Patent ID: 12218345

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in detail with reference to the drawings. 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 properly define the concept of the terms in order to best describe its invention. The terms and words should be construed as meaning and concept consistent with the technical idea of the present invention.

The present invention relates to an electrode sheet of a high-nickel positive electrode, and the electrode sheet according to the present invention includes a non-coated part and a coated part in which a positive electrode mixture layer is applied on at least one surface of a current collector, and the coated part includes: a first positive electrode mixture layer configured to be formed on a central portion along a longitudinal direction of the electrode sheet and to contain a first positive electrode active material of lithium-nickel-cobalt-manganese oxide; and a second positive electrode mixture layer configured to be formed at one or two ends of the first positive electrode mixture layer and to contain a second positive electrode active material having a nickel content lower than that of the first positive electrode active material, and a rolling density (b) of the second positive electrode mixture layer is smaller than a rolling density (a) of the first positive electrode mixture layer.

Generally, the electrode sheet, which is generated by applying an electrode active material slurry on a current collector sheet, and then performing a drying and rolling process, is wound into a roll and stored in a wound state until punching the electrode for the assembly of a battery. At this time, moisture may be more easily permeated in both ends than in the central portion of the coated part, based on the width direction, and thus the moisture content is higher in both ends than in the central portion.

In NCM-based lithium composite transition metal oxide, based on the fact that when the nickel content among the entire transition metal content is less than 70 mol %, the reaction speed with the moisture is low, a second positive electrode mixture layer containing a second positive electrode active material of a nickel-cobalt-manganese transition metal oxide having a nickel content of less than 70 mol % was arranged on one or both edge portions of the electrode sheet coated part to reduce the moisture permeation, and a first positive electrode mixture layer containing a first positive electrode active material of a nickel-cobalt-manganese transition metal oxide having a nickel content of 70 mol % or more was arranged on the central portion of the electrode sheet coated part to implement a high energy density, and the rolling density of the second positive electrode mixture layer was set to be smaller than the rolling density of the first positive electrode mixture layer, thereby implementing a high energy density while suppressing the moisture reaction.

Referring toFIG.1, the electrode sheet of the present invention has a coated part100where the positive electrode mixture layer has been applied on at least one surface of the current collector and a non-coated part200. Further, the coated part100is formed of a first positive electrode mixture layer110formed on the central portion along the longitudinal direction (y axis) of the electrode sheet, and a second positive electrode mixture layer120formed at both ends of the first positive electrode mixture layer110. Herein, both ends mean both sides based on the width direction (x axis) of the electrode sheet.FIG.1illustrates an embodiment in which the second positive electrode mixture layer is formed at both ends of the first positive electrode mixture layer, but not limited thereto, and the second positive electrode mixture layer may be formed at one end of the first positive electrode mixture layer.

The first positive electrode active material of the present invention is a lithium-nickel-cobalt-manganese transition metal oxide, and the content of nickel preferably corresponds to 70 mol % or more of the entire transition metal for the implementation of a high energy density.

The second positive electrode active material of the present invention is a lithium-nickel-cobalt-manganese transition metal oxide, and the content of nickel preferably corresponds to less than 70 mol %, more preferably 45 to 60 mol %, of the entire transition metal in order to suppress reaction with moisture. Conventionally, the reactivity with moisture was suppressed by setting the nickel content of the second positive electrode active material to 40% or less, but according to the present invention, a high energy density was implemented and at the same time, reactivity with moisture was suppressed by increasing the nickel content in the second positive electrode active material through control of rolling density of the first positive electrode mixture layer and the second positive electrode mixture layer.

