The cycling performance of a secondary lithium cell with a 2-methyl THF lithium hectofluorarsenate electrolyte is discussed. Stripping efficiency, dendritization, passivation on standing, and discharge efficiency are considered

Holleck, G. L.

English

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RECHARGEABLE  AMBIENT  TEMPERATURE  LITHIUM  CELLS 
G.  Holleck 
EIC 
As you have  heard  today,  there  are  a  number  of  very  attractive  primary  cells  already  available 
today. With secondary  cells,  progress  has  been  somewhat  more  gradual. 
At EIC, we have  devoted  a  considerable  effort  over  the  past  years  in  developing  ambient 
temperature  lithium cells,  in  particular,  in  solving  the  problem  that is common  to all  Secondary 
lithium  cells,  which is cycling of the  lithium  electrode. 
Now,  the  high  reactivity  of  lithium  requires  very  nonreactive  solvent. Several solvents  appear 
to  be  stable if you  store  lithium in them.  This  stability is not  a  thermodynamic  stability,  but  it is 
a  kinetic  stability. 
You have  protective  films on the  lithium  surface which inhibit  the  reaction. In  general, the 
problem is that  you  cannot  cycle  lithium very well in these  conditions. 
(Figure 2-79) 
Let us quickly review what  the  problems  actually  are. You can  plate  lithium  quite  efficiently 
out  of  menthols  with  100-percent  efficiency.  The  problems  are  that  the  deposit  has  a  bad 
morphology and comes out dendritization and passivates on stand. Especially on cycling, the 
efficiency  deteriorates  rather  rapidly. 
(Figure 2-80) 
In this figure I have indicated a schematic so you can view what happens. But, in plate 
lithium  you  plate  fresh  lithium  which  covers  itself  with  a  protective  layer  consisting  of  electrolytes 
and reaction products with the electrolyte. Some parts might become encapsulated and thus 
isolated  from  the  substrate. 
When you subsequently strip the deposit, part of the lithium will dissolve and part will 
remain at  the  electrode. So your  stripping  efficiencies  are less than 100 percent. When you  continue 
the  cycle,  you  accumulate  more  and  more  of  such  isolated  material  at  the  surface,  and  your  surface 
morphology  continues  to  degrade. 
This  material is predominant  with  lithium  if  you  analyze  it.  It  is  still  lithium,  but  in  a form 
that  you  cannot  get access to  by  stripping. 
(Figure 2-8 1 ) 
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https://ntrs.nasa.gov/search.jsp?R=19800012349 2020-03-21T18:20:49+00:00Z
Now,  we  have  approached  the  system,  this  problem,  in several  ways. Let  me  just  point  out 
the  two  most  successful  approaches. 
We found  out  that  lithium  hectofluorarsenate is the  best  salt  precursor  that we have  found  in 
any  electrolyte so far.  The  reason  for  this is the  reactivity of the  salt  and  the  formation  of  a 
protective film which  has  beneficial  properties.  Most  of  our  experiments  have  been  carried  out  in 
THF,  which  perform  better  than  many  other  solvents we have  looked  at. 
However, THF hectofluorarsenate  solutions  are still too  reactive  for  practical  secondary 
batteries. So, the  second  approach  that we took was to  modify  the  ether  solvent, in  this  case,  by 
introducing  methyl  groups in the  alpha  position  and  making  it less reactive in this  way. 
So we relied  on  a  2-method  THF:  lithium  hectofluorarsenate  electrolyte  as  our  best  present 
electrolyte  for  secondary  lithium  cells;  electrolyte  preparation,  naturally,  as  a big influence on 
performance. 
(Figure  2-82) 
Now,  let  me  quickly  show  you in the  next  three  figures  two  items  which  illustrate  the 
behavior  of  the  lithium  electrolytes.  These  are  half-life  studies  made  by Cook in our  laboratory. 
Here  you  see,  for  example,  a  storage  test  of  lithium  and  various  electrolytes. I would  just  like 
to  point  out  that  in  the  type  of  THF  you see a  matter of  various  days  depending  on  preparation 
methods. In the 2 methyl  THF in excess of 10 months of the year, we have not seen any 
degradation.  This is storage. By the  way,  the  temperature is at 7 1 "C. 
(Figure  2-83) 
This gives you  a  comparison of cycling  efficiency  of  the  lithium  electrode  as a function  of 
cycle  number. In the  figure  you see typical  decay  of  THF  and  the  much  better  performance  of  the  2 
