CA1209202A

CA1209202A - Li surface alloys to reduce voltage delay

Info

Publication number: CA1209202A
Application number: CA000455225A
Authority: CA
Inventors: Ronald J. Ekern; Niles A. Fleischer
Original assignee: Rayovac Corp
Current assignee: Spectrum Brands Inc
Priority date: 1983-06-24
Filing date: 1984-05-25
Publication date: 1986-08-05
Also published as: US4448861A; EP0129882A1; DK217484A; DK217484D0; JPS6091557A

Abstract

Abstract of the Disclosure An improved lithium-thionyl chloride cell is disclosed which includes a lithium anode substrate having a thin lithium-aluminum alloy at the anode surface exposed to electrolyte. As a result voltage delay is reduced.

Description

-The invention is directed to lithium-thionyl chloride cells, and, more par-ticularly, to special lithium anodes which reduce voltage delay in such cell.s.
Lithium-thionyl chloride cells are known frorn such sources as U.S. Patent No~ 3,926,~669, U.K. Patent Specifica-tion No. 1,409~307 and elsewhere. Such systems are considered to offer high energy density and great promise in many ser-vice applications where light weight in proportion to capacity is importan-t. The unique feature of such systems is that the liquid materialr thionyl chloride or its equiva-lent performs the dual function of acting as electrolyte carrier (for an electrolyte salt which is conventionally LiAlC14) and cathode depolarizer. Thus, the cathode depolarizer directly contacts the anode me-tal, which i5 usually highly active lithium metal. It is found that as a result of such direct contact~ a film forms on the lithium anode leading to polarization thereof. Such polarization leads to the phenomenon of voltaye delay, wherein the voltage delivered by the cell when it is placed under load after storage is substantially below cell capability for some time.
This characteristic is a definite handicap, particularly in po~ering electronic devices which require a constant voltage.
According to one aspect of the present invention, there is provided in a lithium-thionyl chloride cell, the improvemen-t for reducing voltage delay which comprises using a lithium anode substrate having a thin layer of lithium-aluminum alloy at the anode surface exposed to electrolyte.
Another aspect of the invention provides a method of reducing voltage delay in a li-thium-thionyl chloride cell, ~'"t which comprises employing as anode in said cell a lithium body having a thin layer of lithium-aluminum alloy a-t the anode sur~ace exposed to electro]yte.
Thus, the problem of voltaye delay in lithiurn--thionyl chloride cells is reduced by -the inven~ion by applying to the surface of the lithium anode a thin covering of aluminum.
Embodiments of the invention will now be described with reference to the accompanying drawings, in which:
Figure 1 is a scanning electron micrograph (SEM) taken at lOOO diameters showing the surface Eilm developed on lithium upon exposure for 5 days at 71C to an electrolyte of LiAlC14 dissolved in thionyl chloride; and Figure 2 is an SEM photograph~ also taken at 1000 diameters showing the surface film developed on lithium having a surface aluminum foil applied thereto after similar exposure.
In accordance with the inven-tion, aluminum foil, which may be as -thick as 0.008 mm, e.g., abou-t 0.008 to about 0.051 mm, is advantageously applied to the lithium anode surface and assembled in contact with the elec-trolyte which usually is LiAlCl4 dissolved in thionyl chloride. Alloying of lithium with the thin aluminum layer readily takes place and it is found that the anode film which produces voltage delay of cells so produced is diminished as compared to similar cells having lithium anodes of commercial puri-ty with no surface layer of aluminum. As noted, the aluminum foil laminated to the lithium is quite thin and may be expressed in terms of about 1% to possibly 14% oE the total ~ ' .
''''.~`!~j thickness of the composite. During formation of the alloy layex, the anode surface becomes roughened and wrinkled.
Examples will now be given:
Experimental anodes were prepared from pure lithiurn strip; from rolled s-trip of lithium~aluminum alloy of uniform composition con-taining 84~ lithium by weight and from lithium strips 0.038 cm thick larnina-ted, respectively, wi-th 0.023 mm A1-Mg Eoil (~lloy 5052 con-taining 2.5% Mg, by weight) and with aluminum foil 0.023 mm thick~ Anodes oE each description were s-tored at 20C and at 71C in thionyl chloride containing 1.6M LiAlC14. The laminated anodes were stored in electrolyte for 5 days with the aluminum or aluminum-magnesium alloy foil layers in contact with electrolyte while the solid anodes were stored for 4 days. Slight pressure was applied to the laminated anodes.
After removal from the electrolyte, the anodes were examined by scanning electron micrography (SEM). The surface films on the lithium, the homogeneous ~i-Al alloy and on the laminate having the Al-Mg layer wera similar in appearance and were characterized by blocky, densely spaced crystals believed to be LiCl. A representative SEM view of the surface film formed on lithium is shown in Figure 1, taken at 1000 diameters~ In contrast, the surface film on the alumi-num laminate displayed smaller crystals which were packed more loosely. This is shown by SEM in Figure 2, also taken at 1000 diameters.
It appears likely that the aluminum-laminated lithium forms a surface alloy by diffusion. The presence of -the alloy at the electrolyte-contacting anode surface ,..
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appears to limit and modify the grow-th oE -the vol-taye delay-causing film. It also appears tha-t, as -the cell is discharyed, the integrity of the surface alloy is preserved while it remains in contact with the lithium substrate. ~ccordingly, the aluminum layer may be quite thin, possibly to the extent of atoms in thickness.
While in accordance with the provisions oE the statute, there is illustrated and described herein specific embodiments of the invention, those skilled in the art will understand that changes may be made in the form of -the invention covered by the claims and that certain features of the invention may sometimes be used to advantage withou-t a corresponding use of the other features.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a lithium-thionyl chloride cell, the improvement for reducing voltage delay which comprises using a lithium anode substrate having a thin layer of lithium-aluminum alloy at the anode surface exposed to electrolyte.

2. A lithium thionyl chloride cell in accordance with claim 1, wherein said anode surface is formed by initially laminating a thin aluminum foil to said lithium substrate.

3. A cell in accordance with claim 2, wherein the aluminum foil has a thickness which is about 1% to about 14%
the thickness of said lithium substrate.

4. A method of reducing voltage delay in a lithium-thionyl chloride cell, which comprises employing as anode in said cell a lithium body having a thin layer of lithium-aluminum alloy at the anode surface exposed to electrolyte.

5. A method in accordance with claim 4, wherein said anode surface is formed by initially laminating a thin aluminum foil to said lithium body.

6. A method in accordance with claim 5, wherein the aluminum foil has a thickness which is about 1% to about 14% the thickness of said lithium body.