JPH11329495A - Flame retardant nonaqueous electrolyte and secondary battery using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To impart fire resistance without loosing the charge and discharge efficiency, energy density, output density and life by comprising an electrolyte, a nonaqueous solvent and the quaternary salt of a compound having an asymmetric chemical structure having a nitrogenous conjugate structure. SOLUTION: As a frame retarder, the quaternary salt of a compound having N,N,N'-substituted amidine group, which is a compound of an asymmetric chemical structure having a nitrogenous conjugate structure of the formula, is used. Examples of the salt include 1-methyl-3-ethylimidazorium/borate tetrafluoride, and it is preferably contained in 30-99 wt.%. The electrolyte consists of a lithium borate tetrafluoride or the like, and the nonaqueous solvent consists of propylene carbonate or the like. Accordingly, a flame retardant nonaqueous electrolyte having a high withstand voltage and excellent in electric conductivity characteristic at low temperature can be provided. In the formula, R<1-2> represent a 1-10C hydrocarbon group which may contain an amino group, nitro group, cyano group, carbonyl group or ether group; R<3-6> represent a 1-10C hydrocarbon group which may contain H or an amino group, nitro group, cyano group, carbonyl group or ether group.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel flame-retardant non-aqueous electrolyte used for an electrochemical element such as a secondary battery, a primary battery, or an electric double layer capacitor, and more particularly to a flame retardant suitable for a secondary battery. To a non-aqueous electrolytic solution. Further, the present invention relates to a flame-retardant non-aqueous electrolyte secondary battery that uses this electrolyte and can be reused by charging, for example, a cordless power supply required for portable equipment and the like, and a power supply for electric vehicles and the like.

[0002]

2. Description of the Related Art A battery using a non-aqueous electrolyte has a high withstand voltage,
Because of its high energy density and excellent storage properties, it is widely used as a power source for consumer electronic devices. However, lithium secondary batteries using metallic lithium for the negative electrode, despite their excellent properties, do not have sufficient charge / discharge cycle life due to the precipitation of dendritic lithium,
It has not been put to practical use yet. Therefore, instead of using metallic lithium as it is, a carbonaceous material that can occlude and release lithium ions has attracted attention and is being actively developed.
In addition, various studies have been made on a non-aqueous solvent constituting an electrolytic solution suitable for the electrolyte. Typical examples of the non-aqueous solvent include a mixture of a high dielectric constant solvent such as propylene carbonate and ethylene carbonate and a low-viscosity solvent such as diethyl carbonate and dimethoxyethane.

[0003]

However, there is a strong demand for higher energy density and higher output density, and further improvement in safety such as flame retardancy and non-flammability is desired. Currently used non-aqueous solvents have relatively low flash points and are flammable.

For this reason, it has been proposed to add flame-retardant phosphoric esters to the electrolyte (Japanese Patent Laid-Open No. 184/1984).
870, JP-A-8-88023). However, when this type of compound is added, flame retardancy can be imparted, but the electrical conductivity is reduced, and the properties of the electrolytic solution are significantly deteriorated. In addition, battery characteristics such as charge / discharge efficiency, energy density, output density, and life are significantly inferior to those before addition.

The present invention has been made in view of the above-mentioned problems, and has a flame-retardant non-aqueous electrolyte having flame retardancy without impairing battery characteristics such as charge / discharge efficiency, energy density, output density, and life. It is intended to provide a liquid. It is another object of the present invention to provide a flame-retardant non-aqueous electrolyte having excellent withstand voltage, electric conductivity characteristics, load characteristics and low-temperature characteristics. Another object of the present invention is to provide a flame-retardant non-aqueous electrolyte secondary battery having excellent charge-discharge cycle characteristics and a long life.

[0006]

Means for Solving the Problems The present inventors have made intensive studies to achieve the above object, and as a result, have reached the present invention. That is, the present invention is characterized in that it comprises an electrolyte (A), a non-aqueous solvent (B), and a quaternary salt (C) of a compound (c) having an asymmetric chemical structure having a nitrogen-containing conjugated structure. A non-aqueous electrolyte. Further, a secondary battery of the present invention is a flame-retardant non-aqueous electrolyte secondary battery comprising the above-described flame-retardant non-aqueous electrolyte.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.