methyl  THF. 
(Figure  2-84) 
Another  aspect  which is important  for  practical  batteries is the  reaction  of  lithium in the 
electrolyte  on  open  circuit  stands.  Again, I show  you  some  data  comparing THF and 2 methyl  THF. 
After  48  hours,  the  plated  THF  has  completely  reacted  or is not  accessible  by  stripping,  which  does 
not  mean  reaction  or  encapsulation.  The 2 methyl  THF is still  accessible. 
There  are  a  few  other  points  that I like to  point  out.  There  are  areas  where we get  clearly  a 
larger amount  of  capacity  back  than we had  before.  This is a  phenomenon which is very important 
also  for  secondary  batteries  and  which we call recontacting.  Some  of  the  material  that  previously 
had  been  isolated  can  be  recontacted  and  can  be  regained.  This  is  of  great  practical  importance. 
156 
(Figure 2-85) 
Now, with these data, we were  very encouraged to go on and to look at  full cells. We built 
prototype cells with various configurations, button cells, D-size cells, and prismatic  cans as you have 
just heard before with cathodes of the intercalation type, titanium disulfide, chromium vanadium 
disulfide, and vanadium oxide, V, 01 3. 
Common to all of these cathodes is that they intercalate lithium reversibly without 
decomposition of the host structure. 
(Figure 2-86) 
This shows you typical cycles for titanium disulfide, lithium titanium disulfide cell. As you 
can see over the cycles, the general shape, the discharge curve which is characterized by a slightly 
sloping plateau a t  a voltage of about 2 volts does not  really change much. Acutally it decreased 
somewhat. The curves have not changed. 
The upturn at the end, these are cathode limited cells and on charge the increase in voltage 
So, you get the voltage increase, and you would get this until some other reaction naturally will 
take over eventually. But  it is fairly distinct. 
Let me say one other thing. Most of our long-term testing has been done with titanium 
disulfide  cells, and maybe I should first look at the next two figures. 
(Figure 2-87) 
This is a typical charge-discharge curve for a lithium commune vanadium disulfide cell. I t  is 
characterized by a similar slightly sloping discharge at  about 2.4 volts, higher voltage than the 
titanium disulfide and similar type of charge. 
(Figure 2-88) 
This is a discharge charge of a lithium vanadium oxide cell. Here you have  actually three 
plateaus; one less than the one up here, 2.5  volts, and at about 2.2 volts, and on  charge followed the 
same multistep process. 
(Figure 2-89) 
Now, most of the extensive  cycling has been carried out with titanium disulfide cells. The 
cycle regime that we have used consists of full depth cycles between preset voltages, 1.6 volts on 
discharge, for example, and 3 volts on charge at about the 6-to 10-hour rate. 
157 
I I l111l1l I1 I I I I I 
This  is  somewhat  different  from  what  you  normally  do in cycle  testing  of  conventional 
cadmium cells. They  constitute  a  much  more severe cycle  regime. I have  summarized  some  cycle 
results  from  small cells. These  are  naturally  cathode-limited  cells,  and  you see a  slight  decrease  in 
capacity  with  cycle  number. 
For  example, we  have  reduced  the  column  density  by  half,  and  the  capacity  has  come  up to 
the  initial  value  which is a  clear  indication  that  the  cathode  structure  is  the  main  determining  factor 
here,  not  the  decomposition  of  the  cathode  material.  But  a  change in cathode  structure  produces 
this  activity. 
(Figure  2-90) 
Now,  having  shown  that  you  can  cycle  such cells, it  naturally  becomes  interesting  at  what 
performance  you  can  expect  from  realistic cell configurations.  Using  realistic  cathode,  formulations, 
and  packaging, I have  calculated  here  energy  densities  for E-size  cells, and  I  have  plotted  it as a 
function  of  the  lithium  anacode  efficient,  which was naturally  a  very  important  factor  for  two 
different cathodes. In the cathodes, naturally the degree of cathode utilization is an important 
factor in determining  your  energy  density. 
You get  these  type  of  curves  where  you see that  with  a  titanium  disulfide  cell,  you  can  expect 
somewhere  between 80 and 140 watt-hours  per  kilogram.  With  a  lithium  vanadium  oxide  cell,  the 