The first positive electrode active material and the second positive electrode active material of the present invention are particles (powder). The average particle diameter D50of the first positive electrode active material particles may be 1 to 30 μm, preferably 3 to 20 μm, and more preferably 5 to 15 μm. In the present invention, the average particle diameter D50may be defined as a particle diameter corresponding to 50% of the volume accumulation amount in the particle diameter distribution curve. The average particle diameter D50may be measured using, for example, a laser diffraction method. For example, according the method of measuring the average particle size (D50) of the positive electrode active material, after dispersing particles of the positive electrode active material in a dispersion medium, the ultrasonic waves of about 28 kHz are irradiated with the output of 60 W by introducing a commercially available laser diffraction particle size measuring apparatus (e.g., Microtrac MT 3000), and then the average particle size (D50) corresponding to 50% of the volume accumulation amount in the measuring apparatus may be calculated.

The average particle diameter D50of the second positive electrode active material particles may be 1 to 30 μm, preferably 3 to 20 μm, and more preferably 5 to 15 μm.

The present invention is characterized in that the rolling density (b) of the second positive electrode mixture layer containing the second positive electrode active material is smaller than the rolling density (a) of the first positive electrode mixture layer containing the first positive electrode active material. Herein, the rolling density is defined as the density of the positive electrode after rolling. The rolling density can be calculated by collecting positive electrodes of a specific area and measuring the mass and thickness (volume) of the positive electrodes.

In an embodiment of the present invention, the rolling density of the first positive electrode mixture layer may be 2.5 g/cm3to 4.3 g/cm3. The rolling density of the second positive electrode mixture layer may be 2.3 to 4.1 g/cm3.

The positive electrode active material particles may be deformed or broken by the force applied during the rolling. As particles are broken, the specific surface area of the positive electrode active material particles increases, which also increases the surface area which may react with moisture. As such, in the present invention, in order to suppress the phenomenon that fine powder is generated during the rolling process, when applying the second positive electrode slurry on the second positive electrode mixture layer which is arranged on the edge portion of the coated part having a high possibility to contact moisture, the loading amount of the second positive electrode slurry is set to be relatively smaller than the loading amount of the first positive electrode slurry, or the rolling rate of the second positive electrode mixture layer is set to be smaller than the rolling rate of the first positive electrode mixture layer, and thus the rolling density of the second positive electrode mixture layer is smaller than the rolling density of the first positive electrode mixture layer.

Further, since the increase of the rolling density of the first positive electrode mixture layer arranged in the central portion of the coated part is desirable for the implementation of a high energy density, the rolling density of the first positive electrode mixture layer is preferably greater than the rolling density of the second positive electrode mixture layer.

in one specific example, a ratio (b/a) of a rolling density (b) of the second positive electrode mixture layer to a rolling density (a) of the first positive electrode mixture layer corresponds to 0.5 to 0.9, and preferably 0.6 to 0.8.

In one specific example, a width of the second positive electrode mixture layer may correspond to 1 to 15% of a width of the first positive electrode mixture layer. Referring toFIG.1, the width length W2of the second positive electrode mixture layer may be defined as the sum of the width length W2aof the second positive electrode mixture layer of the left edge of the coated part and the width length W2bof the second positive electrode mixture layer of the right edge of the coated part, and the sum (W2a+W2b) may correspond to 1 to 15% of the width length W1of the first positive electrode mixture layer. When the width length of the second positive electrode mixture layer satisfies the numerical range, it is preferable in implementing a high energy density while inhibiting the reaction of nickel in the positive electrode active material with moisture. The length of the width of the second positive electrode mixture layer may preferably correspond to 3 to 12%, more preferably 5 to 10%, of that of the first positive electrode mixture layer.

Hereinafter, a method of manufacturing an electrode sheet according to the present invention will be described.

A method of manufacturing an electrode sheet according to an embodiment of the present invention includes: an application process of forming a first positive electrode mixture layer and a second positive electrode mixture layer by applying a first positive electrode slurry containing the first positive electrode active material and a second positive electrode slurry containing the second positive electrode active material on a current collector sheet; a drying process; and a rolling process, wherein, in the application process, the second positive electrode slurry is applied in a direction parallel to an application direction of the first positive electrode slurry at one or both ends of the first positive electrode slurry, based on a width direction of the electrode sheet, and wherein in the rolling process, a rolling density of the second positive electrode mixture layer is smaller than a rolling density of the first positive electrode mixture layer.

