Flexible hybrid carbon nanotube sponges embedded with SnS₂ from tubular nanosheaths to nanosheets as free-standing anodes for lithium-ion batteries

Abstract

Flexible carbon nanotube sponges (CNT sponges) are excellent three-dimensional (3D) porous substrates to fabricate free-standing electrodes applied in lithium-ion batteries, but the low energy density needs to be improved urgently. Hybrid, hierarchical structures designed towards high-performance electrodes have been reported as an efficient route. Here, SnS₂ with controllable mass ratios and various morphologies from tubular nanosheaths to nanosheets has been grown in situ on carbon nanotubes in CNT sponges by a facile solvothermal method, taking thiourea as the medium. We propose the formation mechanism for diverse morphologies of SnS₂. Also, we demonstrate that the tubular SnS₂ nanostructure can be restrictively and directionally grown using CNTs as templates, and has much better reversible capacity and cyclability than its nanoparticle or nanosheet structures on CNTs. Meanwhile, CNT@SnS₂ sponges could be used as free-standing and binder-free electrodes with significantly improved areal capacity than bare CNT sponges.

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1. Introduction

High-performance flexible lithium-ion batteries (LIBs) attract much attention to cater for the huge power demand of modern wearable and portable electronic equipments.^1–4 To realize flexible LIBs, the fabrication of deformable electrode materials with good mechanical properties and high electrochemistry performance is the key challenge.^5–8 Carbonaceous substrates, such as carbon nanotube (CNT) film, graphene paper, graphene foam, carbon cloth, cellulose film, etc.^9–15 play an important role in the manufacture of flexible electrodes because of their excellent mechanical behavior, superior conductivity, light weight, outstanding chemical stability and various assembled structures. Meanwhile, the composites or hybrids built on the carbonaceous substrates significantly improve the electrochemical performance.^16–18 However, the limited amount of active materials loaded on was considered as a crucial reason restricting further enhancement of energy density per unit area/volume.^4,6 Three-dimensional (3D) porous flexible substrates with a fast electron transition pathway, a short ion diffusion length are designed to balance the contradictory relationship between the high mass-loading of the active materials and a proper electrical conductivity of the electrodes, and the use of 3D graphene¹⁹ achieves a higher areal/volumetric energy density. CNT sponges,²⁰ an excellent candidate for 3D flexible porous substrate, possessing a low density about 5–10 mg cm⁻³, a porosity of >99%, a high electrical conductivity of the ∼200 S m⁻¹ and fantastic mechanical flexibility and robustness has been reported in our previous work and applied to make highly compressible supercapacitor electrodes.^21–23 The inherent features of CNT sponges give it potential for free-standing and binder-free electrodes eliminating dead weight caused by metal collectors and binders towards high performance flexible batteries.^24,25

That flexible substrates combine with various functional inorganic materials (LiCoO₂, LiMn₂O₄, transition metal oxides, Si, Sn-based materials, etc.) to form hybrid, hierarchical structures is regarded as an efficient technique toward novel-design and high performance flexible electrodes.^26–30 But till now, the research on metal sulfides decorating flexible electrodes has been rarely reported, neglecting mass of achievements in metal sulfides and its composites hybridized with carbon-based materials as high performance electrode materials.³¹ SnS₂ crystalizing in layered CdI₂-type structure facilitating the lithium ion intercalation and deintercalation processes has been deeply studied as an anode material.^32,33 Its theoretical capacity (value: 645 mA h g⁻¹) is higher than that of commercial graphite anodes (value: 372 mA h g⁻¹) and the defects of its bulk material, large volume expansion, low electronic conductivity and a long ion diffusion length leading to poor electrochemical properties have been improved by nanotechnique to explore SnS₂ nanomaterials or nanostructured SnS₂/carbon composites.^34–37 CNTs as a typical carbonaceous material have been composited with many electroactive materials for enhanced performance, including SnS₂ nanoflakes and nanosheets grown on CNTs which delivery improving lithium-ion storage property.^38–42 Additionally, the porous 3D framework of the CNT sponge which supplies fast ion diffusion channels and efficient electron transport, has been identified to further improve the electrode kinetics.^21,24 Compared with previously studied powder-form mixtures, bulk-form, highly porous CNT–SnS₂ sponges with well-defined microstructure are an alternative promising candidate for developing high-performance lithium-ion battery electrodes. Here, SnS₂ with controllable mass ratios and various morphologies from tubular nanosheaths to nanosheets has been grown in situ on carbon nanotubes in CNT sponges by a facile solvothermal method taking thiourea as media. The formation mechanism and electrochemical performance of diverse morphologies of SnS₂ coated on CNTs have been investigated. CNT@SnS₂ sponges with various weight fractions of SnS₂ all have been applied as free-standing and binder-free anodes of lithium-ion batteries to compare their electrochemical performance. As a contrast, the lithium storage capabilities of CNT sponge and bare SnS₂ also have been tested.

