Engineering microtubular SnO₂ architecture assembled by interconnected nanosheets for high lithium storage capacity

Abstract

Recent attention has been focused on the synthesis and application of hollow and porous structured micro-/nano-materials since this kind of structure exhibits unique advantages for electrode materials. Herein, we report an interesting approach for efficient synthesis of hollow SnO₂ microtubes assembled by interconnected nanosheets. On the basis of self-sacrificing template technology, hollow SnO₂ microtubes were successfully synthesized via a one step facile hydrothermal route using a natural biological substance (here, it is cotton) as template without further post treatment (high temperature or strong acid or base). The as-synthesized hollow SnO₂ microtubes integrate two beneficial features: hollow cavity and porous shell, which are beneficial for accommodating large volume variation and facilitating fast and full electrolyte access to SnO₂ nanosheets. When evaluated as an anode for lithium ion batteries, they exhibit high reversible capacity and good cycle performance. Our findings provide an effective solution for the synthesis of hollow and porous micro-/nano-architectures, which can pave the way to improve the electrochemical performance of electrode materials.

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1. Introduction

Many alternative energy technologies have been developed in an attempt to alleviate the energy and environmental challenges. Lithium ion batteries (LIBs), which are considered to be the most important energy storage and conversion technology with prominent advantages of cost effectiveness and environmentally friendliness, have recently been widely researched.^1–10 The constant strong demand for lithium rechargeable batteries as a power supply for portable electronic devices and electric vehicles has accelerated the research and development of new electrode materials with higher energy density, better stability, longer cycle life and improved safety. The development of new electrode materials for LIBs has always been a focal area of materials science, as the current technology may not be able to meet the high energy demands for electronic devices with better performance. With a high theoretical gravimetric Li storage capacity of 782 mA h g⁻¹, SnO₂ has been considered as an attractive candidate for substitution of the conventional graphite anode in LIBs.^11–19 However, the practical use of this materials is significantly impeded by the poor capacity retention over extended charge/discharge cycling. This problem mainly originates from the large volume change of electrode materials accompanying Li insertion and extraction, which creates large internal stress, leading to the disintegration of the electrode materials. To alleviate this problem, one effective strategy is to design well-designed nanostructures with short lithium/electron diffusion paths, large surface areas and enhanced reactivity.²⁰

For energy storage devices, two dimensional (2D) nanosheets generally exhibit great superiority in electrode materials due to their special structural characteristics of large lateral size and a small thickness.^21–25 Moreover, hollow and porous structures have received great attention owing to well-defined interior voids, low density, large surface area, and surface permeability.^26–31 As electrode materials for LIBs, such structures can provide high specific capacity, superior rate capability, and improved cycling performance.^32–36 Therefore, it would be highly desirable to develop a simple, but general, strategy to effectively synthesize hollow and porous structures for electrode materials assembled from 2D nanosheets. A general procedure towards synthesizing hollow and porous structures involves the growth of a shell of designed materials on various removable templates including hard templates such as monodispersed silica or polymer latex spheres and soft ones, for example, emulsion micelles and even gas bubbles.^37,38 Even thought template-free techniques generally involve fewer synthetic steps and are thus considered quite facile, templating methods offer important advantages, including narrow-size-distribution products with well-defined structural features, which makes the method very straightforward and versatile. There are nevertheless inherent drawbacks in the template removal process, which limit the utility of the approach. For the hard template being removed, the conventional method requires the use of high temperature carbonization process or toxic reagents such as NaOH or HF. It is therefore highly desirable to develop new strategies for one-step synthesizing hollow micro-/nano-structure without complex post treatment, which will build on the strengths of the template methods.

