JPH10270078A - Electrolyte for secondary lithium battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide electrolyte for a secondary lithium battery, for which organic acid lithium salt is used with high conductivity and excellent safety and reliability to restrict the corrosion of a positive electrode aluminum collector for good charging/discharging property. SOLUTION: For solute, an organic acid lithium salt as shown in the formula, (n) and (m) are each integers, 1 through 4} and at least one type of an inorganic acid lithium salt selected out of LiX (X is PF6 , AsF6 , SbF6 or BF4 ) groups are used at a mol or ratio of 95:5-60:40. Carbonic ester compound is contained as an organic solvent.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolyte for a lithium secondary battery, and more particularly to an improvement in the conductivity and electrochemical stability of an organic solvent electrolyte for a lithium secondary battery.

[0002]

2. Description of the Related Art Lithium or lithium is used as a negative electrode active material.
Using a lithium alloy or carbon material, etc., as the positive electrode active material
And a lithium transition metal composite oxide (LiCoO_Two, Li
NiO _Two, LiMn_TwoO_Four) Etc. are especially high
Attracting attention because of its energy density,
Research is being done. However, this type of battery power
The pressure is as high as 4V or more.
Lithium rechargeable batteries generate a lithium anode during charging and discharging.
Electrolysis using dendritic dendrites
The electrolyte reacts with solutes or organic solvents in the
It is difficult to obtain good charge and discharge characteristics because
Development of stable electrolyte with excellent safety and reliability is desired
I have.

In recent years, a lithium secondary battery using an organic electrolyte using LiPF ₆ as a solute has been proposed. In a lithium secondary battery using lithium metal as a negative electrode, a reaction with electrodeposited lithium or the like during charging is performed. Therefore, there is a problem that the charge / discharge efficiency of the lithium negative electrode is low due to high performance. Therefore, for the purpose of developing a higher performance battery, LiN (SO 2) is used as a solute having excellent electrochemical stability and thermal stability.
_An electrolyte using ₂ CF ₃ ) ₂ has been developed and proposed.

[0004] However, LiN (SO ₂
Electrolyte using an organic lithium salt such as CF ₃ ) ₂ corrodes aluminum or an aluminum alloy used as a positive electrode current collector, and fails to provide a practicable battery capacity or cycle characteristics and lacks reliability. there were. JP 5
JP-A-62690 describes the use of a specific lithium inorganic acid salt and a specific lithium organic acid salt as solutes. However, sufficient performance has not yet been obtained.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an electrolyte which is optimal for a lithium secondary battery system using lithium metal for a negative electrode, has high conductivity, suppresses corrosion of a positive electrode current collector, and performs charge / discharge. An object of the present invention is to provide an electrolyte for a lithium secondary battery having excellent characteristics, safety and reliability.

[0006]

According to the present invention, a lithium salt of an organic acid represented by the following general formula (1) is used as a solute.

[0007]

Embedded image

(Where n and m each represent an integer of 1 to 4) and LiX (where X is PF ₆ , AsF ₆ , S
Indicates bF ₆ or BF ₄ . )) At least one lithium inorganic acid salt selected from the group consisting of
0:40 in combination with an electrolyte solution for a lithium secondary battery, characterized by containing a carbonate compound as an organic solvent.

[0009]

The organic acid lithium salt represented by the general formula (1) (hereinafter abbreviated as lithium organic acid salt) is used as a solute.
And LiX (where X is PF ₆ , AsF ₆ , SbF ₆ ,
BF ₄ ) is used, at a specific ratio, at least one kind of lithium inorganic acid salt (hereinafter abbreviated as lithium inorganic acid salt) selected from the group of BF ₄ ), so that the oxidation potential of aluminum during normal charging is reduced. , And the positive electrode aluminum current collector hardly dissolves in the electrolytic solution, so that good charging is performed. In particular, lithium or a lithium alloy or a carbon material is used as a negative electrode active material, and a lithium transition metal composite oxide (LiCoO ₂ , LiNiO ₂ , LiMn ₂ O) is used as a positive electrode active material.
₄ ) In a battery having a voltage of 4 V or more using, for example,
An electrolyte having excellent conductivity, high charge / discharge characteristics, safety, and high reliability can be realized.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION

