JP2981545B2 - Room temperature molten salt - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a room temperature molten salt.

[0002]

2. Description of the Related Art It has long been known that a room-temperature molten salt is produced by mixing 1-ethyl-3-methylimidazolium (EMI) chloride and AlCl ₃ , and the salt is relatively insoluble. Possibility as a battery electrolyte has been studied because it has a high conductivity, a wide potential window, and has unique characteristics such as nonflammability and non-volatility different from conventional electrolyte systems.

In recent years, instead of AlCl ₄ anions, fluorine-containing anion species (N (CF ₃ SO ₂ ) ₂ ⁻ , CF ₃ SO ₃ ⁻ , B
F ₄ ^-..) With more high water resistance, ease of ionic liquid handling have been proposed (Bonhote, P. et al, Inorg C
hem., 35, 1168 (1996)).

On the other hand, as the cation species of the room temperature molten salt,
Conventionally, an imidazolium salt such as EMI chloride has been used. However, this cation decomposes at a potential (about -2.2 V vs. I ⁻ / I ₃ ⁻ ) which is more noble than lithium, and has a withstand voltage. Since the voltage was about 4.5 V, there was a disadvantage that the stability as an electrolyte of the lithium secondary battery was poor.

An object of the present invention is to provide a room temperature molten salt that is stable in a wider potential range and a lithium secondary battery containing the room temperature molten salt.

[0006]

SUMMARY OF THE INVENTION The present invention provides the following room temperature molten salt and lithium secondary battery.

Item 1. Room temperature molten salt having a withstand voltage of 5.8 V or more.

Item 2. The cation component is represented by the following formula (1): NR ¹ R ² R ³ R ⁴ (1) wherein R ¹ , R ² and R ³ are the same or different and are a lower alkyl group, an aryl group, a heterocyclic group, an aralkyl group Is shown. Alternatively, R ¹ and R ² together form a cycloalkyl group.
R ⁴ represents an alkyl group. Item 6. The room temperature molten salt according to item 1, which is a quaternary ammonium represented by the formula:

Item 3. The anion component is N (CF ₃ S
O ₂ ) ₂ ⁻ , CF ₃ SO ₃ ⁻ , BF ₄ ⁻ , Al ₃ Cl ₈ ⁻ , Al ₂ C
l ₇ ^-, AlCl ₄ ^- ambient temperature molten salt according to any one of items selected from the group consisting of 1-2 ^- and PF _6.

Item 4. Item 4. A lithium secondary battery comprising the room-temperature molten salt according to any one of Items 1 to 3 as an electrolyte.

The term “room temperature molten salt having a withstand voltage of 5.8 V or more” in item 1 is, for example, “-3.4 V to 2.4 V v
s. Room-temperature molten salt stably existing in the I ⁻ / I ₃ ⁻ potential region ”.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION
A salt which exists as a liquid at 100 ° C, preferably 20 ° C to 80 ° C, more preferably 20 ° C to 60 ° C, still more preferably 20 ° C to 40 ° C, particularly 20 ° C, and comprises a cationic component and an anionic component.

[0013] Examples of the anionic _{component, N (CF 3 SO 2)} 2 -
(Hereinafter abbreviated as “TF ₂ N”), CF ₃ SO ₃ ⁻ , PF ₆ ⁻ and BF ₄ ⁻ .

The cation component includes the following formula (1): NR ¹ R ² R ³ R ⁴ (1) wherein R ¹ , R ² and R ³ are the same or different and are a lower alkyl group, an aryl group, a heterocyclic ring And an aralkyl group. R
⁴ represents an alkyl group. A quaternary ammonium represented by the formula:

Examples of the lower alkyl group include linear or branched such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, pentyl, hexyl, heptyl, octyl, nonyl and decyl. Examples thereof include a branched lower alkyl group having 1 to 10 carbon atoms.

