KR101278832B1 - Surface-coated lithium titanate powder, electrode, and secondary battery comprising the same - Google Patents

Abstract

The present invention provides a lithium titanium oxide powder whose surface is coated with a metal or a mixture of metal and carbon and a method for producing the same. According to the present invention, by using a lithium titanium oxide whose surface is coated with a mixture of metal and metal and carbon as an electrode active material of a lithium secondary battery, the electronic conductivity of the electrode can be increased and the rate characteristic of the battery can be improved.

Description

Lithium titanium oxide powder coated on the surface, an electrode having the same, and a secondary battery {SURFACE-COATED LITHIUM TITANATE POWDER, ELECTRODE, AND SECONDARY BATTERY COMPRISING THE SAME}

The present invention relates to a lithium titanate whose surface is coated with a metal or a mixture of metal and carbon and which can be used as an electrode active material of a lithium secondary battery.

Recently, with the development of portable devices such as mobile phones, notebook computers, camcorders, etc., demand for secondary batteries such as Ni-MH (Ni-MH) secondary batteries and lithium secondary batteries is increasing. In particular, lithium secondary batteries using lithium and nonaqueous electrolytes have been actively developed due to the high possibility of realizing small, lightweight and high energy density batteries. Generally, a transition metal oxide such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ is used as a cathode material of a lithium secondary battery, and lithium metal or carbon is used as an anode material. This is used, and a lithium secondary battery is comprised using the organic solvent which contains lithium ion as electrolyte between two electrodes. However, a lithium secondary battery using metal lithium as a negative electrode tends to generate dendrite crystals when charging and discharging is repeated, and there is a high risk of short circuit. Lithium secondary batteries that use a nonaqueous solvent containing lithium ions as an electrolyte have been put to practical use. However, since the carbon-based negative electrode material has a large irreversible capacity, there is a problem in that initial charge and discharge efficiency is low and the capacity is reduced. In addition, lithium may be deposited on the surface of carbon during overcharging, thereby causing a problem in safety.

On the other hand, there is an attempt to use lithium titanate as a negative electrode material. Lithium titanium oxide has a voltage of 1.5V based on lithium metal and has a long lifespan. In addition, it is a material that has been successfully used as an electrode active material of a lithium ion battery for watches, and can be disregarded in expansion and contraction during charge-discharge, and thus is an electrode material that is noticed at the time of enlargement of a battery. In addition, since the operating voltage is higher than the reduction potential of lithium, it is possible to prevent a problem that lithium is deposited on the surface of the negative electrode during overcharging. This material has been used conventionally as an anode material and can be utilized as a cathode material.

In particular, Li ₄ Ti ₅ O ₁₂ having a spinel structure is known to have a small change in crystal structure and a small deterioration due to a charge and discharge cycle even when lithium is repeatedly inserted and removed. It is a promising material. However, since the electron conductivity is low, there is a problem that the reaction resistance is high at the time of lithium insertion and desorption, and the characteristics of the rapid charging / discharging are significantly reduced. Therefore, it is difficult to be applied to a battery requiring high power.

Due to the above problems, there were attempts to the carbon coating on the surface of the lithium-titanium oxide to improve the electronic conductivity, Li ₄ Ti ₅ O A by giving ₁₂ into a carbon source (carbon source) in the process of synthesizing a carbon-coated Li ₄ Methods for producing Ti ₅ O ₁₂ and the like are known. However, since Li ₄ Ti ₅ O ₁₂ is synthesized while maintaining an oxidizing atmosphere in air, most carbon sources are burned in this process, and there is a problem that almost no carbon remains on the surface of Li ₄ Ti ₅ O ₁₂ . In addition, when successfully synthesized in a reducing atmosphere such as nitrogen or argon to coat carbon, titanium (titanium) is reduced to make the synthesis of Li ₄ Ti ₅ O ₁₂ difficult.

According to the present invention, after synthesizing Li ₄ Ti ₅ O ₁₂ first, by coating a metal or metal / carbon mixture on the surface of Li ₄ Ti ₅ O ₁₂ , the above problems can be solved. It has been found that the effect of improving the electron conductivity can be further maximized, and therefore, the rate performance of the lithium secondary battery can be improved.

