Low current drain driven by the low ionic conductivity of a solid polymer electrolyte is one of the major obstacles of solid-state battery. In an effort to improve the ionic conductivity of a solid polymer electrolyte membrane (PEM), polyethylene glycol diacrylate (PEGDA) and monofunctional polyethylene glycol methyl ether acrylate (PEGMEA) were copolymerized via photopolymerization to afford a PEGDA network with dangling PEGMEA side chains. By attaching PEGMEA side branches to the PEGDA network backbone, the glass transition temperature (Tg) was found to decrease, which may be controlled by relative amounts of PEGMEA and PEGDA. Concurrently, the ionic conductivity of a co-PEM consisting of lithium bis(trifluoromethane)sulfonylimide (LiTFSI) salt and a succinonitrile plasticizer in the PEGMEA-co-PEGDA copolymer network was enhanced with increasing PEGMEA side branching. The relationship between the network Tg and ionic conductivity of the branched co-PEM was analyzed in the context of the Vogel-Tammann-Fulcher equation. The plasticized branched co-PEM network exhibited room-temperature ionic conductivity at a superionic conductor level of 10-3 S/cm. Of particular importance is the fact that excellent capacity retention at a high current rate (2 C) in charge/discharge cyclings of Li4Ti5O12/co-PEM/Li and LiFePO4/co-PEM/Li half-cells was achieved. This improved charge retention may be attributed to lower frictional surfaces of the electrodes afforded by side brushes, which probably alleviates formation of irreversible reaction byproducts at the electrode/electrolyte interface.