JP5531298B2 - 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.

However, even if a lithium source, an iron source, and a silicate source are mixed with water and the mixture is subjected to a hydrothermal reaction as it is, a single phase of Li ₂ FeSiO ₄ cannot be obtained, and Fe ₃ Si ₂ O ₄ (OH) ₄ etc. were found to be by-produced.
In Non-Patent Document 2, FeCl ₂ is used as the Fe source, which causes problems such as corrosion of the reactor.
Accordingly, an object of the present invention is to provide a method for producing an olivine-type silicate compound such as Li ₂ FeSiO ₄ useful as a positive electrode active material for a lithium ion battery by a hydrothermal reaction without using chloride as a raw material. There is.

  Therefore, the present inventor adopts sulfate as a transition metal source such as an Fe source, and does not add the transition metal sulfate to water together with a lithium source or a silicate source. And an antioxidant are added as a basic aqueous dispersion, transition metal sulfate is added thereto, and then a hydrothermal reaction is performed. It was found that a silicate compound was obtained. Furthermore, it has been found that a lithium ion battery using as a positive electrode material a material obtained by carrying and baking the obtained olivine-type silicate compound on carbon has excellent battery characteristics.

That is, the present invention relates to a basic aqueous dispersion containing a lithium compound, a silicate compound and an antioxidant, and a transition metal represented by MSO ₄ (wherein M represents Fe, Ni, Co or Mn). Li ₂ MSiO ₄ (wherein M is selected from Fe, Ni, Co or Mn), characterized in that one or more sulfates are mixed and the resulting mixture is hydrothermally reacted. A method for producing an olivine-type silicate compound represented by a species or two or more species).

The present invention also relates to an olivine type represented by Li ₂ MSiO ₄ (M is the same as described above), wherein carbon is supported on the olivine type silicate compound obtained by the above hydrothermal reaction, and then calcined. The present invention provides a method for producing a positive electrode active material for a lithium ion battery containing a silicate compound.

  Furthermore, this invention provides the lithium ion battery which has a positive electrode containing said positive electrode active material.

  According to the method of the present invention, a fine olivine-type silicate compound can be efficiently obtained with high purity by a hydrothermal reaction. By using the obtained olivine type silicate compound, a lithium ion battery having excellent battery characteristics can be obtained.

The X-ray diffraction pattern of the lyophilized powder obtained in Example 1 and Comparative Example 1 is shown. The discharge capacity of the 4th cycle of the battery using the positive electrode active material 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.

The present invention is a method for producing an olivine type silicate compound represented by Li ₂ MSiO ₄ (wherein M represents one or more selected from Fe, Ni, Co, or Mn). Specific examples of the olivine silicate compound include Li ₂ FeSiO ₄ , Li ₂ NiSiO ₄ , Li ₂ CoSiO ₄ , Li ₂ MnSiO ₄ , Li ₂ (Fe) _m (Mn) _1-m SiO ₄ (0 <m < 1). Among these, Li ₂ FeSiO ₄ and Li ₂ MnSiO ₄ are preferable from the viewpoint of raw material cost, and Li ₂ FeSiO ₄ is particularly preferable.

In the present invention, it is preferable to prepare a basic aqueous dispersion containing a lithium compound, a silicic acid compound and an antioxidant separately from the transition metal sulfate 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. The content of the antioxidant in the aqueous dispersion is preferably equal to or less than the transition metal in terms of a molar ratio with respect to the transition metal because it suppresses the formation of the olivine-type silicate compound when added in a large amount. 0.5 or less is more preferable.

In order to prevent side reactions and dissolve the silicate compound, it is important that the lithium compound, the silicate compound, and the antioxidant be a basic aqueous dispersion separately from the transition metal sulfate. 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.

Specific examples of the transition metal sulfate MSO ₄ (wherein M represents Fe, Ni, Co, or Mn) used in the present invention include FeSO ₄ , NiSO ₄ , CoSO _4, or MnSO _4. You may use 1 type or in mixture of 2 or more types. Of these, FeSO ₄ and MnSO ₄ are more preferable, and FeSO ₄ is particularly preferable. The amount of transition metal 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.

  Further, the content of Si and Li in the reaction mixture is preferably 2 mol or more with respect to M.

  In the present invention, the aqueous dispersion and the transition metal sulfate 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.

By the hydrothermal reaction, Li ₂ MSiO ₄ (M is the same as above) is obtained in high yield. The obtained LiMSiO _{4 has} an average particle size of 10 to 100 nm and a high crystallinity.

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

The obtained Li ₂ MSiO 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 MSiO ₄ by a conventional method, 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 MSiO ₄ can be obtained. The amount of 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 MSiO 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 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.

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. Polyvinyl alcohol (carbon concentration: 3%) and 10 cm ³ of ultrapure water were added to 4.2 g of the powder obtained by freeze-drying (about 12 hours), and calcined at 700 ° C. for 1 hr in a reducing atmosphere.

Comparative Example 1
8.20 g (0.2 mol) of LiOH.H ₂ O, 3.00 g (0.05 mol) of amorphous silica, and 100 cm ³ of ultrapure water were mixed. Meanwhile, 3.98 g (0.2 mol) of FeCl ₂ .4H ₂ O, 0.59 g (0.0033 mol) of ascorbic acid and 50 cm ³ of ultrapure water were mixed. Both liquid mixtures were mixed, hydrothermal reaction was performed under the same conditions as in Example 1, and the resulting reaction liquid was filtered and freeze-dried (12 hours). The obtained powder was calcined with carbon in the same manner as in Example 1.

Test example 1
X-ray diffraction of the lyophilized powder obtained in Example 1 and Comparative Example 1 was performed. The obtained X-ray diffraction pattern is shown in FIG. As is clear from FIG. 1, the powder obtained in Comparative Example 1 produced Fe ₃ Si ₂ O ₅ (OH) _{4 as a} by-product. On the other hand, it was found that the powder obtained in Example 1 was a single phase of Li ₂ FeSiO ₄ and had 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 obtained discharge capacity at the fourth cycle is shown in FIG. Moreover, the charging / discharging curve of the battery constructed | assembled with the positive electrode material of Example 1 is shown in FIG.
2 and 3, it can be seen that the lithium ion battery using the positive electrode material of the present invention has excellent battery characteristics.

Claims (6)

A basic aqueous dispersion containing a lithium compound, a silicate compound and an antioxidant, and one of transition metal sulfates represented by MSO ₄ (wherein M represents Fe, Ni, Co or Mn) or Li ₂ MSiO ₄ (wherein, M is one or more selected from Fe, Ni, Co or Mn), characterized in that two or more are mixed and the resulting mixture is hydrothermally reacted. A method for producing an olivine-type silicate compound represented by:   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 silicate 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 antioxidant is selected from ammonia, sodium hydrosulfite, and sodium sulfite.   The method for producing an olivine silicate compound according to any one of claims 1 to 4, wherein the pH of the basic aqueous dispersion is 12.0 to 13.5. Li ₂ MSiO ₄ (M is the same as above), characterized in that after completion of the hydrothermal reaction according to any one of claims 1 to 5, carbon is supported on the obtained olivine-type silicate compound and then fired. The manufacturing method of the positive electrode active material for lithium ion batteries containing the olivine type | mold silicate compound represented by these.