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Cite this: RSC Advances, 2013, 3, 3540
Received 26th November 2012,
Accepted 17th January 2013
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Exceptional electrochemical performance of
rechargeable Li–S batteries with a polysulfidecontaining electrolyte3
Shuru Chen, Fang Dai, Mikhail L. Gordin and Donghai Wang*
DOI: 10.1039/c3ra23070h
www.rsc.org/advances
Soluble lithium polysulfides (Li2Sx, x ¢ 6) were used as co-salts/
additives in an ether-based electrolyte for lithium sulfur (Li–S)
batteries. By optimizing the concentration of the polysulfide
species and the amount of electrolyte, the Li–S batteries show
high and stable discharge capacity, outstanding rate capability,
and exceptional cycling performance.
After decades of intensive development, lithium ion (Li-ion)
batteries are still incapable of meeting the energy density
requirements of emerging applications such as electric vehicles.
The exploration of new electrochemistry and new materials is thus
necessary for the creation of high-energy battery systems.1,2 The
rechargeable lithium-sulfur (Li–S) battery is a promising candidate
because sulfur has a high theoretical specific capacity of 1675 mA
h g21 and a high specific energy of 2600 W h kg21.3 The Li–S
system operates by conversion of sulfur through a multistep redox
reaction, forming different lithium sulfide products (Li2Sx, 1 ¡ x
¡ 8).4 Ether-based electrolytes are normally used in Li–S batteries
because of their ability to dissolve insulating polysulfides and thus
improve their reaction kinetics.5–8 However, this dissolution can
also lead to loss of active material from the cathode, causing
capacity fading, and to a shuttle phenomenon that leads to poor
coulombic efficiency.7–10 The formation of insoluble, insulating
Li2S on the surface of both the cathode and the lithium anode also
contributes to poor sulfur utilization and capacity fading because
of its poor reversibility.3,11,12
Considerable effort has been devoted to engineering carbon/
sulfur (C/S) composites that are capable of trapping soluble
polysulfides by physical or chemical adsorption, or of enabling the
reversible reaction of Li2S at the positive electrode.13–25 Electrolyte
additives, e.g. LiNO3 and P2S5, were reported to passivate lithium
metal and suppress the redox shuttle of polysulfides, resulting in
improved coulombic efficiency.26–30 P2S5 was also reported to
promote the dissolution and reversible reaction of Li2S.30
Nevertheless, none of these approaches are sufficient to fully
The Pennsylvania State University, Department of Mechanical and Nuclear
Engineering, University Park, PA, 16802, USA. E-mail: [email protected]
3 Electronic supplementary information (ESI) available: experimental details and
more figures. See DOI: 10.1039/c3ra23070h
3540 | RSC Adv., 2013, 3, 3540–3543
address the dissolution of polysulfides and the accumulation of
Li2S.11,12 Since the dissolution of polysulfides is inevitable, Li–S
liquid batteries that directly use dissolved polysulfides as a
catholyte, as reported decades ago,6–8 have recently been
considered again; their capacity and cyclability are still not
satisfactory.31–33 Alternatively, increasing sulfur loading in the
cathode might be expected to increase the cell capacity and
mitigate the effect of losing active mass through dissolution;
however, even lower sulfur utilization and faster capacity fading
have usually been reported, possibly due to the poorer conductivity
and formation of more insoluble products in the cathode.11,15,17,25
In this communication, we report a simple strategy for high
performance Li–S batteries by combining a conventional C/S
cathode with soluble lithium polysulfide-containing electrolyte
(Scheme 1). The idea of adding polysulfides as co-salts/additives in
electrolyte is to provide extra capacity and compensate for the
active mass loss, and was first tested by C. Barchasz et al. using 0.1
M Li2S6.28 They came to the conclusion that the extra sulfur did
not help to increase the capacity or cycle life. In contrast, we found
that by optimizing the concentration of polysulfide species and the
amount of electrolyte, we can avoid the unfavorable formation of
insoluble Li2S and thereby dramatically improve the capacity,
cyclability, and rate capability of the cell.
Scheme 1 Schematic configuration of a Li–S cell with a polysulfide-containing
electrolyte, sulfur/carbon cathode, and lithium anode.
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Fig. 1 CV curves in 10 mL of both polysulfide-free and -containing (Li2S9, [S] = 2 M)
electrolyte at 0.1 mV s21 scanning rate.