The quaternary salt (C) of the compound (c) having an asymmetric chemical structure having a nitrogen-containing conjugated structure of the present invention is not particularly limited, and examples thereof include linear amidines and cyclic amidines (imidazoles, imidazoline , Pyrimidines, etc.), pyrroles, pyrazoles, oxazoles, thiazoles, oxadiazoles, thiadiazoles,
And quaternary salts such as triazoles, pyridines, pyrazines and triazines. These may be substituted with an alkyl group in a form having an asymmetric structure. The molecular weight of the quaternary salt (C) is usually from 60 to 300.

Among them, a compound (c1) having an N, N, N'-substituted amidine group represented by the following general formula (1) is preferable from the viewpoint of battery performance such as flame retardancy and charge / discharge characteristics. Is a quaternary salt. General formula In the formula, R ¹ and R ² are an amino group, a nitro group,
And represents a hydrocarbon group having 1 to 10 carbon atoms which may contain a cyano group, a carbonyl group or an ether group. For example, an alkyl group such as a methyl group, an ethyl group, a propyl group, a butyl group, a hexyl group, an octyl group; an alkenyl group;
An aryl group such as a phenyl group; a hydrocarbon group such as an arylalkyl group such as a benzyl group; a hydrocarbon group whose terminal or side chain is substituted with an amino group, a nitro group or a cyano group; A hydrocarbon group separated by a carbonyl group or an ether group. R ³ ,
R ⁴ , R ⁵ and R ⁶ each represent a hydrogen atom or a hydrocarbon group having 1 to 10 carbon atoms which may contain an amino group, a nitro group, a cyano group, a carbonyl group or an ether group. For example, methyl, ethyl, propyl,
Alkyl groups such as butyl, hexyl, and octyl;
An alkenyl group; an aryl group such as a phenyl group; a hydrocarbon group such as an arylalkyl group such as a benzyl group; a hydrocarbon group whose terminal or side chain is substituted with an amino group, a nitro group or a cyano group; Is a hydrocarbon group interrupted by a carbonyl group or an ether group. However, ^{two of} R ¹ , R ² , R ³ , R ⁴ , R ⁵ and R ⁶
One or more of them may combine with each other to form a ring. For example, an imidazole ring in which R ¹ and R ⁶ are bonded,
A pyrimidine ring; a diazabicyclo ring in which R ¹ and R ⁶ and R ² and R ³ are bonded.

The compound (c1) may be in the form of a chain or a ring.
N′-alkenyl amidines, specifically, N, N
-Dimethyl-N '-(1,2-dimethylpropenyl) formamidine, N, N-dimethyl-N'-(1,2-
Dimethylpropenyl) acetamidine, N, N-dimethyl-N ′-(1,2-dimethylbutenyl) acetamidine, N, N-diethyl-N ′-(1,2-dimethylpropenyl) acetamidine and the like. . Compound (c
Examples of the cyclic compound in 1) include compounds having an alkyl-substituted asymmetric imidazole ring, benzimidazole ring, and pyrimidine ring.

Among these quaternary salts of the compound (c1), more preferred are compounds containing an imidazolium cation having an asymmetric structure represented by the following general formula (2).

[0012]

Embedded image

In the formula, R ⁷ and R ^{9 each} represent an alkyl group having 1 to 4 carbon atoms (eg, a methyl group, an ethyl group, a propyl group, a butyl group), a phenyl group or a benzyl group. R ⁸ represents a hydrogen atom, an alkyl group having 1 to 4 carbon atoms (eg, a methyl group, an ethyl group, a propyl group, a butyl group), a phenyl group or a benzyl group. R ⁷ , R ⁸ and R ⁹ may be the same or different as long as they have an asymmetric structure as a whole. R ¹⁰ and R ^{11 each} represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms (eg, a methyl group, an ethyl group, a propyl group, and a butyl group). By adopting an asymmetric structure, a flame-retardant non-aqueous electrolyte which imparts flame retardancy and has stable electric conductivity even at a low temperature and is excellent in battery performance such as charge / discharge characteristics can be obtained.

Specific examples of the compound containing an imidazolium cation having an asymmetric structure include 1-methyl-3-ethylimidazolium compound, 1-methyl-3-n-propylimidazolium compound, 1-methyl-3-iso -Propylimidazolium compound, 1-methyl-3-n-butylimidazolium compound, 1-methyl-3-iso-
Butylimidazolium compound, 1-methyl-3-ter
t-butyl imidazolium compound, 1-ethyl-3-n
-Propylimidazolium compound, 1-ethyl-3-i
So-propyl imidazolium compound, 1,2-dimethyl-3-ethyl imidazolium compound, 1,4-dimethyl-3-ethyl imidazolium compound, 1,2-dimethyl-3-n-propyl imidazolium compound, 1, 2-
Dimethyl-3-iso-propylimidazolium compound, 1,2,3,4-tetramethylimidazolium compound, 1-methyl-3-phenylimidazolium compound, 1-methyl-3-benzylimidazolium compound and the like. . These compounds can be used alone or as a mixture of two or more.

