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Formation of SnS nanoflowers for lithium ion batteriesw
Dimitri D. Vaughn II,a Olivia D. Hentz,a Shuru Chen,b Donghai Wangb and
Raymond E. Schaak*a
Received 20th March 2012, Accepted 5th April 2012
DOI: 10.1039/c2cc32033a
SnS nanoflowers containing hierarchically organized nanosheet
subunits were synthesized using a simple solution route, and they
function as lithium ion battery anodes that maintain high
capacities and coulombic efficiencies over 30 cycles.
Lithium ion battery technology has existed for decades, but
large-scale energy storage needs for sustaining a portfolio of
clean-energy technologies—including electric vehicles and
energy generation systems based on the sun and the wind—have
created a growing interest in developing new materials that can
reversibly intercalate lithium.1–3 In this respect, lithium ion
batteries are poised to serve as key energy storage devices that
promise to reduce fossil fuel consumption and CO2 emissions.4
Central to advancing such technologies is the development of
new materials for lithium ion batteries that are cheap and
lightweight while providing high energy density and long service
life.1 Among such materials, hierarchical nanostructures have
been receiving significant attention, because they contain interconnected nanoscale building blocks organized into larger
porous architectures that collectively have high surface areas,
enhanced particle stabilities, shortened lithium ion diffusion
pathways, and extensive percolation networks with increased
electrolyte-electrode interfaces.5–7
Sn-based materials, including SnO2,8 SnSe,9 SnS,10–12 and
elemental Sn,13 have been extensively studied as anode materials
for lithium ion batteries. However, lithium diffusion into these
materials creates a large volume expansion, which leads to
fracture, loss of contact with the current collector, and degraded
performance over time.14 Tin sulfide, SnS, is a particularly
interesting anode material for lithium ion batteries because it
has a high theoretical capacity, is comprised of inexpensive
and abundant elements, and includes sulfur, which can act as a
buffering layer for increased cycling performance.12,15 While a few
reports have demonstrated the utility of SnS nanostructures for
lithium ion battery applications,10–12 discrete hierarchical nanostructures of SnS—which are highly desired for battery applications because of the nanostructure-performance correlations
detailed above—have not been described. Here, we report the
solution-mediated synthesis of SnS nanoflowers comprised of
hierarchical assemblies of single-crystal nanosheets. We also
describe insights into the nanoflower formation pathway,
demonstrate both thermal and environmental morphological
stability, and show that the nanoflowers function as lithium ion
battery anodes with high capacities and coulombic efficiencies
that are maintained for over 30 cycles.
The SnS nanoflowers were synthesized by slowly heating
SnI2 (40 mg), oleylamine sulfide (250 mL), hexamethyldisilazane
(HMDS, 1 mL), and oleylamine (20 mL) at 10 1C min 1 to
200 1C, then holding for 30 min. Transmission electron
microscopy (TEM) images are shown in Fig. 1a and b and
field emission scanning electron microscopy (FESEM) images
are shown in Fig. 1c and S1.w The TEM and SEM images
reveal discrete hierarchical nanostructures that are comprised
of individual nanosheets connected to a central core. The SnS
nanoflowers are uniform with average diameters of 1–2 mm.
Energy dispersive X-ray (EDX) spectra (Fig. S2w) indicate a
a
The Pennsylvania State University, Department of Chemistry and
Materials Research Institute, University Park, PA, 16802, USA.
E-mail: [email protected]
b
The Pennsylvania State University, Department of Mechanical and
Nuclear Engineering, University Park, PA, 16802, USA
w Electronic supplementary information (ESI) available: Full experimental details and additional TEM, FESEM, XRD, and EDX data.
See DOI: 10.1039/c2cc32033a
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Chem. Commun., 2012, 48, 5608–5610
Fig. 1 TEM (a,b) and FESEM (c) images of SnS nanoflowers; SAED
patterns for (d) an ensemble of nanoflowers and (e) a single nanoflower.
