sulfur speciation in li s batteries determined by operando...

Sulfur Speciation in Li−S Batteries Determined by Operando X‑rayAbsorption SpectroscopyMarine Cuisinier,§ Pierre-Etienne Cabelguen,§ Scott Evers,§ Guang He,§ Mason Kolbeck,§ Arnd Garsuch,‡

Trudy Bolin,† Mahalingam Balasubramanian,† and Linda F. Nazar*,§

§Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada‡BASF SE, Ludwigshafen, 67056 Germany†X-ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States

*S Supporting Information

ABSTRACT: Among the most challenging issues in electrochemicalenergy storage is developing insightful in situ probes of redox processes fora working cell. This is particularly true for cells that operate on the basis ofchemical transformations such as Li−S and Li−O2, where the factors thatgovern capacity and cycling stability are difficult to access owing to theamorphous nature of the intermediate species. Here, we investigatecathodes for the Li−S cell comprised of sulfur-imbibed robust sphericalcarbon shells with tailored porosity that exhibit excellent cycling stability.Their highly regular nanoscale dimensions and thin carbon shells allowhighly uniform electrochemical response and further enable directmonitoring of sulfur speciation within the cell over the entire redoxrange by operando X-ray absorption spectroscopy on the S K-edge. Theresults reveal the first detailed evidence of the mechanisms of sulfur redoxchemistry on cycling, showing how sulfur fraction (under-utilization) andsulfide precipitation impact capacity. Such information is critical for promoting improvements in Li−S batteries.

SECTION: Energy Conversion and Storage; Energy and Charge Transport

The Li−S battery possesses many highly desirablecharacteristics for energy storage, but at present, it also

exhibits poor capacity retention by comparison with lithium ioncells.1−3 Moreover, the highest reversible capacities reported forLi−S cells in the literature are typically on the order of 1200mAh·g−1, that is, 75% of the theoretical capacity.4,5 It isreported that the plateau in the electrochemical profile of theLi−S cell at 2.1 V corresponds to conversion of soluble Li2S4 toinsoluble Li2S2

6 in addition to Li2S,5 representing one source of

capacity limitation. However, “Li2S2” has not been isolated nordoes it appear on any phase diagram.7 Other polysulfides Li2Sx(8 ≥ x ≥ 3) are also uncharacterized in the solid state, andsolution studies suggest that they are in rapid equilibrium.8−11

Incomplete electrochemical redox represents a significantshortcoming for practical Li−S cells that can only be rationallyaddressed by uncovering the nature of the electrochemicalprocesses, which is the aim of this work.Here, we realize sulfur speciation in the cell over the full

capacity range using operando X-ray absorption near-edgespectroscopy (XANES) by developing and utilizing a carbon−sulfur composite electrode optimized for both efficienttransport of charge carriers and trapping of soluble polysulfides.The cathode consists of sulfur impregnated in porous hollowcarbon spheres of uniform diameter that minimize significantdissolution into the bulk electrolyte, paired with a nonsulfurous

electrolyte and incorporated in a cell designed to prevent deadzones. It exhibits all of the voltage plateaus and other subtlefeatures of a classic, well-functioning Li−S cell based onmicrometer-sized sulfur, at the typical potentials.2,3,12 Thisdeliberate experimental design, along with well-characterizedreference standards, allows us to utilize synchrotron S K-edgeabsorption spectroscopy.13 We are aware of only one study thathas investigated the electrochemistry of the Li−S battery by thismethod.14 That report, optimized to probe not the cathode butthe electrolyte phase and elucidate its reactivity elegantly,showed that the solvent plays a key role in the electrochemicalperformance. The cells were not studied under real-timeoperando conditions however, and polysulfide spectral featureswere not verified or quantified. Here, not only are we looking atthe bulk electrode, but by constraining the sulfur (andpolysulfide) to 3D nanodimensions, we minimize X-ray self-absorption effects and thus avoid the overwhelming distortionof the XANES spectra that results from bulk particles.15,16 Thisdevelopment has enabled us to develop a clear mechanism ofthe sulfur redox chemistry in the cell that is broadly applicableto Li−S batteries.