Polysulfide-containing electrolytes with the desired sulfur
concentration ([S]) and average polysulfide chain length were
prepared by chemically reacting stoichiometric amounts of sulfur
and Li2S in a polysulfide-free electrolyte of 0.1 M LiTFSI + 0.2 M
LiNO3 in DOL–DME (1 : 1, v : v). To demonstrate the advantages
of these electrolytes, cathodes containing 50 wt% sulfur prepared
by ball milling were used rather than any novel porous carbon/
sulfur composites. The cathodes had an average loading of 0.6 mg
S cm22 with an area of 1.13 cm2 and were tested in coin cells.
Fig. 1 shows the first two cyclic voltammetry (CV) curves of
sulfur cathodes in 10 mL of both polysulfide-free and -containing
(Li2S9, [S] = 2 M) electrolyte. The profiles look similar: they show
two main cathodic peaks at around 2.3 and 2.0 V, which are
related to the change from elemental sulfur to the higher-order
lithium polysulfides (Li2Sx, x . 4), and the reduction of higherorder lithium polysulfides to lower-order lithium polysulfides
(Li2Sx, x ¡ 4), respectively. In the subsequent anodic scans, one
main oxidation peak at 2.35 V and another shoulder peak at 2.45 V
were clearly observed, indicating the conversion from lower-order
lithium polysulfides to elemental sulfur.7 However, all the peak
currents in the polysulfide-containing electrolyte are much higher
than in the polysulfide-free electrolyte, indicating the extra
capacity contribution from the Li2S9 in the electrolyte.
As shown in Fig. 2a, the discharge–charge profiles of cells in 10
mL of polysulfide-free and -containing (Li2S9, [S] = 2 M) electrolyte
at a rate of C/3 (1C = 1680 mA g21 of S in the cathode) have two
voltage plateaus: a higher one at 2.4 V and a lower one at 2.0 V,
which are consistent with the CV results. The initial discharge
capacities in the polysulfide-free and -containing electrolytes are
980 mA h g21 and 1460 (750) mA h g21, respectively (capacity out
of parentheses is based on S in the cathode only while the value in
parentheses is based on total S in both cathode and electrolyte).
Discharge capacities decrease gradually with cycling in polysulfidefree electrolye but show little change in polysulfide-containing
electrolyte. Similar voltage profiles and cycling performance are
found when using other soluble lithium polysulfides such as Li2S8
and Li2S6 with [S] equal to 2 M under the same testing conditions
(Fig. S1, ESI3). This indicates that the performance is largely
independent of polysulfide chain length, likely because the
reduction mechanism for the first reduction step of sulfur and
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Fig. 2 (a) Voltage profiles and (b) cycling performance at a rate of C/3 in 10 mL of
both polysulfide-free and -containing (Li2S9, [S] = 2 M) electrolyte between 1.6 V and
2.6 V.
polysulfides may be the same for all polysulfides with a chain
length of at least 6.34
The cycling performance of sulfur cathodes in polysulfide-free
and -containing electrolyte is presented in Fig. 2b. The coulombic
efficiency for both cells is close to 100%, owing to the protection of
lithium metal by LiNO3 additive, as is widely reported.26–29
Capacity retention is dramatically improved by using 10 mL of
polysulfide-containing electrolyte with [S] = 2 M. The discharge
capacity is stabilized at ca. 1460 (750) mA h g21 in this electrolyte,
while with polysulfide-free electrolyte the capacity decreases to
below 480 mA h g21 after only 50 cycles. Although with
polysulfide-containing electrolyte the capacity is around 750 mA
h g21 of total S, the cell capacity is almost doubled in comparison
to cells with polysulfide-free electrolyte. To find out the capacity
contribution of polysulfide to the cell, Li–S liquid cells with 10 mL
of electrolyte containing 2 M ([S]) polysulfide were tested using
positive electrodes of 1.13 cm2 without any sulfur, including bare
Al foil (low surface area and porosity) and an ordered mesoporous
carbon (OMC as reported in the literature,17 high surface area and
porosity). As illustrated in Fig. S2, ESI,3 the initial discharge
capacity is only around 850 mA h g21 and 160 mA h g21 of
dissolved S for the OMC electrode and the Al foil electrode,
respectively. This indicates that the capacity contribution of the
polysulfide is greatly determined by the surface area and porosity
of the positive electrode. Nevertheless, capacity retention is poor
using the polysulfide-containing electrolyte itself, reiterating the
advantage of combining the C/S cathode with a soluble lithium
polysulfide-containing electrolyte to obtain high performance.
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Fig. 4 (a) Voltage profiles at various rates and (b) rate capability in 10 mL
polysulfide-containing electrolyte with [S] = 2 M.
Fig. 3 Cycling performance at a rate of C/3 in different amounts of electrolyte with
different sulfur concentrations.
