JPH08138733A - Nonaqueous electrolyte - Google Patents

Abstract

PURPOSE: To provide a nonaqueous electrolyte having self fire extinguishing action, high withstand voltage, and excellent application to a battery such as charge/discharge performance. CONSTITUTION: Lithium salt of phosphoric ester in a general formula is added to a nonaqueous electrolyte including preferably chainlike ester and/or annular ester as an organic solvent, and also including the lithium salt, preferably LiPF6 , as an electrolyte. In the formula, R<1> and R<2> can be the same or different to express an alkyl group having a number of carbon of 1-4 or a halogen atom substituting alkyl group having a number of carbon of 2-3 respectively, and any one of the R<1> and R<2> can be a lithium ion.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a novel non-aqueous electrolyte.

[0002]

2. Description of the Related Art Generally, an electrolyte is responsible for transferring ions between a negative electrode and a positive electrode in a battery, and examples thereof include a non-aqueous electrolytic solution, a polymer electrolyte, and an ion conductive glass. Among these, solid electrolytes such as polymer electrolytes and ionic conductive glass have inferior electrical conductivity to non-aqueous electrolytes, but there is no concern about battery leakage and the use of organic solvents that are easy to catch fire does not It has features such as improved safety. On the other hand, the non-aqueous electrolyte is the most widely used because it has the highest conductivity and the excellent adhesion to the electrode, and the battery having the same has the lowest internal resistance.
Therefore, a battery using a non-aqueous electrolyte has a high voltage, a high energy density, and excellent reliability such as storability, and is widely used as a power source for consumer electronic devices.

The non-aqueous electrolyte is generally a solvent obtained by mixing propylene carbonate, γ-butyrolactone, sulfolane, etc., which is a solvent having a high dielectric constant, with dimethoxyethane, tetrahydroethane, 1,3-dioxolane, etc., which are low-viscosity solvents. LiBF ₄ , LiPF _{6 ,} LiClO ₄ , LiAs
F ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiA
A mixture of electrolytes such as lCl ₄ and LiSiF ₆ is used.

However, many of the solvents for such non-aqueous electrolytes have a low withstand voltage, and when an electrolyte with a low withstand voltage is used for a secondary battery, the solvent is electrolyzed when charging and discharging are repeated, so that There is a possibility that the internal pressure of the battery may rise due to the gas generated in the above, or the product may undergo a polymerization reaction and adhere to the electrode. Such a decrease in the charge / discharge efficiency of the battery and a decrease in the battery energy density result in a shortened battery life.

In an attempt to improve the durability of the electrolytic solution, the withstand voltage of conventionally used esters such as γ-butyrolactone and ethyl acetate, and ethers such as 1,3-dioxolane, tetrahydrofuran and dimethoxyethane is used. By using a carbonic acid ester such as diethyl carbonate having a high withstand voltage instead of a low solvent, it is possible to control the decrease in battery energy density after repeated charging and discharging (for example, Japanese Patent Laid-Open No. 184872/1992).

[0006]

By the way, since a battery having a high energy density is desired, research on a high-voltage battery is being promoted from various fields. For example, a secondary battery called a rocking chair type using a composite oxide of lithium and a transition metal such as LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O _{4 for} the positive electrode of the battery and a carbon material for the negative electrode has been studied. Came. In this case, the battery voltage can generate 4V or more, and since there is no deposition of metallic lithium, safety has been confirmed by experiments such as overcharging, external short circuit, nail piercing, crushing, etc. It has become.

With the increase in energy density and the increase in size of such batteries, further improvement in performance such as flame retardancy is required for the electrolytic solution used in the batteries. Therefore, it has been proposed to add phosphoric acid esters, which are known as self-extinguishing compounds, to the electrolytic solution (JP-A-4-184870). However, although an electrolyte containing 15% or more of this type of compound clears flame retardancy, it is not always sufficient in terms of battery charge / discharge efficiency, battery energy density, and battery life.

Further, it has been proposed to obtain a solid electrolyte composed of a reaction product of a phosphoric acid ester and an alkali metal salt for the purpose of preventing liquid leakage of the battery (Japanese Patent Publication No. 6-4232).
No. 5), this electrolyte is solid, and therefore, there is a manufacturing disadvantage in the currently mainstream cylindrical batteries as compared with the electrolytic solution. Further, since unreacted phosphoric acid esters remain in this solid electrolyte, when this electrolyte is used in a rocking chair type battery using carbon capable of doping / undoping with lithium ions in the above-mentioned negative electrode, it is charged. It seems that there are problems in terms of discharge efficiency, energy density, and life.

