Note: Descriptions are shown in the official language in which they were submitted.
1067144
This invention relates to electrochemical cells and
more particularly to such cells utilizing solid electrolytes.
Until recently electrolytes of electrochemical cells
have been based upon liquid systems with all of the attendant
drawbacks of li~uids. Such drawbacks indlude leakage of the
low surface tension liquids, need for electrolyte separators
and electrolyte absorbers to confine the electrolyte and its
ionic components to the desired portions of the cells. To
- overcome these drawbacks it has been proposed to utilize solid
electrolytes. However, most proposed solid electrolytes have
insufficient ionic conductivity for general use.
It is an object of this invention to provide a
solid electrolyte for use in electrochemical cells.
It is another object of this invention to provide a
solid electrolyte having relatively high ionic conductivity.
,
" It is another object of this invention to provide
... . .
electrochemical cells utilizing the solid electrolytes of
this invention.
It is still another object of this invention to ;
provide solid electrolyte cells which can be operated as primary
or secondary cells.
According to the above objects, from a broad -
aspect, the present invention provides an electrochemical
cell comprising an anode, a cathode and a solid electrolyte
; which comprises an alkali metal halide and lead fluoride.
The halide and lead fluoride are in solid solution form and
; in a molar proportion of halide to lead fluoride in the range
of 1:1000 to 1:1.
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1067144
; We have discovered that incorporation of alkali :-
metal halides such as halides of K, Na and Li, and preferably
alkali metal fluorides such as KF, NaF and LiF as dopants
, into lead fluoride (PbF2) increases the ionic conductivity
of the resultant PbF2 composition solids. The PbF2.,
without the inclusion of the alkali metal halide dopant, `
has a room temperature (25C) conductivity of about
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` 106'7~44
7 (+1) x 10-8 ohm~l -cm~l . The solid electrolytes of this
invention are conductors having a conductivity in the range --
of about 10-7 to about 10-2 ohm~l-cm~l and, preferably in the
range of about 10-3 to about 10-6 ohm~l-cm~l at about 25C.
The inclusion of alkali metal halide dopant causes an
increase in the concentration of the mobile or conducting
species in PbF2 , Such species may be either the interstitial - ~
ions or ion vacancies regardless of the alkali metal halide -
used
: ., .
~ 10 As a result of the improved conductivity resulting ?~
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from the addition of the alkali metal halide to the PbF2 ,
' of which an alkali metal fluoride is the preferred species -
with potassium fluoride (KF) being the presently most
preferred species, it is possible to prepare solid electrolytes
for use in electrochemical cells operative over a broad range
of temperatures.
Moreover the PbF2 -alkali metal fluoride solids
have sufficiently high ionic conductivity with very low
electronic conductivity and, consequently, they can be used as
solid electrolyte materials in both primary and secondary electro-
chemical systems.
In general we found that the incorporation of
- an alkali metal halide such as potassium fluoride in amounts
. wherein the molar proportions of KF to PbF2 ranges from
1:1000 to 1:1 provides a sufficient increase in ionic con-
ductivity to furnish a useful solid electrolyte. Within
this range we prefer the concentration of the KF within the
range of 0.5 to 25 mole percent.
Lead fluoride can exist at room temperature in
both orthorhombic (~ -PbF2 ) and cubic (~ -PbF2 ) forms.
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1067~44
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The incorporation of the alkali metal fluoride dopant such
as KF, substantially increases the ionic conductivity of
both ~ -PbF2 and ~-PbF2 . Such a conductivity increase is
caused by the fact that the concentration of the mobile (or
conductive)species (interstitial fluoride ions or fluoride
ion vacancies) is increased by the incorporation of KF in
the PbF2 lattice.
, . . .
The KF doped PbF2 obtained by quenching the melt -
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to room temperature is in the cubic form. However, the !`
orthorhombic o:form PbF2 can be obtained by prolonged grinding
of the cubic form ~ -PbF2 . Furthermore, when ~-PbF2 powder '~
is pressed under a pressure of 30,000 psi or higher, a -
pellet of orthorhombicoc-PbF2 is formed. In order to obtain
;~ a cubic p-PbF2 pellet, the orthorhombic PbF2 pellet is slowly
: - heated to 300-350C where aC-PbF2 transforms into~-PbF2 .
