Carbon nanotube-assisted growth of single-/multi-layer SnS 2 and SnO 2 nano ﬂ akes for high-performance lithium storage †

SnS 2 nanoparticles and SnS 2 nano ﬂ ake/CNTs composite are prepared by a low-cost facile hydrothermal method for their use in rechargeable Li-ion batteries. It is found that the presence of multi-walled CNTs during synthesis greatly a ﬀ ects the morphology of as-formed SnS 2 nanostructures, and circinal single-layer and multilayer SnS 2 nano ﬂ akes enwrapped by CNTs are produced. The composite is further oxidized to porous SnO 2 nano ﬂ ake/CNTs hybrid by annealing at 500 (cid:1) C in air. The formation mechanism of SnS 2 /CNTs and SnO 2 /CNTs composites is examined. All the three materials are used as the anode in Li-ion batteries. The SnS 2 /CNTs composite delivers stronger cycling stability than the pure SnS 2 anode. In tests the former exhibits excellent capacity retention of 91.5% at 100 mA g (cid:3) 1 over 50 cycles, while the latter displays 66.8%. The rate capability of SnS 2 /CNTs composite is much better than pure SnS 2 as well. Redox reaction characteristics and Li-ion transfer kinetics at the two SnS 2 anodes are studied by di ﬀ erential capacity plots and electrochemical impedance spectroscopy. It is discovered that the SnS 2 /CNTs composite with larger surface area allows faster Li-ion transfer kinetics, e ﬀ ective cushion of volume changes, and thus gains the improved Li-ion intercalation behaviours. The capacity of the tin-based anode can be further raised by transformation to a SnO 2 /CNTs hybrid that also delivers excellent rate and cycling performances.


Introduction
Currently, the soaring development of electronic devices and electric vehicles urgently demands novel mobile power sources with high energy and power density, good safety and durability.Rechargeable Li-ion batteries (LIBs) have become a candidate due to their large energy storage density, light weight, low selfdischarge and long cycle life, no memory effects. 1,24][5][6] Among them, tin and tin-based materials such as tin alloys, tin oxides and tin sulphides are considered as potential anode materials for LIBs mostly owing to their large theoretical capacity and good reversibility.Tin disulphide (SnS 2 ) is an extensively-studied anode material with a layered hexagonal CdI 2 -type structure, in which sheets of tin atoms are sandwiched between two close packed sheets of sulphur atoms, and they are bound by weak van der Waals forces. 7A group of SnS 2 nanostructures, such as nanoparticles, 8 nanoplates, 9 nanosheets, 10 nanobelts, 11 nanoower and nanorods, 12,13 have been synthesized by methods including hydrothermal and solvothermal methods, thermal sulfurization, wet chemistry and vapor deposition.In the practice it is found that SnS 2 with a theoretical capacity of 645 mA h g À1 suffers a large volume change ($200%) and particle aggregation related to Liion insertion/extraction process, which leads to electrode pulverization and a loss of effective contact between the active material and in turn results in drastic capacity decay and poor cyclability. 14In addition, low conductivity of SnS 2 (10 À12 -10 À2 S cm À1 ) highly impedes electronic transfer and deteriorates its rate performance.A number of approaches are proposed to alleviate these problems.Among them, carbonaceous materials are oen used to enhance the electrochemical performance of SnS 2 .Kim et al. achieved carbon-coated SnS 2 nanoparticles that show far better Li-ion intercalation behaviour than uncoated SnS 2 . 