Facile Synthesis of Highly Porous Ni–Sn Intermetallic Microcages with

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Facile Synthesis of Highly Porous Ni−Sn Intermetallic Microcages with Excellent Electrochemical Performance for Lithium and Sodium Storage Jun Liu,‡ Yuren Wen,§ Peter A. van Aken,§ Joachim Maier,‡ and Yan Yu*,†,‡ †

CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, 230026, China ‡ Max Planck Institute for Solid State Research, Heisenbergstrasse 1, Stuttgart, 70569, Germany § Max Planck Institute for Intelligent Systems, Heisenbergstrasse 3, Stuttgart, 70569, Germany S Supporting Information *

ABSTRACT: Highly porous Ni3Sn2 microcages composed of tiny nanoparticles were synthesized by a facile template-free solvothermal method (based on Ostwald ripening and etching mechanism) for use as anode materials for highcapacity and high-rate-capability Li-ion and Na-ion batteries. The Ni3Sn2 porous microcages exhibit highly stable and substantial discharge capacities of the amount to 700 mA h g−1 after 400 cycles at 0.2C and 530 mA h g−1 after 1000 cycles at 1C for Li-ion battery anode. For Na-ions storage performance, a reversible capacity of approximate 270 mA h g−1 is stably maintained at 1C during the first 300 cycles.

KEYWORDS: Li-ion batteries, Na-ion batteries, anodes, Ni3Sn2, porous structures

T

intermetallic compounds have been developed, such as ball milling, sintering, and electroplating.17−25 Because the melting point of Sn is much lower than that of Ni, it is difficult to prepare Ni−Sn alloys with defined content and controlled morphology, be it by arc-melting, sintering, or mechanical alloying.20−24 Solution-based hydrothermal or solvothermal methods were considered as a promising way for achieving homogeneous nanosized materials with well-defined shapes26,27 but not yet applied to Ni−Sn alloys. In this paper, we report the successful synthesis of a new type of nanoarchitectured Ni−Sn intermetallic anodes, viz., hierarchical hollow microcages composed of porous nanoparticle units. The well-defined hollow cores and nanopores of microcages can also provide additional elastic buffer spaces with respect to accommodating the volume changes upon Li (Na) ions insertion/extraction.28,29 We focus on Ni3Sn2, which was rarely considered as anode and if then only with modest success. When our nanostructured materials are used as the anode for Li-ion and Na-ion batteries, high rate capability and excellent capacity retention are observed. Highly porous Ni3Sn2 intermetallic microcages were synthesized by a template-free Ostwald ripening-based solvothermal route, in which SnCl2·2H2O and NiCl2·6H2O were used as the

he requirements to be met by rechargeable batteries (Li-ion and Na-ion batteries) in terms of power density, energy density, as well as cyclability have stimulated the search for new high-performance electrode materials.1−3 Although the progress of Li-ion and Na-ion battery development is substantial, energy density, cycle life, and rate capability of these systems remain insufficient.4 As far as the anode side is concerned, replacing graphitic carbon by Sn (theoretical capacity of about 990 and 847 mAh g−1, corresponding to Li22Sn5 and Na15Sn4, respectively) is apt to increase the anode capacity by about three times; the rapid capacity fading during charging/ discharging due to huge volume change (250% for Li-ion batteries and 420% for Na-ion batteries) has severely hindered its applicability in practical Li-ion and Na-ion batteries.5−13 Much research effort has been devoted to overcome this problem.5−13 A promising approach is to move to M−Sn intermetallics, converting in the first cycle to a M/Sn composite. The primary beneficial role of the metal M is to provide mechanical buffer capacity for accommodating volume changes that otherwise would lead to disintegration.14−19 Among others, Ni−Sn intermetallics have received particular attention due to their high charge storage at low cost.17−25 It is now widely recognized that a most efficient way of enhancing the electrochemical performance of alloy-based electrodes is optimizing their morphology, especially in terms of particle size and interparticle spacing.17−19 This can be obtained by refined synthesis methods. Until now, various methods of synthesizing Ni−Sn © 2014 American Chemical Society

Received: July 25, 2014 Revised: September 28, 2014 Published: October 6, 2014 6387

