3 research outputs found
Lithium Species in Electrochemically Lithiated and Delithiated Silicon Oxycarbides
The
work described herein deals with efforts to make a persuasive correlation
between structural characteristics and electrochemical lithium storage
for a silicon oxycarbide prepared from polyÂ(methylhydrogensiloxane)
and divinylbenzene. Structural characterization reveals that the silicon
oxycarbide includes excess free carbon in an amorphous network. The
reversibility of lithiation and delithiation in the silicon oxycarbide
reaches 74% between 0.005 and 3 V relative to lithium at the first
cycle but falls to only ca. 30% between 0.4 and 3 V. We found two
resonances at 0 and 2.4 ppm in the <sup>7</sup>Li magic angle spinning
nuclear magnetic resonance spectrum of the silicon oxycarbide lithiated
to 0.4 V, whose contributions are 67 and 33%, respectively, and thus
are in good agreement with the reversibility observed between 0.4
and 3 V. The fully lithiated silicon oxycarbide shows a single resonance
at ca. 3â9 ppm, which tends to broaden at lower temperatures
to â120 °C, whereas the fully delithiated silicon oxycarbide
has a single resonance at 0 ppm. These results indicate that both
reversible and irreversible lithium species have ionic natures. The
Li K edge in electron energy loss spectroscopy does not show clearly
any identified near-edge fine structures in the inner part of the
silicon oxycarbide after delithiation. Near the surface, on the other
hand, LiF and oxygen- and phosphorus-containing compounds were found
to be the major constituents of a solid electrolyte interface (SEI)
layer. Over repeated lithiation and delithiation, the SEI layer appears
to become thick, which should in part trigger capacity fading