JP3294446B2 - Non-aqueous electrolyte - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a novel non-aqueous electrolyte.

[0002]

2. Description of the Related Art Generally, an electrolyte serves to transfer ions between a negative electrode and a positive electrode in a battery, and examples thereof include a non-aqueous electrolyte, a polymer electrolyte, and ion conductive glass. Among these, solid electrolytes such as polymer electrolytes and ion-conducting glass have poor conductivity compared to non-aqueous electrolytes, but do not have to worry about battery leakage and do not use flammable organic solvents. It has features such as improved safety. On the other hand, non-aqueous electrolytes are widely used because they have the highest electrical conductivity and excellent adhesion to electrodes, and have the lowest internal resistance of batteries using them.
Therefore, a battery using a nonaqueous electrolyte has a high voltage, a high energy density, and is excellent in reliability such as storability, and thus is widely used as a power source for consumer electronic devices.

As a non-aqueous electrolyte, a solvent obtained by mixing a low-viscosity solvent such as propylene carbonate, γ-butyrolactone, or sulfolane with a low-viscosity solvent such as dimethoxyethane, tetrahydroethane, or 1,3-dioxolane is generally used. To 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, the solvent of such a non-aqueous electrolyte often has a low withstand voltage. When an electrolyte having a low withstand voltage is used in a secondary battery, the solvent is electrolyzed when charging and discharging are repeated, and as a result, the solvent is electrolyzed. There is a possibility that the internal pressure of the battery rises due to the generated gas, or the product causes a polymerization reaction and adheres to the electrode. Such a decrease in the charge / discharge efficiency of the battery and a decrease in the energy density of the battery result in a shortened battery life.

[0005] Attempts to improve the durability of electrolytes include the use of conventionally used esters such as γ-butyrolactone and ethyl acetate, and ethers such as 1,3-dioxolan, tetrahydrofuran and dimethoxyethane. By using a carbonate ester such as diethyl carbonate having a high withstand voltage instead of a low solvent, a decrease in the battery energy density after repeated charge / discharge is controlled (for example, JP-A-4-184872).

[0006]

However, since batteries with high energy density are desired, high voltage batteries have been studied from various fields. For example, a rocking chair type secondary battery 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. Have been. In this case, the battery voltage can generate 4V or more, and since there is no precipitation of metallic lithium, safety is confirmed by experiments such as overcharging, external short-circuit, nail sticking, crushing, etc. It has become.

[0007] With the increase in the energy density and the size of such batteries, it has been required to further improve the performance of electrolytes used in batteries, such as flame retardancy. For this reason, it has been proposed to add phosphate esters known as compounds having a self-extinguishing property to an electrolytic solution (Japanese Patent Application Laid-Open No. 4-184870). However, an electrolytic solution containing 15% or more of such a compound is not sufficient in terms of battery charge / discharge efficiency, battery energy density, and battery life, although the flame retardancy is cleared.

It has also been proposed to obtain a solid electrolyte comprising a reaction product of a phosphate ester and an alkali metal salt for the purpose of preventing the battery from leaking (Japanese Patent Publication No. 6-4232).
No. 5), since this electrolyte is a solid, the current mainstream cylindrical battery has a manufacturing disadvantage compared to the electrolyte. In addition, since unreacted phosphate esters remain in this solid electrolyte, when this electrolyte is used in a rocking chair type battery using carbon capable of doping and undoping lithium ions in the above-described negative electrode, the electrolyte 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 which is suitable for a battery having a high energy density, has excellent withstand voltage, load characteristics, low-temperature characteristics, and has self-extinguishing properties. With the goal. Another object of the present invention is to provide a non-aqueous electrolytic solution that does not cause a decrease in battery energy density even by repeated charge and discharge when used in a battery, and that can ensure high battery performance.

[0010]

Means for Solving the Problems In order to produce a non-aqueous electrolyte having a high withstand voltage, excellent electrical conductivity, and excellent applicability to high energy density batteries, the present inventors have developed a phosphor having a high self-extinguishing action. An earnest study was conducted on acid ester compounds. As a result, by adding a phosphate compound in which at least one of the substituents is replaced with lithium ions, a non-aqueous electrolyte exhibiting self-extinguishing properties and having excellent performance as a battery electrolyte such as charge / discharge cycle characteristics. It was found that a liquid was obtained.

