JPH11307120A - Nonaqueous electrolytic solution and nonaqueous electrolytic solution secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolytic solution that is excellent in safety and has high conductivity and low viscosity. SOLUTION: This electrolytic solution contains a fluorinated carbonate, a cyclic carbonate and a chain carbonate. In this case, it should be composed of an electrolyte and a nonaqueous solvent wherein the content of the cyclic carbonate is 2-63 mol.%, the content of the chain carbonate is 2-63 mol.%, and the content of the fluorinated carbonate is 35-96 mol.%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-aqueous electrolyte containing a fluorinated carbonate, and more particularly to a non-aqueous electrolyte capable of providing a non-aqueous electrolyte secondary battery having excellent safety and excellent charge / discharge characteristics. It relates to an electrolytic solution.

[0002] The present invention also relates to a non-aqueous electrolyte secondary battery containing such a non-aqueous electrolyte.

[0003]

BACKGROUND OF THE INVENTION In recent years, secondary batteries utilizing oxidation / reduction reactions of alkali metals have been actively studied. In particular, a battery using a carbon material capable of doping / dedoping lithium ions for a negative electrode and using a composite oxide of lithium and a metal for a positive electrode is called a lithium ion battery, and is small, lightweight, and Due to the high energy density, the field of application is rapidly expanding. By the way, the camera integrated V
With the emergence of new portable electronic devices such as TRs, mobile phones, and laptop computers one after another, in order to further improve the functions of such portable electronic devices, lithium-ion batteries have been required to increase the energy density or reduce the discharge current. Improvements in performance, such as increasing the size, are desired.

In such a lithium ion battery,
A non-aqueous electrolyte is used to exchange lithium ions between a positive electrode and a negative electrode. Lithium ion batteries that use water as a solvent are hydrolyzed because the potential of the electrodes is high. Therefore, a lithium ion battery in which an alkali metal salt is dissolved in a nonaqueous solvent is usually used.

As the non-aqueous solvent, a polar aprotic organic solvent which easily dissolves an alkali metal salt and is hardly electrolyzed is used. Representative examples thereof include ethylene carbonate, propylene carbonate, dimethyl carbonate, and dimethyl carbonate. Methyl ethyl carbonate, carbonates such as diethyl carbonate, γ-butyrolactone,
Esters such as methyl formate, methyl acetate, and methyl propionate; and ethers such as dimethoxyethane, tetrahydrofuran, and dioxolan. The dissolved alkali metal salts include LiPF ₆ and LiB.
F ₄ , LiClO ₄ , LiOSO ₂ CF ₃ , LiN (SO ₂ CF ₃ )
_And lithium salts such as LiN (SO ₂ OCH ₂ CF ₃ ) ₂ and LiC (SO ₂ CF ₃ ) ₃ .

It is desired that such a non-aqueous electrolyte has high conductivity and low viscosity in order to improve the discharge performance of the battery used. Further, it is desired that the positive electrode and the negative electrode be chemically and electrochemically stable so that the performance of the battery is not deteriorated by repeating charge and discharge.

Further, since many of such non-aqueous electrolytes are flammable, when the non-aqueous electrolyte leaks from the battery,
It is expected that the battery may explode or burn, and it is desired to improve the safety of the nonaqueous electrolyte.

For this reason, for example, a non-aqueous electrolyte solution containing a flame-retardant phosphoric acid ester (see JP-A-8-22839) and a non-aqueous electrolyte solution containing a halogen compound (see JP-A-63-2628).
48072) and the like. Among the halogen compounds, fluorine compounds, in particular, have high electrochemical stability and a high flash point (see Japanese Patent Application Laid-Open No. 7-6786).
It has such properties.

