Analytical Detection of Polysulfides in the Presence of Adsorption

Article pubs.acs.org/JPCC

Analytical Detection of Polysulfides in the Presence of Adsorption Additives by Operando X‑ray Absorption Spectroscopy Robert Dominko,*,† Manu U. M. Patel,† Vida Lapornik,† Alen Vizintin,† Matjaž Koželj,† Nataša N. Tušar,† Iztok Arčon,‡,§ Lorenzo Stievano,∥ and Giuliana Aquilanti⊥ †

National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia University of Nova Gorica, Vipavska 13, 5000 Nova Gorica, Slovenia § Institut Jožef Stefan, Jamova 39, 1000 Ljubljana, Slovenia ∥ ICGM−UMR5253, Université de Montpellier, 2 Place Eugène Bataillon−CC 1502, 34095 Montpellier, France ⊥ Elettra-Sincrotrone Trieste S.C.p.A., s.s. 14 km 163.5, 34149 Basovizza, Trieste, Italy

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‡

S Supporting Information *

ABSTRACT: A mechanism for Li−S battery operation with a composite electrode and an adsorption additive obtained by using operando ultraviolet/visible (UV/vis) spectroscopy and X-ray absorption spectroscopy confirms the role of the adsorption additive and reflects the conversion mechanism of sulfur into Li2S. Operando UV/vis spectroscopy shows a reversible appearance of the long-chain polysulfides in the separator in the fifth cycle, whereas the appearance of mid- and short-chain polysulfides suggests a polysulfide shuttle mechanism. By using a nonsulfur-containing electrolyte, a high-precision analysis of sulfur K-edge X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) spectra is possible. The XANES analysis shows that polysulfides reach the maximum concentration at the end of the high-voltage plateau, and the lowvoltage plateau is characteristic of the polysulfides/Li2S equilibrium. The relative amount of Li2S increases linearly until the end of discharge and reaches a relative amount of 75%. This is confirmed by sulfur K-edge EXAFS analysis. Additionally, a quantitative analysis of EXAFS spectra measured during discharge evidences a decrease of the average S−S coordination number. This can be interpreted as a decrease of the chain length of polysulfides. EXAFS analysis showed that there are no specific interactions of the polysulfide species with the matrix or with other species in the electrolyte. protection,19,20 and effective separation between the cathode and lithium anode;21−23 however, these three topics are at least as important as the development of cathode composites. Consideration of all of these areas of research can, in principle, lead to the targeted energy density and cycle life of the Li−S battery. Unlike Li-ion batteries, the laboratory tests of Li−S batteries involve a full-cell configuration, and it is not possible to optimize each electrode separately. Therefore, it is indispensable to have good insight into the mechanism of a Li−S battery during discharge and charge and to understand how changes in some components influence its electrochemical properties. For this purpose, different analytical tools have been developed over the past few years.24−35 By using in situ or operando analytical techniques, different parts of a Li−S battery can be analyzed and, by combining the results from characterization techniques, different mechanisms of polysulfide formation and diffusion can be derived.23,25,28,32,36−41 Analytical tools are gaining in importance and are currently commonly used to explain the differences observed in electrochemical properties. Different

1. INTRODUCTION Lithium sulfur (Li−S) batteries offer a possible gateway for an improvement in energy density compared to the insertionmaterials-based electrodes used in Li-ion batteries.1,2 The Li−S battery concept offers 2−3 times more energy density and is close to being commercialized as a real product by some companies.3 Despite the announced commercialization, there is an open discussion about the mechanism of Li−S batteries and how different components and additives influence the cycling stability, self-discharging, capacity retention, and Coulombic efficiency.4−7 During reduction, sulfur reacts with lithium to form longchain polysulfides that are soluble and have an impact on the conversion of sulfur (by saturation of the electrolyte), on the cycling stability (through the diffusion of polysulfides from a confined space, owing to a concentration gradient), and on the energy density (a higher ratio between electrolyte and sulfur is required).8−10 These properties differ from mechanisms known for Li-ion batteries. A great number of research efforts have been devoted to the development of hierarchical porous-structured carbon- and oxide-based materials in the last five years.11−16 Much less work, in contrast, has concerned electrolyte development,17,18 lithium © 2015 American Chemical Society

Received: June 12, 2015 Revised: July 26, 2015 Published: July 28, 2015 19001

DOI: 10.1021/acs.jpcc.5b05609 J. Phys. Chem. C 2015, 119, 19001−19010

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The Journal of Physical Chemistry C

