Direct Observation of Electrochemical Lithium-Sulfur Reaction inside

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Direct Observation of Electrochemical LithiumSulfur Reaction inside Carbon Nanotubes Koki Urita, Toshihiko Fujimori, Hiroo Notohara, and Isamu Moriguchi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00258 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018

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ACS Applied Energy Materials

Direct Observation of Electrochemical LithiumSulfur Reaction inside Carbon Nanotubes Koki Urita,†* Toshihiko Fujimori,‡* Hiroo Notohara,†and Isamu Moriguchi† †

Graduate School of Engineering, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki-shi,

Nagasaki 852-8521, Japan. ‡

Center for Energy and Environmental Science, Shinshu University, 4-17-1 Wakasato, Nagano-

shi, Nagano 380-8553, Japan.

ABSTRACT: An ongoing challenge for next-generation energy storage systems is to maximize the battery performance of lithium-sulfur (Li-S) systems, which exhibit high theoretical capacity and high energy density. Despite the outstanding effects observed by nano-confinement of sulfur within conductive porous media, few studies have elucidated the ideal nanospace for the Li-S reaction because nanoscale characterization of lithiated sulfur molecules is difficult. We present direct evidence of electrochemically-lithiated sulfur molecules confined inside carbon nanotubes (CNTs) using Cs-corrected high-resolution scanning transmission electron microscopy with electron energy loss spectroscopy. For a certain diameter of CNTs, short sulfur chains were stabilized inside CNTs via the charge transfer interaction, exhibiting a unique electrochemical activity and stable cycle performance compared to those of long sulfur chains. Our findings

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reveal that the optimal CNTs have the one-dimensional channels for smooth progress of the lithiation reaction.

KEYWORDS: lithium-sulfur battery, Carbon Nanotube, (S)TEM, EELS

INTRODUCTION The emerging importance of lithium-sulfur (Li-S) batteries, compared to conventional lithiumion batteries, has substantial potential for next-generation rechargeable high-energy storage devices.1-5 Among the various sulfuric materials (e.g. organosulfur compounds6-9 and metal sulfides10-14), elemental sulfur exhibits considerable advantages because of a high theoretical capacity (1675 mAh g-1) and a high energy density (2600 Wh kg-1). However, there are three major limitations to its practical application: (1) sulfur allotropes are insulators; therefore, conductive scaffolds are required for use in an active material, and the cycle performance is low due to (2) the so-called ‘shuttle effect’, which originates from dissolution of polysulfide intermediates (Li2Sx, 4 ≤ x ≤ 8), and (3) the large volume change of sulfur in discharge/charge processes.15-16 To overcome these inherent issues in Li-S systems, a key strategy is to immobilise electrochemically active sulfur species within conductive nano-porous matrices. Such nanoconfinement suppresses mechanical degradation in other active materials (e.g., SnO2 and Si), resulting in improvements to both capacity and cyclability.17-20 A recent systematic study by Oro et al. suggested that a key factor for improving battery performance in SnO2–mesoporous carbon systems was to design and fabricate nanocomposite electrodes with an appropriate pore filling fraction.20 The steric nano-confinement effect should equally improve the electrochemical performance of Li-S cells (e.g., conducting polymers and porous carbon materials).21-29 Of


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particular interest is the monatomic one-dimensional (1D) chain structure of sulfur encapsulated inside carbon nanotubes (S@CNTs) with diameters on the sub-nanometer scale.30-31 The 1D sulfur chains may be more electrochemically active than cyclic molecules. Yang et al. proposed an intriguing hypothesis whereby desolvated Li ions may diffuse into 1D channels of CNTs filled with 1D sulfur chains due to solid-state electrochemical reactions with a negligible shuttle effect.31 And also, Milroy et al. showed that S@CNT hybrids were excellent candidates for manufacturing high capacity, thin, and flexible micro-batteries owing to the robustness and flexibility of host CNTs. These offer promising applications for next-generation smart-textiles.32 Despite recent progress in the Li-S system using CNTs, clear evidence for the permeation of Li ions into the 1D channels of CNTs remains elusive because sulfur molecules in individual CNTs with known diameter have not directly observed. It is important to determine the size of nanospace that is most favourable for the lithiation reaction in Li-S systems using CNTs with different diameter. To further verify the lithiation reactions occurring within the extremely restricted nanospace of S@CNTs, direct observation of individual lithiated S@CNTs using Cscorrected high-resolution scanning transmission electron microscopy (HR-STEM) equipped with electron energy loss spectroscopy (EELS), which has substantial advantages for characterising light elements in confined nanospace,33 is indispensable. Here, we present first direct observations of lithiated sulfur encapsulated in CNTs using a HRSTEM/EELS technique. We used two types of host CNTs with different effective diameters (d): sub-nanometer scale (d < 1 nm) and d ~2 nm, to understand the nano-confinement effect on the lithiation process. Discharge/charge measurements exhibit a distinct cathodic peak at 1.5 V only in the S@CNT with d ~2 nm, which is currently proposed as a characteristic feature of lithiation in micropores.26, 34-35 The HR-STEM/EELS analysis in this study reveals direct evidence for the


