KR100396491B1 - Lithium secondary battery - Google Patents

Abstract

The present invention relates to a lithium secondary battery, which includes a positive electrode including a lithium metal oxide having a metal oxide layer formed on a surface thereof, a negative electrode including a carbonaceous active material in which lithium ions may be intercalated or deintercalated, and An electrolyte comprising an organic solvent and a lithium salt, the weight ratio of the electrolyte to the weight of the lithium secondary battery is 8 to 10% by weight.

The lithium secondary battery can reduce the amount of the electrolyte, which is economical, and at the same time, the average discharge potential and cycle life of the lithium secondary battery can be improved.

Description

Lithium secondary battery {LITHIUM SECONDARY BATTERY}

[Industrial use]

The present invention relates to a lithium secondary battery, and more particularly, to a lithium secondary battery that is economical and has an excellent average discharge potential retention.

[Prior art]

Lithium secondary batteries are prepared by reversibly inserting and detaching lithium ions as a positive electrode and a negative electrode, and filling an organic or polymer electrolyte between the positive electrode and the negative electrode, and lithium ions are inserted / desorbed at the positive electrode and the negative electrode. When produced, electrical energy is generated by oxidation and reduction reactions.

As a cathode active material of a lithium secondary battery, a chalcogenide compound is used. Examples thereof include LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _1-x Co _x O ₂ (0 <x <1), and LiMnO _2. Composite metal oxides such as are being studied.

Lithium metal is used as a negative electrode active material. However, when lithium metal is used, there is a risk of explosion due to battery short circuit due to the formation of dendrite, and thus it is being replaced by carbon-based materials such as amorphous carbon or crystalline carbon instead of lithium metal. . In particular, in recent years, boron-coated graphite (BOC) is manufactured by adding boron to a carbon-based material to increase the capacity of the carbon-based material.

Research into the lithium secondary battery having the above-described configuration economically and to produce a battery having excellent electrochemical properties is ongoing.

The present invention has been made to solve the above problems, and an object of the present invention is to provide an economical lithium secondary battery.

Another object of the present invention is to provide a lithium secondary battery having excellent cycle characteristics.

Still another object of the present invention is to provide a lithium secondary battery having an excellent average discharge potential retention.

Figure 1a is a graph showing the measurement of the impregnation performance of the positive electrode active material for lithium secondary batteries prepared by the methods of Examples and Comparative Examples of the present invention.

1B is an enlarged view of FIG. 1A.

Figure 2 is a schematic diagram schematically showing the impregnation performance test used in the present invention.

3 is a graph showing the average discharge potential according to the number of cycles of the positive electrode active material of Examples and Comparative Examples of the present invention.

Figure 4 is a graph showing the charge and discharge life of the positive electrode active material of Examples and Comparative Examples of the present invention.

In order to achieve the above object, the present invention is an anode comprising a lithium metal oxide having a metal oxide layer formed on the surface; A negative electrode including a carbonaceous active material in which lithium may be intercalated or deintercalated; And a lithium secondary battery comprising an electrolyte comprising an organic solvent and a lithium salt, the weight ratio of the electrolyte to the weight of the lithium secondary battery provides a lithium secondary battery that is 8 to 10% by weight.

Hereinafter, the present invention will be described in more detail.

The lithium secondary battery of the present invention exhibits improved electrochemical properties while using electrolytes in smaller amounts than conventional batteries.

In the conventional lithium secondary battery, the electrolyte was used in an amount of 11 to 13% by weight based on the total weight of the lithium secondary battery, and in the present invention, 8 to 10% by weight based on the total weight of the lithium secondary battery. As such, even if the amount of the electrolyte is reduced, the electrochemical characteristics of the lithium secondary battery are rather increased, and thus, the effect of reducing the electrolyte and the improvement of the electrochemical characteristics can be simultaneously obtained.

In the present invention, it is possible to reduce the amount of the electrolyte without deteriorating the electrochemical properties by using lithium metal oxide having a metal oxide layer formed on the surface of the cathode active material. It is thought that the positive electrode active material greatly improves the electrolyte impregnation performance due to the metal oxide layer formed on the surface, so that even when a small amount of electrolyte is used, the impregnation effect is excellent and uniform reaction and efficiency in the battery are increased.

