Mix-salts were prepared by mixing lithium borate (salt A or salt B) with lithium aluminate (salt C). Polymer electrolytes were prepared by dissolving lithium salt in PEO. Mix-salt polymer electrolytes exhibited higher ionic conductivities than pure-salt polymer electrolytes. The optimum-mixing ratio was investigated. Conductivity as high as 1×10−4 Scm−1 at 40°C was obtained for the optimized electrolyte system. A potential window of 4.3V was determined for the mix-salt electrolyte. Good charge–discharge performance was observed for the mix-salt electrolyte composed cell, LiNi0.8Co0.2O2//salt A(3)/salt C(11.8) 2/1-PEO22//Li.