Note: Descriptions are shown in the official language in which they were submitted.
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RECHARGEABLE NICKEL-ZINC CELLS
BACKGROUND OF THE INVENTION
The present invention relates to rechargeable nickel-zinc alkaline cells.
Alkaline nickel-zinc cells in the form of plate cells are commonly known but
have not
achieved commercial importance to date, mainly due to the limited life of the
zinc electrode.
Deterioration of the zinc electrode is caused by a change in the shape of the
electrode, the
growth of zinc dendrites, and corrosion of the electrode. In order to reduce
the solubility of
zinc and, thereby reduce any shape change of the electrode, cells have been
formed using
electrolytes with low alkalinity and containing KF and KZC03 and Ca (OH) Z as
anode
additives. In plate type sealed nickel-zinc cells dendrite formation is mostly
eliminated since
any dendrite produced is quickly oxidized by the oxygen present in the system.
The general
characteristics of nickel-zinc cell systems have been summarized by M. Klein
and F.
McLarnon ("Nickel-Zinc Batteries", D. Linden (ed.), Handbook of Batteries,
Chapter 29,
McGraw-Hill, Inc., NY, 1995) and the history and development of Ni-Zn cells is
reviewed by
J. Jindra (J. Power Sources, 66, 15 (1997)). The contents of these
publications are
incorporated herein by reference.
The Ni-Zn cell system follows the following reaction:
2 Ni00H + Zn + 2 H20 ~ 2 Ni(OH)2 + Zn(OH)z
In addition to this main current-generating process, several parasitic
reactions may
occur. Oxygen evolution has been found to occur at the end of a charge cycle
(i.e. at a charge
state of approximately 70-80%) and during overcharging of a cell (which is
necessary for a
better charge acceptance of the nickel electrode). In cases where the negative
electrode can
be easily accessed, oxygen can be directly recombined at the zinc electrode or
an auxiliary
electrode can be incorporated to enhance recombination. After repeated
cycling, hydrogen
evolution can also occur at the zinc electrode. To minimize the amount of
hydrogen
produced, a sufficient excess of Zn0 has to be provided. In general a Zn:Ni
ratio between
about 2 and 3 should be established. Furthermore, to avoid zinc corrosion in
the alkaline
medium, corrosion inhibitors such as In, Pb, Hg, or organic compounds, should
be added.
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In nickel-zinc cells different types of nickel electrodes are used: sintered,
nonsintered
and lightweight substrates. A description of such electrodes is provided in
"Handbook of
Batteries" (David Lindon (ed.), pg. 29.3), the contents of which are
incorporated herein by
reference. Sintered nickel electrodes are prepared by sintering carbonyl
nickel powder into a
porous plaque containing a nickel screen and is then filled with active nickel
hydroxide.
Typically sintered nickel electrodes have a ratio of inactive to active nickel
between 1 to 1.4
:1 providing excellent cycle life and stability, but with the disadvantage of
being very heavy.
Non-sintered nickel electrodes are made by kneading and calendering an
electrode strip
consisting of nickel hydroxide, graphite and plastic binder laminated on both
sides of an
appropriate current collector. Applying lightweight substrates based on a
fiber structure filled
with active electrode mass has the advantage of reducing electrode weight as
well as material
costs.
Cylindrical cells with spirally rolled nickel electrode/separator/zinc
electrode
assemblies, quite similar to Ni-Cd cells, have been tentatively produced by
some
manufacturers, but they suffered from serious short circuit troubles due to
zinc dendrites
growing during the charge cycles across the narrow (open) spiral distances
between cathodes
and anodes.
The objectives of this invention are mainly to produce high current, high
capacity,
cylindrical consumer cells that could be hermetically sealed and showing an
acceptable cycle
life at deep discharge conditions.
SUMMARY OF THE INVENTION
In a preferred embodiment, the present invention provides a rechargeable
electrochemical cell comprising:
a generally cylindrical container having an interior surface and an exterior
surface;
a generally cylindrical cathode contacting the container, the cathode being
coaxial
with the container;
a generally cylindrical anode contained within the cathode and being coaxial
therewith;
a separator for physically separating the anode and the cathode; and,
an electrolyte for electrically contacting the anode and the cathode;
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wherein the anode comprises a zinc material, the cathode comprises a nickel
material.
