JP5611167B2 - Cathode active material for lithium ion battery and method for producing the same - Google Patents

Description

  The present invention relates to an olivine-type silicate compound, a positive electrode active material for a lithium ion battery, and a method for producing the same.

  Lithium ion batteries are a type of non-aqueous electrolyte battery and are used in a wide range of fields such as mobile phones, digital cameras, notebook PCs, hybrid cars, and electric cars. Lithium ion batteries mainly use lithium metal oxide as a positive electrode material and a carbon material such as graphite as a negative electrode material.

As this positive electrode material, lithium cobaltate (LiCoO ₂ ), lithium manganate (LiMnO ₂ ), lithium iron phosphate (LiFePO ₄ ), lithium iron silicate (Li ₂ FeSiO ₄ ) and the like are known. Among these, LiFePO ₄ and Li ₂ FeSiO ₄ have an olivine structure and are useful as a positive electrode material for a high capacity lithium ion battery. Among them, lithium metal phosphate-based positive electrode materials such as LiFePO ₄ are known to exhibit a specific peak intensity ratio in X-ray diffraction in order to reduce impurities in order to further improve the physical properties of the battery obtained. (Patent Document 1).

On the other hand, as a method for producing a lithium silicate metal positive electrode material such as Li ₂ FeSiO ₄ , a solid phase method in which a mixture of a Li source, an iron (metal) source and a silicate source is pulverized and fired at 500 to 900 ° C. Is common (Patent Documents 2 and 3). However, in the solid phase method, it is necessary to perform firing and pulverization in an inert gas atmosphere, which requires complicated operations and it is difficult to control the particle size and crystallinity.
On the other hand, Non-Patent Document 1 describes that Li ₂ Mn _1-y Fe _y SiO ₄ (y = 0 to 1) can be obtained by hydrothermal synthesis.

Japanese Patent No. 4388135 JP 2001-266882 A JP 2002-198050 A

GS Yuasa Technical Report June 2009, Vol. 6, No. 1, p21-26

However, the present inventors can obtain a single phase of Li ₂ FeSiO ₄ by mixing the lithium source, the iron source and the silicate source with water and subjecting the mixture to a hydrothermal reaction as it is. It was found that Fe ₃ Si ₂ O ₄ (OH) _{4 and the} like were by-produced. Therefore, as for the lithium metal silicate-based positive electrode material such as Li ₂ FeSiO ₄ , the fact that a specific peak intensity ratio in X-ray diffraction due to the reduction of impurities is not yet known.

Therefore, an object of the present invention is to provide an olivine-type silicate compound represented by Li ₂ FeSiO ₄ useful as a positive electrode active material for a lithium ion battery, a positive electrode active material for a lithium ion battery, and a method for producing the same. There is to do.

Accordingly, the present inventors have found that an olivine-type silicate compound represented by Li ₂ FeSiO ₄ showing a specific peak intensity ratio in X-ray diffraction is magnetite (Fe ₃ O ₄ ), which is an impurity generated by oxidation of Fe. It has been found to have an excellent battery physical property when used as a positive electrode active material for a lithium ion battery, and the present invention has been completed.

That is, the present invention is expressed by Li ₂ FeSiO ₄ , and in the X-ray diffraction diagram, the peak intensity of the (311) plane of Fe ₃ O ₄ relative to the peak intensity of the (002) plane of Li ₂ FeSiO ₄ is 0.01. The present invention provides an olivine-type silicate compound characterized by being not more than double.
Further, the present invention uses an organic acid iron represented by iron sulfate (FeSO ₄ ) or (R) ₂ Fe (wherein R represents an organic acid residue), and uses a lithium compound, a silicate compound, and an antioxidant. The present invention provides a method for producing the above olivine-type silicate compound, wherein a basic aqueous dispersion containing an agent is subjected to a hydrothermal reaction.
Furthermore, this invention provides the positive electrode active material for lithium ion batteries containing the said olivine type | mold silicate compound.
Moreover, this invention provides the lithium ion battery which has a positive electrode containing the said positive electrode active material for lithium ion batteries.