Examples of the aryl group include phenyl, naphthyl, toluyl, and xylyl. The aryl group includes a halogen atom (F, Cl, Br, I), a hydroxyl group, and a lower alkoxy group (methoxy, ethoxy, propoxy, butoxy). ), 1 to 3 substituents such as a carboxyl group, an acetyl group, a propanoyl group, a thiol group, an alkylthio group (methylthio, ethylthio, propylthio, butylthio), an amino group, a lower alkylamino group, and a di-lower alkylamino group. May be.

Preferred cation components of the present invention include:
The following are exemplified: trimethylhexylammonium; trimethylphenylammonium; trimethylcyclohexylammonium; dimethylethylhexylammonium; trimethylbenzylammonium; trimethylvinylammonium; trimethyl (methoxycarbonylethyl) ammonium; trimethylethyl. Ammonium trimethyl (hydroxyethyl) ammoniumtriethylmethylammoniumdiethylmethylhexylammoniumtrimethyl (pentamethylphenyl) ammoniumtriethylbenzylammoniumN-methylbutylpiperidiniumN-dimethylpiperidinium Triethyl butyl ammonium; triethyl amyl ammonium; Butyl ammonium; - tributylphenyl ammonium; tri-butyl benzyl ammonium; and & trimethyl allyl ammonium.

Examples of the heterocyclic group include pyridyl, thienyl, imidazolyl, pyrazolyl, oxazolyl, isoxazoly, pyrrolidinyl, piperazinyl, morpholinyl and the like.

Examples of the aralkyl group include benzyl, phenethyl and the like.

When the electrochemical stability of the room temperature molten salt of the present invention is measured by cyclic voltammogram, it is -3.2.
V to 2.4 V vs. V. I ⁻ / I ₃ ⁻ (withstand voltage of 5.6 V or more), −3.3 V to 2.4 V vs. I ^- / I
₃ ^- (5.7 V or more voltage resistance), especially -3.4V～2.
4V vs. I ^- / I ₃ ^- is stable at a potential region of (or voltage resistance 5.8 V), high resistance to reduction, as a result of conventional voltage resistance compared with (such as EMI-Tf ₂ N) 4 From 5.5 V or more to 5.6 V or more, preferably 5.7 V
As mentioned above, it improved especially to 5.8V or more.

The room temperature molten salt of the present invention has a temperature of -3.2 Vv.
s. I ^- / I ₃ ^- the potential of the reducing side of the, preferably -3.
3V vs. I ^- / I ₃ ^- following the reduction potential, especially -3.4
V vs. Stable at a reduction potential of I ⁻ / I ₃ ⁻ or less.

The room-temperature molten salt of the present invention is more excellent in reduction resistance than known imidazolium and the like, and thus can be suitably used as an electrolyte of a lithium secondary battery. As the positive electrode, the negative electrode, the separator, and the like of the lithium secondary battery, conventionally known ones can be used as they are.

[0023]

According to the present invention, a room temperature molten salt having excellent reduction resistance can be obtained. The salt is more stable at a potential on the reduction side than the oxidation-reduction potential of lithium (−3.2 V vs. I ⁻ / I ₃ ⁻ ), and is suitable as an electrolyte for a lithium secondary battery.

Since the room temperature molten salt of the present invention is chemically stable and can easily remove water from the system, the lithium secondary battery containing the molten salt is excellent in performance, durability and stability. .

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in more detail with reference to embodiments.

[0026] Example ^{_{1 (1) Tf 2 N -}} · TMHA + ( indicating the Tf ₂ N is bistrifluoromethanesulfonimide _{((CF 3 SO 2) 2} N); TMHA shows trimethyl hexyl ammonium) Preparation of Equimolar amount of trimethylhexyl ammonium bromide
(TMHA ⁺ Br ^-) and a lithium salt (Li ⁺ Tf ₂ N ^-) distilled water, immediately insoluble ambient temperature molten salt in water by mixing stirring (TMHA ⁺ Tf ₂ N ^-) and soluble in water Li ⁺ Br
^- occurs. After the room temperature molten salt is separated by a separating funnel, it is washed several times with distilled water. After washing, vacuum-dried at 100 ° C. with a rotary evaporator to obtain a dehydrated pure room temperature molten salt. The specific gravity of the obtained molten salt is 1.28 g / ml, and the melting point is 25 ° C. The room-temperature molten salt has a feature of maintaining a liquid state due to a supercooled state at -20 ° C or lower at room temperature or less for at least several weeks.

