Note: Descriptions are shown in the official language in which they were submitted.
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ZINC E_ECTRODE FOR ALKALINE BA~TERIES
This invention relates to alkaline batteries and, more
particularly, to a zinc electrode having an active material
containing zinc-magnesium alloy.
BACKGROUND OF T~E INVENTION
An alkaline battery consists of at least a positive electrode, a
zinc negative electrode, separators to keep the electrodes from
touching each other and a cell case and connectors. Ideally, the
zinc electrode should be made of zinc but the fabrication of a
suitable electrode made of zinc has until now not been successful.
Zinc electrodes are presently mainly made of zinc oxide and
additives applied to a substrate. The battery has an energy
density higher than, but a cycle life shorter than that of a lead-
acid battery.
There are two reasons for the short cycle life. When zinc metal
is converted to zinc oxide in the alkaline electrolyte during
discharging, the zinc oxide dissolves into the electrolyte until
the solution is saturated. When the cell is being recharged, that
dissolved zinc oxide is being plated out of solution in the form
of metallic zinc. When zinc is plated out of the alkaline
electrolyte, dendrites tend to form. The dendrites will eventually
penetrate through the separators and cause a short-circuit, and
cause the cell to discharge at a high current. Eventually, the
cell will no longer be able to be recharged or even sustain an open
circuit voltage.
Dendrites tend to form predominantly during charging of the
battery, and especially during the overcharge portion of the charge
cycle. The formation of dendrites is usually avoided by special
charging techniques and by maintaining an excess of zinc oxide in
the zinc electrode.
The second reason for the short cycle life is that zinc electrodes
are made by applying some composition of zinc, zinc oxide and
certain additives to a suitable current collector. The zinc metal
and/or zinc oxide particles are usually held together and are held
to the current collector by some polymer material, such as TeflonTM
or other polymers. During cycling, the active material of the zinc
electrode tends to shift towards the bottom and bottom centre of
the electrode. This not only reduces the surface area of the
electrode but also leads to the formation of dendrites because the
current, especially when charging, is concentrated on a smaller
area.
BRIEF DESt:RIPTION OF PRIOR ART
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In efforts to overcome the above-mentioned problems, metallic
additions to the zinc or zinc oxide have been used. According to
US Patent 3 042 732, an improved anode in a nickel oxide-zinc
secondary alkaline cell is provided which comprises the combination
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of powdered zinc with powdered lead oxide, copper, copper oxide or
magnesium, present in an amount that prevents gassing on deep
discharge, the powdered zinc and powdered additive being intermixed
throughout the anode. The improved anode may also contain a
plurality of zinc-plated magnesium powders. According to French
Patent 1 004 463, the active material of a zinc anode for a nickel
oxide-zinc battery may consist of zinc oxide or zinc powder with
10% to 20% added magnesium powder; the powder mixture is pressed
as a paste onto a grid of zinc or magnesium. Tomassi, W. (Chem.
Abs. 81:138455f, 98440p, 130154m and 98552b) has studied the
electrode potentials, the corrosion, migration kinetics and x-ray
diffractions of negative electrodes for metal-oxygen cells, and,
particularly, of zinc electrodes with additions of magnesium~
aluminum, their alloys and mixtures. The porous electrodes are
made by die-pressing of powdered mixtures of zinc with aluminum
powder and/or magnesium powder in amounts of 5-20% Mg and 5-40% Al,
respectively. Mukherjee, D. etal. (Transactions SAEST, Vol. 22,
No. 1, 1987) studied the polarization behaviour of binary alloys
of zinc with 3% Al, Cd or Mg in potassium hydroxide solution.
20 All these prior art zinc electrodes are made with powdered mixtures
including zinc powder and magnesium powder. None of the prior art
references discloses an active material for a zinc electrode for
an alkaline battery that contains an alloy of zinc and magnesium.
