Lithium-ion intercalation by coupled ion-electron transfer

INTRODUCTION: Lithium-ion batteries power modern portable electronics and electric vehicles by intercalating lithium ions from liquid electrolytes into solid electrode materials. Although predictive models for ion diffusion in solids are well established, the fundamental reaction mechanism for lithium intercalation across the electrode-electrolyte interface remains poorly understood. The Butler-Volmer (BV) equation, a simple model of ion transfer, has been widely used to describe intercalation kinetics in batteries but lacks microscopic details and fails to predict trends in reaction rates across different materials and operating conditions. Moreover, the BV exchange current densities inferred from experiments and simulations for the same electrode-electrolyte pairs can vary by orders of magnitude. Such ambiguities limit the design of batteries with desired energy and power capabilities and reveal the need for an alternative theory of intercalation kinetics. RATIONALE: We hypothesize that lithium intercalation can proceed by coupled ion-electron transfer (CIET), where the insertion of a lithium ion from the electrolyte is facilitated by electron transfer to reduce a neighboring metal cation in the electrode. The mathematical framework of CIET unifies classical ion-transfer models with the Marcus theory of electron transfer and makes predictions that differ substantially from those of existing BV models, such as a strong dependence of the reaction rate on lithium vacancy fraction and the existence of a quantum-mechanical reaction–limited current. RESULTS: Using a charge-adjusted potentiostatic pulse method, we measured intercalation and deintercalation kinetics for common positive electrode materials, including Li_xCoO₂ and Li_xNi_1/3Co_1/3Mn_1/3O₂, in a number of electrolytes and temperatures. The measured current densities increase linearly with increasing lithium vacancy fraction (1 − x) and increasing overpotential (up to 150 mV), which cannot be explained by the prevailing BV model. Instead, the electron transfer–limited regime of CIET theory fits the data well, collapsing hundreds of data points onto a universal current-voltage curve with only a small set of intrinsic material parameters, including the electrode reorganization energy, electronic coupling, and free energies of ion transfer and surface adsorption. These parameters offer guidance for experimental measurements, engineering simulations, and computational screening of faster electrodes and electrolytes. Moreover, rate capability measurements of eight electrode materials revealed a linear decay of battery capacity with increasing current—a signature of CIET reaction limitation in stark contrast to BV kinetics. CONCLUSION: This work presents a unified experimental and theoretical framework for lithium intercalation based on CIET, supported by evidence across a wide range of common electrodes, electrolytes, and operating conditions. The measured intercalation rates are orders of magnitude smaller than those used in computational modeling of batteries and cannot be explained by solid diffusion or empirical film resistances. Instead, the data are consistent with CIET theory, which links the reaction rate to microscopic charge-transfer properties, such as ion-transfer free energy and reorganization energy. This mechanistic understanding challenges the conventional view that lithium-ion batteries are diffusion limited and highlights the electrode-electrolyte interface as a key kinetic bottleneck. By linking interfacial electrochemistry with electrode performance, CIET theory may catalyze new strategies to develop faster-charging, higher-power energy storage technologies.