Accordingly, an object of the present invention is to provide a lithium titanium oxide particles coated with a metal or a mixture of metal and carbon on its surface, a method of manufacturing the same, an electrode including the same as an active material, and a secondary battery having the electrode.

The present invention provides a lithium titanium oxide powder whose surface is coated with a metal or a mixture of metal and carbon.

In addition, the present invention a) a first step of uniformly mixing a lithium titanium oxide and a precursor that can be a metal by reduction; And b) heat treating the mixture in a reducing atmosphere to form fine metal particles on the surface of the lithium titanium oxide particles. It provides a method for producing a lithium titanium oxide powder coated with the surface described above.

And, the present invention a) a first step of preparing a mixture of a metal oxide and lithium titanium oxide; And b) mechanically alloying the mixture under a reducing atmosphere so that the metal fine particles reduced from the metal oxide are coated on the surface of the lithium titanium oxide; It provides a method for producing a lithium titanium oxide powder coated with a surface according to claim 1.

In addition, the present invention comprises the steps of: a) preparing a dispersion in which lithium titanium oxide is dispersed in a solution in which a metal salt is dissolved; And b) a second step of coating the metal fine particles on the surface of the lithium titanium oxide by selectively reducing metal ions by adding a reducing agent to the dispersion; It provides a method for producing a lithium titanium oxide powder coated with a surface according to claim 1.

The present invention also provides an electrode including the lithium titanium oxide powder as an electrode active material and a secondary battery having the electrode.

According to the present invention, by using a lithium titanium oxide whose surface is coated with a mixture of metal and metal and carbon as an electrode active material of a lithium secondary battery, the electronic conductivity of the electrode can be increased and the rate characteristic of the battery can be improved. Therefore, according to the present invention, a lithium secondary battery having a lithium titanium oxide whose surface is coated with a metal or a mixture of metal and carbon as an electrode can be charged and discharged at a high speed, thereby making a consumer lithium ion battery (CLI: Consumer lithium- Applications such as ion battery, power tool, hybrid electric vehicle (HEV) are possible.

Hereinafter, the present invention will be described in detail.

Lithium titanium oxide in the present invention may be represented by the composition formula Li _x Ti _y O ₄ (0.5≤x≤3, 1≤y≤2.5). As a non-limiting example is _{Li 4/3 Ti 5/3} O 4, LiTi 2 O 4, Li 8/7 Ti 12/7 O 4, and the like, _{Li 4/5 Ti 11/5} O 4, preferably Li _4/3 may be a lithium-titanium oxide having Ti _5/3 O ₄ composition. Since the ion radius of lithium and titanium in the lithium titanium oxide crystals is very close to each other, a slight difference in the synthesis method causes mutual substitution of lithium and titanium, resulting in a difference in the position and intensity of the peak when X-ray diffraction analysis is performed. Even though they have stoichiometrically different compositions, they have substantially the same crystal formation and typical peak positions. Thus, the lithium titanium oxides have the same crystal structure, that is, a spinel structure. _{(However, Li 8/7 Ti 12/} 7 O 4 is an exception, having a ramsdellite structure.) However, the ease of manufacturing and chemical stability, and, in terms of charge-discharge capacity, Li _4/3 Ti _{5 /} The composition of ₃ O ₄ is most advantageous as an electrode active material.

The spinel structure is the O ² -face ^- centered cubic (FCC) close packed, and cations filled part of the half octahedral site (1/8 tetrahedral site) and 1/8 tetrahedral site (Tetrahedral site). In a typical spinel AB ₂ O ₄ structure, B ³⁺ ions occupy 1/2 octahedral site, A ²⁺ occupies 1/8 tetrahedral site, and the oxygen atom coordinates with three octahedral and one tetrahedral cations. Therefore, it can be seen that in Li ₄ Ti ₅ O ₁₂ , three of the Li atoms are located at the 1/8 tetrahedral site and the other is located at the 1/2 octahedral site.

If lithium ions are inserted into the spinel structure by an electrochemical reaction, the lithium ions are retained in place without participating in the reaction as constituent atoms, and three lithium ions from the outside are inserted into the spinel structure. It has a composition of Li ₇ Ti ₅ O ₁₂ , and at this time, tetrahedral sites are converted into octahedral sites by lithium ions entering the crystal structure.

