Colloidal tin-germanium nanorods and their Li-ion storage properties.

We report a facile colloidal synthesis of tin-germanium (Sn-Ge) heterostructures in the form of nanorods with a small aspect ratio of 1.5-3 and a length smaller than 50 nm. In the two-step synthesis, presynthesized Sn nanoparticles act as a low-melting-point catalyst for decomposing the Ge precursor, bis[bis(trimethylsilyl)amido]Ge(II), and for crystallization of Ge via solution-liquid-solid growth mechanism. Creation of such Sn-Ge nanoheterodimers can serve as a well-controlled method of mixing these nearly immiscible chemical elements for the purpose of obtaining Sn-Ge nanocomposite electrodes for high-energy density Li-ion batteries. Comparable mass content of Sn and Ge leads to synergistic effects in electrochemical performance: high charge storage capacity above 1000 mAh g(-1) at a relatively high current density of 1 A g(-1) is due to high theoretical capacity of Ge, while high rate capability is presumably caused by the enhancement of electronic transport by metallic Sn. At a current density of 4 A g(-1), Sn-Ge nanocomposite electrodes retain up to 80% of the capacity obtained at a lower current density of 0.2 A g(-1). Temporally separated lithiation of both elements, Sn and Ge, at different electrochemical potentials is proposed as a main factor for the overall improvement of the cycling stability.