It is noted from the data presented that Li/BCX cells lose approximately 8% of their rated capacity in the first 2 months of storage. After this period of time, little difference is noted in the average realized capacity; however, a significant increase in the range is observed. Over the same period of time the OCV falls at a rate of 0.02V per 1,000 hours. After a period of 8,000 hours the OCV appears stabilize at a value of approximately 3.75V. This may be related to changes in Cl2 concentration due to self-discharge or other reactions. These data indicate that no correlation exists between the reduced open circuit voltage and the realized capacity

Krehl, P. W.

Stinebring, R. C.

English

NASA Technical Reports Server

CHARACTERIZATION OF OPEN CIRCUIT VOLTAGE AND CAPACITY 
AS A FUNCTION OF TIME 
Paul W. Krehl and Russell C. Stinebring 
Electrochem Industries 
Div. of Wilson Greatbatch Ltd. 
INTRODUCTION 
Investigations were conducted to determine the effect of halogen/inter- 
halogen addition on the performance characteristics of the Li/SOC12 inorganic 
system. As a result a high energy density battery system was developed: the 
Li/BrCl in SOC12 (L~/BCX) system. 
The open circuit voltage (OCV) of the Li/BCX cell was found to be 3.90 t 
.03V at 24 L 3OC which is more than 0.2V higher than that of Li/SOC12 cells. 
One may speculate that the higher OCV is due to the presence of C12 in solu- 
tion resulting from a partial dissociation of BrC1. It was observed (ref. 1) 
that the addition of By2 in SOC12 also results in an increase in open circuit 
potential. The OCV of the Li/Br2 in SOC12 cell was found to be 3.8V which is 
higher than either Li/SOC12 (3.6V) or Li/Br2 ( 3.5V). It was speculated that 
the higher potential is due to the formation of a complex between Br2 and 
SOC12. Similarly, a complex between BrCl and SOC12 should not be ruled out in 
the case of the Li/BCX cells. 
Figure 1 shows a typical performance curve for fresh Li/BCX D cells 
discharged under a 10 ohm load. The cells exhibited a high voltage plateau 
during the initial stages of discharge and the load voltage decreased to a 
value of 3.5V after approximately 10% of the realized capacity of the cell 
was delivered. 
During investigation into the discharge characteristics under pulsed load 
conditions it was observed that the open circuit potential of the cell changed 
as a function of delivered capacity. Figure 2 shows a comparison of the OCV 
and load voltage versus discharge time for cells subjected to a 0.25A constant 
current discharge at ambient temperature. The current was interrupted period- 
ically so that the OCV could be determined. The initial OCV was 3.93V and 
decreased to approximately 3.67V after 12 hours of discharge. The load 
voltage profile is analogous to the OCV, i.e. the load voltage changes from 
3.83 to 3.27V over the same period of time. The fall of the OCV closer to 
that of one expected for a Li/SOC12 cell suggests that in the initial few 
stages of discharge the concentrations of the more electroactive species are 
reduced. 
Initial room temperature storage tests performed on twelve Li/BCX D cells 
showed that the OCV also varied with time. Figure 3 shows the relationship 
https://ntrs.nasa.gov/search.jsp?R=19840025602 2020-03-22T10:20:14+00:00Z
between OCV and time over a test period of 12,000 hours. The initial OCV was 
3.92 ? .02V and decreased to a value of 3.75 2 O.03V after storage for 8,000 
hours at 24 + 3OC. This corresponds to a rate of decrease in OCV of 0.02 + 
.01V per thousand hours. No additional significant change in OCV was observed 
after 8,000 hours. For example, the values for the OCV after 12,300 hours 
were still approximately 3.75 +- .03V 
In a separate experiment the open circuit potential of BrCl versus Li was 
determined. Benzonitrile was used as supporting solvent for the LiAsF6 elec- 
trolyte, and BrCl was dissolved in the solution. An open circuit potential of 
3.77V was observed. This corresponds well with the observed OCV for Li/BCX 
cell after long-term storage. 
The observed behavior of the open circuit potential of the Li/BCX cell 
