JP3722685B2 - Electrolyte for electrochemical device and battery using the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electrolyte having a novel chemical structure used for electrochemical devices such as lithium batteries, lithium ion batteries, and electric double layer capacitors, and a battery using the same.
[0002]
[Prior art and problems to be solved by the invention]
With the development of portable devices in recent years, the development of electrochemical devices using electrochemical phenomena such as batteries and capacitors as a power source has become active. Further, as an electrochemical device other than the power source, an electrochromic display (ECD) in which a color change is caused by an electrochemical reaction can be given.
[0003]
These electrochemical devices are generally composed of a pair of electrodes and an ionic conductor filling them. As the ionic conductor, a solution in which a salt (AB) composed of a cation (A ⁺ ) and an anion (B ⁻ ) called an electrolyte is dissolved in a solvent, a polymer, or a mixture thereof is used. When this electrolyte is dissolved, it dissociates into a cation and an anion to conduct ions. In order to obtain the ionic conductivity necessary for the device, it is necessary that this electrolyte is dissolved in a sufficient amount in a solvent or a polymer. In practice, a solvent other than water is often used as a solvent, and there are currently only a few types of electrolytes having sufficient solubility in such organic solvents and polymers. For example, LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiN (CF ₃ SO ₂ ) ₂ , LiCF ₃ SO _{3 and the} like are used as the electrolyte for the lithium battery. The cation portion is often determined by the device, such as the lithium ion of a lithium battery, but the anion portion can be used if the condition that the solubility is high is satisfied.
[0004]
While the application range of devices is diversifying, the optimum electrolyte for each application is being searched for, but at present, optimization is reaching its limit because there are few types of anions. Moreover, the existing electrolyte has various problems, and therefore, an electrolyte having a novel anion portion is desired. Specifically, ClO ₄ ions are explosive and AsF ₆ ions are toxic and cannot be used for safety reasons. LiN (CF ₃ SO ₂ ) ₂ and LiCF ₃ SO ₃ are difficult to use because they corrode the aluminum current collector in the battery with a potential applied. The only practically used LiPF ₆ also has problems such as heat resistance and hydrolysis resistance.
[0005]
[Concrete means for solving the problem]
As a result of intensive studies in view of the problems of the prior art, the present inventors have found an electrolyte having a novel chemical structural feature and have reached the present invention.
[0006]
That is, the present invention is an electrolyte for an electrochemical device having a chemical structural formula represented by the general formula (1).
[0007]
[Chemical formula 2]
[0008]
However, M is B or P, A ^{a +} is Li ion, a is 1, b is 1, p is 1 , m is 1 to 2 , n is 1 to 4 , and q is 0 Or R ¹ is C ₁ -C ₁₀ alkylene, C ₁ -C ₁₀ halogenated alkylene, C ₆ -C ₂₀ arylene, or C ₆ -C ₂₀ halogenated arylene (these Alkylene and arylene may have a substituent or a hetero atom in the structure, and m R ¹ may be bonded to each other.), R ² is halogen, X ¹ , X ² are , O, and a battery using the electrolyte.
[0009]
The alkyl, halogenated alkyl, aryl, halogenated aryl, alkylene, halogenated alkylene, arylene, and halogenated arylene used in the present invention include those having other functional groups such as a branch, a hydroxyl group, and an ether bond.
[0010]
Hereinafter, the present invention will be described in more detail.
[0011]
Here, the specific example of a compound shown by General formula (1) of this invention is shown next.
[0012]
[Chemical 3]
[0013]
[Formula 4]
[0015]
[Chemical 6]
[0016]
Here, a lithium ion is mentioned as A ^{<a +>} of the compound shown by General formula (1) of this invention.
[0018]
The electrolyte of the present invention has an ionic metal complex structure, and the central M is selected from transition metals, Group III, Group IV, or Group V elements of the Periodic Table. Preferably, any of Al, B, V, Ti, Si, Zr, Ge, Sn, Cu, Y, Zn, Ga, Nb, Ta, Bi, P, As, Sc, Hf, or Sb, and B or P is preferable. Although various elements can be used as the central M, Al, B, V, Ti, Si, Zr, Ge, Sn, Cu, Y, Zn, Ga, Nb, Ta, Bi, P, As , Sc, Hf, or Sb is relatively easy to synthesize, and in the case of B or P, in addition to the ease of synthesis, it has excellent properties in all aspects such as low toxicity, stability, and cost. .
[0019]
Next, the part of the ligand that is a feature of the electrolyte (ionic metal complex) of the present invention will be described. Hereinafter, the organic or inorganic part bonded to M is referred to as a ligand.
