A free-standing, flexible and bendable lithium-ion anode materials with improved performance

Abstract

Bendable, flexible and self-supported anode materials with excellent electrochemical properties have highly attractive for the high performance lithium-ion batteries (LIBs). We report a simple process to prepare a three dimension structure composed of carbon and SnO₂ nanoparticles (CGN/SnO₂). The non-woven cotton covered with conducting graphene is selected as a flexible and bendable skeleton. Ultrasmall SnO₂ nanoparticles with a size of several nanometers uniformly anchor on both the graphene sheets and cotton fibers. The CGN/SnO₂ composites exhibited a higher Li-battery performance than carbonized cotton/SnO₂ (C/SnO₂) and carbonized cotton covered by glucose/SnO₂ (CG/SnO₂). Especially, the CGN/SnO₂ composites after bending 1000 times can maintain the same lithium storage property as before. Combining with this unique flexible, bendable structure, and the outstanding electrical and electrochemical performance, we demonstrate the great potential of CGN/SnO₂ composites in high performance Li-ion battery field and the flexible, foldable and stretchable conductors application.

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1. Introduction

An important trend of the advanced anode materials in rechargeable lithium-ions batteries (LIBs) for portable electronics and hybrid electric vehicles applications is mechanical flexibility, stability, simple and reproducible preparation process with high energy capacity and power density. Recent studies on the flexible anode materials have been reported.^1–9 Among them, one useful method is to get mixture of the active materials, conductive carbon and binder on a flexible current collector, and then to prepare the anodes.^1,10 However, this method is complicated and expensive, and some detrimental effect between electrolyte and inactive materials inevitably occur, thus influencing the stability of the electrode.

A major scientific challenge for the flexible anode materials with stable performance is developing the self-supported anode materials without polymer binders, carbon conductive and collectors. Some approaches have been exploited including graphene paper,¹¹ graphene/CNT¹² and graphene/SnO₂ (ref. 13 and 14) composites. Liang et al. reported that they used a simple filtration method and following a thermal reduction to prepare the flexible free-standing graphene/SnO₂ nanocomposites with the excellent electrochemical performance for electrode.¹³ Sun et al. vacuum-filtrated graphene oxide and CNT mixtures, and then annealed in Ar–H₂ atmosphere to prepare graphene–CNT hybrid papers.¹² Additionally, they prepared the test samples without the carbon additives, binders and metal current collectors, which would be different from the conventional slurry-casting method to prepare the LIBs anode.^2,15,16 Including the essential prerequisite of flexible and wearable electronic devices. It is important to investigate the electrochemical properties stability before and after bending many times.^17–19 So, to prepare the flexible anode materials keeping the flexible characterization with a simple technology, obtaining the excellent and stable electrochemical performance, especial before and after bending testing still remains a great challenge.

Herein, we report a rational strategy to synthesize well-dispersed ultrasmall SnO₂ particles decorated on 3D non-woven cotton covered by graphene (CGN/SnO₂) as an advanced anode material. The products are prepared by a simple in situ and thermal annealing process. The SnO₂ nanoparticles with several manometers in size homogenously anchor on the graphene sheets, and act as spacers to keep the graphene sheets separate with each other. The graphene sheets can be firmly adsorbed on the cotton fibers to protect the cotton fibers in thermal annealing process. Furthermore, the graphene sheets keep the 3D composite conductivity and the excellent flexibility during the SnO₂ nanoparticles crystallization process. This unique CGN/SnO₂ composite electrode used as free-standing, binder-free anode excluding the current collectors, exhibits the excellent electrochemical performance. Especially, before and after bending many times, this anode material still remains a high reversible capacity, excellent cycling and a high rate capability. This work paves a new way in preparing the flexible anode materials for the flexible and wearable electronic devices application.

