The development and design of the Controlled Microgeometry electrode are described. Advantages of the electrode over others in existance include a higher number of ampere hours per kilogram and the ability to make them over a wide range of thicknesses. The parameters that control the performance of the electrode can be individually controlled over a wide range. Therefore, the electrode may be designed to give the optimum performance for a given duty cycle

Depaul, R. A.

Gutridge, I.

English

NASA Technical Reports Server

THE CMG NICKEL ELECTRODE
R.A. De Paul and I. Gutridge
MPD Corporation
I should like to introduce, to those of you who are not al-
ready aware of it, a new nickel electrode called the Controlled
Microgeometry (CMG) electrode. It is a product of battery re-
search at Inco's European Research and Development Center in
Brimingham, UK. It has been developed to the stage of small scale
production and, since early this year, has been commercially
available from MPD Technololgy both in Europe and the U.S. MPD
Technology is the company set up by Inco to commercialize new
products resulting from research and development.
In this presentation I first want to describe the concept of
the CMG electrode and its advantages over the more conventional
type of electrode, and then to present some results that we have
achieved with CMG electrodes in nickel cadmium cells.
(Figure 18-1)
Figure 18-1 shows diagramatically the construction of the CMG
electrode. The basic element is very thin perforated nickel
foil. The perforations in the foil are both accurate and regu-
lar. Each foil is coated either on one side or both sides with a
layer of nickel hydroxide active material, leaving the holes
clear. An electrode is produced by stacking foils together,
usually 20-100 foils, so that all the small holes are in regis-
ter. There is thus a sandwich-like construction of foil/active
mass/foil, with cylindrical channels passing through the elec-
trodes.
The next two figures show actual electrodes.
(Figure 18-2)
Figure 18-2 shows an ordinary view of a CHG electrode.
(Figure 18-3)
Figure 18-3 shows an expanded view. There are two different
patterns of holes; their significance will appear later.
It is important to get a feel for some of the numbers in-
volved. Typical values for the critical dimensions of the elec-
trode are as follows: the foil thickness is about 4 m; the active
mass thickness is 60 m, about 15 times the foil thickness; the
hole diameter is about 0.5mm, the edge-to-edge hole spacing is
about 0.75mm; the average perforated area is 15%, and the elec-
trode thickness is about 3mm.
183
https://ntrs.nasa.gov/search.jsp?R=19810012980 2020-03-21T13:24:30+00:00Z
(Table 18-1)
Table 18-1 shows the characteristics of a typical electrode
that we are currently producing. The important thing to note is
the ratio of active mass to foil, which is something over 2.5:1.
One advantage of such a construction that will have been ob-
vious from Table 18-1 is a higher number of ampere hours per kilo-
gram. A typical specific capacity for a CMG electrode is
190A h/kg and 350A h11. The best figures obtained to date are
220A h/kg and 450A h/l. I think you will agree that 140A h/kg and
450A h11 would represent a fairly good sintered electrode. The
main reason for this improvement is that we are using this very
thin nickel foil substrate, which combines low weight with good
strength and conductivity. Whereas a typical ratio for a medium
loaded sintered electrode may be equal weights of active mass to
support, we have a ratio of between 2:1 and 3:1.
The second advantage of this construction is really what led
to the coining of the name "controlled microgeometry." All the
parameters that control the performance of the electrode (the size
of the holes, the distance between the holes, thickness of the
foil, thickness of the active mass, the porosity and composition
of the active mass) can be individually controlled over a wide
range. Therefore, the electrode may be designed to give the opti-
mum performance for a given duty cycle. The purpose of the design
may be high energy density or high rate performance. To illus-
trate this, Figure 18-4 shows the effect of changing the pattern
of holes.
(Figure 18-4)
Two 24A h electrodes are shown. In one, the area of holes is
13%. In the second it is 20%. When the electrodes are discharged
at the C/5 rate, there is little difference between the two elec-
trodes, as shown by curves 1 and 2. However, when the discharge
rate is increased to the 2C rate, then the electrode with the
finer 20% hole pattern shows a marked improvement both in capacity
and discharge voltage.
Figure 18-4 just shows the effect of two different hole pat-
terns. When you consider that we can vary the hole area from 5 to
45% and the hole diameter from 0.4 to lmm, I think you will appre-
ciate that we have scope to design for widely different discharge
profiles. In fact, the examination and optimization of this
aspect of electrode design is the subject of a Department of
Energy contract that has recently been awarded to the Inco R&D
center.
The third significant advantage of CMG electrodes is the
ability to make them over a wide thickness range. Conventional
electrodes are usually limited either by practical or by economic
184
considerations to a thickness range of about 0.5-3 mm. CMG elec-
trodes may be made by using just a few foils, or up to about 140
foils to give an electrode 6mm thick. We have an electrode that
