KR101741456B1 - Transition metal oxide-sulfur composite materials for cathodes in lithium-sulfur batteries and preparation method thereof - Google Patents

Abstract

The present invention relates to a transition metal oxide-sulfur composite cathode active material for a lithium-sulfur secondary battery and a method of manufacturing the same, and includes sulfur nanoparticles and α-transition metal oxide nanowires coated on all or a part of the surface of the sulfur nanoparticles Thus, after the first charge / discharge cycle, lithium is inserted into the? -Transition metal oxide nanowire, thereby inhibiting the elution of lithium polysulfide and enhancing the conductivity of sulfur and improving the rate characteristics of the battery.

Description

TECHNICAL FIELD The present invention relates to a transition metal oxide-sulfur composite material for a lithium-sulfur secondary battery, and a method for manufacturing the same.

The present invention relates to a transition metal oxide-sulfur composite cathode active material for a lithium-sulfur secondary battery, which can improve the conductivity of sulfur and thereby improve the battery characteristics, and a method for manufacturing the same.

With the rapid development of portable electronic devices, the demand for secondary batteries is increasing. Particularly, there is a continuing demand for a high energy density battery that can meet the trend of lightweight, thin, short and small size of portable electronic devices, and a battery which is inexpensive, safe and environmentally friendly is required have.

The lithium ion secondary battery uses the electrochemical reaction to change the chemical energy of the electrode material

It is a device that converts directly into electric energy and has higher energy and higher output characteristics than the previous Ni-MH battery.

In particular, in the case of a lithium-sulfur secondary battery, a sulfur electrode is used as an anode and a lithium metal is used as a cathode. In addition, the lithium-sulfur secondary battery is less expensive than a conventional commercialized battery (LiCoO ₂ ) ) And energy density (2,600 Wh / kg), which is expected to increase significantly in the future.

Such a lithium-sulfur battery uses a sulfur-based compound having a sulfur-sulfur combination as a cathode active material and a carbon-based material in which insertion and de-insertion of an alkali metal such as lithium or a lithium ion, A secondary battery in which the SS bond is re-formed during the reduction reaction (discharging), the oxidation number of S decreases and the oxidation number of S increases during the oxidation reaction (charging) The reduction reaction is used to store and generate electrical energy.

In the lithium-sulfur battery, the positive electrode is prepared by preparing a positive electrode active material slurry composition by mixing a positive electrode active material, a binder, a conductive material, and an organic solvent, and coating the slurry composition on a current collector. The binder serves to prevent the active material or the conductive material from separating from the current collector. Examples of the binder used in the lithium-sulfur battery include polytetrafluoroethylene, polyvinylidene fluoride, and polyethylene oxide. However, even when such a binder is used, there is a problem that the positive electrode active material in the lithium-sulfur battery tends to be detached from the current collector.

In addition, the lithium-sulfur secondary battery has a problem caused by a so-called shuttle phenomenon as shown in Fig. 1 for its phenomenon and mechanism. That is, in a conventional lithium-sulfur secondary battery, a polysulfide is dissolved in the sulfur anode during charging and discharging, and a so-called shuttle phenomenon occurs between the anode and the cathode, which causes a serious problem in battery capacity and cycle characteristics.

Korean Patent No. 0382302 Korean Patent No. 1438065

An object of the present invention is to provide a transition metal oxide-sulfur composite cathode active material for a lithium-sulfur secondary battery which can improve the conductivity of sulfur and improve the rate characteristics of the battery.

Another object of the present invention is to provide a working electrode for a lithium ion battery including the above cathode active material.

It is still another object of the present invention to provide a lithium ion battery including the cathode active material.

It is still another object of the present invention to provide an electric vehicle including the cathode active material.

It is still another object of the present invention to provide a method for producing the cathode active material.

In order to accomplish the above object, the cathode active material of the present invention may include sulfur nanoparticles and alpha-transition metal oxide nanowires coated on all or a part of the surfaces of the sulfur nanoparticles.

The? -Transition Metal Oxide nanowire may be of a two-dimensional tunnel structure type in which lithium is inserted during the first cycle charging and discharging.

The? -Transition Metal Oxide nanowire may have a length of 200 to 500 nm and a width of 20 to 60 nm.

The? -Transition metal oxide may be MnO ₂ , SnO ₂ And TiO < ₂ >.

