KR100347882B1 - Conducting Polymer/Manganese Oxide Composite Cathode Material for Lithium Secondary Batteries and Method for Making Same - Google Patents

Abstract

The present invention relates to a method for preparing a composite cathode active material for a lithium secondary battery by synthesizing a conductive polymer on the surface of a manganese oxide. (A) A metal oxide Adding and stirring; (B) adding a monomer having a controlled weight ratio to the reaction mixture of step (A) and allowing the monomer to react for 30 minutes to 2 hours; (C) precipitating the reaction mixture of step (B) in an excess amount of methanol, filtering, washing and then drying in a vacuum oven at 80 DEG C for more than 24 hours. The conductive polymer / manganite oxide composite powder prepared according to the above method can be very usefully used in the fields of automobiles and electronic parts requiring high electrical conductivity and electrical activity at room temperature, such as high performance lithium secondary batteries and batteries.

Description

TECHNICAL FIELD [0001] The present invention relates to a conductive polymer / manganese oxide composite cathode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery using the same. BACKGROUND ART [0002]

The present invention relates to a composite cathode active material for a lithium secondary battery and a method for producing the composite cathode active material, and more particularly, to a method for preparing a composite cathode active material for a lithium secondary battery by synthesizing a conductive polymer on the surface of a manganese oxide.

Generally, cobalt oxide (LiCoO ₂ ), which is a cathode active material used in a commercial small-sized lithium secondary battery, is expensive as a rare element and has a problem that can cause environmental problems. Therefore, low-priced nickel oxide (LiNiO ₂ ) Oxides (MnO ₂ or LiMn ₂ O ₄ ) have been used. Nickel oxide is a two-dimensional layered structure like cobalt oxide and has an operating voltage of 3.6 V and a high discharge capacity, but it has a disadvantage that it is difficult to control the synthesis process, whereas a three-dimensional spinel structure manganese oxide (LiMn ₂ O ₄ ) Low cost, easy to synthesize, and environmentally friendly.

However, the manganese oxide has a low electric conductivity of about 10 ^-4 S / cm and has a disadvantage in that the lifetime of the battery is drastically lowered during charging and discharging due to instability of crystal structure and low thermal stability. Generally, since the electric conductivity of the metal oxide is very low such as 10 ^-3 S / cm or less, carbon powder of about 10 wt% or less per total weight of the anode is used as a conductive material in order to smooth current flow during charging and discharging . In the case of the manganese oxide, the overdispersed crystal structure is changed from cubic to tetragonal and the volume is about 6.5%. In the case of the metal oxide and the carbon conductive material powder, The local short-circuiting of the active material and the carbon conductive material occurs, and the resistance is increased, thereby shortening the life of the battery rapidly. In order to suppress this, manganese ions were replaced with bivalent or trivalent ions. However, it is difficult to expect commercial success.

On the other hand, the conductive polymer can be used both as a cathode and a cathode active material of a secondary battery according to the polarity of a dopant used, but it has been utilized as a cathode active material of a lithium secondary battery mainly by anion doping. In the case of lithium metal and anionically doped conductive polymer battery, the charging reaction of the battery is performed by reduction of lithium ions at the lithium anode and doping of the anion into the polymer chain at the polymer anode, and the discharge reaction consists of the reverse reaction thereof. Conductive polymers such as polyacetylene, polyaniline and polypyrrole can be used as the cathode active material of secondary batteries. Bridgestone / Seiko of Japan and Varta of Germany, / BASF has developed a small lithium battery using polyaniline and polypyrrole in the late 1980s and partially commercialized it.

However, since the lithium / conductive polymer battery has a high self-discharge rate and a slow moving speed of the dopant ions in the polymer chain during the doping / dedoping reaction, the life of the battery is rapidly shortened. Compared to its competitors in terms of price and performance. In addition, the maximum doping level of the conductive polymer is basically a capacity limit of the cathode active material, such as about 35% of polypyrrole, about 25% of polythiophene and about 50% of polyaniline, so that the energy density per weight is higher than the metal oxide .

In order to solve the above problems, Japanese Patent No. 9073920 discloses a nanocomposite material of vanadium oxide and polypyrrole and a method of chemically preparing a polypyrrole / MnO ₂ composite cathode active material ( J. Chem. Soc., Chem. Commun. 198, (1991)), a method of electrochemically or chemically preparing a polypyrrole / LiMn ₂ O ₄ composite cathode active material has been reported ( Chem . Lett ., 1067, (1989)).