with respect to both storage time and depth of discharge suggests that there 
may be a relationship between the OCV and cell performance. A test program 
was initiated in this laboratory to observe the effects of long-term room 
temperature storage on both the open circuit potential and cell capacity, and 
to determine if there was any correlation between the two parameters. 
TEST PROCEDURE 
Approximately 350 Li/BCX D cells were stored under room temperature con- 
ditions. An initial OCV measurement was taken two weeks after the date of 
manufacture and subsequent measurements were taken on a monthly basis. At 
each measurement period a group of 10-12 cells representing the full distri- 
bution of observed open circuit potentials was analyzed for capacity retention. 
The test cells were discharged under 10 ohm loads at room temperature. The 
load voltage measurements were taken through the use of a timed sequential 
electrometer coupled with a Hewlett-Packard (HP85) computer. The realized 
capacities were obtained to a 2V cutoff. The OCV was measured with a Keithley 
(~odel 177 ) electrometer. 
RESULTS AND DISCUSSION 
A plot of the OCV versus time for approximately 350 Li/BCX D cells stored 
under ambient temperature conditions is presented in Figure 4. The average 
OCV for fresh cells ( 2 weeks old) was found to be 3.938. However, as the 
storage time increased the OCV gradually decreased until after approximately 
one year the average OCV was 3.783V. The results shown in Figure 4 also 
emphasize the variability in the open circuit potential at each specific 
measurement interval. Initially the open circuit voltages are grouped in a 
very narrow band ranging from 3.922 to 3.945 volts. However, the spread in 
the open circuit potential values continually increased to a range of 3.755 
to 3.876 volts after one year of storage at room temperature. Comparing the 
data obtained from the original Li/BCX D cells stored for 12,300 hours to that 
obtained in this investigation, it may be speculated that the spread in the 
OCV values may begin to narrow as the actual open circuit potentials level at 
approximately 3.75 volts, 
After each OCV measurement was taken, twelve samples were selected for 
rundown so that the original OCV distribution observed in the D cell popula- 
tion was not distorted. The cells were discharged under a 10 ohm load at room 
temperature. The results of the constant load discharge are presented in 
Figure 5. For fresh cells discharged under 10 ohm loads very little variabi- 
lity in realized capacity exists (14.45 $. 0.45 Ah). However, cells subjected 
to longer storage periods exhibit a lower realized capacity and a greater 
variability in cell performance. For example, the average realized capacity 
for cells stored 7 weeks was approximately 13.2 + 1.5 Ah. The capacity for 
cells discharged after a storage period of 26 and 51 weeks showed a slight 
decrease in capacity (13.1 $. 1.4 and 12.4 rt 1.3 Ah respectively), but were not 
significantly different from the cells discharged after 7 weeks. 
These test results show that both the cell open circuit voltage and cell 
capacity decrease with increasing storage duration. However, no correlation 
was imediately evident between the two parameters. The OCV and discharge 
data shown in Figures 4 and 5, respectively, were compared at individual time 
intervals to determine if the realized capacity was a function of the observed 
OCV. It was noted that cells that exhibited open circuit voltages at the 
extreme ends of the observed range could deliver nearly identical capacities. 
For example, cells stored for a period of 30 weeks showed a range in OCV of 
3.914 to 3.790 volts. A particular ce1l.tha-b exhibited an OCV of 3.79V 
delivered 13.8 Ah to a 2.OV cutoff. Similarly, a cell with an OCV of 3.89V 
delivered 13.4 Ah. 
Examination of a plot of OCV versus cell capacity (Figure 6) shows that 
in general cell capacity data is scattered over a wide range of open circuit 
potentials. It can be seen that cells with an OCV of 3.90V produced capacity 
values between 14.6 and 11.5 Ah. Similarly, cells with an OCV of 3.80V 
