Sponge cadmium electrodes for secondary cadmium-air batterie

Wagner, O. C.

English

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' SECONDARYCADMI'T_-AIRBATTEEIE.8 N _ --I _ 2 _
Otto C. Wagner
Power Sources Division, Electronic Components Laboratory
USAECC_, Fort Monmouth, N. J.
In order to attain a maximum energy density from the cadmium-air
system, sponge cadmium electrodes were developed in this program. The
alr-cathodes were supplied by two manufacturers who are presently
fabricating state of the art electrodes. The separators were supplied
by four manufacturers. No auxiliary charging electrodes were employed
since the air-cathodes are bifunctional; i.e., they are charging and
discharging electrodes.
/
The major failure modes of the cadmium-air system are: loss of
capacity by the cadmium anode (fade-out), shorting by cadmium pene-
tration, poisoning of the air-cathode and water loss.
' Fade-0ut Problem
When an alkaline cadmium electrode is continuously cycled on a
deep discharge regime it loses capacity by a phenomenon referred to
as fade-out. This phenomenon is often observed in nickel-cadmium
and silver-cadmlumbatteries when the cells become cadmium limiting
on discharge. The capacity loss is attributed to a loss in real
surface area by the growth of cadmium crystals and by the subsequent
'." blockage of the electrode poresl,2,3.
The problem of fade-out was to a large extent overcome by the
,- introduction of alpha ferric oxide into the active cadmium powder and
by removal of CO2 from the unit cell. As shown in figure i a control
- .' cadmium-air cell with a sponge negative containing no extender and
exposed to CO2 (black circles) lost 37 percent of its initial capacity
:'_' in 12 cycles. A test cell (open squares) with a negative containing
_'"_' 5% ferric oxide and where CO_ was removed from the unit cell was
;J delivering 80% of theoretical capacity without any loss in capacity
,._ : for 35 cycles. The combination of 5% ferric oxide extender and CO2
'i,... exposure (black squares) resulted in the lost 30% capacity in 35 cycles,
while the combination of no extender and no C_)_exposure (open circles)
resulted in a loss of 25% in capacity in 20 cycles. From these data it
is seen that fade-out can be prevented by the addition of alpha ferric
oxide extender to the active cadmium powder and by the removal of CO2
from the unit cell.
The carbonate effect is again shown in figure 2. The figure shows
the capacity of the sponge cadmium anode (containing 10% Fe203) as"4 .
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percent of theoretical capacity versus cycle number. The prime variable
was cycling in a carbonate free electrolyte (open circles) versus a
carbonate saturate_ electrolyte (solid circles). From the figure it is
seen that carbonate exposure decreased the capacity of the anode by SO
percent in S5 deep°cycles, while the anode in a carbonate free electrolyte
lost only B percen_ of its initial capacity in the same number of cycles.
Since carbonate increases the solubility of cadmium in 7 normal
KOH by about one order of magnitude 4 it appears that carbonate accel-
erates the recrystallizationprocess of the cadmium electrode 5, which
at low current densities of charge and discharge results in a loss of
real surface area and a subsequent blockage of the electrode pore_ 1,2.
Cycling data of C02 scrubbed unit cadmlum-air cells show that
anodes containing 5 to lO percent ferric oxide extender deliver 68 to
70 percent of theoretical capacity after lO0 cycles at i00 percent
depth of discharge (initial efficiency was 75%). These anode effi-
ciencies result in energy densities of 40 to 45 watt-hours per pound
for unit cadmium-air cells of lO ampere-hour capacity.
Cadmium Penetration
In table I are shown the effects of overcharge, carbonate and
separator system on shorting by cadmium penetration. Group A separa-
tor wraps were employed in a carbonate saturated cadmium-air cell
containing a sponge anode. Group B separator wraps were with a
carbonate saturated cell containing a slntered-nickel cadmium anode.
Group C separator wraps were with a CO2 scrubbed cell containing a
sponge anode. After shorting the cells were rebuilt with new separa-
tor combinations, as shown in the table.