2. Experimental section

Materials

Tin chloride pentahydrate (SnCl₄·5H₂O, ≥99.0%) and carbon disulfide (CS₂, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Thiourea (H₂NCSNH₂, ≥99.0%) was purchased from Xilong Chemical Co., Ltd. and ethanol (CH₃CH₂OH, ≥99.7%) was provided by Beijing Chemical Works. All the chemicals were analytical grade without further purification.

Fabrication of CNT@SnS₂ sponge

CNT sponges were synthesized by chemical vapor deposition using ferrocene and 1,2-dichlorobenzene as catalyst and carbon precursor, as described in our earlier report.²⁰ The core–shell CNT@SnS₂ sponge was fabricated by the solvothermal method taking thiourea as media. In a typical procedure, 3.5 g of SnCl₄·5H₂O was dissolved into a mixture of 10 mL deionized water and 10 mL ethanol at room temperature to form the precursor. Subsequently, a 10 mg CNT sponge was soaked into the precursor for 30 min, and then dried at 70 °C for 1 h. Next, the sponge was immersed into thiourea solution melting at 200 °C for 30 min. Then the sponge was transferred into Teflon-lined stainless steel autoclave, sealed and maintained in an oven executing the procedure 180 °C for 10 h and 200 °C for 2 h. After reaction, the sponge was washed with ethanol and deionized water several times to remove the residues, and then rinsed with distilled water for freeze-drying to maintain the porous structure. Finally, the sponge was soaked into CS₂ for 24 h to eliminate sulfur decomposed from thiourea. After air drying, the CNT@SnS₂ sponge was obtained eventually. The schematic illustration of the fabrication process was shown in Fig. 1a.

Fig. 1 (a) Schematic illustration of the fabrication process to the CNT@SnS₂ sponge. (b) Photography of the CNT sponge twisting along a glass bar. (c) Photography of the CNT@SnS₂ sponge under bending. Inset is a photo for a block of the CNT@SnS₂ sponge.

Bare SnS₂ powder was prepared through the same reaction. A mixture of 1.76 g SnCl₄·5H₂O and 0.91 g thiourea was grinded uniformly in an agate mortar for 30 min. Then the mixture was transferred into Teflon-lined stainless steel autoclave, sealed and maintained in an oven executing the same procedure. Ultrasonic cleaning was used to wash the product dispersing in ethanol and deionized water several times. After that, the remain was dried for 12 h at 70 °C.

Characterization

The obtained samples were characterized by X-ray powder diffraction (XRD) using a Bruker D8 Focus X-ray diffractometer with graphite monochromatized Cu K_α radiation (γ = 1.54178 Å). The morphology and structure of the samples were examined by scanning electron microscopy (SEM, Hitachi S-4800), with energy-dispersive X-ray spectrometer (EDX) and transmission electron microscopy (TEM, FEI Tecnai T20). X-ray photoelectron spectroscopy (XPS) analysis was performed on an AXIS-Ultra instrument from Kratos Analytical using monochromatic Al Kα radiation (225 W, 15 mA, 15 kV) and low-energy electron flooding for charge compensation. Raman spectra were collected on a Renishaw inVia Raman Microscope with laser excitation at 532 nm.

