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Li transport characteristics are studied by means of density functional theory (DFT) and molecular dynamics (MD) simulations in order to investigate concentration effects on Li chemical diffusivity and conductivity in TiO2 rutile. Our MD simulations predict one-dimensional diffusion of Li ions via jumps between the octahedral sites along the channels parallel to the c-axis. The diffusion barrier and diffusion coefficient (at room temperature) for the isolated Li, determined by means of DFT calculations, correspond to 60 meV and 9.1 × 10−6 cm2 s−1, respectively. Such a small barrier suggests rapid mass transport along the channels. MD simulations are performed to evaluate the concentration dependent diffusivity profiles. The changes in Li energetics and dynamics are studied as a function of Li content, which is varied primarily between 10% and 50%. In addition, we consider a couple of compositions over 50% although this is above the intercalation limit. Our results suggest that Li diffusivity is strongly dependent on the Li∶TiO2 ratio, and it decreases with increasing Li concentration. For instance, at room temperature, we find Li diffusivity for high concentrations (50% Li) to be three orders of magnitude slower than that for lower concentrations (10% Li). Our analyses on the energetics and dynamics suggest that the changes in the diffusivities originate from successive increases in the barriers with increasing concentration. The decrease in diffusivity as a function of increasing Li content is attributed to the fact that additional Li ions successively block the energetically preferred vacant sites along the channels. Our analyses also show that increasing Li concentration enhances the Li–Li repulsion within the channels, and as a result, diffusion is hindered. We also compare concentration-dependent diffusivities for Li diffusion in anatase, rutile and amorphous TiO2. Interestingly, we find differing concentration dependence of the diffusivity in these chemically identical but structurally non-equivalent TiO2 polymorphs. Our study suggests that these differences result from intrinsic structural characteristics of TiO2 polymorphs, which ultimately contribute to intercalation limit, diffusivity, ionic conductivity, and the electrochemical performance in energy storage applications.
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Physical Chemistry Chemical Physics
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