Sn/carbon nanofibers fabricated by electrospinning with enhanced lithium storage capabilities

Abstract

Sn/carbon nanofibers with well dispersed Sn nanoparticles embedded in carbon nanofibers were fabricated by an electrospinning process. A SnO₂ sol was used as the Sn precursor in order to improve the compatibility of tin/carbon composites and prevent Sn nanoparticles from severe agglomeration upon thermal treatment. When evaluated as a binder-free anode for lithium ion batteries, the Sn/carbon nanofibers deliver a high specific capacity of 583 mA h g⁻¹ after 100 cycles and good cycling performance with coulombic efficiency up to 99%. The high lithium storage capability is mainly attributed to the optimized structure features including the well dispersed tin nanoparticles, high porosity, and the cushion effect of the carbon nanofibers.

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Introduction

As a new type of power source, lithium ion batteries (LIBs) have been widely used in the market of portable electronic devices. However, their application to electric vehicles is still restricted due to the relatively low theoretical capacity (372 mA h g⁻¹) of the commercial graphite anode.^1,2 In recent years, tremendous efforts have been made to develop novel alternative anodic materials to replace conventional graphite.^3–6 Metallic tin has been considered as a promising candidate owing to its high theoretical specific capacity (990 mA h g⁻¹).^7,8 Nevertheless, tin suffers from the severe pulverization and irreversible capacity fading caused by the large volume effect during the charge–discharge process.^9–11

Several strategies have been proposed to improve the cycling performance of tin-based materials, including design of alloys,^12–16 fabrication of nanoscaled tin/carbon composites with various nanostructures.^17–31 Among these materials, tin-embedded carbon nanocomposites have attracted much attention because nanoscaled tin can provide higher Li⁺ diffusion ability and better withstanding of the volume change, while the carbon matrix can act as not only a conductive agent but also a physical buffering layer to relax the mechanical stress and protect tin from severe pulverization, thus leading to the enhanced reversible capacity and excellent cycling performance. For example, tin/carbon composite nanofibers have shown improved cyclability because with the confinement of flexible carbon nanofibers, serious agglomeration of tin is prevented and efficient charge transport is allowed for both Li ions and electrons.⁹

Recently, electrospinning has been widely used to fabricate anode materials consisting of tin/carbon composite nanofibers.^32–44 The electrospun tin/carbon nanofibers possess a porous structure with large surface area and high electrical conductivity, so that the electrochemical performance can be improved. In addition, electrospinning is very suitable to prepare binder-free electrode since the obtained composite fibers can directly form flexible, free-standing, and non-woven mats. Generally, the electrospinning procedure starts from a polymer solution containing Sn salts. The electrospun polymeric composite fibers are then carbonized by thermal treatment, during which Sn salts are converted into SnO₂ followed by the formation of metallic Sn through carbothermal reduction at high temperature. The problem existed in this process is that tin is thermodynamically incompatible with carbon and it is readily to agglomerate into large particles on the surface of the carbon nanofibers upon melting (the melting point of Sn is 210 °C).⁴³ Therefore, it is necessary to control the preparation process carefully in order to disperse nanosized Sn particles in the carbon nanofibers. Duan et al. proposed a facile electrospinning technology to synthesize Sn quantum dots finely embedded in N-doped carbon nanofibers.⁴⁴ The content of Sn in the composite was relatively low to ensure the high dispersity. Zhang et al. reported SnSb alloy-filled porous carbon nanofibers by electrospinning using a new antimony tin oxide precursor.^45,46 The SnSb nanoparticles with a good distribution in carbon nanofibers lead to high capacity, good cycling performance, and high rate capability when used as anode for lithium ion battery and sodium ion battery.

In this paper, well dispersed Sn nanoparticles embedded in carbon nanofibers were synthesized by electrospinning method. In order to improve the thermal stability of tin/carbon composites, a SnO₂ sol was first prepared before mixing with the polymer solution. The SnO₂ sol can enhance the adhesion between Sn and carbon so that the SnO₂ nanoparticles were firmly attached to the polymer nanofibers, even when the composite nanofibers were carbonized at high temperature. Therefore, tin nanoparticles can distribute homogeneously with a relatively high content in the carbon nanofibers. The obtained tin nanoparticle/carbon nanofibers were directly used as binder-free anode for lithium ion battery. The optimized structure features such as the well dispersed tin nanoparticles, large surface area, and the presence of carbon nanofibers as buffer layer endow the materials with enhanced lithium storage capabilities and good cycling performance.

