Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
This invention relates to storage cells
("Batteries") and specifically to a material for use as
a secondary cell cathode. It is an object of the inven-
tion to provide a material which, when incorporated as
a cathode in a secondary cell, will allow a high degree
of reversibility as the cell is repeatedly charged and
discharged. It is anoth~r object of the invention to
provide a relatively inexpensive material that is easily
prepared for use as a cell cathode.
SUMMARY OF THE INVENTION
The inventors have discovered that a lithium
molybdenum disulphide (hixMoS2) compound exhibits several
distinct stages ~"phases") of operation when used as a
cathode in a battery having a lithium anode.
During discharge of a newly constructed battery,
(termed "phase 11' operation by the inventors) lithium
cations intercalate into the cathode thus raising the
concentration of lithium in the cathode. The inventors
have found that battery voltage decreases during battery
discharge to a particular point at which a plateau is
reached. The plateau represents a region in which bat-
tery voltage remains constant while the concentration of
lithium in the cathode continues to increase. Once a
particular concentration of lithium in the cathode is
achieved, the battery will continue to discharge in what
the inventors have termed the "phase 2" region. In the
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phase 2 region, the battery potential may be reversibly
increased or decreased within certain limits as the con-
centration of lithium in the cathode correspondingly de-
creases or increases. The phase 2 reyion overlaps the
plateau on which the phase 1 to phase 2 transition occurs
in that cathode concentrations of lithium are observed
in phase 2 equal to those observed during the transition
from phase 1 to phase 2, but at voltages higher than
that at which the transition occurred. The transition
between phase 1 and phase 2 does not appear to be rever-
sible along the plateau. Phase 2 is a preferred phase
of operation because of the excellent reversibility ob-
served in batteries constructed with cathodes which have
been conditioned to operate in phase 2. As is discussed
in more detail hereinafter, while the initial discharge
into phase 2 may be done at room temperature (e.g. about
20C) if the conversion is done relatively quickly
it is generally preferred (and, in the case of
relatively thick cathodes, may become necessary)to have
a lower temperature (e.g. 0C).
If the potential of a battery which is operating
in phase 2 is allowed to decrease to a particular level,
a second plateau is reached along which cathode concen-
tration of lithium increases at constant battery poten-
tial until a third phase ("phase 3") is reached in which
battery potential may again be reversibly varied as
cathode concentration of lithium increases and decreases.
In phase 3 operation, the cathode concentration of
lithium may be decreased to overlap values of cathode
lithium concentration found in phase 2 and in the plateau
-- 3 --
along which the phase 2 to phase 3 transition occurs.
A battery operating in phase 3 does not appear to be as
highly reversible as a battery in phase 2, and tends to
lose capacity more rapidly on repeated eharge-diseharge
cycling. However, in some applications phase 3 opera-
tion may be considered preferable to phase 2 operation
- because energy density in phase 3 is considerably higher.
Herein, and in the claims, the term "reversible" is used
on the understanding that it does not mean perfeet or
100~ reversibility.
BRIEF DESCRIPTION OF T~IE DRAWING
The figure is a graph showing representative
charaeteristics of a battery having a cathode prepared
by eoating molybdenum disulphide (MoS2) onto an aluminum
foil substrate, a lithium foil anode and a lM LiC104 in
propylene carbonate electrolyte. Battery voltage (mea-
sured in volts) is plotted as the ordinate ~s. an
abscissa "x" where "x" represents the coneentration of
lithium in the cell cathode having the general formula
LixMoS2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The figure shows typieal charaeteristies of
a battery prepared with a lithium anode and a cathode
construeted by coating molybdenum disulphide ~MoS2) onto
an aluminum foil substrate. The quantity x represents
the concentration of lithium in the cathode which in-
ereases as lithium eations interealate into the eathode
during battery diseharge. ~s will become more elearly
apparent hereinafter, while the figure is typical, it
is to be understood that the charaeteristies shown may
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vary somewhat in actual practice depending upon various
parameters.
