Pressure-Induced Capillary Encapsulation Protocol for Ultrahigh

Article pubs.acs.org/JPCC

Pressure-Induced Capillary Encapsulation Protocol for Ultrahigh Loading of Sulfur and Selenium Inside Carbon Nanotubes: Application as High Performance Cathode in Li−S/Se Rechargeable Batteries Dipak Dutta,†,∥ Subhra Gope,†,∥ Devendra S. Negi,§ Ranjan Datta,§ A. K. Sood,‡ and Aninda J. Bhattacharyya*,† †

Solid State and Structural Chemistry Unit, and ‡Department of Physics, Indian Institute of Science, Bangalore-560012, India § International Centre for Materials Science, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore-560064, India S Supporting Information *

ABSTRACT: There has been a paradigm shift in research foci toward elemental electrodes from the conventional intercalation compound-based electrochemical storage. Replacing intercalation transition metal (oxide) compounds with elemental cathodes (e.g., sulfur, oxygen) theoretically raises the storage capacities by more than one order in magnitude. The insulating nature and complexities of the redox reaction associated with electroactive elements necessitates their housing inside an electronic conductor, which has been mainly carbon. Efficiency of the electrochemical storage using such elemental electrodes, besides depending on factors related to the electrolyte, solid-state diffusion, mainly depends on characteristics of the carbon host. We report here a novel, simple, and efficient pressure-induced capillary encapsulation protocol for the confinement of chalcogens, sulfur (S) and selenium (Se), inside carbon nanotubes (CNTs). Confinement led to lowering of the surface tension of molten S/Se, resulting in superior wetting and ultrahigh loading of the CNTs. Higher than 95% of the CNTs is loaded, and very high loading, nearly 85% of S/Se inside the CNTs, is achieved. When assembled at a very high areal loading (∼10 mg cm−2) in the Li−S/Se battery, the S/Se-CNT cathodes exhibited very stable cyclability and high values of specific capacity at widely varying operating current densities (0.1−10 C-rates).

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INTRODUCTION In recent times, lower than three-dimensional extended carbon structures such as the carbon nanotubes (CNTs) have been extensively studied due to their display of unique properties at the nanoscale.1,2 Exhibition of these beneficial properties at small length scales has resulted in their exploration in diverse fields such as molecular electronics, energy storage, automotive parts, sports gears, water filters, thin-film electronics, coatings, actuators, and electromagnetic shields.3,4 The inner cavity of CNTs has proven to be highly beneficial for the encapsulation and confinement of a few technologically relevant materials leading to novel 1-D composites with superior materials properties.5−7 Enhancement in effective composite materials properties has been attributed to the structure and properties of the material(s) confined inside the interior cavity of the CNTs. The materials properties under confinement have often been found to be significantly different as compared to those from the bulk phase.8−11 For example, water encapsulated in the hydrophobic channels of CNTs under axial pressure can exhibit a first-order freezing transition to hexagonal and heptagonal ice nanotubes or a continuous phase transformation into solid-like squares or pentagonal nanotubes.12 Theoretical studies have even predicted well-defined proton transport pathways through the water channels encapsulated inside CNTs.13 It has also © XXXX American Chemical Society

been reported that CNT physical properties undergo modifications as a result of encapsulation of materials inside its hollow space.14 Despite the tremendous technological implications, it is a known fact that CNTs can be encapsulated by only a few materials into its cavity, predominantly by the capillary force action. Hence, the development of new methodologies for encapsulation of diverse materials inside CNTs will be very desirable. Modification of capillary filling method involves diffusion of molecular species such as fullerenes,15 endofullerenes,16 into raw, heat-treated, or vacuum annealed CNTs15−17 and halides via capillary action using both eutectic and noneutectic mixtures.18 These methods again offer very limited functionality including limited loading yields and are applicable only to a few specific materials. In this Article, we demonstrate for the first time a novel and a simple pressure-induced capillary encapsulation of chalcogens, sulfur and selenium, in high yields inside multiwall carbon nanotubes (MWCNTs). This work has both wide ranging as well as task-specific implications. From a general perspective, the work opens a new avenue that would enthuse the Received: November 2, 2016 Revised: December 10, 2016 Published: December 13, 2016 A

DOI: 10.1021/acs.jpcc.6b11017 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Schematic Depiction of the Sulfur and Se Loading Protocol into Open-Ended MWCNTsa

a

M8 = S8 or Se8, and RT stands for room temperature.

of a lack of clear experimental evidence especially from transmission electron microscopy (TEM),34 these nanotubelike structures often resemble more like carbon nanofibers.35 This also implies that the sulfur is not confined inside the CNTs, and the sample represents more a physical mixture of sulfur and CNTs.19,36 Even though a few reports exist describing the loading of Se in CNT and carbon nanocables,37−39 a uniform mass scale loading inside the hollow cavity of CNT is, however, scarce in the literature. Through a synthesis protocol involving cutting and pressure-induced capillary filling of the MWCNTs, the aim is to effectively increase the sulfur and selenium utilization in energy storage and confine the polysulfides/polyselenides within the cathode. The S/Se-CNT cathodes, when assembled in Li−S/Se rechargeable batteries, exhibit very high stability and high current rate capability.