2. Experimental section

2.1 Materials synthesis

Graphene oxide (GO) was synthesized by Hummers method. 1 g of graphite powder and 0.5 g NaNO₃ were added to 40 mL of H₂SO₄ solution and left stirring for 30 min at ambient temperature. Then 3 g of KMnO₄ was slowly added, and stirring was continued for 4 h in the ice water. After increasing the temperature to 100 °C, 100 mL distilled water was added following the 30 min stirring. Finally, 10 mL H₂O₂ was added and the dispersion colour became golden yellow. For purification, the resulting mixture was centrifuged rinsed first with 5% HCl solution and then with deionized water three times. Aqueous dispersion of GO with 7 mg mL⁻¹ was prepared by stock dispersion. After GO dispersion bath ultrasonic for 1 h, the non-woven cotton piece was immersed into the GO dispersion for 30 min. The products were subsequently frozen at −50 °C in cryogenic refrigerator for 24 h, and freeze-drying for 48 h to obtain cotton/graphene oxide (CGO) composite. For comparison, the non-woven cotton piece was immersed into a 0.5 M glucose, and then dried for 12 h at 60 °C to obtain cotton–glucose composite.

In a typical synthesis of carbon/SnO₂ (C–SnO₂) composite, 3 g SnCl₂·2H₂O was dispersed in 100 mL ethanol and stirred for 4 h at 60 °C. Then the non-woven cotton piece, CG and CGO composites were immersed in the above solution for 24 h at room temperature. Afterward, the non-woven cotton piece, cotton–glucose and CGO composites were taken out and dried for 12 h at 60 °C. Finally, cotton/SnO₂ (C/SnO₂), cotton glucose/SnO₂ (CG/SnO₂), and cotton covered by graphene/SnO₂ (CGN/SnO₂) composites were obtained by a tube furnace in N₂ atmosphere for 1 h at 500 °C. For comparison, the non-woven cotton piece, cotton–glucose and CGO with same annealing treatment were carried out to prepare carbonized cotton, carbonized cotton–glucose (CG), and carbonized cotton/graphene (CGN) composite.

2.2 Characterization

Crystallographic information of the as-prepared composites was characterized by X-ray diffraction (XRD) machine (D&A25ADVANCE, BRUKER, Germany), using Cu Kα radiation (λ = 0.15418 nm). The microstructures of the as-prepared samples were observed by scanning electron microscope (SUPRA™55, ZEISS, United Kingdom). Transmission electron microscopy (TEM) images and the corresponding selected-area electron diffraction (SAED) pattern were captured on a microscope (Tecnai G2 F30, FEI, America). The thermal stability, using thermogravimetric analysis (TGA) (TGA/SDTA851^#, METTLER TOLEDO, Switzerland) was carried on a heating rate of 10 °C min⁻¹ in air from room temperature to 800 °C. The microstructures were investigated by Raman spectroscopy (inVia, Renishaw, England) with an excitation wavelength of 532 nm from 800 to 2000 cm⁻¹.

2.3 Electrochemical measurement

All the samples were cut into discs (10 mm in diameter) as the working electrodes without any additional conductive agent, binder and current collector. The weight and thickness of electrodes were 2.0–2.2 mg and 100–120 μm for C/SnO₂, 2.2–2.4 mg and 100–120 μm for CG/SnO₂, and 3.9–4.1 mg and 140–160 μm for CGN/SnO₂, respectively. The electrolyte was 1 mol L⁻¹ solution of LiPF₆ in a mixture of ethylene carbonate (EC)–diethyl carbonate (DMC) (1 : 1, vol%). All the cells were assembled in an Ar-filled glove box with the C/SnO₂, CG/SnO₂, and CGN/SnO₂ electrodes as the working electrode and lithium foil as the counter electrode. Galvanostatic discharge and charge were measured at current density of 260 μA cm⁻² between 0.001 and 3 V using a battery test instrument (LAND CT2001A model, Wuhan SHENGLAN, China). Cyclic voltammogram (CV) tests were conducted on the workstation (CHI760E, Chenhua, China) at a scanning rate of 0.5 mV s⁻¹ in a potential range of 0.005–3 V. A three-electrode cell was used for electrochemical impedance spectroscopy (EIS) test over a frequency range from 100 kHz to 0.01 Hz, where lithium foil acts as both the counter and reference electrodes. All the electrochemical tests were conducted at room temperature.

