Improvements in high rate lithium thionyl chloride power technology achieved by varying the electrolyte composition, operating temperature, cathode design, and cathode composition are discussed. Discharge capacities are plotted as a function of current density, cell voltage, and temperature

Klinedinst, K. A.

English

NASA Technical Reports Server

LITHIUM  THIONYL  CHLORIDE  HIGH  RATE  DISCHARGE 
K. Klinedinst 
GTE  Laboratories 
For  the  past  three  years,  GTE  has  been  involved in  a  program  of  research  and  development  in 
the area  of  high  rate  lithium  thionyl  chloride  power  technology.  This  afternoon I want to quickly 
review the results  of  this  for  you. 
(Figure 2-4 1 ) 
The figure  contains  a  listing  of  the  four  main  parameters  we  have  identified  by  which  you  can 
control  the  high-rate  performance  characteristics  of  this  cell. 
They  include  the  electrolyte  composition,  the  operating  temperature,  cathode  design,  and  the 
cathode  composition.  And I am  going t o  talk  about  each of these  parameters  one  at  a  time,  begin- 
ning  with  the  operating  temperature. 
The  results I am  going to be  showing  you  are  obtained  with  experimental  laboratory  cells, 
and  our  purpose is to  determine  the range of the  limits  of  improvement  that we can  obtain  in 
high-rate  performance  by  the  variation of these  parameters. 
(Figure  2-42) 
The  next slide  contains  some  polarization  curves.  This is a  plot  of  the  logarithm  of  the 
current  density  in  milliampere  per  square  centimeters  as  a  function of the cell  voltage.  And we are 
looking  at 2 mil cathodes  with  the  standard  electrolyte  which is a  11/2  molar  lithium  aluminum 
tetrachloride  solution  in  thionyl  chloride.  LIALCL 4. 
We have  data  representing  three  temperatures: 35,  45,  and 6 5  degrees.  What this  graph  shows 
us is that  over  the  entire  range of current  densities  here,  by  increasing  the cell temperature,  the 
operating  temperature,  we  realize  very  substantial  improvements in operating  voltage. 
(Figure  2-43) 
This next slide contains some complementary data. We are interested in the discharge 
capacity  as  a  function  of  temperature  as  well, so we are  plotting again on  the  same  axis;  logarithm, 
current  density,  milliamperes  per  square  centimeter. 
Now, as  a  function  of  the  discharge  capacity i n  milliamperes  per  hour  per  square  centimeter. 
Each  point  here  corresponds  to  a  point  in  the  preceding slide. 
I should  mention  that all of  this  data was obtained.  Each  point was obtained  by  averaging 
over the discharge  plateau  of  a  separate  lithium  thionyl  chloride  cell. So, we are  not  looking  at 
initial  data,  we  are  looking  at  data  that  is  characteristic  of  the  average  performance  of  the  cell. 
121 
https://ntrs.nasa.gov/search.jsp?R=19800012346 2020-03-21T18:20:47+00:00Z
Again, we have  data  for  25,45,  and  65 degrees. These  curves  show  that again increasing  the 
operating  temperature,  and we gain  very  substantially  in  discharge  capacity,  effectively  doubling  the 
capacity  here  at  rates  of  about  20  milliamperes  per  square  centimeter. 
(Figure 2-44) 
The  next  slide  contains  two  typical  discharge  curves  plotting  the c ll  voltage  as  a function of 
the  discharge  time  in  seconds  of  2  mil  cathodes, again with  the  standard  electrolyte  at  a  rate of 100 
milliamperes  per  square  centimeter. 
We have data for 25 degrees and for 65 degrees, showing the range of temperatures we 
covered, and I think this clearly points out the improvement in voltage and the capacity with 
increasing  temperature. 
I am  going to  turn  now  to  the  second of the  four  parameters I mentioned  initially,  and  that is 
the  electrolyte  composition. 
