Electrochimica Acta 48 (2003) 2029 "/2035
www.elsevier.com/locate/electacta
Polymer electrolytes from plasticized polyMOBs and their gel forms
Wu Xu, C. Austen Angell *
Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, USA
Received 19 May 2002; accepted 8 October 2002
Abstract
Plasticized and crosslinked versions of poly(lithium oligoetherato mono-oxalato borate), called lithium polyMOBs, have been
studied. In heavily plasticized forms of both polyMOB, and a LiBH4-crosslinked polyMOB, the ionic conductivity reaches 10 !3 S
cm !1 at room temperature while single ion conductivity is automatically retained. The electrochemical stability window is up to 5 V,
for both stainless steel (SS) electrodes. The plasticized forms are fluid, not a gel, due to the low molecular weight of the polyanions.
Freestanding gel electrolytes with high single ionic conductivity of 10 !4 S cm !1 at ambient can be obtained by incorporation of
high molecular weight poly(methyl methacrylate) (PMMA) into the solution. Electrochemical cells using these electrolytes will not
suffer from concentration polarization.
# 2003 Elsevier Science Ltd. All rights reserved.
Keywords: Polyanionic electrolytes; PolyMOBs; Gel electrolytes
1. Introduction
Solvent-free polymer electrolytes have ionic conductivities in the range 10 !8 "/10 !4 S cm !1 at ambient
temperatures*/generally too low for use in ambient
temperature electrochemical devices. One simple way to
improve the conductivity is to add molecular solvents
into the polymer electrolytes. Such gel type electrolytes
have now found general application, particularly in
lithium ion rechargeable batteries [1 "/7]. All such
electrolytes to date have had conductivities dominated
by anions and the lithium ion transference number (tLi #)
has proved to be far below 0.5 [2,3].
Recently we reported a new polyanionic electrolyte,
poly(lithium oligoetherato mono-oxalatoborate) named
polyMOB, i.e. P(LiOEGn B) where n represents the
number of repeating oxyethylene units between the inchain anionic moieties. The anionic moieties are very
weakly coordinating [8,9]. These polyanionic materials
showed both high ionic conductivity (for a polymer
electrolyte), 10!5 S cm !1 at 25 8C, and high electrochemical oxidation potential, up to 4.5 V versus Li #/Li.
PolyMOBs were synthesized via simple two-step reac* Corresponding author.
E-mail address: [email protected] (C.A. Angell).
tions from very cheap materials and are benign to the
environment. Since the anions are fixed onto the
polymer chains they naturally have tLi# $/1.
In the continuation of this work, we have studied the
plasticized versions of these polyanions and their crosslinked versions. The plasticizers used are common
solvents such as ethylene carbonate (EC), propylene
carbonate (PC), dimethyl carbonate (DMC) and 1,2dimethoxyethane (DME). The effect of plasticizers on
the ionic conductivity will be discussed. Since no other
lithium salts have been added into these plasticized
polyelectrolytes, tLi # must remain at unity, hence cells
using these electrolytes will not suffer from concentration polarization.
2. Experimental section
2.1. LiBH4 crosslinked polyanion electrolytes
P(LiOEGn B) with different n values was dissolved in
anhydrous THF and cooled in acetone-dry ice bath
(!/78 8C). An amount of LiBH4 in THF solution,
estimated to react with all residual chain ends, was
dropwise added into the above solution with vigorous
stirring. After addition, the solution was stirred at
0013-4686/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0013-4686(03)00182-8
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!/78 8C for 2 h and then at room temperature overnight.
The solvent was then evaporated at reduced pressure
and the residual polymer was dried in a vacuum oven at
ca. 70 8C for 48 h.
