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LONG LIFE NICKEL ELECTRODES FOR A 

NICKEL-HYDROGEN CELL: CYCLE 'LIFE TESTS 

H.S. Lim and S.A. Verzwyvelt 

Research Laboratories 

301 le>alibu Canyon Road 

Malibu, CA 90265 

INTRODUCTION 

Nickel electrodes have been recognized as the lifelimiting components of nickel-hydrogen cells. In order 

to develop a long life nickel electrode for a nickelhydrogen cell, we are engaged in studies of various 

parametric effects on the electrode cycle life. These 

parameters include plaque pore size, plaque mechanical 

strenqth, and active material loading level. Three 

levels of variation were used for each parameter. 

Several fabrication parameters of nickel plaques, 

including nickel powder type, powder" density, and 

sintering time were studied in order to fabricate 

plaques with desired parametric variation (1). Selected 

plaques (7 types) were impregnated in three levels of 

active material loading using the standard Air Force/ 

Huqhes electrochemical deposition process in an 

alcoholic bath. Subsequently, these electrodes were 

tested by a Hughes standard test procedure (200 cycles) 

as a part of their evaluation. For a cycle life 

evaluation of these electrodes, 19 different nickelhydrogen boiler plate test cells were fabricated. This 

initial cell performance has been described earlier (1). 

The present report presents the results of the cycle 

life tests of the boiler plate cells. Details of the 

failure analyses of these test cells will be presented 

later (2). 

TEST CELL MATRIX AND ACCELERATED CYCLE LIFE TEST 

The nickel-hydrogentboiler plate test cell matrix 

includes 19 out of the 21 various designs of electrodes, 

as shown in Table 1. Each electrode type was designated 

as follows for convenience of later discussions: The 

first two digits represent the nickel powder type and 

the median pore size, i.e., 55 for INCO nickel powder 

type 255 and 10 pm pore size, 25 for 255 powder type and 

16 pm pore size, and 87 for 287 powder type and 13 pm 

pore size, respectively. The last two digits represent 
of these cells initially increased with cycling along 

with other cells up to about 1000 cycles and then 

started to decrease, while the pressure of the other 

cells, on average, either remained constant or increased 

with cycling. 

PARAMETRIC EFFECTS OF CYCLE LIFE 

Rated and theoretical capacities (chemical 

capacity) depth-of -discharge (DOD) of cycling with 

respect to the theoretical capacity, and the cycle life 

to 0,9 V of EODV are summarized in Table 2. Although 

the rated capacity was based on the initial measured 

capacity, the theoretical capacity was used in the 

analysis of the results. This is because the measured 

capacity varied with the measurement conditions and 

showed a memory effect of prior cycling. The cycle 

life of nickel electrode depends strongly on the DOD of 

cycling. Therefore, it is necessary to compare the 

cycle life of various cells at a given value of DOD in 

order to evaluate the effects of the electrode 

parameters. A quantitative relationship between cycle 

life and DOD of a nickel-hydrogen cell or a nickel 

electrode is not available. However, the following 

quantitative relationship between the DOD and an 

expansion rate of the nickel electrode active material 

has been established (4). 

where k is the expansion rate and A is a constant. For 

the present data analysis it has been assumed that the 

cycle life of the nickel electrode is inversely 

proportional to this rate, k. The measured cycle life 

was normalized to 70% DOD of the theoretical capacity 

using the relationship in Equation (1 ), combined with 

this assumption. 

