Unraveling Ethylene Carbonate-Propylene Carbonate Disparity at Electrode Interface Using Femtosecond Sum Frequency Generation Vibrational Spectroscopy^†

Ethylene carbonate (EC) is an important electrolyte used in lithium-ion batteries due to its excellent electrochemical performance. However, propylene carbonate (PC) differs from EC by only one methyl substituent and exhibits markedly poor properties. The EC-PC disparity is still poorly understood at the molecular level. In this study, we demonstrated that femtosecond broadband sum frequency generation vibrational spectroscopy (SFG-VS) with simultaneous measurement of multiple polarization combinations provides a powerful probe for investigating the physicochemical processes at the electrode-electrolyte interface during the charge-discharge cycles of lithium batteries. Using monolayer graphene as the working electrode, we observed the distinct reaction outcomes of EC and PC on the electrode surface. The interfacial reaction of EC occurred only in the first charge-discharge cycle, while the interfacial reaction of PC was ongoing along with the charge-discharge cycles, which explains why EC is a better electrolyte choice than PC. This study provides direct experimental evidence in elucidating the differences in interfacial performance between EC and PC, facilitating a deeper understanding of battery interface reactions and guiding the design of high-performance lithium-ion batteries.