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 (<i>E</i>_a) for the Li-ion insertion reaction in a series of 3.0 M LiFSA/solvent solutions, we found that <i>E</i>_a exhibited a strong linear correlation with the calculated binding energy (Δ<i>E</i>_bind) 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.