JP2001006733A - Electrolyte for lithium secondary battery and lithium secondary battery using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrolyte capable of improving charge and discharge characteristics of a secondary battery by dissolving specific lithium salt as a supporting electrolyte in a mixed solvent containing a specific ether compound, a specific cycloalkane compound having an oxygen atom in its cycle and carbonic ester as organic solvents. SOLUTION: In this electrolyte, organic acid lithium salt expressed by formula I (m and n are integers from 1 to 4 independent from each other) is dissolved in a mixed solvent containing 1-60 volume % of a cyclic ether compound having 1 oxygen atom in its circle, 1-60 volume % of cycloalkane or the like having two oxygen atoms in its circle and 10-60 volume % of carbonic ester, the cyclic ether compound is a tetrahydropyran compound expressed by formula II (R1-R5 are hydrogen atoms independent from each other or an alkyl group having 1-6 carbons) and the cycloalkane or the like is a dioxane compound expressed by formula III (R6-R9 is hydrogen atoms independent from each other or an alkyl group having 1-6 carbons).

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 a lithium secondary battery using the same. More specifically, an electrolyte for a lithium secondary battery using a specific lithium salt as a solute using an organic solvent containing a specific cyclic ether compound, a specific cycloalkane compound and a carbonate as a solvent, and a lithium secondary battery using the same About.

[0002]

_2. Description of the Related Art A carbon material such as graphite is used as a negative electrode active material, and LiCoO ₂ , LiNiO ₂ , and LiMn ₂ are used as a positive electrode active material.
A lithium secondary battery using a lithium transition metal composite oxide such as O ₄ is rapidly growing as a new small secondary battery having a high voltage of 4 V and a high energy density.
Lithium metal is a negative electrode active material having a large theoretical energy density per unit weight and unit volume as compared with the above-mentioned carbon material, and a battery using lithium metal as the negative electrode active material is an ultimate high energy secondary battery. Development is expected.

However, in the case of a secondary battery using lithium metal as the negative electrode active material, the active material is consumed by the reaction between the active lithium metal and the electrolyte, and the non-uniform deposition and dissolution of the lithium metal during charging and discharging. , And good charge / discharge characteristics (lithium charge / discharge efficiency, cycle life, etc.) have not been obtained due to generation of dendrites.

For example, when an organic electrolyte using a LiPF ₆ salt, which is preferably used in a lithium ion battery, as an electrolyte and a carbonate-based solvent is applied to a battery using lithium metal as a negative electrode active material, the lithium charge / discharge efficiency becomes lower. It is 90% or less, and 10% or more of lithium is lost in one charge / discharge cycle. As a method for improving the lithium charge / discharge efficiency, LiAsF ₆ salt has been proposed as a preferable supporting electrolyte.

[0005]

SUMMARY OF THE INVENTION However, LiA
In the case of sF ₆ salt, arsenic is a toxic element and is not suitable as a material used for general-purpose batteries.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and it is intended to minimize the reactivity between active lithium metal and an electrolytic solution and the generation of dendrites in a charge / discharge cycle, and to reduce toxicity. A novel lithium secondary battery electrolyte with excellent safety and using the same,
An object of the present invention is to provide a lithium secondary battery having high conductivity, excellent lithium charge / discharge efficiency and charge / discharge cycle characteristics, and having both safety and reliability.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies in view of such circumstances, and as a result, have found that a specific cyclic ether compound as an organic solvent, a cycloalkane compound having an oxygen atom in a specific ring, and a carbonate ester It has been found that by dissolving a specific lithium salt as a supporting electrolyte in a mixed solvent containing, the charge-discharge cycle characteristics of a lithium secondary battery can be dramatically improved, and the present invention has been completed. That is, the present invention provides: An electrolyte for a lithium secondary battery, comprising:
A cyclic ether compound having one oxygen atom in a ring of 0% by volume, (b) a cycloalkane having two oxygen atoms in a ring of 1 to 60% by volume, and (c) 10-60
It is characterized in that a lithium salt of an organic acid represented by the general formula (I) is dissolved in a mixed solvent containing a volume% of a carbonate.

