Post-print accepted by J. Power Sources. Now published at doi: 10.1016/j.jpowsour.2014.04.090

Functional, water-soluble binders for improved capacity and stability of lithium-sulfur
batteries
Matthew J. Lacey*, Fabian Jeschull, Kristina Edström and Daniel Brandell
5
Department of Chemistry – Ångström Laboratory, Uppsala University, Box 538,
Lägerhyddsvägen 1, SE-75121 Uppsala, Sweden
E-mail: matthew.lacey@kemi.uu.se
10

Keywords: lithium-sulfur, binder, poly(vinylpyrrolidone), poly(ethylene oxide)
Note on changes to the manuscript: additions are in red, deletions are struck through
Abstract

15

Binders based on mixtures of poly(ethylene oxide) (PEO) and poly(vinylpyrrolidone) (PVP)
are here shown to significantly improve the reversible capacity and capacity retention oflithium-sulfur batteries compared to conventional binders. This mixed binder formulation
combines the local improvement to the solvent system offered by PEO and the lithium(poly)sulfide-stabilising effect of PVP. Cells with cathodes made of simple mixtures of sulfur

20

powder and carbon black with a binder of 4:1 PEO:PVP exhibited a reversible capacity ofover 1,000 mAh g-1 at C/5 after 50 cycles and 800 mAh g-1 at 1 C after 200 cycles.
Furthermore, these materials are water soluble, environmentally friendly and widely available,
making them particularly interesting for large-scale production and applications in, forexample, electric vehicles.

25
1. Introduction

The demand for ever-higher energy density in portable electrical storage is increasingly
motivating research in systems beyond lithium-ion. Of these, the lithium-sulfur couple, with a30theoretical energy density of 2,600 Wh kg -1, is particularly promising, with the possibility offully-assembled battery packs boasting an energy density two to three times larger than that ofcurrent Li-ion[1,2]. However, the Li-S system suffers from short cycle life and a high degreeof self-discharge, attributable to the well-documented polysulfide redox shuttle[3].

1Post-print accepted by J. Power Sources. Now published at doi: 10.1016/j.jpowsour.2014.04.090

Furthermore, the combination of poor electronic and ionic conductivity of the charge anddischarge products with poor electrochemical reversibility limit sulfur utilization and makethis a system of relatively low rate capability compared to Li-ion[4].
In the last few years, much of the research within this field has considered how cathode5

design can mitigate these drawbacks, with such varied strategies as templated carbon[5–7],
nanotube- or graphene based[8–11] or even conducting polymer[12–14] host structures,control of sulfur distribution in the host and barriers to polysulfide dissolution[15–18].
However, the significance of such strategies is a matter of considerable debate: it has
previously been demonstrated that a sophisticated host structure or even homogeneous mixing

10

is not necessary for high capacity[19], and that complex syntheses of nanomaterials may not
be sufficiently scalable for wider commercialisation[20]. The major electrochemical processes
in the Li-S cell are the reactions of soluble polysulfide anions; the capacity and rate capabilityof the battery are therefore extremely dependent on the electrochemical kinetics andconcentration of polysulfides in solution, and their access (diffusion) to the carbon

15

surface[4,21]. Of course, the benefit of a higher concentration of polysulfides is offset by anenhancement of undesirable reactions at the anode, which contributes to self-discharge andirreversible loss of active mass to the anode[3].
While protection of the anode is clearly of great importance in extending cycle life[22,23], theelectrolyte system is similarly important in ensuring a high capacity and rate capability; the20

latter of these must still merit attention considering the potential applications of the Li-S
system, for example electric vehicles. A fact that is perhaps often overlooked is that, thecathode binder, as well as imparting mechanical stability to the cathode composite, can also
be considered part of the electrolyte system in the Li-S cell. This is because polymers may
swell or dissolve in the electrolyte and, unlike Li-ion systems, polysulfides in solution may

25

interact with functional groups in the polymer. Furthermore, despite various articles on theperformance of different binders in the Li-S system, such phenomena have not been widely2Post-print accepted by J. Power Sources. Now published at doi: 10.1016/j.jpowsour.2014.04.090

nor systematically investigated[1,24–28]. We have touched on this idea in a recent paper[29],in which we investigated the capacity improvement afforded by PEO when used as a binder,and concluded that PEO locally modifies the electrolyte system, suppressing passivation of
the cathode and improving reaction kinetics. The observed performance improvement is
5

consistent with studies elsewhere in which improved capacity is observed with the use of
short-chain PEG as a cathode coating[17] or as an electrolyte solvent[30]. Similarly, Seh etal[31] recently used DFT calculations to evaluate the strength of interaction between
functional polymers and lithium sulfides. These calculations led to the identification ofpoly(vinylpyrrolidone) (PVP) as having a strong interaction with lithium (poly)sulfides;