development of new hybrid materials using CNTs for potential applications in diverse fields. The work also assumes great relevance in the context of specific applications related to secondary batteries where elemental electrodes are used instead of the conventional intercalation compounds. In cases where the electrodes are composed of electroactive insulating elements (e.g., sulfur, selenium), encapsulation inside CNTs should aid in overcoming the limitations of poor electronic conductivity and also facilitate efficient execution of the nontrivial electrochemical process. Li−S batteries has been widely demonstrated as a promising high energy density storage device delivering a capacity (theoretical capacity = 1672 mAh g−1, gravimetric energy density = 2600 Wh kg−1)19 that is nearly an order higher as compared to the lithium-ion battery (387 Wh kg−1). The Li−S batteries offer a lot of opportunities for the integration of rechargeable batteries with renewable energy sources such as solar and wind.20,21 Additionally, the other appealing features are their low cost, high abundance, and nontoxicity.22,23 The gravimetric capacity of selenium (theoretical capacity = 678 mAh g−1) is much higher as compared to any intercalation compound but lower as compared to sulfur. However, lower gravimetric capacity of selenium as compared to sulfur is overwhelmingly compensated by a comparable volumetric capacity and much higher electronic conductivity (1 × 10−5 S cm−1) than sulfur (5 × 10−30 S cm−1).24,25 On the other hand, the usage of selenium for electrochemical storage has advantages over both intercalation compounds as well as sulfur. The active materials’ utilization of Se upon galvanostatic cycling is higher than that of sulfur (ca. 45%),26−28 and the shuttle effect on the electrochemical storage process of Sepolysulfides is weaker than that of S-polysulfides. Hence, effective encapsulation of selenium inside CNTs will also be of potential interest toward the development of alternative elemental-based secondary batteries. To our knowledge, there have been only a few reports discussing the conclusive encapsulation of sulfur inside the single-walled carbon nanotubes (SWCNTs) and double-walled carbon nanotubes (DWCNTs).29 Several past efforts to encapsulate sulfur inside wide diameter MWCNTs have failed, leading to the coating on external surface of the CNT.30−32 Many of these approaches even employ expensive templates to grow carbon nanotube-like structures, which ultimately may not resemble the archetypal SWCNTs or MWCNTs.24,33 Because

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EXPERIMENTAL SECTION

Materials and Methods. Materials. MWCNTs (inner and outer diameter ∼10−30 nm) with purity >90% were obtained from M/s Sun Nanotech Co., Ltd. Selenium (Sigma-Aldrich), lithium bis(trifluoromethanesulfone)imide (LiTFSI, ∼99%), lithium hexafluorophosphate (LiPF6), 1,3-dioxolane (DOL, >99%), cyclopentanone (99%), 1,2-dimethoxyethane (DME, >99%), ethylene carbonate (EC, >99%), and dimethylcarbonate (DMC, >99%) were obtained from Sigma-Aldrich and used as received. Sulfur (assay ≥99.5%) obtained from Sigma-Aldrich was sublimed twice before use. Nitric acid (HNO3, 68% AR grade) was obtained from SD Fine-Chem Ltd. and used as received. Mili-Q water (resistivity ∼18.2 MΩ cm) was used for all synthesis purpose. The conductive carbon paint was obtained from Bare Conductive Ltd. Synthesis of End-Opened CNT. The open-ended CNTs were obtained by a procedure reported elsewhere.40 In a typical procedure, 100 mg of the close-ended CNT was suspended in 2.25 g of concentrated nitric acid and refluxed for 4.5 h in an oil bath maintained at 140 °C. This step not only removed the end-caps of the nanotubes but also led to the surface functionalization. The obtained mass was washed with water until the pH of the solution was around neutral and then dried at 60 °C. This was followed by washing with chloroform and redried. The open-ended MWCNT, henceforth, was represented as “atCNT”. Synthesis of Defunctionalized Open-Ended CNT (atCNT). To remove the surface functionalization, the open-ended CNTs (atCNT) were heated at 600 °C for 2 h under nitrogen in a tubular furnace. The removal of the surface functional groups after this heat treatment was B