[0008]

Embedded image (In the formula, m and n each independently represent an integer of 1 to 4.)

Here, the cyclic ether compound is preferably a tetrahydropyran compound represented by the general formula (II).

[0010]

Embedded image (Wherein, R ¹ , R ² , R ³ , R ⁴ and R ⁵ each independently represent a hydrogen atom or an alkyl group having 1 to 6 carbon atoms)

It is preferable that the cycloalkane is a dioxane compound represented by the general formula (III).

[0012]

Embedded image (Wherein R ⁶ , R ⁷ , R ⁸ and R ⁹ are each independently
Represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms)

2. A lithium secondary battery, comprising: a negative electrode made of lithium metal or a lithium alloy or a compound capable of occluding and releasing lithium; a positive electrode made of a compound capable of occluding and releasing lithium; and the electrolyte solution described above. And

[0014]

The organic solvent includes a cyclic ether compound represented by the above formula (II), a cycloalkane compound having an oxygen atom in the ring represented by the above formula (III), and a mixed solvent containing a carbonate compound. By combining a lithium salt of an organic acid (hereinafter abbreviated as lithium salt of an organic acid) as a solute represented by the above formula (I), an electrolyte for a lithium secondary battery having good charge / discharge cycle characteristics is realized. it can. In particular, lithium metal or a lithium alloy is used as a negative electrode active material, and a lithium transition metal composite oxide (LiCoO ₂ , LiNiO ₂ ,
When applied to a battery using LiMn ₂ O ₄ ) or the like having a voltage of 4 V or more, a battery excellent in lithium charge / discharge efficiency, charge / discharge characteristics, safety, and reliability is realized.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION (Electrolyte for Lithium Secondary Battery) The solvent of the electrolyte according to the present invention consists essentially of a specific cyclic ether compound, a specific cycloalkane compound and a carbonate ester. The proportion of each component in the solvent is such that the cyclic ether is 1 to 60% by volume, the cycloalkane compound is 1 to 60% by volume, and the carbonate is 10 to 60% by volume.
% By volume. The volume of each component is room temperature, that is, 2
It shall be measured at 5 ° C. However, a solid such as ethylene carbonate at room temperature is heated to a melting point and measured in a molten state.

Examples of the cyclic ether compound include a cyclic ether represented by the general formula (II); or 7-oxabicycloheptane, tetrahydrofuran, 2-methyltetrahydrofuran, 3-methyltetrahydrofuran,
Examples thereof include 2,5-dimethyltetrahydrofuran, furan, 2-methylfuran, 3-methylfuran, pyran, 2-methylpyran, and 3-methylpyran. If desired, some of these may be used in combination.

Examples of the cyclic ether represented by the general formula (II) include tetrahydropyran, 2-methyltetrahydropyran, 3-methyltetrahydropyran and 4-methyltetrahydropyran. Among these cyclic ethers, tetrahydropyran, tetrahydrofuran, and 7-oxabicycloheptane are preferable, and tetrahydropyran is particularly preferable.

As the cycloalkane compound, it is preferable to use the compound represented by the above general formula (III).

Examples of the cycloalkane compound represented by the general formula (III) include 1,3-dioxane and 2-methyl-
1,3-dioxane, 2-ethyl-1,3-dioxane, 4-methyl-1,3-dioxane, 4-ethyl-
Examples thereof include 1,3-dioxane, 2,4-dimethyl-1,3-dioxane, and 2,4-diethyl-1,3-dioxane. Among them, those having no alkyl substituent such as 1,3-dioxane are preferable.

As the carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, dipropyl carbonate, diisopropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate,
A single solvent selected from ethyl isopropyl carbonate or the like, or a mixture of two or more thereof is used. Of these, cyclic carbonates are preferred, and ethylene carbonate is particularly preferred. Further, it is preferable that 50% by volume or more of the cyclic carbonate is used in the carbonate component.

