KR20180089059A - Positive Electrode Active Material for Lithium Secondary Battery Comprising Lithium Cobalt Oxide with Core-Shell Structure and Method of Manufacturing the Same - Google Patents

Abstract

The present invention relates to a positive electrode active material for a lithium secondary battery including a lithium cobalt doping oxide with a core-shell structure. A lithium cobalt doping oxide of the core and a lithium cobalt doping oxide of the shell each independently includes three kinds of dopants. The dopants of the core have metal (M1) of an oxidation number of +2, metal (M2) of an oxidation number of +3, and metal (M3) of an oxidation number +4, wherein each content of the M1, M2, and M3 satisfies the following condition (1) based on molar ratios. In addition, the dopants of the shell have metal (M1′) of an oxidation number of +2, metal (M2′) of an oxidation number of +3, and metal (M3′) of an oxidation number of +4, wherein each content of the M1′, M2′, and M3′ satisfies the following condition (2) based on molar ratios. The condition (1) is represented by 2 <= r (molar ratios) = CM1/(CM2 + CM3) <= 3, and the condition (2) is represented by 0.5 <= r′ (molar ratios) = CM1′/(CM2′ + CM3′) < 2, wherein the CM1 is the content of the M1, the CM2 is the content of the M2, the CM3 is the content of the M3, the CM1′ is the content of the M1′, the CM2′ is the content of the M2′, and the CM3′ is the content of the M3.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive electrode active material for a lithium secondary battery including a lithium-cobalt oxide having a core-shell structure, and a method for manufacturing the same. 2. Description of the Related Art Positive Electrode Active Material for Lithium Secondary Battery Comprising Lithium Cobalt Oxide with Core-

The present invention relates to a cathode active material for a lithium secondary battery comprising a lithium-cobalt oxide having a core-shell structure and a method for producing the same.

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

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

At present, LiCoO ₂ , ternary system (NMC / NCA), LiMnO ₄ , LiFePO ₄ and the like are used as a cathode material of a lithium secondary battery. Among them, LiCoO ₂ has advantages such as high rolling density and so on. Therefore, LiCoO ₂ is widely used up to now, and studies are being conducted to increase the operating voltage to develop a high capacity secondary battery. However, LiCoO ₂ has a low charging / discharging current of about 150 mAh / g and has a problem that its crystal structure is unstable at a voltage of 4.3 V or more, resulting in rapid deterioration of lifetime characteristics and a risk of ignition due to reaction with an electrolyte .

To solve this problem, in the prior art, wherein the LiCoO ₂ doped with a metal such as Al, Ti, Mg, Zr, or a technique of coating a metal such as Al, Ti, Mg, Zr on the surface of LiCoO ₂ also used, but , All of these prior arts disclose only a method of doping the doping element within 50 ppm to 8000 ppm and there is still a problem that the structural stability can not be maintained at a high voltage exceeding 4.5 V. In the case of the coating layer made of the metal, Ions are prevented from moving or the capacity of LiCoO ₂ is reduced, and the performance of the secondary battery is deteriorated rather than the stability and life characteristics at high temperature and high voltage.

Therefore, there is a high demand for the development of a cathode active material based on lithium cobalt oxide, which has high lifetime characteristics and enhanced stability even in a high temperature and high voltage environment.

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

As a result of intensive research and various experiments, the inventors of the present application have found that three kinds of dopants having different oxidation numbers are doped into the lithium-cobalt-doped oxide of the core and the lithium-cobalt-doped oxide of the shell , The structural stability of the crystal structure is improved and the crystal structure is maintained even in the operating voltage range exceeding 4.5 V when the content of the dopants satisfies the conditions (1) and (2) of claim 1, And reached the completion of the present invention.

Accordingly, the cathode active material for a secondary battery according to the present invention is a cathode active material for a lithium secondary battery comprising a core-shell structure of lithium cobalt doped oxide,

The lithium cobalt doped oxide of the core and the lithium cobalt doped oxide of the shell each independently comprise three kinds of dopants;

The dopants of the core are a metal (M1) having an oxidation number of +2, a metal (M2) having a +3 oxidation number and a metal (M3) having a oxidation number of +4, and the contents of M1, Satisfy the following condition (1) as a criterion;

The dopants of the shell are metals M1 ', M2' and M2 'having a +2 oxidation number, a +3 metal oxidation number and a +4 oxidation metal M3' 'Is characterized by satisfying the following condition (2) based on the molar ratio.

