JPH0456078A - Nonaqueous electrolyte for lithium secondary battery and lithium secondary battery thereof - Google Patents

Abstract

PURPOSE:To realize nonaqueous electrolyte having chemical stability against a lithium electrode and for giving a long lifetime to a lithium secondary battery by including at least one kind of dithiaalkane group compound. CONSTITUTION:At least one kind of dithiaalkane group which consists of CH3- S-(CH2)n-S-CH3(n=2-4) is included in the non-aqueous electrolyte for lithium secondary battery. Dithiaalkane group can control the reaction of a non-aqueous solvent in the electrolyte with an electrodeposited lithium, and improves cycle characteristic of a negative electrode of a lithium secondary battery comprising this electrolyte, and improves a lifetime of the battery. In this case, a desirable addition quantity of dithiaalkane group is within about 0.1-5 volume % against the non-aqueous solvent, and the non-aqueous electrolyte having an improved conductivity with the described effect can be obtained by using a mixture solvent as solvent.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a non-aqueous electrolyte for lithium secondary batteries and a lithium secondary battery.

[Conventional technology]

In recent years, lithium secondary batteries have attracted attention as batteries that have discharge characteristics of 1f and high energy density. Since the lithium secondary battery uses lithium metal or its alloy as a negative electrode, it cannot use a water-soluble electrolyte and usually
LiCI
O4, LiAsF. A non-aqueous electrolyte in which a lithium inorganic salt such as , LIPF, , LiBF, etc. is dissolved is used.

[Problem to be solved by the invention]

However, in the conventional lithium secondary battery using a nonaqueous electrolyte, the lithium metal, which is the negative electrode active material, gradually deteriorates due to repeated charging and discharging cycles, has a large furnace interior, and has a short lifespan. A major cause of deterioration is that the lithium deposited on the negative electrode during charging is extremely active and reacts with the organic solvent in the electrolyte to form an insulating non-J state film on the surface of the lithium particles. One of the reasons for this is that it becomes unusable as an active material, but it has been found that this is due to the chemical instability of conventional well water solvents towards lithium.

Propylene carbonate, ethylene carbonate, γ-
Well water solvents with C==〇 double bonds such as butyrolactone are high dielectric constant solvents, so they have a high degree of ionic dissociation of solutes and tend to have excellent electrical conductivity. If used as is in a lithium secondary battery, it is likely to react with deposited lithium, resulting in poor cycle characteristics of the lithium electrode and extremely short battery life. On the other hand, tetrahydrofuran, 2-
Ethers such as methyltetrahydrofuran, dimethoxyethylene, and dioxolane generally have slightly better chemical stability toward lithium than the above-mentioned high dielectric solvents, but they have a problem of inferior conductivity due to their low dielectric constants. . In addition, it has been found that the above-mentioned mixed solvent of a high dielectric constant solvent and a low dielectric constant solvent significantly improves the electrical conductivity, and various mixed solvents such as EC-TIIF and PC-Tllr have been investigated. However, the chemical stability against lithium is still insufficient, the battery life is short, and it is not suitable for practical use.

Therefore, it is desired to develop a nonaqueous electrolyte that eliminates the above-mentioned disadvantages of the conventional lithium secondary battery, has good chemical stability with respect to lithium electrodes, and does not require a long-life lithium secondary battery.

[Means to solve the problem]

In view of the above-mentioned conventional problems, as a result of intensive research, the present invention solves the above-mentioned problems and provides a lithium secondary battery with improved battery life that satisfies the above-mentioned demands. It provides an aqueous electrolyte, and the components of the non-aqueous electrolyte include CHi-8-(CH2), -8-CHi
(n=2''-4>) containing at least 1 m of a dithiaalkane compound.

