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3-D vertically aligned few layer graphene – partially reduced
graphene oxide/sulfur electrodes for high performance
lithium–sulfur batteries
Multifunctional nanocomposites with a judicious balance of
surface oxygen functional groups and 3-D nano-architecture
facilitate electronic and ionic transport, and ease of electrolyte
accessibility. The confi nement of the polysulfi de shuttles within
the nanocomposite 3-D matrix leads to excellent capacity and
cycling capabilities.
As featured in:
See D. P. Singh, N. Soin et al.,
Sustainable Energy Fuels,
2017, 1, 1516.
rsc.li/sustainable-energy
3-D vertically aligned few layer graphene – partially
reduced graphene oxide/sulfur electrodes for high
performance lithium–sulfur batteries†
D. P. Singh,
*
ab
N. Soin,
*
cd
S. Sharma,
e
S. Basak,
f
S. Sachdeva,
a
S. S. Roy,
g
H. W. Zanderbergen,
f
J. A. McLaughlin,
d
M. Huijben
b
and M. Wagemaker
a
3-D vertically aligned few-layered graphene (FLGs) nanoflakes syn-
thesised using microwave plasma enhanced chemical vapour depo-
sition are melt-impregnated with partially reduced graphene oxide-
sulfur (PrGO-S) nanocomposites for use in lithium–sulfur batteries.
The aligned structure and the presence of interconnected micro
voids/channels in the 3-D FLG/PrGO-S electrodes serves as template
not only for the high sulfur loading (up to 80 wt%, areal loading of
1.2 mg cm
2
) but also compensates for the volume changes occurring
during charge–discharge cycles. The inter-connectivity of the elec-
trode system further facilitates fast electronic and ionic transport
pathways. Consequently, the binder-free 3-D FLG/PrGO-S electrodes
display a high first-cycle capacity (1320 mA h g
1
at C/20), along with
excellent rate capability of 830 mA h g
1
and 700 mA h g
1
at 2C and
5C rates, respectively. The residual functional groups of PrGO (–OH,
–C–O–C– and –COOH) facilitate fast and reversible capture of Li
+
ions while confining the polysulfide shuttles, thus, contributing to
excellent cycling capability and retention capacity. The 3D electrodes
demonstrate excellent capacity retention of 80% (1040 mA h g
1
at
C/10) over 350 charge–discharge cycles. Comparatively, the 2-D
planar PrGO-S electrodes displayed poor electronic conductivity and
canonlyprovide560mAhg
1
after 150 cycles, thereby further
highlighting the vital role of the electrode morphology in improving
the electrochemical performance of Li–Sbatteries.
1. Introduction
As the next-generation energy storage materials, lithium–sulfur
(Li–S) batteries have become increasingly attractive owing to
their high gravimetric density (2600 W h kg
1
) and specic
capacity (1671 mA h g
1
), in addition, sulfur is a highly cost-
effective and environmentally benign element.
1
However, the
overall performance of current Li–S batteries is impeded by
inherently poor electronic and ionic conductivity of sulfur and
the dissolution of higher-order polysulphides phases (Li
2
S
n
(8 $
n $ 2)) during potential cycling which cause irreversible loss of
active material. This is especially so for the conventional Li–S
battery cathodes, consisting of sulfur, conductive carbon and
binder, where it is difficult to suppress the dissolution of
lithium polysulde in liquid electrolyte and stabilize the active
material in the cathode matrix. During the continuous charge–
discharge cycles, these polysuldes diffuse into liquid electro-
lyte and shuttle through the separator to lithium anode, and
nally precipitate as an insulating layer (Li
2
S
2
and/or Li
2
S) over
the electrodes.
2–5
With increasing current density and charge–
discharge cycles, this further increases the interfacial charge
transfer resistance, lowers the overall columbic effi ciency and
degrades the rate and life cycle performance.
6–10
To expedite the reversible electrochemical reaction and to
achieve high rate performance, both the sulfur and poly-
sulphides must maintain the ionic and electronic conduction
within the electrode matrix.
