An electrochemical cell comprising an anode composed of a molten mixture of non-lithium alkali metals, a cathode comprising an electrically conductive carbonaceous material and an electrolyte comprising an electrolyte salt and a non-aqueous solvent.

Field of the Invention 
The present invention relates to a non-aqueous electrochemical cell which 
has a high discharge capacity and high discharge efficiency in high rate 
discharge and which has excellent size stability. More particularly, the 
invention is concerned with a non-aqueous electrochemical cell using as 
active anode material a molten non-lithium mixture of alkali metals. 
BACKGROUND OF THE INVENTION 
A non-aqueous electrochemical cell using lithium as an active anode 
material has high energy density, good storage characteristics and wide 
operation temperature range. A non-aqueous electrochemical cell is 
therefore often used as a power source for a calculator, a watch or a 
memory back up system. Such a cell comprises an anode, an electrolyte and 
a cathode. In general, such a cell uses as an anode an alkali metal such 
as lithium or sodium; as an electrolyte or electrolytic solution, a 
solution of a solute such as lithium perchlorate or lithium 
tetrafluoroborate in a non-aqueous solvent such as propylene carbonate, 
.gamma.-butyrolactone or diglyme; and as a cathode, manganese dioxide or 
poly-carbonmonofluoride. 
The combination of relatively high theoretical energy density, potentially 
long life, and low cost materials such as reported in the sodium-sulfur 
system high temperature batteries has been reported in the literature as 
suitable for low rate performance work such as electric road vehicle 
propulsion or load leveling of electric power supplies. The sodium-sulfur 
systems, first proposed in 1966, has had a great deal of effort expended 
in trying to develop a practical system. The basic operating principle 
involves the separation of two active molten materials, sodium and sulfur, 
by either a ceramic membrane of beta alumina or sodium glass, which at 
about 300.degree. C. or higher allows the passage of sodium ions that form 
with the sulfur any of the several polysulfides. The open circuit voltage 
of the system is at just over 2 volts, about the same as the lead-acid 
cell. Two formidable problems exist at the present time, viz., cracking of 
the separator and corrosion of the casing and seal. 
Another somewhat similar system is the lithium-iron sulfide system, 
operating at about 450.degree. C. However, insufficient development has 
been done to date to demonstrate the widespread practicality of this 
system. 
Another of the developments being pursued involves a lithium-based cell, in 
which the negative electrode is a lithium alloy (typically either 
lithium-aluminum or lithium-silicon), the positive electrode is an iron 
sulfide, and the electrolyte is a molten salt, such as the eutectic 
composition in the lithium chloride-potassium chloride system. Because of 
the high melting point of such salts, such cells must be operated in the 
temperature range of 400-500 degrees centigrade. 
This requirement to operate at such high temperatures has several important 
disadvantages. One of these is that various degradation processes, such as 
corrosion of the cell container, seals, and other components are 
accelerated by such high temperatures. Another is that a substantial 
amount of energy is lost through heat transfer to the surroundings. Still 
another is that the voltage obtained from such cells is lower at elevated 
temperatures, due to the fundamental property of the negative temperature 
dependence of the free energy of the cell reaction. Furthermore, the 
higher the temperature of operation, the greater the potential problems 
related to damage to the cell during cooling to ambient temperature and 
reheating, whether deliberate or inadvertent. Differences in thermal 
expansion, as well as dimensional changes accompanying phase changes, such 
as the freezing of the molten salt, can cause severe mechanical 
distortions, and therefore damage to cell components. 
Cells involving a lower temperature molten salt electrolyte have been 
investigated where the molten salt is based upon a solution of aluminum 
chloride and an alkali metal chloride. However, such salts are not stable 
in the presence of the respective alkali metals. As a result, an auxiliary 
solid electrolyte must be used to separate the alkali metal and the salt. 
One example of such a cell involves a molten sodium negative electrode, a 
solid electrolyte of sodium beta alumina, a molten aluminum 
chloride-sodium chloride salt, and either antimony chloride or an 
oxychloride dissolved in the chloride salt as the positive electrode 
reactant. 