The organic solvent comprises 1 to 60% by volume, preferably 20 to 60% by volume, more preferably 30 to 60% by volume of the above cyclic ether compound and 1 to 60% by volume, 10 to 60% by volume.
60% by volume, more preferably 10 to 50% by volume of the above cycloalkane compound and 10 to 60% by volume, preferably 10 to 50% by volume, more preferably 20 to 50% by volume
It is used so that it becomes a mixed solvent with the carbonic acid ester.

At this time, an organic solvent which has been conventionally proposed and used as an electrolyte for a lithium secondary battery can be mixed and used. Examples of such organic solvents include carboxylic esters such as methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, and γ-butyrolactone; 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy. -2-methoxyethane, 1,2-
Chain ethers such as dipropoxyethane; amides such as dimethylformamide and dimethylacetamide; sulfites such as dimethyl sulfite, diethyl sulfite, ethylene sulfite and propylene sulfite; dimethyl sulfate;
A solvent selected from sulfuric acid esters such as diethyl sulfate, ethylene sulfate and propylene sulfate; sulfoxides such as dimethyl sulfoxide and diethyl sulfoxide; nitriles such as acetonitrile and propionitrile; It can be used in an amount of less than 10% by volume, preferably less than 10% by volume.

As the electrolyte of the electrolytic solution according to the present invention, an organic acid lithium salt represented by the general formula (I) is used.

[0024]

Embedded image (In the formula, m and n each independently represent an integer of 1 to 4.)

As such an organic acid lithium salt, L
iN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiN
(SO ₂ C ₃ F ₇ ) ₂ , LiN (SO ₂ C ₄ F ₉ ) ₂ , LiN (S
O ₂ CF ₃ ) (SO ₂ C ₂ F ₅ ), LiN (SO ₂ CF ₃ )
(SO ₂ C ₃ F ₇ ), LiN (SO ₂ CF ₃ ) (SO ₂ C
₄ F ₉ ), LiN (SO ₂ C ₂ F ₅ ) (SO ₂ C ₃ F ₇ ), LiN
N (SO ₂ C ₂ F ₅ ) (SO ₂ C ₄ F ₉ ), LiN (SO ₂ C ₃
F ₇ ) (SO ₂ C ₄ F ₉ ). Among them, those having m + n of 3 or more are preferable, and those having m + n of 4 or more are more preferable. Specifically, LiN (SO ₂ C ₂ F
₅ ) ₂ , LiN (SO ₂ CF ₃ ) (SO ₂ C ₄ F ₉ ) and the like are preferable.

At this time, a lithium inorganic acid salt may be used in combination with the lithium organic acid salt. As the lithium salt of an inorganic acid in this case, known ones can be used, and preferably, LiPF ₆ , LiClO ₄ , LiB
F ₄ and LiSbF ₆ are exemplified.

The lithium organic acid salt is used so that the solute concentration in the electrolytic solution is usually 0.5 to 2 mol / dm ³ , preferably 0.5 to 1.5 mol / dm ^3. . 0.
If it is less than 5 mol / dm ³ or more than 2 mol / dm ³ , the conductivity of the electrolyte solution is undesirably reduced. LiPF ₆ , LiCl known as electrolytes for lithium secondary batteries
An electrolytic solution obtained by dissolving an inorganic acid lithium salt such as O ₄ and LiBF ₄ in an organic solvent reduces the lithium charge / discharge efficiency by reacting with lithium metal as a negative electrode active material, but together with the electrolytic solution according to the present invention. If used, the reaction with the lithium metal is suppressed, so that the lithium charge / discharge efficiency can be maintained high.

(Lithium secondary battery) As the negative electrode material constituting the battery, lithium metal or an alloy of lithium and a metal such as aluminum, tin, zinc, silver, lead, etc., graphite capable of occluding and releasing lithium, coke and A carbon material such as an organic polymer compound fired body and a compound capable of occluding and releasing lithium such as SnO ₂ and TiO ₂ can be used. These negative electrode materials may be used as a mixture of two or more. Among these, lithium metal or lithium alloy is preferable.

