JP3856583B2 - Non-aqueous electrolyte for secondary battery and non-aqueous electrolyte secondary battery - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a nonaqueous electrolytic solution containing a fluorine-containing aliphatic cyclic compounds, and more particularly, excellent in safety, and can provide an excellent non-aqueous electrolyte secondary battery in charge and discharge characteristics rechargeable battery for non It relates to a water electrolyte. The present invention also relates to a non-aqueous electrolyte secondary battery including such a non-aqueous electrolyte.
[0002]
TECHNICAL BACKGROUND OF THE INVENTION
In recent years, secondary batteries using oxidation / reduction reactions of alkali metals have been actively studied. In particular, a battery using a lithium-ion-doped / undopeable carbon material for the negative electrode and a composite oxide of lithium and metal for the positive electrode is called a lithium ion battery, and is small and lightweight. Due to the high energy density, the fields of use are expanding rapidly. By the way, as new portable electronic devices such as camera-integrated VTRs, mobile phones, laptop computers, etc. appear one after another, in order to achieve further functional improvements of such portable electronic devices, the lithium ion battery has an increased energy density. Improvement of performance such as increasing the discharge current is desired.
[0003]
In such a lithium ion battery, a nonaqueous electrolytic solution is used to exchange lithium ions between the positive electrode and the negative electrode. Since lithium ion batteries have a high electrode potential and are hydrolyzed when using water as a solvent, a battery in which an alkali metal salt is dissolved in a non-aqueous solvent is usually used.
[0004]
As the non-aqueous solvent, polar aprotic organic solvents that easily dissolve alkali metal salts and are difficult to be electrolyzed are used. Typical examples include ethylene carbonate, propylene carbonate, dimethyl carbonate, and methyl ethyl carbonate. And carbonates such as diethyl carbonate, esters such as γ-butyrolactone, methyl formate, methyl acetate and methyl propionate, and ethers such as dimethoxyethane, tetrahydrofuran and dioxolane. Examples of the alkali metal salt to be dissolved include lithium salts such as LiPF ₆ , LiBF ₄ , LiN (CF ₃ SO ₂ ) ₂ , LiClO ₄ , and LiCF ₃ SO ₃ .
[0005]
Such a non-aqueous electrolyte is desired to have high conductivity and low viscosity in order to improve the discharge performance of the battery used. In addition, it is desired that the battery is chemically and electrochemically stable with respect to the positive electrode and the negative electrode so that the battery performance is not deteriorated by repeated charge and discharge.
[0006]
In addition, since many of these non-aqueous electrolytes are flammable, it is assumed that when the non-aqueous electrolyte leaks from the battery, it may be ignited and explode or burned. Improvement of safety is also desired.
For this reason, for example, those in which a flame retardant phosphoric acid ester is added to a non-aqueous electrolyte (see JP-A-8-22839), those using a halogen compound (see JP-A-63-248072), etc. Has been proposed. Japanese Patent Application Laid-Open No. 10-12272 discloses a nonaqueous electrolytic solution containing a chain perfluoroalkane. When the chain perfluoroalkane is used, a perfluorocarbon having 5 or 6 carbon atoms is disclosed. Fluoroalkane has a low boiling point and is difficult to handle, and the compatibility with an electrolytic solution composed of a chain carbonate or a cyclic carbonate usually used for a lithium battery may be deteriorated.
[0007]
Also, if an accident occurs in the battery and the battery is short-circuited, the electrolyte is electrolyzed due to overcharging, or exposed to high temperatures from the outside, the energy stored in the battery is converted into heat. Released, so-called thermal runaway may occur. For this reason, commercially available batteries have sufficient measures such as overcharge prevention, overcurrent prevention, and shutdown by a separator when the internal temperature rises, but safety can be improved even for non-aqueous electrolytes. Is desired.
[0008]
The process that leads to the thermal runaway of the battery rises to a temperature at which the chemical reaction between the electrode and the electrolyte starts for some reason, and when the heat generation rate of this chemical reaction exceeds the heat dissipation rate of the battery, It is known that the temperature rise stops and thermal runaway occurs. In order to make it difficult for such thermal runaway to occur, it is an effective measure to reduce the heat generation rate between the electrolyte and the electrode.
