Iron-lithium anode for thermal batteries and thermal batteries made therefrom

The invention relates to an iron/lithium anode material for use in thermal batteries which comprises about 15% to 30% by weight lithium. Thermal batteries made from such anodes are also disclosed. The anode comprises particulate iron bound together by the surface tension of the lithium which wets the iron particles. A method is disclosed for the manufacture of the anode material which includes adding iron powder to a molten lithium and mixing to form a homogeneous mixture. The mixture is cooled to form an ingot and rolled into strips for fabrication into anode configurations.

FIELD OF THE INVENTION 
The present invention relates to an anode material comprising iron-lithium 
for use in battery applications and, in particular, to an iron-lithium 
anode for use in lithium anode thermal batteries and to thermal batteries 
made therefrom. 
BACKGROUND OF THE INVENTION 
The present invention is an improvement in thermal batteries of the type 
disclosed and taught in U.S. Pat. Nos. 3,677,822, 3,425,872, 3,527,615, 
3,367,800, 3,891,460, 3,930,888 and 4,119,769. Thermal batteries typically 
comprise a plurality of thermal cells each of which includes an anode, 
cathode, electrolyte and an integral igniteable heat source. The 
electrolyte is usually a eutectic mixture of LiCl and KCl and the cathode 
(depolarizer) comprises a material which is reduced in the electrochemical 
cell such as phosphates, metal oxides, borates and chromates. The most 
common depolarizer material is calcium chromate or vanadium pentoxide. 
Recently, a mixture of iron pyrite and binder have been shown to have 
significant advantages over conventional depolarizer materials, U.S. Pat. 
No. 4,119,769. 
In the present state of the art thermal cells, the anode comprises a 
fusable metal normally selected from the group consisting of alkali 
metals, alkaline earth metals and alloys thereof which melt below the 
operating temperature of the battery. Of these, lithium and lithium alloys 
are preferred. 
The anode, in practice, comprises a metal cup into which the active metal 
is positioned. Positioned adjacent to the active metal is an asbestos 
insulator or separator. The electrolyte, normally in the form of a wafer, 
is positioned adjacent to the separator in the cup. The depolarizer, 
preferably consisting of an analyte and catholyte layered wafer, is 
positioned in stacked array against the electrolyte. 
Use of lithium in thermal batteries provides a number of advantages not the 
least of which is its capability of providing high current densities as a 
liquid metal anode. The reactive nature of lithium and its low melting 
point, however, have caused a number of associated difficulties, the most 
serious of which is leakage of the molten metal. Such leakage causes short 
circuits and premature failure in such batteries. An asbestos separator or 
other fibrous material is typically used to prevent or reduce such leakage 
by direct reaction with the molten metal, but such means occupy precious 
space and expose workers to asbestos material. 
It is, therefore, an object of the present invention to provide an improved 
thermal anode which includes the advantages of lithium without its 
inherent disadvantages. It is a further object to provide an anode which 
results in a thermal cell having decreased weight and thickness by 
eliminating the need for asbestos or other separator materials. A yet 
further object is to achieve the aforementioned benefits and at the same 
time improve the electrical characteristics of the batteries. 
DESCRIPTION OF THE INVENTION 
The invention comprises a pyrometallurically combined iron/lithium anode. 
The ratio of lithium to iron is about 15% to 35% and preferably about 30% 
by weight. Preferably, the lithium is heated to about 500.degree. to 
600.degree. F. and the iron added in particulate form while stirring the 
molten mixture. Preferably, the iron has a particle size of from about 1.3 
to 2.1.mu. with a density of 0.2 to 0.8 gms/cc and a surface area of 
approximately 30 to 70 m.sup.2 /gm. 
As the iron is added to the molten lithium the mixture becomes very 
viscous. Vigorous stirring is desired to wet the iron particles with the 
lithium. The mixture of lithium and wetted iron is preferably poured into 
a mold to cool. While only about 10% by volume of the material is lithium, 
the material retains the essential characteristics of lithium. It is 
easily rolled and shaped, and has reactivity essentially that of lithium. 
Unlike conventional thermal battery lithium anodes, the anode material of 
the present invention softens but does not liquify at normal battery 
operating temperatures. The anode thus maintains its dimensional stability 
during battery operation and does not flow as in conventional thermal 
batteries. 
In the preferred range of lithium to iron described above, it is not 
believed to make a difference if there is no particle to particle contact 
between iron particles. In the present invention, the iron particles are 
held together by the surface tension of the lithium rather than being 
alloyed as is normally the case. Accordingly, it is contemplated that 
materials other than iron which are capable of being wetted by molten 
lithium, for example stainless steel, nickel and nichrome (to which 
lithium alloys slightly) are useful in the present invention. 