In one specific example, in order to suppress the increase of the surface area of the positive electrode active material in the second positive electrode mixture layer after rolling, the loading amount per unit area of the second positive electrode mixture layer is preferably designed to be relatively smaller than the loading amount per unit area of the first positive electrode mixture layer during the application process. When assuming that the same pressure is applied to the second positive electrode mixture layer and the first positive electrode mixture layer during rolling, generation of fine powder of the positive electrode active material in the second positive electrode mixture layer may be relatively suppressed, and the first positive electrode mixture layer may be set to have a predetermined energy density by setting the loading amount per unit area of the second positive electrode mixture layer to be smaller than the loading amount per unit area of the first positive electrode mixture layer. Further, the loading amount has been designed in consideration of the N/P ratio in the portion corresponding to the second positive electrode mixture layer. The precipitation of lithium can be effectively prevented by setting the loading amount per unit area of the second positive electrode mixture layer to be smaller than the loading amount per unit area of the first positive electrode mixture layer. At this time, the ratio (B/A) of the loading amount (B) per unit area of the second positive electrode mixture layer to the loading amount (A) per unit area of the first positive electrode mixture layer may be 0.7 to 0.99, more preferably 0.75 to 0.95, and most preferably 0.8 to 0.9.

In another specific example, the rolling rate of the second positive electrode mixture layer may be set to be different from that of the first positive electrode mixture layer in order to suppress the increase in the surface area of the positive electrode active material in the second positive electrode mixture layer after rolling. By setting the rolling rate of the first positive electrode mixture layer to be greater than that of the second positive electrode mixture layer, it is possible to prevent generation of fine powder during the rolling process while allowing to have a predetermined energy density. In the present invention, the rolling rate may be defined as a rolling strength. That is, it can be defined as the size of the force per unit area applied to the positive electrode mixture layer during rolling, and the measurement method is not particularly limited as long as it is to measure the force applied per unit area.

In the method of manufacturing an electrode sheet of the present invention, the first positive electrode active material contained in the first positive electrode mixture layer, the second positive electrode active material contained in the second positive electrode mixture layer, and each rolling density of the first positive electrode mixture layer and the second positive electrode mixture layer have already been described above.

In the application process, a width of the second positive mixture material layer may correspond to 1 to 15% of a width of the first positive electrode mixture layer. When the width length of the second positive electrode mixture layer satisfies the numerical range, it is preferable in implementing a high energy density while inhibiting the reaction of nickel in the positive electrode active material with moisture.

The manufacturing method according to an embodiment of the present invention may be to simultaneously apply the first positive electrode slurry and the second positive electrode slurry, or to apply the first positive electrode slurry and then apply the second positive electrode slurry. According to the above-described method, the productivity may be improved by simultaneously applying the first positive electrode slurry and the second positive electrode slurry.

Hereinafter, a lithium secondary battery of the present invention will be described. A lithium secondary battery of the present invention includes a positive electrode generated by cutting the coated part and the non-coated part of the electrode sheet according to a shape and a size of a unit electrode.

The positive electrode includes a positive electrode current collector and a first positive electrode mixture layer and a second positive electrode mixture layer formed on the positive electrode current collector.

Further, the porosity of the second positive electrode mixture layer may be greater than the porosity of the first positive electrode mixture layer by adjusting the rolling rate of the second positive electrode mixture layer to become smaller than that of the first positive electrode mixture layer. In the present specification, the porosity can be defined as follows:
Porosity=Pore volume per unit mass/(specific volume+pore volume per unit mass)

The measurement of the porosity is not particularly limited, and it may be measured by Hg porosimetry or Brunauer-Emmett-Teller (BET) measuring method which uses BELSORP (BET equipment) of BEL JAPAN Company using adsorbing gas such as nitrogen.