The  standard  composition, of course, as I mentioned, is the 1.5 t o  I .8 molar  lithium 
aluminum tetrachloride. We found, however, that by modifying the electrolyte by using excess 
amounts  of  aluminum  chloride  from  one  of  the  electrolyte  constituents, we can  greatly  improve the 
high rate  performance  of  the cell. The  next slide demonstrates  this. 
(Figure  2-45) 
We are  back to  polarization  curves  again.  Logarithm  current  density  versus cell  voltage. I have 
data  here  for  two  electrolytes  and  for  two  temperatures  covering  the  range  of  temperatures we 
looked  at.  There is the  data  here  for  the  standard  electrolyte  at  room  temperature,  65 degrees, and 
with  the  excess  aluminum  chloride  electrolyte  at  room  temperature, 6 5  degrees. 
I should  mention  the  optimum  performance  that we found  to be obtained  with  an  electrolyte 
containing 4-1/2 molar  aluminum  chloride  and  thionyl  chloride. 
This graph shows again over the entire range of current densities and over the entire 
temperature range. We gained substantially in cell voltage by the use of electrolytes containing 
excess  aluminum  chloride.  This  seems  to  be  true,  especially  true  at  the  higher  temperatures. 
(Figure 2-46) 
This  figure  contains  the  corresponding  discharge  capacity  data.  Again,  these  points  corres- 
pond  to  the  ones in the  preceding slide where we are  plotting  the  logarithm of the  current  density. 
And,  now,  as  a  function  of  discharge  capacity,  milliamperes  per  hour  per  square  centimeter. 
Here is the  data  for  the  standard  eIectrolyte  at  room  temperature,  65  degrees,  and  for  the 
4-1/2 molar  aluminum  chloride  electrolyte  at  room  temperature,  65  degrees. 
122 
This  graph  shows  that  not  only  does  the  use of the  excess  aluminum  chloride  electrolyte 
mprove  operating  voltage, we also  see  very  great  improvement  in  discharge  capacity  resulting  from 
ihe  use  of  the  excess  aluminum  chloride  electrolyte. 
(Figure  2-47) 
This  figure  again  shows  two  typical  discharge  curves  plotting cell voltage  as  a  function of time 
in seconds  at  a  temperature 65 degrees  and  a  rate  of  100  milliamperes  per  square  centimeter,  fairly 
high rate  for  this cell. 
We have  data  here  for  the  standard 1-1/2 molar  lithium  aluminum  tetrachloride  electrolyte  and 
for  the 4-1/2 molar  aluminum  chloride  electrolyte.  Again  this  graph is meant  to  show in a  practical 
sense  here,  the  improvement in average  voltage  and  the  improvement  in  capacity  associated  with 
this  parameter. 
You can  notice  more  or less this  characteristic - in the  beginning  of  the  discharge  curve, 
associated  with  the  excess  aluminum  chloride.  This  results  from  the  way in which  the  aluminum 
chloride  interacts  with  one  of  the  discharge  products.  And  this is shown in the  figure  which  contains 
the  two overall reactions  that  occur  with  the  two  electrolytes. 
(Figure  2-48) 
In the case of the  standard  aluminum  chloride  electrolyte  with  the  aluminum  tetrachloride 
electrolytes,  the  lithium  thionyl  chloride  produced  sulfur  and  sulfur  dioxide  which  are  soluble in 
the  electrolyte,  and  lithium  chloride  which is insoluble  and  which  precipitates  within  the  porous 
cathode. 
It is the  precipitation of this  product  within  the  porous  cathode  which is responsible  for  the 
eventual  termination of the  discharge  reaction. 
However, in the  presence of the  excess  aluminum  chloride  here,  aluminum  chloride  reacts 
with  this product  producing  the  soluble  lithium  aluminum  tetrachloride  and  prolongs  the  discharge. 