As the shape of the negative electrode, a sheet electrode applied to a current collector after mixing with a binder and a conductive agent, if necessary, and a pellet electrode subjected to press molding can be used.

As the material of the current collector for the negative electrode, metals such as copper, nickel, and stainless steel are used, and among these, copper foil is preferable from the viewpoint of easy processing into a thin film and cost.

As the positive electrode material constituting the battery, a material capable of inserting and extracting lithium, such as a lithium transition metal composite oxide material such as lithium manganese oxide, lithium cobalt oxide, and lithium nickel oxide can be used. ,
The lithium transition metal composite oxide is preferable.

The shape of the positive electrode is not particularly limited. For example, a sheet electrode applied to a current collector after mixing with a binder and a conductive agent as necessary and a pellet electrode subjected to press molding are used. It is possible.

As the material of the positive electrode current collector, a metal such as aluminum, titanium, and tantalum or an alloy thereof is used. Among these, aluminum or its alloy is desirable in terms of energy density because it is lightweight.

Known shapes such as a cylinder type in which a sheet electrode and a separator are formed in a spiral shape, a cylinder type having an inside-out structure in which a pellet electrode and a separator are combined, and a coin type in which a pellet electrode and a separator are laminated can be used. It is.

As the separator constituting the battery, a porous sheet or a nonwoven fabric made of a polyolefin such as polyethylene or polypropylene can be used.

[0036]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to the following examples unless it exceeds the gist of the present invention.

The performance of the electrolyte and the performance of the battery were evaluated by the following methods. 1. Measurement of conductivity of electrolytic solution: Conductivity at 25 ° C. was measured using a conductivity meter CM-30S and a conductivity cell CG-511B manufactured by Toa Denpa Kogyo KK.

2. Measurement of lithium charge / discharge efficiency: Lithium metal foil having a thickness of 100 μm on the working electrode (effective electrode area: 1.
The 23cm ^2), lithium metal foils (effective electrode area of 1mm thick to counter: a 1.23cm ^2), is placed in a coin-type cell with an organic electrolyte solution, respectively, via a separator, BT
Using S-2004 charge and discharge device (Nagano Co. Products Ltd.), charge and discharge test (electrodeposition析電air amount: 7.5C / cm ²⁾ of 20 cycles by a constant current density (0.6 mA / cm ²⁾ was carried out The capacity of the electrochemically active lithium remaining on the working electrode was measured, and the lithium charge / discharge efficiency was calculated using the following equation. Lithium charge / discharge efficiency (%) = 100 × (1-1 / FO)
M) FOM = (sum of charge capacity when charge / discharge is repeated)
/ ((Capacity of charged lithium) − (capacity of remaining electrochemically active lithium))

3. Measurement of charge / discharge cycle characteristics: A cylinder type lithium secondary battery as shown in FIG. 1 was used for the test. The positive electrode of this cylinder type battery uses lithium manganese oxide (LiMn ₂ O ₄ ) as a positive electrode active material.
Then, a carbon material as a conductive agent and polyvinylidene fluoride (abbreviated as PVdF) as a binder are mixed at a predetermined weight ratio, and this is mixed with a solvent (N-methylpyrrolidone).
And then applied to an aluminum foil as a positive electrode current collector, dried, and then pressed under pressure to produce a positive electrode. The negative electrode used was one in which a lithium metal foil having a thickness of 80 μm was pressure-bonded to a copper foil as a negative electrode current collector. A polyethylene porous film was used for the separator. The charge and discharge cycle characteristics were measured at a charge current of 0.252 A up to 4.25 V, and the discharge was performed at a discharge current of 0.919 A.
For 2.59 hours or 2.96 hours. In addition, as a test for confirming the battery capacity every 50 cycles, the battery was discharged to 3.5 V at the above discharge current.