Cell assembly

Coin-type (CR 2032) half-cells were assembled in an argon-filled glove box using CNT@SnS₂ sponges as the positive electrodes, Li metal as the negative electrode and polypropylene (PP) film (Celgard 2400) as separator. The electrolyte is 1 M LiPF6 dissolved in a mixed solution of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methylcarbonate (EMC) with a volume ratio of 1 : 1 : 1. For comparison, bare SnS₂ powder was mixed with acetylene black, and a polyvinylidene fluoride (PVDF) binder in a weight ratio of 80 : 10 : 10 on a copper foil to perform as positive electrode.

Electrochemical characterization

A galvanostatic cycling test of the assembled cells was carried out on a Land CT2001A system in the potential range of 0.01–3 V at a discharge/charge current density of 100 mA g⁻¹. For comparison, the cycling performance of bare SnS₂ powder and CNT sponges were also tested under the same conditions. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted on CHI660E electrochemical workstation. CV tests were recorded at a scan rate of 0.3 mV s⁻¹ from 0.01 to 3 V (versus Li/Li⁺). EIS tests were obtained by applying a sine wave with amplitude of 5 mV over the frequency range from 100 kHz to 0.01 Hz.

3. Results and discussion

Fig. 1a illustrates the fabrication process of the CNT@SnS₂ sponge. Since the CNT sponge is hydrophobic but lipophilic, the precursor was prepared using H₂O/ETOH (1 : 1 by volume) mixed solution and thiourea melt can infiltrate in it. Thiourea melt is to such solvothermal method what water is to hydrothermal method. Moreover, thiourea can freeze the intermediate sponge with Sn resource after solidification and supply sulfur resource during reaction. The re-melting thiourea under heating in autoclave is restrained in the sponge, guaranteeing no Sn resource exchange or diffusion with the surrounding. In addition, the concentration and volume of the precursor absorbed in the CNT sponge are under control according to the expected mass ratio. Such unique process realized controllable mass ratio of SnS₂ growing in situ upon CNTs. And controllable mass ratio of the active materials, especially high weight fraction for high energy density electrodes, is a very important feature of CNT sponge electrodes. By contrast, other techniques such as the traditional hydrothermal synthesis, sol–gel method, CVD, etc., seem difficult to achieve it. As reported in our previous work,²⁰ CNT sponges feature fantastic mechanical flexibility and robustness and here it could be twisted along a glass bar (Fig. 1b). CNT@SnS₂ sponges loading proper active material keeps well flexibility under bending (Fig. 1c). A photo for a block of the CNT@SnS₂ sponge with size 1 cm × 1 cm × 0.4 cm as electrode assembled in coin cell is also supplied in Fig. 1c.

Morphologies of CNT and CNT@SnS₂ sponges are shown in Fig. 2. From scanning electron microscopy (SEM) images (Fig. 2a and c), CNT@SnS₂ is much thicker than CNT, indicating that a sheath is coated on the CNTs. SnS₂ is deposited more uniformly on the inner part than the outer surface of the sponge owing to that the concentration of the reagent outside is disturbed when the sponge soaks in thiourea melt. Transmission electron microscopy (TEM) images (Fig. 2b and d) further reveal uniform core–shell structures with a CNT cavity and a SnS₂ shell. The diameters CNT and CNT@SnS₂ are about 30 nm and 70 nm, respectively. Also, the junction of CNTs is enwrapped by SnS₂ tightly. Such riveting-like structure is an effective route to strengthen the framework.⁴³

Fig. 2 SEM images of CNT (a) and CNT@SnS₂ (c) sponges. TEM images of CNT (b) and CNT@SnS₂ (d) sponges.