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formation of lithiated sulfur inside CNTs in the discharge state. Furthermore, Li ions smoothly diffuse into sulfur-filled CNT nanospaces with diameters over 1.4 nm, providing an ideal steric factor for improving Li-S systems in nanospaces. Our findings can provide a basis for designing nanoporous electrodes for high-performance energy storage systems. RESULTS AND DISCUSSION Structural characterization of nanospace-confined sulfur. One CNT type used in this study consists mainly of double-wall CNTs (DWCNTs) produced by the peapod method.36 We synthesised C60 encapsulated single-wall CNTs (C60@SWCNTs) and subsequently annealed C60@SWCNTs to produce the DWCNT sample. X-ray diffraction (XRD) and Raman analyses reveal the successful filling of C60 and the formation of DWCNTs (Supporting Figures S1 and S3). The other CNT is produced by an enhanced direct injection pyrolytic synthesis (e-DIPS) method and consists of a mixture of SWCNTs, DWCNTs, and few-wall CNTs. The average tube diameter (Figures 1a and b) is defined as the diameter of the innermost tubes because we focus on actual reaction nanospaces for lithiation. The DWCNTs produced by the peapod method show a narrow distribution with an average diameter of 0.5 nm (Figure 1a). The CNTs produced by the e-DIPS method show a wide distribution with an average diameter of 1.8 nm (Figure 1b). The two CNT types are denoted as n-CNT and w-CNT, respectively. We perform HR-TEM observations for an individual S@CNT to verify the relationship between tube diameter and the resulting nano-structures of sulfur chains. Our observations reveal that sulfur chains exhibit three types of distinct nano-structure, which depend on the diameter of the host CNTs: a long single-chain, long double-chains, and a short chain such as S3 molecules. A typical HR-TEM image of a small tube diameter ranging from d = 0.5–0.8 nm shows a long single-chain of sulfur (Figure 1c). The TEM image clearly corresponds to the simulated image constructed from the structural model (Figure 1f). The monatomic chain structure of sulfur is the


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Figure 1. Tube diameter distribution of (a) S@n-CNT and (b) S@w-CNT. The diameter ranges where sulfur exists as a long single-chain (i, blue), long double-chains (ii, black), and short fragment-chains (iii, red) are also indicated in (a) and (b). Typical TEM images of (c) a long single sulfur chain, (d) a long double sulfur chains, and (e) short sulfur chains inside CNTs. Structural models (C: grey, S: green) and simulated images of (f) a long single sulfur chain, (g) long double sulfur chains, and (h) short sulfur chains inside CNTs. Scale bar is 2 nm. dominant component of S@n-CNTs. The presence of 1D sulfur chains is also confirmed by XRD analysis, revealing the long-range order of 1D sulfur chains encapsulated in n-CNTs (Supporting Figure S1).30 The XRD profile of S@n-CNTs exhibits a characteristic Bragg peak at 2θ = 15.14º, which corresponds to 0.304 nm in a 1D lattice constant of sulfur chains, indicating a uniform structural distribution of 1D sulfur chains inside n-CNTs. For a tube diameter ranging from d = 0.8–1.4 nm, long double-chains of sulfur become prominent. Figure 1d shows a typical TEM


