KR100358799B1 - Method of preparing positive active material for lithium secondary battery - Google Patents

Abstract

The present invention relates to a method for producing a cathode active material for a lithium secondary battery, which comprises mixing a lithium salt and a manganese salt and heat-treating the mixture under a reducing gas. This method can solve the problem that the structure can not be maintained as Mn is easily oxidized in the course of manufacturing LiMnO ₂ .

Description

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

[Industrial Applications]

The present invention relates to a method for producing a cathode active material for a lithium secondary battery, and more particularly, to a method for producing a cathode active material for a lithium secondary battery excellent in electrochemical characteristics.

BACKGROUND ART [0002]

The lithium secondary battery is manufactured by using a material capable of reversibly inserting and desorbing lithium ions as a positive electrode and a negative electrode and filling an organic electrolytic solution or a polymer electrolyte between the positive electrode and the negative electrode. And generate electrical energy by oxidation and reduction reactions when they are used.

Lithium metal is used as an anode active material of a lithium secondary battery. However, when a lithium metal is used, there is a risk of explosion due to short circuit of the battery due to the formation of dendrite. Therefore, instead of lithium metal, a carbonaceous material such as amorphous carbon or crystalline carbon It is being replaced.

As a cathode active material is a chalcogenide (chalcogenide) and the compound is used, LiCoO _2, as geuye _{LiMn 2 O 4, LiNiO 2,} LiNi 1-x Co x O 2 (0 <x <1), LiMnO 2 , such as a complex metal of Oxides are being studied. LiNiO ₂ among the above cathode active materials is the cheapest and has the disadvantage of being difficult to synthesize although it exhibits the battery characteristic of the highest discharge capacity. LiCoO ₂ exhibits good electric conductivity, high battery voltage and excellent electrode characteristics. However, LiCoO ₂ is a typical cathode active material commercially available from Sony Corporation and the like. However, LiCoO ₂ has a disadvantage that it is expensive and has a small stability at high rate charging and discharging. Mn-based cathode active materials such as LiMn ₂ O ₄ and LiMnO ₂ are easy to synthesize, have relatively low cost, and have little environmental pollution. Although such Mn-based active materials have disadvantages of small capacity, they are regarded as the most promising cathode active material materials in next-generation large-sized batteries as electric power sources for electric vehicles and electric vehicles due to stability of battery system and environmental friendliness of Mn have.

LiMnO ₂ (space G: Pmmm) among the manganese-based cathode active materials is manufactured by various methods. One of the methods is a method of reacting lithium oxide (Li ₂ O), carbonate (Li ₂ CO ₃ ), or hydroxide (LiOH) with oxides of manganese (MnO ₂ , Mn ₂ O ₃ ) , A precursor such as LiMn ₂ O ₄ or Li ₄ Mn ₅ O ₁₂ can be prepared and reacted with a lithium compound to form a solid phase. The solid-phase reaction is a method in which a desired compound is prepared by mixing raw material powders in a solid state, firing and ball milling several times.

LiMnO ₂ produced by this method is apt to be oxidized and has a problem in that its structure can not be maintained as the oxidation number changes from +3 to +4. Therefore, there is a problem in obtaining an accurate crystal structure of LiMnO ₂ . In order to prevent such problems, a reducing agent such as hydrogen or carbon or an inert atmosphere is used to lower the oxidation number of manganese or to block oxidation. Or γ-MnOOH as a precursor, or LiMnO ₂ was produced by using an ion exchange reaction of γ-MnOOH in a LiOH solution.

Disclosure of Invention Technical Problem [8] Accordingly, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a method for producing a manganese-based cathode active material for a lithium secondary battery which can prevent oxidation of manganese ions.

Another object of the present invention is to provide a method for producing a manganese-based cathode active material for a lithium secondary battery excellent in electrochemical characteristics.

1 is a graph showing an X-ray diffraction pattern of an active material prepared by the method of Example 1 of the present invention and Comparative Examples 1 and 2;

2 is a graph showing an X-ray diffraction pattern of an active material prepared by the methods of Examples 1 and 2 of the present invention.

3 is a graph showing an X-ray diffraction pattern of a cathode active material for a lithium secondary battery produced according to Examples 3 to 5 of the present invention.

4 is a graph showing X-ray diffraction patterns of a cathode active material for a lithium secondary battery manufactured according to Examples 6 to 8 of the present invention.

5A and 5B are SEM photographs of a cathode active material for a lithium secondary battery according to an embodiment of the present invention.

6A and 6B are SEM photographs of a cathode active material for a lithium secondary battery according to another embodiment of the present invention.

