cart; Power storage systems, and the like, but are not limited thereto.

As described above, the electrode assembly according to the present invention includes a plurality of unit cells, wherein at least one of the unit cells is a first unit cell and the remaining unit cells are a second unit cell, The second unit cell has a configuration in which the loading amount of the electrode mixture applied on the current collector is relatively smaller than that of the first unit cell so that the output characteristic is improved in a low temperature condition as compared with the electrode assembly including unit cells having a constant loading amount And the capacity is increased.

FIG. 1 is a graph showing voltage changes according to discharge capacities during discharging at -10 ° C of the batteries according to Examples 1 to 3 and Comparative Examples 1 and 2; FIG.
FIG. 2 shows the definition of the? V value in Experimental Example 2.

Hereinafter, the present invention will be described with reference to Examples. However, the following Examples are intended to illustrate the present invention, and the scope of the present invention is not limited thereto.

≪ Example 1 >

The negative electrode active material (Li₄Ti₅O₁₂), A conductive material (Denka black), a binder, and other additives in a weight ratio of 96.3: 1: 1.5: 1.2 to H₂O to prepare a negative electrode material mixture. The negative electrode material mixture was loaded on an aluminum foil having a thickness of 6 탆, rolled and dried to prepare a negative electrode. At this time, in the cathode of the first unit cell, 256 mg / 25 cm²And the negative electrode of the second unit cell was loaded with 234 mg / 25 cm²of The negative electrode mixture was loaded.

As the anode, LCO was used as a positive electrode active material, and a conductive material and a binder were mixed in NMP at a weight ratio of 96: 2: 2, mixed and loaded on an aluminum foil having a thickness of 10 탆, rolled and dried to prepare a positive electrode . At this time, a positive electrode mixture of 530 mg / 25 cm ² was loaded on the positive electrode of the first unit cell, and a positive electrode mixture of 490 mg / 25 cm ² was loaded on the positive electrode of the second unit cell.

After the first unit cell and the second unit cell were fabricated with a polyethylene porous film (Celgard, thickness: 16 占 퐉) as a separator between the anode and the cathode thus prepared, the separator was provided with eight first unit cells and a second unit cell Five stacked and folded electrode assemblies were prepared by winding and arranging the second unit cells so as to be located on the outermost side of the electrode assembly, and were housed in the battery case.

A liquid electrolyte in which 1 M of LiPF ₆ was dissolved in a solvent in which propylene carbonate (PC): dimethyl carbonate (DMC): ethyl methyl carbonate (EMC) was mixed at 20: 40: 40 vol% The cells were impregnated for 3 days to prepare a battery. After the impregnation for 3 days, a formation process was carried out, and a degassing process and a resealing process were performed to complete the cell.

≪ Example 2 >

A battery was prepared in the same manner as in Example 1, except that 7 stacked unit cells and 6 stacked unit cells were manufactured by winding a separator on the first and second unit cells.

≪ Example 3 >

A battery was prepared in the same manner as in Example 1, except that 6 stacked unit cells and 7 stacked second unit cells were wound up with a separator to prepare stacked and folded electrode assemblies.

≪ Comparative Example 1 &

A battery was prepared in the same manner as in Example 1, except that 13 stacked first unit cells were wound up with a separator to form stacked and folded electrode assemblies.

≪ Comparative Example 2 &

A third unit cell was prepared by using a cathode loaded with a cathode mixture of 256 mg / 25 cm < ² > and a cathode loaded with a cathode mixture of 510 mg / 25 cm < ^{2 &} A battery was prepared in the same manner as in Example 1, except that 13 stacked and folded electrode assemblies were prepared by winding 13 cells through a separator.

<Experimental Example 1>

The batteries of Examples 1 to 3 and Comparative Examples 1 and 2 were discharged at a high rate at 23 캜 to measure the capacity until the output voltage became 3 V and the battery was discharged at a high rate of -10 캜 after the buffering, , And the ratio of the capacity at -10 캜 to the capacity at 20 캜 was calculated. The results are shown in Table 1 below.

