Network single ion conductors based on comb-branched polyepoxide ethers and lithium bis(allylmalonato)borate

Article abstract of DOI:10.1021/ma049331c

The synthesis of network single ion conductors to obtain single ion conductors with high ambient temperature conductivity and mechanical strength is discussed. The single ion conductors were based on comb-branched polyepoxide ethers and lithium (Li) bis(allylmelonato)borate. The single ion conductors possessed good mechanical strength due to the formation of network structure and the Li/Li symmetric cell cycling showed no concentration polymerization. The results show that the backbone structure of the polyepoxides contributes to the total ionic conductivity and it increases with the increasing side chain length.

Full text of DOI:10.1021/ma049331c

Volume 37, Number 14
J uly 13, 2004
© Copyright 2004 by the American Chemical Society
Communications to the Editor
were not stable against lithium metal due to attack of
Netw or k Sin gle Ion Con d u ctor s Ba sed on
Com b-Br a n ch ed P olyep oxid e Eth er s a n d
Lith iu m Bis(a llylm a lon a to)bor a te
metallic lithium on the vulnerable ester bond during cell
cycling.20
In general, polyepoxide ethers have lower glass
transition temperatures than those of corresponding
polyacrylates and polymethacrylate derivatives,21,22 so
higher mobility of the lithium ions is expected. In
addition, the ether bond of polyepoxide is less easily
reduced than the ester bond of polyacrylate and poly-
methacrylate and should be more stable against the
lithium metal when cycled under charge/discharge
conditions. In this Communication we report the results
of network single ion conductors based on LiBAMB and
comb-branched polyepoxide ethers (Scheme 1).
Xia o-Gu a n g Su n , J oh n B. Ker r ,* Cr a ig L. Reed er ,
Ga o Liu , a n d Yon gbon g Ha n
Lawrence Berkeley National Laboratory, MS 62-203,
One Cyclotron Road, Berkeley, California 94720
Received April 5, 2004
Revised Manuscript Received May 21, 2004
In the past several decades polyelectrolyte single ion
conductors, either dry or as gels, have attracted atten-
tion due to the absence of concentration polarization, a
common problem encountered in the conventional solid
polymer electrolytes (SPEs) where both cation and anion
are mobile.1-18
The obtained single ion conductors are all self-
standing transparent films. Figure 1 shows a compari-
son of the ionic conductivities of the polyacrylate and
polyepoxide single ion conductors (SIC) with the same
side chain length of three EO units at the same salt
concentration of EO/Li ) 20 (PAE3XLAE1 and PEPE3,
respectively). The ionic conductivity of polyepoxide ether
based SIC is nearly 1 order of magnitude higher than
that of the polyacrylate ether based SIC. Marchese et
al.21 also noticed that in binary salt systems at the same
side chain length and same salt concentration the
conductivity of polyepoxide ethers was higher than that
of polyvinyl ethers. These results suggest that the
backbone structure of the polyepoxides contributes
significantly to the total ionic conductivity. Figure 1 also
shows the ionic conductivities of the single ion conduc-
tors having the same cross-linker consisting of two
ethylene oxide (EO) units and different side chain
lengths (Scheme 1, n ) 2-5) at the same salt concen-
tration of EO/Li ) 20. The conductivity systematically
increases with increasing the side chain length, as
observed for polyacrylate/polymethacrylate ether based
network single ion conductors,20 which can be explained
by the increased mobility of the ions associated with the
more mobile polymer side chains. This result is consis-
tent with observations in binary salt (LiClO4) solutions
of polyepoxide ethers where it was reported that the
Different strategies have been used to synthesize new
single ion conductors, and the best ambient temperature
conductivities reported are in the range 10-6-10-5
S
cm-1 1-18
However, higher ionic conductivities are usu-
.
ally found for mobile oligomers whose poor mechanical
properties cannot fulfill the requirements of commercial
devices. To obtain single ion conductors with both high
ambient temperature conductivity and appropriate me-
chanical strength, we have recently synthesized and
characterized new network single ion conductors based
on the lithium salt, lithium bis(allylmalonato)borate
(LiBAMB), and polyacrylate and polymethacrylate
ethers.19,20 These single ion conductors possess good
mechanical strength due to the formation of the network
structure, and the Li/Li symmetric cell cycling showed
no concentration polarization as expected for true single
ion conductors. Unfortunately, the conductivity is low
due to the unexpectedly poor dissociation of the Li-
BAMB ion pair and the high glass transition tempera-
ture of polyacrylates and polymethacrylates. It was also
shown that the (meth)acrylate single ion conductors
* Corresponding author: e-mail jbkerr@lbl.gov.
10.1021/ma049331c CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/05/2004

