Conceptualizing Surface-Like Diffusion for Ultrafast Ionic Conduction in Solid-State Materials.

Surface-like diffusion is a recently proposed concept to explain the mechanism of ultrafast ionic conduction in high-rate oxide (e. g., niobium oxides and their alloys with TiO₂ and WO₃) and framework materials (e. g., Prussian blue analogs). This perspective seeks to illustrate the structural origin, theoretical foundation, and experimental evidences of surface-like diffusion. Unlike classical lattice diffusion, which typically involves ionic hopping between adjacent interstitial sites in solids, surface-like diffusion occurs when ions-that are significantly smaller than the interstitials-migrate along the off-center path in the diffusion channel. This mechanism results in an exceptionally low activation energy (E_a) down to 0.2 eV, which is crucial for achieving high-rate performance in electrochemical devices such as lithium-ion and sodium-ion batteries. This concept review also discusses the criteria to identify materials with potential surface-like diffusion and outlines theoretical and experimental tools to capture such phenomenon. Several candidates for further investigation are proposed based on the current understanding of the mechanism.