JP2000113906A - Organic electrolyte - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve dielectric constant, lower freezing point, increase boiling point, improve ion conductivity, by including at least one kind of nitril compounds as solvent of electrolyte salt. SOLUTION: Nitril compound contained in solvent of electrolyte salt is represented by a formula R1COO-(CH2)a-CN. Where, R1 is hydrogen atom, alkyl group where number of carbons is 1-3, alkoxy group where number of carbons is 1-3, a is an integer of 1-3, preferably 2. As electrolyte salt, preferably, lithium salt is used for a lithium secondary battery, and fourth grade ammonium salt or fourth grade phosphonium salt is used for an electric double layer capacitor or an electrolyte capacitor. Organic electrolyte is constituted by adding the electrolyte salt into this nitril compound, and contents of the electrolyte salt is usually selected in a range of about 0.1-2 mol/dm3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic electrolytic solution, more specifically, to a lithium battery, a lithium ion secondary battery, an electric double layer capacitor, an electrolytic capacitor and the like using a specific nitrile compound as a solvent for an electrolyte salt. It relates to a useful organic electrolyte.

[0002]

2. Description of the Related Art An organic electrolyte is composed of a system in which an electrolyte salt is dissolved in an organic solvent, and is generally composed of lithium batteries (primary and secondary batteries), lithium ion secondary batteries, It is used as a constituent material for double-layer capacitors, electric double-layer capacitors, electrolytic capacitors, and the like. By the way, as the requirement of the organic solvent, it is the most basic important matter that the electrolyte salt is dissolved well and the dielectric constant is high in order to increase the degree of ionic dissociation of the electrolyte salt,
On top of that, the viscosity is as low as possible to increase ionic conductivity, the solidification point is low to ensure ionic conductivity at low temperatures, and the high boiling point to lower battery internal workability at high temperature and workability in battery manufacturing Is required. When used for lithium batteries and lithium ion secondary batteries,
It is required to be non-reactive (aprotic) with lithium and to withstand severe oxidizing and reducing environments at the electrode interface due to a high battery voltage.

However, in practice, the above-mentioned required properties often conflict with each other in terms of molecular structure. For example, if a high dielectric constant is to be obtained, the molecule must be a highly polar molecule, resulting in a high viscosity. Easily. In the current lithium batteries and lithium ion secondary batteries, ethylene carbonate (abbreviation: EC, hereinafter similarly referred to) is used as a solvent having a high dielectric constant (hereinafter, referred to as a main solvent).
And only propylene carbonate (PC) are used, and there are the following problems.

Since EC is a solid (freezing point: 37 ° C.) at normal temperature or lower, PC (freezing point: −49 ° C.) or another aprotic solvent having a low freezing point (hereinafter, a solvent excluding PC is referred to as an auxiliary solvent) ) Cannot be used unless the freezing point is lowered. Dimethyl carbonate as co-solvent
(Freezing point: 3 ° C, abbreviation: DMC, hereinafter similarly indicated), diethyl carbonate (freezing point: -43 ° C, DEC), 1,2-dimethoxyethane (freezing point: -58 ° C, DME) and the like are used, These co-solvents have low viscosity but low dielectric constant (DM
C: 3.1, DEC: 2.8, DME: 7.2) and low boiling point (DMC: 90 ° C., DEC: 127 ° C., DME 85)
° C). The freezing point of the electrolytic solution is required to be at most -10 ° C or lower, preferably -20 ° C or lower. For this lowering of the freezing point, it is designed taking into account the effect of lowering the freezing point by dissolving the electrolyte salt. The amount of PC or co-solvent to be used in combination should be at least about 30 to 40% by weight of the total solvent, and it is very difficult to reduce the amount to less than this. The concomitant use of a co-solvent reduces the dielectric constant, but at the same time does not necessarily lower the conductivity due to a decrease in viscosity (however, if the amount of co-solvent is too large, the conductivity will decrease).
At present, the boiling point (238 ° C.) has been greatly reduced. Therefore, a new main solvent having a low freezing point is desired.

