Article
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Cite This: ACS Appl. Polym. Mater. 2019, 1, 1837−1844
A Solid Polymer Electrolyte from Cross-Linked Polytetrahydrofuran
for Calcium Ion Conduction
Jiayue Wang, Francielli S. Genier, Hansheng Li, Saeid Biria, and Ian D. Hosein*
Department of Biomedical and Chemical Engineering, Syracuse University, Syracuse, New York 13244, United States
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S Supporting Information
*
ABSTRACT: Solid networks are produced from polytetrahydrofuran (PTHF) and 3,4-epoxycyclohexylmethyl-3′,4′epoxycyclohexane carboxylate through visible-light-initiated
photo-cross-linking. The networks were loaded with different
quantities of calcium nitrate to create solid polymer
electrolytes (SPEs). The ion conductivity was determined
by impedance spectroscopy, and the thermal properties were
determined by thermogravimetric analysis and differential
scanning calorimetry. All samples were rubber-like and stable
over a temperature range of 30−120 °C. With greater salt
loading, the ion conductivity of the electrolytes first increases
and then decreases. A sample with a molar O:Ca ratio of 1.9
yielded the highest conductivity of 1.14 × 10−4 S/cm at room temperature and transference number of 0.359 at 70 °C. The
response of conductivity to temperature is modeled with both VTF and Arrhenius equations. This copolymer system provides
an approach to calcium ion solid electrolytes for solid-state calcium ion batteries.
KEYWORDS: solid polymer electrolyte, calcium ion conduction, polytetrahydrofuran, ion conductivity, cross-linking
density as compared to lithium, and calcium’s standard
reduction potential is 170 mV above that for lithium, enabling
a substantially greater cell potential. However, multivalent
cations have been reported to have much lower transference
numbers (i.e., 0.005) when compared to monovalent ions,
which indicates that the total conductivity in those electrolytes
is due predominately to anionic transport. Nevertheless,
composition modifications to the electrolytes can address
this issue. For example, Saito et al. observed transference
numbers as high as 0.36 for Mg2+ by adding Lewis acid
compounds to the polymer chain and thereby entrapping the
counteranion and allowing the cation to participate in the
charge carrying.6,11,12 Compared to Mg2+ ion batteries, calcium
batteries have a higher cell voltage, for a given cathode voltage,
because of the 0.5 V lower standard potential of Ca/Ca2+ vs
Mg/Mg2+. Ca2+ also holds promise for faster reaction kinetics
as compared to Mg2+ because of the former’s lesser polarizing
properties. Furthermore, the ionic radius of the calcium cation
(Ca2+) is 33% (1.14 Å) larger, and this smaller charge density
can enable faster solid-state diffusion in cathodes. Calcium
batteries have also recently been shown to be rechargeable.7
We have previously reported a calcium ion SPE produced by
the photo-cross-linking of poly(ethylene glycol) diacrylate
(PEGDA).13 Although the ionic conductivity (3.0 × 10−6 S/
cm at room temperature) and thermal stability (stable up to
INTRODUCTION
Solid polymer electrolytes (SPEs) comprise a polymer matrix
in which salts are included to facilitate ion conduction. These
materials have found use in such critical applications as lithium
ion batteries, fuel cells, and dye-sensitized solar cells. Toward
battery applications, SPEs have received significant research
focus because of their processability, low-cost, suitable
conductivity, and thermal and mechanical properties necessary
to handle the stresses associated with battery operation. SPEs
are attractive over incumbent liquid electrolytes because of
their high thermal stability (in the case of fires, burn outs, etc.),
mechanically solid form, and resistance to leakage. Thereby,
effective SPEs would enable the realization of all solid-state
batteries. SPEs have been examined for several battery systems
(e.g., Li+, Na+, Mg2+, and Zn2+).1−6 However, reports on the
synthesis and study of an SPE for calcium (Ca2+) are limited.
While liquid electrolytes in calcium ion batteries have been
studied,7,8 and the development of calcium anodes and
cathodes is an area of intense research,8−10 it is imperative
that such solid electrolyte studies are conducted, and suitable
polymer systems examined, to realize an all-solid-state calcium
ion battery.
