JP2004303527A - Electrode active material and electrode for nonaqueous electrolyte secondary battery as well as nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-capacity electrode active material for a nonaqueous electrolyte secondary battery effective for improvement of rate characteristics. <P>SOLUTION: The electrode active material for the nonaqueous electrolyte secondary battery is composed of lithium/vanadium/phosphorus complex compound powder having a rhombic LiVOPO<SB>4</SB>type crystal structure. The electrode for the nonaqueous electrolyte secondary battery has an active material layer containing this electrode active material formed on a collector. The nonaqueous electrolyte secondary battery, with this electrode as a positive electrode, is provided with a negative electrode containing a negative electrode active material storing and discharging lithium ion, and an electrolyte. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００５５】
表１より、斜方晶ＬｉＶＯＰＯ_４であれば、三斜晶ＬｉＶＯＰＯ_４よりも著しく高容量の二次電池を提供できることがわかる。
【００５６】
また、実施例１と実施例３で作製した半開放型セルを用いて、各々、充放電の電流値をＣ／５０、Ｃ／２５、Ｃ／１０、Ｃ／５と変えて放電容量を測定することにより求めたレート特性を図１に示す。図１より明らかなように、メジアン径がより小さく、比表面積がより大きい実施例３の正極活物質を使用した場合、実施例１の正極活物質を使用した場合よりもレート特性が向上した。
【００５７】
【発明の効果】
以上詳述した通り、本発明によれば、特定の結晶構造を有するＬｉＶＯＰＯ_４を正極活物質として用いることにより、従来公知の三斜晶ＬｉＶＯＰＯ_４電極活物質と比較して、高容量の非水電解質二次電池を提供することができる。さらに、ＬｉＶＯＰＯ_４の粒径や比表面積を選ぶことにより、レート特性を向上させることも可能となる。
【図面の簡単な説明】
【図１】実施例１及び実施例３で作製した半開放型セルのレート特性を示すグラフである。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an electrode active material for a non-aqueous electrolyte secondary battery that is effective for improving the capacity and rate characteristics of a non-aqueous electrolyte secondary battery, an electrode for a non-aqueous electrolyte secondary battery using the electrode active material, and a non-aqueous electrolyte. The present invention relates to an electrolyte secondary battery.
[0002]
[Prior art]
Conventionally, as electrode active materials for nonaqueous electrolyte secondary batteries, electrode active materials such as LiCoO ₂ , LiMn ₂ O ₄ , and LiVOPO ₄ have been studied. It describes that crystalline LiVOPO ₄ is used as a positive electrode active material of a non-aqueous electrolyte secondary battery.
[0003]
[Non-patent document 1]
Electrochemical and Solid-State Letters, 3 (10)
460-462 '(2000)
[0004]
[Problems to be solved by the invention]
Li-VPO-based composite compounds have a feature that they are excellent in high-temperature characteristics because they do not easily release oxygen at high temperatures, but at present, when used as an electrode active material, a comparative example described later As shown in (1), only low-capacity products have yet been obtained, and improvements have been demanded.
[0005]
Accordingly, the present invention provides an electrode active material for a Li-VPO-based non-aqueous electrolyte secondary battery, which is high in capacity and effective in improving rate characteristics, and a non-aqueous electrolyte secondary battery using the electrode active material. It is intended to provide an electrode for use and a non-aqueous electrolyte secondary battery.
[0006]
[Means for Solving the Problems]
The electrode active material for a non-aqueous electrolyte secondary battery of the present invention is characterized in that it is a lithium-vanadium-phosphorus composite compound powder having an orthorhombic LiVOPO ₄ type crystal structure.
[0007]
LiVOPO ₄ is charged and discharged at a potential of about 4 V with respect to metallic lithium by redox between tetravalent and pentavalent vanadium. As a crystal structure of LiVOPO _4, a crystal structure having a triclinic structure and a crystal structure having an orthorhombic structure are known, and crystal structures giving an X-ray diffraction pattern described in JCPDS cards 72-2253 and 42-0469, respectively. Having. The crystal structure of LiVOPO ₄ used in Non-Patent Document 1 is triclinic, whereas the crystal structure of the electrode active material of the present invention is orthorhombic.
