By dissolving plasticizing lithium borate (Salt A) and LiN(SO2CF3)2 (LiTFSI) at different ratios in poly(ethylene oxide) (PEO), a series of solid-state mix-salt polymer electrolytes were prepared. Higher ionic conductivities were determined for the mix-salt polymer electrolytes than for the pure-salt counterparts. The optimum mixing ratio of the two salts was explored. The electrochemical stability and interfacial performance of the mix-salt polymer electrolytes were also investigated. A battery testing using LiNi0.8Co0.2O2 as cathode material and lithium as anode material was executed to assess the cyclic performance of the electrolyte.