Calcium ion batteries can be a more cost-effective, high
power, and environmentally friendly form of electrochemical
energy storage. Calcium is the fifth most abundant mineral,
and current annual production can supply its battery industry.
The electrolytes from calcium salts are also less expensive (e.g.,
∼$20/100 g of calcium versus ∼$27/100 g of lithium). The 2+
oxidation state of calcium enables it to provide a higher energy
■
© 2019 American Chemical Society
Received: April 20, 2019
Accepted: May 30, 2019
Published: May 30, 2019
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Article
THF at the same time with no salt and weighed after drying as
references. Typical calculated gel fractions were ∼0.76.
Thermal Analysis. The thermal degradation of all samples was
studied by using thermogravimetric analysis (TGA) on a TA
Instruments (TGA Q500). Samples were heated to 600 °C at a
heating rate of 10 °C/min under a nitrogen atmosphere. The thermal
properties of samples were measured by using differential scanning
calorimetry (DSC) on a TA Instruments (DSC Q1000). 3−5 mg of
each sample was first cooled to −90 °C and then heated to 60 °C
under a nitrogen atmosphere. This cool−heat profile was repeated
three times at a rate of 10 °C/min to erase any thermal history. The
fourth cool−heat cycle was used to assess the sample thermal
properties.
AC Impedance Spectroscopy. Ion conductivity was determined
by ac impedance spectroscopy using a Solartron Energy Lab XM
Instrument, as described previously.13,14
Transference Number. The calcium ion transference number of
the SPE sample with highest ionic conductivity (O/Ca = 1.9:1) was
estimated by using a symmetric arrangement, in which the electrolyte
sample was placed between two calcium metal electrodes. The
analysis was performed at 70 °C by dc polarization in conjunction
with ac impedance spectroscopy, as described elsewhere.21 The
calcium transference number was calculated according to
140 °C) were found to be very promising and comparable to
early reports on lithium polymer electrolytes, further improvements should be pursued. Here, we report the preparation and
conductivity of a new solid polymer electrolyte for calcium ion
conduction. The SPE is synthesized through visible-light
initiated cross-linking and copolymerization of a polytetrahydrofuran (PTHF) oligomer (∼3−4 monomer units) and a
cycloaliphatic epoxy. The SPE attains conductivities greater
than 10−4 S/cm and good thermal stability. We are motivated
to explore PTHF as its lower oxygen content and greater chain
flexibility (relative to poly(ethylene oxide)) can allow for
greater ion transport and compensate for the stronger
electrostatic interactions with Ca2+. PTHF is also a reasonable
medium for multivalent systems, as shown for Al 3+
conduction.14
In this work, PTHF is rendered as a solid polymer matrix
through copolymerization with 3,4-epoxycyclohexylmethyl-3,4epoxycyclohexane carboxylate (Epoxy), which provides
mechanical and thermal properties to its network.15−17 Such
an approach can help to prevent crystallization and reduce the
polymer glass transition temperature such that it attains
rubbery properties at room temperature and above, thus
increasing the mobility of the polymer chains and increasing
ionic conductivity.18 Additionally, the cross-linking method has
been broadly applied to SPE development as an approach to
improve mechanical properties. Tong et al. reported improved
storage modulus (20 MPa at 20 °C) and rubber-like stress−
strain behavior due to the electrolyte’s chemically cross-linked
nature.19 Epoxy components have also been employed toward
improved SPE performance.20 A suitable SPE must attain an
ionic conductivity σ ≥ 10−4 S cm−1 at room temperature as
well as high thermal stability and mechanical strength. These
requirements are met by this PTHF−Epoxy system for calcium
ion conduction, demonstrating an SPE candidate toward an allsolid-state calcium ion battery.
■
t Ca 2 + =
Is(ΔV − I0R 0)
I0(ΔV − IsR s)
(1)
in which I0 and IS are the initial and final currents, respectively, and R0
and RS are the cell resistances before and after the polarization,
respectively.
