JPS6286673A - Electrolyte for lithium secondary battery - Google Patents

Abstract

PURPOSE:To secure such an electrolyte that improves charge and discharge coulomb efficiency and life capacity in a lithic negative electrode, by adding a specific organic compound to the electrolyte in which lithic salt is dissolved by an organic solvent. CONSTITUTION:An organic compound having a benzene ring and a carbonyl group in a molecule simultaneously is added to a solution having lithic salt dissolved in an organic solvent. As for the organic compound, diphenyl carbonate, benzophenone, phenyl benzoate, 9-fluonorene, dibenzyl ketone, 3-phenyl acetylacetone, dibenzoylmethane, benzyl, 4-benzoylbiphenyl are used, or one or more than two kinds among derivatives having a substituent in these benzene rings are used. The substituent consists of methyl, ethyl, propyl, butyl, methoxyl, ethoxyl, propoxyl and butoxyl groups, etc., adding content should be set to a range of 0.005-1.0mol/l.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electrolytic solution used in a lithium secondary battery using lithium as a negative electrode active material.

[Prior art]

Batteries that use lithium as the negative electrode active material have high power and
It is expected to have high energy density.

However, although this lithium negative electrode has been successfully applied to primary batteries using manganese dioxide or carbon fluoride as positive electrodes, when applied to secondary batteries, the problem is that the coulombic efficiency of charging and discharging is low. It has become. Possible causes of the low coulombic efficiency of charging and discharging are as follows.

For one thing, the deposited lithium produced when a lithium negative electrode is charged is very active, and this reacts with the electrolyte to form an inert film on the surface of the negative electrode.

This is because the negative electrode becomes passivated. Further, since this film is not uniform and incomplete in two parts, it becomes a growth nucleus of spongy lithium through repeated charging and discharging. If this sponge-like lithium grows long enough, it will penetrate the separator between the two electrodes, causing a short circuit between the two electrodes.

The following proposals have been made to eliminate the above factors:

This method uses aluminum as the negative electrode substrate. When lithium is deposited on aluminum, it becomes a single alloy, and
Since lithium permeates into the aluminum, the reaction between the electrodeposited lithium and the electrolyte is suppressed, thereby preventing the formation of a passivation film. However, in this method, the potential of the lithium negative electrode shifts to the positive electrode side due to 1-alloying.

The electromotive force of the battery decreases, and the aluminum substrate turns into powder due to repeated charging and discharging.

In addition, a solvent such as ether is mixed with a highly dielectric and highly stable solvent such as propylene carbonate.

There is a method of increasing the conductivity of the electrolyte and simultaneously lowering the reactivity toward lithium (Japanese Patent Application Laid-Open No. 59-8279
issue). However, this method has very little effect on improving coulombic efficiency and life performance.

Furthermore, as another method, it has been proposed to add ethylene glycol to the electrolyte to change the reaction between lithium and the electrolyte and the form of lithium precipitation (Japanese Patent Laid-Open No. 130073/1982). .

However, in this case as well, the additive does not exhibit a uniform effect over the entire negative electrode, and therefore sufficient effects are not exhibited.

[Problem that the invention seeks to solve]

The present invention has been made in view of the problems of the prior art described above, and aims to provide an electrolytic solution for a lithium secondary battery that improves the charge/discharge coulombic efficiency and life performance of a lithium negative electrode.

[Means for solving problems]

The electrolytic solution for lithium secondary batteries of the present invention is characterized in that it is made by adding an organic compound having both a benzene ring and a carbonyl group in the molecule to a solution in which a lithium salt is dissolved in an organic solvent. be.

A lithium secondary battery is constructed by partially or completely immersing a negative electrode made of lithium or a lithium alloy and a chargeable/dischargeable positive electrode in a battery containing the electrolytic solution of the present invention. As a positive electrode.

Molybdenum dioxide (MOO3) and vanadium pentoxide (v
Any chargeable and dischargeable electrode may be used, such as oxides such as 20), chalcogen compounds such as titanium sulfide (TiS2) and niobium selenide (NbSez), conductive polymers such as polyacetylene and polypyrrole, or carbon. . Further, other battery components may be used as long as they can be generally used as lithium batteries.

