KR20180006054A - Positive electrode for lithium secondary battery having improved capacity and safety and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a positive electrode for a lithium secondary battery, which comprises: a positive electrode current collector; a first positive electrode active material layer formed on one side of the positive electrode current collector; and a second positive electrode active material layer formed on the other side of the positive electrode current collector, wherein the first and second positive electrode active material layers contain different positive electrode active materials from each other, and the second positive electrode active material contains lithium iron phosphate (LiFePO_4, LFP) as a positive electrode active material. The positive electrode according to the present invention comprises a positive electrode active material layer containing a positive electrode active material which, when used in a lithium secondary battery using a negative electrode having large irreversible capacity, compensates for the large irreversible capacity of the negative electrode, and which at the time of charging/recharging, works as resistance for increasing safety of the lithium secondary battery. Thus, the positive electrode of the present invention may be advantageously used in the production of a lithium secondary battery with excellent capacity characteristics and safety.

Description

TECHNICAL FIELD [0001] The present invention relates to a positive electrode for a lithium secondary battery having improved capacity and safety, and a lithium secondary battery including the positive electrode. [0001] The present invention relates to a positive electrode for a lithium secondary battery,

The present invention relates to a positive electrode for a lithium secondary battery improved in capacity and safety and a lithium secondary battery comprising the negative electrode. More particularly, the present invention relates to a lithium secondary battery using a negative electrode having a large irreversible capacity, And a positive electrode active material layer serving as a resistor capable of improving the safety of the lithium secondary battery during discharging, and a lithium secondary battery including the same.

As technology development and demand for mobile devices have increased, there has been a rapid increase in demand for secondary batteries as energy sources. Among such secondary batteries, lithium secondary batteries, which exhibit high energy density and operating potential, long cycle life, Batteries have been commercialized and widely used.

In recent years, there has been a growing interest in environmental issues, and as a result, electric vehicles (EVs) and hybrid electric vehicles (HEVs), which can replace fossil-fueled vehicles such as gasoline vehicles and diesel vehicles, And the like. Although a nickel metal hydride (Ni-MH) secondary battery is mainly used as a power source for such an electric vehicle (EV) and a hybrid electric vehicle (HEV), a lithium secondary battery having a high energy density, a high discharge voltage, Research is being actively carried out, and some are commercialized.

On the other hand, a metal oxide such as LiCoO ₂ , LiMnO ₂ , LiMn ₂ O _4, or LiCrO ₂ is used as a positive electrode active material constituting the positive electrode of the lithium secondary battery. Examples of the negative electrode active material constituting the negative electrode include metal lithium, a carbon based meterial such as graphite or activated carbon or a material such as silicon oxide (SiO _x ) is used.

The cathode active materials of the lithium secondary battery generally have an initial efficiency of about 90 to 97%, while the anode active materials have an initial efficiency of about 30 to 90% depending on the kind thereof. When a cathode active material having a relatively high initial efficiency and a negative active material having a relatively low initial efficiency as compared with the positive active material are used in constructing the lithium secondary battery, the capacity of the secondary battery due to the negative active material having a larger irreversible capacity And a decrease in cycle life due to a low charge / discharge efficiency for each charge / discharge cycle.

Also, in the first charging process of the lithium secondary battery, the surface of the negative electrode active material reacts with the electrolyte on the negative electrode to form a solid electrolyte interface (SEI) layer. The SEI layer is formed on the surface of the negative electrode active material Decomposing and stabilizing the battery. However, since the amount of lithium is consumed when the SEI film is formed, the amount of reversible lithium is reduced, and the capacity of the battery is eventually reduced.

Particularly, in the present secondary battery system in which the lithium source is on the anode, dead volume increases on the anode side due to the irreversible state of the cathode when the irreversible capacity of the cathode is large. Therefore, And the capacity of the battery is reduced. In addition, due to the low capacity due to the dead volume and the low charge / discharge efficiency in each cycle, degradation of battery life characteristics is also accompanied. This problem is more remarkable when a transition metal (Si, Sn, Ge, Pb, P, As, Sb, Bi, etc.) active material having a low efficiency is used as an anode active material as compared with a high theoretical capacity.

