Eriko Hayashi

Network Polymer Electrolytes with Free Chain Ends as Internal Plasticizer

Novel polymer electrolytes based on network polymers with free chain ends have been prepared, and the effects of the free chain ends on the thermal and mechanical properties, the ionic conductivity, and the charge-transfer resistance at the lithium electrode interface have been explored in detail. Terminal hydroxyl groups of poly(ethylene oxide-copropylene oxide) triol (MW 7940) were partly methylated, and the residual hydroxyl groups were esterifited by acrylic acid. The resulting macromonomers were cross-linked by photoirradiation in the presence of an electrolyte salt to produce the network polymer electrolytes. The free chain ends, caused by the methylation, were proved to function as an immobile internal plasticizer, as demonstrated by the decreases in the glass transition temperature and the elastic modulus with increasing number of free chain ends; however, creep-free mechanical strength was maintained due to the cross-linked structure. The introduction of free chain ends not only increased the bulk ionic conductivity but also reduced the charge-transfer resistance. As a result, network polymer electrolytes having a large number of the free chain ends exhibited an ionic conductivity of 10 -3 S cm -1 and a charge-transfer resistance of 20 cm 2 at 80°C when lithium bis(trifluoromethylsulfonyl)imide was used as an electrolyte salt.

Preparation, Mechanical Properties, and Electrochemical Characterization of Polymer Gel Electrolytes Prepared from Poly(alkylene oxide) Macromonomers

Various kinds of macromonomers have been specially designed and systematically synthesized for their application to polymer gel electrolyte hosts. Poly(alkylene oxide) was selected for the polymer backbone, and its chain ends were modified to form acryloyl groups. The gel electrolytes were prepared by cross-linking the macromonomers dissolved in a liquid electrolyte of 1 mol/L LiClO{sub 4} in propylene carbonate. Mechanical and electrochemical properties of the gel electrolytes were explored in detail. The results of tensile strength, elongation at break, and shear modulus measurements suggested that a favorable cross-linking reaction proceeded in the polymer gel electrolytes. Cross-linked macromonomer films showed excellent swelling characteristics for liquid-electrolyte uptake and this led to stable liquid retention. High ionic conductivity was attained while keeping sufficient mechanical strength, reflecting stability of the liquid uptake process. The potential stability windows of the gel electrolytes, determined by cyclic voltammetry measurements, were as high as 4.5 V vs Li/Li{sup +}.

Chemical and Electrochemical Characterization of Polymer Gel Electrolytes Based on a Poly(alkylene oxide) Macromonomer for Application to Lithium Batteries

Polymer gel electrolytes were prepared from a molecularly designed poly(alkylene oxide) triacrylate macromonomer and different liquid electrolytes. The cross-linking kinetics of the macromonomer in a liquid electrolyte was studied by using differential scanning calorimetry equipped with ultraviolet light irradiation system (UV-DSC). It was revealed that the photoinitiated cross-linking reaction followed the first order kinetics in terms of macromonomer concentration. The cross-linking reaction finished completely under suitable conditions of ultraviolet (UV) light strength and photoinitiator concentrations. The gel electrolytes prepared exhibited a high ionic conductivity of 3 mS/cm at room temperature and kept I mS/cm even at -20°C, when 1 M LiBF 4 /γ-butyrolactone liquid electrolyte was incorporated in the gel. A linear relationship between the ionic conductivity of liquid electrolytes and that of the gel electrolytes containing the corresponding liquid electrolytes was observed, except for the case where LiPF 6 was used as an electrolyte salt. The interfacial resistances at the lithium/gel electrolyte interfaces were largely dependent on the kind of liquid electrolytes incorporated in the gels. Preliminary results of the application of the gel electrolytes to lithium and lithium-ion batteries implied high utilization of cathode material and cycleability.