KR20130134949A - Mixed positive-electrode active material and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a mixed positive electrode active material and a lithium secondary battery comprising the same, more specifically, to a mixed positive electrode active material and a secondary battery comprising the same, capable of being used for a lithium secondary which has excellent lifetime characteristic with low price by adding Li 2NixCu1-xO2(0<=x<=1) whose charging capacity is greater than discharging capacity, to Li4Mn5O12 which is hard to be used for a battery by itself.

Description

Mixed positive electrode active material and lithium secondary battery comprising same {MIXED POSITIVE-ELECTRODE ACTIVE MATERIAL AND LITHIUM SECONDARY BATTERY COMPRISING THE SAME}

The present invention relates to a mixed cathode active material and a lithium secondary battery comprising the same.

2. Description of the Related Art In recent years, lithium secondary batteries have been used in many fields including portable electronic devices such as mobile phones, PDAs, and laptop computers. Especially, as the interest in environmental problems grows, it is one of the main causes of air pollution. As a driving source of electric vehicles that can replace fossil fuel vehicles such as gasoline vehicles and diesel vehicles, lithium secondary batteries Research on batteries has been actively conducted, and some of them are in the commercialization stage. On the other hand, in order to use lithium secondary battery as a driving source of such an electric vehicle, it is necessary to maintain a stable output in a use SOC section in addition to a high output.

On the other hand, in the case of LiCoO ₂ , which is a typical positive electrode material as a cathode material of a high capacity lithium secondary battery, the energy density increases and the practical limit of the output characteristic is reached. Especially, when used in a high energy density application field, In addition to the structural modification at high temperature, oxygen is released in the structure to generate an exothermic reaction with the electrolyte in the cell, which is the main cause of the explosion of the battery. In order to improve the safety problem of LiCoO ₂ , the use of lithium-containing manganese oxides such as LiMnO _{2 having} a layered crystal structure, LiMn ₂ O _{4 having a} spinel crystal structure, and lithium-containing nickel oxide (LiNiO ₂ ) has been considered.

In particular, the lithium-containing manganese oxide having a spinel crystal structure is a material that is highly expected as one of the cathode materials for lithium secondary batteries. Among them, Li ₄ Mn ₅ O ₁₂ has already been reported as a study subject of magnetic behavior in the 1950s (see Non Patent Literature 1 below), but since 1983, MMThackeray reported that it is possible to deposit lithium ions electrochemically. (Refer the following nonpatent literature 2) and examination as a positive electrode material of a lithium secondary battery is performed (for example, the following nonpatent literatures 3 and 4, etc.).

Li ₄ Mn ₅ O ₁₂ is a material having a spinel structure and a theoretical capacity of 163 mAh / g. In Li ₄ Mn ₅ O ₁₂ , since the oxidation number of Mn is +4, charge and discharge are possible in the 3V region. In addition, since Li ₄ Mn ₅ O ₁₂ has a smaller Jahn-Teller distortion effect than LiMn ₂ O ₄ having the same structure, the Li ₄ Mn ₅ O ₁₂ has excellent life and rate characteristics and a low price.

However, since Li ₄ Mn ₅ O ₁₂ has an initial charge capacity smaller than the discharge capacity, there is considerable difficulty in using it alone as a cathode material of a battery. In other words, in order to effectively use the 3V region by applying Li ₄ Mn ₅ O ₁₂ as the cathode material, an additional external lithium source is required.

Accordingly, the development of an external lithium source supply technology for efficiently applying Li ₄ Mn ₅ O ₁₂ , which has excellent life characteristics and low cost, as a cathode material of a lithium secondary battery is urgently needed.

 Journal of American Chemical Society Vol. 78, pp 3255-3260.  M. M. Thackeray, Material Research Bulletin Vol. 18, pp 461-472.  Journal of Electrochemical Society Vol. 136, No. 11, pp3169-3174.  Journal of Electrochemical Society Vol. 138, No. 10, pp2859-2864.

The present invention has been made to solve the above-mentioned requirements and conventional problems, the present invention can effectively supply an external lithium source for using the low cost Li ₄ Mn ₅ O ₁₂ in the lithium secondary battery with excellent life characteristics It is a technical problem to provide a mixed positive electrode active material.