5134 Communications to the Editor
Macromolecules, Vol. 37, No. 14, 2004
Sch em e 1. Syn th esis of Sin gle Ion Con d u ctor s
F igu r e 1. Arrhenius ionic conductivities of single ion con-
ductivities based on polyacrylate and polyepoxide ethers and
AllylE2 cross-linker at the same salt concentration of EO/
Li ) 20.
ionic conductivities increased with increasing side chain
length up to six EO units.21
F igu r e 2. Arrhenius ionic conductivities of single ion con-
ductivities based on PEPE5 with AllylE2 cross-linker at dif-
ferent salt concentrations.
The best ambient temperature conductivity of 1 ×
10-6 S cm-1 is obtained for the sample with a salt
concentration of EO/Li ) 40 (Figure 2). This is impres-
sively high for a self-standing membrane and is appar-
ently due to an optimal balance of the charge carrier
concentration and ionic mobility. An optimal conductiv-
ity at the salt concentration of EO/Li ) 40 was also
observed in previous results regarding comb-branched
and network single ion conductors.20,23 The sample with
the highest conductivity was used to construct a sym-
metrical Li/Li cell, which was cycled at 85 °C under the
current density of 25 µA cm-2 and a cycling sequence
of 1 h relaxation, 2 h charge, 1 h relaxation, and 2 h
discharge. The cycling profile is shown in Figure 3, and
as expected for a true single ion conductor, no concen-
tration polarization and relaxation was observed; i.e.,
when the current is turned on, the cell potential
immediately increases from a couple of millivolts to the
potential determined by the combined bulk and inter-
facial impedance, and once the current is turned off the
cell potential immediately drops back to the initial
value. This cycling behavior is distinctly different from
that of binary salt system in which the cell potential
increases over a time period of minutes to hours due to
the salt concentration polarization.24 The cycling was
stopped after 20 cycles, and the cell impedance was
measured using ac electrochemical impedance spectros-
copy.25 It was found that both bulk and interfacial
impedance increased. The increase of bulk impedance
indicates that the network structure may not have been
fully formed due to the film casting technique, and some
unreacted siloxane groups may continue to react during
cycling. This would result in an increase in the cross-
link density of the electrolyte membrane, which would
lower the mobility of the polymer chains and thus lead
to lower conductivity and higher impedance. The in-
crease of the interfacial impedance suggests a reaction
of the single ion conductor with lithium electrode at the
electrode/electrolyte interface. This would result in the
immobilization of polymer chains at the interface and
thus lower the mobility of the ions passing through the
interface and yield higher interfacial impedance. Such
a process is reflected in the cycling profile by an increase