6,11
This requires: (i) conning the
insulating sulfur and polysulde shuttles in electronically
conductive electrode matrix, (ii) facile ionic network around
encapsulated sulfur formed by the liquid electrolyte in the pores
of the composite electrode matrix and (iii) efficient charge
transfer reaction between the liquid electrolyte and the active
material. To this effect, various matrices including mesoporous
carbons,
11,12
microporous carbon spheres,
13,14
nanotube/
bres,
15,16
activated carbon;
17
polar metal oxides such as TiO
2
,
SiO
2
,Al
2
O
3
,
18,19
have been investigated. More recently, quasi-2-D
metal carbides (such as Ti
2
C) and the 2-D nanosheets of
reduced Graphene Oxide (rGO),
20–22
owing to their superior
a
Faculty of Applied Science, Del University of Technology, The Netherlands. E-mail: d.
p.singh@utwente.nl
b
MESA
+
, University of Twente, Enschede, The Netherlands
c
Institute for Materials Research and Innovation (IMRI), University of Bolton, Bolton
BL3 5AB, UK. E-mail: n.soin@bolton.ac.uk
d
Nanotechnology and Bioengineering Centre (NIBEC), University of Ulster,
Jordanstown, BT37 0QB, UK
e
School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT,
UK
f
Kavli Institute of Nanoscience, Del University of Technology, The Netherlands
g
Department of Physics, School of Natural Sciences, Shiv Nadar University, Gautam
Buddha Nagar, Uttar Pradesh, 201314, India
† Electronic supplementary inform ation (ESI) available. See DOI:
10.1039/c7se00195a
Cite this: Sustainable Energy Fuels,
2017, 1,1516
Received 11th April 2017
Accepted 7th July 2017
DOI: 10.1039/c7se00195a
rsc.li/sustainable-energy
1516 | Sustainable Energy Fuels,2017,1,1516–1523 This journal is © The Royal Society of Chemistry 2017
Sustainable
Energy & Fuels
COMMUNICATION
electrical conductivity, large surface area and surface tailor-
ability, have emerged as promising materials for conning
lithium polysulphides while improving the material utilisation,
rate and cyclic stability.
23
It is well established that the appli-
cations of sp
2
carbon in the area of energy storage are highly
dependent not only on their superior intrinsic physical prop-
erties, such as mechanical strength, electrical and thermal
conductivity, but also on their tunable chemical properties.
24–26
Recent reports have shown that the surface modied carbon
bres, graphene oxide sheets and polymer coatings facilitate
better polysulde anion adhesion on the surface, thereby
enhancing the active material utilisation and life cycle capa-
bility.
27,28
Studies have shown that besides physically conning
elemental sulfur and lithium polysuldes in the meso/
microporous frameworks, the performance of liquid electro-
lyte based batteries is also governed by electrode morphology.
Furthermore, higher performance requires facile ionic (through
liquid electrolyte) and electronic transport (through conductive
additive).
29–33
Pioneering work by Nazar et al. has established
the close relationship between the electrode microstructure and
the subsequent electrochemical performance, which further
necessitates the demand for microstructured electrodes with
enhanced ionic and electronic tortuosity combined with high
sulfur loading.
6,11,19,34,35
Thus, it is still an on-going challenge to
prepare high performance electrodes with high specic capacity
at high rates due to shorter diffusion lengths and reduced
interfacial contact resistance.
Herein, we report novel binder-free 3-D vertically aligned
electrodes of few layered graphene (FLG) nanoakes with
interconnected micro voids/channel, lled with partially
reduced graphene oxide-sulfur (PrGO-S) nanocomposites for
high performance Li–S batteries (schematic shown in Fig. 1).
The melt-inltrated PrGO-S nanocomposites (see ESI †) within
a vertically aligned FLG network, provide several key advantages
over the conventional 2-D planar electrode morphology,
including: (i) facilitation of improved electrical conductivity and
high sulfur loading (80 wt% wrt PrGO/FLG as conrmed by
TGA (ESI, Fig. S1†)) in the interconnected micro-porous FLG
network; (ii) ease of electrolyte accessibility owing to its highly
interconnected nature and (iii) uniform distribution of voids/
channels conning the polysulde shuttle within the cathode
matrix and improving the active material utilisation to facilitate
facile ionic and electronic pathways.