7A and 7B are SEM photographs of LiMn ₂ O ₄ cathode active material which has been conventionally used.

8 is a graph showing charge-discharge characteristics of a lithium secondary battery manufactured using the active material of Example 6 and Comparative Example 1 of the present invention.

9 is a graph showing charge-discharge characteristics of a lithium secondary battery manufactured using the active material of Example 6 and Comparative Example 3 of the present invention.

In order to achieve the above object, the present invention provides a method for preparing a lithium salt, comprising mixing a lithium salt and a manganese salt; And heat-treating the mixture under a reducing gas containing carbon monoxide. The present invention also provides a method for producing a cathode active material for a lithium secondary battery.

Hereinafter, the present invention will be described in detail.

The present invention relates to a method for producing a manganese-based cathode active material which is promising for use as a cathode active material in the field of lithium secondary batteries in the field of lithium secondary batteries. The production method of the present invention uses a reducing atmosphere containing a carbon monoxide gas (CO gas) to prevent the oxidation of LiMnO ₂ , which is likely to be oxidized during the production of the manganese-based cathode active material. This manufacturing method it is possible to prevent oxidation of LiMnO _2, LiMnO ₂ is able to maintain its structure better and thus can be produced at a high yield LiMnO _2.

In addition, the production method of the present invention is based on the fact that the electrochemical characteristics of LiMnO ₂ mixed with other phases can be variously changed, so that the composition of the final product It is a controllable method.

The manufacturing method of the present invention firstly makes the solid phase reaction between the solid phase materials as one stage. That is, the solid lithium salt and the manganese salt are mixed. The mixing ratio of the lithium salt and the manganese salt is 1.3 to 2.3: 2.0 by weight. When the amount of the lithium salt is smaller or larger, a minor phase of Li / Mn / O is generated in addition to LiMnO ₂ .

As the lithium salt, lithium hydroxide, lithium nitrate or lithium acetate can be used. Lithium hydroxide is prepared by dehydrating LiOH.H ₂ O in a hot vacuum oven. The manganese salt may be manganese acetate or manganese dioxide. As the manganese dioxide,? -MnO ₂ may be used, and electrochemical manganese dioxide (EMD) produced by an electric vapor deposition method or chemical manganese dioxide (CMD) produced by a chemical oxidation-reduction reaction may be used. Of these, chemical manganese dioxide having a small surface area and a large particle size is preferable because of its excellent high temperature service life.

The mixture is sufficiently mixed and then heat-treated in a reducing atmosphere to obtain a crystalline powder. The reducing atmosphere is preferably formed using carbon monoxide gas diluted with an inert gas. When hydrogen gas is used in a reducing atmosphere, the reducing property is too strong, and manganese is reduced to divalent ions, so that LiMnO ₂ as a target product can not be produced. The concentration of the carbon monoxide gas is preferably 10 ppm to 10%, more preferably 90 to 300 ppm, relative to the inert gas.

During the heat treatment, carbon monoxide reacts with the surrounding oxygen and is oxidized. The equilibrium formula is as follows.

2CO + O _2? 2CO ₂

This reaction removes the sample and surrounding oxygen and provides a reducing atmosphere to the reactive metal.

The heat treatment is performed at 600 to 900 占 폚 for 3 to 48 hours. When the heat treatment temperature is lower than 600 ° C or higher than 900 ° C, a minor phase or a mixed phase material occurs. In addition, when the heat treatment is performed for less than 3 hours or for more than 48 hours, minor phase or mixed phase material is generated.

As the inert gas, generally used nitrogen or argon gas may be used.

The cathode active material produced by this method has a fibrous or spherical fine particle aggregate and is a cathode active material of LiMnO ₂ .

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

(Example 1)

LiOH · H ₂ O was dehydrated in a vacuum oven and the molar ratio of LiOH and γ-MnO ₂ was 1.5: 2. The obtained mixture was heat-treated at 800 ° C for 5 hours to obtain a crystalline LiMnO ₂ active material powder. The heat treatment was carried out in an oven at a concentration of 45 ppm of carbon monoxide diluted with nitrogen.

(Comparative Example 1)

The heat treatment was carried out in the same manner as in Example 1, except that drying air was poured into the oven. At this time, an oxidizing gas composed of 78 vol.% Of nitrogen, 21 vol.% Of oxygen and 1 vol.% Of other components such as argon was used as dry air.

(Comparative Example 2)

The heat treatment was carried out in the same manner as in Example 1, except that 4% hydrogen diluted with argon was poured into the oven.

