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

Abstract

Silicon is recognized as a promising\nanode material for high-performance lithium ion batteries due to its\nhigh theoretical specific capacity and elemental abundance. Challenges\nrelated to the low electrical conductivity of Si and large volume\nchanges during the lithiation/delithiation cycles, as well as the\nlow rate of lithium diffusion in silicon anodes, hinder practical\napplications. To provide fundamental insights into these issues, silicon\nnanocrystal/graphene aerogel nanocomposites were synthesized by combining\nundecanoic acid-functionalized silicon nanocrystals of various sizes\n(Si<i>X</i>-COOH, where <i>X</i> represents the\nnanocrystal diameter of 3, 5, 8, and 15 nm) with conductive mesoporous\ngraphene aerogels (GAs). The silicon nanocrystals are evenly dispersed\nthroughout the graphene aerogel as shown by energy-dispersive X-ray\n(EDX) mapping. In terms of electrochemical performance, Si<i>X</i>-COOH/GA nanocomposites demonstrated a clear dependence\non the size of the embedded silicon nanocrystals, with the composites\ncomprising the larger silicon nanocrystals showing a higher initial\ncapacity but accompanied by rapid decay of capacity retention over\n100 cycles. To study the effect of thermal processing on the electrochemical\nperformance, Si<i>X</i>-COOH/GA nanocomposites were annealed\nat 600 °C to yield annealed Si<i>X</i>/GA nanocomposites.\nThe annealed nanocomposite composed of the smallest silicon nanocrystals,\nSi3/GA, exhibits a stable specific capacity of ∼1100 mAh/g\nand capacity retention of over 90% after 500 cycles when tested at\na current density of 400 mA/g.