Sulfur Speciation in Li–S Batteries Determined by Operando X-ray

Letter pubs.acs.org/JPCL

Sulfur Speciation in Li−S Batteries Determined by Operando X‑ray Absorption Spectroscopy Marine 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 electrochemical energy storage is developing insightful in situ probes of redox processes for a working cell. This is particularly true for cells that operate on the basis of chemical transformations such as Li−S and Li−O2, where the factors that govern capacity and cycling stability are difficult to access owing to the amorphous nature of the intermediate species. Here, we investigate cathodes for the Li−S cell comprised of sulfur-imbibed robust spherical carbon shells with tailored porosity that exhibit excellent cycling stability. Their highly regular nanoscale dimensions and thin carbon shells allow highly uniform electrochemical response and further enable direct monitoring of sulfur speciation within the cell over the entire redox range by operando X-ray absorption spectroscopy on the S K-edge. The results reveal the first detailed evidence of the mechanisms of sulfur redox chemistry on cycling, showing how sulfur fraction (under-utilization) and sulfide precipitation impact capacity. Such information is critical for promoting improvements in Li−S batteries. SECTION: Energy Conversion and Storage; Energy and Charge Transport

T

electrolyte and incorporated in a cell designed to prevent dead zones. It exhibits all of the voltage plateaus and other subtle features of a classic, well-functioning Li−S cell based on micrometer-sized sulfur, at the typical potentials.2,3,12 This deliberate experimental design, along with well-characterized reference standards, allows us to utilize synchrotron S K-edge absorption spectroscopy.13 We are aware of only one study that has investigated the electrochemistry of the Li−S battery by this method.14 That report, optimized to probe not the cathode but the electrolyte phase and elucidate its reactivity elegantly, showed that the solvent plays a key role in the electrochemical performance. The cells were not studied under real-time operando conditions however, and polysulfide spectral features were not verified or quantified. Here, not only are we looking at the bulk electrode, but by constraining the sulfur (and polysulfide) to 3D nanodimensions, we minimize X-ray selfabsorption effects and thus avoid the overwhelming distortion of the XANES spectra that results from bulk particles.15,16 This development has enabled us to develop a clear mechanism of the sulfur redox chemistry in the cell that is broadly applicable to Li−S batteries.

he Li−S battery possesses many highly desirable characteristics for energy storage, but at present, it also exhibits poor capacity retention by comparison with lithium ion cells.1−3 Moreover, the highest reversible capacities reported for Li−S cells in the literature are typically on the order of 1200 mAh·g−1, that is, 75% of the theoretical capacity.4,5 It is reported that the plateau in the electrochemical profile of the Li−S cell at 2.1 V corresponds to conversion of soluble Li2S4 to insoluble Li2S26 in addition to Li2S,5 representing one source of capacity limitation. However, “Li2S2” has not been isolated nor does it appear on any phase diagram.7 Other polysulfides Li2Sx (8 ≥ x ≥ 3) are also uncharacterized in the solid state, and solution studies suggest that they are in rapid equilibrium.8−11 Incomplete electrochemical redox represents a significant shortcoming for practical Li−S cells that can only be rationally addressed by uncovering the nature of the electrochemical processes, 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-edge spectroscopy (XANES) by developing and utilizing a carbon− sulfur composite electrode optimized for both efficient transport of charge carriers and trapping of soluble polysulfides. The cathode consists of sulfur impregnated in porous hollow carbon spheres of uniform diameter that minimize significant dissolution into the bulk electrolyte, paired with a nonsulfurous © 2013 American Chemical Society

Received: August 17, 2013 Accepted: September 11, 2013 Published: September 11, 2013 3227