In a further embodiment, the cathode material comprises a porous nickel
material
coated a with nickel hydroxide paste.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a cut through a cylindrical AA-size Ni-Zn cell made according
to this
invention.
Figure 2 shows a nickel electrode with two layers from the top and a three-
dimensional view.
Figure 3 shows multiple (three) sleeves of a nickel electrode from the top and
a three-
dimensional view.
Figure 4 shows the discharge capacity of cell A75 and A79 with one nickel
layer as a
function of cycles.
Figure 5 shows the discharge capacity of cell A71 and A86 with two-nickel
layers as a
function of cycles.
Figure 6 shows the discharge capacity of cell A121 containing 2 % and cell A79
with
8.6 % Ni powder / T-210 as a function of cycles.
Figure 7 shows the discharge capacity of cell A128 containing 2 % and cell
A131 with
0 % Co extra-fine powder as a function of cycles.
DESCRIPTION OF PREFERRED EMBODIMENTS
In a preferred embodiment, the invention is directed to fabrication of a
rechargeable
galvanic element with a positive nickel oxide electrode and a negative zinc
electrode
containing an alkaline electrolyte and a separator. The cathode consists of a
nickel foam
structure that is filled with a nickel hydroxide rich paste made of a
polyvinylalcohol (PVA)
slurry. The nickel hydroxide is suitably compressed or compacted into a sheet
or tape of
defined thickness, rolled up into one or more layers and inserted into a
nickel-plated steel can.
In this way the nickel electrode is shaped into a very tight cylindrical
cathode. Alternatively
the filled foam can be compressed into a multiple of sleeves which are
inserted exactly the
same, also forming a cylindrical cathode. Such nickel foam based cathodes are
exhibiting an
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exceptionally low resistance and high efficiency leading to a sharp cut-off
after the capacity is
completely exhausted thereby establishing a cathode limited cell.
The anode consists of zinc powder, zinc oxide and a gelling agent, such as for
example, Carbopol. In rechargeable nickel-zinc cells the anode capacity is
chosen as a
multiple of the cathode capacity. The separator is preferably of the cellulose
type. A brass
nail located in the center of the cell builds the negative terminal. Other
materials for the
negative current collector will be apparent to persons skilled in the art.
The cell is characterized by prevention of excessive swelling of the cathode
due to the
cylindrical design in contrast to plate cells. It is further distinguished by
the use of special
additives to improve recharging. In a preferred embodiment the cathode is
provided with
hydrogen recombination catalysts for eliminating any hydrogen gas that may
evolve. Such
catalysts can comprise those used in mercury-free zinc anodes. A most
preferred catalyst is
silver (Ag). In such embodiment, the silver catalyst may be provided in an
amount of
between about 0.1% to 0.3% (wt.) of the nickel hydroxide. Such Ag catalyst may
be
incorporated into the Ni foam in the form of a colloidal deposit by means of a
spray coating
process as known in the art.
The electrolyte is preferably a solution of potassium hydroxide with lithium
hydroxide
as additive. The rechargeable nickel-zinc cells built according to the
invention can be
manufactured in all conventional cylindrical sizes (e.g. AAA, AA, C and D) but
are not
limited to these formats. Further the cells of the invention are hermetically
sealed and can be
used in all consumer electronic devices.
In a preferred embodiment, the cathode is provided with nickel foil strips to
assist the
can of the cell in its capacity as a current collector.
Figure 1 of the drawings shows a cut through a cylindrical AA-size Ni-Zn cell
embodying the present invention. The cell comprises a Ni-plated steel can 1
housing a porous
nickel oxide cathode 2, a zinc anode 3 and a separator 8 as the main
components of a
rechargeable galvanic element. The cathode 2 may comprise one or several
layers of a porous
nickel substrate filled with nickel hydroxide, additives and a binder, and is
separated from
anode 3, which may comprise zinc powder, zinc oxide and gelling agent, by an
electrolyte
permeable separator 8. The electrolyte, which may consist of aqueous potassium
and lithium
hydroxide, permeates the nickel cathode 2 and zinc anode 3 through separator
8. A current
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collector nail 7, that is connected to the negative cap 5 and embedded into
the plastic top seal
4, is located in the center of the nickel-zinc cell. For safety reasons the
plastic top seal 4 is
provided with a safety vent break area 6.