2.2. Preparation of Electrodes and Battery Assembly. Electrodes from the composite were prepared by making a slurry of the composite, polytetrafluoroethylene (PTFE), and carbon black at a ratio of 8:1:1 in isopropanol solvent. The slurry was then casted onto the surface of aluminum foil using the doctorblade technique. Electrodes with a surface area of 2 cm2 were prepared for galvanostatic cycling and UV/vis experiments, containing approximately 1−2 mg of sulfur/cm2. The dried electrodes were used to assemble the battery. In this work, we used 1 M LiTDI in TEGDME:DOL (tetraethylene glycol dimethyl ether:1,3-dioxolane) as the electrolyte. Galvanostatic cycling was performed in a two-electrode pouch cell using electrodes with a surface area of 2 cm2. For assembly, we used 60 μL of electrolyte per 1 mg of sulfur. Electrodes for XAS measurements were prepared without additional carbon black additive, thus mixing as prepared sulfur/silicalite/Printex XE2 composite with 10 wt % of PTFE. 2.3. UV/Vis Measurements. The UV/vis cell construction and the procedure for the assembly was described in our previous work.28,29 A pouched polymer bag with a sealed glass cover was used to prepare the battery with the composite cathode electrode, the glass wool separator, and the lithium− metal foil, with a hole that was half the size of the cathode. We used 60 μL of electrolyte per 1 mg of sulfur. The battery sandwich was assembled in such a way that there was no lithium present between the glass cover and the cathode, which was present below the separator; therefore, the UV/vis spectra could be obtained without any interference. The same settings were used for the calibration of the standard catholyte solutions. However, in this case, only the separator wetted with 0.1 mL of catholyte was sealed in the cell. For the operando measurements, the cell was attached to a UV/vis spectrometer (PerkinElmer Lambda 950) in such a way that the UV light was directly focused on the glass cover of the cell; the whole setup was completely covered by a thick black plastic bag to avoid interference of light from the surroundings. After the initial spectra of the batteries were recorded, cycling of the batteries was started by using a SP-200 potentiostat/galvanostat (BioLogic). The batteries were run at a charging rate of C/20 for one complete cycle. Meanwhile, UV/vis spectra were recorded every 15 min from the beginning of the discharge until the end of the charge. 2.4. XAS Measurements. The XAS measurements were performed at the XAFS beamline of synchrotron Elettra (Basovizza, Trieste).43 The X-ray absorption spectra were recorded at sulfur K-edge in fluorescence-detection mode. A Si(111) double-crystal monochromator was used with about 0.4 eV resolution at 2.5 keV. Higher-order harmonics were effectively eliminated by using a double-flat silica mirror placed at a grazing angle of 8 mrad. The intensity of the monochromatic X-ray beam before the sample was measured by a 30 cm long ionization chamber detector, filled with a mixture of 30 mbar of N2 and 1970 mbar of He. The fluorescence signal was detected with a silicon drift detector (KETEK GmbH AXAS-M with an area of 80 mm2). The absorption spectra were measured within the interval of −150 to 800 eV relative to the S K-edge (2472 eV). In the XANES region, equidistant energy steps of 0.2 eV were used, whereas for the EXAFS region, equidistant k steps of 0.03 Å−1 eV were adopted with an integration time of 5 s per point. The measuring time for one spectrum was 65 min. The XAS spectra were collected in operando mode. The battery was discharged at C/20 rate per electron. A series of consequent scans of spectra

analytical tools are capable of detecting intermediate polysulfide species and Li2S during discharge and charge processes in different parts of the Li−S battery. Unfortunately, only elemental sulfur and its most-reduced counterpart, Li2S, are crystalline. This limits the use of techniques requiring longrange order, such as X-ray diffraction (XRD), which is often applied in the analysis of Li-ion systems. Additional information about polysulfides can be obtained by using sulfur K-edge X-ray absorption spectroscopy (XAS) and, more specifically, by studying the near-edge structure of the X-ray absorption spectrum (XANES).24−26,36−41 This technique was recently shown to be a precise method for the determination of sulfur species in the cathode during discharge and charge processes. Moreover, it was specifically employed for the detection of radical species such as S3−, forming in specific electrolytes which stabilize them.39−41 On the other hand, operando ultraviolet/ visible (UV/vis) spectroscopy can provide concentration profiles of polysulfides with different chain lengths in the separator.27,28 By combining the results of both techniques, one can obtain a picture of the formation of intermediate species and their diffusion or precipitation in the composite electrode and in the separator. In this work, we present a detailed operando S K-edge XAS study in which we were able to analyze not only the XANES part but also the extended X-ray absorption fine-structure (EXAFS) part of the XAS spectra of a working Li−S battery. This was possible by using an electrolyte system that did not contain any sulfur or chlorine-based components and, simultaneously, a cathode composite with the ability of absorbing polysulfides. In fact, a battery constructed from a positive electrode consisting of a carbon/Mn2O3 doped silicalite/sulfur42 mixture and a 1 M LiTDI TEGDME:DOL electrolyte returned electrochemical properties similar to those of a more common battery with an electrolyte containing a LiTFSI salt. We showed recently42 that by using Mn2O3 doped silicalite concentration of polysulfides within the cathode composite is up to three times higher compared to composite without additive. This provided us the possibility of more accurate detection and analysis of polysulfides with XAS. The diffusion of polysulfides out of the cathode composite was verified by UV/vis spectroscopy, and possible interactions between polysulfides and other components such as host matrices were examined by EXAFS. In summary, the combination of XANES and EXAFS results provides an accurate description of the mechanism of sulfur conversion into Li2S in the given Li−S battery configuration.

