KR101831019B1 - Positive electrode active material for lithium secondary battery, method for preparing the same, and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a core comprising a first lithium cobalt oxide and a shell located on the outer surface of the core and comprising a second lithium cobalt oxide having a molar ratio of Li / Co of less than 1, The positive electrode for a lithium secondary battery further comprising a dopant comprising any one metal selected from the group consisting of W, Zr, Mn, Mg, P, Ni, Al, Sn, V, Cr, Thereby providing an active material.
The cathode active material according to the present invention forms a lithium defect structure in a shell of a lithium-cobalt oxide having a core-shell structure to three-dimensionally convert the two-dimensional lithium migration path, And can exhibit improved capacity and rate characteristics without degrading the capacity. As a result, it is useful as a positive electrode active material of a high voltage battery of 4.4 V or more.

Description

TECHNICAL FIELD The present invention relates to a positive electrode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery including the same. BACKGROUND OF THE INVENTION 1. Field of the Invention [0001]

The present invention relates to a cathode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery comprising the same.

As technology development and demand for mobile devices increase, the demand for secondary batteries as energy sources is rapidly increasing. Among such secondary batteries, lithium secondary batteries having a high energy density and voltage, a long cycle life, and a low self-discharge rate are commercially available and widely used.

As a cathode active material for a lithium secondary battery, a lithium cobalt oxide which is easy to synthesize and has excellent electrochemical performance including life characteristics is mainly used. As handheld devices such as cell phones and tablet PCs are becoming smaller and smaller, there is a demand for batteries to be applied to them, as well as their capacity and energy consumption. In order to increase the energy per unit volume of the battery, the packing density of the active material should be increased or the voltage should be increased. Further, in order to increase the packing density, it is preferable to use a particle having a large particle size. However, since the large particulate active material has a relatively low surface area, the active area in contact with the electrolytic solution is also low. Such a low active area acts kinetically unfavorably, resulting in a relatively low rate characteristic and an initial capacity drop.

Korean Patent Publication No. 2003-0083476 (published on October 30, 2003)

A first technical object of the present invention is to provide a lithium cobalt oxide-based cathode active material having a core-shell structure, which can three-dimensionally convert the two-dimensional lithium migration path in the shell, To improve the capacity and rate characteristics of the battery. The present invention also provides a positive electrode active material for a lithium secondary battery.

A second technical object of the present invention is to provide a manufacturing method for manufacturing the above-mentioned cathode active material.

A third object of the present invention is to provide a positive electrode comprising the positive electrode active material.

A fourth technical object of the present invention is to provide a lithium secondary battery including the positive electrode, a battery module, and a battery pack.

In order to solve the above problems, the present invention provides a lithium secondary battery including a core comprising a first lithium cobalt oxide, and a shell disposed on an outer surface of the core, the shell including a second lithium cobalt oxide having a Li / Co molar ratio of less than 1 And the shell is made of a metal selected from the group consisting of Ti, W, Zr, Mn, Mg, P, Ni, Al, Sn, V, Cr and Mo, The present invention also provides a positive electrode active material for a lithium secondary battery.

The present invention also relates to a method for producing a lithium secondary battery, comprising the step of heat-treating a mixture prepared by mixing a cobalt raw material, a lithium raw material and a dopant raw material, wherein the dopant raw material is selected from the group consisting of a metal which reacts with lithium, Wherein the metal reacting with lithium is selected from the group consisting of Ti, W, Zr, Mn, Mg, P, Ni, Al, Sn, V, Cr, Or a mixture of two or more of them. The present invention also provides a method for producing a cathode active material for a lithium secondary battery.

In addition, the present invention provides a positive electrode comprising the positive electrode active material.

Further, the present invention provides a lithium secondary battery, a battery module, and a battery pack including the positive electrode.