Examples 1 to 8 and Comparative Examples 1 to 3 LiN (SO ₂ C ₂ F ₅ ) ₂ (hereinafter abbreviated as LiBETI) as a lithium salt of an organic acid was dissolved as a solute in a mixed solvent shown in Table 1. Thus, an organic electrolyte having a solute concentration of 1 mol / dm ³ was prepared. Using this electrolyte solution, the conductivity and the lithium charge / discharge efficiency were measured as described above. The results are shown in Table 1. Here, the abbreviations in the table are as follows. LiBETI: LiN (SO ₂ C ₂ F ₅ ) ₂ THP: tetrahydropyran DOX: 1,3-dioxane EC: ethylene carbonate DME: dimethoxyethane DEC: diethyl carbonate THF: tetrahydropyran

[0041]

[Table 1]

Example 9 As an organic solvent, tetrahydropyran (THP) and 1,
A mixture of 3-dioxane (DOX) and ethylene carbonate (EC) at a volume mixing ratio of 30:30:40 is used, and as a solute, LiN (SO
_A mixture of 2C ₂ F ₅ ) ₂ and LiPF ₆ which is an inorganic lithium salt at a molar ratio of 90:10 was used, and the solute concentration was 1 mol.
An organic electrolyte of l / dm ³ was prepared. Using this electrolyte,
The battery of the cylinder type shown in FIG. 1 was produced, and the charge / discharge cycle characteristics were measured.

The battery positive electrode of lithium manganese oxide as a positive electrode active material _{(Li 1. 0 5 Mn 1} . 9 5 O 4) , a conductive agent (KS-6, produced by Lonza) as a binder (PVdF) And
The mixture was mixed at a weight ratio of 86: 10: 4. This was dispersed in a solvent to form a slurry. The slurry was applied to an aluminum foil as a positive electrode current collector, dried, and then pressed under pressure to produce a positive electrode. . The negative electrode used was one in which a lithium metal foil having a thickness of 80 μm was pressure-bonded to a copper foil as a negative electrode current collector. A polyethylene porous film was used for the separator.

The charge / discharge cycle characteristics were measured as follows.
The charging was performed at a charging current of 0.252 A up to 25 V, and the discharging was performed at a discharging current of 0.919 A as a constant capacity discharge for 2.59 hours. The measurement results of the charge / discharge cycle characteristics are shown in Table 2 and FIG. The capacity retention rate is 100% of the initial discharge capacity.
This is the ratio of the discharge capacity after the cycle.

Comparative Example 4 Dimethoxyethane (abbreviated as DME) as an organic solvent
A battery was prepared and the charge / discharge cycle characteristics were measured in the same manner as in Example 9 except that a mixed solvent having a volume mixing ratio of EC and EC of 50:50 was used, and LiPF ₆ was used as a solute. The measurement results of the charge / discharge cycle characteristics are shown in Table 2 and FIG.

The lithium manganese oxide as in Example 10 cathode active material (Li _{1. 0 8}
Mn _{1. 9} to ₂ O _4), a conductive agent (ECP-600JD, manufactured by Lion Corporation) and a binder and (PVdF), a weight ratio of 93.
After mixing the mixture in a ratio of 5: 3.5: 3 in a solvent (N-methylpyrrolidone) to form a slurry, the slurry was applied to an aluminum foil as a positive electrode current collector, dried, and then pressed under pressure. Except that the discharge time in the measurement of the charge-discharge cycle characteristics was 2.96 hours,
A battery was fabricated in the same manner as in Example 9, and the charge / discharge cycle characteristics were measured. The results are shown in Table 2 and FIG.

Comparative Example 5 Dimethoxyethane (abbreviated as DME) as an organic solvent
A battery was prepared in the same manner as in Example 10 except that a mixed solvent having a volume mixing ratio of EC and EC of 50:50 and LiPF ₆ was used as a solute, and the charge / discharge cycle characteristics were measured. The results are shown in Table 2 and FIG.

