Note: Descriptions are shown in the official language in which they were submitted.
~04~i680
BACKC.I~OUN~ OF TIIF. INV~NTION
~ield of the Invention
The invention broadly relates to electrical energy
storage devices and more particularly to a lithium electrode
structure and a secondary electrochemical cell utilizing the
same.
Prior Art
Two approaches generally have been followed in the
construction of a lithium electrode for use in an elec~rical
energy storage device, such as a rechargeable battery,
particularly one employing a molten salt electrolyte. In
one approach, the lithium is alloyed with another metal such
as, for example, aluminum to form a solid electrode at the
operating temperature of the cell. In the other approach,
liquid lithium is retained in a foraminous metal substrate
by capillary action. Heretofore, the latter approach has
been preferred because it offers higher operating cell
voltages and therefore potentially higher battery energy
densities. Certain problems are encountered, however, when
it is attempted to retain molten lithium in a foraminous
metal substrate. More particularly, most metals which are
readily wetted by lithium are too soluble in the lithium to
permit their use as the metal substrate, whereas most metals
structurally resistant to attack by molten lithium are
poorly wetted by the lithium when placed in a molten salt
electrolyte.
It has been suggested that metals structurally
resistant to attack by molten lithium may be wetted by
immersion in molten lithium maintained at a high temperature.
However, the structure so wettcd by lithium at these higher
temperatures usually undcrgocs progrcssivc dc-wetting when
- 2 - ~ ~s
~045680
used ag th~ ne~ativc electrode in a ~econdary battery
containing a molten salt eloctrolyte maintained at the
substantially lower temperatures at which such a battery
operates. Thus after operation of the battery for a number
of cycles, it has been found that lithium no longer prefer-
entially wets the substrate, the electrode progressively
losing capacity. Various methods have been proposed in an
attempt to overcome this problem. See, for example, U. S.
Patents 3,409,465 and 3,634,144. None of the proposed
methods have proven entirely satisfactory.
The use of a solid lithium alloy as taught by the prior
art also is not without problems. More particularly,
lithium-aluminum alloy, for example, is approximately 300
millivolts more positive than liquid lithium. Thus, electro-
chemical cells utilizing lithium-aluminum alloys as elec-
trodes are not able to achieve the same potentials as those
utilizing liquid lithium electrodes. Further, in a molten
salt electrolyte, the lithium-aluminum alloy electrode
expands and contracts greatly during charging and discharg-
ing of the electrochemical cell. Thus, it has been reported
that the lithium-aluminum electrode may change in volume by
as much as 200% during charging and discharging of the cell.
Still further, lithium-aluminum alloys generally are limited
to a lithium content of less than about 30 wt.%.
Various other materials have been suggested for use as
an alloy with lithium to form a solid electrode. In U. S.
Patent 3,506,490, for example, it is suggested that the
lithium be alloyed with either aluminum, indium, tin, lead,
silver, or copper. However, none of these materials have
been proven to be completely satisfactory. Morc particularly,
~04S680
these other suggested materials, such as tin and lead for
example, form alloys containing lesser amounts of lithium
than does aluminum, and thus have a still lower capacity
(ampere-hours) per unit weight of alloy. Further, the
potential of these other alloys-compared with liquid lithium
is more positive than that of a lithium-aluminum alloy;
thus, alloys of such other materials are less desirable.
Other patents relating to solid lithium anodes include U. S.
Patents 3,506,492 and 3,508,967.
Thus a need still exists for a lithium electrode which
would retain its capacity upon continued cycling when used
as a negative electrode in an electrochemical cell, which
preferably would have substantially the same potential as
liquid lithium, and which would maintain its dimensional
stability during charging and discharging of the cell.
SUMMARY OF THE INVENTION
Broadly, the present invention provides an improved-
lithium--electrode and an electrical energy storage device
such as a secondary battery or rechargeable electrochemical
; cell utilizing such electrode. The improved electrode
comprises an alloy of lithium and silicon in intimate
contact with a supporting current-collecting matrix. The
lithium is present in the alloy in an amount from about 80
to 28 wt.~.
