ACS Applied Materials & Interfaces
Research Article
solution, 20 mg of cetyl trimethyl ammonium bromide was added
under stirring. A brown-colored precipitate was obtained after 24 h
and collected after repeated washing with deionized water. The
precipitate was calcined at 550 °C for 2 h at a ramping rate of 5 °C
min−1. After successful synthesis of the rGO and MnxOy nano-
particles, 50 mg of rGO and 20 mg of MnxOy nanoparticles were well
dispersed in 200 mL of deionized water. A solution of the ethylene
diamine−sulfur complex (prepared by adding 400 mg of sublimed
sulfur to 50 mL of ethylene diamine) was added slowly to the above
dispersion of rGO and MnxOy. After incorporation of the ethylenedi-
amine−sulfur solution, whole dispersion was stirred for 12 h to get the
uniform deposition of sulfur nanoparticles. The ternary composite
(designated as rGO/S/MnxOy) was collected by centrifugation and
then vacuum dried. The rGO/S/MnxOy composite was given a heat
treatment at 150 °C in Ar for 2 h to obtain good physical contact
among sulfur, rGO, and MnxOy. Furthermore, to create an
interparticle adhesion and improve overall ionic and electronic
conductivity of the cathode, the rGO/S/MnxOy composite was well
mixed with the Na alginate/polyaniline hybrid matrix. In this process,
the as-prepared rGO/S/MnxOy composite was first well dispersed in
deionized water, followed by the addition of 50 mg of sodium alginate
and 50 μL of aniline. The temperature of the resulting solution was
maintained at 5 °C. While stirring, a precooled ammonium persulfate
aqueous solution was mixed dropwise to the above solution and the
final composite dispersion was stirred overnight at 5 °C. A
freestanding cathode was constructed through vacuum filtration
using AAO membrane, followed by drying at 60 °C in vacuum. The
as-obtained freestanding cathode is designated as rGO/S/MnxOy@
SA−PANI. For a control experiment, the rGO/S/MnxOy composite
with the PVdF binder was taken as an active material. The control
cathode was fabricated by coating the suspension on aluminum foil.
The control cathode is designated as rGO/S/MnxOy@PVdF.
Material Characterization. X-ray diffraction (XRD) measure-
ments were carried out on a Philips X’-pert diffractometer equipped
with Cu Kα radiation (λ = 1.5418 Å). The morphologies of the as-
prepared samples were examined with a field emission gun scanning
electron microscope (FEG-SEM; JEOL-7600F) and a field emission
gun transmission electron microscope (FEG-TEM, JEOL-2100F).
Fourier transform infrared (FTIR) spectra were recorded on a
Thermo Scientific FTIR analyzer (NICOLET 8700). Raman spectra
were obtained using a micro-Raman spectrometer (Renishaw inVia,
U.K.). The X-ray photoelectron spectroscopy (XPS) spectra were
observed using an X-ray photoelectron spectrometer (Kratos
Analytical, AXIS Supra). A thermogravimetric analyzer (TA−DTG
Q600) was used to record thermal degradation responses of the
samples at a ramping rate of 3 °C min−1. The mechanical flexibility of
the freestanding cathode was tested using a Nanoindenter instrument
(TI-900, Hysitron). Six indentations were performed with 100 μN
load.
Here, we perceive this approach by preparing and illustrating
a unique hybrid cathode scaffold for RT Na−S batteries made
of reduced graphene oxide, sulfur nanoparticles, a mixed-
valence manganese oxide (MnxOy), and a hybrid Na alginate/
polyaniline conductive, adhesive matrix. Sulfur nanoparticles
could shorten the diffusion path for both Na+ and electron and
thus facilitate the conversion reaction at room temperature.
Reduced graphene oxide provides excellent electronic con-
ductivity and physically confines the active sulfur particles.
Different classes of polar compounds including metal oxides,
metal sulfides, etc. have been well demonstrated to be excellent
polysulfide immobilizers in lithium−sulfur batteries.19−23 The
stability of metal oxide in the electrolyte after binding to
polysulfides should be considered while selecting an effective
metal oxide for high-performance sulfur-based cathodes.
