Size and Surface Effects of Silicon Nanocrystals in Graphene Aerogel Composite Anodes for Lithium Ion Batteries

Silicon is recognized as a promising anode material for high-performance lithium ion batteries due to its high theoretical specific capacity and elemental abundance. Challenges related to the low electrical conductivity of Si and large volume changes during the lithiation/delithiation cycles, as well as the low rate of lithium diffusion in silicon anodes, hinder practical applications. To provide fundamental insights into these issues, silicon nanocrystal/graphene aerogel nanocomposites were synthesized by combining undecanoic acid-functionalized silicon nanocrystals of various sizes (SiX-COOH, where X represents the nanocrystal diameter of 3, 5, 8, and 15 nm) with conductive mesoporous graphene aerogels (GAs). The silicon nanocrystals are evenly dispersed throughout the graphene aerogel as shown by energy-dispersive X-ray (EDX) mapping. In terms of electrochemical performance, SiX-COOH/GA nanocomposites demonstrated a clear dependence on the size of the embedded silicon nanocrystals, with the composites comprising the larger silicon nanocrystals showing a higher initial capacity but accompanied by rapid decay of capacity retention over 100 cycles. To study the effect of thermal processing on the electrochemical performance, SiX-COOH/GA nanocomposites were annealed at 600 °C to yield annealed SiX/GA nanocomposites. The annealed nanocomposite composed of the smallest silicon nanocrystals, Si3/GA, exhibits a stable specific capacity of ∼1100 mAh/g and capacity retention of over 90% after 500 cycles when tested at a current density of 400 mA/g.