JP2000223151A - Nonaqueous electrolyte, and nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a secondary battery with low heat generating rate due to reaction with a positive electrode and good in safety by including nonaqueous solvent containing fluorine-containing aliphatic cyclic compound and electrolyte. SOLUTION: Fluorine-containing aliphatic cyclic compound is represented by a formula I or a formula II (in the formula I, m is an integer of 1-8, and in the formula II, n is an integer of 1-10). Preferably, nonaqueous solvent contains cyclic carbonic ester and/or chain carbonic ester, the cyclic carbonic ester is a cyclic carbonic ester compound containing 2-5C alkylene, and the chain carbonic ester is a carbonic ester compound containing a 1-5C hydrocarbon group. Preferably, this battery comprises nonaqueous electrolyte, a negative electrode containing, as a negative electrode active material, one of metal Li, Li-containing alloy, and carbon material, tin oxide, silicon, and titan oxide that can dope or de-dope lithium ions, and a positive electrode containing, as a positive electrode active material, composite oxide of Li and transition metal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte containing a fluorine-containing aliphatic cyclic compound, and more particularly to a non-aqueous electrolyte secondary battery having excellent safety and excellent charge / discharge characteristics. To a non-aqueous electrolyte. The present invention also relates to a non-aqueous electrolyte secondary battery containing such a non-aqueous electrolyte.

[0002]

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 achieve further improvements in the functions of such portable electronic devices, lithium-ion batteries have been required to increase energy density or reduce 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. Typical 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 , LiN (CF ₃ SO ₂ ) ₂ , LiClO ₄ , LiCF ₃ SO ₃
And the like.

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 most 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, those in which a flame-retardant phosphate ester is added to a non-aqueous electrolyte (see JP-A-8-22839), those in which a halogen compound is used (see JP-A-63-248072), and the like Has been proposed. Japanese Patent Application Laid-Open No. 10-12272 discloses a non-aqueous electrolyte containing a linear perfluoroalkane. However, when a linear perfluoroalkane is used, a perfluorocarbon having 5 or 6 carbon atoms is used. Fluoroalkanes have low boiling points and are difficult to handle, and may have poor compatibility with electrolytes composed of chain carbonates or cyclic carbonates that are usually used for lithium batteries.

[0007] In addition, when an accident occurs in the battery and the battery is short-circuited, the electrolytic solution is electrolyzed due to overcharging, or exposed to a 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.

[0008] The process by which the battery leads to thermal runaway increases, for some reason, to a temperature at which a chemical reaction between the electrode and the electrolyte starts, and when the heat generation rate of this chemical reaction exceeds the heat radiation rate of the battery. , 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:
To achieve the above object, the present inventors have conducted intensive studies and found that a specific fluorine-containing aliphatic cyclic compound has a high boiling point, is easy to handle, has good compatibility with an electrolytic solution, It has been found that the use of a nonaqueous solvent contains an electrolyte solution having a low reaction rate between the positive electrode and the electrolyte solution, which is one of the causes for increasing the heat generation rate, and has completed the present invention.

[0010]

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 and a non-aqueous electrolyte containing the non-aqueous electrolyte. The purpose is to provide a secondary battery.

[0011]

The non-aqueous electrolyte according to the present invention comprises a non-aqueous solvent containing a fluorine-containing aliphatic cyclic compound represented by the following general formula [1a] or [1b], and an electrolyte. I have.

Embedded image (In the formula [1a], m is an integer of 1 to 8, and the formula [1b]
In the above, n is an integer of 1 to 10. The non-aqueous solvent preferably contains a cyclic carbonate and / or a chain carbonate, and the cyclic carbonate is a cyclic carbonate compound containing an alkylene group having 2 to 5 carbon atoms, The chain carbonate is
It is preferably a chain carbonate compound containing a hydrocarbon group having 1 to 5 carbon atoms.

The electrolyte is LiPF ₆ , LiBF ₄ , Li.
OSO ₂ R ¹ ,

Embedded image (Wherein, R ^{1 to} R ⁸ may be the same or different from each other, and are preferably at least one selected from C ^{1 to} C 6 perfluoroalkyl groups).

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

[0014]

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 fluorine-containing aliphatic cyclic compound as an essential component, and an electrolyte.