On the other hand, unlike EC, PC has a low freezing point.
(-49 ° C.), without the problem of EC in the above,
When used in a lithium ion secondary battery, it can be decomposed by the graphite-based carbon material used for the negative electrode, and therefore can only be used for a battery using an amorphous carbon material for the negative electrode. However, batteries using an amorphous carbon material have a large voltage drop that occurs with discharge (use),
Recently, batteries using a graphite-based material with a small voltage drop due to this discharge (use) have become mainstream.
However, at present, if this amorphous carbon material is the main solvent to which it can be applied, EC at room temperature is the only EC.

[0006]

Means for Solving the Problems The present inventors have proposed an electrolytic solution having various conditions, such as a high freezing point, a high boiling point, and excellent ionic conductivity, in addition to a high dielectric constant which is a characteristic required as a main solvent. After diligent research on solvents, lower aliphatic carboxylic acid ester-based or carbonate ester-based nitrile compounds completely eliminated the above-mentioned drawbacks of EC and PC.
The present inventors have found that they can be used as intended new main solvents or as cosolvents, and have completed the present invention.

That is, the present invention provides a solvent for an electrolyte salt represented by the formula: R ₁ -COO- (CH ₂ ) a-CN [I] (wherein R ₁ is a hydrogen atom, an alkyl group having 1 to 3 carbon atoms) Or an alkoxy group having 1 to 3 carbon atoms; and a is an integer of 1 to 3).
An organic electrolyte solution comprising a seed is provided.

The above nitrile compound [I] includes the following two groups:
They can be roughly classified into a) and b), and can be produced according to the following procedures. a) Lower aliphatic carboxylic acid ester formula: R ₁ '-COOH [II] (wherein R ₁ ' is a hydrogen atom or an alkyl group having 1 to 3 carbon atoms) (i.e., Formic acid, acetic acid, propionic acid, n-butyric acid, isobutyric acid) [II] or an acid anhydride, acid chloride or ester thereof (methyl formate,
Methyl acetate, and methyl propionate), _{formula: HO- (CH 2) a-} CN [III] ( cyano alcohol in formula, a is as defined above) (2
-Cyanoethanol, 3-cyanopropanol, etc.) [I
II] to give a compound of the formula: R ₁ '-COO- (CH ₂ ) a-CN [Ia] (wherein R ₁ ' and a are as defined above) Thus, a nitrile compound [Ia] represented by the following formula, ie, cyanomethyl, 2-cyanoethyl, 3-cyanopropyl ester of formic acid, acetic acid, propionic acid, and n-butyric acid is obtained.

B) Carbonic acid ester system An alkyl chloroformate of the formula COCOOR, where R is methyl, ethyl or n-propyl or isopropyl, is reacted with the above cyano alcohol [III] to give the formula: RO A nitrile compound [Ib] of —COO— (CH ₂ ) a-CN [Ib] (wherein R and a are as defined above), that is, methyl carbonate, ethyl carbonate, n-carbonate
An asymmetric carbonate nitrile compound of propyl, isopropyl cyanomethyl carbonate, 2-cyanoethyl, and 3-cyanopropyl is obtained. Alternatively, the formula:

Embedded image (Wherein R is as defined above)
V] is a lower aliphatic group of the above cyano alcohol [III] or a formula: R ₁ '-COO- (CH ₂ ) a-CN [V] (wherein R ₁ ' and a are as defined above) The nitrile compound [Ib] is obtained by transesterification with a cyanoalkyl carboxylate [V]. In addition,
This transesterification method is superior as an actual industrial production method, but the raw material dialkyl carbonate [IV], cyano alcohol [III] or lower aliphatic carboxylate cyanoalkyl [V] remains as an unreacted product. In addition, since symmetric bis (cyanoalkyl) carbonate is by-produced as a mixture, fractional purification is required.

[0010] The nitrile compound thus produced
In [I], when the number of carbon atoms or a of R ₁ (alkyl group or alkoxy group) becomes 4 or more, the content of the nitrile group (CN) decreases, and the dielectric constant required as the main solvent decreases. . Further, in the case of a = 1 to 3, a = 2 is practically advantageous in terms of production and cost such as raw material acquisition and synthesis conditions. Furthermore, symmetric biscarbonate by-produced by the alternative method of b) above
(Cyanoalkyl), that is, bis (cyanomethyl) carbonate, bis (2-cyanoethyl) carbonate, and bis (3-cyanopropyl) carbonate have a high dielectric constant because they contain two nitrile groups in the molecule. Has a remarkably high viscosity,
On the contrary, it is out of the scope of the present invention because the ionic conductivity is lowered.