is 36mm thick, 15cm by 30cm, with a capacity of just over 100A h
for a single electrode. Thick electrodes have obvious benefits
for high energy density moderate rate batteries, whereas the
ability to make an electrode, say, 0.2mm thick combined with
optimization of hole pattern offers the scope for very high
discharge rates, in the 100 C range.
So much for the idea. Does it work?
During the research and development work on this electrode, we
have made and tested several thousand electrodes, and the proper-
ties I have quoted so far are based on test results of many
hundreds of electrodes. However, tests on single electrodes are
of limited interest, and so I should now like to show some results
achieved in cells. I shall concentrate on nickel cadmium cells,
since I believe they are of most interest to this audience; how-
ever, the electrodes can and are being used in both nickel iron
and nickel zinc systems.
The next few figures show the results of laboratory tests in
nickel cadmium cells using CMG nickel electrodes and commercially
available sintered cadmium electrodes. These results were first
published in a paper presented by Dr. Turner from the Inco Re-
search Labs at the recent Power Sources Symposium in Brighton.
(Table 18-2)
Three cells were built, and the constructions are shown in
Table 18-2. The CMG electrodes varied in thickness from 0.67mm to
just under 2mm. The cadmium electrodes were either 0.6 or 0.8 mm
thick, and we used a number of different separator systems and two
electrolyte concentrations.
Figure 18-5 shows the first few cycles of these cells.
(Figure 18-5)
All three cells require about ten cycles to reach full capac-
ity, but a substantial portion of the capacity is available after
five cycles. Cell A stablized at 111% of theoretical capacity
after 12 cycles. Cells B and C leveled out at about 90% of theo-
retical capacity after about 15 cycles. One of the differences
between cell A and the other two is the electrolyte concentration,
which was 30% in cell A and 20% in the other two. At cycle 26 in
cell C, the electrolyte was changed to 30% KOH, and the capacity
rose to 100% of theoretical.
185
The formation of these cells was done in excess electrolytes.
The cells were cycled without a bottom on the case, in about 20
liters of electrolyte. After cycling in this way for about 50
cycles, the cells were removed from the tank and bottoms were
fixed to them. The electrolyte volume was then 3-7cm 3 A h.
(Figure 18-6)
Figure 18-6 shows the next 40 cycles of cell A. Initially the
cell showed a marked fall in capacity. This highlights an impor-
tant factor in the use of CMG electrodes. It is necessary with
these electrodes to provide sufficient support to the electrode to
prevent swelling. For the first few cycles of this cell, the
plastic cell case was unrestrained and the cell case wall bulged.
At cycle 70 the cell was clamped back to its original dimensions
and see the capacity recovered, showing that this loss was not
permanent. It is therefore very important, in the design of cells
using CN1G electrodes, to ensure that sufficient pressure is exert-
ed to maintain the electrodes within their original dimensions.
This can be done by designing the cell and battery stack to be
close packed and providing the necessary clamping from the battery
box. Alternatively -- and clamping is not possible in all cells
-- for a freestanding electrode, the separator and electrode may
be stitched at regular intervals. A little energy density is
lost, but for some applications this approach is most appropriate.
(Figure 18-7)
Figure 18-7 shows the performance of the cell at different
discharge rates. At 50% of theoretical capacity (132A h) the cell
voltage at 40A drain is 1.26V, falling to 1.21V for 160A drain.
The delivered capacity at 40A was 220, falling to 200A h at 160A.
(Figure 18-8)
Figure 18-8 shows the charge acceptance o' one of these
cells. The upper curve shows the discharge capacity expressed in
both ampere hours and ampere hours per kilogram versus the charge
capacity. The lower curve shows charge capacity versus charge
factor. To maintain a capacity of 120A h, a charge factor of 1.03
is necessary, whereas to maintain 140A h a charge factor of 1.16
is required. Conversely, if a specific battery application re-
quires a maximum overcharge of, say, 10% then the design param-
eters for this cell should include the value of 155A h/kg for CMG
nickel electrodes of this particular construction.
That is a quick survey of the sort of properties that we have
achieved. I shall summarize them in the next two tables.
(Table 18-3)
186
Table 18-3 shows the electrode properties (a) in excess elec-
trolytes, (b)when the electrolyte was restricted to 3 -
7 cm3/Ah, and (c) after 300 deep discharge cycles (by which I
mean 100% depth of discharge every cycle), after which the capac-
ity of the nickel electrodes is 160A h/kg and 320A h/1.
(Table 18-4)
Utilization of the nickel hydroxide is 85 - 90% after deep
cycling for nearly 300 cycles, as shown in Table 18-4.
I hope I have managed to arouse your interest in this elec-
trode, and that you may see possible applications in the aerospace
field.
DISCUSSION
SENDERAK: What are the maximum discharge rates that you can
get out of this type of electrode?
GUTRIDGE: You can design this electrode for very high rate
applications. The standard electrode that we make (15mm thick, 15
by 30), can easily be cycled between C/1 and C/3 rates. However,
with a higher area of holes and the right hole pattern, you can