The cathode active material according to the present invention forms a lithium defect structure in a shell of a lithium-cobalt oxide having a core-shell structure to three-dimensionally convert the two-dimensional lithium migration path, And can exhibit improved capacity and rate characteristics without degrading the capacity. As a result, it is useful as a positive electrode active material of a high voltage battery of 4.4 V or more.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments of the invention and, together with the description of the invention, It should not be construed as limited.
1 is a graph showing initial charging and discharging characteristics of a lithium secondary battery including the cathode active materials prepared in Production Example 1 and Comparative Example 1, respectively.
2 is a graph showing the rate characteristics of a lithium secondary battery including the cathode active materials prepared in Production Example 1 and Comparative Example 1, respectively.

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.

A cathode active material for a lithium secondary battery according to an embodiment of the present invention includes:

A core comprising a first lithium cobalt oxide, and

A shell disposed on the outer surface of the core and comprising a second lithium cobalt oxide wherein the molar ratio of Li / Co is less than 1,

The shell further comprises a dopant comprising any one metal selected from the group consisting of Ti, W, Zr, Mn, Mg, P, Ni, Al, Sn, V, Cr and Mo, do.

The dopant is a metal capable of reacting with lithium to form a lithium compound, and has a property of favoring the outside of the active material energetically. Accordingly, according to an embodiment of the present invention, the dopant reacts with the surrounding lithium at the outer surface of the active material to form a lithium compound. As a result, the molar ratio of Li / Co is less than 1, A lithium deficient structure lacking lithium is generated at 0.95 to 0.99.

The lithium defect structure has a spinel-like structure, and is capable of three-dimensional movement of lithium, such as a spinel structure.

Generally, the rate characteristic of the cathode active material depends on the interfacial reaction rate between the anode active material and the electrolytic solution. In the cathode active material according to an embodiment of the present invention, the outer side (or shell) It has a spinel - like structure that is capable of exhibiting improved rate characteristics because of the high rate of lithium movement in the shell. In addition, the resistance in the shell is small, and an improved capacity characteristic can be exhibited. Particularly, when the cathode active material is an opponent, lifetime characteristics of the battery are improved, and the energy density of the battery can be improved by increasing the anode density.

Specifically, the dopant may be any metal selected from the group consisting of Ti, W, Zr, Mn, Mg, P, Ni, Al, Sn, V, Cr, and Mo, Among them, any metal selected from the group consisting of Ti, P, Mn, and Al, which is excellent in the effect of forming a lithium defect structure due to excellent reactivity with lithium, or a metal of two or more thereof may be more preferable have.

Also, in the cathode active material for a lithium secondary battery according to an embodiment of the present invention, it is preferable that the dopant is included in an amount of 50 to 50,000 ppm based on the total weight of the cathode active material. If the content of the dopant is less than 50 ppm, the effect of increasing the migration rate of lithium is low due to a low generation rate of the lithium defect structure. If the dopant content exceeds 50,000 ppm, the amount of lithium compound produced by the reaction with lithium excessively increases, There is a concern.

Also, in the cathode active material for a lithium secondary battery according to an embodiment of the present invention, the dopant may be discontinuously present in the shell, and as a result, the second lithium cobalt oxide of the lithium defect structure is densely distributed around the dopant can do.

In addition, in the cathode active material for a lithium secondary battery according to an embodiment of the present invention, the dopant may be contained in the coating layer in the form of lithium oxide through reaction with lithium.

Specifically, the lithium oxide may be any one selected from the group consisting of Ti, W, Zr, Mn, Mg, P, Ni, Al, Sn, V, Cr and Mo, Containing lithium oxide. More specifically, the lithium oxide may be any one selected from the group consisting of Li ₂ TiO ₃ , Li ₄ Ti ₅ O ₁₂ , Li ₃ PO ₄ , Li ₂ MnO ₃ , LiMn ₂ O ₄ and LiAlO ₂ , And mixtures thereof.

In the cathode active material for a lithium secondary battery according to an embodiment of the present invention, the core may include a first lithium cobalt oxide represented by the following formula (1), and the shell may include a second lithium cobalt oxide represented by the following formula can do:

[Chemical Formula 1]

Li _a CoO ₂

(2)

Li _{1 - b} CoO ₂

(In the above formulas (1) and (2), a and b are atomic fractions of independent oxide forming elements, 1? A? 1.2 and 0 < b?

