Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to high energy density cells
utilizing solid electrolytes, solid active metal anodes and
novel solid cathodes, and more particularly to such cells
in which the cathodes contain an active material which is
both ionically and electronically conductive.
Recently the state of electronics has achieved a
; high degree of sophistication especially in regard to
devices utilizing integrated circuit chips which have been
proliferating in items such as quartz crystal watches,
calculators, cameras, pacemakers and the like. Miniaturization
of these devices as well as low power drainage and relatively
long lives under all types of conditions has resulted in a
~i demand for power sources which have characteristics of
, rugged construction, long shelf life, high reliability, high
energy density and an operating capability over a wide
range of temperatures as well as concomitant miniaturization
of the power source. These requirements pose problems for
conventional cells having solution or even paste type
electrolytes especially with regard to shelf life. The
electrode materials in such cells may react with the
electrolyte solutions and tend therefore to self discharge
after periods of time which are relatively short when compared to
the potential life of solid state batteries. There may also
be evolu~ion of gases in such cells which could force the
electrolyte to leak out of the battery seals, thus corroding
other components in the circuit which in sophisticated
componentry can be very damaging. Increasing closure
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reliability is both bulky and costly and will not eliminate
; the problem of self discharge. Additionally, solution cells
; have a limitedoperating temperature range dependent upon the
, freezing and boiling points of the contained solutions.
Success in meeting the above demands without the
drawbacks of solution electrolyte systems has been achieved
with the use of solid electrolyte and electrode cells or
~`~ solid state cells which do not evolve gases, self discharge
on long standing or have electrolyte leakage problems. These
,..~
systems however have had their own particular limitations and
drawbacks not inherent in solution electrolyte cells.
Ideally a cell should have a high voltage, a
high energy density, and a high current capability. Prior
art solid state cells have however been deficient in one
or more of the above desirable characteristics.
A first requirement and an important part of the
operation of any solid state cell is the choice of solid
electrolyte. In order to provide good current capability
a solid electrolyte should have a high ionic conductivity
which enables the transport of ions through defects in the
crystalline electrolyte structure of the electrode-electrolyte
system. An additional, and one of the most important require-
ments for a solid electrolyte , is that it must be virtually
solely an ionic conductor. Conductivity due to the mobility
of electrons must be neglible because otherwise the resulting
partial internal short circuiting would result in the consumption
of electrode materials even under open circuit conditions.
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Solution electrolyte cells include an electronically non-
conductive separator between the electrode elements to
prevent such a shortcircuit, whereas solid state cells
utilize the solid electrolyte as both electronic separator
- and the ionic conductive species.
High current capabilities for solid state cells
have been attained with the use of materials which are
. solely ionic conductors such as RbAg4I5 (.27 ohm lcm 1 room
; temperature conductivity). However these conductors are only
,,~,r,:, 10 useful as electrolytes in cells having low voltages and energy
densities. As an example, a solid state Ag/RbAg4Is/RbI3
~i~ cell is dischargeable at 40 mA/cm2 at room temperature but with
about 0.2 Whr/in3 and an OCV of 0.66V. High energy density and
high voltage anodic materials such as lithium are chemically
reactive with such conductors thereby precluding the use
of these conductors in such cells. Electrolytes which are
chemically compatible with the high energy density and high
voltage anode materials,such as LiI, even when doped for
greater conductivity, do not exceed a conductivity of 5 x 10 5
ohm cm at room temperature. Thus, high energy density cells
with an energy density ranging from about 5-10 Whr/in3 and a
voltage at about 1.9 volts for a Li/LiI-doped/PbI, PbS,Pb cell
currently being produced are precluded from having an effective high
' f current capability above 50 ~A/cm 2 at room temperature. A further
; aggravation of the reduced current capability of high energy
density cells is the low conductivity (both electronic and
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ionic) of active cathode materials. Conductivity enhancers
such as graphite for electronic conductivity and electrolyte
for ionic conductivity,while increasing the current capability
of the cell to the maximum allowed by the conductivity of the
~^, electrolyte, reduce the energy density of the cell because
of their volume.