The sulfur nanoparticles may have an average particle diameter of 5 to 50 nm.

The cathode active material may be a mixture of sulfur nano particles and? -Transition metal oxide nanowires in a volume ratio of 1: 0.5 to 1.2.

According to another aspect of the present invention, a working electrode for a lithium ion battery includes the cathode active material.

According to another aspect of the present invention, there is provided a lithium ion battery including the cathode active material.

According to another aspect of the present invention, there is provided an electric vehicle including the cathode active material.

According to another aspect of the present invention, there is provided a method for preparing a cathode active material, comprising the steps of: (A) dispersing a manganese oxide precursor in an acidic aqueous solution and conducting hydrothermal reaction at 100 to 200 ° C;

(B) subjecting the hydrothermal reaction to a first heat treatment at 50 to 130 캜 to obtain an? -Manganese oxide nanowire; And

(C) powdering the α-manganese oxide nanowire and adding sulfur nanoparticles, followed by a second heat treatment at 130 to 200 ° C. to coat all or a part of the surface of the sulfur nanoparticles with α-manganese oxide nanowires ; ≪ / RTI >

After step (C), lithium may be inserted into the α-manganese oxide nanowire coated on all or a part of the surface of the sulfur nano-particles after the first cycle charging and discharging.

In step (C), the sulfur nanoparticles and the α-manganese oxide nanowire powder may be mixed in a volume ratio of 1: 0.5-1.2.

The acidic aqueous solution may be a 50 to 90% aqueous sulfuric acid solution, an aqueous hydrochloric acid solution, or an aqueous nitric acid solution.

Wherein the manganese oxide precursor is more than one member selected from the group consisting of potassium permanganate (KMnO _4), manganese oxide (MnO), manganese hydroxide (Mn (OH) _2), manganese sulfate (MnSO ₄₎ and manganese fluoride (MnF ₄₎ .

When the positive electrode active material of the present invention undergoes a shuttle reaction between lithium and sulfur when charging and discharging proceed in the first cycle, lithium is inserted into the? -Manganese oxide nanowire to inhibit the dissolution of lithium polysulfide, The oxidation number is decreased to improve the conductivity of the sulfur as well as to improve the rate characteristic of the battery.

Also, in the cathode active material of the present invention, the α-manganese oxide nanowire relaxes the shape of the sulfur clusters in the course of the secondary heat treatment in which sulfur is subjected to recrystallization, so that the size is reduced before the recrystallization occurs and becomes uniform as a whole, It alleviates the volumetric expansion of sulfur in the electrode, suppresses the shuttle phenomenon, improves the rate characteristics and improves the conductivity of sulfur. In addition, the surface of sulfur is coated with? -Manganese oxide nanowires to inhibit the dissolution of lithium polysulfide.

Figure 1 shows the operating principle of a lithium-sulfur battery.
FIG. 2 is a graph illustrating the rate characteristics of the positive electrode active material of Example 1 and the positive electrode active material of Comparative Example 1, including the .alpha.-manganese oxide nanowire with lithium inserted therein.
FIG. 3 is a graph illustrating the first and second cycle characteristics of the positive electrode active material of Example 1 and the positive electrode active material of Comparative Example 1 including the lithium-inserted? -Manganese oxide nanowire.
4 is a SEM photograph of a cathode active material prepared according to Example 1 of the present invention.
5 is a TEM photograph of a cathode active material prepared according to Example 1 of the present invention.
6 is a view illustrating a process of coating? -Manganese oxide nanowires on sulfur according to Example 1 of the present invention.
7 is a photograph (TGA) of a component analysis of a cathode active material of Example 1 of the present invention and a cathode active material of Comparative Example 1 and Comparative Example 2 including an? -Manganese oxide nanowire with lithium inserted therein.

The present invention relates to a positive electrode active material of a transition metal oxide-sulfur composite material for a lithium-sulfur battery and a method of manufacturing the same, which can improve the conductivity of a sulfur battery to improve the rate characteristics of the battery.

In particular, the cathode active material of the present invention has lithium inserted into the? -Transition metal oxide nanowire after the first charge and discharge cycle. Further, during the heat treatment (for example, the second heat treatment) during the production of the cathode active material, the recrystallization of sulfur is suppressed, and the size is reduced before the recrystallization occurs to obtain uniformly uniform sulfur nanoparticles.

Hereinafter, the present invention will be described in detail.