In  practice,  it is the  extent  to  which we can  supply  excess  aluminum  chloride  to  the  cell, to  the 
cathode which  determines  the  discharge  duration. 
Now I want  to  turn  and  focus  upon  the  cathode of the  lithium  thionyl  chloride  cell,  and 
wonder  what we can do  to  it  to  improve  the  high  rate  performance of the cell. 
I listed two  parameters  originally:  the  cathode  design  and  the  cathode  composition.  First, we 
are  going to  talk  about  the  design.  Due  to  the  time  limitation,  I am only  going to  talk  about  the 
simplest  of  design  parameters.  That  is  the  cathode  thickness. 
(Figure 2-49) 
123 
In  this  figure  are  shown  two  polorization  curves.  Again,  we  are  looking  at  log  current  density 
versus  cell  voltage, 25 degrees,  which  is  standard  electrolyte.  The  cathode  thicknesses  range  from 2 
mils to 50 mils, so we have  a  fairly  large  variation  in  cathode  thickness. 
You can  see  that  the  effect  of  the  increased  cathode  thickness if not overwhelming,  but  it is 
certainly  significant. I t  is also  understandable  since  the  electrode  reduction  of  the  thionyl  chloride 
has  occurred  within  the  porous  cathode. 
So, as we increase the cathode thickness, we are increasing the internal surface area at 
constant  geometric  current  density. We are  therefore  decreasing  the  specific  rate  of  electrode 
reduction  of  thionyl  chloride. So we are  reducing  the  polarization. 
(Figure 2-50) 
This figure shows the effect of cathode thickness variation upon cathode utilization 
efficiency.  Here I am plotting  the  cathode  thickness  as  a  function  of  the discharge  capacity.  This 
comes  out  to  a  unit  of  volume, so we  are  looking  at  cathode  capacity  for  unit  volume  of  cathode. 
This  can  be  thought  of  as  a  method  of  efficiency  of  the  cathode  utilization.  Again, we are  at 25 
degrees  and  at 30 milliamperes  per  square  centimeter  with  the  standard  electrolyte. 
As we  decrease  the  cathode  thickness,  the  efficiency  of  cathode  utilization  gradually  increases 
until,  for  cathodes  thinner  than  about 6 or 7 mils we get  a  more  rapid  increase in cathode  utilization 
efficiency. 
Why is this  happening?  This  results  because  the  higher  the  current  density  at  which  the  solids 
discharge, the  more  the  lithium  chloride  product  tends  to  be  deposited  toward  the side of the 
cathode  which  faces  the  anode. 
As the  lithium  chloride is deposited  at  the  entrances  to  the  cathode  pores,  the  interior  of  the 
cathode  then  becomes unavailable to  support  further  thionyl  chloride  electrode  reduction. So, in 
practice,  the  cathode,  the  higher  the  discharge  rate,  the less efficient will  be the  utilization  of  the 
cathode overall.  Now,  finally, I turn  to  the last  of  the  four  parameters,  that  of  the  cathode  compo- 
sition.  The  standard  lithium  thionyl  chloride  cathode is composed  of  a  mixture of carbon  black  and 
teflon  as in the case of  the  sulfur  dioxide  battery.  The  ratio f the  materials,  of  course,  can be 
varied within  a  considerable  range.  There  are  a  great  number  of  different  carbon  blacks  which  might 
be used  as cathode  constituents. 
Of  course,  a  great  many  other  materials  and  elements  are  listed in the  table as compounds 
which  could  be  used  possibly to  advantage as cathode  constituents. We examined  a  number  of  these, 
and  some  of  the  results  are  shown in the  next figure. 
(Figure 2-5 1 ) 
1 24 
Here  I  am  back to  the  polarization  curve  where  I  am  plotting  the  logarithm  of  the  current 
density  as  a  constant  of cell voltage at  room  temperature,  with fairly  thick  cathode  standard  electro- 
lyte. We are  looking  at  two  different  cathode  compositions. 