`~ Commercial feasibility in production of the
- electrolyte material is another factor to be considered in the
. construction of solid state cells. Thus, the physical properties
of electrolytes such as BaMg5S5 and BaMg5Se6, which are compatible
with a magnesium but not a lithium anode, and sodium beta
aluminas such as Na20 11 A1203, which are compatible with sodium
:
anodes, will preclude the fabrication of cells having a high
energy density or current capability even when costly production
steps are taken. These electrolytes have ceramic characteristics
making them difficult to work with especially in manufacturing
with
processes involving grinding and pelletization,~such processes
requiring a firing step for structural integrity. Furthermore,
the glazed material so formed inhibits good surface contact
with the electrodes with a result of poor conductivity leading
to poor cell performance. These electrolytes are thus typically
used in cells with molten electrodes.
It is therefore an object of the present invention
to increase the conductivity of the cathode of solid state
cells in conjunction with high energy density anodes and compat-
ible electrolytes such that there is an increase in energy density
without current capability losses, while maintaining chemical
stability between the cell components.
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Generally the present invention involves the incorpora-
tiOll into the cakhode of a solid state cell of a material which
has the characteristics of being both ionically and electronically
conductive as well as being able to function as an active cathode
material. Normally cathodes require the incorporation of sub-
stantial amounts (e.g. over 20 percent by weight) of an ionic
conductor such as that used as the electrolyte in order to
facilitate ionic flow in the cathode during the cell reaction.
This is especially true if the cathodic material is an electronic
conductor since otherwise a reduction product would form at the
cathode-electrolyte interface which would eventually block off
a substantial amount of the ionic flow during discharge. However
the incorporated ionic conductors in prior art cells have not
generally been cathode active materials with the result of
significant capacity loss. Additionally, cathode active materials
which are poor electronic conductors as well require the further
incorporation of electronically conductive materials which
further reduces the cells energy capacity. By combining the
functions of electronic and ionic conductivity with cathode
activity a higher energy density and current capability is
attained with the need for space wasting conductors being
obviated.
Examples of materials having the requisite character-
istics of ionic and electronic conductivity and which are
cathodically active as well as being compatible with electrolytes
used in high energy density cells include the following metal
chalcogenides: CoTe2, Cr2S3, HfS2, HfSe2~ HfTe2~ IrTe2~
MS2~ MSe2~ MTe2~ NbS2, NbSe2, NbTe2, NiTe2, PtS2, PtSe2,
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10917~;3
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PtTe2, SnS2, SnSSe, SnSe2, TaS2, TaSe2, TaTe2, TiS2, TiSe2,
TiTe2, VS2, VSe2, VTe2, WS2, WSe2, WTe2, zrs2, ZrSe2, and ZrTe2,
.
wherein the chalcogenide is a sulfide, selenide, telluride or
a combination thereof.
Also included are the non-stoichiometric metal
chalcogenide compounds such as LixTiS2 where x <1, which
to some extent contain the complexed form of one of the cathode
materials with the anodic cation and which are believed to be
intermediate reaction products durinq cell discharge.
In order for the ionically-electronically conductive
.,
;~ cathode active material to be commercially useful in high
,.
voltage cells with lithium anodes it should be able to provide
a voltage couple with lithium of at least an O.C.V. of 1.5
volts and preferably above 2 volts.
The operating voltage of the ionically-electronically
conductive cathode active material should preferably be roughly
equivalent to the voltage of the higher energy density non-
conductive cathode active material mixed therewith to avoid
detrimental voltage fluctuations.
- 20 A further criteria for the above cathodic material
is that both the ionic and electronic conductivities of the
- cathode active material should range between 10 10and 102 ohm~
cm~l with a preferred ionic conductivity of more than 10-6
and an electronic conductivity greater than 10 1,~ t roome~a~re
In addition, and most importantly, the ionically-
electronically conductive,active cathode material must be
compatible with the solid electrolytes used in the high energy
density cells.