X-ray diffraction (XRD) was employed for phase analysis and the result pattern was shown in Fig. 3a. The diffraction peaks in the pattern of the CNT@SnS₂ sponge are from both SnS₂ and CNTs, indicating hybrid structure of the sponge. And the peaks at 15°, 28°, 32°, 42° and 50° are indexed as (001), (100), (011), (012) and (110) facets in the 2T-type hexagonal SnS₂ (which fits better to JCPDS card file no. 83-1705), respectively. The pattern of the CNT sponge (Fig. S1†) with weak peaks and large half-peak width implies weak crystallinity and just one peak at 22° marked by * in Fig. 3a is legibly displayed. Raman spectra were also recorded to provide the phase analysis (Fig. S2†), in which the CNT@SnS₂ sponge shows obvious D band and G band of CNT. Both the CNT@SnS₂ sponge and pure SnS₂ have the peak at 311 cm⁻¹, which is attributed to A_1g mode of SnS₂. High resolution TEM image (Fig. 3b) of CNT coated few layers SnS₂ shows distinct lattice fringes with the lattice spacing of about 0.59 nm, which agrees well with the interplanar distances of (001) planes of SnS₂ mentioned above. What is more, we can find the tubular SnS₂ nanosheaths outside the CNT. As other nanomaterials with tubular structures (WS₂, MoS₂, NbS₂, etc.) have been restrictively and directionally grown using CNTs as templates,⁴⁴ we demonstrate tubular SnS₂ also can be obtained. Full X-ray photoelectron spectroscopy (XPS) spectrum (Fig. 3c) with strong Sn 3d, S 2p and C 1s peaks confirms that elements Sn, S, C are contained in the CNT@SnS₂ sponge. A weak N 1s peak may come from the remaining thiourea. High-resolution Sn 3d spectra (Fig. S3a†) shows that Sn 3d_3/2 and Sn 3d_5/2 whose binding energies having a spin energy separation of 8.4 eV center at 495.4 and 487.0 eV, respectively, corresponding to the reported data for SnS₂.⁴⁵ High-resolution S 2p spectra at 161.8 eV is displayed in Fig. S3b† and a weak peak at 289.3 eV aside the main peak at 284.8 eV was emerged in C 1s spectra (Fig. S3c†) indicating C–N interaction from thiourea, in accordance with the N 1s peak in the full spectra. Energy-dispersive X-ray spectrometer (EDX) elemental mapping analysis was used to visualize the core–shell structure of the CNT@SnS₂ sponge (Fig. 3d). The SEM image along with the corresponding C, S and Sn maps clearly reveal that Sn and S are distributed along CNTs. The brighter area means a higher concentration.

Fig. 3 (a) XRD pattern of the CNT@SnS₂ sponge. (b) High resolution TEM image of CNT coated few layers SnS₂. The inside dark-contrast portion is the residual Fe catalyst trapped within CNTs during the CVD process. (c) Full XPS spectrum of the CNT@SnS₂ sponge. (d) SEM image and corresponding EDX elemental maps of C, S and Sn.

Fig. 4 shows SEM and TEM images of CNT@SnS₂ sponges loaded with different mass ratios of SnS₂ (CNT@SnS₂-thinner sponge < 60 wt%, “thin” ≅ 65 wt%, “thick” ≅ 75 wt%, “thicker” ≅ 85 wt%, “ultra-thick” > 90 wt%). Through the comparison of five SEM images with the same magnification in Fig. 4a, the diameters of CNT@SnS₂ cables increase apparently accompanying with the raising of the mass ratio. Higher magnification SEM images (Fig. 4b) show morphology change of the coating SnS₂ shell. In the “thinner” sponge, the SnS₂ shell behaves as a tubular sheath tightly wrapping the CNT. As a result, the shell surface is very smooth. With the mass ratio increasing to about 70 wt%, the tubular sheaths cannot become thicker but SnS₂ nanoparticles form outside. Hence, obvious bulges spring up on the surface. The corresponding TEM image (Fig. 4c) illuminates much clearer. The phenomenon reveals excellent synergistic effect of structure between just few layers of SnS₂ crystals and CNTs. The SnS₂ nanoparticles as the crystal seeds would grow to SnS₂ sheets as if the buds evolve to petals, while the mass ratio further increases. Higher mass ratio results in much more seeds to grow up and CNTs are heavily enwrapped by SnS₂ sheets, seeing the images of the “ultra-thick”. Meanwhile, the alterative process of the morphology uncovers the growth mechanism of SnS₂ depositing on CNTs and Fig. 4d provides its schematic illustration. In brief, the morphology of SnS₂ could be altered from tubular nanosheaths to nanosheets through the change of its mass ratios.