16,36,37
This simple and facile
method to fabricate the 3-D FLG/PrGO-S electrodes offers great
exibility in controlling and tuning of electrode thickness and
free volume based on the variation of growth parameters of the
FLGs (see ESI, Fig. S2†). Furthermore, the presence of epoxide
(C–O–C), hydroxyl (–OH) and carboxyl (–COOH) groups on the
PrGO surface is expected to further enhance the sulfur and
lithium polysulde immobilization during the discharge
process. Drop casting of PrGO/S nanocomposite on the FLG is
expected to have additional advantages; rstly, the large surface
area of PrGO will act as a barrier layer between the electrode and
electrolyte; with the functional groups such as epoxide, hydroxyl
and carboxyl on the PrGO physically sandwiching the sulfur
(and its polysuldes) within the micro void/channelled FLG
structure. Secondly, the epoxide groups on the PrGO surface
further enhances the immobilization of sulfur and polysuldes
during the discharge process.
38
2. Experimental
2.1 Synthesis of partially reduced graphene oxide
Graphene oxide was synthesised using a modied Hummers'
method.
39,40
For the preparation of partially reduced graphene
oxide, the as prepared GO was reduced using
L-ascorbic acid
(
L-AA) as reported in the literature.
41,42
Typically, 0.1 mg mL
1
aqueous solution of GO was prepared and L-AA was added
such that the GO :
L-AA ratio was 1 : 10. Upon compl ete
dissolution of
L-AA in the GO s oluti on, the mixtur e was further
heated for 60 minutes at 95
C with continuous stirring. As the
reaction took place, the brown solution was no longer homo-
geneous and black precipitates started to appear. Upon the
completion of reaction, the solution was cooled down and the
precipitate was separated and washed repeatedly with water
using ltration and centrifugation. The m aterial thus ob-
tained (PrGO) was dried overnight in a vacuum oven (50
C) for
further use.
2.2 Synthesis of vertically aligned few layer graphene
nanoakes
The growth of few layered graphene (FLG) nanoakes was
carried out in a 1.5 kW, 2.45 GHz SEKI microwave plasma
enhanced chemical vapour deposition system.
43,44
Stainless
steel substrates (SS316L) cleaned using acetone and
Fig. 1 Schematic illustration of the preparation of PrGO/S composite
and fabrication vertically aligned FLG/PrGO-S electrode via drop
casting and melt infiltration method. Top and cross sectional scanning
electron micrograph of FLG electrode shows the interconnected
network of micro voids/channels and the high aspect ratio of gra-
phene sheets. Furthermore, the proposed facile (electronic and ionic)
charge transport within interconnected porous structures is illustrated.
This journal is © The Royal Society of Chemistry 2017 Sustainable Energy Fuels,2017,1,1516–1523 | 1517
Communication Sustainable Energy & Fuels
isopropanol alcohol were used as substrates, which were placed
on top of a Si wafer to allow plasma irradiation to higher
temperatures. Once the samples were loaded, the chamber was
pumped down to a base pressure of 2 10
3
Torr aer which
nitrogen (N
2
) plasma pre-treatment of substrate was carried out
at 700 W for certain duration of time. CH
4
was then injected at
the end of pre-treatment time while the microwave power was
simultaneously increased to 800 W. During the growth time of
60 seconds, the substrate temperature exceeded 1250
Cas
monitored by an optical pyrometer mounted on top of the
chamber. Post deposition, the samples were allowed to cool
down to room temperature under a N
2
atmosphere. The vertical
alignment of the resulting nanostructures is a unique feature of
the microwave plasma CVD route. Unlike the thermal CVD route
wherein the alignment is due to the crowding and van der Waals
forces, the alignment in microwave CVD route is attributed to
the plasma induced electric eld effects and has been discussed
in detail in our previous works.
43,44
It should be noted that the
microwave plasma deposition route can be used for catalyst free
growth of graphene nanoakes on any substrate which can
sustain the high temperatures and plasma bombardment
encountered during the growth process such as metallic foils,
carbon cloth etc.
