KR100412523B1 - A positive active material for a lithium secondary battery and a lithium secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a cathode active material for a lithium secondary battery and a lithium secondary battery comprising the same, a) a group consisting of a metal oxide, a metal hydroxide, a metal oxyhydroxide, a metal oxycarbonate, a metal hydroxycarbonate, and a mixture thereof. A lithium cobalt chalcogenide compound having a surface treatment layer including at least one compound selected from; And b) a cathode active material for a lithium secondary battery comprising a lithium-manganese chalcogenide compound and a lithium secondary battery comprising the same. The cathode active material of the present invention can achieve a cost reduction by replacing a part of the expensive lithium-cobalt chalcogenide compound with a low cost lithium-manganese chalcogenide compound, and the excellent electrochemical properties of the lithium-cobalt chalcogenide compound It is a cathode active material having both high rate characteristics and thermal stability characteristics of a spinel type lithium-manganese chalcogenide compound.

Description

A positive electrode active material for a lithium secondary battery and a lithium secondary battery including the same {A POSITIVE ACTIVE MATERIAL FOR A LITHIUM SECONDARY BATTERY AND A LITHIUM SECONDARY BATTERY COMPRISING THE SAME}

[Industrial use]

The present invention relates to a positive electrode active material for a lithium secondary battery and a lithium secondary battery including the same, and more particularly, to a positive electrode active material for a lithium secondary battery having excellent electrochemical properties and low manufacturing cost, and a lithium secondary battery including the same.

[Prior art]

Lithium secondary batteries, which are commercially available and currently used, correspond to the heart of the digital era, which is rapidly being applied to portable phones, notebook computers, camcorders, and the like, which have an average discharge potential of 3.7V, that is, 4C.

As a cathode material of such a lithium secondary battery, composite oxides such as LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _1-x Co _x O ₂ , and LiMnO ₂ are used. Mn-based electrode materials such as LiMn ₂ O _{4 and} LiMnO ₂ are easy to synthesize, relatively inexpensive, and have a small capacity because they are attractive materials due to less pollution to the environment. LiCoO ₂ exhibits good electrical conductivity, high battery voltage, and excellent electrode characteristics, and is a representative anode material that is currently commercialized and commercialized by SONY, but has a disadvantage of high price. LiNiO ₂ is the lowest of the cathode materials mentioned above, and exhibits the highest discharge capacity of the battery, but has a disadvantage in that it is difficult to synthesize. At present, more than 95% of the batteries in circulation around the world use high-priced LiCoO ₂ , and efforts to replace such LiCoO ₂ are being made.

As an alternative material or method for supplementing the disadvantages of the conventional cathode active material, Korean Patent Publication No. 1999-049248 discloses an active material having an operating voltage of 3 V for lithium (LiMnO _2, Li ₂ Mn ₄ O ₉ , Li _4). A cathode active material comprising active materials (LiCoO _2, LiNiO _2, LiMn ₂ O ₄ ) 82 to 73% having 5 to 15% of Mn ₅ O ₁₂ ) and 4 V to lithium is described. U. S. Patent 5,429, 890 describes a positive electrode composed of a physical mixture of at least one of spinel type Li _x Mn ₂ O ₄ , Li _x NiO _2, and Li _x CoO ₂ (0 <x ≦ 2). U.S. Patent No. 5,789,110 discloses spinel type Li _x Mn ₂ O ₄ (0 <x ≤ 2) and LiFeO ₃ , αLiFe ₅ O ₈ , βLiFe ₅ O ₈ , LiFe ₃ O ₄ , LiFe ₂ O ₃ , Li _{1 + w} Lithium secondary composed of a mixture of at least one of Fe ₅ O ₈ (0 <w <4), Li _y V ₂ O ₅ (0 <y <2), and Li _z FeV ₂ O ₅ (0 <z <2) The battery is described. U.S. Patent 5,561,007 also describes a lithium secondary battery composed of a mixture of spinel type Li _x Mn ₂ O ₄ (0 <x ≤ 2) and αLi _y MnO ₂ (0 ≤ _y <1).

