Rechargeable Li batteries provide a high energy density to satisfy our daily life. However, a deep understanding of electrochemical electrode/electrolyte interfaces is of crucial importance for designing electrolytes or electrode materials to achieve high-performance Li batteries. In this work, the advantages of sum frequency generation (SFG) spectroscopy are outlined in studying the solvent adsorption manners, the static electric double layer (EDL) structures, the initialization and evolutions of the solid electrolyte interphase (SEI), and the catalytic regulation of interfacial Li$^+$ desolvation dynamics. The fundamental logic to correlate the interfacial molecular information to the transformation, transfer, and diffusion of the ionic charge carriers, and thus to the battery performance, is also highlighted. The technical challenges and solutions for conducting operando SFG on battery systems are discussed, and the physical models beyond the traditional EDL theory for SFG data interpretation are prospected. Finally, efforts to push in situ SFG techniques forward in high time resolution and real-time fine-spectral feature resolution, as well as the applicability in practical rough interface morphologies and chemical heterogeneity, are further prospected.