The first lithium cobalt oxide may have a layered structure, and the second lithium cobalt oxide may have a spinel-like structure.

Also, the cathode active material for a lithium secondary battery according to an embodiment of the present invention may have a structure in which lithium is distributed in a concentration gradient increasing from the surface to the center of the cathode active material.

Specifically, in the particle of lithium cobalt oxide, when 10% or less of the particle radius from the particle surface to the center is referred to as the surface portion, and when the center is within 50% of the particle radius from the particle center to the surface, Li / Co molar ratio is less than 1, or 0.95 to 0.99, and the Li / Co molar ratio in the center portion is 1 or more, or 1 to 1.2. Thus, when lithium is present at a lower concentration on the surface of the particles than the center of grains, lithium can be easily migrated and the rate characteristics of the battery can be improved.

It is preferable that the lithium has a concentration gradient increasing from 1 to 50 atomic% per 10 nm from the surface of the lithium cobalt oxide to the center thereof, and has a concentration gradient increasing from 1 to 20 atomic% per 10 nm May be more preferable. When the lithium concentration is increased by the concentration gradient as described above, a better improvement effect can be exhibited.

On the other hand, the change in the concentration of lithium at the surface portion and the center portion of the particle can be measured according to a conventional method. Specifically, the concentration of each element including lithium in the shell is measured by X-ray photoelectron spectroscopy (XPS), Transmission Electron Microscopy (TEM) or Energy Dispersive X-ray spectroscopy (EDS). The amount of lithium in the lithium cobalt oxide can be measured by an inductively coupled plasma-atomic emission spectrometry Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES). In addition, the shape of the lithium cobalt oxide can be confirmed through the time-of-flight secondary ion mass spectrometry (ToF-SIMS).

Also, the cathode active material for a lithium secondary battery according to an embodiment of the present invention may be distributed in a concentration gradient in which the lithium is increased toward the center of the particle, even in the structure of the core-shell as described above. In this case, the graph of the concentration gradient of lithium in the core and the graph of the concentration gradient of lithium in the shell may have different slope values. As a result, a change in the lithium concentration at the contact surface between the core and the shell An inflection point appearing suddenly appears.

On the other hand, the lithium may be distributed in a uniform concentration in the region of the core portion and the shell, respectively, and the concentration of lithium contained in the core may be higher than the concentration of lithium contained in the shell. In this case, when observing the distribution graph of the lithium in the lithium cobalt oxide particle, a step appears at which the lithium concentration abruptly increases at the contact interface where the core and the shell come in contact with each other.

Further, in the cathode active material for a lithium secondary battery according to an embodiment of the present invention, it is preferable that the shell has a thickness of 1 to 100 nm, or 10 to 50 nm.

The cathode active material for a lithium secondary battery according to an embodiment of the present invention may preferably have an average particle diameter (D ₅₀ ) of 3 to 50 μm in consideration of the specific surface area and the density of the cathode mixture, It may be more preferable to have an average particle diameter of 5 to 20 mu m considering the effect of improving the rate characteristics and the initial capacity characteristics.

In the present invention, the average particle diameter (D ₅₀ ) of the lithium transition metal oxide can be defined as a particle diameter based on 50% of the particle diameter distribution. The average particle diameter (D ₅₀ ) of the cathode active material particles according to an embodiment of the present invention can be measured using, for example, a laser diffraction method. The laser diffraction method generally enables measurement of a particle diameter of several millimeters from a submicron region, resulting in high reproducibility and high degradability. For example, the average particle diameter (D ₅₀ ) of the cathode active material is measured by dispersing lithium transition metal oxide particles in a dispersion medium and introducing the particles into a commercially available laser diffraction particle size analyzer (for example, Microtrac MT 3000) After an ultrasonic wave of about 28 kHz is irradiated at an output of 60 W, the average particle size (D ₅₀ ) at the 50% reference of the particle diameter distribution in the measuring apparatus can be calculated.

The cathode active material having the above structure can be produced by a method including a step of heat-treating a mixture prepared by mixing a cobalt raw material, a lithium raw material and a dopant raw material.

