Revealing Lithium Ion\nTransport Mechanisms and Solvation\nStructures in Carbonate Electrolytes

Optimizing lithium-ion battery (LIB) electrolytes is\nessential\nfor high-current applications such as electric vehicles, yet experimental\ntechniques to characterize the complex structural dynamics responsible\nfor the lithium transport within these electrolytes are limited. In\nthis study, we used ultrafast infrared spectroscopy to measure chemical\nexchange, spectral diffusion, and solvation structures across a wide\nrange of lithium concentrations in propylene carbonate-based LiTFSI\n(lithium bis(trifluoromethanesulfonimide) electrolytes, with the CN\nstretch of phenyl selenocyanate as the long-lived vibrational probe.\nPhenyl selenocyanate is shown to be an excellent dynamical surrogate\nfor propylene carbonate in Li<sup>+</sup> solvation clusters. A strong\ncorrelation between exchange times and ionic conductivity was observed.\nThis correlation and other observations suggest structural diffusion\nas the primary transport mechanism rather than vehicular diffusion.\nAdditionally, spectral diffusion observables measured by the probe\nwere directly linked to the desolvation dynamics of the Li<sup>+</sup> clusters, as supported by density functional theory and molecular\ndynamics simulations. These findings provide detailed molecular-level\ninsights into LIB electrolytes’ transport dynamics and solvation\nstructures, offering rational design pathways to advanced electrolytes\nfor next-generation LIBs.