Chromium (V) compounds as cathode material in electrochemical power sources

A cathode for use in a thermal battery, comprising a chromium (V) compound. The preferred materials for this use are Ca.sub.5 (CrO.sub.4).sub.3 Cl, Ca.sub.5 (CrO.sub.4).sub.3 OH, and Cr.sub.2 O.sub.5. The chromium (V) compound can be employed as a cathode material in ambient temperature batteries when blended with a suitably conductive filler, preferably carbon black.

BACKGROUND OF THE INVENTION 
This invention relates to thermal batteries employing cathodes comprising a 
major amount of a chromium (V) compound as cathode active material. In 
another aspect, the invention relates to a cathode for use in a thermal 
battery. In still another aspect, the invention relates to the use of 
chromium (V) compounds as cathode materials in ambient temperature 
batteries. (NOTE: In discussions of battery technology, the terms cathode, 
depolarized cathode, ad depolarizer are frequently used. Hereinafter, only 
the term "cathode" will be used, and will be understood to embrace the 
meanings carried by any of these terms.) 
Thermal batteries are generally described as batteries which have an 
electrolyte comprising a salt mixture, (i.e., LiCl/KCl eutectic), which is 
solid at ambient temperatures, and nonconductive to ions. When fused, the 
salt mixture becomes fluid and ionically conductive. The development of 
thermal battery technology and details thereof are discussed in more 
detail in a report published by The American Society of Mechanical 
Engineers, entitled A Review of Thermal Battery Technology by B. H. Van 
Domelen and R. D. Wehrle, reprinted from the 9th Intersociety Energy 
Conversion Engineering Conference, and incorporated by reference herein. 
More recent developments in thermal battery technology, although directed 
to a different aspect than the present invention are disclosed in related 
U.S. Patent Application Ser. No. 361,151, filed Mar. 23, 1982 by James R. 
Armijo et al., now U.S. Pat. No. 4,383,014 the disclosure therein being 
incorporated by reference herein. 
The disclosure of said patent application relates to an improvement in 
thermal battery technology wherein an FeS.sub.2 cathode includes an 
additive for stabilization purposes. However, although an improvement over 
the prior art, FeS.sub.2 cathodes, including those of the above-discussed 
application, have a number of disadvantages. For instance, although 
exhibiting longer life and greater voltage stability than prior art 
CaCrO.sub.4 cells, FeS.sub.2 cells require a separator pellet or layer 
between the cathode and the anode resulting in a complicated construction. 
In addition, the FeS.sub.2 cathode has a limited range or maximum 
temperature limit because it thermally decomposes in LiCl/KCl eutectic at 
approximately 550.degree. C. 
The Journal of the Electrochemical Society discloses in Vol. 124, page 968, 
(1977), in an article by J. R. Besenhart et al, that some testing has been 
done with chromium oxides as cathodes for secondary high energy density 
lithium batteries. However, there is no discussion or appreciation therein 
of the use of the specific Cr(V) compounds applied to both thermal 
batteries and ambient temperature batteries as cathodes in accordance with 
the present invention. 
The common cathode materials known to the inventors to be used in thermal 
batteries are CaCrO.sub.4, V.sub.2 O.sub.5 and FeS.sub.2. Of these, 
CaCrO.sub.4 and V.sub.2 O.sub.5 are soluble in molten LiCl/KCl eutectic. 
Therefore, a portion of the active cathode material must be expended in 
the formation of a separator layer to prevent self discharge. Cathodes of 
these materials are also subject to concentration polarization in the 
course of discharge. 
FeS.sub.2 is insoluble in LiCl/KCl eutectic, hence avoiding the two 
difficulties mentioned above. FeS.sub.2, however, possesses limited 
thermal stability, undergoing thermal decomposition in LiCl/KCl eutectic 
at approximately 550.degree. C. 
Like FeS.sub.2, the chromium (V) compounds, subject of this disclosure, are 
insoluble in molten LiCl/KCl eutectic, avoiding the necessity of separator 
formation and concentration polarization. Unlike FeS.sub.2, these 
compounds are not subject to decomposition in LiCl/KCl to at least 
600.degree. C. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved thermal 
battery, having a cathode insoluble in LiCl/KCl eutectic, or other 
suitable molten salt, of greater thermal stability than FeS.sub.2. 
