Carbon Cloth as an

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Three-Dimensional Carbon Nanotubes Forest/Carbon Cloth as an Efficient Electrode for Li-Polysulfide Batteries Xiongwei Wu, Hao Xie, Qi Deng, Hui-Xian Wang, Hang Sheng, Ya-Xia Yin, Wen-Xin Zhou, Rui-Lian Li, and Yu-Guo Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14687 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 23, 2016

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ACS Applied Materials & Interfaces

Three-Dimensional Carbon Nanotubes Forest/Carbon Cloth as an Efficient Electrode for Li-Polysulfide Batteries Xiong-Wei Wu,a,b,d,† Hao Xie,a,b,† Qi Deng,a,b Hui-Xian Wang,a Hang Sheng,a Ya-Xia Yin,b Wen-Xin Zhou,a Rui-Lian Li,*,a and Yu-Guo Guo*,b,c a

College of Science, National Research Center of Engineering Technology for Utilization of Functional Ingredients from Botanica, College of Agronomy, Hunan Agricultural University, Changsha, 410128, P. R. China

b

CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, P.R. China. c

School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, P.R. China

d

Hunan Province YinFeng New Energy Co. LTD, Changsha, 410000, P. R. China

†

These authors contributed equally.

1

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ABSTRACT

The

development

of

a

three-dimensionally

flexible,

large-surface

area,

high-conductivity electrode is important to improve the low conductivity and utilization of active materials and restrict the shuttle of long-chain polysulfides in Li–polysulfide batteries. Herein, we constructed an integrated three-dimensional carbon nanotube forest/carbon cloth electrode with heteroatom-doping and high electrical conductivity. The as-constructed electrode provides strong trapping on the polysulfide species and fast charge transfer. Therefore, the Li–polysulfide batteries with as-constructed electrodes achieved high specific capacities of ~1200 mA h g-1 and ~800 mA h g-1 at 0.1 C and 1 C, respectively. After 300 cycles at 0.5 C, a specific capacity of 623 mA h g-1 was retained.

KEYWORDS

Three-dimensional electrode, carbon nanotubes forest, heteroatom-doping, carbon cloth, Li-polysulfide batteries

2


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1.

Introduction

The fast development of advanced portable electronics, electric vehicles, and smart grids has generated an urgent requirement for high-energy density energy-storage technologies. However, after undergoing two decades of optimization, conventional Li-ion batteries (LIBs) have approached their energy density limit because of the capacity mismatch between cathodes and anodes.1, 2 The conventional insertion compound cathodes with low theoretical energy densities of ~200 Wh kg-1 (e.g., LiCoO2, LiMn2O4, and LiFePO4), will be unable to meet the ever-increasing demand for energy density. 3-5 The relatively lagged progress on cathodes for LIBs has triggered the exploration of new electrochemical systems, and lithium–sulfur (Li-S) batteries, which have overwhelmingly high theoretical energy densities (2,600 Wh kg-1) and utilize abundant and nontoxic elemental sulfur as the cathode, have been widely considered as one of the great potentials for next-generation electrochemical energy storage systems.6 Despite the natural strengths, the practical applications of Li-S batteries still have to circumvent a multitude of basic obstacles. The intrinsically insulating nature of sulfur and its discharged products (Li2S2/Li2S) leads to low utilization of the active material and poor electrochemical reversibility. Meanwhile, the intermediate high-order polysulfides (Li2Sx, 3 ≤ x ≤ 8) inevitably dissolve into the organic liquid electrolyte during the cycling process and diffuse to the anode side, diffusing back after reacting with the metallic lithium to form low-order polysulfides. This process, which is described as the ‘shuttle effect’, gives rise to the irreversible loss of polysulfides, low coulombic efficiency and corrosion of the anode. In addition, 3



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active materials undergo a large volume change (80%) during charge/discharge, fracturing the conductive framework and undesirably delaminating the active materials from the current collector.7, 8 As a result, the capacity deteriorates rapidly after a few cycles.