In order to compensate for the large irreversible capacity of the negative electrode active material, a separate positive electrode active material for previously supplying the lithium to the negative electrode so that excessive lithium is reversibly present in the negative electrode after the first discharge and after each cycle charge / Is mixed with a cathode active material to be used as a main material.

However, when a separate positive electrode active material for supplying lithium to the negative electrode in advance is used, after the positive electrode active material compensates for the irreversible capacity of the negative electrode in the first cycle, it acts as a resistor in the positive electrode active material layer, .

Therefore, it is required to develop a new technology that can compensate the irreversible capacity of the negative electrode in the lithium secondary battery, while not deteriorating the performance of the secondary battery.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a lithium secondary battery in which a negative electrode having a large irreversible capacity is compensated for an irreversible capacity of the negative electrode and a positive electrode active material layer serving as a resistor capable of enhancing the safety of the lithium secondary battery thereafter And a positive electrode for a lithium secondary battery.

It is another object of the present invention to provide a lithium secondary battery including the positive electrode for a lithium secondary battery.

In order to solve the above problems,

Anode collector; A first cathode active material layer formed on one surface of the cathode current collector; And a second positive electrode active material layer formed on the other surface of the positive electrode current collector,

Wherein the first cathode active material layer and the second cathode active material layer include different kinds of cathode active materials,

And the second cathode active material layer comprises lithium iron phosphate oxide (LiFePO ₄ , LFP) as a cathode active material.

Further, in order to solve the above-mentioned other problems,

A lithium secondary battery comprising an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode,

Wherein the electrode assembly comprises the anode according to claim 1 as an outermost shell anode and the second cathode active material layer is located on the outermost electrode surface of the electrode assembly, Thereby providing a battery.

The positive electrode for a lithium secondary battery according to the present invention is characterized in that a negative electrode having a large irreversible capacity is used in a lithium secondary battery to compensate the irreversible capacity of the negative electrode and to improve the safety of the lithium secondary battery thereafter, The lithium secondary battery of the present invention can be effectively used for the production of lithium secondary batteries having excellent capacity characteristics and safety.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view schematically showing a conventional positive electrode active material layer for a lithium secondary battery. FIG.
2 is a diagram schematically showing a positive electrode active material layer in a positive electrode for a lithium secondary battery according to an example of the present invention.
3 is a view schematically showing an example of an electrode assembly including a positive electrode for a lithium secondary battery according to an example of the present invention.
4 is a discharge and resistance profile of 0.2 C rate for the lithium secondary battery manufactured in Example 2 and Comparative Example 2. FIG.

Hereinafter, the present invention will be described in detail in order to facilitate understanding of the present invention.

The terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms and the inventor may appropriately define the concept of the term in order to best describe its invention It should be construed as meaning and concept consistent with the technical idea of the present invention.

The positive electrode for a lithium secondary battery of the present invention comprises a positive electrode collector; A first cathode active material layer formed on one surface of the cathode current collector; And a second cathode active material layer formed on the other surface of the cathode current collector.

Wherein the positive electrode for a lithium secondary battery includes two types of positive electrode active material layers, i.e., a first positive electrode active material layer and a second positive electrode active material layer, wherein the first positive electrode active material layer is formed on one surface of the positive electrode collector, The active material layer is formed on the other surface of the surface of the cathode current collector on which the first cathode active material layer is formed.

The first cathode active material layer and the second cathode active material layer include different kinds of cathode active materials.