In order to solve the above technical problem,

The present invention provides a mixed cathode active material comprising a first cathode active material represented by the following [Formula 1] and a second cathode active material represented by the following [Formula 2]:

[Formula 1]

Li _x Mn _y O _z

(Where x = 1, y = 2, z = 4; x = 2, y = 5, z = 9; or x = 4, y = 5, z = 12; and Mn is Ni, Co, Ti, It may be substituted with one or more elements selected from the group consisting of Cr, Zr, Nb, Cu, V, Mo, W, "Cr" and "Fe.)

(2)

Li ₂ Ni _x Cu _{1 - x} O ₂ (where 0≤x≤1)

In addition, another aspect of the present invention provides a cathode and a lithium secondary battery including the mixed cathode active material.

According to the present invention, by blending a specific lithium metal oxide having a large irreversible capacity with Li ₄ Mn ₅ O ₁₂ and using it as an external lithium source, it is effective to use Li ₄ Mn ₅ O ₁₂ which is excellent in life characteristics and low cost as a cathode material of a lithium secondary battery. To be used.

1 is Li ₄ Mn ₅ O ₁₂ It is a graph showing the first cycle charge and discharge curve when using the cathode material alone (Comparative Example 1).
2 is Li ₂ NiO ₂ It is a graph showing the first cycle charge and discharge curve when using the cathode material alone (Comparative Example 2).
3 is Li ₄ Mn ₅ O ₁₂ + Li ₂ NiO ₂ A graph showing charge and discharge curves when using a mixed cathode material (Example).

Hereinafter, the present invention will be described in detail.

The mixed cathode active material of the present invention includes a first cathode active material represented by the following [Formula 1] and a second cathode active material represented by the following [Formula 2].

[Formula 1]

Li _x Mn _y O _z

Where x = 1, y = 2, z = 4; x = 2, y = 5, z = 9; or x = 4, y = 5, z = 12; and Mn is part or all of Ni It may be substituted with one or more elements selected from the group consisting of, Co, Ti, Cr, Zr, Nb, Cu, V, Mo, W, "Cr" and "Fe.)

Representative examples of the first positive electrode active material represented by [Formula 1] include Li ₄ Mn ₅ O ₁₂ .

The first positive electrode active material (Li ₄ Mn ₅ O ₁₂ ) is a lithium manganese oxide having a cubic symmetrical structure and is one of stoichiometric spinels, such as a cation array structure of Li [Li _0.33 Mn _1.67 ] O ₄ , and has a theoretical capacity of 163 mAh / It has a relatively high capacity as g.

In addition, when the first positive electrode active material is used as an electrode active material of a lithium secondary battery, it exhibits the following electrochemical reaction during charging and discharging.

If this case, the first cathode active material is 2.5 x of Li _{6 .5} Mn ₅ O ₁₂ joseongil only appears in the Jahn-Teller distortion phenomenon, Li x = 3 in a fully charged state, that is rock salt, Li ₅ Mn ₇ At O ₁₂ , the Jahn-Teller distortion effect is weaker than that at Li ₂ Mn ₂ O ₄ .

However, since the first cathode active material has a smaller charging capacity than the discharge capacity (see FIG. 1), there is a limit in using it alone as a cathode material of a battery.

Accordingly, the present invention provides a composite cathode material in which the first cathode active material is blended with a second cathode active material represented by the following [Formula 2] having a large irreversible capacity as an external lithium source.

(2)

Li ₂ Ni _x Cu _{1 - x} O ₂

In Formula 2, 0 ≦ x ≦ 1 (in detail, x = 1).

Unlike the first cathode active material, the second cathode active material is very suitable as an additive capable of providing a lithium source because the charging capacity is larger than the discharge capacity (see FIG. 2). That is, the present invention blends the first positive electrode active material and the second positive electrode active material having different charge and discharge characteristics, so that battery capacity is balanced between the positive electrode and the negative electrode.

In one embodiment, when the second positive electrode active material is charged and discharged in the voltage range of 4.35V to 2.5V, the difference between the charge capacity and the discharge capacity (ie, irreversible capacity) in the first cycle may be 200 mAh / g or more. have. Specifically, the second positive electrode active material has an irreversible capacity of 200 mAh / g or more, and the lithium of the irreversible capacity is inserted into the first positive electrode active material during discharge, thereby serving as a lithium source. The lithium inserted into the first positive electrode active material during the discharge process is inserted / desorbed into the first positive electrode active material during the continuous charge / discharge process and participates in the charge / discharge process of the battery. The first positive electrode active material is the positive electrode material of the battery. It can be used smoothly.

