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Here, the time dependence of the interfacial resistance for Li/polyethylene oxide (PEO)-Li(CF₃SO₂)₂N (LiTFSI)-LiPF₆/LiCoO₂ cells was measured to investigate the stabilization effect of LiPF₆ on the interface between a solid polymer electrolyte (SPE) and a 4-volt class cathode, LiCoO₂. Impedance measurements under the applied potentials between 4.1 V and 4.4 V vs. Li/Li⁺ indicated that the addition of LiPF₆ to LiTFSI was effective in improving the stability at high potentials such as 4.4 V vs. Li/Li⁺. In contrast, the resistance of the non-doped PEO-LiTFSI/LiCoO₂ interface increased with time under the lower potential of 4.1 V vs. Li/Li⁺. Fairly good cycle performance was obtained for the LiPF₆-doped cell, even at a cut-off voltage of 4.5 V vs. Li/Li⁺.