From the data it can be seen that cadmium penetration can be
minimized or prevented by: (a) minimizing overcharge, (b) preventing
carbonate buildup in the cell (compare group C with groups A and B)
and (c) placing a layer of Pellon between the anode and main separator
(compareAI and A2). In addition, sintered-nlckel cadmium anodes
appear more resistant to shorting than s_nge anodes (compare B with
A).
Cadmium Poisoning of the Air-Cathode
Botn types of alr-cathodes evaluated in this program became
severely polarized towards the end of discharge in cells discharged at
current densities greater than iU MA/cm2. On subsequent recharge the
cathodes were reactivated and exhibited a normal E-I discharge re-
sponse versus the fully charged cadmium anodes. However, versus
nearly fully discharged cadmium anodes the alr-cathodes become in-
active. E_en when a fresh uncycled air-cathode was discharged against
a nearly fully discharged cadmium anode the cathode became inactive.
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Chemical analysis indicates the presence of cadmium in the air-cathode
after cycling-the concentration of cadmium in the cathode was O.l mg/cm2.
It is believed that during the end of discharge the cadmium anode increases
the concentration of soluble cadmate ion (Cd(OH)s') in the electrolyte to aquantity sufficient to poison the air cathode. This is presently being
explored by chemical analysis of the electrodes and electrolyte. This
poisoning occurs when the electrolyte is free of carbonate ions or other
impurities. Figure 3 shows that when the electrolyte is saturated with
carbonate ions inactivation of the air-cathode will not occur. For com-
parison, the figure includes the polarization of the air-cathode and
cadmium anodes in pure 7 normal KOH (open circles) and 7 normal KOH satu-
rated with carbonate (solid circles).
It i_ reported that cadmium forms a soluble cadmium carboz_te species
Cd(GOB) -4, _h_ch is about ten times more soluble than the cadm_te ion in7 norma_ KOH*, . Apparently the cadmium carbonate species is beneficial to
the air-cathode in that it prevents poisoning by the cadmate ion. Howe%-er,
as was shown in the Figure 2 carbonate severely decreases the capacity of
the cadmium anode (after 20 cycles) and therefore is not desirable as an
additive to the electrolyte. Consequently, a search was made for electrolyte
additives which would: (a) prevent poisoning of the air-cathode by cadmium
and (b) would be innocuous to the cadmium anode.
One such additive has been found. As shown in Figure 4, when the
electrolyte is saturated with zincate ion the air-cathode will not in-
activate at the end of discharge and the electrical efficiency of the anode
is not impairedafter lO deep discharge cycles. S_nce overcharge did not
show a zinc deposition step it is apparently safe to saturate the electrolyte
with zincate in a rechargeable cadmium-air cell. I might add that aluminate
has also been found to prevent cathode poisoning. At present cadmium-air
cells with zincate electrol_te are being life cycled and other additives
are being screened in an effort to understand and better control the
poisoning effects of the air-cathode.
Water Loss and Unit Cadmium-Air Cell Design
The i0 ampere-hour unit cadmium-air cells lost about O.5cc of water
per cycle at the C/5 rate of charge and discharge. As shown in Figure 5,
a prototype i0 ampere-hour unit cell design includes a sump at the base of
the cell which contains 5cc of reserve electrolyte (as water). The excess
water would prevent drying of the cel_ for about lO cycles (the unit cells
cycled in this program had a sump w_th lOOcc of excess electrolyte). The
battery could be watered every lO cycles through a filling hole in the
cover, after unscrewing the sealing plug. The required amount of water
could be indicated by constructing the cell with a transparent or trans-
lucent plastic frame through which the electrolyte level can be seen and
adjusted.
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Electrolyte adsorbent polyamlde Pelion is placed in both the anolyte
and catholyte compartments to transport electrolyte from the sump to the
electrodes. The main separator consists of two layers of a non-oxidizable
and non-degr_.dablemembrene, such as Astroset 3420-09 or RAI, P-300. The
anode is lO0 mils thick, with an apparent porosity of 62%, and contains 5%
ferric oxide extender and 5% carbonyl nickel conductor. The air-cathodes
are bifunctional with a platinum loading not exceeding 6 rag/era2. The
electrolyte is 30% KOH saturated with zincate. To prevent carbonate buildup
in the cell replaceable ascarite alr spacers can be attached to the air
faces of the air-cathodesor barium hydroxide can be added to the base of
the sump.