First, each component constituting the non-aqueous electrolyte will be described. [Fluorine-containing aliphatic cyclic compound] In the present invention, as the fluorine-containing aliphatic cyclic compound, the following general formula [1a],
The compound represented by [1b] is used.

Embedded image In the formula [1a], m is an integer of 1 to 8, preferably 2
And an integer of 4 to 8, more preferably an integer of 4 to 8. In the formula [1b], n is an integer of 1 to 10, preferably 2 to 10, more preferably 4 to 10.
It is an integer of 10.

As the fluorine-containing aliphatic cyclic compound represented by the formula [1a], specifically, 1-fluorocyclopentane, 1,2-difluorocyclopentane, 1,4-
Difluorocyclopentane, 1,2,3,4-tetrafluorocyclopentane, 1,1,2,3,4,5-hexafluorocyclopentane, 1,1,2,2,3,3
4-heptafluorocyclopentane, 1,1,2,2
3,3,4,5-octafluorocyclopentane and the like. Among these, preferred compounds are 1,1,
2,2,3,3,4-heptafluorocyclopentane,
1,1,2,2,3,3,4,5-octafluorocyclopentane.

As the fluorine-containing aliphatic cyclic compound represented by the formula [1b], specifically, 1-fluorocyclohexane, 1,2-difluorocyclohexane, 1,2,2
3,4,5,6-hexafluorocyclohexane, 1,
Examples thereof include 1,2,2,3,4,5,6-octafluorocyclohexane and 1,1,2,2,3,3,4,4,5,6-decafluorocyclohexane. Of these, preferred compounds are 1,2,3,4,5,6-hexafluorocyclohexane, 1,1,2,2,3,4,
5,6-octafluorocyclohexane, 1,1,2,2
2,3,3,4,4,5,6-decafluorocyclohexane.

Such a fluorine-containing aliphatic cyclic compound has a characteristic that it has a high boiling point and a high flash point, is easy to handle, and is relatively resistant to electrochemical oxidation and reduction.

[Non-Aqueous Solvent] The fluorine-containing aliphatic cyclic compound is preferably used as a mixed solvent with another non-aqueous solvent such as a carbonate ester. In this case, in order to improve the safety of the battery, the fluorine-containing aliphatic cyclic compound is
0.1 to 50% by weight, preferably 0.1 to 50% by weight in the non-aqueous solvent
It is desirable that it be contained in an amount of 40% by weight, more preferably 1 to 30% by weight. In the non-aqueous electrolyte according to the present invention, it is preferable to use a mixed solvent containing the above-mentioned fluorine-containing aliphatic cyclic compound and cyclic carbonate and / or chain carbonate, particularly from the viewpoint of improving ionic conductivity.

[Cyclic Carbonate] As the cyclic carbonate used in the present invention, for example, the following general formula [2a]
Or carbonates represented by [2b].

Embedded image

In the formula [2a] or [2b], R ⁹ and R ¹⁰
May be the same or different from each other, and a hydrogen atom, a linear, branched, or cyclic alkyl group, or part or all of hydrogen may be at least one of fluorine, chlorine or bromine.
Indicates a halogen-substituted alkyl group substituted with a species. Examples of the linear alkyl group include a methyl group, an ethyl group, a propyl group,
A linear alkyl group having 1 to 4 carbon atoms such as a butyl group is preferred. As the branched alkyl group, a branched alkyl group having 3 to 6 carbon atoms such as an isopropyl group, an isobutyl group, a sec-butyl group, and a tert-butyl group is preferable. Examples of the cyclic alkyl group include a cyclopentyl group, a cyclohexyl group,
A C5-C10 cyclic alkyl group such as -methyl-cyclohexyl group is preferred.

The cyclic carbonate may be not only a 5-membered ring compound represented by the above formula [2a] or [2b], but also a 6-membered ring compound. Such a notation [2
As the cyclic carbonate represented by a) or [2b], specifically, ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,3-propylene carbonate, 1,3 -Butylene carbonate, 2,4-pentylene carbonate, 1,3-
Pentylene carbonate, vinylene carbonate and the like. Further, a halogen-substituted cyclic carbonate in which a methyl group such as propylene carbonate or the like has part or all of hydrogen substituted with at least one kind of fluorine, chlorine or bromine can 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 [3].