Cotton, an inexpensive natural product mainly composed of cellulose fibers, has been widely used for textiles and clothing. The cellulose is a natural fiber and the most abundant organic polymer on earth, which make it and its derivatives an exciting addition to the possible templates used for structural control. This is a simple, yet quick and versatile method that produces materials with large pore and hollow structures inherited from the initial cellulose form. In recent years, various approaches based on the cellulose template have been developed for the synthesis of nanotubular and porous structured metal oxides.^39–47 For example, Caruso et al. have reported the fabrication of porous titania and zirconia films based on the acetate cellulose membrane templating techniques and found that the final inorganic shape essentially resulted from a coating of the initial template form.³⁹ Xia et al. prepared SnO₂ nanoparticles@carbon nanofibers composite materials by using a commercial filter paper substance as template.⁴⁰ By varying the sintering temperature, the weight of carbon in the products was tuned, and the carbon nanofibers were demonstrated to be beneficial to improve the cycling performance of LIBs. Guo et al. synthesized hollow SnO₂ nanotubes as anode materials by using kapok fibers as template to improve lithium storage performance.⁴¹ A sonochemical method is developed to fabricate SnO₂ nanotubular materials from biological substances by Zhu et al.⁴² The results show that the as-synthesized nanocrystals have a promising application for the gas sensors with high selectivity, fast response, and quick recovery. Based on the cellulose substrate, Huang et al. prepared a series of nanotubular oxide materials with improved performance based on the cellulose template.^43–46 However, we find that the cellulose matter must be removed from the final materials by heating treatment during the above mentioned experiments. Hence, a facile and one-step methodology to synthesize hollow tubular structured metal oxide materials still remains a challenge based on cellulose template.

Herein, we report a new approach to synthesize hollow SnO₂ microtubes assembled from interconnected nanosheets through one-step hydrothermal process by employing natural cotton as a self-sacrificial template. Our strategy is based on in situ growth of SnO₂ nanosheets on the surface of natural cotton fibers without complex post treatment (high temperature or strong acid and base). We subsequently studied the electrochemical properties of these nanosheet-assembled hollow microtubes as an anode in LIBs, and the results indicate that the as-synthesized sample demonstrates significantly improved cycling and rate performance as an anode materials.

2. Experimental

Materials synthesis

All reagents were analytical grade, commercially available from Beijing Chemical Reagent Co. Ltd, and used as received without further purification. As a typical procedure to prepare SnO₂ microtubes, 7 mmol SnCl₂·2H₂O were dissolved into 50 mL deionized water. The mixture was stirred at room temperature for 30 min, then 14 mmol NH₄F was added into the solution. After being stirred for another 30 min, a clear transparent solution was formed. Commercially available cotton was treated with deionized water, and ethanol in sequence. After that, the as-treated cotton was immersed into above reaction solution, which was transferred into a Teflon-lined autoclave and kept at 180 °C for 24 h. After the autoclave was cooled down to room temperature naturally, the resultant grey white precipitation was rinsed extensively with water and absolute alcohol, and finally dried in air at room temperature for further characterization.

Materials characterization

The crystal structures of the products were confirmed by powder X-ray diffractometry (XRD; Bruker, D8 Advance X-ray diffractometer, Cu-Kα λ = 1.5406 Å). Field-emission scanning electron microscopy (FE-SEM, JEOL S-4800) and transmission electron microscope (TEM, JEM-2010) were used for morphology and structure characterization. Surface area and pore size distribution by nitrogen adsorption/desorption at 77 K using BELSORP-max. The thermogravimetric analysis (TGA) was performed with a DTG-60AH instrument in the air with a heating rate of 5 °C min⁻¹.

Electrochemical measurements

Galvanostatic cycling performances of the as-prepared nanosheet-assembled SnO₂ hollow microtubes were measured by using CR2025 coin cells at room temperature on a multi-channel battery testing system (LAND CT2001A). The working electrode was composed of SnO₂ microtubes, carbon black, and Na-alginate in a weight ratio of 80 : 10 : 10. These materials were ground in a mortar with appropriate amount deionized water as the solvent to make slurry, and the resultant slurry was then uniformly pasted on a Cu foil current collector. The typical electrode was dried at 105 °C for 24 h under vacuum. Finally the treated Cu plates were pressed under a pressure of 20 MPa. A lithium foil was used as the counter electrode and a solution of 1 M LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) (1 : 1 : 1 in volume) was used as electrolyte.

3. Results and discussion

The microtubular SnO₂ sample composed of interconnected nanosheets was synthesized via a facile hydrothermal route based on template method. Its composition has been confirmed by X-ray powder diffraction (XRD) measurement. As shown in Fig. 1, all diffraction peaks can be ascribed to a tetragonal rutile structure of SnO₂ (JCPDS no. 41-1445). No other obvious peaks belonging to SnO or Sn residue are identified in this XRD pattern, indicating this sample has high purity.