However, it is difficult to control the thickness of the conductive polymer to be polymerized, and a composite electrode must be obtained on a specific template. In addition, since an inorganic anion such as ClO ₄ ^- is used as a dopant in the synthesis, a doping / dedoping reaction of anions inside and outside the conductive polymer occurs in the charge / discharge reaction, And the capacity of the lithium ion can be reduced. Since the conductive polymer chains have a dense structure, it is difficult to swell the polymer due to the organic liquid electrolyte. Therefore, migration of the lithium cation through the polymer layer The speed can be reduced. In addition, the conductive polymer doped with an inorganic anion is disadvantageous in that the film is dense and brittleness is strong, so that it is easily broken.

The object of the present invention is to solve the disadvantages of the conventional manganese oxide and the conductive polymer described above, and it is an object of the present invention to provide a polyaniline or polypyrrole And a method for manufacturing an environmentally friendly conductive polymer / manganese oxide composite cathode active material.

1 is a spectrum obtained by measuring the polypyrrole of Example 1-6 prepared by changing the dopant with an X-ray diffractometer.

2 is a graph showing a change in electric conductivity according to a change in manganese oxide content in a composite cathode active material.

3 is a graph showing discharge capacity changes of the composite cathode active materials of Examples 1, 6-7, and Comparative Example 1-2.

4 is a graph showing discharge capacities of the composite cathode active material according to Example 7 and Comparative Example 1 according to charge / discharge rates.

(A) adding a metal oxide to an aqueous solution of a dopant of 0.5 to 1.5 M adjusted at -4 ° C to 4 ° C, followed by stirring; (B) adding a monomer having a controlled weight ratio to the reaction mixture of step (A) and allowing the monomer to react for 30 minutes to 2 hours; (C) precipitating a reaction mixture of step (B) in an excess amount of methanol, filtering, washing and drying in a vacuum oven at 80 ° C for at least 24 hours. Method.

The metal oxide of step (A) may be Li _x Co _1-x Mn ₂ O ₄ , LiCoO ₂ , Mn ₂ O ₄ , LiMn ₂ O ₄ , and the monomer of step (B) may be pyrrole or aniline. have. The weight ratio of monomers to metal oxide is 1: 1 to 1: 3 and can be adjusted according to the purpose of use.

The dopant of step (A) may be selected from the group consisting of camphorsulfonic acid, alkylbenzenesulfonic acid, and alkylbenzenesulfonic acid (sodium salt). The length (n) of the alkyl chain of the dopant is It is adjustable from 0 to 12, and the concentration of the dopant aqueous solution during polymerization is about 0.5 M to 1.5 M, which can be adjusted depending on the purpose of use.

Among the above dopants, the alkylbenzenesulfonic acid has a long alkyl chain and serves to expand the conductive polymer chains at regular intervals. Lithium (Li) is added to a mixed solvent such as ethylene carbonate and dimethyl carbonate, The swelling of the conductive polymer by the organic electrolyte added with lithium hexafluorophosphate is facilitated, and the doped alkylbenzenesulfonic acid anion is fixed in the polymer chain during the charging / discharging reaction due to the macromolecular structure, And only the reduction reaction occurs. Therefore, it is very suitable as a cathode active material for a lithium secondary battery.

Since the polymer having a flexible structure such as alkylbenzenesulfonic acid is coated on the surface of the manganese oxide, the bonding strength with a polymer binder such as polyvinylidene fluoride or polytetrafluoro-ethylene And exhibits excellent electrical conductivity even with a polymer content of 3 to 5% by weight in the composite cathode active material. Therefore, the addition amount of the carbon conductive material can be reduced, and the energy density per weight and volume can be effectively improved.

Examples of the oxidizing agent include ammonium persulfate (APS), Acros, ferric perchlorate, ferric chloride, ferric nitrate, ammonium ferric sulfate, and potassium persulfate (potassium persulfate) can be used. The ratio of monomers to oxidant is 1: 1 to 1: 7.5, and it can be controlled depending on the purpose of use. In the case of manganese oxide (LiMn ₂ O ₄ ), polymerization can be carried out without oxidizing agent.

The present invention will now be described in more detail with reference to the following examples, but it should be understood that the invention is not construed as being limited thereto.