According to another embodiment of the present invention, there is provided a method of manufacturing a cathode active material for a lithium secondary battery.

More specifically, a mixture is prepared by mixing a cobalt raw material, a lithium raw material, and a dopant raw material.

At this time, the cobalt raw material may etc. Specifically, cobalt-containing oxides, hydroxides or oxy-hydroxides, more specifically from _{Co (OH) 2, CoO,} CoOOH, Co (OCOCH 3) 2 and 4H ₂ O, Co (NO ₃ ) ₂ .6H ₂ O, Co (SO ₄ ) ₂ .7H ₂ O, and the like. One or a mixture of two or more of the above compounds may be used.

The lithium source material may specifically be a hydroxide, an oxyhydroxide, a nitrate, a halide, a carbonate, an acetate, a oxalate, a citrate, etc. More specifically, Li ₂ CO ₃ , LiNO ₃ , LiNO ₂ , LiOH, LiOH and _{H 2 O, LiH, LiF,} LiCl, LiBr, LiI, CH 3 COOLi, Li 2 O, Li 2 SO 4, CH 3 COOLi, or Li ₃ C ₆ H ₅ O ₇ or the like. One or a mixture of two or more of the above compounds may be used.

It is preferable that the cobalt raw material and the lithium raw material are mixed in such an amount that the Li / Co molar ratio satisfies the condition of 1 &lt; = Li / Co molar ratio. When mixed in the above-mentioned content range, a core portion containing lithium cobalt oxide of Formula 1 or lithium cobalt oxide having a layered structure is formed. More preferably, the cobalt raw material and the lithium raw material are mixed in such an amount that the Li / Co molar ratio satisfies the condition of 1? Li / Co molar ratio? 1.2.

On the other hand, the dopant raw material may be any one selected from the group consisting of metals capable of reacting with lithium and compounds containing them, or a mixture of two or more thereof.

The metal capable of reacting with lithium may be any one selected from the group consisting of Ti, W, Zr, Mn, Mg, P, Ni, Al, Sn, V, Cr and Mo.

The compound containing a metal capable of reacting with lithium may be a hydroxide, an oxyhydroxide, a nitrate, a halide, a carbonate, an acetate, an oxalate, or a citrate including at least one of the metals described above. The species alone or a mixture of two or more species may be used.

The dopant raw material may be used in an amount such that the dopant is contained in an amount of 50 to 50,000 ppm based on the total weight of the cathode active material.

In addition, the heat treatment for the mixture of the raw materials may be performed at a temperature of 800 to 1100 ° C. If the heat treatment temperature is lower than 800 ° C, the discharge capacity per unit weight, the cycle characteristics, and the operating voltage may decrease due to the residual unreacted starting material. If the temperature exceeds 1100 ° C, There is a fear that the discharge capacity is lowered, the cycle characteristics are lowered, and the operating voltage is lowered.

The heat treatment is preferably carried out in the air or in an oxygen atmosphere, and it is preferable that the heat treatment is performed for 5 to 30 hours since the diffusion reaction between the particles of the mixture can be sufficiently performed.

In the production of the positive electrode active material, it is preferable to appropriately adjust the reaction time and the reaction rate in the heat treatment so that the lithium is distributed in a concentration gradient increasing from the particle surface to the center. Specifically, in order to make a high density state of lithium concentration, it is preferable to lengthen the reaction time and lower the reaction rate, and it is preferable to shorten the reaction time and increase the reaction rate in order to obtain a low density state in which the lithium concentration is low .

The method for producing the cathode active material according to the present invention is a dry method without using a solvent. Generally, the wet process using a solvent in the production of the cathode active material and the surface treatment process is easy to change the pH of the solvent because the metal precursor is dissolved in the solvent and thereby the size of the cathode active material to be finally produced is changed, . In addition, there is a risk that Li ions are eluted from the surface of the positive electrode active material containing Li, and various oxides are formed on the surface as side reaction materials. On the contrary, when the cathode active material is used by the dry method as in the present invention. There is no fear of the occurrence of the above-described problems caused by the use of the solvent, and furthermore, it is more excellent in the production efficiency of the active material and easiness in the process. In addition, since the binder is not used in the surface treatment method by the dry method, there is no fear of occurrence of a side reaction due to the use of the binder.