10

subsequent experiments on cathodes using Li2S as a starting material and PVP as the binder
demonstrated remarkable cycling stability compared to cathodes using the conventionalpoly(vinylidene difluoride) (PVdF) binder. The authors suggested that the performance
improvement from the inclusion of PVP could be attributed to the retention of polysulfide at
the cathode as well as its good properties as a dispersant for cathode preparation. Interestingly,

15

similar calculations and a comparable performance improvement have recently been
demonstrated for nitrogen-doped carbon host materials with related (e.g., pyrrolic N-C=O)
surface functional groups[32].
In this work, we aim to further investigate PEO and PVP as binders, as well as mixtures of thetwo materials. Aside from performance enhancement, these polymers are of interest since

20

they are water-soluble, safe, biodegradable and industrially important. The environmental
friendliness and scalability of water-based cathode preparations with such commodity
polymers is extremely relevant to this system, where the active material is potentially very
cheap. To our knowledge, the PEO:PVP binder system has only been reported once, in a
recent paper by researchers at Hydro-Québec[33], however, the choice of binder was not

25

discussed. There are other comparisons of different binders, including PEO[34] andPVP[9,25,35], in the literature, but these studies generally do not discuss the effect of these3

Post-print accepted by J. Power Sources. Now published at doi: 10.1016/j.jpowsour.2014.04.090

binders on the chemistry or electrochemistry of polysulfides. Additionally, evaluation of the
Li-S system is complex in that cell performance (i.e., sulfur utilisation and capacity retention)
can differ dramatically, not just through different host structures, but also on factors including
sulfur loading[21], mixing[19], choice of binder, electrolyte chemistry[30,36] and5

additives[4,37], electrolyte volume[38] and cycling limits[39]. Because of this, especiallycompared to Li-ion systems, it is difficult to compare the efficacy of one strategy with another.
In this study we will attempt to investigate the effect of the binder by comparison with asimple, easily repeatable and well-performing reference system in which changes to othervariables in the system are minimised as much as possible. Reaction kinetics will be studied

10

by a combination of galvanostatic cycling and impedance spectroscopy (IS). Suchelectrochemical techniques are extremely useful for evaluating the overall behaviour of a cellin a system where the complexity of the reaction mechanism and thermodynamic instabilityof the reaction intermediates make precise characterisation by, e.g., spectroscopic methods,
extremely challenging[40].

15

2. Results and discussion

The effect of changing the binder composition was investigated initially by galvanostatic
20

cycling of standardised test cells with varying cathode composition. The full experimental
procedure is detailed in the experimental section. The cathodes in each case simply comprised
composites of sulfur powder and Super P carbon black mixed by planetary ball milling withthe chosen binder and water. The only exception to this procedure was the PEO binder, where
acetonitrile was used; this is because water-based slurries containing only PEO as the binder25

are extremely viscous, and cathode coatings of acceptable quality were only obtained using
acetonitrile. Cells were assembled using a precise amount of electrolyte relative to the mass of4

Post-print accepted by J. Power Sources. Now published at doi: 10.1016/j.jpowsour.2014.04.090

sulfur in order to minimise differences between cells due to active mass dissolution into theelectrolyte. A binder composition of 2:3 carboxymethylcellulose sodium salt:styrenebutadiene rubber (CMC:SBR) was used as the reference material; CMC:SBR is frequently
encountered as a water-soluble alternative to the commonly used PVdF, especially for Li-ion
5

anode materials[41]. The moderate sulfur loading of the cathode of 50% and the relative
excess of electrolyte ensures that the reference cell cycles with good capacity retention over amoderate number (50 – 200) of cycles and at moderate current densities.

