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

Li-rich lithium titanate (Li4+xTi5O12, x > 0) is known for superior ionic conductivity, yet we show that stoichiometric Li4Ti5O12 (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 8a-16c-8a 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.