However, a battery manufactured by these methods has not yet reached a satisfactory level in terms of electrochemical properties such as lifetime and manufacturing cost.

An object of the present invention is to provide a cathode active material for a lithium secondary battery that can be excellent in electrochemical properties and thermal stability, as well as cost reduction effect.

Another object of the present invention is to provide a lithium secondary battery including the positive electrode active material.

1 is a view showing an XRD pattern of the electrode plate containing a positive electrode active material of Comparative Example.

2A to 2E are SEM images of the electrode plate including the positive electrode active material of Examples and Comparative Examples.

Figure 3 is a graph showing the 0.1C charge and discharge characteristics of the coin battery containing the positive electrode active material of the comparative example.

4 is a graph showing 0.1C charge / discharge characteristics of a coin battery including the cathode active materials of Examples and Comparative Examples.

5 is a graph showing the C-rate capacity characteristics of the coin battery containing the positive electrode active material of the Examples and Comparative Examples.

FIG. 6 is a diagram showing DSC measurement results of a 4.3V rechargeable electrode plate of a coin battery including the positive electrode active materials of Examples and Comparative Examples. FIG.

In order to achieve the above object, the present invention comprises a) at least one compound selected from the group consisting of metal oxides, metal hydroxides, metal oxyhydroxides, metal oxycarbonates, metal hydroxycarbonates and mixtures thereof. A lithium cobalt-based chalcogenide compound having a surface treatment layer formed thereon; And b) a positive electrode active material prepared by mixing a lithium-manganese chalcogenide compound.

The present invention also provides a surface treatment layer comprising at least one compound selected from the group consisting of a) metal oxides, metal hydroxides, metal oxyhydroxides, metal oxycarbonates, metal hydroxycarbonates and mixtures thereof. Formed lithium-cobalt-based chalcogenide compounds; And b) a lithium secondary battery using a mixture of a lithium-manganese chalcogenide compound as a positive electrode active material.

Hereinafter, the present invention will be described in more detail.

The cathode active material for a lithium secondary battery of the present invention is a) a surface treatment with at least one compound selected from the group consisting of metal oxides, metal hydroxides, metal oxyhydroxides, metal oxycarbonates, metal hydroxycarbonates and mixtures thereof. Layered lithium-cobalt chalcogenide compounds; And b) a lithium-manganese chalcogenide compound is prepared.

Lithium surface-treated with at least one compound selected from the group consisting of a) metal oxides, metal hydroxides, metal oxyhydroxides, metal oxycarbonates, metal hydroxycarbonates and mixtures thereof in the cathode active material of the present invention The cobalt chalcogenide compound and b) the lithium-manganese chalcogenide compound are preferably mixed at a weight ratio of 50 to 99.9: 50 to 0.1.

When the weight ratio of the lithium-manganese chalcogenide compound to the weight ratio of the lithium-cobalt chalcogenide compound and b) the lithium-manganese chalcogenide compound is less than 0.1, there is a disadvantage in that it does not contribute to improving the thermal stability at all. In this case, there is a disadvantage in that the charge and discharge capacity, which is an electrochemical characteristic, is severely deteriorated, resulting in deterioration of the electric charge performance.

As the lithium-cobalt chalcogenide compound used for the surface treatment in the present invention, one or more compounds selected from the group consisting of compounds having the following Chemical Formulas 1 to 4 may be preferably used.

[Formula 1]

Li _x CoM _y A ₂

[Formula 2]

Li _x CoO _2-z A ₂

[Formula 3]

Li _x Co _y Ni _1-y O _2-z A _z

[Formula 4]

Li _x Co _y Ni _1-yz M _z A _a

Where 0.95 ≦ x ≦ 1.1, 0 ≦ y ≦ 0.5, 0 ≦ z ≦ 0.5, 0 <a ≦ 2, M is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, and At least one element selected from the group consisting of rare earth elements, and A is an element selected from the group consisting of O, F, S, and P.)