Received: August 17, 2013Accepted: September 11, 2013Published: September 11, 2013

Letter

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© 2013 American Chemical Society 3227 dx.doi.org/10.1021/jz401763d | J. Phys. Chem. Lett. 2013, 4, 3227−3232

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Extremely uniform porous carbon nanospheres (PCNSs)controlled to 220 nm in dimension were fabricated using asimple one-step method to create a hollow porous casing. Thesilica−carbon spheres were synthesized by in situ condensationof tetraethoxyorthosilicate (TEOS) and its self-assembly with acarbon precursor (resorcinol-formaldehyde) that incorporates apore-forming agent (surfactant PolyDADMAC). The designpermits tuning of the pore size, differing from the approachtaken in prior reports of carbon nanospheres.17,18 Carbon-ization at 750 °C and etching of the silica core provide thedesired PCNSs, as shown by the SEM images in Figure 1a,b,

while the pore-former content controls the shell porosity andthickness.19 The ∼20 nm thick shells maintain robustness,while the 4 nm pores are large enough to enable the PCNSs tobe impregnated with sulfur by melt-diffusion and readily permitingress of electrolyte (Figure 1d). The energy dispersive X-ray(EDX) line scan of a representative TEM image (Figure 1c)shows a homogeneous sulfur signal throughout the shell wherethe sulfur is primarily confined and no sulfur contribution fromthe exterior. In the absence of the pore former, no sulfur isincorporated.17,20 The BET isotherm and pore size distributionshown in Figure 1d indicate a sharp decrease in the surface areaupon sulfur impregnation (from to 824 to 108 m2·g−1) and thefilling of much of the shell porosity. The substantial shell porevolume (3.42 cm3·g−1, of which 1.6 cm3·g−1 is within the shell)is sufficient to contain the sulfur mass of the PCNS (∼70 wt %;Figure S1, Supporting Information). The line scan suggests thatthe inner sphere layer is also coated with sulfur.The electrochemical properties of the PCNS/S, optimized

for shell porosity and thickness, were first examined in astandard coin cell configuration at a high rate of 1 C (1672 mA·g−1). The 1st, 50th, and 100th voltage profiles (Figure 1e) showthat the material exhibits not only a typical Li−S voltage profilebut also outstanding capacity retention, remaining at close to

100% after activation for the duration of 100 cycles with acapacity of 730 mAh·g−1. Previous reports have detailed high-capacity retention in excess of 90% over 100 cycles but havefallen short at reporting this benchmark above C/2 rates.17

Higher capacities of 1200 mAh·g−1 were achieved at slowerrates (see below and Figure S2, Supporting Information). Mostimportantly, the cycling stability conferred by the cathodematerial facilitated the operando studies.The preparation of individual polysulfides as references was

essential to achieve sulfur speciation; the characterization ofthese materials was accomplished by a combination of XRDand high-field 7Li MAS NMR. Compositions Li2S, Li2S2, Li2S4,Li2S6, and Li2S8 were initially targeted using the appropriateratio of elemental sulfur and LiEt3BH as a reducing agent inTHF.21 However, most of these compositions were found toconsist of phase mixtures. XRD revealed that α-S8 and Li2Swere the only detectable nanocrystalline phases in Li2Snpowders for n = 1, 2, and 4 and n = 4 and 8, respectively(see Figure S3, Supporting Information), and NMR was usedto elucidate the contribution of amorphous phases (see below).Only Li2S6 appeared fully amorphous by XRD, and the isolationof this new single-phase intermediate was clearly identified by7Li MAS NMR, as shown in Figure 2. To the best of our

