KR20150112338A - Cathode active material having core-shell structure and manufacturing method therof - Google Patents

Abstract

The present invention relates to a cathode active material having a core-shell structure, and a manufacturing method thereof. The present invention may include: a core unit which contains lithium oxides; and an alpha-phased shell unit which is formed by coating transition metal oxides on the surface of the core unit and thermal-treating the outcome. Therefore, the present invention can give high capacity and high power to a lithium secondary battery and improve rate properties and energy density.

Description

[0001] The present invention relates to a cathode active material having a core-shell structure and a cathode active material having a core-

The present invention relates to a core-shell structure cathode active material having an alpha-shell portion on the surface of a core portion and capable of imparting high capacity and high output of a lithium secondary battery and improving cycle characteristics and energy density even in a high temperature state, And a manufacturing method thereof.

Recently, miniaturization of electronic devices has been diversified into mobile phones, notebook computers (PCs), and portable personal digital assistants (PDAs), and the interest in energy storage technology has been increasing.

In addition, in the case of batteries used in hybrid vehicles (HEV) and electric vehicles (EV), not only high capacity and high output, but also stability are a big problem. As the field of application has expanded, research and development on storage technologies have been actively carried out, and therefore there is a growing interest in the development of secondary batteries capable of charging and discharging.

The secondary battery is composed of a positive electrode, a negative electrode, and an electrolyte, and the ratio of the positive electrode is the most important. The cathode material is a cathode active material and generally has a high energy density during charging and discharging, and the structure should not be destroyed by intercalation or desorption of reversible lithium ions. In addition, the electrical conductivity should be high, and the chemical stability of the organic solvent used as the electrolyte should be high. In addition, it should be a material whose production cost is low and environmental pollution problem is minimized.

Examples of the positive electrode active material of the lithium ion secondary battery include lithium nickel oxide (LiNiO ₂ ), lithium cobalt oxide (LiCoO ₂ ), lithium manganese oxide (LiMnO ₄ ), and the like, which are layered compounds capable of intercalating and deintercalating lithium ions. LiNiO ₂ has a high electric capacity, but it has not been put to practical use due to problems such as cycle characteristics and stability during charging and discharging. In addition, LiCoO ₂ has advantages such as high capacity, excellent cycle life and rate capability, and easy synthesis. However, LiCoO ₂ has a disadvantage of high cost of cobalt and harmful to human body and thermal instability at high temperature have.

LiMn ₂ O ₄ , on the other hand, is simpler to synthesize, less expensive, and environmentally friendly. However, Mn is present at an average of 3.5, and the structure of lithium manganese oxide is collapsed due to the Jahn-Teller twist phenomenon for local Mn ^{3 +} ions during charging and discharging, and manganese ions are eluted by the reaction with the electrolyte, There is a disadvantage that it decreases. Disadvantages of such a cathode active material are hindering commercialization of a lithium secondary battery.

On the other hand, the coprecipitation method is a method in which sodium hydroxide used as an aqueous solution containing nickel (Ni), cobalt (Co), and manganese (Mn) and sodium hydroxide used as a co-infiltrant and co- Is mixed with a lithium salt to obtain a cathode active material.

However, the coprecipitation method overcomes the drawbacks of the solid phase method in that it obtains a homogeneous composition in terms of the characteristics of the material. However, since the particle size of the active material is influenced by the particle size of the precursor, Therefore, there is a problem that a lot of effort and time are required in the optimization process.

Korea Patent No. 0515029 Korea Patent No. 0807970 Korea Patent Publication No. 2010-0081456 Korea Patent No. 1147601

An object of the present invention is to provide a cathode-shell structure cathode active material having a shell part on the surface of the core part in alpha.

Another object of the present invention is to provide a method for producing the cathode active material of the core-shell structure.

According to an aspect of the present invention, there is provided a positive electrode active material having a core-shell structure, comprising: a core portion containing lithium oxide; And a crystalline alpha -type shell portion formed by coating a surface of the core portion with a transition metal oxide and then performing heat treatment.

The lithium oxide is _{LiCoO 2, LiMn 2 O 4,} LiNiO 2, and _{Li (Ni x Co y Mn z} ) O 2 (0 <x <1, 0 <y <1, 0 <z <1, x + y + z = 1). &lt; / RTI &gt;

The transition metal oxide may be represented by the following Chemical Formula 1;

[Chemical Formula 1]

MO _x

M is at least one element selected from the group consisting of Ti, Mn, Fe, Co, and Ru, and x is the number of oxygen atoms capable of bonding with the transition metal M.