2.3 Preparation of vertically aligned FLG/PrGO-S electrodes
The fabrication of vertically aligned FLG/PrGO-S electrode
involves the following two steps. Firstly, predetermined
amounts of commercially available sulfur (93 wt%) (Sigma
Aldrich) and synthesised PrGO powders (7 wt%) were mixed in
toluene (0.1 gm of PrGO/S in 50 mL of toluene) and ball milled
using a Fritsch planetary ball-mill under an argon atmosphere
for 60 to 90 minutes to obtain a homogenous mixture. This was
followed by melt inltration at 130
C under argon atmosphere
with subsequent drop casting of PrGO/S on the vertically
aligned FLGs to prepare the binder-free 3-D FLG/PrGO-S
electrodes.
2.4 Microstructural characterisation
Structural studies on the 3-D FLG/PrGO-S electrodes were per-
formed using Titan Cubed Cs corrected electron microscope
with a resolution of 0.08 nm. Inbuilt high resolution EDX was
used for mapping the distribution of carbon and sulphur. X-ray
photoelectron spectroscopy (XPS) was performed on a Kratos
Axis Ultra utilising an Al Ka radiation source (1486.6 eV). XPS
spectra were obtained with a spot size of 200 mm. High resolu-
tion scans of individual elements and the survey spectra were
measured at a pass energy of 50 eV and 200 eV, respectively. The
scans were averaged over 10 sweeps. Thermo Nicolet Nexus 670
FT-IR spectrometer was used for FTIR studies wherein the
spectra was measured for 128 scans (DRIFT mode) at a resolu-
tion of 4cm
1
. The thermogravimetric analysis of the sulfur
content in the PrGO/S electrodes was measured using TGA
carried out on a Netzsch5 STA F3 system. The measurement was
carried out from room temperature until 800
Cat5
C min
1
ramp rate in air.
2.5 Electrochemical measurements
The vertically aligned 3-D FLG/PrGO-S electrodes were tested
within the voltage range of 1.8–2.6 V against lithium metal
using Maccor Battery Tester. Prior to electrochemical testing,
electrodes were dried in vacuum. Cells were assembled using
a homemade vacuum ange type assembly in an argon-lled
Fig. 2 (a) TEM image of pristine FLG showing the variation of graphene layers with a knife-edge like structure, (b and c) low and high magni-
fication TEM image of PrGO-S nanocomposite with the inset showing the diffraction pattern corresponding to crystalline nature of the
composite, inset in (c): selected area diffraction pattern (d) cross-sectional SEM image of the 3-D FLG PrGO/S electrode, (e) dark-field STEM
image of nanocrystalline sulfur wrapped inside the PrGO sheet with the corresponding elemental mapping demonstrating the uniform coating of
sulfur for the PrGO/S nanocomposites. The TEM-EDX is provided in ESI Fig. S4.†
1518
| Sustainable Energy Fuels,2017,1,1516–1523 This journal is © The Royal Society of Chemistry 2017
Sustainable Energy & Fuels Communication
glove box using polypropylene separator. The electrolyte used
was 1.0 M lithium bis-triuoromethanesulfonylimide (LiTFSI)
in 1,3-dioxolane and 1,2-dimethoxyethane (1 : 1 by volume) with
3 wt% LiNO
3
additive.
45
To completely wet the electrode,
approximately 100 ml of electrolyte was used. To compare the
effect of 3D electrode morphology, a standard PrGO/S
composite (85 wt% sulfur) electrode was prepared by mixing
PrGO/sulfur composite with 85 wt% sulphur and carbon black
(5 wt%) with PVDF (10 wt%) binder in NMP and casted onto
carbon coated copper foil.
3. Results and discussion
Fig. 2(a) shows the transmission electron microscopy (TEM)
image of the pristine FLGs, wherein the samples show
a predominantly knife-edge structure with a thick (15–20 nm)
base constantly narrowing down as it goes along the axial
growth direction till it reaches the top with 1–3 layered gra-
phene.
43,44
It should be noted that these highly interconnected
FLGs synthesised (areal mass density (0.073 mg cm
2
)) using
a microwave plasma technique are largely composed of carbon
only (98.8 at%) with a small amount of adsorbed oxygen (1.2
at%) (see Fig. S3†).