In the present invention, the average particle diameter of the lithium titanium oxide particles may be 0.3 μm to 30 μm. When the lithium titanium oxide of the present invention is used as the active material particles of the secondary battery, when the particle size is smaller than the above range, there is a problem that it is difficult to mix with carbon black, a binder, and the like when preparing the electrode slurry, and larger than the above range. There is a problem in that the electrode surface becomes uneven after application of the electrode.

The lithium titanium oxide has a problem in that the electron conductivity is low and the reaction resistance is high at the time of lithium insertion and desorption, and the rate characteristic is significantly lowered.

Rate performance is an indicator of the ability to discharge high current at high speed, and C-rate is defined as the current flowing when the battery capacity is discharged in one hour, and Rated capacity is C / rate. The discharge capacity when discharged by C-rate is expressed in% when the discharge capacity at 5% (current rate when exhausting the self-capacitance after discharge for 5 hours) is 100%. In general, in case of discharging with C-rate, 100% of capacity cannot be discharged. In general, the higher the rated capacity, the better the rate characteristic. This has an important effect on the charging and discharging speed of the battery, and is evaluated as an important performance for applications in applications requiring large power, such as a power tool and a hybrid electric vehicle.

According to the present invention, the surface of the lithium titanium oxide is coated with a metal or a mixture of metal and carbon, whereby the electron conductivity of the lithium titanium oxide used as the electrode active material can be improved and the rate characteristic can be improved.

In the present invention, the metal coated on the surface of the lithium titanium oxide is preferably a material having high electron conductivity, and non-limiting examples thereof include Cu, Ag, Zn, Fe, Co, Ni, Mn, Al, Mg, Cr, Mo Etc. In addition, the metal coated on the surface of the lithium titanium oxide may be in the form of fine particles.

In the present invention, when the material coated on the surface of the lithium titanium oxide is a mixture of metal and carbon, the carbon may be in an amorphous form and may be in the form of fine particles, in which the average size of the carbon particles is 10 nm to 300 nm. Can be.

By coating the surface of the lithium titanium oxide with a metal or a mixture of metal and carbon as described above, the lithium titanium oxide of the present invention can have an electron conductivity of 0.001 Scm ^-1 ~ 0.1 Scm ^{- 1} , due to excellent electronic conductivity characteristics In addition, it is considered that the rate characteristic can be improved by overcoming the problem of polarization of the electrode material due to the high current and the increase of the internal resistance of the electrode, which cause the rate characteristic of the battery to decrease.

The lithium titanium oxide whose surface presented by this invention is coat | covered with the metal or the mixture of metal and carbon can be manufactured by the following method.

a) a first step of uniformly mixing the lithium titanium oxide and the precursor which can be a metal by reduction and optionally the precursor of carbon.

b) a second step of forming the fine metal particles or mixture particles of metal and carbon on the surface of the lithium titanium oxide particles by heat-treating the mixture in a reducing atmosphere.

In this case, in addition to the above step, optionally c) may further comprise a third step of grinding and classifying the heat-treated material.

The lithium titanium oxide used in the mixing of step a) may be a commercial powder or a powder prepared in a laboratory. For example, the lithium titanium oxide may be obtained by dry heat treatment of a mixture of titanium oxide and a lithium compound at a temperature of 700 to 1600 ° C. (See Japanese Patent Application Laid-Open No. 94-275263), and in addition, it can be obtained through a dry or wet powder synthesis method known to those skilled in the art.

The precursor of the metal used in the mixing of step a) is not particularly limited as long as it can be a metal by reduction, and non-limiting examples thereof include metal oxides, metal hydroxides, metal nitride oxides, metal sulfur oxides, metal halides, and the like. There is this.

The precursor of carbon that can be selectively mixed in step a) is not particularly limited as long as it can be carbonized by reduction heat treatment, and non-limiting examples thereof include sucrose, water-soluble polymer, pitch, resin, and the like. .

The mixing method in step a) may be a method known to those skilled in the art as a wet or dry mixing method, non-limiting examples thereof bead-mill, V-mixer, mechno-fusion, etc. There is a way.