One  cathode  contains  carbon  black  and  teflon,  and  the  other  cathode  contains  an  alternative 
cathode  material.  What  it  shows is that we  can gain between  200  and 300 millivolts in operating 
voltage  by  the  simple  replacement  of  one  cathode  material,  carbon  black,  with  another. 
(Figure 2-5 2) 
This  figure  shows  the  corresponding  discharge  capactiy  data,  the  log  current  density  as  a 
function  of  discharge  capacity in milliamperes  per  hour  per  square  centimeter  at  room  temperature 
with a 2-mil cathode standard electrolyte. The figure also shows the data for the standard and 
alternative  cathode  compositon.  In  addition  to  improved  operating  voltage,  improved  cathode 
utilization  can  also  result  from  the  use of alternative  cathode  materials. 
DISCUSSION 
BENNETT:  Does  the  alternate  cathode  material  also give the  same  type of improvement 
performance  at  higher  temperatures  as  evidenced by the original cathode  which  you  used? 
KLINEDINST: Yes, it does. 
BARNARD:  As  a  possible  user of lithium  batteries,  I  would  be  more  concerned  with  the 
degradation  at  the  lower  temperature,  not  improvement  at  the  higher  temperature.  That is a  pretty 
big degradation,  really,  at  the  lower  temperature. 
Do  you  get  the  same  corresponding  degradation in the SO, system? 
KLINEDINST: I am not an  expert  on  the SO, system. I think  that  question  would  probably 
be more  properly  addressed to  someone in the  audience  who is. I  am  sure  there  are  many of them 
here. 
TAYLOR: The answer is, no. I think it is generally agreed that the low temperature 
capability of the SO, system  is  the  best of the  lithium  systems  to  date. 
VASANTH: I am  interested  in  how  far  the  concentration  of  the  aluminum  chloride  affects 
the  improvement  of  the  operating  conditions.  Is  there  a  limiting  concentration,  or  do you feel it is 
indefinite? 
KLINEDINST:  The  four  and  a  half  molar  is  the  maximum  that is the  concentration  at  which 
the  capacity  optimizes  and  maximizes  quite  sharply  at  that  concentration. You can  certainly 
increase  the  concentration  further.  It  decreases  the  performance. 
125 
VASANTH: My second  question  is,  after  the  improvement  of  the  operating  conditions,  was 
the  solution  analyzed,  and was  there  any  component  operation  taking  place? 
KLINEDINST:  In  general,  we  haven’t  analyzed  the  electrolytes. 
GROSS: The  addition of the  aluminum  chloride  caused  some  apparent  voltage  delay  caused 
by  voltage  overshoot.  Can you  explain  the  mechanisms  that  are  involved in this? 
KLINEDINST:  Yes. Well, in the  first  place,  there was not  a  voltage  delay.  What  you  were 
seeing was an experiment in which the electrolyte simply contained aluminum chloride. There 
wasn’t  any  lithium  chloride  in  there  at  all.  This is a  reserve-type cell  in which  the  electrolyte is 
added,  and  then  very  soon  the  discharge is begun. 
Well, if electrolyte  does  not  have  any  lithium  on  it,  then  its  conductivity is going  to  be  pretty 
low  and  you will see a  dip in the  voltage  as  the  electrolyte is added  and  the  discharge  is  begun. 
So, to  answer  the  first  part  of  your  question:  The way to get  around  that is to  simply  add  a 
little  bit  of  lithium  chloride  to  the  electrolyte so that  the  electrolyte will contain  lithium  tetra- 
chloride. The reason that happens if you have pure aluminum chloride, the aluminum chloride 
reacts  with  the  lithium  chloride film that is on  the  anode  and  that very  rapidly  produces  lithium 
tetrachloride,  which  is  the  conductive  electrode  salt. 