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Fig. 2 Powder XRD data for SnS nanoflowers.
Sn : S ratio of 1 : 1 and EDX mapping data (Fig. S3w) confirm
the uniform distribution of Sn and S throughout the
nanoflowers.
Powder X-ray diffraction (XRD) data for the SnS nanoflowers (Fig. 2) indicate an orthorhombic cell with lattice
constants of a = 11.18(9) Å, b = 3.98(5) Å, and c = 4.33(2) Å,
which matches with that of GeS-type SnS.16 No crystalline
impurities are observed. Fig. 1d shows a selected area electron
diffraction (SAED) pattern for an ensemble of SnS nanoflowers, and the ring pattern is also consistent with polycrystalline GeS-type SnS with no crystalline impurities.
Fig. 1e shows an SAED pattern for a single SnS nanoflower,
and the presence of discrete spots rather than complete rings is
consistent with a limited number of crystal domains formed
from the hierarchical arrangement of single-crystal SnS nanosheets. Related synthetic protocols are known to yield SnS,10
SnSe,17 GeS,18 and GeSe nanosheets,18 and given the layered
crystal structure of GeS,15–18 the formation of nanosheet
subunits is not unexpected. Similar SnS nanoflowers form
when SnI4 and SnBr4 are used as precursors (Fig. S4w).
Fig. 3a–d shows TEM images of aliquots taken during the
formation of the SnS nanoflowers using SnBr4. Initially, at early
reaction times (o1 min at 200 1C), an amorphous nanoparticle
background dominates the sample, along with some larger and
irregularly shaped nanoparticle agglomerates (Fig. 3a and S5w).
Fig. 3 TEM images of (a–d) aliquots taken during the formation of
SnS nanoflowers at t = 1, 3, 5, and 15 min at 200 1C, (e) amorphous
nanoparticles and (h) initial crystalline cores, along with (f,g) the
corresponding SAED patterns, and (i,j) nanoparticles attaching to the
growing nanosheet subunits. The SAED patterns in panels (f) and
(g) correspond to the TEM images in panels (e) and (h), respectively,
and panel (j) is an enlargement of panel (i), as indicated.
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The SAED pattern (Fig. 3f) taken from a large region of the
‘‘background’’ nanoparticles (Fig. 3g) confirms that they
are amorphous. As the reaction progresses, the nanoflower
morphology begins to emerge, with branching occurring as
early as 2–3 min at 200 1C. Interestingly, the spots observed on
the SAED pattern (Fig. 3g) taken from one of the early
branched structures (Fig. 3h) indicate that they are, for the
most part, single crystals. As the reaction continues at 200 1C,
the branches around the central core serve as nucleation sites
for the growth of nanosheet protrusions, which ultimately
comprise the final SnS nanoflowers. Growth continues
throughout this nanoflower formation process (Fig. 3a–d),
progressing from the B250 nm core nanostructures at the
early stages to the final B2 mm nanoflowers after 20–30 min.
Given these observations, we hypothesize that the smaller
amorphous nanoparticles are consumed and ultimately add
via coalescence to the growing nanoflowers, in analogy to our
observations of nanosheet formation in the related SnSe
system.17 This is consistent with the gradual depletion of the
amorphous nanoparticles from the TEM images in Fig. 3a–d,
their absence in all final samples, and the observation of
nanoparticles attaching to the growing nanosheets (Fig. 3i, j).
In order to study their applicability in lithium ion batteries,
the SnS nanoflowers were first annealed at 600 1C for 1 h
under nitrogen to remove the surfactants. Importantly, the
nanoflowers show excellent thermal stability with full retention of morphology, composition, and phase (Fig. 4 and S6w).
