Fluoroethylene Carbonate as a Directing Agent in Amorphous Silicon Anodes: Electrolyte Interface Structure Probed by Sum Frequency Vibrational Spectroscopy and Ab Initio Molecular Dynamics

Abstract

Fluorinated 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 (C═O) 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.

Fluorine and Lithium: Ideal Partners for High‐Performance Rechargeable Battery Electrolytes

“…This effect is believed to lead to better Li + diffusion, as lithium ions can diffuse in astraight line through the interface. [68] However,this surface alignment effect is only observable close to the electrode surface,whereas the parallel surface alignment is maintained in the outer interface layers. [69] This effect is illustrated in Figure 6.…”

Dimethylacrylamide, a novel electrolyte additive, can improve the electrochemical performances of silicon anodes in lithium-ion batteries

“…The cathodic peak is near 0 V and is ascribed to lithiation of the Si anode. 29,32 In the following anodic process, the value of anodic peak of the Si anode slightly increases aer addition of 1 wt% and 2.5 wt% DMAA additives in the electrolyte. However, at a high content of DMAA (5 wt% to 10 wt%), a signicant reduction in the 0.3 V delithiation peak represents an increase in the electrode polarization, implying that a thick passivation lm has developed.…”

“…Nowadays, FEC is widely adopted in electrolyte solutions with Si anodes. 15,[20][21][22][26][27][28][29][30][31][32][33][34][35] The main products of FEC decomposition on the electrode are known to be ROCO 2 Li, LiF and polymerized FEC. These products contribute to a uniform and stable SEI layer on the silicon particle surface, thereby preventing further decomposition of EC and LiPF 6 salts at lower potential conditions.…”

Microscopic Elucidation of Solid‐Electrolyte Interphase (SEI) Film Formation via Atomistic Reaction Simulations: Importance of Functional Groups of Electrolyte and Intact Additive Molecules

“…Using transmission electron microscopy (TEM) and scanning electron microscopy (SEM), the SEI film surface was found to become clearly smoother in FEC‐added electrolyte compared to that in the FEC‐free one, i. e., the pristine electrolyte ,. Moreover, the recent experimental studies clarified that the intact FEC molecule plays a direction agent role to create an ordered orientation of electrolyte on anode surface, suggesting that this rearrangement of solvent molecules is effective to produce a better suited SEI film ,. However, it is difficult to identify their microscopic origins solely through experimental investigations since there is still no direct technique to observe the transient processes of the SEI film formation on the anode surface.…”