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C: Energy Conversion and Storage; Energy and Charge Transport
A Novel Li+-Nafion-sulphonated Graphene Oxide Membrane as Single
Lithium-ion Conducting Polymer Electrolyte for Lithium Batteries
Isabella Nicotera, Cataldo Simari, Marco Agostini, Apostolos Enotiadis, and Sergio Brutti
J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b08826 • Publication Date (Web): 31 Oct 2019
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A Novel Li+-Nafion-Sulphonated Graphene Oxide Membrane as Single LithiumIon Conducting Polymer Electrolyte for Lithium Batteries
Isabella Nicotera,a* Cataldo Simari,a Marco Agostini,b Apostolos Enotiadis,c Sergio Brutti d
Department of Chemistry and Chemical Technology, University of Calabria, Via P. Bucci, 87036 Rende
(CS), Italy
a
b
Department of Physics, Chalmers University of Technology, SE41296 Göteborg, Sweden
c
Department of Materials Science and Engineering, University of Ioannina, Ioannina, Greece
d
Department of Chemistry, University of Rome La Sapienza, P.le A.Moro 5, 00185 Rome, Italy
* corresponding author. Email: isabella.nicotera@unical.it
Abstract
Single lithium-ion conducting polymer electrolytes are an innovative concept of solid-state polymer
electrolytes (SPEs) for lithium-battery technology. In this work, a lithiated Nafion nanocomposite
incorporating sulphonated graphene oxide (sGO-Li+), as well as a filler free membrane, have been
synthesized and characterized. Ionic conductivities and lithium transference number, evaluated by
electrochemical techniques after membrane-swelling in organic aprotic solvents (ethylene carbonatepropylene carbonate mixture), display significant values, with σ ≈ 510−4 S cm−1 at 25 °C and 𝑡𝐿𝑖 +
close to unity. The absence of solvent leaching on thermal cycles is also noteworthy.
The description at molecular level of the lithium transport mechanism has been carefully tackled
through a systematic study by 7Li-NMR spectroscopy (Pulsed Field Gradient-PFG and relaxation
times), while the mechanical properties of the film electrolytes have been evaluated by dynamic
mechanical analysis (DMA) in a wide temperature range.
The electrochemical performances of the graphene-based electrolyte in Li/Li symmetric cells and in
secondary cells using LiFePO4 as positive electrode, show good compatibility and functionality with
the Li-metal anode by forming a stable interphase, as well as displaying promising performance in
galvanostatic cells.
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1. Introduction
Solid-state polymer electrolytes (SPEs) are a valuable technology that are capable of replacing
liquid electrolytes in secondary non aqueous Li-ion batteries, thereby enabling the fabrication of
flexible, compact, laminated solid-state structures, free from leaks and easily manufactured in variety
of geometries.1 They are typically based on various polymers, with polyethylene oxide (PEO) the
mostly studied, blended with lithium salts (e.g. lithium triflate, i.e. LiTfO, lithium bis(trifluoromethyl sulphonyl)-imide, etc.) in a variety of ratios.2 These polymer electrolytes are typically dualion conductors, conducting both cations and anions.1–4 The most relevant drawback of such a
conduction mechanism within the solid state polymer phase is the occurrence and rise of
concentration polarizations under the application of a constant electric field, thus leading to the poor
performance of batteries.2,5
More recently, a new concept for polymer electrolytes has been proposed based on a single
lithium-ion conducting (SLIC) SPEs, which have anions covalently bonded to the polymer backbone,
or immobilized by anion acceptors. 5–7 However, the transport of lithium ions across the SLIC-SPEs
membranes is hindered by the formation of strong ionic couples or by the limited concentration of
free ions. 5–7
Polymer electrolytes based on ionomers (e.g. Nafion) with easily ionisable groups (e.g. sulfonic
groups covalently bonded to the polymer side-chains, −CF2SO3−) are promising thank to the high
concentration of weakly coordinating anions (counter-ions).8 Nafion ionomer is a perfluorosulfonated polymer consisting of a tetrafluoroethylene backbone with perfluorovinyl ether sidechains terminating with sulfonic acid groups.9,10 It is widely used in the proton exchange membrane
fuel cells11 or electrolyzers12,13 and is well-known for its excellent stability, high cationic conductivity
and unity transference number. It is commonly used in the H+-form, but can be easily turned to Li+form, and after swelling in non-aqueous solvents, e.g. mixture of propylene carbonate/ethylene
carbonate, it becomes an exploitable ionomer electrolyte for Li-ion batteries.8 Nafion has a robust
and stable chemical structure in either oxidative or reductive environments and an high ionic
conductivity even at low temperatures.14–16 Going beyond Li-ion, lithiated Nafion-based SLIC-SPE
membranes are also particularly interesting for novel battery chemistries such as Li-O2 and Li-S as
well as for lithium metal intercalation batteries such as Li-LiFePO4 (Li/LFP). In fact, lithiated Nafionbased membranes can provide a variety of beneficial effects ranging from the limited oxygen
crossover, the reduced mobility of the lithium poly-sulfides species, the mitigation of the lithium
dendrites growth, and at the same time it can enhance the lithium ion transference number. 14,17
In our laboratory we recently developed and characterized Nafion-nanocomposite membranes for
application in proton exchange fuel cells (PEMFCs).
