Unlocking dormant Li⁺ pathways drives fast ion transport in Li₄Ti₅O₁₂ oxide spinels.

Abstract

Li-rich lithium titanate (Li_4+<i>x</i>Ti₅O₁₂, <i>x</i> > 0) is known for superior ionic conductivity, yet we show that stoichiometric Li₄Ti₅O₁₂ (LTO, <i>x</i> = 0), typically characterized by sluggish ion dynamics, can be transformed into a fast ion conductor without changing its Li content. Local defects, most notably oxygen vacancies, introduced by vacuum treatment activate a previously inaccessible 8<i>a</i>-16<i>c</i>-8<i>a</i> diffusion pathway in stoichiometric LTO, markedly enhancing Li⁺ mobility throughout the bulk. Using a synergistic combination of impedance spectroscopy, solid-state nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR), we resolve the diffusion processes responsible for this transformation. Lithium NMR unambiguously shows that a formerly localized Li⁺ hopping process becomes long-range transport after vacuum treatment, evidencing the activation of extended diffusion pathways. Atomic-scale insights reveal defect-driven structural and dynamical priming that enables rapid Li⁺ insertion, establishing zero-strain LTO as a leading anode for solid-state lithium batteries. Defect-mediated transport emerges as the key mechanism underlying these dynamics, resolving the long-standing conductivity puzzle of stoichiometric spinel LTO (<i>x</i> = 0) and indicating transferable pathways in related spinel-type ion conductors with similar Li⁺ distributions.