X-ray Diffraction. X-ray diffraction (XRD) data were obtained
with a Bruker Kappa Apex Duo with Cu Kα radiation in a sealed Mo
tube and Triumph monochromator, as described previously.13
Raman Spectroscopy. Raman data were obtained with a confocal
Raman microscope consisting of a Raman spectrometer (Renishaw,
InVia) coupled to a Leica DM2700P microscope, as described
previously.13,14
Mechanical Analysis. Dynamic thermomechanical analysis
(DMA) of two samples with different O:Ca ratios was performed
with a dynamic mechanical thermal analyzer (TA Instruments DMA
Q800). A preload force of 0.008 N was applied. Data were collected
from 35 to 105 °C with a heating rate of 2 °C/min. Samples were
stretched to 0.8% strain at a constant frequency of 1 Hz.
EXPERIMENTAL SECTION
Materials. Polytetrahydrofuran (PTHF, Mn 250 g/mol), 3,4epoxycyclohexylmethyl 3′,4′-epoxycyclohexane carboxylate (Epoxy),
calcium nitrate tetrahydrate, photoinitiator camphorquinone (CQ),
and tetrahydrofuran were purchased from Sigma-Aldrich. The cationic
initiator (4-octyloxyphenyl)phenyliodonium hexafluoroantimonate
(OPPI) was obtained from Hampford Research Inc. All chemicals
were used without further purification.
Polymer Network Formation. PTHF and Epoxy precursors
were mixed at a 1:1 proportion by weight with CQ (2.5 wt %) and
OPPI (1.5 wt %). The solution was stirred for 48 h in a dark room,
then injected into a transparent cell, and exposed to incandescent light
from a quartz−tungsten−halogen (QTH) source (12 mW/cm2) for 5
h. The cured samples were washed in THF overnight to remove any
remnant and unreacted components and then vacuum-dried overnight
at 50 °C.
Calcium Nitrate Loading. Different masses of calcium nitrate
were dissolved in 5 mL of THF to make solutions of various
concentrations. The dried polymer samples were soaked in the
solutions for 2 days. THF was a good solvent for calcium nitrate
and easy to remove from samples. The samples after salt loading were
dried under vacuum at 50 °C overnight. Additionally, several samples
were soaked in THF to determine the gel fraction of samples (i.e., the
solid cross-linked portion that is retrieved, less any soluble
components that did not react during photopolymerization) as well
as any mass loss associated with the extended length of time for salt
loading. The amount of salt in samples was determined by the weight
change and gel fraction. From the amount of salt, the O:Ca ratio was
then calculated. Several pure PTHF−Epoxy samples were dissolved in
RESULTS AND DISCUSSION
Scheme 1a shows the molecular structures of the precursor
monomers, PTHF and Epoxy, that comprise the cross-linking
■
Scheme 1. (a) Precursors for PTHF−Epoxy Cross-Linked
Networks; (b) Schematic of the Cross-Linked Structure
copolymer network. Their polymerization proceeds through a
cationic mechanism initiated by the decomposition of a
diaryliodonium salt that is photosensitized to blue light by the
camphorquinone. The end-terminated hydroxyl functions of
the PTHF react with the epoxide functions of the epoxy.22
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Figure 1. (a) Ionic conductivity of PTHF−Epoxy SPEs in Arrhenius plots over a range of O:Ca ratios. (b) Ionic conductivity as a function of O:Ca
ratio at two different temperatures.
Figure 2. Arrhenius plots of the PTHF−Epoxy cross-linked networks over different O:Ca ratios (top right of each graph), fitted to the Arrhenius
(blue) and VTF (red) models.
PTHF only reacts with the epoxy, whereas the epoxy can also
react with other epoxy monomers. However, the rate of the
PTHF−Epoxy reaction is significantly optimized by the
approximately 1:1 molar ratio,23,24 which leads to an
alternating/random copolymer structure with minimal blockiness.25 The PTHF monomers confer the network with the
conductive pathways for ions and the epoxy the mechanical
and thermal properties to render the material suitable for
application as an SPE. Scheme 1b shows a schematic of the
envisioned cross-linking structure, in which PTHF bonds to
only the epoxy, whereas it is possible to have the presence of
some epoxy−epoxy bonds. The reaction of epoxide functions
forms an ethylene oxide group (i.e., monomeric unit of
poly(ethylene oxide)), and hence it can contribute to
coordination and possible ion conduction in the network.
