JP2002003221A - Lithium and iron compound oxide, method for producing it and nonaqueous electrolyte secondary battery using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery which has a high electric discharge capacity, shows a good cycle characteristic of electric charge and discharge, high density charge and discharge performance and moreover is inexpensive and small environmental load. SOLUTION: The nonaqueous electrolyte secondary battery uses as anode activating material, the lithium and iron compound oxide whose characteristic is that it includes alkaline metal and halogen and that according to the X-ray diffraction method using CuKα ray, a half numerical value B of diffraction peak which appears in 15 deg.<2<25 deg., 28 deg.<2<40 deg., 40 deg.<2<45 deg., 58 deg.<2<65 deg. is 2 deg.<B<10 deg. (2).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium iron composite oxide, a method for producing the same, and a non-aqueous electrolyte secondary battery having the same.

[0002]

2. Description of the Related Art In recent years, lithium secondary batteries having a high energy density and a high output density have attracted much attention as small power supplies for portable telephones, video cameras and the like, electric vehicles, and large power supplies for power leveling. Lithium transition metal oxides have been proposed for the positive electrode of lithium secondary batteries, and graphite, amorphous carbon, oxides, lithium alloys and lithium metal have been proposed for the negative electrode.

[0003] Lithium cobalt oxide (LiCoO ₂ ) used as a positive electrode active material is expensive, and in order to cope with the mass consumption of lithium secondary batteries expected in the future, a positive electrode with a lower cost and abundant reserves is required. Development of active materials is important.

[0004] At present, oxides containing manganese, nickel and iron are being vigorously studied as positive electrode active materials for lithium secondary batteries. Of these, iron is the most inexpensive and environmentally friendly material, so iron compounds are very attractive as a positive electrode active material for next-generation lithium secondary batteries.

As iron-containing positive electrode active materials for lithium secondary batteries, various iron compounds such as LiFeO ₂ having a tunnel structure or a layered zigzag structure (J. El.
electrochem. Soc. , 143, 2435 (1
996)), olivine type LiFePO ₄ (J. Elec)
trochem. Soc. , 144, 1609 (199
7)), spinel type LiFe ₅ O ₈ (J. Electro
chem. Soc. , 146, 4371 (199
9)), LiFeO having a hexagonal layered rock salt type structure
₂ (JP-A-10-67519), b-FeOOH (JP
lower Sources, 81-82, 221 (19
99)), amorphous copper-containing iron hydroxide (Abstracts of the 40th Battery Symposium, 3C08) have been proposed.

[0006]

The LiFeO ₂ having the tunnel structure or the layered zigzag structure has a problem that the discharge capacity is reduced to 80% or less of the initial capacity in a 10-cycle life test, and the charge / discharge cycle characteristics are low. . The discharge capacity of olivine-type LiFePO ₄ , spinel-type LiFe ₅ O ₈ , and amorphous copper-containing iron hydroxide is 180 mAh / g or less, which is insufficient as a battery active material. On the other hand, LiFeO ₂ having a hexagonal layered rock salt type structure is disclosed in J. Am. Elect
rochem. Soc. , 144, L177 (199
As shown in 7), there is a problem that the charge / discharge capacity is extremely low (10 mAh / g or less) and the charge / discharge cycle characteristics are low. Further, b-FeOOH has good cycle characteristics and shows a high discharge capacity of 260 mAh / g or more, but cannot have high charge / discharge performance due to its tunnel structure.

Therefore, a lithium iron composite oxide having a high discharge capacity of 200 mAh / g or more, exhibiting good charge / discharge cycle characteristics, and having excellent high rate charge / discharge performance has not been obtained. Therefore, there has been a demand for a lithium iron composite oxide having an unprecedented new structure.

According to the present invention, a novel lithium-iron composite oxide, which has never existed before, is produced and applied as a positive electrode active material. It is an object of the present invention to provide a nonaqueous electrolyte secondary battery having excellent charge / discharge performance, being inexpensive, and having a small environmental load.

[0009]

The invention according to claim 1 relates to a lithium iron composite oxide, wherein the lithium iron composite oxide contains an alkali metal and a halogen, and the X-ray using CuKα ray is used. 15 ° <2θ <by diffraction method
25 °, 28 ° <2θ <40 °, 40 ° <2θ <45
°, 58 ° <2θ <65 °, the half width B of the diffraction peak appearing at 2 ° <B <10 ° (2θ).