  A lithium ion battery using the olivine-type silicate compound of the present invention as a positive electrode material has excellent discharge capacity and is very useful as a positive electrode material for a lithium ion battery.

The X-ray diffraction pattern of the lyophilized powder obtained in Example 1 and Comparative Example 1 is shown. The charging / discharging curve of the battery using the positive electrode active material obtained in Example 1 is shown.

Hereinafter, the present invention will be described in detail.
The olivine-type silicate compound of the present invention is represented by Li ₂ FeSiO ₄ . When the X-ray diffraction of the olivine type silicate compound is measured, the peak intensity of the (311) plane of magnetite (Fe ₃ O ₄ ) relative to the peak intensity of the (002) plane of Li ₂ FeSiO ₄ is It is 0.01 times or less, preferably 0.005 times or less, more preferably 0.001 times or less, and most preferably 0. Fe ₃ O ₄ is an impurity produced by oxidizing Fe is in the Li ₂ FeSiO _4, electrochemically inactive Fe ₃ O ₄ per weight by the formation of Li ₂ FeSiO ₄ is reduced discharge The capacity will be reduced.
In the olivine-type silicate compound of the present invention, this Fe ₃ O ₄ is extremely effectively reduced. By using this as a positive electrode active material, the obtained lithium ion battery can have an excellent discharge capacity.

For example, when Li ₂ FeSiO ₄ is indexed by the space group Pmn2 ₁ , the peak intensity of the (011) plane appears at 2θ = 24.6 °, and the peak intensity of the (010) plane is 2θ = 16.7. Appears at °. In addition, the peak intensity of the (200) plane is 2θ = 28.6 °, the peak intensity of the (210) plane is 2θ = 33.2 °, and the peak intensity of the (020) plane is 2θ = 33.6 °. The peak intensity of the (002) plane appears at 2θ = 36.3 °, and the peak intensity of the (211) plane appears at 2θ = 38.0 °. On the other hand, in Fe ₃ O ₄ , the peak intensity of the (311) plane appears at 2θ = 35.4 °. In addition, the peak intensity of the (220) plane is mainly 2θ = 30.0 °, the peak intensity of the (400) plane is 2θ = 43.0 °, and the peak intensity of the (511) plane is 2θ = 56.9 °. Appear in Therefore, when the peak intensity of the (311) plane of the magnetite (Fe ₃ O ₄ ) relative to the peak intensity of the (002) plane of Li ₂ FeSiO ₄ is 0.01 times or less, the impurity Fe ₃ O ₄ phase is almost all. It does not exist, meaning that Li ₂ FeSiO ₄ is an almost single-phase olivine silicate compound having a very high purity.

The olivine-type silicate compound of the present invention uses an organic acid iron represented by iron sulfate (FeSO ₄ ) or (R) ₂ Fe (wherein R represents an organic acid residue), a lithium compound, a silicate compound And a basic aqueous dispersion containing an antioxidant is produced by hydrothermal reaction. By using the antioxidant, the oxidation of Fe can be suppressed extremely effectively, and the impurity Fe ₃ O ₄ can be reduced.

Examples of the lithium compound include lithium hydroxide (for example, LiOH.H ₂ O), lithium carbonate (Li ₂ CO ₃ ), lithium sulfate, and lithium acetate, and lithium hydroxide and lithium carbonate are particularly preferable. The concentration of the lithium compound in the aqueous dispersion is preferably 0.30 to 3.00 mol / l, more preferably 1.00 to 1.50 mol / l.

The silicic acid compound is not particularly limited as long as it is a reactive silica compound, and amorphous silica and Na ₄ SiO ₄ (for example, Na ₄ SiO ₄ .H ₂ O) are preferable. Of these, the use of Na ₄ SiO ₄ is more preferable because the aqueous dispersion becomes basic. The concentration of the silicate compound in the aqueous dispersion is preferably 0.15 to 1.50 mol / l, more preferably 0.50 to 0.75 mol / l.

