JPH087923A - Electrolyte for lithium secondary cell - Google Patents

Abstract

PURPOSE:To enhance charging/discharging characteristics by adding the specified amount of surface active agent in a fluorine system having perfluoro-alkyl, groups to organic electrolyte, forming an absorption film composed of surface active agent over the surface of lithium, and thereby forming a smooth deposition surface. CONSTITUTION:Surface active agent in a fluorine system having perfluoro-alkyl groups is added in concentration from 0.005 to 0.1% by weight as additives to electrolyte. In this case, an absorption film composed of surface active agent is formed over the surface of lithium, nuclear formation takes precedence to grain growth when lithium electrolytic deposition is carried out, concurrently grain growth becomes uniform in direction, and a smooth deposition surface can thereby be obtained. By this constitution, the reversibility of lithium charging/discharging can be enhanced, concurrently the absorption film composed of surface active agent acts as a protection film so as to control the reaction of constituents in electrolyte with lithium, as a result, a lithium secondary cell can thereby be obtained, the negative electrode of which is excellent in charging/discharging characteristics.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolytic solution for a lithium secondary battery, and more particularly to improvement by an additive for an organic solvent electrolytic solution.

[0002]

2. Description of the Related Art A secondary battery using metallic lithium as a negative electrode active material is expected to be developed as a next-generation high energy-density battery. Conventionally, for example, propylene carbonate (PC) and γ-butyl lactone (γ-B) have been used as an organic solvent suitable for the electrolyte of this type of battery.
L), ester solvents such as ethylene carbonate (EC), and ether solvents such as tetrahydrofuran (THF), 1,2 dimethoxyethane (DME) and 1,3 dioxolane. As a solute, lithium perchlorate (L
iClO ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium hexafluoroarsenate (LiAsF ₆ ), lithium borofluoride (LiBF ₄ ),
Lithium trifluoromethanesulfonate (LiCF ₃ S
A lithium salt such as O ₃ ) is used.

[0003]

Conventionally, a secondary battery using this kind of metallic lithium as a negative electrode active material cannot obtain a sufficient cycle life because the metallic lithium deteriorates rapidly with repeated charging and discharging. There was a problem. As one of the causes of this, when charging and discharging are repeated with a conventionally used electrolytic solution, the surface of the electrically isolated lithium that does not participate in charging and discharging on the negative electrode is caused by the reaction between the electrolytic solution and active lithium metal. It is said that a film is formed. Further, it has been pointed out that the composition of the electrolytic solution has a large influence on the electrodeposition form of lithium, which is considered to be largely involved in the cycle life, and that the electrolytic solution composition that can obtain a smooth lithium deposition surface is Being sought.

In order to solve this problem, it has been attempted to add various additives to the organic electrolyte to control the reaction of lithium / electrolyte and improve the electrodeposition form. Additives that have been found to improve cycle efficiency include 2-methylfuran (2MeF) and pyrrole (P
Examples thereof include y), heterocyclic compounds such as thiophene (Tp), nonpolar compounds such as benzene, inorganic compounds such as hydrogen fluoride (HF) and iodide, but the effects are not sufficient.

[0005]

The present invention has been made to solve the above-mentioned problems and is characterized in that one or more fluorine-containing surfactants are added to the conventional electrolytic solution. To do.

That is, according to the present invention, one or more kinds of fluorine-containing surfactants having a perfluoroalkyl group are added to the organic electrolytic solution conventionally used for lithium secondary batteries in 0.005.
The addition of wt% or more and 0.1 wt% or less provides a lithium secondary battery having excellent charge / discharge cycle characteristics.

[0007]

By adding a fluorinated surfactant to the electrolytic solution, an adsorption film of the surfactant is formed on the lithium surface. With this adsorption film, nucleation is prioritized over grain growth during lithium electrodeposition, the grain growth direction is uniform, a smooth deposition surface is obtained, and reversibility of lithium charge / discharge is improved. It is considered to be improving. Further, the effect that the adsorption film of the surfactant acts as a protective film and the reaction between the components in the electrolytic solution and lithium is controlled can be expected.

The effect of adding a surfactant is 1 mol / lL
The greatest effect was obtained when 0.05 to 0.1 wt% of a surfactant having a perfluoroalkyl group was added to the iClO ₄ / PC electrolyte. When the added concentration of the fluorine-based surfactant to be added is 0.1 wt% or more, the effect of inhibiting the precipitation of lithium is considered to be greater, and when the concentration is 0.005 wt% or less, the added effect is sufficiently obtained. Absent.

