Stabilization of the Interface between a PEO-Based Lithium Solid Polymer Electrolyte and a 4-Volt Class Cathode, LiCoO2, by the Addition of LiPF6 as a Lithium Salt

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+.