Examples of the electrolyte salt include lithium salts such as perchloric acid, tetrafluoroboric acid, hexafluoroarsenic acid, hexafluorophosphoric acid, and trifluoromethanesulfonic acid; and tetraalkyl quaternary ammonium salts (tetramethylammonium, Tetraethylammonium,
Tetraalkyl quaternary phosphonium salts (tetramethylphosphonium, tetraethylphosphonium, tetra-n-propylphosphonium, etc.) and the like are preferable. Among them, lithium or lithium ion secondary batteries are preferred. A lithium salt is used, and a quaternary ammonium salt or a quaternary phosphonium salt is used for an electric double layer capacitor or an electrolytic capacitor.

[0012]

DETAILED DESCRIPTION OF THE INVENTION The organic electrolyte according to the present invention comprises a system in which an electrolyte salt is blended with at least one of the nitrile compounds [I] as described above, wherein the content of the electrolyte salt is usually The concentration may be selected to be in the range of about 0.1 to 2 mol / dm ³ . The nitrile compound [I] used
The use of the obtained organic electrolyte is as follows depending on the type of Cyanoalkyl formate and cyanoalkyl acetate [Ia] are easily decomposed by reaction with a lithium salt or charge / discharge, and a lithium battery and lithium ion 2
It is not suitable for secondary batteries (however, other applications: no problem with electric double layer capacitors, electrolytic capacitors, etc.). On the other hand, in the case of the nitrile compound [I] other than the above two types, a lithium battery and a lithium ion secondary battery can be used without any problem. It has been found that 2-cyanoethyl and ethyl 2-cyanoethyl carbonate [Ib] have preferable characteristics particularly for application to lithium batteries and lithium ion secondary batteries.

In the organic electrolyte of the present invention, in addition to the nitrile compound [I], other polar solvents (main solvent and co-solvent) conventionally used, for example, a lithium battery,
For lithium ion secondary batteries, aprotic solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, 1,2-dimethoxyethane, γ-butyrolactone, γ-valerolactone, dimethyl sulfoxide, sulfolane, tetrahydrofuran, Methyl tetrahydrofuran, etc .; for electric double layer capacitors and electrolytic capacitors, in addition to these aprotic solvents, N-dimethylformamide, N-dimethylacetamide, ethylene glycol and the like can be used in combination, and the formula: (R ₂ O) bR ₃ (OCH ₂ CH ₂ CN) c wherein R ₂ is an alkyl group having 1 to 4 carbon atoms; R ₃ is a residue obtained by removing all hydroxyl groups from a compound having 1 to 4 hydroxyl groups; b is 0 And c is 1-4 (however, b + c is 1-4)], for example, methyl 2-cyanoethyl Ether, ethyl 2-cyano-ethyl ether,
n-propyl-2-cyanoethyl ether, 2-methoxyethyl-2-cyanoethyl ether, ethylene glycol bis (2-cyanoethyl) ether, etc .; or bis
A cyanoethyl ether-based solvent (high dielectric constant solvent) such as (2-cyanoethyl) ether [O (CH ₂ CH ₂ CN) ₂ ] can also be used in combination (these cyanoethyl ether-based high dielectric constant solvents are disclosed in No. 116514).
In this case, the amount of the other polar solvent and / or the cyanoethyl ether-based high dielectric constant solvent may be selected in the range of 5 to 95% by weight based on the total amount of the electrolyte solvent.

[0014]

Next, the present invention will be described more specifically with reference to production examples and examples. Production Example 1 (Production of formic acid 2-cyanoethyl ester) In a four-necked flask equipped with a water separation tube and a reflux condenser, 276.2 g (6 mol) of formic acid and 355.2-cyanoethanol were added.
5 g (5 mol), cyclohexane 300 g and concentrated sulfuric acid 6 g
And conducting an esterification reaction for 8 hours while azeotropically removing water generated under reflux. Since a large amount of water is generated in the initial stage of the reaction, the reaction proceeds at a temperature of about 65 to 70 ° C., and after a lapse of 2 to 3 hours, while flowing dry nitrogen gas,
Further reaction was performed (the temperature was raised to about 85 to 90 ° C.).
After the reaction, the reaction mixture is cooled, 500 ml of pure water and 300 ml of toluene are added, neutralized with an aqueous sodium hydroxide solution and allowed to stand. When the mixture is separated into two layers, the lower layer (aqueous layer) is discarded, leaving the upper layer. After washing with 500 ml of pure water four times and distilling off cyclohexane and toluene under reduced pressure, purification is performed by distillation under reduced pressure to obtain the desired product. The target substance is a colorless and transparent low-viscosity liquid having a dielectric constant of 35.8, a boiling point of 207 ° C., and a freezing point of −
44 ° C.