SUMMARY O_ THE INVENTION
It has now been found that the formation of dendrites in an
alkaline battery can be substantially alleviated and that the cycle
life can be considerably increased by using a zinc electrode with
an active material that comprises zinc-magnesium alloy.
It is an aspect of the present invention to provide a zinc
electrode that does not form dendrites in alkaline batteries. It
is another aspect to provide a zinc electrode that has an active
material containing zinc-magnesium alloy.
Accordingly, there is provided a zinc electrode for use in an
alkaline battery consisting of a current collector and an active
material applied to said current collector, said active material
comprising zinc-magnesium alloy containing magnesium in the range
of from greater than 0% to about 15% by weight.
Accordingly, there is also provided a zinc electrode for use in an
alkaline battery consisting of a current collector, made of metal
chosen from the group consisting of copper, silver, titanium,
stainless steel and magnesium, and an active material applied to
said current collector by compaction, die-pressing, cold-welding,
sintering, or mixing with a binder material, a filler material or
both and heating, said active material comprising zinc-magnesium
alloy containing magnesium in the range of from greater than 0% to
about 15% by weight.
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According to a second embodiment, there is provided a method of
producing a zinc electrode which comprises the steps of forming a
zinc-magnesium alloy containing from greater than 0% to about 15%
by weight of magnesium, forming a particulate of said alloy, and
applying the particulate alloy to a current collector by one of
compaction, die-pressing, cold welding, sintering, and mixing with
a binder material, filler material or both and heating.
The aspects of the invention and the manner in which they are
attained will be apparent from the following detailed description.
DETA I LED DESCR I PT I ON
The negative electrodes for alkaline batteries according to the
invention are prepared by alloying zinc with magnesium and applying
the alloy to a suitable current collector. The alloying of zinc
and magnesium is carried out by conventional means such as by
melting zinc and magnesium in amounts that yield the desired alloy
composition at a temperature of, for example, 500C, and then
solidifying the alloy such as by casting in the form of ribbons,
strips or other suitable forms. The cast alloy is readily
pulverized to desired particle sizes, which are, preferably, in the
range of about 37 to 150 microns.
Suitable zinc-magnesium alloys are alloys that contain magnesium
in the range of from greater than 0% to about 15% by weight.
Preferably, the magnesium content is in the range of about 1% to
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about 10% by weight. Below about 1~ magnesium, the electrode
behaves nearly like pure zinc, which is subject to extensive
dendrite formation, while above about 15~ magnesium, the electrode
has too low a utilization to be of practical use. Above about 10%
magnesium, the zinc electrode becomes increasingly inert.
During the alloying of zinc and magnesium, a small amount of an
additive metal may be added which is effective to reduce the
hydrogen overvoltage of the electrode. Additive metals such as,
for example, lead, indium, thallium, mercury or cadmium added in
an effective amount of, for example, about 1% by weight are
effective in reducing hydrogen evolution in an alkaline battery
during use. If desired an effective amount of an additive metal
such as, for example, aluminum in the range of about 5% to about
25% by weight of zinc, preferably about 22% by weight of the zinc,
may be used to increase the porosity of the zinc electrode. The
aluminum is leached from the zinc-aluminum-magnesium alloy after
fabrication of the electrode. The zinc-magnesium alloy, with or
without additive metals, is applied to a suitable current
collector. Suitable current collectors are made of metals such as,
for example, silver, copper, titanium, stainless steel or
magnesium. The current collector is, preferably, an expanded metal
mesh, although other forms of metal current collectors may be used.
The alloy is applied to the current collector by conventional means
such as, for example, by compaction, die-pressing, cold-welding,
sintering, and mixing alloy with a suitable binder material or
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filler material, or both, and heating.
The electrode so produced has a layer of active material consisting
of zinc-magnesium alloy. The electrode does not have to be
charged, as it is already in the charged state. The electrode of
the invention, when used in an alkaline battery, has a cycle life
longer than conventional zinc oxide-based electrodes, and causes
little or no formation of dendrites.