15He et al. synthesized acetylene black incorporated SnS 2 nanoower with increased Li-ion storage capacities. 16Zhai et al. produced SnS 2 nanosheet anchored MWCNTs and found that the hybrid delivers higher capacity and cyclability than both the MWCNTs and SnS 2 . 17Sun et al. also fabricated SnS 2 nanoake decorated MWCNTs with improved capacity and cycling performance. 18Kang et al. discovered self-supported SnS 2 nanosheet coated MWCNTs with higher capacity and capability than pure SnS 2 nanosheets. 19Tin dioxide (SnO 2 ) is an attractive anode material as well, due to its high theoretical capacity of 781 mA h g À1 , but its huge volume change during Li-ion storage process results in a big loss of reversible capacity.Therefore, various methods are invented to solve this problem, such as SnO 2 coatings on TiO 2 -B nanowires, 20 CNTs and graphene. 21,22hose carbon materials not only give birth to new SnS 2 or SnO 2 nanostructures but also favor charge transfer and electrode stability, yielding improved Li-ion battery performances.So far, a new type of composites consisting of two-dimensional (2D) electrode materials and CNTs network has been reported, including MoS 2 nanosheet/MWCNTs, 3 CoOOH nanoplate/ MWCNTs and Ni(OH) 2 nanoplate/MWCNTs. 23,24The nanosheets or nanoplates embed into the porous matrix and connect conducting MWCNTs to achieve enhanced redox reaction kinetics and electronic conductivity, and nally obtain improved electrochemical properties.To the best of our knowledge, the composite composed of 2D SnS 2 or SnO 2 nanoakes dispersed in such a CNTs network for LIBs applications has not been reported hitherto.We consider that CNTs can not only improve the conductivity of SnS 2 or SnO 2 , but cushion the volume change and retard the agglomeration of the active material, which is possible to achieve novel tin-based anodes with high capacity, good cyclability and rate capability.
In the present work, we synthesized SnS 2 nanoparticles, SnS 2 nanoake/CNTs and SnO 2 nanoake/CNTs hybrids by scalable hydrothermal method and annealing.MWCNTs were found to promote the 2D growth of single-/multi-layer SnS 2 nanoakes, improve the ionic and electronic transfer of electrodes.The thin nanoakes can minimize the volume change during cycling for better cyclability and provide more sites to accommodate Li-ion insertion for increased capacities.A post heat treatment of these SnS 2 /CNTs composite in air yielded a new SnO 2 /CNTs hybrid that inherited the porous, nanoake-like features, and exhibited further increased specic capacities.220) and (112) crystal planes of tetragonal SnO 2 (JCPDS no.41-1445, space group P42/mnm).Again, no diffraction peaks that belong to MWCNTs are identied from the pattern of SnO 2 /CNTs.It is noted that the characteristics of XRD patterns of SnS 2 and SnO 2 (an oxidation product of SnS 2 ) here are in agreement with phase change described in previous reports. 25,26ig. 2 presents SEM images of SnS 2 , SnS 2 /CNTs and SnO 2 /CNTs materials.It is clearly seen that the absence of MWCNTs yields a big number of SnS 2 nanoparticles mostly with irregular shape and size (Fig. 2a).Some particles display an appearance of hexagonal nanoplates, indicating non-uniform growth of SnS 2 nanostructures under the hydrothermal conditions.An enlarged view of one SnS 2 nanoplate reveals its lateral size up to 650 nm and thickness up to $100 nm (Fig. 2b).The presence of MWCNTs during synthesis leads to formation of a distinct nanostructure.As shown in Fig. 2c, considerable 2D SnS 2 nanoakes are enwrapped by MWCNTs to form a porous SnS 2 /CNTs hybrid structure.The nanoakes are dense and thin, with a single layer or multiple layers.The multilayer nanoake is constructed by small akes developing around the centre of large akes or underneath (Fig. 2d and S1a †).MWCNTs either insert into the akes parallel or spread over them to bundle nanoakes into their conductive network, rather than being coated or anchored with SnS 2 . 