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Monodisperse and inorganically capped Sn and Sn/SnO2 nanocrystals for high performance Li-ion battery anodes Kostiantyn V. Kravchyk, Loredana Protesescu, Maryna I. Bodnarchuk, Frank Krumeich, Maksym Yarema, Marc Walter, Christoph Guntlin, and Maksym V. Kovalenko J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja312604r • Publication Date (Web): 15 Feb 2013 Downloaded from http://pubs.acs.org on February 21, 2013
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Journal of the American Chemical Society
Monodisperse and inorganically capped Sn and Sn/SnO2 nanocrystals for high performance Li-ion battery anodes Kostiantyn Kravchyk†‡, Loredana Protesescu†‡, Maryna I. Bodnarchuk†‡, Frank Krumeich†, Maksym Yarema†‡, Marc Walter†, Christoph Guntlin,† and Maksym V. Kovalenko†‡* †
Institute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, CH-8093 Zürich, Switzerland ‡ EMPA-Swiss Federal Laboratories for Materials Science and Technology, CH-8060 Dübendorf, Switzerland Supporting Information Placeholder ABSTRACT: We report a facile synthesis of highly monodisperse colloidal Sn and Sn/SnO2 nanocrystals with mean sizes tunable in the range of 9-23 nm and size distributions below 10%. For testing the utility of Sn/SnO2 nanocrystals as an active anode material in Li-ion batteries, a simple ligand-exchange procedure using inorganic capping ligands was applied to facilitate electronic connectivity within the components of nanocrystalline electrode. Electrochemical measurements demonstrate that 10-nm Sn/SnO2 nanocrystals enabled high Li-insertion/removal cycling stability, in striking contrast to commercial 100-150 nm powders of Sn and SnO2. In particular, reversible Li-storage capacities above 700 mAh g-1 are reported after 100 cycles of deep charging (0.005-2 V) at a relatively high current of 1000 mAh/g. Tin (Sn) is non-toxic, inexpensive and naturally highly abundant element and is amongst technologically most important metals. For instance, it is used not only as corrosion-resistant plating of food cans or in metal alloys such as bronze and solder, but also in emerging photovoltaic materials such as Cu-Zn-Sn chalcogenides and in the next generation rechargeable Li-ion batteries (LIB), to mention just a few applications. In this work, we were driven by two goals: (i) to develop robust synthesis of monodisperse Sn nanocrystals (NCs) and (ii) to study sizedependent cycling performance of nanocrystalline Sn-based LIB anodes. For the latter, finely tunable morphologies and optimal surface chemistries in sub-50 nm size range are considered crucial for achieving high charge/discharge cycling stabilities in the next high-capacity anode materials based on Li alloys with Si, Sn and Ge.1 Furthermore, the potential use of Sn and corresponding oxide SnO2 in a monodisperse colloidal state is by far broader, including solution-deposited transparent conductive oxides for electronics, photovoltaics and sensors, or even as quantum dots (sub-5nm alpha-Tin)2 or as low-temperature catalyst for growing nanowires.1b In addition, quantum-sized effects in Sn nanoparticles have been demonstrated to modify their superconducting properties at cryogenic temperatures.3 Despite numerous reports on rather polydisperse (size deviation σ>15%) and/or relatively large (30-200 nm) and non-uniform Sn colloids,4 robust synthesis of monodipserse sub-30 nm Sn NCs remained undeveloped. The low melting point of bulk Sn (231.9 °C) requires the synthesis to be carried out at sufficiently low temperatures using either highly reactive precursors or strong reducing agents. A new approach to monodisperse Sn NCs reported in this Communication, is inspired by the recent success of metal amides as precursors to monodisperse colloids including metallic Bi,5 Pb,6 In,7 Co,8 Fe,9 FeRh,10 FeCo11 NCs and metal compound NCs such as SnTe12, PbSe and Ag2Se.13
Figure 1. Synthesis of monodisperse Sn NCs: (A) schematics of the reaction, (B) representative size-histograms (σ≈8.3%, 5.9%, 7.7% and 7.6% for 9nm, 12nm, 16nm and 23 nm NCs, respectively), (C, D, E, F) corresponding TEM and (G) SEM image for 16nm NCs. DIBAH: diisobuthylaluminium hydride.
