Facile synthesis of graphene supported FeSn₂ nanocrystals with enhanced Li-storage capability

Abstract

A novel type of graphene supported tin-based alloy, i.e. graphene supported FeSn₂ nanocrystals (G–FeSn₂ nanohybrid), has been designed and synthesized through a chemical reduction route in a polyol system. When examined as an anode material for lithium-ion batteries (LIBs), the as-synthesized G–FeSn₂ nanohybrid displays markedly enhanced Li-storage capabilities in terms of specific capacities and cycling stability compared with bare FeSn₂ nanocrystals.

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Owing to their high safety and theoretical capacities, alloy-type anodes including Si, Ge, and Sn-based materials have been considered to be ideal anodic candidates to replace the commercial insertion-type graphite materials in lithium-ion batteries (LIBs).^1,2 Among them, tin-based alloys (Sn–M, M = Fe, Co, Ni, Cu, Sb, Bi, and In) have received increasing attention over the past 20 years, and have shown promising prospects for practical applications.^3–32 For example, the Sony Nexelion™ battery for camcorder products utilizes a tin-based anode, i.e. amorphous Sn–Co–C alloy, and increases the overall capacity by 30% compared with conventional graphite anodic batteries.³ However, the state-of-the-art anodic performance of tin-based alloys does not meet the requirements in advanced LIBs for electric vehicles and electrical grid.

Up to now, much research effort has been devoted to improve the lithium storage performance of tin-based alloys starting from their micro-structural design.^3–32 Among them, manufacturing nanostructured tin-based alloys and hybridizing these alloys with carbon materials have been regarded as an effective and versatile strategy to improve their Li-storage capabilities.^11–32 For example, carbon spherules,²³ carbon nanofibers,^24,25 and carbon nanotubes (CNTs)^26,27 supported nanosized Sn–Sb and Sn–Co anodes are able to demonstrate markedly improved Li-storage performances owing to the enhanced charge transport and strain accommodation capabilities. Among these carbon matrices, graphene has been proved to be an ideal conducting and buffering matrix for electrodes in LIBs owing to its high flexibility, mechanical strength, and electric conductivity.^28–32 Therefore, the nanohybrids of graphene supported tin-based alloys are able to exhibit further enhanced cycling stability and rate capability, and could serve as promising anode candidates in advanced LIBs.

Herein, a novel type of graphene supported tin-based alloys, i.e. graphene supported FeSn₂ nanocrystals (G–FeSn₂ nanohybrid), has been designed and synthesized through a chemical reduction route in a polyol system. The as-prepared G–FeSn₂ nanohybrid has been applied as an anode material for LIBs, and displays higher specific capacities and enhanced cycling stability compared with bare FeSn₂ nanocrystals.

Fig. 1 reveals the morphological and structural characterizations of the G–FeSn₂ nanohybrid. The observed wrinkles and folds from the scanning electron microscopy (SEM) image are characteristic of graphene matrix, which has been decorated with uniform Fe–Sn alloy nanocrystals during the chemical reduction route in a polyol system (Fig. 1a). The crystalline state of the sample was examined by X-ray powder diffraction (XRD) (Fig. 1b), and the observed crystalline phase can be indexed to tetragonal FeSn₂ (JCPDS no. 65-0374). Fig. 1c shows the transmission electron microscopy (TEM) image of G–FeSn₂ nanohybrid. It can be seen that numerous FeSn₂ nanocrystals have been homogeneously deposited on the surface of graphene matrix. No isolated FeSn₂ nanocrystals are observed in the sample except for the graphene surface due to the strong electrostatic attraction between negatively charged graphene and positively charged Fe³⁺ and Sn²⁺. Moreover, poly (vinyl pyrrolidone) (PVP) as a surfactant can effectively control the grain size of alloy products,³³ resulting in uniform FeSn₂ nanocrystals with narrow size distributions (Fig. 1d).

Fig. 1 Morphological and structural characterizations of G–FeSn₂ nanohybrid: (a) SEM image, (b) XRD pattern, (c–e) TEM image, and (f) HRTEM image.

Additionally, the magnified TEM image (Fig. 1e) demonstrates that the FeSn₂ nanocrystals exhibit typical core-shelled structure with a crystalline core and an amorphous shell, which is due to the slight surface-oxidation of Fe–Sn alloy. Fig. 1f shows the high-resolution transmission electron microscopy (HRTEM) image of the product. The observed lattice fringes with spacings of ca. 0.256 and 0.206 nm can be indexed to the (211) and (202) planes of tetragonal FeSn₂, respectively, further confirming the formation of alloy product.

Fig. 2 displays a typical TEM image of G–FeSn₂ nanohybrid with its elemental mappings of C (yellow), Fe (blue), Sn (red), and their overlap. The observed elemental signals of C, Fe, and Sn originate from graphene matrix and FeSn₂ nanocrystals, respectively. The carbon, iron, and tin elemental signals are uniformly distributed within the selected area, and these elemental distributions are in good agreement with the TEM image. These results demonstrate the homogeneous distribution of FeSn₂ nanocrystals on the surface of graphene matrix.

Fig. 2 TEM-EDX elemental mappings of G–FeSn₂ nanohybrid.

