KR101217461B1 - Composite Comprising Manganese-Based Cathode Active Material and Method of Preparing the Same - Google Patents

Abstract

The present invention is a method for producing a positive electrode active material containing a conductive material so that the lithium manganese oxide exhibits excellent charge and discharge characteristics in the range of 2.5 to 3.5 V, lithium precursor and manganese precursor as a raw material for the synthesis of the lithium manganese oxide, and conductive After mixing a precursor of a material or a conductive material, it provides a method for producing a positive electrode active material comprising the step of firing the mixture in a temperature range in which the conductive material or precursor is not oxidized.
The cathode active material according to the present invention is composed of a composite structure in which particles of a conductive material are located between crystal grains of lithium manganese oxide, and thus have excellent capacity and cycle characteristics.

Description

Composite containing manganese-based positive electrode active material and method for manufacturing the same {Composite Comprising Manganese-Based Cathode Active Material and Method of Preparing the Same}

The present invention is a method for producing a positive electrode active material containing a conductive material so that the lithium manganese oxide exhibits excellent charge and discharge characteristics in the 2.5 to 3.5 V range, more specifically, a lithium precursor which is a synthetic raw material of the lithium manganese oxide and After mixing a manganese precursor, a conductive material or a precursor of a conductive material, and a method for producing a positive electrode active material comprising the step of firing the mixture in a temperature range in which the conductive material or precursor is not oxidized.

As technology development and demand for mobile devices are increasing, the demand for secondary batteries as energy sources is rapidly increasing. Among such secondary batteries, lithium secondary batteries having high energy density and voltage, long cycle life and low self- It has been commercialized and widely used.

In addition, as interest in environmental problems increases, researches on electric vehicles and hybrid electric vehicles, which can replace vehicles using fossil fuels such as gasoline and diesel vehicles, which are one of the main causes of air pollution, are being conducted. . Although a nickel-metal hydride secondary battery is mainly used as a power source of such an electric vehicle and a hybrid electric vehicle, research using a lithium secondary battery having a high energy density and a discharge voltage is being actively conducted, and some commercialization stages are in progress.

As a negative electrode active material of such a lithium secondary battery, a carbon material is mainly used, and use of lithium metal, a sulfur compound, etc. is also considered. In addition, lithium-containing cobalt oxide (LiCoO ₂ ) is mainly used as the positive electrode active material, and a lithium-containing manganese oxide such as LiMnO _{2 having a} layered crystal structure and LiMn ₂ O _{4 having a} spinel crystal structure and a lithium- ₂ ) is also considered.

Among the above cathode active materials, LiCoO ₂ is most widely used because it has excellent lifetime characteristics and charge / discharge efficiency, but it has a disadvantage that its structural stability is poor and its cost competitiveness is limited due to resource limitations of cobalt used as a raw material , Electric vehicles and so on.

The LiNiO ₂ cathode active material exhibits a relatively low cost and high discharge capacity, but exhibits a rapid phase transition of the crystal structure in accordance with the volume change accompanying the charging / discharging cycle, and the safety is significantly lowered when exposed to air and moisture There is a problem.

In addition, lithium manganese oxides such as LiMnO ₂ and LiMn ₂ O ₄ have the advantages of excellent thermal safety and low price, but have a problem of small capacity, poor cycle characteristics, and poor high temperature characteristics.

Among such lithium manganese oxides, spinel-based LiMn ₂ O ₄ is known to be difficult to utilize due to its poor cycle and storage characteristics in the 3V region (2.5 to 3.5V). The reason is that due to the phase transition phenomenon of Jahn-Teller distortion, it exists as a single phase of the cubic phase in the 4V region, but in the 3V region, it is a complex phase of the equiaxed and tetragonal phases. phase), elution to the manganese electrolyte, and the like.

When the spinel lithium manganese oxide is used in the 3V region, the actual capacity and the theoretical capacity are generally low and the C-rate characteristics are also low.

Therefore, studies on the utilization of the 3V region of the spinel-based lithium manganese oxide are known to be very difficult to solve, and some of them have been studied in the 3V region due to the formation of tetragonal phase and the effect of S-doping. It is reported that the cycle characteristics have been improved, but the effect is insignificant or the reason for the improvement has not been found.