Experimental

Materials

SnCl₂·2H₂O was purchased from Sigma-Aldrich (Shanghai, China). Polyacrylonitrile (PAN, Mw = 150 000) was purchased from Aladdin (Shanghai, China). All other chemicals were obtained from Sinopharm Chemical Reagent (Shanghai, China). All solvents and chemicals were of analytical grade and used without further purification.

Preparation of SnO₂ sol

1.68 g of SnCl₂·2H₂O was dissolved in 11 mL of ethanol and refluxed at 78 °C for 3 h. The mixture was cooled to room temperature and then aged for 24 h to obtain the SnO₂ sol.

Synthesis of Sn/carbon nanofibers

The Sn/carbon nanofibers were synthesized by electrospinning method. Typically, the above SnO₂ sol was firstly dissolved in 14 g of N,N-dimethylformamide followed by the addition of 1.28 g of polyacrylonitrile. The mixture was stirred at 50 °C for 8 h to form a homogeneous solution. The electrospinning process was carried out by pumping the solution through a 20 mL syringe fitted with a 12 gauge needle at a flow rate of 1.25 mL h⁻¹. The distance between the needle tip and the collector was 20 cm and the applied voltage was 28 kV. The obtained fiber networks were stabilized at 220 °C for 3 h in air and then carbonized at 700 °C under nitrogen for 2 h with a heating rate of 1 °C min⁻¹.

Characterization

The morphologies of the as-spun nanofibers and Sn/carbon nanofibers were observed using Hitachi S-4800 scanning electron microscope (SEM) and FEI Tecnai G20 Transmission electron microscope (TEM). The crystallization patterns were obtained by Rigaku D/max-2500PC X-ray diffractometer using Cu Kα radiation (λ = 0.1542 nm). The content of carbon in the composite nanofibers was determined by thermogravimetric analysis (TGA) using Netzsch STA409PC analyzer in air with a ramp rate of 10 °C min⁻¹. Nitrogen adsorption–desorption isotherms were obtained using Micromeritics ASAP 2020 analyzer. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method, while the pore size was obtained from the Barrett–Joyner–Halenda (BJH) model.

The electrochemical experiments were carried out using two-electrode Swagelok cells with pure lithium metal sheet as both the counter and the reference electrodes. The Sn/carbon nanofiber networks were directly used as the working electrode without any binder and conductive agent. 1.0 M LiPF₆ was used as electrolyte in a 1 : 1 (w/w) mixture of ethylene carbonate and diethyl carbonate. Cell assembly was performed in an Ar-filled glovebox with concentration of oxygen and moisture below 1.0 ppm. The charge–discharge tests were carried out using a NEWARE battery tester within a voltage window of 0.01–3 V. Cyclic voltammetry (CV) was conducted on a CHI660E electrochemical work station at a scan rate of 0.5 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) was also performed in the frequency range of 100 kHz to 10 mHz at a fixed perturbation amplitude of 5 mV.

Results and discussion

Fig. 1a–d shows the representative scanning electron microscopy images of the as-spun fibers and Sn/carbon nanofibers. Prior to thermal carbonization, the as-spun fibers are smooth with an average diameter of 300 nm (Fig. 1a and b). Upon carbonization at 700 °C, the morphology seemed to be similar to that before thermal treatment except that some fibers were broken (Fig. 1c and d). In addition, the nanofibers show a more uniform diameter in Fig. 1d because some of the fibers might be generated from the same parent fiber. No obvious large Sn particles were found on the surface of the fibers, indicating that Sn nanoparticles were well dispersed in the carbonized fibers. The SnO₂ sol might play an important role for the dispersity of Sn nanoparticles, since it can improve the adsorption affinity between the Sn precursor and carbon, which can prevent the Sn nanoparticles from severe agglomeration. Detailed microstructures of the Sn/carbon nanofibers were observed by transmission electron microscopy as shown in Fig. 1e. The Sn nanoparticles with diameter of 20–50 nm were located at both the outer surface and inner bulk of the carbon nanofibers. No large Sn particles were seen to be separated from the carbon nanofibers, which is consistent with the SEM results. Fig. 1f shows a high-resolution TEM (HRTEM) image of the Sn/carbon nanofibers. The lattice fringe with interplane spacing of approximately 0.28 nm corresponding to (101) plane of Sn is observed, indicating the body centered tetragonal crystal structure of Sn nanoparticles.

