Polaronic States and Their Impact on Charge Carrier Mobility in Lithium Titanium Oxide Battery Electrodes

Lithium-ion batteries are without a doubt a key technology in the coming energy revolution. It is thus surprising that one of the more prevalent Li battery anode materials, reduced lithium titanium oxide (LTO, Li₄Ti₅O₁₂), is still poorly understood on a microscopic level. While recent theoretical and experimental evidence suggests that a polaron hopping mechanism is responsible for the increased electronic conductivity of reduced LTO (compared to pristine LTO), no such explanation exists for the concurrent improvements to the ionic mobility. In this computational study, we show that the presence of polaronic Ti³⁺ centers leads to a significant lowering of Li hopping barriers in both bulk and surface reduced LTO. In agreement with experimental observations our results also suggests that polaron formation upon lithiation of LTO leads to a similar reduction of barrier heights. Further, to gauge polaronic charge mobility, we compute the relative stabilities of different localization patterns and estimate polaron hopping barrier heights. Altogether, our analysis hints at a correlated movement of Li ions and polarons, highlighting LTO’s potential for rational defect engineering.