[0008]
That is, as a result of intensive studies, the present inventors have found that by selecting a specific crystal system even for LiVOPO ₄ type, it is possible to improve the capacity when used as an electrode active material, and to further reduce the particle size and specific surface area. By selecting, it was found that the rate characteristics could also be improved, and the present invention was completed.
[0009]
The reason why the orthorhombic LiVOPO _{4 according} to the present invention exhibits a high capacity is not clear, but it is considered that lithium ions can easily move in the crystal structure more easily than the triclinic LiVOPO ₄ .
[0010]
The electrode for a non-aqueous electrolyte secondary battery of the present invention is characterized in that an active material layer containing such an electrode active material of the present invention is formed on a current collector.
[0011]
Further, the non-aqueous electrolyte secondary battery of the present invention is provided with such a non-aqueous electrolyte secondary battery electrode of the present invention as a positive electrode, a negative electrode containing a negative electrode active material that occludes and releases lithium ions, and an electrolyte. It is characterized by the following.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in more detail.
(1) Electrode Active Material for Nonaqueous Electrolyte Secondary Battery First, the electrode active material for a nonaqueous electrolyte secondary battery of the present invention will be described.
[0013]
The electrode active material for a non-aqueous electrolyte secondary battery of the present invention is characterized in that it is a lithium-vanadium-phosphorus composite compound powder having an orthorhombic LiVOPO ₄ type crystal structure.
[0014]
In the orthorhombic LiVOPO ₄ type lithium-vanadium-phosphorus composite compound, lithium, vanadium and phosphorus in the crystal structure are replaced with other elements to control the charge / discharge capacity and to stabilize the crystal structure. Is also possible.
[0015]
In this case, specific examples of the element that replaces lithium include one or more selected from the group consisting of Na, K, Mg, Ca, Ti, Zr, and Nb, and preferably Ti, Zr, And Nb or one or more selected from the group consisting of Nb. Specific examples of the element that replaces vanadium include Al, Fe, Ga, Bi, Sn, Cr, Cu, Zn, Mg, Ti, Ge, Ta, Mo, W, Nb, Ni, Mn, and Co. One or more selected from the group consisting of, preferably, one or more selected from the group consisting of Ni, Mn, and Co. Further, specific examples of the element that substitutes for phosphorus include one or more selected from the group consisting of Si, N, S, As, Si, and Ge, and preferably include one or more of Si, N, and S. One or more selected from the group consisting of:
[0016]
If the replacement amount of these elements is too large, the battery capacity will be too low. Therefore, each of Li, V, and P before the replacement is usually 0.4 or less, preferably 0.2 or less, more preferably 0.1 or less. It is as follows.
[0017]
In addition, oxygen constituting LiVOPO ₄ may have oxygen deficiency or excess oxygen, but the deviation from the stoichiometric composition ratio is 0.2 or less for the same reason as the above substitution amount. Is preferred.
[0018]
By reducing the particle size of the lithium-vanadium-phosphorus composite compound powder, the diffusion distance of lithium ions in the active material is reduced, and the rate characteristics are improved. Therefore, the particle diameter of the lithium-vanadium-phosphorus compound powder is, for example, the median diameter measured by a laser diffraction type particle size distribution analyzer, usually 20 μm or less, preferably 5 μm or less, more preferably 2 μm or less. If the median diameter is too small, the filling property as an active material tends to decrease, but a lower limit of about 0.1 μm is acceptable.
[0019]
In addition, by increasing the specific surface area of the lithium-vanadium-phosphorus composite compound powder, the contact area between the active material and the electrolyte or the electrolytic solution increases, and the rate characteristics can be improved. Therefore, a lithium - vanadium - a specific surface area of the phosphorus complex compound powder is a value measured by a nitrogen adsorption BET specific surface area meter, usually 0.05 m ² / g or more, preferably ^3m 2 / g or more, more preferably 5m ² / g or more. If the specific surface area is too large, the adhesiveness between the active materials and between the active material and the current collector tends to decrease, but an upper limit of about 100 m ² / g is acceptable.