An actual 24 volt, 25 ampere-hour cadmium-air battery is being con-
structed by General Electric for the US Army Electronics Command. The
battery is being built according to the design parameters that were derived
from this program. It is estimated that the unit cells will have an energy
density of 45 watt-hours per poundat the C/5 rate and the battery plus case
hardware will have an energy density of 35 watt-hours per pound. At present,
a test cell has ex.eeded 300 deep discharge cycles7. Therefore, the goal of
500 cycles for the battery can be anticipated.
Conclusions
I. Loss of capacity by the cadmium anode can be prevented by use of ferric
oxide extender and removal of CO2 from the cell.
2. Cadmium penetration can be prevented by removal of CO2 from the cell,
minimizing overcharge and placing an inert electrolyte absorbent inter-
separator between the cadmium anode and main separator.
3. Poisoning of the air-cathode can be prevented by using KOH electrolyte
saturated with zincate.
4. Water loss can be compensated by providing the unit cell with an
electrolyte sump.
5. Unit cadmium-air cells have been built with a cycle life greater than
300 cycles and an energy density in the range of 40 to 45 watt-hours per
pound.
References
i. Investigation on the Negative Electrode of Nlckel-Cadmium Cells wlth
Sintered Plates, J. P Harivel, B. Morignat and J. Migeon, 8AFT, Fourth
InternationalSymposum on Batteries, Brighton, England, Oct. 64.
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2. Charge Acceptance of Cadmium Hy_roxlde Electrodes at Low Temperatures,
Y. Okinaka, Bell Telephone Laboratories, Extended Abstracts of the Chicago
Meeting of the E. C. S. Oct. 65.
3. Rechargeable Metal-Air Cell, Technical Report ECOM-0257-3 H. H. Hirsch,
W. J. Vander Grlnten, N. N. Carson, P. J. Moran, General Electric,
Contract No. DAABOT-67-C-0257, April 68.
4. Solubility of CdO in KOH, _C03 and Their Mixtures, G. M. Jost, Texas
Instruments, Inc., Extended Abstracts of the Philadelphia Meeting of the
E. C. S., Oct 66.
5. Effect cf Carbonate on the Rate Oxygen Recombination in Sealed Nickel-
Cadmium Batteries, Y. Oklnaka, and D. Turner, BTL, Extended Abatracts of
the Buffalo Meeting of the E. C. S., Oct 65.
6. The Nature of Cadmium Ions in Hydroxide an_ Carbonate Solutions, P. G.
Lake,J.M.sooalngs,can.J.Chem.,7,1089-96(1958)
7. Secondary Cadmlum-Air Cells, O. C. Wagner, UBAECCM, 22nd Annual Power
Sources Conference, May 68.
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Table I - The Effect of Overcharge, Carbonate and Separator System
on Cadmium Penetration.
Separator System Cycles to Total AH of
Short 0vercharge
A _'_admium - Air Cell (CO3 = Saturate_)
AI (+) i X .001_'Cellophane/l X .006" Pellon (-) 58 48
A2 (+) i X .006" Pellon/2 X .001" Cellophane (-) 9 5
.0_ t!A3 (+) i X .006" Pellon/2 X Astroset
/lX .006"Pellon(-) 75 ' 76
B Sinterea Nickel Cadmium-Air Cell (COS = Saturated)
, , = , , -
BI (+) IX .006" Pelion (-) _ 36
C Sponge Cadmium-Air Cell (No C_ = In Electrolyte)
C1 (+) i X .006" Pellon/2 X .002" Astroset
/i X .006" Pellon (-) 120 + 180 +
(still cycling)
02 (+) i X .006" Pellon_l X .O015" RAI P300/l • 006 t!X Pellon (-) 20 + 20 +
(still c_cl_g.)
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Figure i
Capacity Maintenance As A
Function Of Cadmium Mix
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Secondary cadmium-air batteries

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