The battery is shown to discharge ~rom an
initial voltage of above 3 volts along path AB (3.3
volts is generally typical). Along this path, lithium
cations intercalate into the cathode as battery potential
decreases, thus increasing the concentration of lithium
in the cathode as indicated. Path AB shows voltage de-
creasing from theinitial value of about 3.3 volts to a
plateau (path BD) while x correspondingly increases
from 0 to about 0.2. This has been found typical at
room temperature. However, at low temperatures (e.g.,
0C), path ~B has been found to become much steeper
than illustrated with point B lying at a point where x
is only slightly greater than 0. In some cases, point
B has been observed to lie as high as about x = 0.5 on
a room temperature discharge. However, it is speculated
that some electrolytedecomposition may have occurred on
the discharge, or that some impurity may have been
present. The inventors have termed as "phase 1" the
physical structure of a cathode which exhibits a varia-
tion of battery potential with cathode lithium concen-
tration governed by the path AB where point B lies in
the range of x slightly greater than 0 to x ~ 0.5.
If the potential of a battery which is operat-
ing in phase 1 is allowed to decrease along path AB,
then a plateau represented by path BD in the figure is
reached. The plateau is shown at about 1.0 volt - in
practice it will typically fall in the range of about
0.9 to about 1.1 volts at room temperature, but may go
-- 5 --
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as low as about 0.7 volts at very low temperatures.
Although the plateau is shown as beginning at x ~ 0.2,
it is to be understood on the basis of the immediately
preceding discussion that in practice it may begin in
the region of x only slightly greater than 0 to about
x = 0.5. The plateau path BD is shown in the figure as
ending at a point about where x = 1Ø In practice,
this end point has,been observed to occur as high as
about x = 1.5. The reason for such variation is not
clear, but may be attributable to unknown impurities in
the cathode. The plateau path BD shown in the figure
indicates a region in which the battery operates at a
relatively constant potential of about 1.0 volts
while the concentration o~ lithium in the cathode repre-
sented by x increases during battery discharge.
As shown in the figure, if a battery operating
on the plateau path BD is allowed to continue to dis-
charge, then once a cathode lithium concentration repre-
sented by a value of about x = 1.0 has been achieved,
the battery is observed to discharge along path DE.
The battery discharge may be halted at any point along
path DE and the battery may then be substantially re-
versibly recharged along path EC. The inventors have
termed the physical structure of a cathode which exhibits
a variation of battery potential with cathode lithiu~
concentration governed by the path CE shown in the figure
to be "phase 2" and describe the process of reversibly
charging and discharging the battery along path CE as
"phase 2 operation". Once phase 2 operation has been
3Q achieved, the battery will not re-enter the BD plateau
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direc~ly ~rom phase 2.
~lowever, a~ain it is to be noted that while the
figure is representative, variations are observed in prac-
tice. As shown, path DE depicts voltage decreasing from
an initial value of about 1.0 volts to about 0.55 volts
while the value of x increases from about 1 to about 1.5.
Similarly, path CE depicts voltage decreasing from about
2.7 volts to about 0.55 volts as x increases from about
0.2 to about 1.5. In practice, for given voltages the
observed value of x has been somewhat variable. For example,
point C (voltage about 2.7 volts) may range from about a
point where x is only slightly greater than 0 (at low tem-
peratures) to about x = 0.5. Point ~ (voltage about 1.0
volt) may range from about x = 1.0 to about x = 1.6.
Point E (voltage about 0.55 volts) may range from about
x = 1~3 to about x = 2Ø However, in all cases path CE
maintains a slope generally downwardly to the right. The
reason for such variations is not clear, but again may be
attributable to unknown impurities in the cathode. Also,
it is to be noted that such observations are at room tem-
perature. ~t lower temperatures, measured voltages for a
given value o~ x will tend to be somewhat lower~ The 0.55
volts voltage depicted in the figure for point E (and path
EG discussed hereinafter) is typical at room temperature
but generally may range from about 0.4 volts to about 0.6
volts.