knowledge, there is no prior report on 7Li NMR of lithiumpolysulfides. We found that although the range of chemicalshifts in these diamagnetic compounds is extremely narrow,distinct Li environments were only observed for Li2S and Li2S6,with isotropic NMR shifts at 2.3 and 1.0 ppm, respectively(Figure 2a). All other spectra (n = 2, 4, 8) consisted of linearcombinations of these two signals (Figure 2b). Thus, existenceof Li2S2 as a solid metastable phase is not indicated by ourNMR data, and Li2S6 is the only intermediate isolated betweenα-S8 and Li2S. The presence of α-S8, Li2S6, and Li2S in thesample of nominal stoichiometry “Li2S8” illustrates themetastable nature of long-chain polysulfides in the solid state.Demixing of Li2S6 into α-S8 and Li2S was indeed observed ataround 60 °C, so that Li2S8 might require even lowertemperature to be isolated. To complete the reference seriesfor the polysulfide anions S4

2− and S22−, single-phase crystals of

Na2S2 and Na2S4 were used because Li2S2 and Li2S4 cannot beisolated.22 The difference in the cation does not contribute toany significant difference in the spectral shape or position23

(see the Supporting Information and Figure S3 for details).Armed with the unique reference materials (α-S8, S6

2−, S42−,

S22−, and Li2S) and owing to the unique design of the cathode

Figure 1. (a,b) SEM images of PCNSs at both low and highmagnification. (c) STEM image with EDX line scan (red, carbon; blue,sulfur). (d) Nitrogen isotherm prior to sulfur impregnation and insetpore size distribution of bare PCNSs (black) and PCNS/S-70%(blue). (e) The 1st (black), 50th (red), and 100th (blue) cycles at 1 C.

Figure 2. (a) 7Li NMR of the synthesized (poly)sulfides, Li2S (black),Li2S6 (blue), Li2S2 (red), and Li2S8 (dark green) phase mixtures, asillustrated in (b) using a two-component model for Li2S8 (Li2S andLi2S6).

The Journal of Physical Chemistry Letters Letter

dx.doi.org/10.1021/jz401763d | J. Phys. Chem. Lett. 2013, 4, 3227−32323228

material, redox reactions could be monitored in real time in asimilar coin cell using synchrotron-based operando XANES(Figure 3 and Figure S4; see the Supporting Information for

additional details). Distortion was successfully minimized byconstraining the sulfur species to nanodimensions in thespheres (Figure S5, Supporting Information). Figure 3a showsthe operando XANES results during the first discharge at a C/5rate and for the second full cycle at C/10. First and seconddischarge capacities of ∼1200 and 1100 mAh·g−1 were achieved

at these slower rates that were utilized to minimize thecompositional change between consecutive spectra. Figure 3bcompares selected XANES spectra. The S K-edge step does notdecrease during discharge, suggesting that the solvatedpolysulfides are largely contained, reflecting the effective designof the nanospheres. For comparison, Figure 3c displays the exsitu spectra of the standard sulfur species. The strongabsorption peak observed at 2469.5 eV in the cathodecomposite is assigned to the high-intensity “white line” (1s→ 3p transition) of elemental sulfur along with a higher-energyfeature at 2476.8 eV15,16 (Figure 3c, top).The white line intensity is strongly affected by the

electrochemistry, owing to S−S bond cleavage. Contrary tosome previous XRD studies that did not detect crystalline α-S8after one full cycle,24,25 the XANES displays a spectrumcharacteristic of pure elemental sulfur at the end of charge(Figure 3c, top). At intermediate states of charge, a low-energypeak appears at 2467.7 eV. Comparison with the referencespectra (Figure 3c, middle) unequivocally shows that this low-energy feature is associated with linear polysulfides.14 Itsintensity increases with shortening of the polysulfide chain,opposite to the decrease in intensity of the white line. Thepositions of these two (white line and low-energy) bandsremain the same irrespective of the chain length, n, in Li2Sn (2 <n < 8), meaning that the polysulfides are primarily distinguishedby the ratio of their intensities. At the end of discharge, theintensity is minimal at the white line energy, in agreement withthe Li2S reference that exhibits two peaks at 2470.8 and 2473.7eV.16,26 The spectra at the discharged state do not exhibit anylow-energy intensity characteristic of linear polysulfides.Comparison with pure Li2S shows that some α-S8 remains inthe cathode composite (Figure 3c, bottom), which is consistentwith the electrochemistry that shows 75% capacity compared totheoretical results upon discharge. The reason for this isexplained below.Because the standards represent the sulfur species

successively formed by electrochemistry, operando spectracan be described with a linear combination fit (LCF).16

