Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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IHSOLUBLE MIXED HEAVY METAL POLYSULFIDE CATHODES
This invention relates to metal sulfides and particularly to metal
polysulfides utilized as cathode materials in non-aqueous electrochemical
cells.
Elemental sulfur has an extremely high theoretical Plectrochemical capacity
(1.6 A-Hr/g) and in fact has been utilized as a cathode in electrochemical
cells. ffoweYerO such use of sul~ur has been limited by Yarious shortcomings
which severely curtailed the actual attainment of such high capacity and which
further limited utilization of sulfur in many cell applic~tions. Sulfur is
almost insulative with a very low ionic and electronic conductivity, and at
least electronic conductivity of the cathode material is necessary in order to
obtain reasonably full utilization thereof. Thus, sulfur cathodes have required
massive capacity reducing inclusions on non-cathode active electronic
conductors. In solid state cell applications further capacity reducing
non-cathode active or low capacity ionic conductors have been further requlred.
In addition to its low conductivity sulfur has a relatively high vapor
; pressure and dissolution rate with resultant tendency to reduce cell life by
internal cell short circuittng, particularly on storage at elevated
temperatures. Sulfur cathodes have thus been generally utilized only in
elevated temperature cells wherein the sulfur is in the molten state during
operation~ with incredsed conductivity and wherein the molten sulfur is, of
necessity, fully contained.
In srder to at least partially utilize the inherent capacity of sulfur ,
metal sulfides such a~ PbS, AgS, and ~he like were utilized as cathodes,
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particularly in solid state cells. ~hough such materials did nut have the
detrimental high vapor pressure or dissolution of the elemental sul~ur they
also did not provide capacities anywhere near that of the theoretical sulfur
capacity.
Metal disulfides ~uch as ~eS2, CoS2, and NiS2 because of their
relatively higher sulfur content provided higher capacities than the
monosulfide materials and have been effectively utilized in cells, particularly
in elevated temperature operating cells. ~he theoretical capacity of FeS2 for
example is .730 A-Hrlg with about .700 A-Hr/g having been actually obtained.
Capacities of such materials were however still not fdvorably comparative to
that of the elemental sulfur.
Another class of metal sulfides are the transition metal intercalation
compounds. This class is best exemplified by titanium disulfide (TiS2).
Cathodes made from these materials are best suited for rechargeable cells
because of the complete reversibility of intercalation reactions with ~lkali
metal ions. However, such materials provided less primary capacity than other
metal sulfides since the sulfur itself does not enter the electrochemical cell
reaction.
Alkali metal pulysulfides such as Li2SX and Na2Sx with x<l
represent another class of metal sulfides which have been used as cathodes in
electrDchemlcal cells. 5uch materials have in fact provided relatively good
capa ities however seYeral serious disadvantages havP accompanied their use.
The non-aqueous electrolytes of cells having the alkali metal polysulfide
cathodes have had the tendency of beco~ing increasingly more viscous with
accompanying loss of conductivity and severely reduced discharge rate
capability. Additionally the alkali metal polysulfides are at least partially
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soluble in common electrolyte solvents and are accor-
dingly likely to cause cell self discharge over extended
periods o~ time.
Commonly assigned U.S. Patent 4,481,267 discloses a
novel alass of matal polysulfide materials utilized as
cathodes in non-aqueous electrochemical cells. These
metal polysulfides are characterized by haviny an atomic
ratio of sulfur to transition metal o~ greater than 3.5
to 1. Examples of such polysulfides include CoS4 5,
NiS4 5, CUS3 7, and FeS4 5. These materials come the
closest to the theoretical capacity of sulfur, having
theoretical capacities slightly greater than 1.0 A-Hr/g.
These heavy metal polysulfides can be prepared by
precipitation from mixing an aqueous solution of the
metal chloride salt with an aqueous polysul~ide
solution. Suitable polysulfide solutions are prepared
from ammonium polysulfide or sodium polysulfide, for
example. One method of preparing heavy metal poly-
sul~ides using ammonium polysulfide provides products,
except for the heavy metal polysulfide, which are
volatile and can be driven off by heatinq thus
simplifying the separation. When polysulfides prepared
by this method or the method disalosed in U.S. patent
4,481,267 are not heated th~y have a sulfur to metal
ratio from 3.5/1 to as high as 5/1. While these
materials are cathode active as disclosed in U.S. patent
~: 4,481,267 it has been found that there is less degra-
dation on storage if the heavy metal polysulfide are
: first heated to a constant w~ight value before being
made into cathodes. H~ating under vacuum to a constant
weight removes loosely bound ulfur and results in
polysulfides having a sulfur content from betwe~n 3.5/1
~o 4.5/1.