The cathode active material contained in the first cathode active material layer is not particularly limited as long as it is generally used as a cathode active material in a lithium secondary battery. Examples of the cathode active material include lithium-cobalt oxide, lithium-manganese oxide, lithium- - manganese-cobalt oxide and lithium-nickel-manganese-cobalt oxide. Specific examples of the cathode active material include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), Li [Ni _x Co _y Mn _z Mv] O ₂ (where M is a group consisting of Al, X + y + z + v + 1), Li (Li _a M &_lt; _{ba-b 'M' b '} ) O 2 -c a c ( wherein, 0≤a≤0.2, 0.6≤b≤1, 0≤b'≤0.2, 0≤c≤0.2 and; M is Mn and, At least one selected from the group consisting of Ni, Co, Fe, Cr, V, Cu, Zn and Ti; M 'is at least one selected from the group consisting of Al, Mg and B; , F, S, and N), or a compound substituted with one or more transition metals; Lithium manganese oxides such as Li _{1 + y} Mn _{2 - y} O ₄ (where 0 _{? Y?} 0.33), LiMnO ₃ , LiMn ₂ O ₃ and LiMnO ₂ ; 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 ; Formula _{LiNi 1 - y M y O 2} ( where, the M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, 0.01≤y≤0.3 Im) Ni site type lithium nickel oxide represented by a; Formula _{LiMn 2 - y M y O 2} ( where, M = Co, Ni, Fe , Cr, and Zn, or Ta, 0.01≤y≤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; Li ₂ NiO ₂ ; Fe ₂ (MoO ₄ ) ₃ , and the like. However, the present invention is not limited to these. In one embodiment of the present invention, the lithium transition metal oxide included in the first cathode active material layer may be lithium cobalt oxide (LiCoO ₂ ).

The second cathode active material layer includes lithium iron phosphate oxide (LiFePO ₄ , LFP) as a cathode active material, and the cathode active material included in the second cathode active material layer may be specifically lithium iron phosphate oxide (LiFePO ₄ ).

LiFePO ₄ included in the second cathode active material layer compensates for the irreversible portion of the cathode in the activation process of the lithium secondary battery when the cathode for the lithium secondary battery according to an example of the present invention is used as the anode of the lithium secondary battery, And can act as a resistor during discharging to enhance the safety of the lithium secondary battery.

In an exemplary embodiment of the present invention, the second positive electrode active material layer includes a positive electrode for a lithium secondary battery including the positive electrode active material layer disposed at an outermost region of the electrode assembly and forming an outermost electrode surface of the electrode assembly, The LiFePO ₄ of the second positive electrode active material layer may have a charge / discharge rate (LiFePO ₄₎ of about ₅ to 10 times the charge / discharge rate The electrode assembly and the battery case may be damaged due to external impact or needle penetration when the electrode assembly is slowly charged in the activation process of the rechargeable lithium secondary battery and after the irreversible charge of the negative electrode is compensated for by the low electrical conductivity. And thus it is possible to improve the safety of the lithium secondary battery including the lithium secondary battery. .

The positive electrode active material layer is not formed on the surface of the electrode assembly facing the outer side of the electrode assembly because the negative electrode does not face the negative electrode. When the battery positive electrode is used as the outermost positive electrode of the electrode assembly, the second positive electrode active material layer containing LiFePO ₄ is also formed on the surface facing the outward direction of the electrode assembly. Therefore, when the LiFePO 4 ₄ can react with the irreversible capacity of the negative electrode first in comparison with the positive electrode active material contained in the first positive electrode active material layer so that the amount of the positive electrode active material contained in the first positive electrode active material layer And can additionally have an electrode capacity as a cathode active material of LiFePO ₄ In addition, the second cathode active material layer including LiFePO ₄ can exert the effect of acting as a resistor (insulator).

In addition, when LiFePO ₄ is coated on the positive electrode active material layer of the positive electrode by mixing LiFePO ₄ with the positive electrode active material to improve the safety of the battery cell, the capacity of the entire battery cell due to the low capacity and low electric conductivity of LiFePO ₄ However, in the present invention, the second positive electrode active material layer containing LiFePO ₄ may be separated from the first positive electrode active material layer containing a conventional positive electrode active material, By forming the current collector on the other surface of the first positive electrode active material layer side, problems due to low capacity and low electrical conductivity of the LiFePO ₄ can be prevented.

Therefore, the positive electrode for a lithium secondary battery of the present invention can be specifically used as an outermost positive electrode located on the outermost electrode surface of the electrode assembly.

The lithium iron phosphate oxide (LiFePO ₄ ) is typically used by forming a coating layer containing a conductive material such as a carbonaceous material on the surface of lithium iron phosphate oxide in particulate form in order to compensate for low electrical conductivity. LiFePO _{4 may} also be optionally formed with a coating layer containing the conductive material on the surface of the particles.

