NASA Technical Reports Server (NTRS) 20020070846: Solid State Electrolytes Prepared from PEO (360) Silanated Silica

4 Pages·0.24 MB·English

by  NASA Technical Reports Server (NTRS)

Solid State Electrolytes Prepared from PEO(360) Silanated Silica
P. Maitraa, J. Dinga, B. Liua, S.L. Wunder*a, H.-P. Linb, D. Chuah, M. Salomonb
aDepartment of Chemistry, 016-00, Temple University, Philadelphia, PA 19122, USA
bMaxPower, Inc., 220 Stahl Road, Harleysville, PA 19438, USA
*corresponding author: [email protected]
Abstract been shown to improve the stability of the
electrode interface],3.
All solid state composite electrolytes were
prepared Llsing fumed silica (Si02) silanated with In PEa electrolytes, the transport of the Li ion is
an oligomeric polyethylene oxide silane thought to occur through a "hopping" mechanism
containing 6-9 ethylene oxide repeat units, a PEa between vacant sites. Since increased free volume
matrix and LiCI04 (8/1 O/Li). The PEa-silane (Vf) should increase the number of these sites,
covalently attached to the silica ,vas amorphous, ionic transport is expected to increase with Vf.
with a Tg that increased from -90°C to -53°C after One way of introducing free volume into polymers
attachment. The conductivity of films prepared is to increase the number of chains ends, i.e. to
using the PEO-silanated silica increased to ~ decrease the molar mass or introduce branching12•
6xlO·s S/cm at RT compared with ~l x 10.5 S/cm This effect has been utilized in systems in which
for films prepared with unsilanated Sial. low molar mass PEa side chains have been
attached to inorganic and organjc polymer
Introd uction scaffoldsl3. The conductivity of comb-shaped high
molecular weight PEa was observed to increase
All solid state electrolytes (SSEs) that can be with increased content of tri( oxyethylene) side
laminated to the electrodes in Li metal or Li ion chains 10. For pendant PEa groups, those with
rechargeable batteries are expected to improve 11=5-6 ethylene oxide units, under equivalent
their cycle and shelf-life. Polyethylene oxide conditions, have exhibited the highest
(PEa) can be used as a SSE above its melt conductivities 4,14.
temperature of ~ 70°C, but crystallization of the
polymer decreases its room temperature (RT) In the present work, low molar mass PEa, with a
conductivity to -1 O·? S/cm. Composite polymer number average molecular weight of -360g/mol
electrolytes prepared from inorganic fillers and [referred to as PEO(360)] has been attached to
PEa SllOW improved mechanical and temperature fumed silica [referred to as PEO(360)-Si02] and
stability6,8 and enhanced RT conductiviti8. incorporated in composite SPEs prepared from
Recently, composites of PEa with micron and PEa [Mw = 600,000, or a 50/50 mixture of Mw=
nanometer sized ceramic particles such as Si02, 600,000 and 1000] or combinations of PEa and a
Ah03, or Ti02 have been shown to have RT copolymer of polyvinylidene fluoride with
conductivities of 10.5 S/cms,?, predominantly as a hexafluoroisopropanol (PVDF-HFP). Comparative
result of the inhibition of the PEa conductivity (u) studies between PEO(360)-Si02
crystallization2,9, since in these systems, Li iOIl and pure Si0 blended with PEa indicate that ()
2
transport is believed to occur in the amorphous increases with increasing weight fraction of low
phase. The high surface area of the nano-size molar mass PEa, with the greatest effect arising
particles are most effective at inhibition of from composites prepared from PEO(3 60)-Si01. A
crystallization. However, recrystallization of the () of ~ 6 x 10.5 S/cm at R T was obtained for
PEa occurs with time, resulting in lower ionic composites prepared from the silanated silica.
conductivity. A similar inhibition of crystallization
is observed in PEa intercalated in nanoscale Experimental
layered silicates 16. Inorganic particles have also Aerosil 380 (A380) fumed silica, with a nominal
diameter of7nm and surface area of380m2/g, was
This is a preprint or reprint of a paper intended for presentation at a
conference, Because changes may be made before fo.rmal . .
blication this is made available with the understanding that It Will
~~t be cited or reproduced without the permission of the author.
obtained from deGussa. Attachment of the PEO typically 28-34%. The proposed structure, shown
was achieved by a silanization reaction using 2- in Figure 2, contains linkages both to the silica
[methoxy (polyethyleneoxy) propyl] trimethoxy surface (via silanol groups) and between adjacent
silane (Gelest), shown in Figure 1. The nominal silane molecules.
MW is from 458 (n=6) to 590 (n=9), but the MW
of the PEO segment ranges from 295 (n=6) to 427
(11=9), with an average of 360 [referred to as
PEO(360)-silane]. PEO(500) and PEO(350) were
obtained froni Aldrich.