2.1. Comparison of binders by galvanostatic cycling
10
The cycling behaviour of the Li-S cells at a moderate rate of C/5 is presented in Figure 1. For
readability the data is presented as two plots: on the left, the reference CMC:SBR binder, thesingle component binders of PVP and PEO and the 4:1 PEO:PVP mixed binder, which was
determined to have the best performance; the right-hand plot shows other PEO:PVP
15

compositions tested. In agreement with our previous paper, PEO affords a capacity
improvement of over 100 mAh g -1 compared to the reference CMC:SBR system. While theuse of PVP gives an initially smaller capacity increase (ca. 50 mAh g -1) over CMC:SBR
compared with PEO, the capacity is better retained. The capacity of the PVP-containing
cathode matches that of the PEO cathode after 50 cycles. With a combination of PEO and

20

PVP in a high PEO:PVP ratio (e.g., 4:1 and 9:1), the capacity is higher than either of thesingle-component binders – and more than 200 mAh g-1 higher compared to CMC:SBR - withthe same good capacity retention as for PVP. These ratios are similar to the binder
composition used in the previously-referenced paper by Hydro-Québec, i.e., 8:1
PEO:PVP[33]. The capacities after the first few cycles decreases with increasing PVP content,

25

and the 1:1 mixture shows a lower capacity than either of the single component cathodes.

5Post-print accepted by J. Power Sources. Now published at doi: 10.1016/j.jpowsour.2014.04.090

For all cells the coulombic efficiency is relatively similar and reaches a high and stable valueof almost 99% after the first 20 cycles. This data is plotted in Figure 2. The high coulombic
efficiency can be attributed to the efficient passivation of the Li anode by LiNO 3[22,39].
We can attempt to rationalise the effect of the PVP content of the cathode on the cycling
5

performance by consideration of its chemical interaction with polysulfides, as previously
predicted by Seh et al[31]. Addition of 0.3 mL of a saturated solution of Li2S6 in theelectrolyte solvent to 1 mL of a solution of PVP in the same solvent affords a red precipitate
which subsequently remains in a stable suspension over at least 48 hours. Addition of thesame polysulfide solution to an equivalent solution of PEO causes some precipitation of a10

brown solid which decomposed completely within 48 hours, leaving what appeared to be
sulfur powder beneath a layer of PEO gel. The products of these reactions are shown in
Figure 3. This observation is consistent with the strong interaction between PVP and lithium
polysulfide as predicted by Seh et al. It would therefore be expected that, on the firstdischarge of the cell, dissolution of sulfur from the cathode in the form of lithium polysulfide

15

leads to the formation of a similar PVP:Li2Sx complex at the binder. Such a complex might beanalogous to complexes of PVP and iodine currently used as antiseptics[42], or otherpolymer-ion crosslinked materials as reported in the literature[43,44]. Unfortunately, at thetime of writing, we have not been unable to isolate this material as a stable solid forcharacterisation. Nonetheless, this interaction has a clear effect on the stability of the cell

20

through the improved capacity retention, possibly through improved retention of polysulfide
in the vicinity of the cathode as previously suggested. A possible explanation for the poorer
specific capacity for cathodes with mixed binders and higher PVP content (i.e., 1:1, 7:3
PEO:PVP; see Figure 1) may be that a larger proportion of active mass is complexed into an
insoluble form which is then rendered inactive by phase separation of the two binder

25

components.

6

Post-print accepted by J. Power Sources. Now published at doi: 10.1016/j.jpowsour.2014.04.090

Measuring the relaxation of cells under open circuit conditions following cycling, as shown in
Figure 4, gives the clearest indication of the improved stability afforded by the inclusion of
PVP in the binder. Cells were assembled with cathodes containing CMC:SBR, PEO and 4:1
PEO:PVP binders, respectively. After 5 charge-discharge cycles at C/5, the cells were left to5rest at open circuit for exactly 1 week. At the end of the week, the open circuit voltage (OCV)
for the cell containing the 4:1 PEO:PVP binder was 174 mV higher (2.334 V) than the cellwith PEO (2.170 V) and 183 mV higher than the cell with CMC:SBR binder (2.161 V). The
shape of the voltage profile, especially for the cells containing PEO and CMC:SBR binders,appears similar to the voltage profile for the Li-S cell during charge and discharge, with two

10

major plateaus with average voltages of approximately 2.3 V and 2.15 V. The decreasing
voltage of cells over time at open circuit likely indicates a change in the stoichiometry oflithium sulfides at the electrode surface towards shorter chain polysulfides, which is apossible indicator of self-discharge due to the redox shuttle. The inclusion of even a small
proportion of PVP therefore effects a dramatic improvement in the stability of the system,

15

possibly aiding to resist self-discharge, for example through retaining polysulfides near thecathode as suggested by Seh et al. For the best-performing mixed binder of 4:1 PEO:PVP, thehigh reversible capacity of over 1000 mAh g-1, retained over more than 50 cycles, iscompetitive with the state-of-the-art Li-S research, as outlined in a recent review[24].
Furthermore, this result is also an important demonstration that the role of the binder extends

20

beyond ensuring efficient utilisation of the active surface area of the cathode.