Now,  there is first  an  increase  in  voltage,  and  then  it  drops  down.  The  higher  voltage  at  first is 
due  to really a  different overall reaction.  In  other  words, as the  aluminum  chloride  reacts  with  the 
lithium  chloride,  that is as  it  is  produced,  you  are  not  forming solid lithium  chloride. So the overall 
reaction  is  different. 
Then,  as  the  aluminum  chloride  concentration  diminishes,  you  start  to  precipitate  lithium 
chloride  and  that is what  causes  the  subsequent  drop in voltage. 
CLOYD:  It  looks  very  good  that  the  actual  electrolyte  does  include  the  rate  capability. In 
certain  applications,  however,  there is a  requirement of the  intermittent  stop  of  the  current  flow. 
What is the wetstand capability of that lithium system in the electrolyte like that? 
Obviously,  it gives off  protons.  Is  it  at  a  very  fast  rate,  or  what? 
KLINEDINST:  Of  course,  it  would  be  temperature  dependent.  But,  I  would  say  that  the  use 
of  an  electrolyte  like  that  would  pretty  much  be  limited  to  a reserve battery  configuration in which 
you  do  not  have  long  periods of open  circuit  stand,  or  low  rate  discharge  involved.  The  reaction f
the  aluminum  chloride  with  the  lithium  anode  would  prohibit  long  periods  of  stand. 
126 
Li I SOCL,  CELL DESIGN PARAMETERS 
0 ELECTROLYTE COMPOSITION 
0 OPERATING TEMPERATURE 
0 CATHODE DESIGN 
0 CATHODE CQMPOSITION 
Figure 2-41 
CELL  VOLTAGE vs CURRENT  DENSITY AND TEMPERATURE 
looo1 
8 
6 ,  
4 
2 - 
N . E a 
E 
I 
> 
y, l o o .  E 
E 
+ 8  
Le 
3 6  
4 
2 
10  
I 
-1 
CATHODE  THICKNESS.  2 MIIS 
CATHODE AREA: 1 cm2 
ELECTROLYTE: 1.5M L I A I C I ~  IN S0Cl2  
0 TEMPERATURE: 25°C 
0 TEMPERATURE: 45°C 
A TEMPERATURE:  65  C 
I I I 1 I 1 
2.0  2.2 2.4 2.6  2.8 3.0 3.2 
CELL   VOLTAGE IV)  
DISCHARGE CAPACITY vs CURRENT DENSITY AND 
TEMPERATURE 
CATHODE  THICKNESS:  2 Mils 
CATHODE  AREA: 1 em2 
ELECTROLYTE: 1.5M L iA lC l4  IN SOClz 
0 TEMPERATURE:  25°C 
0 TEMPERATURE: 45'C 
A TEMPERATURE:  65°C 
0 1  2 3 4  5  6 
DISCHARGE  CAPACITY  ImAhrlcm21 
Figure 2-42 Figure 2-43 
= 
CELL  DISCHARGE  CURVES AT 25'C AND 65'C 
CATHODE AREA: 1 crn2 \ 
CATHODE THICKNESS: 2 Mllm 
CONSTANT CURRENT DENSITY: 1W rnAlcrn2 
ELECTROLYTE: 1.5 LiAlC14 IN SOC12 
o 1EMPERATURE:  25-C 
TEMPERATURE:  65'C 
I 
O 10 20 30 40 50 60 70 80 90 1W 
DISCHARGE TIME ISECl 
CELL VOLTAGE vs CURRENT  DENSITY AND TEMPERATURE 
USING 1.5M LiAICI, AND 4.5M AICI, IN SOCI, AS 
ELECTROLYTE  SOLUTIONS 
-I 
2 4  2 6  2.0 3.0 3.2 3.4 
CELL  VOLTAGE IVI  
Figure 2-44 Figure 2 4 5  
TEMPERATURE  USING 1.5M LiAICI,  AND 4.5M AICI, IN  SOCI, AS 
DISCHARGE  CAPACITY vs CURRENT  DENSITY  AND 
ELECTROLYTE  SOLUTIONS 
1m - 
8 -  
6 -  CATHODE THICKNESS: 2 Mlls 
CATHODE AREA: 1 cm2 
4 .  