In addition, some studies have proposed a technique for improving the cycle characteristics of the 3V region by mixing the spinel lithium manganese oxide and carbon by milling, in order to utilize the 3V region, but as the inventors of the present application have confirmed, The above-described technical method is complicated in the manufacturing process and did not exhibit the effect of improving the charge and discharge characteristics to the desired level in the 3V region.

Therefore, there is a high need for a manufacturing technology of spinel-based lithium manganese oxide, which is manufactured by a simple method and exhibits excellent charge and discharge characteristics in the 3V region.

SUMMARY OF THE INVENTION Accordingly, the present invention has been made to solve the above-mentioned problems of the prior art and the technical problems required from the past.

The inventors of the present application, after extensive research and various experiments, in order to improve low electrochemical performance in the 3V region, lithium precursors and manganese precursors, which are synthetic raw materials of specific spinel-based lithium manganese oxides, and conductive materials or conductive materials When the lithium manganese oxide is prepared using a precursor of a material, it was confirmed that a simple method can produce a lithium manganese oxide capable of exhibiting excellent charge and discharge characteristics in the 3V region, thus completing the present invention. .

Therefore, the method for producing a positive electrode active material containing a conductive material so that the lithium manganese oxide exhibits excellent charge and discharge characteristics in the range of 2.5 to 3.5 V according to the present invention, a lithium precursor and a manganese precursor as a raw material for the synthesis of the lithium manganese oxide And, after mixing the conductive material or precursors of the conductive material, firing the mixture in a temperature range in which the conductive material or precursor is not oxidized.

In general, spinel-based lithium manganese oxide exists as a single phase of the cubic phase in the 4V region, and is tetragonal phase in the equiaxed phase due to the Jahn-Teller distortion effect in the 3V region (2.5 to 3.5V). As a result of the phase transition phenomenon, the charge and discharge characteristics are greatly reduced. For example, when the secondary battery is manufactured under the same conditions, the actual capacity in the 4V region is close to the theoretical capacity (theoretical capacity is about 130 mAh / g in both the 3V and 4V regions), but in the 3V region The capacity (90 mAh / g) falls far short of the theoretical capacity.

However, the inventors of the present application, through various experiments and in-depth studies, found that the charge-discharge characteristics are greatly reduced by the above phase transition phenomenon in the 3V region, and furthermore, the conductive material is included. When the spinel-based lithium manganese oxide was prepared to improve conductivity, it was surprisingly found that the actual capacity of the 3V region (2.5 to 3.5V) increased to the theoretical capacity level.

As a method of improving the electrical conductivity of lithium manganese oxide, a method of manufacturing a cathode active material by coating a conductive material on the surface of the spinel-based lithium manganese oxide may be considered. However, this method has a problem in that, apart from the manufacturing process of the spinel-based lithium manganese oxide, it has to go through a complicated process of coating a conductive material.

On the other hand, according to the present invention, in the process of manufacturing the spinel-based lithium manganese oxide, the conductive material or a precursor thereof is added to the precursor mixture of the synthetic raw material and the firing atmosphere is adjusted to a temperature range in which the conductive material and the like are not oxidized Because of the method, the overall manufacturing process is very efficient since it makes full use of conventional reaction conditions without the addition of new processes.

In one preferred example, the lithium manganese oxide may be an oxide having a spinel structure of the composition represented by the following formula (1).

Li_{(1 + x)}Mn_2-yA_yO_4-zA '_z (One)

Wherein 0 ≦ x ≦ 1; 0? Y? 0.2; 0 ≦ z ≦ 0.5;

A is one or two or more selected from the group consisting of Al, Mg, Ni, Co, Fe, Ti, V, Zr and Zn;

A 'is one, two or more selected from the group consisting of F, Cl, Br, I, S, chalcogenide compounds, and nitrogen.

Preferably, when x is greater than 0, the content of Mn may be Mn _{2 -xy} which additionally lacks Mn by an excess (x) of Li.

As described above, when a part of Mn is substituted with another metal element and / or a part of O is substituted with another anion, the structural stability of the lithium manganese oxide can be improved and the lifespan characteristics can be further extended.