The present invention relates to the use of mixed
: 35 hea~y metal polysul~ides as very high energy density
: cathode~ in non-aqueou~ electrochemical cells. These
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polysulfides are insoluble in non-aqueous electrolytes
and have the formula M(l)w+nM(2)x+m(Sy)z 2 (wherein M(l)
and M(2) are different heavy metal atoms, n and m are
integer~ representing the. valence states of M~l) and
M(2) respectively, w and x are n~n-zero integers
representing the stoichiometry of M(1) and M(2)
respecti~ely in the polysulfide, S is sulfur, nw + mx =
2z and y i5 greater than 4.5).
Generally speaking the mixed heavy metal poly-
sulfides of the present invention are of the
stoichiometric formula ~l)M~23Sy wherein M(l) and M~2)
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are different heavy metal atoms, S is sulfur, and y is greater than or equal to
4.5. ~hen mixed heavy metals are used in the preparation of the polysulfide it
has been discovered that the polysulfide has a higher sulfur content after
heating under ~acuum than when a single heavy metal is used. This in turn gives
the mixed heavy metal polysulfide a higher electrochemical capacity.
The present invention was discovered when a solution containing salts of
two different heavy metals was mixed with an aqueous solution of sodium
polysulfide. Specifically, when an aqueous equimolar solution of FeS~4 and
CoS04 was mixed with an excess of an aqueous sodium polysulfide solution a
material precipitated which, after ~ashing and drying, analyzed as
C2 3Fe 7S15 . This material has an atomic ratio of sulfur to metal of
5/1 after vacuum drying at 110 C, which is higher than that typically found in
the single heavy metal polysulfides after vacuum drying at 110 C. Changing the
concentration of the salts in the initial solution results in changing the
composition of the mixed heavy metal polysulfide. Also, changing the metals in
the salts would lead tc mixed heavy metal polysulfides of different
composition. Clearly, any metal salt which is soluble in water could be used in
~he preparation of a mixed heavy metal polysulfide. Such salts would inc~ude
the salts of copper9 titanium, vanadium, thromium, molybdenum, tungsten, iron,
ruthenium, cobalt, rhodium, and nickel
~ n addition to preparation from aqueous solutions it is also possible to
prepare mixed heavy metal polysulfides fron organic sol~ents such as dimethyl
formamide or diethyl ether. Instead of ammonium polysulfide, the sulfur can be
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provided from H2S and from elemental sulfur itself.
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It is an advantage of the present invention that mixed heavy metal
polysulfides can be prepared and used as cathode active materials in
electrochemical cells. The electrolyte used in the cells could be aqueous,
non-aqueous, or solid. ~hen the electrolyte is non-aqueous the anode can be
comprised of an alkali or alkaline earth metal such as lithium.
It is an additional advantage that the mixed heavy metal polysulfides can
be made more cheaply than the single heavy metal polysulfides. For example,
Co257 is a polysulfide that is suitable for use as a cathode material but
is expensive due to the cost of cobalt. By replacing some of the cobalt with a
different and cheaper heavy metal such as iron the cost of the cathode material
drops.
It is an additional advantage that the mixed heavy metal polysulfides do
not always exhibit properties similar to the corresponding single metal
polysulfides. For example, Fe3S8 is very air sensitive and decomposes
readily. However a cobalt-iron polysulfide does not suffer from this problem.
It is an object of the present invention to provide mixed heavy metal
polysulfide materials that are useful as cathode active substances in
e1ectrochemical cells .
It is another object of the present invention to provide polysulfide
~ materials that are cheaper tD produce ~han previously known polysulfide; ~ materials but provide at least the same capacity.
These advantages and objects will become clear in ligh~ of the ~ollowing
examples. It is understood that such examples are illustrative in nature and
that other mixed heavy metal polysulfides can be prepared. Accordingly, the
details described in such examples are not to be construed as limitations on
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the present invention. Unless otherwise indicated all parts are parts by
weight.
EXAMPLE l
An aqueous solution that is equimolar in FeS04 and CoS04 is prepared.
To this is added an aqueous solution of sodium polysulfide. The amount of
sodiun polysulfide added is less than the amount needed to precipitate all of
the Co2 and Fe2 ions. A black precipitate is formed which is separated by
~iltration, The black precipitate is washed and then dried under vacuum at
110 C until a constant weight is obtained. The resulting mixed heaYy metal
polysulfide is analyzed and found to have a formula Co2 3Fe 7S15. This
polysulfide has an atomic ratio of su1fur to metal of 5/1, a Yalue which is
greater than that generally found in single heavy metal polysulfides.
