JPH05226002A - Lithium secondary cell gel-like electrolyte - Google Patents

Abstract

PURPOSE:To enhance cycle performance by using a gel-like electrolyte for a lithium secondary cell consisting mainly of an ethelic polyurethane, LiAsF6, and propylene carbonate. CONSTITUTION:A lithium secondary cell contains a gel-like electrolyte 3 made by impregnating propylene carbonate a solvent containing LiAsF6 as an electrolyte in a sheet-like polymer cross-linked body consisting of an ethelic polyurethane intervened between a lithium alloy negative electrode 2 and a positive electrode active substance 4, and is sandwiched by stainless steel plates 1 which serve as both a collector and a case, followed by sealing with a sealing material 5. The electrolyte 3 enhances the cycle life of the lithium negative electrode, and concurrently facilitates assembly even in a thin and large area cell because of being a formed body, and the adhesiveness to the electrodes is good because of being an elastic body so that the fallout of the active substance and the precipitation of a dendrite can be prevented resulting in the enhancement in the cycle life.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gel electrolyte for lithium secondary batteries, which uses lithium or a lithium alloy as a negative electrode active material.

[0002]

As a conventional lithium secondary battery, for example, a separator and an electrolyte solution are interposed between a positive electrode and a negative electrode in the form of a sheet, or a solid electrolyte such as a complex of polyethylene oxide and a lithium salt is interposed. It is known that

[0003]

However, in the above case, the device is thin and has a large area (for example, the unit cell is 1 mm.
There is a problem that the process of injecting the electrolyte solution becomes difficult when manufacturing a laminated battery of about 30 cm × 30 cm or more in area, and the cycle life is shortened due to the loss of active material and the deposition of dendrites. ing. In the above case, the conductivity of the solid electrolyte is 10 ⁻⁴ S · cm ^−1.
In addition to being smaller than the following, there is a problem in that it is difficult to maintain the adhesion between the electrode and the solid electrolyte, so that the interface impedance increases and the polarization increases. The present invention has been made against the background of the above-mentioned conventional technical problems, and facilitates the production of a thin and large-area laminated lithium battery, and prevents the active material from falling off and dendrites from being deposited. It is an object of the present invention to provide a gel electrolyte for a lithium secondary battery, which can improve the cycle life, improve the adhesion between the electrode and the electrolyte, and obtain an interface impedance similar to that of an electrolyte solution.

[0004]

The present invention provides a gel electrolyte for lithium secondary batteries, which comprises (a) ether type polyurethane, (b) LiAsF ₆ and (c) propylene carbonate as main components. ..

The (a) ether type polyurethane constituting the gel electrolyte of the present invention has a polar group having an affinity for a solvent having a relatively large polarity described later, and swells in the solvent, for example, a polyol. Average molecular weight of 1,500 ~
Examples include reaction products of about 4,000 polyfunctional polyols and polyfunctional isocyanates. Specific examples of the ether polyurethane include trifunctional polypropylene glycol and TDI (toluene diisocyanate),
MDI (diphenylmethane diisocyanate) or H
Examples thereof include, but are not limited to, ether polyurethanes obtained by reaction with DI (hexamethylene diisocyanate) and ether polyurethanes obtained by reaction of trifunctional polyethylene glycol with TDI, MDI or HDI.

The electrolyte used in the gel electrolyte is chemically stable with respect to the positive electrode active material and the negative electrode active material, and lithium ions can move in order to cause an electrochemical reaction with the positive electrode active material. Although any non-aqueous substance can be used, the present invention particularly employs LiAsF ₆ (lithium hexafluoroarsenate). As the electrolyte, in addition to the LiAsF _6, for example _{LiPF 6, LiSbF 6, LiBF 4} , L
iClO ₄ , LiI, LiBr, LiCl, LiAlC
It may contain l ₄ , LiHF ₂ , LiSCN, LiSO ₃ CF ₃ or the like in an amount of about 10 mol% or less.

The LiAsF _{6 as} an electrolyte is used in a state of being dissolved in a solvent. In the present invention, propylene carbonate is adopted as this solvent. Of course, as a solvent, in addition to propylene carbonate, ethylene carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane, dioxane, glymes such as dimethoxyethane and diethylene glycol dimethyl ether, lactones such as r-butyrolactone, and phosphoric acid esters such as triethyl phosphate. , Boric acid esters such as triethyl borate, sulfur compounds such as sulfolane and dimethyl sulfoxide, nitriles such as acetonitrile, amides such as dimethylformamide and dimethylacetamide, dimethyl sulfate, nitromethane,
It may contain one or more of nitrobenzene and dichloroethane in an amount of about 10% by weight or less.

The gel electrolyte used in the present application is composed of the above (a) ether type polyurethane, (b) LiAsF ₆ and (c) propylene carbonate, and their composition ratio is 20 to 20 by weight. 80/2 to 10/18 to 7
0, preferably 30 to 60/2 to 10/38 to 60 (however, (a) + (b) + (c) = 100% by weight). Thus, the electrolyte (LiAs) is added to the above-mentioned sheet-like cross-linked polymer made of ether polyurethane.
By impregnating with a solvent (propylene carbonate) containing F ₆ ), the crosslinked polymer swells and becomes a gel electrolyte, and a lithium secondary battery is formed by interposing this gel electrolyte between the positive and negative electrodes. ..

