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