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In order to determine the distribution of Ni and Sn in the Ni−Sn intermetallic anode, elemental mapping of porous nanoparticles units from the microparticles is performed. Figure 2b,c displays the energy-filtered TEM (EFTEM) mapping for Sn and Ni of the bright-field (BF) TEM image (Figure 2a), respectively. Both the Sn and Ni EFTEM maps in Figure 2b,c match well with the STEM image, indicating that Sn and Ni are uniformly distributed throughout the particle. Such uniform elemental distribution was further confirmed by the phase-purity of the current Ni3Sn2 intermetallic, as shown in the XRD pattern (Figure 2d). All the diffraction peaks of Ni−Sn intermetallic can be indexed in terms of orthorhombic Ni3Sn2 (JCPDS No. 65-9650), while the surface area of these highly porous and hollow Ni3Sn2 microcages was measured using the Brunauer− Emmett−Teller (BET) method. As shown in Figure 2e, the N2 adsorption−desorption isotherm at 77 K can be classified as a typical III isotherm with a distinct hysteresis loop, which is consistent with the presence of the distinct mesoporous microstructure with a BET specific surface area of approximately 60 m2 g−1. The pore size distribution (in inset of Figure 2e) according to the Barrett−Joyner−Halenda (BJH) method, indicates that these Ni3Sn2 microcages are mesoporous composites connected with a narrow pore-size distribution in the range of 2−5 nm. Simultaneously, further experiments have been implemented under varied reaction duration to exploit the formation mechanism of Ni3Sn2 hollow microcages, as shown in Figure 3 and Figure S2 in the Supporting Information. Within the first 2 h, Ni3Sn2 nanoparticles self-aggregated into solid microspheres. In the following 2 h, these obtained Ni3Sn2 solid spheres converted into yolk−shell structured particles (Figure 3a−c) and further converted into completely hollow microcages (Figure 1) with prolonging solvothermal time. As the hollow microcages consist of larger particles compared to the aggregates, Ostwald ripening plays a crucial role in the process.30−34 When the solvothermal time was further prolonged to 12 h, more broken hollow microcages with larger size existed in the final products (Figure S3 in Supporting Information), which strongly supports the current hollowing mechanism.35−38 During this process of transforming the dense aggregate structure into hollow structure, NaF is believed to act as etching agent important for the formation of nanoporous metal structures.39 The Li storage performance of as-prepared Ni3Sn2 porous microcages was evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge cycling using two-electrode Swagelok-type cells. Figure 4a show CV curves of Ni3Sn2 porous microcages electrode at a scanning rate of 0.1 mV s−1. The electrochemical process is expected to evolve by a first irreversible activation step18

Figure 1. Microstructure characterization of highly porous Ni3Sn2 microcages: (a,b) low-magnification SEM images showing that these uniform Ni3Sn2 intermetallic microcages have a quasi-spherical shape with size of 2−3 μm; (c,d) high-magnification SEM images display their highly porous shells and hollow structured cores; (e,f) TEM and HRTEM images of a typical intermetallic microparticle, which clearly reveal the porous hollow structure and lattice planes of Ni3Sn2; (g) SAED pattern of the highly porous polycrystalline Ni3Sn2 microcage.

reactant, NaF as the etching agent, and ethylene glycol as the solvent and reducing agent. The detailed synthesis process is shown in one section of Supporting Information. Figure 1a−d shows scanning electron microscopy (SEM) images of Ni3Sn2 intermetallic particles at different magnifications as obtained from a simple low-temperature solvothermal treatment. As depicted by Figure 1a,b, these Ni3Sn2 intermetallic particles show regular quasi-spherical morphology and high monodispersivity. Figure 1b clearly displays that these monodisperse Ni3Sn2 microparticles have a uniform size of 2−3 μm. Local high-magnification SEM images (Figure 1c) reveal the highly porous surface of these intermetallic microparticles, composed of small-sized nanoparticles. The hollow microcage-structured morphology realized by distinct broken hollow particles is shown in Figure 1d. Figure 1e,f displays results from transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) of the as-synthesized intermetallic particles. Figure 1e shows a low-magnification TEM image of a typical Ni3Sn2 microparticle, which confirms that the products are composed of hierarchical hollow microcages. The hollow microcages exhibit diameters of about 2.5 μm and shell thicknesses of about 200 nm, composed of dense nanoparticles with sizes of several nanometers. The continuous nanoporous character and the presence of tiny nanoparticle units constituting shells are corroborated by HRTEM images of a typical microcage-wall (Figure 1f, more HRTEM images of the highly porous Ni3Sn2 intermetallic particles at different selected areas are shown in Figure S1 in Supporting Information). The lattice spacing of 0.33 nm corresponds to the (210) plane of Ni3Sn2 shown in the inset of Figure 1f. Such highly porous polycrystalline Ni3Sn2 microcages have also been confirmed by the selected area electron diffraction (SAED) pattern in Figure 1g.

