JP5709134B2 - Method for producing positive electrode active material for lithium ion battery - Google Patents

Description

  The present invention relates to a method for producing a positive electrode active material for a lithium ion battery.

  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.

As a method for producing a lithium iron silicate-based positive electrode material such as Li ₂ FeSiO _4, a solid phase method in which a mixture of a Li source, an iron source and a silicate source is pulverized and fired at 500 to 900 ° C. is common. (Patent Documents 1 and 2). 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.
In contrast, Non-Patent Documents 1 and 2 have a description that Li ₂ Mn _1-y Fe _y SiO ₄ (y = 0 to 1) can be obtained by hydrothermal synthesis.

JP 2001-266882 A JP 2002-198050 A

GS Yuasa Technical Report June 2009, Vol. 6, No. 1, p21-26 R. Dominiko et al, Journal of Power Sources 184 (2008), p462-468.

Lithium iron silicate and lithium iron phosphate obtained by hydrothermal synthesis are extremely useful as positive electrode materials because of their small particle size and uniformity. However, in order to obtain a positive electrode active material, it is necessary to coat these compounds with a conductive material such as carbon black. In the case of lithium iron phosphate, carbon coating can be performed by adding a carbon source to the hydrothermal synthesis reaction system, but in the case of lithium iron silicate, if a carbon source is added to the hydrothermal synthesis reaction system, It has been found that the problem of generating by-products occurs.
Therefore, an object of the present invention is to provide a new method for producing a positive electrode active material containing carbon-coated lithium iron silicate.

  Therefore, as a result of various studies on the carbon coating means of lithium iron silicate, the present inventor has obtained a positive electrode active material having a sufficient discharge capacity in a method of mixing and grinding lithium iron silicate and a carbon source such as a conductive carbon material. In spite of not being able to obtain, a liquid in which lithium iron silicate, a conductive carbon material, and a solvent-soluble organic compound are dispersed in a solvent is mixed using a ball mill, and then dried and fired. Thus, a positive electrode active material having a high discharge capacity was obtained, and the present invention was completed.

That is, the present invention is a lithium ion battery characterized in that a dispersion containing lithium iron silicate fine particles, a conductive carbon material, a solvent-soluble organic compound and a solvent is mixed using a ball mill, dried and fired. A method for producing a positive electrode active material is provided.
Moreover, this invention provides the lithium ion battery which has a positive electrode containing the positive electrode active material obtained by said manufacturing method.

  The positive electrode active material obtained by the method of the present invention has a high discharge capacity and is useful as a positive electrode active material for lithium ion batteries. In addition, since a positive electrode active material having a high discharge capacity can be obtained with a small amount of carbon source, the cost of the lithium ion battery can be reduced.

The X-ray diffraction pattern of the powder obtained in Example 1 is shown.

  The method for producing a positive electrode active material for a lithium ion battery according to the present invention comprises mixing a dispersion containing lithium iron silicate fine particles, a conductive carbon material, a solvent-soluble organic compound and a solvent using a ball mill, followed by drying. It is characterized by firing.

As the lithium iron silicate fine particles, Li ₂ FeSiO ₄ fine particles are preferable. Li ₂ FeSiO ₄ may be obtained by a solid phase method in which a mixture of a Li source, an iron source and a silicate source is pulverized and fired at 500 to 900 ° C., but a mixture of a silicate source, an iron source and a lithium source It is preferable to use a product obtained by hydrothermal reaction of a material in that a uniform particle size can be obtained.

As a method for producing lithium iron silicate fine particles, (A) a basic aqueous dispersion containing a lithium compound, a silicate compound and an antioxidant and FeSO ₄ are mixed, and the resulting mixture is hydrothermally reacted. Method; or (B) A method in which a basic aqueous dispersion containing a lithium compound, a silicate compound and an organic acid iron salt is hydrothermally reacted is preferable.

  First, the method (A) will be described.

In the method (A), it is preferable to prepare a basic aqueous dispersion containing a lithium compound, a silicate compound and an antioxidant separately from FeSO ₄ from the viewpoint of suppressing side reactions. 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 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.

