Li plating/stripping on Cu and Y₂O₃ (Cu + Y₂O₃) electrodes was examined in a super-concentrated electrolyte of lithium bis(fluorosulfonyl)amide and methylphenylamino-di(trifluoroethyl) phosphate. In principle, Li⁺ ions cannot intercalate into a Y₂O₃ crystal because its intercalation potential obtained from first-principles calculations is −1.02 V vs. Li⁺/Li. However, a drastic decrease in the electrode potential and a subsequent constant-potential region were observed during Li plating onto a Cu + Y₂O₃ electrode, suggesting that Li⁺ interacted with Y₂O₃. X-ray diffraction (XRD) patterns and X-ray absorption fine structure (XAFS) spectra of the Cu + Y₂O₃ electrodes after the Li plating were recorded to verify this phenomenon. The XRD and XAFS results indicated that the crystallinity of Y₂O₃ crystals was lowered because of attack by Li⁺ ions or that the Y₂O₃ crystal structure was broken while the +3 valence state of Y was maintained.