A nonporous membrane based on the polymerization of 2,3-dihydrofuran followed by crosslinking in situ was prepared. The material is compatible with rechargeable Li battery components and, when swollen with an appropriate solvent such as tetrahydrofuran, exhibits separator resistance and Li transport equivalent to Celgard

Moore, J. A.

Willie, R.

English

NASA Technical Reports Server

J. A. Moore and Roice Willie 
Rensselaer Polytechnic Institute 
ABSTRACT 
A non-porous membrane based on the polymerization of 2,3-dihydrofuran 
followed by crosslinking in situ has been prepared. The material is compati- 
ble with &chargeable Li battery component.s and, when swollen with an approp- 
riate solvent such as tetrahydrofuran, exhibits separator resistance and Li+ 
transport equivalent to Celgard. 
INTRODUCTION 
Efforts to develop light-weight batteries have lead to the production of 
ambient temperature, lithium metal-chalcogenide secondary cells (Li, TiS2) 
containing ionic conductors (lithium hexafluoroarsenate) in aprotic, inert 
solvents such as tetrahydrofuran (THF) . The development of these batteries 
will be aided by improvements in separator technology which should signifi- 
cantly enhance cell performance and cycle life. Among the problems which 
exist with microporous separator materials (such as Celgard) is the formation 
of Li dendrites which penetrate through the pores of the separator from the 
anode to the cathode and ultimately short circuit the cell. Thus, a non- 
porous membrane which selectively transports solvated Li+ cations from one 
electrode to the other may well outperform a microporous separator in terms of 
enhancing cell life during charge and discharge. 
Among the requirements for such separators are that the material be 
insoluble in, but wettable by the solvent-electrolyte solution, that it be 
unreactive with metallic lithium and the cathode and that it be capable of ion 
transport. Because a large mass of data indicates that tetrahydrofuran and 
its derivatives are stable under normal battery conditions and fulfill all of 
the above criteria, it is reasonable to believe that incorporation of tetrahy- 
drofuran units into a polymer chain will yield a material which will be an 
effective separator. Such a polymer would be completely compatible with the 
electrolyte solvent systems in use and could be made insoluble by appropriate 
crosslinking reactions. Extensive literature documents the activity of ether 
groups in general, and THF rings, in particular. 
  his work was supported under a grant f,rom the National Aeronautics and 
Space Administration, Lewis Research Center, NAG 3-199. 
https://ntrs.nasa.gov/search.jsp?R=19850023065 2020-03-20T17:23:44+00:00Z
It has been reported that vinyl ethers such as 2,3-dihydrofuran can be 
polymerized to high molecular weight, processable materials. 3a,b Such materi- 
als could fulfill the requirements for a useful separator and therefore we 
have undertaken a reinvestigation of this, and related systems. 
This work describes the preparation and preliminary study of poly(2,3- 
tetrahydrofurandiyl) (P23) separator. This material was subjected to a series 
of compatibility experiments with rechargeable Li battery components as well 
as other physicochemical measurements. The results were compared to those 
from a Celgard 2400 standard. 
EXPERIMENTAL 
GENERAL 
Electrochemical and transport properties were measured by Covalent 
Associates, Inc., Woburn, MA. All electrochemical experiments were conducted 
at 280Ce Compatibility tests were conducted in an oven thermostatted at 
7092"C, Atomic absorption (AA) analyses were accomplished with a Perkin Elmer 
Model 403 spectrometer. Optical microscopy was conducted with an American 
Optical binocular zoon instrument to 40X. Scanning electron microscopy (SEM) 
was run by Photometrics, Inc., Woburn, MA. The P23 Separator, shipped under 
N2 in sealed vials, was opened in an Ar filled glovebox and not exposed to air 
unless otherwise indicated. Infrared spectra were recorded on a Perkin Elmer 
Model 237 spectrometer, ultraviolet spectra were recorded on a Perkin Elmer 
Model 552 spectrometer and 13C and IH nuclear magnetic reonance spectra were 
obtained with a 200 MHz Varian Model 200XL spectrometer. 
