Nickel-cadmium cell apparatus

A gas tightly sealed nickel-cadmium cell is provided which works on the oxygen cycle principle and has a charge and discharge reserve at the negative electrode. Further, the partial pressure of the gases which cannot be reacted in the cell is below 0.4 bar. As a result of this substantial removal of the gases which cannot be reacted from the interior space of the cell, there is a considerable rise in the oxygen consumption rate, i.e., the rate at which the oxygen generated in the cell is reacted at the negative electrode, so that higher charge current densities are possible.

BACKGROUND AND SUMMARY OF THE INVENTION 
The invention relates to substantially hermetically sealed nickel-cadmium 
cells which work on the oxygen cycle principle, and which have a charge 
and discharge reserve at the negative electrode and have a diffusion body 
at the side of the negative electrode facing away from the positive 
electrode. 
Substantially gas tight nickel-cadmium cells usually work on the oxygen 
cycle principle, i.e., the positive capacity oi so designed that the 
positive electrode is completely charged. This avoids a generation of 
hydrogen at the negative electrode. The oxygen is reduced at the negative 
electrode so that, even with continued charging, the negative is never 
fully charged and there is, therefore, no risk of hydrogen generation. The 
oxygen is, however, usually slowly reacted so that, depending on charge 
current level and overcharging state, an oxygen partial pressure of a few 
bar prevails towards the end of charging. Since the cell is filled with 
air during construction, the interior space contains the non-reactible 
constituents of air, in addition to the quantity of oxygen which varies 
considerably in periodic cycles. In particular, some non-reactible 
constituents include nitrogen and small quantities of hydrogen, as well as 
water vapor from the electrolyte. 
Although it is known that after a certain service life or after complete 
discharge (Varta Fachbuchreihe, Vol. 9, "Gas Tight Nickel-Cadmium 
Accumulators", 1978, p. 74), or under certain conditions with charging 
lasting for days (German Published Examined Patent Application No. 
1,127,418), the oxygen in the cell may be reacted to such an extent that a 
slight underpressure prevails in the cell. Thus, the use of an 
over-pressure-resistant housing for such cells has hitherto been 
unavoidable. The cells therefore have a metallic housing in cylindrical or 
prismatic form which is capable of withstanding the internal pressure 
without substantial deformation. 
However, German Patent Application No. 2,742,869 shows a substantially gas 
tight nickel-cadmium cell which has a prismatic, shallow plastic housing. 
The bulging of the housing as a result of the overpressure occurring 
towards the end of charging is counteracted by means of a continuous 
central housing plug which holds the base and lids together. 
It is furthermore known that, as a result of the incorporation of special 
gas diffusion bodies which are arranged on the side of the negative 
electrode facing away from the positive electrode, the oxygen produced 
during charging or overcharging can be fed systematically to the negative 
electrode and reacts there particularly rapidly (for example, German 
Unexamined Patent Application No. 2,907,262). However, in this 
arrangement, the rate of reaction of oxygen (oxygen consumption rate) at 
the negative electrode is still so slow that an overpressure-resistant 
housing continues to be necessary. 
An object of the present invention is to provide a nickel-cadmium cell 
which works on the oxygen cycle principle as discussed above in which the 
oxygen consumption rate is so high that overpressure is no longer formed 
in the cell, and the latter can be constructed with a particularly light 
housing. 
These objects and other objects are achieved by providing a hermetically 
sealed nickel-cadmium cell aparatus which works on the oxygen cycle 
principle and has a charge and discharge reserve at the negative 
electrode. Any gases which cannot be reacted in the cell have a partial 
pressure below 0.4 bar. 
Surprisingly, the present invention has shown that an overpressure no 
longer builds up in the cell, if the partial pressure of the gases which 
cannot be reacted in the cell is below 0.4 bar. The removal of the 
non-reactible gases, in particular, nitrogen, from the gas space of the 
cell, results in a very considerable increase in the oxygen consumption 
rate. As a result, oxygen pressure is reduced by a factor of 3 to 4 
compared with a conventional cell for the same charge current level and 
the same charge condition. This applies even at high charging rates of up 
to 2 C.sub.N A (current in A of double the nominal capacity in Ah). 
The lower the partial pressure is of the gases which cannot be reacted in 
the cell, the better is the oxygen consumption rate. As soon as the 
partial pressure of the gases which cannot be reacted in the cell rises 
above 0.4 bar, the behavior of the cell increasingly approximates a 
conventional cell. According to advantageous features of a particularly 
preferred embodiment, the partial pressure of the gases which cannot be 
reacted in the cell is below 0.1 bar. 