2.2. Preparation of plasticized electrolytes
Different amount of P(LiOEGn B) or crosslinked
P(LiOEGn B) with different n values was weighed into
a 2 ml volumetric tube in a dry glove box filled with
purified nitrogen. Different plasticizers such as pure EC
and PC, or plasticizer mixtures such as EC "/PC (1:1 by
mol, 1:1 by wt. and 4:1 by wt.), EC "/DMC (1:2 by wt.)
and PC "/DME (1:1 by wt.) were added to dissolve the
polyMOB. The compositions are recorded as weight
percent plasticizer in some cases, and as molar concentration, mol l !1, in others as indicated in the figure
captions. The solutions were stored in sealed containers
for conductivity and electrochemical measurements.
2.3. Preparation of PMMA gel electrolytes
The gel electrolytes were prepared in a dry glove box
filled with purified nitrogen. The macromolecular salt
polyMOB was dissolved in a certain amount of EC "/PC
(1:1 by wt.) mixture in a vial. A quantity of poly(methyl
methacrylate) (PMMA) was added. The vial was sealed
and heated to about 130 8C with occasional shaking till
completely homogenized to a clear viscous mass. The
solution was cooled to room temperature and the
fluidity of the solution was evaluated visually. If the
mixture did not flow, even when the vial was left
inverted for some time, the solution was re-heated to
about 130 8C and the hot viscous liquid was pressed
between two stainless steel (SS) plates covered with
Teflon films. After cooling, the self-standing membrane
formed was pealed off.
3. Results and discussion
3.1. Plasticization of polyMOBs by EC "/PC mixture
solvents
Fig. 1 shows the temperature dependence of ionic
conductivity of P(LiOEG3B) plasticized by 1:1 (mol)
EC "/PC mixture. All exhibit super-Arrhenius behavior
as expected for system with T !/Tg. The same is found
for other plasticized polyMOBs with n :/5, 9 and 14.
The isothermal conductivities at room temperature
for this series are plotted in Fig. 2 against lithium ion
concentration. Comparison is made with the conductivity of simple lithium bis(oxalato)borate (LiBOB) [10]
solutions in PC. The important thing to note is that
when EC "/PC is used as plasticizer, the overall conductivity is highest for the polymer with the shortest
spacing, n $/3. s25 8C $/10 !3.40 S cm!1 was obtained for
90 wt.% EC "/PC plasticized P(LiOEG3B). However, in
the vicinity of the maximum there is little difference
between n $/3 and n :/5 spacer cases. The maximum
conductivity at room temperature is located at [Li #]$/
0.4 mol l !1. A factor of ten separates the highest 25 8C
conductivity from that obtainable with free salt solution.
Considering that the polyanion mass is large enough so
to render it immobile in the solution, resulting in the
conductivity being due entirely to Li # cations, these
results are considered encouraging.
The effect of different EC "/PC plasticizer compositions on the conductivity of P(LiOEG5B) was also
studied (Fig. 3). As for the solutions, it is seen that for
T !/25 8C the pure EC plasticized polyMOB has the
highest conductivity, while the pure PC plasticized
2.4. Measurements
All the conductivities of the viscous mixtures and the
gel electrolytes in this work were measured by complex
impedance spectroscopy during slow cooling from 120
to !/50 8C, using dip-type conductance cells with cell
constants between 0.3 and 0.7, calibrated with a 0.1 m
KCl aqueous solution. The automated impedance
spectroscopy system has been described in previous
papers [10,11].
The electrochemical stability of the plasticized electrolytes was measured by cyclic voltammetry, using a
three-electrode dip type cell, as described previously [10].
Fig. 1. Temperature dependence of ionic conductivity of EC "/PC (1:1
by mol) plasticized P(LiOEG3B) with different EC "/PC content which
is shown in the legend as weight percentage.
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moves to lower temperatures as PC is added, but only at
the cost of conductivity decreases.
3.2. Plasticization of polyMOBs by other lower viscosity
solvents
Fig. 2. The room temperature conductivity of P(LiOEGn B)s with
different anionic spacings n$/3, 5, 9 and 14, dissolved in EC "/PC (1:1
by mol) plasticizer as a function of lithium concentration. Comparison
is made with the conductivity of simple LiBOB solutions in PC. Note
that the lowest point of the conductivity "/concentration curve for each
n value is corresponding to the polyMOB without any plasticization.