As the antioxidant, sodium hydrosulfite (Na ₂ S ₂ O ₄ ), aqueous ammonia, sodium sulfite and the like can be used. Since the content of the antioxidant in the aqueous dispersion, which results in suppressing the formation of Li ₂ FeSiO ₄ when added in a large amount, an equimolar amount or less are preferred with respect to Fe, in a molar ratio with respect to Fe 0. 5 or less is more preferable.

It is preferable that the lithium compound, the silicate compound and the antioxidant be a basic aqueous dispersion separately from FeSO ₄ in order to prevent side reactions and dissolve the silicate compound. The pH of the aqueous dispersion may be basic, but it is 12.0 to 13.5 to prevent side reaction (formation of Fe ₃ O ₄ ), solubility of the silicate compound, and progress of the reaction. Particularly preferred in terms. 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 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 concentration of the silicate compound is preferably 0.15 to 1.50 mol / l, more preferably 0.50 to 0.75 mol / l. 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 FeSO ₄ added is preferably 0.15 to 1.50 mol / l in the reaction mixture, and more preferably 0.50 to 0.75 mol / l.

  The content of Si and Li in the reaction mixture is preferably 2 mol or more with respect to Fe.

In the method (A), the aqueous dispersion and FeSO ₄ are then mixed and subjected to a hydrothermal reaction. 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. Moreover, the average particle diameter of the obtained Li ₂ FeSiO ₄ is 10 to 100 nm and its crystallinity is high.

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.

  Next, the method (B) will be described. As the lithium compound and silicic acid compound, those similar to the method (A) are used.

  The method (B) is characterized in that a hydrothermal reaction is performed using an organic acid iron salt as the Fe source. Usually, an organic acid iron salt is a raw material used in a solid phase method, and it was completely unexpected that side reactions can be suppressed by using it in a hydrothermal reaction. As an organic acid used, a C1-C20 organic acid is preferable and a C2-C12 organic acid is more preferable. 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.

  In the method (B), Si and Li are preferably used in a molar ratio of at least twice with respect to Fe, and Si: Li: Fe is more preferably about 1: 1: 2.5 to 1: 1: 3.

  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 concentration of the silicate compound is preferably 0.15 to 1.50 mol / l, more preferably 0.50 to 0.75 mol / l. The concentration of the organic acid iron salt is preferably 0.15 to 1.50 mol / l, more preferably 0.50 to 0.75 mol / l.

In addition, an antioxidant may be added to the aqueous dispersion as necessary. As the antioxidant, sodium hydrosulfite (Na ₂ S ₂ O ₄ ), aqueous ammonia, sodium sulfite and the like can be used. Since the content of the antioxidant in the aqueous dispersion, which results in suppressing the formation of Li ₂ FeSiO ₄ when added in a large amount, an equimolar amount or less are preferred with respect to Fe, in a molar ratio with respect to Fe 0. 5 or less is more preferable.

The aqueous dispersion of these components is preferably basic in order to prevent side reactions and dissolve the silicate compound. The pH of the aqueous dispersion may be basic, but it is 12.0 to 13.5 to prevent side reaction (formation of Fe ₃ O ₄ ), solubility of the silicate compound, and progress of the reaction. Particularly preferred in terms. 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 order of addition of the lithium compound, silicic acid compound and organic acid iron salt is not particularly limited. Moreover, atmospheric conditions may be sufficient.

  In the method (B), the aqueous dispersion is then subjected to a hydrothermal reaction. The hydrothermal reaction is preferably the same as in the method (A).

Li ₂ FeSiO ₄ is obtained in a high yield by the hydrothermal reaction. Moreover, the average particle diameter of the obtained Li ₂ FeSiO ₄ is 10 to 100 nm and its crystallinity is high.

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 average particle size of the lithium iron silicate fine particles used in the present invention is preferably 10 to 1000 nm from the viewpoint of the discharge capacity when a lithium ion battery is used.

