JP2008100910A - Cobalt oxide for lithium secondary battery positive electrode active material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide cobalt oxide as a raw material of lithium cobaltate for a lithium secondary battery excellent in energy density, service capacity and capacity retention. <P>SOLUTION: The cobalt oxide is used as a raw material for producing the positive electrode active material for the lithium secondary battery, which comprises ≤500 ppm (excluding 0 ppm) of silicon (Si) and ≤100 ppm (excluding 0 ppm) of iron (Fe) as impurities, and has 0.1-10 μm average particle diameter. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to cobalt oxide for a positive electrode active material for a lithium secondary battery.

  2. Description of the Related Art In recent years, as home appliances have rapidly become portable and cordless, lithium secondary batteries have begun to be used as power sources for small electronic devices such as laptop computers, mobile phones, and video cameras.

  Regarding this lithium secondary battery, since 1980, when Mizushima et al. Reported that lithium lithium cobaltate was useful as a positive electrode active material of a lithium secondary battery [Non-patent Document 1], a lithium cobaltate-based positive electrode Research and development on active materials has been actively promoted, and many proposals have been made so far.

Conventionally, as a technology for increasing the energy density of the positive electrode active material, for example, a lithium-rich composition by making the composition of lithium cobaltate Li _a CoO ₂ (where 1.05 ≦ a ≦ 1.3), On the contrary, Li _b CoO ₂ (where 0 <b ≦ 1) is used to make cobalt rich, and other metal ions such as Mn, W, Ni, La, Ta, Nb, and Zr are added to lithium cobaltate. Some have been proposed that dope, one that defines the amount of residual Li ₂ CO _{3 in} lithium cobaltate, one that defines residual alkali, and the like.

On the other hand, physical characteristics of the lithium cobaltate-based positive electrode active material, for example, a specific surface area is required, LiCoO ₂ is amorphous, a particle diameter is defined, crystal particles having a specific X-ray diffraction intensity of LiCoO ₂ , etc. Proposed.

In addition, with respect to the raw material, a cobalt oxide having a shape of almost spherical or oval, an average particle diameter of 1 μm or less, and a cobalt oxide in which a plurality of primary particles are directly connected, is used as a mixture with a lithium salt. Firing method (Patent Document 1), average particle diameter D (50%) = 0.5 to l. A method using cobalt oxide in the range of 5 μm (Patent Document 2), cobalt oxide powder having an average particle size of about 0.1, 0.2, 0.5, 1, 5, 10 μm by an atomizing method, and lithium carbonate A method of mixing and baking (Patent Document 3) and the like has been proposed.
Japanese Patent Laid-Open No. 5-54888 Japanese Patent Laid-Open No. 5-94822 Japanese Patent Application Laid-Open No. 5-290832 "Material Research Bulletin" vo 1.15, P783-789 (1980)

However, lithium cobalt oxide as a positive electrode active material for a lithium secondary battery is often affected by impurities contained therein and may deteriorate the performance as a positive electrode.
Therefore, the present inventors have extremely excellent characteristics as a positive electrode active material for a secondary battery, lithium cobalt oxide produced using cobalt oxide from which silica or iron contained in cobalt oxide as a reaction raw material has been removed. As a result, the present invention has been completed.

  The present invention has been developed based on the above findings, and its purpose is to provide a raw material for a lithium secondary battery positive electrode active material that can maintain a stable state with a low viscosity change rate of a lithium secondary battery positive electrode composition liquid. It is to provide a certain cobalt oxide.

  Another object of the present invention is to provide cobalt oxide which is a lithium cobaltate raw material for a lithium secondary battery excellent in energy density, discharge capacity and capacity retention.

  That is, the present invention is cobalt oxide used as a raw material for producing a positive electrode active material for a lithium secondary battery, and the impurity silicon (Si) contained in the cobalt oxide is 500 ppm or less (excluding 0 ppm). The content of Fe is 100 ppm or less (excluding 0 ppm), and the average particle diameter is 0.1 to 10 μm. Is.

The positive electrode active material for a lithium secondary battery is preferably lithium cobaltate.
The cobalt oxide is preferably cobalt oxide as a raw material for lithium cobaltate obtained by mixing cobalt oxide and lithium carbonate and then firing the mixture.

ADVANTAGE OF THE INVENTION According to this invention, the cobalt oxide for lithium secondary battery positive electrode active materials which can maintain the stable state with few viscosity change rates of a lithium secondary battery positive electrode composition liquid can be obtained.
Moreover, the lithium secondary battery excellent in energy density, discharge capacity, and capacity retention rate can be obtained by using the cobalt oxide for lithium secondary battery positive electrode active materials of the present invention.

Hereinafter, the present invention will be described in detail.
The cobalt oxide for a lithium secondary battery positive electrode active material of the present invention is not particularly limited as long as it is industrially available, but is preferably purified as much as possible.

  That is, the feature of the cobalt oxide as a raw material for the positive electrode active material of the lithium secondary battery of the present invention is that the impurity silicon (Si) contained in the cobalt oxide is 500 ppm or less (excluding 0 ppm), preferably 200 ppm. It is as follows. The average particle diameter of the cobalt oxide particles is 0.1 to 10 μm, preferably 0.1 to 5 μm.

As other impurities, the amount of Fe is 100 ppm or less (excluding 0 ppm), preferably 50 ppm or less.
These impurities such as Si and Fe may be measured by dissolving them with an acid such as HC1O ₄ or HCl and quantifying the solution with an ICP measuring device.