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The Journal of Physical Chemistry C confirmed by infrared spectroscopy. The surface defunctionalized open-ended MWCNT, henceforth, was represented as o-CNT. Synthesis of S and Se Confined Inside Open-Ended CNT (o-CNT). Sulfur encapsulated CNT samples were prepared by a novel pressureinduced melt-diffusion strategy, which was quite different from the prevailing techniques. The strategy was shown schematically in Scheme 1. In a typical procedure, 30 mg of the o-CNT (obtained by refluxing MWCNT with acid (step 1) followed by heating at 600 °C (step 2)) was first degassed at 80 °C for 30 min using a suction pump (pressure: 1 × 10−3 mbar, Hind High Vacuum (P) Ltd.), followed by immediately mixing with doubly sublimed sulfur in a ratio of CNT:sulfur = 1:20 (w/w) and then evacuating the mixture for 1 h at ambient temperature on a quartz boat (volume = 1.62 cm3) (step 3). The quartz tube containing the boat with the o-CNT-sulfur mixture was then inserted into a tubular furnace at 150 °C (=T1 in Scheme 1) and equilibrated for 45 min (=t1 in Scheme 1). At this point, sulfur melted and the melt viscosity was minimum.41 However, it still could not enter the nanotube cavity as capillary forces at high vacuum would not be strong enough to drive the melt inside the CNTs. The vacuum was then released by passing nitrogen gas, thus leaving the o-CNT dipped into the sulfur melt. After the release of vacuum, the inward impulse along with the capillary force was expected to result in a strong sucking action of the sulfur inside the CNT, thus filling the CNTs cavity with the sulfur melt. The setup was kept undisturbed for another 4 h (=t2) at 150 °C (=T2) (step 5). Once the setup was removed out of the tubular furnace (T2 < 115 °C), sulfur melt will solidify, thus restricting any further entry of sulfur inside the CNT (step 6). The amounts of sulfur loaded in the S-CNT samples were obtained from the thermogravimetric analysis (TGA) (cf., Figure S1). CHNS elemental analysis was also performed to crosscheck the result obtained from TGA. The loss of weight below 300 °C in the TGA profiles of the S-CNT samples was considered as the loss due to sulfur. For comparison, physical mixtures of end-capped MWCNT with sulfur were prepared by grinding them together in appropriate weight ratios in a porcelain mortar and pestle. The encapsulation of selenium inside o-CNT was done using the same pressure-induced melt diffusion technique as that of sulfur. However, the time of encapsulation and temperature of the reaction were varied. Here, the weight of o-CNT was 10 mg, and the weight ratio Se:o-CNT = 10:1. The inner volume of quartz boat was 0.5 cm3. The quartz tube containing the selenium and CNT mixture was inserted into the tubular furnace preheated and stabilized to 300 °C (=T1), and the mixture was equilibrated for 30 min (=t1) at 300 °C. Similar to sulfur, at this point selenium melts (melting temperature = 220 °C) and o-CNTs remain submerged in the selenium melt. However, molten Se cannot enter the nanotube at high vacuum (1 × 10−3 mbar). The purpose of heating Se melt to 300 °C, which is higher than its melting point, was to generate a low viscosity fluid of selenium that can easily enter the CNT. After 30 min, N2 gas was released such that selenium melt entered the o-CNT and continued to flow for 45 min to 1 h (=t2) followed by cooling the system. If the N2 gas was allowed to flow for more than 1 h at the elevated temperature, then this led to complete loss of Se from the system. Hence, the quartz tube was immediately removed from the tubular furnace after 1 h from the onset of N2 gas flow. However, the gas flow was maintained while the sample cooled. This helped in selectively removing the selenium residing outside the nanotubes. This technique was operationally more effective in case of selenium loading as it caused the loading of selenium inside the CNTs and mitigated the extra cleaning step as employed in the case of removal of residual sulfur. Measurement of Contact Angle of Selenium on Graphene. For measuring the contact angle of selenium droplet on graphene, first a single layer of graphene was peeled out by mechanical exfoliation of highly oriented pyrolytic graphite (HOPG) with a scotch tape. Selenium was liquefied by heating at 350 °C, and a drop of the Se melt was quickly added on the HOPG. After solidification, Se drop on HOPG was analyzed by using the sessile drop analysis method under a Surface Chemistry Analysis Instrument microscope (Elico marketing)

for measuring the contact angle. The contact angle was found to be 40° (Figure S2). Structural and Electrochemical Characterization. The powder X-ray diffraction (PXRD) data were collected on a PANalytical diffractometer using Cu Kα radiation (λ = 1.542 Å). Transmission electron microscopy (TEM) images were recorded on a Technai F30 transmission electron microscope (300 kV). The energy-filtered TEM (EFTEM) measurements (for S-CNT) were performed using the FEI Titan3 80-300 microscope (at 300 kV) equipped with an image aberration corrector. Field effect scanning electron microscopy (FESEM) images were recorded on a Zeiss Gemini Ultra 55 instrument operated at 1−5 keV. Fourier transform infrared spectra were recorded on a PerkinElmer FTIR spectrometer using the Spectrum 2000 software at a spectral resolution of 8 cm−1. The Raman spectra were recorded on a Renishaw Micro-Raman 2000 spectrometer operated at Horiba LabRAM HR, diode pumped solidstate laser excitation (wavelength = 532 nm), with a beam spot size of about 2 μm. The Brunauer−Emmett−Teller (BET) surface areas of MWCNT and o-CNT were measured on a Micrometrics ASAP 2020 accelerated surface area and porosimetry system. Before the N2 adsorption/desorption experiments were started, the CNT samples were degassed at 150 °C for 5 h. The pore size distributions (PSD) of the CNT samples were evaluated via the nonlocal density function theory (NLDFT) method using nitrogen adsorption data and assuming a slit pore model (ASAP 2020 V3.01). The thermogravimetric analysis (TGA) data were recorded on a PerkinElmer Pyris 6000 instrument by heating the samples from room temperature to 700 °C at a heating rate of 5 °C/min under N2. X-ray photoelectron spectroscopy (XPS) data were recorded on an AXIS-Ultra instrument from Kratos using monochromatic Al Kα radiation (225 W, 15 mA, 15 kV). The painting of conducting carbon on one side of the aluminum foil current collector was done by using the Elcometer Doctor Blade Film Applicator 4340. The galvanostatic charge/discharge cycling was performed on an Arbin Instruments (Model BT 2000) Corp., U.S., at different C-rates in the voltage range of (1.3−3) V (versus Li+/Li for S) and (1.3−4.0) V (versus Li+/Li for Se). The CHNS elemental analysis for estimating the total amount of sulfur loading in Sx-CNT samples was performed on the Thermo-Finnigan FLASH EA 1112 CHNS analyzer. Electrochemical Cell Assembly. Electrochemical stability and lithium battery performance studies were tested in Swagelok half-cells with lithium foil (Aldrich) as a counter and reference electrode, Whatman glass fiber as separator, and 1 M LiTFSI in DOL:DME (1:1 by volume) as electrolyte for Li−S battery. Because of the higher compatibility of Se with carbonate electrolytes than the ethers, for the Li−Se battery a solution of 1.0 M LiPF6 in EC-DMC (1:1 by volume) was used as the electrolyte. Around 100 μL of the electrolytes was used to assemble the battery. For an electrode with 5 mg cm−2 aerial loading of sulfur/Se, the LiTFSI:S = 5.74 and LiPF6:Se = 3.04. For electrochemical measurements, a slurry of active material (e.g., CNT, S-CNT, Se-CNT, etc.) was prepared with acetylene carbon black (Alfa Aesar) and polyvinylidene fluoride (PVDF, Kynarflex) in a weight ratio of 80:10:10 in cyclopentanone. This was then cast on a carbon-coated aluminum foil and dried in open air for 24 h. The areal loading of S/Se in the Swagelok cell was up to ∼10 mg cm−2. All cell assembly was done at 25 °C in a glovebox (MBraun) under argon (H2O < 0.5 ppm, oxygen < 0.5 ppm).