range is somewhat  larger  from  about 1 10, 120  to  almost 300 watt-hours  per  kilogram. 
It is  interesting to  note  that  a  lithium  electrode  deficiency  naturally  plays  a  role in  all of  these 
cells, but  it  comes in much  more  strongly  if  you  go  to  a  higher  energy  density  cathode.  There  it 
makes  a large difference  on  what  your  electrode  performance  actually  turns  out  to be. 
(Figure 2-9 1 ) 
This  shows  the  same  type  of  effect  on  radiometric  energy  density,  and  here  again  you  cut  off, 
depending on the lithium electrode efficiency for titanium disulfide to a range of about 180 to 
almost 400 watt-hours  per  liter  and  for  the  vanadium  oxides, 210 to  somewhere  up  here. 
(Figure  2-92) 
Now,  let  me  summarize  the  status  of  the  secondary  lithium cells. 
Two-methyl  THF  lithium  hectofluorarsenate  has  shown  the  best  cycling  performance  to  date, 
and  it  has  exhibited  excellent  behavior  on  stand. 
Laboratory cycling results were verified in complete cells, so we think the feasibility of a 
lithium  titanium  disulfide  cell,  for  example,  with  a  cycle  life  of 100 cycles,  have  been  demonstrated. 
158 
Practical cell configurations  have  attractive  energy  densities,  between 80 and  290  watthours 
per kilo, and  175  to 800 watthours  per  liter,  depending  on  the  system,  specific  system. 
DISCUSSION 
BROADHEAD: How  many  ampere-hours of lithium  did  you  use in your D cells and your 
prismatic cells? 
HOLLECK: Well, we have  built  a  number  of cells, and  it  naturally  depends  on  what  cycle  life 
you design the cell for;  how  much  more  you  put  in.  In  the small  cells that I have  shown  you,  the 
lithium  efficiency is in  the  mid-90  percent. I don’t  quite  remember  how  much  lithium is in  the  large 
cells,  10-mil electrodes,  and I have to  calculate  what  that  comes  out  to  be  with  the  specific  capacities. 
159 
WHAT ARE PROBLEE WITH RECHARGING 
LITHIUM ELECTRODE? 
0 PLATING EFFICIENCY -100% BUT STRIPPING EFFICIENCY <loo%. 
0 DEPOSIT DENDRITIC. 
0 DEPOSIT PASSIVATES ON STAND. 
0 EFFICIENCY OF DISCHARGE DETERIORATES ON EXTENDED CYCLING. 
Figure 2-79 
/ 
W J E  WITH yxI% 
EFFICIENCY 
Figure 2-80 
160 
" IMPROVED LI CYCLING 
a LIAsF~ BEST PRECURSOR 
a MODIFIED ETHER SOLVENT (~ME-THF) 
a ELECTROLYTE PREPARATION 
Figure 2-8 1 
THE ONSET OF LI REACTION  WITH CYCLIC ETHER 
BASED  ELECTROLYTES AT 71'C 
PURIFI- 
CATION M LIAsF~/ 1 M  LIAsF~/ lfl LIC 041 lH LIASFU 1M LIAsF~ 
PROCEDURE TH F Z-ME-THF 2-k-THF 3-HE-THF ~,~-DI"E-THF 
Ab 25(28> 13 Mo - - - 
D A P A ~  4(7> >10 No 6Mo 10 (4) 12 Mo 
a~~~~~~~~ LI CORROSION AFTER 2 DAYS; OBSERVABLE ELECTROLYTE 
C SOLVENT PASSED THROUGH ALUMINA. 
C~~~~~~~ DISTILLED; PASSED  THROW ALUMINA; SALT AWKD IN. THE 
COLORAT I ON AFTER 16 DAYS. 
COLD  FOLLOWED BY PREELECTROLYSIS; ELECTRODE PASSED THROUGH 
ALWIM,  
Figure 2-82 
161 
I I 1111111 11111 I I 1 '   1 '  
80 
60 
40 
20 
80 
60 
40 
20 
5 
t 
IO 15 20 25 
Cycle number 
Figure 2-83 
30 
8 
2-Me-THF 
m II - 
I I I I I (( I 
0 8 16 24  32 40 48 9 6  
Open-circuit stand (hours) 
Figure 2-84 
I' 
I 
PROTOTYPE CELLS 
3.0 
2.5 
2.0 
1.5 
m CATHODES : T I  S2 
"6'13 
m CONFIGURATIONS: BUTTON CELLS 
D-SIZE CANS 
PRISMATIC  CANS 
Figure 2-85 
3.0 - 
2.5 - / CICLE 100 - I \ 1.5 
0 1 2 3 4 / 6 7 8 9 1 0 1 1  
1 1 1 I I ~~ 1 I I t I 
- 
J 
TI& Ms. 
Figure 2-86 
163 
IIIIII I I I I1lll11 Ill1 
0 1 2 3 4 5 6 7 
0 1 2 3 ,  4 5 
TIM (HOURS) 
CYCLING OF Lr/V@u 
6 7 8 
8 
Figure 2-88 
164 
Q 
I 
O a 6  t n CELL 75 CELL77 1 
0 , 5  
0 , 4  
0 , 3  
0 , 2  
0 I 1. 
0 
h 1 I l 
- 
1 I 1 I t 1 I I 1 I I 1 I f 1 1 I 
0 x1 40 60 &o 100 I20 140 160 
OF CYCLES 
Figure 2-89 
30t 
200 
1.01 
\ 
LI ELECTRODE  EFFICIENCY % 
Figure 2-90 
800 
600 
M z 
\ 
3 
I 
> 
I" 
n 
Z 400 
n 
z 
W 
> 
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n 
W 
L A  
z 
200 
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1 E N  
100 98  96 94 92 9 
L I  ELECTRODE  EFFICIENCY % 
Figure 2-91 
STATUS-  CF SECONDARY LI C E L L S  
0 ZME-THF/LI/ASF~ SHOWS THE  BEST  CYCLING  PERFORMANCE TO 
DATE, EXCELLENT BEHAVIOR ON STAND, 
0 LABORATORY CYCLING  RESULTS  VERIFIED I N  COMPLETE  CELLS. 
0 FEASIBILITY OF 100 CYCLE LI/TIS* CELL  DEMONSTRATED, 
0 PRACTICAL  CELL  CONFIGURATION  HAVE  ATTRACTIVE  ENERGY 
DENSITIES (80 TO 290 WH/KG, 175 TO 800 WH/L) 
Figure 2-92 
167 


Rechargeable ambient temperature lithium cells

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