The anion of the quaternary salt (C) is not particularly limited as long as it is used in a usual non-aqueous electrolyte.
For example, halogen anion (fluoro anion, chloro anion, bromo anion, iodine anion), tetrafluoroborate anion, hexafluorophosphate anion, hexafluoroarsenate anion, perchlorate anion, trifluoromethane sulfonate anion, the following general formula A sulfonylimide anion represented by (3), a sulfonylmethide anion represented by the following general formula (4), and an organic carboxylate anion (acetic acid,
Anions such as phthalic acid, maleic acid, and benzoic acid). Formula _{^{(R 12 SO 2) (R}} 13 SO 2) N - (3) formula, R ¹² and R ¹³ are each an ether group 1 or 2 of 1 to 4 carbon atoms which may contain par Shows a fluoroalkyl group. Formula _{^{(R 14 SO 2) (R}} 15 SO 2) (R 16 SO 2) C - (4) wherein, R ^14, R ¹⁵ and R ¹⁶ are each contained one or two ether groups A perfluoroalkyl group having 1 to 4 carbon atoms. Of these, tetrafluoroborate anion, hexafluorophosphate anion, sulfonylimide anion of the above formula (3) and sulfonylmethide anion of the above formula (4) are preferable. It shows battery performance such as flammability and charge / discharge characteristics.

The sulfonyluy represented by the above general formula (3)
Specific examples of the mid anion include (CF_ThreeSO_Two)_TwoN^-,
(C_TwoF_FiveSO_Two)_TwoN^-, (C_ThreeF₇SO_Two)_TwoN^-, (C_FourF₉S
O_Two)_TwoN^-, (CF_ThreeSO_Two) (C_TwoF_FiveSO_Two) N^-, (CF_ThreeSO
_Two) (C_ThreeF₇SO_Two) N^-, (CF_ThreeSO _Two) (C_FourF₉SO_Two) N^-,
(C_TwoF_FiveSO_Two) (C_ThreeF₇SO_Two) N^-, (C_TwoF_FiveSO_Two) (C_FourF
₉SO_Two) N^-, (CF_ThreeOCF_TwoSO_Two)_TwoN^-Etc.
You.

The sulfonic acid compound represented by the above general formula (4)
Specific examples of the tide anion include (CF_ThreeSO_Two)_ThreeC^-, (C_Two
F_FiveSO_Two)_ThreeC^-, (C_ThreeF₇SO_Two)_ThreeC^-, (C_FourF₉SO_Two)_ThreeC
^-, (CF_ThreeSO_Two)_Two(C_TwoF_FiveSO_Two) C^-, (CF_ThreeSO_Two)_Two(C
_ThreeF₇SO_Two) C^-, (CF_ThreeSO _Two)_Two(C_FourF₉SO_Two) C^-, (C
F_ThreeSO_Two) (C_TwoF_FiveSO_Two)_TwoC^-, (CF_ThreeSO_Two) (C_ThreeF₇S
O_Two)_TwoC^-, (CF_ThreeSO_Two) (C_FourF₉SO_Two)_TwoC^-, (C_TwoF_FiveS
O_Two)_Two(C_ThreeF₇SO_Two) C ^-, (C_TwoF_FiveSO_Two)_Two(C_FourF₉SO_Two)
C^-, (CF_ThreeOCF_TwoSO_Two)_ThreeC^-And the like.

The content of the quaternary salt (C) is not particularly limited as long as it is within the range of exhibiting flame retardancy, and is usually 10% by weight or more. ~
99% by weight is preferred. If it is less than 30% by weight, sufficient flame retardancy cannot be provided. On the other hand, if it exceeds 99% by weight, it will not work as an electrochemical element such as a secondary battery because the concentration of the electrolyte is low.

The content of the quaternary salt (C) and the electrolyte (A) is 60% by weight of the quaternary salt (C) / electrolyte (A).
It is preferably in the range of / 40 to 99/1. 60 /
If it is less than 40, the electrolyte (A) cannot be completely dissolved. On the other hand, if it exceeds 99/1, the concentration of the electrolyte is so small that it does not work as an electrochemical element such as a secondary battery.