2? R (molar ratio) = CM1 / (CM2 + CM3)? 3 (One)

0.5? R '(molar ratio) = CM1' / (CM2 '+ CM3') <2 (2)

Here, CM1 is the content of M1, CM2 is the content of M2, CM3 is the content of M3, CM1 'is the content of M1', CM2 'is the content of M2', and CM3 'is the content of M3.

In general, when lithium cobalt oxide is used as a cathode active material for driving a battery of 4.35 V, 4.4 V and 4.45 V at a high voltage, the lithium cobalt oxide is doped with Al, Ti, Zr, Mg, P, Ca, Or coatings have achieved structural durability and surface stability in high-voltage environments. Specifically, lithium cobalt oxide as the essential characteristics from Li _x CoO ₂ in the x <50 situation Co ³⁺ increases the structural stress due to the ionic radius of, small Co ^{4+ 4+} As oxidized to Co, and continue to When the charge is reduced to about x = 20 by charging, a structural change from the O3 structure to the H1-3 structure occurs in the region of 4.53 V based on the coin half cell voltage. These structural changes are irreversibly generated during charging and discharging, and the efficiency, discharge rate characteristics, and lifetime characteristics are remarkably confirmed at over 4.55V. Of course, in the development of 4.45V cell at 4.2V, the charge and discharge were done without any change from O3 structure (there is a change to mono-clinic phase, but it is reversible and has no effect on lifetime) There arises a problem that structural change to H1-3 must be prevented for driving.

Therefore, the inventors of the present application have made intensive research and have found that, as a lithium-cobalt-doped oxide of a core-shell structure, the lithium-cobalt-doped oxide of the core and the lithium-cobalt-doped oxide of the shell have three kinds of dopants When the content ratio is adjusted to have the above range, the structural stability of the cathode active material particles is improved by suppressing the change of the surface structure under high temperature and high voltage, thereby remarkably improving the lifetime characteristics.

In the specification of the present application, the driving voltage was created on the basis of a half-coin cell.

Here, when the r (molar ratio) is out of the range of the condition (1) or when the r '(molar ratio) is out of the condition (2), the irreversible change of the crystal structure occurs a lot. As a result, the desired effect of the present invention can not be obtained. More specifically, the condition of r (molar ratio) is 0.5? R? 1.5 and r '(molar ratio) is 2? R? 2.5.

The crystal structure of the lithium-cobalt-doped oxide of the core-shell structure satisfying these conditions can be maintained without phase change in the range where the positive electrode potential of the full charge display exceeds 4.5 V on the basis of the Li potential.

First, the lithium cobalt doped oxide of the core may have a composition represented by the following formula (1).

Li _a Co _{1- xy z} M _{1 x} M _{2 y} M _{3 z} O ₂ (1)

In this formula,

M1, M2 and M3 are independently selected from the group consisting of Ti, Mg, Al, Zr, Ba, Ca, Ta, Nb, Mo, Ni, Zn, Si, V and Mn;

0.95? A? 1.05;

0 <x? 0.02, 0 <y? 0.02, 0 <z? 0.02, and 2? X / (y + z)?

Similarly, the lithium cobalt doped oxide of the shell may have a composition of the following formula (2).

Li _b Co _1-stw M ₁ ' _s M 2' _t M ₃ ' _w O ₂ (2)

In this formula,

M 1 ', M 2' and M 3 'are independently an element selected from the group consisting of Ti, Mg, Al, Zr, Ba, Ca, Ta, Nb, Mo, Ni, Zn, Si, ;

0.95? B? 1.05;

0 <s? 0.02, 0 <t? 0.02, 0 <w? 0.02, 0.5? S / (t + w) <2.

That is, as shown in the above chemical formula, the composition formulas of the core and the shell are not different from each other in the kind and doping amount of the doping element, in which three kinds of dopants having different oxidation numbers are doped into the cobalt sites. However, in the content ratio, the core has a composition in which +2 is higher than the shell, and the content of +4 and +3 is higher.

Therefore, as can be seen from the above, the concept of the shell is not a completely independent phase separated from the core, but rather a concept such that the composition and / or the content is changed and the surface is doped. Surface doped configurations also fall within the scope of the present invention.

