(680h) Kinetics of Coupled Ion-Electron Transfer for Li-Ion Intercalation

Understanding the microscopic mechanism of ion intercalation is crucial for the design of energy storage devices, such as Li-ion batteries, with optimal power and energy densities. Despite significant advancements in understanding Li-ion diffusion and discoveries of new electrodes and electrolytes, the molecular process of ion intercalation across electrode-electrolyte interfaces is not well established. Li⁺ intercalation kinetics has been traditionally treated by the empirical Butler-Volmer kinetics, but remains poorly measured and understood. In this study, we developed experimental electrochemical methods to probe ion intercalation kinetics, and provided unique experimental evidence to support the microscopic mechanism of Li⁺ intercalation, described by the coupled ion-electron transfer mechanism. Current-voltage responses and reaction-limited capacities, indicative of small and large overpotentials, respectively, were measured across various electrode materials, including common Li_xCoO₂ and NMCs, and were consistent with the proposed theory. A universal dependence of the intercalation rate on the lithium-ion filling fraction was revealed, as well as temperature and electrolyte effects, consistent with the theoretical description that classical ion transfer from the electrolyte is coupled with quantum-mechanical electron transfer from the electrode. Our findings suggest that the proposed mechanism applies to a variety of intercalation materials used in energy storage, and governs the power density at low and moderate applied current densities. The possibility of modifying the reaction-limited current with electrodes and electrolytes opens new directions for interfacial engineering of Li-ion batteries.