Electrochimica Acta 48 (2003) 2037 !/2045
www.elsevier.com/locate/electacta
‘‘PolyMOB’’! lithium salt complexes: from salt-in-polymer to
polymer-in-salt electrolytes
/
Wu Xu, Li-Min Wang, 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
Lithium polyMOB has been investigated as the polymer in a polymer-in-salt type electrolyte incorporating the salts lithium
perchlorate (LiClO4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium tetrafluoroborate (LiBF4). While all salts
give rubbery solids at high salt contents, only LiClO4 provides high conductivity because only in the case of LiClO4 is the lithium
cation motion highly decoupled from the structural relaxation. The crystallization of the salt at high salt contents prevents a
favorable combination of mechanical and electrical properties, but the system provides an excellent example of the principle of the
polymer-in-salt ionic rubber electrolyte and the factors determining its performance.
# 2003 Elsevier Science Ltd. All rights reserved.
Keywords: PolyMOB; Salt-in-polymer electrolytes; Polymer-in-salt electrolytes
1. Introduction
Polymer !/salt complexes or ‘‘salt-in-polymer’’ electrolytes have been studied extensively since the suggestion
by Armand et al. [1] in 1978 that they could be used as
solid electrolytes in electrochemical devices. In these
electrolytes a low polymer glass transition temperature
is required because the ionic mobility is determined by
the polymer segmental motion. The glass transition
temperature (Tg) of this type electrolyte increases
rapidly with salt concentration in the domain. The
ambient conductivity of solvent-free salt-in-polymer
electrolyte is in the range of 10 ! 8 !/10 ! 4 S cm ! 1 and
the cation transport number (t ") is far below 0.5 [2,3].
The observation that many polymer"/salt mixtures
must exhibit a maximum value of Tg at intermediate salt
contents lead to the suggestion [4] that a second domain
of high conductivity may exist at salt-rich compositions
particularly if, in the high salt region, the lithium cation
motions are highly decoupled from the anion matrix.
Thus the conductivity can in principle become much
higher than that of any salt-in-polymer electrolytes at
* Corresponding author.
E-mail address: [email protected] (C.A. Angell).
the same temperature, while adding the advantage that
the lithium ion transport number approaches unity [4,5].
Since only a small amount of high-molecular-weight
polymer is needed to create a solid in polymer-in-salt
electrolytes, these materials should combine the high
conductivity of the fast-ion-conducting glasses and the
good mechanical properties of the polymer. They are
actually rubbery versions of glassy electrolytes. Many
examples of polymer-in-salt type cation-conducting
polymer electrolytes have now been reported [5 !/16],
though cases suitable for battery applications remain to
be developed.
Recently we prepared a novel polyanionic electrolyte,
poly(lithium oligoetherato mono-oxalato orthoborate)
called polyMOBs i.e. P(LiOEGnB) where n represents
the repeating number of oxyethylene units [17]. The
polyMOB with 14 oxyethylene unit spacer showed high
single ionic conductivity of 10 ! 5 S cm ! 1 at 25 8C, and
wide electrochemical stability above 4.5 V versus Li "/
Li.
In a companion paper [18], we have studied the effect
of plasticization by organic solvents on the ionic
conductivities and electrochemical properties of polyMOBs, and also investigated gel forms prepared using
high molecular weight poly(methyl methacrylate)
(PMMA). The conductivity increases greatly on plasti-
0013-4686/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0013-4686(03)00183-X
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W. Xu et al. / Electrochimica Acta 48 (2003) 2037 !/2045
cization with molecular liquid solvents while the single
ion character is maintained since the polyanions cannot
move significantly in the resulting gel. In the present
paper, we report the results of plasticizing the polyMOB
with appropriate lithium salts like LiClO4. The thermal
behavior and ionic conductivities of these polyMOB !/
lithium salt solutions, from low to high salt content, is
reported, and the importance of the particular salt type
used in the plasticization is made clear.
2. Experimental section
2.1. Materials
PolyMOBs with different length of oxyethylene
spacer, i.e. P(LiOEGnB) where n#/3, 5, 9 and 14, were
prepared and dried following the procedures described
in ref. [17]. The numbers 5, 9 and 14 are used to
represent products obtained from syntheses using PEG
200, 400 and 600, respectively, and are not be thought as
integral numbers. The lithium salts, lithium perchlorate
(LiClO4) and lithium tetrafluoroborate (LiBF4) from
Aldrich and lithium bis(trifluoromethanesulfonyl)imide
(LiTFSI) from 3 M as a gratis, were used as received.
