Discriminating the Mobile Ions from the Immobile Ones in Li_4+xTi₅O₁₂: ⁶Li NMR Reveals the Main Li⁺ Diffusion Pathway and Proposes a Refined Lithiation Mechanism

One of the most widely known anode materials with excellent cycling performance in lithium-ion batteries is zero-strain lithium titanate Li₄Ti₅O₁₂ (LTO). This is based on the highly reversible insertion and deinsertion of Li ions varying the composition of Li_4+xTi₅O₁₂ from x = 0 to x = 3. The facile incorporation of Li is tightly connected with bulk Li ion dynamics. Although macroscopic electrochemical properties have been studied in great detail, bulk Li ion diffusion pathways have rarely been probed experimentally from an atomic scale point of view. Here, we evaluated 1D and 2D ⁶Li magic angle spinning (MAS) NMR spectra recorded as a function of x to make the local magnetic changes during Li insertion visible. This enabled us to directly show that rapid Li exchange involves the 8a and 16c sites, forming a 3D network throughout the LTO spinel structure. In contrast, considering the time scale set by our NMR experiments, the Li ions residing on the mixed occupied octahedral positions 16d are quite immobile. Under the MAS conditions applied, they definitely do not participate in rapid Li self-diffusion as evidenced by rather long site-specific longitudinal relaxation times as well as the absence of any NMR coalescence effects. Based on the NMR spectra, we propose a refined Li insertion mechanism which includes the initial formation of a highly conducting quasi solid solution characterized by rapid Li ion exchange.