[0020]
R ¹ in the general formula (1) is selected from C _{1 to} C ₁₀ alkylene, C _{1 to} C ₁₀ halogenated alkylene, C _{6 to} C ₂₀ arylene, or C _{6 to} C ₂₀ halogenated arylene. These alkylenes and arylenes may have a substituent or a hetero atom in the structure. Specifically, instead of hydrogen on alkylene and arylene, halogen, chain or cyclic alkyl group, aryl group, alkenyl group, alkoxy group, aryloxy group, sulfonyl group, amino group, cyano group, carbonyl group, acyl group , An amide group, a hydroxyl group, and a structure in which nitrogen, sulfur, or oxygen is introduced in place of carbon on alkylene and arylene. Furthermore, plural R ^{1 s} may be bonded to each other, and examples thereof include a ligand such as ethylenediaminetetraacetic acid.
[0021]
R ² is preferably a halogen, preferably an electron-withdrawing group, particularly fluorine. When R ² is fluorine, the ion conductivity is very high due to the improvement in dissociation of the electrolyte due to its strong electron-withdrawing property and the effect of improving the mobility due to the reduction in size.
[0022]
X ¹ and X ² are each independently O, and the ligand is bonded to M through these heteroatoms. Here, it is not impossible to combine other than O, but it is very complicated in synthesis. Since this compound has a bond of M by X ¹ and X ² in the same ligand, these ligands constitute a chelate structure with M. Due to the effect of this chelate, the heat resistance, chemical stability, and hydrolysis resistance of this compound are improved. The constant q in this ligand is 0 or 1. Particularly, 0 is preferable because this chelate ring is a five-membered ring, so that the chelate effect is exerted most strongly and the stability is increased.
[0024]
The constants m and n related to the number of ligands described so far are determined by the type of M at the center, and m is preferably 1 to 2 and n is preferably 1 to 4. .
[0025]
The above is an explanation of a novel electrolyte for an electrochemical device comprising the ionic metal complex of the present invention. More specifically, the compound represented by the general formula (1) according to the present invention has a strong electron-withdrawing property. By having a carbonyl group (C═O group), the anion is stabilized, and the charge of the anion and the cation can be easily separated. In other words, the anion and cation are easily dissociated. This is a very important factor when used as an electrolyte in electrochemical devices. There are countless salts called electrolytes, but most of them dissolve and dissociate in water to conduct ions. However, it often does not even dissolve in organic solvents other than water. Such an aqueous solution can also be used for an electrochemical device, but there are many restrictions because the decomposition potential of water as a solvent is low and it is vulnerable to redox. For example, in a lithium battery or the like, since the potential difference between the electrodes of the device is 3 V or more, water is electrolyzed into hydrogen and oxygen. On the other hand, organic solvents and polymers are more resistant to redox than water due to their structures, and are therefore used in devices that require high voltage, such as lithium batteries and electric double layer capacitors.
[0026]
As described above, the electrolyte of the present invention is very soluble in an organic solvent and easily dissociated due to the effect of the C═O group and the effect of increasing the anion size compared to the conventional electrolyte. A solution with a solvent can be used as an excellent ion conductor of a device such as a lithium battery. In general, organic and metal complexes are susceptible to hydrolysis, and are often chemically unstable. Moreover, since the electrolyte of the present invention has a chelate structure, it is very stable and hardly receives hydrolysis. In addition, those having fluorine in the chemical structure represented by the chemical formula (1) are more preferable because of their effects, which improves ionic conductivity and further increases chemical stability such as oxidation resistance.
[0027]
Furthermore, by optimizing the structure of the above chemical formula (1), an electrolyte that dissolves in an organic solvent that does not dissolve in a conventional electrolyte, such as a fluorine-containing organic solvent such as toluene or hexane, or chlorofluorocarbon. You can also get
[0028]
As described above, the electrolyte of the present invention is used as an electrolyte for electrochemical devices such as lithium batteries, lithium ion batteries, and electric double layer capacitors. As other applications, it can be used as a catalyst for organic synthesis reaction, a polymer polymerization catalyst, and an olefin polymerization. And the like.
[0029]
The method for synthesizing these electrolytes is not particularly limited. For example, in the case of a compound having the following chemical formula, after reacting LiBF ₄ with 2 moles of lithium alkoxide in a non-aqueous solvent, It can be synthesized by adding oxalic acid and replacing the alkoxide bonded to boron with oxalic acid.
[0030]
[Chemical 7]
[0031]
When an electrochemical device is constituted using the electrolyte of the present invention, its basic components are composed of an ion conductor, a negative electrode, a positive electrode, a current collector, a separator, a container, and the like.