2.2. Preparation of Li salt!/P(LiOEGnB) solutions
For polyMOBs with spacer of n :/5, 9 and 14, the
solutions were prepared by directly dissolving the Li salt
in the polyMOB at high temperatures. Measured
amounts of polyMOB and Li salt were weighed into
Pyrex vials in a VAC dry box filled with purified
nitrogen. Compositions are reported as mole fraction
of the lithium salt, using the definition Xsalt #/moles salt/
(moles salt"/moles boron). Thus the physical concentration, in mol l ! 1, of Li " will be lower in the solutions
with polyMOBs of higher n value at the same value of
Xsalt. The mole fraction of the Li salt in the solution (XLi
salt) was varied from 0.1 to 0.9 in 0.1 increments. The
vials were sealed with caps and heated in an oven at
around 100 8C for 4 days, during which time the vials
were stirred occasionally (by turning them upside
down), till homogeneous solutions were obtained.
For polyMOB with n #/3, the direct dissolution
method was unsatisfactory as the temperatures needed
to dissolve the LiClO4 completely were too high. In
these cases the samples were prepared by an organic
solution intermediate route. The mixture of LiClO4 and
P(LiOEG3B) with XLiClO4 #/0.1 !/0.9 with 0.1 increments, was first dissolved in anhydrous acetonitrile to
obtain a homogeneous solution. The solvent was then
removed in two stages. The major part was removed by
evaporation on a rotary evaporator under reduced
pressure, and then the residual almost solid material
was further dried in a vacuum oven at 100 8C for 1 week.
We also prepared LiClO4 !/P(LiOEG5B) samples using
the same solution method, in order to compare the effect
of the two preparation methods.
The polymer"/salt materials obtained in this work
ranged from light brown viscous liquids to hard glassy
materials (details below).
2.3. Calorimetric and conductimetric studies
The thermal behavior of the complexes was studied by
differential scanning calorimetry (DSC), in the temperature range of !/100 to 80 8C, using a Perkin !/Elmer
DSC-7 in sub-ambient mode with liquid nitrogen
reservoir. The instrument temperature scale was calibrated using the crystal !/crystal transition of cyclopentane (!/151.16 8C) and melting of indium ("/156.60 8C).
Samples were sealed in aluminum pans, purged with
helium gas, and scanned from !/100 to 80 8C. The
heating rate was 10 K min ! 1. Glass transition temperatures (Tg) were recorded as the onset of the heat capacity
jump on the thermograms during up-scan.
The conductivities of the complexes were measured by
a.c. impedance measurements during cooling from
120 8C to ambient or sub-ambient temperatures, using
(i) dip-type conductance cells containing two parallel
platinum discs when the samples were viscous liquids, or
(ii) block-type cells with two stainless steel rod electrodes compressing a circular disc-shaped film sample,
when the complexes were glassy or stiff rubbery
materials. The measurements were carried out on a
HP 4192A LF Impedance Analyzer in a frequency range
from 5 Hz to 13 MHz. The cell constants of the dip-type
cells were from 0.5 to 0.8 cm ! 1, calibrated by a 0.1 m
KCl aqueous solution. The cell constants of the block
type cells were obtained by measuring the thickness and
radii of the complex films, which typically were about
0.3 cm ! 1. In order to get good contact of the solid films
with the electrodes, the cells were pre-heated in an oven
to a temperature of about 100 8C. Test measurements
during heating and cooling at constant temperature
yielded the same results within the data noise.
3. Results and discussion
3.1. Effect of preparation method
Despite the fact that there are noticeable differences in
the color of the samples obtained by the two procedures
described above, there are generally no systematic
differences in the physical properties provided that the
drying procedure for the acetonitrile is strict enough. In
Fig. 1 the effects of the two different sample preparation
methods on the glass transition temperature and ionic
conductivities of the LiClO4 !/P(LiOEG5B) solutions are
compared. The values of conductivity and Tg for pure
W. Xu et al. / Electrochimica Acta 48 (2003) 2037 !/2045
2039
3.2. Effects of different length of oxyethylene spacer in
the polyMOB component
Fig. 1. Effect of the two preparation methods on the glass transition
temperature and ionic conductivities of LiClO4 !/P(LiOEG5B) complexes. Conductivities lower than 10 $6.4 Scm $1 were obtained by
extrapolations of higher temperature data, using fitted VFT equation.