The improved electrical energy storage device comprises
a rechargeable lithium battery having positive and negative
electrodes spaced apart from one another and in contact with
a suitable lithium-ion-containing electrolyte, preferably a
molten salt electrolyte. The improved lithium electrode of
the present invention is utilized as the negative electrode,
functioning as the cell anode during the discharge mode of
the cell.
, ~
~04S680 -~-
BRI~F DESCI~tPTION OF Tll~ D~ ING
FIG, 1 is a graphical comparative representation of the
typical discharge characteristics of two lithium-alloy electrodes;
FIG. 2 is a pictorial view in perspective of an
electrode of the present invention; and
FIG. 3 is a diagrammatic representation of an elec-
trical energy storage device of the present invention.
DETAILED DESCRIPTION
The present invention relates to a lithium electrode
and an electrical energy storage device using such an elec-
trode.
The lithium electrode of the present invention com-
prises an alloy of lithium and silicon in intimate contact
with a supporting current-collecting matrix, thereby generally
providing a unitary or composite electrode structure. The
term "alloy" as used herein is defined as an intimate mixture
of the two metals in which the metals may form mixed crystals,
solid solutions, or chemical compounds. The metals also may
be present in more than one of these states in the same
alloy. It is an essential feature of the present invention
that the alloy contain from about 80 to 28 wt.~ lithium and
from about 20 to 72 wt.% silicon. A preferred alloy is one
containing from about 60 to 40 wt.% lithium, with the
balance consisting essentially of silicon. It will be
appreciated that the weight percentages referred to herein
refer to the electrode in its formed or fully charged state,
since obviously in operation of the cell the lithium will be
discharged, resulting in an alloy of substantially less or
even no lithium contcnt. The alloy also may contain minor
amounts of impuritics such as, for examplc, iron, calcium,
magncsium, and aluminum.
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The lithium-alloy electrodc structure further includes
a supporting current-collecting matrix in intimatc contact
with the alloy. Suitable materials for the supporting
currcnt-collecting matrix are those materials resistant to
attack by lithium or lithium-silicon mixtures. Examples of
such materials include iron, steel, stainless steel, nickel,
titanium, tantalum, and zirconium. The purpose of providing
a matrix in intimate contact with the alloy is to provide
for substantially uniform current density throughout the
alloy and also to provide structural support for the alloy.
It has been determined that the lithium-silicon alloy
utilized in the present invention lacks structural integrity
when used in an electrical energy storage device as the sole
component of the negative electrode, particularly in a
molten salt electrolyte at its high operating temperature.
To function for any significant length of time without dis-
integration, therefore, it is essential that the lithium
alloy be provided with a supporting matrix. It is contem-
plated and preferred, within the scope of this invention,
that the support and current-collecting capability be
provided by a single structure; however, the support may be
provided by one structure and the current-collecting capa-
bility by another separate structure.
The matrix may be in the form of an electronically
conductive porous substrate having an apparent density of
from about 10 to 30 percent of that of the base material.
Advantageously, the substrate will have a median pore size
within the range of from about 20 to 500 microns and prefer-
ably from about 50 to 200 microns. A particularly preferred
form of such a substratc is formed from woven or non-woven
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wires pressed togethcr to a desired apparent density and
then sintered. Such pressed and sintercd wire structures
are known and commercially available as Feltmetals. The
porous substrate then is impregnated with the alloy, for
example, by immersion in a molten bath of the alloy followed
by removal and cooling. Alternatively, the alloy may be cast
about a matrix formed from a wire screen or expanded metal.
In another variation, the matrix may be in the form of
a perforate container formed from wire screen or the like,
and containing therein a body of the alloy alone. Alter-
natively, the alloy is in intimate contact with a porous
substrate enclosed in the perforate container, it being
desirable that the container and the substrate be in elec-
trical contact with one another. This latter variation is
particularly useful when the porous substrate is formed from
very fine woven or non-woven wires pressed together to form
a body.