Meanwhile, in one embodiment of the present invention, the LiFePO ₄ may have a particle shape, and a coating layer containing a conductive material may not be formed on the particle surface. In this case, due to the low electrical conductivity of the LiFePO ₄ When the positive electrode for a lithium secondary battery of the present invention is used as the outermost positive electrode of the electrode assembly, the safety of the lithium secondary battery can be further improved.

Figs. 1 and 2 schematically show the shapes of the positive electrode active material layer in the outermost positive electrode of the conventional lithium secondary battery and the positive electrode active material layer in the positive electrode for lithium secondary battery according to the example of the present invention. The drawings are only for illustrating the present invention and the scope of the present invention is not limited thereto. In the drawings of the present invention, the size of each component may be exaggerated for illustrative purposes and may differ from the size actually applied.

1, the outermost positive electrode of a conventional lithium secondary battery has a positive electrode active material layer 20 formed on only one side of the positive electrode current collector 10 and an uncoated portion on which the positive electrode current collector 10 is exposed, . The positive electrode active material layer 20 formed on the outermost positive electrode of the conventional lithium secondary battery includes a conventional positive electrode active material (LCO in FIG. 1) and LiFePO ₄ (LFP in FIG. 1) together.

2, a cathode for a lithium secondary battery according to an exemplary embodiment of the present invention includes a cathode active material layer 210 formed on one surface of a cathode current collector 100, A second cathode active material layer 220 is formed. The positive electrode for a lithium secondary battery according to an exemplary embodiment of the present invention includes a positive electrode active material (LCO in FIG. 2) in a first positive electrode active material layer 210, and LiFePO ₄ in a second positive electrode active material layer 220 LFP).

The thickness of the first cathode active material layer may be a thickness corresponding to the thickness of the cathode active material layer of a conventional anode, and the thickness of the first cathode active material layer may be 10 탆 to 100 탆, Lt; / RTI >

The thickness of the second cathode active material layer may be thinner than the thickness of the first cathode active material layer, and the thickness of the second cathode active material layer may be 1 to 80 탆. Specifically, the thickness of the second cathode active material layer may be 5 탆 to 10 탆 have.

The involved in the charge and discharge after the second thickness of the positive electrode active material layer 1 ㎛ or more cases, compensating for the irreversible capacity of the lithium iron phosphate oxide (LiFePO ₄₎ included in the second positive electrode active material layer cathode suitably first The amount of the positive electrode active material contained in the positive electrode active material layer can be relatively increased and the electrode capacity as the positive electrode active material can be additionally provided and the second positive electrode active material layer can exhibit the effect of acting as a resistor. In addition, when the thickness of the second cathode active material layer is 80 탆 or less, the thickness of the second cathode active material layer becomes excessively thick, thereby preventing the battery cell including the second cathode active material layer from becoming thicker than the capacity.

The lithium iron phosphate oxide (LiFePO ₄ ) may be primary particles or secondary particles formed by gathering the primary particles, and may be specifically primary particles. When the LiFePO ₄ is a primary particle, it may be advantageous to reduce the thickness of the second cathode active material layer to 80 μm or less, specifically 10 μm or less due to a small particle size.

The lithium iron phosphate (LiFePO ₄ ) may have an average particle diameter (D ₅₀ ) of 0.1 μm to 30 μm, and more specifically, an average particle diameter (D ₅₀ ) of 0.5 μm to 15 μm.

When the average particle diameter (D ₅₀ ) of the LiFePO ₄ is 0.1 탆 or more, it is possible to prevent the density of the electrode from being lowered so that the capacity per unit volume can be obtained. When the average particle diameter is 30 탆 or less, the thickness of the second cathode active material layer It can be prevented that it becomes too thick.

In the present invention, the average particle diameter (D ₅₀ ) can be defined as a particle diameter based on 50% of the particle diameter distribution. The average particle diameter is not particularly limited, but can be measured using, for example, a laser diffraction method or a scanning electron microscope (SEM) photograph. In the laser diffraction method, it is generally possible to measure the particle diameter of about several millimeters from the submicron region, and high reproducibility and high degradability can be obtained.

