A novel sp3Al-based porous single-ion polymer electrolyte for lithium ion batteries

Article abstract of DOI:10.1039/c5ra01126d

We report synthesis of an Al-based porous gel single-ion polymer electrolyte, lithium poly (glutaric acid aluminate) (LiPGAA), using glutaric acid and lithium tetramethanolatoaluminate as the precursors. The three-dimensional network compound provides short lithium-ion transport pathways and allows organic solvents to be accommodated in the composite for rapid ion transport. The tetraalkoxyaluminate units in the material enable Li-ions to be weakly associated with the polymeric framework, leading to a high ionic conductivity of 1.47 × 10^-4 S cm^-1 at room temperature and a high Li-ion transference number of 0.8. A membrane of the polymer was prepared via a solution cast method with PVDF-HFP poly(vinylidene-fluoride-co-hexafluoropropene) followed by soaking in a solution of ethylene carbonate (EC) and propylene carbonate (PC) (v/v, 1:1). A Li-ion battery assembled with the composite membrane displays remarkable cyclability with nearly 100% coulombic efficiency over a wide temperature range.

Full text of DOI:10.1039/c5ra01126d

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A novel sp3Al-based porous single-ion polymer
electrolyte for lithium ion batteries†
Cite this: RSC Adv., 2015, 5, 32343
ab
Guodong Xu,a Rupesh Rohan,a Jing Liab and Hansong Cheng
*
We report synthesis of an Al-based porous gel single-ion polymer electrolyte, lithium poly (glutaric acid
aluminate) (LiPGAA), using glutaric acid and lithium tetramethanolatoaluminate as the precursors. The
three-dimensional network compound provides short lithium-ion transport pathways and allows organic
solvents to be accommodated in the composite for rapid ion transport. The tetraalkoxyaluminate units in
the material enable Li-ions to be weakly associated with the polymeric framework, leading to a high
ionic conductivity of 1.47 ꢀ 10ꢁ4 S cmꢁ1 at room temperature and a high Li-ion transference number of
Received 20th January 2015
Accepted 30th March 2015
0.8.
A membrane of the polymer was prepared via a solution cast method with PVDF-HFP
poly(vinylidene-fluoride-co-hexafluoropropene) followed by soaking in a solution of ethylene carbonate
(EC) and propylene carbonate (PC) (v/v, 1 : 1). A Li-ion battery assembled with the composite membrane
displays remarkable cyclability with nearly 100% coulombic efficiency over a wide temperature range.
DOI: 10.1039/c5ra01126d
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materials have been shown to exhibit high thermostability, high
ionic conductivity, high Li-ion transference numbers, broad
Introduction
electrochemical
performance.18–24
windows
and
remarkable
battery
Technology development of battery devices with high energy
density and long cycle life is important to meet the ever-
increasing energy storage challenge for various technological
applications.1–4 As one of the essential components in a lithium
ion battery, an electrolyte plays a key role in enabling high
energy density in the device.5,6 For the next generation Li-ion
batteries, which utilize high energy density materials in the
electrodes, an electrolyte with a high voltage tolerance is
required in order to take full advantage of the amount of
available power offered by the electrodes. In addition, the
electrolyte must be able to overcome the chronic problem of
anion concentration polarization during charge and discharge
process, which is oen observed with the typical dual-ion
conductor electrolytes including liquid electrolytes and inor-
ganic lithium salt based polymer electrolytes. The concentra-
tion polarization leads to an increased resistance and ultimate
depletion of the electrolyte.7–10 Recently, many single-ion poly-
mer electrolytes (SIPEs) have been developed to minimize the
cell polarization for high power applications.11–17 Unfortunately,
the majority of these materials display relatively low ionic
conductivity on the order of 10ꢁ5 S cmꢁ1 at room temperature,
which allows batteries to perform only at elevated temperatures.
