High Sulfur Content Material with Stable Cycling in Lithium-Sulfur Batteries

Article abstract of DOI:10.1002/anie.201708746

We demonstrate a novel crosslinked disulfide system as a cathode material for Li-S cells that is designed with the two criteria of having only a single point of S?S scission and maximizing the ratio of S?S to the electrochemically inactive framework. The

Full text of DOI:10.1002/anie.201708746

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Accepted Article
Title: High Sulfur Content Material with Stable Cycling in Li-S Batteries
Authors: Molleigh B. Preefer, Bernd Ochsmann, Craig J. Hawker, Ram
Seshadri, and Fred Wudl
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201708746
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10.1002/anie.201708746
Angewandte Chemie International Edition
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High Sulfur Content Material with Stable Cycling in Li–S Batteries
Molleigh B. Preefer,[a,b,d] Bernd Oschmann,[b,d] Craig J. Hawker,*[a,b,c,d] Ram Seshadri,*[a,b,c,d] and
Fred Wudl*[c, d]
Abstract: We demonstrate a novel crosslinked disulfide system as a
cathode material for Li–S cells that is designed with the two criteria of
having only a single point of S–S scission and maximizing the ratio of
S–S to the electrochemically inactive framework. The material
therefore maximizes theoretical capacity while inhibiting the formation
of polysulfide intermediates that lead to parasitic shuttle. The material
we report contains a 1:1 ratio of S:C with a theoretical capacity of
609 mAh g–1. The cell gains capacity through 100 cycles and has
98 % capacity retention thereafter through 200 cycles, demonstrating
stable, long-term cycling. Raman spectroscopy confirms the proposed
mechanism of disulfide bonds breaking to form a S–Li thiolate species
upon discharge and reforming upon charge. Coulombic efficiencies
near 100% for every cycle, suggesting the suppression of polysulfide
shuttle through the molecular design.
These soluble intermediates migrate between the anode and the
cathode, cause rapid capacity fade and poor Coulombic efficiency,
eventually short-circuiting the cell.
There have been many efforts to combat polysulfide shuttle,
including nanostructured carbon-sulfur cathodes,[11–14] polysulfide
reservoirs,[15,16] encapsulation in carbon[17–23] and TiO2,[24]
polysulfide mediators,[25,26] and altering solvent conditions,[27–30]
among others.[31–33] Our approach exploits the chemical
functionality of disulfides to prevent the initial formation of
polysulfides by only providing one point of electrochemical
scission. De Jonghe et al. have shown impressive
electrochemical reversibility using disulfide-containing solid-state
cathode materials with polyethylene oxide electrolytes versus
lithium, however the sulfur content of the active material is a
maximum of 33 atom percent or less.[34–36] To address this
challenge, we have designed a crosslinked disulfide with 50 atom
percent sulfur content (C6S6 monomer containing roughly
With increasingly widespread use of renewable energy from
intermittent sources such as solar and wind comes a pressing
need for improved energy storage devices. However, currently
used lithium-ion batteries involve intercalation, specifically
layered oxide cathodes with graphite anodes, which limit energy
densities and specific capacities due to the relatively heavy host
structures and state of charge limitations. One of the most
promising systems to overcome this challenge is Li–S batteries,[1]
owing to its high theoretical gravimetric capacity based on a
conversion redox reaction (1675 mAh g–1 assuming reduction of
S0 to S2–), making it especially attractive for applications in
portable electronics and electric vehicles. Additionally, sulfur is
nontoxic and an underutilized, abundant byproduct of oil refining,
making it a sustainable alternative to toxic, expensive transition
metals used in common intercalation materials.[2]
Major challenges of Li–S include the low electronic
conductivity of sulfur,[3] leading to low utilization of active material
and therefore low capacity. Additionally, volume changes during
cycling result in the loss of electrical contact, leading to device
failure.[4–6] Furthermore, when sulfur is cycled with commonly
used ether-based electrolytes, soluble polysulfide intermediates
form[7,8] and create what is known as polysulfide shuttle.[9,10]
50 mole-%
conventional
S
and 50 mole-% C) that operates using the
Li–S electrolyte system,
bis(trifluoromethane)sulfonamide lithium salt (LiTFSI) in
dioxolane (DOL) and dimethoxyethane (DME).