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

The Journal of Physical Chemistry Letters

Letter

100% after activation for the duration of 100 cycles with a capacity of 730 mAh·g−1. Previous reports have detailed highcapacity retention in excess of 90% over 100 cycles but have fallen short at reporting this benchmark above C/2 rates.17 Higher capacities of 1200 mAh·g−1 were achieved at slower rates (see below and Figure S2, Supporting Information). Most importantly, the cycling stability conferred by the cathode material facilitated the operando studies. The preparation of individual polysulfides as references was essential to achieve sulfur speciation; the characterization of these materials was accomplished by a combination of XRD and high-field 7Li MAS NMR. Compositions Li2S, Li2S2, Li2S4, Li2S6, and Li2S8 were initially targeted using the appropriate ratio of elemental sulfur and LiEt3BH as a reducing agent in THF.21 However, most of these compositions were found to consist of phase mixtures. XRD revealed that α-S8 and Li2S were the only detectable nanocrystalline phases in Li2Sn powders for n = 1, 2, and 4 and n = 4 and 8, respectively (see Figure S3, Supporting Information), and NMR was used to elucidate the contribution of amorphous phases (see below). Only Li2S6 appeared fully amorphous by XRD, and the isolation of this new single-phase intermediate was clearly identified by 7 Li MAS NMR, as shown in Figure 2. To the best of our

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

Figure 1. (a,b) SEM images of PCNSs at both low and high magnification. (c) STEM image with EDX line scan (red, carbon; blue, sulfur). (d) Nitrogen isotherm prior to sulfur impregnation and inset pore 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, as illustrated in (b) using a two-component model for Li2S8 (Li2S and Li2S6).

while the pore-former content controls the shell porosity and thickness.19 The ∼20 nm thick shells maintain robustness, while the 4 nm pores are large enough to enable the PCNSs to be impregnated with sulfur by melt-diffusion and readily permit ingress 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 where the sulfur is primarily confined and no sulfur contribution from the exterior. In the absence of the pore former, no sulfur is incorporated.17,20 The BET isotherm and pore size distribution shown in Figure 1d indicate a sharp decrease in the surface area upon sulfur impregnation (from to 824 to 108 m2·g−1) and the filling of much of the shell porosity. The substantial shell pore volume (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 that the 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 a standard coin cell configuration at a high rate of 1 C (1672 mA· g−1). The 1st, 50th, and 100th voltage profiles (Figure 1e) show that the material exhibits not only a typical Li−S voltage profile but also outstanding capacity retention, remaining at close to

knowledge, there is no prior report on 7Li NMR of lithium polysulfides. We found that although the range of chemical shifts 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 linear combinations of these two signals (Figure 2b). Thus, existence of Li2S2 as a solid metastable phase is not indicated by our NMR data, and Li2S6 is the only intermediate isolated between α-S8 and Li2S. The presence of α-S8, Li2S6, and Li2S in the sample of nominal stoichiometry “Li2S8” illustrates the metastable nature of long-chain polysulfides in the solid state. Demixing of Li2S6 into α-S8 and Li2S was indeed observed at around 60 °C, so that Li2S8 might require even lower temperature to be isolated. To complete the reference series for the polysulfide anions S42− and S22−, single-phase crystals of Na2S2 and Na2S4 were used because Li2S2 and Li2S4 cannot be isolated.22 The difference in the cation does not contribute to any 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, S62−, S42−, 2− S2 , and Li2S) and owing to the unique design of the cathode 3228

