Unlocking dormant Li ⁺ pathways drives fast ion transport in Li ₄ Ti ₅ O Bernhard Gadermaier, H. Martin R. Wilkening

Li-rich lithium titanate (Li _{4+ x} Ti ₅ O ₁₂ , x  > 0) is known for superior ionic conductivity, yet we show that stoichiometric Li ₄ Ti ₅ O ₁₂ (LTO, x  = 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 a -16 c -8 a 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 ( x  = 0) and indicating transferable pathways in related spinel-type ion conductors with similar Li ⁺ distributions.