Shear and Structural Relaxation in Superconcentrated Lithium Bis(trifluoromethanesulfonyl)amide Aqueous Solutions

Superconcentrated aqueous electrolyte solutions have been actively studied since the first report on lithium-ion batteries operating with superconcentrated aqueous solutions of lithium salt. One of the drawbacks of superconcentrated electrolytes is their high viscosity, which reduces ionic conductivity and adversely affects battery performance. In this work, we investigated the concentration- and frequency-dependent viscosity and microscopic dynamics of aqueous lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) solutions using shear relaxation and quasi-elastic neutron scattering (QENS) spectroscopies. They are compared with each other to reveal the mechanism of viscosity and to provide novel insights into their fundamental physicochemical behavior. As the concentration increased from 3 to 21 molal, a strong increase in viscosity and a significant retardation of shear relaxation were observed. The comparison between the shear relaxation and QENS shows that the wavenumber of their matching shifts to lower wavenumbers as the concentration is reduced, which suggests that the spatial scale of the liquid structure is enlarged with decreasing concentration. As a result, we observed viscoelastic relaxation even in the low-viscosity region because of a characteristic heterogeneous structure absent in other normal liquids.