JP2003092135A - Electrolyte for lithium secondary battery and lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrolyte for a lithium secondary battery using a room temperature type high-safety molten salt with high cycle characteristics, and having a noncombustible property, suitable for a negative electrode containing a carbon material capable of absorbing/releasing lithium. SOLUTION: This electrolyte for the lithium secondary battery using the combination of a positive electrode capable of absorbing/releasing lithium and a negative electrode containing the carbon material contains room temperature type molten salt comprising (1) either a quaternary imidazolium cation represented by general formula (I) or quaternary pyridinium cation represented by general formula (II) (in the formula, R1 , R3 independently represent a 1-6C alkyl, R2 , R4 , R5 independently represent a 1-6C alkyl, and R6 represents a 1-10C alkyl.) and a halogenated aluminum anion, and (2) a lithium salt, and (3) thionly chloride of 0.02 mol/l-0.15 mol/l.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to storage of lithium
TECHNICAL FIELD The present invention relates to a lithium secondary battery electrolytic solution suitable for a lithium secondary battery including a positive electrode capable of releasing and a negative electrode containing a carbonaceous material capable of absorbing and releasing lithium, and a lithium secondary battery using these electrolytic solutions. .

[0002]

2. Description of the Related Art In recent years, a lithium secondary battery using an organic solvent electrolyte has been widely used as a power source with high energy density. This organic solvent electrolyte is a solvent prepared by mixing a high dielectric constant solvent such as ethylene carbonate, propylene carbonate and butyrolactone with a low viscosity solvent such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate and 1,2-dimethoxyethane. A solution in which a lithium salt is dissolved is used.

However, such an organic solvent is a flammable solvent having high flammability, and there is a risk of flammable combustion when the electrolyte leaks due to a pressure increase in the battery or mechanical breakdown of the battery.

On the other hand, the aluminum halide type room temperature molten salt has no vapor pressure and has no flammability in a wide temperature range.
It is also known as an ionic liquid with a wide potential window and high conductivity (J. Electrochem. Soc., 141, L73 (199
Four)).

A technique has been proposed in which a lithium salt is dissolved in these room temperature molten salts and used as an electrolytic solution for a lithium secondary battery (JP-A-3-225775, JP-A-4-349365 and US Pat. Patent No. 5,552,2
No. 38), the use of imidazolium cations and pyridinium cations is mentioned as the cation component of the room temperature molten salt. However, when a carbonaceous material capable of inserting and extracting lithium is used for the negative electrode, an insertion / desorption reaction of imidazolium cation or pyridinium cation into the carbon occurs before lithium intercalation (insertion reaction), and On the other hand, it has been reported that the battery capacity cannot be obtained because of interfering with the intercalation of the above, and that the carbon layer peels off the negative electrode after repeated cycles (J. Appl. Elecrochem., .26, 1147 (199
6)), but there was a problem in practical application.

Further, in a lithium secondary battery using lithium metal as a negative electrode, an electrolyte solution for a lithium secondary battery, in which a lithium salt is dissolved in a room temperature molten salt and a specific amount of thionyl chloride is further added, is used. A technique has been proposed (Japanese Patent Laid-Open No. 2000-82493). This technology
An imidazolium cation or a pyridinium cation at a potential nobler than the lithium reduction potential inhibits the deposition and dissolution reaction of lithium metal at the electrode (J. Electro
chem. Soc., 140, 1606 (1993)) by adding a specific amount of thionyl chloride. It is considered that this is because thionyl chloride reacts with the lithium metal negative electrode to form a film at the interface, which suppresses the reduction reaction of the imidazolium cation and the pyridinium cation. However, when the same electrolytic solution was used in combination with a negative electrode containing a carbonaceous material as a negative electrode active material, the reductive decomposition reaction of thionyl chloride took place in the negative electrode, resulting in poor battery characteristics.