Fig. 1 XRD patterns of hollow SnO₂ microtubes assembled from interconnected nanosheets and the standard card.

The morphology and microstructure of the as-synthesized SnO₂ sample were first examined with field-emission scanning electron microscopy (FESEM). The natural cotton used in our experiments acts as a self-sacrificial template, which mainly consists of cellulose fibers with diameters ranging from 10 μm to 20 μm (Fig. 2a and inset). When the cotton dipped into the solution containing SnCl₂·2H₂O and NH₄F was treated by hydrothermal method without additional post treatment, black gray product could be collected in the bottom of Teflon-lined stainless steel autoclave. As shown in Fig. 2b, the as-synthesized sample possesses overall morphological characteristics of the initial cotton fibers. The overview image at low magnification clearly reveals that the sample consists of almost fibers, which have lengths of around several hundred microns and diameters of about 10 μm, respectively, and no other morphologies could be detected. Higher magnification FESEM images display that these fibers actually are microtubular structure (Fig. 2c–e). The average outer diameter of the microtubes is several micrometers, which corresponds well to the cotton template size. The thickness of tube wall is estimated to be several hundred of nanometers from the cross-section image (Fig. 2d). From the FESEM image of the tube surface we can also see that the tube wall is constructed by interconnected sheet-like subunits with a thickness of several nanometers. Fig. 2f clearly displays the top-view FESEM image of the interconnected SnO₂ nanosheets, which reveals that the wall of the microtubular SnO₂ is composed of perpendicularly oriented nanosheets. By closer observation, it can also be clearly observed that there exists relatively large open space between SnO₂ nanosheets. Moreover, the SnO₂ nanosheets are interconnected with each other to form a hierarchical network structure. On the basis of the above results, it can be deduced the as-prepared SnO₂ microtubes can be generally classified as a hierarchically porous structure. Such hierarchical structure could efficiently decrease the mass transport resistance and allow the access of the electrolyte to the active surface sites easier, which is beneficial to improve the lithium storage performance. More interesting, from the broken microtube we can find that the inwall of the microtubes is a nanoparticle layer (Fig. 2g). The high magnification FESEM (Fig. 2h) reveals that this nanoparticle layer possesses a porous structure, which is composed of many nanoparticles with several dozens of nanometers. Many irregular pores are uniformly dispersed in the whole inwall and the diameters of the pores are in the range of 5–10 nm. Such nanoarchitectures are of interest for potential applications in lithium storage performance. However, we did not find any trace of cotton template during the FESEM test, indicating that the template might have been etched.

Fig. 2 (a) Photograph of cotton, inset is low-magnification FESEM image of the original cotton template and a single fibre can be clearly observed; (b) low-magnification image of hollow SnO₂ microtubes; (c–e) showing that the as-synthesized SnO₂ sample has a tubular architecture; (f) high magnification FESEM image of microtubes shows nanosheets architecture; (g) FESEM image of a broken hollow tubes; (h) high magnification FESEM of the inwall of the hollow microtubes.

Furthermore, the morphology and microstructure of the as-prepared sample were further characterized by transmission electron microscope (TEM). TEM images at low magnification revealed highly porous wall consisting of interconnected nanosheets and nanoparticles (Fig. 3a and b). The lateral size and thickness of the nanosheets are about 300 and 20 nm, respectively. The magnified TEM image of a flatly lying nanosheet clearly shows that the nanosheets are continuous with a smooth surface. The size of the nanosheets is around 150 nm, consistent with the observation from above FESEM images. To further understand the crystallinity of the SnO₂ nanosheets, high-resolution (HRTEM) image was recorded on an individual nanosheet, as shown in Fig. 3d. The interplanar distance of about 0.335 nm matches well with the inter-plannar spacing of the (110) planes of SnO₂. Fig. S1† shows the selected area electron diffraction (SAED) pattern of nanosheet-assembled hollow microtubes. The diffraction rings composed of diffraction dots confirms their polycrystalline nature. In addition, the TEM and HRTEM images of the NP layer in the inwall have also been obtained (Fig. 3e and f), which clearly demonstrated the porous structure and microstructure of the NP layer. From Fig. 3e, we can see that the inwall is composed of interconnected tetrahedral particles. The present TEM result is in agreement with the analysis of aforementioned XRD and SEM. According to the previous reports, such nanoarchitectures could be beneficial to improve lithium ion storage and gas-sensing properties by the surface-engineering strategy.^15,47

Fig. 3 TEM characterization of hollow SnO₂ microtubes: (a–c) low-magnification TEM images; (d) HRTEM image of hollow SnO₂ microtubes; (e) low-magnification TEM image and (f) HRTEM image of the inwall of hollow SnO₂ microtubes.