OCH oi
3
PEO
CH30-fi-CH2- CH2- CH2- CH2CH20tn-CH3
OCH PEO / /PEO
3
l Si S·
/1"'- ./'
n=6-g Si--O 0 \ 1-
\
PEO"""
Si
Figurel. PEO(360)-silane
\ /PEO
O-Si
The PEO(360)-Si02 was characterized by / \ \
thermogravimetric analyis (TGA), using TA O-Si. ..... PEO
Instruments Hi-Res TGA2950 Thermogravimetric I
Analyzer, and differential scanning calorimetry
(DSC), using a DSC 2920, both at scan rates of Figure 2. Structure ofPEO(360)-Si0
2
10°C/min and under N2 atmosphere. Ionic
conductivity was measured from the complex Films were prepared in a dry glove box purged
impedance spectra obtained using a Schlumberger with argon by dispersing Si0 or PEO(360)-Si0
2 2
HF Frequency Response Analyzer (model SI and LiCl0 in acetonitrile, followed by addition of
4
1255) in combination with an EG&G Princeton PE~. All materials were dried in a vacuum oven
Applied Research (PAR model 273A) for ~24h [PEO at 55°C, LiCl0 at 120°C, Si0 at
4 2
potentiostat/galvanostat in the frequency range 200°C, and PEO(360)-Si0 at 80°C] prior to
2
from 0.01 to 100kHz, using stainless steel preparation of the films. The O/Li ratio was SI1.
blocking electrodes. Films prepared using PEO(600,000)/ PE0(1000)
in a 50/50 wt ratio were cast onto stainless steel
A typical silanization reaction was carried out by film, and the others onto Teflon.
first dispersing, for Ih with stirring, 3 g of A3 80 in
250g hexane, followed by addition of 13 g Results and Discussion
PEO(360)-silane; this mixture was stirred at RT
for ~ 16h. The silanated silica settled to the bottom, DSC traces, shown in Figure 3, of PEO(350) or
and the supernate was decanted. The silanated PEO(500), not shown, are completely crystalline,
silica was evacuated at RT under reduced pressure and exhibit no glass transition temperature, T
g•
overnight, followed by evacuation in a vacuum PEO(360)-silane is semicrystalline, and exhibits a
oven at 90°C overnight. These procedures were Tg at -S9.9°e. When physically adsorbed
adopted to increase the amount of covalently PEO(360)-silane is attached to fumed silica (i.e.
attached silane. In order to remove noncovalently before the heat treatment and rinsing steps), a
bonded silane, the silanated silica was redispersed semi crystalline material with a T g of -79°C is
in methanol, centrifuged at ~3500rpm for ~lh, observed. After the heating and rinsing
and the supernate decanted. This process was procedures, which leave only the covalently
repeated 3x, followed by evacuation at RT. The attached silane, there is almost no crystallization,
amount of grafted PEO(360)-silane, determined by and the Tg increases to -53.3°e. Thus grafting of
the TGA weight loss between 25 to SOO°C, was the silane onto the silica surface results in an
almost complete suppression of the crystallization Table 1* Composition ofPEO in films
and an increase in the glass transition temperature
of -35°C. Even in the case of physically adsorbed Film %PEO %PEO %PEO
silane, there is an increase of 10°C in the T" (360) (600,000) (1000)
compared with the original material. 1 0 90 0
0
2 0 45 45
";) 3.8 86.2 0
*F ilms also contam 10% SI02 and LICI0 at 8/1
4
D O/Li ratio
DSC traces for the dry films, after annealing at
100°C for 48h, exhibited no melt endotherms and
B had Tgs of -18°C. This value is higher than that
r,
reported in the literature I suggesting that the
films are at least as dryas those prepared in other
laboratories. Upon exposure to atmospheric
moisture, Tg decreased.
'j
-120 -70 ·20 30
Temperature (OC)
z; j
'" .. J. Film 3
:~ -4
U
::I
Figure 3: DSC traces of (A) PEO(350); (8) 'c0:
3 -5 I Film 2
PEO(360)-silane; (C) physically adsorbed OJ II
0
..J I
PEO(360)-silane; (D) grafted PEO(360)-Si02 Film J
-61
Increases in T g as a function of Al203 filler loading -7 ,
have previously been observed for polyether based 2.7 29 3.1 3.3 35
composite electrolytes17• However, unlike the case 10001T K-'
of physically mixed high molecular weight PE~,
Figure 4. Temperature dependence of conductivity
Li salt composites, the suppression of
offilms from Table I.
crystallization for the grafted PEO( 460)-silane,
even in the absence of salt, is thermodynamically
Figure 4 shows the temperature dependent
stable. Other investigators have observed an
conductivity data for films 1 and 2. There is a
absence of crystallization of PEO side chains for
small increase in conductivity wilen low molar
chains with Mil < 450 (n=lO) in the case of
mass PEO (1000) is added to the PEO matrix.
