The shear relaxation spectra of the solutions of lithium salts in ionic liquids composed of N-methyl-N-propylpyrrolidinium cation paired with bis(trifluoromethanesulfonyl)amide (TFSA-) or bis(fluorosulfonyl) amide (FSA-) anions are determined from 5 to 205 MHz at various concentrations of lithium salts. The addition of lithium salt retards the shear relaxation, together with the increase in the shear viscosity. The normalized spectra reduce to a single curve when plotted against the product of the frequency and zero-frequency shear viscosity, which indicates that the increase in the shear viscosity by lithium salts is ascribed to the increase in the relaxation time. The difference in the shear viscosity of TFSA- and FSA-based ionic liquids is also elucidated in terms of the shear relaxation time. The relationship with previous studies on ionic mobility and liquid structure is also discussed.
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