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A star-shaped copolymer, poly͑styrene͒-block-poly͓poly͑ethylene glycol͒ methyl ethyl methacrylate ͑PS-block-PPEGMA 2 ͒ 8 , was synthesized by the combination of living anionic polymerization of styrene and ruthenium-catalyzed living radical polymerization of poly͑ethylene glycol͒ methyl ether methacrylate. The prepared star-shaped copolymer was characterized to evaluate its use as a solid polymer electrolyte ͑SPE͒ in lithium-ion batteries. The star polymer comprised a hard, condensed poly͑styrene͒ part at the center, which enhanced the mechanical properties of the solid-state polymer, and a soft, mobile poly͓poly͑ethylene glycol͒ methyl ethyl methacrylate͔ ͑PPEGMA͒ outer part that was responsible for the high ionic conductivity of the SPE. The design of this star polymer resulted in a well-ordered spherical microphase separation structure, in which the individual star polymers were systematically ordered to form the PPEGMA continuous phase distinctly observed in transmission electron microscopy and atomic force microscopy images. The SPE containing the lithium bis͑pentafluoroethanesulfonyl͒ imide salt exhibited high ionic conductivities due to the unique morphology of the polymer; the ionic conductivity of this salt was 10 −4 S cm −1 at 30°C and 10 −5 S cm −1 at 5°C at ͓Li͔/͓EO͔ = 0.03.Lithium-ion rechargeable batteries are energy-storage devices that have several advantages over conventional secondary batteries such as nickel-cadmium ͑Ni-Cd͒ batteries and nickel-metalhydrides ͑Ni-MH͒ batteries. The advantages of lithium-ion rechargeable batteries are their high electrical performance for a long life cycle, high energy density, low weight, and high operational voltage; further, these batteries do not exhibit memory effects. These features have caused an increase in the popularity of the use of lithium-ion batteries as standard power sources in portable devices such as mobile phones and laptop computers. The application of these batteries has further extended to large-scale equipment such as electrical-power storage systems and in-car systems. In such systems the safety of the device is one of the most important criteria to be satisfied before selecting the batteries. However, most commercially available cells contain liquid or liquid-based electrolytes that are made of flammable organic solvents; thus, these cells posses certain risks such as leakage and spontaneous combustion of the electrolyte. Therefore, there is a need to develop efficient polymer electrolytes that do not have any liquids, i.e., solid polymer electrolytes ͑SPEs͒.Poly͑ethylene oxide͒ ͑PEO͒ coupled with a lithium salt is a typical example of an ion conductive material, and its potential use as an SPE has been studied extensively. 1-25 However, the ionic conductivity of the electrolyte of this material ͑approximately 10 −7 S cm −1 ͒ at room temperature was not sufficient for its practical use in batteries. It is desirable for SPEs to exhibit liquidlike ionic conductivity and mechanical properties that are capable of separating the electrode. Alt...