JPH11329491A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve cycle characteristics, by suppressing degradation of a nonaqueous electrolyte caused by decomposition of a solvent in a nonaqueous electrolyte secondary battery. SOLUTION: A nonaqueous electrolyte contains a specific chain ether expressed by a chemical formula, Cn F2n+1 -O-Cm H2m+1 (where, n is 1 to 5 and m is 1 to 4). In this battery, the nonaqueous electrolyte having the chain ether added thereto is used and the reduction of its discharge capacity arising during charge/discharge cycles is suppressed. In other words, the contact of a negative electrode with solvent molecules is eliminated by forming stable coatings of good quality on surfaces of the negative electrode owing to the addition of the chain ether, and therefore a solvent is stabilized, that is, the degradation of the nonaqueous electrolyte is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to an improvement of a non-aqueous electrolyte for the purpose of improving cycle characteristics.

[0002]

2. Description of the Related Art As an electrolyte for a non-aqueous electrolyte secondary battery,
When a non-aqueous electrolytic solution containing a solvent such as tetrahydrofuran is used, the non-aqueous electrolytic solution deteriorates due to deterioration of the electrolytic solution, and thus has a disadvantage that the storage characteristics of the battery deteriorate. In order to improve this point, for example, JP-A-6-176768
As shown in the publication, the use of a specific cyclic ether containing a halogen atom as a solvent has improved the stability of the electrolytic solution.

However, as a result of investigations by the present inventors, it has been found that a non-aqueous electrolyte secondary battery containing a specific cyclic ether as an additive has a problem that the cycle life is short.

[0004]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the above problems, and is directed to a conventional non-aqueous electrolyte secondary battery to which a specific cyclic ether containing a halogen atom is added as a stabilizer. In comparison, it is an object of the present invention to provide a non-aqueous electrolyte secondary battery having excellent cycle characteristics while maintaining good storage characteristics.

[0005]

In order to achieve the above object,
Non-aqueous electrolyte secondary battery of the present invention (hereinafter, “battery of the present invention”
It may be called. ) Is a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode mainly composed of lithium metal or a substance capable of inserting and extracting lithium, a separator for isolating these two electrodes, and a non-aqueous electrolyte. the non-aqueous electrolyte is of the general formula _{C n F 2n + 1 -O-} C m H 2m + 1 ( wherein, n =
1 to 5 and m = 1 to 4. ) Is contained.

As described above, in the battery of the present invention, the non-aqueous electrolytic solution to which the above-mentioned chain ether is added is used, and a decrease in discharge capacity that occurs during a charge / discharge cycle is suppressed.

In other words, by forming a stable and good-quality film on the surface of the negative electrode due to the addition of the chain ether, the contact between the negative electrode and the solvent molecules is cut off, and the solvent is stabilized, that is, the non-aqueous electrolytic solution is formed. This is to prevent the deterioration of the liquid.

Specific examples of the chain ether represented by the above general formula include, for example, trifluoromethyl methyl ether (n = 1, m = 1, and the chemical formula is CF ₃ OC
H ₃ ), pentafluoroethyl methyl ether (n = 2,
m = 1, the chemical formula is C ₂ F ₅ OCH ₃ ), heptafluoropropyl methyl ether (n = 3, m = 1,
The chemical formula is C ₃ F ₇ OCH ₃ ), nonafluorobutyl methyl ether (n = 4, m = 1), and the chemical formula is C ₄ F ₉ OC
H ₃ ), undecafluoropentyl methyl ether (n =
5, m = 1 and the chemical formula is C ₅ F ₁₁ OCH ₃ ), nonafluorobutyl ethyl ether (n = 4, m = 2,
The chemical formula is C ₄ F ₉ OC ₂ H ₅ ), nonafluorobutyl propyl ether (n = 4, m = 3), and the chemical formula is C ₄ F ₉ C
₃ H ₇ ), nonafluorobutyl butyl ether (n = 4,
m = 4, and the chemical formula includes C ₄ F ₉ OC ₄ H ₉ ).

The effect of these chain ethers can be exerted even when the addition amount to the non-aqueous electrolyte is small, but it is particularly equivalent to 0.1 to 30% by volume of all the solvents (excluding chain ethers). It is preferable to include a chain ether.

In the present invention, since the chain ether represented by the above general formula contains an alkyl chain in which all hydrogens are substituted by fluorine, these chain ether molecules are easily decomposed on the surface of the negative electrode, and In addition, it is possible to form a more stable film due to an alkyl group in which all hydrogen atoms are substituted with fluorine.

The present invention relates to an improvement in an additive to a non-aqueous electrolyte, and for a battery material other than an additive, a conventionally known material for a non-aqueous electrolyte battery can be used without any particular limitation. Can be.