Another object of the present invention is to provide a cathode suitable 
for use in batteries operative at ambient temperatures. 
Upon study of the specification and appended claims, further objects and 
advantages of this invention will become apparent to those skilled in the 
art. 
In accordance with the invention, an improved thermal battery containing a 
number of single cells, each comprised of a heat pellet, an intercell 
connector, an anode pellet, a separator, and a cathode pellet is provided 
wherein the cathode is primarily composed of a chromium (V) compound. 
Specifically, the chromium (V) compounds referred to contain, in addition 
to chromium in the plus five oxidation stage, oxygen, sometimes chlorine, 
and generally alkaline earth metals. 
In accordance with another aspect, the invention comprises a cathode useful 
in a thermal battery, the cathode being primarily composed of a chromium 
(V) compound, as discussed above. 
In yet another aspect, the invention comprises an improved cathode for use 
in conventional room or ambient temperature batteries. The cathode is made 
of one of the above-stipulated chromium (V) compounds blended with a 
conductive filler, appropriate for use as a cathode material, and a 
binder. In a preferred aspect, the conductive filler is carbon black. The 
cell is conventional as generally described in The Journal of 
Electroanalytical Chemistry, Vol. 72, (1976), pages 1-31, incorporated by 
reference herein. 
As presently discussed, a number of chromium (V) compounds can be employed 
in the cathodes in accordance with the invention. They are generally 
chromium (V) oxygen containing compounds, and alkaline earth metal salts 
such as the Ca and Ba chromium (V) compounds. Known specific compounds are 
Ca.sub.5 (CrO.sub.4).sub.3 Cl, Ca.sub.2 CrO.sub.4 Cl, Ca.sub.5 
(CrO.sub.4).sub.3 OH, Ca.sub.3 (CrO.sub.4).sub.2, and Cr.sub.2 O.sub.5, 
with Ca.sub.5 (CrO.sub.4).sub.3 Cl, Ca.sub.5 (CrO.sub.4).sub.3 OH and 
Cr.sub.2 O.sub.5 being the most preferred for use with an electrolyte of 
LiCl/KCl eutectic. Other compounds containing Cr(V) may be more suitable 
for use with other molten salts. Such a selection might easily be made by 
one skilled in the art. 
The preparation of these compounds is discussed in The Journal of Inorganic 
Chemistry, Vol. 4, page 78 et seq., (1965), which is incorporated by 
reference herein. 
When employed in thermal batteries, the cathode will consist of about 
70-90% by weight chromium (V) compound and about 10-30% by weight LiCl/KCl 
eutectic which is conventional in nature. 
The LiCl/KCl serves as wetting agent and as a binder, aiding pellet 
formation and improving the mechanical strength of the pellets. 
Alternatively, an electrolyte-binder mix may be used in place of the 
electrolyte alone, provided the binder is unreactive with the Cr(V) 
compound. Magnesium oxide is such a binder. It is also possible to prepare 
pellets of pure Cr(V) compounds with sufficient mechanical integrity to 
allow testing in single cells, as will be subsequently described. 
When chromium (V) compounds are employed in cathodes of ambient temperature 
batteries, the percentage by weight of the chromium (V) compound, filler 
and binder selection is conventional. Typically the cathode will comprise 
5-25% binder, 5-30% conductive filler, with the remainder chromium (V) 
compound. The filler is a material possessing good ambient tempreature 
electrical conductivity. Carbon black is the preferred filler, but 
possible fillers are copper, nickel, graphite, and others which will be 
apparent to those skilled in the art. Selection of binder and conductive 
filler for cathodes incorporating compounds containing reducible moieties 
other than Cr(V) is disclosed in The Journal of the Electrochemical 
Society, Vol. 120, pp. 1214 et seq., (1974); all incorporated by reference 
herein. 
As in thermal batteries, the most preferred compounds for use are Ca.sub.5 
(CrO.sub.4).sub.3 OH, Ca.sub.5 (CrO.sub.4).sub.3 Cl and Cr.sub.2 O.sub.5. 