In thermal battery application, the iron/lithium anode material of the 
present invention provides a number of advantages. In addition to 
affording an anode which is dimensionally stable during battery operation, 
a higher efficiency battery can be fabricated. Such efficiency results 
from being able to utilize more active material within a given volume by 
the elimination of the asbestos separator means. Elimination of the 
asbestos separator also provides a significant health advantage to 
employees making such batteries inasmuch as it does away with a possible 
carcinogen or cocarcinogenic material. When used in a thermal battery 
having a depolarizer of iron pyrite (U.S. Pat. No. 4,119,796) rather than 
chromates, a thermal battery can be manufactured having no suspected 
carcinogen or cocarcinogen present. 
Better electrical parameters have been obtained using the iron/lithium 
anode material of the present invention. Basically, it is believed that 
such improvements are due to a more efficient utilization of lithium. Such 
improvements also result from the elimination of the asbestos separator 
which adds complexity to the design. 
The above-described advantages result in an improved thermal battery as 
well as method for manufacture. However, other advantages will become 
apparent from a perusal of the following detailed description of the best 
mode contemplated for the use and manufacture of the iron/lithium anode of 
the present invention taken in connection with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Preferably, the iron/lithium anode of the present invention is used in the 
thermal battery described in U.S. Pat. No. 4,119,796 incorporated by 
reference herein. With reference to FIG. 1, iron/lithium anode disc 10 is 
positioned in metal cup 11 by means of insulator ring 12, preferably made 
from Fiberfrax.RTM.. A thermal battery 15 utilizing the iron/lithium anode 
of the present invention comprises a metal housing 16, usually in 
cylindrical configuration and a plurality of cells 17 in stacked array. 
Each cell comprises an anode cup 11 in which iron/lithium anode 10 is 
positioned. Between metal housing 16 and cells 17 is a layer 18 of thermal 
and electrical insulation. Positioned between each of the cells 17 is an 
ignitable chemical heat source 20 which is ignited by fuse strip 21 which 
in turn is connected to an electrical match or squib (not shown) to 
activate the battery. 
As described above, the asbestos separators disclosed in U.S. Pat. No. 
4,119,796 can be eliminated. However, the depolarizer binder 
(Cab-O-Sil.RTM.) used in the preferred embodiment of the battery disclosed 
in that patent is not suitable for use with the anode material of the 
present invention. It has been found that SiO.sub.2 reacts with the 
iron/lithium anode creating a self-discharge. Instead of a binder such as 
Cab-O-Sil.RTM. it is desirable to use a material such as MgO as the 
depolarizer binder. In such case approximately 50% by weight of MgO is 
preferred rather than 15% by weight of Cab-O-Sil.RTM. used in U.S. Pat. 
No. 4,119,796. 
Also, it has been determined that it is preferable to use two layer 
anolyte-catholyte depolarizer 22 in each cell 17. The two layer 
depolarizer prevents the cell from short circuiting which has been found 
to be the case with homogeneous single layer depolarizer wafers when used 
without a separator with the anode material of the present invention. 
Thermal batteries comprising a plurality of stacked cells as disclosed in 
U.S. Pat. No. 4,119,796 may thus be made using the iron/lithium anode by 
making the above modifications. The following examples are illustrative of 
the preferred method of making the anode material and the referenced 
modifications. 
EXAMPLE 1 
Method for Making Anode Strip 
1640 grams of iron powder (apparent density 0.3 to 0.4 g/cc and Fisher 
sub-sieve size of 1.8 to 2.1 microns and having a surface area of about 50 
m.sup.2 /gm), 360 grams of lithium metal are weighed and placed in an 
argon purged glove box. The lithium is melted in a stainless steel 
crucible at 316.degree. C. Approximately 700 to 800 grams of powdered iron 
are added to the molten lithium. The lithium is permitted to permeate the 
iron powder and mixed to obtain a homogeneous mass. The remaining iron 
powder is added and mixed until a homogeneous mass is obtained. 
Thereafter the scrap anode strip is added to the homogeneous mixture and 
permitted to melt and mix therein. The materials balance comprises: 
______________________________________ 
Parts by weight 
______________________________________ 
powdered iron 82.0 
lithium metal 18.0 
______________________________________ 
The molten mixture is poured into a graphite or boron-nitride mold and 
permitted to cool. After the material has cooled it is removed from the 
mold as an ingot and stored in a sealed container of argon gas or in an 
atmosphere of less than 5% relative humidity until rolled into sheets or 
strips. 
The chemical analysis of the resulting sheet in this example is: 
______________________________________ 
Iron 82 .+-. 2% 
Lithium 
18 .+-. 1% 
______________________________________ 
with an apparent density of 2.0 to 2.5 g/cc. 
