Visualizing asymmetric phase separation driven by surface ionic diffusion in lithium titanate

Abstract

Phase transition kinetics of battery electrodes is a critical factor affecting power and durability. Li₄Ti₅O₁₂ exhibits high-rate capability despite low bulk lithium diffusivities and kinetic barriers to phase separation. However, the phase transition and high-rate capability of mechanisms continue to remain unclear. Here, we reveal spatially asymmetric phase separation during lithiation and delithiation through real-time monitoring of nanoscale lithium distribution. Surface ionic diffusion, which exhibits intermediate diffusion kinetics for the delithiated Li₄Ti₅O₁₂ and lithiated Li₇Ti₅O₁₂ phases, serves as the primary driving force for asymmetric phase separation. During lithiation, Li₇Ti₅O₁₂ with lower ionic diffusivity forms on the particle shell by surface diffusion even if the particle is not completely covered with an electrolyte; conversely, the core–shell structure does not form during delithiation. Microscale asymmetric phase separation and different ionic diffusivities between the two phases lead to asymmetric polarization in charge–discharge measurements. Preferential Li-ion diffusion at grain boundaries is also observed. This study highlights the feasibility of surface and grain boundary engineering for mediating ionic diffusion.