The X-ray diffraction patterns of the cathode active material prepared by the method of Example 1 and Comparative Examples 1 and 2 were measured and the results are shown in FIG. 1, LiMnO ₂ , Is LiMn ₂ O ₄ , Is γ-Mn ₂ O _3, Is MnO, c represents the Li ₂ O. As shown in Fig. 1, the active material of Comparative Example 1 subjected to heat treatment while flowing air had LiMn ₂ O ₄ produced as a main product in a condition where the ratio of Mn was larger than Li, and Li ₂ MnO ₃ having Mn ⁴⁺ As an adduct. As a result, it can be seen that a composite metal oxide in which Mn ³⁺ and Mn ⁴⁺ are mixed is obtained in an oxidizing atmosphere of dry air.

On the other hand, in the case of Example 1 using carbon monoxide gas, LiMnO ₂ having Mn ³⁺ , which is a more reduced form, was obtained as a main product and LiMn ₂ O ₄ and γ-Mn ₂ O ₃ were obtained as by-products.

In addition, according to the result that LiMnO ₂ can be produced by reducing manganese ions by forming a reducing atmosphere using a carbon monoxide gas in the heat treatment atmosphere, the sample was heat-treated using hydrogen gas having stronger reducibility in Comparative Example 2. In this case, as shown in Fig. 1, a composite metal oxide comprising lithium and manganese could not be produced, and MnO was obtained as a main product and Li ₂ O and the like as an addition product. These results suggest that LiMnO ₂ can not be synthesized because the reductive atmosphere of hydrogen gas is too strong to reduce manganese to bivalent ions. In this case, the remaining lithium remains in an oxide form. As a result, even if the gas gives a reducing atmosphere, LiMnO ₂ can not be synthesized as a hydrogen gas, but it can be synthesized as a carbon monoxide gas.

(Example 2)

The heat treatment was carried out in the same manner as in Example 1, except that carbon monoxide at a concentration of 90 ppm diluted with nitrogen was poured into the oven.

The XRD patterns of the active materials of Examples 1 and 2 while changing the concentration of carbon monoxide are shown in Fig. 2, LiMnO ₂ , Is LiMn ₂ O ₄ , Represents Li ₂ Mn ₃ O ₄ . As shown in FIG. 2, when the carbon monoxide fraction was increased from 45 ppm to 90 ppm twice, it was confirmed that LiMnO ₂ was better developed. With the development of _{LiMnO 2 LiMn 2 O 4, γ} -Mn 2 O 3 it was confirmed to decrease.

(Example 3)

LiOH · H ₂ O was dehydrated in a vacuum oven and the molar ratio of LiOH and γ-MnO ₂ (electrochemical manganese dioxide, Aldrich Co.) was adjusted to 2.1 to 2, and the mixture was thoroughly mixed. The mixture was heat-treated at 600 캜 while flowing carbon monoxide (90 ppm, diluted with nitrogen), thereby preparing a crystalline LiMnO ₂ active material powder.

(Example 4)

The same procedure as in Example 3 was carried out except that the heat treatment was performed at 700 캜.

(Example 5)

The same procedure as in Example 3 was carried out except that the heat treatment was performed at 800 占 폚.

The X-ray diffraction patterns of the active materials prepared by the methods of Examples 3 to 5, which were carried out while changing the heat treatment temperature, are shown in Fig. 3, LiMnO ₂ , Is γ-Mn ₂ O _3, Represents Li ₂ MnO ₃ . As shown in Fig. 3, Li ₂ MnO ₃ was obtained as the main product and γ-Mn ₂ O ₃ and LiMnO ₂ were produced as an addition product in the case of Example 3 which was heat-treated at 600 ° C. It can be seen that when the heat treatment is performed at 600 ° C., LiMnO ₂ starts to be formed. Also, as in Examples 4 and 5, the Li ₂ MnO ₃ phase, which was formed at a lower temperature as the heat treatment temperature was increased, was changed to LiMnO ₂ phase, and the remaining phase was also decreased. In other words, Li ₂ MnO ₃ with Mn ⁴⁺ was first produced by the reaction of LiOH and MnO ₂ during the heat treatment of the reducing atmosphere, and LiMnO ₂ with Mn ³⁺ was produced when the temperature increased.

(Example 6)

The procedure of Example 2 was repeated, except that the molar ratio of LiOH to γ-MnO ₂ (electrochemical manganese dioxide, Aldrich Co.) was changed from 1.7 to 2.0.