Calcium nitrate was chosen as the Ca2+ source to show the
potential of the polymer system under study with relatively
low-cost components (while we note its potential instability
against Ca electrodes); calcium salts with other counterions
will be researched in the future.
The ionic conductivities of PTHF−Epoxy samples with
different O:Ca ratios over the temperature range from 30 to
110 °C are shown in Figure 1a. All SPEs show a monotonic
increase in conductivity with increase salt loading for the
concentrations explored. At the salt loading of O:Ca = 1.9:1,
the SPE shows competitive conductivities of 1.26 × 10−4 S/cm
at 30 °C, reaching the highest conductivity of 0.0157 S/cm at
110 °C. Attaining conductivity values at 104−10−3 S/cm is
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where σ0 is the pre-exponential factor related to the number of
ions31 and Ea the activation energy for ion transport.
This dual temperature dependence model is appropriate for
polymers with low Tg values, such that Tc is in the operating
temperature range of the SPE, as in our case with PTHF−
Epoxy. The dual temperature dependence in the conductivity
data indicates that the Ca2+ conduction at low temperatures is
unrelated to the motion of the chain segments, but rather more
so to rate-activated hopping of calcium ions to nearby oxygen
atoms that are vacant.29,32 This is due to the significantly
reduced segmental mobility, which fundamentally changes the
mechanism by which ions can move through the host network,
and this rate dependence is more suited to an Arrhenius
model. The activation energy obtained from the VTF equation
is also roughly half the magnitude of that determined from the
Arrhenius model, over the entire concentration range explored.
Given the full dissociation of the salt in the polymer host (vide
inf ra), conductivity depends mostly on the ease of ion
transport in the polymer based on the underlying mechanisms
corresponding to the VTF and Arrhenius models (i.e.,
segmental motion or hopping, respectively), which are
indicated by the activation energy. Hence, lower conductivity
is expected at lower temperatures. At temperatures higher than
Tc, measured here to be 60 °C, the ionic conductivity behavior
of the SPEs returns to VTF, as the calcium ions can freely
move along the PTHF chains.
One explanation for the deviation of sample conductivity
from the VTF model at low temperatures comes from the
difference in behavior between the bulk and microscopic “free”
volume expansion, which is not linear as assumed by the VTF
model. Therefore, the ionic mobility is greater than expected
from the system bulk characteristics.29 Furthermore, this
deviation is also well-known for and indicative of amorphous
polymers, which we expect for our system based on the
structural and thermal analysis and X-ray data (vide inf ra). In
the low temperature regime, the ionic conductivities still show
some deviation from the Arrhenius model, with conductivities
being slightly higher than what is predicted from the linear
Arrhenius plots. This could be due to the liquid-like
characteristic of the PTHF, which enables some polymer
segment mobility, whereby conductivity can still be facilitated.
The low Tg values of the samples attest to the rubbery state of
the networks, which can allow for greater conductivity than
that dictated purely from rate-activated ion hopping.