  The antioxidant is not particularly limited as long as it is a compound containing ammonium ions. For example, aqueous ammonia, ammonium sulfate, ammonium chloride, ammonium acetate and the like can be used. As the ammonia water, water in which ammonium sulfate is dissolved may be used. These may be used alone or in combination of two or more. The content of the antioxidant in the aqueous dispersion is preferably 0.001 to 2 in terms of molar ratio with respect to Fe, because if it is added in a large amount, the formation of the olivine silicate compound is suppressed. Is more preferable.

When iron sulfate (FeSO ₄ ) is used as the iron source, a basic aqueous dispersion containing a lithium compound, a silicate compound and an antioxidant is prepared separately from iron sulfate in order to suppress side reactions. It is preferable to leave. In this case, the aqueous dispersion and iron sulfate are mixed and subjected to a hydrothermal reaction. In preparing the aqueous dispersion, the order of addition of the lithium compound, the silicate compound and the antioxidant is not particularly limited, and these three components may be added to water.

The amount of iron sulfate added is preferably 0.15 to 1.50 mol / l in the reaction mixture, and more preferably 0.50 to 0.75 mol / l.
In this case, the Li content in the reaction mixture is preferably 2 mol or more with respect to Fe.

When using organic acid iron (R) ₂ Fe (wherein R represents an organic acid residue) as an iron source, it contains a lithium compound, a silicate compound and an antioxidant, and further contains an organic acid iron A basic aqueous dispersion is prepared. In this case, the addition order of the lithium compound, the silicate compound, the antioxidant and the organic acid iron is not particularly limited. Moreover, atmospheric conditions may be sufficient. Usually, organic acid iron is a raw material used in the solid phase method, but side reactions can be suppressed by using it in a hydrothermal reaction.
In this case, Si and Li in the reaction mixture are preferably used in a molar ratio of at least twice with respect to Fe, and Li: Fe is more preferably about 2.5: 1 to 3: 1.

The organic acid represented by R in the organic acid iron (R) ₂ Fe is preferably an organic acid having 1 to 20 carbon atoms, and more preferably an organic acid having 2 to 12 carbon atoms. More specific organic acids include dicarboxylic acids such as oxalic acid and fumaric acid, hydroxycarboxylic acids such as lactic acid, and fatty acids such as acetic acid.

Making the aqueous dispersion basic is important in preventing side reactions and dissolving the silicate compound. Specifically, the pH of the aqueous dispersion is 12.0 to 13.5, particularly in terms of preventing side reactions (formation of Fe ₃ O ₄ ), solubility of silicate compounds, and progress of the reaction. preferable. The pH of the aqueous dispersion may be adjusted by adding a base such as sodium hydroxide, but it is particularly preferable to use Na ₄ SiO ₄ as the silicate compound.

  The hydrothermal reaction should just be 100 degreeC or more, 130-180 degreeC is preferable and 140-160 degreeC is more preferable. The hydrothermal reaction is preferably performed in a pressure vessel. When the reaction is performed at 130 to 180 ° C, the pressure at this time is 0.3 to 0.9 MPa, and the pressure when the reaction is performed at 140 to 160 ° C is 0. 3 to 0.4 MPa. The hydrothermal reaction time is preferably 1 to 24 hours, more preferably 3 to 12 hours.

Li ₂ FeSiO ₄ is obtained in a high yield by the hydrothermal reaction. The obtained LiFeSiO _{4 has} an average particle size of 10 to 100 nm and a high crystallinity.

The obtained Li ₂ FeSiO ₄ can be isolated by filtration and then drying. As the drying means, freeze drying or vacuum drying is used.

The obtained Li ₂ FeSiO ₄ can be used as a positive electrode active material for a lithium ion battery by carrying carbon and then firing. Carbon support may be obtained by adding a carbon source such as glucose, fructose, polyethylene glycol, polyvinyl alcohol, carboxymethyl cellulose, saccharose, starch, dextrin, citric acid and water to Li ₂ FeSiO ₄ and then baking. The firing conditions are 400 ° C. or higher, preferably 400 to 800 ° C. for 10 minutes to 3 hours, preferably 0.5 to 1.5 hours under an inert gas atmosphere or reducing conditions. By such treatment, a positive electrode active material in which carbon is supported on the surface of Li ₂ FeSiO ₄ can be obtained. The amount of the carbon source used is preferably 3 to 15 parts by mass as carbon contained in the carbon source and more preferably 5 to 10 parts by mass as carbon contained in the carbon source with respect to 100 parts by mass of Li ₂ FeSiO ₄ .

