Strong Li-Content Dependence of Li Diffusivity in TiO2-B

The lithiation process in TiO2-B as a function of Li content x is studied using density functional theory. Considering a variety of possible pathways, we find that stepwise insertion of Li in TiO2-B layers, followed by insertion of Li in 0 layers, is kinetically the most favorable scenario. Diffusion coefficient (D) varies only by 2 orders of magnitude with x for Li contents up to x = 0.75 (3.03 X 10(-10) cm(2)/s <= D <= 7.94 X 10(-8) cm(2)/s), but it becomes almost 6 and 26 orders of magnitude smaller for 0.75 < x <= 1.0 and 1.00 < x < 1.25, respectively. To make a direct comparison to experimental measurements, the maximum diffusion length was estimated for different values of C-rate. It is found that, for any reasonable value of C-rate between 0.1C and 10C, bulk TiO2-B cannot attain capacities larger than 251 mAh g(-1). However, larger capacities up to 335 mAh g(-1) can be obtained by nanosized TiO2-B. In addition, we find that a capacity higher than 335 mAh g(-1). cannot be achieved. Our results suggest that the small discharge capacity of bulk TiO2-B is due to the large increase in the energy barrier of Li diffusion for Li contents above x = 0.75.