Hereinafter, a lithium secondary battery using the gel electrolyte of the present invention will be described in more detail with reference to the drawings. That is, as shown in FIG. 1, this lithium secondary battery is a solvent containing a sheet-like polymer cross-linked body made of ether polyurethane and LiAsF ₆ as an electrolyte between a lithium alloy negative electrode 2 and a positive electrode active material 4. The gel-like electrolyte 3 impregnated with propylene carbonate is sandwiched between the stainless steel plates 1 which also serve as a collector and a case, and a sealing material 5
It is sealed with. As the positive electrode active material used for the positive electrode 4, burned particles such as lithium-containing vanadium pentoxide and lithium-containing manganese dioxide can be used. As the negative electrode active material used for the negative electrode 2, for example, lithium or a lithium alloy capable of absorbing and releasing lithium is used. In this case, the lithium alloy includes lithium containing IIa, IIb, IIIb, IVb, V
A group b metal or an alloy of two or more kinds thereof can be used, and particularly Al, In, Sn, Pb, B containing lithium.
i, Cd, Zn or alloys of two or more of these are preferred.

As described above, in the lithium secondary battery using the gel electrolyte of the present invention, the gel electrolyte composed of ether polyurethane, lithium hexaarsenate and propylene carbonate is used. The cycle life of the lithium negative electrode is improved. Further, since the gel electrolyte is a molded body, it is easy to assemble even when manufacturing a thin, large-area lithium secondary battery, and since the gel electrolyte is an elastic body, it has a compression effect. And improve the adhesion between the electrode and the electrolyte, prevent the active material from falling off and depositing dendrites to improve the cycle life, and because of the gel state, the surface of the electrolyte is in a wet state, Therefore, the contact between the electrode and the electrolyte is as good as the solution, and it is possible to obtain the interface impedance as much as the electrolyte solution.

[00011]

In the lithium secondary battery using the gel electrolyte of the present invention, the cycle life of the lithium negative electrode is particularly improved.
The reason for this is not clear, but it is considered that the gel electrolyte having a specific structure used in the present invention suppresses decomposition of the electrolyte solution due to the strong reducing power of lithium, which is a cause of deterioration of cycle life.

[00012]

EXAMPLES Examples of the present invention will be described below, but the present invention is not necessarily limited to these examples. Example 1 Trifunctional polypropylene glycol (average molecular weight 4,0
00) and toluylene diisocyanate (TDI) in an equivalent ratio of 1 / 1.1 and DABCO (1,4-
Diazabicyclo [2.2.2] -octane) in the presence of a catalyst, the ether-based polyurethane obtained by cross-linking polymerization was molded into a sheet shape with a thickness of 200 μm, and 1 mol / l
Was swollen with a solution of LiAsF ₆ in propylene carbonate to obtain a gel electrolyte. This gel electrolyte was sandwiched between two sheets of lithium foil (16 cm ² ) to give a cell for negative electrode cycle evaluation.
Using this cell, charge and discharge at 10 mA for 1 hour each (pause 1
(0 minutes) When repeated, it was possible to normally charge and discharge up to 144 cycles. Polarization at 144th cycle is 2
It was 0 mV, which was the same as in the first cycle. Also, 14
After 4 cycles, the cell was disassembled and the gel electrolyte and the lithium foil were observed, but no change in appearance was observed in both.

Comparative Example 1 A space between ^two lithium foils (16 cm ² ) was filled with 1.6 mol / l of a solution of LiAsF ₆ in tetrahydrofuran, and 1
When the battery was charged and discharged at 0 mA for 1 hour each, the polarization reached 1 V at the 100th cycle. When disassembled, the lithium foil was covered with a paste.

[00014]

EFFECT OF THE INVENTION Since the gel electrolyte of the present invention is constructed as described above, it facilitates the production of a thin lithium battery having a large area and prevents the active material from falling off and the dendrites from depositing. Thus, it is possible to provide a lithium secondary battery in which the cycle characteristics are improved, the adhesion between the electrode and the electrolyte is improved, and the interface impedance similar to that of the electrolyte solution can be obtained.

[00015]

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a lithium secondary battery using a gel electrolyte of the present invention.

[00016]

[Explanation of symbols]

 1 Stainless Steel Substrate 2 Negative Electrode 3 Gel Electrolyte 4 Positive Electrode Active Material 5 Sealant

Front page continuation (72) Inventor Koichi Miyashita 1-4-1 Chuo 1-4-1 Wako-shi, Saitama, Ltd.

Claims (1)

[Claims] 1. An (a) ether-based polyurethane, (b)
A gel electrolyte for a lithium secondary battery, which comprises LiAsF ₆ and (c) propylene carbonate as main components.