Such a cell can operate in the temperature range 150-250 degrees 
centigrade. It has the disadvantage of having to employ an electrolyte, 
which increases the cell impedance, as well as adding to the cost and 
complexity. 
U.S. Pat. No. 3,844,837 to Bennion et al discloses a nonaqueous battery in 
which the anode may be lithium and/or graphite on which lithium metal is 
deposited and as a positive electrode a platinum cup filled with powdered 
K.sub.2 SO.sub.4 and graphite is utilized. The electrolytes disclosed are 
LiC10.sub.4, LiCF.sub.3 S0.sub.3 or LiBF.sub.4 dissolved in dimethyl 
sulfite. 
SUMMARY OF THE INVENTION 
In accordance with the present invention there is provided a non-aqueous 
electrochemical cell for use in primary-rechargeable storage devices. The 
cell comprises a pair of electroconductive electrodes electrically 
insulated from contact, a solid electrolyte and an organic electrolyte 
solvent which is suitable for use in batteries. 
The cathode or positive electrode may comprise a carbonaceous electrically 
conductive fibrous or sheet material, graphite, a composite of graphite 
and an inert polymer such as Teflon, or any of the conventional metal or 
metal oxide electrodes, for example zinc, cadmium, aluminum, platinum, 
etc. Preferably, the cathode comprises fibrous carbonaceous material which 
is associated with a current collector. Advantageously, the carbonaceous 
material comprises an activated carbon fabric. 
The anode or negative electrode comprises a molten mixture of at least two 
elements selected from the group consisting of cesium, rubidium, 
potassium, and sodium. 
The preferred electroconductive carbonaceous cathode material used in the 
invention is more fully described in copending application Ser. No. 
558,239, entitled Energy Storage Device, filed Dec. 15, 1983, now 
abandoned and Ser. No. 678,186, entitled Secondary Electrical Energy 
Storage Device and an Electrode Therefore, filed Dec. 4, 1984, each by F. 
P. McCullough and A. F. Beale, which applications are incorporated herein 
by reference in their entirety. Simply, the preferred carbonaceous 
material is a fiber spun from stabilized polymeric material such as pitch 
based material or polyacrylonitrile based fibers. These fibers are 
stabilized by oxidation and thereafter made electroconductive by 
carbonization at temperatures of above 850.degree. C., and preferably 
above 1700.degree. C. Advantageously, the carbonaceous fibers have a 
Young's Modulus of greater than about one million psi, and preferably 
about five million psi. The upper limit for practical manufacturing is 
about 100 million psi, although as production techniques improve it may be 
possible to use a material which has a higher Young's Modulus. However, 
such material is at present considered to be much too brittle to withstand 
manufacture into electrodes, as well as, the rigors of use to which a 
battery may be subjected. The carbonaceous material should have sufficient 
strength to withstand the encapsulation without loss of electrical contact 
between the carbon particles. Thus, one can employ a carbonaceous material 
defined in the foregoing application as well as many other forms of 
electroconductive carbons such as GRAFOIL when they are encapsulated in 
the manner hereinafter described. 
The carbonaceous electrode, when constructed as a cloth or sheet, includes 
an electron collector conductively associated with the carbonaceous fibers 
or sheet. The electrode conductor interface is preferably further 
protected by a material to insulate the collector and to substantially 
protect the electron collector from contact with the electrolyte and its 
ions. The protective material must, of course, be unaffected by the 
electrolyte and its ions. 
The current collector intimately contacts the carbonaceous material of the 
electrode. The carbonaceous material may be in the form of an assembly 
such as a planar cloth, sheet or felt. It is also envisioned that the 
electrode may be constructed in other shapes such as in the form of a 
cylindrical or tubular bundle of fibers. It is also apparent that an 
electrode in the form of a planar body of cloth, sheet or felt can be 
rolled up with a separator between the layers of the carbonaceous 
material, and with the opposed edges of the rolled up material, connected 
to a current collector. While copper metal has been used as a current 
collector, any electro-conductive metal or alloy may be employed, such as, 
for example, silver, gold, platinum, cobalt, palladium, and alloys 
thereof. Likewise, while electrodeposition has been used in bonding a 
metal or metal alloy to the carbonaceous material, other coating 
techniques (including melt applications) or electroless deposition methods 
may be employed. 