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RESULTS AND DISCUSSION S and Se are loaded inside MWCNTs using the multistep pressure-induced process as schematically shown in Scheme 1. The amounts of S and Se loaded in S-CNT and Se-CNT samples are evaluated from the thermogravimetric analysis (TGA) (cf., Figure S1). The CNT samples (MWCNT and oCNT) are highly stable and show no weight loss in the temperature range 30−700 °C. Thus, the loss of weight below 300 °C in the case of S-CNT (84%) and below 500 °C in the case of Se-CNT (85% under N2) in the TGA profiles was due C


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The Journal of Physical Chemistry C

Figure 1. (i) Bright-field transmission electron microscope (TEM) images of MWCNT, (ii) high-resolution TEM images showing close-ended MWCNT, (iii) open-ended CNT (o-CNT), and (iv) powder X-ray diffraction patterns of (a) o-CNT, (b) S-CNT, (c) elemental sulfur, (d) SeCNT, and (e) elemental Se. The intense S and Se reflections are also indicated.

by sudden release of pressure by gas flow is expected to provide the required thrust that generates a pumping action and propels the melt of the materials inside the tube. The void created inside the tube due to evacuation causes a massive suction of the melt inside the tube, thereby causing a higher extent of loading. Upon sudden cooling, the melt is left with very little choice other than to attain its native crystallographic state. It is arrested in a state where both the crystalline and the amorphous states may coexist. The method adopted here is unlike the well-established filling technique where the mixture of S/Se and CNT is subjected to high temperatures in sealed capsules. The S/Se-melt enters the host by diffusion, and the degree of infiltration of S/Se in the CNT host is often very uncontrolled and unpredictable.48 The pressure-induced capillary employed in the present study of encapsulation of S/Se inside CNTs is not hypothetical and is guided by the 2γ cos θ following equations,43,44 ΔP = r and ΔP = hρg, where r is the density, γ is the surface tension of the liquid, θ is the solid− liquid contact angle, and g is acceleration due to gravity. Solutions to these equations will give an expression for h, the height up to which a liquid will rise by normal capillary action in a carbon nanotube of radius r. As the density of liquid sulfur (∼1.8 g cm−3 at 155 °C)49 or Se (3.86 g cm−3 at 300 °C)50 is much higher as compared to usual liquids at normal condition of pressure and temperature (water ∼1 g cm−3), the S/Se melt will rise to a minimal height that will not be sufficient to fill the tube. This is the reason why the majority of the reports on CNT-S/Se composite do not show conclusive TEM evidence of sulfur/Se film inside the CNTs. The loading appears as

to the loss of S and Se, respectively. The TGA profile of SeCNT in N2 shows 85% weight loss due to Se; however, when the TGA is performed in air (Figure S1ii(b)), the additional loss between 465 and 610 °C is attributed to the combustion of CNT (∼11%). CHNS elemental analysis also produced similar results. It is noteworthy to mention here that, on the basis of the calculation on around 100 filled CNTs, we have found that more than 95% of the nanotubes are filled. Thus, the pressureinduced melt diffusion procedure used here is expectedly superior leading to ultrahigh loading yields. As described in the Experimental Section, the CNTs are first cut at the tube ends by refluxing in hot nitric acid (atCNT) followed by heating in N2 atmosphere. The heat treatment eliminates the oxygen functionalities, and enhances the hydrophobicity of the CNTs and wetting capability with hydrophobic S and Se. The extent of wetting and capillarity filling of the CNT can be understood42 on the basis of physical parameters, surface tension (γ) and the liquid−CNT contact angle (θ) (related to each other by the Laplace equation43). For effective wetting and filling of the CNT, the surface tension (γ) threshold is 200 mN/m, and the contact angle (θ) should be less than 90°.44,45 For S, γS = 61 mN/m44 and θS (sulfur droplet on graphene) = 4.3°,46 while for Se, γSe = 97 mN/m44 and θSe = 40°.47 This suggests that S and Se melt will be able to effectively wet the CNT surface and is expected to fill the interior cavity of CNT in high yields. However, there might be additional obstacles that may prevent filling of the CNT with S/Se. The pressureinduced capillary technique discussed here effectively overcomes all obstacles to fill the CNTs with requisite amounts of S and Se in the CNT. The purpose of applying vacuum followed D


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Figure 2. (i) Raman Spectra of (a) MWCNT, (b) atCNT, (c) o-CNT, (d) S-CNT and (e) Se-CNT. All spectra in (i) are normalized with reference to G-band before calculating the intensity. (ii) Intensity ratio of D/G, D′/G and 2D/G of MWCNT, atCNT, o-CNT, S-CNT and Se-CNT. (iii) Raman spectra of (a) S-CNT, (b) elemental S, (c) Se-CNT and (d) elemental Se.