The cathode active material produced by the above-mentioned production method includes lithium cobalt oxide having a lithium defect structure having a three-dimensional movement path that facilitates lithium migration on the particle surface, that is, the shell, thereby increasing the migration speed of lithium, The rate characteristics and the capacity characteristics can be improved without concern for degradation.

According to another aspect of the present invention, there is provided a positive electrode and a lithium secondary battery including the positive electrode active material.

Specifically, the anode includes a cathode current collector and a cathode active material layer formed on the cathode current collector and including the cathode active material.

The positive electrode current collector is not particularly limited as long as it has conductivity without causing chemical changes in the battery. For example, carbon, nickel, titanium, , Silver or the like may be used.

In addition, the cathode current collector may have a thickness of 3 to 500 탆, and fine unevenness may be formed on the surface of the current collector to increase the adhesive force of the cathode active material. For example, 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.

Meanwhile, the cathode active material layer may include a conductive material and a binder together with the cathode active material. At this time, the cathode active material is the same as described above.

The conductive material is used for imparting conductivity to the electrode. The conductive material is not particularly limited as long as it has electron conductivity without causing chemical change. Specific examples thereof include graphite such as natural graphite and artificial graphite; Carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, summer black and carbon fiber; Metal powder or metal fibers such as copper, nickel, aluminum and silver; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; And polyphenylene derivatives. These may be used alone or in admixture of two or more. The conductive material may be typically contained in an amount of 1 to 30% by weight based on the total weight of the cathode active material layer.

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

The positive electrode having the above structure can be produced according to a conventional positive electrode manufacturing method, except that the positive electrode active material is used. Specifically, the positive electrode active material layer forming material containing the positive electrode active material, the binder and the conductive material may be coated on the positive electrode current collector, followed by drying and rolling.

At this time, the types and contents of the cathode active material, the binder, and the conductive agent are as described above.

Examples of the solvent include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone ) Or water, and either one of them or a mixture of two or more of them may be used. The amount of the solvent to be used is sufficient to dissolve or disperse the cathode active material, the conductive material and the binder in consideration of the coating thickness of the slurry and the yield of the slurry, and then to have a viscosity capable of exhibiting excellent thickness uniformity Do.

Alternatively, the positive electrode may be produced by casting the positive electrode active material composition on a separate support, and then laminating a film obtained by peeling from the support onto a positive electrode collector.

The lithium secondary battery according to another embodiment of the present invention specifically includes an anode, a cathode, a separator interposed between the anode and the cathode, and a non-aqueous electrolyte, and the anode is as described above.

In the lithium secondary battery, the negative electrode may be manufactured by, for example, preparing a composition for forming a negative electrode including a negative electrode active material, a binder, and optionally a conductive material on the negative electrode collector, and then coating the composition on the negative electrode collector.

At this time, as the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specific examples thereof include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber and amorphous carbon; Metal compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys or Al alloys; Or a composite containing a metallic compound and a carbonaceous material, and the like, alone or in a mixture of two or more of them may be used. Also, a metal lithium thin film may be used as the negative electrode active material.

As the carbonaceous 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 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.

In addition, the binder and the conductive agent may be the same as those described above for the anode.

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 Surface-treated with carbon, nickel, titanium, silver or the like, aluminum-cadmium alloy, or the like can be used.

In addition, the negative electrode collector may have a thickness of 3 to 500 탆, and similarly to the positive electrode collector, fine unevenness may be formed on the surface of the collector to enhance the binding force of the negative electrode active material. For example, 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.

Meanwhile, in the lithium secondary battery, the separator is not particularly limited as long as it is used as a separator in a lithium secondary battery. In particular, it is preferable that the separator is low in resistance against ion movement of the electrolyte and excellent in electrolyte wettability. Specifically, porous polymer films such as porous polymer films made of polyolefin-based polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers and ethylene / methacrylate copolymers, 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.