[0009]
In view of the circumstances as described above, the present inventors diligently studied to achieve the above-mentioned problems. As a result, the specific fluorine-containing aliphatic cyclic compound has a high boiling point and is easy to handle, and is in a phase with the electrolytic solution. By using a non-aqueous solvent containing these fluorine-containing aliphatic cyclic compounds, an electrolyte solution having a low reaction rate between the positive electrode and the electrolyte solution, which is one of the causes of increasing the heat generation rate, is obtained. As a result, the present invention has been completed.
[0010]
OBJECT OF THE INVENTION
The present invention is intended to solve the problems associated with the prior art as described above, and is a non-aqueous electrolyte for a secondary battery excellent in safety and a non-aqueous electrolyte secondary containing the non-aqueous electrolyte. It aims to provide a battery.
[0011]
Summary of the Invention
A non-aqueous electrolyte for a secondary battery according to the present invention is characterized by comprising a non-aqueous solvent containing a fluorine-containing aliphatic cyclic compound represented by the following general formula [1a] or [1b] and an electrolyte. .
[Chemical 3]
(In Formula [1a], m is an integer of 1-8, and in Formula [1b], n is an integer of 1-10.)
The non-aqueous solvent preferably contains a cyclic carbonate and / or a chain carbonate. The cyclic carbonate is a cyclic carbonate compound containing an alkylene group having 2 to 5 carbon atoms, and a chain. The linear carbonate ester is preferably a chain carbonate ester compound containing a hydrocarbon group having 1 to 5 carbon atoms.
[0012]
The electrolytes are LiPF ₆ , LiBF ₄ , LiOSO ₂ R ¹ ,
[Formula 4]
(In formula, R < ¹ > -R < ⁸ > may mutually be same or different, and is a C1-C6 perfluoroalkyl group.)
It is preferably at least one selected from
[0013]
Nonaqueous electrolyte secondary battery according to the present invention,
The non-aqueous electrolyte;
As the negative electrode active material, metallic lithium, lithium-containing alloy, carbon material that can be doped / undoped with lithium ions, tin oxide that can be doped / undoped with lithium ions, silicon that can be doped / undoped with lithium ions, lithium A negative electrode containing any of titanium oxide capable of ion doping and dedoping, and
It consists of a positive electrode containing a complex oxide of lithium and a transition metal as a positive electrode active material.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the non-aqueous electrolyte for a secondary battery and the non-aqueous electrolyte secondary battery according to the present invention will be specifically described.
[0015]
[Nonaqueous electrolyte for secondary batteries ]
The non-aqueous electrolyte for a secondary battery according to the present invention comprises a non-aqueous solvent containing a fluorine-containing aliphatic cyclic compound as an essential component, and an electrolyte.
[0016]
First, each component constituting the non-aqueous electrolyte for a secondary battery will be described.
[Fluorine-containing aliphatic cyclic compound]
In the present invention, compounds represented by the following general formulas [1a] and [1b] are used as the fluorine-containing aliphatic cyclic compound.
[Chemical formula 5]
In the formula [1a], m is an integer of 1 to 8, preferably an integer of 2 to 8, and more preferably an integer of 4 to 8. Moreover, in formula [1b], n is an integer of 1-10, Preferably it is an integer of 2-10, More preferably, it is an integer of 4-10.
[0017]
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-hexafluorocyclo Examples include pentane, 1,1,2,2,3,3,4-heptafluorocyclopentane, 1,1,2,2,3,3,4,5-octafluorocyclopentane and the like.
Of these, preferred compounds are 1,1,2,2,3,3,4-heptafluorocyclopentane and 1,1,2,2,3,3,4,5-octafluorocyclopentane.
[0018]
As the fluorine-containing aliphatic cyclic compound represented by the formula [1b], specifically,
1-fluorocyclohexane, 1,2-difluorocyclohexane, 1,2,3,4,5,6-hexafluorocyclohexane, 1,1,2,2,3,4,5,6-octafluorocyclohexane, 1, 1,2,2,3,3,4,4,5,6-decafluorocyclohexane and the like.
Of these, preferred compounds are 1,2,3,4,5,6-hexafluorocyclohexane, 1,1,2,2,3,4,5,6-octafluorocyclohexane, 1,1,2, 2,3,3,4,4,5,6-decafluorocyclohexane.
[0019]
Such a fluorine-containing aliphatic cyclic compound has a characteristic that it has a high boiling point and flash point, is easy to handle, and is relatively less susceptible to electrochemical oxidation / reduction.