Here, as the negative electrode of the lithium secondary battery, lithium metal is a metal having a high melting point of 180 ° C. and high reaction activity, and is highly dangerous when an abnormality occurs in the lithium secondary battery, and also has cycle characteristics. However, it is known that the carbonaceous material has less such a risk and is excellent in cycle characteristics. Therefore, an electrolytic solution for a lithium secondary battery using a room temperature molten salt, which is used in combination with a negative electrode containing a carbonaceous material, has been expected.

[0008]

DISCLOSURE OF THE INVENTION An object of the present invention is to provide a positive electrode capable of occluding / releasing lithium, and occluding / occluding lithium.
An object of the present invention is to provide a nonflammable electrolytic solution that is most suitable for a lithium secondary battery provided with a negative electrode containing a carbonaceous material that can be released. Another object of the present invention is to provide a lithium secondary battery using these electrolytes, which can improve safety and reliability and which is excellent in discharge capacity and cycle characteristics.

[0009]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies and, by adding a specific amount of thionyl chloride to a room temperature molten salt composed of a specific combination, to absorb lithium
It was found that a nonflammable electrolytic solution suitable for a negative electrode containing a carbonaceous material that can be released can be obtained.

That is, the present invention provides an electrolyte solution for a lithium secondary battery, which is used in combination with a positive electrode capable of inserting and extracting lithium and a negative electrode containing a carbonaceous material, wherein the electrolyte solution for a lithium secondary battery is ( 1) General formula (I):

[0011]

[Chemical 3]

(In the formula, R ₁ and R ₃ are each independently an alkyl group having 1 to 6 carbon atoms, and R ₂ is a hydrogen atom or an alkyl group having 1 to 6 carbon atoms). Quaternized imidazolium cation or general formula (II):

[0013]

[Chemical 4]

(In the formula, R ₄ and R ₅ are each independently a hydrogen atom or an alkyl group having 1 to 6 carbon atoms;
₆ is an alkyl group having 1 to 10 carbon atoms), a room temperature molten salt comprising any one of quaternized pyridinium cations represented by the formula (1), an aluminum halide anion, (2) a lithium salt, and (3) 0.02 mol / l to 0.15 based on the room temperature molten salt in which the lithium salt is dissolved
An electrolyte solution for a lithium secondary battery, characterized by containing mol / l of thionyl chloride.

Further, the present invention is a lithium secondary battery comprising a positive electrode capable of inserting and extracting lithium and a negative electrode containing a carbonaceous material, and the above-mentioned electrolytic solution for lithium secondary battery.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. Cation of the room temperature molten salt used in the electrolytic solution of the present invention,
General formula (I):

[0017]

[Chemical 5]

(In the formula, R ₁ and R ₃ are each independently an alkyl group having 1 to 6 carbon atoms, and R ₂ is a hydrogen atom or an alkyl group having 1 to 6 carbon atoms). Quaternized imidazolium cation or general formula (II):

[0019]

[Chemical 6]

(In the formula, R ₄ and R ₅ are each independently a hydrogen atom or an alkyl group having 1 to 6 carbon atoms;
₆ is an alkyl group having 1 to 10 carbon atoms) is a quaternized pyridinium cation.

In the formula, when R ₁ , R ₂ , R ₃ , R ₄ and R ₅ are alkyl groups, they may be linear or branched, specifically, methyl, ethyl group, i- Propyl group, t-butyl group, n-hexyl group and the like can be mentioned. Similarly for R ₆ ,
It may be linear or branched. R _{1 to} R ₃ or R _{4 to} R ₆
May be the same or different alkyl groups.

The quaternized imidazolium cation represented by the general formula (I) has a total carbon number of R ₁ , R ₂ and R ₃ of 2 to 8.
Are preferred, and specifically, for example, 1,3-dimethylimidazolium, 1-ethyl-3-methylimidazolium, 1-n-butyl-3-methylimidazolium,
1,2,3-Trimethylimidazolium, 1,2-dimethyl-3-ethylimidazolium, 1-butyl-2,3
-Cation such as dimethylimidazolium.
The quaternized pyridinium cation represented by the general formula (II) preferably has a total carbon number of R ₄ , R ₅ and R ₆ of 2 to 10, and specific examples thereof include N-ethylpyridinium and N. -N-butylpyridinium, Ni-butylpyridinium, Nn-propylpyridinium, 1-ethyl-2-methylpyridinium, 1-n-hexyl-2-methylpyridinium, 1-n-butyl-4-methylpyridinium , 1-n-butyl-2,4-dimethylpyridinium and the like.