Fig. 4 (a) SEM images of CNT@SnS₂ sponges from thinner to ultra-thick under the same magnification. (b) Higher magnification SEM images of CNT@SnS₂ sponges. (c) TEM images of CNT@SnS₂ sponges. Images belonged to the same column refer to the same sponge. (d) Schematic illustration for the growth process of SnS₂ deposited on CNTs accompanying the increase of mass ratio. CNT@SnS₂-thinner sponge < 60 wt%, “thin” ≅ 65 wt%, “thick” ≅ 75 wt%, “thicker” ≅ 85 wt%, “ultra-thick” > 90 wt%.

The electrochemical performance of free-standing electrodes made from flexible CNT@SnS₂ sponges was evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge cycling. Cyclic voltammograms of CNT@SnS₂ sponge electrode (Fig. 5a) reflecting cathodic and anodic process of both SnS₂ and CNTs (Fig. S4a and b†) also reveals the composite structure. Three cathodic peaks at about 1.5, 0.8 and 0.3 V are detected during the first sweeping cycle. The intercalation of lithium ions into the SnS₂ layers without phase decomposition generates the peak at 1.5 V. The peak at 0.8 V could be primarily attributed to the decomposition of the SnS₂ into metallic Sn and Li₂S, while the peak at 0.3 V is caused by the formation of Li_xSn alloy. The de-alloying of Li_xSn leads to the first anodic peak at 0.6 V. A broad anodic peak at the potential range of 0.9–2 V is mainly contributed by the deintercalation of lithium ion in Li_xC. The galvanostatic charge/discharge cycling were measured in the voltage window 0.01–3 V under the current density of 100 mA g⁻¹. The result curves of the 1^st, 2^nd, 3^rd, 50^th and 100^th cycles from CNT@SnS₂-45.82 wt% sponge are presented in Fig. 5b. According to the curves of the first three cycles, the voltage plateaus of the electrode are consistent with their corresponding CV plots. The performance of the SnS₂ embedded in CNT sponges has been investigated after artificially eliminating the disturbance of the CNT sponge. The initial discharge capacity of 1522 mA h g⁻¹ is much larger than the charge capacity of 713 mA h g⁻¹ due to the formation of solid electrolyte interface (SEI) film and irreversible Li-ion intercalation into the SnS₂ crystal lattice. The remaining capacity is about 474 and 502 mA h g⁻¹ after 50 and 100 cycles, respectively.

Fig. 5 (a) Cyclic voltammograms of the CNT@SnS₂ sponge electrode depicting the first three cycles. (b) The 1^st, 2^nd, 3^rd, 30^th and 50^th charge/discharge curves of SnS₂-45.82 wt% embedded in CNT sponge (c) cycling performance of SnS₂ tubular nanosheaths (45.82 wt%), nanoparticles (72.60 wt%) and nanosheets (88.37 wt%) deposited on CNTs. Also, bare SnS₂ and SnS₂ purchased are taken as comparison. (d) Nyquist plots of CNT@SnS₂ sponge electrodes. (e) The capacities of the sponges with various ratios of SnS₂ as free-standing electrodes after 10^th, 30^th, 50^th and 100^th cycles. (f) Areal capacity of the CNT@SnS₂-thick sponge electrode and the corresponding CNT sponge.