surface areas for MWCNT and o-CNT are found to be 107.8 and 160.7 m2/g, respectively. The higher surface area of o-CNT may be due to the better accessibility of the inner pores in the open-ended CNT as compared to the MWCNT. The pore size distribution (PSD) pattern (inset of Figure S5) indicates the presence of both the micro- and the mesopores in the CNT. The pore size distribution in the range 5−14 nm may be due to the inner tube pores; still smaller pores, especially, the micropores at 1.24 nm and mesopore at 3.3 nm, may be due to the pores created by CNTs assembly/entanglement. This is also observed from the TEM measurement (Figure 1i−iii). The o-CNT and atCNT show the same powder X-ray diffraction (XRD) patterns with peaks at 26.5°, 42.4°, 54.7°, and 77.4° corresponding to the (002), (100), (004), and (110) reflections, respectively, of the hexagonal graphite structures as that of MWCNTs (Figures 1iv(a) and S6). This indicates that the acid followed by heat treatment does not interrupt the integrity of the CNT. The XRD pattern of the S and Se loaded samples shows the typical reflections of α-octa sulfur (S8, JCPDS no. 08-0247) (Figure 1iv(b) and (c)) and octaselenium (S8, JCPDS no. 06-0362) (Figure 1iv(d) and (e)). The PXRD patterns of S-CNT and Se-CNT indicate that the S and Se retain the same crystallographic elemental form upon encapsulation inside the CNT. However, it is to be noted that the PXRD pattern is a reflection of only the crystalline portion of the encapsulated S/Se inside the CNT. This will be very clear from the discussions based on the TEM images, vide infra. The Raman spectrum of pristine MWCNT along with the spectra of treated samples is shown in Figure 2. The Raman spectra display a sp2-carbon related G mode at 1586 cm−1, disorder-induced modes at 1346 cm−1 (corresponding to Kpoint phonons, called D mode) and 1616 cm−1 (corresponding to near Γ-point phonons, called D′ mode), and a second-order 2D mode at 2685 cm−1 (Figure 2i).54,55 The band at 2924 cm−1

patches lining the inner wall of the tubes, and the samples can at best be described as a physical mixture of S and CNT.51,52 So, to influence loading, huge external pressure should be applied. As an alternative, we demonstrated the pressureinduced capillary technique for loading S/Se inside the CNTs. The vacuum level employed in the present experiments for infiltrating S/Se melt in CNT is 1 × 10−3 mbar, which is sufficiently high, and when the vacuum is released by passing N2 gas to reach a pressure level of 1 atm or higher it is sufficient enough to pull the sulfur/Se melt inside the cavity of CNTs. The removal of oxygen functionalities from atCNT after heating at 600 °C in N2 is confirmed from the FTIR spectroscopy (Figure S3). Following heating, signature bands corresponding to the vibration of oxygen functionalities (1720 cm−1 for CO, 1084 cm−1 for C−O stretch; 1400 cm−1 for O−H bending deformation in carboxylic acids and phenolic groups53) disappear, leaving only the skeletal vibration bands (1575 and 1225 cm−1). The bright-field TEM images of pristine MWCNT clearly show tubular structures with closed ends prior to the acid treatment (Figure 1i and ii). HRTEM image of o-CNT (Figure 1iii) clearly shows that the ends of the nanotubes have been cut open following the acid treatment. It is also clear from the HRTEM image of o-CNT that the endcaps of the nanotubes are not regenerated after heating in nitrogen. The energy dispersive X-ray spectrum (EDS) of pristine MWCNT (Figure S4) shows only the presence of carbon and with negligible fraction of oxygen functionalities adhering to the surface. These observations suggest that the heat treatment definitely improves the hydrophobic nature not only of the external surface of the o-CNT but its interior cavity as well. The Brunauer−Emmett−Teller (BET) specific surface area and pore size distribution (PSD) of the MWCNT (and oCNT) are evaluated from the nitrogen adsorption−desorption isotherm (Figure S5), which shows type-IV behavior. The BET E