make a thin electrode with fewer holes in it, which can be dis-
charged at 15 - 100 C. Thus, you design to determine the internal
resistance of the electrode, and you have control over all the
critical numbers that you need to design to do that.
SENDERAK: Would you cycle these cells as vented or sealed?
GUTRIDGE: The results I just described were in vented cells.
LIM: I have a related question about the rate and the amount
of the electrolyte. I am not sure whether I missed something.
Can you comment about the amount of the electrolyte and the rate
capability? I am asking the question because you have relativ-_ly
large holes, and if you are running the cell in start the condi-
tion, there would be a rather thin electrolyte connection between
the active material and the separator ,act=a.
GUTRIDGE: The numbers I qave were between 3 and 7cm 3/A h of
electrolyte in these vented cells. As far as the discharge rate
in these cells was concerned, we went to the 2 cm 3 rate, and w
were already starting to have problems with the cadmium elec-
trode. Therefore, we were restricted by that in the rates at
which we could discharge the cells.
187
As far as the volume of electrolyte is concerned, the porosity
of the active mass is quite high. We essentially have sufficient
electrolyte within the confines of the electrode to discharge or
to cycle this electrode at moderate rates.
ROGERS: Am I right in thinking that the active material is
packed between the sheets?
GUTRIDGE: Yes.
ROGERS: In that case, as you mentioned, you get a pressure
exerted during expansion of the active materials. What pressure
are we talking about? Is it sufficient, for example, to flatten
the usual polypropylene or felted nylon separator?
GUTRIDGE: Let me make a comment before I answer the question
about expansion. We are not looking at something that will
destroy the strucutre of the electrode. However, the problem you
may get is the one you mentioned. If you do not restrict the
dimensions, you can put pressure on other parts of the cell.
We defined a pressure which we pressed the electrodes before
we start cycling at something like 1.38 - 2.76 x 1044N/m
(2 - 4psi). We do not have figures for the pressure that is
likely to develop in a particular cell arrangement. That depends
on the design of the cell. We try and keep the electrode within
its dimensions and prevent the swelling, rather than let it swell
and see what happens to the cell.
ROGERS: If you do restrict it in, say, battery design as we
do in an aerospace battery, then if the electrode swells the sepa-
rator material is going to flatten out and you are not going to
have a separator any more, you are going to have almost a solid
sheet. It is an extreme case.
GUTRIDGE: We have not seen that sort of problem. One of the
cells that I have described had only two layers of four-mil felted
nylon. That was the only separator material. That cell performed
quite satisfactorily.
BOWERS: For battery C, did the replacement electrolyte con-
tain lithium hydroxide?
GUTRIDGE: Yes.
BOWERS: Have you discharged cells without lithium hydroxide
additive?
GUTRIDGE: We have. We find that the number of cycles for
formation is greater if lithium hydroxide is not present.
188
LEAR: Table 18-3 up there showed 300 cycles. Did you con-
tinue cycle testing?
GUTRIDGE: Yes, these cells are still being cycled, and they
are up to 400, 450 cycles now.
MAURER: The figures raised a lot of interest to those of us
who were involved in those previous three papers. In Figure 18-6,
where you showed the capacity fading with cycling, you said that
was a result of the cell bulging; then you had to squeeze it down
and the capacity went back up. Why was it bulging? Was it !:)e-
cause the active material was falling on the surface and causing
an increase of thickness, and then you squeeze it back so that the
resistance goes down?
GUTRIDGE: The active mass was growing. It was not actually
falling off the structure, because we were able to get the capac-
ity back, but you would finish up with a much lower density elec-
trode if you did not hold it together.
MAURER: Then the active mass is actually increasing and you
are able to squeeze it back?
GUTRIDGE: Yes. If you were to c fit-kinaally cycle without hav-
ing some applied pressure, you would start to lose active mass.
However, that was not what we observed in that short time, because
we were able to get the capacity back.
GARLOCK: have you done any temperature work with this new
electrode?
GUTRIDGE: All our tests have so far been done at room temper-
ature.
FRITTS: I was wondering if you found any problems with severe
overcharge, oxygen pressure between the layers?
GUTRIDGE: No. In fact, the overcharge that we have used in a
lot of our cycling tests is probably a lot higher than you would
choose to use in other electrodes. We have cycled electrodes :it
60, 70% overcharge continually in our early experiments. Tne
overcharge that we use now is a standard, 30% overcharge. High
overcharge is no problem.
BOGNER: How did you deposit the active material, and did it
contain an additive like cobalt?
GUTRIDGE: It is deposited by a slurry coating method. Yes,
it does contain cobalt.
189
Table 18-1
Table 18-2
190
Table 18-3
Table 18-4
191
Figure 18-1
Figure 18-2
192
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Figure 18-4 
193 
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194 
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195


The CMG Nickel Electrode

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