KR100434547B1 - Lithium metal oxide cathode and lithium secondary battery using the same - Google Patents

Abstract

PURPOSE: A lithium cathode having a stable crystal structure is provided to manufacture lithium secondary batteries having high capacity and improved self-discharge characteristics and cycle life characteristics. CONSTITUTION: The lithium cathode comprises a lithium-containing metal composite oxide represented by the formula of LixNiaCobMcO2, a conductive agent and a binder. In the above formula, M is at least one metal element selected from the group consisting of boron, chrome and manganese; x ranges from 0.85 to 1.10; a ranges from 0.50 to 0.95; b ranges from 0.05 to 0.50; c ranges from 0.005 to 0.2; and a+b+c ranges from 0.85 to 1.10. Particularly, the conductive agent is selected from the group consisting of acetylene black, carbon black and graphite, and the binder is polyvinyl difluoride or polytetrafluoroethylene.

Description

Lithium anode and lithium secondary battery employing the same {Lithium metal oxide cathode and lithium secondary battery using the same}

The present invention relates to a lithium positive electrode and a secondary battery employing the same. More particularly, the present invention relates to a lithium positive electrode and a lithium secondary battery employing the same.

Lithium secondary batteries are one of the next generation batteries that have a high capacity density and excellent output characteristics, and thus are increasingly being applied to small electronic devices.

Like a conventional battery, a lithium secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator.

Among them, depending on the type of electrolyte used, it may be roughly classified into a lithium ion secondary battery using an organic electrolyte and a lithium polymer secondary battery using a solid polymer electrolyte. As the separator, polyethylene or polypropylene nonwoven fiber sheet or the like is usually used.

In addition, the positive electrode and the negative electrode each comprise an active material, a conductive agent and a binder. The dual active material is an important component for determining the capacity of each electrode. As the negative electrode active material, a lithium metal or a carbon material which can easily insert and release lithium ions, and a lithium metal oxide having a layered or spinel structure are used as the positive electrode active material. . The present invention relates to a cathode active material.

One of the widely used cathode active materials is LiCoO _{2 having a} layered structure disclosed in US Pat. No. 4,430,518. This active material has a high manufacturing cost and causes environmental problems.

As an active material capable of overcoming this, US Pat. Nos. 4,424,625 and 4828834 disclose spinel structured LiMn ₂ O ₄ and have been put to practical use. This active material is advantageous in that it is inexpensive, pollution-free, and exhibits a high voltage with respect to lithium, but has a disadvantage of poor charging and discharging characteristics.

In addition, all of the above-described active materials have a limitation that the capacity density is only about 130-140 mAh / g, and thus does not meet the recent trend in which high capacity is required.

LiNiO ₂ having a hexagonal structure (JB Goodenough, Mat. Res. Bull., 20, 1137 (1985), JRDahn, J. Electrochem .. Soc.,) As a means to solve the problem of capacity density limitation. 138, 2207-11 (1990)) and LiNi ₁ - _x Co _x O ₂ (0.5 ≦ _x ≦ 1) (T.Ohzuku, Chemistry Express, 5, 733 (1990)) are being researched and developed as positive electrode active materials.

However, in the case of LiNiO ₂ , phase transitions between hexagonal and monoclinic (Monoclinic) are repeated as lithium ions are inserted or released by charging and discharging. As a result, structural stability is lowered, and a sudden decrease in charge / discharge cycle capacity occurs.

In addition, in the case of LiNi ₁ - _x Co _x O _2, the cycle stability is slightly better than that of LiNiO ₂ , but the capacity energy density is considerably low. In particular, when the discharge capacity of the lithium ions is increased to increase the capacity, the electrode potential is very high, and electrolyte decomposition and side reactions are easily generated. Therefore, self-discharge and cycle stability are greatly reduced.

The technical problem to be achieved by the present invention is to provide a lithium anode capable of high capacity and improve the self-discharge characteristics and life characteristics.

Another object of the present invention is to provide a lithium secondary battery employing a lithium positive electrode capable of high capacity and improving self discharge characteristics and lifespan characteristics.

Technical problem of the present invention is a lithium anode comprising an active material, a conductive agent and a binder, the active material may be made by a lithium anode, characterized in that the lithium-containing metal composite oxide of the formula (1):

[Formula 1]

Li _x Ni _a Co _b McO ₂

(Wherein, M is at least one metal element selected from the group consisting of boron, chromium and manganese, 0.85≤x≤1.10, 0.50≤a≤0.95, 0.05≤b≤0.50, 0.005≤c≤0.2, 0.85≤ a + b + c ≦ 1.10).

