Direct Molecular Evidence for Desolvation-Controlled Lithium-Ion Insertion at Graphite Electrodes in Highly Concentrated Electrolytes

Understanding the rate-determining step of lithium (Li)-ion insertion at graphite electrodes is essential for designing fast-charging Li-ion battery electrolyte systems. In this study, we quantitatively investigate how Li-ion solvation affects electrode reaction kinetics in highly concentrated electrolytes. By measuring the activation energy (Ea) for the Li-ion insertion reaction in a series of 3.0 M LiFSA/solvent solutions, we found that Ea exhibited a strong linear correlation with the calculated binding energy (ΔEbind) of Li+–solvent interactions. This result provides direct evidence that, in highly concentrated electrolytes where Li+ is coordinated by both solvent molecules and anions to form ion-ordered structures, the desolvation of solvent molecules, rather than anion decoordination, controls the reaction kinetics. All-atom molecular dynamics (MD) simulations further revealed that, upon electrode polarization, FSA– anions are preferentially excluded from the interfacial electrolyte structure closest to the electrode surface due to electrostatic repulsion, thereby inducing structural relaxation of the Li+ coordination shell. This yields a locally enriched environment of Li+ and solvent molecules, in which the disruption of Li+–solvent interactions (i.e., desolvation), rather than Li+–FSA– interactions, controls the reaction rate and thus determines the activation energy.