18–22
In particular in previous publications we
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demonstrated the preparation of a tailored organo-modified GO nano-layers bearing sulfonated
functional groups (sulphonated graphene oxide, sGO) used as additive for the preparation of Nafion
nanocomposite membranes for PEMFCs that are able to provide enhanced protons mobility at low
temperature and poor humidification.18,23 Graphene oxide is a unique 2D material that shows
remarkable structural, morphological and functional properties: e.g. high ionic conductivity through
the plane, excellent flexibility and high in-plane electronic conductivity only when it is in the reducedform, r-GO
23–25,
otherwise it is a well-known electronic insulator.26,27 The incorporation of its
sulphonated derivative, sGO, in the Nafion ionomer, induces improvements of the hybrid electrolyte
in terms of thermal stability, mechanical strength and barrier properties (mitigation of the reagent
crossover by increased tortuosity and obstruction effect) without hindering proton mobility.18,23
Here our goal is to extend our approach beyond proton conducting membranes to tackle the general
problem of the tuning of the alkaline metal ions mobility in single-ion conducting membranes. Thus,
here we investigate and discuss the Li+ transport properties of Nafion-based membranes for
application in secondary lithium batteries. To this aim, lithiated Nafion and Nafion-nanocomposites
membranes based on sGO were synthesized and their ionic conductivity and lithium transference
number investigated in common non-aqueous solvents. In addition to electrochemical analyses, i.e.
lithium stripping/plating tests and galvanostatic cycling vs. Li/LiFP, a systematic study of the lithiumions transport mechanisms in such systems has been carried out by Pulsed Field Gradient (PFG) NMR
spectroscopy. Finally, the mechanical properties of the film electrolytes have been investigated by
dynamic mechanical analysis (DMA) in a wide temperature range. This work is part of a systematic
investigation of the impact of the incorporation of fillers in Nafion membranes to alter the ionic
transport properties.18,23
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2. Experimental
2.1 Materials
Nafion as 20 wt % dispersion in water and lower aliphatic alcohols was supplied by Aldrich.
Synthetic graphite powder, <20 μm, synthetic was purchased from Aldrich. The organic compound
used for the synthesis of the organo-graphene oxide is 3-amino-1 propanesulfonic acid (acronym:
SULF) was supplied by Aldrich.
2.2. Synthesis of GO and sGO
Graphene Oxide (GO) was synthesized from graphite powder using a modified Staudenmaier’s
method,28 as reported elsewhere.29 In a typical synthesis process, 10 g of powdered graphite were
transferred in a round bottom flask containing a combination of concentrated sulfuric acid (400 mL,
95−97 wt %) and nitric acid (200 mL, 65 wt %). The reaction mixture was then cooled to 0°C by ice
bath. Potassium chlorate powder (200 g) was gradually added to the suspension while stirring and
cooling. After 18 h the reaction was quenched by pouring the mixture into distilled water, and the
oxidation product was washed until the pH reached 6.0, the product was finally dried at room
temperature. For the functionalization of GO with sulphonil groups (sGO), 100 mg of GO was
dispersed in 100 mL of water, followed by the addition of aqueous solution of the 3-amino-1
propanesulfonic acid. After stirring for 24 h, the product was separated by centrifugation.
The successful intercalation of the organic molecules in the interlayer space of the graphene was
confirmed by XRD patterns shown in Figure S1 of the Supporting Information.
2.3 Membranes preparation
Nafion filler-free membrane was prepared from 20 wt% Nafion solution purchased from Aldrich
according to the following processes: (i) 1 g of Nafion solution was heated at about 60 °C to remove
all the solvents (water, 2-propanol, etc.); (ii) Nafion resin was redissolved with 10 mL of DMF until
become a clear solution and finally casting on a Petri dish at about 60 °C overnight until all the solvent
evaporates.
Instead, Nafion-sGO membrane was prepared by different procedure, as reported in the previous
work 23, by dispersing the filler directly in the alcoholic Nafion solution (as purchased), ultrasonicated
for 1 day, and stirred for another day at room temperature until a clear solution was obtained. The
resulting dispersion was then casted on a petri dish, placed in the oven at about 40 °C overnight to
remove the solvents. In order to reinforce both the membranes, these were sandwiched and pressed
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between two Teflon plates and put in oven at 150 °C for about 15 min. The final membranes thickness
is about 50 µm.