Examples of the electrolyte used in the present invention include an organic-based liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, and a molten inorganic electrolyte that can be used in the production of a lithium secondary battery. no.

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

The organic solvent may be used without limitation as long as it can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate,? -Butyrolactone and? -Caprolactone; Ether solvents such as dibutyl ether or tetrahydrofuran; Ketone solvents such as cyclohexanone; Aromatic hydrocarbon solvents such as benzene and fluorobenzene; Dimethyl carbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), propylene carbonate PC) and the like can be used.

Among these, a carbonate-based solvent is preferable, and a cyclic carbonate (for example, ethylene carbonate or propylene carbonate) having a high ionic conductivity and a high dielectric constant, for example, such as ethylene carbonate or propylene carbonate, For example, ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) is more preferable.

The lithium salt can be used without particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium _{salt, LiPF 6, LiClO 4, LiAsF} 6, LiBF 4, LiSbF 6, LiAl0 4, LiAlCl 4, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiN (C 2 F 5 SO 3) 2 _{, LiN (C 2 F 5 SO} 2) 2, LiN (CF 3 SO 2) 2. LiCl, LiI, or LiB (C ₂ O ₄ ) ₂ may be used. The lithium salt is preferably contained in the electrolyte at a concentration of about 0.6 mol% to 2 mol%.

The electrolytes include, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-propylamine, and the like for the purpose of improving lifetime characteristics of the battery, N, N-substituted imidazolidine, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-ethylhexyl glycols, - methoxyethanol or aluminum trichloride may be further included. The additive may be included in an amount of 0.1 to 5% by weight based on the total weight of the electrolyte.

As described above, the lithium secondary battery including the positive electrode prepared using the composition for forming an anode according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention ratio, Portable devices, and electric vehicles such as hybrid electric vehicles (HEV).

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 battery module.

The battery module or the battery pack may include 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 power storage system, as shown in FIG.

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.

[ Manufacturing example  One: Cathode active material  Produce]

Li ₂ CO ₃ powder and Co ₃ O ₄ powder were mixed in an amount such that the Li / Co molar ratio was 1, and 0.04 weight% of Ti powder was further added to the total weight of the mixture to prepare a mixture. The resultant mixture was heat-treated at 900 DEG C for 20 hours, and the resultant powder was pulverized and filtered to prepare a cathode active material (average particle size: 12 mu m).

[ Manufacturing example  2: Cathode active material  Produce]

A cathode active material was prepared in the same manner as in Preparation Example 1, except that a compound containing P as a dopant raw material was used in an amount of 0.25% by weight based on the total weight of the mixture.

[ Manufacturing example  3: Cathode active material  Produce]

A cathode active material was prepared in the same manner as in Preparation Example 1, except that Mn (OH ₂ ) was used in an amount of 0.3% by weight based on the total weight of the mixture as a dopant raw material.

[ Manufacturing example  4: Cathode active material  Produce]

A cathode active material was prepared in the same manner as in Preparation Example 1, except that Al ₂ O ₃ was used in an amount of 0.05% by weight based on the total weight of the mixture as a dopant raw material.

[ Example  1 to 4: The lithium secondary battery  Produce]

Lithium secondary batteries were prepared using the cathode active materials prepared in Preparation Examples 1 to 4, respectively.

Specifically, the cathode active material, the carbon black conductive material, and the PVdF binder prepared in Preparation Examples 1 to 4 were mixed in a N-methylpyrrolidone solvent at a weight ratio of 90: 5: 5 to prepare a composition for forming an anode : 5000 mPa 占 퐏) was coated on an aluminum current collector, followed by drying and rolling to prepare a positive electrode.

In addition, as a negative electrode active material, artificial graphite, MCMB (mesocarbon microbead), carbon black conductive material and PVdF binder were mixed in a N-methylpyrrolidone solvent in a weight ratio of 85: 10: 5 to prepare a composition for forming an anode, This was applied to the entire copper collector to prepare a negative electrode.