A second aspect of the present invention relates to a method for producing a lithium iron composite oxide according to the first aspect, which comprises a step of reducing an iron compound having an oxidation number of four or more. I do.

The invention according to claim 3 relates to the method for producing a lithium iron composite oxide according to claim 2, wherein the iron compound having an oxidation number of 4 or more is M ₄ FeO ₄ ,
M ₃ FeO ₃ , X ₂ FeO ₄ or YFeO ₄ (where M
Is a monovalent element, X is an alkali metal, Y is Mg, Ca, S
r, Ba, Ag, Cu, Pb, Co, and at least one selected from the group of iron compounds represented by Ni).

A fourth aspect of the present invention relates to a non-aqueous electrolyte secondary battery, wherein the positive electrode active material is a non-aqueous electrolyte secondary battery.
It is characterized by using the above-mentioned lithium iron composite oxide.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The lithium-iron composite oxide of the present invention contains an alkali metal and a halogen, and has an angle of 15 ° <2θ <25 °, 28
° <2θ <40 °, 40 ° <2θ <45 °, 58 ° <2
The amorphous compound shows a diffraction peak in which the half width B is 2 ° <B <10 ° (2θ) at θ <65 °.

The lithium-iron composite oxide of the present invention comprises at least one element selected from alkali metals such as sodium and potassium having an ionic radius larger than that of lithium ions;
And at least one element selected from halogens such as iodine and bromine.

As the iron compound having a valence of 4 or more, tetravalent and hexavalent
There is a trivalent compound, but M ₄ Fe
O ₄ or M ₃ FeO ₃ (M is a monovalent element), and X ₆ FeO ₄ (X is an alkali metal such as Na,
K) or YFeO ₄ (Y is Mg, Ca, Sr, Ba,
Ag, Cu, Pb, Co, Ni) can be used. In this case, the product is contaminated with M or X or Y.

The reducing agent used for reducing the iron compound having a valence of 4 or more includes C ₄ H ₉ Li, LiAlH ₄ , LiB
H ₄ and halides can be used. As the halide, a bromide or iodide having a strong reducing power is preferably used.

The bromide includes ZnBr ₂ , AlBr ₃ , NH ₄
Br, IrBr ₃ , CdBr ₂ , KBr, CaBr ₂ , C
oIBr ₂ , CoIBr ₂ , HBr, SrBr ₂ , CsB
r, FeBr ₂ , FeBr ₃ , C ₇ H ₇ Br, NaBr, N
iBr ₂ , VBr ₃ , BaBr ₂ , MgBr ₂ , LiBr,
RbBr can be used, and the iodide includes ZnI ₂ ,
AlI ₃ , NH ₄ I, InI ₃ , CdI ₂ , KI, Zr
I ₄ , HI, CsI, TiI, NaI, BaI ₂ , Li
I and RbI can be used.

As the halide, these hydrates can be used, and each of them can be used alone or in combination. When a halide other than lithium halide is used, the lithium iron composite oxide of the present invention can be obtained by further using a reaction with a lithium salt. Further, when a lithium iron composite oxide is produced using a halide, a halogen is mixed in the product.

In the negative electrode material used in the non-aqueous electrolyte secondary battery of the present invention, a substance capable of inserting and extracting lithium metal and / or lithium ions is used. Examples of the substance capable of inserting and extracting lithium ions include graphite, amorphous carbon, oxides, nitrides, and lithium alloys. As the lithium alloy, for example, an alloy of lithium, aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, and tin can be used.

The non-aqueous electrolyte used in the non-aqueous electrolyte secondary battery of the present invention may be a non-aqueous electrolyte or a solid electrolyte. As the solvent used for the non-aqueous electrolyte, ethylene carbonate, propylene carbonate,
Dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane , Methyl acetate and other polar solvents and mixed solvents thereof are exemplified.
_{F 6, LiBF 4, LiAsF 6} , LiClO 4, LiSC
N, LiCF ₃ CO ₂ , LiCF ₃ SO ₃ , LiN (SO ₂
CF ₃ ) ₂ , LiN (SO ₂ CF ₂ CF ₃ ) ₂ , LiN (CO
Salts such as CF ₃ ) ₂ and LiN (COCF ₂ CF ₃ ) ₂ or mixtures thereof are exemplified.