The present invention has been made in view of the above problems, and provides a non-aqueous electrolyte suitable for a battery having a high energy density, excellent in withstand voltage, load characteristics, low temperature characteristics, and self-extinguishing. With the goal. It is another object of the present invention to provide a non-aqueous electrolyte solution which, when used in a battery, is less likely to cause a decrease in battery energy density even after repeated charging / discharging and ensures high battery performance.

[0010]

The inventors of the present invention have developed a phosphorus having a high self-extinguishing action in order to produce a non-aqueous electrolyte having a high withstand voltage, an excellent electric conductivity and an excellent applicability to a high energy density battery. An intensive study was conducted on the acid ester compound. As a result, by adding a phosphoric acid ester compound in which at least one of the substituents is substituted with lithium ion, non-aqueous electrolysis showing self-extinguishing property and excellent performance as a battery electrolyte such as charge / discharge cycle characteristics. It was found that a liquid was obtained.

In the non-aqueous electrolytic solution of the present invention, the reduction resistance is increased by anionizing the phosphoric acid ester, the reactivity with the negative electrode is lowered, and the electrostatic repulsion prevents the approach with the negative electrode. It is considered that the above performance is improved. That is, the non-aqueous electrolytic solution of the present invention contains an organic solvent, an electrolyte, and a lithium salt of a phosphoric acid ester compound represented by the general formula [1].

[0012]

Embedded image

(Wherein R ¹ and R ² may be the same or different and each represents a lower alkyl group having 1 to 4 carbon atoms, a halogen-substituted alkyl group having 2 to 4 carbon atoms or a lithium ion, ^At least ^one of R ¹ and R ² is a substituent that is not a lithium ion.) In the electrolytic solution of the present invention, the self-extinguishing action of the phosphate ester is such that the higher the phosphorus content in the phosphate ester, that is, the R of the substituent group is ^The smaller the molecular weight of ¹ , R ² is, the larger it is, and the larger the addition amount is, the larger it is. However, it is not preferable to increase the addition amount of the phosphoric acid ester having a large molecular weight because the addition of the phosphoric acid ester increases the viscosity of the electrolytic solution and decreases the conductivity. Thus, R ^1, the number of carbon atoms of R ² is desirably as small as possible, if R ^1, R ² is an alkyl group,
The number is preferably 1 to 4 and, in the case of a halogen atom-substituted alkyl group, preferably 2 to 4.

R ¹ and R ² may be an alkyl group or a halogen-substituted alkyl group, but either one may be a lithium ion. However, it is preferable that both R ¹ and R ² are alkyl groups or halogen-substituted alkyl groups. This makes it possible to obtain the effect of introducing lithium ions into the phosphate ester while maintaining good solubility in the electrolytic solution.

As the alkyl group, a methyl group, an ethyl group,
Examples thereof include n-propyl group, isopropyl group, n-butyl group, isobutyl group and t-butyl group. Examples of the halogen of the halogen-substituted alkyl group include fluorine, chlorine and bromine, and fluorine is most preferable from the viewpoint of stability. Examples of such a fluorine-substituted alkyl group include trifluoroethyl group, difluoroethyl group, monofluoroethyl group, pentafluoropropyl group, 2,2,3,3-tetrafluoropropyl group, 1,1,1-trifluoroisopropyl group. Group, 1,3-difluoro-2-propyl group,
Hexafluoroisopropyl group, 2, 2, 3, 3, 4,
4,4-heptafluorobutyl group, 2, 2, 3, 4,
4,4-hexafluorobutyl group, hexafluoro-2
-Methylisopropyl group, 3,3,4,4,4-pentafluoro-2-butyl group, 4,4,4-trifluorobutyl group, perfluoro-t-butyl group and the like. In addition, chlorine having the same structure as above, instead of fluorine,
Those substituted with bromine are also exemplified. Further, two or more kinds of halogen may be mixed in one substituent.