The electrolytic solution of the present invention must be non-aqueous and liquid-soluble since the lithium negative electrode reacts with an aqueous solution. The organic solvent used in the non-aqueous electrolyte may be any organic solvent that is generally used in electrolytes. Examples include propylene carbonate, sulfolane, acetonitrile, dimethyl sulfoxide, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane, γ-butyrolactone, ethylene carbonate, 1,2-dimethoxyethane, etc., and one or more of them may be used. use.

The lithium salt is a supporting electrolyte that is dissolved in the organic solvent and provides conductivity to the electrolytic solution, and may be one that is generally used as this type of supporting electrolyte). For example, Lx
C10*, L+BP,, Fb to Lt8, LiPFb,
Li [.

Examples include LiBr, and one or more of them are used.

The amount of the lithium salt dissolved in the organic solvent is desirably in the range of 0.01 to 2 mol per organic solvent 12. When the amount is less than 0.01 mol, the resistance of the solution is large, making it difficult to pass current steadily, and furthermore, the charge/discharge capacity may be reduced.

On the other hand, if the amount exceeds 2 moles, the lithium salt becomes saturated in the solution, making it difficult to completely dissolve the lithium salt.

In the present invention, an organic compound having both a benzene ring and a carbonyl group in its molecule is added to a solution prepared by dissolving the above lithium salt in an organic solvent.

It is thought that the following phenomena occur by adding this organic compound. When a lithium secondary battery is charged, lithium is electrodeposited on the negative electrode side.The organic compound is adsorbed onto the surface of this electrodeposited lithium, forming a kind of ion-conductive film.The carbonyl group of the organic compound is The adsorption activity to electrodeposited lithium is large, while the benzene ring has a small adsorption activity.This organic compound has two types of functional groups that have different adsorption activities to electrodeposited lithium.

Forms a film that is adsorbed with uniform strength on the surface of electrodeposited lithium, which has varying energy levels.

The uniformly adsorbed film prevents the reaction between the electrolyte and the deposited lithium. Therefore, passivation of lithium and spongy precipitation are suppressed, and the charge/discharge coulombic efficiency and life performance of the negative electrode are improved.

Examples of the organic compounds include diphenyl carbonate, benzophenone, phenyl benzoate, 9-fluoronolene, dihenzyl ketone, 3-phenylacetylacetone, dibenzoylmethane, benzyl, 4-benzoylbiphenyl, or derivatives thereof having a substituent on the benzene ring. One or more of these may be used. In addition.

The above substituents are a methyl group, an ethyl group, and a propyl group.

butyl group, methoxy group, ethoxy group, propoxy group, butoxy group, etc. For example, an example of the derivative having a substituent on the benzene ring is 4,4°-dimethylbenzophenone, which is obtained by introducing a methyl group into benzophenone.

The concentration of the organic compound added is 0.005 to 1.0 mo
A range of l/l is desirable. Addition concentration is 0.005m.

If it is less than l/1, the effect of adding the organic compound will be very small; on the other hand, 1. If it exceeds Omol/6, the charge/discharge coulombic efficiency and life performance will deteriorate.

〔Effect of the invention〕

According to the present invention, it is possible to provide an electrolytic solution that improves the charge/discharge coulombic efficiency and life performance of a lithium negative electrode of a lithium secondary battery. This is because an organic compound having both a benzene ring and a carbonyl group in its molecule was added to an electrolytic solution in which a lithium salt was dissolved in an organic solvent.

〔Example〕

Hereinafter, nine aspects of the present invention will be described in detail.

Example 1 A Cu wire was used as a sample electrode, a Li foil was used as a counter electrode, and a Li wire was used as a reference electrode, and these were assembled into a beaker-shaped cell. The electrolyte contains 0.5 m of diphenyl carbonate as an additive.

1/L Lithium perchlorate (Li
C104) dissolved in 1.0 mol/1 propylene carbonate? g solution was used. In this electrolytic solution, Li was electrodeposited onto the Cu cotton, and the negative electrode characteristics of Li were investigated.