In the last few years, many researchers have reported SIPEs with
a variety of polymeric structural designs. These electrolyte
Lithium tetraalkoxyaluminates have been considered as
good candidates for electrolytes of Li-ion batteries because the
weak coordination between the lithium ions and the counter-
anions, promoting the solvation of Li-ions in aprotic organic
solvents.25,26 The rst study on tetraalkoxyaluminate unit was to
synthesize trialcohol substituted lithium aluminumhydride,
particularly lithium tri-t-butoxyaluminatehydride, which is
stable up to 300 ꢂC.27 Subsequently, a few tetraalkoxyaluminate
based compounds have been synthesized, among which lithium
tetra (1,1,1,3,3,3-hexauoro-2-propyl) aluminate (LiAl(HFIP))
and lithium tetra (1,1,1,3,3,3-hexauoro-2-phenylpropyl)
aluminate (LiAl(HFPP)) are the most prominent representa-
tives. These materials exhibit a broad electrochemical window
up to 5 V, high ionic conductivity around 10ꢁ3 S cmꢁ1 and
moderate thermal stability up to 100 ꢂC, resulting from the large
anionic size and electron delocalization induced by the four
electron-withdrawing groups attached to the central aluminium
atom.28–30 To circumvent the problems inherently associated
with liquid dual-ion electrolytes, a number of solid single-ion
electrolytes based on tetraalkoxyaluminate and poly-
uoroalkoxyl groups, have also been synthesized.31–33 These
materials were made from LiAlH4 (or partially substituted
LiAlH4) and diols with long aliphatic chains, particularly low
molecular weight poly(ethylene glycol)s. The polymers were
amorphous with a low lithium content. Despite offering a high
Li-ion transference number and a low glass transition temper-
ature, these solid polymers failed to perform as electrolytes
aDepartment of Chemistry, National University of Singapore, 3 Science Drive 3,
Singapore 117543, Singapore. E-mail: chghs2@gmail.com
bSustainable Energy Laboratory, China University of Geosciences Wuhan, 388 Lumo
RD, Wuhan 430074, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra01126d
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when assembled in batteries, largely due to the low ionic
conductivity on the order of 10ꢁ5 S cmꢁ1 at room temperature.
The utilization of a plasticizer (e.g. EC and PC) may provide
an effective way to enhance the exibility of polymer chains and
thus reduce ion pairing and improve electrode/electrolyte
interfacial contact, which ultimately enhances lithium ionic
conductivity and battery performance. In this paper, we present
a protocol to synthesize a novel porous gel single-ion polymer
electrolyte, lithium poly(glutaric acid aluminate) (LiPGAA), as
described in Scheme 1. The use of a short ligand offers two
advantages. One is to increase the polymer crystallinity,
resulting in a porous structure, which facilitates accommoda-
tion of organic solvent molecules. Another is to increase the
lithium content with a higher Al weight ratio. The SIPE
membrane was prepared using a solution cast method with
PVDF-HFP as the binder. The structure, surface morphology,
thermal stability and electrochemical performance of the as-
synthesized compound and its membrane were systematically
investigated.
Fig.
RT-800 ꢂC).
1 ,
TGA thermogram of LiPGAA (nitrogen, 10 ꢂC minꢁ1
corresponding to aluminum oxide and lithium oxide, which
correlates well with the elemental analysis results. The result
suggests that this compound is highly thermally stable.
Results and discussion
The surface morphology of the polymer was examined with
P-XRD and FE-SEM, respectively (Fig. 2). The XRD patterns of
LiPGAA (le) display sharp diffraction peaks, suggesting good
crystallinity as expected. The FE-SEM images (right) show that
the powder exhibits small ake shape structures stacking
The formation of LiPGAA was conrmed by 27Al NMR (in
reference to 1 M Al(NO3)3) with a chemical shi at ꢁ5.72 ppm,
compared with the signal of the starting material LiAl(OCH3)4,
which is at 69.57 ppm. The 13C MAS NMR (in reference to
DMSO) indicates three different carbons with chemical shis at
180.31 ppm, 36.07 ppm, and 21.14 ppm, respectively, which
correlates well with the glutaric acid backbones. The coherence
between the calculated and the found elemental analysis values
also validates the successful synthesis of LiPGAA.
The thermogravimetric analysis of LiPGAA under nitrogen is
shown in Fig. 1. The TGA curve begins with an approximately
5% weight loss before 150 ꢂC, which is attributed to the
absorbed moisture or the trapped solvent molecules. Subse-
quently, no weight loss is observed until the temperature rea-
ches roughly 350 ꢂC at which the compound undergoes obvious
decomposition. The residue weight le is approximately 30% Fig. 2 PXRD (left) and FE-SEM image (right) of LiPGAA.
Scheme 1 Synthesis procedure of LiPGAA.
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together, indicating that some planar structures may also be
formed, similar to the results reported in a previous study.34,35
The surface porosity was characterized by nitrogen isotherm
at 77 K at 1 bar (Fig. 3). The isotherm displays a steep gas uptake
at low pressure and capillary condensation at high pressure.
