Critical Roles of Si‐O‐C and Free‐Carbon Units in High‐Performance SiOC Anodes: Regulating Composition via Precursor Functional Groups

Owing to its low volume expansion and high cycling stability, silicon oxycarbide (SiOC) has gained traction as a Li-ion negative electrode. Nevertheless, the electrochemical roles of its principal building blocks, particularly the Si-O-C tetrahedra units and free-carbon (C_free), have not been fully elucidated. In this study, spherical and blocky SiOC materials were synthesized from silane bearing different organic functional groups via hydrolysis, polycondensation, and pyrolysis. These functional groups dictate the final chemical and microstructural characteristics of the material. The results demonstrate that Si-O-C tetrahedra units act as the primary host for Li⁺. Increasing reversible units improves capacity and promotes pseudocapacitive behavior, enhancing reaction kinetics and Li⁺ transport. Meanwhile, C_free, derived from organic group decomposition, forms a conductive network that facilitates electron transfer and improves active site utilization. Consequently, the spherical SiOC-VTES sample, rich in reversible structural units and an appropriate amount of C_free, delivers a reversible capacity of 1244.2 mAh g^-1 at 0.1 A g^-1. When tested at 2.0 A g^-1, it retains 662.4 mAh g^-1 and demonstrates outstanding stability with 86.4% after 1000 cycles. These findings elucidate composition-structure-performance relationships and provide practical guidance for developing advanced SiOC anodes.