, Once the ~ -PbF2 pellet is formed, the temperature may be
lowered to room temperature or below and the PbF2 pellet
will remain in the cubic form (~ -PbF2 ) without transforming
back to the orthorhombic form. Therefore, both KF doped
oc -PbF2 and KF doped ~-PbF2 pellets suitable for solid
A'',electrolyte use can be made and maintained at temperatures
below 300C.
Figure 3 shows the conductivity of the KF doped
oc -PbF2 as a function of KF concentration up to one mole -
-- percent (1 m/o) at room temper-ature ~25~2C~.
Figure 4 shows the conductivity of the KF doped
-PbF2 as a function of KF concentration at room temperature
` (25+2C).
Figure 5 shows the conductivity of the KF doped
~-PbF2 as a function of temperature.
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. ; ~067144 ` - -
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Figure 6 shows the conductivity of the KF doped
-PbF2 as a function of temperature.
As can be seen from the above, while the ~ form
~cubic) of doped PbF2 has slightly higher conductivity, and
/ is preferred, either the ocor ~ form can be used in this
invention. `~
; The method of preparincJ the ionic conducting
solids suitable for the solid electrolyte of this invention
- includes mixing proper amounts of the alkali metal halide
~ 10 with PbF2 . The mixture is treated such-as by heating at - -
- a temperature where sufficient diffusion of the mixture components
" occùr to provide possible solution and intercrystaIlization ;
; of the alkali metal halide in the PbF2 . The treating step
.; . . ... .
- is preexably continued until such diffusion and/or
solution or intercrystallization is substantially c~mplete. At
higher temperatures the reaction is completed within minutes
and depending upon the composition of the mixture with the ;
formation of a liquiduus. At the lower temperatures the .;
;~ desired lead fluoride solid electrolyte formation is usually -~
completed within several hours but a liquid state is not
always achieved as an alkali metal halide such as KF and
PbF2 (in pure form) melt at about 800C.
Upon completion Of the treatment formation of the
~ ~ solid electrolyte intercrystalline composition, it is rapidly
; cooled (quenched) to room temperature and ground to a powder.
As set forth above the solid electrolyte of this
; invention can be used in either primary or secondary cells.
PRIMARY CELLS
The lead fluoride-alkali metal halide electrolyte
of this invention may be used in various solid state electro- ~
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~ - 1067144
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chemical systems wherein the anode actlve ma~erials are those ' '
which will not reduce ionic lead (Pb++ ) to metallic lead (viz: , ,
Handbood of Cllemistry and Physics-Potentials of Electrochemical ~,
Reactions). Useful cathode active materials are preferably the ' ,
heavy metal fluorides such as AgF, AgF2 , PbF4 , CuF2 ,1~gF2 , , ~
, intercalated fluorides such as SbF5 in graphite, fluorine and ~,
fluorine based materials including carbon with absorbed
'~ fluorine. ' '
SECONDARY SOLID STATE ELECTROLYTE CELLS :~
, 10 Useful rechargeable solid state cells can be con-
', structed using the PbF2 -alkali metal halide solid electrolytes.
, Such different solid electrolyte cells in the discharged ~'
state have the following postulated compositions: '
,
; a) C/PbF~ (KF doped) /Ag
b) Pb/PbF2 ~KF doped) /Ag
c) Pb/PbF2 (KF doped) /Cu
d) C/PbF 2 (KF doped) /Cu ;,
, e) C/PbF 2 ' (KF doped) /C ' , ' '
These cells can be charged to the active states ''
~' having the following postulated compositions: ;
f) Pb/PbF2 (KF doped) /Ac~F/Ag -'
g) Pb/PbF2 (KF doped) /Cuf2 /Cu
'20 h) Pb/PbF2 (KF doped) /PbF4 /C
,' The invention will be more fully described by
reference to the following examples showing procedures for
preparing the composition for the solid electrolyte of this
" ~ invention and'construction of primary and secondary batteries
utilizing'such solid electrolytes. The procedures disclosed
are exemplary of modes for practice of this invention. All
' art recognized equivalent procedures and'materials of those
herein disclosed are intended. For example, the electrolyte can
'', be used in forms other than the pellet form of the Examples.