3. Results and discussion

The fabrication process of the CGN/SnO₂ composite is illustrated in Fig. 1a. The non-woven cotton, a continuous structure with an interconnected 3D scaffold, is selected as the skeleton to prepare the self-supporting and flexible anode materials. Due to the electrostatic interaction, van der Waals' force and hydrogen bond exist between the oxygen-containing groups on GO and cotton.^20,21 The non-woven cotton is homogenously coated with GO immersing into GO aqueous suspension (Fig. S1†). Difference from other reports using oven dry technique,^20,22 the freeze-drying technique is chosen to remove water from a frozen sample by sublimation and desorption under vacuum, maintaining the non-woven cotton original fluffy structure in the CGO. The colour of non-woven cotton changes from white to brown as shown in Fig. 1b and c. When the CGO is introduced into SnCl₂ solution, the Sn²⁺ bonds with the oxygenated groups of graphene oxide and cotton fibers by electrostatic forces with the oxygenated groups. And then, the SnO₂ nanoparticles could nucleate and simultaneously grow in situ on the graphene sheets and cotton fibers. As shown in Fig. 1d, after about 10 min, the colour of the CGO changes from brown to black, indicating the reduction of a certain degree of GO by Sn²⁺. To obtain the CGN/SnO₂, a thermal reduction under N₂ atmosphere is critical to prepare the free-standing CGN/SnO₂. The composites keep the flexible characterization and the SnO₂ nanoparticles homogeneously decorate on the both sides of the graphene and the surface of the original cotton fibers, although a small shrinkage of the non-woven cotton skeleton occurring (Fig. 1e). The CGN/SnO₂ is completely recovered back to its original shape with no mechanical failure even under the excessive bending and warping (ESI Movie†).

Fig. 1 Schematic illustration of synthesis of the CGN/SnO₂ (a), photograph of non-woven cotton (b), CGO (c), non-woven covered by graphene with SnO₂ (d), and CGN/SnO₂ (e).

Fig. 2a shows the XRD patterns of above C–SnO₂ composites. All the XRD patterns display several main diffraction peaks, which could be indexed to the faces of (110), (101), (200), (111), (211), (220), (002), (310), (112), (301) and (321) corresponding to the rutile SnO₂ (JCPDS card no. 41-1445).^23,24 No peaks of possible impurities, such as SnO or Sn, indicating the high purity of the obtained samples. The absence of carbon peaks suggests that carbonized cotton with the lower order structure, and graphene layer is exfoliated completely.^23,25 Fig. 2b–d show the Raman spectra of C–SnO₂ composites with different carbon framework. The Raman spectra in the region 800–2000 cm⁻¹ show the characteristic peaks of carbonaceous materials. The D peak around 1340 cm⁻¹ is associated with the structural defects in the graphitic plane. The G peak around 1580 cm⁻¹ is attributed to the sp² bonded graphitic carbons.^26,27 The G peaks of the C/SnO₂ (1587.6 cm⁻¹), CG/SnO₂ (1586.2 cm⁻¹) and CGN/SnO₂ (1584.8 cm⁻¹) have blue shifts comparing with carbonized cotton (1585.7 cm⁻¹), CG (1582.5 cm⁻¹) and CGN (1583.8 cm⁻¹), respectively, revealing the p-type doping effect on carbon framework.^28,29 The p-type doping effect on carbon framework discloses the electronic interactions between SnO₂ nanoparticles and carbon framework, which facilitates the formation of desirable 3D electronic networks and favours electron transportation between SnO₂ and carbon framework.³⁰ The area intensity ratios of D peak to G peak (I_D/I_G) of C/SnO₂ (2.07), CG/SnO₂ (1.95) and CGN/SnO₂ (1.91) are larger than cotton (1.93), CG (1.84) and CGN (1.76), respectively. Suggesting doping SnO₂ could decrease the average size of the sp² domains upon the carbon framework.³¹ The higher I_D/I_G value indicates the smaller carbon size and more disorder structure of carbon framework.^32,33 Moreover, the disorder structure is caused by vacancies and topological defects, which decreases in the thermal stability of materials at high temperatures. The cotton covered by graphene framework has the lowest value of I_D/I_G, indicating the graphene can decrease the production of vacancies and topological defects, and increase thermal stability.

