Local Structures and Atomic-Scale Ion Dynamics in Li⁺/Na⁺ Mixed Solid Electrolytes

Most solid electrolytes utilize Li compounds, without mixing alkali cations. However, NaI–NaBH4–LiI solid solution with a dilute Li+ concentration exhibits dominant Li+ conduction, presenting an approach for solid electrolyte development. In such Li+/Na+ mixed electrolytes, little is known about the individual Li+/Na+ dynamics or the local structure of each ion. This knowledge is crucial for a fundamental understanding of Li+/Na+ dynamics in mixed-alkali compounds. Reverse Monte Carlo and neural network potential molecular dynamics simulations are used to investigate the local structure and ion dynamics of NaI–NaBH4–LiI. The Li+ exhibits off-center displacement in LiI6 polyhedra, which partially explains the dominant Li+ conduction in NaI with a large space available for hopping. Molecular dynamics simulations confirm preferential Li+ conduction, with a hopping energy of 0.17 eV. Li+ hopping occurs via cation vacancies formed by Schottky defects, but the low concentrations of Li+ and vacancies make it difficult to form a percolating diffusion network. Although Na+ ions dominate the cation sites and act as obstacles, they can still hop via vacancies with a relatively high migration energy (0.33 eV). This Na+ mobility helps bridge Li+ pathways, enabling a connected Li+ conduction network and resulting in high Li+ conductivity. These findings are essential for the future development of Li+/Na+ mixed electrolytes.