The positive electrode active material of the present invention includes sulfur nanoparticles and? -Transition metal oxide nanowires coated on all or part of the surface of the sulfur nanoparticles.

The sulfur nanoparticles have an average particle size of 5 to 50 nm, preferably 10 to 30 nm, which is reduced in size by about 20 to 30% as compared with the sulfur nanoparticles originally used in the second heat treatment during the production of the cathode active material. When the average particle diameter of the sulfur nano-particles is less than the lower limit value or exceeds the upper limit value, the volumetric expansion of sulfur at the time of charging and discharging can not be reduced, the shuttle phenomenon can not be suppressed and the conductivity is not improved.

The? -Transition Metal Oxide nanowire is a two-dimensional tunnel structure type. During the first cycle charging and discharging, lithium is inserted to form? -Transition metal oxide nanowires with lithium inserted therein.

[Chemical Formula 1]

Li _x MO ₂

X is a real number of 0.5? X? 0.9; M is at least one element selected from the group consisting of Mn, Sn And Ti.

Also, the length of the? -Transition metal oxide nanowire is 200 to 500 nm, preferably 250 to 350 nm; The width is 20 to 60 nm, preferably 25 to 50 nm. If the value is less than the lower limit of the numerical range of the length and width of the nanowire or exceeds the upper limit value, lithium is not inserted and the elution of the lithium polysulfide is not inhibited, and the rate characteristic of the battery may be deteriorated.

In the cathode active material, the sulfur nanoparticles and the? -Transition metal oxide nanowires are mixed at a volume ratio of 1: 0.5 to 1.2, preferably 1: 0.7 to 1.0.

The cathode active material of the present invention may be included in a working electrode for a lithium ion battery, a lithium ion battery or an electric vehicle.

In addition, the present invention can provide a method for producing a cathode active material for a lithium-sulfur secondary battery.

The method for producing a cathode active material according to the present invention comprises the steps of: (A) dispersing a manganese oxide precursor in an acidic aqueous solution and subjecting the mixture to hydrothermal reaction at 100 to 200 ° C;

(B) subjecting the hydrothermal reaction to a first heat treatment at 50 to 130 캜 to obtain an? -Manganese oxide nanowire; And

(C) powdering the α-manganese oxide nanowire and adding sulfur nanoparticles, followed by a second heat treatment at 130 to 200 ° C. to coat all or a part of the surface of the sulfur nanoparticles with α-manganese oxide nanowires ; ≪ / RTI >

First, in the step (A), a manganese oxide precursor is dispersed in an acidic aqueous solution and a hydrothermal reaction is performed. If the hydrothermal reaction is omitted, it may be difficult to obtain a nanowire of a two-dimensional tunnel structure type.

The temperature of the hydrothermal reaction is 100 to 200 占 폚, preferably 130 to 160 占 폚. When the temperature of the hydrothermal reaction is lower than the lower limit, the manganese oxide precursor is not dissolved and the yield of the? -Manganese oxide nanowire is lowered and a large amount of by-products can be produced. If the temperature exceeds the upper limit, May not be formed.

Further, the time for performing the hydrothermal reaction is 20 to 30 hours, preferably 22 to 26 hours. When the hydrothermal reaction time is less than the lower limit value, nanowires may not be formed, and when the hydrothermal reaction time is higher than the upper limit value, a large amount of by-products may be produced.

Wherein the manganese oxide precursor is potassium permanganate (KMnO _4), manganese oxide (MnO), manganese hydroxide (Mn (OH) _2), manganese sulfate (MnSO ₄₎ and manganese fluoride least one member selected from (MnF ₄₎ the group consisting of . In this case, when two kinds of manganese oxide precursors are used, they are mixed at a weight ratio of 1: 1 in view of forming a two-dimensional tunnel type nanowire. When the manganese oxide precursor is used alone or in combination, Lithium is inserted quickly and easily compared to mixing at a different weight ratio than the weight ratio.

The acidic aqueous solution is not particularly limited as long as it is a substance capable of evenly dispersing the manganese oxide precursor; Preferably 50 to 90% aqueous sulfuric acid solution, 50 to 90% aqueous hydrochloric acid solution or 50 to 90% aqueous nitric acid solution; And more preferably 50 to 90% aqueous solution of sulfuric acid.

Next, in the step (B), the hydrothermal reaction is followed by a first heat treatment to obtain an α-manganese oxide nanowire.