As such, the second cathode active material serving as a lithium source may further have an irreversible capacity of 250 mAh / g or more. This is because it is preferable to provide a sufficient amount of lithium to the first cathode active material so that all theoretical capacity can be expressed.

However, since the irreversible capacity may vary depending on the composition of the second cathode active material in the cathode active material, the capacity of the second cathode active material may be adjusted according to the capacity of the first cathode active material included in the cathode active material.

The method of forming the mixed cathode active material by mixing the first cathode active material and the second cathode active material is not particularly limited, and various methods known in the art may be adopted.

In this case, the second cathode active material may be included in an amount of 5 to 50 parts by weight, more specifically 20 to 40 parts by weight, based on 100 parts by weight of the total cathode active material. When the content of the second positive electrode active material exceeds 50 parts by weight, it may be difficult to increase the energy of the lithium secondary battery. When the content of the second positive electrode active material is less than 5 parts by weight, the content of the second positive electrode active material may be too small to properly use the first positive electrode active material. The life characteristics and rate characteristics of the battery may be degraded. In addition, while the theoretical capacity of the first cathode active material can be expressed within the range as described above, by reducing the irreversible capacity, all the lithium ions participate in the charge and discharge process, thereby minimizing the formation of dendrites.

Thus, the positive electrode active material of the present invention in which the first positive electrode active material and the second positive electrode active material are mixed, for example, exhibits an equivalent charge capacity and discharge capacity in a voltage range of 4.35 V to 2.5 V, and thus can be harmonically reversible between the positive electrode and the negative electrode. Capacity can be implemented.

In addition, the positive electrode active material according to the present invention is uniform in particle size or shape of the first positive electrode active material and the second positive electrode active material, so that the conductive material coated on the mixed positive electrode material is biased to only one of which the (non) surface area is large. As a result, it is possible to prevent a phenomenon in which the conductivity of the other cathode active material in which the conductive material is relatively distributed is significantly weakened, and as a result, the conductivity of the cathode active material may be greatly improved.

In order to reduce the difference in particle size or specific surface area of two or more positive electrode active materials to be mixed, a method of forming a positive electrode active material having particles of relatively small size as secondary particles as described above, A method of forming a small particle size or a method of applying both simultaneously may be used.

Meanwhile, the cathode active material according to the present invention may include two or more conductive materials having different particle sizes or shapes.

The method of incorporating the conductive material is not particularly limited, and conventional methods known in the art such as coating on the cathode active material can be employed. As described above, in order to prevent a phenomenon in which the conductive material is biased due to particle size difference between the cathode active materials to be mixed, graphite and conductive carbon may be used simultaneously as the conductive material in one embodiment of the present invention. have.

By simultaneously coating graphite and conductive carbon having different particle sizes and shapes as the conductive material on the mixed cathode material, the conductivity of the entire cathode active material is reduced or decreased due to the particle size or surface area difference between the first and second cathode active materials. The problem of output can be improved more effectively, and at the same time, a high capacity cathode material having a wide available SOC section can be provided.

The cathode active material according to the present invention furthermore, in addition to the first cathode active material and the second cathode active material, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium cobalt-nickel oxide, lithium cobalt-manganese oxide, lithium manganese-nickel oxide, One or more lithium-containing metal oxides selected from the group consisting of lithium cobalt-nickel-manganese oxides and oxides substituted or doped with ellipsoid (s) may be further included, and the ellipsoids may be Al, Mg, Ni, Co, Fe, Cr, V, Ti, Cu, B, Ca, Zn, Zr, Nb, Mo, Sr, Sb, W and Bi may be one or more selected from the group consisting of.

In this case, the lithium-containing metal oxide may be included within 50 parts by weight based on 100 parts by weight of the positive electrode active material.

The graphite and the conductive carbon are not particularly limited as long as they are excellent in electrical conductivity and have a conductivity without inducing side reactions in the internal environment of the lithium secondary battery or causing chemical change in the battery.