The electrolyte (A) of the present invention may be the same as that conventionally used as long as it is used for a normal non-aqueous electrolyte. For example, alkali metal salts, quaternary ammonium salts, etc. Is raised. Examples of the alkali metal salt include a lithium salt, a sodium salt, and a potassium salt. For example, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium perchlorate, lithium trifluoromethanesulfonate, and the following general formula (5) A lithium salt of a sulfonylimide represented by the following, a lithium salt of a sulfonylmethide represented by the following general formula (6), lithium acetate, lithium trifluoroacetate, lithium benzoate, lithium p-toluenesulfonate, lithium nitrate, lithium bromide, Lithium salts such as lithium iodide and lithium phenylborate;
Sodium salts such as sodium perchlorate, sodium iodide, sodium tetrafluoroborate, sodium hexafluorophosphate, sodium trifluoromethanesulfonate, and sodium bromide; potassium iodide, potassium tetrafluoroborate;
And potassium salts such as potassium hexafluorophosphate and potassium trifluoromethanesulfonate. General formula (R ¹⁷ SO ₂ ) (R ¹⁸ SO ₂ ) NLi (5) In the formula, R ¹⁷ and R ¹⁸ each represent a perfluoro group having 1 to 4 carbon atoms which may have one or two ether groups. Shows an alkyl group. General formula (R ¹⁹ SO ₂ ) (R ²⁰ SO ₂ ) (R ²¹ SO ₂ ) CLi (6) In the formula, each of R ¹⁹ , R ²⁰ and R ²¹ may have one or two ether groups. A good perfluoroalkyl group having 1 to 4 carbon atoms is shown.

Specific examples of the sulfonylimide lithium salt represented by the above general formula (5) include (CF ₃ SO ₂ ) ₂ N
Li, (C ₂ F ₅ SO ₂ ) ₂ NLi, (C ₃ F ₇ SO ₂ ) ₂ NLi,
(C ₄ F ₉ SO ₂ ) ₂ NLi, (CF ₃ SO ₂ ) (C ₂ F ₅ SO ₂ ) N
Li, (CF ₃ SO ₂ ) (C ₃ F ₇ SO ₂ ) NLi, (CF ₃ S
O ₂ ) (C ₄ F ₉ SO ₂ ) NLi, (C ₂ F ₅ SO ₂ ) (C ₃ F ₇ S
O ₂ ) NLi, (C ₂ F ₅ SO ₂ ) (C ₄ F ₉ SO ₂ ) NLi, (C
F ₃ OCF ₂ SO ₂ ) ₂ NLi.

Specific examples of the lithium salt of sulfonylmethide represented by the above general formula (6) include (CF ₃ SO ₂ ) ₃ CL
i, (C ₂ F ₅ SO ₂ ) ₃ CLi, (C ₃ F ₇ SO ₂ ) ₃ CLi,
(C ₄ F ₉ SO ₂ ) ₃ CLi, (CF ₃ SO ₂ ) ₂ (C ₂ F ₅ SO ₂ ) C
Li, (CF ₃ SO ₂ ) ₂ (C ₃ F ₇ SO ₂ ) CLi, (CF ₃ SO ₂ )
_{2) 2 (C 4 F 9} SO 2) CLi, (CF 3 SO 2) (C 2 F 5 SO 2) 2
CLi, (CF ₃ SO ₂ ) (C ₃ F ₇ SO ₂ ) ₂ CLi, (CF ₃ S
O ₂ ) (C ₄ F ₉ SO ₂ ) ₂ CLi, (C ₂ F ₅ SO ₂ ) ₂ (C ₃ F ₇ S
O ₂ ) CLi, (C ₂ F ₅ SO ₂ ) ₂ (C ₄ F ₉ SO ₂ ) CLi, (C
F ₃ OCF ₂ SO ₂ ) ₃ CLi and the like.

The quaternary ammonium salts include tetramethylammonium / 4-fluoroborate, tetraethylammonium / 4-fluoroborate, tetrapropylammonium / 4-fluoroborate, methyltriethylammonium / 4-fluoroborate Salt, tetraethylammonium / 6
Fluorinated phosphate, tetraethylammonium / perchlorate, etc., or pyridine ring, pyrrolidine ring, morpholine ring, piperidine ring, piperazine ring, pyrimidine ring, 1,
5-diazabicyclo [4,3,0] nonene-5 (DB
N) and quaternary ammonium salts having a cyclic or bicyclic structure such as 1,8-diazabicyclo [5,4,0] undecene-7 (DBU).