The compound forming the surface treatment layer of the lithium-cobalt chalcogenide compound is at least one selected from the group consisting of metal oxides, metal hydroxides, metal oxyhydroxides, metal oxycarbonates, metal hydroxycarbonates and mixtures thereof Compound. These compounds may be amorphous or crystalline. As the metal included in the surface treatment layer, any metal can be used as long as it is dissolved in an organic solvent or water. As such a metal, Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr or a mixture thereof can be preferably used.

As the lithium-manganese chalcogenide compound, at least one compound selected from the group consisting of the following Chemical Formulas 5 to 9 may be preferably used.

[Formula 5]

Li _x Mn _1-y M ' _y A ₂

[Formula 6]

Li _x Mn _1-y M ' _y O _2-z A ₂

[Formula 7]

Li _x Mn ₂ O _4-z A _z

[Formula 8]

Li _x Mn _2-y M ' _y A ₄

[Formula 9]

Li _x Mn _y Ni _1-yz M ' _z A _a

Wherein 0.95 ≦ x ≦ 1.1, 0 ≦ y ≦ 0.5, 0 ≦ z ≦ 0.5, 0 <a ≦ 2, M is Ni or Co, and M ′ is Al, Ni, Co, Mn, Cr, Fe , Mg, Sr, V, and at least one element selected from the group consisting of rare earth elements, A is an element selected from the group consisting of O, F, S and P.)

When the surface treatment layer is a metal oxide, the lithium-cobalt chalcogenide compound is coated with a metal organic solution or an aqueous metal solution, and is manufactured through a process of heat-treating the coated compound.

When the surface treatment layer is a metal hydroxide, a metal oxyhydroxide, a metal oxycarbonate, or a metal hydroxycarbonate, the lithium-cobalt chalcogenide compound is coated by a metal organic solution or a metal aqueous solution, followed by drying. do. That is, it is manufactured by omitting the heat treatment step of the metal oxide process.

In the present invention, the metal organic solution or the metal aqueous solution used as the coating solution of the lithium-cobalt chalcogenide compound is prepared as follows. The metal organic solution may be prepared by dissolving a metal, a metal alkoxide, a metal salt, a metal oxide, or the like in an organic solvent, or by refluxing the mixture. The aqueous metal solution may be prepared by dissolving a metal salt or metal oxide in water, or by refluxing the mixture. Suitable forms of metals that are soluble in organic solvents or water may be selected by one of ordinary skill in the art. For example, the coating solution containing boron may be prepared by dissolving HB (OH) ₂ , B ₂ O ₃ , H ₃ BO ₃ , and the like in an organic solvent or water.

Organic solvents usable in the preparation of metal organic solutions in the coating solution include alcohols such as methanol, ethanol or isopropanol, hexane, chloroform, tetrahydrofuran, ether, methylene chloride, acetone and the like. Such organic solvents can also be easily selected in view of the solubility of the metal.

The metal alkoxide solution prepared by dissolving a metal alkoxide such as metal methoxide, metal ethoxide or metal isopropoxide in the alcohol in the alcohol may be preferably used in the present invention. Among such metal alkoxide solutions, Si alkoxide solutions include tetraethylorthosilicate solutions available from Aldrich, or tetraethylorthosilicate solutions prepared by dissolving in silicate ethanol.

Representative examples of metal salts or metal oxides that may be used in preparing the metal aqueous solution in the coating solution include vanadate such as ammonium vanadate (NH ₄ (VO ₃ )), vanadium oxide (V ₂ O ₅ ), and the like.