Fig. 1 (a and b) SEM images of the as-spun SnO₂/PAN nanofibers; (c and d) SEM images of the Sn/carbon nanofibers carbonized at 700 °C; (e and f) TEM and HRTEM images of the Sn/carbon nanofibers carbonized at 700 °C, respectively.

Fig. 2 shows the XRD pattern of Sn/carbon nanofibers carbonized at 700 °C. The sharp diffraction peaks at 30.6°, 32.0°, 43.8° and 55.7° can be assigned to (200), (101), (220) and (211) planes of tetragonal Sn. No signals of SnO or SnO₂ were detected, indicating that the SnO₂ sol can be completely transformed into metallic Sn by carbothermal reduction at 700 °C. The broad diffraction peaks around 25° are ascribed to the (002) plane of carbon, indicating the formation of amorphous carbon.

Fig. 2 XRD pattern of the Sn/carbon nanofibers.

Fig. 3 shows the thermogravimetric curve of Sn/carbon nanofibers in air. Two distinct weight loss steps were detected. The first weight loss between 30 and 96 °C is ascribed to the evaporation of adsorbed water molecules. The large weight loss in the range from 350 to 500 °C is resulted from the combustion of the carbon. The weight increase occurred between 560 and 800 °C is mainly due to the conversion from metallic Sn to SnO₂ in air. It can be calculated from the TGA curve that the content of Sn in the Sn/carbon nanofibers is about 51.0%.

Fig. 3 TGA curve of the Sn/carbon nanofibers in air with a heating rate of 10 °C min⁻¹.

Fig. 4 shows the nitrogen adsorption–desorption isotherm and pore size distribution curve of the Sn/carbon nanofibers. The materials possess a type IV isotherm with a hysteresis loop, which demonstrates that the nanofibers have a typical mesoporous structure. Such a porous structure exhibits a BET surface area of 498 m² g⁻¹ and a pore volume of 0.26 m³ g⁻¹. The plot (inset) shows a narrow pore size distribution around 3.5 nm according to the BJH method. The large nitrogen uptake at low relative pressure indicates that the nanofibers contain a large fraction of microporosity, which is formed during the carbonization of PAN. Such highly porous Sn/carbon nanofibers can provide a direct expressway for efficient Li⁺ transport between the electrolyte and the Sn nanoparticles encapsulated in the carbon nanofibers, leading to enhanced lithium storage capabilities when applied to the anode of lithium ion batteries.

Fig. 4 Nitrogen adsorption–desorption isotherm and pore size distribution (inset) of the Sn/carbon nanofibers.

In order to evaluate the lithium storage capabilities of the Sn/carbon nanofibers, the fiber networks were directly used as the working electrode without any binder and conductive agent. Fig. 5a shows the typical cyclic voltammetry curves of the electrode in the initial five cycles at a scanning rate of 0.5 mV s⁻¹ between 0 and 3 V. During the first cathodic scan, the broad reduction peak from 0.7 to 0.01 V is attributed to both the formation of solid electrolyte interface (SEI) film and the insertion of lithium into tin nanoparticles. In the subsequent cycles, the oxidation peak at 0.7 V is ascribed to the delithiation of the Li_xSn alloy. Note that this peak becomes stronger in cycles, indicating that the tin nanoparticles, which are enwrapped by amorphous carbon in the nanofibers, gradually make contribution to the capacity.³³ The peaks nearly overlap in the 4^th and 5^th cycle, implying that tin has been almost fully charged/discharged. The anodic peak around 1.3 V corresponds to the oxidation of Sn to SnO₂, which means that this reaction is partially reversible in this case.³⁷ All peaks are reproducible from the second cycle, demonstrating the highly reversible reaction of the Sn/carbon nanofibers electrode.

Fig. 5 (a) Typical CV curves of the Sn/carbon nanofibers at a scanning rate of 0.5 mV s⁻¹; (b) charge–discharge voltage profiles of the Sn/carbon nanofibers for the 1^st, 2^nd, and 100^th cycle at a current density of 200 mA g⁻¹; (c) cycling performance of the Sn/carbon nanofibers at a current density of 200 mA g⁻¹ and the coulombic efficiency; (d) rate capability of the Sn/carbon nanofibers at different current densities; (e) electrochemical impedance spectra of the Sn/carbon nanofibers before and after 100 cycles at 200 mA g⁻¹ in the frequency range from 100 kHz to 10 mHz.