17,18The akes display more uniform shapes, with a diameter up to 600 nm and a thickness of 10 nm (Fig. S1b †), which claims that SnS 2 prefers 2D growth mode along neighbouring CNTs when the two coexist in the superhot solution system.It is believed that the structure of 2D nanoakes and loose CNTs network can possess larger specic surface than SnS 2 nanoparticles that are large and aggregated.In fact, BET measurements demonstrate that the surface area is 15.48 m 2 g À1 for pure SnS 2 and 47.87 m 2 g À1 for the SnS 2 /CNTs (twice larger) (Fig. S2 †).Aer annealing, SnS 2 is oxidized to be thin SnO 2 nanoakes enwrapped in the CNT network (Fig. 2e).The nanoake/CNTs structure is preserved, but the nanoakes become porous and rough, which contain numerous tiny SnO 2 crystallites (Fig. 2f).Hence it is deduced that the oxide particles are supposed to originate from SnS 2 crystals and connect with each other to maintain the nanoake-like shape.Excellent Li-intercalation behaviours of the two composites are expected, because the conductive MWCNTs, thin nanoakes and porous structure are considered to endow fast charge transfer and small electrode resistance. 23,24he two nanoake/CNTs composites are further observed by TEM.Fig. 3a shows a SnS 2 nanoake conjunct CNTs.It has multiple layers, with several slim CNTs across its surface or underneath.An enlarged TEM image reveals that all the threelayer akes are actually composed of nanoparticles (Fig. 3b).These particles are SnS 2 polycrystallites clearly seen in Fig. 3c.A selected area electron diffraction ring pattern of this part shows multiple indices (100), ( 101), ( 102), ( 110) and (201) of SnS 2 (Fig. 3d), which agrees well with the XRD index above.

Results and discussion
Upon annealing SnS 2 is oxidized and a SnO 2 /CNTs composite results.Both the nanoake and CNTs survive (Fig. 4a).The enlarged view of spot C in Fig. 4a shows that the nanoake is also composed of considerable nanoparticles (Fig. 4b), which is corresponding to our SEM observation above.They are SnO 2 polycrystallites with a size of $5 nm (Fig. 4c).The selected area electron diffraction ring patterns reveals the (110), ( 101), ( 211) and (112) crystal planes of SnO 2 in this nanoake (Fig. 4d).
EDS analyses were performed on the SnS 2 /CNTs composite under SEM observation.In the spectrum in Fig. 5, distinct peaks between 3.3 and 4.1 keV correspond to the tin element.Sulfur peak appears at a position of 2.4 keV.Carbon is also found in this pattern, which is believed to be mostly from CNTs.A small peak belonging to oxygen is found, indicative of existence of   oxygen species due to acid treatment.Al peak is from the holder used for placing the sample during observation.A linear scan of a multilayer nanoake vividly tells that tin and sulphur are dominant elements there with a negligible amount of carbon and oxygen (Fig. 5b), which is corresponding to the aforementioned XRD pattern of the SnS 2 /CNTs sample.Aer annealing in air, SnS 2 in the composite is completely oxidized to SnO 2 , as evidenced by the disappearance of sulphur peak and the birth of stronger oxygen peak in the EDS spectrum of the SnO 2 /CNTs composite (Fig. S3 †).