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

The Journal of Physical Chemistry Letters

Letter

at these slower rates that were utilized to minimize the compositional change between consecutive spectra. Figure 3b compares selected XANES spectra. The S K-edge step does not decrease during discharge, suggesting that the solvated polysulfides are largely contained, reflecting the effective design of the nanospheres. For comparison, Figure 3c displays the ex situ spectra of the standard sulfur species. The strong absorption peak observed at 2469.5 eV in the cathode composite is assigned to the high-intensity “white line” (1s → 3p transition) of elemental sulfur along with a higher-energy feature 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 to some previous XRD studies that did not detect crystalline α-S8 after one full cycle,24,25 the XANES displays a spectrum characteristic of pure elemental sulfur at the end of charge (Figure 3c, top). At intermediate states of charge, a low-energy peak appears at 2467.7 eV. Comparison with the reference spectra (Figure 3c, middle) unequivocally shows that this lowenergy feature is associated with linear polysulfides.14 Its intensity increases with shortening of the polysulfide chain, opposite to the decrease in intensity of the white line. The positions of these two (white line and low-energy) bands remain the same irrespective of the chain length, n, in Li2Sn (2 < n < 8), meaning that the polysulfides are primarily distinguished by the ratio of their intensities. At the end of discharge, the intensity is minimal at the white line energy, in agreement with the Li2S reference that exhibits two peaks at 2470.8 and 2473.7 eV.16,26 The spectra at the discharged state do not exhibit any low-energy intensity characteristic of linear polysulfides. Comparison with pure Li2S shows that some α-S8 remains in the cathode composite (Figure 3c, bottom), which is consistent with the electrochemistry that shows 75% capacity compared to theoretical results upon discharge. The reason for this is explained below. Because the standards represent the sulfur species successively formed by electrochemistry, operando spectra can be described with a linear combination fit (LCF).16 Considering the number of reference spectra, multiple solutions exist, among which the LCF constructed from the minimum combination of {α-S8, S62−, S42−, and S2−} gave the best fit and agreement with the electrochemistry. The contribution of S22− was clearly ruled out (see Figure S6, Supporting Information for full discussion and alternative fits). S32− and S82− are not isolable due to complex equilibria in solution, as confirmed recently by operando UV−vis spectrometry of Li−S catholytes; spectra could only be interpreted in terms of an average stoichiometry of dissolved polysulfides, even though the technique should be sensitive to their chain length.11 The use of S42− and S62− here best represents the medium- and longchain polysulfide contributions, as shown by Figure S7 (Supporting Information), which compares the composition of the sulfur cathode upon cycling based on the XANES and on the electrochemistry. The perfect accordance observed upon charge proves that the area and depth probed are representative of the overall cathode and that sulfur speciation is effectively achieved. For clarity, the following omits the first discharge at C/5 to focus only on one full cycle at C/10 (see Figure S8, Supporting Information). As noted above, the discharged cathode contains a fraction of unreacted elemental sulfur estimated at 20−25 wt % from the fit (Figure 3c, bottom). Figure 4a displays the charge from Li2S to S8.

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

Figure 3. Sulfur K-edge XANES upon cycling and reference spectra showing the following: (a) evolution of absorbance as a function of the electrochemical cycling, at C/5 (first discharge) and then C/10; the ends of the (dis)charges are highlighted in red; spectra were acquired continuously to minimize the composition change between two spectra to 50 mAh·g−1sulfur (i.e., 3% of the total capacity); (b) selected spectra during the cycling, labeled using the average composition LixS as estimated by the electrochemistry; (c) reference spectra for elemental sulfur (top), linear polysulfides (S22−, S42−, and S62−, middle) and Li2S (bottom), together with the initial, charged state (red line in the top panel) and discharge states (red lines in the bottom panel).

additional details). Distortion was successfully minimized by constraining the sulfur species to nanodimensions in the spheres (Figure S5, Supporting Information). Figure 3a shows the operando XANES results during the first discharge at a C/5 rate and for the second full cycle at C/10. First and second discharge capacities of ∼1200 and 1100 mAh·g−1 were achieved 3229

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

The Journal of Physical Chemistry Letters

Letter

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, S62−, 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.

is evident by XANES until the second half of the plateau. The final reduction stage signals a steep increase in the fraction of Li2S (also suggested by impedance spectroscopy measurements27). The conversion of all of the available polysulfides results in the sudden voltage drop, thus preventing reduction of the remaining elemental sulfur. The overall discharge as probed by XANES can be summarized as S0 + 3/2 Li+ + 3/2 e− → 3/4 Li2S + 1/4 S0 to account for the persistence of about 25% elemental sulfur. The corresponding calculated specific capacity of 1254 mAh·g−1 is within 5% agreement with the electrochemistry (i.e., 1189 mAh·g−1). The ends of the discharge spectra do not exhibit any low-energy feature but instead only consist of a mixture of S8 and Li2S. Hence, contrary to what has been assumed to date,28,29 the presence of insoluble Li2S2 as a final reduction product and as a source of lower than theoretical capacity is not supported by the XANES data. Importantly, our operando XANES experiments indicate that the discharge capacity is not limited by the precipitation of Li2S but is instead mostly restricted by unreacted sulfur. This cannot be explained by restricted volume expansion upon conversion of S to Li2S. The interior of the sphere is not even half filled as the pore volume allows for all of the sulfur to be accommodated in the shell. Moreover, consumption of α-S8 during the cell discharge is arrested long before Li2S is detected. Therefore, neither steric hindrance nor surface passivation can account for the incomplete sulfur utilization. This is supported by reports of unreacted sulfur at the end of discharge on electrodes prepared from bulk sulfur simply ground with carbon.30 Our data suggest that once S62− is formed through S82− disproportionation, reduction proceeds much faster through these polysulfides in solution than for either the initial or reformed sulfur, leaving elemental sulfur in reserve. A contrario, high discharge capacities (>1200 mAh·g−1) can be attained under dilute configurations,31 where the sulfur load is insufficient, and active material loss is thus severe. We note a delay in the detection of Li2S by XANES upon discharge (based on the fact that charge shows the expected behavior, we believe that cell artifacts are not involved; see Figures S4 and S7,