In the non-aqueous electrolyte of the present invention, since the phosphate ester is anionized, the resistance to reduction increases, the reactivity with the negative electrode decreases, and the approach to the negative electrode is prevented by electrostatic repulsion. It is considered that the above performance is improved. That is, the non-aqueous electrolyte of the present invention contains an organic solvent, an electrolyte, and a lithium salt of the phosphate 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 effect of the phosphate ester increases as the phosphorus content in the phosphate ester increases, that is, the R ¹ , The larger the molecular weight of R ^2, the larger, and the larger the added amount, the larger. However, it is not preferable to increase the amount of the phosphoric acid ester having a large molecular weight, since the addition increases the viscosity of the electrolytic solution and decreases the conductivity. Therefore, it is desirable that the carbon number of R ¹ and R ² be as small as possible. When R ¹ and R ² are alkyl groups,
It 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, and either one of them may be a lithium ion. However, it is preferable that both R ¹ and R ² are an alkyl group or a halogen-substituted alkyl group. 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 an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, and a t-butyl group. Examples of the halogen in 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 a trifluoroethyl group, a difluoroethyl group, a monofluoroethyl group, a pentafluoropropyl group, a 2,2,3,3-tetrafluoropropyl group, and a 1,1,1-trifluoroisopropyl group. Group, 1,3-difluoro-2-propyl group,
Hexafluoroisopropyl group, 2,2,3,3,4,
4,4-heptafluorobutyl groups, 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 in place of fluorine with the same structure as above,
Those substituted with bromine are also exemplified. Further, two or more halogens may be mixed in one substituent.

The lithium salt of the phosphate ester of the present invention having such a substituent includes dimethyllithium phosphate, diethyllithium phosphate, dipropyllithium phosphate, dibutyllithium phosphate, methylethyllithium phosphate, Lithium methyl propyl phosphate, ethyl propyl lithium phosphate, ethyl butyl lithium phosphate, propyl butyl lithium phosphate, lithium di (2,2,2-trifluoroethyl) phosphate, di (2,2,2-trichlorophosphate) Ethyl) lithium, di (2,2,2-tribromoethyl) lithium phosphate, and combinations of the above-described alkyl groups and halogen-substituted alkyl groups.

Since the lithium phosphate ester has the function of making the solvent less flammable, and at the same time has the property of lowering the conductivity of the electrolyte, the nonaqueous electrolyte of the present invention can be used as a practical electrolyte for a secondary battery. In order to use it, it is important to use an appropriate amount. That is, in order to obtain a practical conductivity as an electrolyte for a secondary battery, and to obtain a self-extinguishing action,
The amount of the lithium phosphate ester added may be 0.1 to 2.0 mol / l in volume ratio with respect to the electrolytic solution, preferably 0.2 to 1.0 mol / l. I just need.

Examples of the solvent for the non-aqueous electrolyte of the present invention include chain ethers such as dimethoxyethane, cyclic ethers such as tetrahydrofuran, dimethylformamides, and methyl-N, N-dimethylcarbamate. Carbamates, 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 or a mixture of two or more compounds represented by the general formula [2] can be used.

[0020]

Embedded image

(Wherein R ³ represents a methyl group, an ethyl group, a propyl group, a methoxy group or an ethoxy group, and R ⁴ represents a linear or branched alkyl group having 1 to 3 carbon atoms). By using the chain ester represented by the formula [2], it is possible to obtain an electrolyte solution having a low viscosity and an excellent electric conductivity from a normal temperature to a low temperature. Examples of such a chain ester include esters such as methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, and methyl. Carbonates such as propyl carbonate, methyl isopropyl carbonate and ethyl propyl carbonate can be exemplified. Particularly, as a positive electrode of a battery, LiCoO ₂ , LiMn capable of generating 4 V
When applied to an electrolyte of a battery using O ₂ , LiNiO ₂ , LiMn ₂ O ₄ or the like, carbonates such as dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate are preferable from the viewpoint of oxidation 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 and sulfolane is used, and propylene carbonate and ethylene are preferably used. The cyclic carbonate of the carbonate is selected. Such a cyclic ester can increase the dissociation of the electrolyte and can increase 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 conductivity of the electrolyte is improved, and the load characteristics of the battery and the low temperature It is more preferable because the characteristics can be improved.
When a mixed solvent is used, the concentration of the chain ester in the electrolyte 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 the cyclic ester in the electrolyte solvent is usually 10 to 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, the above-mentioned chain ester and cyclic ester may be used instead or in combination. To form chain ethers such as dimethoxyethane, cyclic ethers such as tetrahydrofuran, amides such as dimethylformamide,
Carbamates such as N, N-dimethyl carbamate,
Cyclic carbamates and amides such as N-methyloxazolidone and N-methylpyrrolidone can also be used.