Also, values of Tg and log s for pure LiCIO4 are obtained by
extrapolations of binary solution data in ref. [19] and refs. [4,5],
respectively.
LiClO4 seen in Fig. 1 were obtained by extrapolation, as
described in refs. [4,5,19]. Since the conductivities less
than 10 ! 6.4 S cm ! 1 cannot be measured directly with the
present instrumentation, data points below this value
were obtained by extrapolation of the conductivitytemperature plots using best fitting to the Vogel !/
Fulcher!/Tamman (VFT) equation.
It is seen from Fig. 1 that the Tg values are almost the
same for the samples with the same LiClO4 content
(XLiClO4), except for the case of XLiClO4 #/0.7 where the
sample from direct dissolution has a Tg about 20 8C
lower than that from the acetonitrile solution preparation. This is difficult to understand since any residual
solvent (acetonitrile) in the solution would be expected
to lower Tg rather than raise it. The difference in the
conductivities for the different preparations is in the
direction expected from the Tg difference. The stepwise
increases in Tg are unexpected but they are reproduced
by samples prepared by each method. The steps in Tg
are generally not reflected in the conductivity behavior,
and are not seen for other n values.
Overall, the variation of isothermal conductivity with
salt concentration for the samples obtained by direct
dissolution shows the form expected from the behavior
of Tg, i.e. the initial decrease of Tg with salt addition is
accompanied by increase in conductivity. The conductivities then decrease as Tg increases and finally increases
again after the Tg passes through its maximum. This
occurs near XLiClO4 #/0.6 after which Tg decreases to the
value given elsewhere for pure LiClO4. Details will be
given in Section 3.3.
Table 1 lists the physical appearances and glass
transition temperatures (Tg) of the LiClO4 !/
P(LiOEGnB) solutions from direct dissolution (n :/5,
9, and 14) and from the acetonitrile solution method
(n #/3). With increasing Li salt content, the solutions
from polyMOB with very short EO spacer i.e. n#/3
change from a rubbery material to a stiff rubbery solid,
then a glassy material and finally a stiff rubbery solid
again. For solutions from polyMOBs with long EO
spacer such as n :/9 and 14, they show changes with salt
content from a very viscous liquid to a sticky rubber, a
soft rubber, then a rubbery material and finally a stiff
rubbery solid. For solutions from polyMOBs with
moderate EO length e.g. n :/5, the physical appearance,
with increasing XLiClO4, changes from a sticky rubber to
a rubbery solid to a stiff rubber. The high salt content
electrolytes evidently would be favored by good mechanical properties.
Fig. 2 shows the variations of Tg with LiClO4 content
for the systems of LiClO4 !/P(LiOEGnB) of variable n . It
is seen that for each P(LiOEGnB) there is a maximum
value of Tg. The maximum shifts to the left with
decreasing n, which is consistent with the idea that it
is the transient crosslinking of oxyethylene chains by
interaction with Li " that raises Tg. The smaller the
linker the lower in concentration the maximum effect is
reached and the sooner the subsequent decrease towards
the low Tg(LiClO4) can commence. For n #/3, the
maximum Tg appears at XLiClO4 #/0.5. For n :/5, the
maximum Tg is located at XLiClO4 #/0.6. The maximum
Tg is found at XLiClO4 :/0.8 for both n :/9 and 14.
The stepwise variation in Tg for the case n :/5 is not
seen with other polymers, but it is found for glasses
prepared by independent methods. On the other hand,
the experimental error of Tg values from our DSC
measurement is within 9/0.2 8C. Therefore, the description of ‘‘stepwise’’ is not just within the experimental
error, but apparently true and observable. However, its
explanation must await further studies.
The temperature dependence of ionic conductivities of
the complexes with different length of spacer n is shown
in Figs. 3!/6. The curved plots are consistent with the
usual VFT behavior, which applies to almost all liquids
approaching their glass temperatures. The curvature is
greatest in the case containing the least polyether (n #/3,
XLiClO4 #/0.8), which emphasizes that this is not merely
a property of polymeric systems.