[0032]
As the ionic conductor, a mixture of an electrolyte and a non-aqueous solvent or polymer is used. If a non-aqueous solvent is used, this ionic conductor is generally called an electrolytic solution, and if a polymer is used, it becomes a polymer solid electrolyte. The polymer solid electrolyte includes those containing a non-aqueous solvent as a plasticizer. As the electrolyte listed here, the electrolyte of the present invention is used in one kind or a mixture of two or more kinds. When two or more types are mixed, the electrolyte of the present invention is always required for one type, and the other is a general lithium salt such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ and LiSbF ₆ can also be used.
[0033]
The non-aqueous solvent is not particularly limited as long as it is an aprotic solvent capable of dissolving the electrolyte of the present invention, and examples thereof include carbonates, esters, ethers, lactones, nitriles, amides, sulfones. Can be used. Moreover, not only a single solvent but 2 or more types of mixed solvents may be sufficient. Specific examples include propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, dimethoxyethane, acetonitrile, propionitrile, tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, nitromethane, N, N-dimethylformamide, dimethyl sulfoxide. , Sulfolane, and γ-butyrolactone.
[0034]
The polymer mixed with the electrolyte is not particularly limited as long as it is an aprotic polymer capable of dissolving the compound. Examples thereof include polymers having polyethylene oxide in the main chain or side chain, homopolymers or copolymers of polyvinylidene fluoride, methacrylic acid ester polymers, polyacrylonitrile and the like. When a plasticizer is added to these polymers, the above-mentioned aprotic non-aqueous solvent can be used. Electrolyte concentration of the present invention in these ion conductors in the, 0.1 mol / dm ³ or more, the saturation concentration or less, preferably, 0.5 mol / dm ³ or more and 1.5 mol / dm ³ or less. When the concentration is lower than 0.1 mol / dm ³ , the ionic conductivity is low, which is not preferable.
[0035]
Although it does not specifically limit as a negative electrode material, In the case of a lithium battery, the alloy of lithium metal and lithium and another metal is used. In the case of a lithium ion battery, a material using a phenomenon called intercalation such as carbon, natural graphite, or metal oxide obtained by firing a polymer, an organic substance, pitch or the like is used. In the case of an electric double layer capacitor, activated carbon, porous metal oxide, porous metal, conductive polymer, or the like is used.
[0036]
As the cathode material is not particularly limited, a lithium battery and a lithium ion battery, for _{example, LiCoO 2, LiNiO 2, LiMnO} 2, lithium-containing oxides such as _{LiMn 2 O 4, TiO 2,} V 2 O 5, MoO Oxides such as ₃ , sulfides such as TiS ₂ and FeS, or conductive polymers such as polyacetylene, polyparaphenylene, polyaniline, and polypyrrole are used. In the case of an electric double layer capacitor, activated carbon, porous metal oxide, porous metal, conductive polymer, or the like is used.
[0037]
【Example】
EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, this invention is not limited by this Example.
[0038]
Example 1
1.37 g of lithium tetrafluoroborate (LiBF ₄ ) was dissolved in 10 ml of acetonitrile at room temperature. Next, 5.09 g of lithium hexafluoroisopropoxide (LiOCH (CF ₃ ) ₂ ) was slowly added to this solution. Then, it was made to react by stirring at 60 degreeC for 3 hours. At this time, lithium fluoride was deposited. To the reaction solution thus obtained, 1.31 g of oxalic acid was added and stirred at 60 ° C. for 1 hour for reaction. Next, this reaction solution was filtered to separate lithium fluoride, and the solvent of the obtained filtrate was removed under reduced pressure conditions of 60 ° C. and 10 ⁻¹ Pa to obtain 1.90 g of a white solid. This solid was dried under reduced pressure conditions at 100 ° C. and 10 ⁻¹ Pa for 24 hours to obtain 1.90 g (yield: 91%) of lithium difluoro (oxalato) borate. The composition of the obtained compound was confirmed by elemental analysis.
[0039]
[Chemical 8]
[0040]
The NMR spectrum of lithium difluoro (oxalato) borate is shown below.
[0041]
Example 2
Next, the electrolyte obtained in Example 1 is dissolved in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC: DMC = 1: 1) to prepare an electrolytic solution having a concentration of 1 mol / dm ^3. did. About this electrolyte solution, the ionic conductivity was measured with the alternating current bipolar cell. As a result, the ionic conductivity was 8.6 mS / cm.
[0042]
Further, when this electrolytic solution was put in a fluororesin container and stored at 100 ° C. for one month, no deterioration such as discoloration of the electrolytic solution was observed. Moreover, when water was added to this electrolyte solution and observed by NMR, it was not hydrolyzed at all.