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The Journal of Physical Chemistry C is a combination of D and D′ and intensifies in the functionalized MWCNT as compared to the pristine MWCNT.55 From MWCNT to atCNT, the ID/IG increased from 1.27 to 1.39 due to the increase in oxygen functionalities, while a further increase to 1.52 in o-CNT is an indication of disorder in the CNT framework following removal of the oxygen functionalities (Figure 2ii). However, the increase in the ID/IG ratio from atCNT to o-CNT by 0.13 is negligible as compared to reported values in literature.55 The ratio of ID′/IG (D′ is negligible in MWCNT) shows no significant change in atCNT and o-CNT. Because the ratio of the intensity of 2D (2685 cm−1) to G bands (I2D/IG) is related to the degree of recovery of sp2 CC bonds in graphitic structures,53,54 we speculate that an increase of I2D/IG from 0.30 in atCNT to 0.59 in o-CNT may be considered as an indication of recovery of sp2 graphitic carbon network and enhancement of the quality of the CNT structure, resulting in increase of the electronic conductivity of the o-CNT sample. The conflicting nature of ID/IG and I2D/IG may be explained on the basis that the heat treatment while removing oxygen functionalities from atCNT creates more defect-free sp2 carbon atoms in o-CNT. However, these sp2 domains are smaller than that in the atCNT, thus increasing the overall structural defects in the former as compared to the latter. Typical sulfur bands56 observed at 153, 219, and 474 cm−1 in the Raman spectrum for S-CNT (Figure 2iii(a)) signify that sulfur in S-CNT is in the S8 cyclic form (Figure 2iii(b)). The Se-CNT (Figure 2iii(c)) shows typical Raman signals of Se−Se vibrations for the cyclo-octa-selenium at 142, 234, and 478 cm−1 and appears at the same position as that of elemental Se (Figure 2iii(d)).57 It is interesting to note that the ratios ID/IG and ID′/IG, which show a minimal increase from MWCNT to o-CNT, show no further increase near Γpoint phonons, called D′ mode, and second-order 2D mode at 2685 cm−1 (Figure 2i).54,55 The band at 2924 cm−1 is a combination of D and D′ and intensifies in the functionalized MWCNT as compared to the pristine MWCNT.55 X-ray photoelectron spectroscopy (XPS) (Figure 3) provides further information about the state of S and Se confined inside the CNT. Two components are obtained after a careful deconvolution of S 2p signal of S-CNT (using XPS Peak 41 software). Two peaks that appear at 163.9 and 164.8 eV, respectively, correspond to the state of sulfur (S8), S 2p1/2 and S 2p3/2 peaks, respectively.58−60 The selenium XPS profile of Se-CNT is deconvoluted into two peaks at 55.6 and 56.6 eV representing, respectively, the Se 3d5/2 and Se 3d3/2 with a spin−orbit splitting of 0.8 eV.58,61 These observations suggest that S and Se inside CNT are in the unreacted elemental state (S8 and Se8) and do not undergo any physical change and may be attached to the CNT surface through hydrophobic interactions only. The incorporation of sulfur and selenium in the CNT is all well evidenced by the TEM studies. A very low contrast difference between sulfur and carbon forbid the visualization of sulfur in the bright-field TEM. However, the energy-filtered transmission electron microscopy (EF-TEM) provides distinctly visible evidence for the incorporation of sulfur in the CNT. Figure 4a−d shows zero loss and EF-TEM images of the representative S-CNT sample. The nanotube shown in Figure 4a (zero loss) appears as such in the carbon mapping (Figure 4b). Figure 4d, where the carbon (Figure 4b) and sulfur (Figure 4c) maps have been overlaid, clearly shows the confinement of sulfur inside the nanotubes. The areas shown by yellow arrows in Figure 4a are the cut open sections of the CNT through

Figure 3. X-ray photoelectron spectra (XPS) of (a) S 2p spectrum of S-CNT and (b) Se 3d spectrum of Se-CNT.

which sulfur has entered the nanotube. It is also interesting to note here that the cusp in the inner tube (indicated by the red arrow, Figure 4a−d) is exactly followed in the sulfur and carbon maps, which further confirms the confinement of sulfur in the interior cavity of the nanotubes by forming a tubular network structure. The EDS pattern (Figure S7) shows the presence of only carbon and sulfur, strongly indicating that the sample contains only these elements with no impurity added in the entire loading procedure. It is noteworthy to mention here that the inner tube of sulfur loaded S-CNT, as seen from the zero loss spectra (Figures 4a), is much darker as compared to the empty MWCNT (Figure 1i). Thus, this is a direct observation of the encapsulation of elemental sulfur (cf., Figure 1, XRD patterns) inside the MWCNT. The large contrast difference between Se and carbon provides clear-cut evidence of the encapsulation of Se in the CNT cavity by normal bright-field TEM (Figure 4e and f). We have obtained several images, at least 95% of which show complete filling of CNTs. Additional TEM images showing the loading of CNT with Se are shown in Figure S8. The field emission scanning electron microscopy (FESEM) image recorded for the Se-CNT (Figure S9) shows that no large micrometer-sized Se particle is present outside the CNT assembly, further supporting the confinement. SEM images obtained at several other points of the same sample also show similar morphology. This is also true for the S-CNT sample. We used the HR-TEM images (Figure 4g) and the corresponding masking technique (Figure 5) to show that the loaded inner darker-contrast material is nothing other than Se. The set of crystallographic planes present in the material is also revealed from the masking technique. Even though the HRTEM image shows only two types of lattice fringes (002) (lattice spacing = 0.34 nm) for the CNT and (101) (lattice F


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Figure 4. (a) Bright-field (zero loss) TEM image of a representative S-CNT. (b) Energy-filtered TEM (EF-TEM) image at the same location as in panel (a), showing white regions containing carbon, (c) EF-TEM image showing sulfur map in white, and (d) the images in panels (b) and (c) are overlaid to show the position of carbon and sulfur. (e,f) Bright-field TEM image and (g) high-resolution TEM (HRTEM) images of Se-CNT.