Finally, membranes were activated in acid in order to get them in H+-form 30 by rinsing in: (i) boiling
HNO3 solution (1 M) for 1 h, (ii) boiling H2O2 (3 vol %) for 1 h, (iii) boiling deionized H2O for 40
min three times, (iv) boiling H2SO4 (0.5 M) for 1 h min and again (v) boiling deionized H2Ofor 40
min twice to remove excess acid.
LiOH (2.0 M aqueous solution) has been used for the lithium-ion exchange reaction in order to
convert Nafion membranes in Li+-form. After 24 h under vigorous stirring at room temperature, the
resulting lithiated Nafion membranes were then rinsed several times in distilled water to remove the
excess of LiOH and thus dried at 120 °C in a convection oven for at least 12 h. With this lithiation
procedure, the sGO dispersed in the nanocomposite membrane also undergo the ion-exchange, as
depicted in the Scheme 1.
Both the Nafion and Nafion-sGO Li-PEMs (polymer electrolyte membranes in lithiated form)
were finally soaked in an anhydrous mixture of ethylene carbonate (EC) and propylene carbonate
(PC), with a molar ratio EC:PC of 1:0.4, for at least 3 days before the measurements. These two
solvents were completely dried before the using by leaving them on 3 Å molecular sieves for at least
one week. The solution uptake of each membrane was determined using a microbalance and recorded
as: uptake % = [(mwet - mdry)/mdry]  100.
Scheme 1. Visual representation of the chemical lithiation of the Nafion membrane and sGO by an ionexchange reaction in 2M LiOH solution.
2.4 Characterization techniques
SEM images were recorded by a Cambridge Stereoscan 360 at 12.5 kV. Membranes were first
frozen and fractured in liquid nitrogen. This guarantees a sharp fracture without modifications of the
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morphology. So, samples were sputter-coated with a thin gold film and finally the membrane cross
sections were observed at SEM.
XRD patterns were recorded with a X’Pert 3710 X-Ray diffrac- tometer (Philips, Eindhoven, The
Netherlands) using a Cu-Kα source operating at 40 kV and 20 mA
2.4 7Li-PFG-NMR spectroscopy
NMR measurements were performed on a Bruker NMR spectrometer AVANCE 300 Wide Bore
working at 116.6 MHz on 7Li. The employed probe was a Diff30 Z-diffusion 30 G/cm/A multinuclear
with substitutable RF inserts. The pulsed field gradient stimulated-echo (PFG-STE) method
31
was
used to measure the lithium self-diffusion coefficients. The sequence consists of three 90°
radiofrequency (rf) pulses (π/2 − τ1 − π/2 − τm − π/2) and two gradient pulses that are applied after the
first and the third rf pulses, respectively. The echo is found at time τ = 2τ1 + τm. Following the usual
notation, the magnetic field pulses have magnitude g, duration δ, and time delay Δ. The FT echo
decays were analyzed by means of the relevant Stejskal–Tanner expression:
𝐼 = 𝐼0𝑒 -𝛽𝐷
Here I and I0 represent the intensity/area of a selected resonance peak in the presence and in
𝛿
absence of gradients, respectively.  is the field gradient parameter, defined as 𝛽 = (𝛾𝑔𝛿)2(∆ ― 3)];
D is the measured self-diffusion coefficient. In these experiments, the used experimental parameters
were: δ = 3 ms, Δ = 30 ms, and the gradient amplitude varied from 400 to 1000 G cm−1. The
uncertainties in D values is estimated to about 3% based on the very low standard deviation of the
fitting curve and repeatability of the measurements. Longitudinal relaxation times (T1) of 7Li were
performed on the same spectrometer by the inversion-recovery sequence (π−τ−π/2). NMR
measurements were run by increasing the temperature step by step from 20 to 80 °C with steps of 10
°C, and leaving the sample to equilibrate for about 20 min at each temperature.
2.5 Electrochemical Impedance Spectroscopy (EIS)
Ionic conductivities of the membranes were measured by electrochemical impedance spectroscopy
(EIS) in the temperature range between 20 °C and 80 °C by using a PGSTAT30
potentiostat/galvanostat equipped with a FRA module. The EIS tests were performed over the
frequency range 1 Hz to 100 KHz. The ohmic resistance was obtained from a high-frequency intercept
on the real axis of the Nyquist plot. The Nyquist spectra were fitted by the equivalent circuit consisting
of a capacitor in parallel with an ohmic resistance, charge transfer resistance (Rct), representing the
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electrodes/sample interface impedance, and in series with an ohmic resistance, representing the
electrolyte resistance (Rel). The cell configuration adopted for these measurements is a trough-plane
2-electrodes system, where the electrolyte membrane is sandwiched between two carbon paper
electrodes and then between graphite current collectors.