The transference number measurement of the sample with
O:Ca ratio of 1.9:1 (highest conductivity) is shown in Figure
S1 of the Supporting Information. Figure S1a shows the final
steady-state (Iss) and initial (I0) currents under a 30 mV
potential bias. The ratio Iss/I0 is adjusted in eq 1 according to
the method of Evans et al. which considers the increase in
resistance at the electrolyte−electrode interface.21 This
expected increase can be observed in Figure S1b. Based on
these data and eq 1, a calcium cation transference number was
calculated to be 0.359. The obtained value is accepted as
sufficient for good electrolyte performance, even in lithium and
sodium gel and solid polymer electrolytes.33,34 The high
calcium loading might be the main reason for the satisfactory
transference number. The experiment was performed at 70 °C
to be above the critical temperature previously discussed and,
thus, to better illustrate the transport mechanism in the
PTHF−Epoxy network. The transference number is an
important indicator of the polarity of the material, which is
associated with its cyclability. Large transference numbers
critical for SPEs to viably replace incumbent liquid electrolytes,
and the SPEs synthesized herein meet this requirement. As we
previously reported, calcium conduction on a more traditional
polymerspecifically poly(ethylene glycol) diacrylatewas
found to reach a maximum conductivity on the order of
magnitude of 10−6 S/cm at a temperature of 30 °C.13 This
indicates substantial improvement in ionic conductivity,
possibly due to enhancement in segmental motion from the
PTHF−Epoxy composition. Figure 1b transposes the conductivity data at temperatures of 40 and 70 °C (as examples)
to reveal that an O:Ca ratio of 1.9:1 was indeed the
approximate value to yield maximal conductivity, as smaller
ratios thereafter (i.e., 1.1:1, higher salt loading) show a drop in
the conductivity. Pure PTHF−Epoxy also showed higher ionic
conductivity with increasing temperature. In this case, the ionic
conductivity perhaps occurs due to proton generation resulting
from ester groups in the epoxy.26 Another cause could be the
presence of impurities intrinsic in the precursors’ manufacturing process, such as catalyst residues.27
The correlation of ionic conductivity to temperature showed
in the Arrhenius plots is often fitted by the Vogel−Tammann−
Fulcher (VTF) model:28
ÅÄÅ
ÑÉÑ
Å
ÑÑ
−B
ÑÑ
σ = AT −1/2 expÅÅÅÅ
ÅÅÇ kB(T − T0) ÑÑÑÖ
(2)
where A is a constant associated with the charge density, B is a
pseudoactivation energy associated with the segmental motion
of the polymer chain, kB is the Boltzmann constant, and T0 is
the equilibrium glass-transition temperature, which is commonly 50 °C lower than the actual glass-transition temperature.29 Figure 2 reveals that the ionic conductivities of the
SPEs over the concentration range explore fit the VTF
equation (red lines) at high temperatures within the range
explored. Fitted parameters for the data are provided in Table
1. However, the ionic conductivities in the low-temperature
Table 1. Activation Energies of PTHF−Epoxy Networks
ionic conductivity
VTF
Arrhenius
0.5
sample O:Ca
T0 (K)
A (S K /cm)
B (eV)
Ea (eV)
no salt
13.8
8.4
4.2
3.8
2.3
1.9
1.1
193
200
201
200
200
201
217
210
1.8
3.6
21.3
181.7
75.3
17347.2
7783.5
2422.8
0.130
0.108
0.126
0.138
0.127
0.171
0.139
0.139
0.221
0.229
0.247
0.244
0.219
0.295
0.310
0.328
range of the Arrhenius plot deviate from the VTF equation.
The temperature at which the ionic conductivity begins to
deviate from the VTF model can be defined as a critical
temperature, Tc,30 which is roughly around 60 °C for our
samples. In the low temperature range, the ionic conductivities
show more of a linear dependence to the reciprocal of
temperature and better fit an Arrhenius model (blue lines):
ÄÅ
É
ÅÅ −Ea 1 ÑÑÑ
Å
ÑÑ
Å
σT = σ0 expÅÅ
Ñ
ÅÅÇ kB T ÑÑÑÖ
(3)
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samples are stable and show no degradation (i.e., mass loss)
from room temperature up to 120 °C, which indicates that the
SPEs are stable within the operating temperature range of
batteries. It also indicates there is no remnant solvent or
moisture during the preparation and handling. Two samples
are an exception and begin to show some mass loss below 100
°C. However, this first drop before 100 °C indicates a loss of
moisture that may be absorbed during the manipulation of SPE
samples for TGA analysis, as observed by others.38,40−42 The
pure copolymer network (i.e., no salt) is itself thermally stable,
showing no mass decrease until 400 °C. With increasing
calcium concentration (i.e., lower O:Ca ratio), the decomposition temperature of samples decreases. Two abrupt mass
drops are observed between 100−200 and >300 °C, which are
attributed to decomposition of calcium nitrate and the polymer
network, respectively. Figure 4 also shows how with increasing
salt loading the onset temperature for the second mass drop
decreases. This might be due to the additional calcium ions
forming additional cross-links with the host network, which
can destabilize it and catalyze its degradation at elevated
temperatures. Above 500 °C the remnant mass increased with
increased salt loading. The composition of this residue is
unclear, but it is believed to be caused by the interaction
between NO3− anions of calcium nitrite.