  The obtained positive electrode active material is excellent in terms of discharge capacity, and is useful as a positive electrode material for lithium ion batteries. The lithium ion battery to which the positive electrode active material of the present invention can be applied is not particularly limited as long as it is a lithium ion secondary battery and has a positive electrode, a negative electrode, an electrolytic solution, and a separator as essential components.

  Here, as long as lithium ions can be occluded at the time of charging and released at the time of discharging, the material structure is not particularly limited, and a known material structure can be used. For example, a carbon material such as lithium metal, graphite, or amorphous carbon. It is preferable to use an electrode formed of an intercalating material capable of electrochemically inserting and extracting lithium, particularly a carbon material.

  The electrolytic solution is obtained by dissolving a supporting salt in an organic solvent. The organic solvent is not particularly limited as long as it is an organic solvent that is usually used for an electrolyte solution of a lithium ion battery. For example, carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, oxolane A compound or the like can be used.

The type of the supporting salt is not particularly limited, but an inorganic salt selected from LiPF ₆ , LiBF ₄ , LiClO ₄ and LiAsF ₆ , a derivative of the inorganic salt, LiSO ₃ CF ₃ , LiC (SO ₃ CF ₃ ) ₂ and LiN (SO ₃ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ and LiN (SO ₂ CF ₃ ) (SO ₂ C ₄ F ₉ ), and organic salt derivatives It is preferable that it is at least 1 type of these.

  The separator plays a role of electrically insulating the positive electrode and the negative electrode and holding the electrolytic solution. For example, a porous synthetic resin film, particularly a polyolefin polymer (polyethylene, polypropylene) porous film may be used.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example, this invention is not limited to these Examples.

[Example 1]
Add 75 cm ³ of ultrapure water to 4.20 g (0.1 mol) of LiOH.H ₂ O, 6.99 g (0.025 mol) of Na ₄ SiO ₄ .nH ₂ O, and 1.70 g (0.025 mol) of aqueous ammonia. Mixed (pH at this time is about 13). To this aqueous dispersion, 6.95 g (0.025 mol) of FeSO ₄ .7H ₂ O was added and mixed. The obtained mixed liquid was put into an autoclave and subjected to a hydrothermal reaction at 150 ° C. for 16 hours. The reaction solution was filtered and then lyophilized. Glucose (carbon concentration: 10%) and ultrapure water (10 cm ³⁾ were added to 8.4 g of the powder obtained by freeze-drying (about 12 hours), and calcined at 600 ° C. for 1 hr in a reducing atmosphere.

[Comparative Example 1]
A lyophilized powder was obtained in the same manner as in Example 1 except that ammonia water was not added.

[Test Example 1]
X-ray diffraction of the lyophilized powder obtained in Example 1 and Comparative Example 1 was performed. The X-ray diffraction at this time is measured by RINT-Ultima II (manufactured by Rigaku Corporation). The measurement conditions are target CuKα, tube voltage 40 kV, tube current 40 mA, scanning range 10 to 80 ° (2θ), step width 0. 0.02 ° and a scan speed of 2.00 ° / min. The obtained X-ray diffraction pattern is shown in FIG. As is clear from FIG. 1, the powder obtained in Comparative Example 1 has a peak intensity of the (311) plane of Fe ₃ O ₄ of 1.5 times that of the (002) plane of Li ₂ FeSiO _4. As a result, Fe ₃ O ₄ was by-produced. In contrast, the powder obtained in Example 1, the Fe ₃ O ₄ peak intensity of (311) plane is not verified, that the Li ₂ FeSiO ₄ (002) plane of Fe ₃ O ₄ to the peak intensity of the It was found that this was a single phase of Li ₂ FeSiO ₄ having a peak intensity of (311) plane of 0 and extremely high purity.