Figure 2 illustrates the embodiment of a nickel electrode made of two layers
of a
nickel foam, pasted with a mixture of nickel hydroxide, nickel powder, cobalt
powder and a
binder (PVA-solution), that is shaped into a very tight cylindrical
arrangement.
The embodiment of Figure 3 differs from that of Figure 2 in that, three or
multiple
sleeves of a nickel foam prepare the nickel electrode filled with nickel
hydroxide mixture.
Separators according to a preferred embodiment of the invention comprise two
overlapping layers of a laminated product comprising one piece of regenerated
high purity
cellulose bonded to a non-woven polyamide synthetic fiber. However, other
separators
known in the art may also be used.
As discussed below in more detail, the process of making the cells of the
present
invention according to a preferred embodiment comprises:
1 ) forming a sheet of Ni foam;
2) applying a paste of nickel powder, cobalt powder, PVA solution, and nickel
hydroxide to the Ni foam;
3) forming a separator in the form of a cylindrical tube open at one end
(referred to
herein as "separator bag");
4) placing the separator bag on a mandrel or other such support;
5) rolling the Ni foam sheet around the bag;
6) placing such Ni foam coated bag within a Ni plated steel can;
7) filling the interior of the bag with a zinc anode material.
As described below, the cathode preferably comprises a nickel foam coated with
8.6%
Ni powder, 4.3 % Co powder, 30% poluvinylalcohol (PVA) solution, and 57.1% Ni
hydroxide. The anode preferably comprises 59% zinc oxide, 10% zinc powder,
0.5%
Carbopol, and 30.5% KOH. The anode is preferably in the form of a gel paste.
The preferred
electrolyte is KOH/LiOH solution. In a preferred embodiment, the electrolyte
comprises
KOH in a concentration in the range of 6 to 9 M and LiOH dissolved in about 1%
to the
saturation poW t.
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The charging of the nickel-zinc cell made according to the preferred
embodiment is
done by a voltage limited charging circuit, constant current charging, or an
electronically
controlled overflow circuit bypassing excess current above 1.95 V.
The following examples serve to illustrate the present invention and are not
intended
to limit the scope thereof.
Example 1
A cylindrical AA-size nickel zinc cell was fabricated which consisted of one
positive
nickel electrode layer and a negative zinc electrode assembled in an
arrangement as shown in
Figure 1. The nickel electrode was prepared by blending a mixture of 8.6 % of
nickel T-210
powder (from Inco Technical Services Ltd., Missisauga, Ontario), 4.3 % of
cobalt extra-fme
powder (UNION MINIERE, INC. - Carolmet Cobalt Products, Laurinburg, N.C.),
30.0 % of
PVA-solution (1.17 % PVA in water/ethanol) and 57.1 % of nickel hydroxide
(from Inco
Technical Services Ltd., Missisauga, Ontario). Some water was added to obtain
a light
suspension. The slurry was pasted into a nickel foam of 38 mm x 36 mm provided
with a
spotwelded nickel foil current collector (36 mm x 4 mm, 0.125 mm thick, 99.98
%, from
Goodfellow Cambridge Ltd.,) at the longitudinal direction. The pasting
procedure was carried
out a few times on both sides of the nickel foam with a spatula to ensure that
the slurry
completely penetrates into the foam. Wet surplus material was removed from the
foam
surface. The nickel electrode was dried at 110°C for one hour. Two
different nickel foam
types were used to prepare a nickel electrode as described above: Retec 80 PPI
(pores per
inch), 1.6 mm thick, (foam from RPM Ventures, ELTEC Systems Corp., Ohio) and
Inco
foam, 2.7 mm thick of the same porosity (from Inco Technical Services Ltd.,
Missisauga,
Ontario).
The zinc electrode was prepared by mixing up 59 % of zinc oxide (from Merck),
10
of zinc / type 004F (from Union Miniere S.A., Overpelt, Belgium), 0.50 % of
Carbopol 940
(from Nacan, Toronto) and 30.5 % of 7 M KOH to a gel paste. Two overlapping
layers of a
laminated product comprising one piece of regenerated high purity cellulose
bonded to a non-
woven polyamide synthetic fiber (from Berec Components Ltd., Co. Durham) were
used to
construct the separator bag. The nickel electrode was rolled up around the
separator bag,
inserted into the nickel plated steel can, filled with 27 % KOH - 10 g/1
LiOHxHZO electrolyte
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and allowed to soak for 24 hours. The zinc anode paste was filled into the
separator bag and
the cylindrical AA-size nickel zinc cell was closed with the negative cap unit
as shown in
Figure 1.