Macromolecules, Vol. 37, No. 14, 2004
Communications to the Editor 5135
important question and will be pursued in future
reports.
Ack n ow led gm en t. The authors are grateful for the
financial support from NASA PERS programs (NASA
Glenn).
Su p p or tin g In for m a tion Ava ila ble: Complete experi-
mental description, physical properties of the prepolymers and
single ion conductors, impedance of Li/Li cell, and postmortem
analysis data. This material is available free of charge via the
Internet at http://pubs.acs.org.
Refer en ces a n d Notes
(1) Gray, F. M., Ed.; Solid Polymer Electrolytes: Fundamentals
and Technological Applications; VCH Publishing: New
York, 1991.
(2) Bannister, D. J .; Davies, G. R.; Ward, I. M.; McIntyre, J . E.
Polymer 1984, 25, 1291.
(3) Ito, Y.; Ohno, H. Solid State Ionics 1995, 79, 300.
(4) Zhou, G. B.; Khon, I. M.; Smid, J . Polym. Commun. 1989,
30, 52.
(5) Kobayashi, N.; Uchiyama, M.; Tsuchida, E. Solid State
Ionics 1985, 17, 307.
F igu r e 3. Charge/discharge profile of the Li/polyelectrolyte/
Li cell before the impedance measurement (SIC with EO/Li )
40) (electrode area is 1 cm2, membrane thickness is 100 µm).
(6) Zhou, G. B.; Khon, I. M.; Smid, J . Macromolecules 1993, 26,
2202.
(7) Tsuchida, E.; Ohno, H.; Kobayashi, N.; Ishizaka, H. Mac-
romolecules 1989, 22, 1771.
(8) Zhang, S. S.; Liu, Q. G.; Yang, L. L. Polymer 1994, 35, 3740.
(9) Benrabah, D.; Sylla, S.; Alloin, F.; Sanchez, J . Y.; Armand,
M. Electrochim. Acta 1995, 40, 2259.
(10) Onishi, K.; Matsumoto, M.; Nakacho, M.; Shigehara, K.
Chem. Mater. 1996, 8, 469.
(11) Fujinami, T.; Tokimune, A.; Mehta, M. A.; Shriver, D. F.;
Rawsky, G. C. Chem. Mater. 1997, 9, 2236.
(12) Watanabe, M.; Suzuki, Y.; Nishimoto, A. Electrochim. Acta
2000, 45, 1187.
(13) Bayoudh, S.; Parizel, N.; Reibel, L. Polym. Int. 2000, 49,
703.
(14) Florjanczyk, Z.; Bzducha, W.; Langwald, N.; Dygas, J . R.;
Kork, F.; Misztal-Faraj, B. Electrochim. Acta 2000, 45, 3563.
(15) Xu, W.; Angell, C. A. Solid State Ionics 2002, 147, 295.
(16) Snyder, J . F.; Ratner, M. A.; Shriver, D. F. J . Electrochem.
Soc. 2003, 150, A1090.
(17) Doyle, M.; Wang, L.;Yang, Z.; Choi, K. J . Electrochem. Soc.
2003, 150, D185.
(18) Sun, X. G.; Angell, C. A. Solid State Ionics 2004, in press.
(19) Sun, X. G.; Liu, G.; Xie, J .; Han, Y.; Kerr, J . B. Solid State
Ionics 2004, in press.
(20) Sun, X. G.; Reeder, C.; Kerr, J . B. Macromolecules 2004,
37, 2219.
(21) Marchese, L.; Andrei, M.; Roggero, A.; Passerini, S.; Pros-
peri, P.; Scrosati, B. Electrochim. Acta 1992, 37, 1559.
(22) Xia, D. W.; Soltz, D.; Smid, J . Solid State Ionics 1984, 14,
221.
(23) Sun, X. G.; Hou, J .; Kerr, J . B., to be published.
(24) Kerr, J . B.; Sloop, S. E.; Liu, G.; Han, Y. B.; Hou, J .; Wang,
S. J . Power Sources 2002, 110, 389.
in the potential with continued cycling. After the cell
cycling was resumed, it quickly failed and signs of
polarization and concentration relaxation were ob-
served. This suggests that some small, mobile anionic
species were produced, and covalent bond breaking was
confirmed from the postmortem gas chromatography
analysis of the dichloromethane extract of the cycled SIC
in which the side chain fragment (pentaethylene glycol
monomethyl ether) and further decomposed smaller
fragments (tetraethylene glycol monomethyl ether and
triethylene glycol monomethyl ether) were observed.
Previously with the polyacrylate SIC’s, such decomposi-
tion was measured qualitatively.20 In this case, the
decomposition was measured quantitatively by use of
an internal standard, and the result showed that the
decomposed product was approximately 1.3 wt % of the
total amount of the SIC.
The postmortem analysis of the cycled Li/Li sym-
metric cells suggests that even the polyepoxide ethers
are not stable to metallic lithium and may require the
use of carbon anodes as used in lithium ion batteries.
The instability may be related to the large interfacial
impedances observed at the lithium electrodes which
are orders of magnitude higher than those observed
with binary salt polymer electrolytes.26 Such large
impedances prevent the use of the cells at practical rates
and may be attributable to the lower conductivity of the
SIC which may be exacerbated by inhibition of the
polymer motion at the electrode surface. Whether this
is simply due to spatial inhibition or is compounded by
some form of self-ordering at the interfaces is an
(25) Doeff, M. M.; Edman, L.; Sloop, S. E.; Kerr, J . B.; De J onghe,
L. C. J . Power Sources 2000, 89, 227.
(26) Persi, L.; Croce, F.; Scrosati, B.; Plichta, E.; Hendrickson,
M. A. J . Electrochem. Soc. 2002, 149, A212.
MA049331C