Control of the carbon shell thickness in Sn70Ge30@carbon core-shell nanoparticles using alkyl terminators: Its implication for high-capacity lithium battery anode materials

This paper describes the synthesis and electrochemical characterization of Sn70Ge30@carbon core-shell nanoparticles prepared by vacuum annealing of the alkyl-capped Sn70Ge30 nanoparticles obtained from the reaction of SnCl4 and GeCl4 with sodium naphthalide in ethylene glycol dimethyl ether (glyme) and RLi (R = butyl, ethyl, methyl). The Sn70Ge30@carbon core-shell nanoparticles have different core sizes and shell thicknesses depending on the alkyl terminator. The annealed narroparticles that terminate with butyl and ethyl groups have core sizes of similar to 14 and similar to 17 nm with carbon shell thicknesses of similar to 16 and similar to 8 nm, respectively. On the other hand, annealed narroparticles that terminate with methyl groups have core sizes of 40 nm with a very thin carbon shell without uniform coverage of the core. Electrochemical characterization shows that nanoparticles prepared using butyl terminators have the highest capacity retention out to 40 cycles (95%) and a first charge capacity of 1040 mAh/g. On the other hand, ethyl- and methyl-capped narroparticles show 82 and 64% capacity retention after 40 cycles. (c) 2008 Elsevier Ltd. All rights reserved.