The normalized cycle life values are summarized in 

Table 2 and plotted against the median plaque pore size, 

the plaque bend strenqth, and the active material 

loading in Figures 3, 4 and 5, respectively. The 

optimum pore size appears to be about 12 pm, regardless 

of the loading level (Figure 3). The 16-pm plaques 

showed much shorter cycle life than the other plaques. 
mechanical strength (bend strength): 40 for 400 psi, 55 

for 550 psi, and 70 for 700 psi. For example, an 8755 

plaque is made of 287 type powder with a bend strength 

of 550 psi and a median pore size of 13 pm, The letters 

L, M, and H indicate the nominal active material loading 

levels of 1.4, 1.55, and 1.7 g/cc void, respectively, 

All test cells contained three standard f light-type 

nickel electrodes in a recirculation stack design (3) 

and 31% KOH electrolyte. The cell capacities were 

rated in three groups: 2.7, 3.0 and 3.3 A-hr, based on 

capacities measured by charging the cells for 80 minutes 

at C rate and then discharging them to 1.00 V at 2.74 C 

rate. The initial measured capacities ranged from 2,8 

to 3.1 A-hr, from 3.1 to 3.5 A-hr and 3.5 to 3.9 A-hr 

for the 2.7, 3.0, and 3.3 A-hr ratinqs, respectively. 

The cycle life tests of all of the 19 boiler plate 

test cells were carried out at 23"~ by a continuous 

cycling to 80% depth-of-discharge of their rated 

capacities using 45-minute cycle regimes. These were 

interrupted periodically for capacity measurements after 

approximately every 1500 cycles. The cycling regime 

included a 2.74 C rate of discharge for 17.5 minutes and 

a 27.5-minute charge at 1.92 C rate for 110% recharge. 

End-of -charge voltages (EOCV) and end-of -discharge 

voltages (EODV) , and end-of-charge pressures (EOCP ) and 

end-of-discharge pressures (EODP) of the cells were 

monitored daily (every 32 cycles ) . 

CELL VOLTAGE AND PRESSURE DURING LIFE TEST 

Plots of EODV and EODP of various cells versus the 

number of life cycles are shown in Figures 1 and 2. The 

EODV decreased gradually with cycling until near the end 

of life where the voltage decrease accelerated, except 

in two anomalous cells; i,e., 8770H (Figure 1) and 

5555M (Figure 2). The end of life was defined as 0,9 V 

of the EODV. The EOCVPs of the cells showed neither any 

appreciable variations with various electrode types nor 

any appreciable change during the cycle life test. 

The EOCP" and EODP's of a given cell changed in 

parallel to each other as expected by the fixed quantity 

of charge and discharge. The cells with short cycle 

life showed overall lower pressure than the other cells. 

These short life cells, which include the 2540 type and 

all L-series, also showed a pressure peak, The pressure 
No definite effect of the bend strength on the 

cycle life was identified, due to the spread data 

points, as shown in Figure 4. 

The active material loading level affected the 

cycle life strongly, as shown in Figure 5, if the values 

of the 2540 plaques were disregarded because of their 

exceptionally short life. The optimum loading level 

appears to be at about 1.6 g/cc void. The preceeding 

conclusions on the parametric effects on the cycle life 

did not change when the data were alternatively analyzed 

using the cycle life values of 0.5 V for the end of 

life, or without using the DOD correction. 

CELL PRESSURE AND ACTIVE MATERIAL UTILIZATION 

Analyses of the cell pressures during the interim 

cell capacity measurements revealed a relationship 

between the cell pressure and the active material utilization. This utilization is defined as the ratio of the 

measured vs the theoretical capacities, A normalized 

cell pressure after full charge and discharge, 

(P-Po)/Co, (where P is the cell pressure, Po is the 

precharge pressure, and Co is the theoretical capacity 

of nickel electrode), is plotted against active material 

utilization of the cells after 1700 life cycles 

(Figure 6). The term (P-P,)/C, is a measure of the 

average oxidation state of the nickel active material. 