The amount of the metal, metal alkoxide, metal salt, or metal oxide added in the preparation of the metal organic solution or the metal aqueous solution is preferably 0.1 to 50% by weight, more preferably 1 to 30% by weight relative to the organic solution or the aqueous solution. When the concentration of the metal, metal alkoxide, metal salt or metal oxide is lower than 0.1% by weight, there is no effect of coating the lithium-cobalt chalcogenide compound with a metal organic solution or an aqueous metal solution, and the metal, metal alkoxide, metal salt, or If the concentration of the metal oxide exceeds 50% by weight, the thickness of the coating layer is too thick, which is not preferable.

The lithium cobalt chalcogenide compound is encapsulated with the metal organic solution or the metal aqueous solution thus prepared.

As the coating method, a general coating method such as sputtering, chemical vapor deposition (CVD), dip coating, or the like may be used. Among these coating methods, the simplest method is a dip coating method, which simply adds a positive electrode active material powder to a coating solution to make a slurry, and then removes the solution.

In addition to the above-described method, the coating method may be performed in a slurry state, and then may be subjected to a one-shot process in which a step of removing a solution and a subsequent drying step may be simultaneously performed. The simplicity of the process has economic advantages and allows the surface treatment layer to be formed more uniformly.

Detailed description of the unification process, while the lithium-cobalt chalcogenide compound and the metal organic solution or metal aqueous solution is added to the mixer and stirred, the temperature of the mixer is increased. At this time, it is preferable to inject a blowing gas to increase the drying speed. As the blowing gas, an inert gas such as nitrogen gas or argon gas may be preferably used as a gas having no CO ₂ or moisture. The drying rate can be increased by maintaining a vacuum instead of the blowing gas.

The metal organic solution or aqueous metal solution is coated on the surface of the lithium-cobalt chalcogenide compound, while the excess metal organic solution or aqueous metal solution is evaporated and removed by increasing external temperature and stirring. Therefore, the slurry production process, the solution removal process, and the drying process can be carried out in a unified process in one mixer. For uniform mixing, the lithium cobalt chalcogenide compound and the metal organic solution or the metal aqueous solution may be added to the mixer, followed by premixing for about 10 to 30 minutes.

The temperature increase of the mixer is carried out by circulating the hot water at which the organic solvent or water, which is a solvent, can be evaporated, preferably from 50 to 100 ° C., outside the mixer, and the hot water cooled through the mixer is generally heat exchanged. The temperature is circulated again by increasing the temperature.

The mixer is capable of mixing the lithium cobalt chalcogenide compound and the metal organic solution or the metal aqueous solution well, and increasing the temperature, there is no particular limitation. In addition, it is preferable that the blowing gas can be injected and the vacuum state can be maintained. As a representative example, a planetary mixer may be used.

When carrying out a general coating process, it is preferable to dry the metal organic solution or the powder coated with the metal aqueous solution for 1 to 24 hours at a temperature of less than 300 ℃.

When carrying out the unification coating process, as mentioned above, since a drying process is performed simultaneously with coating, it is not necessary to perform a drying process separately.

According to this drying process, the metal organic solution or the metal aqueous solution layer formed on the surface reacts with moisture in the atmosphere and is converted into metal hydroxide. As a result, a surface treatment layer containing metal hydroxide is formed on the surface of the positive electrode active material.

In this case, the drying process may be changed to allow the surface treatment layer to include metal oxyhydroxide, metal oxycarbonate, or metal hydroxycarbonate. For example, drying in a carbon dioxide gas atmosphere can form a surface treatment layer containing a metal oxycarbonate or a metal hydroxycarbonate. It may also comprise a mixture of two or more selected from metal hydroxides, metal oxyhydroxides, metal oxycarbonates, and metal hydroxycarbonates.

When the lithium-cobalt chalcogenide compound coated by a general coating method or a unified coating method with a metal organic solution or an aqueous metal solution without undergoing the drying process, a lithium-cobalt chalcogenide compound having a metal oxide formed on its surface can be obtained. have. Such a heat treatment step is preferably performed at 300 to 900 ° C. for 1 to 20 hours, more preferably at 300 to 800 ° C. for 1 to 12 hours. After such a heat treatment step, a slow cooling step may be performed in the furnace, or a quenching step of rapidly cooling to room temperature may be performed. When performing a quenching step, it is preferable to perform at a cooling rate of 0.5 ° C / min or more.