DETAILED DISCUSSION OF THE INVENTION 
For thermal battery applications, wherein a cathode pellet is comprised of 
an active material, a molten salt electrolyte and a binder, the invention 
comprises the use of a chromium (V) compound as the active cathode 
material. As discussed previously, these chromium (V) compounds generally 
contain oxygen, usually chlorine, and alkaline earth metal such as Ca and 
Ba. Preferred chromium (V) compounds for this use are Ca.sub.5 
(CrO.sub.4).sub.3 Cl, Ca.sub.2 CrO.sub.4 Cl, Ca.sub.5 (CrO.sub.4).sub.3 
OH, Ca.sub.3 (CrO.sub.4).sub.2 and Cr.sub.2 O.sub.5, with the most 
preferred being Ca.sub.5 (CrO.sub.4).sub.3 OH, Ca.sub.5 (CrO.sub.4).sub.3 
Cl and Cr.sub.2 O.sub.5. 
In accordance with the invention, the cathode of the battery is typically a 
two gram pellet of the chromium (V) compound. LiCl/KCl eutectic is 
sometimes added as a wetting agent and pelletizing aid. Anodes may be 
calcium, calcium alloy, lithium or lithium alloy or other oxydizable 
material. Separator pellets are LiCl/KCl eutectic with MgO binder. In 
practical applications the chromium (V) compound comprises 70-90% of the 
mass of the cathode pellet, the remainder being, as aforesaid, electrolyte 
or electrolyte-binder mix. Binders other than MgO known to those skilled 
in the art will function effectively. The selection of the binder material 
and method of assembly of materials is conventional as is discussed in 
High Energy Density Li Cells, Pt. II, Cathodes and Complete Cells by G. 
Eichinger and J. O. Besenhard, J. Electrochem Soc. 124, 968 et. seq. 
When the chromium (V) compounds are to be employed in ambient temperature 
batteries, an electrically conductive filler must be blended with the 
chromium (V) compound. Suitably conductive fillers include, carbon black, 
graphite, metal powders or the like. A binder such as Teflon.RTM. is also 
added. Other suitable binders include polyethylene, polypropylene, or 
equivalent polymers. In this type of battery, the conductive filler 
typically comprises 5-30% by weight of the cathode, the binder is 
typically 5-25% by weight, with the remainder being the chromium (V) 
compound. The filler may be a combination of fillers and the binder, a 
combination of binders, as will be evident to those skilled in the art. 
The electrolyte used in the ambient temperature battery is propylene 
carbonate saturated with LiAsF.sub.6. The anode is pure lithium. The 
selection of the electrolyte and anode is conventional, and other 
materials can be substituted therefor as will be obvious to those skilled 
in the art. Such ambient temperature batteries are described in detail as 
noted previously, in, e.g., The Journal of Electroanalytical Chemistry, 
Vol. 72, (1976), pages 1-31, which has been incorporated by reference 
herein. 
In both the thermal batteries and the ambient temperature batteries, the 
cathode is made from finally divided chromium (V) compounds. Cr.sub.2 
O.sub.5 may be prepared by the method set forth in J.A.C.S. 74, (1952), 
P1676 by R. S. Schwartz, I. Fankuchen and R. Ward. Ca.sub.5 
(CrO.sub.4).sub.3 Cl, Ca.sub.5 (CrO.sub.4).sub.3 OH, and Ca.sub.2 
CrO.sub.4 Cl may be obtained by the methods of R. Scholder and W. Klemm. 
Angem. Chem. 66, 461 (1954); R. Scholder and H. Suchy. Z. Anorg. Allgem. 
Chem. 308 295 (1961); R. Scholder and H. Schwarz, ibid., 326, 11 (1963); 
E. Banks and K. L. Jaunarajs, Inorg. Chem. 4, 78 (1965); R. Scholder, F. 
Schwochow, and H. Schwarz ibid., 363, 10, (1968); E. Banks, M. Greenblatt, 
and B. R. McGarvey, J. Sol. St. Chem. 3, 308 (1971); each incorporated by 
reference herein. The powder is generally compacted with or without other 
materials as discussed in the above cited Eichinger and Basenhard article 
into the shape or structure making up the cathode.