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image of double sulfur chains encapsulated in a DWCNT, which is observed in w-CNTs. The simulated image also exhibits double dark contrasts in DWCNTs, which indicate double sulfur chains (Figure 1g). These single- and double-chains are also clearly identified as sulfur from STEM-EELS mapping images (Supporting Figures S4 and S5). To further confirm the observed correlation between the diameter and arrangement of sulfur chains, we perform HR-TEM observations using SWCNTs with an average diameter of 1.2 nm (Supporting Figure S6a). Indeed, sulfur can only form double 1D chains in the SWCNT (Supporting Figure S6b) due to the dominant tube diameter of SWCNTs from 1.1–1.4 nm. In contrast to the formation of long sulfur chains, we observe short fragments of sulfur chains inside CNTs with d = 1.4–2.4 nm, which appear as dark contrasts within a CNT (Figure 1e). We note that these sulfur species easily migrate inside CNTs during electron irradiation; therefore, they are identified as disordered states. Our structural simulation implies that the observed contrast can be qualitatively reproduced using a short S3 chain model (Figure 1h and Supporting Figure S7). Moreover, the presence of S3 chains is confirmed in S@w-CNTs as S3– radical anions using Raman spectroscopy (Supporting Figure S8). The S@w-CNT exhibits a characteristic single Raman peak at 539 cm-1. We assign this Raman peak to the stretching vibration derived from the S3– radical anions (546 cm-1)37 as opposed to that from the neutral S3 molecule (581 cm1 38

) . Due to vapor phase filling, it is believed that small sulfur species (e.g. S2 and S3 radicals) are

infused and subsequently polymerised to form a monatomic chain inside highly restricted 1D nanospaces of CNTs.39 When sulfur is imposed in a weak steric environment, such as 1D nanospaces inside w-CNTs (d = 1.4–2.4 nm), we can expect the stabilisation of small sulfur radicals via charge transfer between host CNTs and the encapsulated sulfur. Free S3 species are


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unstable under ambient conditions; thus, these structural identifications only occur in porous media (e.g., S3– radical inside zeolites40 and neutral S3 molecules inside porous coordination networks41). Thermogravimetric (TG) analysis also reveals that short sulfur chains occur only in S@w-CNTs (Supporting Figure S9). The TG curve of S@n-CNT shows only weight loss (~4 wt%) derived from long chains in CNTs, whose temperature is higher than that of the sulfur powder. On the contrary, weight loss (~25 wt%) at higher temperatures than those of long chains in confined tube spaces is observed in S@w-CNTs; this suggests that radical short chains of sulfur interact strongly with CNTs. Such small sulfur molecules including S3 species are believed to exhibit high-performance in Li-S systems.26 Thus, the sulfur-encapsulated CNT materials produced in this study are revealed to be an ideal system for elucidating the electrochemistry of S3 in conjunction with direct observations of lithiation reactions in nanospaces. Electrochemical reactivity of CNT-confined sulfur chains. Galvanostatic discharge/charge cycle profiles of S@n-CNT and S@w-CNT, and corresponding dQ/dV curves, where Q and V indicate the capacity and reaction potential, respectively, show lithiation reactions of each sulfur sample in CNTs (Figure 2). They are examined in a voltage range of 1.0–2.7 V (vs. Li/Li+) with a current density of 100 mA g-1. Note that the capacities are normalized by the weight of composites. The S@n-CNT does not exhibit a distinct plateau at 1.50 V, which is also observed as a broad feature at the corresponding voltage in the dQ/dV curve (Figures 2a-c). The board feature at the 1st cycle derives not from the reaction of sulfur with Li ions but rather from a solid electrolyte interface (SEI) formation on the CNT surfaces. For the formation of SEI occurs large capacity degradation for S@n-CNTs after 1st discharge process. Moreover, the other reduction peak at 2.37 V is revealed only in the first cycle (Figure 2c). According to the lithiation process of S8 ring-molecules in Li-S systems,34, 42-43 the reduction peaks at 2–2.5 V correspond to the