In addition, when an accident occurs in the battery and the battery is short-circuited, the electrolytic solution is electrolyzed by overcharging, or exposed to high temperature from the outside, the energy stored in the battery is reduced. Is released as heat, and so-called thermal runaway may occur. For this reason, commercially available batteries have taken sufficient measures to prevent overcharge, prevent overcurrent, and shut down the separator when the internal temperature rises.However, the safety of nonaqueous electrolytes should be further improved. Is desired.

In a process in which a battery undergoes thermal runaway, for some reason, the temperature rises to a temperature at which a chemical reaction between the electrode and the electrolyte starts, and the heat generation rate of the chemical reaction increases when the heat radiation rate of the battery increases. , The temperature rise due to heat generation stops,
It is known to lead to thermal runaway. To prevent such thermal runaway from occurring, it is effective to reduce the heat generation rate between the electrolyte and the electrode.

In view of the above circumstances, the present inventors have:
In order to achieve the above object, the present inventors have conducted intensive studies and have found that, by mixing a fluorinated carbonate, a cyclic carbonate and a chain carbonate in a specific ratio, the positive electrode and the electrolytic solution are one of the causes of increasing the heat generation rate. The inventors have found that an electrolytic solution having a low reaction rate with the solution and high conductivity can be obtained, and have completed the present invention.

[0012]

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems associated with the prior art described above, and to provide a non-aqueous electrolyte having excellent safety, high conductivity and low viscosity, and a non-aqueous electrolyte. An object of the present invention is to provide a non-aqueous electrolyte secondary battery including an electrolyte.

[0013]

The non-aqueous electrolyte according to the present invention comprises [A] a fluorinated carbonate, a cyclic carbonate and a chain carbonate, and (i) the content of the cyclic carbonate is 2 to 6;
3 mol%, and (ii) the content of the chain carbonate is 2 to 2.
(Iii) a non-aqueous solvent having a fluorinated carbonate content of 35 to 96 mol%, and [B] an electrolyte.

The cyclic carbonate is a carbonate compound containing an alkylene group having 2 to 5 carbon atoms, and the chain carbonate is preferably a carbonate compound containing a hydrocarbon group having 1 to 5 carbon atoms.

Further, the cyclic carbonate is at least one selected from ethylene carbonate and propylene carbonate, the chain carbonate is at least one selected from dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate, and the electrolyte is , LiPF ₆ , LiBF ₄ , LiOSO ₂ R ¹ ,

[0016]

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(R ^{1 to} R ⁸ may be the same as or different from each other, and are preferably at least one member selected from perfluoroalkyl groups having 1 to 6 carbon atoms).

The non-aqueous electrolyte secondary battery according to the present invention comprises the above-mentioned non-aqueous electrolyte, lithium metal as an anode active material, a lithium-containing alloy, a carbon material capable of doping and undoping lithium ions, and a lithium ion A negative electrode containing either tin oxide that can be doped or undoped, titanium oxide that can be doped or undoped with lithium ions, or silicon that can be doped or undoped with lithium ions, and lithium as a positive electrode active material And a positive electrode containing a composite oxide with a metal.

[0019]

DETAILED DESCRIPTION OF THE INVENTION The non-aqueous electrolyte and the non-aqueous electrolyte secondary battery according to the present invention will be specifically described below.

[Non-Aqueous Electrolyte] The non-aqueous electrolyte according to the present invention comprises a non-aqueous solvent containing a fluorinated carbonate, a cyclic carbonate and a chain carbonate, and an electrolyte.

First, each component constituting the non-aqueous electrolyte will be described. Fluorinated carbonate In the present invention, a compound represented by the following general formula [1] or [2] is preferably used as the fluorinated carbonate.

[0022]

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In the formula [1], R ¹ and R ² may be the same or different from each other, and one of them is a hydrocarbon group having 1 to 4 carbon atoms in which at least one hydrogen atom is substituted by a fluorine atom. And the other is a hydrocarbon group having 1 to 4 carbon atoms or a hydrocarbon group having 1 to 4 carbon atoms in which one or more hydrogen atoms have been substituted with fluorine atoms. In addition, such hydrocarbon groups include oxygen,
Groups containing heteroatoms such as nitrogen are also included.