Referring to Table 1, it can be seen that the batteries of Examples 1 to 3 have a higher capacity ratio of -10 ° C to 20 ° C than the batteries of Comparative Examples 1 and 2. These results show that the capacity of the batteries of Examples 1 to 3 at low temperature conditions is superior to that of the batteries of Comparative Examples 1 and 2. [

<Experimental Example 2>

The batteries of Examples 1 to 3 and Comparative Examples 1 and 2 were subjected to high-speed discharge at -10 ° C to measure the voltage change until the output voltage reached 3 V, and the initial voltage at the maximum voltage measured in each battery Discharge capacity of 100 mAh or less) was subtracted from the lowest voltage (? V). The results are shown in Table 2 below.

Referring to Table 2, it can be seen that the batteries of Examples 1 to 3 have larger ΔV values than the batteries of Comparative Examples 1 and 2. These results show that the batteries of Examples 1 to 3 have better output characteristics at low temperature than those of Comparative Examples 1 and 2.

Those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims (20)

A plurality of unit cells including a positive electrode coated with a positive electrode mixture on a positive electrode collector, a negative electrode coated with a negative electrode mixture on a negative electrode collector, and a separator interposed between the positive electrode and the negative electrode;
At least one of the unit cells is a first unit cell and the remaining unit cells are a second unit cell;
The second unit cell has a relatively small amount of loading of the electrode mixture applied on the current collector than the first unit cell,
Wherein the number of the second unit cells is 35% to 90% of the total number of unit cells. delete The electrode assembly of claim 1, wherein the number of the second unit cells is 40% to 80% of the total number of unit cells. The electrode assembly of claim 1, wherein the first unit cell has a loading amount of at least one of a positive electrode mix and a negative electrode mix relatively larger than a second unit cell. 5. The electrode assembly of claim 4, wherein the loading amount of the positive electrode mixture of the second unit cell is 50% to 95% of the loading amount of the positive electrode mixture of the first unit cell on a weight basis. [6] The electrode assembly of claim 5, wherein the loading amount of the positive electrode mixture of the second unit cell is 65% to 90% of the amount of the positive electrode mixture of the first unit cell. 5. The electrode assembly according to claim 4, wherein the loading amount of the negative electrode mixture in the second unit cell is 50% to 95% of the loading amount of the negative electrode mixture in the first unit cell. The electrode assembly according to claim 1, wherein the positive electrode mixture of the second unit cell and the positive electrode mixture of the first unit cell have the same composition. The electrode assembly of claim 1, wherein the cathode mixture of the second unit cell and the cathode mixture of the first unit cell have different compositions. The electrode assembly according to claim 1, wherein the anode mixture of the second unit cell and the anode mixture of the first unit cell have the same composition. The electrode assembly of claim 1, wherein the anode mixture of the second unit cell and the anode mixture of the first unit cell have different compositions. The electrode assembly according to claim 1, wherein at least a part of the electrode assembly has a first unit cell and a second unit cell arranged alternately. The electrode assembly according to claim 1, wherein the second unit cell is arranged at the outermost side of the electrode assembly. The electrode assembly according to claim 1, wherein the plurality of unit cells are stacked by a separator and a folding structure. The electrode assembly according to claim 1, wherein the electrode assembly has a stacked structure in which a plurality of unit cells are sequentially stacked. The electrode assembly according to claim 1, wherein each of the unit cells is a jelly-roll structure in which an anode, a cathode, and a separator are wound together. A secondary battery comprising the electrode assembly according to claim 1. A battery pack comprising a secondary battery according to claim 17 as a unit cell. A device comprising a battery pack according to claim 18. 20. The device of claim 19, wherein the device is a computer, a mobile phone, a power tool, an electric vehicle (EV), a hybrid electric vehicle, a plug-in hybrid electric vehicle, a motorcycle, Lt; / RTI &gt; system.

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