The as-synthesized G–FeSn₂ nanohybrid was further examined as a potential anode material for LIBs. For comparison, the Li-storage performance of bare FeSn₂ nanocrystals was also investigated under the same conditions. Fig. 3a and b show the first three cyclic voltammetry (CV) curves of bare FeSn₂ nanocrystals and G–FeSn₂ nanohybrid in the potential range of 0.0–2.0 V at a scan rate of 0.1 mV s⁻¹. As observed, the profiles of these curves are in accordance with the Li-storage behavior of Fe–Sn alloy anodes as described previously:^9–13

FeSn₂ + 8.8Li⁺ + 8.8e⁻ → Fe + 2Li_4.4Sn (1)

Li_4.4Sn ↔ Sn + 4.4Li⁺ + 4.4e⁻ (2)

Fig. 3 Lithium storage performance of bare FeSn₂ nanocrystals and G–FeSn₂ nanohybrid: (a and b) CV curves, (c) discharge and charge curves, (d) cycling performance.

The characteristic pair of current peaks observed at (0.0–0.6, 0.2–0.8 V) could be assigned to the alloying and de-alloying processes with high reversibility as described by eqn (2). Fig. 3c displays the corresponding discharge and charge curves of G–FeSn₂ nanohybrid. As observed, the discharge and charge capacities of G–FeSn₂ nanohybrid in the first cycle are 1418.0 and 654.8 mA h g⁻¹, respectively. The large initial capacity loss (53.8%) could be mainly ascribed to the irreversible formation of solid electrolyte interface (SEI) layer and inactive Li₂O component from surface-oxidized layer,^16,34 which are also reflected in its CV curves.

Fig. 3d shows the discharge capacities versus cycle number for bare FeSn₂ nanocrystals and G–FeSn₂ nanohybrid in the potential range of 0.01–2 V at a current density of 100 mA g⁻¹. The hybridizing FeSn₂ nanocrystals with graphene matrix can effectively improve the strain accommodation and charge transport capabilities of the alloy anode, which leads to the enhanced structural stability and reaction kinetics of lithium insertion/extraction. Thus, the G–FeSn₂ nanohybrid is able to exhibit markedly enhanced lithium storage capabilities in terms of cycling stability and specific capacities than bare FeSn₂ nanocrystals. For example, the average Coulombic efficiency from 2 to 40 cycles (Fig. S1†) and the discharge capacity after 40 cycles of G–FeSn₂ nanohybrid are 96.3% and 401.1 mA h g⁻¹, respectively, which are much higher than those of bare FeSn₂ nanocrystals (92.6% and 64.0 mA h g⁻¹). Additionally, the corresponding areal capacity of G–FeSn₂ nanohybrid is 0.6 mA h cm⁻² after 40 cycles (Fig. S2†), and could be further improved by increasing the areal mass density of the active material on copper foam current collectors.

The improved lithium storage performances of G–FeSn₂ nanohybrid could be attributed to its unique structural features, which can be further demonstrated by the characterization of the anode after cycling. Fig. 4 reveals the morphological and compositional characterizations of G–FeSn₂ nanohybrid in a fully de-lithiated state (2.0 V vs. Li⁺/Li) after 40 cycles. It can be seen from the TEM images that numerous nanoparticles are still uniformly anchored on the surface of graphene, indicating the anodic agglomeration and pulverization could be effectively suppressed during cycling (Fig. 4a and b). Moreover, the energy-dispersive X-ray spectrometer (EDX) elemental mappings demonstrate that the C, Fe, and Sn signals are still evenly distributed in the de-lithiated G–FeSn₂ nanohybrid anode (Fig. 4c). Additionally, the F signal originates from the SEI layer, which consists of the decomposition products of electrolytes including LiF, Li₂CO₃, and so forth.³⁵ These results confirm the superior structural stability of the G–FeSn₂ anode during lithium insertion/extraction, which is critical for its enhanced Li-storage performances in terms of cycling stability and specific capacities.

Fig. 4 Morphological and compositional characterizations of G–FeSn₂ nanohybrid in a fully de-lithiated state (2.0 V vs. Li⁺/Li) after 40 cycles: (a and b) TEM images, (c) TEM-EDX elemental mappings of C (yellow), Fe (blue), Sn (red), F (white), and their overlap.

Conclusions

In summary, we have anchored FeSn₂ nanocrystals densely and uniformly on graphene matrix via a facile chemical reduction route in a polyol system. Compared with bare FeSn₂ nanocrystals, the as-synthesized G–FeSn₂ nanohybrid displays much higher specific capacities and markedly enhanced cycling stability by virtue of its unique structural characteristics. Moreover, this synthetic methodology could open up new opportunities for the formation of hybrid tin-based alloy anodes with further improved performance.

Acknowledgements

The authors appreciate the financial supports from the Natural Science Foundation of Jiangsu Province (BK20130900), Natural Science Foundation of Jiangsu Higher Education Institutions of China (13KJB150026), Industry-Academia Cooperation Innovation Fund Project of Jiangsu Province (BY2013001-01), the Priming Scientific Research Foundation for Advanced Talents in Nanjing Normal University (2013103XGQ0008), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedures and Fig. S1. See DOI: 10.1039/c4ra00604f

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