[0027] (2) cyclic voltammogram measurements TMHA ⁺ of N (CF ₃ SO _{2) 2} ^- reduction limits the potential of the molten salt, iodine couple ^(I - / I ₃ ^-) reference electrode (1-ethyl-3- was determined potential for bistrifluoromethanesulfonimide molten salt (EMI-Tf ₂ N abbreviated) to 15mMI _2, 60mM that is immersed platinum wire obtained by dissolving a tetrapropyl ammonium iodide) - methylimidazolium .

FIG. 1 shows the results of the cyclic voltammogram.
Shown in

FIG. 1 also shows the results of cyclic voltammograms of EMI ⁺ Tf ₂ N ⁻ (EMI indicates 1-ethyl-3-methylimidazolium), which is a conventionally used room-temperature molten salt. Show.

As shown in FIG. 1, the room temperature molten salt of the present invention has a temperature of −3.4 V vs. I ^- / I ₃ ^- and stable in the reduction potential, having the above voltage resistance 5.8 V.

As shown in FIG. 2, since a redox peak of lithium is obtained, it is suitable as an electrolyte for a lithium secondary battery.

[Brief description of the drawings]

FIG. 1: TMHA-Tf ₂ N (solid line) and EMI-Tf ₂
The result of the cyclic voltammogram of N (broken line) is shown. The TMHA ⁺ Tf ₂ N ⁻ of the present invention is the same as the conventional EMI ⁺ Tf
₂ N ^- and compared, reduction resistance was found to have greatly improved.

FIG. 2 shows the results of a cyclic voltammogram of a solution obtained by adding 0.5 M LiTf ₂ N to TMHA-Tf ₂ N.
Since TMHA ⁺ Tf ₂ N ⁻ of the present invention is stably present even at a reduction potential of −3.4 V, a lithium peak was observed at −2.0 V or less.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-349365 (JP, A) JP-A-3-2388769 (JP, A) JP-A-62-165879 (JP, A) (58) Investigation Field (Int.Cl. ⁶ , DB name) H01M 10/36-10/40 C07C 211/63 JICST file (JOIS)

Claims (5)

(57) [Claims] 1. A room temperature molten salt comprising a cation component and an anion component, wherein the cation component is represented by the following formula (1): NR ¹ R ² R ³ R ⁴ (1) wherein R ¹ , R ² , R ³ is the same or different and represents a lower alkyl group, an aryl group, a heterocyclic group, or an aralkyl group. Alternatively, R ¹ and R ² together form a cycloalkyl group.
R ⁴ represents a lower alkyl group. A quaternary ammonium represented by the formula: wherein the anion components are N (CF ₃ SO ₂ ) ₂ ⁻ , CF
Room temperature molten salt selected from the group consisting of ₃ SO ₃ ⁻ , PF ₆ ⁻ and BF ₄ ⁻ . 2. The room temperature molten salt according to claim ¹ , wherein R ¹ , R ² and R ³ are the same or different and each represents a lower alkyl group. Wherein the anionic component is _{N (CF 3 SO 2) 2} - a normal temperature molten salt according to claim 1 or 2. Wherein the anionic component is _{N (CF 3 SO 2) 2} - and is,
The room temperature molten salt according to any one of claims 1 to 3, wherein the cation component is trimethylhexyl ammonium. 5. A lithium secondary battery comprising the room-temperature molten salt according to claim 1.