mentioned earlier and also as very clear from the TEM images, the selenium loaded inside the CNT appears in both crystalline (JCPDS no. 06-0362) and amorphous form (Figures 4 and 5). The amorphicity increases toward the CNT meniscus, while toward the interior of the encapsulated film (5−10 nm) it appears to be very crystalline (Figure 6). The contact angle (θB) of bulk Se on graphene is found to be 40°, and the vapor-Se surface tension (γB) = 97 mN/m.44 On the other hand, the contact angle of confined Se (θc) inside the inner cavity of the CNT (cf., Figure 6e) = 24°. Using the Laplace equation, ΔP = 2γ cos θ/r, we can write

spacing = 0.32 nm) for the Se, the masking technique reveals a set of large number of reflections in the Se-CNT. The fast Fourier transform (FFT) pattern generated from the HRTEM image of Se-CNT (Figure 5a and b) shows several spots. Masking the (002) spot from the FFT pattern (Figure 5b and c) and regeneration of inverse FFT (IFFT) pattern for rest of the spots eliminate the CNT lines from the image (Figure 5d). Similar and subsequent masking of the (101) spot eliminates both the CNT and the Se fringes (Figure 5e and f). Masking of all spots from the FFT pattern in Figure 5b and subsequent regeneration of IFFT (Figure 5g) eliminates all lattice fringes from Se-CNT, leaving an amorphous image (Figure 5h). Thus, from the masking procedure, it is clear that all lattice fringes in the HRTEM image of Se-CNT can be assigned to either CNT or Se and match well with the reflections in the XRD pattern of Se-CNT. Masking of spots from FFT patterns and generation of IFFT pattern from several other HRTEM images of Se-CNT produce similar results. The absence of any extra spots other than that of Se-CNT reflections also validates the purity of the samples. While analyzing the surface of the encapsulated S and Se samples (Figure 6), it is interesting to find that both the S and the Se filaments inside the CNT attain a concave meniscus, clearly indicating that the melt of these encapsulated substances wets the surface of CNT. The images also show some unfilled region (nitrogen/air bubbles) along the length of the S/Se filament in the CNT. This is because of condensation and contraction, which takes place upon cooling of the S/Se melt. These free spaces are however very beneficial for these chalcogens filled CNTs for their applications in lithium ion batteries, surfactant chemistry related to foaming,62 as well as nucleating cloud droplets.63 The volume expansion during galvanostatic cycling of S (80%)64 and also for Se can be accommodated by these free spaces. In case of higher loadings, expansion of S/Se may exert an outward force on the host CNT walls, resulting in bulging of the nanotubes outward and loading to detrimental effects on the battery performance. As

γC γB

∝

rC rB

( ), where γ cos θB cos θC

C

and γB are the vapor (air)-selenium

surface tension of the confined and bulk selenium, and rC and rB are the corresponding radii, respectively. Because θC < θB and rC ≪ rB, hence γC ≪ γB. It is known in the literature that the vapor−liquid surface tension of the confined liquid is many folds lower than the bulk surface tension because of the reduction in surface free energy due to confinement.65−67 In SCNT/Se-CNT, the reduction of surface tension of S/Se in their confined state is beneficial for their applications in Li+/Na+ ion batteries because the confined chalcogens may interact more strongly with the incoming Li+/Na+ ions than the CNT host as compared to the physical mixtures of S/Se with CNT where additional Li+/Na+ ions may be store in the CNT. It is noteworthy to mention here that, on the basis of the calculation on around 100 filled CNTs, we have found that more than 95% of the nanotubes are filled. Thus, the pressureinduced melt diffusion procedure used here is expectedly superior and highly effective in filling the CNTs with technologically important materials with high yield. Thus, confined materials are expected to exhibit properties very different from those in the unconfined state. We now probe electrochemical functions of S-CNT and SeCNT for prospective use as cathodes in Li−S and Li−Se batteries, respectively. The electrodes have a very high areal G


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Figure 5. High-resolution transmission electron microscopy (HRTEM) images of Se-CNT and (b) corresponding to fast Fourier transform (FFT) pattern. Parts c, e, g represent subsequent masking of diffraction spots from (b) and (d, f, h) regeneration of the lattice fringes of Se-CNT as inverse FFT (IFFT) patterns for the unmasked spots, respectively.

loading of 5−10 mg cm−2. The electrochemical characterizations are performed in a battery comprised of pure lithium as the anode and S or Se loaded inside o-CNT as the cathode. The first discharge (charge) curves for S-CNT and Se-CNT samples are given in Figure 7i. A reversible capacity of 1443 mAh g−1 is achieved for S-CNT in the first discharge cycle at a rate of 0.1 C (the capacity and C-rates are based on the active mass of sulfur and 1C = 1672 mAh g−1). In the discharge process for S-CNT, two plateaus are observed. The first, contributing nearly ∼162 mAh g−1 to the overall capacity of

1443 mAh g−1 of S-CNT (=1213 mAh g−1 with respect to total electrode mass (Figure 7ii), appears at around 2.4 V and corresponds to the conversion of elemental sulfur to Li2Sx (x = 4−8).68,69 The second plateau at around 2.1 V is due to the conversion of polysulfides to Li2S via Li2S2 (Figure 7i). Cyclability (Figure 7ii) and rate capability are shown, respectively, in Figure 7ii−iv. A capacity of around 822 mAh g−1 (=691 mAh g−1 with respect to total electrode mass, Figure 7ii) is obtained at the end of the 100th cycle at 0.1C. Even though there is a fading of capacity up to 10 cycles (capacity H


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Figure 6. (a) Bright-field (zero loss) TEM image of a representative nanotube of S-CNT composite. (b) Energy-filtered TEM (EF-TEM) image at the same location as in panel (a), showing in white regions containing carbon, and (c) EF-TEM image showing sulfur loaded nanotube in white. (d) Bright-field TEM and HRTEM images of Se-CNT showing concave meniscus of Se filament in the interior core of CNT.

