Slow Li Self-Diffusion in Li Intercalated Nanometer-Sized Needlelike Rutile TiO2 as Probed by Mixing Time Dependent Li Stimulated Echo NMR Spectroscopy

The role of nanocrystalline materials in addressing the current challenges in energy storage has attracted large attention. Besides nanostructured B-type TiO 2 [1], being the fifth polymorph of titanium dioxide, Li intercalated nanocrystalline rutile is considered to act as a potential anode in inherently safe secondary Li ion batteries. In contrast to its coarse-grained counterpart, which is electrochemical-ly inactive, nanocrystalline Li x TiO 2 (rutile) shows a high capacity, high rate performance, and good cycleability when used as an electrode material [2] demonstrating that nanostructuring is a promising strategy to meet the high power and high energy density requirements for Li-based energy storage devices. The advantages of nano-electrodes over those composed of micrometer-sized bulk materials are due to their shorter lengths for both electronic and Li + transport, a higher electrode-electrolyte contact area, and a much better compensation of the strain arising during Li + insertion and extraction. Li diffusion in rutile is reported to be highly anisotropic [3], and Li insertion, which can be carried out chemically, electrochemically or by ball-milling of nanocrystalline TiO 2 with Li foil, is mainly a surface effect which seems to be due to slow Li diffusion in the ab planes preventing the Li ions to reach the thermodynamically preferred octahedral vacancies. Furthermore, repulsive Li-Li interactions in the c-channels seem to play an important role when Li is inserted into the rutile structure. Here, needlelike, nanocrystalline rutile TiO 2 (NanoAmor) of less than 20 nm in diameter and approximately 100 nm in length (see Fig. 1a) was treated with a solution of n-BuLi in hexane at room temperature. SEM images and X-ray diffraction patterns of the so obtained highly air-sensitive powder of Li x TiO 2 (cobalt blue) confirmed that the rutile structure is fully retained during Li insertion. This clearly indicates that a Li content x of less than 0.5 Li atoms per formula unit was obtained, because at higher values of x the material can undergo several phase transformations [4]. In order to study the Li self-diffusion parameters of the mixed conducting product, we carried out 7 Li stimulated echo NMR measurements being sensitive to changes of the electric environment of the Li ions when jumping between distinct crystallographic sites in Li x TiO 2 , see Ref. [5] for details of the method. By utilizing the Jeener-Broekaert pulse sequence, (90°) X – t p – (45°) Y – t m – …