JP5002098B2 - Electrode active material for non-aqueous electrolyte secondary battery, electrode and battery including the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous electrolyte battery, and more particularly to a chargeable / dischargeable non-aqueous electrolyte secondary battery. In particular, the present invention relates to improvement of an electrode active material and aims to improve cycle characteristics.
[0002]
[Prior art]
Non-aqueous electrolyte secondary batteries using an alkali metal such as lithium, an alkaline earth metal such as magnesium, or an alloy or compound thereof, as a negative electrode active material, insert or intercalate negative electrode metal ions into the positive electrode active material. The reaction ensures its large discharge capacity and charge reversibility.
[0003]
Recently, a battery using VOPO ₄ which can be an intercalation host for lithium as a positive electrode material has been proposed as a secondary battery (N. Dupre et al., Solid State Ionics, 140, pp.209-221). (2001), N. Dupre et al., J. Power Sources, 97-98, pp. 532-534 (2001)).
[0004]
[Problems to be solved by the invention]
It is known that there are several polymorphs in the crystal structure of VOPO _{4. In} the above document, α, α _II , δ, and γ type VOPO ₄ is used as a positive electrode active material, and metallic lithium is used as a negative electrode active material. Secondary batteries have been proposed. According to the above document, when the negative electrode is made of metallic lithium, α _II -VOPO ₄ has an initial capacity of 140 mAh / g at a current value of C / 50 and a voltage in the vicinity of 4V. However, any of the positive electrode active materials composed of VOPO _{4 having} various crystal structures described in the above documents has a problem in cycle characteristics. For example, even in the α _II -VOPO ₄ having the most excellent cycle characteristics, a current of C / 50 As a result, the capacity after 9 cycles drops to 118 mAh / g (84%). Therefore, development of an electrode active material with improved cycle characteristics is desired.
[0005]
The present invention has been made in view of the above circumstances, and an object thereof is to provide an electrode active material having improved cycle characteristics, an electrode using the electrode active material, and a nonaqueous electrolyte secondary battery. .
[0006]
[Means for Solving the Problems]
The present inventors have made various studies in order to achieve the above object, and as a result, by using VOPO ₄ having a specific crystal structure as an electrode active material, it is possible to improve the above problems related to cycle characteristics. I found it. That is, the present invention provides a positive electrode active material for a non-aqueous electrolyte secondary battery comprising a vanadium-phosphorus composite compound having a ω-VOPO ₄ type crystal structure.
[0007]
Next, this invention also provides the electrode for nonaqueous electrolyte secondary batteries containing said positive electrode active material.
The present invention also provides a non-aqueous electrolyte secondary battery using the electrode described above. More specifically, a non-aqueous electrolyte secondary battery using the electrode described above as a positive electrode, and further including, as a negative electrode, at least one negative electrode active material selected from the group consisting of alkali metal materials and alkaline earth metal materials The non-aqueous electrolyte secondary battery using an electrode is provided.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in more detail.
(1) Non-aqueous electrolyte secondary battery electrode active material:
As described above, the electrode active material in the present invention is a vanadium-phosphorus composite compound having a ω-VOPO ₄ type crystal structure. As for the crystal structure of VOPO ₄ , the VO ₆ octahedron and the PO ₄ tetrahedron form a two-dimensional or three-dimensional open framework, and have a structure suitable for insertion of alkali metals and alkaline earths. There are crystal structures such as α, α _II , δ, and γ due to the difference in bonding form between the VO ₆ octahedron and the PO ₄ tetrahedron. The compound having the ω-VOPO ₄ type crystal structure in the present invention is It is a kind, specifically, a compound having a crystal structure that gives an X-ray diffraction pattern described in JCPDS card 37-0809.