[0020]
[Nonaqueous solvent]
The fluorine-containing aliphatic cyclic compound is preferably used as a mixed solvent with other non-aqueous solvents such as carbonates. In this case, in order to improve the safety of the battery, the fluorine-containing aliphatic cyclic compound is contained in a non-aqueous solvent in an amount of 0.1 to 50% by weight, preferably 0.1 to 40% by weight, more preferably 1 to It is desirable to be contained in an amount of 30% by weight.
In the nonaqueous electrolytic solution according to the present invention, it is preferable to use a mixed solvent containing the fluorine-containing aliphatic cyclic compound and the cyclic carbonate and / or the chain carbonate in particular from the viewpoint of improving the ionic conductivity.
[0021]
[Cyclic carbonate]
Examples of the cyclic carbonate used in the present invention include carbonates represented by the following general formula [2a] or [2b].
[Chemical 6]
[0022]
In the formula [2a] or [2b], R ⁹ and R ¹⁰ may be the same or different from each other, and may be a hydrogen atom, a linear, branched, or cyclic alkyl group, or a part of hydrogen or A halogen-substituted alkyl group substituted entirely with at least one of fluorine, chlorine or bromine. The linear alkyl group is preferably a linear alkyl group having 1 to 4 carbon atoms such as a methyl group, an ethyl group, a propyl group, or a butyl group. As the branched alkyl group, a branched alkyl group having 3 to 6 carbon atoms such as isopropyl group, isobutyl group, sec-butyl group and 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.
[0023]
Moreover, as cyclic carbonate, not only the 5-membered ring compound represented by the said Formula [2a] or [2b] but a 6-membered ring compound may be sufficient.
Specific examples of the cyclic carbonate represented by the formula [2a] or [2b] include ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,3- Examples include propylene carbonate, 1,3-butylene carbonate, 2,4-pentylene carbonate, 1,3-pentylene carbonate, and vinylene carbonate.
In addition, a halogen-substituted cyclic carbonate in which a methyl group such as propylene carbonate has a part or all of hydrogen substituted with at least one of fluorine, chlorine or bromine can be used.
[0024]
In this invention, what contains a C2-C5 alkylene group as a cyclic carbonate is preferable, and ethylene carbonate and propylene carbonate are especially preferable.
Two or more kinds of such cyclic carbonates can be mixed and used.
[0025]
[Chain carbonate ester]
Examples of the chain carbonate include carbonates represented by the following general formula [3].
[Chemical 7]
[0026]
In the formula [3], R ¹¹ and R ¹² may be the same or different from each other, and a linear, branched, or cyclic alkyl group, or a part or all of hydrogen is at least fluorine, chlorine, or bromine. A halogen-substituted alkyl group substituted with one kind. The linear alkyl group is preferably a linear alkyl group having 1 to 4 carbon atoms such as a methyl group, an ethyl group, a propyl group, or a butyl group. As the branched alkyl group, a branched alkyl group having 3 to 10 carbon atoms such as isopropyl group, isobutyl group, sec-butyl group, and 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.
[0027]
As such a chain carbonate, specifically,
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, 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.
[0028]
A nonaqueous electrolytic solution using a nonaqueous solvent having the above composition has low reactivity between the positive electrode and the electrolytic solution, and can improve battery safety. That is, the non-aqueous electrolyte containing the non-aqueous solvent having the 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. Compared to about 1/5 or less.
[0029]
The maximum exothermic rate represents the maximum exothermic rate in the exothermic reaction (in the present invention, the reaction between the positive electrode and the non-aqueous electrolyte). When the maximum exothermic rate is measured under the same conditions, the maximum exothermic rate is small. The temperature rise is gradual and safe. On the other hand, those with a large maximum heat generation rate have a rapid temperature rise. For example, if sufficient cooling facilities are not provided, the heat generation rate exceeds the endothermic rate, and there is a risk that the reactant will run out of heat. Yes.
[0030]
Such maximum heat generation rate is measured using an accelerating calorimeter (hereinafter referred to as ARC). ARC is one of the methods for evaluating the risk of reactive chemical substances (Thermochimica Acta, 37 (1980), 1-30). When the ARC gradually raises the temperature of the reactive substance and detects the heat of reaction generated from the reactive substance, the ARC increases the ambient temperature in accordance with the temperature rise of the reactive substance and puts the reactive substance in a pseudo-adiabatic state. This faithfully reproduces the self-heating decomposition of the reactive substance. In the present invention, when a mixture of a fluorine-containing aliphatic cyclic compound and the cyclic carbonate and / or chain carbonate is used as the nonaqueous solvent, the quantitative ratio of the cyclic carbonate to the chain carbonate is 0: 100 to 100: 0, preferably 20:80 to 80:20 (all in weight ratio).
