American Chemical Society, Chemistry of Materials, 5(27), p. 1740-1750, 2015
DOI: 10.1021/cm504564k
Full text: Download
Lithium titanate (LTO) is one of the most promising anode materials for large-scale stationary electrochemical storage of energy produced from renewable sources. Besides many other aspects, such as negligible formation of passivation layers and no volume expansion during lithiation, the success of LTO is mainly based on its ability to easily accommodate and release Li ions in a fully reversible way. This feature is tightly connected with Li self-diffusion. As yet, little information are available about microscopic Li diffusion properties and elementary steps of Li hopping at low intercalation levels, i.e., at values of x being significantly smaller than 1. Here, we used Li spin-locking NMR relaxometry to probe absolute hopping rates of LTO (homogeneous) solid solutions in quasi thermodynamic equilibrium. As a result, the largest increase of Li diffusivity is observed when small amounts of Li are inserted. Strong Coulomb repulsions caused by the simultaneous occupation of neighbored 8a and 16c sites serves as an explanation for the enhanced Li diffusivity found. At even larger values of x Li mobility slows down but is still much faster than in the host material with x = 0. Our results experimentally corroborate the outcomes of recently published calculations on DFT level focussing on both dynamic and structural aspects. The findings favour the formation of LTO solid solutions upon chemical lithiation; the steep increase in Li diffusivity found might also help understanding the flat insertion potential observed.