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Figure 2. Galvanostatic discharge (blue)/charge (red) curves of (a) S@n-CNT and (d) S@wCNT from 1st to 10th cycles and (b) S@n-CNT and (e) S@w-CNT after 2nd cycle. Corresponding differential dQ/dV curves of (c) S@n-CNT and (f) S@w-CNT. Insets in (c) and (f) show magnified dQ/dV curves of reductive (blue) and oxidative (red) reactions. dissociation of ring-S8 into linear S8 chains, and the subsequent formation of lithium sulfides (Li2Sn: 4 < n < 8). Considering the low capacity retention of S@n-CNTs (Supporting Figure S10) and the steric barrier for S@n-CNTs with d ~0.5 nm (Figure 1a), we interpret the reduction peak at 2.37 V observed in S@n-CNTs as the lithiation reaction that occurs near the tube-end edges. Moreover, S@n-CNTs show weak oxidation (delithiation) peaks at 2.23 and 2.47 V only in the 1st cycle (inset in Figure 2c), indicating that the edge sulfur reacted with Li ions is dissolved into the electrolytes, whereas the remaining long sulfur chains do not react. Because this steric barrier also appears in S@SWCNTs with d = 1.1–1.4 nm (Supporting Figures S6c and d), we anticipate that permeation of Li ions into 1D sulfur chain-infused CNTs is sterically restricted to a tube diameter below 1.4 nm.


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Conversely, the S@w-CNT exhibits a characteristic plateau at 1.50 V, which is also observed as an intense reduction peak at the corresponding voltage in the dQ/dV curve. Other plateaus indicate weak reduction peaks at 2.07 and 2.41 V in the dQ/dV curve (Figures 2d-f). Based on our experiments, the reduction peak at 1.50 V is derived from the lithiation reaction inside CNTs with diameters of 1.4–2.4 nm. As confirmed by TEM and Raman analyses (Figure 1e and Supporting Figure S8), we assign this reduction peak to the direct lithiation reaction of S3 species into Li2S2 and/or Li2S. This may result from non-formation of intermediate polysulfides (Li2Sn: 4 < n < 8),26,

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which is the dominant feature in conventional S8-based Li-S systems. In the

subsequent charge process, three plateaus continuously appear at 1.82, 2.27, and 2.45 V during discharge/charge cycles, indicating the delithiation process from Li2S2/Li2S to Li2Sn (n = 2–8). Since small molecular chains confined in carbon nanospcaces strongly contribute to enhance the electrochemical performance as mentioned by Xu et al,28 S@w-CNTs containing small S3 species exhibit a higher capacity retention than that of S@n-CNTs (Supporting Figure S10). Detection of lithiated sulfur inside 1D nanospaces. To identify lithiated sulfur inside the CNTs, we perform an ex-situ STEM-EELS imaging technique using S@n-CNT and S@w-CNT cathodes after the discharge process at 1 V (vs. Li/Li+). Fig. 3 displays TEM images, annular dark field (ADF) images, and EELS maps obtained for the rectangular areas depicted in the corresponding ADF images of three representative S@CNTs, with a diameter of 0.6 nm (Figures 3a–c), 0.9 nm (Figures 3d–f), and 1.8 nm (Figures 3g–i). A long single sulfur chain (Figure 3a) and long double sulfur chains (Figure 3d) clearly remain in these monatomic chain structures after the discharge process. The dissociation of S-S bonding, which is expected for the discharge lithiation reaction, is hardly visible in the TEM images. The EELS mapping analysis indicates that the presence of Li is obscured, and only sulfur exists inside the CNTs (Figures 3c and f).


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Figure 3. TEM, ADF, and STEM-EELS chemical mapping images of single chain S@CNTs (a, b, c), double chain S@CNTs (d, e, f) and small chain S@CNTs (g, h, i). EELS spectra of (j) Li K-edge and (k) S L2,3-edge of single chain (blue), double chains (black), and small chains (red).


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This observation is consistent with the weak electrochemical response of S@n-CNTs (Figures 2a-c). Conversely, the presence of both lithium and sulfur is clearly identified inside the hollow core of a CNT with d = 1.8 nm (Figure 3i), indicating the formation of lithiated sulfur after the discharge process. To further support the chemical-bonding nature of lithium and sulfur, we compare the observed Li K-edge EELS spectra (Figure 3j) with those recently verified for Li and its compounds using atomic-level EELS spectroscopy.31 The EELS spectrum of S@n-CNTs after the discharge process shows a Li K-edge peak at ~60 eV. According to a previous study, the Li K-edge peak shifts to higher energy as the coordination numbers (N) decrease for LiI molecules (e.g. N = 6, 58.5 eV; N = 2, 60 eV; N = 0, 63 eV)33, and Li metal shows a K-edge peak at ~55 eV44. Therefore, we conclude that the observed Li K-edge peak originates from chemically bonded Li-S molecules rather than the metal state of Li. To the best of our knowledge, this is the first experimental evidence that directly verifies the reaction-space-size dependence of the electrochemical Li-S reaction in nano-confined environments. CONCLUSION In conclusion, ex-situ HR-STEM/EELS analysis revealed direct evidence for lithiated sulfur molecules inside CNTs. Although direct assignment of Li ions in CNTs is difficult, the presence of Li in the CNTs after the discharge process was unambiguously confirmed. The sulfur molecules form three types of structure in confined 1D nanospaces. The nanospace-infused sulfur molecules have a significant effect on controlling the configuration of sulfur molecules. The smaller the tube diameter, the stronger the confinement effect45: single and double long sulfur chains were formed in CNTs with a diameter below 1.4 nm, and short sulfur chains (e.g. S3) were formed in larger CNTs. Based on our systematic experiments using CNTs with different