The Li-transference number was calculated by using the Bruce-Vincent-Evans method
32
and a
Li/Electrolyte/Li symmetrical cell. Three different configurations were investigated: the two
membranes swelled for 3 days in a EC:PC mixture (EC, ethylene carbonate; PC, propylene carbonate;
Sigma Aldrich, dried for 1 week over 3A molecular sieves) with a molar ratio of 1:0.4, as well as a
benchmark liquid electrolyte, i.e. 1 molar solution of lithium trifluoromethane sulphonate (LiTfO,
Sigma-Aldrich) in EC:PC=1:0.4. The applied polarization was of 50 mV, while the AC impedance
measurements were performed before and after polarization in a frequency range from 100 KHz to
0.1 Hz, using 10 mV amplitude. Lithium deposition stripping tests were performed in a symmetrical
Li/electrolyte/Li cell at a current of 0.2 mA cm-2, room temperature of about 20 °C and at 50°C by
using a Memmert ventilated oven. All benchmark cells with liquid electrolytes have been assembled
using 1.55 mm thick Whatman fiberglass separators. The performance of the Nafion-sGO membrane
were also tested in a lithium cell at room temperature by using commercial LiFePO4 (LFP) positive
electrode (Customcell, areal capacity 1 mAh cm-2, gravimetric practical capacity 150 mAhg-1, active
material mass loading 7.0 mg cm-2) and a lithium disk as negative counterpart (Chemtall Ltd). The
galvanostatic cycling tests were run using various current rates, calculated in respect to the LFP
theoretical capacity (1C=170 mAg-1), in the voltage range of 3.8-2.5 V using the Nafion-sGO
membrane swelled for 3 days in a EC:PC mixture with a molar ratio of 1:0.4. All the above tests were
performed in El-Cell EC-Ref electrochemical cell, using a micro-rod of lithium metal as reference
electrode.
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3. Results and Discussion
The soaking in EC:PC of both Naf-Li+ and Naf-sGO-Li+ PEMs leads to a strong swelling of the
membranes as shown in the Figure 1. The areal surface increase is about +50 % for the Nafion
membrane where the solution uptake is about +100 wt%, whereas slightly smaller values are observed
for Naf-sGO nanocomposite (i.e. the black membrane showed in the right panel of Figure 1): +30
wt% and +80 wt%, respectively.
Figure 1. Pictures of Naf-Li+ film before and after swelling, and swelled Naf-sGO-Li+ on the right.
SEM cross-sectional images of the two PEMs, obtained by cryo-cleavage, are shown in Figure 2. Images
were collected on the membranes both before and after swelling in the EC/PC mixture. Filler free
Nafion (Fig. 2a) is a very smooth and compact membrane, without any significant changing by
absorption of carbonates solvents (Fig. 2b). The presence of graphene platelets dispersed in the
polymer matrix produces micrometer-sized cleavage planes (Fig. 2c) which become even more
visible on the swelled membrane (Fig. 2d and at higher magnification in Fig. 2e). These images
demonstrate the very good dispersion of the graphene sheets without any agglomerates or clustering,
as confirmed also by the SEM-BSE (back scattered electrons) image in Fig. 2f. Actually, BSE
technique is useful to analyze features based on the principle that heavy elements (high atomic
number) backscatter electrons more efficiently than light elements (low atomic number), and thus
appear brighter in the image. For this particular membrane, the filler is based on carbon, therefore we
cannot have fine contrast, however it is sufficient to put in evidence the homogeneity of the film.
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Figure 2. SEM cross-sectional images of Naf-Li+ before (a) and after (b) swelling, and Naf-sGO-Li+ before
(c) and after (d, e, f) swelling. The last image (f) was acquired in BSE mode.
3.1 Ion conductivity
The plasticizing/swelling operation in aprotic organic solvents is necessary to induce lithium-ions
mobility inside the polymer electrolyte that, otherwise, would be almost naught. Generally speaking,
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Nafion membranes displays remarkable Li+ diffusivity and conductivity only after swelling in the
organic carbonate mixtures. We adopted an EC:PC = 1/0.4 molar ratio in analogy with previous
literature 32–34
The lithium conduction properties for the two Nafion and Naf-sGO membranes have been
investigated by EIS. Figure 3 shows the ionic conductivity measured from 20 to 80 °C for the two
swelled membranes.
2x10-3
80
70
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20
(°C)
10-3
-1
 S cm )
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Naf-sGO-Li+
Naf-Li+
10-4
2.8
2.9
3.0
3.1
3.2
3.3
3.4
3.5
1000/T (K-1)
Figure 3: Ionic conductivity measured by EIS (ex-situ test) in through-plane cell configuration, from 20 °C
up to 80 °C, on filler free Naf-Li+ and Naf-sGO-Li+ membranes.