Specific examples of the aluminum halide anion that forms a room temperature molten salt by combining with these cations include, for example, tetrachloroaluminate ion (AlCl ₄ ⁻ ) and tetrabromoaluminate ion (A
LBR ₄ ^-), and the like. The tetrachloroaluminate ion is preferred.

In the present invention, LiCl, LiBr, LiAlCl ₄ , LiAlBr ₄ or the like can be used as the lithium salt dissolved in the room temperature molten salt. The lithium salt is saturated with respect to the room temperature molten salt, and
The concentration is preferably used in an amount of 10 mol% to 33 mol% in the room temperature molten salt. This is because when the lithium salt is saturated, the composition of the electrolytic solution is stable, and when the concentration is within the above range, an appropriate battery voltage can be obtained.

The thionyl chloride used in the electrolytic solution of the present invention is based on a room temperature molten salt in which a lithium salt is dissolved.
It is compounded in the range of 0.02 mol / l to 0.15 mol / l.
When the blending amount is in this range, the reduction reaction of the imidazolium cation or the pyridinium cation is suppressed, the battery capacity is obtained, and the deterioration of the battery characteristics due to the oxidation-reduction reaction of excess thionyl chloride can be avoided.
The content of thionyl chloride is preferably 0.05 mol / l
0.15 mol / l, more preferably 0.08 mol / l-0.
It is 12 mol / l.

The lithium secondary battery of the present invention is constructed by combining the above electrolyte solution with a negative electrode and a positive electrode.

The negative electrode constituting the battery contains, as a negative electrode active material, a carbonaceous material capable of inserting and extracting lithium. Specific examples of the carbonaceous material include graphite, non-graphitizable carbon, and amorphous carbon. These may be used alone or in combination of two or more.

The method for producing a negative electrode using these negative electrode active materials is not particularly limited, and, for example, a binder, a thickener, a conductive material, a solvent, etc. may be added to the negative electrode active material, if necessary. It can be made into a slurry and applied to the substrate of the current collector and dried to obtain a sheet electrode or a press-formed pellet electrode.

The positive electrode constituting the battery can use a material capable of occluding and releasing lithium as a positive electrode active material, and a lithium transition metal composite such as lithium cobalt oxide, lithium nickel oxide and lithium manganese oxide. Oxides are preferred.

The method for producing the positive electrode is not particularly limited, and it can be produced according to the above-mentioned method for producing the negative electrode.

The shape of the battery of the present invention is not particularly limited,
For example, a cylinder type battery or a coin type battery can be used. A cylinder type battery can be manufactured by spirally forming the above-mentioned negative electrode and positive electrode as sheet electrodes and a porous separator made of a material such as polyolefin, injecting the above-mentioned electrolytic solution, and then sealing.

[0032]

EXAMPLES The present invention will now be described in more detail with reference to examples, but the present invention is not limited to the following examples as long as the gist thereof is not exceeded.

[Preparation of Electrolyte Solution] Aluminum chloride (Al
Cl ₃ ) 60 mol% and 1-ethyl-3-methylimidazolium chloride (EMIC) 40 mol% were mixed and melted, and then LiCl 20 mol% was added and dissolved. The reaction formula is shown in formula [1].

[0034]

[Equation 1]

Further, thionyl chloride (SOCl ₂ ) was added at 0.05 mol / l to prepare an electrolytic solution 1.

Thionyl chloride (SOCl ₂ ) 0.1 mol /
Electrolyte solution 2 was prepared in the same manner as electrolyte solution 1 except that the amount was changed to 1.

Electrolysis was performed in the same manner as in Electrolyte 1 except that Nn-butylpyridinium chloride (BPC) was used instead of EMIC, and that thionyl chloride (SOCl ₂ ) was 0.1 mol / l. Liquid 3 was prepared.