The hollow SnO₂ microtubes were obtained under hydrothermal conditions using NH₄F as a morphology control agent and cotton as a self-etching template. A series of control experiments indicated that NH₄F and SnCl₂·2H₂O both have important effect on product morphology (Table S1†). If SnCl₂·2H₂O was not used in the original reactants and just NH₄F and cotton template were subjected to hydrothermal treatment, helical fibers are generated, and more interestingly, many of the fibers display a porous structure (Fig. 4a and S2†). The corresponding XRD pattern (Fig. 4b) is almost completely same as that of the cellulose,⁴⁸ suggesting that the ordered structure of the crystalline cellulose is not disrupted by the hydrothermal process. If SnCl₂·2H₂O and cotton were treated by hydrothermal method in the absence of NH₄F, SnO₂ nanoparticle film instead of microtubes was formed (Fig. 4c and d and S3†) and no cotton fibers were found in SEM images, indicating that the cotton has been etched in the SnCl₂·2H₂O aqueous solution. When NH₄F and SnCl₂·2H₂O both were present in cotton-contained reaction system, nanosheet-assembled SnO₂ microtubes were formed. This result tells us that NH₄F acts as a morphology control agent for the formation of SnO₂ nanosheets and cotton as a template. Furthermore, the effect of the concentration of SnCl₂·H₂O on the formation of the nanosheets was investigated by performing the synthesis with the volume of the initial solution and the molar ratio of SnCl₂·2H₂O to NH₄F both constant. With a lower amount of SnCl₂·2H₂O (1 mmol), the product is one dimensional fibers with an average diameter of ∼100 μm, and also their surface are covered with nanoparticles (Fig. S4a and e†). Fig. S4b and f† show the SEM images of the sample obtained with 3 mmol SnCl₂·2H₂O, from which a small amount nanosheets could be seen, indicating that the nanosheet shell started to form. When the amount of SnCl₂·2H₂O was increased to 5 mmol, the nanosheet shell could be clearly observed on the surface of cotton, and few hollow microtubes had been formed under the present concentration (Fig. S4c and g†). By further increasing the amount of SnCl₂·2H₂O to 7 mmol, perfect hollow microtubes were formed (Fig. 2). However, if the amount of SnCl₂·2H₂O was increased to as high as 15 mmol, the nanosheets do not change much in size but some microspheres were formed on the surface of microtubes (Fig. S4d and h†).

Fig. 4 (a) FESEM image and (b) XRD pattern of the cotton treated by NH₄F at 180°; (c) FESEM image and (d) XRD pattern of SnO₂ nanoparticles film obtained by hydrothermal treatment of SnCl₂·2H₂O and cotton at 180°; (e) photographs of the resultant mixtures after different reaction systems. The above is products after centrifugal separation and the below is products of natural sinking method; (f) schematic illustration of the evolution process of hollow SnO₂ microtubes.

Moreover, we also observed the self-etching of cotton from the macroscopical phenomena of above experiments. For the experiment I after hydrothermal treatment (Fig. 4e-I), we can find a wad of brown cotton with partial carbonization at the bottom of centrifuge tube and the light brown liquid on the top, respectively. However, we did not find cotton in the power precipitate for the only use of SnCl₂·2H₂O reagent (experiment II), and the supernatant liquid has not any changes during the hydrothermal treatment (Fig. 4e-II). For the synthesis of SnO₂ hollow microtubes, a synergistic result could be found in the Fig. 4e-III, which includes brown supernatant liquid and dark gray power and cotton is not found in this system. These results indicate that SnCl₂·2H₂O is one critical factor for the self-etching of cotton template, and NH₄F is another important factor for the formation of the nanosheets. To further understand the self-etching of cotton, we carried out a test on the pH value of above mentioned systems since cellulose could hydrolyze in the acid conditions. As shown in Table S2,† for the NH₄F/cotton system, the pH value has little change before and after hydrothermal treatment, whereas the pH value of the SnCl₂·2H₂O-containing systems significantly decreases after hydrothermal treatment. Thus it can be deduced that the relatively strong acid in our case induces the decomposition of cotton, thus resulting in the self-etching of cotton.