Figs. 7 !/10 show the variations of isothermal conductivities with LiClO4 mole fraction at different
temperatures for the solutions, and also the variation
in Tg. Data points below 10 ! 6.4 S cm ! 1 were obtained by
extrapolation of the conductivity plots shown in Figs.
3 !/6 using best fitting to the VFT equation. Although
W. Xu et al. / Electrochimica Acta 48 (2003) 2037 !/2045
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Table 1
Physical appearances and glass transition temperatures of LiClO4 !/P(LiOEGnB) complexes from direct dissolution except for n#/3
XLiClO4 (mol.%)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Appearance and Tg (in parentheses)
n#/3
n :/5
n :/9
n :/14
Yellow brown, rubber
($/0.3 8C)
Yellow brown, rubber
(1.6 8C)
Yellow brown, rubber
(9.41 8C)
Brown, stiff rubber
(15.8 8C)
Yellow brown, glass
(22.9 8C)
Yellow brown, glass
(31.7 8C)
Yellow brown, glass
(26.3 8C)
Yellow brown, stiff rubber
(15.9 8C)
Dark brown, stiff rubber
($/5.3 8C)
Dark brown, stiff rubber
(not measured)
($/16.1a)
Yellow brown, sticky
rubber ($/14.6 8C)
Brown, rubbery solid
($/20.7 8C)
Brown, rubbery solid
($/7.9 8C)
Brown, rubbery solid
($/5.7 8C)
Brown, rubbery solid
(7.0 8C)
Brown, rubbery solid
(10.0 8C)
Dark brown, rubber
(24.2 8C)
Dark stiff rubber
(13.3 8C)
Dark stiff solid
($/11.4 8C)
Dark stiff solid
(not measured)
($/16.1a)
Yellow brown, very viscous
liquid ($/44.4 8C)
Yellow brown, very viscous
liquid ($/41.9 8C)
Yellow brown, very viscous
liquid ($/38.1 8C)
Yellow brown, very viscous
liquid ($/33.3 8C)
Yellow brown, soft rubber
($/28.3 8C)
Brown, rubber
($/17.7 8C)
Brown, rubber
($/6.2 8C)
Dark brown, rubber
(11.3 8C)
Brown, stiff rubber
(25.7 8C)
Dark brown, stiff rubber
($/10.3 8C)
($/16.1a)
Yellow, viscous liquid
($/49.4 8C)
Yellow, viscous liquid
($/46.4 8C)
Yellow brown, very viscous
liquid ($/42.4 8C)
Yellow, very viscous liquid
($/39.0 8C)
Yellow brown, sticky solid
($/33.8 8C)
Yellow brown, sticky solid
($/25.5 8C)
Brown, rubbery solid
($/14.8 8C)
Brown, rubbery solid
($/2.5 8C)
Brown, rubbery solid
(19.1 8C)
Dark brown, rubber
(6.0 8C)
($/16.1a)
Fig. 2. Variations of Tg with LiClO4 mole percentage for the systems
of LiClO4 !/P(LiOEGnB).
these extrapolated values are subject to uncertainty, the
variation of the conductivity with salt content at low
temperatures is consistent with that at high temperatures
and is very reasonable.
The solutions of LiClO4 in the shortest spacer
polyMOB, n#/3, illustrates most clearly the important
characteristics of this type of system. The variation of
conductivity with LiClO4 content shows the trend:
Fig. 3. Temperature dependence of ionic conductivity of LiClO4 !/
P(LiOEG3B) complexes with different salt concentration. The samples
were obtained from solution method.
increase, decrease and then increase again. This behavior
can be explained in terms of a balance of the opposing
effects, free ion number and microviscosity, on conductivity. However, the final increase (seven orders of
magnitude at room temperature for 0.5 mole fraction of
LiClO4) is so great that a third influence must be
involved. The initial increase, which occurs despite a
W. Xu et al. / Electrochimica Acta 48 (2003) 2037 !/2045
Fig. 4. Temperature dependence of ionic conductivity of LiClO4 !/
P(LiOEG5B) complexes with different salt concentration. The samples
were obtained from direct dissolution.
weak increase in Tg, could be a free carrier number effect
or a decoupling effect. It is not obvious that a free
carrier number effect should be present since the
polyMOB is already quite rich in Li ". However, when
estimates of the conductivity at Tg are made to assess the
decoupling index (see below) it does not seem to provide
an explanation for an increase in conductivity in this low
salt range. Thus the free carrier number must be
responsible for the initial increases.