The BET surface area was calculated to be 93.9 m2 gꢁ1, while the
Langmuir surface area was 149.7 m2 gꢁ1. The pore size distri-
bution gives a wide range of mesopores, centered at approxi-
mately 15 nm.
To calculate the ionic conductivity of the electrolyte
membrane, the following equation was used: s ¼ l/Ra, where s
is the ionic conductivity, l denotes the thickness of the elec-
trolyte membrane, R stands for the bulk resistance (R1 is shown
in the equivalent circuit in Fig. 4) and a represents the surface
area. The EIS response of the membrane at room temperature
with the circuit diagram used for tting is shown in Fig. 4. The
ionic conductivity of the membrane was calculated to be 1.47 ꢀ
10ꢁ4 S cmꢁ1 at room temperature, which is comparable to the
values of most reported gel SIPEs.36,37 The as-synthesized poly-
mer contains a high proportion of aluminium centers, which
gives rise to high rigidity of the framework. As a consequence,
the glass transition temperature of the polymer is raised
substantially (over 300 ꢂC). The high rigidity of the polymer also
results in lower ionic conductivity because the contribution to
ion transport from the segmental motion is reduced. Hence, the
high s value is mainly attributed to the porous structure, which
enables solvent molecules to be accommodated and thus
promotes solvation of Li-ions in the polymer matrix.28
Fig. 4 The EIS plot of LiPGAA/PVDF-HFP composite membrane at
room temperature with the corresponding equivalent circuit.
measurement was carried out between 2.5 V and 6.5 V (versus
Li+/Li) (Fig. 6). The SIPE was found to be stable up to 4.2 V. To
further characterize the membrane, cyclic voltammetry (CV)
with and without EC and PC was conducted (Fig. 7). The
The temperature dependency of the ionic conductivity of the
electrolyte membrane on the inverse of absolute temperature in
the form of an Arrhenius plot is depicted in Fig. 5. The
measurements were conducted between 80 ꢂC and 25 ꢂC
downwards. An important observation is that the plot shows a
small curvature,15,38 unlike the behaviour of liquid electrolytes
and small lithium-salt based dual-ion polymer electrolytes. This
small curvature is commonly observed in other SIPEs, which
reects the mechanical coupling between ion transport and
polꢂymer host mobility.35,37 The highest ionic conductivity at
80 C is 5.32 ꢀ 10ꢁ4 S cmꢁ1
.
The electrochemical stability of the composite membrane
was measured via linear sweep voltammetry (LSV) using a
lithium foil as the counter and reference electrode. The
Fig. 5 Arrhenius plot of log(ionic conductivity) versus inverse absolute
temperature.
Fig. 3 Nitrogen sorption at 77 K (A) and pore size distribution (B).
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salt based electrolytes. The result suggests that the electrolyte
membrane behaves indeed as a single-ion conductor.
To further analyze the electrochemical performance of LiP-
GAA, a battery with LiFePO4 as the cathode and a Li foil as the
anode was assembled using the membrane as the electrolyte.
ꢂ
ꢂ
ꢂ
Cycle tests were conducted at 25 C, 60 C and 80 C, respec-
tively, for various C-rates as shown in Fig. 8. The battery displays
signicant performance at room temperature, while most of the
reported SIPE-based Li-ion batteries are operative only at
elevated temperatures.19 The discharge capacity at 0.1C
increased in the rst few cycles at room temperature, probably
due to the steady formation of ordered channels for Li-ion
conduction. Unfortunately, the battery failed to perform at
higher C rates at room temperature, which is attributed to the
large interfacial resistance as shown in Fig. 4. It is likely that the
high interfacial resistance arises from the high rigidity of the
polymer, leading to poor compatibility between the electrolyte
and the electrodes. With increasing temperature, battery
performance was enhanced with better discharge capacity even
at higher C-rates. The coulombic efficiency of the battery
became nearly 100% from room temperature to 80 ꢂC. Fig. 9
depicts the discharge proles of the Li/LiPGAA/PVDF-HFP/
LiFePO4 battery at 80 ꢂC at different power rates. The discharge
proles were measured to be 147 mA h gꢁ1 at 0.1 C, 140 mA
Fig.
6 The linear sweep voltammetry of the LiPGAA/PVDF-HFP
composite membrane (scan rate 1 mV sꢁ1).