The composition of two composites is further investigated by XPS technique.As for the SnS 2 /CNTs sample, the peaks at 486.86, 495.26 and 161.7 eV in the high-resolution XPS spectra (Fig. 6a and b) are assigned to Sn 3d5/2, Sn 3d3/2 and S 2p, respectively.The energy difference between two tin peaks is 8.4 eV, indicating the existence of Sn(IV).The results conrm the SnS 2 compound in the composite.The broad C 1s peak at 284.8 eV is asymmetric (Fig. 6c), which is consisting of four subpeaks at 283.8, 284.3, 285.4 and 288.4 eV (Fig. S4a †).The peak at 283.8 and 284.3 eV could be assigned to sp 2 C]C bond and sp 3 C-C bond, either in a graphite phase or in an amorphous phase. 16Peaks at 285.4 and 288.4 eV indicate the existence of residual groups such as -OH and -O-C]O, probably due to the acid treatment of CNTs and the adsorbed reactant. 16ikewise, Sn(IV) is detected in the SnO 2 /CNTs composite but sulphur not (Fig. 6d).It can be seen in Fig. 6b  The percentages of CNTs in the SnS 2 /CNTs and SnO 2 /CNTs composites were determined by TG analysis in air atmosphere, as shown in Fig. 7.The data presented in Fig. 7a reveals that the TG plot of SnS 2 /CNTs hybrid shows a three-step mass-loss process.A small weight loss of $1% from room temperature to 200 C can be attributed to the removal of physically adsorbed water.The next weight loss step between 200 C and 560 C is ascribed to the oxidation of SnS 2 into SnO 2 . 18The third weight loss observed from 560 C to 800 C is owing to the burning of CNTs in the composite. 28The total weight loss suffered by the composite is 35.4%.As for the SnO 2 /CNTs hybrid, the drastic weight loss of 16% observed from 560 C to 800 C results from the burning of MWCNTs (Fig. 7b).This conrms that the second weight loss of SnS 2 /CNTs hybrid is truly owing to the oxidation of SnS 2 to SnO 2 .As a result, the weight fraction of CNTs in the SnS 2 /CNTs and SnO 2 /CNTs is estimated to be ca.14% and 16%, respectively.The high ratio of CNTs is likely to contribute to the advantage of constructing conductive network for enhanced tin-based anode performance.
Based on above analyses, the probable growth process of the two composites is discussed with schematic illustration shown in Fig. 8.The formation of tin sulde (SnS) precursors may be described as follows: 12 It is noted that eqn (2) takes place in the presence of H + in the superhot solution. 12We know that increasing the temperature encourages the oxidation reaction of Sn 2+ to Sn 4+ ; when the temperature goes up to 140 C and more, SnS 2 originates from SnS. 29 Thus, we speculate that SnS forms initially and it is then oxidized to SnS 2 in the aid of excessive sulfur sources.Hence, the formation of many SnS 2 nuclei occurs in the hydrothermal process.They grow and start to agglomerate with each other.The further growth of these agglomerates is restricted fairly by dispersed CNTs, so they prefer to develop parallel to the CNTs, eventually yielding uniform thin nanoakes inside the CNTs network (Fig. 8).Upon heat treatment in air, SnS 2 nanoakes are oxidized in situ to form SnO 2 nanoakes in the network.
In this study the Li-ion intercalation behaviour of SnS 2 and SnS 2 /CNTs hybrids are rst compared to investigate the effects of MWCNTs on the electrochemical property.The potential of redox reactions is studied via charge-discharge plots measured at 100 mA g À1 (Fig. 9a and b).Both electrodes have large initial discharge capacity; namely, 1161.2 mA h g À1 for SnS 2 anode and 1430.5 mA h g À1 for SnS 2 /CNTs anode.Aerward, a rapid decline of discharge capacity occurs in the rst two cycles for both electrodes, mostly due to the decomposition of electrolyte components and formation of solid-electrolyte-interface (SEI) layers on electrode surface, or the irreversible Li-ion insertion into the electrode materials (Li + rapping effects). 30In the charge plots, a slowly increasing potential plateau is seen starting from 0.38 V to 0.7 V.It is well known that the lithiation/delithiation reactions in SnS 2 have two steps and can be described by: 16 SnS 2 + 4Li + + 4e À / Sn + 2Li 2 S (4) In the rst step, Li-ion insertion into SnS 2 forms metallic Sn and amorphous Li 2 S. In following discharge-charge cycles, the alloy and dealloy behaviours of the newly formed Sn with Li leads to the birth and extinction of Li 4.4 Sn, and the Li 2 S acts as an inert matrix surrounding the active Sn nanograins. 16By this mechanism, Sn endows SnS 2 with a theoretical capacity of 645 mA h g À1 , which is rather smaller than that of pure Sn (993 mA h g À1 ) and SnO 2 (781 mA h g À1 ).