In the present invention, the thickness of the surface treatment layer of the lithium-cobalt chalcogenide compound is preferably 1 to 100 nm, more preferably 1 to 50 nm. When the thickness of the surface treatment layer formed on the surface is less than 1 nm, the effect of the surface treatment is insignificant, and when the thickness exceeds 100 nm. The thickness of the coating layer is too thick, which is undesirable.

The content of the metal in the surface treatment layer of the lithium-cobalt chalcogenide compound of the present invention is preferably 2x10 ^-5 to 1% by weight, and more preferably 0.001 to 1% by weight based on the positive electrode active material.

Lithium-cobalt chalcogenide compounds in which metal oxides, metal hydroxides, metal oxy hydroxides, metal oxycarbonates, and metal hydroxycarbonates or mixtures thereof are formed on the surface as in the present invention are conventional lithium-cobalt cal Compared with cogenide compound, internal resistance can be made small, and the electric charge of the discharge potential is reduced to maintain high discharge potential characteristics according to the change of current density (C-rate). When the positive electrode active material having such improved surface characteristics is applied to a battery, better life characteristics and lowering discharge potentials are exhibited, thereby showing power improvement characteristics.

The cathode active material of the present invention is prepared by mixing a lithium-cobalt chalcogenide compound having excellent electrochemical properties and a lithium-manganese chalcogenide compound having excellent thermal stability prepared by the surface treatment process as described above.

The positive electrode active material in which the surface treatment layer of the present invention is formed may be used as a positive electrode active material of a battery as it is, or may be used by sieving to obtain a desired average particle diameter. In the case of not classifying, the same powdery material as the material formed on the surface is not removed and remains in the cathode active material slurry. The inventors have found that the powders of metal oxides, metal hydroxides, metal oxyhydroxides, metal oxycarbonates, metal hydroxycarbonates or mixtures of two or more thereof, which are the same materials as the surface treatment layer, are present in the active material slurry to thermally It was found that the stability can be further improved.

In general, as a representative method of evaluating the safety of lithium secondary batteries, an experiment to penetrate into a nail in a charged state is known as one of the most important safety experiments. At this time, there are a number of factors affecting the safety of the charged battery, in particular, the exothermic reaction by the reaction of the charged positive electrode and the electrolyte impregnated in the electrode plate plays an important role.

For example, when a coin battery containing a LiCoO ₂ active material is charged at a constant potential, LiCoO ₂ has a structure of Li _1-x CoO ₂ , which generates heat generated by differential scanning calorimetry (DSC) measurement of the charged state material. The thermal stability of the active material can be determined based on the result of the temperature, the calorific value and the exothermic curve. Since the Li _1-x CoO ₂ active material in the charged state is unstable, when the temperature inside the battery increases, oxygen, which is bonded with cobalt, is released from the metal. The free oxygen reacts with the electrolyte solution inside the cell, giving the cell the possibility of exploding. Therefore, the oxygen decomposition temperature (exothermic temperature) and the calorific value at this time may be an important factor indicating the stability of the battery.

The lithium-cobalt chalcogenide compound used in the positive electrode active material of the present invention has a lower exothermic temperature than the conventional lithium-cobalt chalcogenide compound, and the calorific value is further reduced to show excellent thermal stability. In the present invention, the thermal stability may be further improved by adding the lithium-manganese chalcogenide compound to the surface-treated lithium-cobalt chalcogenide compound having excellent thermal stability. In addition, it is possible to reduce the manufacturing cost of the battery by replacing the expensive lithium-cobalt chalcogenide compound with a low-cost lithium-manganese chalcogenide compound.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only preferred embodiments of the present invention and the present invention is not limited to the following examples.