At this time. The thickness of the shell, which differs in composition from the core, may be from 50 to 2000 nm, and more specifically, from 50 to 200 nm.

If the thickness of the shell is too large beyond the above range, the resistance may be large due to the influence of the shell having a large resistance, and there is a problem that the resistance and the rate characteristics may be negative due to the disconnection of the Li ion migration path. If the thickness is too thin, the high voltage stability due to the shell may not be guaranteed, which is not preferable.

(M1 = M1 ', M2 = M2', M3 = M3 ') and the different (M1? M1'? M2? M2 '? M3? M3 '), and some of them may be the same, for example, M1 = M1', M2 = M2 ', M3 = M3'. This is only one example, and any combination is possible within a range that satisfies the above conditions.

In this case, the dopants are not limited as long as they are metals having different oxidation numbers, and M1 and M1 'are independently selected from the group consisting of +2, Mg, Ca, Ni and Ba; M2 and M2 'are metals having +3 oxidation number and are each independently one element selected from the group consisting of Ti, Al, Ta and Nb; M3 and M3 'are +4 metal oxides independently selected from the group consisting of Ti, Ta, Nb, Mn and Mo and may be different from M2 and M2'.

As described above, when the dopants doped in the core and the shell have oxidation numbers different from those of other elements doped together so that +2, +3, +4 have all the oxidation numbers, the structural stability intended by the present invention is further improved Do.

Specifically, a metal having an oxidation number of +2 can oxidize the doped metal before Co ³⁺ to prevent oxidation to Co ⁴⁺ , thereby preventing structural stress from occurring and improving structural stability, The metal instead of cobalt oxidized by Co ⁴⁺ plays a role in maintaining the structure and improves the surface stability. The +4 oxidation metal suppresses the change of the surface structure under high temperature and high voltage, Thereby preventing deterioration of output characteristics of the secondary battery. By the combination of these dopants, the lithium cobalt doped oxide according to the present invention can maintain structural stability even in the operating range of over 4.5V.

That is, the dopants having different oxidation numbers doped in each of them are partially substituted with the cobalt sites of the lithium cobalt oxide to improve the structural safety according to the respective times and situations.

However, the inclusion of all these dopants in an excessive amount does not result in an increase in the structural stability, and the total content of the doped dopants does not exceed 6% based on the molar ratio in each of the core and the shell, ) And the condition (2) are satisfied, it is possible to exhibit the improved structural stability as described above.

On the other hand, each of the dopants can be uniformly doped throughout the core and shell of the lithium cobalt doped oxide to prevent local structural changes in the grains, although this is not a limitation.

In order to further stabilize the surface structure of the lithium-cobalt-doped oxide, Al ₂ O ₃ having a thickness of 50 nm to 100 nm may be coated on the surface of the lithium-cobalt-doped oxide.

The present invention also provides a method for producing a lithium-cobalt-doped oxide of the core-shell structure of the cathode active material,

(i) preparing a doped cobalt precursor containing three kinds of dopants by coprecipitation; And

(ii) mixing the doping cobalt precursor with a lithium precursor and subjecting the mixture to primary calcination to prepare core particles; And

(iii) preparing a core-shell structure lithium-cobalt doped oxide by mixing the core particles, the cobalt precursor, the lithium precursor, and the three kinds of dopant precursors, and secondary firing to form a shell on the core particle surface;

. &Lt; / RTI &gt;

According to the above production method, the lithium-cobalt-doped oxide of the core-shell structure is prepared by first preparing a doping cobalt precursor containing a dopant by coprecipitation and then firing the doping cobalt precursor and the lithium precursor The dopant is reacted with the lithium precursor in a state where the dopant is uniformly distributed in the cobalt and the byproduct can be reacted with the lithium precursor in a state where the dopant is uniformly distributed in the cobalt. The yield of lithium cobalt doped oxides containing dopants is high.

In step (i), the doped cobalt oxide may be prepared by dissolving the salts including the dopant element and the cobalt salt in water, converting the solution into a basic atmosphere, and coprecipitating it as a doping cobalt precursor. In this case, the salts containing the dopant element may include a salt containing a metal (M1) having an oxidation number of +2, a salt including a metal (M2) having a +3 oxidation number and a metal (M3) , And the mixing ratio of the salts may be determined so as to satisfy the following condition (1).