exhibited a range in realized capacity of 13.8 to 11.7 Ah. 
S W Y  AND CONCLUSIONS 
It is noted from the data presented above that Li/BCX cells lose approxi- 
mately 8% of their rated capacity in the first 2 months of storage. After 
this period of time, little difference is noted in the average realized capa- 
city; however, a significant increase in the range is observed. Over the same 
period of time the OCV falls at a rate of 0.02V per 1,000 hours. After a 
period of 8,000 hours the OCV appears to stabilize at a value of approximately 
3.75V. This may be related to changes in Cln concentration due to self- 
discharge or other reactions. These data indicate that no correlation exists 
between the reduced open circuit voltage and the realized capacity. 
REFERENCES 
1. C. C. Liang, A New High Energy Density Lithium Battery System, Digest of 
the Combined 12th Int. Conf. on Medical and Biological Engineering and 
5th Int . Conf . on Medical Physics, Part I ( Aug . 1979) Paper 1.6. 
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Capacity vs. Time 
Time, Weeks 
Figure  5.  The average and range of r e a l i z e d  c a p a c i t i e s  of c e l l s  d i s -  
charged under 10 ohm loads  a s  a  f u n c t i o n  of s t o r a g e  t ime .  
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Q. Kunigahal 1  i , Bowie S ta te  College: Would you please t e l l  me the  
mechanism t h a t  i s  causinq t h e  ~ l a t e a u ?  I s  i t  the  anode o r  the  
cathode o r  what may be t h e  reac t ion? 
A. Krehl,  Wilson Greatbatch Ltd.: The h igh  vo l tage plateau? 
Q. Kun igaha l l i ,  Bowie Sta te  College: The low vol tage plateau. I saw i n  
one o f  t he  v iewgra f ts  you showed the  discharge curve showing the  
p l  ateau - 1  ower vo l  tage p lateau? 
A. Krehl,  Wilson Greatbatch Ltd.:  Well, we t h i n k  t h a t ' s  j u s t  t he  base 
so lvent  a f t e r  any o f  t he  d i s s o c i a t i o n  products are  depleted and we 
ge t  t o  the  ac tua l  load vo l tage t h a t  t he  c e l l  i s  going t o  see. The 
c e l l  a c t u a l l y  i s  cathode 1  imi ted.  
Q. Yen, JPL: You mentioned about t h e  h igh  open c i r c u i t  vo l tage o f  over 
3.9 v o l t s  due t o  t h e  a d d i t i o n  o f  PCX. Okay, over t h e  standing and 
the  c e l l  vo l tage decl iw?. Have you done any post-mortum ana lys is  
c o r r e l a t i n g  t h e  PCX content - the  vo l tage dec l i ne  and the  capac i ty  
decl ine.  Do you know anyth ing about the  chemistry? 
A. Krehl,  Wilson Greatbatch Ltd. : I ' m  s o r r y  t o  say we r e a l l y  haven ' t  
done too  many s tud ies  w i t h  respect  t o  tha t .  Some o f  our work was 
done w i t h  the  c e l l .  And the  BRCL a c t u a l l y  showed a  peak I t h i n k  
i t  was a c t u a l l y  3.85 and as the  c e l l  discharged we could see t h a t  
peak gradua l ly  decrease and we g o t  a  increase i n  a  peak a t  about 
4.15 which corresponded t o  bromine - f r e e  bromine. So i t  appeared 
tha t ,  i f  there  was some d i s s o c i a t i o n  the re  and g i v i n g  f r e e  ch lor ine ,  
we are  dep le t i ng  t h a t  and s h i f t i n g  th ings  down t o  bromine, bu t  we 
r e a l l y  haven ' t  done too much i n  t h e  way o f  s tud ies  there.  
Q. Yen, JPL: Have you done any measurement about the  d i s s o c i a t i o n  
constant o f  PCX and the  c o r r e l a t i o n  o f  t h e  concentrat ion o f  PCX 
use r e l a t e d  t o  the  vol tage dec l ine? 
A. Krehl, Wilson Greatbatch Ltd.: We got  a  d i s s o c i a t i o n  constant  
from now. It was i n  a  non-polar so lvent  and i t  was .38. That, 
when we ca lcu la ted o u t  how much f r e e  c h l o r i n e  and f r e e  bromine 
would be i n  a  c e l l .  We came up w i t h  t h e  discharge curve showing 
about 70% o f  t h a t  capaci ty  would be under those i n i t i a l  h igh  
vo l tage plateaus and then the  r e s t  would discharge under t h e  
1  ower plateau. 


Characterization of Open Circuit Voltage and Capacity as a Function of Time

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