Considering this fact, we have chosen the mixed-valence
manganese oxide as a polysulfide immobilizer in this study,
based on the fact that Guo et al. demonstrated that in
comparison to MnO2 mixed-valence manganese oxide (i.e.,
Mn3O4) is more stable in a liquid-phase polysulfide environ-
ment in Li−S batteries, resulting in long-term suppression of
the polysulfide shuttle.23 Traditionally, in both lithium−sulfur
and sodium−sulfur batteries, several works were devoted to the
synthesis and engineering of the porous carbonaceous
materials to suppress polysulfide dissolution and unfortunately
less attention was paid to the use of appropriate binders.
Binders always play a crucial role to enhance the performance
of conversion-based electrode materials. It is therefore strongly
believed that the consideration of a suitable binder could
promote the performance of electrode materials, suffering from
substantial volume changes during cycling. Considering the
inherent mechanical stiffness with appropriate swelling
property of the polysaccharides,24 in this work, sodium alginate
is incorporated as a binder to ensure electrode integrity during
insertion and extraction of Na+ ions. Moreover, to obtain an
excellent electronically conductive network along with required
mechanical reinforcement throughout the electrode, polyani-
line was doped into the sodium alginate binder to coalesce a
hybrid conducting Na alginate/polyaniline matrix. The
resulting Na alginate/polyaniline hybrid matrix could serve as
an electronically/ionically conductive network throughout the
cathode to improve the overall conductivity. A three-
dimensionally reinforced freestanding cathode with an average
sulfur loading of 2.05 mg cm−2 was prepared through vacuum
filtration and later vacuum drying of the final composite.
Delivering a nominal discharge voltage of 1.81 V, our Na−S
batteries bestow a high specific energy of 737 W h kg−1 (based
on the active mass of sulfur and sodium) at the second cycle. A
specific energy of 660 W h kg−1 was retained after 50 cycles.
Electrochemical Measurements. The rGO/S/MnxOy@SA−
PANI freestanding electrode was used as a cathode for RT Na−S
batteries. For the rGO/S/MnxOy@PVdF composite, the cathode was
fabricated following the slurry coating technique using 90 wt % rGO/
S/MnxOy composite and 10 wt % PVdF binder. After, the cathodes
containing 2.05 mg cm−2 of sulfur were then coupled with sodium
metal anode in an Ar-filled glovebox (Mbraun) to assemble the
batteries. Celgard 2400 membrane soaked with 40 μL of electrolyte
prepared from 1 M sodium perchlorate (NaClO4) with 0.1 M sodium
nitrate (NaNO3) additive in tetraethylene glycol dimethyl ether
(TEGDME) was incorporated as the separator. The cyclic
voltammetry (CV) response was recorded at 20 μV s−1 between 1.2
and 2.8 V (vs Na+/Na). Charge−discharge cycling performance of the
RT Na−S batteries was recorded within 1.2−2.8 V at the various
current rates. Impedance spectra were recorded from 0.1 Hz to 1
MHz using ΔV = 5 mV. Performances of all cells were carried out at
20 °C. Specific capacity of the cathodes was estimated based on the
weight percentage of sulfur present in the cathode.
EXPERIMENTAL SECTION
■
Synthesis. Reduced graphene oxide (rGO) was prepared through
hydrazine reduction of graphene oxide (GO).25 Graphene oxide
synthesized through Hummer’s route was dispersed in 500 mL of
deionized water by ultrasonication. The GO suspension was then
maintained a pH of 10 by addition of KOH, and 5 mL of hydrazine
hydrate was then added to the suspension. The suspension was
refluxed for 24 h. The resulting rGO was collected by vacuum
filtration followed by repeated wash with deionized water and drying
at 80 °C for 48 h. Mixed-valence manganese oxide (MnxOy)
nanoparticles were prepared following the previously reported
chemical precipitation approach with slight modifications.26 First,
10 mmol (1.98 g) of MnCl2·4H2O was dissolved in 100 mL of
aqueous NaOH solution containing 20 mmol (0.8 g) NaOH. To this
XPS and 55Mn NMR Characterizations of Polysulfide-
Adsorbed MnxOy Sample. To examine the interaction between
MnxOy and intermediate compounds, solid-state 55Mn NMR and XPS
14102
ACS Appl. Mater. Interfaces 2019, 11, 14101−14109