Note: Descriptions are shown in the official language in which they were submitted.
7S'71
The present invention deals with an electric
accumulator cell with at least one cathode, at least one
zinc anode, separator or spacer arranged between cathode
and anode and an alkaline electrolyte. The anode and/or
separator (or spacer) is arranged in such a way that it/
they vibrate in the plane of or parallel to the electrode.
Electric accumulator cells having zinc anodes in
alkaline electrolyte are known in combination with
various kinds of cathodes, for example, nickel oxide or
silver oxide. The zinc anode-containing cell has several
advantages such as, for example, high half cell potential,
a high energy to weight ratio and, in comparison to
alternative anode materials, a low price. Certain problems
are, however, associated with the use of zinc anodes,
particularly regarding the life of the cells and the need
for large amounts of electrolyte. These problems stem
from the special properties of zinc in an alkaline
electrolyte.
The zinc anode is a so-called dissoluble electrode,
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i.e., during the discharge reaction the zinc forms products
soluble in the electrolyte. These products leave the
anode. Zinc forms mainly zincate ions which then may
react further in the electrolyte. The zincate ions then
form zinc oxide which has a much lower solubility in the
~, electrolyte than the zincate ions. The zinc oxide
precipitates as a solid. The main reactions during dis-
charge of the zinc electrode and the precipitation of zinc
oxide are the following:
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, Zn + 4 OH = Zn(OH)4 + 2e
Zn(OH)4 = ZnO + H20 + 2 OH
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Other reactions and other types of ions are present, but the
above are dominating and illustrate the main reactions in
the cell.
The problems with zinc electrodes are caused by
the redeposition of zinc on the electrodes during the
charging process, during which the zinc to a large extent
forms zinc dendrites which have a tendency to grow towards
the counter electrode and cause a short circuit in the cell.
An additional cause of short circuits is the tendency of
the zinc to collect at the edges of the electrodes,
specially at the lower edge. Thus, there occurs a re-
distribution of the zinc active material on the electrode
during the charging-discharging cycling.
Different methods have been tried to solve these
problems. The use of a semipermeable membrane between the
zinc electrode and the counter electrode has become very
common. The membranes are so dense that the growing of
zinc dendrites is made difficult. A large number of
additives of different kinds, organic as well as inorganic
substances have been tried in the electrolyte, the
membranes and the electrode. These endeavours have led to
important improvements of cells with zinc electrodes, but
the results are still not ~atisfactory. Attempts have been
made to redistribute the active material on the surface of
the electrode. One such attempt was to build the cell in
such a way that fluid flow and diffusion in the electrolyte
were obstructed as much as possible. Thus, the redeposition
of zinc should take place on the same surface as from which
the zinc had been dissolved during discharge. That, how-
ever, circumscribes very much the availability of electrolyte
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with consequences which shall be more closely considered
below. Another way to solve the problem of the active
material distribution is to pump the electrolyte around
in the cells. This presupposes, however, cost increasing
and space requiring auxiliary systems including pumps,
tubings etc.
The zinc electrode has also shown a tendency for ~-
passivation. The mechanism and reasons for such passi-
vation are not fully known, but it is generally thought
that one main reason for the passivation is the presence
of zinc oxide particles in the active zinc material. These
zinc oxide particles isolate portions of the zinc material
from participating in the electrochemical process so that
the load on the rest of the electrode increases. To avoid
the passivation effect as far as possible, great importance ~ -
has been put upon using such a large amount of electrolyte
that the zinc dissolved from the electrodes should be
present in a soluble form in the electrolyte. This
requires comparatively large amounts of electrolytes in the
cell, but even then, deposition of zinc oxide was not fully
avoided. Zincate ions are slowly decomposed and form zinc
oxide. The mechanism of the reaction is not fully known,
but the consequence of this decomposition is that a
formation of zinc oxide cannot be totally avoided. It is
possible to oversaturate an electrolyte with respect to
zincate ions and that has been used in accumulator cells,
but the electrolyte volume still remains too large. To
take away the zinc oxide which cannot be avoided in the
electrolyte, it has also been proposed to pump the
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electrolyte through a filter on which the zinc oxide is
filtered off.
The present invention relates to rechargeable
electric accumulator cells containing at least one zinc
anode, at least one cathode, a separator (or spacer)
dispos~d between the zinc anode and cathode, the anode,
cathode and separator being arranged in an alkaline
electrolyte, in which aforementioned cell, the anode and/or
separator between the anode and cathode are arranged to
vibrate either in the plane of or parallel to the electrode
and is characterized in that the zinc concentration in the
electrolyte is so high that the electrolyte contains free
solid zinc oxide in the fully charged accumulator cell.