The lithium precursor which is a synthetic raw material of the lithium manganese oxide may be preferably lithium hydroxide (LiOH). In the present invention, in order to prevent the conductive material or its precursor from being lost while being oxidized in the process of baking the conductive material or its precursor with a manganese precursor, the firing process is performed at a relatively lower temperature than the conventional one. It was confirmed that satisfactory results were not obtained in the firing conditions when lithium carbonate (Li ₂ CO ₃ ) was used as the lithium precursor. Therefore, it is preferable to use lithium hydroxide (LiOH) as a lithium precursor.

The manganese precursor may be preferably manganese oxy hydroxide (MnOOH). As described above, in consideration of the firing conditions of the present invention, manganese dioxide hydroxide (MnOOH) having a relatively smaller binding force of Mn in the compound than manganese dioxide (MnO ₂ ) is easier to decompose during the firing process, and thus lower temperature Enable firing at However, under optimized reaction conditions, satisfactory results may be obtained using MnO ₂ .

The conductive material is not particularly limited as long as the conductive material is excellent in electrical conductivity and does not disappear in the firing conditions without causing side reactions in the internal environment of the secondary battery, but a carbon-based material having high conductivity is particularly preferable. Preferred examples of such highly conductive carbon-based materials include graphite. In some cases, a conductive polymer having high conductivity is also possible.

In addition, the precursor of the conductive material may be used without particular limitation as long as it is a material that is converted into a conductive material in the process of baking at a relatively low temperature in an atmosphere containing oxygen, for example, an air atmosphere.

If the amount of the conductive material or precursor to be coated is too small, it is difficult to expect the desired effect, on the contrary, if the amount is too large, the amount of the active material may be relatively small, thereby reducing the capacity. Accordingly, the amount of the conductive material or the precursor of the conductive material is preferably 0.5 wt% to 15 wt%, more preferably 3 wt% to 10 wt%, based on the total weight of the mixture.

As described above, the firing temperature is a temperature at which the conductive material or its precursor is not lost by oxidation, preferably in the range of 650 ° C to 800 ° C, and if the firing temperature is too low, the yield of lithium manganese oxide decreases and conversely. Too high can lead to oxidation of conductive materials and the like. This firing temperature is in the range lower than 800 ℃ to 1000 ℃ the conventional firing temperature for the synthesis of spinel-based lithium manganese oxide.

The firing may be preferably performed in an atmosphere having an oxygen concentration of 5 to 30%, and more preferably in an air atmosphere.

The firing time may be, for example, 5 hours to 20 hours, preferably 6 hours to 15 hours.

According to the present invention, since the process of firing at a relatively low temperature, preferably, may further comprise the step of grinding the mixture before the firing step.

The present invention also provides a cathode active material prepared by the above method.

The cathode active material prepared by the method according to the present invention has a composite structure in which particles of a conductive material are located between crystal grains of lithium manganese oxide. The cathode active material having a composite structure exhibits more excellent charge / discharge characteristics in the 3V region than simply coating a conductive material.

That is, in general, electrochemical grinding due to the phase transition phenomenon of Jahn-Teller distortion occurs in the 3V region (for example, amorphorization), leading to deterioration of charge and discharge characteristics. As can be seen by the composite structure in which particles of the conductive material are located between the grains of lithium manganese oxide, it is estimated that this can be prevented.

Therefore, the spinel-based lithium manganese oxide in the present invention can exhibit the desired level of charge and discharge characteristics in the 3V region based on the formation of a composite structure with a conductive material.

In the present invention, the spinel-based lithium manganese oxide may include a cubic phase, a tetragonal phase, or both. That is, it may be a composite structure in which particles of a conductive material are located between grains of lithium manganese oxide on an equiaxed crystal system, a composite structure in which particles of a conductive material are located between crystal grains of lithium manganese oxide on a tetragonal system, and in some cases, May be a composite structure in which particles of a conductive material are located between grains of lithium manganese oxide including both an equiaxed and tetragonal phase.

The present invention also provides a cathode mixture comprising the cathode active material as described above.

In addition to the positive electrode active material, the positive electrode mixture may optionally include a conductive material, a binder, a filler, and the like.

In some cases, in addition to the spinel-based lithium manganese oxide as described above may include other active materials, in this case, the spinel-based lithium manganese oxide is preferably 30 to 99%, more preferably based on the total weight of the positive electrode active material May be from 50 to 95%. Here, the other active materials are various active materials known in the art, such as lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium cobalt-manganese oxide, lithium nickel-manganese oxide, lithium cobalt-nickel oxide, lithium cobalt-manganese- Nickel oxides, oxides in which ellipsoid (s) are substituted or doped, etc. are all included.