When the spacer of the polyMOB is short, the high
salt content solution has higher conductivity than the
low salt content solutions. This must be because the
short spacer polyMOB is less able to chelate the Li ions
Fig. 5. Temperature dependence of ionic conductivity of LiClO4 !/
P(LiOEG9B) complexes with different salt concentration. The samples
were obtained from direct dissolution.
2041
Fig. 6. Temperature dependence of ionic conductivity of LiClO4 !/
P(LiOEG14B) complexes with different salt concentration. The samples were obtained from direct dissolution.
added in the plasticizing LiClO4. Then, beyond
XLiClO4 #/0.2 the increase in Tg causes the mobility,
hence conductivity, to decrease until Tg passes its
maximum. A large composition range is found beyond
the Tg maximum in which the addition of LiClO4
systematically increases the conductivity towards the
extrapolated pure salt limit (see in particular, Fig. 7).
3.3. Onset of decoupling in the polymer-in-salt domain
By combining conductivity and Tg data we can learn
the effect of the structural parameters on the freedom of
Fig. 7. Variations of isothermal conductivities and Tg with LiClO4
mole content for LiClO4 !/P(LiOEG3B) complexes. The samples were
prepared by long vacuum drying of acetonitrile solutions.
2042
W. Xu et al. / Electrochimica Acta 48 (2003) 2037 !/2045
Fig. 8. Variations of isothermal conductivities and Tg with LiClO4
mole content for LiClO4 !/P(LiOEG5B) complexes. The samples were
obtained by direct dissolution of LiClO4 in the hot polymer.
Fig. 10. Variations of isothermal conductivities and Tg with LiClO4
mole content for LiClO4 !/P(LiOEG14B) complexes. The samples were
obtained by direct dissolution of LiClO4 in the hot polymer.
very high, even at Tg where the segmental relaxation
time is ca. 100 s. When Rt is near unity, however, the
conductivity at Tg will be of the order of 10 ! 15 S cm ! 1
To estimate the conductivity at Tg we extrapolate the
data of temperature-dependence of conductivities using
the well-known VFT equation:
s #so exp[$Ds To =(T$To )]
Fig. 9. Variations of isothermal conductivities and Tg with LiClO4
mole content for LiClO4 !/P(LiOEG9B) complexes. The samples were
obtained by direct dissolution of LiClO4 in the hot polymer.
Li " cations to migrate independently of the relaxation
of the remainder of the structure. We obtain this
information from the ‘‘decoupling index’’ Rt which is
defined as the ratio of the structural relaxation time (ts)
to the conductivity relaxation time (ts) [20,21]. At a Tg
where the structural relaxation time is 100 s [22], the
decoupling index is given by the approximate relation
log Rt #/14.3"/log sTg [23]. A high value of Rt means
the ionic motion is only weakly controlled by the
immobile elements of the structure, meaning that the
conductivity is highly decoupled from the segmental
motion of the polymer chains and also from the motion
of anionic groups. In this case the conductivity can be
(1)
where so is the pre-exponent conductivity; To, the
vanishing mobility temperature; Ds is inversely proportional to the ‘‘fragility’’ of the liquid [19], provided
conductivity is coupled to viscosity, i.e. Rt #/1 [24] (see
below). The fragility is a measure of how rapidly the
structure changes with increasing temperature above Tg.
High fragility permits a liquid to be very fluid even when
Tg is relatively high. D is normally in the range from 2 to
20, for polymeric and ionic systems.
Values of the VFT parameters such as so, To and Ds
are summarized in Table 2. Also included in Table 2 are
the differences between Tg and To, the estimated
conductivity of the polymer electrolytes at Tg, sTg, and
the corresponding decoupling indexes Rt. We note that
for XLiClO4 !/0.5 the values of some so are unphysically
large ("/100, the value found for lithium superionic
glasses). This would normally imply some additional
phenomenon is involved in the conductivity-temperature dependence. However, the range of data is so small
for these cases that the application of Eq. (1) is probably
inappropriate. Data fit parameters for these compositions are entered in italics in Table 2 and are disregarded
in the analysis.
