The fast evolution of electrical transportation system and smart grid desires advanced energy storage systems. Lithium-ion battery (LIB) is the primary option for energy storage in the global market, but its performance is still subject to improvement. Solid electrolyte interphase (SEI) plays a crucial role in LIB behavior, and electrolyte additives provide a convenient approach for fine-tuning the properties of SEI. It is crucial to obtain detailed structural and chemical information of SEIs and electrolyte soluble byproducts to promote mechanism understanding of the electrolyte additives and formation and function of SEI. Here, we propose to use gradient polarity solvent wash in combination with spectroscopic analysis to characterize both the composition of the electrolyte and layered structure of SEI. A series of model cells with a metal cathode, a lithium anode, and various additives are analyzed to reveal the electrolyte decomposition products in the electrolyte and the SEIs on the electrodes from these test cells.