(Example 1)

1 g of Al-isopropoxide powder was dispersed in 99 g of ethanol, and the mixture was stirred for about 5 hours to prepare an Al-isopropoxide solution at a concentration of 1% by weight. LiCoO ₂ having an average particle diameter of 10 μm was added to the coating solution, followed by coating using the dip coating method. The Al-isopropoxide-coated powder prepared as described above was heat-treated at 500 ° C. for about 10 hours to form a composite metal oxide layer made of Co-Al-O and Al ₂ O ₃ on the surface of the LiCoO ₂ powder. LiCoO ₂ powders were prepared in which properties and shapes were changed. The LiCoO ₂ powder prepared as described above and the LiMn ₂ O ₄ powder having an average particle diameter of 10 μm were mixed at a weight ratio of 93: 1 to prepare a positive electrode active material, and the ratio of the active material / binder / conductor was 94/3/3. After dissolving in NMP solvent to prepare a slurry.

The slurry was coated to a thickness of 100 μm on an Al-foil having a thickness of 25 μm, and cut into a circle having a diameter of 1.6 cm to prepare a positive electrode plate for a coin battery. This positive electrode plate and a mixed solution (1/1 volume ratio) of ethylene carbonate (EC) and dimethyl carbonate (DMC) in which 1M LiPF ₆ was dissolved were used as an electrolyte, and lithium metal was used as a counter electrode. Coin cells were prepared.

(Example 2)

As a positive electrode active material, the surface-treated LiCoO ₂ powder of Example 1 and a LiMn ₂ O ₄ powder having an average particle diameter of 10 μm were added in the same ratio as in Example 1, except that the weight ratio was 89: 5.

(Example 3)

As a positive electrode active material, the surface-treated LiCoO ₂ powder of Example 1 and a LiMn ₂ O ₄ powder having an average particle diameter of 10 μm were added in the same ratio as in Example 1 except that a weight ratio of 84:10 was added.

(Example 4)

As a positive electrode active material, the surface-treated LiCoO ₂ powder of Example 1 and a LiMn ₂ O ₄ powder having an average particle diameter of 10 μm were added in the same ratio as in Example 1 except that a weight ratio of 74:20 was added.

(Example 5)

As a positive electrode active material, the surface-treated LiCoO ₂ powder of Example 1 and the LiMn ₂ O ₄ powder having an average particle diameter of 10 μm were added in the same ratio as in Example 1 except that a weight ratio of 64:30 was added.

(Example 6)

5 g of Al-isopropoxide powder was dispersed in 95 g of ethanol, except that 5 wt% Al-isopropoxide coating solution was prepared.

(Example 7)

1 g of powder was dispersed in 99 g of ethanol with Al-isopropoxide, and the mixture was stirred for about 5 hours to prepare a 1% by weight Al-isopropoxide solution. LiCoO ₂ having an average particle diameter of 10 μm was added to the coating solution for coating. The powder coated with Al-isopropoxide was dried at 100 ° C. for about 10 hours to prepare a LiCoO ₂ powder having an Al (OH) ₃ layer formed on its surface. It was carried out in the same manner as in Example 1 except that LiCoO ₂ powder and LiMn ₂ O ₄ powder having Al (OH) ₃ formed on the surface thereof were mixed with a positive electrode active material at a weight ratio of 93: 1.

(Example 8)

The coating solution was carried out in the same manner as in Example 7, except that 5% by weight of Al-isopropoxide solution was used.

(Comparative Example 1)

Except for using the surface-treated LiCoO ₂ powder prepared in Example 1 as the positive electrode active material was carried out in the same manner as in Example 1.

(Comparative Example 2)

, Which it was carried out in the same manner as in Example 1, except that the average particle diameter is used only 10㎛ of LiCoO ₂ powder as a positive electrode active material.

(Comparative Example 3)

The same procedure as in Example 1 was carried out except that a mixture having a ratio of LiCoO ₂ powder having an average particle diameter of 10 μm and a LiMn ₂ O ₄ powder having an average particle diameter of 93: 1 was used as the positive electrode active material.