P(LiOEG5B) has the lowest conductivity. The sharp
drop of ionic conductivity of EC plasticized
P(LiOEG5B) is due to the crystallization of EC. This
The conductivity can be increased above the maximum value in Fig. 2 especially by use of non-carbonate
plasticizers, e.g. DME. Fig. 4 shows the isothermal
conductivity at 25 8C of PC "/DME (1:1 by wt.) plasticized P(LiOEGn B) with n $/3, 5 and 9. It is found that
ionic conductivity as high as 10!3 S cm !1 at room
temperature can be obtained for polyMOBs with short
spacing of n $/3 and n :/5. Such conductivity is high
enough for the polyMOBs to be applied in practical
electrochemical devices such as lithium rechargeable
batteries. Cells using these electrolytes will not suffer
from concentration polarization, hence should serve
well for higher power applications that are usually
limited by concentration gradients as the cathode"/
electrolyte interface.
EC "/DMC (1:2 by wt.) and pure PC were also used to
plasticize the n $/3 polyMOB, i.e. P(LiOEG3B). Fig. 5
compares the concentration dependence of 25 8C conductivity for plasticization by the above-mentioned four
plasticizers or plasticizer mixtures for this case. It is seen
that the EC "/DMC mixture is also a favorable plastici-
Fig. 3. Temperature dependence of ionic conductivity of plasticized P(LiOEG5B) by different EC "/PC compositions. The plasticizer content is 80
wt.% in all plasticized electrolytes.
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conclude that polyMOB dissociation remains high in
DME- and DMC-containing solutions although either
DME or DMC has a rather low dielectric constant
(o25 8C is 6.99 for DME and 3.1 for DMC) compared
with EC (o40 8C is 89.6) and PC (o25 8C is 64.4). Evidently
the decreased viscosity is the dominant effect on the high
conductivity of the DME- and DMC-containing solutions plasticized systems (h25 8C is 0.42 for DME, 0.59
for DMC, 2.51 for PC and h40 8C of EC is 1.85). On the
other hand, the interesting variation of the conductivity
with salt concentration for the EC "/DMC plasticized
system, which we have not seen before, must be due to
the competition between the dissociation and viscosity
effects, coupled to the non-ideal mixing of EC with
DMC (due to their very different dielectric constants).
Usually s versus c plots are usually smooth curves
passing through a maximum (as for n :/9 in Fig. 4).
Fig. 4. The room temperature conductivity of P(LiOEGn B)s with
different anionic spacings n$/3, 5 and 9, dissolved in PC "/DME (1:1
by wt.) plasticizer mixture with the variation of lithium concentrations.
Comparison is also made with the conductivity of simple LiBOB
solutions in PC. Note that the lowest point of the conductivity "/
concentration curve for each n value corresponds to the plasticizerfree polyMOB.
3.3. Electrochemical properties
The electrochemical properties of EC "/PC plasticized
polyMOB were investigated using cyclic voltammetry.
Figs. 6 and 7 show lithium deposition-stripping process
and electrochemical oxidation of EC "/PC (1:1 wt.)
plasticized P(LiOEG3B) on the different working electrodes platinum (Pt), SS, aluminum (Al) and copper
(Cu), respectively. Except for Pt, all these electrodes
show very good lithium deposition-stripping characteristics. The cyclic voltammogram of P(LiOEG3B)"/EC "/
PC on Pt shows two oxidation peaks between 0 and 1 V,
presumably due to the usual Li "/Pt alloy formation.
The electrochemical oxidation voltages of the plasticized polyMOB is 4.50 V versus Li #/Li for Pt, 4.95 V
for SS, 4.30 V for Al and 3.67 V for Cu, respectively,
according to Fig. 7. It is seen that Al and Cu could be
used as the collector electrodes for anode and cathode,
respectively.