EXAMPLE 2 
A preferred method for making the anolyte and catholyte layer of the 
depolarizer for use in the battery disclosed in U.S. Pat. No. 4,119,769 
with the anode material of Example 1 is described below: 
Anolyte Layer 23 
A mixture of LiCl (45%) and KCl (55%) eutectic is blended with magnesium 
oxide (calcine @ 600.degree. C. for 4 hours) in a ratio of 1 to 1 and 
fused at a temperature of from 380.degree. to 395.degree. C. for about 16 
hours. The fused material is granulated and seived &lt;60 mesh screen. To the 
granulated powder is preferably added 2.5% by weight LiF which is mixed 
into the powder. The resulting mixture is placed in a press cavity 
together with the catholyte material and pressed into a wafer. 
Catholyte Layer 24 
The catholyte layer 24 comprises 25% (by wt.) electrolyte binder mix 
(LiCl-KCl eutectic) and 75% iron pyrite. 
EXAMPLE 3 
Thermal Battery 
A thermal battery (A) was prepared in accordance with the disclosure of 
U.S. Pat. No. 4,119,769. A second battery 15 (B) was prepared in 
accordance with such disclosure, but modified in accordance with this 
disclosure using the iron/lithium anode material 10 prepared in accordance 
with Example 1 and the anolyte/catholyte depolarizer 22 material of 
Example 2. 
Physical Dimensions 
______________________________________ 
Number of Cells = 28 Load = 4 OHMS 
WEIGHT (GMS) HEIGHT (INS) DIAM (INS) 
______________________________________ 
Battery A 
Cell Assem 
8.08 .09 1.73 
Stack 265 2.49 1.73 
Battery 439 2.89 2 
Cell Anode 
.14 lithium 
.48 iron substrate 
screen 
Cathode 1.01 
Asbestos 
.87 .022 
Cup 1.05 
Battery B 
Cell Assem 
7.65 0.7 1.73 
Stack 251 2.25 1.73 
Battery 394 2.63 2 
Cell Anode 
.18 lithium 
.72 iron 
Cathode 1.01 (50% MgO) 
Cup 1.05 
Fiber Ring 
(to position 
anode in 
cup) 
______________________________________ 
Table I compares the voltages of Batteries A and B at various times after 
ignition. Not only does Battery B of the present invention have a higher 
peak voltage, but supplies its specified minimum voltage (38 volts) for a 
greater period of time. 
TABLE 1 
______________________________________ 
A B 
SEC VOLTS SEC VOLTS 
______________________________________ 
15 52.600 15 53.600 
30 45.200 30 51.600 
45 32.000 45 49.800 
60 16.600 60 46.000 
90 39.800 
120 30.600 
______________________________________ 
Tables II and III compare the electrical parameters of the Batteries A and 
B, respectively. 
TABLE II 
______________________________________ 
Cutoff Volts 
48 38 (Specifi- 
16.6 
cation) 
Time, Seconds 
24.3243 38.1818 60 
Avg Volts 51.7183 48.7893 41.1 
Avg Amps 12.9296 12.1973 10.275 
Avg Amps/Sq In 
5.50051 5.18899 .677535 
Watt-Hours 4.51822 6.31165 7.03837 
Watt-Hrs/Lb 
Cell Assem 9.05865 12.6543 14.1113 
Stack 7.73369 10.8035 12.0474 
Battery 4.6684 6.52144 7.27232 
Watt-Hrs/Cu In 
Cell Assem .762755 1.06552 1.1882 
Stack .771945 1.07836 1.20252 
Battery .497645 .695177 .77522 
PCT Efficiency 
Anode 20.5116 30.3736 40.2076 
Cathode 19.3649 28.6755 37.9597 
______________________________________ 
TABLE III 
______________________________________ 
Cutoff Volts 
48 38(specifi- 
30.6 
cation) 
Time, Seconds 
52.1053 95.8696 120 
Avg Volts 51.8364 47.8497 45.125 
Avg Amps 12.9591 11.9624 11.2813 
Avg Amps/Sq In 
5.51306 5.08906 .743887 
Watt-Hours 9.72273 15.2432 16.9689 
Watt-Hrs/Lb 
Cell Assem 20.5889 32.2791 35.9335 
Stack 17.5703 27.5466 30.6651 
Battery 11.1933 17.5487 19.5354 
Watt-Hrs/Cu In 
Cell Assem 2.11033 3.30855 3.68312 
Stack 1.83833 2.88212 3.2084 
Battery 1.17675 1.84489 2.05375 
PCT Efficiency 
Anode 26.9743 45.8135 54.0795 
Cathode 41.5763 70.6183 83.3544 
______________________________________