Embedded image

In the formula [3], R ¹¹ and R ¹² 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 fluorine, chlorine, It is a halogen-substituted alkyl group substituted with at least one kind of bromine. As the linear alkyl group, a linear alkyl group having 1 to 4 carbon atoms such as a methyl group, an ethyl group, a propyl group, and a butyl group is preferable. As the branched alkyl group, a branched alkyl group having 3 to 10 carbon atoms such as an isopropyl group, an isobutyl group, a sec-butyl group, and a tert-butyl group is preferable. As the cyclic alkyl group, a cyclic alkyl group having 5 to 10 carbon atoms such as a cyclopentyl group, a cyclohexyl group, and a 1-methyl-cyclohexyl group is preferable.

Specific examples of such a chain carbonate include dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, dibutyl carbonate, diisopropyl carbonate, methyl ethyl carbonate and the like. Among such chain carbonates, in the present invention, chain carbonates containing a hydrocarbon group having 1 to 5 carbon atoms are preferable, and dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate are particularly preferable.

A non-aqueous electrolyte using a non-aqueous solvent having the above composition has low reactivity between the positive electrode and the electrolyte, and can improve the safety of the battery. That is, a non-aqueous electrolyte containing a non-aqueous solvent having a solvent composition as described above has a maximum heat generation rate when mixed with a positive electrode in a charged state, and a non-aqueous electrolyte containing no fluorine-containing aliphatic cyclic compound. Approximately 1/5
It falls 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 nonaqueous 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.

Such a maximum heat generation rate is measured using an accelerating calorimeter (hereinafter referred to as an 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. When a mixture of the fluorine-containing aliphatic cyclic compound and the cyclic carbonate and / or the chain carbonate is used as the non-aqueous solvent in the present invention, the ratio of the cyclic carbonate to the chain carbonate is 0: 100-100:
0, preferably 20:80 to 80:20 (all by weight).

[Other Solvents] The non-aqueous electrolyte according to the present invention may contain, if necessary, other solvents other than the above as non-aqueous solvents. Other solvents include γ-butyrolactone and γ-butyrolactone. Cyclic esters such as -valerolactone, 3-methyl-γ-butyrolactone, 2-methyl-γ-butyrolactone,
Chain esters such as methyl formate, ethyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, methyl butyrate, methyl valerate, 1,4-dioxane, 1,3-dioxolan, tetrahydrofuran, 2-methyltetrahydrofuran, Cyclic ethers such as 3-methyl-1,3-dioxolan, 2-methyl-1,3-dioxolan, 1,2-dimethoxyethane, 1,2-diethoxyethane, diethyl ether, dimethyl ether, methyl ethyl ether, dipropyl Examples thereof include chain ethers such as ethers, sulfur-containing compounds such as sulfolane and dimethyl sulfate, and phosphorus-containing compounds such as trimethylphosphoric acid and triethylphosphoric acid. 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 any particular limitation as long as it is usually used as an electrolyte for a non-aqueous electrolyte. Specifically, LiPF ₆ , LiBF ₄ , LiCl
O ₄ , LiAsF ₆ , LiAlCl ₆ , Li ₂ SiF ₆ , LiOSO
₂ R ¹ ,

Embedded image Wherein R ^{1 to} R ⁸ may be the same or different and each is a perfluoroalkyl group having 1 to 6 carbon atoms, and the like. Alkali metal salts and the like. These can be used alone or in combination of two or more.

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

Embedded image Is preferred.

[0034] Such electrolytes are usually 0.1 to 3.0.
It is desirable that it is contained in the non-aqueous electrolyte at a concentration of 0.5 mol / l, preferably 0.5 to 2.0 mol / l.

[Non-Aqueous Electrolyte Secondary Battery] The non-aqueous electrolyte secondary battery according to the present invention comprises the above-mentioned non-aqueous electrolyte, lithium as a negative electrode active material, a lithium-containing alloy, and doping / dedoping of lithium ions. A negative electrode containing any of a carbon material capable of doping, tin oxide capable of doping and undoping lithium ions, silicon capable of doping and undoping lithium ions, and titanium oxide capable of doping and undoping lithium ions. And a positive electrode containing a composite oxide of lithium and a transition metal as a positive electrode 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 with an insulating plate 4 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.

As the negative electrode active material constituting the negative electrode 1, any of lithium metal, a lithium alloy, and a carbon material capable of doping / dedoping 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, and any carbon material such as activated carbon, carbon fiber, 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 ,
A composite oxide composed of lithium and a transition metal, such as LiNixCo (1-x) O ₂ , V ₂ O _5, or the like can be used.