The positive electrode current collector is not particularly limited as long as it has conductivity without causing a chemical change in a battery. Examples of the positive electrode current collector include stainless steel, aluminum, nickel, titanium, sintered carbon or aluminum or stainless steel of which the surface has been treated with carbon, nickel, titanium, silver, or the like. Further, the positive electrode current collector may generally have a thickness of 3 to 500 μm, and it is possible to increase the adhesive force of the positive electrode active material by forming minute irregularities on the surface of the positive electrode current collector. It may be used as various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The first positive electrode mixture layer may include a conductive material and a binder together with the above-described first positive electrode active material, and the second positive electrode mixture layer may also include a conductive material and a binder together with the second positive electrode active material.

The conductive material is used to give conductivity to the electrode, and any conductive material may be used without limitation as long as it has electric conductivity and does not cause a chemical change in the battery. Some examples of the conductive material include: graphite, such as natural graphite and artificial graphite; a carbon-based material such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, summer black, or carbon fiber; metal powders or metal fibers such as cooper, nickel, aluminum or silver; conductive whiskey such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; conducting polymers such as polyphenylene derivatives, and one or a mixture thereof may be used. The conductive material may generally be included in a 1 to 30% by weight based on the total weight of the positive electrode material layer.

The binder improves the adhesive force between the positive electrode active material and the positive electrode current collector and attachment between positive electrode active material particles. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one kind or a mixture of two or more kinds of them may be used. The binder may be included in a 1 to 30% by weight based on the total weight of the positive electrode material layer.

The positive electrode may be manufactured according to a general positive electrode manufacturing method except that the first positive electrode mixture layer is applied on the central portion, and the second positive electrode mixture layer is applied on both edges using a dual slot-die coater by using the positive electrode active material, and the rolling rate of the first positive electrode mixture layer and the second positive electrode mixture layer is different at the time of rolling. Specifically, it may be manufactured by supplying a positive electrode slurry for formation of a positive electrode mixture layer including the positive electrode active material and selectively a binder and a conductive material to a dual slot-die coater, applying the positive electrode slurry on the positive electrode current collector using the dual slot-die coater, and drying and rolling the positive electrode current collector. At this time, the type and the content of the positive electrode material, the binder, and the conductive material are as described above.

The solvent of the positive electrode slurry may be a solvent generally used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one of them alone or a mixture of two or more may be used. The amount of use of the solvent is sufficient if it can dissolve or disperse the positive electrode active material, the conductive material and the binder in consideration of the application thickness and production yield, and later allow a viscosity for showing an excellent thickness uniformity at the time of application for preparation of a positive electrode.

Specifically, the lithium secondary battery includes a positive electrode, a negative electrode facing against the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, and the positive electrode has been described above. Further, the lithium secondary battery may selectively further include a battery case for accommodating the electrode assembly of the positive electrode, the negative electrode and the separator, and a sealing member for sealing the battery case.

The negative electrode current collector is not particularly limited as long as it has high electrical conductivity without causing chemical changes in the battery, and examples thereof include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel of which the surface has been treated with carbon, nickel, titanium, silver or the like, aluminum-cadmium alloy, or the like. Further, the negative electrode current collector may generally have a thickness of 3 to 500 μm, and may strengthen the coupling force of the negative electrode active material by forming minute irregularities on the surface of the current collector as in the positive electrode current collector. It may be used as various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The negative electrode mixture layer includes a negative electrode active material, and selectively a binder, and a conductive material. The negative electrode mixture layer may be manufactured by applying a slurry for forming a negative electrode containing a negative electrode active material and selectively a binder and a conductive material on a negative electrode current collector and drying them, or by casting the slurry for forming the negative electrode on a separate support and then laminating a film, obtained by peeling the slurry from the support on the negative electrode current collector.