The first heat treatment is a step of drying the hydrolyzed reactant to obtain an? -Manganese oxide nanowire; At 50 to 130 캜, preferably at 70 to 90 캜 for 10 to 20 hours, preferably 12 to 15 hours.

When the first annealing temperature and time are less than the lower limit value, nanowires having a tunnel structure may not be formed. If the temperature is above the upper limit, nano-oxide may not be coated on the surface of the sulfur nanoparticles.

Next, in step (C), the α-manganese oxide nanowire is pulverized and sulfur nanoparticles are added, followed by a second heat treatment.

Through the second heat treatment, all or a part of the surface of the sulfur nano-particles is coated with the? -Manganese oxide nanowires. In addition, the sulfur nanoparticles relax the shape of the sulfur clusters of the α-manganese oxide nanowires during the course of undergoing recrystallization, and further reduce the size and uniformity of the whole before the recrystallization occurs. As a result, in the electrochemical reaction, the volumetric expansion of sulfur in the electrode is alleviated, the rate characteristic is improved, and the conductivity of sulfur is improved.

The second heat treatment is performed at 130 to 200 ° C, preferably 150 to 180 ° C for 10 to 20 hours, preferably 12 to 15 hours.

When the second heat treatment temperature and time are less than the lower limit value, the particle size of the sulfur nanoparticles is not reduced and becomes uneven, and the nanowire may not coat the sulfur nanoparticles. If the temperature exceeds the upper limit, The wire may not be coated.

The sulfur nanoparticles and the? -Manganese oxide nanowire powder are mixed in a volume ratio of 1: 0.5-1.2.

When the positive electrode active material of the present invention is charged and discharged in the first cycle, a shuttle reaction occurs between lithium and sulfur, and at the same time, lithium is inserted into the a-manganese oxide nanowire. In addition, α- manganese oxide nanowires with the lithium is inserted, so that this manganese Mn (4-x) + e each increase the number of in vitro transformed into manganese in Mn4 + in the structure, an improvement in conductivity than conventional MnO _2.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the present invention. Such variations and modifications are intended to be within the scope of the appended claims.

Example 1.

Potassium permanganate (KMnO ₄ ) and manganese sulfate (MnSO ₄ ) were mixed in an aqueous sulfuric acid solution at a weight ratio of 1: 1, dispersed for 1 hour, hydrothermally synthesized at 150 ° C for 24 hours, The product was recovered and washed sequentially with water and ethanol, respectively. Dried in an oven at 80 DEG C for 12 hours, finely pulverized, and then pulverized .alpha.-manganese oxide nanowire powder and sulfur nanoparticles were mixed at a weight ratio of 1: 1, mixed by hand mill, and heat-treated at 155 DEG C for 12 hours Thereby preparing a cathode active material.

The cathode active material, the conductive material and the binder thus prepared were put in a weight ratio of 6: 2: 2, and a slurry was prepared, and a thin film was formed on an aluminum foil at a height of 100 탆. The cell was assembled using this.

Comparative Example 1

Only sulfur was used. The slurry was prepared by mixing sulfur, a conductive material and a binder at a weight ratio of 6: 2: 2, and a thin film was formed on an aluminum foil at a height of 100 μm.

Comparative Example 2

only? -manganese oxide nanowires were used. The α-manganese oxide nanowire powder, the conductive material, and the binder prepared in the same manner as in Example 1 were mixed in a weight ratio of 6: 2: 2 to prepare a slurry, and a thin film was formed on the aluminum foil at a height of 100 μm.

<Test Example>

In Example 1, a thin film made of a cathode active material was charged and discharged to prepare a lithium-doped a-manganese oxide nanowire, and then a test example was performed. That is, the alpha-manganese oxide nanowire coated with sulfur nanoparticles is lithium-doped.

Test example 1. Rate characteristic measurement

FIG. 2 is a graph showing the rate characteristics of the positive electrode active material of Example 1 and the positive electrode active material of Comparative Example 1, which include? -Manganese oxide nanowires into which lithium is inserted.

As shown in FIG. 2, it was confirmed that Example 1 significantly improved the rate characteristics over the entire period (24 cycles) as compared with Comparative Example 1. [

Test Example 2. Measurement of 1 and 2 cycle characteristics

FIG. 3 is a graph illustrating the first and second cycle characteristics of the positive electrode active material of Example 1 and the positive electrode active material of Comparative Example 1, which include? -Manganese oxide nanowires doped with lithium. FIG.