2? R (molar ratio) = CM1 / (CM2 + CM3)? 3 (One)

Here, CM1 is the content of M1, CM2 is the content of M2, and CM3 is the content of M3.

In addition, the total content of the salts including the dopant element and the content of the cobalt salt can be determined in consideration of the composition of the core, which is the final product.

The salts and cobalt salts containing the dopant element for preparing the doping cobalt precursor of the above process (i) are not limited as long as they are capable of carrying out the coprecipitation process. For example, they may be in the form of carbonate, sulfate or nitrate And in particular, it may be a sulfate.

In the above process (iii), the three kinds of dopant precursors are precursors including a metal (M1 ') having a +2 oxidation number, a precursor including a metal (M2') having a +3 oxidation number, Metal (M3 '), and the mixing ratio of the precursors may be set to satisfy the following condition (2).

0.5? R (molar ratio) = CM1 '/ (CM2' + CM3 ') <2 (2)

Here, CM1 'is the content of M1', CM2 'is the content of M2', and CM3 'is the content of M3'.

In addition, the total content of the cobalt precursor, the lithium precursor, and the three kinds of dopant precursors can be determined in consideration of the composition of the shell.

In order to further coat the metal with the lithium cobalt doped oxide, for example, an oxide such as Al ₂ O ₃ may be dry-mixed or wet-mixed, and the method disclosed in the art is not limited thereto.

In addition, the present invention provides another method of producing a lithium-cobalt-doped oxide of the core-shell structure of the cathode active material,

(i) preparing a core particle by mixing a cobalt precursor, a lithium precursor, and three kinds of dopant precursors and subjecting the precursor to a first calcination; And

(ii) mixing three kinds of dopant precursors independently of the core particle, the cobalt precursor, the lithium precursor, and the above-mentioned process (i) and then performing secondary firing to form a shell on the surface of the core particle, A process for preparing a cobalt doped oxide;

. &Lt; / RTI &gt;

According to the above manufacturing method, a lithium cobalt doped oxide can be prepared by mixing and firing a cobalt precursor, a lithium precursor, and a dopant precursor all at once from the core to the shell.

At this time, in each of the above steps, the mixing ratio of the three kinds of dopant precursors can be determined to satisfy the conditions 1 and 2, and the content of the cobalt precursor and the lithium precursor can be determined in consideration of the final product .

It is also possible to produce lithium cobalt doped oxides of the core-shell structure of the present invention by any of the above methods, wherein the cobalt precursor may be a cobalt oxide, for example, Co ₃ O ₄ , and the dopant precursor Metal oxides or metal salts, and the lithium precursor may also be at least one selected from the group consisting of, but not limited to, LiOH, and Li ₂ CO ₃ .

As described above, the dopants of the three kinds of dopant precursors may be the same or different from each other in the different kinds of dopants, but in order to exhibit more improved structural stability at various voltages at high voltage, in detail, More specifically, +2 may be a metal having an oxidation number, a metal having a +3 oxidation number, and a metal having a +4 oxidation number.

On the other hand, the primary firing to obtain the lithium cobalt doped oxide of the core is performed at a temperature of 850 to 1100 캜 for 8 to 12 hours, and the secondary firing to form the shell is performed at a temperature of 700 to 1100 캜 5 hours to 12 hours.

If the primary firing is carried out at an excessively low temperature beyond the above range or if it is carried out for an excessively short time, the lithium source may not penetrate sufficiently and the cathode active material may not be stably formed. On the other hand, If the temperature is exceeded beyond the above range, or if the lithium cobalt oxide is performed for an excessively long time, the physical and chemical characteristics of the doped lithium cobalt oxide may be changed to cause deterioration in performance, which is not preferable.

Similarly, when the secondary firing is carried out at an excessively low temperature or an excessively short time outside the above range, the precursors constituting the shell may remain in the cathode active material without reacting, On the other hand, when the secondary firing is carried out at an excessively high temperature beyond the above range or for an excessively long time, the dopant component of the shell may be doped into the core portion. In this case, It is difficult to manufacture, which is not preferable.

The resulting lithium-cobalt-doped oxide of the core-shell structure satisfies the above-mentioned conditions (1) and (2) and exerts the intended effect of the present invention.

The present invention also provides a positive electrode, which is produced by applying a slurry containing the positive electrode active material, the conductive material, and the binder to a current collector. If necessary, a filler may be further added to the slurry.