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

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

The present invention also provides a positive electrode for a secondary battery in which the positive electrode mixture is coated on a current collector.

The positive electrode for a secondary battery can be produced, for example, by applying a slurry prepared by mixing the positive electrode mixture to a solvent such as NMP, coating the negative electrode collector, followed by drying and rolling.

The cathode current collector is generally made to have a thickness of 3 to 500 mu m. Such a positive electrode collector is not particularly limited as long as it has conductivity without causing chemical change to the battery, and may be formed on the surface of stainless steel, aluminum, nickel, titanium, sintered carbon or aluminum or stainless steel Carbon, nickel, titanium, silver, or the like may 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 present invention also provides a lithium secondary battery composed of the positive electrode, the negative electrode, the separator, and a lithium salt-containing nonaqueous electrolyte. The lithium secondary battery according to the present invention has an advantage of excellent capacity and cycle characteristics even at 2.5 to 3.5V by coating with a conductive material on Li _{(1 + x)} Mn ₂ O ₄ .

The negative electrode is prepared, for example, by coating a negative electrode mixture containing a negative electrode active material on a negative electrode collector and then drying the mixture. The negative electrode mixture may contain the above-described components as required.

The negative electrode collector is generally made to have a thickness of 3 to 500 mu m. Such an anode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and examples thereof 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, 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 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 separator is generally 0.01 to 10 mu m and the thickness is generally 5 to 300 mu m. 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 lithium salt-containing nonaqueous electrolyte solution is composed of an electrolyte solution and a lithium salt. As the electrolyte solution, a nonaqueous organic solvent, an organic solid electrolyte, and an inorganic solid electrolyte may be used.

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 Nonionic organic solvents such as tetrahydrofuran derivatives, ethers, methyl pyrophosphate, ethyl propionate and the like can be used.

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

As the inorganic solid electrolyte, for example, 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, sulfates, and the like of Li, such as Li ₄ SiO ₄ -LiI-LiOH, Li ₃ PO ₄ -Li ₂ S-SiS ₂ , and the like, may be used.

The lithium salt is a material that is readily soluble in the non-aqueous electrolyte, 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 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide have.

For the purpose of improving the charge / discharge characteristics and the flame retardancy, the electrolytic solution is preferably mixed with an organic solvent such as pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, . 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), FEC (fluoro-ethylene carbonate), and the like.

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

Preferred examples of the medium and large devices include, but are not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and the like.

As described above, according to the present invention, a spinel-based lithium manganese oxide capable of exhibiting excellent charge and discharge characteristics in the 3V region (2.5 to 3.5V) can be manufactured by a simple method. May be used in the manufacture of a lithium secondary battery having superior capacity and cycle characteristics than the prior art.

1 is a graph showing the results of the current-voltage change with increasing cycle for the secondary battery of Example 1 in Experimental Example 1;
Figure 2 is a graph showing the current-voltage change results with increasing cycle for the secondary battery of Comparative Example 1 in Experimental Example 1;
3 is a graph showing a change in capacity with increasing cycles for the secondary battery of Example 1 and Comparative Example 1 in Experimental Example 1.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the following Examples are provided to illustrate the present invention, and the scope of the present invention is not limited thereto.

Example 1

Lithium hydroxide (LiOH) and manganese oxy hydroxide (MnOOH) are mixed in stoichiometric ratio, graphite is added to this mixture, and then uniformly ground and calcined at 750 ° C. in an atmosphere having an oxygen concentration of 20%. Thus, a cathode active material based on a composite of spinel lithium manganese oxide (92 wt%) and graphite (8 wt%) was prepared.

The positive electrode mixture was prepared by adding 87 wt% of the positive electrode active material (containing 80 wt% of spinel-based lithium manganese oxide and 7 wt% of graphite) as a conductive material and 7 wt% of dangka black and 6 wt% of PVDF as a binder.

Comparative Example 1

Lithium hydroxide (LiOH) and manganese oxy hydroxide (MnOOH) are mixed in a stoichiometric ratio and uniformly ground (grinding), and then calcined at 750 ℃ in an oxygen concentration of 20%, the cathode of the spinel lithium manganese oxide An active material was prepared.