As shown in FIG. 3, it was confirmed that Example 1 was superior in specific capacity to Comparative Example 1 in both the first and second cycles.

Test Example 3. Surface Capture of Cathode Active Material

FIG. 4 is a photograph of the cathode active material of Example 1 taken by SEM, and FIG. 5 is a photograph of the cathode active material of Example 1 taken by TEM.

As shown in FIGS. 4 and 5, it was confirmed that the surface of the cathode active material of Example 1 was coated with a-manganese oxide nanowire in which the surface of the sulfur nanoparticles was inserted with lithium.

In particular, as shown in FIG. 6, the size of the sulfur nanoparticles is reduced by heat treatment, and the size is uniformized, which is also confirmed in FIG.

Test Example  4. TGA  Measure

7 is a photograph (TGA) of a component analysis of the cathode active material of Example 1 containing? -Manganese oxide nanowires and the cathode active material of Comparative Examples 1 and 2.

TGA analysis was conducted to investigate the ratio of α-manganese oxide nanowire to sulfur as a cathode active material.

As shown in FIG. 7, in Comparative Example 1 and Comparative Example 2, TGA analysis results for α-manganese oxide nanowires and sulfur, respectively, and α-manganese oxide nanowires showed no change in mass loss rate , The sulfur shows a change in the mass loss rate to 200% or more at the final 0%.

Example 1 shows TGA analysis of sulfur coated α-manganese oxide nanowires. After 200 ° C., the mass loss rate of only α-manganese oxide nanowires in Example 1 was finally shown to be 40%, and sulfur and α- Manganese oxide nanowires were mixed at a ratio of 6: 4.

Claims (14)

Transition metal oxide nanowires coated on all or a part of the surface of the sulfur nano-particles,
Transition metal oxide nanowire is a nanowire having a length of 200 to 500 nm and a width of 20 to 60 nm and is a two-dimensional tunnel structure type in which lithium is inserted during the first cycle charging and discharging. Cathode active material for secondary battery. delete delete The method of claim 1, wherein the? -Transition metal oxide is selected from the group consisting of MnO ₂ , SnO ₂ And TiO &lt; ₂ &gt;. The positive electrode active material according to claim 1, wherein the sulfur nanoparticles have an average particle diameter of 5 to 50 nm. The positive electrode active material according to claim 1, wherein the positive electrode active material is a mixture of sulfur nano particles and? -Transition metal oxide nanowires in a volume ratio of 1: 0.5 to 1.2. 7. A working electrode for a lithium ion battery comprising the cathode active material according to any one of claims 1 to 6. A lithium ion battery comprising the cathode active material according to any one of claims 1 to 6. An electric vehicle comprising the cathode active material according to any one of claims 1 to 6. (A) dispersing a manganese oxide precursor in an acidic aqueous solution and conducting a hydrothermal reaction at 100 to 200 ° C;
(B) a nanotubes having a length of 200 to 500 nm and a width of 20 to 60 nm, subjected to a first heat treatment at 50 to 130 ° C after the hydrothermal reaction, to form a two-dimensional tunnel structure type a-manganese oxide nanowire; And
(C) powdering the α-manganese oxide nanowire and adding sulfur nanoparticles, followed by a second heat treatment at 130 to 200 ° C. to coat all or a part of the surface of the sulfur nanoparticles with α-manganese oxide nanowires The method of manufacturing a cathode active material for a lithium-sulfur secondary battery according to claim 1, delete 11. The method of claim 10, wherein in step (C), the sulfur nanoparticles and the alpha-manganese oxide nanowire powder are mixed at a volume ratio of 1: 0.5-1.2. 11. The method of claim 10, wherein the acidic aqueous solution is an aqueous sulfuric acid solution, an aqueous hydrochloric acid solution, or an aqueous nitric acid solution in an amount of 50 to 90%. 11. The method of claim 10, wherein the manganese oxide precursor consisting of potassium permanganate (KMnO _4), manganese oxide (MnO), manganese hydroxide (Mn (OH) _2), manganese sulfate (MnSO ₄₎ and manganese fluoride (MnF ₄₎ Lt; RTI ID = 0.0 &gt; 1, &lt; / RTI &gt;

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