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 nanoparticles that terminate with butyl and ethyl groups have core sizes of ∼14 and ∼17 nm with carbon shell thicknesses of ∼16 and ∼8 nm, respectively. On the other hand, annealed nanoparticles 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 nanoparticles show 82 and 64% capacity retention after 40 cycles.