The cathode current collector is generally formed to a thickness of 3 to 500 탆 and is not particularly limited as long as it has high conductivity without causing chemical change in the battery. For example, stainless steel, aluminum, nickel, titanium , And a surface treated with carbon, nickel, titanium or silver on the surface of aluminum or stainless steel can be used. Specifically, aluminum can be used. The current collector may have fine irregularities on the surface thereof to increase the adhesive force of the cathode active material, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric are possible.

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

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

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

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

The present invention also provides a secondary battery comprising the positive electrode, the negative electrode, and an electrolytic solution. The secondary battery may be a lithium secondary battery such as a lithium ion battery or a lithium ion polymer battery having advantages such as high energy density, discharge voltage, and output stability, though the kind thereof is not particularly limited.

Generally, a lithium secondary battery is composed of a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte containing a lithium salt.

Hereinafter, other components of the lithium secondary battery will be described.

The negative electrode is manufactured by applying and drying a negative electrode active material on a negative electrode collector, and if necessary, the above-described components may be selectively included.

The negative electrode collector is generally made to have a thickness of 3 to 500 micrometers. The negative electrode current collector is not particularly limited as long as it has electrical conductivity without causing chemical change in the battery. For example, the negative electrode current collector is formed on the surface of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, 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.

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

The separation membrane is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the membrane is generally 0.01 to 10 micrometers, and the thickness is generally 5 to 30 micrometers. Such separation membranes include, for example, olefinic polymers such as polypropylene, which are chemically resistant and hydrophobic; A sheet or nonwoven fabric made of glass fiber, polyethylene or the like is used. When a solid electrolyte such as a polymer is used as an electrolyte, the solid electrolyte may also serve as a separation membrane.

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

Examples of the non-aqueous organic solvent include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma -Butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile , Nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, Tetrahydrofuran derivatives, ether, methyl pyrophosphate, ethyl propionate and the like can be used.

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

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

The lithium salt is a substance that is soluble in the nonaqueous electrolyte and includes, for example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , LiSbF ₆ , LiAlCl ₄ , CH ₃ SO ₃ Li, (CF ₃ SO ₂ ) ₂ NLi, chloroborane lithium, lower aliphatic carboxylate lithium, lithium tetraphenylborate, and imide.

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

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

As described above, in the cathode active material according to the present invention, three types of dopants having different oxidation numbers are doped in the lithium-cobalt-doped oxide of the core and the lithium-cobalt-doped oxide of the shell, respectively, (1) and (2), the structural stability of the crystal structure is improved and the crystal structure is maintained even in the operating voltage range exceeding 4.5 V, which shows high high voltage characteristics and maintains structural stability even at high temperature, The characteristics are improved.

1 is a graph showing a capacity retention rate when the upper limit voltage is charged to 4.55 V at 25 DEG C according to Experimental Example 1. FIG.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings, but it should be understood that the scope of the present invention is not limited thereto.

&Lt; Preparation Example 1 &

Manufacture of cores

2.88 g of MgO, 0.535 g of Al ₂ O ₃ , 0.865 g of TiO ₂ , 200 g of Co ₃ O ₄ and 81.9 g of Li ₂ CO ₃ were dry mixed and then calcined at 1,050 ° C. for 10 hours in a furnace to obtain Mg, Ti-doped lithium cobalt doped oxide Li _1.02 Co _0.96 Mg _0.03 Al _0.005 Ti _0.005 O ₂ was prepared.

&Lt; Preparation Example 2 &

0.48 g of MgO, 1.07 g of Al ₂ O ₃ , 0.865 g of TiO ₂ , 200 g of Co ₃ O ₄ and 81.9 g of Li ₂ CO ₃ were dry mixed and then calcined at 1,050 ° C. for 10 hours in a furnace to obtain Mg, Ti-doped lithium cobalt doped oxide Li _1.02 Co _0.98 Mg _0.005 Al _0.01 Ti _0.005 O ₂ was prepared.

&Lt; Preparation Example 3 &

Co ₃ (SO _{4) 4,} magnesium sulfate (MgSO _4), aluminum sulfate _{(Al 2 (SO 4) 3} ), sulfate titanium _{(Ti (SO 4) 2)} Co: Mg: Al: Ti = 0.96: 0.03: 0.005: 0.005, and coprecipitated using sodium hydroxide to obtain precursor particles of (Co _0.96 Mg _0.03 Al _0.005 Ti _0.005 ) (OH) ₂ .