The evolution probed by XANES and its consistency with the electrochemical profiles allow us to propose the following charge mechanism. Contrary to the assumption that S82− and elemental sulfur are the main oxidation products,10 we observe that the slow and monotonic consumption of Li2S during most of the charge process is instead accompanied by the formation of shorter chains. The contribution from S62− is detected even at the earliest stage and increases along the sloping charge at the expense of S42−, which appears as a transient species allowing the oxidation of Li2S into more stable Li2S6. The late disappearance of Li2S and S42− and hence the maximum of S62− coincide with the voltage rise, signaling the final oxidation of S62− to α-S8. The disappearance of the low-energy peak and the steep increase of the white line on the S K-edge spectra at the end of charge show that the cathode composite completely converts to sulfur, indicating the excellent reversibility within the PCNS host. In terms of the weight of the components and accounting for the ∼1/4 fraction of unreacted sulfur, the overall charge can be summarized 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 in relatively good agreement with the 1085 mAh·g−1 measured experimentally, considering the accuracy of the measurement and the possible error on the LCF. The Li−S cell discharge was similarly investigated by operando XANES (Figure 4b). The first discharge plateau is usually described as the reduction of elemental sulfur to S82−. However, there is strong evidence for the rapid chemical disproportionation of S82− in solution based on UV spectroscopy8 and electrochemical techniques,9 S82− → S62− + 1/4 S80. This would explain the formation of S62− in the initial discharge step, in agreement with the phase mixture obtained when attempting to chemically prepare Li2S8 (Figures 2 and S5, Supporting Information). The contributions of polysulfides including S42− in a second step increase at the expense of elemental sulfur. Then, supersaturation (the “knee” on the voltage profile) marks not only the end of the α-S 8 consumption but also the maximum concentration of polysulfides. While the cell voltage is fixed at 2.1 V, no Li2S 3230

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

The Journal of Physical Chemistry Letters

Letter

monochromator under helium flow, and the data were collected in fluorescence mode using a four-element vortex detector. Details regarding the experimental setup and sample preparation are given in the Supporting Information. 7 Li NMR. The 7Li NMR experiments were carried out at room temperature on a Bruker Avance-500 spectrometer (B0 = 11.8 T, ν0(7Li) = 194 MHz). MAS spectra were obtained by using a Bruker MAS probe with a cylindrical 2.5 mm o.d. zirconia rotor at a spinning frequency of 30 kHz.

Supporting Information). This delayed formation of Li2S could explain why previous operando X-ray diffraction/tomography studies failed to identify Li2S during cell operation even at the lower potential limit32 while suggesting that crystalline Li2S eventually forms upon later equilibration. XANES, as a local probe, detects solid amorphous and crystalline lithium sulfide alike, suggesting rather that the S2− that is formed exists in a supersaturated state before suddenly precipitating on/in the electrode, as proposed earlier by modeling.6 The highly solvated S2− ion might be expected to have a weak and illresolved spectral signature, similar to Li2S, but even more so owing to its noncondensed state, thus explaining the lack of additional spectral intensity in the region between 400 and 800 mAh·g−1. That said, the exact cause for the delayed appearance of Li2S is not yet understood and further studies are underway. The Li2S deposition mechanism will depend, among other factors, on the affinity of Li2S with the substrate (i.e., the cathode composite),33 and on the discharge rate (that will affect the potential). Our results furthermore suggest that molecular components such as anion receptors34 could help tune S2− supersaturation and limit the capacity fading by controlling Li2S precipitation. In summary, our findings shed important light on sulfur speciation in the cell during redox behavior and on the mechanisms that govern dissolution and deposition of the redox end members, S and Li2S. The hysteresis between charge and discharge is also explained by the inherent difference between the steps in the reduction and oxidation processes. Upon reduction, delay in precipitation of Li2S owing to supersaturation of S2− dominates. Upon charge, surface oxidation of Li2S proceeds straightforwardly through the soluble intermediates, S42− and S62−. This is accompanied by an overpotential if the Li2S particles are large, owing to the energy 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 process upon discharge) will give rise to the observed sloping profile even in the absence of a significant overpotential. These results provide new fundamental insight and perspectives crucial to the further development of the Li−S system. Ongoing investigations in our laboratory are exploring operando XANES to unravel atypical sulfur redox behavior in microporous carbons,35 heat-treated carbon−sulfur composites,36 and novel electrolyte systems, which will be reported subsequently.