Suitable techniques for preparing the collector/carbonaceous material 
negative electrode are more fully described in co-pending application Ser. 
No. 729,702, entitled Improved Low Resistance Collector Frame for 
Electro-conductive Organic and Graphitic Materials, filed May 2, 1985, by 
F. P. McCullough and R. V. Snelgrove, now Pat. No. 4,631,116. 
Generally, the mixture of alkali metals which is utilized in fabricating 
the anodes of the invention are molten at ambient temperatures, so that a 
heating element is not required so as to make the cell operational. The 
following table illustrates some of the alkali mixtures which can be 
employed. 
TABLE I 
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Composition Wt. % 
Melting Point .degree.C. 
Cs K Rb Na 
______________________________________ 
-48 77.0 23.0 
-40 87.0 13.0 
-30 95.0 5.0 
-11 78.0 22.0 
-8 92.0 8.0 
33 32.0 68.0 
______________________________________ 
The electrolyte salt which is used in the present invention is an alkali 
metal tetrafluoroborate or a tetraalkyl ammonium tetrafluoroborate, 
especially lithium tetrafluoroborate, sodium tetrafluoroborate, 
tetraethylammonium tetrafluoroborate, tetrabutylammonium 
tetrafluoroborate, etc. 
The electrolyte salt is added to a conventional organic solvent which is 
used in electrochemical storage devices such as ethylene carbonate, 
propylene carbonate, .gamma.-butyrolactone, diethylene oxide diethylether 
(diglyme), and mixture thereof. 
The concentration of the electrolyte salt in the solvent solution is chosen 
according to conventional means to maximize conduction or performance. In 
general, the molarity of the electrolyte salt in the solvent will be about 
0.5 to about 8.0M, preferably from 3.0 to 5.0M. 
The separator which is utilized in the cell of the invention may be any 
inert conventional separator, for example fiberglass, microporous 
polypropylene film such as available from Celanese Corporation under the 
trademark CELGARD.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a cell 10 within a housing (not shown) includes an 
anode assembly comprising an expanded metal matrix 20 in which the molten 
mixture of alkali metals for example, sodium-potassium in the molten state 
are retained. 
A separator 24 encloses the anode assembly with a bipolar connector 25 
passing through the separator 24. The separator 24 preferably comprises a 
fiberglass mat. 
The cathode comprises an activated carbon fiber mat 16 that is placed 
adjacent a bipolar plate 26, which is preferably aluminum. 
In order to bring the battery to its operating temperature when the anode 
comprises a mixture of alkali metals which are not molten at ambient 
temperature, there is optionally provided a heater (not shown). 
FIG. 2 illustrates a multi-cell assembly 30 showing two series connected 
cells separated by a bipolar plate metal connector 32. It is to be 
understood that the assembly 30 can include any number of cell units by 
use of additional bipolar separators and cell units. The assembly 30 
includes cathode and anode plates 33 and 34. The anode 34 comprises a 
molten mixture of alkali metals on a metal screen containing 2.5 times the 
stoichrometric amount of alkali metal. 
The cathode plate 33 comprises a 0.08 cm thick graphite cloth electrode 
having a bulk density of about 0.645 g/cc. The solvent electrolyte 
occuppies the voids of the graphite cloth electrode. 
EXAMPLE 
A. The following table shows the voltage values for the various cells of 
the invention. 