≪ Example 1 >

As a dopant, 23.1 g of dodecylbenzenesulfonic acid (DBSA) (Acros) was dissolved in 350 ml of distilled water, and 10 g of LiMn ₂ O ₄ (KMG co.) Was added to the aqueous solution of the dopant. The solution was stirred for 10 minutes using a magnetic stirrer Lt; / RTI > At this time, the temperature of the aqueous solution is maintained at -4 ° C. to 4 ° C., 10 g of pyrrole monomer (Acros) is added, and the reaction is carried out at a rate of about 380 rpm for 30 minutes to 2 hours. In order to terminate the reaction, the reaction mixture was precipitated in an excess amount of methanol. The precipitate was filtered using a glass filter, washed with distilled water, methanol and acetone until the unreacted dopant of the surfactant system was completely removed, And dried in a vacuum oven to prepare a polypyrrole / manganese spinel composite cathode active material powder.

FIG. 2 shows the electrical conductivity measured by the 4-probe method for the composite cathode active material powder prepared as described above, and the structural characteristics of the conductive polymer were measured using an X-ray diffractometer. FIG.

The charge / discharge test of the battery was performed by using an organic liquid electrolyte of ethylene carbonate / dimethyl carbonate / LiPF ₆ (EC / DMC = 1/1, 1 M LiPF ₆ ) After assembling the lithium ion battery of the composite cathode active material, the battery was charged at a constant current of C / 3 and a constant voltage of 4.2 V at 3.0 to 4.2 V and discharged at a constant current of C / 3. Lithium metal (Chemettal Co.) was used as an anode active material. Polyvinylidene fluoride (PVdF, Altochem Kynar 761) was used as a binder in the preparation of the polypyrrole / LiMn ₂ O ₄ composite electrode, and graphite / acetylene black (sfg6 / acetylene black, Lonza Co.) and acetylene black (Mitsubishi Chemical) were mixed at 7/3. The polypyrrole / LiMn ₂ O ₄ electrode (2 × 2 cm ² ) was prepared by dispersing the active material / binder / conductive material powder in a mixed solvent of n-methyl-pyrrolidinone / acetone, The composition was fixed at a weight ratio of active material / binder / conductive material = 100/6/10, and the weight of the active material in the electrode was adjusted to 0.05 g / cm ² during the production of the secondary battery. The results of charge-discharge experiments are shown in Fig.

≪ Example 2 >

19.2 g of octylbenzenesulfonic acid (OBSA) (Aldrich) as a dopant was dissolved in 350 ml of distilled water, and then the solution was prepared in the same manner as in Example 1. The composite cathode active material powder thus prepared was measured for structural characteristics of the conductive polymer using an X-ray diffractometer, and the result is shown in FIG.

≪ Example 3 >

15.2 g of butylbenzenesufonic acid (BBSA) as a dopant was dissolved in 350 ml of distilled water, and then the solution was prepared in the same manner as in Example 1. The composite cathode active material powder thus prepared was measured for structural characteristics of the conductive polymer using an X-ray diffractometer, and the result is shown in FIG.

<Example 4>

12.2 g of para-toluenesulfonic acid (TSA) (Aldrich) was dissolved in 350 ml of distilled water as a dopant, and then the solution was prepared in the same manner as in Example 1. The composite cathode active material powder thus prepared was measured for structural characteristics of the conductive polymer using an X-ray diffractometer, and the result is shown in FIG.

&Lt; Example 5 >

11.2 g of benzenesulfonic acid (BSA) (TCI) as a dopant was dissolved in 350 ml of distilled water, and then the solution was prepared in the same manner as in Example 1. The composite cathode active material powder thus prepared was measured for structural characteristics of the conductive polymer using an X-ray diffractometer, and the result is shown in FIG.

FIG. 1 shows the X-ray diffraction spectrum of polypyrrole prepared by varying the dopant of Example 1-5, and it was confirmed that the intensity of the peak near 25 to 27 ^o varies according to the dopant change. In particular embodiments the D spacing (d -spacing) of the polymer chains by the Bragg's equation from 2θ = ^3.34. This confirmed the layered structure is 26.4 Å when doped with Example 1 of dodecylbenzenesulfonic acid. From these results, it can be seen that when dodecylbenzenesulfonic acid is used as a dopant, it exhibits excellent electric conduction performance due to intermolecular conduction in the oriented polymer chain, and exhibits fast lithium ion migration upon charging / discharging due to swelling of the polymer layer by the organic electrolytic solution Able to know.