More particularly, it has been found, at least in the
case of iron used as the substrate material, that if the
wire used to form the porous substrate has a diameter of
less than about 10 microns, the substrate tends to break up
and disintegrate upon repeated charging and discharging of
the electrode in a molten salt electrolyte. It is not known
with certainty whether such destruction is the result of
imperceptible expansion and contraction of the electrode, or
a chemical interaction between the lithium-silicon alloy and
iron. In selecting material for use as a substrate, therefore,
consideration should be given to any chemical reaction or cor-
rosion that may occur as a result of the specific electrolyte or
matrix material which is utilized. Further, if the matrix com-
prises wovcn or non-woven wires pressed together to provide a
1045680 - 8-
porou~ su~strate, the wire ~hould have a diametcr of at
least about 10 microt~ dvantageously, the wire diameter
will be from about 10 to about 500 microns and preforably
from about 10 to 200 microns.
In a preferred embodiment of the invention, the elec-
trode is formed by surrounding the matrix with the alloy in
a molten state, for example, by immersing a porous substrate
in a molten body of the alloy. The alloy may be formed by
mixing particulate lithium and silicon and heating such a
mixture to a sufficiently elevated temperature to form a
melt. In accordance with a preferred method however, the
lithium first is heated, in an inert atmosphere, to a tem-
perature above the melting point of lithium, and thereaiter
the silicon is added in an amount to provide the desired
weight percent for the alloy. In such latter method, the
exoth~rmic reaction between the lithium and silicon will
provide substantially all of the heat required to form a
melt of the alloy.
It will be appreciated that the preparation of a
lithium-silicon alloy,per se,is known. The existence of
such an alloy and exemplary methods by which it may be
produced are reported, for example, in U. S. Patents 1,869,494
and 3,563,730. For additional information regarding the
temperature required to form a substantially homogeneous alloy
of lithium and silicon, reference may also be had to the
phase diagram shown in Shank, Constitution of Binary Alloys,
second supplement, McGraw-Hill Book Company, New York (1969).
The lithium electrode may be formed electrochemically
in a molten salt electrolyte in generally the same manner as
known and utilized in forming lithium-aluminum electrodes.
Specifically, silicon in intimato contact with the supporting
current-collccting matrix is immor~cd in moltcn ~alt clcctro-
lyte containing a sourcc of lithium ion~, and tho lithium is
1045680 - ~
coulombically chargcd into thc electrode in an amount to
form the dcsircd alloy.
The present invention also provides an electrical
energy storage device, particularly a secondary cell or
battery, which includes the lithium electrode of the present
invention as the electrically regenerable negative elec-
trode. The electrical energy storage device also includes a
positive electrode and an electrolyte broadly designated as
a non-aqueous lithium-ion-containing electrolyte.
The positive electrode or cathode is an electron
acceptor and contains an active material which is electro-
positive with respect to the lithium electrode. The active
material of the cathode may be sulfur or a metal halide,
sulfide, oxide or selenide. Suitable metals include copper,
iron, tungsten, chromium, molybdenum, titanium, nickel,
cobalt and tantalum. The sulfides of iron and copper are
particularly preferred for use with molten salt electro-
lytes. The cathode may be formed entirely of the active
material or may comprise a composite structure such as a
holder of, for example, graphite containing a body of such
active material.
The lithium-ion-containing non-aqueous electrolyte
utilized in preferred cell embodiments is a molten salt
electrolyte; alternatively, a solid electrolyte or an
organic solvent electrolyte is utilizable.
The term "molten salt electrolyte" as used herein
refers to a lithium halide-containing salt which is main-
tained at a temperature above its melting point during
operation of the electrical encrgy storage device. The
molten salt may be either a singlc lithium halide, a mixturc
of lithium halides, or a autectic mixturc of one or more
lithium halidcs and othar alkali mctal or alkalinc aarth
mctal halido~.
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Typical examples of binary eutectic salts are lithium
chloride-potassium chloride, lithium chloride-magnesium
chloride, lithium chloride-sodium chloride, lithium bromide-
potassium bromide, lithium fluoride-rubidium fluoride,
lithium iodide-potassium iodide, and mixtures thereof. Two
preferred binary salt euteatic mixtures are those of lithium
chloride and potassium chloride (melting point 352C), and
lithium bromide and rubidium bromide (melting point 278C).
Examples of ternary eutectics useful as the molten salt
electrolyte include calcium chloride-lithium chloride-
potassium chloride, lithium chloride-potassium chloride-
barium chloride, calcium chloride-lithium chloride-barium
chloride, and lithium bromide-barium bromide-lithium chloride.
