Electrospun In@C nanofibers as a superior Li-ion battery anode

Abstract

In this communication, we have successfully obtained electrospun indium@carbon (In@C) nanofibers. When applied as the Li-ion battery anode, In@C nanofibers could exhibit a high discharge capacity of 500 mA h g⁻¹ after 200 cycles at 100 mA g⁻¹.

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In the past decade, one-dimensional (1D) nanomaterials have been well developed for potential Li-ion battery electrodes,^1–3 since the 1D structures could provide electron pathways for electrochemical reactions during charge/discharge processes. However, the cycling life of batteries is greatly shortened by the volume change of electrodes due to the repeated lithiation/delithiation processes.^4–6 Exploring nanocomposites is proven to be an effective method to solve the volume-change problem.^7–10 Electrospinning techniques afford us a powerful tool to fabricate 1D carbon-based nanocomposite electrodes.^11,12 To date, some electrospun nanocomposite fibers have been used as Li-ion battery anode materials, such as MoO₃@C,¹³ GeO₂@C,¹⁴ NiO_x@C,¹⁵ Fe₂O₃/C nanofibers.¹⁶ Indium-based materials such as In₂O₃ and In₂S₃, have exhibited high theoretical capacities during high Li alloying with In.^17–21 Theoretically, element electrodes (Sb, Si, Ge, Sn, etc.) have higher theoretical capacities than their corresponding oxides or sulfides, because there exist irreversible reactions between Li₂O or Li₂S and Li for oxide or sulfide electrodes.^8,14,22–27 Thus, we explored the possibilities of In@C nanofibers as superior Li-ion battery anode in this work. Here, we have synthesized In@C nanofibers via electrospinning PVP and In(NO₃)₃ as carbon and In precursors, respectively, followed by a controlled heat treatment. The electrochemical results demonstrated that the as-synthesized In@C nanofibers could reach a discharge capacity of 500 mA h g⁻¹ after 200 cycles at 100 mA g⁻¹.

The overall synthetic procedure of In@C nanofibers in this work was as follows: (i) the In(NO₃)₃ precursor solution containing ethanol and N,N-dimethylformamide (DMF) and polyvinyl pyrrolidone (PVP) was first electrospun into nanofiber framework. (ii) The as-obtained electrospun nanofibers were heated at 200 °C in air for pre-oxidation. (iii) The pre-oxided sample was further carbonized and reduced to form In@C in an argon Ar/H₂ atmosphere at 600 °C. The In content in In@C nanofibers was confirmed to ∼48% by thermogravimetric analysis (TGA, Fig. S1†). In addition, the crystalline phase of In@C nanofibers could be well indexed to the peaks of pure In according to the standard card (JCPDS no. 85-1409), as shown in the pattern (Fig. 1).

Fig. 1 XRD patterns of In@C nanofibers.

Fig. 2 demonstrates the scanning electron microscopy (SEM) images of the In@C nanofibers. As shown in Fig. 2a, the as-obtained In@C nanofibers have a length of several micrometers and a diameter of around 80 nm. It is clearly seen that In@C nanofibers are interconnected with each other to form nanofiber networks. Fig. 2b indicates that the surface of the nanofibers is smooth, implying that In nanocrystals are encapsulated into the carbon nanofibers. The detailed morphological and structural features of the In@C nanofibers were further examined by transmission electron microscopy (TEM). Fig. 3a shows a typical TEM image of In@C nanofibers. No aggregated nanoparticles were found on the surface, and no breaking nanofiber were observed, further indicating that In nanoparticles were embedded into the carbon nanofibers. The presence and distribution of elements are further verified by Energy Dispersive Spectrometer (EDS) elemental mapping analysis, shown in Fig. 3b–d. The edge of indium and carbon EDS maps in Fig. 3c and d are well match the result shown in the STEM image (Fig. 3b), indicating that In and C elements are uniformly distributed throughout the nanofiber. Owing to the long nanofibers structure and amorphous carbon, the as-obtained In@C nanofibers are expected to exhibit superior electrochemical behavior towards lithium storage.

Fig. 2 (a and b) Low-magnification and high-magnification SEM image of the In@C nanofibers.

Fig. 3 (a) TEM image of the In@C nanofibers; (b) STEM image and (c and d) EDX-elemental mapping images of single In@C nanofiber (b).

Galvanostatic charge–discharge cycling, cycling stability measurements, and rate capability tests based on the standard half-cells of In@C nanofibers anodes were conducted to investigate the lithium storage properties. Fig. 4a shows the cycling voltammetry (CV) curves of In@C nanofibers. The first five cycles were tested from 0.01 V to 3 V vs. Li⁺/Li at a scan rate of 0.5 mV s⁻¹. During the first cycle, an initial irreversible capacity loss can be identified, and the reduction peak around 1.3 V is most probably can be formation of the solid-electrolyte interphase (SEI) layer or decomposition of electrolyte.^28,29 The cathodic peak which appear at around 0.5 V in the first cycle corresponded to the reversible reaction (1).³⁰ The two oxidation peaks at 0.5 and 0.7 V could be ascribed to the multi-step dealloying of Li/In which occurs during the anodic scan.