Fig. 5b shows the galvanostatic discharge–charge voltage curves of the Sn/carbon nanofibers for the 1^st, 2^nd and 100^th cycles at a constant current density of 200 mA g⁻¹ within a voltage window of 0.01–3 V. Two obvious slope regions are shown in the first discharge profile. The fast voltage drop to 1.0 V is due to the decomposition of the electrolyte and formation of SEI film, while the slow voltage drop below 1.0 V is attributed to the lithiation of the carbon nanofibers and metallic Sn. Owing to the high lithium storage capacity of Sn, the Sn/carbon nanofibers deliver a high initial discharge capacity of 1217 mA h g⁻¹. In addition, the porous structure of the composite nanofibers is beneficial to shorten the diffusion length of Li ions and enable the electrolyte to reach the Sn nanoparticles rapidly, so that the Sn nanoparticles can be fully activated to achieve a high discharge capacity. The first charge curve shows a capacity of 986 mA h g⁻¹ with a coulombic efficiency of 81%. The irreversible capacity loss is mainly caused by the formation of SEI on the surface of Sn nanoparticles. For the 2^nd cycle, the composite nanofibers exhibit a discharge capacity of 1023 mA h g⁻¹ and a charge capacity of 962 mA h g⁻¹, showing a much higher coulombic efficiency of 94%. A relatively high discharge capacity of 583 mA h g⁻¹ is retained after 100 cycles with a coulombic efficiency of 99%, indicating excellent cycling stability of the Sn/carbon nanofibers.

Fig. 5c shows the cycling performance and the coulombic efficiency of the Sn/carbon nanofibers at a current density of 200 mA g⁻¹. It is found that from the 84^th cycle, the decrease of capacity becomes neglectable. As mentioned above, the Sn/carbon nanofibers possess a high reversible capacity of 583 mA h g⁻¹ after 100 cycles. The relatively good cycling performance is mainly ascribed to the advantages of well dispersed Sn nanoparticles with high lithium storage capability and the presence of the carbon nanofibers, which provide a physical buffering layer to accommodate the volume change of Sn during the lithiation and de-lithiation process and prevent Sn from severe pulverization.

To fully study the electrochemical performance of the Sn/carbon nanofibers, the electrode was cycled at 200 mA g⁻¹, 500 mA g⁻¹, 1000 mA g⁻¹, and finally 200 mA g⁻¹ for 20 cycles, respectively. As shown in Fig. 5d, the nanofibers show a good capacity retention at low rate. Upon cycling at high rate, the reversible capacity could be recovered to a high point of 598 mA h g⁻¹ when the current density was adjusted back to 200 mA g⁻¹, revealing that the Sn/carbon nanofibers can deliver a high specific capacity, good cycling performance and rate capability.

Furthermore, electrochemical impedance spectroscopy of the Sn/carbon nanofibers was carried out to explain the charge and contact resistance of the electrode, as shown in Fig. 5e. The impedance resistance of the electrode decreases after 100 cycles. The lack of electrolyte wetting in the electrode materials before cycling is responsible for the relative low conductivity. Moreover, the electrolyte needs to penetrate into the carbon layer before accessing the embedded Sn nanoparticles, so the formation of SEI over the active Sn is gradual.⁴⁴ The electrical conductivity can be eventually established when the stable SEI is formed, resulting in the decreased impedance resistance after 100 cycles.

Conclusions

In summary, well dispersed Sn nanoparticles embedded in carbon nanofibers were fabricated by electrospinning and subsequent carbonization process. SnO₂ sol was first prepared before electrospinning, which is effective to improve the compatibility of tin/carbon composites and prevent Sn nanoparticles from severe agglomeration upon thermal treatment. When used as binder-free anode for lithium ion battery, the Sn/carbon nanofibers deliver high specific capacity (583 mA h g⁻¹ after 100 cycles) and good cycling performance, owing to the optimized structure features such as the well dispersed tin nanoparticles, high porosity, and the cushion effect of the carbon nanofibers.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (no. 51673090, 51173074); the Key Project of Chinese Ministry of Education (no. 212099); the Promotive Research Funding for Young and Middle Aged Scientists of Shandong Province (no. BS2012CL010).

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