Examples of the lithium salt of the phosphate ester of the present invention having such a substituent include dimethyllithium phosphate, diethyllithium phosphate, dipropyllithium phosphate, dibutyllithium phosphate, methylethyllithium phosphate and phosphorus. Acid methylpropyl lithium, ethyl propyl lithium phosphate, ethyl butyl lithium phosphate, propyl butyl lithium phosphate, di (2,2,2-trifluoroethyl) lithium phosphate, di (2,2,2-trichloro phosphate) Examples include ethyl) lithium, di (2,2,2-tribromoethyl) lithium phosphate, and combinations of the alkyl groups and halogen-substituted alkyl groups exemplified above.

Since the lithium phosphate salt has the function of making the solvent difficult to burn, and at the same time has the property of lowering the electrical conductivity of the electrolytic solution, the non-aqueous electrolytic solution of the present invention is used as a practical electrolytic solution for a secondary battery. It is important to use an appropriate amount for use as. That is, in order to obtain a practical conductivity as an electrolytic solution for a secondary battery, and to obtain a self-extinguishing action,
The phosphoric acid ester lithium salt may be added in a volume ratio of 0.1 to 2.0 mol / liter to the electrolytic solution, and preferably 0.2 to 1.0 mol / liter. Good.

As the solvent of the non-aqueous electrolyte of the present invention, conventionally used chain ethers such as dimethoxyethane, cyclic ethers such as tetrahydrofuran, dimethylformamides, methyl-N, N-dimethylcarbamate. Carbamates such as, chain esters such as diethyl carbonate, and cyclic esters such as propylene carbonate can be used alone or in combination of two or more.

It is particularly preferable to use a chain ester, a cyclic ester or a mixed solvent of both. As the chain ester, one kind of the compound represented by the general formula [2] or a mixture of two or more kinds can be used.

[0020]

[Chemical 4]

(In the formula, R ³ represents a methyl group, an ethyl group, a propyl group, a methoxy group or an ethoxy group, and R ⁴ represents a chain or branched alkyl group having 1 to 3 carbon atoms) By using the chain ester represented by the formula [2], an electrolytic solution having low viscosity and excellent electric conductivity at room temperature to low temperature can be obtained. Examples of such a chain ester include esters such as methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, and ethyl propionate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl. Carbonates such as propyl carbonate, methyl isopropyl carbonate and ethyl propyl carbonate can be exemplified. In particular, as the positive electrode of the battery, LiCoO ₂ , LiMn capable of generating 4 V
When applied to an electrolytic solution of a battery using O ₂ , LiNiO ₂ , LiMn ₂ O _4, etc., carbonates such as dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate are preferable from the viewpoint of oxidation resistance stability.

As the cyclic ester, one or a mixture of two or more selected from propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, γ-butyrolactone or sulfolane is used, and preferably propylene carbonate or ethylene. A cyclic carbonate of carbonates is selected. Such a cyclic ester can enhance the dissociation property of the electrolyte and can enhance the conductivity of the electrolytic solution.

When a chain ester and a cyclic ester are mixed and used, the effect of lowering the viscosity and the effect of improving the dissociation property of the electrolyte are synergized, so that the electric conductivity of the electrolytic solution is improved, the load characteristics of the battery and the low temperature It is more preferable because the characteristics can be improved.
When the mixed solvent is used, the concentration of the chain ester in the electrolytic solution solvent can be usually used in the range of 20 to 90% by volume,
Preferably, it can be used in the range of 40 to 75% by volume. The concentration of cyclic ester in the electrolyte solvent is usually 10-8.
It can be used in the range of 0% by volume, preferably 25 to
It can be used in the range of 60% by volume.

In the case of a battery electrolyte using V ₂ O ₅ , TiS ₂ , polyaniline or the like which generates a voltage of about 3 V as the positive electrode of the battery, instead of or in combination with the above chain ester and cyclic ester. Chain ethers such as dimethoxyethane, cyclic ethers such as tetrahydrofuran, amides such as dimethylformamide, methyl-
Carbamates such as N, N-dimethyl carbamate,
Cyclic carbamates and amides such as N-methyloxazolidone and N-methylpyrrolidone can also be used.