Fig. 2 XRD patterns of C/SnO₂, CG/SnO₂ and CGN/SnO₂ (a), Raman spectra of carbonized cotton and C/SnO₂ (b), CG and CG/SnO₂ (c), CGN and CGN/SnO₂ (d) composites, respectively.

The morphology and microstructure of the as-synthesized C–SnO₂ samples are examined by SEM. The non-woven cotton, as a self-supporting skeleton, mainly consists of flower-shaped hub cotton fibers with diameters ranging from 10 to 20 μm (Fig. S2†). After immersing in SnCl₂ solution and carbonization, the morphologies of the C/SnO₂ cotton fibers have no obvious changes, but some aggregation bulks on the fibers surface can be easily observed (Fig. S3a†). The high magnification SEM image and corresponding EDX mapping show the aggregation bulks mainly consist of Sn and O element from SnO₂ (Fig. S3b–e†). For the cotton fibers covered by glucose, as shown in Fig. S3f–j,† nanosized SnO₂ particles with a much lesser degree of aggregation are uniformly deposited on the surface of CG framework. Glucose is a hydroxyl-enriched molecule, and a plenty of –OH would be available to facilitate the SnO₂ crystallization, which would obstruct the particles aggregation.³⁴ The preparation process shows great versatility in controlling both the macrostructure and the microstructure of CGN/SnO₂ composites. From Fig. 3a, the graphene coating exists not only on the surface of the fibers but also filling the gap. The graphene is an effective connection bridge between the adjacent cotton fibers. The high magnification image (Fig. 3b) can clearly illustrate graphene wraps on the surface of cotton fiber, and many graphene wrinkles connect with each other. Moreover, through the cross section of CGN/SnO₂ fibers, graphene wrinkles with hollow structures can be observed (Fig. S4a and e†), which makes a lot of connecting pipes. Those structures greatly increase the electrode/electrolyte contact areas, and accelerate the electron and Li ions transport speed. Neither bulks nor particles can be observed on the CGN surface. Further investigation of EDS mapping shown in Fig. 3c–e indicates that all C, Sn and O elements are homogeneously distributed on the CGN framework. The as-formed ultra-small SnO₂ nanocrystals may act as an active matrix to stabilize the structure. Compare with glucose, GO has more oxygen containing groups, such as hydroxyl, epoxy, carbonyl and carboxylic on sheets. This oxygen groups not only promote the connection with cotton fibers, but also are used to decorate and separate ultrasmall SnO₂ nanoparticles.³² Element mappings of the CGN/SnO₂ in cross section reveal the uniformly distribution of C, Sn and O elements (Fig. S4b–d†). We can reasonably conclude that the Sn²⁺ diffuses inside the cotton fiber and forms SnO₂ nanoparticles. The SnO₂ nanoparticles with ultrasmall sizes on CGN framework can greatly shorten the solid-state diffusion distance of Li ions, leading to faster charge transfer reaction at the electrode/electrolyte interfaces.^35,36

Fig. 3 SEM images of the CGN/SnO₂ with low (a), and high magnification (b), EDS mapping images of C (c), Sn (d), O (e) elements.