A compound, in which a reversible intercalation and deintercalation of lithium is possible, may be used as the negative electrode active material. Specific examples thereof include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber and amorphous carbon; A metal compound capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy or Al alloy; Metal oxides such as SiOx(0<x<2), SnO2, vanadium oxide, and lithium vanadium oxide that can dope and dedope lithium; or a composite containing the above-described metallic compound and a carbonaceous material such as a Si—C composite or a Sn—C composite, and any one or a mixture of two or more of them. In addition, a metal lithium thin film may be used as the negative electrode active material. As the carbon material, both low-crystalline carbon and high-crystalline carbon may be used. Examples of the low-crystalline carbon include soft carbon and hard carbon. Examples of the highly crystalline carbon include amorphous, flaky, scaly, spherical or fibrous natural graphite natural graphite or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches and high-temperature calcined carbon, such as petroleum or coal tar pitch derived cokes.

In addition, the binder and the conductive material may be the same as described in the positive electrode previously.

Meanwhile, in the lithium secondary battery, the separator is used to separate the negative electrode from the positive electrode and provide a moving path of lithium ions, and any separator generally used in a lithium secondary battery may be used without any special limitation. In particular, a separator having a high electrolyte solution moisturization capability and a low resistance to ion movement of electrolyte solution is preferred.

Specifically, porous polymer films, for example, porous polymer films made of polyolefin-based polymers such as ethylene homopolymers, propylene homopolymers, ethylene/butene copolymers, ethylene/hexene copolymers and ethylene/methacrylate copolymers, or a laminate of two or more thereof may be used. Further, a nonwoven fabric made of a conventional porous nonwoven fabric, for example, glass fiber of high melting point, polyethylene terephthalate fiber, or the like may be used. In order to secure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material may be used, and may be optionally used as a single layer or a multilayer structure.

Further, examples of the electrolyte used in the present invention include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel polymer electrolyte, a solid inorganic electrolyte, and a molten inorganic electrolyte which can be used in the production of a lithium secondary battery, but the present invention is not limited to these examples.

Specifically, the electrolyte may include an organic solvent and a lithium salt.

The organic solvent may be any organic solvent that can act as a medium through which ions involved in an electrochemical reaction of a battery can move. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R—CN (R is a straight, branched or cyclic hydrocarbon group of C2 to C20, which may contain a double bond aromatic ring or ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolane. Among them, a carbonate-based solvent is preferable, and a mixture of a cyclic carbonate (for example, ethylene carbonate or propylene carbonate) having a high ionic conductivity and a high dielectric constant which can increase the charge/discharge performance of a battery, and a linear carbonate compound having a low viscosity (for example, ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) is more preferable. In this case, when the cyclic carbonate and the chain carbonate are mixed in a volume ratio of about 1:1 to about 1:9, the performance of the electrolytic solution may be excellent.

The lithium salt can be used without any particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAl04, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2. LiCl, LiI, or LiB(C2O4)2may be used as the lithium salt. The concentration of the lithium salt is preferably within the range of 0.1 to 2.0M. When the concentration of the lithium salt is within the above range, the electrolyte has an appropriate conductivity and viscosity, so that it can exhibit excellent electrolyte performance and the lithium ions can effectively move.

In addition to the electrolyte components, in order to improve the life characteristics of the battery, inhibit the battery capacity reduction, and improve the discharge capacity of the battery, the electrolyte may contain one or more additives, such as a haloalkylene carbonate-based compound such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, and hexa phosphate triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxy ethanol or, aluminum trichloride. Herein, the additive may be included in an amount of 0.1 wt % to 5 wt % based on the total weight of the electrolyte.

Lithium secondary batteries including a positive electrode according to the present invention are useful for portable devices such as mobile phones, laptop computers, and digital cameras and electric cars such as hybrid electric vehicles because the lithium secondary batteries stably show excellent discharge capacity, output characteristics, and capacity retention rate.