Among them, lithium tetrafluoroborate, 6
The lithium fluorophosphate, the lithium salt of the sulfonylimide represented by the above general formula (5), and the lithium salt of the sulfonylmethide represented by the above general formula (6) show particularly high ionic conductivity and have high thermal stability. Is also preferable because it is an excellent electrolyte. The anion component of the electrolyte (A) and the anion component of the quaternary salt (C) are not necessarily the same, but are preferably the same.

The type of the non-aqueous solvent (B) of the present invention is not particularly limited, and those similar to those used in ordinary non-aqueous electrolytes can be used. Compounds such as carboxylic acid esters, cyclic or chain ethers, phosphoric acid esters, lactone compounds, nitrile compounds, amide compounds, and mixtures thereof can be used.

Examples of the cyclic carbonate include alkylene carbonates such as propylene carbonate, ethylene carbonate and butylene carbonate, and examples of the chain carbonate include dialkyl carbonates such as dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate. .
Examples of the chain carboxylic acid ester include methyl acetate and methyl propionate, and examples of the cyclic or chain ether include tetrahydrofuran, 1,3-dioxolan, and 1,2-dimethoxyethane. Examples of the phosphate ester include trimethyl phosphate, triethyl phosphate, ethyl dimethyl phosphate, diethyl methyl phosphate, tripropyl phosphate, tributyl phosphate, tri (trifluoromethyl) phosphate,
Tri (trichloromethyl) phosphate, tri (trifluoroethyl) phosphate, tri (tripperfluoroethyl) phosphate, 2-ethoxy-1,3,2-dioxaphosphoran-2-one, 2-trifluoro Ethoxy-1,3,2-
Dioxaphospholane-2-one, 2-methoxyethoxy-1,3,2-dioxaphospholane-2-one and the like. Examples of the lactone compound include γ-butyrolactone, examples of the nitrile compound include acetonitrile, and examples of the amide compound include
An example is dimethylformamide. Of these, cyclic carbonates, chain carbonates, phosphates, and mixtures thereof are preferable because they exhibit high battery performance such as high charge / discharge characteristics and output characteristics.

The content of the non-aqueous solvent (B) in the electrolyte depends on the purpose of use of the electrolyte, the quaternary salt (C) and the electrolyte (A).
Can be appropriately selected according to the type of
It is preferably 0% by weight. If it is less than 0.1% by weight, the solubility of the electrolyte (A) is low. On the other hand, when the content exceeds 30% by weight, the flame retardancy of the electrolytic solution decreases.

The flame-retardant non-aqueous electrolyte of the present invention may optionally contain additives such as an activator.

The flame-retardant non-aqueous electrolyte of the present invention comprises a secondary battery,
It can be used for electrochemical devices such as primary batteries and electric double layer capacitors.

The flame-retardant non-aqueous electrolyte secondary battery of the present invention uses an electrolyte having the above composition together with a positive electrode and a negative electrode. The positive electrode has LiCoO ₂ , Li as its active material.
NiO ₂ , Li _x Ni _y Co _{1 -y} O ₂ (where x and y vary depending on the charge / discharge state of the battery, and usually 0 <x <1, 0.7
<Y <1.02), LiMnO ₂ , LiMn ₂ O ₄
Oxides of lithium and one or more transition metals; transition metal oxides such as MnO ₂ and V ₂ O ₅ ;
transition metal sulfides such as oS ₂ and TiS ₂ ; polyaniline,
Conductive polymers such as polypyrrole, polyacene, polythiophene, polyacetylene, poly-p-phenylene, and polycarbazole; compounds that reversibly electrolytically polymerize and depolymerize, such as disulfide compounds, can be used.
Among these, a composite oxide of lithium and a transition metal is
This is preferable in that the battery capacity is improved and the energy density is excellent.

In forming a positive electrode using such a positive electrode active material, a known conductive agent or binder can be added and used in combination.

The negative electrode uses a light metal or light metal ion as its active material. Such light metals include:
Examples include lithium, sodium, potassium, cesium, and aluminum. Similarly, light metal ions include lithium ion, sodium ion, potassium ion, cesium ion, and aluminum ion. Of these, lithium and lithium ion are particularly preferred in terms of battery output and energy density.

The negative electrode is composed of the active material itself or a material capable of inserting and extracting the active material. As a constituent material of such a negative electrode, a light metal itself; a compound itself having light metal ions; an alloy itself containing these light metals may be used, or a material capable of occluding and releasing such light metals or ions thereof. May be used.