Steady and significant strides have been made to overcome the drawbacks of Li-S batteries. One effective and commonly used method is to construct a perfect conductive network by incorporating sulfur into electrically conductive materials, such as porous carbon,9-13 carbon nanofibers,14,

15

carbon nanotubes (CNTs),16-18

graphene,19-24 or some conductive polymers.25, 26 The conductive materials not only can serve as host scaffolds to improve the poor conductivity of S/Li2S but also accommodates sulfur’s volume expansion/shrinkage to a degree. Despite of these efforts, the shuttle effect remains challenging, hindering the improvement of capacity decay. As Peng et al. manifested, a high-performance Li-S cell lies in effectively control over the interaction between the S-host framework and S-containing guests (e.g., S, polysulfide, and Li2S).27 Decorating the surface chemistry of the conductive hosts by introducing heteroatom, such as B,28, 29 N,30-32 and S33 or inorganic anchoring materials as sulfur hosts or coating layers, such as MnO2,34 MoO2,35 TiO2,16, 20, 36 Ni(OH)2,37 Co(OH)2,38 and Nb2O5,39 was effective at suppressing the polysulfides shuttle in a Li-S cell by chemically trapping polysulfides on the cathode hosts and achieved both high capacity and encouraging cycling performance. Although substantial effort and many scientific explorations have been devoted to overcoming

4


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these defects, there is a tortuous way for reaching the commercialization of Li–S batteries.

Moreover, the inactive materials, such as metallic current collectors, conductive agents, and binders, usually occupy a relatively large proportion of a conventional electrode, impeding the full demonstration of Li–S batteries with high energy density. Furthermore, the planar current collectors with a polymer binder are subjected to the challenges of low specific surface area, reduced active sites, inadequate mass and electron-transport ability, and weak connection between the active materials and the conductive current collector after long cycling.40 Alternatively, a three-dimensional (3D) electrode integrating the current collector with active materials without a polymer binder is a promising choice and has been widely adopted as an electrode material in Li-S batteries.41-46 However, solid sulfur is difficult to uniformly anchor to the 3D electrode, which leads to low sulfur utilization and unsatisfactory capacity. Furthermore, as the sulfur content increases, the capacity of sulfur becomes poor and swiftly decays after several cycles.41, 42

Thus, compared with solid sulfur as an active material, the polysulfide-containing liquid catholyte approach is considered to be predominant and demonstrates more uniform active materials distribution, improved sulfur utilization, enhanced redox kinetics of sulfur-containing species, and weak electrode structure variation.34 In recent studies, 3D carbon electrodes with large surfaces and flexible frameworks, such as carbon nanofiber paper,47 CNT paper,48 CNT sponge49 and N, S-doped graphene 5



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sponge,50 have been adopted to serve as super polysulfide reservoirs. These materials not only provide good immobilization of polysulfides but also promote fast charge transfer and enable a long cycle life and high energy density. Furthermore, even with high loading of sulfur on the electrode, excellent cycling stability was observed.50 To the best of our knowledge, 3D electrodes integrating well-aligned carbon nanowire (NW) arrays with current collectors exhibit highly desirable electrocatalytic activity because of their interesting intrinsic merits, which include large exposed surface area, improved electrical conductivity, and high mechanical flexibility.40 Indeed, integrated well-aligned 3D carbon NWs or nanotubes grown on current collectors have been reported.40, 51, 52 Encouraged by the aforementioned studies, herein, an integrated 3D CNTs forest on carbon cloth (CNTs/CC) was engineered and prepared for use in lithium/polysulfide batteries. The 3D CNTs/CC, which have high surface area and heteroatom-doping derived from the carbon source itself, could facilitate better adsorption on the polysulfide species, enhance charge transfer, and promote the reversible conversion of Li2S/polysulfide/S, resulting in Li–polysulfide batteries with high capacity and long cycling stability.

2.

Experimental Section

2.1 Synthesis ZnO NWs/CC was typically prepared via a wet chemical process.40 First, the CC was heated for 2 h at 450 °C in air to improve its wettability. Then, to form a seed layer, the wettable CC was immersed in 0.1 M KMnO4 aqueous solution for 1 h. The 6


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seeded CC (3 × 4 cm2) was then dipped into a sealed bottle with an aqueous solution (20 mL) containing zinc nitrate hexahydrate (3 mM), hexamethylenetetramine (3 mM), and ammonia (2 mL). Subsequently, white-colored ZnO NWs were grown on the CC by placing the sealed bottle into an oven at 90 °C for a day. The white-colored ZnO NWs/CC was obtained after washing with water and drying.