The graphene composite membrane displays slightly higher values with respect to the filler-free
Nafion for the whole temperature range investigated. Additionally, it is worth pointing out that no
solvent leaching was detected during and after the measurements, since after cooling at room
temperature the conductivity value is identical to the initial one, showing total reproducibility of the
data. The absence of leakage is not really trivial in the polymer electrolyte membranes, and this
important outcome implies the solvent mixture is strongly electrostatically coordinated inside the
polymer's clusters. Furthermore, it is worth highlighting that these conductivity values, even only for
Nafion filler free, are quite high if compared to similar studies.17,35,36 This is likely ascribable to the
organic solvent, or solvents mixture, used for the membrane swelling. The EC-PC mixture, in the
molar ratio 1:0.4, was chosen on the basis that it was particularly performant in several gel electrolytes
based on PEO or other polymer matrices.34,37,38
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Estimates of the lithium transference number of the lithium ions in the membranes (both Naf-Li+
and Naf-sGO-Li+) have been obtained by the Bruce-Vincent method. Experimental data are shown
in the figure S2 (supplementary information) where the current response as a function of time is
shown for symmetric Li:PEM:Li cells after a DC polarization of 30 mV. The figure insets show also
the electrochemical impedance spectroscopy (EIS) responses before and after cells polarization.
From the Bruce-Vincent tests, we calculated a lithium transference number (𝑡𝐿𝑖 + ) of 0.31 ± 0.02
for the liquid benchmark electrolyte in line with the literature 7,39 and lithium transport numbers close
to unity for both membranes, i.e. 0.98 ± 0.04 and 0.99 ± 0.05 for the Naf-Li+ and Naf-sGO-Li+
membranes, respectively. These values are slightly larger compared to those obtained on other singleion conducting polymer electrolytes in the literature 7 and, as expected, much larger to those found
for PEO-based polymer electrolytes, i.e. 0.2-0.4,4,40 or PVDF-based polymer membranes,7 where
anions are free to move across the polymer matrix following the electric field. Owing to this, the
outstanding ion conductivities shown in the Figure 3, are almost completely due to the mobility of
lithium ions driven by the electric field thus making these membranes suitable for application in room
temperature in lithium cells. As far as we know, compared to literature, both of our Nafion-based
membranes show the largest conductivity at room temperature ever reported for single-ion conducting
polymer electrolytes. 5,7,36,40
3.2
7Li
7Li
NMR study: Diffusivity (PFG-STE) and Relaxometry (T1)
NMR spectroscopy was used in this work to deeply investigate on the lithium ion dynamics
inside the PEMs through self-diffusion coefficients and spin-lattice relaxation time (T1)
measurements as function of the temperature, in the temperature range of 20-80 °C. The collected
data sets are shown in the Arrhenius plots of Figure 4, a and b, respectively. Diffusion coefficients
represent the long range motions, from 10 nanometers up to micrometers, while T1 is affected by
rotational and short range translational motions in the timescale of the reciprocal of the NMR
frequency ( 1 nm).
It is clear that the lithium diffusion in the Nafion filler-free membrane is higher than in the sGO
composite for the whole temperature range. This outcome, which may seem in contrast with the ionic
conductivity seen above, can be attributed to: (i) the stronger coordination between lithium ions and
2D-graphene platelets, bringing a large number of –SO3- groups; (ii) higher tortuosity of the ions
diffusion path caused by the dispersion of the lamellae in the polymer matrix; and (iii) lower swelling
in the EC/PC mixture of this membrane with respect to the recast one (uptake is about 80 wt% against
100 wt%, respectively).
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Figure 4. (a) Arrhenius plots of the diffusion coefficient (D) and (b) spin-lattice relaxation time (T1) of lithium
ions obtained by 7Li NMR spectroscopy.
The relaxation times in these two samples differ greatly, in particular by increasing the
temperature, meaning that at molecular level there is a different structuring of the lithium, while the
shorter T1 values in the Naf-sGO confirm that Li+ is involved in more stringent interactions with the
lattice. In other words, lithium ions in Nafion membrane have greater roto-translational degrees of
freedom, thus easier movements. Whereas in the sGO-composite the motions are much more impeded
due to stronger electrostatic interactions with both polymer’s side chains and the graphite lamellae,
which are widely functionalized with anionic groups on their surface (suggested also by the
independence of T1 on the temperature); as consequence also the lithium long-range mobility is
reduced, in agreement to the diffusion data observed. It is worth specifying also for these NMR
measurements that after the cooling the measured diffusion value is identical to the initial one, so we
can exclude solvent leaching from the membranes during the measurements and confirm the good
thermo-reversibility of these electrolytes.