The thickness of the second cathode active material layer may be 1% to 80% of the thickness of the first cathode active material layer, and may be 1% to 10% of the thickness of the first cathode active material layer.

When the thickness of the second cathode active material layer satisfies 1% to 80% based on the thickness of the first cathode active material layer, the second cathode active material layer compensates the irreversible capacity of the cathode and functions as a resistor It is possible to prevent the thickness of the second positive electrode active material layer from affecting the entire thickness of the battery cell.

A positive electrode for a lithium secondary battery according to an exemplary embodiment of the present invention may be used as an outermost positive electrode of the electrode assembly. The positive electrode for the lithium secondary battery may include a positive electrode having a thickness of 20 to 50 mu m, It can include a whole house.

The thickness of the cathode current collector is 20 占 퐉 or more, which is thicker than that of a cathode current collector of about 10 占 퐉 or more, which is typically used for manufacturing a cathode having a cathode active material layer on both surfaces thereof. Lt; RTI ID = 0.0 > cathode < / RTI >

The cathode active material, which is included in each of the first cathode active material layer and the second cathode active material layer, may have a weight ratio of 90:10 to 99: 1, and more specifically, a weight ratio of 95: 5 to 99: 1.

If the amount of the positive electrode active material contained in the second positive electrode active material layer is less than 10 parts by weight based on 90 parts by weight of the positive electrode active material layer included in the first positive electrode active material layer, the irreversible capacity of the negative electrode may be adequately compensated, The total capacity of the secondary battery can be increased. On the other hand, when the amount of the positive electrode active material contained in the second positive electrode active material layer is less than 1 part by weight based on 99 parts by weight of the positive electrode active material contained in the first positive electrode active material layer, And the LiFePO ₄ contained therein is difficult to sufficiently compensate for the irreversible capacity of the cathode.

The positive electrode for a lithium secondary battery according to an exemplary embodiment of the present invention can be manufactured by a conventional method known in the art.

For example, a first positive electrode active material slurry is prepared by mixing and stirring a solvent, a binder, a conductive material, and a dispersant, if necessary, with a lithium transition metal oxide included in the first positive electrode active material layer, A second cathode active material slurry is prepared by mixing and stirring a solvent, and optionally a binder, a conductive material and a dispersing agent in the lithium transition metal oxide, and then coating (coating) the anode active material slurry on one surface and the other surface of the cathode current collector After compression, the anode can be manufactured by drying.

The positive electrode current collector is a metal material having high conductivity and is a metal that can easily adhere to the slurry of the positive electrode active material and is not particularly limited as long as it has high conductivity without causing chemical change in the battery in the voltage range of the battery Nickel, titanium, sintered carbon, or a surface treated with carbon, nickel, titanium or silver on the surface of aluminum or stainless steel can be used. In addition, fine unevenness may be formed on the surface of the current collector to increase the adhesive force of the positive electrode active material. The current collector 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, and may have a thickness of 3 to 500 탆.

Examples of the solvent for forming the anode include organic solvents such as NMP (N-methylpyrrolidone), DMF (dimethylformamide), acetone, and dimethylacetamide, and water. These solvents may be used alone or in combination of two or more Can be mixed and used. The amount of the solvent used is sufficient to dissolve and disperse the cathode active material, the binder and the conductive material in consideration of the coating thickness of the slurry and the production yield.

Examples of the binder include polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, Polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM) Sulfonated EPDM, styrene butadiene rubber (SBR), fluorine rubber, poly acrylic acid, and polymers in which hydrogen is substituted with Li, Na, or Ca, or Various kinds of binder polymers such as various copolymers can be used.

The conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; Carbon black such as acetylene black, Ketjen black, channel black, panes black, lamp black, and thermal black; Conductive fibers such as carbon fiber and metal fiber; Conductive tubes such as carbon nanotubes; Metal powders such as fluorocarbon, aluminum and nickel powder; Conductive whiskers 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 conductive material may be used in an amount of 1 wt% to 20 wt% based on the total weight of the positive electrode slurry.