As the electrolyte used in the electrolytic solution of the present invention,
LiBF_Four, LiPF₆, LiClO _Four, LiAsF₆, L
iCF₃SO₃, LiN (CF₃SO₂)₂, LiAlC
l_Four, LiSiF₆Lithium salts such as
Especially LiPF₆Is preferred. LiPF6 as electrolyte₆
Of the lithium salt of the phosphoric acid ester of the present invention,
High self-extinguishing property can be maintained even if the content is lowered. Concrete
LiPF₆When used, phosphate ester
The addition amount of thium salt is less than 1.0 mol / liter
Can also exert sufficient self-extinguishing property, so
Attributed to the addition of a large amount of lithium ester salt
It is possible to prevent a decrease in conductivity. Electrolyte in electrolyte
The concentration is usually within the range of 0.1 to 3 mol / liter.
Can be used, and preferably 0.5 to 2.0 mol /
Used in the concentration range of 1 liter.

The non-aqueous electrolytic solution of the present invention can be preferably used as an electrolytic solution for a lithium secondary battery in particular, but can also be used as an electrolytic solution for other batteries and as an electrolytic solution for electrolytic capacitors and the like. it can.

[0027]

EXAMPLES The present invention will be specifically described with reference to the following examples.
However, the present invention is not limited to these examples.
Not of. In the following description, solvent names are abbreviated as follows.
No. was used. PC: Propylene carbonate, EC: Ethylene carbo
Nate, TMPA: trimethyl phosphate, MEC: methyl
Ethyl carbonate, DEPALi: Diethyl phosphate
Tium 1. Preparation of electrolyte solution Volume ratio of fully dehydrated and distilled / purified EC and MEC 4: 6
LiPF as a mixed solvent of ₆Was dissolved at 1 mol / liter
0.1 mol / liter (implementation of DEPALi in the electrolyte)
Example 1), 0.2 mol / liter (Example 2), 5%
(0.3 mol / liter: Example 3), 10% (0.6
Mol / liter: Example 4), 15% (0.8 mol / liter)
Example 5) Add and stir overnight to dissolve and charge.
A lysate was prepared. In addition, DEPALi is 12 in a vacuum dryer.
The one dried at 0 ° C. for 10 hours was used.

Further, as a comparative electrolyte, LiPF ₆ was dissolved in a mixed solvent of EC and MEC in a volume ratio of 4: ₆ at 1 mol / liter (Comparative Example 1), and the volume ratio of EC, MEC and TMPA was 4: 4: A solution (Comparative Example 2) in which 1 mol / liter of LiPF ₆ was dissolved in the mixed solvent of 2 was prepared. 2. Evaluation of self-extinguishing property of electrolyte solution Manila paper for separator is 1.5cm in width and 30c in length.
m, a 0.04 mm thick strip was cut, and this was dipped in a beaker containing the electrolytic solution prepared as described above for 1 minute or more. Excess sample dripping from the manila paper was wiped with a beaker wall, and the manila paper was stuck at 2.5 cm intervals to a supporting needle of a sample stage having a supporting needle and fixed horizontally. Put the manila paper and sample stand in a 25 cm x 25 cm x 50 cm metal box, ignite with a lighter at one end, and burn the manila paper for the burning length of the manila paper and 30 cm from the first needle to the last needle The time required for the measurement was measured 3 times each. Table 1 shows the average length of burning, the time required for burning, and burning rate.
It was shown to.

[0029]

[Table 1]

The burning velocity "0" in the table means that the burning did not occur. As shown in Table 1, the electrolytic solution to which the lithium salt of the phosphoric acid ester of the present invention is added exhibits self-extinguishing property and flame retardancy, and particularly, in Examples 3 to 5, trimethyl phosphate is 2%.
It showed the same self-extinguishing property and flame retardancy as the electrolytic solution containing 0% by volume. 3. Measurement of Withstand Voltage and Electric Conductivity of Electrolyte Solution The withstand voltage and electric conductivity were measured using the electrolyte solution prepared as described above. The electric conductivity of the electrolytic solution was measured at 10 kHz using an impedance meter. The withstand voltage of the electrolytic solution was 10 mV with a potentiogalvanostat when the electrolytic solution was placed in a triode type withstand voltage measuring cell using glassy carbon for the working electrode, platinum for the counter electrode, and lithium metal for the reference electrode.
The electric potential was swept in sec, and the electric potential at which the oxidative decomposition current did not flow by 0.1 mA or more was set as the withstand voltage region based on the electric potential of the lithium metal. The results are shown in Table 2.