2. EXPERIMENTAL SECTION 2.1. Preparation of Composite Electrodes. In this work, we prepared two sets of electrodes with different ratios of sulfur to the host matrix. Electrodes for galvanostatic tests and UV/vis spectroscopy were prepared by using a composition reported in our previous paper (50 wt % of sulfur in the cathode composite).36 For the XAS experiment, we adjusted the quantity of sulfur to 20 wt % and we kept the ratio between sulfur and silicalite the same as for galvanostatic and UV/vis spectroscopy measurements. Thus, a mixture of MnS-1 (4.5 wt %), Printex XE2 (Degussa) carbon black (70.5 wt %), and sulfur (25 wt %) were ball milled at 300 rpm for 30 min and heated to 155 °C for 5 h with a 0.2 °C min−1 heating ramp under an argon flow. After being cooled to room temperature, both samples were recovered and the content of sulfur was confirmed by elemental analysis (CHNS). 19002


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dominated the EXAFS part and prevented its exploitation in the analysis of the species formed during cycling. The electrochemical cycling tests with the LiTDI-based electrolyte, presented in Figure 1, are very similar to those

were collected during the discharge. Each spectrum roughly corresponded to the change of composition for about Δx = 0.054 in LixS. The exact energy calibration was established with an absorption measurement on native sulfur in transmissiondetection mode. The maximum of the pre-edge peak was set to 2472.0 eV. The calibration spectra were taken before and after the XAS measurements on the samples. Absolute energy reproducibility of the measured spectra was ±0.05 eV or better. The battery prepared in an in situ Swagelok cell,44 which was specifically modified to allow measurements at the low X-ray energy typical of S K-edge for the 13 μm Be window, was mounted on the sample holder in a chamber placed after the first ionization detector. The chamber was filled with He 10% above atmospheric pressure. For fluorescence detection, the sample holder was rotated to obtain an angle of 45° between the surface of the Be window of the Swagelok and the X-ray beam. The fluorescence detector was placed at a distance of about 1−2 cm from the Be window of the Swagelok cell at an angle of 90°, with respect to the incident beam direction, inside the chamber. The distance of the fluorescence detector from the sample was adjusted for each sample to optimize the count rate of the detector and to limit the maximal detector dead time to 97% and dropped to 93−94%, whereas for LiTFSI the final value was 95%. This decrease is probably connected with the diffusion of polysulfides, which were not trapped within the composite cathode, causing more polysulfide shuttling in the latter stage of cycle life. 3.1. UV/Vis Study. To examine the diffusion of polysulfides from the composite cathode used in this work, we performed operando UV/vis spectroscopy. The configuration, assembly procedure, and measuring principles of operando UV/vis spectroscopy experiments have been shown and explained in our previous reports.28,29 Interactions between polysulfide molecules and UV/vis radiation depend on the length of the polysulfide chain, the alkali metal, and the electrolyte in which the polysulfides are dissolved. In this work, a 1 M LiTDI in TEGDME:DOL electrolyte was used; therefore, possible differences from previous results obtained using LiTFSI in the same solvent were first visually inspected in several catholyte solutions containing different polysulfides, as shown by the photographs in Figure S1. The observed differences require a calibration of the catholyte solutions (polysulfides with different stoichiometric ratios between sulfur and lithium atoms). Catholyte solutions in 1 M LiTDI TEGDME:DOL electrolyte with six different concentrations and five different compositions were used for quantitative and qualitative calibrations. Differences between catholyte solutions can be visually observed (Figure S1). To obtain quantitative numbers, we performed the same procedure as detailed in our previous work.28,29 First, we measured UV/vis curves for each composition at different