Considering the number of reference spectra, multiple solutionsexist, among which the LCF constructed from the minimumcombination of {α-S8, S6

2−, S42−, and S2−} gave the best fit and

agreement with the electrochemistry. The contribution of S22−

was clearly ruled out (see Figure S6, Supporting Informationfor full discussion and alternative fits). S3

2− and S82− are not

isolable due to complex equilibria in solution, as confirmedrecently by operando UV−vis spectrometry of Li−S catholytes;spectra could only be interpreted in terms of an averagestoichiometry of dissolved polysulfides, even though thetechnique should be sensitive to their chain length.11 The useof S4

2− and S62− here best represents the medium- and long-

chain polysulfide contributions, as shown by Figure S7(Supporting Information), which compares the compositionof the sulfur cathode upon cycling based on the XANES and onthe electrochemistry. The perfect accordance observed uponcharge proves that the area and depth probed are representativeof the overall cathode and that sulfur speciation is effectivelyachieved.For clarity, the following omits the first discharge at C/5 to

focus only on one full cycle at C/10 (see Figure S8, SupportingInformation). As noted above, the discharged cathode containsa fraction of unreacted elemental sulfur estimated at 20−25 wt% from the fit (Figure 3c, bottom). Figure 4a displays thecharge from Li2S to S8.

Figure 3. Sulfur K-edge XANES upon cycling and reference spectrashowing the following: (a) evolution of absorbance as a function of theelectrochemical cycling, at C/5 (first discharge) and then C/10; theends of the (dis)charges are highlighted in red; spectra were acquiredcontinuously to minimize the composition change between twospectra to 50 mAh·g−1sulfur (i.e., 3% of the total capacity); (b) selectedspectra during the cycling, labeled using the average composition LixSas estimated by the electrochemistry; (c) reference spectra forelemental sulfur (top), linear polysulfides (S2

2−, S42−, and S6

2−,middle) and Li2S (bottom), together with the initial, charged state(red line in the top panel) and discharge states (red lines in thebottom panel).



The evolution probed by XANES and its consistency withthe electrochemical profiles allow us to propose the followingcharge mechanism. Contrary to the assumption that S8

2− andelemental sulfur are the main oxidation products,10 we observethat the slow and monotonic consumption of Li2S during mostof the charge process is instead accompanied by the formationof shorter chains. The contribution from S6

2− is detected evenat the earliest stage and increases along the sloping charge atthe expense of S4

2−, which appears as a transient speciesallowing the oxidation of Li2S into more stable Li2S6. The latedisappearance of Li2S and S4

2− and hence the maximum of S62−

coincide with the voltage rise, signaling the final oxidation ofS6

2− to α-S8. The disappearance of the low-energy peak and thesteep increase of the white line on the S K-edge spectra at theend of charge show that the cathode composite completelyconverts to sulfur, indicating the excellent reversibility withinthe PCNS host.In terms of the weight of the components and accounting for

the ∼1/4 fraction of unreacted sulfur, the overall charge can besummarized as 3/4 Li2S → 3/4 S0 + 3/2 Li+ + 3/2 e−,corresponding to a specific capacity of 1254 mAh·g−1. This is inrelatively good agreement with the 1085 mAh·g−1 measuredexperimentally, considering the accuracy of the measurementand the possible error on the LCF.The Li−S cell discharge was similarly investigated by

operando XANES (Figure 4b). The first discharge plateau isusually described as the reduction of elemental sulfur to S8

2−.However, there is strong evidence for the rapid chemicaldisproportionation of S8

