A triblock copolymer of benzyl methacrylate and oligo(ethylene glycol) methyl ether methacrylate was polymerized to form the general structure PBnMA-POEGMA-PBnMA, using atom transfer radical polymerization (ATRP). The block copolymer (BCP) was blended with lithium bis(trifluoro methylsulfonate) (LiTESI) to form solid polymer electrolytes (SPEs). AC impedance spectroscopy was used to study the ionic conductivity of the SPE series in the temperature interval 30 degrees C to 90 degrees C. Small-angle X-ray scattering (SAXS) was used to study the morphology of the electrolytes in the temperature interval 30 degrees C to 150 degrees C. By using benzyl methacrylate as a mechanical block it was possible to tune the microphase separation by the addition of LiTESI, as proven by SAXS. By doing so the ionic conductivity increased to values higher than ones measured on a methyl methacrylate triblocic copolymer-based electrolyte in the mixed state, which was investigated in an earlier paper by our group. A Li vertical bar SPE vertical bar LiFePO4 half-cell was constructed and cycled at 60 degrees C. The cell produced a discharge capacity of about 100 mAh g(-1) of LiFePO4 at C/10, and the half-cell cycled for more than 140 cycles.