Investigation of lithium diffusion in nano-sized rutile TiO2 by impedance spectroscopy

The kinetics of the electrochemical lithium insertion reaction in nano-sized rutile TiO2 has been investigated using ac impedance spectroscopy. The experimental data are obtained for a rutile compound synthesized via a solution technique and characterized by a morphology corresponding to spherical particles made of a large number of very thin nanorods 20 nm thick. The results are discussed as a function of the Li content x for 0 < x <= 0.8 in LixTiO2, the temperature over the range 10-50 degrees C and the number of discharge-charge cycles. The significant linear decrease of the chemical diffusion coefficient D-Li vs. the lithium content and the high values of Du found in the composition range 0 <= 0.5 are discussed and related with the electrochemical behaviour of the nano-sized material. From comparison with the bulk material, a promoting effect of the morphology on the kinetic characteristics is evidenced. For the first time an experimental evaluation of the activation energy for Li diffusion in nano-sized rutile TiO2 is obtained; the value of 0.35 eV being much lower than that reported from computational experiments for the micro-sized oxide. This work also demonstrates a new system takes place from the second cycle, characterized by a significant improvement of Li diffusion by a factor five and allowing high rates to be used. (C) 2010 Elsevier Ltd. All rights reserved.