■

ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental protocol and supporting Figures S1−S8, showing the determination of sulfur content in the PCNS/S composite, long-term cycling at the 1 C rate with the specific discharge capacity, structural characterization of solid-state lithium and sodium polysulfides prepared as reference materials, a schematic of the operando cell, the sulfur K-edge XANES correction from self-absorption, alternative LCFs of the operando XANES spectra, the composition of the sulfur-based cathode upon cycling based on the XANES and on the electrochemistry, and the LCF of the XANES spectra recorded during the first discharge at C/5. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +1-519-888-4567 ext. 48637. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The research was supported by the BASF International Scientific Network for Electrochemistry and Batteries. We thank Dr. N. Coombs, University of Toronto, for acquisition of the TEM and L. Spencer and G. Goward, McMaster University, for solid-state NMR facilities. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

■

■

REFERENCES

(1) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M. Li−O2 and Li−S Batteries with High Energy Storage. Nat. Mater. 2012, 11, 19−29. (2) Manthiram, A.; Fu, Y.; Su, Y.-S. Challenges and Prospects of Lithium−Sulfur Batteries. Acc. Chem. Res. 2012, 46, 1125−1134. (3) Evers, S.; Nazar, L. F. New Approaches for High Energy Density Lithium−Sulfur Battery Cathodes. Acc. Chem. Res. 2013, 46, 1135− 1143. (4) Elazari, R.; Salitra, G.; Garsuch, A.; Panchenko, A.; Aurbach, D. Sulfur-Impregnated Activated Carbon Fiber Cloth as a Binder-Free Cathode for Rechargeable Li−S Batteries. Adv. Mater. 2011, 23, 5641− 5644. (5) Akridge, J. R.; Mikhaylik, Y. V.; White, N. Li/S Fundamental Chemistry and Application to High-Performance Rechargeable Batteries. Solid State Ionics 2004, 175, 243−245. (6) Kumaresan, K.; Mikhaylik, Y.; White, R. E. A Mathematical Model for a Lithium−Sulfur Cell. J. Electrochem. Soc. 2008, 155, A576−A582. (7) Cunningham, P. T.; Johnson, S. A.; Cairns, E. J. Phase Equilibria in Lithium−Chacogenen Systems. II. Lithium−Sulfur. J. Electrochem. Soc. 1972, 119, 1448−1450.

EXPERIMENTAL METHODS PCNS/S Preparation. The synthetic procedure relied on selfassembly of a silica core and carbon shell that incorporated a pore former (see the Supporting Information for details). The PCNSs were then impregnated with ∼70 wt % sulfur by a meltdiffusion method. Electrochemical Studies. Positive electrodes were constructed from 80% PCNS/S-68%, 10% Kynar Powerflex binder, and 10% Super S carbon using 2325 coin cells with an electrolyte comprised of 1.0 M LiTFSI (lithium bis(trifluoromethanesulfonyl) imide) in DOL (1,3-dioxolane) and DME (1,2-dimethoxyethane) (1:1 volume ratio) and 2 wt % LiNO3 to control surface passivation37 of the lithium foil used as the negative electrode. The discharge/charge rate of 1 C (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 3231