3.4. Crosslinked polyMOBs
Fig. 5. The room temperature conductivity of P(LiOEG3B) plasticized
by different solvents and solvent mixtures with the variation of lithium
concentrations. Comparison is also made with the conductivity of
simple LiBOB solutions in PC. Note that the common low point of the
conductivity "/concentration curve for each plasticizer corresponds to
the polyMOB without any plasticization.
zer as well as the PC "/DME mixture, yielding the room
temperature conductivity very close to 10!3 S cm !1.
Optimization of the EC "/DMC ratio may well yield
s25 8C !/10!3 S cm !1.
From the fact that the ionic conductivity of PC "/
DME and EC "/DMC plasticized P(LiOEG3B) is higher
than that of EC "/PC and PC plasticized electrolyte, we
As mentioned in our previous report [9], polyMOBs
have molecular weight below 5000 Da due to the
exponentially decreasing probability of chain-end with
chain-end reaction. The residual terminal !/OH’s can,
however, react with metallic Li when the polymer is used
in lithium ion batteries, causing poor cell performance.
In this work, we have taken advantages of the chain
ends to crosslink the original polyMOB chains and
convert sticky liquids into stiff rubbers. This is achieved
by using a reaction with a stronger driving force than
that of the original polymerization reaction. This is the
reaction between the !/OH chain ends and the BH4!
anion which joins four chain-ends at a network center,
as shown in the following reaction. The crosslinked
versions of P(LiOEGn B)s have much larger molecular
W. Xu, C.A. Angell / Electrochimica Acta 48 (2003) 2029 "/2035
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Fig. 6. Lithium deposition-stripping process of 80% EC "/PC (1:1 by wt.) plasticized P(LiOEG3B) on platinum, SS, aluminum and copper electrodes.
weights and accordingly different rheological properties.
4OH#BH!
4
0
B(O!)!
4 #4H2 "
On the other hand, due to the reducing effect of BH4!
anion, part of the C #/O groups on the mono-oxalato
borate rings may be reduced to C !/OH groups, even at
very low temperature (!/78 8C). However the ring
structure has not been broken and the C !/OH groups
formed in the above process will immediately react with
the BH4! anion to form new chains. The decrease in the
number of C #/O groups on the mono-oxalato borate
rings decreases the polarity of the ring structure which in
turn decreases the number of free cations. Thus, the
conductivity goes down.
Fig. 7. Electrochemical oxidation voltage of 80% EC "/PC (1:1 by wt.) plasticized P(LiOEG3B) on platinum, SS, aluminum and copper electrodes.
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Fig. 8. Temperature dependence of ionic conductivity of LiBH4crosslinked P(LiOEGn B) with different length of ethyleneoxy repeating units, compared with that of non-crosslinked P(LiOEGn B).
Extensive crosslinking also severely decreases the
ionic conductivity of the polyelectrolytes, as shown in
Fig. 8, due to the decrease in segmental mobility caused
by the decrease in configurational entropy. However,
such decrease in conductivity can be compensated by
plasticization.
The plasticized crosslinked polymers are gel-like
materials at very low plasticization (less than 10 "/20
wt.% plasticizer content depending on the oligoether
spacer length), viscous liquids at moderate plasticization
(20 "/50 wt.% plasticizer content) and again fluid in
heavily plasticized form (above 50 wt.% plasticizer). If
the crosslinking is carried further by addition of excess
LiBH4, even highly plasticized polymers (up to 50 wt.%
plasticizer) remain solid-like (gels).
The ionic conductivity of the plasticized crosslinked
polyMOBs P(LiOEG5B) was studied as a function of
temperature and the results are shown in Fig. 9, where
they are compared with the conductivity for polymers
before crosslinking. It is seen that the gelled crosslinked
polyMOB should have very low ambient conductivity,
less than 10!6 S cm !1. However, a high conductivity of
10!3.08 S cm!1 at room temperature has been achieved
in a 90 wt.% EC "/PC (all carbonates) plasticized crosslinked electrolyte, which is twice the maximum in
plasticized non-crosslinked cases. These show good
potential for applications in lithium ion rechargeable
batteries.