Fig. 9c displays capacities of the two SnS 2 electrodes over 50 electrochemical cycles at 100 mA g À1 in a potential range from 0.01 to 1.2 V versus Li/Li + .The charge capacity of SnS 2 anode rises somewhat in the rst ve cycles, but that of SnS 2 /CNTs electrode does not, indicating that a Li-ion insertion activation process is negligible in the latter due to fast Li-ion transfer in a 2D nanostructure.In general, the SnS 2 /CNTs electrode delivers signicantly better cyclability than the bare SnS 2 electrode.As shown in the gure, the former shows a nal charge capacity of 373.1 mA h g À1 that is 91.5% of its initial charge capacity.On the contrary, the latter exhibits a nal capacity of 200.7 mA h g À1 which is only 66.8% of its initial charge capacity.The poorer cycling performance of pure SnS 2 anode is attributed to its intrinsically low electronic conductivity and internal damage from large volume expansion in the alloying of Li into metallic Sn.This result proves that the strategy of composite with CNTs is as effective as carbon coatings 15 or coatings on CNTs 17,18 to strengthen the stability of SnS 2 anodes in durable LIBs.It is worth noting that the two anodes have close capacities in the early cycling stage, which might be ascribed to the addition of superuous MWCNTs with low capacity (<200 mA h g À1 ) 17 into SnS 2 .As a result, work is in progress to optimize CNTs content in the SnS 2 /CNTs composite, aiming for both larger capacities and better cyclability of SnS 2 anodes.
We also studied the rate performance of SnS 2 and SnS 2 /CNTs anodes.It can be seen from Fig. 9d and e that the electrode polarization is increased faster for the SnS 2 anode than for the SnS 2 /CNTs anode as the current goes up for tests, suggesting better conductivity of the SnS 2 /CNTs electrode.Consequently, the capacity of the latter becomes higher than that of pure SnS 2 at various current densities from 100 to 1000 mA g À1 (Fig. 9f).In particular, the capacity of SnS 2 /CNTs anode is ve times of that of SnS 2 anode at a large current of 1000 mA g À1 .
The Li-ion intercalation behaviors at the two anodes are again examined by their differential capacity curves (Fig. 10a  and b) derived from their discharge-charge plots.Clearly, both  of them have one broad reduction peak in the cathodic scan and one broad oxidation peak in the anodic scan.The position of the peaks is almost the same (0.05-0.39 V, 0.4-0.7 V), but the reduction peak of the SnS 2 /CNTs anode starts about 0.6 V and gradually rises, while that of SnS 2 anode sharply increases at a lower potential of about 0.45 V, suggesting smaller polarization and better reversibility of Li-ion insertion/extraction reaction at the SnS 2 /CNTs anode.In addition, the estimated integral area (representing their specic capacity) between the discharge and charge plots in the whole potential range is clearly larger for the composite electrode than for the SnS 2 , suggesting that more electrons and Li + ions involve the insertion/extraction process in the former.In order to verify these points, EIS technique is employed to explore Li-ion transfer kinetics at the SnS 2 and SnS 2 /CNTs anodes.Fig. 10c shows Nyquist plots and their equivalent circuit aer the cells were discharged and charged for ten cycles.A small depressed semicircle is found at high frequency region in both curves, indicating the formation of a passivating SEI lm on the anode surface.A second depressed semicircle is seen in the high-to-medium frequency region, which denotes the charge transfer progress between the active material and electrolyte.In the low frequency region, the straight line displays characteristics of Warburg impedance (W) that generally reect the Li-ion diffusion behaviours within particles of the active material.In the circuit, R U is the ohmic resistance derived from the organic electrolyte, separator and electrodes, corresponding to the intercept of the rst depressed semicircle with Z-real axis.As for the rst semicircle, R s represents the resistance of Li-ion diffusion through the SEI layer; CPE s is the constant phase-angle element depicting the nonideal capacitance resistance.For the second semicircle, R ct is the charge transfer resistance, and CPE dl is the constant phaseangle element that is related to a double layer capacitance.It is easily found that R ct is signicantly larger for the bare SnS 2 anode than for the SnS 2 /CNTs electrodes, which reects rapider charge transfer or lower charge transfer resistance in the latter.