3. RESULTS AND DISCUSSION Oxide-based additives or host matrices in sulfur composite cathodes can stabilize sulfur in the cathode structure during cycling. As demonstrated in our recent work using ex situ focused ion beam scanning electron microscopy (FIB SEM) analysis assisted by EDX, MnS-1 additive (zeolite silicalite-1 with MFI structure and with incorporated Mn2O3 nanoparticles) is able to retain soluble polysulfides in the cathode matrix, with a global sulfur content three times higher in the cathode with silicalite additive than in cathodes without additives.42 Similar successful approach in terms of polysulfide adsorption has been demonstrated by group of Nazar.36 In this work, the electrolyte salt was changed from LiTFSI to LiTDI to remove sulfur-containing species from the electrolyte. In fact, LiTFSI-based electrolytes contain large quantities of sulfate (S6+), which, in previous investigations, did not interfere in the analysis of the XANES portion of the XAS spectrum but 19003


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Figure 2. Operando UV/vis spectra recorded during the first and fifth cycles: (a) all spectra measured during the first discharge, (b) all spectra measured during first charge, (c) all spectra measured during fifth discharge, and (d) all spectra measured during fifth charge (color in all figures changes from red to green to blue). UV/vis spectra were collected every 15 min.

Figure 3. Recalculated concentrations of short-, mid-, and long-chain polysulfides detected in the separator in the first cycle (left column) and fifth cycle (right column) for the long-chain polysulfides (upper figure) and mid- and short-chain polysulfides (graph in the middle). Recalculated concentration of polysulfides is indicated with scale bars. Cycling curves for the first and fifth cycle are presented on the bottom of each column. Battery was cycled with 167.5 mA/g current density.

concentrations. Typical wavelengths presented in Figure S1 were obtained from the derivatives of UV/vis curves. Measured reflections at selected wavelengths represent different sulfur/lithium ratios, which were obtained from the chemically synthesized polysulfides. The observed trend is very similar to that obtained in 1 M LiTFSI TEGDME:DOL

electrolyte, with only very small deviations. By using the same procedure as in our recent work,29 we measured the UV/vis spectra of all catholytes. The measured spectra were normalized, and the reflectance peaks at preselected wavelengths were used to construct calibration curves. The linear dependence between normalized reflectance values and the natural logarithm of the 19004


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Figure 4. (a) Operando sulfur K-edge XANES spectra of a Li−S battery measured during discharge (only 14 representative spectra, shifted vertically, are shown for clarity). (b) Electrochemical curve obtained during discharge and the relative amount of the three sulfur compounds (sulfur, Li2Sx, and Li2S) determined with a linear combination fit of S K-edge XANES spectra and measured at different operando states of the battery using the three reference XANES profiles (sulfur, Li2Sx, and Li2S).

Looking at the evolution of the concentrations in the fifth cycle, one can see that the concentration of long-chain polysulfides is almost the same at the beginning of discharge and at the end of the charge process. The change in concentration of long-chain polysulfides is rather linear and differs from the evolution in the first cycle. In this measurement, we observed a slight variation of the concentration at the beginning of the discharge and the end of the charge for mid- and short-chain polysulfides. This variation can be attributed to a polysulfide shuttle, as it has been proposed that midchain polysulfides can diffuse back to the lithium surface upon charge and form shorter polysulfides. Additionally, as predicted in our previous work,42 the appearance of shorter polysulfides in the separator can lead to inhomogeneous precipitation of Li2S at the end of discharge, which can become electronically isolated. That is, in our opinion, the main reason for relatively fast capacity fading, although the diffusion of polysulfides out of the cathode was not so severe. 3.2. Sulfur K-Edge XANES Analysis. The sulfur K-edge XANES spectra measured in operando mode during the discharge of the battery (Figure 4) reveal the change of composition of the cathode. Contributions in the XANES spectra of different sulfur compounds formed at the cathode (sulfur, Li-polysulfides, lithium sulfide) can be identified by characteristic energy position of the sulfur edge and pre-edge resonances, which can be ascribed to transitions of sulfur 1s core electrons to unoccupied p-type molecular orbitals in molecules or empty bands in crystalline compounds.24−27,49,50 The elemental state of the sulfur S8 exhibits a dominant edge resonance at 2472 eV. In Li-polysulfides (Li2Sx), where reduced form of sulfur is present at terminal S atoms in the Li− polysulfide chain, an additional pre-edge resonance appears at 2470.2 eV, together with a sulfur peak at 2472 eV, contributed by the internal sulfur atoms. In the case of polysulfide radical anions, additional components are expected at lower energies, around 2468 eV.39−41 In the case of all batteries studied here, however, this peak was never observed, indicating that the formation of radical anion with this electrolyte composition is negligible. For dianionic polysulfides, the relative intensity of the two resonances is proportional to the relative amount of