[0009]
In the compound having a ω-VOPO ₄ type crystal structure in the present invention, charging and discharging are usually performed at a potential of about 4 V with respect to the metal lithium counter electrode by oxidation and reduction between pentavalent and tetravalent vanadium. For this reason, it is possible to control charge / discharge capacity by substituting vanadium and phosphorus in the crystal structure with other elements and controlling the average valence of vanadium. In addition, the crystal structure can be stabilized by substitution. Furthermore, it is possible to stabilize the crystal structure by previously inserting an alkali metal element or an alkaline earth metal element into a part of the site where lithium is inserted.
[0010]
The vanadium-phosphorus composite compound having the ω-VOPO ₄ type crystal structure of the present invention is usually represented by the following general formula (I) based on the composition of VOPO ₄ .
[0011]
[Chemical 1]
_{A x V 1-y M y} O 1+ δ 1 P 1-z Q z O 4+ δ 2 (I)
In the general formula (I), A is at least one element selected from an alkali metal element and an alkaline earth metal element. Specific examples of A include elements such as Li, Na, K, Mg, and Ca. Lithium is preferable. The value of x corresponds to the amount of element A inserted into the lithium site. When the negative electrode is made of a lithium-containing material, if A is other than lithium, if the value of x is too large, the battery capacity tends to decrease too much, so usually 0.4 or less, preferably 0.2 or less, more preferably 0. .1 or less. Of course, the value of x can be 0.
[0012]
M is at least one element selected from the group consisting of Al, Fe, Ga, Bi, Sn, Cr, Cu, Zn, Mg, Ti, Ge, Ta, Mo, W, Nb, Ni, Mn, and Co. is there. The value of y corresponds to the amount of vanadium replaced by the element M. If the value of y is too large, the battery capacity tends to decrease too much. Therefore, it is usually at most 0.4, preferably at most 0.2, more preferably at most 0.1. Of course, the value of y can be set to 0.
[0013]
Q is at least one element selected from the group consisting of S, As, Si, and Ge. The value of z corresponds to the amount of phosphorus substituted by the element Q. On the other hand, if the value of z is too large, the stability of the crystal structure may be lowered. Of course, the value of z can be 0.
[0014]
Further, δ ₁ and δ ₂ correspond to an oxygen deficiency amount or an oxygen excess amount derived from the non-stoichiometry of VOPO ₄ , respectively. δ ₁ is a number that satisfies −0.2 ≦ δ ₁ ≦ 0.2, and δ ₂ is a number that satisfies −0.2 ≦ δ ₂ ≦ 0.2.
The compound which is the active material of the present invention can be produced by a known method, and there are various methods.
[0015]
For example, as one specific example, there can be mentioned a production method in which a raw material aqueous solution having a predetermined molar ratio is stirred while being heated, then dried and fired.
(2) Invention electrode:
In the electrode of the present invention, the above electrode active material is used. In this case, the active material is usually used in powder form, and the average particle size may be about 1-100 μm. The average particle diameter is a value measured by, for example, a laser diffraction particle size distribution measuring apparatus. Moreover, what is necessary is just to set suitably content of the said active material in an electrode according to the usage-amount etc. of the kind of active material to be used, the binder (binder) used as needed, and a electrically conductive material. The electrode of the present invention may be the above electrode active material alone or a mixture with other conventionally known electrode active materials.
[0016]
The production of the electrode of the present invention may be carried out in accordance with a known electrode production method except that the above electrode active material is used. For example, the above active material powder may be added to a known binder (polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, ethylene propylene diene polymer, styrene-butadiene rubber, acrylonitrile-butadiene rubber, fluororubber, poly Vinyl acetate, polymethylmethacrylate, polyethylene, nitrocellulose, etc.) and, if necessary, mixing with known conductive materials (acetylene black, carbon, graphite, natural graphite, artificial graphite, needle coke, etc.) and then mixing obtained The powder may be pressure-molded on a support made of stainless steel or filled in a metal container.
[0017]
Alternatively, for example, the mixed powder is mixed with an organic solvent (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 with a metal substrate such as aluminum, nickel, stainless steel or copper.
[0018]
The thickness of the electrode is usually about 1-1000 μm, preferably about 10-200 μm. If it is too thick, the conductivity tends to decrease, and if it is too thin, the capacity tends to decrease. The electrode obtained by coating and drying may be consolidated by a roller press or the like in order to increase the packing density of the active material.