Preferred ternary eutectic mixtures include those containing
lithi-~m-chloride, lithium fluoride and lithium iodide
(melting point 341C) and lithium chloride, lithium iodide
and potassium iodide (melting point 260C).
The suitable alkali or alkaline earth metal ion should
have a deposition potential very close to or preferably
exceeding deposition potentials of lithium ion in the electro-
lyte. Lithium halide salts can be readily combined with
halides of potassium, barium, and strontium. Halides of
metals such as cesium, rubidium, calcium, or sodium may be
used, but a substantial proportion of these metals may be
co-deposited with the lithium when the electrode is charged,
with a resulting small loss in potential.
Although the ternary eutectic salt mixtures, particu-
larly those containing the iodides, provide lower melting
points, the binary eutectic mixture of lithium chloride-
potassium chloride sometimcs is preferred on the basis of
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104S680
lts lower cost and availability, particularly for batteries
to be used in largc scale applications such as electric
powered vehiclcs and electric utility bulk energy storage.
The solid state electrolytes contemplated herein
include a mixture of lithium sulfate and a lithium halide
such as lithium chloride or lithium bromide or a mixture
thereof. The composition of the mixed salt solid electro-
lyte may vary from 10 to 95 mole % lithium sulfate. Solidelectrolytes having such composition are conductive in what
appears to be a solid phase at temperatures as low as about
400C. Combinations of lithium iodide with a silver halide,
silver mercuric halide, lead halide, copper halide, ammonium
halide or combinations thereof also are suitable for the
solid electrolyte.
The lithium electrode of the present invention also is
useful in electrical energy storage devices which utilize a
lithium-ion source in an organic solvent. The term "organic
` electrolyte" contemplates those non-aqueous electrolytes
which comprise an organic solvent and a solute. The solute
is the source of lithium cations. The solute also is, of
course, miscible or dissolved in the organic solvent. The
solvent is such that it does not attack the electrode
materials and is not affected by them. Obviously the solute
should be stable in its environment at the intended operating
temperature and electrical potential. Organic electrolyte
cells generally are designed to operate at a temperature
below about 12SC., and more specifically, at a temperature
within the range of from about 0 to 80C. It is important
that the solute and the solvcnt be such as to provide a
~04S680
lithium ion-cont~inin~ and conducting mcdium which i9
mobile or liquid undcr thesc conditions. Normally, it is
preferred that the solute bc of hi~h purity.
The solutcs which most nearly meet these requirements
are lithium halide salts. For conductivity purposes, other
metal halides, e.g., aluminum chloride, are often complexed
with the lithium halide. The halide is selected from the
group consisting of chlorine, fluorine, bromine, iodine,
and mixtures thereof. It is envisioned that double anion
complexes also could be used. Examples of suitable solutes
are lithium bromide, lithium chloride, lithium fluoride,
sodium chloride, sodium fluoride, and potassium chloride.
The lithium salt also may be a lithium perchlorate, hexa-
fluophosphate, tetrafluoborate, tetrachloroaluminate, or
hexafluoarsenate.
Preferably, the lithium ion-containing and conducting
medium used i~ in a saturated or supersaturated condition.
The ion-containing and conducting medium should have suffi-
cient salt concentration to permit most economical operation
of the cell. The ion-containing and conducting medium
should have a concentration of solutes greater than about
0.5 molar.
The choice of organic solvent for the ion-containing
and conducting medium is dictated by many of the consider-
ations involving the solute. The solvent of the ion-contain-
ing and conducting medium is any polar material which meets
the requirements of serving as a transfer medium for the
solute and in which the solute is miscible or dissolved.
The solvent also should be of such a material as to be inert
to the electrode materials. The solvent is prcferably a
liquid at from about 0-125C.; operating conditions dictate
such a requircmcnt. For examplc, dimcthylsulfoxide iq an
~045680
excellent ~olvent above its melting point of about 18.5C.
The solvent is desirably one which does not readily release
hydrogen ions. Solvents of high dielectric constants and low
viscosity coefficients are preferred.
Suitable solvents are, for example, the nitriles such
as acetonitrile, propionitrile, suIfoxides such as dimethyl-,
diethyl-, ethylmethyl- and benzylmethylsulfoxide; pyrroli-
dones such as N-methylpyrrolidone, and carbonates such as
propylene carbonate.