In addition to the optimization of MWCNTs content in the SnS 2 /CNTs composite, the oxidation of SnS 2 to SnO 2 could be an alternative route to improve the specic capacity of the anode, due to their theoretical capacity difference.Hence, we for the  rst time achieved a porous thin SnO 2 nanoake/CNTs hybrid readily by annealing the as-obtained SnS 2 /CNTs.Fig. 11a shows the good cycling performance of SnO 2 /CNTs anode at 100 mA g À1 over 20 cycles.Apparently, its capacity of about 500 mA h g À1 exceeds that of SnS 2 /CNTs anode by almost 100 mA h g À1 , let alone graphite with an even lower capacity.The SnO 2 /CNTs electrode also delivers excellent rate capability at various current from 100 to 1000 mA g À1 (Fig. 11b).The results demonstrate that a hybrid structure made of 2D active material (e.g.metal oxides and suldes) and conductive carbon network is an ideal candidate for high-performance LIB electrodes.

Conclusions
SnS 2 nanoparticles and SnS 2 nanoake/CNTs composite were prepared by a simple hydrothermal approach.The addition of conducting MWCNTs for the synthesis was found to affect the morphology of obtained SnS 2 nanostructures.Dispersed CNTs in solutions restrain their growth and yield the formation of 2D SnS 2 nanoakes enwrapped by MWCNTs, rather than irregular SnS 2 nanoparticles, eventually to achieve a 3D porous structure with larger surface area.Firstly, those nanoakes shorten paths for Li-ion transfer and thus reduce the ionic transfer resistance.Secondly, the embedment of SnS 2 nanoakes into a conductive MWCNT matrix allows fast electronic diffusion and effectively cushions the volume change of SnS 2 during cycling.All these unique properties lead to the signicantly better cyclability and rate performance of SnS 2 /CNTs composite than that of the SnS 2 nanoparticle anode.The composite can be fully changed to 2D SnO 2 nanoake/CNTs hybrid via annealing in air, which shows increased Li-ion storage capacities.Our results demonstrate that metal oxides and sulphides that are integrated with conducing MWCNTs and therefore harvest enhanced Li-ion intercalation properties could be a promising material for the anode of high-performance LIBs.

Experimental details
MWCNTs (outer diameter of 6-9 nm, length of 5 mm, Sigma Aldrich) were rst added into nitric acid and stirred overnight, then ltered and washed with de-ionized water and absolute ethanol ($99.5%,Sigma Aldrich), and dried in a vacuum oven.0.009 g CNTs and 1 g sodium dodecylbenzenesulfonate (SDBS) powder were put into a glass beaker containing 60 ml anhydrous ethanol, ultrasonically treated until a homogenous black solution was obtained.Later, 0.001 mol SnCl 2 $2H 2 O and 0.004 mol thiourea powder were added into the CNTs solution.Aer vigorously stirring for 15 min, the solution was transferred into a Teon-lined autoclave of 100 ml capacity, and then the autoclave was sealed and heated at 200 C for 24 h.A black precipitate was yielded and collected by centrifugation, rinsed with distilled water and ethanol for six times.The nal product was dried at 80 C in the vacuum oven overnight.For comparison, SnS 2 particles were produced in the same route but without the addition of CNTs.The obtained SnS 2 /CNTs product was further annealed at 500 C in air for 1 h to gain SnO 2 /CNTs composites.