 Solute: as a lithium salt of an organic acid represented by the general formula (1)
Is specifically LiN (SO _TwoCF_Three)_Two, LiN (S
O_TwoC_TwoF_Five)_Two, LiN (SO_TwoC_ThreeF₇)_Two, Li
N (SO_TwoC_FourF₉)_Two, LiN (SO_TwoCF_Three) (S
O_TwoC_TwoF_Five), LiN (SO_TwoCF_Three) (SO_TwoC_Three
F₇), LiN (SO_TwoCF_Three) (SO_TwoC_FourF₉),
LiN (SO_TwoC_TwoF_Five) (SO_TwoC_ThreeF₇), LiN
(SO_TwoC_TwoF _Five) (SO_TwoC_FourF₉) And LiN (S
O_TwoC_ThreeF₇) (SO_TwoC_FourF₉) Are exemplified.

[0011] Lithium inorganic acid salts include LiP
Examples are F ₆ , LiAsF ₆ , LiSbF ₆ and LiBF ₄ . In the present invention, the molar ratio between the organic acid lithium salt and the inorganic acid lithium salt represented by the formula (1) is 9
The reason for the limitation to the mixed solute within the range of 5: 5 to 60:40 is that the oxidation potential of aluminum is higher than the positive electrode potential at the time of normal charging, and good lithium charge / discharge efficiency is obtained.

That is, when the value of the molar ratio between the lithium organic acid salt and the lithium inorganic acid salt exceeds 95/5, the oxidation potential of aluminum becomes insufficient due to too much lithium organic acid salt, and Due to the corrosion (dissolution) of aluminum in the current collector and the decrease in the conductivity of the electrolyte, a battery having a practically usable charge / discharge capacity cannot be obtained.
Further, when the molar ratio is less than 60/40, the amount of the lithium inorganic acid salt is too large, and the charge / discharge efficiency of the lithium anode decreases due to an increase in the reaction with the lithium metal anode.

The organic acid lithium salt is represented by the general formula (1)
In which n + m is 4 to 8 is preferred. By setting the content in this range, the oxidation potential of aluminum of the lithium organic acid salt can be made higher than the positive electrode potential during normal charging while suppressing the decrease in conductivity due to the increase in the molecular weight of the solute. The organic acid lithium salt and the inorganic acid lithium salt are dissolved in an organic solvent described below, and a total of 0.5 to 1.5 M (mol / liter) is used as a solute concentration in the electrolytic solution.

Organic solvent: As a solvent for the electrolytic solution, a carbonate compound conventionally proposed and used as an electrolytic solution for a lithium secondary battery is contained. In order to improve performance such as conductivity, other non-solvents are used. It is preferable to use a mixture with a water solvent. As the carbonate compound, ethylene carbonate,
Propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate is used, as other non-aqueous solvents, usually, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, esters such as γ-butyrolactone, A solvent selected from ethers such as 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran, or a mixed solvent of a plurality thereof is used. The mixing ratio (volume ratio) when the carbonate compound and another non-aqueous solvent are mixed is usually 1/9 to 9/1, preferably 3/7 to 7 /.
3.

[0015]

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

Examples 1 to 4 Using an equal volume mixed solvent of ethylene carbonate (EC) and 1,2-dimethoxyethane (DME) as an organic solvent,
LiN (SO ₂ CF ₃ ) as solute of lithium organic acid salt
₂ is LiPF ₆ as the solute of lithium inorganic acid salt.
The electrolytes were dissolved at the molar ratios shown in the table to prepare an electrolyte having a solute concentration of 1 mol / liter. The conductivity of the electrolytic solution, the charge / discharge efficiency of lithium, the oxidation potential of aluminum, and the charge / discharge capacity of a coin cell were measured by the following methods. The results are shown in Table 1.