concentrations, plotted in Figure S2, were used to recalculate polysulfide concentrations in the separator during Li−S battery discharge and charge. In this work, UV/vis measurements were performed not only in the first cycle but also in the fifth cycle. Panels a, b, c, and d of Figure 2 show sets of UV/vis spectra measured during first discharge, first charge, fifth discharge, and fifth charge, respectively. Several differences can be observed compared to our previous works. First, the reflectance of the measured UV/ vis spectra in the first discharge cycle is less intense, although the appearance of long-, mid-, and short-chain polysulfides in the separator can be detected. Less intense reflectance peaks measured during battery discharge and charge are directly correlated to lower amounts of polysulfide in the separator (polysulfides diffused out from the cathode). A lower concentration is not attributed to low sulfur conversion because, as shown by cycling curves in Figure 3, the first discharge capacity was above 1200 mAh/g (cells were cycled with a current density corresponding to C/10 per electron). Another important detail from this set of measurements is the evidence of continuous polysulfide diffusion into the separator, which can be observed from the measurements in the fifth cycle. The UV/ vis spectra shown in red always represent starting UV/vis spectra, whereas the blue spectra were measured at the end of the half cycle (either charge or discharge). Already from the visual comparison of the spectra measured during the fifth discharge and fifth charge, a difference could be noted. This was more precisely evaluated after spectral deconvolution, which was performed using the procedure published in our previous report.29 Concentrations were recalculated by using values obtained from calibration curves. In the first cycle, apart from the already-mentioned lower concentrations of polysulfides, we observed a very similar trend in polysulfide formation. As shown in Figure 3a, long-chain polysulfides first appeared in the separator, reaching a maximum concentration at the end of the high-voltage plateau. Such a concentration was about an order of magnitude lower than that reported for the positive electrode made by sulfur impregnation of a simple carbon-black host matrix. The formations of mid- and short-chain polysulfides follow the same evolution, but with much lower concentrations. 19005


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Figure 5. Operando sulfur K-edge XANES spectra of a Li−S battery in the intermediate states during discharge at nominal compositions: (a) Li0.38S, (b) Li1.08S, (c) Li1.34S, and (d) Li1.61S. Solid squares, experiment; magenta line, best fit with linear combination of the three reference XANES profiles (sulfur, Li2Sx, and Li2S), plotted below.

amount (19%) of remaining elemental sulfur. The contribution of elemental sulfur in the XANES spectrum was removed using the spectrum of the initial state of the battery as reference. The extracted reference Li2Sx spectrum is shown in Figure S4. Comparison with previously measured XANES spectra of polysulfides with different stoichiometric compositions from Li2S8 to Li2S224,26 and comparison with theoretically calculated XANES spectra of polysulfides27 shows that the extracted reference Li2Sx spectrum corresponds to a mixture of predominantly midshort-chain polysulfides (S42−, S32−, S22−). Finally, the third reference spectrum corresponding to crystalline Li2S was extracted from the XANES spectrum measured in the final discharge state of the battery after 1933 min, corresponding to the composition of Li1.61S. At this point of the discharge the major part of sulfur is in the form of crystalline Li2S, and the rest in the form of polysulfides (Sx2−). The contribution of polysulfides in the XANES spectrum (28%) was removed using Li2Sx reference spectrum obtained as described before. The obtained reference Li2S spectrum (Figure S4) is in good agreement with previously published Li2S spectra, measured on chemically synthesized Li2S,24,25 and with theoretically calculated Li2S XANES spectrum.27 All other operando XANES spectra measured in other intermediate states of the battery during discharge can be completely described as linear combination of these three reference spectra (Figure S4). The quality of the linear combination fits (LCF) is demonstrated in Figure 5. In this way the evolution of the relative amounts of the three sulfur

terminal versus internal sulfur atoms, i.e., proportional to the length of Sx2− chain.24−27 In shorter chains (for example S32−), the lower-energy peak is more expressed than in longer chains (S82−). Theoretical predictions show that Li2S2 is expected to exhibit only the low-energy peak.27 The lengths of Sx2− polysulfides can thus be distinguished in XANES spectra based on the ratio between the intensities (peak areas) of the two resonances. The relative intensity of the two peaks for different polysulfides, however, does not have a simple linear relationship to the chain length.27 The XANES spectrum of crystalline Li2S with antifluorite structure, on the other hand, was found to be significantly different compared to that of longor short-chain polysulfides or elemental sulfur.24−27 All sulfur K-edge XANES spectra measured on the battery in operando mode can thus be completely described as a linear combination of three components, belonging to the three sulfur compounds formed in the cathode: elemental sulfur, polysulfides, and Li2S. We extracted the reference XANES profiles of the sulfur compounds (Figure S4) directly from the set of operando spectra of the battery. The elemental sulfur XANES spectrum was obtained from the spectrum of the as-prepared battery, which contained only elemental sulfur. The spectrum agrees well with previously published elemental sulfur XANES spectra.24−26,49,50 The reference spectrum relative to polysulfides (Sx2−) was obtained from the battery spectrum measured after 910 min, which corresponded to the composition of Li0.76S. At this intermediate point, the major part of sulfur was in the form of Li polysulfides, with a small 19006