The dispersing agent may be an aqueous dispersing agent or an organic dispersing agent such as N-methyl-2-pyrrolidone.

The present invention also provides a lithium secondary battery comprising the positive electrode for a lithium secondary battery.

The lithium secondary battery includes an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the electrode assembly comprises a positive electrode for a lithium secondary battery according to the present invention, ) As a positive electrode, and the positive electrode for the lithium secondary battery is located on the outermost electrode side of the electrode assembly.

The outermost positive electrode means the positive electrode positioned on the outermost electrode surface when the outer edge of the electrode assembly is divided into the outermost electrode surface parallel to the surface of the electrode and the outermost surface layer on which the electrode is stacked .

Fig. 3 is a view for explaining the position of the outermost positive electrode. Referring to FIG. 3, the outer edge of the electrode assembly may be divided into an outermost electrode surface and an outermost surface layer, where the outermost anode may be located on the outermost electrode surface.

The outermost positive electrode may be positioned so that the second positive electrode active material layer faces the outer direction of the electrode assembly. The first positive electrode active material layer may be positioned toward the outer direction of the electrode assembly, .

The electrode assembly may further include a positive electrode having a first positive electrode active material layer formed on both surfaces of the positive electrode collector, and a positive electrode having a first positive electrode active material layer formed on both surfaces of the positive electrode collector, As shown in FIG. That is, the anode in which the first cathode active material layer is formed on both surfaces of the cathode current collector can be interposed between the cathodes, that is, between the pair of cathodes, with the separator interposed therebetween. Therefore, the anode in which the first cathode active material layer is formed on both surfaces of the cathode current collector may not be positioned on the outermost electrode surface of the electrode assembly.

The electrode assembly may be a stacked or stacked and folded electrode assembly. For example, when the electrode assembly is a stacked electrode assembly or a stacked and folded electrode assembly, the positive electrode for a lithium secondary battery according to an exemplary embodiment of the present invention is a positive electrode and a negative electrode, And the second cathode active material layer may be positioned toward the outer direction of the electrode assembly.

When the second positive electrode active material layer of the positive electrode according to the present invention is positioned toward the outer direction of the electrode assembly, the second positive electrode active material layer functions as a resistor due to its low electrical conductivity, It is possible to prevent a short circuit between the electrode assembly and the battery case in the circumstance, thereby improving the safety of the lithium secondary battery including the electrode assembly.

The lithium secondary battery may be such that the irreversible capacity at the time of initial charge / discharge of the anode is smaller than the irreversible capacity at the time of initial charging / discharging of the cathode.

Here, the irreversible capacity can be expressed by the following equation (1).

[Equation 1]

Irreversible capacity = 1-discharge capacity / charge capacity

The positive electrode for a lithium secondary battery can be suitably used for a lithium secondary battery having a large irreversible capacity difference between the positive electrode and the negative electrode by using a negative electrode having a relatively large irreversible capacity.

As the negative electrode active material used for the negative electrode according to an embodiment of the present invention, a carbon material, lithium metal, silicon, or tin which lithium ions can be occluded and released can be used. Preferably, carbon materials can be used, and carbon materials such as low-crystalline carbon and highly-crystalline carbon can be used. Examples of the low crystalline carbon include soft carbon and hard carbon. Examples of highly crystalline carbon include natural graphite, kish graphite, pyrolytic carbon, liquid crystal pitch carbon fiber high-temperature sintered carbon such as mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches and petroleum or coal tar pitch derived cokes.

The negative electrode collector is generally made to have a thickness of 3 to 500 mu m. Such an anode current collector is not particularly limited as long as it has conductivity without causing a chemical change in the battery, and may be formed on the surface of copper, stainless steel, aluminum, nickel, titanium, fired carbon, copper or stainless steel Carbon, nickel, titanium, silver or the like, an 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.

As the binder and the conductive material used for the cathode, those which can be commonly used in the art can be used as the anode. The negative electrode may be prepared by mixing and stirring the negative electrode active material and the additives, preparing the negative electrode active material slurry, applying the slurry to the current collector, and compressing the same.