The mixture obtained in step a) may be heat treated in a reducing atmosphere as in step b) to form fine particles of a metal or a mixture of metal and carbon on the surface of the lithium titanium oxide. At this time, the heat treatment temperature may be preferably 150 ℃ ~ 500 ℃, the heat treatment may be carried out under a reducing atmosphere in which H ₂ gas 0 to 5 vol% mixed in an inert gas such as N ₂ , Ar, He and the like.

When the precursor of carbon is included in the heat treatment to form a mixture of metal and carbon on the surface of the lithium titanium oxide, a stronger reducing atmosphere is formed by the carbon, which may help the reduction of the metal.

In addition, when the heat treated lithium titanium oxide particles are aggregated, the grinding and classification step may be additionally added. In this case, the grinding and classification may use methods known to those skilled in the art.

In addition to the above method, the present invention provides a method for producing lithium titanium oxide powder coated on a surface of a metal or a mixture of metal / carbon by mechanical alloying or metal reduction in solution.

The manufacturing method of surface coating lithium titanium oxide by the mechanical alloying method is as follows.

a) a first step of preparing a mixture of a metal oxide and a lithium titanium oxide, and optionally a carbon precursor;

b) a second step of mechanically alloying the mixture under a reducing atmosphere so that the metal fine particles reduced from the metal oxide are coated on the surface of the lithium titanium oxide;

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In step b), mechanical alloying is a process in which two or more chemically different powders are mixed and milled together, whereby material movement occurs to obtain a material in which two or more different phases are uniformly distributed instead of a simple mixture. . Milling with such chemical reactions is also referred to as reaction milling or mechanochemical processing.

In the present invention, a method of preparing a lithium titanium oxide powder in which a metal oxide, a lithium titanium oxide, and optionally a carbon precursor is mixed and milled to coat a metal or a metal / carbon mixture on a surface thereof is a method known to those skilled in the art as a mechanical alloying method. Non-limiting examples include relatively high energy for milling, such as SPEX mills, Planetary mills, Attrition mills, and Magneto-ball mills. If it can be applied, it is possible.

On the other hand, the manufacturing method of the surface-coated lithium titanium oxide by the metal reduction method in solution is as follows.

a) a first step of preparing a dispersion in which lithium titanium oxide is dispersed in a solution in which a metal salt is dissolved;

b) a second step of coating the metal fine particles on the surface of the lithium titanium oxide by selectively reducing the metal ions by adding a reducing agent to the dispersion;

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The lithium titanium oxide of the present invention may be used as an active material of an electrode, and may be used as a negative electrode or a positive electrode active material. Preferably, the lithium titanium oxide of the present invention may be used as a negative electrode active material, and since the oxidation potential is applied to the metal in the case of the positive electrode in the secondary battery, the metal formed on the surface of the lithium titanium oxide may be oxidized, but in the case of the negative electrode Since the reduction potential is applied to the surface, it can be maintained in the metal state formed on the surface.

According to the present invention, an electrode having a lithium titanium oxide whose surface is coated with a metal or a composite of metal and carbon as an electrode active material can be manufactured by a method known to those skilled in the art. For example, in addition to using the material as an active material according to the present invention, the electrode may further use a conductive agent for providing electrical conductivity and a binder that enables adhesion between the material and the current collector. The slurry was prepared by adding and stirring a conductive agent in a weight ratio of 1 to 30 wt% and a binder in a weight ratio of 1 to 10% with respect to the electrode active material prepared by the above method, and then adding the mixture to the dispersant and collecting the metal material. It is applied to the whole, compressed and dried to prepare a laminate electrode.

The conducting agent generally uses carbon black. Products currently marketed as conductive agents include acetylene black series (Chevron Chemical Company or Gulf Oil Company), Ketjen Black EC series (Armak Company ), Vulcan XC-72 (manufactured by Cabot Company), and Super P (manufactured by MMM).

Representative examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF) or copolymers thereof, styrenebutadiene rubber (SBR), cellulose, and the like. , N-methylpyrrolidone (NMP), acetone and the like.

The current collector of the metal material may be any metal that has high conductivity and is a metal that can easily adhere the paste of the material and does not have reactivity in the voltage range of the battery. Representative examples include meshes, foils, and the like, such as aluminum, copper, or stainless steel.