After full charge and full discharge of the cell, the 

plots gave straight lines, respectively. The values of 

(P-Po)/CQ at charged state increased, while the values 

at the discharged state decreased as the utilization 

increased. The values at the charged state increased 

more rapidly than those at the discharged state 

decreased when the utilization increased. This result 

appears to indicate that an increased active material 

utilization is more dependent on reaching a high average 

oxidation state than on discharge to a low average 

oxidation state. This observation after 1700 cycles is 

quite contrary to an earlier observation at the 

beginning of life on the relationship between (P-Po)/Co 

and the active material utilization which showed that 

the average oxidation state in the charge state was 

independent of the utilization (I), The utilization was 

entirely dependent on the oxidation state in the 

discharged state. It is speculated that this change of 

the relationship between the utilization and (P-Po)/C 

with cycling may indicate a change of the mechanism 09 

the active material utilization. The value of (PP,)/Co gradually increased with an increased 
number of cycles. This increase appears to be due'to 

either an increase in the average oxidation state of 

active material or a gradual corrosion of the nickel 

sinter to be converted to nickel oxides or hydroxides, 

or both. 

CONCLUDING REMARKS 

o The active material loading level strongly 

affects the cycle life of the nickel electrodes, showing the optimum loading level at 

about 1.6 g/cc void. 

e The plaque pore size affects the cycle life 

significantly, showing that the optimum pore 

diameter was about 12 urn. 

a No noticeable effect of the plaque mechanical 

strength on the cycle life was found in the 

bend strength range of 400 to 700 psi. 

CD As the test cells were cycled, the average 

values of (P-Po)/C gradually increased, and 

the relative depen8ence of these values at the 

charqed and discharqed states on the active 

material utilization was changed, indicat inq a 

potential change of the utilization mechanism 

of active material. 

ACKNOWLEDGMENTS 

This work was supported by NASA-Lewis Research 

Center under Contract No, NAS 3-22238 _(Contract Manager, 

John Smithrick), The authbrs woun like to acknowledge 

Dr. J.D. Margerum and Dr. R.C. Knechtli for many helpful 

discussions during the course of this work, and Mr. S.T. 

Taylor for his monitoring of the life test, 

REFERENCES 

1. H.S. Lim, S.A. Verzwyvelt, C. Bleser, and K.M. 

Keener, "Long Life Nickel Electrodes for a NickelHydrogen Cell: T. Initial Performance", Proc. 18th 

IECEC, Orlando, Florida 1983, p, 1543. 
2. H,S. Lim and S,A. Verzwyvelt, submitted for 

publication at the 19th IECEC, San Francisco, CA, 

August 1984. 

3. G. Holleck, "Failure Mechanisms in Nickel-Hydrogen 

Cell", 1976 Goddard Spaee Flight Center Battery 

Workshop Proceedings, p 297, 

4. H.S. Lim, "Expansion of the Nickel Electrode", 1980 

Goddard Space Flight Center Battery Workshop 

Proceedings, p, 175, and results to be published. 
Table 4, Boiler Plate Test Cell Matrix and ~esignation 

of Cell Types, 
Table 2. Summary of the Boiler Plate Cell Cycle Test 

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MEDIAN PORE DIAMETER,prn 

Figure 3. Plots of the normalized (calc, ) cycle life at 

70% DOD with respect to the theoretical 

capacity versus median plaque pore diameter, 

Curves A (A), B(@), and C (0) represent 

active material loading level of 1-4, 1.55, 

and 1.7 g/ce void, respectively- 
BEND STRENGTH, psi 

Figure 4. Plots of the normalized (calc.) cycle life 

vs plaque bend strength for various plaque 

types and the active material loading 

level, 
PLAQUE TYPES: 

ACTIVE MATERIAL LOADING, g/cc VOID. 

Figure 5. Plots of the normalized (calc. ) cycle life 

vs the active material for various types of 

electrodes, 
13530-3 

CHARGED (SLOPE = 0.53) 

DISCHARGED (SLOPE = -0.19) 

90 95 100 105 110 

ACTIVE MATERIAL UTILIZATION, % 

87XX PLAQUES 

@ 55XX PLAQUES 

2540 PLAQUES 

Figure 6. (P-Po)/C versus active material utilization 

after 1700 cycles. Charged for 16 hours at 

0.1 C rate and discharged to 0.5 V at 0.5 C 

rate,