41 g of LiOH.H ₂ O was added to 100 g of the precursor such that the molar ratio of the total elements in the particle was Li: M (Co, Mg, Al, Ti) = 1.02: 1 and then mixed with the zirconia balls using a ball mill , And the mixture was subjected to a first calcination at 1010 캜 for 12 hours at a high temperature in an air atmosphere to prepare a lithium cobalt doped oxide Li _1.02 Co _0.96 Mg _0.03 Al _0.005 Ti _0.005 O ₂ doped with Mg, Al, and Ti.

&Lt; Preparation Example 4 &

1.248 g of MgO, 0.107 g of Al ₂ O ₃ , 0.346 g of TiO ₂ , 200 g of Co ₃ O ₄ and 81.9 g of Li ₂ CO ₃ were dry mixed and then calcined at 1,050 ° C. for 10 hours in a furnace to obtain Mg, Ti-doped lithium cobalt doped oxide Li _1.02 Co _0.984 Mg _0.013 Al _0.001 Ti _0.002 O ₂ was prepared.

&Lt; Example 1 >

200 g of the lithium-cobalt-doped oxide prepared in Preparation Example 1, 0.204 g of MgO, 0.267 g of Al ₂ O ₃ , 0.216 g of TiO ₂ , 50 g of Co ₃ O ₄ and 20.475 g of Li ₂ CO ₃ were dry mixed And calcined at 950 ° C. for 10 hours in a furnace to obtain a lithium cobalt doped oxide Li _1.02 Co _0.977 Mg _0.008 Al _0.01 Ti _0.005 O ₂ doped with Mg, Al, and Ti in the form of Li _1.02 Co _0.96 Mg _0.03 Al _0.005 Ti _0.005 O ₂ To prepare a cathode-shell structure cathode active material.

&Lt; Example 2 >

Al ₂ O ₃ having an average particle size of 50 nm was further added to the lithium cobalt doped oxide prepared in Example 1 in an amount of 0.05 wt% based on the total mass of the cathode active material and then subjected to secondary calcination at 570 ° C for 6 hours to obtain an aluminum 500 ppm &lt; / RTI &gt; At this time, the aluminum coating layer was formed to have an average thickness of about 50 nm.

&Lt; Comparative Example 1 &

After 200 g of the lithium cobalt doped oxide prepared in Preparation Example 2 was dry-mixed with 0.48 g of MgO, 0.13 g of Al ₂ O ₃ , 0.216 g of TiO ₂ , 50 g of Co ₃ O ₄ and 20.475 g of Li ₂ CO ₃ , And calcined at 950 ° C. for 10 hours in a furnace to obtain a lithium cobalt doped oxide Li _1.02 Co _0.97 Mg _0.02 Al _0.005 Ti _0.005 O ₂ doped with Mg, Al, and Ti in the form of Li _1.02 Co _0.98 Mg _0.005 Al _0.01 Ti _0.005 O ₂ To prepare a cathode-shell structure cathode active material.

&Lt; Comparative Example 2 &

After 200 g of the lithium cobalt doped oxide prepared in Preparation Example 4 was dry-mixed with 0.048 g of MgO, 0.13 g of Al ₂ O ₃ , 0.195 g of TiO ₂ , 50 g of Co ₃ O ₄ and 20.475 g of Li ₂ CO ₃ And then calcined at 950 ° C. for 10 hours in a furnace to obtain a lithium cobalt doped oxide Li _1.02 Co _0.989 Mg _0.002 Al _0.005 Ti _0.004 O ₂ doped with Mg, Al, and Ti in the form of Li _1.02 Co _0.984 Mg _0.013 Al _0.001 Ti _0.002 O ₂ To prepare a cathode-shell structure cathode active material.

Tables 1 and 2 below show the contents and content ratios of the doping elements in Example 1 and Comparative Examples 1 and 2, respectively.