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; ~067 ~4
EXAMPLE 1
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A mixture of PbF2 (75 mole percent) and KF (25~
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; mole percent) is heated to a temperature of about 600~C for a `
period of about 6 hours. After heating, the material is
quenched by pouring it in thin layers upon a cooled metallic -
. ' surface. When the poured material reaches room temperature
it is pulverized. A conductivity cell of this PbF2 -KF
composition is made by pressing a weighed amount of the
electrolyte powder between two Pb discs in a steel die at
100,000 psi. The resistance of a Pb/PbF2 -KF/Pb conductivity
cell is measured by a conductance bridge at 1,000 Hz. The
conductivity of the electrolyte pellet determined from these ;
measurements was 3 (+1~ x 10-5 ohm~~l -cm~l at room temperature.
The conductivity of PbF2 without the alkali metal fluoride
dopant is 7 (+~) x 10-8 ohm~1-cm~1 at room temperature.
EXAMPLE 2 ,~
, A mixture of a composition similar to that of
- Example 1 is heated to a temperature of about 850C, Within
several minutes a liquiduus is noted. The liquiduus is
poured upon a cooled metal sheet and thus quenched rapidly to
room temperature. The resultant solid material is pulverized
.. . ..
- and a conductivity cell is prepared-therewith in the same manner
as in Example 1. The conductivity of the thus obtained
electrolyte pellet is substantially the same as that obtained
in with the pellet of Example 1.
EXAMPLE 3
; ~ A Pb/pbF2~75 mole percent) KF (25]mole percent)
/AgF, Ag solid electrolyte cell is made by pressing the
components as powders in a steel die. The anode,is a mixture
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of Pb and the electrolyte. The cathode is a mixture of
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1067144
~gF, Ag and the electrolyte. The solid electrolyte is '
PbF2 containing 25 mole percent KF. The procedures for
the abrication of the test cells are as follows: a
weighe~ a~ount o solid elec~rolyte'powder is pre-pressed in '~
a 0.6" diameter die at about 3000 psi. The anode mixture
and the cathode mixture are then placed on either side of , ,
the pre-pressed solid electrolyte pellet in the steel die
and the total assembly is pressed at lO0,000 psi. The
resultant solid electrolyte cell has a geometric area of
about 1.8 cm 2 and the thickness of the electrolyte layer is
determined to be about 0.5 mm. The cell exhibits an open
circuit voltage of 1.26 volts at room temperature and is
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discharged at a discharge rate of 22 micro-amperes at room
, ,temperature. The discharge curve of this cell is shown in
Fig~
, ' , EXAMPLE 4 ` .
~ solid electrolyte cell is madé by compressing a
mixture of powdered Pb and 5% powdered electrolyte, (PbF2 and
25 mole percent of KF as dopant) as the anode. A mixture of
20 powdered Ag and 5% electrolyte is used as the cathode. The
solid electrolyte is 75 mole percent of PbF2 doped with 25 ' ~,
, mole percent of K~. The test cell is prepared according to
the procedure set forth Ln Example 3. The geometric area of
, .' ' the cell is about 1.8 cm 2 and the thickness of the electrolyte
:'' ' . -
' layer is approximately 1 millimeter. The cell assembled as
' in Example 3 is charged and discharged at approximately 50
4 micro-amperes. The charge and discharge phases are
accom'plished at about 25C. Fig. 2 shows the charge and -
discharge curves of the rechargea~le 'cell according to this
30 example.
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1067~44
The substitution of PbBr2 , or PbC12 , or PbI2
for PbF2 in a solid electrolyte system that includes an `
alkali metal halide does not appear to result in the high
level o conductivity as does the PbF2 -alkali metal halide
containing solid electrolyte.
"Alkali metals" as used herein means and includes -,
metals of Group IA of the periodic system such as lithium
(Li), sodium (Na), and potassium (K). "Halide" as used
herein with alkali metals means and includes binary compounds
of fluorine, chlorine, bromine, and iodine, and also
astatine wherein the compound when mixed with PbF2 forms a r ' I
! solid eléctrolyte that has a conductivity in the range of
about 10 7to 10 2 ohm l-cm l with high ion conductivity which
can be used in electrochemical cells.
( Thé presence of small amounts of impurity elements
is not believed to play a critical role in the invention.
It should be understood, however, that we contemplate the -
possibility of other elements with PbF2 and the alkali
. . . ;
metal halide to produce a solid electrolyte that do not
affect the desired properties of the electrolyte.
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