Li-ion insertion into cerium dioxide (CeO2) and its subsequent conversion reaction were studied using a CeO2/copper composite electrode in a superconcentrated electrolyte of lithium bis(fluorosulfonyl)amide (LiFSA) and methylphenylamino-di(trifluoroethyl) phosphate (PNMePh) under conditions promoting Li plating/stripping. Since the conversion reaction potential with CeO2 generally lies above the Li plating/stripping level, the conversion ideally occurs first in the cathodic scan. However, the conversion reaction was delayed until after the Li plating in the superconcentrated electrolyte contrary to expectations, whereas this phenomenon was unobserved in a dilute LiFSA/PNMePh electrolyte. Energy-dispersive X-ray spectroscopy and electrochemical impedance analysis indicated that the reversed order of the electrochemical behaviors was caused by the solid electrolyte interphase (SEI) on the CeO2, which had a different material composition and a higher interfacial resistance than the SEI on electrodeposited metallic lithium.