The inventors have observed the most reliably
reversible battery operation to occur in a battery having
a cathode which has been conditioned to operate in phase
2, and have further observed the most reliably reversible
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phase 2 operation to occur along path CD. It has been
found that if a battery which is operating in phase 2 is
discharged along path CE reversibility degrades as bat-
tery potential drops below about 1 volt. Because battery
reversibility in phase 2 degrades as the battery is dis-
charged below approximately 1 volt, the inventors recom-
mend that phase 2 operations be confined to path CD by
monitoring battery voltage to prevent recharging above a
battery potential of about 2.7 volts and by preventing
battery discharge below about 1 volt.
If the potential of a battery having a phase 2
cathode is allowed to decrease along path OE to ap-
proximately 0.55 volts (this voltage being typical as
noted above3, then a second plateau represented by the
path EG is reached (at a cathode lithium concentration
of about x - 1.5 in the representative figure shown)
along which a transition from phase 2 to a third phase
occurs at a relatively constant potential while the con-
centration of lithium in the cathode represented by x
increases to about x = 2.8. It is preferable to maintain
the value of x equal to or below appro~imately 3 in prac-
ticing the present invention.
If a battery which is operating on the EG
plateau is allowed to continue to discharge, then once
a cathode lithium concentration represented by a value
of about x = 2.8 has been achieved, the cell i5 observed
to discharge further along path GH. The cell discharge
may be halted at any point along the path. The battery
may then be recharged along path HF. The inventors have
termed the physical structure of a cathode which exhibits
a variation of battery potential with cath~de lithium
concentration governed by the path FH shown in the figure
"phase 3" and describe the process of reversibly charging
and discharging the battery along path F11 as "phase 3
operation". Once phase 3 operation has been achieved,
the battery will not re-enter the ~G plateau directly from
phase 3.
Point H in the figure does not represent the
lower limit o~ battery discharge capability. However,
the inventors have observed a significant degradation
in the performance of a battery which is discharged in
phase 3 below about 0.3 volts. This degradation is
thought to be related to a diffusion of lithium ions into
the cathode aluminum substrate resulting in the formation
of a lithium-aluminum alloy.
I The inventors believe phase 3 operation not to
be as reliably reversible as phase 2 operation. Further,
the inventors have not been able to achieve battery poten-
tials in phase 3 as high as those achieved in phase 1 or
phase 2 operation of the battery. However, as indicated
previously, this does not mean that phase 3 operation is
invariably undesirable. The energy density of a battery
in phase 3 is considerably higher than the energy den~
sity of a battery in phase 2. Thus, it is contemplated
that in some applications where energy density require-
ments are subservient to improved reversibility and volt-
aye characteristics, phase 3 operation may be selected
as more desirable than phase 2 operation.
The inventors have found that if a battery
operating in phase 3 is slowly recharged, then a trans-
g
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ition from phase 3 to phase 2 will occur when abattery potential of about 2.3 volts is achieved.
The inventors have observed a degradation
o~ battery reversibility in phase 2 and phase 3 oper-
ation when battery potentia:L is allowed to fall below
about 1 volt. It is thought that this degradation
may be due to decomposition of the battery electro-
lyte. The inventors believe that if the problem of
electrolyte instability can be overcome then excellent
reversibility will be observed over the entire phase
2 path and that improved reversibility may be observed
j in phase 3 operation.
X-ray diffraction analysis performed by the
inventors reveals the phase 2 structure to be a
layered compound having a crystal symmetry distinct
from that exhibited by a phase 1 struckure.
Example 1.
A battery was constructed as follows:
The cathode consisted of approximately
6 cm2 of aluminum foil on which was deposited 3 mg/cm2
of MoS2. The anode was a similar sized piece of
lithium foil. The electrodes were separated by a
polypropylene separator soaked with an electrolyte
of .7M LiBr in propylene carbonate (PC). The
battery was discharged at lmA through phase 1 down
through a Eirst voltage plateau of about 1.1 V
until the cathode was converted to phase 2. The
battery was then repeatedly charged and discharged more
than one hundred times at 10 r~ in phase 2 between
-- 10 --
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voltages up to 2.7 V which corresponds to a fully
charged phase 2 cathode and 1.0 V. Battery capa-
city corresponded to 1 electron per MoS2 molecule
(ax= 1~.
Example 2.