[0031]

[Table 2]

As shown in Table 2, the electrolytic solution to which the lithium salt of the phosphoric acid ester of the present invention was added exhibited a high withstand voltage and a high practical level of electrical conductivity. 4. Evaluation of Charge / Discharge of Lithium Ion to Carbon Using a carbon electrode as the positive electrode 1 and lithium metal as the negative electrode 2, the charge / discharge characteristics of the lithium ion to the positive electrode were evaluated using a test cell prepared as follows. As the electrolytic solution, the electrolytic solutions of Examples 3 to 5 prepared as described above were used. [Preparation of Carbon Electrode] Graphite powder having an average particle size of 50 μm was stirred in a Teflon aqueous dispersion to prepare a graphite / Teflon mixture (mixing amount of Teflon to graphite: 2% by weight).
I got This mixture was applied on a copper mesh, punched out into a coin mold, pressed at 1 t / cm ² , and vacuum dried at 120 ° C. to prepare a carbon electrode having a diameter of 13 mm and a carbon weight of 15 mg. [Fabrication of Cell] As shown in FIG. 1, a lithium electrode 2 and a carbon electrode 1 are arranged so as to face each other through a separator (Celguard 3601) 3 impregnated with an electrolytic solution.
A cell was prepared by adding ml to the carbon electrode side, pressing the collector electrode 4 from above and below, and sealing with an insulating packing 5. [Charge / Discharge Test] The above cell was connected to a charge / discharge device 6, and a charge / discharge cycle was performed in a voltage range of 0 V to 1.5 V under a constant current (0.33 mA, 0.25 mA / cm ² ) condition for 50 times. Repeated times. The result is shown in FIG.

As can be seen from FIG. 2, the electrolytic solution containing TMPA (Comparative Example 2) had a lower discharge capacity from the first cycle as compared with the electrolytic solution containing nothing (Comparative Example 1). In the electrolytic solutions (Examples 3 to 5) to which the lithium salt of the phosphoric acid ester of the present invention was added, almost no decrease in capacity was recognized as compared with the electrolytic solution of Comparative Example 1.

[0034]

As is apparent from the above description, according to the present invention, by adding a lithium salt of a phosphoric acid ester to a non-aqueous electrolytic solution containing an organic solvent and an electrolyte,
It is possible to provide a non-aqueous electrolyte that exhibits a self-extinguishing action, is flame-retardant, can withstand high voltage, and has excellent battery charge / discharge performance.

[Brief description of drawings]

FIG. 1 is a view showing a coil-type test cell used for a charge / discharge test of a non-aqueous electrolyte solution of the present invention.

FIG. 2 is a diagram showing charge / discharge cycle characteristics of the non-aqueous electrolyte solution of the present invention.

[Explanation of symbols]

 1 ・ ・ ・ ・ Positive electrode 2 ・ ・ ・ ・ Negative electrode

Claims (6)

[Claims] 1. A non-aqueous electrolytic solution obtained by dissolving a lithium salt as an electrolyte in an organic solvent, which contains a lithium salt of a phosphoric acid ester represented by the general formula [1]. Embedded image (In the formula, R ¹ and R ² may be the same or different and each is a lower alkyl group having 1 to 4 carbon atoms or 2 to 4 carbon atoms.
Represents one halogen atom-substituted alkyl group or lithium ion, and at least one of R ¹ and R ² is a substituent other than lithium ion. ) 2. The non-aqueous electrolyte solution according to claim 1, wherein the lithium salt of the phosphoric acid ester is diethyl lithium phosphate. 3. The organic solvent is a compound represented by the general formula [2], ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, γ.
1 selected from butyrolactone and sulfolane
The non-aqueous electrolytic solution according to claim 1 or 2, which is a solvent of one kind or a mixture of two or more kinds. Embedded image (In the formula, R ³ represents a methyl group, an ethyl group, a propyl group, a methoxy group or an ethoxy group, and R ⁴ represents a chain or branched alkyl group having 1 to 3 carbon atoms.) 4. The non-aqueous electrolytic solution according to claim 1, wherein the concentration of the lithium salt of the phosphoric acid ester is in the range of 0.1 to 2.0 mol / liter. . 5. The non-aqueous electrolytic solution according to claim 1, wherein the electrolyte is LiPF ₆ . 6. The non-aqueous electrolyte solution according to claim 1, wherein the electrolyte concentration is in the range of 0.1 to 3.0 mol / liter.