Lithium-ion intercalation by coupled ion-electron transfer

Lithium-ion batteries are powering a revolution in electrification, but the underlying intercalation mechanism at the electrified interface remains poorly understood. Here, we provide experimental and theoretical evidence that lithium intercalation occurs by coupled ion-electron transfer (CIET), in which classical ion transfer from the electrolyte is coupled with quantum-mechanical electron transfer from the electrode to form an ion-electron pair in the reduced state. Current-voltage responses and reaction-limited capacities, corresponding to small and large overpotentials, respectively, were measured for common electrode materials and linked by the theory. The experiments showed universal dependence of the (de-)intercalation rate on Li-ion filling fraction, as well as temperature and electrolyte effects consistent with the theory. These results could be used to guide the design of high-rate battery interfaces that maximize the CIET reaction-limited current.