(3) Nonaqueous electrolyte secondary battery of the present invention:
The nonaqueous electrolyte secondary battery of the present invention can employ components in a nonaqueous electrolyte secondary battery such as a known lithium secondary battery, except that the electrode (2) of the present invention is used as an electrode.
[0019]
The electrode of the present invention can usually be used as a positive electrode. In this case, as the negative electrode, a known negative electrode active material can be used as the electrode active material, but it is preferable to use at least one selected from the group consisting of alkali metal materials and alkaline earth metal materials.
The alkali metal material referred to in the present invention is an alkali metal such as lithium, sodium or potassium, a compound of an alkali metal, an alloy, etc., or a material capable of occluding and releasing alkali metal ions (for example, Li _2.5 Co _0.5 N, Li ₄ Ti ₅ O ₁₂ , carbon materials, etc.).
[0020]
Alkaline earth metal materials include materials that can occlude and release alkaline earth metal ions in addition to alkaline earth metals such as magnesium and calcium, compounds and alloys of alkaline earth metals, and the like (for example, Mg _z Ti ₂ (PO ₄ ) ₃ (0 <z <4) and the like are also included.
The negative electrode may be manufactured by a known method, for example, by the same method as described in (2) above. That is, for example, the negative electrode active material powder is mixed with the known binder described in (2) as necessary, and the known conductive material described in (2) as necessary. What is necessary is just to shape | mold in a sheet | seat shape and to crimp | bond this to conductor networks (collector), such as stainless steel and copper. Moreover, for example, it can also be produced by applying a slurry obtained by mixing the mixed powder with the known organic solvent described in (2) on a metal substrate such as copper.
[0021]
As another component, what is used for a well-known nonaqueous electrolyte secondary battery can be used as a component. For example, the following can be illustrated.
The electrolytic solution usually includes an electrolyte and a solvent. The solvent of the electrolytic solution is not particularly limited as long as it is non-aqueous, for example, carbonates, ethers, ketones, sulfolane compounds, lactones, nitriles, chlorinated hydrocarbons, ethers, amines, esters. Amides, phosphate ester compounds, and the like can be used.
[0022]
These representatives are listed as follows: 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-dioxolane, 4-methyl-1,3-dioxolane, diethyl Use ether, sulfolane, methyl sulfolane, propionitrile, benzonitrile, butyronitrile, valeronitrile, 1,2-dichloroethane, trimethyl phosphate, triethyl phosphate, etc. It can be. These can be used alone or in combination of two or more.
[0023]
As an electrolytic solution, the alkali metal ions or the alkaline earth metal ions in the negative electrode active material can be transferred to these solvents so as to electrochemically react with the positive electrode active material or the positive electrode active material and the negative electrode active material. Possible electrolyte materials, for example, 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) can be used ₂ or the like. In the present invention, a known solid electrolyte such as LiTi ₂ (PO ₄ ) ₃ having a NASICON structure can also be used.
[0024]
In the battery of the present invention, conventionally known various materials can be used for elements such as a separator, a battery case, and other structural materials, and there is no particular limitation.
For example, when using a separator between the positive and negative electrodes, a microporous polymer film is used, such as nylon, cellulose acetate, nitrocellulose, polysulfone, polyacrylonitrile, polyvinylidene fluoride, polypropylene, polyethylene, polybutene, etc. Those made of a polyolefin polymer are used. Polyolefin polymers are preferable from the viewpoint of the chemical and electrochemical stability of the separator, and are preferably made of polyethylene from the viewpoint of the self-closing temperature, which is one of the purposes of the battery separator.
[0025]
In the case of a polyethylene separator, ultrahigh molecular weight polyethylene is preferable from the viewpoint of maintaining high-temperature shape, and the lower limit of the molecular weight is preferably 500,000, more preferably 1,000,000, and most preferably 1.5 million. On the other hand, the upper limit of the molecular weight is preferably 5 million, more preferably 4 million, and most preferably 3 million. This is because if the molecular weight is too large, the pores of the separator may not close when heated because the fluidity is too low.