[0020]
Such lithium-vanadium-phosphorus composite compound powder constituting the electrode active material of the present invention can be produced by a known method as long as an orthorhombic LiVOPO ₄ type crystal structure can be obtained, and the method is also various. There is a way. As an example of the production method, for example, a Li source, a V source, a P source, and a desired substitution element source as described below are obtained by using (sum of substitution elements of Li and Li): (sum of substitution elements of V and V). ): A method in which a raw material aqueous solution containing a molar ratio corresponding to (total of P and P substitution elements) = 1: 1: 1 is stirred while being heated, and then dried and fired.
[0021]
In this case, each element source may contain the target element and may be any element that can remove unnecessary elements by firing, but when manufactured via an aqueous solution, it is preferably a water-soluble raw material. Specific examples include the following.
Li source: usually, lithium hydroxide, lithium nitrate, lithium carbonate, etc., preferably lithium hydroxide, lithium nitrate, etc. V source: usually, V source such as vanadium dioxide, vanadium trioxide, vanadium pentoxide, etc., preferably vanadium pentoxide, etc. : Usually, a substitution element source such as ammonium phosphate, phosphoric acid, phosphoric anhydride and the like, preferably phosphoric acid: For example, when Mn is a substitution element, manganese nitrate, manganese chloride, manganese dioxide or the like, preferably manganese nitrate or the like May be used alone or in combination of two or more.
[0022]
The firing conditions are, in the air, usually 400 ° C. or higher, preferably 500 ° C. or higher, usually 670 ° C. or lower, preferably 650 ° C. or lower, usually 1 hour or longer, preferably 10 hours or longer, usually 100 hours or shorter, preferably Is preferably 50 hours or less.
[0023]
The treatment after firing is not particularly limited as long as the required median diameter and specific surface area can be obtained. For example, the median diameter can be reduced by mechanical pulverization and the specific surface area can be increased. When performing mechanical pulverization, it is preferable to perform mechanical pulverization in a state where air is shut off in order to prevent precipitation of Li ₂ O, Li ₃ N, and the like. As a device for performing mechanical pulverization, a known device such as a ball mill, a planetary ball mill, a roller mill, an atomizer, a pin disk mill, and a jet mill can be used.
[0024]
(2) Electrode for Nonaqueous Electrolyte Secondary Battery Next, the electrode for a nonaqueous electrolyte secondary battery of the present invention using such an electrode active material of the present invention will be described.
[0025]
When the electrode active material of the present invention is used for an electrode, the above-mentioned lithium-vanadium-phosphorus composite compound may be usually used in a powder form, and its average particle size may be about 1 to 100 μm. The average particle size is, for example, a median particle size measured by a laser diffraction type particle size distribution analyzer. The content of the active material in the electrode may be appropriately set according to the type of the active material to be used, the amount of the binder (binder) and the conductive material used as needed, and the like. In the electrode of the present invention, the electrode active material of the present invention may be used alone or, if necessary, as a mixture with another conventionally known electrode active material.
[0026]
The production of the electrode of the present invention may be performed according to a known method for producing an electrode, except that the above-mentioned electrode active material of the present invention is used. For example, if necessary, a powder of the above-mentioned active material may be mixed with a known binder (for example, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, ethylene propylene diene polymer, styrene-butadiene rubber, acrylonitrile-butadiene rubber, fluorine rubber). , Polyvinyl acetate, polymethyl methacrylate, polyethylene, nitrocellulose, etc.) and, if necessary, mixed with a known conductive material (eg, acetylene black, carbon, graphite, natural graphite, artificial graphite, needle coke, etc.) The obtained mixed powder may be formed by pressing on a support made of stainless steel or the like or filled in a metal container. Alternatively, the above mixed powder is mixed with an organic solvent (for example, N-methylpyrrolidone, toluene, cyclohexane, dimethylformamide, dimethylacetamide, methyl ethyl ketone, methyl acetate, methyl acrylate, diethyltriamine, N, N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran) The electrode of the present invention can also be produced by a method such as applying a slurry obtained by mixing the above with a metal substrate such as aluminum, nickel, stainless steel, or copper, followed by drying. In this case, the electrode obtained by coating and drying may be compacted by a roller press or the like in order to increase the packing density of the active material.
