Design of polymer electrolytes based on a lithium salt of a weakly coordinating anion to realize high ionic conductivity with fast charge-transfer reaction

To design polymer electrolytes with high ionic conductivity as well as fast charge-transfer reaction at the electrode interface, electrolyte properties of a novel lithium salt of a weakly coordinating anion, lithium tetra(1,1,1,3,3,3-hexafluoro-2-propyl)aluminate, LiAl[OCH(CF3)(2)](4), have been studied in the bulk, in aprotic solvents, and in a polyether. Although the lithium salt melts at fairly low temperature, it shows poor conductivity even in the molten state because of its strong ionic association. However, in aprotic solvents, LiAl[OCH(CF3)(2)](4) exhibits a relatively high degree of dissociation because of weak coordination ability of the anion toward the cation. This is reflected in the higher ionic conductivity than that of common lithium salts, LiN(SO2CF3)(2) and LiBF4, at an identical concentration in the low polar solvents. In a polyether, an increase in the glass-transition temperature (T-g) of the polymer electrolytes with salt concentration is less marked in the LiAl[OCH(CF3)(2)](4) system. The lithium salt can be incorporated in the matrix polyether at high concentrations without a loss in the ionic conductivity. The interface between the polyether electrolyte containing LiAl[OCH(CF3)(2)](4) and a metallic lithium electrode is statically stable for a long time, and the charge-transfer resistance decreases with increased salt concentration. These results indicate that an increase in LiAl[OCH(CF3)(2)](4) concentration in the polyether facilitates not only an increase in the ionic conductivity but also a decrease in the interfacial resistance.