The electrolyte is generally used in a lithium secondary battery, and includes an organic solvent and a lithium salt. As the organic solvent, cyclic carbonate, linear carbonate, ester, ether or ketone may be used. The ring carbonate may be a ring carbonate selected from the group consisting of ethylene carbonate, propylene carbonate and mixtures thereof, and the linear carbonate is selected from the group consisting of dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate and methylpropyl carbonate. One or more linear carbonates may be used. In addition, the ester may be an ester selected from the group consisting of n-methyl acetate, n-ethyl acetate and n-propyl acetate. As the ketone, polymethyl vinyl ketone may be used.

As the lithium salt dissolved in the organic solvent of the electrolyte, any one capable of promoting the movement of lithium ions between the positive electrode and the negative electrode can be used, and representative examples thereof include LiPF ₆ , LiAsF ₆ , LiCF ₃ SO ₃ , and LiN ( CF ₃ SO ₂ ) ₃ , LiBF ₆ or LiClO ₄ can be used.

The lithium metal oxide used in the present invention is selected from Chemical Formulas 1 to 14, and the metal oxide layer formed on the surface includes a metal selected from the group consisting of Mg, Al, Co, K, Na, Ca, Si, Ti, and V. do.

[Formula 1]

Li _x MnA ₂

[Formula 2]

Li _x MnO _2-z A _z

[Formula 3]

Li _x Mn _1-y M ' _y A ₂

[Formula 4]

Li _x Mn _1-y M ' _y O _2-z A _z

[Formula 5]

Li _x Mn ₂ O ₄

[Formula 6]

Li _x Mn ₂ O _4-z A _z

[Formula 7]

Li _x Mn _2-y M ' _y A ₄

[Formula 8]

Li _x BA ₂

[Formula 9]

Li _x BO _2-z A _z

[Formula 10]

Li _x B _1-y M " _y A ₂

[Formula 11]

Li _x Ni _1-y Co _y A ₂

[Formula 12]

Li _x Ni _1-y Co _y O _2-z A _z

[Formula 13]

Li _x Ni _1-yz Co _y M " _z A ₂

[Formula 14]

Li _{x '} Ni _1-y' Mn _{y '} M _z' A _α

(Wherein, 0.95 ≦ x ≦ 1.1, 0.01 ≦ y ≦ 0.1, 0.01 ≦ z ≦ 0.5, 0.95 ≦ x '≦ 1, 0.01 ≦ y ′ ≦ 0.5, 0.01 ≦ z' ≦ 0.1, 0.01 ≦ α ≦ 0.5, M 'is at least one of transition metals or lanthanide metals selected from the group consisting of Al, Cr, Mn, Fe, Mg, La, Ce, Sr and V, and M "is Al, Cr, Co, Mg, La At least one of transition metals or lanthanide metals selected from the group consisting of Ce, Sr and V, A is selected from the group consisting of O, F, S and P, and B is Ni or Co.)

The cathode active material for a lithium secondary battery of the present invention is prepared by encapsulating a compound selected from Chemical Formulas 1 to 14 with a metal alkoxide solution or a metal aqueous solution.

The coating method may be a general coating method such as sputtering method, CVD (Chemical Vapor Deposition) method, dip coating method, but as the simplest coating method, the dip coating method to simply dip the powder in the coating solution and then take it out Preference is given to using. The metal alkoxide solution is prepared by mixing an alcohol and a metal in an amount corresponding to 0.1 to 10% by weight based on the alcohol, and then refluxing it. Mg, Al, Co, K, Na, Ca, Si, Ti, and V may be used as the metal. As a solution containing Si, tetraorthosilicate commercially available from Aldrich can be used. As the metal alkoxide, metal methoxide, metal ethoxide or metal propoxide may be used, and the alcohol may be methanol, ethanol or isopropanol. The metal aqueous solution may use a vanadium oxide (V ₂ O ₅ ) aqueous solution. The vanadium oxide aqueous solution may be prepared by mixing water and vanadium oxide corresponding to 0.1 to 10% by weight based on the water, and then refluxing it.

When the concentration of the metal is lower than 0.1% by weight, there is no effect of coating a compound selected from the group consisting of the compounds of Formulas 1 to 14 with a metal alkoxide solution or an aqueous metal solution, and the concentration of the metal exceeds 10% by weight. If the coating layer is too thick, it is not preferable.

The powder coated with the metal alkoxide solution or the metal aqueous solution is dried in an oven at about 120 ° C. for about 5 hours. This drying process serves to more evenly distribute the lithium salt in the powder.