A battery of similar construction to the
cell in Example 1 except that the cathode had only
.3 mg/cm2 of MoS2 deposited on it. The cathode was
converted into phase 3 by discharging the battery
through both the first voltage plateau into phase 2
and through a second voltage plateau of about 0.55 V
into phase 3. The discharge current was 1 mA. The
battery was then repeatedly charged and discharged
between 2.4 V (which corresponds to a fully charged
phase 3 cathode) and 1.0 V more than one hundred
times at 1 mA. Battery capacity corresponded to
1.5 + 0.2 electrons per MoS2 molecule (~x ~ 1.5 + 0.2).
The battery was repeatedly charged and discharged a
few times between 2.4 V and 0.5 V. Battery capacity
in this case corresponded to 2 + 0.2 electrons per
MoS2 molecule ~x ~ 2 + 0.2).
Example 3.
The battery of Example 2 was recharged slowly
(at about 100 microamps) to 2.7 ~. It was found that
the cathode was reconverted to phase 2 operation
after being recharged in this manner.
Example 4.
A battery was constructed as follows:
The cathode consisted of 1.3 cm2 of alum-
inum foil on which was deposited 0.5 mg/cm2 of MoS2.
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The anode consisted of a similar area of lithium foilpressed onto an expanded nickel grid. The electrodes
were suspended in an electrolyte of .7M LiBr and
PC contained in a 50 ml glass beaker. An argon
atmosphere was contained within the beaker with a
neoprene stopper. This battery was conditioned by
a 100 microamp initial discharge, then repeatedly
cycled in phase 2 eighty-two times between 2.7 V
: and 1 V at 100 microamps. ~he battery was then
further discharged into phase 3 where it cycled
repeatedly ten times between 2.4 V and .5 V.
~ lthough the initial conditioning discharges
in the foregoing examples were done at room temperature
(about 20C) and good results were achieved, it is
generally considered desirable to cool a cell for
the conditioning discharge. Otherwise, problems with
electrolyte decomposition may be encountered. The
cathodes of the above examples were relatively thin
and it was possible to perform the conditioning
discharges relatively quickly at room temperature
without development of significant temperature grad-
ients in the cathode. ~owever, with thicker cathodes,
the desirability of cooling becomes important. At
10 mg/cm of MoS2, it appears that cooling is essen-
tial. Although cooling temperatures as low as -20C
have been used, a temperature of 0C has been found
quite satisfactory for cathode thicknesses ranging up
to about 20 mg/cm2 of MoS2.
Phase operation may also be achieved where the
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MoS~ is partially oxidized to MoO2. Partial
oxidization to MO2 can improve conductivity without
serious loss of capacity.
Exam~le 5.
A battery having a cathode which included
MoS2 partially oxidized to MO2 was constructed as
follows:
(a) MoS2 powder having an average
particle diameter of about 20
microns was mixed in a 1 to 1
volume ratio with propylene glycol
and a film of the resulting slurry
applied to the aluminum foil
substrate.
(b) The substrate with applied film
was baked at 580C in an atmos-
phere containing about 0.4 mole
percent oxygen in nitrogen for
about 10 minutes to form a cathode
containing approximately 20 mole
percent MO2 and approximately
80 mole percent MoS2.
A cell was constructed using two stainless
steel flanges separated by a neoprene O-ring sealer.
The anode consisted of a 6 cm2 sheet of lithium. A
6 cm2 piece of the prepared cathode (on which had
been deposited approximately 43 milligrams of the
partially oxidized MoS2) was used as the cell cathode.
A porous polypropylene separator sheet which had
been soaked in a 1 m solution of lithium perchlorate
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in propylene carbonate was inserted between the
anode and the cathode.
The newly constructed cell was conditioned
by înitially discharging it at 4 mA to a lower
cutoff voltage of about 0.85 V. During this initial
discharge, the cell voltage dropped in about 20 minutes
: to a plateau of about 1 V alnd then decrea~ed approx-
imately linearly to about 0.85 V in a further 2 hours.
The cell thus prepared and conditioned was cycled
through 66 discharge-charge cycles at about 4 mA
between a minimum voltage of about 0.85 V and a
maximum voltage of about 2.7 V.
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