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The Journal of Physical Chemistry C species (sulfur, Sx2−, and Li2S) during the first discharge of the battery can be determined with high precision (Figure 4b), with an uncertainty below 1% for all components. We need to emphasize that presented results represent average of the back side of the cathode approximately 10 μm in depth, and we assume this is representative of the whole bulk volume because of the presence of polysulfide adsorption additive. The result shows that in the initial state, only pure elemental sulfur is detected. During the first part of the reduction process, corresponding to the high-voltage plateau of the discharge curve, the relative amount of elemental sulfur decreases steeply and the relative amount of the Li−polysulfides increases accordingly. At the composition Li0.76S, about 80% of the total sulfur is in the form of Li−polysulfides, Li2Sx, and the remainder is elemental sulfur. In this process, two different rates of sulfur consumption and polysulfide formation can be observed. Sulfur reduction is faster at the beginning than at the end of the plateau. The faster rate of sulfur reduction at the beginning is mainly attributed to the formation of long-chain polysulfides. In the following part of the high-voltage plateau, the reaction of elemental sulfur is accompanied by the simultaneous reduction of long-chain polysulfides into shorter chains. Consequently, the apparent ratio of polysulfides is not increasing as fast as in the first part, as it corresponds to a combination of two processes (sulfur reduction and polysulfide reduction). The maximum ratio of polysulfides is obtained at the end of the high-voltage plateau, corresponding to the composition Li0.76S. At the beginning of the low-voltage plateau, the formation of lithium sulfide (Li2S) starts, and its relative amount linearly increases until the end of the discharge, where it becomes the dominant discharge product. This measurement clearly shows that the low-voltage plateau starts when the precipitation process of Li2S begins, and a flat voltage curve is attributed to the polysulfides− Li2S equilibrium. This observation is in perfect agreement with some previous reports24,31 showing that Li2S starts forming at the beginning of the low-voltage plateau. The almost linear increase of Li2S relative to the amount of reacted lithium leads to about 75% of Li2S in the final discharged state, with the remaining sulfur in the form of short-chain Li−polysulfides, Sx2−. Linear increase of Li2S amount was not detected in the recent detailed XRD work,31,51 indicating that kinetics plays an important role in the formation of Li2S (different sulfur/carbon ratios were used in two studies). Conversion in our setup is consistent with the measured electrochemistry, and the detected amount of sulfur species at the end of the discharge proves the role of silicalite as an adsorbing agent in the composite electrode, as was proposed using UV/vis spectroscopy and FIBequipped SEM.42 In conclusion, we can divide the discharge curve into three regions characterized by different equilibria among sulfur compounds. At the high-voltage plateau, solid−liquid equilibrium is accompanied by a conversion of sulfur into long-chain polysulfides. The low-voltage plateau represents liquid−solid equilibrium because polysulfides directly precipitate into Li2S, and transition voltage between two plateaus can be viewed as liquid−liquid equilibrium corresponding to shortening of longchain polysulfides. 3.3. EXAFS Study. A sulfur-free electrolyte opened the possibility for quantitative EXAFS data analysis, allowing us to study the transformation of the local environment of the sulfur centers in the electrode and to highlight possible interactions between polysulfides and the host matrix.