The present invention also provides a secondary battery comprising the electrode of the present invention. The secondary battery of the present invention can be produced using a method known in the art, and is not particularly limited. For example, the separator may be placed between the positive electrode and the negative electrode to add a nonaqueous electrolyte. In addition, the electrode, the separator and the nonaqueous electrolyte and, if necessary, other additives, may be those known in the art.

In addition, a porous separator may be used as a separator in manufacturing a battery of the present invention, and for example, a polypropylene-based, polyethylene-based, or polyolefin-based porous separator may be used, but is not limited thereto.

The nonaqueous electrolyte of the secondary battery that can be used in the present invention may include a cyclic carbonate and / or a linear carbonate. Examples of the cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), gamma butyrolactone (GBL), and the like. Examples of the linear carbonates include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), methyl propyl carbonate (MPC), and the like. In addition, the nonaqueous electrolyte of the secondary battery of the present invention contains a lithium salt together with the carbonate compound. Specific examples of the lithium salt include LiClO ₄ , LiCF ₃ SO ₃ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , and LiN (CF ₃ SO ₂ ) ₂ .

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. However, the present invention is not limited thereto.

Example 1

1) Lithium titanate (Li ₄ Ti ₅ O ₁₂ ) and CuO are added in a weight ratio of 95: 5, and the mixture is uniformly heat treated by planetary milling for 5 hours in a reducing atmosphere. Heat treatment conditions are performed in 300 degreeC and nitrogen + hydrogen (hydrogen 4vol%) atmosphere. After heat treatment, the agglomerated particles are crushed and classified by ball-milling.

2) The material prepared in 1) was used as an electrode active material. 5 parts by weight of acetylene black as a conductive agent and 5 parts by weight of PVDF with a binder were added to 90 parts by weight of the material, and added to NMP (N-methyl-2-pyrrolidone) to prepare an electrode slurry, which was then coated on an aluminum (Al) current collector. It is apply | coated to and dried, and an electrode is manufactured.

Using a lithium foil as the counter electrode, a coin half cell was formed together with the electrode manufactured by the above method.

A secondary battery is manufactured by interposing a polyolefin-based separator between the prepared electrodes and injecting the electrolyte solution.

[Example 2]

1) Lithium titanate (Li ₄ Ti ₅ O ₁₂ ), CuO and Super P in a weight ratio of 90: 5: 5 and synthesized in the same manner as in Example 1), and the same as in Example 1 2) The half cell is constructed by the method.

Comparative Example 1

A half cell was constructed in the same manner as in Example 1, 2) using uncoated lithium titanate (Li ₄ Ti ₅ O ₁₂ ).

Claims (8)

A lithium titanium oxide is coated with a metal or a mixture of metal and carbon,
The lithium titanium oxide is represented by the formula of Li _x Ti _y O ₄ (0.5≤x≤3, 1≤y≤2.5),
The metal is selected from the group consisting of Cu, Ag, Zn, Fe, Co, Ni, Mn, Al, Mg, Cr and Mo,
The metal and carbon are in the form of particles,
The average size of the carbon particles is 10nm ~ 300nm,
The lithium titanium oxide powder, characterized in that the metal or a mixture of metal and carbon is coated on the outer peripheral surface of the particulate lithium titanium oxide. A metal or a mixture of metal and carbon is coated on the surface of the lithium titanium oxide particles,
The lithium titanium oxide is selected from the group consisting of Li _4/3 Ti _5/3 O ₄ , LiTi ₂ O ₄ , Li _8/7 Ti _12/7 O ₄ and Li _4/5 Ti _11/5 O ₄ ,
The metal is selected from the group consisting of Cu, Ag, Zn, Fe, Co, Ni, Mn, Al, Mg, Cr and Mo,
The metal and carbon are in the form of particles,
Lithium titanium oxide powder, characterized in that the average size of the carbon particles is 10nm ~ 300nm. delete Claim 1 or claim 2, wherein the electron conductivity is 0.001 Scm ^-1 ~ 0.1 Scm ^{- 1} which is characterized by a lithium titanium oxide powder. The lithium titanium oxide powder according to claim 1 or 2, which is used as an electrode active material capable of occluding and desorbing lithium ions. An electrode comprising the lithium titanium oxide powder of claim 1 or 2 as an active material. The secondary battery provided with the electrode of Claim 6. The secondary battery of claim 7, wherein the electrode is a negative electrode.