The electrolyte used in the electrolytic solution of the present invention includes:
LiBF_Four, LiPF₆, LiClO _Four, LiAsF₆, L
iCF_ThreeSO_Three, LiN (CF_ThreeSO_Two)_Two, LiAlC
l_Four, LiSiF₆Lithium salts such as
Especially LiPF₆Is preferred. LiPF as electrolyte₆
When used, the lithium salt of the phosphate of the present invention
High self-extinguishing properties can be maintained even when the content is reduced. Concrete
LiPF₆When using
The addition amount of the titanium salt is 1.0 mol / liter or less;
Can exhibit sufficient self-extinguishing properties.
Caused by adding a large amount of lithium phosphate
Lowering of the electrical conductivity can be prevented. Electrolyte in electrolyte
The concentration is usually used in the concentration range of 0.1 to 3 mol / l.
And preferably 0.5 to 2.0 mol /
Use in the liter concentration range.

The nonaqueous electrolytic solution of the present invention can be suitably used particularly as an electrolytic solution for a lithium secondary battery. However, the nonaqueous electrolytic solution may be used as an electrolytic solution for other batteries and further as an electrolytic solution for electrolytic capacitors and the like. it can.

[0027]

EXAMPLES The present invention will be specifically described below with reference to examples.
However, the present invention is not limited by these Examples.
Not. In the following description, the solvent names are as follows:
No. was used. PC: propylene carbonate, EC: ethylene carbonate
Nate, TMPA: trimethyl phosphate, MEC: methyl
Ethyl carbonate, DEPALi: diethyl phosphate
Cium 1. Preparation of Electrolyte Solution The volume ratio of EC and MEC that has been sufficiently dehydrated and distilled and purified is 4: 6.
LiPF in the mixed solvent of ₆Was dissolved at 1 mol / l
0.1 mol / l of DEPALi in the electrolyte (implement
Example 1), 0.2 mol / liter (Example 2), 5%
(0.3 mol / liter: Example 3) 10% (0.6
Mol / l: Example 4), 15% (0.8 mol / l)
Tolu: Example 5) Add, stir overnight to dissolve
A solution was prepared. In addition, DEPALi is 12 in a vacuum dryer.
What was dried at 0 degreeC for 10 hours was used.

As a comparative electrolyte, LiPF ₆ was dissolved at 1 mol / l in a mixed solvent of EC and MEC at a volume ratio of 4: 6 (Comparative Example 1), and the volume ratio of EC, MEC and TMPA was 4: 4: A solution prepared by dissolving 1 mol / liter of LiPF _{6 in} the mixed solvent of Comparative Example 2 (Comparative Example 2) was prepared. 2. Self-extinguishing property evaluation of electrolyte solution Manila paper for separator is 1.5cm wide and 30c long
m, and cut into strips having a thickness of 0.04 mm and immersed in a beaker containing the electrolyte solution adjusted 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 horizontally fixed by piercing the support needle of the sample stand having the support needle at 2.5 cm intervals. Put the manila paper and the sample stand in a metal box of 25 cm x 25 cm x 50 cm, ignite with a lighter at one end and burn the manila paper for the burned length of the manila paper and 30 cm from the first needle to the last needle The time required for the measurement was measured three times. Table 1 shows the average value of the burned length, the time required for burning, and the burning speed.
It was shown to.