When the lithium-iron composite oxide of the present invention is used as a positive electrode active material of a lithium secondary battery, it has a high discharge capacity of 200 mAh / g or more, exhibits good charge / discharge cycle characteristics, and has a high rate of charge / discharge. Excellent performance.

The reason for this is not clear yet, but it is presumed that the lithium-iron composite oxide of the present invention is an amorphous compound, and the change in the crystal structure of the active material with cycling is small, and therefore the cycle characteristics are excellent.

Further, the oxide contains an alkali metal ion and a halogen having an ionic radius larger than that of a lithium ion, and since these ions serve as pillars in the active material, the structure is stabilized, and thus the cycle characteristics are improved. It is considered to be excellent.

Further, the lithium iron composite oxide of the present invention is an amorphous compound, is capable of three-dimensional diffusion of lithium ions in the active material, and is considered to be excellent in high-rate charge / discharge performance.

[0025]

EXAMPLES A nonaqueous electrolyte secondary battery comprising the lithium iron composite oxide of the present invention as a positive electrode active material will be described in more detail with reference to examples. However, the present invention is not limited to the following examples.

Example In this example, a lithium-iron composite oxide was obtained by reducing a highly oxidizable hexavalent iron compound in a solution. First, the hexavalent iron compound K
_A process for producing an aqueous solution in which ₂ FeO ₄ is dissolved will be described.

10 g of iron (III) nitrate hydrate was dissolved in 100 ml of sodium hypochlorite solution (antiformin), and sodium hydroxide was added to the solution while maintaining the temperature at 30 ° C. until the solution was saturated, and mixed well. The precipitate was filtered, 150 ml of a saturated potassium hydroxide solution was added to the filtrate, and the mixture was stirred for 5 minutes and filtered. The filtrate was completely dissolved in 4 or 5 times with 10 ml of a 3M potassium hydroxide solution.

Next, the solution was poured into 100 ml of an ice-cooled saturated potassium hydroxide solution, and 100 ml of an ice-cooled saturated potassium hydroxide solution was further added, followed by stirring at 20 ° C. for 5 minutes. After the precipitate was filtered, the filtrate was completely dissolved again with 10 ml of a 3M potassium hydroxide solution four or five times. As a result, an aqueous solution in which K ₂ FeO ₄ was dissolved was obtained.

Next, 3 g of lithium iodide powder was added to the aqueous solution in which K ₂ FeO ₄ was dissolved, the mixture was stirred for 30 minutes, and the precipitate was filtered. The filtrate was thoroughly washed with ethanol to remove excess lithium iodide. Finally, by drying, a lithium iron composite oxide was obtained. The X-ray diffraction pattern of the final product is shown in FIG. In addition, a known β as a comparative example
-FeOOH (J. Power Sources, 81
-82, 221 (1999)) is shown in FIG.

As is apparent from FIG. 1, the lithium iron composite oxide of the present invention has a temperature of about 18 °, 31 °, 43 °, and 62 °.
In addition, a broad diffraction peak was shown, indicating that the compound was an amorphous compound.

Further, ICP emission spectroscopy confirmed that the obtained lithium iron composite oxide contained potassium and iodine.

Next, 20 parts by weight of acetylene black as a conductive agent and 5 parts by weight of polyfukkavinylidene as a binder were added to 75 parts by weight of the lithium iron composite oxide as a positive electrode active material, and the mixture was used as a solvent. A slurry was obtained by wet mixing with N-methyl-2-pyrrolidone. The slurry was applied to both sides of an aluminum mesh as a current collector, pressed under a pressure of 1 t / cm ² , and dried under vacuum to produce a positive electrode having a size of 15 mm × 15 mm × 0.5 mm.

Finally, using the above positive electrode, a battery of the present invention comprising a lithium iron composite oxide as a positive electrode active material was manufactured.
Lithium metal for the negative electrode, ethylene carbonate and dimethyl carbonate in the non-aqueous electrolyte were mixed at a volume ratio of 1: 1.
Using a 1 mol / l LiPF ₆ electrolyte, a flooded battery was manufactured.