Figure 6 shows the experimental EXAFS spectra in the k and R space during discharge. The measurement can be divided into

Figure 6. Experimental EXAFS data in k space: (a) spectra obtained during the discharge of the battery at the high-voltage plateau, (b) spectra obtained during the discharge of the battery at the high-voltage plateau (black curves) and at the start of the low-voltage plateau (from Li0.76S to Li0.89S), (c) all EXAFS measurements (blue curves correspond to measured spectra during composition change between Li1.15S and Li1.58S), and (d) all experimental EXAFS data in R space.

three different sections. During sulfur reduction at the highvoltage plateau, no changes in the main frequency of the EXAFS oscillations were observed, apart from an overall decrease in the intensity of the signal, which was also evidenced by the decrease in the intensity of the first peak of the modulus of the Fourier transform (Figure 6a,d, black curves). As we demonstrated with the LCF analysis of the XANES part of the spectra, during the discharge at the high-voltage plateau, about 80% of the sulfur is reduced to polysulfides and the remainder is elemental sulfur, whereas no Li2S is formed yet. The decrease in the intensity of the first peak of the modulus of the Fourier transform of the EXAFS spectra is compatible with the average decrease in the number of nearest sulfur neighbors. The EXAFS data recorded at the low-voltage plateau show, at k = 3.5 Å−1, an additional signal at high frequency superimposed on the main frequency. This is shown in Figure 6b (red curves) and is highlighted by the blue arrow. The occurrence of such a high-frequency signal must be assigned to the onset of the occurrence of the Li2S phase, which is in agreement with the LCF analysis. Such a high frequency should give rise to a high R value contribution in the Fourier transform, which is, nonetheless, hidden in most spectra because of the statistical noise and therefore cannot be quantified unequivocally by a quantitative EXAFS fitting in the whole series of spectra corresponding to the low-voltage 19007


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the fitting procedure, and the fitting parameters have been constrained in order to have the number of independent points, as calculated according to the Nyquist criterion, much higher than the number of variables. The relative fraction of the two species has been used as a free parameter in the fitting procedure. The ratio obtained from the best-fit calculation was S/Li2S = 30(5)/70(5). Figure 7 shows the comparison of the experimental EXAFS spectrum recorded at a composition of Li1.47S (dots) with the best-fit calculation (solid curve).

plateau. Panels c and d of Figure 6 show the experimental EXAFS data in k and R space, respectively, for the whole discharge cycle. As expected, the high-frequency signal increases with time, but only at Li1.26S is there a detectable signal above the statistical noise in the modulus of the Fourier transform at R values between 3 and 4 Å. This corresponds to the second and third coordination shell of S and Li atoms, which are characteristic of the crystalline structure of Li2S. Quantitative analysis was performed for the S−S first coordination shell within the single-scattering framework. Backscattering amplitudes and phase shifts, as well as the photoelectron mean-free path, were calculated by FEFF646 for the structure of S8,47 and a nonlinear best fit to the experimental data was performed by using the Artemis program.45 The spectrum of the pristine material was used as a reference. The fitting of the first spectrum is shown in Figure S5. The fitting interval in R space was 1−2.2 Å. Data were interpreted in terms of two S nearest neighbors at a distance of 2.046 Å, which is typical of elemental sulfur,47 with a σ2 of 0.003(1) Å2 where exp(−2k2σ2) is the EXAFS Debye−Waller factor. Two further nonstructural parameters were treated as follows: the amplitude reduction factor, S02, was set to 0.8 by imposing a coordination number N = 2, whereas the energy mismatch, E0, between experimental and theoretical scales was found to be 6.4 eV. The structural values found from the EXAFS analysis are in their typical range for bulk solids and correspond to the presence of elemental sulfur. For the fits of the spectra recorded during discharge, the nonstructural parameters S02 and E0 were kept constant at the values found for the initial state, together with the value of σ2. Comparison of the experimental EXAFS spectra for the battery in the initial state (Figure S5a) and at the end of the high-voltage plateau (Figure S5b) shows a general decrease in the coordination number, N, to 1.6(2), which would correspond to the average coordination number typical of S62−. At this point of the discharge, the amount of reacted lithium is 0.75, corresponding to an average composition between S42− and S22−. Such a discrepancy could be attributed to an inhomogeneous distribution of the sulfur in the electrode. In fact, one should take also into account that the spectra are taken from the back of the electrode, where the reaction could be somewhat slowed compared to the side of the electrode facing the negative lithium electrode. Another possible explanation for this effect, however, is the partial migration of polysulfides formed during the whole high-voltage plateau, leading to a general increase of long-chain species in the electrode and thus an overestimation of their concentration. Such a loss of sulfur would be in good agreement with the UV/vis experiment presented above. On the other hand, LCF fitting of the XANES spectra suggested that about 20% of the total sulfur is still in the form of elemental sulfur in the probed part of the electrode. Taking into account this remaining amount, the number of nearest neighbors in the polysulfides decreases to about 1.6, that is, between the expected values for S62− and S42−. Although the above explanation is plausible, it must be stressed that the EXAFS data analysis is not sufficiently sensitive to detect the simultaneous presence of 0.8 S−Li bonds at about 2.5 Å, as suggested by the XANES LCF, owing to the very weak scattering power of the lithium atoms compared to that of sulfur atoms. The data recorded at the end of discharge, on the contrary, could be modeled using the structural model relative to elemental sulfur47 and to antifluorite Li2S.48 All of the single scattering signals contributing up to 5 Å have been included in

Figure 7. Comparison between the experimental EXAFS spectrum at nominal composition Li1.47S (dots) and the best-fit calculation (solid curve).