The polydopamine (PDA) precursor solution was prepared with dopamine hydrochloride (80 mg) as a carbon source and Tris (60 mg) as the PH buffer agent separately dissolved in 36 mL and 4 mL of ethanol/water (1:1 v/v), respectively. Then, the Tris solution was dropped into the dopamine hydrochloride solution to obtain the PDA precursor. The as-prepared ZnO NWs/CC was immersed in the PDA precursor for 24 h. After washed with water and ethanol, and dried at 60 °C, the brown-colored PDA/ZnO NWs/CC was calcinated at 800 °C for 1 h under a H2/Ar atmosphere (1:9 v/v). Finally, the substrate was etched in 1 M HCl overnight, followed by washing with water and ethanol and drying at 80 °C under vacuum for 12 h to obtain the CNTs/CC.

2.2 Material Characterization

X-ray diffraction (XRD) patterns were collected between 10° and 70° at a scan rate of 5°/min with a Cu Kα radiation source (λ = 1.5418 Å) operating at 40 KV and 200 mA on a model D/max-2500 X-ray diffractometer. Raman spectra were collected with a laser wavelength of 532 nm. X-ray photoelectron spectroscopy (XPS) was conducted on a thermos Scientific ESCA Lab 250Xi with 200-W monochromatic Al 7



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Kα radiation. The morphology and microstructure were measured by field-emission scanning electron microscopy (FE-SEM, JEOL 6710) and field-emission transmission electron microscopy (FE-TEM, Tecnai G2 F20 U-TWIN). Brunauer–Emmett–Teller (BET) isotherms were obtained using an Autosorb-1 analyzer.

2.3 Electrochemical Measurements

CR2032 coin cells were assembled to evaluate the electrochemical performance of the electrode materials using the CC (heat treatment at 450 °C) and CNTs/CC as cathodes, lithium metal as an anode and Celgard 2400 film as a separator. A blank electrolyte

was

obtained

by

mixing

an

appropriate

amount

of

lithium

trifluoromethanesulfonate (LiCF3SO3, 98%, Acros Organics, 1 M) and LiNO3 (99 +%, Acros

Organics,

0.1

M)

into

1,3-dioxolane

(DOL)

(99.5%,

Acros

Organics)/dimethoxyethane (DME) (99+%, Acros Organics) (1:1 by volume). The polysulfide electrolyte was prepared by dissolving a certain proportion of sublimed sulfur powder (98%) and Li2S (99.9%, Acros Organics) into the blank electrolyte at 50 °C for 10 h to obtain 1 M Li2S6 catholyte. The CC and CNTs/CC cathodes were cut into wafers with diameters of 12 mm，which has a weight of 10 mg and 12 mg, respectively. 10 µL of the Li2S6 catholyte (1 M) was dropped onto the electrodes with a sulfur loading of 1.7 mg cm-2. Then, the electrodes were covered with Celgard 2400 film, followed by 40 µL of the blank electrolyte. Finally, the lithium anode was placed on the top of the separator. All these steps were performed in a glovebox with H2O/O2 < 0.1 ppm. Cyclic voltammetry (CV) and galvanostatic charge/discharge experiments 8


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were performed at 1.8-2.8 V versus Li+/Li on the Princeton Applied Research instrument and Land battery tester, respectively. Electrochemical impedance spectra (EIS) were measured from 105 Hz to 10 mHz using the Princeton Applied Research instrument. In our work, the specific capacity is calculated based on S.

3.

Results and Discussion

Scheme 1 demonstrates the synthesis process of the CNTs/CC by a facile template-assisted method. Initially, a ZnO sacrificial template was successfully grown on the CC. Figure 1a and 1b show the morphologies and microstructures of the ZnO arrays. ZnO arrays were uniformly and densely grown on the carbon fibers, which had diameters of approximately 100~300 nm (as shown in Figure S1a). Then, PDA coating layers were formed on the ZnO arrays through dopamine self-polymerization. Subsequently, the PDA coating layers became carbon shells after calcination in an Ar/H2 atmosphere followed by acid etching with 1 M HCl for 6 h to remove the sacrificial template. Finally, the 3D CNTs/CC substrate was attained, on which CNTs were in-situ uniformly grown on carbon fiber like ZnO arrays, as noted in Figure 1c and 1d. The well-aligned hollow CNTs had a wall thickness of ~17 nm and lengths of approximately 1~2 µm (Figures 1e and S1b). Figure 1f also shows the hollow structure of the CNTs, as noted in Figure 1e.