As the solvent of the non-aqueous electrolyte, ethylene carbonate (EC), propylene carbonate (P
C), organic solvents such as vinylene carbonate (VC) and butylene carbonate (BC), and dimethyl carbonate (DMC) and diethyl carbonate (DE)
C), a mixed solvent with a low boiling point solvent such as methyl ethyl carbonate (EMC), 1,2-diethoxyethane (DEE), 1,2-dimethoxyethane (DME) and ethoxymethoxyethane (EME). . Among them, the compatibility with the additives specified in the present invention is good, and particularly preferable solvents for improving cycle characteristics are one or more cyclic carbonates and one or more chain carbonates. Is a mixed solvent having a volume ratio of 1: 4 to 4: 1.

The positive electrode materials include manganese dioxide, manganese oxide containing lithium, cobalt oxide containing lithium, vanadium oxide containing lithium, nickel oxide containing lithium, and iron oxide containing lithium. Oxides, chromium oxides containing lithium, and titanium oxides containing lithium are exemplified.

On the other hand, as the negative electrode material, metallic lithium, lithium-aluminum alloy, lithium-lead alloy,
Lithium alloys such as lithium-tin alloys, carbon materials such as graphite, coke, and fired organic materials, SnO ₂ , SnO, TiO ₂ , N
A metal oxide having a potential such as b ₂ O ₃ lower than that of the positive electrode active material is exemplified.

The battery of the present invention is excellent in cycle characteristics because the chain ether added to the non-aqueous electrolyte forms a stable and high quality film on the surface of the negative electrode.
This is for stabilizing the solvent by breaking contact between the negative electrode and the solvent molecules, which is presumed to suppress the decomposition reaction of the electrolytic solution that occurs at the time of charge and discharge, and to improve the reversibility in charge and discharge. In addition, since the electrolyte can exist stably not only at room temperature but also at high temperature, a non-aqueous electrolyte secondary battery having excellent high-temperature storage characteristics can be obtained.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and may be appropriately modified within the scope of the invention. It can be implemented by

(Experiment 1) In Experiment 1, the relationship between the type of chain ether to be added and the cycle characteristics of the battery was examined.

Example 1 In producing a battery, a positive electrode was prepared as follows. First, 90 parts by weight of a lithium-containing cobalt oxide (LiCoO ₂ ) powder as a main material of a positive electrode, 5 parts by weight of an artificial graphite powder, and 5 parts by weight of polyvinylidene fluoride
A slurry was prepared by mixing parts by weight of an N-methyl-2-pyrrolidone (NMP) solution. This slurry was applied to both sides of an aluminum foil by a doctor blade method to form an active material layer. Then, it vacuum-dried at 150 degreeC for 2 hours, and produced the positive electrode.

Next, for the negative electrode, 95 parts by weight of natural graphite as the main material of the negative electrode and 5 parts by weight of polyvinylidene fluoride in N
The slurry was prepared by mixing with the MP solution. This slurry was applied to both surfaces of a copper foil by a doctor blade method to form a carbon layer. Then, vacuum drying at 150 ° C for 2 hours,
A negative electrode was manufactured.

Then, LiN (C ₂ F ₅ SO ₂ ) ₂ is added to an equal volume mixed solvent of ethylene carbonate and diethyl carbonate.
A non-aqueous electrolyte dissolved in 0.5 mol / liter is prepared. Further, trifluoromethyl methyl ether was prepared as a chain ether in an amount of 10% by volume with respect to the volume of the above equal volume mixed solvent, and added to and mixed with a non-aqueous electrolyte to prepare a non-aqueous electrolyte.

Using the above positive electrode, negative electrode and non-aqueous electrolyte, a non-aqueous electrolyte secondary battery of AA size (battery size: diameter
A battery A1 of the present invention (14 mm, height 50 mm) was produced. Note that a porous film made of polypropylene is used as the separator.

Example 2 A battery A2 of the present invention was produced using pentafluoroethyl methyl ether as a chain ether instead of the trifluoromethyl methyl ether used in the battery A1.

Example 3 A battery A3 of the present invention was produced using heptafluoropropyl methyl ether as a chain ether instead of the trifluoromethyl methyl ether used in the above battery A1.

Example 4 A battery A4 of the present invention was produced by using nonafluorobutyl methyl ether as a chain ether instead of the trifluoromethyl methyl ether used in the battery A1.

Example 5 A battery A5 of the present invention was produced by using undecafluoropentyl methyl ether as a chain ether instead of the trifluoromethyl methyl ether used in the battery A1.

Example 6 A battery A6 of the present invention was produced by using nonafluorobutyl ethyl ether as a chain ether instead of the trifluoromethyl methyl ether used in the battery A1.

Example 7 A battery A7 of the present invention was produced by using nonafluorobutyl propyl ether as a chain ether instead of the trifluoromethyl methyl ether used in the above battery A1.

