Elucidating Relationships between Structural Properties of Nanoporous Carbonaceous Shells and Electrochemical Performances of Si@Carbon Anodes for Lithium-Ion Batteries

The encapsulation of silicon in hollow carbonaceous shells (Si@C) is known to be a successful solution for silicon anodes in Li-ion batteries, resulting in many efforts to manipulate the structural properties of carbonaceous materials to improve their electrochemical performance. In this regard, we demonstrate in this work how both the shell thickness and pore size of nanoporous carbonaceous materials containing silicon anodes influence the electrochemical performance. Structurally well-defined Si@C materials with varying carbon-shell thicknesses and pore sizes were synthesized by a nanocasting method that manipulated the carbon shell and by a subsequent magnesiothermic reduction that converted the amorphous silica cores into silicon nanocrystals. When these materials were employed as anodes, it was verified that two opposite effects occur with respect to the thickness of carbon shell: The weight ratio of silicon and the electrical conductivity are simultaneously affected, so that the best electrochemical performance is not obtained from either the thickest or the thinnest carbon shell. Such countervailing effects were carefully confirmed through a series of electrochemical performance tests and the use of electrochemical impedance spectroscopy. In addition, the effect of pore size was elucidated by comparing Si@C samples with different pore sizes, revealing that larger pores can further improve the electrochemical performance as a result of enhanced Li-ion diffusion.