Ni3Sn2 + 8.8Li+ + 8.8e− → 2Li4.4Sn + 3Ni

(1)

followed by converting the alloy into a Sn/Ni composite. In the composite, the usual lithiation of Sn occurs reversibly Li4.4Sn → Sn + 4.4Li+ + 4.4e− +

−

Sn + 4.4Li + 4.4e → Li4.4Sn

(2) (3)

The galvanostatic discharge/charge process of these Ni3Sn2 porous microcages anode was performed in a voltage range of 0.01−2.0 V at a current density of 0.2C (5 h per half cycle) as shown in Figure 4b. The first discharge and charge steps deliver a specific capacity of 1309 and 771 mA h g−1, respectively. Note that all the capacity values in the graph were calculated based 6388

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Figure 2. (a−c) Bright field TEM (a) and corresponding EFTEM elemental mapping of the Ni3Sn2 porous nanoparticles units obtained by ultrasonication of microcages for Sn-M4,5 edges (b) and Ni-L2,3 edges (c), respectively; (d) XRD pattern of the obtained Ni3Sn2 porous microcages, showing that all the diffraction peaks were indexed as the orthorhombic Ni3Sn2 (JCPDS No. 65-9650); (e) N2 adsorption/desorption isotherms and the corresponding pore size distribution (the inset) of mesoporous Ni3Sn2 porous microcages, calculated using the BJH method.

Figure 3. (a−c) SEM images of the intermediate of yolk−shell structured Ni−Sn intermetallic microparticles obtained at 4 h. The inset of panels b and c show SEM and TEM images of a typical yolk−shell microparticle, indicating that hollowing is ascribed to a typical inward Ostwald ripening; (d) XRD patterns of the intermediates obtained at different crystallization times (2, 3, and 4 h); (e) schematic illustration of the formation process of hollow structured Ni3Sn2 porous microcages based on Ostwald ripening and etching. 6389

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Figure 4. Excellent electrochemical performance of Ni3Sn2 porous microcages for lithium batteries: (a) CV curves at a scanning rate of 0.1 mV s−1 in the voltage range of 0.01−2.0 V; (b) voltage-capacity curves at 0.2C rate; (c) voltage-capacity curves at different rates (increased from 0.25C to 10C); (d) rate capability at different rates (increased from 0.25C to 10C); (e) cycling performances of Ni3Sn2 porous microcages anode at 0.2C rate.

respectively. As shown in this figure, these porous Ni3Sn2 anode materials display a high steady value even under these demanding conditions. Ni3Sn2 porous microcages also show excellent electrochemical performances when used as Na-ion battery anode. CV curves (Figure 5a) in NaClO4/PC electrolyte at 0.1 mV s−1 show four peaks, located at 0.88, 0.69, 0.45, and 0.02 V in the first sodiation process. Two sodiation peaks at higher voltage disappeared in the subsequent cycles, they were probably concerned with the formation of SEI film caused by the decomposition of electrolyte, contributing to the irreversible capacity.13 The desodiation process is characterized by only one welldefined peak at 0.10 V, which could attributed to the alloy compounds of Na15Sn4.9,13,40 The detailed sodiation and desodiation mechanisms of the Ni−Sn intermetallic are described by the following equations

on Sn present in the intermetallic particles. As the current intermetallic contains about 57.4% Sn, the capacity data for the whole alloy mass are superior as well. (The values referring to the alloy and Sn at various current densities are listed in the Supporting Information S4.) The large initial capacity loss can be partly attributed to the formation of a thick solid electrolyte interphase (SEI) layer on the electrode surface during the first discharge step and the storage of Li+ in nanoporous voids, which are difficult to be extracted.6,7 Though the first discharge capacity decayed to 808 mA h g−1, the porous Ni3Sn2 microcages showed a very stable cycling performance after the first cycle (Figure 4b). The discharge capacity remained on the very high value of 696 mA h g−1 even after 400 cycles with high average Coulombic efficiency (Figure 4e). Discharge/charge curves of these porous Ni3Sn2 microcages at different current densities show that they exhibit excellent rate capability as anode materials for Li-ion batteries (Figure 4c). Remarkably, at current densities as high as 5C and 10C, this material can still deliver reversible capacities of 465 and 404 mA h g−1, respectively. Figure 4d demonstrates this superior rate capability of porous Ni3Sn2 microcages with discharge capacities of 620, 596, 542, 465, and 404 mA h g−1 at 0.25, 1, 2, 5, and 10C 6390

Ni3Sn2 + 7.5Na + + 7.5e− → 2Na3.75Sn + 3Ni

(4)

Na3.75Sn → Sn + 3.75Na + + 3.75e−

(5)

Sn + 3.75Na + + 3.75e− → Na3.75Sn

(6)

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Figure 5. Excellent electrochemical performance of Ni3Sn2 porous microcages for sodium batteries: (a) CV curves at a scanning rate of 0.1 mV s−1 in the voltage range of 0.01−2.0 V; (b) voltage-capacity curves at 1C rate; (c) voltage-capacity curves at different rates (increased from 0.25C to 10C); (d) rate capability at different rates (increased from 0.25C to 10C); (e) cycling performances of Ni3Sn2 porous microcages anode at 1C rate.

