Note: Descriptions are shown in the official language in which they were submitted.
3 ~'7~Z~
This invention relates to an electrolyte for use
in electrochemical cells, current producing cells or electric-
al accumulators of various types, including fuel and solar
cells. The electrolyte contains a complexing agent which
reduces electrode solubility, improves electrolyte conductance
and takes part in the charge/discharge electrochemical reaction.
In the continuing search for improved power sources
considerable work has been done in investigating means for
improving the performance of various electrochemical cells
and storage batteries particularly in respect of size, weight
and capacity. Important applications for these being in such
fields as hearing aids, portable e~uipment, space and satellite
applications and ultimately electrically driven non-polluting
vehicles. One aspect of this work has naturally been to
investigate also the electrolyte used in such cells. Factors
of interest here are conductance, current-carrying capacity,
corrosion effects on electrodes and other cell parts, etc.
These factors vary with the concentration of the electrolyte
temperature, solubility of the electrodes, and additives.
Other important considerations are rate of chemical reaction,
charge transfer and, in the case of many rechargeable cells,
such problems as dendrite growth, deterioration of electrode
structure, loss of capacity and resultant short serviceable
life.
Various additives have been tested in electrochemical
cells of different types and inorganic as well as organic
additives as sometimes also used in the electroplating in-
dustry in an attempt to combat dendrite growth, improve charge
transfer and reduce internal resistance, or to generally improve
cell performance and life.
Additives have also been added to electrode structures
to help reduce loss of capacity, electrode poisoning effects, or
'7~
to improve electrolyte penetration further into the electrode
structure to better utilize the available material.
For the main part, the modifications and additives
so far used and proposed have not shown adequate long-term
stability on continued recycling use in electrochemical cells
as they tend to be oxidized, rejected or react irreversibly
with the electrode or other cell parts or are dissipated and
lose their efficacity. The present invention solves many
of the previously mentioned problems and in degree dependent
upon the cell type, electrode structure and construction in
which the electrolyte is used, such a cell is improved in
efficiency, performance and economic desirability.
The chemical reactions, during charge and discharge
of various electrochemical cells, are not fully known and
there are often a number of competing reaction schemes taking
part. To one skilled in the art of electrochemical cell re-
actions, be it as applied to electroplating or to electrical
or rechargeable cells, it is not essential to fully understand
all details of the reactions taking part to make use of the
invention. Furthermore the invention should of course be
measured solely by means of the claims herein whereas the
reactions taking place in a given electrochemical cell com-
bination when using the electrolyte of this invention should
not in any way be construed as defining or limiting the in-
vention.
The electrolyte of this invention is particularly
advantageous when used in pre-charged, dry stored, high
capacity cells in metal-air cells and in rechargeable cells
such as silver-zinc (~g-Zn), nickel~cadmium (Ni-Cd), nickel-
zinc (Ni-Zn), nickel-iron (Ni-Fe), lead oxide (PbO) and
metal-air cells and other cell systems.
In accordance with a particular embodiment of
- 2 -
~ 7~2-~3
the invention, there is provided an electrochemical cell, a
current producing cell or an accumulator. These comprise
active electrodes and an electrolyte which contains, in
accordance with the invention, a dilute solution of potassium
or sodium ferro- or ferricyanide.
The constituent parts of the electrolyte and the
optional initial additive of fluoride in suitable form can
equally well be used in cells having different electrolytes
as also for the formation and activation of various electrodes
prior to assembly into a cell as also for certain electro-
plating and electrochemical surface treatment processes. The
electrolyte of this invention has in general improved conduct-
ivity, particularly at low temperatures, reducing cell internal
resistance. Acting as a charge transfer medium, it increases
cell potential and improves cell stored capacity and efficiency
by ~etter utilization of the electrode electrochemical capa-
city. In the case of rechargeable cells, it will in addition
combat dendrite formation by attenuation of electrode solubi-
lity and alteration of electrode conditions. In cells of the
silver-zinc type cell degradation due to Ag migration to the
zinc electrode is also greatly attenuated and recycling life
considerably extended even without recourse to special separa-
tors.
The electrolyte comprlses a potassium or sodium-ferro
or ferri-cyanide with or without a cyanide or ferricyanide of
the electrode materials of the cell as electrolyte in the form
of a solution or as an addition in solution or salt form to an
electrolyte for electrochemical cells, current producing cells
or accumulators. The metallic (Fe) link retains the (CN) and
avoids loss during recycling the (Fe(CN)) complex actually
taking part in the charge/discharge reaction. Dependent upon
cell type electrodes and electrolyte composition, a reduction
~-~s - 3 -
. . , ~ . ,.
7fl~2~
of the electrode solubility follows which reduces or eliminates
,effects such as shape change, loss of electrode capacity,
dendrite growth, etc.
In certain cases, particularly in rechargeable cells
with zinc electrodes, an additional small addition of a suit-
able fluoride NaF or KF is also advantageous. Saturating the
electrolyte with zincate ZnO can also be useful. 'rhe electro-
lyte of this invention provides the following improvements
when used in electrochemical cells as prescribed. These
advantages are present in all aspects of the invention here-
after listed to a greater or lesser degree and will not be
listed again separately in each example.