MEMBRANE PREPARATION 
The polymerization of 2,3-dihydrofuran in bulk and in solution has been 
effected using BF3eEt20 as initiator. Typically, a solution of BF3.Et20 in 
CH2CI.2 was added to the monomer in bulk and in CH2C12 solution at -78OC. 
Anhydrous conditions and a N2 atmosphere were maintained. At the completion 
of the reaction the initiator was deactivated with MeOH/CH2C12 solution, and 
the white, ropey polymer was precipitated into methanol. 
Membranes were prepared by d i s s o l v i n g  t h e  r e p r e c i p i t a t e d  polymer [reduced 
v i s c o s i t y  = 0.77dL/g] i n  chlorobenzene a t  a  concen t r a t i on  of about 30% and 
adding 1% t r i p h e n y l  phosphi te  and 5% benzoyl peroxide. Films were c a s t  on a  
Teflon block with a  draw k n i f e  t o  t h e  nominal dimensions and, a f t e r  drying f o r  
9 h r  a t  room temperature  under N2, then heated i n  an oven under vacuum a t  78OC 
f o r  12 hr .  Af te r  cool ing  under vacuum, t h e  f i l m s  were peeled from t h e  c a s t i n g  
s u r f a c e ,  so lven t  e x t r a c t e d  t o  remove byproducts ,  c u t  t o  s i z e  and s t o r e d  under 
N2 
COMPATIBILITY TESTS 
Measurements of phys ica l  dimensions were made on two 1.0 cm x  2  m i l  P23 
p i eces  cu t  from t h e  membrane a s  rece ived  and a f t e r  a  2  h r  soak i n  te t rahydro-  
fu ran  [(THF) Burdick and Jackson] and 2 m e t h y l t e t r a h y d r o f u r a n  [(2-MeTHF) 
Aldr ich]  d i s t i l l e d  from CaH2 under A r .  Addi t iona l  1  c m  x  4  c m  x 2  m i l  P23 
samples and 1  cm x  4  cm x  1  m i l  Celgard 2400 samples were c u t  f o r  t h e  compati- 
b i l i t y  t e s t s .  Weight changes of P23 were assessed  a f t e r  7  days s to rage  a t  
70°C i n  both 2-MeTHF and 1.4M LiAsFg (U.S. S t e e l  Agrichemicals)/2-MeTHF. 
A l l  compa t ib i l i t y  t e s t s  were conducted i n  d u p l i c a t e  i n  Teflon-taped screw 
cap v i a l s .  The components comprised Li f o i l  (L i thcoa ,  10 m i l ) ,  TiS2 (Cerac, 
b a t t e r y  g rade ) ,  a s  w e l l  a s  2-MeTHF and 1.4M LiAsF6/2-MeTHF prepared a t  d ry  i c e  
temperature  followed by p r e - e l e c t r o l y s i s  between 2  Li  e l ec t rodes .  
The fo l lowing  q u a n t i t i e s  of m a t e r i a l s  were used: 
L i  f o i l :  1  cm x  4  cm 
Celgard 2400 o r  P23: 1  cm x  4  cm 
Solvent o r  e l e c t r o l y t e :  5.0 m l  
A series of 20 v i a l s  was prepared and numbered a s  fol lows:  
V ia l  Number Contents 
s o l v e n t ,  P23 
e l e c t r o l y t e ,  P23 
e l e c t r o l y t e ,  P23, Li 
e l e c t r o l y t e ,  P23, L i ,  TiS2 
s o l v e n t ,  Celgard 
e l e c t r o l y t e ,  Celgard 
e l e c t r o l y t e ,  Celgard, L i  
e l e c t r o l y t e ,  Celgard, L i ,  TiS2 
e l e c t r o l y t e  x 
e l e c t r o l y t e ,  Li 
d u p l i c a t e  of 1  
d u p l i c a t e  of 2 
d u p l i c a t e  of 3 
d u p l i c a t e  of 4 
d u p l i c a t e  of 5 
d u p l i c a t e  of 6 
d u p l i c a t e  of 7 
d u p l i c a t e  of 8 
d u p l i c a t e  of 9 
d u p l i c a t e  of 10 
CYCLIC VOLTAMMETRY AND INFRARED ANALYSIS 
C y c l i c  voltammetry (CV) was conducted d i r e c t l y  i n  V i a l s  2, 3, and 6 a f t e r  
t h e  7 day s t o r a g e  per iod .  A 3 mm d i a .  p o l i s h e d  v i t r e o u s  C working e l e c t r o d e  
a l o n g  w i t h  L i  c o u n t e r  and r e f e r e n c e  e l e c t r o d e s  were i n s e r t e d  i n t o  t h e  v i a l  
i n s i d e  t h e  glovebox. Voltammograms were g e n e r a t e d  by a BAS CV-1B ins t rument  
and recorded  on an HP 7015B XY r e c o r d e r .  I n f r a r e d  s p e c t r o s c o p y  was conducted 
i n  s e a l e d  matched and p o l i s h e d  KC1 c e l l s  (Wilmad) w i t h  a 0 , l  mm pa th  l e n g t h .  
