Note: Descriptions are shown in the official language in which they were submitted.
10~3~33
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
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 evolution 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
~093~i33~
reliability is both bulky and costly and will not eliminate
the problem of self discharge. Additionally, solution cells
have a limi~edoperating 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 solld 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.
1093~i33~
Solution electrolyte cells inc]ude an electronically non-con-
ductive separator between the electrode elements to prevent such
a short circuit, whereas solid state cells utilize the solid
electrolyte as both electronic separator and the ionic conductive
species.
~ Iigh current capabilities for solid state cells have
been attained with the use of materials which are solely ionic
conductors such as RbAg4Is (.27 ohm 1 cm 1 room temperature con-
ductivity). However these conductors are only useful as electro-
lytes in cells having low voltages and energy densities. As an
example, a solid state Ag/RbAg4I5/RbI3 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 con-
ductors thereby precluding the use of these conductors in such
cells. Electrolytes which are chemlcally 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 1 cm 1 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 current capability above 50t~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 - ----------- ---- -
M-332~D
1~3~i33
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 electro-
lyte material is another factor to be considered in the con-
struction 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 A12O3, 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
processes involving grinding and pelletization with 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 increasethe 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 den-
sity without current capability losses resulting from the
- addition of inert conductive materials, while maintaining
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^
10~3633
chemical stability between the cell components.
Generally the present invention involves the formation
of the cathode of a solid state cell with a material which has
the characteristics of being both ionically and electronically
conductive as well as being able to function as an active
ca~hode material. Normally cathodes require the incorporation
of substantial 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. How-
ever 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, MoS2,
1093~c;33
MoSe2, MoTe2, NbS2, NbSe2, NbTe2, NiTe2, PtS2, PtSe2, PtTe2,
SnS2, SnSSe, SnSe2, TaS2, TaSe2, TaTe2, TiS2, TiSe2, TiTe2,
S2, VSe2, VTe2, WS2, WSe2, WT2, zrS2, Zrse2~ and ZrTe2, wherein
the chalcogenide is a sulfide, selenide, telluride or combination.
Also included are the non-stoichiometric metal chal-
cogenide compounds such as LixTiS2 where x~l, whieh 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 during cell discharge.
In order for the ionically-electronically conductive
cathode active material to be commercially useful in high voltage
cells such as 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.
A further criteria for the above cathodic material is
that both the ionic and electronic conductivities of the cathode
active material should range between lO~l~nd 102 ohm~l cm~l with a
preferred ionic conductivity of more than 10-6 and an electronie
eonduetivity greater than 10-1,all at room temperature.
In addition, and most importantly, the ionically-
eleetronieally eonduetive,aetive eathode material must be eom-
patible with the solid electrolytes used in the high energy
density eells.
The solid electrolytes used in high energy density
lithium eells are lithium salts andhave room temperature ionie con-
duetivities greater than 1 x 10-9 ohm~l cm~l These salts can eithe
be in the pure form or combined with conductivity enhancers such
1093633
that the current capability is improved thereby. Examples of
lithium salts having the requisite conductivity for meaningful
cell utilization include lith:ium iodide (LiI) and lithium iodide
admixed wi-th lithium hydroxide (LioH) and aluminum oxide (A12O3)
with the latter mixture being referred to as LLA and disclosed
in U.S. patent no. 3,713,897.
High energy density solid electrolyte cells may have
as their anodes materials similar to lithium which have high
voltage and low electrochemical equivalent weight character-
istics. Suitable anodic materials include metals from Groups
IA and IIA of the Periodic Table such as sodium, potassium,
beryllium, magnesium and calcium as well as aluminum from Group
IIIA and other metals above hydrogen in the EMF series.
Cells with other anodes can utilize correspondingsalts as electrolytes such as sodium salts for a cell with
a sodium anode. Additionally, electrolyte salts with useful
conductivities and having a cation of a metal~lower EMF than
that of the anode metal may also be useful.
It is postulated that the aforementioned ionically-
electronically conductive cathode active materials react with
the ions of the anode (e.g. 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.
R limiting factor in solid state cell performance is
the conductivity of the cell reaction product. A low conductivity
M~
- ~0S~3~ 3
product results in large internal resistance losses which
effectively terminate cell usefulness. Thus a further
advantage of cells having the above ionically-electronically
conductive cathode active material is that the complexed
reaction product retains conductivity thereby enabling full
utilization of Lhe cathode.
A small amount of electrolyte can also be included
in the cathode structure in order to blur the interface between
cathode and electrolyte thereby providing moreintimate
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 incl-lsion
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. Accordingly the cathode should include at least 90%
by weight of the ionically-electronically conductive cathode
active material.
In order that the present invention be more completely
understood the following examples are given with all parts
being by weight unless otherwise specified. The examples are
only for illustrative purposes and should not be taken as
limitations of either cell construction or of materials con-
tained therein.
5 G ~--J
` ` 10~3~;33
EXAMPLE 1
A solid state electrochemical cell is formed using a
lithium metal disc having dimensions of about 1.47 cm2 contact
surface area by about 0.01 cm thickness; a cathode disc having
dimensions of about 1.71 cm2 contact surface area by about 0.02
cm thickness consisting of titanium disulfide (TiS2) and weighing
about 100 mg, and a solid electrolyte with the same dimensions
as the cathode and consis~lng of LiI, LioH, and A1203 in a 4:1:2
ratio. The electrolyte is first pressed with the cathode at a
pressure of about 100,000 psi. The anode is then pressed to the
other side of the electrolyte using about 50,000 psi. The
resulting cell is discharged at a temperature of 72C under a load
of lOk~. The cell realizes 14 milliamp hours (mA~) to 2 volts,
21 mAH to 1.5 volts, and about 24 mAH to 1 volt.
The titanium disulfide in the above Example is both
a good ionic and electronic conductor (10-5 ohm~l cm~l ionic
conductivity and greater than 10~1 ohm~l cm~l electronic con-
ductivity at room temperature) and thus constitutes the cathode
without conductive additives. The titanium disulfide functions
as a reactive species in the cell reaction with the lithium cations
to form the non-stoichiometric LixTiS2 which is also ionically
and electronically conductive thus further ameliorating the
problem of incomplete cell discharge resulting from non-conductive
reaction products choking off further cell reaction.
The ionically-electronically conductive, cathode active
materials can be admixed with one another to form a cathode as
in the following EXAMPLES.
1093633
EXAMPLE 2
A solid state cell :is made in accordance with
EXAMPLE 1 but with the cathode having a contact surface area
of 1.82 cm2 and comprising a 1:1 mixture of titanium disulfide
and molybdenum disulfide weighing about 50 mg. The cell is
discharged at 27C under a load of 18~A. The cell realizes
2.2 mAH to 2 volts, 5 mAH to 1.5 volts and 5.9 mAH to 1 volt.
EXAMPLE 3
A cell identical to the cell in EXAMPLE 2 is discharged
at 27 C under a load of 36JUA. The cell realizes about 1 mAH
to 2 volts, about 3 m~H to 1.5 volts and about 5 mAH to 1 volt.
It is understood that other disclosed conductive metal
chalcogenides can function similarly whether without any further
conductive enhancers or with a maximum of 10% of conductive
;:
, materials and such materials also fall within the scope of the
,
~ present invention as defined by the following claims.
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