In addition to the foregoing representative list of
suitable electrolytes and positive electrode materials, many
others will be apparent to those versed in the art. It is
not intended that the invention be limited, therefore, only
to those specifically identified.
Referring now to the drawing and FIG. 1 in particular,
curve A depicts a typical discharge plot of an electrode of
the present invention (60 wt% Li - 40 wt% Si) versus a
liquid lithium counter electrode. From that plot it is seen
that substantially 100% of the theoretical capacity (amp-hr)
is recoverable from the electrode of the present invention.
It also will be noted that the potential versus liquid
lithium varies during discharge. Specifically, the electrode
of the invention discharges through five distinct voltage
plateaus.
The first plateau is at substantially the same pctential
as liquid lithium. Each of the four succeeding plateaus is
at a potential progressively more positive with respect to
liquid lithium, namely, about 70, 170, 250, and 310 mv more
positive than liquid lithium. The reason for such series of
different potentials is not known with certainty, and the
present invcntion is not to be considcred as limited by any
particular thcory. It is bclicvcd, howcver, that thc fir~t
-13-
~045680
plateau, which is substantially tlle same as that for liquid
lithium, occurs during dischargc of unalloycd lithium,
and each succeeding plateau thereafter represents a speeific
speeies of lithium-silicon eompounds. It must be appreeiated,
however, that the known phase diagrams for lithium-silieon
show only two species, i.e., Li4Si and Li2Si. It is not
known, therefore, whether FIG. 1 is indieative of the exist-
ence of an as yet unidentified lithium-silicon speeies or
whether some other meehanism is involved. Obviously,
knowledge of the precise meehanism involved is not necessary
for the praetiee of the present invention.
For comparative purposes, also shown in FIG. 1, is
eurve B, a discharge eurve for a typical lithium-aluminum
electrode (30 wt% Li - 70 wt% Al) of equal weight. Comparing
eurves A and B, it is seen that the lithium eleetrode of the
present invention has a substantially greater eapaeity than
an equivalent prior art lithium-aluminum eleetrode. Sueh
eapaeity is attained by virtue of the greater amount of
lithium which ban be eontained in the lithium-silicon alloy.
The prior art lithium-aluminum alloys generally are limited
to a maximum of less than about 30 wt.% lithium. In actual
practice, lithium-aluminum alloys are rarely used which
contain more than about 20 wt.% lithium, since alloys con-
taining greater percentages of lithium tend to release free
lithium into the electrolyte with a resulting loss in coulombic
efficiency, in much the same manner as occurs with the
prior art liquid lithium electrodes.
It also will be noted in comparing the two curves that
both alloys start off with a potential substantially the
same as that of liquid lithium. At such a potential it is
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possible that the release of free lithium into the electro-
lyte may occur. When it is desired to eliminate such a
possibility, the electrodes advantageously are discharged
against a liquid lithium counterelectrode until the potential
reaches the second plateau (about 70 mv for lithium-silicon
and 300 mv for lithium-aluminum). Even after an initial
discharge of the electrodes to bring them down to the second
plateau, it will be seen that the electrode of the present
invention has a more favorable potential than that of lithium-
aluminum and still retains a substantially greater capacity
(amp-hrs). Moreover, after such initial discharge, even if
the electrode of the present invention is discharged com-
pletely to the end of the last plateau (310 mv), the time
averaged potential will be substantially nearer that of
liquid lithium than the prior art lithium-aluminum electrode.
The two comparative curves shown in F~G. 1 clearly demon-
strate the superiority and advantages of the lithium electrode
of the present invention over that of the prior art lithium-
aluminum electrodes.
Referring now to FIG. 2, a lithium electrode 10 of the
present invention is shown. The electrode 10 includes a
conductor wire 12 and a matrix which comprises a cage or
perforate container formed from a wire screen 14 and a
porous substrate impregnated with a lithium silicon alloy
16.