The non-aqueous electrolyte secondary battery according to the present invention comprises:
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.

[0041]

The non-aqueous electrolyte according to the present invention contains a fluorine-containing aliphatic cyclic compound and uses a non-aqueous solvent having a specific solvent composition. Excellent safety. The fluorine-containing aliphatic cyclic compound used here has a higher boiling point than that of a linear perfluoroalkane having the same carbon number, and is therefore easy to handle. In addition, such a non-aqueous electrolyte has a practical level of conductivity, and does not separate the electrolyte. Such a non-aqueous electrolyte can be suitably used as an electrolyte for a lithium ion secondary battery.

[0042]

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

[0043]

Example 1 <Preparation of non-aqueous electrolyte> Ethylene carbonate (EC), dimethyl carbonate (DMC) and 1,2-difluorocyclopentane were prepared by mixing ethylene carbonate: dimethyl carbonate: 1,2-difluorocyclopentane = 40. : LiPF ₆ was dissolved in a non-aqueous solvent mixed at a ratio of 40:20 (weight ratio) to 1 mol / liter to prepare a non-aqueous electrolyte.

<Preparation of Positive Electrode for Measuring Maximum Heating Rate> LiC
oO ₂ , PVDF (poly (vinylidene fluoride)) and graphite were mixed in a weight ratio of 91: 3: 6, slurried with NMP, applied to an aluminum foil,
After drying, pressing was performed to produce a positive electrode. The positive electrode thus obtained, a Li negative electrode, and a non-aqueous electrolyte for charging prepared by dissolving LiPF ₆ at a molar ratio of 1: 1 in a solvent in which ethylene carbonate and dimethyl carbonate are mixed at a volume ratio of 1: 1. Then, constant voltage charging was performed at 4.4V. 2 hours after charging, the potential is 4.3
7V. This electrode was sufficiently washed and dried with dimethyl carbonate to remove dimethyl carbonate.
This electrode was cut into about 2 mm square to produce a positive electrode for measuring the maximum heat generation rate.

<Measurement of Maximum Heating Rate> Under an argon atmosphere, 0.3 ml of the nonaqueous electrolyte prepared above and 1.00 g of the positive electrode for measuring the maximum heating rate were mixed to prepare a measurement sample. The measurement was performed using ARCTM (Accele
rating Rate Calorimeter). The measurement temperature range was 40 to 350 ° C. In addition,
The heat generation rate represents the amount of self-temperature rise of the sample per unit time, and the maximum heat generation rate is the maximum value of the heat generation rate during the measurement period.

[0046]

Examples 2 to 7 A non-aqueous electrolyte was prepared in the same manner as in Example 1 except that the fluorine-containing aliphatic cyclic compound used was changed to that shown in Table 1, and the maximum heat generation rate was measured. did.

[0047]

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

Table 1 shows the results.

[Table 1]

[0049]

[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 (5)

[Claims] 1. A non-aqueous solvent containing a fluorine-containing aliphatic cyclic compound represented by the following general formula [1a] or [1b]:
A non-aqueous electrolyte comprising an electrolyte. Embedded image (In the formula [1a], m is an integer of 1 to 8, and the formula [1b]
In the above, n is an integer of 1 to 10. ) 2. The non-aqueous electrolyte according to claim 1, wherein the non-aqueous solvent contains a cyclic carbonate and / or a chain carbonate. 3. The cyclic carbonate is a cyclic 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. 3. The method according to claim 2, wherein
3. The non-aqueous electrolyte according to 1. 4. The electrolyte according to claim 1, wherein the electrolyte is LiPF ₆ , LiBF ₄ , LiO.
SO ₂ R ¹ , (Wherein, R ^{1 to} R ⁸ may be the same or different from each other and are perfluoroalkyl groups having 1 to 6 carbon atoms). Item 4. The non-aqueous electrolyte according to any one of Items 1 to 3. 5. 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. A negative electrode containing any of undoped tin oxide, lithium ion-doped / undoped silicon, and lithium-ion doped / undoped titanium oxide; and lithium and transition metal as the positive electrode active material A non-aqueous electrolyte secondary battery comprising a positive electrode containing a composite oxide of