Understanding fast ion dynamics in sodiated Li₄Na _x Ti₅O₁₂: from interfacial to extended Li⁺ and Na⁺ dynamics in its mixed-conducting solid solutions

Climate change and energy crises require the development of new sustainable materials to realise reliable electrochemical energy storage devices. Spinel-type Li₄Ti₅O₁₂ (LTO) is one of the most promising anode materials not only for Li-based batteries, but also for those relying on sodium. While Li⁺ ion dynamics at the early stages of lithiation has been studied already previously, almost no data on the diffusion properties of Na⁺ ions can be found in the literature. Here, we used nucleus-specific ⁷Li and ²³Na nuclear magnetic resonance (NMR) spectroscopy to quantify the motional processes in mixed-conducting Li₄Na _x Ti₅O₁₂ with x = 0.1, 0.5 and 1.5 on the angstrom length scale. Most importantly, our results reveal a strong increase in Li⁺ diffusivity in the early stages of chemical sodiation that is accompanied by a sharp decrease in activation energy when x reaches 0.5. The two-component ⁷Li NMR spectra point to the evolution of an interfacial solid solution at very low sodiation levels (x = 0.1). At x = 0.5, these regions emerge over almost the entire crystallite area, enabling rapid 8a-16c-8a Li⁺ exchange (0.4 eV), which leads to facile long-range ion transport. We direct the attention of the reader towards the initial formation of solid solutions in LTO-based anode materials and their capital impact on overall ion dynamics. In contrast to macroscopic electrochemical testing, NMR is uniquely positioned to detect and to resolve these exceptionally fast ion dynamics during the initial stages of sodiation. As these processes crucially determine the fast-charging performance of LTO-type batteries, our study lays the atomistic foundations to establish a general understanding of why two-phase materials such as LTO can act as an impressive insertion host for both Li and Na ions.