Furthermore, the morphology and crystal structure of the C–SnO₂ samples are characterized by TEM and HRTEM. As shown in Fig. 4a and c, some large particles or particles agglomerations are deposited on the fiber of C/SnO₂ and CG/SnO₂. The ring-like mode in the selected-area electron diffraction (SAED) pattern (inset images of Fig. 4a and c) and HRTEM images (Fig. 4b and d) confirm the presence of polycrystalline SnO₂ with a tetragonal rutile-like crystal structure. It may be caused by less attractive force between cotton or glucose and Sn²⁺, and the particles are more likely to deposit or aggregate on the fibers surface. The aggregation of the SnO₂ particles will be further reunited during Li ions insertion/extraction, causing volume expansion and capacity fading. The TEM image and SAED pattern of CGN/SnO₂ in Fig. 4e and g show that the cotton fibers and graphene sheets are uniformly covered by ultrasmall SnO₂ nanoparticles. Before test, the composite is treated under a strong ultrasonic treatment for preparing the TEM samples. The SnO₂ nanoparticles can be firmly decorated on the CGN framework, indicating the SnO₂ good contact with CGN. The HRTEM image shown in Fig. 4h reveals graphene sheets with multiple layers. Ultrasmall SnO₂ nanoparticles with several nanometers in size are uniformly dispersed on the cotton and graphene sheets (Fig. 4f and h). Clear lattice fringes with d spacing of 0.21 and 0.33 nm are observed, which can be attributed to the (210) and (110) planes of rutile SnO₂, respectively.³⁷ The graphene has a beneficial effect on the nucleation, growth and formation of ultrasmall SnO₂ nanoparticles with a uniformly mono-dispersion on cotton fibers and graphene layers. The nanostructures of SnO₂ are helpful for the electrolyte permeating into the inner SnO₂ particles and further increasing the contact areas of the electrode/electrolyte. In this way, more active materials could effectively participate in the faradic reactions at the interface and significantly improve the specific capacity.^35,37 The theoretical specific capacity of the free-standing C/SnO₂, CG/SnO₂ and CGN/SnO₂ composite can be calculated by TGA (Fig. S5†). The SnO₂ content in the C/SnO₂, CG/SnO₂ and CGN/SnO₂ determined by TGA is determined to be 40.11 wt%, 36.11 wt% and 45.80 wt%, and the carbon content is 59.89 wt%, 63.89 wt% and 54.20 wt%, corresponding to the theoretical specific capacity of 403.0, 340.8 and 454.0 mA h g⁻¹, respectively. The specific capacities of carbonized cotton, CG, and capacity = (carbon content × related capacity + SnO₂ content × 782 mA h g⁻¹). The specific capacities of carbonized cotton, CG, and CGN are shown in Fig. S6.†

Fig. 4 TEM images of C/SnO₂ (a), CG/SnO₂ (c), CGN/SnO₂ cotton part (e), and CGN/SnO₂ graphene part (g), and HRTEM images of C/SnO₂ (b), CG/SnO₂ (d), CGN/SnO₂ cotton part (f), and CGN/SnO₂ graphene part (h).

The sheets resistance of fabrics were measure using a standard four-point probe method in previous reports, which should be called the surface resistance.^20,38 Because the non-woven cotton is fluffy, it is hard to use four-point probe method test the resistance. We test the bulk resistance of the flexible bulk CGN/SnO₂ materials resistance using two copper electrodes in the upper and lower position as shown in Fig. 5a. According to the equation: ρ = RS/L, the R, L and S are the resistance, the depth of carbon chip and the area of the carbon chip, respectively. The calculated resistivity of C/SnO₂, CG/SnO₂ and CGN/SnO₂ is 5.26 × 10⁹, 6.01 × 10⁴ and 3.31 × 10² Ω cm, respectively (Fig. 5b). Compared with the C/SnO₂, the resistivity of the CG/SnO₂ decreases five orders of magnitude due to the cotton covered by a carbon film after heat treatment. Compared with CG/SnO₂, the CGN/SnO₂ resistivity is even smaller than two orders. There may be three possible reasons for the improved conductivity. First, graphene with excellent electrical conductivity coating on the cotton fibers is an important reason for improving the resistance of material. Secondly, the change in the contacts modes between fibers is another important factor for improving the electrical properties. In the C/SnO₂ and CG/SnO₂, the fibers contacts with each other should be point to point. However, in the CGN/SnO₂, graphene acting as a buffer layer covers on the cotton fibers to increase the contact area with the contact mode of surfaces to surfaces, like Fig. 5c. Third, as shown in Fig. 5d, for those parallel fibers, there are no direct contact with each other, so the electron transport kinetics of the C/SnO₂ and CG/SnO₂ composite can only be in one direction. In contrast, for the CGN/SnO₂ composite, graphene sheets fill in the cotton fibers interspace to connect the parallel fibers, resulting in the electron transportation in any dimension with increased conductivity. As can be seen from Fig. 5b, the resistance of the CGN/SnO₂ even after bending 1000 times is still low, which will have a positive influence on the electrochemical performance.