RESISTIVITY MEASUREMENTS 
A two-compartment polypropylene c e l l  (des igned  by Covalent A s s o c i a t e s )  
was used f o r  a l l  r e s i s t a n c e  measurements, The membrane was suppor ted  by Vi ton 
"0"-rings which s e a l e d  t h e  c e l l  and f i x e d  t h e  a r e a  exposed t o  t h e  e l e c t r o l y t e  
a t  6.0 cm2. R e s i s t i v i t y  d a t a  was t aken  w i t h  a GenRad Model 16506 impedance 
b r idge .  
LI  I O N  TRANSPORT STUDY 
The f l u x  of ~ i +  through  t h e  P23 membrane was conducted i n  t h e  same ce l l  
used f o r  r e s i s t i v i t y  measurements. One compartment con ta ined  2,OM LiClO4/THF 
w h i l e  t h e  o t h e r  con ta ined  2.OM NaC104/THFe Because t h e  P23 m a t e r i a l  swe l led  
t o  a much g r e a t e r  degree  i n  THF t h a n  i n  2-MeTHF ( v i d e  i n f r a ) ,  THF was t h e  s o l -  
v e n t  of cho ice .  The c e l l  was f i l l e d  and a l lowed t o  e q u i l i b r a t e ,  Subse- 
q u e n t l y ,  1.0 m l  a l i q u o t s  were removed from t h e  NaC104/THF compartment and 
ana lyzed  f o r  ~ i +  c o n t e n t  w i t h  AA spec t roscopy .  Th is  exper iment  was t h e n  
r e p e a t e d  w i t h  Celgard 2400 s e p a r a t o r  m a t e r i a l .  
RESULTS AND DISCUSSION 
COMPATIBILITY TESTS 
The s e a l e d  t u b e s  were opened i n s i d e  t h e  glovebox and t h e  2 m i l  and 5 m i l  
P23 membranes were measured f o r  u n i f o r m i t y  of t h i c k n e s s .  The normal 2 m i l  
membrane ranged from 1.5 t o  2.5 m l  i n  t h i c k n e s s ,  w h i l e  t h e  5 m i l  membrane 
ranged from 2,0 to 3,O mil in thickness, Both samples were flexible and 
cellophane-like in mechanical behavior. The P23 samples could be folded and 
unfolded many times without structural damage. 
Under the 40X microscope, neither bubbles nor cracks were evident in 
either the 2 mil or the 5 mil sample. Two pieces of P23 were deliberately 
exposed to air for 3 days. Microscopic examination revealed no visible struc- 
tural changes nor was any embrittlement detected. 
All of the 70°C compatibility tests employed 2 mil P23. All visual 
observations were identical for their corresponding duplicates. No loss of 
solvent volume was detected over the 7 day storage period. 
After 20 hr of storage at 70°C, a yellowing of the P23 membrane in vials 
containing LiAsFg was observed, The yellowing was enhanced in vials contain- 
ing LiAsFg and Li foil, 
In Vial 1 containing only 2-MeTHF, the P23 sample was clear and color- 
less, but in the presence of LiAsF6 (Vial 2), P23 turned uniformly yellow. 