The nanoflowers are also environmentally stable, retaining
their morphology after six months of storage in powder form
under ambient conditions (Fig. S7w). To evaluate their performance, the SnS nanoflowers were used to prepare a lithium
ion battery anode (details in the Supplementary Information)
and tested against a lithium metal counter electrode and with
LiPF6 containing an ethylene carbonate, diethyl carbonate,
and dimethyl carbonate electrolyte solution mixture (volume
ratio of 1 : 1 : 1). The coin-type cell was cycled on a battery
testing system under 50 mA g 1 with charge and discharge
cutoff potentials set at 1.1 V and 0.01 V vs. Li+/Li,
respectively.
Fig. 4 Charge/discharge profile of SnS nanoflowers annealed at
600 1C for 1 h in N2 along with a TEM image of the annealed sample
(inset).
Chem. Commun., 2012, 48, 5608–5610
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Fig. 5 Cycling performance for SnS nanoflowers tested at a current
density of 50 mA g 1 with charge and discharge potentials of 1.1 V and
0.01 V, respectively.
The discharge-charge profiles (Fig. 4) revealed an initial
discharge capacity of approx. 1050 mAh g 1, which is close to
the sum of the theoretical irreversible capacity (356 mAh g 1)
and the maximum theoretical reversible capacity (780 mAh
g 1), based on the formation of metallic Sn and subsequent
generation of a Li4.4Sn alloy:
SnS(s) + 2 Li+ + 2 e - Sn(s) + Li2S(s)
+
Sn(s) + 4.4 Li
+ 4.4 e - Li4.4Sn(s)
(1)
(2)
The efficiency of the first cycle is inevitably low because of
reaction (1). However, after the first discharge plateau at
1.3 V, the discharge-charge profiles become stable. The second
discharge capacity was found to be 600 mAh g 1, which is
77% of the maximum theoretical reversible capacity, and the
cycle life performance remained stable and reversible up to at
least 30 cycles (Fig. 5). The average discharge capacity was
approx. 580 mAh g 1, and the overall coulombic efficiency was
greater than 96%. This performance compares favourably with
other reports of SnS anodes, achieving higher discharge capacity
than commercial graphite anodes over 30 cycles and showing the
longest cyclability reported to date for colloidal SnS nanostructures.1,10,11 We attribute the high charge/discharge capacity
and electrochemical reversibility to the unique morphology of
the SnS nanoflowers. The large surface area and small size
yield a large electrochemical interfacial region that facilitates
highly efficient Li+ ion insertion, and the open and lowdensity morphology in the discrete hierarchical nanostructures
facilitates efficient Li+ ion diffusion through the anode material
and tolerance of phase changes during cycling.
In summary, SnS nanoflowers consisting of a hierarchical
arrangement of nanosheet subunits surrounding a central core
have been synthesized using a simple one-pot heat-up method
in oleylamine. The as-synthesized particles show excellent
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Chem. Commun., 2012, 48, 5608–5610
thermal and environmental stability, and they were found to
be active as anodes in lithium ion batteries, exhibiting high
charge/discharge capacity and excellent reversibility over
30 cycles. Given the insights into how the SnS nanoflowers
form and how their morphology impacts their performance in
lithium ion batteries, we anticipate that the morphology could
be further tuned and that additional nanostructure-property
correlations will emerge.
This work was supported primarily by the U.S. Department
of Energy, Office of Basic Energy Sciences, Division of Materials
Sciences and Engineering, under award #DE-FG02-08ER46483
(R.E.S., materials synthesis and characterization). D.D.V.
acknowledges support from an NSF Graduate Research
Fellowship. O.D.H. was supported by an NSF REU program
under grant #CHE-1004641. S.C. and D.W. thank the
Assistant Secretary for Energy Efficiency and Renewable
Energy, Office of Vehicle Technologies of the U.S. Department
of Energy under Contract No. DE-AC02-05CH11231,
Subcontract No. 6951378 under the Batteries for Advanced
Technologies (BATT) Program for funding (battery testing).
TEM and FESEM imaging were performed in the Electron
Microscopy Facility of the Huck Institutes of the Life Sciences
and in the Materials Characterization Facility at the Penn
State Materials Research Institute.
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