Anomalous temperature dependence of self-interstitial diffusivity in metallic lithium and sodium

Abstract

Despite the abundance and importance of alkali metals for various applications, the behavior of point and dimensional defects in them has remained terra incognita. At the same time, the growing interest to high-capacity Li-metal batteries and the study of Li electrodeposition mechanisms dictates the need to investigate the defect dynamics. Here, we report the results of MD simulations of atomic motions in bulk Li and Na, including vacancy and interstitial (particularly <111>-crowdion) defect dynamics. Our main finding is an anomalous temperature dependence of the apparent diffusion coefficient for self-interstitial atoms (SIA), namely, the diffusivity decrease with the temperature rise. To uncover the underlying mechanism, we analyzed the interplay between 1D motions in terms of mean squared displacement (MSD) and 3D motions due to SIA rotations or switching. We found that the coefficient for 1D crowdion migration along the 〈111〉 axes is virtually temperature independent in case of both Li and Na. This implies linear temperature dependence of the drag coefficient γ, in contrast to other BCC metals. High rotation frequency reaching several THz at 300 K seems to drive the 1D diffusion into a semi-ballistic mode, therefore also affecting the temperature dependence of the apparent 3D diffusion coefficient. We believe that our results contribute to the atomistic understanding of low-barrier processes in a crystal lattice.