Fig. 7 The cyclic voltammetry of the LiPGAA/PVDF-HFP composite
membrane with and without EC and PC.
electrolyte CV with EC and PC is consistent with the LSV result,
indicating a broad electrochemical window. However, there is
no current signal for the CV of the composite membrane
without EC and PC, suggesting an open circuit due to lack of an
organic solvent to solvate the Li+ ions. Consequently, it is
difficult for the Li+ ions to transport from the membrane to the
Li foil.
Fig.
8 The cycle performance of Li|LiPGAA/PVDF-HFP|LiFePO4
battery in the temperature range of 25 ꢂC to 80 ꢂC and C/n rates.
The Li-ion transference number (tLi+) was measured by
sandwiching the prepared membrane between two lithium
electrodes.39 The value was derived using the equation proposed
by Evans and co-workers (Table 1).40 The tLi+ value was found to
be 0.80 at room temperature, modestly smaller than unity but
substantially higher than the values for small inorganic lithium
Table 1 The measured intial and steady-state currents, the initial and
steady-state resistances of the passivation layers on the Li electrode
and the Li-ion transference number
+
DV/mV
I0/mA
Is/mA
R0/U
Rs/U
tLi
Fig. 9 The charge (red)/discharge (black) profiles at 80 ꢂC and at C/n
10
6.50
5.20
7.54
9.22
0.80
rates.
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h gꢁ1 at 0.2 C and 120 mA h gꢁ1 at 0.5 C. Obviously, improving 1 M Al(NO3)3): 69.57 (br) ppm; 13C: 48.54 ppm; 1H: 3.18 ppm (s,
the compatibility between the membrane and the electrodes is 12H).
essential for enhancement of the battery performance.
Overall, LiPGAA exhibits high thermostability and ionic Preparation and characterization of silylated glutaric acid
conductivity comparable to the properties of other SIPEs. The
The silylation was carried out under argon atmosphere by
lithium ion transference number of the material is somewhat
reacting glutaric acid 5.2848 g (40 mmol) with 20.8 mL freshly
inferior to the values of other SIPE materials, chiey resulting
distilled hexamethyldisilazane (HMDS) (100 mmol) as well as
from the low solubility of three dimensional framework in
0.5 mL trimethylsilyl chloride in anhydrous 1,2-dichloroethane
common organic solvents, hindering the formation of a high
molecular weight polymer in the synthesis. Detail comparison
of the properties is shown in Table S1 (ESI†).
ꢂ
(DCE) at 100 C for 6 h. Upon cooling down to room tempera-
ture, DCE and excess HMDS were removed under a reduced
pressure. 1H NMR spectra in CDCl3: 2.23 ppm (t, 4H), 1.75 ppm
(m, 2H), 0.15 ppm (s, 18H).
Conclusions
Preparation and characterization of LiPGAA
We have demonstrated an approach to synthesize an
aluminium based porous single-ion polymer electrolyte, LiP-
GAA. The method to prepare tetraalkoxyaluminates can be
employed to synthesize a family of this type of compounds.
LiPGAA is the rst tetraalkoxyaluminate-based polymeric elec-
trolyte membrane ever made with good battery performance to
our knowledge. The structure and properties of LiPGAA and the
composite membrane have been well characterized via
elemental analysis, nuclear magnetic resonance, thermogravi-
metric analysis, nitrogen isotherm and electrochemical
impedance spectroscopy. The mesoporous polymer electrolyte
displays excellent thermal stability up to 350 ꢂC and high ionic
conductivity of 1.47 ꢀ 10ꢁ4 S cmꢁ1 at room temperature. The
measured Li-ion transference number of 0.80 is substantially
higher than the values of liquid electrolytes and inorganic
lithium-salt based dual-ion polymer electrolytes. Furthermore,
good operability of the battery assembled with the electrolyte
membrane in a wide temperature range was well demonstrated
and signicant performance at elevated temperatures was
observed. The coulombic efficiency was found to be nearly
100% with the wide temperature range. Further enhancement
of electrochemical performance can be envisaged through
improvement of the compatibility between the electrolyte and
the electrodes.