Preparation Example 2 (2-cyanoethyl acetate) Preparation Example 3 (2-cyanoethyl propionate) In Preparation Example 1, the esterification reaction was carried out under the same conditions except that acetic acid or propionic acid was used instead of formic acid. And obtain the respective objectives. The properties and characteristic values of each object are as follows. Production Example 2 Production Example 3 Properties Colorless transparent low-viscosity liquid Colorless transparent low-viscosity liquid Dielectric constant 18.7 16.7 Boiling point 210 ° C. 217 ° C. Freezing point −32 ° C. −16 ° C.

Production Example 4 (Methyl 2-cyanoethyl carbonate) A molecular sieve 3 was previously placed in a four-necked flask.
A, 2-cyanoethanol 142.2 dehydrated
g (2 mol), 300 g of toluene and triethylamine 2
73 g (2.7 mol) were charged and, under vigorous stirring under ice-cooling (10 ° C. or lower), 236.3 g (2.7 g) of methyl chloroformate.
5 mol) is gradually added dropwise over 3 hours to react. After completion of the dropwise addition, the temperature was raised to 40 ° C., the reaction was continued for another 2 hours, 32 g (1 mol) of methanol was added, the mixture was stirred for another 2 hours, then cooled, 500 ml of pure water was added, and after stirring for about 5 minutes, Then, the upper layer is separated, washed four times with 500 ml of pure water, toluene is distilled off, and the residue is purified by distillation under reduced pressure to obtain the desired product. The target product was a colorless, transparent, low-viscosity liquid having a dielectric constant of 24.6, a boiling point of 85 ° C (20 mmHg), and a freezing point of -11 ° C.

Examples 1 to 4 Using the nitrile compounds obtained in Production Examples 1 to 4 as solvents, 1-molar solutions (electrolyte solutions) of lithium hexafluorophosphate (LiPF ₆ ) were prepared.
z, conductivity at 20 ° C. was measured. The results are as follows. Conductivity nitrile compound (Siemens / cm) 2-cyanoethyl formate 5.8 × 10 ^-3 2. 2-cyanoethyl acetate 3.1 × 10 ⁻³ 3. 3. 2-cyanoethyl propionate 2.5 × 10 ^-3 Methyl carbonate 2-cyanoethyl 8.1 × 10 ^-4

Continuation of the front page (72) Inventor Shinji Bessho 7-1 Akita-cho, Takatsuki-shi, Osaka Sunstar Giken Co., Ltd. F-term (reference) 4H006 AA03 AB91 5H024 AA01 DD17 EE09 FF14 FF18 FF19 5H029 AJ07 AL07 AL08 AM03 AM04 AM05 AM07 DJ09 HJ02

Claims (6)

[Claims] 1. A solvent for an electrolyte salt having the formula: R ₁ —COO— (CH ₂ ) a —CN (wherein, R ₁ is a hydrogen atom, an alkyl group having 1 to 3 carbon atoms, or an alkyl group having 1 to 3 carbon atoms) An alkoxy group; and a is an integer of 1 to 3). 2. The organic electrolyte according to claim 1, wherein a is 2 in the nitrile compound. 3. The nitrile compound according to claim 1, wherein R ₁ is a hydrogen atom, methoxy or ethoxy and a is 2. 4. The organic electrolyte according to claim 1, wherein the electrolyte salt is a lithium salt. 5. The organic electrolyte according to claim 1, wherein the electrolyte salt is a tetraalkyl quaternary ammonium salt and / or a tetraalkyl quaternary phosphonium salt. 6. The organic electrolyte according to claim 1, wherein another solvent is used in combination with the nitrile compound.