[0043]
A corrosion test of the aluminum current collector was performed using this electrolytic solution. The test cell used was a beaker type having aluminum as a working electrode, a counter electrode and lithium metal as a reference electrode. When the working electrode was held at 5 V (Li / Li ⁺ ), no current flowed. After the test, the surface of the working electrode was observed with SEM, but no change was observed compared to before the test.
[0044]
Example 3
A battery charge / discharge test was actually performed using the electrolytic solution of Example 2. The test cell was produced as follows. The positive electrode was prepared by mixing 90% by weight of LiCoO ₂ powder, 5% by weight of polyvinylidene fluoride (PVDF) as a binder, and 5% by weight of acetylene black as a conductive material, and further adding N, N-dimethylformamide to form a paste. The paste was applied on an aluminum foil and dried to obtain a test positive electrode body. Lithium metal was used for the negative electrode. Then, using the glass fiber filter as a separator, the cell was assembled by immersing the electrolytic solution of Example 2 in this separator.
[0045]
Next, a constant current charge / discharge test was performed under the following conditions. Both charging and discharging were performed at a current density of 0.35 mA / cm ² , charging was performed at 4.2 V, and discharging was performed up to 3.0 V (vs. Li / Li ⁺ ). As a result, the initial discharge capacity was 125 mAh / g. Moreover, although charging / discharging was repeated 20 times, the 20th capacity | capacitance obtained the result of 88% of the first time.
[0046]
Example 4
A battery charge / discharge test was actually performed using the electrolytic solution of Example 2. The test cell was produced as follows. A half cell was produced using natural graphite as a negative electrode material, and a charge / discharge test was performed. Specifically, 90% by weight of natural graphite powder was mixed with 10% by weight of polyvinylidene fluoride (PVDF) as a binder, and N, N-dimethylformamide was further added to form a slurry. This slurry was applied on a nickel mesh and dried at 150 ° C. for 12 hours to obtain a test negative electrode body. Lithium metal was used for the counter electrode. And the electrolytic solution of Example 2 was immersed in this separator by using a glass fiber filter as a separator, and a half cell was assembled. A constant current charge / discharge test was conducted under the following conditions. Both charging and discharging were performed at a current density of 0.3 mA / cm ² , charging was performed at 0.0 V, and discharging was performed up to 1.5 V (vs. Li ^/ Li ⁺ ). As a result, the initial discharge capacity was 320 mAh / g. Moreover, although charging / discharging was repeated 20 times, the 20th capacity | capacitance obtained the result of 95% of the first time.
[0047]
Comparative Example 1
LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC: DMC = 1: 1) to prepare an electrolyte solution having a concentration of 1 mol / dm ³ . Next, when this electrolytic solution was put in a fluororesin container and stored at 100 ° C. for 1 month to conduct a heat resistance test, the electrolytic solution was yellow.
[0048]
Moreover, when water was added to the electrolyte before this heat test and observed by NMR, various hydrolysis products were observed. Hydrogen fluoride, phosphorus oxychloride and the like were detected as hydrolysis products.
[0049]
Comparative Example 2
LiN (CF ₃ SO ₂ ) ₂ was dissolved in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC: DMC = 1: 1) to prepare an electrolytic solution having a concentration of 1 mol / dm ³ . Next, a corrosion test of the aluminum current collector was performed using this electrolytic solution. The test cell used was a beaker type having aluminum as a working electrode, a counter electrode and lithium metal as a reference electrode. When the working electrode was held at 5 V (Li / Li ⁺ ), current flowed and the current value increased with time. When the working electrode surface was observed by SEM after the test, severe pitting corrosion was observed on the aluminum surface.
[0050]
【The invention's effect】
The electrolyte of the present invention is an electrolyte having higher heat resistance and hydrolysis resistance than conventional electrolytes used for electrochemical devices such as lithium batteries, lithium ion batteries, and electric double layer capacitors. The battery used is made possible.

Claims (2)

An electrolyte for an electrochemical device having a chemical structural formula represented by the general formula (1).
M is B or P,
A ^{a +} is Li ion,
a is 1,
b is 1,
p is 1,
m is 1-2 ,
n is 1 to 4 ,
q represents 0 or 1 respectively;
R ¹ is C ₁ -C ₁₀ alkylene, C ₁ -C ₁₀ halogenated alkylene, C ₆ -C ₂₀ arylene, or C ₆ -C ₂₀ halogenated arylene (these alkylene and arylene are in the structure) May have a substituent or a hetero atom, and m R ¹ may be bonded to each other.)
R ² is halogen,
X ¹ and X ² each represents O.   A battery comprising at least a positive electrode, a negative electrode, and an electrolytic solution, wherein the electrolytic solution includes an electrolyte having a chemical structural formula represented by the general formula (1) according to claim 1.