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Understanding the complex interfacial phenomena at electrolyte/electrode interfaces is crucial for optimizing the electrochemical performance and durability of lithium-ion batteries. Traditional characterization techniques such as X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy provide valuable information but often lack molecular-level specificity or true in situ capability under operating conditions. Sum frequency generation (SFG) spectroscopy, a second-order nonlinear optical technique, offers unique advantages by selectively probing interfaces with molecular sensitivity, surface specificity, and non-contact detection. It enables real-time monitoring of vibrational modes, molecular orientations, and dynamic evolution at buried interfaces, thereby revealing key insights into interfacial reactions and structural transformations. This review summarizes recent advances in applying SFG spectroscopy to investigate electrolyte/electrode interfaces in lithium-ion batteries. Emphasis is placed on the characterization of anode and cathode interphases, including solvent adsorption, formation of solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI), and high-voltage interfacial reactions. Comparative analysis with complementary surface-sensitive techniques is also discussed. Finally, current challenges and future opportunities for SFG in electrochemical systems are highlighted, providing perspectives for advancing interfacial understanding and guiding rational design of next-generation battery materials.
Zhang et al. (Sat,) studied this question.