The compound powder selected from the group consisting of the compounds of Formulas 1 to 14 coated with the metal alkoxide solution or the metal aqueous solution is heat-treated at 300 to 800 ° C. In order to produce a more uniform crystalline active material, the heat treatment process is preferably performed under conditions of blowing dry air or oxygen. At this time, if the heat treatment temperature is lower than 300 ℃ coated metal alkoxide solution is not crystallized, the application of this active material to the battery may interfere with the movement of lithium ions. In addition, when the heat treatment temperature is higher than 800 ° C, lithium is evaporated in the oxide selected from Chemical Formulas 1 to 14, thereby changing the crystal structure, which is not preferable.

In the heat treatment process, the metal alkoxide solution or the metal aqueous solution is changed into a metal oxide, thereby preparing a cathode active material of a compound selected from the group consisting of the compounds of Formulas 1 to 14 coated with a metal oxide on the surface.

The compound of Formulas 1 to 14 used in the present invention may be a commercially available compound of Formulas 1 to 14, or may be a compound prepared by the following method.

In order to synthesize the compounds of Formulas 1 to 14, lithium salts and metal salts are mixed at a desired equivalent ratio. As the lithium salt, any one generally used to prepare a cathode active material for a manganese-based lithium secondary battery may be used, and representative examples thereof may include lithium nitrate, lithium acetate, lithium hydroxide, and the like. Manganese salt, cobalt salt, nickel salt or nickel manganese salt may be used as the metal salt. The manganese salt may be used, such as manganese acetate, manganese dioxide, the cobalt salt may be used cobalt oxide, cobalt nitrate or cobalt carbonate, and the like, nickel hydroxide, nickel nitrate or nickel Acetates and the like can be used. The nickel manganese salt may be prepared by precipitating nickel salt and manganese salt by coprecipitation method. As the metal salt, fluorine salt, sulfur salt or phosphorus salt may be precipitated together with manganese salt, cobalt salt, nickel salt, or nickel manganese salt. Manganese fluoride, lithium fluoride, etc. may be used as the fluorine salt, and manganese sulfide, lithium sulfide, etc. may be used as the sulfur salt, and H ₃ PO ₄ may be used as the phosphate salt. The manganese salt, cobalt salt, nickel salt, nickel manganese salt, fluorine salt, sulfur salt and phosphorus salt are not limited to the compound.

The mixing method may use, for example, mortar grinder mixing, in which an appropriate solvent such as ethanol, methanol, water, acetone is added and there is little solvent in order to promote the reaction of lithium salt and metal salt. It is desirable to carry out mortar grinder mixing until solvent-free.

The obtained mixture is heat-treated at a temperature of about 400 to 600 ° C. to prepare compound precursor powders of formulas 1 to 14 in a semi-crystalline state. If the heat treatment temperature is lower than 400 ℃ there is a problem that the reaction between the lithium salt and the metal salt is not sufficient. In addition, after drying the precursor powder prepared by heat treatment or after the heat treatment process, the precursor powder may be remixed at room temperature while blowing dry air to uniformly distribute the lithium salt.

The semi-crystalline precursor powder obtained may further be subjected to secondary heat treatment at a temperature of 700 to 900 ° C. for about 10 to 15 hours. If the secondary heat treatment temperature is lower than 700 ° C, there is a problem that the crystalline material is difficult to form. The heat treatment process is carried out by raising the temperature at a rate of 1 to 5 ℃ / min under the conditions of blowing (dry) air or oxygen, and consists of natural cooling after maintaining for a predetermined time at each heat treatment temperature.

Subsequently, it is preferable to remix the powder of the compound selected from the group consisting of the compounds of Formulas 1 to 14 at room temperature to distribute the lithium salt more uniformly.

In order to manufacture the positive electrode with the positive electrode active material, first, a binder such as polytetrafluoroethylene or polyvinylidene fluoride is dissolved in a solvent for preparing a slurry, and then the conductive material is dispersed. The solvent for preparing the slurry can uniformly disperse the positive electrode active material, the binder, and the conductive material, and it is preferable to use one that is easily evaporated. Typically, acetonitrile, methanol, ethanol, tetrahydrofuran, water, and the like are used. Can be used. Next, a lithium metal oxide selected from Chemical Formulas 1 to 14 coated with a metal oxide on a surface of the cathode active material is uniformly dispersed in the slurry in which the conductive material is dispersed to prepare a cathode active material slurry. The amount of the solvent contained in the slurry does not have a particularly important meaning in the present invention, and it is sufficient only to have an appropriate viscosity to facilitate the coating of the slurry.