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diameters, we found that the lithiation reaction at ~1.5 V during the discharge process occurs continuously with short sulfur chains (S3– radicals) in CNTs with effective diameters over 1.4 nm. The lithiation reaction progresses smoothly inside the 1D channels of CNTs, forming short sulfur chains. Considering the importance of the electrochemical Li-S reaction coupled with nano-confinement, we predict that our findings will facilitate the future design of cathodic electrodes for high performance all-solid-state Li-S systems. EXPERIMENTAL SECTION Synthesis of S@CNTs. The DWCNT sample was produced by the C60-fused reaction.36 We used SWCNTs produced by the arc-discharged method (SO-P grade, Meijo Nano Carbon Co., Ltd.) as a template. To remove the end-cap of the SWCNT, the SWCNT sample was first oxidised at 723 K under dry air (300 ml min-1) for 1 h. The open-ended SWCNT sample and fullerene C60 (99.9%, Aldrich) were sealed in a grass tube in vacuo (~10-2 Pa), and then maintained at 873 K for 48 h. The prepared C60 encapsulated SWCNT sample (C60@SWCNT) was washed with toluene several times to remove excess C60 attached to the outside of the SWCNT. The purified C60@SWCNT was annealed at 1773 K under Ar gas flow (1 L min-1) for 12 h. The resulting DWCNTs were used to encapsulate 1D sulfur chains. The end-cap of the DWCNT was removed by the same procedure used for the SWCNT. To fill 1D sulfur chains, the open-ended DWCNT sample and sulfur (99.9999%) were sealed in a glass tube in vacuo (10-2 Pa) and subsequently kept at 873 K for 48 h. The prepared sample was washed with carbon disulfide, which is a good solvent for dissolving bulk sulfur attached to the outside of the DWCNT. Finally, we obtained S@DWCNTs (denoted as S@n-CNTs). S@w-CNTs and S@SWCNTs were synthesised by the same procedure used to produce S@DWCNTs. We used CNTs produced by the e-DIPS method (EC-P grade, Meijo Nano Carbon Co., Ltd.) and the


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above-mentioned open-ended SWCNT sample as host CNTs to obtain S@w-CNTs and S@SWCNTs, respectively. Analytical techniques. The sulfur and lithium molecules introduced in the carbon nanotubes were directly observed using a high-resolution scanning transmission electron microscope (HRSTEM; ARM-200CF, JEOL Ltd.) equipped with electron energy loss spectroscopy (EELS) at an accelerating voltage of 80 kV. The diameter distributions of CNTs are determined from 50 – 60 individual CNT images. To precisely measure diameter of CNTs, we used the calibration curve which plots the relationship between a diameter measured by using simulated images and that calculated by geometrical structure of corresponding CNTs (e.g. The diameter calculated from chirality of a (11, 11) SWCNT is 1.49 nm. On the other hand, the length between darkest contrasts in the simulated image is 1.40 nm.). S@CNT samples before and after the discharge process were dispersed in ethanol or propylene carbonate solution and dropped on Cu grids covered by a carbon-deposited collodion membrane. The discharge/charge properties were measured with an electrochemical analyser (HJ-SD-8, Hokuto Denko) at room temperature in a three-electrode Swagelok-cell equipped with metallic Li foil on Ni mesh as a reference and counter electrodes, and 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1,3dioxolane/1,2-dimethoxymethane (DOL/DME 1:1 by volume) solution was used as the electrolyte solution. The S@CNT electrodes without a binder (mass ca. 0.7 mg) were pressed onto the Ni mesh (100 mesh, φ = 6 mm) as the working electrode. Charging and discharging were performed in constant current (CC) mode. The current density was set to 0.1 C (1 C = 1675 mA g-sulfur-1), and the potential range was 1.0–2.7 V vs. Li/Li+. Furthermore, the discharged samples for TEM observation were prepared under CC mode where the potential was set to 1 V in the same system. XRD profiles were measured using the synchrotron X-ray source at SPring-8


