Li-ion diffusion in Li4Ti5O12 and LiTi2O4 battery materials detected by muon spin spectroscopy

Lithium diffusion in spinel Li4Ti5O12 and LiTi2O4 compounds for future battery applications has been studied with muon spin relaxation (mu+SR). Measurements were performed on both thin-film and powder samples in the temperature range between 25 and 500 K. For Li4Ti5O12 and above about similar to 200 K, the field distribution width (Delta) is found to decrease gradually, while the field fluctuation rate (nu) increases exponentially with temperature. For LiTi2O4, on the contrary, the Delta(T) curve shows a steplike decrease at similar to 350 K, around which the nu(T) curve exhibits a local maximum. These behaviors suggest that Li+ starts to diffuse above around 200 K for both spinels. Assuming a jump diffusion of Li+ at the tetrahedral 8a site to the vacant octahedral 16c site, diffusion coefficients of Li+ at 300 K in the film samples are estimated as (3.2 +/- 0.8) x 10(-11) cm(2)/s for Li4Ti5O12 and (3.6 +/- 1.1) x 10(-11) cm(2)/s for LiTi2O4. Further, some small differences are found in both thermal activation energies and Li-ion diffusion coefficients between the powder and thin-film samples.