Excluding the above data sets, the values of sTg show
some significant trends. They indicate that as the Li "
ion concentration increases, the logarithmic decoupling
index (log Rt) first decreases and then increases to
W. Xu et al. / Electrochimica Acta 48 (2003) 2037 !/2045
2043
Table 2
VFT parameters from best fitting for LiClO4 !/P(LiOEGnB) complexes
n Value
XLiClO4
so (S cm ! 1)
To (K)
Ds
Tg !/To (K)
log sTg
log Rt
3
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.17
0.08
0.20
0.04
0.08
0.66
0.27
5.93
0.66
0.09
0.98
1.64
0.30
0.31
26.7
2.4 %/104
20.9
5.41
0.05
0.21
0.83
0.57
0.96
3.21
12.0
63.7
408
1.01
0.02
0.11
0.14
0.17
0.47
0.93
0.55
5.50
17.1
0.63
4.23
215.0
240.6
242.6
264.0
272.1
246.4
254.5
206.6
234.5
209.9
194.7
201.2
231.9
242.1
199.7
155.4
214.5
206.2
195.8
192.5
184.3
199.3
204.7
195.4
191.9
194.1
186.3
210.3
208.5
197.2
198.1
202.3
197.5
200.7
196.1
205.8
213.5
235.2
233.6
7.02
4.03
4.15
2.92
3.00
5.48
4.06
8.72
3.88
5.93
8.47
8.65
5.46
5.23
12.84
31.24
9.16
8.26
5.00
5.61
7.43
6.18
6.49
8.61
10.80
12.86
16.40
6.59
3.24
4.54
4.79
4.76
5.98
6.54
6.23
8.65
9.68
4.82
2.29
57.9
34.1
40.0
24.9
23.9
58.4
45.0
82.4
33.4
48.6
57.8
64.1
35.6
38.0
83.4
142.0
71.9
78.4
33.4
38.8
50.8
40.5
40.2
60.0
75.1
90.4
112.5
52.6
15.3
29.6
32.7
31.8
41.9
47.0
62.2
64.8
78.8
44.0
23.4
!/12.10
!/13.47
!/11.63
!/14.82
!/15.88
!/10.22
!/10.54
!/8.72
!/12.01
!/12.17
!/12.40
!/11.57
!/15.96
!/15.95
!/11.92
!/10.46
!/10.54
!/8.69
!/14.05
!/12.75
!/11.79
!/13.45
!/14.35
!/11.67
!/10.90
!/10.20
!/9.18
!/11.43
!/20.96
!/14.10
!/13.46
!/13.93
!/12.57
!/12.16
!/8.79
!/11.18
!/10.15
!/11.40
!/9.30
2.20
0.83
2.67
!/0.52
!/1.58
4.08
3.76
5.58
2.29
2.13
1.90
2.73
!/1.66
!/1.65
2.38
3.84
3.76
5.61
0.25
1.55
2.51
0.85
!/0.05
2.63
3.40
4.10
5.12
2.87
!/6.669
0.20
0.84
0.37
1.73
2.14
5.51
3.12
4.15
2.90
5.00
5
9
14
Pure LiClO4
values of order 3!/6 at high LiClO4 contents, with
fluctuations that reflect the uncertainties of Eq. (1)
extrapolations. A rapid increase in log Rt was seen in
other salt-in-polymer electrolytes [17,25] beyond the
transition from salt-in-polymer to polymer-in-salt domains, and is a feature essential for successful polymerin-salt electrolytes. It is only by such decoupling that a
high ambient conductivity can be obtained when glass
temperatures lie above !/50 8C. In the present case the
data show that, to obtain superior conductivites, (s at
25 8C !/10 ! 4 S cm ! 1) the salt would need to remain
uncrystallized up to LiClO4 contents above 90 mol.%.
Unfortunately, this cannot happen with pure LiClO4 as
salt, so the development of successful polymer-in-salt
electrolytes will require the use of eutectic salt mixtures,
or the discovery of new lithium salts that combine noncrystallizing characteristics with high melt conductivity.
This point is emphasized by the results we have obtained
for two other polyMOB systems using different lithium
salts described in the next section.