TABLE II 
__________________________________________________________________________ 
Load/Voltage (Volts) 
Solvent Electrolyte 
Anode 
Cathode 10M 
1M 100K 
10K 
1K 100 
10 
__________________________________________________________________________ 
0.5 wt. % ET.sub.4 NBF.sub.4 
Na/k 
ACF 2.7 
2.7 
2.6 
2.1 
0.6 0.08 
0.006 
in Diglyme on on 
Steel 
Pt 
25 wt. % Bu.sub.4 NBF.sub.4 
Na/k 
ACF 3.1 
3.1 
3.1 
3.0 
2.5 0.6 
0.06 
in Diglyme on on 
Steel 
Pt 
1.5 wt. % LiBF.sub.4 
Na/k 
ACF 3.1 
3.1 
3.1 
2.9 
1.8-0.2 
0.02 
0.002 
in Diglyme on on 
Steel 
Steel 
10 wt. % LiBF.sub.4 
Na/k 
ACF 3.1 
3.1 
3.1 
2.7 
-- 0.9 
0.09 
in Diglyme on on 
Steel 
Pt 
Diglyme Na/k 
ACF 3.1 
3.0 
2.7 
1.3 
0.2 0.02 
0.002 
on on 
Steel 
Pt 
10 wt. % NaBF 
Na/k 
ACF 2.2 
2.2 
2.2 
2.2 
1.8 0.6 
0.1 
(saturated) in 
on on 
Propylene Pt Pt 
carbonate 
13 wt. % LiBF.sub.4 
Na/k 
Carbon 2.6 
2.6 
2.5 
1.7 
0.7 0.16 
0.02 
in Propylene 
on fibers 
Carbonate Steel 
(basket 
weave) Cu plated 
15 wt. % LiBF.sub.4 
Na/k 
Thornel 3.0 
3.0 
2.9 
2.4 
1.3 0.25 
0.025 
in Propylene 
on Cu.degree. 
carbonate Steel 
Plated 
15 wt. % LiBF.sub.4 
Na/k 
ACF 2.6 
2.6 
2.6 
2.5 
1.9 1.8 
0.5 
in Propylene 
on on 
carbonate Steel 
Pt 
23 wt. % Bu.sub.4 NBF.sub.4 
Na/k 
ACF 2.6 
2.6 
2.6 
2.5 
2.2 0.9 
0.17 
in Propylene 
on on 
carbonate Pt Pt 
25 wt. % ET.sub.4 NBF.sub.4 
Na/k 
ACF 2.9 
2.9 
2.9 
2.9 
2.8 0.7 
0.09 
(Sat) in Propylene 
on on 
carbonate Pt Pt 
11.5 wt. % ET.sub.4 NBF.sub.4 
Na/k 
ACF 2.7 
2.7 
2.7 
2.5 
2.1 1.1 
0.2 
(Sat) in Propylene 
on on 
carbonate Steel 
Pt 
2.5 wt. % ET.sub.4 NBF.sub.4 
Na/k 
ACF 2.0 
2.0 
2.0 
1.9 
1.6 0.7 
0.1 
in Propylene 
on on 
carbonate Steel 
Pt 
Propylene carbonate 
Na/k 
ACF 2.8 
2.8 
2.8 
2.5 
2.0-0.8 
0.1 
0.01 
on on 
Steel 
Pt 
30 wt. % LiBF.sub.4 
Na/k 
ACF 2.4 
2.4 
2.3 
1.6 
0.6 0.09 
0.01 
in Ethylene carbonate 
on on 
Steel 
Pt 
30 wt. % LiBF.sub.4 
Na/k 
ACF 2.6 
2.6 
2.6 
2.6 
2.5 1.6 
0.5 
in Ethylene carbonate 
on on 
T = 0.degree. C. (Skin) 
Steel 
Pt 
__________________________________________________________________________ 
B. As a comparison the non-equilibrium voltage values of a commercially 
available size C ni-Cd cell (Model G.C.2 of General Electric Corporation) 
was determined. The results are as follows: 
______________________________________ 
Load 
10M lM 100K 10K lK 100 10 
______________________________________ 
Volts 1.27 1.27 1.27 1.27 1.27 1.26 1.21 
______________________________________ 
The principles, preferred embodiments and modes of operation of the present 
invention have been described in the foregoing specification. The 
invention which is intended to be protected herein, however, is not to be 
construed as limited to the particular forms disclosed, since these are to 
be regarded as illustrative rather than restrictive. Variations and 
changes may be made by those skilled in the art without departing from the 
spirit of the invention.