The concentration of sulfur species and the amount of
electrolyte show significant effects on the performance of Li–S
batteries, though the performance is independent of polysulfide
chain length. In Fig. 3, it is clear that discharge capacities are
stable for the first 40 cycles as the amount of polysulfidecontaining electrolyte is kept at 10 mL for each cell. The stable
discharge capacity increases from 1050 (700) mA h g21 to 1450
(750) mA h g21 when [S] increases from 1 M to 2 M, indicating the
extra capacity provided by adding more polysulfides. When the
amount of electrolyte is doubled, although the mass of S added to
each cell is also doubled, the initial discharge capacity hardly
changes. The capacity drops fairly quickly in the first 10 cycles,
similar to previously reported results,35 and stabilizes at 760 (500)
mA h g21 and 1250 (650) mA h g21 when [S] is 1 M and 2 M,
respectively. When [S] is further increased to 4 M while the
amount of electrolyte is kept at 10 mL, the stable capacity increases
from 1460 mA h g21 to around 1700 mA h g21 based on S in the
cathode; but the capacity decreases from 750 mA h g21 to 570 mA
h g21 based on total S in both cathode and electrolyte, compared
with that of the cell with [S] of 2 M. This indicates decreased
utilization of total S with [S] higher than 2 M because the capacity
becomes mainly limited by the surface area and porosity of the
carbon electrode when a sufficient amount of sulfur species are
present in the liquid electrolyte. The optimal concentration of
polysulfides in the electrolyte may be close to 2 M for achieving the
highest utilization of total S.
Another interesting phenomenon is that all cells tested in the
polysulfide-containing electrolyte stabilized within about 10 cycles
with capacities below 836 mA h g21 based on total S from the
cathode and electrolyte (50% utilization of S with a capacity of
1672 mA h g21). This suggests there is less than 1e2 per S
transferred during cycling. The ultimate product during discharge
may primarily be slightly soluble Li2S2 and some higher-order
polysulfide such as Li2S4. Conductive surfaces in the positive
electrode may be passivated by significant precipitation of Li2S2
during discharge, induced by the polysulfides added to the
electrolyte, leading to huge polarization and causing the cell to
reach the cut-off voltage before much Li2S2 can be further reduced
to insoluble Li2S.10 This may be confirmed by the sharp drop of
3542 | RSC Adv., 2013, 3, 3540–3543
the discharge curves at the end of the discharge cycle (Fig. 2a), as a
slope at the end of the discharge is attributed to conversion of
Li2S2 to Li2S, which is kinetically slow and normally suffers from
high polarization.32 By avoiding the irreversible formation of Li2S,
the cell can be reversibly cycled between elemental sulfur and
Li2S2 through multiple soluble polysulfides. These reactions are
dominated by the interfacial charge transfer and are highly
reversible and kinetically fast.36 As a preliminary proof, we
controlled the depth of discharge (DOD) of cells with a
polysulfide-free electrolyte to avoid formation of Li2S. Cyclability
was improved, as expected, when an appropriate capacity cut-off of
600 mA h g21 was selected – however, the improvement came at
the expense of cell capacity (Fig. S3, ESI3). We believe that the
similar cycling performance of this cell to that of cells with the
polysulfide-containing electrolyte implies that a similar mechanism is at work. However, further studies to directly confirm this
hypothesis are still needed.
The rate performance of Li–S batteries using the polysulfidecontaining electrolyte was also tested. When the rate is increased
to 5C (8.4 A g21), the sample still delivers a capacity of more than
600 (310) mA h g21 with a coulombic efficiency of close to 100%,
and additionally shows relatively low polarization with a second
voltage plateau at y1.8 V, indicating a remarkable high-rate
capability (Fig. 4a and b). Moreover, the discharge capacity can be
recovered when the rate is returned to C/3, showing great
reversibility.
In summary, we used soluble lithium polysulfides as co-salts/
additives along with LiNO3 in an ether-based electrolyte for Li–S
batteries and found that: 1) the cell capacity can be increased and
stablized by adding polysulfides; 2) the extra capacity and
improved cyclability provided by the addition of polysulfides
relates to the concentration of sulfur in the electrolyte rather than
the total amount of sulfur in the cell; and 3) increasing the
amount of electrolyte may cause more polysulfides to dissolve and
thus decrease the cell capacity. By optimizing the concentration of
polysulfides and the amount of electrolyte, we successfully
improved the cycling performance and rate capability of Li–S
batteries. The excellent performance may be due to the
polysulfides helping to prevent formation of insoluble Li2S. This
mechanism may in turn provide a new framework for achieving
better performance with Li–S batteries.
This work was supported by the Assistant Secretary for Energy
Efficiency and Renewable Energy, Office of Vehicle Technologies
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