2.2. High rate cycling and kinetic effects

The performance of the four binders of principal interest in this study, i.e., CMC:SBR, PEO,
25

PVP and 4:1 PEO:PVP, was studied further by a combination of higher rate galvanostatic
cycling with impedance spectroscopy. The full electrochemical experiment is detailed in the7Post-print accepted by J. Power Sources. Now published at doi: 10.1016/j.jpowsour.2014.04.090

experimental section. The program essentially comprised sequences of galvanostatic cycling
at a rate of 1C (1,672 mA g-1) with periodic rest intervals, during which impedance spectra
were obtained with the cell in a fully charged or discharged state. The cycling data over 200
cycles is presented in Figure 5, with selected Nyquist plots showing cell impedance in the5charged and discharged states after 10, 100 and 200 cycles presented in Figure 6. At thisrelatively high cycling rate, the cell containing the 4:1 PEO:PVP binder combination stillretains the best capacity compared to the other compositions tested, with a reversible capacityof over 800 mAh g-1 after 200 cycles. Observing that the cells using 4:1 PEO:PVP and PEO
alone as binders exhibit the same initial discharge capacity of 1170 mAh g -1, this result is

10

further evidence for the ability of PVP to effect an improvement in capacity retention despite
a slightly increased cell impedance for the mixed binder system.
The PEO-containing cells show significantly higher capacities than the CMC:SBR-containingreference cell in agreement with the previous result at moderate cycling rates. Interestingly,
while showing superior capacity when cycled at C/5 compared to the CMC:SBR reference,15

the cell containing PVP alone as the binder performs considerably worse when cycled at 1C.
This is an indicator of slower reaction kinetics compared to other cells. Such effects are easily
seen by examination of the voltage profiles, which are presented for the first and 197th cycles(the latter being the last full cycle in the loop of 10 cycles before the 200th cycle) for thesecells in Figures 7 and 8 respectively. The voltage profiles show generally decreased hysteresis

20

for PEO-containing cells with an elongation of the lower discharge plateau and suppression of
the voltage peak seen at the beginning of the charge cycle, as we have previously reported. By
comparison, the cell containing PVP alone as the binder shows a very large hysteresis, despite
good performance at lower charge-discharge rates. However, an interesting feature of note is
the suppression of the voltage peak at the beginning of the discharge cycle for PVP-

25

containing cells, compared to CMC:SBR and PEO. This feature may be related to initial
complexation of polysulfide to PVP in the binder as suggested above. After 197 cycles, the8Post-print accepted by J. Power Sources. Now published at doi: 10.1016/j.jpowsour.2014.04.090

trends are broadly the same; reduced hysteresis, especially for the upper plateau, and better
retention of both major plateaus for the cells with PEO-containing cathodes. The voltage
profiles of the PEO and 4:1 PEO:PVP cells appear to be essentially the same apart fromproportional increases in the length of the major plateaus in the latter case. This could be an
5indication of higher availability of active sulfur in the 4:1 PEO:PVP cell, supporting theargument for improved sulfur retention near the cathode.
Further insight into the effect of the binders on reaction kinetics can be gained by study of the
impedance response of the cells (Figure 6). In all cases the major features of the Nyquist plot
are a major semi-circle, which can be interpreted as representing the reactions of polysulfide

10

at the cathode, and low-frequency diffusional impedance. Coupled with this at higher
frequency is the porosity There is an additional high-frequency semicircle representing
dissolution and plating of Li at the anode. This feature has a small charge transfer resistance(Rct) for all binders except for PVP; this higher resistance may indicate an effect on thecomposition of the solid-electrolyte interface (SEI) due to PVP, perhaps as a result of PVP

15

dissolution in the electrolyte contributing to a more resistive layer, but also possibly reflecting
a lower content of sulfur species in the SEI.
Analysis of the major semi-circle provides information about the electron transfer kinetics ofpolysulfide independent of processes on other timescales, for example, diffusion. Suchanalysis by measuring Rct from the diameter of this semi-circle alone may be misleading since20

the actual electrochemical surface area of the cathode is unknown and may change
continuously throughout operation of the cell. It is also further complicated in view of the factthat Rct is also in series with the porosity of the cathode, and so the true R ct may be obscured.
This therefore makes it difficult to distinguish kinetic effects from changes in porosity.
However, for these cells, we believe that the true Rct is the dominant element in the semi-

