At an electrode interface, redox mechanisms associated with electrolyte solvent breakdown into components of the solid-electrolyte interphase (SEI) layer are often invoked to rationalize the cycling performance of battery materials. Mechanisms of SEI formation frequently include the release of gas, whose composition serves as an indicator of accompanying chemical structures comprising the SEI. Recently, our group monitored reduction of fluoroethylene carbonate (FEC) and ethylene carbonate (EC) electrolyte solvents on silicon nanoparticles using interface-sensitive vibrational sum frequency generation (SFG) spectroscopy in tandem with electrochemistry. We observe SFG signatures that suggest carbon monoxide (CO) evolution on nano-Si proceeds at distinct lithiation potentials for different electrolyte solvents. These results appear to indicate that FEC undergoes this reduction pathway beginning at very modest degrees of lithiation (potential) of silicon. However, EC appeared to be more sensitive to the degree of Si lithiation, reducing to CO only at the highest lithiation state of Si. Upon prolonged reduction of EC, we additionally observe the formation of poly(EC), which is soluble in carbonate-based electrolytes and appears to form through the release of CO<sub>2</sub>. The results of this study validate previous computational descriptions of FEC and EC reduction, as well as provide insight on a failure mechanism EC-based electrolytes impart upon Si anodes. Implications for the prospects of monitoring and manipulating interfacial electric fields to control interfacial redox processes in electrochemical energy storage devices will also be discussed.

Solvent reduction at Si electrode/electrolyte junctions: SEI evolution probed by vibrational spectroscopy

2019 Spring National Meeting

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