The new, highly stable Li–S cell we report has a theoretical
gravimetric capacity of 609 mAh g–1 based on 6 moles of lithium
per C6S6 monomer unit. The high sulfur content benefits the
performance two-fold: there is potential for high gravimetric
capacity and, if full conversion occurs, creates a highly charged
discharged product. In its discharged state, the active material is
expected to form C6S66– with six Li+ counterions, creating a highly-
charged structure that is insoluble in the ether-based electrolyte.
This allows the material to undergo a reversible redox reaction
over hundreds of charge cycles without shuttle or capacity fade.
Additionally, if full reduction does not occur, the monomer units
will be bound in the crosslinked framework, also preventing
dissolution into the electrolyte.
The crosslinked disulfide polymer was obtained in a three-
step synthesis described in Scheme 1. C6(SLi)6 was synthesized
from a modified procedure previously reported by Harnisch and
Angelici,[37] which was subsequently oxidized in purified air to
[a]
[b]
M. B. Preefer, Prof. Dr. C.J. Hawker, Prof. Dr. R. Seshadri,
Department of Chemistry and Biochemistry, University of California
Santa Barbara, CA 93106 (USA)
M. B. Preefer, Dr. B. Oschmann, Prof. Dr. C.J. Hawker,
Prof. Dr. R. Seshadri
Materials Research Laboratory, University of California
Santa Barbara, CA 93106 (USA)
E-mail: seshadri@mrl.ucsb.edu
[c]
[d]
Prof. Dr. C. J. Hawker, Prof. Dr. R. Seshadri, Prof. Dr. F. Wudl
Materials Department, University of California
Santa Barbara, CA 93106 (USA)
M. B. Preefer, Dr. B. Oschmann, Prof. Dr. C.J. Hawker,
Prof. Dr. R. Seshadri, Prof. Dr. F. Wudl
Mitsubishi Chemical Center for Advanced Materials, University of
California
Santa Barbara, CA 93106 (USA)
Scheme 1. Synthesis scheme of crosslinked disulfide active material.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201708746
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obtain the crosslinked structure. The thiol form of the intermediate
structure, benzenehexathiol (BHT), is well-studied and is
commonly used as a ligand in MOF and coordination polymer
chemistry.[38,39] Lithium was used as the counterion in the thiolate
intermediate so that the starting material would be assembled in
a partially discharged state for an Li–S cell if the material does not
fully crosslink at every site.
Solid state 13C NMR with magic angle spinning shows a
signal at 127 ppm arising from aromatic C. The presence of a
7
shoulder at 137 ppm (Figure 1a) and a Li NMR signal at 0 ppm
(Figure S1) suggest the presence of residual thiolate. However,
the sulfur high-resolution X-ray photoelectron spectrum (XPS)
indicates the presence of only one sulfur oxidation state (Figure
1c), and the lithium high-resolution spectrum shows no signal
(Figure 1d). Therefore, the disulfide active material is almost
entirely crosslinked prior to cycling.
Figure 2. Electrochemistry of the disulfide material with LiTFSI in
DOL/DME as the electrolyte system, and Li is both the anode and the
reference electrode. (a) Cyclic voltammetry from 3.2 V to 0.75 V at a
sweep rate of 0.1 mV/s. (b) Galvanostatic cycling with potential limitation
(GCPL) at a rate of C/10 from 3.0V to 1.0V shown for 200 cycles. (c)
Capacity for each cycle (charge and discharge) from GCPL increases
through cycle 100 and maintains 150 mAh/g through 200 cycles. The error
bar at cycle 100 represents the standard deviation of capacities observed
over 10 different cells. (d) Coulombic efficiency, defined as charge divided
by discharge capacity, hovers around 100% for all 200 cycles.
oxidation at 2.5 V. GCPL also reflects the same redox potentials
from the presence of plateaus at the corresponding potentials
(Figure 2b). The Galvanostatic cycling was performed with
identical coin cells for 200 cycles. The gravimetric capacity grows
over the first 100 cycles and stabilizes through cycle 200 with
98 % capacity retention (Figure 2c). The Coulombic efficiency
nears 100 % for all cycles following cycle 1 (Figure 2d),
suggesting the crosslinked disulfide material circumvents one of
the most detrimental processes in classical Li–S batteries,
parasitic polysulfide shuttle. All electrochemical testing was done
without any additives to the electrolyte or the cathode material,
demonstrating the exceptional stability of the disulfide material.