In the active material for lithium positive electrode according to the present invention, the added molar ratio of x is 0.85-1.10, preferably about 1.0, which is not preferable because the capacity of the active material is lowered outside this range.

In addition, in the addition molar ratio of Ni and Co, since Ni is inexpensive compared to Co and a high capacity active material can be obtained, it is advantageous to add Ni in a larger amount than Co. Therefore, it is preferable that a is 0.50-0.95, and b is 0.05-0.50. If the amount of Co added is larger than Ni, it is not preferable because the capacity of the active material is reduced.

In addition, the value of c, the addition molar ratio of at least one metal element selected from the group consisting of boron, chromium and manganese is in the range of 0.005-0.2, if c is less than 0.005, the addition effect is almost lost, whereas it exceeds 0.2. It is not preferable because the capacity density decrease phenomenon of the active material occurs.

The manufacturing method of the active material for lithium positive electrode according to the present invention is not particularly limited, and may be obtained by uniformly mixing the salts of the constituent metals using a ball mill and the like, and then putting them in a heating furnace to perform heat treatment.

In addition, the positive electrode according to the present invention may be prepared by mixing the lithium-containing metal composite oxide of Chemical Formula 1, which is an active material, with a conductive agent and a binder, and then molding the same as in manufacturing a normal electrode plate. Here, the conductive agent and the binder may be any one that can be used in the field of the present invention, the conductive agent is acetylene black, carbon black, graphite, etc., the binder is polyvinyl difluoride, polytetrafluor Ethylene and the like can be used, and the content of the conductive agent and the binder is 3-12% by weight and 5-15% by weight, respectively, based on the total weight of the active material.

Another technical problem of the present invention is a lithium secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode is a lithium secondary battery including a lithium-containing metal composite oxide represented by Chemical Formula 1, a conductive agent, and a binder. Can be made by.

In the lithium secondary battery according to the present invention, the negative electrode and the electrolyte may be any one commonly used in the art, and as the negative electrode, a carbon material capable of inserting and releasing lithium metal or lithium ions (graphite, Coke or the like) or an amorphous lithium metal oxide. In addition, as an electrolyte, an organic particle in which inorganic particles such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , and LiCF ₃ SO ₃ are dispersed in a single or mixed organic solvent selected from propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, and the like. Electrolyte solution or normal solid polymer electrolyte can be used.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples.

<Example 1-6>

LiNO ₂ , Ni (OH) ₂ , B ₂ O ₃ , MgCO ₃ , Cr (NO) ₃ .9H ₂ O, and Mn (NO) _{3 ·} 6H ₂ O were prepared as raw materials for synthesizing the positive electrode active material. Each mixture was mixed in the molar ratio shown in the following table, and the mixed powder was placed in an alumina crucible and heat-treated at 800 ° C. for about 10 hours under an oxygen atmosphere. The mixed powder was heat-treated and then pulverized to obtain an active material powder. 8 g of acetylene black as a conductive agent and 7 g of a polyvinyl difluoride (PVdF) solution (8% solution in N-methyl-2-pyrrolidone) as a binder were added to 85 g of the obtained active material powder to prepare a cathode active material slurry. Subsequently, the prepared slurry was applied to aluminum foil, then dried and molded into a diameter of 16 mm to prepare a lithium anode.

A lithium film having a thickness of 8 mm and a diameter of 16 mm was used as the cathode, and 1 mol of LiPF 6 was dissolved in 1 L of a mixed solution of propylene carbonate and diethyl carbonate. In addition, the polypropylene film was used as a separator.

The coin-type battery 1-6 (diameter: 20 mm, thickness: 1.6 mm) was manufactured using the positive electrode, the negative electrode, the electrolyte, and the separator thus prepared.

Thus, the capacity of the battery 1-6 was measured under the conditions of 0.5 mA / cm 2 of charging current density, 4.2 V of charging voltage, 0.5 mA / cm 2 of discharge current density, and 2.7 V of stop voltage, and 100 charge / discharge cycles were performed. The charge and discharge characteristics of the battery were evaluated by measuring the capacity retention rate during the repetition. In addition, the magnetic discharge characteristics were evaluated by measuring the magnetic discharge rate after leaving at room temperature for 7 days in a 4.2V state of charge.

These evaluation results are shown in the table below.