(Example 7)

The procedure of Example 2 was repeated, except that the molar ratio of LiOH to γ-MnO ₂ (electrochemical manganese dioxide, Aldrich Co.) was changed from 2.0 to 2.0.

(Example 8)

The procedure of Example 2 was repeated, except that the molar ratio of LiOH to γ-MnO ₂ (electrochemical manganese dioxide, Aldrich Co.) was changed from 2.2 to 2.0.

The X-ray diffraction patterns of the cathode active materials prepared by the methods of Example 2 and Examples 6 to 8 were measured and the results are shown in FIG. 4, LiMnO ₂ , Li ₂ MnO ₃ , Is LiMn ₂ O ₄ , Represents? -Mn ₂ O ₃ . As shown in FIG. 4, all LiMnO ₂ was obtained as the main product, and as the molar ratio of LiOH was increased, the LiMn ₂ O ₄ phase as an additive product developed into a Li ₂ MnO ₃ phase. And manganese which is not sufficiently mixed with lithium appears to form γ-Mn ₂ O.

5A-5B, 6A-6B, and 7A-7B show the active materials of Examples 2 and 6 and SEM photographs of LiMn ₂ O ₄ of Nikki, which is conventionally used. FIGS. 5A and 5B are SEM photographs of the active material of Example 2 enlarged to 2000 times and 5000 times, respectively, and FIGS. 6A and 6B are SEM photographs of the active material of Example 6 enlarged to 2000 times and 5000 times, respectively. 7A and 7B are SEM photographs of LiMn ₂ O ₄ active material of Nikki Co., which are magnified 300 times and 10000 times, respectively. As shown in Fig. 5A, the active material of Example 2 represents a fiber type, and as shown in Fig. 6A, the active material of Example 6 has a shape which is close to a minute sphere. In addition, as shown in Figure 7a, because it appears having a shape close to Fig LiMn ₂ O ₄ Nikki's release of conventionally used spherical shape, but can also be thought of as having a structure similar to that of Figures 6a, a further expanded it 6b and Fig. in the 7b, example 6, while the active material is a particulate of less than 1㎛ mungchyeojyeo, Nikki's LiMn ₂ O ₄ of it can be seen that it is formed of a single particle of about 2㎛.

(Comparative Example 3)

Comparative Example 1 was carried out except that the molar ratio of LiOH to? -MnO ₂ (electrochemical manganese dioxide, Aldrich Co.) was changed from 1.7 to 2.

In order to examine the charge / discharge characteristics of the active material of the present invention, a lithium secondary battery was prepared using the active material prepared in the method of Example 6, Comparative Example 1, and Comparative Example 3. The cathode active material prepared by the method of Example 6, Comparative Example 1 and Comparative Example 3 was added to N-methylpyrrolidone solvent together with a carbon black conductive agent and a polyvinylidene fluoride binder to prepare a slurry. This slurry was cast in an aluminum foil to prepare a positive electrode. Lithium secondary semiconductors were fabricated by using metal lithium as the anode and the counter electrode. At this time, a mixture of ethylene carbonate and dimethyl carbonate in which 1 M LiPF ₆ was dissolved was used as the electrolyte solution.

The results of measuring the charge-discharge characteristics of the produced battery are shown in FIG. 8 and FIG. 9, respectively. As shown in Fig. 8, the charge and discharge characteristics of the cells of Example 6 and Comparative Example 1 are similar. As shown in Fig. 9, the battery using the active material of Example 6 has a charge / discharge capacity and a potential higher than that of Comparative Example 3.

As described above, the method of producing the positive electrode active material of the present invention can solve the problem that the structure can not be maintained as Mn is easily oxidized in the course of manufacturing LiMnO ₂ using a carbon monoxide reducing atmosphere. In addition, it is a synthesis method that can obtain LiMnO ₂ having various mixed phases by controlling various conditions in the synthesis process, and it is a method to examine various electrochemical characteristics of the mixed phase.

Claims (6)

(Correction) a lithium salt and a manganese salt are mixed; The mixture is heat-treated under a reducing gas containing carbon monoxide (CO) Wherein LiMnO ₂ is a positive electrode active material for a lithium secondary battery. The method according to claim 1, wherein the heat treatment step is performed at 600 to 900 占 폚 for 3 to 48 hours. (delete) The production method according to claim 1, wherein the reducing gas is a mixed gas of a CO gas and an inert gas, and the concentration of the CO gas is 10 ppm to 10% of the inert gas. The process according to claim 1, wherein the ratio of lithium to manganese in the mixture of lithium salt and manganese salt is 1.3 to 2.3: 2.0. (delete)