Specifically, the graphite is not limited to natural graphite or artificial graphite, and the conductive carbon is particularly preferably a carbonaceous material having high conductivity. Specifically, carbon black, acetylene black, ketjen black, channel black, Carbon black such as lamp black, thermoplastic black or the like, or a mixture of one or more materials selected from the group consisting of materials including crystal graphene and graphite. It goes without saying that a conductive polymer having high conductivity may also be used in some cases.

Here, the conductive material made of graphite and conductive carbon may be included in an amount of 0.5 to 15 parts by weight based on 100 parts by weight of the mixed cathode material. If the content of the conductive material is less than 0.5 parts by weight, the effect as described above can not be expected. If the content of the conductive agent is more than 15 parts by weight, the amount of the cathode active material is relatively small, .

In this case, the content of the conductive carbon may be included in an amount of 1 to 13 parts by weight, specifically 3 to 10 parts by weight, based on 100 parts by weight of the cathode material.

According to another aspect of the present invention, there is provided a lithium secondary battery comprising a positive electrode comprising the positive electrode active material and the positive electrode material coated on the current collector, and a positive electrode including the positive electrode.

Generally, a lithium secondary battery is composed of a positive electrode composed of a positive electrode material and a current collector, a negative electrode composed of a negative electrode material and a current collector, and a separator capable of blocking electron conduction and conducting lithium ions between the positive electrode and the negative electrode, The void of the membrane material contains an electrolyte solution for the conduction of lithium ions.

The positive electrode and the negative electrode are usually prepared by applying a mixture of an electrode active material, a conductive agent and a binder on a current collector, followed by drying. If necessary, a filler may be further added to the mixture.

The lithium secondary battery of the present invention can be manufactured according to a conventional method in the art. Specifically, it can be produced by putting a porous separator between the anode and the cathode and introducing a non-aqueous electrolyte.

The present invention further provides a battery module or a battery pack including two or more of the lithium secondary battery. In this case, the battery module or the battery pack may be a small device such as a cell phone or a notebook computer, a power tool; An electric vehicle including an electric vehicle (EV), a hybrid electric vehicle (HEV), and a plug-in hybrid electric vehicle (PHEV); Electric two-wheeled vehicles including E-bikes and E-scooters; Electric golf cart; Electric truck; It may be used as a power source of any one of medium and large devices such as electric commercial vehicles or power storage systems.

Hereinafter, the present invention will be described in more detail with reference to specific examples.

Example

Manufacture of anode

As a positive electrode active material, 90% by weight of a mixture consisting of Li ₄ Mn ₅ O ₁₂ (70% by weight) and Li ₂ NiO ₂ (30% by weight) was added to NMP together with 6% by weight of denca black as a conductive material and 4% by weight of PVDF as a binder. Was added to make a slurry. This was coated on an aluminum (Al) foil as a positive electrode collector, rolled and dried to prepare a positive electrode for a lithium secondary battery.

The lithium secondary battery  Produce

A polymer electrolyte was manufactured by injecting a lithium electrolyte solution through a separator of porous polyethylene between the positive electrode and the graphite negative electrode prepared as described above.

Comparative Example  One

Except for using only Li ₄ Mn ₅ O ₁₂ as the positive electrode active material, it is the same as in the embodiment.

Comparative Example  2

Except for using only Li ₂ NiO ₂ as the positive electrode active material, the same as in Example.

Experimental Example

Evaluating the capacity of the battery while charging and discharging in the voltage range of 4.35V to 2.5V (4.6V to 2.0V in Comparative Example 1) with respect to the lithium secondary batteries according to Examples and Comparative Examples 1 and 2 (C-rate = 1C) is shown in Figures 1 to 3, respectively.

As can be seen from the charge / discharge curve shown in FIG. 1, when the positive electrode was formed by Li ₄ Mn ₅ O ₁₂ alone, the charge capacity was much smaller than the discharge capacity (that is, the irreversibility of the negative electrode became very large) and thus the application to the battery was virtually impossible. I can see the difficulty.

As can be seen from the charge / discharge curve shown in FIG. 2, when Li ₂ NiO ₂ alone was used to form a positive electrode, the charge capacity was much larger than the discharge capacity (that is, the irreversibility of the positive electrode became very large), thus making it difficult to apply to a battery. Able to know.