CM1 CM2 CM3 r Example 1 3 0.5 0.5 3 Comparative Example 1 0.5 One 0.5 0.33 Comparative Example 2 1.3 2 One 4.33

CM1 ' CM2 ' CM3 ' r ' Example 1 0.8 One 0.5 0.53 Comparative Example 1 2 0.5 0.2 2.85 Comparative Example 2 0.2 0.5 0.4 0.22

<Experimental Example 1>

The oxide particles prepared in Example 1 and Comparative Examples 1 and 2 were used as a cathode active material, and PVdF as a binder and natural graphite as a conductive material were used. The positive electrode active material: binder: conductive material was mixed well with NMP so as to have a weight ratio of 96: 2: 2, and then applied to Al foil having a thickness of 20 탆 and dried at 130 캜 to prepare a positive electrode. Lithium foil was used as the cathode, and half-coin cells were prepared using an electrolyte containing 1M of LiPF ₆ in a solvent of EC: DMC: DEC = 1: 2: 1.

The prepared half-coin cells were charged at 4.5 DEG C at an upper limit voltage of 4.5 DEG C at 25 DEG C and 0.5 DEG C, respectively, and discharged again to 1.0 V at a lower limit voltage of 3 V to measure the capacity retention rate of 50 cycles. The results are shown in Fig.

Referring to FIG. 1, the capacity retention ratio of the battery using the cathode active material according to the present invention is 90% or more, while the capacity retention rate of the battery using the cathode active material of the comparative example is about 70% It can be seen that the embodiments satisfying the conditions of the present invention have higher high voltage and high temperature lifetime characteristics because the characteristics are poor and it is expected that the difference accelerates as the cycle progresses.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims (19)

