JP2013089298A - Positive electrode active material for secondary battery and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an olivine type silicate compound such as LiFeSiO, exhibiting crystal orientation with extremely high uniformity and useful as a positive electrode active material for a secondary battery, and to provide a positive electrode active material for a secondary battery and a method for producing the positive electrode active material for a secondary battery.SOLUTION: An olivine type silicate compound is represented as LiMSiO(in the formula, M represents one or two or more kinds of elements selected from among Fe, Ni, Co and Mn), and has a peak intensity ratio of a (010) surface to a (011) surface of 0.6 times or more, in an X-ray diffraction pattern.

Description

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

  A secondary battery such as a lithium ion battery is a kind of non-aqueous electrolyte battery, and is used in a wide range of fields such as a mobile phone, a digital camera, a notebook PC, a hybrid vehicle, and an electric vehicle. 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 phosphate metal-based positive electrode materials such as LiFePO ₄ are known to have a uniform crystal orientation that exhibits a specific peak intensity ratio in X-ray diffraction in order to further improve the physical properties of the obtained battery. (Patent Documents 1 and 2).

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 3 and 4). 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.

JP 2007-207637 A Special table 2008-541364 gazette JP 2001-266882 A JP 2002-198050 A

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

However, even if the present inventors mix three of a lithium source, an iron source, and a silicate source with water and subject the mixture to a hydrothermal reaction as it is, the crystal orientation of Li ₂ FeSiO ₄ obtained can be improved. It was found that the battery physical properties could not be sufficiently controlled and the physical properties of the battery might be lowered when used as a positive electrode active material for a secondary battery. Therefore, as for the lithium metal silicate-based positive electrode material such as Li ₂ FeSiO ₄ , it is a fact that a controlled and highly uniform crystal compounding property is not yet known.

Accordingly, an object of the present invention is to provide a highly uniform crystal orientation and an olivine-type silicate compound such as Li ₂ FeSiO ₄ useful as a positive electrode active material for a secondary battery, a positive electrode active material for a secondary battery and its It is to provide a manufacturing method.

  Therefore, the present inventors have an excellent olivine type silicate compound showing a specific peak intensity ratio in an X-ray diffraction diagram, which has an extremely high crystal compoundability and is excellent when used as a positive electrode active material for a secondary battery. The present inventors have found that the battery has physical properties and have completed the present invention.

That is, the present invention is represented by Li ₂ MSiO ₄ (wherein M represents one or more selected from Fe, Ni, Co, or Mn), and in the X-ray diffraction diagram, (011) plane The olivine type silicate compound is characterized in that the peak intensity of the (010) plane with respect to the peak intensity is 0.6 times or more.

Further, the present invention provides a transition metal sulfate represented by MSO ₄ (wherein M represents Fe, Ni, Co or Mn) or (R) ₂ M (wherein R represents an organic acid residue). , M represents Fe, Ni, Co, or Mn), and a basic aqueous dispersion containing a lithium compound, a silicic acid compound and an alignment controller is subjected to a hydrothermal reaction using an organic acid transition metal salt represented by: The present invention provides a method for producing the above olivine-type silicate compound.
Furthermore, this invention provides the positive electrode active material for secondary batteries containing the said olivine type | mold silicate compound.

Further, the present invention provides a transition metal sulfate represented by MSO ₄ (wherein M represents Fe, Ni, Co or Mn) or (R) ₂ M (wherein R represents an organic acid residue). , M represents Fe, Ni, Co, or Mn), and a basic aqueous dispersion containing a lithium compound, a silicate compound, and an alignment controller is hydrothermally reacted to form water. The present invention provides a method for producing a positive electrode active material for a secondary battery, characterized in that after the thermal reaction, carbon is supported on the obtained olivine type silicate compound and then fired.

  A secondary 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 secondary 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. The charging / discharging curve of the battery using the positive electrode active material obtained by the comparative 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 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 more preferable.