DSC measurements (Figure 5a) from −80 to 50 °C show
the SPEs have no clear crystallization or melting peak within
this temperature range, which indicates their amorphous
structure. We reason that the short PTHF chain in
combination with cross-linking with the epoxy prevents any
crystallization from occurring and is thus not a determining
factor for conductivity. As consequence, rather than crystalline
peaks (large and sharp), there exist very broad glass transitions
(Tg) that end at ∼0 °C. All electrolytes show Tg values
between −30 and −5 °C, well below room temperature, and
hence are expected to have elastic properties over temperatures
explored for conductivity. Figure 5b shows the determined
glass transition temperature values for SPEs as a function of
salt loading. At an O:Ca ratio of 1.9:1, the highest Tg value of
−7 °C is attained. The SPEs show a relatively stable, constant
glass transition temperature with increased salt loading, except
for the sudden increase at a O:Ca ratio of 1.9:1, and finally a
drop for O:Ca = 1:1. Generally, the glass-transition temperature is drastically affected by salt concentration, as observed in
lithium ion studies, because polyether-based systems lose
flexibility which leads to a higher Tg when the ion content
increases.43 However, this issue is not observed in our samples,
except at very high salt concentrations. When the calcium
loading was low, because of the rubbery state of the samples,
Tg was not significantly affected by an increase in calcium
concentration. This effect was only observed at very high
calcium loadings. We can explain the changes in Tg at high salt
loadings as follows. From an O:Ca ratio of 1.9, samples have
more salt than polymer, which causes the polymer system to
transition from a salt-in-polymer to a polymer-in-salt regime44
with increased salt loading. In the latter regime, Tg decreases
with further increase of salt. With salt as the main component,
the PTHF−Epoxy chains are saturated, and further increase in
salt quantities will not yield an increase in Tg and conductivity.
Calcium may then begin to aggregate, consequently decreasing
coordination of polymer chains and softening the polymer,45
which causes the decrease in Tg when the O:Ca ratio is 1.1.
The trends in conductivity and glass transitions reveal how
the salt interactions with the polymer host enable the increase
show lower concentration polarization and therefore can
maintain its power density through several charge−discharge
cycles.33
Figure 3 presents the X-ray diffraction patterns of PTHF−
Epoxy SPEs, without any calcium salt and for several O:Ca
Figure 3. X-ray diffraction spectra of PTHF−Epoxy SPEs acquired at
room temperature (22 °C).
ratios. The pure PTHF−Epoxy sample shows a broad
amorphous peak centered at ∼20° and a lower intensity
broad peak centered at ∼44°. The presence of two such broad
peaks is common for amorphous polymer samples. The latter
peak does not correspond to either PTHF or epoxy X-ray
diffraction,35,36 which indicates that both XRD peaks are
produced from the PTHF−Epoxy copolymer. Furthermore,
the XRD spectra indicate the large-scale homogeneity of the
polymer amorphous phase (i.e., no indication of secondary
phases or crystalline phases) as well as the homogeneity of the
polymerized network. The increase in salt loading results in the
both XRD peaks (at 20° and 44°) decreasing in intensity and
broadening in width, and both disappear completely at higher
O:Ca ratios (i.e., 3.1 and 1.6). The disappearance of both
peaks signifies complexation of the salt within the polymer
matrix, which increases amorphousness in the system.37−39 No
crystalline peaks associated with Ca(NO3)2 powder were
observed, which confirms full dissolution and dissociation of
salt in the host network, which is an important feature toward
achieving higher conductivity.
Thermogravimetric analysis (TGA) of SPEs with different
salt loadings was performed. As shown in Figure 4, most
Figure 4. TGA plots of a pure PTHF−Epoxy sample and salt-loaded
SPEs.
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Figure 5. (a) DSC traces of a pure PTHF−Epoxy cross-linked network and calcium-loaded cross-linked samples (second heating cycle shown).
