Nanosized Si-based materials have been extensively investigated because of their high gravimetric capacity and stable cycle performance. However, the tap density of nanosized materials is poor, leading to poor volumetric capacity. In this regard, micrometer-sized Si nanoparticles and carbon composites have been introduced to improve the volumetric energy density of Li-ion cells. However, most synthesismethods for these Si/C composites are complex, and thus, only a few methods among them are scalable for mass production. Herein, a scalable solid-state synthesis through self-assembly due to the relative miscibility of hydrophobic and hydrophilic precursors is introduced to obtain micrometer-sized porous carbon spheres containing nanosized Si particles. The self-assembly synthesis uses hydrophilic Si/SiO2 core-shell nanoparticles, hydrophilic phenolic resins, and hydrophobic fumed silica. Because phenolic resin melts and Si/SiO2 core-shells are miscible, the Si/SiO2 core-shells are embedded in the phenolic resins. Immiscible phenolic resin melts and fumed silica lead to the formation of spherical resins. Eventually, the self-assembled micrometer-sized Si/C composite spheres are obtained after heating and HF etching. The tap density of the self-assembled Si/C spheres is much higher than that of the bare Si nanoparticles. In addition, the self-assembled Si/C composite shows excellent cycle performance because of voids in the composite. (C) 2019 The Electrochemical Society.