In FIG. 3 is depicted an electrical energy storage
device 20 which utilizes the lithium alloy electrode of the
present invention. The storage device 20 includes a positive
electrode 22 and a negative electrode 24, the latter com-
prising a porous metal substrate impregnated with a lithium-
silicon alloy. Electrodes 22 and 24 are provided with
. 15._
~04S680
electrical conncctors 26 and 28, respectively. The electrical
energy storage d~vice ~190 includcs a housing 30 and a cover
32. The cover 32 i9 provided with aperturcs therethrough
for electrical connectors 26 and 28. Located within the
apertures are electrically nonconductive insulators 34. The
electrical energy storage device also includes a non-aqueous
electrolyte 36. When the non-aqueous electrolyte is a solid
electrolyte or a molten salt electrolyte, both of which must
operate at relatively high temperatures, housing 30 also
may be provided with heating means such as a plurality of
electrical resistance heaters 38.
The following examples are set forth for the purpose of
further illustrating the present invention. For convenience,
most of the examples relate to use of the electrode of the
present invention in a molten salt electrolyte which is
preferred. However, the invention should not be construed
as limited to electrical energy storage devices utilizing a
molten salt electrolyte, for, as herein disclosed, it will
have equal utility in an electrical energy storage device
(primary or secondary cell or battery) utilizing either a
solid electrolyte or an organic electrolyte.
Example 1
The following example demonstrates the preparation and
testing of the lithium electrode of the present invention.
The supporting current-collecting matrix utilized was a
commercially available material known as Feltmetal comprising
fine low-carbon steel fibers ~approximately 10 microns
in diameter) compacted into the flat porous plate (~.32 cm~
and sintered. The matrix had a surface arca of about 6.45
cm2 pcr sidc and an apparcnt density of 10~. The mcdian
porc size was in thc rangc from about 50 to 150 microns.
104S680
The matri~ was w~ighed and then impregnatcd with lithium
alloy by immersing it in an alloy comprising 45 wt.~ lithium
and 55 wt.% silicon at a temperature between about 720 and
820C for about 15 minutes. The matrix then was removed
from the molten lithium, alloy bath, allowed to cool, and
weighed again. It was determined that the matrix had
retained approximately 1.32 grams of the alloy, thus forming
a lithium electrode having a total theoretical capacity of
2.29 ampere hours.
The electrode was tested by placing it in a molten salt
bath containing 58.~ mole % lithium chloride and 41.2 mole %
potassium chloride. The molten salt electrolyte was main-
tained at a temperature of about 400C, and the test electrode
was alternately charged and discharged at a constant current
of about 600 milliamps against a larger liquid lithium
counter-electrode. The voltage potential between the two
electrodes and the current flow were continuously recorded
versus time on a strip chart recorder. From the chart it
was determined that the average coulombic efficiency (amp-hr
discharge/amp-hr charge x 100) for 4 cycles was about 100~.
The voltage potential versus the liquid lithium counter-
electrode and the discharge characteristics were substan-
tially the same as those depicted in FIG. 1. The test was
terminated after four cycles when the fine individual fibers
of the supporting matrix broke up, since this resulted in a
loss of coulombic efficiency.
Example 2
The foregoing procedure of Example 1 was repeated using
a porous substrate of titanium metal for the matrix, the alloy
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1~45680
comprisillg 60 wt.~ Li and 40 wt.~ Si. The titanium sub-
strate had fibers approximatcly 100 microns in diameter, an
apparent density of 19~, and a median pore size in the range
of from 400 to 500 microns. It was tested in substantially
the same manner as hereinbefore described, and the test was
voluntarily terminated after 34 cycles. Coulombic efficiency
averaged between 95 and 100%, and again the voltage versus
the liquid lithium counterelectrode and the discharge
characteristics were substantially the same as those depicted
in FIG. 1, thus, demonstrating the advantage of the larger
fiber size and the utility of titanium as the material for
the matrix.
Example 3
A porous substrate composed of iron fibers approxi-
mately 10 microns in diameter, having an apparent density of
10% and a median pore size within the range of from 50 to
150 microns was obtained. The porous substrate was enclosed
in a perforate screen formed from steel wires having a
diameter of 230 microns and a mesh size of 60 (U. S. standard
sieve size) to form the matrix ~similar to FIG. 2). The
electrode was formed and tested in su~stantially the same
manner as hereinbefore described but using a 60 wt.% Li - 40
wt.% Si alloy. Again a coulombic efficiency between 95 and
100% was obtained, and the discharge characteristics were
substantially the same as those depicted in FIG. 1. This
test also was voluntarily terminated after 17 cycles, thus
demonstrating the utility of the two-structure combined form
of supporting and currcnt-collecting matrix, viz., separate
supporting and current-collecting structures.