Vial 13 revealed an amber color permeating the membrane with the edges being 
the most intensely colored. It is important to note that the electrolyte 
solutions in Vials 2 and 13 were clear and colorless. Only the membrane was 
affected. Similarly, the Celgard 2400 samples in Vials, 5, 6, and 7 were 
unchanged as were the blanks (Vial 19 and 20). 
Experience with this membrane suggests that the yellowing may be caused 
by oxidation of the polymer by residual 02, and that this oxidation is exacer- 
bated by the presence of halide, Over the following 6 days no further changes 
were noted with the exception of a slight yellowing of the electrolyte in 
contact with P23, 
At the end of the 7 day storage tests the samples were removed from the 
vials and examined, As the samples dried they shriveled to a form which makes 
microscopic examination or dimensional measurements difficult. P23 from vial 
12 seemed to be markedly yellow when compared to P23 from vial 11. The 
Celgard 2400 standard was unchaged after storage for 7 days at 70°C. When 
rinsed with methanol to remove small particles of TiS2, the P23 membrane from 
Vials 4 and 14 was as yellow in color as the P23 from Vials 3 and 13. Even 
after solvent evaporation in the glovebox, the membranes retained their plas- 
ticity -similar to commercial Saran Wrap rather than cellophane. None of the 
P23 samples manifested any sign of brittleness, 
Infrared scans on 2-MeTHF from Vial 11 and 1,4M LiAsF612-MeTHF from Vial 
12 were obtained and compared to electrolyte from Vial 16 which contained 
Celgard 2400. All spectra were identical, The spectrum of electrolyte from 
Vial 12 compared to the spectrum of electrolyte from Vial 16 showed no extran- 
eous absorbances. Thus the spectral evidence indicates that P23 is not dis- 
solving in the solvent or solvent-electrolyte solutions. Attempts by 
reweighing the test samples to determine if dissolution was occuring were 
inconclusive. 
To probe further the possibility of P23 degradation, cyclic voltammetry 
scans were made on electrolyte from Vial 6 (Celgard 2400), Vial 2 (P23), and 
Vial 3 (P23 + Li). With the exception of a slight increase in cathodic 
current from electrolyte in Vial 3, no large quantities of electroactive 
materials are present between 0.8 and 4.4 V vs. Li. A slightly enhanced 
current from Vial 3 may well be caused by small amounts of electrolyte-Li 
reaction products. 
RESISTIVITY MEASUREMENTS 
A 2 mil sample of P23 was placed in the resistivity cell after which the 
cell was filled with 1.4M LiAsF6/2-MeTHF electrolyte. After 5 min, the total 
cell resistance was 41,000 ohms; after 30 min, 2150 ohms; after 1.5 hr, 520 
ohms; after 3 hr, 204 ohms; after 5 hr, 170 ohms. The total cell resistance 
stabilized at this last value. In the absence of the P23 separator, the cell 
manifested a resistance of 165 ohms. Thus, the separator resistance multipled 
by the 6 cm2 geometric area gave a value of 30 ohm cm2. 
When Celgard 2400 was substituted for P23, the cell resistance was found 
to be below the sensitivity of the impedance bridge after just 5 minutes. 
Thus, the P23 separator resistance 'is at least 30 times greater than that of 
Celgard in 2-MeTHF. 
To ascertain whether the P23 might swell more readily in THF rather than 
in 2-MeTHF and thereby become less resistive, the resistance of a P23 sepa- 
rator in 1.4M LiAsF6/THF was measured. After 5 min, the total cell resistance 
was essentially equivalent to the cell resistance without P23 as in the case 
for Celgard 2400. This result clearly demonstrates that P23 is a good Li+ 
transporter when it can be made to swell by an appropriate solvent. 
LI ION TRANSPORT STUDY 
Based on the results with THF, it was decided to conduct this experiment 
with 2.0 M LiC1041THF and 2.0 M NaC1041THF electrolytes. Over a 3.6 hr period 
the average rate was found to be 2.2 x 10-I mmole Li+/h/cm2. Under exactly 
the same conditions, Celgard 2400 was also found to give a rate of 2.2 x 10-1 
mmole ~i+/hr/cm~. This result corroborates the resistivity data acquired in 
LiAsF6/THF for P23 and Celgard 2400. It should be noted that because of the 
propensity of P23 to swell in THF (vide infra), the geometric area and actual 
swollen area may differ by up to 100%. However, this uncertainty should not 
prevent P23 from being used as an effective battery separator. 