As the separator, a conventional porous polymer film conventionally used as a separator, such as a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene-butene copolymer, an ethylene-hexene copolymer, and an ethylene-methacrylate copolymer The produced porous polymer film can be used alone or in a laminated form, or a nonwoven fabric made of a conventional porous nonwoven fabric such as glass fiber of high melting point, polyethylene terephthalate fiber or the like can be used, but is not limited thereto.

Examples of the organic solvent included in the electrolytic solution include propylene carbonate (PC), ethylene carbonate (EC), ethylene carbonate (EC), and the like. ), Diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane , Vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite, and tetrahydrofuran, or a mixture of two or more thereof. Specifically, ethylene carbonate and propylene carbonate, which are cyclic carbonates in the carbonate-based organic solvent, can be preferably used because they have high permittivity as a high viscosity organic solvent and dissociate the lithium salt in the electrolyte well. The cyclic carbonates include dimethyl carbonate and di Low-dielectric-constant linear carbonates such as ethyl carbonate can be mixed in an appropriate ratio to form an electrolytic solution having a high electrical conductivity.

Alternatively, the electrolytic solution stored in accordance with the present invention may further include an additive such as an overcharge inhibitor or the like contained in an ordinary electrolytic solution.

The external shape of the lithium secondary battery of the present invention is not particularly limited, but may be a cylindrical shape, a square shape, a pouch shape, a coin shape, or the like using a can.

The lithium secondary battery according to the present invention can be used not only in a battery cell used as a power source of a small device but also as a unit cell in a middle- or large-sized battery module including a plurality of battery cells.

Preferable examples of the above medium and large-sized devices include, but are not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and electric power storage systems.

Example

Hereinafter, the present invention will be described in more detail with reference to Examples and Experimental Examples, but the present invention is not limited by these Examples and Experimental Examples. The embodiments according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to enable those skilled in the art to more fully understand the present invention.

Example 1: Preparation of positive electrode for lithium secondary battery

, 94 wt% of lithium cobalt oxide having a composition of LiCoO ₂ , 3 wt% of carbon black as a conductive material, and 3 wt% of polyvinylidene fluoride (PVdF) as a binder were dissolved in N-methyl- (NMP) to prepare a first cathode active material slurry.

Further, 94 wt% of lithium iron phosphate oxide having a composition of LiFePO ₄ , 3 wt% of carbon black as a conductive material, and 3 wt% of polyvinylidene fluoride (PVdF) as a binder were dissolved in N-methyl- -Pyrrolidone (NMP) to prepare a second cathode active material slurry.

The first cathode active material slurry was coated on one side of an aluminum (Al) thin film as a cathode current collector having a thickness of 20 탆 and dried, and then a second cathode active material slurry was applied on the other side of the aluminum thin film and dried.

A roll press was performed to prepare a positive electrode.

Comparative Example 1: Preparation of positive electrode for lithium secondary battery

94 wt% of lithium cobalt oxide having a composition of LiCoO ₂ , 3 wt% of carbon black as a conductive agent, and 3 wt% of PVdF as a binder were added to N-methyl-2-pyrrolidone (NMP) . The positive electrode mixture slurry was coated on one side of an aluminum (Al) thin film as a positive electrode current collector having a thickness of about 20 탆 and dried, followed by roll pressing to prepare a positive electrode.

Example 2: Preparation of lithium secondary battery

≪ Preparation of negative electrode &

A negative electrode mixture slurry was prepared by adding 96 wt% of natural graphite, 1 wt% of Denka black (conductive agent), 2 wt% of SBR (binder), and 1 wt% of CMC (thickener) as an anode active material to water. The prepared negative electrode mixture slurry was coated on one surface of the copper collector to a thickness of 65 탆, dried and rolled, and punched to a predetermined size to prepare a negative electrode.

≪ Production of lithium secondary battery >

A porous polyethylene membrane having a thickness of 17 탆 was interposed between the anode prepared in Example 1 and the cathode thus prepared, and then a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 30:70 Was charged with an electrolyte in which 1 M LiPF ₆ was dissolved to prepare a lithium secondary battery.