A lithium secondary battery was prepared by preparing an electrode assembly between the positive electrode and the negative electrode prepared as described above with a separator of porous polyethylene interposed therebetween, placing the electrode assembly inside the case, and injecting an electrolyte into the case. The electrolyte solution was prepared by dissolving lithium hexafluorophosphate (LiPF ₆ ) at a concentration of 1.15 M in an organic solvent composed of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (mixed volume ratio of EC / EMC / DEC = 3/4/3) Respectively.

[ Comparative Example  1: Production of lithium secondary battery]

A lithium _secondary battery was produced in the same manner as in Example 1 except that LiCoO ₂ (average particle diameter: 12 탆) was used as the cathode active material.

[ Comparative Example  2: Production of lithium secondary battery]

LiCoO ₂ was impregnated into the Li ₂ TiO ₃ -containing slurry, dried, and heat-treated at 350 ° C. to form a coating layer of Li ₂ TiO ₃ on the surface, and LiCoO ₂ Based anode active material (average particle diameter: 12 占 퐉).

A lithium secondary battery was fabricated in the same manner as in Example 1 except that the negative active material prepared above was used.

[ Experimental Example  1: Evaluation of cathode active material]

The change in the molar ratio of Li / Co according to the depth profile from the surface to the inside of the active material was observed using a transmission electron microscope (TEM) and EDS (EDS) for the cathode active material prepared in Preparation Example 1. The results are shown in Table 1 below.

The depth (nm) from the cathode active material particle surface The molar ratio of Li / Co 20 0.98 40 1.0 60 1.02

As shown in Table 1, it was confirmed that the cathode active material prepared in Preparation Example 1 contained a concentration gradient structure in which the molar ratio of Li / Co gradually increased from the surface of the particles to the center thereof.

In addition, the dopant and content in the coating layer were confirmed through TEM-EDS analysis of the active materials of Production Examples 1 to 4 above. The results are shown in Table 2 below.

The dopant form contained in the coating layer Content (ppm) Production Example 1 Li ₂ TiO ₃ , Li ₄ Ti ₅ O ₁₂ 350 Production Example 2 Li ₃ PO ₄ 2300 Production Example 3 Li ₂ MnO ₃ , LiMn ₂ O ₄ 3000 Production Example 4 LiAlO ₂ 450

[ Experimental Example  2: Cathode active material  Electrochemical property evaluation]

A coin cell (Li metal cathode) was prepared using the cathode active material prepared in Preparation Example 1 and charged and discharged at a temperature of 0.1 C / 0.1 C at room temperature (25 캜). The charge / discharge characteristics and rate Respectively. At this time, LiCoO ₂ was used as a cathode active material for comparison. The results are shown in Fig. 1 and Fig.

1, the lithium _secondary battery comprising the cathode active material of Production Example 1 having a lithium-deficient structure in the shell of the lithium cobalt oxide particles, the lithium _secondary battery comprising the cathode active material of LiCoO ₂ having no lithium defect structure The charge / discharge characteristics of the lithium secondary battery were almost the same as those of the lithium secondary battery of Comparative Example 1. However, in the case of one positive electrode active material having a lithium defect structure on the surface of the lithium cobalt oxide particle, a section in which the profile was bent between 4.05 V and 4.15 V during charging and discharging was observed (see the circled portion in Fig. 1) .

Further, as shown in Fig. 2, the lithium secondary battery comprising the cathode active material of Production Example 1 having a lithium-deficient structure in the shell of the lithium cobalt oxide particles, the lithium secondary battery comprising the cathode active material of LiCoO ₂ not having a lithium defect structure The lithium secondary battery of Comparative Example 1 exhibited improved rate characteristics.

[ Experimental Example  3: Evaluation of battery characteristics of lithium secondary battery]

The properties of the lithium secondary batteries prepared in Examples 1 to 4 and Comparative Examples 1 and 2 were evaluated in the following manner.