The XRD pattern of the samples reveals a characteristic signal of amorphous carbon with two broad and low-intensity peaks at 2θ ≈ 23° (002) and 2θ ≈ 43° (100) in Figure 2a. The characteristic peaks of the CNTs/CC become sharper and higher 9



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than that of the CC, indicating the partial graphitization of the CNTs. The Raman spectra of the CNTs/CC (Figure 2b) show a low ID/IG ratio of 0.65 at 800 °C. In general, in Raman spectra for carbon materials, the D band is a characteristic feature of a disordered graphitic lattice, whereas the G band corresponds to graphitic layers. The integral intensity ratio of the G (1587 cm-1) band to the D (1351 cm-1) band indicates the graphitic degree of a carbon material.50 Therefore, in agreement with the XRD results, the Raman results of the CNTs/CC demonstrate that the CNTs were partly graphitized.

The specific surface area of the carbon host is crucial for battery performance because the high surface area could provide more adsorption sites for S and polysulfide species. The CNTs, which have porous and hollow structures, should possess high specific surface areas. To measure the size of the surface area of the CNTs/CC, a nitrogen-adsorption (BET) test was conducted, as shown in Figure S2. The CNTs/CC yields a high BET surface area of 506 m2 g-1 (no adsorption–desorption isotherm could be measured for CC). To confirm the chemical composition and types of nitrogen doping in the carbon framework, the elemental spectrum was examined by XPS. Three peaks can be identified in Figure 2c, which are ascribed to O1s, N1s, and C1s. The C1s XPS spectrum (Figure S3) shows different carbon-containing groups, such as C=O, O-C=O and C-O. The O-C=O group is mainly from the CC after heated in the air. The high-resolution N1s spectrum (Figure 2d) verifies the presence of different types of N species, including pyridinic N at 397.9 eV, pyrrolic N at 400.4 eV. The atomic percentage of N and O in CNTs/CC was approximately 2.4% and 2.7% 10


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(2.3% of O in CC). Doping N and O atoms are believed to improve the surface wettability and activity of the carbon matrix，and meanwhile play a role for promoting facile electron transfer.

To investigate the redox kinetics of the electrochemical reactions, EIS was performed. The Nyquist plots of the CNTs/CC and CC electrodes at the open-circuit voltage (OCV) after a 12-hour quiescence are shown in Figure 3a. Meanwhile, the corresponding equivalent circuit model and fitting resistances are displayed in Figure S4 and Table S1. The Nyquist plot consists of two semicircles located in the high−medium-frequency region, representing the surface film impedance (Rf) and the charge-transfer resistance (Rct).53 The curves and table show that the CC and CNTs/CC electrodes both produce large Rct values (40.49 and 37.35 Ω, respectively) due to no chemical activation process and incomplete electrolyte infiltration.54 However, no semicircle in the high-frequency region reflecting Rf for the CNTs/CC is observed, whereas CC demonstrate a large Rf. This observation could be ascribed to the integrated CNTs forest grown on carbon fiber providing a large surface area and N-doping and, thus, effectively entrapping polysulfide species and avoiding the formation of Li2S2/Li2S on the lithium anode. To further explore the electrochemical performance of the CNTs/CC electrode, CV was conducted within the voltage range of 2.8 V to 1.8 V after discharging to 1.8 V. Figure 3b depicts the CV curves of CC and CNTs/CC at a scan rate of 0.1 mV/s, both of which show two pairs of redox peaks for the conversion between S and Li2S2/Li2S. Furthermore, the CNTs/CC electrode exhibits higher reversibility than the CC electrode, which displays higher anodic 11



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peaks at 2.35 and 2.43 V and increasing cathodic peaks at 1.95 and 2.26 V. Moreover, a smaller polarization is observed in the CV shape of the CNTs/CC. The CV profiles (Figure 3c, d) overlap very well from the second cycle to fourth cycle and show almost no peak position or intensity changes, indicating the excellent electrochemical stability of the CC and CNTs/CC.