As can be seen from the charge / discharge curve shown in FIG. 3, when the Li ₄ Mn ₅ O ₁₂ + Li ₂ NiO ₂ composite positive electrode was formed according to the present invention, the charge capacity and the discharge capacity were equal to each other, thereby balancing the reversible capacity of the battery between the positive and negative electrodes. It can be seen that it is very suitable for use as a lithium secondary battery.

(The data shown in Figures 1 to 3 is just an example, and the detailed values will vary depending on the specification of the cell, so it can be said that the trend of the graph is more important than the detailed values.)

The foregoing description is merely illustrative of the technical idea of the present invention, and various changes and modifications may be made by those skilled in the art without departing from the essential characteristics of the present invention. The scope of the present invention should be interpreted based on the scope of the following claims, and all technical ideas within the scope of equivalents thereof should be interpreted in accordance with the following claims: It is to be understood that the invention is not limited thereto.

Claims (18)

A mixed cathode active material comprising a first cathode active material represented by the following [Formula 1] and a second cathode active material represented by the following [Formula 2]:
[Chemical Formula 1]
Li _x Mn _y O _z
(Where x = 1, y = 2, z = 4; x = 2, y = 5, z = 9; or x = 4, y = 5, z = 12; and Mn is Ni, Co, Ti, It may be substituted with one or more elements selected from the group consisting of Cr, Zr, Nb, Cu, V, Mo, W, Cr and Fe.)
(2)
Li ₂ Ni _x Cu _{1 - x} O ₂ (Where 0 ≦ x ≦ 1)
The method of claim 1,
The first cathode active material is a cathode active material, characterized in that Li ₄ Mn ₅ O ₁₂ .
The method of claim 1,
The second cathode active material is a cathode active material, characterized in that Li ₂ NiO ₂ .
The method of claim 1,
The second cathode active material is a cathode active material, characterized in that the difference between the charge capacity and the discharge capacity (non-reversible capacity) in the first cycle when the charge and discharge in the voltage range of 4.3V to 2.5V is 200mAh / g or more.
5. The method of claim 4,
The second cathode active material is a cathode active material, characterized in that having an irreversible capacity of 250mAh / g or more.
The method of claim 1,
The second positive electrode active material is a positive electrode active material, characterized in that it comprises 5 to 50 parts by weight based on 100 parts by weight of the positive electrode active material.
The method according to claim 6,
The second cathode active material is a cathode active material, characterized in that it comprises 20 to 40 parts by weight based on 100 parts by weight of the cathode active material.
The method of claim 1,
Wherein the cathode active material further comprises a conductive material.
9. The method of claim 8,
Wherein the conductive material is made of graphite and conductive carbon.
9. The method of claim 8,
Wherein the conductive material is contained in an amount of 0.5 to 15 parts by weight based on 100 parts by weight of the cathode active material.
10. The method of claim 9,
The conductive carbon is one selected from the group consisting of carbon black, carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black, or a material whose crystal structure includes graphene or graphite. A cathode active material, characterized in that the above mixture.
The method of claim 1,
Wherein the cathode active material is at least one selected from the group consisting of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium cobalt-nickel oxide, lithium cobalt-manganese oxide, lithium manganese-nickel oxide, lithium cobalt- Wherein the lithium-containing metal oxide further comprises at least one lithium-containing metal oxide selected from the group consisting of lithium, substituted or doped oxides.
The method of claim 12,
The other element is at least one element selected from the group consisting of Al, Mg, Ni, Co, Fe, Cr, V, Ti, Cu, B, Ca, Zn, Zr, Nb, Mo, Sr, Sb, And a cathode active material.
The method of claim 12,
The lithium-containing metal oxide is a positive electrode active material, characterized in that contained within 50 parts by weight based on 100 parts by weight of the mixed cathode active material.
A positive electrode comprising the positive electrode active material according to any one of claims 1 to 14.
A lithium secondary battery comprising the positive electrode according to claim 15.
A battery module comprising two or more lithium secondary batteries according to claim 16.
18. The method of claim 17,
The battery module may include a power tool; An electric vehicle including an electric vehicle (EV), a hybrid electric vehicle (HEV), and a plug-in hybrid electric vehicle (PHEV); Electric two-wheeled vehicles including E-bikes and E-scooters; Electric golf cart; Electric truck; Battery module, characterized in that used as a power source of any one of a commercial vehicle or a power storage system.

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