MISCELLANEOUS TESTS 
P23 Wetability and Swelling Measurements. It was found that methanol, 
2-MeTHF, and THF readily wet P23 while H20 does not. In 2-MeTHF (7 days), P23 
went from 2-3 mil to 3-3.5 mil in thickness. In THF (5 min) P23 went from 2-3 
mil to 3-4 mil in thickness. These numbers are grossly approximate because 
of the difficulty of measuring thickness accurately on a gelatinous material. 
Dimensional changes for P23 in THF and 2-MeTHF were roughly assessed in 
their respective vials because removing the membranes from the solvent 
resulted in rapid shrinkage. Thus, P23 in 2-MeTHF increased in length and 
width by about 13% after 2 hr at 28OC. But in THF, P23 appeared to undergo a 
90% increase in length and width within 5 min of contact with THF at 280Ce No 
further changes were noted with time. 
Heat Sealing. A sample of P23 was heat sealed to itself satisfactorily 
with a jaw impulse heater at a setting of 5. P23 requires more heat than 
Celgard 2400, but this fact presents no problem in suitably bagging battery 
electrodes with P23. 
SEM Study of P23 Structure. All attempts at obtaining SEM images of the 
THF-swollen membrane failed because of the shriveling phenomenon as the sol- 
vent evaporated. The dry membrane surface (shiny side) is remarkably smooth 
and featureless at 11,000X. To obtain a cross section free of distortion from 
a cutting implement, a sample of 2 mil P23 was fractured at liquid M2 tempera- 
ture. The cross section is amorphous and pinhole-free at 4,440X. 
CONCLUSIONS 
P23 is pliable, insoluble in 2-MeTHF and in THF, and manifests adequate 
conductivity in THF-electrolyte systems, Further work and extended compati- 
bility studies in THF-based electrolytes would be fruitful, Indeed, the 
latest EIC electrolyte ( 50:50 THF:2-MeTHF plus 2lnethylfuran) is an obvious 
choice. Finally, P23 must be tested as a separator in rechargeable Ef cells 
to ascertain its resistance to dendrite penetration and long term chemical and 
electrochemical stability. The very promising results obtained from these 
initial efforts bode well that continued study of novel polymers containing 
heterocyclic rings may lead to separator systems which are superior to those 
currently in use. 
REFERENCES 
1. "Status of the DOE Battery and Electrochemical Technology Program", Mitre 
Corp., report #MTR-8026, September, 1979. 
2a. C. J. Pedersen and H. K. Frensdorff, Angew. Chem, Int. Ed. Englo - 11 ,  16 
(1972). 
2b. J. J. Christensen, D. J. Eatough and Re M. Izatt, Chem. Rev. - 74, 351 
( 1974). 
2c. W. J. Schultz, M. C. Etter, A. V. Pocius 'and S. Smith, J, Ame Chem, Soc. 
102, 7981 (1980). 
-
2c. W. J.  Schultz,  M. C. Etter ,  A. V. Pocius and S. Smith, J .  Am. Chem. Soc. 
102, 7981 (1980).  
-
3a. D. A. Barr and J. B.  Rose, J. Chem. Soc. 1954, 3766. 
3b. G. J. Schmitt and C. Schuerch, J. Polym. Sc i .  - 49, 287 (1961).  
TOTAL CELL RESISTANCE 
Membrane 
THF 2MTHF 
5min. 30 90 180 300 5min. 30 90 180 300 
- - -
P23 "JO - - - - 41,OOOil 2150 520 204 170 
Celgard 
2400 "JO - "Jo - 
Electrolyte = 1.4M LiAsF6 
Membrane 
P2 3 
Celgard 2400 
Average Rate (MMole ~ i ' / h r / ~ r n ~ )  
2.2 x 10-1 


Development of a lithium secondary battery separator

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