When the X-ray diffraction of the olivine-type silicate compound represented by Li ₂ MSiO ₄ of the present invention is measured, the peak intensity of the (010) plane is 0.6 times the peak intensity of the (011) plane in the obtained diffraction diagram. It is above, Preferably it is 0.7 to 5.0 times, More preferably, it is 1.0 to 2.5 times. The peak intensity means the highest value indicated by a peak due to a specific Miller index surface.

  Thus, when the peak intensity of the (010) plane is 0.6 times or more than the peak intensity of the (011) plane, the orientation of the olivine type silicate compound in which lithium ions easily diffuse inside the crystal is controlled. It is estimated that In addition, the charging and discharging of a lithium ion battery facilitates the entry and exit of lithium ions having such a specific direction having anisotropy. Therefore, the olivine is used as a positive electrode active material having extremely uniform crystal orientation. It is considered that a secondary battery having an excellent discharge capacity can be obtained by using the type silicate compound.

For example, when Li ₂ MSiO ₄ is Li ₂ FeSiO ₄ and 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 Appears at 2θ = 16.7 °. 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 °. In view of further improving the uniformity of crystal orientation, the peak intensity of the (011) plane is preferably 1.0 to 2.5 times the peak intensity of the (200) plane, and 1.1 to 2. It is more preferably 0 times.

The olivine-type silicate compound of the present invention has, as a transition metal (M) source, a transition metal sulfate represented by MSO ₄ (wherein M represents Fe, Ni, Co or Mn) or (R) ₂ M ( In the formula, R represents an organic acid residue, M represents an organic acid transition metal salt represented by Fe, Ni, Co, or Mn), and a base containing a lithium compound, a silicate compound, and an alignment controller The aqueous dispersion is subjected to a hydrothermal reaction. By using the orientation control agent, the crystal orientation in Li ₂ MSiO ₄ can be made extremely effective.

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 alignment control agent is not particularly limited as long as it is a compound containing dithionite ion (S ₂ O ₄ ²⁻ ). For example, sodium dithionite (Na ₂ S ₂ O ₄ ), potassium dithionite, Ammonium dithionite and the like can be used. These may be used alone or in combination of two or more. When the content of the alignment control agent in the aqueous dispersion is added in a large amount, the formation of the olivine-type silicate compound is suppressed, so 0.001-1 is preferable in terms of molar ratio to the transition metal. 0.8 is more preferable.

When transition metal sulfate MSO ₄ (wherein M represents Fe, Ni, Co, or Mn) is used as the transition metal source, a lithium compound, silica, and silica are separated from the transition metal sulfate from the viewpoint of suppressing side reactions. It is preferable to prepare in advance a basic aqueous dispersion containing an acid compound and an alignment controller. In this case, the aqueous dispersion and the transition metal 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 alignment control agent is not particularly limited, and these three components may be added to water.

Specific examples of the transition metal sulfate MSO ₄ include FeSO ₄ , NiSO ₄ , CoSO _{4, and} MnSO ₄ , and these may be used alone or in combination of two or more. Of these, FeSO ₄ and MnSO ₄ are more preferable, and FeSO ₄ is more 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.
In this case, the Li content in the reaction mixture is preferably 2 mol or more with respect to M.

When using an organic acid transition metal salt (R) ₂ M (wherein R represents an organic acid residue and M represents Fe, Ni, Co or Mn) as the transition metal source, a lithium compound, silicic acid A basic aqueous dispersion containing a compound and an orientation control agent and further containing an organic acid transition metal salt is prepared. In this case, the addition order of the lithium compound, the silicic acid compound, the alignment control agent, and the organic acid transition metal salt is not particularly limited. Moreover, atmospheric conditions may be sufficient. Usually, an organic acid salt is a raw material used in a solid phase method, but side reactions can be suppressed by using it in a hydrothermal reaction.
In this case, Li in the reaction mixture is preferably used in a molar ratio of 2 times or more with respect to the transition metal, and Li: M is more preferably about 2.5: 1 to 3: 1.

The organic acid represented by an organic acid transition metal salt (R) of ₂ M R, preferably an organic acid having 1 to 20 carbon atoms, 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.