[0026]
What is necessary is just to assemble the battery of this invention according to a well-known method using these battery elements. In this case, the shape of the battery is not particularly limited, and various shapes and sizes such as a cylindrical shape, a square shape, and a coin shape can be appropriately employed.
[0027]
【Example】
EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.
Example 1
ω-VOPO ₄ was obtained as follows.
[0028]
V ₂ O ₅ and NH ₂ OH · HCl were weighed so as to have a molar ratio of 1: 1, and an 85% H ₃ PO ₄ aqueous solution was added so that the molar ratio of P / V was 1.1 / 1. Water was further added and stirred at 80 ° C. The solution was evaporated to dryness, and the resulting solid was dried at 110 ° C. overnight, washed twice in boiling water, filtered, dried at 60 ° C. overnight, pulverized, and VO (precursor) HPO ₄ ) · 0.5H ₂ O was obtained.
[0029]
The obtained VO (HPO ₄ ) · 0.5H ₂ O was calcined in nitrogen at 500 ° C. for 3 hours, and further calcined in oxygen at 600 ° C. to obtain the intended ω-VOPO ₄ . .
The X-ray diffraction result of the obtained ω-VOPO ₄ is shown in FIG. From the powder X-ray diffraction pattern shown in FIG. 1, it was confirmed to be ω-VOPO ₄ described in JCPDS card 37-0809. The valence of vanadium determined by oxidation-reduction titration of Fe and Zn was 4.95.
[0030]
Next, the obtained ω-VOPO ₄ was used as a positive electrode active material (25 mg), and a conductive material (12.5 mg) composed of 95% by weight of acetylene black and 5% of polytetrafluorethylene was added with ethanol and mixed. A positive electrode was bonded to a stainless steel mesh. Prior to battery preparation, it was dried at 200 ° C. for 4 hours.
Metal lithium as the negative electrode, EC (ethylene carbonate): DMC (dimethyl carbonate) = 1: 2 as the electrolyte, polypropylene as a separator, and a constant current of 0.08 mA (C / 50) using a semi-open cell The battery was charged / discharged in the range of 4.3 to 3.2V. As a result, the reversible capacity (charge capacity) at the first cycle was 85 mAh / g, and good cycle characteristics were exhibited as shown in FIG. That is, the known literature (N. Dupre et al., Solid State Ionics, 140, pp. 209-221 (2001), N. Dupre et al., J. Power Sources, 97-98, pp. 532-534 ( 2001)), even in α _II -VOPO ₄ having the most excellent cycle characteristics, the capacity after 9 cycles decreased to 84% at a current value of C / 50, but according to FIG. In the case of ω-VOPO ₄ , the capacity of the ninth cycle under the same conditions is maintained at 99%.
[0031]
【Effect of the invention】
According to the present invention, since a specific electrode active material is used, it is possible to provide a non-aqueous electrolyte secondary battery having better cycle characteristics as compared with a conventionally known VOPO ₄ electrode active material.
[Brief description of the drawings]
FIG. 1 shows an X-ray diffraction pattern of ω-VOPO ₄ which is an embodiment of the present invention.
FIG. 2 is a characteristic diagram showing cycle characteristics of ω-VOPO ₄ which is an embodiment of the present invention.

Claims (4)

A positive electrode active material for a non-aqueous electrolyte secondary battery comprising a vanadium-phosphorus composite compound having a ω-VOPO ₄ type crystal structure. The electrode for nonaqueous electrolyte secondary batteries containing the positive electrode active material of Claim 1.   A nonaqueous electrolyte secondary battery using the electrode according to claim 2 as a positive electrode.   The nonaqueous electrolyte secondary battery according to claim 3, wherein an electrode including at least one negative electrode active material selected from the group consisting of an alkali metal material and an alkaline earth metal material is used as the negative electrode.

Images