Fig. 5 Schematic of bulk resistance test system (a), resistance of C/SnO₂, CG/SnO₂ and CGN/SnO₂ composites (b), contact fibers schematic of C/SnO₂ and CG/SnO₂ (upper), and CGN/SnO₂ (lower) (c), parallel fibers schematic of C/SnO₂ and CG/SnO₂ (upper), and CGN/SnO₂ (lower) (d).

To identify the electrochemical process and then assembled into coin cells as a free-standing and binder-free working electrode without any additional conductive agent, binder or current collector. The CV measurement is performed as shown in Fig. 6. The CG/SnO₂ and CGN/SnO₂ have the similar CV curves (Fig. 6b and c). In the first cycle, two well defined cathodic peaks around 0.5 V and 0 V are observed. The peak around 0.5 V corresponds to the solid electrolyte interface (SEI) formation and the reductive transformation of SnO₂ to Sn (4Li⁺ + SnO₂ + 4e⁻ = Sn + 2Li₂O).³⁹ This cathodic peak shifted to higher voltages (around 1.0 V) in the subsequent cycles. The CG/SnO₂ cathodic peak is more intensive than CGN/SnO₂, which indicates to produce more SEI layer. The other cathodic peak nearly 0 V can be attributed to the formation of alloys (xLi⁺ + Sn + xe⁻ = Li_xSn) and lithium reaction with carbon framework (xLi⁺ + C + xe⁻ = Li_xC).⁴⁰ In the anodic process, two oxidation peaks around 0.7 and 1.1 V are observed, representing the dealloying process of Li_xSn and partially reversible reaction of SnO₂.³⁵ There are no obvious SnO₂ lithiation process peaks appears for C/SnO₂ composite, only lithium reaction with carbon oxidation and reduction peaks in the CV curve (Fig. 6a), indicating the SnO₂ of C/SnO₂ contribute for lithium storage is negligible. From second cycle onwards, the CV curves of CG/SnO₂ and CGN/SnO₂ mostly overlap, indicating the good reversibility of the electrochemical reactions.

Fig. 6 CV curves of C/SnO₂ (a), CG/SnO₂ (b), and CGN/SnO₂ (c) composites.

To further comprehend the electrochemical performance of as-prepared samples, galvanostatic discharge/charge measurements are investigated at the current density of 260 μA cm⁻² in the potential windows of 0.001–3.0 V. The typical discharge/charge curves in first cycle are shown in Fig. 7a, the CGN/SnO₂ composites deliver a capacity of 5848.0 μA h cm⁻² for the first discharge and a reversible capacity of 3087.9 μA h cm⁻². The first coulombic efficient of CGN/SnO₂ is calculated to be 52.8%, higher than that of C/SnO₂ (16.1%) and CG/SnO₂ (36.2%). The high capacity can be attributed to a synergetic effect between CGN and the SnO₂ in the 3D framework, where Li ions are interspacing between neighbouring SnO₂ nanoparticles,⁴¹ and graphene sheets are the most candidate to consume large lithium to form much SEI.⁴² The initial irreversibility of the electrode composite could be due to SEI formation and reduction of SnO₂ to Sn, as the same analysis with CV.^40,43,44

Fig. 7 Charge–discharge curves (a), cycling performance (b), EIS impedance (c) of C/SnO₂, CG/SnO₂ and CGN/SnO₂ composites; original and bending 1000 times of CGN/SnO₂ rate test properties (d).

