Revealing the Sodium Storage Mechanism in High-Temperature-Synthesized Silicon Oxycarbides

Silicon oxycarbides (SiOCs) are considered promising anode materials for sodium-ion batteries. However, the mechanisms of Na⁺-ion storage in SiOCs are not clear. In this study, the mechanism of Na⁺-ion storage in high-temperature-synthesized SiOCs (1200–1400 °C) is examined. Phase separation of the oxygen (O)-rich and carbon (C)-rich SiO_xC_y domains of SiOC during synthesis was accompanied by the evolution of micropores, graphitic layers, and a silicon carbide (SiC) phase. The high-temperature-synthesized SiOCs exhibited a large voltage plateau capacity below 0.1 V (45–63% of the total capacity). Ex situ measurements and density functional theory simulations revealed that within the sloping voltage region, Na⁺-ion uptake occurs mainly in the defects, micropores, C-rich SiO_xC_y phase, and some O-rich SiO_xC_y phases. In contrast, in the voltage plateau below 0.1 V, Na⁺-ion insertion into the O-rich SiO_xC_y phase and formation of Na-rich Si compounds are the main Na⁺-ion uptake mechanisms. The generated SiC phase confers excellent long-term cyclability to the high-temperature-synthesized SiO_xC_y.