(Measurement of Conductivity) The conductivity of the organic electrolyte was measured by the following method. The conductivity at 25 ° C. was measured using a conductivity meter CM-30S manufactured by Toa Denpa Kogyo KK and a conductivity cell CG-511B.

(Measurement of Lithium Charge / Discharge Efficiency) The measurement of lithium charge / discharge efficiency (Eff) was performed by placing an organic electrolyte in a three-electrode cell in a dry box under a dry air atmosphere (amount of electrolyte: 15 ml). ) And using a potentiostat / galvanostat (1287 manufactured by Solartron)
Nickel electrode for working electrode (effective electrode area: 0.44c
m ² ), lithium metal as the counter electrode (effective electrode area: 0.79
cm ² ), using lithium metal as the reference electrode, and calculating from the potential change of the working electrode accompanying the constant current discharge (current density: 0.5 mA / cm ² ) using the following equation. The discharge end point was +1.0 V vs Li / Li ⁺ . Eff = Qs / Qp × 100 (%) In the above equation, Qp represents the amount of electricity required to deposit lithium on the Ni working electrode, and Qs represents the amount of electricity required to dissolve the deposited lithium as lithium ions. Amount (discharge amount).

(Measurement of Aluminum Oxidation Potential) The aluminum oxidation potential was measured by placing an organic electrolytic solution in a three-electrode cell (amount of electrolytic solution: 15 ml) in a dry box under a dry argon atmosphere. Using a galvanostat (1287 manufactured by Solartron), an aluminum electrode (electrode area: 7.0 mm ² ) was used as a working electrode, and lithium metal was used as a counter electrode and a reference electrode. The oxidation potential of aluminum is such that the reaction current density corresponding to the oxidation reaction when scanning the potential at 50 mV / sec is 2.
The potential was defined as the lithium reference potential when the voltage reached 0 μA / cm ² .

(Measurement of charge / discharge capacity by coin cell)
FIG. 1 is a cross-sectional view of the lithium secondary batteries (coin type; diameter: 20 mm, thickness: 1.6 mm) manufactured in Examples and Comparative Examples. In this coin-type cell, a stainless steel case 1 also serving as a positive electrode terminal and a stainless steel sealing plate 2 also serving as a negative electrode terminal are insulated and sealed with a polypropylene gasket 3. The positive electrode 4 is obtained by mixing lithium cobalt composite oxide (LiCoO ₂ ) as a positive electrode active material, acetylene black as a conductive agent, and a fluororesin as a binder in a weight ratio of 90: 5: 5, This was dispersed in a solvent (N-methylpyrrolidone) to form a slurry, which was then applied to an aluminum foil as a positive electrode current collector, dried, and then a positive electrode having a diameter of 12.5 mm was produced. The negative electrode 5 is made of a lithium metal foil having a diameter of 16 mm and a thickness of 0.5 mm, and includes a porous polypropylene film separator 6 immersed in an organic solvent electrolyte. The capacity of the battery is 1.7 mA in a voltage range from 4.2 V to 2.5 V.
h.

Comparative Example 1 As a solute, a lithium salt of an organic acid was LiN (SO ₂ C
In the same manner as in Example 1 except that F ₃ ) ₂ was used alone, the conductivity of the electrolytic solution, the charge / discharge efficiency of lithium, the oxidation potential of aluminum, and the charge / discharge capacity of a coin cell were measured.
Table 1 shows the obtained results.

Comparative Example 2 In the same manner as in Example 1 except that LiPF ₆ was used alone as the inorganic acid lithium salt as the solute, the conductivity of the electrolytic solution, the charge and discharge efficiency of lithium, the oxidation potential of aluminum, and the The charge / discharge capacity was measured. The obtained result is
It is shown in the table.