[0024]

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In the formula [2], R ³ is an alkylene group in which at least one hydrogen atom having 2 to 4 carbon atoms is substituted by a fluorine atom. Such alkylene groups include groups containing hetero atoms such as oxygen and nitrogen.

Examples of the fluorinated carbonate represented by the general formulas [1] and [2] include methyl trifluoroethyl carbonate, ditrifluoroethyl carbonate, methyl pentafluoropropyl carbonate, methyl hexafluoroisopropyl carbonate, and ethyl trifluoroethyl carbonate. And methylpentafluoropropyl carbonate.

Such a fluorinated carbonate is physically safe, hardly thermally decomposed, flame-retardant and hardly subjected to electrochemical oxidation and reduction. Cyclic carbonate Examples of the cyclic carbonate used in the present invention include carbonates represented by the following general formula [3].

[0028]

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Here, R ⁴ and R ⁵ may be the same or different from each other, and a hydrogen atom, a linear, branched or cyclic alkyl group, or a part or all of hydrogen is chlorine or It represents a halogen-substituted alkyl group substituted with at least one kind of bromine. Examples of the linear alkyl group include a methyl group, an ethyl group, a propyl group, and a butyl group. Examples of the branched alkyl group include an isopropyl group, an isobutyl group, a sec-butyl group, and a tert-butyl group.
Examples of the cyclic alkyl group include a cyclopropyl group and a cyclobutyl group. Further, the cyclic carbonate may be not only a 5-membered ring compound represented by the above formula [3] but also a 6-membered ring compound.

Specific examples of such a cyclic carbonate include ethylene carbonate, propylene carbonate, 1,3-propylene carbonate, 1,2-butylene carbonate, 1,3-butylene carbonate, 2,3-butylene carbonate, and vinylene. Carbonate, 2,4-pentylene carbonate, 1,3-pentylene carbonate and the like. Further, a halogen-substituted cyclic carbonate in which part or all of hydrogen of a methyl group such as propylene carbonate is substituted with at least one of chlorine and bromine can also be used.

In the present invention, as the cyclic carbonate, those containing an alkylene group having 2 to 5 carbon atoms are preferable, and ethylene carbonate and propylene carbonate are particularly preferable.

Such cyclic carbonates can be used as a mixture of two or more kinds. Chain carbonate Examples of the chain carbonate include carbonates represented by the following general formula [4].

[0033]

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In the formula [4], R ⁶ and R ⁷ may be the same or different from each other, and a part or all of a linear, branched, or cyclic alkyl group or hydrogen may be replaced with fluorine, chlorine, It is a halogen-substituted alkyl group substituted with at least one kind of bromine.

As such a chain carbonate ester,
Specific examples include dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, dibutyl carbonate, diisopropyl carbonate, and methyl ethyl carbonate.

Among such chain carbonates, in the present invention, a chain carbonate containing a hydrocarbon group having 1 to 5 carbon atoms is preferable, and dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate are particularly preferable.

Other Solvents The non-aqueous electrolyte according to the present invention may contain other solvents other than the above as non-aqueous solvents.
γ-butyrolactone, γ-valerolactone, 3-methyl-γ-
Cyclic esters such as butyrolactone and 2-methyl-γ-butyrolactone; chain esters such as methyl formate, ethyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, methyl butyrate, and methyl valerate; and 1,4-dioxane , 1,3-dioxolan, tetrahydrofuran, 2-methyltetrahydrofuran, 3-methyl-1,3-dioxolan, 2-
Cyclic ethers such as methyl-1,3-dioxolan, chain ethers such as 1,2-dimethoxyethane, 1,2-diethoxyethane, diethyl ether, dimethyl ether, methyl ethyl ether, dipropyl ether, and sulfolane Sulfur-containing compounds can be mentioned.