These non-reactive gases can be removed before the final sealing of the 
cell. According to certain preferred embodiments, these gases are removed 
by thoroughly flushing with oxygen, for example. However, in particularly 
preferred embodiments, the removal of these non-reactive gases is achieved 
by evacuation of the cell by means of a vacuum pump. It was discovered 
that if partial pressures of less than 0.4 bar are maintained, an 
overpressure does not occur in any operating state of the cell, even with 
high charging currents. Such cells can therefore be given plastic 
housings, preferably in the prismatic or shallow construction, without any 
particular further pressure-resistant measures. The external atmospheric 
pressure forces the assembly of plates together via the plastic housing 
and ensures a good hydraulic contact. The essential technical advantages 
of such cells are: the lack of risk of explosions with serious 
consequences in the event of incorrect use or incorrect operation in mines 
or in manned space travel, a saving in weight compared with metal 
housings, a very simple manufacture of the housing by injection molding 
processes and a simple insulation of the terminal lead throughs. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWING FIGURE 
The gas tight housing 10 can be made of plastic and is provided with a 
layer 12 which inhibits the diffusion of nitrogen, on at least 85% of its 
surface. Suitable plastic materials for the housing are, in particular, 
polyolefins and polyamides. However, any plastics can be used provided 
they are inert towards the alkaline electrolytes. It was discovered that 
the coating with a layer which inhibits the diffusion of nitrogen is 
necessary, since the plastics have a relatively high permeability for 
gases, in particular for atmospheric nitrogen. The driving force for this 
permeation is increased by the underpressure which in general prevails in 
the cell. Without the coating which inhibits the diffusion of nitrogen, a 
long term increase in pressure occurs in the cell, and the advantages 
mentioned, such as high oxygen consumption rate or the forcing together of 
the plate assembly by the external atmospheric pressure are lost. 
It was found that it is often not necessary to coat the entire housing 
surface. On the contrary, in the case of thick-walled housing parts, in 
particular, such as the lid and base, a coating can be dispensed with or 
the coating can be applied only partially. However, in order to suppress 
the diffusion of nitrogen into the cell even in the long term, the coating 
should cover at least 85% of the surface of the housing. 
According to certain preferred embodiments, the layer which inhibits 
diffusion is preferably composed of metal. It is comtemplated that the 
layer be produced by vapor coating the housing or by the decomposition of 
gaseous metal compounds in vacuum or by electrochemical metallization of 
the housing. According to certain preferred embodiments, the 
electrochemically produced metal layer is reinforced further by 
electroplating. However, in particularly preferred embodiments, the metal 
layer is composed of a stuck-on metal foil. For this purpose, metal foils 
are advantageously used which are provided with a contact adhesive layer 
and can therefore be very simply applied to the housing surface. 
Suitable metals for the metallic coating are, for example, aluminum, steel, 
zinc, nickel, copper etc. The metal layer should be at least 15 .mu.m 
thick since it is difficult to manufacture thinner layers without fairly 
high porosity. Further, excessively thick layers are expensive and heavy, 
and thus, for optimum operating conditions, a layer thickness range of 
about 20 .mu.m to 70 .mu.m is suitable. 
According to certain preferred embodiments, an organic coating, in 
particular a lacquer coating composed of a suitable material, is used as 
the diffusion-inhibiting layer. Particularly suitable is a layer composed 
of polyvinylidene chloride or fluoride which is at least 50 .mu.m thick. 
It is also contemplated that the layer which inhibits the diffusion may 
additionally be provided with a protective lacquer or plastic layer which 
increases the abrasion resistance, and if the diffusion-inhibiting layer 
is composed of metal, improves the corrosion resistance and makes possible 
an insulation of the cell housing in the battery unit. In the case of very 
thin metal layers which may still have a certain porosity, very 
particularly good results are achieved if a protective lacquer layer is 
also provided to prevent the diffusion of gases. The lacquer layer is 
composed of polyvinylidene chloride or fluoride, for example. As a result 
of the coating, the additional advantage is achieved that the permeation 
of water vapor through the housing material composed of plastic is 
considerably restricted, as a result of which the quantity of electrolyte 
in the cell remains substantially constant even under extreme climatic 
conditions. 
The following example is offered by way of illustration only and should not 
be construed as limiting the scope of the invention in any way. 
EXAMPLE 
Two prismatic substantially gas tight nickel-cadmium cells were constructed 
with a nominal capacity (5-hour) of 19 ampere-hours according to the 
teaching of German Unexamined Patent Application No. 2,907,262, including 
metallic gas diffusion bodies at the rear sides of the negative 
electrodes. The cells had a 2 mm thick polypropylene housing. A 
self-adhesive 50 .mu.m thick aluminum foil was glued onto the outer 
surface of one of these cells. The aluminum foil covered 86% of the 
surface of the housing. The other cell was left untreated. Before being 
sealed, both cells were evacuated with a vacuum pump to 0.01 bar residual 
pressure. An electrical pressure sensor on the cell made it possible to 
track the variation in pressure during cyclization (charging and 
discharging) of the cells. 
With a charging current of 17 ampere and a charging factor of 1.2 relative 
to the previous discharge (exhausting discharge) a periodically repeating 
pressure play (pressure variation) of p=0.3 bar was measured. 
After a running period of 6 months, the pressure variations in the uncoated 
cell were already 0.7 bar and the cell had a residual pressure of 0.5 bar 
in the discharged cell. This residual gas atmosphere consisted of nitrogen 
which got into the cell by permeation through the housing wall. The 
capacity of the uncoated cell was dropping and the walls bulged slightly 
outwards at the end of charging as a consequence of the internal pressure 
produced of 1.2 bar (residual pressure 0.5 bar+pressure play 0.7 bar). 
Conversely, the cell coated with aluminum foil exhibited virtually no 
change in it properties. 
Although the present invention has been described and illustrated in 
detail, it is to be clearly understood that the same is by way of 
illustration and example only, and is not to be taken by way of 
limitation. The spirit and scope of the present invention are to be 
limited only by the terms of the appended claims.