Lithium-metal batteries, which use lithium metal as the anode, have the advantage of much higher energy density over the commercially used lithium-ion batteries with graphite as the anode. However, during repeated charge-discharge cycles, lithium dendrites may form due to uneven deposition of lithium on the anode, and lithium-dendrite-growth-induced short-circuits are always a problem. Using solid polymer electrolyte (SPE) for lithium-metal batteries has the benefit of using the electrolyte as the electrode separator while inhibiting growth of lithium dendrites. Poly(ethylene glycol) (PEG) has been extensively used for SPE systems due to its strong lithium ion solvating ability and high dielectric constant. In this study, crosslinked PEG polymer electrolyte membranes were synthesized, and low molecular weight PEG were introduced to the system. With the increased amount of small PEG molecules added, the ionic conductivities of the SPEs showed significant increase. Chemical structure, thermal and electrochemical properties of the SPEs were also studied.