[For production]

When the non-aqueous electrolyte of the present invention is used in a lithium secondary battery, the dithiaalkane compound contained suppresses the reaction between the active electrodeposited lithium and the well water solvent, improves the cycle characteristics of the negative electrode, and improves the battery life. results in an improvement in

In this case, any of the above dithiaalkane compounds,
About 0.1-5 for normal 111 or 2 or more solvents
A lithium secondary battery equipped with a non-aqueous electrolyte containing a mixed solvent of % by volume has particularly excellent charge/discharge characteristics.
It has no effect on battery life.

〔Example〕

Next, embodiments of the present invention will be described below.

Dithiaalkane compound C113-S-(Ctb)--S added as a component of the above-mentioned non-aqueous electrolyte of the present invention
-Clh has high chemical stability towards lithium,
This is considered to be one of the reasons why the reaction between the electrodeposited lithium and the well water solvent is suppressed.

Next, 2,5-dithiahexane will be explained as an example.

2,5-dithiahexane C1 et al.-8-(CI et al.), -
S -CH2 is dimethoxyethane CH, -Q-(C1
12) t -0-c Although the oxygen atom in the Hi molecule is replaced with a sulfur atom, it is chemically more stable than dimethoxyethane, as described later.

11U, dimethoxyethane is used as an electrolyte solvent for lithium batteries, and one of the reasons for this is that it has a large number of donors and is easily soluble in lithium cations. This is because the oxygen atom present in the dimethoxyethane molecule has a paralingimia negativity of 3.5 that is larger than that of the adjacent carbon atom, 2.5.
This is due to the fact that more electrons used in covalent bonds are attached to oxygen atoms, and because the electron density is larger on oxygen atoms, they are less likely to be attached to lithium ions, which are cationic species. It is thought that it has become easier to place. However, if the electrons are biased toward the oxygen atom, the bond in 0-crJJ becomes easy to break, which tends to affect the stability.For example, Reference J, EleCtrOChe
According to Ill, Soc, 135 1863, dimexyethane is not stable towards lithium and is said to react as follows to produce lithium methoxide.

II, C= C1□+LIOC113 In this way, the reaction of dimethoxyethane to produce lithium methoxide as described above indicates that the O-C bond is easily broken, and the reason for this is that the oxygen atom It is presumed that this is due to the bias of electrons toward . By the way, when comparing the electronegativity of sulfur and oxygen atoms, the electronegativity of sulfur atom is 2.5, which is smaller than that of oxygen atom, which is 3.5.On the other hand, the electronegativity of carbon atom is 2, as mentioned above.
5, it is substantially equal to that of a sulfur atom.

Therefore, the electronic polarization of the S--C bond is small and the bond is broken, so 2,5-dithiahexane is more stable than dimethoxyethane.

For the same reason, Cfiz-5-(CL)-S-C
Even when n=3 or 4 in Hi, the chemical stability against lithium is excellent. When n is 5 or more, the viscosity of the electrolytic solution increases, resulting in a decrease in electrical conductivity, which is unsuitable. Therefore, it is necessary to limit n to a range of 2 to 4.

Next, the effect of adding the additive of the present invention will be clarified in the following test example in which a non-aqueous electrolyte prepared by adding 2,5-dithiahexane to a PC-D14E mixed melt is used.

Test Example A test cell consisting of a working electrode, a counter electrode, and a reference electrode was prepared, and a charge/discharge cycle test was conducted. That is, as a working electrode, a lithium foil with a thickness of 0.2 mm was punched out into a disc shape with a diameter of 40 mm, which was then crimped to a current collector punched out of stainless steel expanded metal in the same shape. there was. The capacity of this working electrode corresponds to 510 mAh. As the counter electrode, a lithium foil with a thickness of 0.75 m was punched out into a disk shape with a diameter of 40 mm, and similarly to the working electrode, a piece was used which was press-bonded to stainless steel expanded metal.

A lithium wire was used as a reference electrode. The working electrode and the counter electrode were placed opposite to each other with a separator in between, and the well water electrolyte of the present invention was injected into the space between them to form a cell.