According to another embodiment of the present invention, there is provided a battery module including the lithium secondary battery as a unit cell and a battery pack including the same. The battery module or the battery pack may be used as a middle or large size device power source of one or more of a power tool; an electric vehicle including an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV); or a system for power storage.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily carry out the present invention. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

Example 1

(Preparation of First Positive Electrode Slurry for Formation of First Positive Electrode Mixture Layer)

A first positive electrode active material (NCM811, D50is 11 μm) containing the nickel content of 80 mol % based on the total transition metal as the transition metal oxide of lithium-nickel-cobalt-manganese was prepared, which was then mixed with the carbon black conductive material and PVDF binder at the weight ratio of 96.5:1.5:2 in the solvent of N-methyl pyrrolidone, to thereby manufacture a first positive electrode slurry (viscosity: 5000 Pa·s).

(Preparation of Second Positive Electrode Slurry for Formation of Second Positive Electrode Mixture Layer)

A second positive electrode active material (NCM622, D50is 11 μm) containing the nickel content of 60 mol % based on the total transition metal as the transition metal oxide of lithium-nickel-cobalt-manganese was prepared, which was then mixed with the carbon black conductive material and PVDF binder at the weight ratio of 96.5:1.5:2 in the solvent of N-methyl pyrrolidone, to thereby manufacture a second positive electrode slurry (viscosity: 5000 Pa·s).

(Preparation and Storage of Positive Electrode Sheet)

The first positive electrode slurry and the second positive electrode slurry were supplied to the dual slot-die coater, and then the first positive electrode mixture layer and the second positive electrode mixture layer were formed by simultaneously applying the first positive electrode slurry and the second positive electrode slurry on one surface of the aluminum current collector sheet using the coater. At this time, the first positive electrode slurry was set to be applied on the coated portion of the slurry coating portion, and the second positive electrode slurry was set to be applied on both edge regions of the coating portion of the first positive electrode slurry. The total sum of the width length of the coating portion of the second positive electrode slurry was set to correspond to 8% of the width length of the coating portion of the first positive electrode slurry. At this time, the loading amount per unit area of the first positive electrode slurry is 700 mg/25 cm3, and the loading amount per unit area of the second positive electrode slurry is 560 mg/25 cm3. Herein, the ratio (B/A) of the loading amount per unit area (B) of the second positive electrode mixture layer to the loading amount per unit area (A) of the first positive electrode mixture layer was set to be 0.8. Thereafter, a positive electrode sheet including a first positive electrode mixture layer and a second positive electrode mixture layer was dried at 130° C., and rolled. As a result of measuring each rolling density of the first positive electrode mixture layer and the second positive electrode mixture layer after rolling, the rolling density of the first positive electrode mixture layer was 3.8 g/cm3and the rolling density of the second positive electrode mixture layer was 2.47 g/cm3, and the ratio (b/a) of the rolling density (b) of the second positive electrode mixture layer to the rolling density (a) of the first positive electrode mixture layer was 0.65. Herein, the rolling density was calculated by obtaining the positive electrode of each specific area from the first positive electrode mixture layer and the second positive electrode mixture layer of the electrode sheet, and then performing measurement of the mass and volume.

Likewise, the prepared positive electrode sheet was wound in the form of a roll, which was then stored in a room temperature of 25° C. for 7 days, and then a positive electrode tab was notched in the non-coated part of the positive electrode sheet, and the positive electrode sheet was slitted, to thereby manufacture a positive electrode.

(Battery Preparation)

A porous polyolefin separator was interposed between the positive electrode and the lithium metal as the negative electrode, to thereby prepare an electrode assembly, which was then located inside a case, and an electrolyte solution was injected into the case, to thereby manufacture a lithium secondary battery. At this time, lithium hexafluorophosphate (LiPF6) of 1.0M concentration was dissolved in an organic solvent consisting of ethylene carbonate/dimethyl carbonate/ethyl methylcarbonate (EC/DMC/EMC volume ratio=3/4/3) to manufacture the electrolyte solution.

Examples 2 to 3

A battery was manufactured in the same manner as in Example 1 except that the loading amount per unit area of the second positive electrode slurry and the rolling density of the second positive electrode mixture layer in Example 1 were changed as shown in Table 1.