Comparative Example 2: Production of lithium secondary battery

A lithium secondary battery was produced in the same manner as in Example 2, except that the positive electrode prepared in Comparative Example 1 was used instead of the positive electrode prepared in Example 1 as the positive electrode in Example 2 above.

Experimental Example 1: Electrochemical Characteristic Evaluation Experiment

The lithium secondary batteries obtained in Example 2 and Comparative Example 2 were charged at a constant current (CC) of 25 at 0.5 C until the voltage reached 4.4 V and then charged at a constant voltage (CV) current was achieved. After that, it was left to stand for 20 minutes and then discharged at a constant current (CC) of 0.2 C until it reached 3.0 V. [

A discharge profile of 0.2 C rate was obtained and is shown in FIG.

4, the positive electrode for a lithium secondary battery according to Example 1 of the present invention and the positive electrode for a lithium secondary battery according to Comparative Example 1 include the same positive electrode active material. However, the positive electrode for a lithium secondary battery according to Example 1 includes LiFePO ₄ (The second positive electrode active material layer) was formed on the other surface of the positive electrode collector. Thus, in the case of the lithium secondary battery of Example 2 using the same as the positive electrode of the lithium secondary battery, the lithium secondary battery of Comparative Example 2 It was confirmed that the discharge capacity was larger.

10: collector 20: cathode active material layer
110: positive current collector plate
120: positive electrode tab portion
210: First cathode active material layer
220: second cathode active material layer

Claims (13)

Anode collector; A first cathode active material layer formed on one surface of the cathode current collector; And a second positive electrode active material layer formed on the other surface of the positive electrode current collector,
Wherein the first cathode active material layer and the second cathode active material layer include different kinds of cathode active materials,
Wherein the second cathode active material layer comprises lithium iron phosphate oxide (LiFePO ₄ , LFP) as a cathode active material.
The method according to claim 1,
The positive electrode active material contained in the first positive electrode active material layer may be at least one selected from the group consisting of lithium-cobalt oxide, lithium-manganese oxide, lithium-nickel-manganese oxide, lithium-manganese-cobalt oxide and lithium- And at least one selected from the group consisting of lithium and lithium.
The method according to claim 1,
Wherein the cathode active material contained in the second cathode active material layer is lithium iron phosphate oxide (LiFePO ₄ ).
The method according to claim 1,
Wherein the lithium iron phosphate oxide (LiFePO ₄ ) is in the form of a particle, and a coating layer containing a conductive material is not formed on the surface of the particle.
The method according to claim 1,
Wherein the thickness of the first cathode active material layer is 10 占 퐉 to 100 占 퐉.
The method according to claim 1,
And the thickness of the second cathode active material layer is 1 占 퐉 to 80 占 퐉.
The method according to claim 1,
Wherein the thickness of the second positive electrode active material layer is 1% to 80% of the thickness of the first positive electrode active material layer,
The method according to claim 1,
Wherein the thickness of the positive electrode collector is 20 占 퐉 to 50 占 퐉.
The method according to claim 1,
Wherein each of the first and second cathode active material layers has a weight ratio of 90:10 to 99: 1.
A lithium secondary battery comprising an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode,
Wherein the electrode assembly comprises the anode according to claim 1 as an outermost shell anode and the second cathode active material layer is located on the outermost electrode surface of the electrode assembly, battery.
11. The method of claim 10,
Wherein the electrode assembly further comprises a positive electrode having a first positive electrode active material layer formed on both surfaces of the positive electrode collector,
Wherein a positive electrode having a first positive electrode active material layer formed on both surfaces of the positive electrode collector is sandwiched between negative electrodes with a separator interposed therebetween.
11. The method of claim 10,
Wherein the electrode assembly is a stacked or stacked and folded electrode assembly.
11. The method of claim 10,
Wherein the irreversible capacity at the time of first charging and discharging of the positive electrode is smaller than the irreversible capacity at the time of first charging and discharging of the negative electrode, wherein the irreversible capacity is represented by the following formula (1).
[Equation 1]
Irreversible capacity = 1-discharge capacity / charge capacity

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