Silicon-nanoparticles isolated by in situ grown polycrystalline graphene hollow spheres for enhanced lithium-ion storage

A silicon-based anode material offers extremely high lithium storage capacity, but suffers from severe volume expansion during lithiation, which causes a drastic capacity decay. Embedding and isolating Si nanoparticles (SiNPs) into sealed amorphous carbon hollow spheres with sufficient voids is a promising strategy to accommodate the volume effect of Si. However, the created voids significantly compromise the volumetric energy density. Conversely, if insufficient voids are introduced, the inferior mechanical property of the amorphous carbon turns into the decisive factor destroying the structural integrity of the composites. Graphene is more suitable as a protective shell material due to its excellent mechanical strength. However, there still remains a formidable challenge of constructing closed graphene hollow spheres owing to their unique two-dimensional structure. Herein, we first propose a bottom-up route to controllably synthesize a polycrystalline graphene hollow sphere isolated SiNP nanocomposite (Si@void@graphene) through an in situ pyrolysis and metal-catalyzed graphitization reaction, in which glucose and metal sulfate with strictly controlled content and ratio are employed as the carbon source and catalyst precursor, respectively. The obtained graphene hollow spheres with superb mechanical properties offer a permanent structural and electrical environment for the inner SiNPs even insufficient voids are introduced while maintaining a reasonable volumetric energy density. When the void space is designed based on the assumption that Si has only 250% volume change, the Si@void@graphene electrode exhibits high reversible capacity, superior rate capability and much prolonged cycle life as compared to those of the Si@void@amorphous carbon electrode.