To verify the absence of cotton in hollow SnO₂ microtubes obtained by hydrothermal treatment, the thermogravimetric analysis (TGA) of hollow SnO₂ microtubes was carried out from room temperature to 700 °C at a ramp rate of 5 °C min⁻¹ in the air and that of cotton was also done for comparison. As shown in Fig. S5 (see ESI†), thermal degradation of cotton proceeds essentially through two types of reactions.^49,50 At lower temperatures in the range of 247 °C to 400 °C, there is a gradual degradation which involves depolymerization and dehydration, while at higher temperatures, a rapid volatilization occurs, which is often accompanied by formation of levo-glucosan. Under such conditions, levo-glucosan breaks down to give smaller molecular species. Repolymerization of these volatiles leads to the formation of residual carbon (the yield of the residual carbon is 0.297% at 750 °C).⁴⁹ However, a larger weight loss was not found for the hollow SnO₂ microtubes, and just a little weight loss of 3% could be obtained over 25–750 °C. The first slight weight loss of about 1% can be attributed to the removal of crystalline and adsorbed water between 25 to 200 °C. The second weight loss may correspond to smaller molecular species adsorbed on the hollow SnO₂ microtubes (the inset in Fig. S5†).

Fig. 4f gives a schematic diagram of the formation process of the hollow microtubes. At the beginning of the reaction, Sn²⁺ was absorbed on the surface of the cotton fibers. Subsequently, under the hydrothermal conditions, SnO₂ nanocrystal layer was first formed on the surface of the cotton fibers and then further grew into nanosheets based on the oriented attachment and Ostwald ripening, during which the cotton template was completely eched under the acid conditions. As a result, hollow SnO₂ microtubes were formed.

To examine the porous structure of the nanosheet-assembled SnO₂ hollow microtubes, the nitrogen adsorption–desorption measurements were carried out. Fig. 5a shows the N₂ adsorption and desorption isotherms and the pore size distribution of the as-obtained hollow SnO₂ microtubes. The nitrogen sorption isotherms can be classified as type IV according to IUPAC classification, i.e. typical mesoporous structure. The pore size distribution data indicates that the size of the pores mainly centers at 20 nm with a relatively narrow distribution (Fig. 5b), in well agreement with the deduction from the isotherms. Such a porous structure results in a relatively high Brunauer–Emmett–Teller (BET) specific surface area of 70.41 m² g⁻¹. This structure not only can keep the nano effect of the electrode, but also can alleviate the volume changes of SnO₂ electrode during electrochemical reaction.

Fig. 5 (a) N₂ adsorption–desorption isotherms of hollow SnO₂ microtubes. (b) Pore size distribution calculated from the desorption branch.

The electrochemical reaction mechanism of Li with SnO₂ has been well studied and can be described in following equations:

Li⁺ + e⁻ + electrolyte → SEI (1)

SnO₂ + 4Li + 4e⁻ → Sn + 2Li₂O (2)

Sn + xLi⁺ + xe⁻ ↔ Li_xSn (0 ≤ x ≤ 4.4) (3)

It is generally accepted that the first two reactions are partially reversible and should be responsible for the large irreversible capacity of the first cycle. The third one is widely known to be reversible and the lithium ions can be repeatedly alloyed and dealloyed with Sn formed in situ, which contributes to the dominant capacity of the present system.^{17,18,51–53} However, this reversible process is also accompanied by the dramatic structural change, which leads to pulverization of the electrodes and becomes significant drawback of SnO₂-based electrode materials. To alleviate this problem, one effective strategy is to design desired nanostructures, where the concept is to utilize the local empty space and porous shell, to partly buffer the large volume change, thus improving the capacity retention of the electrode materials upon extended cycling.²⁰ The present nanosheet-assembled SnO₂ microtubes create such a sheet-like structure, which can provide a short diffusion path for more efficient lithium transport. At the same time, the presence of the voids in the tubes will not only relieve the structural alterations caused by the charge/discharge process, thus improving the cycling performance, but also help to store more lithium. To reveal the advantage of the present hollow and porous structure, we compared the lithium storage properties of the as-prepared interconnected nanosheet-assembled SnO₂ microtubes with those of other solid counterparts, namely, commercial SnO₂ solid particles (SEM and XRD could be obtained from the Fig. S6†).