[0027]
The thickness of the electrode is usually 1 μm or more, preferably 10 μm or more, usually 1000 μm or less, and preferably about 200 μm or less. If the thickness is too large, the conductivity tends to decrease, and if it is too small, the capacity tends to decrease.
[0028]
Such an electrode of the present invention is charged and discharged at a potential of about 4 V with respect to metallic lithium by oxidation and reduction of tetravalent and pentavalent vanadium, and thus is usually used as a positive electrode of a nonaqueous electrolyte secondary battery. It is possible to use.
[0029]
(3) Non-aqueous electrolyte secondary battery Next, a non-aqueous electrolyte secondary battery of the present invention using such an electrode of the present invention as a positive electrode will be described.
[0030]
The nonaqueous electrolyte secondary battery of the present invention can employ the components of a known nonaqueous electrolyte secondary battery, except that the above-described electrode of the present invention is used as a positive electrode.
[0031]
The negative electrode active material used for the negative electrode of the secondary battery of the present invention may be a lithium alloy such as lithium or a lithium aluminum alloy, but has higher safety and can occlude and release lithium, a carbonaceous material, Metals, metal oxides, or metal nitrides are preferred. That is, in a known VOPO ₄ as another positive electrode active material, since the positive electrode active material does not contain lithium in an initial state, it is essential to use a lithium-containing material such as lithium or lithium aluminum anode, in LiVOPO ₄ If so, since the positive electrode active material contains lithium, a negative electrode active material containing no lithium can be used.
[0032]
The carbonaceous material as the negative electrode active material is not particularly limited, and graphite, coal-based coke, petroleum-based coke, coal-based pitch carbide, petroleum-based pitch carbide, or carbide obtained by oxidizing these pitches, needle coke And carbon materials such as pitch coke, phenolic resin, and crystalline cellulose, and carbon materials partially graphitized thereof, furnace black, acetylene black, pitch-based carbon fibers, and the like.
[0033]
Examples of the metal material include Si, Sn, Ti, Sb, Al, Ge, Pb, In, and alloys thereof. As the metal oxide, _{e.g., SnO, SnO 2, Sn 1} -x M x O (M = Hg, P, B, Si, Ge or Sb, but _{0 ≦ x <1), Sn} 3 O 2 (OH) 2
, Sn _3-x M _x O ₂ (OH) ₂ (M = Mg, P, B, Si, Ge, Sb or Mn, where 0 ≦ x <3), LiSiO ₂ , SiO ₂ or LiSnO ₂ Can be. Examples of the metal nitride include Li _2.5 Co _0.5 N and the like.
[0034]
One of these active material materials may be used alone, or two or more may be used in combination.
[0035]
The negative electrode may be manufactured according to a known method. For example, the negative electrode can be manufactured in the same manner as the method for manufacturing the electrode of the present invention described in the above section (2). That is, for example, after the powder of the negative electrode active material is mixed with the known binder exemplified in the above section (2) as needed, and further as necessary, the known conductive material exemplified in the above section (2), This mixed powder may be formed into a sheet shape, and this may be pressed onto a conductive net (collector) such as stainless steel or copper. Further, it can also be produced by applying a slurry obtained by mixing the above mixed powder with the known organic solvent exemplified in the above section (2) on a metal substrate such as copper and drying.
[0036]
As other components of the secondary battery of the present invention, those used in known nonaqueous electrolyte secondary batteries can be used. For example, the following can be exemplified.