The variation of Ds value with LiCIO4 content shows
a maximum, and this maximum appears at the salt
content where the maximum Tg is located. A similar and
more precisely defined maximum was observed in an
earlier study of salt-in -polymer systems [26]. There it
was interpreted in terms of increased intermediate range
order due to Li " crosslinking of chains, and the same
interpretation can be applied to the present case.
3.4. Dependence of polymer-in-salt phenomenology on
lithium salt
The dependence of Tg on salt type, in systems
incorporating LipolyMOB as polymer, is shown in
W. Xu et al. / Electrochimica Acta 48 (2003) 2037 !/2045
2044
Fig. 11. Variations of the glass transition temperatures with salt
content for different Li salt !/P(LiOEG5B) complexes.
Fig. 12. Variations of the isothermal conductivities at different
temperatures for different Li salt !/P(LiOEG5B) complexes with salt
content.
Fig. 11. Data are for solutions in the n :/5 polyMOB.
Data for LiClO4 from earlier figures are compared with
those for systems in which the LiClO4 is replaced by
LiTFSI or LiBF4. In each case the Tg values of the pure
Li salts were obtained by an extrapolation method
described in ref. [19].
The Tg plot for the LiTFSI solutions shows monotonous increase with salt content. It is seen that no
maximum exists and Tg is always above ambient.
Accordingly this system can exhibit none of the rubbery
electrolyte properties desirable in a polymer-in-salt
electrolyte.
In the LiBF4 system, the value of Tg for the pure salt
lies well below the Tg maxima of Fig. 2 (and also 25 8C)
so a viable ionic rubber range should exist. Only one
composition has been prepared in this system and it has
a Tg comparable to that of LiClO4 !/P(LiOEG5B)
solutions prepared by the acetonitrile solution route
(Fig. 1).
The variations of the isothermal conductivities for
these solutions of different lithium salts are plotted in
Fig. 12. It is seen that it is only at high salt content that
the LiClO4 !/P(LiOEG5B) system yields the highest
conductivity of these three salt systems. The LiTFSI !/
P(LiOEG5B) system shows a monotonous decrease in
conductivity at ambient temperature after a weak
maximum conductivity at XLiTFSI #/0.1. This variation
in conductivity is as expected from the variation of Tg
with salt content and confirms that LiTFSI cannot
provide the basis for an ionic rubber solid electrolyte.
By employing the VFT equation, we also obtained the
VFT parameters for LiTFSI !/P(LiOEG5B) and LiBF4 !/
P(LiOEG5B) systems and the results are listed in Table
3. Compared with LiClO4 !/P(LiOEG5B) system, these
two systems have lower so and Ds values at each salt
concentration. The LiTFSI !/P(LiOEG5B) also shows
lower decoupling index. For LiBF4 !/P(LiOEG5B) system, the range of conductivity data available (B/1 order
of magnitude) is too small to extract meaningful
parameters.
It cannot be decided from the data in Fig. 12 whether
or not LiBF4-rich solutions have the decoupling char-
Table 3
VFT parameters from best fitting for other Li salt !/P(LiOEGnB) complexes
Salt
XLi
LiTFSI
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.7
LiBF4
salt
so (S cm ! 1)
To (K)
Ds
Tg !/To (K)
log sTg
log Rt
0.09
0.31
0.21
0.66
0.25
2.20
0.11
0.83
0.06
0.45
209.9
225.2
224.8
217.1
228.8
206.6
241.4
224.4
258.5
255.3
5.93
5.25
5.16
6.49
5.20
8.67
4.35
7.19
3.68
4.49
48.6
32.8
34.3
44.4
38.9
62.1
35.0
58.6
32.6
!/6.9
!/12.17
!/16.15
!/15.36
!/13.96
!/13.88
!/12.18
!/13.98
!/12.04
!/13.91
71.59
2.13
!/1.85
!/1.06
0.34
0.42
2.12
0.33
2.26
0.39
85.59
W. Xu et al. / Electrochimica Acta 48 (2003) 2037 !/2045
acteristics needed for successful polymer-in-salt electrolytes, though the high melting point of LiBF4 itself must
eliminate systems based on the pure salt from consideration. It is possible that a system in which the melting
point of LiBF4 is lowered by complexing with suitable
Lewis acids, or mixing with suitable low melting second
components, could provide a suitable liquid salt for
these purposes, and the investigation of this possibility is
being investigated in current work.