Example 8 A battery A8 of the present invention was produced by using nonafluorobutyl butyl ether as a chain ether in place of the trifluoromethyl methyl ether used in the above battery A1.

Comparative Example 1 A comparative battery V as a comparative example was prepared using a chain ether, tridecafluorohexylmethyl ether, instead of the trifluoromethyl methyl ether used in the battery A1.

The tridecafluorohexyl methyl ether used herein is a chain ether, but is a compound of the general formula wherein n = 6 and m = 1, and the chain ether (n = 1 to 1) according to the present invention. 5) Not.

Comparative Example 2 A comparative battery W was produced as a comparative example using nonafluorobutylpentyl ether as the chain ether instead of the trifluoromethyl methyl ether used in the battery A1.

The nonafluorobutylpentyl ether used herein is a chain ether, but is a compound of the general formula wherein n = 4 and m = 5, and is a chain ether (m = 1 to 4) according to the present invention. )is not.

Comparative Example 3 A comparative battery X was prepared as a comparative example using epifluorohydrin instead of the trifluoromethyl methyl ether used in the battery A1. This epifluorohydrin is not a chain ether but a cyclic ether.
No. 176768.

Comparative Example 4 A non-aqueous electrolyte was prepared without using the trifluoromethyl methyl ether (chain ether) used in the battery A1, and a comparative battery Y as a comparative example was prepared.

A cycle test was performed using these batteries. The experimental conditions were as follows: each battery was charged at 200 mA at room temperature (25 ° C).
After charging at a constant current of 4.2V, repeat the charge / discharge cycle test, in which the process of discharging at a constant current of 200mA to 2.75V is one cycle, and calculate the number of cycles until the discharge capacity falls below 90% of the initial discharge capacity. Was.

The results are shown in Table 1. In Table 1,
The names and chemical formulas of the additive, the chemical formula _{(C n F 2n + 1 -O} -C m
H _{2m + 1} ) are specified in the order of n and m and the number of cycles.

[0037]

[Table 1]

From Table 1, it can be seen that the batteries A1 to A8 of the present invention have a lower discharge capacity retention ratio after cycling than the comparative battery Y without additives and the comparative battery X with epifluorohydrin as an additive. It is clear that the cycle characteristics are high and the cycle characteristics are good.

The batteries A1 to A8 of the present invention have 6 carbon atoms.
Compared to the comparative battery V or the comparative battery W to which the above-mentioned fluoroalkyl chain or chain ether having an alkyl chain having 5 or more carbon atoms was added, there was a tendency that the cycle characteristics were excellent. This is probably because addition of a fluoroalkyl chain having a large number of carbon atoms or a chain ether having an alkyl chain increases the viscosity of the electrolyte and decreases the solubility in the electrolyte.

Here, the characteristics of the comparative battery V and the comparative battery W
Is not excellent in any of the comparative batteries,
It is considered that the molecular weight is increased by increasing the alkyl chain length, that is, n and m, and the viscosity of the electrolytic solution is increased and the conductivity is decreased.

In particular, the comparative battery X uses the cyclic ether as an additive, but it is considered that the cyclic ether molecule itself is not easily decomposed and it is difficult to form a film on the surface of the negative electrode, so that it is difficult to obtain excellent characteristics. .

Of the batteries A1 to A8 of the invention, the batteries A4 and A6 of the invention have particularly good cycle characteristics.
This fact shows that it is preferable to use nonafluorobutyl methyl ether (C ₄ F ₉ OCH ₃ ) or nonafluorobutyl ethyl ether (C ₄ F ₉ OC ₂ H ₅ ) as a chain ether.

(Experiment 2) In Experiment 2, the charge storage characteristics of the batteries A1 to A8 of the present invention prepared in Experiment 1 and the comparative batteries X and Y were examined.

The experimental conditions were as follows: each battery was operated at 4.2 mA at 200 mA.
And then discharge to 2.75V at 200mA. Thereafter, the battery is charged at 200 mA to 4.2 V, and a storage test is performed at 60 ° C. for 20 days in this state. Further, the change in discharge capacity due to storage of each battery was measured by discharging at 200 mA to 2.75 V.

FIG. 1 shows the result. FIG. 1 shows the discharge capacity of each battery after storage, with the vertical axis representing the discharge capacity (mAh) and the horizontal axis representing the storage period (days).

As shown in FIG. 1, the batteries A1 to A5 of the present invention
It can be seen that A8 has a larger discharge capacity after storage than the comparative batteries X and Y and has excellent storage characteristics.
This is presumably because the chain ether forms a film on the interface between the negative electrode and the electrolytic solution, and the film stably exists in the charged state.

From the results of the above Experiments 1 and 2, the battery of the present invention has excellent cycle characteristics and storage characteristics, indicating the superiority of a specific chain ether.