Figure 4a displays the charge/discharge curves of lithium-polysulfide batteries with the CNTs/CC and CC electrodes at 0.1 C (1C = 1675 mA h g-1). Clearly, the reversible capacity of the lithium/polysulfide battery with the CNTs/CC electrode (approximately 1200 mA h g-1) is almost double that of the lithium/polysulfide battery with the CC electrode. Specifically, compared with the CC electrode, the charge-discharge profile of the CNTs/CC electrode has a higher and longer discharge plateau at ~2.12 V, corresponding to the transition from Li2S4 to Li2S2/Li2S, and a corresponding lower and longer charge plateau at ~2.29 V. The longer charge-discharge plateaus and their lower polarization indicate better reaction kinetics and electrochemical reversibility. The optimal performance demonstrated by the CNTs/CC may be attributable to their better electrical conductivity, high surface area, and heteroatom-doping. The cell with the CNTs/CC electrode manifests outstanding reversible capacities at the various charge-discharge rates. The reversible capacities of the cell with the CNTs/CC are 1188, 1073, 993, 881, and 788 mA h g-1, respectively, at 0.1, 0.3, 0.5, 0.8, and 1 C (Figure 4b). Subsequently, when set back to 0.1 C, the discharge capacity still recovers to 1194 mA h g-1, corroborating the electrochemical stability of the integrated 3D heteroatom-doped CNTs/CC configuration. Although 12


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larger overpotentials are produced as the current density increases, large reversible capacities are still achieved (Figure 4c). The cycling stability of the cell with the CNTs/CC electrode was tested at 0.5 C after the first cycle at 0.1 C (Figure 4d). In the initial 50 cycles, the capacity is approximately 900 mA h g-1. Even up to 300 cycles, a capacity of 623 mA h g-1 is still retained. Notably, the voltage hysteresis between the discharge and charge plateaus (△E) is a vital evaluation index for the electrochemical reversibility of a cell.38 Therefore, Figure 4e further demonstrates the reversibility of the cell with the CNTs/CC electrode. Before 100 cycles, the cell nearly retains its low polarization with a △E of ~0.3 V, except in the initially activated cycles. The excellent electrochemical stability and reversibility can be attributed to fast electron and ion transport, which result from the immobilization of polysulfide species by the CNTs/CC with heteroatom doping and a 3D conductive framework. However, up to 300 cycles, the voltage gap slowly increased to ~0.45 V, and the reversible capacity decreased. This polarization growth and capacity degradation are mainly attributable to two reasons. On the one hand, the aggregation of solid sulfur species occurred after long cycling, as noted in Figure S5, thereby reducing the activity of the solid sulfur species and the conductivity of the electrode. On the other hand, the formation of a passivation film produced by the reaction between the long-chain polysulfides (Li2S4-8) and the lithium anode implies the existence of the shuttle effect as the cycling continued. The formation of the passivation film was confirmed by conducting EIS after the rate performance tests of the cells. The CNTs/CC shows a smaller Rf (2.49 Ω) than the CC (3.48 Ω) (Figure 4f and Table S1), manifesting the improved suppression 13



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of dissolved polysulfide species in the 3D CNTs/CC. Remarkably, a fairly small Rct (1.35 Ω) was achieved in the CNTs/CC compared with that of the CC (15.64 Ω), of CC, which means much faster charge transfer for the surface reactions of the CNTs/CC than that for the CC.

An integrated 3D CNTs/CC was engineered to have a high conductivity, large surface area and strongly coupled interface between heteroatom-doped CNTs and lithium polysulfides, resulting in superior electrochemical performance. The interaction mechanism between the carbon host and the S species is suggested as follows. First, the CNTs/CC is a highly 3D conductive scaffold with an ohmic resistance of less than 5 Ω (Table S1). Such a highly conductive 3D cathode electrode can compensate for the weak conductivity of S and Li2S/Li2S2. Second, the CNTs, which have high surface areas, can provide a large quantity of adsorption sites for lithium polysulfides on both the outside and internal surfaces of the CNTs. This avoids the aggregation of S (charging product) and Li2S/Li2S2 (discharging products) and the leakage of long polysulfides resulting in the loss of active materials and decay of reversible capacity. Obvious bulk S particles are observed on the CC electrode after rate discharge-charge in Figure 5b and Figure S6 compared with the CC before rate discharge-charge (Figure 5a), whereas no aggregation of S is noted on the CNTs/CC electrode in Figure 5c-e after rate discharge-charge. From the energy-dispersive X-ray spectroscopy (EDS) mapping corresponding to Figure 5e, S is also uniformly distributed on the CNTs in Figure 5f. In addition, the incorporation of N and O atoms into the carbon lattice can tune the electron distribution and enhance the ability to trap 14


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S species.27 The Li cation in polysulfides tends to bond with electronegative pyridinic N and O functional groups (as noted in Scheme 2), thereby alleviating the shuttle effect of lithium polysulfides and enhancing electron transfer. All of these factors contribute to the excellent electrochemical performance of the CNTs/CC.