The cycling performance of the C/SnO₂, CG/SnO₂ and CGN/SnO₂ samples at a current density of 260 μA cm⁻² is shown in Fig. 7b. The CGN/SnO₂ showed the best electrochemical performance, delivering a high reversible capacity of 2512.5 μA h cm⁻² after 60 cycles, which is about 80.0% capacity retention compared with that in the second cycle. It is worth to note that from the third cycle onwards, the coulombic efficient is higher than 97%, demonstrating that the CGN/SnO₂ composite of quick reaching stable cycling. The specific capacity of CGN composite is only 307.5 μA h cm⁻² after 60 cycles (Fig. S7†), indicating the high capacity of CGN/SnO₂ is major contribute from SnO₂ nanoparticles. The SEM image in Fig. S8(c)† demonstrate that the CGN/SnO₂ maintains their overall morphology after cycling. The CG/SnO₂ composites show higher than 3000 μm A cm⁻² in first cycle, however, relatively capacity drastic decay to 275.2 μm A cm⁻² after 60 cycles, only 23.1% capacity retention compared with the second cycle. The loss of capacity after extended cycling may be due to the severe volume variation of the aggregated particles and electrode pulverization.⁴⁵ As seen from Fig. S8b,† the CG/SnO₂ electrode materials become small breaks after cycling, this is caused by huge volume expansion/contraction up cycling.^46,47 Although the C/SnO₂ have a good cycling stability, but the reversible capacity is only 294.9 μA h cm⁻² after 60 cycles, which is much lower than that of CGN/SnO₂. As shown in Fig. S8a,† there is obvious deformation on the cotton fibers, where SnO₂ aggregation bulks have been split, so the storage capacity is only contributed from carbonized cotton framework. In another point, the CGN/SnO₂ composites areal capacity is much higher than that of materials reported, such as CoO nanowires/carbon cloth (1950 μA h cm⁻² after 90 cycles),⁴⁸ SnO₂@Si nanowire/carbon cloth (1390 μA h cm⁻² after 50 cycles),⁴⁹ Cu–Sn nanowires (70 μA h cm⁻² after 100 cycles),⁵⁰ Si-CNT nanocomposites (1000 μA h cm⁻² after 50 cycles),⁵¹ CoNiO nanowire/TiO₂ nanotubes (360 μA h cm⁻² after 40 cycles),⁵² SnO₂/Fe₂O₃ nanotubes (730 μA h cm⁻² after 50 cycles),⁵³ and carbon-coated porous silicon anodes (180 μA h cm⁻² after 70 rate cycles).⁵⁴

To better understand the beneficial effect of composites, the electrochemical impedance measurements of C/SnO₂, CG/SnO₂ and CGN/SnO₂ are performed. The spectra are shown in Fig. 7c, and a simplified equivalent circuit model is constructed to analyze the impedance spectra shown inset of Fig. 7c. It is worth to note that the CG/SnO₂ and CGN/SnO₂ have two semicircles, a high-frequency semicircle and a medium-frequency semicircle. However, the C/SnO₂ only has one semicircle from high to medium frequency due to the high-frequency and medium-frequency semicircle partially coincidence. The hard distinguish high-frequency and medium-frequency may be caused by the low conductive and SnO₂ bulks hard for electron transport and charge-transfer of C/SnO₂. An intercept at the Z_real axis in high frequency corresponds to the ohmic resistance (R_Ω), which represents the resistance of the electrolyte. The high-frequency semicircle is attributed to the Li-ion migration resistance (R_SEI) through the SEI film. The semicircle in the medium-frequency region is assigned to charge-transfer resistance (R_ct). The inclined line is associated with Warburg impedance (W) corresponding to the lithium-ion diffusion process. The equivalent series resistance, R_Ω and R = R_SEI + R_ct, for the CGN/SnO₂ anode is thus estimated to be 7.3 Ω and 119.1 Ω, which is much smaller that the CG/SnO₂ (12.1 Ω and 155.2 Ω) and C/SnO₂ (24.6 Ω and 169.1 Ω). The decrease in R resistance of CGN/SnO₂ electrode can be attributed to the high electronic conductivity of CGN and fine contact of SnO₂ nanoparticles on the CGN framework. This result is also in agreement with the superior cycling performance of CGN/SnO₂, which is beneficial for lithium ions insertion/extraction into the anodes under the high charge–discharge cycling conditions.^23,45,55,56