25

circle under consideration, for three reasons: first, that the major semi-circle and Warburg
features fit the Randles circuit relatively well in most cases, and we were unable to obtain a9Post-print accepted by J. Power Sources. Now published at doi: 10.1016/j.jpowsour.2014.04.090

sensible fit when taking into account the porosity; secondly, that these cathodes are thin andSuper P carbon is relatively non-porous, and thirdly; that we have previously demonstrated
that the addition of PEO as a cathode coating or PEGDME as an electrolyte additive effected
a similar improvement in performance when compared with the use of PEO as a binder, and5

that these strategies would not be expected to influence cathode porosity[29].
Based on this assumption, kinetic information can be obtained by examining the relaxation
frequency, f*, which is the maximum of the semi-circle in the imaginary part of the
impedance, -Z’’. This relaxation frequency relates to a time constant given by:1𝜏 = 𝑅𝑐𝑡 𝐶𝑑𝑙 = 2𝜋𝑓 ∗10(1)Where Cdl is the double layer capacitance. We can also assume that, on the time scale of theseprocesses (< 1 second), the fastest reactions (i.e., the process of lowest impedance) are that ofpolysulfides in solution, and that insoluble discharge products (S 8, Li2S) are effectively
inactive. There are several such reactions but the overall equilibrium can be expressed as:𝑛(𝑥 − 2 ) Li2 S𝑥 + 𝑛e− + 𝑛Li+ ⇌ xLi2 Sx−n⁄215(2)This can be considered as a simple electrochemical reaction with slow electron transfer
kinetics, especially for cells in the discharged state. For such reactions at low overpotentials
the Butler-Volmer equation approximates to a linear relationship:𝜕𝜂𝑅𝑇

𝑅𝑐𝑡 = 𝜕𝐼 = 𝑛𝐹𝐼(3)0

While the AC peturbation of 50 mV used in this experiment, in order to ensure good quality
20

data, would ordinarily be considered high for this approximation, the result should not deviatesignificantly from the relationship expressed in Equation 3. We can further express theexchange current density I0 as:
𝐼0 = 𝑛𝐹𝐴𝑐∞ 𝑘𝑠(4)

Where A is the electrochemical surface area, 𝑐∞ is the bulk concentration of polysulfide and ks
25

is a pseudo-rate constant representing the heterogeneous rate constant for the reaction
10

Post-print accepted by J. Power Sources. Now published at doi: 10.1016/j.jpowsour.2014.04.090

described in Equation 2. By combination of Equation 1, 3 and 4 we arrive at an expression for
the relaxation time constant 𝜏:
𝑅𝑇

𝐶

1

𝜏 = (𝑛2𝐹2 ) ( 𝐴𝑑𝑙) 𝑐 𝑘

(5)

∞ 𝑠

Since the ratio Cdl/A is assumed to be a constant, the relaxation time constant is approximately
5

inversely proportional to the product of the polysulfide concentration and the pseudo-rate
constant ks. While accurate values for 𝑐∞ and ks are unmeasurable by this technique, the time
constant 𝜏 at least gives an indication of polysulfide reaction kinetics. Average values for 𝜏
have been determined for these cells in the charged and discharged states and are tabulated inTable 1.

10

The estimated time constants for CMC:SBR and PEO-containing cathodes are higher (i.e.,
slower kinetics) in the discharged state. The average relaxation times indicate 𝑐∞ 𝑘𝑠 products
approximately 2 times larger in the discharged state and approximately 3 to 5 times higher inthe charged state for PEO-containing cathodes compared with CMC:SBR. These observations
are consistent with the previous interpretation of the voltage profiles. Similarly, the estimated15

time constant for the PVP cathode in the charged state is significantly higher than for theother cathodes, further reflecting the poor performance as a result of this binder. However, the
PVP cathode is unique in that the time constant in the discharged state is lower than that forthe charged state. This unusual trend could be interpreted as evidence of stabilisation oflithium sulfides by PVP.

