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Sn0.9Si0.1 core/carbon shell nanoparticles, with the sizes of 16 and 10 nm, were prepared by annealing as-prepared butyl-capped Sn0.9Si0.1 particles with an average particle size of 1 μm. Even though as-prepared samples were severely encapsulated by butyl terminators, annealing led to pulverization of the bulky particles into core−shell nanoparticles with a shell thickness dependent on the annealing temperature. The core Sn0.9Si0.1 size was estimated to be constant at 6 nm, and the carbon shell thickness decreased from 10 to 4 nm with increasing annealing temperature from 600 to 700 °C, respectively. In addition, the carbon shell was found to be more ordered at 700 °C than at 600 °C. Sn0.9Si0.1 core/carbon shell nanoparticles exhibited excellent lithium storage ability at a high current rate, resulting in a value of 964 mA·h/g at a rate of 0.3 C (1 C = 1200 mA/g), and demonstrated good capacity retention after 50 cycles.