Specifically, the lithium secondary batteries prepared in Examples 1 to 4 and Comparative Examples 1 and 2 were subjected to charging and discharging at a temperature of 2 C / 0.1 C in a driving voltage range of 3 to 4.4 V at room temperature (25 캜) Rate characteristics and a charge / discharge cycle of 50 cycles at a high temperature (45 ° C) under the condition of 0.5C / 1C within a drive voltage range of 3 to 4.4 V, and then the cycle capacity The retention rate (capacity retention) was measured and shown in Table 3 below.

Room temperature (25 ℃) Rate characteristic (2C / 0.1C%) 50 cycle capacity retention rate (%) at high temperature (45 DEG C) Comparative Example 1 92.5 95.1 Comparative Example 2 92.2 95.5 Example 1 94.9 96.9 Example 2 95.0 97.5 Example 3 94.2 96.7 Example 4 94.0 98.3

As a result of the tests, the batteries of Examples 1 to 4 including the positive electrode active material having a lithium-deficient structure in the shell of Comparative Example 1 including lithium cobalt oxide having no lithium defect structure as a positive electrode active material, The battery showed improved cycle characteristics as compared with the battery of Comparative Example 2 including the cathode active material having a coating layer of Li ₂ TiO ₃ on its surface.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, And falls within the scope of the invention.

Claims (23)

A core comprising a first lithium cobalt oxide, and
A shell disposed on the outer surface of the core and comprising a second lithium cobalt oxide wherein the molar ratio of Li / Co is less than 1,
The shell further comprises a dopant comprising any one metal selected from the group consisting of Ti, W, Zr, Mn, Mg, P, Ni, Al, Sn, V, Cr and Mo, In addition,
Wherein the first lithium cobalt oxide is a compound represented by the following formula (1)
Wherein the second lithium cobalt oxide is a compound represented by the following formula (2).
[Chemical Formula 1]
Li _a CoO ₂
(2)
Li _1-b CoO ₂
(In the above formulas (1) and (2), a and b are atomic fractions of independent oxide forming elements, 1? A? 1.2 and 0 < b?
The method according to claim 1,
Wherein the dopant is contained in an amount of 50 to 50,000 ppm based on the total weight of the cathode active material.
The method according to claim 1,
The dopant is discontinuously present in the shell,
And the second lithium cobalt oxide is densely distributed around the dopant.
The method according to claim 1,
Wherein the dopant reacts with lithium and is contained in the form of a lithium compound.
5. The method of claim 4,
The lithium compound is _{Li 2 TiO 3, Li 4 Ti} 5 O 12, Li 3 PO 4, Li 2 MnO 3, containing any one or a mixture of two or more of them selected from the group consisting of LiMn ₂ O ₄ and LiAlO ₂ And a cathode active material for a lithium secondary battery.
delete The method according to claim 1,
Wherein the first lithium cobalt oxide of Formula 1 has a layered structure and the second lithium cobalt oxide of Formula 2 has a spinel-like structure.
The method according to claim 1,
Wherein the positive electrode active material has a structure in which lithium is distributed in a concentration gradient increasing from the surface to the center of the positive electrode active material.
The method according to claim 1,
Wherein a concentration gradient of lithium in the core and the shell increases with increasing distance from the center of the cathode active material to the center of the cathode and the shell, respectively.
The method according to claim 1,
Wherein a slope of the concentration gradient of lithium in the core and a gradient slope of concentration gradient of lithium in the shell have different slope values.
The method according to claim 1,
The core comprises a higher concentration of lithium than the shell,
Wherein the lithium is distributed at a uniform concentration in a region of each of the core and the shell.
The method according to claim 1,
Wherein the shell has a thickness of 1 to 100 nm.
The method according to claim 1,
And having an average particle diameter (D ₅₀ ) of 3 to 50 탆.
delete delete delete delete A positive electrode for a lithium secondary battery comprising the positive electrode active material according to claim 1.
19. A lithium secondary battery comprising a positive electrode according to claim 18.
A battery module comprising the lithium secondary battery according to claim 19 as a unit cell.
A battery pack comprising the battery module according to claim 20.
22. The method of claim 21,
A battery pack that is used as a power source for mid- to large-sized devices.
23. The method of claim 22,
Wherein the middle- or large-sized device is selected from the group consisting of an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and a power storage system.

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