0.15 mol of thionyl chloride (SOCl ₂ )
Electrolyte solution 4 is the same as electrolyte solution 1 except that
Was prepared.

Comparative electrolytic solution 1 was prepared in the same manner as electrolytic solution 1 except that thionyl chloride (SOCl ₂ ) was not added. In addition, thionyl chloride (SOC
Comparative electrolytic solution 2 was prepared in the same manner as electrolytic solution 1 except that the l ₂ ) was changed to 0.6 mol / l.

[Cyclic voltammetry (CV)
Evaluation] Cyclic voltammetry (hereinafter referred to as CV) was measured with the combination of the electrolytic solution and the working electrode in Table 1. FIG. 1 shows the CV results, and Table 1 shows the CV efficiency. In addition,
The current density on the vertical axis in FIG. 1 is a relative display, but both are within the range of −10 mA · cm ^{−2 to} 10 mA · cm ⁻² .

The CV was measured by using a working electrode, a reference electrode and a counter electrode at a running speed of 2 mV / s from 0 V to 3.5 V. The working electrode comprises a carbonaceous material and a binder in a weight ratio of 9 :.
1 was mixed and dispersed in N-methylpyrrolidone to form a slurry, which was applied to a tungsten foil and dried,
The electrode area was defined to be 1 cm × 1 cm square. Metal lithium was used for the reference electrode and the counter electrode. Furthermore, from the obtained CV results, the potential range for occlusion / release of lithium (0V-1.0V for graphite electrode, 0V-1.5V for amorphous carbon electrode)
CV to obtain the oxidation reaction charge / reduction reaction charge in
It was efficient. All of the above evaluations were performed under an argon atmosphere.

[0042]

[Table 1]

[Evaluation of Single-Pole Charge / Discharge Cycle] A combination of the electrolytic solution and the working electrode shown in Table 2 was used to evaluate the negative electrode and single pole (Examples 5 to 5).
6, Comparative Example 2) and positive electrode single electrode evaluation (Examples 7 to 8, Comparative Example 3) were performed. The evaluation was performed based on the discharge capacity and charge / discharge efficiency (discharge capacity / charge capacity) at the 10th cycle. Also,
Changes in discharge capacity with respect to cycles are shown in FIGS. 2 and 3 for Example 6 and Comparative Example 2, and Example 7 and Comparative Example 3, respectively.

In the negative electrode single electrode evaluation, a graphite electrode prepared in the same manner as the working electrode used in the CV evaluation, metallic lithium was used for the counter electrode, and the current density was 0.1 mA · cm ⁻² (0.2 C), 0.
Charge and discharge was performed at ~ 2V. In the positive electrode single electrode evaluation, lithium cobalt oxide, a conductive agent, and a binder were used in a weight ratio of 85: 1.
The mixture was mixed at 0: 5, and this was dispersed in N-methylpyrrolidone to form a slurry, which was applied to a tungsten foil and dried to prepare an electrode with an electrode area of 1 cm × 1 cm square. Metal lithium is used for the counter electrode, and current density is 0.1mA · cm.
Charging / discharging was performed at ^-2 (0.2C) and 4.1 to 3.5V.
All of the above evaluations were performed under an argon atmosphere.

[0045]

[Table 2] * Examples 5-6 and Comparative Example 2 are negative electrode single pole evaluations, Examples 7-
8, Comparative Example 3 is positive electrode single pole evaluation

[0046]

As shown in FIG. 1 and Table 1, in Examples 1 to 3 using the electrolytic solution within the scope of the present invention, the graphite electrode 0 to
0.6 V vs. Storage / release of lithium was observed in Li / Li +. In Example 4 using the amorphous carbon electrode,
0-1.5V vs. Storage / release of lithium was observed in Li / Li +. Moreover, the CV efficiency was good in all the examples. Further, from FIGS. 2 to 3 and Table 2, it was observed that in Examples 5 to 8 using the electrolytic solution within the scope of the present invention, good cycle characteristics were obtained for both the negative electrode and the positive electrode.