EXAMPLE 2
A solution of (N~4)2MoS4 in dimethyl formamide(DMF) is prepared. TD
this solution is added a solution of elemental sulfur in DMF. Enough sulfur is
added to react the MoS4 2 anion to the MoS~ 2 anion. The polysulfide
onlon precipltates as ~NH4~zMoS9 and this is separated by filtration. The
(NH4)2MoSg is then added to a ;solution of CuCl2 in DMF. The CuCl2 is
n~molar~excess of the (NH4~2MoSg. Diethyl ether is added until the
so1uti~on becomes cloudy. This solution is then chilled and crystals of CuMoSg
are~formed. This mixed heavy metal polysulfide has an atomic ratio of sulfur to
metal of 4.5/l.
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EXAMPLE 3
An ammoniacal aqueous solution of NH4V03 is preparedr This solution is
then saturated with H2S to for~ (NHq)Y3S4 which precipitates out as
dark crystals. These dark crystals are separated from the solution by
filtration. The separated crystals are then dissolved in a dilute aqueous
sodium hydroxide solution. Sulfur is then added with stirring to form
(NH4)3VS8. A solution of cobalt(II) chloride complexed with ammonia is
then added to give Co3(VSB~2.
COMPARATIVE EXAMPLE A
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Button type cells were made with the dimensions of .95"(24.5mm) outside
diameter by 0.12"(3~m) height with each containing a lithium foil anode(440
mA-Hr) pressed on a nickel grid welded to the inside bottom of the cell
container. The cell contained an electrolyte of .75M LiC104 in l:l~volume) of
propylene carbonate and dimethoxy ethane. The cathode was comprised of 100 mg
of Co2S7 formed into a disk shape having a surface area of 3cm2. One cell
was discharged at high rate (500 ) and gave 800mA-Hr/g to a lV cutoff. Another
cell was discharged at low rate (2k ) and gave lOOOmA-~rlg to a lY cutoff. Both
discharges have one major voltage plateau at 1.8Y and a shorter plateau at
~ ~1.4V.
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COMPARATIVE EXAMPLE B
Button type cells were constructed identical to those in Comparative
Example A except that the cathode was comprised of 100 mg of Fe3S8. One
cell was discharged at high rate (500 ) and one cell at low rate (~k ). Each
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discharge gaYe rough1y the same capacity of 900 mA-Hrlg to a ~Y cutoff. The
discharges had two voltage plateaus of roughly equal duration, the first at
1.6Y and the second at 1.4V.
EXAMPLE 4
Button type cells were constructed identical to those in C~parative
examples A and B except that the cathode material was comprised of 100 mg of
C2 3Fe 7S15. One cell was discharged at high rate (500 ) and gave 110~
mA-Hr/g to a lV cutoff. A second cell was discharged at low rate (2k ) and gave
1300 mA-Hr/g to a lV cutoff, Both discharges had two voltage plateaus of
roughly equivalent duration, the first at 1.8Y and the second at 1.4V. This
data provides two pieces of evidence that Co2 3Fe 7S15 is a discrete new
material and not a mixture o~ the cobalt and iron polysulfides. The first is
that the capacity is greater than the sum of the capacities o~ the requisite
amount of cobalt and iron polysulfides. The second is the absence of a voltage
plateau at 1.6V which would be present of there was any iron pDlysulfide
present.
The mixed heavy metal polysulfide materials of the present inYention are
sultaùle for use in both aqueous and non-aqueous electrochemical cells since
they are insoluble in th~ c~mmon aqueous and non-aqueous solYents. 5uch
solvents include propylene carbon.a~e, acetonltrile, dimethoxyethane, dioxolaneu~gamma-butyrolactone, tetrahydrofuran, methyl formate, dimethylsulfoxide, sul~urdioxide, aqueous alkaline solutions and the like. In addition, such ~ixed heaYy
metal::polysul~i~es are useful as high capacity cathsdes in solid state cells
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wherein the electrolyte is comprised of ionically tonductive metal sal~s in the
solid state, such as Lil.
In order to take full advantage of the high energy densities of the
polysulfide materials of the present invention it is preferred that they are
utilized in non-aqueous cells having alkali or alkaline earth metal anodes such
as lithium wherein they provide cells with typi al volta~es between about 1.5
and 2 volts.
Although the mixed heavy metal polysulfides given in the examptes were
limited to just two different heaYy metals it would be possible to prepare
mixed heavy metal polysulfides c~mprised of three or more di~ferent heavy
metals. This would be accomplished by preparing a solution of three or more
heavy metal salts from which the mixed heavy metal polysu7fide is precipitated.
The above examples were given for the purposes of illustrating the present
invention. Changes may be made in particular heavy metals, ratios of
components, cell structure, components of such cell and the like without
departing frGm the scope of the present invention as defined in the following
claims:
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