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Figure 2. Proposed mechanism of precursors-to-metallic Sn conversion derived from NMR studies in oleylamine. Regardless of the starting Sn(II) compound (chloride, acetate, triflate or silyalmide), addition of lithium dimethylamide or silylamide leads to the same Tin-oleylamido derivatives. Monodisperse 9-23 nm large Sn NCs (Figure 1, see details in Supporting Information and in Table S1)14 were obtained by injecting LiN(SiMe3)2 into hot oleylamine solution containing SnCl2 or Sn[N(SiMe3)2]2 at 180-210 °C, followed by the reduction with diisobutylaluminium hydride (DIBAH). The use of inexpensive SnCl2 and LiN(SiMe3)2 as initial precursors was conceived from the possibility of the convenient in-situ formation of Sn[N(SiMe3)2]2. Instead, 119Sn NMR spectra (Figures 2 and S1), taken at various stages of the synthesis, showed that in-situ or exsitu prepared Sn[N(SiMe3)2]2 is not an actual precursors. A different reaction mechanism is plausible – a fast in-situ formation of metal-oleylamide species. An important role of LiN(SiMe3)2 is to act as a base for deprotonating oleylamine. Formation of Lioleylamide isfacilitated by the volatility of HNSiMe3. Lioleylamide then quickly reacts with SnCl2 generating Snoleylamido complex, an actual precursor for the final reduction step. Very similar results, judging from 119Sn NMR spectra, reaction rates and in terms of the size and shapeuniformity of Sn NCs, were obtained using sodium and potassium silylamides, other strong bases such as lithium hydride and LiNMe2 (Figure S2), as well as various oleylamine-soluble Sn(II) precursors such as Sn[N(SiMe3)2]2, Sn(Ac)2 and Sn(CF3SO3)2 (Figure S3). In agreement with the proposed role of organic bases, no NCs could be formed in their absence: only polydisperse 100-300 nm Sn particles are collected after injecting DIBAH directly into Sn(II) salts in oleylamine (Figure S4). Very likely Sn-oleylamido complexes and/or Li-oleylamide may act not only as a precursor but also as surface capping layer during the NC nucleation and growth (rather than neutral OLA molecules). These observations may also explain the “magic role” of the excess LiN(SiMe3)2 in the recent preparations of Bi,5 In,7 Ag2Se, CdSe and PbSe NCs.13 Further mechanistic studies on the in-situ formed of metal/long chain amide complexes for the NC synthesis are underway and will be reported elsewhere. Following the nucleation and growth under airless conditions, the isolation and purification of Sn NCs was carried out in air. Oleic acid was added to replace weakly bound oleylamide/oleylamine species (see FTIR spectra in Figure S5). Highresolution BF-STEM and HAADF-STEM images of Sn NCs (Figure 3A, B) indicated a single-crystalline core covered by the amorphous oxide shell formed upon sample exposure to air. The oxide thickness of 2-3 nm is retained after the storage of samples in air for 6 months. Powder X-ray diffraction patterns (Figures 3C, S6) confirmed that NC cores are single-crystalline beta-Sn (I41/amd space group, a = 0.58308, c = 0.31810 nm)15 without detectable crystalline Sn-oxide phases. Magic angle spinning (MAS) 119Sn NMR spectra of powdered samples contained
Figure 3. (A) Bright field- and (B) high-angle annular dark-field scanning transmission microscopy images (BF-STEM and HAADF-STEM, respectively), (C) powder XRD pattern and (inset in C) 119Sn MAS NMR spectrum of colloidal Sn/SnO2 NCs. Vertical lines in (C) mark the diffraction pattern of bulk beta-Sn; blue curve is the fit obtained by Rietveld refinement.
Figure 4. (A) Photograph illustrating phase transfer of Sn/SnO2 NCs from hexane into water induced by the ligand-exchange reaction and corresponding (B) TEM image, (C) electrophoretic mobility and (D) ATR-FTIR spectra of HS-/SO42- functionalized Sn/SnO2 NCs. single broad peak at ca. -600 ppm, commonly reported for amorphous SnO2, and no signs of SnO (expected at -208 ppm).16 The peak broadening of up to 100 ppm is similar to the reports for sub10nm SnO2 nanoparticles and hollow SnO2 nanospheres,17 while much narrower peaks (