The zinc anode and/or separator is vibrated in the direction
of the plane of the anode or separator at a frequency and
amplitude during the charging process sufficient to inhibit
dendrite growth on the zinc anode e.g., at a frequency of
between about 1 and 500 hz and an amplitude of between
about 0.1 and 10 mm during the charging process. The
vibrating part is mounted so that all portions thereof
move substantially the same distance during each cycle o~
vibration and all portions of the vibrating part move in a
reciprocating manner to have at the same moment the same
rectilinear movement during all portions of the vibration
cycle.
The difficulties involved with maintaining the
electrolyte free from zinc oxide have been described above.
It is, however, theoretically possible in known cells (and
ohould also be praotioally po~sible in fully charged
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relatively new cells) to keep the electrolyte free from
zinc oxide. Such cells are not included in the invention
according to which the zinc concentration shall be so high
that the presence of solid zinc oxide theoretically or --
practically cannot be avoided.
The electrolyte preferably consists of potassium
hydroxide dissolved in water to a solution containing 20
to 45 percent by weight of the hydroxide. It has been
found that it is possible with cells according to the
present invention to use electrolytes containing such large
quantities of zinc oxide that the electrolyte becomes
viscous and slurrylike. Suitably, the electrolyte contains
at least 200 grams of zinc oxide, preferably 250 to 400
grams zinc oxide, per liter of electrolyte when the cell is
discharged. However, amounts of zinc oxide above 600 grams
per liter have been used successfully.
Especially good results were obtained when the
vibrating components were constructed so that the
electrolyte was pumped between the electrodes. This
pumping action ensures that the electrolyte not only ls
moving back and forth and bac~ but also has a movement
which results in an electrolyte circulation between the
upper and lower parts of the electrode surfaces.
, . . . .
Figure 1 shows a cell according to the present
invention and Figure 2 shows different possible constructions
~ of a separator ~or spacer) which i8 intended to vibrate
`~ between the electrodes of the cell. In the cell shown in
`' Figure 1 the anodes are arranged for vibration although it
will be understood that the separator may also or
alternatively be vibrated.
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As shown in Figure 1, the cell contains a plurality
of dissoluble zinc electrodes 1 and counter electrodes 2
between which are disposed separators (or spacers) 3. The
dissoluble zinc electrodes 1 are connected by connecting
devices 4 to a common bridge 5. The zinc electrodes 1 are
arranged to be vibrated by means of a shaft 6 journaled
between bearings 8 in the cell wall 12. The shaft 6 is
eccentrically formed. Circulatory motion may be imparted
by shaft 6 by suitable means outside the cell (not shown)
and vibratory movement is imparted to the zinc electrodes 1
by yoke 7 and suspension and connecting devices 18 and 19.
The shaft 6 is electrically insulated from the electrodes
by using connecting devices formed of an insulating
material. Through a flexible cord 17, the zinc electrodes
may be connected to post 10 on the outside of the cell.
The electrodes 2 may similarly be connected to post 11 by
conventional means not shown.
The separator shown in Figure 2 is provided with an
outer edge 21, and attaehment 22 and distance ribs 23, 24,
25 and 26. These ribs are suitably but not necessarily
formed likewise on the entire separator and may have, for
example, one o the four different configurations shown in
the Figure. These ribs 23, 24, 25 or 26 impart a circu-
lation to the electrolyte in and up and down direction as
~, well as a back and forth movemept and thus enhance the
1 circulation or pumpting of the electrolyte.
The vibration frequency imparted to the zinc anode
and/or separator is suitably 1 - 500 Hz and the amplitude
is chosen in accordance herewith, in such a way that the
linear velocity reaches about 20 mm/second or more, at
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least at some instance during the up and down movement.
The amplitude may thus be on the order of from about G.l to
10 mm or more. Preferably, the amplitude is about 4 mm and
the frequency about 50 Hz.
The principles, preferred embodiments and modes of
operation of the present invention have been described in
the ~oregoing specification. The invention which is
intended to be protected herein, however, is not to be
construed as limited to the particular forms disclosed,
since these are to be regarded as illustrative rather than
restrictive. Variations and changes may be made by those
skilled in the art without departing from the spirit of the
invention.
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