-18-
10~5680
Example 4
The following example is set forth to dcmonstrate the
applicability and utility of the present lithium-silicon
alloy electrode as an anode in an electrical energy storage
device which utilizes an organic electrolyte. The negative
lithium-silicon alloy electrode was prepared in substan-
tially the same manner as the electrode of Example 1 but
using a 60 wt.% Li - 40 wt.% Si alloy. The positive elec-
trode comprised a porous carbon structure impregnated with
iron sulfide and retained in a dense graphite holder. The
organic electrolyte consisted essentially of a propylene
carbonate solvent containing 100 gm lithium perchlorate per
liter of solvent.
The electrical energy storage device was alternately
charged and discharged at a predetermined current and for a
preselected time interval. The voltage potential between
the two electrodes and the current flow were continuously
recorded vs. time on a strip chart recorder. From the chart
it was determined that a coulombic efficiency of from about
95 to 100 percent was attained. The time-averaged voltage
during discharge of the electrical energy storage device
utilizing the electrode of the present invention was about
2.1 volts, whereas with a comparable lithium-aluminum
electrode the time-averaged voltage would only have been
about 1.8 volts. Further, no significant change or de-
terioration of the electrode performance was observed, thus
demonstrating the utility of the fine woven iron fibers as a
supporting current-collecting matrix for use in an organic
electrolyte.
--19--
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Example 5
The ~ollowing example is set forth to domonstrate the
applicability and utility of the electrode of the present
invention when used in an electrical energy storage device
as the negative electrode. The electrode utilized was pre-
pared using a commercially available substrate as the
matrix. The substrate comprised a plurality of woven
nickel wires having a diameter of 16 microns compressed and
sintered to provide a substrate with an apparent density of
15%. The electrode was prepared by immersing the matrix in
a bath of molten (700C) lithium-silicon alloy (60 wt%
lithium-40 wt~ silicon). The electrode then was discharged
against a liquid lithium counterelectrode down to the +70 mv
potential plateau (see FIG. 1).
The positive electrode or cathode comprised a dense
graphite holder containing a quantity of iron sulfide as the
active material. This was covered with a porous graphite
separator to retain the active material in the holder while
still permitting the free passage therethrough of charged
ions. The negative and positive electrodes were immersed in
a molten salt electrolyte comprising 58.8 mol % lithium
chloride and 41.2 mol % potassium chloride. The electrolyte
was maintained at a temperature of about 400C.
The electrical energy storage device was alternately
charged and discharged at a predetermined current and for a
preselected time interval. The voltage potential between
the two electrodes and the current flow were continuously
recorded vs. time on a strip chart recorder. From the chart
it was determined that a coulombic efficioncy of from about
96 to 100% was obtainod. Further, the time-avoraged voltage
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~045680
during discllarge of the electrical energy storage dev~ce
utilizing the electrode of the present invention was about
1.48 volts utilizing the second and third plateaus, whereas
with a comparable lithium aluminum electrode the time-
averaged voltage would only have been about 1.3 volts. Italso was noted that there was substantially no perceptible
change in size of the lithium-silicon electrode during
cycling. Further, no perceptible amount of lithium was lost
to the electrolyte. The test was terminated after four
cycles because of a failure of one of the cathode compo-
nents. Nonetheless, the results clearly demonstrate the
efficacy of the lithium electrode of the present invention
and the advantages obtainable therewith.
It will of course be realized that various modifications
can be made in the design and operation of the lithium
electrode and cell of the present invention without de-
parting from the spirit thereof. Thus, while the lithium
electrode structure has been illustrated and described with
respect to certain exemplary embodiments relating to particular
preferred constructions and materials for the supporting
current-conducting matrix, and while preferred embodiments
of secondary cells utilizing molten salt electrolytes and
metal sulfide cathodes have been illustrated and described,
it should be understood that within the scope of the ap-
pended claims the invention may be practiced otherwise thanas specifically illustrated and described.