Among the constituent materials of such a negative electrode, for example, materials that can occlude and release lithium or its ions include, for example, (1) graphites, fired organic polymer compounds (phenol resins, furan resins, etc.). Carbonized materials such as fired at an appropriate temperature and carbonized, cokes (pitch coke, needle coke, petroleum coke, etc.), carbon fiber, glassy carbons, pyrolytic carbons, activated carbon, etc .; (2) lithium For example, polymers such as polyacetylene and polypyrrole, which exhibit conductivity by absorbing ions, can be used. As the light metal alloy, for example, a lithium-aluminum alloy or the like can be used.

As a constituent material of the negative electrode, among these, it is preferable to use the carbonaceous material (1) from the viewpoint of improving charge / discharge characteristics and self-discharge characteristics.

In forming a negative electrode from such a material, a known binder or the like can be added.

The flame-retardant non-aqueous electrolyte secondary battery of the present invention contains the above-described flame-retardant non-aqueous electrolyte as an electrolyte. 62-90863, JP-A-8-306364, JP-A-63-32870, JP-A-6-60
No. 906 and “Battery Technology” [Vol. 6, p. 129 (1994)], etc., without impairing battery characteristics such as charge / discharge efficiency, energy density and output density. A flame-retardant nonaqueous electrolyte secondary battery having flame retardancy and having a long service life and excellent in practicality can be obtained. The shape, form, and the like of the flame-retardant nonaqueous electrolyte secondary battery of the present invention are not particularly limited,
It can be arbitrarily selected within the scope of the present invention, such as a cylindrical shape, a square shape, a coin shape, a card shape, and a large size.

[0038]

Next, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples. <Preparation of electrolyte> 1-methyl-3 as a quaternary salt (C)
-Ethyl imidazolium / 4-fluoroborate (MEI
B), 1-methyl-3-ethylimidazolium / 6-fluorophosphate (MEIP), 1-methyl-3-ethylimidazolium / bistrifluoromethanesulfonylimide salt (MEITFSI), 1-methyl-3-ethylimidazo Lium / perchlorate (MEIC) and 1-methyl-3
Using ethyl imidazolium trifluoromethanesulfonate (MEIMS) as lithium salt (A):
Lithium tetrafluoroborate (LiBF ₄ ), lithium _6- fluoroperphosphate (LiPF ₆ ), lithium bistrifluoromethanesulfonylimide (TFSILi), lithium perchlorate (LiClO ₄ ), and lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ) Using non-aqueous solvent (B)
As propylene carbonate (PC), diethyl carbonate (DEC) and trimethyl phosphate (TMP)
Were blended in the proportions shown in Table 1 to prepare Examples 1 to 3 of the present invention.
7 was prepared. As a comparative example, LiPF
An electrolyte comprising only ₆ and PC (Comparative Example 1), an electrolyte comprising 1,3-dimethylimidazolium / 4-fluoroborate (DMIB) having a symmetric chemical structure, LiBF ₄ and PC (Comparative Example 2), An electrolytic solution (Comparative Example 3) consisting only of LiBF4 and MEIB was prepared.

[0039]

[Table 1]

<Evaluation of Flame Retardancy> A manila paper sheet for separator having a width of 1.5 cm, a length of 30 cm and a thickness of 0.04 mm was immersed in a beaker containing an electrolytic solution for 5 minutes. After wiping the liquid dripping from the manila paper, the manila paper is stabbed onto the support needles spaced at 5 cm intervals and fixed horizontally. I ignite one end of the manila paper with a lighter in a windless state and wait for it to extinguish naturally. Its burning length (cm) and burning rate (cm / sec)
c) was measured three times, and the average value was determined. Table 2 shows the results.

<Evaluation of Withstanding Voltage and Electric Conductivity of Electrolyte> The withstand voltage and electric conductivity of the electrolytes of Examples 1 to 7 and Comparative Examples 1 to 3 were measured. The withstand voltage was measured by putting the above-mentioned electrolyte into a three-electrode withstand voltage measuring cell using glassy carbon for the working electrode, lithium metal for the reference electrode, and platinum for the counter electrode, and sweeping at 10 mV / sec with a potentiostat. The withstand voltage was defined as the upper limit voltage at which the oxidative decomposition current did not flow by 0.1 mA or more based on the potential of the metal. The electrical conductivity was measured using an AC impedance meter.
It was measured at 0 kHz at 25 ° C and -20 ° C. The results are shown in Table 2.