As the electrolytic solution of the present invention, 1iclo4 is added 1 mol/J to a molten mixture of propylene carbonate and dimethoxyethane.
Dissolved and further added 2,5-dithiahexane in different amounts were used. For comparison, a similar cell using an additive-free electrolyte was fabricated.

Each test cell constructed in this way was heated for 1 time at 25°C.
Charging and discharging were repeated at a constant capacity of 511 Ah at the current value of the axis ^. The lifespan was determined based on the potential change of the working electrode.

The cycle characteristics of the lithium working electrode were evaluated by calculating the average charge/discharge efficiency per cycle using the following formula. Here n represents the number of cycles.

E = 1-(51-(510-51)/n)15
1x 100 The results are shown in the drawings.As is clear from the results, it was confirmed that the average charge/discharge efficiency of the cells to which 2,5-dithiahexane was added was improved.

In this case, addition 1 of 2,5-dithiahexane is effective in the range of about 0.1% to 5% by volume, especially about 0.1% to 5% by volume.
．． It was found that the effect was significant in the range of 5 to 4%.

In addition, C113-5-(CI+7), -5-CI+3
Dithiaalkanes represented by (n = 3 to 4) also had the same effect as the above-mentioned 2,5-dithiahexane.

Thus, the above-mentioned CH3-8-(Clb)-
A well water electrolyte containing at least one dithiane alkane compound represented by S-Clli (n = 2 to 4) is prepared using metal lithium or an alloy capable of occluding and releasing lithium ions, a carbon agent, and a conductive material. By combining it with the upper component of a lithium secondary battery, which consists of a camellia made of a neutral polymer or an a-free oxide, and a positive electrode made of a substance that can electrically react reversibly with lithium ions, charging and discharging efficiency can be improved. The lithium secondary battery of the present invention can be constructed with improved long life.

In this case, as the non-aqueous electrolyte of the lithium secondary battery,
For example, the present invention can be applied to a solvent system in which a solvent consisting of ethylene carbonate or propylene carbonate and a solvent consisting of tetrahydrofuran or 2-methyltetrahydrofuran or 1,2-dimethoxyethane are mixed in a volume ratio of 1=1. When using a non-aqueous electrolyte comprising a non-aqueous solvent to which at least Im of a dithiaalkane compound is added in a volume ratio of about 0.1% to 5%, conductivity can also be improved. It's nice to have it.

〔Effect of the invention〕

As described above, according to the present invention, a lithium secondary battery l
) As an aqueous electrolyte, CH, -8-(C11□), -8
When using a lithium secondary battery containing at least one type of dithialkane consisting of -C1h(n-2 to 4), the dithialkane is a non-aqueous solvent in the electrolytic solution and the electrodeposited lithium. Therefore, the cycle characteristics of the negative electrode of a lithium secondary battery equipped with the electrolyte of the present invention are improved, and the battery it
It has no life-improving effect.

In this case, it is preferable that the amount of the dithiaalkanes added is in the range of about 0.1 to 5% by volume based on well water/B, and by using a mixed solvent as the solvent, the above effects can be achieved. In addition, a well water electrolyte having improved conductivity can be obtained, which is preferable.

[Brief explanation of the drawing]

The drawing is a graph showing the relationship between the amount of the additive and the average charge/discharge efficiency of the lithium electrode when a non-aqueous electrolyte containing the additive of the present invention is used. Patent applicant Furukawa Battery Co., Ltd.

Claims (1)

[Claims] 1. As a component of the non-aqueous electrolyte, CH_3-S-(CH_
2) A nonaqueous electrolyte for a lithium secondary battery, which contains at least one dithiaalkane compound represented by ___■-s-CH_3 (n=2 to 4). 2. At least one of the dithiaalkane compounds is added to a conventional solvent consisting of one or more kinds in an amount of about 0.1 to
The non-aqueous electrolyte for a lithium secondary battery according to claim 1, wherein 5% by volume is added. 3. A lithium secondary battery comprising the non-aqueous electrolyte according to claim 1 or 2 as a non-aqueous electrolyte.