Fig. 6a shows the representative cyclic voltammograms (CVs) of the nanosheet-assembled SnO₂ hollow microtubes for the first three consecutive cycles. CV curves were recorded on a CHI-660D potentiostat at a scanning rate of 0.5 mV s⁻¹ in the voltage window of 3–0.01 V. The present CV behavior is consistent with those of SnO₂ reported in the literatures, suggesting the same electrochemical reaction pathway. As can be clearly seen, an apparent reduction peak in the first cycle is presented around 0.74 V, which can be ascribed to the partially reversible decomposition of SnO₂ to Sn [see eqn (2) above] and the formation of the solid–electrolyte interface (SEI) film [see eqn (1) above], and in the subsequent cycles it shifts to a higher potential of about 1.15 V. The other redox pairs (cathodic, anodic) shown at the potentials of 0.01 and 0.64 V can be attributed to the alloying and dealloying processes [see eqn (3) above], which contribute to the main capacity of the cell. From the second cycle onwards, the CV curves mostly overlap, indicating the good reversibility of the electrochemical reactions.

Fig. 6 (a) Cyclic voltammograms (CVs) of hollow SnO₂ microtubes; (b) discharge/charge voltage profile between 0.01–1.2 V at a rate of 1 C of hollow SnO₂ microtubes; (c) coulombic efficiency (CE) and cycling performance and of hollow SnO₂ microtubes as well as commercial SnO₂; (d) cycling performance of hollow SnO₂ microtubes and commercial SnO₂ at different rates (1–5 C).

Fig. 6b shows the discharge/charge voltage profiles of the as-synthesized samples at a constant current density of 1 C over the voltage range of 0.1–1.2 V vs. Li⁺/Li. The sample delivers the initial discharge and charge specific capacities of 1661.7 and 712.8 mA h g⁻¹. The large initial Li-storage capacity might be associated with the unique structure of the nanosheet-assembled hollow and porous structure, which probably is beneficial for storing more lithium, and such features have also been observed in many other SnO₂ hollow or porous nanostructures.^8–17 Although we also observed a significant irreversible capacity loss after the first cycle, such behavior virtually is common to all systems based on alloy or conversion reactions. Nonetheless, a perfect reversibility of the capacity was still obtained during the following cycle test, with an average columbic efficiency of higher than 99% for up to 50 cycles after the second cycle (Fig. 6c).

Fig. 6c presents the cycling performance of the hollow SnO₂ microtubes electrode and the commercial SnO₂ materials with a voltage window of 0.01–1.2 V at a current rate of 1 C. It can be seen that the cycling of hollow SnO₂ microtubes is quite stable, which exhibits a high discharge capacity of 598.6 mA h g⁻¹ after 50 cycles that retains 84% of the capacity in the second cycle. The capacity fading per cycle from 2^nd to 50^th is 0.32% under the current density of 1 C. We also examined the morphology of the hollow SnO₂ microtubes electrode after 50 cycles. The morphology of the hollow microtubes and nanosheet shell can be basically preserved during the insertion/extraction process of the lithium ions ever under the rate of as high as 1 C, indicating a high structural stability of the materials (Fig. S7 in ESI†). For comparison, the lithium storage performance of commercial SnO₂ materials was also studied. It can be clearly seen from Fig. 6c that great capacity fading occurs during the test cycles in the case of commercial SnO₂ materials electrode. After 50 cycles, the discharge capacity only remained 161 mA h g⁻¹ (52% of discharge capacity in the second cycle). When the cycling test was prolonged to 100 times, the hollow SnO₂ electrodes still deliver a discharge capacity of about 400 mA h g⁻¹, which is higher than the theoretical capacity of graphite and commercial SnO₂ power electrode (Fig. S8†). From these results, we can see that compared with the commercial SnO₂ materials electrode, the hollow SnO₂ microtubes electrode exhibits significantly improved capability.