  The conductive carbon material used in the present invention is preferably carbon black, more preferably acetylene black or ketjen black. The amount of the conductive carbon material used is preferably 0.01 to 20 parts by mass, more preferably 0.1 to 10 parts by mass with respect to 100 parts by mass of the lithium iron silicate fine particles from the viewpoint of good discharge capacity and economy. preferable.

  The solvent-soluble organic compound may be either a water-soluble organic compound or an organic solvent-soluble organic compound, more preferably a water-soluble organic compound, and particularly preferably a water-soluble alcoholic organic compound or a water-soluble fatty acid. Specific examples include glucose, fructose, polyethylene glycol, polyvinyl alcohol, carboxymethylcellulose, saccharose, starch, dextrin, citric acid and the like, and sugars such as glucose, fructose, saccharose, dextrin are more preferred. The amount of the solvent-soluble organic compound used is preferably 0.01 to 20 parts by mass, more preferably 0.1 to 10 parts by mass with respect to 100 parts by mass of the lithium iron silicate fine particles from the viewpoint of good discharge capacity and economy.

  In the present invention, the mass ratio of the conductive carbon material and the solvent-soluble organic compound is preferably 1:10 to 10: 1, and preferably 1: 5 to 5: from the viewpoint of obtaining a synergistic effect by the combined use of these components. 1 is more preferable, and 1: 3 to 3: 1 is more preferable. The total amount of the conductive carbon material and the solvent-soluble organic compound is preferably 0.02 to 30 parts by mass, more preferably 0.2 to 100 parts by mass of lithium iron silicate fine particles, from the viewpoint of discharge capacity and economy. -20 mass parts is preferable, and 0.2-15 mass parts is especially preferable.

  The solvent may be either water or an organic solvent, but water is preferred.

  The lithium iron silicate fine particles, the conductive carbon material, and the solvent-soluble organic compound are dispersed in a solvent. The amount of the solvent used is preferably 200 to 500 parts by mass, more preferably 200 to 300 parts per 100 parts by mass of the lithium iron silicate fine particles. Part by mass is preferred.

The ball mill apparatus used for mixing the obtained dispersion liquid may be an apparatus used for ordinary ball milling. Examples of the container used include steel, stainless steel, and nylon, and examples of the inner wall include alumina brick, magnetic material, natural silica, rubber, and urethane. As the ball, alumina sphere, natural silica, iron ball, zirconia ball or the like is used. The size of the ball is preferably ³ mm to 20 mm, and the amount of the ball used is preferably 0.5 to 1.0 g / cm ³ with respect to the container capacity.

  Ball mill mixing is preferably performed for 10 minutes or more, more preferably for 3 hours or more, and even more preferably for 6 hours or more under the condition of 10 to 100 revolutions / minute. Further, from the viewpoint of economy, it is preferably 24 hours or less.

The obtained mixture is dried after filtration if necessary. As the drying means, freeze drying or vacuum drying is used. Next, the firing is performed under an inert gas atmosphere or a reducing condition at 400 ° C. or higher, preferably 400 to 800 ° C. for 10 minutes to 3 hours, preferably 0.5 to 1.5 hours. preferable. 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 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 secondary battery. For example, carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, An oxolane 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 Next, an Example is given and this invention is demonstrated in detail.

Reference example 1
LiOH.H ₂ O 4.20 g (0.1 mol), Na ₄ SiO ₄ .nH ₂ O 6.99 g (0.025 mol), Na ₂ S ₂ O ₄ 4.35 g (0.025 mol) and ultrapure water 75 cm ³ was added and mixed (the pH at this time was about 12.5). 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. Lyophilized (about 12 hours) to obtain 4.2 g of Li ₂ FeSiO ₄ powder.

Reference example 2
LiOH.H ₂ O 4.20 g (0.1 mol), amorphous silica 1.50 g (0.025 mol), iron oxalate (FeC ₂ O ₄ .2H ₂ O) 5.00 g (0.025 mol) 75 cm ³ of pure water was added and mixed (the pH at this time was about 12). The obtained liquid mixture was thrown into the autoclave and the hydrothermal reaction was performed at 140 degreeC for 48 hours. The reaction solution was filtered and then lyophilized. Lyophilized (about 12 hours) to obtain 4.2 g of Li ₂ FeSiO ₄ powder.