The rate properties of CGN/SnO₂ composite characterized at various current densities are shown in Fig. 7d. The discharge capacity after 5 cycles is 2060.4 μA h cm⁻² at a current density of 220 μA cm⁻². As obvious increase in current density from 440 to 3520 μA cm⁻², the discharge capacities of 1744.1, 1491.2, 1124.4 and 575.0 μA h cm⁻² are obtained, respectively. Even under such rigorous testing conditions, as the current density is reduced to 220 μA cm⁻², the capacity almost recovers to the original value, indicating the CGN/SnO₂ composite is quite suitable for large current charge and discharge. Of importance for the flexible electronic devices is that performance stability of the capacitive characteristic before and after bending many times for the samples. The CGN/SnO₂ composite is bent from 0° to 180° as shown in the video file,† and back to the initial state for 1000 times. The capacity shows a slight decrease at different current density after 1000 times bending (Fig. 7d). It is reasonable to conclude that the CGN/SnO₂ composite electrode should be a good candidate for textile flexible and wearable electronic devices applications.

The superior reversible capacity, excellent cycling, high coulombic efficiency, and good rate capacity performance of CGN/SnO₂ composite can be attributed to the following several merits: (1) the non-woven cotton covered by the flexible graphene acts as flexible 3D carbon supports serving as a buffer to release the stress during cyclic processes. In addition, the CGN framework showing the excellent electronic conductivity decreases the inner resistance of the LIBs, facilitate good transport of electrons ions, resulting in a higher specific capacity. (2) The ultrasmall sizes of the SnO₂ nanoparticles uniformly disperse on the graphene sheets and cotton fibers. Due to the short transport length and more accessible sites for the active materials, which improves the coulombic efficient, excellent cycling and high reversible capacity performance. (3) The SnO₂ nanoparticles tightly decorate on the cotton covered by graphene framework, which can significantly reduce the strain and agglomeration during the lithiation/delithiation processes. (4) The interconnected pipeline structure can provide large electrode/electrolyte contact area, and rapid mass and electron transport of Li ion, which facilitates the lithium diffusion kinetics and could lead to excellent rate capability. So it can be concluded that CGN/SnO₂ composites have great potential applicable for flexible electrochemical devices application.

4. Conclusions

In summary, using the non-woven cotton as a flexible and bendable skeleton, we developed a facile strategy to synthesize a self-supporting 3D structure of electrically conductive graphene anchored with SnO₂ nanoparticles as an advanced anode material for high performance LIBs. The graphene coating on the non-woven cotton protects cotton fibers and shows the excellent electronic conductivity, which facilitates electrons and electrolyte transport. The ultrasmall SnO₂ nanoparticles are uniformly anchored on the graphene sheets and cotton fibers, and reduce the strain and agglomeration in lithium ions insertion/extraction. Even after bending 1000 times, the capacitive performance of the CGN/SnO₂ composites is almost no obvious loss. Most importantly, this synthesize method presented in this work may be extended to other active materials with CGN framework for large-scale promising anode material in high-performance energy storage field.

Acknowledgements

This project was supported by the National Natural Science Foundation of China (51172050, 51102060, 51102063, 51302050, and 51372052), Shandong Province Young and Middle-Aged Scientists Research Awards Fund (BS2013CL003), and the Fundamental Research Funds for the Central Universities (HIT. ICRST. 2010009).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19347a

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