[0031]
[Other solvents]
In the nonaqueous electrolytic solution according to the present invention, if necessary, it may contain a solvent other than the above as a nonaqueous solvent.
cyclic esters such as γ-butyrolactone, γ-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,
Cyclic ethers such as 1,4-dioxane, 1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 3-methyl-1,3-dioxolane, 2-methyl-1,3-dioxolane,
Chain ethers such as 1,2-dimethoxyethane, 1,2-diethoxyethane, diethyl ether, dimethyl ether, methyl ethyl ether, dipropyl ether,
Sulfur-containing compounds such as sulfolane and dimethyl sulfate,
Examples thereof include phosphorus-containing compounds such as trimethyl phosphoric acid and triethyl phosphoric acid.
These solvents can be used alone or in combination of two or more.
[0032]
[Electrolytes]
The electrolyte used in the present invention is not particularly limited as long as it is normally used as an electrolyte for a non-aqueous electrolyte. _{Specifically, LiPF 6, LiBF 4, LiClO} 4, LiAsF 6, LiAlCl 6, Li 2 SiF 6, LiOSO 2 R 1,
[Chemical 8]
(In formula, R < ¹ > -R < ⁸ > may mutually be same or different, and is a C1-C6 perfluoroalkyl group.)
And alkali metal salts in which these lithiums are substituted with alkali metals. These can be used alone or in combination.
[0033]
Of these, LiPF ₆ , LiBF ₄ , LiOSO ₂ R ¹ ,
[Chemical 9]
Is preferred.
[0034]
Such an electrolyte is desirably contained in the non-aqueous electrolyte at a concentration of usually 0.1 to 3.0 mol / liter, preferably 0.5 to 2.0 mol / liter.
[0035]
[Nonaqueous electrolyte secondary battery]
The non-aqueous electrolyte secondary battery according to the present invention includes the non-aqueous electrolyte for a secondary battery , metallic lithium as a negative electrode active material, a lithium-containing alloy, a carbon material capable of doping and dedoping lithium ions, and lithium ions. A negative electrode including any one of tin oxide that can be doped and dedoped, silicon that can be doped and dedoped with lithium ions, titanium oxide that can be doped and dedoped with lithium ions, and lithium as a positive electrode active material It is comprised from the positive electrode containing complex oxide with a transition metal.
[0036]
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, the cylindrical non-aqueous electrolyte secondary battery includes a negative electrode 1 obtained by applying a negative electrode active material to a negative electrode current collector 9 and a positive electrode obtained 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 through 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 comprised so that it may function as a positive electrode. The separator is a porous film.
[0037]
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 internal pressure of the battery rises, the current interrupting thin plate 8 is pushed up and deformed, and the positive electrode lead 12 is cut leaving a portion welded to the thin plate 8 to interrupt the current.
[0038]
As the negative electrode active material constituting such a negative electrode 1, any of metallic lithium, a lithium alloy, and a carbon material that can be doped / undoped with lithium ions can be used. Among these, it is preferable to use a carbon material that can be doped / undoped with 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.
[0039]
Further, the positive electrode active material constituting the positive electrode 2 includes LiCoO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNixCo (1-x) O _{2 and} other complex oxides composed of lithium and transition metals, V ₂ O ₅ etc. can be used.
[0040]
The non-aqueous electrolyte secondary battery according to the present invention includes the non-aqueous electrolyte described above as the electrolyte, and the shape and form of the battery are not limited to those shown in FIG. It may be a mold or the like.
[0041]
【The invention's effect】
The non-aqueous electrolyte for a secondary battery according to the present invention contains a fluorine-containing aliphatic cyclic compound and uses a non-aqueous solvent having a specific solvent composition. Excellent in properties. The fluorine-containing aliphatic cyclic compound used here is easy to handle because its boiling point is higher than that of a chain perfluoroalkane having the same carbon number. Further, such a non-aqueous electrolyte for a secondary battery has a practical level of conductivity and does not separate the electrolyte. Such a nonaqueous electrolytic solution can be suitably used as an electrolyte for a lithium ion secondary battery.
