The dynamic behavior of 1.0 mol dm−3 LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) mixed solvent (1:1 in volume) on lithium iron phosphate (LiFePO4) thin film electrode fabricated by RF-sputtering was investigated by in-situ Fourier transform infrared (FT-IR) spectroscopy. The solvation and desolvation reactions of EC and DEC with Li+ ion were observed in the charge and discharge processes of thin film electrode, respectively. In addition, the adsorption of solvent molecules on the electrode surface was suggested by polarized IR analyses. Lithium alkylcarbonate, lithium carboxylate and lithium carbonate were formed as decomposition products of the electrolyte solution at the anodic potential more than 4.5 V vs. Li/Li+.