Finally, the diffusion activation energies calculated from the Arrhenius plots are: 7.15 kcal mol-1
for Naf-Li+ and 7.08 kcal mol-1 for Naf-sGO-Li+. These values are almost doubled that of the proton
diffusion in Nafion-H+ swelled in water, i.e. about 3.5 kcal mol-1 15, while they are analogous to the
activation energies typically observed in PEO-based polymer electrolytes. Therefore, the lithium ions
transport mechanism hypothesized in such systems, sketched in the Scheme 2, should be similar to
that in PEO-salt gel-type complex: one lithium ion can form four to six coordination bonds
simultaneously with sulfonic groups (of Nafion and/or of sGO) and with the carboxylic oxygen of
the EC/PC molecules. These electrostatic interactions favor molecular complexes and produce a
transitory physical crosslinking, wherein lithium ion can move from one site to another through a
conformational transformation assisted by the segmental motion of the polymer chains and the solvent
molecules.
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Scheme 2. Visual representation of the lithium ions transport mechanism in the Lithiated Nafion and NafionsGO films.
Finally, in Table 1 the conductivity of the two PEMs measured by EIS is compared to conductivity
values calculated from the DLi self-diffusion coefficients using the Nernst-Einstein equation, which,
in this single-ion conduction PEMs, considers only lithium cation as charge carrier:
𝜎𝑁𝑀𝑅 =
𝐹2
𝑐 + × 𝐷𝐿𝑖 +
𝑅𝑇 𝐿𝑖
( )
The concentration of lithium ions inside the two samples, and the details of this calculations are
reported in the Supporting Information.
Generally, this equation was applied to dual-ion conductor systems and allows us to understand the
entity of the ionic association.41–43 In fact, NMR values are always higher than EIS due to the presence
of mobile ion pairs that contribute to the spin-echo decay in the PFG-SE experiment but do not
involve a net charge transport. For these type of systems, no formation of ion pairs or ionic association
are possible, but only lithium ions more or less "blocked" in their movement by the electrostatic
coordination with the polymer and filler.
As we can see from the Table 1, the calculated values are very close to the experimental ones,
especially for the Naf-sGO-Li+ membrane for which they are almost superimposable.
This outcome strengthens our hypothesis on the ionic transport mechanism exposed above and
highlight as in the composite PEM, the presence of sGO favors the formation of an appropriate
network which promotes the ion transport.
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Table 1. Comparison between conductivities (expressed in mS cm-1 ) measured by
EIS and calculated by Nerst-Einstein equation.
Naf-Li+
T°C
Naf-sGO-Li+
EIS
NMR
EIS
NMR
20
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0.40
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30
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0.53
0.54
40
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0.62
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0.74
0.78
0.80
60
0.77
0.87
0.87
0.88
70
0.91
1.01
0.99
1.00
80
0.89
1.07
1.11
1.15
3.3 Dynamic mechanical analysis
All PEMs have been investigated by DMA in order to highlight the lithiation effect and the sGO
incorporation on the mechanical properties, also in comparison with the protonated Nafion membrane
(Naf-H+). Figure 5a shows the elastic modulus (E') in the temperature sweep tests performed from 20
°C to 300 °C on the two membranes, Naf-Li+ and Naf-sGO-Li+, both before swelling (dry form) and
after swelling in the EC/PC solution, in order to investigate also the effect of swelling on the
mechanical properties of the polymeric films.
Nafion-H+
(a)
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107
106
swelled films
Naf-sGO-Li
Naf-Li
105
0
50
dry membranes
+
Naf-sGO-Li
+
Naf-Li
100
+
+
150
200
250
300
Temperature (°C)
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1.0
swelled films
Naf-sGO-Li
Naf-Li
Naf-sGO-Li
+
Naf-Li
+
25
(b)
dry membranes
+
)
0.6
Nafion-H+
0.4
(c)
dry membranes
Naf-sGO-Li+
Naf-Li+
20
+
stress (MPa)
0.8
tan 
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Naf-sGO-Li+
Naf-Li+
10
5
0.2
0.0
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0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
strain (%)
Temperature (°C)
Figure 5. Experimental DMA investigation, from 20 °C up to 300 °C, of the membranes both dry and swelled
in the EC:PC mixtures: (a) Storage modulus (E’) vs. T and (b) tan  vs. T. and (c) stress-strain curves conducted
at 30 °C.