<Comparative Example 1-3>

Each raw material was mixed in a molar ratio as shown in the following table, and then positive electrodes were prepared in the same manner as in Example 1-6, and coin-type batteries 7-9 employing these were prepared, respectively. The capacity density, charge and discharge characteristics, and self discharge characteristics of the batteries thus prepared were evaluated under the same conditions as in Example 1-6, and the results are shown in Table 1 below.

Mixed molar ratio of raw materials Capacity density (㎃h / g) Capacity retention rate for 100 cycles (%) Self discharge rate (%) Li Ni Co M Example 1 (battery 1) 1.0 0.85 0.1 B: 0.05 158 97.5 5.1 2 (battery 2) 1.0 0.8 0.1 Mg: 0.1 151 96.1 4.7 3 (battery 3) 1.0 0.8 0.1 Cr: 0.1 150 94.3 6.5 4 (battery 4) 1.0 0.85 0.1 Mn: 0.05 157 95.8 5.9 5 (battery 5) 1.0 0.75 0.15 B: 0.02Cr: 0.08 151 99.1 3.8 6 (battery 6) 1.0 0.8 0.1 B: 0.02Cr: 0.08 153 98.7 4.1 Comparative example 1 (battery 7) 1.0 1.0 - - 161 82.6 12.2 2 (battery 8) 1.0 0.8 0.2 - 148 90.7 7.8 3 (battery 9) 1.0 0.4 0.3 Mn: 0.3 137 95.5 8.3

As can be seen from the results of Table 1, in the case of LiNiO2 (Comparative Example 1), the capacity density was very good, but the cycle stability and the self-discharge rate were very poor, and in the case of LiNi ₁ - _x Co _x O ₂ (comparative) In Example 2), the cycle stability is better than that of Comparative Example 1, but the characteristics are generally unsatisfactory, such as poor capacity density and self-discharge rate. In addition, in the case of Comparative Example 3, the cycle stability is quite good, but since the molar ratio of nickel is relatively lower than that of cobalt and the content of manganese exceeds the desired range, the capacity density is notably decreased, and the self discharge rate is also poor. .

On the contrary, the batteries employing the lithium positive electrode included in the scope of the present invention (Examples 1-6) showed satisfactory results in which the capacity density, the cycle stability, and the self discharge rate were all satisfactory.

Since the positive electrode active material according to the present invention has a stable crystal structure, a lithium secondary battery employing a lithium positive electrode including the same has excellent capacity density and cycle stability as well as a low self discharge rate.

Claims (10)

A lithium anode comprising a lithium-containing metal composite oxide, a conductive agent and a binder represented by the following formula (1). [Formula 1] Li _x Ni _a Co _b McO ₂ Wherein M is at least one metal element selected from the group consisting of boron, chromium and manganese, 0.85 ≦ x ≦ 1.10, 0.50 ≦ a ≦ 0.95, 0.05 ≦ b ≦ 0.50, 0.005 ≦ c ≦ 0.2, 0.85 ≦ a + b + c ≦ 1.10). The lithium anode of claim 1, wherein the conductive agent is selected from the group consisting of acetylene black, carbon black, and graphite. The lithium positive electrode of claim 1, wherein the binder is polyvinyldifluoride or polytetrafluoroethylene. The lithium positive electrode of claim 1, wherein the content of the conductive agent and the binder is 3-12% by weight and 5-15% by weight based on the total weight of the lithium-containing metal composite oxide. A lithium secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode is a lithium secondary battery including a lithium-containing metal composite oxide, a conductive agent, and a binder represented by Chemical Formula 1 below. [Formula 1] Li _x Ni _a Co _b McO ₂ Wherein M is at least one metal element selected from the group consisting of boron, chromium and manganese, 0.85 ≦ x ≦ 1.10, 0.50 ≦ a ≦ 0.95, 0.05 ≦ b ≦ 0.50, 0.005 ≦ c ≦ 0.2, 0.85 ≦ a + b + c ≦ 1.10). The lithium secondary battery according to claim 5, wherein the conductive agent is selected from the group consisting of acetylene black, carbon black and graphite. The lithium secondary battery according to claim 5, wherein the binder is polyvinyldifluoride or polytetrafluoroethylene. The lithium secondary battery according to claim 5, wherein the content of the conductive agent and the binder is 3-12% by weight and 5-15% by weight, respectively, based on the total weight of the lithium-containing metal composite oxide. The lithium secondary battery according to claim 5, wherein the negative electrode is a lithium metal, a carbon material or an amorphous lithium metal oxide. The lithium secondary battery according to claim 5, wherein the electrolyte is an organic electrolyte or a solid polymer electrolyte.