&Lt; Example 6 >

And 20 g of manganese oxide (LiMn ₂ O ₄ ) were used. Electric conductivity measurement and charge / discharge tests of the composite cathode active material powder thus prepared were carried out in the same manner as in Example 1, and the results are shown in FIGS. 2 and 3, respectively.

&Lt; Example 7 >

And 30 g of manganese oxide (LiMn ₂ O ₄ ) were used. The composite cathode active material powder thus prepared was subjected to electrical conductivity measurement and charge / discharge tests in the same manner as in Example 1, and the results are shown in FIGS. 2, 3 and 4, respectively.

FIG. 2 is a graph showing changes in electric conductivity according to changes in manganese oxide content in the composite cathode active material including Examples 1 and 6-7. Compared to the physical mixture powder of conductive polymer and LiMn ₂ O ₄ at the same weight ratio, . From the results of FIG. 2, it can be seen that the composite cathode active material produced by the present invention exhibits excellent conductivity as compared with the physical mixed powder.

&Lt; Comparative Example 1 &

The performance of the lithium metal / LiMn ₂ O ₄ battery was investigated using LiMn ₂ O ₄ as the cathode active material. The electrode manufacturing method and the charge-discharge test were the same as in Example 1, and the results are shown in FIG. 3 and FIG.

&Lt; Comparative Example 2 &

The lithium metal / LiCoO ₂ battery was manufactured using LiCoO ₂ (Seimi Co.) as a cathode active material, and the performance of the battery was examined. The electrode manufacturing method and the charge-discharge test were the same as in Example 1, and the results are shown in FIG.

FIG. 3 shows the discharge capacity change of Examples 1, 6-7 and Comparative Example 1-2, showing that the composite cathode active material of 4 wt% of polypyrrole of Example 7 exhibited excellent discharge capacity as compared with conventional LiMn ₂ O ₄ . FIG. 4 shows the discharge capacity according to the charging and discharging speeds of Example 7 and Comparative Example 1. It can be seen that the composite cathode active material of Example 7 shows a high capacity even at a high rate.

As described above, the composite cathode active material for a lithium secondary battery composed of the conductive polymer and the metal oxide of the present invention facilitates the transfer of electrons as the electric conductivity increases, so that it is possible to charge and discharge at a high rate. In the cathode active material slurry, The amount of the conductive material to be added can be reduced, so that the energy density per volume and the energy density per weight can be improved, so that the cell performance is superior to that of the conventional manganese oxide. The composite cathode active material prepared by the composition of the present invention can be very usefully used in the fields of automobiles and electronic parts requiring high electric conductivity and electrical activity at room temperature such as high performance lithium secondary batteries and batteries, A circuit design technique for manufacturing a memory card, and an effect applicable to an ultra thin film battery.

Claims (6)

(A) adding a metal oxide to an aqueous solution of a dopant of 0.5 to 1.5 M adjusted at -4 캜 to 4 캜, followed by stirring; (B) adding a monomer having a controlled weight ratio to the reaction mixture of step (A) and allowing the monomer to react for 30 minutes to 2 hours; (C) precipitating the reaction mixture of step (B) in an excess amount of methanol, filtering, washing and then drying in a vacuum oven at 80 ° C for at least 24 hours. A method for producing a conductive polymer / manganese oxide composite powder to be used. The method according to claim 1, Wherein the metal oxide of step (A) is selected from the group consisting of Li _x Co _1-x Mn ₂ O ₄ , LiCoO _2, Mn ₂ O ₄ and LiMn ₂ O ₄ . Wherein the conductive polymer / manganese oxide composite powder is used as a cathode active material of the conductive polymer / manganese oxide composite powder. The method according to claim 1, Wherein the monomer of step (B) is at least one selected from the group consisting of pyrrole and aniline, and is used as a cathode active material of a lithium secondary battery. The method according to claim 1, The dopant of step (A) is selected from the group consisting of camphorsulfonic acid, alkylbenzenesulfonic acid and alkylbenzenesulfonic acid sodium salt. The conductive polymer / manganese oxide complex used as the cathode active material of the lithium secondary battery Gt; A conductive polymer / manganese oxide composite powder used as a cathode active material of a lithium secondary battery, which is produced according to the method of claim 1. A lithium secondary battery using the conductive polymer / manganese oxide composite powder of claim 5 as a cathode active material.