1) Lower internal cell resistance.
2) Better low temperature performance.
3) Higher output potential.
4) Better efficiency.
5) Better utilization of electrode material.
6) Combatting of dendrite formation.
7) Longer cell life.
The electrolyte of this invention can be prepared as
follows: Potassium hydroxide (KOH) solution is prepared to a
density of 1.12 g/ml 20C. (2.6 molar or 13 wt%~. Potassium
ferricyanide (K3Fe(CN)6) is then added till a density of 1.14
to 1.2 g/ml is attained. Alternatively 15 grams of (KOH)
granules 85%KOH and 4 grams of potassium ferricyanide
(K3Fe(CN)6) can be dissolved per 100 ml of distilled water.
The absolute ratios are not at all critical and in many cases
even a very small amount of potassium ferricyanide added to an
existing electrolyte of many cells is sufficient to improve
performance considerably.
'rhe electrolyte density used in various types of
cells differs widely and is chosen to suit the particular
~"~ 4 -
i", .
4~
operating temperature and conditions as well as the cell com-
ponents and electrodes concerned. The criteria for the choice
of electrolyte composition or electrolyte additive used as
per this invention by any manufacturer for a given applica-
tion or set of conditions may likewise vary and could well
fall outside the preferred range here indicated as a guide.
This in no way detracts from the value of the invention or
the benefits claimed herein.
Examples:
1. In accordance with one aspect o~ this invention a
normal silver-zinc (Ag-Zn) pre-charged, dry stored cell is
supplied with the electrolyte of this invention consisting of
potassium hydroxide (KOH) solution of density 1.12 g/ml 20C.
to which is added potassium ferricyanide (K3Fe(CN)6) till a
density of 1.14 g/ml is attained. The cell will provide a
higher output, have a lower internal resistance, give better
efficiency and capacity.
2. In accordance with a further aspect of this invention
a normal cell of the nickel-cadmium (Ni-Cd) type is provided
with the electrolyte of this invention prepared as in Example
1 above instead of its more conventional electrolyte of KOH
or NaOH. The cell will similarly provide improved character-
istics as well as a higher output level than usual for
conventional cells. The extent of other improvements will
depend upon details of the electrode and cell construction.
3. In accordance with a further aspect of this invention
a cell of the normal nickel-iron (Ni-Fe) type is likewise
improved by the use of the electrolyte.
4. In accordance with a further aspect of this invent-
ion a cell of metal-air type, preferably of the zinc-air type,
is similarly improved by the use of the electrolyte of this
invention.
~'~7 ~JZ ~
5. In accordance with a further aspect of this invention
a cell of the nickel-zinc (Ni-Zn) ty~e is similarly improved
by the use of the electrolyte.
6. In accordance with a further aspect of this invention
a cell of the nickel-zinc (Ni-Zn) rechargeable type using the
electrolyte of this invention will also have zincate ZnO added
to the electrolyte and a very little fluoride KF or NaF and
boron in the form of Na2B407, K3sO3 or orthoboric acid H3B03.
In a preferred embodiment of this invention the
electrolyte of this invention is used in a nickel-cadmium (Ni-
Cd) rechargeable cell of normal construction, the electrode
structure of which can be of the pocket or the sintered or
other type and with suitable separators. The electrolyte of
this invention being made up as previously described of
potassium hydroxide KOH to a density of 1.12 g/ml to which is
added potassium -ferricyanide (K3Fe(CN)~) to increase the
density to around 1.14 to 1.18 g/ml. The absolute ratios are
not at all critical as even a very small amount of (K3Fe(CN)6)
added to an electrolyte is sufficient to considerably improve
performance.
In another preferred embodiment of the invention the
electrolyte is used in a nickel-zinc (Ni-Zn) cell of normal
construction and can be composed as in the previous preferred
embodiment of KOH solution to 1.12 g/ml to which is added
potassium ferricyanide to bring the density to 1.14 to 1.18
g/ml. In the case of zinc based electrodes in rechargeable
cells, it has been found beneficial to add a small amount of
potassium fluoride or sodium fluoride KF or NaF and boron in
the form of Na2B407, K3B02 or orthoboric acid H3B03 and to
saturate the electrolyte with zincate ZnO so as to
further arrest the solubility of the zinc electrode. The
electrolyte of this invention is in no way limited to use
~,.
~'7~Z~3
with the above examples and can also be used with other
electrode systems and additives. Such additives are often
used to modify electrode conductivity or overpotential or
to reduce gassing and self discharge, etc., some such
examples being graphite, carbon, Ni, Pb, Mn, Mo 9 V, Ti, Zr,
Hf, Nb, Hg, etc. When using the electrolyte of this invention
some such additives can to advantage be added in the form of a
cyanide or ferricyanide such as p~tassiumcyanomercurate
(K2Hg(CN)4).
The principles, preferred embodiments, examples,
various advantages and improvements obtained from the use of
the present invention have been described in the foregoing
specification. The invention which is to be protected herein
is however not to be construed as limited to the particular
examples of its use described or disclosed as these are to
be regarded only as illustrative rather than restrictive.
Variations and changes may well be made by those skilled in
the art without in any way departing from the spirit of the
invention.
- 7 -