The heat treatment may be performed at 500 to 900 占 폚, and the content of the transition metal oxide may be 0.1 to 20% by weight based on the lithium oxide.

According to another aspect of the present invention, there is provided a method of manufacturing a cathode-shell structure cathode active material, comprising the steps of: (a) mixing a lithium oxide and a transition metal oxide precursor in a solution; (b) obtaining a compound coated with a transition metal oxide on the surface of the lithium oxide in a reducing atmosphere; And (c) heat-treating the compound at 500 to 900 占 폚 to form an alpha-phase transition metal oxide on the surface of the lithium oxide.

In the step (a), the transition metal oxide precursor may be a metal alkoxide solution, a metal salt organic solution, or an aqueous metal solution, and the metal may be at least one selected from the group consisting of Ti, Mn, Fe, Co and Ru.

In the step (a), the lithium oxide is LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , and Li (Ni _x Co _y Mn _z ) O ₂ (0 <x <1, 0 <y <1, , x + y + z = 1).

In the step (b), the reducing agent may be hydrazine or ethylene glycol. In the step (c), the heat treatment may be performed for 1 to 7 hours.

According to another aspect of the present invention, there is provided a lithium secondary battery comprising: a cathode including the cathode active material and a binder; A negative electrode comprising a negative electrode active material; A separation membrane for preventing a short circuit between the anode and the cathode; And an electrolyte comprising a lithium salt.

The binder may be at least one selected from the group consisting of polyamide-imide, polyimide, and polyvinylidene fluoride, and the anode may further include a conductive agent that is conductive carbon, a conductive metal, or a conductive polymer.

The negative active material may include activated carbon, graphite carbon, or lithium ions are inserted into the graphite-based carbon, a lithium salt is composed of _{LiPF 6, LiBF 4, LiClO 4} , LiCF 3 SO 3, LiSbF 6 and LiAsF ₆ Lt; / RTI &gt;

The cathode active material of the core-shell structure of the present invention is formed by coating a transition metal oxide on the surface of lithium oxide to form a more stable structure to prevent the structure of lithium oxide from being destroyed, It is possible to provide a lithium secondary battery having improved energy density and rate characteristics at the time of charging and discharging.

Also, the lithium secondary battery can be provided with a high capacity and a high output power at a high temperature, and the cycle characteristics can be improved.

1 is a SEM photograph of a cathode active material and an alpha phase MnO ₂ prepared according to one embodiment of the present invention and a comparative example.
2 is a TEM photograph of a cathode active material manufactured according to an embodiment of the present invention.
3 is a graph showing charge-discharge voltage characteristics of a half-cell manufactured according to an embodiment of the present invention and a comparative example.
FIG. 4 is a graph showing discharge capacity of a half-cell manufactured according to an embodiment of the present invention and a comparative example when charging / discharging is performed at a high temperature.

The present invention relates to a positive electrode active material having a core-shell structure having an alpha-shell portion on the surface of a core portion capable of imparting a high capacity and a high output of a lithium secondary battery at a high temperature and improving cycle characteristics and energy density, .

Hereinafter, the present invention will be described in detail.

The cathode active material of the core-shell structure of the present invention comprises a core portion containing lithium oxide; And an alpha -type shell portion formed by coating a surface of the core portion with a transition metal oxide and then performing heat treatment.

The core portion _{LiCoO 2, LiMn 2 O 4,} LiNiO 2, and _{Li (Ni x Co y Mn z} ) O 2 (0 <x <1, 0 <y <1, 0 <z <1, x + y + z = 1). &Lt; / RTI &gt; The lithium oxide is destroyed due to the progress of charging and discharging and elution of manganese, cobalt or nickel ion occurs. Since the lithium oxide is weak in a high temperature environment, the shell portion is formed to enclose the core portion to prevent elution of manganese, cobalt or nickel ions, Can be improved.

The shell portion can be converted into a crystalline phase (alpha phase) on an amorphous phase by heat-treating the transition metal oxide represented by the following Chemical Formula 1 at 500 to 900 占 폚, preferably 600 to 700 占 폚, for 1 to 7 hours. Generally, the shell portion not only easily prevents elution of manganese, cobalt or nickel ions of lithium oxide, but also protects the core portion without being destroyed even at a high temperature. The alpha phase of the shell portion can be confirmed by TEM analysis through SAED pattern or FTT image.

