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

Abstract

Climate change and energy crises require the development of new sustainable materials to realise new electrochemical energy storage devices. Spinel-type Li4Ti5O12 (LTO) is one of the most promising anode material 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 earlier, almost no data on the dynamic properties of Na+ ions can be found in literature. Here, we used nucleus-specific 7Li and 23Na 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 sodiation that is accompanied by a sharp decrease in activation energy when x reaches 0.5. The two-component 7Li NMR spectra revealed 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 highly 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 early 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 universal understanding of why two-phase materials such as Li4Ti5O12 can act as an impressive insertion host for Li and Na.

Impact of thermal treatment on the Li-ion transport, interfacial properties, and composite preparation of LLZO garnets for solid-state electrolytes

“…In general, the linewidth (FWHM) of 7 Li NMR signals is directly related to ion dynamics, as faster ion motions can effectively average out the anisotropic interactions present in the solid state, 38,39 reducing the observed linewidths. Fig.…”