[0042]
【Example】
EXAMPLES Hereinafter, although this invention is demonstrated further more concretely based on an Example, this invention is not limited at all by these Examples.
[0043]
[Example 1]
<Preparation of non-aqueous electrolyte>
Ethylene carbonate (EC), dimethyl carbonate (DMC) and 1,2-difluorocyclopentane are mixed so that ethylene carbonate: dimethyl carbonate: 1,2-difluorocyclopentane = 40: 40: 20 (weight ratio). LiPF ₆ was dissolved in the non-aqueous solvent so as to be 1 mol / liter to prepare a non-aqueous electrolyte.
[0044]
<Production of positive electrode for measuring maximum heat generation rate>
LiCoO ₂ , PVDF (poly (vinylidene fluoride)) and graphite are mixed so that the weight ratio is 91: 3: 6, and slurried with NMP, applied to aluminum foil, dried and pressed. A positive electrode was produced. A non-aqueous electrolyte for charging in which LiPF ₆ is dissolved in a solvent in which a volume ratio of 1: 1 is mixed with a positive electrode, a Li negative electrode, and ethylene carbonate and dimethyl carbonate obtained in this manner is used. Then, constant voltage charging was performed at 4.4V. After charging, the potential after 2 hours was 4.37V. This electrode was thoroughly washed with dimethyl carbonate and dried to remove dimethyl carbonate.
This electrode was cut to about 2 mm square to produce a positive electrode for measuring the maximum heat generation rate.
[0045]
<Maximum heat generation rate measurement>
Under an argon atmosphere, 0.3 ml of the non-aqueous electrolyte prepared above 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 by an ordinary method using ARCTM (Accelerating Rate Calorimeter) manufactured by COLUMBIA SCIENTIFIC. The measurement temperature range was 40 to 350 ° C.
The exothermic rate represents the amount of increase in the self-temperature of the sample per unit time, and the maximum exothermic rate is the maximum value of the exothermic rate during the measurement period.
[0046]
Examples 2-7
A non-aqueous electrolyte was prepared in the same manner as in Example 1 except that the fluorine-containing aliphatic cyclic compound used in Example 1 was changed to that shown in Table 1, and the maximum heat generation rate was measured.
[0047]
[Comparative Example 1]
In Example 1, except that the solvent composition used was changed to that shown in Table 1, a non-aqueous electrolyte was prepared in the same manner as in Example 1, and the maximum heat generation rate was measured.
[0048]
The results are shown in Table 1.
[Table 1]
[0049]
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing one embodiment of a non-aqueous electrolyte secondary battery of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ...... Negative electrode 2 ... Positive electrode 3 ... Separator 4 ... Insulating plate 5 ... Battery can 6 ... Sealing gasket 7 ... Battery lid 8 ...・ Thin plate for current interruption 9... Negative electrode current collector 10... Positive electrode current collector 11.

Claims (5)

A non-aqueous electrolyte for a secondary battery comprising a non-aqueous solvent containing a fluorine-containing aliphatic cyclic compound represented by the following general formula [1a] or [1b] and an electrolyte.
(In Formula [1a], m is an integer of 1-8, and in Formula [1b], n is an integer of 1-10.) The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the non-aqueous solvent includes a cyclic carbonate and / or a chain carbonate. The cyclic carbonate is a cyclic carbonate compound containing an alkylene group having 2 to 5 carbon atoms, and the chain carbonate ester is a carbonate ester compound containing a hydrocarbon group having 1 to 5 carbon atoms. The nonaqueous electrolyte for secondary batteries according to claim 2. The electrolyte is LiPF ₆ , LiBF ₄ , LiOSO ₂ R ¹ ,
(Wherein R ^{1 to} R ⁸ may be the same as or different from each other, and are perfluoroalkyl groups having 1 to 6 carbon atoms). Item 4. A nonaqueous electrolytic solution for a secondary battery according to any one of Items 1 to 3. The non-aqueous electrolyte for a secondary battery according to any one of claims 1 to 4, a metallic lithium as a negative electrode active material, a lithium-containing alloy, a carbon material that can be doped / undoped with lithium ions, a lithium ion doped A negative electrode containing any one of tin oxide capable of undoping, silicon capable of doping and dedoping lithium ions, titanium oxide capable of doping and dedoping lithium ions, and lithium and transition metals as positive electrode active materials A non-aqueous electrolyte secondary battery comprising a positive electrode containing a composite oxide of