A Hitachi S-4800 scanning electron microscope (SEM) and a Hitachi H9000NAR transmission electron microscope (TEM) were used to observe these materials.Their crystallographic structure was determined by X-ray diffraction (XRD) using a Scintag XDS 2000 X-ray diffractometer with Cu K a radiation at a scan rate of 2 min À1 .Chemical composition of these particles was characterized by an energy dispersive X-ray spectroscopy (EDS) with an XFlash detector (Bruker AXS), and an X-ray photoelectron spectroscope (XPS, Perkin-Elmer PHI 5400 ESCA System) using a monochromatized Mg K a X-ray radiation source.Thermogravimetry analysis (TGA) was carried out on a simultaneous thermal analyzer (TA Instruments, SDT 2960) in an air atmosphere from room temperature to 1000 C at a rate of 20 C min À1 .The Brunauer-Emmett-Teller (BET) surface area were measured by a surface area and porosity analyzer (MICROMERITICS, ASAP 2020).
Cu foils (12 mm thick) were cleaned with acetone and dried in air.A slurry was made from a mixture of the prepared material, carbon black (99.5%,Alfa Aesar), and polyvinylidene uoride (4 wt% in N-methyl-2-pyrrolidone) in a weight ratio of 80 : 10 : 10.The slurry was then spread on the copper foil using a blade casting method, followed by drying in air at room temperature and in vacuum at 100 C for 12 h.Later, the Cu foil was cut into disks with a diameter of 3/8 inch that were then pressed by a Carver Lab Press with a pressure of 6 MPa.Masses of the active material are about 1 mg.CR2032 coin cells were assembled to investigate battery behaviours of the samples acting as the working electrode.A lithium disc is used as the counter and reference electrode in the cells, which was separated from the working electrode by a Celgard-2320 separator.The organic electrolyte was 1 mol L À1 LiPF 6 dissolved in a mixed solvent of ethylene carbonate and diethylene carbonate with 1 : 1 volume ratio (Novolyte Technologies, Inc.).All the coin cells were assembled in an Ar-lled glove box with oxygen and moisture levels lower than 1 ppm.The galvanostatic charge-discharge was carried out on LANHE automatic battery testers at 100 mA g À1 current for 50 cycles or 100-1000 mA g À1 for 30 cycles.AC impedance measurements were also conducted on these cells at a potential of $0.4 V from 100 kHz to 0.01 Hz with a perturbation of 5 mV applied.

Fig. 3
Fig. 3 TEM observation of SnS 2 /CNTs composite: (a) SnS 2 nanoflake under low magnification, (b) SnS 2 nanoflake under high magnification of A spot in (a), (c) crystallites seen from B spot in (a), and (d) electron diffraction ring pattern taken from B spot with Miller indices of SnS 2 .

Fig. 4
Fig. 4 TEM observation of SnO 2 /CNTs composite: (a) SnO 2 nanoflake under low magnification, (b) SnO 2 nanoflake under high magnification of spot C in (a), (c) multiple crystallites in the nanoflake, and (d) electron diffraction rings taken from this spot with Miller indices of SnO 2 .
that broad O 1s peak at 531.3 eV is composed of three subpeaks at 530.5 eV (Sn-O bond), 531.6 eV (Sn-O-C bond) and 532.9 eV (C-O bond) 27 (Fig. S4b †).C 1s XPS peak from the SnO 2 /CNTs is asymmetric as well at 284.6 eV (Fig. 6f), which includes three subpeaks at 283.8 eV (C]C bond), 284.6 eV (C-C bond), 285.3 eV (C-O bond) and 288.4 eV (O-C]O bond) as shown in Fig. S4c.† Thereby, it can be easily concluded that SnS 2 and SnO 2 nanoakes have been successfully produced and incorporated into the CNTs network by hydrothermal means and annealing in oxidative atmosphere.

Fig. 5
Fig. 5 EDS spectrum of SnS 2 /CNTs composite.The inset is elemental linear scan across a single SnS 2 nanoflake.

Fig. 9
Fig. 9 Charge-discharge plots of (a) SnS 2 and (b) SnS 2 /CNTs composite and (c) their cycling at 100 mA g À1 ; charge-discharge plots of (d) SnS 2 and (e) SnS 2 /CNTs composite when cycled at different current densities and (f) their rate capability.