(Comparative Example 4)

The average particle diameter of 10㎛ of LiCoO ₂ powder with an average particle diameter of 10㎛ of LiMn ₂ O ₄ powder ratio is 89: 5, except for using a mixture as a positive electrode active material, which was carried out in the same manner as in Example 1.

(Comparative Example 5)

The same procedure as in Example 1 was carried out except that a mixture having a ratio of 84:10 of LiCoO ₂ powder having an average particle diameter of 10 μm and a LiMn ₂ O ₄ powder having an average particle size of 10 μm was used as the positive electrode active material.

XRD Pattern

XRD patterns of the electrode plates made of the cathode active materials of Comparative Examples 2, 4, and 5 were examined, and the results are shown in FIG. 1. In Figure 1 a, b and c in turn are XRD patterns of the electrode plate prepared according to Comparative Examples 2, 4 and 5. As shown in FIG. 1, the electrode plates of Comparative Examples 4 and 5 prepared by adding LiMn ₂ O ₄ to LiCoO ₂ showed a spinel Fd3m peak found in LiMn ₂ O ₄ .

SEM photo comparison

The difference of the surface shape of the electrode plate containing the positive electrode active material of an Example and a comparative example was confirmed by SEM photograph. 2A, 2B, 2C, 2D and 2E are in turn SEM images of electrode plates prepared according to Comparative Examples 1, 2, 4, 5 and Example 3. FIG.

Charge / discharge characteristic evaluation

Charge and discharge of the 2016 type coin cell manufactured according to the Examples and Comparative Examples at 0.1C between 4.3 and 2.75V is shown in Figures 3 and 4. In FIG. 3, a, b, c, and d sequentially show charge and discharge characteristics of the coin batteries prepared according to Comparative Examples 3, 4, 2, and 5. In Figure 4 a, b and c in turn shows the charge and discharge characteristics of the coin battery prepared according to Comparative Example 1, Example 2 and Example 3.

As shown in FIG. 3 and FIG. 4, it can be seen that there is no significant difference in charge and discharge characteristics at a low rate (0.1C) of the coin batteries manufactured according to the examples and the comparative examples.

However, it was found that the charging and discharging characteristics of the battery according to the embodiment of the present invention are more excellent at higher rates (1C). The capacity of the coin cells of Examples and Comparative Examples was measured while varying the C rate, and is shown in FIG. 5. As shown in Figure 5, it can be seen that the capacity characteristics of the battery according to the embodiment of the present invention is much superior to the comparative example.

Thermal stability evaluation

The thermal stability of the positive electrode active material prepared according to Examples and Comparative Examples was evaluated by the following method. The coin cells of Examples and Comparative Examples were charged to 4.3 V and then disassembled in a drying chamber to separate the electrode plates. Only about 10 mg of the active material coated on the Al-foil was collected from the separated electrode plate, and thermal stability was evaluated using a 910 DSC (manufactured by TA instrument). DSC analysis was performed by scanning at an elevated rate of 3 ° C./min in a temperature range of 25 to 300 ° C. under air atmosphere. DSC results are shown in FIG. 6.

In Figure 6 a, b and c in turn shows the DSC thermogram of the positive electrode active material prepared according to Examples 2, 3 and Comparative Example 1. As shown in Figure 6, LiCoO ₂ of Comparative Example 1 of the present invention exhibited a large exothermic peak in the range of about 190 to 220 ℃. LiCoO ₂ active material in the charged state was I of the structure of the Li _1-x CoO _2, decomposes the combination of CoO of the Li _1-x CoO ₂ causes a weak O _2. The exothermic peak is caused by O ₂ from which the Co—O bond is decomposed to react with the electrolyte to generate a large exotherm. A large exothermic peak means that the stability of the battery is low. However, the active materials of Examples 2 and 3, which are cathode active materials mixed with surface-treated LiCoO ₂ and LiMn ₂ O ₄ of the present invention, shifted to a temperature near 240 ° C. at which an exothermic peak is higher than an exothermic peak of Comparative Example 1. It can be confirmed. This shows that the thermal stability of the positive electrode active material of Examples 2 and 3 is excellent. From these results, it can be seen that the positive electrode active material in which the surface-treated LiCoO ₂ and LiMn ₂ O ₄ of the present invention are mixed has superior thermal stability than the conventional positive electrode active material.