Example 1
10 g of water was added to 3.34 g of the Li ₂ FeSiO ₄ powder obtained in Reference Example 1, 0.19 g of ketjen black, and 0.45 g of glucose. The obtained dispersion was put into a ball mill pulverizer having a capacity of 400 cm ³ , and 320 g of zirconia balls (φ5 mm) was added thereto and mixed for 12 hours under the condition of 60 rpm. The obtained mixture was vacuum-dried at 80 ° C. and baked at 600 ° C. for 1 hour under a reducing atmosphere condition.

Comparative Example 1
3.34 g of Li ₂ FeSiO ₄ powder obtained in Reference Example 1 and 0.9 g of glucose were put into a ball mill pulverizer having a capacity of 400 cm ³ , and 320 g of zirconia balls (φ5 mm) were put into this, under the conditions of 60 rpm. Mix for 12 hours. The obtained mixture was baked at 600 ° C. for 1 hour under a reducing atmosphere condition.

Comparative Example 2
10 g of water was added to 3.34 g of Li ₂ FeSiO ₄ powder obtained in Reference Example 1 and 0.38 g of Ketjen Black. The obtained dispersion was put into a ball mill pulverizer having a capacity of 400 cm ³ , and 320 g of zirconia balls (φ5 mm) was added thereto and mixed for 12 hours under the condition of 60 rpm. The obtained mixture was vacuum-dried at 80 ° C. and baked at 600 ° C. for 1 hour under a reducing atmosphere condition.

Test example 1
The fired product obtained in Example 1 was subjected to X-ray diffraction. The obtained X-ray diffraction pattern is shown in FIG. FIG. 1 shows that the powder obtained in Example 1 is a single phase of Li ₂ FeSiO ₄ and has high purity.

Test example 2
Using the fired products obtained in Example 1 and Comparative Examples 1 and 2, a positive electrode of a lithium ion secondary battery was produced. The fired product obtained in Example 1 and Comparative Examples 1 and 2, ketjen black (conductive agent), and polyvinylidene fluoride (binding agent) were mixed at a weight ratio of 75:15:10. -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 obtained discharge capacity is shown in Table 1.
From Table 1, it can be seen that the lithium ion battery using the positive electrode material of the present invention has excellent battery characteristics despite a small amount of carbon.

Example 2
The discharge capacity of a lithium ion battery obtained in the same manner as in Test Example 2 using Li ₂ FeSiO ₄ powder obtained in the same manner as in Example 1 except that the ratio of glucose and ketjen black (KB) was changed. Is shown in Table 2.

Claims (7)

Silicic acid source, an iron source and mixture lithium iron silicate powder obtained by hydrothermal reaction of the lithium source, a conductive carbon material, a dispersion containing the solvent-soluble organic compounds and solvent were mixed using a ball mill, A method for producing a positive electrode active material for a lithium ion battery, characterized by firing after drying.   The method according to claim 1, wherein the conductive carbon material is carbon black.   The method according to claim 1 or 2, wherein the solvent-soluble organic compound is a compound selected from a water-soluble alcoholic organic compound and a water-soluble fatty acid.   The method according to any one of claims 1 to 3, wherein the conductive carbon material is carbon black, the solvent-soluble organic compound is a water-soluble alcoholic organic compound, and the solvent is water.   The manufacturing method according to any one of claims 1 to 4, wherein the mass ratio of the conductive carbon material and the solvent-soluble organic compound is 1:10 to 10: 1. The manufacturing method according to any one of claims 1 to 5, wherein the total amount of the conductive carbon material and the solvent-soluble organic compound is 0.1 to 20 parts by mass with respect to 100 parts by mass of the lithium iron silicate powder . The production method according to any one of claims 1 to 6 , wherein the firing condition is 10 minutes to 3 hours at 400 ° C or higher under an inert gas atmosphere or a reducing condition.

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