Synthesis of Hyperbranched Polyether Macromonomers and Ion-conducting Behavior of Their Polymer Electrolytes

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Progress of the study of ion-conducting polymers stimulates to alter conventional electrochemical systems, which consist of electrolytes and liquid electrolytes, to solid electrochemical systems. Polymer electrolytes have, thus, occupied an important position in solid state electrochemistry, because of their unique properties, such as thin film forming property, good processability, flexibility, light weight, elasticity (plasticity), and transparency as well as relatively high ionic conductivity and wide potential window in the solid states. Especially, it has been considered to be important and promising to apply polymer electrolytes to solid-state lithium/polymer batteries, which ensure safety, high energy density, freedom in shape geometry, and processability in large-scale-production. The development of large-scale high-energy-density rechargeable lithium/polymer electrolyte batteries, applicable to electric vehicles, is one of the most challenging science and technology in solid s … More tate electrochemistry.Polymer electrolytes are solid solutions of electrolyte salts in ion-conducting polymers and exhibit relatively high ionic conductivity at ambient temperatures. Ion transport in the polymer electrolytes is considered to be cooperative with local segmental motion of the polymers. In this study, polyether-based ion-conducting polymers having free chain ends in high densities have been prepared as matrixes for electrolyte salts. Our working concept of this study to achieve highly conducting polymer electrolytes is that fast molecular motion of short and flexible ether side chains in the matrix polymers would contribute to fast ion transport. 2-(2-Methoxyethoxy)ethyl glycidyl ether (MEEGE) has been copolymefized with ethylene oxide (EO) to obtain P(EO/MEEGE) as the matrix polymers. EO and MEEGE were copolymerized by KOH-catalyzed ring-opening anionic polymerization in the presence of 2-(2-methoxyethoxy)ethanol to give OH-terminated oligomers, followed by esterification reaction of the OH groups by acrylic acid to give P(EO/MEEGE) macromonomers. The macromonomer/salt solutions containing a photo-initiator were cast on glass plates and irradiated with UV light, resulting to from network polymer electrolytes. The conductivity for the network polymer electrolytes, which give the best conductivity in this study, reaches 10ィイD1-4ィエD1 ScmィイD1-1ィエD1 and 10ィイD1-3ィエD1 ScmィイD1-1ィエD1 at 30℃ and at 80℃, respectively, and even at 0℃ it is close to 10ィイD1-5ィエD1 ScmィイD1-1ィエD1. Although LiィイD1+ィエD1 transport number of the polymer electrolytes is lower than 0.5, as is generally seen polyether based polymer electrolytes, the electrochemically stable domain is wider than 4 V vs. Li/LiィイD1+ィエD1. The presence of the conductivity maximum as a function of the macromonomer molecular weight, irrespective of the constant Tg, indicates that the dendritie-side-chain motion that can not be scaled by Tg and mainly affects the fast ion transport. The introduction of hyper-branched structure is quite effective to achieve fast ion transport in polymer electrolytes. Less