20

It is interesting to note that the addition of PVP, which, as a single binder, performs relatively
poorly at high rate, still effects an improvement in capacity when combined with PEO. As
previously stated, a possible interpretation is that the effect of PVP is to improve the retentionof polysulfides in the vicinity of the cathode. Another interpretation might be that PVP could
partially dissolve from the cathode and aid in the formation of a more efficient SEI layer,

25

curtailing the redox shuttle. However, while such an effect cannot be ruled out, based on our

11Post-print accepted by J. Power Sources. Now published at doi: 10.1016/j.jpowsour.2014.04.090

results and the previous work of Seh et al on this binder, it is more plausible that the major
effect of PVP involves stabilisation of the cathode environment.
A further unusual trend in the impedance spectra which should be addressed is that Rct for
CMC:SBR and PEO-containing cathodes in the discharged state decreases on cycling. Such5

behaviour is unexpected and difficult to explain since it would appear to indicate an increasein the rate of the reaction. Based on Equation 3 and 4, a relatively unchanged 𝑐∞ 𝑘𝑠 productwould appear to imply an increasing electrochemical surface area, which could be a result of
dissolution of passivating solids (e.g., binder or Li2S) out of the cathode over time. However,such a phenomenon is not supported by a significant stabilisation of capacity and has not, to10

our knowledge, been reported in the literature. The origin of this trend in the impedance could
be a basis for a future study.

12

Post-print accepted by J. Power Sources. Now published at doi: 10.1016/j.jpowsour.2014.04.090

3. Conclusion

By the use of a mixed binder composed of 4:1 PEO:PVP we have demonstrated relatively
stable cycling of our Li-S test cells which retain a capacity of over 1000 mAh g-1 at C/5 after
5

50 cycles and over 800 mAh g-1 at 1C after 200 cycles, without the use of a sophisticatedcarbon host structure. Such performance is, to our knowledge, considerably improved
compared to any other reported binder for this system and is competitive with theperformance of any Li-S cells in the literature to date.
Based on analysis of cycling behaviour and impedance spectroscopy we conclude that this10binder system combines the complementary benefits of faster reactions of polysulfides andenhanced capacity due to the inclusion of PEO and an improvement in capacity retention from
PVP. These observations are strong evidence that the binder should be considered as achemically active component of the electrolyte system in a Li-S cell, and that manipulation of
the functionality of such materials is a potentially significant strategy in the search for

15

improved cycle life of these cells. Differences in electrochemical kinetics may also allow for
improved rate capability as we have touched upon in this work; however, the structure of thehost composite is still likely to have the largest influence on capacity and rate capability, and
it is not clear that the trends observed for the simplified system described here will remain
when using different carbon hosts with very different properties.

20

It is also worth stressing that a considerable benefit of this binder system is that it allows fordispersion of carbon-sulfur composites in water. The addition of PVP, even in the small
amounts used in this work, significantly reduces the viscosity of water-based slurries
containing PEO as a binder. While PEO and PVP are just two examples of possible functional
binders, both of these materials are produced on a large scale and are environmentally

25

compatible. Considering the key advantage of the low cost of sulfur and that perhaps thebiggest markets for Li-S in decades to come could be in large-scale applications, it is surely

13

Post-print accepted by J. Power Sources. Now published at doi: 10.1016/j.jpowsour.2014.04.090

important to investigate simple and scalable strategies such as these. Such strategies arecompatible with efforts made elsewhere to optimise the structure of the carbon host andfurther stabilize the anode, and may bring the Li-S system considerably closer to wider
commercialisation.
5
4. Experimental Section

Materials: Lithium bis(trifluoromethyl)sulfonimide (LiTFSI, Novolyte) and LiNO 3 (Aldrich)were dried at 120 °C overnight prior to use. 1,2-dimethoxyethane (DME, Novolyte), CMC
10binder (Leclanché) were used as received. 1,3-dioxolane (DOL, anhydrous), poly(ethyleneoxide (PEO, Mw = 4,000,000), poly(vinyl pyrrolidone (PVP, Mw = 360,000) and lithium
sulfide (Li2S) were used as received from Aldrich. A sample of SBR binder (Targray PSBR100) was kindly provided by Fraunhofer IWS, Dresden, Germany.
Cathode preparation: Cathode coatings were prepared by mixing of sulfur powder, Super P

15

carbon black and binder in a 50:40:10 ratio respectively in the appropriate solvent by
planetary ball-milling for two hours. The solvent used for PEO as the binder was acetonitrile,water was used for all other slurry compositions. The slurries were bar-coated onto Al foil andallowed to dry at ambient conditions. Cathodes were cut into 2 cm diameter discs with anaverage sulfur loading of approx. 1.5 – 2.5 mg (approx. 0.5 – 0.8 mg cm-2). The cathodes

20

were transferred to an argon-filled glove box and dried under vacuum at ambient temperature
overnight. Vacuum-sealed pouch cells were prepared using Li foil (125 µm, Cyprus Foote
Minerals) as the counter-reference electrode separated with a 2.5 mm diameter glass-fibre
filter separator soaked with 100 µL per mg of sulfur of the electrolyte. The electrolyte wascomposed of 1 M LiTFSI, 0.25 M LiNO3 in 1:1 DME:DOL. The cells were cycled under25

galvanostatic conditions at C/5 (334 mA g-1, based on a theoretical capacity of sulfur of 1,672mAh g-1) between 2.6 V and 1.8 V vs Li/Li+ with an Arbin battery cycler.

