Solvation structure dependent ion transport and desolvation mechanism for fast-charging Li-ion batteries.

The solvation structures of Li in electrolytes play prominent roles in determining the fast-charging capabilities of lithium-ion batteries (LIBs), which are in urgent demand for smart electronic devices and electric vehicles. Nevertheless, a comprehensive understanding of how solvation structures affect ion transport through the electrolyte bulk and interfacial charge transfer reactions remains elusive. We report that the charge transfer reaction involving the desolvation process is the rate-determining step of the fast charging when ion conductivity reaches a certain value as determined by investigating electrolytes with eight conventional solvents (linear/cyclic carbonate/ether). The physicochemical characteristics of solvent molecules can result in strong ion-ion, moderate ion-dipole, strong ion-dipole, and weak ion-dipole/ion-ion interactions, respectively, in which the speed of the charge transfer reaction follows the above order of interactions. Among all solvents, dioxolane (DOL) is found to enable strong ion-ion interactions in electrolytes and thus exhibits exceptional fast-charging performance and it can still retain 60% of the initial capacity at 20C (1C = 170 mA g) with a polarization of merely 0.35 V. Further experimental characterization and theoretical calculation reveal that the aggregates in DOL electrolytes contribute to hopping assisted ion transport and facilitate the desolvation process of Li. Our results deepen the fundamental understanding of the behavior of Li solvation and provide an effective guiding principle for electrolyte design for fast-charging batteries.