A complete fit of the series of spectra collected through the first discharge with the two S−S components, the first one at 2.04 Å representing sulfur and polysulfides and the other one at 4.04 Å representing Li2S, was also tried. It must be stressed that even though the presence of Li2S is detected in the EXAFS signal at the beginning of the low-voltage plateau, the intensity remains very low and becomes clearly visible only in the last six spectra (starting at the average composition Li1.34S). The variation of the average coordination number of the S−S component during the whole process is shown in Figure 8. A gradual decrease in the coordination number is shown in the first part of the discharge, varying from 2 to about 1.6(2) at the end of the high-voltage plateau. As already stated above, such a

Figure 8. Variation of the average S coordination number during the first discharge. The average coordination of the most important polysulfides is reported for comparison. The vertical line represents the end of the high-voltage plateau. 19008


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The Journal of Physical Chemistry C value would correspond to the presence of about 20% of elemental sulfur and to polysulfides of an average composition between S62− and S42−. A steeper decrease is observed at the beginning of the low-voltage plateau, where the average coordination number varies rapidly, down to values close to those typical for S42− and S22−. At this point, the presence of Li2S becomes clearly visible in the Fourier transform of the EXAFS spectra and can be consistently fitted. The average coordination number of the component representing polysulfides does not vary during this part of the discharge, even though their relative amount decreases. At the same time, the amount of Li2S increases rapidly to about 70% of the total amount of sulfur at the end of the discharge. It is important to stress that, apart from these two components, no other signal was clearly detected in the EXAFS spectra throughout the whole series. Taking into account the sensitivity of EXAFS, which is usually above 5%, no specific interaction of the polysulfide species with the matrix or with other species in the electrolyte could be detected.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Access to synchrotron radiation facilities of Elettra is acknowledged. We thank Luca Olivi of Elettra for expert advice on beamline operation. This research has received funding from the Slovenian Research Agency research programs P1-0112 and P20148 and the European Union Seventh Framework Programme under Grant Agreement 314515 (EUROLIS).

4. CONCLUSION Using a combination of analytical tools, we derived a mechanism for polysulfide formation and diffusion from the composite sulfur-based electrode with silicalite as the adsorption additive. Operando UV/vis spectroscopy confirmed the role of silicalite, as the quantity of polysulfides detected in the separator was approximately ten times lower than in composites without any adsorption additive, and the evolution of different polysulfides followed the same sequence as in the composites without any adsorption additive. We measured and derived information from the fifth cycle, where we observed no changes at the beginning of discharge and the end of charge for long-chain polysulfides; however, the concentration linearly decreased during the discharge and linearly increased during the charge processes. Changes in the concentrations of midchain and short-chain polysulfides suggest a polysulfide shuttle mechanism. The use of the cathode composite in combination with a sulfur-free electrolyte enabled us to obtain a detailed and precise XANES analysis. With LCF, we observed the complete conversion of sulfur in more reduced states (polysulfides or Li2S). The concentration of polysulfides reached a maximum value at the end of the high voltage, which is in agreement with the concentration profile of polysulfides in the separator detected by UV/vis spectroscopy. The low-voltage plateau is characteristic of the polysulfides/Li2S equilibrium, and the ratio of Li2S increased linearly until the end of discharge, giving a final composition of 75% Li2S and the rest as short-chain polysulfides. A nonsulfur-containing electrolyte enabled, for the first time, the complete analysis of sulfur EXAFS spectra. From the EXAFS part of the XAS spectra, we were able to clearly detect sulfur and Li2S components, whereas the polysulfides were determined from the variation in the average coordination number of the S− S component during the discharge process. We need to emphasize that, apart from sulfur and Li2 S, no other components were detected in the EXAFS spectra throughout the whole series and we were not able to detect any specific interaction of the polysulfide species with the matrix or with other species in the electrolyte.

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Photographs of five different catholyte solutions, linear fits of the normalized intensities measured from catholyte solutions, experimental setup, sulphur K-edge XANES spectra of sulfur compounds present in a Li−S battery, and comparison of the modulus and imaginary part of the Fourier transform of the experimental EXAFS spectrum of the cathode and the best-fit calculation for the battery. (PDF)

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REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05609. 19009


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Analytical Detection of Polysulfides in the Presence of Adsorption

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