[0042]

[Table 2]

As is clear from Table 2, the electrolytes of Examples 1 to 7 of the present invention all exhibited excellent flame retardancy, high withstand voltage and high electrical conductivity. Among these, it can be seen that the systems using tetrafluoroborate anion, hexafluorophosphate anion and bistrifluoromethanesulfonylimide anion as electrolyte anions (Examples 1 to 3) are particularly excellent in flame retardancy. In addition, the electrolytes of Examples 1 to 7 using the imidazolium compound having an asymmetric structure had lower electric conductivity than the electrolytes of Comparative Examples 2 and 3 using the imidazolium compound having a symmetric structure. It can be seen that the degree characteristics are excellent. In addition, it can be seen that the electrolyte solutions of Examples 1 to 7 containing a non-aqueous solvent are excellent in electric conductivity especially at low temperatures, as compared with Comparative Example 3 containing no non-aqueous solvent.

<Preparation of Secondary Battery> A coin-type nonaqueous electrolyte lithium secondary battery as shown in FIG. 1 was prepared. In FIG. 1, 1 is graphite, 2 is a positive electrode active material molded body, 3 is a porous separator, 4 is a negative electrode can, 5 is a positive electrode can, and 6 is a gasket. The non-aqueous electrolyte lithium secondary battery shown in FIG. 1 was prepared in the following procedure. A positive electrode active material molded body 2 prepared by mixing LiCoO ₂ with acetylene black as a conductive agent and polyvinylidene fluoride powder as a binder and press-molding the mixture was put on a nickel net placed on the bottom surface of a stainless steel positive electrode can 5. Crimped. Next, after placing the polypropylene porous separator 3 on the molded body,
Was injected, and a gasket 6 was inserted. Thereafter, the stainless steel negative electrode can 4 having the graphite 1 adhered thereto was placed on the polypropylene porous separator 3, the opening end of the positive electrode can 5 was bent inward, and the sealing portion was glass hermetically sealed. The nonflammable nonaqueous electrolyte lithium secondary battery of Example 8 shown in FIG.

Example 8 except that the flame-retardant non-aqueous electrolyte having the composition of Example 2, Example 3, Example 6, or Example 7 was used.
By operating in the same manner as in the above, flame-retardant non-aqueous lithium secondary batteries of Examples 9 to 12 having the same configuration as in FIG. 1 were produced.

As a comparative example, the same operation as in Example 8 was carried out except that the electrolytic solution used in Comparative Examples 1 and 3 was used instead of the flame-retardant nonaqueous electrolytic solution. Non-aqueous electrolyte lithium secondary batteries of Comparative Examples 4 and 5 were produced.

<Evaluation of Battery Characteristics> The charge / discharge characteristics of the non-aqueous electrolyte lithium secondary battery prepared as described above were compared as follows. Set the upper limit voltage to 4.2V and set 1
The battery was charged at a constant current and a constant voltage of 10 mA at a current of 10 mA, then discharged at a low current of 1 mA to a final voltage of 3.0 V, and this charging and discharging was repeated as a predetermined number of cycles. FIG. 2 is a graph in which the charge / discharge efficiency at that time is plotted against the number of cycles. As shown in FIG. 2, Examples 8 to 12 exhibited better charge / discharge than Comparative Examples 4 and 5, indicating that they exhibited excellent charge / discharge characteristics.

[0048]

As described above, according to the present invention, in a non-aqueous electrolyte for a secondary battery comprising an electrolyte and a non-aqueous solvent,
Furthermore, by containing a quaternary salt of an asymmetric compound having one or more nitrogen atoms and having a conjugate structure as a flame retardant, it has excellent flame retardancy, and has high withstand voltage and electric conductivity characteristics at low temperatures. An excellent flame-retardant non-aqueous electrolyte can be provided. Further, according to the present invention, by using such a flame-retardant non-aqueous electrolyte, a flame-retardant non-aqueous electrolyte secondary battery having excellent charge / discharge characteristics can be provided. It is a great thing.

[Brief description of the drawings]

FIG. 1 is a half sectional view of a lithium secondary battery prepared in an example of the present invention.

FIG. 2 is a diagram showing a comparison of charge and discharge characteristics of lithium secondary batteries prepared using various non-aqueous electrolytes.