Extended electrochemical testing is demonstrated with fewer cells
to confirm its longevity (Figure S2).
Figure 1. Characterization of the disulfide active material prior to
electrochemical testing. (a) 13C NMR spectrum with MAS at 16 kHz show two
peaks in the aromatic region, and the asterisks label the spinning sidebands.
(b) Raman spectrum with labelling of characteristic peaks. (c) High resolution
XPS in the sulfur 2p region with fits for the 2p1/2 and 2p3/2 components of one
oxidation state. (d) High resolution XPS in the lithium 1s region of the pristine
material (no peak detected) and the discharged material at 1.0 V.
Raman spectroscopy (Figure 1b) provides significant
structural insight into the materials, both in the pristine state and
at different states of charge. The pristine disulfide powder displays
a strong disulfide peak at 480 cm–1, assigned from previous
literature.[40,41] Also seen is a strong aromatic C–S mode at 1050
cm–1, as well as an aromatic C–C mode at 1450 cm–1 (Figure 1b).
These three peaks signify the major structural components of the
active material, notably benzene rings connected through
disulfide bonds.
Electrochemical testing of the disulfide material reflects stable
cycling through both cyclic voltammetry (CV) and Galvanostatic
cycling with potential limitation (GCPL) with lithium as both the
anode and the reference electrode. CV was performed in a coin
cell at 0.1 mV/s and indicates reversibility is established after the
first cycle (Figure 2a). The main reduction occurs at 2.25 V with a
less pronounced reduction at 2.18 V. There is a one-step
Ex situ analysis provides confirmation for and insight into
how the active material performs at the molecular level and
microstructural level. Raman spectroscopy is particularly useful
for confirming the mechanism by which redox occurs. The
disulfide bond, which is the only electrochemically active part of
the material, can be tracked by the peak at 480 cm–1, starting in
the pristine material. Once cast, the Raman spectrum shows the
emergence of the graphitic bands at 1350 cm–1 and 1580 cm–1
from the Ketjen Black carbon additive (Figure 3a, shaded region).
Spectra were taken after 100 cycles at the most charged and
discharged states, 3.2 V and 1.0 V, respectively. The charged
product clearly shows a strong disulfide peak that even grows in
intensity from the pristine material, suggesting an increase in
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10.1002/anie.201708746
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crosslinking. The peak at 480 cm–1 recedes in the spectrum of the
discharged product, and a new peak at 270 cm–1 emerges. By
comparison with a less easily oxidized analogous material, lithium
benzene-1,3,5-tris(thiolate), the peak at 270 cm–1 can be assigned
to -S–Li (Figure S3). Therefore, the presence of the disulfide peak
increased optimization, particularly engineering the electrodes to
incorporate improved ionic and electronic properties, the cathode
material reported herein has potential for practical applications
with capacities close to the theoretical amount.
for the charged state and its suppression upon discharging Experimental Section
indicate the electrochemical cleavage and reformation of the
Preparation and characterization of disulfide active material:
disulfide bonds in the network. A larger sampling of Raman
spectra (Figure S4) suggests that complete conversion is not
taking place during cycling, which is consistent with the material
not reaching theoretical capacity during cycling. Different regions
in the electrodes display different extents of charge or discharge,
suggesting not all particles undergo redox with every cycle.
Complete characterization of intermediate products can be found in
the Supporting Information.
Synthesis of hexakis(benzylthio)benzene:
Hexakis(benzylthio)benzene was synthesized by a modified procedure
reported by Harnisch and Angelici.[37] NaH (60 % dispersion in mineral oil),
hexachlorobenzene (analytical grade), benzyl mercaptan (99 %), Li chips,
and NH3 were used as received from Sigma Aldrich. All solvents were dried
at a solvent system before use. After washing 8.0 g of NaH (60%
dispersion in oil, 0.20 mol) with 3 x 20 mL of hexanes and with 2 x 20 mL
of DMF, NaH was suspended in 150 mL of DMF and cooled to 0 °C. Benzyl
mercaptan (23.5 mL, 0.20 mol) was added slowly to the suspension
followed by the addition of hexachlorobenzene (4.76 g, 16.7 mmol). The
reaction mixture was stirred for 10 min at 0 °C before it was allowed to
warm to room temperature. The mixture was stirred at room temperature
overnight. 10 mL of isopropyl alcohol was added to the reaction mixture
and stirred for 15 minutes, followed by the addition of 75 mL of deionized
water. The mixture was extracted with chloroform and dried with
magnesium sulfate. The organic layer was concentrated to 200 mL using
reduced pressure, and the product was precipitated with 400 mL of
methanol. 1H NMR (500 MHz, CD2Cl2): δ 7.22 (m, 5H, Ar-H), 4.07 (s, 2 H,
CH2).13C NMR (500 MHz, CD2Cl2): δ 147, 138, 129, 128, 127, 42. FDMS
TOF m/z: Calcd for C48H42S6 810.16; Found 810.13.