[0048]

[Table 2]

[0049]

The electrolytic solution for a lithium secondary battery of the present invention and the secondary battery using the same can suppress the reactivity between lithium metal and the electrolytic solution and the generation of dendrites in the charge / discharge cycle, and provide a good charge. The present invention has excellent unique effects such as high discharge cycle characteristics, no toxicity and high safety and reliability.

[Brief description of the drawings]

FIG. 1 is a sectional view of a cylinder type battery.

FIG. 2 is a diagram showing charge / discharge cycle characteristics of a cylinder type battery manufactured using the electrolyte solutions prepared in Example 9 and Comparative Example 4.

FIG. 3 is a diagram showing charge / discharge cycle characteristics of a cylinder-type battery manufactured using the electrolyte solutions prepared in Example 10 and Comparative Example 5.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Wang, Tetsuaki 8-3-1 Chuo, Ami-cho, Inashiki-gun, Ibaraki Prefecture Within the Tsukuba Research Laboratory, Mitsubishi Chemical Corporation (72) Inventor Shigeaki Kasuya 8 Ami-cho, Inashiki-gun, Ibaraki Prefecture 3-1, 1-1 in Tsukuba Research Laboratories, Mitsubishi Chemical Corporation (72) Inventor Hirohiko Saito 1-1-1, Showa-cho, Kariya-shi, Aichi Prefecture Inside Denso Corporation (72) Kyohei Usami 1-1-1, Showa-cho, Kariya-shi, Aichi Address DENSO Corporation F-term (reference) 4C022 GA01 4C062 AA01 4H048 AA03 AB91 VA11 VA20 VA30 VA40 VA50 VB30 5H029 AJ02 AJ05 AJ12 AJ14 AK03 AK18 AL06 AL07 AL12 AM02 AM03 AM04 AM05 AM07 BJ02 HJ07 HJ02

Claims (6)

[Claims] 1. A cyclic ether compound having (a) 1 to 60% by volume of one oxygen atom in a ring, and (b) 1 to 6% by volume.
In a mixed solvent containing 0% by volume of a cycloalkane having two oxygen atoms in a ring and (c) 10 to 60% by volume of a carbonate, a lithium organic acid represented by the general formula (I) is mixed. An electrolyte for a lithium secondary battery, wherein a salt is dissolved. Embedded image (In the formula, m and n each independently represent an integer of 1 to 4.) 2. The electrolyte for a lithium secondary battery according to claim 1, wherein the cyclic ether compound is a tetrahydropyran compound represented by the general formula (II). Embedded image (Wherein, R ¹ , R ² , R ³ , R ⁴ and R ⁵ each independently represent a hydrogen atom or an alkyl group having 1 to 6 carbon atoms) 3. The cycloalkanes represented by the general formula (II)
3. The dioxane compound represented by I).
The electrolytic solution for a lithium secondary battery according to the above. Embedded image (Wherein R ⁶ , R ⁷ , R ⁸ and R ⁹ are each independently
Represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms) 4. The carbonic acid ester is ethylene carbonate,
Propylene carbonate, dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, diisopropyl carbonate, methyl ethyl carbonate, methyl-n-propyl carbonate, methyl isopropyl carbonate, ethyl-n-propyl carbonate, ethyl isopropyl carbonate and -n-propyl carbonate The electrolyte for a lithium secondary battery according to any one of claims 1 to 3, wherein the electrolyte is selected from the group consisting of isopropyl. 5. The electrolyte for a lithium secondary battery according to claim 1, wherein the concentration of the lithium organic acid salt dissolved in the electrolyte is 0.5 to 1.5 mol / dm ^3. liquid. 6. The electrolytic cell according to claim 1, wherein the negative electrode comprises a lithium metal or lithium alloy or a compound capable of occluding and releasing lithium; the positive electrode comprises a compound capable of occluding and releasing lithium; A rechargeable lithium battery comprising a liquid.