[0037]
The electrolyte typically contains an electrolyte and a solvent. The solvent of the electrolytic solution is not particularly limited as long as it is a non-aqueous solvent.For example, carbonates, ethers, ketones, sulfolane compounds, lactones, nitriles, chlorinated hydrocarbons, ethers, amines, esters , Amides, phosphate compounds and the like can be used. When these representatives are listed, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, ethylene carbonate, vinylene carbonate, methyl formate, dimethyl sulfoxide, propylene carbonate, acetonitrile, γ-butyrolactone, dimethylformamide, dimethyl carbonate, diethyl carbonate, sulfolane, ethyl methyl carbonate, 1,4-dioxane, 4-methyl-2-pentanone, 1,3-dioxolan, 4-methyl-1,3-dioxolan, diethyl Ether, sulfolane, methylsulfolane, propionitrile, benzonitrile, butyronitrile, valeronitrile, 1,2-dichloroethane, trimethyl phosphate, triethyl phosphate and the like. It is. These solvents may be used alone or as a mixture of two or more.
[0038]
As the electrolyte, in these solvents, the alkali metal ions or alkaline earth metal ions in the negative electrode active material can be moved to perform an electrochemical reaction with the positive electrode active material or the positive electrode active material and the negative electrode active material. Possible electrolyte materials such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiAsF ₆ , LiB (C ₆ H ₅ ) ₄ , LiCl, LiBr, CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, LiN ( SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiN (SO ₃ CF ₃ ) _{2 and the} like can be used. They may be used alone or as a mixture of two or more.
[0039]
In the present invention, a known solid electrolyte, for example, LiTi ₂ (PO ₄ ) ₃ having a NASICON structure can also be used.
[0040]
Other components such as the separator, the battery case, and other structural materials are not particularly limited, and various conventionally known materials can be used.
[0041]
【Example】
Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples as long as the gist is not exceeded.
[0042]
Example 1
Orthogonal LiVOPO ₄ was synthesized as follows.
[0043]
LiNO ₃ , V ₂ O ₅ , and H ₃ PO ₄ were dissolved in water at a molar ratio of 2: 1: 2, and the mixture was stirred at 80 ° C. This solution was evaporated to dryness, dried at 110 ° C. overnight, pulverized, and calcined in air at 600 ° C. for 14 hours. As a result of measuring the X-ray diffraction pattern of the obtained powder, it was confirmed that the obtained powder was orthorhombic showing the X-ray diffraction pattern described in JCPDS card 42-0469.
[0044]
The median diameter of the orthorhombic LiVOPO ₄ was 12.5 μm as measured by a laser diffraction type particle size distribution analyzer. The BET specific surface area measured by nitrogen adsorption was 0.1 m ² / g.
[0045]
Next, a mixture of 60% by weight of the above orthorhombic LiVOPO ₄ as a positive electrode active material, and a mixture of 95% by weight of acetylene black and 40% by weight of a conductive material composed of 5% by weight of polytetrafluoroethylene with addition of ethanol to a stainless steel mesh. It was crimped to form a positive electrode. This was dried at 200 ° C. for 4 hours before assembling the battery.
[0046]
Lithium metal was used as a negative electrode, a solution in which LiPF ₆ was dissolved at a concentration of 1 mol / L in a solvent of EC (ethylene carbonate): DMC (dimethyl carbonate) = 1: 2 (volume ratio) was used as an electrolytic solution, and polypropylene was used as a separator. An open cell was prepared and charged at a constant current of C / 50 (1 C is a current value for discharging a rated capacity based on a discharge capacity of one hour in one hour) in a range of 4.5 to 3.0 V. Discharged. As a result, the reversible capacity (discharge capacity) was 85 mAh / g. The reversible capacity (discharge capacity) when the current value was C / 5 was 4 mAh / g.
[0047]
Comparative Example 1
In Example 1, a solution in which LiNO ₃ , V ₂ O ₅ , and H ₃ PO ₄ were dissolved in water at a molar ratio of 2: 1: 2 was obtained by evaporating to dryness, drying overnight, and pulverizing. Triclinic LiVOPO ₄ was synthesized in the same manner except that the calcination temperature of the obtained pulverized product was 700 ° C. From the X-ray diffraction pattern of the obtained powder, it was confirmed that the powder was triclinic because it was an X-ray diffraction pattern described in JCPDS card 72-2253. This triclinic LiVOPO _{4 had} a median diameter of 1.3 μm and a BET specific surface area of 1.9 m ² / g.