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 secondary battery by carrying carbon and then firing it. Carbon support may be obtained by adding a carbon source such as glucose, fructose, polyethylene glycol, polyvinyl alcohol, carboxymethylcellulose, saccharose, starch, dextrin, citric acid, and water to Li ₂ MSiO 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 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 a secondary battery. The secondary battery to which the positive electrode active material of the present invention can be applied may be a lithium ion secondary battery, and is not particularly limited as long as it 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 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 Hereinafter, although this invention is demonstrated concretely based on an Example, this invention is not limited to these Examples.

[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. 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 Na ₂ S ₂ O ₄ 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 peak intensity of the (010) plane relative to the peak intensity of the (011) plane was 0.5 times that of the powder obtained in Comparative Example 1, whereas that obtained in Example 1 was obtained. It can be seen that the resulting powder is 1.3 times and will be controlled to have crystal orientation in a certain direction. In addition, the peak intensity of the (010) plane with respect to the peak intensity of the (200) plane was 1.2 times in Comparative Example 1 and 3.4 times in Example 1.

[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. 2, and the charge / discharge curve of the battery constructed with the positive electrode material of Comparative Example 1 is shown in FIG.

  2 to 3, it can be seen that the lithium ion secondary battery using the controlled positive electrode material of the present invention having high uniformity has excellent battery properties.

Claims (13)

Li ₂ MSiO ₄ (wherein M represents one or more selected from Fe, Ni, Co, or Mn), and in the X-ray diffraction diagram, (0101) with respect to the peak intensity of the (011) plane ) An olivine type silicate compound having a peak intensity of 0.6 times or more. The olivine-type silicate compound according to claim 1, wherein Li ₂ MSiO ₄ is Li ₂ FeSiO ₄ .   The positive electrode active material for secondary batteries containing the olivine type | mold silicate compound of Claim 1 or 2.   The secondary battery which has a positive electrode containing the positive electrode active material for secondary batteries of Claim 3. Transition metal sulfate represented by MSO ₄ (wherein M represents Fe, Ni, Co or Mn) or (R) ₂ M (wherein R represents an organic acid residue, M represents Fe, Ni And a basic aqueous dispersion containing a lithium compound, a silicic acid compound and an alignment controller is subjected to a hydrothermal reaction using an organic acid transition metal salt represented by: Co or Mn). A method for producing the olivine-type silicate compound according to 1 or 2.   The method for producing an olivine-type silicate compound according to claim 5, wherein the orientation control agent is a compound containing dithionite ion.   The method for producing an olivine-type silicate compound according to claim 5 or 6, wherein the content of the orientation control agent in the basic aqueous dispersion is 0.001 to 1 in terms of molar ratio to the transition metal. A basic aqueous dispersion and one or more transition metal sulfates represented by MSO ₄ (wherein M represents Fe, Ni, Co or Mn) are mixed, and the resulting mixture is obtained. The manufacturing method of the olivine type | mold silicate compound of any one of Claims 5-7 made to make a hydrothermal reaction. The basic aqueous dispersion further contains an organic acid transition metal salt represented by (R) ₂ M (wherein R represents an organic acid residue and M represents Fe, Ni, Co or Mn). The manufacturing method of the olivine type | mold silicate compound of any one of Claims 5-7.   The method for producing an olivine-type silicate compound according to any one of claims 5 to 9, 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 any one of claims 5 to 10, 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 5 to 11, wherein the pH of the basic aqueous dispersion is 12.0 to 13.5. Transition metal sulfate represented by MSO ₄ (wherein M represents Fe, Ni, Co or Mn) or (R) ₂ M (wherein R represents an organic acid residue, M represents Fe, Ni Is obtained after hydrothermal reaction of a basic aqueous dispersion containing a lithium compound, a silicic acid compound and an alignment controller. 4. The method for producing a positive electrode active material for a secondary battery according to claim 3, wherein carbon is supported on the olivine-type silicate compound and then fired.