1. A cathode active material for a lithium secondary battery comprising a core-shell structure of lithium cobalt doped oxide,
The lithium cobalt doped oxide of the core and the lithium cobalt doped oxide of the shell each independently comprise three kinds of dopants;
The dopants of the core are a metal (M1) having an oxidation number of +2, a metal (M2) having a +3 oxidation number and a metal (M3) having a oxidation number of +4, and the contents of M1, Satisfy the following condition (1) as a criterion;
The dopants of the shell are metals M1 ', M2' and M2 'having a +2 oxidation number, a +3 metal oxidation number and a +4 oxidation metal M3''Satisfies the following condition (2) based on the molar ratio:
2? R (molar ratio) = CM1 / (CM2 + CM3)? 3 (1)
0.5? R '(molar ratio) = CM1' / (CM2 '+ CM3') <2 (2)
Here, CM1 is the content of M1, CM2 is the content of M2, CM3 is the content of M3, CM1 'is the content of M1', CM2 'is the content of M2', and CM3 'is the content of M3. The positive electrode active material according to claim 1, wherein the lithium-cobalt-doped oxide of the core-shell structure retains its crystal structure without phase change in a range where the positive electrode potential of full-bombardment exceeds 4.5 V on the basis of Li potential. The cathode active material according to claim 1, wherein the lithium-cobalt-doped oxide of the core has a composition represented by the following Formula 1:
Li _a Co _{1- xy z} M _{1 x} M _{2 y} M _{3 z} O ₂ (1)
In this formula,
M1, M2 and M3 are independently selected from the group consisting of Ti, Mg, Al, Zr, Ba, Ca, Ta, Nb, Mo, Ni, Zn, Si, V and Mn;
0.95? A? 1.05;
0 <x? 0.02, 0 <y? 0.02, 0 <z? 0.02, and 2? X / (y + z)? The cathode active material according to claim 1, wherein the lithium cobalt doped oxide of the shell has a composition represented by the following formula (2)
Li _b Co _1-stw M ₁ ' _s M 2' _t M ₃ ' _w O ₂ (2)
In this formula,
M 1 ', M 2' and M 3 'are independently an element selected from the group consisting of Ti, Mg, Al, Zr, Ba, Ca, Ta, Nb, Mo, Ni, Zn, Si, ;
0.95? B? 1.05;
0 <s? 0.02, 0 <t? 0.02, 0 <w? 0.02, 0.5? S / (t + w) <2. The method according to claim 3 or 4, wherein M1 and M1 'are metals having an oxidation number of +2, M2 and M2' are metals having +3 oxidation number, M3 and M3 ' And a positive electrode active material. The method according to claim 1,
Each of M1 and M1 'is independently an element selected from the group consisting of Mg, Ca, Ni and Ba;
M2 and M2 'are each independently an element selected from the group consisting of Ti, Al, Ta and Nb;
M3 and M3 'are independently selected from the group consisting of Ti, Ta, Nb, Mn and Mo, and are different from M2 and M2'. The cathode active material according to claim 1, wherein the thickness of the shell is 50 to 2000 nm. The cathode active material according to claim 1, wherein Al ₂ O ₃ is coated on the surface of the shell in a thickness of 50 nm to 100 nm. 9. A method of producing a lithium-cobalt-doped oxide of a core-shell structure of a cathode active material for a secondary battery according to any one of claims 1 to 8,
(i) preparing a doped cobalt precursor containing three kinds of dopants by coprecipitation; And
(ii) mixing the doping cobalt precursor with a lithium precursor and subjecting the mixture to primary calcination to prepare core particles; And
(iii) preparing a core-shell structure lithium-cobalt doped oxide by mixing the core particles, the cobalt precursor, the lithium precursor, and the three kinds of dopant precursors, and secondary firing to form a shell on the core particle surface;
&Lt; / RTI &gt; The method according to claim 9, wherein in step (i), the salt containing the dopant element and the cobalt salt are dissolved in water, and the solution is converted into a basic atmosphere to prepare a doped cobalt oxide as a doping cobalt precursor by coprecipitation &Lt; / RTI &gt; 11. The method of claim 10, wherein the salts comprising the dopant element each comprise a salt comprising a metal (M1) having an oxidation number of +2, a salt comprising a metal (M2) having a +3 oxidation number, (M3), and the mixing ratio of the salts satisfies the following condition (1): &lt; EMI ID =
2? R (molar ratio) = CM1 / (CM2 + CM3)? 3 (1)
Here, CM1 is the content of M1, CM2 is the content of M2, and CM3 is the content of M3. 11. The process according to claim 10, wherein the salts and cobalt salts comprising the dopant element are in the form of a carbonate, sulphate or nitrate salt. The method of claim 9, wherein the three dopant precursors of step (iii) are selected from the group consisting of a precursor comprising a metal (M1 ') having a +2 oxidation number, a precursor comprising a metal (M2' +4 is a precursor containing an oxidized metal (M3 '), and the mixing ratio of the precursors satisfies the following condition (2):
0.5? R (molar ratio) = CM1 '/ (CM2' + CM3 ') <2 (2)
Here, CM1 'is the content of M1', CM2 'is the content of M2', and CM3 'is the content of M3'. 9. A method of producing a lithium-cobalt-doped oxide of a core-shell structure of a cathode active material for a secondary battery according to any one of claims 1 to 8,
(i) preparing a core particle by mixing a cobalt precursor, a lithium precursor, and three kinds of dopant precursors and subjecting the precursor to a first calcination; And
(ii) mixing three kinds of dopant precursors independently of the core particle, the cobalt precursor, the lithium precursor, and the above-mentioned process (i) and then performing secondary firing to form a shell on the surface of the core particle, A process for preparing a cobalt doped oxide;
&Lt; / RTI &gt; 15. The method of claim 14,
The three kinds of dopant precursors of the above-mentioned process (i) include a salt containing a metal (M1) having an oxidation number of +2, a salt including a metal (M2) having a +3 oxidation number and a metal having a oxidation number of +4 ), Wherein the mixing ratio of the salts satisfies the following condition (1);
The three kinds of dopant precursors of the above process (iii) are a precursor including a metal (M1 ') having a +2 oxidation number, a precursor including a metal (M2') having a +3 oxidation number, (M3 '), and the mixing ratio of the precursors satisfies the following condition (2): &lt; EMI ID =
2? R (molar ratio) = CM1 / (CM2 + CM3)? 3 (1)
0.5? R (molar ratio) = CM1 '/ (CM2' + CM3 ') <2 (2)
Here, CM1 is the content of M1, CM2 is the content of M2, CM3 is the content of M3, CM1 'is the content of M1', CM2 'is the content of M2', and CM3 'is the content of M3'. 10. The method of claim 9 or 14, wherein the cobalt precursor is a cobalt oxide, wherein the dopant precursors dopant metal, metal oxide or metal salt for the lithium precursor is one selected from the group consisting of LiOH, and Li ₂ CO ₃ Or more. The method according to claim 9 or 14, wherein the first firing is performed at a temperature of 850 ° C to 1100 ° C, and the second firing is performed at 700 ° C to 1100 ° C. A positive electrode, which is produced by applying a slurry containing a positive electrode active material, a conductive material and a binder according to any one of claims 1 to 8 to a current collector. A secondary battery comprising a positive electrode according to claim 18.

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