TEMPER4TURE: 25°C ELECTROLYTE: 
TEMPERATURE. 65'C 1 A1c13 IN soc12 
w- 2- 
.t! 
4 - 
5 
2 8 -  
z : 1 0 0 .  
r .  
3 -  " 
2 -  
10-  
i 
0 1 4 6 8 10 12 
1 8  
DISCHARGE CAPACITY IrnAhrkm21 
Figure 2 4 6  
CELL DISCHARGE CURVES AT 65'C USING 1.5M LiAICI, 
AND 4.5M *AlCl, IN SOCI, AS ELECTROLYTE  SOLUTIONS 
CATHOOE THICUNESS: 2 Mils 
CONSTANT CURRENT DENSITY: 100mAlcm2 
o ELECTROLYTE: 1.5M LiAlC14 in SOC12 
ELECTROLYTE: 4.5M AIC13 in SOC12 
I 
0 60 120 1ao I I 240 300 
DISCHARGE  TIME  (SECI 
Figure 2 4 7  
130 
Li I SOCl, CELL DISCHARGE REACTIONS 
WITH LiALCL, ELECTROLYTE 
4 Li + 2 SoCL,-4 LiCL + S + SO, 
WITH ALCL, ELECTROLYTE 
ALCL, 
4 Li + 2 SOCL, -4  LiALCL, + S + SO, 
Figure 2-48 
CELL  VOLTAGE vs CURRENT  DENSITY  AND CATHODE THICKNESS 
TEMPERATURE: 25'C 
CATHODE A R E A :  1 cm2 
ELECTROLYTE: 1.5M LiAICI4 mSOCI2 
0 2 rnd THICK  CATHODE 
50 md THICK CATHODE 
'1 
0.1 , 2.2 
2.4  2.6  2.8 3.0 3.2 3.4 3.6 
CELL  VOLTAGE  (VI  
Figure 2 4 9  
CATHODE  UTILIZATION vs CATHODE  THICKNESS 
TEMPERATURE:  25'C 
CATHODE AREA:  1 mn2 
ELECTROLYTE:  I S M  LiAICI4 in SOC12 
CURRENT DENSITY:  30rnAlcrn2  
1 I 1 I 1 I I 7 -r - I 
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 
CATHODE THICKNESS IMMI 
Figure 2-50 
132 
CELL  VOLTAGE vs CURRENT  DENSITY  WITH  STANDARD 
AND ALTERNATIVE  CATHODE  MATERIALS 
TEMPERATURE-  25'C \ 
CATHODE THICKNESS 50 Mdl 
CATHODE AREA, 1 crn2 
ELECTROLVTE 15M L8AlCI4 INSOC12 
0 STANDARDCATHODE MATERIAL 
ALTERNATIVE CATHODE MATERIAL 
01 
2 9  3 0  31 3 2  33 3 4  3 5  
CELL VOLTAGE IVI 
Figure 2-5 1 
DISCHARGE  CAPACITY vs CURRENT  DENSITY  WITH  STANDARD 
AND ALTERNATIVE  CATHODE  MATERIALS 
1 w O -  
8 -  
6- 
4- 
TEMPERATURE 25 C 
CATHODE THICKNESS 2 MIIS 
CATHODE AREA 1 m2 
ELECTROLVTE I5 L8AIC14 IN 
0 STANDARDCATHODE  MATERIAL 
2- 
1 w- 
8- 0 ALTERNATIVE CATHODE MATERIAL - 
0.1 I 
0 2 6 8 I 
I 1 
10  12 
DISCHARGE  CnPAClTY  lmAhr/crn2i 
Figure 2-52 
133 


Lithium thionyl chloride high rate discharge

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