Synergistic O^2-/Li⁺ Dual Ion Transportation at Atomic Scale

The ion migration during electrochemical process is a fundamental scientific issue for phase transition behavior and of technical importance for various functional devices, where cations or anions are active under electrical bias. Usually only one type of functional ion, O^2- or Li⁺, is activated due to their different migration energy barriers, cooperated by the valence change of other immobile ions in the host lattice matrix, e.g., Co²⁺/Co³⁺ and Mn³⁺/Mn⁴⁺ redox couples, owing to the charge neutralization. Here we select spinel Li₄Ti₅O₁₂ as anode and construct an all-solid-state battery under a transmission electron microscope; a synergistic transportation of O^2- and Li⁺ driven by an electrical bias was directly observed at the atomic scale. A small amount of oxygen anions was extracted firstly as a result of its lowest vacancy formation energy under 2.2 V, leading to the vertical displacement of oxygen. Up to 2.7 V, an ordered phase with both Li- and O- deficiency formed. The Li⁺ and O^2- ions are simultaneously extracted out from the [LiO₄] tetrahedra due to the electroneutrality principle. The migration paths of O and Li have been proposed and verified by first-principles calculations. These results reveal a brand new synergistic ion migration manner and may provide up-to-date insights on the transportation process of lithium ion conductors.