Examples 5 to 7 Ethylene carbonate (EC) and 1,2-dimethyl
Using an equal volume mixed solvent with toxicethane (DME),
The solute of the organic acid lithium salt is LiN (SO_TwoCF _Three) (S
O_TwoC_FourF₉) And the solute of lithium inorganic acid salt is LiPF₆
At a molar ratio of the mixed solute with 95: 5 to 70:30
Mix and dissolve, solute concentration is 1mol / dm^ThreeOrganic electricity
Except for preparing the lysate, the procedure of Example 1 was repeated to prepare the electrolyte solution.
Power, lithium charge / discharge efficiency, aluminum oxidation potential
And the charge / discharge capacity of the coin cell was measured. Got
The results are shown in Table 2.

Examples 8 to 10 The solute of the lithium organic acid salt is LiN (SO ₂ C ₂ F ₅ ) ₂
In the same manner as in Example 1 except that the mixed solute of LiPF ₆ and the solute of the lithium salt of inorganic acid and LiPF ₆ were used in the same manner as in Example 1, the conductivity of the electrolytic solution, the charge and discharge efficiency of lithium,
The oxidation potential of aluminum and the charge / discharge capacity of the coin cell were measured. Table 2 shows the obtained results.

Example 11 The solute of a lithium salt of an organic acid was LiN (CF ₃ SO ₂ ) (C
₄ F ₉ SO ₂ ) and the lithium salt of inorganic acid are solutes of LiBF ₄
The conductivity of the electrolytic solution, the charging and discharging efficiency of lithium, the oxidation potential of aluminum, and the charging and discharging capacity of a coin cell were measured in the same manner as in Example 1 except that the molar ratio of the mixed solute with the mixture was 60:40. Table 2 shows the obtained results.

Example 12 The solute of the lithium salt of an organic acid is LiN (CF ₃ SO ₂ ) (C
₄ F ₉ SO ₂₎ and solute lithium inorganic salt is LiSbF
The conductivity of the electrolytic solution, the charge and discharge efficiency of lithium, the oxidation potential of aluminum, and the charge and discharge capacity of the coin cell were measured in the same manner as in Example 1 except that the molar ratio of the mixed solute with ₆ was 60:40. . Table 2 shows the obtained results.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

The electrolytic solution for a lithium secondary battery of the present invention comprises:
It has excellent electrical conductivity, does not corrode (dissolve) aluminum in the positive electrode current collector during charging, and provides high lithium charge / discharge efficiency.
The present invention has excellent unique effects such as high reliability.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view schematically showing a coin-type cell used in Examples and Comparative Examples.

[Explanation of symbols]

 1: stainless steel case 2: stainless steel sealing plate 3: polypropylene gasket 4: positive electrode 5: negative electrode 6: separator

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Asato Kominato 8-3-1 Chuo, Ami-cho, Inashiki-gun, Ibaraki Prefecture Within the Tsukuba Research Laboratory, Mitsubishi Chemical Corporation (72) Kunihisa Shima, Chuo-Hachi, Ami-cho, Inashiki-gun, Ibaraki Mitsubishi Chemical Co., Tsukuba Research Laboratories (72) Shoichiro Mori 8-3-1 Chuo, Ami-cho, Inashiki-gun, Ibaraki Pref. Mitsubishi Chemical Corporation Tsukuba Research Laboratories

Claims (2)

[Claims] 1. A lithium salt of an organic acid represented by the following general formula (1) as a solute: (Where n and m each represent an integer of 1 to 4) and L
iX (where X is PF ₆ , AsF ₆ , SbF ₆ or B
Shows the F _4. A) using at least one lithium salt of an inorganic acid selected from the group of) in a molar ratio of 95: 5 to 60:40, and containing a carbonate compound as an organic solvent. liquid. 2. The electrolyte for a lithium secondary battery according to claim 1, wherein a lithium salt of an organic acid represented by the general formula (1) is used in which the sum of n and m is 4 to 8. .