The invention will now be illustrated by the following non-
limitative examples. In the examples the electrodes were
cyclically charged at a constant potential of 1.93 V with a 25%
overcharge, and were discharged at a constant current in the range
of about 1 to 10 mA/cm2.
Example 1
Solid discs of ZillC, zinc-magnesium alloys and zinc-magnesium-lead
alloys were each assembled into a cell to become a zinc electrode.
Located 0.5 mm above the zinc electrode was a silver metal screen,
and 2 cm above that was a platinum counter-electrode. Using a
programmable potentiostat and a strip recorder, the zinc electrode
was cycled within the boundaries for a zinc electrode when used in
a conventional zinc electrode-containing battery. A 35% KOH,
zincate free electrolyte was used, and cycling of the electrode
consisted of a charge and a discharge; no open circuit times were
used. As cycling progressed, zinc dissolved into the electrolyte
during discharging and replated onto the electrode during charging.
When replating, æinc tends to form dendrites which will contact the
silver grid just above the zinc electrode. The potential between
the zlnc electrode and silver screen was monitored by a strip
recorder. When a dendrite touched the silver screen, that
potential dropped to zero. The time when the drop in voltage
occurred was determined. The results are shown in
Table I.
Table I
ALLOY ?IME TO ov (Zn VS. Ag
Zinc (pure) 25.5 min - shorted
Zinc ~ 1.5%)Mg 37.5 min - H2 evolved - no short
Zinc - 2.2% Mg 31 min - H2 evolved - no short
Zinc - 2.2~ Mg, 1~ Pb 43 min - H2 evolved - no short
15 The pure zinc electrodes all failed after 25.5 minutes due to a
dendrite contacting the silver screen. None of the alloy samples
shorted, but the amount of hydrogen evolved increased with cycling
until the recorder trace became erratic.
Example 2
An alloy was made from zinc and 4% by weight of magnes$um. A
cleaned piece of copper expanded mesh was dipped into the melt.
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About 10 g of the melt adhered to the mesh upon retraction from the
melt. The solid zinc-4% magnesium electrode was wrapped in two
layers of CelgardTM separator (Celanese Corp.), and cycled in a Ni-
Zn cell. The electrode was cycled as in Example 1. The results
are shown in Table II.
Table II
Cycles No. Observation
1-15 Time to reach maximum capacity of 3.5 A.h
16-31 Capacity maintained 3.5 Aoh
10 32-75 Capacity decreased gradually to 1.2 A.h
The cycling was carried out to full depth discharge, and the
charging was carried out with a 25% overcharge each cycle. Cycling
started with a di~charge as the zinc electrode was already in the
charged state. Cycling was terminated becau~e of low capacity; no
sign~ of cell shoxting were evident.
Example 3
Three zinc electrodes were made by encasing active material in a
flat tube fabricated from a commercial lead-acid battery separator.
An expanded copper grid was inserted in the tube, the active
material was evenly distributed over the grid, and the tube was
heat-sealed. The electrodes contained zinc powder, zinc oxide
powder or zinc-4~ magnesium powder, particle sizes < 74 microns,
as the active material. The electrodes were cycled in a Ni-Zn cell
as in Example 2. The results are given in Table III.
Table III
Electrode No. of Cycles
Zinc (started with discharge; 35% KOH) 5
Zinc Oxide (started with 2 conditioning
cycles; 35~ KOH saturated with ZnO) 5
Zinc-4% Magnesium (started with discharge;
35% KOH) 9
All electrodes shorted after the number of cycles shown in Table
III. The low cycle life was due to the relatively large pores in
the separator through which zinc dendrites could pass easily.