For heavily plasticized polyanionic electrolytes, the
LiBH4-crosslinked electrolytes have slightly higher conductivities than non-crosslinked cases. This may be due
to their higher lithium ion concentration, which causes
from both polyMOB and the crosslinker LiBH4.
Fig. 9. Temperature dependence of ionic conductivity of EC "/PC (1:1
by wt.) plasticized LiBH4-crosslinked P(LiOEG5B) with different
content of plasticizer. The 30 wt.% EC "/PC plasticized electrolyte is
a very viscous liquid at room temperature and when the EC "/PC
content is above 50 wt.% the electrolyte is fluid.
3.5. Gel electrolytes
The liquid state of the heavily plasticized (non-crosslinked) polyanionic electrolytes can be transformed into
gel electrolytes by adding certain amount of high
molecular weight polymers. In this work, PMMA with
molecular weight of 996 000 (from Aldrich) and high
boiling point plasticizer mixture EC "/PC (1:1 by wt.)
were employed. It is known from the above results that
the maximum conductivity at room temperature is
10!3.40, i.e. 4.0 %/10!4 S cm !1, for 90 wt.% EC "/PC
plasticized P(LiOEG3B). Therefore, the gel formation
Fig. 10. Temperature dependence of ionic conductivities of n %
PMMA "/(100!/n )% [10% P(LiOEG3B) "/90% EC "/PC] gel electrolytes,
compared with that of 10% P(LiOEG3B) "/90% EC "/PC (1:1 by wt).
W. Xu, C.A. Angell / Electrochimica Acta 48 (2003) 2029 "/2035
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4. Conclusions
In heavily plasticized forms of polyMOB and LiBH4crosslinked polyMOB, the ionic conductivity reaches
10!3 S cm !1 at room temperature. The electrochemical
stability window is up to 5 V on SS electrodes. However,
these plasticized electrolytes are not a gel, but fluid
because the polyanions are of low molecular weight.
This may be compensated by incorporation of high MW
PMMA into the solution. Then freestanding gel electrolytes can be obtained and the single ionic conductivity is
still as high as 2%/10!4 S cm !1 at room temperature.
By choosing different plasticizers or plasticizer mixtures
and different kind of high molecular weight polymers,
and optimizing the composition of electrolyte systems,
the gel electrolytes with high single ionic conductivity
and good cell performance can be obtained.
Fig. 11. Effect of PMMA content on isothermal conductivities of
PMMA "/P(LiOEG3B) "/EC "/PC system, which was formed by addition of PMMA into 90% EC "/PC plasticized P(LiOEG3B) solution.
and conductivity of the P(LiOEG3B) "/EC "/PC system
were studied by adding different PMMA content into
the 90 wt.% EC "/PC plasticized P(LiOEG3B) solution. It
is found that when PMMA content is less than 13 wt.%,
such as 2, 4, 7 and 10 wt.% studied, the system is not a
homogeneous but a phase separation mixture with the
polymer coagulation floating in the solution. When the
PMMA content is higher than 13 wt.%, a gel is formed.
Fig. 10 shows the temperature dependence of the
PMMA "/P(LiOEG3B)"/EC "/PC gels and Fig. 11 shows
the isothermal conductivity of the gel system with the
content of PMMA. When 13 wt.% PMMA is added, the
room temperature conductivity decreases from 10 !3.40,
i.e. 4.0 %/10!4 S cm !1 to 10 !3.66, i.e. 2.2 %/10!4 S
cm !1. It should be noted that this gel electrolyte is
actually a single lithium ionic conductor. Such a
conductivity value is relatively high for single ion
conductors. Therefore, it has been shown that this
kind of gel electrolytes can be used in lithium and
lithium ion batteries requiring single cation conduction.
Acknowledgements
This work was supported by Mitsubishi Chemical
Corporation of Japan.
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