On the other hand, in Comparative Example 1 using Comparative Electrolyte 1 without addition of thionyl chloride, reductive decomposition reaction of EMI cation occurred, and lithium occlusion / desorption was not observed at all (FIG. 1). In Comparative Example 2 using the same Comparative Electrolyte 1, no discharge capacity was obtained at the negative electrode (FIG. 2, Table 2). Furthermore, in Comparative Example 3 using Comparative Electrolyte Solution 2 in which thionyl chloride was added in an amount exceeding the range of the present invention, the discharge capacity and charge / discharge efficiency of the positive electrode decreased (FIG. 3, Table 2). Therefore, the superiority of the present invention is clear.

The lithium secondary battery electrolytic solution of the present invention is suitable for use in combination with a negative electrode containing a carbonaceous material which is nonflammable and capable of inserting and extracting lithium.
It is possible to provide a lithium secondary battery having improved safety and reliability and excellent battery characteristics such as cycle characteristics.

[Brief description of drawings]

FIG. 1 is a CV diagram using a carbonaceous electrode as a working electrode in Examples 1 to 4 and Comparative Example 1.

FIG. 2 is a graph showing the relationship between the discharge capacity and the number of cycles in the graphite negative electrodes of Example 6 and Comparative Example 2.

FIG. 3 is a diagram showing the relationship between the discharge capacity and the number of cycles for the LiCoO ₂ positive electrodes of Example 7 and Comparative Example 3.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Futoshi Matsumoto             1-14 Yamazaki Kaizukacho, Noda City, Chiba Prefecture             Bird B-1 (72) Inventor Nobuyuki Koura             2-2-1-110 Satsukidaira, Misato City, Saitama Prefecture (72) Inventor Asano Kominato             3-3-1 Chuo 8-chome, Ami Town, Inashiki District, Ibaraki Prefecture             Within Mitsubishi Chemical Corporation (72) Inventor Eisuke Yasukawa             3-3-1 Chuo 8-chome, Ami Town, Inashiki District, Ibaraki Prefecture             Within Mitsubishi Chemical Corporation F term (reference) 5H029 AJ03 AJ05 AJ12 AK03 AL06                       AL07 AL18 AM02 AM07 AM09                       CJ08 HJ02 HJ10

Claims (5)

[Claims] 1. An electrolytic solution for a lithium secondary battery, which is used in combination with a positive electrode capable of inserting and extracting lithium and a negative electrode containing a carbonaceous material, wherein the electrolytic solution for a lithium secondary battery is (1) general. Formula (I): (In the formula, R ₁ and R ₃ are each independently a carbon number of 1 to
Is an alkyl group of 6 and R ₂ is a hydrogen atom or 1 to 1 carbon atoms
A quaternized imidazolium cation represented by the general formula (II): (In the formula, R ₄ and R ₅ are each independently a hydrogen atom or an alkyl group having 1 to ₆ carbon atoms, and R ₆ is 1 to 1 carbon atoms.
A quaternized pyridinium cation represented by 10), a room temperature molten salt composed of an aluminum halide anion, (2) a lithium salt, and (3) a dissolved lithium salt. An electrolyte solution for a lithium secondary battery, comprising 0.02 mol / l to 0.15 mol / l of thionyl chloride based on the room temperature molten salt. 2. The thionyl chloride is from 0.08 mol / l to
The electrolytic solution for a lithium secondary battery according to claim 1, which is 0.12 mol / l. 3. The electrolyte solution for a lithium secondary battery according to claim 1, wherein the cation of the room temperature molten salt is a 1-ethyl-3-methylimidazolium cation or an Nn-butylpyridinium cation. 4. The electrolytic solution for a lithium secondary battery according to claim 1, wherein the aluminum halide anion of the room temperature molten salt is tetrachloroaluminate ion. 5. A lithium secondary battery comprising a positive electrode capable of inserting and extracting lithium and a negative electrode containing a carbonaceous material, and the electrolytic solution for a lithium secondary battery according to any one of claims 1 to 4.