Example 4
Three zinc electrodes were fabricated. The electrodes consisted
of active material, a filler material (PbTiO4) and a polymer binder
(TeflonTM). A paste was made from these components and spread onto
a copper mesh qrid. The electrodes were encased in 2 layers of
CelgardTM separator. The electrodes contained zinc powder, zinc
oxide powder or zinc-4~ magnesium powder, particle sizes < 74
microns, as the active material. The electrodes were cycled in
nickel-zinc cells as in Example 2. The results are shown in Table
IV.
Table IV
Electrode No._of Cycles
Zinc (started with discharge;
35% KOH) 5
Zinc Oxide (started with 2 conditioning
cycles; 35~ KOH saturated with ZnO) 5
Zinc-4% Magnesium (35% KOH) 53
The results shown in Table IV indicate that zinc electrodes having
an active material of zinc-magnesium alloy have a much longer cycle
life than a zinc- or zinc-oxide-based electrode.
Example 5
Three electrodes were made by applying a mixture of two zinc-
magnesium alloys, having particle sizes < 500 microns, to a
magnesium or copper grid substrate and sintering at 375C for a
time sufficient to produce the electrode with a coherent, hard
metallic layer of zinc-magnesium alloy. For comparative purposes,
two conventional ZnO-based electrodes were tested. The electrodes
were cycled, as described in Example 1, in a cell containing a 35%
KOH electrolyte.
The electrode compositions and cycling results are given in Table
V.
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Table V
Electrode Cycling Capacity
No. Alloys Weight No. Duration Initial Final
1 Zn - 5% Mg 30 g3006 months 30% 15%
Zn - 50% Mg 15 g
2 Zn - 7% Mg 30 g75016 months 30% 35-40%
Zn - 50% Mg 15 g
3 Zn - 10% Mg 30 g110016 months 30% 8%
Zn - 50% Mg 15 9
4 ZnO on Cu grid 350 7 months 75% 8%
ZnO on Mg grid 350 7 months 78% 10%
percentage of theoretical
Electrode 1 failed after 6 months due to a corroded connector.
Electrode 2 did not show failure after 16 months and continued to
cycle. Electrode 3 failed after 16 months due to very low
capacity. The conventional electrodes 4 and 5 substantially failed
after 350 cycles due to high resistance and shorting, respectively.
Example 6
Two electrodes were fabricated to evaluate the effect of the
inclusion of aluminum in the active material of a zinc-magnesium
electrode on the electrode capacity. An electrode containing no
aluminum in the active mix was fabricated for comparison.
The electrodes were made by inserting and packing particles of
active material into perforated copper tubes. One tube contained
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Zn-5%Mg, another contained Zn-10%Al-5%Mg and the third contained
Zn-22%Al-5%Mg. The copper tubes acted as the current collector and
the active material container. The two electrodes containing
aluminum were leached overnight in 30% KOH to remove the aluminum
and to leave a porous Zn-5%Mg active material in the copper tubes.
The results are shown in Table VI.
Table VI
.
Electrode Maximum CapacityCycle Life Final Capacity
(% of theoretical) (% of theoretical)
Zn-5%Mg 30 33 25
Zn-10%Al-5%Mg 43 70 25
Zn-22%Al-5%Mg 63 45 25
The electrodes were removed from cycle tests when the capacity
dropped to 25% of theoretical, not because of electrode failure.
This example shows how the inclusion of aluminum in the electrode
active material alloy, and subsequent leaching therefrom, improves
the electrode performance by increasing the electrode porosity.
From the foregoing examples it is clear that zinc electrodes
fabricated from an active material of zinc and magnesium will
result in zinc electrode-containing batteries that have an extended
14
cycle life. Electrodes made from blends of zinc powder and
magnesium powder do not have the intimate blend of zinc and
magnesium as an alloy. It is also clear that the formation of
dendrites is suppressed for electrodes that have an active material
comprising zinc-magnesium alloy.
It is understood that changes and modifications may be made in the
embodiments of the invention without departing from the scope and
purview of the appended claims.
