Mechanics-based optimization of yolk-shell carbon-coated silicon nanoparticle as electrode materials for high-capacity lithium ion battery

• The fracture of yolk-shell carbon-coated silicon nanoparticles is found to be geometrical dimension-dependent during lithiation.
• The fracture mechanism is analyzed by mechanics-based theoretical modeling.
• An optimal design guideline is proposed and verified to ensure structural integrity and maximize capacity for the nanoparticle anode.

Yolk-shell carbon-coated silicon nanoparticles (Si@void@C NPs) have been demonstrated to have a great promise in solving the problem of significant volume change of silicon-based anode materials during lithiation and delithiation cycling. However, our in situ lithiation experiments show that Si@void@C NPs may still subject to fracture upon lithiation, depending on their characteristic structural features such as the size of Si yolk, the thickness of carbon shell, and the interspace between the yolk and shell. Given the size of Si yolk, to ensure structural integrity of Si@void@C NPs during lithiation and delithiation, thicker carbon shell and larger yolk-shell interspace are preferred. On the other hand, from the perspective of attaining higher effective capacity, thinner carbon shell and smaller yolk-shell interspace are favored. To find the optimal structural design which yields the maximum capacity and meanwhile ensure the integrity of Si@void@C NPs during lithiation, mechanics-based theoretical modeling is carried out. A diagram for structural optimizations is obtained, by which the optimized Si@void@C NPs are synthesized and found to have improved capacity and capacity retention compared to the unoptimized ones. The results of this paper provide a guideline for the design of Si@void@C NPs as anode materials for high-capacity lithium ion battery.