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The solid electrolytes used in high energy density
lithium cells are lithium salts and have ionic conductivities
at room temperature.
greater than 1 x 10 9 ohm 1 cm 1~ These salts can either be
in the pure form or combined with conductivity enhancers such
that the current capability is improved thereby. Examples of
lithium salts having the requisite conductivity for meaningful
cell utilization include lithium iodide (LiI) and lithium
iodide admixed with lithium hydroxide (LioH) and aluminum oxide
(A1203), with the latter mixture being referred to as LLA and
disclosed in U.S. patent no. 3,713,897.
It is postulated that the aforementioned ionically-
electronically conductive,cathode active materials react with
the ions of the anode (i.e. lithium cations) to form a non-
stoichiometric complex during the discharge of the cell. This
complexing of cations allows them to move from site to site
.
thereby providing ionic conductivity. Additionally the above
compounds provide the free electrons necessary for electronic
conductivity.
The above compounds are admixed with other compounds
or elements which provide a greater energy density but which
cannot be utilized in and of themselves because of their
inability to function as ionic and/or electronic conductors.
.. .
The inclusion of the ionically-electronically conductive,cathode
active material thereby increases the capacity of the cell by
obviating the need for non-dischargeable conductive materials.
Furthermore, when the conductive active material is homogeneously
admixed with the higher energy density compound the
realizable utilization of the so formed cells is approximately
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' equal to the theoretical. A limiting factor in solid state cell
~: performance is the conductivity of the cell reaction product.
,
; A low conductivity product results in large internal resistance
r''' losses which effectively terminate cell usefu~ess . Thus
~:;
in cells having the above ionically-electronically conductiv~,
cathode active material the complexed reaction product retains
conductivity thereby enabling full utilization of other active
- cathode materials which are in proximity therewith.
Accordingly,high energy density cathodic materials
such as sulfur and iodine as well as other solid chalcogens,
Se and Te, and halogens such as bromine can be effectively
utilized to greater potential. Solid state cells utilizing
; sulfur in conjunction with lithium anodes and lithium salt
solid electrolytes have shown great promise in terms of voltage
obtainable and total energy density.However, one of the draw-
, `:;
backs has been the formation of the low ionically conductive
lithium sulfide (Li2S~ as the cell reaction product and
, especially at the cathode electrolyte interface. This build
up has effectively choked off the further utilization of these
~;i 20 cells. However the inclusion of the ionically-electronically
~,............ . .
~conductive,cathode active materials provides a more uniform
distribution of the reaction product throughout the cathode,:
structure because of their ionically conductive characteristics.
Since the reaction productsof the ionically conductive materials
retain conductivity,further utilization of the cell is also
- possible with the non-conductive active material in conductive
proximity to the conductive active material.
A small amount of electrolyte can also be included
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in the cathode structure in order to blur the interface between
cathode and electrolyte thereby providing more intimate
electrical contact between the cathode and the electrolyte.
This enables the cell to operate at higher current drains for
: longer periods of time. Additionally,the electrolyte inclusion
can increase the ionic conductivity of the cathode should the
ionically conductive cathode active material have a lower
conductivity than that of the electrolyte. This inclusion
however, if made, should not exceed 10% by weight since greater
amounts would merely decrease the energy density of the cell
with little if any further tradeoff in terms of current drain
capacity.
The following examples illustrate the high energy
density and utilizability of a sulfur containing cathode in a
` solid state cell with the abovementioned ionically and electron-
ically conductive cathode active metal chalcogenides. Sulfur
in and of itself cannot be used as a cathode in a solid state
cell unless it contains susbstantial amounts of ionic and
electronic conductors which constitute 60% or more of the
total cathode by weight. Thus the inclusion into a cathode
of sulfur of an ionically and electronically conductive
metal chalcogenide such as titanium disulfide enables the use
of sulfur without the concomitant severe losses of energy
capacity. Titanium disulfide is both a good ionic and electronic
conductor (10 50hm 1cm room temperature ionic conductivity and
--1 -1 --1
greater than 10 ohm cm room temperature electronic conductivity)
and also functions as a reactive species in the cell reaction with
the lithium cations to form the nonstoichiometric LiXTiS2 which is alsc
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ionically and electronically conductive thus further ameliorating
the other problem of non-conductive reaction products choking off
further cell reaction. In addition TiS2 generally discharges at
a voltage similar to that of sulfur i.e. 2.3 volts and thus the
cell voltage is steady without cell voltage fluctuations.
