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 SiOxCy 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 SiOxCy phase, and some O-rich SiOxCy phases. In contrast, in the voltage plateau below 0.1 V, Na+-ion insertion into the O-rich SiOxCy 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 SiOxCy.