CNT sponges embedded with other mass ratios of SnS₂ were also assembled in coin cells to compare the electrochemical performance of SnS₂ featuring various morphologies. Three SnS₂ ratios of 45.82 wt%, 72.60 wt% and 88.37 wt% were obtained and their morphologies correspond to tubular nanosheaths, nanoparticles and nanosheets, respectively. Also, bare SnS₂ and SnS₂ purchased are taken as comparison. Their cycling performance measured in the voltage window of 0.01–3 V under the current density of 100 mA g⁻¹ after 100 cycles are shown in Fig. 5c (just supply the curves of discharge capacity versus cycle numbers) and Fig. S5† (coulombic efficiencies versus cycle numbers). All initial coulombic efficiencies are far less than 100% attributing to SEI film and irreversible Li-ion intercalation, but at subsequent cycles, the coulombic efficiencies stabilize at higher than 90%. As a result, SnS₂-45.82 wt% shows best reversible capacity and cyclability. The initial discharge capacities of SnS₂-45.82 wt%, SnS₂-72.60 wt%, SnS₂-88.37 wt%, bare SnS₂ and purchased SnS₂ are 1522 mA h g⁻¹, 1430 mA h g⁻¹, 1427 mA h g⁻¹, 1169 mA h g⁻¹ and 1010 mA h g⁻¹, respectively. After 100^th cycles, the remaining capacities are 502 mA h g⁻¹, 285 mA h g⁻¹, 123 mA h g⁻¹, 16 mA h g⁻¹ and 115 mA h g⁻¹, respectively. The result of SnS₂-72.60 wt% agrees well with the reported work in which MWCNTs were coated with SnS₂-76 wt% nanosheets.⁴² Agglomeration of the SnS₂ particles is considered as the main factor resulting in bad performance and has been verified by SEM images (Fig. S6†) of bare and purchased SnS₂ powder. A unique synergy between the CNTs core and the tubular SnS₂ sheaths at the nanoscale possibly contributes to the outstanding performance of SnS₂-45.82 wt%, which also highlights the crucial role of CNTs on enhancing electronic conductivity, maintaining thermodynamic/kinetic stability, and hindering the agglomeration.

To characterize the evolution of the core–shell structure and analyze why the cyclability of the electrodes dramatically reduce, SEM analysis were executed after the cells were taken to the absolutely charged state. The electrodes bringing out from the cells were examined after washing and drying. The coarse shells (SnS₂ sheets) in the original CNT@SnS₂-88.37 wt% sponge have been polished obviously into a smooth layer after charge/discharge cycling, as shown in Fig. S7a.† The smooth layer is constructed from Sn nanoparticles confirmed by the XRD pattern of the electrode (Fig. S8†). The SnS₂ sheets protruding away from the growing substrate with high specific surface energy have strong tendency to be corroded when the electrode was soaked in the electrolyte during charge/discharge process. The SEM image of the CNT@SnS₂-45.82% sponge electrode (Fig. S7b†) after cycling shows the shell is as smooth as its initial state. Such phenomenon is consistent with its cycling stability.