As expected, the swelling in the organic carbonates blend has a plasticizing effect on the PEMs:
E' is smaller for both Naf-Li+ and Naf-sGO-Li+ after swelling and therefore, macroscopically, the
membranes are “softer” than the dried ones. Additionally, it is observed that the E' trend has a sigmoid
pattern with an evident inflection point: this shape denotes a glass transition state, better visible from
the tan  trend shown i
n Figure 5b. This inflection point occurs at about 220 °C in the swelled membranes, while rising
to about 270 °C for the dry ones. Overall the presence of the sGO filler improves the mechanical
properties of the membrane, preserving its elasticity. In fact, the Naf-sGO-Li+ membrane has a larger
elastic modulus compared to the Naf-Li+ both in the dry and the swelled states. Turning to the effect
of lithiation on the mechanical properties of the Nafion membrane, it is important to underline the
shift towards much higher temperatures of the typical transitions of this ionomer: the -relaxation,
i.e. the Tg of the ionic clusters, which typically is at about 120 ° C, now is higher than 200°C, and the
-relaxation of the hydrophobic chains, which generally is below 20 °C, now moves to being between
80 and 100 °C. 44,45 Our hypothesis is that lithium ions trigger a greater degree of cross-linking within
the ionic clusters of the polymer, possibly by modifying the local meso-morphology.
Finally, Figure 5c displays the stress-strain behavior of the membranes both swelled and dry, up
to the limit of the linearity region for each one. Dry membranes exhibit similar elongation at yield
point (strain of about 0.35 %) but with a tensile stress markedly higher for Naf-sGO (21.5 MPa) than
Nafion filler-free film (14 MPa). Due to the softening of the membranes upon swelling in the
carbonate solvents, the maximum stress decreases in both samples, i.e. the deformability of the films
increases, but again the presence of graphene platelets produces higher elongation at break.
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3.4. Electrochemical lithium stripping/deposition tests
The galvanostatic performance of Li/Li symmetric cells with Naf-Li+ and Naf-sGO-Li+ PEMs
have been recorded in order to evaluate the Li-stripping/deposition electrochemical activity in these
cell configurations. The Li-stripping/deposition tests have been carried out at room temperature and
at 50 °C for both PEMs and compared to the performance in EC:PC=1:0.4 LiTfO 1M benchmark
liquid electrolyte, as shown in the Figure 6. It is important to underline that the overvoltage fluctuation
observed for all cell at room temperature is due to the small temperature night/day oscillations.
The cell polarizations are moderate for both membranes slightly below 200 mV and 100 mV at
room temperature and at 50 °C, respectively. Compared to the benchmark liquid electrolyte soaked
on a 0.2 mm thick glass-fiber separator, both membranes show polarizations larger in absolute value.
However, the Naf-sGO-Li+ membrane shows an activated trend in the cell polarization and after 200
hours of stripping/deposition cycles, it shows the smallest overvoltage and the smallest overall
hysteresis both at room temperature and 50 °C. This trend demonstrates the once a stable interphase
between the Li-metal anode and the membrane electrolyte is formed, the larger conductivity of the
Naf-sGO-Li+ membrane discloses, as expected, smaller overvoltage and better performance.
Moreover, the different trends of the overvoltage upon cycling, decreasing for the Naf-sGO-Li+
membrane and increasing for the Naf-Li+ one, call for a more stable and less reactive Naf-sGO-Li+/Li
interface. This different trend possibly calls for a different evolution of the solid-electrolyteinterphase (SEI) that forms and evolves upon cycling at the lithium/membrane interface as already
discussed in the literature by several authors. 3,46 However, our clear experimental evidence does not
provide details about the chemical nature of the long-term performance enhancement provided by the
sGO. In order to shed light ex situ analyses by spectroscopy or microscopy are required. Such
investigation is however beyond the scope of this manuscript and the current adopted experimental
approach.
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Figure 6. Stripping/deposition overvoltage responses for the Li/electrolyte/Li symmetric cells, performed at
0.2 mA cm-2 current, at room temperature and at 50 °C: (a) comparison of the three electrolytes at room
temperature; (b) EC:PC=1:0.4 LiTfO 1m; (c) Naf-Li+ membrane and (d) Naf-sGO-Li+ membrane.
3.5. Preliminary tests of a solid Li/ Naf-sGO-Li+ /LFP cell configuration
The outperforming Naf-sGO-Li+ membrane has been preliminary tested in galvanostatic
experiments with LiFePO4 commercial electrodes (nominal capacity 1.0 mAh cm-2) in order to
demonstrate its use in a full lithium cell. The cell voltage profiles as well as the experimental specific
capacities are shown in the Figure 7. We would like to stress that no liquid electrolyte has been added
to these cells and that the LiFePO4-electrode formulation has not been optimized for example by
adding some lithiated-Nafion polymer as co-binder in the film preparation.