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(λ = 0.08 nm). Raman spectra were measured using 785-nm laser excitation with a singlemonochromator micro-Raman spectrometer in back-scattering configuration (Via Raman Microscope, Renishaw). TG analysis of S@CNTs and sulfur powder was examined under Ar flow 300 mL min-1 up to ~ 800 K (EXSTAR TG/DTA 7300, Hitachi High-Tech Science).

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XRD profiles of SWCNTs, C60@SWCNTs, DWCNTs (n-CNTs) and S@DWCNTs (S@nCNTs) (Figures S1, S3). Raman spectra of SWCNTs, DWCNTs, S@w-CNT and w-CNT (Figures S2, S8). TEM, STEM and EELS mapping images of S@w-CNT and S@n-CNT (Figures S4, S5). Tube diameter distribution, TEM image and discharge/charge curve of S@SWCNT (Figure S6). Simulated images of S3@SWCNT (Figure S7). TG curves of S@nCNT, S@w-CNT and sulfur powder. Capacity retention of S@n-CNT and S@w-CNT (Figure S9). Capacity retention of S@n-CNT and S@w-CNT (Figure S10) [PDF] AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ORCIDs


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Koki Urita: 0000-0002-2647-8702 Toshihiko Fujimori: 0000-0003-0070-9696 Author Contributions K.U. and T.F. conceived and supervised the research. T.F. synthesized the S@CNT samples. K.U. performed (S)TEM observation and analysed the data. H.N. performed and analysed the electrochemical experiments. All authors contributed equally to discussion and writing the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT K.U. and T.F. acknowledge support from JSPS KAKENHI Grant Numbers 16H05967 and 17H03390, respectively. T.F. was in part supported by JST PRESTO Grant Number JPMJPR131A. The synchrotron radiation experiments were performed at the BL02B2 of SPring8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2014A1274 and 2014B1491). 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 (1), 19-29. 2. Chen, L.; Shaw, L. L. Recent advances in lithium–sulfur batteries. J. Power Sources 2014, 267, 770-783. 3. Manthiram, A.; Fu, Y.; Chung, S. H.; Zu, C.; Su, Y. S. Rechargeable lithium-sulfur batteries. Chem. Rev. 2014, 114 (23), 11751-87. 4. Manthiram, A.; Chung, S.-H.; Zu, C. Lithium-sulfur batteries: progress and prospects. Adv. Mater. 2015, 27 (12), 1980-2006.


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Table of Contents Graphics


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Figure 1. Tube diameter distribution of (a) S@n-CNT and (b) S@w-CNT. The diameter ranges where sulfur exists as a long single-chain (i, blue), long double-chains (ii, black), and short fragment-chains (iii, red) are also indicated in (a) and (b). Typical TEM images of (c) a long single sulfur chain, (d) a long double sulfur chains, and (e) short sulfur chains inside CNTs. Structural models (C: grey, S: green) and simulated images of (f) a long single sulfur chain, (g) long double sulfur chains, and (h) short sulfur chains inside CNTs. Scale bar is 2 nm. 257x166mm (143 x 143 DPI)



Figure 2. Galvanostatic discharge (blue)/charge (red) curves of (a) S@n-CNT and (d) S@w-CNT from 1st to 10th cycles and (b) S@n-CNT and (e) S@w-CNT after 2nd cycle. Corresponding differential dQ/dV curves of (c) S@n-CNT and (f) S@w-CNT. Insets in (c) and (f) show magnified dQ/dV curves of reductive (blue) and oxidative (red) reactions. 254x152mm (300 x 300 DPI)


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Figure 3. TEM, ADF, and STEM-EELS chemical mapping images of single chain S@CNTs (a, b, c), double chain S@CNTs (d, e, f) and small chain S@CNTs (g, h, i). EELS spectra of (j) Li K-edge and (k) S L2,3-edge of single chain (blue), double chains (black), and small chains (red). 266x418mm (143 x 143 DPI)


Direct Observation of Electrochemical Lithium-Sulfur Reaction inside

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