Example 4

A battery was manufactured in the same manner as in the example 1 except that the second positive electrode active material contained in the second positive electrode slurry in Example 1 was changed to one having ⅓ nickel content (NCM 111, D50is 11 μm), and the loading amount per unit area of the second positive electrode slurry and the rolling density of the second positive electrode mixture layer were changed as in Table 1.

Comparative Examples 1 to 2

A battery was manufactured in the same manner as in Example 1 except that the loading amount per unit area of the second positive electrode slurry and the rolling density of the second positive electrode mixture layer in Example 1 were changed as shown in Table 1.

TABLE 1Loading amount per unit areaFirstSecondRatioRolling densitypositivepositive(B/A) ofFirstSecondelectrodeelectrodeloadingpositivepositiveslurryslurryamountselectrodeelectrodeRatio (b/a)loadingloadingper unitmixturemixtureof rollingamount (A)amount (B)arealayer (a)layer (b)densitiesExample 1700 mg/25 cm2560 mg/25 cm20.83.8 g/cm32.47 g/cm30.65Example 2595 mg/25 cm20.852.66 g/cm30.7Example 3630 mg/25 cm20.93.04 g/cm30.8Example 4595 mg/25 cm20.852.85 g/cm30.75Comparative770 mg/25 cm21.14.0 g/cm31.05Example 1Comparative350 mg/25 cm20.51.71 g/cm30.45Example 2

Experimental Example: Evaluation of Capacity and Lifespan Characteristics

The initial (first) charge/discharge was performed using the electrochemical charge-discharge device for the lithium secondary battery of examples 1 to 4 and comparative examples 1 to 2. At this time, the charge was performed by applying electric current with the current density of ⅓ C-rate until reaching the voltage of 4.2V, and the discharge was performed with the same current density until reaching 2.5V. This charge/discharge was carried out a total of 500 times.

The capacity of each battery was measured in the charge/discharge process as described above.

The capacity retention rate of each battery was calculated therefrom as follows, and the result is shown in Table 2.
Capacity retention rate (%)=(capacity in 500 cycles/initial capacity)×100

TABLE 2Initial dischargeCapacity retentioncapacityrate(mAh/g)(%/500cycle)Example 139.488.5Example 240.490.8Example 342.191.5Example 440.989.9Comparative44.285.1Example 1Comparative35.887.2Example 2

Referring to Table 2, the initial discharge capacity in the comparative example 1 was greater than that in the examples 1 to 4, but the capacity retention rate after 500 cycles in the comparative example 1 was smaller than that in the examples 1 to 4. The reason why the initial capacity of the comparative example 1 was great seemed to be because the rolling density of the second positive electrode mixture layer in the comparative example 1 was relatively greater than that in the examples 1 to 4. Further, the capacity retention rate of the comparative example 1 is worst because in the second positive electrode mixture layer of the comparative example 1, a lot of coarse powder is generated during rolling, thereby becoming vulnerable to moisture, and thus as the charge/discharge is repeated, gas generation increases and resistance increases due to the nickel reacting with moisture, thereby deteriorating the lifespan performance.

Further, the initial discharge capacity and capacity retention rate in the comparative example 2 were smaller than those in examples 1 to 4. The reason why the initial capacity of the comparative example 2 was the smallest seemed to be because the rolling density of the second positive electrode mixture layer of the comparative example 2 was the smallest. Further, when the ratio (b/a) of the rolling density (b) of the second positive electrode mixture layer to the rolling density (a) of the first positive electrode mixture layer was too small, it was not desirable in terms of the capacity retention rate.

In the above, the present invention has been described in more detail through the drawings and examples. Accordingly, the embodiments described in the specification and the configurations described in the drawings are only the most preferred embodiments of the present invention, and do not represent all of the technical ideas of the present invention. It is to be understood that there may be various equivalents and variations in place of them at the time of filing the present application.