[Chemical Formula 1]

MO _x

M is at least one element selected from the group consisting of Ti, Mn, Fe, Co, and Ru, and x is the number of oxygen atoms capable of bonding with the transition metal M.

When a metal oxide containing a metal such as Si, Al, Zr, or Mg is used without using the transition metal oxide as the shell portion, heat treatment is performed at a temperature of 1000 ° C or more to cause phase transformation. Are formed in large proportions and are easily separated from lithium oxides at high temperatures. In addition, when the transition metal oxide itself not converted to the alpha phase is used as the coating layer, the capacity and output of the lithium secondary battery are lowered as compared with the case where the phase is changed to the alpha phase, and the energy density and rate characteristics And the lifetime of the lithium secondary battery is reduced.

When the heat treatment temperature and time are less than the lower limit value, the phase change may not occur. If the temperature is higher than the upper limit value, the characteristics of the cathode active material may be deteriorated.

The content of the transition metal oxide is 0.1 to 20% by weight, preferably 5 to 10% by weight, based on the lithium oxide. When the content of the transition metal oxide is less than the lower limit, the shell part does not act as a protective film for inhibiting the elution of manganese, cobalt or nickel ions contained in the core part, so that the reaction of manganese, cobalt or nickel ions with the electrolyte may occur, The increase in the capacity is not sufficient. If the upper limit is exceeded, the thickness of the shell portion becomes thick, and the characteristics of the lithium oxide may be deteriorated.

The present invention also provides a method for producing a cathode-shell structure cathode active material by coprecipitation.

The method for producing the cathode active material of the core-shell structure of the present invention comprises the steps of: (a) mixing lithium oxide and a transition metal oxide precursor in solution; (b) obtaining a compound coated with a transition metal oxide on the surface of the lithium oxide in a reducing atmosphere; And (c) heat treating the compound at 500 to 900 &lt; 0 &gt; C.

In the step (a), the lithium oxide is stirred in distilled water for 0.5 to 2 hours, and then the transition metal oxide precursor is added and stirred together for 0.5 to 1 hour.

The transition metal oxide precursor may be a metal alkoxide solution, a metal salt organic solution or an aqueous metal solution, and the metal may be one selected from the group consisting of Ti, Mn, Fe, Co, and Ru.

Next, in the step (b), a reducing agent is added and the mixed solution prepared in the step (a) is stirred under a reducing atmosphere to obtain a compound coated with a transition metal oxide on the surface of the lithium oxide.

The content of the coated transition metal oxide precursor may be controlled according to the yield of the transition metal oxide to be coated, not by controlling the time for synthesizing the transition metal oxide precursor.

The reducing agent is not particularly limited, but hydrazine or ethylene glycol may be preferably used. Normally, sufficient agitating force is required to form high-density particles, but unstable manganese, cobalt or nickel ions are partially oxidized due to air entrainment or the like, so that high-density products can not be obtained. A reducing agent is added to suppress such oxidation.

Next, in step (c), the compound prepared in step (b) is heat-treated at 500 to 900 ° C for 1 to 7 hours to convert the phase of the coated transition metal oxide.

The thickness of the thus prepared alpha metal phase transition metal oxide is 2 to 50 nm.

In addition, the present invention provides a lithium secondary battery including the above cathode active material.

The lithium secondary battery according to the present invention comprises a positive electrode comprising the positive electrode active material and a binder; A negative electrode comprising a negative electrode active material; A separation membrane for preventing a short circuit between the anode and the cathode; And an electrolyte comprising a lithium salt.

The binder contained in the positive electrode may include at least one selected from the group consisting of polyamide-imide, polyimide, and polyvinylidene fluoride. Further, the anode may further include a conductive agent, and examples of the conductive agent include conductive carbon, conductive metal, and conductive polymer.

The negative electrode active material is a compound capable of reversibly intercalating and deintercalating lithium, and specifically may be a graphite carbon having activated carbon, graphite carbon, or lithium ion inserted therein.

The lithium salt may be at least one selected from the group consisting of LiPF ₆ , LiBF ₄ , LiClO ₄ , LiCF ₃ SO ₃ , LiSbF ₆ and LiAsF ₆ .