Without further elaboration, it is believed that one skilled in the art 
can, using the preceding description, utilize the present invention to its 
fullest extent. The following preferred specific embodiments are, 
therefore to be construed as merely illustrative, and not limitative of 
the remainder of the disclosure in any way whatsoever. In the following 
examples, all temperatures are set forth uncorrected in degrees Celsius; 
unless otherwise indicated, all parts and percentages are by weight. 
EXAMPLE 1 
A cell was constructed employing a 2-gram pellet of Ca.sub.5 
(CrO.sub.4).sub.3 Cl as the cathode, LiCl/KCl eutectic with a binder as 
the electrolyte, and a Ca-stainless steel bi-metal as the anode. The 
cathode composition was substantially pure Ca.sub.5 (CrO.sub.4).sub.3 Cl. 
The cell was heated to 500.degree. C. under argon and discharged through a 
resistive load of 7.9 ohms. At approximately 90 second intervals, the 
resistive load was decreased to 0.89 ohms for 5 seconds. For 25 minutes, 
the cell voltage remained above 2 volts while under the 7.9 ohm load. 
EXAMPLE 2 
A cell was reconstructed employing a 2-gram pellet of Ca.sub.5 
(CrO.sub.4).sub.3 OH as the cathode. LiCl/KCl eutectic with a binder acted 
as the electrolyte, and Li-Si alloy, as the anode. The cathode composition 
was substantially pure Ca.sub.5 (CrO.sub.4).sub.3 OH. The cell was heated 
to 500.degree. C. under argon and discharged at a current density of 32 
mA/cm.sup.2. At approximately 90-second intervals, the current density was 
increased to 126 mA/cm.sup.2 for 5 seconds. For 61 minutes the cell 
voltage remained above 2 volts while discharging at 32 mA/cm.sup.2. 
EXAMPLE 3 
A cell was constructed employing a 2-gram pellet of Ca.sub.5 
(CrO.sub.3).sub.3 OH as the cathode, LiCl/KCl eutectic with a binder as 
the electrolyte, and stainless steel metal felt saturated with CaLi.sub.2 
alloy as the anode. The cathode composition was substantially pure 
Ca.sub.5 (CrO.sub.4).sub.3 OH. The cell was heated to 500.degree. C. under 
argon and discharged at a current density of 32 mA/cm.sup.2. At 
approximately 90-second intervals, the current density was increased to 
126 mA/cm.sup.2 for 5 seconds. For 73 minutes, the cell voltage remained 
above 2 volts while the cell was discharged at 32 mA/cm.sup.2. 
EXAMPLE 4 
A cell was constructed employing a 2-gram pellet of Cr.sub.2 O.sub.5 as the 
cathode. LiCl/KCl eutectic with a binder acted as the electrolyte, and 
Li-Si alloy was the anode. The depolarizer composition was substantially 
pure Cr.sub.2 O.sub.5. The cell was heated to 500.degree. C. under argon 
and discharged at a current density of 32 mA/cm.sup.2. At 55 second 
intervals the current density was increased to 126 mA/cm.sup.2 for 5 
seconds. For 23 minutes the cell voltage remained above 2 volts while 
discharged at 32 mA/cm.sup.2. 
In another example, while nonconductive at room temperature, the Cr(V) 
compounds can serve as cathodes in ambient temperature batteries when 
blended with a suitably conductive filler, for example, carbon black. 
EXAMPLE 5 
A cell was constructed employing a cathode plaque consisting of 66.7% 
Ca.sub.5 (CrO.sub.4).sub.3 OH, 26.6% carbon black, and 6.7% Teflon.RTM. 
filler. Propylene carbonate saturated with LiA.sub.s F.sub.6 acted as the 
electrolyte, and the anode was pure Li. An open circuit voltage of 2.6 
volts was obtained. When the cell was discharged at a current density of 
0.5 mA/cm.sup.2, a nominal voltage of 2.2 volts was observed; when 
discharged at a current density of 3 mA/cm.sup.2, a nominal voltage of 
1.30 volts was observed. 
The preceding examples can be repeated with similar success by substituting 
the generically or specifically described reactants and/or operating 
conditions of this invention for those used in the preceding examples. 
From the foregoing description, one skilled in the art can easily 
ascertain the essential characteristics of this invention, and without 
departing from the spirit and scope thereof, can make various changes and 
modifications of the invention to adapt it to various usages and 
conditions.