The lithium poly(glutaric acid aluminate) (LiPGAA) was
synthesized via polymerization between the silylated glutaric
acid (40 mmol) and 3.1612 g of LiAl(OCH3)4 (20 mmol) in an
anhydrous DMF, stirring at 50 ꢂC for 24 h and subsequently at
90 ꢂC for 72 h. A white solid was rst collected by ltration, then
puried by Soxhlet extraction using acetonitrile as the solvent
and nally by drying at 120 ꢂC under a reduced pressure for
24 h. Elemental analysis on (C10H12AlLiO8)n: calculated: C,
40.84; H, 4.11; Al, 9.17; Li, 2.36%; found: C, 36.12; H, 4.54; Al,
10.53; Li, 2.44%. CP-MAS NMR: 27Al (in reference to 1 M
Al(NO3)3) ꢁ5.72 ppm; 13C (in reference to DMSO) 180.31 ppm,
36.07 ppm, 21.14 ppm.
1
The H NMR and 13C NMR spectra of LiAl(OCH3)4 and sily-
lated glutaric acid were recorded on a Bruker AVANCE 500
spectrometer. The 13C and the 27Al magic angle spinning (MAS)
NMR spectra of LiPGAA were recorded on a Bruker DRX-400
spectrometer, in reference to DMSO and 1 M Al(NO3)3, respec-
tively. The thermal stability of LiPGAA was investigated under
nitrogen with a ramp of 10 ꢂC minꢁ1 from room temperature to
800 ꢂC in the Thermo Gravimetric Analyzer (model TGA Q 50) of
TA, Inst., USA. Powder X-ray Diffraction (XRD) of the polymer
was performed on a D5005 Bruker AXS diffractometer with Cu-
Ka radiation (l ¼ 1.5410) in the scanning range between 1.4ꢂ
and 60ꢂ at room temperature. The surface morphology of the
compound and its composite membrane were imaged using
JEOL JSM-6701F eld emission scanning electron microscopy
(FE-SEM). The samples were prepared by platinum-sputtering
(30 s, 20 mA). Nitrogen sorption isotherms were measured up
to 1 bar using a Micromeritics ASAP 2020 surface area and pore
size analyzer. Before the measurements, the samples (ꢃ100 mg)
were degassed under a reduced pressure (<10ꢁ2 Pa) at 150 ꢂC for
12 h. The UHP grade N2 was used for the measurement. Oil-free
vacuum pumps and oil-free pressure regulators were used to
prevent contamination of the samples during the degassing
process and isotherm measurement.
Experimental
Materials
Glutaric acid (Sigma-Aldrich), lithium aluminium hydride 1.0 M
in diethyl ether (Sigma-Aldrich), anhydrous N,N-dime-
thylformamide (DMF) (Sigma-Aldrich), and acetonitrile (Tedia)
were used as purchased. Hexamethyldisilazane (HMDS) (VWR
international), 1,2-dichloroethane (DCE) (Merck), methanol
(Fisher) were distilled before use. All chemicals used were of
reagent grade.
Electrolyte membrane preparation
Preparation and characterization of LiAl(OCH3)4
PVDF-HFP and LiPGAA in a ratio of 3 : 1 (w/w) were added into a
ꢂ
LiAl(OCH3)4 was synthesized following the procedure proposed DMF solvent stirred at 80 C to obtain a homogeneous disper-
by Bakum.41 Elemental analysis on LiAl(OCH3)4: calculated: C, sion. Subsequently, the dispersion was cast onto a teon petri
ꢂ
30.40; H, 7.65; Al, 17.07; Li, 4.39%; found: C, 27.11; H, 5.96 Al, dish and kept in an oven at 80 C for 12 h. The obtained elec-
17.86; Li, 4.73%. NMR spectra in DMSO-d6: 27Al (in reference to trolyte membrane was dried further in a vacuum oven at 80 ꢂC
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Briey, 80 wt% of LiFePO4, 10 wt% of acetylene black, and
10 wt% of poly(vinylidene uoride) (PVDF) were mixed in NMP
and magnetically stirred to form a slurry. The homogenous
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foil was employed as a counter and reference electrode. The
lithium ion transference number was measured using the
method proposed by Evans40 by sandwiching the membrane
between two lithium foil electrodes. Aer the initial resistance
was measured, a potential of 10 mV was applied until a steady
state was reached. Subsequently, the nal resistance was
measured by EIS. The galvanic charge–discharge experiments
were carried out with the voltage in the range of 2.5–3.8 V, using
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The authors gratefully acknowledge support of this study by a
start-up grant from NUS, a Tier 1 grant from Singapore Ministry
of Education, a POC grant from National Research Foundation
of Singapore, a Singapore DSTA grant and the grants from the
National Natural Science Foundation of China (no. 21233006
and 21473164).
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