(Experiment 3) In Experiment 3, a suitable amount of chain ether added to the non-aqueous electrolyte was examined.

First, LiN (C ₂ F ₅ SO ₂ ) ₂ was added to an equal volume mixed solvent of ethylene carbonate and diethyl carbonate in an amount of 0.5%.
A solution in which mol / liter was dissolved was prepared. Next, nonafluorobutylethyl ether as a chain ether was prepared in each volume% with respect to the volume of the equal volume mixed solvent (see Table 2), and the solution was added to and mixed with the solution to prepare a non-aqueous electrolyte solution. Prepared. A cycle test was performed under the same conditions as in Experiment 1, except that these non-aqueous electrolytes were used.

Table 2 shows the results. In Table 2,
Inventive battery A6 and comparative battery Y (without chain ether)
Are also transcribed from Table 1.

[0051]

[Table 2]

As shown in Table 2, the battery A6 of the present invention and the batteries B2 to B5 have particularly good cycle characteristics. From this, it is understood that it is preferable to use nonafluorobutyl ethyl ether by adding and mixing so as to be 0.1% by volume to 30.0% by volume with respect to the nonaqueous electrolytic solution.

When a chain ether other than nonafluorobutylethyl ether is used, the range of the addition amount is also 0.
It was separately confirmed that it is preferable that the content be 1% by volume to 30.0% by volume.

(Experiment 4) In Experiment 4, the relationship between the type of electrolyte salt added to the non-aqueous electrolyte and the cycle characteristics of the battery was examined.

First, a solution prepared by dissolving 0.5 mol / l of various electrolyte salts shown in Table 3 in a mixed solvent of equal volumes of ethylene carbonate and diethyl carbonate is prepared.
Next, nonafluorobutyl ethyl ether was prepared at 10% by volume of a mixed solvent of equal volume, and added to and mixed with the above solution to prepare a non-aqueous electrolyte. Except that these non-aqueous electrolytes were used, batteries C1 to C8 of the present invention were produced in the same manner as in Experiment 1, and then a cycle test was performed under the same conditions as in Experiment 1.

Table 3 shows the results. In Table 3, the results of Battery A6 of the present invention and Comparative Battery Y are also transcribed from Table 1.

[0057]

[Table 3]

As shown in Table 3, the cycle characteristics of the batteries A6 and C4 of the present invention are particularly good. From this, as the electrolyte salt, LiN (C ₂ F ₅ SO ₂ ) ₂ or LiN (CF ₃ SO ₂ ) (C ₄ F ₉ S
It can be seen that it is preferred to use O ₂ ).

This is because LiN (C ₂ F ₅ SO ₂ ) ₂ or LiN (CF ₃ S
It is considered that the electrolyte salt of O ₂ ) (C ₄ F ₉ SO ₂ ) is stably present in the film formed on the negative electrode surface by the addition of the chain ether.

[0060]

As described above, by using a non-aqueous electrolyte containing a specific additive, deterioration of the non-aqueous electrolyte caused by decomposition of the solvent in the non-aqueous electrolyte is suppressed, A non-aqueous electrolyte secondary battery having excellent cycle characteristics can be provided, and its industrial value is extremely large.

[Brief description of the drawings]

FIG. 1 Charging and storage of a battery of the present invention and a comparative battery (60 ° C.)
4 is a graph showing a change in discharge capacity according to the above.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Hiroshi Watanabe 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (72) Toshiyuki Noma 2-chome Keihanhondori, Moriguchi-shi, Osaka No.5-5 Sanyo Electric Co., Ltd. (72) Inventor Koji Nishio 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd.

Claims (4)

[Claims] 1. A non-aqueous electrolyte secondary comprising a positive electrode, a negative electrode mainly composed of lithium metal or a substance capable of inserting and extracting lithium, a separator for isolating these two electrodes, and a non-aqueous electrolyte. in the battery, the nonaqueous electrolytic solution, the general formula _{C n F 2n + 1 -O-} C m H 2m + 1 ( wherein, a n = 1 to 5, and a m = 1 to 4.) in A non-aqueous electrolyte secondary battery comprising the chain ether shown below. 2. The chain ether is C ₄ F ₉ OCH ₃ or C 4
Nonaqueous-electrolyte secondary battery according to claim 1, wherein the a ₄ F ₉ OC ₂ H _5. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous electrolyte contains 0.1 to 30% by volume of the chain ether based on the total volume of the solvent. . 4. The method according to claim 1, wherein the non-aqueous electrolyte is LiN (CF ₃ SO ₂ ) (C ₄ F ₉ ).
SO ₂₎ or LiN (C ₂ F ₅ SO ₂₎ non-aqueous electrolyte secondary battery according to claim 1, characterized in that it contains at least one electrolyte salt selected from _2.