Figure 7. (i) Voltage versus capacity, (ii) specific capacity versus cycle number with respect to S (and CNT-S) mass (current rate = 0.1C), (iii) specific capacity versus cycle number with respect to Se (and CNT-Se) mass (current rate = 0.1C), and (iv) the current rate capability of S-CNT (red symbols) and Se-CNT (blue symbols). Filled and open symbols designate the discharge (DC) and charge (C), respectively. M signifies mass of active electrode materials S and Se.

discharge plateau at 3.5 V contributing ∼130 mAh g−1 to the total discharge capacity (=630 mAh g−1) is due to the conversion of Se to polyselenides (Li2Sex) (Figure 7i). The second plateau (contributing ∼79% to the total capacity) at around 2.2 V is due to the conversion of polyselenides to Li2Se. In the charging profile, however, a single prominent plateau at

fading from second to 10th cycle = 25%), it stabilized rapidly showing a capacity of 822 mAh g−1 at the end of 100 cycles. SCNT shows retention per cycle capacity of 99.41% from the 10th to 50th cycle and 99.8% from the 50th to 100th cycle. The specific capacity versus voltage plot of Se-CNT also produced characteristics similar to those of S-CNT. The first I



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3.7 V indicates a single step conversion of Se2− to Se0. The first discharge specific capacity of Se-CNT is around 630 mAh g−1 (=534 mAh g−1 with respect to total mass of electrode), which is ∼92% of the theoretical capacity of Se (=678 mAh g−1) (Figure 7iii). Even though the specific capacity decreases in initial cycles exhibiting a value of 460 mAh g−1 (retention of 73% of initial capacity) and 409 mAh g−1 (retention of 65% of initial capacity), at the end of the fifth and 10th cycles, respectively, it stabilizes at the later cycles. At the end of the 100th cycle, the capacity stabilizes to 353 mAh g−1 exhibiting extremely stable cycling of Se-CNT over 100 cycles. The relatively weaker performance of Se-CNT and S-CNT in terms of capacity as compared to the literature report of Abouimrane et al.26 and Zhao et al.,70 respectively, may be explained on the basis that the active materials in the deep interior of the entangled CNTs are not accessible to the electrolyte and hence the Li+-ions. However, the cyclability is not compromised, and the cell shows high stability up to 100 cycles. At a high current rate of 1C, 2C, and 10C, the S-CNT exhibits a capacity of 664, 509, and 210 mAh g−1 (Figure 7iv). After the cell is charged to rather high rate of 10C, capacities of 566 and 398 mAh g−1 are recovered at 1C and 2C, respectively. The capacity calculated on the basis of the total mass of the electrode does not differ appreciably from that calculated on the basis of sulfur mass (Figure 7ii). The contribution of CNT to the total capacity of S-CNT or Se-CNT is almost negligible (∼10 mAh g−1, Figure S10), and hence the capacity exhibited by the hybrid S-CNT/Se-CNT material is only due to the encapsulated material. The Se-CNT cathode exhibited stable capacities of 333 and 231 mAh g−1 at current rates of C/5 and C/2, respectively (Figure 7iv). At still higher current rates of 1C and 2C, stable capacities of 153 and ∼100 mAh g−1 are obtained with a successful recovery of specific capacity to 198 and 367 mAh g−1 for current rates of C/2 and C/10. In comparison to S/Se-CNT, the cyclability of physical mixtures (PM) of S/Se with CNT (Figure S11) is much poorer. This again suggests that the encapsulation protocol of S/Se inside MWCNTs demonstrated here is highly effective in producing high performance S/Se-CNT cathodes leading to very stable cyclability at widely varying current densities.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b11017. Thermogravimetric analysis of S-CNT/Se-CNT; selenium on graphene contact angle; infrared spectra of MWCNT, o-CNT, S-CNT, and Se-CNT; EDS profile of CNT; N2 adsorption isotherm and pore-size distribution of MWCNT and o-CNT; PXRD patterns of MWCNT, atCNT, and o-CNT; EDS profiles of S-CNT and SeCNT; TEM and SEM images of Se-CNT; and galvanostatic cycle performance of MWCNT, o-CNT, S-PM, and Se-PM (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-80-22932616. E-mail: [email protected]. ORCID

Dipak Dutta: 0000-0001-5400-8313 Aninda J. Bhattacharyya: 0000-0002-0736-0004 Author Contributions ∥

D.D. and S.G. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Centre for Nanoscience and Engineering (CENSE), Solid State and Structural Chemistry Unit (Mr. I. S. Jarali), Department of Organic Chemistry, and The Society for Innovation and Development (SID), Indian Institute of Science (IISc), for providing various instrumentation facilities. We thank the Nanomission under the Department of Science and Technology, India, India-Taiwan Programme of Cooperation in Science and Technology, for financial support. D.D. and S.G. thank the DST Nano Mission for Research Associateship and University Grant Commission (UGC) Senior Research Fellowship (SRF) for financial support.

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REFERENCES

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CONCLUSION

In summary, we have reported here a unique and efficient pressure-induced capillary filling method for confinement of sulfur and selenium in the interior core of the MWCNTs. This method results in ultrahigh loading yields of the chalcogens inside the MWCNTs. The ensuing composites S-CNT and SeCNT have been convincingly demonstrated as prospective cathodes in Li−S and Li−Se rechargeable batteries, respectively. The high efficiency of the Li−S/Se electrochemical reaction observed here is directly attributed to the efficacy of the encapsulation protocol of S/Se inside the CNTs. The polyselenides/polysulfides are completely confined within the precincts of the CNT cavity, leading to an exceptionally stable battery performance at widely varying current densities. It is foreseen that the synthesis protocol discussed here has tremendous potential for further explorations in terms of both materials and diversity of applications related to energy, sensing, catalysis, and drug delivery. J


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