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Fluoroethylene Carbonate as a Directing Agent in Amorphous Silicon Anodes: Electrolyte Interface Structure Probed by Sum Frequency Vibrational Spectroscopy and Ab Initio Molecular Dynamics
journal contribution
posted on 2017-12-18, 00:00 authored by Yonatan Horowitz, Hui-Ling Han, Fernando A. Soto, Walter T. Ralston, Perla B. Balbuena, Gabor A. SomorjaiFluorinated compounds are added to carbonate-based electrolyte solutions
in an effort to create a stable solid electrolyte interphase (SEI).
The SEI mitigates detrimental electrolyte redox reactions taking place
on the anode’s surface upon applying a potential in order to
charge (discharge) the lithium (Li) ion battery. The need for a stable
SEI is dire when the anode material is silicon as silicon cracks due
to its expansion and contraction upon lithiation and delithiation
(charge–discharge) cycles, consequently limiting the cyclability
of a silicon-based battery. Here we show the molecular structures
for ethylene carbonate (EC): fluoroethylene carbonate (FEC) solutions
on silicon surfaces by sum frequency generation (SFG) vibrational
spectroscopy, which yields vibrational spectra of molecules at interfaces
and by ab initio molecular dynamics (AIMD) simulations at open circuit
potential. Our AIMD simulations and SFG spectra indicate that both
EC and FEC adsorb to the amorphous silicon (a-Si) through their carbonyl
group (CO) oxygen atom with no further desorption. We show
that FEC additives induce the reorientation of EC molecules to create
an ordered, up-right orientation of the electrolytes on the Si surface.
We suggest that this might be helpful for Li diffusion under applied
potential. Furthermore, FEC becomes the dominant species at the a-Si
surface as the FEC concentration increases above 20 wt %. Our finding at open circuit potential can now initiate additive
design to not only act as a sacrificial compound but also to produce
a better suited SEI for the use of silicon anodes in the Li-ion vehicular
industry.
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vibrational spectroscopychargecarbonyl groupsilicon anodessum frequency generationelectrolyte interphaseSum Frequency Vibrational SpectroscopyLi diffusionup-right orientationElectrolyte Interface Structure Probedcarbonate-based electrolyte solutionsDirecting AgentcircuitEC moleculesAIMD simulationsAb Initio Molecular Dynamics Fluorinated compoundsSEIFluoroethylene CarbonateAmorphous Silicon Anodesa-Si surfaceFEC concentration increaseselectrolyte redox reactionssilicon cracksanode materialyields vibrational spectraion batterysilicon surfacessilicon-based batterySFG spectraadditive designethylene carbonateFEC adsorbFEC additivesab initioSi surface
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