2− in solution based on UV spectros-copy8 and electrochemical techniques,9 S8

2− → S62− + 1/4 S8

0.This would explain the formation of S6

2− in the initial dischargestep, in agreement with the phase mixture obtained whenattempting to chemically prepare Li2S8 (Figures 2 and S5,Supporting Information). The contributions of polysulfidesincluding S4

2− in a second step increase at the expense ofelemental sulfur. Then, supersaturation (the “knee” on thevoltage profile) marks not only the end of the α-S8consumption but also the maximum concentration ofpolysulfides. While the cell voltage is fixed at 2.1 V, no Li2S

is evident by XANES until the second half of the plateau. Thefinal reduction stage signals a steep increase in the fraction ofLi2S (also suggested by impedance spectroscopy measure-ments27). The conversion of all of the available polysulfidesresults in the sudden voltage drop, thus preventing reduction ofthe remaining elemental sulfur. The overall discharge as probedby XANES can be summarized as S0 + 3/2 Li+ + 3/2 e− → 3/4Li2S + 1/4 S0 to account for the persistence of about 25%elemental sulfur. The corresponding calculated specific capacityof 1254 mAh·g−1 is within 5% agreement with the electro-chemistry (i.e., 1189 mAh·g−1). The ends of the dischargespectra do not exhibit any low-energy feature but instead onlyconsist of a mixture of S8 and Li2S. Hence, contrary to what hasbeen assumed to date,28,29 the presence of insoluble Li2S2 as afinal reduction product and as a source of lower than theoreticalcapacity is not supported by the XANES data.Importantly, our operando XANES experiments indicate that

the discharge capacity is not limited by the precipitation of Li2Sbut is instead mostly restricted by unreacted sulfur. This cannotbe explained by restricted volume expansion upon conversionof S to Li2S. The interior of the sphere is not even half filled asthe pore volume allows for all of the sulfur to beaccommodated in the shell. Moreover, consumption of α-S8during the cell discharge is arrested long before Li2S is detected.Therefore, neither steric hindrance nor surface passivation canaccount for the incomplete sulfur utilization. This is supportedby reports of unreacted sulfur at the end of discharge onelectrodes prepared from bulk sulfur simply ground withcarbon.30

Our data suggest that once S62− is formed through S8

2−

disproportionation, reduction proceeds much faster throughthese polysulfides in solution than for either the initial orreformed sulfur, leaving elemental sulfur in reserve. A contrario,high discharge capacities (>1200 mAh·g−1) can be attainedunder dilute configurations,31 where the sulfur load isinsufficient, and active material loss is thus severe. We note adelay in the detection of Li2S by XANES upon discharge (basedon the fact that charge shows the expected behavior, we believethat cell artifacts are not involved; see Figures S4 and S7,

Figure 4. Evolution of sulfur K-edge XANES upon electrochemical cycling based on linear combination analysis shown upon (a) charge and (b)discharge at a C/10 rate. Spectra are reproduced with a linear combination of four reference compounds, {α-S8, S6

2−, S42−, and Li2S}, whose weights

are represented in the top panels. The persistence of a fraction of elemental sulfur is highlighted by the horizontal dotted line.



Supporting Information). This delayed formation of Li2S couldexplain why previous operando X-ray diffraction/tomographystudies failed to identify Li2S during cell operation even at thelower potential limit32 while suggesting that crystalline Li2Seventually forms upon later equilibration. XANES, as a localprobe, detects solid amorphous and crystalline lithium sulfidealike, suggesting rather that the S2− that is formed exists in asupersaturated state before suddenly precipitating on/in theelectrode, as proposed earlier by modeling.6 The highlysolvated S2− ion might be expected to have a weak and ill-resolved spectral signature, similar to Li2S, but even more soowing to its noncondensed state, thus explaining the lack ofadditional spectral intensity in the region between 400 and 800mAh·g−1. That said, the exact cause for the delayed appearanceof Li2S is not yet understood and further studies are underway.The Li2S deposition mechanism will depend, among otherfactors, on the affinity of Li2S with the substrate (i.e., thecathode composite),33 and on the discharge rate (that will affectthe potential). Our results furthermore suggest that molecularcomponents such as anion receptors34 could help tune S2−