14

Post-print accepted by J. Power Sources. Now published at doi: 10.1016/j.jpowsour.2014.04.090

Polysulfide:polymer reactions: PVP was dried under vacuum at 80 °C prior to use. A
saturated solution of lithium polysulfide with a nominal stoichiometry of Li2S6 was preparedby mixing the appropriate ratios of Li2S and sulfur powder in DME:DOL over 24 hours.
Polymer solutions of PEO, PVP and were prepared by dissolving approximately 15 mg of5

polymer in 1 mL DME:DOL. The reactions of lithium polysulfide were observed by additionof 0.3 mL of the saturated polysulfide solution to each of the polymer solutions.
Impedance spectroscopy: Impedance spectroscopy combined with galvanostatic cycling was
performed with a VMP2 (Princeton Applied Research – Bio-Logic Science Instruments).
Cells were cycled according to a program of 11 discharges and 10 charges within the10

aforementioned cycling limits at 1 C. Following the 11th discharge, the cell was allowed torelax at OCV for 1 hour. The impedance spectrum was obtained with the cell in the same twoelectrode configuration at open circuit (Eoc ~ 2.4 V in the charged state, ~2.15 V in the
discharged state) over the range 200 kHz – 10 mHz with Vpp = 50 mV. The cell was thencharged to 2.6 V at C/5, allowed to relax for 1 hour and the impedance spectrum again

15

measured. The experiment was continued in this fashion for at least 20 loops.

Acknowledgements
The authors wish to thank the Era Net Transport project “MaLiSu” and Vinnova in Sweden
for financial support.
20
References

25

[1]

X.L. Ji, L.F. Nazar, J. Mater. Chem. 20 (2010) 9821.

[2]

P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Nat. Mater. 11 (2012) 19.

[3]

Y. V Mikhaylik, J.R. Akridge, J. Electrochem. Soc. 151 (2004) A1969.

[4]

S.S. Zhang, J. Power Sources 231 (2013) 153.

[5]

X. Ji, K.T. Lee, L.F. Nazar, Nat. Mater. 8 (2009) 500.

15

Post-print accepted by J. Power Sources. Now published at doi: 10.1016/j.jpowsour.2014.04.090[6]

N. Jayaprakash, J. Shen, S.S. Moganty, A. Corona, L.A. Archer, Angew. Chem. Int. Ed.
Eng. 50 (2011) 5904.

[7]

C. Liang, N.J. Dudney, J.Y. Howe, Chem. Mater. 21 (2009) 4724.

[8]

S.M. Zhang, Q. Zhang, J.Q. Huang, X.F. Liu, W.C. Zhu, M.Q. Zhao, W.Z. Qian, F.
Wei, Part. Part. Syst. Charact. 30 (2013) 158.

[9]

Y.-J. Choi, K.-W. Kim, H.-J. Ahn, J.-H. Ahn, J. Alloy. Compd. 449 (2008) 313.

5[10] H.L. Wang, Y. Yang, Y.Y. Liang, J.T. Robinson, Y.G. Li, A. Jackson, Y. Cui, H.J. Dai,
Nano Lett. 11 (2011) 2644.

10

15

[11]

S. Evers, L.F. Nazar, Chem. Commun. 48 (2012) 1233.

[12]

J. Wang, J. Yang, J. Xie, N. Xu, Adv. Mater. 14 (2002) 963.

[13]

Y. Zhang, Z. Bakenov, Y. Zhao, A. Konarov, T.N.L. Doan, M. Malik, T. Paron, P.
Chen, J. Power Sources 208 (2012) 1.

[14]

X. Yu, J. Xie, J. Yang, H. Huang, K. Wang, Z. Wen, J. Electroanal. Chem 573 (2004)
121.

[15]

Y.S. Su, A. Manthiram, Nat. Commun. 3 (2012) 1166.

[16]

X. Wang, Z. Wang, L. Chen, J. Power Sources 242 (2013) 65.

[17]

J.-Q. Huang, Q. Zhang, S.-M. Zhang, X.-F. Liu, W. Zhu, W.-Z. Qian, F. Wei, Carbon
N. Y. (2013) 1.