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

The Journal of Physical Chemistry Letters

Letter

(27) Yuan, L.; Qiu, X.; Chen, L.; Zhu, W. New Insight Into the Discharge Process of Sulfur Cathode by Electrochemical Impedance Spectroscopy. J. Power Sources 2009, 189, 127−132. (28) Mikhaylik, Y. V.; Akridge, J. R. Polysulfide Shuttle Study in the Li−S Battery System. J. Electrochem. Soc. 2004, 151, A1969−A1976. (29) Zheng, G.; Yang, Y.; Cha, J. J.; Hong, S. S.; Cui, Y. Hollow Carbon Nanofiber-Encapsulated Sulfur Cathodes for High Specific Capacity Rechargeable Lithium Batteries. Nano Lett. 2011, 11, 4462− 4467. (30) Elazari, R.; Salitra, G.; Talyosef, Y.; Grinblat, J.; Scordilis-Kelley, C.; Xiao, A.; Affinito, J.; Aurbach, D. Morphological and Structural Studies of Composite Sulfur Electrodes upon Cycling by HRTEM, AFM and Raman Spectroscopy. J. Electrochem. Soc. 2010, 157, A1131− 1138. (31) Liang, C.; Dudney, N. J.; Howe, J. Y. Hierarchically Structured Sulfur/Carbon Nanocomposite Material for High-Energy Lithium Battery. Chem. Mater. 2009, 21, 4724−4730. (32) Nelson, J.; Misra, S.; Yang, Y.; Jackson, A.; Liu, Y.; Wang, H.; Dai, H.; Andrews, J. C.; Cui, Y.; Toney, M. F. In Operando X-ray Diffraction and Transmission X-ray Microscopy of Lithium Sulfur Batteries. J. Am. Chem. Soc. 2012, 134, 6337−6343. (33) Seh, Z. W.; Zhang, Q.; Li, W.; Zheng, G.; Yao, H.; Cui, Y. Stable Cycling of Lithium Sulfide Cathodes through Strong Affinity with Bifunctionnal Binder. Chem. Sci. 2013, 4, 3673−3677. (34) Xie, B.; Lee, H. S.; Li, H.; Yang, X. Q.; McBreen, J.; Chen, L. Q. New Electrolytes Using Li 2 O or Li 2 O 2 Oxides and Tris(pentafluorophenyl) Borane as Boron Based Anion Receptor for Lithium Batteries. Electrochem. Commun. 2008, 10, 1195−1197. (35) Wang, J.; Liu, L.; Ling, Z.; Wan, C.; Jiang, C. Polymer Lithium Cells with Sulfur Composites as Cathode Materials. Electrochim. Acta 2003, 48, 1861−1867. (36) Xin, S.; Gu, L.; Zhao, N.-H.; Yin, Y.-X.; Zhou, L.-J; Guo, Y.-G.; Wan, L.-J. Smaller Sulfur Molecules Promise Better Lithium−Sulfur Batteries. J. Am. Chem. Soc. 2012, 134, 18510. (37) Aurbach, D.; Pollack, E.; Elizari, R.; Salitra, G.; Kelley, C. S.; Affinito, J. On the Surface Chemical Aspects of Very High Energy Density, Rechargeable Li−Sulfur Batteries. J. Electrochem. Soc. 2009, 156, A694−A702.