supersaturation and limit the capacity fading by controlling Li2Sprecipitation.In summary, our findings shed important light on sulfur

speciation in the cell during redox behavior and on themechanisms that govern dissolution and deposition of theredox end members, S and Li2S. The hysteresis between chargeand discharge is also explained by the inherent differencebetween the steps in the reduction and oxidation processes.Upon reduction, delay in precipitation of Li2S owing tosupersaturation of S2− dominates. Upon charge, surfaceoxidation of Li2S proceeds straightforwardly through thesoluble intermediates, S4

2− and S62−. This is accompanied by

an overpotential if the Li2S particles are large, owing to theenergy necessary for activation, a feature observed in many Li−S electrochemical profiles. Slower kinetics of the solid →solution process (as compared to the solution → solid processupon discharge) will give rise to the observed sloping profileeven in the absence of a significant overpotential. These resultsprovide new fundamental insight and perspectives crucial to thefurther development of the Li−S system. Ongoing inves-tigations in our laboratory are exploring operando XANES tounravel atypical sulfur redox behavior in microporouscarbons,35 heat-treated carbon−sulfur composites,36 and novelelectrolyte systems, which will be reported subsequently.

■ EXPERIMENTAL METHODSPCNS/S Preparation. The synthetic procedure relied on self-assembly of a silica core and carbon shell that incorporated apore former (see the Supporting Information for details). ThePCNSs were then impregnated with ∼70 wt % sulfur by a melt-diffusion method.Electrochemical Studies. Positive electrodes were constructed

from 80% PCNS/S-68%, 10% Kynar Powerflex binder, and10% Super S carbon using 2325 coin cells with an electrolytec om p r i s e d o f 1 . 0 M L i T F S I ( l i t h i um b i s -(trifluoromethanesulfonyl) imide) in DOL (1,3-dioxolane)and DME (1,2-dimethoxyethane) (1:1 volume ratio) and 2wt % LiNO3 to control surface passivation37 of the lithium foilused as the negative electrode. The discharge/charge rate of 1C (1672 mA·g−1) corresponds to a current density of 1.25 mA·cm−2.XANES Studies. XANES studies were carried out at the sector

9-BM-B in the Advanced Photon Source using a Si(111) crystal

monochromator under helium flow, and the data were collectedin fluorescence mode using a four-element vortex detector.Details regarding the experimental setup and sample prepara-tion are given in the Supporting Information.

7Li NMR. The 7Li NMR experiments were carried out atroom temperature on a Bruker Avance-500 spectrometer (B0 =11.8 T, ν0(

7Li) = 194 MHz). MAS spectra were obtained byusing a Bruker MAS probe with a cylindrical 2.5 mm o.d.zirconia rotor at a spinning frequency of 30 kHz.

■ ASSOCIATED CONTENT*S Supporting InformationDetailed experimental protocol and supporting Figures S1−S8,showing the determination of sulfur content in the PCNS/Scomposite, long-term cycling at the 1 C rate with the specificdischarge capacity, structural characterization of solid-statelithium and sodium polysulfides prepared as referencematerials, a schematic of the operando cell, the sulfur K-edgeXANES correction from self-absorption, alternative LCFs of theoperando XANES spectra, the composition of the sulfur-basedcathode upon cycling based on the XANES and on theelectrochemistry, and the LCF of the XANES spectra recordedduring the first discharge at C/5. This material is available freeof charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected]. Tel: +1-519-888-4567 ext.48637.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe research was supported by the BASF InternationalScientific Network for Electrochemistry and Batteries. Wethank Dr. N. Coombs, University of Toronto, for acquisition ofthe TEM and L. Spencer and G. Goward, McMaster University,for solid-state NMR facilities. Use of the Advanced PhotonSource was supported by the U.S. Department of Energy,Office of Science, Office of Basic Energy Sciences, underContract No. DE-AC02-06CH11357.

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sulfur speciation in li s batteries determined by operando...

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