In + 4.33Li ↔ Li_4.33In (1)

Fig. 4 (a) CV profiles of In@C nanofibers at the scan rate of 0.5 mV s⁻¹; (b) charge–discharge curves of In@C nanofibers.

The discharge/charge profiles of In@C nanofibers at a rate of 100 mA g⁻¹ in the firstly five cycles was as shown in Fig. 4b. The initial discharge and charge specific capacities of In@C nanofiber anodes are 1033 and 610 mA h g⁻¹, respectively, corresponding to the coulombic efficiency of around 60%. There is about 40% capacity loss of the initial cycling capacity of the In@C, which is generally attributed to the irreversible formation of the SEI layers in the surface of the electrode due to electrolyte decomposition during discharge process.

A highly reversible and stable cycling performance is one of the most important factors in measuring lithium storage properties. Therefore, cycling measurements were conducted to characterize the lithium storage behavior of In@C nanofibers. Fig. 5a shows the cycling performances and coulombic efficiency curves of the synthesized materials at 100 mA g⁻¹ in the voltage range of 0–3.0 V. All reported capacities in this work are based on the total mass of In@C nanofibers. After 200 charge/discharge cycles, the In@C nanofiber electrode affords remarkable battery performance with over 500 mA h g⁻¹ capacity remained. From the coulombic efficiency curves, we calculated that the coulombic efficiency of In@C electrode is near 99% after 200 cycles. Rate capabilities of the samples up to 4000 mA g⁻¹ were also tested and the result is presented in Fig. 5b. As the current densities increase stepwise from 100 to 300, 500, 1000, 2000, and 4000 mA g⁻¹, the electrode delivers stable capacities at each of these rates, varying from 510 to 453, 410, 361, 310, and 248 mA h g⁻¹, respectively. When the current density finally returns to 100 mA g⁻¹, the charge capacity can recover to 485 mA h g⁻¹. Meanwhile, the In@C nanofiber electrode has an excellent cycling ability and stability at high current density, as shown in Fig. 5c. The reversible capacity of the In@C is over 350 mA h g⁻¹ at 4000 A g⁻¹ after 400 cycles. Such high cycling stabilities can be attributed to the fact that the carbon nanofibers encapsulating In nanocrystals can effectively limit the volume expansion and reformation of SEI on the outer surface instead of on individual nanocrystals.

Fig. 5 (a) Cycling performance of In@C within a voltage range of 0.01–3.0 V vs. Li/Li⁺ at a current density of 100 mA g⁻¹. (b) Rate capability of In@C between 0.01 and 3.0 V. (c) Cycling performance of In@C at a current density of 4 A g⁻¹, respectively. (d) Impedance Nyquist plots of the In@C electrodes at the 1^st and 200^th cycle.

In order to understand the reasons for the improved high rate performance, electrochemical impedance spectroscopy (EIS) measurements were carried out for the In@C electrodes at first and 200^th cycle at a current density of 100 mA g⁻¹. As depicted in Fig. 5d, every plot is characteristic of one semicircle in the high frequency region and a straight sloping line in the low frequency region. In general, the semicircle was attributed to the summation of the contact, the SEI and the charge-transfer resistance. At the first discharge/charge process, the In@C anode has smaller semicircle diameter. The In@C possessed the high electrical conductivity and the fast charge-transfer reaction for lithium ion insertion and extraction.

The excellent electrochemical performance of carbon nanofibers encapsulating In anodes might be related to their unique structural features in multiple advantages. First, the carbon nanofibers effectively accelerate the electron and ion transport. Second, the ultra-uniform carbon nanofibers encapsulating the nanocrystals can endure volume expansion during consecutive cycling test. Meanwhile, the carbon nanofibers effectively limit the formation of SEI films on the outer surface instead of on individual particles. Third, the carbon nanofibers can greatly improve the physical connection and electrical contact with the 1D frameworks, thereby maximizing the effective electrochemical utilization of the active materials and ensuring a reversible lithium intake/removal process even under high current density.

In conclusion, we have successfully synthesized In@C nanofibers by the electrospinning/annealing techniques. The as-obtained In@C nanofibers display a discharge capacity of 500 mA h g⁻¹ after 200 cycles at a current density of 100 mA g⁻¹ due to carbon encapsulate the In nanoparticle, which could contribute to high lithium storage capacity. The as-obtained In@C nanofibers are promising for advanced battery application in the future.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 51302079).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17681f

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