43,44
For the PrGO/S composites, Fig. 2(b–d)
shows the sub-micrometer sulfur particles wrapped by partially
reduced graphene oxide (PrGO) sheets with the elemental
mapping revealing a homogenous dispersion of sulfur on the
PrGO sheets. Owing to the melt inltration process utilised for
the 3-D FLG/PrGO-S synthesis, a thin layer of sulfur nano-
particles as well as larger discrete particles were observed
embedded in the matrix as evident from the elemental mapping
and energy dispersive X-ray (EDX) analysis, which further
conrms the high sulfur/carbon ratio (see Fig. S4, ESI†). For the
3-D FLG/PrGO-S electrodes, the mild
L-ascorbic acid reduction
(starting GO, C : O ¼ 2.1) led to the removal of a signicant
amount of oxygen functional groups (PrGO, C : O ¼ 4.6) leaving
behind residual C–O–C, –COOH and –C]O groups
(Fig. 3(a)).
41,43–47
These residual groups have been previously
suggested to immobilise sulfur and subsequently free lithium
polysuldes during the charging–discharging process.
24,38,48
Furthermore, peaks attributed to –S–O– bonding were observed
in both the S 2p (164.6 and 165.5 eV) and O 1s spectrum
(530.6 eV, Fig. S3, ESI†), thus conrming the electronic
interactions occurring between sulfur and the PrGO matrix
(Fig. 3(b)).
49
The presence of the –S–O– bonding was further
corroborated by Fourier Transform Infra-red Spectroscopy
(FTIR) studies, which revealed peak at 1030 cm
1
(Fig. S5, ESI†).
Similar results of interaction between GO and sulfur have been
reported by Li et al.
49
and Guo et al.
50
wherein the –S–O bonding
helps in anchoring sulfur and intermediate polysulde prod-
ucts during the cycling process. Recent reports have utilised the
epoxide and hydroxyl groups on the basal plane of GO in
addition to the edge dominant carbonyl and carboxyl groups for
the immobilisation of sulfur and its discharge products.
51
However, the presence of these functional groups also leads to
the poor electronic conductivity of GO.
24,51
Thus, for most of the
previous reports with GO/S composites, the achieved rate
performances are generally below the rate of 2C (1C ¼ 1675 mA
g
1
).
24,52
On the contrary, reduced GO (RGO) or graphene with
intrinsically higher electronic conductivity has also been
employed to prepare RGO/S or G/S composites for improving
rate performance.
24
However, RGO or graphene does not have
the abundant functional groups that can bind sulfur and its
discharge products. Thus, to obtain the optimal balance of the
presence of functional groups and the electronic conductivity of
the graphene matrix, the partially reduced graphene oxide
(PrGO) is ideal. The electrochemical performance of the 3-D
FLG/PrGO-S electrode was evaluated using galvanostatic
discharge–charge measurements at variable current densities
from 80 mA g
1
(C/20) to 8000 mA g
1
(5C) within the voltage
window of 1.8 to 2.6 V versus Li/Li
+
. As shown in Fig. 4, the rst
discharge of PrGO/FLG-S electrode at C/20 illustrates the typical
two step charge discharge behaviour shown by Li–S batteries
(corresponding to the conversion of elemental sulfur into long-
chained (Li
2
S
n
,4# n # 8), and short chained (Li
2
S
n
, n <4)
polysuldes), and a signicantly high discharge capacity of 1350
mA h g
1
at 80 wt% sulfur loading was achieved. This is
higher than the value reported for electrochemically syn-
thesised vertically aligned sulfur-graphene nanowalls (1261
mA h g
1
at C/8).
49
Similarly, Wang et al. reported a rst
discharge capacity of 1611 mA h g
1
albeit at a much slower rate
of 50 mA g
1
(C/33).
53
Ni et al. reported sulfur-rGO nano-
composites with a high initial discharge capacity of 1473 mA h
g
1
at 0.1C but aer subsequent cycles at the same C-rate, the
discharge capacity of their electrodes dropped to 1230 mA h
Fig. 3 X-ray photoelectron spectra of 3-D FLG PrGO/S electrode, (a) deconvolution of C 1s core level spectra of PrGO, inset shows the C 1s
spectra of pristine GO, (b) deconvolution of the S 2p spectra showing the –S–O– bonding for the PrGO-S nanocomposites.
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Communication Sustainable Energy & Fuels