[Explanation of symbols]

 1 Graphite 2 Positive electrode active material molded body 3 Porous separator 4 Negative electrode can 5 Positive electrode can 6 Gasket ○ Number of cycles and measured charge / discharge efficiency of Example 8 △ Number of cycles and measured value of charge / discharge efficiency of Example 9 □ Example 10 Cycle number and charge / discharge efficiency measurement value ▲ cycle number and charge / discharge efficiency measurement value of Example 11 ■ Cycle number and charge / discharge efficiency measurement value of Example 12 × cycle number and charge / discharge efficiency measurement value of Comparative Example 4 ● Comparison Cycle number and charge / discharge efficiency measurement value of Example 5

Claims (11)

[Claims] 1. An electrolyte (A), a non-aqueous solvent (B), and a compound (c) having an asymmetric chemical structure having a nitrogen-containing conjugated structure.
A flame-retardant non-aqueous electrolyte comprising a quaternary salt (C). 2. The flame-retardant non-flammable compound according to claim 1, wherein the compound (c) is a compound (c1) having an N, N, N′-substituted amidine group represented by the following general formula (1). Water electrolyte. General formula (Wherein, R ¹ and R ² each represent a C 1-10 hydrocarbon group which may contain an amino group, a nitro group, a cyano group, a carbonyl group, or an ether group.
³ , R ⁴ , R ⁵ and R ⁶ each represent a hydrogen atom or a hydrocarbon group having 1 to 10 carbon atoms which may contain an amino group, a nitro group, a cyano group, a carbonyl group or an ether group. However, R ¹ , R ² , R ³ , R ⁴ , R ⁵ and R ⁶
Or two or more of may bond to each other to form a ring. ) 3. The flame-retardant non-aqueous electrolyte according to claim 1, wherein the quaternary salt (C) is a compound having an imidazolium cation represented by the following general formula (2). Embedded image (Wherein, R ⁷ and R ⁹ are an alkyl group having 1 to 4 carbon atoms,
Indicates a phenyl group or a benzyl group. R ⁸ is a hydrogen atom,
It represents an alkyl group having 1 to 4 carbon atoms, a phenyl group or a benzyl group. R ⁷ , R ⁸ and R ⁹ may be the same group or different. R ¹⁰ and R ^{11 each} represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. ) 4. The flame-retardant non-aqueous electrolyte according to claim 1, wherein the content of the quaternary salt (C) is 30 to 99% by weight. 5. The anion of the quaternary salt (C) is a tetrafluoroborate anion, a hexafluorophosphate anion, a sulfonylimide anion represented by the following general formula (3) or a general formula (4): The flame-retardant non-aqueous electrolyte according to any one of claims 1 to 4, which is a sulfonyl methide anion. Formula _{^{(R 12 SO 2) (R}} 13 SO 2) N - (3) ( wherein, R ¹² and R ¹³ are each an ether group one or two which may from 1 to 4 carbon atoms optionally containing . shows a perfluoroalkyl group) in formula _{^{(R 14 SO 2) (R}} 15 SO 2) (R 16 SO 2) C - (4) ( wherein, R ^14, R ¹⁵ and R ¹⁶ are each ether group Represents a perfluoroalkyl group having 1 to 4 carbon atoms which may contain 1 or 2 6. The electrolyte (A) is lithium tetrafluoroborate, lithium hexafluorophosphate, a lithium salt of a sulfonylimide represented by the following general formula (5), or a sulfonylmethyl group represented by the following general formula (6): The flame-retardant nonaqueous electrolyte according to any one of claims 1 to 5, which is a lithium salt of a metal. General formula (R ¹⁷ SO ₂ ) (R ¹⁸ SO ₂ ) NLi (5) (wherein, R ¹⁷ and R ¹⁸ each have 1 to 4 carbon atoms which may have one or two ether groups. Represents a fluoroalkyl group.) General formula (R ¹⁹ SO ₂ ) (R ²⁰ SO ₂ ) (R ²¹ SO ₂ ) CLi (6) (wherein, R ¹⁹ , R ²⁰ and R ²¹ each represent one ether group. Or a perfluoroalkyl group having 1 to 4 carbon atoms which may contain 2). 7. The flame-retardant non-aqueous electrolyte according to claim 1, wherein the non-aqueous solvent (B) is a cyclic carbonate and / or a chain carbonate and / or a phosphate. 8. A flame-retardant non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and the flame-retardant non-aqueous electrolyte according to claim 1. 9. A flame-retardant non-aqueous electrolyte lithium secondary battery comprising a positive electrode, a negative electrode and the flame-retardant non-aqueous electrolyte according to claim 1. 10. The flame-retardant non-aqueous electrolyte lithium secondary battery according to claim 8, wherein the active material of the negative electrode comprises lithium or lithium ion. 11. The active material of the positive electrode comprises a composite oxide of lithium and one or more transition metals.
The flame-retardant non-aqueous electrolyte lithium secondary battery according to the above.