Furthermore, rate performance of the SnS₂-56.78 wt% electrode was investigated through altering the charge/discharge rates programmably from 100 to 400 mA g⁻¹ and then falling back to 100 mA g⁻¹ for 10 cycles. The capacities are varied obviously from average 700 mA h g⁻¹ to 470 mA h g⁻¹, 260 mA h g⁻¹ and back to 460 mA h g⁻¹ along with the change of the charge/discharge rates (Fig. S9a†). Electrochemical impedance spectroscopy (EIS) measurements of the electrodes mentioned above were performed after 100 cycles and the Nyquist plots (Fig. 5d and S9b†) illustrate the comparison of electronic conductivity among the electrodes. The sponge electrode with lower mass ratio of loaded SnS₂ possesses higher electronic conductivity. The charge transfer resistances (R_ct) of the electrodes are 56 Ω, 89 Ω, 119 Ω, 596 Ω and 430 Ω, respectively. As the loading of SnS₂ increases, the morphology of SnS₂ grown on CNTs also changes from tubular nanosheaths to nanoparticles and then flower-like nanosheets (as shown in Fig. 4), resulting in less perfect interface and reduced adhesion strength between the SnS₂ coating and the underlying CNTs. As a result, the electron transfer across the SnS₂-CNT interface will be influenced and the charge-transfer resistance becomes more severe with higher SnS₂ loading. Compared with the SnS₂ electrodes directly pasted on copper foil (Fig. S9b†), the CNT sponge with close contact to SnS₂ improves electrical conductivity greatly.

The electrochemical performance of CNT@SnS₂ sponges as free-standing electrodes also was investigated. The sponges with various mass ratios of loaded SnS₂ from “thinner” to “ultra-thick” were assembled in cells for testing under the current density of 100 mA g⁻¹. Fig. 5e illuminates the capacities of the electrodes after 10^th, 30^th, 50^th and 100^th cycles. The capacity of CNT@SnS₂ sponges (based on total mass) are mostly increased significantly compared with that of bare CNT sponge at the initial cycles, due to the higher capacity of SnS₂. After 100^th cycles, the enhancement is negligible. Much more loaded SnS₂ does not lead to higher capacity considering the serious decay of the capacity from SnS₂ part. CNT@SnS₂-thin and CNT@SnS₂-thinner sponge electrodes show lower capacity, yet with improved cyclability. The “ultra-thick” sponge electrodes perform the worst like bare SnS₂ prepared by the same method mentioned above, ascribing to serious agglomeration and low porosity. Actually, the sponges lose flexibility and feel hard when the mass ratio of SnS₂ is larger than 85%. CNT sponge electrodes scarcely decayed with fantastic cycling stability possess a disappointing capacity of 160 mA h g⁻¹ for application. Detailed cycling behavior of the sponges are shown in Fig. S10.†

Since the CNT sponge features a porous and highly electronic conductive 3D framework, the areal capacity of the sponge electrodes could be improved by increasing the sponge thickness. However, the light weight of the CNT sponge (thus the active electrode mass) limits the areal capacity. Here, the areal capacity of the CNT@SnS₂-thick sponge electrode is calculated in Fig. 5f. The area of the electrode was obtained through disassembling the cell and calculated the effective area (Fig. S11†). The areal capacity of the CNT sponge is enhanced significantly after decorating with SnS₂ and that of the CNT@SnS₂-thick sponge electrode is 20-fold larger for the first cycle and 5-fold larger after 100 cycles. In addition, such result is just built on that the thickness of an electrode is less than 0.5 mm and could be amplified larger.

4. Conclusion

In summary, flexible hybrid CNT sponges with controllable mass ratios and various morphologies of embedded SnS₂ from tubular nanosheaths to nanosheets have been synthesized by a facile solvothermal method taking thiourea as media. The reversible capacity and cyclability of tubular SnS₂ nanosheaths (SnS₂-45.82 wt%) are much better than those of nanoparticles (SnS₂-72.60 wt%) or nanosheets (SnS₂-88.37 wt%) which are more easily corroded and dissolved during cycling. CNTs play the role on hindering the agglomeration and enhancing electronic conductivity. These CNT@SnS₂ sponges could be used as free-standing and binder-free electrodes with significantly improved areal capacity than bare CNT sponge.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This work was financially supported by the National Nature Science Foundation of China (no. 51325202).

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Footnote

† Electronic supplementary information (ESI) available: Additional XRD patterns, Raman spectra, XPS spectra, SEM images, electrochemical performance data and a photo of electrode. See DOI: 10.1039/c6ra01143h

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