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  One.

1-1. Preparation of cathode-shell structure cathode active material

LiMn ₂ O ₄ as a lithium oxide was thoroughly stirred in distilled water at room temperature (25 ° C) for 1 hour, and KMnO ₄ was added as a MnO ₂ precursor. The mixture was stirred together, and then ethylene glycol as a reducing agent was added dropwise And the mixture was reacted for 3 hours to prepare a cathode-shell structure cathode active material. The cathode active material was heat-treated at 600 ° C for 3 hours to convert the coated MnO ₂ to an alpha phase.

At this time, the content of MnO ₂ coated on the basis of LiMn ₂ O ₄ is 5 wt% (LiMn ₂ O ₄ content is 95 wt%). The synthesis of MnO ₂ formed on the surface of LiMn ₂ O ₄ indicates that KMnO _{4, which} is a precursor of manganese oxide, is violet and the resulting MnO ₂ (s) is black, This is possible.

1-2. Manufacture of anode and manufacture of half-cell

The synthesized core-shell structure cathode active material, Denka Black (DB) and polyvinylidene fluoride (PVdF) were mixed at a weight ratio of 92: 4: 4, and NMP was used as a dispersion medium. (Agate Morta), and dried at 80 DEG C for 8 hours to form a positive electrode active material layer, thereby preparing a positive electrode of a lithium secondary battery. In order to improve the conductivity of the prepared anode, it was vacuum dried in a vacuum oven at 80 캜 for 12 hours.

The prepared anode was used as the working electrode of the half-cell and the counter electrode was a lithium metal, a separator is a polypropylene, the electrolyte is wet, as the electrolyte is 1 M LiPF ₆ salt of ethylene carbonate, ethyl dissolved -Methyl carbonate and dimethyl carbonate in a volume ratio of 1: 1: 1 was used to prepare a half-cell. The fabricated half cell was fabricated in Coin 2032 type, and all processes in the cell assembly proceeded in a dry room where the relative humidity was kept below 3%.

Comparative Example  One.

But the same manner as in Example 1, without the use of LiMn ₂ O ₄ MnO ₂ precursor A positive electrode and a half cell were fabricated using the prepared positive electrode active material.

Test Example  One. Example  And Comparative example SEM  shooting

FIG. 1 is a SEM photograph of a cathode active material and alpha-phase MnO ₂ prepared according to one embodiment of the present invention and a comparative example.

As shown in FIG. 1, FIG. 1A is a photograph of a plate-shaped alpha phase MnO ₂ , FIG. 1B is a photograph of a LiMn ₂ O ₄ cathode active material prepared in Comparative Example 1, FIG. 1 is a photograph of a cathode active material coated with an alpha phase MnO ₂ on the surface of LiMn ₂ O ₄ prepared in FIG. 1B, it can be seen that LiMn ₂ O ₄ has a particle size larger than that of the plate-like alpha phase MnO ₂ . 1C, it can be seen that the surface of LiMn ₂ O ₄ is coated with alpha phase MnO ₂ .

The alpha phase MnO ₂ photographed in FIG. 1A was prepared by dissolving KMnO ₄ as a MnO ₂ precursor in distilled water for 30 minutes and then reacting MnO ₂ (s) for 3 hours while adding dropwise ethylene glycol as a reducing agent. And then annealed at 600 ° C for 3 hours to prepare alpha phase MnO ₂ . Synthesis if the MnO ₂ of KMnO ₄ is a manganese oxide precursor denotes a purple, MnO ₂ (s) produced exhibits a black color was confirmed by a change in color of the mixed solution.

Test Example  2. Example TEM  shooting

2 is a TEM photograph of a cathode active material manufactured according to an embodiment of the present invention.

As shown in FIG. 2, it can be confirmed that the surface of 4V spinel type LiMn ₂ O ₄ is coated with alpha phase MnO ₂ .

Test Example  3. Example  And Comparative example  Measurement of output characteristics

3 is a graph showing charge-discharge voltage characteristics of a half-cell manufactured according to an embodiment of the present invention and a comparative example.