Figure 5b shows the 1st, 2nd, 50th, 100th, 150th, 200th, 250th, and 300th cycle of the Ni3Sn2 intermetallic anode electrode in NaClO4/PC electrolyte at a current rate of 1C. Ni3Sn2 intermetallic porous microcages show an initial large discharge capacity of 1082 mA h g−1 and reversible charge capacity of about 384 mA h g−1. The cycling stability during Na-ion insertion/ extraction in the Ni3Sn2 intermetallic porous microcages was also investigated at a current density of 1C for 300 cycles (Figure 5e). After the first five charge/discharge cycles, a reversible capacity of approximate 350 mA h g−1 is stably maintained. Remarkably the reversible and stable capacities at various discharge/charge rates are retained at about 540, 447, 351 mA h g−1 for current densities of 0.25, 2, 5C respectively (Figure 5c,d). Even at a high current rate of 10C, a stable capacity of 276 mA h g−1 is still maintained, thereby making the 3D hollow microcage-sample a promising anode for Na-ion batteries. After long cycling process, such hollow microstructure of anode can be still kept, as shown in Supporting Information Figures S4 and S5. As far as the cycling behavior of these intermetallic anode materials for Li-ion batteries is concerned, we examined their cycling performance at a current density of 1C for as many as 1000 cycles. Over the whole period, excellent cycling performance is observed, with high stable discharge capacities of about 534 mA h g−1 after 1000 cycles at 1C (Figure 6a).

Figure 6. (a) Charge−discharge capacities of Ni3Sn2 porous microcages for lithium batteries over 1000 cycles at 1C; (b) schematic representation of the first lithiation and sodiation of porous Ni3Sn2 intermetallic microcages, forming 0D electroactive M−Sn (M = Li, Na) particles embedded in 3D conducting Ni hollow matrix. 6391

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The outstanding electrochemical performance of these porous and hollow Ni3Sn2 microcages for Li-ion and Na-ion batteries may be attributed to the combination of the in situ formed 0D electroactive M−Sn (M = Li, Na) particles and 3D conducting Ni hollow matrix formed after the first lithiation or sodiation of Ni3Sn2 porous microcages (Figure 6b, Supporting Information Figures S6 and S7). The schematic illustration referring to the diffusion of Li ions, Na ions, and electrons during the discharge process of Ni3Sn2 porous microcages is given in Figure 6b. The mechanical strain of the Sn during charge/discharge processes is obviously effectively suppressed by the hollow cores structure and the presence of the Ni matrix in the hollow microcages. Moreover, homogeneously encapsulated Ni converted from lithiation or sodiation of Ni3Sn2 is certainly beneficial for the necessary electron transport. In conclusion, we have successfully developed a large-scale, facile method to fabricate highly porous Ni3Sn2 microcages composed of tiny nanoparticles (based on Ostwald ripening and etching mechanism). Owing to the shortened ion-diffusion distance, high contact surface area, good electronic conductivity, together with the mechanical capacity to accommodate volume variations of Sn, the Ni3Sn2 porous microcages exhibit highly stable and substantial discharge capacities of the amount to 700 mA h g−1 after 400 cycles at 0.2C and 530 mA h g−1 after 1000 cycles at 1C for Li-ion battery anode. For Na-ions storage performance, a reversible capacity of approximate 270 mA h g−1 is stably maintained at 1C during the first 300 cycles.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information and figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Sofja Kovalevskaja award of the Alexander von Humboldt Foundation, by the National Natural Science Foundation of China (Nos. 21171015, 21373195, and 11202177), the “Recruitment Program of Global Experts” and program for New Century Excellent Talents in University (NCET), the Fundamental Research Funds for the Central Universities (WK2060140014, WK2060140016) and the Max Planck Society. The research leading to these results has received funding from the European Union Seventh Framework Programme [FP7/2007-2013] under Grant Agreement 312483 (ESTEEM2).

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REFERENCES

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dx.doi.org/10.1021/nl5028606 | Nano Lett. 2014, 14, 6387−6392