[Test Example 2]
Using the fired product obtained in Example 1 and Comparative Example 1, a positive electrode of a lithium ion secondary battery was produced. The fired product obtained in Example 1 and Comparative Example 1, ketjen black (conductive agent), and polyvinylidene fluoride (binding agent) were mixed at a mixing ratio of 75:15:10, and this was mixed with N-methyl. -2-Pyrrolidone was added and sufficiently kneaded to prepare a positive electrode slurry. The positive electrode slurry was applied to a current collector made of an aluminum foil having a thickness of 20 μm using a coating machine, and vacuum dried at 80 ° C. for 12 hours. Thereafter, it was punched into a disk shape of φ14 mm and pressed at 16 MPa for 2 minutes using a hand press to obtain a positive electrode.

Next, a coin-type lithium ion secondary battery was constructed using the positive electrode. A lithium foil punched to φ15 mm was used for the negative electrode. As the electrolytic solution, a solution obtained by dissolving LIPF ₆ at a concentration of 1 mol / l in a mixed solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 1: 1 was used. As the separator, a known one such as a polymer porous film such as polypropylene was used. These battery components were assembled and housed in a conventional manner in an atmosphere with a dew point of −50 ° C. or lower to produce a coin-type lithium secondary battery (CR-2032).

  Charging / discharging at a constant current density was performed for 4 cycles using the manufactured lithium ion secondary battery. The charging conditions at this time were constant current and constant voltage charging with a current of 0.1 CA (33 mA / g) and a voltage of 4.5 V, and the discharging conditions were constant current discharging with a current of 0.1 CA and a final voltage of 1.5 V. All temperatures were 30 ° C. The charge / discharge curve of the battery constructed with the positive electrode material of Example 1 is shown in FIG.

  FIG. 2 shows that the lithium ion secondary battery using the positive electrode material of the present invention has excellent battery properties.

Claims (5)

An organic acid iron represented by iron sulfate (FeSO ₄ ) and (R) ₂ Fe (wherein R represents an organic acid residue) is used, and a molar ratio of 0 to a lithium compound, a silicate compound, and Fe is used. It contains an antioxidant , which is a compound containing ammonium ions of 0.001 to ₂ , and further contains an organic acid iron represented by (R) ₂ Fe (wherein R represents an organic acid residue). A basic aqueous dispersion and iron sulfate (FeSO ₄ ) are mixed, and the resulting mixture is hydrothermally reacted.
An olivine type represented by Li ₂ FeSiO _{4 and} having a peak intensity of (311) plane of Fe ₃ O ₄ of 0.01 times or less with respect to the peak intensity of (002) plane of Li ₂ FeSiO ₄ in the X-ray diffraction diagram A method for producing a silicate compound. The method for producing an olivine-type silicate compound according to claim 1 , wherein the lithium compound is selected from lithium hydroxide, lithium carbonate, lithium nitrate, lithium acetate, and lithium sulfate. The method for producing an olivine-type silicate compound according to claim 1 or 2 , wherein the silicic acid compound is selected from amorphous silica and Na ₄ SiO ₄ . The method for producing an olivine-type silicate compound according to any one of claims 1 to 3 , wherein the pH of the basic aqueous dispersion is 12.0 to 13.5. An organic acid iron represented by iron sulfate (FeSO ₄ ) and (R) ₂ Fe (wherein R represents an organic acid residue) is used, and a molar ratio of 0 to a lithium compound, a silicate compound, and Fe is used. It contains an antioxidant , which is a compound containing ammonium ions of 0.001 to ₂ , and further contains an organic acid iron represented by (R) ₂ Fe (wherein R represents an organic acid residue). A basic aqueous dispersion and iron sulfate (FeSO ₄ ) are mixed, the resulting mixture is subjected to hydrothermal reaction, and after the hydrothermal reaction, carbon is supported on the obtained olivine-type silicate compound, and then calcined. The manufacturing method of the positive electrode active material for lithium ion batteries containing the olivine type | mold silicate compound obtained by the manufacturing method of any one of Claims 1-4 .