(8) Barchasz, C.; Molton, F.; Duboc, C.; Leprêtre, J.-C.; Patoux, S.; Alloin, F. Lithium/Sulfur Cell Discharge Mechanism: An Original Approach for Intermediate Species Identification. Anal. Chem. 2012, 84, 3973−3980. (9) Badoz-Lambling, J.; Bonnaterre, R.; Cauquis, G.; Delamar, M.; Demange, G. La Reduction du Soufre en Milieu Organique. Electrochim. Acta 1976, 21, 119−131. (10) Yamin, H.; Gorenshtein, A.; Penciner, J.; Sternberg, Y.; Peled, E. Lithium−Sulfur Battery. Oxidation/Reduction Mechanisms of Polysulfides in THF Solutions. J. Electrochem. Soc. 1988, 135, 1045−1048. (11) Patel, M. U. M.; Demir-Cakan, R.; Morcrette, M.; Tarascon, J.M.; Gaberscek, M.; Dominko, R. Li−S Battery Analyzed by UV/Vis in Operando Mode. ChemSusChem 2013, 6, 1177−1181. (12) Su, Y.-S.; Manthiram, A. A Facile In Situ Deposition Route to Obtain Carbon-Wrapped Sulfur Composite Cathodes for Lithium− Sulfur Batteries. Electrochim. Acta 2012, 77, 272. (13) Totir, D. A.; Antonio, M. R.; Schilling, P.; Tittsworth, R.; Scherson, D. A. In Situ Sulfur K-Edge X-ray Absorption Near Edge Structure of an Embedded Pyrite Particle Electrode in a Non-Aqueous Li-Based Electrolyte Solution. Electrochim. Acta 2002, 47, 3195−3200. (14) Gao, J.; Lowe, M. A.; Kiya, Y.; Abruña, H. D. Effects of Liquid Electrolytes on the Charge−Discharge Performance of Rechargeable Lithium/Sulfur Batteries: Electrochemical and In-Situ X-ray Absorption Spectroscopic Studies. J. Phys. Chem. C 2011, 115, 25132−25137. (15) Pickering, I. J.; George, G. N.; Yu, E. Y.; Brune, D. C.; Tuschak, C.; Overmann, J.; Beatty, J. T.; Prince, R. C. Analysis of Sulfur Biochemistry of Sulfur Bacteria Using X-ray Absorption Spectroscopy. Biochemistry 2001, 40, 8138−8145. (16) Manceau, A.; Nagy, K. L. Quantitative Analysis of Sulfur Functional Groups in Natural Organic Matter by XANES Spectroscopy. Geochim. Cosmochim. 2012, 99, 206−223. (17) Jayaprakash, N.; Shen, J.; Moganty, S. S.; Corona, A.; Archer, L. A. Porous Hollow Carbon@Sulfur Composites for High-Power Lithium−Sulfur Batteries. Angew. Chem., Int. Ed. 2011, 50, 5904−5908. (18) Zhang, C.; Wu, H. B.; Yuan, C.; Guo, Z.; Lou, X. W. Confining Sulfur in Double-Shelled Hollow Carbon Spheres for Lithium−Sulfur Batteries. Angew. Chem., Int. Ed. 2012, 124, 9730−9733. (19) Bruno, M. M.; Cotella, N. G.; Miras, M. C.; Barbero, C. A. A Novel Way to Maintain Resorcinol-Formaldehyde Porosity During Drying; Stabilization of the Sol−Gel Nanostructure Using a Cationic Polyelectrolyte. Colloids Surf., A 2010, 362, 28. (20) Fuertes, A.; Valle-Vigón, P.; Sevilla, M. One-Step Synthesis of Silica@Resorcinol-Formaldehyde Spheres and Their Application for the Fabrication of Polymer and Carbon Capsules. Chem. Commun. 2012, 48, 6124. (21) Gladysz, J. A.; Wong, V. K.; Jick, B. S. Introduction of Sulfur into Organic Molecules via Lithium Triethylborohydride Reduction of S8. J. Chem. Soc., Chem. Commun. 1978, 19, 838−839. (22) Rosen, E.; Tegman, R. A Preparative and X-ray Powder Diffraction Study of the Polysulfides Na2S2, Na2S4 and Na2S5. Acta Chem. Scand. 1971, 25, 3329−3336. (23) Mori, R. A.; Paris, E.; Guili, G.; Eeckhout, S. G.; Kavicic, M.; Zitnik, M.; Bucar, K.; Pettersson, L. G. M.; Glatzel, P. Electronic Structure of Sulfur Studied by X-ray Absorption and Emission Spectroscopy. Anal. Chem. 2009, 81, 6516−6525. (24) Cheon, S.-E.; Ko, K.-S.; Cho, J.-H.; Kim, S.-W.; Chin, E.-Y.; Kim, H.-T. Rechargeable Lithium Sulfur Battery I. Structural Change of Sulfur Cathode during Discharge and Charge. J. Electrochem. Soc. 2003, 150, A796−A799. (25) Ryu, H.-S.; Ahn, H.-J.; Kim, K.-W.; Ahn, J.-H.; Lee, J.-Y. Discharge Process of Li/PVdF/S Cells at Room Temperature. J. Power Sources 2006, 153, 360−364. (26) Takahara, H.; Tabuchi, M.; Takeuchi, T.; Kageyama, H.; Ide, J.; Handa, K.; Kobayashi, Y.; Kurisu, Y.; Kondo, S.; Kanno, R. Application of Lithium Metal Electrodes to All-Solid-State Lithium Secondary Batteries Using Li3PO4-Li2S-SiS2 Glass. J. Electrochem. Soc. 2004, 151, A1309−A1313. 3232

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