The graph of FIG. 3 shows the charge / discharge voltage characteristics of the anode with a cut-off voltage of 3.5 to 4.3 V. The graphs of <0.1C, 0.1D>, <0.1C, 0.2D> The discharge when the processes of <0.1C, 0.5D>, <0.1C, 1D>, <0.1C, 2D>, <0.1C, 3D>, <0.1C, 4D> The dose was confirmed. Basically, in order to test the rate characteristics, the c-rate of the charge was constantly maintained at 0.1 C, and the tendency of the c-rate of the discharge was observed.

As shown in FIG. 3, the half-cell manufactured according to Example 1 showed that the anode made of the cathode-shell structure of the core-shell structure exhibited a more stable rate characteristic without causing a side reaction with respect to the electrolyte.

Test Example  4. Example  And Comparative example  High temperature characteristic measurement

FIG. 4 is a graph showing the discharge capacity of a half-cell manufactured according to an embodiment of the present invention and a comparative example when charging and discharging are performed at a high temperature of 55 ° C at 1C.

4, the Example 1, the positive electrode active material prepared in accordance with the will, and the coated MnO ₂ to maintain the harsh environment structural stability even in a high temperature condition by protecting the LiMn ₂ O _4, for example, compared to non-coated with MnO ₂ 1 &lt; / RTI &gt; of the positive electrode active material. In addition, the MnO ₂ coating layer is structurally superior in stability because it protects LiMn ₂ O ₄ from HF formed by reacting with a small amount of water in the electrolyte and LiPF _{6 in the} electrolyte solution.

Claims (16)

A core portion containing lithium oxide; And
And a crystalline alpha-phase shell portion formed of a transition metal oxide on the surface of the core portion. The method of claim 1, wherein the lithium oxide is _{LiCoO 2, LiMn 2 O 4,} LiNiO 2, and _{Li (Ni x Co y Mn z} ) O 2 (0 <x <1, 0 <y <1, 0 <z < 1, x + y + z = 1). The positive electrode active material of the core-shell structure according to claim 1, The positive electrode active material of claim 1, wherein the transition metal oxide is represented by the following formula (1).
[Chemical Formula 1]
MO _x
M is at least one element selected from the group consisting of Ti, Mn, Fe, Co, and Ru, and x is the number of oxygen atoms capable of bonding with the transition metal M. The cathode active material according to claim 1, wherein the heat treatment is performed at 500 to 900 占 폚. The cathode active material according to claim 1, wherein the content of the transition metal oxide is 0.1 to 20% by weight based on the lithium oxide. (a) mixing lithium oxide and a transition metal oxide precursor in solution;
(b) obtaining a compound coated with a transition metal oxide on the surface of the lithium oxide in a reducing atmosphere; And
(c) heat-treating the compound at a temperature ranging from 500 to 900 占 폚, wherein an alpha-phase transition metal oxide is formed on the surface of the lithium oxide. [7] The method of claim 6, wherein the transition metal oxide precursor in step (a) is a metal alkoxide solution, a metal salt organic solution, or an aqueous metal solution. The method according to claim 7, wherein the metal is at least one selected from the group consisting of Ti, Mn, Fe, Co, and Ru. The lithium _secondary battery according to claim 6, wherein the lithium oxide is LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ and Li (Ni _x Co _y Mn _z ) O ₂ (0 <x < , 0 < z < 1, x + y + z = 1). The method of claim 6, wherein the reducing agent in step (b) is hydrazine or ethylene glycol. [7] The method of claim 6, wherein the heat treatment in step (c) is performed for 1 to 7 hours. A positive electrode comprising a positive electrode active material and a binder according to any one of claims 1 to 5;
A negative electrode comprising a negative electrode active material;
A separation membrane for preventing a short circuit between the anode and the cathode; And
A lithium secondary battery comprising an electrolyte comprising a lithium salt. The lithium secondary battery of claim 12, wherein the binder is at least one selected from the group consisting of polyamide-imide, polyimide, and polyvinylidene fluoride. 13. The lithium secondary battery of claim 12, wherein the anode further comprises a conductive agent that is conductive carbon, a conductive metal, or a conductive polymer. 13. The lithium secondary battery of claim 12, wherein the negative electrode active material comprises activated carbon, graphite carbon, or graphite carbon having lithium ions inserted therein. The lithium secondary battery according to claim 12, wherein the lithium salt is at least one selected from the group consisting of LiPF ₆ , LiBF ₄ , LiClO ₄ , LiCF ₃ SO ₃ , LiSbF ₆ and LiAsF ₆ .

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