Overall the Li/ Naf-sGO-Li+ /LiFePO4 cell is able to supply stable and reversible performance in
galvanostatic cells at room temperature with high coulombic efficiencies of over 98.5% at C/20 and
shows promising rate performances. At C/20 more than 67% of the nominal specific capacity of the
LiFePO4 electrode is reversibly cycled in a constant plateau at about 3.5 V with a voltage hysteresis
between charge and discharge that is less than 100 mV. The Li/ Naf-sGO-Li+ /LiFePO4 cell outperform
the benchmark Li/ Naf -Li+ /LiFePO4 in particular at high rates due to the increase of the ohmic drops.
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In fact as expected both cells suffer a decrease in the specific capacity while the current rates: however
the smaller conductivity of the Naf-Li+ membrane (see above) hinders the performance leading to a
≈5% of capacity retention at C/5 compared to C/20, whereas Naf-sGO-Li+ cell keeps approximately
80% of the original capacity. On passing it may be of interest to highlight that both Naf-Li+ and NafsGO-Li+ cells show a so-called activated trend in terms of capacity vs. cycle number at C/20. One
may speculate about a possible slow activation of the active material in the electrodes due to a slow
wetting from the swelled membrane to the electrode sheet.
Figure 7. (a) Cell voltage profiles and (b) specific capacity at different current rate upon cycling for a Li/ NafsGO-Li+/LiFePO4 cell compared to a Li/Naf-Li+/ LiFePO4 one. (30°C; 1C=170 mA g-1 = 1.0 mA cm-2)
The obtained preliminary performance at room temperature of the Li/Naf-sGO/LiFePO4 cell are
comparable with the existing literature for other single-ion conducting polymer electrolytes 7. One
may recall that comparison is not easily straightforward as the electrode mass loading have a direct
impact on the real current density for identical current rates.
4. Conclusions
In this work we discuss the preparation and use in solid state lithium cells of new SLIC-SPEs
based on Nafion and sulphonated GO, where protons are ion-exchanged with Li+ ions. Two new
lithium conducting membranes, i.e. Naf-Li+ and Naf-sGO-Li+, have been prepared and swelled in
EC:PC carbonate blend to activate the lithium ion mobility: remarkably high thermo-reversibility of
the membranes and no solvent leaching was observed on thermal cycles. Furthermore, the mechanical
properties of the sGO added membrane is enhanced compared to the Naf-Li+ one, as well as the
lithiation of the ionomer produce significant shift of the thermal transitions.
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The ionic transport mechanism of both membranes has been characterized by impedance
spectroscopy to evaluate the thermal trend of the conductivity and the Li+ transport number.
Apparently 𝑡𝐿𝑖 + values are close to unity for both membranes and the total ionic conductivities are
very high, in particular for the graphene-composite where a value of approximately 1 mS cm-1 is
exceeded at 80 °C.
The lithium ion transport mechanism has been further analyzed by NMR spectroscopy. Overall
the lithium motion in the swelled membrane is similar to that in PEO-salt gel-type ones: molecular
complexes of lithium ions with sulfonic groups (Nafion and/or sGO) and carboxylic oxygens (EC/PC)
produce a transitory physical crosslinking, wherein ions can move through a conformational
transformation assisted by the segmental motion of the polymer chains and by the solvent molecules.
Having established the beneficial effect of the sGO addition in the lithiated Nafion membranes on the
mechanical, thermal and ion transport properties, these two PEMs have been assembled in solid state
lithium cells. Both Naf-Li+ and Naf-sGO-Li+ membranes are capable of galvanostatic
stripping/deposition reactions with limited overvoltage. In particular, the formation of a remarkably
stable interphase between the Li-metal anode and the Naf-sGO-Li+ electrolyte has been highlighted
both at room temperature and at 50 °C. As a final point the outperforming Naf-sGO-Li+ membrane
has been successfully used as SLIC-SPE in a Li/LiFePO4 battery to demonstrate its preliminary
performance in galvanostatic cycling tests.
Supporting Information.
XRD characterization of the Graphene and Graphene Oxide materials. Bruce-Vincent test for the
evaluation of the lithium transference number of the electrolytes. The method used for the calculation
of the density (geometrical method) and lithium concentration in the Nafion-based electrolytes to be
used in the Nerst-Einstein Equation.
Acknowledgments
The authors would like to thank Prof. Vito di Noto (University of Pavia) for the fruitful discussion,
Dr. Mariano Davoli (Univesity of Calabria) for his help in the acquisition of the SEM images.
One of us (SB) would like to thank MISE and ENEA for the funding of this research activity through
the “Accordo di Collaborazione tra ENEA e Dipartimento di Scienze dell'Università degli Studi della
Basilicata-PAR2017”.
At the University of Calabria (I.N.) this work was supported by the European Community’s Seventh
Framework Program (FP7 2007-2013) through the MATERIA Project (PONa3_00370).
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