The slurry thus prepared is applied to a current collector, and vacuum dried to form a positive electrode plate, which is then used for battery production. It is sufficient that the slurry is coated on the current collector to an appropriate thickness depending on the viscosity of the slurry and the thickness of the positive electrode plate to be formed, and the current collector is not particularly limited, but an aluminum current collector is generally used.

In the lithium secondary battery of the present invention, the negative electrode has a shape of KMFC (Japan Kawasaki, Japan), which is a material used as a negative electrode active material of a lithium secondary battery, such as graphite and carbon, which can deintercalation-intercalation of lithium ions. And manufactured with a carbonaceous negative electrode active material such as graphite).

In the lithium secondary battery of the present invention, a porous polymer film made of polypropylene or polyethylene, which is a polymer film widely used in lithium ion secondary batteries, may be used as a separator having a function of physically separating electrodes.

The produced cell may be rectangular, cylindrical or coin type, and the case material may be a can battery or a pouch battery. A brief introduction of the square-type pouch cell manufacturing process is to produce positive and negative electrode plates, and then the adcoated tabs are welded to the positive and negative electrodes. The anode / separator / cathode is wound to produce a jelly roll type electrode plate group. Pressing and compressing the winding jelly roll-type electrode plate group is put into a pouch that is already formed, and the remaining portion except the electrolyte injection hole is sealed by heat fusion. After the electrolyte is injected into the electrolyte inlet (in the case of direct injection of electrolyte), vacuum and atmospheric pressure are repeated 2-3 times to complete the electrolyte injection. Finally, the electrolyte inlet is heat-sealed to completely seal the entire battery to manufacture the pouch battery. do.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only one preferred embodiment of the present invention and the present invention is not limited to the following examples.

(Example 1)

Al-isopropoxide was dissolved in ethanol to prepare an Al-isopropoxide ethanol solution at 5% concentration. LiCoO ₂ powder (Nippon Chem, trade name: C-10) was added to the Al-isopropoxide ethanol solution and stirred to coat LiCoO ₂ powder with the ethanol solution. Heat-treating the resulting product at 300 ℃, to prepare a LiCoO ₂ of aluminum oxide formed on the surface was used as a positive electrode active material.

A cylindrical battery having a capacity of 1800 mAh was manufactured by a conventional method using KMFC (Kawasaki, Japan, fiber-shaped graphite) as the positive electrode active material and the negative electrode active material. In this case, 4.5 g of a mixed solvent (3: 3: 1 volume ratio) of ethylene carbonate, dimethyl carbonate, and diethyl carbonate in which 1.15 M LiPF ₆ was dissolved was used.

(Example 2)

The same procedure as in Example 1 was carried out except that the amount of the electrolyte was reduced from 4.5 g to 4.2 g.

(Example 3)

The same process as in Example 1 was carried out except that the heat treatment was changed from 300 ° C to 700 ° C.

(Example 4)

The same procedure as in Example 3 was conducted except that the amount of the electrolyte was reduced from 4.5 g to 4.2 g.

(Comparative Example 1)

LiCoO ₂ (Nippon Chem, trade name: C-10) was used as a positive electrode active material, and KMFC was used as a negative electrode active material to prepare a cylindrical battery having a capacity of 1800 mAh in a conventional manner. In this case, 4.5g of a mixed solvent (3: 3: 1 volume ratio) of ethylene carbonate, dimethyl carbonate, and diethyl carbonate in which 1.15M LiPF ₆ was dissolved was used.

(Comparative Example 2)

Except that the amount of the electrolyte was reduced from 4.5g to 4.2g was carried out in the same manner as in Comparative Example 1.

The wettability of the positive electrode active material used in Examples 1 to 3 and Comparative Example 1 was measured, and the results are shown in FIGS. 1A and 1B. Impregnation performance experiment was carried out by placing an electronic scale on one side of the positive electrode plate, the other side was immersed in the electrolyte, and weighed at regular intervals as shown in FIG. 2. At this time, the greater the weight increase, the better the electrolyte impregnation performance. FIG. 1B is an enlarged view of FIG. 1A. As shown in Figure 1a and Figure 1b, it can be seen that the positive electrode active material of Examples 1 to 3 in which the metal oxide layer is formed on the surface may be impregnated with more electrolyte solution than Comparative Example 1.