poly( ethylene oxide )-polyacrylates and
When 13.8% PEO(360)-silanated silica is added to
polymethacrylates 19, and for n < 5 in the case of
PEO(600,OOO), film 3, the conductivity increases
polyepoxides 14.
by at least a factor of 5 compared with film 1,
although the amount of low molecular weiaht
In order to investigate the effects of free volume ::0
PEO(360) is considerably less than that achieved
on conductivity, films were prepared with
by addition of PEO(lOOO). Although the number
increasing amounts of low molar mass but solid (at
of chain ends is approximately 3 x greater for
RT) PE~. Table I presents the composition (wt%)
PEO(360) compared with the same weight percent
of PEO in the films. In all cases the amount of
PEO(lOOO), this cannot account for the difference
SiOl was 10%. The amount of the PEO(360) from
the PEO(360)-silane was determined from TGA in v between films (2) and (3). Przyluski et al 15
have suggested a model in which the increase in
weight loss data.
conductivity in composite electrolytes compared
with polyether based electrolytes was due to the (10) Ikeda, Y.; Wad a, Y.; Matoba, Y.; Murakami,
formation of highly conductive layers at the S.; Kohjiya, S. Electrochim. Acta 2000, 45, 1 167-1174.
polyether matrix=-fi ller interface. In addition, PEO (11) Kim, Y. W.; Lee, W.; Choi, B. K.
Electrochim. Acta 2000,45, 1473-1477.
segments of n 6 have been most effective in
(12) Kono, M.; Hayashi, E.; Watanabe, M. J.
enhancing Li ion transport.
Electrochem. Soc. 1998,145, 1521-1526.
(13) Lauter, U.; Meyer, W. H.; Wegner, G.
The films prepared using PEO(360)-silanated
Macromolecules 1997, 30, 2092-2101.
silica and PEO(600,OOO) were strong and self (14) Marchese, L.; Andrei, M.; Roggero, A.;
supporting. Preliminary data for films prepared Passerini, S.; Prosperi, P.; Scrosati, B. Electrochimica
with PVDF-HFP, PEO(360)-Si02, PEO and Acta 1992,37,1559-1564.
LiCl0 indicate that they are tougher, but that the (15) Przyluski, 1.; Sierkierski, M.; W.Wieczorek.
4
conductvities are only reasonable if the total wt% Electrochim. Acta 1995, 40, 2101-2108.
(16) Vaia, R. A.; Vasudevan, S.; Krawiec, W.;
of inert material (i.e. Si0 and PVDF-HFP) is less
2
Scanlon, L.; Giannelis, E. P. Advanced Materials 1995,
than ~ 30%. Further work is in progress to
7, 154-156.
improve both the conductivity and mechanical
(17) Wieczorek, W.; Florjancyk, Z.; Stevens, J. R.
properties of the films.
Electrochim. Acta 1995, 40, 2251-2258.
(18) Wieczorek, W.; Such, K.; Wycislik, H.;
Conclusions
Plocharski, J. Solid State Ionics 1989, 36, 255.
(19) Yan, F.; Dejardin, P.; Frere, Y; Gramain, P.
Films prepared using PEO(360)-silanated silica, Makromol. Chem. 1990,191,1197-1207.
LiCl0 and a PEO matrix exhibited conductivities
4
of ~6x 1 0.5 at RT. Reasonable mechanical
properties were obtained from processable films
without the necessity of forming a crosslinked
network.
Acknowledgements
The partial support of this research by Temple
University and NASA (NAS3-01160) is gratefully
acknow ledged.
References
(1) Appetecchi, G. B.; Dautzenberg, G.; Scrosati,
8.J. Electrochem. Soc. 1996,143,6-12.
(2) Best, A. S.; Ferry, A.; MacFarlane, D. R.;
Forsyth, M. Solid State ionics 1999, 126,269-276.
(3) Borghini, M.; Mastragostino, M.; Passerini, S.;
Scrosati, B. J Electrochem. Soc. 1995, 142, 2118.
(4) Cowie, 1. M. G.; Sadaghianizadeh, K. Solid
State 10nics 1990, 42, 243-249.
(5) Croce, F.; Appetecchi, G. 8.; Persi, L.;
Scrosati, B. Nature 1998, 394,456-458.
(6) Croce, F.; Bonino, F.; Panero, S.; Scrosati, B.
Philosophical Magazine 1989, B59, 161.
(7) Croce, F.; Curini, R.; Martinelli, A.; Persi, L.;
Ronci, F.; B.Scrosati; Caminiti, R. J Phys. Chern. B
1999,103,10632-10638.
(8) Croce, F.; Panero, S.; Passerini, S.; Scrosoti,
B. Electrochim. Acta 1994, 39,255.
(9) Dai, Y.; Greenbaum, S.; Golodnitsky, D.;
Ardel, G.; Strauss, E.; Peled, E.; Rosenberg, Y. Solid
State ionics 1998,106,25.