4. Conclusions
Using a polyanionic polymer and LiClO4 as ionic
plasticizer, a wide range of ‘‘polymer-in-salt’’ (ionic
rubber) electrolyte behavior has been made available for
study, particularly by using polyMOBs with short
spacer groups. High conductivity and high decoupling
index ca. 106, have been measured at high salt content.
The increase in conductivity is a direct consequence of
decreased glass transition temperature and increased
decoupling index.
These electrolytes are physically robust materials,
with rubbery solid characteristics in most cases because
the glass transition temperature is below ambient
temperature. The mechanical properties of these materials appear to be excellent. If lithium salt that had similar
decoupling characteristics to LiClO4 but lower Tg were
available, a superior solid electrolyte could be obtained.
Acknowledgements
This work was supported by a grant from Mitsubishi
Chemical Corporation of Japan.
2045
References
[1] M.B. Armand, J.M. Chabagno, M. Dulcot, in: Extended Abstracts, The Second International Conference on Solid Electrolytes, St. Andrews, Scotland, September 1978.
[2] P.G. Bruce, M.T. Hardgrave, C.A. Vincent, Solid State Ionics
53 !/56 (1992) 1087.
[3] W. Gorecki, M. Jeannin, E. Belorizky, C. Roux, M. Armand, J.
Phys.: Condens. Matter 7 (1995) 6823.
[4] C.A. Angell, C. Liu, E. Sanchez, Nature 362 (1993) 137.
[5] C.A. Angell, J. Fan, C. Liu, Q. Lu, E. Sanchez, K. Xu, Solid State
Ionics 69 (1994) 343.
[6] J. Fan, R.F. Marzke, C.A. Angell, J. Non-Crystallogr. Solid 172 !/
174 (1994) 178.
[7] K. Xu, C.A. Angell, Mater. Res. Soc. Symp. Proc. 369 (1995) 505.
[8] J. Fan, C.A. Angell, Electrochim. Acta 40 (1995) 2397.
[9] (a) M. Watanabe, K. Yamada, K. Sanui, N. Nogata, J. Chem.
Soc. Chem. Commun. (1993) 929.;
(b) M. Watanabe, T. Mizumura, Solid State Ionics 86 !/88 (1996)
353.
[10] L. Feng, H. Cui, J. Power Sources 63 (1996) 145.
[11] M. Forsyth, J. Sun, D.R. MacFarlane, Solid State Ionics 112
(1998) 161.
[12] D.R. MacFarlane, F. Zhou, M. Forsyth, Solid State Ionics 113 !/
115 (1998) 193.
[13] A. Ferry, L. Edman, M. Forsyth, D.R. MacFarlane, J. Sun, J.
Appl. Phys. 86 (1999) 2346.
[14] A. Ferry, L. Edman, M. Forsyth, D.R. MacFarlane, J. Sun,
Electrochim. Acta 45 (2000) 1237.
[15] M. Forsyth, D.R. MacFarlane, A.J. Hiller, Electrochim. Acta 45
(2000) 1243.
[16] Z. Wang, W. Gao, X. Huang, Y. Mo, L. Chen, Electrochem.
Solid-State Lett. 4 (2001) A148.
[17] W. Xu, M.D. Williams, C.A. Angell, Chem. Mater. 14 (2002) 401.
[18] W. Xu, C.A. Angell, Electrochim. Acta, in press.
[19] M. Videa, C.A. Angell, J. Phys. Chem. B 103 (1999) 4185.
[20] C.T. Moynihan, N. Balitactac, L. Boone, T.A. Litovitz, J. Chem.
Phys. 55 (1971) 3013.
[21] C.A. Angell, Solid State Ionics, 9 !/10 (1983) 3 and 18 !/19 (1986)
72.
[22] C.T. Moynihan, P.B. Macedo, C.J. Montrose, P.K. Gupta, M.A.
DeBolt, et al., Annu. New York Acad. Sci. 279 (1976) 15.
[23] C.A. Angell, Annu. Rev. Phys. Chem. 43 (1992) 693.
[24] C.A. Angell, C.T. Imrie, M.D. Ingram, Polym. Int. 47 (1998) 9.
[25] M.G. McLin, C.A. Angell, Solid State Ionics 53 !/56 (1992) 1027.
[26] M.G. McLin, C.A. Angell, J. Phys. Chem. 95 (1991) 9464.