These solvents can be used alone or in combination of two or more. Electrolyte The electrolyte used in the present invention can be used without particular limitation as long as it is usually used as an electrolyte for a non-aqueous electrolyte. Specifically, L
iPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiAlCl
₆ , Li ₂ SiF ₆ , LiOSO ₂ R ¹ ,

[0039]

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(R ^{1 to} R ⁸ may be the same or different and are each a perfluoroalkyl group having 1 to 6 carbon atoms), and the like. And the like. These can be used alone or in combination of two or more.

Among these, in particular, LiPF ₆ and LiB
F ₄ , LiOSO ₂ R ¹ ,

[0042]

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Is preferred. Such electrolytes are usually
0.1-3.0 mol / l, preferably 0.5-2.0
It is desirable that the compound is contained in the non-aqueous electrolyte at a concentration of mol / liter.

Solvent Composition of Nonaqueous Electrolyte In the nonaqueous electrolyte according to the present invention, the fluorinated carbonate as described above, a cyclic carbonate,
Chain carbonate, (i) the content of the cyclic carbonate is 2 to 63 mol%, preferably 10 to 55 mol%, and (ii) the content of the chain carbonate is 2 to 63 mol%, preferably A non-aqueous solvent having a content of 5 to 50 mol% and (iii) a fluorinated carbonate content of 35 to 96 mol%, preferably 40 to 85 mol% is used.

When the content of the cyclic carbonate and the chain carbonate is in the above range, the conductivity of the electrolytic solution is improved, and the characteristics of the battery using the same are also excellent. When the content of the fluorinated carbonate is in the above range, the conductivity of the electrolytic solution is higher than the practical level, the reactivity between the positive electrode and the electrolytic solution is low, and the safety of the battery can be improved.

The non-aqueous electrolyte having the above-described solvent composition has a maximum heat generation rate when mixed with the positive electrode in a charged state, about 1/10 of that of the non-aqueous electrolyte containing no fluorinated carbonate. It has dropped below.

The maximum heat generation rate represents the maximum heat generation rate in the exothermic reaction (in the present invention, the reaction between the positive electrode and the non-aqueous electrolyte). When the maximum heat generation rate is measured under the same conditions,
If the maximum heat generation rate is small, the temperature rise is slow and safe. On the other hand, those having a large maximum heat generation rate have a sharp rise in temperature. For example, if sufficient cooling equipment is not provided, the heat generation rate exceeds the heat absorption rate, and there is a danger that the reactants may run out of heat. I have.

The maximum heat generation rate is measured using an accelerating calorimeter (hereinafter, referred to as ARC). ARC is one of the methods to evaluate the danger of reactive chemical substances (Thermochimica Acta, 3
7 (1980), 1-30). The ARC gradually raises the temperature of the reactive substance, and when detecting the reaction heat generated from the reactive substance, raises the surrounding temperature in accordance with the temperature rise of the reactive substance,
The reactive substance is placed in a pseudo-adiabatic state, whereby the exothermic decomposition of the reactive substance is faithfully reproduced.

Non-Aqueous Electrolyte Secondary Battery The non-aqueous electrolyte secondary battery according to the present invention is capable of doping and undoping metallic lithium, a lithium-containing alloy, and lithium ions as the negative-electrode active material and the negative electrode active material. Negative electrode containing any one of a carbon material, silicon capable of doping and undoping lithium ions, titanium oxide capable of doping and undoping lithium ions, and tin oxide capable of doping and undoping lithium ions; and a positive electrode. The positive electrode contains a composite oxide of lithium and a transition metal as an active material.