A positive electrode mixture was prepared by adding 7 wt% graphite, 7 wt% dangka black as a conductive material, and 6 wt% PVDF as a binder to 80 wt% of the prepared positive electrode active material.

[Experimental Example 1]

The positive electrode mixture prepared in Example 1 and Comparative Example 1 was added to NMP to make a slurry, and the resultant was rolled and dried to apply a positive electrode current collector to prepare a secondary battery positive electrode. A coin-type lithium secondary battery was manufactured by interposing a porous polyethylene separator between the positive electrode and the negative electrode based on graphite and injecting a lithium electrolyte.

The secondary battery thus produced was repeatedly charged and discharged at 0.1 C, and the change in capacity according to the cycle was measured, respectively, and the results are shown in FIGS. 1 to 3.

1 shows a change in current-voltage according to an increase in cycle in the region of 3 to 4 V according to the secondary battery of Example 1, and FIG. 2 shows an increase in cycle in the region of 3 to 4 V according to the secondary battery of Comparative Example 1 The change of the current-voltage according to the present invention is disclosed, and FIG. 3 also discloses the change in capacity with increasing cycles for the secondary battery of Example 1 and the secondary battery of Comparative Example 1. FIG.

Referring to these figures, although the graphite content for the spinel-based lithium manganese oxide is the same condition, the secondary battery of Example 1, when compared with the secondary battery of Comparative Example 1, the initial capacity is large, even when the cycle increases The decrease is small and the length of the flat potential section is kept substantially constant, indicating that the charge and discharge performance is very good.

Those skilled in the art to which the present invention pertains will be able to perform various applications and modifications within the scope of the present invention based on the above contents.

Claims (19)

As a method for preparing a positive electrode active material containing a conductive material so that the lithium manganese oxide having a spinel structure of the composition represented by the following formula (1) exhibits charge and discharge characteristics in the 2.5 to 3.5 V range, a synthetic raw material of the lithium manganese oxide After mixing a phosphorus lithium precursor and a manganese precursor, a conductive material or a precursor of a conductive material, and firing the mixture in a temperature range where the conductive material or precursor is not oxidized:

Li (1 + x) Mn _2-y A _y O _4-z A ' _z (1)
In this formula,
0 ≦ x ≦ 1;
0.01 ≦ y ≦ 0.2;
0 ≦ z ≦ 0.5;
A is one or two or more selected from the group consisting of Al, Mg, Ni, Co, Fe, Ti, V, Zr and Zn;
A 'is one, two or more selected from the group consisting of F, Cl, Br, I, S, chalcogenide compounds, and nitrogen. delete 2. The method of claim 1, wherein when x is greater than 0, the content of Mn is Mn _2-xy , in which Mn is additionally deficient by an excess (x) of Li. The method of claim 1, wherein the lithium precursor is lithium hydroxide (LiOH). The method of claim 1, wherein the manganese precursor is manganese oxy hydroxide (MnOOH). The method of claim 1, wherein the conductive material is a carbon-based material. The method of claim 6, wherein the carbon-based material is graphite. The method of claim 1, wherein the amount of the conductive material or precursor added is 0.5 wt% to 15 wt% based on the total weight of the mixture. The method of claim 1, wherein the firing temperature is 650 ° C. to 800 ° C. 6. The method of claim 1, wherein the firing is performed in an atmosphere having an oxygen concentration of 5 to 30%. The method of claim 1, wherein the firing is performed for 5 hours to 20 hours. The method of claim 1, further comprising grinding the mixture prior to the firing step. The cathode active material prepared by the method according to any one of claims 1 and 3 to 12. The cathode active material of claim 13, wherein the cathode active material has a composite structure in which particles of a conductive material are located between crystal grains of lithium manganese oxide. A cathode mixture comprising the cathode active material according to claim 13. The positive electrode mixture for secondary batteries is coated on a current collector according to claim 15. A lithium secondary battery comprising a positive electrode for a secondary battery according to claim 16. 18. The lithium secondary battery of claim 17, wherein the lithium secondary battery is used as a unit cell of a battery module which is a power source of a medium and large device. The lithium secondary battery of claim 18, wherein the medium-to-large device is an electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle.

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