[0048]
A positive electrode was produced in the same manner as in Example 1, except that the obtained triclinic LiVOPO ₄ was used as a positive electrode active material. Using this positive electrode, a semi-open cell was fabricated in the same manner as in Example 1, and charged and discharged at a constant current of C / 50 in the range of 4.5 to 3.0 V. As a result, the reversible capacity (discharge capacity) was 6 mAh /. g.
[0049]
Example 2
The orthorhombic LiVOPO ₄ obtained in Example 1 was sealed in a Teflon container in an argon atmosphere and pulverized by a ball mill for 96 hours. The median diameter of the pulverized LiVOPO ₄ was 2.1 μm, and the BET specific surface area was 7.1 m ² / g.
[0050]
A positive electrode was prepared in the same manner as in Example 1 except that the ball milled orthorhombic LiVOPO ₄ was used as a positive electrode active material. Using this positive electrode, a semi-open cell was fabricated in the same manner as in Example 1, and charged and discharged at a constant current of C / 5 in the range of 4.5 to 3.0 V. As a result, the reversible capacity (discharge capacity) was 56 mAh / g.
[0051]
Example 3
The orthorhombic LiVOPO ₄ obtained in Example 1 was sealed in a stainless steel container in an argon atmosphere, and pulverized by a planetary ball mill for 5 hours. The median diameter of the pulverized LiVOPO ₄ was 0.73 μm, and the BET specific surface area was 23.7 m ² / g.
[0052]
A positive electrode was produced in the same manner as in Example 1, except that orthorhombic LiVOPO ₄ pulverized by this planetary ball mill was used as a positive electrode active material. Using this positive electrode, a semi-open cell was produced in the same manner as in Example 1, and charged and discharged at a constant current of C / 50 in the range of 4.5 to 3.0 V. As a result, the reversible capacity (discharge capacity) was 123 mAh / g. The reversible capacity (discharge capacity) when the current value was C / 5 was 83 mAhg /.
[0053]
These results are summarized in Table 1.
[0054]
[Table 1]

[0055]
Table 1 shows that orthorhombic LiVOPO ₄ can provide a secondary battery with a significantly higher capacity than triclinic LiVOPO ₄ .
[0056]
In addition, using the semi-open cells manufactured in Example 1 and Example 3, the discharge capacity was measured by changing the charge / discharge current value to C / 50, C / 25, C / 10, and C / 5, respectively. FIG. 1 shows the rate characteristics obtained by the above operation. As is clear from FIG. 1, when the positive electrode active material of Example 3 having a smaller median diameter and a larger specific surface area was used, the rate characteristics were improved as compared with the case where the positive electrode active material of Example 1 was used.
[0057]
【The invention's effect】
As described above in detail, according to the present invention, by using LiVOPO ₄ having a specific crystal structure as a positive electrode active material, a non-aqueous solution having a higher capacity than a conventionally known triclinic LiVOPO ₄ electrode active material is obtained. An electrolyte secondary battery can be provided. Furthermore, by selecting the particle size and specific surface area of LiVOPO ₄ , the rate characteristics can be improved.
[Brief description of the drawings]
FIG. 1 is a graph showing rate characteristics of semi-open cells manufactured in Example 1 and Example 3.

Claims (5)

An electrode active material for a non-aqueous electrolyte secondary battery, which is a lithium-vanadium-phosphorus composite compound powder having an orthorhombic LiVOPO ₄ type crystal structure. 2. The electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the median diameter is 20 μm or less. An electrode for a non-aqueous electrolyte secondary battery, comprising an active material layer containing the electrode active material according to claim 1 formed on a current collector. A non-aqueous electrolyte secondary battery comprising: a positive electrode comprising the electrode according to claim 3; a negative electrode including a negative electrode active material that occludes and releases lithium ions; and an electrolyte. The non-aqueous electrolyte secondary battery according to claim 4, wherein the negative electrode active material contains one or more selected from the group consisting of a carbonaceous material, a metal, a metal oxide, and a metal nitride. .

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