Such a non-aqueous electrolyte secondary battery can be applied to, for example, a cylindrical non-aqueous electrolyte secondary battery. As shown in FIG. 1, a cylindrical nonaqueous electrolyte secondary battery has a negative electrode 1 formed by applying a negative electrode active material to a negative electrode current collector 9 and a positive electrode formed by applying a positive electrode active material to a positive electrode current collector 10. 2 is wound through a separator 3 into which a non-aqueous electrolyte is injected, and is housed in a battery can 5 in a state where insulating plates 4 are placed above and below the wound body. A battery lid 7 is attached to the battery can 5 by caulking via a sealing gasket 6, and is electrically connected to the negative electrode 1 or the positive electrode 2 via a negative electrode lead 11 and a positive electrode lead 12, respectively. It is configured to function as a positive electrode. The separator is a porous film.

In this battery, the positive electrode lead 12 may be electrically connected to the battery lid 7 via the current interrupting thin plate 8. In such a battery, when the pressure inside the battery rises, the current interrupting thin plate 8 is pushed up and deformed, and the positive electrode lead 12 is cut off leaving the portion welded to the thin plate 8 to cut off the current.

Examples of the negative electrode active material constituting the negative electrode 1 include lithium metal, a lithium alloy, a carbon material capable of doping and undoping lithium ions, and a tin oxide capable of doping and undoping lithium ions. Any of silicon capable of doping and undoping lithium ions and titanium oxide capable of doping and undoping lithium ions can be used. Among these, it is preferable to use a carbon material capable of doping and undoping lithium ions. Such a carbon material may be graphite or amorphous carbon, activated carbon, carbon fiber,
Any carbon material such as carbon black and mesocarbon microbeads can be used.

The positive electrode active materials constituting the positive electrode 2 include LiCoO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNiO ₂ ,
LiNi _x Co _(1-x) composite oxide comprising lithium such as O ₂ and a transition metal, or the like can be used V ₂ O _5.

The non-aqueous electrolyte secondary battery according to the present invention
The electrolyte contains the non-aqueous electrolyte described above, and the shape and form of the battery are not limited to those in FIG.
It may be a coin type or a square type.

[0055]

The nonaqueous electrolyte according to the present invention contains a fluorinated carbonate and uses a nonaqueous solvent having a specific solvent composition, so that the rate of heat generation by the reaction with the positive electrode is low,
Excellent safety. In addition, such a non-aqueous electrolyte has high conductivity and does not separate the electrolyte.
Therefore, the nonaqueous electrolyte according to the present invention can be suitably used as an electrolyte for a lithium ion secondary battery.

[0056]

EXAMPLES Hereinafter, the present invention will be described more specifically based on examples, but the present invention is not limited to these examples.

[0057]

Example 1 Preparation of positive electrode for measuring maximum heat generation rate LiCoO ₂ , PVDF (polyvinylidene fluoride) and graphite were mixed in a weight ratio of 91: 3: 6, and a slurry was formed with N-methylpyrrolidone. It was applied to an aluminum foil, dried and pressed to produce a positive electrode. The positive electrode thus obtained, the Li negative electrode, and a nonaqueous electrolyte for charging (LiPF ₆ in a solvent in which propylene carbonate and dimethyl carbonate were mixed at a volume ratio of 1: 1) were added.
(Dissolved so as to be mol / liter), and was charged at a constant voltage of 4.4 V. After charging, 2
The potential at the passage of time was 4.38V. This electrode was sufficiently washed and dried to produce a positive electrode for measuring a maximum heat generation rate.

Preparation of Electrolyte Solution Ethylene carbonate (EC), methyl ethyl carbonate and methyl trifluoroethyl carbonate (MFEC)
LiPF _{6 is} added to a non-aqueous solvent mixed to a predetermined ratio.
Was dissolved to 1 mol / liter to prepare a non-aqueous electrolyte.

Measurement of maximum heat generation rate In an argon atmosphere, 0.3 ml of the above prepared non-aqueous electrolyte was
And 1.00 g of the positive electrode for maximum heat generation rate measurement were mixed to prepare a measurement sample.