As described above, the present invention is a cathode active material for a lithium secondary battery made of a mixture of a metal-coated lithium-cobalt chalcogenide compound and a lithium-manganese chalcogenide compound and is inexpensive and thermally stable to a lithium-cobalt chalcogenide compound. When the composite positive electrode active material electrode plate is prepared by adding the excellent lithium-manganese chalcogenide compound, the life characteristics are excellent, and when the battery is manufactured by adding the lithium-manganese chalcogenide compound, an expensive lithium-cobalt chalcogenide compound is used. Compared to the case of use, the spinel type lithium-manganese chalcogenide compound possesses high rate characteristics and thermal stability characteristics, resulting in thermally stable and cost-effective effects by the amount of the added lithium-manganese chalcogenide compound.

Claims (8)

a) a lithium-cobalt knife having a surface treatment layer comprising at least one compound selected from the group consisting of metal oxides, metal hydroxides, metal oxyhydroxides, metal oxycarbonates, metal hydroxycarbonates and mixtures thereof Cogenide compounds; And b) lithium-manganese chalcogenide compounds A cathode active material for a lithium secondary battery comprising a. The positive electrode active material for lithium secondary battery according to claim 1, wherein the weight ratio of the lithium cobalt chalcogenide compound: lithium manganese chalcogenide compound on which the surface treatment layer is formed is 50 to 99.9: 50 to 0.1. The cathode active material of claim 1, wherein the lithium-cobalt chalcogenide compound is at least one compound selected from the group consisting of Chemical Formulas 1 to 4 below. [Formula 1] Li _x CoM _y A ₂ [Formula 2] Li _x CoO _2-z A ₂ [Formula 3] Li _x Co _y Ni _1-y O _2-z A _z [Formula 4] Li _x Co _y Ni _1-yz M _z A _a Where 0.95 ≦ x ≦ 1.1, 0 ≦ y ≦ 0.5, 0 ≦ z ≦ 0.5, 0 <a ≦ 2, M is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, and At least one element selected from the group consisting of rare earth elements, and A is an element selected from the group consisting of O, F, S, and P.) The positive active material of claim 1, wherein the lithium-manganese chalcogenide compound is at least one compound selected from the group consisting of Chemical Formulas 5 to 9 below. [Formula 5] Li _x Mn _1-y M ' _y A ₂ [Formula 6] Li _x Mn _1-y M ' _y O _2-z A ₂ [Formula 7] Li _x Mn ₂ O _4-z A _z [Formula 8] Li _x Mn _2-y M ' _y A ₄ [Formula 9] Li _x Ni _1-yz Mn _y M ' _z A _a Wherein 0.95 ≦ x ≦ 1.1, 0 ≦ y ≦ 0.5, 0 ≦ z ≦ 0.5, 0 <a ≦ 2, M is Ni or Co, and M ′ is Al, Ni, Co, Mn, Cr, Fe , Mg, Sr, V, and at least one element selected from the group consisting of rare earth elements, A is an element selected from the group consisting of O, F, S and P.) The metal of claim 1, wherein the metal included in the surface treatment layer is formed of Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, As, Zr, and mixtures thereof. At least one metal selected from the group of positive electrode active material for lithium secondary batteries. The cathode active material of claim 1, wherein the metal content of the surface treatment layer is 2 × 10 ⁻⁵ to 1 wt% based on the cathode active material. The cathode active material of claim 6, wherein the metal content of the surface treatment layer is 0.001 to 1 wt% based on the cathode active material. A lithium secondary battery comprising the cathode active material according to any one of claims 1 to 7.