Cell cycling was carned out with constant voltage taper charging at 1.90 Volts
for
approximately 500 minutes followed by the discharge process at 3.9 Ohms to a
cut-off
voltage of 800 mV. Figure 4 shows the discharge capacity of each cycle of
cylindrical AA-
size nickel zinc cells A75 (Retec 80, 1.6 mm thick) and A79 (Inco, 2.7 mm
thick) containing
one layer of nickel electrode consisting of the above mentioned nickel foam
types and a
pasted zinc electrode as a function of cycle life. The results obtained show a
stable discharge
behavior for at least 100 cycles with a relatively flat discharge profile and
a small capacity
decline during cycling. The first few cycles are formation cycles that run
under the cycling
condition described above.
Example 2
A cell was assembled as described above with the exception that the positive
electrode
was made of two nickel layers and the appropriate dimension of the nickel foam
was 38 mm x
70 mm. In the case of cylindrical cell design the assembly is volume limited
and therefore
cells with two layers contain less zinc. The nickel foam types used in this
example were Retec
80 PPI and Retec 110 PPI both of the same thickness of 1.6mm but of different
porosity's as
indicated by PPI (pores per inch). Inco foam, 2.7 mm thick, could not be used
in a double
layered arrangement because of its high thickness resulting in a deficiency of
positive zinc
electrode.
In Figure 5 the discharge capacity of each cycle of cylindrical AA-size nickel
zinc
cells A71 (Retec 80) and A86 (Retec 110) with 2 layers of nickel electrode and
a pasted zinc
electrode is shown. It turned out that the cells had high values of discharge
capacity (600-500
mAh) for the first twenty cycles but due to the Zn/Ni ratio of only 1.2 the
discharge capacity
decreased with increasing cycles.
Example 3
A cell, A121, was assembled as described in Example 1 except with a thinner
Inco 2.2
mm nickel foam but of the same porosity as the 2.7 mm Inco foam and with 2 %
of nickel T-
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210 powder and 63.7 % of nickel hydroxide. The other components of nickel
hydroxide slurry
were the same as in Example 1. In this case, it was not necessary to add water
to this light
suspension that easy penetrates into the Inco foam, 2.2 mm thick.
In Figure 6, the discharge capacity of each cycle of cylindrical AA-size
nickel zinc cell
A121 and A79 (from Example 1) can be seen. In comparison with cell A79, that
is also
constructed with one nickel layer, cell A121 delivers a 150-50 mAh higher
discharge capacity
up to 50 cycles due to its composition with more active nickel hydroxide (63.7
% instead of
57.1 %) and to a larger amount of pasted cathode mass as indicated in the
following table:
Cell No. Nickel Foam Type Nickel Cathode (g)
A79 Inco / 2.7 mm 2.88
A121 Inco / 2.2 mm 4.44
Example 4
Two cells were built as described in Example 1 but with 2% (A128) and 0 %
(A131)
of cobalt (Co) extra fine powder and with 59.4 % and 61.4 % of nickel
hydroxide. The other
components of nickel hydroxide slurry were the same as in Example 1 and Inco
foam, 2.2 mm
thick was used as the foam material.
Figure 7 shows the discharge capacity of each cycle of cylindrical AA-size
nickel zinc
cell A128 and A131. The discharge capacity of cell A128 with 2 % cobalt is
approximately
200 mAh higher than that of cell A131 containing 0 % cobalt since the addition
of cobalt
increases electronic conductivity of nickel electrode mass. The following
table summarizes
foam type and nickel cathode mass of both cells:
Cell No. Nickel Foam Type Nickel Cathode (g)
A128 (2% Co) Inco / 2.2 mm 3.44
A131 (0% Co) Inco / 2.2 mm 3.51
Although the invention has been described with reference to certain specific
embodiments, various modifications thereof will be apparent to those skilled
in the art
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without departing from the spirit and scope of the invention as outlined in
the claims
appended hereto.