The lithium secondary batteries manufactured by the methods of Examples 1 and 2 and Comparative Examples 1 and 2 were charged with a current of 1C = 1800 mAh at a charge and discharge potential of 4.2 V to 2.75 V and charged and discharged for 200 cycles. While performing charging and discharging, the average discharge potential and cycle life were measured according to the number of cycles, and the results are shown in FIGS. 3 and 4, respectively. The average discharge potential and cycle life characteristics show all the results of several measurements of the batteries of Examples 1 and 2 and Comparative Example 1 and the same experimental method.

As shown in FIG. 3, it can be seen that the lithium secondary batteries of Examples 1 to 2 have a higher average discharge potential according to the number of charge and discharge cycles than Comparative Example 1. FIG.

In addition, as shown in FIG. 4, in the lithium secondary batteries of Comparative Examples 1 to 2 in which the metal oxide layer was not formed on the surface, the cycle life was almost the same or increased slightly as the amount of the electrolyte decreased. On the other hand, the lithium secondary battery in which the metal oxide layer is formed on the surface of the lithium secondary battery can be seen that the cycle life is significantly improved as the amount of the electrolyte decreases.

The reason for obtaining the results shown in FIGS. 3 and 4 is that LiCoO ₂ having a metal oxide layer formed on its surface has the same impregnation capability with respect to the electrolyte, compared to LiCoO ₂ without a metal oxide layer formed. In the case where a positive electrolyte solution is used, a larger amount of powder is uniformly impregnated to induce an improvement in efficiency and reactivity of the electrochemical reaction.

According to the results of FIGS. 3 and 4, the lithium secondary battery of the embodiment in which the metal oxide layer is formed on the surface has improved economical efficiency, cycle characteristics, and average characteristics, since the average discharge potential and cycle life characteristics are improved by using a small amount of electrolyte. An electrochemical characteristic improvement effect, such as a discharge potential retention rate, can be acquired simultaneously.

As described above, the lithium secondary battery of the present invention can reduce the amount of the electrolyte solution, which is economically advantageous, and can also improve the average discharge potential and cycle life of the lithium secondary battery.

Claims (3)

An anode comprising lithium metal oxide having a metal oxide layer formed on the surface thereof; A negative electrode including a carbonaceous active material in which lithium ions may be intercalated or deintercalated; And A lithium secondary battery comprising an electrolyte solution containing an organic solvent and a lithium salt, The weight ratio of the electrolyte to the weight of the lithium secondary battery is 8 to 10% by weight of a lithium secondary battery. The lithium secondary battery of claim 1, wherein the lithium metal oxide is selected from Chemical Formulas 1 to 14. [Formula 1] Li _x MnA ₂ [Formula 2] Li _x MnO _2-z A _z [Formula 3] Li _x Mn _1-y M ' _y A ₂ [Formula 4] Li _x Mn _1-y M ' _y O _2-z A _z [Formula 5] Li _x Mn ₂ O ₄ [Formula 6] Li _x Mn ₂ O _4-z A _z [Formula 7] Li _x Mn _2-y M ' _y A ₄ [Formula 8] Li _x BA ₂ [Formula 9] Li _x BO _2-z A _z [Formula 10] Li _x B _1-y M " _y A ₂ [Formula 11] Li _x Ni _1-y Co _y A ₂ [Formula 12] Li _x Ni _1-y Co _y O _2-z A _z [Formula 13] Li _x Ni _1-yz Co _y M " _z A ₂ [Formula 14] Li _{x '} Ni _1-y' Mn _{y '} M _z' A _α (Wherein, 0.95 ≦ x ≦ 1.1, 0.01 ≦ y ≦ 0.1, 0.01 ≦ z ≦ 0.5, 0.95 ≦ x '≦ 1, 0.01 ≦ y ′ ≦ 0.5, 0.01 ≦ z' ≦ 0.1, 0.01 ≦ α ≦ 0.5, M 'is at least one of transition metals or lanthanide metals selected from the group consisting of Al, Cr, Mn, Fe, Mg, La, Ce, Sr and V, and M "is Al, Cr, Co, Mg, La At least one of transition metals or lanthanide metals selected from the group consisting of Ce, Sr and V, A is selected from the group consisting of O, F, S and P, and B is Ni or Co.) (Correction) The lithium secondary battery according to claim 1, wherein the metal oxide is an oxide of a metal selected from the group consisting of Mg, Al, Co, K, Na, Ca, Si, Ti, and V.