Comparative Example As a positive electrode active material, b-FeOOH, which is conventionally known for having a large discharge capacity, is described in J. Am. Power Sources, 81-82, 221
(1999). A comparative battery was manufactured in the same manner as in the example except that this positive electrode active material was used.

The battery of the present invention and the comparative battery manufactured as described above were subjected to a 10-cycle charge / discharge test at a constant current. Charge and discharge end-to-end voltages are 4.5
V and 1.5 V.

FIG. 3 is a graph showing the relationship between the initial discharge capacity (mAh / g) per 1 g of the positive electrode active material and the current density of the battery of the present invention and the comparative battery. In FIG. 3, the symbol ● indicates the battery of the present invention, and the symbol ▲ indicates the comparative battery.

At a low current density of 0.05 mA / cm ² , both the battery of the present invention and the comparative battery exhibited a capacity of 200 mAh / g or more. However, it can be seen that the battery of the present invention maintains a high discharge capacity even at a higher current density, and is superior in high-rate charge / discharge performance as compared with the comparative battery. Note that the battery of the present invention has a current density of 0.05 mA / cm ² ,
In the cycle life test, 80% or more of the initial discharge capacity was maintained, and the cycle characteristics were excellent.

In this embodiment, a method for reducing a hexavalent iron compound, especially K ₂ FeO ₄ , was selected as a method for producing a lithium iron composite oxide, but a method using another hexavalent iron compound was used. In the same case, the battery had high discharge capacity, excellent cycle characteristics, and excellent high-rate charge / discharge performance.

Although the lithium iron composite oxide of the present invention contains potassium and iodine, it also has a high discharge capacity and a good cycle characteristic even when it contains other alkali metals or halogens. Excellent, high rate charge / discharge performance.

[0040]

As described above, according to the present invention, an alkali metal and a halogen are contained, and 15 ° <2θ <25 ° and 28 ° <2θ <by X-ray diffraction using CuKα ray.
40 °, 40 ° <2θ <45 °, 58 ° <2θ <65 °
The half width B of the diffraction peak appearing at 2 ° <B <10 ° (2
The non-aqueous electrolyte secondary battery provided with the lithium iron composite oxide (θ) as the positive electrode active material has a high discharge capacity, good charge / discharge cycle characteristics, and excellent high rate charge / discharge performance. is there.

[Brief description of the drawings]

FIG. 1 is a diagram showing an X-ray diffraction pattern of an active material obtained in an example.

FIG. 2 is a view showing an X-ray diffraction pattern of β-FeOOH.

FIG. 3 is a diagram showing the relationship between the initial discharge capacity and the current density of the battery of the present invention and the comparative battery.

 ──────────────────────────────────────────────────の Continued on the front page F term (reference) 4G002 AA06 AA08 AA10 AB02 AE05 4G048 AA03 AA04 AA05 AB02 AC06 AD03 AE05 AE07 5H029 AJ00 AJ02 AJ03 AJ05 AK03 AL07 AL08 AM02 AM03 AM04 AM05 AM07 DJ17 HJ00HJA A07 A13 A07 BA16 BA17 CA07 CA08 CB01 CB02 CB05 CB08 CB09 CB12 HA00 HA02 HA13

Claims (4)

[Claims] Claims: 1. An alkali metal and a halogen,
15 ° <2θ <25 by X-ray diffraction using CuKα ray
°, 28 ° <2θ <40 °, 40 ° <2θ <45 °, 5
When the half width B of the diffraction peak appearing at 8 ° <2θ <65 ° is 2
° <B <10 ° (2θ), wherein the lithium iron composite oxide is characterized in that: 2. The method for producing a lithium iron composite oxide according to claim 1, further comprising a step of reducing an iron compound having an oxidation number of four or more. 3. An iron compound having an oxidation number of 4 or more is represented by M
₄ FeO ₄ , M ₃ FeO ₃ , X ₂ FeO ₄ or YFeO
₄ (where M is a monovalent element, X is an alkali metal, Y is Mg, Ca, Sr, Ba, Ag, Cu, Pb, Co, N
at least one selected from the group of iron compounds represented by i
3. The method for producing a lithium iron composite oxide according to claim 2, wherein 4. A non-aqueous electrolyte secondary battery using the lithium iron composite oxide according to claim 1 as a positive electrode active material.