That is, first, after charging by electrodepositing Li at a constant current of 1 mA/cJ for 30 minutes, the battery was immediately discharged by dissolving Li at the same current density. Note that the discharge ended when the potential of the sample electrode with respect to the reference electrode reached 0.5V. Repeat this charge/discharge cycle.

The discharge capacity at each cycle was measured. Then, the ratio of the discharge capacity to the amount of charged electricity, that is, the coulomb efficiency, was determined.

Figure 1 is a diagram showing the relationship between the coulombic efficiency of Li electrodes and the number of cycles, and in Figure 2, a is the case where the above electrolyte of the present invention is used, and Sl is a comparative example that does not contain additives. 1 mol/7! This is the case when LiClO4/propylene carbonate is used as the electrolyte.

As is clear from FIG. 1, by using the electrolytic solution of the present invention to which diphenyl carbonate is added.

It can be seen that the coulombic efficiency and life performance of the Li electrode are significantly improved.

Example 2 0.0% of the organic compounds shown in the table were used as additives.
Five types of electrolytic solutions were prepared in the same manner as in Example 1 except that 02 mol/E was added (samples mb to " in the table). Using these electrolytic solutions, Li electrodes were charged in the same manner as in Example 1. The discharge characteristics were measured.

Table 2 shows the relationship between the Coulombic efficiency of Li electrodes and the number of cycles. In the figure, b to f are the cases when the above electrolyte of the present invention is used (corresponding to the sample scale in the table). Yes, also Sl
is 1 mo+/12 without additives as a comparative example.
This is the case when LiClO4/propylene carbonate is used as the electrolyte.

As is clear from FIG. 2, it can be seen that by using the electrolytic solution of the present invention containing additives, the Coulombic efficiency and life performance of the Li electrode are significantly improved.

Example 3 0.5 mol/l of diphenyl carbonate and 1.0 mo+/l of lithium perchlorate (LiC104) in sulfolane
1 was dissolved to prepare an electrolytic solution of the present invention. Using this electrolytic solution, the charging and discharging characteristics of the Li electrode were measured in the same manner as in Example 1.

Figure 3 is a diagram showing the relationship between the coulombic efficiency of Li electrodes and the number of cycles. This is a case where 1 mol/l LiClO4/sulfolane is used as the electrolyte.

As is clear from FIG. 3, by using the electrolytic solution of the present invention, the Coulombic efficiency and life performance of the Li electrode are significantly improved.

Example 4 0.0'l of benzophenone in propylene carbonate
mol/l and lithium tetrafluoroborate (LiB
F.

) 1.0mol/j! An electrolytic solution of the present invention was prepared by dissolving the above. Using this electrolytic solution, the charging and discharging characteristics of the Li electrode were measured in the same manner as in Example 1.

Figure 4 is a diagram showing the relationship between the coulombic efficiency of Li electrodes and the number of cycles. This is the case when 1 mol/e of LiBF4/propylene carbonate is used as the electrolyte.

As is clear from FIG. 4, it can be seen that by using the electrolytic solution of the present invention, the Coulombic efficiency and life performance of the Li electrode are significantly improved.

[Brief explanation of drawings]

FIGS. 1 to 4 are diagrams showing the relationship between the Coulombic efficiency of Li electrodes and the number of charge/discharge cycles when two electrolytic solutions of the present invention are used.

Claims (1)

[Claims] (1) An electrolytic solution for a lithium secondary battery, characterized in that an organic compound having a benzene ring and a carbonyl group in the molecule is added to a solution of a lithium salt dissolved in an organic solvent. (2) The above organic compound is diphenyl carbonate,
Benzophenone, phenyl benzoate, 9-fluoronolene, dibenzyl ketone, 3-phenylacetylacetone,
The electrolyte for lithium secondary batteries according to claim (1), which is dibenzoylmethane, benzyl, 4-benzoylbiphenyl, or one or more derivatives thereof having a substituent on the benzene ring. liquid.