[18]

Y.-J. Choi, Y.-D. Chung, C.-Y. Baek, K.-W. Kim, H.-J. Ahn, J.-H. Ahn, J. Power
Sources 184 (2008) 548.

[19]

S.S. Zhang, Electrochem. Comm. 31 (2013) 10.

[20]

A. Manthiram, Y. Fu, Y.-S. Su, Acc. Chem. Res. 46 (2013) 1125.

[21]

X.L. Li, Y.L. Cao, W. Qi, L. V Saraf, J. Xiao, Z.M. Nie, J. Mietek, J.G. Zhang, B.
Schwenzer, J. Liu, J. Mater. Chem. 21 (2011) 16603.

[22]

Y. V. Mikhaylik, I. Kovalev, R. Schock, K. Kumaresan, J. Xu, J. Affinito, in:, ECS
Trans., The Electrochemical Society, 2010, pp. 23–34.

[23]

D. Aurbach, E. Pollak, R. Elazari, G. Salitra, C.S. Kelley, J. Affinito, J. Electrochem.
Soc. 156 (2009) A694.

[24]

D. Bresser, S. Passerini, B. Scrosati, Chem. Commun. 49 (2013) 10545.

[25]

Y. Jung, S. Kim, Electrochem. Comm. 9 (2007) 249.

20

25

30

16

Post-print accepted by J. Power Sources. Now published at doi: 10.1016/j.jpowsour.2014.04.090

5

10

[26]

Y.Q. Huang, J. Sun, W.K. Wang, Y. Wang, Z.B. Yu, H. Zhang, A.B. Wang, K.G.
Yuan, J. Electrochem. Soc. 155 (2008) A764.

[27]

J. Sun, Y.Q. Huang, W.K. Wang, Z.B. Yu, A.B. Wang, K.G. Yuan, Electrochim. Acta
53 (2008) 7084.

[28]

J. Sun, Y.Q. Huang, W.K. Wang, Z.B. Yu, A.B. Wang, K.G. Yuan, Electrochem.
Comm. 10 (2008) 930.

[29]

M.J. Lacey, F. Jeschull, K. Edström, D. Brandell, Chem. Commun. 49 (2013) 8531.

[30]

C. Barchasz, J.-C. Leprêtre, S. Patoux, F. Alloin, Electrochim. Acta 89 (2013) 737.

[31]

Z.W. Seh, Q. Zhang, W. Li, G. Zheng, H. Yao, Y. Cui, Chem. Sci. 4 (2013) 3673.

[32]

J. Song, T. Xu, M.L. Gordin, P. Zhu, D. Lv, Y.-B. Jiang, Y. Chen, Y. Duan, D. Wang,
Adv. Func. Mater. (2013) n/a.

[33]

C.-S. Kim, A. Guerfi, P. Hovington, J. Trottier, C. Gagnon, F. Barray, A. Vijh, M.
Armand, K. Zaghib, J. Power Sources 241 (2013) 554.

[34]

S.E. Cheon, J.H. Cho, K.S. Ko, C.W. Kwon, D.R. Chang, H.T. Kim, S.W. Kim, J.
Electrochem. Soc. 149 (2002) A1437.

[35]

N.-I. Kim, C.-B. Lee, J.-M. Seo, W.-J. Lee, Y.-B. Roh, J. Power Sources 132 (2004)
209.

[36]

C. Barchasz, J.-C. Leprêtre, S. Patoux, F. Alloin, J. Electrochem. Soc. 160 (2013)
A430.

[37]

S.S. Zhang, Electrochim. Acta 97 (2013) 226.

[38]

S. Urbonaite, P. Novák, J. Power Sources 249 (2014) 497.

[39]

S.S. Zhang, Electrochim. Acta 70 (2012) 344.

[40]

S. Wenzel, H. Metelmann, C. Raiß, A.K. Dürr, J. Janek, P. Adelhelm, J. Power Sources
243 (2013) 758.

[41]

W.-R. Liu, M.-H. Yang, H.-C. Wu, S.M. Chiao, N.-L. Wu, Electrochem. Solid-State
Lett. 8 (2005) A100.

[42]

F. Haaf, A. Sanner, F. Straub, Polym. J. 17 (1985) 143.

[43]

S. Ghosh, J. Rasmusson, O. Inganäs, Adv. Mater. 10 (1998) 1097.

[44]

I. Iliopoulos, R. Audebert, Macromolecules 24 (1991) 2566.

15

20

25

30

17