[0029]

[Table 1]

In the table, the burning speed "0" indicates that no combustion was performed. As shown in Table 1, the electrolyte solution to which the lithium salt of the phosphate of the present invention was added exhibited self-extinguishing properties and flame retardancy.
It exhibited self-extinguishing properties and flame retardancy similar to those of an electrolyte 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 electrolytic solution adjusted 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 measured by putting the electrolytic solution into a three-electrode withstand voltage measuring cell using glassy carbon for the working electrode, platinum for the counter electrode and lithium metal for the reference electrode, and using a potentiogalvanostat at 10 mV /
The potential was swept in sec, and the potential at which the oxidative decomposition current did not flow by 0.1 mA or more based on the potential of the lithium metal was defined as the withstand voltage range. The results are shown in Table 2.

[0031]

[Table 2]

As shown in Table 2, the electrolytic solution to which the lithium phosphate of the present invention was added exhibited a high withstand voltage and a practically high level of 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, lithium ion charge / discharge characteristics of the positive electrode were evaluated using a test cell prepared as follows. As the electrolyte, the electrolytes of Examples 3 to 5 adjusted as described above were used. [Preparation of carbon electrode] Graphite powder having an average particle size of 50 μm was stirred in an aqueous dispersion of Teflon, and a mixture of graphite and Teflon (mixing amount of Teflon to graphite: 2% by weight) was used.
I got The mixture was applied to a copper mesh, punched out in a coin shape, pressed at 1 t / cm ² , and dried in vacuum at 120 ° C. to prepare a carbon electrode having a diameter of 13 mm and a carbon weight of 15 mg. [Preparation of Cell] As shown in FIG. 1, a lithium electrode 2 and a carbon electrode 1 were arranged to face each other via a separator (Cell Guard 3601) 3 impregnated with an electrolytic solution.
ml was added to the carbon electrode side, pressed from above and below with the collecting electrode 4 and sealed with an insulating packing 5 to prepare a cell. [Charge / Discharge Test] The above cell was connected to a charge / discharge device 6, and a charge / discharge cycle was performed at a voltage range of 0 V to 1.5 V under a constant current (0.33 mA, 0.25 mA / cm ² ) condition. Repeated times. The result is shown in FIG.

As can be seen from FIG. 2, the discharge capacity of the electrolyte solution to which TMPA was added (Comparative Example 2) decreased from the first cycle as compared with the electrolyte solution to which nothing was added (Comparative Example 1). In the electrolytic solution to which the lithium salt of the phosphoric acid ester of the present invention was added (Examples 3 to 5), almost no decrease in the 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 electrolyte comprising an organic solvent and an electrolyte,
It is possible to provide a non-aqueous electrolyte which exhibits a self-extinguishing effect, is flame-retardant, can withstand a high voltage, and has excellent battery charge / discharge performance.

[Brief description of the drawings]

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

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

[Explanation of symbols] 1 ... Positive electrode 2 ... Negative electrode

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-60-23973 (JP, A) JP-A-4-184870 (JP, A) JP-A-8-111238 (JP, A) JP-A-58-1983 206078 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H01M 10/40 H01M 6/16

Claims (6)

(57) [Claims] 1. A non-aqueous electrolytic solution comprising a lithium salt of a phosphate represented by the general formula [1] in a non-aqueous electrolytic solution in which a lithium salt is dissolved as an electrolyte in an organic solvent. 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, 2 to 4 carbon atoms.
Represents a halogen atom-substituted alkyl group or a lithium ion, and at least one of R ¹ and R ² is a substituent other than a lithium ion. ) 2. The non-aqueous electrolyte according to claim 1, wherein the lithium salt of the phosphate 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 electrolyte according to claim 1, wherein the non-aqueous electrolyte is a solvent obtained by mixing species or two or more species. 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 electrolyte according to claim 1, wherein the concentration of the lithium phosphate salt is in the range of 0.1 to 2.0 mol / liter. . 5. The non-aqueous electrolyte according to claim 1, wherein the electrolyte is LiPF ₆ . 6. The non-aqueous electrolyte according to claim 1, wherein the electrolyte concentration is in the range of 0.1 to 3.0 mol / liter.