[0009]

Embodiments of the present invention will be described below.

(Example 1) 1 mol / l LiClO ₄ was dissolved in propylene carbonate, and 0.1% of sodium perfluoroalkylcarboxylate, perfluoroalkyltrimethylammonium salt, and perfluoroalkylethylene oxide were added, respectively. Liquids 1, 2 and 3 were used. A battery is constructed by using a nickel substrate as a working electrode (negative electrode), lithium as a counter electrode (positive electrode), and lithium as a reference electrode (for potential measurement). First, 1 mA / cm ²
After depositing lithium on the nickel substrate at a constant current of 30 minutes (Q = 1.8 C / cm ² ), the deposited lithium was dissolved until the electrode potential reached 1.6 V (vs. Li / Li ⁺ ). It was The current efficiency of charging / discharging was calculated from the amount of electricity at this time.

FIG. 1 is a diagram showing a change in charge / discharge efficiency (%) with respect to the number of cycles when the charge / discharge cycle is repeated. For comparison, a change in charge / discharge efficiency when an electrolyte solution containing no surfactant is used is also shown in the same figure (comparative example). As can be seen from FIG. 1, in the case of no addition, the charge / discharge efficiency is seen to decrease as the number of cycles is increased, whereas the number of cycles is increased by adding the fluorosurfactant having a perfluoroalkyl group. After repeating 50 times, there was almost no decrease in efficiency.

In particular, in the electrolytic solution (electrolytic solution 1) containing sodium perfluoroalkylcarboxylate, the charge / discharge efficiency was high, and stable characteristics were obtained even after repeated cycles.

Example 2 A battery was constructed in the same manner as in Example 1, 1 mol / l LiClO ₄ was dissolved in propylene carbonate as an electrolytic solution, and sodium perfluoroalkylcarboxylate was added in an amount of 0.001 to 0.5 wt. %
What was added and changed to was used. With this,
The current efficiency of charging / discharging was obtained in the same manner as in Example 1.

Table 1 shows the amount of sodium perfluoroalkylcarboxylate added and the current efficiency after repeating the charge / discharge cycle 30 times.

[0015]

[Table 1]

FIG. 2 shows the change in charge / discharge efficiency with respect to the number of cycles when the electrolytic solutions 1, 4, 5, 8 were used.

As can be seen from Table 1 and FIG. 2, the effect of adding the surfactant is most apparent in the electrolytic solutions 1, 5, 6, 7
Then, it was the case where the added amount was 0.005 to 0.1 wt%. In particular, the range of electrolytes 1 and 5 is 0.05 to 0.1.
It was found that the effect was greatest when the addition was wt%. In the case of the electrolyte solution 8, since the amount added was too low, the effect did not appear so much, and as in the case of no addition (see the comparative example in FIG. 1), the efficiency decreased as the cycle was repeated. On the other hand, in the case of the electrolytic solution 4, it is considered that the reason why the current efficiency is lowered is that the surfactant tends to inhibit the dissolution and precipitation of lithium because the addition amount is too large.

The solvent and lithium salt used in the electrolytic solution of the lithium secondary battery of the present invention are basically not limited to those used in this embodiment.

[0019]

As described above, according to the present invention, the fluorine-containing surfactant having a perfluoroalkyl group is added to the organic electrolyte conventionally used in lithium secondary batteries in an amount of 0.005 to 0.005. By adding 1 wt%, it is possible to provide a lithium secondary battery having excellent negative electrode charge-discharge cycle characteristics.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing the relationship between charge / discharge efficiency and the number of charge / discharge cycles of a lithium secondary battery using an electrolytic solution according to the present invention.

FIG. 2 is a characteristic diagram showing the relationship between charge / discharge efficiency and the number of charge / discharge cycles when the concentration of surfactant added is changed.

[Explanation of symbols]

 1 Electrolyte 1 2 Electrolyte 2 3 Electrolyte 3 4 Electrolyte 4 5 Electrolyte 5 8 Electrolyte 8

Claims (2)

[Claims] 1. An electrolytic solution for a lithium secondary battery in which a lithium salt is dissolved in an organic solvent, wherein, as an additive to the electrolytic solution, at least one of fluorochemical surfactants having a perfluoroalkyl group is used as the electrolytic solution. An electrolyte solution for a lithium secondary battery, which is added in the inside. 2. The electrolyte for a lithium secondary battery according to claim 1, wherein the concentration of the added fluorine-containing surfactant having a perfluoroalkyl group is 0.005 to 0.1 w.
An electrolytic solution for a lithium secondary battery, which is t%.