Curves are offset on the Y-axis. (b) Summary of the calculated glass transition temperatures (Tg).
to salt loading. The results indicate that Ca2+ complexation
occurs, and thus reasonable solvation of the calcium nitrate in
the SPE, up to salt loadings that provide available Ca2+ to
attain good conductivity values.
Figure 7 presents the DMA analysis of a salt-free PTHF−
Epoxy sample and a sample with O:Ca ratio of 1.6. The higher
in conductivity. The most conducting sample is the one with
O:Ca ratio of 1.9. The high conductivity is attributed to the
transition to a salt-in-polymer regime.44 The addition of salt
creates a tighter electrolyte structure and provides more free
volume to the polymer chain as well as more free ions, which is
beneficial for increasing conductivity. However, a further
increase in salt, higher than the 1.9:1 ratio, yields a lower Tg
because for a greater concentration of calcium ions, due to
saturation of the chain, the ionic mobility might be
compromised, which consequently decreases conductivity.
Additional DSC measurements of select samples from 50 to
200 °C are shown in Figure S2 in the Supporting Information.
Those samples were chosen to obtain a broad range of
concentrations but also considering the similarities in behavior
of the samples with intermediary O:Ca ratios. These results
show the absence of any thermal transition at higher
temperatures, confirming the polymer amorphousness. Moreover, the lack of a second Tg indicates that phase separation
unlikely occurred.46,47
Figure 6 presents Raman spectra of PTHF−Epoxy electrolytes for O:Ca = 1.9 and 8.4 as well as for a pure PTHF−Epoxy
Figure 6. Raman spectra of PTHF−Epoxy SPEs.
Figure 7. DMA plots of (a) storage modulus and (b) tan δ versus
temperature of PTHF−Epoxy cross-linked networks with and without
salt.
material. The small Raman peak centered at ∼932 cm−1 is
related to the vibration modes of the C−O−C bond48 in
PTHF and the ethylene oxide groups in the epoxy after ring
opening. Notably, with an increase in salt loading these Raman
bands shift, which indicates complexation of Ca2+ to oxygen.
The Raman band at ∼1050 cm−1 is attributed to dissociated
nitrate anions,49,50 the concentration of which is proportional
storage modulus (E′) observed in the salt loaded sample
indicates that the complexation of calcium within the polymer
matrix increased the stiffness of the resulting electrolyte,
possibly due to the additional cross-linking effect of salt
loading.51,52 Shim et al. also reported a similar storage modulus
of their polysiloxane/modified gallic acid cross-linking electrolyte as a sign of robustness and stability.53 The smooth
decrease in E′ in the salt loaded sample shows the relaxation of
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This is confirmed by the absence of peaks in the tan δ,
indicating that the samples do not present any glass transitions
between 35 and 105 °C.55 DSC results provided in Figure S2
also confirm the lack of a Tg from 50 to 200 °C. In addition,
both samples maintained tan δ below unity over the
temperature range studied, indicative of maintaining rubbery-like and elastic behavior as stable solid samples.44
CONCLUSION
We have shown the preparation and properties of a solid
polymer electrolyte for calcium ions. The SPE is synthesized
through the copolymerization of PTHF with a cycloaliphatic
epoxy, which is loaded with calcium nitrate as the Ca2+ source.
The SPEs show promising conductivity, as well as suitable
thermal and mechanical stability. Future work will focus on
exploring other salts of calcium as well as prototyping calcium
ion batteries toward the goal of achieving an all-solid-state
calcium ion battery.
■
■
ASSOCIATED CONTENT
* Supporting Information
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsapm.9b00371.
Transference number measurements and additional
DSC (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail idhosein@syr.edu.
ORCID
Francielli S. Genier: 0000-0002-9466-8727
Ian D. Hosein: 0000-0003-0317-2644
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Funding is acknowledged from the American Chemical Society
(PRF# 57332-DNI7) and the College of Engineering and
Computer Science at Syracuse University. Funding in part is
also acknowledged from the New York State Department of
Economic Development (DED) through the Syracuse Center
of Excellence. F.S.G. is grateful to Syracuse University for
Graduate Fellowship support.
■
■
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