Synthesis of crosslinked lithium benzene hexathiolate:
496 mg of Li (71 mmol) was added to 70 mL of liquid ammonia at -78 °C.
A solution of hexakis(benzylthio)benzene (4.8 g, 6.0 mmol) in 15 mL of dry
THF was added. After stirring for one hour at -78 °C, 30 mL of degassed
methanol were added and the reaction mixture was warmed to room
temperature over 2 hours. 50 mL of degassed water were added and side
products were removed by the extraction with diethyl ether. Lithium
benzene hexathiolate (1.2 g, 65 % yield) was concentrated by freeze
drying. The hexathiolate is extremely air-sensitive, so crosslinking was
conducted immediately by dissolving lithium benzene hexathiolate (250
mg, 0.8 mmol) in water (10 mL) and by purging with air purified through a
bubbler of sulfuric acid for 48 hours. The crosslinked material precipitated
during purging. After 48 hours, the reaction mixture was centrifuged to
isolate the precipitated material. The material was redispersed in water
and centrifuged followed by two more redispersion and centrifugation
steps. The final product was isolated by freeze drying over night with a
yield of 180 mg (84%).
Figure 3. Raman spectroscopy of the pristine powder, pristine electrode,
charge, and discharged products compare disulfide bonds breaking and
reforming. The highlighted region from 1350 cm–1 to 1580 cm–1 indicates the
graphitic contribution from the Ketjen Black additive.
Scanning electron microscopy (SEM) provides further
understanding of the capacity gain over the first 100 cycles, which
is an unexpected occurrence in the Li–S system. Comparing SEM
images from the cast electrodes before and after cycling reveals
the disulfide particles undergo self-microstructuring, creating
porosity over many cycles (Figures S5, S6). The increased
surface area may account for the capacity gain seen in the
Galvanostatic cycling over the first 100 cycles, as lithium can
access the densely-crosslinked active material more readily at the
surface. Once the particles have undergone this microstructural
change, the electrochemical performance reaches a steady state,
and the capacity does not change from cycle 100 through 200.
In summary, we report a new crosslinked disulfide material
that exhibits stable cycling over hundreds of cycles and good
Coulombic efficiency with no electrolyte additives and minimal
device engineering. The use of a disulfide active material with
maximized sulfur content prevents polysulfide shuttle by providing
Preparation and characterization of electrodes:
The active disulfide material and conductive carbon additive (AzkoNobel
Ketjenblack EC-600JD) were mixed in an aqueous dispersion of
carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR)
(CMC:SBR = 1:1 wt/wt) in a weight ratio of disulfide:KB:CMC/SBR =
50:35:15. The slurry was cast on carbon coated aluminium foil as the
current collector (Toya-Carbo-50G01, Toyal) and dried at 60°C for 12 h
under vacuum. Disc electrodes with a diameter of 10 mm were punched
after drying. Active material mass loadings were in the range of 0.8 mg and
1.2 mg per electrode. Average thickness of the electrodes was 30 μm.
Electrochemical measurements:
Galvanostatic cycling was conducted using 2032-type coin cell (MTI
Corporation) with metallic Li as the counter electrode. 19mm disks of
polypropylene (Celgard 3501) were used as the separator, and the
only one point of electrochemical scission,
a crosslinked
electrolyte
was
composed
of
1.0
M
lithium
framework, and a highly charged discharged product. With
bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in 1,3-dioxolane
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Put a Ring on It: A
crosslinked disulfide
material with high sulfur
content displays stable
cycling in a Li–S battery
with no evidence of the
detrimental polysulfide
shuttle.
Molleigh B. Preefer,
Bernd Oschmann, Prof.
Dr. Craig J. Hawker,* Prof.
Dr. Ram Seshadri,* and
Prof. Dr. Fred Wudl*
Page No. – Page No.
Title
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