In the following examples as throughout the en~ire
specification and claims all parts and percentages are parts by
weight unless otherwise specified. The examples are given for
illustrative purposes only, and specific details are not to be con-
strued as limitations.
EXAMPLE I
A solid state cell is made from a lithium metal disc
having dimensions of about 1.47 cm2 surface area by about 0.01 cm
thickness; a cathode disc having dimensions of about 1.82 cm2
surface area by about 0.02 cm thickness, consisting of 80% TiS2
and 20% S, and weighing 100 mg; and a solid electrolyte there-
- between with the same dimensions as the cathode and consisting
of LiI, LioH, and A1203 in a 4:1:2 ratio. The electrolyte is
first pressed to the cathode at a pressure of about 100,000 psi,
- 20 and then the anode is pressed thereto at about 50,000 psi. The
resulting cell is discharged at room temperature under a load of
100 k~b The cell provides 26 milliamp hours (mAH) to 2 volts,
about 41 mAH to 1.5 volts, and in excess of 46 mAH to 1 volt.
The cell has a realizable capacity in excess of 12 watt hours/in3.
The following Table illustrates the results obtained from
cells tested under condition of different loads or temperatures and
having differing cathode weight, cathode-electrolyte interface
surface area or relative percentages of TiS2 to S with resulting
capacity limits to 2, 1.5 and 1 volt.
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M-3324-C
~09-1763
It should be noted from the Examples that cells dis-
charged at 37C show even greater capacity than those cells
discharged at room temperature and under the same load. Thus,
since 37C is human body temperature, cells utilizing the present
; invention can be used in pacemakers with the capability of
lasting in excess of 10 years,thereby greatly reducing the
` need for surgery necessitated by the need for implanting fresh
batteries.
Additionally, the 80:20 ratio of TiS2 to S by weight,
roughly equivalent to a mole-to-mole ratio, provides a greater
useful capacity than the 60:40 ratio despite the increased
; amount of the higher energy density sulfur in the latter.
With the mole-to-mole ratio,three lithiums can react stoichio-
metrically in the cell reactions i.e. 2Li + S -~Li2S and
Li + TiS2-~ LiTiS2. These reactions provide a three electron
change with both a high voltage and a high capacity. The mole -
to-mole ratio of TiS2 to S provides for complete stoichiometric
utilization and is thus highly preferred.
EXAMPLE 23
A cell made with the materials of Example l
the dimensions of 1.258" OD and 0.085" thickness, a
cathode of 1.5 grams is made as the back cover of a tritium
illuminated liquid crystal display (LCD) watch. A limiting
resistor of 330k~ limits the voltage applied to the watch.
The operating current for the abovementioned watch ranges
between 1 and 3~A. Thus,with a stoichiometric capacity of
750 mAH and assuming a conservative utilizability of 2/3
capacity the cell is theoretically capable of powering the
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10~1763
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watch at an average drain rate of 2~lA and a voltage in excess
of 2.2 volts for about 28.5 years. The lifetime of such cells
is in excess of the lifetime of the currently produced watches
themselves. Accordingly, with~the stability of solid state
.,
cells in general and the capacity of the present cell in
particular, batteries can be made as integral parts of electrical
componentry such as watches rather than as a part requiring
constant replacement.
~'t`
EXAMPLE 24
A cell made in accordance with that of Example 1
is made but with tantalum disulfide (TaS2) in place of titanium
disulfide (TiS2) and a wcight ratio to sulfur of 87.5:12.5.
Upon discharge of the cell at 72C under a load of lOk.~the cell
realizes 6 mAH to 2 volts, 18 mAH to 1.5 volts and 24 mAH to
volt.
EX~MPLE 25
A cell made in accordance with the previous Example
is discharged at 72C under a load of 20k~. The cell realizes
14 mP~H to 2 volts, 25 mAH to 1.5 volts and about 28 mAH to 1 volt.
It is understood that changes and variations of the
invention as described herein can be made without departing
from the scope of the present invention as defined in the
following claims.
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