Moreover, the measuring method of an average particle diameter is not specifically limited, For example, it calculates | requires with the laser method.
In the present invention, a cobalt oxide raw material outside the above range is used to react with the cobalt oxide and lithium carbonate to produce lithium cobalt oxide. The raw material impurities are directly affected by the product. Therefore, when it is used as a positive electrode active material for a lithium secondary battery, the initial capacity and cycle characteristics are lowered, which is not preferable.

  In addition, when using such cobalt oxide as a lithium secondary battery positive electrode active material raw material, in addition to lithium cobaltate, for example, the cobalt part of lithium cobaltate is nickel, manganese, vanadium, chromium, titanium, aluminum, boron, etc. It goes without saying that it can also be used as a raw material for compounds substituted with.

  The method for producing lithium cobaltate using the cobalt oxide of the present invention is, for example, a combination ratio of cobalt oxide and lithium carbonate in the vicinity of 1, preferably 0.99 to 1.10 as the atomic ratio of Li / Co. And the mixture is fired at a temperature of 600 to 1100 ° C. The firing time is at least 2 hours, and after firing, the fired product can be cooled and pulverized to such an extent that it can be loosened.

EXAMPLES Next, the present invention will be described with reference to examples, but this is merely illustrative and does not limit the present invention.
Examples 1 to 3 and Comparative Example 1
The physical properties of cobalt oxide as a raw material are shown in Table 1 below.

  However, in the following examples, Example 1 using cobalt oxide (raw material 1) in which the contents of impurities silicon (Si) and Fe are both 0 ppm is a reference example.

(Lithium cobaltate)
The cobalt oxide powders of the raw materials 1 to 4 and lithium carbonate were weighed so that the Li / Co atomic ratio was 1 to 1.05, and were sufficiently mixed in a mortar to prepare a uniform mixture. Next, the mixture was filled in an alumina crucible, placed in an electric heating furnace, heated in an air atmosphere, and held at a temperature of 700 to 1000 ° C. for 10 hours to be fired to obtain a fired product of lithium cobalt oxide. The obtained fired product is cooled in the air, then pulverized, the average particle size is measured by a laser method, and the results of measuring the impurities such as Si and Fe and the alkali amount are shown in Table 2.

(Lithium secondary battery)
Production of lithium secondary batteries;
91% by weight of each lithium cobaltate, 6% by weight of graphite powder, and 3% by weight of polyvinylidene fluoride (PVDF) were mixed to prepare a positive electrode material, which was dispersed in 2-methylpyrrolidone to prepare a kneaded paste. The kneaded paste was applied to an aluminum foil, dried, repressed with a pressure of 2000 kg / cm ² , and punched into a 2 cm square to obtain a positive electrode plate.

Further, by using the lM-LiC1O ₄ / EC (ethylene carbonate) + DEC (diethylene carbonate) in the electrolyte, the negative electrode using Li metal, to produce a lithium secondary battery.

(Slurry viscosity test)
The paste viscosity was increased when Si was present in the raw material, and was evaluated as follows.
5 g of lithium cobaltate having various physical properties, 1.0 g of graphite, 0.3 g of PVDF (polyvinylidene fluoride) and 4 ml of N-methyl-2-pyrrolidone were placed in a ball mill having a capacity of 45 ml, and at room temperature at a rotation speed of 2500 rpm. Kneading was performed for a minute. After completion of the kneading, the obtained mixed slurry was allowed to stand for 30 minutes and 2 hours, and each solution was measured with a B-type viscometer at a low rotation speed of 1.5 rpm and a measurement time of 20 seconds.

(Measurement method of alkali amount)
30 g of the sample was dispersed in 100 g of distilled water and stirred for 30 minutes. After stirring, 60 ml of the filtered back solution was added to 0. The amount of alkali was measured by titration with 1N-HCI.

The results are shown in Table 2.
When the binder, the solvent, and the positive electrode active material are kneaded with the constituent materials, the viscosity ratio (X) defined below of the kneaded product is used as a viscosity evaluation parameter.

X = B / A
[Wherein, X is the viscosity ratio of the positive electrode composition, A is the viscosity (cp) after 30 minutes of homogenization at 20 ° C. of the positive electrode active material, and B is the viscosity after 2 hours of homogenization at 20 ° C. of the positive electrode active material ( cp)]

As shown in the results of Table 2 above, it can be seen that the paste viscosity increases as the amounts of Si and Fe in the cobalt oxide increase.
Further, it can be seen that when the amount of Si exceeds 500 ppm at the above value, the viscosity value increases, causing troubles in electrode application.

  On the other hand, when Si was hardly contained in the raw material, the initial capacity cycle characteristics showed good values, but the battery performance was deteriorated as Si and Fe increased.

  The present invention can obtain a cobalt oxide for a lithium secondary battery positive electrode active material that can maintain a stable state with a small viscosity change rate of the lithium secondary battery positive electrode composition liquid, so that energy density, discharge capacity, and capacity retention can be obtained. It can be used as a raw material for a positive electrode active material for a lithium secondary battery having an excellent rate.

Claims (3)

  Cobalt oxide used as a raw material for producing a positive electrode active material for a lithium secondary battery, wherein the impurity silicon (Si) contained in the cobalt oxide is 500 ppm or less (excluding 0 ppm), and the content of Fe Is 100 ppm or less (excluding 0 ppm), and the average particle diameter is 0.1 to 10 μm, and the cobalt oxide for a positive electrode active material for a lithium secondary battery is characterized in that:   The cobalt oxide for a lithium secondary battery positive electrode active material according to claim 1, wherein the positive electrode active material for a lithium secondary battery is lithium cobaltate.   The cobalt oxide for a lithium secondary battery positive electrode active material according to claim 1, which is a cobalt oxide of a raw material for lithium cobaltate obtained by mixing cobalt oxide and lithium carbonate and then firing the mixture.