The measurement was performed using an ARC manufactured by COLUMBIA SCIENTIFIC.
(Accelerating Rate Calorimeter) using a routine method. The measurement temperature range was 40 to 350 ° C. Note that the heat generation rate represents the amount of increase in the temperature of the sample per unit time, and the maximum heat generation rate refers to the maximum value of the heat generation rate during the measurement period.

Table 1 shows the results.

[0062]

Examples 2 and 3 and Comparative Examples 1 to 3 A non-aqueous electrolyte was prepared and the maximum heat generation rate was measured in the same manner as in Example 1 except that the solvent composition of the non-aqueous solvent was as shown in Table 1. .

Table 1 shows the results.

[0064]

[Table 1]

As is evident from Table 1 above, the non-aqueous electrolyte comprising a non-aqueous solvent containing at least 40 mol% of fluorinated carbonate has a low maximum heat generation rate.

[0066]

Example 4 Preparation of Electrolyte Solution In the same manner as in Example 1, a non-aqueous solvent obtained by mixing ethylene carbonate (EC), methyl ethyl carbonate and methyl trifluoroethyl carbonate (MFEC) at a predetermined ratio was added to a non-aqueous solvent. LiPF ₆ was dissolved at 1 mol / liter to prepare a non-aqueous electrolyte.

The conductivity of the obtained non-aqueous electrolyte was measured. The conductivity of the electrolyte was measured at a measurement frequency of 3 kHz using a CM-40S radiometer manufactured by Toa Denpa Kogyo.

Table 2 shows the results.

[0069]

Examples 5 to 10 and Comparative Example 4 A non-aqueous electrolyte was prepared in the same manner as in Example 1 except that the solvent composition of the non-aqueous solvent was as shown in Table 2, and the conductivity was measured.

Table 2 shows the results.

[0071]

[Table 2]

As is apparent from Table 2, the amount of the cyclic carbonate and the chain carbonate in the non-aqueous solvent was 5%.
When it is at least mol%, a highly conductive non-aqueous electrolyte can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing one embodiment of a non-aqueous electrolyte secondary battery of the present invention.

[Explanation of symbols]

 1 ... Negative electrode 2 ... Positive electrode 3 ... Separator 4 ... Insulating plate 5 ... Battery can 6 ... Sealing gasket 7 ... Battery cover 8 ... · Current interrupting thin plate 9 ··· Negative electrode current collector 10 ··· Positive electrode current collector 11 ··· Negative electrode lead 12 ··· Positive electrode lead

Claims (4)

[Claims] (A) a fluorinated carbonate, a cyclic carbonate, and a chain carbonate, (i) the content of the cyclic carbonate is 2 to 63 mol%, and (ii) the content of the chain carbonate. (Iii) a non-aqueous solvent having a fluorinated carbonate content of 35 to 96 mol%, and [B] an electrolyte. 2. The cyclic carbonate is a carbonate compound containing an alkylene group having 2 to 5 carbon atoms, and the chain carbonate is a carbonate compound containing a hydrocarbon group having 1 to 5 carbon atoms. Claim 1
3. The non-aqueous electrolyte according to 1. 3. The cyclic carbonate is at least one selected from ethylene carbonate and propylene carbonate.
The chain carbonate is at least one selected from dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate, and the electrolyte is LiPF ₆ , Li
BF ₄ , LiOSO ₂ R ¹ , Wherein R ^{1 to} R ⁸ may be the same or different and are at least one selected from the group consisting of perfluoroalkyl groups having 1 to 6 carbon atoms. Or the non-aqueous electrolyte according to 2. 4. A non-aqueous electrolyte according to claim 1, wherein the negative electrode active material is metallic lithium, a lithium-containing alloy, a carbon material capable of doping / dedoping lithium ions, and a lithium ion doping. Undoped tin oxide, lithium ion doped and undoped titanium oxide,
A non-aqueous electrolyte secondary battery comprising a negative electrode containing any of silicon capable of doping / dedoping lithium ions and a positive electrode containing a composite oxide of lithium and a transition metal as a positive electrode active material.