The cycling performance at various current rates is also evaluated at the same voltage window, which is important for practical applications. Fig. 6d shows representative discharge–charge profiles of SnO₂ microtubes at current rates of 1 C, 2 C, 3 C, 4 C and 5 C. Benefiting from the unique structure, the hollow SnO₂ microtubes electrode exhibits exceptional cycling response to a continuously varying current rate. Similar to most of reports, the specific capacity slightly decreases as the current density increases. Even at the current rate as high as 5 C, it still deliver a discharge capacity of above 480 mA h g⁻¹ with nearly 100% coulombic efficiency. More importantly, the specific capacity of the electrode at current rate of 1 C could recover to the initial reversible value after the high-rate measurements, indicating the excellent rate capability of the hollow SnO₂ microtube architectures. To demonstrate the great advantage of this structure for LIBs, the rate capability of commercial SnO₂ materials is also investigated under the same conditions as shown in Fig. 6d. Compared to the hollow SnO₂ microtubes, the commercial materials deliver the specific capacity as low as 19 mA h g⁻¹ at a current rate of 5 C, which probably is as a result of sluggish ionic adsorption/diffusion kinetics and poor electronic conductivity.

To further comprehend the improved electrochemical performance of the hollow SnO₂ microtubes, we carried out electrochemical impedance spectroscopy (EIS) analysis. Both Nyquist plots consist of one depressed semicircle at high frequency region and a straight line at low frequency region (Fig. 7). The high frequency region can be attributed to the charge transfer process, and the value of semicircle diameter gives an approximate indication of the charge transfer resistance. The diameter of semicircle at high frequencies is remarkably reduced in the plot of the SnO₂ microtube electrode, compared with that of commercial SnO₂ electrode, indicating the greatly decreased charge-transfer resistance at the electrode/electrolyte interface. The enhanced conductivity facilitates the electron and Li⁺ transfer in the electrode.

Fig. 7 The electrochemical impedance spectroscopy analysis of the hollow SnO₂ microtube and commercial SnO₂ electrodes.

An effective strategy is proposed for the improved Li-storage of SnO₂ anode through engineering nanosheets-assembled microtubes. This superior electrochemical performance of the as-synthesized hollow SnO₂ microtubes could be ascribed to their special structure, that is, hollow cavity, porous wall and hierarchical network, which not only provides short Li ion and electrolyte pathways and high electronic, ionic conductivity, and but also be able to accommodate large volume variation. All these factors mentioned above are responsible for the stable electrochemical performance of hollow SnO₂ microtubes. However, we also know that the present cycling and rate performance are still far from the demands in their practical application. In the future, several effective strategies could be proposed to improve the electrochemical performance, including (1) alloying Sn with other metal elements, such as Sb, Cu;^54,55 (2) hybrizing the SnO₂ nanoparticles with carbon materials;^56,57 (3) designing unique micro- and nano-architectures.^32,58–60

4. Conclusions

In summary, an effective method was employed to prepare hollow SnO₂ microtubes by using cotton as self-sacrificing template. The as-synthesized microtubes retained the 1D morphology of cotton fibers, and the wall of each microtube was composed of interconnected SnO₂ nanosheets. The cotton used in our experiments acts as self-sacrificing template, around which the SnO₂ nanosheets are formed, thereby passing on its highly porous and hollow structure and influencing the overall dimensions of the inorganic materials. When evaluated as an anode for LIBs, the hollow SnO₂ microtubes manifest significantly improved capacity retention. It exhibits a stable reversible capacity of 598.6 mA h g⁻¹ at a current of 1 C, and capacity retention keeps over 99% after 50 cycles. The hollow and porous structure offers a sufficient void space, which effectively alleviates the mechanical stress caused by volume change. The nano-scale sheets ensure fast Li-ion and electrolyte diffusion in the electrode. Our strategy is simple, cheap and mass-productive, which can be extended to the synthesis of other hollow materials for LIBs as well as gas sensors.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21471016 and 21271023) and the 111 Project (B07012).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra02452h

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