4.

Conclusions

An integrated three-dimensional CNTs forest/CC was constructed as a Li–polysulfide battery electrode using a facile template-assisted method. This hierarchical 3D CNTs/CC, which exhibits high electrical conductivity and heteroatom-doping, provides fast electron transfer, strong adsorption abilities for dissolved polysulfide species and better accommodation of the volume changes of S and undissolved Li2S2/Li2S. As expected, the CNTs/CC demonstrates fast charge transfer, reduced polarization and highly reversible conversion of Li2S/polysulfide/S. The Li–polysulfide battery with a CNTs/CC electrode shown a high capacity of ~900 mA h g-1 at 0.5 C and retained a capacity of 623 mA h g-1 after 300 cycles. Our work demonstrates that the CNTs/CC is an excellent three-dimensional electrode for Li–polysulfide batteries and can be applied to other energy-storage and electro-catalysis systems.

ASSOCIATED CONTENT

Supporting Information

15



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Supporting Information Available: High magnification SEM of ZnO NWs and CNTs, nitrogen adsorption–desorption isotherms image, C 1s XPS spectrum of CC and CNTs/CC, equivalent circuit and fitting data of Nyquist profiles, SEM image of CNTs/CC and CC after charge-discharge.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Y.G.G.); [email protected] (R.L.L)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors greatly acknowledge the financial supports from China National Funds for Distinguished Youth Scientist (Grant No. 51225204), the Hunan provincial Natural Science Foundation of China (Grant No. 2015JJ3074), the Science and Technology project of Changsha (Grant No. KL403147-11), the Research Foundation of Education Bureau of Hunan Province (Grant No. 15A091), and the Beijing National Laboratory for Molecular Sciences (Grant No. BNLMS20150118).

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Scheme 1. Schematic illustration of the construction procedure of the CNTs/CC electrode. (a) CC electrode, (b) ZnO grown CC after a wet chemical process，(c) Carbon shell coating ZnO/CC after PDA coating and calcinated at 800 °C. (d) Carbon nanotubes forest grown CC.

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Figure 1. (a) and (b) different magnification SEM image of ZnO NWs/CC. (c) and (d) different magnification SEM image of the CNTs/CC obtained at 800℃. (e) High magnification SEM image of the CNTs grown on the CC. (f) High magnification TEM of the CNTs.

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Figure 2. . (a) XRD pattern, (b) Raman spectrum, (c) XPS spectra of the surface chemical composition of the CC and CNTs/CC. (d) N 1s XPS spectrum of the CNTs/CC.

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Figure 3. . (a) The Nyquist plots of the CC and CNTs/CC before cycling from 105 Hz to 100 mHz at room temperature. (b) CV profiles of the CC and CNTs/CC at 0.1 mV s-1 after discharging to 1.8 V. CV profiles of (c) the CC and (d) CNTs/CC with the first four cycles at scan rate of 0.1 mV s-1 after discharging to 1.8 V.

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Figure 4. . (a) Galvanostatic charge/discharge curves of the CC and CNTs/CC electrodes at 0.1 C. (b) rate performance of the CC and CNTs/CC electrodes at different rates. (c) Galvanostatic charge/discharge profiles of the CNTs/CC at various rates. (d) Cycling performance of the CNTs/CC electrode at 0.5 C. (e) Galvanostatic charge/discharge profiles of the CNTs/CC for different cycles at 0.5 C. (f) The Nyquist plots of the CC and CNTs/CC after rate performance test from 105 Hz to 100 mHz at room temperature.

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Figure 5. . SEM images of carbon fiber of the CC (a) Before discharge-charge and (b) after rate discharge-charge. (c), (d), (e) Different magnification SEM images of the CNTs/CC after rate discharge-charge. (f) Element mapping for sulfur distribution on the CNTs Corresponding to (e).

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Scheme

2. .

Schematic

illustration

of

interaction

mechanism

heteroatom-doped carbon tubes surface and lithium polysulfides.

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between


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For Table of Contents only

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Carbon Cloth as an

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